Advances in Macromolecules: Perspectives and Applications

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Advances in Macromolecules

Maria Vittoria Russo Editor

Advances in Macromolecules Perspectives and Applications

123

Editor Prof. Maria Vittoria Russo Universit`a di Roma Sapienza Dipto. Chimica Piazzale Aldo Moro, 5 00185 Roma Italy [email protected]

ISBN 978-90-481-3191-4 e-ISBN 978-90-481-3192-1 DOI 10.1007/978-90-481-3192-1 Springer Dordrecht Heidelberg London New York Library of Congress Control Number: 2009944206 © Springer Science+Business Media B.V. 2010 No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Preface

Macromolecular science attracted the worldwide interest since the last century and has grown dramatically nowadays due to the novel challenges and perspectives foreseen for emerging fields such as optoelectronics, biology, medicine and catalysis where these materials provide unexpected applications. Original advances are reported in a wide number of papers dealing with polymerization methods, physical phenomena, advances in chemico-physical characterization, theoretical approaches and modelling, which witness the importance of the macromolecules in a variety of interdisciplinary fields. Macromolecules fulfil many needs of the society facing global challenges, for example reuse of polymeric materials, health care, novel technologies for the development of security and defence. Macromolecules show up many tuneable properties arising from their chemical structure and functionalization, that can be modulated through the synthesis, and from their versatility for manifold applications. This book wishes to emphasize some peculiarities of the macromolecules which are considered the premises of future advanced developments. The aim is to give a glance on the opportunities offered in basic science and in most leading and sophisticated technologies, focussing mainly on research topics. On the other hand, the vastness of features and subjects related to the research in macromolecular science implies that important fields such as composites, hybrids, carbon nanotubes, plastic materials etc. will not be considered in this book, although their importance and interest is obviously paramount. Chapter 1 highlights one of the most intriguing challenges for macromolecules, that is the extraordinary properties associated to the nanostructure. Many fields of materials science are successfully applied to the macromolecules and are reviewed including novel synthetic approaches, emulsion polymerizations, self-assembly, templating and grafting techniques, electrochemistry and electrospinning, all of them investigated with the aim of achieving nanosized materials. The peculiar properties associated to the nanostructure are underlined with examples of the most cited macromolecules and in particular polymers. Chapter 2 focuses on an important property of macromolecules, i.e., nonlinear optical (NLO) behaviour, with promising applications in development of faster and highly performing communication devices such as computers and fiber optic telecommunications. An introduction to the basic physical principles that undergo v

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Preface

the NLO properties is followed by the discussion of the principal techniques actually preferred for the detection of this optoelectronic behaviour in the case of macromolecules. The molecular structure requirements for the showing of NLO properties of macromolecules are also addressed and discussed with the aid of numerous examples, thus giving a deep insight into this promising field. Experimental methods which allow the detection of NLO properties and devices based on materials having NLO properties are thoroughly described in Chapter 3. In particular, the main experimental techniques for polymer orientation are presented and the limitations of the different poling configurations are discussed. Patterning and lithographic techniques applied to macromolecular systems are described as well as some of the most up to date devices based on such materials. Devices such as Mach-Zehnder modulators, microring resonators, optical filters and switches are discussed. This chapter emphasizes the role of modern technology in the field of novel applications for macromolecules. Advanced synchrotron based characterization techniques of solid state applied to macromolecules are reported in Chapter 4. After an introduction to the physics and principles of NEXAFS and XPS spectroscopy, the main features of these techniques that allow a non conventional assessment of the electronic and chemical structure are depicted. The study of macromolecular organization and self-assembly can be nicely obtained by these spectroscopic tools. For example the formation of SAMs (Self Assembled Monolayers) of a variety of molecules arranged in supramolecular assemblies can be detected as well as the behaviour of biomolecules bound to surfaces mimicking biological substrates. Many examples of macromolecules studied with NEXAFS and XPS highlight the potential of these spectroscopic methods to give insight into the molecular and supramolecular structure which in turn determine the most desired properties. A class apart of macromolecules is represented by biomolecules. Meaningful examples addressing new insight in this multidisciplinary area are proposed in Chapter 5, in which the breakthrough of these materials in different fields of science and technology is highlighted. Macromolecules of natural source and bioconjugates, with a particular attention to their nanostructured morphology, of interest in future applications such as catalysts, membranes or energy conversion devices, biosensors, and advances in the field of drug delivery and intelligent therapeutics are in particular discussed. In summary, the book aims to answer to the demand of scientists who foresee a promising future in various areas of basic research and applications concerning macromolecules in general and nanosized ones in particular. Although not exhaustive, the book is intended to encourage the research in the exciting future of macromolecular science. Moreover, the text is enriched with many figures and references, especially reviews, and with appendixes dealing with widely used characterization techniques which are proposed as useful tools for the education of graduate and PhD students. Rome, Italy September 2009

Maria Vittoria Russo

Contents

1

Nanostructured Macromolecules . . . . . . . . . . . . . . . . . . . . Maria Vittoria Russo, Ilaria Fratoddi, and Iole Venditti

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Macromolecular Systems with Second Order Nonlinear Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . Roberto Centore and Antonio Roviello

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Macromolecular Systems with Nonlinear Optical Properties: Optical Characterization and Devices . . . . . . . . . . Paolo Prosposito and Fabio De Matteis

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Functional and Nanostructured Materials Investigated by XPS and NEXAFS Spectroscopies . . . . . . . . . . . . . . . . . . . Giovanni Polzonetti and Chiara Battocchio

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Hybrid Systems Biomolecule-Polymeric Nanoparticle: Synthesis, Properties and Biotechnological Applications . . . . . . . Cleofe Palocci and Laura Chronopoulou

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5

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Appendix Principal Characterization Techniques of Nanostructured Macromolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Contributors

Chiara Battocchio Laboratory of Materials Chemistry, Department of Physics, University “Roma Tre”, Via della Vasca Navale 79, Rome 00146, Italy, [email protected] Roberto Centore Department of Chemistry “Paolo Corradini”, University of Naples “Federico II”, Via Cinthia, Naples 80126, Italy, [email protected] Laura Chronopoulou Department of Chemistry, University of Rome “Sapienza”, Piazzale Aldo Moro 5, Rome 00185, Italy, [email protected] Fabio De Matteis Micro- and Nano-Structured Systems laboratory - MINASlab, Dipartimento di Fisica, Consorzio Interuniversitario per la Scienza e Tecnologia dei Materiali – INSTM, Università degli Studi di Roma “Tor Vergata” Via della Ricerca Scientifica 1, Roma 00133, Italy, [email protected] Ilaria Fratoddi Department of Chemistry, University of Rome “Sapienza” P.le A. Moro 5, Rome 00185, Italy, [email protected] Cleofe Palocci Department of Chemistry, University of Rome “Sapienza”, Piazzale Aldo Moro 5, Rome 00185, Italy, [email protected] Giovanni Polzonetti Laboratory of Materials Chemistry, Department of Physics, University “Roma Tre”, Via della Vasca Navale 79, Rome 00146, Italy, [email protected] Paolo Prosposito Micro- and Nano-Structured Systems Laboratory – MINASlab, Dipartimento di Fisica, Consorzio Interuniversitario per la Scienza e Tecnologia dei Materiali – INSTM, Università degli Studi di Roma “Tor Vergata” Via della Ricerca Scientifica 1, Roma 00133, Italy, [email protected] Antonio Roviello Department of Chemistry “Paolo Corradini”, University of Naples “Federico II”, Via Cinthia, Naples 80126, Italy, [email protected] Maria Vittoria Russo Department of Chemistry, University of Rome “Sapienza” P.le A. Moro 5, Rome 00185, Italy, [email protected] Iole Venditti Department of Chemistry, University of Rome “Sapienza” P.le A. Moro 5, Rome 00185, Italy, [email protected] ix

List of Abbreviations

6FDA AFM AHB AIBN AK ALS AOP AOT AP APC Arg AT ATR ATRA ATRP BCB BE BOC BSA CD CLD-1 CMC CPW CSA CTAB CyD DANS DAST DBCP DBR DBSA DCDA DFT

4,4 -(Hexafluoro-Isopropylidene)Diphthalic Anhydride Atomic Force Microscopy Angular Hole Burning 2-20-Azobisisobutyronitrile L -Alanine– L -Lysine Advanced Light Source All-Optical Poling Sodium Bis(2-Ethylhexyl) Sulfosuccinate Atmospheric Plasma Amorphous Polycarbonate Arginin Anthracene Attenuated-Total-Reflection Atom Transfer Radical Addition Atom Transfer Radical Polymerization Benzocyclobutene Binding Energy Tertbutoxycarbonyl Bovine Serum Albumin Circular Dichroism Aminophenyltetraene-bridge Chromophore Critical Micelle Concentration Coplanar Waveguide Camphorsulfonic Acid Cetyltrimethylammonium Bromide Cyclodextrin N,N-Dimethylamino-Nitrostilbene 4-Dimethylamino-N-Methylstilbazolium 4-Toluene-Sulphonate Diblock Copolymers Distributed Bragg Reflector Dodecil-Benzenic Sulfonic Acid 10,12-Docosadiyndioic Acid Density Functional Theory xi

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DIA DLIP DMF DMSO d-OSC DPD DPPH DR1 DR19 DX EA EB ECM E-DPN EF EFISH Ek Em EO EOAS EOM EPR ES FC FET FITC FLAG FRGP FWHM Gly GMA HAS HEC HETE His HOMO HOPG HRS HST HV IP IR ITO LB LBL

List of Abbreviations

Dimethylaminoindoaniline Laser Interference Micro-Nanopatterning Dimethylformamide Dimethylsulphoxide Digital Oscilloscope Dissipative Particle Dynamics 1,1-Diphenyl-2-Picrylhydrazyl Dispersed Red 1 Dispersed Red 19 Dextran Electroauxiliary Emeraldine Base Extracellular Matrix Electrochemical Dip-Pen Nanolithography Fermi Level Electric Field Induced Second Harmonic Kinetic Energy Electromagnetic Electro-Optical Electro-Optical Absorption Spectroscopy Electro-Optic Modulators Enhanced Permeation and Retention Emeraldine Salt Ferrocene Field-Effect Transistors Fluorescein Isothiocyanate Fiber-Optic Link Around the Globe Free Radical Graft Polymerisation Full Width at Half Maximum Glicyne Glycidyl Methacrylate Human Serum Albumine Hydroxyethyl Cellulose 5-Hydroxyeicosatetraenoic Acid Histidine Highest Occupied Molecular Orbital Highly Ordered Pyrolytic Graphite Hyper-Rayleigh Scattering 4-Hydroxystyrene High Voltage Ionization Potential Infrared Indium Tin Oxide Langmuir Blodgett Layer-By-Layer

List of Abbreviations

LS LUMO MBE MLD MMA MMONS MNBA MO MOCVD MRI MS MTh MWCNT MZ NaPS NEXAFS NIL NLO NMP NMR NP Np NPAO NSA NSC ODPA OG 146 OLED OPE OR PA PAA PAB PAC PAMAM PAMAMOS PANANA PANI PAP Pc PC PCL PDA PDDA PDMS

Light Scattering Lowest Unoccupied Molecular Orbital Molecular Beam Epitaxy Molecular Layer Deposition Methyl Methacrylate 3-Methyl-4-Methoxy-4 -Nitrostilbene 4 -Nitrobenzylidene-3-Acetamido-4-Methoxyaniline Molecular Orbitals Metal Organic Chemical Vapor Deposition Magnetic Resonance Imaging Mass Spectrometry 3-Methylthiophene Multiwall Carbon Nanotube Mach Zehnder Sodium Persulfate Near Edge X-Ray Absorption Fine Structure Spectroscopy Nanoimprint Lithography Nonlinear Optical N-methyl-2-Pyrrolidinone Nuclear Magnetic Resonance Nanoparticle Particle Number Density Nanoporous Alumina Oxide Naphthalenesulfonic Acid Neural stem cell 3,3 ,4,4 -Oxydiphthalic Anhydride UV epoxy provided by the EPOXY TECK Company Organic Light-Emitting Diodes Oligo(Phenyleneethynylene) Optical Rectification Polyacetylenes Polyacrylic Acid Poly(Azobenzene) Polyacrylate Polyamidoamine Poly(Amidoamine-Organosilicon) Poly(Aniline-Co-Anthranilic Acid) Polyaniline Photo-Assisted Poling Phthalocyanines Polycarbonate Polycaprolactone Polydiacetylene Poly(Diallyldimethylammonium Chloride) Poly(Dimethylsiloxane)

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PDPSA PDPTh PDTEC PE PEDOT PEG PEL PEO Pf PFcS Phe PHS PI-DAIDC PIGP PIP PLA PLED Ply PMANI PMMA PMPy PMT PMY POEP POMA PP PPA PPE PPED PPPA PPPT PPV PPy PPZ PS PSC PSD PSi PSS PTFE PTh PVA PVAC PVF

List of Abbreviations

3-Pentadecyl Phenol-4-Sulphonic Acid Poly(Dimethyl-3,4-Propylenedioxythiophene) Poly(Desaminotyrosyl-Tyrosine Ethyl Ester Carbonate) Pass Energy Poly(3,4-Ethylenedioxythiophene) Polyethylenglicol Polyelectrolytes Polyethylenoxide Porphyrins Polyferrocenylsilane Phenylalanine Poly(4-Hydroxystyrene) Polyetherimide Double Amino Isophorone Dicyanide Plasma Surface Treatment and Plasma-Induced Graft Polymerization Polyisoprene Poly-Lactic Acid Polymer Light-Emitting Diodes Poly L-Lysine Poly(N-Methylaniline) Polymethylmethacrylate Poly(N-Methylpyrrole) Photomultiplier Tube Polymetallayne Poly(N-Octadecyl-2-Ethynylpyridinium Bromide) Poly(o-Methoxyaniline) Photopolymerization Polyphenylacetylene Poly(para-Phenylene Ethynylene) Poly(o-Phenylenediamine) Poly(meta-Phenylene Isophthalamide) 6-(5-Pyridin-2,Yl-Pyrazin-2-Yl)Pyridine-3-Thiol Poly(para-Phenylvinylene) Polypyrrole Polyphosphazenes Polystyrene Polysaccharides Particle Size Distribution Polysilanes Poly(Sodium 4-Styrenesulfonate) Polytetrafluoroethylene Polythiophene Polyvinylalcohol Polyvinylacetate Polyvinylidenefluoride

List of Abbreviations

PVK PVP PVPY Py-C60 QD RAFT RGW RIE RT S SA SAM SANS scCO2 SEM SERR SFG SH SHG SR SR&NI STEX STXM SU8 SWCNT TBAA TBU TCVDPA TEM TEY Tg THAHFP TMC TMT TPT Trp TSA TS-CuPc Tyr UHV UHVC UV VL VP

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Polyvinylcarbazole Polyvinylpyrrolidone Poly(4-Vinylpiridine) N-Methyl-2-(P-Pyridyl)-3,4-Fulleropyrrolidine Quantum dot Reversible Addition-Fragmentation Chain Transfer Technique Resonant Grating Waveguide Reactive Ion Etching Room Temperature Styrene Self-Assembly Self-Assembled Monolayer Small-Angle Neutron Scattering Supercritical Carbon Dioxide Scanning Electron Microscopy Surface Enhanced Raman Resonance Sum-Frequency Generation Second Harmonic Second-Harmonic Generation Synchrotron Radiation Simultaneous Normal and Reverse Initiation Static-Exchange Approximation Scanning Transmission X-Ray Microscopy Negative, Epoxy-type, Near-UV Photoresist (365 nm) US Patent No. 4882245 (1989) Single Wall Carbon Nanotube Tetrabutylammonium Acetate Tert-Butyl Tricyanovinylidenediphenylaminobenzene Transmission Electron Microscopy Total Electron Yield Glass Transition Temperature Tetrahexylammonium Hexafluorophosphate Transition Metal Catalyzed Teng and Man Technique 1,1 ;4 ,1 -Terphenyl-4-Thiol Tryptophane p-Toluene Sulfonic Acid Tetrasulfonate Copper Phtalocyanine Tyrosine Ultrahigh Vacuum Ultrahigh Vacuum Chambers Ultraviolet Vacuum Level 1-Vinyl-2-Pyrrolidone

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WDM Wp X-PEEM XPS ZnPf

List of Abbreviations

Wavelength Division Multiplexers Weight Fraction of Polymer X-Ray Photoemission Electron Microscopy X-Ray Photoelectron Spectroscopy Zn Diethynyl Porphyrin

Chapter 1

Nanostructured Macromolecules Maria Vittoria Russo, Ilaria Fratoddi, and Iole Venditti

Abstract Macromolecules with nanoscale size are actually object of dramatic interest due to the expectations in several technological applications ranging from optoelectronics to biomedicine. In this chapter the most investigated methods suitable for the achievement of nanostructured macromolecules are reported together with a variety of examples of chemical structures and properties. Self-assembly, template assisted, grafting, electrochemical and emulsion polymerizations, as well as electrospinning technique are described highlighting the variety of materials, mainly polymers, that are prepared in a range of shapes and dimensions which are most appropriate for a desired property. The same macromolecule can be obtained, for example, with the structure of a nanosphere or of a nanorod, which in turn can be hollow or solid, thus being promising for different applications. The structure-property correlation will be outlined for many of the cited macromolecules throughout the chapter. Moreover, the mix of methods based on different approaches to generate nanostructures is also reported since often there is not a defined line of separation between them. Finally, a sub-chapter is dedicated to the advances in several fields of emerging technology and to the perspectives of future applications for nanostructured macromolecules.

1.1 Introduction In the last decade, nanoscience and nanotechnology have been object of an outstanding burst owing to expectations of benefits for health and quality of life in a variety of fields: nanoelectronics, nanodevices, nanocomposite materials, alternative energy resources, biotechnology and nanomedicine, besides breakthroughs in basic science. The interface between science and technology is a peculiar feature for this field of the research, involving the expertise of scientists of different education.

M.V. Russo (B) Department of Chemistry, University of Rome “Sapienza”, P.le A. Moro 5, Rome 00185, Italy e-mail: [email protected]

M.V. Russo (ed.), Advances in Macromolecules, DOI 10.1007/978-90-481-3192-1_1, C Springer Science+Business Media B.V. 2010

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The focus of this chapter is mainly devoted to the most suitable procedures for the preparation of nanostructured macromolecules with particular emphasis on polymeric materials of synthetic origin. Another chapter of the book will deal with the methods for the attainment of nanostructured polymers of natural and biological origin. Several challenges in the field of technological applications are mentioned in appropriate contexts, highlighting the role of the nanostructure on the properties and performances of polymers and macromolecules with a glance to structural features. In general, two main approaches can be envisaged for the attainment of nanostructures: bottom-up (i.e. growth induced from the monomer to the macromolecule) and top-down (i.e. nano size induced from bulk material). To the bottom-up methods belong the self-assembly procedure, electrochemical, template assisted, grafting, emulsion, gamma-radiation induced, and chemical oxidation, among the preferential ones reported in a wide number of papers dealing with nano polymers. The top-down methods involve electrospinning technology, Langmuir-Blodgett deposition, osmosis and laser micro/nanopatterning. However, this classification is sometimes ambiguous, because the two methods are often complementary and the techniques to achieve nanoparticles are usually borderline or overlapping. One of the main goals of the recent research relies on the preparation and application of materials with the desired nanoscale morphology [1]. In fact, the direct control of morphology is a fundamental request for the fine tuning of the size, shape and extension of the nano-feature and this has to be in turn combined to the achievement of desired optical and electronic properties. Furthermore, the obtained morphology should be stable in time and thermally. The nanoparticles of conjugated and non-conjugated polymers exhibit a variety of morphologies, i.e. spheres, rods, fibers, ribbons, flakes and other ones which resemble the architectures in nature, such as cauliflowers, raspberries, fractals, that have inspired the fantasy of scientists. The nanoparticles are in turn able to build 1D, 2D and 3D structures. Nanotechnologies are essential to fabricate highly integrated, tiny, and lightweight electronic devices with high performance and nanostructured materials also endow with intrinsically exceptional properties such as the energy conversion and storage [2]. The precise control over the nanostructure formation is often obtained through indirect methods, as for instance, thermal or solvent annealing [3, 4]. A good control of the morphology at nanometric scale is also accessible with methods based on the ability of certain classes of materials to self-assembly or crystallize with the desired shape, i.e. spheres or rods [5] or organize themselves in emulsions with a solvent such as water, where the composition rules the predominant phase in a predictable arrangement [6]. A different approach based on templates has been widely explored, where the nanostructure is generated by using an organic or inorganic sacrificial material that is removed at a later stage of the process [7]. Tethered polymer phases can be formed either by polymer grafting (“grafting to”) or graft polymerization (“grafting from”) and dense surface coverage are generally obtained. Emulsion polymerization has proved to be effective for the formation of spherical nanoparticles and experimental parameters drive regularity of shapes and

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sizes. Electrochemical methods were also applied for the preparation of nanotubes and nanowires by using template and template free methods. Electrospinning has been extensively used for the preparation of nanofibres and nanotubes, by using an high voltage source to induce fibres formation from natural and synthetic polymers. The chapter has not the ambition of being exhaustive because the current research on this topic is rapidly evolving by the publication of hundreds of papers. However, the aim is to offer a glance on the efforts and challenges in the constantly growing field of macromolecular nanostructures with some related applicative perspectives. As an example, a variety of nanostructured synthetic and natural polymers, with different chemical and physical properties is reported in Fig. 1.1. Hybrid systems, carbon nanotubes and composites are not reviewed in this chapter, our attention was mainly devoted to the preparation of different macromolecules by means of the most investigated methods.

Fig. 1.1 SEM images of different morphologies obtained for synthetic polymers (a: polymethylmethacrylate; b: polystyrene; c: polyphenylacetylene; d, e: poly(N,N-dimethylpropargylamine derivatives; f: Pt-polymetallayne) and biopolymers (g: chitosan; h, i, l, m: hyaluronic acid derivatives; n: dextran) (Reprinted with permission from Chronopoulou et al. [8]. Copyright 2009 American Chemical Society)

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Self-assembly, template, grafting, electrochemical, emulsion polymerization and electrospinning are reported with particular attention to new applications and perspectives for nanoscale materials with unforeseen properties due to the nanosize.

1.2 Self-Assembly 1.2.1 General Features Self-organization of macromolecules is one of the most popular way to achieve nanostructured features because it can be in principle applied to every kind of polymer, natural or synthetic [8]. The recent advances in design criteria for the attainment of well-defined polymers and nanostructures allow to produce macromolecules with specific functionalities which are tailored for potentials in development of capsules, drug delivery systems and nanoscale electro-optical devices [9]. Upon this premise, the methods that are able to induce the self-assembly of macromolecules are related to the chemico-physical properties of the selected polymer, of the substrate on which the nanostructure grows and on their combination. Obviously, this premise envisages the variety of different morphologies, nanostructures and related applications that can be obtained by the versatility of self-assembly. The concept of self-assembly was introduced in a pioneering paper, where the idea was applied to biomolecules [10]; the authors report “Molecular selfassembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures”. This statement can be straightforwardly transferred to non-biological macromolecules. A selfassembling system consists of a group of molecules or segments of a macromolecule that interact each other. These molecules or molecular segments may be similar or different. Their interaction flows from some less ordered state (a solution, disordered aggregate, or random coil) to a final more ordered state (a crystal or folded macromolecule). Aggregation occurs when there is a net attraction and an equilibrium separation between the components. The equilibrium separation normally represents a balance between attraction and repulsion. The following Fig. 1.2 simply illustrates this concept. In a wide context, two-dimensional (2D) and three-dimensional (3D) structures are built with the self-assembly of macroscopic components of different nature via capillary interactions. The main topics proving the versatility of the method are reported in a comprehensive paper [11] with many examples; open hexagonal arrays and hexagonal lattices are formed around circular templates self-assembled from poly(dimethylsiloxane) plates; spherical structures grow by self-assembly of hexagonal metal plates on the surface of a drop of perfluodecalin in water; compact 3D structures are obtained by self-folding of a string of tethered, polymeric polyhedra and large crystals self-assemble from micrometer-sized hexagonal metal plates; aggregates with electrical connectivity can be produced and

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A E

r

d a eq

B

random

D

C

aggregation equilibria

Fig. 1.2 (a) Schematic picture of aggregation equilibria: (a) The equilibrium curve (eq) represents a balance between attraction (a) and repulsion (r); (b), (c) and (d) represent the aggregation equilibria from random to ordered assembling

assembled from polyhedral, polymer components bearing solder patterns of wires and dots. According to the above cited general principles, self-assembly of macromolecules relies on some universal features and is mainly concerned with chemistry principles, design and selection of molecules and looks at the world of biological processes. It is interesting the comparison with the top-down “size-shrinking” (e.g. nanolithography), based on physical approach and particularly suitable for the development of the microelectronics technology, that lowers the limits of the size of components and devices, and with the nanofabrication and nano-manipulation bottom-up approach to molecular nanotechnology that also relies on physical methods (e.g., near-field scanning microscopes). An attempt to provide a rationale to the features related to the self-assembly has been reported in a recent paper that has faced the investigation of thermodynamic parameters, in particular entropic terms, which are drivers for the polymer nanoparticle self-assembly, with a theoretical approach based on fluids density functional theory (DFT) calculations [12]. It is note worthy, however, that the concepts and methods and principles which are on the basis of the self-assembly process often overlap each other and are complementary, so as it will be described in the text.

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1.2.2 1D, 2D and 3D Self-Assembled Macromolecular Structures: General Methods and Examples 1.2.2.1 Supramolecular Chemistry and Hierarchical Self-Organization; Polymers and Block Copolymers Quite often, nanoparticles assembling is based on evaporation procedures, i.e. the drying on a substrate of a drop of suspension or solution containing the nanoparticles thus inducing the particles organization [13]. This is a dynamic process governed by a variety of factors, such as interactions between nanoparticles, substrates and solvents [14, 15], drying kinetics [16–18], hydrodynamic effects [19, 20] and diffusion processes [21], which lead to unusual transitory structures [22]. The morphologies as well as the size of the domains of these self-assembled particles depend on the solvents used for the procedure, the evaporation time, temperature and particle diffusion, and can be qualitatively evaluated with theoretical simulations. The drying-mediated self-assembly of nanoparticles on diblock copolymer substrates was deeply investigated with the aid of a coarse-grained lattice gas model and Monte Carlo simulation techniques [23]. The “bottom-up” approach, that in principle is based on the ability of functional building blocks to assemble into defined superstructure arrays, is one of the most widely used method to achieve materials at the scales between 1 and 100 nm. Supramolecular chemistry and self-organization is a fundamental topic of this approach “where the goal is not smaller size or individual addressing but complexity through self-processing, which strives for self-fabrication by the controlled assembly of ordered, fully integrated, and connected operational systems by hierarchical growth” as reported by J. M. Lehn [24] and is a convenient alternative to nanofabrication and nanomanipulation. In the framework of hierarchical self-organization it is note worthy the synthesis and self-assembly of polymer coated ferromagnetic nanoparticles, where the use of dipolar nanoparticles as building blocks with inherent dipole moment enables the preparation of organized hierarchical materials in one- and two-dimensional assemblies, which represent a promising area of application in materials chemistry. As an example, a review on this topic reports on polymer-coated ferromagnetic cobalt nanoparticles (core shell nanoparticles self-assembled in aligned chains, reported in Fig. 1.3) that were synthesized by using end-functionalized polystyrene (PS) surfactants with amine, carboxylic acid or phosphine oxide end-groups as stabilizing agents of the ferromagnetic Co nanoparticles [25]. A different approach to the synthesis of nanosized macromolecules through hierarchical self-assembly is based on Layer-by-Layer (LbL) chemistry. LbL allows the deposition of ultra thin films whose thickness can be controlled by the chemical structure of the molecules and number of deposited layers. The interactions between layers can be ionic, covalent, hydrogen-bonding, and charge-transfer, depending upon the nature of the polymer used in the preparation.The layer-bylayer assembly of an electroactive polymer nanocomposite thin film of cationic linear poly(ethyleneimine) and Prussian Blue nanoparticles, has been exploided

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Fig. 1.3 TEM images of self-assembled ferromagnetic PSCoNPs (DPS-CoNPs 21–31 nm) at low (a) and high magnification (b), prepared from a mixture of PS-NH2 (3) and PSCOOH (6) in the thermolysis of Co2 (CO)8 . The PS-CoNPs were cast onto supporting surfaces from a particle dispersion in toluene (Reprinted with permission from Keng et al. [25]. Copyright 2009 American Chemical Society)

showing mechanical and swelling properties [26]. Also the biomedical purposes take advantage of the LbL technique for the assembling of nanomaterials; for example a hydrophobic drug can be deposited by the sequential adsorption of oppositely charged polyelectrolytes onto a charged substrate [27]. This technique is a suitable tool for the self assembly of other materials e.g. polydiacetylene (PDA) films and nanotubes organized on flat surfaces and inside of nonporous alumina templates [28] and porphyrin arrays [29]. The supramolecular self-assembly approach in the solid state from solution, leading to well defined nanostructures has been discussed in a comprehensive paper that describes the main features related to this method; interactions of macromolecules with the substrate surface, design of well defined molecular structure, and use of block copolymers have been considered in a joint experimental-theoretical approach, in view of understanding the structure-property relationship of conjugated nanostructures [30]. The hierarchical self-assembly approach has been proposed as a valuable method in many examples of macromolecules nanodesign. Nanoparticles assembly at

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liquid-liquid interface can be controlled by tuning the size, the volume fraction and the chemical characteristics of the ligands; this method is suited to generate nanoparticle-polymer composites, whose spatial distribution can be controlled by enthalpy or entropy, thereby producing auto responsive materials. An interesting paper illustrates how the self-assembly of polymeric supramolecules induces the synthesis of functional materials with peculiar properties and shows nice examples of the way macromolecules are induced to self-assembly [31]: flexible polymers, such as comb-shaped supramolecules, are assembled through hydrogen bonds, while rod-like polymers require a combination of bonds (recognition); the connection of amphiphiles to one of the blocks of a diblock copolymer induces selforganization in hierarchical structures; a lamellae-within-cylinders structure can be cleaved to produce nanoporous materials, can lead to disk-like morphology by cross linking the slices within the cylinders or can deliver nanorods by cleaving the side chains. The self-assembly of polymer nanoparticles (spheres and wire-like threads) can occur in solution by using dendrimer macroinitiators in atom transfer radical polymerization (ATRP) [32] that will be extensively discussed in Section 1.4.4. Ring opening polymerization (ROP) is successfully used for the self-assembly of amphiphilic graft polyphosphazenes with different mole ratios of hydrophobic groups to hydrophilic segments to yield supramolecular aggregates (nanospheres, high-genus particles, macrophage-like) [33]. These examples show the role of the chemical structure on designing nanoscale objects through supramolecular self-assembly. Among macromolecules, porphyrins are particularly attractive building blocks because the intimate packing of these aromatic macrocycles can lead to new photophysical and photochemical properties. Self-assembling of porphyrin molecules into hollow hexagonal nanoprisms with uniform size and shape and controllable aspect ratio was recently achieved by the self-assembly technique assisted with surfactant. Nanoprisms can readily self-organize into an ordered, smectic three-dimensional (3D) architecture through simple evaporation of the solvent [34]. Free-standing porphyrin nanosheets with high aspect ratios were recently obtained by reprecipitation method [35]. These results should be significant in porphyrin crystallization and porphyrin application in optoelectronic devices, catalysis, drug delivery, and molecular filtration. About a decade ago, the main features (experimental and applicative) governing the self-organization of nanostructured macromolecules were highlighted, with particular emphasis on block copolymers, envisaging the future perspectives for these materials [36]. Since then, the research has dramatically grown and many goals have been achieved. A review reports an organized and detailed overview on theoretical aspects and basic principles of self-assembly and micellization of block copolymers in solution, together with a wide number of examples concerning the methods for the stabilization of macromolecular aggregates and their applications, mainly focused on biomedical field, in the perspective of “smart” nano-objects production [37]. The self-organization of block copolymers in different shapes is depicted in Fig. 1.4.

