Adsorption on Surfaces and Surface Diffusion of Adsorbates (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Condensed Matter)

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Landolt-Börnstein Numerical Data and Functional Relationships in Science and Technology New Series / Editor in Chief: W. Martienssen

Group III: Condensed Matter Volume 42

Physics of Covered Solid Surfaces Subvolume A Adsorbed Layers on Surfaces Part 1 Adsorption on Surfaces and Surface Diffusion of Adsorbates

Editor H.P. Bonzel Authors E.I. Altman, M. Bienfait, H.P. Bonzel, H. Brune, R. Diehl, M.Y.L. Jung, V.G. Lifshitz, M.E. Michel, R. Miranda, R. McGrath, K. Oura, A.A. Saranin, E.G. Seebauer, P. Zeppenfeld, A.V. Zotov

13

ISSN 1615-1925 (Condensed Matter) ISBN 3-540-41223-9 Springer-Verlag Berlin Heidelberg New York

Library of Congress Cataloging in Publication Data Zahlenwerte und Funktionen aus Naturwissenschaften und Technik, Neue Serie Editor in Chief: W. Martienssen Vol. III/42A1: Editor: H.P. Bonzel At head of title: Landolt-Börnstein. Added t.p.: Numerical data and functional relationships in science and technology. Tables chiefly in English. Intended to supersede the Physikalisch-chemische Tabellen by H. Landolt and R. Börnstein of which the 6th ed. began publication in 1950 under title: Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik und Technik. Vols. published after v. 1 of group I have imprint: Berlin, New York, Springer-Verlag Includes bibliographies. 1. Physics--Tables. 2. Chemistry--Tables. 3. Engineering--Tables. I. Börnstein, R. (Richard), 1852-1913. II. Landolt, H. (Hans), 1831-1910. III. Physikalisch-chemische Tabellen. IV. Title: Numerical data and functional relationships in science and technology. QC61.23 502'.12 62-53136 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in other ways, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution act under German Copyright Law. Springer-Verlag Berlin Heidelberg New York a member of BertelsmannSpringer Science+Business Media GmbH © Springer-Verlag Berlin Heidelberg 2001 Printed in Germany The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Product Liability: The data and other information in this handbook have been carefully extracted and evaluated by experts from the original literature. Furthermore, they have been checked for correctness by authors and the editorial staff before printing. Nevertheless, the publisher can give no guarantee for the correctness of the data and information provided. In any individual case of application, the respective user must check the correctness by consulting other relevant sources of information. Cover layout: Erich Kirchner, Heidelberg Typesetting: Authors and Redaktion Landolt-Börnstein, Darmstadt Printing: Computer to plate, Mercedes-Druck, Berlin Binding: Lüderitz & Bauer, Berlin SPIN: 10689660

63/3020 - 5 4 3 2 1 0 – Printed on acid-free paper

Editor H.P. Bonzel Forschungszentrum Jülich Institut für Grenzflächenforschung und Vakuumphysik (IGV) 52425 Jülich Germany

Authors E.I. Altman Department of Chemical Engineering Yale University New Haven, CT 06520 USA 3.4.3 Halogens on metals and semiconductors M. Bienfait CRMC2/CNRS Faculté de Luminy Physique - Case 910 F-13288 Marseille Cedex 9 FRANCE 3.1.2 Noble gases on graphite, lamellar halides, MgO, NaCl H.P. Bonzel Forschungszentrum Jülich Institut für Grenzflächenforschung und Vakuumphysik (IGV) 52425 Jülich Germany 1 Introduction to physical and chemical properties of adlayer/substrate systems 3.7.1 CO and N2 on metals W.A. Brown Department of Chemistry University College London London WC1H 0AJ U.K. 3.7.2 NO, CN, O2 on metals H. Brune Institut de Physique Expérimentale (IPE) École Polytechnique Fédérale de Lausanne (EPFL) PHB-Ecublens CH-1015 Lausanne 3.3.1 Metals on metals

K. Christmann Institut für Physikalische und Theoretische Chemie Freie Universität Berlin 14195 Berlin Germany 3.4.1 Chemisorbed hydrogen on metals and semiconductors R. Denecke Department of Physics Uppsala University S-751 21 Uppsala Sweden 4.3 Electron binding energy of metals and semiconductors R.D. Diehl Department of Physics Pennsylvania State University University Park, PA 16802 USA 3.2.1 Alkali metals on metals M. Enachescu Candescent Technologies 6320 San Ignacio Ave. San José, CA 95119 USA 3.4.4 P, S, As, Sb on metals and semiconductors N. Esser Institut für Festkörperphysik Technische Universität Berlin D-10623 Berlin Germany 4.6 Surface optical properties J.E. Fieberg Hillsdale College Hillsdale, MI 49242 USA 3.8.9 Halogen-substituted hydrocarbons on metals and semiconductors A. Föhlisch II. Institut für Experimentalphysik Universität Hamburg D-22761 Hamburg Germany 3.7.1 CO and N2 on metals H.-J. Freund Fritz-Haber-Institut der Max Planck Gesellschaft (MPG) D-14195 Berlin Germany 3.9 Adsorption on oxides

H.J. Grabke Max-Planck Institut (MPI) für Eisenforschung GmbH D-40074 Düsseldorf Germany 3.5 Surface segregation of atomic species (non-metal on metal) M. Grunze Institut für Angewandte Physikalische Chemie Universität Heidelberg D-69120 Heidelberg Germany 3.8.10 Polyatomic chain-like hydrocarbons on metals and semiconductors E. Hasselbrink Institut für Physikalische und Theoretische Chemie Universität Essen D-45117 Essen Germany 3.8.3 NH3 and PF3 on metals and semiconductors J. Heidberg Institut für Physikalische Chemie und Elektrochemie Universität Hannover D-30167 Hannover Germany 3.10 Adsorption on alkali halides G. Held Institut fuer Physikalische und Theoretische Chemie Universität Erlangen-Nürnberg D-91058 Erlangen Germany 3.8.7 Cyclic hydrocarbons on metals and semiconductors K. Hermann Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) Abteilung Theorie D-14195 Berlin Germany 4.1 Surface structure on metals and semiconductors K. Horn Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) Abteilung Theorie D-14195 Berlin Germany 3.7.4 Diatomic molecules on semiconductors

H. Ibach Institut für Grenzflächenforschung und Vakuumphysik (IGV) Forschungszentrum Jülich D-52425 Jülich Germany 4.4 Surface free energy and surface stress K. Jacobi Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) D-14195 Berlin Germany 4.2 Electron work function of metals and semiconductors W. Jaegermann Fachbereich Materialwissenschaft Fachgebiet Oberflächenforschung Technische Universität Darmstadt D-64287 Darmstadt Germany 3.8.2 H2O and OH on semiconductors M.Y.L. Jung Deppartment of Chemical Engineering University of Illinois Urbana, IL 61801 USA 3.11 Surface diffusion on metals, semiconductors and insulators B.E. Koel Department of Chemistry, SSC 606 University of Southern California Los Angeles, CA 90089-0482 USA 3.8.4 CO2, NO2, SO2, OCS, N2O, O3 on metals and semiconductors H. Kuhlenbeck Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) Abteilung Chemische Physik D-14195 Berlin Germany 3.9 Adsorption on oxides V.G. Lifshits Institute of Automation and Control Processes 690041 Vladivostok Russia 3.3.2 Metals on semiconductors

A.C. Luntz Fysik Institut Odense Universitet 5230 Odense M Denmark 3.12 Dynamics of activated adsorption N. Martensson Department of Physics Uppsala University S-751 21 Uppsala Sweden 4.3 Electron binding energy of metals and semiconductors T. Mayer Fachbereich Materialwissenschaft Fachgebiet Oberflächenforschung Technische Universität Darmstadt D-64287 Darmstadt Germany 3.8.2 H2O and OH on semiconductors R. McGrath Surface Science Research Centre and Department of Physics The University of Liverpool Liverpool L69 3BX U.K. 3.2.1 Alkali metals on metals E.G. Michel Departimento Fisica de la Materia Condensada C-III Instituto Universitario de Ciencia de Materiales "Nicolas Cabrera" Universidad Autonoma de Madrid 28049 Madrid Spain 3.2.2 Alkali metals on semiconductors R. Miranda Departimento Fisica de la Materia Condensada C-III Instituto Universitario de Ciencia de Materiales "Nicolas Cabrera" Universidad Autonoma de Madrid 28049 Madrid Spain 3.2.2 Alkali metals on semiconductors D.R. Mullins Oak Ridge National Laboratory Oak Ridge, TN 37831-6201 USA 3.8.5 Substituted hydrocarbons on metals

B.E. Nieuwenhuys Gorlaeus Laboratory Leiden University NL 2300 Ra Leiden The Netherlands 3.7.3 Diatomic molecules on alloys A. Nilsson Department of Physics Uppsala University S-751 21 Uppsala Sweden 3.7.1 CO and N2 on metals 4.3 Electron binding energy of metals and semiconductors K. Oura Department of Electronic Engineering Faculty of Engineering Osaka University Osaka 565-0871 Japan 3.3.2 Metals on semiconductors H. Over Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) D-14195 Berlin Germany 3.4.2 C, N, O on metals and semiconductors G. Pirug Institut für Grenzflächenforschung und Vakuumphysik (IGV) Forschungszentrum Jülich D-52425 Jülich Germany 3.8.1 H2O and OH on metals W. Richter Institut für Festkörperphysik Technische Universität Berlin D-10623 Berlin Germany 4.6 Surface optical properties M. Rocca Centro di Fisica delle Superfici e Basse Temperature del CNR Istituto Nazionale di Fisica della Materia I-16146 Genova Italy 4.5 Surface phonon dispersion

G. Rupprechter Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) Abteilung Chemische Physik D-14195 Berlin Germany 3.8.6 Linear hydrocarbons and CH4 on metals and semiconductors M. Salmeron Lawrence Berkeley Laboratory Materials Science Bldg. 66/208 Berkeley, CA 94720 USA 3.4.4 P, S, As, Sb on metals and semiconductors D. Sander Max-Planck Institut (MPI) für Strukturphysik D-06120 Halle Germany 4.4 Surface free energy and surface stress A.A. Saranin Institute of Automation and Control Processes 690041 Vladivostok Faculty of Physics and Engineering Far Eastern State University 690000 Vladivostok Russia 3.3.2 Metals on semiconductors E.G. Seebauer Deppartment of Chemical Engineering University of Illinois Urbana, IL 61801 USA 3.11 Surface diffusion on metals, semiconductors and insulators M. Sokolowski Institut für Physikalische und Theoretische Chemie Universität Bonn D-53115 Bonn Germany 3.8.11 Large aromatic adsorbates on metals and semiconductors G.A. Somorjai Department of Chemistry University of California Berkeley, CA 94720 USA 3.8.6 Linear hydrocarbons and CH4 on metals and semiconductors

H.-P. Steinrück Institut fuer Physikalische und Theoretische Chemie Universität Erlangen-Nürnberg D-91058 Erlangen Germany 3.8.7 Cyclic hydrocarbons on metals and semiconductors J. Suzanne Departement de Physique CRMC2 - Centre National de la Recherche Scientifique (CNRS) Faculte des Sciences de Luminy F-13288 Marseille, Cedex 9 France 3.6 Molecules on graphite, BN, MgO (except noble gases) W.T. Tysoe Department of Chemistry and Laboratory for Surface Studies University of Wisconsin - Milwaukee Milwaukee, WI 53211 USA 3.8.5 Substituted hydrocarbons on metals Ch. Uebing Department of Physics and Astronomy Rutgers, The State University of New Jersey Piscataway, NJ 08854-8019 USA 3.5 Surface segregation of atomic species (non-metal on metal) E. Umbach Lehrstuhl für Experimentelle Physik II Physikalisches Institut der Universität D-97074 Würzburg Germany 3.8.11 Large aromatic adsorbates on metals and semiconductors H. Viefhaus Max-Planck Institut (MPI) für Eisenforschung GmbH D-40074 Düsseldorf Germany 3.5 Surface segregation of atomic species (non-metal on metal) J.M. Vohs Department of Chemical Engineering University of Pennsylvania Philadelphia, PA 19104-6315 USA 3.8.8 Oxygenated hydrocarbons on metals and semiconductors

M.A. Van Hove Lawrence Berkeley National Laboratory Materials Science 66 Berkeley, CA 94720 and Department of Physics University of California-Davis Davis, CA 95616 USA 4.1 Surface structure on metals and semiconductors P.R. Watson Department of Chemistry Oregon State University Corvallis, OR 97331 USA 4.1 Surface structure on metals and semiconductors J.M. White Department of Chemistry and Biochemistry University of Texas at Austin Austin, TX 78712 USA 3.8.9 Halogen-substituted hydrocarbons on metals and semiconductors H. Wiechert Institut für Physik der Johann Gutenberg-Universität D-55099 Mainz Germany Molecules on graphite, BN, MgO (except noble gases) Ch. Wöll Lehrstuhl für Physikalische Chemie I Ruhr-Universität Bochum D-44801 Bochum Germany 2 Characterization of adsorbate overlayers: Measuring techniques P. Zeppenfeld Institut für Experimentalphysik Atom- und Oberflächenphysik Johannes-Kepler-Universität Linz A-4040 Linz, Austria 3.1.1 Noble gases on metals and semiconductors A.V. Zotov Faculty of Electronics Vladivostok State University of Economics and Service 690600 Vladivostok Institute of Automation and Control Processes 690041 Vladivostok Russia 3.3.2 Metals on semiconductors

Landolt-Börnstein Editorial Office Gagernstr. 8, D-64283 Darmstadt, Germany fax: +49 (6151) 171760 e-mail: [email protected] Internet http://www.landolt-boernstein.com Helpdesk e-mail: [email protected]

Preface Surface Science is understood as a relatively young scientific discipline, concerned with the physical and chemical properties of and phenomena on clean and covered solid surfaces, studied under a variety of conditions. The adsorption of atoms and molecules on solid surfaces is, for example, such a condition, connected with more or less drastic changes of all surface properties. An adsorption event is frequently observed in nature and found to be of technical importance in many industrial processes. For this reason, Surface Science is interdisciplinary by its very nature, and as such an important intermediary between fundamental and applied research. The beginning of Surface Science may be placed around the years 1900-1928, connected with seminal works by J.W. Gibbs, I. Langmuir, M. Knudsen, O. Stern, M. Volmer, C. Davisson and L. Germer, H.S. Taylor, J.E. Lennard-Jones, I.N. Stranski and others. In modern times, research activities in this field have literally exploded worldwide. Consequently, enormous progress can be recognized and it is no exaggeration, to note a high degree of maturity in this well-established scientific discipline. Tribute is being paid to this effect by the renowned Series of Landolt-Börnstein whose editor-in-chief Werner Martienssen, Frankfurt am Main, has initiated several volumes of collected scientific data in the field of Surface Science. Indeed, the point in time has arrived where all quantitative data, that have been generated over so many years, are worth being collected in tables and figures for critical review and reference purposes as well. The beginning has been made with Landolt-Börnstein volume 24, entitled Physics of Solid Surfaces. This volume, consisting of four subvolumes, appeared in 1993-96 and covers the properties of clean solid surfaces. The present volume 42 is devoted to Covered Solid Surfaces and, in particular, to Adsorbed Layers on Surfaces. It is as such a collection of data obtained for adsorbates on well-defined crystalline surfaces. "Well-defined" means surfaces of known crystallographic structure and chemical composition. These conditions can in most cases be realized by careful sample preparation in ultra-high vacuum. Work on the present volume started in late 1997 when I was first contacted by W. Martienssen. An initial outline of the volume was written in January of 1998. At this point I want to express my sincere gratitude to George Comsa, Bonn, and Gerhard Ertl, Berlin, for their support by making valuable suggestions, concerning both the outline and the choice of possible authors. In fact, the choices made at the time proved to be excellent ones, and the consulting of G. Comsa and G. Ertl turned out to be extremely helpful for the evolution of the present volume. It was almost clear at the beginning, that the amount of general information and quantitative data on Adsorbed Layers on Surfaces is enormous, too large to fit into a single volume. Hence, again several subvolumes had to be planned. Unfortunately, the chapters anticipated for each of the subvolumes did not arrive synchronously with the production schedule, such that the sequence of chapters actually printed in the subvolumes deviates from that in the general outline of the whole volume. We apologize for this inconvenience, but in the age of electronic information distribution this problem will be solved, once all volumes are available electronically. Search routines will guide the reader to the data of his desire. Until that time, the index of each subvolume will have to do. Finally and most importantly, I would like to extend my deep appreciation to all authors of this volume for their excellent contributions, and to the editing and production offices of Springer-Verlag for efficient cooperation and general support. Jülich, May 2001

The Editor

Survey of Volume III/42 Physics of Covered Solid Surfaces Subvolume A: Adsorbed Layers on Surfaces 1

2

Introduction to physical and chemical properties of adlayer/substrate systems

subvolume A1

Characterization of adsorbate overlayers: Measuring techniques

subvolume A2

3

DATA: Adsorbate properties of

3.1 3.1.1 3.1.2

Adsorption of noble gases Noble gases on metal substrates and semiconductors Noble gases on graphite, lamellar halides, MgO, and NaCl

subvolume A1 subvolume A1

3.2 3.2.1 3.2.2

Adsorption of alkali metals Alkali metals on metals Alkali metals on semiconductors

subvolume A1 subvolume A1

3.3 3.3.1 3.3.2

Adsorption of metals Metals on metals Metals on semiconductors

subvolume A1 subvolume A1

3.4 3.4.1 3.4.2 3.4.3 3.4.4

Non-metallic atomic adsorbates on metals and semiconductors Chemisorbed hydrogen on metals and semiconductors C, N, O on metals and semiconductors Halogens on metals and semiconductors subvolume A1 P, S, As, Sb on metals and semiconductors subvolume A3

3.5

Surface segregation of atomic species (non-metal on metal)

subvolume A3

3.6

Adsorption of molecules on graphite, BN, MgO (except noble gases)

subvolume A3

Molecular diatomic adsorbates CO and N2 on metals NO, CN, O2 on metals Diatomic molecules on alloys Diatomic molecules on semiconductors

subvolume A3 subvolume A3 subvolume A3 subvolume A3 subvolume A3

3.7 3.7.1 3.7.2 3.7.3 3.7.4

3.8 3.8.1 3.8.2 3.8.3 3.8.4 3.8.5 3.8.6 3.8.7 3.8.8 3.8.9

3.8.11

Molecular polyatomic adsorbates on metals and semiconductors H2O and OH on metals H2O and OH on semiconductors NH3 and PF3 on metals and semiconductors CO2, NO2, SO2, OCS, N2O, O3 on metals and semiconductors Substituted hydrocarbons on metals Linear hydrocarbons and CH4 on metals and semiconductors Cyclic hydrocarbons on metals and semiconductors Oxygenated hydrocarbons on metals and semiconductors Halogen-substituted hydrocarbons on metals and semiconductors Polyatomic chain-like hydrocarbons on metals and semiconductors Large aromatic adsorbates on metals and semiconductors

3.9

Adsorption on oxide surfaces

subvolume A3

3.10

Adsorption on alkali halide surfaces

subvolume A3

3.8.10

3.11

Surface diffusion of adsorbates on metals, semiconductors and insulators

subvolume A3 subvolume A3 subvolume A3 subvolume A3 subvolume A3 subvolume A3 subvolume A3 subvolume A3 subvolume A3 subvolume A3 subvolume A3

subvolume A1

3.12

Dynamics of activated adsorption

subvolume A3

4

Data: Adsorbate-induced changes of substrate properties

4.1

Surface structure on metals and semiconductors

subvolume A2

4.2

Electron work function of metals and semiconductors

subvolume A2

4.3

Electron binding energy of metals and semiconductors

subvolume A3

4.4

Surface free energy and surface stress

subvolume A2

4.5

Surface phonon dispersion

subvolume A2

4.6

Surface optical properties

subvolume A3

Ref. p. 59]

1 Introduction

1

1 Introduction to physical and chemical properties of adlayer/substrate systems H.P. BONZEL

1.1 General background The Physics and Chemistry of solid surfaces has been studied ever since the early work of Langmuir [13Lan, 18Lan], Lennard-Jones [28Len, 32Len, 36Len, 37Len], Davisson and Germer [27Dav], Stranski [28Str, 31Str], Stern [29Ste], Volmer [21Vol, 39Vol] and others. Prior to those, J.W. Gibbs defined important thermodynamic properties of solid surfaces for the first time with unprecedented rigor in the framework of thermodynamic theory [1876, 1878]. Further interest in surfaces, particularly in chemical reactions at surfaces, evolved from the discovery and technical implementation of heterogeneous catalytic reactions at the turn of the 18th century [39Mit, 72Sch]. Numerous studies of adsorption and surface reactions on metal surfaces, mostly on finely dispersed particles, appeared in the field of catalytic chemistry. In the late 1940s, intense work on semiconductor surfaces and interfaces provided a new powerful impetus for the growth of surface physics [47Bar, 94Gat]. In later years, the development of numerous quantitative surface analytical tools played a major role for a further rapid advancement of the new field of surface science. These new analytical techniques which often required good vacuum for their operation, enabled a widespread research of clean and covered surfaces of different materials, such as metals and alloys, semiconductors, alkali halides, oxides, etc. The additional basic desire to prepare a surface of a solid in a well-defined state and keep it in that state for a time long enough to carry out a microscopic or spectroscopic investigation, called for most experiments to be run under clean ultra-high vacuum (UHV) conditions (i.e. < 10–9 mbar). The latter was made possible by advances in vacuum technology, such as the development of new vacuum pumps and pressure measuring devices. Thus UHV of < 1x10–10 mbar became routine for many surface science experiments, increasing the lifetime of a "clean" surface and supporting electron, ion and atom beam scattering studies from surfaces. In general, the study of surfaces can be divided into those of clean surfaces where foreign atoms or molecules are totally undesirable, and into those of covered surfaces where atoms or molecules are intentionally added. The latter process is that of adsorption, and depending on the strength of interaction between the added species and the surface, we distinguish physisorption (weak) and chemisorption (strong) [60Ada, 64Hay, 70Cla, 72Som2]. The adsorbed species is called the adsorbate while the material with the exposed surface is called either substrate or adsorbent. The term adsorbent is verbally closely linked to adsorption and appears frequently in the older literature dealing strictly with the phenomenon of adsorption. Nowadays one prefers to use the term substrate because of the obvious link to vapor deposition of thin films on a substrate. Vapor deposition is also an adsorption process but there the object of interest is usually a structurally well defined multi-layer film. At this point it may be helpful to address briefly the distinction between adsorbed layers and thin films. First of all, there is no rigorous difference from a physics point-of-view but traditionally a distinction can be made on the grounds of energetics and coverage. Adsorbed layers, in the sense of this Landolt-Börnstein volume at least, are defined as ensembles of adsorbed atoms or molecules at effective coverages up to one monolayer, weakly or strongly bound to a substrate. Intra-layer interaction can be repulsive or attractive. All physical and chemical aspects of the adsorption process and of the adsorbate itself, often in relationship to the equivalent gas phase species, are of interest. The present volume will therefore cover the relevant research results obtained in this area of adsorption on structurally and chemically well-defined crystalline surfaces. Thin films, on the other hand, are mostly multi-layer configurations of several nanometer to micrometer thickness on a given substrate. Interfacial as well as intra-layer bonding is generally strong (always

Landolt-Börnstein New Series III/42A

2

1 Introduction

[Ref. p. 59

attractive interaction). Structural questions, such as the formation of thin films (including nucleation and growth), its crystallographic orientation with respect to the substrate (epitaxy) and its compactness, are of primary interest. Technical aspects, such as the crystallinity and possibly grain size, the mechanical and chemical resistance of thin films play an important role. The properties of thin films, in contrast to adsorbed layers as defined above, will not be considered in this volume. The distinction between physi- and chemisorption is historical and not rigid in a scientific sense. Once atoms or molecules are adsorbed, their properties are distinctly different from those of their gaseous counterpart. The stronger they are adsorbed, the more significant are these differences. For example, their intra-molecular bonds are weakened or strengthened, hence bondlengths are changed, the energies of molecular orbitals and molecular vibrational frequencies are changed, etc. If the interaction between adsorbate and substrate is very strong, certain intra-molecular bonds may weaken to the extent that they rupture resulting in the partial (or total) dissociation of the adsorbed species. Then we have a case of a heterogeneous surface reaction. It may happen that one part of the reaction products desorbs from the surface, leaving the other adsorbed as a fragment (radical). An adsorption process of this kind is more complex and may consist of several steps: (1) physisorption of the molecule; (2) surface diffusion and transition into a chemisorbed state; (3) breakup of the molecule into fragments; (4) chemisorption of tightly bound fragment(s); (5) diffusion, association and desorption of weakly bound fragments. The physisorbed molecule in this chain of surface events is called a precursor state to the chemisorbed state. The rate of surface diffusion in the weakly bound precursor state is generally high, enabling the molecule to find an empty site for chemisorption. This search is limited by the finite lifetime in the precursor state. As indicated by this brief description, the process of adsorption of atoms and molecules on solid surfaces involves kinetic as well as static aspects. Obviously, the sequence of steps (1)-(5) above is a complex kinetic phenomenon. On the other hand, measuring the physical properties of an adsorbed atom, molecule or fragment concerns the static nature of that species. In both cases the structure and chemical composition of the clean surface is of importance, because the properties of the adsorbed species depend sensitively on the local structure and chemistry of the adsorption site. Thus the description of adsorbed layers on surfaces is not thinkable without a detailed knowledge of clean surfaces. It is therefore no coincidence that the current volume of Adsorbed Layers follows the Landolt-Börnstein volume on Clean Surfaces. Important data characterizing clean surfaces of metals, semiconductors etc. are collected in the Landolt-Börnstein volumes III/24, subvolumes A-D. The current volume on Adsorbed Layers will introduce and summarize the knowledge obtained on all aspects of adsorption of atoms and molecules on solid surfaces of well-defined structure and chemical composition. A brief introduction to the surface thermodynamic concepts and equations in relationship to adsorption on solid surfaces will be given [66Def, 73Hir, 73Wag, 96Des]. The physical and chemical properties of the adsorbed species as well as the substrate related to thermodynamics will be introduced in this context. A more complete introduction, by means of examples, into the important physical and chemical properties of adsorbates, of both static and dynamic origin, follows. Adsorbate-induced changes of substrate properties will also be introduced. The following major chapter is devoted to a short description of surface analytical techniques for the investigation of adsorption. Modern experimental methods will be preferred over older less reliable ones which have been applied at high pressure or poor vacuum to samples with ill-defined structure and composition. This chapter will complement the description of techniques given in the Landolt-Börnstein volume III/24. The subsequent presentation of data will be grouped into two main parts: (1) properties of adsorbates, and (2) adsorbate-induced changes of substrate properties. In the first part, the presentation of properties is divided into groups of increasing molecular size, followed by more general chapters on material aspects, such as oxide, alkali halide, graphite and boron nitride surfaces, or certain property aspects, such as surface diffusion of adsorbed species, and the dynamics of activated adsorption. In the second part, various substrate properties, that are significantly changed by adsorbed species are described in six chapters. These changed substrate properties are: surface structure, electron work function, electron binding energies, surface free energy and surface stress, surface phonons, and optical properties.

Landolt-Börnstein New Series III/42A

Ref. p. 59]

1 Introduction

3

1.2 Surface thermodynamics From a historical and conceptual point of view it is useful, at the beginning of a survey of adsorption experiments and data, to introduce the concepts of surface thermodynamics and to define the fundamental quantities associated with adsorption. The origin of these concepts goes back to Gibbs [1876, 1878]. The following equations will be given for a two-phase system (solid/gas or solid/liquid or liquid/gas) of constant volume V, consisting of N chemical components with concentrations ci [in number of molecules/unit volume], immersed in a heat bath at temperature T, and connected with reservoirs of particles of each component. The interfacial area between both phases shall be A. The change in Helmholtz free energy of the total system for an infinitesimal deviation from equilibrium is dF = −SdT − PdV + γ dA + ∑ µ i dn i ,

(1)

i

where γ is the specific surface energy, equal to the work required to reversibly increase the surface area, µi the chemical potential of components i, and ni = ciVi the total number of species. The specific surface energy and chemical potentials of each component are thus defined as  ∂F  γ = and   ∂A  T,V , n i

 ∂F  . µi =    ∂n i  T,V,A ,n

(2)

j

The operational definition of γ via a small change in surface area may conceptually be connected with a concomitant change in the concentration of one or several of the chemical components, ∆ni, in the second phase. To keep all concentrations (intensive variables) constant, the amount of ∆ni has to be replenished in that phase by adding it to the system through a leak, keeping T and V constant. The only place where the small amount of component i can go is the incremental increase in interfacial area, ∆A. This operation therefore defines the adsorption of component i on the interface between the two phases 1 and 2. The adsorbed amount per unit area is called the surface excess, equal to  ∆n  . Γi =  i   ∆A  T,P ,V,n

(3)

j

Gibbs views the interface with the adsorbed material as a "third phase" but without treating this phase in the same thermodynamic rigor as the other two phases. Instead he proposes a mathematical "dividing surface" which is placed arbitrarily but parallel to and near the geometric interface. Formally, a free energy, entropy and particle concentrations, but no volume, are attributed to the surface phase, which is indicated by the superscript (S). Differences are defined as follows [73Hir] F(S ) = F − F(1) − F( 2 ) , S(S ) = S − S(1) − S( 2 ) , (2) n (i S ) = n i − n (1) . i − ni Due to the mathematical form of the dividing surface, the surface phase has no volume, i.e. V(S) = 0.

Landolt-Börnstein New Series III/42A

(4a) (4b) (4c)

1 Introduction

4

[Ref. p. 59

Fig. 1a, b. Plot of particle density c profiles vs. distance x vertical to an interface for a two component system A, B to illustrate a possible position of the dividing surface between solid phase and gas phase. (a) Component A in substrate solid phase; (b) component B: adsorbate. Note that densities are not drawn to scale [73Hir].

Figure 1 illustrates graphically the interfacial region of a two-phase system for two components A and B, with low concentrations of A,B in the gas phase (2) and also of B dissolved in the solid phase (1). The dividing surface is placed parallel to the interface, such that the excess concentration ΓA is zero. The extent of the adsorbed layer is just one monolayer of B in this simple picture, and its actual volume is practically insignificant compared to the volume of either phase (1) or (2). Even if the adsorbed layer would consist of two or three monolayers, this would still be true. The excess concentration ΓB is nonzero and equal to the coverage of B. Based on Eqs. (1)-(4), the free energy change for the surface phase is dF(S ) = −S(S )dT + γdA + ∑ µ i dn (i S ) .

(5)

i

The corresponding internal energy change is obtained by dE (S ) = dF(S ) − TdS(S ) − S(S ) dT . The expression for the differential internal energy can be integrated, yielding E(S ) = TS(S ) + γA + ∑ µ i n (i S ) .

(6)

i

Taking the total differential of this equation and subtracting it from the previous equation produces the following Gibbs-Duhem relationship [73Hir] S(S )dT + Adγ + ∑ n (i S )dµ i = 0 .

(7)

i

Subtracting Eq. (7) from Eq. (5), we obtain dF(S ) = d ( γA ) + d  ∑ n (i S )µ i  . Integrating this equation and i  dividing by the area A, an important definition of the system's specific surface free energy, f(S), results: f (S ) = γ + ∑ Γi µ i , with Γi = i

n (i S ) . A

(8)

Landolt-Börnstein New Series III/42A

Ref. p. 59]

1 Introduction

5

The excess concentration Γi of the adsorbed component i may also be called the surface coverage of i in units of molecules/cm2. Equation (8) shows that the specific surface free energy is only equal to the surface energy γ, if the sum over all chemical potentials on the r.h.s. of Eq. (8) is zero; for a onecomponent system this requires Γ1 = 0. In other words, for a clean surface, free of adsorbates, the first layer of the substrate has no surface excess, hence f (S ) = γ . Converting the extensive variables in Eq. (7) to quantities per unit surface area yields the Gibbs adsorption equation dγ = − s ( S ) dT − ∑ Γi dµ i ,

(9)

i

which governs the dependence of the surface energy on temperature and chemical potential. The following partial derivatives are equal to the specific surface entropy and the surface excess, −Γi, respectively:  ∂γ  (S )   = −s ,  ∂T  n

 ∂γ  = − Γi .    ∂µ i  T,µ

and

i

(10)

j

Note that the surface entropy is itself not a constant. For a two-component system, under isothermal conditions, we have dγ = − Γ1dµ1 − Γ2dµ 2 . Imagine a solid (1) in equilibrium with the gas phase (2), and a certain amount of gas adsorbed. The chemical potential of the adsorbing ideal gas at pressure p2 shall be µ 2 = µ 20 + kT ln p 2 ,

(11)

such that  ∂γ  ∂γ  ∂µ 2  = − Γ2 =     ∂p 2  ∂p 2   ∂µ 2  T,µ1

−1

=

1  ∂γ  .   kT  ∂ ln p 2  T,µ

(12)

1

If the dividing surface is placed at the geometric interface as shown schematically in Fig. 1, the excess concentration Γ1 is zero, and Γ2 is just the amount of adsorbed gas on the solid surface. A similar case would be the segregation of solute to the surface of a solid. This corresponds to a single phase system with two components. If the solute is component 2, the same equation (12) describes the variation of surface energy with surface concentration of solute. The negative sign of the surface excess indicates the lowering of γ with adsorption or segregation. Vice versa: only those particles will adsorb, either from the gas phase or the interior of the solid, that cause a lowering of the surface energy. It follows from Eqs. (10) and (12) that there is an intimate connection between the surface energy γ and the phenomenon of adsorption at the surface. This is the reason why thermodynamic relationships involving the surface energy term are important for the discussion of adsorption. Any type of adsorbed layer on a solid surface will change the surface energy, and when the latter is anisotropic, i.e. dependent on crystallography, this change will also imply morphological changes at the surface. This may be particularly significant for small particles, such as present on a finely dispersed catalyst used for heterogeneous catalysis. The interplay between adsorption, changing surface energy, morphology changes, and concomitant changes in surface reactivity is one of the most important ways to understand the complexity of heterogeneous catalytic reactions. Along the same lines of reasoning it is necessary to also consider the effect of curved surfaces which alter the surface energy as a function of curvature [52Her, 63Mul, 66Def]. Thus curvature at the surface introduces another dimension in the relationship between surface energy, adsorption and reactivity. Finally, we introduce the concept of intrinsic surface stress [52Her, 1876] which is equal to the work required to deform a surface. It is directly related to the surface energy and its derivative with respect to strain. For crystalline surfaces, strain is a tensor, and thus the surface stress, τij, is also a tensor of second Landolt-Börnstein New Series III/42A

1 Introduction

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[Ref. p. 59

rank. It may be defined by the derivative of the surface energy per atom, γA, with respect to the strain tensor, εij which leads to the Shuttleworth equation [50Shu] τ ij =

∂γ 1 d( γA ) = γ δ ij + . ∂ε ij A dε ij

(13)

On surfaces of two-fold symmetry the surface stress is expected to be anisotropic. This has been verified theoretically for Pd(110) and Pt(110) surfaces where the surface stress in the close-packed [1 1 0] direction is higher than in the [001] direction [95Fei]. Any change in γ will cause a change in surface stress, and vice versa. Thus adsorption will change both quantities [97Iba]. Curved surfaces The role of curved surfaces is very important in context with solid crystals and adsorption, regardless whether isotropic or anisotropic surface energy is considered [52Her, 63Mul]. Since the surface energy enters all expressions which govern the change of a physical quantity with the radius of curvature at the surface, these dependencies play also a role in context with adsorption because the latter influences the surface energy as well as its possible anisotropy. When the surface energy is isotropic, most relationships for liquid/liquid and liquid/gas two-phase systems can be transferred to the solid/liquid and solid/gas systems. The curved surface portion of a solid is characterized by an additional chemical potential relative to that of the flat surface, µ0. The result is the Gibbs-Thomson equation: 1 1 µ( M ) = µ 0 + Ω γ  +  ,  r1 r2 

(14)

where r1 and r2 are the principal radii of curvature at point M, and Ω is the atomic volume of the solid(liquid) phase. As a consequence of this dependence, several physical properties change with the radii of curvature, such as the vapor pressure of small spherical particles (droplets): ln

p( r ) Ω 2γ . =− p0 kT r

(15)

Secondly, the melting temperature of small crystals will be a function of their radii, i.e. they will melt at a lower temperature than the bulk material, Tm, according to: ln

T( r ) Ω 2γ sl , =− Tm ∆H f r

(16)

with ∆Hf = heat of fusion and γsl = interfacial energy at the liquid/solid interface. Also, small crystals have a heat of sublimation smaller than the value for the bulk solid [66Def]. The surface energy of solid crystals is generally anisotropic, i.e. it depends on the crystallographic orientation of the surface. In that case a generalisation of the Gibbs-Thomson equation (14) was given by Herring [52Her]:  ∂2 γ  1  ∂2 γ  1  µ( M ) = µ 0 + Ω   γ + 2  +  γ + 2   . ∂θ1  r1  ∂θ 2  r2   

(17)

Landolt-Börnstein New Series III/42A

Ref. p. 59]

1 Introduction

7

Here the sum of γ and its second derivative with respect to orientation is called the surface stiffness. For the special case of a 2-dimensional monatomic island on a single crystal surface, the surface energy is replaced by the step energy, and the equation for the chemical potential of the island edge, equivalent to Eq. (17), is d γ step  1  µ step ( M ' ) = µ '0 +  γ step +  . dθ 2  r 

(18)

Here the expression in parentheses is called the step stiffness. Both surface and step stiffnesses may be changed by adsorption. Adsorption heats and energies Different types of heats of adsorption have been defined in classical thermodynamics but they are numerically similar. Their relationship to experimental determinations is more or less straightforward [68Cer, 83Cer]. The molar differential heat of adsorption, Qdiff, of a component i from the gas phase (1) on a solid (2) is defined as the difference in enthalpy associated with the transfer of one mole of i to the surface of the substrate at constant T, P and other components nj. Assuming ideal gas behavior, the differential heat of adsorption is defined as [66Def]  ∂H   ∂ ln p i  = − kT2  Q diff =  (S )   .  ∂T  Γ  ∂n i  T,P ,n ,n i i

(19)

j

The value of Qdiff is thus defined for a particular coverage Γi. The r.h.s. of Eq. (19) is analogous to the Clausius-Clapeyron equation and contains a directive for the experimentalist to measure the differential heat of adsorption as a function of coverage [69Tra]. The isosteric heat of adsorption is defined very similarly and differs from the differential heat only by kT (about 50 meV at 600 K), Q iso = Q diff + kT . The name isosteric heat comes from the r.h.s. of Eq. (19) where the partial derivative of ln pi with respect to temperature is taken at constant coverage. The function pi(T) at constant Γi is called isoster and can be obtained experimentally, e.g. by measuring the coverage as a function of temperature, keeping the pressure constant (isobars), and subsequently cross-plotting the data as isosters [69Tra]. Both, differential and isosteric heats of adsorption, evaluated at several coverages of adsorbate, describe in principal the coverage dependence of the adsorption heat. This variation of Q with coverage can be very substantial, in particular for chemisorption systems, as many investigations have shown. There is usually a sharp drop in the heat of adsorption when the first chemisorbed layer is filled and a second layer begins to be populated. Finally, we mention the calorimetric heat of adsorption, Qcal, which is also frequently measured, recently even for single crystal samples [98Bro]. The calorimetric heat can be thought of as an integral heat of adsorption, because the measurement may extend over a larger range of increasing coverage of the adsorbate. Only if the calorimeter has sufficient resolution, in terms of temperature change versus time, can the coverage dependence also be determined. Then it is possible to measure differential calorimetric heats, as the coverage increases from zero to a monolayer, for example. The exact correlation of changes in temperature to coverage is then a necessary requirement. Generally, the calorimetric heat of adsorption can not be expected to be equal to the isosteric or differential heats of adsorption. Thermodynamically, an expression for Qcal has been derived which relates it to the differential heat of adsorption as follows [66Def, 70Cla]:  ∂p (1)   ∂p (1)  dT   . Q cal = Q diff + V (1)  (iS )  +  i   ∂n i  T  ∂ T  n( S ) dn (i S )  i  

Landolt-Börnstein New Series III/42A

(20)

1 Introduction

8

[Ref. p. 59

Here the coverage dependence is contained in the derivatives inside the parentheses. These are difficult to measure, such that Eq. (20) is not an easy directive to follow. Nevertheless, many different experimental approaches to measuring the calorimetric heat of adsorption have been proposed and tested. If the calorimeter can be operated differentially, e.g. by sending pulses of gas to the surface and by measuring the temperature rise due to adsorption simultaneously, the calorimetric heat can be determined for a certain coverage window. Under adiabatic conditions the calorimetric heat of adsorption is then equal to the isosteric heat at a given coverage [51Kin, 66Def, 70Cla]: Q cal ( n (i S ) ) = Q diff ( n (i S ) ) + kT = Q iso ( n (i S ) ) .

(21)

Experiments of this kind have recently been carried out with single crystal samples under vacuum conditions, using a molecular beam for gas dosing [91Bor, 98Bro]. An example for the differential heat of adsorption versus coverage measured calorimetrically is shown in Fig. 2 for CO on a Pd(100) surface [97Yeo]. The adsorption heat decreases with increasing coverage, indicative of repulsive interactions between adsorbed CO molecules. This result is in good agreement with a previous measurement of the isosteric heat of adsorption for the same system [69Tra]. Most older data in this field of adsorption calorimetry suffer from an insufficient characterization of the substrate surface structure and composition. 180

CO/Pd (100)

–1

Heat of adsorption Qdiff [kJ mol ]

160 140

0 ML C 0.05 ML C 0.07 ML C 0.20 ML C 0.45 ML C

120 100

Fig. 2. Calorimetric heat of adsorption Qdiff for CO, adsorbed at 300 K on clean Pd(100) (0 ML C) as well as Pd(100) precovered with various amounts of carbon (0.05-0.45 ML C), as a function of CO coverage θ [97Yeo].

80 60 0

0.2

0.4 0.6 CO coverage θ [ML]

0.8

1.0

Landolt-Börnstein New Series III/42A

Ref. p. 59]

1 Introduction

9

1.3 Kinetics of adsorption/desorption and adsorption isotherms 1.3.1 Thermal rates of adsorption / desorption Kinetic phenomena, such as rates of adsorption and desorption of atoms or molecules, measured as a function of pressure, temperature (substrate or gas), surface structure, etc., contain important information on the energetics of adsorption. Unambiguous relationships between kinetics and energetics are, however, at least in the field of adsorption, frequently not available. The simplest case is the rate of adsorption, rad, of atoms or simple molecules onto a uniform surface [64Hay]: rad (θ, T) =

p

(2πmkT)1 2

s (θ, T) ,

(22)

where p is the partial pressure of the adsorbing gas, m the atomic (molecular) weight, and s(θ,T) the sticking coefficient. The quantity θ is from now on the relative surface coverage, defined as θ = n ad N 0 , equal to the ratio of the number of adsorbed species, nad, over the number of surface atoms, N0. The surface coverage is thus equivalent to the formerly defined surface excess, Γ. The sticking coefficient, defined as the rate of adsorption divided by the rate of impinging molecules, may be written as a product of a coverage dependent function and a temperature dependent Boltzmann term, containing the activation energy of adsorption, Eac: E  s( θ, T) = c f (θ ) exp ac   kT 

(23)

The quantity c is the condensation coefficient (of the order of unity) which describes the energy accomodation of the adsorbing species. The activation energy of adsorption is illustrated by the onedimensional adsorption potential shown in Fig. 3.

Potential energy E

barrier Eac

precursor state

chemisorbed state

Fig. 3. Illustration of the activation energy E of adsorption between a physisorbed precursor and a chemisorbed state in a one-dimensional potential energy diagram.

Reaction coordinate z

This potential shows two minima, one characterizing a weakly adsorbed, physisorbed state, the other a more strongly adsorbed, chemisorbed state. The transition from one to the other shows a barrier, the activation energy of chemisorption, Eac. For many adsorbates, in particular atomic species, there is no Landolt-Börnstein New Series III/42A

1 Introduction

10

[Ref. p. 59

indication of an activation barrier. The rate of adsorption is then only weakly temperature dependent. For molecular species, that chemisorb intact, the activation barrier is generally very small. To note an effect of this barrier on the rate of adsorption, one has to perform the experiment at very low temperature [78Nor]. This was demonstrated for CO and O2 on Ni(111), for example, at 6 K [84Sha, 85Sha, 86Sha]. Electron spectroscopy and work funktion measurements show these species to populate exclusivly the physisorbed state. Only after raising the temperature to about 20 K, these molecules begin to transfer into the chemisorbed state, whereby O2 dissociates into atoms. A very low activation energy of 0.4 kJ/mol is estimated for CO chemisorption [85Sha]. For molecular species which adsorb dissociatively, the activation barrier can be substantial. A pronounced temperature dependence of the rate of adsorption is found in such cases [85Ste, 90Ber]. Furthermore, the angular dependence of the rate of adsorption (and desorption) responds sensitively to the presence of an activation energy of adsorption. This topic will receive more attention further below and in chapter 3.11 of this volume. The coverage dependent function, f(θ), includes details of the adsorption reaction (e.g. molecular or dissociative) and the statistics of site occupation on the surface, including possible surface diffusion prior to localized adsorption. The simplest case is f(θ) = 1 – θ, which means that empty sites can readily be occupied. For dissociative adsorption A2 → 2 A one has approximately a (1 – θ)2 dependence. More complicated cases have been proposed [55Ehr, 56Ehr, 57Kis, 58Kis]. It is frequently observed that the sticking coefficient does not follow a simple θ-dependence of the kind mentioned above. Figure 4 shows the measured sticking coefficient for CO on Pd(100) at 300 K [97Yeo]. Although CO adsorbs molecularly, the data do not support the simple site occupation formula. The explanation of this dependence rests on the existence of a precursor state to chemisorption, such as schematically shown in Fig. 3, which allows CO molecules bound in this state to migrate over a small portion of the surface looking for an empty chemisorption site. In addition, the transition from this precursor site to the chemisorption site is activated, causing the sticking coefficient to exhibit a complex temperature dependence. 0.90

CO/Pd (100)

0 ML C 0.05 ML C 0.07 ML C 0.20 ML C 0.45 ML C

Sticking probability

0.75

0.60

0.45

0.30

0.15 0

0.2

0.4 0.6 CO coverage θ [ML]

0.8

1.0

Fig. 4. Sticking probability of CO on clean Pd(100) (0 ML C) as well as Pd(100) precovered with various amounts of carbon (0.05-0.45 ML C), vs. CO coverage θ at 300 K [97Yeo].

The rate of desorption, rd, is always temperature dependent since atoms or molecules are transferred from a bound state to an unbound state in the gas phase. Nevertheless, this rate is coverage dependent and given by [75Kin, 75Men]  E  rd (θ, T) = θ n k d ( T) = θ n ν d ( T) N 0 exp − d  ,  kT 

(24)

Landolt-Börnstein New Series III/42A

Ref. p. 59]

1 Introduction

11

where kd(T) is the rate constant of desorption (in units of molecules/(cm2 s)), νd ( T) the pre-exponential frequency factor, N0 the density of surface sites, and Ed the activation energy of desorption. The activation energy of desorption is in general also coverage dependent, just as the heat of adsorption. At constant coverage, the activation energy of desorption is equal to the isosteric heat of adsorption plus the activation energy of adsorption. First order desorption implies that atoms or molecules desorb directly from their molecular physisorbed or chemisorbed state. Second order desorption occurs when a second order surface reaction, such as A + A → A2, preceeds the desorption. Complicated kinetics may be involved for combined surface reaction/desorption processes, resulting in other apparent order exponents of desorption [78Gor, 81Cas]. The experimentally determined order of desorption can not be straightforwardly interpreted in terms of a simple mechanism. A common technique for studying desorption kinetics is that of temperature programmed desorption (TPD) [53Bec, 56Ehr, 62Red]. A crystalline substrate containing a certain coverage of adsorbate is heated with a linear temperature ramp. Simultaneously the partial pressure of desorbing molecules is monitored by a mass spectrometer (or pressure gauge). For a sufficiently high pumping speed of the system one can show that the desorption rate is proportional to the partial pressure. A maximum in the rate of desorption is observed at a certain temperature T*, as illustrated in Fig. 5.

θ2

Pressure p

θ1

T* 250

300

350

400 450 500 Temperature T [K]

550

600

Fig. 5. Example of TDS traces of partial pressure p vs. substrate temperature T for two different initial coverages θ.

This temperature depends on the order of desorption, the activation energy of desorption, the initial coverage, etc. Under simple conditions, i.e. assuming Ed and νd to be constants, and for a first order desorption process, the peak temperature T* is related to the desorption energy: Ed ν T* = ln d − 3.64 . kT * dT dt

(25)

In general, however, the coverage dependence of Ed and νd has to be considered for almost any desorption process. One reason is adsorbate-adsorbate interactions which may not be negligible. In particular, the pre-exponential frequency factor may strongly depend on coverage and vary over many orders of magnitude [88See]. Advanced methods of experimentation, e.g. by relating the work function change of the substrate measured during desorption directly to the rate of desorption [83Pfn], and detailed data evaluation enable very accurate measurements of the coverage dependence of the isosteric heat of adsorption and the pre-exponential factor, respectively. A good example is CO on Ru(001) [82Men, 83Pfn] for which the results are shown in Fig. 6. The sudden decrease in the heat of adsorption and the

Landolt-Börnstein New Series III/42A

1 Introduction

12

[Ref. p. 59

frequency factor near the coverage of 0.3 is related to repulsive interactions within the adlayer causing a phase transition from an ordered 3 × 3 R 30° structure to a poorly ordered compressed CO layer.

Fig. 6a, b. Heat of adsorption Ead (a) and preexponential factor ν (b) vs. coverage θ for CO on Ru(001). Various quasi-equilibrium and kinetic measurements contributed to this data. The sudden decrease at 0.33 coverage is due to a structural change in the adsorbed layer [83Pfn].

Angular and kinetic energy dependence The principal questions concerning the angular and kinetic energy dependence of the rate of adsorption (or desorption) has been of great interest from early on [15Knu, 29Ste, 1879]. The starting point of this discussion is the velocity distribution of gas molecules under the condition of thermal equilibrium [85Com]. Here Maxwell's law is valid:  m  dn = 4πn    2πkT 

32

 mv 2  2 exp −  v dv ,  2kT 

(26)

where dn is the number density of molecules per unit volume, having a velocity between v and v+dv, and m the mass of the molecule. Introducing a solid surface into the gas phase creates a new situation in that directions of impinging and desorbing molecules relative to that surface can be defined. The differential flux of molecules impinging per unit time and unit solid angle in a given direction ( ϑ, ϕ ) , where ϑ is the polar angle relative to the surface normal and ϕ the azimuthal angle, and per unit time is then given by [85Com]:

Landolt-Börnstein New Series III/42A

Ref. p. 59]  m  df = n    2π kT 

1 Introduction 32

 mv 2  3 exp −  v cos ϑ dv .  2 kT 

13

(27)

Note the third power in velocity in Eq. (27) as compared to the square dependence in Eq. (26). This indicates that molecules in a flux directed onto a surface or leaving the surface will have an average kinetic energy different from that of molecules in the homogeneous gas phase. The average kinetic energy per molecule is (3/2)kT in the latter case, but 2kT for molecules directed towards or away from the surface if T is the gas temperature (here equal to the surface temperature). The angular distribution is cosine ("Knudsen distribution"). Based on the principle of microscopic reversibility ("detailed balance of fluxes"), the angular and kinetic energy distribution directed towards the surface, Eq. (27), and that of the molecules leaving the surface are the same in thermal equilibrium. The angular distribution of molecules leaving the surface are thus slightly peaked in the direction of the surface normal and the velocity distribution is Maxwellian. It was believed for a long time that Eq. (27) would be generally valid to describe the angle and velocity distribution of adsorbing and desorbing molecules. However, it was shown by Clausing [30Cla] and elaborated by Comsa [68Com] that this is not the case. The fundamental reason is simply that the sticking coefficient of adsorbing species is generally not unity, i.e. the probability of adsorption is not unity. This means that only a fraction of impinging molecules adsorb on the surface while others are scattered back. Vice versa, the probability of desorption must be also effected under equilibrium conditions, in that only some molecules desorb while others are scattered elastically or inelastically within the potential well without actually leaving the surface. Adsorption as well as desorption fluxes may therefore exhibit angle and energy dependencies which are markedly different from Eq. (27). Of course, the reason for sticking coefficients < 1 is the existence of an activation barrier of adsorption, Eq. (23). Molecules impinging in directions closer to normal incidence and/or with higher kinetic energy have a substantially higher probability of overcoming the barrier of adsorption, as their velocity component normal to the surface is larger. A similar argument holds for the desorption process. Thus the angular and velocity distributions of adsorbing and desorbing molecules are expected to deviate significantly from Eq. (27). In a qualitative sense, they will be more peaked in the normal direction than a cosine distribution and the mean kinetic energy of desorbing species will be higher than 2kT. There is then a two-fold task for the experimentalist. For adsorption systems with a unit sticking coefficient one may want to prove Eq. (27) and the principle of detailed balancing, i.e. the validity of Eq. (27) for adsorption and desorption. On the other hand, for systems with sticking coefficients < 1, one still expects to be able to prove equal adsorption and desorption rates as a function of polar angle, according to the principle of detailed balancing, but the actual measured dependencies should differ from that in Eq. (27). Strictly speaking, a rigorous proof of Maxwell and Knudsen distributions (or the deviations therefrom) is not possible under the conditions of thermal equilibrium because adsorbing and desorbing molecules can not be distinguished experimentally. Instead, separate experiments testing either fluxes have to be carried out utilizing the concept of molecular beams. Pressure or flux detectors selecting a small solid angle are used to probe the details of desorption [68Wil, 77Com, 79Com]. Molecular beams with a narrow angular cross section (and a narrow energy distribution) serve to probe the angular dependence of the sticking coefficient [85Ste]. Time-of-flight spectroscopy of desorbing molecules is used to measure the velocity dependence at a particular polar angle. The conditions are not those of a true equilibrium experiment, and it is therefore not a priori certain that the results will reflect the principle of detailed balancing [63Hir].

Landolt-Börnstein New Series III/42A

1 Introduction

14

Normalized integral flux

0.8

[011]

0.6

0.4

0 – 60°

Adsorption S0(ϑ) × cosϑ Desorption D(ϑ)

– 40°

– 20°

0° Angle ϑ

20°

40°

H2/Ni (111)

0.8

[011]

cosϑ

0.2

a

1.0

CO/Ni (111)

Normalized integral flux

1.0

[Ref. p. 59

[011]

0.6

0.4 Adsorption S0(ϑ) × cosϑ Desorption D(ϑ)

0.2

60°

Fig. 7a. Angular dependence of adsorption S and desorption D probability of CO from a Ni(111) surface. Equal probabilities are expected on the basis of detailed balancing. Gas and surface temperatures are 300 K [85Ste].

b

[011]

cos4.5ϑ

0 – 60°

– 40°

– 20°

0° Angle ϑ

20°

40°

60°

Fig. 7b. Angular dependence of adsorption S and desorption D probability of H2 from a Ni(111) surface. Equal probabilities are expected on the basis of detailed balancing. Non-cosine distributions for H2 indicate an activation barrier of adsorption. Gas temperature is 300 K, surface temperature 190 K [85Ste].

Examples of experimental angular distributions of adsorption and desorption are presented in Fig.7 for CO and H2 on a Ni(111) surface [84Ste, 85Rob, 85Ste]. In both cases the angular adsorption and desorption rates are clearly the same, confirming first of all the validity of detailed balancing and hence a quasi-equilibrium situation. Secondly, an excellent cosine distribution is found in the case of CO, in agreement with Eq. (27) and consistent with the sticking coefficient being close to unity for CO on Ni(111) [74Chr, 80Iba]. On the other hand, for H2 the distributions are non-cosine, i.e. in conflict with Eq. (27). They are more strongly peaked along the surface normal and can be described by cos4.5 ϑ . The sticking coefficient for H2 on Ni(111) was reported to be 0.025-0.07 [85Rob, 85Ste], and therefore the finding of non-cosine angular distributions is fully consistent with these low sticking coefficients. From measurements of the sticking coefficients of the two gases one knows that the adsorption of H2 is activated [85Rob] while that of CO on Ni(111) is probably not, or at least with a very small activation barrier [85Sha]. In the case of H2, measurements of the velocity distributions of desorbing D2 from Ni(111) at a surface temperature of 1134 K have shown that they are non-Maxwellian, with the gas temperature being substantially higher ("hot molecules") than the surface temperature, for polar angles of 0 - 55° [77Com, 79Com]. A particularly drastic example of this kind is shown in Fig. 8 by time-of-flight spectra of desorbing D2 from a Cu(100) surface at 1000 K and normal emission [82Com, 85Com]. The spectrum calculated for a Maxwell distribution at a gas temperature of 1000 K is shown for comparison. The measured distribution is very narrow and almost totally to the left of the Maxwell distribution, i.e. almost all desorbing molecules are much "hotter" than 1000 K. The mean kinetic energy of desorbing D2 is in this case about 4kT. Therefore the existence of an activation barrier of adsorption was also concluded from these results.

Landolt-Börnstein New Series III/42A

Ref. p. 59]

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15

Fig. 8. Time-of-flight spectra of D2 molecules desorbed from a clean (ο) and sulfur (x) (θS = 0.3) contaminated Cu(100) surface at 1000 K at normal emission. Short flight time corresponds to high kinetic energy. The dashed curve is a caculated spectrum based on a Maxwell distribution at a gas temperature of 1000 K [82Com]. The non-maxwellian experimental distribution for D2 indicates an activation barrier of adsorption.

Indeed, a number of experiments measuring the angular and velocity distributions have provided ample evidence for the correlation of non-cosine, non-Maxwellian distributions with activation barriers of adsorption and sticking coefficients less than unity [85Com]. Non-cosine distributions can be described phenomenologically by a coskϑ function, with values of k commonly between 10 and 1. Measured velocity distributions correspond to mean kinetic energies much higher than the surface temperature of the substrate, they are non-cosine and much narrower than the Maxwell distribution. All of these findings are relevant for chemisorption systems. By comparison, physisorption is not activated since there is no precursor state to physisorption. Further considerations in the context of activated adsorption can be made concerning the internal energy distributions of adsorbing and desorbing molecules. More detail about this and the effect of an activation barrier of adsorption on the characteristics of adsorption and desorption can be found in chapter 3.12.

1.3.2 Isotherms The relationship between the surface coverage and gas pressure p, at a constant temperature T, is called an isotherm, θ(p). The isotherm is a typical function characterizing the interaction between surface and gas molecules, under the conditions of thermodynamic equilibrium between solid and gaseous phases. A simple case is the Langmuir isotherm which is readily obtained from equating the rate of (non-activated) adsorption, assuming f(θ) = 1 – θ, and the rate of desorption, assuming first order. The result is θ( p ) =

p  E  12 , κ ( T) = ν d N 0 (2πmkT) exp − d  ,  kT  p + κ ( T)

(28)

where κ(T) (in units of pressure) depends exponentially on the activation energy of desorption, N0 the density of surface sites, and νd is the temperature dependent "frequency" of desorption, Eq. (24). A more detailed form of κ(T), which may be derived in the framework of statistical thermodynamics, depends on a number of assumptions concerning the degrees of freedom of the adsorbed species as well as their possible mutual interaction. No interactions are included in the derivation of Eq. (28). If nearest-neighbor interactions between the adsorbed species are taken into account, the same principal form of the isotherm, Eq. (28), is derived but κ(T) includes now a coverage dependent term containing the interaction energy, ε [39Fow, 96Des]. Therefore this isotherm is written as p(θ):

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θ (2πmkT) k F ( θ, T), k F (θ, T) = s 1− θ

12

p( θ ) =

 E + zθε νd exp − d  ,  kT 

[Ref. p. 59

(29)

where z is the average lateral coordination number between adspecies in the 2-dimensional lattice. If ε > 0, interactions are attractive, and below a critical temperature Tc < zε/2k, a two-phase adlayer can form for a certain range of coverage, consisting of a 2d ordered phase and a diluted disordered phase. For repulsive interactions, ε < 0, there is no critical temperature, i.e. a phase transition does not occur. For a case of non-activated adsorption, Ed + zθε = Qiso, we see that the isosteric heat of adsorption increases for attractive interaction between adspecies and decreases for repulsive interaction. The Langmuir isotherm shows that, at low pressure, the coverage is directly proportional to the pressure, while it approaches unity at high pressure. Single layer adsorption is frequently observed for chemisorbing molecules under vacuum conditions. However, this does not mean that isotherms are easily measured for single crystal samples of well-defined structure in vacuum. For example, if the heat of adsorption is > 20 kJ/mol, a measurement of an isotherm at low T would require low pressure and long equilibration times; on the other hand, at high T high pressures are needed which is not compatible with vacuum. Hence one prefers to measure a group of isobars θ(T)p which are replotted as isotherms. Multilayer adsorption can occur when the heat of adsorption of the second and higher layers are still appreciable compared to that of the first layer. This problem was considered by Brunauer, Emmet and Teller (BET) and led to the formula of the respective isotherm [38Bru]: θ( p ) =

k BET ( T) p p 0  Q − QL  , k BET ( T) = k 0 ( T) exp − iso  .  kT  (p 0 + k BET ( T)p − p)(p 0 − p)

(30)

The value p0 is the saturation pressure of the adsorbing gas at which an infinitely thick multilayer would form (condensation); QL is the heat of condensation. The model assumes a constant heat of adsorption for all layers beyond the first which is more strongly adsorbed. Interactions within each layer are not included. If the heat of adsorption varies from layer to layer, a more complex formula would result.

1.3.3 Non-thermal desorption In this introduction to adsorbate/substrate systems we have largely relied on the framework of thermodynamics for the phenomenological description of adsorption and desorption. Rates are governed by partial pressures in the gas phase and by gas and substrate temperatures, respectively. On the other hand, there is a group of adsorption/desorption phenomena where temperature is of secondary importance. Instead, rates are influenced decisively by non-thermal, non-equilibrium processes, induced by incident radiation with high energy photons, electrons or ions. Although both, the rate of adsorption as well as the rate of desorption, can be effected by energetic radiation, the process of desorption has received more attention in this context. Thus one speaks of ion-induced, electron-induced etc. desorption of atoms, molecules or fragments. The detailed mechanism of non-thermal desorption (and fragmentation) has been subject of intense investigation. In the case of infra-red radiation the distinction between thermal and non-thermal effects is particularly challenging, and the term laser-induced thermal desorption (LITD) should not be taken as an indication of a purely thermal mechanism. In many cases radiation induced electronic transitions to nonbonding states are the most important mechanistic step of this kind of desorption [64Men]. This gave rise to the term desorption induced by electronic transitions (DIET), a process that was subject of numerous conferences. DIET has become a sub-field of adsorption/desorption which will not be covered in this volume in any detail [83Tol, 85Bre, 88Stu, 90Bet, 93Bur].

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1.4 Physical and chemical properties of adsorbed species After the introduction to the general phenomenology of adsorption/desorption of gases on well-defined crystalline surfaces we will now give an overview of the main physical and chemical properties of adsorbed species. The adjective main relates to those properties which have been specifically analyzed in greater detail and with sufficient accuracy, for a large number of molecules, such that a listing and comparison in tables appears to be reasonable. Naturally, the number of investigations will vary widely from molecule to molecule, depending on several factors: ease of operation with gases in various environments; achieving stable adsorbate coverages under realizable conditions; applicability of several analytical techniques; relevance to technical processes, e.g. heterogeneous catalysis; corrosion, thin film formation, semiconductor device technology, etc. We will introduce and briefly discuss a few selected experimental results for each adsorbate property by referring to simple examples. One of the preferred systems to be used for the purpose of illustration is the reversible adsorption of CO on a Ni(110) surface. This is also one of the best studied systems in this field (compare section 3.7.1). For reference purposes we list here some general properties of gaseous CO. The electronic configuration is (1σ)2(2σ)2(3σ)2(4σ)2(1π)4(5σ)2, with the last three as valence molecular orbitals at 19.7, 16.8 and 14.0 eV binding energy, respectively [78Plu, 84Kra]. The bond length is 1.13 Å, the dissociation energy 11.2 eV, the static dipole moment 0.1 debye, and the C–O stretch vibration has a value of 2143 cm–1 [79Hub].

1.4.1 Sticking coefficient The sticking coefficient, defined as the rate of adsorption divided by the rate of impinging molecules, at a certain coverage and substrate temperature, is a quantity involving dynamic aspects as well as the energetics of adsorption. It is measured by following the coverage of the adsorbate as a function of gas exposure, either in the presence of a constant background pressure of this particular gas or in a molecular beam experiment where the flux of impinging molecules is known. The surface coverage is obtained by a surface sensitive chemical probe or a suitable physical indicator which has been calibrated, e.g. the change of work function versus coverage of adsorbate. A common way to determine coverage is also by stopping the exposure, desorbing all molecules by raising the temperature, recording and integrating the pressure pulse versus temperature. 0.8

1.0

CO/Ni

CO/Ni (110) 0.8

(110)

0.4 (100)

0.2

a

Rel.sticking coeff. s/s0

Sticking coeff. s

0.6

0

0.4 0.6 0.8 Apparent CO coverage θ [ML]

1.0

Fig. 9a. Sticking coefficient s of CO vs. apperent coverage θ on Ni(111) (- - -), Ni(100) ( ) and Ni(110) (....) surfaces at 300 K [93Stu]. Measured differences indicate surface structural sensitivity.

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0.4

0.2

(111)

0.2

0.6

b

0

0.2

0.4 0.6 CO coverage θ

0.8

1.0

Fig. 9b. Relative sticking coefficient s/s0 of CO vs. coverage θ on Ni(110) at 130 K [85Beh]. Comparison with Fig.9a illustrates the temperature dependence.

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[Ref. p. 59

Figure 9 compares the results of both approaches for CO on Ni(110) at two different temperatures, 130 K [85Beh] and 300 K [93Stu], respectively. In Fig. 9a the sticking coefficient is determined by the molecular beam technique for the low-index surfaces (111), (100) and (110). Its value at zero coverage is 0.75, independent of surface orientation, and decreases with increasing coverage until it reaches 0.3 at a monolayer of CO. Figure 9b shows the coverage variation of the relative sticking coefficient for the (110) surface (normalized to unity at zero coverage) determined continuously from the change in electron work function versus CO exposure. The behavior is qualitatively quite different than in Fig. 9a, partly due to the lower temperature of 130 K which may be responsible for an extended range of nearly constant sticking, compared to the faster decrease at 300 K. The observation of s0 < 1 and the rather pronounced temperature dependence of the sticking coefficient versus coverage point consistently towards the existence of a physisorbed CO precursor state and an activation barrier of adsorption.

1.4.2 Heat of adsorption The heat of adsorption is in general a function of the adsorbate coverage. Its value at a certain coverage is a direct measure of the strength of interaction between substrate and adsorbate and of the inter-molecular interaction. Therefore the heat of adsorption depends on the adsorption site and on the surface structure in general. If the adsorption of molecules is fully reversible, i.e. if no dissocation occurs, and if there is no significant activation barrier for adsorption, the heat of adsorption and the activation energy of desorption should be approximately the same. This comparison has been made for CO on Ni(110) for which calorimetric, isosteric and desorption measurements exist [73Mad, 90Fei, 92Dea, 93Stu]. 150

CO/Ni (110)

–1

Adsorption heat Q [kJ mol ]

140 130 120 110 100 90 0

0.2

0.4 0.6 0.8 Apparent CO coverage θ [ML]

1.0

Fig. 10. Heat of adsorption Q vs. apparent coverage θ for CO adsorbed on Ni(110) at 300 K. Comparison of data obtained by Stuckless et al. [93Stu] (full circle), Madden et al.[73Mad] (open circle), DeAngelis et al. [92Dea] (down triangle) and Feigerle et al. [90Fei] (up triangle).

Figure 10 shows a plot of these energies versus CO coverage between zero and one monolayer. The initial heat of adsorption is near 130 kJ/mol, decreasing towards higher coverages to about 100 kJ/mol. There are certain systematic differences in these results: The plateau of constant heat of adsorption extends up to a coverage of about 0.6 for the isosteric and desorption energies, respectively, while the calorimetric heat shows a decrease already at 0.3 coverage leveling off at 0.55. These differences may be due to rather different temperature ranges playing a role at these measurements. The calorimetric measurement is also an almost isothermal measurement while the measurements of the isosteric heat and the desorption energy involve a rather broad temperature menu. Measurements of isotherms as a source of the heat of adsorption are rare for single crystal substrates. Special techniques of experimentation are required to record isotherms under vacuum conditions. One such measurement is known for CO on Ni(110) [70Kli] which shall illustrate the case. The coverage of

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radioactively labelled 14CO is monitored [54Dil, 69Kli] at constant substrate temperature as a function of CO pressure. Pressure p [Torr] for T = 298 K –7 –5 10 10

–9

1.2

10

–3

10

CO/Ni (110)

–2

CO surface density [1015 cm ]

1.0

0.8

0.6 T = 298 K 182 K

0.4 0.2 16

19

22

25 ln (p/kT)

28

31

34

Fig. 11. Surface density N [1015cm−2] vs. pressure p (respectively ln(p/kT)) for CO adsorbed on Ni(110) measured at two temperatures of 182 and 298 K [70Kli].

The isotherms shown in Fig. 11 for T = 182 K and 298 K are then fitted to a theoretical isotherm based on the Devonshire cell model [37Dev]. Lateral interaction between adsorbed CO molecules as well as the substrate CO interaction are accounted for in this model. Under special assumptions, values of 106 kJ/mol and 15 kJ/mol are separately obtained for the heat of adsorption and the CO–CO interaction energy, respectively. These energies are not coverage-specific. Because of the separation of the heat of adsorption into an energy of adsorption of an isolated CO and the energy of lateral CO interaction, the value of 106 kJ/mol should be compared to the heat of adsorption at zero coverage in Fig. 10.

1.4.3 Orientation of adsorbed molecules With regard to the structure of an adsorbed molecule or radical on a crystalline metal or semiconductor surface, answers on different levels of complexity can be sought. One of the primary questions concerns the adsorption bond(s) itself, i.e. which molecular group(s) bonds to surface atoms of the substrate. Secondly, how is the molecule oriented relative to the surface normal? The first question can often be answered by referring to the cluster-surface analogy. There is a large group of molecular metal clusters which are ideal model systems of chemisorption [78Plu, 79Mue]. For example, the group of metal carbonyls consists of several metal atoms and CO molecules which are bonded through the carbon atom to the metal. In analogy, one expects CO to bond (involving the 5σ and 1π molecular orbitals of CO) via the carbon atom to a metal surface. Investigations of CO bonding to a large variety of metal surfaces have confirmed this cluster-surface analogy in many respects. From a techniques point of view three examples will be given here which demonstrate how adsorbed CO is oriented on a metal surface. The first technique rests on the phenomenon that linearly polarized light couples differently to molecular orbitals of π and σ symmetry [76Lie]. CO has three molecular orbitals of interest here: 4σ, 1π and 5σ (in order of decreasing binding energy). When a CO covered surface is irradiated with uv-photons, photoelectrons at characteristic kinetic energies are emitted. The emitted intensity is a measure of the interaction probability between photon and bound electron state (orbital). Figure 12 shows a series of spectra for p- and s-polarized light (28 eV) recorded for various polar angles of emission from a Ni(100) surface covered with adsorbed CO [76Smi]. Two peaks are observed at about 8 eV and 11 eV binding energy. The peak at 11 eV is only intense for p-polarized light at normal emission. Comparing the experimental results to theoretical calculations for oriented CO

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[Ref. p. 59

molecules [76Dav, 78Dav], agreement is found for perpendicularly adsorbed CO oriented and bound with the carbon atom towards the surface [76Smi, 77All]. The accuracy of determining the bond angle by this technique is not very high. ϑA = 47.7° A

hν ϑp

hν

ϑA = 90°

ϑp p

p ϕp A

ϕp

CO/Ni (100) clean ϑp = 0° N(E) [counts per incident photon]

N(E) [counts per incident photon]

clean ϑp = 0°

3L CO ϑp = 0°

3L CO ϑp = 0°

3L CO ϑp = 30°

a

– 14

– 12

– 10

–8 –6 –4 Initial energy Ei [eV]

3L CO ϑp = 23°

–2

0

b

– 14

– 12

– 10

–8 –6 –4 Initial energy Ei [eV]

–2

0

Fig. 12a, b. Angle resolved photoemission spectra of CO adsorbed on Ni(100) at a photon energy of 28 eV. (a) Nearly p-polarized light; (b) s-polarized light [76Smi].

The second technique utilizes the effect of stimulated ion desorption from adsorbates when the covered surface is irradiated with medium to high energy electrons. We consider the particular system CO adsorbed on Ni(110) at low and high CO coverage. The measurement of the angular distribution of desorbing ions (ESDIAD, electron stimulated desorption ion angular distribution) is then related to the orientation of the adsorbed species which supply the ions [79Mad1, 79Mad2, 85Rie, 86Alv]. ESDIAD distributions of mostly O+ ions, generated by 1000 eV incident electrons, from adsorbed CO at coverages of 0.5 and 1.0, respectively, are shown in Fig. 13 [86Alv]. A single emission peak perpendicular to the surface is observed at the lower coverage, attributed to perpendicularly adsorbed CO. A double peak, measured at polar angles of about 19° relative to normal, is clearly resolved at the saturation coverage of CO for which the well ordered (2×1)p2mg layer exists. This double peak is interpreted in terms of a tilted CO molecule whereby the projected tilt occurs along the [001] and [00 1 ] surface directions [85Rie, 86Alv]. The fact that these angular distributions are measured for O+ emission suggests that CO is Landolt-Börnstein New Series III/42A

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oriented with the carbon atom next to the surface (Ni–C bonding). The quantitative evaluation of the CO tilt is somewhat hampered by the attractive force between the emitted ion and its image charge, resulting in bent ion trajectories emitted at a polar angle larger than that of the tilted molecule.

Fig. 13. ESDIAD intensity distribution for CO adsorbed on Ni(110) showing perpendicularly adsorbed CO at low coverage and tilted CO at saturation coverage. The incident electron energy was 1000 eV [86Alv].

A third technique for locating the orientation of bond directions of adsorbed molecules is x-ray photoelectron diffraction (XPD) in the forward scattering mode [79Pet, 82Ord, 84Ege, 88Wes2, 89Wes]. Recording the angle-resolved intensities of core level photoelectrons, here either C1s or O1s from the carbon or oxygen atom of CO, respectively, allows the spatial localization of the CO bond axis and furthermore, an unambiguous distinction of the molecule's bonding through either the carbon or oxygen atom [79Pet]. Typical results for CO adsorbed on Ni(110) are presented in Fig. 14, for two different coverages [92Kna, 94Fri]. The angular distribution of C1s intensity versus polar angle at a CO coverage of about 0.6 shows a large narrow peak at normal emission and smaller maxima at 44° and 64° polar angles. The main peak at normal emission proves, firstly, that CO is bonded through the carbon atom to the Ni surface, since the corresponding O1s distribution is isotropic, and secondly, that CO is perpendicularly adsorbed. Checking the azimuthal variation of the C1s forward scattering peak width shows rather different values at half height, indicative of anisotropic amplitudes of "wagging" vibrations of CO [88Wes1, 94Fri]. The smaller peaks in Fig. 14 arise from inter-molecular scattering and are thus a sign of short range order within the CO layer. The second C1s intensity distribution shown in Fig. 14 was recorded along the [001] azimuth at 120 K, for the CO coverage of unity. This distribution shows a total of four peaks, located at polar angles of 20° and 67°, respectively. The peaks at 20° are due to intra-molecular scattering of C1s photoelectrons and indicate therefore the tilt angle of adsorbed CO molecules. The outer peaks at 67° are due to intermolecular scattering, i.e. scattering between neighboring CO molecules, and are consistent with a well ordered arrangement of CO on the Ni(110) surface (compare 1.4.5).

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CO/Ni (100) C1s in <110>

[Ref. p. 59

Intensity I

T = 300 K θCO = 0.55

a

90°

75°

60°

45°

30°

0° 15° 15° Polar angle ϑ

30°

CO(2×1) p2mg/Ni (110) C1s in <001>

45°

60°

75°

90°

T = 120 K θ=1 experiment theory

Intensity I

Fig. 14a, b. XPD C1s intensity distributions of CO adsorbed on Ni(110) at (a) 300 K and a coverage of 0.55, (b) 120 K at a coverage of unity. The dotted curve in (b) is the result of a multiple scattering calculation [92Kna, 94Fri].

b

90°

75°

60°

45°

30°

0° 15° 15° Polar angle ϑ

30°

45°

60°

75°

90°

1.4.4 Surface diffusion coefficients Adsorbed species, atoms and molecules, are mobile in their physisorbed precursor states and, depending on the surface temperature, also in their chemisorbed states. The mobility is generally higher, the lower the adsorption energy. Rates of surface diffusion have been measured for many chemisorbed species but only for a few physisorbed species [59Gom, 90Gom, 93Bon, 95See]. A number of different experimental techniques exist for the measurement of surface diffusion coefficients (compare section 3.11). Some techniques rely on rather macroscopic diffusion, i.e. over distances of the order of several µm or more. Others determine the local mobility, on the scale of nm. Hence the characteristic quantitative parameters describing the rate of surface diffusion can be different, depending on the technique. The main reason for observed differences in absolute values of measured surface diffusion coefficients and activation energies is the inhomogeneous potential that the adsorbed species encounters by diffusing across the surface [65Gjo, 66Gjo]. Even on a well oriented low-index single crystal surface there are defects sites, such as steps, kinks, small adatom or vacancy clusters, dislocation emerging points, which constitute in general higher binding energy sites than those on a flat terrace. Therefore, if a technique measures diffusion over a distance large compared to the averge separation between these defect sites, the activation energy of surface diffusion will include contributions due to transient binding to these sites. In this case one speaks Landolt-Börnstein New Series III/42A

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of mass transport surface diffusion. On the other hand, if a technique measures diffusion in an area small compared to the inter-defect separation, an activation barrier characteristic of terrace surface diffusion will be measured, provided the measured area is defect-free. In this case the diffusion coefficient is defined by statistical random walk and one speaks of tracer or intrinsic surface diffusion, if the coverage of the diffusing species is very low [83Gom]. For larger coverages, where interactions between diffusing species become important, a chemical surface diffusion coefficient has been defined that contains a thermodynamic derivative factor of the adlayer's chemical potential with respect to particle density [77Ehr]. In that case the surface diffusion coefficient may depend strongly on coverage, especially where ordered adsorbed layers exist. More about these distinctions can be found in section 3.11. A corresponding chemical mass transport surface diffusion coefficient can also be defined. Another complication in comparing data from different techniques may arise from the possibility that several surface diffusion mechanisms may be operative. Evidence for different surface diffusion mechanisms may be revealed by large differences in measured activation energies and by large differences in absolute values of measured surface diffusion coefficients at the same temperature. Hence experimental data, especially if they originate from different techniques, need to be be compared systematically in order to decide whether the effective rates of diffusion (at the same or at different temperatures) may be indicative of different diffusion mechanisms. Rates of surface diffusion of CO adsorbed on Ni(110) and Ni(111), for example, were investigated by several rather different techniques: second harmonic diffraction (SHG)[88Zhu, 91Xia] and linear optical diffraction [93Xia, 95Xia, 97Xia] from CO "gratings", quasi-elastic He atom scattering [96Ber], and by field emission fluctuations from CO layers at different coverages on a field emitter tip [90Lin]. It is interesting to compare the various data sets obtained for Ni(110) but also those for Ni(110) with the ones for Ni(111) [88Zhu, 90Lin]. Table1 summarizes the former in terms of pre-exponential factor, ν0, and activation energy of surface diffusion, Edif, according to the phenomenological Arrhenius expression:  E  DS = ν 0 exp − dif  .  kT 

(31)

As expected, surface diffusion of CO is anisotropic on the (110) surface, characterized by a lower activation energy in the close-packed [1 1 0] direction than perpendicular to it. The measured activation energies for CO on clean Ni(110) are in the range of 1-4.8 kcal/mol and pre-exponential factors vary over many decades. Additional measurements on the effect of surface impurities have shown that a small amount of coadsorbed sulfur (θ = 0.05) leads to an increase in the activation energy to 6-8 kcal/mol [97Xia]. This effect is attributed to the prediminant adsorption of impurities at steps thus increasing the apparent activation energy of CO surface diffusion. Figure 15 shows a plot of surface diffusion coefficients of CO versus 1/T for clean Ni(110) calculated on the basis of the data in Table 1 by using Eq. (31). Two well separated data groups can be recognized, one with high diffusion coefficients near 10–5 cm2/s, the other in a range between 10–9 and 10–14 cm2/s, located at lower temperatures. The diffusion coefficients of data set (a) obtained by He atom scattering [96Ber] at a CO coverage of 0.15 are larger by a factor of more than 104 relative to the other group. Also, the activation energies are low for both diffusion directions, with the [001] direction still having a lower activation energy than the [1 1 0] direction, opposite to all measurements in the other data group. Since He atom scattering is a very local probe of diffusional motion, it has been argued that the activation energies are truly representing barriers between adjacent adsorption sites, i.e. independent of the presence of steps on the surface [98Gra]. On the other hand, if CO diffusion along steps is faster than on terraces, the activation energies could also be characteristic of the step mode of surface diffusion. In any case, the physical meaning of data group (a) is not well understood at this time. With regard to coverage dependence, low activation energies are found at near saturation coverage of CO, as shown by the data sets (b) and (c) [91Xia, 93Xia]. An increase in activation energies for both diffusion directions if found at lower CO coverages, as seen by the example (d) [93Xia]. By comparison, surface diffusion of CO on clean Ni(111) is characterized by an activation energy of 6.8-6.9 kcal/mol which is higher than on Ni(110). The calculated diffusion coefficients fall into the same range as those of

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[Ref. p. 59

the lower group in Fig. 15 [88Zhu, 90Lin]. It is interesting to note that two very different techniques, one of them also a local probe of diffusion [90Lin], have produced a fairly consistent set of data for CO/Ni(111). Hence the apparent discrepancy displayed in Fig. 15 for Ni(110) can not easily be associated with local versus non-local measurements of surface diffusion. –4

10

CO/Ni (110)

a –6

2 –1

Diff.coeff. Ds [cm s ]

10

[110] [001] Fig. 15. Surface diffusion data of CO adsorbed on Ni(110) obtained by various authors, techniques and coverages. (a) θCO = 0.15 [96Ber]; (b) θCO near saturation [91Xia]; (c) θCO = 0.98 [93Xia]; (d) θCO = 0.5 [93Xia]. Each data set consists of two lines, corresponding to the [1 1 0] and [001] diffusion directions, respectively.

–8

10

d

–10

10

b

c

–12

10

–14

10

2.5

3.5

4.5

5.5

6.5

–1

–3 –1

7.5

8.5

9.5

Inv. temperature T [10 K ]

Diffusion coefficients of CO adsorbed on Ni(110) and Ni(111) at room temperature are about 10–9 cm2/s [88Zhu, 90Lin, 91Xia, 93Xia], corresponding to a mean diffusion path of 0.63 µm in the time of 1 s. This means that a large area can be sampled in 1 s allowing an equilibrated layer to be established, i.e. ordering of the CO molecules to occur if this is compatible with energy minimization. Indeed, long range ordered adsorbate structures are frequently encountered in the case of adsorbed CO (compare section 3.7.1). Table 1. Survey of surface diffusion data for CO on Ni(110) surface. Edif [kcal/mol]

Range ∆T [K]

Coverage

Ref.

3.8 × 10–9

1.1

105-182

unspec.

[91Xia]

[001]

4.8 × 10

3.1

110-182

[1 1 0]

2 × 10–8

2.1

140-204

0.98

[93Xia]

[001]

2 × 10–8

Direction

ν0 [cm2/s]

[1 1 0]

–6

2.8

140-204

0.98

[1 1 0]

4.5 × 10

–6

4.05

150-220

0.5

[001]

1.5 × 10–6

4.75

180-244

0.5

[1 1 0]

1 × 10–4

6

180-230

0.5 + S

[001]

1 × 10–3

8

180-230

0.5 + S

[1 1 0]

1.8 × 10

–4

1.31

200-360

0.15

[001]

7.3 × 10–5

0.81

240-360

0.15

[93Xia]

[95Xia, 97Xia]

[96Ber]

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1.4.5 Long-range order Lateral interactions between adsorbed species on single crystal surfaces lead frequently to well ordered arrays. The ordered layer may sometimes include substrate atoms, equivalent to a reconstruction of the original surface, or may only consist of the adsorbed species. If the order is of sufficient long range character, diffraction patterns from elastically scattered low-energy (LEED) or high energy (RHEED) electrons, x-rays at grazing incidence, or atoms (such as thermal energy He atom scattering, TEAS) will result. The kinematic features of the diffraction patterns allow to propose a unit cell of the ordered array, and if the coverage is known, also a corresponding structure model. Quantitative analysis of the diffracted intensities versus energy may lead, if the unit cell is not too large, to quantitative structural information of the surface-adsorbate complex, i.e. the adsorbate itself and the adsorption site (compare 1.4.7). Most recently, it has become possible to image adsorbed molecules directly by scanning tunneling microscopy (STM) and to see long range order in the adlayer. Although the area covered in those images is small (about 40×40 Å2) there is no doubt that some layers are excellently ordered over much larger distances. We will illustrate here the long range order effect by qualitative LEED [85Beh], XPD [98Emu] and real space STM images [95Spr] for CO adsorbed on Ni(110). The observations with LEED show that three ordered CO arrays can be noted with increasing coverage at 130 K, with the notations c(8×2), c(4×2) and (2×1)p2mg. Schematic LEED patterns are shown in Fig. 16, together with corresponding structure models [85Beh]. The latter are based on measured CO coverages and the conversion of the unit cell from reciprocal to real space. The corresponding ideal coverages are 2/3, 3/4 and 1.0, respectively. The (2×1)p2mg pattern at unity coverage is sharper than the others, indicative of good long range order. The diffraction spots ( n + 12 , 0) are missing at normal incidence of the electrons. This behavior is consistent with a glide plane along the [1 1 0] surface direction and a mirror plane along the [001] direction, hence the symmetry notation p2mg of the unit cell. Since the unit cell of the (2×1)p2mg structure contains two CO molecules, which must be structurally inequivalent, a tilt of these CO molecules in opposite directions is suggested [82Ban]. The corresponding (2×1)p2mg structure model based on LEED, STM and other experimental work is shown schematically in Fig. 17. The long range order of the (2×1)p2mg-CO layer has also been verified by He atom diffraction [91Par] which probes the outer most atoms at the surface, in this case the oxygen atoms of the adsorbed CO.

Fig. 16a - c. Structure models and schematic LEED patterns for CO on Ni(110) observed at the coverages of (a) θ = 0.67, (b) θ = 0.75 and (c) θ = 1.0 [85Beh]. The unit cell is indicated in each case.

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[Ref. p. 59

Fig. 17a, b. (a) Structure model and unit cell of (2×1)p2mg CO layer on a fcc (110) surface; (b) corresponding 2D Brillouin zone.

Fig. 19a, b. STM images of CO adsorbed on Ni(110) at 300 K in the (2×1)p2mg configuration. The (2×1)p2mg unit cell is indicated in the lower figure. Image sizes are 60×60 Å (upper part) and 30×30 Å (lower part) [95Spr]. Fig. 18a, b. Large solid angle XPD intensity patterns for CO/Ni(110) in the (2×1)p2mg configuration. Azimuthal surface orientation same as in fig. 17. (a) upper part: experimental data, 2-fold averaged, (b) lower part: single scattering calculation [98Emu].

Angle-resolved XPD measurements of C1s intensities from the well ordered (2×1)p2mg-CO layer over a large solid angle are in good agreement with the structure model [75Lam]. Figure 14 showed already four C1s peaks for the [001] azimuth in a single polar angle scan. If the C1s intensity is recorded for the same layer over a large solid angle, other forward scattering peaks arising from intermolecular

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forward scattering can be detected outside the [001] azimuth, such as shown in Fig. 18a by the stereographic plot of C1s intensity. The result of model calculations of elastic photoelectron scattering for an unsupported (2×1)p2mg-CO layer, illustrated in Fig. 17 for CO molecules tilted by 20° relative to normal, is shown in Fig. 18b as a stereographic projection. Excellent agreement between the experimental and theoretical C1s forward scattering peaks is found in position and relative intensity which fully supports the structure model of the (2×1)p2mg CO layer [98Emu]. A real space verification of the long range ordered CO structure on Ni(110) at the coverage of unity was obtained by STM [95Spr]. This coverage was achieved at room temperature by maintaining a background pressure of CO of 1×10–6 mbar in the vacuum chamber (compare the isotherm in Fig. 11). Figure 19 shows two images of the (2×1)p2mg-CO layer, taken at two different bias voltages. The (2×1) unit cell is indicated and it can be clearly seen that there are two CO molecules per unit cell. The CO molecules show a zig-zag arrangement along the [1 1 0] glide direction and a mirror plane perpendicular to it. These are exactly the characteristic features of the (2×1)p2mg unit cell. Refinements of this model, such as quantitative structural data, tilt angle of CO, adsorption site, etc. are based on other techniques (compare 1.4.3 and 1.4.6). The STM images of adsorbed CO in Fig. 19 are only possible because of the high CO coverage which does not permit a diffusional motion of CO across the surface. The packing density of CO is so high that even vibrational motions are hindered to some extent, a fact which explains the sharpness of these images.

1.4.6 Vibrational frequencies Vibrational frequencies of molecules adsorbed on single crystals can be measured by infra-red absorption spectroscopy (IRAS) in reflection, Raman spectroscopy, or by energy loss spectroscopy of scattered lowenergy electrons or He atoms. The vibrational eigenmodes of the adsorbates are characteristic of their bonding to the substrate and their internal structure and bond strengths. Measured vibrational frequencies are always compared to known frequencies of equivalent gaseous species as a reference. Frequency shifts relative to the gas phase species can be interpreted in terms of bond and even structural or chemical changes. The number of observed modes as well as their group symmetries are important guides to structural changes and the orientation of the adsorbate relative to the surface. Depending on the technique, specific selection rules for the excitation of vibrational modes have to be considered. More detail about this aspect can be found in chapter 2.

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[Ref. p. 59

Fig. 21. Dispersion of CO vibrational frequencies for the main directions of the surface Brillouin zone, measured under non-specular conditions for the well ordered (2×1)p2mg CO structure [90Voi]. Fig. 20a, b. Electron energy loss spectra of CO on Ni(110) in (2×1)p2mg structure. The scattering geometry is specular and the energy resolution is 15.5 and 28 cm–1 for (a) and (b), respectively [90Voi].

Since the accessible frequency range is largest in electron energy loss spectroscopy (EELS) of adsorbates, as compared to infra-red absorption spectroscopy (IRAS), a sample EELS spectrum for CO adsorbed on Ni(110) is shown in Fig. 20 [90Voi]. The coverage of CO is at saturation, such that an ordered (2×1)p2mg structure results [75Lam, 85Beh, 85Gur]. The intra-molecular vibration of the C–O stretch is found at 1984 cm–1, indicative of a weakened C–O bond compared to the gas phase molecule which has a stretch frequency of 2143 cm–1. Vibrations characterizing the Ni–C adsorption bond are observed at 114 cm–1 and 421 cm–1, corresponding to the "frustrated" translation/rotation and the Ni–CO stretch modes, respectively. The detected frequency values are not fixed but dependent on the parallel component of the wave vector of the scattering electrons and on the orientation of the scattering plane. This dependence has been investigated, and Fig. 21 shows the dispersion of the CO related frequencies for the two main directions of the unit cell of the (2×1)p2mg structure. The degree of dispersion is a direct measure of the repulsive CO–CO interactions within the adsorbed layer. Measured vibrational frequencies of adsorbed CO contain also information on the adsorption site. Considering the scattering geometry and the group specific selection rules for observing particular modes, it should in principle be possible to elucidate the site [79Ric]. For CO adsorbed on Ni(110), both atop and bridge adsorption sites have been proposed on the basis of EELS data [82Ban, 90Voi]. Bandy et al. deduced a mixture of CO bonding to a single (atop) and two adjacent Ni surface atoms (bridge site) at low CO coverage and exclusively to bridge sites at high CO coverage. Voigtländer et al. favored the atop site Landolt-Börnstein New Series III/42A

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at high coverage. Structural investigations proved the bridge site to be correct at the high CO coverage of the ordered CO (2×1)p2mg layer [88Han, 93Hua, 93Pan1, 93Pan2].

2058

0.545

2055

~ 0,57

2049

1905

1908 1912

1905

T = 90 K (annealed to 240 K) θCO = ~ 0.57

1914

CO/Ni (111)

1902

0.525

0.35 0.33

2049

0.405

2052

2055

Absorbance A

0.435

2054

1887

0.50

1831

0.175 0.14 0.11 0.07 0.03 0.015

1816

1818

0.3

1800

1850

1900 1950 –1 Wavenumber ν [cm ]

2020

0.003 A

resolution = 7 cm

2060

–1

Fig. 22. Infra-red absorption spectra of CO adsorbed on Ni(111) for CO coverages between 0.015 and 0.57. All peaks correspond to C–O stretch vibrations [88Sur].

2100

The correlation between adsorbate eigenfrequencies and adsorption site assignment is in general not unambiguous. The problem is further demonstrated by the example of CO on Ni(111) which has been studied by IRAS [88Sur, 89Che] as well as EELS [79Erl1, 81Ber]. Figure 22 shows a sequence of IRA spectra of adsorbed CO for increasing coverages at 90 K and annealed at 240 K [88Sur]. Two main bands are observed, one at low frequencies of 1816-1914 cm–1, the other at about 2050 cm–1. The low frequency band shifts significantly with increasing coverage. This band was assigned to 3-fold hollow and 2-fold bridge sites, the latter being occupied at higher coverages. The high frequency band was attributed to atop adsorption sites. A well ordered CO layer of c(4×2) symmetry appeared at a coverage of 0.5 at which only one kind of CO was observed, at the time associated with CO in 2-fold bridge sites [79Erl1, 88Sur]. Landolt-Börnstein New Series III/42A

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It was shown later by quantitative photoelectron diffraction work that this site assignment was incorrect [93Sch, 96Dav]. Instead CO was found to be adsorbed in 3-fold hollow sites, both fcc and hcp, at low and intermediate coverages up to 0.5, which is the coverage of the c(4×2)-CO structure. The observed frequency shifts are believed to be largely caused by lateral CO–CO interactions. This example shows that the frequency of an internal mode of an adsorbate is not necessarily a reliable guide to the adsorption site [93Sch].

1.4.7 Structure and adsorption site Quantitative structure investigations of the adsorbate/substrate complex are possible by a number of techniques, such as LEED, photoelectron diffraction, x-ray diffraction and x-ray absorption spectroscopy, to name the most frequently used ones. Quantitative LEED in the form of recorded beam intensities versus energy requires long range ordered adsorbate structures. Photoelectron diffraction and x-ray absorption, on the other hand, are typical short range order techniques. A new variant of LEED rests on the holographic interpretation of elastic electron scattering [90Sal, 95Sal] where the primary scattering event generates the reference wave and the secondary scattering events at neighboring atoms represent the object waves. Superposition of both, reference and object waves, causes interference and produces a diffuse intensity distribution in space (hologram). A suitable Fourier transform of the intensity pattern yields the three-dimensional atomic structure around the primary scattering atom, adsorbed randomly on the surface. This holographic approach works for single atom adsorption in an ordered array, whereby this atom serves as the beam splitter [97Reu]. However, holographic LEED does not work for adsorbed molecules. We stay with the example of CO adsorbed on Ni(110) and present results of a quantitative LEED intensity investigation of the structure of the adsorbed CO [88Han]. In fitting the experimental data of beam intensity versus electron energy to theoretical profiles, a large number of trial structures were considered. Five structural parameters, such as the C–O and the Ni–C bond lengths, the Ni–Ni interlayer spacing, the surface-C and the C–O tilt angles, and the adsorption site (atop, 2-fold short and long bridge) were varied. The results were: CO is adsorbed in the short bridge site, the C–O and Ni–C bond length are 1.12 Å and 1.95 Å, respectively, and the Ni–C and C–O tilt angles in the [001] directions are 27°±3° and 17°±3°, respectively [88Han]. The optimum values are found by the usual R-factor (residual) analysis. For example, Fig. 23 shows the R-factor contour plot for the C–O bond length versus Ni–C bond length, assuming a common Ni–C–O tilt of 20°. A rather shallow broad minimum is achieved.

Fig. 23. LEED R-factor plot for the variables C–O bond length and CO tilt angle. A minimum is found near 22° and 1.9 Å bond length [88Han].

A second quantitative structure determination for the same system is based on energy scanned C1s photoelectron intensities of the adsorbed CO for several directions and a kinetic energy range of 60-400 eV [93Hua]. The data were evaluated by Fourier analysis as well as by fitting to theoretical

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intensity versus energy profiles, based on a multiple scattering spherical wave approach. The results were that CO is adsorbed in the short bridge site, tilted along the [001] azimuth. The tilt angle for the direction between the center of the Ni–Ni bridge and the C-atom is 16°±2°, and the tilt angle for the C–O bond is 19°. The interlayer spacing between the first and second layer is 1.27 Å, and the Ni–C and C–O bond lengths are 1.94 Å and 1.16 Å, respectively [93Hua]. A comparison of best fit experimental and theoretical C1s intensity, in the form of the modulation amplitude, χ(k) = [I(k)–I0(k)]/I0(k), where I0(k) is atomiclike diffractionless C 1s intensity, versus electron momentum, for three measured directions is shown in Fig. 24 for the short brige site and atop site, respectively. The bridge site is clearly favored in this comparison.

Fig. 24a, b. Energy dependent C1s photoemission fine structure for (2×1)p2mg-CO on Ni(110) taken at three polar angles. Comparison of experiment (solid) and theory (dashed; multiple scattering calculation) for two different CO adsorption sites, (a) 2-fold short-bridge and (b) atop [93Hua].

X-ray absorption studies of CO adsorbed on Ni(110) yield the position of the oxygen atom of CO relative to the nearest neighbor Ni atoms (SEXAFS) as well as the inclination of CO relative to the Ni surface (NEXAFS) [93Pan1]. The results are that CO is bonded to the short bridge and tilted by 21°±5° along the [001] azimuth. Finally, we quote a theoretical study of tilted CO on Pt(110) and Ni(110) surfaces [85Bau]. An interesting material dependence is found in that CO tilts 20° along the [001] azimuth for Ni(110), but 35°- 40° in a completely different direction, namely the [433] azimuth, for Pt(110). The latter case would be consistent with a p1g1 symmetry of the unit cell [82Hof1, 82Hof2, 87Wes]. The case CO adsorbed on Pt(110) is from an experimental point of view still controversial since a p1g1 as well as a p2mg symmetry have been reported [84Rie] [92Win]. Table 2 summarizes and compares the quantitative structural data collected so far by various techniques for the (2×1)p2mg CO layer adsorbed on Ni(110).

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[Ref. p. 59

Table 2. Structure data for the (2×1)p2mg CO layer adsorbed on Ni(110). Ref.

[85Rie] [86Alv] [88Han] [88Wes1] [92Kna, 98Emu] [93Hua] [93Pan1] [85Bau]

Adsorption site Bond length Bond length

[001] tilt

[001] tilt

d12 [Å]

of CO

C–O [Å]

Ni–O [Å]

Ni–C

C–O

Ni–Ni

short bridge short bridge short bridge (theory)

1.12 1.16 (1.138)

1.95 1.94 (1.852)

27° 16° -

19° 19° 17° 21° 20° 19° 21° 20°

1.25 1.27 -

1.4.8 Molecular orbital energies, dispersion and 2d band structure Molecular orbital (MO) energies of adsorbed species, detectable by photoelectron spectroscopy, are dependent on the type and strength of bonding to the substrate surface and on the intermolecular interactions within the layer. Angle-resolved measurements of photoemission using linearly polarized ultra-violet light (ARUPS) allows to detect the energetic positions of the orbitals and to determine their symmetry character, provided the adsorbed species have a unique orientation in space. It is also possible to measure the dispersion of MO-states with electron wavevector, provided there is long range order in the layer. Structural details of the layer can be derived in connection with a model calculation of the 2d band structure. The well ordered (2×1)p2mg structure of CO adsorbed on Ni(110) was subject of a detailed ARUPS study [86Kuh] which may serve as an example. The 2d Brillouin zone for the (2×1)p2mg unit cell is shown in Fig. 17b. A single CO species bonded perpendicularly to a high symmetry site on a surface is characterized by three occupied MOs, called the 4σ, 1π and 5σ (in order of decreasing binding energy). Since the symmetry of CO adsorbed on Ni(110) is only 2-fold, the 1π orbital is split into an x- and ycomponent. Furthermore, there are two CO molecules per unit cell (compare 1.4.4), which doubles the number of molecular orbitals compared to the that of an isolated adsorbed CO. Hence a total of eight MOs are expected. Depending on the geometry of the photoemission experiment and the polarization of the incident photons, excitation from different orbitals is enhanced. This effect is illustrated, for example, in Fig. 25 by a series of spectra recorded for different polar angles in the ΓΧ azimuth and for a photon energy of 35 eV. A systematic variation of the kinetic energy of the photoelectron, via the variation of the photon energy, and its detection angle relative to normal, the dispersion of the MO states versus the parallel component of the electron wave vector, kII, has been mapped out for the two high symmetry directions ΓΧ ( [1 1 0] direction) and ΓΥ ( [001] direction) of the 2d Brillouin zone [86Kuh]. Here kII is given by k II =

2mE kin sin ϑ . h2

(32)

The results are summarized in Fig. 26 as a 2d band structure plot of all 8 molecular orbital states of adsorbed CO on Ni(110). A large dispersion of molecular orbitals, in particular of the 1πx and 4σ states, is observed at Γ along the [1 1 0] glide plane. The full lines in Fig. 26 represent the results of a tight binding calculation [79Hor, 83Gre] for an unsupported CO layer of (2×1)p2mg symmetry where some metal–CO bonding was accounted for by renormalized CO wavefunctions. The agreement between experiment and theory has been optimized by choosing a value of the CO tilt angle. The best agreement is

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achieved for a tilt of 17° (error ±2°) and supports the assumed real space structure of Fig. 17a. Larger tilts tend to decrease the dispersion while a zero tilt would cause an unrealistically large dispersion of 4.2 eV for the 1πx states. Geometrically, the tilt of CO minimizes the intermolecular separation between all CO molecules in this ordered structure. In simple terms, a chemisorption bond involves the hybridization of substrate and adsorbate orbitals (electron states), i.e. there are new bonding and antibonding states, some of which may be empty. The empty states are normally not seen by photoemission spectroscopy. They can be analyzed by inverse photoemission spectroscopy (IPES) in a manner analogous to PES. Angle, energy and polarization dependent measurements can be carried out to determine the 2d electron band structure of unoccupied states [86Fre, 89Mem].

CO (2×1) p2mg/Ni (110) ΓX

ϑ = 80° 70° 60°

50°

40°

Intensity I

30°

20°

10° 0° –10°

Fig. 25. Photoemission spectra of CO on Ni(110) taken at several polar angles ϑ in the ΓΧ azimuth, photon energy at 35 eV [86Kuh]. 5

6

Landolt-Börnstein New Series III/42A

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9 8 10 Binding energy Eb [eV]

11

12

13

1 Introduction

34 5.5 6.0

CO (2×1) p2mg/Ni (110)

6.5

1

7.5

Binding energy Eb [eV]

+

1p x 1

7.0

5

8.0

[Ref. p. 59

1

+ y – y

– – x

8.5 9.0

5

+

9.5 10.0

4

–

10.5 11.0 11.5 Y

4

Fig. 26. Experimental (filled circles) and theoretical (lines) 2d electron band structure of (2×1)p2mg-CO on Ni(110) along the ΓΧ and ΓΥ directions [86Kuh].

+

Wavevector k

X

1.4.9 Core level binding energies The electron binding energies of core levels are a sensitive indicator of intra-molecular as well as surface bonding. Depending on the chemical environment of an atom, its element-specific core level binding energy can vary significantly mainly because of changes in the local valence electron charge. Hence the core level binding energies of an atom within a molecule will change with the surface-molecule bonding and also with different chemical neighbors of an adsorbed molecule. Furthermore, the core level binding energy of a specific atom within a molecule depends on its immediate neighbors, e.g. the O1s binding energy is expected to be different for O2, CO, NO, H2O, etc. Therefore, measurements of core level binding energies by photoemission are the basis of a surface sensitive chemical analysis of adsorbed layers, atoms and molecules, known as electron spectroscopy for chemical analysis (ESCA) [67Sie]. Also, a molecularly adsorbed state (e.g. N2) can be clearly distinguished from a dissociated state (atomic N) of the same species. Beyond chemical analysis, even subtle structural distinctions can be made, e.g. between physisorption and chemisorption of the same molecule, or between different adsorption sites of the same molecule. However, an effect interfering with this useful correlation between core electron binding energy and chemical/physical state of the adsorbed species is the electronic screening of the hole generated by the photoemission process [82Sie]. The hole screening effects the kinetic energy of the departing photoelectron and hence its apparent binding energy. This final state effect therefore influences the measured electron binding energy in a way that is not straightforwardly associated with the local substrate-adsorbate bond.

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To give an example for subtle distinctions of several adsorbed molecular states and also between molecular and atomic adsorbed states, we present O1s core level data for molecular and atomic oxygen adsorbed on Pt(111) in Fig. 27. Four spectra are shown in this figure, one for the physisorbed molecular O2 state at 25 K, two for chemisorbed molecular O2 at 90 K and 138 K, respectively, and one for the dissociated atomic O state at > 150 K [95Pug]. The two chemisorbed molecular states are associated with superoxo and peroxo configurations, respectively [82Ste]. The characteristic O1s binding energies and structural parameters are summarized in Table 3. The physisorbed O2 state is very similar to the gas phase oxygen state, as judged by valence photoelectron spectra [95Pug]. It is not clear whether this state is equivalent to an intrinsic precursor state for O2 chemisorption or not.

O2/Pt(111)

O1s

hν = 1487 eV

atomic phase

Intensity I

chemisorbed phase II

chemisorbed phase I

Fig. 27. O1s core level spectra of molecular O2 and atomic O adsorbed on Pt(111). Different phases exist at different temperatures: Physisorbed oxygen at 25 K; chemisorbed O2 at 90 K; second chemisorbed O2 phase at 138 K; atomic oxygen at > 150 K. All spectra excited with monophysisorbed phase

540

538

Landolt-Börnstein New Series III/42A

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534 532 Binding energy Eb [eV]

chromatized Al K α 1487 eV [95Pug]. 530

528

526

radiation at

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36

CO/Pt(111)

O1s

[Ref. p. 59

C1s

c(5× 3) θ = 0.6 on top 0.4 bridge 0.2

Intensity I

c(4×2) θ = 0.5 on top 0.25 bridge 0.25

(4×4) θ = 0.2

Fig. 28. C1s and O1s core level spectra of three ordered CO layers at different coverages of 0.2, 0.5 and 0.6 adsorbed on Pt(111). Excited with monochromatized

on top 0.2

Al K α

radiation at 1487 eV

[94Bjö].

536

534

532

530 528 290 Binding energy Eb [eV]

288

286

284

Table 3. Characteristic data of various adsorbed oxygen states on Pt(111).

Chemical state O–O bond order Adsorption T [K] O–O bond length [Å] O1s EB [eV] O–O stretch, ν [cm–1]

Physisorbed O2

Chemisorbed O2

Chemisorbed O2

Chemisorbed O

gas like 2 25-30 (1.208) *) 536.88; 535.80 (1556) *)

superoxo 1.5 90 1.37 530.54 -

peroxo 1 135-150 1.43 529.63 700-875

atomic 0 > 150 529.83 Pt–O: 470

*) Gas phase O2 values. Another illustration concerns the change of binding energy of an adsorbate core level with the adsorption site. Although there is no general correlation between binding energy and site, it appears that a decrease in binding energy is associated with increased coordination of the adsorption site [78Plu]. The example is shown in Fig. 28 for CO on Pt(111) at three different coverages of 0.2, 0.5 and 0.6 [79Nor, 83Kis, 94Bjö]. All layers are well ordered and exhibit a (4×4), c(4×2) and c(5× 3 )rect LEED pattern, respectively. Single O1s and C1s peaks are recorded for the (4×4) phase in which CO adsorbs only in atop sites [87Tüs]. Double O1s and C1s peaks are seen for the other two phases for which both atop and bridge sites are occupied [87Tüs]. Hence it is clear in this case that the higher and lower binding energy peaks correspond to the atop and bridge adsorbed CO, respectively. The quantitative intensities seem to correlate well with the relative site occupations known from the structure models. Another example is CO Landolt-Börnstein New Series III/42A

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adsorbed on a partially hydrogen covered Ni(100) surface [92Til]. For the ordered c(2 2 × 2 )R45°CO/H structure two well separated O1s and C1s peaks of CO are seen which are attributed to CO adsorption in atop and 4-fold coordinated sites. The effect of coadsorbed hydrogen is minor in this case. Table 4 summarizes CO core level binding energies correlated with different adsorption sites. Table 4. Core level binding energies of CO in different adsorption sites. System

Structure

Site

O1s EB [eV]

C1s EB [eV]

Ref.

CO/Ni(100)

c(4×2) p(3 2 × 2 )R45° c(2×2) c(2 2 × 2 )R45°

atop

532.2

285.9

[92Til]

bridge atop

531.3 532.9

285.5 286.3

[92Til]

atop 4-fold bridge atop atop atop

532.8 530.3 531.2 533.0 532.7 532.7

286.2 285.1 285.4 286.3 286.7 286.7

bridge

531.0

286.0

CO/H/Ni(100)

CO/Cu(100) CO/Pt(111)

disordered c(2×2) (4×4) c(4×2) c(5× 3 )rect

[90Ant] [94Bjö]

A detailed study of core level line shapes, satellites, peak splitting and peak shifts is a wide subfield in photoelectron spectroscopy. For example, CO bonds to transition metal surfaces via electron donation and back-donation, involving the anti-bonding CO 2π orbital which becomes partially occupied. Photoexcitation of an O1s or C1s core level creates a positive hole which needs to be screened. Screening may involve charge transfer from metal states to the CO 2π orbital or an intra-molecular 1π→2π transition. The latter occurs also with gas phase CO and is seen as a satellite of the main core level line. It is found at 3.9 eV higher binding energy and therefore called a shake-up peak. A systematic comparison of core level and shake-up peaks for adsorbed CO and carbonyl compounds has been carried out to understand the bonding of CO to transition metals [78Plu, 83Fre2]. Quantitative core line intensities can be used to measure relative coverages of adsorbed species. To get reliable intensities one should integrate the electron emission over a larger solid angle to become insensitive towards effects of angle dependence. Also, a spectrum should be recorded over a sufficiently large energy range, such that all possible satellite peaks are included in the integration. Angle dependent measurements yield information on the orientation of intra-molecular bonds (compare 1.4.3).

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1.4.10 Two-dimensional phase diagrams and order-disorder transitions The study of ordered adsorbate layers on single crystal surfaces is a subfield of adsorption on surfaces. Depending on the relative magnitude of inter-molecular interaction to adsorbate-substrate interaction, a sequence of ordered two-dimensional structures can be formed with increasing coverage. Important are competing nearest and next-nearest neighbor pairwise as well as three-body interactions within the adsorbate layer. Perfectly ordered regions (commensurate structures) may be separated by anti-phase domain boundaries which by themselves may exhibit order (incommensurate structures). Each ordered phase has a limited temperature range of stability. A 2d phase diagram, i.e. a plot of temperature versus coverage, describes the stability regions of all involved phases. Order-disorder as well as order-order transitions may occur at the phase boundaries. Many adsorbate phases are "floating" on the substrate surface, in the sense that the adsorbate-substrate interaction is sufficient to cause commensurability, but insufficient to cause a significant structural change at the surface, such as a reconstruction. However, adsorbate induced surface reconstructions are also known to occur (compare 1.5.1 and 5.1). Such a phase may be part of a complex phase diagram, e.g. for H adsorbed on Ni(110) [81Eng, 84Pen]. The subject will be briefly discussed by the example of H adsorbed on Fe(110) [77Boz, 82Imb, 83Sel]. Two well ordered surface structures of H, (2×1) and (3×1), have been found at 140 K by LEED at the coverages of about 0.5 and 0.67, respectively. Less sharp streaky patterns were seen for intermediate coverages. Recording the intensities of the fractional order (i.e. superstructure) LEED spots versus temperature in the range 150-300 K, an order-disorder transition was monitored at nearly constant H coverages. The critical temperatures were determined from the inflection points of the intensity versus temperature curves. A plot of these transition temperatures versus coverage is shown in Fig. 29a as a phase diagram, complemented by hypothetical two-phase regions (dashed) with anti-phase boundaries [82Imb]. These (2×1) and (3×1) H superstructures apparently do not involve a surface reconstruction. A theoretical modelling of the H/Fe(110) system was carried out by Monte Carlo and transfer matrix techniques [83Sel]. The best agreement with experiment, shown in Fig. 29b, was obtained by including repulsive first and second nearest neighbor (pairwise) and also attractive three-body (nearest neighbor) interactions between adsorbed H. An additional (3×1) H structure at a coverage of 0.33 is found theoretically which was not detected in the experiment. The observation of multiple ordered phases of adsorbates at submonolayer coverages is no rare event. Adsorbed species with large dipole moments, such as alkali metals, CO, H or O atoms, have a particularly strong tendency to order. A second example of a rather complex 2d phase diagram is given in Fig. 30 for CO on Pt(111) [84Bib, 89Per, 90Per]. This somewhat schematic phase diagram is the result of experimental and theoretical studies. Experimental LEED work had shown a series of ordered patterns [77Ert, 82Ste, 90Per] with increasing coverage: ( 3 × 3 ) R 30° = c ( 3 × 5)rect at a CO coverage of 0.33, c(4×2) = ( 3 × 2)rect at 0.5, ( 3 × 5)rect at 0.6, ( 3 × 3)rect at 0.67 and ( 3 × 7)rect at 0.71 coverage. Monte Carlo simulations reproduce all of the observed ordered structures at the ideal coverages of 1/3, 1/2, 3/5, 2/3 and 5/7, respectively. The adsorption sites of CO are close to the ideal atop and 2-fold bridge sites in all cases. This theoretical result is also in good agreement with vibrational studies showing C–O stretch frequencies at 2100 cm–1 and 1840-1887 cm–1 for atop and bridge adsorbed CO, respectively, for the whole coverage range [82Ste, 90Per]. At intermediate coverages incommensurate domain wall structures exist which are equivalent to two-phase regions in the phase diagram in Fig. 30. The critical temperatures for order-disorder transitions, experimentally not yet determined, rise with increasing coverage.

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2.5

Tc = 265 K Tc = 245 K

250

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225

lattice gas

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Temperature T [K]

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175

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0.3

0.4

0.5 0.6 H coverage θ

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Fig. 29a, b. Experimental (a) and theoretical (b) 2d phase diagram of adsorbed H on Fe(110). The low coverage 3×1 phase was not observed experimentally [82Imb, 83Sel]. 400

CO/Pt(111)

350 fluid

250 200 150

( 3× 3)R30°

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I

II

III

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Fig. 30. Proposed 2d phase diagram for adsorbed CO on Pt(111). Phases I, II and III correspond to ordered structures ( 3 × 5) rect at coverage 0.6,

( 3 × 3) rect at 0.67 and

( 3 × 7) rect at 0.71, respectively. Shaded areas in between correspond to incommensurate phases [84Bib, 90Per].

50 0 0.3

multi-layer-growth

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1.4.11 Chemical state One of the important issues in analyzing an adsorbed molecular species, also from a more technical point of view, is the question of its chemical state. Does the molecule stay intact when it adsorbs or does it dissociate into fragments? Which bonds fracture under what conditions? Processes such as bond fracture, dissociation or molecule fragmentation are a direct link to possible surface reactions involving those fragments, formed in the adsorption step, and other coadsorbed molecular or radical species. Therefore, studying adsorption and the chemical state of an adsorbed species, also as a function of temperature, is Landolt-Börnstein New Series III/42A

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always a study of surface reactivity, and hence of direct relevance to the technical field of heterogeneous catalysis. A simple example is the adsorption and dissociation of CO on metal surfaces. Various systematic studies have indicated that the noble metals Cu, Ag and Au and their neighbors in the periodic table do not dissociate CO, while Fe, Mo and W and the elements to their left in the periodic table easily dissociate CO [76Bro]. Metals, such as Co, Ru and Re, seem to be "border line" cases where CO is molecularly adsorbed on some low-index surfaces (at room temperature) while it dissociates on others. Obviously, for these metals the surface structure becomes important for the dissociation of CO, in addition to the electronic structure of the substrate. Similar systematics have been noticed for N2 and NO [76Bro]. Simple theoretical and thermodynamic arguments have been forwarded to explain this material dependence [80Ben, 80Miy, 80Ros]. Another interesting example is the adsorption of ethylene (C2H4, ethene) on Pt(111). This molecule adsorbs intact at 90 K in a di-sigma configuration, i.e. with its C–C axis parallel to the surface [77Iba, 78Iba, 81Bar]. At temperatures around 270 K it partially desorbs and partially dehydrogenates to a species that has a single C-bond to the surface and a concomitant structural reorientation of the C–C axis to perpendicular. The species was later shown to be ethylidyne (–C–CH3) [78Kes, 79Kes] which is stable to about 440 K. Above that temperature it further decomposes and dehydrogenates to –CH radicals. The same ethylidyne species can be obtained by adsorbing acetylene (C2H2) at 120 K and warming this adlayer to 270 K [78Iba, 78Kes, 79Kes]. Under those conditions some molecules dehydrogenate, others react with the adsorbed hydrogen to ethylidyne which is stable up to 440 K. These adsorption/decomposition reactions are element and structure specific as a comparison of Ni, Cu, Pd and Pt surfaces has shown [83Gat, 93Bao]. The foremost spectroscopic techniques that yield information on the chemical state of adsorbates are optical (IRS, Raman, SFG, SHG), electron (EELS, XPS, UPS) and ion (SIMS) spectroscopies. Quantitative structural techniques such as LEED and XPD have also contributed to solving the question of the chemical state of the primary and secondary intermediate species. In this context, sections 1.4.3, 1.4.5-1.4.8 should also be consulted.

1.4.12 Influence of surface structural defects The important fundamental concept of structure sensitivity in heterogeneous catalysis has been proposed on the basis of a supposedly inhomogeneous surface of the active catalyst [25Tay, 29Bal]. So called active centers of either crystallographic [25Tay] or compositional character were believed to be responsible for an increased rate and the selectivity of the reaction. These hypotheses have found support by numerous more or less well defined experiments. For example, the rate of ethylene hydrogenation was found to be more active on preferentially oriented Ni thin films than on unoriented Ni films [40Bee]. Numerous experiments on supported metal catalysts produced evidence of structure sensitive reactions [70Cla, 73Sin]. Therefore it had long been suspected that the adsorption of gases on metals, alloys and semiconductors was structure sensitive, in the sense, that differently coordinated surface atoms of a substrate exhibited a different adsorption energy of a particular species. Along with such a difference, the kinetics of adsorption and also the final adsorbed state may be influenced by surface defects. Dissociation of the molecule may or may not occur, depending on whether the adsorption site is a high coordination defect or a low coordination terrace site. Experimental work on flat single crystal surfaces, either highly oriented or slightly misoriented relative to a low-index orientation, proved to be an excellent way to check the ideas about structure sensitivity at low pressures. Misoriented crystals were prepared to exhibit surfaces with a known defect structure, such as step and kink sites, and with a known density of these defects [77Som, 81Som]. Furthermore, the structural dependence of adsorbate properties, once established by well defined single crystal experiments, is being used to characterize the structure of small metal particles supported on an oxide support [96Rai, 97Rai]. Infra-red spectroscopy of adsorbed CO as a probe molecule yields information on the heterogeneity of the surface via the frequencies of the stretch vibration, for example. This information is very useful in relationship to realistic supported catalysts used in commercial catalytic processes [94Xu].

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The first experiments with adsorption on stepped single crystal surfaces were carried out in 1971/72 for Cu and Pt [71Per, 72Lan]. It was found that dissociative adsorption of H2 and O2, for example, occurred with higher rates at 300 K on stepped Pt(111) and Pt(100) surfaces than on their smooth counterparts. A similar observation was made for the adsorption of oxygen on stepped Si surfaces [73Iba]. A clear demonstration for the influence of step sites on adsorption and reactivity was given for CO on a stepped Ni(111) surface. Experimental results are summarized in Fig. 31 [78Erl]. Thermal desorption spectra of CO from a flat Ni(111) crystal show a single desorption peak at 433 K [74Chr, 78Erl]. For CO desorption from a stepped Ni[5(111)×( 1 1 0 )] crystal, however, a second peak is observed at 820 K which is attributed to associative desorption from C and O atoms. These atoms are originally formed by CO dissociation occurring exclusively at step sites during increasing of the temperature. The fact that they remain located at the step sites was concluded from an additional observation [78Erl]. If CO adsorbed at low temperature was dissociated by electron bombardment and then thermally desorbed, the associative desorption from C and O gives rise to a peak at 620 K, Fig. 31, which must be assigned to C and O atoms located on terrace sites. Hence the desorption peak at 820 K was clearly differentiated from the peak at 620 K due to associative desorption from C and O adsorbed on terrace sites. Additional vibrational spectroscopy showed step-adsorbed CO molecules at 150 K to be characterized by a very low C–O stretch frequency of 1520 cm–1 [79Erl2]. Such a low stretch frequency is indicative of substantial C–O bond weakening and thus consistent with the high propensity of this CO to dissociate on heating. Raising the temperature to 300 K led to a considerable lowering of the intensity of this 1520 cm–1 peak relative to those at higher frequencies characteristic of CO adsorbed on regular terrace sites. This could have already indicated the dissociation of this CO.

Fig. 31a - d. Thermal desorption spectra of CO from a smooth Ni(111) (a-b) and a stepped Ni[5(111)×( 1 1 0 )] crystal (c-d). The α-peak corresponds to molecular CO adsorption. (b) Electron irradiation of the CO layer on Ni(111) causes a new desorption peak β1 due to dissociated CO. (c) Observed β2-peak from stepped Ni[5(111)×( 1 1 0 )] is attributed to thermally dissociated CO at step sites. (d) Electron irradiation of the CO layer on stepped Ni[5(111)×( 1 1 0 )] causes both desorption peaks β1 and β2, due to dissociated CO on Ni(111) terrace sites and step sites, respectively, to appear [78Erl].

Numerous other examples dealing with adsorbed CO or other gases are known where surface defects are special adsorption sites in the sense that the adsorption energy is higher there than on terrace sites [81Som, 89Cam]. Related to this energy difference, step sites are preferentially occupied, and the reactivity of these sites is generally higher, often leading to localized dissociation of adsorbed species. Landolt-Börnstein New Series III/42A

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It is now widely recognized that this important result of systematic surface research constitutes a firm base of Taylor's idea, expressed in 1925, of active sites and structure sensitivity in heterogeneous catalysis [25Tay]. Moreover, atomic steps are not only crystallographic features but also electronic defects since their local electronic structure, characterized by the local electron charge density, differs from that of terrace sites. This was experimentally demonstrated by work function measurements of stepped surfaces which led to the assignment of a dipole moment to the step [77Bes]. In this context the so-called structural effect of adsorption (and catalysis) is at the same time also an electronic effect. Another kind of varying the surface structure is by imposing a lateral stress, such as encountered for thin epitaxial films on a single crystalline substrate. Depending on the lattice mismatch between film and substrate surface, the film may be strained relative to its normal bulk state. Positive as well as negative strain can occur. Due to the strain, the surface stress will also be changed. Investigations of adsorption on such strained (stressed) surfaces of thin films have shown [92Rod, 94Kam, 98Gse] that the properties of the adsorbed species are altered more or less significantly. The variation of the C–O stretch frequence, for example, of CO adsorbed on a number of differently strained surface films shows a linear correlation between frequency shift and strain [94Kam]. Self-consistent density functional calculations of the effect of strain on the reactivity of surfaces has shown by the example of oxygen and CO adsorption on Ru(001) that the heats of adsorption of both species increase with increasing strain in a range from –2.5 to +2.5% [98Mra]. A corresponding decrease was found for the activation energy of CO dissociation. Overall, the imposed stress causes strain, which changes the surface structure (e.g. via slight expansion or compression), and furthermore the surface electronic structure, such as the width and center position of the d-band of transition metals, for example. Mavrikakis et al. demonstrate that the changes in the energetic adsorbate properties can also be correlated with changes of the center position of the d-band [98Mra]. Summarizing the influence of crystallographic surface orientation, surface structural defects, surfacs strain on reactivity, one can argue qualitatively that the more "open" the surface (in the sense of a lower bond order of surface atoms), the more reactive it will be.

1.4.13 Influence of chemical surface additives ("promotors") Modyfying surface properties by alloying or simply by adding second surface components, sometimes called the "promotor" of a reaction, to a clean surface is an issue that is aimed at probing the electronic factor in heterogeneous catalysis. The general idea is to change the local electron concentration at the surface and to thereby influence its reactivity. The prime example in catalysis is the addition of alkali metal species in various forms [83Mro]. This idea has been carried over to theory and experiments of adsorption, where single crystal surfaces have been modified by adsorbing atomic species, such as alkali metals or other species, to investigate induced changes in the properties of subsequently added adsorbates. The study of the mutual influence of two (or more) different adsorbates constitutes the field of coadsorption. Obviously, two groups of additives may be distinguished, electron donors and electron acceptors. While alkali metals are typical electron donors, lowering the work function of a metal substrate [23Lan], species such as C, O, P, S, Se etc. are considered to be electron acceptors, associated with an increase in the work function of the substrate. As a rule, the electron donating species enhance the propensity of adsorption, the electron accepting species retard adsorption (and reactions), i.e. they act as "poisons" to a surface reaction. Early electron band theories of this effect were aimed at explaining the electronic factor in catalysis [38Wag]. Numerous systematic studies of the alkali metal promoter effect on well defined single crystal surfaces have been reported. Although coadsorption phenomena in general shall not be elaborated on in this volume of LANDOLT-BÖRNSTEIN, two examples will be mentioned that illustrate the effect. These are the K promoted adsorption of CO and N2 on Fe single crystal surfaces [79Bro, 79Ert]. The first bears some relevance to the Fischer-Tropsch synthesis of hydrocarbons from CO and H2 [56And, 72Dry, 82Bon1], and the second to the synthesis of NH3 from N2 and H2 [75Emm, 80Ert], both on K promoted Fe-based catalysts. For example, adsorbed K on Fe(110) enhances the propensity for CO dissociation at 310 K and low pressure, as shown in Fig. 32. While the total coverage of C and O increases with increasing K coverage, the ratio of dissociated to molecular CO rises from about 0.2 to 1 at a monolayer Landolt-Börnstein New Series III/42A

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of K. The initial sticking coefficient of CO, on the other hand, decreases with increasing K coverage. This behavior has been explained in terms of an increased adsorption energy of CO/K compared to CO on clean Fe(110) and a concomitant decrease in the activation barrier of dissociation [79Bro]. An analogous result and interpretation was found for the K-promoted adsorption and dissociation of N2 on Fe(110) [79Ert]. Further studies by vibrational spectroscopies at 150 K have demonstrated the C–O bond weakening of K coadsorbed CO [92Zhu], a feature that parallels an increased backdonation of electrons into the anti-bonding 2π* molecular orbital of CO and hence an increased metal–CO bond strength [81Kis, 87Bon1].

Fig. 32a - c. Effect of K-induced dissociation of CO on Fe(110) at 310 K. (a) Increase in total C,O coverage with increasing K coverage. (b) Initial sticking coefficient of CO vs. K coverage. (c) Fraction of dissociated over molecular CO, determined by photoemission spectroscopy, vs. K coverage [79Bro].

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1.5 Adsorbate-induced changes of substrate properties The adsorption of atoms and molecules on crystalline surfaces implies some degree of physical and/or chemical interaction with the substrate. A pure physical van der Waals-type interaction is relatively weak and will not change the structural environment near the adsorption site to any significant extent. However, apparent electronic changes of the substrate, such as electron work function or core and valence level binding energies, may be expected. On the other hand, when chemical bonding between certain adsorbate orbitals and localized or delocalized electron orbitals of surface atoms occurs, the redistribution of valence charges near the adsorption site may be substantial and thus effect nearest and next nearest neighbor bonds in the substrate (as well as in the adsorbate). In that case it is evident that structural, electronic and vibrational changes in the substrate, usually in the local environment of the adsorbed species, will occur, effecting surface energy and surface stress as well. These effects are generally summarized as adsorbate-induced changes of substrate properties. A systematic investigation of such changes yields information on the adsorbate and also on the adsorbate-substrate interaction. The most important adsorbate-induced changes of substrate properties will be briefly introduced in this section.

1.5.1 Changes of structure: surface relaxation and surface reconstruction The structure of clean single crystal surfaces is frequently not equal to the structure of an equivalent cut through a bulk crystal [78Mül, 84Est]. Quantitative structural studies have shown that the separation between atomic layers near the surface is often smaller (seldom larger) than in the bulk ("relaxation"). In some cases also the lateral structure deviates from the bulk terminated structure, equivalent to a change in the structural unit cell. These surfaces are then called "reconstructed". Examples of reconstructed surfaces were found at first on low-index semiconductor surfaces [59Sch] and later on metal surfaces [65Hag, 66Fed, 67Fed1, 67Lyo]. A large number of structural data is now available on relaxation and reconstruction phenomena of clean surfaces [88Jon, 93Fas]. Adsorbate-induced changes of clean surface structures are divided into three categories: (1) Changes in surface relaxation, i.e. in interlayer separations vertical to the surface. (2) Surface reconstruction, i.e. changes in interatomic bond lengths parallel (and perpendicular) to the surface, causing a periodic lateral shifting of atoms or a density change of surface atoms, connected with a superstructure different from that of the clean surface. Two examples of reconstructed fcc (110) surfaces are shown schematically in Fig. 33, the missing row and the pairing row structures, respectively. Both exist in nature, as we will see below. (3) Surface faceting, i.e. breakup of a flat surface of a certain crystallographic orientation into a rough hill and valley structure, consisting of two or more new surface orientations ("facets") appearing in a more or less irregular fashion. There are also cases where the reconstruction of a clean surface is destabilized ("lifted") by adsorption, e.g. the missing row (1×2) reconstruction of Pt(110) by adsorbed CO [73Bon, 76Com]. The structural transformation (1×2) → (1×1)-CO → (2×1)p1g1-CO is driven by a change in total surface free energy which is lower for (1×2) in the clean state but lower for the non-reconstructed surface in the CO covered state [82Bon2, 82Fer]. That is equivalent to the adsorption energy of CO being lower on the reconstructed than on the non-reconstructed surface [83Beh, 84Bar, 88Imb]. The adsorbate-induced destabilization of a reconstructed surface requires a certain minimum temperature because structural changes are thermally activated. Thus the (1×2) → (1×1)-CO transition occurs at T > 240 K [82Fer] while below this temperature the adsorption of CO on the stable (1×2) surface can be studied. Historically, the phenomenon of surface reconstruction was closely related to adsorption, which occurred unintentionally in the sense of contamination. The principal question, whether reconstruction could be a genuine phenomenon of a clean surface or be due to impurities on that surface, was, at least for metals, a matter of serious debate [67Fed2, 67Som]. It was later clarified that surface reconstruction was indeed a rather common phenomenon, both for clean and adsorbate covered surfaces [89Som, 94Sta]. However, even the adsorbate-induced reconstruction of surfaces was a controversial issue in the beginning, best illustrated by early investigations of H2 and O2 adsorption on Ni(110) [62Ger]. A (2×1)-H superstructure had been observed by LEED, and it was argued by Germer that the extra LEED beams were indicative of a reconstructed Ni surface since H-atoms were known to be poor electron scatterers Landolt-Börnstein New Series III/42A

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and not capable of causing intense extra beams [66Ger]. This was questioned by Bauer [66Bau]. On the other hand, adsorbed oxygen caused a (1×2)-O LEED pattern which Germer attributed to an ordered array of O-atoms on the surface [62Ger]. Years later it was shown that the (2×1)-H superstructure is indeed due to H ordering on the non-reconstructed Ni(110) surface [81Eng, 83Rie, 84Pen, 87Rei] while the (1×2)-O LEED pattern is due to a reconstructed Ni(110) surface with an ordered array of O-atoms [81Nie, 81Sme, 90Kle, 90Voi].

Fig. 33a - c. Perspective view of fcc (110) surfaces. (a) non-reconstructed surface; (b) reconstructed (1×2) missing row surface; (c) reconstructed (1×2) pairing row surface [74Chr].

The system H/Ni(110) is most interesting from another point of view. More than five H-related superstructures were later detected by elastic He scattering and LEED [83Rie, 84Pen]. The one with the highest H-coverage of 1.5 was a (1×2)-H structure on a reconstructed Ni surface which involved a pairing of atomic Ni rows in the first layer, such as illustrated in Fig. 33c, and additional changes in interlayer spacings beneath the surface [87Kle]. Whereas the interlayer spacing between the first and second layer of the clean Ni(110) surface is contracted by 8.5% [87Rei], it is slightly expanded for the (1×2)-H structure [87Kle]. This behavior is typical for adsorbate-induced changes of relaxation in that the interlayer bond distances at the surface increase in the presence of an adsorbate, tending to assume a nearbulk value [89Som]. This tendency is also visible for the non-reconstructed (2×1)-H superstructure where the change in interlayer spacing goes from 8.5% contraction (clean) to 4.5% contraction for the H covered surface [87Rei]. Overall, the adsorbate-induced reduction of the clean surface interlayer relaxation seems to be a general phenomenon, at least for surfaces where the first-to-second interlayer spacing exhibits a contraction [89Som]. The interplay between adsorption, in particular chemisorption, and surface structural changes is of principal importance in heterogeneous catalysis [72Som1, 95Som]. Since many catalytic reactions are structure sensitive, it is of great interest to know the most active surface orientation for a particular reaction. This knowledge is in general very difficult to obtain because comprehensive structural investigations during an ongoing reaction are not possible. The third kind of structural change, faceting, is much more violent and involves a massive lateral transport of matter. Adsorption of alkali metals on some fcc (110) surfaces causes the missing row structure of Fig. 33b. This structure is characterized by (111) oriented nanoscale facets. Thus reconstruction can in some cases be viewed as a nucleus to faceting. Facets of µm dimension are often Landolt-Börnstein New Series III/42A

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seen after oxygen or chlorine treatment or after surface reactions involving oxygenates or halogenated molecules [79Fly]. Furthermore, monolayer coverages of a deposited metal, such as Rh, Pd, Au, Pt, on W(111) or Mo(111) surfaces may cause a complete breakup of the surface into small {211} and {110} facets when heated to more than 800 K [97Nie, 99Mad]. This behavior is possibly significant in relationship to heterogeneous reactions on bimetallic catalysts. One of the critical questions concerns the connection between coverage and faceting. Is there a critical coverage of an adsorbate beyond which faceting of a surface will occur? The basic reason for larger scale faceting is a change in the anisotropy of the substrate's surface free energy caused by adsorption. However, quantitative information on this dependence is scarce. This topic will be dealt with in section 1.5.4.

1.5.2 Changes of electron work function The electron work function of a crystal is the difference between bulk chemical potential (Fermi level), µ , and the total electrostatic potential barrier at the surface. Adsorption of atoms or molecules on the surface can not change the bulk chemical potential but certainly the surface barrier if the adsorbed species develops a dipole moment in the process of adsorption or has a permanent dipole moment which becomes oriented in the electrostatic field at the surface. The work function change, ∆φ, is therefore equal to the electrostatic surface potential barrier connected with a dipole moment via the classic Helmholtz equation: ∞

∆φ = − µ + 4πe ∫ dz z δn( z) .

(33)

−∞

Here δn( z ) is the local redistribution of (valence) charge density that accompanies adsorption. The concomitant formation of an adsorbate dipole moment with adsorption can occur via charge transfer between this species and the surface or by charge polarization in the surface-adsorbate complex. The former is common for more strongly chemisorbed species while the latter is important for physisorption. The classic example of a work function change in context with physisorption is the adsorption of noble gases, e.g. xenon on a metal surface. Theoretically the interaction can be described as van der Waals (long range) or, in the framework of local density theory, as due to electron correlation forces between the valence electrons of Xe and those of the metal [82Lan]. In the latter, an exchange-correlation potential outside the surface was calculated which is of longer range than the electrostatic (image) potential of the surface, shown in Fig. 34. The adsorbed Xe atom is localized entirely in the field of the exchange-correlation potential where it is polarized. The resulting dipole moment of adsorbed Xe is then responsible for the heat of adsorption and for a change in work function measurable outside the surface. Work function changes of up to 0.8-1.0 eV may result from Xe adsorption [79Küp, 82Jac]. Furthermore, electrons bound in valence and core levels of adsorbed Xe may experience binding energy shifts which depend on the total magnitude of the potential [82Lan]. Numerous measurements of the 5p1/2 level binding energy of adsorbed Xe atoms on various clean metal substrates indicate that the observed shifts in this binding energy agree closely with the measured work function differences of these surfaces [80Hul, 84Wan]. A similar study for stepped surfaces shows two well separated Xe 5p1/2 levels which can be correlated with Xe adsorption on terrace and step sites, respectively [81Wan]. The binding energy difference was interpreted in terms of a corresponding local difference in electrostatic potential ("local work function") of step sites and terrace sites. Thus photoemission measurements of adsorbed Xe (PAX) were proposed to be a technique to study local work function differences due to crystallographic defects or pre-adsorbed foreign atoms. It should be mentioned, however, that the correlation between local work function and Xe 5p1/2 binding energy has been discussed controversially in the literature [80Kai, 86Wan, 87Kai, 87Ste]. The chemisorption of atoms or molecules, on the other hand, is generally characterized by chemical bond formation between adsorbate and substrate surface atoms, i.e. some degree of hybridization between electronic states of both partners is involved. The direction of the charge transfer determines in many cases the sign of the work function change and the orientation of the surface dipole. For example, Fig. 35 shows measured work function changes for adsorbed Cl and Cs atoms on a Cu(111) surface, respectively [77God, 80Lin]. Since the electronegative Cl accepts charge from Cu, the work function increases, and vice versa for Cs. The behavior for Cs is typical of adsorbed alkali metals on metal surfaces in that there Landolt-Börnstein New Series III/42A

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is a strong initial decrease in work function followed by a shallow minimum and a small rise near a saturated monolayer [89Lan]. The latter features are due to depolarization of closely packed dipoles. The final work function denotes a metallic alkali metal overlayer. The strong initial decrease indicates a substantial charge transfer from the alkali metal to the substrate, equivalent to a large dipole moment of the adsorbed alkali species. Dipole moments of single adsorbed species can be evaluated from the slope of ∆φ versus coverage in the limit of vanishing coverage.

Fig. 35a, b. Electron work function changes due to (a) chlorine and (b) cesium adsorption on a Cu(111) surface, respectively [77God, 80Lin].

Fig. 34a-c. Results of a model calculation of a Xe atom adsorbed on a high electron density jellium surface (e.g. aluminium). (a) Contours of constant electron density in a cut perpendicular to the surface through the center of the Xe atom. (b) Xe valence p-electron density vs. distance (difference density between metal-adatom system and sum of clean metal plus single Xe atom except 5p level). (c) Effective single particle potential energy contributions due to electrostatic dipole, Ves, and the exchange-correlation interaction, Vxc, respectively, [82Lan].

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1.5.3 Electron binding energy changes, core and valence levels The adsorption of atoms and molecules on surfaces can not only be studied by analyzing their valence and core level spectra (sections 1.4.7/1.4.8) but also by evaluating changes in substrate levels although these are more subtle and not so easy to recognize. For example, core level spectra of clean semiconductor and metal single crystal surfaces consist of (at least) two contributions, one due to surface atoms and the other due to bulk atoms. The energetic difference is called a surface core level shift, ∆Es,b, defined as the difference EB(surface atom) – EB(bulk atom). This difference can be positive or negative. When there is more than one kind of surface atom, such as for fcc (110) or bcc (111) oriented surfaces, separate surface contributions can be attributed to surface atoms of different local coordination. The separation of a surface peak from the bulk core level peak can be achieved by comparing spectra taken at normal and grazing (more surface sensitive) emission angles, particularly for low kinetic photoelectron energies. When atoms or molecules are adsorbed on a single crystal surface, the energetic situation of surface atoms involved in the adsorption bond changes and so does the core level binding energy of that atom. Thus there is a new surface core level shift for surface atoms bonding to an adsorbate, ∆Es,ad, defined as EB(surface atom with adsorbate) – EB(clean surface atom). The adsorbate-induced surface core level shifts can be as large as 1.4 eV. The well studied system of CO on Pt(111) shall serve as an example [82Apa, 86Düc, 94Bjö]. Figure 36 illustrates the results for the Pt 4f7/2 core level studied with monochromatized synchrotron radiation of 125 eV [94Bjö]. This low photon energy, which is equivalent to a low kinetic energy of about 50 eV for a Pt 4f7/2 photoelectron, enhances the surface sensitivity, such that for clean Pt(111) the surface and bulk contributions are readily detected at binding energies of 70.50 and 70.90 eV, respectively, corresponding to ∆Es,b = –0.40 eV [94Bjö]. This is in good agreement with previous reports [82Apa, 83Apa, 86Düc]. Adsorption of CO at a coverage of 0.2, causing a (4×4) LEED pattern, reduces the intensity of the clean surface peak and causes a new CO-related peak at 71.9 eV, corresponding to a surface core level shift of ∆Es,CO = 1.4 eV. Increasing the CO coverage to 0.5 and 0.6 produces sequentially two more ordered CO arrays with both atop and bridge adsorbed CO. The clean surface peak has become quite small under these conditions but now two CO-related surface peaks at 71.2 and 71.9 eV binding energy are needed to fit the measured spectrum. Those peaks are attributed to atop and bridge adsorbed CO, with resolved shifts of 1.4 and 0.7 eV, respectively. Previous studies had not resolved the adsorption-site-specific core level shifts, and therefore "average" values of ∆Es,CO = 1.3 eV [83Apa] and 1.0 eV [86Düc] had been reported. The relatively large magnitude of > 1 eV for a CO-induced surface core level shift, i.e. for a shift beyond the position of the bulk Pt peak, is not easily explained by the charge distribution in this Pt–CO surface complex. Instead, details of the photoemission process have to be invoked to rationalize this number. A phenomenological way of understanding the magnitude is by means of a Born-Haber energy cycle which at the same time shows the relationship of surface core level measurements to thermodynamic surface properties. The principal of the cycle is illustrated in Fig. 37 [83Ege]. CO adsorbed on Pt(111) is shown in the lower left corner. Desorption of CO costs 1.38 eV [84Poe]. Removing an electron from the 4f7/2 core level of the clean and CO covered surface costs 70.5 and 71.9 eV (atop CO), respectively, leaving a Au surface atom behind provided the core hole is fully screened (so-called Z → Z+1 approximation). Finally, desorbing a CO molecule from a Au surface atom embedded in a Pt surface costs approximately the desorption energy of CO from Au. At the end we have a relationship for estimating this desorption energy: E ad , CO ( Au ) = ∆Es, CO − E ad , CO ( Pt ) .

(34)

Inserting the measured values (e.g. for atop CO) the estimate for the adsorption energy of CO on Au is 0.02 eV which is a little too low compared to literature data [76Mce]. If we take the average value of ∆Es,CO = 1.05 eV for a mixture of atop and bridge CO, we find a more reasonable desorption energy of CO from Au of 0.33 eV. Another useful relationship, based on a Born-Haber cycle similar to the one illustrated in Fig. 37, connects surface core level shifts with the energy of surface segregation (section 3.6) [80Joh, 83Ros]. Emitting a 4f7/2 core electron from a surface and bulk Pt atom, for example, creates a Au surface and Au Landolt-Börnstein New Series III/42A

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bulk atom in a Pt matrix. Moving the Au dissolved "impurity" atom to the Pt surface requires the energy of segregation of Au to the Pt surface. Hence we have Esegr(Au,Pt) = ∆Es,b(Pt). Of course, such a simple relationship holds only for neighboring elements in the periodic table, since the Z → Z+1 approximation is used. This relationship between surface core level shifts and energies of surface segregation has been critically discussed by Feibelman [83Fei]. It is also known that the energy of segregation of a Z+1 element in a Z matrix is related to the energy of solution of Z+1 in Z and the difference in surface free energies of the clean Z and Z+1 materials [77Wyn, 78Mie, 83Fei]. Thus there exists a connection between surface core level shifts ∆Es,b(Z) and surface free energies [87Bon2]. Feibelman points out that differences in the energy of adsorption of a species "ad" on surfaces of Z and Z+1 can be related to the work function change due to adsorption on Z, ∆φ ad , and the adsorbate induced surface core level shift [83Fei]:

[

]

∆Ead ( Z, Z + 1) = κ ∆φad ( Z ) + ∆Es, ad ( Z ) ,

(35)

where κ is a constant. These various equations show how adsorption induced changes of substrate properties, such as the work function, surface core level binding energies and surface energies, are connected to energies of adsorption.

Fig. 37. Born-Haber energy cycle depicting CO desorption from Pt and Au surface atoms (vertical arrows) and core level photoemission from a clean and CO covered Pt(111) surface (horizontal arrows) [83Ege, 86Düc].

Fig. 36. High resolution Pt 4f7/2 photoemission spectra (photon energy of 125 eV) of a clean and CO covered Pt(111) surface at several CO coverages of 0.2, 0.5 and 0.6. Note the shifted Pt surface atom peak at 70.9 eV (clean) due to adsorbed CO. Compare also fig. 28 [94Bjö].

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[Ref. p. 59

1.5.4 Changes of surface free energy and surface stress The most simple and fundamental thermodynamic relationship governing surface energetics is Gibbs' adsorption equation (9) in section 1.2. Accordingly, adsorption always causes a decrease in the surface free energy of a clean crystalline surface. The rate of change in surface energy with the change in chemical potential in the gas phase is a means to determine the surface excess, i.e. the amount of material adsorbed:  ∂γ  1  ∂γ  = = − Γ2 .     ∂µ kT  2  T, µ1  ∂ ln p 2  T, µ1

(12)

Measurements to prove Eq. (12) and to determine the surface excess have been made before the times of routine surface analysis by modern spectroscopies. For example, the absolute value of the surface energy of Cu (predominantly of (100) orientation) and its changes with O2 chemical potential at a high temperature of 1200 K, have been determined quantitatively under equilibrium conditions by the zero creep technique [70Hon1]. The results are shown in Fig. 38. There is a plateau at 1410 erg/cm2 for very low partial O2 pressures, representative of the clean surface, and a sharp decrease by about 600 erg/cm2 in a range of 10–23 to 10–18 bar O2 pressure. Here oxygen adsorption takes place causing the decrease according to Eq. (12). A quantitative evaluation of the surface excess yields 4 × 1014 oxygen atoms/cm2 which is a coverage of 0.25 on the Cu(100) surface. These results have been well reproduced by Bauer et al. [76Bau]. A further increase in O2 chemical potential leads to a sudden rise in surface energy between about 10–16 to 10–14 bar. This increase is associated with a surface phase change: beginning of oxidation of Cu and severe surface faceting. Evaluation of this data by Eq. (12) is no longer sensible. 16

O2/Cu(100)

2

–2

Surface energy Es [10 erg cm ]

14

12

10

bulk oxide

8

6 –30 10

10

–26

–22

–18

–14

10 10 10 10 O2 partial pressure p [bar]

–10

10

–6

10

–2

Fig. 38. Change of absolute surface free energy of Cu(100) vs. logarithm of oxygen partial pressure at 1200 K under equilibrium conditions [70Hon2].

Hondros and McLean have also studied the relative anisotropy of the free surface energy (γ-plot) of Cu in the clean and oxygen covered state by evaluating grain boundary groove angles [70Hon2]. Figure 39 shows a selection of their results as contours of constant surface energy plotted in a unit triangle of a stereographic projection. The surface energy values, obtained at T = 1200 K, are normalized to unity at the (100) orientation. Two γ-plots are shown, one for the clean surface at 2.5 × 10–24 mbar of O2 and the other for an O-covered surface at 1 × 10–14 mbar of O2. The total anisotropy of surface energy for the clean surface is about 2.1% with a minimum at the most densely packed (111) orientation. For the O-covered surface the total anisotropy has risen to 3.25%, and the absolute minimum is now at the (100) orientation; even the (110) orientation has a lower surface free energy than (111). Thus (111) as well as (110) oriented surfaces are expected to facet under the influence of oxygen adsorption, exposing

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preferentially (100) facets. This is indeed observed [70Hon2]. The change in anisotropy of the surface free energy by adsorption is due to different adsorption energies of the same species on differently oriented faces. For example, if the adsorption energy on the (111) surface is lower than on the (100), the corresponding coverage and hence the decrease in surface energy is lower for that orientation, such that the surface energy of the covered (100) may now be smaller than that of the covered (111) orientation. The adsorbate-induced change in surface energy is related to the energy of adsorption in a non-linear way. The relationship can be obtained, for the simple case of non-activated Langmuir adsorption (Ead = Ed), for example, by inserting Eq. (28) into Eq. (12):  ∂γ  p , = − kT Γ2 = − kTN 0   p + κ ( T)  ∂ ln p 2  T, µ

(36)

 E  12 with κ ( T) = νd N 0 (2πmkT) exp − ad  .  kT 

(37)

1

Integration yields for the adsorption-induced change in surface energy:  p  ∆γ( p, T) = − kTN 0 ln 1 +  . κ ( T)  

(38)

The ratio p/κ(T) can become very large compared to 1 for typical values in a vacuum chemisorption experiment, e.g. for p = 10–7 mbar, νd = 1012 s–1, Ead = 1 eV, and T = 300 K. Under these conditions p/κ(T) = 1890, and we recover a simple linear relationship between ∆γ and the energy of adsorption:   p ∆γ( p, T) = − N 0  kT ln + E ad  . 1/ 2 νd N 0 ( 2πmkT)  

(39)

This equation depends weakly on temperature and pressure, i.e. it holds for the saturation limit of an adsorbate's isotherm. One obtains under the above mentioned conditions ∆γ = 313 erg/cm2. At 10–5 mbar the decrease would amount to 500 erg/cm2. Higher values of p are unrealistic at the low temperature of 300 K and the chosen high adsorption energy, in particular since Eqs.(38) and (39) do not give an asymptotic limit of ∆γ. Adsorbate-induced changes in surface energy are also related to changes in surface stress via the Shuttleworth Eq. (13). In a qualitative comparison to the data of Fig. 39, changes of surface stress due to oxygen adsorption on a Ni(100) surface have been measured by the single crystal cantilever bending technique [92San, 97Iba]. The results, obtained at room temperature and in a coverage range up to 0.5, are shown in Fig. 40, exhibiting large values of ∆τ up to –5400 erg/cm2 at the maximum oxygen coverage. Compared to the intrinsic (tensile) surface stress of 1440 erg/cm2 for clean Ni(100) [91Gum], this is a very large change. In fact, the adsorption of oxygen changes the absolute surface stress from tensile to a high compressive stress (τ < 0). Unfortunately, a more quantitative comparison of changes of the surface energy and the surface stress for a certain oxygen coverage and a possible evaluation of the gradient term ∆(dγ/dε) in the spirit of the Shuttleworth Eq. (13) is not possible since measurements of both quantities for the same adsorbate/substrate system are not available. High values of adsorbate-induced changes in surface stress, such as measured for adsorbed carbon, oxygen and sulfur on Ni(100) [97Iba], may lead to surface reconstruction. In fact, it is noteworthy that under the conditions of an adsorbed layer only changes in surface stress will cause reconstruction. The reason is that changes in surface energy due to adsorption are always negative, i.e. reducing the surface energy of the clean surface. However, the latter is already minimized from a structural point of view, and a further reduction, beyond that caused by the adsorbate, does not seem physically possible, as long as the Landolt-Börnstein New Series III/42A

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0 -1

∆ τ (10

-2

Surface stress

3

erg/cm

2)

coverage stays below a value needed for faceting. On the other hand, a high compressive surface stress is equivalent to a weakening of lateral surface bonds favoring a lateral surface expansion. This bond weakening has been directly detected by a softening of certain surface phonon modes [83Sze, 86Mül]. As a response to this driving force, the surface tries to increase the nearest neighbor bond distances in the first layer, either by a surface reconstruction in which the original density of surface atoms is maintained, or by reducing the surface density, in that a certain number of atoms is kicked out. These types of reconstruction are then driven by a relief of surface stress and not by a lowering of the surface free energy. Famous examples are the carbon-induced reconstructions on Ni(100) [79Onu] and Ni(111) [93Gar]. In the case of oxygen on Ni(100), although the maximum change in surface stress is very large (Fig. 40), a surface reconstruction does apparently not take place.

-4

-3

-5 0.0

O/Ni(100) 0.1

0.2

0.3

0.4

0.5

Oxygen coverage θ Fig. 40. Changes in the surface stress of Ni(100) induced by oxygen adsorption at room temperature [92San, 97Iba]. Reference state is the clean surface at ∆τ = 0.

Fig. 39a, b. Contour plot of relative surface free energy of Cu measured at T = 1200 K. Data are shown as a stereographic projection in the unit triangle of low-index orientations. All values are normalized to unity at the (100) orientation. (a) Clean Cu surfaces; (b) oxygen covered Cu surfaces at a O2 partial pressure of 10–14 mbar [70Hon2].

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1.5.5 Changes of surface phonon dispersion

Frequency

The dispersion of surface phonons of clean solid surfaces has been measured for a large number of materials [91Kre]. The technique is quasi-elastic scattering of either He atoms at energies of 30-50 meV or of low-energy electrons at 100-300 eV from single crystal surfaces. The scattered particles are energy analyzed with highest resolution to determine the phonon frequencies. The scattering occurs under nonspecular conditions to obtain the variation of phonon frequency as a function of parallel momentum transfer, with the scattering plane located in a high symmetry surface direction [82Iba, 92Iba, 94Ben]. Surface phonons are quantized vibrational eigenstates of surface atoms and exhibit a well defined energy versus momentum dependence. In many cases the dispersion curves of surface phonons are separated from the continuum of bulk phonon states. Depending on the wave vector, the decay length of a surface phonon amplitude into the bulk can be extremely short (couple of layers), i.e. it is localized at the surface, whereas the decay length for surface resonances is infinite by definition but with a large amplitude at the surface [92Iba]. The dispersion of surface resonances is fully embedded in the projected dispersion of bulk phonons. In general, the dispersion of surface phonons depends sensitively on the interatomic bond strengths of atoms in the surface layer and the interaction between first and second layer. In the presence of adsorbed species the bonding of surface atoms to all its neighbors is changed, i.e. the bond strength within the first layer and also between first and second (less to the third etc.) layer is altered. Accordingly, all the interatomic separations at the surface may change, an effect that is measurable as a change in the amount of surface relaxation and also surface stress (compare 1.5.4). Sometimes the energetic changes are so large that the surface will reconstruct. Of course, the dispersion of surface phonons is altered as well. One of the prominent surface phonon modes is the Rayleigh phonon, a vibration of surface atoms in a plane normal to the surface (sagittal plane) and with a k-vector parallel to the surface. This typical surface vibration depends sensitively on the lateral and vertical bonding of surface atoms, and hence changes with the presence of adsorbed species. To illustrate the expected changes, a phonon dispersion diagram is shown schematically in Fig. 41 where the bulk phonon bands are indicated as a projected area (hatched) and the Rayleigh surface phonon is drawn as a line starting from the Γ -point to the zone boundary X [97Iba].

Bulk phonon band ∆τ > 0 clean surface ∆τ < 0

Surface phonon 0.0

Γ

0.2

0.4

0.6

q-vector

0.8

1.0

Fig. 41. Schematic plot of the dispersion of a Rayleigh surface phonon (solid line) and of a bulk phonon band (hatched area). Atoms or molecules adsorbed on the surface can change the surface phonon dispersion, visible as an increase or decrease of the corresponding frequencies relative to those of the clean surface. These shifts are equivalent to a strengthening or weakening of the interatomic surface bonds, respectively, and correspond also to a change in surface stress, being tensile (∆τ > 0) or compressive (∆τ < 0), respectively [97Iba].

X

The surface phonon has a certain frequency at this boundary for the clean surface, related to the intrinsic stress of that surface. Adsorbing atoms or molecules may now strengthen or weaken the intra-layer surface bonds (depending e.g. on the direction of charge transfer between surface and adsorbate). Accordingly, there is a force for the intra-layer bond length to shorten or elongate, or in other words, the surface wants to laterally compress or dilate, respectively. Normally neither of this can happen since the surface layer stays in registry with the bulk, as long as we do not consider surface reconstruction. Hence these forces are equivalent to a change in surface stress, ∆τ, which is positive (tensile) or negative (compressive), respectively, corresponding to either stronger or weaker interatomic bonds relative to the clean surface. The restoring force for the vibrational motion of surface atoms (e.g. vertical to the surface) will increase (stiffen) or decrease (relax) leading to an increase or decrease in the frequency of the Rayleigh mode, as schematically indicated in the figure. Landolt-Börnstein New Series III/42A

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[Ref. p. 59

c(2×2)O/Ni(100)

450 400 350

–1

Frequency ν [cm ]

300 250 200

Fig. 42. Comparison of experimental and theoretical (solid lines) surface phonon dispersion data (frequency vs. parallel component of wave vector) for an ordered c(2×2)-O Ni(100) surface. The calculations are based on the position of O-atoms at 0.9 Å above the surface and the inclusion of lateral dipole-dipole interactions. For comparison, the experimental data for the clean surface Rayleigh phonon (solid points) are also included [83Leh, 84Rah].

150 clean surface

100 50 0 Γ

0.2

0.4

0.6 0.8 1.0 –1 Wave vector qII [Å ]

1.2

X

An interesting example for the change in surface phonon dispersion due to an adsorbate is presented in Fig. 42 for the system oxygen on Ni(100) characterized by a c(2×2) LEED pattern [83Sze, 84Rah]. Dispersion curves for several surface phonon modes are shown for the oxygen covered surface. The branch of the Rayleigh phonon dispersing from zero energy at Γ to a high value at X , is compared to that measured for the clean Ni(100) surface [83Leh, 86Roc]. The Rayleigh phonon for the c(2×2)-O surface is clearly shifted to lower frequencies over the whole range, at the zone boundary ( X -point) from 132 cm–1 down to 80 cm–1. Such a big change is indicative of substantially weakened intermolecular Ni–Ni surface bonds and a corresponding decrease in surface stress, such as measured independently by Sander et al. [92San] and displayed in Fig. 40. A theoretical simulation of the oxygen induced change in surface phonon dispersion yields quantitative values for the position of the oxygen atom above the surface and changes in lateral and vertical bond strength [84Rah]. These results are qualitatively consistent with changes in the interlayer separation between the first and second Ni layer of the c(2×2)-O-Ni(100) surface. The surface was found to expand relative to the bulk for the oxygen covered surface, in contrast to a contraction determined for the clean Ni(100) surface [83Fre1]. The total change of 8.4% is very substantial and indicative of a large change in the Ni–Ni force constant. Other examples of adsorbate induced changes in surface phonon dispersion show even stronger effects, such as soft phonon modes, Kohn anomalies and surface reconstructions.