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Fig. 1.4 Examples of structures obtained from block copolymers: (i) direct micelle, (ii) vesicles, and (iii) other morphologies: (iiia) inverse micelle, (iiib) lamellar structures, and (iiic) cylindrical or tubular micelle (Reprinted from Rodríguez-Hernández JR et al. [37], with permission from Elsevier)

Examples of structures obtained from block copolymers range from micelle, vescicles to lamelle or cylindrical and tubular structures, suitable for drug delivery systems and, in general, as host-guest systems. In fact, an emerging field of nanoscale science is envisaged in molecular capsules which can host guest molecules through noncovalent interactions. These synthetic molecular receptors exert their peculiar activity upon the conjugation of parameters such as size, shape, and chemical complementarity and are proposed for applications in catalysis of chemical reactions and for the stabilization of reactive species [38]. For example, hollow hydrophilic metal functionalized nanostructures can be produced from an amphiphilic metallic diblock copolymer which supramolecularly self-assemble into monodisperse noncovalently connected micelle and can be used as nanocages [39]. Interestingly, functionalized block copolymers in solution can provide the ordering of nanoparticles in a variety of distinct phases, i.e. cubic, layered hexagonal, hexagonal columnar, gyroid and square columnar, as developed by molecular dynamics studies and by experimental investigations based on the solvent composition and valence of the organic counter ion, respectively [40, 41]. A peculiar example of colloidal stable micelle formation is represented by core-shell organometallic 1D nanocylinders obtained from the self-assembly of polyferrocenylsilane cores and polyisoprene coronas crosslinked block copolymers; these micelle are suited

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to be used in microfluidic alignment, nanoceramic fabrication and other advanced technologies [42]. Recently, self-assembled block copolymers have attracted the interest of scientists as masks for nanolithography, templates for the synthesis of nanoparticles [43] and membranes for ultrafiltration [44]. Block copolymers films made from polystyrene-block-poly (4-vinylpiridine) and 2-(4 -hydroxybenzeneato) benzoic acid form cylinders with hexagonal order aligned along the normal direction to the substrate and embedded into the PS matrix. The nanoporous films are obtained by removing the benzoic acid from the cylinders, and periodic hexagonal moiré superstructures are obtained when the films with long range order are superimposed to small misorientation angles, producing labyrinth-like patterns [45]. Similar moirè-type superstructures are reported for partly tert-butoxycarbonyl (BOC) and tert-butyl (TBU) protected block copolymers based on 4-hydroxystyrene with varying block ratios; these materials give rise to the transformation of a partly BOC-protected block copolymer into the homopolymer poly(4-hydroxystyrene) by annealing at moderate high temperature [46]. In a different context, complex coacervate core micelle can be obtained by the reaction of a polyion-neutral diblock copolymer with an oppositely charged polyelectrolyte. These micelle are formed upon hierarchical self-assembly in water of the two polymeric components and, more interestingly, upon self-assembly of metal ion coordination polymers [47]. Self-organization of block copolymers into regular patterns has been investigated with the aim of finding high performance applications in microelectronics [48]. In this review most of the reported studies deal with polystyrene (PS) and polymethylmethacrylate (PMMA) diblock copolymers (PS-b-PMMA) that are materials compatible with the semiconductor fabrication infrastructure and also suitable for the understanding of materials properties. PS-b-PMMA, alike diblock copolymers, spontaneously form patterns at molecular scale dimensions through microphase separation. For lithography applications, it is important the control of the orientation of the self-assembled pattern, e.g. cylindrical and lamellar phases, which can be obtained by coupling the self-assembly process with an external bias. The methods for the control of pattern orientation and pattern transfer processes, together with examples of device fabrication such as shallow-trench-array capacitors, of controlled optical index materials, of nanoporous membranes and nanocrystal Flash memories are reported. A list of features that must be considered as a guide for the development of polymer self-assembly-based high–resolution patterning methods for high-performance semiconductor electronics at the nanoscale is provided and can be observed that the polymer self-assembly procedure is a challenging substitute for high-resolution lithography. It is noteworthy that a theoretical approach, i.e. the dissipative particle dynamics (DPD) method provides the understanding of the self-assembling behavior of block copolymers with two molecular architectures made from an A-homopolymer block combined with a BC-comb block or a BC-alternating block; hierarchical structures, such as spheres-within-lamellae, cylinders-within-lamellae, gyroid-within-lamellae,

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lamellae-within-lamellae, lamellae-within-cylinders, and lamellae within-spheres, can be foreseen for the development of photoelectron based devices [49]. 1.2.2.2 Self-Assembly of Dendrimers Dendritic molecules, which are three-dimensional branched compounds, have the property of self-assembling into complex arrays by non covalent (supramolecular) interactions, giving rise to controllable nanomaterials. Recent reviews emphasize the use of these peculiar macromolecules as building-blocks to generate highly branched complex nanoscale assemblies, represented in Fig. 1.5, and highlight the potentials of these assemblies in chemistry and biology [50, 51]. Dendrons may exhibit the self-assembly ability trough hydrogen bonds in solution to produce controlled geometries (i.e. well defined assemblies of buildingblocks) [52]. An alternative way to induce non covalent interactions of individual dendritic branches is the assembly mediated by templates, which can be organic molecules interacting with the dendrons trough hydrogen bonds or acid-base

Fig. 1.5 Schematic illustration of the self-assembly of dendritic building blocks. (a) Untemplated assembly of dendrons. (b) Templated assembly of dendrons. (c) Nanoparticles with assembled dendritic surface groups. (d) One-dimensional, fibrous, gel-phase assemblies of dendritic molecules. (e) Liquid crystalline assemblies of dendritic molecules.(Reprinted from Smith et al. [50], with permission from Elsevier)

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reactions [53, 54]. Dendrimers functionalized with rotaxane or dibenzo-24-crown8 macrocycle are also favored for a spontaneous assembly into interlocked architectures [55, 56]. The assembly of dendritic superstructures can be carried out also by means of metal coordination chemistry, by a number of different key strategies. Since the pioneering work of Balzani group [57], the research has been developed by introducing the idea that metal centers act as “building block connectors” and some selected, but not exhaustive, examples of literature reports are given [58–62]. The properties of these assembled dendritic superstructures range from electrochemical, light-harvesting, phosphorescent and electroluminescent to biochemical ones. Clusters of metals are also cores for the assembly of dendrimers which show electrochemical and biomimetic properties [63, 64]. A peculiar case is represented by the stabilization of gold nanoparticles with sulfur containing dendritic ligands which provide the control of nano-architecture dimensions for stable assemblies [65]. In the field of bio-nanotechnology, dendritic disulfides made from biocompatible L-lysine building-blocks were also found as useful ligands for the controlled assembly of gold nanoparticles [66] with anion sensing properties [67]. The synthesis of CdSe dendron stabilized nanoclusters with high stability and biocompatibility (box-nanocrystals) is also noteworthy [68]. Gene vectors which can deliver DNA to target cells are object of wide scientific interest for the development of gene therapy. In particular, polyamidoamine (PAMAM) dendrimers belong to a class of nano polymers with highly branched spherical structure and a unique surface of primarily positively charged amino groups. PAMAM can transport DNA into a large variety of cell types and has emerged as a promising non-viral gene vector [69]. The increasing number of papers on this topic highlights the importance of this field of research and only some representative ones will be hereafter reported. Since the pioneering work of Tomalia and co-workers [70] who demonstrated that PAMAM-DNA complex dendrimers exhibit the highest in vivo gene transfer efficiency, the research developed the formation of nanoscale complexes which provide DNA protection and enhanced activity of bioconjugates [71, 72]. Globular nanostructures were achieved from plasmid DNAcopolymers (dendritic poly L-lysine and linear PEG blocks) self-assembly [73], and a poly(azobenzene) dendrimer based on a calyx-4-arene core functionalized with peripheral L-lysine units provides a UV-switch able framework, thus showing that the affinity of the system for DNA can be controlled by using UV irradiation [74]. Amphiphilic dendrimers are reported to be vectors for gene delivery with an inherent self-assembling potential with DNA [75]. Other morphologies can be obtained, i.e. dendritic nanoclusters and nanotubes, with different chemical approaches which generate a wide variety of different nanoscale architectures and have a promising potential in host-guest chemistry and nanotechnology [76, 77]. Asymmetrically functionalized dendritic blocks, e.g. dendrons with polar and apolar groups, self-assemble to produce macromolecules with surfactant properties [78–81].

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Many other features and properties are typical of dendrimers; assembly of large aggregates is achieved when the dendrimer is linked to a different multi-functional system, for example thiolated phosphorous dendrimers are suitable stabilizers of Au55 clusters leading to the formation of gold cluster superstructures [82]. Supramolecular fibrillar architectures, of prominent interest for applications in neurodegenerative diseases, are achieved through hydrophobic and hydrophilic contacts in solvents which promote the aggregation in gel-phase material [50]. Percec and coworkers have developed a dramatic amount of research on dendritic self-ordering that gives rise to supramolecular dendromesogens packed in hexagonal or cubic structures of nanoscale dimensions with liquid crystal properties [83, 84]. The self-assembly of dendritic molecules into liquid crystalline materials is favored also by the presence of mesogenic groups; nice examples of this approach were reported by Ponomarenko [85], Serrano [86] and Hult [87] research groups. In this framework, an interesting paper reports the role of dendritic selfcomplementary hydrogen-bonding units that are used as noncovalent cross-linking agents who promote the chain entanglement of linear polymers (PMMA derivatives) into polymeric nanoparticles [88].

1.2.3 Self-Assembly Induced in π-Conjugated Polymers Among one dimensional nanomaterials, synthetic procedures, properties and applications of polymers have been extensively reported in a dedicated chapter of a recent review [89]. A class apart of macromolecules is represented by π-conjugated polymers, which are the basis of the development of organic electronics whose performance is governed by their degree of order. The main properties related to conductive polymers in their nanozize morphology and the more reliable methods to induce nano sized structures based on intermolecular and intramolecular effects are extensively reviewed by Wessling [90] and by Kim [91]. The self-assembly of these materials is then an important topic in the field of nano macromolecules. In the following sub-chapters, the main features concerning the most investigated π-conjugated polymers will be described, with examples on different synthetic methodologies for the attainment of nanostructures for several applications. 1.2.3.1 Polyaniline Polyaniline (PANI) represents one of the most cited examples of nanostructured polymers, due to its outstanding electronic properties and technological applications that have promoted a wide number of studies and publications. Although a variety of different and peculiar morphologies have been reported, such as brain like [92], cauliflowers [93], nanoflakes, nanospheres and nanorods [94], chrysanthemum flower-like [95], plate-like structures and flower-like superstructures [96],

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the nanorods morphology seems to be the favorite one and has been obtained with a multiplicity of methods, some of which will be hereafter reported as examples. A simple dispersion polymerization in a PVA matrix allows the formation of PANI assembled nanorods with a tubular orientation. The electrical conductivity of the nanorods is interpreted by the two-dimensional variable-range hopping model, due to the fact that the rods in the film are not strictly aligned in one particular direction [97]. Polyaniline nanostructures with controlled morphology of different shapes (sheets-, fiber- and spherical-like) are synthesized by using p-toluene sulphuric acid (p-TSA) aqueous solutions and a rational mechanism based on the self-assembly of micelle is proposed for the formation of PANI nanostructures [98]. Bundles of a PANI copolymer, i.e. poly(aniline-co-anthranilic acid) (PANANA), can be assembled by using proper amounts of anthranilic acid that plays the roles of monomer, acid-media provider, and dopant in the reaction system [99]. Emeraldine base (EB) and emeraldine salt (ES) forms of poly(o-methoxyaniline) (POMA), are able to construct biomolecular hybrids with DNA showing a fibrillar network structure of invariant fibrillar diameter for different hybrid compositions. An approximate model of the Na-DNA/POMA-ES system indicates nanostructured self-organized assembly of the components in the hybrid [100]. However, the most desired property of fibers for the electronic devices technology is their orientation in a definite direction. Self-assembly of oriented PANI arrays can be achieved in the presence of inorganic acids and by changing the PANI/acid concentration ratio, (Fig. 1.6) [101, 102]. The choice of acids has also other effects on the nanostructure; for example, the use of tetrachloroaurate as an efficient oxidant of aniline in the presence of a chiral inducing agent, i.e. (1S)-(+)-10-camphorsulfonic acid ((S)-(+)-CSA) or its enantiomer (R)-(–)-CSA, allows the formation of optically active PANI nanorods, together with the further self-assemblies into monodispersed hierarchical Au (0) microspheres [103]. Recently, aniline oligomers have also received attention, because they can be envisaged as the building blocks of block architectures

Fig. 1.6 A large number of PANI arrays with average diameter of 1.2 μm and highly ordered structure are produced by change of the aniline/acid concentration ratio (Reprinted from Wu et al. [101], with permission from Elsevier)

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with promising structure-function properties based on supramolecular construction principles [104].

1.2.3.2 Polypyrrole Among conjugated polymers, polypyrrole (PPy) and its derivatives represent a class of technologically important macromolecules mainly due to their conducting properties and applications in molecular electronics. A remarkable review accounts for the advanced research on the mono and multilayer deposition on different surfaces of these polymers in their nanometer-size with one-dimensional resolution and hybrids formation with gold nanoparticles [105]. In the framework of this research topic, PPy chains self-assembled in nanowires with a coral-like shape can be obtained by FeCl3 induced oxidative polymerization and dodecil-benzenic sulphonic acid (DBSA) dopant [106]; oxidative polymerization is a widely used method for the attainment of polymeric nanostructures. For example, bundles of self-assembled PPy nanotubes have been fabricated by polymerization reaction with bis(2-ethylhexyl) sulfosuccinate reverse (water-in-oil) emulsions [107] and rods with enhanced electrical conductivity and thermal stability are reported to be formed via a self-assembly process of micelle obtained from a oxidative polymerization in the presence of p-toluensulfonic acid used as surfactant and doping agent [108, 109]. A further example of PPy nanotubes synthesized by oxidative polymerization in octane is reported in Fig. 1.7 [107]. A variety of synthetic procedures for the achievement of PPy nanostructures (spheres, rods, tubules, core-shells) are reported in the literature where the concept of self-assembly is mixed with that of template synthesis and composites fabrication, because the methods often show overlapping features. Most of the examples deal with the template assisted procedure and are reported in the subchapter “Templates”.

Fig. 1.7 TEM images of PPy nanotubes prepared in octane (a) and enlargement (b) (Adapted with permission from Jang and Yoon [107]. Copyright 2009 American Chemical Society)

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1.2.3.3 Polythiophene The surface morphology of polythiophene (PTh) films is important for the mechanical and electrical properties of this widely investigated material. Different synthetic strategies were carried out for the preparation of nanostructured PTh, for example, nanotubules of PTh were obtained with electrochemical polymerization giving rise to self-ordered nanostructures with fractal dimensionality and nanowires with diameters in the range 50–100 nm have been produced with gamma radiation-induced oxidative polymerization [110]. Hollow spheres of PEDOT, poly(3,4-ethylenedioxythiophene), can be self-assembled through a “grow from membrane” process; the self-assembly is promoted by the hydrogen bond between 3,4-ethylenedioxythiophene monomers and acetic acid used as dopant agent during the oxidative polymerization [111]. Spherical PEDOT particles can be achieved by functionalization of the polymeric structure with specifically designed PEObased reactive stabilizers in aqueous media. These self-assembled PEDOT particles exhibit high conductivity for applications ranging from PLEDs to flexible organic solar cells [112]. PEDOT aggregates of hollow microspheres were also obtained and SEM and TEM images, representatives of these morphologies, are reported in Fig 1.8 [113]. Different morphologies, i.e. vesicles and lamelle are formed by an amphiphilic conjugated diblock copolymer made from polyfluorene-b-polythiophene units; this material shows the property of forming aggregates at the air-water interface induced by the Langmuir-Blodgett (LB) technique and of tuning the optical properties upon modification of the aggregation state [114]. When 3,4-ethylenedioxythiophene (EDOT) is chemically polymerized in the presence of polyacrylic acid (PAA) as a template, conducting nanowires can be assembled from smaller nanowires in a side-by-side manner and exhibit excellent conductivity [115]. The electronic properties of PTh have promoted a wide interest in the development of organic/polymer light-emitting diodes (OLEDs/PLEDs) and it is note worthy that the performance of these devices is dramatically enhanced by the

Fig. 1.8 SEM (a) and TEM (b) micrographs of PEDOT hollow aggregates (the exterior size distribution of hollow microsphere is shown in inset of (b) (Reprinted from Xia et al. [113], with permission from Elsevier)

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use of integrated self-assembled nanowires of a PEDOT-organic molecule (highly substituted condensed benzothiophene) [116]. A further important application is the fabrication of polymer based sensors and indeed the chemical sensing responses of organic field-effect transistors (FET) based on nanostructured regioregular polythiophene have been recently investigated reporting an in depth study of the sensing mechanism [117]. 1.2.3.4 Polyacetylenes and Polyynes Functional polyacetylenes show a variety of properties such as liquid crystallinity, photoconductivity, light emission, ionic susceptibility, photoresistance, chromism, helical chirality, optical nonlinearity, self-assembly, cytocompatibility, and bioactivity [118] and have been the object of thousands of scientific reports. Quite recently, the research has been extended to the study of these materials in nanostructured fashion with the aim of improving their performance and the few examples hereafter reported will give a glance on this emerging topic. Polyphenylacetylene (PPA) is a π-conjugated semiconducting polymer, it is highly stable and can be synthesized in cis or trans configurations, depending on the polymerization procedure [119]; it shows NLO properties [120] and is a suitable matrix for the immobilization of lipolytic enzymes [121]. Upon emulsion polymerization it self-assembles into nanospheres with photonic crystal properties [122]. Mono-substituted helical poly(phenyl)acetylene structures have been prepared through living polymerization; the molecular structure looks like a narrow spiral with a conjugated electron system and with the attached side groups spiraling in the opposite sense. This polymer feature is able to self-assemble and may be deposited in an oriented fashion, showing electrical conductivity [123]. An amphiphilic PPA carrying L-leucine pendants was self-assembled into nanospheres and nanorods and it was assessed that the morphology depends on the polarity or solvating power of the solvent mixture, i.e. on the affinity or likeness of the solvent molecules with the hydrophobic PPA backbone or the hydrophilic Leu pendant group; morphological transition processes from micellar nanopearls, via rings, globules, loops and cages to extended nanofibers are detected on the course of the self-assembly process [124]. Amphiphilic polyacetylenes, such as poly(N-octadecyl-2-ethynylpyridinium bromide), self-assemble through layerby layer deposition within aluminosilicate (saponite) nanosheets, leading to a double layer of polymer where the alkyl chains are arranged in interdigitated features, thus producing a hydrophobic barrier that hinders the transport of water molecules [125]. Other conjugated materials with optoelectronic properties give rise to nanostructures. Uniaxially ordered films of a rigid rod conjugated polymer, namely poly(para-phenylene ethynylene) with thioacetyl end groups (TA-PPE), are aligned onto friction-transferred poly(tetrafluoroethylene) substrates; the achievement of highly ordered structures self-assembled by simply drop casting of the polymer solution, dramatically enhances the performance of the photoswitcher devices due to the efficient charge transfer along the aligned polymer structure [126].

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Metal containing polyynes are multifunctional materials which combine the properties of organic polymers with those of metal centers coordinated to the organic moiety and are able to form nanotemplates, colloidal photonic crystals, multilayer capsules and hollow vesicles [127, 128]. An example of a rod-like polymetallayne self-assembly in hollow nanorods has been recently reported [129]; the computer simulations of the nanostructure show that the polymer chains are ordered in parallel lines that give rise to a tubular morphology rather unusual for these materials, but promising for sensor devices applications.

1.3 Templates 1.3.1 General Features The word “template” in the contest of polymer science means that a structure directed agent is able to replicate a shape into another under structural inversion. A quite widely used method for the achievement of nanostructured polymers deals with the assistance of templates. Direct templating is particularly suited for getting mesostructures of organic and soft materials such as polymers that, in general, can be easily replicated by adopting hard templates which allow a great synthetic flexibility. If direct templating is carried out, the templated material is an inverse copy of the original template structure and this technique is then useful for the achievement of nanostructured or porous materials. Moreover, the dimensions and structures can be tuned or modified by a proper choice of the template. A simple scheme of the template technique is reported in Fig. 1.9. Due to the feasibility of the technique, dramatic efforts have been recently explored by researchers to exploit templating methods which can give rise to structure controlled materials with functional advanced properties. The templating strategies that induce the nano morphology build up include a variety of polymerization procedures such as photopolymerization, linear polymer chain templating, particle dispersion templating, molecularly imprinted polymers, templating in vesicles, and templating within liquid crystal surfactant assemblies [130].

1.3.2 Template Techniques Most of the template techniques for the achievement of nanostructured macromolecules are described more extensively in the sub-chapters 3 “Grafting polymerization” and 4 “Electrochemical methods”. Hereafter, some examples of polymeric materials obtained in nano-size dimension through the use of different templateassisted polymerization methods will be shown. Among the variety of procedures, photopolymerization [131] is widely adopted to perform templating reactions due to its characteristic of control on the structural evolution of the templated polymer structure, through the kinetic parameters.

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Fig. 1.9 Schematic presentation of templating approaches toward nanostructured soft materials by using endo and exotemplates (Reprinted with permission from Thomas et al. [130]. Copyright 2009 American Chemical Society)

When the polymerizations are carried out in highly organized mesoscale templates, well defined and controlled network architectures can be achieved. The combination of templating and photopolymerization is of peculiar interest in the field of biomedicine, where nanostructured materials play a relevant role in tissue engineering and drug delivery applications. In general, due to a greater control over the template process, this approach is most suitable for linear chain templating or catalytic polymerization, organized particle templating, molecular imprinting, templating of assembled vesicles and polymer templating in liquid crystals. Mesostructured inorganic solids, originated from self-assembling of supramolecular structures, represent a class of suitable templates for the achievement of nanostructured materials. Ordered mesoporous polymers and carbonaceous frameworks

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supramolecular aggregates act as templates for block copolymers introduction [132]. Polymeric templates are also widely described. For example, a porous polymeric membrane can be obtained by the bombardment (irradiation) of a polymeric film with high energy heavy ions, followed by chemical etching. The pore density (number of pores per square unit) depends on the intensity and duration of the irradiation and the diameter of the pores is related to the intensity of the etching process [133]. A further way to accomplish nanostructured conducting polymers through template technique is the use of “soap-bubble” template; for example pyrrole is electrochemically polymerized along the walls of soap bubbles giving patterned conducting microcontainers for biomolecules encapsulation [134]. In a similar method, resorcinol-formaldehyde nanopolymers, precursors of carbon nanoparticles, can be obtained by using surfactant-templated vesicular assemblies [135]. The oxidative chemical polymerizations of polypyrrole (PPy), poly(N-methylpyrrole) (PNMPy), polythiophene (PTh) and poly(3,4-ethylenedioxythiophene) (PEDOT), performed with polycarbonate and alumina membranes as templates, lead to highly oriented nanofibers and nanotubes whose diameter can be tailored with the pore size of the membrane [136] and linear aggregates of nano PPy blobs were achieved by alumina-membrane templated polymerization [137]. PTh nanostructures have been also produced by using metal nanoparticle template, i.e. copper nanoparticles are mixed with soluble PTh to yield thin films that are further subjected to thermal treatment so that insoluble PTh films with Cu nanoparticles included are obtained. The Cu nanoparticles are removed with a proper solvent, leaving voids that can be filled with spherical molecules such as fullerene derivatives [138]. PANI and PPy with controlled nano-morphologies are achieved by manipulating the length of the hydrophobic surfactant or by changing the chemical structure of the template adsorbing substrate [139]. The formation of PPy wires and ribbons is induced by lamellar inorganic/organic mesostructures as templates that are shaped in situ during the polymerization between surfactant cations, such as cetyltrimethylammonium bromide (CTAB), and oxidizing anions, while by using short chain or nonionic surfactants sphere-like nanostructures are produced [140, 141]. The same procedure can be applied for the controlled growth of poly(N-methylaniline) nanowires and microspheres [142]. CTAB can also be used for the modification of a fibrillar complex made by FeCl3 and methyl orange, acting as reactive selfdegraded template that induces the formation of nanotubular structures of PPy [143]. Quite recently, biomolecules have became promising templates for the synthesis of 1D nanostructures; for example DNA promotes the assembling of Au and Ag particles in nanotubes, nanowires and nanorods and proteins and polypeptides are also candidates for analogous purposes. In this contest, heparin and sodium alginate are morphology-directing agents for the achievement of PPy and PANI nanowires and fibers; likewise starch is a convenient template for the electrochemical polymerization leading to PPy nanowires [144]. The synthesis of PPy and hybrid (Au-PPy-Au) nanowire arrays of controlled dimension can be performed by an all electrochemical template method, within the pores of homemade polycarbonate membranes [145].

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Among the biomolecules, diacetylenic phospholipidic tubules (500 nm wide and several micrometers long) are suitable templates for the growth of PPy strands on the edges/seams of these peculiar materials made by twisted bilayer ribbons closed to yield an open helix [146]. Langmuir-Blodgett technique can be used to produce nanopatterns of aligned ribbons from the supramolecular assembly of tri-L(glutamic diethyl esther)-1,3,5-benzenetricarboxamide; this nanopattern can readily accomplish a templated chemical polymerization of PPy nanoparticles [147]. The use of electron beam lithography is also a suitable method for the fabrication of nanochannel templates of desired width and length that allow any kind of polymerization to obtain polymer filaments [148] and, in a different way, nanochannel templates favoring the formation of polymer nanotubes and nanowires are often made from nanoporous anodic aluminum oxide (AAO) [149]; polymer nanotubular structures obtained from the latter method are represented in Fig. 1.10.

Fig. 1.10 FESEM images of non-polar polymer nanotubes and nanowires fabricated by using nanoporous Anodic Aluminum Oxide (AAO) template (Reprinted with permission from She et al. [149]. Copyright 2009 The Society of Polymer Science, Japan)

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1.3.3 Materials in Template Techniques The template assisted synthesis is based, among the others, on three main categories of materials, i.e. diblock copolymers [150–152], anodized alumina layers [153] and organic or inorganic colloidal particles [154–156] which act as scaffolds or supporting structures with desired size for the polymer that has to be templated and can be afterwards removed by dissolution in common solvents. Polystyrene templating particles are particularly suited for the preparation of nanostructured materials and, among these, for the synthesis of PANI nanostructures. The PS template assisted electrochemical preparation of PANI is based on a general procedure that proceeds firstly with the formation of a PS template on a conducting substrate, followed then by electropolymerization of aniline and finally with the removal of the PS template [157]. By using oppositely charged PS nanoparticles (i.e. charged by addition of negatively charged or positively charged polyelectrolytes, i.e. poly(sodium-4-styrenesulfonate, PSS, or poly(diallyldimethylammonium chloride), PDDA, respectively) as templates, PS/PANI core/shell particles, PANI hollow spheres, PANI/PS nanocomposite and nanoporous PANI can be obtained due to the different growth mechanism [158]. A modification of this procedure, i.e. by using templating PS nanoparticles selfassembled onto a PANI modified screen-printed electrode, leads to the formation of PANI nanostructures with the shape of cauliflowers. These peculiar PANI nanoparticles have found an interesting application in an amperometric enzyme biosensor for hydrogen peroxide [159]. As a curiosity, an unusual brain-like morphology of PANI is obtained by using aniline/citric acid salts as the template in a gas/solid reaction using chlorine gas as the oxidant [160]. Amphiphilic micelle of azobenzenesulfonic acid are often used as templates for tuning PANI morphology, obtaining nanofibers, rods, spheres, and tubes, depending on the polymerization conditions; the solid state properties of PANI are highly dependent on the size and shape of the polymerization templates employed for the synthesis [161]. Analogous procedure has been proposed for the preparation of PANI micro/nanostructures through the supramolecular self-assembly attained by protonated PANI intercalated nanoclays; inter-chain hydrogen bonding, inter-plane phenyl stacking and electrostatic layer by layer self-assembling between polarized alkyl chains aided by dopant anions (3-pentadecyl phenol-4-sulphonic acid, PDPSA) lead to PANI nanostructures [162, 163]. In a wide context, the synthesis of inherently conducting polymers, PTh, PANI and PPy, and their properties and applications (capacitance, sensors, artificial muscles, biomolecular interactions, cell growth) related to the attainment of nanodimension, have been reviewed and a section is dedicated to physical templates (pore sized membranes, synthetic opals) that induce the doped polymer fibrillar morphology [164]. A peculiar type of templates is represented by phospholipids. Lipid tubules were introduced by Schnur and coworkers [165] a couple of decades ago and more recently have been investigated as promising templating materials for the selective growth of PPy nanostructures which surprisingly self-assemble at the edges (not at the surface) of the phospholipidic tubules [146].

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PPy inverse opal patterns (ordered two-dimensional rings, hexagonal or honeycomb monolayers) over wide areas are accomplished by using a colloidal template method. The templates are made of poly(styrene/sodium p-styrene sulphonate) latex particles that drive the opal structure upon modulation of their packing density, thus inducing a modulation of the polymer properties [166, 167]. Unlike hard templates (alumina, zeolites, etc.) which require many synthetic steps, surfactant templates may be a convenient alternative. The morphology of PANI and PPy (spheres, wires, flat films) can be modulated though the use of adsorbed surfactants aided by co-adsorbing molecules; aligned nanowires of PANI produced by this template assisted method can be self-assembled over large areas for the improvement of microelectronic and sensor devices, as depicted in Fig. 1.11 [139]. In this framework, reverse micelle also show easy feasibility for template self-assembly. For example, reverse cylindrical micelle systems were prepared from aggregates of sodium bis(2-ethylhexyl) sulfosuccinate, containing a nanometer-sized water pool in the oil phase, and have been successfully exploited for the template oxidative polymerization of PPy nanotubes [107].