1.5.6 Changes of surface optical properties The scattering of light at surfaces (in a range of near visible and low uv frequencies) has been used in a large variety of experimental approaches to probe surfaces and interfaces [95Mcg], in particular also changes of surfaces due to the adsorption of atoms and molecules. One of the main interests in pursuing optical techniques is the advantage that they do not require vacuum environment. Hence they are applicable in a wider range of technologically relevant situations, such as material processing steps, e.g. during molecular beam epitaxial growth of semiconductor thin films, or in electrochemistry. The Landolt-Börnstein New Series III/42A

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penetration depth of light into the solid is large (could be an advantage for studying burried interfaces) and therefore the techniques have in general a poor surface sensitivity. The low surface sensitivity is at least valid for linear optical scattering experiments whereas non-linear second order scattering, such as important in second harmonic (SHG) and sum frequency generation (SFG) spectroscopies, can be highly surface sensitive [84She, 86She, 89Lie]. In general, the optical response of a solid to an electromagnetic field of optical frequency depends on the first, second, and higher order dielectric susceptibilities. If the bulk of the solid is centrosymmetric, the bulk second order dipole response is zero, and the dominant second order contribution originates from the surface (or a nearby interface) where the inversion symmetry is broken. Therefore the study of second harmonic generated intensity is sensitive to the electronic properties (expressed e.g. by the charge density profile) of the outermost layer [92Urb, 95Wol] and intrinsically surface sensitive. It is useful for the study of adsorbing species on the surface or of clean surface structural changes induced by varying the temperature, for example. Most light scattering techniques, however, depend on the first order dielectric response which is measured through the reflectivity of (linearly) polarized monochromatic light. Here the surface sensitivity is enhanced by subtracting the dominant isotropic bulk contribution. Relatively simple techniques are the differential reflectance (DRS) [71Chi] and reflectance anisotropy (RAS) [85Asp, 87Asp] spectroscopies. In the first the reflectance is measured as a function of frequency for two different surface conditions, clean and adsorbate covered, for example. Taking the difference between both signals removes the bulk contribution, with the remainder being characteristic of changes at the surface. In the second, the reflectance is measured for linearly polarized light at near normal incidence for optically anisotropic surfaces, with the E-vector oriented in two orthogonal high symmetry surface directions a and b. The normalized difference, ∆r / r = ( ra − rb ) / ( ra + rb ) [85Asp], is then recorded either as a function of time, to monitor changes at the surface, or versus frequency, in the sense of a true spectroscopy. Another similar but older technique is spectroscopic ellipsometry where the reflection induced changes in the state of polarization of a linearly polarized incident light is measured [68Arc, 71Mey, 83Gij]. The ratio of reflectivity, measured with the polarization parallel and perpendicular to the plane of incidence, can be expressed as a function of two parameters, ∆ and Ψ, the phase shift and relative attenuation of the field vectors parallel and perpendicular to the plane of incidence. Through Maxwell's theory, these parameters are related to the refractive index of the ambient and the optical constants of the reflecting system. For a clean isotropic surface these are the real and imaginary parts of the refractive index. In the case of an adsorbed layer, the changes in ∆ and Ψ are related in a complex way to the optical constants of both the film and the substrate as well as the thickness of the film. It is common to nearly all optical techniques that the physical interpretation of measured signals and even of the changes of physical parameters evaluated from the primary signal is difficult. As far as adsorption induced changes of clean surface properties are concerned, the information obtained from optical techniques is therefore rather indirect. In some cases spectral resonance features associated with the surface (interface) may be detected, and their changes with adsorbing atoms or molecules may be useful to follow the kinetics of this process but fundamental information on the properties of the adsorbed species is rarely obtained. To illustrate the information extracted from some of the more frequently used techniques, we present examples for adsorbates on Cu(110) analyzed by ellipsometry [79Hab, 82Han], RAS [95Hof] and SHG [95Wol]. Due to the two-fold symmetry of Cu(110), the surface optical properties are anisotropic. The results for the ellipsometric investigation are shown in Figs. 43a and b. The data in Fig. 43a correspond to differences of ∆ and Ψ, each measured for the two azimuth [1 1 0] and [001] at three temperatures, respectively, that characterize the clean Cu(110) surface. Obviously δ∆ = ∆( [1 1 0] ) – ∆( [001] ) and, to a lesser extent, δΨ show a "resonance" near 600 nm (2 eV). In Fig. 43b the dependence of the relative oxygen Auger peak intensity (ratio of the oxygen 510 eV peak to the Cu 920 eV peak) versus δ∆ (measured at a wavelength of 632.8 nm) is presented for oxygen covered Cu(110) and Cu(111) surfaces at 296 K and on removal of adsorbed oxygen at 473 K [79Hab]. There is a linear relationship between both quantities which shows that δ∆ can be used under non-vacuum conditions to monitor the oxygen coverage. The absolute coverage of oxygen or surface structural information is not readily obtained from the optical data.

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0.6

3.0

2.5

Photon energy hν [eV] 2.0

[Ref. p. 59

1.5

Cu(110)

0.5 0.4

δ∆ [deg]

0.3 0.2 0.1 0 – 0.1 – 0.2 400

0.2

450 500 550 600 650 700 750 800 850 Wavelength λ [nm] Photon energy hν [eV] 3.0 2.5 2.0 1.5

0.6

O/Cu

0.1

δΨ [deg]

0 – 0.1 – 0.2 – 0.3 – 0.4

T = 295 K 520 K 600 K

Auger peak ratio O510 eV/Cu920 eV

0.5

– 0.5 – 0.6 400

a

450 500 550 600 650 700 750 800 850 Wavelength λ [nm] b

0.4 0.3 0.2

Cu(110) T = 296 K Cu(111) 520 K Cu(111) 600 K

0.1

0

0.1

0.2

0.3 0.4 δ∆ [deg]

0.5

0.6

0.7

Fig. 43a, b. (a) Spectral ellipsometry from clean Cu(110) showing the differences δψ and δ∆ vs. wavelength of light at three different temperatures [82Han]. (b) Dependence of the relative oxygen Auger peak intensity (ratio of the oxygen 510 eV peak to the Cu 920 eV peak) vs. ellipsometric parameter δ∆ measured at a wavelength of 632.8 nm. Data are plotted for oxygen covered Cu(110) and Cu(111) surfaces at 296 K and on removal of adsorbed oxygen from Cu(111) at 473 K [79Hab].

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Reflectance Re(∆r/ r )

Cu(110)

(2×1)-O/Cu(110)

5 × 10

–3

(2×1)-CO/Cu(110)

1.5

2.0

2.5

3.0

3.5

4.0

Photon energy E [eV]

4.5

5.0

5.5

6.0

Fig. 44. Real part of the optical reflectance anisotropy spectra for clean Cu(110), oxygen covered (2×1)-O-Cu(110) and CO covered (2×1)-CO-Cu(110) surface. Horizontal lines indicate zeros of each spectrum [95Hof] .

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Fig. 45. Dependence of the s- and p-polarized SHG intensity vs. oxygen exposure for a Cu(110) surface. SHG radiation generated by a fundamental wave at 630 nm at normal incidence in a plane parallel to the

Γ X [1 1 0] azimuth [95Wol].

The second example in Fig. 44 shows the real part of the reflectance anisotropy versus photon energy for the clean, oxygen and CO covered Cu(110) surfaces, respectively [95Hof]. Substantial differences in the spectra are obvious. The resonance at 2.1 eV of the clean surface is nearly quenched by adsorbed oxygen. It is totally quenched by adsorbed CO and a large peak near 4.4 eV emerges for that case. The big changes at 2.1 eV allow time resolved (exposure dependent) measurements of the adsorption kinetics to be made. The resonance at 2.1 eV is explained by a transition involving electronic surface states of Cu(110) [91Jia, 95Hof]. Thirdly, the SHG study of the same system also showed the same resonance at 2.06 eV for clean Cu(110) and a shifted resonance at 2.09 eV for the oxygen covered Cu surface [95Wol]. The resonance at 2.06 eV was identified as a surface state transition at the Y point of the surface Brillouin zone (compare also [92Urb]). The occupied surface state is strongly attenuated with increasing oxygen coverage, thus effecting the intensity of the SHG resonance. The dependence of the SHG intensity versus oxygen exposure is illustrated in Fig. 45 for s- and p-polarized light, incident at the fundamental wavelength of 630 nm (1.97 eV) along the [1 1 0] azimuth. The drastic change in intensity for the s-polarized case can be correlated with the known structural changes of the Cu(110) surface during oxygen exposure. The steep decrease up to 1 L exposure corresponds to the loss of inward relaxation (–8.5%) and changes of the electronic surface structure while the smaller increase for higher exposures is related to the formation of the (2×1)-O surface reconstruction and outward relaxation of 17% [95Wol]. Note that this detailed interpretation would not be possible without referring to the results of extensive studies carried out with other quantitative spectroscopies and structural techniques. In comparing the spectral features of all three optical experiments dealing with the Cu(110) surface it is obvious that the resonance at 2.1 eV is present and dominant in all data sets. The sensitivity of electronic surface states (of clean Cu(110)) to adsorbed oxygen (and probably other adsorbates) causes in this case the optical techniques to be quite sensitive to adsorbates, enabling their use to monitor the kinetics of adsorption/desorption kinetics, for example. Since electronic surface states are quite common for a number of semiconductor surfaces, it is understandable that optical response investigations are sensitive to adsorbates especially on these surfaces. Hence they are frequently employed to study kinetic phenomena involving adsorption or thin film growth.

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References for 1 1876 1878 1879 13Lan 15Knu 18Lan 21Vol 23Lan 25Tay 27Dav 28Len 28Str 29Bal 29Ste 30Cla 31Str 32Len 36Len 37Dev 37Len 38Bru 38Wag 39Fow 39Mit 39Vol 40Bee 47Bar 50Shu 51Kin 52Her

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83Ege 83Fei 83Fre1 83Fre2 83Gat 83Gij 83Gom 83Gre 83Kis 83Leh 83Mro 83Pfn 83Rie

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Imbihl, R., Ladas, S., Ertl, G.: Surf. Sci. 206 (1988) L903. Jona, F., Marcus, P.M., in: Structure of Surfaces II. Springer Series in Surf. Sci., Vol. 11, Berlin: Springer-Verlag, 1988, p. 90. 88See Seebauer, E.G., Kong, A.C.F., Schmidt, L.D.: Surf. Sci. 193 (1988) 417. 88Stu Stulen, R.H., Knotek, M.L.: "Desorption Induced by Electronic Transitions - DIET III", in: Springer Series in Surf. Sci., Vol. 13. Ertl, G., Gomer, R. (eds.), Berlin: Springer-Verlag, 1988. 88Sur Surnev, L., Xu, Z., Yates jr., J.T.: Surf. Sci. 201 (1988) 1; 14. 88Wes1 Wesner, D.A., Coenen, F.P., Bonzel, H.P.: Surf. Sci. 199 (1988) L419. 88Wes2 Wesner, D.A., Coenen, F.P., Bonzel, H.P.: Phys. Rev. Lett. 60 (1988) 1045. 88Zhu Zhu, X.D., Rasing, T., Shen, Y.R.: Phys. Rev. Lett. 61 (1988) 2883. 89Cam Campbell, C.T.: Adv. Catalysis 36 (1989) 1-54. 89Che Chen, J.G., Erley, W., Ibach, H.: Surf. Sci. 223 (1989) L891. 89Lan Lang, N.D.: "Theory of Alkali Adsorption on Metal Surfaces", in: Physics and Chemistry of Alkali Metal Adsorption. Bonzel, H.P., Bradshaw, A.M., Ertl, G. (eds.), Amsterdam: Elsevier, 1989, p. 11-24. 89Lie Liebsch, A., Schaich, W.L.: Phys. Rev. B 40 (1989) 5401. 89Mem Memmel, N., Rangelov, G., Bertel, E., Dose, V., Kometer, K., Rösch, N.: Phys. Rev. Lett. 63 (1989) 1884. 89Per Persson, B.N.J.: Solid State Commun. 70 (1989) 211. 89Som Somorjai, G.A., Hove, M.A. v.: Progr. Surf. Sci. 30 (1989) 201. 89Wes Wesner, D.A., Coenen, F.P., Bonzel, H.P.: Phys. Rev. B 39 (1989) 10770. 90Ant Antonsson, H., Nilsson, A., Martensson, N., Panas, I., Siegbahn, P.E.M.: J. Electron Spectrosc. Relat. Phenom. 54/55 (1990) 601. 90Ber Berger, H.F., Leisch, M., Winkler, A., Rendulic, K.D.: Chem. Phys. Lett. 175 (1990) 425. 90Bet Betz, G., Varga, P.: "Desorption Induced by Electronic Transitions - DIET IV." Springer Series in Surf. Sci., Ertl, G., Gomer, R. (eds.), Berlin: Springer-Verlag, 1990. 90Fei Feigerle, C.S., Desai, S.R., Overbury, S.H.: J. Chem. Phys. 93 (1990) 787. 90Gom Gomer, R.: Rep. Progr. Phys. 53 (1990) 917. 90Kle Kleinle, G., Wintterlin, J., Ertl, G., Behm, R.J., Jona, F., Moritz, W.: Surf. Sci. 225 (1990) 171. 90Lin Lin, T.S., Lu, H.J., Gomer, R.: Surf. Sci. 234 (1990) 251. 90Per Persson, B.N.J., Tüshaus, M., Bradshaw, A.M.: J. Chem. Phys. 92 (1990) 5034. 90Sal Saldin, D.K., Andres, P.L. d.: Phys. Rev. Lett. 64 (1990) 1270. 90Voi Voigtländer, B., Bruchmann, D., Lehwald, S., Ibach, H.: Surf. Sci. 225 (1990) 151. 91Bor Borroni-Bird, C.E., Al-Sarraf, N., Andersson, S., King, D.A.: Chem. Phys. Lett. 183 (1991) 516. 91Gum Gumbsch, P., Daw, M.: Phys. Rev. B 44 (1991) 3943. 91Jia Jiang, M.Y., Pajer, G., Burstein, E.: Surf. Sci. 242 (1991) 306. 91Kre Kress, W., Wette, F.W. d.: "Surface Phonons", in: Springer Series in Surf. Sci. Vol. 21. Ertl, G., Gomer, R. (eds.), Berlin: Springer-Verlag, 1991. 91Par Parschau, G., Rieder, K.H.: Surf. Sci. 257 (1991) L628. 91Xia Xiao, X.D., Xu, X.D., Daum, W., Shen, Y.R.: Phys. Rev. Lett. 66 (1991) 2352. 92Dea DeAngelis, M.A., Glines, A.M., Anton, A.B.: J. Chem. Phys. 96 (1992) 8582. 92Iba Ibach, H.: Phys. Blätter 9 (1992) 705. 92Kna Knauff, O., Grosche, U., Bonzel, H.P., Fritzsche, V.: Mol. Phys. 76 (1992) 787. 92Rod Rodriguez, J.A., Goodman, D.W.: Science 257 (1992) 897. 92San Sander, D., Linke, U., Ibach, H.: Surf. Sci. 272 (1992) 318. 92Til Tillborg, H., Nilsson, A., Martensson, N.: Surf. Sci. 273 (1992) 47. 92Urb Urbach, L.E., Percival, K.L., Hicks, J.M., Plummer, E.W., Dai, H.L.: Phys. Rev. Lett. 45 (1992) 3769. 92Win Wintterlin, J., Behm, R.J., in: Scanning Tunneling Microscopy I. Springer Series in Surf. Sci. 20. Güntherodt, H.H., Wiesendanger, R.(eds.), Berlin: Springer-Verlag, 1992, p. 39-81. 92Zhu Zhu, L., Bao, S., Xu, C.Y., Xu, Y.B.: Surf. Sci. 260 (1992) 267. 93Bao Bao, S., Schindler, K.M., Hofmann, P., Fritzsche, V., Bradshaw, A.M., Woodruff, D.P.: Surf. Sci. 291 (1993) 295. Lando lt -Börnst ein New Ser ies III/42A

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93Stu 93Xia 94Ben 94Bjö 94Fri 94Gat 94Kam 94Sta 94Xu 95Fei 95Hof 95Mcg 95Pug 95Sal 95See 95Som 95Spr 95Wol 95Xia 96Ber 96Dav 96Des 96Rai 97Iba 97Nie 97Rai 97Reu 97Xia 97Yeo

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Bonzel, H.P.: "Surface Diffusion on Metals", in: Diffusion in Solid Metals and Alloys. Landolt-Börnstein (New Series) III/26. Mehrer, H. (ed.), Berlin: Springer-Verlag, 1993, p. 717747. Burns, A.R., Stechel, E.B., Jennison, D.R.: "Desorption Induced by Electronic Transitions DIET V." Springer Series in Surf. Sci., Ertl, G., Gomer, R. (eds.), Berlin: Springer-Verlag, 1993. Fasolino, A., Selloni, A., Shkrebtii, A.: "Surface reconstruction and relaxation", in: Physics of Solid Surfaces: Structure. Landolt-Börnstein (New Series) III/24a. Chiarotti, G. (ed.), Berlin: Springer-Verlag, 1993, p. 125-224. Gardin, D.E., Batteas, J.D., Hove, M.A. v., Somorjai, G.A.: Surf. Sci. 296 (1993) 25. Huang, Z., Hussain, Z., Huff, W.T., Moler, E.J., Shirley, D.A.: Phys. Rev. B 48 (1993) 1696. Pangher, N., Haase, J.: Surf. Sci. 292 (1993) L791. Pangher, N., Haase, J.: Surf. Sci. 293 (1993) L908. Schindler, K.M., Hofmann, P., Weiss, K.U., Dippel, R., Gardner, P., Fritzsche, V., Bradshaw, A.M., Woodruff, D.P., Davila, M.E., Asensio, M.C., Conesa, J.C., González-Elipe, A.R.: J. Electron Spectrosc. Relat. Phenom. 64/65 (1993) 75. Stuckless, J.T., Al-Sarraf, N., Wartnaby, C., King, D.A.: J. Chem. Phys. 99 (1993) 2202. Xiao, X.D., Xie, Y., Shen, Y.R.: Phys. Rev. B 48 (1993) 17452. Benedek, G., Toennies, J.P.: Surf. Sci. 299/300 (1994) 587-611. Björneholm, O., Nilsson, A., Tillborg, H., Bennich, P., Sandell, A., Hernnäs, B., Puglia, C., Martensson, N.: Surf. Sci. 315 (1994) L983. Fritzsche, V., Knauff, O., Bonzel, H.P.: Phys. Rev. B 49 (1994) 10643. Gatos, H.C.: Surf. Sci. 299/300 (1994) 1. Kampshoff, E., Hahn, E., Kern, K.: Phys. Rev. Lett. 73 (1994) 704. Starke, U., Hove, M.A. v., Somorjai, G.A.: Progr. Surf. Sci. 46 (1994) 305. Xu, X., Szanyi, J., Xu, Q., Goodman, D.W.: Catal. Today 21 (1994) 57. Feibelman, P.J.: Phys. Rev. B 51 (1995) 17867. Hofmann, P., Rose, K.C., Fernandez, V., Bradshaw, A.M., Richter, W.: Phys. Rev. Lett. 75 (1995) 2039. McGilp, J.F.: "Optical Characterization of Semiconductor Surfaces and Interfaces." Progr. Surf. Sci. 49 (1995) 1-106. Puglia, C., Nilsson, A., Hernnäs, B., Karis, O., Bennich, P., Martensson, N.: Surf. Sci. 342 (1995) 119. Saldin, D.K., Chen, X.: Phys. Rev. B 52 (1995) 2941. Seebauer, E.G., Allen, C.E.: Progr. Surf. Sci. 49 (1995) 265-330. Somorjai, G.A., Hove, M.A. v.: Acta Crystallogr. B 51 (1995) 502. Sprunger, P., Besenbacher, F., Steensgaard, I.: Surf. Sci. 324 (1995) L321. Woll, J., Meister, G., Barjenbruch, U., Goldmann, A.: Appl. Phys. A 60 (1995) 173. Xiao, X.D., Xie, Y., Jakobsen, C., Galloway, H., Salmeron, M., and Shen, Y.R.: Phys. Rev. Lett. 74 (1995) 3860. Bertino, M.F., Hofmann, F., Steinhögl, W., Toennies, J.P.: J. Chem. Phys. 105 (1996) 11297. Davis, R., Woodruff, D.P., Hofmann, P., Schaff, O., Fernandez, V., Schindler, K. M., Fritzsche, V., Bradshaw, A.M.: J. Phys. Condens. Matter 8 (1996) 1367. Desjonquères, M.C., Spanjaard, D.: Concepts in Surface Physics. Berlin: Springer-Verlag, 1996. Rainer, D.R., Wu, M.C., Mahon, D., Goodman, D.W.: J. Vac. Sci. Technol. A 14 (1996) 1184. Ibach, H.: Surf. Sci. Rep. 29 (1997) 193-264. Nien, C.H., Madey, T.E.: Surf. Sci. 380 (1997) L527. Rainer, D.R., Xu, C., Holmblad, P.M., Goodman, D.W.: J. Vac. Sci. Technol. A 15 (1997) 1653. Reuter, K., Bernhardt, J., Wedler, H., Schardt, J., Starke, U., Heinz, K.: Phys. Rev. Lett. 79 (1997) 4818. Xiao, X.D., Xie, Y., Jakobsen, C., Shen, Y.R.: Phys. Rev. B 56 (1997) 12529. Yeo, Y.Y., Vattuone, L., King, D.A.: J. Chem. Phys. 106 (1997) 1990.

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1 Introduction Brown, W.A., Kose, R., King, D.A.: "Femtomole Adsorption Calorimetry on Single-Crystal Surfaces." Chem. Rev. 98 (1998) 797-832. Emundts, A., Pirug, G., Werner, J., Bonzel, H.P.: Surf. Sci. 410 (1998) L727. Graham, A.P., Toennies, J.P.: "Determination of the Lateral Potential Energy Surface of Single Adsorbed Atoms and Molecules on Single Crystal Surfaces Using Helium Atom Scattering", in: Int. Conf. on Vibrations at Surfaces. Tokyo, Japan, 1998. Gsell, M., Jakob, P., Menzel, D.: Science 280 (1998) 717. Mravikakis, M., Hammer, B., Norskov, J.K.: Phys. Rev. Lett. 81 (1998) 2819. Madey, T.E., Nien, C.H., Pelhos, K., Kolodziej, J.J., Abdelrehim, I.M., Tao, H.S.: Surf. Sci. 438 (1999) 191.

2 Characterization of adsorbate overlayers: measuring techniques (CH. WÖLL) See subvolume III/42 A2

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3 Data: Adsorbate properties

3.1 Adsorption of noble gases

3.1.1 Noble gases on metals and semiconductors P. ZEPPENFELD

3.1.1.1 Introduction 3.1.1.1.1 General remarks After 30 years of continuing investigation, the adsorption properties of the noble gases on metal and semiconductor surfaces have recently attracted renewed interest. On the one hand, some fundamental aspects have come within the reach of modern experimental and theoretical techniques, such as the very nature of physisorption and the noble gas - substrate interaction, the possibility to study growth and surface kinetics at the atomic scale, and the recent interest in nanoscale surface friction and related tribological issues, where noble gas adlayers serve as model systems [99P]. On the other hand, noble gas adsorption is being used as a non-destructive and quantitative surface analytical tool as, for instance, in photoemission of adsorbed xenon (PAX) [97W] and for titration analysis of heterogeneous surfaces based on the site specificity of the interaction strength [96S, 98W]. Hence, it may be expected that the noble gases will continue to serve as model systems for solving fundamental issues in surface science as well as to provide a powerful analytical probe for surface characterization. 3.1.1.1.2 The interaction of noble gases at surfaces The surface potential energy V can be decomposed into the contribution of the substrate holding potential VAS (i.e., the interaction of an isolated noble gas atom with the surface) and the effective lateral interaction VAA between the adsorbed atoms. The holding potential is a function of the lateral position x,y of the adsorbate and of its height z above the surface. The interaction between the noble gases and metal surfaces is attractive at larger distances (van der Waals attraction) and thus proportional to -z-3 (-z-4 at very large distances, where retardation effects have to be taken into account). Close to the surface, the interaction becomes strongly repulsive (Pauli repulsion) and can be described by an exponential or algebraic decay. The combination of both the repulsive and the attractive part gives rise to a shallow physisorption well in the holding potential whose depth ranges from a few meV for He up to about 300 meV for Xe. The potential minimum determines the binding distance z0 as well as the adsorption site (x0,y0). The lateral variation of the well depth VAS(x ,y,z0) reflects the atomic and electronic structure of the substrate lattice; its peak-to-peak amplitude ∆VAS is called the surface corrugation. Various theoretical models have been proposed to describe the atom-surface interactions for the noble gases [91V1]. In the simplest approach, the holding potential is obtained as the pairwise sum of atomic

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pair-potentials, e.g., of the Lennard-Jones or Morse type. Alternatively, the potential may be expanded into a Fourier series with the first few terms (V0(z) and VG(z)) being taken into account. Recently, a more sophisticated type of potential has been constructed by Barker [91B] for the case of Xe/Pt(111) in which non-central terms are added to the usual pairwise contributions in order to mimic the repulsive interaction with the delocalized metal electrons. In this case, the potential minimum and, hence, the Xe adsorption site is located on top of the Pt surface atom in contrast to the high coordination (hollow sites) obtained with pair potentials. The lateral interaction between the adsorbed noble gas atoms is, to a large extend, determined by the dispersion-repulsion usually described by the well known gas phase potentials. However, significant contributions may also arise from surface induced dipole repulsions, many-body and substrate mediated interactions as discussed in detail in Ref. [97B2]. These contributions tend to weaken the attractive interactions with respect to the gas phase value. For instance, according to the gas phase Xe Lennard-Jones potential, the lateral interaction in a hexagonal close packed 2D solid phase would be 72 meV per Xe atom, whereas a value of 43 meV has been reported for Xe/Pt(111) [88K2]. In a few but notable cases, the repulsive terms may even outweigh the attractive van der Waals potential, resulting in an overall repulsive lateral interaction, as observed for Xe on the strongly binding substrates Pd, W and Ni. Such strong repulsions have been attributed to surface induced dipole-dipole interactions and initiated further investigations on the precise nature of the noble gas - metal interaction (see below). Modifications of the electron density distribution within the metal were revealed in LDA calculations of Xe adsorped on a Pt25 cluster [90M] and have been interpreted in terms of hybridization of atomic orbitals. This has led to an ongoing debate on the ‘chemical contributions’ in the noble gas - metal interaction (which, by the way, were already suggested in the early work of Xe adsorption on W and Ni in the 1970s). Further support on the role of hybridization of the noble atom orbitals with the substrate electronic states have been obtained from the experimental evidence of ‘anticorrugation’ effects in He vs. Ne scattering from surfaces [93R] and related density functional calculations [84A, 96P]. Furthermore, intrinsic surface electronic states have been suggested to play an important role in the binding of the noble gases on metals affecting both VAS and VAA [96B]. 3.1.1.1.3 Sticking and accommodation A large number of studies has dealt with the scattering (adsorption-desorption) of noble gas atoms from surfaces as a function of kinetic energy, incident angle, and surface temperature. From a fit to these data, noble gas - surface potentials have been derived, such as the aforementioned Xe-Pt(111) potential [91B]. In the present context, we will limit ourselves to those experiments which are relevant to the adsorption properties, i.e., we will address the sticking probabilities for noble gas atoms incident with thermal energies, but not the angular and energetic distributions of the backscattered atoms. For an overview of the latter issues we refer to [91R]. At thermal kinetic energies and low enough surface temperature, the heavier noble gases (Xe, Kr, Ar) usually have a sticking probability close to unity although on some surfaces such as Ru(0001) [92S2] and Cu(110) [94Z2] values as low as 20% have been reported. These small values are due to the high elastic reflection probability which is a consequence of the inefficient energy transfer (phonon mismatch) and the phonon quantization [92S2]. The situation is most dramatic for the lighter noble gases which exhibit pronounced quantum behaviour. For instance, Ne on Ru(0001) has an initial sticking probability below 1% at low incident kinetic energy [88S]. The sticking coefficient generally depends on the surface coverage θ. Especially in those cases, where the initial sticking coefficient s0 = s(θ→0) is significantly smaller than one, surface areas already covered with the noble gas allow a more efficient trapping of the incident noble gas atoms due to the more efficient energy transfer on these areas [91H]. The trapping efficiency of the noble gas covered areas strongly depends on the surface temperature as well as on the adlayer morphology. If, e.g., 2D islands are formed on the surface during the adsorption process, noble gas atoms impinging on these islands may be either trapped on top of an island and become incorporated at the island edge, or they may desorb from the island before reaching the island edge, if the surface temperature is too high for the bilayer to be stable and if the islands have grown sufficiently large. In this case a more complex variation of the Lando lt Bö rnst ein New Ser ies III/42A1

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sticking coefficient is observed (Fig. 4) [94Z2]. In addition, the presence of surface defects or impurities as well as the ‘transient’ mobility of a noble gas atom upon adsorption [85T, 94Z2] may strongly affect the sticking coefficient. 3.1.1.1.4 Adsorption energies Experimentally, adsorption energies can be inferred from adsorption isotherms in which the surface coverage is recorded as a function of the equilibrium pressure and temperature. The surface coverage θ can be monitored in various ways, such as by the appearance of adlayer diffraction spots (or, alternatively, the intensity decrease of the substrate related spots) using low energy electron, X-ray or thermal He atom diffraction; the height of the noble gas Auger peaks; the photoemission signal, or the work function change ∆φ upon adsorption. For a quantitative evaluation, the isosteric heat Qiso, which is the difference between the enthalpies of the adsorbed and the 3D gas phase, is evaluated from the Clausius-Clapeyron equation as described in chapter 1.2. It should be noted that in most experiments the 3D gas and the physisorbed adlayer are not in true thermodynamic equilibrium since the temperature of the 3D gas (usually at room temperature) and the surface temperature (typically between 20 and 100 K) are not the same. The corresponding corrections, however, are small and usually disregarded. In many experiments, the isosteric heat of adsorption (latent heat) is determined for adsorption into the 2D solid monolayer phase. This value denoted by Q1, can be assumed to be constant over the coverage range of coexistence between a 2D solid and a 2D gas phase, and before any phase transition or compression of the monolayer solid phase takes place. At low coverage, where only a dilute phase (2D gas) is formed, the isosteric heat may increase or decrease with coverage depending on whether the lateral interactions between the adsorbed noble gas atoms are attractive or repulsive. In this context the isosteric heat of adsorption approaching zero coverage Q0 = Qiso(θ → 0) is another important quantity. Its experimental determination is more delicate and may be obscured by the fact that the initial adsorption may preferentially occur at surface defects (steps) or impurities due to a stronger binding of the noble gas atoms to these defects. In fact, the increase of Qiso at very low coverage observed, e.g., on stepped or high index surfaces allows to extract the adsorption energy of the defect sites and can even be used as a quantitative measure to determine the defect concentration. The isosteric heat of adsorption Qiso(θ) is related to the (differential) potential energy V(θ) of the adsorbate on the surface and the (temperature dependent) kinetic degrees of freedom f in the adsorbed and the 3D gas phase, respectively: Qiso (θ ) = H ads (θ ) − H 3 D ≈ U ads (θ ) − H 3 D = V (θ ) + [ f ads − ( f 3 D + 2)] kT / 2 . Assuming an ideal, mono-atomic gas, the kinetic correction becomes −kT / 2 for adsorption into an (ideal) 2D gas phase and + kT / 2 for adsorption into a 2D solid. A detailed account of the thermodynamics of adsorption can be found in [97B2]. The individual contributions to the binding energy V are not all independently accessible to the experiment. Nevertheless, the lateral interaction energy VAA can be inferred from the variation of the isosteric heat with coverage or simply from Q1 – Q0. It can also be measured directly by determining the critical coverage θc at which the transition from the 2D dilute into the 2D solid phase occurs. The 2D condensation can be detected, e.g., in a helium scattering experiment due to different effective cross sections for diffuse scattering from the 2D dilute and the 2D solid phase, respectively [83P, 88K2]. The 2D heat of condensation Q2D is then deduced from the 2D analogon of the Clausius-Clapeyron equation, where the 2D spreading pressure p2D is related to the critical coverage θc by p 2 D = N 0θ c kT (assuming an ideal 2D gas). Taking into account the kinetic contributions, the lateral interaction is given by VAA = Q2D − kT.

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3.1.1.1.5 Desorption energies and desorption kinetics Aside from isothermal adsorption experiments, the binding energy of the noble gases can also be inferred from thermal desorption measurements. The desorption rate is recorded as a function of time during a linear temperature ramp (temperature programmed desorption, TPD) or at constant temperature (isothermal desorption). If the adlayer remains in quasi 2D-equilibrium during the desorption process the rate can be expressed by the Polanyi-Wigner equation (see Eq. 24):  E  rd (θ , T ) = ν d (T )θ n N 0 exp − d  .  kT  The exponent n denotes the order of desorption, related to the nature and the statistics of the desorption pathway, and Ed is the desorption energy. In accordance with the shape of the holding potential (see 3.1.1.1.2), the desorption of the noble gases is not activated and Ed is a direct measure of the binding energy. More precisely, it is the difference between the differential enthalpy of an atom in the adsorbed phase and in the 3D gas phase (generally not in thermodynamic equilibrium with the adsorbed phase). Up to kinetic corrections of the order of kT the desorption energy Ed is thus identical to the isosteric heat of adsorption Qiso defined above. Therefore, in the tables in section 3.1.1.2.2 no distinction between adsorption and desorption energies is made, but the experimental method is indicated. The prefactor νd appearing in the Polanyi-Wigner equation is related to the entropy term in the relevant free enthalpy balance ∆G = µ = ∆H - T∆S, i.e., νd = exp(∆S/k). For the adsorbed noble gases the prefactor is usually of the order of 1012 to 1013 s−1and can be interpreted as an ‘attempt frequency’ for desorption associated with the (perpendicular) atom-surface vibration frequency. As far as the desorption kinetics is concerned, desorption orders of zero and one have been observed for the noble gases on different surfaces and in different coverage regimes. In the case of phase coexistence (2D equilibrium) between a 2D dilute (gas) and a 2D solid phase as well as for noble gas multilayers zero order desorption is expected, i.e., the desorption rate is independent of coverage and follows a simple exponential increase with temperature. This implies that in a series of TPD curves recorded for different initial coverages the leading edges of these curves will coincide. The TPD curves are, therefore, conveniently plotted in an Arrhenius type diagram in which the desorption energy and the prefactor are readily obtained from the slope and offset of the common leading edge [92S1]. First order desorption kinetics is observed if the noble gas adlayer is in a single phase. In particular, this situation occurs at low coverage (or high desorption temperature) where a pure 2D gas phase is stable. An extended coverage regime of first order desorption (even up to monolayer completion) has been observed in those cases where repulsive lateral interactions between the adsorbed noble gas atoms prohibit the condensation of the dilute phase into a 2D solid phase, such as for Xe on Pd, W and Ni surfaces. In most other cases the regime of first order desorption and, hence, the stability of a 2D gas phase is limited to a small fraction of a monolayer. Finally, the presence of surface defects (steps or impurities) may significantly alter the desorption kinetics, affecting the desorption order as well as the desorption energy and prefactor [90B]. 3.1.1.1.6 Structure and phase diagram Noble gas monolayers are often considered as model systems in the study of two-dimensional phases and phase transitions. As in bulk matter the analog of a gas, fluid and solid phase also exist in purely two dimensional ‘floating’ adlayers, although the translational ordering of a floating solid is not of long range order - with important consequences on the nature of the 2D solid-fluid (melting) transition. The adsorption on corrugated crystalline substrates differs from the ideal 2D case, e.g., by the appearance of lattice gas and fluid phases due to the discrete spacing of the adatoms on preferred lattice sites located at the potential minima of the holding potential. In addition, several solid phases may exist beside the floating 2D incommensurate phase, such as commensurate or high-order commensurate phases in registry with the 2D substrate lattice. Which type of solid phase is formed depends on the relative size of the lateral interaction VAA between adsorbed atoms and the corrugation Vc = ∆VAS of the holding potential as Lando lt Bö rnst ein New Ser ies III/42A1

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well as on the natural misfit m between the 2D floating adlayer and the substrate lattice. Perfectly commensurate (C) monolayer solids are expected for strongly corrugated substrates, weak lateral interactions (Vc/VAA ≥ 1) and small lattice misfit. On the other hand, incommensurate (I) structures are favored for (Vc/VAA « 1) or large lattice mismatch. For intermediate cases, high-order commensurate (HOC) phases characterized by a commensurate superstructure whose unit cell contains several atoms on different lattice sites can be obtained. Experimentally, all these cases have actually been observed (see Table 7). On a few close packed substrates, namely Pt(111), Pd(111) and Cu(111), Xe may form perfectly commensurate phases with a ( 3 × 3 ) R30° structure and for Xe on W(100) a c(2×2) structure was reported. On most other surfaces with hexagonal or square lattice symmetry, such as Ag(111), Al(111), Ni(100), Pd(100), etc., the noble gases form purely incommensurate, hexagonal close packed structures with an interatomic spacing close to the corresponding bulk value. Nevertheless, these adlayers are usually orientationally aligned along a high symmetry direction of the substrate. This has led to the conclusion in the early literature, that ‘the substrate may influence the orientation but not the structure and internal spacing of physisorbed adlayers’ [75P]. The (111) faces of various fcc metals can stabilize uniaxially commensurate (UC) phases due to the high corrugation along the [001] direction, i.e., the adsorbate may be localized within the potential troughs parallel to the [1 1 0] direction. The noble gas atoms are then arranged in a quasi-hexagonal c(α×2) structure with a more or less significant distortion between the interatomic spacings along (a) and across (b) the troughs. In some cases, such as Kr and Xe on Cu(110), α can be a rational number giving rise to (n×2) HOC phases. Another frequent observation is the occurence of azimuthally broadened ‘sickle-like’ diffraction spots or, in some cases, a finite azimuthal rotation angle. Such a ‘Novaco-McTague’ rotation out of the high symmetry direction can occur as a result of strain energy minimization. The rotation seems to be quite susceptible to the presence of structural imperfections (steps) or impurities (such as K in the case of Ag(111) [97L]). As another example, Kr and Ar adlayers on Pt(111) can be prepared in both R0° and R30° orientation depending on whether the substrate steps, involved in the 2D island nucleation and growth, are clean or pre-covered with, e.g., H or CO [86K3, 88K2]. Transitions between the various 2D phases can occur as a function of surface temperature and coverage, giving rise to rather complex structural phase diagrams. For instance, the variation of the lattice misfit m with temperature (thermal expansion) or coverage (lateral compression) can induce a change of the equilibrium structure among the various I, C, UC and HOC phases. As an example, the phase diagram for the Xe monolayer on Pt(111) presented in Fig. 11 includes a ( 3 × 3 ) R30° commensurate (C), an incommensurate (I) phase, their transition to a uniaxially compressed (striped incommensurate - SI) phase, a hexagonal incommensurate (HI) compressed phase and a Novaco-McTague R(30°±3.3°) rotated phase [87K1, 87K2]. This phase diagram and the sequence of the observed 2D phase transitions are in agreement with theoretical predictions [79B]. 3.1.1.1.7 Adsorption site In the earlier literature the preferential adsorption sites for noble gas atoms on surfaces were generally assumed to be those with highest coordination (i.e., hollow sites on terraces and bottom sites at the substrate steps). Only recently, theory [90M, 90G] and experiment [92W, 94Z3, 98S] have pointed out the possibility of an on-top adsorption site. In particular, Xe adsorbed on the fcc(111) surfaces of Pt, Pd, Cu [98S] as well as Xe on Ru(0001) [97N] were shown to be most strongly bound atop of the substrate atoms and at the upper edge of the substrate steps [94Z3]. This feature has been ascribed to the peculiar interaction of the noble gas atoms to the metal substrate which obviously cannot be described in terms of a simple sum of pair potentials which, instead, would predict high coordination sites. Both the on top adsorption site and the increased repulsion between the adsorbed noble gas atoms have been correlated with the strength of the holding potential VAS [86J]. Strongly binding substrates like Pd or adsorption sites with enhanced binding energy (such as the step edges on Pt(111)) seem to promote on-top adsorption and repulsive interactions between neighboring adatoms. A recent DFT study [96P] reveals that the nature of the interaction of the noble gases (He, Ne) is determined by the subtle interplay between electron Lando lt Börnst ein New Ser ies III/42A1

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polarization and hybridization [96P]. In addition, the contribution of intrinsic surface states to the binding of the noble gases on metals has been proposed to explain the stronger binding of Xe on Pd(111) than on Pt(111), the different lateral interactions of Xe on the two surfaces and the preferential decoration of the upper step edge on the Pt(111) surface. Nevertheless, a quantitative understanding of the noble gas substrate interaction and of the relation between binding strength, adsorption site and lateral repulsive interactions is still lacking. Quantitative results on the adsorption geometry have been obtained from LEED I-V curves for the aforementioned cases. The best agreement in all these cases is found for an on-top adsorption geometry (see Table 8). 3.1.1.1.8 Electronic structure Early work has shown that upon noble gas adsorption the surface potential (work function) of the substrate is lowered by as much as 1 eV (Table 9). These large values can be ascribed to the large polarizability α of the noble gas atoms (in particular Xe) and possible chemical contributions to the surface bond. The work function change ∆φ has also been used to measure the surface coverage, e.g., in isothermal adsorption experiments. An analytic relation, accounting for dipole-dipole interactions and mutual depolarization effects, is given by the Topping formula: ∆φ =

4πµ 0 n ad 3/2 1 + 9αn ad

,

where µ0 is the adsorption induced atomic dipole moment which is related to the work function change via 4πµ 0 = ∂ (∆φ ) / ∂n ad . For noble gas adsorption, the Topping formula has been checked to hold in several (but not in all) cases. The initial Xe dipole moment on the low index Pd (fcc) planes has been found to increase in the order µ0(110) < µ0(100) < µ0(111) and, hence, to exhibit the reverse face specificity as the initial desorption energies [84W3]. This observation was explained in the framework of the ‘s-resonance model’ [84W2]. The electronic structure of the adsorbed noble gases is determined in the valence band by the occupied np1/2 and np3/2 atomic orbitals (n = 3, 4, 5 for Ar, Kr and Xe, respectively) and by the unoccupied (n+1)s orbitals which are usually located far above the Fermi level. Due to the coupling with the substrate and the Xe-Xe lateral interactions, the original atomic states are broadened and the np3/2 photoemission peak is split into two peaks due to the lifting of the degeneracy of the mj = ±1/2 and ± 3/2 states. It has been argued that the binding energy for a given bound electronic state of an adsorbed noble gas atom is ‘pinned’ to the vacuum level [84W1, 90W]. In the case of Xe adsorption this would imply that EV(5p1/2) = EF(5p1/2) + φ, where the superscripts V and F denote the vacuum and the Fermi reference levels, respectively. The photoemission derived binding energies, however, are smaller (by about 1.1 eV) than those of the corresponding atomic states, which has been attributed to final state relaxation (electron hole screening). For a large number of substrates (mainly transition metals) the relaxation effect was found to be essentially substrate independent [90W], whereas on the sp-metal Pb(111), for instance, layerdependent shifts of the photoemission peaks attributed to changes in the final state relaxation have been reported [88J]. It has further been argued that the photoemission derived binding energies depend on the local surface potential of the immediate adsorption site i = (xi,yi) [97W]. Hence, the electron energy difference between two unlike sites i and j reflects the difference in the local work function., i.e., for Xe: ∆EF(5p1/2) (i,j) = ∆φ(i,j). This ‘local work function concept’ together with the site specificity of the noble gas adsorption is the basis of the Photoemission-spectroscopy of Adsorbed Xe (PAX), which has become a powerful tool for the characterization of heterogeneous surfaces, such as stepped surfaces or surface alloys [90W, 97W]. In Table 10 we have listed the values for the measured binding energies EF(np) for flat and homogeneous surfaces, only. Upon 2D island formation, the noble gas np derived states develop a 2D band structure (Table 12) with typical dispersion amplitudes (band widths) ∆ε of the order of 0.1 to 0.5 eV, related to the strength

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of the lateral interaction [80H]. At step edges or on anisotropic surfaces, such as Pt(110)-(1×2), the noble gas atoms aggregate into chain-like structures exhibiting a 1D band-structure [95W, 97T]. Finally, noble gas multilayers have been shown to exhibit quantized electronic states depending on the layer thickness [95S, 95P1]. Their number and energies can be explained within a simple potential well model [99G2]. 3.1.1.1.9 Diffusion kinetics Surface diffusion data for the noble gases on surfaces are scarce and microscopic studies (using FEM and STM) on the diffusion processes, if they exist, are not always quantitative. The experimental determination of the surface mobility is complicated by two effects: (i) noble gas atoms may exhibit an initial hyperthermal mobility related to the binding energy gained by the noble gas atom upon adsorption and the inefficient energy dissipation via lateral adsorbate-substrate phonon coupling [85T]. This ‘transient mobility’ must be separated from the ‘true’ thermal mobility of the thermalized noble gas atom. (ii) Surface defects act as efficient traps or nucleation sites for the diffusing noble gas atoms. They have to be included in calculating the diffusion constants from macroscopic measurements or from an analysis of the island density or size distribution in adsorption experiments. (iii) The diffusion process may be complicated by the discrete nature of the vibrational levels within the shallow diffusion potential, especially for the lighter noble gases. Laser induced desorption experiments for Xe and Kr on Pt(111) which probe the diffusion on a length scale of a few hundred µm, reveal a significant coverage dependence (adatom diffusion vs. chemical diffusion) yielding diffusion constants which vary by an order of magnitude between 5% and 25% of the monolayer saturation coverage [93M2]. The diffusion barrier extracted from these experiments (Edif = 52(±9) meV for Xe/Pt(111) in the intermediate coverage range) was assigned to the detachment of Xe atoms from island edges and, therefore, involves the corrugation energy of the holding potential as well as the lateral interaction between the noble gas atoms. This value is consistent with thermodynamic data of adsorption [88K1]. A similar value (48(±12) meV) was found for Xe diffusion on W(110) using the field emission current fluctuation method [80C]. STM data for Xe/Pt(111) at 4 K [92W] show a preferential decoration of the step edges during adsorption and, at the same time, very small thermal mobility of isolated thermalized Xe adatoms. This observation has been interpreted in terms of a large transient mobility and a sizeable surface corrugation amplitude. From STM experiments at variable temperature, the corrugation amplitude (diffusion barrier for a Xe adatom) and the activation energy for detachment from a 2D island edge can be estimated to be of the order of 31 and 64 meV, respectively, at low temperatures [95H2]. In contrast, recent quasi-elastic He scattering experiments performed on the Xe 2D-gas phase on Pt(111) at surface temperatures around 105 K [99E] seem to indicate an almost vanishing barrier for adatom diffusion. It should be noted, however, that the effective or ‘dynamic’ barrier probed at these large thermal energies is smaller than the ‘static’ diffusion barrier or surface corrugation [99G1], which has been estimated to be < 9.6 meV. 3.1.1.1.10 Lattice dynamics The lattice dynamics (phonon spectrum) of a physisorbed noble gas adlayer is characterized by three vibration modes, one perpendicular to the surface with no or very weak dispersion (Einstein mode) and two other modes polarized within the surface plane. The vibration energy of the Einstein mode is related to the second derivative of the surface holding potential (perpendicular force constant). In the harmonic approximation the only lateral coupling is due to the stress within the noble gas monolayer (first derivative of VAA) which may be non-zero but is usually quite small. This explains the weak energy variation of this mode with wavevector parallel to the surface. As a consequence, sharp multiple phonon losses can be observed for this mode [99S2]. Experimentally, the surface phonon spectrum of noble gas monolayers has been investigated on several systems using high resolution inelastic He atom scattering. Although the cross section is largest for the perpendicular vibration mode, longitudinally and transversely polarized modes have also been detected, namely on the Cu(100), (110) and (111) surfaces and on Pt(111) [00B]. The vibration energy for the perpendicular mode ranges between 2.5 meV for the heaviest species (Xe) and 5 meV for Ar and is expected to increase with the strength of the holding potential. The Lando lt Börnst ein New Ser ies III/42A1

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vibration energies can be used to fit the holding potential VAS (perpendicular mode), the interatomic potential VAA (longitudinal mode) as well as the surface stress [94Z1]. The phonon modes may couple to the substrate phonons where they overlap with the substrate bulk phonons and where the perpendicular adlayer mode crosses the edges of the projected bulk phonon bands or the surface Rayleigh mode of the substrate [85H]. A linewidth broadening in the overlap region could first be detected for the noble gases on Ag(111) [88G1] and later for the noble gases on Pt(111). In the latter case, the hybridization between the substrate Rayleigh wave and the noble gas Einstein-mode [89H, 90Z1] as well as the coupling to the longitudinal bulk band edge (Van Hove anomaly) [90Z2] has also been evidenced. With increasing thickness, the phonon spectrum continuously evolves towards the one expected for a semi-infinite noble gas crystal in a layer-by-layer fashion [88G2, 90Z1]. The phonon signal can thus be used to characterize the adsorption and growth of noble gas multilayers.

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3.1.1.2 Data In the tables heats of adsorption and desorption energies are given in meV (per atom). Translation into other commonly used units is achieved with the following conversion table: 1 kJ/mol 1 kcal/mol 1000 K

= = =

10.36427 meV 43.38484 meV 86.17386 meV

The following standard acronyms are used to denote the experimental techniques: AES: ARUPS: FE: FEM: ICISS: LEED: LEED-IV: PES: SEXAFS: SPLEED: TEAS: TPD: UPS: XPS: XRD: ∆φ:

Auger Electron Spectroscopy Angularly Resolved UPS Field Emission Field Emission Microscopy Impact Collision Ion Scattering Spectroscopy Low Energy Electron Diffraction LEED I (intensity) vs. V (voltage, energy) profiles PhotoEmission Spectroscopy (used here in connection with synchrotron radiation) Surface Extended X-ray Absorption Fine Structure Spin Polarized LEED Thermal Energy (He) Atom Scattering Temperature Programmed Desorption UV-light PES X-ray PES X-ray Diffraction Workfunction measurements

The following tables are intended to provide a complete as possible compilation of the literature data, whereas the figures only serve as an illustration of some of the characteristic properties.

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3.1.1.2.1 Sticking coefficients of the noble gases See Figs. 1 – 5. Table 1. Initial sticking coefficient s0 of the noble gases (for thermal incident energies and surface temperatures typical for adsorption experiments). Abbreviations used: Tg: gas temperature, Ts: surface temperature during adsorption, : coverage in monolayer units. (Ad. = adsorbate) Substrate

Ad.

Initial sticking coefficient s0

Remarks

Ref.

Ag(111) Cu(100) Cu(110)

Ni(111)

Xe Xe Ar Kr Xe Ar Kr Xe Xe Ar Kr Xe Xe

Pd(100) Pd(110) Pt(111)

Xe Xe Ar

~1 ~ 0.9 ... 1 0.15 ± 0.06 0.24 ± 0.1 0.37 ± 0.15 0.052 0.15 0.20 ~1 0.23 0.30 0.46 0.35 0.6 > 0.9 ~1 0.9

86B 79G 94Z2 94Z2 94Z2 93F 93F 93F 82C2 87M 87M 87M 96W 91W 71P 79K1 90R2

Xe

0.90 ± 0.03

TPD, XPS, Ts = 30 K AES TEAS, Tg = 300 K, Ts = 30 K TEAS, Tg = 300 K, Ts = 43 K TEAS, Tg = 300 K, Ts = 55 K TPD, Tg = 300 K, Ts = 6.5 K TPD, Tg = 300 K, Ts = 6.5 K TPD, Tg = 300 K, Ts = 6.5 K decreasing with TPD, Tg = 295 K, Ts = 25 K TPD, Tg = 295 K, Ts = 25 K TPD, Tg = 295 K, Ts = 25 K optical differential reflectance, Ts = 38 K TPD, ∆φ, AES, at Ts = 77 K trapping probability, Ts = 80 K, low incident energy for Ts = 95 K, low incident energy (smaller and coverage dependent at higher incident energies) Ts = 87 K, coverage dependent trapping probability, Ts = 80 K, low incident energy for thermal beams, significant angular and energy dependence TPD, Tg = 300 K, Ts = 6.5 K TPD, Tg = 300 K, Ts = 6.5 K TPD, Tg = 300 K, Ts = 6.5 K TPD, Tg = 300 K, Ts = 6.5 K Tg = 110 ... 300 K; Ts = 60 ... 65 K trapping probability, Ts = 80 K, low incident energy Tg = 105 K, Ts = 62 K Tg = 110 K, Ts = 20 K 0 Ts = 104 K

K1)

Ni(100) Ni(111)

0.48 1 Pt(110)-(1x2) Xe

≥ 0.9

Ru(0001)

W(100) W(100)-2H

Ne Ar Kr Xe Xe Ar

0.0042 0.13 0.25 0.71 0.95 ± 0.05 0.9

W(110)

Xe

W(111)

Xe

0.9 ± 0.1 1 ~ 0.5

1

90A, 91A

90S2 90R2 93S3 92S2 92S2 92S2 92S2 80W2 90R2 79W 80W2 74D

) K multilayers (about 10 ML thick) deposited on a Ru(0001) substrate.

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3.1.1.2.2 Binding energies, thermodynamic quantities and phase diagrams See Figs. 6 – 10. Table 2. Heats of adsorption and desorption energies of the noble gases in the low coverage limit. The values correspond to either a 2D gas phase (on terraces) or the initial binding strength to surface defects (step edges). Abbreviations used: T: surface temperature during adsorption, : coverage in monolayer units. Percentage values: maximum 2D-gas density in units of the density of the monolayer solid. (Ad. = adsorbate) Substrate

Ad.

Energy [meV]

Remark

Ref.

Ag(111)

Ar Kr Xe

74 ± 7 110 ± 10 215 ± 15

Al(110) Cu(100) Cu(110) Cu(111) K

Xe Xe Xe Xe Ar Kr Xe Xe Xe Kr Xe

190

LEED, 1 ... 11% LEED, 1 ... 9% LEED, 1 ... 7% UPS, 6% at T = 58 K TPD, 1 ... 10%, νd = 5×1013 s−1 AES, ~ 10% at T ~ 80 K TEAS, from Q1 – Q2D UPS, 11% at T = 58 K TPD, → 0, nonwetting TPD, → 0, nonwetting TPD, → 0, nonwetting TPD, 1st order, νd = 1012 s−1 TPD, st order regime) LEED, → 0 LEED, 0 ∆φ, LEED, → 0 TPD, → 0, using νd = 1015 s−1 UPS, 100% at T = 58 K TPD, 1st order, using νd = 1013 s−1 TPD, → 0, using νd = 1015 s−1 TPD, → 0, using νd = 1015 s−1 AES, θ = 0 ... 1 (terraces) AES, θ = 0 ... 1 (step edges)

82U 82U 82U 86E 91S 79G 94R 86E 93F 93F 93F 82C2 91C1 84M3 84M3 71P 84W3 86E 79K1 84W3 84W3 83M2

Ni(100) NiAl(110) Pd(100)

Pd(110)

Xe

Pd(111) Pd(810)

Xe Xe

Pt(110)-(1×2) Pt(111)

Xe Xe

Pt(112) Pt(557) Pt(997) Ru(0001) 1

Xe Xe Xe Xe

180 ± 10 28 56 83 226 200 ~200 273 ± 5 ~330 408 > 425 442 360 356 ± 13 443 ± 13 347 277 278 245 ± 15 1) 410 ± 40 239 250 293 249 ± 22 373 ± 22 369 ± 22 370

TEAS TEAS, νd = 2.95×1013 s−1, θ on terrace, TPD, νd = 1.4×1012 s−1 at defects, TPD, νd = 9×1015 s−1 TPD, on terraces TPD, at defects TPD, νd = 1012.5± 0.9 s−1 TPD, νd = 1014.2±1 s−1 at steps (θ = 0.04) TPD, νd = 1014.1±0.9 s−1 at steps (θ = 0.04) TPD, at step edges (θ < 0.16) UPS, 14% at T = 58 K ~ 5% at T = 20 K

83K 86K2 85P 90R1 83K 98W 90S2 90S2 90S2 98W 86E 97N

) Obtained from an Arrhenius plot. In a subsequent analysis [90B] the true holding potential minimum was suggested to be 255 meV (due to the temperature dependence of the desorption prefactor νd ).

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Table 3. Heats of adsorption and desorption energies for (unconstrained) monolayers of the noble gases. The values are characteristic of the monolayer latent heat in the 2D gas-solid coexistence regime, Q1, for systems with attractive adsorbate-adsorbate interactions and for a dense 2D gas phase in the case of repulsive interactions. In the first case the 2D heat of condensation, Q2D, is indicated if known. In the second case (‘rep.’) the coverage or coverage range of the measurement is specified. Abbreviations used: T: surface temperature during adsorption, : coverage in monolayer units. (Ad. = adsorbate) Substrate

Ad.

Energy [meV]

Ag(111)

Ar Kr Xe

99 ± 7 151 ± 5 225 ± 5 187 ± 21 224

Al(110) Au(100)-rec Cu(100) Cu(110)

Xe Xe Xe Kr Kr Xe

Cu(110)-(2×1)O Xe Cu(211) Cu(311) Cu(610)

Xe Xe Xe

Ge(100)-(2×1)

Xe

Ge(111)-c(2×8) Ir(100) Ir(110) Ir(111) Ir(210) Ir(321) Ir(511) Ir(531)-(731)

Kr Xe Xe Xe Xe Xe Xe Xe

Ni(100)

Xe

207 ± 4 190 ± 3 180 224 ± 10 ... 236 ± 8 270 ± 2 130 ± 5 114 ± 10 226 ± 6 218 ± 6

Ref. Q2D [meV] 81U 81U 79U 76R 86B 88G2 98I 91S 76M 79G 81G 00S 47 ± 9 94R 98P

256 ± 9 ... 174 ± 5

89B

200 ± 5

81G

223 ± 6 174 ± 6 197 ± 21 197 ± 21 278 ± 13 282 ± 4 233 ± 8 175 ± 8 166 ± 10, 163, 118 325 ... 282 304 ... 273 325 ... 282 312 ... 291 338 ... 304 312 ... 282 312 ... 291 248 ± 10, 232, 164 226

98P 76R 75P 80B 93S2

rep. ?

88P 74N2 74N2 74N2 74N2 74N2 74N2 74N2 88P 82C2

Remark LEED LEED LEED ∆φ TPD, 0th order, νd = 3×1015 s−1 TEAS, isothermal desorption ellipsometry TPD ∆φ AES LEED, AES, ~ 1 LEED isobars, ~ 0.5 TEAS TPD, 0th order, νd = 1013± 1 s−1 LEED, decrease upon monolayer compression AES, LEED, c(2x2) high temperature, low density phase. TPD, A-sites TPD, B-sites, νd = 4⋅1011± 1 s−1 ∆φ ∆φ AES, < 0.4 AES, 0.4 < < 0.8 two steps in LEED isotherms, T < 77 K LEED, three substeps FEM FEM FEM FEM FEM FEM FEM LEED, three substeps ∆φ, 0.1 < < 0.5, TPD: 1st order

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3.1.1 Noble gases on metals and semiconductors Ref. Q2D [meV]

Substrate

Ad.

Energy [meV]

Ni(111)

Xe

191 ± 8

96W

Xe

317 ± 26

Xe Xe

217 ... 270 228 ... 259 259 ... 155 ~200 ... ~250 ~330 ... ~280 408 ... 364

89F, 83D2 87M 90C, 91C1 84M3 71P 84W3

NiAl(110) Pd(100)

Kr Xe

273 ± 5 Pd(110)

Xe

Pd(111)

Xe

Pd(810)

Xe

Pt(100) Pt(110)-(1×2)

Xe Xe

Pt(110)-(1×2)H Xe Pt(111)

Ar Kr

Xe

Pt(210) Pt(321) Pt(997)

Xe Xe Xe

Ru(0001)

Ne Ar Kr Xe

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rep. rep. rep. core

84M3

Remark optical differential reflectance, isothermal desorption, νd = 3.4⋅1011 s−1 UPS isotherms, 80 K ≤ T ≤ 90 K TPD, νd = 1011 ... 1014, θ = 0.2 ... 1 TPD, state 1, θ = 0.3 ... 0.5 TPD, state 2, θ = 0.5 ... 1 LEED, θ = 0 ... 0.7 ∆φ, LEED, θ = 0 ... 1 TPD, θ = 0 ... 0.35, using νd = 1015 s−1 LEED, 0 < < 0.8

TPD, 1st order, using νd = 1013 s−1 TPD, θ = 0 ... 1, using νd = 1015 s−1 360 ... 334 84W3 TPD, θ = 0 ... 0.7, using νd = 1015 s−1 2 356 ± 13 ... 295 ± 13 rep. 83M2 ) AES, θ = 0 ... 1 (terraces) 443 ± 13 ... 356 ± 13 rep. AES, θ = 0 ... 1 (step edges) 325 ... 278 74N1 FEM 338 88D TPD, A-sites 208 TPD, B-sites 310 99W TPD, A-sites 190 TPD, B-sites 95 17 91K, TEAS 88K2 154 26 91K, TEAS 88K2 161 ± 9 93M2 TPD, νd = 4.4⋅1011± 0.4 s−1 310 43 91K, TEAS 88K2 336 ± 4 48 ± 4 85P, 83P TEAS 284 ± 22 90S2 TPD, νd = 1012.5± 0.9 s−1 270 ... 280 98W TPD, νd = 2 ... 8⋅1012 s−1, θ = 0.1 ... 0.7 286 ± 9 93M1 TPD, νd = 1.3⋅1013± 0.4 s−1 330 ... 273 74N1 FEM 312 ... 295 74N1 FEM 338 ... 299 74N+1 FEM 370 98W TPD, at step edges (θ on terraces 280 28 92S1 TPD 103 92S1 TPD 152 92S1 TPD 233 92S1 TPD

> 425 ... 373 442 ... 399

79K1 84W3

79

80

3.1.1 Noble gases on metals and semiconductors

Substrate

Ad.

Ru(0001)-sk

Xe

Energy [meV]

Ref. Q2D [meV] 81W

Si(100)-(2×1)

Xe

Si(111)-(7×7)

Kr

369 ... 321 252 ... 174 202 (site 1) 175 (site 2) ~ 300 ... ~ 250

Xe

~ 330 ... ~ 280

83C

W(100)

Ar Kr Xe

W(110)

Ar Kr Xe

W(111)

Ar Kr Xe

82 1) 236 1) 255 1) 269 1) 94 1) 319 1) 399 1) 230 187 ... 174 83 1) 220 1) 255 1) ~350 217 402 ± 2

70E 70E 70E 80W2 70E 70E 70E 80W2 81O 70E 70E 70E 59E 62R 74D

W(210)

Ar Kr Xe Ar Kr Xe Xe

93 1) 246 1) 279 1) 82 1) 239 1) 283 1) 325

70E 70E 70E 70E 70E 70E 62R

W(211)

W(411)

92M 83C

[Ref. p. 111

Remark TPD, using νd = 1013 s−1 kink and step sites terraces TPD, using νd = 1011 s−1 AES - Fowler isotherms, small (strongest binding sites) AES - Fowler isotherms, small (strongest binding sites) FEM FEM FEM ∆φ(T) FEM FEM FEM ∆φ(T) TPD FEM FEM FEM FEM FEM TPD, 1st order, νd = 1015 s−1, absolute accuracy ± 56 meV FEM FEM FEM FEM FEM FEM FEM

1

) The values obtained in [70E] are model dependent and possibly erroneous [80W2]. In fact, the results in [80W2] and [74D] suggest that the adsorption energies on the low index planes of bcc W decrease in the order (111) > (100) > (110) in agreement with the coordination argument in [84W3]. 2 ) In a subsequent paper [84M2] coverage independent values were given: 438 meV (at step edges) and 347 ± 9 meV (on terraces).

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Table 4. Heats of adsorption and desorption energies for bilayers of the noble gases. The values are characteristic of the bilayer latent heat in the 2D gas-solid coexistence regime, Q2 (adsorbate-adsorbate interactions in the second layer are always attractive). Abbreviations used: T: surface temperature during adsorption, : coverage in monolayer units. Substrate

Adsorbate

Binding energy [meV]

Ref.

Remark

Ag(111)

Ar Kr Xe

Cu(110) Pd(100) Pd(810)

Xe Kr Xe Xe

81U 81U 79U 86B 88G2 98I 88Q 98P 84M3 84M3 83M2 84M2

Pt(111)

Xe

Pt(997) Ru(0001)

Xe Ne Ar Kr Xe

89 ± 4 118 ± 4 173 ± 5 187 153 ± 6 171 ± 3 186 182 ± 6 122 ± 5 145 ± 15 252 ± 13 243 221 205 ± 12 170 170 22 85 122 173

LEED LEED LEED TPD, 0th order, νd = 9×1013 s−1 TEAS, isothermal desorption ellipsometry surface plasmon resonance TPD, νd = 1015± 1 s−1 LEED, isosteres LEED, isosteres AES step edges terraces TEAS TPD, νd ~ 1013 s−1 TPD, νd ~ 1013 s−1 TPD TPD TPD TPD

86K2 98W 98W 92S1 92S1 92S1 92S1

Table 5. Heats of adsorption and desorption energies for multilayer (≥ 3ML) noble gas films. At the top of the table the heats of sublimation for the bulk crystals of Ne, Ar, Kr, and Xe are given as a reference. (Ad. = adsorbate) Substrate

Ad.

Binding energy [meV]

Layer thickness Ref. [ML]

Remark

Ne Ar Kr Xe

21.5 81.3 115.0 163.3

∞ ∞ ∞ ∞

93S1 93S1 93S1 93S1

T = 7.5 K T = 28 K T = 38 K T = 52 K

Ag(111) Ag(111) Cu(110) Pd(810)

Xe Xe Xe Xe

TEAS, isothermal desorption ellipsometry TPD, νd = 5⋅1013± 1 s−1 AES step edges terraces

Xe Ne Ar Kr Xe

3 3 ≥3 ≥3 3 3 ≥4 >3 ≥5 ≥5 ≥5 ≥5

88G2 98I 98P 83M2 84M2

Pt(111) Ru(0001)

115 ± 10 174 ± 3 167 ± 6 165 ± 13 200 187 156 171 20 77 112 162

Lando lt Börnst ein New Ser ies III/42A1

84M2 86K2 92S1, 90S1 92S1, 90S1 92S1, 90S1 92S1, 90S1

TEAS TPD, νd TPD, νd TPD, νd TPD, νd

= 1⋅1012 s−1 = 5⋅1012 s−1 = 1⋅1013 s−1 = 2⋅1013 s−1

82

3.1.1 Noble gases on metals and semiconductors

[Ref. p. 111

Table 6. Other thermodynamic quantities of noble gas monolayers. Abbreviations used: T: surface temperature, : coverage in monolayer units, d: nearest neighbor adsorbate spacing. (Ad. = adsorbate) Substrate Ad.

Ag(111) Ar Kr Xe

thermal expansion coefficient α [10–4 K–1] 22 ± 4 8±1 4.4 ± 5 (above 60 K)

thermal expansion coefficient γ [10–3 K–1]

2.0 ± 2 (ML step) 1.3 ± 2 (BL step)

Cu(100) Xe

Cu(110) Xe Pt(111)

Xe

Substrate Ad.

Cu(100) Xe Pd(810) Xe Pt(111)

Xe

~ 1 ... 8 (25 K ... 60 K)

isothermal compressibility

Ref.

KT [10–3 Å2/K] 81U 81U 1.68 ± 0.02, unconstrained ML 79U (d = 4.515 Å, T = 76 K) 0.97 ± 0.16, compressed ML (d = 4.366 Å, T = 56 K) 1.8 ± 7 97B1, (d = 4.41 ... 4.50 Å, 00B T = 62 ... 78 K) 1.5 ± 2 84G (d = 4.56 Å, T = 71.5 K) ~ 2.0 ... 3.8 88K1 (d = 4.45 ... 4.57 Å, T = 88 K)

roughening Remark critical temperature triple point Ttr temperature [K] TR [K] [meV/K] Tc [K] 0.26 ± 0.04 T = 77 ... 111 K 0.61 ± 0.09 66.4 (2. ML) 68.2 ± 0.7 67.3 (3. ML) ~ 1.4 ... 0.65 120 98 Entropy

Ref.

79G 84M2 85P, 88K1

3.1.1.2.3 Monolayer structures and phase diagrams See Fig. 11 Table 7. Structure of the 2D solid phase of noble gas adlayers in the monolayer coverage regime ( ≤ 1). Abbreviations used: T: surface temperature, p: 3D equilibrium pressure in mbar, : coverage in monolayer units, θs: coverage in substrate atom units; C: commensurate phase, HI: hexagonal incommensurate phase, SI: striped incommensurate phase, UC: uniaxially commensurate phase, HOC: high-order commensurate phase; R denotes the adlayer azimuthal orientation with respect to the substrate surface lattice; a, (b): adsorbate nearest neighbor spacings. If not stated otherwise, the structures were obtained from LEED. (Ad. = adsorbate) Substrate

Ad. Structure

unit cell dimensions atomic area Remark [Å] [Å2]

Ag(110)

Xe

UC-R0°

Ag(111)

Ar

HI-R0°

c(α×2) α ≈ 1.53 a = 4.42 ± 0.05 (b = 4.65 ± 0.05) a = 3.78...3.93 a = 3.79 ± 0.02

18.0

Ref.

T = 77 K, p > 10−6

73C

T = 34...45 K, p ~ 4×10−6 T = 21 K

82U 88G2

Lando lt Bö rnst ein New Ser ies III/42A1

Ref. p. 111] Substrate

Ad. Structure

unit cell dimensions atomic area Remark [Å] [Å2]

Ref.

Kr

a = 4.04 ... 4.15 a = 4.19 ± 0.05 a = 4.02 ± 0.02 a = 4.50 ± 0.03 a = 4.38 ± 0.02 a = 4.42...4.54 a = 4.529

17.5 ± 0.1

82U 76R 88G2 73C 88G2 79U 87G

a = 4.42 ± 0.05 a = 4.41, b = 4.61 a = 4.38 ± 0.03

16.9 ± 0.5 17.9 16.6 ± 0.2

Xe

HI-R0°

HI-R0° HI-R0° HI-R0°

Ag(211) Al(110) Al(111) Al(332)facetted Au(100)-rec Cr(110)

83

3.1.1 Noble gases on metals and semiconductors

Xe HI-R0° Xe UC-R0° Xe HI-R0° Xe Xe Xe C

p(2×2) a = 4.99 b = 5.76 Cu(100) Xe HI-R0° a = 4.5 ± 0.1 a = 4.52 ± 0.01 a = 4.40 ± 0.01 Cu(110) Ar I (R~4°?) a = 3.90 ± 0.07 b = 3.88 ± 0.07 Kr UC-R0° a > 4.08, b > 4.15 HOC (8×2), a = 4.08, b = 4.15 HI-R0° a = 4.03 ± 0.05 Xe UC, HOC-R0° (α×2) α ≥ 12/7 a ≥ 4.37, b ≥ 4.22 HOC (26×2), a = 4.42, b = 4.23 HOC (12×2), a = 4.37, b = 4.15 C (?) c(2×2) a = 5.1, b = 4.42 Cu(110)Xe UC (2×α) (2×1)O a = 5.1, b ≥ 4.33 HOC (2×3) ( 3 × 3 )R30° Cu(111) Xe C a = 4.42 I a = 4.3 ... 4.4 Cu(211) Xe HI-R0° a = 4.42 ± 0.05 Kr HI-R0° a = 4.18 ± 0.05 Cu(311) Xe HI-R0° a = 4.45 ± 0.05 Cu(610) Xe HOC-R45° (2×6), a ≈ 4.33 ( 3 × 3 )R30° Cu3Pt(111) Xe C a = 4.52 HI ?

Lando lt Börnst ein New Ser ies III/42A1

15.2 ± 4

16.9...17.9 17.762

23.5

17.5 ± 0.8

13.1 ± 0.2

T = 5...60 K T = 55 K, p = 6.5×10−4 T = 22 K T = 77 K, p > 10−6 T = 24 K T = 5...85 K XRD, at melting point TM = 131.8 K, p = 1.5 torr T = 77 K, p > 10−6 T = 45 K, θ > 0.1 T ≥ 40 K aligned R0° (step direction) no new LEED spots T = 80 K, p > 2×10−6 T = 77 K, p > 10−6 TEAS, T = 77 K TEAS, T < 65 K T = 24 K

73C 91S 85M 90H 76M 89K

71C, 79G 97G 82H

> 14.7 14.7

T = 53 K, p < 3×10−5

81G 81G, 82H

14.1 15.8...17.5 ≥ 15.8

T = 42 K LEED, TEAS, T = 20...70 K

82H 73C, 89B, 81G, 96Z

16

θ = 0.1...1, T ≤ 61 K

96Z

15.8 18.4

T < 45 K, θ → 1 T ≥ 70 K, p ~ 10−7... 10−6 small θ only TEAS, θs ≤ 0.42

96Z, 89B 73C, 81G

> 22.1 13.8 16.9

16.9 15.2 17.1 ± 0.4 17.7 < 17.7

θs = 0.67 T = 77 K, p > 10−6 T = 50 K SEXAFS, T = 18 ... 47 K T = 77 K, p = 3×10−6 T = 55 K, p = 6.5×10−4 T = 77 K, p = 3×10−6 T = 84 K, p = 3.9×10−7 T = 50 K, θ = 0.1...1.0 T = 50 K, θ = 0.2...0.6 T = 50 K, θ > 0.6

98P

73C 98S 90J 76R 76R 75P 80B 94S 92S3

84

3.1.1 Noble gases on metals and semiconductors

[Ref. p. 111

Substrate

Ad. Structure

unit cell dimensions atomic area Remark (Å) (Å2)

Ref.

Ge(100)(2×1) Ge(111)c(2×8) InSb(100)rec Ir(100)(1×1) Ir(100)(1×5)

Xe

C

(2×1)

no extra LEED spots

93S2

Xe

C

c(2×8)

88P

Xe

C

Xe

HI-R45° UC –R45°(?) HOC

sequential population of three different sites In registry with ×4 reconstructed surface T = 55 K, θ ≥ 1 T = 55 K, θ = 0.2

72I

T = 55 K, θ ≥ 1 thick Ar films T = 30 K, 2.5 L, diffuse ring T = 65...84 K Transient during heating, only in coexistence with HI-R30° T = 38 K, saturation coverage no extra spots in LEED T = 45 K, θ > 0.5 sickle shaped spots θ = 0.4...1, diffuse ring T = 13 K, θ = 0.7...0.9 θ = 0.6...1, diffuse ring T = 77 K T ~ 100 K, p >10−7 T ~ 100 K, p >10−6 saturation at T = 80 K 3.8 L at 70 K T = 50 K

72I 70D 82C2

84M3 71P 79K1, 84W3 84W3 91V2 95H1, 92H2 91V2, 92H2

4.6 L at 55 K

95H1, 92H2

Xe

Nb(100) Ni(100)

HI-R90° Ar UC Xe HI

Ni(111)

Xe

Xe Ni(hkl) NiAl(110)

Xe Xe

Pd(100)

Kr

HI-R30° C (?)

a = 4.44 ± 0.01 a = 4.48, b = 4.64 a ~ 4.4 ... 4.02 a ~ 4.06 a = 4.8 ... 4.35 a = 4.48 a = 4.49 ( 3 × 3 )R30° a = 4.76

17.1 ± 1

HI-R33.8° C HI-R0°, 30° Pt(100)-hex Xe C I Xe

Xe

16.8 ± 0.2 p(2×2)

HI-R0±8° UC (?) HI

HI UC-R45° (?) Xe UC-R0° Xe C

Kr

17.7 ± 0.2

16.1

C

Pt(110)(1×2)

a = 4.35 c(α×2) a = 4.52 ± 0.03

( 3 × 3 )R30° a = 4.31

C

Pd(810)

a = 4.36 a = 4.42 (3×5)

C

Xe

Pd(110) Pd(111)

13.9

UC

HOC

20...16.5 17.4 17.5 19.6

( 7 × 7 )R19.2° 15.3 a = 4.20 ( 19 × 19 )R23.4° 17.8 a = 4.53 a ~ 4.25 15.6 (2×1)-1D a = 4.48 17.4 ( 3 × 3 )R30°

(α×2), α ≈ 10/7 a ≈ 3.95 a = 3.93 ± 0.15 (8×2) (3×2)

28.9 16.3

at step edges on terraces, T = 73 K θ < 0.8 θ ≥ 0.8 C-I transition similar to Xe/Pt(111) LEED, 0.3 ≤ θ ≤ 1 ARUPS, two different Kr-species T ≤ 97 K T ≤ 77K, θ = 1

88M 72I

89F

96W 92F 91C1 90C 84M3

92H2 83M2 99G2

95W 95W 88D, 95W

Lando lt Bö rnst ein New Ser ies III/42A1

Ref. p. 111] Substrate

Ad. Structure UC

Pt(111)

Ar

Kr

HOC HOC, HI-R0° HI(?)-R30° HI-R30° HOC

unit cell dimensions atomic area Remark (Å) (Å2) (α×2) a = 4.5 ... 4.2 (4×4) (n×n), n = 7,11,18, ...

11.9 ≥ 12.6

a = 4.10 ... 4.14 14.6...14.8 (5×5) – with R30° 13.9 layer orientation

HI-R0° 1) ( 3 × 3 )R30° a = 4.80 a = 4.79...4.50 b = 4.80 a = 4.49...4.45 a = 4.44...4.33

19.9

HOC-R33.3° HI-R0°

a = 4.33

16.2

HI-R0° HI-R0° C-R0° (steps)

a = 4.43 a = 4.4 a = 5.54 (1D)

16.8

I-R0°

a = 4.1, b = 4.4

15.9

Ar Kr

HOC HOC

Xe

C

(4×3) (3×3) a = 4.07 ( 3 × 3 )R30° a = 4.69

Xe

C SI HI-R30° HI-R30°±α

Xe Pt(332) Pt(997)

Ru(0001)

Xe Xe

Xe Xe

...

95W

TEAS, T < 33 K T ≥ 33 K

92Z

after step decoration with 88K2 CO (< 0.01 ML), TEAS 87K3 T = 25 ... 55 K, θ ≤ 0.8 T < 50 K, θ ≥ 0.9 TEAS after step decoration with H 86K3 or CO (< 0.01 ML) 83P, 85P, TEAS, 91K T ≥ 62 K, θs ≤ 0.33 θs ≤ 0.37 (if not C) 87K2 0.37 ≤ θs ≤ 0.38 0.38 ≤ θs ≤ 0.40, α = 0 ... 3.3° θs = 0.40 only at high dosing pressures (10−6 mbar) T < 70 K, θ → 1 T = 25 K, θs = 0.40 (sat.) step decoration, θ

86K1 88C

12.7 14.3

89C2 98W (see also 97P2, 92W) T = 45 K, ML completion, 97P2 row-by-row growth 98N, 00N2 annealed to 55 K 97N, 98N

19.1

annealed to 77 K at 1 K/s

98N, 92S1

(7×7) ? HI-R45°

18.2 23.8 ( 2 × 2 )R45°

Ref.

θ = 0.3 ... 1

16.4

HI-R0°, HOC(?) Si(111)(7×7) W(100)

85

3.1.1 Noble gases on metals and semiconductors

90S1 84D, 89M 79W

(measured: 22.2 at 66 K, 80W2 imperfections) 79E W(110) Xe C p(2x2) 28 T = 77 ... 90 K, p ~ 10−7 T ~ 20 K 80C 79E UC, I, a = 6.32 ... 5.53 20.2 ... 17.6 T = 77 ... 90 K, p > 10−6 HOC (?) b = 4.49 ... 4.22 18.2 saturation at T = 62 K 79W 1 ) A rotation of the Kr adlayer by 5-6° out of the R30° orientation have been observed by Zeppenfeld and [00B] and could be due to the influence of residual defects or impurities on the surface. C (?)

Lando lt Börnst ein New Ser ies III/42A1

20

3 different states (local adsorption sites) T ~ 60 K T ~ 65 K

87K1, 91K

86

3.1.1 Noble gases on metals and semiconductors

[Ref. p. 111

Table 8. Details of noble gas 2D monolayer structures. Abbreviations used: T: surface temperature, HI: hexagonal incommensurate phase. If not stated otherwise, the structures were obtained from LEED. < u ⊥2 >1/2 denotes the vertical mean square displacement of the noble gas atom. (Ad. = adsorbate) adsorption site adlayer-substrate < u ⊥2 >1/2 spacing d [Å] [Å]

Substrate

Ad.

Ag(111)

Kr

3.3 ± 0.1

Xe

3.55 ± 0.1

Cu(110) Cu(111)

Ir(100)-(1×1) Ir(100)-(1×5) Ni(100) Pd(111)

Pt(111)

Ru(0001)

Kr Xe

2

3.36 ± 0.07 3.60 ± 0.08

Xe

3.45 ± 0.1

Xe Xe Ar Kr Xe

on top

2.95 ± 0.03 3.16 ± 0.03 1.6 ± 0.5 3.5 ± 0.8 3.4 ± 0.1

on top

3.5 ± 0.1

fcc and hcp hollow 1) on top

4.0 ± 0.1

Xe

Kr Xe

1

top of rows on top

3.4 ± 0.2

fcc and hcp 4.2 ± 0.1 hollow 1) preference for 3.70 ± 0.03 2) on top sites (average height) 3.54 ± 0.06

Remark

Ref.

0.08 ± 0.01 LEED-IV, T = 30 K HI phase 0.11 ± 0.01 LEED-IV, T = 45 K HI phase 0.67 LEED-IV, T = 25 K 0.22 ± 0.11 LEED-IV, T = 50 K ( 3 × 3 )R30° phase SEXAFS HI phase LEED-IV LEED-IV ICISS ICISS LEED-IV, T= 80K, 110 K ( 3 × 3 )R30° phase SPLEED, T = 55 K dilute phase SPLEED, T = 70K ( 3 × 3 )R30° phase 0.3 ± 0.1 LEED-IV, T = 80 K, 110K ( 3 × 3 )R30° phase SPLEED, T = 75 K ( 3 × 3 )R30° phase LEED-IV, T = 25 K (3×3) HOC phase 0.28 LEED-IV, T = 25 K ( 3 × 3 )R30° phase

81U 81U 00S 98S 90J 72I 72I 93S4 93S4 99S1 93H, 95H1 93H, 95H1 99S1

95P2 97N, 98N 97N, 98N

) In contrast, the more recent LEED-IV experiments seem to rule out the hollow sites. ) Closest approach to the metal is found for atoms closest to on top sites.

Lando lt Bö rnst ein New Ser ies III/42A1

Ref. p. 111]

87

3.1.1 Noble gases on metals and semiconductors

3.1.1.2.4 Electronic structure See Figs. 12 – 18. Table 9. Substrate work function changes ∆φ induced by noble gas monolayers. The initial dipole moment µ 0 per atom and the polarizability α were obtain from the intitial slope ∂ (∆φ ) / ∂θ and from a fit to the Topping formula, respectively. (Ad. = adsorbate) Substrate

Ad.

∆φ [V]

µ0 [D]

α Remark [10−24 cm3]

Ref.

Al(111)

Ar Kr Xe Xe Xe Xe

0.143 ± 0.015 0.185 ± 0.015 0.290 ± 0.015 0.1 ... 0.2 0.45 0.44 0.45 0.48 ± 0.01 0.5 0.29 0.45

0.096 0.134 0.235

2.66 2.76 3.20

82C1 82C1 82C1 90H 73C 73C 76R 86B, 86J, 85J 87F 76R 73C 76R 76M 71C, 73C 99G2 73C, 82M1 73C 86E 76R 76R 75P 82L 88P 74N2 74N2 74N2 74N1 74N1 82C2 92F

Al(332) Ag(110) Ag(111)

Ag(211)

Kr Xe

Au(100)-rec Cu(100)

Xe

Cu(110) Cu(111)

Xe Xe

Cu(211) Cu(211) Cu(311) Gd(0001) Ge(111)-(2×8) Ir(100) Ir(110) Ir(111) Ir(210) Ir(321) Ni(100)

Kr Xe Xe Xe Xe Xe Xe Xe Xe Xe Xe

0.45 0.47 0.33 0.61 ± 0.02 0.48 0.58 0.35 0.53 0.58 0.60 ± 0.03 0.22 ± 0.01 1.6 0.8 1.8 1.3 1.0 0.376

Ni(110) Pb(111)

Kr Ar Xe Ar Kr Xe

0.25 0.165 1.00 0.085 0.100 0.130

Lando lt Börnst ein New Ser ies III/42A1

∆φ ∆φ UPS Inverse PES ∆φ ∆φ ∆φ ∆φ ∆φ

0.20

0.24

0.53 0.14

0.29 0.2

XPS XPS XPS UPS

3.67

∆φ, ARUPS ∆φ UPS ∆φ ∆φ ∆φ PES secondary electron cutoff FEM FEM FEM FEM FEM ∆φ per step atom on Ni(h10) vicinal surfaces, UPS

ARUPS ARUPS ARUPS ARUPS

82C2 82C2 82J2 88J 88J 88J

88

3.1.1 Noble gases on metals and semiconductors

Substrate

Ad.

∆φ [V]

µ0 [D]

α Remark [10−24 cm3]

Pd(100)

Kr Xe

0.36 ± 0.05 0.94

0.17 ± 0.05 0.95

1.7 ± 0.08 8.2

0.86 ± 0.02

0.61

8.4

0.45 ± 0.03 0.44 0.42 0.70

3.6 ± 0.08

Pd(110)

Xe

Pd(111)

Xe

Pd(810)

Xe

Pt(100) Pt(111) Pt(111)

Xe Xe Xe

Pt(210) Pt(311) Pt(321) Pt(997)

Xe Xe Xe Xe

Ru(0001)

Xe

0.65 0.70 ± 0.02 0.96 ± 0.02 0.92 ± 0.01 0.85 ± 0.02 0.60 0.27 ± 0.02 1.03 ± 0.02 1.0 0.6 0.29 0.26 1.1 0.9 0.9 0.47 0.32 0.72 ± 0.02

1.12 0.49 0.53 0.3 ± 0.1

1.4 0.34 0.30 1.0

∆φ ∆φ, µ 0 at steps ? see [83M2] ∆φ µ ave = 0.36 D initial slope (small ) ∆φ UPS ∆φ ARUPS UPS, step edges full monolayer FEM full monolayer ( 3 × 3 )R30° phase hex-R33.3° phase FEM FEM FEM step decoration terrace saturation on terraces at step edges

[Ref. p. 111 Ref. 84M3 71P 84W3 86E 80K 84M3 84W3 79K1 84W3 88C 83M2 74N1 86S, 98W 96O 74N1 74N1 74N1 98W 86J, 86E 81W 99G2 70E 70E 70E 75W 80W2 70E 70E 70E 80W2 80C 83O 70E 70E 70E 74D 70E 70E 70E 70E, 62R 70E 70E

0.62 FE 0.46 ... 0.60 0.31 1) FE 0.83 0.38 1) FE 1.35 0.97 1) 0.99 ± 0.05 UPS 1.05 ± 0.02 0.98 ~6 ∆φ FE W(110) Ar 0.85 ... 1.05 0.29 1) FE Kr 1.97 1.93 1) FE Xe 2.4 1.67 1), 2) 0.45 ± 0.02 0.35 6.6 ∆φ 2.0 FE 2.1 FE FE W(111) Ar 0.40 0.16 1) FE Kr 0.83 0.50 1) FE Xe 1.13 0.41 1) 1.1 ± 0.01 0.66 ... 0.75 3.5 ... 4.9 retarding potential FE W(120) Ar 0.70 0.40 1) FE Kr 0.95 0.43 1) FE Xe 1.4 0.60 1) FE W(211) Ar 0.34 0.29 1) FE Kr 0.58 0.44 1) FE Xe 0.92 0.81 1) 1 ) Assuming monolayer densities [1014 atoms/cm2] of 7.9 (Ar), 7.2 (Kr) and 6.0 (Xe). 2 ) Based on Fowler-Nordheim equation which was later noted to break down for Xe/W(110) [80W2]. W(100)

Ar Kr Xe

Lando lt Bö rnst ein New Ser ies III/42A1

Ref. p. 111]

89

3.1.1 Noble gases on metals and semiconductors

Table 10. Energetic positions of the 5p orbital states of adsorbed Xe as derived from photoemission. If not stated otherwise, the values are for the low coverage limit. The energies are referred to the Fermi level but the workfunction values for the clean substrates, φC , are also given. Abbreviations used: T: surface temperature, : coverage in monolayer units, Γ: center of the Brillouin zone (electron emission normal to the surface plane)- see also Table 12. Substrate

φC [eV] 1)

Ag(100) Ag(111)

4.65 4.72

E(5p3/2) [eV]

Al(110) Al(111)

4.45 4.48 4.30 Al(332)-facetted 4.4 Au(111) Cu(100) Cu(110)

5.68 4.59 4.74 4.48 4.5

Cu(111)

4.90

Cu3Pt(111)

5.4 5.65

6.1 ~ 6.02 5.7 (mj = ± 3/2) 6.1 (mj = ± ½)

5.7 2) 5.8 3) 6.6 (mj = ± 3/2) 7.2 (mj = ± ½)

Co(0001) Fe(110) GaAs(110)

5.56

Gd(0001) Ir(100) Ir(100)-(1×1) Ir(100)-(1×5) Ir(110) Ir(111) Ir(210) Ir(321) Ir(531)-(731) Ni(100)

3.3 ± 0.1 5.67 ± 0.05 6.15 6.00 5.0 5.79 ± 0.05 5.0 5.4 4.9 5.30 5.56 5.2 ± 0.1 5.1 ± 0.1

5.5 ± 0.1 5.75 ± 0.1

Ni(310) Ni(410) Ni(810) Ni(110)

Lando lt Börnst ein New Ser ies III/42A1

4.55 4.55

6.4 ~ 6.2

E(5p1/2) [eV] 7.68 7.62 7.27 7.56 ... 7.76 7.74 7.55 7.5 7.8 6.72 7.3 ~ 7.15 7.80 7.3 7.30 7.10 6.9 6.65 6.9 2) 7.0 3) 8.1

Remarks

2D-solid 2D-gas θ = 0 ... 1, T = 45K ARUPS at Γ terrace and steps or (111) and (221) facets UPS ARUPS at Γ, θ = 1 ARUPS at Γ, θ = 1 2D-solid 2D-gas ordered surface disordered surface ARUPS at Γ, θ = 1 ARUPS at Γ, θ = 1 PES

PES FEM UPS 6.24 4) UPS 6.38 4) FEM FEM FEM FEM FEM 6.83 (6.90) PES, θ = 1 but coverage independent 6.75 ± 0.1 UPS 7.00 ± 0.1 UPS θ = 0.1 7.5 ± 0.1 θ > 0.1 7.4 ± 0.1 7.6 ± 0.1 at step edges, θ = 0.4 7.45 ± 0.1 θ = 0.89 7.5 ± 0.1 at step edges, θ = 0.11 on terraces, θ ≥ 0.17 6.8 ± 0.1 7.75 UPS ~ 7.5 ARUPS at Γ

Ref. 90W 86J 91S 90W 85M 90H 90W 97W 99G2 90W 82M1 86E 92S3 93G 93G 92H1 82L 74N2 90W, 79K3 90W, 79K3 74N2 74N2 74N2 74N2 74N2 82C2 92F 76T 92F 92F 92F 82J1 82J2

90 Substrate

φC [eV] 1)

Ni(111)

5.40 5.3 ± 0.1

NiAl(100) Pb(111)

4.05 ± 0.02

3.1.1 Noble gases on metals and semiconductors

[Ref. p. 111

E(5p3/2) [eV]

Ref.

5.2 ± 0.1 5.3, 5.7 5.55 ± 0.1 6.05 6.4

Pd(100) Pd(110)

5.65 5.65 5.20

Pd(111)

5.95

Pd(810)

5.55

Pt(100)-(1×1) Pt(100)-hex

5.77 5.80 5.72 5.7 5.8

Pt(100) Pt(110)-(1×2) Pt(110)-(1×2)H Pt(111)

6.40

5.9

5.85 Pt(210)

Pt(311) Pt(321) Pt(332) Pt(997) Re(0001) Ru(0001)

5.83 ... 6.21 5.82 ... 6.18 5.27 ... 5.62 6.0 splitting: < 0.1 0.40 ± 0.03 0.58 ± 0.03

4.9 ~ 4.9 4.7-4.8 5.5 4.7 (mj = ± 3/2) 4.1 (mj = ± ½) 5.1

E(5p1/2) [eV] 6.80 6.6 ± 0.1 6.9 6.80 ± 0.1 7.27 7.60 7.65 6.75 ... 6.98 6.2 7.03 ... 7.36 6.97 ... 7.33 6.47... 6.82 7.2

6.88 ... 6.99 6.88 ... 7.25 6.08 6.10 6.1 ~ 6.1 5.9-6.0 6.8 5.8 5.90 6.88 6.03

~ 4.15 (mj = ± 3/2) ~ 4.75 (mj = ± ½) 4.2, 4.70 5.95 6.2 6.95

5.2 5.5 5.4 ~ 5.0 5.4 6.65 5.52

5.4 6.4 ~ 5.22

6.22 ... 6.28 6.80 6.8 5.40 6.76 7.6 6.48

Remarks

UPS, low coverage ARUPS at 30°, θ = 1 UPS UPS, θ = 0 ... 1 ARUPS at Γ, θ = 1 ARUPS, no dispersion, low coverage (< 0.3L) UPS, θ = 0 ... 1 ARUPS at Γ UPS, θ = 0 ... 1 UPS, θ = 0 ... 1 UPS, θ = 0 ... 1 at step edges dilute phase ( 3 × 3 )R30° ( 7 × 7 )R19.2° on terraces at step edges ARUPS at Γ ARUPS at Γ UPS ARUPS at Γ, θ = 1 FEM UPS, ARUPS at Γ 2 sites even for θ → 0 ARUPS at Γ on terraces at step edges θ →0 FEM spin resolved ARUPS at Γ, θ = 1 ARUPS at Γ on terraces at step edges FEM FEM FEM UPS, on terraces at step edges 5) ARUPS at Γ on terraces at step edges ARUPS at Γ, θ = 1

90W 89F 76T 90C 88J 88J 84W3 90W 84W3 79K2 84W3 80W1 91V2

83M2 90W 97W 95S 74N1 88D, 95W 97T, 99W 83D1 89A 74N1 85S 88C 97W 74N1 74N1 74N1 89C2 97T 90W 85J 80W1, 81W 99G2

Lando lt Bö rnst ein New Ser ies III/42A1

Ref. p. 111]

91

3.1.1 Noble gases on metals and semiconductors

φC [eV] 1)

Substrate

E(5p3/2) [eV]

E(5p1/2) [eV]

Remarks

Ref.

5.0 (mj = ± 3/2) 5.4 (mj = ± 1/2)

6.65

ARUPS at Γ, ( 3 × 3 )R30°

00W

Rh(111) 5.6 n-Si(100) 4.60 n-Si(111)-(7×7)

6.7 7.70 7.40 7.70 7.53 7.70

4.5 p-Si(111)-(7×7)

5.95 6.05 5.6 5.4 4.50 4.65 ± 0.03 6.8 5.10

W(100) W(110)

corner holes, θ = 0...0.05 terrace sites, θ = 0.05...0.7 adatoms, θ = 0.7...1 UPS, T = 60 K site 1, θ = 0...0.04 site 2, θ = 0.04...0.8 site 3, θ = 0.8...1 UPS, T = 60 K

6.75 7.90 8.1 7.15

UPS, 5L at 80 K

94J 90W 89M

97P1 84

97P1 90W 75W 90W

1

) If not given in the indicated reference, values were taken from Table 1 in [90W]. ) Thin and magnetized Co film on W(110). Spin resolved measurements reveal a magnetic splitting of ∆E(5p1/2) = 0.05 ± 0.03 eV and ∆E(5p3/2) = 0.10 ± 0.03 eV. 3 ) Thin and magnetized Fe film on W(110). Spin resolved measurements reveal a magnetic splitting of ∆E(5p1/2) = 0.18 ± 0.03 eV and ∆E(5p3/2) = 0.24 ± 0.03 eV. 4 ) The peak positions which can be extracted from the spectra in [79K3] differ considerably from those listed in [90W]. 5 ) Adsorption at step edges can be suppressed after pre-adsorption of CO or H2 2

Table 11. Energetic positions of the np orbital states of adsorbed Kr (n = 4), Ar (n = 3) and Ne(n=2) as derived from photoemission. If not stated otherwise, the values are for the low coverage limit. The energies are referred to the Fermi level but the workfunction values for the clean substrates, φC , are also given. Abbreviations used: : coverage in monolayer units, Γ: center of the Brillouin zone (electron emission normal to the surface plane). (Ad. = adsorbate) – see also Table 12. Substrate

Ad.

φC [eV] E(5p3/2) ) [eV]

1

Ni(110)

Ne Ar

Remark

15.05 15.05 UPS, no splitting 9.15 9.6 UPS ~ 9.2 ~ 9.2 ARUPS at Γ, no splitting at low θ Kr 7.85 8.5 UPS ~ 7.65 ~ 8.35 ARUPS at Γ 6.9 ARUPS Pt(110)-(1×2) Kr 5.9 (mj = ± 3/2) 6.3 (mj = ± 1/2) spin resolved Pt(111) Ar ~ 7.75 (mj = ± 3/2) ARUPS at Γ, θ = 1 ~ 7.65 (mj = ± 1/2) spin resolved Kr ~ 5.80 (mj = ± 3/2) ~ 6.95 ARUPS at Γ, θ = 1 ~ 6.15 (mj = ± 1/2) Rh(111) Ar 5.6 8.7 UPS, no splitting at low θ Kr 5.6 7.7 UPS 1 ) If not given in the indicated reference, values were taken from Table 1 in [90W].

Lando lt Börnst ein New Ser ies III/42A1

4.55 4.55 4.55 4.55 4.55

E(5p1/2) [eV]

Ref. 82J1 82J1 82J2 82J1 82J2 95W 86S 85S, 86S 94J 94J

92

3.1.1 Noble gases on metals and semiconductors

[Ref. p. 111

Table 12. Band structure (dispersion) of the p valence levels of the noble gas monolayers obtained in angular resolved photoemission. The energies at Γ are referred to the Fermi level. If not explicitly given in the indicated reference, the positions EF and band widths ∆ε were extracted from graphs of the dispersion curves. In this case the values are given with a ~ sign. Abbreviations used: : coverage in monolayer units, Γ: center of the Brillouin zone (electron emission normal to the surface plane). (Ad. = adsorbate) EF [eV] at Γ

Substrate

Ad.

Al(111)

Xe 1) ~ 5.8 ~ 6.3 ~ 7.55 Ar ~ 9.2 ~ 9.7 Kr ~ 7.2 ~ 7.5 ~ 8.4 Xe 2) ~ 5.4

Cu(110)

~ 5.95

Cr(110)

Xe

Pb(111)

Ar

Kr

Xe

Pd(100)

Ar Kr

Xe

7.3 5.7 7.0 ~ 9.1 ~ 9.15 ~ 9.70 ~ 7.75 ~ 8.1 8.83 ~ 5.9 ~ 6.35 7.60 8.50 8.90 6.35 6.7 7.55 4.6 5.1 6.4

Derived from Orbital ∆ε [eV]

Remarks

Ref.

0.44 4p3/2, mj = ± 3/2 0.40 4p3/2, mj = ± 1/2 0.36 4p1/2, mj = ± 1/2 3p3/2, mj = ± 1/2, ± 3/2 0.35 3p1/2, mj = ± 1/2 4p3/2, mj = ± 3/2 4p3/2, mj = ± 1/2 0.37 4p1/2, mj = ± 1/2 0.57 5p3/2, mj = ± 3/2 0.80 5p3/2, mj = ± 1/2 0.55 0.60 5p1/2, mj = ± 1/2 0.40 3p3/2, mj = ± 1/2, ± 3/2 ~ 0.2 ~ 0.2 3p1/2, mj = ± 1/2 ~ 0.4 3p3/2, mj = ± 3/2 ~ 0.4 3p3/2, mj = ± 1/2 ~ 0.3 3p1/2, mj = ± 1/2 ~ 0.5 4p3/2, mj = ± 3/2 ~ 0.45 4p3/2, mj = ± 1/2 ~ 0.4 4p1/2, mj = ± 1/2 ~ 0.6 5p3/2, mj = ± 3/2 ~ 0.5 5p3/2, mj = ± 1/2 ~ 0.5 5p1/2, mj = ± 1/2 3p3/2, mj = ± 1/2, ± 3/2 3p1/2, mj = ± 1/2 4p3/2, mj = ± 3/2 4p3/2, mj = ± 1/2 0.4 4p1/2, mj = ± 1/2 0.51 5p3/2, mj = ± 3/2 0.72 5p3/2, mj = ± 1/2 0.55 0.60 5p1/2, mj = ± 1/2 0.43 0.45

along ΓM along ΓM along ΓM along ΓM, ΓK along ΓM, ΓK along ΓM, ΓK along ΓM, ΓK along ΓM, ΓK along ΓM along ΓK along ΓM along ΓK along ΓM, ΓK

85M

82H 82H

82M1

89K along ΓH along ΓH along ΓH along ΓH along ΓH along ΓH along ΓH along ΓH along ΓH along ΓH along ΓH along ΓH along ΓH along ΓH along ΓM along ΓK along ΓM along ΓK along ΓM along ΓK

88J

88J

88J

80H 80H

78H

Lando lt Bö rnst ein New Ser ies III/42A1

Ref. p. 111]

93

3.1.1 Noble gases on metals and semiconductors

Substrate

Ad.

EF [eV] at Γ

Derived from Orbital ∆ε [eV]

Remark

Ref.

Pt(110)(1×2)

Kr

5.9 6.3 6.9 ~ 4.7 ~ 5.9 4.7 4.1 5.8 4.2

4p3/2, mj = ± 3/2 4p3/2, mj = ± 1/2 4p1/2, mj = ± 1/2 5p3/2 5p1/2 5p3/2, mj = ± 3/2 5p3/2, mj = ± ½ 5p1/2, mj = ± 1/2 5p3/2, mj = ± 3/2

dilute layer, θ = 0.4 3)

95W

4.70

5p3/2, mj = ± ½

5.95

5p1/2, mj = ± 1/2

5.4 6.8

5p3/2 5p1/2

Xe Pt(110)(1×2)H

Xe

Pt(111)

Xe

Pt(997)

Xe

0.20 0.25 0.80 0.24 0.22 0.33 0.65 0.35 0.41 0.32 0.35 ~0 ~0

along ΓX dilute layer, θ = 0.34 3) along ΓX dilute layer, θ = 0.45 3, 4 ) along ΓX ~ along ΓM ~ along ΓK ~ along ΓM ~ along ΓK ~ along ΓM ~ along ΓK dilute layer, θ = 0.15 5) along ΓK (and ΓM)

95W 97T, 99W

88C

97T

) Using EF = EV - φ with φ = 4.05 eV such that E(5p1/2) = 7.55 eV according to private communication cited in [88C]. The values for ∆ε were evaluated from Fig. 4 in [85M] and are smaller than those given in Tab. 2 of [88C]. 2 ) A broadening of the band width upon uniaxial compression is observed - here the values for ‘full coverage’ are given. 3 ) Under these conditions, Kr and Xe form quasi 1D structures along the Pt-[11 0] direction. Indeed, no dispersion is observed in the orthogonal ΓY-direction. 4 ) Note that the energetic position of the mj = ± 1/2 and mj = ± 3/2 subbands of the 5p3/2 level at the Γ-point is reversed as compared to the usual energetic ordering of the subbands in 2D. 5 ) In this case, a single Xe chain decorates the (111)-step edge with a large spacing between neighboring atoms being 2aPt = 5.56 Å. No splitting of the 5p3/2 subbands and no dispersion is observed along the chain direction. 1

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94

3.1.1 Noble gases on metals and semiconductors

[Ref. p. 111

3.1.1.2.5 Surface diffusion coefficients

See Fig. 19.

Table 13. Diffusion barriers Edif for the noble gases on various metal surfaces. The prefactor D0, if indicated, is related to the measured diffusion coefficient D via D = D0 exp(-Edif /kT). Abbreviations used: T: surface temperature, : coverage in monolayer units, θs: coverage in substrate atom units. (Ad. = adsobate) Substrate

Ad.

Edif [meV] D0 [cm2/s]

Ni(111)

Xe

14 ± 1 17 ± 2

Pt(111)

Kr Xe

~ 48 52 ± 9 56 ± 4 31 64 < 9.6 48 ± 13 48 ± 13

7×10−8 10−8 ... 10−9

W(110)-vicinals Xe

239 252 108

6×102 2×103 2×10−6

W(111) W(310)-(100)

65 ... 148 < 39 > 48 ~ 167

Xe

W(110)

Xe Xe

Xe Ar Kr Xe

2×10−9±0.2 3×10−8±0.3 3.4×10−4±0.5 1.1×10−4±0.2

Remarks

Ref.

θs = 0.04 θs = 0.16 first order linear optical diffraction θ = 1, assuming D0 = 10−9 cm2/s θ = 0.25 θ =1 adatom diffusion (θ → 0) + detachment from 2D islands 2D gas (θ = 0.017), T = 105 K θ = 0.3, T = 54 ... 72 K θ = 0.9, T = 67 ... 83 K FE current fluctation method θ = 0.3 θ = 0.5 θ = 0.7 FE current fluctation method FEM FEM FEM FEM

00N1

93M2 93M1, 93M2 95H2 99E 80C

83O

59E 59E 59E 59E

Lando lt Bö rnst ein New Ser ies III/42A1

Ref. p. 111]

95

3.1.1 Noble gases on metals and semiconductors

3.1.1.2.6 Vibration frequencies and phonon dispersion See Figs. 20 – 23. Table 13. Perpendicular vibration frequencies !ω⊥ and characteristics of the phonon dispersion curves for the noble gas monolayers. The sound velocities cl and ct were obtained from the initial slope of the dispersion curves for the longitudinal (L) and shear-horizontal (SH) modes, respectively. Where complete or partial dispersion curves are available, only the value at the boundary of the surface Brillouin zone is indicated. Abbreviations used: Γ, M, K: high symmetry points of the 2D adlayer Brillouin zone (BZ); [001] , [1 1 0] and [112 ] : crystallographic directions of the substrate surface. All data were obtained using inelastic He-atom scattering. (Ad. = adsorbate) Substrate

Ad.

!ω⊥ (meV)

Ag(110)

Kr Xe Ar Kr Xe Ar Kr Xe Kr Xe Xe Kr Xe Xe

~ 2.7 ~ 2.3 3.67 ± 0.15 2.92 ± 0.14 2.79 ± 0.15 3.9 ... 4.1 3.3 ... 3.5 2.8 ... 2.9 ~3 ~ 2.5 2.62 ~ 2.8 ~ 2.5 2.55 ± 0.05 0.7 2.71 0.4 4.70 ± 0.05 (Γ) 5.00 ± 0.05 (M) 5.00 ± 0.05 (K)

Ag(111)

Au(111)

Cu(100)

Cu(110)

Cu(111)

Xe

Pt(111)

Ar

Kr

Xe

Lando lt Börnst ein New Ser ies III/42A1

cl (m/s)

ct (m/s)

Ref. 84M1 84M1 88G1 88G1 88G1 86M 86M 86M 82M2 81M, 82M2 97G, 99S2 83M1 83M1, 94Z1 94Z1, 97R

along [001] 99S2 98B 00B, 90Z2 1400 ± 50 1460 ± 60

880 ± 50 900 ± 60

4.05 ± 0.05 (Γ) 3.85 ± 0.05 (M) 3.85 ± 0.05 (K)

1100 ± 50 1170 ± 60

620 ± 50 670 ± 30

3.80 ± 0.05 (Γ) 3.40 ± 0.05 (M) 3.40 ± 0.05 (K)

1230 ± 50 1280 ± 50

750 ± 30 780 ± 30

Remark

at BZ boundary

at BZ boundary gap energy (L) at BZ boundary gap energy (SH)

[112] [1 1 0] 00B, 89H

[1 1 0] [112] 00B, 89H

[1 1 0] [112]

3.1.1 Nobel gases on metals and semiconductors

96

[Ref. p. 111

Figures for 3.1.1 1.0

Sticking coeff. s0

0.8 Ne Ar Kr Xe

0.6

0.4

0.2

100

200 300 400 Gas temperature Tg [K]

500

600

1.0

5

0.8

4

Sticking coeff. s0 [10–3]

Sticking coeff. s0

0

Fig. 1. Initial sticking coefficient s0 of Ne, Ar, Kr and Xe on Ru(0001) as a function of the gas temperature Tg (surface temperature during adsorption Ts = 6.5 K) [92S2].

0.6

0.4

Ar Kr Xe

0.2

0 0

15

30 45 Surface temperature Ts [K]

60

3

2

Ne

1

75

0

5

6

7 8 9 Surface temperature Ts [K]

10

11

Fig. 2. Initial sticking coefficient s0 of Ne, Ar, Kr and Xe on Ru(0001) as a function of the surface temperature Ts (gas temperature Tg = 300 K). The usable temperature range is limited by the thermal desorption rate [92S2].

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 111]

3.1.1 Nobel gases on metals and semiconductors

1.0

1.0

Ne/Ru (0001)

Ar/Ru (0001) 0.8

Sticking coeff. s

Sticking coeff. s

0.8

0.6

0.4

0.2

a

0

0.6

0.4

0.2

0.5

1.0 1.5 Coverage θ [ML]

2.0

2.5

b

1.2

0

1.0

0.8

0.8

Sticking coeff. s

Sticking coeff. s

1.0 1.5 Coverage θ [ML]

2.0

2.5

2.0

2.5

Xe/Ru (0001)

1.0

0.6

0.6

0.4

0.4

0.2

0.2

0

0.5

1.2

Kr/Ru (0001)

c

97

0.5

1.0 1.5 Coverage θ [ML]

2.0

2.5

d

0

0.5

1.0 1.5 Coverage θ [ML]

Fig. 3. Coverage dependence of the sticking coefficient s() of Ne, Ar, Kr and Xe on Ru(0001) at a surface temperature Ts = 6.5 K and gas temperature Tg = 300 K. Measurements for adsorption at 6.5 K (filled circles) and for adsorption onto an annealed monolayer of the noble gas (open circles) are shown [90S1].

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3.1.1 Nobel gases on metals and semiconductors

98 1.0

1.0

Ar/Cu (110)

Kr/Cu (110) 0.8

Sticking coeff. s

Sticking coeff. s

0.8

0.6

0.4

0.2

0

[Ref. p. 111

0.6

0.4

0.2

0.2

0.4 0.6 Coverage θ [ML]

0.8

1.0

0.4 0.6 Coverage θ [ML]

0.8

1.0

0

0.2

0.4 0.6 Coverage θ [ML]

0.8

1.0

1.0

Xe/Cu (110)

Sticking coeff. s

0.8

0.6

0.4

0.2

0

0.2

Fig. 4. Coverage dependence of the sticking coefficient s() of Ar, Kr and Xe gas on Cu(110) at a surface temperatures Ts of 30, 43 and 55 K, respectively; gas temperature Tg = 300 K [94Z2].

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 111]

3.1.1 Nobel gases on metals and semiconductors

1.0

1.0

Pt (111)

Pt (557)

Rel.sticking coeff. s/sm

0.8

Rel.sticking coeff. s/sm

0.8

0.6

0.4

0.2

0

99

0.6

0.4

0.2

0.2

0.4 0.6 Coverage θ [ML]

0.8

1.0

0.4 0.6 Coverage θ [ML]

0.8

1.0

0

0.2

0.4 0.6 Coverage θ [ML]

0.8

1.0

1.0

Pt (112)

Rel.sticking coeff. s/sm

0.8

0.6

0.4

0.2

0

0.2

Fig. 5. Relative sticking coefficient of Xe on Pt(111), Pt(557) and Pt(112) as a function of coverage at a surface temperature Ts = 87 K [90S2].

Lando lt -Bö rnst ein New Ser ies III/42A1

3.1.1 Nobel gases on metals and semiconductors

100 7

10

2D solid [( 3× 3)R30°] + 2D gas

Xe coverage θ [%]

5

2D liquid + 2D gas

8

Thermal expansion α [10–4 K–1]

6

4 3 Ttr

2

2D gas

1

Tc

0 80

85

90

95 100 105 110 Surface temperature T [K]

115

320

Xe/Pt(111)

6 Xe/Ag(111)

4 bulk Xe

2

0 15

120

Fig. 6. 2D triple point Ttr, critical temperature Tc and phase coexistence lines for monolayer Xe on Pt(111) [85P]. Further details (2D solid phases) of the phase diagram are shown in Fig. 11.

25

35 45 Surface temperature T [K]

55

65

Fig. 7. Linear thermal expansion coefficient α of the unconstrained monolayer for Xe/Pt(111), Xe/Ag(111) and bulk Xe [88K1].

1.50

Molar adsorption entropy Sad [meV K–1]

1.25

Adsorption heat Qiso [meV]

310

1.00

300

0.75

290

0.50

280

270

[Ref. p. 111

0.25

0

0.1

0.2 Coverage θs

0.3

0.4

Fig. 8. Isosteric heat of adsorption Qiso of the Xe adlayer on Pt(111) as a function of Xe coverage θs [88K1]. The coverage is given in substrate atom units, i.e., θs = 1/3 corresponds

to

saturation

of

the ( 3 × 3 )R30°

commensurate phase corresponds to θs = 1/3.

0

0.1

0.2 Coverage θs

0.3

0.4

Fig. 9. Differential molar entropy of the Xe adlayer on Pt(111) as a function of Xe coverage. Data points (circles) and theoretical curves assuming a completely mobile 2D ideal gas (solid line), a completely mobile Volmer gas (dashed line) and a localized adlayer (dashdotted line) [88K1]. The coverage is given in substrate atom units, i.e., saturation of the ( 3 × 3 )R30° commensurate phase corresponds to θs = 1/3.

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 111]

3.1.1 Nobel gases on metals and semiconductors

–4

–3

10 8

10 8

6 4

6 4

2

2 –4

–5

bulk Argon

6 4 2

–5

10 8

bilayer

6 4

2D solid

2 –6 6 4

2 –6 10 8 6 4

20

bilayer 2D solid

2

2D gas

2

–7

a

6 4

–7 10 8 6 4

10 8 2

bulk Krypton

10 8

Vapor pressure p [Torr]

Vapor pressure p [Torr]

10 8

10

101

2D gas

–8

22

–5

10 8

24 26 28 30 –1 –3 –1 Inv. temperature T [10 K ]

32

10

34

b

14

16

18 20 22 24 –1 –3 –1 Inv. temperature T [10 K ]

26

28

6 4

Vapor pressure p [Torr]

2 –6 10 8 6 4

bulk Xenon

2 –7 10 8 6 4

bilayer 2D solid

2 –8

10 8 6 4

2D gas

2 –9

10

8

10

c

12 14 16 18 –1 –3 –1 Inv. temperature T [10 K ]

20

22

Fig. 10. (a) Phase diagram of Ar on Ag(111) [81U]. The dashed line is the vapour pressure of bulk Ar. Circles, triangles and squares mark the onset of formation of the first, second and third layer, respectively. From the slope of the coexistence lines (solid lines through the symbols) the latent heats of adsorption (Tables 3–5) were determined. (b) Phase diagram of Kr on Ag(111) [81U]. Lines and symbols: same meaning as in (a). (c) Phase diagram of Xe on Ag(111) [79U]. Lines and symbols: same meaning as in (a).

Lando lt -Bö rnst ein New Ser ies III/42A1

3.1.1 Nobel gases on metals and semiconductors

102

130

0.90

G L

unrelaxed

0.75

~3%

90

6.5%

( 3× 3)R30°

70

Workfunction change ∆φ [eV]

Temperature T [K]

1.05

Tc

110

7.2%

0.60

HI

C

(4.49... 4.45 Å)

(4.8 Å)

0.45

HIR

(4.44... 4.33 Å)

50

SI

0.1

0.30

HOC

relaxed

(4.79 ... 4.5 Å)

30 0

[Ref. p. 111

0.15

9.6%

0.2 0.3 Coverage θs

0.4

0.5

Fig. 11. Structural phase diagram of monolayer Xe on Pt(111) (schematic) [91K]. C, SI, HI and HIR denote the commensurate ( 3 × 3 )R30° structure, the striped incommensurate, the hexagonal incommensurate and the hexagonal incommensurate rotated 2D solid phases. G and L denote the 2D gas and liquid phases, respectively.

HOC denote the high-order commensurate phase. Percentages indicate the lattice misfit defined as m = (a0-a)/a0 with a0 = 4.80 Å being the commensurate lattice spacing.

0

0.15

0.30

0.45 0.60 0.75 Coverage θ [ML]

0.90

1.05

Fig. 13. Comparison of the work function change for Xe on Pd(100) as a function of Xe coverage from different experiments [84M3]. Open symbols: Moog and Webb [84M3] with fit to Topping formula (solid line); dashed line: Wandelt and Hulse [84W3]; dotted line: stepped surface, Miranda et al. [83M2]; dash-dotted line: Palmberg [71P]. The work function change found by Kaindl et al. [80K] for monolayer coverage is also indicated (filled circle).

For Fig. 12 see next page 4.0 5p3/2, mj = ± 3/2

Binding energy Eb [eV]

4.5

F

5.0 5p3/2, mj = ± 1/2

5.5 6.0

Fig. 14. Dispersion curves for the 5p Xenon levels along the ΓKMΓ direction of the surface Brillouin zone of a Xe layer adsorbed on Pt(111). Circles refer to the

5p1/2

6.5 H K K'

7.0

0

0.25

0.50

M M'

0.75 1.00 1.25 –1 Wavevector kII [Å ]

1.50

1.75

( 3 × 3 )R30° commensurate layer with a Xe-Xe distance of 4.8 Å, triangles refer to an incommensurate rotated phase with a Xe-Xe distance of 4.38 Å. Data from [88C], figure from [90W].

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Ref. p. 111]

3.1.1 Nobel gases on metals and semiconductors

0

0

Al(111)

– 0.025

Ar dφ = – 0.30(3) eV dθ 0

Workfunction change ∆φ [eV]

Workfunction change ∆φ [eV]

– 0.025 – 0.050 – 0.075 – 0.100 – 0.125 – 0.150 – 0.175 0

103

Kr dφ = – 0.36(4) eV dθ 0

– 0.050 – 0.075 – 0.100 – 0.125 – 0.150 – 0.175

0.25

0

0.50 0.75 1.00 1.25 Coverage θ [ML]

1.50

1.75

– 0.200 0

0.25

0.50

0.75

1.00

1.25

1.50

1.75

Workfunction change ∆φ [eV]

– 0.05 Xe dφ = – 0.54(4) eV dθ 0

– 0.10 – 0.15 – 0.20 – 0.25 – 0.30 0

0.25

0.50 0.75 1.00 1.25 Coverage θ [ML]

1.50

1.75

Fig. 12. Measured work function changes as a function of coverage θ for Ar, Kr and Xe adsorbed on Al(111) at 40 K. The solid and dashed lines are theoretical curves based on the Topping model [82C1].

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3.1.1 Nobel gases on metals and semiconductors

104 9.0

K

Ar/Pb(111)

H

3p

Binding energy Eb [eV]

9.2

F

[Ref. p. 111

K M

Γ

9.4

9.6

5.8

9.8

6.0

10.0 0

0.25

7.6

0.50 0.75 1.00 –1 Wave vector kII [Å ]

1.25

Xe/Pb(111) 5p

6.2

1.50

6.4

5p3/2, mj = ± 3/2

Kr/Pb(111) 4p

7.8

6.6

8.0

Binding energy Eb [eV]

6.8

F

mj = ± 1/2

8.2

F

Binding energy Eb [eV]

7.0

7.2

8.4

8.6

7.4

8.8

7.6

9.0

7.8

9.2 0

0.25

0.50 0.75 1.00 –1 Wave vector kII [Å ]

1.25

1.50

8.0 0

5p1/2

0.25

0.50 0.75 1.00 –1 Wave vector kII [Å ]

1.25

1.50

Fig. 15. Band dispersion (along the ΓH direction) of the valence p-levels of Ar, Kr and Xe at monolayer coverage on Pb(111) [88J].

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Ref. p. 111]

3.1.1 Nobel gases on metals and semiconductors

Γ – 9.5

Γ – 9.5

M

105 M

–10.0

–10.0

–10.5

Binding energy Eb rel. to Evac [eV]

Binding energy Eb rel. to Evac [eV]

j = 3/2

–11.0 Γ

M

1st layer 2nd layer indirect transitions

–11.5

–10.5

–11.0

–11.5

j = 1/2

–12.0

–12.0

–12.5 0 0.2 a Monolayer

0.4

0.6 0.8 1.0 1.2 –1 Wave vector kII [Å ]

1.4

1.6

–12.5 0 b Bilayer

0.2

0.4

0.6 0.8 1.0 1.2 –1 Wave vector kII [Å ]

1.4

1.6

Fig. 16. Band dispersion along ΓM for monolayer and bilayer Xe on Al(111). Solid and dashed lines represent the theoretical single layer band dispersion curves [80H] displaced by layer dependent binding energy shifts [85M].

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3.1.1 Nobel gases on metals and semiconductors

106

4.25

4.25

4.50

4.50

5p3/2 mj = ± 1/2

Binding energy Eb [eV]

4.00

Binding energy Eb [eV]

4.00

4.75

5p3/2 mj = ± 3/2

F

F

4.75 5.00

5.00

5.25

5.25

kII along ΓX

5.50

5.75

6.00

6.00 2.0

1.5 1.0 –1 Wave vector kII [Å ]

0.5

kII along ΓY

5.50

5.75

6.25 2.5

[Ref. p. 111

6.25

0

5p1/2 mj = ± 1/2

0

0.5

1.5 1.0 –1 Wave vector kII [Å ]

2.0

2.5

Fig. 17. Band dispersion (along the ΓX and ΓY directions) of Xe adsorbed on a hydrogen modified Pt(110)-(1×2) surface (Xe coverage θ = 0.45 ML). The symbols correspond to experimental data derived from ARUPS spectra with different photon energies (26 and 30 eV) and different directions of light incidence (normal and 45° off-normal). The solid lines are the results of a fully relativistic Korringa-Kohn-Rostocker (KKR) calculation for freestanding Xe chains [99W].

For Fig. 18 see next page. 2.5 T = 45 K

Xe/Pt(111)

T = 80 K

2.0

1.2

2 –1

0.8

0.4

0

a

Kr/Pt(111)

Diff. coeff. D [10–6 cm s ]

2 –1

Diff. coeff. D [10–7 cm s ]

1.6

1.5

1.0

0.5

0.2

0.4 0.6 Coverage θ [ML]

0.8

1.0

0

b

0.2

0.4 0.6 Coverage θ [ML]

0.8

1.0

Fig. 19. Surface diffusion coefficient D for Kr (at 45 K) and Xe (at 80 K) on Pt(111) as a function of coverage. Symbols: data extracted from experiment, solid line: prediction based on the quasi-chemical approximation [93M2].

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 111] 10

3.1.1 Nobel gases on metals and semiconductors 10

Xe/Cu(100)

Xe/Ru(0001)

8 Xe coverage θ [ML]

Xe coverage θ [ML]

8

107

6

4

2

6

4

2

0 4.4 10

5.2

6.0

6.8 7.6 F Binding energy Eb [eV]

0 4.4

8.4

5.2

6.0

6.8 F

Binding energy Eb [eV]

7.6

8.4

Xe/Pt(100)-hex

Xe coverage θ [ML]

8

6

4

Fig. 18. Quantum well electronic states of Xe multilayer films adsorbed on Cu(100), Ru(0001) and Pt(100)-hex as a function of the film thickness. Dots and triangles are the experimental results, the vertical bars are theoretical predictions of a tight binding calculation [99G2].

2

0 4.4

5

5.2

6.0

6.8

Binding energy EbF [eV]

7.6

8.4

Γ

K

Phonon energy hω [meV]

4 ML

3 BL

2

1

0

Fig. 20. Phonon dispersion curves for monolayer (ML), bilayer (BL), trilayer (TL) and 25 ML Xe on Pt(111) along the adlayer ΓK direction (substrate

TL

[112] direction). Beside the evolution of the mode dispersion with increasing film thickness the hybridization between the adlayer mode and the surface Rayleigh wave (solid line) is apparent [90Z1].

25ML

0.2

Lando lt -Bö rnst ein New Ser ies III/42A1

0.4 0.6 –1 Wave vector q [Å ]

0.8

1.0

3.1.1 Nobel gases on metals and semiconductors

108 8.0 R

LR

8.0

Ar/Pt(111) [110]

SH

6.0

7.0

5.0

Phonon energy hω [meV]

Phonon energy hω [meV]

7.0

S

4.0 LA

LA

3.0 2.0

SH

0

0.5

1.0 1.5 –1 Wave vector q [Å ]

8.0

[110]

R

LR

4.0

SH

2.0

0

0.5

8.0

1.0 1.5 –1 Wave vector q [Å ]

0.5

1.0 1.5 –1 Wave vector q [Å ]

2.0

2.5 [112]

R

LR

5.0 S

4.0

SH

LA

3.0 2.0

0

2.5

7.0

[110] LA

5.0 4.0

S

3.0 2.0 SH

0.5

2.0

2.5

1.0 1.5 –1 Wave vector q [Å ]

2.0 [112]

R

L

2.5

6.0 5.0

LA

4.0 S

3.0 2.0 1.0

1.0 1.5 –1 Wave vector q [Å ]

0.5

8.0

Xe/Pt(111)

R

L

2.0

6.0

0

2.0

1.0

Phonon energy hω [meV]

Phonon energy hω [meV]

c

SH

3.0

6.0

S

LA

3.0

1.0

S

4.0

7.0

5.0

7.0

5.0

8.0

1.0

b

LA

6.0

0

2.5

Phonon energy hω [meV]

Phonon energy hω [meV]

2.0

Kr/Pt(111)

7.0 6.0

[112]

R

LR

1.0

1.0

a

[Ref. p. 111

0

SH

0.5

1.0 1.5 –1 Wave vector q [Å ]

2.0

2.5

Fig. 21. For caption see next page.

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 111]

3.1.1 Nobel gases on metals and semiconductors

109

Fig. 21. (a) Phonon dispersion curves for a HI-R0° Ar monolayer on Pt(111) along the substrate [1 1 0] and [112] directions at T = 23 K. (b) Phonon dispersion curves for a HI-R30±523° Kr monolayer on Pt(111) along the substrate [1 1 0] and [112] directions at T = 50 K. (c) Phonon dispersion curves for a hexagonal incommensurate rotated (HI-R30±2.6°) Xe monolayer on Pt(111) along the substrate [1 1 0] and [112] directions at T = 50 K. Notations used: R, L: Pt(111) surface Rayleigh wave and longitudinal resonance, respectively; S, LA and SH: perpendicular, longitudinal and shear horizontal Xe adlayer modes, respectively. The lines are calculations based on McLachlan modified HFD-B2 gas-phase potentials (solid lines) [00B].

10

Γ

K

X

10

R 6

4

⊥2 H1

Γ

Y

8

⊥3

Phonon energy [meV]

Phonon energy [meV]

8

M

L

⊥3 R

6

4

⊥2 L*

H

⊥ 2

a

⊥ 2

H2

0

G 0.4

0.8 1.2 –1 Wave vector q [Å ]

1.6

0

b

0.2

0.4 0.6 –1 Wave vector q [Å ]

0.8

1.0

Fig. 22. Phonon dispersion curves for the (26×2) HOC monolayer of Xe on Cu(110) along the ΓX (a) and ΓY (b) directions. Notations used: R: Cu(110) surface Rayleigh wave; ⊥, ⊥2, ⊥3: single and multiple excitation of the perpendicular Xe vibration mode; H, H1, H2: branches of the hybridized ⊥ and R modes. L, L*: longitudinal Xe modes, G: gap mode. Dashed lines are the result of a lattice dynamics calculation [97R].

Lando lt -Bö rnst ein New Ser ies III/42A1

3.1.1 Nobel gases on metals and semiconductors

110

[Ref. p. 111

4.0 3.5

R

Phonon energy [meV]

3.0

⊥

2.5

Fig. 23. Phonon dispersion curves for a Xe monolayer on Cu(100) along the [100] substrate direction (after [97G]). Notations used: R: Cu(100) surface Rayleigh wave; ⊥ and SH: perpendicular and shear horizontal Xe adlayer modes, respectively. The SH mode was first explained by assigning it to the longitudinal branch [97G,99S2], which required a large softening of the inplane Xe-Xe force constant. Later it was suggested [97B1,00B] that it should be assigned to the SHpolarized mode.

2.0 SH

1.5 1.0 0.5 0

0.2

0.4 0.6 –1 Wave vector q [Å ]

0.8

1.0

Lando lt -Börnst ein New Ser ies III/42A1

3.1.1 Nobel gases on metals and semiconductors

References for 3.1.1 59E 62R 70D 70E 71C 71P 72I 73C 74D 74N1 74N2 75P 75W 76M 76R 76T 77K 78H 79B 79E 79G 79K1 79K2 79K3 79U 79W 80B 80C 80H 80K 80W1 80W2 81G 81M 81O 81U 81W 82C1 82C2 82H 82J1 82J2 82L 82M1 82M2 82U 83C 83D1

Ehrlich, G., Hudda, F.G.: J. Chem. Phys. 30 (1959) 493. Rootsaert, W.J.M., van Reijen, L.L., Sachtler, W.M.H.: J. Catalysis 1 (1962) 416. Dickey, J.M., Farrell, H.H., Strongin, M.: Surf. Sci. 23 (1970) 448. Engel, T, Gomer, R.: J. Chem. Phys. 52 (1970) 5572. Chesters, M.A., Pritchard, J.: Surf. Sci. 28 (1971) 460. Palmberg, P.W.: Surf. Sci. 25 (1971) 598. Ignatiev, A., Jones, A.V., Rhodin, T.N.: Surf. Sci. 30 (1972) 573. Chesters, M.A., Hussain, M., Pritchard, J.: Surf. Sci. 35 (1973) 161. Dresser, M.J., Madey, T.E., Yates jr., J.T.: Surf. Sci. 42 (1974) 533. Nieuwenhuys, B.E., Meijer, D.Th., Sachtler, W.M.H.: Phys. Status Solidi A 24 (1974) 115. Nieuwenhuys, B.E., Sachtler, W.M.H.: Surf. Sci. 45 (1974) 513. Papp, H., Pritchard, J.: Surf. Sci. 53 (1975) 371. Waclawski, B.J., Herbst, J.F.: Phys. Rev. Lett. 35 (1975) 1594. McElhiney, G., Pritchard, J.: Surf. Sci. 60 (1976) 397. Roberts, R.H., Pritchard, J.: Surf. Sci. 54 (1976) 687. Tapping, R.L., Norton, P.R.: Chem. Phys. Lett. 41 (1976) 252. Klein, M.L., Venables, J.A. (eds.): Rare Gas Solids, Vol. II. London: Academic 1977. Horn, K., Schefffler, M., Bradshaw, A.M.: Phys. Rev. Lett. 41 (1978) 822. Bak, P., Mukamel, D., Villain, J., Wentowska, K.: Phys. Rev. B 19 (1979) 1610. Engel, T., Bornemann, P., Bauer, E.: Surf. Sci. 81 (1979) 252. Glachant, A., Bardi, U.: Surf. Sci. 87 (1979) 187. Küppers, J., Nitschké, F., Wandelt, K., Ertl, G.: Surf. Sci. 87 (1979) 295. Küppers, J., Nitschké, F., Wandelt, K., Ertl, G.: Surf. Sci. 88 (1979) 1. Küppers, J., Michel, H., Nitschké, F., Wandelt, K., Ertl, G.: Surf. Sci. 89 (1979) 361. Unguris, J., Bruch, L.W., Moog, E.R., Webb, M.B.: Surf. Sci. 87 (1979) 415. Wang, C., Gomer, R.: Surf. Sci. 84 (1979) 329. Bardi, U., Glachant, A., Bienfait, M.: Surf. Sci. 97 (1980) 137. Chen, J.-R., Gomer, R.: Surf. Sci. 94 (1980) 456. Hermann, K., Noffke, J., Horn, K.: Phys. Rev B 22 (1980) 1022. Kaindl, G., Chiang, T.-C., Eastman, D.E., Himpsel, F.J.: Phys. Rev. Lett. 45 (1980) 1808. Wandelt, K., Hulse, J., Küppers, J., Ertl, G.: Proc. IV ICSS and III ECOSS (Le Vide, Les Couches Minces, 1980), Vol. 1, p. 104. Wang, C., Gomer, R.: Surf. Sci. 91 (1980) 533. Glachant, A., Jaubert, M., Bienfait, M., Boato, G.: Surf. Sci. 115 (1981) 219. Mason, B.F., Williams, B.R.: Phys. Rev. Lett. 46 (1981) 1138; Surf. Sci. 111 (1981) 609. Opila, R., Gomer, R.: Surf. Sci. 112 (1981) 1. Unguris, J., Bruch, L.W., Moog, E.R., Webb, M.B.: Surf. Sci. 109 (1981) 522. Wandelt, K., Hulse, J, Küppers, J.: Surf. Sci. 104 (1981) 212. Chiang, T.C., Kaindl, G., Eastman, D.E.: Solid State Commun. 41 (1982) 661. Christmann, K., Demuth, J.E.: Surf. Sci. 120 (1982) 291. Horn, K., Mariani, C., Cramer, L.: Surf. Sci. 117 (1982) 376. Jacobi, K., Hsu, Y.-P., Rotermund, H.H.: Surf. Sci. 114 (1982) 683. Jacobi, K., Rotermund, H.H.: Surf. Sci. 116 (1982) 453. Lang, N.D., Williams, A.R., Himpsel, F.J., Reihl, B., Eastman, D.E.: Phys. Rev. B 26 (1982) 1728. Mariani, C., Horn, K., Bradshaw, A.M.: Phys. Rev. B 25 (1982) 7798. Mason, B.F., Caudano, R., Williams, B.R.: J. Chem. Phys. 77 (1982) 562. Unguris, J., Bruch, L.W., Webb, M.B., Phillips, J.M.: Surf. Sci. 114 (1982) 219. Conrad, E., Webb, M.B.: Surf. Sci. 129 (1983) 37. Daiser, S., Wandelt. K.: Surf. Sci. 128 (1983) L213.

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111

112 83D2 83M1 83M2 83K 83O 83P 84A 84D 84G 84M1 84M2 84M3 84W1 84W2 84W3 85H 85J 85M 85P 85S 85T 86B 86E 86J 86K1 86K2 86K3 86M 86S 87F 87G 87K1 87K2 87K3 87M 88C 88D 88G1 88G2 88J 88K1 88K2 88M 88P 88S 88Q

3.1.1 Nobel gases on metals and semiconductors Dolle, P., Alnot, M., Ehrhardt, J.J., Thomy, A., Cassuto, A.: C. R. Acad. Sci. (Paris) 296 (1983) 529. Mason, B.F., Williams, B.R.: Surf. Sci. 130 (1983) 295. Miranda, R., Daiser, S., Wandelt, K., Ertl, G.: Surf. Sci. 131 (1983) 61. Küppers, J., Seip, U.: Appl. Surf. Sci. 16 (1983) 313. Okano, T.: Jpn. J. Appl. Phys. 22 (1983) 1496. Poelsema, B., Verheij, L.K., Comsa, G.: Phys. Rev. Lett. 51 (1983) 2410. Annett, J.F., Haydock, R.: Phys. Rev. Lett. 53 (1984) 838. Demuth, J.E., Shell-Sorokin, A.J.: J. Vac. Sci. Technol. A 2 (1984) 808. Glachant, A., Bienfait, M., Jaubert, M.: Surf. Sci. 148 (1984) L665. Mason, B.F., Williams, B.R.: Surf. Sci. 139 (1984) 173. Miranda, R., Albano, V.E., Daiser, S., Wandelt, K., Ertl, G.: J. Chem. Phys. 80 (1984) 2931. Moog, E.R., Webb, M.B.: Surf. Sci. 148 (1984) 338. Wandelt, K.: J. Vac. Sci. Technol. A 2 (1984) 802. Wandelt, K., Gumhalter, B.: Surf. Sci. 140 (1984) 355. Wandelt, K., Hulse, J.E.: J. Chem. Phys. 80 (1984) 1340. Hall, B., Mills, D.L., Black, J.E.: Phys. Rev. B 32 (1985) 4932. Jablonski, A., Eder, S., Wandelt, K.: Appl. Surf. Sci. 22/23 (1985) 309. Mandel, T., Kaindl, G., Domke, M., Fischer, W., Schneider, W.D.: Phys. Rev. Lett. 55 (1985) 1638. Poelsema, B., Verheij, L.K., Comsa, G.: Surf. Sci. 152/153 (1985) 851. Schönhense, G., Eyers, A., Friess, U., Schäfers, F., Heinzmann, U.: Phys. Rev. Lett. 54 (1985) 547. Tully, J.D.: Faraday Discuss. Chem. Soc. 80 (1985) 291. Behm, R.J., Brundle, C.R., Wandelt, K.: J. Chem. Phys. 85 (1986) 1061. Eder, S., Markert, K., Jablonski, A., K., Wandelt, K.: Ber. Bunsenges. Phys. Chem. 90 (1986) 225. Jablonski, A., Eder, S., Markert, K., Wandelt, K.: J. Vac. Sci. Technol. A 4 (1986) 1510. Kern, K., David, R., Palmer, R.L., Comsa, G.: Phys. Rev. Lett. 56 (1986) 620. Kern, K., David, R., Palmer, R.L., Comsa, G.: Surf. Sci. 175 (1986) L669. Kern, K., Zeppenfeld, P., David, R., Palmer, R.L., Comsa, G.: Phys. Rev. Lett. 57 (1986) 3187. Mason, B.F., Williams, B.R.: Surf. Sci. 177 (1986) L925. Schönhense, G.: Appl. Phys. A 41 (1986) 39. Frank, K.H., Horn, K., Wilder, J., Koch, E.E.: Appl. Phys. A 44 (1987) 97. Greiser, N., Held, G.A., Frahm, R., Greene, R.L., Horn, P.M., Suter, R.M.: Phys. Rev. Lett. 59 (1987) 1706. Kern, K.: Phys. Rev. B 35 (1987) 8265. Kern, K., David, R., Zeppenfeld, P., Palmer, R.L., Comsa, G.: Solid State Commun. 62 (1987) 391. Kern, K., Zeppenfeld, P., David, R., Comsa, G.: Phys. Rev. Lett. 59 (1987) 79. Menzel, D.: Kinetics of Interface Reactions, Springer Series in Surface Sciences Vol. 8. Berlin: Springer Verlag 1987, p. 1. Cassuto, A., Ehrhardt, J.J., Cousty, J., Riwan, R.: Surf. Sci. 194 (1988) 579. Ducros, R., Ehrhardt, J.J., Fusy, J., Mutafschiev, B.: Phys. Rev. B 38 (1988) 10035. Gibson, K.D., Sibener, S.J.: J. Chem. Phys. 88 (1988) 7862. Gibson, K.D., Sibener, S.J.: J. Chem. Phys. 88 (1988) 7893. Jacobi, K.: Phys. Rev. B 38 (1988) 5869. Kern, K., David, R., Zeppenfeld, P., Comsa, G.: Surf. Sci. 195 (1988) 353. Kern, K., Zeppenfeld, P., David, R., Comsa, G.: J. Vac. Sci. Technol. A 6 (1988) 639. Mason, B.F., Williams, B.R.: Z. Phys. D 10 (1988) 369. Packard, W.E., Webb, M.B.: Surf. Sci. 195 (1988) 371. Schlichting, H., Menzel, D., Brunner, T, Brenig, W., Tully, J.C.: Phys. Rev. Lett. 60 (1988) 2515. Qian, X., Betz, M.: Phys. Rev. Lett. 61 (1988) 1497. Lando lt Bö rnst ein New Ser ies III/42A1

3.1.1 Nobel gases on metals and semiconductors 89A 89B 89F 89H 89K 89M 90A 90B 90C 90G 90H 90J 90M 90R1 90R2 90S1 90S2 90W 90Z1 90Z2 91A 91B 91C1 91C2 91E 91K 91H 91R 91S 91V1 91V2 91W 92F 92H1 92H2 92M 92S1 92S2 92S3 92W 92Z 93F

113

Alnot, A., Ehrhardt, J.J., Barnard, J.A.: Surf. Sci. 208 (1989) 285. Berndt, W.: Surf. Sci. 219 (1989) 161. Fargues, D., Dolle, P., Alnot, M., Ehrhardt, J.J: Surf. Sci. 214 (1989) 187. Hall, B., Mills, D.L., Zeppenfeld, P., Kern, K., Becher, U., Comsa, G.: Phys. Rev. B 40 (1989) 6326. Komeda, T., Sakisaka, Y., Onchi, M., Kato, H., Suzuki, S., Edamoto, K., Aiura, Y.: Phys. Rev. B 40 (1989) 3344. Markert, K., Pervan, P., Heichler, W., Wandelt, K.: Surf. Sci. 211/212 (1989) 611. Arumainayagam, Ch., Madix, R.J., McMaster, M.C., Suzawa, V.M., Tully, J.C.: Surf. Sci. 226 (1990) 180. Bethune, D.S., Barker, J.A., Rettner, C.T.: J. Chem. Phys. 92 (1990) 6847. Castro, G.R., Isern, H., Schneider, U., Stöcker, M., Wandelt, K.: Vacuum 41 (1990) 393. Gottlieb, J.M.: Phys. Rev. B 42 (1990) 5377. Hinch, B.J., Lock, A., Madden, H.H., Toennies, J.P., Witte, G.: J. Electron. Spectrosc. 51 (1990) 673. Jupille, J., Ehrhardt, J.-J., Fargues, D., Cassuto, A.: Faraday Discuss. Chem. Soc. 89 (1990) 323. Müller, J.E.: Phys. Rev. Lett. 65 (1990) 3021. Rettner, C.T., Bethune, D.S., Schweizer, E.K.: J. Chem. Phys. 92 (1990) 1442. Rettner, C.T., Mullins, C.B., Bethune, D.S., Auerbach, D.J., Schweizer, E.K., Weinberg, W.H.: J. Vac. Sci. Technol. A 8 (1990) 2699. Schlichting, H.: Dissertation, Technical University Munich (1990). Siddiqui, H.R., Chen, P.J., Guo, X., Yates, J.T.: J. Chem. Phys. 92 (1990) 7690. Wandelt, K.: Chemistry and Physics of Solid Surfaces VIII, Springer Series in Surface Sciences Vol. 22. Berlin: Springer Verlag 1990, p. 289. Zeppenfeld, P., Becher, U., Kern, K., Comsa, G.: J. Electron Spectrosc. 54/55 (1990) 265. Zeppenfeld, P., Becher, U., Kern, K., David, R., Comsa, G.: Phys. Rev. B 41 (1990) 8549. Arumainayagam, Ch., Stinnett, J.A., McMaster, M.C., Madix, R.J.: J. Chem. Phys. 95 (1991) 5437. Barker, J.A., Rettner, C.T., Bethune, D.S.: Chem. Phys. Lett. 188 (1991) 471. Castro, G.R., Isern, H., Schneider, U., Stöcker, M., Wandelt, K.: J. Vac. Sci. Technol. A 9 (1991) 1676. Cathrine, B., Fargues, D., Alnot, M., Ehrhardt, J.J.: Surf. Sci. 259 (1989) 162. Eigler, D.M., Weiss, P.S., Schweizer, E.K., Lang, N.D.: Phys. Rev. Lett. 66 (1991) 1189. Kern, K., Comsa, G.: Phase Transitions in Surface Films, NATO ASI Series 261. New York: Plenum Press, 1991, p. 41. Head-Gordon, M., Tully, J.C., Schlichting, H., Menzel, D.: J. Chem. Phys. 95 (1991) 9266. Rettner, C.T., Ashfold, M.N.R. (eds.): Dynamics of Gas-Surface Interactions, Advances in GasPhase Photochemistry and Kinetics. Cambridge: The Royal Society of Chemistry, 1991. Schneider, U., Castro, G.R., Isern, H., Janssens, T., Wandelt, K.: Surf. Sci. 251/252 (1991) 551. Vidali, G., Ihm, G., Kim, H.-Y., Cole, M.W.: Surf. Sci. Rep. 12 (1991) 133. Vogt, B., Kessler, B., Müller, N., Schönhense, G., Schmiedeskamp, B., Heinzmann, U.: Phys. Rev. Lett. 67 (1991) 1318. Weiss, W., Umbach, E.: Surf. Sci. 249 (1991) L333. Fargues, D., Dolle, P., Ehrhardt, J.J., Sotto, M., Bouillard, J.C.: Surf. Sci. 274 (1992) L494. Haugstad, G., Raisanen, A., Yu, X., Vanzetti, L., Franciosi, A.: Phys. Rev. B 46 (1992) 4102. Hilgers, G.: Dissertation, University Bielefeld, 1992. Michel, E.G., Pervan, P., Castro, G.R., Miranda, R., Wandelt, K.: Phys. Rev. B 45 (1992) 11811. Schlichting, H., Menzel, D.: Surf. Sci. 272 (1992) 27. Schlichting, H., Menzel, D., Brunner, T, Brenig, W.: J. Chem. Phys. 97 (1992) 4453. Schneider, U., Castro, G.R., Busse, H., Janssens, T., Wesemann, J., Wandelt, K.: Surf. Sci. 269/270 (1992) 316. Weiss, P.S., Eigler, D.M.: Phys. Rev. Lett. 69 (1992) 2240. Zeppenfeld, P., Becher, U., Kern, K., Comsa, G.: Phys. Rev. B 45 (1992) 5179. Friess, W., Steinacker, E., Brunner, T., Menzel, D.: Surf. Sci. 298 (1993) L173.

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114 93G 93H 93M1 93M2 93M3 93R 93S1 93S2 93S3 93S4 94J 94R 94S 94Z1 94Z2 94Z3 95H1 95H2 95P1 95P2 95S 95W 96B 96O 96P 96S 96W 96Z 97B1 97B2 97G 97L 97N 97P1 97P2 97R 97T 97W 98B

3.1.1 Nobel gases on metals and semiconductors Getzlaff, M., Bansmann, J., Schönhense, G.: Phys. Rev. Lett. 71 (1993) 793. Hilgers, G., Potthoff, M., Müller, N., Heinzmann, U.: Surf. Sci. 287/288 (1993) 414. Meixner, D.L., George, S.: J. Chem. Phys. 98 (1993) 9115. Meixner, D.L., George, S.: Surf. Sci. 297 (1993) 27. Menzel, D., Brenig, W., Brunner, T., Frieß, W., Schlichting, H.: J. Electron Spectrosc. 64/65 (1993) 583. Rieder, K.H., Parschau, G., Burg, B.: Phys. Rev. Lett. 71 (1993) 1059. Schlichting, H., Menzel, D.: Rev. Sci. Instrum. 64 (1993) 2013. Schwennicke, C., Schimmelpfennig, J., Pfnür, H.: Phys. Rev. B 48 (1993) 8928. Schroeder, S., McMaster, M.C., Stinnett, J.A., Madix, R.J.: Surf. Sci. 297 (1993) L148. Soszka, W.: Surf. Sci. 287/288 (1993) 69. Janssens, T.V.W., Castro, G.R., Wandelt, K., Niemantsverdriet, J.W.: Phys. Rev. B 49 (1994) 14599. Ramseyer, C., Girardet, C., Zeppenfeld, P., Goerge, J., Büchel, M., Comsa, G.: Surf. Sci. 313 (1994) 251. Schneider, U., Busse, H., Linke, R., Castro, G.R., Wandelt, K.: J. Vac. Sci. Technol. A 12 (1994) 2069. Zeppenfeld, P., Büchel, M., David, R., Comsa, G., Ramseyer, C., Girardet, C.: Phys. Rev. B 50 (1994) 14667. Zeppenfeld, P., Goerge, J., Büchel, M., David, R., Comsa, G.: Surf. Sci. 318 (1994) L1187. Zeppenfeld, P., Horch, S., Comsa, G.: Phys. Rev. Lett. 73 (1994) 1259. Hilgers, G., Potthoff, M., Müller, N., Heinzmann, U.: Surf. Sci. 322 (1995) 207. Horch, S., Zeppenfeld, P., Comsa, G.: Surf. Sci. 331-333 (1995) 908. Paniago, R., Matzdorf, R., Meister, G., Goldmann, A.: Surf. Sci. 325 (1995) 336. Potthoff, M., Hilgers, G., Müller, N., Heinzmann, U., Hauner, L., Braun, J., Borstel, G.: Surf. Sci. 322 (1995) 193. Schmitz-Hübsch, T., Oster, K., Radnik, J., Wandelt, K.: Phys. Rev. Lett. 74 (1995) 2595. Weinelt, M., Trischberger, P., Widdra, W., Eberle, K., Zebisch, P., Gokhale, S., Menzel, D., Henk, J., Feder, R., Dröge, H., Steinrück, H.-P.: Phys. Rev. B 52 (1995) R17048. Bertel, E.: Surf. Sci. 367 (1996) L61. Ostertag, Ch., Oelsner, A., Schicketanz, M., Schmidt, O., Fecher, G.H., Schönhense, G.: Surf. Sci. 352-354 (1996) 179. Petersen, M., Wilke, S., Ruggerone, P., Kohler, B., Scheffler, M.: Phys. Rev. Lett. 76 (1996) 995. Stichler, M., Zebisch, P., Weinelt, M., Steinrück, H.-P.: Surf. Sci. 348 (1996) 370. Wong, A., Zhu, X.D.: Appl. Phys. A 63 (1996) 1. Zeppenfeld, P., Büchel, M., Goerge, J., David, R., Comsa, G., Ramseyer, Ch., Girardet, C.: Surf. Sci. 366 (1996) 1. Bruch, L.W.: J. Chem. Phys. 107 (1997) 4443. Bruch, L.W., Cole, M.W., Zaremba, E.: Physical Adsorption: Forces and Phenomena, Oxford: Clarendon Press, 1997. Graham, A.P., Bertino, M.F., Hofmann, F., Toennies, J.P., Wöll, Ch.: J. Chem. Phys. 106 (1997) 6194. Leatherman, G.S., Diehl, R.D., Karimi, M., Vidali, G.: Phys. Rev. B 56 (1997) 6970. Narloch, B., Menzel, D.: Chem. Phys. Lett. 270 (1997) 163. Pervan, P., Markert, K., Wandelt, K.: Appl. Surf. Sci. 108 (1997) 307. Pouthier, V., Ramseyer, C., Girardet, C., Kuhnke, K., Marsico, V., Blanc, M., Schuster, R., Kern, K.: Phys. Rev. B 56 (1997) 4211. Ramseyer, C., Pouthier, V., Girardet, C., Zeppenfeld, P., Büchel, M., Diercks, V., Comsa, G.: Phys. Rev. B 55 (1997) 13203. Trischberger, P., Dröge, H., Gokhale, S., Henk, J., Steinrück, H.-P., Widdra, W., Menzel, D.: Surf. Sci. 377-379 (1997) 155. Wandelt, K.: Appl. Surf. Sci. 111 (1997) 1. Braun, J., Fuhrmann, D., Siber, A., Gumhalter, B., Wöll, Ch.: Phys. Rev. Lett. 80 (1998) 125. Lando lt Bö rnst ein New Ser ies III/42A1

3.1.1 Nobel gases on metals and semiconductors 98I 98N 98P 98S 98W 99E 99G1 99G2 99P 99S1 99S2 99W 00B 00N1 00N2 00S 00W

115

Igarashi, S., Abe, Y., Irie, Y., Hirayama, T., Arakawa, I.: J. Vac. Sci. Technol. A 16 (1998) 974. Narloch, B., Menzel, D.: Surf. Sci. 412-413 (1998) 562. Pouthier, V., Ramseyer, C., Girardet, C., Diercks, V., Halmer, R., David, R., Zeppenfeld, P.: Phys. Rev. B 57 (1998) 13149. Seyller, Th., Caragiu, M., Diehl, R.D., Kaukasoina, P., Lindroos, M.: Chem. Phys. Lett. 291 (1998) 567. Widdra, W., Trischberger, P., Frieß, W., Menzel, D., Payne, S.H., Kreuzer, H.J.: Phys. Rev. B 57 (1998) 4111. Ellis, J., Graham, A.P., Toennies, J.P.: Phys. Rev. Lett. 82 (1999) 5072. Graham, A.P., Toennies, J.P.: Surf. Sci. 427-428 (1999) 1. Grüne, M., Pelzer, T., Wandelt, K., Steinberger, I.T.: J. Electron Spectrosc. 98-99 (1999) 121. Persson, B.N.J.: Surf. Sci. Rep. 33 (1999) 83. Seyller, Th., Caragiu, M., Diehl, R.D., Kaukasoina, P., Lindroos, M.: Phys. Rev. B 60 (1999) 11084. Siber, A., Gumhalter, B., Braun, J., Graham, A.P., Bertino, M.F., Toennies, J.P., Fuhrmann, D., Wöll, Ch.: Phys. Rev. B 59 (1998) 5898. Widdra, W., Trischberger, P., Henk, J.: Phys. Rev. B 60 (1999) R5161. Bruch, L.W., Graham, A.P., Toennies, J.P.: J. Chem. Phys. 112 (2000) 3314. Nabighian, E., Zhu, X.D.: Chem. Phys. Lett. 316 (2000) 177. Narloch, B.: Dissertation, Technical University Munich (1996) and in preparation. Seyller, Th., Caragiu, M., Diehl, R.D.: Surf. Sci. 454-456 (2000) 55. Widdra, W., Fink, A., Menzel, D.: Appl. Phys. A, in preparation.

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3.1.2 Noble gases on graphite, lamellar halides, MgO and NaCl M. BIENFAIT The experimental studies of the adsorption of monolayers of rare gases on graphite, BN, MgO and lamellar halides have been very active during the last 30 years. These adsorbed monolayers were thought to be physical realizations of two-dimensional matter. Experimentalists wanted to determine to which extend the physical properties and the phase transitions in two dimensions were comparable to their bulk counterparts. The obtained results could be tested against theoretical predictions on the influence of reduced dimensionality on the physical behaviour of matter. In addition, the influence of the surface potential on the structure, dynamics and phase transitions could be comprehensily studied. A lot of results have been obtained so far and the major part of them have been reviewed or collected in proceedings [1-10]. This chapter reviews the thermodynamics, the structure, the thermal vibrations and the kinetics of adsorption of the noble gases condensed in the monolayer range on the basal planes of graphite, BN and lamellar halides, and on the (100) surface of MgO and NaCl.

3.1.2.1 Thermodynamics The measurements have been performed on uniform powders by adsorption volumetry [6], or calorimetry [1], or on single crystal surfaces by Auger spectroscopy [11], LEED [9,12] and ellipsometry [13]. 3.1.2.1.1 Rare gases adsorbed on graphite (0001) 3.1.2.1.1.1 Xenon on graphite The two-dimensional phase diagram of the monolayer of Xe adsorbed on graphite is displayed in Figs. 1 [6,8,14] and 2 [11,15,16] in two usual representations, coverage θ vs. pressure and pressure vs. T−1. Fig. 2 extends the data of Fig. 1 to lower pressures and temperatures and allows the presentation of the second layer formation and of the bulk sublimation curve, as well. The phase diagram exhibits 2D gas, liquid and solid phase coexistence, i.e. first order phase transitions between gas ↔ solid, gas ↔ liquid, and liquid ↔ solid. The corresponding triple point Tt and critical temperature Tc are Tt = 99 ± 1K and Tc = 117 ± 1K, respectively (see Table I). The liquid-solid phase transition study has been extended to higher temperatures [17-21]. There is still a debate to know whether the first order transition becomes continuous above a tricritical temperature located between 125 and 150K. From the Arrhenius plots (Fig. 2), the isosteric heats of adsorption can be measured. Their values can be found in Ref. 14 and 15 and in Table II. 3.1.2.1.1.2 Krypton on graphite Many papers and discussions [14,22-36] were devoted to determination of the phase diagram of the first monolayer of Kr adsorbed on graphite. It is now well accepted that there is no liquid-gas coexistence domain in this diagram. The surface corrugation of graphite with respect to krypton has such a large effect that it stabilizes the 2D solid to high temperatures and does not allow any clear distinction between gas and liquid. It is said that the system has an incipient triple point. The phase diagram is represented in Figs. 3 and 4. It also exhibits a tricritical point at about 115K. The surface corrugation also induces structural effect like the commensurate-incommensurate transition and rotational epitaxy (see paragraph II). The values of the isosteric heats of adsorption for this system can be found in Ref. 14 and in Table II.

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3.1.2.1.1.3 Argon on graphite The phase diagram of the adsorbed monolayer of Ar/graphite is displayed in Fig. 5 and 6 in the coverage temperature and log p – T−1 variables respectively. It has been drawn from the combinaison of thermodynamics and scattering experiments [37-47] ; it has 2D gas, liquid and incommensurate hexagonal solid phases. There is a triple point at Tt = 49.7K according to Ref. 45 or at 47K according to Ref. 42, and a critical point at Tc = 59 or 55K according to Ref. 22 and 42, respectively. The melting is either weakly first order [42] or continuous [44] ; the liquid phase is a strongly correlated fluid close to the melting temperature ; it becomes an isotropic liquid at higher T [46].

3.1.2.1.1.4 Neon on graphite A phase diagram collecting all the thermodynamic information on the neon/graphite system [48-57] is shown in Fig. 7. Coexistence in the monolayer range between liquid-solid, liquid-gas and solid-gas has been observed ; the triple point Tt is 13.6K and the critical temperature Tc is 15.8K. 3.1.2.1.1.5 Rare gas mixtures on graphite A few studies have been devoted to the co-adsorption of different rare gases on graphite. They are reported in paragraph II.5 below.

3.1.2.1.1.6 Helium on graphite The monolayer adsorption of 4He and 3He on graphite has been reviewed comprehensibly in several monographs [1,10].

3.1.2.1.2 Rare gases on BN The hexagonal boron nitride has been used as a substrate for the adsorption of Kr and Xe. The surface of its basal plane displays the same hexagonal structure than graphite with a lattice parameter difference of 2%. The published data are scarce [58,59] ; they were obtained by volumetric adsorption isotherm measurements. The critical temperatures for Kr and Xe and the triple point for Xe are reported in Table I. The heat of adsorption can be found in Ref. 59 and in Table II.

3.1.2.1.3 Rare gases on lamellar halide surfaces One of the major questions raised in the study of two-dimensional phase transitions in adsorbed layers was to check the validity of the corresponding state law for van der Waals, i.e. rare gases interactions. This law is fairly well obeyed for rare gases adsorbed on graphite [22]. It was tempting to determine the influence on the reduced 2D critical temperatures of the corrugation potential and of the lattice parameter of the surface, as well. A set of measurements have been carried out on the (111) surface of lamellar halides like CoCl2, CoBr2, FeCl2, NiCl2, CdCl2 [60-63]. They are reported in Fig. 8. The results show that the localization of the inert gases on the substrate potential wells (dimensional incompatibility zero) stabilizes the layer above the theoretical reduced critical temperature T2c/T3c = 0.38. Conversely the corresponding state law is well obeyed when the lattice misfit is large or small. In that case, the rare gas atoms can experience an "average" interaction potential.

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3.1.2.1.4 Rare gases on MgO(100) and NaCl(100) A major stimulus to recent studies of physical adsorption on ionic crystals has been the desire to examine the frustration resulting from the competition between the square symmetry of their (100) surface and the natural hexagonal packing of the rare gases ; new universality classes could be accessible for the square two-dimensional lattice [64]. The phase diagram of the first layer of Xe, Kr and Ar adsorbed on MgO(100) has been measured by adsorption volumetry [64,65]. Phase coexistence has been observed and the corresponding triple points and critical temperatures have been determined. They are collected in Table I. The phase diagram are represented in Fig. 9, 10, 11 for Xe, Kr and Ar/MgO, respectively. The heat of adsorption can be found in Ref. 64-68 and in Table II. A first order two-dimensional gas

→ ←

solid phase transition has also been observed in the monolayer of

the Xe/NaCl(100) system [69].

3.1.2.2 Structure Physical adsorption includes a range of phenomena in the monolayer regime which arise from the competition between the adsorbate-adsorbate and adsorbate-substrate interactions. Depending on the relative strength of these interactions and on temperature and chemical potential, the rare gases structure can be either commensurate or incommensurate with the graphite substrate. The incommensurate solid can be a floating 2D crystal, or a striped or hexagonal phase ; it can exhibit either static distortion waves and orientational epitaxy at small misfit or domain-walls at larger dimensional incompatibility. There are examples where the domain-walls are mobile ; they stabilize a domain-wall fluid, also called a reentrant fluid. At high temperature, the substrate induces strong correlation in the adsorbate fluid. More details on the theory of these fascinating surface phase can be found, for instance, in Ref. 10 (see also "Zeppenfeld" for the incommensurate-commensurate transition in rare gas layers adsorbed on metals).

3.1.2.2.1 Xenon on graphite The various hexagonal structures measured by different diffraction techniques [16,20,34,70-79] have been reviewed in Ref. 10 (p. 218-220). In brief, the monolayer of Xe/graphite condenses in the major part of the solid phase diagram in a hexagonal incommensurate structure exhibiting an orientational alignment depending on lattice misfit (Fig. 12) ; the commensurate ¥[¥ ODWWLFH LV UHDFKHG MXVW EHIRUH WKH VHFRQG layer condensation. At melting, the dense fluid exhibits strong orientational and positional correlations.

3.1.2.2.2 Krypton on graphite The structure of the adsorbed monolayer is dominated by effects of the substrate corrugation which imposes a commensurate solid in a large part of the phase diagram. A commensurate-incommensurate transition is observed ; it is associated at low and medium temperature to an orientational epitaxy of the layer (see Fig. 12). At higher temperature, the commensurate and incommensurate regions are separated by a domain-wall fluid [25-30, 80-81].

3.1.2.2.3 Argon on graphite The monolayer of solid argon is hexagonal incommensurate. Its lattice parameter varies with temperature and coverage ; furthermore, the layer exhibits an epitaxial alignment which depends on lattice misfit as represented in Fig. 12. This was the first example in which the predicted Novaco-McTague orientational alignment [83] was observed for the first time [47]. Just above the monolayer melting, the liquid is a strongly correlated fluid [41,43-45,47,82]. Landolt-Börnstein New Series III/42A1

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3.1.2.2.4 Neon on graphite The solid layer at low temperature has a ¥[¥ 5 KLJKHU RUGHU an important (12-13°) orientational alignment is observed [54-56].

VXSHUODWWLFH $W KLJKHU WHPSHUDWXUH

3.1.2.2.5 Rare gas mixtures on graphite The introduction of Xe in a layer of Kr stabilizes the ¥[¥ FRPPHQVXUDWH VWUXFWXUH >@ 7KH DQDO\VLV RI the corresponding binary phase diagram shows a first order transition between the commensurate and incommensurate solids [85,86]. Mixtures of Ar and Xe exhibit a large tendency to form commensurate alloys [87]. A first order commensurate-incommensurate phase transition is also observed in that case. Although outside the scope of this review, interesting features have been found in the study of mixed layers of one rare gas and another gas. For instance, Ar + CO [88,89] and Ar + N2 [89] mixtures exhibit pinwheel and herringbone structures; on the other hand, displacement transitions have been observed in Kr + SF6 [90], Xe + SF6 [91], Kr + C6H12 [92,93,95], Kr + CCl4 [94,95], Kr + CH4 [96], Xe + CH2Cl2 [97], Kr + CH2Cl2 [98] mixtures. 3.1.2.2.6 Rare gas on lamellar halides A few diffraction studies have been performed on the rare gas monolayers adsorbed on graphite ; they are devoted to the Kr/NiCl2 and Xe/NiCl2 systems [62]. The observed structure is hexagonal incommensurate.

3.1.2.2.7 Rare gases on MgO(100) and NaCl(100) The argon and krypton monolayers adsorbed on MgO(100) exhibit typical frustration phenomenon induced by the troughs along the [110] Mg ions rows on the MgO surface [67]. This frustration leads for argon to a 2x3 lattice which evolves to a pseudo-hexagonal incommensurate structure at monolayer completion [65-67]. An interesting feature is observed for the 2x3 structure when its temperature is raised; a unidirectional order-disorder transition is measured both by neutron diffraction [65] and quasielastic neutron scattering [99]. The krypton layer displays a coexistence between a 2xn and a hexagonal incommensurate phase [67,68,100]. The solid layer of Xe/NaCl(100) is hexagonal incommensurate [101].

3.1.2.3 Influence of structural defects 3.1.2.3.1 Thermodynamics and phase transitions The shape of adsorption isotherms measured on lamellar halides [102], on graphite [103] and on NaCl [101] strongly depends on surface homogeneity. Poor substrates exhibit broad distributions of adsorption energies whereas uniform surfaces allow the observation of step wise isotherms. The steps are clear signatures of first order two-dimensional phase transitions [6,8,14].

3.1.2.3.2 Structure Few adsorption studies have been devoted to surface with controlled defects. A typical example relates the influence of atomic steps on the adsorbate structure [101]. Vicinal (610) NaCl surfaces have been used as a substrate. Heterogeneities do not modify the lateral structure of the two-dimensional solid xenon which remains hexagonal close-packed, but they reduce the size of the two-dimensional crystallites.

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3.1.2.4 Vibrations Only a few lattice dynamics studies have been carried out on rare gases adsorbed on lamellar compounds or on MgO. To our best knowledge, the only adsorbate studied so far was Ar. The experiments took advantage of the large neutron cross section of the 36Ar isotope. The phonon density of states of Ar adsorbed on graphite has been measured by inelastic neutron scattering [43,54]. Transverse and longitudinal in-plane modes and one out-of-plane mode were observed. The same kind of experiments was performed for the two structures (2x3 and hexagonal) of 36Ar adsorbed on MgO(100) [99]. The measured density of states is different for the two atomic arrangements as shown in Fig. 13 and 14 ; they correspond to in-plane and out-of-plane modes. The all set of experimental results has been used to test the accuracy of the interaction potentials [43,54,99].

3.1.2.5 Kinetics of adsorption and desorption Three regimes of condensation, previously observed in thermodynamics measurements (paragraph I.1.1), have been measured in the adsorption of a submonolayer of xenon adsorbed on graphite [104]. The first one corresponds to the condensation or evaporation of a two-dimensional (2D) "gas", the second one to the growth of 2D crystal in the presence of the 2D gas, and the third one to the completion of the 2D crystal on the (0001) graphite face. Zero order kinetics for both adsorption and desorption is found in the large range of coverage (0.3 < 0 < 0.9) where the two phases coexist on the surface. The activation energy of desorption of the 2D crystal is measured ; its value (~ 6 kcal mole−1) is in fair agreement with the value of the latent heat of evaporation of this phase (5.5 or 5.7 kcal mole−1) determined previously. The interpretation of this zeroth order surface kinetics can be understood as due to the coexistence of twodimensional gas and islands and to a very rapid exchange between the atoms in the gas and those of the island edges [105,106]. Table I. Twodimensional triple points Tt and critical temperatures Tc of the first layer of noble gases adsorbed on graphite, BN and MgO.

Tt Tc

Tt Tc

Xe/graphite

Kr/graphite

Ar/graphite

Ne/graphite

99 ± 1 117 ± 1

incipient triple point

47 - 50K 55 - 59K

13.6 15.8

Xe/BN

Kr/BN

Xe/MgO

Kr/MgO

Ar/MgO

87

101 ± 1 119 ± 2

67 ± 1 87 ± 2

38 ± 2 63 ± 2

102 119

Table II Isosteric heat of adsorption in kcal mole−1 of rare gases adsorbed on graphite, BN and MgO in the monolayer range.

solid-gas liquid-gas

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Kr/BN

Xe/Mgo

5.5 5.1

3.4

3.67 (65) 2.74 (65) 2.0

(15) (14)

4.0

Kr/MgO

Ar/MgO

(59) 4.4

(59)

(59)

(65)

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Figures for 3.1.2

Xe/graphite

B1

1.0

D'1

D'1

L

Coverage θ

0.8

+S

S A'1

L

D1

0.6 G +S

0.4

G

0.2

8 10

–4

+L G

A1

0 –5 4⋅10 6

Fig. 1. Phase diagram showing the evolution of xenon adsorption isotherms between 97 and 117 K on a homogeneous graphite surface (from Ref. 6). G: twodimensional gas; L: twodimensional liquid; S: twodimensional solid.

A1 2

4

6

–3

8 10 2 4 6 Vapor pressure p [Torr]

–2 8 10

2

4

6

–1 810

–3

10

"liquid 2D" 1st layer

crystal 3D

–4

10

transition

1st layer

2nd layer

–5

Vapor pressure pXe [Torr]

10

crystal 2D 1st layer

–6

10

"gas 2D" 1st layer

–7

10

Fig. 2. Logarithm of the threedimensional pressure of xenon of the first and second layer adsorbed graphite vs. reciprocal temperature (from Ref. 15).

–8

10

–9

10

9

11

13 15 –1 –3 –1 Inv. temperature T [10 K ]

17

19

Landolt-Börnstein New Series III/42A1

3.1.2 Noble gases on graphite, lamellar halides, MgO and NaCl

2.00 1.75 1.50 Fractional coverage θ

2c

IC

A2

2b

1.25

A1

1.00

2a

C2

–16

C1

–17

B7

C

F

IC

L

–22

70

80

1.2

90 100 110 Temperature T [K]

120

130

140

–23 70

F

G 80

90

100 110 120 Temperature T [K]

130

140

150

Fig. 4. Reentrant fluid of Kr/graphite. High T region of the Kr/graphite phase diagram (from Ref. 30). The reentrant fluid (RF) phase is a domain wall fluid bounded by commensurate (C) and incommensurate (IC) regimes. S(23D) and L(3D) are the bulk solid and liquid phases. Solid lines denote first-order transitions and dashed lines denote continuous transitions.

S

1.0

Fractional coverage θ

RF

C

–21

Fig. 3. Phase diagram of Kr/graphite, in the monolayer regime, in terms of fractional coverage and temperature (from Ref. 24). C, F and IC denote commensurate, fluid, and incommensurate phases, respectively. Dashes indicate paths of specific heat scans. There is a tricritical point at the extrapolated intersection of the IC to C + F and C to F boundaries and a multicritical point at C1.

0.8 S+V

L+V

0.6 F

0.4 0.2 0 40

IC-A

–20

C+F

L(3D)

IC-R

–19

B6

0.25 0 60

S(3D)

–18

0.75 0.50

123

–15

2d

Chemical potential µ [102 K]

Ref. p. 128]

45

Landolt-Börnstein New Series III/42A1

50 55 60 Temperature T [K]

65

70

Fig. 5. Proposed phase diagram of Ar on graphite. S, L, V and F represent, respectively, solid, liquid, vapor and fluid phases. Solid circles near 55 K at submonolayer coverages correspond to positions of heat-capacity anomaly arising from liquid-vapor transition. Other circles are signatures of melting. Dashed lines are speculative. At submonolayer coverages, Ar solid melts via a weak first-order transition at a triple-point temperature (47.2 K) to a liquid-vapor coexistence region. The liquid phase appears to be orientationally ordered below 54 K. The positions of the broad anomalies centering near 49.5 K, due to the gradual decrease of this order, are not shown. Unless indicated otherwise, the uncertainty in the peak position of the heat-capacity anomalies is comparable to or smaller than the size of the circles (from Ref. 42).

124

3.1.2 Noble gases on graphite, lamellar halides, MgO and NaCl

2

10

[Ref. p. 128

0.55

two or more layers

10

0.50

d

c

–2

Crit. temperature ratio T2C / T3C

Vapor pressure p [Torr]

1

b a

–4

10

0.45

2D solid monolayer

2D gas

–6

10

3.78 Å

Ar Kr Xe

0.40

–8

10

12

16

20 24 28 –1 –3 –1 Inv. temperature T [10 K ]

0.20 – 0.25

32

4

10

Temperature T [K] 22 18 20

26 25 24 3D liquid

Tt

16

1.5

0.25

15

Neon/graphite

3D solid

Parameter: coverage in ml

2

10

– 0.05 0.0.5 0.15 Dimensional incompatibility

Fig. 8. 2D critical temperatures as a function of misfit. Ratio of 2D to 3D critical temperatures for Ar, Kr and Xe on various lamellar halide surfaces (from Ref. 63). The dimensional incompatibility is the excess above one of the ratio of the spacing between neighboring anions of the substrate and the interatomic spacing of the (111) plane of the bulk adsorbate.

Fig. 6. Phase diagram of Ar/graphite (from Ref. 41).

30 28

– 0.15

2D critical point in second layer

1.0

Vapor pressure p [Torr]

10 0.75 0.88

–2

10

1.0

1.5

0.5 0.813

–4

10

2D hypercritical fluid

2D rotated incommensurate solid

–6

10

0.767

2D –8

10

3.0

3.5

4.0

Tc = 15.80 K Tt = 13.57 K

4.5 5.0 5.5 6.0 –1 –2 –1 Inv. temperature T [10 K ]

6.5

7.0

Fig. 7. Phase diagram of Ne/graphite (from Ref. 56). Not shown are the ¥ RUGHUHG SKDVH DW lower T and the 2D critical and triple points, which occur at pressures that are too low to measure; the corresponding temperatures are indicated, however.

Landolt-Börnstein New Series III/42A1

Ref. p. 128]

3.1.2 Noble gases on graphite, lamellar halides, MgO and NaCl

2

10

60 40

Xe/MgO

20 6 4

bulk

2

fluid

18

Tc(2D)

6 4 2

sub vapor

–1

10

Vapor pressure p [Torr]

Vapor pressure p [Torr]

8 6 4

Kr/MgO

2

10

10 8

8 6 4

second solid

first

75

85 90 95 100 –1 –4 –1 Inv. temperature T [10 K ]

105

Fig. 9. Arrhenius plot diagram showing features for vapor pressure of 3D Xe. Various monolayer critical and triple points are shown (from Ref. 64).

1 (2D) dense phase A

10–1

bulk

2

1

8 6 4

fluid

Tc(2D)

sub solid

vapor first

10 –3 6⋅10 10

Tt(2D)

11

15

13 14 12 –1 –3 –1 Inv. temperature T [10 K ]

Fig. 10. P vs 1/T diagram for Kr/MgO and vapor pressure of 3D Kr. The monolayer critical and triple points and various adsorbed phases are indicated (from Ref. 64).

3.0°

Ar/MgO

2.5°

(2D) dense phase B

Ar

2.0°

Rotation angle ϕ

h.c.p. solid

10–2

second

2 –2

Tt(2D) 80

8 6 4

2 –1 10 8 6 4

–2

2⋅10

Vapor pressure p [Torr]

125

1.5°

–3

10

Kr

1.0°

10–4 10–5 14

Xe

(2D) gas 0.5°

16

18 20 22 24 –1 –3 –1 Inv. temperature T [10 K ]

26

28

Fig.11. Clapeyron lines representing the pressure dependence of the two (2d) transitions, (2d) gas ⇔ (2d) dense phase A and (2d) dense phase A ⇔ (2d) dense phase B, vs. 1/T. The intersection of the two lines defines the triple point temperature T2t ; T2t = 38 ± 2 K (neutron diffraction results show that the dense phases A and B are, respectively, a (2d) "liquid crystal" like phase and a h.c.p. (2d) solid) (from Ref. 65).

Landolt-Börnstein New Series III/42A1

0°

2

3

4

5 Misfit [%]

6

7

8

Fig. 12. Epitaxial orientation. Rotation angle as a function of fractional misfit for Ar, Kr and Xe on graphite (from Ref. 10). The angle shown is the increment from the 30° axis of the graphite. The misfits are negative for Ar and Kr and positive for Xe.

126

3.1.2 Noble gases on graphite, lamellar halides, MgO and NaCl

[Ref. p. 128

500 450 400

INS intensity [counts]

350 300 b

250 200 150 × 0.1

100 50 a

0 – 6.0

– 4.5

– 3.0

–1.5

0 1.5 Energy [meV]

3.0

4.5

6.0

Fig. 13. Inelastic neutron-scattering spectra of 36Ar/MgO(100) at 10 K. (a) Experimental spectra at incident energy of 7 meV for a 1.16 layer (hexagonal structure); the vertical bars and the triangle represent experimental errors and the experimental broadening (reduced by a factor of 1/10), respectively; (b) calculated spectrum for the hexagonal incommensurate structure after convoluting with the instrumental line shape of 0.3 meV at a neutron gain of 5 meV. Units are arbitrary and the baseline is shifted with respect to curve (a) (from Ref. 99).

Landolt-Börnstein New Series III/42A1

Ref. p. 128]

3.1.2 Noble gases on graphite, lamellar halides, MgO and NaCl

1000.0 937.5 875.0 812.5 750.0 687.5

INS intensity [counts]

625.0 562.5 c

500.0 437.5 375.0 312.5

b

250.0 187.5 125.0 62.5

a

× 0.03

0 – 6.0

– 4.5

– 3.0

–1.5

0 1.5 Energy E [meV]

3.0

4.5

6.0

Fig. 14. Inelastic neutron-scattering spectra of 36Ar/MgO(100) at 10 K. (a) Experimental spectra at incident energy of 7 meV for a 0.8 layer 2x3 structure; note the difference between the experimental errors (vertical bars) in the gain and the loss energy ranges; the triangle represents the experimental line shape reduced by a factor of 1/30. (b) Calculated spectrum for the 2x3 structure, in arbitrary units, after convoluting with the instrumental resolution of 0.3 meV at a neutron gain of 5 meV. (c) Calculated spectrum without broadening. The width of the peaks is due to the numerical step in the calculation of the dispersion branches. Baselines of curves (b) and (c) are shifted with respect to curve (a) (from Ref. 99).

Landolt-Börnstein New Series III/42A1

127

128

3.1.2 Noble gases on graphite, lamellar halides, MgO and NaCl

References for 3.1.2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Dash, J.G.: Films on Solid Surfaces. New York: Academic Press, 1975. Phases bidimensionnelles adsorbées. Bienfait, M., Suzanne, J. (eds.): J. Phys. (Paris) Colloq. 10 C4 (1977). Phase transitions in Surface Films, NATO-ASI-BSI. Dash, J.G., Ruvalds, J. (eds.), New York: Plenum, 1980. Bienfait, M., in: Current Topics in Materials Science, Vol. 4. Kaldis, E. (ed:), Amsterdam: North-Holland, 1980, p. 361. Proceedings of the Intern. Conf. on Ordering in Two Dimensions, Lake Geneva, Wisconsin. Sinha, S.K. (ed.), Amsterdam: North-Holland, 1980. Thomy, A., Duval, X., Regnier, J.: Surf. Sci. Rep. 1, 1 (1981). Phase transitions in Surface Films 2, NATO-ASI, Vol. 267. Taub, H., Torzo, G., Lauter, H.J., Fain, S. (eds.), New York: Plenum, 1991. Thomy, A., Duval, X.: Surf. Sci. 299/300 (1994) 415. Suzanne, J., Gay, J.M.: The structure of physically adsorbed phases in Handbook of Surface Science, Vol. 1, chap. 10. Unertl, W.N. (ed.), Elsevier, 1996, p. 503. Bruch, L.W., Cole, M.W., Zaremba, E.: Physical Adsorption: Forces and Phenomena, Oxford Science Publ. 1997. Suzanne, J., Coulomb, J.P., Bienfait, M.: Surf. Sci. 40 (1973) 414; 44 (1974) 141. Calisti, S., Suzanne, J., Venables, J.A.: Surf. Sci. 115 (1982) 455. Quentel, G., Rickard, J.M., Kern, R.: Surf. Sci. 50 (1975) 343. Thomy, A., Duval, X.J.: Chim. Phys. 67 (1970) 1101. Suzanne, J., Coulomb, J.P., Bienfait, M.: Surf. Sci. 47 (1975) 204. Hamichi, M., Faisal, A.Q.D., Venables, J.A., Kariotis, R.: Phys. Rev. B 39 (1989) 415. Colella, N.J., Suter, R.M.: Phys. Rev. B 34 (1986) 2052. Gangwar, R., Colella, N.J., Suter, R.M.: Phys. Rev. B 39 (1989) 2459. Jin, A.J., Bjurstrom, M.R., Chan, M.H.W.: Phys. Rev. Lett. 62 (1989) 1372. Dimon, P., Horn, P.M., Sutton, M., Birgeneau, R.J., Moncton, D.E.: Phys. Rev. B 31 (1985) 437. Nuttal, W.J., Noh, D.Y., Wells, B.O., Birgeneau, R.J.: J. Phys. Condens. Matter 7 (1995) 4337. Larher, Y., Gilquin, B.: Phys. Rev. A 20 (1979) 1599. Larher, Y.: J. Chem. Soc. Faraday Trans. I 70 (1974) 320. Butler, D.M., Litzinger, J.A., Stewart, G.A., Griffiths, R.B.: Phys. Rev. Lett. 42 (1979) 1289; Butler, D.M., Litzinger, J.M., Stewart, G.A.: Phys. Rev. Lett. 44 (1980) 466. Kramer, H.M., Suzanne, J.: Surf. Sci. 54 (1976) 659. Chinn, M.D., Fain, S.C.: Phys. Rev. Lett. 39 (1977) 146. Price, G.L., Venables, J.A.: Surf. Sci. 59 (1976) 509. Fain, S.C., Chinn, M.D., Diehl, R.D.: Phys. Rev. B 21 (1980) 4170. Suter, R.M., Colella, N.J., Gangwar, R.: Phys. Rev. B 31 (1985) 627. Specht, E.D., Mak, A., Peters, C., Sutton, M., Birgeneau, R.J., D'Amico, K.L., Moncton, D.E., Nagler, S.E., Horn, P.M.: Z. Phys. 69 (1987) 346. Migone, A.D., Chan, M.H.W., Niskanen, K.J., Griffiths, R.B.: Journal of Physics 16 (1983) L.1115. Niskanen, K.J., Griffiths, R.B.: Phys. Rev. B 32 (1985) 5858. Larher, Y., Terlain, A.: J. Chem. Phys. 72 (1980) 1052. Schabes-Retchkiman, P.S., Venables, J.A.: Surf. Sci. 105 (1981) 536. Guryan, C.A., Lee, K.B., Stephens, P.W., Goldman, A.I., Larese, J.Z., Heiney P.A., Fontes, E.: Phys. Rev. B 37 (1988) 3461. Chung, S., Kara, A., Larese, J.Z., Leung, W.Y., Frankl, D.R.: Phys. Rev. B 35 (1987) 4870. Rouquerol, J., Partyka, J., Rouquerol, F.: J. Chem. Soc. Faraday Trans. I 73 (1977) 306. Lahrer, Y.: J. Chem. Phys. 68 (1978) 2257; Surf. Sci. 134 (1983) 469. Prenzlow, C.F., Halsey, G.D.: J. Phys. Chem. 61 (1957) 1158. Millot, F.: J. Phys. (Paris) Lett. 40 (1979) L 9.

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3.1.2 Noble gases on graphite, lamellar halides, MgO and NaCl 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74

75 76 77 78 79 80 81 82

129

Shaw, C.G., Fain, S.C.: Surf. Sci. 83 (1979) 1; Surf. Sci 91 (1980) L 1. Migone, A.D., Li, Z.R., Chan, M.H.W.: Phys. Rev. Lett. 53 (1984) 810. Taub, H., Carneiro, K., Kjems, J.K., Passell, L., McTague, J.P.: Phys. Rev. B 16 (1977) 4551. Nielsen, M., Als-Nielsen, J., Bohr, J., McTague, J.P., Moncton, D.E., Stephens, P.W.: Phys. Rev. B 35 (1987) 1419. D'Amico, K.L., Bohr, J., Moncton, D.E., Gibbs, D.: Phys. Rev. B 41 (1990) 4368. Zhang, Q.M., Larese, J.Z.: Phys. Rev. B 43 (1991) 938. Shaw, C.G., Fain, S.C., Chinn, M.D.: Phys. Rev. Lett. 41 (1978) 955. Thomy, A., Duval, X., Regnier, J.: C. R. Acad. Sci. (Paris) C268 (1969) 1416. Lerner, E., Hedge, S.G., Daunt, J.G.: Phys. Lett. 41A (1972) 239. Huff, G.B., Dash, J.G. J.: Low Temp. Phys. 24 (1976) 155. Antoniou, A.A.: J. Chem. Phys. 64 (1976) 4901. Lerner, E., Hanono, F.: J. Low Temp. Phys. 35 (1979) 363. Rapp, R.E., de Souza, E.P., Lerner, E.: Phys. Rev. B 24 (1981) 2196. Tiby C., Lauter, H.J.: Surf. Sci. 117 (1982) 277. Calisti, S., Suzanne, J.: Surf. Sci. 105 (1981) L255. Calisti, S., Suzanne, J., Venables, J.A.: Surf. Sci. 115 (1982) 68. Demetrio de Souza, J.L.M., Rapp., R.E., de Souza, E.P., Lerner, E.: J. Low Temp. Phys. 55 (1984) 273. Régnier, J., Thomy, A., Duval, X.: J. Colloid Interface Sci. 70 (1979) 105. Dupont-Pavlovsky, N., Bockel, C., Thomy, A.: Surf. Sci. 160 (1985) 12. Teissier, C., Larher, Y., in: Ordering in Two-dimensions. Sinha, S.K. (ed.), New-York: NorthHolland, 1980, p. 163. Nardon, Y., Larher, Y.: Surf. Sci. 42 (1974) 299. Morishige, K., Hanayama, M., Kittaka, S.: J. Chem. Phys. 88 (1988) 451. Millot, F., Larher, Y., Tessier, C.: J. Chem. Phys. 76 (1982) 3327. Coulomb, J.P., Sullivan, T.S., Vilches, O.E.: Phys. Rev. B 30 (1984) 4753. Coulomb, J.P.: in Ref. 7, p. 113. Meichel, T., Suzanne, J., Gay, J.M.: C. R. Acad. Sci. Paris 303 (1986) 989. Meichel, T., Suzanne, J., Girard, C., Girardet, C.: Phys. Rev. B 38 (1988) 3781. Angot, T., Suzanne, J., in: The Structure of Surfaces III. Tong, S.Y., Van Hove, M.A., Takayanagi, K., Xie, X.D. (eds.), Springer Series Surf. Sci. 24. Berlin: Springer-Verlag, p. 671. Glachant, A., Bardi, U.: Surf. Sci. 87 (1979) 187. Hong, H., Peters, C.J., Mak, A., Birgeneau, R.J., Horn, P.M., Suematsu, H.: Phys. Rev.B 40 (1989) 4797. Nuttall, W.J., Fahey, K.P., Young, M.J., Keimer, B., Birgeneau, R.J., Suematsu, H.: J. Phys. Condens. Matter 5 (1993) 8159. Faisal, A.Q.D., Hamichi, M., Raynerd, G., Venables, J.A.: Phys. Rev. B 34 (1986) 7440. Hammonds, E.M., Heiney, P.A., Stephens, P.W., Birgeneau, R.J., Horn, P.M.: J. Phys. C 13 (1980) L 301. Heiney, P.A., Stephens, P.W., Birgeneau, R.J., Horn, P.M., Moncton, D.E.: Phys. Rev.B 28 (1983) 6416; Nagler, S.E., Horn, P.M., Rosenbaum, T.F., Birgeneau, R.J., Sutton, M., Mochrie, S.G.J., Moncton, D.E., Clarke, R.: Phys. Rev. B 32 (1985) 7373. Zerrouk, T.E.A., Hamichi, M., Pilkington, J.D.H., Venables, J.A.: Phys. Rev. B 50 (1994) 8946. Nuttall, W.J., Noh, D.Y., Wells, B.O., Birgeneau, R.J.: Surf. Sci. 307-309 (1994) 768. Mowforth, C.W., Rayment, T., Thomas, R.K.: Faraday Symp. Chem. Soc. 20 (1985). Ellis, T.H., Iannotta, S., Scoles, G., Valbusa, U.: Phys. Rev. B 24 (1981) 2307; Bracco, G., Cantini, P., Glachant, A., Tatarek, R.: Surf. Sci. 125 (1983) L 81. Grimm, B., Hövel, H., Pollmann, M., Reihl, B.: Phys. Rev. Lett. 83 (1999) 991. D'Amico, K.L., Moncton, D.E., Specht, E.D., Birgeneau, R.J., Nagler, S.E., Horn, P.M.: Phys. Rev. Lett. 53 (1984) 2250. Stephens, P.M., Heiney, P.A., Birgeneau, R.J., Horn, P.M., Moncton, D.E., Brown, G.S.: Phys. Rev. B 29 (1984) 3512. Tiby, C., Lauter, H.J.: Surf. Sci. 117 (1982) 277.

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130 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106

3.1.2 Noble gases on graphite, lamellar halides, MgO and NaCl Novaco, A.D., McTague, J.P.: Phys. Rev. Lett. 38 (1977) 1286; J. Phys. (Paris) Colloq. 38 C4-116. Régnier, J., Bockel, C., Dupont-Pavlovsky, N.: Surf. Sci. 112 (1981) L770. Ceva, T., Goldmann, M., Marti, C.: J. Phys. (Paris) 47 (1986) 1527. Stephens, P.W., Goldman, A.I., Heiney, P.A., Bancel, P.A.: Phys. Rev. B 33 (1986) 655. Bohr, J., Nielsen, M., Als-Nielsen, J., Kjaer, K., McTague, J.P.: Surf. Sci. 125 (1983) 181. You, H., Fain, S.C.: Phys. Rev. B 34 (1986) 2840. You, H., Fain, S.C., Satija, S., Passell, L.: Phys. Rev. Lett. 56 (1986) 244. Bouchdoug, M., Menaucourt, J., Thomy, A.: J. Phys. (Paris) 47 (1986) 1797. Menaucourt, J., Bockel, C.: J. Phys. (Paris) 51 (1990) 1987. Razafitianamaharavo, A., Convert, P., Coulomb, J.P., Croset, B., Dupont-Pavlovsky, N.: J. Phys. (Paris) 51 (1990) 1961. Razafitianamaharavo, A., Dupont-Pavlovsky, N., Thomy, A.: J. Phys. (Paris) 51 (1990) 91. Abdelmoula, M., Ceva, T., Croset, B., Dupont-Pavlovsky, N.: Surf. Sci. 272 (1992) 167. Dupont-Pavlovsky, N., Abdelmoula, M., Rakotozafy, S., Coulomb, J.P., Croset, B., Ressouche, E.: Surf. Sci. 317 (1994) 388. Hamdi-Alaoui, M.A., Coulomb, J.P., Dupont-Pavlovsky, N., Mirebeau, I., Mutaftschiev, B.: Surf. Sci. 295 (1993) L1031. Rakotozafy, S., Dupont-Pavlovsky, N., Croset, B., Duval, X.: Surf. Sci. 350 (1996) 206. Madih-Ayadi, K., Dupont-Pavlovsky, N., Ceva, T., Convert, P., Croset, B., Ayadi, A.: Surf. Sci. 405 (1998) 192. Layet, J.M., Bienfait, M., Ramseyer, C., Hoang, P.N.M., Girardet, C., Coddens, C.: Phys. Rev. B 48 (1993) 9045. Jordan, J.L., McTague, J.P., Hastings, J.B., Passell, L.: Surf. Sci. 150 (1985) L 82. Bardi, U., Glachant, A., Bienfait, M.: Surf. Sci. 97 (1980) 137. Bonnetain, L., Duval, X., Letort, M.: C. R. Acad. Sci. Paris 234 (1952) 1363. Duval, X., Thomy, A.: C. R. Acad. Sci. Paris 259 (1964) 4007. Bienfait, M., Venables, J.A.: Surf. Sci. 64 (1977) 425. Venables, J.A., Bienfait, M.: Surf. Sci. 61 (1976) 667. Opila, R., Gomer, R.: Surf. Sci. 112 (1981) 1.

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3.2.1 Alkali metals on metals

131

3.2 Adsorption of alkali metals

3.2.1 Alkali metals on metals R.D. DIEHL, R. McGRATH

3.2.1.1 Introduction The study of alkali metal adsorption on metal surfaces has been of interest in surface science for nearly 70 years, and there have been many reviews and monographs addressing different aspects of the subject [77B1, 82B1, 84N1, 84N2, 86B1, 89A1, 89B2, 89M1, 90C1, 91D1, 92K3, 93B1, 93N1, 93P2, 94B1, 94N1, 94N3, 94P1, 95A1, 95D1, 95F1, 95O1, 95S1, 96D1, 97D1, 98O2, 98T1, 99G2]. The interest in this topic is partly due to the importance of technological applications (promotion of catalytic reactions, enhancement of oxidation, increases in electron and ion emission rates, electrochemistry and battery production), but also due to the fundamental interest in how these “simple” adsorbates interact with each other, with coadsorbates and with the surface itself. Alkali atoms have a simple valence electronic structure (one single s electron) and hence are attractive systems to model using quantum mechanical calculations. The following table indicates some physical parameters for the alkali metals. Physical properties of alkali metals: Ionic radii Rion and metallic radii Rmet of alkali metals, in Å, their melting temperatures Tm, their ionization potentials (IP) and their nearest-neighbor (NN) distances in the bulk metals [86K1]. Metal

Rmet [Å]

Rion [Å]

Tm [K]

IP [eV]

NN [Å]

Li Na K Rb Cs

0.68 0.97 1.33 1.48 1.67

1.56 1.91 2.38 2.55 2.73

453.7 371.0 336.3 312.6 301.6

5.39 5.14 4.34 4.18 3.89

3.02 3.66 4.53 4.84 5.24

In this compilation we address several quantifiable aspects of alkali adsorption on metal surfaces encompassing electronic, thermodynamic, and structural results. After listing known physical properties of the bulk alkalis for reference, we start with one of the classic manifestations of alkali adsorption: the adsorbate-induced change in system work function. We tabulate the large number of studies of changes in work function as a function of adsorbed alkali coverage. We then tabulate the coverage dependence of energy losses due to alkali valence electron excitation measured using electronic energy loss spectroscopy (EELS). Finally, we list studies of occupied and unoccupied states close to the Fermi level. Next we turn to quantification of characteristic heats of adsorption of alkali atoms. In general these are coverage dependent so we list mainly the heat of adsorption at zero coverage. We then list coveragedependent values for alkali adatom vibrational mode energies which have been obtained using HREELS, HAS and LEED. Structural aspects of alkali adsorption account for the remainder of the chapter. Much excitement was generated in the early 1990's by the discovery of unexpected adsorption sites for alkalis on metal surfaces [96D1, 97D1]. After tabulating the known adsorption sites for simple structures we address several structural phenomena which have been observed with alkali adsorption systems: incommensurate overlayer rotation, monolayer compression relative to the bulk and condensation transitions at low coverages. Known temperature-coverage and isothermal phase diagrams are then presented. In recent

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132

[Ref. p. 169

years several alkali systems have been discovered where the alkali adatoms intermix with the substrate top layers. These are listed in the last section. Coadsorption of alkali atoms with other species, on which there is a vast literature (e.g. [86B1, 92K3, 93B1] and references therein), will not be considered in this chapter. The results of DFT calculations will not be considered here either. Throughout this chapter we consider the alkali coverage θalkali as the ratio of the number of alkali atoms to the number of substrate atoms in the first substrate layer, rather than considering the coverage in units of a saturated monolayer of alkali atoms. Acronyms used in the text DFT: ESDIAD: FE(M): HAS: (HR)EELS: IPES: IRAS: LEED: MDS: MEIS: NEXAFS: (NI)SXW: PhD: RHEED: SCLS: SEC(E/P): SEXAFS: STM: SXRD: TDS: UPS: VC:

Density-functional theory Electron-stimulated desorption ion-angular distribution Field emission (microscopy) Helium-atom scattering (High-resolution) electron energy-loss spectroscopy Inverse photoemission spectroscopy Infrared reflection absorption spectroscopy Low-energy electron diffraction Metastable deexcitation spectroscopy Medium-energy ion scattering Near-edge extended x-ray absorption fine structure (Normal incidence) standing x-ray wavefield absorption Photoelectron diffraction Reflection high-energy electron diffraction Surface core-level spectroscopy Secondary electron method (electron/photon) Surface extended x-ray-absorption fine structure Scanning tunneling microscopy Surface x-ray diffraction Thermal desorption spectroscopy Ultraviolet photoemission spectroscopy Vibrating capacitor

3.2.1.2 Adsorbed alkali metals: Data 3.2.1.2.1 Adsorption bonds The nature of the chemisorption bond formed when alkali metal atoms adsorb on metals has been a point of contention and much discussion [97D1]. The earliest measurements of alkali metal adsorption were of the large work function decrease which occurs when alkali metals are adsorbed on metal surfaces [23L1, 25L1, 29L1, 31L1, 32L1]. The early models of alkali metal chemisorption invoked some element of charge transfer from the alkali adatom to the metal substrate in order to explain this large decrease in work function [32L1, 35G1], although they did not quantitatively reproduce the experimental results. Later proposals were made to suggest that the decrease in work function might also be consistent with covalent bonding [88I1, 90R1, 94W3]. Many of these later discussions were based on the results of core-level photoemission studies. Some confusion in determining the nature of the alkali chemisorption bond arose from the difficulty in interpreting the core-level shifts which can be observed using high-resolution photoemission. It has now been established [93A2] that core-level photoemission cannot be used to directly probe the amount of charge transfer between the adsorbed alkali atoms and the substrate. The reason for this that the initial state and final state effects are entangled in core-level photoemission; there is no simple way to disentangle them to get the information on the initial-state properties. There are comparisons of the core-

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3.2.1 Alkali metals on metals

133

level shifts observed for the different alkali on the same substrate, notably for adsorption on Al(111) [93A2], but little if any quantitative information on the chemisorption bond itself has been obtained from these measurements. For these reasons we have not attempted to tabulate the results of core-level shift experiments in this work. Core-level photoemission results have provided extremely useful qualitative information regarding the adsorption geometries of adatoms, and much of this work is summarized in two reviews [93A2, 95A1]. The ensuing discussion of the issue of charge transfer provoked many experimental and theoretical studies of alkali metal adsorption. In the aftermath of this discussion, it is recognized that the picture of charge transfer is useful when discussing alkali metal adsorption, but that the terms "ionic" and “covalent” as used in molecular bonding may not be appropriate. Recent first-principles calculations have been carried out which quantitatively reproduce the work function measurements for Na adsorption on Al(100) and Al(111) [91S1, 94B2], and while they do indicate some degree of charge transfer, it is not possible to quantify the amount of charge transfer, nor to measure it directly. Work function measurements, however, give some quantitative comparison of the polarization induced in the different adsorption systems, while electron energy loss measurements provide information on how the chemisorption bond affects the electron states within the adatoms. 3.2.1.2.2 Work function changes Because they are highly electropositive, the adsorption of alkali metals causes large decreases in the work functions of surfaces. The adsorption of alkali metals onto metal surfaces causes a characteristic work function change: at low alkali coverage there is a sharp decrease in the system work-function, which goes through a minimum and then reaches a saturation value. This behavior is interpreted generally as being due to the large charge transfer per adatom at low coverages where the adatoms are essentially isolated from each other (thus decreasing the work function at low coverages), and a counteracting continual decrease in adatom polarization due to their mutual interaction as the density increases (thus increasing the work function at higher coverages) [89B2]. There has been a debate on whether the dipole moment reflects transfer of charge from the alkali to the metal surface or merely polarization within the alkali atom [93N2]. The characteristic work-function curve is illustrated in Fig. 1, which also defines the measured parameters. The initial dipole moment µ 0 can be evaluated from the work function curve. Defining the dipole moment as µ = q2d, where we approximate the dipole as point charges, 2d being the distance between the center of alkali charge and its image charge in the metal, then the change in work function is given by the Helmholtz equation: ∆φ = 2πµ(θ)θ where θ is the alkali coverage. Then µ 0 can be determined either by evaluating the initial slope of the curve:

µ=

1  d (∆φ )    2  dθ  θ → 0

or by fitting the curve to the Topping/Miller formula [27T1, 46M1] ∆φ = 2πθ

µ0 1 + 9αθ 3 / 2

where α is the polarizability. Studies where the results have been analyzed using both methods show only very small variations in the numerical values obtained. The unit of µ 0 is the Debye (1 D = 3.33564x10−30 coulomb meter).

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134

[Ref. p. 169

As discussed by several authors [79H1, 89A1, 89W1] there is an inconsistency in the literature in the usage of µ. Some authors have defined µ = qd which leads to a difference of a factor of 2 in µ 0 from that considered here. The values presented in the following table have been corrected where necessary to satisfy the definition µ = q2d. The technique of photoemission of adsorbed xenon (PAX) has been used in a number of cases [86M1], [85M1, 89W1, 92J2] to address the question of the extent of the lateral electronic perturbation of the substrate around an absorbed alkali atom. By measuring Xe 5p1/2 core-level shifts of Xe atoms at, close to, or removed from the alkali adsorbate, the local change in surface potential (not local change in work function, see discussion in [89A1]) can be measured at these locations. These studies indicate a strong short-range variation in surface potential as a function of distance from the alkali. These results have not been tabulated in this chapter. The following tables summarize the results of experimental work function data. Tsub is the substrate temperature in K during deposition. RT denotes room temperature. The methods of work function measurement are: VC: Vibrating capacitor method; RP: Retarding potential method; SEC(E) and SEC(P): Electron- and photon-induced secondary electron method respectively; FE: Field emission method. ∆φmin is the work function change at the minimum, ∆φML is the work function change at one saturated monolayer. µ 0 is the initial dipole moment. Where * appears in the table, a minimum was not observed. The tables are adapted and updated from [89A1]. Experimental work function data for lithium. Substrate

T [K]

Method

∆φmin [eV]

∆φML [eV]

Ag(111) Al(111) Cu(001) Cu(111) Mo(211) Ni(100) Ru(0001) W(110) W(110) W(110) W(211) W(211) W(100) W(111)

RT 140 180 160 77 308

VC VC SEC(E) RP RP RP VC RP RP FE VC RP/FE VC VC

−1.86 * −2.1 −2.4 * −2.8 −3.5 −2.9 −2.8 −3.0 * * * *

−1.76 −3.22 −1.7 −1.85 −1.5 −2.5 −2.5 −1.9 −2.1 −2.3 −1.5 −1.5 −1.4 −1.4

300 77 77 300 77/300 300 300

µ0 [D]

2.0

4.9

Ref. [86P1] [89H2] [94M2] [96L1] [79G1] [88G1] [96E1, 98E1] [74M1] [74M1] [74M1] [74M1] [73M1] [74M1] [74M1]

Experimental work function data for sodium. Substrate

T [K]

Method

∆φmin [eV]

∆φML [eV]

Ag(100) Ag(110) Al(111) Al(111) Al(111) Al(100) Al(100) Al(100)

300

SEC(P) SEC(P) SEC(E) VC VC SEC(E) SEC(E) SEC(E)

* * * −2.85 −2.85 * −2.0 *

−2.4 −2.0 −1.6 −2.55 −2.77 −1.6 −1.6 −1.6

RT 300 140 RT 100 350

µ0 [D]

6.8 3.6 4.2 6.4 6.4

Ref. [85G1] [77B2] [74P1] [89H2] [89H2] [74P1] [87P1] [87P1]

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3.2.1 Alkali metals on metals

Substrate

T [K]

Method

∆φmin [eV]

∆φML [eV]

Au(100) Au(100) Cu(111)

300 130 RT

VC VC RP

* −2.8 −3.4

−2.6 −2.6 −3.0

Cu(111) Cu(111) Cu(111) Cu(110) Cu(100) Cu(001) Mo(211) Ni(111) Ni(111) Ni(100) Ni(100) Ni(100) Ni(110) Pt(111) Ru(0001) Ru(0001) Ru(0001) Ru(0001) Ru(1010) Ta(110) W(110) W(110) W(110) W(100) W(112)

RT RT RT RT 100 180 77 RT RT RT RT 323 RT

RP SEC(E) RP/SEC(P) SEC(P)/RP RP SEC(E) RP VC RP/SEC(P) VC RP RP VC SEC(P) SEC(P) SEC(E) RP RP RP RP RP RP RP SEC(P) RP

−2.4 −2.75 −2.3 −2.15 −2.2 −2.5 −1.9 −3.1 −3.25 −3.1 −3.3 −3.2 −2.1 −4.0 −3.5 −3.5 −3.6 −3.23 −3.1 −2.65 −3.4 −3.5 −2.9 −3.0 −2.2

−2.0 −2.25

120 300 300

300 300 77 300 300 RT 200

−1.9 −1.8 −2.15 −1.8 −2.2 −2.5 −2.5 −2.6 −2.4 −2.0 −3.0 −3.0 −2.7 −2.8

135 µ0 [D]

22.6

7.4 7.2 6.8 3.2 7.2 6.8 8.6 6.9

−2.8 −2.48 −2.6 −2.6 −2.6 −2.7 −2.1

2.9 8.5±0.6

µ0 [D]

Ref. [95N2] [95N2] [80L1] [91T1] [93S2, 93S4] [91D2] [93S3] [83W2] [94M2] [80G1] [69G1] [93S2, 93S4] [69G1] [74A1, 75A2] [88G1] [69G1] [91B2] [91P1] [87R1] [85K1] [89D1] [87K2] [71F1] [70M1] [70M1] [89M2] [85C2] [71C1]

Experimental work function data for potassium. Substrate

T [K]

Method

∆φmin [eV]

∆φML [eV]

Au(100) Au(100) Au(100) Ag(111) Ag(100) Al(111) Al(111) Al(100) Al(100) C(0001) Co(0001) Cu(100) Cu(100) Cu(100)

300/100 300 130

SEC(E) VC VC RP SEC(P) SEC(P)/RP VC SEC(E) SEC(E) RP SEC(P) SEC(E)

* * -3.4 -2.9 -2.5 * -2.61 -2.8 * * -3.3±0.1 -3.0 -3.6 -2.85

-3.1 -3.1 -3.1 -2.4 -2.4 -2.2 -2.07 -2.3 -2.3 -1.8 -2.8±.1 -2.5 -2.3 -2.0

RT 140 100 350 90 200 330 120 RT

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VC

8.2 9.2 9.2 12±2 11±1 15.7

Ref. [92O1] [95N2, 95S3] [95N2, 95S3] [82F1] [90N1] [86P1] [89H2] [87P1] [87P1] [93H2] [79H1] [86A1] [87D1] [92A3]

3.2.1 Alkali metals on metals

136 Substrate

T [K]

Method

∆φmin [eV]

∆φML [eV]

Cu(110) Fe(110) Fe(110) Fe(100) Fe(111) Mo(110) Mo(100) Mo(211) Mo(111) Ni(111) Ni(111) Ni(100) Ni(100) Ni(100) Ni(100) Ni(110) Pd(100) Pt(111) Pt(111) Pt(111) Pt(111) Re(001) Rh(111) Ru(0001) Ru(0001) Ru(0001) Ru(0001) Ru(0001) Ru(0001) Ru(0001) Ru(0001) Ru(0001) Ru( 10 10 )

RT RT RT RT RT 77 77 77 77 80 90 RT 120 100 323 RT 300

SEC(P) SEC(P) VC VC VC FE FE FE FE SEC SEC(P) RP SEC(P) SEC(P) RP VC

-3.1 -3.7 -3.5 -2.4 -2.5 -3.1 -2.4 -2.5 -2.1 * -4.0 -3.5 -3.6 -3.5 -3.7 -2.7 -4.15 -4.7 -4.6 -4.6 -4.5 -4.35 -3.8 -4.3 -4.0 -4.4 -3.9 -4.0 -3.9 4.0 -3.9 -3.76 -2.6

-2.1 -2.8 -3.0 -2.1 -2.4 -2.3 -2.1 -2.1 -1.7 -4.2 -3.5 -2.8 -2.9 -2.9 -3.3 -3.2 -4.0 -4.1 -4.0 -3.7 -3.0 -3.2 -2.4 -3.3 -2.4 -3.0 -3.1 -3.2 -3.4 -3.2 -3.2 -2.4

Ru( 10 10 ) Ta(110) Ta(100) Ta(211) Ta(111) W(110) W(110) W(100) W(100) W(100) W(100) W(211) W(211) W(211) W(111) W(111)

300 300 77 77 77 RT 77 300 RT RT

-3.3 -3.02 -2.3 -2.1 -1.6

-2.9 -2.68 -2.1 -1.8 -1.3

300 300 <140 RT 80 60-80 85 120
RT 245 77 RT 77

SEC(P) SEC(P) SEC(P) RP VC SEC(P)-

SEC(P) SEC(P) SEC(P) RP SEC(P) SEC(E) RP RP RP RP FE FE FE FE FE RP FE SEC(P) FE FE

-2.77

-2.64

-3.1

-2.7

µ0 [D] 12.0 14.0 8.8 7.8

7.4 13.5

5.3 12.8 15.0 18.8 18.8

13.8 7.3 14.6 15.6 13.6 7.8 13±0.8

15.7 17.2 13.5±0.7 9.5 9.7 13.6

-2.9 FE FE FE

[Ref. p. 169

-2.7 8.2 11.5 6.2

Ref. [85W1] [79B1] [81L1] [81L1] [81L1] [76B1] [76B1] [76B1] [76B1] [85L2] [86U1] [74A1, 75A2] [84L1] [84S1] [88G1] [69G1] [87B3] [91B2] [83K1] [85L1] [96L4] [97V1] [92C1] [85D1] [85M1] [87D2] [91P1] [86U2] [88S2] [85W3] [87R1] [88D1] [86H2] [88S2] [71F1] [75B1] [75B1] [75B1] [66S1] [76B1] [71F1] [66S1] [85C2] [75B1] [66S1] [74M1] [75B1] [66S1] [75B1]

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3.2.1 Alkali metals on metals

137

Experimental work function data for rubidium. Substrate

T [K]

Method

∆φmin [eV]

∆φML [eV]

µ0 [D]

Ref.

Al(111)

140

VC

−2.52

−1.99

8.2

[89H2]

µ0 [D]

Ref.

Experimental work function data for cesium. Substrate

T [K]

Method

∆φmin [eV]

∆φML [eV]

Ag(110) Al(111) Al(111) C(0001) Cu(111) Cu(100) Cu(110) Fe(110) Mo(100) Mo(110) Mo(211) Ni(100) Ni(100) Ni(100) Ni(110) Ru(0001) Ru(0001) Ru(0001) Ru(0001)

300 140 RT 80 RT RT 300 RT

SEC(P) VC SEC(E)/RP RP RP RP RP SEC(P)

77 77 RT RT 323 RT 120 300 300

RP RP SEC(P) RP VC SEC(P) SEC(E) RP SEC(E)

−2.5 −2.44 −2.6 −2.44 −3.4 −2.9 −3.1 −4.1 −3.6 −3.6 −2.8 −3.0 * −3.8 * −4.0 −4.1 −4.1 −4.1

−2.4 −1.8 −2.3 −2.2 −3.0 −2.5 −2.6 −3.4 −3.4 −3 −2.7 −2.6 −3.1 −3.3 −3.1 −3.6 −3.4 −3.4 −2.9

RP RP FE RP SEC(P) RP VC VC VC RP RP SEC(E) RP VC FE FE FE RP FE FE

−3.4 −3.18±0.03 −2.8 −3.7 −3.9 −3.6 −3.85 −2.9 −3.13 −3.1 −2.88 −3.1 −3.1 −3.04 −2.9 −3.9 −3.2 −3.0 * *

−3.1 −2.94 −2.7 −3.2

Ru( 10 10 ) Ta(110) W(111) W(110) W(110) W(110) W(110) W(100) W(110) W(100) W(100) W(100) W(100) W(100) W(100) W(110) W(211) W(211) W(103) W(115)

300 300 77 77 300 250 RT RT RT 300 RT RT 300 77 77 77 RT 77 77

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−3.2 −3.1 −2.88 −3.03 −2.4 −2.83 −2.4 −2.6 −2.82 −2.8 −3.6 −3.1 −2.9 −2.95 −2.5

8.2 18±0.8 19.2

8.8

28.2 7.0 13.8 22.0

15.3±0.7 17.4

15.8±0.8 19.9±1 15.0±0.8

18.1 34 23 17.8 15.8

[83H1] [87H1, 89H2] [86P1] [86H1] [78L1, 80L1] [82P1] [97A1] [99S1] [82S1] [94G1] [81G1] [75P1] [77K1] [88G1] [69G1] [91P1] [87R1] [88S2] [85H1] [88S2] [71F1] [69S1] [70F1] [97P1] [89M2] [80D1] [71L1] [71L1] [73P1, 73P2] [71F1] [69M1] [72V1] [73D1] [69S1] [69S1] [69S1] [71P1] [69S1] [69S1]

3.2.1 Alkali metals on metals

138

[Ref. p. 169

3.2.1.2.3 Electronic structure of adatoms In order to understand the mechanism of the alkali-metal-induced work-function variation, it is necessary to know the valence electronic structure of the adatoms. When low-energy electrons impinge on a solid surface, some of them undergo characteristic energy losses due to inelastic scattering in the solid. Some of these are due to the excitation of valence electrons. The energy losses for single-particle excitations are generally in the range 1-20 eV. For alkali metal adsorption, it is generally found that these loss peaks appear at very low coverages and shift to lower energies with increasing coverage, up to the work function minimum. At higher coverages, the losses shift back to higher energies. In the following table we have tabulated the results of many EELS studies for alkali metal adsorption, including the energy loss at very low coverage ( E10 ), at the work function minimum ( E1min ) and at the completion of the monolayer ( E1ML ). There are some general features of these results. First, the energy losses extrapolated to zero coverage lie in the range of 3-4 eV. Second, the minimum energy losses are close to the excitation from the ns level to the np level in a free alkali atom. This suggests that at this point, the ns resonance of the adsorbed alkali-metal atom is filled with almost one electron, a fact which has relevance to the condensation transitions observed for some systems. Third, with the exception of Cs/Al(111), the coverages corresponding to the minimum energy loss correspond closely to those of the work function minimum. And fourth, the loss energies shift to higher energies at higher coverages. This shift is most prominent for Na and decreases for the heavier alkali metals. EELS Results for alkali-metal covered surfaces. The experimental numbers indicate the energy loss at very low coverage ( E10 ), at the work function minimum ( E1min ) and at the completion of the monolayer ( E1ML ). Alkali

Substrate

E10 [eV]

E1min [eV]

E1ML [eV]

Ref.

Na Na Na Na Na Na Na Na Na Na K K K K K K K K K Rb Cs Cs Cs Cs Cs Cs Cs

Al(111) Al(111) Al(111) Cu(111) Mo(100) Ni(100) Ni(100) Pt(111) Ru(0001) W(100) Al(111) Ag(111) Ag(100) Cu(100) Ni(100) Ni(100) Pd(100) W(100) W(100) W(100) Al(111) Ag(100) Cu(111) Mo(100) Mo(110) W(100) W(100)

2.6-3.1

2.0

3.0 2.8

2.5 3.2 3.2 3.4

2.0 2.3 2.8 2.1

4.0 3.6 3.2

2.0 3.0 2.8 1.5 1.5

3.7 3.5

1.7 1.5

2.8

2.1

1.8

0.95

3.1 2.9

1.25 1.6 2.2

2.9

1.5

[87K1] [89H2] [91G2] [80L1] [96L3] [75A1, 75A2, 79J1] [73A1] [88H1] [87R1, 87S1] [85C2] [91G2] [82F1] [91M1] [86A1] [75A1, 75A2, 77S1, 79J1] [73A1] [87B3] [85C2] [72T1] [72T1] [87H1] [91M1] [78L1, 80L1] [82S1, 83S1, 85S1] [94G1] [69M1] [83S1, 85C2, 85S1]

3.2 3.1 3.7 3.5 3.7 3.7 1.65 2.3 1.8 2.25 2.3 2.5 2.6 2.3 2.0 1.8 0.95 1.3 1.55 1.7 1.5 1.5

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3.2.1 Alkali metals on metals

139

Further results on the electronic structure of the adatom–substrate complex have been obtained using other spectroscopic techniques. These techniques probe the occupied and unoccupied alkali-induced states near the Fermi level. We have tabulated the results of UPS and MDS experiments probing the occupied density of states. In these studies, evidence for an occupied ns resonance is observed. In many of these studies, this resonance has only been observed at coverages close to or above the work-function minimum and some workers have inferred that this is evidence for an ionic-to-metallic phase transition of the alkali layer. However in other work, this resonance has been observed at very low alkali coverages. It appears that the width of the resonance and experimental resolution considerations must be taken into account in considering these results in the context of alkali metal bonding [88H2, 89H3, 89I1]. States observed in UPS and MDS experiments for alkali metal covered surfaces. θ is the coverage range over which the adsorbate-induced occupied state is reported.

Alkali

Substrate

Technique

θ

Assignment Ref.

Li Na K K K K Cs

Cu(111) Cu(111) Al(111) Al(111) Ag(110) Cu(110) Cu(111)

UPS UPS UPS UPS UPS MDS UPS

2s 3s 4s 4s 4s 4s 5s

[93D1] [93D1] [88H2] [89F1] [90N1] [85W1] [93D1]

Complementary to these studies are results obtained with IPES and two-photon photoemission, which probe the unoccupied density of states above the Fermi level. Interpretation of these studies is somewhat complicated by the presence of image charge states, but in general for the lighter alkalis Li, Na and K the unoccupied density of states for adsorbed layers is derived primarily from np orbitals. For Cs, the unoccupied electronic states have been variously assigned to 6p [89F1, 93C2, 93D1] or 5d orbital character [97A1]. The general behavior of adsorbate induced unoccupied p and d states is a continuous shift towards the Fermi level as a function of alkali coverage. The results are presented below. Some workers do not assign the adsorbate-induced features to specific atomically-derived orbitals [91M2, 93F1, 94F1, 94S5, 94W1] and these results have been omitted. States observed in IPES experiments for alkali metal covered surfaces. θ is the coverage range over which the adsorbate-induced occupied state is reported. Alkali

Substrate

θ

Assignment Reference

Li Na Na Na Na Na K K K Cs Cs Cs Cs

Cu(111) Al(111) Cu(111) Cu(111) Cu(110) Ni(111) Al(111) Ag(110) Cu(111) Al(111) Cu(111) Cu(111) Cu(100)

0.25 >0.09 0.15-0.25 0.25

2p 3p 3p 3s 3p 3p 4p 4p 4p 6p 6p 5d 5d

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0.08-0.34 0.25

0.25

[93D1] [88H1] [93D1] [91D2, 93D1] [93T2] [93T1] [89F1] [87J1] [93D1] [89F1] [93D1] [97A1] [97A1]

3.2.1 Alkali metals on metals

140

[Ref. p. 169

3.2.1.2.4 Heats of adsorption/desorption Most of the direct measurements of the adsorption energies of alkali metals adsorbed on metal surfaces are those deduced from thermal desorption measurements. This can be observed directly in the zerocoverage limit using field ion microscopy. Otherwise, families of desorption curves at different starting coverages can be obtained from flat single crystals. The general features observed in families of thermal desorption curves from alkalis adsorbed on metal surfaces are first-layer desorption peaks which shift to lower temperature as the coverage is increased, and frequently peaks at successively lower temperatures due to further adsorbed layers. As discussed in the Section 1.3.1 of the Introduction to this Volume, it is possible to deduce the thermal desorption energy from such curves. In general, desorption energies decrease as the coverage increases for alkali metals on metal surfaces. An example of such measurements for Cs on Ru(0001) are shown in Fig. 2. Desorption energies can also be deduced from trapping measurements. In these measurements, the initial trapping probability of alkali adatoms is measured, along with the temperature dependence of the mean residence time of those trapped adatoms. From those parameters, the zero-coverage desorption energy can be derived [76H1]. An alternative method for determining these energies is to measure a family of adsorption or desorption isotherms and then to extract the heats of adsorption from the slope of the adsorption points on an Arrhenius plot of the isosteres. This is discussed in detail in Section 1.3.2 of the Introduction to this Volume. These measurements are somewhat more complex to carry out since they require an accurate assessment of both the vapor pressure at the surface and the coverage during the measurement. The one reported measurement of this type was carried out using He-atom scattering [94R1]. A list of measured heats of desorption are given in the following table for several alkali metal adsorption systems. Measured heats of adsorption/desorption for alkali metals on metal surfaces. The values given are the zero-coverage limit unless stated otherwise. (FEM) indicates that the measurement was done on an FEM tip. Alkali

Substrate

Method

Coverage

Energy [eV]

Ref.

Li Li Li Li Na Na Na Na Na Na Na Na Na Na Na K K K K K

Ru(0001) Ru(0001) Ru(0001) W(110) Au(100) Ag(110) Ni(111) Ni(100) Ni(110) Ru(0001) Ru(0001) Ru(0001) Ru (10 10) W (FEM) W(110) Cu(100) Au(100) Fe(111) Fe(100) Fe(110)

TPD TPD TPD Trapping TPD TPD TPD TPD TPD TPD TPD TPD TPD TPD Trapping HAS Isotherms TPD TPD TPD TPD

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.26-0.37 0 0 0 0

2.8 2.9 3.2 2.8 2.4 2.0 2.54 2.52 2.18 2.5 2.5 2.8 2.64 2.4 2.6 1.0±0.1 2.6 2.25 2.34 2.47

[96P1] [86D1] [98E1] [76H1] [95N2] [76M1] [70G1] [70G1] [70G1] [83D1] [87R1] [89D1] [88K1] [70G1] [76H1] [94R1] [95N2] [81L1] [81L1] [81L1]

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3.2.1 Alkali metals on metals

141

Alkali

Substrate

Method

Coverage

Energy [eV]

Ref.

K K K K K K K K K K K

Ni(110) Pt(111) Pt(111) Pt(111) Re(001) Ru(0001) Ru(0001) Ru(0001) Ru(0001) Ru(0001) Ru (10 10)

TPD TPD TPD TPD TPD TPD TPD TPD TPD TPD TPD

0 0 0.36 0 0 0 0.33 0 0 0 0

2.56 2.6 1.1 2.6 2.97 2.8 1.1 2.9 3.5 2.8 2.9

[70G1] [82G1] [82G1] [82C1] [97V1] [85D1] [85D1] [87R1] [88D1] [89H5] [86H2]

K K Cs Cs Cs Cs Cs Cs Cs Cs Cs Cs Cs Cs Cs

Ru (10 10) W(110) Ni(110) Ni(FEM) Ru(0001) Ru(0001) Ru(0001) Ru(0001) Ru(0001) Ru (10 10) W(100) W(110) W(110) W(110) W(110)

TPD Trapping TPD TPD TPD TPD TPD TPD TPD TPD TPD TPD TPD TPD TPD

0 0 0 0 0 0 0 0.33 0 0 0 0 0 0 0

2.84 2.1 2.72 3.1 2.9 3.2 2.6 0.82 3.2 3.16 2.8 3.2 3.2 3.06 3.1

[88K1] [76H1] [70G1] [70G1] [90D1] [96P1] [85H1] [85H1] [87R1] [88K1] [67G1] [67G1] [70F1, 72K1] [74T1] [80D1]

3.2.1.2.5 Adatom vibrational frequencies A common method used to obtain information about chemisorption bonds is to measure the vibrational frequencies of adsorbed adatoms. There have been many such studies for adsorbed alkalis, most of them employing either inelastic He-atom scattering or high-resolution electron energy loss spectroscopy. The modes which have been observed have been identified as vibrations of the adatoms either perpendicular or parallel to the surface. In general, the perpendicular vibrations have been found to have higher energies than the parallel vibrations, for on-surface adsorption. In the cases of intermixed alkalis, the situation is more complex because there are many types of intermixed geometries. The following paragraphs summarize the observed trends. The perpendicular vibration modes for on-surface adsorption scale approximately with the inverse square root of the mass of the alkali for any given substrate, indicating that the perpendicular force constant does not vary strongly with alkali (or with the adsorbate-substrate spacing). Because the HREELS measurements of these modes have relied primarily on their high cross-sections for dipole scattering, the numbers given in the following table are zone-center values, and their dispersion has not been measured (apart from the Na-Al(111) system, Reference [95N1, 97N1]). Their coverage dependence has been studied for several systems, and there are a few trends which appear to be common to most of these systems. The characteristic features in the HREELS measurements are a nearly-coverageindependent perpendicular vibration frequency and a strongly-coverage-dependent loss-peak intensity. The intensity grows to a maximum at a relatively low coverage, and then decreases to a very low level for higher coverages, reflecting the changing character of the electronic structure of the adlayer as the coverage changes. The perpendicular vibrational frequency usually remains constant or increases slightly

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142

[Ref. p. 169

with coverage. This shift is generally at least partially attributed to the change in the dipole-dipole interaction as the coverage increases. The expression which describes this dependence is

ω 2 = ω 02 +

Q2Σ M(1 + α e Σ)

where Q is the dynamic charge, αe is the electronic polarizability and M is the reduced mass. Σ is a lattice sum over the r−3 term in the dipole-dipole interaction, and for a hexagonal lattice is equal to 8.9n3/2, where n is the surface density of atoms. In some cases, this equation appears to not account for the full shift in vibrational frequency, and this has been attributed to a delocalization of the mode and an increasing participation of the substrate atoms in the mode as the coverage increases, but this explanation cannot fully explain all of the observations [96L1]. The parallel vibration modes typically have very low energies which may be difficult to resolve in a HREELS experiment, in addition to having very small dipole moments which would result in low intensities. The lower energy range, however, is ideal for He-atom scattering measurements. Because Heatom scattering does not rely on dipole scattering, the dispersion curves for the parallel modes have been mapped out for many systems using this technique. The modes are usually dispersionless, or nearly so. No clear trends have been observed in the coverage dependence of these modes due to lack of coveragedependent data. At coverages near the saturated monolayer, the energies of the parallel modes for alkalis on graphite have been found to scale with the inverse square root of the mass of the alkali, similar to the case of the perpendicular vibrations. However, for low coverages of alkalis on Cu(100), the energy of the parallel mode is found to decrease more strongly as mass increases. This has been attributed to the difference in the lateral potentials (due to the substrate) experienced by the different adatoms [98S1]. The parallel mode energies can also be extracted from LEED intensity measurements, if the perpendicular mode energy is already known. Such an analysis has been performed for Cs/Ru(0001) [95O2]. The vibration frequencies for the systems studied thus far basically are similar for similar geometries. For example, the following table shows that the perpendicular vibration energy for Li is around 33-43 meV for systems where the adatom are on the surface (on the Cu surfaces), where as it is much less, about 18 meV, for intermixed Li (on Al). Likewise, the perpendicular vibration frequencies for Na are around 18 meV except for the intermixed Na/Al which is about 12 meV, and the parallel vibration frequencies are around 5-6 meV. For K, the perpendicular modes are around 12-14 meV and the parallel vibration frequencies are in the 2-6 meV range, whereas an intermixed species has a perpendicular mode energy of 28 meV. There have been no studies which specifically measure differences in vibrational frequencies due to adsorption geometry, e.g. a site dependence, although small geometrical effects should be expected [92N1]. Fig. 3 shows the on-surface perpendicular and parallel frequencies for adsorbed alkalis plotted against M−1/2.

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3.2.1 Alkali metals on metals

143

Experimental values for alkali adatom vibrational mode energies. Alkali Substrate Li Li Li Li Li Li Na Na Na Na Na Na Na Na Na Na K K K K K K K K K K K Rb Cs Cs Cs Cs Cs Cs

Technique Coverage

Al(111) HREELS Cu(111) HREELS Cu(111) HREELS Cu(110) HREELS Cu(110)-(1x2) HREELS Cu(110)-(1x2) HREELS Al(111) HREELS Al(111) HREELS Cu(111) HREELS Cu(100) HREELS Cu(100) HAS Cu(100) HAS Cu(110) HREELS Cu(110)-(1x2) HREELS Mo(100) HREELS Pt(111) HAS Cu(111) HREELS Cu(111) HREELS Cu(100) HAS Cu(100) HREELS Cu(100) HREELS Cu(110) HREELS Cu(110)-(1x2) HREELS Ni(110) HREELS Pt(111) HREELS Graphite(0001) HAS Graphite(0001) HAS Graphite(0001) HAS Cu(100) HAS Cu(100) HAS Ru(0001) HREELS Ru(0001) HREELS Ru(0001) LEED Graphite(0001) HAS

0.03-1.0 0.025-0.3 0.3-0.5 0.08-0.80 0.04-0.40 0.40-0.80 0.067-0.33 0.5 0.075-0.3 0-0.4 0-0.13 0.18-0.5 0.13 0.6 0-0.45 0.05 0.02-0.27 0.08-0.4 0.07 0.0-0.02 0.02-0.11 0.02-0.32 0.02-0.56 0.2 0.02-0.15 0.04 0.25 0.25 0.08 0.27 0.03-0.19 0.19--0.24 0.25 0.25

Polarization

Energy [meV]

Geometry

Ref.

Perpendicular Perpendicular Perpendicular Perpendicular Perpendicular Perpendicular Perpendicular Perpendicular Perpendicular Perpendicular Parallel Parallel Perpendicular Perpendicular Perpendicular Parallel Perpendicular Perpendicular Parallel Perpendicular Perpendicular Perpendicular Perpendicular Perpendicular Perpendicular Parallel Parallel Parallel Parallel Perpendicular Perpendicular Perpendicular Parallel Parallel

17.6-18.1 38-43 43 35-37 33-36 36-35 11.6-12.6 15 21 18 5.6-6.3 6.3-2.0 18 18 19 5.2 12 13 2.86 10-13 13 14 10-13 14.9 17-22 6.5 4.4 2.8 0.0-0.65 6.8 6.7-9.0 9.0-8.3 1.0-1.3 2.2

Intermixed On-surface On-surface On-surface On-surface On-surface Intermixed Intermixed On-surface On-surface On-surface On-surface On-surface On-surface On-surface On-surface On-surface On-surface On-surface On-surface On-surface On-surface On-surface Reconstructed On-surface On-surface On-surface On-surface On-surface On-surface On-surface On-surface On-surface On-surface

[95N1] [96L1] [96L1] [91R1] [91R1] [91R1] [95N1, 97N1] [95N1, 97N1] [90L1, 93L1] [90A1] [97G1] [97G1] [90R2] [90R2] [96L3] [99G1] [90A1] [93L1] [98S1] [90A1] [90A1] [90R2] [90R2] [85W2] [97H1, 99K1] [93C1] [93C1] [93C1] [98S1] [99S2] [96H1] [96H1] [95O2] [93C1]

3.2.1.2.6 Adsorption sites and bond lengths (commensurate structures) The adsorption sites determined for relatively simple (primitive) commensurate overlayer structures are given in the tables below. The large number of measured structures allows some trends to be observed. First, alkalis adsorbed on the substrates of square and rectangular symmetry are in the high-coordination hollow sites. All available evidence indicates that these are the preferred sites over the whole coverage range [93M1, 93W1, 94M1]. The situation for the hexagonal substrates is more interesting. In some cases the equilibrium adsorption site is the highest-coordination hollow site (either hcp or fcc), while for others it is the lowestcoordination top site. In the LEED studies of the top-site systems where substrate relaxations are included in the analysis, all of them show some degree of rumpling. The main effect of this rumpling is to move the alkali adatoms closer to the surface, thus slightly decreasing their nearest-neighbor distance as well as the next-nearest-neighbor distance. It has been shown using DFT calculations that a necessary component Lando lt -Bö rnst ein New Ser ies III/42A1

144

3.2.1 Alkali metals on metals

[Ref. p. 169

of the top-site structure for K/Al(111) is a small rumpling of the substrate [95S1]. Therefore it has been proposed that whether an atom occupies a top site may depend on how easily the substrate can be deformed to accommodate the rumpling [95O1], and that therefore a substrate which cannot be easily deformed is less likely to exhibit top-site adsorption. DFT calculations have predicted that for K/Al(111) there is a crossover from top site to hollow site as the coverage decreases, and that a single adatom should be in the hollow site [92S1]. It has also been suggested that top-site structures (and rumpling) are related to a possible increased screening effect for certain symmetries [92O2], implying that top site occupation is a collective effect and would not happen for a single adatom. Of the lowest-coverage site determinations for such a system, the NISXW study of Rb/Al(111) [92K2, 93K2], indicates that the top site is still occupied at a coverage of 0.12 in the disordered phase. A PhD experiment for K/Al(111) indicates that the top-site is occupied in the coverage range 0.05-0.40 [95C1]. This is not in agreement with the DFT result [92S1]. At higher coverages, a change in adsorption site with coverage has been observed on three substrates, Ru(0001) [92O2], Ag(111) [97K1] and Rh(111) [96S1]. In the cases of Cs/Ru(0001) and Cs/Rh(111), the site changes from the top site in the p(2x2) structure to the hcp site in the (¥[¥ 5 VWUXFWXUH ,W ZDV proposed [92O2] that for Cs/Ru(0001) this change is due to the screening afforded by the surface geometry of the atop site in the p(2x2) phase but not the atop site in the (¥[¥ 5 SKDVH ,Q WKH FDVHV of K and Rb on Ru(0001), and K, Rb and Cs on Ag(111), the adatoms are adsorbed in the fcc sites in the p(2x2) phase and in the hcp sites in the (¥[¥ 5 SKDVH )RU DGVRUSWLRQ RQ 5X LW ZDV SURSRVHG that the hcp site at higher coverage occurs because the hcp site is the natural "metallic" site on Ru(0001), but this is clearly not the case on Ag(111). Since this site change has been observed now on both an hcp and an fcc substrate, a likely cause of the change may be coordination. In the p(2x2) structure, adatoms in either hollow site effectively increase their coordination when the substrate rumples, and the choice of fcc site may be related to the rumpling energy. In the (¥[¥ 5 VWUXFWXUH KRZHYHU UXPSOLQJ FDQQRW occur by symmetry for either hollow site, therefore the coordination is a maximum when the second-layer substrate atom is directly beneath the adatom. On other surfaces, notably Pt(111), Pd(111) and K on Rh(111), the adsorption site is hcp for both the p(2x2) and the (¥[¥ 5 SKDVHV ,Q WKH FDVH RI 3W ')7 FDOFXODWLRQV [98M1] have suggested that the origin for the hcp site preference is due to the tilting of the electron density due to the d-states toward the hcp sites, leading to an added attraction in those sites which is not present for the fcc sites. This picture is consistent with the results for alkali adsorption on Rh(111) and Pd(111), where hcp sites are also occupied in the p(2x2) structure. It is also consistent with the observations of fcc-site occupation for the p(2x2) structure in the systems of K and Rb on Ru(0001), and K, Rb and Cs on Ag(111), where the effect of the d-electrons may be significantly less. An issue which has been discussed with respect to the nature of the adsorption bond is the chemisorption bond length. The chemisorption bond length was initially expected to increase as the alkali coverage increases due to large depolarization which occurs as the coverage increases, as similar changes had been observed for Cl and S adsorption [88L1, 88S1, 89W2]. Changes have been observed in the chemisorption bond lengths for alkalis, but only when their coordination to the substrate changes. The data in the tables below show that for any given alkali-substrate combination, when the coordination of the alkali increases, the chemisorption bond length increases. If one corrects for the different radii of the various substrates by subtracting the substrate metallic radius from this number it can be seen that generally this "effective r" increases with coordination, irrespective of the substrate. There are a few notable exceptions, particularly for Cs adsorption. There the difference in the chemisorption bond strength is believed to play a role, as discussed in reference [99K2]. The other surprising adsorption sites for alkali metals are the so-called substitutional sites. At the simplest, these structures involve adatoms which occupy vacancies that are induced by the alkali adsorption [94S3]. For the case of Al(111), DFT calculations have indicated that the reason a substitutional structure is so favorable is that the energy for vacancy formation on Al(111) is so small (0.36 eV) [93P1, 95S1]. It was also suggested that these substitutional structures provide a way for the alkali adatom to form a more ionic bond with the substrate, the evidence of this being the increase of largely-substrate charge density between the alkali adatoms [95S1]. A low vacancy-formation energy was also predicted for Ag(111) (0.67 eV) and Cu(111) (0.92 eV) [93P1], but so far no substitutional sites have been observed on those surfaces except for Li adsorption. Lando lt -Börnst ein New Ser ies III/42A1

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145

More complex multilayer intermixed structures have also been observed and characterized for alkali adsorption on the (100) surfaces of Cu, Ag, Al, Ni and Au, and the (111) surfaces of Al and Cu [98T1]. The fcc(110) surfaces of many metals also have been observed to undergo significant reconstructions upon alkali adsorption, and the geometries of many of these substrate reconstructions have been measured [96D1, 98T1]. The following tables contain experimentally-determined commensurate structure parameters for alkali metal adsorption systems (only simple structures are listed). The temperatures quoted are the measurement temperature. The bond length quoted is the chemisorption bond length. Effective r (Eff. r) is the chemisorption bond length minus the metallic radius of the substrate atom. Excess r (Exc. r) is the Effective r minus the ionic radius of the alkali metal atom. N is the coordination number of the alkali adatom. A coordination number denoted as * indicates that due to surface reconstruction, an unambiguous assignment cannot be made.

Commensurate structure parameters for lithium adsorption. Surface

Structure

T [K]

Al(100) Cu(100) Ni(100) Ru(0001)

c(2x2) 300/100 c(2x2) 180 c(5¥[¥ 5 230 (¥[¥ 50

Site

Method Bond length [Å]

Eff. r [Å]

Exc. r N [Å]

Ref.

subst. hollow hollow hcp

LEED LEED LEED LEED

1.43±0.05 1.39±0.08 1.33±0.05 1.39±0.05

0.76 0.71 0.65 0.71

[99P1] [93M2] [97J3] [95G1]

2.87±0.05 2.67±0.08 2.58±0.05 2.74±0.05

4 4 4 3

Commensurate structure parameters for sodium adsorption. Surface

Structure

T [K]

Site

Method Bond length [Å]

Eff. r [Å]

Exc. r N [Å]

Ref.

Al(111) Al(111) Al(111) Al(100) Al(100) Al(100) Cu(100) Cu(100) Ni(100) Ni(100) Rh(111) Ru(0001) Ru(0001)

(¥[¥ 5 (¥[¥ 5 (¥[¥ 5 c(2x2) c(2x2) c(2x2) c(2x2) c(2x2) c(2x2) c(2x2) (¥[¥ 5 p(2x2) (¥[¥ 5

100/300 300 170/300 140 100 100/300 100 300 300 320 30 50 50

subst. subst. subst. hollow hollow subst. hollow hollow hollow hollow hcp fcc hcp

LEED 3.21±0.01 SEXAFS3.31±0.03 NISXW 3.10±0.06 SEXAFS3.20±0.03 LEED 3.27±0.01 LEED 3.07±0.01 LEED 2.98±0.03 LEED 3.03±0.04 LEED 2.84±0.08 LEED 2.95±0.04 LEED 2.84±0.09 LEED 2.95±0.04 LEED 2.95±0.04

1.78±0.01 1.88±0.03 1.67±0.06 1.76±0.03 1.83±0.01 1.63±0.01 1.71±0.03 1.76±0.04 1.59±0.08 1.72±0.04 1.49±0.09 1.60±0.04 1.60±0.04

0.81 0.91 0.70 0.79 0.86 0.66 0.74 0.79 0.62 0.75 0.52 0.63 0.63

[94B3] [91S2] [92K1] [92A1] [95B3] [95B3] [99M1] [99M1] [75A1, 75D1] [94N4] [94V1] [94H1] [94H1]

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3.2.1 Alkali metals on metals

146

[Ref. p. 169

Commensurate structure parameters for potassium adsorption.

Surface

Structure

T [K]

Site

Method

Ag(111) Ag(111) Ag(111) Al(111) Al(111) Co(0001) Co (10 10) Cu(111) Cu(110) Fe(100) Ni(111) Ni(111) Ni(111) Ni(111) Ni(100) Pd(111) Pd(111) Pd(100) Pt(111) Pt(111) Rh(111) Rh(111) Ru(0001) Ru(0001)

p(3x3) p(2x2) (¥[¥ 5 (¥[¥ 5 (¥[¥ 5 p(2x2) c(2x2) p(2x2) (1x2) (3x3) p(2x2) p(2x2) p(2x2) p(2x2) c(4x2) p(2x2) (¥[¥ 5 c(2x2) p(2x2) (¥[¥ 5 p(2x2) (¥[¥ 5 (¥[¥ 5 P(2x2)

40/300 40/300 40/300 90 300 160/400 300 65/300 100/300 300 120/400 70/300 130/300 100/300 80 50/604 50/470 300 100 100 50 50 80 80

fcc LEED fcc LEED hcp LEED top LEED subst LEED top LEED hollow LEED top SEXAFS subst. PhD subst. SXW top LEED top SEXAFS top PhD top PhD hollow LEED hcp LEED hcp LEED hollow LEED hcp LEED hcp LEED hcp LEED hcp LEED hcp LEED fcc LEED

Bond length [Å]

Effective r [Å]

Exc. r [Å]

N

Ref.

3.27±0.03 3.27±0.03 3.29±0.02 3.23±0.05 3.58±0.03 2.93±0.11 3.12±0.05 3.05±0.02 3.27±0.27 2.92±0.04 2.82±0.04 2.92±0.02 3.02±0.01 2.86±0.03 3.20±0.05 3.10±0.07 3.10±0.07 3.20±0.05 3.12±0.04 3.15±0.02 3.15±0.05 3.15±0.05 3.29±0.05 3.25±0.05

1.82±0.03 1.82±0.03 1.84±0.02 1.80±0.05 2.10±0.03 1.68±0.11 1.87±0.05 1.77±0.03 1.99±0.27 1.66±0.04 1.57±0.04 1.67±0.03 1.77±0.01 1.62±0.03 1.96±0.05 1.72±0.07 1.72±0.07 1.82±0.05 1.74±0.04 1.77±0.02 1.80±0.05 1.80±0.05 1.98±0.05 1.94±0.05

0.49 0.49 0.51 0.47 0.73 0.35 0.54 0.44 0.66 0.33 0.24 0.34 0.44 0.29 0.62 0.39 0.39 0.49 0.41 0.44 0.46 0.46 0.65 0.61

3 3 3 1 6 1 * 1 * 8 1 1 1 1 4 3 3 4 3 3 3 3 3 3

[97K1] [95L2] [95L2] [92S1] [92S1] [99L1] [91B1] [93A1] [94H2] [97C1] [92F1] [93A1] [93H1] [94D1] [92M1] [99K2] [99K2] [96B1] [98M1] [98M1] [96S1] [96S1] [92G1] [92G1]

Commensurate structure parameters for rubidium adsorption.

Surface

Structure

T [K]

Site

Method

Bond length [Å]

Effective r [Å]

Exc. r [Å]

N

Ref.

Ag(111) Ag (111) Ag(111) Al(111) Al(111) Al(111) Al(111) Al(111) Al(111) Al(111) Cu(111) Rh(111) Ru(0001) Ru(0001)

p(3x3) p(2x2) (¥[¥ 5 p(2x2) p(2x2) (¥[¥ 5 (¥[¥ 5 (¥[¥ 5 (¥[¥ 5 (¥[¥ 5 p(2x2) (¥[¥ 5 p(2x2) (¥[¥ 5

40/300 40/300 40/300 170 100 170 150 100 300 100/300 300 50 50 50

hcp fcc hcp top top top top top subst. subst. top hcp fcc hcp

LEED LEED LEED SXW LEED SXW SXW LEED SXW LEED SXW LEED LEED LEED

3.43±0.03 3.39±0.03 3.40±0.03 3.13±0.10 3.21±0.26 3.13±0.10 3.23±0.10 3.36±0.03 3.72±0.10 3.74±0.02 3.07±0.01 3.20±0.05 3.41±0.05 3.41±0.05

1.98±0.03 1.94±0.03 1.95±0.03 1.70±0.10 1.78±0.26 1.70±0.10 1.80±0.10 1.93±0.03 2.29±0.10 2.27±0.02 1.79±0.01 1.85±0.05 2.07±0.05 2.07±0.05

0.50 0.46 0.47 0.22 0.30 0.22 0.32 0.45 0.81 0.79 0.31 0.37 0.59 0.59

3 3 3 1 1 1 1 1 6 6 1 3 3 3

[97K1] [95L2] [95L2] [92K2] [99A1] [92K2] [94S2] [94N2] [94S2] [94N2] [94S1] [96S1] [94H3] [94H3]

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3.2.1 Alkali metals on metals

147

Commensurate structure parameters for cesium adsorption.

Surface

Structure

T [K]

Site

Method Bond length [Å]

Effective r [Å]

Exc. r [Å]

N

Ref.

Ag(111) Ag(111) Ag(111) Ag(111) Ag(111) Ag(100) Al(111) C(0001) Cu(111) Pd(111) Pd(111) Rh(111) Rh(111) Rh(100) Ru(0001) Ru(0001)

(2¥[¥ 5 p(3x3) (¥[¥ 5 p(2x2) (¥[¥ 5 c(2x4) (¥[¥ 5 p(2x2) p(2x2) p(2x2) (¥[¥ 5 p(2x2) (¥[¥ 5 c(4x2) p(2x2) (¥[¥ 5

40/100 40/100 40/200 40/200 40/300 170 100 80 300 50/592 50/445 50 50 120 80 80

fcc fcc fcc fcc hcp hollow top hollow top hcp hcp top hcp hollow top hcp

LEED LEED LEED LEED LEED SXD LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED

2.12±0.03 2.11±0.04 2.07±0.04 2.08±0.03 2.08±0.03 1.78±0.16 2.03±0.03 2.41 1.73±0.05 1.79±0.07 1.81±0.07 1.62±0.05 1.94±0.05 2.10±0.06 1.90±0.08 2.17±0.02

0.45 0.44 0.40 0.41 0.41 0.11 0.36 0.74 0.06 0.12 0.14 -0.05 0.27 0.43 0.23 0.50

3 3 3 3 3 4 1 6 1 3 3 1 3 4 1 3

[99C1] [97K1] [99C1] [97K1] [97K1] [97M1] [98N1] [89H4] [83L1] [99K2] [99K2] [99K2] [99K2] [89V1] [92O2] [92O2]

3.57±0.03 3.55±0.04 3.51±0.04 3.52±0.03 3.52±0.03 3.72±0.16 3.45±0.03 3.1 3.01±0.05 3.17±0.07 3.19±0.07 3.25±0.05 2.95±0.05 3.44±0.06 3.25±0.08 3.52±0.02

3.2.1.2.7 Incommensurate overlayer rotations The rotational epitaxy of incommensurate layers has been studied for many years in a diverse range of systems, and many models have been devised to describe these systems [91G1]. In this section we will describe a few of the models which are possibly applicable to alkali overlayers. A geometrical model developed by Bohr and Grey [91G1] and applied with success to various metal interfaces, proposes that the overlayer will tend to align itself so that its domain walls (which arise from the static distortion of the lattice) are oriented along a symmetry direction of the substrate. Thus various trajectories of rotational angle vs. lattice misfit are predicted for incommensurate overlayers, depending on their symmetry. A second geometrical model, first applied to alkali metals by Doering [85D2], proposes that the overlayer may form higher-order commensurate (HOC) structures, where the repeat distance of the overlayer relative to the substrate is large compared to the lattice spacing. For a finite system, there are several main trajectories of HOC structures in angle-misfit space, which an overlayer might follow quasicontinuously as its density (and misfit) changes. A more extensive theory was developed by Novaco and McTague (NM) [77N1, 79M1] in which the adatom-adatom interactions are explicitly taken into account in the form of elastic constants. This firstorder theory calculates the response of an elastic overlayer to a small-amplitude substrate potential corrugation. Since, in general, transverse overlayer deformations cost less energy than longitudinal ones, the incommensurate overlayer relaxes by rotating relative to the substrate, i.e. the rotation is essentially a static transverse distortion wave in the overlayer. For any given misfit there is at least one minimumenergy overlayer angle. Since elastic properties depend on the details of the adatom-adatom interactions, one might expect different rotation angles for different adatom-adatom interactions. Fig. 4 shows plots of the angle-misfit trajectories for these models. The reason for the correspondence between the geometrical models and the harmonic NM theory is not completely obvious since the models have quite different origins, but in essence, the HOC and domain-wall alignment models pick out low-energy trajectories, just as the NM theory does. However, the NM theory actually selects the lowest-energy trajectory for a given interadsorbate interaction, whereas the geometric models cannot distinguish between the different trajectories.

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[Ref. p. 169

The NM theory and the geometric models are expected to break down close to commensurate structures where the periodic effects of the substrate on the overlayer are more important. For this regime, a domainwall theory was developed by Shiba [79S1, 80S1] which predicts that close to a commensurate structure where domain walls in the overlayer are relatively sharp, the overlayer (and the domain walls) will remain aligned along a substrate direction until a certain critical misfit is reached. The critical misfit occurs at a point where the domain walls become too weak for the overlayer to sustain the alignment. At this point the overlayer crosses over from a “domain wall” to a “modulated” regime, and the equilibrium angle of rotation moves toward that predicted by the NM theory for a “uniform” overlayer. In the geometrical models, this crossover behavior of the overlayer could be accommodated by the jumping from one trajectory to another, but of course there is no way in these models to predict when or how this might occur. The following table indicates which alkali overlayers have been observed to form rotated incommensurate structures and the density range and angle range over which they have been observed. In general all of the systems behave similarly at the same coverages. On the hexagonal substrates the data in most cases qualitatively follow the predictions of the NM theory for a repulsive dipole interaction. There are deviations from this behavior though, and in some cases (notably Li/Ru(0001)), the geometrical theories may provide a better description. It should be noted, however, that a marked temperature dependence was observed in the rotation angle for alkalis on Ag(111) [96L2] and Cu(100) [85A1], and such a change with temperature may make comparisons to the models difficult. It does rule out a simple application of the geometrical models, though, since a continuous off-trajectory change would not be consistent with their predictions. There are fewer data for rotated alkali layers on square-symmetry substrates, but the data which are available indicate that these systems also agree qualitatively with the NM prediction for repulsive dipoles. Rotational parameters for alkali metal overlayers. Alkali

Substrate

Coverage range Angle range [deg]

Ref.

0.16-0.22 0.33-0.59 0.33-0.44 0.33-0.46 0.33-0.53 0.14-0.20 0.14-0.21 0.14-0.23

5-0 30-18 30-20 30-13 30-14 19.1-0 19.1-0 19.1-0

[91W1] [88C2] [88P1] [86D1] [83D1] [96L2] [96L2] [96L2]

0.33-0.37 0.32-0.38 0.33-0.43

3.3-6.0 3.3-6.0 0-12

[85A1] [92F2] [87B2]

hexagonal on hexagonal: Cs Na K Li Na K Rb Cs

C(0001) Pt(111) Pt(111) Ru(0001) Ru(0001) Ag(111) Ag(111) Ag(111)

hexagonal on square: K K Cs

Cu(100) Ni(100) Rh(100)

The temperature dependence of the overlayer rotation angle for alkalis on Ag(111) was studied in some detail [96L2]. For most coverages, the rotation angle decreases toward 0° as the temperature is increased. The move toward alignment with the substrate is a consequence of the loss of adatom-adatom correlations as the temperature increases. The temperature dependence of this rotation angle has been shown to be consistent with the power law behavior predicted by a mean-field theory of epitaxial rotation [92A2]. This theory could not be applied to the data at low coverages, however. Near the (¥[¥ 5 SKDVH the rotation angle was found to increase, possibly toward 30°, but this could not be confirmed because the rotation transition was pre-empted by melting of the overlayer.

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3.2.1.2.8 Monolayer saturation lattice parameters The nearest-neighbor distances of alkali metals in their saturated monolayers are generally smaller than their metallic diameters derived from the bulk solids. The following table shows the lateral nearestneighbor distances at monolayer saturation for many alkali overlayers along with the ratios of the monolayer nearest-neighbor spacing to the bulk metallic alkali diameter. While it has been occasionally suggested that this is due to charge transfer in the chemisorption bond which makes the effective size of the alkali smaller, calculations for a free-standing Cs monolayer indicate that it is due to the strong intralayer metallic bonding [83W1]. Such a compression is a consequence of the reduced coordination of the atoms in the 2D solid, similar to the shorter bond lengths observed for the lower coordination bonds in general [60P1]. It should be noted, however, that for systems which form islands at low coverages, the equilibrium spacing in these “unconstrained” layers below saturation coverage is generally larger than the bulk spacing [97L1]. This is an indication that the adsorbate-substrate interaction is considerably more important at the lower coverages than it is at monolayer saturation. This compression effect at monolayer saturation has been observed in other metallic adsorption systems, for example Pb/Cu(100) [91L1] or Hg/Cu(100) [92L1]. Looking at overlayers which are commensurate at saturation coverage, it is clear that the presence of a commensurate phase can stabilize the structure at a density extraordinarily compressed relative to the normal bulk alkali spacing, or, in some cases, expanded relative to the bulk. However, it is the incommensurate structures which give us a clearer view of the "true" two-dimensional compression effect. There is some scatter in the measured values for the incommensurate lattice spacings, but there do not seem to be any clear trends which depend on either the substrate material or its symmetry, which is consistent with the notion that this compression is more a property of the 2D layer than of the chemisorption bond. However, the heavier alkali metals experience a larger compression than the lighter ones. From the data we have available, the mean measured compressions (for incommensurate monolayers) are 2% for Li, 5% for Na, 8% for K, 7% for Rb and 15% for Cs. The calculated value for the free-standing Cs monolayer was 11% [83W1]. Monolayer-saturation lattice constants for alkali overlayers. C or I denotes whether the monolayer saturation structure is commensurate, incommensurate, or higher-order commensurate. NN denotes the nearest-neighbor distance and dmetallic denotes the metallic diameter. Alkali

Substrate

NN [Å]

NN/dmetallic

C/I/HOC

Ref.

Li Li Na Na Na Na Na Na Na K K K K K K K K K

Cu(100) Ru(0001) Al(111) Cu(100) Cu(111) Ni(111) Ni(100) Ru(0001) W(112) C(0001) Pd(100) Al(111) Cu(111) Cu(100) Cu(110) Fe(110) Ni(111) Ni(100)

2.94 3.06 3.82 3.61 3.78 3.54 3.52 3.67 3.54 4.92 3.89 4.26 4.4 4.57 4.3 4.31 4.47 4.35

0.94 0.98 1.00 0.95 0.99 0.93 0.92 0.94 0.93 1.03 0.82 0.90 0.92 0.96 0.90 0.91 0.94 0.91

C I C C HOC I C I I C C I I I I I I I

[92T1] [95G2] [93A2] [93E1, 97G3] [91T1] [69G1] [69G1] [83D1] [70P1] [91L2] [87B3] [93A2] [88F1] [84A1] [89F2] [99S1] [88C1] [92F2]

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[Ref. p. 169

Alkali

Substrate

NN [Å]

NN/dmetallic

C/I/HOC

Ref.

K K K K K K Rb Rb Rb Rb Rb Cs Cs Cs Cs Cs Cs Cs Cs Cs Cs Cs Cs Cs Cs

Pd(111) Pd(100) Pt(111) Ru( 10 1 0 ) Ag(111) Ag(100) Al(111) Cu(111) C(0001) Ag(111) Ru(0001) Al(111) C(0001) Ag(111) Cu(111) Cu(100) Cu(110) Ir(100) Mo(110) Ni(111) Ni(100) Pt(111) Ru(0001) W(100) W(110)

4.58 4.30 4.16 4.02 4.54 4.08 4.96 5.10 4.92 4.76 4.69 4.95 4.92 5.01 4.86 4.57 4.7 4.04 4.74 4.71 5.0 4.36 4.69 4.81 4.70

0.96 0.90 0.87 0.84 0.95 0.86 0.97 1.00 0.96 0.93 0.97 0.91 0.90 0.92 0.89 0.84 0.86 0.74 0.87 0.86 0.92 0.80 0.89 0.88 0.86

I C I I I C C C C I C C C C I I I I I I I I C I I

[87M1] [87B3] [88P1] [85D1] [96L2] [90M1] [93A2] [94S1] [94W2] [96L2] [94H3] [93A2] [94W2] [96L2] [88F1] [85C1] [89F2] [85H2] [84R1] [89C1] [72A1] [88C2] [92O2] [89M1] [70F1]

3.2.1.2.9 Condensation transition parameters For certain alkali-substrate combinations, the alkali overlayers undergo a condensation transition at a relatively low coverage instead of compressing uniformly as the coverage is increased. This condensation transition is intriguing because it resembles a nonmetal to metal transition such as those observed in other metal adsorption systems [93P2, 94P1]. (It should be noted, however, that "metalization" transitions have also been observed in the absence of condensation in alkali overlayers [96L5, 99F1].) The structural result of the condensation is the formation of 2D islands of alkali metal, presumably in equilibrium with a 2D vapor phase. In all cases but one, these islands are solid at room temperature, and their melting has not been studied. The alkali adsorption systems for which condensation has been observed are shown in the following table. This table neglects any condensation phenomena which might be associated with large reconstructions such as those which occur on fcc (110) surfaces.

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Alkali adsorption systems which undergo condensation. The information given includes the coverages at which they condense, the structure they condense into, and the temperature. LT denotes a low temperature (usually 80-150 K) and RT denotes room temperature. System

Coverage

Structure

Temp

NN[Å]

Ref.

Na/Al(111) Na/Al(111) Na/Al(111) K/Al(111) Rb/Al(111)

0.15 0.06 0.15 0.1 0.08

LT RT RT LT,RT LT

~4.3 4.96 4.96 4.96 5.73

[93A2] [95B2] [93A2] [93A2] [93A2]

Rb/Al(111)

0.1

"3x3" HOC (√3x√3)R30° (√3x√3)R30° (√3x√3)R30° p(2x2) 1 1    HOC 5 − 6

RT

5.5

[93A2]

Na/Al(100) Na/Al(100)

0.25 0.2

RT LT

4.05 4.05

[95B3] [97L1]

K/Al(100)

0.18

c(2x2) c(2x2) 2 0   HOC   1 5/3 

LT

5.56

[97L1]

Li/Be(0001) Na/Cu(111) Li/Cu(100) K/Cu(100) K/C(0001) Rb/C(0001) K/Ag(100)

0.20 0.18 0.25 0.18 0.02 0.02 0.08

(√3x√3)R30° (3x3)HOC c(2x2) ring p(2x2) p(2x2) c(2x2)

RT LT LT RT LT LT RT

3.97 6.01 3.61 5.6 4.92 4.92 4.09

[94W1] [94F1] [92T1] [85A1, 86A1] [91L2] [94W2] [90M1]

The condensation of Na on Al(111) has been studied theoretically using density-functional theory [93N2, 94S3]. This DFT study indicates that at low coverages, the adsorbate-substrate interaction dominates the overlayer, while at higher coverages the attractive, metallic adsorbate-adsorbate bonds are more important and cause the condensation of the overlayer [93N2, 94S3] The reason that condensation occurs for some alkali overlayers and not others is apparently a result of the balance between the adsorbate-adsorbate and adsorbate-substrate interactions. The stronger dipole moments of the heavier alkali atoms upon adsorption and their lower cohesive energies presumably are related to the fact that they have less of a tendency to condense, compared to the lighter ones. In all cases reported here, alkali metals condense into commensurate or higher-order commensurate phases, suggesting that the extra energy gained by forming a commensurate structure may tip the balance in favor of condensation. This energy must be quite large in the case of alkalis on Al(111) at room temperature where the site is substitutional. As discussed in Section 3.2.1.2.8, the alkali-alkali distance is usually expanded relative to the nearest-neighbor distance in the bulk alkali metal, upon condensation into islands [97L1].

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[Ref. p. 169

3.2.1.2.10 Two-dimensional phase diagrams Equilibrium structures Coverage-temperature phase diagrams provide a useful tool for understanding and comparing alkali metal adsorption systems. The usual method for mapping out these phase diagrams is to measure diffraction intensities of the observed overlayer phases as a function of temperature, over the entire submonolayer coverage range (and sometimes above). The inflection points of the curves obtained in this way are used to identify the disordering temperatures, and these temperatures comprise the order-disorder (or melting) phase boundary. The intermixing of alkalis with some substrates, in particular aluminum and graphite, have complicated the mapping of equilibrium phase diagrams of those systems. Since many of the structures formed on these substrates at low temperatures are metastable, isothermal phase diagrams, such as those shown in the next section, provide a useful alternative. Only a few coverage-temperature phase diagrams have been mapped for alkali metal adsorption on reconstructed surfaces and these are rather complex. The systems for which equilibrium phase diagrams have been determined are shown in the following table. Equilibrium phase diagrams. All phase diagrams were determined using LEED. Alkali

Substrate

Figure

Reference

K K Cs K Cs K K Cs K Cs Na K Cs K Rb Cs Cs

Co(0001) Co (10 10) Cu(111) Cu(110) Cu(110) Ni(111) Ni(100) Ni(111) Pt(111) Rh(100) Ru(0001) Ru(0001) Ru(0001) Ag(111) Ag(111) Ag(111) W(100)

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

[98V1] [92M2] [88F1] [88F3, 89F2] [88F3, 89F2] [93K1] [92F2] [89C1] [88P1] [87B2] [83D1] [85D1] [92O2] [96L2] [96L2] [96L2] [89M1]

The phase diagrams for alkali metal overlayers on flat metal surfaces have several common elements. In the next few paragraphs we will over-generalize slightly in order to describe several aspects which are common to many of these adsorption systems. At coverages significantly lower than monolayer saturation, ordered phases only occur below room temperature. The exceptions to this arise in cases where the adsorption potential is more corrugated, producing a higher degree of site stabilization. At very low coverages the dominant alkali-alkali interaction is repulsive, causing phases with widely-spaced adatoms. Typically, if the temperature is low enough, diffraction rings are observed at low coverages. These rings indicate that there is a reasonably well-defined nearest-neighbor distance but no long-range rotational correlations in these dispersed overlayers. Rings have been observed at very low coverages, even in most cases where the alkali overlayer condenses at some critical coverage into islands. The formation of these uniform-density overlayers depends on a strong repulsive interaction between the alkali adatoms, and rings corresponding to adatom-adatom spacings as large as of 60 Å have been observed [91L2]. All of the available evidence indicates that these dispersed "ring" phases are repulsive fluids except at very low temperatures. The high mobility of the adatoms in these dispersed phases has been the bane of experiments seeking to determine low-coverage adsorption sites on

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fcc (111) surfaces [93A1]. On substrates of somewhat higher corrugation such as fcc(100), some site stabilization has been observed for very low coverages at temperatures near 100 K [93W1]. At intermediate coverages these low-density fluids solidify. In some cases there is a condensation into a dense phase (see section 3.2.1.2.9), but in most cases the overlayer forms a low-density solid phase. The temperature stability of these solids increases rapidly as their density increases, suggesting that if the temperature is lowered sufficiently, these solid phases can be produced at extremely low densities. The symmetry of alkali metal overlayers, even on surfaces having square or rectangular symmetry, is almost always hexagonal or quasi-hexagonal, except in cases of exceptional substrate corrugation. Commensurate structures are usually more stable than incommensurate structures of a similar density. In general, more open substrate surfaces have a larger corrugation for alkali adsorption than closepacked ones, and the larger alkali metal atoms are affected less by any given substrate periodicity than the smaller ones. The refractory metals provide a larger corrugation than the lighter transition metals. The importance of the substrate corrugation for any particular system is evident by the relative stability of the commensurate phases, both to changes in coverage and to increasing temperature. Since the adatom-adatom and adatom-substrate interactions change with the coverage, the corrugation of the chemisorption potential can also change with coverage, even causing the equilibrium adsorption site to change with coverage (see section 3.2.1.2.6). On more corrugated substrates, the coverage-driven transitions between commensurate phases tend to be first-order transitions, in which the intermediate coverage phases consist of coexisting commensurate structures and the diffraction pattern indicates the presence of both commensurate phases. On very smooth substrates such as the fcc(111) surfaces, incommensurate phases tend to dominate the phase diagram, and the stability of these incommensurate phases depends largely on the strength of the adatom-adatom interactions which stabilize them. Intermediate substrate corrugation causes the formation of density modulations, or domain walls in these incommensurate phases, and a further increase in the corrugation can lead eventually to the formation of microscopic domains of two (or more) commensurate structures, and finally to the type of phase coexistence described above [94N1, 94N3]. On fcc(100) surfaces, the overlayers are frequently distorted uniaxially, i.e. they form a quasihexagonal structure in which domain walls form in one direction (e.g. for K/Ni(100) [92S2]). An interesting variation on these structures are the uniaxial structures formed by Na adsorption on Cu(100) [97G3]. These structures consist of rows of adatoms only a few atoms wide, separated by several lattice spacings. Since the corrugation experienced by Na on Cu(100) is only about 50 meV [97G2], the mechanism behind the formation of these structures seems to be different from those observed in other uniaxial structures, and apparently includes a condensation transition. These 1D structures are similar to the 1D structure observed for Tl on Cu(100) [91B3], which is proposed to exhibit a Peierls distortion [64P1]. The corrugation of the substrate also provides the field which leads to the non-symmetry rotation of incommensurate overlayers, as described in Section 3.2.1.2.7. Other substrate effects can arise from defects in the substrate structure. Substrate steps can cause pinning of the epitaxial rotation of the overlayer. Steppinning of incommensurate phases seems to be more prevalent at low coverages than at high coverage, and it has been demonstrated that alkali metal atoms at low coverage tend to adsorb at step sites [95L1]. This tendency to align along the steps is usually overridden at higher coverages, presumably by the increasingly important collective interactions as the density increases. Step pinning not only causes incommensurate overlayers to align along the step direction, it can cause or prevent the formation of particular commensurate phases. For instance, in the low-coverage potassium overlayer on Cu(111) or Ni(111), the (¥[¥ 5 phase was not observed, possibly due to step pinning along the 0° direction. The saturation of the monolayer for most of these overlayers occurs when the adatom-adatom spacing is smaller than that dictated by their metallic diameters, as discussed in Section 3.2.1.2.8. In several cases it has been observed that the density of the monolayer does not saturate until part of the second layer is adsorbed. There have been few studies on the disordering of alkali metal overlayers. The low-density incommensurate structures on Cu(111) and Ni(111) were found to melt into fluids which are highly correlated [88F2, 89C2]. The highly correlated nearest-neighbor distance in these fluids should not come as a surprise since the appearance of diffraction rings at even lower coverages indicates strong adatom-adatom interactions. However, rotational correlations are observed in these overlayers over a wide temperature range and are interesting in light of the theories for 2D melting in systems having repulsive interactions [89C2].

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[Ref. p. 169

Non-equilibrium phase diagrams In cases where equilibrium phase diagrams are difficult to determine, it is useful at least to have a nonequilibrium structure vs. coverage diagram for a specific temperature. These diagrams are typically measured by observing the diffraction pattern as a function of coverage for a specified temperature. Non-equilibrium phase diagrams. All were determined using LEED with the exception of Na/Cu(100) at 50 K and 100 K, which was determined by He-atom diffraction. Alkali

Substrate

Temp [K]

Figure

Reference

Na K Rb Cs Na K Li Na Na K Cs Li Na K Rb Na

Al(111) Al(111) Al(111) Al(111) Al(111) Al(111) Cu(100) Cu(100) Cu(100) graphite(0001) graphite(0001) Ni(100) Ag(100) Ag(100) Ag(100) Pt(111)

100/300 100/300 100/300 100/300 100 100 180/300 50/100 180 90 90 230/370 180/300 180/300 180/300 85

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

[93A2] [93A2] [93A2] [93A2] [97L1] [97L1] [98T1] [97G3] [94M2] [91L3] [96H2] [98T1] [98T1] [98T1] [98T1] [88C2]

3.2.1.2.11 Adsorbate substrate intermixing Alkali metals on certain metal surfaces have a propensity to intermix with the substrate surface atoms, often producing complex surface alloy structures. A recent review of the complex structures formed by alkali adsorption on fcc surfaces describes many of these systems [98T1]. These structures will be discussed in chapter 4.1 in this volume; here we provide a summary. The intermixed structures that have been observed are all activated structures; i.e. their formation requires some thermal energy. The temperatures required to form these structures range from about 200 K to 400 K. Some general features of the intermixed structures studied thus far were summarized in Reference [98T1] as: 1) The intermixed structures reflect the symmetry of the substrate. 2) The formation of intermixed structures does not depend on miscibility of the alkalis and the substrate metals. 3) The alkali adatoms tend to occupy the highest coordination sites. 4) The alkali adatoms tend to be located closer to the vacuum side of the surface than substrate atoms. 5) The substrate atoms displaced in the intermixing process migrate to and from substrate step sites. The simplest cases of intermixing are those that involve the substitution of alkali atoms for top-layer substrate atoms, i.e. the alkali adatoms reside in vacancies in the substrate surface. Experimental measurements of these simple intermixed structures are included in Section 3.2.1.2.6, and are given below again for completeness. In general, these structures involve a higher adatom coordination than is possible without intermixing. The substitutional (¥[¥ 5 VWUXFWXUH IRUPHG E\ alkalis on Al(111) has been studied in some detail theoretically. DFT calculations have found that the formation of these structures is related to the low energy required to form vacancies on the Al(111) surface [95S1], as well as the lowering of the total energy which results from the adatoms sitting closer to the surface and having a higher coordination [95S1]. Lando lt -Börnst ein New Ser ies III/42A1

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More complex intermixed structures involving a larger disturbance of the surface layers have also been observed and characterized. The structures that have been characterized are listed in the table below. Theoretical studies of these systems generally have found that the observed structures are in fact the lowest-energy structures [97Q1, 98O1]. The alkali-induced missing-row reconstructions of the fcc(110) can also be viewed as intermixed structures, although in these cases, the alkali adatoms do not necessarily form commensurate overlayers. These structures always involve missing single substrate rows, and the alkali adatoms reside in the troughs. The fcc(110) missing-row reconstructions were the first alkaliinduced reconstructions to be observed [83H1], and were ascribed to the instability of the fcc(110) surfaces since such structures were also observed on clean Au, Pt and Ir (110) surfaces. However, similar alkali-induced reconstructions have now been observed on stable fcc(100) surfaces. The stability of the missing-row structures has been attributed to the polar nature of the alkali bond to the surface which is maximized when the alkali atoms are embedded in the surface [87B1]. The evidence for this is that at higher alkali coverages where the bond formed is less polar, the number of alkali adatoms in substitutional sites decreases, sometimes to zero [87B1, 98T1]. In the following table, the intermixed structures have been classified according to the scheme employed in the recent review [98T1]. The first class, fcc(001)-(2x1)-xAM refers to structures in which the substrate undergoes a missing-row reconstruction, and alkali atoms occupy the resulting troughs. The "x" corresponds to a fractional number of alkali atoms per unit cell, typically between 0.6 and 0.8. A similar situation exists for the fcc(001)-(3x3)-xAM and fcc(110)-(1x2)-xAM structures. The notation 1x3L corresponds to "light" missing row structures in which there are double missing rows separated by single rows, while 1x3H corresponds to "heavy" missing row structures in which there are double rows separated by single missing rows. In some cases, the intermixing involves just one substrate layer, but in other cases several layers may be involved. These structures are discussed in more detail in Chapter 4.1 of this volume.

Intermixed structures All geometry determinations were made using LEED, with the exception of the SEXAFS study of Al(111)-(¥[¥ 51D Structure

Adatom geometry

Ref.

Cu(100)-(2x1)-xLi Ag(001)-(2x1)-xNa Ag(001)-(2x1)-xK Ag(001)-(2x1)-xRb

incommensurate incommensurate incommensurate incommensurate

[93M3] [98T1] [98T1] [98T1]

Al(001)-c(2x2)-2Na Al(001)-c(2x2)-2Li

substitutional substitutional

[95B3] [99P1]

Cu(001)-(3x3)-5Li Cu(001)-(4x4)-10Li Ag(001)-(3x3)-5Na Ag(001)-(4x4)-10Na Fe(001)-(3x3)-K Ni(001)-(4x4)-10Li Ni(001)-(5x5)-17Li

complex complex complex complex substitutional complex complex

[95M2] [95M3] [98T1] [97J2] [96B2] [97J1] [97J1] cont.

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[Ref. p. 169

Intermixed structures (cont.) Structure

Adatom geometry

Ref.

Ag(001)-(3x1)-xK Ni(001)-c(6x2)-xNa Ag(110)-(1x2)-K Ag(110)-(1x2)-Cs Ag(110)-(1x2)-Cs Au(110)-(1x3) Cu(110)-(1x2)-K Cu(110)-(1x2)-Cs Cu(110)-(1x3L)-Cs Cu(110)-(1x3H)-Cs Ni(110)-(1x2)-K Pd(110)-(1x2)-Cs

incommensurate incommensurate incommensurate incommensurate incommensurate incommensurate incommensurate incommensurate incommensurate incommensurate incommensurate incommensurate

[91O1, 96I1] [98T1] [87F1] [89B1, 95S2] [95S2] [89H1] [90H1, 94H2] [90H1] [96M1] [96M1] [92J1, 93S1] [88B1]

Au(110)-c(2x2)-K

complex

[89H6]

Cu(110)-(4x2)-3Li

complex

[97M2]

Al(111)-(¥[¥ 5/L Al(111)-(¥[¥ 51D Al(111)-(¥[¥ 5. Al(111)-(¥[¥ 55E

substitutional substitutional substitutional substitutional

[96N1] [91S2, 94B3] [94S4] [94N2]

Al(111)-(2¥[¥ 5&V complex Al(111)-(2x2)-2Na complex Cu(111)-(2x2)-3Li complex

[96A1] [95B1] [95M1]

Al(111)-(2x2)-(Na+K) Al(111)-(2x2)-(Na+Rb) Al(111)-(2x2)-(Na+Cs)

[96C1] [96C1] [96C1]

complex complex complex

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 169]

3.2.1 Alkali metals on metals

157

Figures for 3.2.1 3.0

Cs/Ru(0001)

2.5

Heat of adsorption Q [eV]

Work function φ

∆φML

∆φmin

2.0

1.5

1.0

0.5 0

0

0.2

0.4 0.6 Coverage θ

Alkali coverage

0.8

Fig. 2. The heat of Cs adsorption on Ru(0001) as a function of the coverage as derived from TPD measurements [96P1, 98O2]. The open points are from theory and the arrow on the right indicates the calculated cohesive energy of Cs [96P1].

Fig. 1. Typical behavior of the work function for alkali atoms adsorbed on a metal surface.

50

Li

45

frequency (meV)

40 35 30 25

K

Na perpendicular

20 15

Cs Rb

10

parallel

5 0

0

0.1

0.2

M-1/2

0.3

0.4

(amu-1/2 )

Fig. 3. On-surface alkali-metal perpendicular and parallel vibration energies plotted against the inverse square root of the mass number of the alkali. The lines are to guide the eye.

Lando lt -Bö rnst ein New Ser ies III/42A1

1.0

3.2.1 Alkali metals on metals

158

[Ref. p. 169

NM, dipole potential NM, Lennard-Jones potential Shiba HOC positions

25°

Rotation Angle

20°

15°

10°

5°

0° 0.0

0.1

0.2

0.3

Misfit

Fig. 4. The predictions of the various models for rotational epitaxy, for a hexagonal overlayer on a hexagonal substrate. Misfit is defined as (LO-2LS)/2LS, where LO is the overlayer lattice spacing and LS is the substrate lattice spacing. NM denotes the Novaco-McTague model, HOC denotes the higher-order commensurate positions.

700

K/Co(0001) desorption

(2×2) rotated

100

0

0.1

0.2

0.3 0.4 Coverage θ

450 350

p(1x1) lattice gas

distributed vacancy structure

250

ring

200

550

0.5

0.6

Fig. 5. Coverage-temperature phase diagram for K on Co(0001) [98V1].

multilayers

multilayers

c(2x2) high density domain wall

Temperature (K)

θsat = 0.31 ± 0.01

400 300

K/Co(1010)

650

disorder

low density domain wall

500

split

Temperature T [K]

600

150 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Coverage Fig. 6. Phase diagram for K on Co( 10 10 ) [92M2]. The phase boundaries are approximate.

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 169]

3.2.1 Alkali metals on metals

159

500

Cs/Cu(111) p(2×2)

Temperature T [K]

400 disordered

300 "hexatic" fluid

200

100

incommensurate solid 2nd layer onset

Fig. 7. The phase diagram determined by LEED for Cs on Cu(111) [88F1]. 0

0.1

0.2 Coverage θ

400

(W

Temperature T [K]

(LR)+(D)

320

(1×3R)+(D)

360

2 1×

R)+

0.3

0.4

K/Cu(110)

(S) (W1×3R)+(S)

(QHR)

(1×2R)+(S) (1×3R)+(S)

280 240 (RT)+(D)

200

(R

T )+

160

(SO

) (QHU)

(DU) 120 80

(QHU)+(IG) 0

0.1

0.2

0.3 0.4 Coverage θ

0.5

0.6

0.7

Fig. 8. Phase diagram for K adsorption on Cu(110). QHU indicates the quasihexagonal structure on the unreconstructed substrate. IG denotes the immobile gas. DU indicates the disordered Cs on the unreconstructed substrate. RT indicates the reconstructing phase transition region. SO indicates short-range-ordered quasihexagonal overlayer. D indicates disordered atoms on the reconstructed substrate. S denotes regions where there are streaks in the diffraction pattern. LR indicates a local substrate reconstruction. 1x2R and 1x3R indicate the reconstructed substrate. W1x2R and W1x3R indicated a well-ordered reconstructed substrate. QHR indicates the quasihexagonal layer on the 1x3 reconstructed substrate.

Lando lt -Bö rnst ein New Ser ies III/42A1

3.2.1 Alkali metals on metals

160

400

Cs/Cu(110)

(W1×2R)+(S)

(W1×3R)+(S)

Temperature T [K]

(LR)+(D)

(1×3R)+(D)

360 320

[Ref. p. 169

(1×2R)+(S) (1×3R)+(S)

280 240 (RT)+(D)

200

( RT

)+(

SO)

160

(QHU)

(DU) 120 80

(QHU)+(IG) 0

0.1

0.2

0.3 Coverage θ

0.4

0.5

0.6

Fig. 9. Phase diagram for Cs adsorption on Cu(110). QHU indicates the quasihexagonal structure on the unreconstructed substrate. IG denotes the immobile gas. DU indicates the disordered Cs on the unreconstructed substrate. RT indicates the reconstructing phase transition region. SO indicates short-range-ordered quasihexagonal overlayer. D indicates disordered atoms on the reconstructed substrate. S denotes regions where there are streaks in the diffraction pattern. LR indicates a local substrate reconstruction. 1x2R and 1x3R indicate the reconstructed substrate. W1x2R and W1x3R indicated a well-ordered reconstructed substrate. QHR indicates the quasihexagonal layer on the 1x3 reconstructed substrate.

Temperature (K)

500

p(2x2)

K/Ni(111)

incommensurate solid

400

2nd layer onset

modulated fluid

300 200

isotropic fluid 0

0.1

0.2

0.3

0.4

Coverage Fig. 10. Submonolayer coverage-temperature phase diagram for K/Ni(111) [93K1].

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 169]

3.2.1 Alkali metals on metals

161

400

K/Ni(100)

th des erma orp l tion

isotropic fluid + c(4x2) c(4x2)

( 21 5/30 )

0.25

0.3

diffusion or desorption

2nd layer formation

uniaxially incommensurate phase

200

100 0.2

uniaxial incommensurate

300

isotropic fluid

Temperature (K)

disordered phase

epitaxial rotation

rectangular

hex

0.35

Fig. 11. Submonolayer coveragetemperature phase diagram for K/Ni(100) [92F2].

0.4

Coverage 500

Cs/Ni(111)

K/Pt(111)

400 400

200

150 100

IS 0

0.05

0.10

0.15 0.20 Coverage θ

HFD + second layer 0.25

0.30

0.35

Fig. 12. Submonolayer coverage-temperature phase diagram for Cs/Ni(111) [89C1].

Lando lt -Bö rnst ein New Ser ies III/42A1

100

0

0.1

0.2

0.3

MULTIPHASE

IR + second layer

(√3x√3)R30˚

AS or F1

F2

2x2

250

DISORDER

300

MIXED PHASE

300

coexistence

Temperature T [K]

IS IR + second layer

(√7x√7)R19.1˚

C

Temperature (K)

disordered

350

200

1st LAYER DESORPTION

RINGS

450

0.4

Coverage Fig. 13. Phase diagram for K adsorption on Pt(111) [88P1]. In the multiphase region a continuous compression of the overlayer results in rotational epitaxy or aligned structures.

3.2.1 Alkali metals on metals

162

400

Cs/Rh(100) (333±5) K

Temperature (K)

Lattice Gas 300

(Desorption)

(258±5) K 7 8

200

(155±5) K

4

5

3

(145±5) K

6

2

1

100

0 0

0.1

0.2

0.3

0.4

0.5

Coverage

500

[Ref. p. 169 Fig. 14. Experimental phase diagram for the system Cs/Rh(100) [87B2]. The encircled numbers denote the different phases observed: (1): c(2√2x4√2)R45º, (2): (√5x√5)Rarctan1/2, (3): c(4x2), (4): Incommen-surate phase with superstructure spots shifting from positions of phase (3) to positions of phase (5), (5): (3x2)-2Cs, (6): (√5x√5)Rarctan1/2-2Cs, (7): hexagonal incommensurate phase with rotational epitaxy, (8): hexagonal incommensurate phase c(2√2x2√2/3)R45º.

FIRST LAYER DESORPTION

Na/Ru(0001)

Temperature (K)

400 DISORDERED (3/2x3/2)

300

LARGE RING+ (3/2x3/2)

IRREVERISBLE TR

200 SMALL RING

100

(2x2)

TR

SPLIT (√3x√3)R30˚ (√3x√3) R30˚

TR

LARGE RING

(2x2) + (√3x√3)R30˚

0

0.2

0.3

0.4

Fig. 15. Phase diagram for Na adsorbed on Ru(0001) at 80 K [83D1]. Dashed lines indicate approximate phase boundaries. TR indicates transition region.

0.5

Coverage

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 169]

3.2.1 Alkali metals on metals

500

K/Ru(0001)

Cs/Ru(0001)

400

Desorption

Ring

(2×2)

Ring

(2x2)

0.1

0.2

Ring

Multilayers

200

(√3x√3)R30˚

3

200

300

Rotated Structures

Temperature (K)

300

100

Desorption

Disorder

400 Temperature T [K]

163

100

0

0.1

0.2 Coverage θ

0.3

0.4

0

0.3

0.4

0.5

Coverage Fig. 16. Phase diagram for the adsorption of K on Ru(0001) [85D1].

Fig. 17. Phase diagram for the adsorption of Cs on Ru(0001). The hash-marked area represents a qualitative result [92O2].

400

75

K/Ag(111)

desorp

tion

F 60

0

IR 0

a

0.1

IA

0.2 Coverage

p(2×2)

IA

0.3

3

30

Co-

100

Co-(IA & IA30°)

200

45

p(3×3)

Temperature T [K]

F

Co-(IA & IA30°)

Temperature T [K]

300

IR

IR

HOC

0.14 0.16 Coverage θ

0.18

7

15

IA

0.4

b

0 0.10

0.12

0.20

Fig. 18a, b. (a) Coverage-temperature phase diagram for K/Ag(111) [96L2]. The open points indicate the inflection points of the disordering curves (LEED intensity vs. temperature plots). Regions which are shaded denote commensurate phases. IA denotes an incommensurate phase, aligned with the substrate, IA30° denotes an incommensurate phase, aligned at 30° with respect to the substrate, Co- denotes a coexistence of two phases, F denotes a fluid, ¥ GHQRWHV WKH ¥[¥ 5 VWUXFWXUH ¥ GHQRWHV WKH ¥[¥ 5 VWUXFWXUH +2& GHQRWHV D higher-order commensurate phase. (b) Provides an enlarged view of the coverage range 0.10 to 0.20.

Lando lt -Bö rnst ein New Ser ies III/42A1

3.2.1 Alkali metals on metals deso

Rb/Ag(111)

90 n

a

0

0.1

0.2 Coverage

3

0.3

60 IA

IA30°

45 30

2 3

15 Co-

IA

Co-(IA & IA30°)

IR

0

p(2×2)

IA

Co-(IA & IA30°)

200

Temperature T [K]

HOC

100

F

75

F

300 Temperature T [K]

rptio

0.4

b

0 0.06

0.10

7

IR

HOC

IR

Co-

400

[Ref. p. 169

p(3×3)

164

0.14 Coverage

0.18

0.22

Fig. 19a, b. (a) Coverage-temperature phase diagram for Rb/Ag(111) [96L2]. The open points indicate the inflection points of the disordering curves (LEED intensity vs. temperature plots). Regions which are shaded denote commensurate phases. IA denotes an incommensurate phase, aligned with the substrate, IA30° denotes an incommensurate phase, aligned at 30° with respect to the substrate, Co- denotes a coexistence of two phases, F denotes a fluid, 2¥ GHQRWHV WKH ¥[¥ 5 VWUXFWXUH ¥ GHQRWHV WKH ¥[¥ 5 VWUXFWXUH ¥ GHQRWHV WKH (¥[¥ 5 VWUXFWXUH +2& GHQRWHV D KLJKHURUGHU FRPPHQVXUDWH SKDVH (b) provides an enlarged view of the coverage range 0.06 to 0.22. (Error: the arrow IA30o denotes the small region left of the Co- region !).

400

de

Cs/Ag(111)

sor

75 pti

F

on

60

IR 0

0.1

0.2 Coverage θ

0.3

IR

7

IR

30

IA Co-

IA

IA & IA30°

IA

3

45

2 3 IA30° CoIA p(3×3)

200

100

a

Temperature T [K]

F

p(2×2)

Temperature T [K]

300

15

0.4

b

0 0.06

0.08

0.10 0.12 Coverage θ

0.14

0.16

Fig. 20a, b. (a) Coverage-temperature phase diagram for Cs/Ag(111) [96L2]. The open points indicate the inflection points of the disordering curves (LEED intensity vs. temperature plots). Regions which are shaded denote commensurate phases. IA denotes an incommensurate phase, aligned with the substrate, IA30° denotes an incommensurate phase, aligned at 30° with respect to the substrate, Co- denotes a coexistence of two phases, F denotes a fluid, 2¥ GHQRWHV WKH ¥[¥ 5 VWUXFWXUH ¥ GHQRWHV WKH ¥[¥ 5 VWUXFWXUH ¥ GHQRWHV WKH (¥[¥ 5 VWUXFWXUH (b) provides an enlarged view of the coverage range 0.06 to 0.16.

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 169]

3.2.1 Alkali metals on metals

Na/Al(111)

500

Cs/W(100)

(1×1)W + 400 disordered Cs

"3×3"

Csdesorption

4×4

1×1

T = 100 K

300

c(2×2)W + 200 disordered Cs 130 K

c(2×2)W + incommens. Cs(II)

"ring"

0

0.1

0.2

2×2

1×1

RT

( 3× 3)R30°

c(2×2)W c(2×2)W + + incommens. p(2×2)Cs Cs(I)

100

second layer growth

Temperature T [K]

165

hex.

0.3 0.4 Coverage θ

0.5

0.6

0

0.1

0.2

0.3 0.4 Coverage θ

0.5

0.6

0.7

Fig. 21. Experimental phase diagram for the adsorption of Cs on W(100) [89M1].

Fig. 22. Na/Al(111)-structures observed following deposition at 100K and at 300K, respectively [93A2].

K/Al(111)

Rb/Al(111) ( 3× 3)R30° split 3

1×1

T = 100 K

( 3× 3)R30°

1×1 2×2

T = 100 K

( 3× 3)R30° RT

1×1

0

0.1

0.3 0.4 0.5 0.6 Coverage θ Fig. 23. K/Al(111)-structures observed following deposition at 100K and at 300K, respectively [93A2].

Lando lt -Bö rnst ein New Ser ies III/42A1

0.2

1×1

0

( 0.1

1 1 5 –6

)

( 3× 3)R30° RT

0.2 0.3 0.4 0.5 Coverage θ Fig. 24. Rb/Al(111)-structures observed following deposition at 100K and at 300K, respectively [93A2].

3.2.1 Alkali metals on metals

166

Cs/Al(111)

[Ref. p. 169

Na/Al(100) T = 100 K

( 3× 3)R30°

ring 1×1

T = 100 K

2×2

split c(2×2)

1×1

c(2×2)

ring

(

1×1

1 1 5 –6

)

( 3× 3)R30° RT

0.2 0.3 0.4 Coverage θ Fig. 25. Cs/Al(111)-structures observed following deposition at 100K and at 300K, respectively [93A2].

0.7 0.3 0.4 0.5 0.6 Coverage θ Fig. 26. Na/Al(100)-structures observed following deposition at 100 K.

K/Al(100)

Li/Cu(100)

T = 100 K

T = 180 K

0

0.1

0

0.1

0.2

complex * ** structures

c(2×2) compression

"ring"

T = 300 K

( 5/30 –12 )

1×1

2×1 c(2×2)

3×3

4×4

disorder

0.6

0.7

streak*** 0

0.1

0.2

0.3 0.4 0.5 0.6 Coverage θ Fig. 27. K/Al(100)-structures observed following deposition at 100 K.

0

0.1

0.2

0.3

0.4 0.5 Coverage θ

0.8

* c(7 2× 2)R45° ** c(5 2× 2)R45° *** 1/2Θ 0 –1/4Θ 2 Fig. 28. Surface structures observed for Li/Cu(100) at 180 K and 300 K as a function of Li coverage [98T1].

(

)

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 169]

3.2.1 Alkali metals on metals

167

Na/Cu(100)

Na/Cu(100) T = 50 K

T = 180 K

c(2×2)

3×1

c(4 2×2 2)R45°

4×1

p(4×2)

( 10× 10)R18.4°

( –23 02 )

"ring"

"ring"

c(2×2) c(14×2)

"ring" T = 100 K 0

0.1

0.2 0.3 0.4 0.5 Coverage θ Fig. 29. Surface structures observed for Na/Cu(100) a 50 K and 100 K as a function of Na coverage [97G3].

0.2 0.3 0.4 0.5 Coverage θ Fig. 30. Surface structures observed for Na/Cu(100) a 180 K as a function of Na coverage [94M2]

K/C(graphite)

Li/Ni(001)

T = 90 K

"ring"+ p(2×2)

0

T = 230 K

p(2×2)

1×1

"ring" 0.22 0.25 Coverage θ Fig. 31. Schematic representation of the progression of equilibrium phases as a function of coverage for K on graphite at 90 K (the lable "ring" + p(2x2) applies to the whole section from 0.02 to 0.22) [91L2] . 0.02

0

rotated incomm. p(2×2)

"ring" 0.015

0.14 0.16 0.22 0.25 Coverage θ Fig. 32. Schematic representation of the progression of equilibrium phases as a function of coverage for Cs on graphite at 90 K [96H2].

Lando lt -Bö rnst ein New Ser ies III/42A1

0.2

0.3

* **

0.4 0.5 Coverage θ

0.6

q-hex. 0.7

0.8

** c(5 2× 2)R45°

T = 370 K c(2×2)

1×1

( 7× 7)R19.1° disorder

0.1

c(2×2)

* c(9 2× 2)R45°

Cs/C(graphite) T = 90 K

0.1

0

0.1

0.2

0.3

0.4 0.5 Coverage θ

4×4 0.6

* disorder 0.7

0.8

* 5×5 Fig. 33. Schematic diagram of the observed structures for Li adsorption on Ni(100) at 230K and 370K.

3.2.1 Alkali metals on metals

168

[Ref. p. 169

K/Ag(001)

Na/Ag(001) T = 300 K

T = 300 K

2×1

2×1

1×1

4×4 4×2 2×1

3×3

1×1

3×1

streak

streak

T = 180 K

T = 180 K

1×1

0

0.1

c(2×2)

0.2

0.3

0.4 0.5 Coverage θ

*

** *** q-hex.

0.6

0.7

0.8

* c(9 2× 2)R45° ** c(7 2× 2)R45° *** c(5 2× 2)R45°

q-hex.

1×1

0

0.1

0.2

0.3

c(2×2)

0.4 0.5 Coverage θ

hex.

0.6

0.7

0.8

Fig. 35. Structures observed for K/Ag(100) as a function of coverage at 180 K and 300 K, respectively [98T1].

Fig. 34. Structures observed for Na/Ag(100) as a function of coverage at 180 K and 300 K, respectively [98T1].

Rb/Ag(001)

Na/Pt(111) T = 85 K

T = 300 K 2×1 1×1

p(2×2) 0

T = 180 K

qc c (4×2) (3×2) hex.

1×1

0.1

incommensurap(2×2) te

3×3 streak

0

coexistence

"ring"

0.2

0.3

0.4 0.5 Coverage θ

0.7

0.2

0.3 Coverage θ

0.4

0.5

0.6

Fig. 37. Structures observed for Na/Pt(111) as a function of coverage at 85 K [88C2].

2nd layer

0.6

0.1

( 3× 3)R30°

0.8

Fig. 36. Structures observed for Rb/Ag(100) as a function of coverage at 180 K and 300 K, respectively [98T1].

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3.2.1 Alkali metals on metals

169

References for 3.2.1 23L1 25L1 27T1 29L1 31L1 32L1 35G1 46M1 60P1 64P1 66S1 67G1 69G1 69M1 69S1 70F1 70G1 70M1 70P1 71C1 71F1 71L1 71P1 72A1 72K1 72T1 72V1 73A1 73D1 73M1 73P1 73P2 74A1 74M1 74P1 74T1 75A1 75A2 75B1 75D1 75P1 76B1 76H1 76M1 77B1

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91D2 91G1 91G2 91L1 91L2 91L3 91M1 91M2 91O1 91P1 91R1

3.2.1 Alkali metals on metals Barnes, C.J., Lindroos, M., Holmes, D.J., King, D.A.: Surf. Sci. 219 (1989) 143. Bonzel, H.P., Bradshaw, A.M., Ertl, G.: Physics and Chemistry of Alkali Metal Adsorption. Amsterdam: Elsevier, 1989. Chandavarkar, S., Diehl, R.D., Faké, A., Jupille, J.: Surf. Sci. 211/212 (1989) 423. Chandavarkar, S., Diehl, R.D.: Phys. Rev. B 40 (1989) 4651. Duszak, R., Prince, R.H.: Surf. Sci. 216 (1989) 14. Frank, K.H., Sagner, H.-J., Heskett, D.: Phys. Rev. B 40 (1989) 2767. Fan, W.C., Ignatiev, A.: J. Vac. Sci. Technol. A 7 (1989) 2115. Häberle, P., Fenter, P., Gustafsson, T.: Phys. Rev. B 39 (1989) 5810. Hohlfeld, A., Horn, K.: Surf. Sci. 211/212 (1989) 844. Horn, K., Hohlfeld, A., Somers, J., Lindner, T., Hollins, P., Bradshaw, A.M.: Phys. Rev. Lett. 63 (1989) 1536. Hu, Z.P., Li, J., Wu, N.J., Ignatiev, A.: Phys. Rev. B 39 (1989) 13201. Hrbek, J., Shek, M.-L., Sham, T.K., Xu, G.-Q.: J. Chem. Phys. 91 (1989) 5786. Häberle, P., Gustafsson, T.: Phys. Rev. B 40 (1989) 8218. Ishida, H.: Phys. Rev. Lett. 63 (1989) 1535. Müller, K., Besold, G., Heinz, K.: Structures and phases of adsorbed alkali metals, in: Physics and Chemistry of Alkali Metal Adsorption. Bonzel, H.P., Bradshaw, A.M., Ertl, G. (eds.), Amsterdam: Elsevier, 1989, p. 65. Maus-Friedrichs, W., Hoermann, H., Kempter, V.: Surf. Sci. 224 (1989) 112. Von Eggeling, C., Schmidt, G., Besold, G., Hammer, L., Heinz, K., Müller, K.: Surf. Sci. 221 (1989) 11. Wandelt, K.: Work-function changes due to alkali metal adsorption, in: Physics and Chemistry of Alkali Metal Adsorption. Bonzel, H.P., Bradshaw, A.M., Ertl, G. (eds.), Amsterdam: Surface Science,1989, p. 25. Winograd, N., Chang, C.-C.: Phys. Rev. Lett. 62 (1989) 2568. Astaldi, C., Rudolf, P., Modesti, S.: Solid State Commun. 75 (1990) 847. Campbell, C.T.: Ann. Rev. Phys. Chem 41 (1990) 775. Duszak, R., Prince, R.H.: Surf. Sci. 226 (1990) 33. Hu, Z., Pan, B., Fan, W., Ignatiev, A.: Phys. Rev. B 41 (1990) 9692. Lindgren, S.Å., Svensson, C., Walldén, L.: Phys. Rev. B 42 (1990) 1467. Modesti, S., Chen, C.T., Ma, Y., Meigs, G., Rudolf, P., Sette, F.: Phys. Rev. B 42 (1990) 538 1. Nielsen, H.B., Burghaus, U., Broström, G., Matthias, E.: Surf. Sci. 234 (1990) L271. Riffe, D.M., Wertheim, G.K., Citrin, P.H.: Phys. Rev. Lett. 64 (1990) 571. Rudolf, P., Astaldi, C., Modesti, S.: Phys. Rev. B 42 (1990) 1856. Barnes, C.J., Hu, P., Lindroos, M., King, D.A.: Surf. Sci. 251/252 (1991) 561. Bonzel, H., Pirug, G., Ritke, C.: Langmuir 7 (1991) 3006. Binns, C., Norris, C.: J. Phys.: Condens. Matter 3 (1991) 5425. Diehl, R.D.: Structures and phase transitions in alkali metal overlayers on transition metal surfaces, in: Phase Transitions in Surface Films 2, Taub, H., et al. (eds.), New York: Plenum Press, 1991, p. 97. Dudde, R., Johansson, L.S.O., Reihl, B.: Phys. Rev. B 44 (1991) 1198. Grey, F., Bohr, J.: Epitaxial rotation and rotational phase transitions, in: Phase Transitions in Surface Films 2, Taub, H., et al. (eds.), New York: Plenum Press, 1991, p. 83. Gaspar, J.A., Eguiluz, A.G., Tsuei, K.-D., Plummer, E.W.: Phys. Rev. Lett. 67 (1991) 54. Li, W., Lin, J.-S., Karimi, M., Moses, C., Vidali, G.: Appl. Surf. Sci. 48/49 (1991) 160. Li, Z.Y., Hock, K.M., Palmer, R.E.: Phys. Rev. Lett. 67 (1991) 1562. Li, Z.Y., Hock, K.M., Palmer, R.E., Annett, J.F.: J. Phys.: Condens. Matter 3 (1991) S103. Matthew, J.A.D., Netzer, F.P., Astl, G.: Surf. Sci. 259 (1991) L757. Memmel, N., Rangelov, G., Bertel, E., Dose, V.: Phys. Rev. B 43 (1991) 6938. Okada, M., Tochihara, H., Murata, Y.: Phys. Rev. B 43 (1991) 1411. Pirug, G., Ritke, C., Bonzel, H.P.: Surf. Sci. 257 (1991) 50. Rudolf, P., Astaldi, C., Cautero, G., Modesti, S.: Surf. Sci. 251/252 (1991) 127. Lando lt -Börnst ein New Ser ies III/42A1

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Scheffler, M., Droste, C., Fleszar, A., Máca, F., Wachutka, G., Barzel, G.: Physica B 172 (1991) 143. Schmalz, A., Aminpirooz, S., Becker, L., Haase, J., Neugebauer, J., Scheffler, M., Adams, D.L., Bøgh, E.: Phys. Rev. Lett. 67 (1991) 2163. Tang, D., McIlroy, D., Shi, X., Su, C., Heskett, D.: Surf. Sci. Lett. 255 (1991) L497. Wu, N.J., Hu, Z.P., Ignatiev, A.: Phys. Rev. B 43 (1991) 3805. Aminpirooz, S., Schmalz, A., Becker, L., Pangher, N., Haase, J., Nielsen, M.M., Batchelor, D.R., Bøgh, E., Adams, D.L.: Phys. Rev. B 46 (1992) 15594. Abernathy, D.L., Mochrie, S.G.J., Zehner, D.M., Grübel, G., Gibbs, D.: Phys. Rev. B 45 (1992) 9272. Argile, C., Rhead, G.E.: Surf. Sci. 279 (1992) 244. Castro, G.R., Busse, H., Janssens, T., Wandelt, K.: Phys. Scr. T41 (1992) 208. Fisher, D., Chandavarkar, S., Collins, I.R., Diehl, R.D., Kaukasoina, P., Lindroos, M.: Phys. Rev. Lett. 68 (1992) 2786. Fisher, D., Diehl, R.D.: Phys. Rev. B 46 (1992) 2512. Gierer, M., Bludau, H., Hertel, T., Over, H., Moritz, W., Ertl, G.: Surf. Sci. 279 (1992) L170. Jiang, Q.T., Statiris, P., Gustafsson, T., Häberle, P., Zehner, D.M.: J. Vac. Sci. Technol. A10 (1992) 2197. Janssens, T., Castro, G.R., Busse, H., Schneider, U., Wandelt, K.: Surf. Sci. 269/70 (1992) 664. Kerkar, M., Fisher, D., Woodruff, D.P., Jones, R.G., Diehl, R.D., Cowie, B.C.C.: Surf. Sci. 278 (1992) 246. Kerkar, M., Fisher, D., Woodruff, D.P., Jones, R.G., Diehl, R.D., Cowie, B.C.C.: Phys. Rev. Lett. 68 (1992) 3204. Kiskinova, M.P.: Poisoning and Promotion, in: Catalysis Based on Surface Science Methods; Studies in Surface Science and Catalysis Vol. 70. Amsterdam: Elsevier, 1992. Li, W., Lin, J., Karimi, M., Dowben, P.A., Vidali, G.: Phys. Rev. B 45 (1992) 3708. Muschiol, U., Bayer, P., Heinz, K., Oed, W., Pendry, J.B.: Surf. Sci. 275 (1992) 185. Masuda, T., Barnes, C.J., Hu, P., King, D.A.: Surf. Sci. 276 (1992) 122. Neugebauer, J., Scheffler, M.: Phys. Rev. B 46 (1992) 16067. Okada, M., Iwai, H., Klauser, R., Murata, Y.: J. Phys. Condens. Matter 4 (1992) L593. Over, H., Bludau, H., Skottke-Klein, M., Ertl, G., Moritz, W., Campbell, C.T.: Phys. Rev. B 45 (1992) 8638 . Stampfl, C., Scheffler, M., Over, H., Burchhardt, J., Nielsen, M.M., Adams, D.L., Moritz, W.: Phys. Rev. Lett. 69 (1992) 1532. Shrimpton, N.D., Fisher, D., Diehl, R.D.: Phys. Rev. B 45 (1992) 12118. Tochihara, H., Mizuno, S.: Surf. Sci. 279 (1992) 89. Adler, D.L., Collins, I.R., Liang, X., Murray, S.J., Leatherman, G.S., Tsuei, K.-D., Chaban, E.E., Chandavarkar, S., McGrath, R., Diehl, R.D., Citrin, P.H.: Phys. Rev. B 48 (1993) 17445. Andersen, J.N., Lundgren, E., Nyholm, R., Qvarford, M.: Surf. Sci. 289 (1993) 307. Bonzel, H.P., Pirug, G.: Interaction between Alkali metal adsorbates and adsorbed molecules (CO;H2O), in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis Vol. 6. King, D.A., Woodruff, D.P. (eds.), Amsterdam: Elsevier, 1993, p. 51. Cui, J., White, J.D., Diehl, R.D.: Surf. Sci. Lett. 293 (1993) L841. Cassuto, A., Schmidt, S., Mane, M.: Surf. Sci. 284 (1993) 273. Dudde, R., Reihl, B.: Surf. Sci. 287/288 (1993) 614. Ellis, J., Toennies, J.P.: Phys. Rev. Lett. 70 (1993) 2118. Fischer, N., Schuppler, S., Fisher, R., Fauster, T., Steinmann, W.: Phys. Rev. B 47 (1993) 4705. Huang, Z., Wang, L.Q., Schach von Wittenau, A.E., Hussain, Z., Shirley, D.A.: Phys. Rev. B 47 (1993) 13626. Hock, K.M., Palmer, R.E.: Surf. Sci. 284 (1993) 349.

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94B2 94B3 94D1 94F1 94G1 94H1 94H2 94H3 94M1 94M2 94N1

94N2 94N3 94N4 94P1

3.2.1 Alkali metals on metals Kaukasoina, P., Lindroos, M., Diehl, R.D., Fisher, D., Chandavarkar, S., Collins, I.R.: J. Phys.: Condens. Matter 5 (1993) 2875. Kerkar, M., Fisher, D., Woodruff, D.P., Jones, R.G., Diehl, R.D., McConville, C.F., Cowie, B.C.C.: J. Vac. Sci. Technol. A 10 (1992) 2148. Lindgren, S., Svensson, C., Walldén, L.: J. Electron. Spectrosc. Relat. Phenom. 64-65 (1993) 483. Meyerheim, H.L., Robinson, I.K., Jahns, V., Eng, P.J., Moritz, W.: Z. Kristallogr. 208 (1993) 73. Mizuno, S., Tochihara, H., Kawamura, T.: Surf. Sci. 293 (1993) 239. Mizuno, S., Tochihara, H., Kawamura, T.: Surf. Sci. 292 (1993) L811. Nørskov, J.K.: Adsorbate-adsorbate interactions on metal surfaces, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis Vol. 6. King, D.A., Woodruff, D.P. (eds.), Amsterdam: Elsevier, 1993, p. 1. Neugebauer, J., Scheffler, M.: Phys. Rev. Lett. 71 (1993) 577. Polatoglou, H.M., Methfessel, M., Scheffler, M.: Phys. Rev. B 48 (1993) 1877. Plummer, E.W., Dowben, P.A.: Prog. Surf. Sci. 42 (1993) 201. Statiris, P., Häberle, P., Gustafsson, T.: Phys. Rev. B 47 (1993) 16513. Shi, X., Tang, D., Heskett, D., Tsuei, K.-D., Ishida, H., Morikawa, Y.: Surf. Sci. 290 (1993) 69. Su, C., Shi, X., Tang, D., Heskett, D., Tsuei, K.-D.: Phys. Rev. B 48 (1993) 12146. Shi, X., Tang, D., Heskett, D., Tsuei, K.-D., Ishida, H., Morikawa, Y., Terakura, K.: Phys. Rev. B 47 (1993) 4014. Tang, D., Heskett, D.: Phys. Rev. B 47 (1993) 10695. Tang, D., Su, C., Heskett, D.: Surf. Sci. 295 (1993) 427. Wedler, H., Mendez, M.A., Bayer, P., Löffler, U., Heinz, K., Fritzsche, V., Pendry, J.B.: Surf. Sci. 293 (1993) 47. Barnes, C.J.: Adsorbate induced reconstruction of f.c.c.{110} surfaces, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis Vol 7. King, D.A., Woodruff, D.P. (eds.), Amsterdam: Elsevier, 1994, p. 501. Bormet, J., Neugebauer, J., Scheffler, M.: Phys. Rev. B 49 (1994) 17242. Burchhardt, J., Nielsen, M.M., Adams, D.L., Lundgren, E., Andersen, J.N.: Phys. Rev. B 50 (1994) 4718. Davis, R., Xu, X.-M., Woodruff, D.P., Weiss, K.-U., Dippel, R., Schindler, K.-M., Hofmann, P., Fritzsche, V., Bradshaw, A.M.: Surf. Sci. 307-309 (1994) 632. Fischer, N., Shuppler, S., Fauster, T., Steinmann, W.: Surf. Sci. 314 (1994) 89. Gorodetsky, D.A., Melnik, Y.P., Usenko, V.A., Yas'ko, A.A., Yarigin, V.I.: Surf. Sci. 315 (1994) 51. Hertel, T., Over, H., Bludau, H., Gierer, M., Ertl, G.: Surf. Sci. 301 (1994) 1. Hofmann, P., Bao, S., Schindler, K.-M., Schaff, O., Polcik, M., Fritzsche, V., Bradshaw, A.M., Davis, R., Woodruff, D.P.: Surf. Sci. 319 (1994) L7. Hertel, T., Over, H., Bludau, H., Gierer, M., Ertl, G.: Phys. Rev. B 50 (1994) 8126. Meyerheim, H.L., Wever, J., Jahns, V., Moritz, W., Eng, P.J., Robinson, I.K.: Surf. Sci. 304 (1994) 267. Mizuno, S., Tochikara, H., Kawamura, T.: Phys. Rev. B 50 (1994) 17540. Naumovets, A.G.: Two-dimensional phase transitions in alkali-metal adlayers, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis Vol 7. King, D.A., Woodruff, D.P. (Eds.), Amsterdam: Elsevier, 1994, p. 163. Nielsen, M.M., Burchhardt, J., Adams, D.L., Lundgren, E., Andersen, J.N.: Phys. Rev. Lett. 72 (1994) 3370. Naumovets, A.G.: Surf. Sci. 299/300 (1994) 706. Nielsen, M.M., Burchhardt, J., Adams, D.L.: Phys. Rev. B 50 (1994) 7851. Plummer, E.W., Carpinelli, J.M., Weitering, H.H., Dowben, P.A.: Phys. Low-Dim. Struct. 4/5 (1994) 99.

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Reichmuth, A., Graham, A.P., Bullman, H.G., Allison, W., Rhead, G.E.: Surf. Sci. 307-309 (1994) 34. Shi, X., Su, C., Heskett, D., Berman, L., Kao, C.C., Bedzyk, M.J.: Phys. Rev. B 49 (1994) 14638. Scragg, G., Cowie, B.C.C., Kerkar, M., Woodruff, D.P., Daimellah, A., Turton, S., Jones, R.G.: J. Phys.: Condens. Matter 6 (1994) 1869. Stampfl, C., Neugebauer, J., Scheffler, M.: Surf. Sci. 307-309 (1994) 8. Stampfl, C., Scheffler, M., Over, H., Burchhardt, J., Nielsen, M.M., Adams, D.L., Moritz, W.: Phys. Rev. B 49 (1994) 4959. Sandl, P., Bertel, E.: Surf. Sci. Lett. 302 (1994) L325. Van Hove, M.A.: Surf. Rev. Lett. 1 (1994) 9. Watson, G., Brühwiler, P.A., Sagner, H.J., Frank, K.H., Plummer, E.W.: Phys. Rev. B 50 (1994) 17678. White, J.D., Cui, J., Strauss, M., Diehl, R.D., Ancilotto, F., Toigo, F.: Surf. Sci. 307-309 (1994) 1134. Wertheim, G.K., Riffe, D.M., Citrin, P.H.: Phys. Rev. B 49 (1994) 4834. Andersen, J.N.: Surf. Rev. Lett. 2 (1995) 345. Burchhardt, J., Nielsen, M.M., Adams, D.L., Lundgren, E., Andersen, J.N., Stampfl, C., Scheffler, M., Schmalz, A., Aminpirooz, S., Haase, J.: Phys. Rev. Lett. 74 (1995) 1617. Brune, H., Wintterlin, J., Behm, R.J., Ertl, G.: Phys Rev. B 51 (1995) 13592. Berndt, W., Weick, D., Stampfl, C., Bradshaw, A.M., Scheffler, M.: Surf. Sci. 330 (1995) 182. Christensen, S.V., Neilsen, K.T., Nerlov, J., Batchelor, D.R., Adams, D.L.: Surf. Sci. 339 (1995) L919. Diehl, R.D., McGrath, R.: Surf. Rev. Lett. 2 (1995) 387. Fasel, R., Osterwalder, J.: Surf. Rev. Lett. 2 (1995) 359. Gierer, M., Over, H., Bludau, H., Ertl, G.: Surf. Sci. 337 (1995) 198. Gierer, M., Over, H., Bludau, H., Ertl, G.: Phys. Rev. B 52 (1995) 2927. Leatherman, G., Diehl, R.D.: Surf. Sci. 380 (1997) 455. Leatherman, G.S., Diehl, R.D., Kaukasoina, P., Lindroos, M.: Phys. Rev. B 53 (1996) 10254. Mizuno, S., Tochihara, H., Barbieri, A., Van Hove, M.A.: Phys. Rev. B 51 (1995) 7981. Mizuno, S., Tochihara, H., Barbieri, A., Van Hove, M.A.: Phys. Rev. B 51 (1995) 1969. Mizuno, S., Tochihara, H., Barbieri, A., Van Hove, M.A.: Phys. Rev. B 52 (1995) R11658. Nagao, T., Iizuka, Y., Umeuchi, M., Shimazaki, T., Oshima, C.: Surf. Sci. 329 (1995) 269. Neumann, A., Schroder, S.L.M., Christmann, K.: Phys. Rev. B 51 (1995) 17007. Over, H., Bludau, H., Gierer, M., Ertl, G.: Surf. Rev. Lett. 2 (1995) 409. Over, H., Gierer, M., Bludau, H., Ertl, G.: Phys. Rev. B 52 (1995) 16812. Stampfl, C., Scheffler, M.: Surf. Rev. Lett. 2 (1995) 317. Schuster, R., Eng, P.J., Robinson, I.K.: Surf. Sci. 326 (1995) L477. Schröder, S.L.M., Neumann, A., Solomun, T., Lenz-Solomun, P., Christmann, K.: Surf. Sci. 337 (1995) 285. Adams, D.L.: Appl. Phys. A 62 (1996) 123. Burchhardt, J., Lundgren, E., Nielsen, M.M., Andersen, J.N., Adams, D.L.: Surf. Rev. Lett. 3 (1996) 1339. Bergmans, R.H., Brands, A.L.G.P., Denier van der Gon, A.W., Ceelen, W.C.A.N., Brongersma, H.H., Bielen, P., Creemers, C.: Surf. Sci. 350 (1996) 1. Christensen, S.V., Nerlov, J., Nielsen, K., Burchhardt, J., Nielsen, M.M., Adams, D.L.: Phys. Rev. Lett. 76 (1996) 1892. Diehl, R.D., McGrath, R.: Surf. Sci. Rep. 23 (1996) 43. Ebinger, H.D., Jänsch, H.J., Polenz, C., Polivka, B., Preyss, W., Saier, V., Veith, R., Fick, D.: Phys. Rev. Lett. 76 (1996) 656. He, P., Jacobi, K.: Phys. Rev. B 53 (1996) 3658. Hunt, M.R.C., Durston, P.J., Palmer, R.E.: Surf. Sci. 364 (1995) 266.

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3.2.1 Alkali metals on metals Imaki, M., Mizuno, S., Tochihari, H.: Surf. Sci. 357-358 (1996) 145. Lindgren, S.-A., Svensson, C., Walldén, L., Carlsson, A., Wahlström, E.: Phys. Rev. B 54 (1996) 10912. Leatherman, G.S., Diehl, R.D.: Phys. Rev. B 53 (1996) 4939. Lopinski, G.P., Estrup, P.J.: Surf. Sci. 365 (1996) 149. Lehmann, J., Roos, P., Bertel, E.: Phys. Rev. B 54 (1996) R2347. Lu, Q.B., O'Connor, D.J., King, B.V., MacDonald, R.J.: Surf. Sci. 347 (1996) L61. Meyerheim, H.L., Moritz, W., Schulz, H., Eng, P.J., Robinson, I.K.: Phys. Rev. Lett. 76 (1996) 1671. Nielsen, M.N., Christensen, S.V., Adams, D.L.: Phys. Rev. B 54 (1996) 17902. Payne, S.H., McKay, H.A., Kreuzer, H.J., Gierer, M., Bludau, H., Over, H., Ertl, G.: Phys. Rev. B 54 (1996) 5073. Schwegmann, S., Over, H.: Surf. Sci. 360 (1996) 271. Arena, D.A., Curti, F.G., Bartynski, R.A.: Phys. Rev. B 56 (1997) 15404. Ceelan, W.C.A.N., van der Gon, A.W.D.., Falta, J., Hille, A., Materlik, G.: Surf. Sci. 391 (1997) 59. Diehl, R.D., McGrath, R.: J. Phys.: Condens. Matter 9 (1997) 951. Graham, A.P., Hofmann, F., Toennies, J.P., Chen, L.Y., Ying, S.C.: Phys. Rev. B 56 (1997) 10567. Graham, A.P., Hofmann, F., Toennies, J.P., Chen, L.Y., Ying, S.C.: Phys. Rev. Lett. 78 (1997) 3900. Graham, A.P., Toennies, J.P.: Phys. Rev. B 56 (1997) 15378. Hannon, J.B., Giesen, M., Klünker, C., Schulze Icking-Konert, G., Stapel, D., Ibach, H., Müller, J.E.: Phys. Rev. Lett. 78 (1997) 1094. Jiang, H., Mizuno, S., Tochihara, H.: Surf. Sci. 380 (1997) L506. Jiang, H., Imaki, M., Mizuno, S., Tochihara, H.: Surf. Rev. Lett. 4 (1997) 1227. Jiang, H., Mizuno, S., Tochihara, H.: Surf. Sci. 385 (1997) L930. Kaukasoina, P., Lindroos, M., Leatherman, G., Diehl, R.D.: Surf. Rev. Lett. 4 (1997) 1215. Lundgren, E., Beutler, A., Nyholm, R., Andersen, J.N., Heskett, D.: Surf. Sci. 370 (1997) 311. Meyerheim, H.L., Pflanz, S., Schuster, R., Robinson, I.K.: Z. Kristallogr. 212 (1997) 327. Mizuno, S., Jiang, H., Tochihara, H.: Surf. Rev. Lett. 4 (1997) 1221. Nagao, T., Iizuka, Y., Shimazaki, T., Oshima, C.: Phys. Rev. B 55 (1997) 10064. Pi, T.-W., Hong, L.-E., Wu, R.-T.: Surf. Rev. Lett. 4 (1997) 1197. Quassowski, S., Hermann, K.: Surf. Rev. Lett. 4 (1997) 1209. Verhoef, R.W., Zhao, W., Asscher, M.: J. Chem. Phys. 106 (1997) 9353. Ebinger, H.D., Arnolds, H., Polenz, C., Polivka, B., Preyss, W., Veith, R., Fick, D., Jansch, H.J.: Surf. Sci. 413 (1998) 586. Moré, S., Berndt, W., Bradshaw, A.M., Stumpf, R.: Phys. Rev. B 57 (1998) 9246. Nielsen, M.M., Burchhardt, J., Adams, D.L., Andersen, J.N.: Phys. Rev. B 58 (1998) 12655. Ohsaki, I., Oguchi, T.: J. Phys. Soc. Jpn. 67 (1998) 978. Over, H.: Prog. Surf. Sci. 58 (1998) 249. Senet, P., Toennies, J.P., Witte, G.: Chem. Phys. Lett. 299 (1998) 389. Tochihara, H., Mizuno, S.: Prog. Surf. Sci. 58 (1998) 1. Vaara, T., Vaari, J., Lahtinen, J.: Surf. Sci. 395 (1998) 88. Adams, D.L., Nielsen, M.M., Burchhardt, J., Andersen, J.N.: Phys. Rev. B 60 (1999) 11078. Caragiu, M., Leatherman, G.S., Diehl, R.D., Kaukasoina, P., Lindroos, M.: Surf. Sci. 441 (1999) 84. Fouquet, P., Witte, G.: Phys. Rev. Lett. 83 (1999) 360. Graham, A.P., Menzel, A., Over, H., Silvestri, W., Toennies, J.P.: (1999) Gierer, M., Over, H.: Z. Kristallogr. 214 (1999) 14. Klünker, C., Steimer, C., Hannon, J.B., Giesen, M., Ibach, H.: Surf. Sci. 420 (1999) 25. Lando lt -Börnst ein New Ser ies III/42A1

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Kim, Y.D., Schwegmann, S., Over, H.: Phys. Chem. Chem. Phys. 1 (1999) 2001. Lahtinen, J., Vaari, J., Vaara, T., Kauraala, K., Kaukasoina, P., Lindroos, M.: Surf. Sci. 425 (1999) 90. Mikkelsen, A., Adams, D.L.: Phys. Rev. B 60 (1999) 2040. Petersen, J.H., Mikkelsen, A., Nielsen, M.M., Adams, D.L.: Phys. Rev. B 60 (1999) 5963. Seyller, Th., Haseneder, R., Reinhart, E., Borgmann, D., Wedler, G.: Surf. Sci. 424 (1999) 278. Senet, P., Toennies, J.P., Witte, G.: Chem. Phys. Lett. 299 (1999) 389.

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3.2.2 Alkali metals on semiconductors E.G. MICHEL, R. MIRANDA

3.2.2.1 Introduction 3.2.2.1.1 Historical layout and general background The investigation of the electronic and crystallographic properties of metal-semiconductor junctions was prompted by their rectifying properties [74Bra, 87Flo, 88Rho, 90Mön, 91Lan, 91Lel, 92Bri, 93Bec, 94Flo]. This rectifying behaviour is due to the formation of a potential barrier (Schottky barrier) at the metal-semiconductor interface [38Mot, 38Sch, 87Sti, 88Ke1, 88Ke2, 88Pr2, 88Spi, 88Sti, 89Bat, 89Pri, 89Spi]. This interface has been intensively investigated in order to understand the mechanisms controlling the properties of the barrier [38Sch] (see also section 3.3). Several different models have been proposed in order to explain the main properties of the Schottky barrier, as barrier height, dependence with the nature of the metal and the semiconductor, and dependence with the thickness of the metallic layer. The first model was proposed by Schottky [39Sch, 40Sch] and Mott [38Mot], who suggested that the bands of the metal and the semiconductor in contact adjust so as to bring both vacuum levels in coincidence. In this case, the barrier height results as the energy difference between the metal work function and the semiconductor electron affinity (Schottky-Mott rule). This model does not reproduce the experimental results. Bardeen [47Bar] then was the first to consider electronic interface states in metal-semiconductor contacts. The barrier heights are still expected to vary linearly as a function of metal work-function but at a reduced slope [65Cow]. Such interface states are implying an interfacial dipole-layer [47Bar]. In a conceptually simple approach, Heine [65Hei] identified those interface states as the wave-function tails of the meta1 electrons in the energy region between the top of the semiconductor valence-band and the Fermi level, where the meta1 conduction-band overlaps the band gap of the semiconductor. That induced density of interface states (IDIS, [77Tej]) or continuum of meta1-induced gap states (MIGS, [76Lo1]) is then derived from the virtual gap states (ViGS) of the complex band structure of the semiconductor, which are solutions of Schrödinger's equation for complex wave vector. It is in this sense that the label "virtual" is used. Depending on the boundary conditions, intrinsic surface states at clean surfaces [35Mau, 39Goo], adsorbate-induced surface states on surfaces covered with adsorbates in submonolayer quantities, and meta1-induced gap states in meta1semiconductor interfaces [76Lo1, 76Lo2, 84Ter1, 84Ter2] are derived from those ViGS. Initially, this MIGS or IDIS model was not widespread accepted since it predicts, as the Schottky-Mott rule did, a linear relationship of the barrier height on the metal work function, but with a slope S less than unity. Meanwhile, there was, however, growing evidence that the chemical trends of barrier heights observed with different metals on a specific semiconductor, as well as of the slopes S reported for different semiconductors, are well described by the MIGS or IDIS model [86Mön, 87Mön, 88Mön]. In these circumstances and based on specific experimental results, which were obtained by using photoemission spectroscopy, Spicer et al. [79Spi] proposed that during the formation of metal-semiconductor contacts native defects are generated in the semiconductor at or close to the interface. The energy levels of those defects were supposed to determine the interface position of the Fermi level within the semiconductor band-gap. This defect model attracted much attention (for a review see, for example, [83Mön]). Nowadays, a consensus has been reached in the sense that both virtual gap states and defects are needed to explain the complete set of experimental data on real interfaces. Only when the experimental conditions can be controlled to reduce the defect density to below 1013 cm−2, the Schottky barrier height is determined by the virtual gap states. In highly defective interfaces, defect states play a role in the control of the barrier height [87Mön, 90Mön].