Fig. 1.11 Illustration of the process to fabricate morphologically controlled nanostructures of electrically conducting polymers on surfaces by using surfactant templates. This particular schematic draw represents the proposed scheme of wire formation on (a) chemically treated HOPG and (b) HOPG (Reprinted with permission from Carswell et al. [139]. Copyright 2009 American Chemical Society)

1.3.4 Nanopatterning of Polymers (Top Down Methods) From the point of view of the applicability of nanopatterning to a wide range of materials, patterns of selected shape can be fabricated by assembly of nanoparticles without covalent interactions, as stated in a paper that provides the main features related to this topic; the assembly is performed: (i) in the absence of specific

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interactions, depending on the relation between the particle size and the pattern feature size, shape of the confining features and type of confinement; (ii) in the presence of electrostatic interactions which act trough chemical functionalization on the non-covered areas of the substrate to link nanoparticles with the appropriate complementary functionalization; (iii) in the presence of supramolecular interactions that involve host-guest chemistry [168]. In the case of polymeric materials, the patterning is in general based on two nanolithografic methods: reactive ones and non reactive ones; in the first approach the polymer is synthesized during the patterning, in the latter one the polymer is deposited or modified by a local perturbation. The nanopatterning of conjugated polymers is based on general requirements such as the control of the dimension and position of the structures that are deposited. The hierarchical organization of the macromolecules across multiple length scales allows supramolecular charge transport and integration in electronic devices [89]. The patterning reactive techniques are extensively mentioned in the literature as Area Selected Polymerization, Chemical Amplified Soft-Lithography, Photochemical Patterning (where the patterning feature is a defined stamp) and non reactive patterning as well, Microcontact Printing (by using the polymer as “ink”), Microtransfer Molding, Lithographically Induced Self-Construction, Grid Assisted Self-Organization, Inkjet Printing, Lithographically Controlled Wetting and Nanorubbing, among the most popular. Diblock copolymers self-organize to form patterns through minimization of free energy, i.e. trough a procedure called microphase separation. The up to date features and methods leading to high resolution patterning performed with polymer self-assembly at IBM are reviewed [48]; the processes for pattern orientation and transfer, the engineering of polymer based patterns for the development of optical waveguides, fabrication of nanoporous membranes, the improvement of patterns for high resolution lithography and flash memory transistors are extensively presented with a particular emphasis to the expectations in future technology advances. Colloidal nanolithography, deep silicon etching and nanomolding are the techniques used to achieve fibrillar polymer structures which mimic the gecko foot hairs; these nanofibrils are densely packed, perpendicular and strongly adhesive to a synthetic surface, and due to these characteristics are promising materials for integration in flexible membranes and exploitation of new adhesives [169]. Direct Laser Interference Micro-Nanopatterning (DLIP) has been used to build nanometer sized PANI arrays (as thin as 600 nm) self-assembled on dielectric polymers; the width of the polymer lines can be modulated by changing the laser beam intensity, without loss of the chemical and electronic properties of PANI. It is interesting that the dielectric substrate can be ablated, exploiting its optical properties at the working wavelength. The authors believe this technique relevant for the development of polymer based sensors [170]. The Electrochemical Dip-Pen Nanolithography (E-DPN) leads to direct writing of PTh nanowires (diameter less than 100 nm) on the surface of semiconducting or insulating materials, thus allowing the fabrication of complex structures which are proposed for the design of devices with multipurpose applications (electronics, defense, pharmaceutics, and biotechnology) [171].

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1.4 Grafting Polymerization 1.4.1 General Features The nanotechnology applied to chemistry provides new tools for the research in the field of macromolecules and emerged from the desire of control on the physical and biological functions of materials at the molecular level and of radically improving the physical properties of traditional materials. Chemical surface modification with grafted polymers is a well known method for the tailoring of the surface properties of polymeric and ceramic membranes, thus improving their performance. In recent years, porous inorganic oxide substrates (e.g. silica, zirconia, or alumina) have been proposed as chemically and thermally stable materials for the graft polymerization processes. The covalent bonding and the nanostructure of the polymeric phase deposited onto inorganic substrates has been used to create membranes that can resist to swelling effects and operate at high temperatures. A useful technique to produce nanostructured polymers, such as block, graft and star polymers [172], typically involves two routes, i.e. graft polymerization and/or polymer grafting (schematically shown in Fig. 1.12); the main difference in these two ways is the possibility of performing in situ polymerization on the surface after (graft polymerization) or before (polymer grafting) the graft. For example, periodic nanostructures of poly(glycidyl methacrylate) (pGMA) were grafted onto poly(ethylene-alt-tetrafluorethylene) (PTFE) films by reversible additionfragmentation chain transfer (RAFT) polymerization [173]. In general, the purpose is to obtain structural and morphological changes in the active and supporting layers

Fig. 1.12 Schematic illustration of graft polymerization and polymer grafting

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of the material upon chemical modification. Apart from the chemical modifications, the research was devoted to assess the effect of modifications on the surface roughness by using nanomorphology. Fundamental for the characterization of polymer grafting surface are microscopy techniques, i.e. Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Attenuated Total Reflection– Fourier transform infrared spectroscopy (ATR-FTIR spectroscopy). AFM is often chosen for topological characterization of the modified surface; Fig. 1.13 shows the surface graft polymerization of 1-vinyl-2-pyrrolidone onto a silicon surface, accomplished by atmospheric pressure hydrogen plasma surface activation, followed by graft polymerization in both N-methyl-2-pyrrolidone (NMP)

Fig 1.13 Tapping mode AFM images (1 × 1 μm2 ) of polymer grafted silicon at [M]0 = 30% (v/v) in a mixture of aqueous solvent (where [M]0 is the surface initiation at a plasma treatment time of 10 s and rf power of 40 W) and (a) [NMP] = 15% (v/v), (b) [NMP] = 40% (v/v), (c) [NMP] = 60% (v/v), and (d) [NMP] = 100% (v/v) (no water) (Adapted with permission from Lewis et al. [182]. Copyright 2009 American Chemical Society)

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and in an NMP/water solvent mixture. SEM is often a complement of AFM and points out the attention on the mesoscopic morphology of the modified surface. TEM seems to be the most suitable technique to visualize the internal structure of the non-modified and modified active layer of a surface, due to its high resolution and chance to achieve contrast between the areas having different chemical structure: the latter property is most easily achieved through selective incorporation of heavy elements. ATR–FT-IR spectroscopy provides an easy and convenient way for the determination of the relative amounts of different polymeric species present in the outmost part of a polymer grafting surface; the depth of penetration of the reflected IR beam in the ATR technique is typically somewhat below 1 μm and therefore the observed spectrum represents the average composition of this layer. Since the thickness of the active layer in the polymer grafting surface is far below 1 μm, the method proves to be highly suitable for analyzing the active layer of modified and non-modified polymer grafting surfaces. The relative amount of the polymers is determined by analyzing the IR adsorption bands specific for each polymer. This specificity renders the method sensitive even to relatively small amounts of the polymers attached to the surface. Other typical features of grafted materials are investigated by studying the hydrophilic modification of the surface of the samples assessed, for example, by measuring the water droplet contact angle and the water droplet adsorption time. The graft copolymerization plays a key role in the field of nanotechnology mainly because of the synthetic flexibility. In graft copolymerization the control over the polymerization reaction is driven by important features: when the number of growing chains is constant and chain transfer or termination reactions are avoided, the functional groups at the polymer terminus will be maintained, allowing for additional chemistry to take place. Such transformation reactions result in the production of a macroinitiator which can initiate the polymerization of a different monomer, thereby producing block copolymers; if a difunctional initiator is used, the same technique can be applied toward triblock copolymers synthesis. Furthermore, the presence of a functional group on a monomer in conjunction with another monomer in a statistical copolymerization results in pendant species which can be transformed to initiate the polymerization of graft copolymers. Of particular interest are copolymers which contain an inorganic block that allows to exploit the best properties of the individual materials and to generate new classes of compounds. For example, polysiloxanes show high oxygen permeability and favorable water and weather resistance, and polyphosphazenes exhibit a broad range of physical properties leading to applications in biomedicine as well as flame retard, based on the substituents bonded to phosphorus. For carbon-based vinyl monomers, controlled polymerization has been traditionally achieved by ionic mechanisms [174]. The living anionic polymerizations of styrene and methyl methacrylate are quite common, resulting in preservation of the polymer functionality. However, alike the inorganic analogues the ionic polymerization mechanism is limited to a rather narrow class of monomers, under conditions of the most stringent purity. Therefore, the aim to develop a controlled free radical

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polymerization system has driven the research in this area during the last decade and atom transfer radical polymerization (ATRP) has been proposed as a favourable route for graft polymerizations. If an activated alkyl halide is stirred with a vinyl monomer in the presence of a copper catalyst, well-defined polymers are obtained with predetermined functionality and molecular weight, making them ideal for the synthesis of a variety of copolymers. The key of the control is the achievement of a rapid equilibrium between active and dormant propagating species. The maintenance of a low steady-state concentration of radicals ensures that termination reactions are limited to nearly insignificant values until very high monomer conversions are attained. ATRP has demonstrated to provide controlled polymerizations of monomers such as styrenes, acrylates and methacrylates [175]. A large number of different methods for the achievement of surface graft polymerization were developed including several patented techniques (often based on surface activation with organosilanes), and controlled free-surface graft polymerization or plasma-assisted graft polymerization. The yield of polymer graft, as well as chain length and density, are sensitive functions of the reaction conditions. The engineering of the surfaces, consisting of a terminally anchored polymer layer, requires a careful control of the surface density and molecular weight of the polymeric chains. The resulting physicochemical and transport properties of the grafted polymer layer depend on the conformation and topology of the tethered polymer phase. The polymer chain configuration will vary from the extreme brush like configuration (for high density coverage) to separated chains in the so-called mushroom regime. In order to control the structure of the grafted polymer phase (both chain density and chain length) in nanosize dimension, various graft polymerization methods have been proposed and hereafter reported: • Free radical graft polymerization • Plasma surface treatment and Plasma-Induced Graft Polymerization (PIGP) • Atom Transfer Radical Polymerization (ATRP)

1.4.2 Free Radical Graft Polymerization Grafted polymers offer unique opportunities to tailor and manipulate interfacial properties and produce nanostructured devices while retaining the basic mechanical strength and geometry of the supporting solid substrate. For example, a substrate can be modified with a polymer, which is completely miscible with the surrounding fluid medium, mean while the polymer detachment is prevented by the covalent attachment of the polymer chains to the substrate. Surface engineering can be achieved by either physically adsorbing or chemically bonding functional polymer chains [176]. A tethered polymer phase can be formed either by polymer grafting (“grafting to”) or graft polymerization (“grafting from”) [177, 178]. Surface chain coverage and spatial uniformity achieved by polymer grafting may be limited by steric hindrance.

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In contrast, graft polymerization proceeds by sequential monomer addition, thereby allowing for the formation of a denser surface coverage. Among several methods for covalent bonding of polymer chains onto a substrate, free-radical surface graft polymerization is a simple strategy to obtain a high surface coverage. Free-radical graft polymerization typically involves the formation of both free polymer chains (in the solution) and grafted polymer chains (on the substrate). In this approach, the sequential monomer addition to the surface occurs through the propagation growth of terminally anchored surface chains (surface propagation) and coupling termination reactions between free polymer chains and growing surface chains (polymer grafting). In polymer grafting, the homopolymer radicals must diffuse toward the solid surface to react with the grafted polymer radicals. As a result, the diffusion limitations of macromolecules may reduce the contribution of polymer grafting to the overall polymer graft yield. In contrast, in surface propagation, diffusion and steric limitations are diminished because of the smaller size of the monomer molecules. Free-radical method usually requires a surface activation by a direct attachment of initiator molecules or by the introduction of surface active sites (i.e., vinyl groups in surface graft polymerization of vinyl monomers). In particular, organosilane coupling agents (i.e., chloro- and alkoxysilanes) are commonly employed to introduce active sites onto inorganic oxide surfaces. For example, modification of amorphous silica surfaces with organosilanes has been well studied in both gas and liquid phases for applications such as adsorption, adhesion and chromatography [179, 180]. Gas-phase silylation typically results in a lower conversion and is cumbersome when a large scale silylation is desired. Liquidphase silylation, can be performed in water or in anhydrous environment, and the choice of solvent greatly affects the resulting silylation coverage. Specifically, the chloro and alkoxy groups of multifunctional organosilanes undergo bulk hydrolysis and condensation, forming polysilane networks in an aqueous environment prior to depositing onto the substrate. As a result, the fraction of initial surface silanols that reacts with the functional organosilane is quite small, and the silylation process is usually non-uniform and difficult to control [181]. In contrast, in an anhydrous silylation reaction (i.e., in xylene) of a hydrated silica substrate, condensation and hydrolysis between one or more functional groups of neighboring silane molecules occur mainly on the surface with a minimal intercondensation between silane molecules in the bulk phase. As a consequence, this latter technique leads to a more dense and uniform silylation coverage. After the whole activation, the vinylsilane-modified substrate can be free-radical graft polymerized with a desired functional monomer, producing polymer chains that are chemically bonded to the substrate along with homopolymer chains in solution. In this step, the formation of grafted polymer chains is typically attributed to both propagation of growing surface chains (surface propagation) and coupling termination between growing homopolymer chains and growing surface chains (polymer grafting). Grafting with poly(vinyl acetate) is of particular interest since the grafted polymer layer can render the modified substrate hydrophobic or hydrophilic (by a

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post-grafting hydrolysis converting poly(vinyl acetate) into poly(vinyl alcohol) at a desired degree of hydrolysis). There has been a growing interest in the surface modification of inorganic oxide substrates with covalently bonded polymer phases for a variety of practical applications, such as filler–polymer control in polymer composites, support packing for liquid and gas chromatography, biocompatible surfaces, colloid stability and modified inorganic membranes, and for fundamental studies of interfacial phenomena.

1.4.3 Plasma Surface Treatment and Plasma-Induced Graft Polymerization (PIGP) Surface nanostructuring by grafting functional polymers to a substrate surface is a surface modification approach that provides the enhancement of the chemical functionality and alters the surface topology of native inorganic and organic materials [182]. Plasma surface treatment, which is used for metal oxide surface etching/cleaning in microelectronics, [183] has been proposed in several studies as a suitable approach to both alter the surface chemistry and potentially supplant previous solution-phase initiator strategies with high-density surface activation. Early studies have focused on the use of plasma treatment to modify the surfaces in order to reduce the adsorption of organics and biofoulants in separation membranes, to improve the surface wettability in microcontact printing for poly(dimethylsiloxane) (PDMS) stamps, and to enhance the adhesive bonding strength in advanced materials [184]. It was demonstrated that this versatile and environmentally benign technique has the propensity of modifying the surface chemistry with high efficiency for both organic and inorganic materials. Plasma treatment alone, however, proved to be an insufficient surface modification tool, and it has been noted that polymeric plasma-treated surfaces do not retain their modified chemical properties over time and to air exposure. Vapor-phase plasma polymerization, in which the monomer feed through plasma is initiated in the gas phase and then the monomer polymerizes on a substrate surface, has also been investigated as a surface modification method [185]. Furtherly, surface-adsorbed radical monomer species, which are designed to polymerize with condensing monomer radicals from the vapor phase, may be modified by continuous plasma bombardment, leading to highly cross-linked, chemically and physically heterogeneous polymer films, noncovalently adsorbed to the surface. It must be considered that the local concentration of monomer species in the plasma afterglow is highly dependent on the radial dimensions of the plasma source, and the resulting spatial variations in monomer deposition rate may lead to non uniform film structure and morphology. Plasma-induced graft polymerization (PIGP) is an alternative surface modification approach in which plasma is used to activate the surface; the monomer

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in the liquid phase is sequentially grafted to the initiation sites via free radical graft polymerization. This approach allows to engineer a grafted polymer phase characterized by a high surface density of polymer chains that are initiated and polymerized directly from the substrate surface, thus minimizing polydisperse chain growth and improving the stability under chemical, thermal, and shear stresses [186]. To date, PIGP has focused primarily on low-pressure plasma initiation and surface grafting onto polymeric materials, with limited studies on inorganic oxides [187]. Various studies have inferred and quantified, through surface binding assays using radical scavengers such as 1,1-diphenyl-2-picrylhydrazyl (DPPH), the presence of surface radicals that serve as initiators for graft polymerization. These studies have also reported that the surface radical number density that results from plasma treatment can be controlled and optimized by tuning the plasma treatment time and the radio frequency (rf) power of the plasma generator. Moreover, an excessive treatment time and/or rf power results in poor surface activation, plausibly, as argued by Choi, because of the formation of stable inactive species [188]. A notable limitation for the achievement of PIGP on inorganic substrates, unlike polymeric materials, has been the requirement of a sufficiently dense layer of surface activation sites, created through silylation and macroinitiator grafting, that may form surface radicals for the polymer initiation upon plasma treatment [189]. Given the complex surface chemistry and limited lifetime of reactive plasma-initiated surface species, the exact chemical nature of these plasma-generated organic moieties needs to be established yet. The surface preparation required for such technique combined with the difficulties related to surface hydroxyl chemistry limits the large-scale adaptation of this method and the level of chain density that can be achieved. Direct plasma initiation and grafting without the use of surrogate surfaces has been demonstrated qualitatively on titanium oxide particles and silicone rubber materials [190] with characteristic surface radical formation noted as a function of treatment time and rf power, similar to that assessed for organic materials. Kai and coworkers [191] demonstrated that, under low-pressure plasma surface treatment of Shirasu porous glass, a direct correlation between silanol density and grafted polymer density is observed. This suggests that the number density of surface radicals that may be produced in the low pressure plasma surface activation of inorganic oxide substrates, may be limited by the native oxide surface chemistry. These findings, combined with the added requirement of ultrahigh vacuum chambers necessary for low-pressure plasma processing, indicate that the current approach is insufficient for achieving high-density surface activation and graft polymerization for large surface area modification of inorganic substrates. In a recent study [182], an atmospheric plasma (AP) composed of a mixture of hydrogen (1 vol%) and helium was used to activate silicon substrates directly, creating surface-bound radicals that can then initiate the liquid-phase graft polymerization from these anchoring sites (see Fig. 1.14).

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Fig. 1.14 Illustration of multistep process plasma-induced graft polymerization (Reprinted with permission from Lewis et al. [182]. Copyright 2009 American Chemical Society)

The AP plasma source selected in this study operated at a low breakdown voltage, produced a highly uniform glow discharge, and maintained low processing gas temperatures (<80◦ C), which is advantageous for the graft polymerization onto thermally sensitive materials. The monomer chosen for this study, 1-vinyl-2-pyrrolidone (VP), is of interest because poly(vinyl pyrrolidone) has excellent biocompatible properties, has been proposed as a surface modifier to reduce membrane fouling, and is miscible in both aqueous and organic media. Experimental results are reported for both plasma surface initiation and VP graft polymerization, focusing on the control and optimization of the surface initiator density and on the impact of surface graft polymerization conditions on the resulting surface topology of the terminally anchored polymer surface layer.

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Moreover, active polymer radicals generated by electric discharges initiate the graft polymerization of a vinyl monomer, and a composite structure can be formed on the polymer surface. Glycidyl methacrylate (GMA) [192] has a pendant epoxy group, through which the grafted polymer can be chemically coupled with a selected compound, having a functional group such as –OH; cyclodextrin (CyD) is composed of a cyclic ring structure with six to eight glucopyranose units, and has a unique chemical property holding a molecule in the ring cavity. CyD contains many –OH groups, and the molecule can be chemically bound to the GMA grafted polymers through the opening of the epoxy group. More interests have been paid to the applications of the CyD-immobilized materials such as the expected ability of selective adsorption by formation of inclusion compounds. The reported examples highlight how the material’s function is determined by the chemistry of the grafted polymers.

1.4.4 ATRP: Atom Transfer Radical Polymerization ATRP or Atom Transfer Radical Polymerization is a powerful synthetic technique in polymer science and involves free radicals; it was introduced as an extension to ATRA or Atom Transfer Radical Addition by Jin-Shan Wang and Mitsuo Sawamoto in the nineties. As a living radical polymerization which is a form of living polymerization, it allows the reaction to be carried out in a controlled way, and can be used to obtain polymers with high molecular weight and low polydispersity index, besides long range order of the nanophases. This control is accomplished by the use of a transition metal based catalyst. This catalyst provides an equilibrium between active, and therefore propagating, polymer and an inactive form of the polymer known as the dormant form. Since the dormant state of the polymer is vastly preferred in this equilibrium, side reactions are suppressed. By lowering the concentration of radicals, the termination is suppressed and the control is achieved. Similarly to the other controlled-living radical polymerization (CRP) methods, ATRP allows the synthesis of polymers with desired composition and molecular architecture. The polymers prepared by ATRP are highly chain end-functionalized and can therefore participate in various post-polymerization modifications and serve as macroinitiators in the synthesis of block copolymers. Several nanostructured polymers and organic/inorganic nanocomposites have also been synthesized by this technique [193, 194]. For example Fig. 1.15 shows a series of well-defined liquid crystalline (LC) homopolymers and amphiphilic LC-coil diblock copolymers with functional azobenzene units and narrow polydispersities, synthesized by using the ATRP method. Among block copolymers, cylinder and/or spherical morphologies of PEG block dispersed into LC block were clearly observed after annealing at 105◦ C (smectic phase) for 24 h. The sizes of the separated structures are in the range of 10–20 nm, increasing with the increase of LC fraction [195].

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Fig. 1.15 TEM micrographs of the amphiphilic LC-coil diblock copolymers with a defined length of a flexible poly(ethylene glycol) segment as the hydrophilic coil, prepared by the ATRP method. PEG blocks exhibit as cylinders and/or spheres with a width/diameter about 2–3 nm dispersed into the LC matrix: PEG block appeared as dark and the solid lines in a, b, and c represent the domain boundaries (Reprinted with permission from Tian et al. [195]. Copyright 2009 American Chemical Society)

A general mechanism for ATRP is shown in Fig. 1.16. The homolytic cleavage of the alkyl (pseudo)halogen bond of (Pn X) by a transition metal complex in the lower oxidation state (Mt m Lz ) generates an alkyl radical (Pn •) and a transition metal complex in the higher oxidation state (X Mt m+1 Lz ). The formed radical can initiate the polymerization through addition across the double bond of a vinyl monomer, promote the propagation and induce the termination by either coupling or disproportionation reactions, or can be reversibly deactivated by the transition metal complex in the higher oxidation state. The formation of radicals during the ATRP process is reversible. An example is the ATRP for the polymerization of styrene, initiated with alkyl halide 1-phenylethyl chloride, giving Pn X, in the presence of the activator CuBr complexed by 2,8 -bipyridine (Mt m Lz ). Furthermore, their stationary concentration is low because the equilibrium between the activation (ka) and deactivation (kd) processes is shifted to the

Fig. 1.16 Mechanism for the metal catalyzed ATRP

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left-hand side, which reduces the termination reactions. As a result of persistent radical effect, polymers with predictable molecular weights, narrow molecular weight distributions and high functionalities have been synthesized [175]. ATRP technique allows the controlled polymerization of a wide range of vinyl monomers including styrenics, acrylates, acrylonitrile, vinyl acetate. To obtain consistent results, special handling procedures are required, and the preformed catalysts must be stored under an inert atmosphere; oxygen or other oxidants should be removed from the system prior to the addition of the catalyst in the lower oxidation state because the process of catalyst complex handling can be challenging. Although the ATRP is a convenient method for the synthesis of block copolymers, however it undergoes some limitations, such as the easy oxidation of the transition metal complex (Fig. 1.17a). To rise above this drawback, the reverse ATRP was developed (Fig. 1.17b); it uses the more stable Cu(II) complexes in the initiating step. However, a drawback occurs also in reverse ATRP: since the transferable atom or group (X) is added to the reaction as part of the copper salt, therefore highly active catalysts should still be used in the amount comparable to the concentration of the radical initiator; for this reason, complex concentration cannot be independently reduced and block copolymers cannot be formed.

Fig. 1.17 Methods for conducting ATRP (Reprinted with permission from Jakubowski and Matyjaszewski [175]. Copyright 2009 American Chemical Society)

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Simultaneous normal and reverse initiation (SR&NI) ATRP (Fig. 1.17c) was developed to allow the precursors of highly active catalytic complexes to be added to the reaction in the higher oxidation state and at lower concentration. SR&NI ATRP comprises a dual initiation system i.e. standard free radical initiators and initiators comprising a transferable atom or group in conjunction with the stable precursor of an active catalyst complex. This initiation system can be used to prepare any type of polymer that can be obtained by normal ATRP, and can be conducted in bulk, solution, emulsion, miniemulsion, and by heterogeneous polymerization. Problems arise for SR&NI ATRP as well, since a standard free radical initiator is still added to the polymerization mixture to form radicals that reduce Cu(II). A new method for the formation of an active catalyst was recently introduced by Jakubowski and Matyjaszewski to induce chain initiation [175]. The procedure consists of the preparation of an “activator generated by electron transfer” for ATRP (AGET ATRP, Fig. 1.17d), which overcomes the previously cited problems by using an electron transfer rather than organic radicals to reduce the higher oxidation state transition metal. This procedure has all the benefits of normal ATRP plus the benefits of adding a more stable catalyst complex to the reaction mixture. The use of oxidatively stable catalyst precursors can allow the more facile preparation, storage, and shipment of ATRP catalytic systems. The universal character of this approach was proved by applying the new activation/initiation process to a wide range of polymers.

1.5 Electrochemical Methods 1.5.1 General Features The electron transfer process is one of the most important synthetic methods in organic and polymer chemistry [196, 197] and among several pathways for electron transfer reactions, the electrochemical method is a straightforward and powerful one requiring mild polymerization conditions. The reversible interchange between the redox states in the conductive polymer gives rise to the changes in its chemico-physical properties including polymer conformation, doping level, conductivity and color. New compounds have been synthesized in order to enhance the stability and optimize and tune the electroluminescent properties of macromolecules [198, 199]. Among the materials prepared by electrochemical synthesis, conducting polymers, such as poly(para-phenylenevinylene) (PPV), polythiophene (PTh), polypyrrole (PPy), polyaniline (PANI), polyacetylene (PA) and poly(3,4ethylenedioxythiophene) (PEDOT) are nothe worthy. After the pioneering studies on the electroluminescence properties of PPV [200], conducting polymers have been deeply investigated in view of their potential applications in optoelectronics, for example as light-emitting diodes [201] or displays, energy storage devices, actuators, sensors, etc [202]. However, the interchange rate is usually slow, due to the rate-determining process of counter-ion transport into the polymer layer

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for charge balance [203]. It usually takes a few 100 ms or more to complete the charge in a regular conductive polymer film [204]. This slow interconversion rate is the main obstacle for applications of these polymer films to devices requiring fast charge/discharge capability such as electrochromic devices and supercapacitors. Electrochemistry has recently emerged as a powerful tool in a different field of research, i.e. the preparation of nanostructured conducting polymers [205]; in fact electrochemical synthesis offers several advantages with respect to chemical polymerization, first of all the easy control of the thickness of a deposited polymer layer on any conducting surface. In modern synthesis, mild conditions such as low temperature and anhydrous atmosphere are often employed in order to achieve selectivity and high yields and electrochemical reactions generally fit this request. Electrosynthesis is based on the use of an electrochemical cell and a requisite for this technique is the use of electroactive molecules, for example thiophene, pyrrole and aniline among organic monomers. The experimental setup is based on a galvanic cell, a potentiostat and two electrodes [206], as schematized in Fig. 1.18. The monomer is solubilized typically into organic solvents or in aqueous media and usually an electrolyte (e.g. lithium perchlorate or tetrabutylammonium acetate) is added. Platinum, carbon rods, magnesium, mercury, stainless steel can be used as electrodes and the electrosynthesis can be carried out with constant potential or constant current. The choice of the electrodic material, its shape and size play a crucial role in many electrochemical reactions. The initial reaction takes place at the surface of the electrode and then the intermediates diffuse into the solution where they participate to secondary reactions. The oxidations take place at the anode with initial formation of radical cations as reactive intermediates and the reductions occur at the cathode, with formation of radical anions. At a sufficiently high positive (i.e. anodic) electrode potential, monomers undergo electrochemical oxidation, the polymerization process starts and cation radicals or other reactive species are formed.