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3.2.2.1.2 Alkali metal/semiconductor interfaces The perspective on the metal-semiconductor interfaces that has allowed this consensus, changed in the late seventies, when synchrotron radiation was applied to these interfaces [85Bri, 89So1]. It was found that most interfaces were reactive, with a significant interdiffusion between the metal and the semiconductor [78Bri, 82Bri]. In order to understand the physics of the metal-semiconductor junctions, much work was devoted since then to a class of system that was considered close to ideal, the alkali-metal (AM)-semiconductor interface. These interfaces are non-reactive, at least at low temperature. AM's do not form three-dimensional islands but have a tendency to build well ordered one- or two-dimensional structures. There is also no interdiffusion into the substrate (with the possible exception of Li [95Gre2]). The most characteristic physical property of AM/semiconductor interfaces is the strong decrease in the surface work function induced by AM adsorption (see section 4.2). This feature is behind several important technical applications of AM/semiconductor interfaces. We may cite topics such as thermionic emitters, photocathodes, spin-polarized electron guns, and catalysts [75Dry, 77Wan, 79Hat, 82His, 82Leu, 87Cap]. Thus, the interest on AM/semiconductor interfaces goes far beyond the fundamental aspects mentioned above. 3.2.2.1.3 Experimental techniques The geometry of the first AM atoms deposited on a semiconductor surface has received great attention due mainly to the application to this field of several different structural techniques that include: STM SXRD AED PhD LEED RHEED MEIS TDS SEXAFS PEXAFS XSW THS

scanning tunneling microscopy surface X-ray diffraction Auger electron diffraction photoelectron diffraction low-energy electron diffraction reflection high-energy electron diffraction medium energy ion scattering thermal desorption spectroscopy surface extended X-ray absorption fine structure photoemission extended X-ray absorption fine structure X-ray standing waves thermal helium scattering

The electronic and vibrational structure has been investigated using techniques such as: BEEM PES

XPS EELS PAX

ballistic-emission electron microscopy, an application of STM that yields the Schottkybarrier height directly in space [88Kai] photoemission spectroscopy, either direct (UPS, ultraviolet photoemission spectroscopy, or IPS, inverse photoemission [85Bri], to study the valence band electronic structure or the core-levels. This includes also ARUPS (angle-resolved ultraviolet photoemission spectroscopy) X-ray photoemission spectroscopy electron energy loss spectroscopy photoemission of adsorbed xenon

3.2.2.2 Adsorption on covalent semiconductors: physical and chemical properties Much theoretical and experimental work has been performed on the systems Na/Si(100)2x1, K/Si(100)2x1, K/Si(111)7x7 and Cs/Si(111)7x7. We describe in the following the properties of AM atoms adsorbed on group IV semiconductors (Si and Ge), with special emphasis in the above mentioned interfaces.

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3.2.2.2.1 Adsorption site, structure and long range order Table 1. High-symmetry adsorption sites of Na, K, and Cs on the (2x1) reconstructed surfaces of Si(001) and Ge(001) as proposed on the basis of various experimental and theoretical investigations. Sites are labeled as follows: P (pedestal), T (top), T3 (top third layer or valley), and T4 (top fourth layer or cave). Depending on coverage, either one site (four first columns) or a two-site combination (last three columns) were proposed. P

T3

T4

T

P/T3

P/T4

P/T3-T4

Experiment Method LEED

73Lev, 90Wei

91Ura, 93Ura

SEXAFS ARUPS

95Lin 94Abu, 89Ent, 96Seg 88Abu

XPD STM XPS UPS PAX TDS XSW

92Sou1 92Sou1 92Sou1

90Has1

92Mic 92Mic 95Cas

AED FI-STM RHEED MEIS/AES SXRD

91Ete1, 93Les

89Ase 91Has1

91Ete1

90Has1 91Mak 91Smi

91Smi

94Mey, 95Mey, 98Mey

PEXAFS

93Spi

Theory 88Cir 90Bat

89Ram 89Lin 92Kob

92Kob 93Spi

91Mor

91Mor

The most important structural property is the adsorption site. The adsorption site of alkali metals on Si(100)2x1 has been controversial for many years. There are several different possible adsorption geometries. The adsorption energies are not much different (see section 3.2.2.2.2), and the position found depends in part on the coverage and preparation method. Fig. 1 shows the Si(100)2x1 reconstructed surface, with the Si-Si dimers (side view), and the high symmetry sites where the AM atoms can be adsorbed on (top view). High-symmetry AM adsorption sites are labeled as follows: P (pedestal), T (top), D (dimer-bridge), T3 (top third layer or valley), and T4 (top fourth layer or cave). T3 and T4 are often referred to as cave sites leading to some confusion in the literature. We define one monolayer (ML) for the Si(100)2x1 surface as a density of 6.78 x 1014 atoms cm−2, i.e. one atom per Si surface atom) Lando lt -Bö rnst ein New Ser ies III/42A1

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The structural properties of AM's on Si(100)2x1 have been observed with several different techniques. Table I gives an overview of some investigations that were carried out at different AM coverages on Si(100)2x1 and Ge(100)2x1. Besides the occupation of only a single adsorption site, also the combination of two positions such as P/T3, P/T4 (the so called "double-layer models") and T3/T4 has been suggested at AM saturation coverage, which generally is found to be about one monolayer (ML) in the literature [86Oel, 87Ort, 91Smi] (see also 3.2.2.2.2). The first structural model for 0.5 ML K or Cs on Si(100)2x1 is due to Levine [73Lev], who assumed AM adsorption in the P only. Later on it was proposed that for the room temperature saturation coverage (see also 3.2.2.2.2), there should be a combination of two adsorption sites ("double-layer model") [88Abu]. The results of these studies are summarized in Table 2. Most of the discrepancies found for AM adsorbate structures listed in Table I can be attributed to differences in the surface preparation conditions [92Mic, 92Sou2] or in the actual AM coverage, both affecting strongly the adsorption geometry. Auger electron diffraction [89Ase] (AED) and photoelectron diffraction [88Abu] (PhD) make use of the strong forward scattering of the electrons by an intervening atom [90Egg]. These techniques favored T3 sites at 0.5 ML (AED) or a combination of P/T3 ("double layer model") for more than 0.5 ML. Table 2. Structural properties of AM/Si(100)2x1: experimental values. θ is the AM (alkali metal) coverage, T is the adsorption temperature, R is the ripple of the AM layer (distance from upper to lower AM atom in the case of the occupation of two sites), d is the vertical distance from AM atoms to the closest Si dimer atom (in the case of occupation of two sites by AM atoms, d1 (d2) denotes the distance from the upper (lower) AM atom). For bond length values see Table 11. AM θ [ML]

Site

T [K]

R [Å]

Na 0.5-0.7 0.5-0.7

P P/T3

465 465

1.05

K

0.6

P/T3

300

1.25

1.2

D/T3

300

0.08

RT sat

P/T3

300

0.66

d1=2.94 d2=2.28

0.29

D/T3

300

0.1

d1=2.76±0.15-0.3 d2=2.65±0.15-0.3

Cs

d [Å] d=1.85 d1=1.90 d2=0.85 d1=2.28 d2=1.03 d1=2.20±0.15-0.3 d2=2.12±0.15-0.3

Occupation

T3:0.50(3) D:0.25(3) other:0.34(4)

T3:0.11(3) D:0.14(3) other:0.04(1)

Method

Ref.

LEED

90Wei

RHEED

91Mak

SXRD

98Mey

LEED

98Ham

SXRD

98Mey

STM has been applied for different AM coverages [90Has1, 92Sou2, 92Sou3]. At very low coverage K atoms are adsorbed on Si(100)2x1 on various coexisting sites, and tend to form linear chains perpendicular to the < 011 > direction [90Has1]. At intermediate coverages, lower than 0.5 ML but probably larger than θ = 1/3 ML, an ordering transition appears, with the formation of one-dimensional linear chains along the <011> direction [92Eff, 92Sou2, 92Sou3] (see Fig. 2). STM images for K do not have enough resolution to discriminate clearly where the AM atoms sit on. However, the evidence found [92Sou2, 92Sou3] supports that the K linear chains formed at high coverages are bonded to the Si surface dangling bonds. This supports that the adsorption site is T4 for θ = 0.5 ML. This result was confirmed in the case of Na/Si(100)2x1 by the PEXAFS technique, that allowed to conclude that the Na linear chain is adsorbed on the cave site [93Spi]. Low-energy electron diffraction (LEED) has been applied by measuring beam intensity versus electron energy curves (I/V curves) [67Pal, 69Web, 73Lev, 77Hol, 90Wei, 91Ura, 93Ura]. It was concluded [91Ura, 93Ura] that the double-layer model (P /T3) is the most likely at AM saturation Lando lt -Börnst ein New Ser ies III/42A1

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coverage. SXRD structure analysis of the K/Si(001)2x1 and the Cs/Si(001)2x1 surfaces [98Mey] supported that the AM atoms simultaneously adsorb on different sites on clean Si(001)2x1. The main adsorption sites are found to be the D site above the dimer rows and the T3 site between the dimers as well, as "asymmetric" sites close to D and T3. The occurrence of multiple adsorption sites is not surprising, since the adsorption was done at room temperature and it is not likely to produce an equilibrium structure. The interface Rb/Si(100)2x1 has been investigated with XSW also [91Ete1]. For coverages of 0.5 ML a T4 site was found (annealed surface), while for coverages exceeding this value a combination of P/T3 was proposed (room temperature deposition). Another XSW study found a combination of sites with predominance of T4 for evaporation at room temperature [95Cas]. Table 3. Long-range order of AM/Si(100)2x1 surfaces. SD denotes adsorption on single-domain Si(100)2x1 surfaces. θ is the AM (alkali metal) coverage, T is the adsorption temperature. LEED was used to observe all the structures. AM

θ [ML]

Structure

T [K]

Ref.

Na

<0.1 0.15

2x1 2x1 and 2x3 or 3x2 2x3 or 3x2 incomm. 2x1 2x3

300 300

89Gla 90Gla

<0.1 0.1-0.2

2x1

( ), ( )

300 80-150

89Gla 98Sar3 91Abu 92Sas

0.25 0.33 0.5 0.5

4x5 2x3 2x1 2x1

80-150 320 300 300

87Ort

0.2-0.3 ~0.4 0.5 0.33 K/SD

Cs

2 1 0 3

2 −1 0 3

300 300 300 300

Relying on theoretical calculations (see Table 1) to decide the adsorption site presents difficulties related to the small difference between the chemisorption energies of different adsorption sites (see also 3.2.2.2.2). It has been proposed that for θ = 0.5 ML, the linear chains along the < 011 > direction are adsorbed on the cave sites [89Lin, 89Ram], but the valley bridge site has been proposed as well [89Ish, 91Mor, 91Zha, 92Kob, 94Kru]. All these calculations discard Levine's pedestal position as the preferred site of adsorption [73Lev]. In conclusion, it is established that for θ <0.5 ML and carefully prepared surfaces, AM atoms occupy T3 and/or T4 sites. Once the coverage exceeds 0.5 ML, pedestal sites are also occupied. The long-range order of AM/semiconductor interfaces has been investigated by LEED mainly. The results are summarized in Table 3. While the 2x1 structure at saturation coverage has been investigated in great detail (see above), there exist also atomic models for the other reconstructions based in LEED [91Abu, 92Sas] and STM [93Bro] investigations. The adsorption geometry of AM's on Si(111) surfaces has received also attention. Reflection high energy electron diffraction (RHEED) studies at K/Si(111) surfaces [85Dai, 88Miz, 89Koh] demonstrated a conservation of the 7x7 periodicity. The long-range order as investigated by LEED reveals also that only small modifications of the 7x7 pattern of the clean surface take place, for adsorption at room temperature. The adsorption site has been investigated with several structural techniques (XSW, STM, FISTM) [91Ete2, 91Has3, 92Lag1, 92Lag2, 93Ete, 93Mic]. At very low coverages, adsorption on top of the center adatom of the 7x7 unit cell has been reported [91Has3]. STM results showed preferential adsorption on faulted-halves of the 7x7 unit cells [90Has3, 91Has2, 91Has3, 96Jeo].These studies have Lando lt -Bö rnst ein New Ser ies III/42A1

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revealed that the first stage of adsorption of K on Si(111)7x7 corresponds to occupation of the faulted halves of the 7x7 reconstruction. At higher coverages (second stage), K atoms tend to form clusters, preferentially in faulted regions [98Wat]. A complicated situation is found in the case of Cs or Rb on Si(111)7x7 at high coverage. This is mainly due to the complex surface reconstruction, where several adsorption sites are possible. XSW experiments show that the adsorption site changes from threefold hollow at high coverage to on top at low coverages [91Ete2, 92Lag1, 92Lag2, 93Ete, 93Mic]. The adsorption of most AM's on Si(111) under adequate conditions (elevated temperature), gives rise to the formation of a 1x3 reconstruction. This reconstruction has been also found for other monovalent adsorbates (Ag). Its structure has been characterized using RHEED [85Dai, 87Has], LEED [90Fan, 91Tik], AES [78Lel, 90Fan, 94Fuk, 94Wei], STM [92Jeo, 92Wan, 93Wan, 94Ohn, 95Car, 95Olt, 95Pag, 98Sar1, 98Sar2], ISS [86Aon, 93Has], SXRD [94Won, 98Lot], PAX [96Kim] and theoretical calculations [95Erw, 95Jeo, 96Jeo2]. The reconstruction has several interesting features. First, the 1x3 reconstruction is of one-dimensional character. Second, it passivates the surface against reaction with gasses such as oxygen [91Tik], at variance with all other AM/semiconductor interfaces, that are highly reactive [89Bon]. All 3x1 reconstruction are similar for all metals. The reconstruction is predominantly a Si(111) substrate reconstruction [90Fan]. The absolute metal coverage for this phase is 1/3 of ML [1 ML = 7.8 x 1014cm−2 for Si(111)] [93Has, 94Fuk]. The 3x1 surface is semiconducting [92Jeo, 94Oku, 94Sak, 97Oku]. π-bonded chains formed by top Si atoms are present on the surface [94Oku, 94Sak]. However, the atomic structure still remains controversial. Several structural models have been proposed [90Fan, 92Jeo, 93Wan, 94Ohn, 94Oku, 94Sak, 94Wei, 95Erw, 98Lot, 98Sar1, 98Sar2], but no consensus has been reached. The two most successful models were the Seiwatz model [94Sak, 94Wei, 94Won] and the extended Pandey model [94Oku, 95Erw]. The Seiwatz model consists of parallel π-bonded chains formed by fivefold rings of Si atoms in < 1 1 0 > projection, separated by empty channels, with a top -site adsorbate saturating the surface dangling bonds. The extended Pandey model consists of a sevenfold ring carrying the π-bonded chain, alternating with a five and six-member ring of Si. Recently, two other models have been proposed on the basis of detailed SXRD experiments [98Lot] and STM results [98Sar1, 98Sar2], respectively. Lottermoser et al. [98Lot] found a surface configuration consisting of consecutive fivefold and sixfold Si rings in < 1 1 0 > projection, separated by channels containing the alkali atoms. Saranin et al. [98Sar1, 98Sar2] proposed a double π-bonded chain model, incorporating two Si π-bonded chains in faulted and unfaulted orientations, and connected through AM atoms. The adsorption of AM's on Si(111)2x1, that is obtained by cleaving in ultra high vacuum, presents some interesting features. Details are given in Table 4. In the case of Na/Si(111)2x1 [94Man], two adsorption sites were found together with clear evidence of an ionic-covalent transition.

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Table 4. Long-range order of AM/Si(111)2x1 surface. θ is the AM (alkali metal) coverage. The adsorption was done at room temperature. AM

θ range (ML)

Structure

Method

Reference

Na K

0.5 0.0-0.15 0.15-0.35

1x1 2x1 2x1+ (2√3x2√3)R30º 2x1+3x1 3x1 Two domain √3x2 1x1 1x1 (√3x√3)R30º

LEED LEED LEED

92Rei2 97Nel 97Nel

LEED LEED LEED

97Nel 97Nel 97Nel

LEED LEED LEED

97Nel 90Rei 89Rei2

0.35-0.47 0.47-0.6 0.6-0.7

Cs

0.7-1.7 0.1-1 0.3

3.2.2.2.2 Adsorption energy and saturation coverage The adsorption energy of different AM atoms has been analyzed using Thermal Desorption Spectroscopy (TDS). The most studied elements have been Na and K, aiming at determining the adsorption energy, the number of chemisorption sites, and the nature of the most favorable one. The adsorbed state of K on Si(100)2x1 has been analyzed using TDS [90Tan]. Two desorption peaks were observed (α and β, see Fig. 3), which were attributed to the desorption of K atoms from two different chemisorbed states, associated with the upper and lower sites on the Si(100)2x1 surface The results are summarized in Table 5. Table 5. Thermodynamic properties of AM/Si(100)2x1 and AM/Ge(100)2x1: experimental values. TD, desorption temperature; Ead, adsorption energy. The "Method" column refers to the theoretical approach used to extract the Ead values from the experimental data. Interface

Coverage [ML]

Na/Si(100)

0-0.5 0.5-0.7 0 limit 0.5 limit 0.1-0.5 <0.5 0.5-0.58 <0.5 0.5-0.75 0 limit 0.5 limit 1.0 limit

K/Si(100)

Cs/Si(100)

Na/Ge(100)

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Site

T3/T4 P T3/T4

TD [K]

Ead [eV/atom]

Method

Reference

710 620 880-700 630-600 500-800 750 470 750 600 800 729 650

2.2-1.3 1.3-0.4 2.4 1.6 2.0-1.0 2.4 1.3

82Raw

91Tik2

62Red

90Tan

82Raw 62Red

92Mic 89Pap 89Ken

1.7 1.6 1.2

84Hab

97Nay

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Table 6. Thermodynamic properties of AM/Si(100)2x1: adsorption energies (Ead) calculated in the local density approximation (for details see cited references). System

Na/Si(100)2x1

K/Si(100)2x1

Ead [eV/atom] P T3 2.22 1.94 1.85 2.12

2.05 1.76 1.81 1.74

Reference T4 1.92 1.84

P/T3

P/T4

2.06 2.04

1.99 2.11

93Spi 95Ko 99Gra 91Mor

The adsorption energy of AM atoms has been investigated also by theoretical calculations (see Table 6). The close values found explain the observation of coexisting sites for AM/Si(100) interfaces (see also 3.2.2.2.1). The saturation coverage for AM atoms is not θ = 0.5 ML. A further deposition can be reached (although it was suggested that impurities play an important role in the process) [92Sou2], with the next AM atoms deposited on the ridge between the linear chains [86Oel, 87Ort, 91Smi, 92Mic]. The results are summarized in Table7. Theoretical calculations [91Zha, 92Kob, 94Kru] and experimental evidence collected with X-ray standing waves [91Ete1], show that at saturation coverage (θ = 1), AM atoms adsorb simultaneously on the pedestal and the valley-bridge sites (see also section 3.2.2.2.1).

Table 7 Saturation coverage (θ sat) of AM/Si(100)2x1 and AM/Si(111)7x7 at room temperature. System

θsat [ML]

Method

Reference

Na/Si(100)2x1 K/Si(100)2x1

0.68 1.2(3) 1 0.98(5) 0.3(1) 0.68 0.97(5) 0.75 0.68 0.3-0.4 0.61 1.0-1.1

AES-TDS SXRD LEED MEIS SXRD AES MEIS AES-TDS AES-TDS AES XSW XSW

91Tik2 98Mey 86Oel 91Smi 98Mey 87Ort 91Smi 89Ken 89Pap 93Wei 93Ete 91Ete2

Cs/Si(100)2x1

K/Si(111)7x7 Rb/Si(111)7x7 Cs/Si(111)7x7

3.2.2.2.3 Metallization and electronic structure The metallization sequence of AM/Si interfaces is probably the most important and debated topic related to these systems. No metallization of the AM-Si(100)-(2x1) interface has been found for θ < 0.5 ML, in contrast to the theoretical predictions of [92Kob] for 0.5 ML, that reproduce quite well the band topology (see Fig. 4). The main evidences collected about the metallization of the AM-Si(100)2x1 have been obtained from direct and inverse photoemission experiments. Early results [87Ent, 89Ent, 89Kon] showed that the K/Si(100)2x1 and Cs-covered surfaces have a band gap, with no density of states at the Fermi energy for Lando lt -Börnst ein New Ser ies III/42A1

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θ ≤ 0.5 ML. Johansson and Reihl [91Joh, 92Joh] confirmed these results, and also found the K/Si(100)2x1 interface insulating at low coverages (θ < 0.5 ML). These authors found however, that for a slightly further deposition, θ > 0.5 ML, metallization appears with a finite density of states at the Fermi energy (see Figs. 5 and 6). A more detailed analysis of these results was presented later on in Refs. [91Joh, 92Joh, 94Mar, 96Seg, 98Joh]. The interfaces K/Si(100)2x1 and Rb/Si(100)2x1 were analyzed using high-resolution angle-resolved photoemission spectroscopy and inverse photoemission spectroscopy. The photoemission spectra were collected for a series of coverages close to 0.5 ML. The picture that is obtained from these data is the following: the surface is semiconducting below θ = 0.5 ML. For a coverage below 0.5 ML, but close to it, the AM atoms undergo an ordering transition, giving rise to the formation of well ordered atomic chains along the <110> direction (i.e. the dimer rows) [91Kub, 92Sou2]. As the coverage increases during this stage, an unocuppied state moves closer to the Fermi energy [91Joh, 92Joh]. For a critical coverage slightly above 0.5 ML this state crosses the Fermi energy, being its minimum binding energy at Γ points. When the band crosses the Fermi energy, there is a semiconductor-metal transition. The observation of a metallic Fermi edge (associated to the crossing of the Fermi energy by this state, F2 in Fig. 5), is related to the existence of excess AM atoms, once the AM atomic rows are completed [96Seg]. Thus, the quasi-one dimensional rows are semiconducting and the metallization process is of two-dimensional nature, related to the interaction between adjacent atomic chains through excess AM atoms. The semiconducting character of the AM chains at 0.5 ML was evidenced by the observation of another electronic state (F1 in Fig. 5), associated with the well-ordered K atomic rows [92Sou1, 96Seg]. This electronic state exhibits one-dimensional character, and it does not cross the Fermi energy. Further AM deposition does not change the metallic character of the surface.

Table 8. Metallic character and metallization onset of AM/Si interfaces. 0.5* denotes a coverage of 0.5+0.05 ML. θ sat denotes the saturation coverage at room temperature. System

Metallic onset

Method

Reference

K/Si(100)2x1

0.5* 0.6-0.7 0.5* 0.5* 0.5*

ARUPS+AES ∆Φ+AES THS O2 reaction ARUPS

Rb/Si(100)2x1 K/Si(111)7x7

RT saturation RT saturation 1.5 0.66 RT saturation

IPS+UPS IPS+UPS UPS IPS-UPS

96Seg 89Per, 92Mic 95Fou 92Mic 94Far 91Joh, 92Joh, 94Mar, 96Seg, 98Joh 98Joh 90Mag 93Wei 90Mag, 91Mag

Cs/Si(111)7x7

The Fermi surface of the K/Si(100)2x1 surface has been investigated using angle-resolved photoemission at 21.2 eV [98Mar] and it is displayed in Fig. 7. It consists in a series of bright areas centered at surface Γ points and exhibiting full 2x1 periodicity. The Fermi surface comes from the surface band that crosses the Fermi energy, and that is observed only for coverages above 0.5 ML, in agreement with other findings [91Joh, 92Joh, 94Mar, 96Seg, 98Joh]. The surface state dispersion found [94Mar, 98Mar] (opposite to other findings), lead to a different interpretation of the metallization process, consisting in the overlap of the Si electron clouds around the K adsorption sites [94Ref, 96Ref, 98Mar]. In addition to the direct observation of a band crossing the Fermi energy for coverages slightly above 0.5 ML, the metallization has also been monitored using other techniques (see Table 8). The experiments disagree with the simplest theoretical predictions for the metallic character of the surface. The discrepancies found are due to electron-correlation effects in the adsorbed monolayer [90DiN, 93Plu]. The reduced dimensionality of the electronic states associated with the deposited AM

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atoms (which are of one-dimensional or two-dimensional character), and their reduced band width are relevant for the understanding of correlation effects. Both features favour the electron localization, and give rise to a non-metallic overlayer [81Sol, 92Lou]. It has been suggested [90DiN] that the high intrasite Coulomb interaction associated with the localized Wannier wave function (induced by the deposited AM atom) introduced a Mott transition, that explains the insulating behaviour of the adsorbed layer [89Ort, 92Flo, 93Bec, 93Ged, 94Flo2]. The surface bands have been calculated in Ref. [92Kob], and are reproduced in part in Fig. 4a and 4b. This figure shows the band structures of the K/Si(100)2x1 surface calculated [92Kob] for the case of 1 ML of K atoms occupying both valley-bridge and pedestal sites. The electronic structure of the clean surface presents two dangling-bond related surface bands labeled S1 and S2 [92Kob]. S1 is fully occupied, while S2 is unoccupied.. For θ = 0.5 ML, the surface bands are pushed down in energy [92Kob] due to the formation of a K-Si bond. For θ = 1 ML, the S2 surface state is fully occupied, and the two surface bands, S1 and S2, are shifted even further down in energy. The bands calculated for 1.0 ML reproduce very nicely the experimental results (see Fig. 4), except for an overall shift of approximately 0.3 eV attributed to the fact that exchange correlation effects are not taken into account properly in the local density approximation. The predicted metallization sequence is not correct. K on Ge(100)2x1 presents a behaviour similar to the one found for Si. STM results [94Cur] show at low coverage two distinct adsorption sites for K, and only for higher coverages, the K atoms form clear one-dimensional chains along the < 011 > direction, as seen in the K/Si(100) surface [92Sou2, 92Sou3]. The K/Ge(100) interface develops a metallic character for a coverage close to the saturation limit θ = 1. These results are similar to [92Eff] for K/Si(100), that seem to indicate a metallic behaviour for θ = 1, the saturation limit for K deposition. Metallization appears on these surface as soon as a geometry with local coverage of θ = 1 is reached. This is not in contradiction with the photoemission data that show metallization as soon as θ is larger than 0.5 ML, since the formation of surface islands having locally a saturation coverage, θ = 1, is possible. Other AM/silicon interfaces, as AM/Si(111)7x7 and AM/Si(111)-(√3x√3)R30º-B have been studied in several articles [89Dit, 90Mag, 91Mag, 93Wei, 94Wei, 97Wei]. There are also some studies on Ge(111) [90Sou]. A full metallicity of the AM/Si(111) surfaces is observed only after the saturation coverage of the first monolayer, with the exception of Cs/Si(111)7x7, where a semiconductor/metal transition is observed at submonolayer coverage [90Mag, 91Mag, 94Wei]. The electronic properties of the clean Si(111)7x7 and Si(111)(√3x√3)R30º-B surfaces are distinctly different. Whereas the clean Si(111)7x7 surface is metallic [81Bac, 86Ham], the Si(111)(√3x√3)R30º-B surface is semiconducting [89Lyo, 89Kax]. In this surface, trivalent B occupies a fivefold-coordinated subsurface site, directly below a Si adatom at the T4 position [89Bed, 89Hea, 89Kax, 89Lyo]. The presence of B stabilizes the (√3x√3)R30º surface reconstruction with respect to the 7x7 reconstruction and strongly modifies the electronic properties of the Si(111) surface. A Si 2p core-level study [90Ma] concluded that the K-Si bond is predominantly ionic at the K/Si(111)(√3x√3)R30º-B interface while is it covalent at the K/Si(111)7x7 interface [92Ma]. The surface states appearing at the K/Si(111)7x7 and Cs/Si(111)7x7 were analyzed in Refs. [90Mag, 91Mag]. While in the case of K the interface became metallic only after saturation at room temperature, the interface with Cs metallized after a coverage of 2/3 of saturation. A detailed study using angle-resolved photoemission spectroscopy, LEED, and work function measurements [93Wei] found that the interfaces of K with Si(111)7x7 and Si(111)(√3x√3)R30º-B are clearly nonmetallic. A non-dispersing K-induced surface state is observed just below the Fermi energy at the RT saturated K/Si(111)7x7 and K/Si(111) (√3x√3)R30º-B interfaces. At the K/Si(111)(√3x√3)R30º-B interface, a second interface state was observed at 2.2 eV binding energy. This state was identified as the backbond surface state of the Si adatoms in the B-induced (√3x√3)R30º surface reconstruction. The persistence of this state upon K adsorption indicates that the K atoms directly bond to the dangling bonds of the Si adatoms, leaving all Si-Si bonds intact. Valence-band photoemission, Si 2p core-level spectroscopy and work function measurements indicate that both interfaces are semiconducting at the room temperature saturation coverage. The observation of a nonmetallic ground state for the K/Si(111)(√3x√3)R30º-B interface could be understood on the basis of single-particle considerations and it was proposed that this interface represents a two-dimensional Mott insulator [93Wei, 97Wei]. Metallization occurs during the growth of a second K layer at low temperature, as evidenced by the development of an intraband plasmon satellite in Lando lt -Börnst ein New Ser ies III/42A1

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the K 3p core-level spectrum above 1.5 ML, by the appearance of a Fermi edge in valence-band photoemission near 2 ML, and by sudden quenching of the surface photovoltage effect at a coverage of approximately 2.5 ML (onset of long-range conductivity in the overlayer). The behaviour of the analogous system Cs/Si(111)(√3x√3)R30º-B is different [95Gre]. It has been found that at low coverage the interaction of Cs with the substrate is weak, and the Cs 6s electrons remain localized at Cs atoms. At higher coverages some evidences of surface reaction were found. The 3x1 reconstructions of AM/Si(111) interfaces are semiconducting [92Jeo, 94Oku, 94Sak, 96Wei, 99Oku]. After forming the 3x1 AM-induced reconstruction, the surface becomes passivated against reaction with gasses [91Tik] (see Fig. 8). This feature suggests that there are no unsaturated dangling bonds at the surface, and that AM atoms are also in a completely reacted state. The electronic band structure has been determined, but a complete understanding is difficult in view of the lack of a detailed atomic model of the surface. Most authors report significant exchange and correlation effects in these reconstructions [96Wei, 97Oku].

3.2.2.2.4 Overlayer plasmons and electron excitations Early work on K/Si(100)2x1 reported the observation of overlayer plasmon dispersion. The plasmon loss was attributed to the formation of well ordered K chains along the <110> surface direction [83Toc, 84Aru, 84Tsu, 85Ish]. Two overlayer plasmon modes in K chains adsorbed on Si(100)2x1 were reported by angle-resolved EELS. The dispersion of the more intense mode showed an azimuth-dependent anisotropy. At q|| → 0 this mode was assigned to the transverse oscillation, and the other to the longitudinal oscillation along the K chains. Experiments performed under strict vacuum and cleanliness conditions [92Eff, 92Sou2] showed a high quality AM layer with an ordering transition at a coverage close to 0.5 ML [91Kub, 92Sou2] (see also 3.2.2.2.3). The observation of a broad contribution appearing on the high binding energy side of Rb 4p has been related to the excitation of overlayer plasmon losses in the already metallic interface [98Cha]. The existence of free AM valence band electrons has been evidenced also by the observation of quantum-well type resonances for K and Na on Si(100)2x1 [93Ham, 94Ham]. This type of resonances may be formed by the confinement of AM valence electrons in the overlayer. This type of quantum confinement will produce states or resonances with more or less well-defined energy depending on the thickness homogeneity, and on how well the electrons propagating back and forth in the overlayer are reflected at the interfaces.

3.2.2.2.5 Core level binding energies and components The core level binding energies and components of AM/silicon surfaces has been investigated in detail [92Rif, 93Wei, 95Cha, 96Cha, 97Ch1, 97Ch2, 97Ch3, 97Seg, 98Cha, 98Gru], in particular for Si(100)2x1 substrates. Fig. 9 shows a series of Si 2p spectra for increasing amounts of K in the submolayer range. The AM induced band bending change is responsible of the overall shift of the spectra. The prominent Su feature at the low binding energy side in the Si 2p spectrum of the clean Si(100)2x1 surface, is a surface component that has been associated with the upper atoms of asymmetric dimers [92Lan]. A small amount of AM results in a broadening of the Si 2p line shape, probably due to the existence of several components and an inhomogeneous surface. At saturation coverage, the surface structure S is again well resolved in the Si 2p spectrum (see Table 9 for details).

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Table 9. Energy splitting of the S surface component of Si 2p core levels measured with respect to the bulk for different AM/Si(100)2x1 interfaces and clean Si(100)2x1 (see Fig. 10). AM

Energy splitting [eV]

Reference

Clean Si(200)2x1 Na K Rb Cs

0.50 0.33 0.42 0.48 0.585

92Lan 97Ch2 95Cha 98Cha 96Cha

A comparison between the Si 2p spectra of the different Si(100)2x1-AM surfaces is presented in Fig. 10. The intensity of the surface component S suggests that it corresponds to emission from the whole uppermost Si layer for Na and K interfaces, while it seems to correspond to half a monolayer for the Rb and Cs interfaces. The values of the S energy splitting with respect to the bulk is presented in Table 9 for different AM. Theoretical calculations predict the formation of symmetric dimers after AM adsorption. This conclusion was supported from the corresponding Si 2p core levels from Na and K interfaces. On the contrary, the Si 2p core levels obtained for Rb and Cs interfaces do not differ much from those of the clean surface, which suggests a weaker modification of the surface by the adsorbed AM atoms, and the Si dimers may remain asymmetric in this case. The difference in the modification on the Si dimers by the different AM atoms must be related to the Si-AM or AM-AM interaction. A concomitant surface band gap decrease when the size of the AM atom increases has also been detected by inverse photoemission [93Joh]. Fig. 11 shows the evolution of Rb 4p spectra as a function of Rb coverage. This behaviour is representative for other AM [98Cha]. For small Rb coverages the 4p spectrum consists of a broad doublet (A component). A second component B is resolved at a coverage of 0.67 θ sat (θ sat, saturation coverage). It grows faster in the later stage of adsorption and becomes well resolved with an emission intensity similar to A. The AM core levels obtained for the different RT saturated Si(100)2x1-AM surfaces are shown in Fig. 12. Note the existence of two components (A and B), in agreement with other results supporting the existence of two adsorption sites on the surfaces (see also 3.2.2.2.1) under adequate coverage and temperature conditions (in the case of Na the second component is observed also for coverages larger than 0.5 ML [97Ch2, 98Cha]). The spin orbit splitting and the A-B energy separation for different AM core levels on Si(100)2x1 are summarized in Table 10 [95Cha, 96Cha, 97Ch2]. Thicker AM layers have been investigated by deposition at low temperatures [97Ch4, 97Ch5]. The adsorption of Li gives rise to silicide formation above submonlayer coverages [95Gre2].

Table 10. Spin-orbit splitting and energy separation between the A and B components (see text) observed for AM core levels on AM/Si(100)2x1 room temperature saturated surfaces. Core level

Spin orbit splitting [eV]

A-B splitting [eV]

Reference

Na 2p K 3p Rb 4p Rb 3d Cs 4d

0.16 0.24 0.8 1.48 2.27

0.38 0.46 0.58 0.55 0.45

97Ch2 95Cha 98Cha 98Cha 96Cha

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The evolution of the K 3p core level binding energy as a function of K coverage in K/Si(100)2x1 has been analyzed in Ref. [97Seg] (see Figs. 13a and 13b). Three different regimes were found, labeled (a), (b), and (c) in Fig. 13a. In the first regime, the binding energy decreases linearly with K coverage, until a minimum is reached at 0.70 θ sat. In the second regime, the BE abruptly increases until it reaches a value of 18.75 eV at 0.778 θ sat. Finally, for coverages above 0.778 θ sat, the binding energy remains approximately constant. Regime (a) reflects an overall trend in alkali-metal adsorption, both on metallic [79Bro, 89Ste] and semiconducting [88Oel, 93Wei] surfaces. Several mechanisms can be invoked to explain this binding energy decrease, the simplest being a coverage-dependent charge transfer towards the substrate, which would be also responsible of the K-induced surface-dipole change. Nevertheless, theoretical calculations have shown that the work-function change is due to a strong polarization of the charge at the K site [89Ish, 92Kob]. The binding energy decrease should then mainly reflect the change in coordination as the coverage increases (i.e. adatom-adatom interactions) [93Wei]. This mechanism explains the rather universal decrease observed for alkalis on metallic or semiconducting surfaces. This trend is broken in Fig. 13a when a coverage of 0.70 θ sat is reached. Then, an abrupt change in the binding energy is detected (regime (b)). The change takes place at the same time as a metallic peak is observed in the valence band. The shift was attributed to an ordering transition concomitant with the metallization process [92Sou2]. For coverages above 0.778 θ sat (regime (c)) the binding energy remains constant. The Si and K core level line shape and binding energies of K/Si(111)7x7 have been investigated in Ref. [93Wei]. The observation of an asymmetric line shape in the K 3p core level was not related to metallic losses. It was rather attributed to a finite number of shakeup excitations in the nonmetallic interface. The evolution of the K 3p binding energy vs K coverage in K/Si(111)7x7 and K/Si(111)(√3x√3)R30º-B was reported in Ref. [93Wei] (see Fig. 13b). The binding energy exhibits a shift to lower binding energies, attributed to adsorbate-adsorbate interaction. The adsorption of K on H:Si(111)1x1 has been investigated as a way to interpret the K/Si(111)7x7 core levels and analyze the Schottky barrier formation mechanisms [98Gru].

3.2.2.2.6 Chemical state and nature of the bond The nature of the AM-semiconductor bond as a function of coverage has been investigated in detail. In the case of adsorption on metals (see also section 3.2.1), Langmuir [24Lan], Gurney [35Gur] and Lang [71Lan] developed a model involving an ionic to metallic transition as a function of AM coverage. At low coverages, the AM atoms are largely ionized with the alkali s level (broadened into a resonance) above the Fermi energy. With increasing coverage, the depolarization effects shift the resonance downwards leading to partial occupancy of the adatom s level. At sufficiently high coverage, the AM overlayer turns metallic with a partially filled s band [85Wor]. It has been established that the AM-metal bond has some covalent component even at low coverages and it should more precisely described as a polarized bond, as opposed to a strictly ionic bond. This model was taken over for the AM-semiconductor interface as well [85Toc, 86Oel, 88Oel]. Recent detailed calculations support that the image of a polarized covalent bond for AM/Si(100)2x1 [89Ish, 89Lin, 91Mor, 92Kob].

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3.2.2 Alkali metals on semiconductors

[Ref. p. 210

Table 11. Bond length for Na, K, Rb and Cs adsorbed on different silicon surfaces. 1 ML is defined as 6.78 x 1014 atoms cm−2 for the Si(100) surface and as 7.85 x 1014 atoms cm−2 for the Si(111) surface. Interface

Method

Coverage [ML]

Bond length [Å]

Reference

Na/Si(100)2x1

PEXAFS PEXAFS PEXAFS LEED SXRD SEXAFS SEXAFS XSW XSW PEXAFS PEXAFS SXRD SEXAFS XSW

0.5 0.5 0.2 0.5 1.2±0.3 0.5 0.55±0.1 0.19 0.09 0.15 0.5 0.3±0.1 1.0 0.07

2.67±0.05 2.80±0.1 2.80±0.1 2.975 2.90-3.58 3.14±0.10 3.20±0.03 3.03±0.02 3.3±0.1 3.54±0.05 3.57±0.06 3.33-3.50 3.50±0.04 3.5±0.1

93Man 91Kim 91Kim 90Wei 98Mey 88Ken 95Lin 95Cas 91Ete 95Man 95Man 98Mey 91Bat 93Ete1

K/Si(100)2x1

Rb/Si(100)2x1 Rb/Si(111)7x7 Cs/Si(100)2x1

Cs/Si(111)7x7

Table 12. Covalent radius (rcov), metallic radius (rmet), ionic radius (rion) and the sum of both for alkali metals. From [73Pau]. To be compared with bond length values in Table 11. Element rcov [Å]

rmet [Å]

rion [Å]

rcov + rcov (Si) [Å]

Na K Rb Cs Si

1.572 2.025 2.16 2.35

0.95 1.33 1.48 1.69

2.71 3.13 3.28 3.42

1.54 1.96 2.11 2.25 1.17

The nature of the bond has been investigated by determining the bond length. The AM-Si distance has been measured using different techniques (SEXAFS, XSW, PEXAFS), see Table 11. In all cases the bond length is very close to the sum of covalent radii (see Table 12). It is not easy to draw a definitive conclusion on the bond nature based only on the bond length, but one can conclude that the AM-Si bond is weak. This is further confirmed by the rather low desorption temperatures of AM from Si surfaces [86Sou, 87Oel] as compared to e.g. metal surfaces [80Des]. The image of a covalent bonding is also supported by an analysis of the Si 2p core level. Very large chemical shifts (in the range of 1 eV) to higher or lower binding energies (for electronegative or electropositive species, respectively) have been previously reported for ionic adsorbates (with very high or very low electronegativity) on Si surfaces [84McF, 85Fra]. Instead, a systematic comparison of the Si 2p level for Na, K, Rb and Cs (see Fig. 10) 2Sou1, 98Cha] shows only a small core level shift. Metastable deexcitation spectroscopy (MDS) experiments have shown for Cs or K covered Si surfaces that the alkali s electron remains occupied even at low coverage [89Nis, 90Nis]. This technique is very sensitive to the s orbital, at variance with photoemission spectroscopy. Finally, the existence of plasmons within the adsorbate overlayer has been reported for several AM-Si systems [84Aru, 86Toc, 87Fra], also in agreement with the metallization of the overlayer and the covalent nature of the bond. In summary, all experimental results support a covalent type of bonding. Its exact nature has been clarified by theoretical calculations [89Ish, 89Lin, 91Mor, 92Kob] showing that, as a result of Si-AM hybridization, a polarized covalent Si-AM bond is formed

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193

3.2.2.3 Adsorption on III-V semiconductors: physical and chemical properties 3.2.2.3.1 Long range order, structure and adsorption site The structural properties of AM/III-V semiconductors have been studied using LEED for the system Cs/GaAs(110) [74Bom, 76Bom, 76Der, 77Der]. This system has been also investigated in detail using STM [89Fir]. These studies have revealed that the adsorption of Cs on GaAs(110) exhibits three different structural phases: atoms are initially deposited either isolated or forming one-dimensional linear chains [89Fir] as shown in Fig. 14, along the < 1 1 0 > direction, with the AM atom adsorbed on Ga. The chains are hundreds of Å apart from each other, and provide a model system for testing theories concerning the electronic properties of one-dimensional wires. For a further deposition [91Whi, 91Whi2], triple chains are formed (Fig. 14), and for higher coverages two-dimensional islands appear with a c(4x4) symmetry and five atoms per unit cell (Fig. 14). Additional Cs deposition results in a saturated surface, with a coverage close to θ = 0.45. At this stage the Cs layer clusters and it becomes three-dimensional. This corresponds to roughly a Cs atom per semiconductor unit cell [89Hes] and a Cs-Cs bond length similar to the bulk Cs [94Fre]. These different structures show that Cs atoms tend to attract each other. This is at variance with the simplest expectation, i.e. that they would repel each other due to the dipole-dipole interaction that appears when electronic charge is transferred from the alkali atom to the semiconductor. The effective attraction has been explained as consequence of the semiconductor surface relaxation [91Heb, 92Heb, 93Bec, 93Pa1, 93Pa2]. The deposition of AM atoms tends to eliminate the relaxation of the clean semiconductor surface. Then the interplay between the relaxation energy and the adatomsemiconductor interaction creates an attractive potential between the adsorbed adatoms. The formation of the attractive potential has been described calculated for Na on GaAs(110) in Refs. [91Heb, 92Heb]. It was found that the adsorption energy of isolated Na atoms (1.60 eV/atom) was larger than the geometry of a zig-zag (or triple) chain (1.58 eV/atom) (see Fig. 14). This result suggests [91Heb, 92Heb] the existence of an effective Na-Na attraction along the surface. The local atomic structure of Cs/InP(110), Cs/GaAs(110), and Na/InP(110) at low coverage has been studied using the PEXAFS technique [88Ke1, 88Ke2]. Figs. 15 and 16 respectively show the Cs 4d core level for the Cs/GaAs(110) and Cs/InP(110) at various Cs coverages. Below 0.4 ML the Cs core level exhibits two components. A similar behaviour is observed for the Na 2p core level in the system Na/InP(110).The appearance of two components indicates the existence of two inequivalent adsorption sites. The easiest explanation for the two components assigns one of them to the AM atom bonded to one cation and two anions on one site, and the second one to the AM atom bonded to two cations and one anion on the other (see Fig. 14). For each site the charge transfer would be different, since the dangling bonds have a different occupancy on the cation and the anion. This structural model [88Ken, 88Ke2] is consistent with previous LEED data [74Bom, 76Bom, 76Der, 77Der] as well as with inverse photoemission experiments, where two surface states were observed just above the Fermi level [89Mag, 89Ma2]. An interesting point is the possible structural changes of the surface induced by AM adsorption. In the case of InP(110), Na deposition induces structural changes in the surface, with removal of the surface relaxation and surface reconstruction [89Cho]. 3.2.2.3.2 Metallization and electronic structure AM/GaAs(110) systems have been considered for a long time as a model systems for the investigation of the Schottky barrier formation. Due to this fact they have received a lot of attention [88Eva, 88Ke1, 88Ke2, 88Pr1, 89Lau, 89Pri], in particular the system Cs/GaAs(110) (see also section 3.2.2.3.1). Work-function measurements (see also section 4.2) [92Ort] have shown three different regimes for the deposition of Cs on GaAs(110), that correspond to the three kinds of structures discussed above: linear chain, triple rows, and c(4x4) structure. The c(4x4) structure appears for θ > 0.22, the triple row for 0.99 < θ < 0.22, and the linear chain structure for θ < 0.09 [89Hes, 94Fre]. The AM-induced work function change is similar for other AM's (e.g. K and Rb) [86Ruc, 88Kin, 89Cha, 89Rei, 90Dud, 91Pri]. Theoretical calculations predict that K atoms should occupy the same adsorption site as Cs atoms, that corresponds experimentally to adsorption on the Ga atoms. The metallization of the Cs/GaAs(110) interface has been analyzed using STM spectroscopy as well [91Whi, 91Whi2]. An analysis of the Intensity-Voltage (I-V) curves show that the phase corresponding to linear chains presents a band gap of 1.10 eV. Further Cs deposition gives rise to the formation of triple Lando lt -Bö rnst ein New Ser ies III/42A1

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3.2.2 Alkali metals on semiconductors

[Ref. p. 210

rows, that have a reduced band gap of 0.65 eV. Finally, additional Cs deposition produces the c(4x4) structure, with a band gap of 0.6 eV. A metallic interface is found only for larger coverages, in the range of 0.5 ML. Direct and inverse photoemission studies [89Mae, 89Mag, 92Rei1] have found an occupied state 0.25 eV above the valence-band maximum at low coverages, probably in the range of linear chains. This state shifts downwards in energy at saturation coverage by 0.45 eV. The empty states above the Fermi energy have been also studied. Their behaviour suggests that metallization appears after the saturated monolayer is formed [89Mag, 92Rei1]. EELS has also been used to analyze the onset of metallization [90DiN, 94Com]. The elastic peak shows no losses for θ < 0.45. For θ > 0.45 metallization was observed as a broadening of the elastic peak intensity. This coverage range was achieved by depositing Cs at low temperature (150 K) on the Cs/GaAs(110) interface, saturated with Cs at room temperature. This procedure allows the growth of further layers above the room-temperature saturation coverage. Since energy losses are mainly coupled to interface dipolar transitions, that do not exist for the excitations created within a metal monolayer, these results only show that there is metallization when a second layer is deposited. Much of the work performed on AM/GaAs(110) interfaces has been devoted to understanding the details of the different effects that control the Fermi-level behavior reproduced in Figs. 17 and 18. Fig. 17 is a compilation of data from Refs. [88Pr1, 89Pri] shows the shift of the interface Fermi level with AM deposition on p and n doped GaAs(110). Fig. 18 presents similar results for GaSb(110) [94Sch]. In both sets of data the Fermi levels behave in the same way: there is a large initial shift for p-doped semiconductors and a smoother evolution for the n-doped samples. The final Fermi level position is the same for both dopings, and it is reached for coverages close to one monolayer. The results of Figs. 17 and 18 were obtained by analyzing the PES core-level line shapes as a function of the AM coverage. Notice that the data of Fig. 17 were taken at low temperature. The experiments of Fig. 17 were repeated at room temperature, and it was found that the Fermi levels for n- and p- doping coincided with each other above a rather low coverage, θ = 0.1 [91Bau]. This suggests a high mobility for the AM atoms (see also section 3.11) on the surface, and the formation of metallic islands that pin the Fermi energy at a fixed position. It has been shown by analyzing the photoemission core-level shifts, that a Cs-covered surface at room temperature is metallic [88Ke1]. Due to the surface photovoltage effect [90Alo, 90Ald] substantial modifications in the semiconductor band bending may be introduced by a photon field, thus altering the results of the measured interface Fermi level. It has been shown, however, that the results of Fig. 17 are not altered by this effect [91Bau]. It is interesting to compare the modification of the overlayer band gap with the evolution of the interface Fermi level pinning, for n- and p-semiconductor doping. The results of Fig. 17 seem to underestimate the values found with STM spectroscopy, although both results indicate a closing of the overlayer band gap with increasing coverage. This is not surprising, as the core-level photoemission experiments leading to the results of Fig. 17 are averaged over a large interface area. The case of Cs on InSb(110) is very similar to the GaAs(110) case [91Whi, 91Whi2]. In particular, zig-zag chains are initially observed at low coverage, with a band gap of 0.2 eV, followed by the formation of a c(2x6)-Cs overlayer with four atoms in the unit cell [91Whi2]. Although the linear chains show a band gap of 0.2 eV (probably due to the substrate band structure), the c(2x6)-Cs overlayer has a band gap of 0.6 eV. The Cs density for this two-dimensional structure corresponds to θ = 1/3. For further deposition, metallization is quickly observed, probably for θ around 0.5 [91Whi]. The transition from a one-dimensional to a two-dimensional Cs layer is reflected in the existence of two phases with different electronic properties [96Bet]. We complete our discussion of experimental results found for AMs on III-V semiconductors by commenting on the evidence given for Na on GaP(110) [93Eva, 94Cha]. Na was deposited on GaP(110) at 100 K and the photoemission core levels and valence band were analyzed. The data for the valence band spectra, recorded at normal emission, are collected in Fig. 19. These results are similar to the Cs/GaAs(110) case; this shows again the great similarity between different AM atoms and semiconductor III-V (110) surfaces. As in the Cs/GaAs(110) case, at low Na coverages an occupied level appears located 0.2 eV above the valence band maximum. Further deposition of Na shifts this level downwards until it finally merges with the valence band.

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195

Figures for 3.2.2

Fig. 1. Right: different adsorption sites for alkali metal atoms on Si(100)2x1: T3 (valley), T4 (cave), P (pedestal), D (dimer bridge) and T (top). Left: side view of a Si(100)2x1 surface (small white circles, small black circles denote Si dimer atoms) covered with 1 monolayer of alkali metal atoms (large circles) that occupy P and T3 sites ("double layer model").

Fig. 2. STM images of 0.5 ML of K on Si(100)2x1 (4º stepped in order to obtain a single-domain surface). Tunneling voltage = 1.2 V (left) and −1.2 V (right). The distance between two potassium chains is 7.68 Å. From [91Sou].

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3.2.2 Alkali metals on semiconductors

Si(100)-K

heating rate : 7.7 K s

α0

[Ref. p. 210

–1

α

β

θ>1

Desorption rate D

g

1.0 f

0.7 e

0.5 d 0.3 c 0.1 b

0.03

a

100

200

300

400

500 600 700 Temperature T [K]

800

900

1000

1100

Fig. 3. Desorption rate (D) vs sample temperature (T) for K atoms adsorbed on the Si(100)2x1 surface. K atoms were deposited at 100 K. Solid curves represent a theoretical model. The peaks α and β correspond to two different adsorption sites at the 2x1 surface. The peak α0 is due to bulk K desorption. From [90Tan].

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3.2.2 Alkali metals on semiconductors

197

2.0 1.5 1.0 S3 Energy E [eV]

0.5 [100]

0 S2

– 0.5 – 1.0

S1

– 1.5 – 2.0 Γ

J

K

J'

Γ

J'

– 0.50 – 0.25 0

S1

Binding energy Eb [eV]a

0.25

S1

S1

S'1 0.50

S'1

0.75

S'1

S2

S2

1.00 1.25 1.50 1.75 2.00 Γ

Lando lt -Bö rnst ein New Series III/42A1

J

K Wave vector q

J'

Γ

Fig. 4. Band structure for adsorption of 1 monolayer of K atoms on a Si(100)2x1 surface in a P-T3 geometry (continuous lines, calculations from [92Kob]. Upper part: experimental results from inverse photoemission spectroscopy (shifted to lower energy to agree with theoretical bands at Γ ) from [91Joh]. The origin of energy is set at the top of the occupied band. From Ref. [93Mor]. Lower part: a similar comparison with experimental results (shades) from angleresolved photoemission. The origin of energy is at the Fermi energy, and the theoretical curves have been shifted 0.6 eV towards the Fermi energy. From Ref. [92Abu].

198

3.2.2 Alkali metals on semiconductors [011]

J' K

[Ref. p. 210

J'

[011]

[011]

Γ0

J Γ1

[011]

hν = 12 eV

(×4)

28 eV

θ = 0 ML

12 eV

Intensity I

0.2

(×4)

28 eV

F1

12 eV

(×4)

0.5

F2

28 eV

F1

0.5*

EF –6

–5

–4

–3 –1 –2 Binding energy Eb [eV]

0

1

2

Fig. 5. Coverage dependence of the surface states F1 and F2. The spectra are taken at the resonance conditions of F2 (hν = 12 eV, Γ 0 ) and F1 (hν = 28 eV, Γ1 ). The two bottom spectra were taken by adding approx. 0.05 θsat to the spectra at 0.5 ML. Note the abrupt appearance of F2 at this coverage. From [96Seg].

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3.2.2 Alkali metals on semiconductors

Si(100) 2×1-K single-domain Γ- J

199

ϑi = 35°

hν = 9.5 eV

32.5° 30° 25° 20°

Intensity I

15°

U3

10° 5° 0° J'

K

Γ

J

U2

EF 0

1

2 Energy E [eV]

3

4

5

Fig. 6. Inverse photoemission spectra recorded on single-domain K/Si(100)2x1 (approx. 0.5 ML coverage) recorded for different incidence angles ϑi along the Γ − J direction of the surface Brillouin zone. U2 corresponds to the unoccupied counterpart of F2 from Figure 5. From [91Joh].

Fig. 7. Experimental Fermi surface map of the K/Si(100)2x1 system at saturation coverage (hν = 21.2 eV). The surface Brillouin zones have been drawn superimposed. Bright areas denote regions where a band crosses the Fermi energy. From [98Mar].

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3.2.2 Alkali metals on semiconductors

0.6

2 1

0.5

O(KLL)/(Si(KLL) Auger ratio

[Ref. p. 210

0.4 3'

0.3

3

0.2

Fig. 8. Oxygen uptake for oxygen adsorption on 1: a clean Si(111)7x7 surface; 2: a 0.1 ML Na/Si(111)7x7 surface; 3: a high-quality 0.1 ML Na/Si(111)3x1 surface; 4: a low quality 0.1 ML Na/Si(111)3x1 surface. Adsorption temperature: 300 K. From [91Tik].

0.1

0

5

10 15 Oxygen exposure [L]

20

25

hν = 130 eV ϑe = 60°

Si 2p S

Emission intensity

f

∆Φ = –3.46 eV

e

–3.39

d

–3.32

c

–2.65

b

–1.86

a

–0.94 eV Su

clean

101.5

101.0

0 100.5

100.0

99.5 99.0 98.5 Binding energy Eb [eV]

98.0

97.5

97.0

Fig. 9. Si 2p core-level spectra recorded with a photon energy of 130 eV at 60º emission angle for increasing K coverage. The coverage was determined indirectly by measuring the work-function change. From [95Cha].

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3.2.2 Alkali metals on semiconductors

Si(100) 2×1-AM hν = 130 eV RT ϑe = 0°

Si 2p

B

S Cs

S

Emission intensity

Rb S

K B

S

Na

Su clean

101.5

101.0

100.5

100.0 99.5 99.0 Binding energy Eb [eV]

98.5

98.0

97.5

Fig. 10. Normal emission Si 2p spectra recorded from 2x1 clean and 2x1-AM surfaces. From [98Cha].

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201

202

3.2.2 Alkali metals on semiconductors

Rb/Si(100) 2×1

Rb 4p

B

hν = 32 eV RT ϑe = 0°

[Ref. p. 210

A

Rb(θs) = 1.00

Emission intensity

B

0.83 0.67 0.50 0.42 A

0.33 0.17 0.06 0

8

9

10

11 12 Kinetic energy Ekin [eV]

13

14

15

Fig. 11. Normal emission Rb 4p spectra recorded with a photon energy of 32 eV from Si(100) surfaces for increasing Rb coverage at room temperature. From [98Cha].

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3.2.2 Alkali metals on semiconductors

Cs 4d

Si(100) 2×1-Cs hν = 110 eV

82

80

78

76 B

Emission intensity

Rb 3d

116

114

112

A

Si(100) 2×1-Rb hν = 150 eV

108

110

K 3p

Si(100) 2×1-K hν = 70 eV

22

20

18

Na 2p

35

74

16

14

Si(100) 2×1-Na hν = 100 eV

34

33

32 31 Binding energy Eb [eV]

30

29

Fig. 12. Normal emission AM core level spectra obtained from roomtemperature saturated AM/Si(100)2x1 surfaces. The temperature of the sample was 100 K during recording. From [98Cha].

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203

204

3.2.2 Alkali metals on semiconductors

1

0

0.2

0.4

Coverage θK /θsat 0.6 0.8 1.0

1.2

1.4

19.6

0

19.4

–1

K 3p ∆φ –2

7×7 3× 3

Intensity of K 3p

Work funct. change ∆φ [eV]

[Ref. p. 210

19.2

19.0

–4 19.0

K 3p binding energy EB [eV]

–3

18.8

K 3p hν = 31 eV

18.9

5 ML bulk

Binding energy Eb [eV]

18.6

3p1/2

18.8

18.4

18.7

3p3/2

18.6

18.2 18.5 18.4

a

I 0

2

4 6 Evaporation time [min]

8

10

18.0

b

0

0.2

II 0.4

III

0.6 0.8 1.0 Rel. coverage θ/θsat

Fig. 13a, b. (a) Upper panel: work-function change (empty circles) and K 3p core level intensity (filled circles) as a function of K evaporation time and saturation coverage (θsat). Lower panel: K 3p binding energy versus K coverage. Three different regimes are shown (see text). From [97Seg]. (b) Coverage dependence of the K 3p core level binding energies for the K/Si(111)7x7 and K/Si(111)(√3x√3)R30º-B interfaces. The data from regime III were recorded from a thick K film, grown at 77 K. From [93Wei].

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3.2.2 Alkali metals on semiconductors

205

S

S P

T

S

P S

P

T

S

P S

P

T

P

T

S

P S

S

P S

P

T

P

T

S

P S P

[110]

Ga

As

Cs

[001]

[001]

5

4 3 1

[110]

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2

As

Cs

Fig. 14. Upper panel: schematic diagram showing a proposed geometry for the adsorption of Cs on GaAs(110). Ga and As are represented by small filled and open circles, respectively, while large circles locate Cs atoms. The enclosed letter identifies primary (P), secondary (S) and tertiary (T) maxima, as seen with STM. The left structure represents the low-coverage zig-zag chain, and the right structure the triple chain observed at higher coverage. From [89Fir]. Lower panel: schematic drawing of the c(4x4) Cs overlayer observed on GaAs(110). Only the substrate As lattice is shown for clarity (black circles). Segments of a Cs(110) lattice are superimposed over the two polygons on the right. From [91Whi].

206

3.2.2 Alkali metals on semiconductors

[Ref. p. 210

Cs 4d

0.04 ML Cs on GaAs(110)

Intensity I

P2 P1

34

33

35

36 37 38 Kinetic energy Ekin [eV]

39

40

41

Fig. 15. Curve fitting of the Cs 4d spectrum for 0.04 ML of Cs with two doublets corresponding to different adsorption sites. The dots represent the data and the upper solid line is the sum of the components P1 and P2. From [88Ken1].

0.8

0.8

0.6

0.6

0.4

p-GaSb n-GaSb

0.2

0

– 0.2

Surface Fermi level

Surface Fermi level

For Figures 16 and 17, see next pages

0.4

p-GaSb n-GaSb

0.2

0

– 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 0 Cs coverage θ [ML] Rb coverage θ [ML] a b Fig. 18a, b. (a) Band bending for the interface formation of Rb on p- and n-type GaSb(110) at room temperature. (b) Band bending for the interface formation of Cs on p and n-type GaSb(110) at room temperature. From [94Sch]. 0

0.2

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3.2.2 Alkali metals on semiconductors

Intensity I

Ref. p. 210]

33

P1

P2

34

35

36 38 37 Kinetic energy Ekin [eV]

39

40

41

34

35

36 37 38 Kinetic energy Ekin [eV]

39

40

41

Intensity I

a

33 b

Fig. 16a, b. Curve fitting for the Cs 4d spectra for (a) 0.1 ML and (b) 0.4 ML Cs on p-InP(110), with two doublets which are associated with two different adsorption sites. The dots represent the data and the upper solid line is the sum of the components denoted P1 and P2. From [88Ke2].

Lando lt -Bö rnst ein New Series III/42A1

207

208

3.2.2 Alkali metals on semiconductors

1.6

1.2 n

K/GaAs(110)

CBM

T = 85 K

Rel.binding energy Eb [eV]

Rel.binding energy Eb [eV]

CBM

1.6

Na/GaAs(110)

0.95 eV

0.8 p

0.4

[Ref. p. 210

T = 85 K

1.2 n

0.93 eV

0.8 p

0.4

0.53 eV

0.55 eV

VBM

VBM 1 2 3 Alkali coverage θ [Monolayers]

1.6

Rel.binding energy Eb [eV]

CBM n

1.2

4

0

1 2 3 Alkali coverage θ [Monolayers]

1.6

Rb/GaAs(110)

CBM

T = 85 K

Rel.binding energy Eb [eV]

0

0.82 eV

0.8 p

0.4

1.2

n

Cs/GaAs(110) T = 85 K

0.81 eV

p

0.8

0.4

0.66 eV

0.67 eV

VBM 0

4

VBM 1 2 3 Alkali coverage θ [Monolayers]

4

0

1 2 3 Alkali coverage θ [Monolayers]

4

Fig. 17. Concurring experimental variations of As 3d and Ga 3d core level binding energies of substrate atoms as a function of alkali coverage for interfaces of Na, K, Rb, and Cs with n- and p-GaAs(110) at 85 K (solid lines and left hand scale). The bars represent the Fermi level positions at the interface relative to the valence band maximum (right hand scale), obtained for n- and p-doped GaAs(110) substrates from the measured core level binding energies, after correcting for finite photoemission sampling depth. From [89Pri].

Landolt -Bö rnst ein New Series III/42A1

Ref. p. 210]

3.2.2 Alkali metals on semiconductors

Na/GaP(110) T = 100 K hω = 61 eV

209

θ = 1.0 0.5

Intensity I

0.4 0.22 0.14

VBM 0

10

8

6

4 2 Energy below VBM [eV]

Na/GaP(110) top of valence band hω = 61 eV

0

–2

SPV

θ = 1.9

Intensity I

1.0 0.63

0.50

VBM – 1.0

– 0.5

0

0.37 0.5 1.0 Energy below VBM [eV]

1.5

2.0

2.5

Fig. 19. Valence band spectrum of GaP(110), recorded at normal emission at a photon energy of 61 eV, for different coverages of Na as indicated. Upper part: spectra including the region of the valence band. Lower part: Close-up spectra featuring the Na-induced state in the band gap, as a function of Na deposition. The 1.9 ML spectrum exhibits a Fermi cutoff, shifted with respect to the reference Fermi level (top) due to the stabilization of a surface photovoltage. From [93Eva].

Lando lt -Bö rnst ein New Series III/42A1

3.2.2 Alkali metals on semiconductors

210

References for 3.2.2 24Lan 35Gur 35Mau 38Mot 38Sch 39Goo 39Sch 40Sch 47Bar 62Red 65Cow 65Hei 67Pal 69Web 71Lan 73Lev 73Pau 74Bom 74Bra 75Dry 76Bom 76Der 76Lo1 76Lo2 77Der 77Hol 77Tej 77Wan 78Bri 78Lel 79Bro 79Hat 79Spi 80Des 81Bac 81Sol 82Bri 82His 82Leu 82Raw 83Mön 83Toc 84Aru 84Hab 84McF 84Ter 84Ter 84Tsu

Langmuir, I.: Phys. Rev. 23 (1924) 112. Gurney, R.W.: Phys. Rev. 47 (1935) 479. Maue, A.W.: Z. Phys. 94 (1935) 717. Mott, N.F.: Proc. Cambridge Philos. Soc. 34 (1938) 568. Schottky, W.: Naturwissenschaften 26 (1938) 843. Goodwin, E.T.: Proc. Cambridge. Philos. Soc. 35 (1939) 205. Schottky, W.: Z. Phys. 113 (1939) 367. Schottky, W.: Phys. Z. 41 (1940) 570. Bardeen, J.: Phys. Rev. 71 (1947) 717. Redhead, P.A.: Vacuum 12 (1962) 203. Cowley, A.M., Sze, S.M.: J. Appl. Phys 36 (1965) 3212. Heine, V.: Phys. Rev. 138 (1965) 1689. Palmberg, P.W., Peria, W.T.: Surf. Sci. 6 (1967) 57. Weber, R.E., Peria, W.T.: Surf. Sci. 14 (1969) 13. Lang, N.D.: Phys Rev. B 4 (1971) 4234. Levine, J.D.: Surf. Sci. 34 (1973) 90. Pauling, L.: The Nature of the Chemical Bond. Cornell University Press, 1973. Bommel, A.J. van, Crombeen, J.E.: Surf. Sci. 45 (1974) 308. Brawn, F.: Papp. Ann. 153 (1974) 556. Dry, M.E., Shingles, T., Boshoff, L.J., Ousthuizen, G.J.: J. Catalysis 15 (1969)190. Bommel, A.J. van, Crombeen, J.E.: Surf. Sci. 57 (1976) 109. Derrien, J., Arnaud d’Avitaya, F., Bienfait, M.: Solid State Commun. 20 (1976) 557. Loui, S.G., Cohen, M.L.: Phys. Rev. B 13 (1976) 2461. Louis, E., Ynduráin, F., Flores, F.: Phys. Rev B 13 (1976) 4408. Derrien, J., Arnaud D’Avitaya, F.: Surf. Sci. 65 (1977) 668. Holtom, R., Gundry, P.M.: Surf. Sci. 63 (1977) 263. Tejedor, C., Flores, F., Louis, E.: J. Phys. C: Solid State Phys. 10 (1977) 2163. Wang, Chen Chou: J. Appl. Phys. 48 (1977) 1477. Brillson, L.: J. Phys. Rev. Lett. 40 (1978) 260. LeLay, G., Manneville, M., Kern, R.: Surf. Sci. 72 (1978) 405. Brodén, G., Bonzel, H.P.: Surf. Sci. 84 (1979) 106. Hatsopoulos, G.N., Gyftopou1os, E.P.: Thermionic Energy Conversion. Cambridge, Massachusetts: MIT Press, 1979. Spicer, W. E., Lindau, I., Skeath, P., Yu, C Y.: J. Vac. Sci. Technol. 17 (1979) 1019. Desplat, J.L.,, Papageorgopoulos, C.A.: Surf. Sci. 92 (1980) 97. Backes, U., Ibach, H.: Solid State Commun. 40 (1981) 575. Del Sole, R., Chadi, D.J.: Phys, Rev. B 24 (1981) 7430. Brillson, L.J.: Surf. Sci. Rep. 2 (1982) 123. Hiskes, J.R., Schneider, P.J.: Phys. Rev. B 23 (1980) 949. Leung, K.N., Ehlers, K.W.: Rev. Sci. Instrum. 53 (1982) 803. Rawlings, K.J., Foulias, S.D., Price, G.G., Hopkins, B.J.: Surf. Sci. 118 (1982) 47 Mönch, W.: Surf. Sci. 132 (1983) 92 Tochihara, H.: Surf. Sci. 126 (1983) 523. Aruga, T, Tochihara, H., Murata, Y.: Phys. Rev. Lett. 53 (1984) 372. Habenchaden, E., Küppers, J.: Surf. Sci. 138 (1984) L147. McFeely, F.R., Morar, J.F., Shinn, N.D., Landgren, G., Himpsel, F.J.: Phys. Rev. B 30 (1984) 764. Tersoff, J.: Phys. Rev. B 30 (1984) 4874. Tersoff, J.: Phys. Rev. Lett. 52 (1984) 465. Tsukada, M., Ishida, H., Shima, N.: Phys. Rev. Lett. 53 (1984) 376. Landolt -Börnst ein New Series III/42A1

3.2.2 Alkali metals on semiconductors 85Bri 85Dai 85Fra 85Ish 85Toc 85Wor 86Aon 86Ham 86Mön 86Oel 86Ruc 86Sou 86Toc 87Cap 87Ent 87Flo 87Fra 87Has 87Mön 87Oel 87Ort 87Sti 88Abu 88Cir 88Eva 88Kai 88Ke1 88Ke2 88Kin 88Miz 88Mön 88Oel 88Pr1 88Pr2 88Rho 88Spi 88Sti 89Ase 89Bat 89Bed

211

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212 89Bon 89Cha 89Cho 89Dit 89Ent 89Fir 89Gla 89Hea 89Hes 89Ish 89Kax 89Ken 89Koh 89Kon 89Lau 89Lin 89Lyo 89Ma2 89Mae 89Mag 89Nis 89Ort 89Pap 89Per 89Pri 89Ram 89Rei 89Rei2 89So1 89Spi 89Ste 90Ald 90Alo 90Bat 90Cas 90DiN 90Dud 90Egg 90Fan 90Gla

3.2.2 Alkali metals on semiconductors Bonzel, H.P., Bradshaw, A.M., Ertl, G. (eds.), Physics and Chemistry of Alkali Metal Adsorption. Amsterdam: Elsevier, 1989. Chang,Y., Hanson, J., Hwu, Y., Margaritondo, G.: J. Vac. Sci. Technol. A 7 (1989) 717. Choudhary, K.M., Mangat, P.S., Starnberg, H.I., Hurych, Z., Kilday, D., Soukiassian, P.: Phys. Rev. B 39 (1989) 567. Ditzinger, U.A., Lunau, Ch., Schieweck, B. Tosh, St., Neddermeyer, H., Hanbüken, M.: Surf. Sci. 211-212 (1989) 707. Enta, Y., Suzuki, S., Kono, S., Sakamoto, T.: Phys. Rev. B 39 (1989), 5524. First, P.N., Dragoset, R.A., Stroscio, J.A., Celotta, R.J., Feenstra, R.M.: J. Vac. Sci. Technol. A 7 (1989) 2868. Glander, G.S., Webb, M.B.: Surf. Sci. 222 (1989) 64. Headrick, R.L., Robinson; I.K.,V1ieg, E., Feldman, L.C.: Phys. Rev. Lett. 63 (1989) 1253. Heskett, D., Maeda-Wong, T., Smith, A.J., Graham, W.R., DiNardo, N.J., Plummer, E.W.: J. Vac. Sci. Technol. B 7 (1989) 915. Ishida, H., Terakura, K.: Phys, Rev, B 40 (1989) 11519. Kaxiras, E., Pandey, K.C., Himpsel, F.J., Tromp, R.M.: Phys. Rev. B 41 (1989) 1261. Kennou,S., Kamaratos, M., Ladas, S., Papageorgopoulos, C.A.: Surf. Sci. 216 (1989) 462. Kohmoto, S., Mizuno, S., Ichimiya, A.: Appl. Surf. Sci. 41-42 (1989) 107. Kono, S., Enta, Y., Abukawa, T., Kinoshita, T., Sakamoto, T.: Appl, Surf. Sci. 41/42 (1989) 75. Laubschat, C., Prietsch, M, Domke, M., Weschke, E., Mandel, T., Remmers, G., Ortega, J.E., Xue, C., Kaindl, G.: in Ref. [89Bat]. Ling, Y., Freeman, A.J., Delley, B.: Phys. Rev. B 39 (1989) 10144. Lyo, I.W., Kaxiras, E., Avouris, Ph.: Phys. Rev. Lett. 63 (1989) 1261. Magnusson, K.O., Reihl, B.: Phys, Rev. B 40 (1989) 5864. Maeda Wong, T., Heskett, D., DiNardo, N.J., Plummer, E.W.: Surf. Sci. 208 (1989) 11. Magnusson, K.O., Reihl, B.: Phys, Rev. B 40 (1989) 7814. Nishigaki, S., Oishi, N., Matsuda, S., Kawanishi, N., Sasaki, T.: Phys. Rev. B 39 (1989) 8048. Ortega, J., Flores, F.: Phys. Rev. Lett. 63 (1989) 2500. Papageorgopoulos, C.A., Kamaratos, M.: Surf. Sci. 221 (1989) 263. Pervan, P., Michel, E.G., Castro, G.R., Miranda, R., Wandelt, K.: J. Vac. Sci. Technol. A 7 (1989)1885. Prietsch, M., Domke, M., Laubschat, C., Mandel, T.,. Xue, C., Kaindl, G.: Z. Phys. B74 (1989) 21. Ramirez, R.: Phys. Rev. B 40 (1989) 3962. Reih, B., Magnusson, M.O., Nichols, J.M., Perfetti, P., Salvon, F.: in Ref. [89Bat]. Magnusson, K.O., Reihl, B.: Phys. Rev. B 39 (1989) 10456. Soukiassian, P., Kendelewicz, T.: in Ref. [89Bat]. Spicer, W.E.: Appl. Surf. Sci. 41/42 (1989) 1. Stenborg, A., Bjorneholm, O., Nilsson, A., Martensson, N., Andersen, J.N., Wigren, C.: Surf. Sci. 211/212 (1989) 470. Aldao, C.M., Waddlill, G.D., Benning, P.J., Caposso, C., Weaver, J.H.: Phys. Rev. B 41 (1990) 6092. Alonso, M., Cimino, R., Horn, K.: Phys. Rev. Lett. 64 (1990) 1947. Batra, I.P.: J. Vac. Sci. Technol. A 8 (1990) 3426. Castro, G.R., Pervan, P., Michel, E.G., Miranda, R., Wandelt, K.: Vacuum, 41 (1990) 564. DiNardo, N.J., T. Maeda Wong,, Plummer, E.W.: Phys. Rev. Lett. 65 (1990) 2177. Dudde, R., Bernhoff, H., Reihl, B.: Phys. Rev. B 41 (1990) 12029. Eggelhof, W.F.: CRC Crit. Rev. Solid State Mater. Sci. 16 (1990) 213. Fan, C., Ignatiev, A.: Phys. Rev. B 41 (1990) 3592. Glander, G.S., Webb, M.B.: Surf. Sci. 224 (1989) 60.

Landolt -Bö rnst ein New Series III/42A1

3.2.2 Alkali metals on semiconductors 90Has1 90Has2 90Has3 90Ma 90Mag 90Mön 90Nis 90Rei 90Sou 90Tan 90Wei 91Abu 91Bat 91Bau 91Ete1 91Ete2 91Has1 91Has2 91Has3 91Heb 91Joh 91Kim 91Kub 91Lan 91Lel 91Mag 91Mak 91Mor 91Pri 91Smi 91Sou 91Tik 91Tik2 91Ura 91Whi 91Whi2 91Zha 92Abu

92Bri

213

Hasegawa, Y., Kamiya, I., Hoshizume, T., Sakurai, T., Tochihara, H., Kubota, M., Murata, Y.: Phys. Rev. B 41 (1990) 9688. Hashizume, T., Hasegawa, Y., Kamiya, I., Ide,T., Sumita, I., Hoydo, S., Sakurai, T., Tochihara, H., Kubota, M., Murata, Y.: J. Vac. Sci. Technol. A 8 (1990) 233. Hasegawa, Y., Kamiya, I., Hashizume T., Sakurai, T.: J. Vac. Sci. Technol. A 8 (1990) 238. Ma, Y., Rowe, J.E., Chaban, E.E., Chen, C.T., Headrick, R.L., Meigs, G.M., Modesti, S., Sette, F.: Phys; Rev. Lett. 65 (1990) 2173. Magnusson, K.O., Reihl, B.: Phys. Rev. B 41 (1990) 12071. Mönch, W.: Electronic Structure of Metal-Semiconductor Contacts. Dordrecht: Kluwer, 1990. Nishigaki, S., Matsuda, S., Sasaki, T., Kawanishi, N., Ikeda Y., Takeda, H.: Surf. Sci. 231 (1990) 271. Reihl, B., Magnusson, K.O.: Phys. Rev. B 42 (1990) 11839. Soukiassian, P., Kendelewicz, T., Kim, S.T., Papageorgopoulos, C.A., Kapoor, S., Baros, Y., Hurych, Z.D.: Vacuum 41 (1990) 571. Tanaka, S., Takagi, N., Minami, N., Nishijima, M.: Phys Rev. B 42 (1990) 1868. Wei, C.M, Huang, H., Tong, S.Y., Glander, G.S., Webb, M.B.: Phys. Rev. B 42 (1990) 11284. Abukawa, T., Okane, T., Kono, S.: Surf. Sci. 256 (1991) 370. Barchelor, D.R., King, D.A.: Chem. Phys. Lett. 186 (1991) 19. Bauer, A., Prietsch, M., Molodtsov, S., Laubschat, C., Kaindl, G.: Phys. Rev. B 44 (1991) 4002. Eteläniemi, V., Michel, E.G., Materlik, G.: Surf. Sci. 251/252 (1991) 483. Eteläniemi, V., Michel, E.G., Materlik, G.: Phys. Rev. B 44 (1993) 4036. Hashizume, T., Sumita, I., Murata, Y., Hoydo, S., Sakurai, T.: J. Vac. Sci. Technol. B (1991) 742. Hashizume, T., Motai, K., Hasegawa, Y., Sumita, I., Tanaka, H., Amano, S., Hyodo, S., Sakurai, T.: J. Vac. Sci. Technol. B 9 (1991) 745. Hashizume, T., Hasegawa, Y, Sumita, I., Sakurai, T.: Surf. Sci. 246 (1991) 189. Hebenstreit, J., Heinemann, M., Scheffler, M.: Phys. Rev. Lett. 67 (1991) 1031. Johansson,L.S.O., Reihl, B.: Phys. Rev. Lett. 67 (1991) 2191. Kim, S.T., Soukiassian, P., Barbier, L., Kapoor, S., Hurych. Z.: Phys. Rev. B 44 (1991) 5622. Kubby, J.A., Greene, W.J., Soukiassian, P.: J. Vac. Sci. Technol. B 9 (1991) 739. Lannoo, M., Friedel, F.: Atomic and Electronic Structure of Surfaces. Theoretical Aspects. Berlin: Springer- Verlag, 1991. Lelay, G.: J. Vac. Sci. Technol. B l (1983) 354. Magnusson, K.O., Wiklund, S., Dudde, R., Reihl, B.: Phys. Rev. B 44 (1991) 5657. Makita, T., Kohmoto, S., Ichimiya, A.: Surf. Sci. 242 (1991) 66. Morikawa, Y., Kobayashi, K., Terakura, K., Blügel, S.: Phys. Rev. B 44 (1991) 3459. Prietsch, M., Ludeke, R.: Phys.Rev. Lett. 66 (1991) 2511. Smith, A.J., Graham, W.R., Plummer, E.W.: Surf. Sci. 243 (1991) L37. Soukiassian, P., Kubby, J.A: Springer Series in Surface Science, Vol. 24, The Structure of Surfaces III. Tong, S.Y., Van Hove, M.A, Takayanagi, K., Xie, X.D. (eds.) Tikhov, M., Surnev, L., Kiskinova, M.: Phys. Rev. B 44 (1991) 3222. Tikhov, M., Boishin, G., Surnev, L.: Surf. Sci. 241 (1991) 103. Urano, T., Uchida, Y, Hongo, S., Kanaji, T.: Surf. Sci. 242 (1991) 39. Whitman, L.J., Stroscio, J.A., Dragoset, R.A., Celotta, R.J.: Phys. Rev. Lett. 66 (1991) 1338. Whitman, L.J., Stroscio, J.A., Dragoset, R.A., Celotta, R.J.: Phys, Rev. B 44 (1991) 5951. Zhang, B.L., Chan, C.T., Ho, K.M.: Phys. Rev. B 44 (1991) 8210. Abukawa,T., Kashiwakura, T., Okane, T., Sasaki, Y., Takahashi, T., Enta, Y., Suzuki, S., Kono, S., Sato, S., Kinoshita, T., Kakizati, A., Ishii, T., Park, C.Y., Yu, S.W., Sakamoto, K., Sakamoto, T.: Surf. Sci. 261 (1992) 217. Brillson, L.J.: Surfaces and Interfaces: Atomic-Scale Structure, Band Bending and Band-Offset, in: Handbook on Semiconductors. Moss, T.S. (ed.), Elsevier, 1992.

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214 92Eff 92Flo 92Heb 92Jeo 92Joh 92Kob 92Lag1 92Lag2 92Lan 92Lou 92Ma 92Mic 92Ort 92Rei1 92Rei2 92Rif 92Sas 92Sou1 92Sou2 92Sou3 92Wan 93Bec 93Bro 93Ete 93Eva 93Ged 93Ham 93Has 93Joh 93Les 93Man 93Mic 93Mor 93Pa1 93Pa2 93Plu 93Spi 93Ura 93Wan 93Wei

3.2.2 Alkali metals on semiconductors Effner, U.A., Badt, D., Binder, J., Bertrams, T., Brodde, A., Lunau, Ch., Neddermeyer, H., Hanbücken, M.: Surf. Sci. 27 (1992) 207. Flores, F., Ortega, J.: Europhys. Lett. 17 (1992) 619. Hebenstreit, J., Schefller, M.: Phys. Rev. B 46 (1992) 10134. Jeon, D., Hashizume, T., Sakurai, T., WilIis, R.F.: Phys. Rev. Lett. 69 (1992)14l9. Johansson, L.S.O., Reihl, B.: Appl. Surf. Sci. 56/58 (1992) 486. Kobayashi, K., Morikawa, Y., Terakura, K., Blügel, S.: Phys. Rev. B 45 (1992) 3469. Lagomarsino, S., Scarinci, F., Castrucci, P., Giannini, C., Fontes, E., Patel, J.R.: Phys. Rev. B 46 (1992) 13631. Lagomarsino, S., Scarinci, F, Castrucci, P, Giannini, C., Fontes, E., Patel, J.R.: Appl. Surf. Sci. 56-58 (1992) 402. Landemark, E., Karlsson, C.J., Chao, Y.C., Uhrberg, R.I.G.: Phys. Rev. Lett. 69 (1992) 1588. Louis, E., Flores, F., Guinea, F., Tejedor, C.: J. Phys. C: Solid State Phys. 16 (1983) L39. Ma, Y., Chen, C.T., Meigs, G., Setts, F., Illing, G., Shigakawa, H.: Phys.Rev. B 46 (1992) 5961. Michel, E.G., Pervan, P., Castro, G.R., Miranda, R., Wandelt, K.: Phys. Rev. B 45 (1992) 11811. Ortega, J.E., Miranda, R.: Appl. Surf. Sci. 56/ 58 (1992) 211. Reihl, B., Dudde, R., Johansson, L.S.O., Magnusson, K.O., Sorensen, S.L., Wiklund, S.: Appl. Surf. Sci. 56/58 (1992) 123. Reihl, B., Sorensen, S.L., Dudde, R., Magnusson, K.O.: Phys. Rev. B 46 (1992) 1838. Riffe, D.M., Wertheim, G.K., Rowe, J.E., Citrin P.H.: Phys. Rev. B 45 (1992) 3532. Sasaki, Y., Enta, Y., Suzuki, S., Kono, S.: Surf. Sci. 276 (1992) 205. Soukiassian, P., in: Ordering at Surfaces and Interfaces, Springer Series in Materials Science, Vol 17. Yoshimori, A., Shinjo, T., Watanabe, H. (eds.), Berlin: Springer, 1992. Soukiassian, P., Kubby, J.A, Mangat, P., Hurych, Z., Schirm, K.M.: Phys. Rev. B 46 (1992) 13471. Soukiassian, P., Kim, S.T., Hurych, Z., Kubby, J.A.: Surf. Sci. 56/58 (1992) 394. Wan, K.J., Lin, X.F., Nogami, J.: Phys. Rev. B 46 (1992) 13635. Bechstedt, F., Schemer, M.: Surf. Sci. Rep. 18 (1993) 145. Brodde, A., Bertrams, Th., Neddermeyer, H.: Phys. Rev. B 47 (1993) 4508. Eteläniemi, V, Michel, E.G., Materlik, G.: Phys. Rev. B 48 (1993) 12023. Evans, D.A, Lapeyre, J., Horn, K.: Phys. Rev. B 48 (1993) 1939. Gedik, Z., Ciraci, S., Batra, I.P.: Phys Rev. B 47 (1993) 1639. Hamawi, A., Walldén, L.: Surf. Sci. 285 (1993) 93. Hashizume, T., Katayama, M., Jeon, D.R., Aono, M., Sakurai, T.: Jpn. J. Appl. Phys. 32 (1993) L1263. Johansson, L.S.O., Reihl, B.: Surf. Sci. 287/288 (1993) 524. Lessmann, A.: Ph.D. Thesis. University of Hamburg, 1993. Mangat, P.S., Soukiassian, P., Schirm, K.M., Spiess, L., Tang, S.P., Freeman, A.J., Hurych, Z., Delley, B.: Phys. Rev. B 47 (1993) 16311. Michel, E.G., Eteläniemi, V., Materlik, G.: J. Phys. Condens. Matter 5 (199 3) A85. Morikawa, Y., Kobayashi, K., Terakura, K.: Surf. Sci. 283 (1993) 377. Pankratov, O., Scheffler, M.: Surf. Sci. 287/288 (1993) 584. Pankratov, O., Scheffler, M.: Phys. Rev. Lett. 70 (1993) 351. Plummer, E.W., Dowben, P.A.: Prog. Surf. Sci. 142 (1993) 201. Spies, L., Tang, S.P., Freeman, A.J., Delley, B., Mangat, P.S., Schirm, K.M., Hurych, Z., Soukiassian, P.: Appl. Surf. Sci. 65/66 (1993) 690. Urano, T., Hongo, S., Kanaji, T.: Surf. Sci. 287/288 (1993) 294. Wan, K.J., Lin, X.F., Nogami, J.: Phys. Rev. B 47 (1993) l3700. Weitering, H.H., Chen, J., DiNardo, N.J., Plummer, E.W.: Phys. Rev. B 48 (1993) 8119.

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94Cha 94Com 94Cur 94Far 94Flo 94Flo2 94Fre 94Fuk 94Ham 94Kru 94Man 94Mar 94Mey 94Ohn 94Oku 94Ref 94Sak 94Sch 94Wei 94Won 95Car 95Cas 95Cha 95Erw 95Fou 95Gre 95Gre2 95Jeo 95Ko 95Lin 95Man 95Mey 95Olt 95Pag 96Bet

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Abukawa, T., Kashiwakura, T., Okine, T., Takahashi, H., Suzuki, S., Kono, S., Sato, S., Kinoshita, T., Kakizaki, A., Ishii, T., Parl, C.Y., Kang, K.A., Sakamoto, K., Sakamoto, T.: Surf. Sci. 303 (1994) 146. Chassé, Th., Paggel, J., Neuhold, G., Theis, W., Horn, K.: Surf. Sci. 307/309 (1994) 295. Compañó, R., del Pennino, U., Mariani, C., in: Proceedings of the 4th Intenational Conference on the Formation of Semiconductor Interfaces, 1994, p. 2l1. Curson, N.J., Walton, J.M., Tear, S.P., McCash, E.M., Allison, W.: Surf. Sci. 307/309 (1994) 639. Faraci, G., La Rosa, S., Pennisi, A.R., Margaritondo, G.: Phys. Rev. B 49 (1994) 2943. Flores, F., Miranda, R.: Adv. Mater. 6 (1994) 540. Flores, F., Levy-Yeyati, A., Martin-Rodero, A., Merino, J.: Phys. Low-Dimens. Struct. 1 (1994) 23. Freeman, G.R., March, N.H.: J. Phys. Chem. 98 (1994) 9486. Fukuda, T.: Phys. Rev. B 50 (1994) 1969. Hamawi, A.: Phys. Rev. B 50 (1994) 10910. Krüger, P., Pollman, J.: Appl. Phys. A 59 (1994) 487. Mangat, P.S., Soukiassian, P., Huttel, Y., Hurych, Z.: J. Vac. Sci. Technol. B 12 (1994) 2694. Martin-Gago, J.A., Asensio, M.C., Soria, F., Aebi, P., Fasel, R., Naumovic, D., Osterwalder, J.: Surf. Sci. 307/309 (1994) 995. Meyerheim, H.L., Sawitzki, R.: Surf. Sci. 301 (1994) L203. Ohnishi, H., Yamamoto, Y., Katayama, I., Ohba, Y., Oura, K.: Appl. Surf. Sci. 82/83 (1994) 444. Okuda, T., Shigeoka, H., Daimon, H., Suga, S., Kinoshita, T., Kakizaki, A.: Surf. Sci. 321 (1994) 105. Refolio, M.C., López-Sancho, J.M., López-Sancho, M.P., Rubio, J.: Phys, Rev. B 49 (1994) 2629. Sakamoto, K., Okuda, T., Nishimoto, H., Daimon, H., Suga, S., Kinoshita, T., Kakizaki, A.: Phys. Rev. B 50 (1994) l725. Schirm, K.M., Soukiassian,P., Mangat, P.S., Soonckindt, L.: Phys. Rev. B 49 (1994) 5490. Weitering, H.H., DiNardo, N.J., Pérez-Sandoz, R., Chen, J., Mele, E.J.: Phys. Rev. B 49 (1994) 16837. Wong, G.C.L., Lucas, C.A., Loretto, D., Payne, A.P., Fuoss, P.H.: Phys. Rev. Lett. 73 (1994) 991. Carpinelli, J.M., Weitering, H.H.: Surf. Sci. 331/333 (1995) 1015. Castrucci, P., Lagomarsiono, S., Scarinci, F., Franklin, G.E.: Phys. Rev. B 51 (1995) 5043. Chao,Y.C., Johansson, L.S.O., Karlsson, C.J., Landemark, E., Uhrberg, R.I.G.: Phys. Rev. B 52 (1995) 2579. Erwin, S.C.: Phys. Rev. Lett. 75 (1995) 1973. Foulias, S., Curson, N.J., Cowen, M.C., Allison, W.: Surf. Sci. 331-333 (1995) 522. Grehk, T.M., Göthelid, M., Karlsson, U.O., Johansson, L.S.O., Gray, S.M., Magnusson, K.O.: Phys. Rev. B 52 (1995) 11165. Grehk, T.M., Johansson, L.S.O., Gray, S.M., Johansson, M., Flodström, A.S.: Phys. Rev. B 52 (1995) 16593. Jeong, S., Kang, M.H.: Phys. Rev. 51 (1995) 17635. Ko, Y.J., Chang, K.J., Yi, J.Y.: Phys. Rev. B 51 (1995) 4329. Linsday, R., Dürr, H., Wincott, P.L., Colera, I., Cowie, B.C., Thornton, G.: Phys. Rev B 51 (1995) 11140. Mangat, P.S., Soukiassian, P.: Phys. Rev. B 52 (1995) 12020. Meyerheim, H.L., Sawitzki, R., Moritz, W.: Phys, Rev. B 52 (1995) 16830. Olthoff, S., McKinnon, A.W., Welland, M.E.: Surf. Sci. 326 (1995) 113. Paggel, J.J., Neuhold, G., Haak, H., Horn, K.: Phys. Rev. B 52 (1995) 5813. Betti, M.G., Biagi, R, del Pennino, U., Mariani, C., Pedio, M.: Phys. Rev. B 53 (1996) 13605.

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98Mar 98Mey 98Sar1 98Sar2 98Sar3 98Wat 99Gra

3.2.2 Alkali metals on semiconductors Chao,Y.C., Johansson, L.S.O., Uhrberg, R.I.G.: Phys. Rev. B 54 (1995) 5901. Jeon, D., Hashizume, T., Sakurai, T.: Appl. Surf. Sci. 94-95 (1996) 493. Jeong, S., Kang, M.H.: Phys. Rev. B 54 (1996) 8196. Kim, J.W., Seo, J.M., Kim, S.: Surf. Sci. 351 (1996) L239. Refolio, M.C., López Sancho, J.M., López Sancho, M.P., Rubio, J.: Phys. Rev. B 53 (1996) 4791. Segovia, P., Castro, G.R., Mascaraque, A., Prieto, P., Kim, H.J., Michel, E.G.: Phys. Rev. B 54 (1996) 14277. Weitering, H.H., Shi, X., Erwin, S.C.: Phys. Rev. B 54 (1996) 10585. Chao,Y.C., Johansson, L.S.O., Uhrberg, R.I.G.: Surf. Sci. 372 (1997) 64. Chao,Y.C., Johansson, L.S.O., Uhrberg, R.I.G.: Phys. Rev. B 55 (1997) 7198. Chao,Y.C., Johansson, L.S.O., Uhrberg, R.I.G.: Phys. Rev. B 55 (1997) 7667. Chao,Y.C., Johansson, L.S.O., Uhrberg, R.I.G.: Surf. Sci. 391 (1997) 237. Chao,Y.C., Johansson, L.S.O., Uhrberg, R.I.G.: Phys. Rev. B 55 (1997) 15446. Naydenov, B., Surnev, L.: Surf. Sci. 370 (1997) 155. Nelson, E.J., Kendelewicz, T., Liu, P., Pianetta, P.: Surf. Sci. 380 (1997) 365. Okuda, T., Sakamoto, K., Nishimoto, H., Daimon, H., Suga, S., Kinoshita, T., Kakizaki, A.: Phys. Rev. B 55 (1997) 6762. Segovia, P., Castro, G.R., Mascaraque, A, Michel, E.G.: Surf. Sci. 377-379 (1997) 220. Weitering, H.H., Shi, X., Johnson, P.D., Chen, J., DiNardo, N.J., Kempa, K.: Phys. Rev. Lett. 78 (1997) 1331. Chao,Y.C., Johansson, L.S.O., Uhrberg, R.I.G.: Appl. Surf. Sci. 123/124 (1998) 76. Grupp, C., Taleb-Ibrahimi, A.: Surf. Sci. 408 (1998) 160. Hamamatsu, H., Yeom, H.W:, Yokoyama, T., Kayama, T., Ohta, T.: Phys. Rev. B 57 (1998) 11883. Johansson, L.S.O., Dütemeier, T., Reihl, B.: Phys. Rev. B 58 (1998) 5001. Lee, K.D., Chung, J.: Phys. Rev. B 57 (1998) 2053. Lottermoser, L., Landemark, E., Smilgies, D.M., Nielsen, M., Feidenhans’l, R., Falkenberg, G., Johnson, R.L., Gierer, M., Seitsonen, A.P., Kleine, H., Bludau, H., Over, H., Kim, S.M., Jona, F.: Phys. Rev. Lett. 80 (1998) 3980. Martin-Gago, J.A., Asensio, M.C., Aebi, P., Fasel, R., Naumovic, D., Osterwalder, J., Refolio, M.C., López-Sancho, J.M., Rubio, J.: Phys. Rev. B 57 (1998) 9201. Meyerheim, H.L., Jedrecy, N., Sauvage-Simkin, M., Pinchaux, R.: Phys. Rev. B 58 (1998) 2118. Saranin, A.A., Zotov, A.V., Ryzhkov, S.V., Tsukanov, D.A., Lifshits, V.G., Ryu, J.T., Kubo, O., Tani, H., Harada, T., Katayama, M., Oura, K.: Phys. Rev. B 58 (1998) 7059. Saranin, A.A., Zotov, A.V., Lifshits, V.G., Ryu, J.T., Kubo, O., Tani, H., Harada, T., Katayama, M., Oura, K.: Phys. Rev. B 58 (1998) 3545. Saranin, A.A., Zotov, A.V., Ryzhkov, S.V., Tsukanov, D.A., Lifshits, V.G., Ryu, J.T., Kubo, O., Tani, H., Harada, T., Katayama, M., Oura, K.: Phys. Rev. B 58 (1998) 4972. Watanabe, A., Naitoh, M.,, Nishigaki, S.: Jpn. J. Appl. Phys. 37 (1998) 3778. Gravila, P., Meier, P.F.: Phys. Rev.B 59 (1999) 2449.

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3.3 Adsorption of metals 3.3.1 Metals on metals H. BRUNE

3.3.1.1 Introduction The following chapter gives an account of our present experimental knowledge on the adsorption of metal atoms onto solid metal surfaces of well defined structure and chemical composition. Only chemically clean and structurally well prepared low index surfaces of single crystals are considered. The samples are composed of a single element. The metallic adsorbates are condensed on the surface from the vapor phase in an ultra-high vacuum (UHV) chamber. As adsorbate materials we consider all metallic elements, apart from alkalies, to which Chap. 3.2. of the present volume is devoted. According to the definition of adsorbed layers on surfaces given in Chap. 1 of the present volume we concentrate on coverages of up to one atomic monolayer (ML, 1 ML is defined as one adsorbate atom per substrate unit cell). We refer to the literature for issues related to multilayer thin film growth and epitaxy [82Bau, 82Voo, 84Bau1, 98Pim, 98Zha], to theory and experiment of island nucleation [73Ven, 84Ven, 87Ven, 98Bru, 00Pol], to thermodynamic growth modes [84Bau1], or to strain relief [97Kin] in thin films comprising more than one atomic layer in thickness. In the spirit of the Landolt-Börnstein Series the data compilation focuses on experimental results. In the case of adsorption energies of monomers on atomic terraces we list theoretical results derived from ab-initio calculations for comparison. Adsorption of metals from the vapor onto solid metal surfaces starts with the condensation of metal atoms at their impact site, which is in most cases located on an atomically flat substrate terrace. The energy Eads, by which they are adsorbed, is topic of Sect. 2. It has long been a debate whether the substantial energy associated with the adsorption of metal adatoms is transferred instantaneously to excitations of the substrate lattice, or whether the adsorbate performs transient, non-thermal, motion over a few lattice sites until its adsorption energy is dissipated. Sect. 3 is devoted to this so-called transient (non-thermal) motion. The atoms, once in thermal equilibrium with the substrate, perform thermally activated two-dimensional (2D) diffusion, an issue treated in Chap. 3.11 of the present volume. As the coverage is increased beyond a few isolated monomers, the adsorption energy can become coverage- and site- dependent due to adsorbate-adsorbate interactions, which might span over several lattice sites. These lateral interactions are treated in Sect. 4. The last Sect. 5 treats the structure of submonolayer islands and of the first monolayer. Depending on the lattice mismatch and on the stiffness of lateral adsorbate interactions as compared to the corrugation of substrate adsorption potential, the adsorbed layer is either pseudomorphic with the substrate or it forms weakly incommensurate phases. For many combinations of metallic elements there is a tendency towards an exchange of adsorbate atoms with substrate atoms. This site exchange can be followed by the dissolution of the adsorbate into the bulk or by the formation of surface alloys where mixing is confined to the first atomic layer. The tables of Sec. 5 include information on the exchange of single atoms and on whether adlayer, surface alloys, or bulk alloys are formed. Acronyms of experimental methods ABS ADE ADT AES AFM ARXPS FD

Atomic Beam Scattering Adsorption Desorption Equilibrium Adsorption Desorption Transient Auger Electron Spectroscopy Atomic Force Microscopy Angle Resolved X-ray Photoelectron Spectroscopy Field Desorption cont.

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[Ref. p. 251

Field Emission Microscopy High Energy Ion Scattering Low Energy Ion Scattering Field Ion Microscopy Low Energy Electron Diffraction Medium Energy Ion Scattering Perturbed γ-γ Angular Correlation Photoelectron Diffraction Imaging Primary-Beam Diffraction Modulated Electron Emission Reflection High Energy Electron Diffraction Rutherford Backscattering Spectroscopy Surface Extended X-ray Absorption Fine Structure Spot Profile Analysis Low Energy Electron Diffraction Scanning tunneling microscopy Surface X-ray Diffraction Thermal Desorption Spectroscopy Ultra High Vacuum Scanning Electron Microscope X-ray Photoelectron Diffraction

Acronyms of computational techniques DFT FP-LMTO GGA LAPW LDA

Density functional theory Full-potential linear-muffin-tin-orbital Generalized gradient approximation (for the exchange correlation) Linearized augmented plane wave (method) Local density approximation (for the exchange correlation)

3.3.1.2 Adsorption energies Condensation of metal atoms on metal surfaces is fairly simple, as compared to the chemisorption of molecules. There is no activation energy for adsorption (Eac = 0 in Eq. (23) of Chap. 1) and consequently the metal atoms approaching the surface are directly attracted towards the adsorption minimum. The adsorption energy Eads is of the order of a few eV/atom and thus bonding is substantial. The absence of Eac and the high value of Eads lead to temperature independent sticking coefficients of s = 1 up to elevated substrate temperatures. The adsorbates considered here range from the electropositive alkaline earth and rare earth atoms (Groups IIA and IIIA, for the Group IA alkali metals see Chap. 3.2) via the Group 1B atoms to d-electron metals. The electropositive adsorbates behave very much as alkali metals, although less pronounced. They form large dipole moments, detected by work function measurements, and generally have repulsive lateral interactions. For the earth alkalines, there is generally a sharp drop in the adsorption energy upon monolayer completion reflecting itself also in a drop of s. For the rare earths, whether there is a change in s upon monolayer completion depends on the elements; on W(110), e.g., Eu shows a change whereas Gd and Tb do not [86Kol2]. For the d-electron metals, the adsorbate-substrate bond is of the same covalent nature as the adsorbate-adsorbate bond. Nevertheless, there is a layer dependence of Eads for heteroepitaxial metal systems, but the variations are small compared to the absolute values of Eads and thus the sticking coefficient stays s = 1 independent of coverage for a large temperature regime. On the atomic terraces there are competing adsorption sites. Single metal atoms favorize highly coordinated hollow or sometimes even substitutional sites. For illustration of the adsorption sites and surface symmetry we show the low index surfaces of bcc, fcc and hcp crystals in Fig. 1. Only few experimental techniques directly reveal the adsorption site of single adatoms. Field ion microscopy (FIM) can accomplish this for refractory metals, which are strongly enough bound to sustain the imaging field. Scanning tunneling microscopy (STM) has also been used to reveal adsorption sites, which is not trivial as it requires simultaneous imaging of adsorbate and substrate with atomic resolution. Table 1 gives an Lando lt -Börnst ein New Ser ies III/42A1

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overview of the metal adsorption sites that have been unambiguously determined in the zero-coverage limit. Note that for Ir/Ir(111) single atoms adsorb preferably on stacking fault positions; the bulk stacking sequence is energetically favorable only for islands comprising a few atoms. This is not the case, however, for Pt/Pt(111) where fcc-sites are favored right away from the zero coverage limit on. Note also that STM-inferred adsorption sites are often substitutional sites or adatoms in troughs. For isolated adatoms on flat surfaces it is often impossible to unambiguously localize the adsorbate with respect to the substrate lattice. (One exception is the chemisorption system C/Al(111), where the hcp-site could be determined by simultaneous imaging of the two uppermost atomic substrate layers and of the adsorbate [90Bru]). Due to adsorbate-adsorbate interactions, Eads also depends on coverage. Ideally one would like to measure Eads starting from isolated adatoms on terrace sites and then follow the evolution of Eads all the way up to the completion of a monolayer. There are only a few ways to experimentally accomplish this. The first and most important is via the kinetics of desorption, which is only possible for systems with reversible adsorption, i.e., for heteroepitaxial systems that don’t form bulk alloys (for binary phase diagrams see [58Han], for the tendency toward exchange and the stability of surface alloys see [97Chr]). In thermal desorption spectroscopy (TDS), or temperature programmed desorption (TPD), the surface is heated at a constant rate (dT/dt) while monitoring the partial pressure of the desorbing species with a mass spectrometer. The heating rate is kept low as compared to the pumping rate. The desorption rate rdes is given by the following expression rdes = dn/dt = -nxνdes(θ)exp(-Edes(θ)/kT) ,

(1)

where Edes is the activation energy for desorption and νdes its attempt frequency. The desorption order is x ∈ {0;1} for metals/metals, n is the density of adatoms, n = θsn0, with n0 being the density of substrate atoms per m2, θs is the adsorbate coverage expressed with respect to n0. The absolute desorption rate in Eq. (1) is obtained by multiplying rdes with the sample area seen by the mass spectrometer. Quantitative determination of νdes(θ) and Edes(θ) requires analysis of desorption curves for a series of different initial coverages using Eq. (1). This is the "model-independent" or "complete" analysis of TDS data [74Bau, 75Bau, 75Fal, 75Kin, 84Hab]. Frequently the following more simple procedure has been applied to derive an estimate for Edes. For submonolayer to monolayer coverages rdes has a single desorption peak located at T*. For first order desorption, which is often the case for metal/metal systems, and under the simplifying assumption that νdes and Edes are coverage independent, one can derive Edes directly from T* [62Red] via Edes /kT* = ln(νdes T*/( dT/dt)) - 3.64 .

(2)

However, this is a potentially dangerous way to proceed since νdes contains an entropy term exp(¨S/k) which can change νdes by several orders of magnitude as a function of θ. Also Edes depends on coverage, due to adsorbate-adsorbate interactions. Thus the Edes values solely derived from Eq. (2) are subject to systematic errors, such that, not only absolute values of Edes, but also coverage trends may come out wrong [87Nie]. Despite these systematic problems the simplicity of the linear relationship Eq. (2) found by Redhead, coupled with assumed constant prefactor, forms the basis of many analyses of TDS spectra. At constant coverage, the activation energy for desorption derived from TDS equals the isosteric heat of adsorption since Eac = 0 for the metal/metal systems considered here. Under adiabatic conditions the isosteric heat equals the calorimetirc heat of adsorption, which we label adsorption energy Eads. For systems showing no intermixing at all, desorption takes either place from a 2D gas of atoms on the atomic terraces, and/or from the phase boundary between this 2D-gas and 2D condensed islands. Accordingly, the TDS derived values correspond to the binding energy at ideal terrace sites and/or at step sites of adsorbate islands. In quantitative TDS experiments desorption from both phases can be discerned such that binding energies for both adsorption sites can be derived separately. This is illustrated in Fig. 2 for Ni desorption from a W(110) surface. There are two branches for θ < 0.4 ML, the upper is caused by desorption from the 2D-gas–2D-solid interface and thus from steps; the lower is due to desorption from the 2D-gas and thus to terrace sites. The strong coverage dependence of νdes and Edes is evident. The Lando lt -Bö rnst ein New Ser ies III/42A1

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[Ref. p. 251

increase of Edes with increasing θ is a signature for attractive adsorbate-adsorbate interactions (see Sect. 3). The data points are sufficiently numerous that a sensible extrapolation to the coverage limit of single adsorbed adatoms is possible. Table 2 lists the adsorption energies inferred from TDS. In favorable cases where a complete analysis has been performed and thus Edes (θ) data are available, we list the zero coverage extrapolation, a value at the intermediate coverages where Edes is often constant, and the value at saturation coverage. In the cases where only T* values have been reported we list these values and the reader may derive estimates for Edes from Eq. (2) using the experimental heating rate and assuming νdes = 1013 Hz. For some systems, even though they do not form bulk alloys, annealing can lead to the formation of so-called surface alloys, where mixing is confined to the first atomic layer [97Bes]. An example is Ag/Pt(111), where annealing to T = 600 K leads to the formation of a real mixture where small Ag clusters are dissolved in the first Pt layer (θ < 0.5 ML) and vice versa (0.5 ML < θ < 1 ML) [93Röd]. The Eads values derived for this system consequently do not correspond to the adsorption energy of Ag atoms on ideal Pt(111) terraces but presumably to desorption from a Ag-Pt adatom gas or from substitutional sites. A test of absolute binding energies derived from TDS can be accomplished for homoepitaxial systems or for multilayers by comparing the TDS-result to the tabulated vaporization energy of the corresponding element [78Wea]. It is illustrative to compare the cohesive energy Ecoh with the vaporization energy. If both energies are equal, atoms evaporate directly from kinks since Ecoh is the binding energy at kink sites. If there is a difference between both energies, desorption takes place from terrace sites and the energy difference can be associated with the adatom formation energy, i.e., the binding energy difference between terrace and step sites. For Cu we find Ecoh – Evap = 3.49 eV - 3.16 eV = 0.33 eV, and for Al Ecoh – Evap = 0.34 eV , which compares well with computed values [96Stu]. For heteroepitaxial systems forming bulk alloys, the TDS method to derive Eads is not applicable and the only experimental technique allowing to deduce adsorption energies is calorimetry. This technique has recently been established for single crystal surfaces [91Bor, 98Bro] and gives access to adsorption energies on terrace sites. Generally, these values are integrated over a coverage interval, however, when the calorimeter is operated with sufficient time resolution in combination with a pulsed doser, Eads can also be measured as a function of coverage [97Yeo2]. However, measuring the temperature increase of a bulk single crystal sample upon adsorption of a few percent of a monolayer of atoms from the vapor is experimentally challenging. Therefore this technique has so far only been applied to measure heats of adsorption for chemisorbed molecules as CO, O2 and hydrocarbons on a few selected metal surfaces [93Stu, 96Stu, 96Yeo, 97Yeo1, 97Yeo2], but not yet to measure metal/metal adsorption energies. Other sources of adsorption energies are the TDS-related adsorption desorption transient (ADT) and adsorption desorption equilibrium (ADE) measurements which can equally be extrapolated to the zerocoverage limit [84Bau1]. Differences in TDS- and ADE-derived Eads(0 ML)-values are often ascribed to a barrier to desorption which manifests itself only in the inherently non-equilibrium thermal desorption process, but not in the quasi-equilibrium isosteric heat of adsorption obtained from ADE [92Kim]. Finally, there is the field desorption (FD) technique where the required field for adatom desorption in a FIM is used to draw conclusions on Eads. Absolute values are certainly due to systematic errors since the field considerably perturbs the electronic binding configuration. However, the variation of Eads between different elements is believed to be correct. We show results for the 5d-elements on various low index surfaces of W in Fig. 3. It is seen that the Re-atom having the half-filled 5d shell has the highest binding energy, whereas Eads decreases in a symmetric fashion for elements having fewer or more 5d electrons than Re, respectively. For lanthanides there is a similar behavior, Gd, with its half-filled 4f shell, is most strongly bound.

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For comparison with experiment we list in Table 3 binding energies for terrace sites computed with abinitio methods. This Table shows that the LDA approximation for the exchange correlation functional leads to strong overbinding. This is reflected in the much too large Ecoh-value calculated for Al. An approximation for the exchange correlation doing well for high coordinated slab atoms can do less well for low coordinated atoms, and thus for the isolated atom, taken as reference when calculating total binding energies. The overbinding of LDA is mostly due to difficulties in describing the isolated atom, whereas it is believed to be more accurate for the energetics of the slab. The absolute values of Eads derived from GGA are expected to do better but yet they have to be taken with care. Despite these difficulties, relative energy values derived from density functional theory can be quite precise, as seen by the accord between measured and computed diffusion barriers recently obtained even for close-packed metal surfaces (see the example of Pt/Pt(111) with Em,exp = 0.260±0.003 eV [98Kyu] compared with Em,th = 0.29 eV [99Fei]; in this example also the preference of fcc-sites for single atoms comes out right from theory, compare Tables 1 and 3). Therefore, at the present state, the computed Eads values can predict the favored adsorption site, and by this discriminate exchange from surface adsorption (see case of Co/Cu(100) in Table 3 for which an exchange at low θ and an ad-layer at 1 ML has been calculated in agreement with experiment), but not yet its absolute binding energy. Absolute Eads values for terrace sites can be derived for homoepitaxial systems from theoretical adatom formation energies and the experimental cohesion energy. Table 1. Experimentally determined adsorption sites for single adatoms (ordered after substrate according to periodic table) System Au/Ni(110) Au/Ni(111) Pd/Cu(100) Ag/Cu(100) Ag/Cu(111) Pb/Cu(111) W/W(111) Pd/W(110) W/W(110) Re/W(110) Ir/Ir(111) Re/Ir(111)

Site(s)

st

Method

Ref.

Au in 1 layer Ni rows STM [93Nie] Au in phase with Ni lattice STM [96Hol] Pd in phase with Cu lattice STM [96Mur] Ag in phase with Cu lattice STM [96Spr] Ag in phase with Cu lattice STM [97Bes] Pb in phase with Cu lattice STM [94Nag] 94% on L-site, 6% on F FIM [74Gra] 3-fold coord. surface site S FIM [84Bau1] 3-fold coord. surface site S FIM [84Bau1] 2-fold coord. bulk lattice site L FIM [80Fin] FIM [89Wan] hcp favored by 16 meV a) atoms move from 50% hcp 50% fcc FIM [91Wan] to hcp by ann. to 200 K W/Ir(111) hcp atoms move from 50% hcp 50% fcc FIM [91Wan] to hcp by ann. to 200 K Pd/Ir(111) fcc atoms move from 50% hcp 50% fcc FIM [91Wan] to fcc by ann. to 60 K Co/Pt(111) substitution Co in phase with Pt lattice STM [99Lun1, 99Lun1] FIM [96Göl] Pt/Pt(111) fcc/hcp fcc favored by > 60 meV b) Pt/Pt(110)-(1x2) 4-fold hollow in missing row trough STM [97Lin] Pt/Au(111) substitution Pt in phase with Au lattice STM [99Ped] Au/Au(110)-(1x2) 4-fold hollow in missing row trough STM [97Gün] a ) The two non-equivalent sites are discerned by the non-uniform intensity distribution in the FIM images leading to triangles which apex point along, [ 211 ] or in the opposite direction, for hcp- and fcc-sites, respectively. The preference for hcp-sites is unique to single atoms and small clusters; with increasing cluster size the Ir atoms occupy fcc-sites. b ) At 20 K deposition temperature both sites are occupied at random, upon annealing to 45 K hcp atoms diffuse onto fcc-sites. The binding energy difference is inferred from the threshold T for diffusion out of the two sites, and from the known migration barrier between fcc-sites.

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substitution substitution substitution substitution substitution substitution L/F S S L hcp/fcc hcp

Observations

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[Ref. p. 251

Table 2. Adsorption energies (order according to periodic system, first priority substrate, second priority adsorbate). System

Eads [eV]

Hg/Fe(100) Hg/Ni(100) Ag/Ni(111)

1.13±0.12 1.19±0.15 3.03±0.06 f) 3.24±0.08 f) 1.14 0.83 2.07 0.70±0.03 0.74±0.03 0.5

Hg/Ni(111)

Hg/Cu(100)

Ni/Mo(110) Cu/Mo(110) Mg/Ru(0001) Ni/Ru(0001) Cu/Ru(0001) Zn/Ru(0001) Pd/Ru(0001) Ag/Ru(0001)

Au/Ru(0001)

Au/Ru(1000) Cu/Rh(100) Hg/Ag(100) Ni/Ta(110) Cu/Ta(110) Pd/Ta(110) Fe/W(100) Ni/W(100) Hg/W(100) Fe/W(110)

Ni/W(110)

Cu/W(110)

2.4 d) 2.0 d)

1.6 2.49±0.05 e) 3.00±0.05 f) 2.3 - 2.7 d) 4.04±0.05 e) 3.58±0.07 e) 3.42±0.10 4.15±0.10 3.63±0.10 3.6 e) 3.9 e)

T* [K]

1 2

1435 1222 883/787 830/730 1350 1210 620-720 1410-1440

p(2x2) (¥[¥ 5 0, c(4x4) 0, c(2x2) 0, c(2x2) 1 1 0.2 1.0 1 1 0.1-1.3 0-1 0.0

Method

Ref.

ADE ADE ADE

[81Jon] [87Jon] [00Mró]

ADE

[90Sin1, 90Sin2]

TDS h)

[92Kim]

ABS TDS TDS TDS

[90Dow] [90He, 90Tik] [90He] [99Hua]

TDS TDS TDS c) TDS TDS a) b)

[88Ber2] [85Yat] [92Rod2] [92Cam] [87Nie] [93Rod]

900-1050

1262 0.63±0.03 0.25±0.03 1520 1258 1540 1480 1480 2.15±0.12 1.91±0.14 4.1 g) 3.6±0.1 e) 4.1±0.1 f) 4.24±0.06 c) 4.35 e) 5.00 f) 4.93 e) 3.20 e) 3.85 e) 3.85 f)

θ [ML]

0-1 0.0 0.5 0.1 0.6 1.0 0.1 0.2-0.5 1 0 1 1 1 1 0-1 0-1

0-1 0-0.8 1475

1 0 0.2-0.4 0.6 0 0.2-0.4 0.2-0.8

TDS TDS a) b)

[87Nie] [86Har]

TDS a) TDS a)

[95Pou]

TDS TDS g)

[91Jia] [92Kim]

TDS TDS TDS TDS TDS ADE TDS TDS b) TDS a)

[97Cha] [97Cha] [90Koe] [90Ber] [87Ber] [78Jon, 79Jon]

TDS TDS a)

[87Ber] [86Kol2]

TDS a)

[86Kol2]

[90Ber] [00Kol]

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3.3.1 Metals on metals Eads [eV]

Pd/W(110)

Ag/W(110)

Au/W(110)

Eu/W(110) Gd/W(110) Tb/W(110) Co/W(111) Ni/W(111) Ag/W(111) Cu/Re(0001)

Pd/Re(0001) Ag/Re(0001)

223

T* [K]

θ [ML]

Method

Ref.

1510

1 0 0.6 0 0.8 1.0-5.0 0.0 0.2-0.8 0.15-0.9 0.0 0.2 0.8 0.08-0.26 0.42

TDS TDS a)

[88Ber1] [80Sch]

TDS a)

[86Kol2]

TDS a)

[86Kol2]

TDS a)

[86Kol1]

TDS a) TDS TDS TDS TDS TDS a)

[86Kol1] [86Kol1] [95Gua] [95Gua] [95Gua] [99Wag]

TDS TDS TDS a)

[92Rod1] [92Cam] [98Sch]

3.6±0.1 e) 4.1±0.1 e) 4.1±0.1 f) 4.4±0.1 f) 4.2±0.1 2.80 e) 3.35-3.55 e) 3.55 f) 3.30 e) 3.67 e) 4.10 e) 2.85 e) 2.10 e) 2.85 e) 1480-1440 1480 1180 2.1 f) 3.4 1180 1350-1500

0.2-1.0 <1 <1 0.1 0.8 1 0-1 0.01 0.10 0.50 0.75 1

2.57±0.08 f) 2.76±0.04 f) 3.01±0.04 f) 3.11±0.08 f) Ag/Pt(111) 2.88 1080 TDS c) [82Dav] 2.63 970 [82Dav] [86Har] 3.2 0.0 TDS a) 4.1 0.6 3.6 1.0 a ) Complete analysis of a family of desorption curves with different initial coverages. b ) Excellent agreement is obtained for Edes from multilayers with bulk heat of vaporization. c ) ln rdes vs. 1/T analyzed, but no θ-dependence given. d ) Application of Eq. (2) and assumption of νdes = 1x1013 Hz. e ) Desorption from 2D gas, Eads on terrace site. f ) Desorption from interface between 2D gas – 2D solid adsorbate islands, Eads on step. g ) Arrhenius behavior of leading edge was analyzed. h ) Simulation of TDS-spectra with exponential mobility model; this analysis also yields lateral interaction energies, e.g., for Hg/Cu(100)-c(4x4) Edes = E0 - θ W with W = -0.13 eV, indicative of attractive lateral interactions. The results on E0 and W are strongly model dependent; this is not included in the error bars above.

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[Ref. p. 251

Table 3. Computed adsorption energies at terrace sites System

Eads [eV]

Site(s)

Ecoh/Ecoh-th.

Al/Al(111)

3.06/3.10 x/x+0.04 3.75 3.03 2.93 3.89 2.82/2.84 2.99 2.94 3.33/3.31 3.71

fcc/hcp fcc/hcp 4-fold 4-fold 4-fold 4-fold fcc/hcp 4-fold fcc fcc/hcp 4-fold fcc 4-fold/exch 4-fold fcc/hcp 4-fold fcc/hcp

3.39/4.15

Al/Al(100)

Al/Al(110) Ag/Ag(111) Ag/Ag(100) Ag/Pt(111) Au/Au(111) Au/Au(100) Cu/Cu(111) Co/Cu(100) 3.92/5.25 5.38 Ir/Ir(111) 7.68/7.69 Ir/Ir(100) 9.07 Pt/Pt(111) 5.35/5.17

θ [ML]

<1/9 1/56 3.39/4.15 1/16 ? ? 3.39/4.15 1/12 1.0 1.0 ? 1.0 1.0 hcp unstable 1/48 0.1 1.0 1.0 1.0 ?

Slab xxyxz

Method

Ref.

>3x>3x5 d) 14x4x6 e) 4x4x7 d) ?x?x2 b) ?x?x5 3x4x8 d) 1x1x5 a) 1x1x5 a) ?x?x7 d) 1x1x5 a) 1x1x5 a) 12x4x4 d) 2x2x7 d) 2x2x7 d) 1x1x5 a) 1x1x5 a) ?x?x7 d)

DFT-LDA DFT-LDA DFT-LDA LDA LDA DFT-LDA FP-LMTO FP-LMTO FP-LDF FP-LMTO FP-LMTO DFT-GGA LDA-LAPW LDA-LAPW FP-LMTO FP-LMTO FP-LDF

[96Stu] [00Bog] [96Stu] [87Fei] [90Fei] [96Stu] [95Boi] [95Boi] [94Fei1] [95Bo] [95Boi] [00Bog] [00Pen] [00Pen] [95Boi] [95Boi] [94Fei1]

a

) rigid slab ) 1. layer relaxed in z-direction but kept rigid in xy-plane c ) 1. layer relaxed d ) 1. and 2. layer relaxed e ) first 4 layers relaxed b

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3.3.1.3 Atom condensation 3.3.1.3.1 Transient mobility A metal atom approaching a metal surface from the vapor is captured by the strongly attractive adsorption potential and attracted by it towards the surface where it gains a binding energy of several electron volts. Transient mobility is the motion of atoms in the course of the dissipation of this adsorption energy. The possibility of this non-thermal motion between the initial site of impact and the final adsorption site has frequently been addressed in the literature since the early days of FIM investigations of atomic diffusion [64Ehr, 65Gur, 65You]. This interest was perhaps motivated by intuition suggesting that the several electron volts gained upon adsorbing a metal atom onto a metal surface would lead to a few jumps onto adsorption sites adjacent to the impact point before eventually reaching thermal equilibrium with the substrate. However, up to now, direct experimental proofs for transient mobility do not exist for metal on metal condensation. In contrast, every atomic scale experiment performed to study transient motion in detail, clearly excluded this type of motion for metals/metals (for FIM results see the overview given in ref. [91Wan]). In the following we discuss quantitative experiments addressing the issue of transient mobility. Experimental methods permitting direct conclusions on transient motion examine the outcome of adatom deposition onto a substrate held at very low temperatures where thermally activated diffusion is frozen. This leads to statistical growth if transient mobility is absent, if present deviations from statistical growth are detected. In FIM experiments the population of adsorption sites is examined, whereas in STM experiments one measures the mean island size or island distance- and size-distributions. FIM experiments can distinguish fcc- from hcp-adsorption on trigonal lattices [89Wan]. For Pt/Pt(111) these experiments revealed that the fcc-site is energetically favored by at least 60 meV (see Table 1) and therefore annealing of monomers to 45 K, promoting their thermal motion, leads to exclusive adsorption onto these sites. However, deposition at 20 K, where thermal motion is frozen for this system, led to adatoms randomly distributed over hcp- and fcc-sites [96Göl]. This implies that transient motion is absent since any mobility would lead to a preferred population of the energetically more favorable fcc-sites. Analog observations have been made for Ir, Re, W, and Pd on the Ir(111) surface (see Table 4). STM measurements allow the determination of the mean island size <s> as quotient of coverage and island density. Both numbers are known with sufficient absolute precision that conclusions on transient mobility can be reached. This is achieved by experimentally following the mean island size with increasing coverage and making comparison to numbers expected from models for presence and absence of transient motion. The mean island size expected for the various lattice types for pure statistical growth can be estimated from percolation theory [76Syk1, 76Syk2]. However, this estimate yields slightly too small values since percolation theory neglects deposition onto filled sites. In kinetic Monte-Carlo (KMC) simulations one can account for impingement onto, and subsequent downward diffusion from islands or monomers. Such simulations yield <s> = 1.48 atoms (percolation theory yields 1.35 atoms) without, and <s> = 2.3 atoms with transient motion over one lattice site (values for θ = 0.1 ML on a triangular lattice). This difference is clearly outside the experimental error bars and allowed to rule out transient mobility for Ag/Pt(111). Similar conclusions have been drawn from STM images acquired for Ni and Au/Au(110), where transient motion was excluded based on the chain lengths and distances, and on island size distributions, respectively (see Table 4). There are diffraction experiments, from which transient motion has indirectly been concluded. RHEED-oscillations suggested 2D growth of Fe and Cu/Ag(100) at 77 K [89Ege] and diffraction rings implied a mean-island spacing of 10 atoms for Fe, Co, and Cu/Cu(100) at 80 K [93Nyb]. These observations are suggestive of transient motion. Nevertheless, explanations of these observations without the assumption of transient motion have been put forward in the literature [90Eva, 91San] such that these diffraction results cannot be considered as hard evidence for transient motion. From the compilation of data in Table 4 we can conclude that transient mobility is precluded for all systems where direct atomic-scale observations exist. These results therefore suggest that generally for metals/metals the adsorption energy is instantaneously transferred to the substrate lattice and adatoms thermalize and condense at their impact point. The only exception is a "knock-on" exchange process Lando lt -Bö rnst ein New Ser ies III/42A1

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[Ref. p. 251

observed for Ir/Rh(100) by means of FIM [96Kel]. For this system thermally activated exchange sets in at 330 K, however, a fraction of 10% of the Ir atoms already exchanged with Rh at low T. This can be interpreted as transient vertical motion, i.e., part of Eads is used to overcome the energy barrier of exchange. Table 4: Experimental investigations of transient motion (TM, order periodic table for substrate) System

TM

T [K]

Observations

Method

Fe/Cu(100) Co/Cu(100) Cu/Cu(100) Ir/Rh(100)

+ a)

80

diffraction rings

SPA-LEED [93Nyb]

+

77

FIM

[96Kel]

+ a)

77

10% ballistic exchange at 77 K, thermal exchange starts at 330 K RHEED-oscillations -> 2D-growth

RHEED

[89Ege]

15

Pd/Ir(111) W/Ir(111) Re/Ir(111) Ir/Ir(111) Ag/Pt(111) Pt/Pt(111)

-

FIM FIM FIM FIM FIM FIM STM FIM

[80Fla] [86Fin] [91Wan] [91Wan] [91Wan] [91Wan] [99Bru] [96Göl]

Ni/Au(110) Au/Au(110)

-

random distr. over different biding sites condensation on-top of 3-atom-sized W(111) plane random distr. over fcc and hcp sites random distr. over fcc and hcp sites random distr. over fcc and hcp sites random distr. over fcc and hcp sites <s> shows absence of trans. mob. atoms randomly on hcp and fcc atoms solely on fcc-sites Ni chain length and spacing comparing experimental and theoretical cluster size distributions

STM STM

[97Hit] [97Gün]

Fe/Ag(100) Cu/Ag(100) W/W(111)

a

20 20 20 20 35 20 45 130 125

Ref.

) Only indirect evidence for transient motion.

3.3.1.3.2. Neighbor-driven mobility Apart from the absence of transient motion upon adatom condensation inmidst of flat terraces there are several observations suggesting that the motion of an adatom towards a neighboring adatom or island is different from thermal diffusion of isolated adatoms. Wang and Ehrlich reported denuded zones, bare of Ir atoms, around Ir islands on Ir(111) [93Wan1, 93Wan2]. In the first set of FIM experiments Ir was condensed at 20 K and subsequently its diffusion was followed after annealing periods at 105 K. Far away from the island, atomic jumps could be traced in the usual way, but as soon as the lateral distance to the island was less than 3 nearest neighbor spacings, the atom was "funneled" in a single diffusion event toward the island [93Wan1]. In a second experiment the same group observed the denuded zones to be somewhat narrower (2 atoms wide) but to persist down to 20 K, i.e., in the absence of regular terrace diffusion [93Wan2]. These observations were interpreted as follows. Adatoms are stronger bound to ascending steps than they are at terraces. Therefore, the adatom adsorption potential becomes pulled down upon approaching a step, which also reduces its corrugation for motion toward the step, see Fig. 4. While the first set of data can be reconciled by a 10% reduction of the diffusion barrier, the depletion at 20 K would imply a reduction of that barrier by a factor of five! This reduction seems large for attachment to a one-fold coordinated step site. At this site the binding energy is increased by approximately the dimer bond energy of 0.32 eV [90Wan]. This can reduce the terrace diffusion barrier of 0.27 eV [89Wan] over a distance of two sites down to 0.19 eV (Fig. 4a), which is not sufficient to explain the observed motion at 20 K. Two- or three-fold lateral coordination to step atoms increases the binding energy approximately by 0.61 and 0.76 eV, respectively [89Wan]. This pulls the diffusion barrier down over two lattice sites to 0.12 eV (Fig. 4b) and 0.08 eV, respectively. However,

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227

even the lower value of 0.08 eV is still too large to explain thermal motion at 20 K. One can only rationalize the FIM observations of denuded zones by thermal motion toward islands if strongly reduced barriers for the last few nearest neighbor distances are assumed. These barriers can not be attained by interactions pulling the potential linearly down over two sites. An alternative interpretation of the experiments would be transient motion toward the step over the strongly smoothened potential energy surface close to a step for the direction of step approach. A growth experiment at variable low temperature, as the one carried out by STM for Ag/Pt(111) (Table 4), would reveal whether and how much the barrier is reduced for motion toward monomers being one nearest neighbor distance apart. For Pd/Ag(100) He scattering experiments indicated such a "neighbor driven mobility" over one lattice site to be activated down to substrate temperatures of half the onset temperature of ordinary terrace diffusion [94Van]. This implies a reduction of the activation barrier for the attachment to an other monomer by a factor of two, as compared to the diffusion barrier for isolated adatoms. The experiments of neighbor and island driven mobility could alternatively be explained as attraction of the atoms to neighbors or island step bottoms during their trajectory of arrival from the gas phase. However, such direct attraction to higher coordinated sites can be excluded for other systems as Ag/Pt(111), Au and Ni/Au(110) (see Table 4), which makes the explanation also unlikely for Ir/Ir(111) and Pd/Ag(100). Therefore, the currently most consistent interpretation is that metal atoms strike the surface perpendicularly, condense at the point of impact and then thermally diffuse. The adatom adsorption potential adjacent to other adatoms or islands is significantly smoothened for direction of approach by the increase in binding energy at laterally higher coordinated sites. Whether this smoothening suffices to explain the experimental observations of denuded zones and neighbor driven mobility reported so far remains to be clarified. Transient motion can alternatively be considered for the last one or two jumps toward monomers or islands over the smoothened potential energy surface, however, we emphasize that transient mobility is clearly excluded for individual monomers on the corrugated adsorption potential inmidst atomic terraces. The present discussion shows that while condensation of metal atoms onto terraces is well understood, their condensation close to steps and other adatoms is not yet as clear, encouraging further experimental and theoretical investigation. The details of the adsorption energy landscape in the vicinity of an island or a neighboring atom are expected to become relevant in the understanding of thin film and nanostructure growth.

3.3.1.4 Adsorbate – adsorbate interactions The lateral interactions between adsorbed species play an important role in the formation of overlayers. They enter in their nucleation and growth as well as in the thermodynamically most stable structure taken on as function of coverage. Lateral adsorbate-adsorbate interactions have attracted theoretical [78Ein] and experimental [91Ehr] interest since decades. The interactions have several origins which can be divided according to the adsorbate separation. At small interatomic distances direct electronic interactions dominate and localized chemical dimer bonds are formed. This interaction falls off exponentially and is therefore of very short range. At larger separations adsorbate interactions are predominantly indirect and mediated in three ways: first electrostatically (dipole-dipole) and second elastically (deformation of substrate), which both decay monotonically with separation r as 1/r3, and third they are mediated by Friedel oscillations resulting from screening. Friedel oscillations are oscillations in the total substrate electron density around the adsorbates and lead to oscillatory interactions going as cos(2kFr)/r5 for bulk electrons and as cos(2kFr)/r2 for surface state electrons [78Ein, 78Lau, 00Hyl]. The important consequences of these laws are that surface state mediated interactions are expected to be of extremely long range, and that they oscillate with significantly larger wavelength than bulk electron mediated interactions since kF(surface) >> kF(bulk) (see, e.g., Ag where kF((111)-surface state) = 0.083 Å-1 [99Jea] and kF(bulk) = 1.2 Å-1 [87Ash]). Direct experimental evidence of long range surface state mediated interactions has very recently been obtained [00Kno, 00Rep]. In the following we treat adsorbate-adsorbate interactions. First for the simplest case of adsorbed dimers, and then we discuss the present knowledge on lateral interactions spanning over more than one lattice site.

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[Ref. p. 251

3.3.1.4.1 Dimer bond energies The experimental methods for determination of dimer bond energies and lateral interactions can be divided into those relying on indirect observations and those where adatoms or dimers are directly traced. In the first case integral techniques as TDS and ¨Φ, as well as local techniques (STM) have been used. In the case of TDS the dimer bond energies and/or lateral interactions are derived from the coverage dependence of the desorption energy. Often Edes(θ) is linear in coverage and written as Edes(θ) = Edes(θ) - wθ. Thus w > 0 signifies attraction and w < 0 repulsion. Typical values for w derived in this manner are w = -0.41 eV/ML for Hg/Cu(100)-c(2x2) [90Dow] and w = 0.38 eV/ML for the denser (1x1) phase of Hg on the same surface [92Kim]. See also Hg on Fe(100) and on W(100) with w = 0.065±0.005 eV/ML [81Jon] and w = -0.092 eV/ML [78Jon, 79Jon], respectively. We note that these values stem from a macroscopic measurement and can not be associated to dimer or cluster bond energies. For some systems Edes does not vary linearly with θ but shows plateaus, as revealed by careful TDS experiments. It is believed that the levels of these plateaus correspond to the energy difference between atoms desorbing from the 2D adatom gas and those desorbing from the edges of adatom islands or small adatom clusters which coexist with the gas. An example was reported for Ag/W(110) by Kolaczkiewicz and Bauer [86Kol1]. The Edes(θ) data in Fig. 5 initially show a linear increase revealing attractive interactions between adatoms. At a coverage of 0.1 ML there is a first plateau, which has been associated with desorption from silver dimers leading to a dimer bond energy of 0.45 eV for that system. From similar observations, estimates of the dimer bond energies have been derived also for Ni, Cu and Au on W(110) (see Table 5). We note that the Eb-values derived that way are based on a particular view of what is happening on the surface - namely desorption from dimers - for which there is no direct substantiation. Assuming that the plateau corresponds to desorption from larger clusters would lead to smaller dimer bond energies - see trimer hypothesis and the values derived from Roelofs and Bellon cited in Table 5. Interactions between adsorbates can lead to phase transformations in the adsorbed layer. At low coverages and high temperatures adatoms are expected to behave like a 2D lattice gas. If the coverage is increased, or the temperature lowered, one expects the formation of a condensed phase coexisting with the 2D lattice gas. Since the dipole moments per atom decrease rapidly with increased lateral coordination work function measurements (¨Φ) clearly discern the monomer gas from the condensed phase. Therefore ¨Φ-measurements can reveal the boundary line between coexistence and pure 2D gas in the phase diagram of the adsorbed layer. Kolaczkiewicz and Bauer were able to deduce this phase boundary for Ni, Cu, Pd, Ag, and Au on W(110) [84Kol, 85Kol1, 85Kol2]. Their results for Ag/W(110) are shown in Fig. 6. The lateral interactions are inferred by fitting the experimental phase diagram with Monte Carlo simulations using nearest and next-nearest neighbor, as well as many atom interactions as input. Fig. 7 shows the pair and many atom interactions considered. Their large number gives rise to a multidimensional parameter space and it is likely that several points in this space may produce the observed phase diagram. This is suggested by the different results reported for the immediate nearest neighbor interactions by different authors, all analyzing the very same data shown in Fig. 6. Stoop obtains Eb = 0.074 eV [83Sto], which is much smaller than the value derived from TDS under the dimer hypothesis, whereas Ehrlich and Watanabe [91Ehr] obtain with the method from Roelofs and Bellon [89Roe] 0.18 eV. Kolaczkiewicz and Bauer derived pair interaction Lennard-Jones potentials from their ¨Φ data [84Kol, 85Kol2], which yielded the Eb values marked as trimer hypothesis in Table 5 since they were derived under the assumption that desorption takes place from trimers [86Kol1]. This comparison for Ag2/W(110) shows that macroscopic measurements can only be interpreted unambiguously if one knows what the rate limiting processes really are. This often requires information on the microscopic level. Another indirect way to derive dimer bond energies recently evolved from variable temperature STM. The method relies on the fact that the density of nucleated islands is quite sensitive to the stability of small clusters. If the stable cluster changes from a dimer to a trimer, dimer dissociation enters in addition to monomer diffusion as a rate limiting step in cluster nucleation. Consequently, the Arrhenius plot of saturation island densities shows a marked change in slope as the dimer becomes unstable. Whereas the slope in the stable dimer regime only contains the monomer migration barrier, the slope at higher T contains in addition the dimer bond energy. Such nucleation studies have been carried out for only few

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systems so far, however, they yield precise dimer bond energies (Table 5). The experimental methods permitting direct exploration of monomer diffusion and dimer dissociation are FIM and STM. Dimer bond energies can be derived from FIM and STM by observing the dimer stability as a function of T. In the case of STM this has been done by determining the threshold temperature for the onset of 2D Ostwald ripening which yielded Eb = 0.15±0.02 eV for Ag/Pt(111), consistent with the nucleation results [99Bru]. STM has also been used to directly observe dimer association and dissociation events for Pt/Pt(110) [00Lin]. This study revealed that Eb is not exactly Ediss - Em, as often assumed. A number of FIM studies explore the T-dependence of the dimer dissociation rate and derive Eb based on the known Em values, using this assumption. Since the error of Em also reflects itself in the one of Eb we only tabulate the recent Eb values, where Em is known with high precision. For a review including also former studies see [91Ehr]. Table 5. Dimer bond energies (order after periodic table, first priority substrate and second adsorbate). System Cu/Ni(100) Ni/W(110) Cu/W(110)

Pd/W(110)

Ag/W(110)

Au/W(110)

Re/W(110) Ir/W(110) Ir/Ir(111) Ag/Pt(111) Pt/Pt(110) Pt/Pt(111)

Eb [eV] a

0.46 ± 0.19 ) 0.34 ± 0.03 b) 0.30 0.35 c) 0.22 d) 0.15 0.22 0.18 (0.05-0.12/bond) 0.087±0.001 0.45 c) 0.18 d) 0.074 0.18 0.35 c) 0.24 d) 0.29 −0.032±0.009 0.065±0.003 0.32±0.02 0.15±0.02 0.07±0.03 0.23±0.01

Method

Ref.

STM, nucleation STM, nucleation TDS TDS

[96Mül1] [98Bru] [86Kol1] [86Kol1]

TDS ¨Φ data -MC analysis FIM TDS FIM TDS

[75Bau] [89Roe] [75Bas] [80Sch] [99Koh] [86Kol1]

¨Φ

¨Φ

data -MC analysis data -MC analysis TDS

[83Sto] [91Ehr] [86Kol1]

¨Φ data -MC analysis FIM FIM FIM STM, nucleation and Ostwald ripening STM, νdiss(T) and νass(T) e) FIM

[89Roe] [92Wat] [92Wat] [90Wan] [99Bru] [00Lin] [99Kyu]

) Deriving different ν0 for monomer diffusion and dimer dissociation (4 x 1011±1 Hz and 5 x 1012±2 Hz). ) Assuming a common attempt frequency of ν0 = 5 x 1011±1 Hz for monomer diffusion and dimer dissociation. c ) Dimer hypothesis. d ) Trimer hypothesis. e ) Analysis of dimer dissociation and association rates show that Ediss < Em + Eb since Em gets lowered for the last step towards another monomer (see discussion of neighbor driven mobility above). a

b

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3.3.1.4.2 Long range interactions Long range adsorbate-adsorbate interactions can only be investigated by microscopic techniques. FIM has been used extensively for this purpose [91Ehr, 94Kel], and very recently also low temperature STM was applied [00Kno]. In both techniques the occupation of lattice sites by diffusing monomers close to another monomer or to an adisland is monitored as a function of temperature and time. Boltzman statistics then yields the differences in free energy between the different sites and thus one can in principle map out the atom adsorption potential. The results obtained with FIM show that the interactions are element specific, oscillatory, of several atomic distances in range, and they can be anisotropic. Instead of reviewing all interaction studies [91Ehr, 91Wat, 92Wat] we will only discuss a few examples to show the wealth of behavior found for different systems. For example, the atoms in a stretched Re dimer formed across adjacent atomic channels on W(211) repel each other at their smallest distance and attract each other if they are more than two neighbor distances apart. In contrast, two Re atoms positioned along one such channel on the same surface attract each other starting from 3 nearest neighbor distances until the dimer is formed [91Ehr]. The task of data recording and analysis becomes easier for heteropairs, since then, due to the different diffusion barriers, only one atom moves while the other atom stands still. The experiments of Watanabe et al., addressing Pd-Re on W(110), showed long range interactions, which were repulsive at short distances along [001] and [ 110 ], and attractive along [ 111 ] [91Wat]. The same trends as for Pd-Re were observed for W-Pd, for Ir-Ir [91Wat], and recently for Pd-Pd [99Koh]. A different behavior was observed for Re-Re where interactions are repulsive at close distances in all directions, attraction only begins to appear beyond 7.5 Å, but then it is again strongest along [ 111 ] [92Wat]. These examples illustrate that the details of long range pair interactions vary from one atom to another, but the long range (> 10 Å) over which two atoms feel each other, and the orientational dependence, are common features. The first STM results concerned with long range interactions revealed only indirectly that there was a long range repulsion between metal adatoms. In several nucleation studies too large island densities were systematically observed for metal/metal systems with migration barriers below 0.10 eV [00Bar]. These island densities can only be reconciled with a standard attempt frequency for diffusion of ν0 = 1013 s-1 if long-range repulsion between the diffusing monomers delays island nucleation and thus increases densities as compared to standard nucleation with purely attractive interactions [00Bog, 00Fic]. An STM study showing long range ordering of surface segregated impurities on Cu(111) already gave some evidence to the existence of long range interactions on fcc(111) surfaces [98Wah]. However, these studies showed atoms placed only at some of the expected neighbor distances. Recent observations directly revealed the existence of such long range interactions for Cu diffusion on Cu(111) [00Kno, 00Rep] and also for Co/Cu(111) as well as for Co/Ag(111) [00Kno]. The interactions were found to oscillate with λF/2 and thus to be mediated by the surface state electrons; the interaction strength was only in the meV range and decayed as 1/r2 [00Kno] as predicted from theory for mediation by a partly filled Shockley type surface state [78Lau]. The short range repulsion for Cu/Cu(111) has been predicted in recent large scale DFT calculations [00Bog].

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3.3.1.5 Overlayer structure Adsorbed layers on single crystal surfaces are generally strained due to their lattice mismatch with the substrate. These layers are commensurate (C) when the corrugation of the substrate potential acting on the adsorbed atoms is large compared to the stiffness of their lateral bonds. If the lateral bonds are dominant, an incommensurate (I) layer results. In this case the adlayer lattice forms moiré patterns with the substrate lattice, which might be rotated to minimize their energy [77Nov, 79McT, 84Doe, 85Doe]. For the intermediate case, i. e., when both interaction potentials become comparable, weakly incommensurate phases are observed. They consist of large areas, which are nearly commensurate, separated by relatively narrow domain walls where the strain resulting from the lattice mismatch is locally relieved. The situation can be more complicated if the substrate participates in the relaxation. Vertical buckling of the first few substrate layers, for instance, can lead to adatoms on bridge sites being lower than atoms on hollow sites and thus to an inversion of adsorption heights as compared to expectation [95Hwa, 96Nag]. Finally, the adatoms can perform exchange processes and thereby substitute substrate atoms. For bulk inmiscible elements the structures formed that way are surface alloys, which are confined to one monolayer. For adsorbates forming a bulk alloy with the substrate, alloy layers might form at the surface as metastable intermediates before dissolution of the adatoms into the bulk. The interest in thin metal films on metal substrates is mainly due to the possibility of using the interaction with the substrate to stabilize and create crystallographic structures of the adsorbate elements, which are inexistent or only stable under elevated temperature or pressures in its bulk [85Pri]. The most famous example is the stabilization of fcc-Fe down to low temperature by the growth on Cu(100) [67Jes, 68Jes] (bulk Fe exists in the fcc-phase only from 1200 – 1650 K, otherwise it crystallizes in a bcc lattice). Many experimental techniques are allowing access to the structure of overlayers. However, each technique only reveals certain aspects of it and complete structural knowledge generally only evolves from a combination of several techniques. Conventional LEED reports the surface periodicity, whereas LEED-IV-curves give access to the vertical height of the adsorbate and to relaxations in the first few substrate layers, as does glancing incidence X-ray diffraction. Real-space imaging with STM directly shows whether the periodic structures inferred from diffraction are caused by a superposition of stacking domains. Ion scattering (LEIS, MEIS, HEIS) can reveal the chemical composition of the first few layers, which allows to distinguish adlayers from alloys and to determine the chemical composition of the latter. For surface alloys this can also be done with careful STM measurements. STM also shows the morphology of islands. For instance, a monolayer of Co on Cu(111) can be present in the form of three atomic layer high islands, the lowest one of which is embedded into the first Cu(111) plane and the middle layer is surrounded by a Cu rim resulting from the extracted substrate atoms [97Ped]. Such information can hardly be obtained from diffraction techniques – LEED only shows a (1x1) pattern for Co/Cu(111). On the other hand, the exact vertical positions of ad- and substrate atoms in an overlayer can only be deduced from diffraction experiments and their comparison with models. Valuable input for these models can come from STM images. Altogether, only the complementary use of several techniques unravels the entire structural information. For reasons of space, we will restrict ourselves in the Tables below to the lateral positions of the atoms. We will report if and from which temperature on there is exchange, but we will not table the exact vertical positions of the adatoms, which have been deduced for a few cases from LEED-IV or X-ray diffraction. Before presenting the Tables, we now discuss typical examples of monolayer structures illustrated in Figs. 8 - 14. The trivial case is a pseudomorphic overlayer realized for small misfit m and/or for substrates with highly corrugated adsorption potential. The example reproduced in Fig. 8 shows a monolayer of Cu strained by 5.8% into registry with the Ru(0001) lattice. (Note that the strain s is defined with reference to the lattice constant of an adsorbate bulk plane with corresponding symmetry, in our case s = (2.70 Å - 2.55 Å)/2.55 Å, whereas the misfit m is defined with respect to the substrate, i.e., m = (2.55 Å - 2.70 Å)/2.70 Å = -5.5%). The STM image of Cu/Ru(0001) reveals an atomic arrangement with a periodicity and symmetry identical to that of the substrate, the image is bare of long-range height modulation by which a weakly incommensurate phase would manifest itself.

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For systems with large misfit, or with relatively stiff lateral bonds as compared to the corrugation of the adsorbate-substrate potential, there are several ways to accommodate strain in weakly incommensurate phases. These are shown for the case of hexagonally close-packed substrates in Fig. 9. These substrates have the unique property of two highly coordinated adsorption sites, the fcc and hcp hollows sites, whereas for instance on a square lattice there is only one four-fold hollow site. Therefore on hexagonal surfaces strain relief can proceed through formation of fcc- and hcp- stacking domains separated by domain walls, also called surface partial dislocations. The textbook example for strain relief by stacking transitions is the Au(111)-reconstruction where, driven by the tensile stress, a 4% compression of the first layer is achieved by two domain walls per (¥[ XQLWFHOO HDFK LQVHUWLQJ RQHKDOI H[WUD DWRP WKXV leading to 23 atoms adsorbed on 22 second-layer atoms along the close-packed [ 110 ]-directions [85Har, 85Tak, 90Bar, 91San]. Due to the difference in binding energy more fcc- than hcp-sites are populated giving rise to a pairwise arrangement of the [ 112 ]-oriented domain-walls. Locally the compression is unidirectional, however, on large terraces a mesoscopic order of the domain walls is established: The domain walls bend by ±120° with a period of 250Å [90Bar] forming the so called herringbone structure which reduces the anisotropy of the surface stress tensor [92Nar]. The herringbone structure is schematically shown in Fig. 9a. Examples for such incommensurate striped phases (IS) of metal monolayers are Ag and Au/Ru(0001) and Ag/Pt(111) (see Fig. 10a). Isotropic strain relief is achieved on a smaller length scale by trigonal networks as shown in Figs. 9b and c. These trigonal incommensurate phases (TI) involve domain wall crossing which can be costly due to their generally repulsive mutual interaction [79Bak], and due to the fact that the crossing points often involve unfavorable on-top adsorption sites. The threading dislocations shown in Fig. 9b are realized in the system Ag/Ru(0001) for θ < 0.85 ML [95Ste], or for the alloy formed by Al on Au(111) [97Fis]. The trigonal network shown in Fig. 9c has so far only be seen for films comprising two or more monolayers (for strain relief and orientational relationships in multilayers, which is not the subject here, we refer to [97Kin, 98Zha] and to [82Bau], respectively). Both TI-phases (Figs. 9b and c) exhibit different areas of fcc- and hcp-stacking and thereby account for the energy difference of these two lattice sites. In the case shown in Fig. 9b this is realized by bending of the DL's forming convex (larger) fcc-areas and concave (smaller) hcp-areas. For straight DL's (Fig. 9c) different areas of the two stacking domains are created by shifting one class of domain walls relative to the crossing point of the two others; this also avoids crossing of all three domain walls in a single point [94Bru] (see 2. ML areas in Fig. 10b for an example). Figure 10b shows a peculiarity of the system Ag/Pt(111) induced by the chemical potential of adatoms on-top of the first layer [97Bro]. A full Ag monolayer is pseudomorphic as seen in Fig. 10b by the absence of DL's in the first monolayer region. This is highly counterintuitive. The domain walls present in islands (Fig. 10a) should persist up to a full layer since the stress caused by lattice misfit is larger in a full monolayer than in adatom islands where stress can partly be relieved at the edges [97Bro]. Ag is under compressive stress on Pt(111) (m = 4.3%) and therefore the partial surface dislocations (DL) represent areas of locally lower atomic density. Before completion of the first layer, second layer Ag islands form, and due to the low 2D vapor pressure of Ag, these adatoms islands are in equilibrium with a 2D adatom gas created by lateral evaporation from the islands at 300 K. These adatoms fill in the first-layer DL's to gain coordination at the cost of stress increase in this layer. The structure of overlayers is certainly a function of the preparation conditions; these have to be included for sensible comparison of the literature. Ag/Cu(111) is an example where several metastable structures are created as a function of increasing deposition temperature. Figure 11 shows that this system forms a moiré overlayer (Tdep = 225 K), while deposition at 300 K leads to a trigonal lattice of dislocation loops in the underlying substrate, substitution takes place at 775 K deposition temperature [97Bes]. On square lattices the substrate adsorption potential is typically highly corrugated (see the large monomer diffusion barriers on square lattices compared to close-packed surfaces reported in Chap. 3.11 of the present Volume). Therefore the most frequent structure of metal adlayers on surfaces with 4-fold symmetry is a pseudomorphic (1x1)-unit-cell. However, also on square lattices there are mechanisms of strain relief. Despite the fact that there are no stacking domains on such lattices (see above), there is a mechanism of strain relief which resembles the one on close-packed lattices. Single atomic rows can shift by half a lattice constant as seen for 1 ML Cu/Ni(100) (m = 2.56%) in Fig. 12. Thereby atoms in this row are displaced onto bridge sites where they appear higher in the STM topographs. Bridge site adsorption

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has less coordination to the substrate which is counterbalanced by an increase of lateral coordination from 4 to 6 and the energy gain associated with strain relief [96Mül2]. This mechanism of strain relief continues in thicker films of Cu/Ni(110). In the n-th monolayer n-rows are shifted leading to internal {111}-facets between the pseudomorphic and the shifted stacking areas. On fcc(110) surfaces there are close-packed rows in the [ 110 ]-direction whereas the interatomic distance is ¥WLPHV DV PXFK LQ WKH [001]-direction. This open structure can lead to (1x2)-reconstructions for the clean surfaces where they rearrange into small close-packed {111}-facets. Substrate atoms in the uppermost rows are possible candidates for exchange processes as substantiated by the observation that diffusion on these surfaces often involves exchange (diffusion takes place by exchange on fcc(110) surfaces, e.g., for Pt/Ni(110) [91Kel], Au/Ni(110) [93Nie], Ir/Ir(110) [91Che], Re/Ir(110) [92Che], and W/Ir(110) [80Wri], for more examples see Chap. 3.11). In addition, many of the investigated heteroepitaxial metal systems with fcc(110) surfaces involve substitutional atoms as thermodynamically stable adsorbate structures (see for instance Pd/Cu(110) and In/Cu(110), see Table 6 for references). Exchange is also realized in the overlayer structures of Pb formed on Cu(110) at room temperature (Fig. 13), however, due to the large lattice constant of Pb, atomic rows along the less dense packed direction of the substrate are replaced by close packed Pb rows. The p(4x1) adlayer is energetically practically degenerate with structures where one Cu-row is replaced by a Pb row. This degeneracy leads to a coexistence of ad- and substitutional structures involving p(8x1) and p(12x1) unit-cells [95Nag]. The exchange on fcc(110) substrates can even lead to so-called subsurface growth where the adsorbate grows in the second layer and is capped by substrate atoms during deposition [92Rou, 97Hug, 97Mur]. The surfaces of the refractive metals generally participate only little in the structure of the adlayer as much as relaxations (large elastic modulus) and exchange processes (large surface free energy) are concerned. Therefore they often can be seen as a rigid surface onto which the adsorbate atoms accommodate for a given coverage in a way, which corresponds best to the lateral interactions between them and to the period and corrugation of the substrate adsorption potential. An example where adsorbate-adsorbate interactions are repulsive, much as for alkali metals (see Chap. 3.2), is Gd/W(110). This system stands for many rear earth elements on W surface and undergoes a sequence of commensurate structures with increasing adatom density as shown in Fig. 14. In Tables 6-8 we have listed the submonolayer and monolayer structures of metals on the principal low index surfaces of metal substrates from 2- to 4-fold rotational symmetry. This compilation comprises heteroepitaxial systems only since the structure of homoepitaxial systems is in most cases trivial. For unreconstructed surfaces the bulk stacking is pseudomorphically continued and for reconstructed ones the reconstruction is lifted below the adlayer and at the same time taken on by the adsorbate layer. Only few homoepitaxial cases are worth mentioning since their reconstructions can metastably be lifted, as seen for Au/Au(110)-(1x2) [97Gün], or a reconstruction can be induced at a lower temperature by homoepitaxial adsorption, as seen for Pt/Pt(111) [93Bot]. We have only included systems, for which the original literature could be consulted by the author. For systems where several groups reported contradictory results, and these contradictions have now been removed, only the surviving truth is given and referred to; in cases where contradictions persist we list the different results. Due to the progress in surface science instrumentation and surface preparation during the recent decades clear preference is given to the more recent results. The former studies are often discussed and referred to in these more recent investigations. For references to the early literature, and for complementation of the Tables below, the reader may also consult the list of metal/metal structures given in the book of G. Somorjai [94Som]. The review article of E. Bauer [84Bau1] is also recommended as valuable collection of references, which partly complement the Tables given here on adsorption on refractory metals, in particular by references to the Russian literature.

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Table 6. Overlayer structure on substrates with 2-fold rotational symmetry (order according to periodic system, first priority substrate, second priority adsorbate) System

Structure

θ [ML]

Tdep/Tann [K]

Method

Ni/Al(110) Cu/V(110)

intermixing NiAl (1x1)

0-1 0-2

300 300

Ag/Fe(110) Al/Ni(110)

SK, 2 wetting ML 0-2 3D islands 0-2 dissolution into bulk (1x1) a) 0 (1x1) a) < 0.5 phase separation b) < 0.4 (5x3) > 0.4 0.93 (1x1) overlayer c) 1.0 (2x2), c(2x4) alloy c) 1.0 Pd-Cu-chains 0 subsurface growth > 0.1 substitution 10−4 0.75 p(4x1) overl. p(4x1) a) 0.778 p(9x1) a) 0.80 p(5x1) a) (1x1) 0-1 (2x8) (1x1) 0-1 chains along <111> 0-0.2 chain order (2x1) 0.2-0.5 chain order (3x1) 0.5-1 dist. hexagonal 0-0.8 liquid < 0.4 hex 0.6-0.7 c(6x2) 0.7 (2x3) and c(2x2) 0.09-0.26 close-packed hex 0.31 c(2x2), (1.6x2), 0-1 (1.5x1.8) chains along <111> 0-1 (1x1) < 0.68  10 0  −3 4   

HEIS, XPS [00Shu] LEED, XPS, [99Kra] AES UHV-SEM [96Nor] AES, STM, TDS,[98Hah] AES FIM [91Kel] LEIS [95Dor] STM [93Nie] STM, RBS [95Nie]

Pt/Ni(110) Au/Ni(110)

Co/Cu(110) Pd/Cu(110) In/Cu(110) Pb/Cu(110)

Fe/Mo(110) Ni/Mo(110) Cu/Mo(110) Pd/Mo(211)

Ag/Mo(110) Ba/Mo(110)

Pt/Mo(211) Pb/Mo(110)

1 0  1 3    1 0  3 10  1 − 1 1 2 

     

600 300 Tan > 600 > 105 570 f) 300 300/700 300 350

Ref.

300

HAS STM STM, theory

[98Töl, 99Töl, 00Töl] [00Lin] [97Mur]

77 300

PAC STM

[89Kla] [95Nag]

300-370

STM RHEED LEED LEED

[00Osi] [99Tsu] [84Bau2, 87Kol] [98Ste]

LEED LEED, ¨Φ, EELS

[84Bau2, 87Kol] [98Gor]

300

77

[00Jo1]

300/<900 300 1300 300-1000

< 0.67

300-1000

< 0.66

300-930

> 0.33

1020-1070

RHEED LEED

[98Ste]

RHEED, SEM

[00Jo2]

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System

Structure

θ [ML]

Tdep/Tann [K]

Method

Ref.

Ce/Mo(110)

7 0 1 2    5 −1  1 2   

0-0.6

300

RHEED

[96Tan]

800

RHEED, AFM

[99Jo1, 99Jo2]

300

LEED

[95Pou]

250-300 < 600 > 750 300 300 300 130 300 300

STM

[94Buc, 97Li] [94Hah]

STM, theory MEIS STM STM STM,LEIS STM, LEIS

[97Mur] [90Fen, 91Fen] [92Rou] [97Hit] [96Hug] [97Hug]

LEED, AES, TDS, ¨Φ STM, LEED LEED

[00Kol]

LEED, AES, TDS, ¨Φ STM

[00Kol] [99Kah]

LEED

[95Fri]

LEED UHV-SEM LEED

[87Ber] [83Spi] [86Kol1]

3 0 1 2     0.62 − 1  1 2   Nd/Mo(110)

(14x2), (13x2), 0-1 (11x2), (8x2), (6x2), c(6x2), c(4.5x2), c(5x3) Au/Ru(1000) d) (1x1) 0.0-0.5 (1x3), Au-chains 0.5-1.3 along [0100] Cu/Pd(110) atomic Cu chains 0-0.1 pseudomorphic 0-1 intermixing Pd/Ag(110) subsurface growth > 0.1 Au/Ag(110) bilayer (1x1) 0.05-0.8 subsurface growth 1.0 Ni/Au(110) adatoms 0-1 (1x2) surface alloy Cu/Au(110) (1x2) 1 subsurface growth V/W(110) disordered surf. alloying Mn/W(110) (1x1) pseudom. e) 1.0 Fe/W(110) (1x1) < 1.0 (10x10) epitaxial Fe(110) isl. (1x1) Co/W(110)

Ni/W(110) Ag/W(110) Eu/W(110)

(1x1) 1-layer isl. relaxed 2-layer isl. pseudomorphic with NW-orientation (1x1) 2 ML wet substr. (3x2) (7x2) hex hex

300 > 700 300 100/900 100/720 Tann = 950 300

0-1 0-1 0-1

300/460 300/610

100 470-820 300

0-5 0.33 < 0.43 > 0.31 < 0.53

[99Bod] [90Ber]

cont.

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[Ref. p. 251

Table 6 (cont.) System

Structure

θ [ML]

Tdep/Tann [K]

Method

Ref.

Gd/W(110)

(10x2) (8x2) (7x2) (6x2) (5x2) c(5x3) hex random (nx2) stripes moiré

0.2 0.25 0.29 0.33 0.40 0.53 0.64

300/1200 300/720 300/1200

LEED STM LEED

[86Kol1] [97Pas] [86Kol1]

300

STM, LEED

[00Pia]

Ho/W(110)

0.1-0.5 0.5

a

) Adsorbate atoms substitute substrate atoms. ) At 0.4 ML 0.16 ML Au that were initially alloyed into the Ni surface suddenly "pop out", for θ > 0.4 ML part of the Au is present in Au-chains forming on the Au-Ni alloy surface, part also goes back into the alloy surface such that θchain/θalloy § FRQVW c ) O acted as surfactant. d ) 1-fold substrate symmetry. e ) Stabilization of bcc δ-Mn up to a local thickness of 3 ML. f ) Adatoms were obtained by segregation from Ni(110) crystal containing 0.8% Au. b

Table 7. Overlayer structure on substrates with 3-fold rotational symmetry (order according to periodic system, first priority substrate, second priority adsorbate) System

Structure

θs [ML]

Tdep/Tann [K]

Method

Ref.

Fe/Al(111) Ag/Al(111) Sn/Al(111) Pb/Al(111) Cd/Ti(0001) Mn/Co(0001) Pt/Co(0001) Fe/Ni(111) Cu/Ni(111) Pd/Ni(111) Ag/Ni(111)

disordered alloy (1x1) f) dense hex. overl. dense hex. overl. (1x1) ordered alloy (1x1) (1x1) (1x1) (13x13)-moiré moiré hex rot ±2° substitution (9x9) moiré DL loops in Ni surface alloy k) (¥[¥ 5 p(2x2) (1x1) (1x1) intermixing m) (1x1) fcc

0-1 1

300, 470 300 300 300

LEED-IV LEIS LEED, AES LEED, AES LEED-IV LEED LEED, XPS, STM LEED, EELS TPD, XPS STM STM AES, LEED STM STM

[93Beg] [00Los] [80Arg] [80Arg] [77Shi, 77Shi] [98Cho, 99Cho1] [00Cab] [00D'Ad] [00Kos] [99Ter] [97Bes] [00Mró] [96Hol, 98Bes] [95Jac]

STM LEED

[97Bes] [90Sin1, 90Sin2]

STM LEED STM, LEED SEXAFS

[92Bro] [76Gra] [00Pas] [00But]

Au/Ni(111)

Hg/Ni(111) Fe/Cu(111)

0.3-0.8 0-1.0 0-4 1.0

300 300 300 100/800 300 0.74 300 0-2 180-900 0-0.1 500 0.79 170 300/400 0.35-0.7 > 425

0-0.8

300/570

0–1 0-1

300 300-350

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 251]

3.3.1 Metals on metals

237

System

Structure

θs [ML]

Tdep/Tann [K]

Method

Ref.

Co/Cu(111)

substitution, Cucapping Co-bilayer isl. and Cu-vacancy islands (1x1) Co- and Cutermination (1x1)-Co Cu-capping pres. no exchange n) (1x1) (1x1) 8 1   9 −1  -moiré l)  

1.0

300/< 700

LEIS

[94Rab]

0.6

300

STM

[93Fig]

1.5

300

STM, LEED-IV

[96Fig]

1.0

STM

[97Ped]

1.0

150/300 300 300

0-1.0 0.8

300 225

LEED-IV, STM LEED-IV LEED STM

[00Pri] [82Tea] [79Fuj, 83Pes] [97Bes]

< 0.08 10−4

300 > 425 77

PAC LEED

[89Kla] [77Erl]

0.25

300/620

0-0.21 0-0.21 0.53 0.64 0.73 0-1 1 0-1

300/523

SEXAFS LEED STM

[82Com] [77Fuj] [94Nag, 96Nag]

300

LEED-IV

[96Sch]

300 600-1170 200 Tan 300/600 300

STM, ¨Φ , LEIS

[96Kop], [95Wu]

AES, LEED, ¨Φ

[99Kol2]

STM, LEED STM

[93Vri] [95Mey]

Tan 300/500 300 300/520 300

AES, LEED, ¨Φ STM LEED STM AES, LEED, ¨Φ

[99Kol2] [95Gün] [87Par] [91Pöt] [99Kol2]

350/500 Tan

LEED AES, LEED, ¨Φ

[92Cam] [99Kol2]

300

STM

[95Ste]

300 300

STM STM

[95Hwa] [95Ste]

Ni/Cu(111) Pd/Cu(111) Ag/Cu(111)

In/Cu(111) Sn/Cu(111)

Te/Cu(111) Au/Cu(111) Pb/Cu(111)

Mg/Ru(0001) Al/Ru(0001) Fe/Ru(0001) Co/Ru(0001) Ni/Ru(0001)

Cu/Ru(0001)

Rh/Ru(0001) Pd/Ru(0001)

Ag/Ru(0001)

DL-loops in Cu substitution substitution  2/3 2/3 − 2/3 4/3   1 − 1  1 2    p(2x2) a) (2x2) (¥[¥ 5 c) (2x2) c) (4x4)-Pb d) (5x5) (7x7) (1x1) surface alloy (1x1) adlayer until des. (1x1) pseudom. (1x1) pseudom. part. DL loops adlayer until des. (1x1) pseudom.

1.1-1.3

DL loops 1.1-1.3 (1x1) adlayer until des. (1x1) < 1.0 pure adlayer until desorption unidirectionally = 0.90 strained hex. buckl. of underl. Ru threading DL’s

cont.

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3.3.1 Metals on metals

238 Table 7 (cont.) System Pt/Ru(0001) Au/Ru(0001)

Mg/Pd(111) V/Pd(111)

Fe/Pd(111) Ag/Pd(111) Au/Pd(111) Fe/Ag(111) Ni/Ag(111)

Sb/Ag(111) Au/Ag(111) Tl/Ag(111) Pb/Ag(111)

Structure

θs [ML]

(1x1) adlayer isl. < 0.5 surface alloy 0.4 (1x1) unidirextionally 0.9 strained hex. only local order 0-0.7 substitution V exchanges with 1. < 0.5 Pd layer, Pd isl. (¥[¥ 5 ZLWK 1. layer pure Pd 2. layer V0.33Pd0.66 (1x1), likely c) 1 1.0 (1x1) f, g) (1x1) isl. <1 (1x1) surface alloy 1 (1x1) h) island growth 0.1 substitution moiré islands with 0.1 dnn = 2.5 Å, α = 5.9° 0.08-0.3 substitution e) substitution 1.0 (¥[¥ 5 followed by moiré (¥[¥ 5 LVO 0 - 0.33 R30° isl. coex. with 0.33 moiré, α = ±(3 -5)° 0.67 R30° and moiré 0.67 with moiré

Fe/W(111) Co/W(111) Ni/W(111)

(1x1) (1x1) (1x1)

Cu/W(111) Rh/W(111)

(1x1) 0-1 (1x1) 0.9 {211}-subst faceting 1.0 (1x1) 1.0 (1x1) 0-1.1 {211}-subst faceting {211}-subst faceting 0.3 - 2.0 (1x1) 0-1 (1x1) 0–1 no intermixing 0-1 (1x1) with little 0-1 intermixing (2x2) surf. alloy in isl. and substrate

Pd/W(111) Ag/W(111) Ir/W(111) Pt/W(111) Ti/W(111) Gd/W(111) Co/Re(0001)

1.0 1.2 1.0

[Ref. p. 251

Tdep/Tann [K]

Method

Ref.

Tan < 920 300/1200 300 300/1200

STM

[98Mon]

STM

[92Hwa, 92Sch]

<250

XPD, LEED

[93Fis]

300

STM, LEIS, XPD

[00Kon]

570

300, 570 300 100-300 300/925 300, 470

LEED-IV

[93Beg]

300-500 130-285

STM STM

[95Mey] [98Hir]

300 300

STM ARXPS LEED, RHEED, AES TEM

[98Veg] [94Eis] [78Raw]

300

300 - 400

300 Tann > 1200 300 300/1100 800-1300 Tann > 900 300 Tann = 900 Tann > 1100 Tan > 750 Tann < 1500 300-1500 300 300

LEED-IV [93Beg] ARXPS, LEED-IV [93Eis] STM [97Gle]

LEED, RHEED LEED

[81Tak , 82Tak, 83Tak]

LEED, AES, ¨Φ LEED LEED LEED, AES, ¨Φ LEED LEED

[78Raw] [84Rol] [99Kol1] [95Gua] [95Gua] [99Kol1] [95Gua] [95Gua]

LEED, AES, ¨Φ LEED LEED LEEM LEED LEED FEM LEED, STM

[99Kol1] [95Gua] [95Gua] [99Pel] [95Gua] [95Gua] [99Sha] [99Par]

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 251]

3.3.1 Metals on metals

θs [ML]

System

Structure

Cu/Re(0001)

(1x1) 0-0.8 (14x14) 0.8-2.0 (1x1) 0-1 (1x1) < 0.93 adislands < 0.5 intermixing (2x2) Pt3Al alloy dissolution of Al into Pt bulk bcc(111) phase 1 (1x1) adatom isl. < 0.5 on hcp and fcc sites exchange induces > 0 Pt(111) reconstr. fcc -> hcp alloying dissolution into bulk (10x10) 1.1 (1x1) pseudo 1 1 (1x1) adlayer 1 exchange j) alloy islands <1 (1x1) 0-1

Pd/Re(0001) Ag/Re(0001) Al/Pt(111)

V/Pt(111) Co/Pt(111)

Ni/Pt(111) Cu/Pt(111)

Pd/Pt(111) Ag/Pt(111)

La/Pt(111) Re/Pt(111)

Al/Au(111)

Fe/Au(111) Co/Au(111)

Ni/Au(111) Mo/Au(111)

(1x1) pseud. islands 0-0.5 isl. ø > 200Å DL’s 0.2-0.9 back to pseudomor. 1.0 surface alloy 0-0.99 surface alloy (1x1) < 0.5 (2x2) surface alloy > 1 (1x1) adislands with 0 - 0.8 edge alloy 3-layer alloy adatoms 0.1 exchange surface alloy, 0.2 distorted hex. phase 0.4-1 nucl. at elbows i) subst. at elbows <10−3 monolayer islands 10−3-10−2 bilayer islands 0.02-0.8 these sink by 1 layer Co diff. into bulk dissolution into bulk 0.1-1 nucl. at elbows i) nucl. at elbows 0.1

239

Tdep/Tann [K]

Method

Ref.

300 300/970 350/500 300-600 300 Tan = 500 Tan = 800 Tan > 1000

LEED, TDS, XPS

[99Wag]

LEED STM, LEED STM

[92Cam] [97Par] [00Lee]

300 < 200

XPD STM

[99Sam] [00Var]

300

STM

Tann > 450 Tann > 550 Tann > 750 300 300 340 450 600 300

SXD STM, AES LEED XPD STM, HAS STM, HAS STM LEED

[95Grü, 99Lun1] [97Fer] [95Atr, 97Fer] [94Grü] [98Tsa, 99Hen]

300

STM

[98Sam] [97Hol] [98Hol] [00Sch1] [91Att, 94Att] see also [00Mar] [97Bro]

Tann

STM PAX, HREELS LEED, XPD

[93Röd, 93Str] [00Fey] [00Ram]

LEED, XPS, STM

[99Ram]

STM

[99Fis]

STM

[97Fis]

STM STM HAS STM STM HAS STM STM STM

[91Voi2, 92Str] [96Mey] [97Töl] [91Voi1] [99Pad] [97Töl] [99Pad] [91Cha2, 93Cha] [00Hel]

300/ <900 300/900 300 Tann = 1000 < 230 > 230 350/450 300 300 300 300 Tann > 400 Tdep > 450 Tann = 750 300 300

cont.

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3.3.1 Metals on metals

240 Table 7 (cont.) System Rh/Au(111)

Pd/Au(111) Ag/Au(111) Pt/Au(111)

Pb/Au(111)

Structure

θs [ML]

Tdep/Tann [K]

coex. of 1 and 20.2 300 layer islands dissolution into bulk Tann = 670 adatoms 0.1 150 intermixing 300/240 (1x1) 0-1 300 rec. locally lifted substitution 0.00 - 0.03 300 mixed PtAu islands 0.03 - 1.0 & subsurface Au isl. pure Pt islands > 1.0 < 0.68 5% exp. 30° rot. Pd(111) - isl. 0.68 moiré, α = 5°

Cu/Pb(111)

(1x1) subsurface isl. 0-3

300

[Ref. p. 251

Method

Ref.

STM

[94Alt]

LEIS

[92Koe]

STM STM

[89Dov, 91Cha1, 93Cha] [99Ped]

LEED, AES

[78Bib]

TEM STM

[82Tak] [95Nag]

a

) Adsorbate atoms substitute substrate atoms. b ) O acted as surfactant. c ) Surface alloy. d ) de-alloying. e ) Sb segregation upon annealing or Ag deposition, this gives rise to surfactant effect of Sb in Ag growth. f ) 1. ML is pseudomorphic, i.e., continues the substrate stacking, g ) 2. layer grows with stacking fault. h ) LEED-IV-curves remain unchanged upon adsorption. i ) Likelihood of substitution at elbows leading to the observed heterogeneous nucleation. j ) Cu adlayer and exchange from the adlayer into Pt induces substrate reconstruction. k ) the lattice constant of the surface alloy changes smoothly with composition following Vegards law in 2D. l ) The STM images obtained after 300 K deposition by McMahon et al. [92McM] were falsely interpreted as (9x9) moiré pattern. After the study of Besenbacher et al. they are due to partial DL loops in the underlying Cu. m ) Intermixing can be suppressed with Pb as surfactant, also Fe is in fcc state up to 4 ML when Pb is used. n ) Presumably abrupt interface between Co and Cu when Pb is used as surfactant.

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Ref. p. 251]

3.3.1 Metals on metals

241

Table 8. Overlayer structure on substrates with 4-fold rotational symmetry (order according to periodic system, first priority substrate, second priority adsorbate)

θ [ML]

System

Structure

Sn/Al(100) Pb/Al(100) Fe/Cr(100)

c(2x2) c(2x2) ordered surf. alloy 0-0.5 Fe0.5Cr0.5 disordered alloy with 0-1 0-0.1 ML surf Cr 1.0 (1x1), bcc δ-Mn (1x1) 0-1

Cr/Fe(100) Mn/Fe(100) Co/Fe(100) Ni/Fe(100) Hg/Fe(100) Al/Ni(100) Mn/Ni(100) Fe/Ni(100) Cu/Ni(100) Ag/Ni(100) V/Cu(100)

(1x1) (1x1) (1x1) poor order Ni3Al(100) alloy c(2x2) c) (1x1) intermixing (1x1) e) c(2x8) hexagonal

0-1 0-1 1.0

Method

Ref.

300/470

LEED, AES LEED, AES STM

[78Arg1] [78Arg1] [99Cho2]

570

STM

[96Dav]

570 300

MEIS, LEED XPD LEED, PDMEE XPD LEED LEED-IV

[97Pfa] [93Zha] [00Ber] [93Zha] [81Jon] [88Lu]

LEED-IV PDMEE, LEED STM STM LEED LEED, XPD

[93Wut] [99Luc] [96Mül1] [91Bro] [99Tod] [00Moo]

LEED STM LEED-IV MEIS STM

[94Hau] [96Law]

Tdep/Tann [K]

300

300 > 300 0.5 270 0-1 300 0-1 200-400 0.17-1.0 300

Mn/Cu(100)

(1x1) < 0.5 (2x1) 0.5-1.5 (1x1) 1.0 pseudom. 3D islands 0.5-3 0.5 c(2x2) c)

300 285-575 270/470

Fe/Cu(100)

c(2x2) c)

0.5

300

Co/Cu(100)

(1x1) b ) Co-Cu alloy and pure Co coexisting c(2x2) c)

1.0 0 0-2

Cr/Cu(100)

Pd/Cu(100)

Ag/Cu(100)

In/Cu(100) Te/Cu(100) Pt/Cu(100)

0.5

(2x2)p4g clock rec. b) 1.0 c(10x2) hex ad.-layer = 0.9 = 0.9 (1x1) surf. alloy < 0.9 coex. with c(10x2) layer in alloy layer  2 0 −1 5    c(2x10) hex substitution (2¥[¥ 5 c(2x2) b)

10-4 0.33 1.0

300

300 300 300 300 250 300/425

[93Wut] [99Bro] [92Cha, 93Joh, 94Cha] [68Jes, 83Mir, 84Fal] [99Nou] [00Kim]

LEED STM XPS, UPS, PAX, LEED STM, LEED, RBS [96Mur] LEED-IV [88Wu] MEIS [96She1, 96She2] STM [96Spr, 97Bes] [67Bau, 68Pal]

300

LEED

[93Nak]

300/420 77 300/620 300/470

LEED, MEIS PAC SEXAFS LEIS, LEED

[89Kla] [82Com] [96She] cont.

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3.3.1 Metals on metals

242

[Ref. p. 251

Table 8 (cont.) System

Structure

θ [ML]

Tdep/Tann [K]

Method

Ref.

Au/Cu(100)

c(2x2) c)

0.5

300

c(14x2)

1.0

300

de-alloying (3x3) & c(2x6) c(2x2) c(4x4) (¥[¥ 5 2 2  2−2   

1.2

LEED-IV STM MEIS PDI LEED, XPD MEIS MEIS LEED

[87Wan] [92Cha1] [99Bro] [93Tob, 95Tob] [93Nau] [93Nak] [96She] [92Kim]

LEED LEED

[87One] [91Bin]

HAS, LEED LEIS

[93Li] [98Pla]

STM, LEED LEEM

[98Boc] [00Kel]

LEED, AES

[78Arg2]

STM, LEED LEED LEED-IV STM LEED SEXAFS LEED LEIS STM

[00Kis] [00Oni] [90Tia] [95Hah] [94Hau] [99Sch] [00Sch2] [97Lan] [00Deg]

UPS

[96Pat] [82Smi] [96Sch1] [92Kim] [90Ber] [87Ber]

Hg/Cu(100)

Tl/Cu(100)

 4 0 2 7  

Pb/Cu(100)

Bi/Cu(100) Al/Pd(100) Mn/Pd(100) Cu/Pd(100) Cr/Ag(100) Mn/Ag(100) Fe/Ag(100) Co/Ag(100) Pd/Ag(100) Pt/Ag(100) Hg/Ag(100) Fe/W(100) Ni/W(100)

120 0.1 - 0.5 200 0.2 - 0.62 0.5 300 100-300 0.67

0.67 6 6  2 − 2    (¥61x¥61)Rtan−1(5/6) 0.1-0.5 0.38-0.45 c(4x4) surf. alloy 0.45-0.53 c(2x2) adlayer 0.57-0.68 c(5¥[¥ 5 consisting of antiphase domainwalls in c(2x2) dispersed << 0.67 dense hex. overlayer 0.67 (2x2)-p4g bilayer 0.5 - 2 surface alloy 0.5 c(2x2) c) 0-1 (1x1) pseudom. f) (1x1) 1.0 surf. alloy 0-3 c(2x2) 0-1 substitution 0.1 adislands 0.1 incorporated islands substitution 0.1 (1x1) substitution 0.1 (1x1) < 0.7 (1x1) (1x1)

300 100 150-250 250-450 250-450 250-450 160–220 300/570

300/700

300-400 300 210-340 >130 275/425 425 >135 >200 120 100/700 100

UPS LEED LEED LEED

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 251]

3.3.1 Metals on metals

θ [ML]

System

Structure

Cu/Ir(100)

anisotropic Cu islands 0-0.6 Ir chains from lifting of (5x1) reconstruction phase separation (1x1) 1.0 (1x1) 1.0 Fe in subst. sites, 0-0.16 place-exchange lifts "hex"-rec. and leads to Au islands c(2x2) (7¥[¥ 5

Cr/Au(100) Fe/Au(100)

Pb/Au(100)

243

Tdep/Tann [K]

Method

Ref.

300/1000

STM, LEED, ISS

[00Gil]

300 300

LEED HRLEED STM

[94Hau] [94Jia, 95Jia] [98Her]

LEED

[78Gre]

a

) Ru(0001) substrate was generated by depositing thick Ru films onto a Mo(110) crystal. ) Clear evidence for intermixing. c ) Surface alloy. e ) In islands reaching a critical diameter of 30 atoms one atomic row shifts by half a lattice constant onto bridge sites. This is the beginning of internal faceting continuing as strain relief mechanism up to 20 ML. f ) There is an abrupt structural transition to bct for θ > 1 ML at 300 K T K, and for θ > 3 ML at 370 K T K. b

Acknowledgments The author gratefully acknowledges help from S. Jans and C.-L. Bandelier in preparing the figures. He is also indebted to U. Heiz for carefully reading the manuscript.

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244

3.3.1 Metals on metals

[Ref. p. 251

Figures for 3.3.1 (110 )

bcc

(100)

[110 ]

[111 ]

[211 ]

F

L [001]

-

[001]

(111 )

[011 ]

(110 )

hcp

-

[211 ]

L

-

[011 ]

(100)

3

-

-

[010]

fcc

1

2

S

-

(111 )

(211 )

-

[011 ]

[110 ]

fcc

-

-

[011 ]

hcp

[011 ]

(0001)

-

(1100 ) 3

-

[1200]

[001]

(1100 ) 1

[1000]

3 2

-

2 4

[1100 ]

[1100 ]

[0001]

[0001]

(1000) 1

4

[0001]

[-1200]

Desorption frequency ν (s-1)

Fig. 1. The principal low-index surfaces of bcc, fcc and hcp crystals. For the hcp lattice the ideal packing with c/a = 2¥ LV DVVXPHG 7ZR VWDFNLQJ VHTXHQFHV RI WKH KFS SODQH DUH VKRZQ ,Q VRPH FDVHV WKH DWRPV DUH numbered according to the sequence of atomic planes. In the case of bcc(110) L stands for lattice and S for surface site; on the bcc(111) surface F labels the faulted site in contrast to the lattice continuation L; on fcc(111) the lattice site is labeled fcc and the faulted site hcp.

1017

Desorption energy E (eV)

1016 Ni/ W(110)

10

15

1014

5.0 4.8 4.6 4.4 0

0.2

0.4 0.6 0.8 Coverage θ (ML)

1.0

Fig. 2. Desorption energies Edes(θ ) and pre-exponential factors νdes(θ ) derived from TDS for Ni/W(110) [86Kol1]. Upper branches correspond to desorption from the 2D solid – gas interface, whereas lower branches are desorption from the 2D gas.

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Ref. p. 251]

3.3.1 Metals on metals

245

11 10

BINDING ENERGY (eV)

9

a) E

8 7 6 5

x

4

b) E

3 2 1

0

Hf Ta W Re Os

Ir

Pt Au

2

7

8

3

4

5

6

9

x

n5d Fig. 3. Binding energies of 5d transition metal atoms on various W surfaces derived from critical voltages for field desorption. Crosses signify (110) surface orientation, squares (100), circles (112), and triangles (111); open and closed symbols indicate different evaluation procedures [84Bau1].

Fig. 4a, b. Attachment of an adatom to one- (a) and two- (b) fold step sites assuming attraction over two lattice sites. The energy barriers are drawn to scale for Ir/Ir(111).

3.6

Frequency factor

10 15

3.4 Desorption energy

3.2

10 13

Dimer energy

3.0

10 14

Ag/W(110)

Frequency factor ν (s -1)

Desorption energy E des (eV)

10 16

2.8 0

0.1

0.2 0.3 Coverage θ

0.4

Fig. 5. Parameters for desorption from the 2D gas phase of Ag/W(110) [86Kol1].

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246

3.3.1 Metals on metals

[Ref. p. 251

1000

-[110]

q 900

Temperature T (K)

2D-Gas

[001]

800

ε4

2D-Gas & condensed

700

t1

600

ε2 ε1

Ag/W(110) 500 0

0.1 0.2 Coverage θ

ε5 ε3

t2

0.3 Fig. 7. Illustration of pair- and many-atom interac-tions contributing to binding in clusters on W(110) [89Roe].

Fig. 6. Phase boundary separating pure lattice gas region from coexistence region of lattice gas and condensed phase in T-θ -diagram for Ag/W(110) [85Kol1].

100 Fig. 8. STM image showing the pseudomorphic (1x1) monolayer of Cu on a Ru(0001) substrate [95Gün].

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Ref. p. 251]

3.3.1 Metals on metals

247

δ hcp fcc

hcp

a

fcc

fcc hcp

hcp

(a)

(b)

(c)

Fig. 9a - c. Possible domain wall structures for strain relief on hexagonally close packed surfaces: (a) unidirectional compression or expansion along the close packed atomic rows. For isotropic strain relief on a mesoscopic scale often two of the three possible rotational domains alternate leading to a herringbone pattern. (b) and (c) trigonal networks with wall crossings [97Bru].

a)

b) 2nd ML [11

0]

1st ML

Substr. 1st ML

Substr. 1st ML 500

2nd ML 500

Fig. 10a, b. The first monolayer of Ag/Pt(111) forms (a) a pattern of partial surface dislocations for θ < 1.0 ML and (b) a pseudomorphic layer as soon as 2. layer islands and thus a 2D Ag adatom gas are present on-top of the first layer ((a) θ = 0.5 ML, Tdep = 500 K, (b) θ = 1.5 ML, Tdep = 300 K, Tann = 800 K) [97Bro].

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248

3.3.1 Metals on metals

a)

b)

[Ref. p. 251

c)

20

20

20

Fig. 11a - c. STM images of Ag deposited on Cu(111) showing the effect of substrate temperature during deposition. (a) Deposition of 0.8 ML at 225 K leads to a moiré pattern, while (b) deposition of the same amount at 300 K results in a triangular misfit dislocation structure. (c) Deposition of Ag submonolayers at 775 K leads to alloying of Ag atoms into the first Cu layer (dark spots) [97Bes].

a)

b)

250

∆h

Fig. 12a, b. (a) Strain relief for a square lattice: Cu/Ni(100). (b) Single atomic Cu rows are shifted from 4-fold hollow to 2-fold bridge sites leading to the bright stripes in the STM image [96Mül2].

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Ref. p. 251]

3.3.1 Metals on metals

249

Pb overlayer Pb substitutional Cu 1st layer Cu 2nd layer

[001] [110]

p(4x1) overlayer Θ = 0.75

p(1

2x1

)

p(4 p(8

p(12x1)

x1

)

p(12x1) substitutional + overlayer Θ = 0.75

x1

) p(8x1)

ste

p(8

x1

)

30

p(8

x1

)

p(4

x1

) p(12x1)

pe

dg

e

p(4x1)

p(4x1) substitutional Θ = 0.75

p(4x1) substitutional side view

Fig. 13. STM image of 0.75 ML Pb on Cu(110). The superstructures depend critically on coverage. Upon a slight increase in θ (= Θ) the p(4x1) overlayer structure transforms into a structure with identical surface symmetry, but where one substrate [001]-row per unit-cell is substituted by a Pb row. Coexistence of ad- and substitutional structures lead to the p(12x1) and p(8x1) structures seen in the STM image. The substitutional rows lie lower and appear dark; the bright line is a step running across the image [95Nag1].

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250

3.3.1 Metals on metals a)

b)

[Ref. p. 251

c)

[001]

d)

- ] [110

e)

f)

Fig. 14. Sequence of overlayer structures observed by means of LEED for Gd/W(110) with increasing coverage. (a) (10x2), (b) (7x2), (c) (5x2), (d) (6x2), (e) c(5x3), and (f) hexagonal for θ s = 1/5, 2/7, 2/5, 1/3, 8/15, and 5/8, respectively [86Kol2].

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251

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3.3.1 Metals on metals 90He 90Koe 90Sin1 90Sin2 90Tia 90Tik 90Wan 91Att 91Bin 91Bor 91Bro 91Cha1 91Cha2 91Che 91Ehr 91Fen 91Jia 91Kel 91Pöt 91San 91San 91Voi1 91Voi2 91Wan 91Wat 92Bro 92Cam 92Cha1 92Cha2 92Che 92Hwa 92Kim 92Koe 92McM 92Nar 92Rod1 92Rod2 92Rou 92Sch 92Str 92Wat 93Beg 93Bot 93Cha 93Eis 93Fig 93Fis 93Joh 93Li 93Nak 93Nau

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3.3.1 Metals on metals Nielsen, L.P., Besenbacher, F., Lægsgaard, E., Stensgaard, I., Engdahl, C., Stoltze, P., Jacobsen, K.W., Nørskov, J.K.: Phys. Rev. Lett. 71 (1993) 754. Nyberg, G.L., Kief, M. T., Egelhoff, W.F.: Phys. Rev. B 48 (1993) 14509. Röder, H., Schuster, R., Brune, H., Kern, K.: Phys. Rev. Lett. 71 (1993) 2086. Rodriguez, J.A.: Surf. Sci. 296 (1993) 149. Strüber, U., Küppers, J.: Surf. Sci. 294 (1993) L924. Stuckless, J.T., Al-Sarraf, N., Wartnaby, C., King, D.A.: J. Chem. Phys. 99 (1993) 2202. Tobin, J.G., Waddill, G.D., Li, H., Tong, S.V.: Phys. Rev. Lett. 70 (1993) 4150. Vrijmoeth, J., Günther, C., Schröder, J., Hwang, R.Q., Behm, R.J., in: "Magnetism, Structure in Systems of Reduced Dimension", Farrow, R.F.C. (ed.), New York: Plenum Press, 1993, p. 55. Wang, S.C., Ehrlich, G.: Phys. Rev. Lett. 70 (1993) 41. Wang, S.C., Ehrlich, G.: Phys. Rev. Lett. 71 (1993) 4174. Wuttig, M., Flores, T., Knight, C.C.: Phys. Rev. B 48 (1993) 12082. Wuttig, M., Gauthier, Y., Blügel, S.: Phys. Rev. Lett. 70 (1993) 3619. Zhang, J., Han, Z. L., Varma, S.: Surf. Sci. 298 (1993) 351. Altman, E.I., Colton, R.J.: Surf. Sci. 304 (1994) L400. Attard, G.A., Price, R., Al-Akl, A.: Electrochim. Acta 38 (1994) 1525. Brune, H., Röder, H., Boragno, C., Kern, K.: Phys. Rev. B 49 (1994) 2997. Bucher, J.P., Hahn, E., Fernandez, P., Massobrio, C., Kern, K.: Europhys. Lett. 27 (1994) 473. Chambliss, D.D., Johnson, K.E.: Phys. Rev. B 50 (1994) 5012. Eisenhut, B., Stober, J., Rangelov, G., Fauster, T.: Phys. Rev. B 49 (1994) 12980. Feibelman, P.J.: Surf. Sci. 313 (1994) L801. Feibelman, P.J., Nelson, J.S., Kellogg, G.L.: Phys. Rev. B 49 (1994) 10548. Grütter, P., Dürig, U.T.: Phys. Rev. B 49 (1994) 2021. Hahn, E., Kampshoff, E., Fricke, A., Bucher, J.P., Kern, K.: Surf. Sci. 319 (1994) 277. Haugan, M.E., Qibiao, C., Onellion, M., Himpsel, F.J.: Phys. Rev. B 49 (1994) 14028. Jiang, Q., Chan, A., Wang, G.C.: Phys. Rev. B 50 (1994) 11116. Kellogg, G.L.: Surf. Sci. Rep. 21 (1994) 1. Nagl, C., Haller, O., Platzgummer, E., Schmid, M., Varga, P.: Surf. Sci. 321 (1994) 237. Rabe, A., Memmel, N., Steltenpohl, A., Fauster, T.: Phys. Rev. Lett. 73 (1994) 2728. Somorjai, G.A.: "Introduction to Surface Chemistry and Catalysis", New York: John Wiley & Sons, 1994. Vandoni, G., Félix, C., Monot, R., Buttet, J., Harbich, W.: Surf. Sci. 320 (1994) L63. Atrei, A., Bardi, U., Galeotti, M., Rovida, G., Torrini, M.,anazzi, E.: Surf. Sci. 339 (1995) 323. Boisvert, G., Lewis, L.J., Puska, M.J., Nieminen, R.M.: Phys. Rev. B 52 (1995) 9078. Dorenbos, G., Boerma, D.O., Buck, T.M., Wheatley, G.H.: Phys. Rev. B 51 (1995) 4485. Fritzsche, H., Meyer, G., Amer, N.M.: Phys. Rev. B 51 (1995) 15933. Grütter, P., Dürig, U.: Surf. Sci. 337 (1995) 147. Guan, J., Campbell, R.A., Madey, T.E.: Surf. Sci. 341 (1995) 311. Günther, C., Vrijmoeth, J., Hwang, R.Q., Behm, R.J.: Phys. Rev. Lett. 74 (1995) 754. Hahn, E., Kampshoff, E., Wälchli, N., Kern, K.: Phys. Rev. Lett. 74 (1995) 1803. Hwang, R.Q., Hamilton, J.C., Stevens, J.L., Foiles, S.M.: Phys. Rev. Lett. 75 (1995) 4242. Jacobsen, J., Nielsen, L.P., Besenbacher, F., Stensgaard, I., Lægsgaard, E., Rasmussen, T., Jacobsen, K.W., Nørskov, J.K.: Phys. Rev. Lett. 75 (1995) 489. Jiang, Q., Wang, G.C.: Surf. Sci. 324 (1995) 357. Meyer, J.A., Behm, R.J.: Surf. Sci. 322 (1995) L275. Meyer, J.A., Schmid, P., Behm, R.J.: Phys. Rev. Lett. 74 (1995) 3864. Nagl, C., Pinczolits, M., Schmid, M., Varga, P., Robinson, I.K.: Phys. Rev. B 52 (1995) 16796. Nagl, C., Platzgummer, E., Schmid, M., Varga, P., Speller, S., Heiland, W.: Phys. Rev. Lett. 75 (1995) 2976. Nielsen, L.P., Besenbacher, F., Stensgaard, I., Lægsgaard, E., Engdahl, C., Stoltze, P., Nørskov, J.K.: Phys. Rev. Lett. 74 (1995) 1159. Poulston, S., Tikhov, M., Lambert, R.M.: Surf. Sci. 331-333 (1995) 818. Stevens, J.L., Hwang, R.Q.: Phys. Rev. Lett. 74 (1995) 2078. Lando lt -Börnst ein New Ser ies III/42A1

3.3.1 Metals on metals 95Tob 95Wu 96Dav 96Fig 96Göl 96Hol 96Hug 96Kel 96Kop 96Law 96Mey 96Mül1 96Mül2 96Mur 96Nag 96Nor 96Pat 96Sch1 96Sch2 96She1 96She2 96Spr 96Stu 96Stu 96Tan 96Yeo 97Bes 97Bro 97Cha 97Chr 97Fer 97Fis 97Gle 97Gün 97Hit 97Hol 97Hug 97Kin 97Lan 97Li 97Lin

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Tobin, J.G., Waddill, G.D., Hua, L., Tong, S.Y.: Surf. Sci. 334 (1995) 263. Wu, Y., Tao, H.S., Garfunkel, E., Madey, T.E., Shinn, N.D.: Surf. Sci. 336 (1995) 123. Davies, A., Stroscio, J.A., Pierce, D.T., Celotta, R.J.: Phys. Rev. Lett. 76 (1996) 4175. Figuera, J. d. a., Prieto, J.E., Kostka, G., Müller, S., Ocal, C., Miranda, R., Heinz, K.: Surf. Sci. 349 (1996) L139. Gölzhäuser, A., Ehrlich, G.: Phys. Rev. Lett. 77 (1996) 1334. Holmblad, P.M., Larsen, J.H., Chorkendorff, I., et al.: Catalysis Letters 40 (1996) 131. Hugenschmidt, M.B., Hitzke, A., Behm, R.J.: Phys. Rev. Lett. 76 (1996) 2535. Kellogg, G.L.: Phys. Rev. Lett. 76 (1996) 98. Kopatzki, E., Keck, H.G., Baikie, I.D., Meyer, J.A., Behm, R.J.: Surf. Sci. 345 (1996) L11. Lawler, J.F., Kraan, R.G.P. v. d., Kempen, H. v., Quinn, A.J.: Phys. Rev. B 53 (1996) 11159. Meyer, J.A., Baikie, J.D., Kopatzki, E., Behm, R.J.: Surf. Sci. 365 (1996) L647. Müller, B., Fischer, B., Nedelmann, L., Brune, H., Kern, K.: Phys. Rev. B 54 (1996) 17858. Müller, B., Fischer, B., Nedelmann, L., Fricke, A., Kern, K.: Phys. Rev. Lett. 76 (1996) 2358. Murray, P.W., Stensgaard, I., Lægsgaard, E., Besenbacher, F.: Surf. Sci. 365 (1996) 591. Nagl, C., Schmid, M., Varga, P.: Surf. Sci. 369 (1996) 159. Noro, H., Persaud, R., Venables, J.A.: Surf. Sci. 357-358 (1996) 879. Patthey, F., Massobrio, C., Schneider, W.D.: Phys. Rev. 53 53 (1996) 13146. Schneider, W.D., Patthey, F., Roy, H.V., Schaffner, M.H., Delley, B.: Mod. Phys. Lett. B 10 (1996) 1161. Schwegmann, S., Over, H., Gierer, M., Ertl, G.: Phys. Rev. B 53 (1996) 11164. Shen, Y.G., Yao, J., O'Connor, D.J., King, B.V., MacDonald, R.J.: J. Phys. Cond. Mat. 8 (1996) 4903. Shen, Y.G., Yao, J., O'Connor, D.J., King, B.V., MacDonald, R.J.: Solid State Commun. 100 (1996) 21. Sprunger, P.T., Lægshaard, E., Besenbacher, F.: Phys. Rev. B 54 (1996) 8163. Stuck, A., Wartnaby, C.E., Yeo, Y.Y., Stuckless, J.T., Al-Sarraf, N., King, D.A.: Surf. Sci. 249 (1996) 229. Stumpf, R., Scheffler, M.: Phys. Rev. B 53 (1996) 4958. Tanaka, Y., Kamei, M., Gotoh, Y.: Surf. Sci. 360 (1996) 74. Yeo, Y.Y., Vattuone, L., King, D.A.: J. Chem. Phys. 104 (1996) 3810. Besenbacher, F., Nielsen, L.P., Sprunger, P.T., in: "Growth and Properties of Ultrathin Epitaxial Layers", Vol. 8, King, D.A., Woodruff, D.P. (eds.), Amsterdam: Elsevier, 1997, p. 207. Bromann, K., Brune, H., Giovannini, M., Kern, K.: Surf. Sci. 388 (1997) L1107. Champbell, C.T.: Surf. Sci. Rep. 27 (1997) 1. Christensen, A., Ruban, A.V., Stoltze, P., Jacobsen, K.W., Skriver, H.L., Nørskov, J.K., Besenbacher, F.: Phys. Rev. B 56 (1997) 5822. Ferrer, S., Alvarez, J., Lundgren, E., Torrelles, X., Fajardo, P., Boscherini, F.: Phys. Rev. B 56 (1997) 9848. Fischer, B., Barth, J.V., Fricke, A., Nedelmann, L., Kern, K.: Surf. Sci. 389 (1997) 366. Gleich, B., Ruff, M., Behm, R.J.: Surf. Sci. 386 (1997) 488. Günther, S., Hitzke, A., Behm, R.J.: Surf. Rev., Lett. 4 (1997) 1103. Hitzke, A., Hugenschmidt, M.B., Behm, R.J.: Surf. Sci. 389 (1997) 8. Holst, B., Hohlen, M., Wandelt, K., Allison, W.: Surf. Sci. 377-379 (1997) 891. Hugenschmidt, M.B., Ruff, M., Hitzke, A., Behm, R.J.: Surf. Sci. 388 (1997) L1100. King, D.A., Woodruff, D.P., in: The Chemical Physics of Solid Surfaces, Vol. 8. Amsterdam: Elsevier Science, 1997. Langelaar, M.H., Boerma, D.O., in: "Surface Diffusion: Atomistic and Collective Processes", Vol. 360, Tringides, M.C. (ed.), New York: Plenum Press, 1997, p. 67. Li, Y., Bartelt, M.C., Evans, J.W., Waelchli, N., Kampshoff, E., Kern, K., DePristo, A.E.: Phys. Rev. B 56 (1997) 12539. Linderoth, T.R., Horch, S., Lægsgaard, E., Stensgaard, I., Besenbacher, F.: Phys. Rev. Lett. 78 (1997) 4978.

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3.3.1 Metals on metals Murray, P.W., Thorshaug, S., Stensgaard, I., Besenbacher, F., Lægsgaard, E., Ruban, A.V., Jacobsen, K.W., Kopidakis, G., Skriver, H.L.: Phys. Rev. B 55 (1997) 1380. Parschau, M., Schlatterbeck, D., Christmann, K.: Surf. Sci. 376 (1997) 133. Pascal, R., Zarnitz, C., Bode, M., Wiesendanger, R.: Phys. Rev. B 56 (1997) 3636. Pedersen, M.Ø., Bönicke, I.A., Lægsgaard, E., Stensgaard, I., Ruban, A., Nørskov, J.K., Besenbacher, F.: Surf. Sci. 387 (1997) 86. Pfandzelter, R., Igel, T., Winter, H.: Surf. Sci. 389 (1997) 317. Tölkes, C., Zeppenfeld, P., Krzyzowski, M.A., David, R., Comsa, G.: Phys. Rev. B 55 (1997) 13932. Tölkes, C., Zeppenfeld, P., Krzyzowski, M.A., David, R., Comsa, G.: Surf. Sci. 394 (1997) 170. Yeo, Y.Y., Vattuone, L., King, D.A.: J. Chem. Phys. 106 (1997) 392. Yeo, Y.Y., Vattuone, L., King, D.A.: J. Chem. Phys. 106 (1997) 1990. Besenbacher, F., Chorkendorff, I., Clausen, B.S., Hammer, B., Molenbroek, A.M., Nørskov, J.K., Stensgaard, I.: Science 279 (1998) 1913. Bocquet, F., Gauthier, S.: Surf. Sci. 416 (1998) 1. Brown, W.., Kose, R., King, D.A.: Chem. Reviews 98 (1998) 797. Brune, H.: Surf. Sci. Rep. 31 (1998) 121. Choi, B.C., Bode, P.J., Bland, J.A.C.: Phys. Rev. B 58 (1998) 5166. Gorodetsky, D.A., Melnik, Y.P., Proskurin, D.P., Sklyar, V.K., Usenko, V.A.,, Yas'ko, A.A.: Surf. Sci. 416 (1998) 255. Hahn, P., Bertino, M.F., Toennies, J.P., Ritter, M., Weiss, W.: Surf. Sci. 412-413 (1998) 82. Hernán, O.S., Parga, A.L.V. a. d., Gallego, J.M., Miranda, R.: Surf. Sci. 415 (1998) 106. Hirstein, A.: Ph.D.-Thesis, Lausanne: Ecole Polytechnique Fédérale, 1998. Holst, B., Nohlen, M., Wandelt, K., Allison, W.: Phys. Rev. B 58 (1998) R10195. Kyuno, K., Gölzhäser, A., Ehrlich, G.: Surf. Sci. 397 (1998) 191. Mongeot, F.B. d., Scherer, M., Gleich, B., Kopatzki, E., Behm, R.J.: Surf. Sci. 411 (1998) 249. Pimpinelli, A., Villain, J.: "Physics of Crystal Growth", Cambridge: Cambridge University Press, 1998. Platzgummer, E., Borell, M., Nagl, C., Schmid, M., Varga, P.: Surf. Sci. 412-413 (1998) 202. Sambi, M., Granozzi, G.: Surf. Sci. 400 (1998) 239. Schlatterbeck, P., Parschau, M., Christmann, K.: Surf. Sci. 418 (1998) 240. Stepanowskyi, S., Ubogy, I., Kolaczkiewicz, J.: Surf. Sci. 411 (1998) 176. Tölkes, C., Struck, R., David, R., Zeppenfeld, P., Comsa, G.: Phys. Rev. Lett. 80 (1998) 2877. Tsay, J.S., Shern, C.S.: Surf. Sci. 396 (1998) 319. Vegt, H.A. v. d., Vrijmoeth, J., Behm, R.J., Vlieg, E.: Phys. Rev. B 57 (1998) 4127. Wahlström, E., Ekvall, I., Olin, H., Walldén, L.: Appl. Phys. A 66 (1998) S1107. Zhang, Z., Lagally, M.G., in: Series in Direction in Condensed Matter Physics, Vol. 14, Singapore: World Scientific, 1998. Bode, M., Hennefarth, M., Haude, D., Getzlaff, M., Wiesendanger, R.: Surf. Sci. 432 (1999) 8. Brown, D., Noakes, T.C.Q., Woodruff, D.P., Bailey, P., Le-Goaziou, Y.: J. Phys. Cond. Mat. 11 (1999) 1889. Brune, H., Bales, G.S., Boragno, C., Jacobsen, J., Kern, K.: Phys. Rev. B 60 (1999) 5991. Choi, B.C., Bode, P.J., Bland, J.A.C.: Phys. Rev. B 59 (1999) 7029. Choi, Y.J., Jeong, I.C., Park, J.-Y., Kahng, S.-J., Lee, J., Kuk, Y.: Phys. Rev. B 59 (1999) 10918. Feibelman, P.: Phys. Rev. B 60 (1999) 4972. Fischer, B., Brune, H., Fricke, A., Barth, J.V., Kern, K.: Phys. Rev. Lett. 82 (1999) 1732. Henzler, M.: Surf. Sci. 419 (1999) 321. Huang, H.H., Jiang, X., Siew, H.L., Chin, W.S., Sim, W.S., Xu, G.Q.: Surf. Sci. 436 (1999) 167. Jeandupeux, O., Bürgi, L., Hirstein, A., Brune, H., Kern, K.: Phys. Rev. B 59 (1999) 15926. Jo, S., Gotoh, Y.: Jap. J. Appl. Phys. 38 (1999) 6878. Jo, S., Gotoh, Y.: Surf. Sci. 435 (1999) 652. Kahng, S.J., Park, J.V., Kuk, Y.: Surf. Sci. 442 (1999) 379. Koh, S.J., Ehrlich, G.: Phys. Rev. B 60 (1999) 5981. Lando lt -Börnst ein New Ser ies III/42A1

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Kolaczkiewicz, J., Bauer, E.: Surf. Sci. 420 (1999) 157. Kolaczkiewicz, J., Bauer, E.: Surf. Sci. 423 (1999) 292. Kralj, M., Pervan, P., Milun, M.: Surf. Sci. 423 (1999) 24. Kyuno, K., Ehrlich, G.: Surf. Sci. 437 (1999) 29. Luches, P., Gazzadi, G.C., Bona, A. d., Marassi, L., Pasquali, L., Valeri, S., Nannarone, S.: Surf. Sci. 419 (1999) 207. Lundgren, E., Stanka, B., Koprolin, W., Schmid, M., Varga, P.: Surf. Sci. 423 (1999) 357. Lundgren, E., Stanka, B., Leonardelli, G., Schmid, M., Varga, P.: Phys. Rev. Lett. 82 (1999) 5068. Nouvertné, F., May, U., Bamming, M., Rampe, A., Korte, U., Güntherodt, G., Pentcheva, R., Scheffler, M.: Phys. Rev. B 61 (1999) 14382. Padovani, S., Scheurer, F., Bucher, J. P.: Euophys. Lett. 45 (1999) 327. Parschau, M., Christmann, K.: Surf. Sci. 423 (1999) 303. Pedersen, M. Ø., Helveg, S., Ruban, A., Stensgaard, I., Lægsgaard, E., Nørskov, J. K., Besenbacher, F.: Surf. Sci. 426 (1999) 395. Pelhos, K., Hannon, J.B., Kellogg, G.L., Madey, T.E.: Surf. Sci. 432 (1999) 115. Ramstad, A., Strisland, F., Raaen, S., Worren, T., Borg, A., Berg, C.: Surf. Sci. 425 (1999) 57. Sambi, N., Granozzi, G.: Surf. Sci. 426 (1999) 235. Schieffer, P., Tuillier, M.H., Hanf, M.C., Krembel, C., G, G.: Surf. Sci. 422 (1999) 132. Shakirova, S.A., Serova, E.V.: Surf. Sci. 422 (1999) 24. Terada, S., Yokoyama, T., Saito, N., Okamoto, Y., Ohta, T.: Surf. Sci. 433-435 (1999) 657. Todorov, S.S., Bu, H., Boyd, K.J., Rabalais, J.W., Gilmore, C.M., Sprague, J.A.: Surf. Sci. 429 (1999) 63. Tölkes, C., David, R., Tscherisch, K.G., Comsa, G., Zeppenfeld, P.: Europhys. Lett. 46 (1999) 589. Tsunematsu, H., Aita, M., Tanaka, A., Suzuki, S., Sato, S., Gotoh, Y.: J. El. Spectr. & Rel. Phen. 103 (1999) 281. Wagner, R., Schlatterbeck, D., Christmann, K.: Surf. Sci. 440 (1999) 231. Barth, J.V., Brune, H., Fischer, B., Weckesser, J., Kern, K.: Phys. Rev. Lett. 84 (2000) 1732. Bertacco, R., Isella, G., Duò, L., Ciccacci, F., Bona, A. d., Luches, P., Valeri, S.: Surf. Sci. 454-456 (2000) 671. Bogicevic, A., Ovesson, S., Hyldgaard, P., Lundqvist, B.I., Brune, H., Jennison, D.R.: Phys. Rev. Lett. 85 (2000) 1910. Butterfield, M.T., Crapper, M.D.: Surf. Sci. 454-456 (2000) 719. Cabeza, G.F., Légaré, P., Sadki, A., Castellani, N.J.: Surf. Sci. 457 (2000) 121. D'Addato, S., Pasquali, L., Gazzadi, G.C., Verucchi, R., Capelli, R., Nannarone, S.: Surf. Sci. 454456 (2000) 692. Degroote, B., Dekoster, J., Langouche, G.: Surf. Sci. 452 (2000) 172. Feydt, J., Elbe, A., Meister, G., Goldmann, A.: Surf. Sci. 445 (2000) 115. Fichthorn, K. A., Scheffler, M.: Phys. Rev. Lett. 84 (2000) 5371. Gilarowski, G., Méndez, J., Niehus, H.: Surf. Sci. 448 (2000) 290. Helveg, S., Lauritsen, J.V., Lægsgaard, E., Stensgaard, I., Nørkov, J.K., Clausen, B.S., Topsøe, H., Besenbacher, F.: Phys. Rev. Lett. 84 (2000) 951. Hyldgaard, P., Persson, M.: J. Phys. Condens. Matter 12 (2000) L13. Jo, S., Gotoh, Y.: Surf. Sci. 464 (2000) 145. Jo, S., Gotoh, Y., Nishi, T., Mori, D., Gonda, T.: Surf. Sci. 454 (2000) 729. Kellogg, G.L., Plass, R.: Surf. Sci. 465 (2000) L777. Kim, S.K., Kim, J.S., Han, J.Y., Seo, J.M., Lee, C.K., Hong, S.C.: Surf. Sci. 453 (2000) 47. Kishi, K., Oka, A., Takagi, N., Nishijima, M., Aruga, T.: Surf. Sci. 460 (2000) 264. Knorr, N., Brune, H., Epple, M., Hirstein, A., Schneider, A.M., Kern, K.: (2000) to be published. Kolaczkiewicz, J., Bauer, E.: Surf. Sci. 450 (2000) 106. Konvicka, C., Jeanvoine, Y., Lundgren, E., Kresse, G., Schmid, M., Hafner, J., Varga, P.: Surf. Sci. 463 (2000) 199. Koschel, H., Held, G., Steinrück, H. P.: Surf. Sci. 453 (2000) 201. Lee, A.F., Wilson, K., Lambert, R.M.: Surf. Sci. 446 (2000) 145.

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3.3.1 Metals on metals Linderoth, T.R., Horch, S., Petersen, L., Helveg, S., Schønning, M., Lægsgaard, E., Stensgaard, I., Besenbacher, F.: Phys. Rev. B 61 (2000) R2448. Ling, W.L., Takeuchi, O., Ogletree, D.F., Qiu, Z.Q., Salmeron, M.: Surf. Sci. 450 (2000) 227. Losch, A., Niehus, H.: Surf. Sci. 446 (2000) 153. Markovic, N.M., Lucas, C.A., Climent, V., Stamenkovic, V., Ross, P.N.: Surf. Sci. 465 (2000) 103. Moore, D.P., Ozturk, O., Schumann, F.O., Morton, S.A., Waddill, G.D.: Surf. Sci. 449 (2000) 31. Mróz, S., Jankowski, Z., Nowicki, M.: Surf. Sci. 454-456 (2000) 702. Onishi, H., Sakama, H., Aruga, T., Kawazu, A., Iwasawa, Y.: Surf. Sci. 444 (2000) 7. Osing, J., Murphy, S., Shvets, I.V.: Surf. Sci. 454-456 (2000) 280. Passeggi, M.C.G., Prieto, J.E., Miranda, R., Gallego, J.M.: Surf. Sci. 462 (2000) 45. Pentcheva, R., Scheffler, M.: Phys. Rev. B 61 (2000) 2211. Piaszenski, G., Göbel, R., Jensen, C., Köhler, U.: Surf. Sci. 454-456 (2000) 712. Politi, P., Grenet, G., Marty, A., Ponchet, A., Villain, J.: Phys. Rep. 324 (2000) 271. Prieto, J.E., Rath, C., Heinz, K., Miranda, R.: Surf. Sci. 454-456 (2000) 736. Ramstad, A., Raaen, S., Barrett, N.: Surf. Sci. 448 (2000) 179. Repp, J., Moresco, F., Meyer, G., Rieder, K.H., Hyldgaard, P., Persson, M.: Phys. Rev. Lett. 85 (2000) 2981. Schaefer, B., Nohlen, M., Wandelt, K.: Surf. Sci. (2000) in press. Schieffer, P., Hanf, M.C., Krembel, C., G, G.: Surf. Sci. 446 (2000) 175. Shutthanandan, V., Saleh, A.A., Smith, R.J.: Surf. Sci. 450 (2000) 204. Tölkes, C., David, R., Krzyzowski, M.A., Zeppenfeld, P.: Surf. Sci. 454-456 (2000) 741. Vargoz, E., Rusponi, S., Boragno, C., Kern, K., Brune, H.: (2000) to be published.

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3.3.2 Metals on semiconductors

3.3.2 Metals on semiconductors V.G. LIFSHITS, K. OURA, A.A. SARANIN, A.V. ZOTOV

List of Abbreviations AED AES ARAES AR-EELS ARPES ARUPS CAICISS CDW CHCT CITS '& DAS EELS HCC HCT HREELS HT IAES ICISS IET IPES IR ISS KRIPES LEED LEEM LEIS LT LRO MBE MDS MEED MEIS ML MTL MTLTT NICISS PAC PEH PEM PES PYS QMS RAS RBS

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Auger electron diffraction Auger electron spectroscopy Angle-resolved Auger electron spectroscopy Angle-resolved electron energy loss spectroscopy Angle-resolved photoelectron spectroscopy Angle-resolved ultraviolet photoelectron spectroscopy Co-axial impact collision ion scattering spectroscopy Charge density wave Conjugate honeycomb chained trimer (model) Current imaging tunneling spectroscopy 'RXEOH ERQGHG FKDLQ PRGHO Dimer-adatom-stacking-fault (model) Electron energy loss structure Honeycomb chain-channel (model) Honeycomb chained trimer (model) High resolution electron energy loss spectroscopy High temperature Ion-induced Auger electron spectroscopy Impact-collision ion-scattering spectroscopy Inequivalent trimers (model) Inverse photoemission spectroscopy Infra-red Ion scattering spectroscopy K-Resolved inverse photoelectron spectroscopy Low-energy electron diffraction Low-energy electron microscopy Low-energy ion spectroscopy Low temperature Long range order Molecular beam epitaxy Metastable-atom de-exitation spectroscopy Medium-energy electron diffraction Medium-energy ion spectroscopy Monolayer Missing top layer (model) Missing top layer twisted trimer (model) Neutral impact collision ion scattering spectroscopy PerturEHG DQJOH FRUUHODWLRQ Photoelectron holography Photoelectrom microscopy Photoelectron spectroscopy Photoelectron yield spectroscopy Quadrupole mass spectroscopy Reflectance anisotropy spectroscopy Rutherford backscattering spectroscopy

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List of Abbreviations (cont.) RC RDS REM RHEED RMEED RS RT SAM SDR SE SEM SEXAFS SHG SIMS SPA-LEED SPE SPEM SRO SSM STM STS TDS TED TEM TPD TRAXS UHV UPS WFM XAS XPD XPS XRD XRR XSW

Ring cluster Reflectance difference spectroscopy Reflection electron microscopy Reflection high-energy electron diffraction Reflection medium-energy diffraction Raman scattering Room temperature Scanning Auger microscopy Surface differential reflectivity Spectroscopic ellipsometry Scanning electron microscopy Surface extended X-ray absorption fine structure Second harmonic generation Secondary ion mass spectrometry Spot profile analysis - low energy electron diffraction Solid phase epitaxy Scanning photoemission microscopy Short range order Surface stress measurements Scanning tunneling microscopy Scanning tunneling spectroscopy Thermal desorption spectroscopy Transmission electron diffraction Transmission electron microscopy Temperature-programmed desorption Total reflection angle X-ray spectroscopy Ultra high vacuum Ultra-violet photoelectron spectroscopy Work function measurements X-ray absorption spectroscopy X-ray photoelectron diffraction X-ray photoelectron spectroscopy X-ray diffraction X-ray reflectivity X-ray standing wave spectroscopy

3.3.2.1 Introduction 3.3.2.1.1 Outline The structures formed by submonolayers of metal adsorbates on the clean semiconductor surfaces under ultrahigh vacuum conditions have been studied extensively for about 40 years. A great body of various information has been accumulated in this field. This chapter presents an updated report of the structural data obtained for metal submonolayer films adsorbed on elemental semiconductors, Si and Ge. The data are limited to the early stages of adsorption, up to coverage of one or two monolayers. We consider mostly the ordered structures of adsorbates with a well defined periodicity. The formation of the disordered adsorbate layers, as well as the questions of growth of thicker silicide or metal films are not addressed in this chapter. The present data are based on publications which use well established experimental and theoretical techniques. Structural data for both low and high-Miller-index surfaces, if available, are presented. The chapter is divided into three main parts. The first part is represented by Table 2 which gives a list of the known structures with quoting the Landolt -Börnst ein New Series III/42A1

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adsorbate/semiconductor systems for which these structures have been observed. This information is complemented by the sketches of LEED patterns (reciprocal lattices) of the structures, as such data are used conventionally for their identification. The second and third parts contain the detailed data on the particular metal adsorbates on Si and Ge crystalline surfaces, respectively. Within sections metal adsorbates are listed in alphabetic order and surfaces are listed in order of increasing sum of Miller indices. The data for a given metal/semiconductor system are organized in the individual general table which has eight entries at most. These entries give, in order: the surface periodicity, usually deduced from LEED or RHEED patterns or in case of local domains from STM images; very schematic description of the preparation conditions, including typically adsorbate coverage and formation temperature; the structure model if available; the techniques by which the structural data have been obtained, i.e. particular experimental methods or theory; corresponding references; the numbers of the figures and additional tables where detailed structural parameters are given; possible remarks. The metal/semiconductor systems for which the data are present are listed in Table 1 with the references to the appropriate general tables. Beside the data containing in the general tables, the complementary information is given by figures and additional tables. Figures represent typically either the phase diagrams or the sketches of the structural models. In selected cases, mostly when the structure has not been yet established conclusively, the LEED patterns and/or STM images are given to illustrate the structure reciprocal lattice and/or real space features, respectively. The detailed structural parameters, e.g. coordinates of atoms or their displacements from ideal bulk positions, given in the additional tables, correspond typically only to the well established structures. 3.3.2.1.2 Metal adsorption and surface phase formation The specific feature of the metal adsorption on semiconductor surfaces is that in the most cases the substrate surface does not remain intact during adsorption. Only when adsorption occurs at relatively low temperatures, e.g. at room temperature, the basic structure of the clean semiconductor surface might be preserved under the adsorbate layer and the adsorbate atoms just decorate the original semiconductor surface. The 2×2 structure formed upon adsorption of Group III metals, Al, Ga or In, on the clean Si(100)2×1 surface at about 300-400 K furnishes such an example: The adsorbed metal atoms form dimers which reside in the troughs between original Si dimer rows of the Si(100)2×1 surface. However, metal adsorption at elevated temperatures, i.e. when the kinetical limitations are absent, is accompanied usually by substantial reconstruction of a semiconductor sustrate surface. As a result, the specific two-dimensional (1–2 ML thick) layer built of atoms of adsorbate and substrate is formed. As this layer has all main attributes of the phase, i.e. definite structure, composition, range of stability, etc., it is convenient to define it as a metal/semiconductor surface phase [94L1]. To characterize the composition of the surface phases on elemental semiconductors, Si or Ge, two parameters are used, metal coverage and density of substrate top atoms. The latter value is essentially the coverage of the Si or Ge layer above the substrate bulk. It reflects the fact that only a certain fraction of atoms constituting the top atomic layer of the substrate might be involved into the surface phase formation, while the surplus substrate atoms are liberated and migrate over the surface until they become trapped by the step edges or form the islands. An example is Si(100)4×3-In surface phase in which half of the top Si(100) substrate layer is missing, i.e. it has a Si coverage (top Si atom density) of 0.5 ML.

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Table 1. List of metal/semiconductor systems for which the data are present in this chapter (marked by bullets) with the references to the appropriate general tables. (hkl) denotes the high-Miller-index surfaces.

Adsorbate Ag Al As Au Ba Bi Ca Ce Co Cu Eu Ga Gd Hg In Mg Ni Pb Pd Pt Sb Sm Sn Sr Te Tl Yb

Substrate Si (100) (110) • • • • • •

• • • •

• •

• •

•

•

• • • • • • • • • • •

•

•

• • • •

(111) • • • • • • • • • • • • • • • • • • • • • • • • •

(hkl) • •

•

•

Table 3 6 10 14 16 17 21 22 23 24 25 26 31 32 33 37 38 39 43 44 45 48 49 51 52 53 54

Ge (100)

(110)

(111) • • • •

•

(hkl) •

Table 55 56 57 58

•

60

•

63

•

•

•

64

•

•

•

65

•

•

66

•

69

•

71

•

•

Each surface phase exists in a certain temperature and adsorbate coverage interval where it basically preserves its structure. Though an ideal surface structure is associated with a definite metal coverage, it remains stable with a certain variation of its composition through the formation of such structural defects as vacancies, adatoms and domain boundaries. However, when the variation of the composition is beyond the region of stability of a given surface phase, another structure might form. This process usually takes place in selected local regions. So, it is quite a common situation that the domains of different surface phases coexist at the surface. Variation in temperature can also induce the phase transitions. These transitions might be reversible (e.g., Ge(111) 3 × 3 -Pb ↔ Ge(111)3×3-Pb at about 200 K) or irreversible (e.g., Si(100)2×2-In → Si(100)4×3-In at about 500 K). In the most cases the irreversible transitions involve besides a reordering of the adsorbate atoms also a substantial reconstruction of the top substrate layer. Surface phases manifest themselves in many surface-related phenomena. The most evident example is the Stranski-Krastanov mode of thin film growth which is typical for majority of the metal adsorbates on semiconductors. According to this growth mode, the first stage of growth consists in the formation of the twodimensional adsorbate layer (or Stranski-Krastanov layer) usually 1–2 ML thick which is essentially the surface phase. The sequential formation of several surface phases with increasing of adsorbate coverage is also possible (and indeed occurs often) at this stage. In the latter case, the completion of the two-dimensional layer growth Landolt -Börnst ein New Series III/42A1

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corresponds to the formation of the last surface phase with the maximum possible coverage of adsorbate atoms. Afterwards, i.e. when no additional adsorbate atoms can be adopted into the surface phase, the growth of threedimensional islands of adsorbate takes place. In case of the growth at relatively high temperatures, the completion of the last surface phase is accompanied usually by the steep decrease of a sticking probability of adsorbate atoms to the surface. Another glowing example refers to a surface diffusion since surface diffusion of adsorbate is often accompanied by the formation of surface phases with diffusion coefficient being different for various surface phases. The adsorbate bonding energy also usually varies from one surface phase to another which should influence the such surface process as desorption: The desorption proceeds through the formation of the surface phases in the sequence opposite to that observed during adsorption and desorption kinetics and/or desorption activation energy might be noticeably different for various surface phases. The difference in the composition and atomic structure of surface phases results naturally in the variation of electronic structure, surface reactivity and different physical properties.

3.3.2.1.3 Notations for surface structures and adsorption sites The geometric structure is used conventionally for identification of a particular metal/semiconductor surface phase. The surface structures are usually labeled in accordance with their periodicity with respect to the underlying semiconductor crystal plane. Two methods for the description of the two-dimensional lattices are used conventionally. The first one was proposed by Park and Madden [68P] and it consists in the determination of the matrix which establishes a link between the basic translation vectors of the surface under consideration and those of the ideal (unreconstructed) substrate surface. That is, if a and b are the basic translation vectors of the substrate lattice, while as and bs are the basic translation vectors of the surface phase, than they can be linked by the equations as = G11a + G12b bs = G21a+ G22b where four coefficients, G i j , form the required matrix G12  G  . G =  11  G 21 G 22  The values of G i j determine whether the structure of the surface phase is commensurate or incommensurate. The commensurability means that a rational relationship between the vectors as, bs and a, b can be established. If there is no rational relationship between the unit vectors of the surface phase and the substrate, the surface phase is incommensurate one. In other words, the incommensurate superstructure is registered in-plane incoherently with underlying substrate lattice. The second method for the description of the two-dimensional lattices was proposed by Wood [64W]. This method is the most-used one now though it is less versatile than the above matrix notation. In the Wood description, the ratio of the surface periods and those of the unreconstructed substrate are given as well as an angle of rotation which makes the unit mesh of the surface to be aligned with the basic translation vectors of the substrate. That is, if a certain adsorbate A induces on the B(hkl) surface the reconstruction with the basic translation vectors of |as| = m |a| and |bs| = n |b| and with the rotation angle of ϕ, then this surface structure is labeled as B(hkl)m×n–R ϕ°–A The surface mesh may be centered, in which case the notation would be c(m×n).

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[Ref. p. 396

Fig. 1. Basic translation vectors for the (a) (100), (b) (110) and (c) (111) surfaces of Si and Ge.

It should be remarked that the Wood description is appropriate only in the cases when the rotation angle is the same for the both basic translation vectors (i.e. for as and bs with relation to a and b, respectively). Otherwise, the matrix notation only can provide an accurate description. However, in the literature sometimes less rigorous notation (of the Wood's-like type) is used with the addition of the necessary comments or even without them when it concerns the well-known structures. An example is the clean Si(110) surface. The reconstruction of this   surface is properly described in matrix notation as Si(110)  172 12  (or as Si(110)(2,2)×(17,1)). However it is   often labeled as Si(110)"2×16" with quotes indicated that it is not exactly the 2×16 structure (the unit mesh of the "2×16" structure is non-rectangular in contrast to the rectangular (1×1) mesh). Another example of using quotes to outline that the notation of the structure is not strict is the "1×1" structure. This notation corresponds usually to the surface which is lack of well-defined long-range order and displays an apparent 1×1 LEED (RHEED) pattern. The possible choice of the basic translation vectors is not unique. Therefore, for the sake of definiteness Fig. 1 shows the basic translation vectors for the unreconstructed (100), (110) and (111) planes of Si and Ge as used in the present chapter. These vectors are a = 1/2[011 ] and b = 1/2[011] for Si(100) and Ge(100); a = 1/2[1 10] and b = [001] for Si(110) and Ge(110); As a = 1/2[1 01] and b = 1/2[11 0] for Si(111) and Ge(111). the lengths of the basic translation vectors for the (110) plane are not equal to each other (unlike the cases of the (111) and (100) surfaces), the sequence of writing the unit vectors is especially critical for the definite description of the surface structures on the Si(110) and Ge(110) surfaces. The ordered surface structures are formed by adsorbate atoms occupying the regular sites on the semiconductor surfaces. The great variety of atomic arrangements exists even for the structures with a similar periodicity. For example, in the Si(111) 3 × 3 -R 30° structure of Group III atoms, Al, Ga or In, there is one adatom in each ( 3 × 3 ) unit cell as shown in. The arrangement of the Si(111) 3 × 3 -R30° of Sb or Bi includes the adsorbate trimers. The Si(111) 3 × 3 -R30°Ag surface has a complicated honeycomb-chainedtrimer structure. So there are a lot of possible bonding sites for adatoms on the semiconductor surfaces and it seems unlikely to be possible to label them all. However there are several main bonding sites which have the conventions. In case of the (111) surface, these are "on-top", H3 and T4 sites (see Fig. 2). In the "on-top" position, adatom resides directly above the substrate atom of the top (111) layer. In both H3 and T4 sites, the adatoms is situated in the center of the triangle formed by three top substrate atoms, but, in the case of H3 site, it is above the atom in the fourth substrate layer and, in the case of T4 site, it is above the atom in the second layer of the substrate. Adatom has three neighbor substrate atoms in the H3 position and four ones in the T4 site. The labeled adsorption sites on the reconstructed Si(100)2×1 and Ge(100)2×1 surface are "bridge", "valley bridge", "cave" and "pedestal" sites. These adsorption positions are shown in Fig. 2. To our knowledge, there are no yet commonly accepted terms for the adsorption sites on the Si(110) surface.

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Fig. 2. The various possible adsorption sites on (a) reconstructed (100)2×1 and (b) unreconstructed (111)1×1 surfaces of Si and Ge.

3.3.2.1.4 Preparation The most crucial requirement for the preparation and characterization of adsorbate structures on semiconductors is the ultra-high vacuum conditions with vacuum of the order 10–10 Torr or better. Another essential requirement is the cleanliness of the original semiconductor surface on an atomic level. There have been a great number of investigations devoted to the design of cleaning procedures and this question is widely discussed in the literature. Three main techniques are typically employed for cleaning semiconductor surfaces, namely, annealing, ion bombardment followed by annealing and cleavage. It should be remarked that except for the case of cleavage, before the insertion into the UHV chamber the sample has undergone several stages of ex situ treatment, including mechanical and chemical polishing to produce the mirror smooth surface, chemical cleaning to remove the organic contaminations, etc. Once the atomically-clean surface is obtained, the deposition of small amounts of certain adsorbate is conducted to form the desired surface phase. The most of solid substances are deposited by thermal evaporation in vacuum. Depending on many circumstances (the temperature of evaporation of material, the reactivity of material, the experimental design, etc.) the great variety of different sources for deposition of adsorbates are employed. The tungsten filament or basket with a melted drop of the material to be evaporated represented the simplest deposition sources. These kinds of sources, though quite sufficient for qualitative investigations, do not provide possibility to maintain the reproducible calibrated rate of deposition. So, in modern UHV installations, the effusion cells are employed conventionally which enable the stability and control of the deposition rate. Selected elements have the sufficiently high vapor pressure at temperatures below the melting point (e.g. Pd, Fe, Ni) and they can be evaporated via sublimation by passing electrical current directly through the wires or bars made of these materials. In some cases, electron-beam and laser-radiation sources are employed for deposition of adsorbates. The most essential parameters for the preparation of a given surface phase are the adsorbate coverage and the substrate temperature during deposition or annealing of the adsorbate layer. (It should be remarked however that in selected cases the deposition rate and/or duration of annealing might play essential role also). The information on the surface structure formation is presented generally in the form of the phase diagrams built in the "coveragetemperature" coordinates, like the schematic on shown in Fig. 3 as an example. The formation phase diagrams can be determined in the several kinds of experiments. First, the adsorbate can be deposited onto the Si substrate held at a fixed temperature. After each deposition step, the surface reconstruction is elucidated (usually, by means of LEED or RHEED). The set of experimental dots in a such measurement occupies the pass A on Fig. 3. This procedure if being repeated at various temperatures provides the possibility for the determination of the coverage and temperature ranges at which the certain surface phases exist. The graphic presentation of these data produces the required formation phase diagram.

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[Ref. p. 396

Fig. 3. Schematic "formation phase diagram". The regions of the occurrence of the surface structures 1, 2, 3, 4 are shown by solid lines. The passes A, B, B′, C (dashed lines) illustrate the different kinds of experiments (see the text).

Second, the adsorbate film of a fixed coverage can be formed at room temperature and then annealed at the progressively higher temperatures. In this case, the LEED (RHEED) patterns are recorded and indexed after each annealing step (see pass B on Fig. 3). The annealing at high temperatures might result in the noticeable desorption of the adsorbate atoms from the surface (sometimes, in the diffusion of adsorbate into the bulk) and the actual pass (marked as B′) should deviate from the vertical pass B towards the lower coverages. So, it is essential to distinguish between the case when the "coverage" axis shows the adsorbate coverage of a RT deposit before annealing (e.g., determined by means of quartz-crystal thickness monitor) and the case when the coverage of adsorbate left at the surface after annealing (e.g., determined by means of AES) is shown. In the latter case, the experiments might involve the RT deposition of a "thick" film followed by annealing to cause the isothermal desorption of the adsorbate. It is obvious that the corresponding pass (marked by C) is opposite to the pass A. Another essential remark is as follows: In the most investigations, before the LEED (RHEED) observations the heating is interrupted and the sample is cooled to room temperature. However, sometimes the LEED (RHEED) observations are performed with the "hot" sample. The ability to detect the reversible phase transformations on the surface upon heating/cooling is unique to this kind of experiments. Thus, when giving the particular formation phase diagram, we present it with the brief comments which clarify its origin.

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3.3.2.1.5 Techniques of surface characterization A great variety of methods is used at present for characterization of the structure and properties of the adsorbate/semiconductor structures. Each one of the methods is specifically sensitive to one (or several) features of the surface phase. Therefore, the combination of several complementary techniques is usually required for obtaining conclusive reliable information. Among other techniques, LEED is the most widely used routine method for structural analysis. LEED technique primarily gives information about the size and shape of the twodimensional surface unit mesh and, thus, serves often for initial identification of a given surface structure. Consideration of the surface atomic arrangement on the basis of the LEED data is also possible. Usual procedure involves in the latter case the comparison of the I-V (reflection intensity versus electron energy) curves calculated for a given atomic model with those recorded in the LEED experiment. However, the kinematics theory is no longer valid for the LEED due to the multiple-scattering effects. This imposes the serious computation problems associated with the dynamical calculations. Another kind of electron diffraction widely used for surface analysis is RHEED. The usual geometry of RHEED, when the electron gun and phosphorescent screen are positioned remotely from the substrate, makes it possible to observe the RHEED patterns during growth processes. In the first approximation, the results of RHEED experiments can be considered by utilize the kinematic theory. However the more accurate consideration of the surface atomic geometry requires the complicated dynamic analysis, as in the case of LEED. These difficulties are essentially avoided in the TED and XRD since the singlescattering approximation of diffraction can be more safely assumed in TED and XRD than in LEED and RHEED. As a result, TED and XRD are widely used now for determination of the precise coordinates of atoms constituting the surface structure. The surface chemical composition is characterized in the most cases by AES. It provides possibility to detect all except H and He elements of the Periodic Table presented in a few near-surface atomic layers. Thus, AES is widely employed for quantitative estimations of the adsorbate coverages. The changes in the electronic structure of the system with adsorption are also manifested in the changes in Auger spectra. For characterization of the surface electronic structure a great variety of spectroscopy techniques are employed, e.g. UPS, XPS, ARPES, IPES, surface optical spectroscopies, etc. These data are used for structural analysis indirectly by comparing them with the results of theoretical calculations. ISS and its modifications are powerful methods for the surface structure determination. The primary advantages of ion scattering methods is that, first, they provide detailed structural information in real space rather than in reciprocal space as for diffraction or photoemission, and, second, long-range order at the surface is not necessary for ISS analysis. However, it should be noted that the interpretation of results from ISS is strongly model-dependent. In recent years STM has steadily established its footing and is now becoming an indispensable tool for surface analysis. It provides the possibility for real-space imaging of the geometric and electronic properties of surfaces on a scale which extends to atomic dimensions. The structural information obtained by this method is unique as the most surface probes average the information on a large number of atoms. STM combined with other complementary surface-sensitive techniques has permitted the reliable elucidation of a great body of the surface structures. However, it is well understood now that, as STM is sensitive to the electronic structure of the atoms, the atomic features seen by STM do not always simply allow the determination of atom positions in the outermost layer of a sample surface. The structural data reported in the chapter are based in the most cases on the results obtained by a variety of techniques. When presenting a particular structural model of a given surface phase we specify the data of which techniques have been used for the model construction. The references on the information obtained with a particular experimental technique are given in the general tables. Experimental techniques are quoted generally with the names used in the original papers. Very often different nomenclatures correspond to very similar, if not identical, techniques. All theoretical results on the structure and properties of the surface phases are referred to simply as "theory", without specification of the different approaches.

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[Ref. p. 396

3.3.2.2 List of structures

Table 2. List of the ordered commensurate structures formed at adsorption of metallic adsorbates on the main Si and Ge surfaces, (100), (110) and (111). The sketches of the corresponding LEED patterns are shown, since they are used conventionally for the identification of the surface structure. The sketch of a LEED pattern from a single-domain structure coincides with the sketch of the superstructure reciprocal lattice superposed on the reciprocal lattice of the corresponding unreconstructed surface. Thus, the extrareflections (shown by solid circles) correspond to the superstructure reciprocal rods, while the main reflections (shown by open circles) to those of the unreconstructed 1×1 surface. The sketch of a LEED pattern from a multi-domain structure represents the superposition of the LEED patterns produced by domains in all possible equivalent orientations. N denotes the number of possible equivalent domains. Structure

Si surface

Ge surface

Single-domain

Multi-domain

N

(100) substrate orientation 2

2×1

Ag, As, Ba, Sb Bi, Pb, Sb, Sr

2×2

Ag, Al, Au, In, Pb Bi, Cu, Ga, In, Mg, Pb, Sn

3×1

Sr

2

3×2

Ag, Al, Ba, Ga, In, Mg, Sm, Sr

2

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Table 2 (cont.) Structure

Si surface

Ge surface

Single-domain

Multi-domain

N

In

3×3

2

4×1

Al, Pb, Sn

4×2

Ba

Pb

2

4×3

In

In

2

5×1

Au, Sn, Sr

Pb

2

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[Ref. p. 396

Table 2 (cont.) Structure

Si surface

5×2

Al, Ga, In

Ge surface

Multi-domain

N 2

In

5×4

Single-domain

2

6×2

Sn

2

7×1

Al

2

7×5

Ga

2

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Table 2 (cont.) Structure

Si surface

Ga

7×6

8×2

Bi

c(4×2)

Ag, Pd, Pt

c(4×4)

As, Bi, Pb, Sb, Sn

c(6×2)

Ag, Ba

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Ge surface

Single-domain

Multi-domain

N 2

2

Pb

2

2

272

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 2 (cont.) Structure

Si surface

Ge surface

c(6×4)

Pd, Pt

2

c(8×2)

Au

2

c(8×4)

Pb, Sn

c(12×4)

Al

2

c(12×10)

Cu

2

Pb

Single-domain

Multi-domain

N

2

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273

Table 2 (cont.) Structure

Si surface

3 0   1 2

Ge surface Pb

(110) substrate orientation 1×2

Al, Au, Co, Cu, Ni, Sb

2×5

Au

3×2

Bi, Sb

3×4

Cu, In

5×4

Ag, Cu, Ni

6×6

Cu

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Sb

Single-domain

Multi-domain

N 4

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[Ref. p. 396

Table 2 (cont.) Structure

Si surface

Ge surface

Single-domain

Multi-domain

N

8×5

Co

11×2

Co

c(2×8)

Sn

c(4×4)

Sn

c(18×6)

Sn

 2 1    − 3 3

Al, Ga, Sn

2

 3 0    − 1 3

Sn

2

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275

Table 2 (cont.) Structure

Si surface

 2 1   − 4 4

Ga

2

 3 0    −1 4

Al

2

 4 0    − 1 3

Au

2

 4 3   − 2 2

In, Pb, Sn

2

 14 0     − 2 1

Sb

2

 3 3    0 5

As

2

 3 3   − 4 2

Sn

2

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Ge surface

Single-domain

Multi-domain

N

276

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 2 (cont.) Structure

Si surface

 6 4   − 3 2

In

Ge surface

Single-domain

Multi-domain

N 2

(111) substrate orientation 1×1

As

As

2×1

Sb, Yb

Sb, Cu

2×2

Ca, Ce, Eu, Gd, In

Al, Au, Sn

3×1

Ag, Ba, Eu

Ag

3

3

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277

Table 2 (cont.) Structure

Si surface

3×2

Ca, Mg, Sm, Yb

Ge surface

Single-domain

Multi-domain

N 6

Sn, Pb

3×3

4×1

Cu, In

3

4×2

In

3

4×4

In

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Ag, In

278

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 2 (cont.) Structure

Si surface

Ge surface

5×1

Ba, Ca, Eu, Sm, Yb

Ag

5×2

Ag, Au

Single-domain

Multi-domain

N 3

6

Sn

5×5

6×1

Ag

6×6

Ag, Au, Tl

3

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Ref. p. 396]

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279

Table 2 (cont.) Structure

Si surface

7×1

Ca, Sm, Yb

8×2

Ba

Single-domain

Multi-domain

N 3

Ag

3

Al

10×10

11×11

Ga

c(6×2)

Ca, Mg

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Ge surface

6

280

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 2 (cont.) Structure

3 × 3 -R30°

Si surface

Single-domain

Multi-domain

N

Ag, Al, Au, Ag, Au, Bi, Bi, Ce, Ga, Pb, Sn Hg, In, Ni, Pb, Pd, Pt, Sb, Sn

2 3 ×3 − 4 0    −1 2 

Sn

2 3 × 2 3 -R30°

Au, Pd, Sn

3 3 × 3 3 -R30°

Pd

4 3 × 4 3 -R30°

Ge surface

3

In

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Ref. p. 396]

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281

Table 2 (cont.) Structure

Si surface

5 3 × 5 3 -R30°

Sb

7× 3  3 1    2 1

In

7 ×4  2 1    0 4

7 × 7 R ± 19.1°

Ag, Al, Co, Pt

Single-domain

Multi-domain

N

3

Sn

3 7 × 3 7 R ± 19.1° Sn

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Ge surface

3

2

2

282

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 2 (cont.) Structure

Si surface

Ge surface

Single-domain

Multi-domain

N

13 × 13 R ± 13.9° Co

2

19 × 19 R ± 23.4° Ni

2

21 × 21 R ± 10.9° Ag

2

2 21 × 2 21 R ± 10.9°

31 × 31 R ± 8.9°

2

Au

In

In

2

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Ref. p. 396]

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283

Table 2 (cont.) Structure

Si surface

61 × 61 R ± 26.3°

Ge surface

In

Single-domain

Multi-domain

N 2

91 × 3 10 1    1 2

Sn

3

2 91 × 2 91 R ± 5.2°

Sn

2

133 × 4 3 9 13   4 − 4  

Sn

3

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[Ref. p. 396

3.3.2.3 Metallic adsorbates on silicon 3.3.2.3.1 Ag (Silver) on Si

Table 3. Structures formed by Ag on Si(100), Si(110) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

< 0.1 ML, RT

isolated Ag RHEED adatoms AES PES STM THEORY

88S 88S 88S 89S3, 90H3 93Z3, 97O

4a

0.8 ML, RT

parallel Ag LEED dimers STM

93L6, 96G3 93L6, 96G3

4b

0.5 ML, RT

orthogonal RHEED Ag dimers ? STM

88S 94W1

Table Remarks

Ag/Si(100) 2×1

2×2

0.8–1.0 ML, RT parallel Ag LEED dimers ? STM

93L6, 96G3 93L6, 96G3

1.0 ML, RT

98P1

c(4×2)

< 0.5 ML, RT

3×2

∼ 0.5 ML, ∼ 770 K

c(6×2)

≥ 0.6 ML RT deposit. + ann. ∼ 350 K

orthogonal CAICISS Ag dimers ? STM Ag adatoms LEED on Si rows ? AES STM LEEM MEIS XPD XPS ARPES LEED XPD XPS ARPES

small domains <30 Å, no LRO

94W1 93L5, 94M3, 94W1 94W1, 94W3 93L5, 94W1 94M3 94M3 95S7, 98S8 95S7 98Y1 95S7 95S7 95S7 99M6

small domains 5

lack of mirror symmetry in the period doubled direction; top Si atom density 0.5 ML [94M3]

single domain

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Table 3 (cont.) Structure

Preparation

Model

Technique Ref.

Fig.

0.14–0.44 ML, 770 K

RHEED TRAXS

92Y6 92Y6

6

5×4

0.44–1.0 ML, 770 K

RHEED TRAXS

92Y6 92Y6

6

1×1

> 0.14 ML, > 900 K

RHEED TRAXS

92Y6 92Y6

6

Table Remarks

Ag/Si(110)  9 3    − 9 3

irreversibly transforms to 1×1 at 930 K

Ag/Si(111)

3× 3

1 ML, 520–820 K

HCT

IET

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LEED

72B, 78L, 78W, 7, 8 81L1, 81S3 LEED I-V 89F3, 93J2, 93O4 RHEED 78G, 83H, 83L1, 85H, 87H1, 89I5 TRAXS 87H1 AES 78L, 78W, 81S3, 83L1, 87H1, 91R2, 92R1 PES 78W, 81H2, 87K3, 89M1, 93H8 ISS 81S3, 88O1, 93C4 CAICISS 88P3, 89C1, 89S1, 89W, 91K2 MEIS 84L, 89C2 STM 87W1, 91T2, 92S6, 92W3, 93O2, 93W6, 99S3 SEXAFS 82S, 83S XRD 88T, 89V1, 91T3, 93T2 XRR 96T3 XSW 91V, 96W2 XPD 89K2, 93H6, 96C3, 96C4 THEORY 91D1, 94K3, 95K1 STM 99A4 THEORY 99A4 9

4

5

saturating phase; top Si atom density 1 ML

286

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 3 (cont.) Structure

Preparation

6×1

0.33 ML, HCC or desorption of Ag DπC ? from 3 × 3 at 870–970 K

Model

Technique Ref.

Fig.

LEED 78L, 81S3 7, 10 LEED I-V 90F1, 93J2 RHEED 77I2, 78G, 80I, 87H1 TRAXS 87H1 AES 78L, 81S3, 87H1, 90F1, 91R2, 92R1, 94F2 PES 99L2 ISS 81S3, 93C4 STM 93W6, 94F2, 95C3, 99S2, 99S3 TED 98C3 THEORY 98E2

7× 7

0.7–0.85 ML RT deposit. onto cleaved Si(111)2×1

LEED AES PYS UPS

21 × 21

0.14–0.2 ML deposit. onto

RHEED 95Z4, 96L1, 97T2 11, 12 STM 98T1 THEORY 99A3

3 × 3 -Ag at 150–250 K 6×6

0.18–0.5 ML deposit. onto

Table Remarks transforms to 3×1 at T > 490 K; top Si atom density 4/3 ML [99S3]

81B, 83L1, 83L2 81B, 83L1, 83L2 81B 82R

RHEED

95Z4, 97T2

LEED STM

93W6 93W6

11, 13

3 × 3 -Ag at 90–150 K 5×2

0 3 ML, 870–1020 K

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Ref. p. 396]

3.3.2 Metals on semiconductors

287

Table 4. Structural parameters of the HCT model of the Si(111) 3 × 3 -Ag reconstruction obtained by various surface techniques. Symbols refer to Fig. 8, lengths are in Å. XRD [93T2] z-Ag z-Si1 Z(Ag-Si) ∆z-Si2 ∆z-Si3a ∆z-Si3b ∆z-Si4a ∆z-Si4b x-Ag ∆x-Si1 ∆x-Si2 Ag-Ag (a) Ag-Ag(b) Ag-Si1(c) Ag-Si1(d) Si1-Si1(e)

3.05(2) 2.25(2) 0.80(2) –0.00(2) 0.09(2) –0.26(5) 0.05(2) –0.18(4) 2.85(1) –0.88(3) –0.09(4) 4.94(2) 3.43(1) 2.59(3) 2.60(1) 2.31(5)

a

XRD/XSW CAICISS RHEED LEED [89V1, 91V] [91K2] [89I5] [93O4] 3.05(2) 2.26 a) 0.80

2.87(1) –0.88(3) –0.08(2) 4.97(2) 3.42(1) 2.57(5) a) 2.63(6) a) 2.32(4) a)

) From Table 1 of Ref. [91D1]. ) ∆z-Si3a – ∆z-Si3b. c ) ∆z-Si4a – ∆z-Si4b. d ) From the Si(111) reflecting-plane. b

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2.90 2.15(7) 0.75(7) –0.08 0.19 0.39 0.17 –0.34 2.94 –0.20 5.10(2) 3.39 2.61

2.95(5) 3.00(3) 2.20(10) 2.31(4) 0.75(10) 0.69(3)

LEED XPS SEXAFS [93J2] [96C3] [96W2] 3.35 d) 2.41 d)

3.14 2.35

THEORY [91D1] 3.15 2.30

0.85(6)

0 20(5) b) 0.14(7) c) 3.06

5.38 3.36

2.70(50)

2.80(12)

3.45(12) 2.41(20)

2.80 –0.77 4.98 3.42 2.54 2.61 2.35

4.93(10)

2.77(25)

4.84 3.45 2.54 2.60 2.51

288

3.3.2 Metals on semiconductors

[Ref. p. 396

Si(100)2x1-Ag

Si(100)3x2-Ag

Fig. 4. Top view of the structures formed by RT Ag deposition onto the Si(100)2×1 surface. The Si substrate dimerization is assumed to remain intact during RT Ag/Si(100) interface formation [92K2]. (a) structural model for isolated Ag adsorption. Ag atom occupies position between two Si dimers. Buckling of Si dimers is shown by variation in the size of the dimer atoms [93L6]. Ag-Si bond length is 2.93 Å and adsorption height 1.12 Å [93Z3]. (b) Possible structural model for Si(100)2×1Ag reconstruction at θAg = 0.5 ML [94W1]. Ag atoms are shown in black and Si atoms in white.

Fig. 5. Possible structural model for the Si(100)3×2-Ag reconstruction as proposed on the basis of STM and LEEM data. The unit cell containing 3 Si and 3 Ag atoms is composed of a well ordered 2×1 part due to the Si in it and a decorating Ag part, which induces the periodtripling and the unit cell asymmetry [94M3]. However, recent data indicate that Ag coverage for the 3×2-Ag phase should be larger than 0.6 ML [95S7, 98Y1].

Ag/Si(110)

Ag/Si(111)

Fig. 6. Phase diagram of the Ag/Si(110) system as determined by RHEED-TRAXS [92Y6].

Fig. 7. Phase diagram of the Ag/Si(111) interface as determined using LEED-AES [83L1].

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Ref. p. 396]

3.3.2 Metals on semiconductors

Si(111) 3 × 3 -Ag

289

Si(111) 3 × 3 -Ag

Fig. 9. Schematic illustrations of the (a) HCT and (b) inequivalent triangle (IET) models for the atomic structure of the Si(111) 3 × 3 -Ag surface. Solid lines indicate the unit cell, and dashed lines represent chained Ag triangles. In (c) the definition of ϕAg and ϕSi are illustrated. Structural parameters are given in Table 5 [99A4].

Fig. 8. Top and side views of the HCT (honeycomb

3 × 3 -Ag chained trimer) model for the Si(111) surface. Ag atoms (large black circles) with 1 ML coverage replace the top Si layer and form HCT with the interatomic distance of 3.43±0.01 Å. The Si atoms with 1 ML coverage in the layer below are displaced by 0.88±0.03 Å to form trimers. The formation of the Si trimers satisfies two of the three dangling bonds while the remaining dangling bond is satisfied by bonding with an Ag atom. The Si trimer formation greatly distorts the substrate lattice and large relaxation down to the seventh atomic layer are present to relieve the strain. A vertical buckling of the third and fourth Si layers are 0.35±0.05 and 0.23±0.04 Å, respectively. Detailed structural parameters of the HCT model are presented in Table 4. The third and forth Si layers are split into two planes, a and b , in normal direction [93T2].

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Table 5. Structural parameters calculated for the HCT (Fig. 9a) and IET (Fig. 9b) models of the Si(111)

3 × 3 -Ag surface. It is found that a structural model consisting of two inequivalent Ag triangles is energetically more favorable than well established HCT model. IET model is confirmed by low-temperature STM observations. For the definition of ϕAg and ϕSi, see (Fig. 9c). Symbols refer to Fig. 8 [99A4]. Parameter

HCT

IET

ϕAg [°] ϕSi [°] Ag-Ag (a) [Å] Si1-Si1 (e) [Å] z-Ag [Å] z-Si [Å]

60 60 4.88 2.55 3.02 2.29

54 66 4.88 2.58 3.01 2.29

290

3.3.2 Metals on semiconductors

[Ref. p. 396

Si(111)3x1-Ag

Ag/Si(111)

Fig. 10. Structural models proposed for the Si(111)3×1Ag reconstruction: (a) honeycomb chain-channel (HCC) model [98E2] and (b) double π-bonded chain (DπC) model [99S2]. In the HCC model, the top Si atoms are almost in the same plane and form a planar honeycomb chain stabilized by the Si double bonds. The metal atoms are suggested to be located in the channels and are assumed to be in the two-fold coordination. DπC model incorporates two Si π-bonded chains in the faultedunfaulted orientations connected through Ag atoms. Both models have the same Ag coverage of 1/3 ML and top Si atom density of 4/3 ML, but differ in the metal adsorption site. 6×1 periodicity is explained by a distortion of the surface-layer Si atoms in which the chain hexagons are rotated by 7° in the HCC model or by interaction between edge Si atoms belonging to the neighboring rows in the DπC model [99S2].

Fig. 11. A phase diagram determined by RHEED observations during Ag deposition onto the Si(111) 3 × 3 -Ag surface at fixed temperatures. The (1/3, 1/3) spots were always observed in the ranges covered by this diagram [97T2].

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Ref. p. 396]

3.3.2 Metals on semiconductors

291

Si(111) 21 × 21 -Ag

Fig. 12. Tentative models of the Si(111) 21 × 21 R(±10.89°)-Ag reconstruction. This reconstruction is

3 × 3 -Ag formed by Ag adatoms on the intact surface. Adatom coverage is (a) 0.19 ML (4 adatoms per 21 × 21 (5 adatoms per

unit

cell),

[98T1],

(b)

0.24

ML

21 × 21 unit cell) [94N1], (c) 0.14

ML (3 adatoms per 21 × 21 unit cell), [94I5]. Models (b) and (c) were proposed originally for Au induced

21 × 21 phase.

Si(111)6x6-Ag

Fig. 13. Tentative models of the Si(111)6×6-Ag reconstruction. This reconstruction is formed by Ag adatoms on the intact

3 × 3 -Ag surface. Adatom

coverage is 0.33 ML (12 adatoms per cell) [95Z4].

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21 × 21 unit

292

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.2 Al (Aluminum) on Si Table 6. Structures formed by Al on Si(100), Si(110), Si(111) and Si(112) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Al/Si(100) 0.5 ML, 2×2 300–620 K

Model

Technique

parallel symmet- LEED ric Al dimers LEED I-V AES XPS ARPES STM THEORY

3×2

0.15–0.35 ML, parallel symmet- LEED 470–620 K ric Al dimers AES XPS STM THEORY

Ref.

Fig.

Table Remarks

89I3, 91N2, 92M, 95Y3, 95Y6, 96Y4 92M, 93S8, 94S7 89I3 95Y3, 95Y6 91N5, 94Y4, 95Y6, 96Y7 91N2, 93I3, 94I1, 95Z1 88R3, 91N5, 92A, 92B2, 93B2, 94B1, 94K6

14, 15, 7, 8 16

89I3, 91N2, 92M, 95Y3, 95Y6, 96Y4 89I3 95Y3, 95Y6 91N2, 93I3, 94I1, 95Z1 88R3, 91N5, 92A, 92B2, 93B2, 94B1, 94K6

14, 15, 7, 8 16

5×2

0.35–0.45 ML, parallel symmet- LEED 470–620 K ric Al dimers AES XPS STM THEORY

89I3, 91N2, 92M, 95Y3, 95Y6, 96Y4 89I3 95Y3, 95Y6 93I3 88R3, 91N5, 92A, 92B2, 93B2, 94B1, 94K6

14, 15, 7, 8 16

4×1 (4×5)

0.3–0.4 ML, 670 K

LEED LEED AES

92M 89I3 89I3

14

7×1

0.3–0.4 ML, 770 K or 0.4–0.45 ML, 970 K

LEED AES

89I3, 92M 89I3

14

4×5 (LEED) corresponds probably to 4×1 + 5×2

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Ref. p. 396]

3.3.2 Metals on semiconductors

293

Table 6 (cont.) Structure

Preparation

Model

Technique

Ref.

Fig.

c(12×4)

0.5 ML, 870 K

ordered array of Al (Al-Si) clusters containing 4–8 Al atoms?

LEED AES STM

89I3, 95Z2 89I3 94I1, 94I3, 95S1, 95S8, 99Z2

14

LEED AES

92Z4, 94L5 92Z4, 94L5

17

Al/Si(110) 0.5–1.0 ML, 2×1 720 K  3 0    −1 4

0.15–0.3 ML, 770–1050 K

LEED AES

92Z4, 94L5 92Z4, 94L5

17

 3 3    − 2 1

0.3–0.5 ML, 770–1050 K

LEED AES

92Z4, 94L5 92Z4, 94L5

17

RHEED

92Y4

LEED

64L, 65L, 73B, 84C2, 18, 85U, 86K2, 88H5, 19, 92N3, 94K7 38 90H1, 92N3 95H2, 97H3 84C2, 89I4, 94K7 87A 81H1, 81H2, 91A2 85U, 86K2 81H1, 81H2, 85K2, 85U, 86H5, 86K3 87N2 85K3, 86K2, 89L3, 91G3 91L2, 93A3 89D4 88H4, 88H6, 88H7, 89H4, 93T3, 94Y3 88H4, 88H6, 88H7, 89H4 93W5 77C1, 84N, 88N6, 88R2, 89N2, 90R3, 91T4, 92I2, 93I2, 97K3, 99W1

Table Remarks in STM: c(2n×4), mostly n = 6, but also c(16×4), c(22×4), c(24×4)

Al/Si(111)

3× 3

1/3 ML, ≥ 870 K Al-adatoms or 1/3 ML RT deposited onto cleaved Si(111)2×1

LEED I-V RHEED AES IAES PES XPS ARPES KRIPES EELS AR-EELS PED STM STS PEH THEORY

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9

order-disorder transition at 1050 K

294

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 6 (cont.) Structure

7× 7

Preparation

Model

3/7 ML, ≥ 870 K

Technique

Ref.

Fig.

LEED

81H1, 81H2, 86K2, 92N3, 94K7 92N3 94K7 81H1, 81H2 91G3 89H4 89H4

18

LEED I-V AES ARPES EELS STM STS α-7×7

0.1–0.3 ML, 20–700 K

LEED AES EELS STM

"γ-phase"

0.6–0.7 ML > 720 K

Al/Si(112) 0.4 –1.0 ML, 5×1 > 770 K 6×1

≥ 1 ML, > 770 K

Al atoms in substitutional sites ?

LEED

Table Remarks usually coexists with

3 × 3 or "γ-phase"

64L, 65L, 77R, 92N3, 18, 94K7 20, 94K7 21 77R 93Y2, 94Y2 18, 20

LEED I-V AES EELS STM STS

64L, 65L, 77C2, 94K7, 94Z1, 96M 92N3 94K7, 94Z1 77C2, 85K3, 91G3 89H4, 94Y2 89H4

LEED AES

94J 94J

52

LEED AES

94J 94J

52

periodicity still uncertain: 7×7 [64L, 89H4], 8×8 [94K7, 94Z1], 9.4×9.4 [96M]

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Ref. p. 396]

3.3.2 Metals on semiconductors

295

Al/Si(100)

Si(100)2x2-Al Si(100)3x2-Al Si(100)5x2-Al

Fig. 14. (a) General phase diagram for the submonolayer Al/Si(100) system as determined by LEED-AES [89I3]. (b) Refined phase diagram for the low-temperature range [95Y3].

Fig. 15. Formation of the 2×2, 3×2 and 5×2 structures by chains of parallel Al dimers in the Al/Si(100) submonolayer system. The similar structures are formed in the Ga/Si(100) and In/Si(100) systems.

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296

3.3.2 Metals on semiconductors

Si(100)2x2-Al

[Ref. p. 396

Table 7. The coordinates of 18 atoms in the Si(100)2×2-Al structure as determined by tensor LEED [93S8]. The atom numbers and axes are defined in Fig. 16.

Fig. 16. (a) Top and (b) side views of the parallel symmetric Al dimer on the Si(100) surface. The coordinates of 18 atoms as determined by tensor LEED [93S8] are listed in Table 7. Experimentally determined and calculated values of Si dimer bond length (LSi), Al dimer bond length (LAl), Al-Si back bond length (RAl -Si) and separation (∆Z) between the planes containing Al dimers and Si dimers are listed in Table 8.

Atom No

X [Å]

Y [Å]

Z [Å]

1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8

0.00 0.00 1.80 –1.80 1.80 –1.80 3.84 0.00 3.84 0.00 3.84 0.00 3.84 0.00 2.08 –2.08 2.08 –2.08

1.25 –1.25 2.62 2.62 –2.62 –2.62 1.84 1.87 –1.84 –1.87 0.00 0.00 –3.84 –3.84 0.00 0.00 –3.84 –3.84

0.00 0.00 1.08 1.08 1.08 1.08 2.22 2.60 2.22 2.60 3.43 4.08 3.37 4.00 4.95 4.95 5.17 5.17

Table 8. Experimentally determined and calculated values of Si dimer bond length (LSi), Al dimer bond length (LAl), Al-Si back bond length (RAl-Si) and separation (∆Z) between the planes containing Al dimers and Si dimers in the Si(100)2×2-Al structure. LSi [Å]

LAl [Å]

RAl-Si [Å]

∆Z [Å]

Method

2.44 2.44 2.95 2.46 2.36

2.50 2.69 2.54 2.71 2.67

2.50 2.47 2.46

1.08 1.10

LEED I-V [93S8] THEORY [91N5] THEORY [92A] THEORY [94K6] THEORY [94B1]

2.48

0.98 1.07

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Ref. p. 396]

3.3.2 Metals on semiconductors

297

Al/Si(110)

Al/Si(111)

Fig. 17. Phase diagram for the submonolayer Al/Si(110) system as determined by LEED-AES [92Z4].

Fig. 18. Phase diagram for the submonolayer Al/Si(111) system as determined by LEED-AES [94K7].

Si(111) 3 × 3 -Al

Table 9. Experimentally determined and calculated values of displacements of Si atoms from ideal positions, the distance between Al adatom and the second-layer Si atom and bond lengths in the structure. Si(111) 3 × 3 -Al ∆r corresponds to lateral displacement, ∆z corresponds to vertical displacement. The numbers in parentheses denote the atoms as numbered in Fig. 19. The bond names and directions of displacements are also shown in Fig. 19. The displacements and lengths are given in Å units. LEED I-V [90H1]

Fig. 19. Side view of the Si(111) 3 × 3 -Al structure with Al occupying the T4 site. Arrows indicate the atomic displacements from the bulk sites. Experimentally determined and calculated values of displacements, the distance between Al adatom and the second-layer Si atom and bond lengths are listed in Table 9.

Lando lt -Bö rnst ein New Series III/42A1

∆r(2) ∆z(2) ∆z(3a) ∆z(4a) ∆z(3b) ∆z(4b) d1 -3a B1 B2 B3 B4 B5

0.15±0.2 0.02±0.1 −0.44±0.1 −0.32±0.15 0.16±0.1 0.08±0.15 2.63±0.1 2.41 2.23 2.38 2.43 2.49

LEED I-V THEORY THEORY [92N3] [84N] [91T4]

2.52 2.37 2.07 2.37 2.42 2.49

0.12 −0.02 −0.33 −0.17 0.11 0.09 2.45

0.16, 0.11 −0.09, −0.02 −0.37 −0.25 0.11 0.06 2.50 2.38 2.23 2.32 2.40 2.50

298

3.3.2 Metals on semiconductors

[Ref. p. 396

Al/Si(111)

Si(111)α-7x7-Al

Fig. 20. (a) LEED pattern from the Si(111)α7×7-Al surface (0.25 ML, 400 °C). The LEED pattern displays the 7×7 periodicity like that for the clean Si(111)7×7 surface, but the spot intensity distribution is essentially different. (b) LEED pattern from the Si(111)-Al ‘‘γphase’’ (0.6 ML, 600 °C). The electron energy is 55 eV [94K7].

Fig. 21. Possible atomic geometry of the Si(111)α-7×7 surface proposed on the basis of STM observations [94Y2]. Al atoms are shown by black circles, Si atoms are shown by white circles and black dots.

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

299

3.3.2.3.3 As (Arsenic) on Si Table 10. Structures formed by As on Si(100), Si(110) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

Table Remarks

As/Si(100) 2×1

symmetric As LEED flash ann. to dimers 1120–1320 K + slow cooling to RHEED 620 K in As4 flux or As deposit. at AES 620–670 K PES ARPES XPS XSW STM XRD SHG NICISS RDS QMS IMS

86B3, 86U2, 87B, 22 88N7, 88Z2, 89O2, 90J, 90O2, 94F1 90U 89O2, 90O2, 92E, 94C 87B, 89B 86B3, 86U1, 86U2 88N7 88Z2, 94F1 88B4, 95I2, 96K1 90J 91K1 88N7 96K1 89O2, 90O2 98O1

11, 12

As coverage ∼ 1 ML; saturating phase; chemically passive

THEORY 86U1, 86U2, 91A3, 93K3, 93T5, 94K8, 94L4, 95C1, 96K1, 97C4, 97P8, 98G2, 98G7, 98P2, 99C2, 99T1 c(4×4)

1 ML ann. 1070 As dimers or LEED K missing dimers AES ?

94W2, 95W 94W2, 95W

As deposit. 870 K As chains, As LEED adatoms and XPS As atoms in STM substitutional THEORY sites ?

93B1 93B1 93B1 93B1

As coverage∼ 1/4 ML

As/Si(110)  3 3    0 5

Lando lt -Bö rnst ein New Series III/42A1

23

As coverage 0.67±0.05 ML

300

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 10 (cont.) Structure

Preparation

Model

Technique Ref.

Fig.

Table Remarks

As/Si(111) 1×1

flash ann. to ideal 1×1 As LEED 1120–1320 K+ termination slow cooling to 620 K in As4 flux or As deposit at RHEED 620–670 K AES

86O2, 87U, 87B, 24 88C1, 88C5, 88H9, 88H10, 88Z3, 89O1, 90D2, 90O2, 96J2 94C 88C1, 88C5, 88H9, 88H10, 89O1, 90D2, 90O2, 94C PES 86O2, 87B, 89B, 94C ARPES 87U KRIPES 95B3 EELS 94S6 XSW 87P2, 89P2, 93S9 MEIS 88C1, 88C5, 88H9 RBS 88Z3 STM 88C1, 88B4, 88B5 He 89D5, 90D2, 90D3, scattering 92S7 PED 96G1, 96J2 PEH 98L SHG 91K1 SE 93R RS 91W5, 93R QMS 90O2 THEORY 86O2, 87U, 88H11, 89M4, 89M7, 89P1, 92S7, 94E, 94R, 96C5, 96H1, 98G2, 98P2

13

ideal As coverage 1 ML, actually, ∼ 0.9 ML; saturating phase; chemically passive

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Ref. p. 396]

3.3.2 Metals on semiconductors

301

Table 11. Displacements from the ideal positions of the Si atoms in the four uppermost layers for the Si(100)2×1-As structure. The directions and atom numbers are defined by Fig. 22. Atom No.

∆X [Å]

1 2 3 4 5 6 7 8

0.12 0.12

[86U1] ∆Z [Å] 0.09 0.09 0.07 0.18 0.03 0.12

∆X [Å]

[95C1] ∆Z [Å]

0.14 0.14

∆X [Å]

0.08 0.08 0.08 0.19 0.04 0.10

0.18 0.18

0.02 0.02

0.05 0.05

[98G7] ∆Z [Å] –0.01 –0.01 0.20 0.15 0.13 0.09 0.01 0.01

Table 12. Experimentally determined and calculated values of As-As dimer bond length (L), As-Si back bond length (R) and vertical distance (h) between the As dimers and the first Si(100) layer for the Si(100)2×1-As structure. The vertical distance of the dimer relative to the bulk position of the first Si layer is also given in parentheses. L As-As [Å] 2.55±0.01 2.58±0.04 2.55 2.52 2.52 2.54 2.51

R As-Si [Å]

(1.40±0.01) 2.44 2.45 2.42 2.44 2.42

h [Å]

1.41 (1.32) 1.43 (1.35) (1.41)

Method

Ref.

XRD XSW THEORY THEORY THEORY THEORY THEORY

[90J] [94F1] [86U1] [93T5] [93K3] [97C4] [98G7]

Table 13. Si(111)1×1-As surface: Silicon-arsenic bond length, silicon-arsenic-silicon bond angle and outward displacement of the As atom from the position of a Si atom in the first layer of an unreconstructed surface (as shown in Fig. 24). R Si-As [Å]

∠Si-As-Si [°]

∆d [Å]

Method

Ref.

2.44±0.03 2.42±0.04 2.41±0.1(5) 2.42 2.42 2.45

103.7±1.6 105.0±2.5 105.5±0.8 104.9 105.0 103.2

0.24±0.06 0.2±0.1 0.17±0.03 0.2 0.19 0.25

MEIS MEIS XSW PED THEORY THEORY

[88H10] [88C1] [87P2] [96G1] [86O2] [89P1]

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302

3.3.2 Metals on semiconductors

[Ref. p. 396

Si(100)2x1-As

Fig. 22. Side view of the atomic structure of Si(100)2×1As surface. Displacements of Si atoms from bulk positions are listed in Table 11. The values of As-As dimer bond length (L), As-Si back bond length (R) and vertical distance (h) between the As dimers and the first Si(100) layer are listed in Table 12. As atoms are shown in black and Si atoms in white.

 3 3  -As  0 5

Si(110) 

Si(111)1x1-As

Fig. 23. Possible atomic geometry of the Si(110) (3,3)×(0,5)-As surface. Filled circles represent As atoms and empty circles represent Si. Largest filled circles lie in the top layer, intermediate sized circles represent the capping adatoms intermediate between layers, and the smallest filled circles represent substitutional As atoms in the lower layer [93B1].

Fig. 24. (a) Top and (b) side views of the atomic structure of Si(111)1×1-As. The values of silicon-arsenic bond length (R), silicon-arsenic-silicon bond angle and outward displacement of the As atom from the position of a Si atom in the first layer of an unreconstructed surface (∆d) are listed in Table 13. As atoms are shown in black and Si atoms in white.

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

303

3.3.2.3.4 Au (Gold) on Si Table 14. Structures formed by Au on Si(100), Si(110), Si(111), Si(775) and Si(995) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

Table Remarks

2×2

∼ 1.0 ML, RT

orthogonal LEED Au dimers STM

c(8×2)

0.1–0.5 ML, 870–1020 K

LEED AES STM LEIS

76O, 79O, 99C1 76O, 79O, 99C1 93L4 99C1

5×1 or

∼ 0.5 ML, 1070 K

LEED AES REM RBS PES

76O, 79O 76O, 79O 92Y2 93L3 93L3

3 ML RT deposit. + ann. 870 K

REM STM

92Y2 98S3

forms at Au electromigration when DC heating current is in the [110] direction

LEED AES STM LEIS TED REM LEEM RAS

98H, 98S3, 99C1, 99M3 99C1 93L4, 98S3, 99C1 99C1 95J2 99C1, 99O, 99M3 98M4 98P3

has early been misinterpreted to be a combination of

Au/Si(110) 0.08–0.26 ML, 1×2 770–1240 K

RHEED TRAXS

92Y3 92Y3

26

2×5

0.28–0.46 ML, 770–1020 K

RHEED TRAXS

92Y3 92Y3

26

 4 0    − 1 3

0.46–1.23 ML, 670–1120 K

RHEED TRAXS LEED

92Y3 92Y3 81G

26

1×1

> 1020 K

RHEED TRAXS

92Y3 92Y3

26

Au/Si(100)

26 × 1

c(18×2)

5× 10 R5. 0.7–1.0 ML 1020–1270 K 7° or 5×3.2R5.7°

Lando lt -Bö rnst ein New Series III/42A1

IC phase

94L3 94L3

25

poor LRO

26 × 1 and 26 × 3

304

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 14 (cont.) Structure

Preparation

Model

Technique Ref.

0.2–0.5 ML, 970–1070 K

chains of Au atoms

LEED

Fig.

Table Remarks

Au/Si(111) 5×2

RHEED AES ISS ICISS EELS UPS ARUPS PES IPES REM STM XRD XSW TED LEEM SHG RBS RAS

3× 3

0.7–1.0 ML, 970–1070 K

CHCT or MTLTT

LEED

69B, 70H, 72B, 74L, 27, 30 77L, 86H1, 86T1 83L1, 85I1, 90D1, 93H7, 93H10 77L, 81L2, 83L1, 86T1 83Y2 88H3 81L2, 85L2 81L2 95C2 86C2, 96O2 97H1 89T1, 90T, 98A3 90B3, 92H5, 94O2, 96Y1 91S2 86D2, 88B1 91T1, 95M3 91B1, 91S3 92O, 93O1 92Y5 97P7

77L, 86H1, 86T1, 98N1 LEED I-V 92Q, 96H2 RHEED 85I1, 93H7, 93H10, 98K1, 99K4 AES 77L, 81L2, 86T1 ISS 85O1 MEIS 90C2, 91C2 ICISS 88H1, 88H2 EELS 81L2, 85L2 UPS 81L2 ARUPS 90K1 PES 86C2, 96O2 IPES 86N2 REM 80O1 STM 88D, 90H2, 90N3, 92S4, 94T2, 95F2, 98N1, 99K4 XRD 89F2, 91D4, 94K4 XSW 95F2, 98S5 TED 91T1, 95P2 LEEM 91S3

27, 28, 29, 31

top Si atom density 1.1–1.3 ML; variable gold content, "surface solution"

15

top Si atom density 1 ML; variable gold content, "surface solution"

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

305

Table 14 (cont.) Structure

Preparation

Model

Technique Ref.

Fig.

Table Remarks

SHG 92O, 93O1 RBS 92Y5, 92Y8 THEORY 92D, 94D1 2D pseudopentagonal glass

XRD TED LEEM SHG RBS

70H, 77L, 86H1, 27, 28, 92Y5, 98N1 29, 32 93H7, 98N1, 99K4 77L, 81L2 88H1, 88H2 81L2 81L2 86C2, 96O2 86N2 80O1 90N3, 94T2, 95F2, 98N1, 99K4 91D4, 98G5, 98M3 91T1, 95P2 91S3 92O 92Y8

LEED RHEED AES LEEM REM TED STM

83L1 93H7, 98K4 83L1 91S3 98A3 91T1 98K4

top Si atom density 1.3–1.7 ML at 0.3 ML of Au [98A3]

2 3 × 2 3 1.7–1.8 ML, 700 K

LEED

92Y5

2 21 × 2 2 ∼ 1.2 ML, 520 K

RHEED STM

97K6, 98S7, 99K4 98S7, 99K4

6×6

1.0–1.6 ML, 540–640 K

LEED RHEED AES ICISS EELS UPS PES IPES REM STM

"1×1"

0–1.5 ML, > 1070 K

metastable; formed during the first slow cooling after formation of

3 × 3 at 870 K Au/Si(775) 2×1

∼ 0.7 ML, 770–1020 K

Au/Si(995) 3×1 ∼ 0.7 ML, ∼ 1020 K

Lando lt -Bö rnst ein New Series III/42A1

LEED STM

95S3 95S3

Si(775) facets stabilized by Au

LEED STM

95S3 95S3

Si(995) facets stabilized by Au

306

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 15. Structural parameters of the gold-trimer-based models (MTLTT and CHCT) of the Si(111) 3 × 3 Au reconstruction obtained from various surface techniques. 2/3 ML honeycomb-based models are excluded. Symbols refer to Fig. 31 [95P2]. TED XRD XRD [95P2] [94K4] [91D4] III-Si III-Au

MEIS [91C2] Str. 1 Str. 2

LEED [92Q]

LEED [96H2]

RHEED XSW THEORY [98K1] [98S5] [92D]

2.80 0.00

0.00

2.81 0.00

Trimer values Au-Au [Å] 2.71 ϕAu [°] 1.94

2.70 0.00

2.80 0.00

2.80 0.00

3.00 12.00

2.90 12.90

Si-Si [Å] ϕSi [°]

2.89

2.88 4.84

3.20 5.64

3.20 18.00

3.50 –17.00 11.66

3.50 11.40

2.85 2.27 2.07

1.73 2.09 3.20

2.34 2.34 2.34

2.86 3.39

2.93 4.40

2.83 0.00

Gold-to-Silicon x-y projected distances Au-Si1 [Å] 2.07 Au-Si2 [Å] 2.50 Au-Si3 [Å] 2.55

2.06 2.56 2.45

2.08 2.57 2.43

2.34 2.35 2.34

Vertical distances z-Au [Å] 3.09 z-Si [Å] 2.16 z(Au-Si) [Å] 0.93

1.60

1.80

0.56

2.89 2.33 0.56

3.08

2.92 2.36 0.56

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Si(100)2x2-Au

Au/Si(111)

Fig. 25. Possible structural model for the Si(100)2×2-Au reconstruction. Au atoms are shown in black and Si atoms in white. 2×2 unit cell is outlined [94L3].

Au/Si(110)

Fig. 27. The two-dimensional phase diagram of Au/Si(111) system based on the RHEED observations. STM observations show that the Si(111) 3 × 3 -Au structure is broken up into sub 100 Å domains that decrease in size with increasing Au coverage. A continuous change from the α- 3 × 3 pattern to the βFig. 26. The two-dimensional phase diagram of Au/Si(110) system as determined using RHEED-TRAXS [92Y3].

3 × 3 pattern in diffraction at RT is correlated with the increase in domain wall density as illustrated in Figs. 28 and 29. The domain walls are found to decompose completely at around 893 K resulting in apparent large domains of 3 × 3 structure and ‘‘sharp’’ RHEED pattern. Above 0.96 ML Au coverage, the domain walls are proposed to arrange with LRO of 6×6 periodicity when the sample is annealed at around 600 K and slowly cooled. On the contrary, quenching of the sample results in the formation of the metastable β- 3 × 3 phase. [98N1].

Lando lt -Bö rnst ein New Series III/42A1

307

308

3.3.2 Metals on semiconductors

[Ref. p. 396

Au/Si(111)

Si(111)5x2-Au

Fig. 28. A schematic phase diagram of the domain walls of Au/Si(111) system at gold coverage above 0.76 ML

Fig. 30. Possible structural model for the Si(111)5×2-Au reconstruction. Model includes four gold atom sites per 5×2 unit cell forming two gold rows. Not yet clear is whether there is a fifth, partially occupied gold site which may appear as protrusion in STM images. Alternatively, this protrusion is ascribed to the Si adatoms. Both are shown in figure. Si atom density is 1.5 ML which is slightly higher than the value of 1.1-1.3 ML determined by REM [90T]. Au atoms are shown in black and Si atoms in white. 5×2 unit cell is outlined. [95M3, 97P3].

[98N1]. Corresponding STM images of the α- 3 × 3 , β- 3 × 3 , ‘‘sharp’’ shown in Fig. 29.

3 × 3 and 6×6 phases are

Fig. 29. Empty state STM images of the (a) α- 3 × 3 Au, (b) β- 3 × 3 -Au (c) "sharp" 3 × 3 -Au and (d) 6×6-Au phases. The size of the images: (a) and (b) 200×200 Å2, (c) and (d) 100×100 Å2. Insert in (a) shows 22×22 Å2 image of the commensurate 3 × 3 region. Triangles in (d) express the domain walls between outof-phase

3 × 3 domains [98N1].

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Si(111) 3 × 3 -Au

Si(111)6x6-Au

Fig. 31. Top and side views of the (a) missing top layer twisted trimer (MTLTT) [91C2] and (b) conjugate honeycomb chained trimer (CHCT) [92D] models for the Si(111) 3 × 3 -Au surface. Both models have a missing silicon top layer and gold trimers. The CHCT model is essentially the MTLTT model without any twists and first layer silicon atoms equidistant from all the nearest gold atoms. Ideal Au coverage of either model is 1.0 ML. However, the ‘‘stable’’ Si(111) 3 × 3 -Au that has been annealed for long periods has a saturation coverage of 0.85 ML. STM studies reveal that below this coverage the average

3 × 3 -Au domain size is 50 Å. This suggests that the 3 × 3 -Au domain walls are gold deficient. Ideally an MTL structure has a Si density of 1.0 ML but the presence of Si filled vacancy domain walls would bring this value up to about 1.13 based on 50 Å sized domains. Detailed structural parameters of the models are presented in Table 15.

Lando lt -Bö rnst ein New Series III/42A1

Fig. 32. Possible atomic arrangement of the Si(111)6×6Au structure. This surface structure is very complicated, with 14 independent gold atoms (shown in black circles), relaxations in 24 independent silicon sites and three partially occupied gold sites (larger black circles). The second double layer silicon atoms are only slightly displaced from the bulk sites. In one sense the structure can be described as micro-domains of the parent

3 × 3 -Au on Si structure. A better description is in terms of a tiling of incomplete pentagonal and trimer units, essentially a pseudopentagonal glass, or the two gold ring configurations A and B. In terms of this structural units it appears possible to explain all the gold structures in the coverage range of 0.8–1.5 ML as pseudoglasses with strong SRO but varying degree of LRO. One of the rectangular units observed in domain boundaries of the 3 × 3 -Au structure is shown. 6×6 unit cell is outlined [98G5, 98M3].

309

310

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.5 Ba (Barium) on Si Table 16. Structures formed by Ba on Si(100) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique

Ref.

Fig.

Table Remarks

Ba/Si(100) 3×2

0.15–0.4 ML, Ba-atom chains LEED 820–1320 K RHEED AES

91F, 94V, 96U2, 98T7, 99H2 91M2, 93M 91F, 94V, 96U2, 98T7, 99H2

33

2×1

0.25–0.5 ML, Ba-atom chains LEED 820–1170 K RHEED AES

91F, 94V, 96U2, 98T7, 99H2 91M2, 93M 91F, 94V, 96U2, 98T7, 99H2

33

4×2

> 0.5 ML, 820–1070 K

33

TDS MDS

91F, 94V, 96U2, 98T7, 99H2 91F, 94V, 96U2, 98T7, 99H2 94H2 94H2

0.35–0.4 ML, 1170–1320 K

LEED AES

98T7, 99H2 98T7, 99H2

3×1

∼ 0.35 ML, 1120 K

LEED AES WFM STM

96W1, 99K2 96W1, 99K2 96W1, 99K2 99K2

5×1

∼ 0.5 ML, 1120 K

LEED AES WFM

96W1, 99K2 96W1, 99K2 96W1, 99K2

8×2

∼ 0.65 ML, 1120 K

LEED AES WFM

96W1, 99K2 96W1, 99K2 96W1, 99K2

Ba-atom chains LEED AES

c(6×2)

in LEED: ×4 streaks

coexists with 3×2

Ba/Si(111)

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

311

Ba/Si(100)

Fig. 33. Phase diagram of the submonolayer Ba/Si(100) system. [96U2]. In addition to the phases present at phase diagram, the c(6×2) phase was reported to coexist with 3×2 phase [98T7, 99H2].

3.3.2.3.6 Bi (Bismuth) on Si Table 17. Structures formed by Bi on Si(100), Si(110) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

2×2

≤ 0.5 ML, 420 K

symmetric Bi dimers

LEED STM XSW THEORY

94N3 94N3 96Q1 94T3, 96Q1

34

2×1

0.5–0.8 ML, symmetric 420–770 K Bi dimers

LEED AES STM XSW EELS UPS TDS THEORY

81K, 94N3, 97N1 34, 35 81K, 95K3 94N3, 97N1 95F3, 96Q1 95K3, 98G3 98G3 95K3 95F3, 94T3, 96Q1, 98G8

2×n

> 1 ML ann. symmetric LEED 470–770 K Bi dimers and RHEED missing Bi STM dimers HREELS XPS UPS XRD

90H4, 93P3 36, 37 90H4, 91H2 93P3, 94P2, 99M4 97G4 91H2, 99J 97G4 99J

Table Remarks

Bi/Si(100)

Lando lt -Bö rnst ein New Series III/42A1

in LEED: faint 2×2 spots; in STM: local; 2×2 regions; Metastable: Transforms irreversibly to 2×1 at 420 K

5 ≤ n ≤ 13 ; Bi coverage is related to n by Θ = (n – 1)/n

312

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 17 (cont.) Structure

Preparation

Model

Technique Ref.

Fig.

Table Remarks

1×1 (LEED)

1 ML, 290–470 K

disordered LEED Bi layer on AES Si(100)1×1 ? LEED I-V PES

90F2, 91H3, 92H2 90F2 92F 91H3, 92H2

2×n, 2×8

1 ML ann. 780–810 K

Si dimers LEED and missing STM Si dimers

93P3 67 93P3, 99M4, 99N2

6 ≤ n ≤ 10 , mostly n = 8; Si reconstruction stabilized by Bi traces

c(4×4)

prolonged Si dimers STM low-rate and missing LEED deposit. at Si dimers AES 870 K or1 ML ann. 920 K

99M4 95W 95W

Si reconstruction stabilized by Bi traces

3×2

0.07–0.2 ML, 700 K

81O2 81O2 81O2 93S5

in STM: removal of updown terraces of clean Si(110)"16×2"

1×1 (LEED)

> 0.3 ML, 580–720 K

Bi/Si(110) LEED AES TDS STM disordered Bi LEED layer on AES Si(110)1×1 ? TDS LEED I-V

81O2 81O2 81O2 95S5

Bi/Si(111) α- 3 × 3 1/3 ML, 630–820 K

Bi adatoms

LEED

79K1, 81S1, 81S2, 38 93P2, 93S7, 94B2, 95N2 AES 79K1, 81S1, 81S2 STM 93P2, 93S7, 94B2 LEED I-V 91W4, 92W5 XRD 95N2, 96T3 THEORY 97C5

18, 20

β- 3 × 3 1 ML, 570–820 K

Bi trimers

LEED

19, 20

87P3, 93P2, 93S7, 39 94B2, 95N2 STM 93P2, 93S7, 94B2 LEED I-V 91W4, 92W5 XRD 85T1, 87T2, 87T3, 89T3, 95N2, 96T3 PED 87P3 PES 87P3, 92H2 ARUPS 87K1 PEH 97R, 98R1 THEORY 90N2, 91N3, 97C5

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Si(100)2x2-Bi

Si(100)2x1-Bi

Si(100)2x1-Bi

Fig. 34. Structural models of (a) metastable low-coverage Si(100)2×2-Bi and (b) stable Si(100)2×1-Bi surface phases. Si atoms are shown in white and Bi atoms in black. Unit cells are outlined [94N3, 94T3, 96Q1].

Fig. 35. Side view of the atomic structure of Si(100)2×1Bi as determined using ab initio pseudopotential calculations [98G8]. Si atoms are shown in white and Bi atoms in black. Distances are given in Å.

Si(100)2xn -Bi Si(100)2xn -Bi

Fig. 36. The dependence of the order n in the Si(100) 2×n-Bi structure as a function of annealing temperature [93P3].

Lando lt -Bö rnst ein New Series III/42A1

Fig. 37. Structural model of the Si(100)2×n -Bi surface as revealed by STM [94P2]. Bi atoms forming Bi dimers (black circles), the missing Bi dimer rows (MR) and the dislocation-type defects (in the middle of the upper three rows) are shown. A similar structure is formed in Bi/Ge(100) system.

313

314

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 18. Si(111) 3 × 3 -Bi α-phase with 1/3 ML Bi coverage: Atomic coordinates as determined using LEED I-V analysis [92W5] and displacement of Si atoms from bulk positions as determined using LEED I-V analysis [92W5] and ab initio calculations [97C5]. Atoms are numbered as in Fig. 38. Atom No.

Coordinates X Y [Å] [Å]

1 2 3 4 5 6 7 8

0.000 1.058 –2.117 3.326 0.000 3.326 0.000 1.109

[92W5] Z [Å]

0.000 1.833 0.000 1.920 0.000 1.920 0.000 1.920

0.000 1.106 1.106 1.712 2.445 4.237 4.684 5.468

Displacement ∆X ∆Y [Å] [Å] –0.053 0.100

[92W5] ∆Z [Å]

0.078

Displacement ∆X ∆Y [Å] [Å] –0.10 0.20

[97C5] ∆Z [Å]

0.17

–0.177 0.554 –0.004 0.443

–0.22 0.41 –0.14 0.30

Table 19. Si(111) 3 × 3 -Bi β-phase with 1 ML Bi coverage: Atomic coordinates as determined using LEED I-V analysis [92W5] and displacement of Si atoms from bulk positions as determined using LEED I-V analysis [92W5] and ab initio calculations [97C5]. Atoms are numbered as in Fig. 39. Atom No.

Coordinates X Y [Å] [Å]

1 1' 2 3 4 5 6 7 8

0.845 –1.690 1.049 –2.099 3.326 0.000 3.326 0.000 1.109

[92W5] Z [Å]

1.464 0.000 1.818 0.000 1.920 0.000 1.920 0.000 1.920

Displacement ∆X ∆Y [Å] [Å]

0.000 0.000 2.210 –0.062 2.210 0.118 2.864 3.326 5.185 5.648 6.6467

[92W5] ∆Z [Å]

0.093

Displacement ∆X ∆Y [Å] [Å]

–0.03 0.06 –0.129 0.3335 –0.160 0.303

0.05

[97C5] ∆Z [Å]

0.06 –0.03 0.15 –0.01 0.11

Table 20. Experimental and calculated values of the distances h12 and b11′ (defined as in Figs. 38 and 39) for α- and β-Si(111) 3 × 3 -Bi phases. Structure

Length

LEED I-V XRD [91W4, 91W1] [95N2]

THEORY [97C5]

Si(111) 3 × 3 -Bi (α-phase, 1/3 ML Bi)

h12

1.107

1.60

1.98

Si(111) 3 × 3 -Bi (β-phase, 1 ML Bi)

h12 b11′

2.210 2.928

2.70

2.64 3.05

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Si(111)α- 3 × 3 -Bi

Si(111)β- 3 × 3 -Bi

Fig. 38. (a) Top and (b) side views of the Si(111)α-

Fig. 39. (a) Top and (b) side views of the

3 × 3 -Bi (α-phase, 1/3 ML Bi): T4 Bi-adatom model.

Si(111) 3 × 3 -Bi ( β-phase, 1 ML Bi): Bi-trimer model. The coordinates of atoms, displacement of Si atoms from bulk positions and h12 and b11′ lengths are listed in Tables 18, 20. Bi atoms are shown in black and Si atoms in white.

The coordinates of atoms, displacement of Si atoms from bulk positions and h12 length are listed in Tables 18, 20. Bi atoms are shown in black and Si atoms in white. A similar structure is formed by Al, Ga, In, Pb and Sn on Si(111) and by Bi, Pb and Sn on Ge(111).

Lando lt -Bö rnst ein New Series III/42A1

315

316

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.7 Ca (Calcium) on Si Table 21. Structures formed by Ca on Si(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model Technique

Ref.

Fig.

Table Remarks

Ca/Si(111) 2×2

0.2–0.6 ML, 770–950 K

LEED AES

00S 00S

40

5×1

0.2–0.5 ML, 950–1050 K

LEED AES

00S 00S

40

7×1

0.2–0.5 ML, 1050 K

LEED AES

00S 00S

40

LEED AES REM STM

86O1, 99S2, 00S 00S 93K4 99S2

40

3×1 (LEED) 0.2–0.4 ML, 1050–1240 K

top Si atom density 4/3 ML; in STM: local structure 3×2 and c(6×2)

Ca/Si(111)

Fig. 40. Formation phase diagram for the Ca/Si(111) system as determined using LEED-AES [00S].

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

317

3.3.2.3.8 Ce (Cerium) on Si Table 22. Structures formed by Ce on Si(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique

Ref.

Fig.

Table Remarks

0.3–1 ML ann. 620 K

Ce adatoms ? LEED AES PES

87F 87F 87F

actual coverage 0.35±0.07 ML (1/3 ML)

0.3–1 ML ann. 720 K

LEED AES PES

87F 87F 87F

actual coverage 0.20±0.03 ML (1/4 ML)

Ce/Si(111)

3× 3

2×2

3.3.2.3.9 Co (Cobalt) on Si Table 23. Structures formed by Co on Si(100), Si(110) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig. Table Remarks

0.01–0.2 ML, 300–870 K, quench

Si dimers and missing Si dimers

LEED AES STM

94D2, 96O4, 97S2 67 94D2, 96O4 97S2

Co/Si(100) 2×n

Co/Si(110) 1×2

0.1–0.5 ML, 670–770 K

LEED AES

95D3 95D3

41

8×5

0.25–0.4 ML, 970–1070 K

LEED AES

95D3 95D3

41

11×2

0.15–0.25 ML, 770 K

LEED AES

95D3 95D3

41

Lando lt -Bö rnst ein New Series III/42A1

6 ≤ n ≤ 10 ; Si reconstruction stabilized by Co traces

318

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 23 (cont.) Structure

Preparation

Model

Technique Ref.

Fig. Table Remarks

Co/Si(111) "1×1"

7× 7

0.02–0.1 ML, ann. random RC's 1000–1130 K with one Co atom

STM ISS PES LEEM SPEM

0.12–0.2 ML closed- packed ann. 870–1000 K RC’s with one Co atom

LEED

13 × 13 0.5–1.0 ML ann. 770 K

92B3, 95P1 42 92B3 99P2 97P5, 98P5, 99P2 99P2

84P, 86W, 96O4, 42 98P5 AES 86W, 96O4 XPS 84P PES 99P2 ARUPS 84P, 85P ISS 92B3 STM 92B3, 93B3, 93T4, 95P1 LEEM 97P5, 98P5, 99P2 SPEM 99P2 THEORY 93T4 LEED AES

ideal Co coverage 0.14 ML

96O4 96O4

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Co/Si(110)

319

Si(111) 7 × 7 -Co

Fig. 42. The structure of the Si(111) 7 × 7 -Co which comprises the array of the closed-packed ring clusters (RC) [92B3, 93T4]. Each RC contains 6 Si adatoms (shaded circles) and one Co atom (closed circle) substituting top Si atom. The center of the RC is located in the on-top site. The ideal Co coverage of the Si(111) 7 × 7 -Co is 1/7 (∼ 0.14) ML. At lower Co coverages, the random distribution of the RC's produces the ‘‘1×1’’-RC phase which is typical for many transition metals, Co, Ni, Fe, Pd, Pt, Ir [95P1]. Fig. 41. Phase diagram of the submonolayer Co/Si(110) system as determined using LEED-AES for the (a) MBE mode (Co deposition onto the heated Si(110) surface) and (b) SPE mode (RT Co deposition followed by annealing). The annealing duration in the SPE mode was determined by the time required to complete the surface structure formation and was in the range of 10–30 min. The region on the phase diagram labeled ‘‘8×5 → 1×2’’ indicates the region where the continuous displacement of the 1/5-order spots towards 1/2 position is observed in the LEED pattern with the Co deposition. The region labeled ‘‘1×2 → 8×5’’ represents the reverse transition. The coverage axis shows the amount of deposited Co. Actually surface concentration of Co is essentially lower due to Co/Si intermixing (e.g. Co concentration required for the formation of the 8×5 structure is about 0.4 at.% and that of the 1×2 structure is about 0.9 at.% [95D3].

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320

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.10 Cu (Copper) on Si Table 24. Structures formed by Cu on Si(100), Si(110) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique

Ref.

Fig.

Cu/Si(100) 2×n

n = 6–8; 0.3–2 ML, ann. 770–820 K, quench

Si dimers LEED and missing Si dimers AES EELS STM He scattering

2×2

≥ 1 ML, ann. 770–820 K, quench

LEED AES EELS STM He scattering

c(12×10)

Table Remarks

≥ 1 ML, ann. 770–820 K, quench

LEED

90B4, 95I1, 67 96I, 98I3 95I1, 96I, 98I3 95I1, 96I 98I3 90B4 90B4, 95I1, 43 96I, 98I3 95I1, 96I, 98I3 95I1, 96I 98I3 90B4 43

EELS STM

95I1, 96I, 98I3 95I1, 96I, 98I3 95I1, 96I 98I3

AES

Si reconstruction stabilized by Cu traces

Cu/Si(110) 1×2

0.05–10 ML, ann. 1090–1170 K, quench

LEED AES THEORY

95I3, 96I 95I3, 96I 90S

44, 45

5×4

0.1–10 ML, ann. 1020–1090 K, quench

LEED AES THEORY

95I3, 96I 95I3, 96I 90S

44, 45

3×4

0.3–10 ML, ann. 920–1020 K, quench

LEED AES THEORY

95I3, 96I 95I3, 96I 90S

44, 45

6×6

0.7–10 ML, ann. 570–920 K, quench

LEED AES SEM

95I3, 96I 95I3, 96I 95I3, 96I

44, 45

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

321

Table 24 (cont.) Structure

Preparation

Model

Technique

Ref.

Fig.

LEED AES PYS

85T2 85T2 85T2

LEED

83R, 85D2, 46 85D3, 87K2, 89M2, 94I2, 96I 89T4 83R, 85D2, 85D3, 86D1, 87K2, 91K3, 94I2, 96I, 97S9 85C2, 86C1 83R 88C3, 90C4 86N1, 86N2 85D2, 85D3 85C1, 85C2, 85C3 92Z5 91S1, 91S4, 92S2 89D3, 89D5 88W, 89D1, 89T2, 91M1, 95K2, 97S9 87K2 91K3, 95K2 94I2 89M2, 92B1 85C2, 96I 89T4 89M2 85C2 88C3, 90C4

Table Remarks

Cu/Si(111) 4×1

0.5–1.0 ML RT deposit. onto cleaved Si(111)2×1

5.5×5.5 ("5×5")

∼ 1 ML, 670–870 K

IC phase

RHEED AES

ARAES XPS ARUPS KRIPES EELS AED XSW XAS He scattering STM

TDS ISS RBS LEEM SEM REM PEM SAM THEORY

Lando lt -Bö rnst ein New Series III/42A1

Cu coverage 1 ML, excess Cu dissolves into the bulk [94I2]; top Si atom density 1–1.3 ML [90C4, 95K2]; Cu:Si stoichiometry is ∼ 1:1 [90C4]; 5.55×5.55 is a periodicity of the most top layer [89D3, 89D5]

322

3.3.2 Metals on semiconductors

[Ref. p. 396

Cu/Si(100)

Fig. 43. Phase diagram for the submonolayer Cu/Si(100) system as determined using LEED-AES-STM [98I3]. Cu coverage denotes the deposited amount of Cu and temperature corresponds to the temperature from which the sample was quenched. Deposited Cu atoms diffuse into the bulk above 570 K and segregate to the surface by quenching. Always the mixed structures of two or three phases are observed. Cu does not adsorb on Si(100) and desorbs at temperatures higher than 600 °C. (In the early works [95I1, 96I], c(12×10) structure was erroneously defined as the 6×5 and 10×5 structures.)

Cu/Si(110)

Fig. 45. Phase diagram for the submonolayer Cu/Si(110) system as determined using LEED-AES [95I3]. Cu coverage denotes the deposited amount of Cu and temperature corresponds to the temperature from which the sample was quenched.

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Si(111)5.5x5.5-Cu

Cu/Si(110)

Fig. 44. The Cu M23VV/Si L23VV Auger intensity ration versus temperature for heating (solid curve) and quenching (dashed curve) Si(110) surface with 1.2 ML of Cu. The surface structures observed by LEED are indicated as a function of Auger ratio. The increase of the surface Cu concentration by quenching is caused by the Cu segregation due to supersaturation. The decrease in the Cu Auger intensity above 700 °C is caused by thermal desorption. The dotted curve shows the decrease in Cu segregation due to increase in annealing time [96I].

Lando lt -Bö rnst ein New Series III/42A1

Fig. 46. (a) Schematic representation of the LEED pattern from the Si(111)"5×5"-Cu surface. The open circles correspond to the first-order Si(111) reflections, the closed circles correspond to the "5×5" reflections. The size of the circles approximately indicates their intensities. The ratio of the reciprocal lattice vectors a : b is 0.816±0.03 [87K2]. (b) STM image of the Si(111)"5.5×5.5"Cu. The surface shows up as an array of round shaped clusters with 5.4-5.7 periodicity [97S9].

323

324

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.11 Eu (Europium) on Si Table 25. Structures formed by Eu on Si(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique

Ref.

2×2

1–3 ML ann. 870–1020 K

EuSi2 Silicide Phase ?

LEED AES PES IPES EELS

91H4, 97M 91H4, 97M 91H4 91H1, 97M 97M

5×1

1–3 ML ann. 1040–1070 K

LEED AES PES IPES EELS

91H4, 97M 91H4, 97M 91H4 91H1, 97M 97M

3×1

1–3 ML ann. 1100–1200 K

LEED AES PES IPES EELS

91H4, 97M 91H4, 97M 91H4 91H1, 97M 97M

Fig.

Table Remarks

Eu/Si(111)

3.3.2.3.12 Ga (Gallium) on Si Table 26. Structures formed by Ga on Si(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique

Ref.

Fig.

Table Remarks

0.4–0.55 ML, 350–770 K

Parallel Symmetric Ga dimers

RHEED LEED LEED I-V AES ARUPS XSW STM

81S4 15, 47, 27, 28 88B2 48 94S7 88B2 91E1 94Q, 95Q, 95T1 90B5, 90N1, 91N2, 96S6, 97N2, 99E1 96S6 98C2 91N5, 94Q, 95T1, 97N2

Ga/Si(100) 2×2

STS RAS THEORY

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

325

Table 26 (cont.) Structure

Preparation

Model

Technique

Ref.

Fig.

3×2

0.15–0.35 ML, 350–770 K

Parallel Symmetric Ga dimers

RHEED LEED AES ARUPS STM

15, 47

THEORY

81S4 88B2 88B2 91E1 88N3, 90B5, 90N1, 91N2 91N5

RHEED LEED AES THEORY

81S4 88B2 88B2 91N5

15, 47

RHEED LEED AES STM

81S4 88B2, 96S6 88B2 90B5, 95L2, 96S6, 98N2, 99N1

47

5×2

∼ 0.4 ML, 350–770 K

8×n

0.7–0.9 ML, 720–850 K

Parallel Symmetric Ga dimers

Table Remarks

Ga/Si(110)  2 1    − 3 3

0.08–0.11 ML, 800–870 K

Ga adatoms ? LEED AES QMS

92S5 92S5 92S5

49

 2 1   − 4 4

0.11–0.14 ML, 755–800 K

Ga adatoms ? LEED AES QMS

92S5 92S5 92S5

49

Ga/Si(111)

3× 3

Lando lt -Bö rnst ein New Series III/42A1

1/3 ML, 720–820 K Ga adatoms LEED or 1/3 ML RT LEED I-V deposit. onto cleaved RHEED Si(111)2×1 AES PES ARUPS KRIPES PYS EELS XSW XPD AED MEIS

84B 38, 50, 29, 30 87K5, 88K4 51 85O2, 89M3, 95H2 84B 81F, 87B, 87H2 85K1, 86K3 87N2, 87N3 84B 95S2 89P2, 89Z, 91Z 86H2 86H2 92C2

mainly n = 5, but also 3 and 4

326

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 26 (cont.) Structure

Preparation

Model

Technique

Ref.

STM

88C4, 88N4, 88P2, 89S2, 89Z, 96F1, 96F2, 98F1, 98I2, 99L1 92K1 89M4, 89Z, 90R3, 95S2, 96C2

SHG THEORY

0.65–1.0 ML, 570–820 K

Ga atoms substituting top Si atoms; IC phase

RHEED LEED XSW STM

Fig.

Table Remarks

SSM THEORY

85O2 50 88C4 88Z1, 89P2, 91Z 88C4, 88P2, 89P2, 99L1 90M2 89M4

6.3 3 ×6.3 3 ∼ 1.0 ML, 720–820 K

RHEED LEED

85O2 88C4

50

not detected in STM

∼ 1.0 ML, 720–820 K

RHEED LEED

85O2 88C4

50

not detected in STM

LEED

82K, 93J1, 94J, 52 99B3, 99P3 82K, 93J1, 94J, 99P3 95Y5 96B3, 99B3, 99E2 97G5, 99P3 95Y5, 97G5, 99B3, 99E2, 99P3

6.3×6.3

11×11

Ga/Si(112) 6×1 5×1

≥ 1.0 ML, 720–820 K

Ga-atom chains with periodic vacancies

AES ARAES STM RAS THEORY

in STM: n×1 structures (n = 4 –7) with 5×1 and 6×1 unit cells most common

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Ga/Si(100)

Si(100)2x2-Ga

Fig. 47. (a) General phase diagram for the submonolayer Ga/Si(100) system as determined by RHEED assuming the Ga sticking coefficient to be unity [81S4]. (b) Phase diagram for the high-coverage range refined on the basis of STM observations [99N1].

Fig. 48. (a) Top and (b) side views of the parallel symmetric Ga dimer on the Si(100) surface. The coordinates of 18 atoms as determined by tensor LEED [94S7] are listed in Table 27. Experimentally determined and calculated values of Si dimer bond length (LSi), Ga dimer bond length (LGa), Ga-Si back bond length (RGa -Si) and separation (∆Z) between the planes containing Ga dimers and Si dimers are listed in Table 28.

Ga/Si(110)

Fig. 49. Phase diagram for the submonolayer Ga/Si(110) system as determined by LEED-AES- QMS [92S5].

Lando lt -Bö rnst ein New Series III/42A1

327

328

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 27. The coordinates of 18 atoms in the Si(100)2×2-Ga structure as determined by tensor LEED [94S7]. The atom numbers and axes are defined in Fig. 48. Atom No

X [Å]

Y [Å]

Z [Å]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

0.00 0.00 1.87 –1.87 1.87 –1.87 3.84 0.00 3.84 0.00 3.84 0.00 3.84 0.00 2.12 –2.12 1.99 –1.99

1.31 –1.33 2.65 2.65 –2.65 –2.65 1.93 1.80 –1.93 –1.80 0.00 0.00 –3.84 –3.84 0.00 0.00 –3.84 –3.84

0.00 0.00 1.07 1.07 1.07 1.07 2.28 2.56 2.28 2.56 3.53 4.01 3.66 3.84 5.07 5.07 5.11 5.11

Table 28. Experimentally determined and calculated values of Si dimer bond length (LSi), Ga dimer bond length (LGa), Ga-Si back bond length (RGa-Si) and separation (∆Z) between the planes containing Ga dimers and Si dimers in the Si(100)2×2-Ga structure. LSi [Å]

LGa [Å]

RGa-Si [Å]

∆Z [Å]

Method

2.38

2.62 2.58 2.63 2.65

2.53

1.07

2.47 2.54

1.09 1.18

LEED I-V [94S7] XSW [95Q] THEORY [91N5] THEORY [94Q]

2.46 2.49

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

329

Si(111) 3 × 3 -Ga

Ga/Si(111)

Fig. 50. Phase diagram of the Ga/Si(111) system at T = 550 °C as determined using RHEED [85O2], LEED [88C4] and STM [88C4, 88P2].

Fig. 51. Structure of the Si(111) 3 × 3 -Ga: optimized surface geometry from dynamical analysis of LEED I-V spectra [88K4].

Table 29. The coordinates of 9 atoms in the Si(111) 3 × 3 -Ga structure as determined using I-V LEED analysis [94S7]. The atom numbers are defined in Fig. 51. The X, Y and Z axes are parallel to the [1 1 1 ], [1 21 ] and [1 01] directions, respectively. Atom No

X [Å]

Y [Å]

Z [Å]

1 2 3 4 5 6 7 8 9

0.00 1.35 1.35 1.93 2.57 4.37 4.71 5.22 5.22

0.00 1.05 –2.10 3.33 0.00 3.33 0.00 2.27 –1.13

0.00 1.83 0.00 1.92 0.00 1.92 0.00 0.00 1.97

Lando lt -Bö rnst ein New Series III/42A1

330

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 30. Experimentally determined and calculated values of displacements of Si atoms from ideal positions, the height of the Ga adatom above the Si surface (zGa-Si) and Ga-Si back bond length (dGa-Si) in the Si(111) 3 × 3 -Ga structure. ∆r corresponds to lateral displacement, ∆z corresponds to vertical displacement. The numbers in parentheses denote the atoms as numbered in Fig. 51. The displacements and lengths are given in Å. LEED I-V [88K4] dGa-Si zGa-Si ∆r(2) ∆z(2) ∆z(5) ∆z(4) ∆z(7) ∆z(6)

2.50 1.35 0.12 –0.05 –0.49 0.15 –0.28 0.06

XSW [89Z] 2.55 1.49

THEORY [89Z] 2.48 1.34 0.15 –0.04 –0.35 0.12 –0.23 0.08

THEORY [89M4] 2.54 1.36 0.14

MEIS [92C2]

RHEED [95H2]

1.45 0.13 –0.03 –0.46 0.19 –0.26 0.07

1.51 0.07 –0.02 –0.24 0.14 –0.08 0.18

Si(112)5x1-Ga

Fig. 52. Plausible structure of the Si(112)5×1-Ga surface. In the model of the 5×1 reconstruction, a chain of four Ga atoms located along the step edge is interrupted by a vacancy at which two Si edge atoms form a dimer. Similarly, the 6×1 surface structure consists of chains of only five Ga atoms which are interrupted by vacancies [99E2].

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

331

3.3.2.3.13 Gd (Gadolinium) on Si

Table 31. Structure formed by Gd on Si(111) surface; its preparation conditions and techniques used for its investigation. Structure

Preparation

Model

Technique

Ref.

LEED AES EELS IPES

89H3, 97M 89H3, 97M 97M 97M

Fig.

Table Remarks

Gd/Si(111) 2×2

1 ML ann. 870–1020 K

3.3.2.3.14 Hg (Mercury) on Si

Table 32. Structures formed by Hg on Si(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Technique

Ref.

Fig.

Table Remarks

10–20 L, 150 K

LEED PES He scattering

92L2, 93L2 92L2, 93L2 93L2

stable upon ann. to 1020 K

≥ 30 L, 150 K

LEED PES He scattering

92L2, 93L2 92L2, 93L2 93L2

stable upon ann. to 1520 K

Hg/Si(111)

3× 3

1×1

Lando lt -Bö rnst ein New Series III/42A1

332

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.15 In (Indium) on Si Table 33. Structures formed by In on Si(100), Si(110) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique

Ref.

Fig.

Table Remarks

0.4–0.55 ML, 300–420 K

parallel symmetric In dimers

LEED

STS PAC THEORY

86K1, 95Y3, 15, 53, 34, 35 96Y3, 97Z 54 86K1 96S3 86K1 96Y3, 96Y7, 97Y1 87R, 96C8, 96Y6 95Y3 95Y4, 96C8 93S4 91B2, 91B3, 93S4, 94Z2, 97D, 98S6, 99K3 97D 92K4, 93K2 91N5

In/Si(100) 2×2

RHEED LEED I-V AES ARPES PES XPS PED ICISS STM

3×2

0.15–0.35 ML, 300–420 K

parallel symmetric In dimers

LEED PES XPS XPD STM STS

95Y3, 95Y4 15, 53 96Y6 95Y3 95Y4 91B2, 91B3, 97D 97D

5×2

∼ 0.4 ML, 300–420 K

parallel symmetric In dimers

LEED XPS

95Y3 95Y3

4×3

0.5–0.6 ML, 470–720 K

In adatoms LEED and Si dimers RHEED ? AES PES XRD ICISS STM

n×3

0.5–1.0 ML, 300–420 K

15, 53

top Si atom density 0.5 ML [98S6]; In coverage7/12 ML [98S6]

SEM TPD PAC

84K3, 86K1, 97Z 53, 55 84K2, 86K1 84K3, 86K1 96Y6 98B2 94S4, 99R1 91B3, 94L2, 94S4, 98B2, 98R2, 98S6, 98Z, 99R1 86K1 89K3, 90O1 92K4, 93K2

LEED STM

95Y3 53 98E1, 99K3, 99R2

n = 2, 3 and 4; coexists with 2×2

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

333

Table 33 (cont.) Structure

Preparation

Model

Technique

Ref.

Fig.

Table Remarks

In/Si(110) 3×4

0.02–0.2 ML, 700 K

LEED LEED I-V

93S6 93S6

56

 4 3   − 2 2

0.2–0.35 ML, 650 K

LEED LEED I-V

93S6 93S6

56

two mirror domains

 6 4   − 3 2

0.8–1.0 ML, 650 K

LEED LEED I-V

93S6 93S6

56

two mirror domains

LEED

64L, 65L, 84B, 38, 57, 36 86K2 58 79B2, 79K2, 79K3, 82B, 93H11, 95H2, 95Y2 92N1 84B, 93O3 84B 93O3, 93W8 85N, 86H5 86K3, 87K4 87K4, 87N2, 88O2 86H3, 86K2, 88H8, 93O3 93F 89I1 90C1, 90C3 93W7, 93W8 87N1, 88N5, 96O1, 96T4, 97K4, 97S7, 97S11 97K4 93W7 86H4 85N, 86H5, 87N2

In/Si(111)

3× 3

1/3 ML, 720–870 K or 1/3 ML RT deposit. onto cleaved Si(111)2×1

In adatoms

RHEED

MEED AES PYS PES ARPES ARUPS KRIPES EELS XRD LEIS ICISS XSW STM

STS SEXAFS WFM THEORY

31 × 31

0.4–0.6 ML, 720–770 K

In adatoms RHEED and Si chains EELS STM STS WFM

Lando lt -Bö rnst ein New Series III/42A1

79A, 79B2, 79K2, 57, 59 79K3, 82B 86H3, 88H8 88P2, 96T4, 97K4, 98S2 97K4 86H4

absolute In coverage 0.53±0.02 ML [98S6]; top Si atom density 0.88±0.02 ML [98S6]

334

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 33 (cont.) Structure

Preparation

4×1

0.6–1.0 ML, 520–720 K

Model

Technique

Ref.

LEED

64L, 65L, 97S5, 57, 60 97S6 79K2, 79K3, 82B, 82Z, 91N1, 93H11 91N1, 93O3, 97S6, 97S8 93O3, 94O3 93O3, 94O3, 97H2, 99H1 95A2, 96A1, 99H1 86H3, 86K2, 93O3 90C3, 91C1, 93S3 91N1 94F3, 99B4 97C3, 98M2 87N1, 88P2, 93S3, 96T4, 97S4, 97S6, 97S8, 97S11, 98S2, 98S6 98P4 86H4

top Si atom density 0.5 ML [98S6]; In cover-age 0.75 ML [98S6]

RHEED AES UPS IPES PES EELS ICISS AED XRD TED STM

RAS WFM

Fig.

Table Remarks

4×2 (8×2)

0.9–1.2 ML CDW phase RHEED ann. 570–670 K, STM cool < 100 K ARPES

99Y1 99Y1 99Y1

transforms reversibly to 4×1 at > 170 K

2×2

overlapping RHEED 0.5–1.0 ML RT deposit. onto In trimers ? LEED AES cleaved PYS Si(111)2×1 EELS or 0.2–0.7 ML STM RT deposit.

80B, 93H11 61 84B, 97S1, 97S7 84B, 97S1, 97S7 84B 86K2 96O1, 97S1, 97S3, 97S7, 97S10, 97S11, 98S2

converts irreversibly to 4×1 at > 470 K

in STM: coexisting domains of quasihexagonal and quasirectangular phases

STS WFM

85L1, 95K4 57, 62 85L1, 93O3, 95S4 85L1, 93O3 93O3, 94O3 93O3, 94O3 95K4, 95S4, 97K1, 97K4 97K4 86H4

LEED RHEED AES EELS STM

86K2, 97S7 93H11 97S7 86K2 96O1, 97S7

in STM: striped structure

onto 3 × 3 -In

7× 3 "HT"

7× 3 "RT"

2–5 ML ann. 670 K

0.6–1 ML RT deposit. onto

3 × 3 -In

LEED AES EELS UPS IPES STM

63

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

335

Table 34. The displacements of Si atoms in the Si(100)2×2-In structure as determined using LEED I-V analysis [96S3]. The atom numbers and directions of displacements are defined in Fig. 54. Atom No

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

Displacement [Å]

0.01 0.01 0.01 0.01 0.06 0.07 0.06 0.07 0.04 0.05 0.02 0.04 0.03 0.03 0.03 0.03

Table 35. Experimentally determined and calculated values of In dimer bond length (dIn-dimer), Si dimer bond length (dSi-dimer), In-Si back bond length (dIn-Si) and separation (∆Z) between the planes containing In dimers and In dimers in the Si(100)2×2-In structure.

dIn-dimer [Å] dSi-dimer [Å] dIn-Si [Å] ∆Z [Å]

LEED I-V [96S3]

THEORY [91N5]

2.78 2.35 2.61 1.22

2.82 2.40 2.60 1.38

Table 36. The values of vertical coordinates of Si atoms counted from the In adatom and radial displacements of Si atoms in the 2nd and 5th layers in the Si(111) 3 × 3 -In structure as determined using XRD [93F] and RHEED [95H2]. All distances are given in Å. The atom numbers and directions of displacements are defined in Fig. 58.

d1-2 d1-3a d1-3b d1-4a d1-4b d1-5 ∆r(2) ∆r(5)

XRD [93F]

RHEED [95H2]

1.85±0.05 2.98±0.07 2.63±0.08 5.33±0.07 4.98±0.08 5.45±0.17 0.20±0.1 0.14±0.1

1.83±0.10 2.82±0.14 2.52±0.15 5.24±0.23 4.87±0.14

Lando lt -Bö rnst ein New Series III/42A1

0.02±0.13

336

3.3.2 Metals on semiconductors

[Ref. p. 396

In/Si(100)

Si(100)2x2-In

Fig. 53. (a) General phase diagram for the submonolayer In/Si(100) system as determined by LEED-RHEED-AES [86K1]. (b) Refined phase diagram for the lowtemperature In deposition (at RT and 150 °C) as determined by LEED [95Y3]. The subsequent STM observations [98E1, 99K3] have shown that the RT deposited In layer in the 0.5–1 ML range comprises the mixture of the 3×3, 4×3 and 2×3 domains coexisting with the 2×2 phase. Note that the low-temperature 4×3 phase is different from the 4×3 phase formed at high temperatures.

Fig. 54. (a) Top and (b) side views of the parallel symmetric In dimer on the Si(100) surface. The directions of displacements of Si atoms from their bulk positions are indicated by arrows. The magnitudes of the displacements as determined by LEED I-V analysis [96S3] are listed in Table 34. Experimentally determined and calculated values of Si dimer bond length (dSi-dimer), In dimer bond length (dIn-dimer), In-Si back bond length (dIn-Si) and separation (∆Z) between the planes containing In dimers and Si dimers are listed in Table 35. According to LEED data [96S3], ϕ = 28.0°, α = 119.2° and β = 94.7° for the optimized geometry

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Si(100)4x3-In

In/Si(110)

Fig. 55. Possible structural model of the Si(100)4×3-In surface phase with 7 In atoms and 6 Si atoms per 4×3 surface unit cell as suggested on the basis of the STM data [98S6]. Si atoms are shown by open and shaded circles, In atoms are shown by dark circles. The 4×3 unit cell is outlined. Fig. 56. Phase diagram for the submonolayer In/Si(110) system as determined using LEED [93S6]. α corresponds to 3×4-In phase, β corresponds to (4,3)×(–2,2)-In phase and γ to (6,4)×(3,3)-In phase.

In/Si(111) Si(111) 3 × 3 -In

Fig. 57. Phase diagram for the submonolayer In/Si(111) system as determined using RHEED [88H8]. In the early works the 7 × 3 structure was defined erroneously as the 1×1-R30°.

Lando lt -Bö rnst ein New Series III/42A1

Fig. 58. Side view of the Si(111) 3 × 3 -In structure with In occupying the T4 site. Arrows indicate the atomic displacements from the bulk sites. The values of vertical coordinates of Si atoms counted from the In adatom and radial displacements of Si atoms in the 2nd and 5th layers are listed in Table 36.

337

338

3.3.2 Metals on semiconductors

[Ref. p. 396

Si(111)4x1-In

Si(111) 31 × 31 -In

Fig. 60. Tentative structural model of the Si(111)4×1-In surface phase as proposed on the basis of the STM observations and quantitative determination of the surface composition [98S6]. The Si atoms forming πbonded chain (2 atoms per 4×1 unit cell) are shown by dashed circles. In adatoms (3 atoms per 4×1 unit cell) are shown by large black circles.

Si(111)2x2-In

Fig. 59. (a) STM image and (b) structural model of the Si(111) 31 × 31 -In surface phase as proposed on the basis of the STM observations and quantitative determination of the surface composition. The top Si atoms of the ideal unreconstructed Si(111) surface are shown by small open circles. The Si atoms forming the reconstructed topmost Si layer (28 atoms per

31 × 31 unit cell) are shown by gray circles. In adatoms (17 atoms per

31 × 31 unit cell) are shown

by large black circles. Three 31 × 31 unit cells are displayed. In the top left unit cell, the In atoms are removed to visualize the atomic arrangement of the topmost Si layer. Fig. 61. (a) Filled state STM image and (b) tentative structural model of the Si(111)2×2-In surface phase as proposed on the basis of the STM observations [97S7]. Open circles represent Si atoms of the top bulk-like Si(111) layer. In atoms (0.75 ML) are shown by black circles. The position of protrusions in the STM image is shown by hatched areas. Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Si(111) 7 × 3 -In

Si(111) 7 × 3 -In

Fig. 63. STM image of the room-temperature (‘‘RT’’) striped Si(111) 7 × 3 -In (more precisely (2,–1)× (1,2)) surface structure. The inset shows the surface

at a greater magnification [97S7].