Fig. 1.18 Initial steps in the electropolymerization of thiophenes

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The tendency of the substrates or monomers to undergo electron transfer is represented by their oxidation and reduction potentials. The electron-transfer process is more favorable when the oxidation potential is less positive or the reduction potential is less negative. In order to achieve electron-transfer-driven reactions selectively, this process should occur choosy at the position that is needed for the subsequent chemical process, i.e. to cleave the specific bond or make a bond in the specific position in the starting molecule. From the viewpoint of synthetic efficiency and atom economy, the control of the electron transfer reactions can be achieved by the use of functional groups and templates to drive and facilitate the selectivity in predictable manner. For example, in the case of polymerization of thiophene, scheme reported in Fig. 1.18, the oxidation of the monomer produces a radical cation which can couple with a second radical cation or with another monomer to produce a dication dimer or a radical cation dimer, respectively. By successive steps, polymeric chains grow up and can be recovered. The electron density in the π-system of the thiophene ring drives the potential required to oxidize the monomer itself; electron-donating groups lower the oxidation potential, while electron-withdrawing groups increase the oxidation potential. For example, unsubstituted thiophene polymerizes at about 1.7 V vs. SCE (saturated calomel electrode), while 3-methylthiophene polymerizes at lower potential due to electron-donor effects and steric hindrance. This trend can be observed also for polythiophene with respect to thiophene monomer: the oxidation potential of the monomer is higher than the oxidation potential of the resulting polymer. This is one of the critical points of the electrochemical polymerization, that makes the process difficult to be performed in terms of regioregularity, stability and solubility of the product. Functional groups that make the electron-transfer process more favorable are usually called electroauxiliary (EA) [207] and drive the oxidation potential to become less positive or the reduction potential less negative. From a molecular orbital point of view, the oxidation process can be explained by electron transfer from the highest occupied molecular orbital (HOMO) of a monomer to the electrode. The increase of the HOMO enegy level by the introduction of an EA is the most straightforward method for activating the substrate toward oxidation. Alternatively it is possible to stabilize the radical cation generated by one-electron oxidation of the substrate. The choice of the best EA depends on the nature of the substrate and the reagent. Silyl derivatives, sulphides, stannyl or arylthio (ArS) groups serve as electroauxiliaries for the oxidation of heteroatom compounds [208]. Electroauxiliaries based on intramolecular coordination can also be advantageously used: if a substrate molecule has a specific coordinating site that can be stabilized by the developing of a charge, the electron transfer could be assisted by intramolecular coordination [209]. As an example, the pyridyl group is effective as a coordinating group for the oxidation of compounds containing heteroatoms such as O, S, and Se [210]. An advantage of the electrochemical polymerization is that the polymer does not need to be isolated and purified, but it produces structures onto the electrode

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surface with various degrees of structural irregularities. The nucleation and growth mechanism leading to the deposition of polymeric chains onto the anode has been widely studied; depending on the monomer concentration, electrolyte used, current density, temperature, solvent, electrode material, electrode potential and other experimental parameters, different morphologies for conducting polymers can be isolated. Nanostructured morphologies were obtained for example with the aid of templates during the polymerization process or, in general, with a proper control of the experimental parameters. In fact, the control of the growth process, starting from the nucleation of colloidal particles can be achieved by a fine tuning of the experimental conditions through which complex structures can be obtained. For example, disc-shaped nanoparticles of polyaniline have been prepared with the use of a pulse potentiostatic method at a highly ordered pyrolytic graphite (HOPG) electrode, from low concentration of aniline solution. The diameters of the obtained discs were in the range 20–60 nm depending on the variation of the electropolymerization charge applied, from 5.7 to 19.3 μC cm−2 [211]. The use of ultrasounds in chemical synthesis has also attracted significant research interest and has been applied in electrochemical polymerization. It is suggested that, in homogeneous solutions, the mass-transport process is accelerated by ultrasounds because of the macroscopic streaming [212] and microscopic cavitations phenomena [213]. The effect of ultrasounds on homogeneous electrolysis processes has been studied and a characteristic change in the distribution from oneelectron to two-electron oxidation products was observed together with a faster conversion under ultrasound conditions, although the detailed mechanism is not clear at present. For example, Kolbe electrolysis was found to be enhanced by ultrasound under biphasic conditions, in emulsion systems [214]. Recent progress was achieved by using electrochemical reactors enabled to perform electrochemical reactions under high-temperature and/or high-pressure conditions [215], opening also perspectives to the use of supercritical fluids as reaction media. Supercritical fluids have attracted significant research interest as efficient reaction media and in particular, supercritical carbon dioxide (scCO2 ) has significant potential as an environmentally benign solvent. Supercritical conditions for CO2 can be readily attained (Tc = 31◦ C, Pc = 7.3 MPa) [216] and scCO2 might replace hazardous organic solvents because it is non-toxic, inexpensive, miscible with organic compounds, and non-flammable. In addition, scCO2 can be recovered and reused after the reaction. Early experimental studies on electrolysis in scCO2 by Silvestre and coworkers showed that carbon dioxide is poorly conducting under supercritical conditions [217]. It was found, however, that the use of a small amount of water as co-solvent led to sufficient conductivity and the voltammetry of ferrocene in scCO2 containing tetrahexylammonium hexafluorophosphate could be achieved by the addition of water [218]. It has been reported that the scCO2 /water emulsion system is also effective for the electrochemical polymerization of pyrrole to form PPy and PTh films [219, 220]. Moreover, the use of scCO2 in electrochemical polymerizations has other actractive features because it is well-known that carbon dioxide penetrates into the polymeric materials and often reduces interchain interactions. Recently, homogeneous scCO2 /cosolvent systems have been applied

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to electrochemical polymerization. For example, electrochemical polymerization of pyrrole to form PPy films was successfully achieved in the scCO2 /acetonitrile system [221].

1.5.2 Role of Reaction Media Reaction media such as solvents and supporting electrolytes play a fundamental role in the control of electrochemical reactions; in fact they serve as an environment for molecular events in the electron transfer processes. Among others, trifluoroethanol or trifluoroacetic acid and in general unconventional fluorine-containing organic solvents, have recently received significant research interest and exhibited interesting features in the anodic oxidation of organic compounds. Ionic liquids and supercritical fluids can also be used for organic electrochemical reactions. Ionic liquids are salts that form a stable fluid at or near room temperature and are expected to replace hazardous and volatile organic solvents because they have low vapor pressure, non flammability, high polarity and relative inertness. Most ionic liquids consist of bulky organic cations, for example N,N-dialkylimidazolium, N-alkylpyridinium, quaternary ammonium, quaternary phosphonium, and common weakly coordinating anions such as AlCl4 − , BF4 − , PF6 − , CF3 SO3 − , TfO− , (CF3 SO3 )2 N− and some of them, such as N,N-dialkylimidazolium salts, show excellent conductivity [222]. Typical chemical structures of cations and anions are reported in Fig. 1.19. Ionic liquids can be recovered and reused after the completion of reactions, although there are some practical problems associated with these processes [223]. Ionic liquids themselves play the role of supporting electrolyte, and therefore, electrolysis can be conducted without any intentionally added electrolyte. The use of ionic liquids as electrolytes, doping agents, and practical recycling media for electrochemical polymerization has been developed. It is evident that the electrochemical synthesis of π-conjugated polymers takes also advantage of the fact that the anion component of the electrolyte can be introduced into the polymer as a doping agent to improve the conductivity. An early investigation reports about reactions carried out in chloroaluminate ionic liquids and π-conjugated polyarenes, polythiophene and polyaniline films were prepared in ionic liquids [224]. However, the use of chloroaluminate ionic

R N

+

N R'

+

pyridinum cations

1-alkyl-3-alkylmidazolinum cations

–

AlCl4

BF4

–

–

PF6

Cl

N R

–

(CF3SO3)2N

–

typical anions

Fig. 1.19 Typical chemical structures of cations and anions for ionic liquids

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liquids is problematic because their handling requires a special apparatus due to their inherent moisture sensitivity. An important improvement has been achieved by using N,N-dialkylimidazolium compounds having stable counter anions such as BF4 − , PF6 − , and CF3 SO3 − , which exhibit air and moisture stability [225]. For example, electrochemical polymerization of pyrrole was carried out by using 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, with a good control of the morphological structure of the film formed on the anode, and marked differences between the morphology of the upper and lower surface of the polymer film were observed. SEM analysis of the polymer films grown by using voltage pulses (Fig. 1.20) indicate that the polymer appears first as a series of fibrils, which can extend over a significant portion of the film before filling to form a complete film. This fine structure imparts a larger surface area to the polymer that can be prepared as a solid, homogeneous film [226]. The electrochemical synthesis of conductive polymers is accompanied by the doping of counterions into the polymer network. Considering that the conductive polymer properties are mainly affected by their doping states, it is of particular importance the choice of an appropriate reaction medium for the electropolymerization in order to attain conductive polymers with the desirable properties for different applications.

Fig. 1.20 SEM images of different fine-structured poly(pyrrole) films formed on the ionic liquid surface by using voltage pulses (100 ms pulses, top; 10 ms, lower four) (Reprinted with permission from Pringle et al. [226]. Copyright 2009 American Chemical Society)

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For example, aniline monomer can be easily oxidized in the presence of various electrolytes but good conductivities can be achieved only at low pH values, suggesting the use of acidic media for the electrochemical synthesis of polyaniline. Different acids have been used for this purpose and among others, perchloric or sulphoric acids are common examples [227]. Less attention has been paid to the use of phosphoric acid; however, a mixed electrolyte based on H3 PO4 and H2 SO4 mixtures has been used for the electropolymerization of aniline, giving rise to films with good stabilities and grain like morphologies. It has been assessed that H2 SO4 acts as a required agent for successful polymer growth, whereas H3 PO4 has an action as doping agent [228]. Environmentally friend and electrochemically stable waterproof ionic liquids like imidazolium cations with stable anions were also used. It is known that, unlike highly polar media, a hydrophobic medium facilitates the deposition of uniform well-adherent thick films. For example, the ionic liquid used to synthesize poly(3,4-ethylenedioxythiophene) (PEDOT) films leads to the formation of randomly oriented nanofibers and particles confined to submicrometer-sized domains in the film microstructure [229]. The role of the electrosynthesis parameters (monomer concentration and electrolyte nature) on the kinetics and on the morphology of the polymeric nanomaterials has been deeply investigated with particular attention to the incorporation of polyelectrolytes into conducting polymers, in view of potential application in the biomedical field. Composite films of polypyrrole and polysaccharides (heparin, hyaluronic acid and derivatives) were electrosynthesized as nanotubes and it was demonstrated that the conductivity and the morphology of the resulting polymers strongly depend on the negative charge distributions in the polysaccharide macromolecules backbone [230].

1.5.3 Nanowires and Nanotubes by Template and Template Free Methods Conductive polymeric nanostructures can be prepared by using hard or soft templates or with template-free methods. The template method has been extensively used because of its simplicity, versatility and controllability. Some further features on this topic are reported in Section 1.3. A typical hard template material can be a thin porous film of aluminum oxide or polycarbonate and polymeric materials can be deposited into the pores to form nanotubes or nanowires. The electrochemical template method enables a better control of the dimensions compared with the chemical methods. In addition, the nanostructures produced by the electrochemical method are in solid contact with a base electrode that is beneficial for further processing steps when building an electrochemical device. Since the first report of nanowire synthesis by Possin in 1970, many nanowires have been made, but the synthesis of nanotubes requires a delicate control of experimental parameters, such as concentration and reaction times. Only in the

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90thies Martin and co-workers prepared nanotubes in template pores [231]. They have shown that PPy nanofibers (diameters ca. 200 nm) have higher charge transport rates with respect to a conventional film under the same conditions. Recently, Demoustier-Champagne and co-workers chemically and electrochemically synthesized nanotubes and nanowires of polyaniline, polypyrrole, and poly(3,4ethylenedioxythiophene) [232]. Joo and co-workers have also electrochemically synthesized partially filled, long (up to 40 μm) nanotubes of conductive polymers like PANI, PPy and PEDOT, by controlling the polymerization time and the current, for potential applications of these materials as nanotip emitters in field emission displays and polymer-based transistors [233]. Usually, when a template is used, the electrodic surface is covered with an electrically insulating template having cavities with different shapes and sizes through which the solute species get through and catch up with the electrode surface. The electropolymerization starts at the electrodic surface, and propagates through the pores towards the outside solution. Nanowires or nanofibrils of different length, thickness and shape can be obtained although the nature of the nanosized templates and the control of the electrodic potential and solution concentration are fundamental for the attainment of nanofeatured films. It has been observed that an increase of electropolymerization rate occurs at a controlled electrode potential with increase of the feed solution concentration [234] and the thickness of PPy nanotubules strongly depends on the pore size of the template and on the kind of used electrolyte [235]. Moreover, the increasing of the electric conductivity of polypyrrole nanotubules with respect to the bulk material has been observed, probably due to an increased conjugation length in polypyrrole molecules packed into nanotubules. The increase in conductivity was observed also for other conducting polymer based nanotubes, probably because the polymerization inside the confined space of the pores, combined with electrostatic interaction, ensures the alignment of the resulting polymers on the walls of the pores [236]. In depth SEM studies of electrochemically prepared polyaniline suggested that the formation of hollow nanotubes can be driven by the deposition of polyaniline on the surface of the pore walls during the electropolymerization process [237]. Among oxides, alumina template has been widely used because of its easy preparation by anodic treatment of aluminum metal. Nanofibrils with uniform and well aligned structure made of copolymers based on PANI and PPy have been prepared on this template [238] and aligned nanotubular heterojunctions of poly(p-phenylene) and PTh have been isolated; it has been demonstrated that the length and the wall thickness of the nanotubes can be controlled by varying experimental parameters such as the electropolymerization current, the duration, and the nature of the doping anion present in the electropolymerization solution [239, 240]. Martin’s group proposed a mechanism based on electrostatic and solvophobic interactions between the growing polymer and the pore walls to explain the nanotube growth in template pores [241]. They observed that the interactions induce the growing polymer to nucleate and the chains grow preferentially along the pore wall to form tubular structures. As the polymerization proceeds further, the polymer grows inwardly and nanowires can be isolated.

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Wire-shaped growth of nanostructured PPy with diameter ≤10 nm, has been obtained by electropolymerization at naturally occurring step defects and artificially formed pit defects of HOPG, in a template assisted electropolymerization where the size of the nanostructures could be controlled by limiting the pyrrole polymerization time at anodic potentials [242]. Electrochemical polymerization of pyrrole within the confines of anodized alumina templates and subsequent metal nanoparticles immobilization on the surface of polymer pillars has been used to make surfaces that show roughness on two independently controllable levels: submicroscopic roughness from polymer pillar dimensions and nanoscopic roughness from the appropriate size selection of metal NPs [243]. A combined two-step method of preparing PPy nanowires was reported, consisting of (i) electrochemical grafting of a thin polyacrylate film at a carbon electrode and (ii) electropolymerization of pyrrole, resulting in polypyrrole wires with diameter 600 nm and length 300 μm, growing through the pores of a polyacrylate film [244]. Polystyrene nanospheres can be also used as a specific template for the preparation of nanocomposites with organic materials. In fact, the electropolymerization can be carried out within the voids of polystyrene nanospheres giving rise to PANI honeycomb films with desirable pore size, pore wall widths, and film thickness [245]. The use of template assisted electropolymerization is based on the preparation of nanosized cavities, channels or holes, on the electrodic surface, which drives the deposition of the resulting polymeric layer. Recent papers deal with aspects of template-based and templateless electropolymerization [246, 247]. Although the template based electropolymerization allows the preparation of well defined conducting polymers with different dimensions and morphologies (nanotubes or nanowires), the costs of the template synthesis are high and the handling and removing processes are relatively complex. In the case of templateless electropolymerization, the choice of experimental conditions such as monomer concentration, reaction time and working electrode material and porosity is crucial for the controlled growth of conducting polymer colloids. Electrochemical assembly method has been reported for the preparation of close packed ordered nanostructures of polyaniline in the presence of p-aminobenzenethiol [248] and nanodot based arrays of poly(o-phenylenediamine) were grown on Au(111) surfaces and on a p-aminobenzenethiol-modified Au surfaces [249]. PANI nanowires-gold nanoparticles hybrid networks were recently prepared and used as chemiresistive sensors. Initially, polyaniline nanowires with a diameter of 250–320 nm, bridging the gap between a pair of microfabricated gold electrodes, were synthesized by using templateless electrochemical polymerization with a two step galvanostatic technique. Polyaniline nanowires were then electrochemically functionalized with gold nanoparticles by using the cyclic voltammetry technique [250] and tested as hydrogen sulphide sensors with excellent limit of detection (0.1 ppb). Conductive polymer nanotubes with different inner morphologies ranging from hollow nanotubes to solid nanowires were prepared, to develop fast color-switching

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electrochromic devices. Since the switching rate depends on the ion-transport rate occurring in the polymer layer, an easy pathway to slow interchange rate is to reduce the ion-transport resistance in the polymer. A suitable approach is to introduce molecular-scale porosity, by a proper tailor of the polymer structure [251]. Up to now, the Reynolds group obtained the fastest electrochromic response (ca. 90 ms) by using a bulky conductive polymer such as poly(dimethyl3,4-propylenedioxythiophene) [252]. These nanostructured materials show intrinsically high surface area and can lead to high charge/discharge capacities and short diffusion distances for ion transport, which in turns leads to fast charge/discharge rates. Among the others, the nanotubular structure is particularly attractive because it is possible to control the charge/discharge rate and charge/discharge capacity, by a proper choice of the wall thickness and length of the nanotubes. Furthermore, the inside surface of hollow tubes can be chemically modified to further enhance their functions [253]. A further controllable approach for preparing conducting polymer nanostructures is the molecular template-assisted electrosynthesis, where some of the doping anions or other active species used in a “templateless” synthesis can themselves act like templates with molecular dimensions. In addiction, the electrode surface can be modified with an adsorbate and the electropolymerization is driven to proceed in a template-like manner. For example, a gold electrode modified with two component self-assembled monolayers consisting of p-aminobenzenethiol and n-octadecanethiol has been used for the growth of polyaniline nanostructures on paminobenzenethiol islands embedded into an octadecanethiol layer [254]. Similarly a gold electrode modified with thiolate β-cyclodextrin self-assembled monolayers have been exploited for the preparation of PPy films in acetonitril, where the polymer growth sites are restricted within the β-cyclodextrin cavities [255]. PPy and PANI based nanodots and nanowires have also been prepared by using a gold electrode modified with self-assembled monolayers of either β-cyclodextrin or p-aminobenzenethiol [256].

1.5.4 Aligned Nanostructures One of the key factors to obtain high performance functional nanomaterials is their ordering at a large scale, providing the alignment and orientation of the nanostructures. Oriented PANI nanowires, deposited in large arrays have been prepared by a proper control of the nucleation and growth process, by using a high initial current density in a first step. A large number of polyaniline nuclei were firstly deposited onto the electrode and in a successive step, with a reduction of the polymerization current density, the growth of oriented nanowires was achieved [257]. PPy nanowires have been obtained by one-dimensional growth at a graphite-paraffin composite electrode from bidimensional nucleation sites onto electrodic surface [258]. Oriented PPy nanofibers with fairly uniform diameters (in the range of 120–200 nm) and average length of up to microns were produced on a pre-roughed platinum plate electrode. Tubules of PPy with diameters ranging from 0.8 to 2.0 μm

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and length 15–30 μm were prepared in the presence of either β-naphthalenesulfonic acid or p-toluenesulfonic acid as a doping agent, on a stainless steel electrode. The growth mechanism has been studied and it was hypothized that an assembling of micelle of doping molecules or pyrrole–dopant clusters occurs, acting like templates on the electrode surface and giving rise to the tubules [259, 260]. By using a biphasic aqueous–organic system, one-step electrochemical strategy has been fixed up to synthesize polypyrrole nanofiber arrays. The experimental protocol ensures a very low concentration of pyrrole monomer in the polymerization process and provides good control on the deposition of nanofibers with diameters ranging from hundreds of nanometers to micrometers. In this procedure, the pyrrole monomer was dissolved in the organic phase and slowly allowed to diffuse into an aqueous phase through the interface, and then PPy was deposited on the electrode by electropolymerization. Owing to the very low concentration of the monomer near the electrode controlled by the interface, the lengths of PPy nanofibers could be easily tailored [261]. Aligned polypyrrole nanofibers were produced with successive CV electropolymerizations as schematized in Fig. 1.21.

Fig. 1.21 Schematic illustration of biphasic electropolymerization. Pyrrole diffused from organic phase into aqueous phase slowly (indirect diffusion). Then polypyrrole was after deposited on the electrode by electropolymerization by the diffusion of pyrrole to the electrode (direct diffusion). Aligned polypyrrole nanofibers were produced with successive CV electropolymerizations (Reproduced with permission of The Royal Society of Chemistry, from Li et al. [261]. Copyright 2009)

1.5.5 Coaxial Nanowires and Nanotubes Coaxial nanowires and nanotubes have attracted great attention due to the potential synergic properties or functionalities arising from the combination of different materials in core/shell systems [262]. Among others, coaxial nanowires based on

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conductive polymers in the presence of transition metal oxides could be promising electroactive materials for electrochemical energy storage [263]. As an example, due to its high energy density and low cost, MnO2 is one of the most used as electrochemical energy storage material, and to prepare coaxial nanowires with conducting polymers. MnO2 /poly(3,4-ethylenedioxythiophene) coaxial nanowires were prepared by co-electrodeposition in a porous alumina template. While MnO2 provides high energy storage capacity, the highly conductive and flexible PEDOT shell facilitates the electron transport and ion diffusion into the core MnO2 [264]. Controlled electrochemical synthesis of conductive polymer nanotubes in a porous alumina template has been studied as a function of monomer concentration and potential; in the case of PEDOT the electropolymerization leads either to solid nanowires or to hollow nanotubes depending on the template pore diameter, the applied oxidation potential and the monomer concentration [265]. Nanowires are formed at slow reaction rate and high concentration monomer supply; in fact monomeric molecules should have enough time to diffuse into and fill the pores, from the bulk solution. On the other hand, nanotubes are predominantly formed with fast reaction rate and low monomer concentration, because the monomers that diffuse from the bulk solution can be deposited along the pore wall thanks to the interaction of the polymer with the wall surface. Deep investigation on electrodeposition of 3,4-ethylenedioxythiophene (EDOT) as a model monomer and inner morphologies of PEDOT nanostructures synthesized at various monomer concentrations (10−100 mM) and applied potentials (1.0−1.8 V) have been reported and the growth mechanism is depicted in Fig. 1.22 [251]. At potentials higher than 1.4 V the reaction is limited by the diffusion of the monomers and tubular portions increase along with the applied potentials and against monomer concentrations. If low potential and high monomer concentration are used (e.g., 1.4 V, 100 mM EDOT), the polymerization reaction occurs on the whole electrode surface without notable preference, because the monomer can diffuse into the pore bottom under the slow reaction rate, leading to rigid, dense

Fig. 1.22 Growth mechanism of PEDOT nanostructures based on diffusion and reaction kinetics for high oxidation potential region (≥1.4 V). Two extreme cases are considered: (a) slow reaction rate under sufficient monomer supply and (b) fast reaction rate under insufficient monomer supply (Reprinted with permission from Cho et al. [251]. Copyright 2009 American Chemical Society)

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nanowires. On the other hand, if high potential and low monomer concentration (e.g., 1.8 V, 10 mM EDOT) is used, the polymerization reaction becomes very fast, and monomer supply is not sufficient to fill out the pores. Long, porous, thin-walled nanotubes are obtained because the monomer is immediately consumed to elongate the polymer chain. Since the reaction initiates along the electrode surface at the pore bottom, the conductive polymers are continuously deposited along the pore wall. Other parameters, such as electrolyte concentration, pore diameter, template thickness, stirring, temperature, also have influence on the inner morphology of the nanotubes. The growth mechanism of nanotubes was also studied at very low oxidation potentials (<1.4 V) where the reaction occurs very slowly. It was observed that nanotubular structures are favored and are nearly independent of monomer concentrations at potentials lower than 1.4 V. In this case, the morphology of base electrodes at the pore bottom becomes critical for the polymerization and the synthesis of PEDOT on the flat-top electrodes at the very low oxidation potential of 1.2 V leads to solid nanowires.

1.6 Emulsion Polymerization 1.6.1 General Features Emulsion polymerisation is a chemical technique widely used to produce particles with various colloidal and physicochemical properties and with nanodimensions. This is in general a heterogeneous free radical polymerization that involves the emulsification of the relatively hydrophobic monomer in water and sometimes an organic phase-in-water emulsifier, followed by the initiation reaction with either a water soluble initiator (e.g. sodium persulfate (NaPS)) or an oil-soluble initiator (e.g. 2-20-azobisisobutyronitrile (AIBN)) [266]. Typical monomers used in the emulsion polymerization include butadiene, styrene, acrylonitrile, acrylate ester and methacrylate ester, vinyl acetate, and vinyl chloride, but also biopolymers are now obtained by this versatile technique in several mesodimensionate morphologies [267]. In the common procedure extremely large oil–water interfacial area is generated and the particle nuclei grow in size with the progress of the polymerization. Thus, effective stabilizers such as ionic and non-ionic surfactants and protective colloids (e.g. hydroxyethyl cellulose and polyvinyl alcohol), which can be physically adsorbed or chemically incorporated onto the particle surface, are often required to prevent the interactive latex particles from coagulation. Under the circumstances, satisfactory colloidal stability can be achieved via the electrostatic stabilization mechanism [268], the steric stabilization mechanism [269] or both. The emulsion polymerization process is rather complex because nucleation, growth and stabilization of the polymer particles are controlled by the free radical polymerization mechanisms in combination with various colloidal phenomena.

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Perhaps, the most striking feature of emulsion polymerization is the segregation of free radicals among the discrete monomer-swollen polymer particles. This effect will greatly reduce the probability of bimolecular termination of free radicals and, thereby, results in a faster polymerization rate and yields polymers with higher molecular weight. This advantageous characteristic of the emulsion polymerization cannot be achieved simultaneously in bulk or solution. Although the nucleation period is quite short, generation of particle nuclei during the early stage of the polymerization plays a crucial role in determining the final latex particle size and particle size distribution and it has also a significant influence on the quality of the latex products. The way to effectively control the particle nucleation process represents a very challenging task to those who are involved in this fascinating research area. The transport of monomer, free radicals and surfactant to the growing particles and partition of these reagents among the continuous aqueous phase, the emulsified monomer droplets (monomer reservoir), the monomer swollen polymer particles (primary reaction loci) and the oil–water interface are the key factors that govern the particle growth stage. The batch emulsion polymerization is commonly used in the laboratory to study the reaction mechanisms, to develop new latex products and to obtain kinetic data for the process development and the reactor scale-up. Most of the commercial latex products are manufactured by semibatch or continuous reaction systems due to the very exothermic nature of the free radical polymerization and the rather limited heat transfer capacity in large-scale reactors. One major difference among the above reported polymerization processes is the residence time distribution of the growing particles within the reactor. The broadness of the residence time distribution in decreasing order is continuous>semibatch>batch. As a consequence, the broadness of the resultant particle size distribution in decreasing order is continuous>semibatch>batch, and the rate of polymerization generally follows the trend: batch>semibatch>continuous. Furthermore, the versatile semibatch and continuous emulsion polymerization processes offer the operational flexibility to produce latex products with controlled polymer composition and particle morphology. This may have an important influence on the application properties of latex products [270]. Moreover the miniemulsion, microemulsion and conventional emulsion polymerizations techniques show quite different particle nucleation and growth mechanisms and kinetics [271] Despite several advantages (water as the dispersion medium is environmentally friendly) [272], i.e. higher polymerization rates [273] and the relative simplicity of the process, the emulsion polymerization involves many mechanistic events, and the understanding of the events that dictate the rate of formation and the growth of polymer particles is difficult [274]. The mechanism of emulsion polymerization is shown in Fig. 1.23. This topic is still studied and in recent papers Thickett and Gilbert focused their investigations on the aspects concerning the mechanism of the emulsion polymerization, for both electrostatically and electrosterically stabilized particles [275, 276]; various kinetic limits has been described to explain an emulsion polymerization

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Fig. 1.23 Full scheme of kinetics process in a typical emulsion polymerization reaction (Reprinted from Thickett and Gilbert [274]. Copyright 2009, with permission from Elsevier)

system and their applicability under various conditions, and this allows accurate determination of the rate parameters. The colloidal properties of latex products are of great importance from both academic and industrial points of view. Some representative characteristics include the particle size and particle size distribution, the particle surface charge density (or zeta potential), the particle surface area covered by one stabilizer molecule, the conformation of the hydrophilic polymer physically adsorbed or chemically couplet onto the particle surface, the type and concentration of functional groups on the particle surface, the particle morphology, the optical and rheological properties and the colloidal stability.

1.6.2 Theoretical Overview A typical batch emulsion polymerization reaction contains three distinct intervals, labeled Interval I, II and III, such as reported in a pioneering paper by Harkins [277]. Interval I is that where the particle formation takes place and monomer droplets, surfactant micelle (if above the critical micelle concentration, CMC) and precursor particles (a small, colloidally unstable particle that upon further propagational growth, coagulation and adsorption of surfactant will eventually grow to a colloidally stable “mature” particle) are present.

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Interval II occurs after the conclusion of the particle formation period whereby only mature latex particles now exist; the particle number density (Np, the number of particles per unit volume of the continuous phase) remains constant and the particles grow by propagation in the presence of monomer droplets. As the diffusion of monomer from a droplet to a particle is rapid on the timescale of polymerization, the droplets act as monomer stores that ensure the monomer concentration within a particle to be essentially constant. Upon the exhaustion of these monomer droplets, Interval III starts, where the remaining monomer contained within the particles is polymerized. This often, but not always, corresponds to an increase in the polymerization rate and above a certain weight fraction of polymer (wp) within the particle a “gel” effect exists where the effective termination rate is reduced [278]. These three intervals are shown graphically in Fig. 1.24. The rate varies as a function of Np, of the particle size and of the initiator concentration [I]; in the ideal experiment for the understanding of the mechanism, each of these can be changed independently while all other quantities are kept constant. However, the early studies used systems where both Np and size changed together as [I] was changed. The complicated nature of this process means that the rate coefficients for entry and exit could not be determined unambiguously. Then the method chosen for the kinetic experiments devoted to the understanding of mechanisms consists in seeded experiments that begin in Interval II (by-passing particle formation), wherein Np and particle size can be controlled independently. It is essential in such experiments that the particle formation be avoided during Interval II, since otherwise Np will change during the experiment, which sometimes creates difficult experimental constraints. The value of Np is obtained from the size measurements of the latex by using: Np = mp /(4/3 π r3 dp )

(1.1)

Fig. 1.24 The three intervals of a typical emulsion polymerization reaction, showing surfactant ), large monomer droplets, micelle (indicated by clusters of surfactant molecules molecules ( within Interval I), radicals (R ), initiator (I) and surfactant-stabilized latex particles (Reprinted from Thickett and Gilbert [274]. Copyright 2009, with permission from Elsevier)

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where mp is the mass of the polymer per unit volume of the continuous phase, 4/3 π r3 the volume-average, t the unswollen radius of the seed latex and dp the density of the polymer. A detailed understanding of the particle formation mechanism is complex but crucial for a complete evaluation of the emulsion polymerization, and the knowledge of the conditions that avoid the formation of new particles in seeded systems (secondary nucleation) is vital for a number of industrial procedures. The rate of a polymerization is normally defined as the rate of consumption of the monomer: d[M]/dt = −Kp [M][R]

(1.2)

where [M] is the concentration of the monomer and [R] the total radical concentration. In an emulsion where the polymerization only takes place within the particle interior, [M] is replaced by Cp (concentration of monomer in the particle phase); the total radical concentration is ñNp = NA, where ñ is the average number of radicals per particle, Np is the number of particles per unit volume of the continuous phase, and NA is Avogadro’s constant. Since it is experimentally convenient to measure the fractional conversion of monomer into polymer (denoted by x, where 0≤ x ≤1), a change in variable is made and the rate of fractional conversion is now considered, giving: dx/dt = n˜ [(Kp Cp Np )/(n◦M NA )]

(1.3)

where n◦ M is the initial number of moles of monomer per unit volume of the continuous phase (all other parameters as defined previously). Equation (1.3) shows that ñ, including its time dependence, can be obtained experimentally via accurate monitoring of the polymerization rate. The synthesis of well-characterized electrosterically stabilized latexes presents a more difficult challenge. Electrosteric stabilization (by using ionizable watersoluble polymers grafted to the particle surface to impart colloidal stability) is a common technique in the synthesis of industrial polymers to be used in surface coatings and adhesives. While easy to synthesize, the characterization of the “hairy layer” on the particle surface is extremely complicated. A new route to the synthesis of well-defined electrosteric latexes has recently been developed through the advent of the successful controlled-radical polymerization in emulsion [279], in particular of the reversible addition-fragmentation chain transfer (RAFT) technique. Ferguson et al. [280, 281] developed the first electrosterically stabilized emulsion under complete RAFT control through the use of an amphipathic RAFT agent that allows the synthesis of relatively monodisperse hydrophilic block polymers in water as the first step. Subsequent starved-feed addition of a hydrophobic monomer into the aqueous phase eventually results in the self-assembly of diblock copolymer chains (the beginning of particle formation), after which the particles continue to grow to any size. The reaction scheme of this procedure is shown in Fig. 1.25.