Fig. 62. STM images of the high-temperature (‘‘HT’’) Si(111) 7 × 3 -In (more precisely (2,–1)×(1,2)) surface structure. Two structures with different atom configurations are observed to coexist at the surface: (a) quasihexagonal arrangement with local In coverage of 1.0 ML and (b) quasirectangular atomic configuration with a local In coverage of 1.2 ML. The corresponding unit cells are sketched on the right-hand side where the different sizes of circles are scaled approximately according to their apparent height in the STM images [97K4].

Lando lt -Bö rnst ein New Series III/42A1

339

340

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.16 Mg (Magnesium) on Si Table 37. Structures formed by Mg on Si(100) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

Table Remarks

Mg/Si(100) 3×2

∼ 6 ML, RT deposit. Mg-atom chains + ann. 540–580 K

LEED AES EELS

94K5 94K5 94K5

64, 65

Mg coverage ∼ 1/3 ML

2×2

∼ 6 ML, RT deposit. Mg-atom chains + ann. 580–610 K

LEED AES EELS

94K5 94K5 94K5

64, 65

Mg coverage ∼ 1/4 ML

3×2

∼ 6 ML, RT deposit. Mg-atom chains + ann. 610–640 K

LEED AES EELS

94K5 94K5 94K5

64, 65

Mg coverage ∼ 1/6 ML

Mg/Si(111) 3×1 (LEED) ∼ 0.4 ML, 620–720 K

LEED

91Q, 93W2, 95A3, 95A4, 98K3 LEED I-V 91Q AES 91Q PES 93W2, 95A3, 95A4 STM 98K3, 98S6, 99S2

in STM: local structure 3×2 and c(6×2); top Si atom density 4/3 ML [98K3]

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Mg/Si(100)

Si(100)3x2-Mg, 1/3 ML

Si(100)2x2-Mg, 1/4 ML

Fig. 64. The variation of Auger peak ratio and surface structure upon annealing of the 10 Å of Mg deposited onto Si(100)2×1 surface at RT. The surface of 10 Å of Mg thick with absent LEED pattern changes to 1×1 by annealing at temperatures from 200 to 260 °C. The 1×1 structure of 1 ML coverage changes to the 3×2 structure of 1/3 ML, and the 3×2 structure changes to the 2×2 structure of 1/4 ML at 310 °C. The 2×2 structure changes further to the second phase of the 3×2 structure of 1/6 ML at 340 °C and the 3×2 structure becomes a clean 2×1 structure above 370 °C [94K5].

Si(100)3x2-Mg, 1/6 ML

Fig. 65. Possible structure models for (a) Si(100)3×2-Mg (1/3 ML Mg coverage), (b) Si(100)2×2-Mg (1/4 ML Mg coverage) and (c) second Si(100)3×2-Mg (1/6 ML coverage) built of Mg-atom chains. The models are suggested on the basis of LEED-AES data [94K5].

Lando lt -Bö rnst ein New Series III/42A1

341

342

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.17 Ni (Nickel) on Si Table 38. Structures formed by Ni on Si(100), Si(110), Si(111), Si(210), Si(320), Si(113), Si(510), Si(115) and Si(331) surfaces; their preparation conditions Structure

Preparation

Model

Technique

Ref.

Fig.

Table Remarks

Ni/Si(100) 2×n

86A, 86M, 88K2, 66, 67 89D2, 91D2, 96O4 88K2, 89D2, 91D2, 96O4, 96U1 UPS 82C ARUPS 90E He scattering 89R4 STM 88N2, 95Z3, 95K5, 95M2, 96U1, 96S5, 96K2, 98K2, 98Y2, 98O3, 98F2 SEM 89D2, 91D2 THEORY 95Z3, 95M2, 99N3

6 ≤ n ≤ 10 , n decreases with Ni concentration; Si reconstruction stabilized by Ni traces

LEED

65J, 77O, 85I2, 88N8, 89D2, 93S2 92Y7 85I2, 89D2 88L3, 92Y7, 93S2 93S2 88L3 88N1, 88L3, 98O4, 99Y2 89D2 88N8 90S

transforms reversibly to 1×2 at 870–920 K

65J, 77O, 85I2, 88N8, 89D2, 93S2 92Y7 85I2, 89D2 92Y7, 93S2 93S2 99Y2 88N8 90S

transforms reversibly to 1×5 through 1×7 and 1×9 at 990–1070 K

0.3–2 ML, Si dimers and LEED ann. 1100–1500 K, missing Si quench dimers AES

Ni/Si(110) 5×4

0.5–1.0 ML ann. 970 K, quench

RHEED AES XPS RMEED RBS STM SEM WFM THEORY

1×2

0.5–1.0 ML ann. 920 K, cooling at 10 K/s

LEED RHEED AES XPS RMEED STM WFM THEORY

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

343

Table 38 (cont.) Structure

Preparation

1×n

n×5

Model

Technique

Ref.

Fig.

Table Remarks

0.5–1.0 ML ann. 1070 K, cooling at 10 K/s

LEED RHEED AES XPS UPS RBS STM WFM THEORY

65J, 77O, 85I2, 88N8, 89D2 92Y7 85I2, 89D2 88L3, 92Y7 76S 88L3 88B3, 88L3, 99Y2 88N8 90S

mostly n = 5, but also 7 and 9 for intermediate phases between 1×2 and 1×5; with increase in Ni concentration structures form a sequence1×5, 1×9, 1×7, 1×2

0.5–1.0 ML ann. 1720 K, quench

LEED RHEED AES XPS STM

89D2 92Y7 89D2 92Y7 99Y2

mostly n = 4, but also 8 and 9; with increase in Ni concentration structures form a sequence8×5, 9×5, 4×5, 1×5

Ni/Si(111) random RC’s LEED I-V with one Ni AES atom PES ISS STM LEEM

83Y1 42 83Y1, 95L3 81C, 99G 81C 95P1, 95B4, 96Y5 95P1, 95B4

19 × 19 0.2–0.5 ML, ann. hexagonally- LEED 1070–1270 K, packed RC’s quench with 3 Ni LEED I-V atoms RHEED AES

SEM

59S, 67B, 68B, 68 69T, 89D2 67B 77I1 69T, 70C, 89D2, 89I2, 91D2, 95L3, 96P 81C, 99G 81H1, 81H2 81C 87W2, 88C2, 96P, 96Y2, 96Y5 89D2, 91D2

LEED LEED I-V STM

84C1 85Y 96P, 96Y2

"1×1"

0.1 ML, ann. 1130 K, quench

PES ARPES ISS STM

3× 3

0.2 ML ann. 470 K or 0.3–1 ML ann. 620–720 K

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ideal coverage 0.16 ML

344

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 38 (cont.) Structure

Preparation

Model

Technique

Ref.

Fig.

Table Remarks

0.004–0.01 ML ann. 1200–1300 K, quench

LEED AES

94O1 94O1

Si reconstruction stabilized by Ni traces

∼ 0.02 ML ann. 1200–1300 K, quench

LEED AES

94O1 94O1

Si reconstruction stabilized by Ni traces

0.002–0.004 ML ann. 900–1100 K, quench

LEED AES ARUPS

81O1, 89M5, 90X, 94O1 89M5, 94O1 89M

Si reconstruction stabilized by Ni traces

2×2

0.008–0.02 ML ann. 1200–1300 K, quench

LEED AES ARUPS

89M5, 90X, 94O1 89M5, 94O1 89M5

Si reconstruction stabilized by Ni traces

4×2

0.015–0.025 ML ann. 1200–1300 K, quench

LEED AES

94O1 94O1

Si reconstruction stabilized by Ni traces

1×4

0.003–0.005 ML ann. 1200–1300 K, quench

LEED AES

94O1 94O1

Si reconstruction stabilized by Ni traces

1×2

0.005–0.01 ML ann. 1200–1300 K, quench

LEED AES

94O1 94O1

Si reconstruction stabilized by Ni traces

3×3

0.003–0.005 ML ann. 1200–1300 K, quench

LEED AES

94O1 94O1

Si reconstruction stabilized by Ni traces

3×1

0.01–0.013 ML ann. 1200–1300 K, quench

LEED AES

94O1 94O1

Si reconstruction stabilized by Ni traces

Ni/Si(210) 2×2

Ni/Si(320) 2×1

Ni/Si(113) 3×2

Ni/Si(510)

Ni/Si(115)

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

345

Table 38 (cont.) Structure

Preparation

7×1

Model

Technique

Ref.

Fig.

Table Remarks

0.013–0.018 ML ann. 1200–1300 K, quench

LEED AES

94O1 94O1

Si reconstruction stabilized by Ni traces

c(13×2) or ∼ 0.04 ML ann. 1200–1300 K, 13×1 quench

LEED AES

91W2, 94O1 94O1

Si reconstruction stabilized by Ni traces

0.01–0.015 ML ann.1200–1300 K, quench

LEED AES

94O1 94O1

Si reconstruction stabilized by Ni traces

Ni/Si(331)

c(17×2) or 17×1

Lando lt -Bö rnst ein New Series III/42A1

346

3.3.2 Metals on semiconductors

Si(100)2xn-Ni

Fig. 66. The formation of the Ni-induced Si(100)2×n structure is governed by Ni concentration and heat treatment, as Ni atoms diffuse into the bulk during annealing and segregate to the surface upon quenching. (a) Variation of the AES peak ratio, R = I Ni-LMM / I Si -LVV, for Ni/Si(100) samples with annealing temperature (closed circles, solid curves) and after quenching from various temperatures to room temperature (open circles, broken curves). Samples I and II have different initial concentration. The region of the observation of the Si(100)2×n structure is indicated. (b) Variation of n for Si(100)2×n structures with AES peak ratio, R = I Ni-LMM /I Si-LVV as determined using LEED [88K2]

[Ref. p. 396

Si(100)2xn-Ni

Fig. 67. The structure of the Ni-induced Si(100)2×n surface comprises the Si dimer rows interrupted by the line defects composed of dimer-vacancy defects. (a) STM image of the Si(100)2×n surface [95K5]. The atomic arrangement of the most frequently observed line defects involves (b) double dimer vacancy and (c) sequence of double dimer vacancy, "split-off" dimer and single vacancy. These configuration were shown to have the lowest energy [95M2]. In the figure, open and filled circles denote the top- and second-layer Si atoms, respectively. The similar structures are induced on Si(100) also by small amounts of Bi, Co and Cu.

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

347

Si(111) 19 × 19 -Ni

Fig. 68. The structure of the Si(111) 19 × 19 -Ni which comprises the array of the hexagonally-packed ring clusters (RC) [96P]. Each RC contains 6 dimerized Si adatoms (shaded circles) and 3 Ni atoms (closed circles) substituting top Si atoms. The center of RC of this type is located in H3 site. The ideal Ni coverage of the Si(111) 19 × 19 -Ni is 3/19 (∼ 0.16) ML. At lower Ni coverages, the RC’s of another type are formed. These RC’s are smaller and look more bright in the filled state STM images. They incorporate one Ni atom each and are centered at on-top site. The random distribution of these RC’s produces the ‘‘1×1’’-RC phase similar to that observed in Co/Si(111) system.

3.3.2.3.18 Pb (Lead) on Si Table 39. Structures formed by Pb on Si(100), Si(110) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

Tab.

Remarks

Pb/Si(100) 2×2

0.3–0.6 ML, parallel 300–670 K asymmetric Pb dimers

LEED AES STM THEORY

92Z3 92Z3 94I4, 94L6, 96V 98G6

69, 70 40

c(8×4)

0.6–0.9 ML, asymmetric 300–800 K Pb dimers

LEED AES STM THEORY

92Z3 92Z3 94I4, 94L6 98T3

69, 90

Lando lt -Bö rnst ein New Series III/42A1

transforms irreversibly to c(8×4) at 720 K

348

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 39 (cont.) Structure

Preparation

4×1

Model

Technique Ref.

Fig.

Tab.

0.9–1.5 ML, 300–670 K

LEED AES

92Z3 92Z3

69

2×1

1.0–2.0 ML, symmetric 300–600 K Pb dimers

LEED AES UPS EELS STM

92Z3 92Z3 89L1, 89L2 93Z4 94I4, 94L6

69

c(4×4)

1.5–2.1 ML, Pb dimers 300–570 K

LEED AES STM LEEM

92Z3, 94L6 92Z3 94I4, 94L6 94L6

69, 71

1×1

∼ 1 ML, 300 K

LEED AES RHEED

94N5, 95N1 94N5, 95N1 96O3

 4 3   − 2 2

0.20–0.55 ML ann. 680 K

RHEED

96O3

LEED LEED I-V RHEED AES PES ISS STM PEH SSM

64E, 85S, 88L2 91D3 88Y1, 88Y2, 92C1 88L2 91H3, 92C1 85S 91G2 95R 92R2

38, 72, 41, 42 73

64E, 85S, 91G2 88Y1, 88Y2, 92C1, 92W4 92C1 91W3 92W2 91G2, 94S5, 95S6 92R2

72, 74

Remarks not detected in STM [94L6, 94I4]

two orthogonal domains on the same terrace; at 570 K transforms reversibly to 2×1

Pb/Si(110)

two domains; transforms reversibly to 1×1 at 470 K

Pb/Si(111) α- 3 × 3

1/3 ML, 670–770 K

β- 3 × 3

4/3 ML, ann. compressed LEED closepacked RHEED ≥ 600 K, cool to RT IC Pb layer PES UPS ARPES STM SSM

Pb adatoms

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

349

Table 39 (cont.) Structure

Preparation

γ- 3 × 3 ("mosaic")

0.2–0.3 ML, Pb and Si 670–770 K adatoms

7×7 (lowcoverage)

Model

0.06–0.12 ML, Pb adatoms 300 K on Si(111)7×7

Technique Ref.

Fig.

LEED 91G1, 91G2 LEED I-V 95J1 RHEED 92C1

116

PES ARPES RBS STM STS SDR SSM

92C1, 92K3 92K3 91G1, 91G2 91G1, 91G2, 97G3 97G3 97C6 92R2

LEED STM

91G1, 91G2 91G1, 91G2, 95T2, 96G2 91G1, 91G2

75

64E, 85S, 91G2, 92W4 88Y1, 88Y2, 92C1, 92W4 91H3, 91W3, 92C1 92W2 85S 91G2 91G2, 92H3, 95H1 92H3

72, 75

RBS 7×7 (highcoverage)

1.0–1.3 ML, closedpacked LEED 300 K Pb layer on Si(111)7×7 RHEED PES ARPES ISS RBS STM STS

Lando lt -Bö rnst ein New Series III/42A1

Tab.

Remarks SixPb1–x/Si(111) solid solution with ideal x = 0.5

350

3.3.2 Metals on semiconductors

Pb/Si(100)

[Ref. p. 396

Si(100) c(4x4)-Pb

Fig. 69. Phase diagram for the Pb/Si(100) system as determined using LEED-AES observations [92Z3]. All phases except for the 4×1 one have been detected by STM.

Si(100)2x2-Pb

Fig. 70. Possible structural model of the Si(100)2×2-Pb surface as proposed on the basis of STM data [94I4]. The structure consists of parallel asymmetric Pb dimers (shown by closed circles of different size) on still dimerized Si(100)2×1 surface. Structural parameters are presented in Table 40.

Table 40. Bond lengths and inclination angle for the buckled Pb dimer obtained for the Si(100) 2×2Pb phase (Fig. 70) using first-principles total energy calculations [98G6]. Parameter

value

Pb-Pb ad-dimer [Å] Pb-Si (upper Pb) [Å] Pb-Si (lower Pb) [Å] Si-Si dimer [Å] Buckling angle [°]

3.13 2.80 2.74 2.40 12.57

Fig. 71. (a) Filled state STM image (300×200 Å2) of two orthogonal domains of the Si(100) c(4×4)-Pb surface [94L6]. (b) Possible structure of the Si(100)c(4×4)-Pb phase as proposed on the basis of STM and LEEM observations [94L6]. The structure consists of the firstlayer Pb dimers (shown by large hatched circles) which reside above the complete layer of buckled Pb dimers (up and down Pb atoms are shown by black and white circles, respectively). Small white circles represent Si atoms. The model structure has a saturation coverage of 1.25 ML. The two orthogonal domains occur on the same terrace as a result of the placement of the top layer Pb addimers with respect to the buckling direction of the Pb dimers in the lower layer. Ovals correspond to the bright features seen in STM image.

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Fig. 72. Phase diagram for the Pb/Si(111) system as determined using RHEED [88Y2]. All phases shown in the phase diagram are stable at RT, except for the 1×1 one. In accordance with notation accepted in this chapter the low-coverage

Pb/Si(111)

3 × 3 structure is labeled as α-

phase and the high-coverage 3 × 3 structure as βphase. In contrast, in the original paper and several other publications, the opposite notation was used (i.e. the lowcoverage

3 × 3 structure was labeled as β-phase and

the high-coverage

3 × 3 structure as α-phase).

Table 41. Displacement of Si atoms from ideal positions in the Si(111) 3 × 3 -Pb α-phase (T4 adatom model) as determined using LEED I-V analysis [91D3]. ∆r corresponds to lateral displacement towards adatom, ∆z corresponds to vertical displacement into the bulk. Atoms are numbered as in Fig. 73. Displacement

[Å]

∆r(2) ∆z(2) ∆z(5) ∆z(4) ∆z(7) ∆z(6) ∆z(8)

0.11 –0 08 0.11 –0 27 0.11 –0 11 –0 05

Table 42. Distances between Pb adatom and second-layer Si atoms and the bond lengths in the

Si(111)α- 3 × 3 -Pb

Fig. 73. Side view of the Si(111) 3 × 3 -Pb (α phase, 1/3 ML Pb): T4 Pb-adatom model. The values of displacements of Si atoms from ideal positions, the distance between Pb adatom and second-layer Si atom and the bond lengths are listed in Tables 41 and 42. Pb adatom is shown in black and Si atoms in white.

Si(111)β- 3 × 3 -Pb

Si(111) 3 × 3 -Pb α-phase (T4 adatom model) as determined using LEED I-V analysis [91D3]. Atoms are numbered as in Fig. 73. Distance

[Å]

d1-5 b1-2 b2-5 b2-4 b5-7 b4-6 b7-8 b6-8

2.40 2.55 2.32 2.40 2.34 2.52 2.30 2.37

Lando lt -Bö rnst ein New Series III/42A1

Fig. 74. Top view of the Si(111) 3 × 3 -Pb β-phase with 4/3 ML Pb. Pb atoms are represented by large hatched circles and Si atoms are shown by open circles. The unit cell contains one Pb atom in H3 cite and three Pb atoms displaced from the bridge site towards the T1 site (labeled off-centered T1 Pb).

351

352

3.3.2 Metals on semiconductors

[Ref. p. 396

Si(111)7x7-Pb

Fig. 75. Structural models of the structures formed at RT deposition of Pb onto Si(111)7×7. (a) and (b) lowcoverage Si(111)7×7-Pb phases with 0.06 ML and 0.12 ML Pb, respectively, as proposed on the basis of STM observations [91G1]. (c) high-coverage (1.22 ML) Si(111)7×7-Pb phase as proposed on the basis of STM observations [95H1]. Small circles represent the first and second Si(111) substrate, large open and filled circles represent Si and Pb adatoms, respectively. Pb dimers are shown in overlapping large open circles. The faulted and unfaulted halves of the 7×7 unit cell are indicated by F and U, respectively.

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

353

3.3.2.3.19 Pd (Palladium) on Si

Table 43. Structures formed by Pd on Si(100) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

Table Remarks

Pd/Si(100) c(4×2)

20 ML Si dimers and ann. 1170–1470 K embedded Pd atoms

LEED AES EELS STM

80O3, 93I1 80O3, 93I1 80O3, 93I1 93I1, 94I3

76

ideal coverage 0.25 ML

c(6×4)

20 ML Si dimers, ann. 1270–1470 K missing Si dimers and embedded Pd atoms

LEED AES EELS STM

80O3, 93I1 80O3, 93I1 80O3, 93I1 93I1, 94I3

76

coexists with c(4×2)

0.03–0.1 ML ann. 1270 K, quench

random RC’s

LEED 86Y1 LEED I-V 86Y2 STM 95P1

0.5–3 ML ann. 550–770 K

Pd trimers ?

LEED

Pd/Si(111) "1×1"

3× 3

42

RHEED AES ISS STM TEM TED REM

82Y2, 84C1, 86Y1 82O, 88A1 82Y2 82Y2 95C4, 97Y2 88A1 88A1 88A1, 89T4

2 3 × 2 3 0.5–3 ML ann. 820–1270 K

LEED RHEED AES

80O3, 84C1 82O 80O3

< 0.1 ML left at the surface

3 3 × 3 3 2–3 ML ann. 550–770 K

STM

95C4, 97Y2

Si-segregated

Lando lt -Bö rnst ein New Series III/42A1

3 × 3 -Pd ?

354

3.3.2 Metals on semiconductors

[Ref. p. 396

Pd/Si(100)

Fig. 76. Possible structure of the reconstructed Pd/Si(100) surface as suggested on the basis of the STM observations. The Si(100) c(4×2)-Pd surface phase is formed by regular arrangement of Pd atoms embedded in the fourfold hollow sites surrounded by two Si dimers. The Si(100) c(4×6)Pd surface phase is formed by regular arrangement of the missing dimer defects superimposed on the c(4×2) structure [93I1]. The similar structures are formed in the Pt/Si(100) system.

3.3.2.3.20 Pt (Platinum) on Si Table 44. Structures formed by Pt on Si(100) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

Table Remarks

Pt/Si(100) c(4×2)

4–10 ML Si dimers ann. 820–1270 K and embedded Pt atoms

LEED AES EELS STM

80O2, 92I1 92I1 92I1 92I1, 94I3

76, 77

0.16–0.33 ML left at the surface; coexists with c(4×6)

c(4×6)

4–10 ML Si dimers, ann. 820–1270 K missing Si dimers and embedded Pt atoms

LEED AES EELS STM

80O2, 92I1 92I1 92I1 92I1, 94I3

76, 77

0.16–0.33 ML left at the surface; coexists with c(4×2)

0.03–0.1 ML ann. 1270 K, quench

LEED 86Y1 LEED I-V 86Y2 STM 95P1

42

LEED

80O2, 86Y1, 88M

78

AES

80O2, 88M, 89M6

LEED

80O2, 86Y1, 88M 80O2, 88M

Pt/Si(111) "1×1"

3× 3

7× 7

0.3–3 ML ann. 920–1270 K

1–3 ML ann. 600–920 K

random RC’s

AES

78

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

355

Pt/Si(100)

Pt/Si(111)

Fig. 77. Phase diagram of the submonolayer Pt/Si(100) system [92I1] as a function of the Pt-to-Si Auger-peak ration or the Pt coverage derived from the Auger-peak ratio. Here I0 (Si LVV) is the Si LVV Auger signal from a clean Si(100) surface [92I1].

Fig. 78. Temperature range in which the Si(111)

7 × 7 -Pt and Si(111) 3 × 3 -Pt surface phases are observed and the changes in Auger peak heights of Pt (open circles) and Si (closed circles) during annealing. Both Auger peaks are normalized to their pure states [80O2].

3.3.2.3.21 Sb (Antimony) on Si

Table 45. Structures formed by Sb on Si(100), Si(110) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Sb/Si(100) 0.7–1.0 ML, 2×1 620–920 K

Lando lt -Bö rnst ein New Series III/42A1

Model

Technique

symmetric Sb dimers

LEED RHEED SEXAFS STM AES PES

Ref.

Fig.

Tab.

90R4, 90Z, 91R1 79, 80 46 89G, 97K7 90R4, 91R1 89R1, 90R4, 91N4 90Z, 97K7 89R1, 89R2, 89R3, 96J1, 98P6 ARUPS 93C5, 93C6 XSW 95L1, 96Q2 ISS 96N3 MEIS 92S3, 98S4 Ion channeling 92G2, 95B5 THEORY 93T1, 93T5, 94Y6, 95C1, 96J1, 98T4, 98T6, 99M5

Remarks diffuse 1/2-order spots in LEED and RHEED: high density of defects

356

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 45 (cont.) Structure

Preparation

Model

Technique

c(4×4)

0.2–0.3 ML, 840–1040K

overlaid Sb LEED dimers AES RAS ISS THEORY

Ref.

Fig.

95W, 97P1, 98D1 95W, 98D1 97P1 98D1 98D1, 98J1, 98J2

81

Tab.

Remarks observed at Sb desorption in narrow Sb coverage interval

Sb/Si(110) β-3×2

1 ML, 920 K Sb trimers ? LEED PES AES SDR

89R2, 92Z2 82, 83 89R2, 95P3, 96C6, 97C2 92Z2 95P3, 96C6

1×2

0.6–0.85 ML, disordered ? LEED 920–1120 K AES

92Z2 92Z2

82

α-3×2

0.2–0.6 ML, 920–1120 K

Sb adatoms LEED ? AES PES

92Z2 92Z2 98C4

82, 83

 14 0    − 2 1

< 0.3 ML, 920–1120 K

LEED AES

92Z2, 94Z3, 94L5 92Z2, 94Z3, 94L5

82

LEED RHEED AES XPD XPS STM

88P4, 90Z 84, 85 47 91A1 90Z 88A2, 96C3 88K3, 88P4 90M1, 91E2, 94N4, 97K2 92N2, 96N2, 97K5 91A1 97L3 90M1, 93N, 93W1, 98T5

intermediate structure between β-3×2 and α3×2

two domains; coexists with α-3×2

Sb/Si(111)

3× 3

1 ML, 870–1020 K

Sb trimers

XRD SE PEH THEORY

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

357

Table 45 (cont.) Structure

3× 3

2×1

Preparation

Model

0.33 ML, 970 K

Sb adatoms STM

0.8–0.95 ML, Sb-atom 870–1020 K zigzag chains

5 3 × 5 3 0.5–0.7 ML, 870–1020 K

1×1 (LEED)

∼ 1 ML RT deposit. onto 7×7 or 2×1 Si(111) surfaces

Technique

LEED RHEED XPS STM SE

Sb(Si)LEED adatoms on RHEED 5×5 surface XPS STM

disordered

Ref.

Fig.

Tab.

91E2

88P4 91A1 88P4 91E2, 97K2 91A1

Remarks in STM: small domains

84, 86

SE THEORY

88P4 84, 87, 91A1 88 88P4 97K2, 97P4, 97P6, 99K1 91A1 99K1

LEED AES PES STM

84M, 86T2 84M, 86T2 86T2 96C7

2×2 LEED: three domains; in STM: short chains with poor LRO

Sb clusters ?

Table 46. Structural parameters of the Si(100)2×1-Sb, the Sb-Sb dimer length (L) and the height of Sb dimer above the first Si(100) layer (h).

L[Å] h[Å]

THEORY [93T1,93T5]

THEORY [94Y6]

THEORY [95C1]

SEXAFS [90R4]

Ion channeling [92G2]

XSW [95L1]

2.93±0.05 1.73±0.05

2.96 1.70

2.94 1.72

2.88±0.03 1.74±0.05

2.8±0.1 1.7±0.1

2.81±0.09 1.74±0.05

Lando lt -Bö rnst ein New Series III/42A1

358

3.3.2 Metals on semiconductors

Si(100)2x1-Sb

[Ref. p. 396

Si(100)2x1-Sb

Fig. 80. Structural parameters of Sb dimer on the Si(100) surface as determined using ab initio pseudopotentional calculations [98T4]. The data for the Sb-Sb dimer length (L) and the height of Sb dimer above the first Si(100) layer (h), determined using various experimental and theoretical techniques are listed in Table 46. Fig. 79. The structure of the Si(100)2×1-Sb surface. (a) 50×50 Å2 empty state STM image of the Si(100)2×1Sb surface formed by annealing of 0.6 ML Sb at 820 K. The surface comprises the array of the Sb dimer rows. The brighter areas are Sb dimers of the second Sb layer. The surface is characterized by a high density of defects, voids (seen as dark regions) and anti-phase domain boundaries. (b) The registry of the Sb dimers observed by STM relative to the clean Si(100)2×1 surface. The Sb dimers are aligned perpendicular to the Si dimers and form rows that run parallel to the Si dimer direction [90R4]. Similar structure is formed in Te/Si(100) system.

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Si(100)c4x4-Sb

Sb/Si(110)

Fig. 81. Possible structure of the Si(100)-c(4×4)Sb surface with the Sb dimers (0.25 ML) lying above Si dimer rows and oriented perpendicular to the direction of the Si dimers in the uppermost substrate layer. The Sb-Sb dimers are shown in black, with high and low-lying firstlayer Si atoms indicated by shaded and open circles, respectively. Both primitive and conventional unit cells are shown. The model was chosen as having the minimal free energy and yielding the best fit to the CAICISS data as compared to the other candidate models (with Sb dimers substituting Si dimers and with Sb dimers sitting in between the underlying Si dimer rows) [98D1].

Fig. 82. Phase diagram of the submonolayer Sb/Si(110) system as determined using LEED-AES. The Si(110)β3×2-Sb structure is obtained by saturation adsorption of 1 ML at 920 K. The other structures are observed during Sb desorption at 920–1020 K [92Z2].

Sb/Si(111)

Si(110)α-2x3-Sb

Si(110)β-2x3-Sb

Fig. 84. (a) adsorption and (b) desorption phase diagrams of Sb on Si(111). At Sb adsorption and desorption, the same structures are observed, but at different Fig. 83. Possible atomic arrangement of the (a) Si(110)α3×2 structure built of Sb trimers (Sb coverage is 1.0 ML) [92Z2] and (b) Si(110)β-3×2 structure built of Sb adatoms (Sb coverage is 0.33 ML) [98C4]. Sb atoms are shown in black and Si atoms in white.

Lando lt -Bö rnst ein New Series III/42A1

temperatures. The Si(111) 3 × 3 -Sb structure is formed at saturation deposition of 1 ML. For temperatures above 650 °C, 3 × 3 converts to 2×1 just after the Sb deposition is interrupted [91A1].

359

360

3.3.2 Metals on semiconductors

Si(111) 3 × 3 -Sb

[Ref. p. 396

Table 47. Bond lengths between atoms, coordinates of Sb atoms and displacement parameters for Si atoms in the first three near surface layers in the Si(111) 3 × 3 -Sb structure. Numbers are all in Å units.

Sb-Sb Sb-Si r1 d12 δ2 δ4 δ5 δ6 δ7 δ8

XRD [97K5]

THEORY [90M1]

XRD [92N2]

2.92±0.01 2.47±0.04 1.688±0.003 2.437±0.017 0.123±0.014 0.048±0.009 –0.127±0.030 –0.028±0.017 0.027±0.020 –0.011±0.008

2.82 2.57 1.63 2.51 0.06 –0.07 –0.16 –0.16 –0.10 –0.10

2.82 2.74 1.63 2.63 –0.18

Fig. 85. (a) Top and (b) side views of the Si(111)

3 × 3 -Sb structure. Vertical displacement vectors are shown for the first three top Si layers. Lateral displacements from the ideal positions are shown for the topmost Si layer. d12 and r1 are absolute distances, not a displacement. The displacement parameters are listed in Table 47. Sb atoms are shown in black and Si atoms in white [97K5].

Si(111)2x1-Sb

Fig. 86. Structure of the Si(111)2×1-Sb zigzag chains (top view) [91E2]. Sb atoms are shown in black and Si atoms in white.

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Si(111) 5 3 × 5 3 -Sb

Si(111) 5 3 × 5 3 -Sb

Fig. 88. Possible model of the atomic arrangement of the Fig. 87. The structure of the Si(111) 5 3 × 5 3 -Sb surface. The periodic arrangement of the triangular subunits of three types, namely, unfaulted 5×5 unit cell halves with a single adatom (SAUH), unfaulted halves with three adatoms (TAUH) and faulted halves without adatoms (FH), produces the surface with a 5 3 × 5 3 periodicity. (a) 170×110 Å2 filled state STM image of the Si(111) 5 3 × 5 3 -Sb surface. (b) Schematic presentation of the "ideal" 5 3 × 5 3 -reconstructed surface.

Lando lt -Bö rnst ein New Series III/42A1

Si(111) 5 3 × 5 3 -Sb structure. The unit cell is composed of six triangular regions with three faulted and the other three unfaulted unit cell halves of the Si(111)5×5 DAS structure. Three faulted units (FH) have no adatoms, while each of two unfaulted units (TAUH) has three adatoms and one unfaulted unit (SAUH) has a single adatom. In the SAUH, the shaded circles represent two unoccupied sites among three possible Sb adsorption sites. The Sb atoms substitute Si atoms in the rest-layer sites, except for those bonded to adatoms. Adatoms are considered to be Si atoms in Ref. [97K2] and Sb atoms in Ref. [99K1]. During Sb desorption, substitution of Sb adatoms by Si adatoms plausibly takes place.

361

362

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.22 Sm (Samarium) on Si Table 48. Structures formed by Sm on Si(100) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model Technique

Ref.

Fig.

Table Remarks

Sm/Si(100) 3×2

0.8 ML ann. 750 K

LEED AES PES

91O 91O 91O

LEED STM PES

93W3, 93W9 93W9 93W9

Sm/Si(111) 3×1 (LEED) 0.2–0.3 ML ann. 770 K

5×1

∼ 0.4 ML ann. 770 K

LEED STM PES

93W3, 93W9 93W9 93W9

7×1

∼ 0.5 ML ann. 770 K

LEED STM PES

93W3, 93W9 93W9 93W9

in STM: local structure 3×2

3.3.2.3.23 Sn (Tin) on Si Table 49. Structures formed by Sn on Si(100) and Si(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

2×2

0.5 ML, RT deposit.

parallel Sn dimers STM

c(4×4)

0.2–0.375 ML ann. 820–920 K

Fig.

Table Remarks

Sm/Si(100)

LEED RHEED AES PES STM

91B4, 98G4

84U 84U, 88R1 84U 88R1, 90R2 91B4

not seen by LEED and RHEED; in STM: local domains with poor LRO 89

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

363

Table 49 (cont.) Structure

Preparation

Model

6×2

0.375–0.5 ML, Sn-Si dimers ? ann. 820–920 K

Technique Ref.

Fig.

LEED

89

PES IPES ARUPS XSW STM

84U, 94P1, 97L2, 97P2 84U, 88R1 84U, 94P1, 97L2, 97P2 88R1, 90R2 94P1 97P2 97L2 91B4

RHEED AES

4×1

0.6 ML, ann. 920 K

LEED AES ARUPS XRD

97P2 97P2 97P2 98I1

c(8×4)

0.5–1.0 ML, asymmetric ann. 820–920 K Sn dimers

LEED

84U, 94P1, 97L2, 97P2 84K2, 84U, 88R1 84U, 94P1, 97L2, 97P2 88R1, 90R2 94P1 97P2 97L2 98I1 91B4 97P2

89, 90

84U, 94P1, 97L2, 97P2 84K2, 84U, 88R1 84U, 94P1, 97L2, 97P2 88R1, 90R2 94P1 97P2 97L2 98I1 91B4 97P2

89

RHEED AES PES IPES ARUPS XSW XRD STM SDR 5×1

1.0–1.5 ML, double Sn layer ann. 820–920 K

LEED RHEED AES PES IPES ARUPS XSW XRD STM SDR

Lando lt -Bö rnst ein New Series III/42A1

Table Remarks transforms reversibly to 2×1 at 770 K

364

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 49 (cont.) Structure

Preparation

Model

Technique Ref.

Fig.

Table Remarks

Sn/Si(110)  4 3   − 2 2

0.14–0.44 ML, Sn trimers ? ann. 1000–1070 K

RHEED TRAXS UPS STM

93Y1 93Y1 93Y1 98A2

91

 3 0    − 1 3

0.4–0.63 ML, Sn trimers and ann. 1000–1070 K Sn adatoms ?

RHEED TRAXS UPS STM

93Y1 93Y1 93Y1 98A2

91

 3 3    − 2 1

0.63–0.92 ML, ann. 1000–1070 K

RHEED TRAXS STM

93Y1 93Y1 98A2

91

 3 3   − 4 2

2–3 ML, ann. 890 K

STM

98A2

 2 0    −1 4

2–3 ML, ann. 1000 K

STM

98A2

c(4×4)

2–3 ML ann. 1000 K

STM

98A2

in STM: local areas  2 0  between   −1 4  3 3  and   − 2 1

Sn/Si(111)

3× 3

1/3 ML, Sn adatoms 620–770 K or 0.3–0.5 ML RT deposit. onto cleaved Si(111)2×1

LEED RHEED AES PES

64E, 85T3 84I2 85T3 93G2, 95G4, 98A1 ARUPS 86K3, 87K4, 88O2 XRD 89C3 STM 89N1, 90K2, 94T1, 98Y3 STS 90K2, 96L3 THEORY 90R5, 93A2, 93G2, 96A2, 96A3

92, 93, 50

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

365

Table 49 (cont.) Structure

Preparation

3× 3 0.3 ML ("mosaic") ann. 1070 K

Model

Technique Ref.

Sn and Si adatoms STM STS PES

2 3 × 2 3 1.0–1.2 ML double Sn layer ann. 470–770 K

133 × 4 3 ≥ 1.1 ML, ann. 470–570 K

90K2, 94T1, 98Y3 90K2 95G4

LEED RHEED PES

Fig. 116

64E 84I2 93G2, 94T1, 98A1 ARUPS 87K4, 88O2 XRD 96L2 ICISS 92W6 RBS 94T1 STM 89N1, 90K2, 91T5, 92W6, 94T1 STS 90K2, 96L3 THEORY 93A2, 93G2, 96A3

92, 94

RHEED STM

92

84I2 98Y3

Table Remarks SixSn1–x/Si(111) solid solution with ideal x = 0 5

coexists with

3 7 × 3 7 and 2 91 × 2 91

3 7 × 3 7 ≥ 1.1 ML, ann. 470–570 K

RHEED STM

84I2

92

133 × 4 3 and 98Y3

2 91 × 2 91≥ 1.1 ML, ann. 470–570 K

RHEED STM

coexists with

84I2 98Y3

2 91 × 2 91 92

coexists with

133 × 4 3 and 3 7 ×3 7

Lando lt -Bö rnst ein New Series III/42A1

366

3.3.2 Metals on semiconductors

Sn/Si(100)

[Ref. p. 396

Si(100)c(8x4)-Sn

Fig. 89. Phase diagram for the Sn/Si(100) system as determined using RHEED-LEED-AES observations [84U, 88R1]. In addition to the shown structures, the 4×1 structure has been reported to be observed at about 0.6 ML annealed to 650 °C [97P2, 98I1].

Sn/Si(110)

Fig. 91. Phase diagram for the Sn/Si(110) system as determined using RHEED-TRAXS observations [93Y1]. In the STM study, in addition to the shown structures, the (3,3)×(–4,2), (2,0)×(–1,4) and c(4×2) structures have been detected [98A2].

Fig. 90. (a) Filled state STM image of Si(100) c(8×4)-Sn surface. (b) Structural model as suggested on the basis of the STM data [91B4] and first-principles total-energy calculations [98T3]. The structure is built of asymmetric Sn dimers (large open and filled circles representing the "up" and "down" Sn atoms, respectively). The smaller open circles represent Si atoms. The same model can be applied to the Si(100) c(8×4)-Pb and Ge(100) c(8×4)-Pb surfaces.

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Sn/Si(111)

367

Si(111) 2 3 × 2 3 -Sn

Fig. 92. Phase diagram showing structures detected by RHEED during annealing of the Sn/Si(111) system [84I2]. The Sn coverage is that before heating. A, B and C denote the

2 91 × 2 91

133 × 4 3 , 3 7 × 3 7 and

structures. In the coverage range

0.3–1.0 ML, 2 3 × 2 3 coexists with

3 × 3 , while

the ideal coverage for the 2 3 × 2 3 is within the range 1.0–1.2 ML as determined using STM-RBS [94T1].

Upon

successively to

heating,

2 3×2 3

transforms

3 × 3 at 190 °C and 1×1 at 860 °C.

Fig. 94. Possible structure of the Si(111) 2 3 × 2 3 Sn as proposed on the basis of the data obtained by STM [91T5], ICISS [92W6] and RBS [94T1]. The model corresponds to Sn coverage of 1.17 ML and suggests the presence of two layers of Sn. Relative heights are given with respect to the topmost Sn atoms depicted by the largest circles. The first layer Si atoms are shown by small open circles [91T5].

Both transitions are reversible. The 3 × 3 structure can be produced also by deposition of 0.3–0.5 ML of Sn onto the cleaved Si(111)2×1 surface held at RT [85T3].

Table 50. Structural parameters for the Si(111) Si(111) 3 × 3 -Sn

Fig. 93. (a) Top and (b) side views of the Si(111) 3 × 3 -Sn structure: T4 Sn-adatom model. The directions of displacements are shown by arrows. The values of displacements of Si atoms from ideal positions are listed in Table 50. Sn adatoms are shown in white and Si atoms in black.

Lando lt -Bö rnst ein New Series III/42A1

3 × 3 -Sn structure as determined using total energy calculations [90R5] and XRD [89C3]. The ∆ represents vertical displacements (+ is up, – is down) and a represents lateral displacements (– is towards Sn atoms, + is away). The displacements are given in Å. Displacement

THEORY [90R5]

XRD [89C3]

∆2 ∆2′ ∆3 ∆3′ a1 a4

+0.29 –0.48 +0.07 –0.15 –0.13 +0.05

+0.20 –0.42 +0.17 –0.35 –0.21 +0.10

368

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.24 Sr (Strontium) on Si Table 51. Structures formed by Sr on Si(100) surface; their preparation conditions and techniques used for their investigation Structure

Preparation

Model

Technique

Ref.

Fig.

Table Remarks

Sr/Si(100) 3×2

Sr-atom chains ∼ 0.3 ML, 1070–1470 K

LEED AES STM

90F3, 95B6, 96B2, 96B4 90F3 95B6, 96B2, 96B4

95, 96

2×1

0.5–1.0 ML, Sr-atom chains 1070–1270 K

LEED AES

90F3 90F3

95

5×1

1.0–1.2 ML, Sr-atom chains 1070–1270 K

LEED AES

90F3 90F3

95

3×1

Sr-atom chains ≥ 1.2 ML, 1070–1170 K

LEED AES STM

90F3, 95B6, 96B2, 96B4 90F3 95B6, 96B2, 96B4

95

Sr/Si(100)

Si(100)3x2-Sr

Fig. 95. Phase diagram of the submonolayer Sr/Si(100) system as determined by LEED-AES [90F3]. Subsequent STM observations [95B6, 96B4, 96B2] have confirmed formation of the 3×2 and 3×1 phases, but have not detected the 2×1 and 5×1 phases.

Fig. 96. Possible structure of the Si(100)3×2-Sr surface phase with 1/3 ML Sr coverage as proposed on the basis of LEED-AES-STM observations [95B6, 96B4, 96B2].

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

369

3.3.2.3.25 Te (Tellurium) on Si Table 52. Structures formed by Te on Si(100) surface; their preparation conditions and techniques used for their investigation Structure

Preparation

Model

Technique Ref.

Fig.

Tab.

Remarks

Te/Si(100) "1×1"

≥ 1 ML, ann. Te atoms in LEED 470–620 K bridge sites AES

92H4, 95D1, 95D2, 97 96S4, 98O2, 98T2 95D1, 95D2, 98O2, 98T2 XPS 95D1, 95D2, 96S4 UPS 95D1, 95D STM 94Y5 AFM 96S4 SEXAFS 96B5 THEORY 99T2

Te coverage ∼ 1 ML; in LEED: 1×1 and faint streaks; in STM: chains of 5–6 atoms

2×1

≥ 1 ML, ann. Te dimers 770–870 K

LEED AES STM

98O2, 98T2 79 98O2, 98T2, 99W2 99W2

Te coverage ∼ 1 ML

3×1

LEED ≥ 1 ML, ann. Te dimers and missing AES 950 K Te dimers ? STM

98O2, 98T2 98O2, 98T2, 99W2 99W2

Te coverage ∼ 2/3 ML

Si(100)1xn -Te

Fig. 97. Possible structure of about 1 ML of Te grown on Si(100) at 470–620 K as suggested on the basis of STM observations [94Y5] and supported by first-principles total-energy calculations [99T2]. Te atoms reside in near bridge sites. The missing Te rows are formed to release the stress due to the larger covalent radius of Te compared to that of Si. Along a direction parallel to the missing row, the surface is perfectly ordered. In the perpendicular direction, Te atoms are shifted from their ideal bridge sites toward the missing rows in such a way that the spacing of the Te atoms is increased to 4 Å. Due to the poor LRO this surface is seen in LEED as 1×1 with faint streaks.

Lando lt -Bö rnst ein New Series III/42A1

370

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.26 Tl (Thallium) on Si Table 53. Structures formed by Tl on Si(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

LEED AES UPS STM STS

99V3 99V3 99V3 99V2, 99V3 99V2, 99V3

98, 99

1–2 ML 7×7 high coverage RT deposit

LEED AES UPS STM

99V3 99V3 99V3 99V3

98, 100

1×1

0.5–1 ML, 530–670 K

LEED AES UPS STM

99V3 99V3 99V3 99V3

98

6×6

1–2 ML, 470–670 K

LEED AES UPS STM

99V3 99V3 99V3 99V3

98

Table Remarks

Tl/Si(111) 7×7 low coverage

0.2 ML Tl clusters RT deposit.

IC phase

Tl forms a superlattice of metallic nanoclusters consisting of 9 Tl adatoms

6×6 hexagonal domain superlattice with 1×1 local order

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Tl/Si(111)

Si(111)7x7-Tl

Fig. 98. Surface phase diagram of Tl on Si(111) as determined using LEED-AES-STM [99V3]. Both RT low- and high-coverage 7×7-Tl phases do not destroy 7×7 substrate periodicity. They have different appearance in STM images as shown in Figs. 99 and 100.

Si(111)7x7-Tl

Fig. 99. STM images of Si(111)7×7 covered with ∼ 0.2 ML Tl at RT (low coverage 7×7-Tl). (a) 1500×1500 Å2; (b) 500×500 Å2. Tl adatoms form metallic nanoclusters which exclusively cover the faulted halves of the 7×7 unit cells. The Fourier transform of the STM image in the inset confirms the high degree of order and the hexagonal symmetry of the nanostructure array [99V2].

Fig. 100. STM images (500×500 Å2) of 1.5–1.7 ML Tl on Si(111)7×7 deposited at RT (high coverage 7×7-Tl). [99V3].

Lando lt -Bö rnst ein New Series III/42A1

371

372

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.3.27 Yb (Ytterbium) on Si Table 54. Structures formed by Yb on Si(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model Technique Ref.

Fig.

Table Remarks

Yb/Si(111) 3×1 (LEED) 1 ML, ann. 1000–1100 K or 0.2–0.3 ML, 770 K

LEED AES PES ISS STM

84K1, 85C4, 93W9 84K1, 85C4 85C4, 93W4, 93W9 84K1, 85C4 93W4, 93W9

5×1

1 ML ann. 800–900 K

LEED AES PES ISS STM

84K1, 93W9 84K1 93W9 84K1 93W9

7×1

∼ 0.5 ML ann. 770 K

LEED PES STM

93W9 93W9 93W9

2×1

0.5–1 ML, ann. 700–800 K

LEED

84K1, 85C4, 93W4, 93W9 84K1, 85C4 85C4, 93W4, 93W9 84K1, 85C4 93H9, 93W4, 93W9 93H9

AES PES ISS STM STS

in STM: local structure 3×2

3.3.2.4 Metallic adsorbates on germanium 3.3.2.4.1 Ag (Silver) on Ge Table 55. Structures formed by Ag on Ge(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

0.2–0.4 ML, 520–720 K

Ag adatoms LEED

Fig.

Tab.

Remarks

Ag/Ge(111) 4×4

79B1, 93L1, 95G3, 101 98S1 AES 79B1 PES 81R, 93L1, 95G3 STM 93H2, 98S1 THEORY 97V

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Ref. p. 396]

3.3.2 Metals on semiconductors

373

Table 55 (cont.) Structure

Preparation

Model

Technique Ref.

Fig.

3× 3

> 0.67 ML, 520–720 K

HCT

LEED 79B1, 88K1, 98S1 LEED I-V 89F1, 94H1 SPA-LEED 90B2 AES 79B1, 88K1 PES 81R, 88K1, 93L1, 95G3 ARPES 89K1 STM 93H2, 98S1 THEORY 97V

8

Tab.

Remarks ideal coverage 1 ML; top Si atom density 1 ML

3×1

∼ 0.2 ML, 500–720 K

STM

93H2, 98S1

in STM: local regions at the boundaries of 4×4 domains; never covers extended areas

5×1

∼ 0.2 ML, 500–720 K

STM

98S1

in STM: narrow stripes adjacent to 3×1 domains

6×6

RT deposition

LEED PES

93L1, 95G3 93L1, 95G3

forms in a wide coverage range

onto

3× 3

Ge(111)4×4-Ag

Fig. 101. Possible model of the Ge(111)4×4-Ag structure based on STM results. Six Ag atoms in on-top sites seen as six bright protrusions in the filled state STM images; three Ge atoms situated in H3 sites and the Ge atoms forming trimers at the corners of the "Ge" half of the unit cell are visualized in empty state STM images [98S1].

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374

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.4.2 Al (Aluminum) on Ge Table 56. Structures formed by Al on Ge(111) and Ge(113) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique

Ref.

Fig.

104, 105

Table Remarks

Al/Ge(111) 2×2

1 ML ann. 1120 K, Ge adatoms 0.05–0.1 ML left

LEED AES

82Y1 82Y1

10×10

5–6 ML ann. 920 K, IC phase 1–5 ML left

LEED AES

82Y1 82Y1

0.2–0.8 ML ann. 770 K

LEED

97J

in LEED: split 1/2 spots

periodicity 10 is not exact: estimated to be between 9 and 11

Al/Ge(113) 2×1

3.3.2.4.3 As (Arsenic) on Ge Table 57. Structures formed by As on Ge(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

Table Remarks

As/Ge(111) 1×1

Ann. at 670 K in Ideal 1×1 As LEED As4 flux, ∼ 104 L termination ARPES PES STM THEORY

85B, 86B1, 86B2, 24 87B, 93H4 85B, 86B1, 86 87B, 93H4, 96R1 88B4, 90B1 86B2, 87H3, 88H11, 96C5

saturating phase with 1 ML As coverage

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

375

3.3.2.4.4 Au (Gold) on Ge Table 58. Structures formed by Au on Ge(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

Tab.

Remarks

Au/Ge(111) 2×2

3× 3

0.07–0.4 ML, Ge adatoms LEED 670–770 K RHEED STM

94G, 94S2 94S2 94G, 94S2

104, 105

0.2–1.0 ML, CHCT 670–770 K

83L1, 88K1, 94S2 95O 94S2 83L1, 94S2 94G 89K1 88K1 93H1 94G, 94N2, 94S2

102

LEED LEED I-V RHEED AES PES ARPES KRIPES XRD STM

in LEED: split 1/2 spots; honeycomb structure of 2×2 domains with c(4×2) domain walls 59

ideal coverage 1 ML; top Si atom density 1 ML

Ge(111) 3 × 3 -Au

Table 59. Structural parameters of the CHCT model of the Ge(111) 3 × 3 -Au reconstruction obtained by LEED and SXRD. The parameters are defined in Fig. 102. Parameter

SXRD [93H1]

LEED I-V [95O]

xAu [Å] xGe1 [Å] xGe2 [Å] zAu [Å] zGe1 [Å] zGe2 [Å] zGe3 [Å] zGe4 [Å] zGe5 [Å] zGe6 [Å] zGe7 [Å]

1.625±0.006 4.108±0.019 4.586±0.016 0.00 0.420±0.034 3.022±0.045 3.794±0.056 3.794±0.056 6.244 6.244 7.061

1.62±0.07 4.07±0.07 4.58±0.09 0.00 0.51±0.05 2.95±0.08 3.63±0.12 3.87±0.07 6.10±0.07 6.32±0.05 7.06±0.07

Lando lt -Bö rnst ein New Series III/42A1

Fig. 102. Top and side views of the conjugateed honeycomb chained trimer (CHCT) model for the Ge(111) 3 × 3 -Au surface, showing distortions from bulk positions down to the sixth atomic layer [95O]. Ideal Au coverage and top Si atom density both are 1.0 ML. Detailed structural parameters of the model are presented in Table 59.

376

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.4.5 Bi (Bismuth) on Ge Table 60. Structures formed by Bi on Ge(100) and Ge(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique

Ref.

Fig.

1 ML RT deposit. + ann. 500 K

Bi dimers and missing dimers

STM

97L1

37

LEED I-V AES THEORY

91W1 91W1 97C5

103

Table Remarks

Bi/Ge(100) 2×n

Bi/Ge(111)

3× 3

1.5 ML RT deposit. Bi adatoms + ann. 600 K

61, 62 Bi coverage 1/3 ML

Table 61. Ge(111) 3 × 3 -Bi phase with 1/3 ML Bi coverage: Atomic coordinates as determined using LEED I-V analysis [91W1] and displacement of Si atoms from bulk positions as determined using LEED I-V analysis [91W1] and ab initio calculations [97C5]. Atoms are numbered as in Fig. 103. Atom No.

Coordinates [91W1] X Y Z [Å] [Å] [Å]

1 2 3 4 5 6 7 8

0.000 0.808 –1.617 3.465 0.000 3.465 0.000 1.155

0.000 1.400 0.000 2.001 0.000 2.001 0.000 2.001

0.000 1.324 1.324 2.186 2.585 4.726 5.014 5.831

Displacement [91W1] ∆X ∆Y ∆Z [Å] [Å] [Å]

Displacement [97C5] ∆X ∆Y ∆Z [Å] [Å] [Å]

–0.346 0.693

–0.11 0.22

0.600 –0.046 0.445 –0.135 0.423

0.19

–0.01 –0.01 0.23 0.44 –0.14 0.33

Table 62. Experimental and calculated values of the height h12 (defined as in Fig. 103) for Ge(111)

3 × 3 -Bi phase.

Structure

Height

LEED I-V [91W4, 91W1]

XRD [95N2]

THEORY [97C5]

Ge(111) 3 × 3 -Bi

h12

1.324

2.70

1.99

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Ref. p. 396]

3.3.2 Metals on semiconductors

377

Ge(111) 3 × 3 -Bi

Fig. 103. (a) Top and (b) side views of the Ge(111)

3 × 3 -Bi: T4 Bi-adatom model. The coordinates of atoms, displacement of Ge atoms from bulk positions and h12 length are listed in Tables 61, 62. Bi atoms are shown in black and Ge atoms in white.

3.3.2.4.6 Cu (Copper) on Ge Table 63. Structures formed by Cu on Ge(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique

Ref.

Fig.

Table Remarks

5.2×5.2-R30°

≤ 0.27 ML, 300–520 K

IC phase

LEED AES KRIPES ARPES

89H2 89H2 89H2 89H2

8.8×8.8-R30°

2 ML ann. 370–770 K

IC phase

LEED AES STM

96B1 96B1 96B1

in STM: hexagonal domain superlattice

2×1

2 ML ann. ≥ 520 K

Cu germanide ? LEED AES STM

96B1 96B1 96B1

coexists with 8.8×8.8-R30° ; occurs in three domains

Cu/Ge(111)

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378

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.4.7 Ga (Gallium) on Ge Table 64. Structures formed by Ga on Ge(100), Ge(111) and Ge(113) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

Table Remarks

Ga/Ge(100) "7×5.5"

0.3–1.0 ML ann. 770 K

LEED AES STM

98G1 98G1 98G1

in STM: stacking of 7×5 and 7×6 units

Ga/Ge(111) 2×2

0.05–0.5 ML 570–1050 K

Ge adatoms (and Ga adatoms ?)

7.5×7.5 γ-phase

0.5–0.7 ML ann. 770 K

IC phase, LEED Ga atoms STM substitute top Ge atoms PES XRD XSW THEORY

13.5×13.5β- 0.7–0.9 ML ann. 770 K phase

IC phase, LEED Ga atoms STM substitute top Ge atoms PES XRD XSW THEORY

13.5×13.5 βH 3-phase

0.9–2.0 ML ann. 770 K

Ga adatoms in H3 sites on β-phase

93P1, 94M2, 95M1 STM 93P1, 94M2, 95M1 THEORY 93C1, 94M1 LEED

104, 105

in LEED split 1/2 spots; honeycomb structure of 2×2 domains with c(4×2) domain walls

92Z1, 95M1 92Z1, 95A1, 95B1, 95M1 95M1 95M1 95A1 95A1

106

in LEED: 1×1 with 7.5×7.5 satellites; in STM: domain superlattice 7×7 to 8×8

95M1 95A1, 95B1, 95M1 95M1 95M1 95A1 95A1

106

in LEED: 1×1 with 13.5×13.5 satellites; in STM: domain superlattice13×13 to 14×14

LEED STM XSW

95M1 95M1 95A1

LEED

97J

Ga/Ge(113) 2×1

0.2–0.8 ML ann. 720 K

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Ge(111)2x2-Ga

Fig. 104. (a) LEED pattern and (b) its scheme of the Ga-stabilized Ge(111)2×2 surface [95M1]. The half-order reflections split perpendicular to the <1, 0>-directions.

Ge(111)2x2-Ga

Fig. 105. Plausible arrangement of the Ga-induced Ge(111)2×2 honeycomb structure consisting of 2×2 domains of Ge adatoms (shown by open circles) separated by c(4×2) domain boundaries also built of Ge adatoms (shown by filled circles). All adatoms occupy T4 sites. Small open circles show Ge atoms of the top layer of the unreconstructed Ge(111) surface [94M1]. The surface is plausibly similar to the high-temperature 2×2 phase of the clean Ge(111) surface. The presence of Ga (or Al, Au, In) impurities stabilizes it at room temperature.

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379

380

3.3.2 Metals on semiconductors

[Ref. p. 396

Ga/Ge(111)

Fig. 106. Schemes of the LEED patterns of (a) γ and (b) β-phases in Ga/Ge(111) system. The spot size corresponds roughly to the LEED intensities. The exact positions were obtained from the XRD data [95M1].

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

381

3.3.2.4.8 In (Indium) on Ge Table 65. Structures formed by In on Ge(100), Ge(111) and Ge(103) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

Tab.

Remarks

2×2

0.5 ML, 300–370 K

parallel In dimers

LEED RHEED PES STM

96S2 90R1 90R1 96S2, 97F3

15, 107

3×3

0.5–1.0 ML, 300–370 K

In trimers ?

LEED STM

96S2 96S2, 97F3

107, 108

ideal coverage presumably 2/3 ML; coexists with 4×3

4×3

0.5–1.0 ML, 300–370 K

In trimers ?

LEED STM

96S2 96S2, 97F3

107, 108

ideal coverage presumably 3/4 ML; coexists with 3×3

5×4

1.2 ML ann. 470–620 K

LEED STM XRD

96S2 96S2, 97F3 94S1

107, 109

n×4

1.2 ML ann. 770 K or 0.5 ML ann. 520 K

LEED STM

96S2 96S2, 97F3

107

in STM: stacking of 4×3 or 3×3 units

95B2 95B2

104, 105

in LEED: split 1/2 spots; in STM: honeycomb structure of 2×2 domains with c(4×2) domain walls

RHEED LEED AES STM

81I1 96G4 96G4 96G4

110

RHEED LEED AES STM

81I1 95B2, 96G4 95B2, 96G4 95B2, 96G4

110

In/Ge(100)

In/Ge(111) 2×2

0.05–0.1 ML ann. 670 K

4×4

1 ML, 300 K

n×2 3

0.1–0.4 ML ann. 650 K

Lando lt -Bö rnst ein New Series III/42A1

Ge adatoms LEED AES

IC phase

n decreases with In coverage from 13 to 10; in STM: striped IC phase with 2×2 local order

382

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 65 (cont.) Structure

Preparation

Model

Technique Ref.

Fig.

4 3×4 3

0.5–0.6 ML, ≥ 520 K

IC phase

RHEED LEED AES STM

81I1 95B2, 96G4 95B2, 96G4 95B2, 96G4

110

RHEED LEED AES

81I1 95B2, 96G4 95B2, 96G4

59, 110

periodicity increases with In coverage from 4.43 3 ×4.43 3 ; in STM: hexagonal domain superlattice with 1.15×1.15 local order

0.5 ML ann. 640 K

61 × 61

0.7 ML ann. 650 K

RHEED

81I1

110

≥ 0.7 ML ann. IC phase 470 K

RHEED

81I1

110

2 ML ann. 420 K or 6 ML ann. 570 K

LEED AES STM

95G2, 96S1 95G2 95G2, 96N1, 96S1 99B1

111

4.3×4.3

Remarks

4 3 × 4 3 to

31 × 31

In adatoms and Ge chains

Tab.

In/Ge(103) 1×1

In adatoms

XRD

In/Ge(100)

Fig. 107. Phase diagram of the In/Ge(100) system as determined using LEED and STM [96S2].

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Ge(100)4x3-In Ge(100)3x3-In

Fig. 108. Possible model of the coexisting Ge(100) 4×3In and Ge(100)3×3-In structures as suggested on the basis of the STM data [97F3]. The first-layer In atoms are indicated by large filled circles while the second(third-) layer Ge substrate atoms are shown as large (small) empty circles. The 4×3 and 3×3 unit cells are outlined.

Ge(100)5x4-In

Fig. 109. Possible structural model of the Ge(100) 5×4In structure as suggested on the basis of the data of XRD [94S1] and STM [97F3]. In atoms are shown in filled circles and Ge atoms are shown as empty circles.

In/Ge(111) Ge(103)1x1-In

Fig. 110. Phase diagram of the submonolayer In/Ge(111) system as determined using RHEED observations [81I1]. Except for the order-disorder transitions to the hightemperature 1×1 structure, all transformations are irreversible, i.e. only structural changes from lower to upper structure are observed. The subsequent STM observations [96G4] confirmed the formation of all structures except for the 61 × 61 . In addition, the Ininduced 2×2 structure was detected by STM at low In coverage (∼ 0.05–0.1) [95B2].

Lando lt -Bö rnst ein New Series III/42A1

Fig. 111. Possible structure of the Ge(103)1×1-In surface phase as proposed on the basis of the data obtained by LEED-STM [96N1, 96S1] and XRD [99B1]. The model contains 2 In adatoms per 1×1 unit cell. The alternative model proposed in [95G2] suggests the presence of In adatom and Ge adatom in the 1×1 unit cell. Ge atoms are shown by open circles with circles of greater radius representing the Ge atoms closer to the surface. In atoms are shown by large solid circles.

383

384

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.4.9 Pb (Lead) on Ge Table 66. Structures formed by Pb on Ge(100) and Ge(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

2×2 c(4×2)

≤ 0.5 ML, 300–370 K

parallel LEED symmetric and AES asymmetric STM Pb dimers

c(8×4)-α

0.5–1.0 ML, asymmetric 300–370 K Pb dimers

3 0   1 2

1.3–1.7 ML, 300–370 K

c(8×4)-β

≥ 1.5 ML 300–370 K

5×1

∼ 1 ML ann. 570 K

Fig.

Tab.

Remarks

Pb/Ge(100)

IC phase

93Z1 112 93Z1 94Y1, 95Y1, 97F2

poor LRO: dim LEED pattern; in STM: mixture of 2×2, 4×2, c(4×2) and c(8×2) domains

LEED AES EELS STM THEORY

93Z1 90, 93Z1 112 93Z1 94Y1, 95Y1, 97F2 98T3

ideal coverage 0.75 ML

LEED AES EELS STM RHEED

93Z1 112 93Z1 93Z1, 93Z2 94Y1, 95Y1, 97F2 97F2

ideal coverage 1.6±0.1 ML; RT transformation to c(8× SRVVLEOH

LEED AES EELS STM

93Z1, 97F2 112 93Z1 93Z1, 93Z2 94Y1, 95Y1, 97F2

seen in LEED as 4×1; chemically passive

LEED AES EELS STM

93Z1 93Z1 93Z1 95Y1, 97F2

112

Pb/Ge(111) α- 3 × 3 1/3 ML, Pb adatoms 300 K (+ ann. 370–470 K)

LEED

83M, 86F, 97G2 113, 88L1, 89H1 114 LEED I-V 83I, 93C3 RHEED AES 83M, 93C3, 97G2 PES 93C3, 97G2 ARPES 87T1, 93C2, 97G2, 99M2 EELS 99P1 XRD 86F, 87P1 STM 93S1 PEH 96R2 THEORY 93A1

67

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

385

Table 66 (cont.) Structure

Preparation

Model

Technique Ref.

Fig.

Tab.

Remarks

β- 3 × 3 4/3 ML, close-packed 300 K Pb layer (+ ann. 370–470 K)

LEED LEED I-V RHEED AES PES EELS XRD XSW STM

γ- 3 × 3 0.2-0.3 ML, Pb adatoms and ("mosaic") 300 K (+ ann. Ge adatoms 370–470 K)

RHEED AES PES ARPES EELS STM STS THEORY

93C3 116 93C3 93C3 93C2 98C1 93S1, 98C1, 99S1 98C1 99S1

GexPb1–x/Ge(111) solid solution with 0.1 ≤ x ≤ 0.4

CDW phase

LEED AES PES ARPES EELS STM THEORY

96C1, 97G2 97G2 98M1 97G2, 99M2 96C1 96C1 96C1

transforms reversibly

LEED RHEED XRD XSW STM THEORY

83M, 95F1 83I, 84I1 90G, 99V1 95F1 93H5 94A

3×3

cooling of α- 3 × 3 to ≤ 200 K

"1×1"

heating of β- 3 × 3 to 450–570 K

striped IC phase

83M, 95F1, 95S6 113, 88L1, 89H1 115 83I, 93C3, 95S6 83M, 93C3 93C3 93Z2 86F 95F1 93S1, 94S3, 94S5, 95S6 THEORY 93A1, 94A

STM 1.333 – β- 3 × 3 1.354 ML ann. with stripe-like 370 K domain walls

hexagonal 1.37 ML IC phase ann. 370 K

Lando lt -Bö rnst ein New Series III/42A1

STM β- 3 × 3 with hexagonal network of domain walls

93S1, 94S3, 94S5, 95S6

94S5, 95S6

to α- 3 × 3 at 200 K

113

reversible transition; transition temperature 450 K below 4/3 ML, 570 K above 4/3 ML

386

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 67. Atomic coordinates and displacement of Ge atoms from bulk positions in the Ge(111) 3 × 3 -Pb α-phase with 1/3 ML Pb coverage as determined using LEED I-V analysis [89H1]. Atoms are numbered as in Fig. 114. Atom No.

1 2 3 4 5 6 7 8

Coordinates X Y [Å] [Å] 0.000 1.637 0.000 2.001 0.000 2.001 0.000 0.000

Z [Å]

2.310 1.365 4.201 –1.155 2.310 –1.155 2.310 0.000

5.785 4.085 4.085 3.368 2.918 0.867 0.617 0.000

Displacement ∆X ∆Y [Å] [Å] –0.364

0.21 –0.42

∆Z [Å] –0.05 –0.05 0.10 –0.35 0.05 –0.20

Table 68. Atomic coordinates for the Ge(111) 3 × 3 -Pb β-phase with 4/3 ML Pb coverage as determined using x-ray-diffraction structural analysis [99V1]. Positions are given by R= c1a1 + c2a2 + c3a3, where a1 and a2 are the lattice vectors of the unreconstructed surface and have a length a0/ 2 , while a3 is perpendicular to the surface and has a length a0 3 (a0 = 5.658 Å is the bulk lattice constant). Atoms are labeled as in Fig. 115. Atom

c1

H3 Pb OC T1 Pb OC T1 Pb OC T1 Pb Top Ge 2nd Ge

0.333 –0.223 1.447 0.777 0.667 0.000

c2 0.667 1.223 1.223 1.554 0.333 0.000

c3 0.377 0.365 0.365 0.365 0.083 0.000

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Pb/Ge(100)

Pb/Ge(111)

Fig. 113. Phase diagram of the Pb/Ge(111) system as determined using RHEED [83I]. The α- and β- 3 × 3 are stable structures, while 4×4 structure is a metastable and transient one.

Fig. 112. (a) Phase diagram of the Pb/Ge(100) system as determined using LEED-AES [93Z1]. The reversible and irreversible transformations induced by heating of RT phases are illustrated. The "3×6" denotes the (3,0)×(1,2) structure. (b) Phase diagram of the Pb/Ge(100) system as determined using RT STM observations [97F2].

Lando lt -Bö rnst ein New Series III/42A1

387

388

3.3.2 Metals on semiconductors

Ge(111)α- 3 × 3 -Pb

[Ref. p. 396

Ge(111)β- 3 × 3 -Pb

Fig. 115. (a) Top and (b) side views of the Ge(111)

Fig. 114. (a) Top and (b) side views of the Ge(111)

3 × 3 -Pb (α-phase, 1/3 ML Pb): T4 Pb-adatom model. The coordinates of atoms and displacement of Si atoms from bulk positions are listed in Table 67. Pb atoms are shown in black and Ge atoms in white.

3 × 3 -Pb β-phase with 4/3 ML Pb. Pb atoms are represented by large hatched circles and Ge atoms are shown by open circles. The unit cell contains one Pb atom in H3 cite and three Pb atoms displaced from the bridge site towards the T1 site (labeled off-centered T1 Pb). In (b) the distances between atoms and the Pb (covalent) radius are shown to scale. The coordinates of atoms as determined by XRD [99V1] are listed in Table 68. Ge(111)γ- 3 × 3 -Pb

Fig. 116. Filled state STM image of the Ge(111)γ-

3 × 3 -Pb ("mosaic" γ-phase, Ge0.37Pb0.63/ Ge(111)) built of Pb and Ge adatoms. Pb adatoms are seen as bright and Ge as dark [98C1]. The similar structure is observed in Pb/Si(111), Sn/Si(111) and Sn/Ge(111) systems.

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

389

3.3.2.4.10 Sb (Antimony) on Ge Table 69. Structures formed by Sb on Ge(100), Ge(110) and Ge(111) surfaces; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique

Ref.

Fig.

1 ML, 670–770 K

symmetric or asymmetric Sb dimers ?

LEED RHEED ARPES STM THEORY XRD

92H1, 92Y1, 97F1 117 92L1 92H1 97F1 94K1, 97T1 92L1

Table Remarks

Sb/Ge(100) 2×1

saturating phase

Sb/Ge(110) 1×1

1 ML, 470 K ideal LEED Sb-terminated AES surface ? PES STM

91L1 91L1 91L1 91L1

3×2

0.67 ML, 470 K

91L1 91L1 91L1 91L1

LEED AES PES STM

Sb/Ge(111) 2×1

1 ML, 770–970 K

1×1 (LEED)

0.5–1.0 ML, disordered ? 270–670 K

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Sb-atom zigzag RHEED chains LEED XSW SEXAFS XRD THEORY RHEED LEED I-V LEED XSW THEORY

92S1 94K2 93K1, 94K2 93K1 92S1 96T1, 96T2 92S1 92W1 94K2 94K2 96T2

118

70

2×2 LEED pattern: three domains

Sb in substitutional positions ?

390

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 70. Atomic positions of the Ge(111)2×1–Sb surface. Atoms are numbered as in Fig. 118. Positions are given by R= c1a1 + c2a2 + c3a3, where a1 and a2 are the lattice vectors of the unreconstructed surface and have a length a0/ 2 (see Fig. 118), while a3 is perpendicular to the surface and has a length a0 3 (a0 = 5.658 Å is the bulk lattice constant). Atom No. 1 2 3 4 5 6 7 8

Bulk c1

0.667 1.667 0.000 1.000 0.000 1.000

c2

0.333 0.333 0.000 0.000 0.000 0.000

c3

1.083 1.083 1.000 1.000 0.750 0.750

THEORY [96T2] c1 c2 0.870 1.456 0.684 1.636 –0.007 0.994 –0.006 0.994

0.435 0.229 0.343 0.318 0.003 –0.003 –0.003 –0.003

c3 1.342 1.348 1.076 1.084 1.010 0.984 0.755 0.738

XRD [92S1] c1 c2 0.842 1.421 0.670 1.660 –0.030 0.990 –0.020 0.980

c3

0.421 0.210 0.330 0.330 –0.020 –0.010 –0.010 –0.010

1.353 1.373 1.086 1.104 1.011 1.001 0.750 0.751

Ge(100)2x1-Sb

Fig. 117. Side view of the atomic structure of Ge(100) 2×1-Sb as determined (a) by first-principles total-energy calculations [97T1] (symmetric Sb-dimer structure) and (b) experimentally by x-ray diffraction [92L1] (asymmetric Sb-dimer structure) Sb atoms are shown in black and Ge atoms in white. The displacements from the ideal bulk-like positions are shown in Å.

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

Ge(111)2x1-Sb

Fig. 118. Atomic structure of the fully relaxed 2x1 zigzag chain structure of one monolayer of Sb on Ge(111). Atoms are labeled as in Table 70. (a) Top view of the surface showing Sb atoms and the first layer of Ge atoms. The lattice vectors of the hexagonal 1×1 unit cell are shown, together with the 2×1 surface unit cell. (b) Side view of the surface in the (1 1 0) plane.

Lando lt -Bö rnst ein New Series III/42A1

391

392

3.3.2 Metals on semiconductors

[Ref. p. 396

3.3.2.4.11 Sn (Tin) on Ge Table 71. Structures formed by Sn on Ge(111) surface; their preparation conditions and techniques used for their investigation. Structure

Preparation

Model

Technique Ref.

Fig.

LEED RHEED AES PES

119

Tab.

Remarks

Sn/Ge(111) 2×2

3× 3

0.1–0.2 ML, 300–720 K

0.4 ML RT deposit. + ann. 450–620 K

Sn adatoms

LEED LEED I-V RHEED AES PES ARPES EELS XRD

Sn and Ge adatoms

STM

92G1

116

0.28–0.43 ML Sn adatoms, ann. 480–570 K CDW phase cool < 200 K

LEED LEED I-V AES PES ARPES EELS STM XRD THEORY

97C1, 97G1 99Z1 97G1 99A1, 99A2 97G1, 98B1 97C1 97C1, 99M1 98U, 99B2, 99Z1 97C1, 99A1

121

0.8–1.0 ML RT deposit. + ann. 770 K

LEED RHEED AES PES STM XRD

85D1, 95G1 119 81I2 85D1 85D1, 95G1 92G1, 93H3, 95G1 88P1

3× 3 0.1 ML ("mosaic") RT deposit. + ann. 770 K 3×3

5×5

85D1 81I2 85D1 85D1, 99A2

DAS

95G4, 97C1, 97G1 119, 99Z1 120 81I2 97G1 95G4, 99A1, 99A2 97G1, 98B1 97C1 87P1, 98U, 99B2, 99Z1 STM 92G1, 97C1, 99M1 THEORY 97C1, 99A1

diffuse LEED and RHEED patterns

72

metastable: converts to 7×7 at 620 K; reversible transition to 3×3 at 200 K

GexSn1–x /Ge(111) solid solution

73

transforms reversibly to

3 × 3 above 200 K

GexSn1–x /Ge(111) solid solution

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Ref. p. 396]

3.3.2 Metals on semiconductors

393

Table 71 (cont.) Structure

Preparation

Model

Technique Ref.

7×7

0.4–0.5 ML RT deposit. + ann. 770 K

DAS

LEED RHEED AES PES ARUPS STM

Fig.

STS XRD

85D1, 95G1 119 81I2 85D1 85D1, 93G1, 95G1 84Y 88B3, 92G1, 93H3, 95G1 88B3 88P1

2 ML ann. 920 K or 1.2 ML ann. 520–670 K

LEED RHEED PES STM

95G1 84I2 95G1 95G1

91 × 3

1.2–1.4 ML RT deposit. + ann. 500 K

RHEED

84I2

7 ×4

0.7 ML RT deposit. + ann. 820 K

STM

95G1

2 3 ×3

Tab.

Remarks GexSn1–x /Ge(111) solid solution

not seen by LEED; in STM: small domains in disordered regions adjacent to 5×5

Table 72. The vertical distance (dSn) of the Sn adatom from the first Ge layer, the vertical distances (∆di ) of the i-th Ge plane with respect to the positions in an ideally truncated surface and lateral displacements of Ge atoms in 1st and 4th layers in the Ge(111) 3 × 3 -Sn structure as determined using XRD, LEED I-V and combined LEED/XRD [99Z1]. All lengths are given in Å. Parameters

XRD

LEED I-V

LEED/XRD

dSn ∆d12 ∆d23 ∆d34 ∆d45 Lateral slide1 Lateral slide4 Vertical ripple2 Vertical ripple4

+1.910±0.20 +0.088±0.10 +0.028±0.05 +0.085±0.03 +0.052±0.02 0.050±0.02 0.040±0.02 +0.609±0.04 +0.410±0.04

+1.750±0.11 –0.024±0.04 –0.042±0.05 –0.067±0.05 –0.027±0.06 0.030±0.15 0.182±0.17 +0.562±0.04 +0.463±0.04

1.750 –0.018 –0.030 –0.059 –0.030 0.050 0.040 0.556 0.426

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394

3.3.2 Metals on semiconductors

[Ref. p. 396

Table 73. Structural distortion for the low-temperature Ge(111)3×3-Sn surface phase compared to the roomtemperature Ge(111) 3 × 3 -Sn phase as determined using LEED I-V and the combination of LEED and XRD. The atoms are labeled as in Fig. 121 Parameters

LEED I-V

LEED/XRD

∆Sn (A and C)

(A) –0.101 (C) –0.061 +0.294 –0.077 +0.086 0.199 0.260

–0.015

∆Sn (B) ∆d2 (A and C) ∆d2 (B) Lateral d2 (B) Lateral d3

+0.355 –0.036 +0.134 0.117 0.148

Sn/Ge(111)

Fig. 119. Phase diagram of the Sn/Ge(111) system summarizing the results of the observations using RHEED [81I2, 84I2] and LEED-AES [85D1]. The 7×7, 5×5 3× 3

and

91 × 3 are stable, while the

3 × 3 structure is metastable and converts upon annealing to 7×7. Not shown in the phase diagram is the reversible transition from below 200 K.

3 × 3 to 3×3 upon cooling

Landolt -Börnst ein New Series III/42A1

Ref. p. 396]

3.3.2 Metals on semiconductors

395

Ge(111) 3 × 3 -Sn

Ge(111)3x3-Sn

Fig. 120. Structural model of Ge(111) 3 × 3 -Sn surface. (a) is a atop view and (b) is a side view. Ge atoms are drawn in a bulk-truncated positions, with arrows to show the direction of distortion induced by Sn. Detailed structural parameters are presented in Table 72 [98B1].

Fig. 121. Structural model of Ge(111)3×3-Sn surface. The arrows show the direction of the distortion-induced

Lando lt -Bö rnst ein New Series III/42A1

charge ordering transition with respect to the

3× 3

phase, i.e. the difference between 3×3 and structures [99Z1].

3× 3

3.3.2 Metals on semiconductors

396

References for 3.3.2 59S 64E 64L 64W 65J 65L 67B 68B 68P 69B 69T 70C 70H 72B 73B 74L 76O 76S 77C1 77C2 77I1 77I2 77L 77O 77R 78G 78L 78W 79A 79B1 79B2 79K1 79K2 79K3 79O 80B 80I 80O1 80O2 80O3 81B 81C 81F 81G

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Landolt -Börnst ein New Series III/42

3.3.2 Metals on semiconductors 81H1 81H2 81I1 81I2 81K 81L1 81L2 81O1 81O2 81R 81S1 81S2 81S3 81S4 82B 82C 82K 82O 82R 82S 82Y1 82Y2 82Z 83H 83I 83L1 83L2 83M 83R 83S 83Y1 83Y2 84B 84C1 84C2 84I1 84I2 84K1 84K2 84K3 84L 84M 84N 84P 84U

397

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Lando lt -Bö rnst ein New Series III/42

398 84Y 85B 85C1 85C2 85C3 85C4 85D1 85D2 85D3 85H 85I1 85I2 85K1 85K2 85K3 85L1 85L2 85N 85O1 85O2 85P 85S 85T1 85T2 85T3 85U 85Y 86A 86B1 86B2 86B3 86C1 86C2 86D1 86D2 86F 86H1

3.3.2 Metals on semiconductors Yokotsuka, T., Kono, S., Suzuki, S., Sagawa, T.: J. Phys. Soc. Jap. 53 (1984) 696. Bringans, R.D., Uhrberg, R.I.G., Bachrach, R.Z., Northrup, J.E.: Phys. Rev. Lett. 55 (1985) 533. Chambers, S.A., Anderson, S.B., Weaver, J.H.: Phys. Rev. B 32 (1985) 581. Chambers, S.A., Weaver, J.H.: J. Vac. Sci. Technol. A 3 (1985) 1929. Chambers, S.A., Greenlee, T.R., Howell, G.A., Weaver, J.H.: J. Vac. Sci. Technol. A 3 (1985) 1291. Chorkendorff, I., Kofoed, J., Onsgaard, J.: Surf. Sci. 152/153 (1985) 749. DiCenzo, S.B., Bennett, P.A., Tribula, D., Thiry, P., Wertheim, G.K., Rowe, J.E.: Phys. Rev. B 31 (1985) 2330. Daugy, E., Mathiez, P., Salvan, F., Layet, J.M., Derrien, J.: Surf. Sci. 152/153 (1985) 1239. Daugy, E., Mathiez, P., Salvan, F., Layet, J.M.: Surf. Sci. 154 (1985) 267. Horio, Y., Ichimiya, A.: Surf. Sci. 164 (1985) 589. Ichikawa, M., Doi, T., Hayakawa, K.: Surf. Sci. 159 (1985) 133. Ichinokawa, T., Ampo, H., Miura, S., Tamura, A.: Phys. Rev. B 31 (1985) 5183. Kinoshita, T., Kono, S., Sagawa, T.: Solid State Commun. 56 (1985) 681. Kinoshita, T., Kono, S., Sagawa, T.: Phys. Rev. B 32 (1985) 2714. Kelly, M.K., Margaritondo, G., Anderson, J.A., Frankel, D.J., Lapeyre, G.J.: J. Vac. Sci. Technol. A 3 (1985) 1481. Lifshits, V.G., Akilov, V.B., Churusov, B.K., Gavriljuk, Yu.L., Zavodinskii, V.G.: Solid State Commun. 55 (1985) 717. Li, S.-T., Hasegawa, S., Iwasaki, H., Mizokawa, Y., Nakamura, S.: Appl. Surf. Sci. 22/23 (1985) 275. Nicholls, J.M., Mårtensson, P., Hansson, G.V., Northrup, J.E.: Phys. Rev. B 32 (1985) 1333. Oura, K., Katayama, M., Shoji, F., Hanawa, T.: Phys. Rev. Lett. 55 (1985) 1486. Otsuka, M., Ichikawa, T.: Jpn. J. Appl. Phys. 24 (1985) 1103. Pirri, C., Peruchetti, J.C., Gewinner, G., Derrien, J.: Surf. Sci. 152/153 (1985) 1106. Saitoh, M., Oura, K., Asano, K., Shoji, F., Hanawa, T.: Surf. Sci. 154 (1985) 394. Takahashi, T., Takayama, I., Ishikawa, T., Ohta, T., Kikuta, S.: Jpn. J. Appl. Phys. 24 (1985) L727. Taleb-Ibrahimi, A., Mercier, V., Sebenne, C.A., Bolmont, D., Chen, P.J.: Surf. Sci. 152/153 (1985) 1228. Taleb-Ibrahimi, A., Sebenne, C.A., Proix, F., Maigne, P.: Surf. Sci. 163 (1985) 478. Uhrberg, R.I.G., Hansson, G.V., Nicholls, J.M., Persson, P.E.S., Flödstrom, S.A.: Phys. Rev. B 31 (1985) 3805. Yang, W.S., Wu, S.C., Jona, F.: Surf. Sci. 155 (1985) L292. Aruga, T., Murata, Y.: Phys. Rev. B 34 (1986) 5654. Bringans, R.D., Uhrberg, R.I.G., Bachrach, R.Z.: Phys. Rev. B 34 (1986) 2373. Bringans, R.D., Uhrberg, R.I.G., Bachrach, R.Z., Northrup, J.E.: J. Vac. Sci. Technol. A 4 (1986) 1380. Bringans, R.D., Uhrberg, R.I.G., Olmstead, M.A., Bachrach, R.Z.: Phys. Rev. B 34 (1986) 7447. Calliari, L., Marchetti, F., Sancrotti, M.: Phys. Rev. B 34 (1986) 521. Cao, R., Yen, J.-J., Nogami, J., Lindau, I.: J. Vac. Sci. Technol. A 4 (1986) 845. Dallaporta, H., Cros, A.: Surf. Sci. 178 (1986) 64. Durbin, S.M., Berman, L.E., Batterman, B.W., Blakely, J.M.: Phys. Rev. B 33 (1986) 4402. Feidenhans’l, R., Pedersen, J.S., Nielsen, M., Grey, F., Johnson, R.L.: Surf. Sci. 178 (1986) 927. Higashiyama, K., Kono, S., Sagawa, T.: Jpn. J. Appl. Phys. 25 (1986) L117. Landolt -Bö rnst ein New Series III/42

3.3.2 Metals on semiconductors 86H2 86H3 86H4 86H5 86K1 86K2 86K3 86M 86N1 86N2 86O1 86O2 86T1 86T2 86U1 86U2 86W 86Y1 86Y2 87A 87B 87F 87H1 87H2 87H3 87K1 87K2 87K3 87K4 87K5 87N1 87N2 87N3 87P1 87P2 87P3 87R

399

Higashiyama, K., Kono, S., Sagawa, T.: Surf. Sci. 175 (1986) L794. Hirayama, H., Baba, S., Kinbara, A.: Jpn. J. Appl. Phys. 25 (1986) L452. Hirayama, H., Baba, S., Kinbara, A.: J. Vac. Sci. Technol. A 4 (1986) 1416. Hansson, G.V., Nicholls, J.M., Mårtensson, P., Uhrberg, R.I.G.: Surf. Sci. 168 (1986) 105. Knall, J., Sundgren, J.-E., Hansson, G.V., Greene, J.E.: Surf. Sci. 166 (1986) 512. Kelly, M.K., Margaritondo, G., Anderson, J.A., Frankel, D.J., Lapeyre, G.J.: J. Vac. Sci. Technol. A 4 (1986) 1396. Kinoshita, T., Kono, S., Sagawa, T.: Phys. Rev. B 34 (1986) 3011. Martin, J.A., Savage, D.E., Moritz, W., Lagally, M.G.: Phys. Rev. Lett. 56 (1986) 1936. Nicholls, J.M., Salvan, F., Reihl, B.: Surf. Sci. 178 (1986) 10. Nicholls, J.M., Salvan, F., Reihl, B.: Phys. Rev. B 34 (1986) 2945. Olmstead, M.A., Uhrberg, R.I.G., Bringans, R.D., Bachrach, R.Z.: J. Vac. Sci. Technol. B 4 (1986) 1123. Olmstead, M.A., Bringans, R.D., Uhrberg, R.I.G., Bachrach, R.Z.: Phys. Rev. B 34 (1986) 6041. Taleb-Ibrahimi, A., Sebenne, C.A., Proix, F.: J. Vac. Sci. Technol. A 4 (1986) 2331. Taleb-Ibrahimi, A., Sebenne, C.A.: Surf. Sci. 168 (1986) 114. Uhrberg, R.I.G., Bringans, R.D., Bachrach, R.Z., Northrup, J.E.: Phys. Rev. Lett. 56 (1986) 520. Uhrberg, R.I.G., Bringans, R.D., Bachrach, R.Z., Northrup, J.E.: J. Vac. Sci. Technol. A 4 (1986) 1259. Wu, S.C., Wang, Z.Q., Li, Y.S., Jona, F.: Phys. Rev. B 33 (1986) 2900. Yang, W.S., Wu, S.C., Jona, F.: Surf. Sci. 169 (1986) 383. Yang, W.S., Wu, S.C., Zhao, R.G.: Phys. Rev. B 33 (1986) 919. Aizawa, T., Tsuno, T., Daimon, H., Ino, S.: Phys. Rev. B 36 (1987) 9107. Bringans, R.D., Olmstead, M.A., Uhrberg, R.I.G., Bachrach, R.Z.: Phys. Rev. B 36 (1987) 9569. Fujimori, A., Grioni, M., Joyce, J.J., Weaver, J.H.: Phys. Rev. B 36 (1987) 1075. Hasegawa, S., Daimon, H., Ino, S.: Surf. Sci. 186 (1987) 138. Higashiyama, K., Kinoshita, T., Miyahara, T., Kato, H., Kono, S., Ohsawa, H., Enta, Y., Maeda, F., Yaegashi, Y.: Surf. Sci. 186 (1987) L568. Hybertsen, M.S., Louie, S.G.: Phys. Rev. Lett. 58 (1987) 1551. Kinoshita, T., Kono, S., Nagayoshi, H.: J. Phys. Soc. Japn. 56 (1987) 2511. Kemmann, H., Muller, F., Neddermeyer, H.: Surf. Sci. 192 (1987) 11. Kono, S., Higashiyama, K., Kinoshita, T., Miyahara, T., Kato, H., Ohsawa, H., Enta, Y., Maeda, F., Yaegashi, Y.: Phys. Rev. Lett. 58 (1987) 1555. Kinoshita, T., Ohta, H., Enta, Y., Yaegashi, Y., Suzuki, S., Kono, S.: J. Phys. Soc. Jap. 56 (1987) 4015. Kawazu, A., Yokohama, I., Suzuki, H., Ohsaki, A., Takeuchi, K., Sakama, H.: Phys. Rev. B 36 (1987) 9809. Nogami, J., Park, S., Quate, C.F.: Phys. Rev. B 36 (1987) 6221. Nicholls, J.M., Reihl, B., Northrup, J.E.: Phys. Rev. B 35 (1987) 4137. Nicholls, J.M., Reihl, B.: Phys. Rev. B 36 (1987) 8071. Pedersen, J.S., Feidenhans’l, R., Nielsen, M., Kjær, K., Grey, F., Johnson, R.L.: Surf. Sci. 189/190 (1987) 1047. Patel, L.R., Golovchenko, J.A., Freeland, P.E., Gossmann, H.-J.: Phys. Rev. B 36 (1987) 7715. Park, C.Y., Abukawa, T., Higashiyama, K., Kono, S.: Jpn. J. Appl. Phys. 26 (1987) L1335. Rich, D.H., Samsavar, A., Miller, T., Lin, H.F., Chiang, T.-C., Sundgren, J.-E., Greene, J.E.: Phys. Rev. Lett. 58 (1987) 579.

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400 87T1 87T2 87T3 87U 87W1 87W2 88A1 88A2 88B1 88B2 88B3 88B4 88B5 88C1 88C2 88C3 88C4 88C5 88D 88H1 88H2 88H3 88H4 88H5 88H6 88H7 88H8 88H9 88H10 88H11 88K1 88K2 88K3 88K4 88L1 88L2 88L3 88M 88N1 88N2 88N3 88N4 88N5 88N6

3.3.2 Metals on semiconductors Tonner, B.P., Li, H., Robrecht, M.J., Onellion, M., Erskine, J.L.: Phys. Rev. B 36 (1987) 989. Takahashi, T., Nakatani, S., Ishikawa, T., Kikuta, S.: Surf. Sci. 191 (1987) L825. Takahashi, T., Izumi, K., Ishikawa, T., Kikuta, S.: Surf. Sci. 183 (1987) L302. Uhrberg, R.I.G., Bringans, R.D., Olmstead, M.A., Bachrach, R.Z., Northrup, J.E.: Phys. Rev. B 35 (1987) 3945. Wilson, R.J., Chiang, S.: Phys. Rev. Lett. 59 (1987) 2329. Wilson, R.J., Chiang, S.: Phys. Rev. Lett. 58 (1987) 2575. Akiyama, K., Takayanagi, K., Tanishiro, Y.: Surf. Sci. 205 (1988) 177. Abukawa, T., Park, C.Y., Kono, S.: Surf. Sci. 201 (1988) L513. Berman, L.E., Batterman, B.W., Blakely, J.M.: Phys. Rev. B 38 (1988) 5397. Bourguignon, B., Carleton, K.L., Leone, S.R.: Surf. Sci. 204 (1988) 455. Becker, R.S., Swartzentruber, B.S., Vickers, J.S.: J. Vac. Sci. Technol. A 6 (1988) 472. Becker, R.S., Klitsner, T., Vickers, J.S.: J. Microscopy 152 (1988) 157. Becker, R.S., Swartzentruber, B.S., Vickers, J.S., Hybertsen, M.S., Louie, S.G.: Phys. Rev. Lett. 60 (1988) 116. Copel, M., Tromp, R.M., Köhler, U.K.: Phys. Rev. B 37 (1988) 10756. Chiang, S., Wilson, R.J., Gerber, Ch., Hallmarc, V.M.: J. Vac. Sci. Technol. A 6 (1988) 386. Chambliss, D.D., Rhodin, T.N., Kasowski, R.V.: J. Vac. Sci. Technol. A 6 (1988) 1499. Chen, D.M., Golovchenko, J.A., Bedrossian, P.J., Mortensen, K.: Phys. Rev. Lett. 61 (1988) 2867. Copel, M.W., Tromp, R.M.: Phys. Rev. B 37 (1988) 2766. Dumas, Ph., Humbert, A., Mathieu, G., Mathiez, P., Mouttet, C., Rolland, R., Salvan, F., Thibaudau, F., Tosch, St.: Phys. Scr. 38 (1988) 244. Huang, J.H., Williams, R.S.: J. Vac. Sci. Technol. A 6 (1988) 689. Huang, J.H., Williams, R.S.: Phys. Rev. B 38 (1988) 4022. Huang, J.H., Williams, R.S.: Surf. Sci. 204 (1988) 445. Hamers, R.J.: J. Vac. Sci. Technol. B 6 (1988) 1462. Hwang, R.Q., Williams, E.D., Park, R.L.: Surf. Sci. 193 (1988) L53. Hamers, R.J., Demuth, J.E.: J. Vac. Sci. Technol. A 6 (1988) 512. Hamers, R.J., Demuth, J.E.: Phys. Rev. Lett. 60 (1988) 2527. Hirayama, H., Baba, S., Kinbara, A.: Appl. Surf. Sci. 33/34 (1988) 193. Headrick, R.L., Graham, W.R.: J. Vac. Sci. Technol. A 6 (1988) 637. Headrick, R.L., Graham, W.R.: Phys. Rev. B 37 (1988) 1051. Hybertsen, M.S., Louie, S.G.: Phys. Rev. B 38 (1988) 4033. Knapp, B.J., Tobin, J.G.: Phys. Rev. B 37 (1988) 8656. Kato, K., Ide, T., Miura, S., Ichinokawa, T.: Surf. Sci. 194 (1988) L87. Kinoshita, T., Enta, Y., Ohta, H., Yaegashi, Y., Suzuki, S., Kono, S.: Surf. Sci. 204 (1988) 405. Kawazu, A., Sakama, H.: Phys. Rev. B 37 (1988) 2704. Li, H., Tonner, B.P.: Surf. Sci. 193 (1988) 10. Le Lay, G., Peretti, J., Hanbucken, M., Yang, W.S.: Surf. Sci. 204 (1988) 57. van Loenen, E.J., Dijkkamp, D., Hoeven, A.J.: J. Microscopy 152 (1988) 487. Morgen, P., Szymonski, M., Onsgaard, J., Jørgensen, B., Rossi, G.: Surf. Sci. 197 (1988) 347. Neddermeyer, H., Tosch, St.: Phys. Rev. B 38 (1988) 5784. Niehus, H., Köhler, U.K., Copel, M., Demuth, J.E.: J. Microscopy 152 (1988) 735. Nogami, J., Park, S., Quate, C.F.: Appl. Phys. Lett. 53 (1988) 2086. Nogami, J., Park, S., Quate, C.F.: Surf. Sci. 203 (1988) L631. Nogami, J., Park, S., Quate, C.F.: J. Vac. Sci. Technol. B 6 (1988) 1479. Nagayoshi, H.: J. Phys. Soc. Jap. 57 (1988) 2105.

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3.3.2 Metals on semiconductors 88N7 88N8 88O1 88O2 88P1 88P2 88P3 88P4 88R1 88R2 88R3 88S 88T 88W 88Y1 88Y2 88Z1 88Z2 88Z3 89B 89C1 89C2 89C3 89D1 89D2 89D3 89D4 89D5 89F1 89F2 89F3 89G 89H1 89H2 89H3 89H4 89I1 89I2 89I3 89I4 89I5

401

Niehus, H., Mann, K., Eldridge, B.N., Yu, M.L.: J. Vac. Sci. Technol. A 6 (1988) 625. Nesterenko, B.A.: Appl. Surf. Sci. 33/34 (1988) 21. Oura, K., Watamori, M., Shoji, F., Hanawa, T.: Phys. Rev. B 38 (1988) 10146. Ohta, H., Kinoshita, T., Yaegashi, Y., Suzuki, S., Kono, S.: J. Phys. Soc. Jap. 57 (1988) 4452. Pedersen, J.S., Feidenhans’l, R., Nielsen, M., Grey, F., Johnson, R.L.: Phys. Rev. B 38 (1988) 13210. Park, S., Nogami, J., Quate, C.F.: J. Microscopy 152 (1988) 727. Porter, T.L., Chang, C.S., Tsong, I.S.T.: Phys. Rev. Lett. 60 (1988) 1739. Park, C.Y., Abukawa, T., Kinoshita, T., Enta, Y., Kono, S.: Jpn. J. Appl. Phys. 27 (1988) 147. Rich, D.H., Miller, T., Samsavar, A., Lin, H.F., Chiang, T.-C.: Phys. Rev. B 37 (1988) 10221. Rubio, J., Illas, F., Ricart, J.M.: Phys. Rev. B 38 (1988) 10700. Russo, N., Toscano, M.: J. Vac. Sci. Technol. A 6 (1988) 1559. Samsavar, A., Miller, T., Chiang, T.-C.: Phys. Rev. B 38 (1988) 9889. Takahashi, T., Nakatani, S., Okamoto, N., Ishikawa, T., Kikuta, S.: Jpn. J. Appl. Phys. 27 (1988) L753. Wilson, R.J., Chiang, S., Salvan, F.: Phys. Rev. B 38 (1988) 12696. Yaguchi, H., Baba, S., Kinbara, A.: J. Vac. Soc. Jap. 31 (1988) 399. Yaguchi, H., Baba, S., Kinbara, A.: Appl. Surf. Sci. 33/34 (1988) 75. Zegenhagen, J., Hybertsen, M.S., Freeland, P.E., Patel, J.R.: Phys. Rev. B 38 (1988) 7885. Zegenhagen, J., Patel, J.R., Kincaid, B.M., Golovchenko, J.A., Mock, J.B., Freeland, P.E., Malik, R.J.: Appl. Phys. Lett. 53 (1988) 252. Zinke-Allmang, M., Feldman, L.C., Patel, J.R., Tully, J.C.: Surf. Sci. 197 (1988) 1. Bringans, R.D., Olmstead, M.A.: J. Vac. Sci. Technol. B 7 (1989) 1232. Chang, C.S., Porter, T.L., Tsong, I.S.T.: J. Vac. Sci. Technol. A 7 (1989) 1906. Copel, M., Tromp, R.M.: Phys. Rev. B 39 (1989) 12688. Conway, K.M., MacDonald, J.E., Norris, C., Vlieg, E., van der Veen, J.F.: Surf. Sci. 215 (1989) 555. Demuth, J.E., Köhler, U.K., Hamers, R.J., Kaplan, P.: Phys. Rev. Lett. 62 (1989) 641. Dolbak, A.E., Olshanetsky, B.Z., Stenin, S.I., Teys, S.A., Gavrilova, T.A.: Surf. Sci. 218 (1989) 37. Doak, R.B., Nguyen, D.B.: Phys. Rev. B 40 (1989) 1495. Daimon, H., Nagano, S., Hanada, T., Ino, S., Suga, S., Murata, Y.: Surf. Sci. 221 (1989) 244. Doak, R.B.: J. Vac. Sci. Technol. B 7 (1989) 1252. Fan, W.C., Ignatiev, A.: Phys. Rev. B 40 (1989) 5479. Feidenhans’l, R., Grey, F., Bohr, J., Nielsen, M., Johnson, R.L.: J. Phys. Colloq. 50 (1989) C7. Fan, W.C., Ignatiev, A., Huang, H., Tong, S.Y.: Phys. Rev. Lett. 62 (1989) 1516. van Gorkum, A.A., Nakagawa, K., Shiraki, Y.: J. Appl. Phys. 65 (1989) 2485. Huang, H., Wei, C.M., Li, H., Tonner, B.P., Tong, S.Y.: Phys. Rev. Lett. 62 (1989) 559. Hansen, J.C., Knapp, B.J., de Souza-Machado, R., Wagner, M.K., Tobin, J.G.: J. Vac. Sci. Technol. A 7 (1989) 2083. Henle, W.A., Ramsey, M.G., Netzer, F.P., Cimino, R., Braun, W.: Solid State Commun. 71 (1989) 657. Hamers, R.J.: Phys. Rev. B 40 (1989) 1657. Izumi, K., Takahashi, T., Kikuta, S.: Jpn. J. Appl. Phys. 28 (1989) 1742. Ichinokawa, T., Tani, T., Sayama, A.: Surf. Sci. 219 (1989) 395. Ide, T., Nishimori, T., Ichinokawa, T.: Surf. Sci. 209 (1989) 335. Ide, T., Nishimori, T., Tani, T., Ichinokawa, T.: Surf. Sci. 216 (1989) 189. Ichimiya, A., Kohmoto, S., Fujii, T., Horio, Y.: Appl. Surf. Sci. 41/42 (1989) 82.

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402 89K1 89K2 89K3 89L1 89L2 89L3 89M1 89M2 89M3 89M4 89M5 89M6 89M7 89N1 89N2 89O1 89O2 89P1 89P2 89R1 89R2 89R3 89R4 89S1 89S2 89S3 89T1 89T2 89T3 89T4 89V1 89W 89Z 90B1 90B2 90B3 90B4 90B5

3.3.2 Metals on semiconductors Knapp, B.J., Hansen, J.C., Wagner, M.K., Clendening, W.D., Tobin, J.G.: Phys. Rev. B 40 (1989) 2814. Kono, S., Abukawa, T., Nakamura, N., Anno, K.: Jpn. J. Appl. Phys. 28 (1989) L1278. Knall, J., Barnett, S.A., Sundgren, J.-E., Greene, J.E.: Surf. Sci. 209 (1989) 314. Le Lay, G., Hricovini, K., Bonnet, J.E.: Appl. Surf. Sci. 41/42 (1989) 25. Le Lay, G., Hricovini, K., Bonnet, J.E.: Phys. Rev. B 39 (1989) 3927. Li, S.-T., Hasegawa, S., Yamashita, N., Nakashima, H.: Appl. Surf. Sci. 41/42 (1989) 118. Markert, K., Pervan, P., Heichler, W., Wandelt, K.: J. Vac. Sci. Technol. A 7 (1989) 2873. Mundschau, M., Bauer, E., Telieps, W., Swiech, W.: J. Appl. Phys. 65 (1989) 4747. Maehashi, K., Hasegawa, S., Nakashima, H.: Appl. Surf. Sci. 41/42 (1989) 567. Meade, R.D., Vanderbilt, D.: Phys. Rev. Lett. 63 (1989) 1404. Myler, U., Jacobi, K.: Surf. Sci. 220 (1989) 353. Morgen, P., Jørgensen, B.: Surf. Sci. 208 (1989) 306. Morse, D.C., Mele, E.J.: Phys. Rev. B 40 (1989) 3465. Nogami, J., Park, S., Quate, C.F.: J. Vac. Sci. Technol. A 7 (1989) 1919. Northrup, J.E.: Phys. Rev. B 39 (1989) 1434. Ohno, T.R., Williams, E.D.: Appl. Phys. Lett. 55 (1989) 2628. Ohno, T.R.: Jpn. J. Appl. Phys. 28 (1989) L2061. Patterson, C.H., Messmer, R.P.: Phys. Rev. B 39 (1989) 1372. Patel, J.R., Zegenhagen, J., Freeland, P.E., Hybertsen, M.S., Golovchenko, J.A., Chen, D.M.: J. Vac. Sci. Technol. B 7 (1989) 894. Rich, D.H., Leibsle, F.M., Samsavar, A., Hirschorn, E.S., Miller, T., Chiang, T.-C.: Phys. Rev. B 39 (1989) 12758. Rich, D.H., Franklin, G.E., Leibsle, F.M., Miller, T., Chiang, T.-C.: Phys. Rev. B 40 (1989) 11804. Rich, D.H., Samsavar, A., Miller, T., Leibsle, F.M., Chiang, T.-C.: Phys. Rev. B 40 (1989) 3469. Rohlfing, D.M., Ellis, J., Hinch, B.J., Allison, W., Willis, R.F.: Surf. Sci. 207 (1989) L955. Sumimoto, K., Tanaka, K., Izawa, Y., Katayama, I., Shoji, F., Oura, K., Hanawa, T.: Appl. Surf. Sci. 41/42 (1989) 112. Salvan, F., Thibaudau, F., Dumas, Ph.: Appl. Surf. Sci. 41/42 (1989) 88. Samsavar, A., Hirschorn, E.S., Leibsle, F.M., Chiang, T.-C.: Phys. Rev. Lett. 63 (1989) 2830. Tanishiro, Y., Takayanagi, K.: Ultramicroscopy 31 (1989) 20. Tosch, St., Neddermeyer, H.: Surf. Sci. 211/212 (1989) 133. Takahashi, T., Nakatani, S., Okamoto, N., Ishikawa, T., Kikuta, S.: Rev. Sci. Instrum. 60 (1989) 2365. Takayanagi, K., Tanishiro, Y., Ishitsuka, T., Akiyama, K.: Appl. Surf. Sci. 41/42 (1989) 337. Vlieg, E., Denier van der Gon, A.W., van der Veen, J.F., MacDonald, J.E., Norris, C.: Surf. Sci. 209 (1989) 100. Williams, R.S., Daley, R.S., Huang, J.H., Charatan, R.M.: Appl. Surf. Sci. 41/42 (1989) 70. Zegenhagen, J., Patel, J.R., Freeland, P.E., Chen, D.M., Golovchenko, J.A., Bedrossian, P., Northrup, J.E.: Phys. Rev. B 39 (1989) 1298. Becker, R.S., Vickers, J.S.: J. Vac. Sci. Technol. A 8 (1990) 226. Busch, H., Henzler, M.: Phys. Rev. B 41 (1990) 4891. Baski, A.A., Nogami, J., Quate, C.F.: Phys. Rev. B 41 (1990) 10247. Barbier, L., Lapujoulade, J.: J. Vac. Sci. Technol. A 8 (1990) 2662. Baski, A.A., Nogami, J., Quate, C.F.: J. Vac. Sci. Technol. A 8 (1990) 245.

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3.3.2 Metals on semiconductors 90C1 90C2 90C3 90C4 90D1 90D2 90D3 90E 90F1 90F2 90F3 90G 90H1 90H2 90H3 90H4 90J 90K1 90K2 90M1 90M2 90N1 90N2 90N3 90O1 90O2 90R1 90R2 90R3 90R4 90R5 90S 90T 90U 90X 90Z 91A1 91A2 91A3 91B1

403

Cornelison, D.M., Chang, C.S., Tsong, I.S.T.: Nucl. Instrum. Meth. Phys. Res. B 45 (1990) 394. Chester, M., Gustafsson, T.: Phys. Rev. B 42 (1990) 9233. Cornelison, D.M., Chang, C.S., Tsong, I.S.T.: J. Vac. Sci. Technol. A 8 (1990) 3443. Chambliss, D.D., Rhodin, T.N.: Phys. Rev. B 42 (1990) 1674. Daimon, H., Chung, C., Ino, S., Watanabe, Y.: Surf. Sci. 235 (1990) 142. Doak, R.B., Nguyen, D.B.: Phys. Rev. B 41 (1990) 3578. Doak, R.B.: J. Electron. Spectros. Relat. Phenom. 54/55 (1990) 281. Enta, Y., Suzuki, S., Kono, S., Sakamoto, T.: J. Phys. Soc. Jpn. 59 (1990) 657. Fan, W.C., Ignatiev, A.: Phys. Rev. B 41 (1990) 3592. Fan, W.C., Ignatiev, A., Wu, N.J.: Surf. Sci. 235 (1990) 169. Fan, W.C., Wu, N.J., Ignatiev, A.: Phys. Rev. B 42 (1990) 1254. Grey, F., Feidenhans’l, R., Pedersen, J.S., Nielsen, M., Johnson, R.L.: Phys. Rev. B 41 (1990) 9519. Huang, H., Tong, S.Y., Yang, W.S., Shih, H.D., Jona, F.: Phys. Rev. B 42 (1990) 7483. Hasegawa, T., Takata, K., Hosaka, S., Hosoki, S.: J. Vac. Sci. Technol. A 8 (1990) 241. Hashizume, T., Hamers, R.J., Demuth, J.E., Markert, K., Sakurai, T.: J. Vac. Sci. Technol. A 8 (1990) 249. Hanada, T., Kawai, M.: Vacuum 41 (1990) 650. Jedrecy, N., Sauvage-Simkin, M., Pinchaux, R., Massies, J., Greiser, N., Etgens, V.H.: Surf. Sci. 230 (1990) 197. Karlsson, C.J., Landemark, E., Johansson, L.S.O., Uhrberg, R.I.G.: Phys. Rev. B 42 (1990) 9546. Kubby, J.A., Wang, Y.R., Greene, W.J.: Phys. Rev. Lett. 65 (1990) 2165. Mårtensson, P., Meyer, G., Amer, N.M., Kaxiras, E., Pandey, K.C.: Phys. Rev. B 42 (1990) 7230. Martinez, R.E., Augustyniak, W.M., Golovchenko, J.A.: Phys. Rev. Lett. 64 (1990) 1035. Nogami, J., Baski, A.A., Quate, C.F.: J. Vac. Sci. Technol. A 8 (1990) 3520. Nagayoshi, H.: Surf. Sci. 234 (1990) 371. Nogami, J., Baski, A.A., Quate, C.F.: Phys. Rev. Lett. 65 (1990) 1611. Oostra, D.J., Smilgys, R.V., Leone, S.R.: Surf. Sci. 226 (1990) 226. Ohno, T.R., Williams, E.D.: J. Vac. Sci. Technol. B 8 (1990) 874. Rich, D.H., Miller, T., Chiang, T.-C.: Phys. Rev. B 41 (1990) 3004. Rich, D.H., Samsavar, A., Miller, T., Chiang, T.-C.: Phys. Scr. 41 (1990) 83. Ricart, J.M., Rubio, J., Illas, F.: Phys. Rev. B 42 (1990) 5212. Richter, M., Woicik, J.C., Nogami, J., Lindau, I., Miyano, K.E., Quate, C., Baski, A.A., Kendelewicz, T., Bouldin, C.E., Spicer, W.E., Pianetta, P.: Phys. Rev. Lett. 65 (1990) 3417. Ramchurn, S.K., Bird, D.M., Bullett, D.W.: J. Phys. Condens. Matter 2 (1990) 7435. Shkrebtii, A.I., Bertoni, C.M., Del Sole, R., Nesterenko, B.A.: Surf. Sci. 239 (1990) 227. Tanishiro, Y., Yagi, K.: Surf. Sci. 234 (1990) 37. Uneta, M., Watanabe, Y., Fukuda, Y., Ohmachi, Y.: Jpn. J. Appl. Phys. 29 (1990) L17. Xing, Y.R., Zhang, J.P., Wu, J.A., Liu, C.Z., Wang, C.H.: Surf. Sci. 232 (1990) L215. Zotov, A.V., Saranin, A.A., Lifshits, V.G., Khramtsova, E.A.: Surf. Sci. 230 (1990) L147. Andrieu, S.: J. Appl. Phys. 69 (1991) 1366. Andersen, J.N., Wigren, C., Karlsson, U.O.: J. Vac. Sci. Technol. B 9 (1991) 2384. Alerhand, O.L., Wang, J., Joannopoulos, J.D., Kaxiras, E., Becker, R.S.: Phys. Rev. B 44 (1991) 6534. Bauer, E.: Surf. Sci. 250 (1991) L379.

Lando lt -Bö rnst ein New Series III/42

404 91B2 91B3 91B4 91C1 91C2 91D1 91D2 91D3 91D4 91E1 91E2 91F 91G1 91G2 91G3 91H1 91H2 91H3 91H4 91K1 91K2 91K3 91L1 91L2 91M1 91M2 91N1 91N2 91N3 91N4 91N5 91O 91Q 91R1 91R2 91S1 91S2 91S3 91S4 91T1 91T2 91T3

3.3.2 Metals on semiconductors Baski, A.A., Nogami, J., Quate, C.F.: J. Vac. Sci. Technol. A 9 (1991) 1946. Baski, A.A., Nogami, J., Quate, C.F.: Phys. Rev. B 43 (1991) 9316. Baski, A.A., Quate, C.F., Nogami, J.: Phys. Rev. B 44 (1991) 11167. Cornelison, D.M., Worthington, M.S., Tsong, I.S.T.: Phys. Rev. B 43 (1991) 4051. Chester, M., Gustafsson, T.: Surf. Sci. 256 (1991) 135. Ding, Y.G., Chan, C.T., Ho, K.M.: Phys. Rev. Lett. 67 (1991) 1454. Dolbak, A.E., Olshanetsky, B.Z., Stenin, S.I., Teys, S.A.: Surf. Sci. 247 (1991) 32. Doust, T.N., Tear, S.P.: Surf. Sci. 251/252 (1991) 568. Dornisch, D., Moritz, W., Schulz, H., Feidenhans’l, R., Nielsen, M., Grey, F., Johnson, R.L.: Phys. Rev. B 44 (1991) 11221. Enta, Y., Suzuki, S., Kono, S.: Surf. Sci. 242 (1991) 277. Elswijk, H.B., Dijkkamp, D., van Loenen, E.J.: Phys. Rev. B 44 (1991) 3802. Fan, W.C., Ignatiev, A.: Surf. Sci. 253 (1991) 297. Ganz, E., Xiong, F., Hwang, I.-S., Golovchenko, J.A.: Phys. Rev. B 43 (1991) 7316. Ganz, E., Hwang, I.-S., Xiong, F., Theiss, S.K., Golovchenko, J.A.: Surf. Sci. 257 (1991) 259. Glander, G.S., Akavoor, P., Kesmodel, L.L.: Phys. Rev. B 44 (1991) 5893. Hofmann, R., Netzer, F.P.: Phys. Rev. B 43 (1991) 9720. Hanada, T., Kawai, M.: Surf. Sci. 242 (1991) 137. Hricovini, K., Le Lay, G., Kahn, A., Taleb-Ibrahimi, A., Bonnet, J.E., Lassabatere, L.: Surf. Sci. 251/252 (1991) 424. Henle, W.A., Ramsey, M.G., Netzer, F.P., Horn, K.: Surf. Sci. 254 (1991) 182. Kelly, P.V., Tang, Z.-R., Woolf, D.A., Williams, R.H., McGilp, J.F.: Surf. Sci. 251/252 (1991) 87. Katayama, M., Williams, R.S., Kato, M., Nomura, E., Aono, M.: Phys. Rev. Lett. 66 (1991) 2762. Katayama, I., Hanawa, T., Shoji, F., Oura, K.: Appl. Surf. Sci. 48/49 (1991) 361. Leibsle, F.M., Hirschorn, E.S., Samsavar, A., Miller, T., Chiang, T.-C.: Phys. Rev. B 44 (1991) 8115. Li, S.-T., Hasegawa, S., Nakamura, S., Nakashima, S.: Jpn. J. Appl. Phys. 30 (1991) L1671. Mortensen, K.: Phys. Rev. Lett. 66 (1991) 461. McKee, R.A., Walker, F.J., Conner, J.R., Specht, E.D., Zelmon, D.E.: Appl. Phys. Lett. 59 (1991) 782. Nakamura, N., Anno, K., Kono, S.: Surf. Sci. 256 (1991) 129. Nogami, J., Baski, A.A., Quate, C.F.: Phys. Rev. B 44 (1991) 1415. Nagayoshi, H.: Surf. Sci. 242 (1991) 239. Nogami, J., Baski, A.A., Quate, C.F.: Appl. Phys. Lett. 58 (1991) 475. Northrup, J.E., Schabel, M.C., Karlsson, C.J., Uhrberg, R.I.G.: Phys. Rev. B 44 (1991) 13799. Onsgaard, J., Christiansen, M., Ørskov, F., Godovski, P.J.: Surf. Sci. 247 (1991) 208. Quinn, J., Jona, F.: Surf. Sci. 249 (1991) L307. Richter, M., Woicik, J.C., Pianetta, P., Miyano, K.E., Kendelewicz, T., Bouldin, C.E., Spicer, W.E., Lindau, I.: J. Vac. Sci. Technol. A 9 (1991) 1951. Raynerd, G., Hardiman, M., Venables, J.A.: Phys. Rev. B 44 (1991) 13803. Sancrotti, M., Rossi, G.: Phys. Rev. B 44 (1991) 1958. Schamper, Ch., Moritz, W., Schulz, H.: Phys. Rev. B 43 (1991) 12130. Swiech, W., Bauer, E., Mundschau, M.: Surf. Sci. 253 (1991) 283. Sacchi, M., Sancrotti, M., Sakho, O., Rossi, G.: Surf. Sci. 251/252 (1991) 301. Takahashi, S., Tanishiro, Y., Takayanagi, K.: Surf. Sci. 242 (1991) 73. Tong, S.Y., Huang, H.: Surf. Sci. 243 (1991) L46. Takahashi, T., Nakatani, S., Okamoto, N., Ishikawa, T.: Surf. Sci. 242 (1991) 54.

Landolt -Bö rnst ein New Series III/42

3.3.2 Metals on semiconductors 91T4 91T5 91V 91W1 91W2 91W3 91W4 91W5 91Z 92A 92B1 92B2 92B3 92C1 92C2 92D 92E 92F 92G1 92G2 92H1 92H2 92H3 92H4 92H5 92I1 92I2 92K1 92K2 92K3 92K4 92L1 92L2 92M 92N1 92N2 92N3 92O 92Q

405

Tsuge, H., Arai, M., Fugiwara, T.: Jpn. J. Appl. Phys. 30 (1991) L1583. Törnevick, C., Hammar, M., Nilsson, N.G., Flodström, S.A.: Phys. Rev. B 44 (1991) 13144. Vlieg, E., Fontes, E.F., Patel, J.R.: Phys. Rev. B 43 (1991) 7185. Wan, K.J., Ford, W.K., Lapeyre, G.J., Hermanson, J.C.: Phys. Rev. B 44 (1991) 6500. Wei, J., Williams, E.D., Park, R.L.: Surf. Sci. 250 (1991) L368. Weitering, H.H., Hibma, T., Heslinga, D.R., Klapwijk, T.M.: Surf. Sci. 251/252 (1991) 616. Wan, K.J., Guo, T., Ford, W.K., Hermanson, J.C.: Phys. Rev. B 44 (1991) 3471. Wilhelm, H., Richer, W., Rossow, U., Zahn, D., Woolf, D.A., Westwood, D.I., Williams, R.H.: Surf. Sci. 251/252 (1991) 556. Zegenhagen, J.: Phys. Scr. T. 39 (1991) 328. Adams, G.B., Sankey, O.F.: J. Vac. Sci. Technol. A 10 (1992) 2046. Bauer, E.: Appl. Surf. Sci. 60/61 (1992) 350. Brocks, G., Kelly, P.J., Car, R.: Surf. Sci. 269/270 (1992) 860. Bennett, P.A., Copel, M., Cahill, D., Falta, J., Tromp, R.M.: Phys. Rev. Lett. 69 (1992) 1224. Carlisle, J.A., Miller, T., Chiang, T.-C.: Phys. Rev. B 45 (1992) 3400. Chester, M., Gustafsson, T.: Surf. Sci. 264 (1992) 33. Ding, Y.G., Chan, C.T., Ho, K.M.: Surf. Sci. 275 (1992) L691. Evans, J.A., Laine, A.D., Weightman, P., Matthew, J.A.D., Woolf, D.A., Westwood, D.I., Williams, R.H.: Phys. Rev. B 46 (1992) 1513. Fan, W.C., Wu, N.J., Ignatiev, A.: Phys. Rev. B 45 (1992) 14167. Göthelid, M., Hammar, M., Törnevik, C., Karlsson, U.O., Nilsson, N.G., Flodström, S.A.: Surf. Sci. 271 (1992) L357. Grant, M.W., Lyman, P.F., Hoogenraad, J.H., Seiberling, L.E.: Surf. Sci. 279 (1992) L180. Håkansson, M.C., Karlsson, U.O., Kanski, J., Nilsson, P.O., Khazmi, Y., Hricovini, K.: Surf. Sci. 278 (1992) L131. Hricovini, K., Le Lay, G., Kahn, A., Taleb-Ibrahimi, A., Bonnet, J.E.: Appl. Surf. Sci. 56/58 (1992) 259. Hadley, M.J., Tear, S.P., Doust, T.N.: Appl. Surf. Sci. 56/58 (1992) 137. Higuchi, S., Nakanishi, Y.: J. Appl. Phys. 71 (1992) 4277. Hasegawa, T., Hosaka, S., Hosoki, Sh.: Jpn. J. Appl. Phys. 31 (1992) L1492. Itoh, H., Narui, S., Sayama, A., Ichinokawa, T.: Phys. Rev. B 45 (1992) 11136. Illas, F., Ricart, J.M., Casanovas, J., Rubio, J.: Surf. Sci. 275 (1992) 459. Kelly, P.V., O’Mahony, J.D., McGilp, J.F.: Appl. Surf. Sci. 56/58 (1992) 453. Kimura, Y., Takayanagi, K.: Surf. Sci. 276 (1992) 166. Karlsson, C.J., Landemark, E., Chao, Y.-C., Uhrberg, R.I.G.: Phys. Rev. B 45 (1992) 6321. Krausch, G., Jacobs, K., Fink, R., Lohmüller, J., Luckscheiter, B., Runge, B.-U., Wöhrmann, U., Schatz, G.: Europhys. Lett. 19 (1992) 611. Lohmeier, M., van der Vegt, H.A., van Silfhout, R.G., Vlieg, E., Thornton, J.M.C., MacDonald, J.E., Scholte, P.M.L.O.: Surf. Sci. 275 (1992) 190. Li, D., Zhang, J., Lee, S., Dowben, P.A.: Phys. Rev. B 45 (1992) 11876. Murakami, K.-I., Nishikata, K., Yoshimura, M., Kawazu, A.: Appl. Surf. Sci. 60/61 (1992) 146. Nakamura, N., Anno, K., Kono, S.: Surf. Sci. 262 (1992) L101. Nakatani, S., Saito, A., Kuwahara, Y., Takahashi, T., Aono, M., Kikuta, S.: Jpn. J. Appl. Phys. 31 (1992) L426. Nishikata, K., Murakami, K., Yoshimura, M., Kawazu, A.: Surf. Sci. 269/270 (1992) 995. O’Mahony, J.D., Kelly, P.V., McGilp, J.F.: Appl. Surf. Sci. 56/58 (1992) 449. Quinn, J., Jona, F., Marcus, P.M.: Phys. Rev. B 46 (1992) 7288.

Lando lt -Bö rnst ein New Series III/42

406 92R1 92R2 92S1 92S2 92S3 92S4 92S5 92S6 92S7 92W1 92W2 92W3 92W4 92W5 92W6 92Y1 92Y2 92Y3 92Y4 92Y5 92Y6 92Y7 92Y8 92Z1 92Z2 92Z3 92Z4 92Z5 93A1 93A2 93A3 93B1 93B2 93B3 93C1 93C2 93C3 93C4 93C5 93C6 93F

3.3.2 Metals on semiconductors Raynerd, G., Doust, T.N., Venables, J.A.: Surf. Sci. 261 (1992) 251. Rao, K., Martinez, R.E., Golovchenko, J.A.: Surf. Sci. 277 (1992) 323. van Silfhout, R.G., Lohmeier, M., Zaima, S., van der Veen, J.F., Howes, P.B., Norris, C., Thornton, J.M.C., Williams, A.A.: Surf. Sci. 271 (1992) 32. Sacchi, M., Sancrotti, M., Sakho, O., Rossi, G.: Appl. Surf. Sci. 56/58 (1992) 563. Slijkerman, W.F.J., Zagwijn, P.M., van der Veen, J.F., Gravesteijn, D.J., van de Walle, G.F.A.: Surf. Sci. 262 (1992) 25. Shibata, A., Kimura, Y., Takayanagi, K.: Surf. Sci. 273 (1992) L430. Sakama, H., Kawazu, A.: Appl. Surf. Sci. 60/61 (1992) 159. Shibata, A., Kimura, Y., Takayanagi, K.: Surf. Sci. 275 (1992) L697. Santini, P., Ruggerone, P., Miglio, L., Doak, R.B.: Phys. Rev. B 46 (1992) 9865. Wan, K.J., Ford, W.K., Hermanson, J.C.: Surf. Sci. 268 (1992) L253. Weitering, H.H., Ettema, A.R.H.F., Hibma, T.: Phys. Rev. B 45 (1992) 9126. Wan, K.J., Lin, X.F., Nogami, J.: Phys. Rev. B 45 (1992) 9509. Weitering, H.H., Heslinga, D.R., Hibma, T.: Phys. Rev. B 45 (1992) 5991. Wan, K.J., Guo, T., Ford, W.K., Hermanson, J.C.: Surf. Sci. 261 (1992) 69. Worthington, M.S., Stevens, J.L., Chang, C.S., Tsong, I.S.T.: J. Vac. Sci. Technol. A 10 (1992) 657. Yang, X., Cao, R., Terry, J., Pianetta, P.: J. Vac. Sci. Technol. B 10 (1992) 2013. Yamanaka, A., Tanishiro, Y., Yagi, K.: Surf. Sci. 264 (1992) 55. Yamamoto, Y.: Surf. Sci. 271 (1992) 407. Yamamoto, Y.: Jpn. J. Appl. Phys. 31 (1992) L53. Yuhara, J., Inoue, M., Morita, K.: J. Vac. Sci. Technol. A 10 (1992) 334. Yamamoto, Y.: Jpn. J. Appl. Phys. 31 (1992) 2241. Yamamoto, Y.: Jpn. J. Appl. Phys. 31 (1992) 2544. Yuhara, J., Inoue, M., Morita, K.: J. Vac. Sci. Technol. A 10 (1992) 3486. Zegenhagen, J., Molinás-Mata, P.: Ultramicroscopy 42 (1992) 952. Zotov, A.V., Lifshits, V.G., Demidchik, A.N.: Surf. Sci. 274 (1992) L583. Zhao, R.G., Jia, J.F., Yang, W.S.: Surf. Sci. 274 (1992) L519. Zotov, A.V., Khramtsova, E.A., Lifshits, V.G., Kharchenko, A.T., Ryzhkov, S.V., Demidchik, A.N.: Surf. Sci. 277 (1992) L77. Zegenhagen, J., Fontes, E., Grey, F., Patel, J.R.: Phys. Rev. B 46 (1992) 1860. Ancilotto, F., Selloni, A., Car, R.: Phys. Rev. Lett. 71 (1993) 3685. Anyele, H.T., Cafolla, A.A., Matthai, C.C.: Appl. Surf. Sci. 70/71 (1993) 433. Akavoor, P., Glander, G.S., Kesmodel, L.L., Burke, K.: Phys. Rev. B 48 (1993) 12063. Biegelsen, D.K., Bringans, R.D., Northrup, J.E., Schabel, M.C., Swartz, L.-E.: Phys. Rev. B 47 (1993) 9589. Brocks, G., Kelly, P.J., Car, R.: Phys. Rev. Lett. 70 (1993) 2786. Bennett, P.A., Parikh, S.A., Cahill, D.G.: J. Vac. Sci. Technol. A 11 (1993) 1680. Cheng, C., Kunc, K.: Appl. Surf. Sci. 65 (1993) 603. Carlisle, J.A., Miller, T., Chiang, T.-C.: Phys. Rev. B 47 (1993) 10342. Carlisle, J.A., Miller, T., Chiang, T.-C.: Phys. Rev. B 47 (1993) 3790. Chaudhury, S., Williams, R.S., Katayama, M., Aono, M.: Surf. Sci. 294 (1993) 93. Cricenti, A., Selci, S., Felici, A.C., Ferrari, L., Contini, G., Chiarotti, G.: Phys. Rev. B 47 (1993) 15745. Cricenti, A., Bernhoff, H., Reihl, B.: Phys. Rev. B 48 (1993) 10983. Finney, M.S., Norris, C., Howes, P.B., van Silfhout, R.G., Clark, G.F., Thornton, J.M.C.: Surf. Sci. 291 (1993) 99.

Landolt -Bö rnst ein New Series III/42

3.3.2 Metals on semiconductors 93G1 93G2 93H1 93H2 93H3 93H4 93H5 93H6

93H7 93H8 93H9 93H10 93H11 93I1 93I2 93I3 93J1 93J2 93K1 93K2 93K3 93K4 93L1 93L2 93L3 93L4 93L5 93L6 93M 93N 93O1 93O2 93O3 93O4 93P1 93P2 93P3

407

Göthelid, M., Grehk, T.M., Hammar, M., Karlsson, U.O., Flodström, S.A.: Phys. Rev. B 48 (1993) 2012. Griffiths, C.L., Anyele, H.T., Mathai, C.C., Cafolla, A.A., Williams, R.H.: J. Vac. Sci. Technol. B 11 (1993) 1559. Howes, P.B., Norris, C., Finney, M.S., Vlieg, E., van Silfhout, R.G.: Phys. Rev. B 48 (1993) 1632. Hammar, M., Göthelid, M., Karlsson, U.O., Flodström, S.A.: Phys. Rev. B 47 (1993) 15669. Higashiyama, K., Lewis, S.K., Ynzunza, R.X., Tober, E.D., Kim, Y.J. Fadley C.S.: Surf. Sci. 291 (1993) 47. Haight, R., Peale, D.R.: Phys. Rev. Lett. 70 (1993) 3979. Hwang, I.-S., Golovchenko, J.A.: Phys. Rev. Lett. 71 (1993) 255. Herman, G.S., Bullock, E.L., Yamada, M., Kaduwela, A.P., Friedman, D.J., Thevuthasan, S., Kim, Y.J., Tran, T.T., Fadley, C.S., Lindner, Th., Riken, D.E., Robinson, A.W., Bradshaw, A.M.: Surf. Sci. 284 (1993) 23. Hasegawa, S., Ino, S.: Surf. Sci. 283 (1993) 438. Herman, G.S., Woicik, J.C., Andrews, A.B., Erskine, J.L.: Surf. Sci. 290 (1993) L643. Hofmann, R., Netzer, F.P., Patchett, A.J., Barrett, S.D., Leibsle, F.M.: Surf. Sci. 291 (1993) 402. Hasegava, S., Nagai, Y., Oonishi, T., Ino, S.: Phys. Rev. B 47 (1993) 9903. Hasegawa, S., Ino, S.: Int. J. Mod. Phys. B7 (1993) 3817. Itoh, H., Narui, S., Tanabe, H., Ichinokawa, T.: Surf. Sci. 284 (1993) 236. Illas, F., Ricart, J.M., Rubio, J., Casanovas, J.: Phys. Rev. B 47 (1993) 2417. Itoh, H., Itoh, J., Schmid, A., Ichinokawa, T.: Phys. Rev. B 48 (1993) 14663. Jung, T.M., Kaplan, R., Prokes, S.M.: Surf. Sci. 289 (1993) L577. Jia, J.F., Zhao, R.G., Yang, W.S.: Phys. Rev. B 48 (1993) 18109. Kendelewicz, T., Woicik, J.C., Miyano, K.E., Herrera-Gomez, A., Cowan, P.L., Pianetta, P., Spicer, W.E.: J. Vac. Sci. Technol. B 11 (1993) 1449. Krausch, G., Fink, R., Jacobs, K., Luckscheiter, B., Lohmüller, J., Runge, B.-U., Wöhrmann, U., Schatz, G.: Surf. Sci. 285 (1993) 81. Krüger, P., Pollmann, J.: Phys. Rev. B 47 (1993) 1898. Krasilnikov, A.B., Latyshev, A.V., Aseev, A.L.: Surf. Sci. 290 (1993) 232. Le Lay, G., Johnson, R.L., Seemann, R., Grey, F., Feidenhans’l, R., Nielsen, M.: Surf. Sci. 287/288 (1993) 539. Li, D., Lin, J., Li, W., Lee, S., Vidali, G., Dowben, P.A.: Surf. Sci. 280 (1993) 71. Lu, Z.H., Sham, T.K., Norton, P.R.: Solid State Commun. 85 (1993) 957. Lin, X.F., Wan, K.J., Glueckstain, J.C., Nogami, J.: Phys. Rev. B 47 (1993) 3671. Lin, X.F., Wan, K.J., Nogami, J.: Phys. Rev. B 47 (1993) 10947. Lin, X.F., Wan, K.J., Nogami, J.: Phys. Rev. B 47 (1993) 13491. McKee, R.A., Walker, F.J., Conner, J.R., Raj, R.: Appl. Phys. Lett. 63 (1993) 2818. Nagayoshi, H.: Surf. Sci. 282 (1993) 163. O’Mahony, J.D., McGilp, J.F., Verbruggen, M.H.W., Flipse, C.F.J.: Surf. Sci. 287/288 (1993) 713. Ohnishi, H., Katayama, I., Ohba, Y., Shoji, F., Oura, K.: Jpn. J. Appl. Phys. 32 (1993) 2920. Öfner, H., Surnev, S.L., Shapira, Y., Netzer, F.P.: Phys. Rev. B 48 (1993) 10940. Over, H., Huang, H., Tong, S.Y., Fan, W.C., Ignatiev, A.: Phys. Rev. B 48 (1993) 15353. P.Molinás-Mata, P., Zegenhagen, J., Höpner, A., Bender, H., Schönherr, E., Labitzke, H.: J. Phys. Condens. Matter 5 (1993) 4687. Park, C., Bakhtizin, R.Z., Hashizume, T., Sakurai, T.: Jpn. J. Appl. Phys. 32 (1993) 1416. Park, C., Bakhtizin, R.Z., Hashizume, T., Sakurai, T.: Jpn. J. Appl. Phys. 32 (1993) L528.

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408 93R 93S1 93S2 93S3 93S4 93S5 93S6 93S7 93S8 93S9 93T1 93T2 93T3 93T4 93T5 93W1 93W2 93W3 93W4 93W5 93W6 93W7 93W8

93W9 93Y1 93Y2 93Z1 93Z2 93Z3 93Z4 94A 94B1 94B2 94C 94D1 94D2 94E 94F1

3.3.2 Metals on semiconductors Rossow, U., Frotscher, U., Richter, W., Zahn, D.R.T.: Surf. Sci. 287/288 (1993) 718. Seehofer, L., Falkenberg, G., Johnson, R.L.: Surf. Sci. 290 (1993) 15. Shimaoka, G.: Appl. Surf. Sci. 65/66 (1993) 569. Stevens, J.L., Worthington, M.S., Tsong, I.S.T.: Phys. Rev. B 47 (1993) 1453. Steele, B.E., Li, L., Stevens, J.L., Tsong, I.S.T.: Phys. Rev. B 47 (1993) 9925. Sakama, H., Kawazu, A., Sueyoshi, T., Sato, T., Iwatsuki, M.: Jpn. J. Appl. Phys. 32 (1993) 2929. Sakama, H., Watanahe, K., Kawazu, A.: Surf. Sci. 298 (1993) 63. Shioda, R., Kawazu, A., Baski, A.A., Quate, C.F., Nogami, J.: Phys. Rev. B 48 (1993) 4895. Sakama, H., Murakami, K., Nishikata, K., Kawazu, A.: Phys. Rev. B 48 (1993) 5278. Sakata, O., Hashizume, H., Kurashina, H.: Phys. Rev. B 48 (1993) 11408. Tang, S., Freeman, A.J.: Phys. Rev. B 47 (1993) 1460. Takahashi, T., Nakatani, S.: Surf. Sci. 282 (1993) 17. Takaoka, K., Yoshimura, M., Yao, T., Sato, T., Sueyoshi, T., Iwatsuki, M.: Phys. Rev. B 48 (1993) 5657. Tsai, M.-H., Dow, J.D., Bennett, P.A., Cahill, D.G.: Phys. Rev. B 48 (1993) 2486. Tang, S., Freeman, A.J.: Phys. Rev. B 48 (1993) 8068. Watanabe, S., Aono, M., Tsukada, M.: Surf. Sci. 287/288 (1993) 1036. Wigren, C., Andersen, J.N., Nyholm, R., Karlsson, U.O.: Surf. Sci. 289 (1993) 290. Wigren, C., Andersen, J.N., Nyholm, R., Karlsson, U.O.: Surf. Sci. 293 (1993) 254. Wigren, C., Andersen, J.N., Nyholm, R., Karlsson, U.O., Nogami, J., Baski, A.A., Quate, C.F.: Phys. Rev. B 47 (1993) 9663. Wu, H., Lapeyre, G.J., Huang, H., Tong, S.Y.: Phys. Rev. Lett. 71 (1993) 251. Wan, K.J., Lin, X.F., Nogami, J.: Phys. Rev. B 47 (1993) 13700. Woicik, J.C., Kendelewicz, T., Herrera-Gomez, A., Miyano, K.E., Cowan, P.L., Bouldin, C.E., Pianetta, P., Spicer, W.E.: Phys. Rev. Lett. 71 (1993) 1204. Woicik, J.C., Kendelewicz, T., Herrera-Gomez, A., Andrews, A.B., Kim, B.S., Cowan, P.L., Miyano, K.E., Bouldin, C.E., Karlin, B.A., Herman, G.S., Erskine, J.L., Pianetta, P., Spicer, W.E.: J. Vac. Sci. Technol. A 11 (1993) 2359. Wigren, C., Andersen, J.N., Nyholm, R., Göthelid, M., Hammar, M., Törnevik, C., Karlsson, U.O.: Phys. Rev. B 48 (1993) 11014. Yamamoto, Y.: Surf. Sci. 281 (1993) 253. Yoshimura, M., Takaoka, K., Yao, T., Sato, T., Sueyoshi, T., Iwatsuki, M.: Phys. Rev. B 47 (1993) 13930. Zhang, Y., Zhao, R.G., Yang, W.S.: Surf. Sci. 293 (1993) L821. Zhao, R.G., Zhang, Y., Yang, W.S.: Phys. Rev. B 48 (1993) 8462. Zhou, R.-H., Cao, P.-L., Lee, L.-Q.: Surf. Sci. 290 (1993) L649. Zhao, R.G., Jia, J.F., Yang, W.S.: Phys. Rev. B 48 (1993) 5333. Ancilotto, F., Selloni, A., Car, R.: Phys. Rev. B 50 (1994) 15158. Brocks, G., Kelly, P.J., Car, R.: J. Vac. Sci. Technol. B 12 (1994) 2705. Bakhtizin, R.Z., Park, C., Hashizume, T., Sakurai, T.: J. Vac. Sci. Technol. B 12 (1994) 2052. Cole, R.J., Evans, J.A., Weightman, P., Matthew, J.A.D., Woolf, D.A., Westwood, D.I.: Phys. Rev. B 49 (1994) 7528. Dobrodey, N.V., Ziegelman, L.I., Zavodinsky, V.G., Kuyanov, I.A.: Surf. Rev. Lett. 1 (1994) 273. Dolbak, A.E., Maslov, A.A., Olshanetsky, B.Z.: Phys. Low-Dim. Struct. 8 (1994) 39. Engle, W.P., Sulston, K.W., Bose, S.M.: Phys. Rev. B 50 (1994) 10880. Franklin, G.E., Fontes, E., Qian, Y., Bedzyk, M.J., Golovchenko, J.A., Patel, J.R.: Phys. Rev. B 50 (1994) 7483. Landolt -Bö rnst ein New Series III/42

3.3.2 Metals on semiconductors 94F2 94F3 94G 94H1 94H2 94I1 94I2 94I3 94I4 94I5 94J 94K1 94K2 94K3 94K4 94K5 94K6 94K7 94K8 94L1 94L2 94L3 94L4 94L5 94L6 94M1 94M2 94M3 94N1 94N2 94N3 94N4 94N5 94O1 94O2 94O3

409

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3.3.2 Metals on semiconductors Sakama, H., Murakami, K., Nishikata, K., Kawazu, A.: Phys. Rev. B 53 (1996) 1080. Santucci, S., Di Nardo, S., Lozzi, L., Passacantando, M., Picozzi, P.: Surf. Sci. 352/354 (1996) 1027. Smith, A.R., Men, F.K., Chao, K.-J., Zhang, Zh., Shih, C.K.: J. Vac. Sci. Technol. B 14 (1996) 909. Sakama, H., Kawazu, A., Sueyoshi, T., Sato, T., Iwatsuki, M.: Phys. Rev. B 54 (1996) 8756. Takeuchi, N.: J. Vac. Sci. Technol. A 14 (1996) 1652. Takeuchi, N.: Phys. Rev. B 53 (1996) 7996. Takahashi, T., Nakatani, S.: Surf. Sci. 357/358 (1996) 69. Tanishiro, Y., Kaneko, K., Minoda, H., Yagi, K., Sueyoshi, T., Sato, T., Iwatsuki, M.: Surf. Sci. 357/358 (1996) 407. Ukraintsev, V.A., Yates, jr., J.T.: Surf. Sci. 346 (1996) 31. Urano, T., Tamiya, K., Ojima, K., Hongo, S., Kanaji, T.: Surf. Sci. 357/358 (1996) 459. Veuillen, J.-Y., Gómez-Rodríguez, J.-M., Cinti, R.C.: J. Vac. Sci. Technol. B 14 (1996) 1010. Weitering, H.H.: Surf. Sci. 355 (1996) L271. Woicik, J.C., Kendelewicz, T., Yoshikawa, S.A., Miyano, K.E., Herman, G.S., Cowan, P.L., Pianetta, P., Spicer, W.E.: Phys. Rev. B 53 (1996) 15425. Yagi, Y., Kakitani, K., Yoshimori, A.: Surf. Sci. 356 (1996) 47. Yoshimura, M., Shinabe, S., Yao, T.: Surf. Sci. 357/358 (1996) 917. Yeom, H.W., Abukawa, T., Takakuwa, Y., Mori, Y., Shimatani, T., Kakizaki, A., Kono, S.: Phys. Rev. B 53 (1996) 1948. Yeom, H.W., Abukawa, T., Takakuwa, Y., Nakamura, M., Kimura, M., Kakizaki, A., Kono, S.: Surf. Sci. 365 (1996) 328. Yao, T., Shinabe, S., Yoshimura, M.: Appl. Surf. Sci. 104/105 (1996) 213. Yeom, H.W., Abukawa, T., Takakuwa, Y., Mori, Y., Shimatani, T., Kakizaki, A., Kono, S.: Phys. Rev. B 54 (1996) 4456. Yeom, H.W., Abukawa, T., Takakuwa, Y., Nakamura, M., Kimura, M., Shimatani, T., Mori, Y., Kakizaki, A., Kono, S.: J. Electron. Spectros. Relat. Phenom. 80 (1996) 177. Carpinelli, J.M., Weitering, H.H., Bartkowiak, M., Stumpf, R., Plummer, E.W.: Phys. Rev. Lett. 79 (1997) 2859. Cricenti, A., Le Lay, G., Nesterenko, B., Perfetti, P., Sebenne, C.A.: J. Vac. Sci. Technol. A 15 (1997) 133. Collazo-Davila, C., Marks, L.D., Nishii, K., Tanishiro, Y.: Surf. Rev. Lett. 4 (1997) 65. Cho, J.-H., Kang, M.-H., Terakura, K.: Phys. Rev. B 55 (1997) 15464. Cheng, C., Kunc, K.: Phys. Rev. B 56 (1997) 10283. Cricenti, A., Gothelid, M., Le Lay, G.: Surf. Sci. 382 (1997) 182. Dong, Z.-C., Yakabe, T., Fujita, D., Jiang, Q.D., Nejo, H.: Surf. Sci. 380 (1997) 23. Falkenberg, G., Seehofer, L., Johnson, R.L.: Surf. Sci. 377 (1997) 75. Falkenberg, G., Seehofer, L., Rettig, R., Johnson, R.L.: Surf. Sci. 372 (1997) 155. Falkenberg, G., Seehofer, L., Johnson, R.L.: Surf. Sci. 371 (1997) 86. Goldoni, A., Modesti, S.: Phys. Rev. Lett. 79 (1997) 3266. Goldoni, A., Cepek, C., Modesti, S.: Phys. Rev. B 55 (1997) 4109. Gómez-Rodríguez, J.M., Veuillen, J.-Y., Cinti, R.C.: Surf. Sci. 377/379 (1997) 45. Gavioli, L., Betti, M.G., Mariani, C.: Surf. Sci. 377/379 (1997) 215. Glembocki, O.J., Prokes, S.M.: Appl. Phys. Lett. 71 (1997) 2355. Hill, I.G., McLean, A.B.: Phys. Rev. B 55 (1997) 15664. Hill, I.G., McLean, A.B.: Phys. Rev. B 56 (1997) 15725. Horio, Y.: Surf. Rev. Lett. 4 (1997) 977.

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3.3.2 Metals on semiconductors Tong, X., Hasegawa, S., Ino, S.: Phys. Rev. B 55 (1997) 1310. Venables, J.A., Metcalfe, F.L., Sugawara, A.: Surf. Sci. 371 (1997) 420. Yeom, H.W., Abukawa, T., Takakuwa, Y., Mori, Y., Shimatani, T., Kakizaki, A., Kono, S.: Phys. Rev. B 55 (1997) 15669. Yoshida, S., Yamamoto, N., Nagamura, T., Oyama, M., Okazaki, S.: Surf. Sci. 393 (1997) L84. Zotov, A.V., Saranin, A.A., Ignatovich, K.V., Lifshits, V.G., Katayama, M., Oura, K.: Surf. Sci. 391 (1997) L1188. Anyele, H.T., Griffiths, C.L., Cafolla, A.A., Matthai, C.C., Williams, R.H.: Appl. Surf. Sci. 123/124 (1998) 480. An, T., Yoshimura, M., Ueda, K.: Appl. Surf. Sci. 130/132 (1998) 118. Aoki, K., Minoda, H., Tanishiro, Y., Yagi, K.: Surf. Rev. Lett. 5 (1998) 653. Baddorf, A.P., Jahns, V., Zhang, J., Carpinelli, J.M., Plummer, E.W.: Phys. Rev. B 57 (1998) 4579. Bunk, O., Falkenberg, G., Seehofer, L., Zeysing, J.H., Johnson, R.L., Nielsen, M., Feidenhans’l, R., Landemark, E.: Appl. Surf. Sci. 123/124 (1998) 104. Carpinelli, J.M., Weitering, H.H., Plummer, E.W.: Surf. Sci. 401 (1998) L457. Chandola, S., Power, J.R., Farrell, T., Weightman, P., McGilp, J.F.: Appl. Surf. Sci. 123/124 (1998) 233. Collazo-Davila, C., Grozea, D., Marks, L.D.: Phys. Rev. Lett. 80 (1998) 1678. Cricenti, A., Ottaviani, C., Comicioli, C., Perfetti, P., Le Lay, G.: Phys. Rev. B 58 (1998) 7086. Dixon, R.J., McConville, C.F., Jenkins, S.J., Srivastava, G.P.: Phys. Rev. B 57 (1998) R12701. Evans, M.M.R., Glueckstein, J.C., Nogami, J.: Surf. Sci. 406 (1998) 246. Erwin, S.C., Weitering, H.H.: Phys. Rev. Lett. 81 (1998) 2296. Fujita, K., Kusumi, Y., Ichikawa, M.: Phys. Rev. B 58 (1998) 1126. Fukumizu, H., Kurokawa, S., Sakai, A., Hasegawa, Y.: Jpn. J. Appl. Phys. 37 (1998) 3785. Gai, Z., Zhao, R.G., Ji, H., Yang, W.S.: Surf. Sci. 405 (1998) L484. Gräschus, V., Mazur, A., Krüger, P., Pollmann, J.: Phys. Rev. B 57 (1998) 13175. Gavioli, L., Grazia, Betti M., Mariani, C.: Surf. Sci. 409 (1998) 207. Glueckstein, J.C., Evans, M.M.R., Nogami, J.: Surf. Sci. 415 (1998) 80. Grozea, D., Landree, E., Marks, L.D., Feidenhans’l, R., Nielsen, M., Johnson, R.L.: Surf. Sci. 418 (1998) 32. González-Méndez, M.E., Takeuchi, N.: Phys. Rev. B 58 (1998) 16172. Gay, S.C.A., Jenkins, S.J., Srivastava, G.P.: J. Phys. Condens. Matter 10 (1998) 7751. Gay, S.C.A., Jenkins, S.J., Srivastava, G.P.: Surf. Sci. 402/404 (1998) 641. Horn-von Hoegen, M., Minoda, H., Yagi, K., Meyer zu Heringdorf, F.-J., Kähler, D.: Surf. Sci. 402/404 (1998) 464. Imperatori, P., Evans-Lutterodt, K., Ippoliti, A., Capozi, M., Pedio, M., Felici, R.: Appl. Surf. Sci. 123/124 (1998) 636. Ichikawa, M., Fujita, K., Kusumi, Y.: Surf. Rev. Lett. 5 (1998) 665. Itoh, H., Ann, T., Kawasaki, T., Ichinokawa, T.: Surf. Rev. Lett. 5 (1998) 747. Jenkins, S.J., Srivastava, G.P.: Appl. Surf. Sci. 123/124 (1998) 48. Jenkins, S.J., Srivastava, G.P., Dixon, R.J., McConville, C.F.: Surf. Sci. 402/404 (1998) 645. Khramtsova, E.A., Ichimiya, A.: Phys. Rev. B 57 (1998) 10049. Koo, J.-Y., Yi, J.-Y., Hwang, C., Kim, D.-H., Lee, S., Ko, Y.-J., Chang, K.J., Shin, D.-H.: Surf. Rev. Lett. 5 (1998) 1. Kubo, O., Saranin, A.A., Zotov, A.V., Ryu, J.-T., Tani, H., Harada, T., Katayama, M., Lifshits, V.G., Oura, K.: Surf. Sci. 415 (1998) L971. Khramtsova, E.A., Sakai, H., Hayashi, K., Ichimiya, A.: Jpn. J. Appl. Phys. 37 (1998) 6511. Luh, D.-A., Miller, T., Chiang, T.-C.: Phys. Rev. Lett. 81 (1998) 4160. Landolt -Bö rnst ein New Series III/42

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3.3.2 Metals on semiconductors Zotov, A.V., Saranin, A.A., Lifshits, V.G., Ryu, J.-T., Kubo, O., Tani, H., Katayama, M., Oura, K.: Phys. Rev. B 57 (1998) 12492. Avila, J., Mascaraque, A., Michel, E.G., Asensio, M.C., Le Lay, G., Ortega, J., Perez, R., Flores, F.: Phys. Rev. Lett. 82 (1999) 442. Avila, J., Huttel, Y., Mascaraque, A., Le Lay, G., Michel, E.G., Asensio, M.C.: Surf. Sci. 433/435 (1999) 327. Aizawa, H., Tsukada, M.: Phys. Rev. B 59 (1999) 10923. Aizawa, H., Tsukada, M., Sato, N., Hasegawa, S.: Surf. Sci. 429 (1999) L509. Bunk, O., Falkenberg, G., Zeysing, J.H., Lottermoser, L., Johnson, R.L., Nielsen, M., BergRasmussen, F., Feidenhans’l, R.: Appl. Surf. Sci. 142 (1999) 88. Bunk, O., Zeysing, J.H., Falkenberg, G., Johnson, R.L., Nielsen, M., Feidenhans’l, R.: Phys. Rev. Lett. 83 (1999) 2226. Baski, A.A., Erwin, S.C., Whitman, L.J.: Surf. Sci. 423 (1999) L265. Bunk, O., Falkenberg, G., Zeysing, J.H., Lottermoser, L., Johnson, R.L., Nielsen, M., BergRasmussen, F., Baker, J., Feidenhans’l, R.: Phys. Rev. B 59 (1999) 12228. Ceelen, W.C.A.N., de Ridder, M., Moest, B., van der Gon A.W. Denier, Brongersma, H.H.: Surf. Sci. 430 (1999) 146. Che, J.G., Zhang, K.M., Xie, X.D.: Phys. Rev. B 60 (1999) 4784. Evans, M.M.R., Nogami, J.: Phys. Rev. B 59 (1999) 7644. Erwin, S.C., Baski, A.A., Whitman, L.J., Rudd, R.E.: Phys. Rev. Lett. 83 (1999) 1818. Gregoratti, L., Günther, S., Kovac, J., Marsi, M., Phaneuf, R.J., Kiskinova, M.: Phys. Rev. B 59 (1999) 2018. Hill, I.G., McLean, A.B.: Phys. Rev. B 59 (1999) 9791. Hu, X., Peterson, C.A., Sarid, D., Yu, Z., Wang, J., Marshall, D.S., Droopad, R., Hallmark, J.A., Ooms, W.J.: Surf. Sci. 426 (1999) 69. Jedrecy, N., Gavioli, L., Mariani, C., Corradini, V., Betti, M.G., Croset, B., de Beavais, C.: Surf. Sci. 433/435 (1999) 367. Ko, Y.-J., Park, K.-H., Ha, J.S., Yun, W.S.: Phys. Rev. B 59 (1999) 4588. Komai, M., Sasaki, M., Ozawa, R., Yamamoto, S.: Appl. Surf. Sci. 146 (1999) 158. Kubo, O., Ryu, J.T., Tani, H., Harada, T., Katayama, M., Oura, K.: Jpn. J. Appl. Phys. 38 (1999) 3849. Khramtsova, E.A., Sakai, H., Hayashi, K., Ichimiya, A.: Surf. Sci. 433/435 (1999) 405. Lai, M.Y., Wang, Y.L.: Phys. Rev. B 60 (1999) 1764. Le Lay, G., Layet, J.M., Cricenti, A., Ottaviani, C., Perfetti, P.: Surf. Sci. 438 (1999) 97. Melechko, A.V., Braun, J., Weitering, H.H., Plummer, E.W.: Phys. Rev. Lett. 83 (1999) 999. Mascaraque, A., Avila, J., Asensio, M.C., Michel, E.G.: Surf. Sci. 433/435 (1999) 337. Minoda, H., Yagi, K., Meyer zu Heringdorf F.-J., Meier, A., Kähler, D., Horn-von Hoegen, M.: Phys. Rev. B 59 (1999) 2363. Miki, K., Owen, J.H.G., Bowler, D.R., Briggs, G.A.D., Sakamoto, K.: Surf. Sci. 421 (1999) 397. Miwa, R.H.: Appl. Surf. Sci. 142 (1999) 52. Matsuda, I., Yeom, H.W., Tono, K., Ohta, T.: Phys. Rev. B 59 (1999) 15784. Nakada, Y., Aksenov, I., Okumura, H.: J. Vac. Sci. Technol. B 17 (1999) 1. Naitoh, M., Shimaya, H., Nishigaki, S., Oishi, N., Shoji, F.: Appl. Surf. Sci. 142 (1999) 38. Natori, A., Kawabe, T.: Surf. Sci. 433/435 (1999) 600. Ottaviano, L., Lozzi, L., Santucci, S.: Surf. Sci. 431 (1999) 242. Petaccia, L., Grill, L., Zangrando, M., Modesti, S.: Phys. Rev. Lett. 82 (1999) 386. Phaneuf, R.J., Bennet, P.A., Marsi, M., Günther, S., Gregoratti, L., Casalis, L., Kiskinova, M.: Surf. Sci. 431 (1999) 232. Prokes, S.M., Glembocki, O.J.: J. Vac. Sci. Technol. A 17 (1999) 1410. Landolt -Bö rnst ein New Series III/42

3.3.2 Metals on semiconductors 99R1 99R2 99S1 99S2 99S3 99T1 99T2 99V1 99V2 99V3 99W1 99W2 99Y1 99Y2 99Z1 99Z2 00S

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Ryu, J.T., Fuse, T., Kubo, O., Fujino, T., Tani, H., Harada, T., Saranin, A.A., Zotov, A.V., Katayama, M., Oura, K.: J. Vac. Sci. Technol. B 17 (1999) 983. Ryu, J.T., Kubo, O., Tani, H., Harada, T., Katayama, M., Oura, K.: Surf. Sci. 433/435 (1999) 575. Stumpf, R.R., Carpinelli, J.M., Weitering, H.H.: Phys. Rev. B 59 (1999) 15779. Saranin, A.A., Zotov, A.V., Lifshits, V.G., Katayama, M., Oura, K.: Surf. Sci. 426 (1999) 298. Saranin, A.A., Zotov, A.V., Lifshits, V.G., Ryu, J.-T., Kubo, O., Tani, H., Harada, T., Katayama, M., Oura, K.: Surf. Sci. 429 (1999) 127. Tütüncü, H.M., Gay, S.C.A., Srivastava, G.P.: Physica B 263/264 (1999) 424. Takeuchi, N.: Phys. Rev. B 60 (1999) 4796. de Vries, S.A., Goedtkindt, P., Steadman, P., Vlieg, E.: Phys. Rev. B 59 (1999) 13301. Vitali, L., Ramsey, M.G., Netzer, F.P.: Phys. Rev. Lett. 83 (1999) 316. Vitali, L., Leisenberger, F.P., Ramsey, M.G., Netzer, F.P.: J. Vac. Sci. Technol. A 17 (1999) 1676. Wang, S., Radny, M.W., Smith, P.V.: Phys. Rev. B 59 (1999) 1594. Wiame, F., Mathot, G., Sivananthan, S., Rujirawat, S., Caudano, R., Sporken, R.: Appl. Surf. Sci. 142 (1999) 475. Yeom, H.W., Takeda, S., Rotenberger, E., Matsuda, I., Horikoshi, K., Schaefer, J., Lee, C.M., Kevan, S.D., Ohta, T., Nagao, T., Hasegawa, S.: Phys. Rev. Lett. 82 (1999) 4898. Yoshimura, M., An, T., Ono, I., Ueda, K.: Surf. Sci. 433/435 (1999) 470. Zhang, J., Ismail, Rous, P.J., Baddorf, A.P., Plummer, E.W.: Phys. Rev. B 60 (1999) 2860. Zhu, C., Kawazu, A., Misawa, S., Tsukahara, S.: Phys. Rev. B 59 (1999) 9760. Saranin, A.A., Ignatovich, K.V., Lifshits, V.G., Bethge, H., Kayser, R., Goldbach, H., Klust, A., Wollschläger, J., Henzler, M.: Surf. Sci. 448 (2000) 87.

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3.4 Non-metallic atomic adsorbates on metals and semiconductors 3.4.1 Chemisorbed hydrogen on metals and semiconductors (K. CHRISTMANN) See subvolume III/42A3

3.4.2 Adsorption of C, N, O on metals and semiconductors (H. OVER) See subvolume III/42A3

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3.4.3 Halogens on metals and semiconductors E. I. ALTMAN

3.4.3.1 Introduction The adsorption of halogens on a wide range of metal and semiconductor surfaces has been extensively studied over the last thirty years. The interest in halogen adsorption stems from practical problems in such disparate areas as microelectronics fabrication, heterogeneous catalysis, electrochemistry, and corrosion. In the microelectronics industry, etching reactions are widely used to pattern interconnects and gates in integrated circuits. These reactions are based upon halogen surface chemistry and are generally carried out in halogen containing plasmas [92W]. Molecular halogens, halocarbons such as CF4, hydrogen halides such as HBr, and other halogen containing species such as XeF2 have all been used as the halogen source; regardless of the source, the reactive species are halogen molecules, atoms, and ions. Under reaction conditions, a saturated chemisorbed halogen layer is rapidly formed which then further reacts to form either a bulk halide or a subhalide layer. These halides and subhalides are sufficiently volatile, or weakly bound to the surface, to sublime or desorb, thus etching the surface. Both the reaction to form the halide, or subhalide, and its subsequent desorption can be promoted by ion bombardment due to the plasma [92W, 95B1, 97C]. Since halogen adsorption is fast compared to the subsequent etching chemistry [92W, 94S, 97N] characterizing the interaction of halogen ions, atoms and molecules with adsorbed halogen layers is essential to understanding and modeling the etching process, thus motivating the study of halogen adsorption on materials of interest in integrated circuits such as silicon [90B1, 93G, 94S, 95J, 95R, 9T], tungsten [80B1, 81K1, 85W3], aluminum [85W1, 85W2], and copper [85W1, 97N1, 97N2, 98N1, 98N2]. In addition, for many materials, including Si, halogenation beyond the adsorbed layer results in a disordered subhalide layer with a diffuse interface to the underlying bulk [92W], thus attempts to develop atomic-level etching schemes have focused on etching the adsorbed layer [93P1, 94R, 95C, 98M, 99N1]. In heterogeneous catalysis, halogens generally decrease the activity of transition metal catalysts thus acting as "poisons". The halogens, however, can also be used to advantage to „promote“ certain reactions over others. The best known example of halogen promotion is ethylene epoxidation which uses a Agbased catalyst. In this case, addition of a small amount of a chlorocarbon to the feed leads to a chlorinated catalyst with a higher selectivity to ethylene oxide versus CO and CO2 [73K, 83B1, 87B]. On a more fundamental level, dissociation of halocarbons has been shown to be an effective method for generating adsorbed alkyl species that are important intermediates in catalysis under ultra-high vacuum (UHV) conditions [92L1, 92L2, 92C]. The alkyl species can then be studied using a wide range of UHV surface science techniques. An important question in these studies is how the adsorbed halogen affects the chemistry of the adsorbed alkyl groups. In electrochemistry, halogen ions are ubiquitous in electroplating, electrodissolution, and electropolishing processes, thus it is important to understand how halogen adsorption can affect chemistry at the electrode surface. In general, the halogens adsorb so strongly to the electrode surfaces that all other species are blocked [80G]. This typically occurs without preventing electrolysis of species from the solution, thus favoring reactions that occur in solution over those that proceed through adsorbed intermediates on the electrode surface [80G]. Understanding how these effects can be exploited to manipulate electrochemistry has motivated studies of halogen adsorption on common electrode materials such as Pt and Au. It is also well known that adsorption of halogens from both aqueous solutions and the gas phase can promote oxidation and corrosion. These phenomena have motivated studies of the adsorption of HCl [89W], HBr [82K] and HI [80G], and the coadsorption of halogens and oxygen [83B2, 91Z, 96S1, 98H], both from the gas phase and from aqueous solutions. The remainder of this chapter will focus on halogen chemisorption on transition metal and semiconductor surfaces, with particular emphasis on adsorption-desorption kinetics, adsorption energetics, and the structure of the chemisorbed layers. Following a description of the general features of halogen adsorption, a review of results for specific surfaces will be presented, grouped by crystallographic structure and orientation.