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Fig. 1.25 Synthesis of an electrosterically stabilized seed latex by using RAFT (Reprinted from Thickett and Gilbert [274]. Copyright 2009, with permission from Elsevier)

1.6.3 Critical Parameters for Emulsion Polymerization Some typical features of the nanobeads and an experimental parameter wich allow to control and modulate the chemical physical properties are shown in Fig.1.26a, b respectively. In particular the shape and dimensions of the spheres are influenced by the reaction time, by cosolvent and initiator concentration. In the emulsion technique for copolymerization the monomer/comonomer ratio plays the main role to determinate the chemical features and dimensions [282, 283].

Fig. 1.26 (a) SEM image of PPA nanobeads with d = 280 nm and larger ordered domain of about 12 × 12 μ, obtained using toluene/PA= 2/1, KPS = 0.008 g, and reaction time = 90 min.; (b) average diameter of PPA () (toluene/PA= 2/1; reaction time 90 min) and P(PA/HEMA) (•) (PA/HEMA = 10/1; toluene/PA= 2/1; reaction time 150 min) nanobeads, as a function of KPS amount. Adapted from Venditti et al. [283]. Copyright 2009, with permission from Elsevier

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The particle size (often nano) and polydispersion are usually confirmed via scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS) or hydrodynamic chromatography, a separation technique that keeps apart the particles on the basis of the size; sometimes DLS and SEM data are not directly comparable (SEM images are collected from dried samples while DLS measurements are in solution and a swelling effect occurs) but the trends of growth of the nanoparticles are similar for the two methods. The cosolvent effect on the dimensions and shape of the particles is due to the formation of a layer around the monomer droplets and this layer controls the transfer of the monomer molecules from inside the droplets to the surrounding aqueous phase and can absorb the monomer molecules that are dispersed in the solvent; e.g. in the presence of toluene, the size of the drop limits the dimension of the particle and also reduces the probability of collision, responsible of the coalescence. Moreover, the cosolvent acts as a hydrophobic layer and reduces the potential energy of the interface, leading to an overall stabilization of the emulsion and hence to regularly shaped particles; a high polydispersity and irregular particle shapes were found for those samples prepared in the absence of toluene, while very low polydispersity and a regular round shape were found for the others; this behavior is consistent with the study of Tanrisever et al. [284] on the polymerization kinetics of PMMA. The role of the cosolvent is analogous to the role of nonionic and ionic/nonionic emulsifiers in the radical polymerization of unsaturated monomers (styrene, alkyl(methacrylates), etc.) in aqueous medium, as reported by Capek [285] where the increased stability of the emulsions containing a nonionic emulsifier was attributed to the effects produced by the presence of a layer of the emulsifier around the monomer droplets. The effect of KPS concentration in the emulsion polymerization is shown in Fig. 1.26b for a hydrofobic monomer, phenylacetylene; the increase of the initiator amount produces larger polymeric particles, because in this situation the polymerization rate is high and the polymer chain growth is completed in short times, so that the growth rate of the particles is controlled by the coalescence. In the emulsion copolymerization, when a hydrophilic monomer is involved, the initiator concentration has a different influence on the particles size, such as in P(PA/HEMA) emulsion copolymerization (see Fig. 1.26b); when the initiator concentration increases, the particles dimension decreases, because the density of polymerization sites of nucleation increases. In general, it is assessed that the initiator is important not only for the tuning of the rate of polymerization and of the molecular weight but also the particle size and latex viscosity can be tuned by changing the concentration and the kind of initiator [286]. Finally an important experimental parameter to be considered in the emulsion copolymerization is the concentration of hydrophilic monomer; it has several effects on the particles features, in particular on the particle size, particle size distribution and superficial charges, liable for swelling behavior and adhesion properties.

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1.7 Electrospinning 1.7.1 General Overview Self-assembly has been defined in a stretchable fashion and it is possible to simply describe it as the process by which spontaneous arrangements of ordered structures are generated, based on several different interactions at various length scales. For example, if we consider a length scale ranging from the angstrom to the millimeter we would experience interactions ranging from covalent and hydrogen bonds, to Van der Waals and electrostatic force and finally surface tensions. The control of these forces and interactions is fundamental for the achievement of the desired deposition [287]. There are several jet-based approaches, which have been demonstrated to have a significant impact on the preparation of materials at nanometric level, ranging from ink-jet printing to electrospraying. Ink-jet printing has contributed a great deal to this endeavor, although it shows limitations in the processing of highly concentrated suspensions from which nanoscaled deposits are derived. Electrospray explores high intensity electric fields to generate droplets and form self-assemblies from a wide range of conducting nanoparticles. Polymeric fibers having diameters ranging from a few nanometers up to microns can be successfully prepared by means of the electrospinning based method [288]. The use of polymeric fibers has several applications, varying from the tissue engineering [289] to reinforcements in nanocomposites [290], nanowires and nanotubes [291]. The pioneering work of Martin reported the extraordinary increase in the electronic conductivity of polymeric nanofibers, due to the confinement of dimension and size in the nanometric range [292]. Electrospinning is a suitable technique for the production of fibers with small diameter, and was introduced for the first time by Formhals in 1934 by developing a method for the production of artificial filaments, applied to spun cellulose acetate fibers from an acetone/alcohol solution [293]. Electrospinning easily tunes the possibility of assembling polymeric fibers with diameters less than 100 nm and length up to microns for a wide choice of polymeric materials with different molecular weights and functionalities. This technique was in depth reinvestigated in the recent years [294, 295] and applied to a variety of functional polymeric materials [296] and to biopolymers, suited for tissue engineering applications [297, 298]. This method is based on the feed of a polymeric solution or melt, maintained at a high positive potential, through a thin metallic needle. A typical electrospinning setup consists of a capillary through which the liquid to be electrospun is forced, a high voltage source with positive or negative polarity which injects charge into the liquid and a grounded collector; the set up is shown in Fig. 1.27. In a prototypical electrospinning experiment, a characteristic jet path is created when a fluid polymer solution, supplied to a drop attached to an orifice by surface tension and viscoelastic stresses, is electrified by a sufficiently high electrical potential applied between the drop and a collector placed some distance away. The shape of the drop approaches a cone and an electrically charged jet of fluid emanates from

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V

Taylor cone

needle Liquid jet

collector Fig. 1.27 Schematic representation of the electrospinning technique

the tip of the cone. The electrical forces from the charge carried within the jet, induce the jet to continue to elongate as it cools and finally the thin fluid jet solidifies into a nanofibre onto the collector. A syringe pump, gravitational forces, or pressurized gas are typically used to force the liquid through a small-diameter capillary, forming a pendant drop at the tip. An high voltage electrode is then immersed in the liquid or can be directly attached to the capillary if a metal needle is used. The voltage source (typically direct currents, in the range 1–30 kV) is then turned on and the charge is injected into the polymer solution. The increase of the electric field strength gives origin to the repulsive interactions between homologue charges in the liquid and the attractive forces between the oppositely charged liquid and the collector begins to exert tensile forces on the fluid, elongating the pendant drop at the tip of the capillary. As the electrostatic attraction between the oppositely charged liquid and the collector and the electrostatic repulsions between homologue charges in the liquid become stronger, the leading edge of the solution changes from a rounded meniscus to a cone (the Taylor cone) [299]. By applying a high voltage difference between the polymeric droplet coming out of the metallic needle and a collecting cathode, it is possible to force the formation of a controlled stream, and to prepare polymeric fibers. Examining how the polymer droplet at the end of a capillary behaves when an electric field is applied, it was found in further literature studies that the pendant droplet develops into a cone and the fiber jet is emitted from the apex of the cone [300]. This is one reason why electrospinning can be used to generate fibers with diameters significantly smaller than the diameter of the capillary from which they are ejected. At critical values of the applied voltage, the drop on the tip of the Taylor cone becomes a jet and this effect is strictly connected to the overcoming of the surface charge on the surface tension of the droplets. The fibers are formed during the flight of the stream towards the collecting plate, as a consequence of

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solvent evaporation. The just formed fibers can be collected on the cathode and usually highly disordered depositions are observed, depending on several parameters such as splitting of the stream, applied voltage, rate of the stream and chemicophysical characteristics of the polymers such as viscosity, solubility, surface tension. Fibers produced by using this process typically have diameters in the order of a few micrometers down to the tens of nanometers. Despite a relatively easy access to this operation, a precise control of the parameters affecting the morphology of the fibers is necessary to obtain specificity in the product shape, size or properties. It is note worty that nanofibres with small diameters have a large surface area per unit mass and different molecules, particles, and biomolecules can be sequestered and protected inside or on the surface of the nanofibers, remaining accessible for use when needed. Moreover, nanofibers can be used as convenient packages and supports for reagents and catalysts.

1.7.2 Process and Mechanism Typically, molecular weights of polymers suitable for electrospinning are in the range from 100,000 to several millions u.m.a. and the concentrations in the range 5–15 wt%. The electrical charge that is important in electrospinning is an excess or uncompensated charge, usually in the form of positive or negative ions. Although all ionic solutions contain charged molecules or ions, the solution is electrically neutral because the number of positive and negative ions is exactly equal. The essential excess ions are usually created near the interface between a metallic conductor and the molecules in the solution. The electrons moving into the solution from the metal electrode create excess negative ions in the polymeric solution and the electrons moving from the solution into the metal leave excess positive ions. Once created, the ions move by diffusive and convective processes to reduce the repulsive interactions between the similarly charged excess ions and to maintain the same electrical potential everywhere on the surface of the fluid body. The addition of a salt to the uncharged solution preserves the electrical neutrality, although the salt molecules may dissociate into positive and negative ions which move independently and thereby increase the electrical conductivity. The process of collecting fluid jets involves coils formed by electrical bending, branching, conglutinated networks of fibers and garlands and the solidification of the thin jets produces nanofibers with the occurrence of nucleation and crystallization inside the nanofibers. Phase separation of both polymer blends and block copolymers can be observed. The formulation proposed by Taylor to explain the conical shape of the droplet before the jet issues a mathematical singularity at the tip of the cone where the jet begins. The singularity is avoided assuming a parabolic shape for the tip and the shape and the size of the drop may vary with time so that the flow into the drop and the flow out of the drop are not necessarily equal at every instant. For a constant feed rate, if the voltage is too high, the drop becomes smaller and the beginning of the jet often moves to the edge of the orifice before the jet finally stops. If the voltage

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is too small, the size and the shape of the drop and the diameter of the jet tend to oscillate in time. If the voltage is far too low, the drop grows large and changes the shape as the gravitational forces became important. Ultimately, a drop flows over its support and a pendant drop drips. The length of a short, essentially straight segment, measured along its own axis, increases in response to the Coulomb repulsion between the excess charge distributed along and carried with the segment. The repulsive Coulomb forces between the charges of homologue sign on adjacent segments of the jet work against the axial component of the viscoelastic stress and elongate the jet in the direction of its axis. Polymer solutions which support the motion of free electrons as in a metallic conductor are not commonly encountered. PANI provides an interesting exception in which the electrons move through dry polyaniline nanofibers [301]. The electrical conductivity of the PANI fibers becomes evident during the collection of the dry nanofibers, because of the tendency of the network of nanofibers to extend in the direction of the applied electric field rather than forming a flat non-woven mat typical of insulating polymers. This extension happens because the characteristic redistribution time for the electrons along the length of the electronically conducting polyaniline nanofibres is short. The electronic charge quickly accumulates at favorably oriented ends or bends of the fibers and stretches the dry fiber network in the direction of the applied field.

1.7.3 Parameters Affecting the Electrospinning Process 1.7.3.1 General Features The choice of the flow rate of the polymer, of the distance between needle and collecting cathode, of the applied potential, of the temperature and humidity should go with the account of the concentration and viscosity of the solution, of the molecular weight and molecular weight distribution of the polymer and of the functionalities on the polymeric chains, which in turn affect the dielectric constant and the solubility of the polymers. For example, it was observed that the polymer solution should have a concentration high enough to cause the entanglement of the chains but not so high to reach very high viscosity values that can inhibit the polymer motion in the electric field. Systematic researches on the influence of the parameters were carried out [302] showing that, for instance, the size of the polymeric fibers is strongly affected by the concentration i.e. the diameters increase by increasing the concentration. In recent years, a greater understanding of the processing parameters has developed the formation of fibers with diameters in the range of 100–500 nm, typically referred to as nanofibers. The achievement of nanofibers has led to reneved interest on the electrospinning process due to potential applications in filtration, protective clothing, and biological purposes in tissue engineering scaffolds and drug delivery devices [303]. Polyaniline-based nanofibers with diameter below 30 nm were obtained with the electrospinning process [304] which is suitable not only for conjugated polymers, but also for non conjugated, water soluble polymers such as poly-ethylen oxide (PEO), polyvinyl alcohol (PVA) and poly-lactic acid (PLA). In fact, one of the

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Fig. 1.28 Optical and FE-SEM images of electrospun fibers (c) Optical micrograph of aligned PEO nanofibers 20×; (d) FE-SEM image of aligned PEO nanofibers (5,000×); (e) FE-SEM image of isotropic PEO nanofibers (5,000×) (Adapted with permission from Kakade et al. [306]. Copyright 2009 American Chemical Society)

advantages of this technique is that it can be applied to a wide variety of polymers, bearing polar or non-polar pending groups [305]. In Fig. 1.28 patterns of aligned PEO electrospun fibers are shown [306]. Polymeric porous and wrinkled fibers can also be obtained, as in the case of polystyrene electrospun from DMF solutions [307]; moreover, polystyrene microand nanospheres with low dimensional polydispersion have been produced by electrospray and the effect of PS molecular weight and concentration on the beads morphology has been investigated [308].The interior of these fibers, when electrospun in a high-humidity environment, was found to be highly porous rather than polished, despite the smooth and nonporous appearance of the fibers surfaces. The formation of interior porosity is attributed to the miscibility of water, a nonsolvent for the polymers in solution, with DMF. The resulting morphology is a consequence of the relatively rapid diffusion of water into the jet, leading to a liquid−liquid phase separation that precedes the solidification due to the evaporation of DMF from the jet. The fibers exhibit a wrinkled morphology that can be explained by a buckling instability, when they are electrospun in a low-humidity environment. Understanding which morphology is formed under a given set of conditions is achieved through the comparison of three characteristic times: the drying time, the buckling time, and the phase separation time. The morphology has important consequences for the main properties of the fibers, among the others their mechanical strength and stiffness. 1.7.3.2 Applied Voltage, Distance Between Capillary Tip and Collector and Flow Rate An increase of the applied voltage alters the shape and the morphology of the drop at which the Taylor cone and fiber jet are formed, as in the case of polyethylene oxide (PEO)/water system studied by Deitzel et al. [309]. At low applied voltages the Taylor cone is formed at the tip of the pendent drop and as the applied voltage is increased, the volume of the drop decreases until the Taylor cone is formed at the tip of the capillary. Meechaisue et al. [310] examined the effects of the processing parameters, including the applied electric field strength, on electrospun poly(desaminotyrosyl- tyrosine ethyl ester carbonate) (poly(DTE carbonate). The

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authors investigated the behavior of a solution of poly(DTE carbonate) at concentration 15% w/v, varying the applied electric field strength from 10 to 25 kV/10 cm. Beaded fibers were primarily observed when the applied electric field strength was below 20 kV/10 cm, while mostly smooth fibers were obtained above this field strength. The increase of the electric field strength from 10 to 15 kV/10 cm decreased the bead density, while by increasing the field strength from 20 to 25 kV/10 cm the average fiber diameter increased from 1.9 to 2.2 mm. The authors attribute these latter results to the increase of the mass flow rate, related to the enhancement of the electrostatic force. Based on literature reports, it is evident that there is an optimal range of electric field strengths for a certain polymer/solvent system, since either a too weak or a too strong field will lead to the formation of beaded fibers. While playing a much smaller role, the distance between capillary tip and collector can also influence the fiber size by 1–2 orders of magnitude. The fiber diameter generally decreases with increasing the distance from the Taylor cone, as in the case of electrospun fibers from a PEO/water solution; examining the fiber diameter as a function of the distance from the Taylor cone, it was observed that the diameter of the fiber jet decreased approximately 2-fold, from 19 to 9 mm after traveling distances of 1 and 3.5 cm, respectively [311]. Beaded morphology for electrospun polystyrene fibers upon shortening the distance between the capillary tip and the collector was observed, attributed to inadequate drying of the polymer fiber prior to reaching the collector [312]. The polymeric fibers are deposited onto a grounded collector and, depending on the application, a number of collector configurations can be used, including a stationary plate, rotating mandrel, solvent (e.g. water), etc, and different morphologies can be obtained. Typically the use of a stationary collector will lead to the formation of a randomly oriented fiber mat. A rotating collector can be used to generate mats with aligned fibers, with the rotation speed playing an important role in determining the degree of anisotropy. The use of rotating collector plates allows the deposition of fibers with a higher degree of alignment [309]. Additionally, both the conductivity and the porosity of the collector play also a role in determining the packing density of the collected fibers [313]. The polymer flow rate has an impact on the fiber size, and additionally can influence the fiber porosity as well as the fiber shape and morphology. The effect of the flow rate on the structure of the electrospun fibers of polystyrene in tetrahydrofuran solution was studied, demonstrating that both fiber diameter and pore size increase with increasing the flow rate. Moreover, at high flow rates significant amounts of bead defects become noticeable, due to the inability of the fibers to dry completely before reaching the collector. Incomplete fiber drying also leads to the formation of ribbon like (or flattened) fibers as compared to fibers with a circular cross section [314]. 1.7.3.3 Choice of Solvent, Polymer Concentration and Solution Conductivity The choice of solvent is also a critical parameter, influencing whether the fibers are capable of forming as well as the fiber porosity. In order to have solvent evaporation

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during the jet flight between capillary tip and collector, a volatile solvent is required. For example, polystyrene fibers electrospun from solutions containing various ratios of DMF and THF were examined. The produced electrospun PS fibers contained irregular beads and the electrospinning certainly was enhanced with increasing the DMF content. The bead concentration was also controlled by DMF content. The aspect ratio of the formed beads and the diameter of the fibers were increased with increasing the solution concentration and when PS was dissolved in THF only, an unexpected half hollow spheres (HHS) structure appeared. Further different shape forms of PS non-woven mats have been prepared by controlling the electrospinning parameters [315]. Between these two extremes it was observed that as the solvent volatility decreases, the pore size increases with decreased pore depth (thus decreasing pore density). A phase separation occurs as the polymer jet is traveling through the atmosphere. This phase separation can be vapor-induced, which occurs when the non-solvent from the vapor phase penetrates the polymer solution. However, transport of the non-solvent into the polymer solution is limited by the slow diffusion of the non solvent adjacent to the fiber surface. For very volatile solvents, the region adjacent to the fiber surface can be saturated with solvent in the vapor phase, which further limits the penetration of non-solvent. This can hinder the skin formation leading to the development of a porous surface morphology. The polymer concentration is a fundamental parameter and determines the spinnability of a solution, namely whether a fiber forms or not. The polymer concentration influences either the viscosity and the surface tension of the solution, both of which are very important parameters in the electrospinning process. In fact, if the solution is too dilute, the polymer fiber will break up into droplets before reaching the collector due to the effects of the surface tension. However, if the solution is too concentrated, fibers cannot be formed due to the high viscosity, which makes it difficult to control the solution flow rate through the capillary. The solution must have a high enough polymer concentration for the chain entanglements to occur, although it cannot be either too dilute or too concentrated. Thus, an optimum range of polymer concentrations exists in which fibers can be electrospun when all other parameters are held constant. In many experiments it has been shown that within the optimal range of polymer concentrations, the fiber diameter increases with increasing the polymer concentration; for example, increasing the concentration of polystyrene in THF, the fiber diameter increased and the distribution of pore sizes became narrower [312]. The solution conductivity plays a less important role although it can influence the fibers size within 1–2 orders of magnitude. If a solution possesses high conductivity, it will have a greater charge carrying capacity with respect to solutions with low conductivity and as a consequence, the fiber jet of highly conductive solutions will be subjected to a greater tensile force in the presence of an electric field than a fiber jet will from a solution with a low conductivity. In general, the radius of the fiber jet is inversely related to the cube root of the solution conductivity [316]. Highly conductive solutions are extremely unstable in the presence of strong electric fields, which lead to a dramatic bending instability as well as a broad diameter distribution [317].

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The effect of adding ions to PVA/water solution on the diameters of the electrospun fibers was studied and it was observed that by adding increasing concentrations of NaCl (ranging from 0.05 to 0.2 wt%) to a PVA/water solution, a decrease of the mean fiber diameter from about 200 to 160 nm occurs [318]. The authors attribute this decrease in the mean fiber diameter to the increased net charge density imparted by NaCl, which increases the electric force exerted on the jet. Some efforts were made also to incorporate biomolecules like bovine serum albumin (BSA) into dextran fibers for potential drug delivery or tissue engineering applications [319]. It was observed that adding 5% BSA, a decrease of the mean fiber diameter occurs from approximately 2.5 mm–500 nm and that the viscosity of the dextran solution is unchanged by the addition of up to 10% BSA, indicating that the decrease in fiber diameter is due to the increased net charge found in the polymer jets. Another possible technique is the direct electrospinning of a polymer melt. In general, the solution spinning results in a greater range of fiber sizes, while melt spun fibers are typically limited to micron size or larger [320]; however, there are specific advantages and disadvantages for each method. Melt electrospinning eliminates the need for harsh organic solvents, which is ideal for scaled-up processes, but melts must be kept at elevated temperatures to be electrospun, whereas stable solutions can typically be electrospun at room temperature. The melt electrospinning of block copolymers of PEG and PCL (polycaprolactone) with various molecular weights was studied and it was observed that the optimum melt temperatures ranged between 60 and 90◦ C [321]. These high temperatures may preclude their use for tissue engineering or drug delivery applications. The use of polymer melts also eliminates the problem of inadequate solvent evaporation between the capillary tip and the collector; however, the polymer must be able to cool suitably over this distance in order to generate fibers with a cylindrical morphology. 1.7.3.4 Nozzle Configuration In addition to adjusting the solution or processing parameters, the type of electrospinning process can greatly influence the resulting product. This can include choices in nozzle configuration such as single, side-by-side, or coaxial nozzles. A number of nozzle configurations have been employed, and perhaps the simplest and most common is the single nozzle technique. In this configuration a charged polymer solution or melt flows through a single capillary. This configuration is very versatile and has been used to electrospin single polymer solutions [322] as well as polymer blends out of polymers soluble in a common solvent [323] and was used also for the electrospinning of composite fibers containing bovine serum albumin (BSA) loaded Ca-alginate micro spheres microencapsulated in poly(L-lactic acid) (PLLA) fibers [324]. While the electrospinning of polymer blends is often desirable in order to achieve the proper combination of properties, it may not be possible by using a single needle configuration if the polymers of interest are not soluble in a common solvent. Thus, it may be necessary to use a side-by-side mode. In this configuration two separate polymer solutions flow through two different capillaries, which are set side-by-side.

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This configuration was used to electrospin bicomponent systems out of poly(vinyl chloride)/segmented polyurethane and poly(vinylchloride)/poly(vinylidene fluoride) [325] and it was observed that the solution conductivity plays a more important role in the ability to form a single fiber jet under a strong electric field in the sideby-side configuration. The conductivity of the PVC solution was significantly higher than either of the other two solutions, and thus two distinct Taylor cones, one from each solution, were formed when subjected to a strong electric field and the ratio of the two components varied along the length of the fiber, which was attributed to fluctuations of the jet on the surface of the Taylor cone. A relatively new nozzle configuration is the coaxial configuration, which allows the simultaneous coaxial electrospinning of two different polymer solutions. In this set up two separate polymer solutions flow through two different capillaries, which are coaxial with a smaller capillary inside a larger capillary. This technique, called coaxial-electrospinning, has received great interest because by using this nozzle configuration smaller fibers can essentially be encapsulated into larger fibers, leading to what is known as core-shell morphology. For example, PEO–PEO and PEO–PDT type core–shell nanofibers were produced by this method [326]. In general, two coaxial capillary tips are used to simultaneously feed the two liquids. At a short distance from this two-capillary nozzle, typically several centimeters, a metallic plate is placed as counter-electrode or collector [327]: in Fig. 1.29 the process is schematically represented.

Fig. 1.29 Two immiscible liquids (red and blue in this Figure) are injected through two concentric electrified needles which are placed a few centimeters away from a grounded electrode (the collector). A compound Taylor cone is developed from whose tip a coaxial nanojet is emitted. Upon solidification of the outer liquid, a sheathed fiber, or a liquid-filled hollow fiber, is formed (Reprinted with permission from Loscertales et al. [327]. Copyright 2009 American Chemical Society)

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1.7.4 Core-Shell Nanofibers and Nanotubes by Coaxial Electrospinning Modifications of the electrospinning technique have been proposed in order to obtain oriented fibers or core-shell nanofibers and nanotubes. The coelectrospinning allows the preparation of core-shell fibers or tubes: different polymeric solutions can be injected thought concentric annular needles and electrospun through the formation of a double Taylor cone. The mixing of the two solutions can be avoided by means of a very fast electrospinning process by which coaxial nano and micro fibers can be obtained; with a proper choice of the polymeric materials, the inner core can be removed and nanotubes isolated [328], as shown in Fig. 1.30. This technique was proved to be very versatile for the encapsulation of biorelevant molecules and nanocomposites: for example, coaxial electrospinning of aligned PCL nanofibers encapsulated with bovine and platelet-derived growth factor-bb was carried out for the demonstration of controlled release and bioactivity retention [329]. The encapsulation of a model protein, (fluorescein isothiocyanateconjugated bovine serum albumin, fitcBSA), along with PEG within the biodegradable poly(caprolactone) PCL nanofibers was achieved by using a coaxial electrospinning technique [330]. By varying the inner flow rate with a constant outer flow rate, the loading of fitcBSA could be controlled. The coaxial electrospinning has also been used for the encapsulation of active biological threads and scaffolds. In the cell/polymer electrospinning, a coaxial needle arrangement is used with the flow of highly concentrated cellular suspension in the inner needle and medical grade biocompatible polymer in the outer needle. After electrospinning, the collected cells have been cultured, have been found to be viable and showed no evidence of having incurred any cellular damage during the bionanofabrication process. Electrospun poly(vinyl alcohol) nanofibers were tested for the encapsulation of bacteria (Escherichia coli, Staphylococcus) and viruses that remained viable for months at low temperatures [331]. In general, complex biological objects can be prepared directly without total loss of biological functionality.

Fig. 1.30 (a) Simple nanofiber, (b) core-shell nanofiber, (c) nanotubes

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As a curiosity, spider and caterpillar silk were electrospun into nanofibers and characterized by electron microscopy [332, 333]. The diameter of these silk nanofibers ranged from about 6.5 to 300 nm, making them several orders of magnitudes smaller than the silk fibers spun by silkworms or by many kinds of spiders. Electron diffraction patterns of annealed electrospun nanofibers exhibited diffraction peaks with the molecules aligned along the axis of the fiber with a crystalline order comparable to that of naturally spun silks.

1.7.5 Electrospun Fiber Properties One of the most important advantages of the electrospinning technique is the production of very thin fibers with large surface areas together with the ease of functionalization for various purposes, the superior mechanical properties and the effortlessness of the process. These advantages provide a wide range of opportunities for their use in many different applications, ranging from tissue engineering, drug release, implants, to biotransformation, reinforced composites etc. [334]. The potentiality of the electrospinning is that the method can be varied in different ways to combine materials properties with different morphological structures suitable for these applications. A large number of biodegradable and biocompatible polymers have been electrospun by conventional electrospinning or modified electrospinning methods and loaded with different bioactive molecules. Moreover, the faculty to adjust the fiber size from micro to nano size is one of the strengths of electrospinning, since fibers with diameters in the nanometer size range closely mimic the size scale of fibrous proteins of the natural extra cellular matrix (ECM), such as collagen. This ability of electrospun nanofibers to mimic the ECM is vital, since previous studies have shown that both the size scale of the structure and the topography play important roles in cell proliferation and adhesion, respectively [335]. Additionally, the porosity of electrospun mats aids in nutrient transport. Although many researches have encountered limitations concerning the cell infiltration into nanofiber mats, due to the relatively small pores associated with such matrices, nanofibrous mats have the potential to overcome mass transfer limitations seen in other polymer drug delivery systems due to their high surface to volume ratio. Additionally, nanofiber systems can afford greater drug loading as compared to other techniques. In addition to these bio-properties, more usual chemical and mechanical properties should also be considered in relation with the electrospinning processes.

1.8 Applications and Perspectives Nanostructured macromolecules, synthetic and natural, functional or structural, have been prepared by using several chemical, physical, and even biomimetic approaches. New ways dedicated to investigate the growth of nanostructures

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continue to be developed in order to obtain the control over size, shape, and distribution [8]. The great challenge of organizing the nanostructures, assembling them into larger complexes or connecting them into devices, becomes every day more realistic. Nanostructured polymers currently represent one of the most active areas of research in the whole word of polymer science, represented not only by synthetic approaches, but also by theory, structural analysis and applications developments. At the interface of science and technology, electronic and optoelectronic devices impact many areas of business and society, i.e. communications, computing, and medical devices and the demand for ever more compact and powerful systems is strong, exciting a growing interest in the development of nanoscale devices that could combine new functions with greatly enhanced performances. The extraordinarily large surface area of the nanoparticles and the opportunity of placing different functional groups on the surface, offers new and unespected perspectives and applications. Nanostructured polymers can expand/contract with changes of pH, or interact with biomolecules, enzymes, anti-bodies, cells, virus and bacteria in peculiar ways to provide rapid ex-vivo medical diagnostic tests; an overview on the emerging biotechnological and medical applications is presented in Chapter 5. Synthetic macromolecules are inherently capable of organizing into a variety of periodic and nonperiodic morphologies, i.e spheres, wires, tubes, grains, sponges with a characteristic length scale typically of the order of 10–1,000 nm. Significant technological applications for these materials can be envisaged in nanofiltration membranes, in ionomer-membranes for fuel cells and electrodialysis, and in elastomeric multi-block-copolymer fibers. For example, the lotus effect in macromolecules and in general superhydrophobic surfaces can be achieved in the case of nanostructured polymers likewise superhydrophobic surfaces, that occur naturally in some plant leaves and insect wings, eyes, and legs, and are characterized by a high contact angle (usually >150◦ ) and low sliding angle less than 5◦ (low flow resistance). The water-repellening properties of superhydrophobic surfaces strongly depend on the micro- and nanoscaled approach and on the control of the wetting behavior of protecting layers and surfaces toward the detrimental effects of environmental water and moisture; it was recently demonstrated the formation of superhydrophobic poly(dimethylsiloxane) (PDMS), by exposing the surface to ultra fast laser pulses [336]. The development of nanostructured materials for ultra sensitive detection of organic, inorganic and biological species has also received great attention concerning conjugated macromolecules [337]. For example, porphyrin colorimetric indicators were prepared in molecular and nano-architectures for the development of well-integrated systems for the sensing of particular chemical species [338]. Nanoscale π-conjugated organic and organometallic polymers can be used as well for sensors [339, 340], biosensors [341], electrochemical devices, single electron transistors [342], nanotips of field emission displays [343] and a recent review reports the technological advances in these important topics [344].