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[Ref. p. 450

3.4.3.2 General features of halogen adsorption Studies of halogen adsorption have revealed many features that are common to all metal and semiconductor surfaces. Above liquid nitrogen temperature, molecular halogens dissociatively adsorb, regardless of the surface. The only exception is the ability to form physisorbed molecular multilayers at temperatures less than ~230 K [85D, 92K, 92L3]. In general, dissociative adsorption of molecular halogens is irreversible. With few exceptions, chemisorbed halogen atoms desorb either atomically or as reaction products with the substrate such as SiCl2 and CuCl; for a given surface, the desorption product is determined by the relative strength of the substrate-halogen bond and the cohesive energy of the substrate. Dissociative adsorption of molecular halogens is a uniformly fast process. For all halogens on all metal and semiconductor surfaces studied, estimates of the initial sticking coefficient fall between 0.2 and 1.0. Further, the sticking coefficient tends to remain constant to high coverages, resulting in rapid saturation of the surface, and suggesting that adsorption proceeds through a precursor mechanism. In general, halogen uptake does not stop when the chemisorbed layer is saturated. Rather, continued halogen exposure leads to either bulk halide formation or the formation of a mixed halogen-substrate layer that extends into the bulk. There are a number of signatures that delineate chemisorption from bulk halogenation. These include a sharp drop in the sticking coefficient upon saturation of the chemisorbed layer [83B1, 85W1, 97N1, 97N2], the appearance of new, lower-temperature desorption peaks that can be associated with halide sublimation [83B1, 85W1, 87B, 97N1, 97N2], diffraction patterns that can be referenced to bulk halide phases [88T, 96G, 97N1, 97N2, 99A], and spectroscopic evidence of subsurface halogen atoms and bulk halogenation [92W, 94V]. Thus a distinction can be readily drawn between chemisorption and bulk halogenation; this chapter is restricted to halogen chemisorption. Since the halogens are highly electronegative, halogen chemisorption is accompanied by charge transfer from the substrate to the halogen. Thus halogen adsorption is expected to create a dipole directed towards the surface, resulting in an increase in the work function. While work function increases are generally observed during halogen adsorption, there are some exceptions. At low coverages, work function increases are observed for Au(100) [93N]. For this surface, the decrease in work function can be explained in terms of adsorption induced lifting of the clean surface reconstruction [93N]. The (1×1) surface has a lower work function than the reconstructed surface thus tending to reduce the work function. When this contribution is taken into account, it is found that the halogens increase the work function relative to the clean (1×1) surface. Other cases where work function decreases are observed include I on Pt(111) [92J] and several of the halogens on the (110) surfaces of bcc transition metals [74A, 80B1, 83S, 83F, 87F1]. These increases are more difficult to understand; explanations offered in the literature will be discussed in the sections on these surfaces. Finally, although the solvent and other electrolytes present in solution would be expected to significantly affect adsorption, in virtually every case studied, adsorption of halide ions from solution produces the same surface structures observed when gas phase molecular halogens are exposed to clean surfaces in UHV [80B2, 83B3, 89S, 96W1, 97T2, 98N2, 98V]. Further, recent studies have shown little difference between structural changes induced by halogen adsorption and etching in the gas phase and in solution [98N2,98V].

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3.4.3.3 Halogen adsorption on metal surfaces 3.4.3.3.1 FCC Transition metals 3.4.3.3.1.1 FCC (100) surfaces With the exception of Au and Pt, halogen adsorption on the (100) surfaces of fcc transition metals does not depend strongly on either the metal or the halogen, as summarized in Table 1. On the (100) surfaces of Ag [80K, 83B3, 87B], Cu [81R, 85W1, 97N1, 97N2], Ni [81J, 81K2, 85D, 87J] and Pd [90W] the four-fold hollow is the favored adsorption site [82C1, 87L, 90Y], and at saturation c(2×2) halogen layers are formed in which every other four-fold hollow is occupied. Thus the saturation coverage is 0.5 ML (halogen atoms/surface metal atom). Except for I on Cu [80C1] and Ni(100) [81J, 87J], no other ordered structures have been observed at lower coverages at room temperature. Iodine on Ni(100) is unique in that the (2×3) and c(2×8) structures dictate that not all the adsorbed I atoms occupy four-fold hollow sites. This difference between I and the other halogens on Ni(100) can be understood in terms of the larger size and lower electronegativity of I. The lower electronegativity results in less charge transfer from the metal to the adsorbed I atom and thus less of a driving force for I atoms to occupy the highest coordinated fourfold sites. Meanwhile, the larger size suggests that at similar distances, repulsive interactions between adsorbed halogen atoms may be more significant for I than Cl or Br. Thus, the repulsive interactions can be large enough to force the I atoms onto sites other than the four-fold hollows. In the other studies of halogens on Ag, Cu, Ni, and Pd (100) surfaces, at low coverages either disordered [80K, 87B, 88H, 90W] or c(2×2) LEED patterns were observed at room temperature [97N1, 97N2]. Interestingly, for Br and Cl on Cu(100), c(2×2) LEED patterns were observed starting at less than 0.1 ML while ordered c(2×2) structures could only be observed with scanning tunneling microscopy (STM) when the coverage approached saturation [98N1, 98N2]. This discrepancy can be explained by thermal diffusion in the adsorbed layer that is fast compared to the STM scan rate (~1 sec) but slow compared to the electron-ion interaction time for diffraction (~10−15 sec) [98N1]. This suggests that at least on Cu(100), adsorbed halogens tend to form c(2×2) islands at low coverages and that adatom diffusion between four-fold sites is rapid at room temperature. STM images also showed very large c(2×2) domains at saturation at room temperature, also indicative of fast diffusion [98N1]. Gold and Pt (100) behave considerably differently towards the halogens than the other (100) fcc transition metal surfaces. As indicated in Table 1, on Pt and Au the halogens tend to form a number of ordered structures as the coverage increases; the saturation coverages are higher than on the other metals; the halogen desorption temperatures tend to be lower suggesting that the halogens are more weakly bound; and molecular desorption is observed at high coverages. While Au and Pt (100) are the only surfaces of this group that reconstruct, halogen adsorption lifts the reconstruction [80B2, 90G, 93N, 95I, 96W1]. Therefore, adsorption takes place on the (1×1) surface and differences between Pt and Au and the other fcc transition metals are not related to the surface reconstructions. With the exception of Cl on Au(100), the series of structures observed when halogens are adsorbed on Au and Pt (100) can be explained in terms of the tendency to form hexagonal adlayers as the coverage is increased. This is illustrated in Fig. 1 for Br on Au(100) [80B2, 96W1]. When bromine is adsorbed from the gas phase, a c(2×2) structure initially forms. When the coverage reaches 0.5 ML, this structure transforms to a c(2√2×√2)R45o with the same nominal coverage; an intermediate (4√2×√2)R45o has also been observed. Increasing the coverage compresses the c(2√2×√2)R45o layer along the [010] direction producing a series of c(p√2×√2) R45o with p<2 structures that at the highest coverages can be pictured as distorted hexagonal arrangements of adsorbed Br atoms. Similar pictures can be drawn for Cl [90G], Br [82K], and I [80G, 84S, 91V] on Pt(100) and I [93N] on Au(100). For Cl on Au(100), the observed series of LEED patterns have been interpreted in terms of one-dimensional chains of Cl atoms that become increasingly more densely packed as the coverage approaches 1.0 ML [95I].

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[Ref. p. 450

Table 1. Summary of experimental results for halogen adsorption on (100) surfaces of fcc transition metals. The "coverage" refers to the maximum coverage obtained for the corresponding adsorbate structure. The initial sticking coefficient is denoted by S0, the temperature programmed desorption (TPD) peak temperature Tp, and the maximum work function change by ∆φ. For surfaces where more than one adsorbate structure is observed, the desorption peak temperature refers to new desorption features observed when the structure changes.

Metal Halogen Adsorbate Structure

Coverage S0 [ML]

Desorption Product

Tp [K]

∆φ [eV]

Ag

AgCl

800

AgI

680

Cl

820

+1.4 80K,87B,87L,88H +1.6 89H1,89K,97O +0.95 83B3,89H1,89K, 97T2 +1.2 95I

Cl2 Br Br Br Br

750 815 +0.8 600-770 600-770 460

0.42

I

~600, 755 +0.08 93N

0.5 0.56

I I

450 435

CuCl CuBr

950 870, 1000

Au

Cl Br I

c(2×2) c(2×2) c(2×2)

0.5 0.5 0.5

Cl

(m√2×√2)R45o m>1 c(2×2)-3 Cl c(2×2) (m√2×√2)R45o c(2√2×√2)R45o c(p√2×√2)R45o p<2 c(2√2×n√2)R45o n>1 c(2√2×√2)R45o c(p√2×√2)R45o p<2 c(2×2) c(2×2) (2×2) Distorted c(2×2) (2×2) *) c(2×2) c(2×2) (2×3) c(2×8) c(2×2) c(2×2) c(2×2) (1×1) c(2×2)

<0.8

Br

I

Cu

Cl Br I

Ni

Cl Br I

Pd Pt

Cl I F Cl

I

(3×2) c(4×2) c(2√2×n√2)R45o n>1 c(2√2×√2)R45o (5√2×√2)R45o

1.0 0.5 0.5 0.5 0.56

0.5 0.5 0.25 ~0.5 0.25 0.5 0.5 0.33 0.375 0.5 0.5 0.5 1.0 0.5

1.0 1.0 1.0

0.2

0.3 0.3

Ref.

80B2,96O,96W1

82C1,97N2,98N2 81R,97N1,97N2 80C 81K2 85D,85L 81J,87J

~1.0

~1.0

Cl

1100

>0.3 >0.4

F Cl2, Cl

925 +0.5 850, 1000 (Cl) 650 950, 980

0.67 0.25 <0.5

Cl2 I

0.5 0.6

I I2

90W,97T2, 92M1 85B1 90G

80G,84S,91V

700 <500, 650, 750

*) Only observed at 80 K.

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Figure 1 also suggests that the halogens tend to occupy bridge sites on Au and Pt (100). This assignment is based upon splitting of the fractional order diffraction spots in LEED which can be due to domain boundaries between adsorbate domains with a lower symmetry than the substrate; i.e. the adsorbate sitting in two-fold bridge sites on a four-fold symmetric surface [80B2]. While this does not prove that the Br atoms sit on bridge sites on Pt(100), the periodicity of the higher coverage structures of the halogens on Au and Pt(100) dictates that the atoms sit in sites other than the four-fold hollows, distinctly different from the saturation structures on Ag, Cu, Ni, and Pd where all the adatoms occupy four-fold hollow sites. The major difference between Au and Pt(100) and the (100) surfaces of the other fcc transition metals is that once the halogen coverage reaches 0.5 ML on Ag, Cu, Ni, and Pd(100), continued halogen exposure results in the slow growth of bulk halides, while on Au and Pt(100) continued halogen exposure results in the slow formation of higher coverage, compressed chemisorbed layers. This difference can be understood in terms of the lower driving force for halide formation on Au and Pt compared to the other fcc transition metals. As shown in Table 2, the heats of formation of Au(I) and Pt(II) halides per halogen atom are considerably less exothermic than those of the corresponding Ag(I), Cu(I), and Ni(II) halides. Thus for Au and Pt(100), the situation can be reached where the adsorption energy exceeds the heat of formation of the halide, even at high coverages when the adsorption energy is greatly reduced due to the repulsive interactions in the compressed adsorbate phases that lead to the reduced desorption temperatures at high coverages. This argument suggests that the formation of compressed chemisorbed halogen layers on Au and Pt(100) rather than bulk halides is not due to kinetic constraints. It has been shown for Cu(100) that initial halide growth can be described by the Cabrera-Mott model of oxidation in which initial oxidation proceeds through unactivated place-exchange, indicating that initial halide formation is not kinetically limited [85W1, 97N1, 97N2]. Table 2. Physical properties of transition metal halides. The heat of formation data was taken from [99C] and the structural data from [65W]. The structures listed are for the most stable phase at 300 K. The column labeled "Minimum Interhalogen Distance" lists the halogen nearest neighbor separations for those halides that form structures with close-packed halogen planes. Halide

∆Hf [kJ/mol]

Crystal Structure

Lattice Constants [nm]

Minimum Interhalogen Distance [nm]

AgF AgCl AgBr AgI AuCl AuBr AuI CuF CuCl CuBr CuI NiF2 NiCl2 NiBr2 NiI2 PdF2 PdCl2 PtCl2 PtBr2

−204.6 −127 −100.4 −61.8 −34.7 −14.0 0.0

Rock Salt Rock Salt Rock Salt Zinc Blende

0.492 0.555 0.577 0.647

0.348 0.392 0.408 0.456

Tetragonal Zinc Blende Zinc Blende Zinc Blende Zinc Blende

0.435, 13.73 0.425 0.541 0.569 0.604

0.410 0.301 0.382 0.402 0.427

Cadmium Chloride Cadmium Chloride Cadmium Chloride Rutile Orthorhombic Orthorhombic

0.354, 17.34 0.371, 17.80 0.389, 19.63 0.493, 0.334 0.381, 1.100, 0.334 0.386, 1.105, 0.335

0.334 0.350 0.367

−137.2 −104.6 −67.8 −651.4 −305.3 −212.1 −78.2 −123.4 −82

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[Ref. p. 450

3.4.3.3.1.2 FCC (111) surfaces Similar to the (100) surfaces of transition metals, the lowest energy adsorption site on the (111) surfaces is the highest coordinated site, in this case the three-fold hollow [86L, 89S, 91W, 92T, 93B1]. Numerous studies using techniques such as X-ray standing wave methods [92T], SEXAFS [86L, 91W], photoelectron diffraction [87C], and tensor LEED analysis [93B1] all indicate that the fcc type hollows are favored over the hcp hollows. Also similar to the (100) surfaces, repulsive interactions between neighboring adsorbed halogen atoms limit the occupancy of the three-fold hollows. With the possible exception of Cl on Ag [83B1, 86L] and Au(111) [93K], the coverage of the fcc hollows is limited to 1/3 resulting in (√3×√3) R30o structures. As indicated in the summary of experimental results for halogen adsorption on fcc(111) surfaces in Table 3, almost all the halogens on all the fcc (111) surfaces studied form this (√3×√3) R30o structure at a coverage of a third of a monolayer. Unlike the (100) surfaces, however, on the (111) surfaces, halogen chemisorption does not stop when the coverage of the highest coordinated sites reaches a maximum. Rather, prior to bulk halide formation, the halogens tend to form series of higher coverage adsorbed phases in which the adatoms must occupy sites other than the fcc hollows. These higher coverage phases are generally described as compression phases of the (√3×√3) R30o phase. Three types of compression are observed: 1) uniaxial compression which creates distorted hexagonal adlayers [77E, 78G, 80C2, 83B3, 89H2]; 2) biaxial compression [81S1, 93K, 95M, 97J]; and 3) biaxial compression plus rotation which gives a rotated hexagonal or "rot-hex" unit cell [94O, 95Y, 96Y]. Each type of compression is illustrated in Fig. 2. A variety of terminology has been used in the literature to describe the compression phases. Some have used terms such as "triad" and "rosette" that describe the appearance of the LEED pattern [80C2], while others have referred to the phases with larger unit cells as "incommensurate" because the coincidence lattices are large and the halogen-halogen separation can vary continuously with coverage [95M]. In Table 3, we adopt the terminology of Yamada et al. who use a modification of Wood's notation to describe the structure of the adlayers [95Y, 96Y]. In this terminology, (n Rβ × m Rα) means that the magnitude of the lattice vectors of the adlayer are n and m times the magnitude of the lattice vectors of the substrate surface while α and β refer to the angles between the adlayer and substrate surface lattice vectors. For the uniaxially compressed phases, it is convenient to refer to the (√3×√3) R30o structure in terms of the equivalent centered rectangular unit cell described by c(3×√3 R30o). As illustrated in Fig. 2.b, uniaxial compression of the c(3×√3 R30o) unit cell along the [ 110 ] direction results in adlayers that can be referred to as c(p×√3 R30o) where p<3; I on Ag [83B3, 96Y] and Au(111) [80C2, 94O, 95Y], Br on Ag(111) [78G, 89H2] and Cl on Ni(111) [77E] all form such adlayers. For the biaxially compressed phases, the primitive unit cell described by (√3×√3) R30o is used as the starting point and the compressed phases are referred to as (r√3 × r√3) R30o, where r<1; Cl and Br on Au [81S1, 93K, 95M, 96M] and Cu(111) [77G, 93M, 96W2, 97J] form these sorts of compressed phases. The rot-hex structures are referred to by (r√3 × r√3) R30 ± αo, where r is again less than one and α is the rotation angle; I on Ag [83B3, 96Y] and Au(111) [80C2, 94O, 95Y], and Br on Au [95M, 96M] Cu(111) [97J] form phases that can be described by this notation. The advantage to the Yamada terminology is that the distortion from hexagonal symmetry, the degree of compression, and the rotation are all obvious from the parameters p, r, and α. It should be noted, however, that the notation does not adequately describe the variation in adsorption site dictated by the mismatch between the adlayer and substrate surface lattices; as shown in Fig. 2, it is clear that the points on the adatom lattices described by the Yamada notation do not all fall on the same surface sites. When this variation is taken into account, the surface unit cells obviously become much larger. The dimensions of these coincidence unit cells have been accurately determined for several systems, where this is the case this alternate description of the surface structure is also provided in Table 3. In addition to the structures described above, Table 3 also lists several (n x n) structures that form following the (√3×√3) R30o structure [78G, 80E, 81C, 84W]. These structures can also be considered "rot-hex" phases with respect to the uncompressed 1/3 monolayer structure. In these structures, n-1 halogen atoms are placed between n three-fold hollow sites in the [ 110 ] and [ 011 ] directions. This results in a coverage of ( n − 1) n 2 halogen atoms per surface metal atom. 2

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427

Table 3. Summary of experimental results obtained for halogen adsorption on the (111) surfaces of fcc transition metals. The column headings are the same as defined for Table 1. Desorption Product

Tp [K]

∆φ (eV)

Ref.

0.4

AgCl

760

+1.8

77G, 83B1, 86L, 98A

0.75

Br

850

+1.4

78G, 89H2

AgI

630

Cl Cl2

790 640

Metal Halogen Adsorbate Structure Coverage S0 [ML] Ag

Cl Br

I

Au

Cl

Br

I

Cu

Cl

Br

Ni

I Cl

Pd

Cl

Pt

(√3×√3)R30o (17×17) (√3×√3)R30o c(p×√3)R30o p<3 (3×3) (√3×√3)R30o C(p×√3)R30o p<3 (0.9√3×0.9√3)R27o (√3×√3)R30o

0.5 0.33 0.44 0.33 0.43 0.49

0.53 (r√3×r√3)R30o 0.79 < r < 0.82 (r√3×r√3)R30±αo ~0.5 0.81 < r < 0.85 3.3o < α <4.7o 0.33 (√3×√3)R30o c(p×√3 R30ο) p<3 0.40 (r√3×r√3)R30±αo 0.45 r = 0.86-0.90 α = 2.9-3.3o 0.33 (√3×√3)R30o (0.92√3×0.92√3)R30o 0.39 or (4√7×4√7)R19.2o (0.86√3×0.86√3)R30o 0.45 or (6√3×6√3)R30o 0.33 (√3×√3)R30o (0.9√3×0.9√3)R30o 0.41 or (9√3×9√3)R30o 0.33 (√3×√3)R30o o 0.33 (√3×√3)R30 o 0.4 c(2.5×√3)R30 o 0.33 (√3×√3)R30

Br I F Cl

(√3×√3)R30o (√3×√3)R30o

0.33 0.33

(3×3)

0.33

Br

(√3×√3)R30o (3×3) Distorted (3×4) (4×4) (√3×√3)R30o

0.33 0.44 0.50 0.56 0.33

I

83B3, 96Y

+0.9

81S1, 93K, 95M

95M, 96M

~600

Cl

885

80C2, 94O, 95Y

+1.2

77G, 93M, 96W2

+0.8

97J

~1.0

NiCl2, NiCl < 1250 +0.5

~1.0

Cl, PdCl PdCl2 Br

0.8

80C1, 82D 77E, 91W, 92T

F, PtF2 Cl Cl2 Br Br

1050 +0.7 80E, 88T 580, 800 1000 +0.46 82T 95S 890 81B 880 +0.35 80E, 95B2, 97S1 650, 750 850 470

I

850

−0.5

84D, 84W, 89S, 92J cont.

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[Ref. p. 450

Table 3 (cont.) Metal Halogen Adsorbate Structure Coverage S0 [ML]

Rh

Cl I

(√7×√7)R19.1ο (3×3) (3√3×9√3)R30ο (√3×√3)R30o (4×4) (√3×√3)R30o

0.43 0.44 0.64 0.33 0.56 0.33

>0.2

Desorption Product

Tp [K]

I I I2 Cl

650 600 ~550 1150

∆φ (eV)

Ref.

81C 93B1

The major difference between halogen adsorption on (100) and (111) surfaces is the tendency to form higher coverage phases in which the adatoms are not all in the highest coordinated sites. This difference can be understood by comparing the adatom-adatom separations in the c(2×2) and (√3×√3) R30o structures with the halogen-halogen distances in the corresponding metal halide, these numbers are provided in Table 4. As indicated in Table 2, the halides tend to form zinc blende and rock salt structures that can be pictured as alternating (111) hexagonal planes of metal cations and halogen anions [65W]. The CdCl2 structure formed by the nickel (II) halides can also be pictured as a layered structure; in this case the (0001) plane is the hexagonal plane [65W]. On both the (100) and (111) surfaces, the halides tend to grow with their (111) or (0001) planes parallel to the substrate [83B1, 83J, 96G, 97N1, 97N2, 99A1, 99A2, 99N2]. Table 4 shows that the distances between the adatoms in the c(2×2) structures tend to be roughly equal to the halogen-halogen distances in the halide, while the (√3×√3) R30o structures result in considerably larger distances between the adatoms. Therefore compressing the c(2×2) phases pays a penalty in both shifting the adatoms to lower coordinated sites and in adatom-adatom repulsion, thus favoring bulk halide formation over the formation of higher coverage adsorbed phases. In contrast, Table 4 indicates that the compression structures on the (111) surfaces still result in roughly equal or larger halogen-halogen separations than in planes of the same symmetry in the bulk halide. Thus for the (111) surfaces, repulsion in the (√3×√3) R30o phases is not severe enough to prevent additional halogen adsorption leading to the compressed phases. Using similar arguments, Jones and Kadodwala showed that the structures of the highest coverage adsorbed phases on Cu(111) could be understood in terms of compression limited by the van der Waal’s diameters of the halogen atoms which are similar to the halogen-halogen separations in the Cu(I) halides [97J]. Scanning Tables 3 and 4 reveals that Cl appears to behave considerably differently on Ag and Au(111) than the other halogens on the other (111) surfaces. Based on SEXAFS measurements, the Cl (√3×√3) R30o structure on Ag(111) was assigned to an ordered vacancy structure in which 2/3 of the fcc hollows are occupied [86L], consistent with Auger electron spectroscopy (AES) measurements that suggested a Cl coverage much higher than 1/3 of a monolayer [83B1]. Along the same lines, based on AES estimates of the Cl coverage on Au(111), it has been suggested that a structure with a Cl coverage of 1.3 ML accounts for the observed (√3x√3) R30o LEED pattern [93K]. There are a couple of difficulties with these models. First, both models give halogen-halogen distances that are 25% smaller than any other distances observed in halogen adlayers or transition metal halides. Chlorine is expected to bond more weakly to Ag and Au surfaces than the other fcc transition metals and so for these metals it should be even more difficult to form structures with halogen separations significantly smaller than the 0.36 nm van der Waal’s diameter of Cl [99C]. Also, if the Cl atoms were so densely packed, one would expect repulsion to lead to significantly lower Cl desorption temperatures on the (111) surfaces than on the (100) surfaces; however, Cl desorbs as AgCl at roughly similar temperatures on Ag(111) and Ag(100) [83B1, 87B]. Finally, structural models for the higher coverage phases suggest Cl coverages of roughly 0.5 ML [98A], so clearly there is an inconsistency. It has been suggested that AuCl and AgCl formation occurs simultaneously with chemisorption which could account for the intense Cl signals in AES [98A]. Based on photoelectron spectroscopy data, however, Kastanas and Koel found no evidence of AuCl formation [93K]. Thus the structure of Cl adlayers on Au and Ag(111) remains unresolved.

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Table 4. Comparison of halogen nearest neighbor distances for fcc (100) surfaces at saturation with the (√3x√3)R30o phase and saturation adsorbate phases on fcc (111) surfaces, and the interhalogen distances in the close-packed halogen planes of the corresponding halides. Metal

Halogen X-X (100) [nm]

Ag

Cl Br I Cl Br I Cl Br I Cl Br I Cl Br I F Cl Br I Cl I

Au

Cu

Ni

Pd

Pt

Rh

0.409 0.409 0.409 <0.288 0.385 0.385 0.362 0.362 0.362 0.352 0.352 0.352 0.389 0.389 0.389 0.278 0.339 0.359

X-X (111)-(√3×√3)R30o [nm]

X-X (111)-sat [nm]

0.501 0.501 0.499 0.499 0.442 0.442 0.442 0.431

0.432 0.450 0.395 0.400 0.450 0.381 0.399 0.442 0.378

0.476 0.476

0.476 0.476

0.480 0.480 0.465 0.465

0.371 0.350 0.359 0.465

Halide X-X [nm] 0.392 0.408 0.458

0.410 0.382 0.402 0.427 0.334 0.350 0.367

Table 3 also reveals that I on Pt(111) is unique in that adsorption decreases the work function [92J]. Jo and White explain this decrease in terms of internal and external contributions to the work function change [92J]. The external component is due to the dipole formed by the adatom which is positive for an electronegative adsorbate such as iodine. Jo and White suggest that this contribution is small for I on Pt(111). Further, they suggest that the internal component is due to polarization within the metal due to dp rehybridization. For I, this component is negative and large because the I adsorbate level is close in energy to the edge of the d-band leading to a large splitting on hybridization. Thus a situation is reached where the negative internal contribution dominates and a decrease in work function is observed even though charge is transferred to the adsorbate.

3.4.3.3.1.3 FCC (110) surfaces Experimental results for halogen adsorption on fcc(110) transition metal surfaces are summarized in Table 5. The results suggest that adsorption on (110) surfaces is similar to adsorption on (111) surfaces. The halogens dissociatively adsorb with an initial sticking coefficient close to one, and the highest coordinated hollow site is the favored adsorption site [82E, 85B3, 86Z]. For Au and Pt(110), the clean surface reconstructions are lifted at low halogen coverages [82E, 94G, 97S1]. Adatom-adatom repulsion generally limits the coverage of the hollow sites to 50% leading to either (2x1) [85B2, 85B3, 94K] or c(2x2) [82E, 83B3] structures; both structures have been observed on Pt(110) [82E, 97S1]. Continued halogen exposure results in the formation of compression structures followed by slow bulk halide growth [82E, 85B2, 85B3, 97S1].

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[Ref. p. 450

Table 5. Summary of experimental results obtained for halogen adsorption on the (110) surfaces of fcc transition metals. The column headings are the same as defined for Table 1. Metal

Halogen Adsorbate Structure

Coverage S0 [ML]

Desorption Product

Tp [K]

Ag

F Cl

(2×1) (2×1)

0.5 0.5

AgF AgCl

775 850

~c(4×2) (2×1) c(4×2) c(2×2) Pseudo-hex (3×2) c(2×2) (3×2) c(2×2) c(2×2) (10×1) c(4×2) (2×2) (3×2) c(16×2) c(2×8) c(2×2) Pseudo-hex (2×1) c(2×2) (4×1) (4×2)

0.75 0.5 0.75 0.5 0.6-0.7 0.67 0.5 0.67 0.5 0.5 0.7 0.75

AgCl AgBr AgBr

780 900 850

Br I Au Cu

I Br

Ni

I Cl

Br Pd

Cl I

Pt

Cl

~1.0

∆φ [eV]

Ref. 94K 85B3, 87H, 96S1, 96S2

+1.4

85B2, 86Z 83B3

~1.0

NiCl2

<1150

+1.0

94G 83B

+1.1

79C 82E, 97F1

99F 0.5 0.56 0.63 0.5 0.6-0.7 0.5 0.5 0.71

~1.0

Cl

1020

+1.2

82E 97T1

~1.0

Cl

950

Cl2

670

0.55

82E, 97S1

In the compression structures, the halogen atoms are considered to remain in the troughs that run along the [ 110 ] direction of the surface. Thus the compression structures are obtained by uniaxial compression of the adatoms in the troughs to distances less than two times the substrate periodicity. When the (2×1) structure is compressed, the adatoms in adjacent troughs also stagger to create the distorted hex-agonal arrangement as pictured in Fig. 3. For I on Ag [83B3] and Pd(110) [97T1], between 0.6-0.7 ML the adatom separation along [ 110 ] varies continuously with coverage, resulting in a structure that is incommensurate in one dimension creating a phase referred to as "pseudo-hexagonal" in the literature. The compression on the (110) surfaces can be understood in terms of the same arguments used to explain the halogen adsorption structures observed on the (111) surfaces. The 2× periodicity in the troughs in the (2×1) and c(2×2) structures is considerably larger than both the halogen-halogen distances in the corresponding halides and the halogen van der Waal's diameters. Thus additional adatoms can be squeezed into the troughs without a severe penalty due to adatom-adatom repulsion. The maximum coverage occurs when the adatom-adatom separation approaches that in the close-packed planes of the bulk halide. As in the case of Cl adsorption on Ag(111), there is some debate over the structure of Cl layers on Ag(110). Based on LEED results, Bowker and Waugh [85B3] suggested that Cl forms a distorted hexagonal structure similar to Fig. 3 c with a periodicity close to c(4×2). More recent SEXAFS data, however, has been explained in terms of a more complicated "domain wall" model [87H]. The difficulty with the domain wall model is that it suggests Cl-Cl distances of only 0.314 nm, considerably less than any other Cl-Cl separation observed on fcc transition metal surfaces and the Cl-Cl distance in AgCl(111). In contrast, the distorted hexagonal model gives Cl-Cl distances of 0.393 nm, essentially the same as the Lando lt -Börnst ein New Ser ies III/42A1

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0.392 nm separation in AgCl(111). Finally, a recent STM study of Br2 adsorption on Ni(110) indicates that this system is unique amongst halogen adsorption on transition metal surfaces [99F]. It was found that Br2 adsorption removes Ni atoms from the (110) terraces forming added Ni-Br rows on the surface. Thus the reconstruction produces a mixed surface layer containing both Br and Ni atoms. While similar reconstructions have been observed for O adsorption on (110) surfaces [93B4], bromine on Ni(110) is the only known case for any of the lowindex planes where the halogen does not form a simple chemisorbed layer where the adatoms sit atop a (1x1) surface. 3.4.3.4 BCC transition metals In general, halogen adsorption on bcc transition metal surfaces has not been as thoroughly studied as halogen adsorption on fcc transition metal surfaces. The work that has been done suggests that adsorption-desorption kinetics and adsorption strength do not depend strongly on either the metal or the crystal plane. On all bcc surfaces studied, chemisorption is dissociative with an initial sticking coefficient of the order of one [82F, 83R1, 79D1, 79D2, 80D, 80R1, 80R2, 86H]. On Cr [82F, 83R1] and Fe [86H], chemisorbed halogens desorb as metal halides, while on V [80D], Nb [84B], Mo [84B], Ta [84B] and W [80B1, 81K1] the halogens desorb as atoms. Using a pulsed ion beam technique, Bolbach and Blais performed a systematic study of halogen desorption kinetics from the group VB and VIB metals in the zero coverage limit [84B]. Desorption was found to be first order and the kinetic parameters are listed in Table 6. As expected, the table shows that the desorption activation energies, and thus the heats of adsorption, go in the order F>Cl>Br. The table also shows that while the desorption preexponential depends on the metal and the crystal plane, for a given metal the desorption activation energies are essentially independent of crystal plane and show only a small variation between metals. Table 6. Kinetic parameters for first order desorption of halogens from surfaces of bcc tranistion metal at low coverages [84B]. Desorption Activation Energy Metal Surface Halogen Pre-exponential [eV] [sec−1] Nb

(110)

(100)

(111)

Mo

(110) (100)

(111)

Ta

(110) (100)

F Cl Br F Cl Br F Cl Br F Cl F Cl Br F Cl Br F Cl F

2×1013 3×1013 1×1013 3×1012 3×1012 3×1012 1×1012 1×1013 7×1012 1×1013 2×1013 2×1013 5×1013 1×1013 3×1013 3×1013 1×1013 2×1013 3×1013 7×1013

4.85 4.3 3.8 4.75 4.0 3.6 4.75 4.2 3.6 4.6 4.15 4.6 4.15 3.65 4.65 4.15 3.65 4.75 4.0 5.1 cont.

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[Ref. p. 450

Table 6 (cont.) Metal

W

Surface

(110)

Halogen

Pre-exponential [sec−1]

Desorption Activation Energy [eV]

Cl Br F Cl

5×1013 7×1013 1×1014 2×1013

4.3 4.0 4.8 4.1

On both the (100) and (110) surfaces the halogens tend to form series of compression phases as the halogen coverage increases. These phases will be described in the following sections.

3.4.3.4.1 BCC (100) surfaces Experimental results for the (100) surfaces of the bcc transition metals are summarized in Table 7. Similar to the fcc (100) surfaces, the halogens tend to form c(2x2) structures with a coverage of 0.5 ML [79D1, 79D2, 79J, 80R1, 80R2, 82F, 83R1, 83R2, 83S]. The favored adsorption site for the bcc surfaces is not known. Unlike the fcc (100) surfaces, on the bcc (100) surfaces chemisorption does not generally stop at saturation of the c(2×2) layer, rather higher halogen doses lead to continuous compression of the c(2×2) net [79D1, 79D2, 79J, 80R1, 80R2, 82F, 83R1, 83R2, 83S]. The continuous compression suggests that adatom-adatom repulsion dominates over the variation in adsorption strength with surface site in determining the structure of the adlayers. The compression of the c(2×2) structure can take several forms. As shown in Figure 4, compressing the c(2×2) structure uniaxially along the [010] direction produces a distorted hexagonal layer. The compression tends to be continuous producing structures that can be referred to in the matrix notation as:  1 cot α    ,  − 1 cot α  where α is the angle shown in Figure 4.a. Note that when α = 45o, this reduces to the c(2×2) structure. This sort of uniaxial compression has been observed for Cl and Br on Cr(100) [82F, 83R1], Cl and Br on Fe(100) [79D1, 82G], I on Ta(100) [83S], and Br and I on W(100) [80R1, 80R2]. The maximum compression, and thus the maximum coverage, is determined by the size of the halogen atom and the lattice constant of the metal. Therefore, Br has a saturation coverage of 0.875 ML on W(100) compared to 0.8 ML for I, and the saturation coverage of Br on the (100) surface of Cr, which has a lattice constant of 0.288 nm compared to 0.316 nm for W, is 0.8 ML. For Br and I on Fe(100), biaxial compression is accompanied by rotation and the adlayer maintains a square symmetry as shown in Figure 4.c [79D2, 79J, 82G]. These structures have been referred to as (2sinβ × 2sinβ)Rβ, where β is the angle between the lattice vectors of the adlayer and the substrate. When β = 45o, this is simply the c(2×2) structure; reducing β below 45o leads to compression and rotation. For I on Fe(100), β = 40.6o at saturation, while for Br, the structure switches to a non-rotated uniaxially compressed phase when β reaches 37.5o. For Cl on Ta and W(100), the rows of adatoms along the [ 011 ] and [ 011 ] directions compress [80K1, 83S]. Simultaneously, adjacent adatom rows shift with respect to one another as shown in Figure 4.d. In this way, a c(2×2) structure can be converted into the (1x1) structures observed at saturation. Again, the compression is continuous. Using the matrix notation, the surface periodicity can be described by the matrix: 1 − x  1 − x  , 0 1   1− x  where x is one minus the coverage.

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Table 7. Summary of experimental results obtained for halogen adsorption on the (100) surfaces of bcc transition metals. The column headings are the same as defined for Table 1. Metal

Halogen Adsorbate Structure Coverage S0 [ML]

Desorption Product

Tp [K]

∆φ [eV]

Ref.

Cr

Cl

0.9

CrCl

1175

+1.4

82F

0.8 0.5 0.75

0.9

CrBr

1250

0.75 0.5 0.75

CrBr2 FeCl2

900

~1.0

0.75 0.5 0.675

~1.0

Br

Fe

Cl

Br

I

Ta

Cl

I

V W

Cl Br Cl

c(2×2)  1 cot α   − 1 cot α    45o < α < 58o (5×2) c(2×2)  1 cot α   − 1 cot α    o 45 < α < 56.3o c(4×2) c(2×2)  1 cot α   − 1 cot α    53o < α < 56.3o c(4×2) c(2×2) (2sinα × 2sinα)Rα 45o < α <37.5o  1 cot α   − 1 cot α    o 53.5 < α < 55.6o c(2×2) (2sinα × 2sinα)Rα α = 45-40.6o 1 − x  1− x  0 1   1− x  0 < x < 0.15 (1×1) c(2×2)  1 cot α   − 1 cot α    o 45 < α < 60.9o c(2×10) c(2×2) c(2×2) c(2×2) 1 − x  1− x  0 1   1− x  0.17 > x > 0

0.5 0.8

83R1

+1.43

79D1, 82G, 86H

+1.1

79D2, 82G

0.5 0.675

+0.55

79J, 82G

1.0

+1.0

83S

1.0 0.5 0.9

+0.8

83S

+1.9 +0.75

83R2 80D 81K1

0.73

0.9 0.5 0.5 0.5 1.0

~1.0 1.0

Br Cl

1240 1440

cont.

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[Ref. p. 450

Table 7 (cont.) Metal

Halogen Adsorbate Structure Coverage S0 [ML] Br

I

(1×1) c(2×2) (3√2×√2)R45o c(4×2)  1 cot α   − 1 cot α    56.3o < α < 60.2o c(8×2) c(2×2)  1 cot α   − 1 cot α    o 45 < α <58o (2×5)

1.0 0.5 0.67 0.75 0.875

0.875 0.5 0.8

Desorption Product

Tp [K]

∆φ [eV]

Ref.

80R1

80R2

0.8

3.4.3.4.2 BCC (110) surfaces For the bcc crystal structure, the (110) crystal plane is the most densely packed plane. The bcc(110) surface can be pictured as a distorted hexagonal structure. It is often convenient to describe this surface using the centered rectangular unit cell highlighted in Fig. 5a rather than the primitive unit cell; the primitive unit cell can be transformed into the centered rectangular cell by multiplying the primitive lattice vectors by the matrix:  1 −1    . 1 1 When the centered rectangular unit cell is used, the clean, unreconstructed surface forms a c(1×1) structure. In the summary of experimental results in Table 8, the centered rectangular unit cell is used except where noted. On Cr and Fe(110), the halogens tend to form c(3×1) structures at a coverage of a third of a monolayer [84F, 87F1, 91M]. Increasing the coverage results in continuous compression of the adlayer accompanied by rotation. This results in a series of complex structures that can be described by the matrices listed in Table 8. For Br on Cr(110) a (2×1) layer is formed at a saturation coverage of 0.5 ML. LEED patterns revealed the presence of glide lines in the [001] direction indicating that all the Br atoms must sit in equivalent sites in this structure. Further, the only high-symmetry site that produces such glide lines is the quasi three-fold hollow site, suggesting a preference for the halogens to occupy the highest coordinated sites on the Fe and Cr(110) surface. If we take the quasi three-fold hollow site as the favored adsorption site, then as shown in Fig. 5b, the c(3×1) structure closely resembles the (√3×√3)R30ο structures observed at 1/3 ML on the fcc (111) surfaces. Thus halogen adsorption on Cr and Fe (110) can be viewed as analogous to adsorption on the fcc (111) surfaces. Unlike Fe and Cr (110), on Mo and Ta (110) the halogens tend to form discrete series of coincidence lattices rather than continuous compression phases. These structures are illustrated in Fig. 6 for Cl on Mo(110) [82C2], similar structures are observed for Cl on Ta (110) [81S2]. The formation of discrete coincidence lattices was taken as evidence that the adlayer registry with the substrate plays a more important role than repulsive adatom interactions in determining the adsorbate structure on Mo and Ta [82C2]. This also suggests that the variation in adsorption strength with adsorption site is greater on Mo and Ta (110) than on Cr and Fe (110). As indicated in Table 8, I adsorption on W(110) produces a number of ordered structures [74A,81D]. Lando lt -Börnst ein New Ser ies III/42A1

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435

At coverages between 1/4 and 1/3 of a monolayer, the LEED patterns can be explained by compression of rows of adatoms running parallel to the close-packed direction of the substrate. As shown in Fig. 7a, the spacing between the rows is twice that of the substrate. At 1/3 of a monolayer, the coincidence structure shown in Fig. 7b forms and compression stops. When the coverage reaches 0.4 ML, a new coincidence structure is observed. Between 0.41-0.5 ML, another series of continuous compression phases are observed, terminating in the structure shown in Fig. 7c; the lower coverage structures are obtained by expanding the 0.5 ML structure along the close-packed direction of the substrate. Note that the I atoms form a square lattice in Fig. 7c; at 0.5 ML the distorted hexagonal (2×1) structure shown in Fig. 7d is also observed. This (2x1) structure is continuously, uniaxially compressed along [ 110 ] ultimately leading to the c(2×6) structure shown in Fig. 7e at saturation. Table 8. Summary of experimental results obtained for halogen adsorption on the (110) surfaces of bcc transition metals. Except where noted, the adsorbate structures are based on a non-primitive centered rectangular substrate unit cell. The column headings are the same as defined for Table 1. Metal Halogen Adsorbate Structure Cr

Cl

Br

1 3 1  2 −1 3

  

∆φ [eV]

Ref.

0.4

CrCl

1200

−0.3

87F1

0.9 CrBr

1250

-0.4

84F, 87F1

0.5

c(3×1)

0.33

 x  2(1 + x )  − 3 − x Cl

Desorption Tp [K] Product

(2×1)

1

Fe

Coverage S0 [ML]

2 +x *) x − 1 

0.44

(2×1)

0.5

c(3×1)

0.33

 1− Q − P    1+ P − Q − 2 PQ  − Q 1+ P  1

CrBr2

~1.0 FeClx

900

>1050

86M2, 91M, 92L3

0.43

0< P < 0.085 Q = 0.33 – 1.67P  1− S − R  0.5   1 + P − S − PS − RQ  − Q 1 + P  1

Br

P = 0.11, Q = 0.27, R = 0.08, S = 0.16 At saturation c(3×1)  4 +3 x 0    4 + 3x  − 2 − 6 x 6  0 < x < 4/15 1

1  4 5y    4 + 3y  − 3 5 

0.33

86M2, 89J, 91M

0.4

0.42

cont.

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[Ref. p. 450

Table 8 (cont.) Metal Halogen Adsorbate Structure I

Coverage S0 [ML]

c(3×1)

0.33

 4 + 3x 0    4 + 3 x  −2 − 6 x 6 

0.4

1

Desorption Tp [K] Product

∆φ [eV]

Ref. 86M1, 86M2, 88J, 91M

0 < x < 4/15 Mo

Ta

Cl

Cl

I

(2×1)

0.5

(1×1)

0.5

(1×2)

0.75

(1×3)

0.83

(1×1)

0.5

c(1×5)

0.6

c(1×7)

0.71

(1×2)

0.75

82C2

−0.75 81S2, 83S

 3 0  −1 1   

−0.9

83S

−0.5

80B1

c(1×3) (1×1) W

Cl

0.4

(5×2)  0   −2 

3



2 x  1 

0.33

0.5 Cl

1250

74A, 81D

*)

1 > x > 3/4  1 2  −2 1  *)     1    −1 

(2 (2

2 − tan θ 3 tan θ 2 − tan θ 3 tan θ

0.4

)

0.5



) *) 

38o < θ <43.3o (3×2) *)

0.5

(2×1)

0.5

 1 tan θ   2  −1 tan θ     2

0.58

c(2×6) *) Based on primitive unit cell.

0.58

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It should be noted that the LEED patterns observed for halogen adsorption on the bcc surfaces can also be explained by regularly spaced antiphase domain boundaries [82B]. Such models, however, yield unrealistically small distances between the adsorbed halogen atoms and require complicated concerted motion of the adatoms to obtain continuously varying LEED patterns [82J]. Further, electronic band structure studies performed on chemisorbed halogen layers on Fe(110) support the formation of the incommensurate compression phases described above [91M]. Finally, the most striking feature of halogen adsorption on bcc (110) surfaces is the observed decreases in work function. Based on electronic structure calculations for Cl on Ta(110), Wu and Klepeis [97W] have offered the following explanation for the work function decreases. They consider work function changes to be due to the sum of three contributions: 1) a dipole due to polarization of the adsorbate due to the metal surface; 2) a dipole due to charge transfer between the metal and the adsorbate; and 3) a dipole due to electronic or Smoluchowski smoothing within the surface region of the metal. The first and third components decrease the work function while the second component increases the work function since charge is transferred to the halogens. For the fcc surfaces (with the exception of I on Pt(111) [92J]) and the other bcc surfaces, the second term dominates and the work function increases. The calculations of Wu and Klepeis indicate that the decrease due to electronic smoothing for Ta(110) is sufficiently large to lead to an overall decrease in the work function. They argue that the smoothing effect is unusually large for bcc(110) surfaces because of the high charge density and large interplanar spacing of the closest-packed bcc crystal plane, both of which tend to increase the magnitude of the dipole directed towards the surface. One could also argue that the higher coordination of the surface atoms on the (110) surfaces versus the (100) surfaces results in less charge transfer to the halogens on the (110) surfaces.

3.4.3.4.3 BCC(111) surfaces Detailed studies of halogen adsorption on the (111) surfaces of bcc transition metals have been limited to Cl on W(111) [80B1]. At Cl coverages between 0.35 and 1.1 ML, streaking in the LEED pattern in the (01) and (10) directions was observed. This streaking was attributed to Cl adsorption in rows parallel to the <110> directions of the substrate, with the substrate periodicity within the rows but no long-range order between the rows. Above 1.1 ML, a (4x4) LEED pattern was observed that was explained in terms of a hexagonal Cl layer in which five Cl atoms are squeezed into four sites along the [ 110 ] and [ 011 ] directions. The (4×4) structure saturates at a coverage of 1.56 ML, higher Cl coverages led to faceting. The chlorine desorbed as Cl in a peak at 1300 K with a shoulder developing at 1100 K at high coverages. Chlorine adsorption increased the work function by 0.7 eV at saturation.

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3.4.3.5 HCP transition metals The most interesting aspect of halogen adsorption on hcp transition metal surfaces is the emission of electrons, photons, and ions upon dissociative adsorption of the halogens. The electron emission is referred to as „exoelectron“ emission while the photon emission is referred to as "chemiluminescence". These processes have been studied theoretically by Nørskov, et al [79N] and more recently by Kasemo [96K], and experimentally by Cox, et al [83C]. The picture that emerges is that as the halogen molecule approaches the surface, the electron affinity of the molecule can equal the Fermi energy of the metal. When this happens, an electron can tunnel to the molecule causing it to dissociate. This creates a negative halogen ion and a neutral halogen atom with an affinity level well below the Fermi level. The energy released when this affinity level is filled by an electron near the Fermi level can be coupled to an emitted photon or electron. Alternatively, the affinity level can be filled by resonant tunneling of an electron in the valence band of the metal. This creates a hole that can be filled by an Auger process in which an electron drops down from the Fermi level and the released energy is transferred to a secondary electron which is emitted. The difference between the Fermi energy and the energy of the affinity level must be greater than twice the work function for electrons to be emitted by the Auger process. Consistent with the Auger model, exoelectron emission has been observed for the low work function metals Sc (φ = 3.4 eV), Y (φ = 3.1 eV), and Hf (φ = 3.9 eV) but not for the higher work function metals Zr (φ = 4.05 eV) or Ti (φ = 4.33 eV) [83C1]. In addition, Cl− emission has been observed for Y but not Hf, Zr, or Ti and this result has also been attributed to the low work function of Y [83C1]. Table 9. Summary of experimental results obtained for halogen adsorption on the surfaces of hcp transition metals. The column headings are the same as defined for Table 1. Surface

Halogen Adsorbate Coverage S0 Structure [ML]

Desorption Product

Tp [K]

Ti(0001)

Cl

Ti( 1010 ) Ru( 1010 )

∆φ [eV]

Ref.

0.668 0.5

TiClx

>1020

Cl

(16×16) (n×1)

87W, 92R 87W, 92R

Cl

(2×3)

0.67

Cl

950, 1200 0.55

83G

 2 0  −1 3   

0.83

 2 0  −1 4    c(2×1)

0.88 1.0

In other ways, halogen adsorption on the hcp transition metals is similar to adsorption on the other transition metal surfaces. Adsorption is dissociative with sticking coefficients close to one and halogen adsorption increases the work function [83C2, 83G, 87W, 92R]. For Y [83C2] and Ti [83C2, 87W, 92R], Cl desorbs as metal chlorides while Cl desorbs atomically from Ru [83G]. Structural studies of halogen adsorption on hcp transition metal surfaces have been limited to Cl on Ti(0001) [87W, 92R], Ti( 1010 ) [87W, 92R] and Ru ( 1010 ) [83G], the results are summarized in Table 9. The (16×16) structure on Ti(0001) was attributed to a hexagonal adlayer with 13 Cl atoms for every 16 Ti atoms in the <1000> directions [87W]. This yields Cl-Cl distances of 0.362 nm, equal to both the van der Waal’s diameter of Cl and the Cl-Cl distance in the basal plane of TiCl3. At low coverages, it was found that Cl favors three-fold hollow sites on Ti(0001), however, a preference for hcp versus fcc type hollows could not be distinguished [92R]. The hcp( 1010 ) surface consists of close-packed rows separated by troughs, similar to the fcc(110) surface. For Cl on Ti( 1010 ), (n×1) LEED patterns were observed that were attributed to Cl atoms in the troughs [87W]. For Ru( 1010 ), the c(2×1) LEED pattern observed at saturation was attributed to the structure pictured in Figure 9 [83R]. The patterns observed at lower coverages could be explained by either ordered vacancy structures, or expansion of the c(2×1) structure in the [0001] direction. Gudde and Lambert [83G] favored the ordered vacancy model because the alternate model requires compression of the adlayer perpendicular to, rather than parallel to the troughs. Lando lt -Börnst ein New Ser ies III/42A1

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3.4.3.6 Semiconductor surfaces 3.4.3.6.1 Diamond cubic semiconductors 3.4.3.6.1.1 Diamond cubic (100) surfaces The (100) surfaces of Ge and Si form dimer-row reconstructions [96N1]. In these recon-structions, pairs of adjacent surface atoms tilt towards one another, forming a bond between them. This reduces the number of dangling bonds per surface atom from two for the bulk terminated surface to one for the reconstructed surface. In STM images of Ge and Si(100), the most prominent features are rows of the dimers that run parallel to the [011] and [ 011 ]directions. The dimers are tilted so that one of the dimer atoms is closer to the underlying substrate than the other. On Ge(100), ordered patterns of tilted dimers form creating domains with (2×2) and c(4×2) periodicities [98Z]. On Si(100), however, the dimers rapidly flip between equivalent orientations at room temperature, thus a (2×1) periodicity that reflects the ordering of the dimers is observed [96N1]. On both Ge and Si(100), the halogens dissociate and attach to the dimer dangling bonds [85S, 94R, 95R, 97Y, 99T]. The σ bond between the dimers remains intact, thus preserving the dimer row reconstruction of the clean surface as indicated in Table 10. At saturation of the chemisorbed layer, a halogen atom is attached to all the dangling bonds and so the saturation coverage is 1 ML. Halogen adsorption is thought to eliminate the dimer tilt and thus adsorption on Ge(100) converts the c(4×2) structure into a (2×1) structure [85S]. For F and Cl on Si(100), only the (2x1) structure is observed [93B3, 93G, 99T]. For Br, a (3x1) structure has been observed after annealing, however, this is due to etching [94R] and so Br2 chemisorption can be considered to result in only the (2×1) structure. Iodine on Si(100) is unique in that a c(4×2) structure is observed at 0.5 ML in which every other dimer is terminated by two I atoms [95R]. It is thought that this structure is only seen for I because the larger I atoms sterically hinder occupation of neighboring dimers. At higher coverages, I atoms attach to all the dimers and chemisorption saturates at 1 ML. Defects can markedly affect halogen adsorption on Si(100). Early electron stimulated desorption ion angular dependence (ESDIAD) and high resolution electron energy loss spectroscopy (HREELS) studies suggested that Cl atoms could bridge bond to Si dimers [93G]. More recent studies have shown that the ESDIAD signature originally attributed to bridge bonded Cl disappeared on surfaces with low defect densities [97Y]. Further, when defects such as dimer vacancies were intentionally introduced by sputtering and Ni contamination, the ESDIAD feature reappeared [97D, 97Y]. Thus it was concluded that Cl bonds only terminally to Si dimers. The defects were also found to influence TPD results; surfaces with defects exhibited an additional SiCl2 desorption peak at 600 K [97Y]. Similar observations have been made for Br2 adsorption on Si(100). When Br2 is adsorbed at 220 K, an SiBr2 desorption peak at 700 K is observed that is not seen when Br2 is adsorbed at 380 K [95F]. The 700 K peak has been assigned to formation of a defective corrosive layer at coverages exceeding 1 ML, thus Br2 chemisorption on low defect density surfaces yields a SiBr2 desorption peak at 940 K [95F]. For F2 on Si(100), lower temperature SiF2 desorption peaks were also observed that may be due to either defects present prior to adsorption or to a corrosive layer rather than simple chemisorption onto Si dimers [99T]. Table 10. Summary of experimental results obtained for halogen adsorption on diamond cubic semiconductor surfaces. The column headings are the same as defined for Table 1. Surface

Halogen Adsorbate Coverage Structure [ML]

Ge(100) Ge(111)

Cl Cl Br I F

Si(100)

(2×1) (1×1) (1×1) (1×1) (2×1)

1.0 1.0 1.0 1.0 1.0

S0

Desorption Tp [K] Product

0.5-0.9 *) SiF2

800

Ref. 85S 85S 85S, 97F2 89B, 97G 89E, 93B, 99T cont.

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Table 10 (cont.) Surface

Halogen Adsorbate Coverage Structure [ML] Cl Br I

S0

Desorption Tp [K] Product

1.0 0.5 *) SiCl2 960 (2×1) 1.0 SiBr 940 (2×1) 2 0.5 c(4×2) 1.0 (2×1) 700, 950 Si(111) Cl 1.0 0.5 *) SiCl2 (1×1) Br 1.0 (1×1) I 1.0 (1×1) *) The sticking coefficients depend on the incident angle, the lower values glancing incidence [89E, 94S, 99T].

Ref. 93B3, 93G, 94S, 97Y 89T, 94R, 95F 95R 90B1, 90B2, 91G, 94S 97I 97I of 0.5 were obtained at

A distinctive feature of dissociative halogen adsorption on semiconductor surfaces is atom abstraction in which one of the atoms from the dissociated molecule remains on the surface while the other scatters into the gas phase. This mechanism has been observed for F2 on Si(100) [99T]. For F2 molecules at normal incidence on a clean Si(100) surface at 250 K, the dissociation probability is 96% and roughly 14% of the dissociation events result in abstraction. The abstraction probability reaches a maximum at a F coverage of about 0.5 ML and then falls to zero as the coverage approaches saturation. Scanning tunneling microscopy images of Cl2 [98L2], Br2 [94R], and I2 [95R] on Si(100) at 300 K revealed solely pairs of halogen atoms on a single dimer or on adjacent dimers indicating that the adsorbed halogen atoms are immobile and that abstraction does not occur for these halogens. 3.4.3.6.1.2 Diamond cubic (111) surfaces Bulk terminated diamond cubic (111) surfaces contain one dangling bond per surface atom and the (111) surfaces of both Ge and Si reconstruct to reduce the number of dangling bonds. The Ge(111) surface forms the c(2×8) reconstruction pictured in Figure 9.a [91H]. In this reconstruction, Ge adatoms are placed in three-fold hollows on the surface. Each adatom bonds to the three Ge atoms beneath it and has one dangling bond. In addition, each unit cell of the reconstructed surface contains four "rest atoms" with one dangling bond. The Si(111) surface forms the complicated (7×7) "dimer-adatom-stacking fault" reconstruction pictured in Figure 9.b [85T, 96D, 96N1]. This reconstruction also contains adatoms in three-fold hollows. In addition, there is a stacking fault that covers half the surface unit cell. The faulted and unfaulted halves of the unit cell are connected by Si dimers in the second layer. Finally, tiling the surface unit cell creates the deep corner vacancies shown in Fig. 9b. The surface unit cell contains twelve adatoms each with one dangling bond and six rest atoms with one dangling bond. As with transition metals that reconstruct, halogen adsorption on Ge and Si(111) tends to lift the reconstruction. Thus, (1×1) surfaces with a halogen atom bound to each Si or Ge atom on the surface are favored [85S, 89B, 90B1, 97F2, 97G, 97I]. For Ge, lifting the reconstruction is facile and exposure to Cl2, Br2, and I2 at room temperature produces (1x1) halogen terminated surfaces [85S, 97F2]. For Si, lifting the reconstruction requires not only removal of the adatoms but also restructuring to remove the stacking fault. Thus, lifting the (7×7) reconstruction is a slow process and formation of the (1×1) halogen terminated surfaces requires either halogen adsorption at 670-770 K, or annealing the adsorbed halogen layer above 740 K [90B1, 90B2, 97I]. At lower temperatures, the adatoms are the initial adsorption sites with the halogen atoms tying up the adatom dangling bonds [90B1, 93C, 95J]. Further, STM studies have shown a preference for adsorption at the "center" adatoms as opposed to the adatoms that are adjacent to the corner holes [96J]. Continued halogen exposure results in addition of halogen molecules across the adatom-rest layer backbonds [90B1, 90B2, 93C]. This creates adatoms bound to two and three halogen atoms, and rest layer atoms terminated by halogen atoms. This general adsorption mechanism has been observed for Cl [90B1, 90B2], Br [87F2, 93C] and I [93C]. Since adsorption at room temperature effectively produces adsorbed SiX2 and SiX3, the halogen coverage can exceed 1 ML and saturation coverages of 2.03 ML for Cl and 1.57 ML for I [93C] have been reported for adsorption temperatures of Lando lt -Börnst ein New Ser ies III/42A1

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300 K and lower [93C]. Halogen atom abstraction is far more prevalent on Si(111) than on Si(100). In STM studies of F2, Cl2, and Br2 adsorption on Si(111), single halogen adatoms attributed to abstractive adsorption were observed [95J,96J]. Further, based on these results, abstraction probabilities (percent of adsorption events resulting in abstraction) ranging between 30% for Br2 with an incident translational energy of 0.5 eV to 100% for F2 with an incident translational energy of 0.05 eV were calculated. For all three halogens, the abstraction probability was observed to decrease with increasing translational energy. In comparison, on Si(100) abstraction was only observed for F2, and the abstraction probability for F2 was more than a factor of five lower than on Si(111) [94R, 95R, 98L2, 99T]. 3.4.3.6.2 Zinc blende III-V semiconductor surfaces 3.4.3.6.2.1 Zinc blende (110) surfaces Unlike zinc blende (100) and (111) surfaces, the (110) surface exposes equal numbers of both components and so zinc blende (110) surfaces are non-polar. Thus for III-V semiconductors, the lowest energy low-index surface is the (110) surface, and the materials cleave to yield (110) surfaces [96D]. Studies of Cl2 and Br2 adsorption on GaAs at 300 K indicate that initial adsorption occurs within the rectangle formed by four surface As atoms [93P2]. This creates a (1×1) surface with a coverage of one halogen atom per Ga atom or 0.5 ML. With increasing exposure, adsorption on top of the As atoms was observed [93P2,96S3]. The maximum coverage of the As atoms was 50% leading to domains of the (2×1) and c(2×2) structures pictured in Fig. 10 [93P2]. Continued halogen exposure resulted in etching and the formation of a halogenated layer containing Ga and As mono and dihalides [92G, 93P2, 94P, 96S3, 96S4]. Fluorine was found to behave somewhat differently than the other halogens [89M, 96N2]. When XeF2 was exposed to GaAs(110), F atoms formed by dissociation initially attached to both Ga and As atoms on the surface. Before the chemisorbed layer saturated, i.e. before all the surface Ga atoms were terminated by an adsorbed F atom, bulk GaF3 was observed on the surface; the low vapor pressure of GaF3 prevented sublimation and Ga etching. The As was believed to be etched via the formation of volatile AsF3. There is little data on halogen adsorption on the (110) surfaces of the other III-V semiconductors. 3.4.3.6.2.2 Zinc blende (100) surfaces The bulk-terminated zinc blende (100) surface contains only one of the two components. Thus, the (100) surface is polar and is unstable towards reconstruction. The (100) surfaces of the III-V semiconductors form a number of reconstructions depending on the surface preparation. For GaAs(100), these include the Ga-rich c(8×2) and (6×6) reconstructions and the As-rich c(2×8), (2×4), and c(4×4) reconstructions; the other III-V semiconductors exhibit similar reconstructions [90B3, 96D, 96V1, 96V2, 99M]. The reconstructions consist of Ga and As dimers with the Ga-rich surfaces containing more Ga dimers and the As-rich surfaces more As dimers [90B3, 99M]. Scanning tunneling microscopy studies of Cl2 and Br2 adsorption on GaAs(100) have shown that initial adsorption occurs onto the Ga atoms, even on the Asrich surfaces [98L3, 99M]. On the As-rich (2×4) surface, continued Cl or Br exposure results in halogen insertion into the backbonds between the As dimers on the surface and the Ga atoms in the trenches between the dimers [99L]. Simultaneously, As mono and dihalides form; etching occurs when the trihalide forms [96S5, 97S2, 99L]. In general, F2, Cl2, and Br2 adsorption onto surfaces rich in the group V element produces disordered surfaces [94V, 96S5, 98L3]. On the Ga-rich c(8×2) surface, continued Cl2 exposure causes the Ga dimer bonds to break and the formation of GaCl2 on the surface; when GaCl3 forms the surface etches, even at room temperature [97S2]. This creates a (1×1)-As surface terminated by one Cl atom per As atom [97S2]. Continued Cl2 exposure then roughens the surface.

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Table 11. Summary of experimental results obtained for halogen adsorption on zinc blende III-V semiconductor surfaces. The column headings are the same as defined for Table 1. Material Surface (Orientation)

Halogen Adsorbate Coverage S0 Structure [ML]

GaAs(110)

Cl

(1×1)

Br

GaAs(100)

GaAs(111) GaAs(111)

c(8×2)

Cl

I I (4×6) I c(4×4) Cl c(2×8) I Cl (1×1)-As Ga-terminated Cl I (2×2)-As I (√19×√19)

(1×1) (2×1) c(2×2) (1×1) (2×1) c(2×2) (4×1)

0.5 0.75 0.75 0.5 0.75 0.75

(1×1) (1×1) (1×1) (1×1)

1.0 1.0 1.0 1.0

(1×1) (1×1) (1×1) (1×1) (1×1)

1.0 1.0 1.0 1.0 1.0

(1×1) (1×1)

1.0 1.0

Desorption Product

Tp [K]

Ref. 93P2, 96S3

93P2

0.2-0.8 GaCl

625

0.2-0.8 GaCl

625

92L4, 92M2, 94F, 97S2 96V1, 96V2 96V1, 96V2 96V1, 96V2 94F 96V1, 96V2 92M2 98L1 76J 76J

R23.4o-As

InSb(100) InAs(100)

c(8×2) c(8×2)

I I

96V1,96V2 96V1, 96V2

Iodine behaves differently than the other halogens. Iodine exposure to GaAs(100) surfaces produces (1×1) surfaces at 1.0 ML, independent of the initial surface reconstruction [96V1, 96V2]. The iodine atoms bond to the outermost atom, whether it be Ga or As. Thus, for example, when I2 is exposed to the c(4×4) As-rich surface, XPS data show only As-I bonds. Work on I2 adsorption on the In-rich c(8×2) reconstructed surfaces of InAs(100) and InSb(100) has also been reported [96V1,96V2]. Both also exhibit (1×1) LEED patterns after I2 exposure, however, the InSb surface tends to etch with preferential loss of In. The initial sticking coefficient for Cl2 on c(8×2) and c(2×8) GaAs(100) has been measured as a function of incident energy, angle, and surface temperature [94F]. The initial sticking coefficient was found to vary between 0.2-0.8 and to be largely independent of the surface structure. Since the sticking coefficient was determined by measuring the percentage of Cl2 reflected from the clean GaAs surface with a mass spectrometer, the sticking coefficient includes the probability of both dissociation and abstraction. In STM studies of the initial adsorption of Br2 on the (2×4) As-rich GaAs(100) surface, abstraction probabilities varied between 19% for adsorption at Ga atoms between the As dimers, to 64% for adsorption at As dimer vacancies [98L3]. 3.4.3.6.2.3 Zinc blende (111) surfaces The bulk terminated zinc blende (111) surface also contains only one of the two components, and like the (100) surface, several reconstructions have been observed. For GaAs(111), the As-terminated or B surface forms (√19×√19) R23o and (2×2) reconstructions [76J] and the Ga-terminated or A surface a (2×2) reconstruction [90H]. When I2 is exposed to either As-terminated reconstructed surface at 300 K, a (1×1) surface is obtained and Ga and As containing etch products are observed; the etching is continuous [76J]. There are few other reports on halogen adsorption on III-V (111) surfaces. One study on Cl layers on GaAs(111) suggests that a (1×1) Ga surface terminated by one Cl atom per Ga atom can be formed by treatment in aqueous HCl [98L1]. Lando lt -Börnst ein New Ser ies III/42A1

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Figures for 3.4.3

Fig. 1a - c. Structural models of adsorbed Br phases on Au(100). The dark shaded balls represent the Au substrate atoms, the larger, partially transparent circles the adsorbed Br atoms, and the black rectangles highlight the adsorbate unit cells. The arrows show the atomic motion required to transform the structures in (a) and (b) to those in (b) and (c). (a) The c(2×2) structure with Br adsorbed in bridge sites. (b) The c(2√2×√2)R45o structure. (c) An example of a c(p√2×√2) R45o with p<2 structure.

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Fig. 2a - d. Models illustrating compression phases observed for halogen adsorption on the (111) surfaces of fcc transition metals. The partially transparent circles represent the adsorbed atoms and the black quadrilaterals highlight the adsorbate unit cells; both the primitive and centered rectangular unit cells are shown. (a) The (√3×√3) R30o or c(3×√3 R30o) structure. (b) Uniaxial compression of the c(3×√3 R30o) structure along [ 110 ] leads to c(p×√3 R30o) where p<3 structures. (c) Biaxial compression creates (r√3×r√3) R30o structures. (d) A "rot-hex" structure created by simultaneous compression and rotation.

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Fig. 3a - c. Models of halogen adsorbate phases observed on the (110) surfaces of fcc transition metals. The partially transparent circles represent the adsorbed halogen atoms and the black rectangles highlight the adsorbate unit cells. (a) The (2×1) structure. Shifting the adatoms in the adjacent troughs that run along the [ 110 ] direction creates the c(2×2) structure shown in (b). Compressing the c(2×2) structure along [ 110 ] creates the c(4×2) structure in (c).

Fig. 4a - c. Models of halogen adsorbate structures observed on the (100) surfaces of bcc transition metals. The partially transparent circles represent the adsorbed halogen atoms, and the black quadrilaterals highlight the adsorbate unit cells. (a) Structure created by uniaxial compression of a c(2×2) structure along [010]. (b) Square structure created by simultaneous compression and rotation of a c(2×2) structure. (c) Compression of rows of adatoms along with staggering of adjacent adatom rows creates the structure pictured in (c).

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Fig. 5a, b. (a) Model of bcc (110) surface with centered rectangular unit cell highlighted. (b) Model of c(3×1) structure observed for halogen adsorption on Fe and Cr(110) surfaces. The partially transparent circles represent the adsorbed halogen atoms.

Fig. 6a - d. Models of series of adsorbate structures observed for increasing coverages of Cl on Mo(110). The dark shaded balls represent the Mo atoms, the larger, partially transparent circles the Cl atoms, and the black rectangles highlight the adsorbate unit cells. (a) (2×1) (b) (1×1) (c) (1×2) and (d) (1×3) structures.

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Fig. 7a - d. Models of commensurate structures observed for I adsorption on W(110). The partially transparent circles represent the I atoms and the black quadrilaterals highlight the unit cells. The solid lines represent the commensurate surface unit cell while the dashed lines the adsorbate unit cell. (a) The structure observed at 0.25 ML. Compressing this structure uniaxially as indicated by the arrows creates incommensurate structures with coverages up to 0.33 ML. (b) The (3×2) structure observed at 0.5 ML. The arrows show how expanding this structure leads to lower coverage structures. (c) The (2×1) structure also observed at 0.5 ML. Compression of the (2×1) unit cell as indicated by the arrows ultimately leads to the c(2×6) structure shown in (d) observed at the saturation coverage of 0.58 ML.

Fig. 8. Model of the c(2×1) structure observed at saturation coverages of Cl on Ru( 1010 ). The shaded balls represent the Ru atoms and the partially transparent circles the Cl atoms.

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Fig. 9a, b. Structural model of (a) the c(2×8) Ge(111) reconstruction and (b) the (7×7) Si(111) reconstruction. The small, dark shaded balls represent second layer atoms and the larger shaded balls, first or rest layer atoms; the lighter shaded large balls represent rest atoms that have a dangling bond. The partially transparent circles represent the Si or Ge adatoms. The heavy solid lines highlight the surface unit cells.

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Fig. 10a, b. Models of Cl and Br adsorption on GaAs(110). The dark shaded balls represent As atoms, the lighter shaded balls Ga atoms, and the partially transparent circles, Cl or Br atoms. The rectangles highlight the surface unit cells. (a) The (2×1) and (b) the c(2×2) structure.

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References for 3.4.3 65W 73K 74A 76J 77G 78G 79C 79D1 79D2 79J 79N 80B1 80B2 80C1 80C2 80D 80E 80G 80K 80R1 80R2 81B 81C 81D 81J 81K1 81K2 81R 81S1 81S2 82B 82C1 82C2 82D 82E 82F 82G 82J 82K 82T 83B1 83B2 83B3 83B4 83C1 83C2 83F 83G 83J 83R1 83R2

Wyckoff, R.W.G.: Crystal Structures. New York: Interscience Publishers, 1965. Kilty, P.A., Rol, N.C., Sachtler, W.M.H.: Proc. 5th Intern Congr. on Catalysis Vol. 2, 1973, p. 929. Avery, N.R.: Surf. Sci. 43 (1974) 101. Jacobi, K., Steinert, G., Ranke, W.: Surf. Sci. 57 (1976) 571. Goddard, P.J., Lambert, R.M.: Surf. Sci. 67 (1977) 180. Goddard, P.J., Schwaha, K., Lambert, R.M.: Surf. Sci. 71 (1978) 351. Citrin, C.H., Eisenberger, P., Hewitt, R.C.: Surf. Sci. 89 (1979) 28. Dowben, P.A., Jones, R.G.: Surf. Sci. 84 (1979) 449. Dowben, P.A., Jones, R.G.: Surf. Sci. 88 (1979) 348. Jones, R.G., Perry, D.L.: Surf. Sci. 88 (1979) 331. Nørskov, J.K., Newns, D.M., Lundqvist, B.I.: Surf. Sci. 80 (1979) 179. Bonczek, F., Engel, T., Bauer, E.: Surf. Sci. 97 (1980) 595. Bertel, E., Netzer, F.P.: Surf. Sci. 97 (1980) 409. Citrin, P.H., Eisenberger, P., Hewitt, R.C.: Phys. Rev. Lett. 45 (1980) 1948. Cochran, S.A., Farrell, H.H.: Surf. Sci. 95 (1980) 359. Davies, P.W., Lambert, R.M.: Surf. Sci. 95 (1980) 571. Erley, W.: Surf. Sci. 94 (1980) 281. Garwood jr., G.A., Hubbard, A.T.: Surf. Sci. 92 (1980) 617, and references therein. Kitson, M., Lambert, R.M.: Surf. Sci. 100 (1980) 368. Rawlings, K.J., Price, G.G., Hopkins, B.J.: Surf. Sci. 100 (1980) 289. Rawlings, K.J., Price, G.G., Hopkins, B.J.: Surf. Sci. 95 (1980) 245. Bechtold, E.: Appl. Surf. Sci. 7 (1981) 231. Cox, M.P., Lambert, R.M.: Surf. Sci. 107 (1981) 547. Dowben, P.A., Jones, R.G.: Surf. Sci. 105 (1981) 334. Jones, R.G., Woodruff, D.P.: Vacuum 31 (1981) 411. Kramer, H.M., Bauer, E.: Surf. Sci. 107 (1981) 1. Kiskinova, K, Goodman, D.W.: Surf. Sci. 108 (1981) 64. Richardson, N.V., Sass, J.K.: Surf. Sci. 103 (1981) 496. Spencer, N.D., Lambert, R.M.: Surf. Sci. 107 (1981) 237. Stott, Z.T., Hughes, H.P.: Vacuum 31 (1981) 487. Biberian, J.P.: Surf. Sci. 116 (1992) L223. Citrin, P.H., Hamann, D.R., Mattheis, L.F., Rowe, J.E.: Phys. Rev. Lett. 49 (1982) 1712. Clarke, L.J., Morales de la Garza, L.: Surf. Sci. 121 (1982) 32. DiCenzo, S.B., Wertheim, G.K., Buchanan, D.N.E.: Surf. Sci. 121 (1982) 411. Erley, W: Surf. Sci. 114 (1982) 47. Foord, J.S., Lambert, R.M.: Surf. Sci. 115 (1982) 141. Grunze, M., Dowben, P.A.: Appl. Surf. Sci. 10 (1982) 209. Jones, R.G., Dowben, P.A.: Surf. Sci. 116 (1982) L228. Katekaru, J.Y., Garwood jr., G.A., Hershberger, J.F., Hubbard, A.T.: Surf. Sci. 121 (1982) 396. Tysoe, W.T., Lambert, R.M.: Surf. Sci. 115 (1982) 37. Bowker, M., Waugh, K.C.: Surf. Sci. 134 (1983) 639. Bange, K., Grider, D., Sass, J.K.: Surf. Sci. 126 (1983) 437. Bardi, U., Rovida,G.: Surf. Sci. 128 (1983) 145. Bange, K., Döhl, Grider, D.E., Sass, J.K.: Vacuum 33 (1983) 757. Cox, M.P., Foord, J.S., Lambert, R.M., Prince, R.H.: Surf. Sci. 129 (1983) 399. Cox, M.P., Foord, J.S., Lambert, R.M., Prince, R.H.: Surf. Sci. 129 (1983) 375. Foord, J.S., Lambert, R.M.: Surf. Sci. 138 (1984) 258. Gudde, N.J., Lambert, R.M.: Surf. Sci. 134 (1983) 703. Jones, R.G., McConville, C.F., Woodruff, D.P.: Surf. Sci. 127 (1983) 424. Reed, A.P.C., Lambert, R.M., Foord, J.S.: Surf. Sci. 134 (1983) 689. Reed, A.P.C., Lambert, R.M., Foord, J.S.: Vacuum 33 (1983) 707.

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3.4.3 Halogens on metals and semiconductors 83S 84B 84D 84F 84S 84W 85B1 85B2 85B3 85D 85L 85S 85T 85W1 85W2 85W3 86H 86L 86M1 86M2 86Z 87B 87C 87F1 87F2 87H 87J 87L 87W 88H 88J 88T 89B 89E 89H1 89H2 89K 89M 89S 89T 89W 90B1 90B2 90B3 90G 90H 90Y 90W 91G 91H 91M 91V 91W 91Z

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452 92C 92G 92J 92K 92L1 92L2 92L3 92L4 92M1 92M2 92R 92T 92W 93B1 93B2 93B3 93B4 93C 93K 93M 93N 93G 93P1 93P2 94F 94G 94K 94O 94P 94R 94S 94V 95B1 95B2 95C 95F 95I 95J 95M 95R 95S 95Y 96D 96G 96J 96K 96M 96N1 96N2 96O 96S1 96S2 96S3 96S4

3.4.3 Halogens on metals and semiconductors Chiang, C.M., Wentlaff, T.H., Jenks, C.J., Bent, B.E., J. Vac. Sci. Technol. A 10 (1992) 2185. Gu, C., Chen, Y., Ohno, T.R., Weaver, J.H.: Phys. Rev. B 46 (1992) 10197. Jo, S.K., White, J.M.: Surf. Sci. 261 (1992) 111. Kastanas, G.N., Koel, B.E.: Appl. Surf. Sci. 64 (1993) 235. Lin, J.L., Bent, B.E.: J. Vac. Sci. Technol. A 10 (1992) 2202. Lin, J.L., Bent, B.E.: Chem. Phys. Lett. 194 (1992) 208. Linsebigler, A.L., Smentkowski, V.S., Ellison, M.D., Yates jr., J.T.: J. Am. Chem. Soc. 114 (1992) 465. Ludviksson, A., Xu, M., Martin, R.M.: Surf. Sci. 277 (1992) 282. McBride, J.R., Schimpf, J.A., Soriaga, M.P.: J. Am. Chem. Soc. 114 (1992) 10950. Mokler, S.M., Watson, P.R., Ungier, L., Arthur, J.R.: J. Vac. Sci. Technol. B 10 (1992) 2371. Ri, C.S., Watson, P.R.: J. Vac. Sci. Technol. A 10 (1992) 2535. Takata, Y., Yagi, S., Yokoyama, T., Ohta, T., Kitajima, Y.: Surf. Sci. 265 (1992) 111. Winters, H.F., Coburn, J.W.: Surf. Sci. Rep. 14 (1992) 161. Barnes, C.J., Wander, A., King, D.A.: Surf. Sci. 281 (1993) 33. Bennett, S.L., Greenwood, C.L., Williams, E.M.: Surf. Sci. 290 (1993) 267. Boland, J.J.: Science 262 (1993) 1703. Besenbacher, F., Nørskov, J.K.: Prog. Surf. Sci. 44 (1993) 5. Chakarian, V., Shuh, D.K., Yarmoff, J.A., Håkansson, M.C., Karlsson, U.O.: Surf. Sci. 296 (1993) 383. Kastanas, G.N., Koel, B.E.: Appl. Surf. Sci. 64 (1993) 235. Motai, K., Hashizume, T., Lu, H., Jeon, D., Sakurai, T., Pickering, H.W.: Appl. Surf. Sci. 67 (1993) 246. Neumann, A., Christmann, K., Solomun, T.: Surf. Sci. 287-288 (1993) 593. Gao, Q., Cheng, C.C., Chen, P.J., Choyke, W.J., Yates jr., J.T.: J. Chem. Phys. 98 (1993) 8308. Patrin, J.C., Li, Y.Z., Chander, M., Weaver, J.H.: Appl. Phys. Lett. 62 (1993) 1277. Patrin, J.C., Weaver, J.H.: Phys. Rev. B 48 (1993) 17913. Flaum, H.C., Sullivan, D.J.D., Kummel, A.C.: J. Chem. Phys. 100 (1994) 1634. Gao, X., Weaver, M.J.: Phys. Rev. Lett. 73 (1994) 846. Krasnopler, A., Stuve, E.M.: Surf. Sci. 303 (1994) 355. Ocko, B.M., Watson, G.M., Wang, J.: J. Phys. Chem. 98 (1994) 897. Pankratz, J. Neinhaus, H., Monch, W.: Surf. Sci. 308-309 (1994) 211. Rioux, D., Chander, M., Li,Y.Z., Weaver, J.H.: Phys. Rev. B 49 (1994) 11071. Szabó, A., Farrall, P.D., Engel, T.: Surf. Sci. 312 (1994) 284. Varekamp, P.R., Simpson, W.C., Shuh, D.K., Durbin, T.D., Chakarian, V., Yarmoff, J.A.: Phys. Rev. B Barone, M.E., Graves, D.B.: J. Appl. Phys. 78 (1995) 6604. Bittner, A.M., Wintterlin, J., Beran, B., Ertl, G.: Surf. Sci. 335 (1995) 291. Chander, M., Goetsch, D.A., Aldao, C.M., Weaver, J.H.: Phys. Rev. B 52 (1995) 8288. Flowers, M.C., Jonathan, N.B.H., Liu, Y., Morris, A.: Surf. Sci. 343 (1995) 133. Iwai, H., Okada, M., Fukutani, K., Murata, Y.: J. Phys. Condens. Matter. 7 (1995) 5163. Jensen, J.A., Yan, C., Kummel, A.C.: Science 267 (1995) 493. Magnussen, O.M., Ocko, B.M., Adzic, R.R., Wang, J.X.: Phys. Rev. B 51 (1995) 5510. Rioux, D., Stepniak, F., Pechman, R.J., Weaver, J.H.: Phys. Rev. B 51 (1995) 10981. Soriaga, M.P., Schimpf, J.A., Carrasquillo jr., A., Abreu, J.B., Temesghen, W., Barriga, R.J., Jeng, J.J., Yamada, T., Batina, N., Itaya, K.: Surf. Sci. 335 (1995) 204. Duke, C.B.: Surface Structures of Elemental and Compound Semiconductors, in: Handbook of Surface Galeotti, M., Cortigiani, B., Torrini, M., Bardi, U., Klimov, A., Eltsov, K.: Surf. Sci. 349 (1996) L164. Jensen, J.A., Yan, C., Kummel, A.C.: Phys. Rev. Lett. 76 (1996) 1388. Kasemo, B.: Surf. Sci. 363 (1996) 22. Magnussen, O.M., Ocko, B.M., Wang, J.X., Adzic, R.R.: J. Phys. Chem. 100 (1996) 5500. Neddermeyer, H.: Rep. Prog. Phys. 59 (1996) 701. Nienhaus, H. Monch, W.: Appl. Surf. Sci. 104-105 (1996) 95. Ocko, B.M., Magnussen, O.M., Wang, J.X., Wandlowski, Th.: Phys. Rev. B 53 (1996) 7654. Stietz, F., Elbe, A., Meister, G., Schaefer, J.A., Goldmann, A.: J. Phys.: Condens. Matter. 8 (1996) 7699. Stietz, F., Elbe, A., Meister, G., Schaefer, J.A., Goldmann, A.: Surf. Sci. 365 (1996) 278. Simpson, W.C., Shuh, D.K., Yarmoff, J.A.: J. Vac. Sci. Technol. B 14 (1996) 2909. Simpson, W.C., Tong, W.M., Weare, C.B., Shu, D.K., Yarmoff, J.A.: J. Chem. Phys. 104 (1996) 320. Lando lt -Börnst ein New Ser ies III/42A1

3.4.3 Halogens on metals and semiconductors 96S5 96V1 96V2 96Y 96W1 96W2 97C 97D 97F1 97F2 97G 97I 97J 97N1 97N2 97O 97S1 97S2 97T1 97T2 97W 97Y 98A 98H 98L1 98L2 98L3 98M 98N1 98N2 98V 98Z 99A1 99A2 99C 99F 99L 99M 99N1 99N2 99T

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Simpson, W.C., Shuh, D.K., Hung, W.H., Håkansson, M.C., Kanski, J., Karlsson, U.O., Yarmoff, J.A.: Varekamp, P.R., Håkansson, M.C., Kanski, J., Shuh, D.K., Björkqvist, M., Gothelid, M., Simpson, Varekamp, P.R., Håkansson, M.C., Kanski, J., Björkqvist, M., Gothelid, M., Kowalski, B.J., He, Z.Q., Yamada, T., Ogaki, K., Okubo, S., Itaya, K.: Surf. Sci. 369 (1995) 321. Wandlowski, Th., Wang, J.Z., Magnussen, O.M., Ocko, B.M.: J. Phys. Chem. 100 (1996) 10277. Walter, W.K., Manolopoulos, D.E., Jones, R.G.: Surf. Sci. 348 (1996) 115. Chang, J.P., Arnold, J.C., Zau, G.C.H., Shin, H.S., Sawin, H.H.: J. Vac. Sci. Technol. A 15 (1997) 1853. Dohnálek, Z., Yang, W., Ukraintsev, V.A., Choyke, W.J., Yates jr., J.T.: Surf. Sci. 392 (1997) 17. Fishlock, T.W., Pethica, J.B., Jones, F.H., Egdell, R.G., Foord, J.S.: Surf. Sci. 377-379 (1997) 629. Fouchier, M., McEllistrem, M.T., Boland, J.J.: Surf. Sci. 385 (1997) L905. Göthelid, M., LeLay, G., Wigren, C., Björkvist, M., Karlsonn, U.O.: Surf. Sci. 371 (1997) 264. Itchkawitz, B.S., McEllistrem, M.T., Boland, J.J.: Phys. Rev. Lett. 78 (1997) 98. Jones, R.G., Kadodwala, M.: Surf. Sci. 370 (1997) L219. Nakakura, C.Y., Altman, E.I.: Surf. Sci. 370 (1997) 32. Nakakura, C.Y., Phanse, V.M., Altman, E.I.: Surf. Sci. 370 (1997) L149. Ocko, B.M., Wang, J.X., Wandlowski, T.: Phys. Rev. Lett. 79 (1997) 1511. Schennach, R., Bechtold, E.: Surf. Sci. 380 (1997) 9. Stietz, F., Schaefer, J.A., Goldmann, A.: Surf. Sci. 383 (1997) 123. Temesghen, W.F., Abreu, J.B., Barriga, R.J., Lafferty, E.A., Soriaga, M.P., Sashikata, K., Itaya, K.: Surf. Teshima, T., Ogaki, K., Itaya, K.: J. Phys. Chem. B 101 (1997) 2046. Wu, C.J., Klepeis, J.E.: Phys. Rev. B 55 (1997) 10848. Yang, W., Dohnálek, Z., Choyke, W.J., Yates jr., J.T.: Surf. Sci. 392 (1997) 8. Andryushechkin, B.V., Eltsov, K.N., Yurov, V.Y.: Surf. Sci. 407 (1998) L633. Hohenegger, M., Bechtold, E., Schennach, R.: Surf. Sci. 412-413 (1998) 184. Lu, Z.H., Tyliszczak, T., Hitchcock, A.P.: Phys. Rev. B 58 (1998) 13820. Lyubinetsky, I., Dohnálek, Z, Choyke, W.J., Yates jr., J.T.: Phys. Rev. B 58 (1998) 7950. Liu, Y., Komrowski, A.J., Kummel, A.C.: Phys. Rev. Lett. 81 (1998) 413. Matsuura, T., Sugiyama, T., Murota, J.: Surf. Sci. 402 (1998) 202. Nakakura, C.Y., Altman, E.I.: Surf. Sci. 398 (1998) 281. Nakakura, C.Y., Zheng, G., Altman, E.I.: Surf. Sci. 401 (1998) 173. Vogt, M.R., Lachenwitzer, A., Magnussen, O.M., Behm, R.J.: Surf. Sci. 399 (1998) 49. Zandvliet, H.J.W, Swartzentruber, B.S., Wulfhekel, W., Hattink, B.J., Poelsema, B.: Phys. Rev. B 57 Andryushechkin, B.V., Eltsov, K.N., Shevlyuga, V.M., Tarducci, C., Cortigiani, B. Bardi, U., Atrei, A.: Andryushechkin, B.V., Eltsov, K.N., Shevlyuga, V.M.: Surf. Sci. 433-435 (1999) 109. Chemical Rubber Company: CRC Handbook of Chemistry and Physics, 80th Edition, Cleveland: CRC Fishlock, T.W., Pethica, J.B., Oral, A., Egdell, R.G., Jones, F.H.: Surf. Sci. 426 (1999) 212. Liu, Y., Komrowski, A.J., Kummel, A.C.: J. Chem. Phys. 110 (1999) 4608. McLean, J.G., Kruse, P., Kummel, A.C.: Surf. Sci. 424 (1999) 206. Nakakyama, K., Aldao, C.M., Weaver, J.H.: Phys. Rev. B 59 (1999) 15893. Nakakura, C.Y., Altman, E.I.: Surf. Sci. 424 (1999) 244. Tate, M.R., Gosalvez-Blanco, D., Pullman, D.P., Tsekouras, A.A., Li, Y.L., Yang, J.J., Laughlin, K.B.,

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3.11 Surface diffusion on metals, semiconductors, and insulators E.G. SEEBAUER, M.Y.L. JUNG

3.11.1 Introduction 3.11.1.1 General remarks Diffusion of adsorbates on surfaces plays an important role in a wide variety of physical and technological processes, including crystal growth, sintering, corrosion, heterogeneous catalysis, and microelectronic fabrication. Published experimental data for thermally-activated diffusion of metals compose most of the surface diffusion literature, probably because of significant applications in heterogeneous catalysis as well as the relative robustness of metal surfaces. Experimental data for semiconductors are far less numerous. Most work on semiconductors has been performed on Si, Ge, and GaAs and is comparatively recent (< 15 years old). The reasons for this stem in part from of the miniaturization of electronic devices and the corresponding importance of interfacial effects as the surface-to-volume ratio has increased. Semiconducting surfaces are also intrinsically more fragile than those of metals, effectively prohibiting the use of important surface diffusion techniques such as field ion microscopy and field emission microscopy. Work on insulators is still less prevalent. The work has been published at a fairly even rate over the past three decades and spans a wide variety of substrates, including oxides, salts, molecular solids and others. Here the lack of effective methods for studying wellcharacterized surfaces has been rather pronounced, primarily because of difficulties with charging and changes in surface composition that the complicate use of electron and ion spectroscopies. Pre-adsorbed species or impurities can strongly influence diffusional effects in metal catalysis. Similar effects take place in semiconductor thin film growth, where impurities also function as surfactants by affecting the surface free energy. Impurities can either enhance or impede diffusion. Enhancements are attributed to reductions in surface free energy or in the Schwoebel-Ehrlich (SE) barriers that sometimes exist at steps. Inhibitions are attributed to site-blocking effects or to increases in SE barriers. Direct adsorbate-adsorbate interactions cause either enhancements or reductions, and can act at either short or long range [87Mak1, 88Bra1]. Tunneling diffusion is a low-temperature phenomenon that has been studied for many years. For lowmass adsorbates such as hydrogen, quantum mechanical tunneling dominates diffusion at temperatures below the crossover temperature Ttn, which is generally of order 120-170K. Above Ttn, surface diffusion follows thermally-activated Arrhenius behavior. In practice, adsorbates heavier than hydrogen or its isotopes cannot diffuse by tunneling at measurable rates because the tunneling probability decreases quickly according to exp(-M1/2), where M denotes the adsorbate mass. This dependence predicts a significant isotope effect for the tunneling diffusion of atomic hydrogen. However, a quantitative description is actually quite complicated. In older reports, diffusion is slightly faster for heavy hydrogen isotopes, which suggests an anomalous inverse isotope effect [82Dif1, 85Wan1]. More recent reports suggest that isotope effect is negligible [95Dan1, 95Won1]. The theoretical issue remains unsettled, in part because of difficulties in describing phonon-adsorbate interactions [87Aue1, 93Lee1]. Results have appeared recently that quantify directly the non-thermal effects of photons and lowenergy ions on surface diffusion. Such work has important applications in thin film deposition. Diffusion under liquid ambients has important applications in electrochemistry, corrosion, and separating of miscible chemical solutions. Most experimental measurements have appeared only in the past decade or so. Diffusion effects are complicated by interactions among adatoms, the surface, and electrolytes in H2O. The electrolytes may fill available sites for adatom motion or weaken adatom-surface bonding. Most computational work to model surface diffusion has appeared only in the past decade, driven mainly by the increased availability of inexpensive computational power together with the corresponding

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[Ref. p. 517

appearance of general-purpose commercial software codes for ab initio and molecular dynamics calculations. Computer simulations offer the opportunity to examine diffusion processes on an atomic scale under conditions where experiments are prohibitively difficult, time-consuming, or expensive. The literature has now matured to the point where some systematic listing of results seems in order. 3.11.1.2 Intrinsic and mass transfer diffusion Under most conditions where tunneling is negligible, the diffusivity D obeys Arrhenius behavior with respect to temperature T: D = Doe

− Qdiff kT

,

(1)

where Do represents the pre-exponential factor, Qdiff the activation energy for diffusion, and k the Boltzmann constant. For comparison of activation energies between systems, the diffusion energy is often scaled to the desorption energy Qdes through the definition of a corrugation ratio Ω = Qdiff / Qdes [90Gom1]. There exist several ways to distinguish surface diffusion coefficients. In this article, data are subdivided mainly according to whether the diffusion takes place by mass transfer or intrinsic diffusion. The mass transfer and intrinsic diffusion respectively describe situations in which the number of mobile particles varies or remains constant as temperature is varied [73Bon1]. Sometimes in the literature "intrinsic" refers to mass transport over short (atomic scale) distances, while "mass transfer" suggests motion over micrometers or more. However, these connotations seem to reflect historical vestiges of the days when intrinsic diffusivities DI were measured experimentally by various probe tip methods (short length scale), whereas mass transfer diffusivities DM were measured by profile decay methods (long length scale). Real surfaces are almost never perfectly flat but instead support various defect structures such as steps, kinks, and vacancies. Such structures appear schematically in Fig. 1. Surprisingly, a growing body of evidence suggests that intrinsic diffusion parameters are often relatively insensitive to small concentrations of steps, kinks or other surface defects. For example, in principle diffusion over long distances can be limited by motion over steps. Such feature sometimes present Schwoebel-Ehrlich barriers to "downstairs" motion [66Ehr1, 66Sch2] and often bind adsorbates more strongly than do terraces [73Bon1]. Indeed, for Ag on Ge(111), as shown in Fig. 2, minor variations in crystallographic orientations cause major changes in Do and Qdiff [83Sul1]. There exist relatively few systems for which direct comparisons can be made between short- and long-distance measurements. The few systems that do exist appear in Fig. 3, which compares Qdiff measured by both short-range (2-20 nm) and long-range (2-20 µm) methods for gases on solids. The numbers are identical within the experimental error. Unfortunately, almost no comparable data exist for Do. Furthermore, almost all the data are for metals; only one semiconductor (Si) and no insulators are represented. Nevertheless, these data point to the significant utility of macroscopic measurements for probing microscopic processes. DM and DI are formally related by [73Bon1]: DM =

N DI , Ns

(2)

where N denotes the actual number of mobile particles, while Ns represents a maximal number. For selfdiffusion (A moving on A), Ns corresponds to the areal density of particles in the substrate, whereas for heterodiffusion (A moving on B), Ns represents the areal density of adsorbed heteroatoms. In selfdiffusion, for which N/Ns typically falls far below unity, this ratio may be strongly temperaturedependent. The temperature dependence may arise from adatom-vacancy pair formation, for example. In heterodiffusion, it is common for all adsorbed particles to be mobile so that N = Ns, implying that DM = DI. However, some cases (especially at low coverages) adsorbates may become trapped at defects or may incorporate substitutionally into terrace sites. In such cases, N / Ns may again become small and temperature-dependent. Good examples include Group III and V elements on Group IV semiconductors. For example, scanning tunneling microscopy demonstrates that boron and phosphorous can substitute for top-layer silicon atoms [94Wan1].

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The distinction between intrinsic and mass transfer diffusion is often ignored in the published literature. However, total mass transport across the surface is governed by DM, with critical implications for applications involving particle sintering, scratch filling, and other practical situations. Since N/Ns can be a strong function of temperature in such practical applications, the distinction between DM and DI should not be ignored. For both intrinsic and mass transfer diffusion, it is useful to analyze the surface diffusion coefficient terms of enthalpies and entropies [91Lom1]: D=

ν λ 2 ∆S k − ∆H e e 2n d

kT

.

(3)

Here, ν denotes the attempt frequency for hopping, λ the average hop length, nd the dimensionality (1, 2 or 3). The thermodynamic quantities ∆S and ∆H respectively represent the entropy and enthalpy changes associated with hopping and possibly the formation of the adsorbed particle. Comparison of Eq. 1 with Eq. 3 shows that Do = (1/2nd)νλ2exp(∆S/k) and that Qdiff = ∆Hm. In the case of intrinsic diffusion, ∆S and ∆H are associated with site-to-site hopping only. A firstprinciples estimate for the prefactor Do can be made by assuming ∆S = 0. Then with the attempt frequency of ν ~1013 s−1 (corresponding to a vibrational frequency) and the hop length λ ~ 0.3 nm (corresponding to an atomic diameter), Do falls near 10−7 m2/s. This value agrees at least loosely with most experiments. The activation energy Qdiff, however, is not so easily estimated a priori. At equilibrium, N/Ns is determined by the affinity of adatoms for the kinks, steps and terraces into which adatoms can be incorporated. An expression for the equilibrium concentration of adatoms may be written in terms of the enthalpies ∆Hf and entropies ∆Sf of adatom formation [51Zen1]: N − ∆H f , kink =e NS

kT + ∆S f ,kink k

+e

− ∆H f , step kT + ∆S f , step k

+e

− ∆H f ,terrace kT + ∆S f ,terrace k

.

(4)

Typically only one of the three terms in this sum dominates. Combination of Eqs. 2 and 4 yields simple relations between the activation energies and pre-exponential factors for intrinsic and mass transfer diffusion:

Qdiff , M = Qdiff , I + ∆H f D oM = DIo e

∆S f / k

(5) (6)

Let us now consider the sign and the magnitude of ∆Hf and ∆Sf. Since ∆Hf is a positive quantity for adatom formation from all surface structures (i.e., kinks, steps, terraces), it is clear that Qdiff,M must be greater than Qdiff,I. For terraces, the energy for vacancy formation can be quite large [95See1], leading to a value for Qdiff,M that is a great deal larger than Qdiff,I. The difference between Qdiff,M and Qdiff,I should be smaller for steps and kinks because fewer bonds need to be broken to form an adatom-vacancy pair. Like ∆Hf, ∆Sf is a positive quantity because of lattice mode softening near the vacancy [51Zen1]. In the bulk, ∆Sf is often fairly large [82Lan1, 86Lan1]. Similar effects seem to occur for terrace vacancies on surfaces [95See1], although the effects are probably smaller at steps and kinks. At low temperatures, experimental evidence for semiconductors indicates that the primary source of adatoms is from kink sites. New high-temperature STM methods permit the real-time measurement of step fluctuations due to rapid diffusion of kinks along the steps. The activation energy for this fluctuation (controlled by mass transfer diffusion) can be readily obtained. Two groups [94Bar1, 95Swa1] have reported nearly identical activation energies near 130 kJ/mol for Si(100). This value falls between those for intrinsic and high-temperature mass transfer diffusion (85 and 220 kJ/mol respectively) [87Ich1, 93Voi1, 94Kee1]. At high temperatures, terraces appear to produce most adatoms. Activation energies for mass transfer diffusion are therefore higher than at low temperatures. Evidence for this effect comes from molecular dynamics simulations for self-diffusion on Lennard-Jones solids [94Sun1] and semi-conductors [96Sun1, 96All1]. The simulations show that the number of adatom-vacancy pairs produced from terraces increases

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dramatically with temperature according to the formation energy ∆Hf. Experimental evidence comes from surface roughening measurements. The transition temperature between an atomically flat surface and one composed of many adatoms and vacancies is closely related to the roughening temperature [51Bur1, 94Con1]. Experimental roughening temperatures fall between 60 and 75% of the substrate melting temperature Tmelt [94Con1, 94Lap1], coinciding closely with the transition temperature measured between the two mass transfer diffusion regimes that appear in the tables of data below. Curiously, there exists no evidence that adatom creation from steps ever controls mass transfer diffusion. Only two temperature regimes can be discerned in the tables of data, suggesting that kinks or terraces account for nearly all mobile adatoms. The apparent inactivity of steps may originate from one of two causes. First, the energy for adatom creation from steps might be so close to that for kinks that the transition between the two is indistinguishable experimentally. Second, the combination of ∆Hf and ∆Sf for adatom formation from steps may conspire in such a way that the contribution from steps is always overshadowed by that from kinks or terraces. At temperatures approaching the melting point of the substrate, one might expect the surface to become so disordered that the adatom concentration N approaches Ns. DM should then approach DI. That this situation occurs is demonstrated in Fig. 4, which shows DM [94Kee1] and DI [91Mo1] for selfdiffusion on Si(100). The extrapolated data indeed meet near Tmelt.

3.11.2 Methodology 3.11.2.1 Experimental techniques There has arisen over the years a large collection of over fifty methods for measuring surface diffusion, which are described briefly below. Unfortunately, most can be applied only to a very restricted set of adsorption systems that fortuitously have favorable properties. Most methods can be grouped at least loosely into eight categories: single-atom imaging, field emission, electron diffraction, chemical state relaxation, spin axis relaxation, profile evolution, capillarity-driven effects, and island growth. 3.11.2.1.1 Single-atom imaging Some techniques track the motion of individual adsorbed atoms, in which case intrinsic diffusion can be measured. Scanning tunneling microscopy (STM) is most commonly used for this purpose. In STM [91Mo1, 95Swa1], a very sharp probe tip is placed within about 0.5 nm of the surface and is electrically biased to yield a tunneling current between the tip and surface. Although many forms of STM exist, a common method requires that the tunneling current be held constant while the tip is slowly scanned across the surface. Because of the spatial corrugation in electronic potential, the scanning tip must be moved perpendicularly to the surface using a piezoelectric actuator in order to maintain constant current. A surface image can then be constructed by tracking the voltage on the actuator versus the lateral position of the tip. STM is limited to relatively slow diffusion at low adsorbate coverages because following single atoms becomes quite difficult when the mobility is large and when many atoms are present. Field ion microscopy (FIM) also involves a very sharp probe tip, but the tip itself forms the surface over which diffusion occurs [94Kel1]. Huge electric fields of a few volts/nm are produced near the tip by surrounding it with a biased imaging screen. He or Ne gas is introduced and becomes ionized near protruding atoms on the tip. The ions are accelerated to the surrounding screen and form an image that has atomic-scale resolution. Only refractory metals that form sharp tips may be studied. Scanning transmission electron microscopy (STEM) is a standard microscopy technique that images single atoms with a tightly focused beam [80Utl1]. The same restrictions apply as for STM, with the additional constraint that only fairly heavy atoms (e.g., uranium) can be successfully imaged.

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3.11.2.1.2 Field emission Field electron microscopy (FEM) has been employed for numerous surface diffusion studies [90Gom1]. The technique resembles FIM in that a sharp tip is biased with respect to a surrounding screen, resulting in high electric fields at the tip surface. Electrons are emitted from the tip form an image. Adsorption changes the local work function, affecting image intensity. FEM has a poorer spatial resolution (2.5 to 3 nm) than FIM due to the large transverse velocity spread of the substrate electrons. In the older shadowing method, adsorbate is evaporated onto one side of the tip and permitted to diffuse. However, many crystal planes are necessarily sampled. To avoid these complications, a newer fluctuation method monitors random changes in the emission current from a small, single-plane portion of the tip. The relaxation time for fluctuation relates quantitatively to D. As with FIM, only substrates capable of forming very sharp tips may be used. Ion thermoemission (IT) resembles the FEM fluctuation method except that fluctuations in the current due to thermally desorbing ions are monitored [88Gla1]. Only species that naturally desorb as ions may be studied. 3.11.2.1.3 Electron diffraction Low-energy electron diffraction (LEED) examines long-range surface periodicity by the diffraction of low-energy electrons (typically 50-100 eV) from the surface onto an imaging screen [74Bed1]. Phase transitions that affect the periodicity are followed easily in real time with LEED. To the extent that such transitions are mediated by surface diffusion, the diffusion coefficient can be estimated. However, surface reconstruction is a very complicated process whose dynamics are poorly understood. It is unclear how to relate the energetics of atomic motion through a fixed energy corrugation (classical surface diffusion) to reconstruction effects that induce dramatic changes in corrugation. Reconstructions can involve substantial collective motions and be influenced by subsurface lattice strains, thereby introducing difficult complications. Reflection high-energy electron diffraction (RHEED) examines surface periodicity using much higher electron energies (10-20 keV). For surface diffusion studies, RHEED monitors primarily the atomic smoothness of the surface during deposition from the gas phase [90Gib1]. Generally the substrate is misoriented a few degrees from a low-index crystallographic plane in order to create a regular array of atomic steps. If diffusion during deposition is fast, atoms adsorbing on terraces incorporate at the steps, inducing "step-flow" growth. The surface remains smooth, and RHEED diffraction spots remain unchanged. If diffusion is too slow for easy travel between the steps during a characteristic time scale set by incident flux, two-dimensional island nucleation on the terraces occurs. As the surface roughens and smoothens repetitively (that is, as new atomic layers are completed), the RHEED pattern changes in oscillatory fashion. With extensive mathematical modeling of the growth, the onset of RHEED oscillations can be used to extract the diffusivity. This method has been used mostly for diffusion of Ga on GaAs, but the results are quite scattered, probably due to the complexity of the physical situation and the consequent uncertainties in modeling. Reflection electron microscopy (REM) is used to observe denuded zones created at the terrace edges. By analyzing the denuded zones as a function of temperature and time, the diffusivity may be calculated. This technique is limited to studying surfaces where islands may be easily distinguished, as on the Si(100) surface where the 2x1 and 1x2 reconstructions that alternate in successive layers are clearly visible [95Doi1]. The resolution of the technique is on the order of 0.5 µm.

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3.11.2.1.4 Chemical state relaxation In chemical state relaxation, a spectroscopy sensitive to chemical binding state is used to follow the evolution of an initially random adsorbate distribution into a preferred chemical state. Such techniques have quite limited application for surface diffusion because (a) two easily distinguished chemical states must exist, and (b) it must be established that the transformation is diffusively mediated. Electron spin resonance (ESR) has been used to follow diffusion of NO2 and Al2O3 [95Sch1]. ESR detects unpaired electron spins, such those of lone NO2 molecules. In a magnetic field, these spins absorb tuned microwave radiation due to resonant spin flipping. However, on Al2O3 the NO2 tends to form dimers with paired spins. Thus, diffusion of initially random NO2 molecules into the dimer state can be monitored by a decrease in ESR signal. Fourier-transform infrared spectroscopy (FTIR) monitors changes in the IR vibrational absorption spectrum as a molecule diffuses from a terrace site to a step or other surface defect [92Hei1]. The temporal change in the vibrational mode of interest can be used to deduce the diffusivity. Perturbed γγ angular correlation (PAC) is a technique that tracks the temporal coincidence of the two γ quanta emitted during radioactive decay of 111In to 111Cd [92Kra1]. This coincidence is affected by the interaction of the electric quadrupole moment of an excited-state 111Cd nucleus with an extranuclear electric field gradient. The gradient varies with bonding site for In on Si (111). Thus, randomlydistributed adsorbate that is subject to a variety of gradient fields can diffuse into a single well-defined state and be followed by PAC during the process. Surface second harmonic generation (SHG) is an optical reflectance technique that employs pulsed lasers [88Ric1]. Because of the lack of inversion symmetry and the large electric field gradients present at a surface, a nontrivial second-order optical susceptibility may exist there, leading to frequency doubling that can be detected in reflection. SHG is especially useful for substrates with high crystal symmetries, so that second harmonic contributions from the bulk can be completely suppressed. The second harmonic yield can be dramatically affected by adsorption, although quantitative coverage calibration requires a measurement by some independent technique. In the case of hydrogen adsorbed on silicon, SHG is sensitive to H bonded to terraces but not certain types of defects [96Han1]. Therefore, diffusion between the defects and the terraces may be monitored, generally during an adsorption experiment in which adsorption is defect-mediated. Helium scattering (HeS) examines the specular reflection of low-energy helium atoms from the surface [82Poe1]. In the case of CO on Pt, the reflected intensity decreases dramatically as the coverage on terraces increases, although CO at defects has no effect. Therefore, the intensity changes as CO migrates at from the terraces to the thermodynamically preferred defects. A related technique, low-energy ion scattering (LEIS), uses Ne+ ions. On Cu(100) [92Bre1] the signal is proportional to the population of Cu adatoms. By monitoring the rate of adatom incorporation at steps, the diffusivity may be determined. Fluorescence emission (FE) has been employed to examine the diffusion of organosiloxane ligands on silica under aqueous solution [94Wan2]. In this case, the ligands diffuse together to form an excimer whose optical emission can be monitored to follow the rate of association. 3.11.2.1.5 Spin axis relaxation Atomic-spin sensitive techniques exist that monitor surface diffusion through changes in spin axis that occur during diffusion. Nuclear magnetic resonance (NMR) examines the spin states of atoms that have a magnetic moment. By pulsing techniques, an assembly of such nuclei can be prepared with parallel spins, and the relaxation of these spins into random directions can be measured in time [72Bod1]. Several processes contribute to the relaxation, but on a small spherical particle (in a powder), surface diffusion contributes by reorienting the diffusing species in space as motion occurs. However, the mathematical modeling is very complicated, and the surfaces of powders present many crystallographic orientations over which diffusion must occur. Avoided-level crossing muon spin resonance (ALC-µSR), a derivative of the NMR technique, can be applied to systems in which adsorbed free radicals are formed by loss of H [94Sch1]. When bombarded with highly-polarized positive muons (µ+), these radicals can combine with muonium

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(Mu = µ+e-, like a light H isotope) that forms in the system. The radioactive decay of µ+ results in a highly anisotropic particle signature. Thus, as the spatial orientation of incorporated Mu changes by diffusion of the host molecule over a powder particle, spatially resolved detection of the decay products measures the extent of depolarization of the initially-injected µ+. Although the sensitivity of the technique is superb (one radical in 1020 molecules), the nature of the chemistry required, the requirement of a µ+ source, and the extensive mathematical modeling needed to obtain D restrict the application greatly. Nuclear spin relaxation (NSR) does not require small particles because in certain cases nuclear spin depolarization occurs by coupling of the nuclear electric quadrupole moment of the adsorbate to fluctuations in the substrate electric field gradient as the atom moves [95Chr1]. The depolarization of an initially prepared set of nuclear spins is monitored by thermal desorption into a special detector. Mathematical modeling is complicated, and only a small set of substrates and adsorbate nuclei can avoid competing depolarization processes. Spatial resolution depends on the length of diffusion before desorption, but lies near 10 nm. 3.11.2.1.6 Profile evolution Numerous techniques exist in which adsorbate is deposited on some portion of a surface to yield a sharp initial concentration profile, which is then allowed to broaden diffusively. The deposition usually involves either a tightly focused molecular beam or a diffuse source together with a retractable mask. However, sometimes spatially resolved pulsed laser desorption from a uniform adsorbed layer can yield the desired profile [91Rei1]. Unfortunately, surface damage is a problem in many cases [86See1, 86See2]. Profile evolution techniques measure only diffusion driven by chemical gradients (as opposed to tracer diffusion) for heterodiffusion, but in suitable profile geometries can do so very directly with a minimum of complicated modeling. The utility of a given method is limited by (a) the variety of adsorbates it can monitor without major surface perturbation, (b) the spatial resolution it can attain (including initial profile formation), and (c) the suitability of the initial profile geometry for quantitative analysis. Scanning electron microscopy (SEM) can be used when the adsorbate and substrate have good contrast for imaging by electron reflection. This generally holds only for metallic adsorbates on semiconductors and insulators, and even then only in special cases. For example, multilayers of adsorbate may be required, thereby complicating the diffusion process because multiple surface corrugations are present. Concentration calibration is difficult. Better results are obtained when the adsorbate strongly modifies the surface contact potential [94Mil1]. The maximum resolution for SEM is about 3 nm, although in practice results as poor at 15 µm have been reported. Electron beam damage is a possible problem. Scanning Auger microscopy (SAM) has better, more general sensitivity for a wide variety of adsorbates, but has somewhat poorer spatial resolution (~50 nm). This technique has been employed very effectively with a one-dimensional initial step suitable for analysis in the Boltzmann-Matano framework [83Sul1]. This approach is particularly suited for obtaining the concentration dependence of D. Again, however, electron beam damage can be an issue. Techniques based on spatially-resolved surface second harmonic generation (SHG) can sometimes better avoid the damage problem. Two methods such exist for measuring surface diffusion. One involves measuring second harmonic diffraction (SHD) from adsorbate grating initially set up in a grating pattern by laser desorption at the antinodes of a two-laser interference pattern [91Rei1]. As the profile decays by diffusion, the SH signal in first-order diffraction decreases. The technique has diffraction-limited resolution (~0.4 µm), but measures the concentration dependence of D with difficulty. The method is also restricted to systems in which the adsorbate is bonded weakly enough to be desorbed without surface damage for grating formation. Second harmonic microscopy (SHM) is an alternate technique that images the profile in real space rather than reciprocal space [92Sch1]. Here, a one-dimensional profile (in the Boltzmann-Matano geometry) is set up by masked deposition, and the entire profile is illuminated and imaged at the second harmonic wavelength into a photodiode array. The concentration dependence of D is obtained easily, and the damage problems associated with laser desorption are avoided. However, the spatial resolution tends to be a bit poorer (~1-3 µm).

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Linear laser diffraction (LLD) works very similarly to SHD but examines the linear diffraction of the probe beam instead of the second harmonic [92Zhu1]. Signals are considerably stronger than in SHD, but suffer from diffuse background scattering. Surface ion microscopy (SIM) can be used for species that desorb naturally as ions [92Sto1]. Its use has therefore been restricted so far to strongly electropositive elements such as potassium, rubidium, and cesium. The desorbing ions may be manipulated using appropriate ion optics to produce an enlarged image with a spatial resolution of about 10 µm. Contact potential difference (CPD) measurements may be performed in scanning mode using a special vibrating wire capacitor method [77But1]. This method avoids all surface damage but has a spatial resolution of only about 50 µm. The surface work function also affects secondary electron emission (SEE) in response to a focused electron beam [80Bow1]. A spatial resolution of 100 µm has been reported. Care must be taken to avoid electron stimulated desorption. Photoemission electron microscopy (PEEM) images the entire surface at once using electrons emitted in response to ultraviolet illumination [94Oer1]. The photoemission yield varies according to the local work function, and therefore adsorbate coverage, in a way that must be calibrated independently for a given adsorbate. Spatial resolution improves to near 0.3 µm. Laser-induced thermal desorption (LITD) has proven very useful for studies of gas diffusion on metals. A pulsed laser heats the surface to desorb adsorbate in a small, well-defined surface patch. The amount desorbed can be measured with a quadrupole mass spectrometer. The beam is then blocked, permitting diffusional refilling of the spot that is in turn monitored with subsequent laser pulses. Spatial resolution is limited by heat conduction at the spot edge to about 4 mm [86See2], although beam imperfections generally degrade this number to 10-20 µm. Surface damage can be a problem, especially for strongly bonded adsorbates on fragile surfaces. The method also averages the profile over the entire spot, although a variable aperture technique can potentially circumvent this effect [88See1]. Autoradiography (AR) monitors radioactive adsorbates by placing a photographic plate above a diffused one-dimensional profile [71Tan1]. However, the resulting radioactively-exposed photograph has a spatial resolution of only about 0.1 µm. In serial sectioning (SS), the substrate is thinly sliced normal to the diffusion direction, and the total radioactivity in each slice is then measured [85Stu1]. Spatial resolution is even poorer in this case. Scanning low-energy electron diffraction (SLEED) involves performing standard LEED in scanning mode. Spatial resolution of 0.1 µm has been achieved [70Pin1]. Scanning high resolution electron energy loss spectroscopy (SHREELS) involves measuring the energy loss of a beam of electrons due to energy transfer into discrete surface vibrational modes [94Fro1]. The data obtained form an adsorbate vibrational spectrum, which may be used to scan a diffusion profile with a spatial resolution of about 0.3 µm. Rutherford backscattering (RBS) monitors the backscattering of high-energy helium nuclei off the surface [93Coh1]. Different adsorbates yield different backscattering signatures, and uniform coverages may be calibrated against a standard. Scanning the probe beam gives a spatial resolution near 0.5 µm. Auger electron spectroscopy (AES) in standard nonscanning mode has been used for diffusion measurements by employing the incident beam for two purposes [75Jan1]. First, at short beam exposures, the adsorbate concentration is measured. At longer exposures, either a) beam-induced desorption cleans a small spot on the surface, or b) beam induced adsorption deposits a small spot on the surface (by cracking CO to adsorb carbon, for example), defining a circular concentration profile. By intermittent short measurements following initial cleaning, the rate of diffusive spot refilling or depleting can be followed. In an actual practice, beam damage and poor definition of spot edges have restricted this method to two reports in each regime. Resolutions fell near 0.5 µm. Secondary ion emission (SIE) involves bombarding the profile with high energy (4 keV) ions and monitoring the sputtered adsorbate atoms with a mass spectrometer [71Abr1]. A broad-area beam is employed for bombardment; a single slit placed at some distance from the initial step admits sputtered ions from only one location on the surface. The data fit to the time dependence expected from the diffusion equation with constant D. Spatial resolution of only 1 µm has been achieved.

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3.11.2.1.7 Capillarity-driven effects Capillarity effects arise in physical situations where significant geometrical curvatures are present. Because of the existence of surface energy, the local chemical potential becomes a function of the local radius of curvature K according to the Gibbs-Thompson equation [93Bon1]:

µ (Κ ) = µ o + γΛΚ ,

(7)

where Λ denotes the atomic volume and γ the (orientation-dependent) specific surface energy. Spatial gradients in K and/or γ therefore give rise to gradients in µ(Κ), which in turn cause diffusion. These effects can form the basis for diffusion measurements when small scratches, grooves, voids, etc., are formed artificially on a surface and are then permitted to relax by diffusion. Scratch decay (SD) [94Kee1] and surface smoothing (SSm) [81Gav1] make use of this idea using the optical scattering pattern from a single scratch or a random array of scratches, respectively. Grain boundary grooving (GBG) [81Rob1] examines the widening of grain boundaries on the initially flat surface of a polycrystalline material. Tip blunting (TB) [85Bin1] follows the thermal rounding of a field emitter tip. Spheroidization of tubular voids (STV) [72Yen1] examines (with SEM) how high-aspect-ratio voids, induced in a solid by thermal or mechanical shock, break up and round into spherical shapes. Sintering [87Saj1] examines the necks that form between individual grains of powders annealed at high temperatures. All of these methods work best for self-diffusion, and because of the relatively large amounts of mass transfer required, generally find use only at high temperatures. Thus, they are used extensively for studies of mass-transfer diffusion. The mathematical modeling is fairly extensive, and the results are generally averaged over a large range of crystallographic orientations. Modeling difficulties are most severe when the surface energy varies significantly (and often in unknown fashion) with orientation. However, the experiments are generally easy to set up. Spatial resolution varies with the detection technique, but ranges from about 10 nm (for SEM) to about 1 µm (for optical methods). An in-plane variant of the typical capillarity measurement involves examining the time-dependent shape fluctuations of monatomic steps. In this case, γ becomes a step energy, and K the local radius of curvature of a step. Fluctuations are diffusionally mediated, and in favorable cases can be monitored with electron microscopy. Low-energy electron microscopy (LEEM) involves forming a real image from a diffraction spot in a LEED experiment, and has been used on Si(100) where alternating terraces between steps have LEED patterns rotated 90° from each other, resulting in strong contrast in the resulting LEEM images [94Bar1]. Spatial resolution is about 15 nm. 3.11.2.1.8 Island growth Several methods rely on the diffusional formation of particle islands of adsorbate having poor wettability on the underlying substrate. Such systems often obey transformation kinetics governed by Ostwald ripening, in which large particles having large radii of curvature grow by virtue of Eq. 7 at the expense of small particles with small radii. The means of mass exchange between particles is surface diffusion. Rutherford backscattering/electron microscopy (RBS/EM) uses two methods to monitor particle growth [87Zin1]. RBS involves monitoring the scattering of high-energy helium nuclei off the surface. The backscattering can be used to obtain the average particle height, but electron microscopy is required to determine particle shape (from which the RBS data can yield volume) and to ensure that the shape is constant during growth. With this method, fairly extensive mathematical modeling is required to obtain D, and the pre-exponential factor cannot be obtained. It is also generally unclear which adsorbate concentrations characterize the diffusional process. Surface acoustic wave (SAW) detection involves monitoring the attenuation and changes in velocity of surface sound waves (Rayleigh) in response to the evolution of crystalline size [95Cla1]. Photoemission (PE) monitors the change in average surface work function as islands form from a uniform layer [90Sen1]. This change appears indirectly in the yield of electrons emitted in response to ultraviolet radiation.

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Island boundary motion (IBM) examines the disappearance of islands in the vicinity of a region having no islands. Masked deposition followed by weak annealing delineates a clean region from an islandcovered region. Islands near the boundary slowly lose mass to the clean region by diffusion, while the other islands remain in diffusional equilibrium with each other [95Bek1]. 3.11.2.1.9 Miscellaneous Quasi-elastic neutron scattering (QENS) examines the energy broadening in a beam of cold monoenergetic neutrons induced by surface atomic motion [86Cou1]. The mathematical modeling is generally complex, and the adsorbate coverage sometimes exceeds one monolayer, making the physical picture rather unclear due to presence of two interfaces. Spatial resolution falls in the nm range, but the technique is restricted to hydrogen-containing adsorbates, since only H possesses a large enough scattering cross section to induce measurable scattering from adsorbates. A related technique, quasielastic helium atom scattering (QHAS) uses He atoms [90Fre1, 91Ton1]. The stronger interaction of He atoms with matter makes QHAS more surface sensitive than QENS (allowing for measurements at very low adsorbate coverages), and also permits diffusion measurements for adsorbates that do not contain H. Both QENS and QHAS possess the rare capability for monitoring intrinsic diffusion on an atomic scale at high temperatures. However, as pointed out by H. P. Bonzel in the introduction to this volume, QHAS seems to give appreciably higher diffusivities than other methods where direct comparisons can be made (e.g., see Fig. 15 of the Introduction). Molecular beam (MB) techniques have been used to obtain activation energies for surface diffusion by examining adsorption [93Fel1] or desorption [75Fox1] kinetics. In either case, the beam is modulated. The adsorption or desorption must take place only at sites that are fairly widely spaced, with surface diffusion occurring to deliver adsorbate to or from the sites. Mathematical modeling then yields an expression for either a sticking probability or a desorption probability that contains the difference (Qdes – Qdiff) as a net activation energy. When Qdes can be obtained independently, as is usually possible, Qdiff can then be computed. Unfortunately, the physical situation is generally not defined well enough to determine Do; in fact, a value must generally be assumed to obtain Qdiff. Helium bubble formation (HBF) uses electron microscopy to measure the distribution of bubble sizes that form when a solid implanted with He ions is annealed [73Ait1]. For He in Nb, bubble formation occurs by migration and coalescence, not by He diffusion. Thus, the size distribution can be related to the surface diffusion coefficient, which governs bubble migration. Spatial resolution is about 10 nm. Electrocrystallization (EC) is an electrochemical technique for use under liquids [93Pro1]. It measures the variations in current associated with layer-by-layer growth of a dissolved ionic metal being reduced and deposited at an electrode surface. This method requires isolated surface sites that can reduce aqueous ions, and sites to which the reduced atoms can diffuse to nucleate the formation of a new atomic layer. When deposition occurs under critical potentials, layer growth is nearly instantaneous, yielding a current spike that marks the arrival of an atom at the nucleation site. This method works only for thoroughly studied surfaces that can be proven to grow by this mechanism. Spatial resolution has been reported at 5 µm. Fludized bed chromatography (FBC) indirectly measures the intraparticle diffusion under liquid. Breakthrough curves from the chromatograph from fixed beds of either ion-exchange resin or solids are fit to mathematical models in order to deduce surface diffusion parameters from chromatographic peaks from pulse responses [98Koh1]. 3.11.2.2 Classification of experimental techniques The experimental techniques described above can sometimes be used to determine either intrinsic or mass transfer diffusivities depending on whether the measurements are sensitive to the temperature-dependent concentration N in Eq. 2. For example, under some conditions STM can resolve individual atom motions and therefore measure intrinsic diffusion. However, STM is sometimes used to follow the shape evolution of larger surface features like steps or trenches. Since the rates of these processes depend upon N, such methodology measures mass transfer diffusion. As another example, LITD determines intrinsic or mass transfer diffusivities for gases adsorbed on metals depending on the surface defect concentration. For Lando lt -Börnst ein New Ser ies III/42A1

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465

surfaces with few defects for adsorbate bonding, mobile adatom concentrations remain nearly independent of temperature, making LITD sensitive to intrinsic diffusion. However, on highly defected surfaces, adsorbed atoms can bond strongly to the defects (especially at low temperatures and coverages) and desorb onto the terraces at higher temperatures. A probable example of such behavior is H atoms adsorbed on Pt(111) [86See1]. In such cases, LITD becomes sensitive to mass transfer diffusion. The following tables group experimental methods according to what kind of diffusion they typically measure. Some methods can be used to measure both kinds of diffusion, but have greater accuracy or ease of use for only one kind. Historical usage frequency gives some insight into such considerations. Thus, we have grouped the methods represented in the data tables according to their relative frequency of appearance in the intrinsic or mass transfer categories. If an experimental technique appears at least 80% of time in the intrinsic tables (4 through 18 and 24), the method appears in Table 1 below for intrinsic diffusion. Similarly, methods that appear at least 80% of the time in the mass transfer data tables (19 through 21 and 25) appear in Table 2 for mass transfer diffusion. Techniques that do not meet these criteria, and therefore have found significant use for measuring both kinds of diffusion, appear in Table 3.

Table 1. Experimental techniques for intrinsic diffusion Abbreviation AFM ALC-µSR AR EC ESR FE FEM FIM FTIR HeS IT LEIS LITD LLD MB NMR NSR PAC PEEM QENS QHAS RBS/EM REM SAM SEE SEM SHD SHG SIE STM TEAS TIM TSRLS

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Full Name Atomic force microscopy Avoided-level crossing muon spin resonance Autoradiography Electrocrystallization Electron spin resonance Fluorescence emission Field emission microscopy Field ion microscopy Fourier transform infrared spectroscopy Helium scattering Ion thermoemission Low energy ion scattering Laser-induced thermal desorption Linear laser diffraction Molecular beam Nuclear magnetic resonance Nuclear spin relaxation Perturbed γγ angular correlation Photoemission electron microscopy Quasi-elastic neutron scattering Quasi-elastic helium atom scattering Rutherford backscattering/electron microscopy Reflection electron microscopy Scanning Auger microscopy Secondary electron emission Scanning electron microscopy Second harmonic diffraction Second harmonic generation Secondary ion emission Scanning tunneling microscopy Thermal energy atom scattering Thermally desorbed ion microscopy Time- and spectrally-resolved luminescence spectroscopy

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3.11 Surface diffusion on metals, semiconductors, and insulators

[Ref. p. 517

Table 2. Experimental techniques for mass transfer diffusion Abbreviation FBC GBG HBF IBM LEEM MEIS PE RHEED SAW SD SHM SHREELS SS SSm STEM STV TB TEM

Full Name Fludized bed chromatography Grain boundary grooving Helium bubble formation Island boundary motion Low energy electron microscopy Medium energy ion backscattering spectroscopy Photoemission Reflection high-energy electron diffraction Surface acoustic waves Scratch decay Second harmonic microscopy Scanning high resolution electron energy loss spectroscopy Serial sectioning (radioactive) Surface smoothing Scanning transmission electron microscopy Spheroidization of tubular voids Tip blunting Transmission electron microscopy

Table 3. Experimental techniques for both intrinsic and mass transfer diffusion Abbreviation AES CPD LEED SIM

Full Name Auger spectroscopy Contact potential difference Low energy electron diffraction Scanning ion microscopy

3.11.2.3 Computational techniques Computer simulations have increasingly begun to provide significantly new insights into surface diffusion. Several tools have emerged whose utility depends upon the complexity of the system and the amount of computational time and effort one has to expend. The most common methods include molecular dynamics, Monte Carlo techniques, and ab initio calculations.

3.11.2.3.1 Molecular dynamics Molecular dynamics (MD) integrates the classical Newtonian differential equations of motion for surface and substrate atoms. These equations for an adatom trajectory Y(t) can be written in the form: m ⋅ Y = − ∂V (Y ) ∂Y ,

(8)

where m denotes the adatom mass and V(Y) the potential energy. Solution for Y(t) yields the surface diffusion coefficient through a velocity autocorrelation function or through an ensemble average of the mean-square displacement of adatoms: D = lim

t →∞

[Y (t ) − Y (t = 0)]2

4t .

(9)

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Here the bracketed term represents the ensemble average of the mean-square displacement [91Lom1]. Clearly the accuracy of the adatom potential energy surface (PES) (Fig. 5) governs the quality of MD simulations. Both classical (empirical) and semi-empirical PES's can be employed in MD simulations. 3.11.2.3.2 Monte Carlo method The Monte Carlo (MC) method gives a qualitative description of collective diffusive motion in terms of probabilities. MC calculations are simple, yet powerful enough to describe non-equilibrium adatomsurface interactions. An MC simulation begins by placing adatoms from a canonical ensemble at random on a specified PES. An adatom is chosen at random, and its nearest-neighbor sites are examined for occupancy. If a neighbor site is filled, hopping is forbidden. If a neighbor site is empty, a hopping probability P is calculated according to:  ∆E  P ∝ exp − .  k BT 

(10)

Here ∆E represents the energy difference between the saddle point and initial adsorption site and corresponds to the activation barrier for adatom jumps. P is then compared to a random number between 1 and 0 [53Met1]. If P is greater than the random number, then the adatoms jumps into the new site. Each time an adatom is selected, the time is increased by an amount ∆t=τ/N, where 1/τ represents the attempt frequency of hops, and N denotes the number of adatoms in the ensemble [91Lom1]. The surface diffusion coefficient is then calculated from Eq. 9. Prior to starting a full MC simulation, a thermodynamic equilibrium must be established by running a large number of initial MC steps. Final MC run may require well over 105 steps. Surface diffusion on metals in the framework of lattice gas potentials is outlined by Uebing et al. [97Ueb1]. For semiconductors, either a three-body PES or ab initio PES can used [96Kax1]. 3.11.2.3.3 Ab initio methods The classically-based methods described above yield adequate results for many surface diffusion problems; however, there are important problems in which quantum-mechanical effects play a substantial role (e. g., tunneling and bond rehybridization in surface reconstruction). Solving the self-consistent, many-body Schrodinger equation is theoretically ideal but computationally intractable. Even with the Born-Oppenheimer approximation that decouples electronic and nuclear motions, obtaining a solution becomes exceedingly difficult as numbers of nuclei and electrons increase [96Kax2]. In pursuit of computational tractability, density functional theory (DFT) abandons the calculation of wavefunctions and instead focuses upon total electron density [64Hoh1]. Even with the DFT approximation, it remains difficult to study dynamics of surface diffusion by ab initio methods. In most cases, pre-exponential factors remain inaccessible; one can obtain only static diffusion activation barriers. Fortunately, such barriers are often quite accurate. 3.11.2.3.4 Classical interatomic potentials All classical PES's employ an empirical functional form with free parameters that require fitting to experimental data. Depending on the nature of adatom-surface interactions (e. g. Van der Waals, covalent, ionic, etc.), PES's are functionalized differently. The relative computational time requirements for various methods are outlined in Fig. 6. A lattice gas model generally produces a highly-idealized periodic potential from which only a qualitative description of adatom-surface interaction is possible. The minima of the periodic potential constitute adsorption sites or traps (Fig. 7). The relative depth of the potential minima or traps is influenced by perturbations from nearest-neighbor interactions. The lattice-gas Hamiltonian is expressed as:

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3.11 Surface diffusion on metals, semiconductors, and insulators

V =−

n

∑

ci ε i +

[Ref. p. 517

n

∑ ci c jφ

,

(11)

i< j

i

where εi is the adsorption energy. If site i is occupied, then ci is 1; otherwise, ci is 0 [97Ueb1]. The second term describes the lateral interactions with φ equal to the energy difference between shallow and deep traps. Lennard-Jones (LJ) pair potential, utilized mainly for weak metal-adatom interacting systems (e. g. Xe/Pt(110) [84Val1]), contains sum of repulsive and attractive interaction contributions:  r  12  r  6  V = 4 ε  o  −  o   ,  r  ri   i = 1  i   n

∑

(12)

where ri is the distance between the adatom and the ith metal atom. The two parameters ε and ro represent the depth of potential well and the equilibrium bond distance, respectively. An alternative to LJ potential is the Morse potential: V = VoA

∑ (xi2 − 2 xi ), n

(13)

i =1

xi = exp[− (ri − ro ) a ] where xi is the Pauling bond order. The two parameters VoA and a are the minimum potential energy and scaling factor, respectively [91Lom1]. Classically-based simulations do not work for metals because of the very delocalized bonding characteristic of nearly-free electrons in the substrate. Pair potentials also fail to work for semiconductors that have strongly directional covalent bonding. For example, a pair potential would predict a closepacked structure for Si instead of the experimentally-observed diamond structure. Only three-body or nbody interatomic PES's can yield the diamond lattice structure, although that structure may contain zeroenergy lattice defects [88Ter1]. Since n-body interatomic potentials contain too many free parameters, in practice potential expansions usually incorporate terms with no more than three bodies. The two most popular three-body interatomic PES's are the Stillinger-Weber (SW) and Tersoff potentials. The SW potential [85Stil1] represents the total potential energy as a sum of two- and three-body terms: V=

n

n

∑ V2 (rij ) + ∑ V3 (rij , rik , r jk ) .

i< j

(14)

i< j
The two-body potential V2 depends only on the distance rij between neighboring atoms and contains an exponentially-decaying function that terminates the potential at a cutoff distance rc: V2 (rij ) = A(

B rij

p

[

]

1 − q ) exp (rij − rc ) −1 . rij

(15)

Here A, B, p, and q are parameters that are fitted to reproduce important physical properties of bulk semiconductor such as bond length, melting point, etc. Beyond rc, V2 vanishes. This cutoff feature permits the use of smaller ensembles and greatly reduces computational effort. The three-body potential V3 depends upon the distances between three neighboring atoms as well as the angles between them: V3 (rij , rik , rjk ) = h(rij , rik , θ jik ) + h( r ji , r jk , θ ijk ) + h( rki , rkj , θ ikj ),

(

)

[

]

2 , −1 −1   h(rij , rik , θ jik ) = λ exp γ rij − rc + γ ( rik − rc )  ⋅ cos θ jik + β  

(16)

where λ, γ, and β (~ 1/3) are fitted parameters and θ is the angle centered on atom i. The V3 term is responsible for maintaining the tetrahedral angle appropriate to the diamond lattice. Lando lt -Börnst ein New Ser ies III/42A1

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In contrast to SW approach, which is originally derived by expanding the total potential energy into contributions from one, two, three, and higher-body interactions, the Tersoff [88Ter1] potential abandons the n-body form and instead expresses potential in nominally two-body form: V =

∑

1 VR (rij ) − bijV A (rij ) , 2 i, j

(17)

where VR and VA are the repulsive and attractive portions of a pair-like Morse interaction [95Gar1]. The bij term formulated by Abell [85Abe1] explicitly describes the local bond order. The Tersoff potential parameterizes the bij terms according to: bij = (1 + β nζ ij n ) −1 2 n ,

ζ ij =

∑ 1 + c2

d 2 − c 2 [d 2 + (h − cos θ ijk ) 2 ]

,

(18)

i≠k

where β, n, c, d, and h are fitted parameters. The angular dependence of bij contains the three body interactions that correctly reproduces the ground-state diamond structure. However, the Tersoff potential is also useful for qualitative descriptions of nontetrahedral lattice configurations [96Kax2]. Many other functional forms exist for semiconductor potentials, but most derive from SW and Tersoff formulations. Examples include those of Pearson et al. [84Pea1], Biswas-Hamann [85Bis1], BoldingAnderson [90Bol1], Wang-Rockett [91Wan1]. A major difficulty arises in constructing interatomic PES's for insulators. This problem originates from the complex ionic lattice interactions (e.g., dielectric polarization); consequently, only a few PES's exist to date. PES's derived from the Shell model [58Dic1] by Catlow [79Cat1] and Jackson et al. [86Jac1] are most popular. The Shell model accounts for strong attractive Coulombic interactions among lattice ions, short-range repulsion from ionic charge-density overlap, and the dielectric response of the crystal [96Sto1]. 3.11.2.3.5 Semi-empirical potentials While pair potentials succeed in describing the energetics of weak metal-adatom interactions, they fail to describe stronger interactions in metals [96Foi1]. For strong metal-adatom systems, many-body potentials such as the embedded atom model (EAM) and corrective effective medium (CEM) are commonly utilized. The many-body Rosato-Guillope-Legrand (RGL) potential has been tested for some metal systems [99Mac1]. The EAM potential [83Dawl] is based on effective medium theory (EMT) [80Nor1] (alternately called the quasiatom approach [80Sto1]) and approximates the energy of ith atom in a lattice as a sum of two terms: the equilibrium energy required to embed the atom in a local electron density and a two-body electrostatic component [95Gar1]: Vi =

∑ F ( ρ i ) + 2 ∑ Φij (rij ), 1

i, j

i

ρ i = ∑ ρ atom (rij )

(19)

j

where local electron density ρi is summed over n-atomic densities, ρatom , and F and Φij represent empirical functional forms of embedding energy and coulombic interactions between atoms i and j. Φij is an explicit function of the effective charge on individual atoms in lattice. Other approximations to EMT exist such as the corrected embedded model (CEM) [90Sta1], the tightbinding (TB) model [84Fin1], and modified Morse potentials [80Fla1]. CEM differs slightly from the EAM approach by utilizing an ab initio two-body electrostatic component and empirical embedding energy term. The TB model [84Cha1] includes a crude description of bond rehybridization that classical models cannot account for. TB simplifies the quantum mechanical description of bonding with energy

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3.11 Surface diffusion on metals, semiconductors, and insulators

[Ref. p. 517

bands. Unfortunately, this approximation fails for most real systems that can exhibit electron transfer between bonds, electron-density localization, and related effects. The TB potential comprises two terms:

V = Vband + Vrep ,

(20)

where Vrep is the sum of short-range repulsive pair potentials. Vband represents a band energy derived from the collective overlap of localized bonds of lattice atoms. A complete outline of the TB formulation is given by Khan et al. [89Kha1]. 3.11.2.3.6 Quantum mechanical potentials One particular self-consistent approximation to the Schrodinger equation yields a very popular class of PES's called pseudopotentials [90Rap1]. The approximation is called density functional theory (DFT) and expresses the total energy of the system as a sum of two terms:

∫

V = F ( ρ (r)) + Vion ρ (r ) dr ,

(21)

where F(ρ(r)) accounts for the Coulombic interactions and Vion represents the external nuclear-electron potential. The first term implicitly contains three contributions: kinetic energy, Coulombic interactions and exchange correlations. Finding a suitable exchange correlation function is not a trivial task. One common approach makes an ad hoc local density approximation (LDA), but other approximations also exist. Despite these difficulties, DFT calculations usually do not clash too strongly with experimental results [96Kax2]. There are no free parameters to fit to experimental data for ab initio calculations.

3.11.3 Tables of data 3.11.3.1 Commentary to tables Tables 4 through 26 do not contain all data available in the literature, but represent the results of a critical survey. Some words of caution are in order about the results presented. Deficiencies in methodology become particularly pronounced in the calculation of Qdiff and Do, which are derived respectively from the slope and intercept (extrapolation to infinite temperature) of an Arrhenius plot. In experiments where the rate of diffusion can be determined with only modest accuracy, gross errors in Qdiff and Do may result. Moreover, in many cases D was not measured directly but was obtained from some other physical observable influenced by D. Nontrivial physical and mathematical modeling was then required to obtain D. Sometimes only the temperature dependence of D could be retrieved. Thus, Do was often not available. Furthermore, sometimes the diffusivity could be obtained only at a single temperature. In these cases, either the literature report or the present authors assumed a value of Do in order to estimate Qdiff. As a result of these various problems, the spread in reported results was enormous for the several adsorption systems in which comparisons were possible. In such cases, particularly outlandish reports for the magnitudes of Qdiff and Do were eliminated outright. In a few cases, Do was not reported in the original work, but was readily calculated from the raw data by the present authors. These cases are marked with footnotes. Tables 4 through 25 list experimental data. The tables of diffusion parameters are organized into the major categories of intrinsic diffusion and mass transfer diffusion, and are further subdivided by substrate type: metallic, semiconductor, or insulator. Intrinsic diffusion on metals encompasses such a large data set that data are further subdivided according to metal and adsorbate type. For intrinsic diffusion, Qdiff and Do (where available) are reported for each entry, together with Ω ( = Qdiff/Qdes) when the desorption energy Qdes could be determined. Where Qdes (for Ω) is taken from a reference not containing the surface diffusion data, both references are given. The method and adsorbate coverage are indicated, together with the temperature range. As indicated in earlier sections, diffusion behavior is closely tied to Tmelt. Thus, the temperatures have all been reported as fraction T/Tmelt.

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For mass transfer diffusion, Qdes is so difficult to find for most insulators that Ω could not be computed. An indication is given of whether each system appears to be in the high-temperature or low-temperature mass transfer regime (described further in the Discussion). Coverages are not reported because selfdiffusion is often involved, or because coverages are often so poorly defined. For several systems, the original authors reported a product of a Do times the thickness δ of a hypothetical diffusional layer. This layer is typically conceived to have a thickness between 0.3 and 1 nm [76Fur1]; to convert the reported values into standard units of Do for Table 21, δ = 1.0 nm was used by the present authors. Some cautions need to be issued with respect to surface cleanliness. Most of the experiments for insulators were not conducted under ultrahigh vacuum conditions. Hence, many of the surfaces were contaminated to varying degrees with atmospheric oxidation products and other species. Contamination can have major effects on surface diffusion on metals [91Bon1], and may be similarly important on insulators. However, since much of the methodology for diffusion studies on insulators has been rather crude and indirect, the contamination effects are probably rarely worse than the inherent weaknesses of the methodology. Impurity-mediated surface diffusion data is categorized in Table 22 according to whether the impurity increased, decreased, or did not affect diffusion. Most available data come from gas-adsorbate/metal alloy and metal-adsorbate/gas-metal systems. A common practice of assuming a value for Do to estimate Qdiff was rejected here since Do changes with impurity type. For this reason, many results were omitted because only D was given. We did not report Ω since impurities can affect desorption kinetics in an unpredictable way. Most reports of impurity-mediated diffusion give the influence of impurity concentration on D itself (as opposed to effects on Qdiff and Do), as seen in Fig. 8. In the tables we list Do and Qdiff for the lowest reported coverage of adsorbate and the highest reported coverage of impurity, thereby highlighting impurity effects. Table 23 shows data for tunneling diffusion. In all cases the adsorbate is an isotope of hydrogen, and the two lone substrates include W and Ni. The temperature-independent tunneling diffusivity Dtn is reported together with the crossover temperature Tt n, where classically activated thermal diffusion begins to dominate atomic motion. Data are provided for low coverages only. Tables 24 and 25 contain intrinsic and low-T mass transfer diffusion data under liquids. No high-T masstransfer data were available. Most solvents include H2O, and in some cases include a solid solute. Solute concentrations are reported by molarity; an aqueous solvent should be assumed if not listed explicitly. All experiments were performed either at room temperature Tr or at a small range of T near Tr; consequently, most yielded only D. In order to estimate Qdiff, we assumed a value for Do using the average value obtained from low-T, mass transfer diffusion of metals in vacuum. More than half of the mass transfer data are connected with diffusion of organic compounds on oxides measured using fluidized bed chromatography. Table 26 compiles data from computer computations. Where possible, the table compares the computational results to available experimental data. We report a range of experimental data where there are multiple reports. Computational method is listed along with the specific potential energy surface used.

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[Ref. p. 517

3.11.3.2 Surface diffusion tables 3.11.3.2.1 Thermal diffusion in vacuum 3.11.3.2.1.1 Intrinsic diffusion Table 4. Refractory metals on W. System

Do [m2/s]

Qdiff Ω [kJ/mol] [Qdiff/Qdes]

T/Tmelt

Method, Coverage

Ref.

Mo/W(211)|| Mo/W(211) Mo/W(211)|| Mo/W(311)|| Ta/W(110) Ta/W(110) Ta/W(211) Ta/W(321) W/W(100) W/W(110) W/W(110) W/W(110) W/W(110) W/W(110) W/W(110) W/W(111)ex W/W(111) W/W(211)|| W/W(211)|| W/W(211)|| W/W(211)|| W/W(211)⊥ W/W(211)|| W/W(211)⊥ W/W(211)|| W/W(211)⊥ W/W(321)|| W/W(321)|| W/W(321) W2/W(211) W2/W(110)

9x10−11 2.4x10−10 2x10−7 1.2x10−11 1x10−7 b) 4x10−6 9x10−12 2x10−9 1x10−8 1x10−7 b) 2.6x10−7 2.1x10−7 6.2x10−7 1x10−7 b) 1x10−7 b) 7.7x10−7 3.8x10−11 1.5x10−6 7.1x10−11 1.8x10−13 3x10−9 5x10−8 7.7x10−7 4.2x10−13 4x10−8 1.2x10−7 1x10−7 b) 7x10−4 1.4x10−3

54 54 69 53 67 75 47 64 130 67 84 88 84 88 115 172 75 80 54 82 55 56 70 100 80 28 84 81 63 80 89

0.06...0.08 0.06…0.075 0.06...0.08 0.06...0.08 0.06 0.06…0.075 0.06…0.07 0.07…0.08 0.39…0.45 0.17…0.23 0.08 0.075…0.09 0.08…0.09 0.33…0.38 0.055…0.14 0.07 0.07…0.09 0.065…0.08 0.07…0.08 0.14…0.19 0.14…0.19 0.22…0.25 0.22…0.25 0.15…0.22 0.15…0.22 0.08…0.09 0.075…0.09 0.06 0.08…0.1

FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FEM, FEM, FIM, 0 FIM, 0 FIM, 0 FIM, 0 FEM, FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FEM, FEM, FEM, FEM, FEM, FEM, FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0

75Sak1, 68Plu1 70Bas1, 68Plu1 88Wan1,68Plu1 75Sak1, 68Plu1 75Tso1, 68Plu1 70Bas1, 68Plu1 70Bas1, 68Plu1 70Bas1, 68Plu1 89Ste1, 68Plu1 90Cho1, 68Plu1 75Tso1, 68Plu1 75Gra1, 68Plu1 70Bas1, 68Plu1 78Kel1, 68Plu1 89Ste1, 68Plu1 75Gra1, 68Plu1 80Fla1, 68Plu1 88Wan1,68Plu1 70Bas1, 68Plu1 80Fla1, 68Plu1 92Gon1, 68Plu1 92Gon1, 68Plu1 90Cho2, 68Plu1 90Cho2, 68Plu1 90Cho2, 68Plu1 90Cho2, 68Plu1 75Gra1, 68Plu1 70Bas1, 68Plu1 80Fla1, 68Plu1 78Kel1, 68Plu1 78Kel1, 68Plu1

0.068 0.067 0.086 0.072 a) 0.10 a) 0.11 a) 0.075 0.096 a) 0.16 0.087 0.11 0.11 0.11 0.11 0.15 0.27 0.12 0.12 0.082 0.12 0.082 0.039 0.11 0.15 0.12 0.041 0.12 0.11 0.088 0.10 c) 0.12 c)

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. ex : Exchange mechanism. a ) Qdes estimated by present authors in comparison to other crystal faces. b ) Estimate by original authors. c ) Estimate obtained by comparison to similar chemical systems.

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3.11 Surface diffusion on metals, semiconductors, and insulators

Table 5. Transition metals on W. System

Do [m2/s]

Qdiff Ω [kJ/mol] [Qdiff/Qdes]

Ti/W(110)-(211) Ti/W(211)-(100) Ti/W(310) V/W(100) Cr/W(100) Ni/W(100) Ni/W(100) Ni/W(110) Ni/W(110) Ni/W(111) Ni2/W(111) Cu/W(110)-(210) Cu/W(100) Rh/W(211)|| Rh/W(110) Rh3/W(110) Rh4/W(110) Pd/W(100) Pd/W(110) Pd/W(211) Pd/W(311)-(100) Pd/W(332)-(311) Ag/W(110) Hf/W(100) Re/W(110) Re/W(110) Re/W(211)|| Re/W(211) Re/W(211)|| Re/W(321) Re/W(321) Re2/W(211)|| Ir/W(110) Ir/W(110) Ir/W(211)|| Ir/W(211) Ir/W(211)|| Ir2/W(211)|| Pt/W(110) Pt/W(110) Pt2/W(110) Pt3/W(110) Pt4/W(110) Ag/W(110) Au/W(110)-(100)

1x10−9 1x10−9 1.2x10−10 1x10−6 2x10−7 b) 2x10−7 b) 7x10−8 1x10−12 1x10−12 3.3x10−7 2.5x10−7 6x10−8 3.1x10−7 6x10−8 4x10−8 1.7x10−5 2x10−8 3x10−8 1x10−7 1.5x10−6 2.2x10−7 1.1x10−6 7.3x10−7 4.8x10−8 4.5x10−8 8.9x10−9 1x10−9 5x10−11 2.7x10−9 6.1x10−8 9x10−10 1x10−7 3x10−7 9x10−8 1.6x10−7 5x10−7 -

92 105 81 121 88 142 97 47 47 85 85 50 84 52 71 75 88 100 49 31 134 71 29 50 92 100 83 85 80 85 85 75 75 67 51 56 64 65 63 65 65 77 84 14 146

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0.17 c) 0.19 c) 0.15 0.17 c) 0.11 c) 0.32 0.22 0.11 0.11 0.19 a) 0.19 c) 0.11 0.15 0.26 0.11 a) 0.11 c) 0.13 c) 0.23 a) 0.11 a) 0.071 a) 0.31 0.16 0.095 0.088 c) 0.10 a) 0.10 a) 0.10 0.11 0.10 0.10 a) 0.089 a) 0.094 c) 0.13 a) 0.12a) 0.090 0.10 0.16 0.16 c) 0.12 a) 0.12 a) 0.12 c) 0.14 c) 0.16 c) 0.05 0.44

T/Tmelt

Method, Coverage

Ref.

0.19 0.19 0.26…0.39 0.13…0.14 0.13…0.15 0.16…0.19 0.10…0.11 0.05…0.06 0.05 0.105…0.11 0.05…0.11 0.14…0.17 0.14…0.16 0.05…0.06 0.15…0.17 0.05 0.03…0.04 0.12…0.20 0.12…0.20 0.13…0.21 0.10…0.13 0.085 0.09…0.10 0.08…0.095 0.07…0.09 0.08…0.10 0.09…0.10 0.08…0.095 0.07…0.095 0.075…0.09 0.06…0.09 0.065…0.07 0.06…0.7 0.07…0.085 0.07…0.08 0.06 0.06…0.07 0.06…0.07 0.08…0.09 0.08…0.09 0.03…0.08 -

FEM, 0.2 FEM, 0.15 FEM, 0.2 FEM, 0.2 FEM, 0.2 FEM, all FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FEM, FIM, 0 FIM, 0 FIM, 0 FEM, FIM, 0 FIM, 0 FEM, all FEM, all SEM, >2.0 FEM, FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 SEM, >2 FEM, 0.25

70Vla1 70Vla1 99Bie1 94Vla1 94Vla1 76Rou1 87Kel1 87Kel2, 78Bas1 78Bas1 78Fla1, 76Rou1 78Fla1, 76Rou1 66Mel1, 86Kol1 66Mel1, 86Kol1 88Wan1,68Plu1 90Dha1, 68Plu1 90Dha1, 68Plu1 90Dha1, 68Plu1 84Rou1 78Bas1, 78Rou1 95Sen1 78Rou1 78Rou1 83Spi1 94Beb1 75Tso1, 68Plu1 70Bas1, 68Plu1 76Sto1, 68Plu1 70Bas1, 68Plu1 88Wan1,68Plu1 72Tso1, 68Plu1 70Bas1, 68Plu1 76Sto1, 68Plu1 70Bas1, 68Plu1 75Tso1, 68Plu1 75Ree1, 68Plu1 70Bas1, 68Plu1 88Wan1,68Plu1 75Ree1, 68Plu1 75Tso1, 68Plu1 76Bas1, 68Plu1 76Bas1, 68Plu1 76Bas1, 68Plu1 76Bas1, 68Plu1 90Jon1 70Vla1, 86Kol1

474

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[Ref. p. 517

Table 5 (cont.) System

Do [m2/s]

Qdiff Ω [kJ/mol] [Qdiff/Qdes]

T/Tmelt

Method, Coverage

Ref.

Au/W(100) Au/W(110) Au/W(211) Hg/W(100) Hg/W(111) Hg/W(321) Nd/W(110)-(100) Nd/W(111)

-

46 42 33 96 50 38 71 63

0.02…0.12 0.02…0.12 0.02…0.12 0.04…0.06 0.04…0.06

FEM, 0.5 FEM, 0.5 FEM, 1 FEM, 0…0.2 FEM, 0…0.2 FEM, 0…0.2 FEM, 1 FEM, 1

76Jon1, 86Kol1 76Jon1, 86Kol1 76Jon1, 86Kol1 68Swa1 68Swa1 68Swa1 70Mar1 70Mar1

0.14 0.13 0.1 0.53 0.27 0.2 0.16 0.14

|| : Parallel to atomic rows, step edges. a ) Qdes estimated by present authors in comparison to other crystal faces. b ) Estimate by original authors. c ) Estimate obtained by comparison to similar chemical systems.

Table 6. Alkali and alkaline earth metals on W. System

Do [m2/s]

Qdiff Ω [kJ/mol] [Qdiff/Qdes]

T/Tmelt

Method, Coverage

Ref.

Li/W(poly) Li/W(110) Li/W(111) Li/W(211) Li/W(211)-[110] Na/W(poly) Na/W(110) K/W(poly) K/W(100) K/W(110) K/W(211) K/W(211)|| K/W(211)⊥ K/W(211) K/W(211)|| K/W(211) K/W(710)|| K/W(710)⊥ K/W(11,1,0)|| K/W(11,1,0)⊥ K/W(19,1,0)|| K/W(19,1,0)⊥ Rb/W(poly) Cs/W(poly) Cs/W(poly) Cs/W(110) Cs/W(110) Cs/W(321)

3x10−7 d) 5.7x10−7 9.8x10−9 5x10−11 e) 3x10−12 8.7x10−6 5x10−9 5x10−9 3x10−5 3x10−5 4x10−5 6.9x10−6 5.7x10−5 9.1x10−6 3x10−5 6.4x10−6 6.4x10−6 2x10−5 5x10−9 -

63 12 46 42 16 54 26 39 41 33 29 44 73 49 40 75 43 67 41 60 37 37 42 25 28 46 40 73

0.46…0.60 0.02…0.05 0.11 0.11 0.06…0.07 0.43…0.54 0.07…0.08 0.33…0.43 0.20…0.30 0.05…0.07 0.05…0.07 0.20…0.35 0.20…0.35 0.09…0.13 0.11…0.13 0.10 0.20…0.30 0.20…0.30 0.20…0.30 0.20…0.30 0.20…0.30 0.20…0.30 0.32…0.42 0.18…0.22 0.31…0.32 0.05…0.06 0.07…0.11 <0.06

FEM, CPD, 0.02 FIM, 0.02 FIM, 0.02 FEM, 1.2 FEM, FEM, 0.02 FEM, SIM, 10−3 FEM, 0.04 FEM, 0.04 SIM, 10−3 SIM, 10−3 FEM, 5x10−3 FEM, 2x10−6 FEM, 0.04 SIM, 10−3 SIM, 10−3 SIM, 10−3 SIM, 10−3 SIM, 10−3 SIM, 10−3 FEM, IT, 0.03 FEM, FEM, 0.2 FEM, 0.05 FEM, 0.2

90Gla1 82Lob1,74Med1 92Bie1, 74Med1 92Bie1, 74Med1 95Bie1 90Gla1 84Mor1, 64Ehr1 90Gla1 84Bay1 65Sch1, 66Sch1 65Sch1, 83Bay1 83Bay1 83Bay1 91Bla1, 83Bay1 87Beb1, 83Bay1 92Bie1, 83Bay1 84Bay1 84Bay1 84Bay1 84Bay1 84Bay1 84Bay1 90Gla1 32Lan1, 90Gla1 90Gla1 68Swa1, 67Gav1 85Mor1, 67Gav1 68Swa1, 67Gav1

0.24 0.047 0.18 a) 0.16 a) 0.062 0.22 0.098 0.20 0.17 0.13 a) 0.12 0.18 0.30 0.20 0.16 0.30 0.18 0.28 0.17 0.25 0.15 0.15 0.24 0.15 0.17 0.15 0.13 0.32

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475

3.11 Surface diffusion on metals, semiconductors, and insulators

System

Do [m2/s]

Qdiff Ω [kJ/mol] [Qdiff/Qdes]

Ba/W(100) Ba/W(110) Ba/W(110) Ba/W(110) Ba/W(111) Ba/W(210) Ba/W(211) Ba/W(221) Ba/W(321)

1x10−11 1x10−11 b) 3x10−11 1x10−11 b) 1x10−11 b) 1x10−11 b) 1x10−11 b) 1.6x10−8

63 18 40 89 45 44 70 21 80

0.13 a) 0.040 0.086 0.19 a) 0.094 a) 0.092 a) 0.15 a) 0.044 a) 0.17 a)

T/Tmelt

Method, Coverage

Ref.

0.22 0.07 0.13…0.15 0.16 0.16 0.08 0.24 0.13…0.15

FEM, 0.65 FEM, 0.65 FEM, <0.08 FEM, 0.07 FEM, 0.65 FEM, 0.65 FEM, 0.65 FEM, 0.65 FEM, <0.08

54Dre1, 72Fed1 54Dre1, 72Fed1 62Uts1, 72Fed1 74Vla1, 72Fed1 54Dre1, 72Fed1 54Dre1, 72Fed1 54Dre1, 72Fed1 54Dre1, 72Fed1 62Uts1, 72Fed1

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. a ) Qdes estimated by present authors in comparison to other crystal faces. b ) Estimate by original authors. d ) Do estimated from reported attempt frequency ν using Eq. (3) by present authors (assuming hop length λ of 0.3 nm). e ) Calculation by present authors from original work (assuming that data fit Arrhenius's law).

Table 7. Other solids on W. System

Do [m2/s]

Qdiff Ω [kJ/mol] [Qdiff/Qdes]

T/Tmelt

Method, Coverage

S/O-W(poly) S/W(100) S/W(110) S/W(211) S/W(721) Ge/W(110)-(111) Ge/W(110)-(100) Si/W(110) SiO/W(111)-(110) SiO/W(110)-(100) SiO/W(110)-(111) SiO/W(211)-(100) In/W(100) WSi/W(110) Pb/W(100) Pb/W(111) Ce/W(poly) Eu/W(poly) Yb/W(110)-(100) Yb/W(211)

5x10−7 2x10−5 3.1x10−8 3x10−8 6.4x10−5 7x10−6 3x10−5 -

100 80 63 75 105 130 155 67 251 289 126 167 105 59 134 134 31 17 42 67

0.27…0.40 0.12 0.09 0.1 0.11…0.13 0.07…0.08 0.19…0.20 0.06…0.08 0.11…0.15 0.09…0.15 0.49…0.70 0.49…0.65 0.03…0.05 0.03…0.05

AES, 84Ish1 FIM, 80Bec1, 88Kel FIM, 80Bec1, 88Kel FIM, 80Bec1, 88Kel FIM, 80Bec1, 88Kel FEM, 0.6 69Sok1 FEM, 0.6 69Sok1 FIM, 0 81Tso1, 64Ehr1 FEM, 1st layer 69Sok1 FEM, 1st layer 69Sok1 FEM, 1st layer 69Sok1 FEM, 0.4 69Sok1, 71Vla1 SAM, 0.25 86Mor1 FIM, 0 85Wri1 FEM, 0.3 81Mor1, 77Jon1 FEM, 0.15 81Mor1, 77Jon1 IT, 94Gla1 IT, 88Gla1 FEM, 1 70Mar1 FEM, 1 70Mar1

c

0.17 0.13 0.16 0.22 0.31 0.37 0.12 0.31 c) 0.36 c) 0.16 0.21 0.31 0.09 c) 0.48 0.55 0.17 0.097 0.20 0.31

) Estimate obtained by comparison to similar chemical systems.