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Biosensors based on nanomaterials exploit many new signal transduction technologies in their manufacture [345]. In molecular electronics and sensors, conducting polymers represent innovative systems for the immobilization of enzymes [346, 347]. The entrapment of enzymes in polymeric films provides a controlled method to fasten biologically active molecules in a defined area on the electrodes. These examples show that conducting polymers in the area of bioanalytical sciences are of great interest since their biocompatibility opens up the possibility of using them as in vivo biosensors. Conducting polymers particularly in the form of thin films or blends or composites have also been widely investigated as membranes in sensors for air-borne volatiles (alcohols, NH3 , NO2 , CO) detection. In particular, polyaniline and polypyrrole nanofibres have received a great deal of attention because of their vast surface area that allows fast diffusion of gas molecules into the structure [348] and polythiophene based sensors allowed the detection of ppb of hydrazine gases [349]. Nanostructured organometallic polymers containing Pd(II) or Pt(II) sites and palladium dispersed nanoparticles were studied for the development of devices, Surface Acoustic Wave sensors, which show response towards relative humidity and hydrogen [350]; the mode of interaction of these polymers with H2 S gas was also deeply investigated [351]. Among the variety of morphologies, nanowires are emerging as a powerful class of materials that, through controlled growth and organization, are opening up substantial opportunities for novel nanoscale photonic and electronic devices [352]. In this framework, nanostructured polyaniline is a most promising material for chemical sensors since significantly enhanced performance of nanofiber films over conventional materials can be observed in both sensitivity and response time [353]. Nanostructured polyaniline is highly sensitive to many chemical vapors at the parts per million levels or less through several different mechanisms including doping/dedoping, reduction/oxidation, swelling, and by the modulation of the polymer conformation. The modification of polyaniline nanofibers with selected additives is an effective strategy to improve the gas sensitivity; for example the mixing of CuCl2 with polyaniline nanofiber films leads to an improved response (about 4 orders of magnitude) towards hydrogen sulfide gas [354]. Another interesting property that foresees advances in technology is flash welding. When exposed to light, polyaniline converts most of the absorbed energy into heat and if the polymer is in the form of nanofibers, the generated heat is trapped inside the individual nanofibers and, since the surrounding air is a very poor heat conductor, the heat dissipates slowly. If a camera flash intense light is used, a rapid temperature rise occurs which can lead to instantaneous welding of the nanofibers, while under irradiation of moderate intensity, polyaniline nanofibers will rapidly “melt” to form a smooth and continuous film. This phenomenon, called flash welding will open new applications for monolithic actuators, i.e. devices that make use of different forms of energy to induce motion [355]. The potentiality of nanofibers is not limited to electronic devices; in fact nanoscale support materials for catalysis could be also developed. For example,

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palladium nanoparticles supported on polyaniline nanofibers behave as an active catalyst for Suzuki coupling at relatively low temperatures and as water-dispersible catalyst supports to produce and stabilize Pd nanoparticles [356]. Nanostructured semiconducting block copolymers containing triphenylamine as hole transport moiety and perylene bisimide as dye and electron transport, have been investigated in view of applications in photovoltaic devices. The polymers show nanowire like structure which formation is driven by the crystallization of perylene bisimides via π- π stacking and since this self-assembly gives rise to domains size comparable to the exciton diffusion length, these materials offer perspectives for the implementation of organic solar cells [357]. Another fascinating property of conducting polymers is electrochromism that envisages applications to smart windows, rear-view mirrors, electronic paper, displays, stealth technology, etc. A prerequisite, especially for display applications, is a fast color switching, property showed by the nanotubular structure of PEDOT with an extremely fast electrochromic response without sacrificing the color contrast [358]. Electrochromic devices based on PEDOT nanotubes can be switched from the oxidized to the reduced states by applying alternating square potentials between 1.0 and −1.0 V and show strong coloration and flexibility. The color is yellowish at the oxidized state due to the background of the sputtered gold electrode, while it is dark blue at the reduced state [359]. Conductive polymer nanotubes show further interesting applications for the constructions of high-power energy storage devices such as supercapacitors and batteries, by using their fast charge/discharge characteristic [360]. Devices based on nanostructured materials can provide high power density by enhancing the charge/discharge rates [2]. Redox-type supercapacitors provide high specific capacitances, as in the case of PEDOT-nanotube-based supercapacitor, which maintains at least 80% of its maximum energy density even when the power density is boosted to 25 kW/kg [361], getting high power density without significant loss of its energy density. Nanostructured macromolecules play also an important role in the development of materials for photonics [362]. In fact they can possess useful optical properties such as electroluminescence, photoluminescence, nonlinear optical properties [363] or can be used as matrices for optically active species [364]. Moreover, polymers that show compositional patterns can coherently scatter light [365] or be exploited for producing photonic materials [366]. Photonic crystals are materials where the coherent scattering of light or modification of the modes of light propagation occur [367, 368]. Photonic crystals obtained from polymers possess a relatively low refractive index (n) and the intrinsically low value of n gives rise to materials with full or incomplete bandgaps. On this basis, polymeric photonic crystals are able to suppress the propagation of light in specific directions and wavelength range. The self-assembly of polymeric nanospheres into colloid crystals with different packing, i.e. hexagonal close-packed (hcp) or face-centered cubic (fcc), is a typical approach for the achievement of materials with periodic structures. Changes in the spectral position of the stop band originate from a change in the average refractive index and/or the lattice constant. For example, these changes occur in response to a change

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in pH, temperature and in the presence of analytes [369, 370]. Polymeric photonic crystals can also show inverse opals structure and it is possible to obtain a two state color switch in the case of poly(methacrylic-acid-co-N-isopropylacrylamide) hydrogel or functionalized polyacrylamide [371]. One of the potential applications of colloidal crystals based on polymeric nanostructures is the production of recording media for optical data storage [372] which open new perspectives, for example, in the fabrication of protecting secure documents. The reported topics aim to give just several insights into the emerging fields of nanomaterials and related technology devoted to meet social needs such as communication and security, health and energy, here briefly outlined.

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Chapter 2

Macromolecular Systems with Second Order Nonlinear Optical Properties Roberto Centore and Antonio Roviello

Abstract Polymers for applications in second order nonlinear optics (NLO) have been an hot topic in applied macromolecular research since the middle of 1980s. These polymers contain organic or metallorganic NLO active fragments, typically π-conjugated molecules end capped with electron donor and acceptor groups, incorporated within the polymer. The NLO active fragments can be physically dispersed in the polymer matrix or they can be covalently bounded to the polymer chain as side pendants or along the main chain; finally, cross-linked NLO polymers can be considered too. In this Chapter, starting from the basic physical aspects of molecular NLO activity, the principal classes of NLO active molecules will be described and the different ways in which the NLO active fragments can be incorporated within a polymer will be examinated with emphasis on the chemical features allowing optimization of NLO performances of the final materials.

2.1 Introduction Our present way of life is based on fast exchange of information. A large number of operations that, some years ago, would have required that a person went out of home to an office are now easily performed by the person sitting in front of its computer connected to Internet. Buying travel tickets or books, doing bank transactions, reading newspapers or scientific journals, attending university lessons or conferences are only some examples of the change that has occurred. The change is far from being exhausted and a fundamental need of our society is the possibility of exchanging increasing amounts of information at continuously increasing rates. Up to now, the change has been made possible by the impressive development of information technology and telecommunication of the last years. The developments occurred R. Centore (B) Department of Chemistry “Paolo Corradini”, University of Naples “Federico II”, Via Cinthia, Naples 80126, Italy e-mail: [email protected]

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along different lines, i.e. development of faster computer processors, development of memory supports of high capacity and reduced dimensions and development of communication lines of increasing performances in terms of amount of information processed per unit of time and unit of length. In the latter field the breakthrough event was the introduction of fiber optic telecommunication. It can be traced back to 1988 the placement of the first submarine transatlantic fiber optic communications cables (TAT8) between Europe and USA, while the FLAG system (Fiber-Optic Link Around the Globe) working since 1997, extends without interruptions for 28,000 km from UK to Japan connecting 12 countries at 10 Gbit/s full-duplex. As compared with traditional copper wire connections, but also with satellitar connections, the optical fiber allows to reach rate of transmission higher by far, that can approach, in principle, the limit of Tbit/s. Of course, since the two points connected by the fiber optic transmission line are in general electronic devices, a relevant issue is the electric-to-optic conversion of the information and viceversa. The devices for the electroptic conversion presently in use (electroptic modulators) are based on acentric inorganic crystals, such as Lithium Niobate and can work properly up to rate of transmission not higher than 10 Gbit/s. The electroptic conversion is therefore the bottleneck of the optical communications at the present state-ofthe-art level and the need of developing new materials is urging in order to reach the limit of rate of transmission of Tbit/s allowed by the fiber. Polymeric materials having highly polarized conjugated organic moieties incorporated within have in principle the capability of satisfying the requirements for high rate electroptic conversion and from the middle of 1980s basic and applied research in this field started which is still active at present. The efforts of the research activity in the field were addressed toward two basic points: synthesis of new conjugated molecules with high electrooptic activity and development of macromolecular systems with these molecules incorporated within. Macromolecular chemistry has gained another field of application-oriented research that will be outlined in the present chapter.

2.2 Optical Nonlinearity in Materials and Molecules 2.2.1 Optical Nonlinearity in Materials When an external electric field is applied to a dielectric material, the material gets a dipole moment (polarization of the dielectric). The phenomenon can be characterized by the dipole moment per unit volume, also known as the polarization density, that the material gains. For weak applied fields the response of the material is linear in the applied field but if the field is very strong, nonlinear contributions can play a role. At microscopic level, nonlinear contributions can be thought of as due to the interaction of the strong external field with the strong electric field inside atoms. Since the material property (dipole moment per volume unit) is a vector, and the external perturbation (electric field) is a vector as well, the mathematical relation between them is in general expressed by a tensor.

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As a general approach we can express the electric dipole moment of the material in the external field as a Taylor series of the applied field, taking the unperturbed material as the zero point of the series [1, 2]: We get

P =P i

(0)i

+

∂P i ∂Ej

j

2 i 1 ∂ P E + Ej E k j ∂E k 2! ∂E 0 0 j k 1 ∂ 3 Pi + Ej Ek El + . . . 3! ∂Ej ∂E k ∂E l 0 j

j

k

(2.1)

l

In (2.1) apices i, j, k, l etc. indicate (contravariant) components of the three vectors under consideration, i.e. P, P(0) and E. They are, respectively, the dipole moment per volume unit of the perturbed material, the (permanent) dipole moment per volume unit of the unperturbed material, and the perturbing external electric field. Of course, apices run from 1 to 3 and we can assume that 1 stands for the x component, 2 for the y component and 3 for the z component of each vector with respect to a common reference system, R = {O, (x,y,z)}. Partial derivatives in (2.1) depends on two or more indices. These derivatives are the components, in R = {O, (x,y,z)}, of the various susceptibility tensors. In particular, first order derivatives, which depend on two indices, are 32 = 9 in total and are the components of the linear or 1st order susceptibility tensor, χ (1) ; second order derivatives depends on three indices, they are 33 = 27 in total and are the components of the 2nd order (or quadratic) susceptibility tensor, χ (2) ; third order derivatives depends on four indices, are 34 = 81 and identify the components of the 3rd order (or cubic) susceptibility tensor, χ (3) , and so on. With this notations, (2.1) becomes

Pi = P(0)i +

(1)i j

χj

E +

j

1 (2)i j k 1 (3)i j k l χjk E E + χjkl E E E + . . . 2! 3! j

k

j

k

l

(2.2) in which pedices j, k, l indicate co-variant components of the tensors. By introducing the compact Einstein’s convention for tensors (in the product of quantities bearing indices, summation is intended over each equal index put up and down, i.e. over equal covariant and contravariant indices), Equation (2.2) becomes (1)i

Pi = P(0)i + χj

Ej +

1 (2)i j k 1 (3)i χ E E + χjkl E j E k E l + . . . 2! j k 3!

(2.3)

For clarity and as example we give the expression of some components of χ (1) , χ (2) and χ (3) tensors.

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R. Centore and A. Roviello

(1)1

χ3

=

∂Px ∂Ez

(2)1

0

, χ23 =

∂ 2 Px ∂Ey ∂Ez

(3)1

0

, χ231 =

∂ 3 Px ∂Ey ∂Ez ∂Ex

(2.4) 0

We remember that if the reference system is orthonormal (as often it is) there is no difference between contravariant components (indicated by apices) and covariant components (pedices) so the components can be all indicated with pedices, as it is usually done. Also, it should remembered that some authors include the numeric coefficients of the Taylor series in the definition of tensor components [1]. With reference to the above expressions, it is important to specify that in the case of an oscillating electric field as that associated with an incident electromagnetic wave, which is a common experimental situation in nonlinear optics, the components of the tensors χ (1) , χ (2) and χ (3) also depends on the frequency of the oscillating electric field (dispersion). In this context, it is also important to note that the electric field in Equations (2.1), (2.2), (2.3) and (2.4) is the total applied field: this can be a superposition of several fields of different frequencies. It is important to acknowledge symmetry properties of the susceptibility tensors. Let us consider first non-dispersive media, in which the frequency dependence of susceptibility components can be omitted. Basic theorems of differential calculus tell us that for partial derivatives of order higher than one, the order of deriva(2)1 (2)1 tion doesn’t change the derivative, so, for instance, χ23 = χ32 . This property, which is called intrinsic permutation [3], has the effect, as an example, that of the 27 components of χ (2) , only 18 are really independent. There is another relevant symmetry property of the χ (2) tensor, known as Kleinman symmetry [3], that allows permutation of all three indices. In this case the independent components drop to 10. For dispersive media, the frequency dependence of the tensors must be included. In this case, the nonlinear coefficient of the third term of Equation (2.3) becomes, for a total field consisting of two waves oscillating at frequencies ω1 and ω2 , (2)i

χjk (−ω3 ,ω1 ,ω2 ) Ej (ω1 )Ek (ω2 )

(2.5)

in which polarization at the new frequency ω3 = ω1 + ω2 is produced. In this case we can exchange indices j and k (intrinsic permutation), but only if the frequencies are exchanged as well. In addition, when dispersion can be ignored, the frequencies can be freely permuted without permuting the corresponding Cartesian indices and viceversa, and the susceptibilities will remain unchanged (Kleinman symmetry). Notations introduced in Equation (2.5) are very important. The two incident fields of frequency ω1 and ω2 are called pump-waves, while the field of frequency ω3 is called signal-wave. The minus sign for the frequency of the signal wave accounts formally for the energy conservation in the second order nonlinear process, i.e. the three frequencies must satisfy the relation ω 1 + ω2 = ω3

(2.6)

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Macromolecular Systems with Second Order Nonlinear Optical Properties

83

i.e. ω 1 + ω2 − ω3 = 0

(2.7)

Depending on the features of the electric fields involved, several second-order NLO effects can be distinguished. For ω1 = ω2 = 0 and ω2 > ω1 , pump fields of frequency ω1 and ω2 can produce a new field of frequency ω3 = ω1 + ω2 (sum-frequency generation, SFG) or ω3 = ω2 − ω1 (difference-frequency generation, DFG). For ω1 = ω2 = ω = 0 the pump field of frequency ω can produce a new field of frequency ω + ω = 2ω (second-harmonic generation, SHG) or ω − ω = 0, i.e. a static electric field (optical rectification, OR). In the linear electrooptic effect (EO or Pockels effect), the two pump fields have frequency ω and 0 (i.e. the second is a static field), the signal wave has also frequency ω but a change n in the index of refraction of the material is produced. Let now Rˆ be a symmetry operation of the material. All χ (n) tensors, being intrinˆ With reference, in sic properties of the unperturbed material, are invariant under R. (2) particular, to χ we have ˆ (2)i = χ (2)i Rχ jk jk

(2.8)

So, from (2.3) it is ˆ i = Rˆ P(0)i + χj(1)i Ej + RP (1)i ˆ

= P(0)i + χj

REj +

+ ··· =

1 (2)i j k 2! χjk E E

+

1 (3)i j k l 3! χjkl E E E

1 (2)i ˆ j k 2! χjk RE E

+

1 (3)i ˆ j k l 3! χjkl RE E E

(2.9) + ···

Let us now suppose that Rˆ is the inversion center. In this case, taking the inversion center at the origin of the reference system R = {O, (x,y,z)}, and remembering that the application of the inversion center changes the sign to all components of vectors, it is ˆ j = −E j ; RE ˆ j E k = E j E k ; RE ˆ j E k E l = −E j E k E l ˆ i = −P i ; RE RP

(2.10)

so we have (1)i j

− Pi = P(0)i − χj

E +

1 (2)i j k 1 (3)i χjk E E − χjkl REj Ek El + . . . 2! 3!

(2.11)

On the other hand, changing sign in (2.3) it is (1)i j

− Pi = −P(0)i − χj

E −

1 (2)i j k 1 (3)i χ E E − χjkl REj Ek El + . . . 2! jk 3!

(2.12)

It is now clear that (2.11) and (2.12) can hold simultaneously if, and only if,

84

R. Centore and A. Roviello (2)i

P(0)i = 0, χjk = 0, . . . That is, in a centrosymmetric medium all even order susceptibilities are null; in particular, the tensor χ (2) is null in a centrosymmetric medium. Other symmetry rules for the components of χ (2) in the various crystal classes can be found in standard textbooks of nonlinear optics [3, 4]. It is interesting to remark that χ (2) has the same symmetry properties of the piezoelectric tensor [5]. Hystorically, many second order NLO data have been obtained by SHG measurements (see Chapter 3) and, even presently, SHG measurements give the easiest way to screen second order NLO active materials. SHG coefficients are indicated by the simbol dijk . The relation between dijk coefficients and the components of susceptibility tensor χ (2) is given by (2)i

i χjk = 2djk

(2.13)

In a SHG process the two pump waves have the same frequency (Fig. 2.1) and the intrinsic permutation rule holds rigorously. This allows to express dijk coefficients using a simpler two-index notation, dil : the first index is equal in the two notations, and its values 1, 2, 3 correspond, respectively, to x, y and z components, as already stated. The second index, in the two-index notation, corresponds to specific values j and k of the three-index notation, according to Table 2.1 [3]. So, d33 coefficient corresponds to dzzz , and d31 to dzxx . The linear electro-optic effect or Pockels effect (Fig. 2.1) can be defined as a rotation or deformation of

ω

χ(2) (−2ω, ω, ω)

ω

χ(2) (−ω, ω, 0)

2ω

Second harmonic generation (SHG)

ω

Linear electro-optic (EO) effect

Δn E (ω = 0)

Fig. 2.1 Schematic illustration of SHG and Pockels effects Table 2.1 Relation between two-index and three-index notation for SHG coefficients

i

jk = l

x→1 y→2 z→3

xx→1 yy→2 zz→3 yz = zy→4 xz = zx→5 xy = yx→6

2

Macromolecular Systems with Second Order Nonlinear Optical Properties

85

the optical indicatrix (i.e. of index of refraction) upon application of a static electric field to a noncentrosymmetric medium. This effect is used for fabrication of electrooptic modulators which are key devices of modern telecommunication nets. These systems exploit the phase change induced in an optical wave by the static electric field, that can also be converted in a change of intensity. In this way an electric modulated signal can be converted in an optical modulated signal, which is a frequent task in modern fiber-optic telecommunications systems. A more detailed description of electro-optic modulators will be given in Chapter 3. EO coefficients are indicated by the simbol rijk . The relation between rijk coefficients and the components of susceptibility tensor χ (2) is given by (2)

rijk = −

2χijk

(2.14)

n2i n2j

Also for electro-optic coefficients a simpler two-index notation can be used, in a similar way as for SHG coefficients. In the case of poled polymer films (vide ultra) r33 and r31 components are relevant. From (2.1) it is clear that, dimensionally, a component of χ (2) is given by the ratio of the polarizability density and squared electric field. SI units of P are C · m/m3 and of E are (V/m). Remembering that the electric potential can also be expressed (2)i as C/m, it follows that SI units for χjk are C·m V 2 C · m m2 C C m m / = · 2 = 2 = · = m V C V m3 m3 V V

(2.15)

In m/V units, typical susceptibilities are very small so, in practice, the submultiple pm/V is used.

2.2.2 Molecular Nonlinearities Molecular optical nonlinearities can be introduced in a similar way to bulk nonlinearities. The ground state electric dipole moment of a molecule upon application of an external electric field can be expanded as a Taylor series of the applied field, taking the unperturbed molecule as the zero point of the series [2]: μi = μig + + 3!1

∂μi ∂Ej

j

j

k

l

0

Ej +

1 2!

∂ 2 μi Ej Ek ∂Ej ∂Ek 0

j k ∂ 3 μi Ej Ek El ∂Ej ∂Ek ∂El 0

+ ...

(2.16)

In (2.16), which is the microscopic analogue of (2.1), apices i, j, k, l etc. indicate (contravariant) components of the three vectors μg , μ ed E, which are, respectively, the ground state electric dipole moment of the unperturbed molecule,

86

R. Centore and A. Roviello

the ground state electric dipole of the perturbed molecule and the perturbing external electric field. Partial derivatives in (2.16) define the components of the various polarizability and hyperpolarizability tensors. In particular, first order derivatives are the components of the linear polarizability tensor, α; second order derivatives are the components of the first order (or quadratic) hyperpolarizability tensor, β; third order derivatives identify the components of the second order (or cubic) hyperpolarizability tensor, γ , and so on. With this notations, (2.15) becomes (Einstein’s convention) μi = μig + αji Ej +

1 i j k 1 i j k l E E E + ... β E E + γjkl 2! jk 3!

(2.17)

Again, it has to be specified that in the case of an oscillating electric field, the components of the tensors α, β and γ also depends on the frequency of the oscillating electric field (dispersion). Intrinsic permutation and Kleinman symmetry rules hold for the components of the microscopic tensors as already seen for susceptibility tensors. As we have seen above, the β tensor has components with three indices, βjki . For such a tensor it is possible to perform a mathematical operation, called contraction, that gives a co-vector as the result. In other words, it can be shown that suitable linear combinations of components of the β tensor behave as the components of a vector. For the β tensor it is possible to contract over the contravariant index and over one of the covariant indices, and the result is a (co)-vector, indicated as β or h (Einstein convention). Therefore, the β vec , whose generic (i-nth) component is βhi explicit form of the three components of β in the reference system R are: h x y z = βxx + βyx + βzx x component of β also indicated as βx βhx h x y z βhy = βxy + βyy + βzy y component of β also indicated as βy h x y z = βxz + βyz + βzz z component of β also indicated as βz βhz

(2.18)

From (2.15) it follows that, dimensionally, βjki is the ratio between dipole moment and squared electric field. In SI system, dipole moment is measured in C · m and electric field in V/m. So, dimensionally, βjki is expressed in C · m3 C·m = 2 V2 V m

(2.19)

3

is Since in this units typical βjki values are too small, the submultiple 10−50 C·m V2 generally used. An other quantity which is relevant for the study of quadratic NLO active molecules, as we shall see below, is the scalar product between μ g and β,

2

Macromolecular Systems with Second Order Nonlinear Optical Properties

87

i.e. μg · β. Let R = {O, (x,y,z)} be a fixed reference system with z axis along the direction of the permanent dipole moment of the molecule, μg . In this system it is μg = μg k

(2.20)

having indicated with k the unit vector along z, as usual. If the components of β are also given in this reference system it is x y z + βyz + βzz μg · β = μg βz = μg βxz

(2.21)

otherwise, for any reference system but orthonormal, it is μg · β = μx β x + μy β y + μz β z

(2.22)

that, taking into account (2.18), becomes x y z x y z x y z μg · β = μx (βxx + βyx + βzx ) + μy (βxy + βyy + βzy ) + μz (βxz + βyz + βzz ) (2.23)

SI units for dipole moment are C · m, and taking into account (2.19), SI units of μg · β are C2 · m4 V2

(2.24)

Also in this case, an exponential factor is used (10−80 ). Gaussian units, also known as esu units, are frequently used for expressing molecular nonlinearities. The transformation from SI to Gaussian units is simple. Gaussian units for βjki are statC·cm3 . statV2

This combination of units is given the name of esu. The conversion factor from SI to Gaussian system is 10−50

3 C · m3 −30 statC · cm = 2.694 · 10 = 2.694 · 10−30 esu V2 statV2 2

(2.25)

4

cm Gaussian units for μg · β are statC . Again, this combination of units is given statV2 the name of esu. The conversion factor from SI to Gaussian is

10−80

2 4 C2 m4 −48 statC cm = 0.8076 · 10 = 0.8076 · 10−48 esu V2 statV2

(2.26)

Theoretical expressions of hyperpolarizability coefficients can be obtained by quantum mechanical methods (time dependent perturbation theory). In fact, the oscillating electric field of the incident electromagnetic wave, E0 cos ωt, (E0 is the amplitude and ω the frequency) can be considered as a time dependent perturbation acting on the molecule. Using second order perturbation theory, the following expression is obtained for β ijk (SHG effect is considered) [6]

88

R. Centore and A. Roviello

βijk (ω,ω, − 2ω) = 21 2

∞ ∞

ω ω +2ω2 j j μign μnl μklg + μknl μlg 2 ng 2lg 2 2 ωng −4ω ωlg −ω

n=0 l=0 ωng ωlg −ω2 j i k +μgn μnl μlg 2 2 2 2 ωng −ω ωlg −ω

(2.27) where

μlpq = ψp |μˆ l |ψq in which μˆ l is the component of the dipole moment operator along the axis l, ψ p and ψ q are stationary wavefunctions for the p and q states of the molecule (the subscript g refers to the ground state), and ωpq is the frequency of transition from state q to state p. Equation (2.27) contains complicated summations over all stationary states of the molecule. If one is interested in theoretical calculations, the equation is acceptable as it stands, since all frequencies and transition moments can be calculated (in principle) from the wavefunctions of the molecule which, in turn, can be computed by molecular orbital calculations. To obtain a more tractable equation, the two-level model can be applied [6, 7]. In this model, all terms in Equation (2.27) beyond the first excited state are ignored; so, the infinite sums (series) over all eigenstates are replaced by a sum over the ground and the first excited states only. For most organic molecules of interest in second order NLO, the two-level model is a good approximation because the energy difference between the ground and first excited state is by far less than that between the ground and any other excited state; i.e. there is a HOMO-LUMO transition low in energy, and in many cases falling in the visible or near-UV region. Furthermore, both the fundamental and doubled laser frequencies are usually well below the frequencies of transition to the higher excited states. As a consequence, all terms in Equation (2.27) involving these higher excited states are quite small and can be wiped out. Typical molecules of interest have an electron-donor and an electron-acceptor group linked through a π -electron system. As a result, the first excited state is often a low-lying charge transfer state. Also, most of these molecules have the donor and acceptor groups located such that the charge transfer takes place primarily along the axis of the permanent dipole moment of the molecule (i.e. the z axis). As a consequence, the dominant component of the β tensor is β zzz , often referred to in the literature as β CT (charge transfer). Applying the two-level model to Equation (2.24) and setting i, j and k equal to z, the following relatively simple expression for the second-order polarizability is got

βzzz (2ω) =

2 μ2eg · μ · ωeg 3 2 ω2 − ω2 ω2 − 4ω2 eg eg

(2.28)

where ωeg is the frequency of transition from the ground to the first excited state, μeg is the transition dipole moment between the ground and the first excited state and μ is the difference between the permanent dipole moment of the first excited state

2

Macromolecular Systems with Second Order Nonlinear Optical Properties

89

and of the ground state. Equation (2.28) clearly shows a dependence of β zzz from the frequency of the incident wave (dispersion). In particular, a strong enhancement of β zzz is observed when the frequency of the incident wave (fundamental wave) is close to the frequency of the HOMO-LUMO transition of the molecule (resonance enhancement). The dispersion free expression of β zzz , also indicated as β zzz (0), is the limit of the above expression for ω→0 and it is equal to βzzz (0) =

3 μ2eg · μ 2 2 Eeg

(2.29)

This quantity provides a very useful quantity for comparing the quadratic NLO activity of different molecules. Rearranging (2.28) taking into account (2.29), the following expression is obtained βzzz (2ω) = βzzz (0)

4 ωeg 2 − ω2 )(ω2 − 4ω2 ) (ωeg eg

(2.30)

which is useful to correct for dispersion β zzz values measured at frequency ω, within the two-level model. Equations (2.27), (2.28), (2.29) and (2.30) show that nonlinear optical effects in organic π-conjugated molecules (i.e. electric field dependent polarizablity) are basically due to electronic rearrangements; so it is expected that organic molecules exhibit extremely fast response times leading to the possibility of electro-optic modulation in frequency domains that are prohibited for inorganic crystals presently available in the market.

2.2.3 Experimental Techniques for the Measurement of Microscopic Nonlinearities Quadratic molecular nonlinear coefficients, i.e. β ijk or quantities related can be determined by different techniques. It is useful to distinguish between nonlinear optical methods and spectroscopic methods. EFISH and HRS techniques belong to the first class, EOAM to the second. In the EFISH technique (Electric Field Induced Second Harmonic) a solution of the (dipolar) chromophore under consideration is put into a uniform static electric field, obtained for example with parallel metal electrodes. In practice, instead of putting metal electrodes in the solution, the solution is put in a wedge shaped cell bounded by glass windows at which metal electrodes are applied [8]. Owing to the dipolar nature of the chromophores, they statistically orient under the electric field and the whole solution becomes a second order NLO-active polar medium. When a laser radiation crosses the solution, second harmonic radiation is generated and collected. By comparison with a standard nonlinear medium (usually a quartz slab) it is possible to extract the value of the dot product μg .β of the chromophore. As we have

90

R. Centore and A. Roviello

seen above, the full expression of the scalar product is given by (2.23). However, for most push-pull chromophores, only the β zzz component of the β tensor is relevant. By comparing with (2.23), this means that only the z component of the contracted β vector is relevant and it is equal to β zzz again, so the dot product μg · β, in this case, is simply equal to μg β zzz . Clearly, the independent measurement of the permanent dipole moment of the molecule is required in order to extract β CT from EFISH measurements. EFISH technique works only on dipolar molecules, moreover ionic chromopores cannot be handled with this technique. As we have seen above values of β CT vary with the wavelength of the incident radiation; for this reason good EFISH data are generally measured at 1.9 μm incident wavelength which, in most cases, ensures off-resonance data to be obtained. Hyper-Rayleigh scattering (HRS) is a technique allowing determination of different components of first molecular hyperpolarizability β without molecular orientation [8]. HRS is due to orientational fluctuations of acentric chromophore molecules in solution, which give rise to an average asymmetry, on a microscopic scale, in an isotropic liquid. So the light scattered by such system has a component at the second harmonic of the incident light. This component depends on the β coefficients of the solute compound and varies quadratically with the incident intensity. Advantages of the HRS technique are the possibility of handling ionic chromophores or octupolar chromophores and a relative simplicity of the experimental apparatus. It has also some drawbacks: the scattered second harmonic is in general weak and sensitive detection is required for significant measurements; high fundamental intensity is required; problems can be given by stimulated Raman or Brillouin scattering as well as from two-photon-absorption phenomena. Dipole moments of the ground state μg and the dipole moment differences μ = μe –μg (μe excited state dipole moment) of chromophores can be determined by means of the Electro-Optical Absorption (EOA) spectroscopy [9, 10]. In this technique, the difference of absorption of a solution with (ε E (ϕ,˜v)) and without (ε (˜v) ) an externally applied electric field E is measured with light parallelly (ϕ =0◦ ) and perpendicularly (ϕ = 90◦ ) polarized to the direction of E. For uniaxial phases, induced in a solution by both an alternating and a constant electric field (order of 106 Vm−1 ) the dichroism εE (ϕ,˜v) − ε (˜v) depends on the orientational order of the molecules due to their ground state dipole moment and on the electric field dependence of the electric transition dipole moment μeg (E). A multilinear fitting of the electro-optical dichroism

1 L (ϕ,˜v) = ε E (ϕ,˜v) − ε (˜v) 2 E for ϕ = 0◦ and ϕ = 90◦ , as a function of shape parameters of the absorption bands, leads to six coefficients from which the molecular parameters μg and μ can be derived. Standard UV/VIS spectra are required for the evaluation of the integral absorption. Within the assumption of one-dimensional chromophore, the static hyperpolarizability tensor coordinate along the direction of μg , can be calculated according to

2

Macromolecular Systems with Second Order Nonlinear Optical Properties

91

the two-level-model using Equation (2.29). Values of β 0 and μg β 0 evaluated with EOA spectroscopic measurements are generally in good agreement with EFISH values for unidimensional linear chromophores [11]. In the case of unidimensional bent chromophores or 2-D chromophores (vide ultra) the agreement is less satisfactory.

2.2.4 Optimizing Molecular Nonlinearity Equation (2.29) provides the basic theoretical tools for improving molecular quadratic hyperpolarizability within the framework of the two-level model. In fact β zzz (0) can be enhanced by reducing Eeg , i.e. the energy gap between HOMO and LUMO levels, and/or by increasing the difference of dipole moment between the ground and the first excited states, or by increasing the transition dipole moment. As we have already stated, good candidates are molecules containing a low energy excited state (i.e. small Eeg ) with charge transfer (i.e. high μ) and these features hold in molecules containing electron donor and acceptor groups at the ends of a π conjugated bridge (Fig. 2.2). Ideally, the best π conjugated bridge would be a polyene chain; actually, chromophores having donor-acceptor groups linked by extended polyene chains show high β zzz but suffer the drawback of poor chemical stability in the poling conditions (see below) [2]. Better thermal stability is obtained using phenylene bridges. In this case, however, the excited state is quinoid, so the resonance energy is lost in the HOMO-LUMO transition, and this corresponds to higher values of Eeg in Equation (2.29). An improvement, as compared with phenyl bridges, is got using low aromaticity heterocycles which give a better compromise between chemical stability and reduced resonance energy. A large number of heterocycles have been considered for NLO chromophores [12]. They include six membered heterocycles with one or more heteroatoms (e.g. pyridine, triazine), five membered heterocycles with one or more heteroatoms (thiophene, imidazole, oxadiazole, thiadiazole), 10-electron fused heterocycles (benzimidazole, benzoxazole, benzothiazole, triazolo-thiadiazole).

+

D

D

–

A

Fig. 2.2 The two limiting formulae of molecules containing an electron donor (D) and an electron acceptor (A) group separated by a π conjugated bridge

D

A

A

+

D

–

A

92

R. Centore and A. Roviello

The choice of the donor and acceptor groups is also critical in view of their incidence on the parameters contained in (2.29). While the donor used is, almost invariably, the dialkylamino group directly attached to a phenyl ring, which gives the best compromise between donor power and chemical stability, different choices are available for the electron withdrawing group. Nitro and cyano groups are typical examples of strong acceptor groups; however, more powerful groups have been developed in recent times based on five-membered heterocycles substituted with groups capable of withdrawing electron density both by resonance and inductive effects. Some examples of acceptor groups are shown below (Fig. 2.3). A list of representative chromophores is shown in Fig. 2.4, while their molecular nonlinearity, in the form of the μg · β product, is given in Table 2.2 [2, 13, 14]. From the examples discussed above it can be argued that the choice of chromophores with aromatic ground state is a factor that can limit the magnitude of β for a given couple of donor-acceptor groups and a given bridge length. The strategy to overcome this is the development of chromophores that do not lose aromaticity upon

NC NC NC NO2

CN

Nitro

Cyano

CN

NC CN

NC

CN

CN

NC

O CN

DCV

TCV

H3C

O

CH3

F3C

CN

NC

O

R

S O SDS

CF3–TCF

TCF

Fig. 2.3 Electron withdrawing groups used in NLO chromophores HO HOEt N

N

NO2

N N

N

N

N

NO2

N

HOEt

O

1 (DR-1)

5

HOEt N

N

N NO2

N

HOEt

2 (DANS)

HOEt

NO2

S

6

nBu N

N

N

nBu

N

HOEt

N NC

N NO2

CN

S

3

7

CN

tBuMe2SiO(CH2)2

HOEt N

N

N

HO

N

CN

tBuMe2SiO(CH2)2

N

CN N 4

NO2

O 8

Fig. 2.4 Chemical diagrams of representative dipolar NLO chromophores

CN

2

Macromolecular Systems with Second Order Nonlinear Optical Properties

Table 2.2 Molecular nonlinearity of chromophores listed in Fig. 2.4

Chromophore

μg · β (10−48 esu at 1,907 nm)

1 2 3 4 5 6 7 8

580 662a 950 1,400 900 1,350 6,200 35,000

a

(CH3)2N

93

Measured at 1,300 nm.

N

(CH3)2N

+

O

N

O

–

DIA

Fig. 2.5 Limiting resonance formulae for the chromophore DIA

charge transfer [15]. One possibility is given by chromophore molecules that have equal aromatic and quinoidal character both in the ground and in the charge transfer state; an example is the chromophore DIA (dimethylaminoindoaniline) shown in Fig. 2.5. For this relatively simple chromophore which, as compared with DANS, is two atoms shorter and is bent, a β0 three times higher was measured [15]. The other possibility is represented by compounds in which the structure of HOMO and LUMO states is reversed as compared with conventional chromophores, i.e. the ground state is quinoid neutral while the first excited state is charge-separated and aromatic (zwitterionic chromophores): in this case the resonance energy is gained upon excitation; a low Eeg and a high μ are expected for these molecules and this should correspond to high quadratic nonlinearity (Fig. 2.6). Actually, for these chromophores, the structure of the ground state (i.e. quinoidal or aromatic) depends on the twist angle around the central C – C bond. It has been shown by the group of Professor Marks that twist angles around 80◦ , that can be stabilized by proper substitution with bulky groups in ortho position, result in exceptionally high molecular hyperpolarizability and in a substantially zwitterionic ground state [16]. The approach of zwitterionic chromophores has been widely pursued and in some cases chromophores with very high nonlinearity have been prepared, but the translation to polymeric materials has not been successful to date. Infact, zwitterionic chromophores owing to their very high ground state dipole moment suffer of low solubility in polymeric matrices and have strong tendency to associate via dipole-dipole interactions [17].

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H3C

N

O

H3 C

+

N

O

–

–

O N

O

CH3

+

N

CH3

Fig. 2.6 Limiting resonance forms for two examples of zwitterionic chromophores

The chromophores we have discussed up to this point can be defined as one dimensional (1-D), in the sense that they have only one conjugated path joining the donor and acceptor groups and allowing the charge transfer. Chromophores of higher dimensionality possess different geometric paths joining the donor and acceptor groups. A relevant example is given by C2ν symmetric chromophores, Fig. 2.7. It is evident that in these molecules there are two different geometric paths for the charge transfer (2-D chromophores). Owing to the symmetry of the molecule, the two (hypothetical) excited states corresponding to the two charge transfer paths should possess the same energy, but electronic degeneration is not allowed in C2ν symmetric molecules. Actually, these chromophores have two low energy excited states both involving electronic rearrangements from the two different conjugation paths [18, 19]. The two low-lying transitions contribute to β CT of the molecule in additive way and this accounts for the higher observed nonlinearities as compared with 1-D chromophores containing the same donor-acceptor groups and the same CH3

CH3

N

N

AcO

OAc N

N

N

N

O

NC

CN

μβ= 2500 • 10– 48 esu at 1907 nm. Fig. 2.7 Chemical diagram of a 2-D chromophore, after [20]

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Macromolecular Systems with Second Order Nonlinear Optical Properties

95

conjugation path. 2-D chromophores are a typical example in which the two level model cannot be applied.

2.3 Second Order Nonlinear Optically Active Materials In a preceding paragraph we have shown that a necessary condition for second order NLO activity in a material is the absence of the symmetry center. This a stringent requirement and it is perhaps the most difficult feature the chemistry of these materials must face with. To get a non-centrosymmetric arrangement of molecules on a macroscopic scale is a difficult task, and even more difficult is to get a non-centrosymmetric polar structure. A polar structure is that in which there is at least one macroscopic direction (polar axis) which is not changed in the opposite direction by symmetry transformations allowed for the material. With reference to crystals, out of the 32 crystal classes only the following 20 correspond to noncentrosymmetric crystals 1, 2, m, 222, mm2, 4, −4, 422, 4mm, −42m, 3, 32, 3m, 6, −6, 622, 6mm, −62m, 23, −43m Among non-centrosymmetric crystal classes only the following 11 correspond to polar crystals, which are of particular relevance for second order NLO 1, 2, m, mm2, 3, 3m, 4, 4mm, 6, 6mm Actually, it has been shown that the highest second order susceptibility coefficients (macroscopic nonlinearity), for a given chromophore (microscopic nonlinearity), can be reached in the (polar) crystal classes 1, 2, m and mm2, while other polar crystal classes are less favourable [21]. As a matter of fact, among achiral organic compounds only 10–15% crystallize in non-centrosymmetric space groups. The reason for the high frequency of centrosymmetric space groups in organic crystals can be partly explained on the basis of the fact that, for non-ionic compounds, the most relevant intermolecular forces are the dipole-dipole. For these forces, the minimum energy configuration of adjacent dipoles is antiparallel and this can be simply obtained in a crystal by the inversion center. This feature is even more cogent in the case of NLO push-pull chromophores which have, in general, a high permanent dipole moment. Clearly, an obvious way to get a non-centrosymmetric crystal would be recoursing to chiral compounds. However, also in this case the probability of getting a polar crystal is low, as witnessed by the fact that the most frequent acentric space group for chiral compounds is P21 21 21 (class 222) which is non-polar; and even if the 1-D NLO chromophores does crystallize in a polar space group potentially highly NLO active (e.g. P21 , Pca21 , Pna21 ), in many cases the chromophore is actually placed with its long axis (charge transfer axis) perpendicular to the polar binary screw axis, giving rise to a pseudo-centrosymmetric antiparallel orientation of the dipoles [22–24] and vanishing bulk nonlinearities [21]. Actually, this introduces an additional and probably far more difficult task in the design of noncentrosymmetric crystals for quadratic NLO applications. In fact, not only the whole crystal must be acentric and possibly polar, but also, within the unit cell, molecules must be properly oriented with respect to relevant symmetry

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elements, in particular for what concerns the orientation of molecular directions corresponding to optimized β molecular coefficients [21]. Chemical strategies used for increasing the probability of getting an acentric polar crystal are all based on overriding dipole-dipole interactions. Ionic chromophores in which the packing is mainly driven by the stronger Coulomb interactions among ions are an approach [25–28]. Relevant results have been obtained with N-methyl-stilbazolium salts or N-phenyl-piridinium salts. In particular, DAST (4-dimethylamino-N-methylstilbazolium 4-toluene-sulphonate) is presently known as one of the best organic nonlinear optical crystal (SHG efficiency 1,000 times urea). An other possibility is the lowering of the rod-like symmetric shape of the 1-D chromophore by introducing lateral substituents [29–31]; this has also the effect of increasing the distance between molecules so reducing the driving force of dipole-dipole interactions towards centrosymmetric crystals. Examples of this approach include early observations of the relatively high frequency of acentric space groups in meta-substituted anilines which have led, more recently, to the discovery of 4 -nitrobenzylidene-3-acetamido-4-methoxyaniline (MNBA) and 3-methyl-4-methoxy-4 -nitrostilbene (MMONS) showing SHG efficiency respectively 230 and 1,250 times the urea standard. Acentric structures observed in chromophores bearing bulky lateral ferrocenyl substituents also are based on this approach (Fig. 2.8) [32]. Other approaches to get acentric crystals are based on crystal engineering strategies. These include exploiting directional intermolecular interactions (e.g. H-bonding [33], halogen–halogen [34]), or supramolecular approaches as inclusion compounds and co-crystals. It has been shown that polarizable materials whose crystal structure is centrosymmetric can give strong SHG signal by

+

H3C

N CH3

N

H3C

N N

+

N

H3C

H3C

–

PF6 H3C

SO3

–

Cc (m)

Cc (m)

Cl H3CO O2N

NH2

Pna2 1 (mm2)

N NO2

H3CCONH Cc (m)

Fig. 2.8 Some examples of chromophores forming acentric crystals. The space group and the crystal class are reported for each compound

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Macromolecular Systems with Second Order Nonlinear Optical Properties

97

inclusion into a host lattice structure which imparts a polar alignment of the molecular dipoles. Inclusion hosts as thiourea, tris(o-thymotide), deoxycholic, as well as clathrate hosts as cyclodextrins have been used, with a high frequency of noncentrosymmetric structures [35, 36]. Co-crystals based on merocyanine dyes and phenol derivatives are also potentially interesting having shown high frequency of non centrosymmetric space group occurrence [37]. Autoassembling systems are another example of quadratic NLO active materials. The principle of this approach is the anchoring of a NLO active chromophore on a properly functionalized surface exploiting complementary chemical functions on the surface and on the chromophore. In this way a mono-layer is obtained. By proper chemical functionalization of the cap of chromophore molecules of the monolayer, a second layer of chromophore molecules can be added with the same orientation, and so on, thus obtaining a multi-layer assembly, intrinsically polar. The propensity of halosilanes to react with hydroxy-rich surfaces, including silanol terminated glass surfaces, enables layers to be deposited. Terminal functional groups on the absorbed silanes can react with complementary functions on the chromophore allowing the formation of a mono-layer. The procedure can be iterated (Fig. 2.9) [38–40]. An other approach to self-assembled multilayers is based on the zirconium phosphate/phosphonate system [41, 42]. Examples of chromophores incorporated in these superlattices include stilbazolium, azo-dyes and heterocycle based systems [38–42]. NLO polymers are the other relevant class of quadratic NLO materials [2, 8, 43]. In general, they are organic or metallorganic polymers containing high β chromophores physically dispersed or covalently bounded to the polymer chain (Fig. 2.10).

CH2Cl2

O

SiCl3 +

OH OH

N

N

OH

N N

i

HO

Si

Si

O O

O O

OH

N

N

N N

N N

OH

O

O

Si

O

HO

HO

OH

N N

OH N

CH2Cl2

CH2

CH2Cl2

CH2

CH2

CH2

N Cl – +

N+ Cl –

N N N O

Si O

Si O

O

O

O N

ii

+

Cl

–

Si O

O

N

+

Cl

Si O O

–

O Si3O2Cl8

Si O

Si O

O

O Repeat

iii

Fig. 2.9 Example of construction of a self-assembled multilayers. Adapted from [39]

Multilayers

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R. Centore and A. Roviello guest - host

side - chain

main - chain

cross - linked

Fig. 2.10 Some examples of NLO polymers

The various classes of NLO polymers will be described in detail in the next paragraph, here we discuss some general features. Polymers must satisfy several requirements in order to be suitable for quadratic NLO applications. First of all, they must be amorphous, so as to allow waveguiding without light scattering: this, automatically, rules out the whole class of semicrystalline polymers. They must be chemically, thermally and photochemically stable in the working conditions and for very long times. The latter requirement can be a problem, given that push-pull chromophores generally bear chemical groups potentially sensible to the oxygen of air. Polymers must be as much transparent as possible in the windows generally used for fiber optical transmission (i.e. 1.3 and 1.5 μm). Given that all preceding requirements be fulfilled there remains the general problem of getting an acentric polar arrangement of active chromophores. In fact, in an amorphous polymer containing highly dipolar chromophores, both physically dispersed and covalently bonded to the chain, the minimum free energy structure corresponds almost invariably to a statistically centrosymmetric arrangement of the chromophores that makes the material NLO inactive [44]. The typical procedure for inducing acentric order in NLO polymers is the electric poling procedure, Fig. 2.11. In practice, a thin film (thickness of the order of microns) of the polymer, spin coated onto a substrate (glass or ITO) is heated up to the Tg and put into an intense electric field, for instance that generated by a gold needle or wire electrode charged at some kV potential and set close (e.g. 1 cm) to the film surface; this leads the dipolar chromophores to orient, on average, along the direction of the field which, in general, is normal to the film surface, so inducing a statistically C∞ν polar symmetry. On cooling down the film, below the Tg , the acentric order is frozen and a NLO active material is obtained.

2

Macromolecular Systems with Second Order Nonlinear Optical Properties

99

mA

NEEDLE ELECTRODE HV METALLIC GRID

mA HV

+++++++++++++++++++++++ THIN FILM

ELECTRODE

SUBSTRATE

HEATING BLOCK

Fig. 2.11 Schematic diagram of a poling apparatus (From Rau et al. [45]. Reprinted with permission of Taylor & Francis Ltd)

Clearly, the poling procedure, which is a fundamental step for the preparation of a NLO active polymer, raises other issues from the chemical side. Actually, the poling procedure is a very severe one. The intense electric poling field is actually generated by the ions produced by atmosphere ionization and deposed on the film surface. So, high chemical stability is required to the polymers to resist ion shelling. In some cases the combination of high Tg of the polymer with the strong electric poling field leads to the chemical decomposition of the material during poling. The chemical decomposition of the film is in many cases quite evident (whitening of the initially coloured film, or dielectric breakdown) but in other cases the partial chemical decomposition is more subtle to see. So, a fundamental issue is the careful cheking the stability of the material in the actual poling conditions. The statistically non-centrosymmetric polar order of the chromophore units reached during the poling procedure is thermodynamically stable only with the electric field on. Once cooled down at room temperature and switched off the field, that polar order becomes metastable and it will relax to the more stable centrosymmetric arrangement, which is not NLO active. So, an other fundamental issue is to reach an high time stability of the polar order. Industrial standards for device testing require stability of NLO performances for 10 years at 85◦ C, that are quite severe. The various techniques of electric poling for NLO polymers will be described in detail in the next chapter, as well as the experimental details of the NLO measurements and the theory of poling relaxation. Here we anticipate some golden rules for quality SHG NLO data: the laser fundamental wavelength should be far from any absorbing wavelength of the chromophore (1,550 nm is in general a good choice, 1,064 nm is not); the technique used for measuring the index of refraction should be clearly stated: possibly, the index of refraction at both wavelengths (fundamental and second harmonic) should be measured by the ellipsometric technique; thermal and chemical stability of the

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R. Centore and A. Roviello

polymer in the poling conditions should be carefully checked; UV-VIS spectrum of the polymer before and after the poling should be recorded; time stability of the NLO signal should be studied by normal or accelerated ageing tests.

2.4 Second Order NLO Active Polymers 2.4.1 Guest-Host Systems Historically, guest-host systems have been the first polymeric NLO active materials to be developed. In these systems a high β CT chromophore is physically dispersed in a suitable, amorphous polymeric matrix. The non centrosymmetry is induced by the usual poling procedure. Polymer matrices most frequently used include poly(methylmethacrylate), PMMA, because of its excellent optical properties, as well as other amorphous polymers with high Tg as amorphous polycarbonate (APC), polyquinoline, polyimides. Many reports have dealt initially with guest-host systems containing DR1 or DANS as the dispersed chromophore. The nonlinear response of these materials has been the subject of theoretical investigations, based on the oriented gas model. In this model it is assumed that (i) chromophores don’t interact with the matrix and with each other; (ii) they have cylindrical shape, are free to rotate under the influence of the poling field and have only one significant beta component, i.e. that along the molecular axis β zzz ; (iii) the permanent dipole moment of the molecule is oriented along the z axis. On this basis, a relation between the macroscopic nonlinearity of the material (i.e. r33 or d33 coefficient) and the molecular properties of the chromophore can be deduced [2]

r33 = N cos3 ϑ 2βf (ω)/n4 = N(μE/5kT)2βf (ω)/n4 in which N is the chromophore number density, cos3 ϑ is the polar order parameter, n is the refractive index, f(ω) is the local field factor arising from the host dielectric permittivity, μ is the ground state permanent dipole moment of the chromophore and E is the external poling field. From the above relation a linear dependence of the nonlinear coefficient upon the chromophore number density is expected. However, experimental studies have shown that the behavior of d33 or r33 versus N is linear only for small N; by increasing N a maximum of r33 is reached for a value of N around 10 · 1020 molecules/cm3 depending on electronic and geometric features of the chromophore [43]; by further increasing N, a reduction of the nonlinear coefficient is observed. This behaviour has been attributed to the dipolar interactions between chromophores which become significant by increasing N and favour the antiparallel arrangement so reducing the acentric order parameter; actually the behaviour has been reproduced by more complete theoretical analyses which include dipole–dipole interactions between chromophore units in the model [46] and by Monte Carlo simulations [43].

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Macromolecular Systems with Second Order Nonlinear Optical Properties

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Table 2.3 Electro-optic activity of selected guest-host polymers Guest-host polymer

r33 (pm/V) at 1,330 nm

30 wt% 1 in PMMA 25 wt% 7 in Polyimide 30 wt% 8 in PMMA

13 30 60

This behaviour has been found experimentally also for systems in which the chromophore is covalently bounded to the polymer chain (vide ultra). In particular, the theoretical analyses have predicted higher NLO responses for chromophores having more oblate ellipsoid shape than prolate. Some representative examples of performances for guest-host systems are reported in Table 2.3, with reference to chromophores of Fig. 2.4 and Table 2.2. Values of r33 up to 73 pm/V have been reported. Although easy to assemble in principle, guest-host systems generally suffer two main drawbacks. First, the amount of NLO active chromophore that can be dispersed in the matrix is generally small (of the order of 10–30 wt %); higher amounts induce phase segregations and optical anisotropy and light scattering. The second is the generally poor time stability of the NLO performances. The second drawback, however, can be efficiently managed using polymer matrices with high Tg . Guest-host systems showing NLO performances stable, at 80◦ C, over a period of more than 2,000 h have been developed. However, the use of polymer matrices with very high glass transition temperatures is limited by the thermal stability of the chromophores at the severe poling conditions. Exploiting secondary intermolecular interactions (H bonding) seems a more promising strategy.

2.4.2 Side Chain NLO Polymers The use of chromophores covalently bonded to the polymer chain (NLO polymer) is a natural evolution of guest-host systems. In this case the polymer is obtained from a monomer (or a co-monomer) which is a chromophore too. There are clearly different choices for the way the chromophore is oriented with respect to the polymer chain. One possibility is to put the chromophore unit as a side pendant of the polymer chain (side-chain NLO polymer). This, in turn, can be accomplished by connecting the chromophore to the chain through suitable spacer groups, for instance conformationally flexible polymethylenic units (polymer type I); in these polymers, no atoms of the chromophore unit, between the donor and acceptor groups, formally belong to the polymer chain. The prototype of this class of polymers is given by amorphous polyacrylates or polymethacrylates obtained by radical polymerization of chromophore acrylic monomers (Fig. 2.12) [47–56]. Homopolymers acrylate or methacrylate are hardly considered, because of very low solubility and low molecular weight. Copolymers are normally studied. In particular many researches have studied the NLO activity of the copolymers as a

102

R. Centore and A. Roviello CH3

CH2

C

CH3 COOCH3

+

CH2

C

AIBN, DMF COO

80˚C

CH3

(

CH2 O

CH3

)

C

1–x

(

CH2 O

O

)

C

x

O

CH3

N CH3

N co-polymethacrylate

CH3

N N x = 0.10 PMA10 d33 = 7.4 pm / V at 1368 nm

N N

x = 0.20 PMA20 d33 = 11.5 pm / V

N

S

N N

N

S N

N N

N

N N

Fig. 2.12 Synthetic procedure for a side-chain NLO polymer (type I), after [56]

function of the molar amount of the chromophore comonomer. These studies indicated that the NLO performances (i.e. d33 ) increase with the chromophore amount up to a maximum corresponding to a content of NLO active monomer around 20–30% by mol, depending on the chromophore dimensions and polarity. For higher concentrations, the NLO activity decreases (Fig. 2.13). This behaviour, which is closely related to that already discussed for guesthost systems, has been theoretically investigated and is related to several factors.

70 60

d33 (pm/V)

50 40 30 20 10 0 0

5

10 15 20 25 30 35 Chromophore molar content (%)

40

Fig. 2.13 Typical behaviour of d33 as a function of the chromophore molar content for side-chain NLO polymers (type I) (From Persico et al. [53]. Reprinted with permission of John Wiley & Sons, Inc)

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Macromolecular Systems with Second Order Nonlinear Optical Properties

103

However, the overcrowding of the chromophore units which favours locally antiparallel coupling between highly polar chromophores and that hampers the motion of the chromophore units during the poling are the most relevant effects [43, 46]. An other feature of type I NLO polymers is related to the time stability. Polyacrylate NLO polymers generally show poor time stability of NLO performances [57]; this can be increased switching to methacrylate polymers, in view of the higher Tg . Further improvement of time stability can be achieved through secondary interactions (H bonding), in particular when H bonding donor or acceptor groups are placed directly on the chromophore unit [58], or with bent polar chromophores inducing high Tg [56]. An other relevant class of type I polymers is given by polyimides [59– 65]. As compared with polymethacrilates they show higher Tg and, in general, better time stability of NLO performances. The synthetic route to polyimides is versatile. They can be obtained by polymerization between bis-anhydrides and amino-chromophores. High performances are obtained using bis-phthalic anhydrides such as 3,3 ,4,4 -oxydiphthalic anhydride (ODPA) or 4,4 -(hexafluoroisopropylidene)diphthalic anhydride (6FDA). The polyimide of Fig. 2.15, having Tg = 238◦ C showed no decay of d33 at room temperature; after 600 h at 100◦ C it retained 90% of the initial d33 and after 300 h at 150◦ C it retained more than 82% of the initial d33 [60]. An other approach to the synthesis of type I side-chain NLO polymers is the post-functionalization of a preformed polymer chain with a NLO chromophore (Fig. 2.16). Examples include post-functionalization by azo coupling, Knoevenagel condensation, Mitsunobu condensation between imide or carboxy functions of the polymer chain and alcohol functions of the chromophore [66–68]. This approach to NLO polymers has several advantages as the possibility of easy modulation of the chromophore content in the functionalized polymer and, in some cases, the possibility of using commercially available polymers as substrates for the post-functionalization.

1,2

normalized d33

1,0

Fig. 2.14 Time stability of the d33 coefficient for the two copolymers of Fig. 2.12 (Tg = 136◦ C, triangles, Tg = 150◦ C, squares). Baking temperature 80◦ C. Data for a polyurethane based on the same chromophore (Tg = 170◦ C, circles) are also shown (Reprinted from Fusco et al. [56], with permission from Elsevier)

0,8 0,6 0,4

PMA10 PMA20 PUSF4

0,2 0,0

0

5

10

15 time (days)

20

25

30

104

R. Centore and A. Roviello O

H2N

NH2

O N

N

O

O O

CF3

F3C

O

n

O

O

N

N O

O

+

O N

CF3

F3C

Tg = 238˚C

O N

N

NO2

r33= 27 pm/V at 780 nm

N

NO2

Fig. 2.15 One step synthesis of a NLO polyimide

CH3 CH2

Ph3/DEAD

CH CH

C

C O

DR1/THF, 25˚C

C N

O

n

CH3 CH2

C

CH CH C O

C O

N

n

H N

N r33 = 16 pm/Vat 632.8 nm

N

NO2

Fig. 2.16 Synthetic approach for type I side-chain NLO polymers (polyimide) by postfunctionalization, Mitsunobu reaction, from [67]

Recently, Diels-Alder reaction between maleimide-containing NLO chromophores and pendant anthracenyl moieties of the polymer chain has been used in the context of post-functionalization approach [69]. This is particularly interesting in view of the mild reaction conditions, quantitative yield and absence of ionic species and catalyst. Metallorganic, type I side-chain polymers have also been reported. In a relevant class of these polymers, shown in Fig. 2.17, the NLO active chromophore unit is a metallorganic fragment which is grafted to a pre-formed polymer chain through coordinative bond between the metal and donor sites (typically N atoms) present in the side groups of the polymer. Easily available, low cost commercial polymers can be used in this approach (poly(4-vinylpyridine), poly

2

Macromolecular Systems with Second Order Nonlinear Optical Properties

CH Et

CH CH2 N

N

Ni

O

O

Ni

Et

R

100 - n

N N

Et O

C N

CH CH2

n

N N

CH CH2

n

R

C

N O

105

N Et

Et

Et

CH

O

O

Ni

N N CH

R=

C

R

N

n = 20

NO2

d33 = 20 pm/V at 1368 nm

Fig. 2.17 An example of side-chain type I metallorganic NLO polymer (A. Roviello, unpublished results)

(1-vinylimidazole) etc.) and the amount of grafted chromophore can be easily varied so as to tune relevant properties of the final polymer such as solubility and glass transition temperature [24, 70–72]. Also the metal can be varied: polymer containing Ni(II), Cu(II) an Pd(II) have been reported up to now. In another relevant class of side-chain NLO polymer, the chromophore unit is attached to the chain as a side pendant without spacers (type II), in such a way that one atom of the chromophore does belong to the polymer chain; generally that atom is the N donor of the chromophore (donor embedded polymers). The most common way to get this is to attach two hydroxyalkyl groups to the donor N atom of the chromophore. An example is given by chromophores based on bis(2-hydroxyethyl)aniline [73–82]. Chromophores of this type can react with acid dichlorides to give polyesters, or with diisocyanate to give polyurethanes. Polyurethanes based on 2,4-tolylendiisocianate are very frequently considered (Fig. 2.18).