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Ref.

476

3.11 Surface diffusion on metals, semiconductors, and insulators

[Ref. p. 517

Table 8. Gaseous adsorbates on W. System

Do [m2/s]

H/W(100)|| 2.8x10−11 H/W(100)⊥ 1.7x10−9 H/W(110) 1.7x10−11 H/W(211)|| 2x10−8 H/W(211)⊥ 1x10−9 H/W(320)|| 4x10−7 H/W(320)⊥ 5x10−7 H/W(321)|| 1x10−7 H/W(321)⊥ 5x10−9 D/W(100)|| 4.4x10−11 D/W(100)⊥ 7.2x10−9 D/W(110) 1.3x10−9 D/W(211)|| 9x10−10 D/W(211)|| 3x10−10 D/W(320)|| 3x10−10 D/W(320)⊥ 1x10−9 D/W(321)|| 1x10−9 D/W(321)⊥ 2x10−10 3 H /W(110) 3.3x10−7 C/W(110) 5x10−5 C/W(211) 5x10−5 N/W(110) 1.4x10−6 O/W(110) 3.8x10−5 O/W(110) 2x10−11 O/W(110) 2x10−5 O/W(110) 4.5x10−8 O/W(411) 3x10−7 O/W(320) H2O/W(121)-(111) 5x10−3 e) CO/W(110) 1.3x10−9 Xe/W(100) 7x10−12

Qdiff T/Tmelt Ω [kJ/mol] [Qdiff/Qdes]

Method, Coverage

15 28 17 33 28 23 23 17 16 23 32 20 28 27 13 14 15 15 20 230 222 88 113 59 96 100 209 100 35 93 5

FEM, 0.06 95Dan1 FEM, 0.06 95Dan1 FEM, 0.1 85Wan1, 71Tam1 FEM, 0.2 88Dan1, 71Tam1 FEM, 0.2 88Dan1, 71Tam1 FEM, 0.2 91Ueb1 FEM, 0.2 91Ueb1 FEM, 0.2 91Cho1 FEM, 0.2 91Cho1 FEM, 0.06 95Dan1 FEM, 0.06 95Dan1 FEM, 0.5 85Tri1, 71Tam1 FEM, 0.2 88Dan1, 71Tam1 FEM, 0.2 88Dan1, 71Tam1 FEM, 0.2 91Ueb1 FEM, 0.2 91Ueb1 FEM, 0.2 91Cho1 FEM, 0.2 91Cho1 FEM, 0.1 85Wan1 FEM, 76Rad1, 91Ben1 FEM, 76Rad1, 91Ben1 AES, 77Pol1 CPD, 0.4…0.9 77But1, 72Goy1 FEM, 0.15…0.25 79Che1, 72Goy1 SEE. 0.25 77But1, 72Goy1 FEM, 0.6 85Tri1, 72Goy1 SEE, 0.67 80Bow1, 72Goy1 SEE, 0.5 80Bow1, 72Goy1 FEM, >1 99Bry1 FEM, 0.3 79Che2, 72Win1 FEM, 0.3 80Che1

0.11 0.21 0.13 0.24 a) 0.21 a) 0.17 a) 0.17 a) 0.12 a) 0.12 a) 0.17 0.24 0.14 0.21 a) 0.20 a) 0.10 a) 0.10 a) 0.11 0.11 0.15 0.21 0.20 0.26 0.21 0.11 0.18 0.19 0.39 0.19 0.29 0.25

0.04…0.06 0.04…0.06 0.04…0.05 0.02…0.07 0.02…0.07 0.03 0.03 0.03 0.03 0.04…0.06 0.04…0.06 0.02…0.04 0.02…0.07 0.02…0.07 0.03 0.03 0.03 0.03 0.04…0.05 0.22…0.26 0.22…0.26 0.19…0.24 0.28…0.31 0.08…0.22 0.025…0.04 0.16…0.20 0.28…0.40 0.33 0.032…0.035 0.18…0.21 <0.01

Ref.

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. a ) Qdes estimated by present authors in comparison to other crystal faces. e ) Calculation by present authors from original work (assuming that data fit Arrhenius's law).

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477

3.11 Surface diffusion on metals, semiconductors, and insulators

Table 9. Metal adsorbates on Ni. System

Do [m2/s]

Qdiff Ω [kJ/mol] [Qdiff/Qdes]

K/Ni(111)-(100) K/Ni(111)-(110) K/Ni(110)-(100) Ni/Ni(100) Ni/Ni(110)|| Ni/Ni(110) ⊥ Ni/Ni(110)⊥ Ni/Ni(111) Ni/Ni(311)|| Ni/Ni(331)|| Ag/Ni(poly) Pt/Ni(100)ex Pt/Ni(110)ex Pt/Ni(311)

1x10−13 1x10−6 e) x) 1x10−11 2x10−10 2x10−7 1x10−5 1x10−7 b) 1x10−7 b) 1x10−7 b)

15 26 19 61 22 52 31 32 29 43 100 67 27 37

0.10 c) 0.17 c) 0.12 c) 0.15 0.044 a) 0.10 a) 0.077 a) 0.079 0.072 a) 0.11 c) 0.25 c) 0.12 c) 0.050 c) 0.064

T/Tmelt

Method, Coverage

Ref.

0.05…0.08 0.05…0.08 0.05…0.08 0.08…0.12 0.08…0.12 0.52…0.70 0.08…0.12 0.08…0.12 0.08…0.12 0.08…0.12 0.46…0.66 0.08…0.09 0.06 0.08…0.09

FEM, 1.5 FEM, 1.5 FEM, 1.5 FIM, 0 FIM, 0 QHAS, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 LEED, FIM, 0 FIM, 0 FIM, 0

92Bla1 92Bla1 92Bla1 80Tun1, 94Ald1 80Tun1, 94Ald1 97Gra1 80Tun1, 94Ald1 80Tun1, 94Ald1 80Tun1, 94Ald1 80Tun1, 94Ald1 70Pin1 92Kel1 91Kel1 92Kel1

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. ex : Exchange mechanism. a ) Qdes estimated by present authors in comparison to other crystal faces. b ) Estimate by original authors. c ) Estimate obtained by comparison to similar chemical systems. e ) Calculation by present authors from original work (assuming that data fit Arrhenius's law). x ) Average values obtained by present authors.

Table 10. Non-metal adsorbates on Ni. System

Do [m2/s]

Qdiff T/Tmelt Ω [kJ/mol] [Qdiff/Qdes]

Method, Coverage

Ref.

H/Ni(poly) H/Ni(100) H/Ni(100) H/Ni(100) H/Ni(100) H/Ni(111) H/Ni(111) D/Ni(100) D/Ni(100) D/Ni(100) D/Ni(111) D/Ni(111) C/Ni(poly) CO/Ni(100) CO/Ni(100) CO/Ni(110)|| CO/Ni(110)⊥ CO/Ni(110)|| CO/Ni(110)⊥ CO/Ni(111) CO/Ni(111)

4.5x10−7 2.5x10−7 2x10−8 1.7x10−9 3x10−8 5x10−7 8.5x10−7 2x10−9 5x10−7 e) 3x10−8 3.5x10−9 1x10−5 2.5x10−5 4.5x10−10 2x10−12 7x10−9 e) x) 2x10−8 e) x) 1.2x10−9 1x10−7

29 17 15 15 13 14 13 18 18 15 22 14 29 21 27 17 9.2 3 6 29 28

FEM, LITD, 0…0.1 LITD, 0…1 LLD, FEM,0.25 SHD, 0.3 FEM, 0.08 LITD, 0…0.5 LITD, 0…1 FEM, 0.15 SHD, 0.3 FEM, 0.05 AES, LITD, 0…0.3 LITD, 0.25 LITD/SHD, 0.05 LITD/SHD, 0.03 QHAS, 0.15 QHAS, 0.15 SHD, 0…0.5 FEM, 0.05

57Wor1, 85Geo1 85Geo1 87Mul1, 85Geo1 92Zhu1 91Lin1 93Lee1, 74Chr1 91Lin1 86Mul1, 85Geo1 87Mul1, 85Geo1 91Lin1 95Won1, 74Chr1 91Lin1 76Moj1, 75Ise1 86Mul1, 81Kis1 87Roo1, 81Kis1 93Xia1, 90Fei1 95Xia1 96Ber2 96Ber2 88Zhu1, 74Chr2 90Lin1, 74Chr2

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0.30 0.16 0.14 0.14 0.13 0.14 0.13 0.18 0.18 0.15 0.23 0.14 0.04 0.18 0.22 0.13 0.069 0.026 0.042 0.27 0.26

0.14…0.16 0.12…0.16 0.12…0.16 0.09 0.042…0.082 0.06…0.09 0.042…0.082 0.12…0.15 0.12…0.16 0.035…0.082 0.06…0.09 0.042…0.082 0.36…0.49 0.12…0.15 0.12…0.17 0.11…0.14 0.08…0.11 0.12…0.16 0.10…0.18 0.13…0.16 0.09…0.13

478

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[Ref. p. 517

Table 10 (cont.) System

Do [m2/s]

Qdiff T/Tmelt Ω [kJ/mol] [Qdiff/Qdes]

CO/Ni(511) S/Ni(poly) S/Ni(111)

1x10−6 2x10−8 b)

27 84 28

0.22 a) 0.24 0.080

0.08…0.12 0.23…0.29 0.06

Method, Coverage

Ref.

FEM, 0.2 FEM, FIM, 0

90Lin1, 81Kis1 83Bla1, 82Cap1 85Kel1, 82Cap1

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. a ) Qdes estimated by present authors in comparison to other crystal faces. b ) Estimate by original authors. x ) Average values obtained by present authors.

Table 11. Metal adsorbates on Pt. System

Do [m2/s]

Qdiff Ω [kJ/mol] [Qdiff/Qdes]

T/Tmelt

Method, Coverage

Ref.

Li/Pt(100) Na/Pt(111) Pd/Pt(100) Pt/Pt(100)ex Pt/Pt(100)-hex Pt/Pt(110)-1x2 k) Pt/Pt(110)|| Pt/Pt(110)⊥ Pt/Pt(111) Pt/Pt(111) Pt/Pt(311)|| Pt/Pt(331)|| Pt2/Pt(100)ex Pt2/Pt(111) Pt3/Pt(100)ex Ir/Pt(110) Ir/Pt(110) Ir/Pt(311) Au/Pt(110) Au/Pt(311) Ag/Pt(111) W/Pt(111)

6x10−12 e) 2x10−7 e) x) 1x10−7 b) 1x10−7 b) 2x10−7 d) 2x10−9 d) 8x10−7 b) 1x10−7 b) 2x10−7 1x10−7 d) 1x10−10 b) 4x10−8 b) 1x10−7 b) 1.9x10−8 1x10−7 b) 1x10−7 b) 1x10−7 b) 1x10−11 b) 3x10−5 2x10−7 d) 1x10−7 b)

24 3 67 45 41 78 80 75 25 25 50 80 42 36 54 75 75 71 59 54 15 29

0.14…0.20 0.049…0.17 0.13 0.04 0.16…0.24 0.14…0.19 0.05…0.06 0.05…0.06 0.042…0.048 0.054…0.078 0.05…0.06 0.05…0.06 0.04 0.069…0.078 0.04 0.05…0.06 0.05…0.06 0.05…0.06 0.05…0.06 0.05…0.06 0.054…0.10 0.05…0.06

FEM, 0 QHAS, 0.05 FIM, 0 FIM, 0 STM, 0.075 STM, FIM, 0 FIM, 0 FIM, 0 STM, 4x10−3 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 STM, 0.05 FIM, 0

98Suc1 99Gra1 91Kel2 94Kel1 98Mor1 98Lin1 78Bas2 78Bas2 98Kyu1 96Bot1 78Bas2 78Bas2 94Kel1 98Ky1 94Kel1 78Bas2 78Bas2 78Bas2 78Bas2 78Bas2 95Bru1 78Bas2

0.16 c) 0.098 0.089 0.17 0.17 0.16 0.054 a) 0.05 0.10 0.16 0.086 c) 0.083 a) 0.11 c) 0.15 0.15 0.13 0.15 0.13 0.047 c)

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. ex : Exchange mechanism. a ) Qdes estimated by present authors in comparison to other crystal faces. b ) Estimate by original authors. c ) Estimate obtained by comparison to similar chemical systems. d ) Do estimated from reported attempt frequency ν using Eq. (3) by present authors (assuming hop length λ of 0.3 nm). e ) Calculation by present authors from original work (assuming that data fit Arrhenius's law). k ) Qdiff for double hop diffusion is 8 kJ/mol higher than for single hop. x ) Average values obtained by present authors.

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Ref. p. 517]

479

3.11 Surface diffusion on metals, semiconductors, and insulators

Table 12. Non-metal adsorbates on Pt. System

Do [m2/s]

H/Pt(poly) 2x10−8 b) NaO/Pt(111) 2x10−7 e) x) H/Pt(111) 1x10−7 e) x) D/Pt(111) 2x10−7 e) x) C/Pt(111) 2x10−5 CO/Pt(poly) 1x10−7 CO/Pt(110)-1x2|| 1.5x10−4 CO/Pt(110)-1x2⊥ 6x10−6 CO/Pt(110)-1x1|| 5x10−7 CO/Pt(110)-1x1⊥ 7x10−8 CO/Pt(110) 10−6…10−5 b) CO/Pt(111) 7x10−5 CO/Pt(111) CO/Pt(111) 4x10−11 d) CO/Pt(111) 1.6x10−8 e) x) CO/Pt(111) 6x10−11 Supported on Al2O3 (100 Å clusters) CO/Pt(111) 6x10−11 Supported on Al2O3 (12 Å clusters) CH4/Pt(111) 3x10−8 b) NO/Pt(111) 2.4x10−3 NO/Pt(111) 1x10−6 O/Pt(100) 2x10−8 b) O/Pt(110) 2x10−8 b) O/Pt(111) 2x10−8 b) Kr/Pt(111) 1.5x10−8 b) Xe/Pt(111) 3.4x10−8 Xe/Pt(111) Xe/Pt(111) 5x10−3 Xe/Pt(11,11,9)|| Xe/Pt(11,11,9)⊥ 1x10−5

Qdiff T/Tmelt Ω [kJ/mol] [Qdiff/Qdes]

Method, Coverage

Ref.

19 3 7 7 130 61 55 46 42 37 10 52 29 17 13 27

0.26 0.10 0.10 0.21 0.45 0.37 0.32 0.29 0.25 0.065 0.42 0.22 0.13 0.10 0.20

0.05…0.06 0.049…0.15 0.069…0.12 0.069…0.12 0.42…0.49 0.05…0.06 0.16…0.19 0.16…0.19 0.16…0.19 0.16…0.19 0.07 0.16…0.18 0.06…0.20 0.20 0.14…0.20

FEM,1 QHAS, 0.04 QHAS, 0.2 QHAS, 0.2 AES, FEM, all PEEM, PEEM, PEEM, PEEM, LITD, 0 LITD, 0.1 HeS, 0.1 FTIR, 0.05 QHAS, 0.03 NMR, 0.3…1.0

69Lew1, 74Nis1 99Sil1 98Gra1 98Gra1 78Mar1 67Lew1, 86See2 94Oer1, 80Fai1 94Oer1, 80Fai1 94Oer1, 80Fai1 94Oer1, 80Fai1 82Une1 92Kwa1, 86See2 82Poe1, 86See2 90Reu1, 86See2 98Gra2 93Bec1, 86See2

44

0.33

0.14…0.20

NMR, 0.3…1.0 93Bec1, 86See2

2 49 21 142 113 130 5 5 5 3 12 12

0.14 0.40 0.20 0.40 0.47 0.47 0.30 0.18 0.18 0.47 0.49 0.49

0.022 0.17…0.19 0.20…0.23 0.05…0.06 0.22…0.24 0.22...0.24 0.022 0.03…0.04 0.03 0.010…0.017 0.03…0.04 0.03…0.04

LITD, 0.2 HeS, 0.12…0.16 MB, <0.01 FEM, 0 FEM, 1 FEM, 1 LITD, 1 LITD, 0.25 FEM, STM, 0.3…0.7 LITD, 0.05 LITD, 0.05

93Mei1 94Cro1 83Ser1 68Lew1 68Lew1, 70Fus1 68Lew1, 70Fus1 93Mei1 93Mei2, 80Che1 95Hor1 94Sne1 94Sne1

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. b ) Estimate by original authors. d ) Do estimated from reported attempt frequency ν using Eq. (3) by present authors (assuming hop length λ of 0.3 nm). e ) Calculation by present authors from original work (assuming that data fit Arrhenius's law). x ) Average values obtained by present authors.

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[Ref. p. 517

Table 13. Adsorbates on Rh. System

Do [m2/s]

Qdiff [kJ/mol]

T/Tmelt Ω [Qdiff/Qdes]

Method, Coverage

Ref.

Rh/Rh(100)|| Rh/Rh(110)|| Rh/Rh(111) Rh/Rh(311)|| Rh/Rh(311)|| Rh/Rh(331)|| Rh2/Rh(100) Rh3/Rh(100) Rh4/Rh(100) Rh5/Rh(100) Rh6/Rh(100) Rh7/Rh(100) Rh8/Rh(100) Rh9/Rh(100) Rh10/Rh(100) Rh11/Rh(100) Pt/Rh(100) Pt/Rh(110)ex Pt/Rh(111)ex Pt/Rh(311)|| Pt/Rh(331) Pt2/Rh(100) Pt3/Rh(100) Pt4/Rh(100) Pt5/Rh(100) Pt6/Rh(100) H/Rh(111) H/Rh(111) H/Rh(332)|| H/Rh(332)⊥ D/Rh(111) D/Rh(111) D/Rh(332)|| D/Rh(322)⊥ CO/Rh(poly) CO/Rh(111)

1x10−7 b) 3x10−5 2x10−8 2x10−7 1x10−7 b) 1x10−6 1x10−7 b) 1x10−7 b) 1x10−7 b) 1x10−7 b) 1x10−7 b) 1x10−7 b) 1x10−7 b) 1x10−7 b) 1x10−7 b) 1x10−7 b) 2x10−7 1x10−7 b) 1x10−7 b) 1.6x10−8 b) 1x10−7 b) 1x10−7 b) 1x10−7 b) 1x10−7 b) 1x10−7 b) 1x10−7 b) 6.5x10−7 1x10−7 4x10−6 8x10−6 1x10−7 8x10−8 3x10−7 4x10−6 2x10−7

80 58 15 52 56 62 77 98 101 98 110 100 110 118 103 118 89 63 21 42 69 84 98 98 98 112 13 18 21 22 14 15 16 21 29 30

0.14 0.10 0.027 0.093 0.10 0.11 0.14 c) 0.18 c) 0.18 c) 0.17 c) 0.20 c) 0.18 c) 0.20 c) 0.21 c) 0.19 c) 0.21 c) 0.16 c) 0.11 c) 0.04 c) 0.076 c) 0.12 c) 0.16 c) 0.17 c) 0.17 c) 0.17 c) 0.20 c) 0.23 0.23 0.26 a) 0.28 a) 0.18 0.20 0.20 a) 0.27 a) 0.23 0.23

FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 FIM, 0 LITD, 0.84 LITD, 0.33 LITD, 0.84 LITD, 0.84 LITD, 0.33 LITD, 0.02 LITD, 0.84 LITD, 0.84 NMR, LITD, 0.01

96Kel1, 62Hon1 73Ayr1, 62Hon1 73Ayr1, 62Hon1 73Ayr1, 62Hon1 96Kel1 73Ayr1, 62Hon1 96Kel1, 62Hon1 94Kel3, 62Hon1 94Kel3, 62Hon1 94Kel3, 62Hon1 94Kel3, 62Hon1 94Kel3, 62Hon1 94Kel3, 62Hon1 94Kel3, 62Hon1 94Kel3, 62Hon1 94Kel3, 62Hon1 93Kel1 93Kel1 93Kel1 93Kel1 93Kel1 93Kel2 93Kel2 93Kel2 93Kel2 93Kel2 92Man1, 79Yat1 88See1, 79Yat1 95Hoo1, 79Yat1 95Hoo1, 79Yat1 92Man1, 79Yat1 88See1, 79Yat1 95Hoo1, 79Yat1 95Hoo1, 79Yat1 89Dun1, 85Roo1 88See1, 85Roo1

0.13 0.08…0.09 0.02…0.03 0.08…0.09 0.16 0.09…0.10 0.16 0.16 0.16 0.16 0.18 0.18 0.18 0.20 0.17 0.20 0.14…0.15 0.11 0.03 0.11…0.12 0.11…0.12 0.14…0.15 0.16…0.17 0.16…0.17 0.16…0.17 0.18…0.20 0.08…0.10 0.09…0.12 0.07…0.09 0.07…0.09 0.08…0.10 0.09…0.12 0.07…0.09 0.07…0.09 0.13 0.12…0.18

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. ex : Exchange mechanism. a ) Qdes estimated by present authors in comparison to other crystal faces. b ) Estimate by original authors. c ) Estimate obtained by comparison to similar chemical systems.

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3.11 Surface diffusion on metals, semiconductors, and insulators

Table 14. Adsorbates on Ru. System

Do [m2/s]

Qdiff T/Tmelt Ω [kJ/mol] [Qdiff/Qdes]

K/Ru(100) H/Ru(100) H/Ru(100) H/Ru(S)-[15(100)x2(100)]|| H/Ru(S)-[15(100)x2(100)] ⊥ D/Ru(100) Li/Ru(100) N/Ru(100) CO/Ru(100) CO/Ru(100) CO/Ru(S)-[15(100)x2(100)]|| CO/Ru(S)-[15(100)x2(100)] ⊥ C3H8/Ru(100) n-C4H10/Ru(100) n-C5H12/Ru(100) n-C6H14/Ru(100) n-pentane/Ru(100) Isopentane/Ru(100) Cyclopentane/Ru(100) Neopentane/Ru(100)

4x10−5 6.3x10−8 6.8x10−9 3.9x10−8 3.8x10−7 4.6x10−8 2x10−7 d) 2x10−6 3.8x10−5 4x10−4 3.6x10−4 0.14 1.1x10−5 1.1x10−5 3x10−5 1.6x10−5 3x10−5 5.5x10−6 6x10−8 4x10−6

38 17 12 14 25 17 44 91 46 40 39 75 13 15 19 20 18 17 14 13

0.21 0.24 0.17 0.19 0.36 0.25 0.14 0.17 0.29 0.25 0.25 0.25 0.48 0.27 0.29 0.32 0.32 0.30 0.28 0.28

Method, Coverage

Ref.

0.09…0.10 LITD, 0.15 93Wes1 0.10…0.13 LITD, 0.15 86Mak1, 80Shi1 0.081…0.11LITD, 0.25 95Bro1 0.06…0.09 LITD, 0.60 95Are1 0.06…0.09 LITD, 0.60 95Are1 0.08…0.09 LITD, 0.25…0.30 87Mak1, 80Shi1 0.08 NMR, 0.02 98Ebi1, 96Ebi1 0.12…0.14 STM, 0 96Zam1 0.10…0.13 LITD, 0.27 89Dec1, 83Pfn1 0.097…0.12LITD, 0.25 95Bro1 0.09…0.11 LITD, 0.27 94Wes1, 83Pfn1 0.09…0.11 LITD, 0.27 94Wes1, 83Pfn1 0.03…0.04 LITD, 0.2 90Bra1 0.04…0.05 LITD, 0.2 90Bra1 0.04…0.06 LITD, 0.2 90Bra1 0.05…0.07 LITD, 0.2 90Bra1 0.05…0.06 LITD, 0.1 90Are1 0.05…0.06 LITD, 0.1 90Are1 0.05…0.06 LITD, 0.1 90Are1 0.04…0.05 LITD, 0.1 90Are1

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. d ) Do estimated from reported attempt frequency ν using Eq. (3) by present authors (assuming hop length λ of 0.3 nm).

Table 15. Adsorbates on Cu. System

Do [m2/s]

Qdiff T/Tmelt Ω [kJ/mol] [Qdiff/Qdes]

Method, Coverage

Ref.

Ag/Fe(110) Na/Cu(100) Cu/Cu(100) Cu/Cu(100) Cu/Cu(100) Cu/Cu(100) Cu/Cu(1,1,13) Cu/Cu(1,1,17) Cu/Cu(1,1,79) Ag/Cu(100) Ag/Cu(110)|| Ag/Cu(110)⊥ Ag/Cu(110)|| Ag/Cu(110)⊥ Ag/Cu(111)

1x10−7 d) 2x10−7 1x10−9 1.4x10−8 6.2x10−6 7.3x10−4 2.8x10−4 1.8x10−9 1.3x10−9 4.6x10−5

15 5 27 38 38 35 31 31 31 73 84 71 29 29 73

SEM, 3 QHAS, 0.1 HeS, TEAS, LEIS, <0.05 LEED, STM, STM, STM, RBS/EM, <0.5 RBS/EM, <0.5 RBS/EM, <0.5 SAM, 0…2.0 SAM, 0…2.0 RBS/EM, <0.5

96Nor1 93Ell1, 97Gra2 92Ern1 87Mig1, 92Ern1 92Bre1, 92Ern1 95Dur1, 92Ern1 95Gie1, 92Ern1 95Gie1, 92Ern1 95Gie1, 92Ern1 73Rou1, 92Ern1 73Rou1 73Rou1 85Gha1 85Gha1 73Rou1

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0.068 0.057 c) 0.093 0.13 0.13 0.12 0.11 0.11 0.11 0.27 c) 0.30 c) 0.26 c) 0.11 c) 0.11 c) 0.27 c)

0.31…0.35 0.13…0.29 0.13…0.18 0.21…0.32 0.06…0.11 0.13…0.16 0.23…0.28 0.21…0.28 0.25…0.46 0.18…0.36 0.18…0.33 0.18…0.33 0.24…0.36 0.24…0.36 0.18…0.36

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[Ref. p. 517

Table 15 (cont.) System

Do [m2/s]

Qdiff T/Tmelt Ω [kJ/mol] [Qdiff/Qdes]

Ag/Cu(311)|| Ag/Cu(311)⊥ Pb/Cu(100) CO/Cu(100) CO/Cu(100) S/Cu(111) Cl/Cu(111)

2.1x10−9 1.1x10−9 1x10−8 b) 10−5…10−6 b) 1x10−7 7x10−11

29 31 66 8…13 f) 3 25 19

0.11 c) 0.11 c) 0.22 c) 0.14…0.22 0.051 0.13 c) 0.08

0.24…0.36 0.24…0.36 0.35…0.50 0.10 0.074…0.18 0.60 0.08…0.11

Method, Coverage

Ref.

SAM, 0…2.0 SAM, 0…2.0 RBS, <0.1 LITD, QHAS, 0.3 QHAS, <0.16 AES, all

85Gha1 85Gha1 93Coh1 82Vis1, 72Tra1 98Gra3 91Hin1 96Wal1

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. a ) Qdes estimated by present authors in comparison to other crystal faces. b ) Estimate by original authors. c ) Estimate obtained by comparison to similar chemical systems. d ) Do estimated from reported attempt frequency ν using Eq. (3) by present authors (assuming hop length λ of 0.3 nm). f ) Data recalculated by present authors assuming a pre-exponential factor for kink-formation of 107 s−1; compared to those measured in Refs. [95Pea1] and [95Swa1].

Table 16. Adsorbates on other metals. System

Do [m2/s]

Qdiff T/Tmelt Ω [kJ/mol] [Qdiff/Qdes]

Na/Al(111) 1x10−8 d) 53 Cu/Mo(poly) 8.7x10−8 52 Ba/Mo(110) 67 24 Ga/Mo(110) 2x10−9 −7 60 Rh/Mo(110) 1x10 −9 29 In/Mo(110) 3x10 −9 48 Sn/Mo(110) 4x10 −7 21 CO/Mo(110) 1x10 Ag/Pd(100) 7x10−11 d) 34 K/Pd(111) 1.3x10−8 6.4 72 Cu/Pd(110)⊥ 49 Cu/Pd(110)|| Pd/Pd(100) 1x10−7 b) 54 Pd/Pd(111) 1x10−6 b) 73 50 CO/Pd(111) 1x10−4 CO/Pd(111) 2.2x10−7 17 CO/Al2O3 supported Pd 2x10−7 d) 25 C6H6 radical/Silica 1.7x10−8 15 supported Pd Au/Au(poly) 5x10−12 39 Au/Au(110) 71 2x10−7 d) 50 Ni/Au(110)-1x2 ⊥ 59 CH3O2C(CH2)15SH/Au(111) In/Ag(100) 30 Ni/Ag(poly) 2.5x10−6 109 Ag/Ag(100) 2x10−8 b) d) 39

Method, Coverage

Ref.

0.14 c) 0.14 0.097 c) 0.10 c) 0.099 c) 0.12 c) 0.091 c) 0.046 0.29 c) 0.20 c) 0.14 0.19 0.34 0.11 0.17 0.30 c)

0.32 0.27…0.37 0.13…0.17 0.06…0.11 0.06…0.11 0.06…0.11 0.04…0.08 0.09…0.12 0.06…0.10 0.16…0.23 0.11…0.16 0.05…0.40 0.21 0.19 0.09…0.15 0.11…0.16 0.10…0.20

STM, 0 95Bru2 SIE, 71Abr1 CPD, 0.01 79Ved1 FIM, 0 94Saa1 FIM, 0 90Dha1 FIM, 0 94Saa1 FIM, 0 94Saa1 FEM, 0.1 93Son1 HeS, 0 95Fel1 PEEM, <0.05 96Sna1 STM, 94Buc1 STM, 94Buc1 LEED, 93Eva1, 62Hon1 PAC, 90Hun1, 62Hon1 PEEM/LITD, 0.1 93Rot1, 69Tra1 PEEM, 0.33 97Sna1 NMR 87Sho1, 69Tra1 ALC-µSR, 0.08 94Sch1

0.11 0.19 0.1 c) 0.27 c) 0.14

0.22…0.58 0.097…0.13 0.46…0.66 0.070…0.18

AFM STM STM, 0.22 STM, PAC, 10−4 LEED, LEIS, <0.03

95Gob1 94Fre1, 62Hon1 97Hit1 94Stra1 90Fin1 70Pin1 96Lan1 Lando lt -Börnst ein New Ser ies III/42A1

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3.11 Surface diffusion on metals, semiconductors, and insulators

Table 16 (cont.) System

Do [m2/s]

Ag/Ag(111) 5x10−9 d) Ag/Ag(111) Polydimethylsiloxane/Ag Perfluorinated polyether krytox/Ag Gd/Gd(poly) Dy/Dy(poly) Er/Er(0110) Er/Er(1000) Pd/Ta(110) 2x10−7 b) Hg/Ta(111) W-Re/W-Re(poly) K/Re(001) 5.6x10−7 ReSi/Re(111) S/Re(100) 2x10−8 NH3/Re(100) 2.8x10−7 0.01 O/Re(2021) Re-Ir/Ir(100) 2x10−6 b) Re/Ir(100) Ir/Ir(100) ex 6.3x10−6 Ir/Ir(110)-1x1 ex 4x10−7 Ir/Ir(110)-1x1 || 6x10−6 Ir/Ir(110)-1x1 || 1.2x10−7 Ir/Ir(111) 8.8x10−7 Ir/Ir(111) 1x10−8 Ir/Ir(311) 1x10−7 Ir/Ir(331) 1x10−6 Ir2/Ir(110) || 3.7x10−9 Ir2/Ir(100) ex 2.6x10−8 Ir2/Ir(111) 1.2x10−9 Ir3/Ir(100) 2x10−8 Ir4/Ir(111) 7.5x10−10 Ir5/Ir(111) 3.1x10−10

Qdiff T/Tmelt Ω [kJ/mol] [Qdiff/Qdes]

Method, Coverage

Ref.

9.4 17 15 l) 21 l)

0.033 0.060 -

0.041…0.11 0.11…0.16 0.24 0.24

STM, 0.03 LEED, SIMS SIMS

95Bru1 95Luo1, 62Hon1 98Dei1 98Dei1

100 159 109 146 47 88 146 21 71 76 14 92 70 92 80 69 77 83 21 26 69 88 100 113 41 60 44 64

0.29 0.53 0.34 0.46 0.11 c) 0.50 0.18 c) 0.19 0.17 0.093 c) 0.12 c) 0.13 0.11 a) 0.12 a) 0.13 a) 0.034 a) 0.041 a) 0.11 c) 0.14 c) 0.16 a) 0.18 c) 0.065 c) 0.096 c) 0.07 c) 0.10 c)

0.29…0.39 FEM, 0.34…0.41 FEM, FEM, FEM, 0.05…0.06 FIM, 0 0.03…0.05 FEM, 0.05 0.53…0.66 FEM, 0.040…0.093 LITD/SHG, 1 0.08 FIM, 0 0.09 STM, 0.03…0.04 SHD, 0.05 0.10…0.12 FEM, 0.01 0.08…0.10 FIM, 0 0.08…0.12 FIM, 0 0.10 FIM, 0 0.08…0.18 FIM, 0 0.08…0.18 FIM, 0 0.11…0.12 FIM, 0 0.02…0.03 FIM, 0 0.03…0.04 FIM, 0 0.11…0.24 FIM, 0 0.11…0.24 FIM, 0 0.15…0.19 FIM, 0 0.15…0.19 FIM, 0 0.06…0.07 FIM, 0 0.08…0.10 FIM, 0 0.06…0.08 FIM, 0 0.10…0.12 FIM, 0

84Cis1, 62Hon1 84Cis1, 62Hon1 86Koz1, 62Hon1 86Koz1, 62Hon1 88Sch1 75Bob1 71Abr1 99Zha1 85Wri1 93Dun1, 88Kel1 93Far1, 90Ros1 96Son1 94Che1 92Che1 90Che1 91Tso1 91Tso1 91Che1 90Che2 89Wan1 97Tso1 97Tso1 91Tso1 91Tso1 89Wan2 89Wan2 89Wan2 89Wan2

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. ex : Exchange mechanism. a ) Qdes estimated by present authors in comparison to other crystal faces. b ) Estimate by original authors. c ) Estimate obtained by comparison to similar chemical systems. d ) Do estimated from reported attempt frequency ν using Eq. (3) by present authors (assuming hop length λ of 0.3 nm). l ) Do estimated by present authors to calculate Qdiff (assumed a ν = 1013 s−1 and λ = 0.3 nm).

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[Ref. p. 517

Table 17. Adsorbates on semiconductors. System

Do [m2/s]

Qdiff T/Tmelt Ω [kJ/mol] [Qdiff/Qdes]

Method, Coverage

H/Si(111)-7x7 H/Si(111) H/Si(100) Li/Si(100)-7x7 C70/Si(100) C84/Si(100) O/Si(111) Al/Si(111) Al/Si(100)-2x1 Si/Si(111) Si/Si(111) Si/Si(100) Si/Si(100) Si/Si(100) Si/Si(100) Si2/Si(100) Ga/Si(111) Ga/Si(100) Ge/Si(100) In/Si(111) Sn/Si(111) Sn/Si(100) Sb2/Si(100) Ag/Si(111) Ag/Si(111)-7x7 Ag/Si(100)-2x1 Cs/Si(100) Pb/Si(111)-7x7 Pb/Si(111) C6H6/Si(111)-7x7 O/Ge(100)-2x1 Ge/Ge(111) Ag/Ge(111) Pb/Ge(111)-c(2x8) NO2/GaAs(100) As4/GaAs(100)

1x10−7 8x10−8 7x10−16 2x10−5 e) 1x10−5 1x10−7 1x10−7 d) 1x10−8 1x10−5 3x10−14 d) 5x10−9 3.4x10−9 8x10−7 e) x) 1x10−13 -

145 146 75 141 13 155 89 68 17 105 71 65 93 126 109 91 46 73 81 69 31 96 117 38 32 68 45 62 117 91 3.9 34 44 52 25 23

SHD, 0.15 91Rei1 SHG, 0 96Han1 SHG, 0 96Han1, 89Sin1 NSR/NMR, 1x10−5 95Chr1 MB 95Moa1 MB 95Moa1 STM, 0.1 95Loz1 SEM, >1 78Nes1 STM, 0.05 96Zhu1 LEED, 74Bed1 STM, 93Voi1 STM, 0.07 91Mo1, 89Mo2 STM, 95Pea1 STM, 95Swa1 STM, 92Zan1 STM, 0 96Swa1 RBS/EM, all 87Zin1, 87Zin2 RBS/EM, all 87Zin1, 87Zin2 RBS/EM, all 90Zin1, 92Kra1 PAC, 0 92Kra1 RBS/EM, all 87Zin1, 87Zin2 RBS/EM, all 87Zin1, 87Zin2 STM, 0 95All1, 93Mo1 SEM, >1 92Ray1 SEM, 84Han1 SEM, 84Han1 SEM, 0.01 94Per1, 94Mil1 SEM, 0.01 96Gom1,95Hib1 STM, 0.13 95Hib1 STM, all 95Wol1 LITD, all 96Mor1 QHAS 99Gle1 SEM, 94Ven1 STM, 0 92Gan1,83Met1 MB, all 93Fel1 MB, 75Fox1

0.60 0.60 0.38 0.56 0.070 0.63 0.22 c) 0.060 c) 0.23 0.16 0.14 0.20 0.28 0.24 0.20 0.24 0.35 0.20 c) 0.37 0.090 0.23 0.35 0.16 0.16 0.34 0.23 0.35 0.67 1.0 0.091 0.18 0.40 0.67 0.63

0.40…0.43 0.31…0.42 0.33…0.44 0.67…0.73 0.34…0.45 0.34…0.45 0.24…0.30 0.40…0.58 0.17…0.34 0.48…0.53 0.38…0.47 0.20…0.33 0.30…0.41 0.29…0.36 0.42 0.18…0.24 0.37…0.48 0.37…0.48 0.47…0.56 0.13…0.33 0.37…0.48 0.37…0.48 0.24…0.32 0.40…0.50 0.29…0.47 0.29…0.47 0.19…0.21 0.18…0.20 0.27…0.33 0.18 0.18…0.54 0.87…0.95 0.52…0.64 0.25…0.29 0.17…0.30 0.21…0.29

Ref.

c

) Estimate obtained by comparison to similar chemical systems. ) Do estimated from reported attempt frequency ν using Eq. (3) by present authors (assuming hop length λ of 0.3 nm). e ) Calculation by present authors from original work (assuming that data fit Arrhenius's law). x ) Average values obtained by present authors. d

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3.11 Surface diffusion on metals, semiconductors, and insulators

Table 18. Adsorbates on insulators. System

Do [m2/s]

Qdiff T/Tmelt Ω [kJ/mol] [Qdiff/Qdes]

C2H6/C(poly) Ag/C(poly) CdCl2/C(poly) In/C(poly) Au/C(poly) AuCl3/C(poly) UO2Cl/C(poly) UO2Ac2/C(poly) HD/HD H2O/H2O Kr/Ar(100) NO2/Al2O3(111) C6H6/Al2O3 C6H6/SiO2 C6H6 radical/ carbosil C6H6 radical/Silica gel 2,4,6 TrimethylCyclohexadienyl/Silica CH4/MgO(100) CO2/MgO(100) Na/MgO(100) Cu/Yba2Cu3O7−x CO2/NaCl(100) Au/NaCl Ag/NaCl Au/KCl Ag/KCl Au/KBr Ag/KBr Ag/AgBr(111) Li2F2/CaF2(111) Au/NaF

1.6x10−6 e) 2x10−8 e) 8x10−8 e) 5x10−8 e) 8x10−8 e) 6x10−8 e) 9x10−8 e) 1x10−7 e) 5x10−9 e) 1.4x10−8 e) 3x10−11 1x10−8 3x10−9 1x10−8 7.2x10−9

5.9 80 83 81 85 85 86 83 0.13 24 0.58 23 l) 12 25 10 11 13

0.24 0.13 0.52 0.60 0.5 0.20

0.020…0.030 QENS, 0.54 0.078 STEM, 0.078 STEM, 0.078 STEM, 0.078 STEM, 0.078 STEM, 0.078 STEM, 0.078 STEM, 0.42…0.93 QENS, <1 0.68…0.73 NMR, 0.060…0.13 TSRLS, 0.04 ESR, 0.07…0.12 NMR, 0.16 NMR, 0.1 0.11…0.18 ALC-µSR, 0.05 0.11…0.18 ALC-µSR, 0.05 0.11…0.18 ALC-µSR, -

86Cou1 80Utl1 80Utl1 80Utl1 80Utl1 80Utl1 80Utl1 80Utl1 93Mar1 87Miz1 98Zim1 95Sch1 89Bod1 70Bod1, 72Bod1 94Sch1 94Sch1 95Sch2

1.3x10−11 5x10−8 4x10−7 6x10−8 1x10−8 1x10−6 1x10−9 2x10−8 e) 1x10−7

3.7 l) 11 l) 4.8 146 3.6 26 23 20 30 34 17 33 70 8

0.24 0.30 0.10 0.40 0.38 0.32 0.58 0.44 0.39 0.59 c) 0.13

0.03 QENS, 0.8 0.03 LITD, 0 0.050…0.080 AES, 0.35…0.38 SEM, 0.040…0.090 FTIR, 0.01 0.35…0.63 SEM, 0.35…0.53 SEM, 0.35…0.60 SEM, 0.36…0.55 SEM, 0.37…0.57 SEM, 0.37…0.57 SEM, 0.52…0.61 AR, 0.55…0.69 TD 0.33…0.49 SEM, -

87Bie1 92Mei1 75Jan1 94Han1 92Hei1, 93Hei1 74Rob1 76Don1 74Rob1 76Don1 74Rob1 76Don1 71Tan1 98Klu1 74Rob1

c

Method, Coverage

) Estimate obtained by comparison to similar chemical systems. ) Calculation by present authors from original work (assuming that data fit Arrhenius's law). l ) Do estimated by present authors to calculate Qdiff (assumed a ν = 1013 s−1 and λ = 0.3 nm). e

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[Ref. p. 517

3.11.3.2.1.2 Mass transfer diffusion

Table 19. Mass transfer diffusion on metals. System

Do [m2/s]

Qdiff [kJ/mol]

Tregime Ω [Qdiff/Qdes]

T/Tmelt

Method, Coverage

Ref.

Ni (three faces) Ni(110) Cu Ag/Cu(110) || Ag/Cu(110) ⊥ Ag/Cu(111) Pt Ba/Pt(poly) H/Pt(111) D/Pt(111) O/Pt(110)-1x2 || CO/Pt(111) Au Au Re Nb Ta Mo Mo Mo/Mo(110) Dy/Mo(211) W W W W W Fe/W Cs/W(110) Th/W(311)

1x10−6 9x10−7 3x10−6 1.3x10−6 4x10−7 4x10−7 1x10−4 0.1 0.2 0.075 1x10−8 5x10−6 4.3x10−5 3x10−5 1.6x10−4 1x10−6 e) 1x10−3 4x10−4 2.4x10−5 6.3x10−5 4x10−8 1.5x10−9 2.3x10−5 5x10−5 -

84 73 71 39 39 96 109 130 30 46 126 52 88 84 218 192 197 201 234 231 145 299 301 276 264 297 151 54 243 193

0.17 0.15 0.21 0.14 0.14 0.35 0.23 0.27 0.40 0.63 0.52 0.39 0.24 0.23 0.28 0.27 0.25 0.30 0.36 0.35 a) 0.43 0.39 0.39 0.36 0.34 0.38 0.33 c) 0.18 0.33 -

Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low Low

0.64…0.80 0.45…0.67 0.50…0.86 0.41…0.45 0.41…0.45 0.41…0.45 0.57…0.77 0.63…0.69 0.10…0.12 0.10…0.12 0.30…0.33 0.14…0.21 0.30…0.33 0.40…0.75 0.65…0.80 0.53…0.73 0.35…0.45 0.50…0.65 0.58…0.90 0.35…0.42 0.25…0.28 0.24…0.34 0.50…0.66 0.50…0.75 0.51…0.88 0.45…0.59 0.44…0.48 0.15…0.20 0.41…0.49 0.32…0.48

SD SD GBG PE PE PE SD FEM LITD LITD PEEM SHREELS GBG SD, SGB GBG GBG TB TB GBG FEM CPD FEM TB TB GBG AR AR PE CPD SEM

69Mil1, 94Ald1 78Bon1, 94Ald1 64Bra1, 62Hon1 95Kur1 95Kur1 95Kur1 62Bla1, 78Bas2 76Kul1 86See1 86See1 94Oer1, 70Fus1 94Fro1, 86See2 89Lin1, 62Hon1 64Geg1,62Hon1 72All1, 62Hon1 72All1, 62Hon1 74Wan1,62Hon1 76Bin1, 62Hon1 72All1, 62Hon1 95Ste1 98Lob1 73Pic1 60Bar1, 68Plu1 85Bin1, 68Plu1 69All1, 68Plu1 73Koz1, 68Plu1 73Koz1 69Lov1,67Gav1 69Ved1 95Zha1

0.53 0.55 0.61 0.57 0.40 0.33 0.38 0.44…0.59 0.50 0.59 0.93 0.47

High High High High High High High High High High High High High High

0.79…0.92 0.59…0.64 0.78…0.91 0.78…0.91 0.80…0.98 0.70…0.93 0.67…0.91 0.81…0.98 0.64…0.94 0.80…0.89 0.79…0.95 0.50…0.59 0.50…0.59 0.75…0.91

SD GBG SD SD SD SD SD SD GBG GBG GBG AR AR SD, GBG

74Mil1, 62Hon1 79Nii1,62Hon1 70Mil1,62Hon1 70Mil1,62Hon1 69Mil1,62Hon1 67Mai1, 62Hon1 78Bon1,62Hon1 66Col1,62Hon1 61Gjo1,62Hon1 79Yos1,62Hon1 67Per1,62Hon1 83Dan1 82Dan1 64Geg,62Hon1

Fe/Zr( 10 1 0 ) Co Fe Fe(110) [3% Si] Fe(100) [3% Si] Ni(three faces) Ni(three faces) Ni(110) Cu(eight faces) Cu(100) Ag Ag Br/Ag(poly) I/Ag(poly) Au

0.14 222 10 230 10 253 2.5 237 0.02 198 −3 163 1x10 0.047 188 0.01…0.3 150…200 0.065 170 0.25 167 264 1x104 e 5x10−3 e) 98 ) e 157 e) 200 ) e 167 0.3 )

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3.11 Surface diffusion on metals, semiconductors, and insulators

System

Do [m2/s]

Qdiff [kJ/mol]

Tregime Ω [Qdiff/Qdes]

T/Tmelt

Method, Coverage

Ref.

Au Pb(110) Pb(110) Pb(111) V(111) Cr Nb Ta Mo Mo Mo(110) Mo(100) Cu/Mo(110) Cu/Mo(100) Mo-Re,1:33 W W(100)

800 120 e) x) 6.6x10−3 8.3x10−3 0.3 2.2x10−3 0.069 8.2x10−3 0.04 2.9 0.2 0.1 1.5x10−3 1 7.6

272 125 96 87 201 209 339 270 322 285 288 402 155 155 238 536 536

0.76 0.64 0.49 0.44 0.39 0.53 0.47 0.34 0.49 0.43 0.44 0.61 0.46 0.46 0.36 0.70 0.70

0.90…0.97 0.50…0.55 0.74…0.92 0.50…0.55 0.64…0.70 0.69…0.78 0.50 0.49…0.73 0.65…0.85 0.53…0.81 0.76…0.83 0.66…0.83 0.38…0.42 0.38…0.42 0.53…0.81 0.75…0.90 0.74…0.84

SD TB QHAS TB SD GBG HBF TB TB GBG SD SD IBM IBM GBG TB SD

68Mcl1,62Hon1 95Kui1, 93Kui1 90Fre1,62Hon1 95Kui1 83Yu1,62Hon1 69All1,62Hon1 73Ait1,62Hon1 81Hok1,62Hon1 76Bin1,62Hon1 81Sri1,62Hon1 70Sin1,62Hon1 70Sin1,62Hon1 95Bek1,62Hon1 95Bek1,62Hon1 81Sri1,62Hon1 85Bin1, 68Plu1 69Bow1,68Plu1

High High High High High High High High High High High High High High High High High

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. a ) Qdes estimated by present authors in comparison to other crystal faces. c ) Estimate obtained by comparison to similar chemical systems. e ) Calculation by present authors from original work (assuming that data fit Arrhenius's law). x ) Average values obtained by present authors.

Table 20. Mass transfer diffusion on semiconductors. System

Do [m2/s]

Qdiff Tregime Ω [kJ/mol] [Qdiff/Qdes]

T/Tmelt

Method

Si/Si(100) Si/Si(100) Si/Si(111) Fe/Si(111) Ga/Ge(100) o) Ge/Ge(100) Ge/Ge(111) Ag/Ge(111) g) Ga/GaAs(100) Ga/GaAs(100)  n) Ga/GaAs(100) ⊥ n) Al/AlAs(100) In/Si(111) Sb/Si(111) Ge/Si(111) Si/Si(poly) Si/Si(100) Si/Si(100) Si/Si(100) Si/Si(100)

7x10−8 e) m) 2x10−6 d) 1x10−4 8x10−6 d) 3x10−7 8.5x10−10 6x10−9 e) 6x10−9 e) 0.3 1 0.06 94 0.1 9.5x10−4 0.15

135 212 125 73 125 183 106 144 126 39 68 167 176 251 238 298 116 222 213 260

0.46…0.61 0.43…0.45 0.37 0.51…0.59 0.40…0.45 0.43…0.49 0.35…0.38 0.48…0.62 0.47…0.50 0.65…0.68 0.60…0.66 0.51…0.54 0.43…0.51 0.52…0.57 0.57…0.66 0.86…0.98 0.66…0.86 0.63…0.80 0.77…0.86 0.61…0.67

REM 95Doi1 STM/LEED 91Web1 RHEED 87Ich1 MEIS 96Woh1 FEM 92Hon1 STM 96Che1 RHEED 93Fuk1 SAM 83Sul1, 79Ber1 RHEED 85Nea1, 90Gib1 RHEED 90Hat1 RHEED 90Hat1 RHEED 93Shi1 SHM 96All1 SHM 95All1 SHM 96All2 GBG 81Rob1 LEEM 94Bar1 SD 94Kee1 SSm 81Gav1 REM 95Doi1

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0.30 0.47 0.28 0.48 0.28 0.58 0.36 0.11 0.19 0.43 0.64 0.75 0.58 c) 0.66 0.26 0.50 0.47 0.58

Low Low Low Low Low Low Low Low Low Low Low Low High High High High High High High High

Ref.

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[Ref. p. 517

Table 20 (cont.) System

Do [m2/s]

Si/Si(111) 0.9 −5 Au/Si(111)- 3X 3 7x10 Au/Si(111)-5x1 4.8x10−3 Au/Si(111)-7x7 0.12 K/Si(111) 8 Rb/Si(111) 100 Cs/Si(111) 1x103 Ge/Ge(111) 0.87 Sb/Ge(111) g) 0.12 In/Ge(111) g) Ga/GaAs(100) 0.47 i) Ga/GaAs(111)B -

Qdiff Tregime Ω [kJ/mol] [Qdiff/Qdes]

T/Tmelt

Method

Ref.

347 125

0.77 0.46

High High

0.51…0.57 0.55…0.64

LEED AES

78Ols1 84Gav1

154 192 155 168 179 368 199 138 268 272

0.57 0.71 0.88 0.87 0.93 0.98 0.73 0.55 0.77 0.78 i)

High High High High High High High High High High

0.64…0.70 0.66…0.71 0.47…0.66 0.47…0.66 0.47…0.66 0.38…0.41 0.68…0.76 0.53…0.63 0.54…0.62 0.47…0.53

AES AES SIM SIM SIM RHEED SHM SHM RHEED RHEED

84Gav1 84Gav1 92Sto1 92Sto1 92Sto1 93Fuk1 92Sch1 94Sun2 90Gib1, 89Oht1 90Shi1

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. c ) Estimate obtained by comparison to similar chemical systems. d ) Do estimated from reported attempt frequency ν using Eq. (3) by present authors (assuming hop length λ of 0.3 nm). e ) Calculation by present authors from original work (assuming that data fit Arrhenius's law). compared to those measured in Refs. [95Pea1] and [95Swa1]. g ) Extrapolated to zero coverage. i ) Along [ 1 1 0 ] direction. Do along [110] is a factor of 4 smaller. m ) Diffusion on the 1x2 surface. Do on the 2x1 surface is a factor of 10 slower. n ) Diffusion near the edge of the (111)A surface. o ) Diffusion in [110] direction. Do along [ 1 1 0 ] direction is a factor of 10 slower.

Table 21. Mass transfer diffusion on insulators. System

Do [m2/s]

Sb/C(poly) 1.6 CO2/Porous glass 1x10−8 e) (96% SiO2/4% B2O3) C3H3/Porous glass 2x10−8 e) (96% SiO2/4% B2O3) Al2O3 4.8x10−5 Ag/Al2O3 8.1x10−3 Cr/Al2O3 6x10−8 e) j) NiO/Al2O3 7.4x10−6 Co/MgO(100) 8x10−9 e) j) Cr/MgO(100) 1.5x10−9 e) j) Co/NiO 2.2x10−5 e) j) Cr/MgAl2O4 1.3x10−8 e) j) 0.43 e) j) Pu/UO2 s Bi2Sr2CaCu2O8+x (2212) ) 1x10−6 b) YBa2Cu3O7−x s) 5x10−13 148 YBa2Cu3O7−x(100) 42 YBa2Cu3O7−x(001) H2

Qdiff

Tregime

T/Tmelt

Method

Ref.

68 4.3

Low Low

0.070…0.12 1996 K p)

TEM FBC

95Bar1 98Tuc1

4.0

Low

1996 K p)

FBC

98Tuc1

256 192 119 147 59 110 138 121 285 212 68 39 66 0.18

Low Low Low Low Low Low Low Low Low Low Low Low Low Low

0.55…0.75 0.41…0.44 0.43…0.69 0.66…0.74 0.33…0.43 0.31…0.42 0.45…0.65 0.40…0.49 0.53…0.62 903…1013 K p) 993 K p) 670…720 K p) 670…720 K p) 0.18…0.25

GBG SEM SS q) SEM SS q) SS q) SS q) SS q) AR r) RHEED RHEED MEIS MEIS SAW

94Tso1 95Bek1 85Stu1 77Hir1 93Stu1 93Stu1 93Stu1 85Stu1 76Fur1 95Ish1 92Kar1 99Kar1 99Kar1 95Cla1

[kJ/mol]

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3.11 Surface diffusion on metals, semiconductors, and insulators

System HD D2 CaTiO3 Si3N4 Al2O3 Al2O3 Al2O3 Al2O3(0001) Al2O3(1120) Al2O3 Al2O3 Al2O3 Al2O3(1120)-<0001> Al2O3(1120)-<1100> MgO CO/MgO(100) Cr/MgO(100) Cr/MgAl2O4(111) UO2 UO2 UO2 Pu/UO2

Do [m2/s]

[kJ/mol]

0.18 41 1.5x105 j) 1.5x106 105 277 0.029 24 400 48 430 3.7x105 j) 40 e) p) 0.37 30 e) j) 34 4x104 500 3x1011 e) j)

0.28 0.38 330 987 452 519 544 448 498 385 397 494 397 444 408 318 321 335 452 548 301 720

Qdiff

Tregime

T/Tmelt

Method

Ref.

Low Low High High High High High High High High High High High High High High High High High High High High

0.19…0.25 0.19…0.25 0.65…0.72 0.74…0.78 0.76…0.98 0.72…0.80 0.80…0.87 0.73…0.85 0.73…0.85 0.62…0.66 0.83…0.90 0.74…0.87 0.68…0.94 0.68…0.94 0.76…0.83 0.43…0.50 0.43…0.50 0.49…0.57 0.56…0.63 0.53…0.63 0.72…0.85 0.62…0.67

SAW SAW GBG GBG GBG Sintering SD SD SD Sintering STV STV Sintering Sintering Sintering SS q) SS q) SS q) GBG GBG AR AR

95Cla1 95Cla1 89Ceh1 81Rob1 86Gad1 87Saj1 75Mar1 75Hua1 75Hua1 78Mor1 72Yen1 78Gup1 93Lan1 93Lan1 87Saj1 93Stu1 93Stu1 85Stu1 71Mai1 73Hod1 84Zho1 76Fur1

b

) Estimate by original authors. ) Calculation by present authors from original work (assuming that data fit Arrhenius's law). compared to those measured in Refs. [95Pea1] and [95Swa1]. p ) Melting temperature not known to present authors. q ) Edge-source method. r ) Point-source method. s ) Diffusion on SrTiO3(100) substrate. e

3.11.3.2.1.3 Impurity-influenced diffusion

Table 22. Impurity-influenced diffusion. Enhancers System

Impurity, Coverage

Do [m2/s]

Qdiff T/Tmelt [kJ/mol]

Method, Coverage

Ref.

W/W W/W Si/Si(111)-7x7 Si/Si(111) t)

Ni, 0 Ni, 1 Ga, 0 Ga, 0.33

-

299 87 193 48

FEM,FEM,RHEED, RHEED, -

73Pic1 73Pic1 92Nak1 92Nak1

t

) 0.33 ML of Ga induced Si(111)- 3 x 3 structure.

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0.24…0.34 0.24…0.34 0.49…0.58 0.46…0.52

490

3.11 Surface diffusion on metals, semiconductors, and insulators

[Ref. p. 517

Inhibitors System

Impurity, Coverage

Li/W(110) O, 0 Li/W(110) O, >1 K/W(110) O,0 K/W(110) O, >1 H/Ru(100) Cu, 0 H/Ru(100) Cu, 0.15 CO/Ru(100) Cu, 0 CO/Ru(100) Cu, 0.08 CO/Ni(110)-3x1 ⊥ S, 0 CO/S-Ni(110)-3x1 ⊥ S, 0.2 K/Re(100) CO, 0 K/Re(100) CO, 0.095

Do [m2/s]

Qdiff T/Tmelt [kJ/mol]

Method, Coverage

Ref.

3x10−7 d) 5x10−7 d) 2x10−5 d) 1x10−5 d) 6.8x10−9 3.7x10−4 4x10−4 1.7x10−7 2x10−12 3x10−8 5.6x10−7 0.03

12 125 41 72 12 41 40 23 9.2 31 21 63

CPD, 0.02 NSR, 10−3 TIM, TIM, LITD, 0.25 LITD, 0.25 LITD, 0.25 LITD, 0.25 LITD/ SHD, 0.03 LITD/ SHD, 0.03 LITD/ SHD, 1 LITD/ SHD, 1

82Lob1, 74Med1 91Rie1 82Bay1 82Bay1 95Bro1 95Bro1 95Bro1 95Bro1 95Xia1 95Xia1 99Zha1 99Zha1

0.02…0.05 0.29…0.31 0.23…0.28 0.23…0.28 0.081…0.11 0.11…0.13 0.097…0.12 0.10…0.12 0.08…0.11 0.12…0.14 0.040…0.093 0.040…0.093

⊥ : Perpendicular to atomic rows, step edges. d ) Do estimated from reported attempt frequency ν using Eq. (3) by present authors (assuming hop length λ of 0.3 nm). Neutrals System

Impurity, Coverage

Do [m2/s]

Qdiff T/Tmelt [kJ/mol]

Method, Coverage

Ref.

H2O/W(121)-(111) H2O/W(121)-(111)

Au, 0 Au, 2.5

5x10−3 e) 5x10−3 e)

35 35

FEM,>1 FEM, >1

99Bry1 99Bry1

0.088…0.095 0.088…0.095

e

) Calculation by present authors from original work (assuming that data fit Arrhenius's law).

3.11.3.2.2 Non-thermal diffusion Table 23. Tunnelling diffusion. System

Dtn [m2/s]

Ttn [K]

Method, Coverage

Ref.

H/W(100) || H/W(100) ⊥ D/W(100) || D/W(100) ⊥ H/W(110) D/W(110) 3 H/W(110) H/Ni(100) H/Ni(100) H/Ni(111) H/Ni(111) D/Ni(100) D/Ni(100)

3x10−16 8x10−17 1x10−17 7x10−17 2x10−9 9x10−10 5x10−10 2x10−16 2x10−15 1x10−15 2x10−14 2x10−16 2x10−15

140 140 140 140 150 72 72 100 160 125 100 100 170

FEM, 0.06 FEM, 0.06 FEM, 0.06 FEM, 0.06 FEM, 0.1 FEM, 0.1 FEM, 0.1 FEM, 0.25 LLD, 0.05 LLD, 0.05 FEM, 0.08 FEM, 0.15 LLD, 0.05

95Dan1 95Dan1 95Dan1 95Dan1 82Dif1,85Wan1 82Dif1,85Wan1 82Dif1,85Wan1 91Lin1 92Lee1 95Won1, 93Lee1 91Lin1 91Lin1 92Lee1 Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 517]

491

3.11 Surface diffusion on metals, semiconductors, and insulators

System

Dtn [m2/s]

Ttn [K]

Method, Coverage

Ref.

D/Ni(111) D/Ni(111)

1x10−14 1x10−15

100 125

FEM, 0.05 LLD, 0.05

91Lin1 95Won1, 93Lee1

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges.

3.11.3.2.3 Diffusion under liquids Table 24. Intrinsic Diffusion under liquids. System

Ambient

Do [m2/s] Ag/Ag(111) H2O 1x10−7 e) Pt/Pt(111) 1 M H2SO4 Cr/Fe2O3(100) 10−6 M Cr(NO3)3 2x10−7 b) C-18 ligands/silica H2O 1x10−7 b) (3-(1-pyrenyl) propyl)- H2O 2x10−9 e) demthyl-chlorosilane/silica

Qdiff [kJ/mol] 28 19 84 27 68

T/Tmelt Ω [Qdiff/Qdes] 0.10 0.22...0.27 0.041 0.13…0.16 0.16 0.14 0.14…0.15

Method Ref. EC REM STM FE FE

93Pro1 88Two1 93Egg1 95Han1 94Wan2

b e

) Estimate by original authors. ) Calculation by present authors from original work (assuming that data fit Arrhenius's law).

Table 25. Mass transfer diffusion under liquids. System Au/Au Au/Au Au/Au(100) Au/Au Au/Au(111) Au/Au Au/Au Au/Au Au/Au(111) Pt/Pt C6H6/NaX zeolite C6H6/NaX zeolite Phenol/XAD-7 Phenol/XAD-4 Benzaldehyde/XAD-7 Benzaldehyde/XAD-4 Acetone/Al2O3 Ethylacetate/Al2O3 1-Nitropropane/Al2O3 Propionic acid/C(poly) Pentachlorophenol/ C(poly)

Lando lt -Bö rnst ein New Ser ies III/42A1

Ambient 0.5 M H2SO4 0.5 M H2SO4 0.05 M H2SO4 1 M HClO4 10-5 M KCN 0.05 M NaF 0.05M K2SO4 0.05 M KCl 0.1 M HClO4 0.5 M H2SO4 Cyclohexane n-hexane H2O H2O H2O H2O Cyclohexane Cyclohexane Cyclohexane H2O H2O

Do [m2/s]

Qdiff

6x10−8 e) 2x10−9 e) 5x10−7 e) 1x10−9 e) 10-8…10−5 b) 10-8…10−5 b) 10-8…10−5 b) 10-8…10−5 b) 10-8…10−5 b) 10-8…10−5 b) 10-8…10−5 b) 10-8…10−5 b) 10-8…10−5 b) 10-8…10−5 b) -

59 54 74 c) u) 63 73 u) 65 u) 61 u) 57 u) 75 80 27 25 13 18 16 17 8 6 9 8 40 u)

T/Tmelt

Method Ref.

0.20…0.24 0.22…0.25 0.22 0.23 0.22 0.22 0.22 0.22 0.22 0.13…0.16 313…348 K p) 313…353 K p) 303…328 K p) 303…328 K p) 298…348 K p) 298…348 K p) 0.13…0.15 0.13…0.15 0.13…0.15 0.080…0.084 0.08

SSm SSm AFM SSm SSm SSm SSm SSm STM SSm FBC FBC FBC FBC FBC FBC FBC FBC FBC FBC FBC

[kJ/mol]

90Alo1 93Don1 95Ike1 92Don1 92Mcc1 93Gar1 93Gar1 93Gar1 91Tre1 90Alo2 88Awu1,98Miy1 88Awu1,98Miy1 98Miy1 98Miy1 74Kom1,98Miy1 74Kom1,98Miy1 88Ma1,98Miy1 88Ma1,98Miy1 88Ma1, 98Miy1 82Suz1,98Miy1 98Sla1

492

3.11 Surface diffusion on metals, semiconductors, and insulators

[Ref. p. 517

Table 25 (cont.) System

Ambient

Do [m2/s]

Qdiff

T/Tmelt

Method Ref.

[kJ/mol]

CH3OH/H2O 10−8…10−5 b) 3 95Miy1,98Miy1 288…308 K p) FBC −7 e hexa-n-pentyl- 3x10 ) 96Mao1 30 348…380 K p) FBC oxytriphenylene b ) Estimate by original authors. e ) Calculation by present authors from original work (assuming that data fit Arrhenius’s law). p ) Melting temperature not known to present authors. u ) Do equated to 3x10−6 m2/s (from the average Do value from mass transfer diffusion on metals). C6H6/ODS-silica gel 4-phenylazo phenol

3.11.3.2.4 Computational data Abbreviations for computational methods CEM DFT EAM EMT MC MD RGL SW S-Morse Shell-Morse TB Tersoff

Corrected embedded model Density functional theory Embedded atom model Effective medium theory Monte Carlo Molecular dynamics Rosato-Guillope-Legrand potential Stillinger-Weber potential Semi-empirical Morse potential Shell-Morse potential Tight binding model Tersoff potential

Table 26. Calculated diffusion parameters compared to available experimental data. System

W/W(110)

Do [m2s] Calc.

Qdiff Computational Ref. [kJ/mol] method Calc. Calc. −7

Do [m2/s] Exp.

Qdiff [kJ/mol] Exp.

95

MD (EMT)

91Lom1, 82Dol1 0.1…6.2 67…88 x10−7

W/W(211)  2.2x10−7

86

MD (EMT)

91Lom1, 82Dol2 7.1x10−11 54…80 …1.5x10−6

Xe/W(110) Ni/Ni(100) Ni/Ni(110)

2.5x10−7 3.2x10−7 -

4.6 59 17

MD (LJ) 84Val1 MD-MC (CEM) 92San1 MD-MC (CEM) 94Per2

Ni/Ni(311) Ni/Ni(331) Ag/Ni(100) Pt/Ni(100) Pd/Pt(100) Ag/Pt(111)

1.9 34 ex 20 14 2.3x10−7 d) 41 8.3x10−8 d) 53 7.4x10−8 d) 47 8 v)

3.6x10

MD (S-Morse) 80Fla1 MD-MC (CEM) 94Per2 MD (S-Morse) 80Fla1 MD (S-Morse) 80Fla1 MD-MC (CEM) 92San1 MD-MC (CEM) 92San1 MD-MC (CEM) 92San1 MD (EMT) 95Bru1

5 7x10−12 61 1x10−13… 22…52 1x10−9 2x10−10 2x10−7 1x10−5 w) 1x10−7 ex 1x10−7 2x10−7 d)

29 43 100 w) 67 ex 67 15

Ref. Exp. 75Tso1, 75Gra1, 70Bas1, 90Cho1, 78Kel1 88Wan1, 70Bas1, 80Fla1, 92Gon1, 90Cho2 80Che1 h) 80Tun1, 94Ald1 97Gra1, 80Tun1

80Tun1 80Tun1 70Pin1 92Kel1 91Kel2 95Bru1

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 517] System

Do [m2s] Calc.

Computational Ref. Qdiff [kJ/mol] method Calc. Calc.

Pt/Pt(110)

-

56

MD (S-Morse) 80Fla1

Pt/Pt(111)

Pt/Pt(331)

-

C/Pt(111) O/Pt(111)

3.4x10−7 1.5x10−7

37 v) 32 v) 13 v) 15 s) 16 15 v) 36 48 61 55 61 52 51 109 76

DFT DFT MD (EMT) MD (EMT) MD (RGL) MD (EMT) DFT MD (S-Morse) MD (EAM) MD (EAM) MD (S-Morse) MD (EAM) MD (EAM) MD (EMT) MD (EMT)

Pt2/Pt(111) Pt/Pt(311)

Xe/Pt(111) Rh/Rh(100)  4x10−7 6.6x10−7 1.0x10−7 Rh/Rh(110) Rh/Rh(111) 7.1x10−8 Rh/Rh(311) Rh/Rh(331) Pt/Rh(100) 8.3x10−8 H/Ru(100) 1x10−7 Cu/Cu(100)

Cu/Cu(111) Pd/Cu(100) Ag/Cu(100) Pd/Pd(100) Ag/Pd(100) Ni/Ag(100) Ag/Ag(100)

Ag/Ag(110)

493

3.11 Surface diffusion on metals, semiconductors, and insulators

3.6x10−7

5 87 100 77 68 46 26 4.8 42 60 64 16 64

Qdiff [kJ/mol] Exp.

−9

2x10 … 75…80 1x10−7 1x10−7 d) 25

94Fei1 96Boi1 95Liu1 94Sto1 99Mac1 98Kyu1, 95Liu1 1.9x10−8 99Boi1 80Fla1 1x10−10 91Liu1 94Vil1 80Fla1 4x10−8 91Liu1 94Vil1 91Lom1, 83Dol1 2x10−5 91Lom1, 83Dol1, 2x10−8 b) 86Lev1 MC (DFT) 95Sho1 3.4x10−8 MD (EMT) 91Lom1, 82Mcd1 1x10−7 b) MC 91Lom1, 85Vot1 MD-MC (CEM) 92San1 MD (S-Morse) 80Fla1 MD (S-Morse) 80Fla1 3x10−5 MD (EMT) 91Lom1, 82Mcd2 2x10−8 MD (S-Morse) 80Fla1 MD (S-Morse) 80Fla1 1…2x10−7 MD (S-Morse) 80Fla1 1x10−6 MD-MC (CEM) 92San1 2x10−7 MC 91Lom1, 87Mak2, 0.63…6.8 90Tru1 x10−7 MD-MC (CEM) 92San1 0.1…1.4 x10−8

MD (EAM) 96Eva1 3.4x10−7 41 DFT 96Eva1, 94Lee1 67 v) 96Eva1, 91Liu1 2x10−6 ex 69…76 ex MD (EAM) 8.7 MD (EMT) 96Kal1 MD-MC (CEM) 92San1 1.0x10−7 56 MD-MC (CEM) 92San1 1.6x10−7 35 MD-MC (CEM) 92San1 9.7x10−8 59 MD/MC-CEM 92San1 1.8x10−7 41 MD (EMT) 95Fel1 2x10−7 d) 40 MD-MC (CEM) 92San1 1.1x10−7 69 MD-MC (CEM) 92San1 2.3x10−8 d) 23 MD (TB) 98Pan1 1.4x10−7 41 MD-EAM 96Boi1 3.1x10−7 46 DFT 96Yu1 48 v) MD (TB) 98Pan1 4x10−6 ex 75 ex 25 MD-MC (CEM) 94Per2 MD-MC (CEM) 94Per2 33 ex

Lando lt -Bö rnst ein New Ser ies III/42A1

Do [m2/s] Exp.

1x10−8 6x10−6 1x10−7 7x10−11 d)

Ref. Exp. 98Lin1, 78Bas2 98Kyu1, 96Bot1

36

98Kyu1

50

78Bas2

80

78Bas2

130 130

78Mar1, 88Pal1 68Lew1

5 80

93Mei2 96Kel1, 73Ayr1

58 15

73Ayr1 73Ayr1

52…56 62 89 12…17

96Kel1, 73Ayr1 73Ayr1 93Kel1 95Bro1, 86Mak1

27…35

92Ern1, 87Mig1, 92Bre1, 95Bur1, 95Gie1

66 73 54 34

93Coh1 73Rou1 93Eva1 95Fel1

w 2.5x10−6 w) 109 ) −8 b d 2x10 ) ) 39

70Pin1 96Lan1

494

3.11 Surface diffusion on metals, semiconductors, and insulators

[Ref. p. 517

Table 26 (cont.) System

Ag/Ag(111)

H/Si(100)2x1  H/Si(100)2x1 ⊥ Si/Si(100)2x1  Si/Si(100)2x1 ⊥ Si/Si(100)2x1 ⊥ Si/Si(100)2x1 ⊥ Si2/Si(100)2x1 Si/Si(111) Ge/Si(111) Al/Si(100)2x1  Al/Si(100)2x1 ⊥ Al/Si(111) Pb/Si(111)7x7 Pb/Ge(111)c(2x8)

Do [m2s] Calc.

Computational Ref. Qdiff [kJ/mol] method Calc. Calc.

Do [m2/s] Exp.

2.8x10−8 1.5x10−8 1x10−3

6.7 5.3 12 v) 6 v) 159

MD (TB) MD (EAM) MD (EMT) MD (EMT) MC

98Pan1 96Boi1, 98Pan1 90Jon1 95Bru1 94Wu1

5x10−9 d) 9.4

95Bru1

7x10−6

75

96Han1

4x10−5

125 v) 263

DFT MC

93Vit1 94Wu1

-

173 v) 58 v)

DFT DFT

93Vit 92Bro1

1x10−7

65

91Mo1

4.8x10−7

116

92Sri1

-

111

MD-MC (Tersoff) MC

91Els1

-

96 v)

DFT

92Bro1

-

118 v)

DFT

99Lee1

1x10−7

91

96Swa1

6.4x10−8 0.02 y) -

122 v) 15 231y) 29 v)

DFT MD MD (SW) DFT

97Gor1 91Lom1, 84Noo1 96All2 0.06 92Bro1 -

-

9.6 v)

DFT

92Bro1

-

87 v) 68 v)

DFT DFT

99Hos1 98Son1

5x10−8

54 v)

DFT

96Kax1

68 78Nes1 1x10−5 −14 62…117 96Gom1, 95Hib1 3x10 … 5x10−9 52 92Gan1 1x10−13

82

MC DFT

96Kax1 96Ohn1

6x10−9 e) 39

90Hat1

101

DFT

96Ohn1

6x10−9 e) 68

90Hat

17

MD Morse)

6x10−8

74Rob1

5x10−14 Ga/GaAs(100) 2.1x10−6 -2x4 || Ga/GaAs(100) 2.5x10−6 -2x4 ⊥ Au/KCl -

(Shell- 97Nak1

Qdiff [kJ/mol] Exp.

Ref. Exp.

71…105 93Voi1, 74Bed1 238 96All2 17 96Zhu1

20

|| : Parallel to atomic rows, step edges. ⊥ : Perpendicular to atomic rows, step edges. ex : Exchange mechanism. b ) Estimate by original authors. d ) Do estimated from reported attempt frequency ν using Eq. (3) by present authors (assuming hop length λ of 0.3 nm). e ) Calculation by present authors from original work (assuming that data fit Arrhenius's law). h ) Sample cut 5 degrees from normal. v ) Static energy barrier. w ) Polycrystalline surface. y ) Mass-transfer diffusion

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Ref. p. 517]

3.11 Surface diffusion on metals, semiconductors, and insulators

495

3.11.4 Discussion and correlations 3.11.4.1 Thermal diffusion in vaccum 3.11.4.1.1 Intrinsic diffusion on metals The data for intrinsic diffusion on metals in Tables 4 through 16 allow for some statistical analysis. No significant differences appeared for the prefactor Do when metallic and nonmetallic adsorbates were compared. Therefore, Do for the entire body of intrinsic diffusion on metals is displayed in Fig. 9 A nearGaussian distribution emerges with a centroid at log(Do) = −3.0 (Do = 10−7 m2/s) with standard deviation σ of 10−2 m2/s. Somewhat surprisingly, at the rather coarse level of this survey, diffusion parameters are not greatly influenced by the details of the diffusion mechanism. Two identifiable subgroups emerges in the corrugation ratio Ω ( = Qdiff/Qdes), depending on whether or not the adsorbate is metallic. A histogram of Ω for metallic adsorbates yields a roughly Gaussian distribution as shown in Fig. 10, centered at Ω = 0.15 with σ = 0.09. Since Qdes for dimers, trimers, etc. are unknown, these systems were not included in Fig. 10. The distribution moves substantially upward and becomes wider for nonmetals, as shown in Fig. 11. Although this distribution is less well peaked, the centroid lies near 0.21 with σ = 0.10. The average corrugation ratio for nonmetallic adsorbates lies distinctly higher than for metals, probably due to more localized bonds for nonmetals that do not smear out charge density variations as do metallic bonds. The correlation for nonmetals holds not only for simple gases (H, CO, O), but also for strongly bound materials (C, Si, SiO, Ge) as well as physisorbed species (noble gases). Note that the average values for Ω have shifted slightly from previously reported values of 0.13 and 0.23 [95See1] obtained with a smaller data set. Although the mean value of Do lies at 10−7 m2/s, as expected using the simple argument detailed above, the wide standard deviation in log (Do) deserves some attention. Insight for such measurements can be found from plots of Ω vs. log (Do) (Figs. 12 and 13). Low values of Ω correlate strongly with low values of log (Do) and vice versa. The corrugation ratio is chosen as the plotting parameter rather than Qdiff since measurements of Ω cluster more closely about a central value than those for Qdiff. This property accentuates the correlations with Do that might otherwise be obscured. As an example of a related phenomenon, the data of Figs. 12 and 13 show strong correlations, but when the two data sets are plotted together, correlations virtually disappear because of the differing averages in Ω for metallic and nonmetallic adsorbates. Although such compensation effects arise in many applications and often point to interesting physics [86See2, 75Tso1], we believe that the effect seen in surface diffusion arises mostly from uncertainties in extrapolating an Arrhenius plot to infinite temperature. That true compensation effects sometimes occur in the data is unquestioned, as in the cases of CO on Ru and sulfided Ru; 1H, 2H, 3H on W(110); and H on Si(100) and Si(111). Here, the covariance of Qdiff and Do extends over a range far exceeding the likely experimental error. However, in most homologous adsorption series, such as n-alkanes on Ru(100), pentanes on Ru(100), 1H and 2H on Rh(111), 1H and 2H on Ni(100), Rhx clusters on W(110) and Ptx clusters on W(110), inspection of the tables reveals no clear relation between Qdiff (or Ω) and Do. Moreover, a fairly simple estimate shows that the slopes in Figs. 12 and 13 may be explained entirely by experimental error. To make this estimate, we assume that the actual Do is incorrectly measured by an amount ∆ln(Do) at temperature T. Because ln(Do) and Qdiff are related by: ln( D ) = ln( D o ) − Qdiff kT

(22)

a corresponding error ∆Qdiff arises in the activation energy. If the actual D is approximately correct, it is easy to show that: ∆ ln( D o ) = ∆Q diff kT or, including Qdes:

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(23)

496

3.11 Surface diffusion on metals, semiconductors, and insulators ∆Ω =

2.303kT ∆ log( D o ) . Qdiff

[Ref. p. 517

(24)

When the average values of Qdes and T characterizing a large data set are employed, the coefficient 2.303kT/Qdes predicts a slope for the lines in Figs. 12 and 13. The average values characterizing Qdes and T in both figures are roughly 313 kJ/mol and 400 K, respectively, yielding a predicted slope of 0.024. this number nearly matches with the slopes of 0.019 and 0.023 obtained by multiple regression for metals and nonmetal adsorbates, respectively. To avoid such problems, many authors who lack D as a function of T assume Do of 10−7 m2/s in order to estimate Qdiff. This practice has been extended by Kellogg [94Kel4], who in his review of FIM results on metals recalculates Qdiff using this assumption for all data considered even when Arrhenius plots were presented. Our presentation supports his practice in a broad sense, especially when considerable uncertainty exists in the slope of the Arrhenius plot. That experimental uncertainties in both Ω and Do can be very large is obvious from Tables 4 through 16. For example, sets of results from CO/Pt(111) and W/W(211) show Ω and Do varying by up to a factor of 2 and 106, respectively. 3.11.4.1.2 Intrinsic diffusion on semiconductors A histogram plotting the frequency f of reported values vs. log Do is shown in Fig. 14. Too few data exist to draw strong conclusions about patterns in Do. Several reports cluster about the "expected" value of about 10−7 m2/s. However, the significant outliers deserve some attention. In each of these cases, Do is surprisingly low, near 10−14 m2/s for Pb/Si(111), 10−13 m2/s for Pb/Ge(111) [92Gan1] and 10−16 m2/s for H/Si(100) [96Han1]. It should be noted that in none of these cases does migration occur by simple site-tosite hopping; rather, diffusion involves complicated concerted motions. Since concerted motions can only occur when atoms are appropriately aligned, the attempt frequency in Eq. (3) decreases from a typical atomic vibrational frequency (typically 10−13/s) to a much smaller value (here, 10−7/s or less), resulting in small values of Do. A corresponding histogram for Ω is shown in Fig. 15. More data exist than for Do, but the range of values is very broad. Interestingly, many of the data exceed Ω = 0.35, which is a rough upper limit for diffusion on metallic substrates [91Zhd1, 95See1]. The data for Ω on semiconductors in this temperature regime may be correlated more profitably with a simple relation based on bond counting. Using singlebonded H on Si(111) as a base case, where a Ω of 0.6 has been determined both experimentally [88Rei1, 96Han1] and theoretically [93Vit1] one finds:

Ω = 0.6/M bond ,

(25)

where M bond is the number of covalent bonds the adsorbed atom has to the surface. The correspondence between the data and this relation is shown in Fig. 16. Systems with the concerted diffusive motions described above are excluded. For monatomic adsorbates, determining Mbond is straightforward. However, for dimers, it is a bit more difficult. In the case of Sb2/Si(100), for example [93Mo1], the Sb forms dimers that align themselves perpendicularly to the Si dimer rows. Diffusion was proposed to occur via a twostep process in which the Sb dimer first rotates parallel to the Si rows by pivoting about one of the Sb atoms, followed quickly by a second pivot, resulting again with the dimer perpendicular to the rows. In such a case, the dimer diffuses when one of the two bonds between a single Sb atom and the Si(100) surface is broken. Hence, for diffusion of a dimer on Si(100), Mbond equals 2. Ga, which also forms dimers on the Si(100) surface, is treated in a similar manner. The desorption energy in Table 17 for the dimers was estimated to be that measured experimentally for single atoms with no correction associated with the dimerization. Data excluded from Fig. 16 include the study of benzene on Si(111), as the number of bonds to the surface is indeterminate, and studies where complicated concerted motions were found to occur. This correlation is expected to work only for species making strong directional bonds to the surface. There is no reason to expect that weakly bonded adsorbates held by Van der Waals attraction should obey this rule, as demonstrated by benzene on Si/(111). Furthermore, if site-to-site hopping is not the mechanism of diffusion, deviations may be expected, as evidenced by the concerted exchange studies. Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 517]

3.11 Surface diffusion on metals, semiconductors, and insulators

497

3.11.4.1.3 Intrinsic diffusion on insulators The data for Do in Table 18 are summarized as a histogram of report frequency in Fig. 17. The data cluster fairly closely about an average value of roughly 2x10−8 m2/s. This average is lower by a factor of about 5 than the "expected" value for intrinsic diffusion. However, the data are so few that it is difficult to say whether the deviation is real. Surface contamination tends to be a problem in studies on insulators, as most experimental work has been conducted outside of ultrahigh vacuum. Also, the data set includes many measurements on surfaces having many crystallographic orientations exposed. Few results for Ω exist because of the lack of data for Qdes on insulators. No statistically significant trend for Ω could be discerned from the small set of results. 3.11.4.1.4 Mass transfer diffusion on metals Currently, nearly all mass transfer diffusion studies on metal surfaces describe self-diffusion. Two distinct temperature regimes separated by a critical temperature Tc emerge from an analysis of mass transfer data on metals, semiconductors, and insulators. Data for the low-T and high-T regimes, collated in Table 19, are separated by Tc ~ 0.75 Tmelt. At low T, Ω and Do average 0.28 and 3x10−6 m2/s, respectively, as shown in Figs. 18 and 19. At high T, the Ω and Do average higher at 0.52 and 0.3 m2/s, respectively. The enhancements for high-T mass transfer are enormous: a factor of 2 in Qdiff and a factor of 105 in Do. We can also define a different scaling parameter α (= Qdiff/kTmelt), that is useful in correlating data where no desorption data are available and also relates more clearly to melting-temperature scaling. Fig. 20 clearly shows that the scaled Qdiff falls into two distinct T regimes. Practical estimates of diffusion parameters in new systems can be made with some success by analogy to diffusion in the bulk. The mechanics and physical basis for this semiempirical approach has been discussed previously by Seebauer and Allen [95See1]. The method works best for self-diffusion, although good results can be obtained for heterodiffusion at low adsorbate coverages where surface structure and diffusion dynamics are largely unaffected by adsorbate-adsorbate interactions. While some of the specific numbers here have shifted somewhat from the earlier report due to the new data from recent years, the overall level of agreement between the correlations and experimental data is comparable to that presented in [95See1]. In brief, the mass tranfer activation energy Qdiff ,M comprises of the energy of migration ∆Hm and the energy of vacancy formation ∆Hf. The first term equals Qdiff,I for intrinsic adatom diffusion in Eq. 5. The second term comprises of the chemical work ∆Hfc required for an atom to break bonds with its neighbors as well as possible ionization work ∆Hfi. If the vacancy forms from a terrace site, simple bond counting suggests that ∆Hfc should be a large fraction of the enthalpy for vacancy in the bulk ∆Hfcb. For calculation with metallic substrates, ∆Hm should be set at the intrinsic value of 0.15Qdes for metallic adsorbates and 0.21Qdes for nonmetallic adsorbates as discussed in Sect. 3.11.4.1.1 above. At high temperatures, the experimental data are best fit by setting the chemical formation energy ∆Hfc for terraces exactly equal to the bulk value ∆Hfcb. Since no ionization effects arise for lack of a bandgap, the chemical formation entropy can be estimated directly from bulk values. A detailed discussion of the physical basis for estimating prefactors Do appears in previous work by Seebauer and Allen [95See1]. However, for practical purposes it is probably best merely to use the mean value from Table 20. Putting these results together, we obtain the following guidelines for a typical case: Above Tc Qdiff,M = ∆Hfcb + (0.15 or 0.21)Qdes Do = 0.3 m2/s

(26) (27)

Below Tc Qdiff,M = 1/3 ∆Hfcb + (0.15 or 0.21)Qdes Do = 3x10−6 m2/s

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(28) (29)

498

3.11 Surface diffusion on metals, semiconductors, and insulators

[Ref. p. 517

3.11.4.1.5 Mass transfer diffusion on semiconductors For all nonmetallic substrates, Qdiff,M and Do for mass transfer diffusion are also greater than those for intrinsic diffusion. Two mass transfer diffusion regimes exist, dependent on substrate temperature. Tc also appears to vary appreciably with substrate. However, values lying between 60 and 75% of Tmelt are typical. Tc is generally difficult to determine precisely, since any particular experiment rarely exhibits both regimes. A rare, clear example of this transition is shown in Fig. 21 for self-diffusion on the Si(100) surface. Below Tc, Qdiff,M lies at approximately twice the value for that of intrinsic diffusion while Do lies very slightly above the intrinsic value. As mentioned above, adatom equilibrium with kinks probably governs this behavior. Reports in this regime on semiconductors typically involve self-diffusion. Above Tc, Qdiff,M and Do increase dramatically in response to adatom formation from terraces. Reports in this regime include both self-diffusion and heterodiffusion. Fig. 22 shows a histogram for Do reports for mass transfer on semiconductors. The data taken at high T/Tmelt average to roughly Do = 0.4 m2/s. For the low T regime, Do averages to 2×10−7 m2/s. Fig. 23 shows data for Ω plotted in similar fashion. A fairly large data set exists, since values for Qdes are relatively abundant for both self-diffusion and heterodiffusion. Again, the data fall into two clear groups. At high T/Tmelt, Ω averages 0.67, while at low T/Tmelt , Ω is near 0.32. For α (= Qdiff/kTmelt), substantial intermixing of data points near α = 15 occurs (Fig. 24). For Qdiff,M calculation, the migration enthalpy ∆Hm should be set at the intrinsic value of 0.6 Qdes/Mbond. At high temperature, the data are best fit by setting the chemical formation energy ∆Hfc to 2/3 of the bulk ∆Hfcb; at low temperature, equate ∆Hfc to 2/9 of ∆Hfcb [95See1]. A typical prefactor Do comes from merely averaging the data from Table 20. Combining these results, we obtain the following guidelines for a typical case: Above Tc Qdiff,M = 2/3 ∆Hfcb + 0.60 Qdes/Mbond (± up to Eg if strong ionic effects) Do = 0.4 m2/s (up to 102 if strong ionic effects)

(30) (31)

Below Tc Qdiff,M = 2/9 ∆Hfcb + 0.60 Qdes/Mbond Do = 2x10−7 m2/s

(32) (33)

3.11.4.1.6 Mass transfer diffusion on insulators For insulators, typical value of Tc also varies between 60 and 75% of Tmelt. Tc is generally difficult to determine precisely, since any particular experiment rarely exhibits both regimes. Rare, clear examples of this transition are shown in Fig. 25 for Co and Cr on MgO [93Stu1]. Below Tc, Qdiff,M and Do lie just slightly above the values for intrinsic diffusion. As mentioned above, adatom equilibrium with kinks probably governs this behavior. Reports in this regime typically involve either heterodiffusion of adatoms originating from an inexhaustible source, such as in point-source or edge-source methods or self-diffusion where an infinite source is implied. Above Tc, Qdiff,M and Do increase dramatically in response to adatom formation from terraces. Reports in this regime include both self-diffusion and hetero-diffusion with both inexhaustible and finite sources. Fig. 26 shows a histogram for reports of Do for mass transfer diffusion on insulators. The data clearly fall into two sets depending on T/Tmelt, with intermixing at the border near Do = 0.1 m2/s. At lower T, Do averages to a much lower value near 1x10−5 m2/s. Unfortunately, so few data exist for Qdes on insulators that a corresponding plot for Ω cannot be usefully drawn; consequently α was plotted. The reason for the lack of data is that many insulators decompose before they (or the adsorbates studied) evaporate. Fig. 27 clearly shows the scaled activation energy falling into two distinct temperature regimes.

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Ref. p. 517]

3.11 Surface diffusion on metals, semiconductors, and insulators

499

Calculation for insulating materials presents even more difficulty than for semiconductors due to the enormous diversity of possible substrates. For covalent substrates, Eqs. 32 and 33 should suffice as a first estimate. Since Eg is larger in insulators, ionic effects will be more pronounced. The data for the contribution from intrinsic diffusion are so sparse and scattered as to be nearly useless for prediction; vacancy formation energies are also difficult to interpret. A good estimate for insulators comes from the heat of formation of compound ∆Hf, suitably normalized to a single atom. At high T, the data are best fit by setting the vacancy formation energy ∆Hfc to 6/5 of ∆Hf. At low T, the kink formation energy ∆Hfc comes out to be 8/15 of ∆Hf [95See1]. A typical prefactor Do comes from merely averaging the data from Table 21. Combining these results, we obtain the following guidelines for a typical case: For covalent insulators: use Eqs. 32 and 33 For ionic insulators: Above Tc Qdiff,M = 6/5 ∆Hf Do = 5x103 m2/s

(34) (35)

Below Tc Qdiff,M = 8/15 ∆Hf Do = 1x10−5 m2/s

(36) (37)

3.11.4.1.7 Comparison of mass transfer diffusion on various substrates A striking trend appears in the high-T regime for mass transfer diffusion on metals, semiconductor, and insulators (Figs. 28 and 29). Both average values of α ( = Qdiff/kTmelt) and Do show clear regression in the value of the peak as one moves from metals (α =15) to semiconductor (α =17) to insulators (α =22). While the progression in Do from metals to semiconductor is minimal, the Do increases by a factor of 103 from semiconductor to insulator. No similar trend appears in the low-T results. 3.11.4.1.8 Adsorbate concentration dependence In general, diffusivity D often exhibits a strong dependence on surface concentration. Numerous examples of this behavior have been documented on metal surfaces [93Bon1]. Using a Monte Carlo model, Bowker and King [78Bow1] qualitatively showed that the lateral interactions can lead to three limiting cases for the coverage dependence of D [91Lom1] (Fig. 30). With increasing coverage θ, D decreases for repulsive interactions and increases for attractive interactions. D becomes independent of θ without lateral interactions. For cases where both repulsive and attractive interactions exists, D may exhibit more complex dependence on θ (Fig. 31). Many examples of examples of θ dependence of D have been reported for metals (Figs. 32 through 35). Relatively few examples of θ dependence of D have been reported for nonmetals, and all existing measurements are for mass-transfer diffusion on semiconductors (Fig. 2 and Figs. 36 through 38). The data reported in the tables for these systems are quoted in the low-coverage limit. The behavior ranges from that of In on Si(111) [95All1], where both Qdiff and Do are independent of coverage over a wide range, to that of In on Ge(111) [94Sun1], where Qdiff varies by 40% and Do by six orders of magnitude. These variations can be strongly peaked, and in such cases are almost certainly the result of surface phase transitions. Since mass transfer diffusion is involved, it is unclear in general whether the variations result from changes in DI, N/Ns, or both. However, in the case of Sb on Ge (111) [92Sch1], changes in N/Ns were thought to be responsible, due to an adsorbate islanding mechanism. Various surface reconstructions were induced by higher θ of Au on Si(111), greatly altering Do [84Gav1]. Small changes in crystallographic orientation can have enormous effects, as in the case of Ag on Ge(111) [83Sul1].

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500

3.11 Surface diffusion on metals, semiconductors, and insulators

[Ref. p. 517

3.11.4.1.9 Impurity-influenced diffusion Few general conclusions can be developed from Table 22 due to insufficient number of reported data. While surface diffusion in some systems is enhanced in the presence of impurities, most systems show reductions in D. With only one exception, systems with impurity inhibition also exhibited an increase in Qdiff. Brown et al. [95Bro1] attributed the increase in Qdiff to both short- and long-range trapping of adatoms by impurities. A minute decrease in Do exhibited by many systems shows that site blocking generally does not limit surface diffusion on the terrace; in some cases, Do actually increased by a factor of 105 [82Bay1]. The diffusional enhancement appears to occur for systems in which steps limit diffusion. For example, the activation energy for mass-transfer self-diffusion of W [73Pic1] and Si [92Nak1] decrease by more than a factor of 3 with the presence of impurities. While no Do is given for either cases, an increase in D can be observed in the STM images [94Vri1]. The induced transition from 3-D islanding growth (Volmer-Weber) to layer-by-layer growth (Frank-Van der Merwe) also implies an increase in adatom diffusion over Schwoebel-Ehrlich step barriers. A neutral effect of impurities on diffusion was observed in H2O/Au-W system [99Bry1]. Neither Do nor Qdiff changed, even at high coverage of Au. Bryl et al. attributed this observation to the weak adsorbate interation between H2O and Au. 3.11.4.2 Non-thermal diffusion 3.11.4.2.1 Tunneling diffusion Few tunneling surface diffusion data exist because experimental observations of quantum motion of are rare [92Zhu1]. No correlation could be deduced from such limited data set. Dtn sometimes depends on the adsorbate coverage. Figs. 39 and 40 reveal that adsorbate-surface as well as adsorbate-adsorbate interactions play a strong role in tunneling surface diffusion. Only LLD and FEM were used to measure Dtn. LITD generally yielded both high values of Dtn and crossover temperature from the activated to non-activated diffusion Ttn. LITD generally gives higher Dtn and Ttn by factors of 10 and 1.5, respectively. The origin of this difference is unclear. 3.11.4.2.2 Ion- and photon-influenced diffusion Very recently the first reports of direct measurements of ion-influenced [99Dit1] and photon-influenced [98Dit1] surface diffusion have appeared. Both measurements involved Ge diffusing on Si(111) and employed SHM as the measuring technique. While the basic phenomenology for the ion influences probably generalize to metallic and insulating substrates, the photon influences depend on mechanisms that operate only for semiconductors. Some sample results of the ion-influenced diffusion appear in Fig. 41. Curiously, D obeys a conventional Arrhenius expression; the ion influences enter in only through modified activation energies and prefactors and through the appearance of two distinct temperature regimes. Even more surprising, bombardment causes a decrease in D at sufficiently high temperatures. Effects generally vary with the square root of ion mass. The mechanism underlying all these effects is still incompletely understood, but seems to involve collisionally-induced surface vacancy formation. Sample results for photon-influenced diffusion appear in Fig. 42. Here the results depend on doping type of the underlying substrate. D increases for n-type doping but decreases by about the same amount for p-doping. The effects of photons on activation energies and prefactors are relatively insensitive to adsorbate type (for adsorbates from Groups III, IV and V of the Periodic table). The effects are not yet fully understood but almost certainly arise from charging of surface vacancies. The population statistics of the charge states change due to the excess carriers generated by photons having energies greater than the bandgap energy of the substrate.

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Ref. p. 517]

3.11 Surface diffusion on metals, semiconductors, and insulators

501

3.11.4.3 Diffusion under aqueous liquids The data set for diffusion under liquids is relatively small (Tables 24 and 25). However, a qualitative trend appears for mass-transfer self-diffusion of metals: D is generally higher than in vacuum. In some cases, the enhancement can be quite large. For example, D for Pt self-diffusion increases by a factor of 103 in aqueous H2SO4 [88Two1]. The effects of liquids seem to be mediated mainly by the interaction between the metal and the any dissolved anions. For example, Garcia et al. [93Gar1] found that strongly bonding anions enhance D by a factor of 10 over weakly bonding ions. Applying a positive bias to the metal seems to amplify this effect so that D can be further enhanced [92Don1]. More than half of the mass transfer data under liquids focuses on organic compounds on oxides. Miyabe et al. [98Miy1] offers an empirical correlation for estimating D for a small group of liquid chromatographic systems based on heat of adsorption Qads. D decreases with an increase in Qads. 3.11.4.4 Computational results The comparisons between the computational and experimental results in Table 26 are limited but instructive. For intrinsic diffusion on metals, the embedded atom model (EAM) and other semi-empirical approaches generally predict lower activation energy Qdiff than what is observed experimentally [98Kyu1]. On the other hand, Qdiff from ab initio results generally exceed those from experiment. (However, recent ab initio results for Pt2/Pt(111) [99Boi1] and Xe/Pt(111) [95Sho1] exactly match the experimental Qdiff). The prefactors Do from EAM generally agree well with experimental Do with only a few exceptions. For example, for Ag/Pd(100) the computed Do is a factor of 103 too low. For semiconductors, both ab initio (DFT) and classical approaches generally predict higher Qdiff than seen experimentally; ab initio approaches, however, generally get closer. Unfortunately, more broadlybased comparisons with experiment prove difficult due to the insufficient number of systems that have both experimental and computational data. Kaxiras [96Kax1] has discussed the general issues relating to computations and has suggested that there is still much room for improvement. In particular, semiempirical and classical methods wrongly predict the positions of potential minima and saddle points of the potential energy surface, leading to incorrect ground and transitional states for adsorbates. Classical potentials such as the Stillinger-Weber and Tersoff also cannot describe the vacancy ionization phenomena in mass transfer diffusion [98Dit1] or certain surface reconstruction effects in intrinsic diffusion. As an example of the latter problem, surfaces like the dimerized Si(100)-(2x1) and GaAs(100)(4x2) contain highly-distorted, non-tetrahedral covalent bonds. Diffusion on such surfaces sometimes involves a concerted exchange mechanism with a complex bonding scheme that requires quantum methods to model accurately. Thus, methods like MD that employ classical potentials can offer quantitatively reliable results only in special cases where these effects play no important role [96All2]. Otherwise, one should probably expect to reproduce experiment with only qualitative accuracy. For this purpose no classical potential appears to be superior to the others under all circumstances; each has its peculiar balance of strengths and limitations [95Gar1].

Acknowledgements This work was partially supported by the US National Science Foundation (CTS 98-06329). EGS acknowleges support of the US Department of Energy (DEFG02-91ER45439) through the Materials Research Laboratory at the University of Illinois.

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3.11 Surface diffusion on metals, semiconductors, and insulators

502

[Ref. p. 517

–9

10

Figures for 3.11

8 6

Ag on Ge (111) ϕ

4

Step Adatom

2

Step-adatom

–10

10

8 6 4

2 –1

Kink

Chem.diff.coeff. D [m s ]

Terrace

–11

10

Terrace vacancy Adatom Fig. 1. Model of a surface showing various typical features such as terraces, steps, kinks, vacancies, and adatoms [93Bon1].

8 6 4 2

–12

10

8 6

175

ϕ=0°

2

O/W(100)

–13

10

0

125 CO/Ni(111)

100

H/Ni

0.1

0.2

0.3 0.4 Coverage θ

0.5

0.6

Fig. 2. Variations in diffusivity D as a function of step density for Ag/Ge(111). Inclination with respect to the (111) face is 5.9° (•), 0.9° (x) and 0° (ο) [83Sul1].

75

25

0

10

H/Pt

O/Pt CO/Rh 25

Tmelt

–6

H/Si(111)

50 75 100 125 –1 Activ.energy Qmac [kJ mol ]

Si on Si (100)

–8

2 –1

50

–4

10

CO/Pt(111)

Chem.diff.coeff. D [m s ]

–1

ϕ = 0.9 °

4

150 Activ.energy Qmic [kJ mol ]

ϕ = 5.9 °

2

150

Fig.3. Diffusion activation energies Qmic (from microscopic methods) and Qmac (from macroscopic methods) are compared. Data are for O/W(100) [80Bow1, 79Che1], H/Ni [57Wor1, 87Mul1], CO/Ni(111) [90Lin1, 88Zhu1], H/Pt [69Lew1, 86See2], CO/Pt(111) [92Kwa1, 82Poe1], O/Pt [68Lew1, 94Oer1], CO/Rh [88See1, 89Dun1], and H/Si(111) [91Rei1, 95Han1]

10

–10

10

exp. extropolated

–12

10

–14

10

Dt

–16

10

DI

–18

10

0

0.5

1.0 1.5 2.0 2.5 –3 –1 Inv. temp. 1/T [10 K ]

3.0

3.5

Fig. 4. Arrhenius plot for the intrinsic [91Mo1] and mass transfer self-diffusivities on Si(100), showing convergence near the Tmelt [94Kee1].

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Ref. p. 517]

3.11 Surface diffusion on metals, semiconductors, and insulators

503

Fig. 5a-c. 3-D representation of PES's for (a) P/Si(100), (b) Si/Si(100) and (c) Al/Si(100) in a p(2x1) unit cell. The dimer rows of he substrate are oriented along the y axis M and H denote the potential minima [92Bro1].

Code size (lines) Basis set size CPU time/MD step Physical time/ MD step

Ab initio (DFT)

Semi- empirical (e.g. TB) Classical potentials

104 100 – 500 1h 1 fs

103 4 1 min 1 fs

102 1s 1 fs

Fig. 6. Comparison of computational effort of classical interatomic potentials, semi-empirical TB approximation and ab initio calculations]. In all cases order-of magnitude estimates are given. The code size corresponds to the part that calculates the energy and forces in each approach, excluding the part that performs MD, which should be common to all approaches. For CPU time estimates, a system of 250 Si atoms is assumed to be simulated on a modern workstation with a rating of 100 Mflops [96Kax2].

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3.11 Surface diffusion on metals, semiconductors, and insulators

504

[Ref. p. 517

εs ϕ*

ε0s

ϕ

ε

Fig. 7. Schematic diagram of the periodic potentials. The minima of the potential represent adsorption sites with energy ε. Nearest and more distant neighbor interaction, ϕ, are assumed to modify ε. The saddle point energies εs are modified by ϕ* [97Ueb1].

–14

10

8 6

CO on Ni(110)

4

[001]

42.0

33.5 –1

–15

10

Diffusivity D [m s ]

2 –1

Activ. energy Qdiff [kJ mol ]

2

8 6

0%S

4

2

2

5

–16

10

8 6

20 % S

4

10

8

25.0

16.5

8.0

2 –17

10

4.0 –4

10

–6

6.5

0

0.05

0.10 Coverage θ [ML]

0.15

0.20

0

2 –1

Prefactor D [m s ]

10

4.5 5.5 5.0 6.0 –1 –3 –1 Inv. temperature T [10 K ]

10

–8

10

–10

10

–12

Fig. 8a-c. (a) CO diffusion along [001] direction on Ni(110) for a number of S coverages θ. (b) activation energy Qdiff and (c) prefactor Do as a function of S impurity θ [95Xia1]. 0

0.05

0.10 Coverage θ [ML]

0.15

0.20

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Ref. p. 517]

3.11 Surface diffusion on metals, semiconductors, and insulators

100

505

80 70 60 50

60

Frequency f

Frequency f

80

40

40 30 20

20

10

0 –16 10

10–12

10–8 10–4 Prefactor D0 [m2 s–1]

1

30

0.6

25

0.5

20

0.4

15 10 5

0

0.2

0.4 0.6 Corrugation ratio Ω

0.8

1.0

Fig. 10. Frequency f of literature reports vs. Ω (=Qdiff/Qdes) for intrinsic diffusion of metallic adsorbates on metals.

Corrugation ratio Ω

Frequency f

Fig. 9. Frequency f of literature reports vs. log Do (m2/s) for intrinsic diffusion of all adsorbates on metals.

0

0.3 0.2 0.1

0.2

0.4 0.6 Corrugation ratio Ω

0.8

1.0

Fig. 11. Frequency f of literature reports vs. Ω (=Qdiff/Qdes) for intrinsic diffusion of non-metallic adsorbates on metals.

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0 –14 10

10–12

10–10 10–8 10–6 0 2 –1 Prefactor D [m s ]

10–4

10–2

Fig. 12. Correlation between Ω (=Qdiff/Qdes) and log (Do [m2/s]) for metallic adsorbates diffusing on metals. Multivariate regression yields a best fit of Ω = 0.019 log (Do) + 0.27.

3.11 Surface diffusion on metals, semiconductors, and insulators

506 0.6

[Ref. p. 517

4

0.5 0.4 Frequency f

Corrugation ratio Ω

3

0.3 0.2

2

1

0.1 0 –14 10

10

–12

10

–10

–8

–6

10 10 10 Prefactor D0 [m2 s–1]

–4

10

–2

0 –16 10

1

Fig. 13. Correlation between Ω (=Qdiff/Qdes) and log (Do [m2/s]) for non-metallic adsorbates diffusing on metals. Multivariate regression yields a best fit of Ω = 0.023 log (Do) + 0.38.

10–6

10–4

1.0

5

experiment theoretical fit

0.8

Corrugation ratio Ω

4 Frequency f

10–12 10–10 10–8 0 2 –1 Prefactor D [m s ]

Fig. 14. Frequency f of literature reports vs. log Do (m2/s) for intrinsic diffusion on semiconductors.

6

3 2

0.6

0.4

0.2

1

0

10–14

0.2

0.4 0.6 Corrugation ratio Ω

0.8

1.0

Fig. 15. Frequency f of literature reports vs. Ω (=Qdiff/Qdes) for intrinsic diffusion on semiconductors.

0

1

2 Bond number M

3

4

Fig. 16. Dependence of Ω (=Qdiff/Qdes) on the number of adsorbate-surface bonds Mbond for intrinsic diffusion on semiconductors.

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Ref. p. 517]

3.11 Surface diffusion on metals, semiconductors, and insulators 10

12 10

high T low T

8

Frequency f

8

Frequency f

507

6

6

4

4 2

2 0 –12 10

10–10

10–8 10–6 10–4 0 2 –1 Prefactor D [m s ]

10–2

0

1

Fig. 17. Frequency f of literature reports vs. log Do (m2/s) for intrinsic diffusion on insulators.

0.2

0.4 0.6 Corrugation ratio Ω

Fig. 18. Frequency f of literature reports vs. Ω (=Qdiff/Qdes) for mass transfer diffusion on metals. The data group into high and low T regimes.

10

8 high T low T

high T low T 6

6

Frequency f

Frequency f

8

4

10–8

10–4 Prefactor D0 [m2 s–1]

1

104

Fig. 19. Frequency f of literature reports vs. log Do (m2/s) for mass transfer diffusion on metals. The data group into high and low T regimes.

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4

2

2

0 –12 10

1.0

0.8

0

5

10 15 20 Norm. activ. energy α

25

30

Fig. 20. Frequency f of literature reports vs. α (=Qdiff/kTmelt) for mass transfer diffusion on metals. The data group into high and low T regimes.

3.11 Surface diffusion on metals, semiconductors, and insulators

508 –13 8 6 4

10

10

high T low T

Si on Si (100)

2

8

–14 8 6 4

10

2

–15 10 8 6 4

a

2

Frequency f

2 –1

Chem.diff.coeff. D [m s ]

[Ref. p. 517

b

–16 8 6 4

6

4

10

2

2 –17

10

0.8

0.9

1.0 1.1 –3 –1 Inv.temp. 1/T [10 K ]

1.2

1.3

Fig. 21. Arrhenius plot of diffusivity D for mass transfer self-diffusion of Si/Si(100). Two regimes appear, separated by the break in the plot near 1025 K. Diffusion on the 1x2 and 2x1 terraces are represented by closed (•) and open (ο) symbols, respectively [95Doi1].

5

0 –10 10

10–8

10–6 10–4 10–2 1 Prefactor D0 [m2 s–1]

102

104

Fig. 22. Frequency f of literature reports vs. log Do (m2/s) for mass transfer diffusion on semiconductors. The data group into high and low T regimes.

high T low T

Frequency f

4

3

2 Fig. 23. Frequency f of literature reports vs. Ω (=Qdiff/Qdes) for mass transfer diffusion on semiconductors. The data group into high and low T regimes.

1

0

0.2

0.4 0.6 Corrugation ratio Ω

0.8

1.0

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Ref. p. 517]

3.11 Surface diffusion on metals, semiconductors, and insulators

509

5 high T low T

4

Frequency f

3

2

1

0

5

–16

10

–18

15 20 Norm. activ. energy α

Temperature T [°C] 1200 1100 1000 900 800

25

30

700

Co on MgO (100)

2 –1

αDδ [m s ]

10

10

Fig. 24. Frequency f of literature reports vs. α (=Qdiff/kTmelt) for mass transfer diffusion on semiconductors. The data group into high and low T regimes.

10

–20

10

–22

10

–24

6

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Cr on MgO (100)

7 8 9 10 –1 –4 –1 Inv. temperature T [10 K ]

11

Fig. 25. Arrhenius plot of the product of diffusivity D, isotope segregation factor a (near 1), and layer thickness δ for mass transfer diffusion of Co and Cr on MgO(100). Two regimes appear, separated by the break in the plot near 1400 K and 1350 K for Co (ο) and Cr (∆), respectively.

3.11 Surface diffusion on metals, semiconductors, and insulators

510

[Ref. p. 517

7 high T low T

6

Frequency f

5

4

3

2

1 0 –16

–12

–8

–4 0 4 Prefactor D0 [m2 s–1]

8

12

16

Fig. 26. Frequency f of literature reports vs. log Do (m2/s) for mass transfer diffusion on insulators. The data group into high and low T regimes.

2.0 high T low T

Frequency f

1.5

1.0

0.5

Fig. 27. Frequency f of literature reports vs. α (=Qdiff/kTmelt) for mass transfer diffusion on insulators. The data group into high and low T regimes. 0

5

10

15 20 Norm. activ. energy α

25

30

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Ref. p. 517]

3.11 Surface diffusion on metals, semiconductors, and insulators

511

7 insulators semicond. metals

6

Frequency f

5

4

3

2

1

0

5

10

15 20 Norm. activ. energy α

25

30

Fig. 28. Frequency f of literature reports vs. α (=Qdiff/kTmelt) for high-T, mass transfer diffusion on metals, semiconductors and insulators.

9 insulators semicond. metals

8 7

Frequency f

6 5 4 3 2 1 0 –12 –10 –8 10 10 10 10–6 10–4 10–2 1 102 104 0 Prefactor D [m2 s–1]

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106

108

1010 1012

Fig. 29. Frequency f of literature reports vs. log Do (m2/s) for highT, mass transfer diffusion on metals, semiconductors and insulators.

3.11 Surface diffusion on metals, semiconductors, and insulators

512

[Ref. p. 517

0.6

Diffusivity D [arb. units]

0.5 0.4 0.3 0.2 0.1

0

0.2

0.5

0.4 0.6 Coverage θ

0.8

1.0

Fig. 30. Variation of diffusivity D with coverage θ for repulsive (), attractive (▲), and no nearest-neighbor interactions (•) [78Bow1, 91Lom1].

–9

10

O on W(100)

0.3

2 –1

0.1

a

0.2

0.4 0.6 Coverage θ

0.8

1.0

8 6 4

Diffusivity D [m s ]

2 –1

–10

725 K

2 –13

10

0

0.1

0.2

0.3 0.4 Coverage θ

0.5

0.6

0.7

8 6 4 2

1033 K

–11

10

8 6 4 2

–12

10 b

2 –12 8 6 4

Fig. 32. Diffusivity D of Dy on Mo(112) at T = 725 K () and 800 K (•) as a function of coverage θ [98Lob1].

T = 1153 K

2

10

T = 800 K

10

–9

10

2 –11 8 6 4

10

0.2

0

Dy on Mo(112)

2 –10 10 8 6 4

Diffusivity D [m s ]

–4

2 –1

Diffusivity D [10 m s ]

0.4

8 6 4

0

0.2

0.4 0.6 Coverage θ

0.8

1.0

Fig. 31a, b. (a) Simulation of the coverage θ dependence of diffusivity D for repulsive nearestneighbor and attractive next-nearest-neighbor interactions [78Bow2]. (b) Experimental variation in D with θ for O/W(100) [77But1 ]. T = 1153 K (o) and T = 1033 K (•).

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Ref. p. 517]

3.11 Surface diffusion on metals, semiconductors, and insulators

108

O on Re(2021)

29

–3

25

10

84

10

–5

Q

D

0 –6

10

76

2 –1

92

0

–4

Prefactor D [m s ]

–1

–1

Activ. energy Qdiff [kJ mol ]

10

100 Activ. energy Qdiff [kJ mol ]

–2

10

[001]

21

17 [110]

13 –7

68

10

60

10 1.0

9

–8

0

0.2

0.4 0.6 Coverage θ

0.8

a

2 –1

210 200

0.4 0.6 Coverage θ

0.8

1.0

8 6

[110]

2 –10

10

8 6 4

0

–1

Sb on Ge (111)

0.2

4

Prefactor D [m s ]

220

0

–9

10

Fig. 33. Activation energy Qdiff (•)and prefactor Do (▲) vs. relative coverage θ of O on Re(2021) [96Son1].

Activ.energy Q [kJ mol ]

513

[001]

2 –11

10

190

8 6 4 2

–12

10

8 6

2 –1

Pre-exp.factor D0 [m s ]

180 1 b

4

0.2

0.4 0.6 Coverage θ

{

–1 8 6 4 2 –2

10

0.8

1.0

Fig. 34a, b. (a) Activation energy Qdiff and (b) prefactor Do as a function of CO coverage θ for CO diffusion along [ 11 0 ] (z) and [001] ( ) directions [93Xia1].

2

10

0

0

0.1

0.2

0.3 0.4 Coverage θ

0.5

0.6

Fig. 36a, b. (a) Activation energy Qdiff and (b) prefactor Do vs. coverage θ for Sb diffusion on Ge(111) [92Sch1].

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For Fig. 35, see next page

3.11 Surface diffusion on metals, semiconductors, and insulators

514

[Ref. p. 517

200

1

H on Pt(111) –1

180

–3

10

0

2 –1

Prefactor D [m s ]

–1

Activ.energy Q [kJ mol ]

10

–5

10

140 120

–7

10

100 2 10

–9

10

160

0

0.05

0.10

0.15 0.20 Coverage θ

0.25

0.30

0.35

1

2 –1

Pre-exp.factor D0 [m s ]

63.0

–1

Activ. energy Qdiff [kJ mol ]

52.5

42.0

–4

10

In on Si (111) In on Ge (111)

31.5

21.0 0

–2

10

–6

10

0.05

0.10

0.15 0.20 Coverage θ

0.25

0.30

0.35

Fig. 35. Activation energy Qdiff and prefactor Do as a function of coverage θ for H diffusion on Pt(111) [86See1].

0

0.1

0.2

0.3 0.4 Coverage θ

0.5

0.6

Fig. 37a, b. (a) Activation energy Qdiff and (b) prefactor

Do vs. coverage θ for In diffusion on Ge(111) (ο) [94Sun2] and Si(111) (•) [95All1].

Lando lt -Börnst ein New Ser ies III/42A1

Ref. p. 517]

1.5

Ge on Si (111)

250

230 220 –1 10 8

2 –1

H on Ni(111)

1.2

–15

2 –1

240

Pre-exp.factor D0 [m s ]

515

T = 110 K

Diffusivity Dtn [10 m s ]

–1

Activ.energy Q [kJ mol ]

260

3.11 Surface diffusion on metals, semiconductors, and insulators

6

0.9

0.6

4

0.3 0

2

1.5

–2

0.4 0.6 Coverage θ

0.8

Fig. 38a, b. (a) Activation energy Qdiff and (b) prefactor Do vs. coverage θ for Ge diffusion on Si(111) [96All1].

–12

10

860

Temperature T [°C] 770 690

620

8 6 4

0.4

0.5

0.2 0.3 Coverage θ

0.4

0.5

D on Ni(111) T = 110 K

1.0 2 –1

0.2

0.2 0.3 Coverage θ

1.2

–15

0

Diffusivity Dtn [10 m s ]

10

0.1

0.9

0.6

2

10

Diffusivity D [m s ]

2 –1

–13 8 6

0.3 0

4 2

65 eV

–14

10

8 6

54 eV 35 eV

4 2 –15

10

0.84

0.1

Fig. 39. Tunneling diffusivity Dtn of hydrogen (upper part) and of deuterium (lower part) on Ni(111) vs. coverage θ measured at T = 110 K [95Won1].

thermal 0.88

0.92 0.96 1.00 1.04 1.08 –1 –3 –1 Inv. temperature T [10 K ]

1.12

Fig. 41. Arrhenius plots for Ge diffusion on Si(111) under Ar+ bombardment of 3x1012 ions/cm2 s at several energies. Initial Ge coverages θ lie near 0.6, though diffusivity D remain independent of θ in all cases. The angle of incidence of Ar+ is 60 ° off normal. Thermal + diffusion (∇), Ar -influenced diffusion: 35 eV (), 54 eV (∆) and 65 eV (•) [99Dit1].

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3.11 Surface diffusion on metals, semiconductors, and insulators

516

–15 8 6 4

10

1

H

2 –1

2

2

H

–12

10

3

2

2

0.2

4⋅10

0.4 0.6 Coverage θ 1

H

1.0

T = 27 K

1.10

1.15

1.20 1.25 1.30 1.35 –1 –3 –1 Inv. temperature T [10 K ]

1.40

(•, ο) and n-doped (, ) Si(111) under thermal and illuminated (1.9x10−4 W/m2 ) conditions [98Dit1].

8 6

2 –1

–14

10

Fig. 42. Arrhenius plot for In diffusion on p-doped

–16

Diffusivity D [m s ]

p n

2

H on W(110)

2

illuminated

8 6 4

H

0.8

thermal

–13

–19

–15

8 6 4

10

–18 8 6 4

10

10

In on Si(111)

2

2 –17 10 8 6 4

0

8 6

440

4

Diffusivity D [m s ]

2 –1

Diffusivity D [m s ]

2 –16 10 8 6 4

10

Temperature T [°C] 560 500

640

–11

10

[Ref. p. 517

4

2 2 –17

10

8 6

3

4 –18

2⋅10

H

0

0.2

0.4 0.6 Coverage θ

H

0.8

1.0

Fig. 40 (Upper part) Predicted (ab initio) variation in tunneling diffusivity Dtn with coverage θ of H isotopes [1H, 2H, 3H] on a metal surface at 27 K [86Wha1]. (Lower part) Experimental (FEM) variation in Dtn with θ for H isotopes diffusion on W(110) at 27 K [85Wan1].

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517

Special references: review articles for 3.11 Gjostein, N.A.: Surface Self-diffusion in FCC and BCC Metals: A Comparison of Theory and Experiment, in: "Surfaces and Interfaces I," Chap. 11, Burke, J.J., Reed, N.L., Weiss, V. (eds.), New York: Syracuse University Press, 1967. Robertson, W.M.: Surface Diffusion of Oxides, a Review. J. Nucl. Mater. 30 (1969) 36. Neumann, G., Neumann, G.M.: Surface Self-diffusion of Metals. Diffusion Monograph Series 1 (1972) 1. Bonzel, H.P.: Surface Diffusion of Metals, in: "Structure and Properties of Metal Surfaces," Vol. 1, Shimodiara, S. (ed.), Tokyo: Maruzen, 1973. Rhead, G.E.: Diffusion on Surfaces. Surf. Sci. 47 (1975) 207. Ehrlich, G., Stolt, K.: Surface Diffusion. Ann. Rev. Phys. Chem. 31 (1980) 603. Gomer, R.: Surface Diffusion. Vacuum 33 (1983) 537. Naumovets, A.G., Vedula, Yu.S.: Surface Diffusion of Adsorbates. Surf. Sci. Rep. 4 (1985) 365. Doll, J.D., Voter, A.F.: Recent Developments in the Ttheory of Surface Diffusion. Ann. Rev. Phys. Chem. 38 (1987) 413. Atkinson, A.: Surface and Interface Mass Transport in Ionic Materials. Solid State Ionics 28/30 (1988) 1377. Rhead, G.E.: Atomic Mobility at Solid Surfaces. Int. Mater. Rev. 34 (1989) 261. Gomer, R.: Diffusion of Adsorbates on Metal Surfaces. Rep. Prog. Phys. 53 (1990) 917. Baetzold, R.: Surface Diffusion of Atomic and Molecular Adsorbates, in: "Metal-Surface Reaction Energetics," Chap. 3, Shustorovich, E. (ed.), New York: VCH, 1991. Ehrlich, G.: Direct Observations of the Surface Diffusion of Atoms and Clusters. Surf. Sci. 246 (1991) 1. Lombardo, S.J., Bell, A.T.: A Review of Theoretical Models of Adsorption, Diffusion, Desorption and Reaction of Gases on Metal Surfaces. Surf. Sci. Rep. 13 (1991) 1. Bonzel, H.P.: Landolt-Börnstein, New Series III/26, O. Madelung (ed.), New York: Springer-Verlag, 1993, 717. Ehrlich, G.: Diffusion of Individual Adatoms. Surf. Sci. 299/300 (1994) 628. Kellogg, G.L.: Field Ion Microscope Studies of Single-atom Surface Diffusion and Cluster Nucleation on Metal Surfaces. Surf. Sci. Rep. 21 (1994) 1. Venables, J. A.: Atomic Processes in Crystal Growth. Surf. Sci. 299/300 (1994) 798. Seebauer, E.G., Allen, C.E.: Estimating Surface Diffusion Coefficients. Progr. Surf. Sci. 49 (1995) 265. Smith, A.P., Wiggs, J.K., Jonsson, H., Yan, H., Corrales, L.R., Nachtigall, P., Jordan, K.D.: Si Adatom Binding And Diffusion on the Si(100) Surface: Comparison of ab initio, Semi-empirical, and Empirical Potential Results. J. Chem. Phys. 102 (1995) 1044. Lando lt -Bö rnst ein New Ser ies III/42A1

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Kaxiras, Efthimios: Review of Atomistic Simulations of Surface Diffusion and Growth on Semiconductors. Comp. Mater. Sci. 6 (1996) 158. Miyabe, K., Takeuchi, S., Tezuka, Y.: Estimation of Surface Diffusion Coefficient in Reversed-phase Liquid Chromatography Using Methanol/Water Mobile Phases. J. Chem. Eng. Jpn. 31 (1998) 347.

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519

References for 3.11 32Lan1 51Bur1 51Zen1 53Met 54Dre1 56Log1 57Wor1 58Dic1 60Bar1

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3.11 Surface diffusion on metals, semiconductors, and insulators Vladimirov, G. G., Medvedev, B. K., Sokol’skaya, I. L.: Sov. Phys. Solid State. 12 (1970) 1118. Vladimirov, G. G., Sokol’skaya, I. L.: Sov. Phys. Solid State. 12 (1970) 1224. Abramenkov, A. D., Slezov, V. V., Tanatarov, L. V., Fogel, Ya. M.: Sov. Phys. Solid State. 12 (1971) 2365. Maiya, P. S.: J. Nucl. Mater. 40 (1971) 57. Tamm, P. W., Schmidt, L. D.: J. Chem. Phys. 54 (1971) 4775. Tan, Y. T., Perkett, W., Powell, R.: J. Appl. Phys. 42 (1971) 4752. Vladimirov, G. G., Shakirova, S. A., Sokol’skaya, I. L.: Sov. Phys. Solid State. 12 (1971) 2748. Allen, B. C.: Met. Trans. 3 (1972) 2544. Boddenberg, B., Haul, R., Oppermann, G.: Advan. Mol. Relaxation Processes 3 (1972) 61. Fedorus, A. G., Naumovets, A. G., Vedula, Yu. S.: Phys. Status Solidi A13 (1972) 445. Goymour, C. G., King, D. A.: J. Chem. Soc. Faraday Trans. I. 68 (1972) 280. Tracy, J. C.: J. Chem. Phys. 56 (1972) 2748. Tsong, T. T., Phys. Rev. B. 6 (1972) 417. Winterbottom, W. L.: J. Vac. Sci. Technol. 9 (1972) 936. Yen, C.F., Coble, R.L.: J. Am. Ceram. Soc. 55 (1972) 507. Aitken, D., Goodhew, P. J., Waldron, M. B.: Nature Phys. Sci. 244 (1973) 15. Ayrault, G.: Ph. D. Thesis, Dept. of Physics, University of Illinois/Urbana (1973). Bonzel, H. P.: in "Structure and Properties of Metal Surfaces," Vol. 1, Shimodiara, S., (ed.), Tokyo: Maruzen Co., Ltd., 1973. Hodkin, E. N., Nicholas, M.G.: J. Nucl. Mater. 47 (1973) 23. Johnson, R. A.: in “Diffusion”, American Soc. for Metals, Menlos Park, OH (1973). Kozman, L., Bartha, L.: Isotopenpraxis. 9 (1973) 203. Pichaud, M., Drechsler, M.: Surf. Sci. 36 (1973) 813. Roulet, C.: Surf. Sci. 36 (1973) 295. Ayrault, G., Ehrlich, G.: J. Chem. Phys. 60 (1974) 281. Bedair, S. M.: Surf. Sci. 42 (1974) 595. Christmann, K., Schober, O., Ertl, G., Neumann, M.: J. Chem. Phys. 60 (1974) 4528. Christmann, K., Schober, O., Ertl, G.: J. Chem. Phys. 60 (1974) 4719. Komiyama, H., Smith, J. M.: AIChE J. 20 (1974) 728. Medvedev, V. K., Smereka, T. P.: Sov. Phys. Solid State 16 (1974) 1046. Mills, B., Howey, J. R.: Metal Science 8 (1974) 125. Nishiyama, Y., Wise, H.: J. Catal. 32 (1974) 50. Robinson, V. N. E., Robins, J. L.: Thin Solid Films 20 (1975) 155. Vladimirov, G. G., Lobanova, S. S.: Sov. Phys. Solid State. 15 (1974) 2117. Wang, J. L., Vanselow, R.: Surf. Sci. 43 (1974) 21. Bobev, K., Mireva, Z.: Surf. Sci. 51 (1975) 513. Foxon, C. T., Joyce, B. A.: Surf. Sci. 50 (1975) 434. Graham, W. R., Ehrlich, G.: Thin Solid Films 25 (1975) 85. Huang, F.H., Henrichsen, R.A., Li, C.Y.: Mater. Sci. Res. 10 (1975) 173. Isett, L. C., Blakely, J. M.: Surf. Sci. 47 (1975) 645. Janssen, A.P.: Surf. Sci. 52 (1975) 230. Maruyama, T., Komatsu, W.: J. Am. Ceram. Soc. 58 (1975) 338. Reed, D. A., Ehrlich, G.: Philos. Mag. 32 (1975) 1095 Sakata, T., Nakamura, S.: Surf. Sci. 51 (1975) 313. Tsong, T. T., Kellogg, G.: Phys. Rev. B. 12 (1975) 1343. Van Vechten, J. A.: Phys. Rev. B 11 (1975) 3910. Basset, D. W.: J. Phys. C. 9 (1976) 2491. Binh, V. T., Uzan, R., Drechsler, M.: Surf. Sci. 57 (1976) 118. Donohoe, A.J., Robins, J.L: Thin Solid Films 33 (1976) 363. Furuya, H., Koizumi, M.: Nucl. Tech. 28 (1976) 226. Jones, J. P., Jones, N. T.: Thin Solid Films. 35 (1976) 83. Lando lt -Börnst ein New Ser ies III/42A1

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3.11 Surface diffusion on metals, semiconductors, and insulators Tsong, T. T.: in “Surface Mobilities on Solid Materials,” Binh, V. T., (ed.), New York: Plenum, 1981, 109. Bayat, B., PhD Thesis, Marburg, 1982. Daniels, E. A., Pandit, M. Y.: Z. Phys. Chem. Neue Folge. 133 (1983) 215. DiFoggio, R., Gomer, R.: Phys. Rev. B 25 (1982) 3490. Doll, J. D., McDowell, H. K.: J. Chem. Phys. 77 (1982) 479. Doll, J. D., McDowell, H. K.: Surf. Sci. 123 (1982) 99. Capehart, T. W., Seabury, C. W., Graham, G. W., Rhodin, T. N.: Surf. Sci. 120 (1982) L441. McDowell, H. K., Doll, J. D.: Surf. Sci. 121 (1982) L537. Lannoo, M., Allan, G.: Phys. Rev. B. 25 (1982) 4089. Loburets, A. T., Naumovets, A. G., Vedula, Yu. S.: Surf. Sci. 120 (1982) 347. Poelsema, B., Verheij, L. K., Comsa, G.: J. Am. Phys. Soc. 49 (1982) 1731. Suzuki, M., Fujii, T.: AIChE J. 28 (1982) 380. Unertl, W. N., Jackman, T. E., Norton, P. R., Jackson, D. P., Davies, J. A.: J. Vac. Sci. Technol. 20 (1982) 607. Viswanathan, R., Burgess, D. R., Stair, P. C. Jr., Weitz, E.: J. Vac. Sci. Technol. 20 (1982) 605. Bayat, B., Wassmuth, H. -W.: Surf. Sci. 133 (1983) 1. Blaszczyszyn, M., Blaszczyszyn, R., Meclewski, R., Melmed, A. J., Madney, T. E.: Surf. Sci. 131 (1983) 433. Daniels, E. A., Pandit, M. Y.: Z. Phys. Chem. Neue Folge. 137 (1983) 83. Daw, M. S., Baskes, M. I.: Phys. Rev. Lett. 50 (1983) 1285. Doll, J. D., Freeman, D. L.: Surf. Sci. 134 (1983) 769. Gomer, R.: Surface Mobilities on Solid Materials, Binh, Vu Thien (ed.), NATO ASI Series B, 86, New York: Plenum Press, 1983, 1. McDowell, H. K., Doll, J. D.: J. Chem. Phys. 78 (1983) 3219. Metois, J.J., Le Lay, G.: Surf. Sci. 133 (1983) 422. Pfnür, H., Feulner, P., Menzel, D.: J. Chem. Phys. 79 (1983) 4613. Serri, J. A., Tully, J. C., Cardillo, M. J.: J. Chem. Phys. 79 (1983) 1530. Spiller, G. D. T., Akhter, P., Venables, J. A.: Surf. Sci. 131 (1983) 517. Suliga, E., Henzler, M.: J. Phys. C 16 (1983) 1543. Yu, J. M., Trivedi, T.: Surf. Sci. 125 (1983) 396. Bayat, B., Wassmuth, H. W.: Surf. Sci. 140 (1984) 511. Binder, K.: Applications of the Monte Carlo Method in Statistical Physics, Springer-Verlag: NY (1984). Chadi, D. J.: Phys. Rev. B 29 (1984) 785. Ciszewski, A., Melmed, A. J.: Surf. Sci. 145 (1984) L509. Finnis, M. W., Sinclair, J. E.: Philos. Mag. A 50 (1984) 45. Gavrilyuk, Yu.L., Lifshits, V.G.: Phys. Chem. Mech. Surf. 2 (1984) 1091. Hanbücken, M., Futamoto, M., Venables, J.A.: Surf. Sci. 147 (1984) 433. Ishikawa, K., Tanemura, M.: Jpn. J. Appl. Phys. 23 (1984) 842. Morin, R.: J. Physique Colloq. C9 (1984) 17. NoorBatcha, I., Raff, L. M., Thompson, D. L.: J. Chem. Phys. 81 (1984) 3715. Pearson, E., Takai, T., Halicioglu, T., Tiller, W. A.: J. Cryst. Growth 70 (1984) 33. Roux, H., Piquet, A., Uzan, R., Drechsler, M.: Surf. Sci. 141 (1984) 301. Valone, S. M., Doll, J. D.: Surf. Sci. 139 (1984) 478. Zhou, S. Y., Olander, D. R.: Surf. Sci. 136 (1984) 82. Abell, G. C.: Phys. Rev. B 31 (1985) 6184. Binh, V. T., Melinon, P.: Surf. Sci. 161 (1985) 234. Biswas, R., Hamann, D. R.: Phys. Rev. Lett. 55 (1985) 2001. George, S. M., DeSantolo, A. M., Hall, R. B.: Surf. Sci. 159 (1985) L425. Ghaleb, D., Perraillon, B.: Surf. Sci. 162 (1985) 103. Kellogg, G. L.: J. Chem. Phys. 83 (1985) 852. Morin, R.: Surf. Sci. 162 (1985) 109. Lando lt -Börnst ein New Ser ies III/42A1

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Neave, J.H., Dobson, P.J., Joyce, B.A., Zhang, J.: Appl. Phys. Lett. 47 (1985) 100. Root, T. W., Schmidt, L. D., Fisher, G. B.: Surf. Sci. 150 (1985) 173. Stillinger, F. H., Weber, T. A.: Phys. Rev. B 31 (1985) 5262. Stubican, V.S., Huzinec, G., Damjanovic, D.: J. Am. Ceram. Soc. 68 (1985) 181. Tringides, M., Gomer, R.: Surf. Sci. 155 (1985) 254. Valone, S. M., Voter, A. F., Doll, J. D.: Surf. Sci. 155 (1985) 687. Wrigley, J. D., Ehrlich, G.: J. Vac. Sci. Technol. A3 (1985) 1572. Wang, S. C., Gomer, R.: J. Chem. Phys. 83 (1985) 4193. Coulomb, J.P., Bienfait, M.: J. Physique 47 (1986) 89. Foiles, S. M., Baskes, M. I., Daw, M. S.: Phys. Rev. B 33 (1986) 7983. Gaddipati, A.R., Scott, W.D.: J. Mater. Sci. 21 (1986) 419. Jackson, R. A., Murray, A. D., Harding, J. H., Catlow, C. R. A.: Philos. Mag. A 53 (1986) 27. Kolaczkiewicz, J., Bauer, E.: Surf. Sci. 175 (1986) 508. Kozlowski, G., Ciszewski, A., Swiech, W.: J. Physique Colloq. C2 (1986) 337. Lannoo, M., Allan, G.: Phys. Rev. B 33 (1986) 8789. Levine, S. M., Garofalini, S. H.: Surf. Sci. 167 (1986) 198. Morris, M. A., Barnes, C. J., King, D. A.: Surf. Sci. 173 (1986) 618. Mak, C. H., Brand, J. L., Deckert, A. A., George, S. M.: J. Chem. Phys. 85 (1986) 1676. Mullins, D. R., Roop, B., White, J. M.: Chem. Phys. Lett. 129 (1986) 511. Seebauer, E. G., Schmidt, L. D.: Chem. Phys. Lett. 123 (1986) 129. Seebauer, E. G., Kong, A. C. F., Schmidt, L. D.: Surf. Sci. 176 (1986) 134 Sendecki, S.: Surf. Sci. 165 (1986) 402. Whaley, K. B., Nitzan, A., Gerber, R. B.: J. Chem. Phys. 84 (1986) 5181. Auerbach, A., Freed, K. F., Gomer, R.: J. Chem. Phys. 86 (1987) 2356. Beben, J., Kleint, Ch., Meclewski, R.: J. Physique Colloq. C6 (1987) 545. Bienfait, M., Coulomb, J.P., Palmari, J.P.: Surf. Sci. 182 (1987) 557. Doll, J. D., Voter, A. F.: Annu. Rev. Phys. Chem. 38 (1987) 413. Ichikawa, M., Doi, T.: Appl. Phys. Lett. 50 (1987) 1141. Kellogg, G. L.: Surf. Sci. 192 (1987) L879. Kellogg, G. L.: Surf. Sci. 187 (1987) 153. Mak, C. H., Brand, J. L., Koehler, B. G., George, S. M.: Surf Sci. 188 (1987) 312. Mak, C. H., George, S. M.: Chem. Phys. Lett. 135 (1987) 381. de Miguel, J. J., Sánchez, A., Cebollada, A., Gallego, J. M., Ferròn, J., Ferrer, S.: Surf. Sci. 189/190 (1987) 1062. 87Miz1 Mizuno, Y., Hanafusa, N.: J. Physique Colloq. C1 48 (1987) 511. 87Miz2 Mizusaki, J., Amano, K., Yamauchi, S., Fueki, K.: Solid State Ionics 22 (1987) 323. 87Mul1 Mullins, D. R., Roop, B., Costello, S. A., White. J. M.: Surf. Sci. 186 (1987) 67. 87Roo1 Root, B., Costello, S. A., Mullins, D. R., White, J. M.: J. Chem. Phys. 86 (1987) 3003. 87Saj1 Sajgalik, P., Pánek, Z., Uhrik, M.: J. Mater. Sci. 22 (1987) 4460. 87Sch1 Schaefer, H. E.: Phys. Stat. Sol. A 102 (1987) 47. 87Sho1 Shore, S. E., Ansermet, J. P., Slichter, C. P., Sinfelt, J. H.: Phys. Rev. Lett: 58 (1987) 953. 87Zin1 Zinke-Allmang, M., Feldman, L.C.: Surf. Sci. 191 (1987) L749. 87Zin2 Zinke-Allmang, M., Feldman, L.C., Nakahara, S.: Appl. Phys. Lett. 51 (1987) 975. 88Awu1 Awum, F., Narayan, S., Ruthven, D.: Ind. Eng. Chem. Res. 27 (1988) 1510. 88Bor1 Borg, R. J., Dienes, G. J.: in “An Introduction to Solid State Diffusion,” New York: Academic Press, Inc., 1988. 88Bra1 Brand, J. L., Deckert, A. A., George, S. M.: Surf. Sci. 194 (1988) 457. 88Dan1 Daniels, E. A., Lin, J. C., Gomer, R.: Surf. Sci. 204 (1988) 129. 88Gla1 Gladyszewski, L.: Surf. Sci. 200 (1998) 386. 88Kel1 Kelly, D. G., Gellman, A. J., Salmeron, M., Somorjai, G. A., Maurice, V., Huber, M., Oudar, J.: Surf. Sci. 204 (1988) 1. 88Ma1 Ma, Y. H., Lin, Y. S., Fleming, H. L.: AIChE Symp. Ser. 84 (1988) 1. 88Rei1 Reider, G. A., Höfer, U., Heinz, T.F.: Phys. Rev. Lett. 66 (1988) 2883. 88Ric1 Rice, B. M., Raff, L. M., Thompson, D. L.: J. Chem. Phys. 8 (1988) 7221.

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3.11 Surface diffusion on metals, semiconductors, and insulators Seebauer, E. G., Kong, A. C. F., Schmidt, L. D.: J. Chem. Phys. 88 (1988) 6597. Schwoebel, P. R., Kellogg, G. L.: Phys. Rev. B. 38 (1988) 5326. Tersoff, J.: Phys. Rev. B 37 (1988) 6991. Twomey, T., Uchida, Y., Lehmpfuhl, G., Kolb, D. M.: Z. Phys. Chem. Neue Folge 160 (1988) 1. Wang, S. C., Ehrlich, G.: Surf. Sci. 206 (1988) 451. Zhu, X. D., Rasing, Th., Shen, Y. R.: Phys. Rev. Lett.: 61 (1988) 2883. Boddenberg, B., Beerwerth, B.: J. Chem. Phys. 93 (1989) 1440. Ceh, M., Kolar, D.: J. Mat. Sci. 24 (1989) 4307. Deckert, A. A., Brand, J. L., Arena, M. V., George, S. M.: Surf. Sci. 208 (1989) 441. Duncan, T. M., Thayer, A. M., Root, T. W.: Phys. Rev. Lett. 63 (1989) 62. Kahn, F. S.: Broughton, J. Q.: Phys. Rev B 39 (1989) 3688. Lin, T., Chung, Y.: Surf. Sci. 207 (1989) 539. Ohta, K., Kojima, T., Nakagawa, T.: J. Cryst. Growth. 95 (1989) 71. Sinniah, K., Sherman, M. G., Lewis, L. B., Weinberg, W. H., Yates, J. T., Janda, K. C.: Phys. Rev. Lett. 62 (1989) 567. Stepien, Z. M., Kukulka, J.: Surf. Sci. 216 (1989) 505. Wang, S. C., Ehrlich, G.: Phys. Rev. Lett. 62 (1989) 2297. Wang, S. C., Ehrlich, G.: J. Chem. Phys. 91 (1989) 6535. Alonso, C., Salvarezza, R. C., Vara, J. M., Arvia, A. J: Electrochim. Acta 35 (1990) 1331. Alonso, C., Salvarezza, R. C., Vara, J. M., Arvia, A. J., Vazquez, L., Bartolome, A., Baro, A. M.: J. Electrochem. Soc. 137 (1990) 2161. Arena, M. V., Deckert, A. A., Brand, J. L., George, S. M.: J. Phys. Chem. 94 (1990) 6792. Bolding, B. C., Andersen, H. C.: Phys. Rev. B 41 (1990) 10568. Brand, J. L., Arena, M. V., Deckert, A. A., George, S. M.: J. Chem. Phys.: 92 (1990) 5136. Chen, C., Tsong, T. T.: Phys. Rev. Lett. 26 (1990) 3147. Chen, C., Tsong, T. T.: Phys. Rev. B. 41 (1990) 12403. Choi, D. -S., Kim, S. K., Gomer, R.: Surf. Sci. 234, (1990) 162. Choi, D. -S., Gomer, R.: Surf. Sci. 230, (1990) 277. Dhanak, V. R., Bassett, D. W.: Surf. Sci. 238 (1990) 289. Feigerle, C. S., Desai, S. R., Overbury, S. H.: J. Chem. Phys. 93 (1990) 787. Fink, R., Wesche, R., Klas, T., Krausch, G., Platzer, R., Voigt, J., Wöhrmann, U., Shatz, G.: Surf. Sci. 225 (1990) 331. Frenken, J. W. M., Hinch, B. J., Toennies, J. P., Wöll, Ch.: Phys. Rev. B 41 (1990) 938. Gibson, E. M., Foxon, C. T., Zhang, J., Joyce, B. A.: Appl. Phys. Lett. 57 (1990) 1203. Gladyszewski, L.: Surf. Sci. 231 (1990) 120. Gomer, R.: Rep. Prog. Phys. 53 (1990) 917. Hata, M., Isu, T., Watanabe, A., Katayama, Y.: J. Vac. Sci. Technol. B 8 (1990) 692. Hunger, E., Haas, H.: Surf. Sci. 234 (1990) 273. Jones, G. W., Marcano, J. M., Norskov, J. K., Venables, J. A.: Phys. Rev. Lett. 65 (1990) 3317. Lin, T. -S., Lu, H. -J., Gomer, R.: Surf. Sci. 234 (1990) 251. Proter, J. D., Robinson, T. O.: J. Phys. Chem. 97 (1993) 6696. Rappe, A. M., Rabe, K. M., Kaxiras, E., Joannopoulos, J. D.: Phys. Rev. B 41 (1990) 1227. Reutt-Robey, J. E., Doren, D. J., Chabal, Y. J., Christman, S. B.: J. Chem. Phys. 93 (1990) 9113. Rosenzweig, Z., Asscher, M.: Surf. Sci. 225 (1990) 249. Sendecki, S.: Surf. Sci. 231 (1990) 125. Shitara, T., Kondo, E., Nishinaga, T.: J. Cryst. Growth 99 (1990) 530. Stave, M. S., Sanders, D. E., Raeker, T. J. DePristo, A. E.: J. Chem. Phys. 93 (1990) 4413. Truong, T. N., Truhlar, D. G., Chelikowsky, J. R., Chou, M. Y.: J. Phys. Chem. 94 (1990) 1973. Zinke-Allmang, M., Stoyanov, S.: J. Appl. Phys. 29 (1990) L1884. Benzinger, J. B.: in “Metal-Surface Reaction Energetics,” Shustorovich, E., (ed.), New York: VCH Publishers, 1991, 65. Lando lt -Börnst ein New Ser ies III/42A1

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Blaszczyszyn, R., Kleint, Ch.: Surf. Sci. 253 (1991) 129. Chen, C., Tsong, T. T.: Phys. Rev. Lett. 66 (1991) 1610. Choi, D. -S., Uebing, C., Gomer, R.: 259 (1991) 139. Elswijk, H. B., Hoeven, A. J., van Loenen, E. J., Dijkkamp, D.: J. Vac. Sci. Technol. B 9 (1991) 451 Hinch, B. J., Frenken, J. W. M., Zhang, G., Tonnies, J. P.: Surf. Sci. 259 (1991) 288. Iwanari, S., Takayanagi, K.: Jpn. J. Appl. Phys. 30 (1991) L1978. Kellogg, G. L.: Phys. Rev. Lett. 67 (1991) 216. Kellogg, G. L., Wright, A. F., Daw, M. S.: J. Vac. Sci. Technol. A 9 (1991) 1757. Lin, T.-S., Gomer, R.: Surf. Sci. 255 (1991) 41. Liu, C. L., Cohen, J. M., Adams, J. B., Voter, A. F.: Surf. Sci. 253 (1991) 334. Lombardo, S.J., Bell, A.T.: Surf. Sci. Rep. 13 (1991) 1. Mo, Y.W., Kleiner, J., Webb, M.B., Legally, M.G.: Phys. Rev. Lett. 66 (1991) 1998. Reider, G.A., Höfer, U., Heinz, T.F.: Phys. Rev. Lett. 66 (1991) 1994. Riehl-Chudoba, M., Memmert, U., Fick, D.: Surf. Sci. 245 (1991) 180. Toennies, J. P.: in “Surface Phonons,” Vol. 21., Kress, W., de Wette, F. W., (eds.), Berlin, Heidelberg: Springer-Verlag Berlin, 1991. Trevor, D. J., Chidsey, C. E. D.: J. Vac. Sci. Technol. B 9 (1991) 964. Tsong, T. T., Chen, C.: Surf. Sci. 246 (1991) 13. Uebing, C., Gomer, R.: Surf. Sci. 259 (1991) 151. Wager, J. F.: J. Appl. Phys. 69 (1991) 3022. Wang, J., Rockett, A.: Phys. Rev. B 43 (1991) 12571. Webb, M. B., Men, F.K., Swartzentruber, B. S., Kariotis, R., Legally, M. G.: Surf. Sci. 242 (1991) 23. Zhdanov, V.P.: Surf. Sci. Rep. 12 (1991) 183. Biernat, T., Beben, J., Meclewski, R.: Surf. Sci. 266 (1992) 11. Blaszczyszynowa, M., Blaszczyszyn, R.: Acta Physica Polonica. A 81 (1992) 285. Breeman, M., Boerma, D. O.: Surf. Sci. 269/270 (1992) 224. Brocks, G., Kelly, P. J., Car, R.: Surf. Sci. 269/270 (1992) 860. Chen, C., Tsong, T. T., Zhang, L., Yu, Z.: Phys. Rev. B 46 (1992) 7803. Dona, J. M., Gonzalez-Velasco, J.: Surf. Sci. 274 (1992) 205. Ernst, H. J., Fabre, F., Lapujoulade, J.: Phys. Rev. B. 46 (1992) 1929. Ganz, E., Theiss, S.K., Hwang, I., Golovchenko, J.: Phys. Rev. Lett. 68 (1992) 1567. Gong, Y. M.: Surf. Sci. 266 (1992) 30. Heidberg, J., Kampshoff, E., Kühnemuth, R., Schönekäs, O.: Surf. Sci. 272 (1992) 306. Honda, T., Okano, T.: Appl. Surf. Sci. 60/61 (1992) 260. Krausch, G., Detzel, T., Fink, R., Lucksheiter, B., Platzer, R., Wöhrmann, U., Shatz, G.: Phys. Rev. Lett. 68 (1992) 377. Kellogg, G. L.: Surf. Sci. 266 (1992) 18. Kwasniewski, V. J., Schmidt, L. D.: Surf. Sci. 274 (1992) 329. Lee, A., Zhu, X. D., Deng, L., Linke, U.: Phys. Rev. B 46 (1992) 15742. Mann, S. S., Seto, T., Barnes, C. J., King, D. A.: Surf. Sci. 261 (1992) 155. McCarley, R. L., Bard, A. J.: J. Phys. Chem. 96 (1992) 7410. Meixner, D. L., Arthur, D. A., George, S. M.: Surf. Sci. 261 (1992) 141. Nakahara, H., Ichikawa, M.: Appl. Phys. Lett. 61 (1992) 1531. Raynerd, G., Doust, T.N., Venables, J.A.: Surf. Sci. 261 (1992) 251. Sanders, D. E., DePristo, A. E.: Surf. Sci. 260 (1992) 116. Schultz, K. A., Seebauer, E. G.: J. Chem. Phys. 97 (1992) 6958. Srivastava, D., Garrison, B. J.: Phys. Rev. B 46 (1992) 1472. Storch, R., Stolz, H., Wassmuth, H. W.: Ann. Physik 1 (1992) 315. Wu, C. J., Carter, E. A.: Phys. Rev. B 46 (1992) 4651. Zhu, X. D., Lee, A., Wong, A., Linke, U.: Phys. Rev. Lett. 68 (1992) 1862. Zandvliet, H. J. W., Elswijk, H.B., van Loenen, E. J.: Surf. Sci. 272 (1992) 264. Becerra, L. R., Klug, C. A., Slichter, C. P., Sin, J. H.: J. Phys. Chem. 97 (1993) 12014.

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Stoneham, M., Harding, J., Harker, T.: MRS Bulletin 21 (1996) 29. Suni, I. I., E. G. Seebauer: Thin Solid Films 272 (1996) 229. Sutton, A. P., Godwin, P. D., Horsfield, A. P.: MRS Bulletin 21 (1996) 42. Swartzentruber, B. S.: Phys. Rev. Lett. 76 (1996) 459. Walter, W. K., Manolopoulous, D. E., Jones, R. G.: Surf. Sci. 348 (1996) 115. Wohllebe, A., Hollander, B., Mesters, S., Dieker, C., Crecelius, G., Michelsen, W., Mantl, S.: Thin Solid Films 287 (1996) 93. 96Yu1 Yu, B. D., Scheffler, M.: Phys. Rev. Lett. 77 (1996) 1095. 96Zam1 Zambelli, T., Trost, J., Wintterlin, J., Ertl, G.: Phys. Rev. Lett. 76 (1996) 795. 96Sto1 96Sun1 96Sut1 96Swa1 96Wal1 96Woh1

97Gra1

Graham, A. P., Silvestri, W., Toennies, J. P.: Surface Diffusion, in: “Atomistic and Collective Processes,” Tringides, M., (ed.), New York: Plenum Press, 1997. 97Gra2 Graham, A. P., Hofmann, F., Toennies, J. P., Chen, L. Y., Ying, S. C.: Phys. Rev. B 56 (1997) 10567. 97Hit1 Hitzke, A., Hugenschmidt, M. B., Behm, R. J.: Surf. Sci. 389 (1997) 8. 97Nak1 Nakamura, J., Kagawa, T., Osaka, T.: Surf. Sci. 389 (1997) 109. 97Sna1 Snabl, M., Borusik, O., Chab, V., Ondrejcek, M., Stenzel, W., Conrad, H., Bradshaw, A. M.: Surf. Sci. 385 (1997) L1016. 97Tso1 Tsong, T. T., Fu, T. -Y.: Appl. Surf. Sci. 121/122 (1997) 34. 97Ueb1 Uebing, C. Gomer, R.: Surf. Sci. 381 (1997) 33. 98Dei1 Deimel, M., Rulle, H., Liebing, V., Benninghoven, A.: Appl. Surf. Sci. 134 (1998) 271. 98Dit1 Ditchfield, R.: Llera-Rodríguez, D, Seebauer, E. G.: Phys. Rev. Lett. 81 (1998) 1259. 98Ebi1 Ebinger, H. D., Arnolds, H., Polenz, C., Polivka, B., Prey, B. W., Veith, R., Fick, D., Jansch, H. J.: Surf. Sci. 412/413 (1998) 586. 98Gra1 Graham, A. P., Menzel, A., Toennies, J. P.: J. Chem. Phys. 111 (1998) 1676. 98Gra2 Graham, A. P., Hofmann, F., Toennies, J. P.: Europhys. Lett. 42 (1998) 449. 98Gra3 Graham, A. P., Hofmann, F., Toennies, J. P., Williams, G. P., Hirschmugl, C. J., Ellis, J.: J. Chem. Phys. 108 (1998) 7825. 98Klu1 Klumpp, S., Dabringhaus, H.: Surf. Sci. 417 (1998) 323-336. 98Koh1 Koh, J. H., Wankat, P. C., Wang, N. -H, L.: Ind. Eng. Chem. Res. 37 (1998) 228. 98Kyu1 Kyuno, K., Golzhauser, A., Ehrlich, G.: Surf. Sci. 397 (1998) 191. 98Lin1 Linderoth, T. R., Horch, S., Laegsgaard, E., Stensgaard, I., Besenbacher, F.: Surf. Sci. 402/404 (1998) 308. 98Lob1 Loburets, A. T., Naumovets, A. G., Vedula, Yu. S.: Surf. Sci. 399 (1998) 297. 98Miy1 Miyabe, K., Takeuchi, S.: Can. J. Chem. Eng. 76 (1998) 887. 98Mor1 Mortensen, J. J., Linderoth, T. R., Jacobsen, K. W., Laegsgaard, E., Stensgaard, I., Besenbacher, F.: Surf. Sci. 400 (1998) 290. 98Mus1 Mussawisade, K., Wichmann, T., Kehr, K. W.: Surf. Sci. 412/413 (1998) 55. 98Pan1 Papanicolaou, N. I., Evangelaskis, G. A., Kallinteris, G. C.: Comp. Mat. Sci. 10 (1998) 105. 98Suc1 Suchorski, Yu., Beben, J., Imhihl, R.: Ultramicroscopy 73 (1998) 67. 98Sla1 Slaney, A. J., Bhamidimarri, R.: Water Sci. Technol. 38 (1998) 227. 98Son1 Sonnet, Ph., Stauffer, L., Minot, C.: Surf. Sci. 407 (1998) 121. 98Tuc1 Tuchlenski, A., Uchytil, P., Seidel-Morgenstern, S.: J. Membrane Science 140 (1998) 165. 98Zim1 Zimmerer, G.: J. Low Temp. Phys. 111 (1998) 629. 99Bie1 Biernat, T., Dabrowski, A. M. Vacuum 54 (1999) 99. 99Boi1 Boisvert, G., Lewis, L.: Phys. Rev. B 59 (1999) 9846. 99Bry1 Bryl, R., Blaszczyszyn, R.: Vacuum 54 (1999) 103. 99Dit1 Ditchfield, R., Seebauer, E. G.: Phys. Rev. Lett. 82 (1999) 1185. 99Gle1 Glebov, A. L, Toennies, J.P., Vollmer, S.:Phys. Rev. Lett 82 (1999) 3300. 99Gra1 Graham, A. P., Menzel, A., Over, H., Silvestri, W., Toennies, J. P.: in preparation. 99Hos1 Hoshino, T., Okano, K., Enomoto, N., Hata, M., Tsuda, M.: Surf. Sci. 423 (1999) 117. 99Kar1 Karmanenko, S. F.: Supercond. Sci. Technol. 12 (1999) 36. 99Lai11 Laird, S. K., Buttry, D. A., Sommerfield, R. A.: Geophys. Res. Lett. 26 (1999) 699. 99Lee1 Lee, G. D., Wang, C. Z., Lu, Z. Y., Ho, K. M.: Surf. Sci. 426 (1999) L427. Lando lt -Bö rnst ein New Ser ies III/42A1

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Lando lt -Börnst ein New Ser ies III/42A1

Diffusion in Condensed Matter

3d, 4d and 5d Elements, Alloys and Compounds (Landolt-Bornstein: Numerical Data and Functional Relationships in Science and Technology - New Series Condensed Matter)

Interactions of Photons and Electrons with Atoms (Numerical Data and Functional Relationships in Science and Technology - New Series)

Laser Applications (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Advanced Materials and Technologies)

Virial Coefficients of Pure Gases (Landolt-Bornstein: Numerical Data and Functional Relationships in Science and Technology - New Series Physical Chemistry)

Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology Part 2

Densities of Aromatic Hydrocarbons (Landolt-Bornstein, Numerical Data and Functional Relationships in Science and Technology , Vol 8)

Collisions of Electrons with Atomic Ions (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Elementary Particles, Nuclei and Atoms)

Photochemistry on Solid Surfaces (Studies in Surface Science and Catalysis)

Selected Systems from Al-Fe-V to Al-Ni-Zr (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Physical Chemistry)

Adsorption on Surfaces and Surface Diffusion of Adsorbates (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Condensed Matter)

Diffusion in Condensed Matter

3d, 4d and 5d Elements, Alloys and Compounds (Landolt-Bornstein: Numerical Data and Functional Relationships in Science and Technology - New Series Condensed Matter)

Interactions of Photons and Electrons with Atoms (Numerical Data and Functional Relationships in Science and Technology - New Series)

Laser Applications (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Advanced Materials and Technologies)

Virial Coefficients of Pure Gases (Landolt-Bornstein: Numerical Data and Functional Relationships in Science and Technology - New Series Physical Chemistry)

Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology Part 2

Adsorption and Diffusion

Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces (Studies in Surface Science and Catalysis)

Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces (Studies in Surface Science and Catalysis)

Densities of Aromatic Hydrocarbons (Landolt-Bornstein, Numerical Data and Functional Relationships in Science and Technology , Vol 8)

Adsorption, Surface Area and Porosity

Adsorption and Diffusion in Nanoporous Materials

Diffusion in condensed matter: methods, materials, models

Diffusion in condensed matter methods materials models

Adsorption on Silica Surfaces

Collisions of Electrons with Atomic Ions (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Elementary Particles, Nuclei and Atoms)

Photochemistry on Solid Surfaces (Studies in Surface Science and Catalysis)

Adsorption on New and Modified Inorganic Sorbents (Studies in Surface Science and Catalysis)

Symmetry and Condensed Matter Physics

Selected Systems from Al-Fe-V to Al-Ni-Zr (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Physical Chemistry)

Springer Handbook of Condensed Matter and Materials Data

Springer Handbook of Condensed Matter and Materials Data

Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces

Understanding Soft Condensed Matter Via Modeling and Computation (Series in Soft Condensed Matter 3)

Adsorption on New and Modified Inorganic Sorbents

Time and Intimacy : A New Science of Personal Relationships (Lea's Series on Personal Relationships)

Spillover and Mobility of Species on Solid Surfaces ,Volume 138 (Studies in Surface Science and Catalysis)

Optical Properties of Condensed Matter and Applications

Optical Properties of Condensed Matter and Applications

Surface Diffusion

Adsorption on Surfaces and Surface Diffusion of Adsorbates (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Condensed Matter)

Diffusion in Condensed Matter

3d, 4d and 5d Elements, Alloys and Compounds (Landolt-Bornstein: Numerical Data and Functional Relationships in Science and Technology - New Series Condensed Matter)

Interactions of Photons and Electrons with Atoms (Numerical Data and Functional Relationships in Science and Technology - New Series)

Laser Applications (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Advanced Materials and Technologies)

Virial Coefficients of Pure Gases (Landolt-Bornstein: Numerical Data and Functional Relationships in Science and Technology - New Series Physical Chemistry)

Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology Part 2

Adsorption and Diffusion

Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces (Studies in Surface Science and Catalysis)

Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces (Studies in Surface Science and Catalysis)

Densities of Aromatic Hydrocarbons (Landolt-Bornstein, Numerical Data and Functional Relationships in Science and Technology , Vol 8)

Adsorption, Surface Area and Porosity

Adsorption and Diffusion in Nanoporous Materials

Diffusion in condensed matter: methods, materials, models

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