HO

OH NH

+

O2N

O

O OCN

O

O

NH

N

N

N

NCO

O

Py

n

O

N

O 2N Polyurethane xx, r33 = 6.5 pm/V at 1552 nm.

Fig. 2.18 Synthesis of a side chain NLO polyurethane (type II) from diol chromophore and 2,4tolylendiisocianate. Data are taken from [79]

106

R. Centore and A. Roviello

As compared with polyesters, polyurethanes generally show higher Tg , because of H bonding formation, and good solubility in common solvents such as py, DMF or NMP; moreover, the pattern of substitution in the diisocyanate induce constitutional disorder in the chain which further reduce the tendency to crystallize. A large number of polyurethanes have been prepared starting from diolchromophores and 2,4-tolylendiisocyanate. The time stability of NLO performances of polyurethanes is in general good because of fairly high glass transition and H-bonding formation, see Fig. 2.14; it is even better in the case of chromophores having H bonding donor and acceptor sites [81].

2.4.3 Main-Chain NLO Polymers In main-chain NLO polymers, chromophore units are embedded in the polymer chain. This means that two atoms of the chromophore, at least, must belong to the polymer chain. Also in this case, several different architectures can be considered in principle and many of them have been actually studied. Given that the chromophore is fully fitted in the chain, one fundamental difference is if the orientation of the chromophore, namely of the chromophore’s dipole, with respect to the chain, is (locally) transverse (T-type) or parallel (P-type). Within T-type polymers [83, 84] a relevant example was reported by the group of Suter [83] (Fig. 2.19). These polyamides have relevant properties: they are well soluble in common solvents (DMSO, NMP), moderate to high Tg (from 125 to 206◦ C depending on the spacer) and exhibit a relevant NLO activity (d33 = 40 pm/V at 1,542 nm and r33 = 16 pm/V at 1,300 nm for x = 12). Poling of these polymers should imply mainly the rotation of the chromophore around the local chain axis. The transverse orientation of the chromophore, which implies shorter lateral arms of the chain, and the low torsional potential around the bond N-Phenyl, should make easier the

N

CONH

NHCO NO2

Fig. 2.19 A class of main-chain NLO polymers with transverse orientation of chromophores

x = 4, 6, 8, 10, 12

(CH2)

x n

2

Macromolecular Systems with Second Order Nonlinear Optical Properties

107

orientation process. Also the reported time-stability of NLO performances is good for these polymers (no relaxation at ambient temperature after 120 days). Folded accordion polymers are another example of main-chain polymers with transverse orientation of the chromophore [85, 86]. An other class of polymers that can be considered to pertain to T-type is based on Y-shaped chromophores. Some examples have been recently reported [20]. The chemical diagram of a representative polyurethane is shown in Fig. 2.20. The mainchain nature of the polyurethane is evident, given that most of the atoms of the chromophore are actually embedded in the chain. Moreover, owing to the Y shape, the resultant dipole moment of the chromophore is locally perpendicular to the chain. The NLO performances of these polymer are interesting. The off-resonance SHG activity of polyurethane PUY3 is quite fair, while the time stability can be considered very good, as shown in Fig. 2.21. P-type main-chain polymers, in which the direction of the dipole moments of the chromophores is locally parallel to the polymer chain, can be further divided in two classes: polymers in which the sequence of dipoles is random and those in which the sequence is regular. Polymers of the first type are potentially less interesting, basically because the poling procedure is expected to be difficult for them. In fact, chromophores are picked at both ends to the chain and, moreover, their disposition along the chain is statistically centrosymmetric. Yet, for a polymer of this type, a fair SHG activity has been reported upon poling [87]. Regioregular P-type polymers are more appealing by far (Fig. 2.22) [88–94]. These polymers, in fact, have the advantage that, within each chain, the polar orientation of chromophores, given by the synthesis, cannot be altered because it is due to covalent bonds; moreover, the poling procedure should be efficient in this case because of the increased dipolar moment of chain segments.

CH3 O NH

CH3

CH3

N

N

O

O O

N

N

N

N

NH

O

NC

CN

PUY3

d33 = 15 pm/ V at 1368 nm

Fig. 2.20 A main-chain T-type polyurethane based on a Y-shaped chromophore (from [20])

n

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Fig. 2.21 Normalized d33 coefficient of the polyurethane PUY3 at three different baking temperatures, as a function of the time (From Carella et al. [20]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission)

Fig. 2.22 Idealized scheme of a regioregular P-type main-chain NLO polymer

Interesting results have been reported recently for a regioregular polyester obtained by self-polymerization of an hydroxy-acid chromophore in mild conditions (Fig. 2.23) [94]. Since the ester bonds are formed by reaction between the carboxy and the alcoholic groups, the sequence of the monomeric units along the chain is strictly regioregular (head-to-tail). Within regioregular polymers reported to date, REG3 shows the best performances. Regioregular polymers like REG3 have another potentially interesting feature. In fact, each polar chain has two chemically different end-groups: hydroxy and carboxy. Moreover, the end-groups are placed in fixed orientation with respect to the average polar vector of the chain. So, by selective reaction of the end-groups on suitably activated surfaces, a polar arrangement of polymer chains, without electric poling, could be obtained in principle. HOCH2CH2 N H3C

N

N

OCH2CH2

N N CH2CH2COOH

N

NO2 H3C

N

N N N

NO2

CH2CH2C O REG3 d33= 21 pm/V at 1368 nm.

Fig. 2.23 Synthesis of a regioregular main-chain P-type polymer

n

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2.4.4 Cross-Linked Systems and Organic Molecular Glasses One of the main requirements of a NLO polymer, for device-fabrication, is a high time stability of the acentric order, which means high time stability of the performances given by the device in working conditions. In each of the classes of NLO polymers described above we have seen that this requirement can be substantially fulfilled by suitable chemical choices that affect properly Tg , shape of the chromophore, secondary interactions etc. It is obvious, however, that from the side of the time stability, the best performances are expected for cross-linked systems, in which the allowed mobility of molecular segments is minimum. This is the reason why cross-linked systems have been widely studied in NLO field. Different chemical strategies can be devised in order to get an organic crosslinked NLO active system. One possibility is the synthesis of NLO polymers or co-polymers (side-chain or main-chain) in which the monomeric units also bear chemical functionalities allowing thermal or photo-crosslinking [95, 96]. In the co-polymer system shown below (Fig. 2.24), cross-linking is got by reaction between epoxy and carboxy groups. The best procedure developed by the

CH3 CH2

CH3 CH2

C

C

x C

O

O CH2 CH O CH2

x = 0.7

1–x C

O

O CH2 CH2 N

CH2CH3

N N

d33 = 29 pm / V at 1320 nm. Fig. 2.24 A side-chain NLO type I co-polymer containing epoxy cross-linkable moiety in the co-monomeric unit [96]

COOH

r33 = 12.6 pm / V at 1320 nm.

NO2

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authors is a two-step one. The polymer is first poled at a relatively low temperature, 70◦ C, for a relatively low time (1 h), without cross-linking. Then, with the electric field still on, the temperature is raised up to 130◦ C for 3 h, so inducing cross-linking of the system. So, poling and cross-linking are performed as separate steps in this approach. As compared with similar copolymers without cross-linking (i.e. only methyl-methacrylate as the co-monomer) the nonlinear r33 coefficients are similar, while the time stability of the cross-linked system is better by far. Another approach is based on multi-component systems [97–101]. It has been pursued, in particular, by the group of Professor Dalton. In many cases they have used a two components system formed by a polyurethane NLO oligomer with isocyanate end groups that is reacted with a cross-linking agent, triethanolamine, as example [102, 103]. In this approach, the oligomer (or prepolymer) is prepared first; then a solution of the oligomer and of the cross-linker is spin-coated on the substrate (Fig. 2.25). The film obtained undergoes a simultaneous process of poling/cross-linking. Since poling and cross-linking take place simultaneously, temperature ramp and poling conditions are critical for the performances of the final material. In fact, as the cross-linking reaction goes on, the Tg of the material increases and the mobility of the chromophore units goes down; so, as higher is the degree of cross-linking, as harder is to orient the chromophores. Generally an initial step of slow cross-linking at low temperature is performed, in order to reach a high degree of orientation, followed by a hardening process at increasingly higher temperatures for long times. The results are in many cases very good. A step forward is represented by one-component cross-linkable systems [104–107]. Ideally, they are formed by multi-functional chromophores (i.e. capable of cross-linking by self-reaction) that, even if chemically pure, do not crystallize and, therefore, can be reduced in amorphous transparent thin films of optical quality by spin-coating. The molecular amorphous film of the chromophore (organic molecular glass) can be cross-linked and poled simultaneously. H

H OCN

N

O

O

N

NCO n

O

N

H3C

O

3D Thermoset OH

CH3

r33 = 27 pm/V at 1064 nm. N HO

N O

OH

O

Fig. 2.25 A cross-linked NLO system obtained by reaction of an isocyanate end-capped prepolimer and triethanolamine, from [103]

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The advantages of the approach are evident. The monomeric nature of the compound allows, initially, maximum freedom for the chromophore units that can orient, under electric poling, in the easiest way, i.e. by rotation around the baricentre; the number density of the chromophore is maximum since there is no need of inactive binders in the chemical formulation; the one component nature of the system should reduce phase separation processes and inhomogeneities. After cross-linking, the time stability of acentric order is expected to be high as in the other cross-linking approaches. Few examples of cross-linkable organic glasses have been reported to date. A system with interesting performances has been recently described (Fig. 2.26) [106]. This chromophore has a very low tendency to crystallize. After melting (102◦ C) on cooling the liquid phase, no crystallization occurs, and the amorphous liquid phase is stable at room temperature over days in a glassy state (Tg = 37◦ C). Glassy amorphous films can be easily obtained by spin coating from DMF solution, and

O O

N

N

N N N NO2

O O

DIMET

1,2

1,2

1,0

1,0

0,8

0,8

ISHG

ISHG

Fig. 2.26 A chromophore forming quadratic NLO cross-linkable glasses

0,6

0,6

0,4

0,4

0,2

0,2 0,0

0,0 20

40

60

80

100

Temperature (°C)

120

140

20 40 60 80 100 120 140 160 180 200

Temperature (°C)

Fig. 2.27 Depoling behaviour of the cross-linked chromophore of Fig. 2.26, at 1 K/min heating rate (Reprinted from Carella et al. [106], with permission from Elsevier)

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by addition of few weight percent of radical initiator (2% benzoyl peroxide or t-butylperoxide) the amorphous film can be cross-linked and poled. Of course, radical photoinitiators can be used as well in this approach; thermal initiators, however, are preferable because photoinitiation often induces photobleaching in the case of azo-chromophores. After poling/cross-linking the films are no more soluble in DMF and they show a fair NLO activity (d33 = 14 pm/V at 1.9 μm) and a very good time stability (Fig. 2.27).

2.4.5 NLO Dendrimers Dendrimers are a class of macromolecules whose application in NLO field has been considered recently [108]. Ideally, dendrimers are perfect monodisperse macromolecules with a regular and highly branched three-dimensional architecture. Their monodisperse, tree-like nature determines interesting properties such as the formation of voids and nanostructures [109]. A NLO active dendrimer is sketched in the figure below, in which the arrows stand for the NLO active fragments, typically push-pull moieties. Dendrimers are prepared in an iterative sequence of reaction steps, in which each additional iteration leads to a higher generation material, the number of branches increasing from a generation to another (divergent approach) [109]. The dendrimer sketched in Fig. 2.28, for instance, is a fourth generation dendrimer. NLO dendrimers corresponding to the diagram of Fig. 2.28, in which the 15 push-pull moieties are typical amino-nitroazobenzene chromophores have been successfully prepared [108]. Owing to the topology of the dendrimeric structure, each chromophore unit contributes coherently to the macroscopic NLO activity, and the measurements have shown that the NLO activity of the dendrimer is

Fig. 2.28 Schematic diagram of a NLO dendrimer

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more than 20 times greater than the individual azobenzene monomer [108]; this is due to the fact that in NLO dendrimers the chromophore units are oriented noncentrosymmetrically along the molecular axis according to a cone-shape, instead of being spreaded according to a spherical shape. One possibility with NLO dendrimers is to use them as stand-alone materials; in fact, because of the high molecular weight, their solutions have viscosity high enough to produce optical quality films by spin coating. Eventually, peripheral alkyl tails of the dendrimer can be functionalized with cross-linkable groups allowing, through a post poling hardening process, a higher poling stability and better NLO performances. Another possibility is to covalently incorporate a NLO dendrimer within a polymer. Interesting results have been recently reported for block copolymers dendrimer/linear polymer [110]. It has been shown that the block copolymer is nanostructured with the dendrimeric blocks forming domains of average diameter 17–24 nm dispersed in the linear polymer matrix. The dimensions of the chromophore domains are not very high, so allowing easier orientation in the electric field and a reduced tendency to the disorientation with the electric field off, because the domains are well separated with respect to each other; on the other hand, in typical guest-host systems, chromophore aggregates are larger, because of the low compatibility chromophore-polymer matrix, and their orientation is reduced.

2.5 Perspectives We have stated in the Introduction of this Chapter that the research on organic and polymeric compounds for second order NLO applications started in the middle of 1980s, on the basis of theoretical issues suggesting that polymeric materials, having highly polarized conjugated organic moieties incorporated within, could have in principle the capability of satisfying the requirements of high rate electroptic conversion needed in fiber-optic telecommunications. Actually, electroptic modulation in the THz regime has been demonstrated in prototype devices containing organic polymers as the active components [111], nevertheless the industrial development of these devices has been hindered up to now by several factors. Some are of physico-chemical nature and are related with the difficulties in tailoring materials having all at once a lot of properties in some cases hardly compatible with each other: large electroptic coefficients and high thermal and photochemical stability, easy orientation under electric field and good time stability of the orientation in the absence of the electric field, high optical transparency, low absorption at telecommunication wavelenghts. However, as now it is clear for the reader of the present Chapter, these difficulties can be considered substantially overcome, thanks to the work and ideas of several chemists that have developed molecular and macromolecular architectures capable to circumvent the gross and subtle difficulties behind the production of high performance NLO active materials.

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Other hindering factors are more technological (see the next chapter for a more detailed exposition) and are related with the need of processing poled NLO polymers into buried channel waveguides surrounded by materials with lower refractive index. Multicolor photolithography (MCP), laser ablation or reactive ion etching (RIE) have been successfully used to this purpose and prototype polymeric waveguides have been prepared keeping optical losses at a very low level (e.g. 0.01 dB/cm) [112]; development of suitable cladding materials compatible with the active NLO polymers, as well as the coupling of polymer waveguides to silica fibers are other relevant technological topics that have to, and actually can be, successfully addressed [112]. The other hindering factors, perhaps the most relevant ones, are mainly economic in their nature and are related with the difficulty of implementing a new technology in the face of specifications written for an older technology. In fact, the production techniques of NLO active devices are now optimized for inorganic materials (e. g. Lithium Niobate) and are not suitable for organic-polymeric active components.

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Chapter 3

Macromolecular Systems with Nonlinear Optical Properties: Optical Characterization and Devices Paolo Prosposito and Fabio De Matteis

Abstract In the last years, the request of optoelectronic devices with wide bandwidth and fast response has strongly promoted the research on innovative second order nonlinear optical materials. A suitable alternative to traditional crystalline inorganic materials is the exploitation of polymeric and hybrid organic-inorganic materials doped or functionalized with nonlinear optical chromophores. The basic requirement is the noncentrosimmetric alignment of the active molecules. In this chapter we will review the main techniques for such purpose. Moreover some of the main optoelectronic devices important for high bit rate optical communication and optical computing systems will be illustrated. In particular Mach-Zehnder modulators, microring resonators, switches and filters based on innovative materials will be reviewed.

3.1 Introduction During the last 2 decades, applied physics has developed an increasing interest for the nonlinear optical (NLO) properties of organic materials due to the possible use of such materials for active waveguide systems and telecommunication devices. Electro-optic modulators (EOM) which encode signals on the light beams to carry data along optical fibers, are essentially based on the second-order nonlinear materials [1]. A class of materials intrinsically lacking of a center of symmetry, basic requirement for second-order nonlinearity, is represented by non centrosymmetric crystals like Lithium Niobate (LiNbO3 ). The most firmly established technologies for interferometric modulators on EOM market are based on such crystals. An optical pattern, comprising a Mach-Zehnder interferometer, is realized on an P. Prosposito (B) Micro- and Nano-Structured Systems Laboratory – MINASlab, Dipartimento di Fisica, Consorzio Interuniversitario per la Scienza e Tecnologia dei Materiali – INSTM, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, Roma, 00133, Italy e-mail: [email protected]

M.V. Russo (ed.), Advances in Macromolecules, DOI 10.1007/978-90-481-3192-1_3, C Springer Science+Business Media B.V. 2010

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Fig. 3.1 Sketch of an integrated Mach-Zehnder

Input Light

RF electrode

Modulated Light Output

Substrate

electro-optic substrate (LiNbO3 ), so that application of an electric signal to one arm of the interferometer can induce a phase shift. Recombination of optical radiation from the two arms enables all results from constructive to total destructive interference, thus achieving light intensity modulation. A schematic picture of an integrated Mach-Zehnder modulator is shown in Fig. 3.1. A lot of efforts have been done to improve overall performance, namely the transmission speed and bandwidth. Unfortunately the large dispersion of the dielectric constant from microwave to optical frequency produces a delay between optical and electrical RF signals that travel in the material, putting intrinsic limitations on the possibility of improvements. In front of the need for new and faster devices able to send large amount of information at high bit rates, the well established technology of Lithium Niobate modulators seems to have reached its performance plateau. High production costs due to vacuum based deposition and crystal growth techniques such as molecular beam epitaxy (MBE) or metal organic chemical vapour deposition (MOCVD) constitutes another severe limitation. Growth and patterning of LiNbO3 devices involve expensive and time consuming methods. Moreover this “traditional” material has further drawbacks consisting in its low integrability with substrates and external components like fibers and integrated optical components [2]. Polymers can be envisaged as suitable alternatives to crystalline inorganic materials for EOM [3]. The speed difference between carrier wave and modulation signal in polymeric materials is only of 10–15%. New concept holds for EO modulator based on organic material where single molecules, endowed with very high secondorder nonlinearity [4, 5], are embedded in a inert polymeric matrix [6–8]. The need of macroscopic effect is reached with a post synthesis alignment procedure [9]. This strategy allows to decouple two well distinct problems: on one side the necessity of high optical quality matrix suitable for low cost processing and waveguide fabrication; on the other side, the synthesis of high hyperpolarizability active molecules [10, 11]. Moreover the costs (including processing) of such materials are very competitive. In principle, organic optoelectronics can supersede the traditional inorganic optoelectronics in communication technology, with low cost and improved performances [12, 13]. Potential advantages for electro-optic (EO) applications, like optical modulators and switching devices, become possible [14]. The low values of the dielectric

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constant of organic NLO materials, for instance, yield large bandwidth in data transmission. However such benefits are obtained at expenses of some additional fabrication procedures. After deposition, organic materials are centrosymmetric on a macroscopic scale and they can not be endowed of second order nonlinear properties. Poling, i.e. the orientation of the microscopic molecular dipoles, is required in order to break this symmetry. One of the major challenges concerns the effective translation of high molecular nonlinearities (μβ), where μ is the chromophore dipole moment and β is the first molecular hyperpolarizability, into large macroscopic EO activities (r33 ) in poled polymers with high alignment temporal stability [15, 16]. The device design, the fabrication processes and the operating environments impose severe requirements which are to be met in order to allow a commercial exploitation of organic materials. A high thermal stability of the second-order nonlinear optical susceptibility χ(2) is one of the main demand: devices must be able to operate continuously around RT for sufficiently long time (years) and to sustain short periods of excursion up to higher temperatures during the various fabrication and packaging steps and occasionally in their lifetime period. These practical considerations have led investigators away from simple guesthost polymer systems, perceived to have less intrinsic orientation stability, toward systems in which the nonlinear chromophore mobility is hindered. This can be achieved by chemically bounding the active molecules, in side- or in main-chain, or alternatively promoting cross-linking after orientation [17]. In this chapter we will shortly summarize the nonlinear optical properties of macromolecular systems and some of the main experimental techniques for their optical characterization. Some basic optoelectronic patterns will be reported in order to give a brief account of the advances in the realization of active waveguide systems and telecommunication devices based on organic materials. The main optoelectronic devices based on nonlinear optical properties of chromophores in polymeric and hybrid matrices will be illustrated. In particular Mach-Zehnder modulators, microring resonators, switches and wavelength filters will be reviewed.

3.2 Optical Characterization The basic physical aspects of molecular NLO activity have been presented in Chapter 2 of this book. A comprehensive theoretical summary of the optical nonlinearity on a molecular level can be found therein. Here, we will focus on the optical response of systems containing a nonlinear active moiety on macroscopic level. Partial overlaps between the two chapter have been maintained for ease of comprehension. The nonlinear properties of such a macroscopic system depend on the orientational average of the microscopic contribution of the single nonlinear molecule. If the spatial orientation of the nonlinear molecules is isotropically distributed, the overall NLO response will be null no matter how strong the single molecule’s nonlinear response would be. Therefore the study of the macroscopic distribution of

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Fig. 3.2 Molecular axes (x,y,z) and laboratory axes (1,2,3) modulator

z θ μind

Ep

2 1

φ

the molecular orientation is essential to understand the NLO behavior of a system. The poling procedure induces changes in the linear and nonlinear optical properties of the systems. In order to study such changes, let us refer to a system containing N (chromophore number density) quasi-one-dimensional molecules, i.e. the chromophore is a polar molecule of axial symmetry along z molecular axis (see Fig. 3.2). When a molecule is subjected to an electromagnetic-wave, an electric dipole μ

ind is induced which can be expressed as a power series of the inducing local electric

(f is the so called local field factor [7]) field f E

+ βf 2 E

∗E

+ γ f 3E

∗E

∗E

+ ... μ

ind = αf E

(3.1)

The first term is the linear contribution and α is called (linear) polarizability tensor while the following terms account for the nonlinear contribution and the corresponding constants are said higher order hyperpolarizability tensors. On a macroscopic point of view one can write:

+ χ (2) E

∗E

+ χ 3E

∗E

∗E

+ ...

= χ (1) E P

(3.2)

Again, χ (n) is an n+1-rank tensor describing the electrical susceptibility of the

. material when subjected to an external electric field E A simple relationship between microscopic hyperpolarizability (β, γ , etc.) of the active molecules and NLO macroscopic properties of the materials (χ (n) ) can be established under the assumption of weak intermolecular interaction. This approximation is known as the oriented gas model [18]. For instance, the second order bulk susceptibility can be regarded as the statistical average of individual molecular hyperpolarizability β (2)

χijk = Nfi fj fk βIJK ijk

(3.3)

Here subscripts IJK refer to the molecule-based coordinate system (x,y,z), ijk refer to the coordinate system of the bulk material (1,2,3), N is the number density of NLO molecules, fi , fj , fk represent local field factors and the brackets denote the statistical average over all orientations of NLO molecules.

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Macromolecular Systems with Nonlinear Optical Properties

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Let us start examining the first contribution to the induced dipole μ

ind in Eq. (3.1)

. The linear molecular polarizability which is linear in the inducing electric field E tensor in the molecular coordinate system can be written, within the oriented gas model, as: α⊥ ≡ αxx = αyy α// ≡ αzz

(3.4)

The macroscopic linear polarization P3 (1) along the poling axis 3 which is the direction of the poling field Ep , is: ε−1 (1) (1) P3 = [μz cos ϑ + (μy sin ϕ + μx cos ϕ) sin ϑ]·G()d = χ33 E3 = E3 4π 33 (3.5) Here G() is a molecular normalized distribution with respect to the solid angle which defines the average over all the possible orientations. Substituting the expression for the induced electric dipole μ

ind , one gets: ε−1 (1) (1) E3 P3 = N (α// − α⊥ ) cos2 ϑ + α⊥ E3 = χ33 E3 = 4π 33

(3.6)

Since the G() value equals to 1/8π 2 in the √ case√of isotropic distribution of molecular orientation; the refractive index n = ε = 1 + 4π χ is given by: nIsotr =

1 + 4πN[(α// − α⊥ )1/3 + α⊥ ]

(3.7)

with N the chromophore density [19]. After the poling, the molecule orientation will be no more isotropic and the overall optical response has to be described within a tensorial formalism. For linear non-absorbing media, it is always possible to orthogonalize the dielectric tensor εij . The principal axis (optical axis) will coincide with the 3-axis and the corresponding refractive index is called extraor√ dinary index ne = n3 = ε33 [20]. In the plane orthogonal to the principal axis, the refractive index n0 is called ordinary. The refractive index can be written as ni = nIsotr. + ni , with i = e,o If the change Δni is small with respect to the value of the refractive index nIsotr. , one can write n2i = n2Isotr. + 2nIsotr. ni with: 1 2πN 2 ∼ ne = (α// − α⊥ ) · cos ϑ − n1 3 1 n0 = − ne 2

(3.8)

The latter represents the induced birefringence of the systems and shows the dependence of ni on cos2 ϑ . For an isotropic system cos2 ϑ = 1/3 and no birefringence is expected.

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3.2.1 Second Harmonic Generation Now it should be necessary to study some of the formal symmetry properties of the nonlinear susceptibility. At the first nonlinear term, one has to consider the mutual interaction of three waves of frequencies ω1 , ω2 and ω3 = ω1 +ω2 . Therefore one need to determine the six tensors (2)

(2)

χijk (ω1 , ω3 −ω2 ), (2)

χijk (ω2 , −ω1 , ω3 ),

(2)

χijk (ω1 , −ω2 , ω3 ),

χijk (ω2 , ω3 ,−ω1 )

(2)

χijk (ω3 , ω2 , ω1 )

χijk (ω3 , ω1 , ω2 ),

(2)

and six additional tensors in which each frequency is replaced by its negative. In these expressions, the indices i, j and k can independently take on the values x, y and z. Since each of these 12 tensors thus consists of 27 Cartesian components, as many as 324 different (complex) numbers need to be specified in order to describe the interaction. Fortunately, there are a number of restrictions resulting from symmetries that relate the various components of χ (2) and hence far fewer that 324 numbers are usually needed in order to describe the nonlinear coupling. In Chapter 2 of this book, such symmetry restrictions have been introduced in the nonlinear expansion of the polarization vector with greater details. Here, we will only recall few important simplifications in the formal representation of the second order nonlinear contribution for SHG. A full formal treatment would constitute a considerable diversion and can be found elsewhere [21, 22]. In literature, the nonlinear polarization term for SHG is commonly expressed by using the notation: PNL (2ω) = 2d(2ω) E(ω)E(ω)

(3.9)

The SHG coefficient d(2ω) is introduced which is related to the nonlinear susceptibility χ (2) by the relation: (2ω)

dijk =

1 (2) χ (2ω,ω,ω) 2 ijk

(3.10)

Commonly practiced assumption of symmetry in its last two indices (jk) is valid in lossless media whenever Kleinmann’s symmetry condition is fulfilled [21, 22]. However, the assumption is fully valid in the case of SHG, since in this case the frequency of the two electric fields are equals. Then we can use a contracted notation according to the prescription: jk:

11

22

33

23,32

31,13

12,21

l:

1

2

3

4

5

6

The relationship between microscopic and macroscopic SHG properties can be simply written for systems containing N quasi-one-dimensional molecules, i.e. for

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Macromolecular Systems with Nonlinear Optical Properties

125

molecules with a single non vanishing β component. In such a case, it is given by only two independent non-vanishing components of macroscopic nonlinear coefficient dij d33 = d31 =

1 μind cos θ 1 = Nf ω f ω f 2ω βzzz cos3 θ N 2 E3 E3 2

1 μind cos θ 1 = Nf ω f ω f 2ω βzzz [ cos θ − cos3 θ ] N 2 E1 E1 2

(3.11) (3.12)

In the above equations β zzz is the component of the molecular first-order hyperpolarizability tensor β along the principal molecular axis, and are angular averages which describe the degree of polar order, f ω f 2ω are the local field correction factors at ω and 2ω respectively. To model SHG in poled materials, the first and third moments, and , must be evaluated. The time evolution of the orientation distribution of dipoles G(θ , t), after the poling field has been switched off, will determine the decay of the d33 value and, as a consequence, the SHG signal. Understanding the mechanisms of polar order decay is crucial for the tailoring of new exploitable active materials. In contrast with crystals, polymers containing oriented molecules tend to evolve towards a randomization of dipole orientation when the field is removed. Relaxation in such poled systems, following the molecular statistical model, arises from thermal reorientation whose rate is governed by the mobility of the molecules in the matrix. The mobility is in turn determined by a number of parameters including, in particular, glass transition temperature (Tg ) and the amount of free volume in the polymer. SHG is certainly one of the most direct techniques able to characterize NLO properties of second order materials. By using polarized radiation incident on a nonlinear material, being the direction 3 the optical axis of the nonlinear material with reference to Fig. 3.3, it can be shown that SH intensity for the two polarizations (S and P) is given by [23]:

ω Fig. 3.3 SHG measurement for poled materials. The fundamental beam is incident at angle ϕ on a nonlinear material of thickness L. The angles of fundamental and SH beams are ϕ 2ω and ϕ ω respectively

ϕ

ϕ'2ω ϕ'ω 3

2ω

ω L

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P. Prosposito and F. De Matteis S I2ω

L ∝ sin π 2lC L P 2 2 I2ω ∝ deff Iω sin2 π 2lC 2 2 d31 sin2 ϕ2ω Iω

2

(3.13)

where the effective coefficient deff is + d cos2 ϕ sin ϕ + deff = d33 sin2 ϕω sin ϕ2ω 31 2ω 2ω +2d15 sin ϕω cos ϕω cos ϕ2ω

(3.14)

IS 2ω refers to the SH intensity measured with intensity Iω of the fundamental beam polarized perpendicular to the plane formed by the direction of the incident fundamental beam and the NLO film surface normal (S-polarized). IP 2ω is the SH intensity with the fundamental beam polarized in this plane (P-polarized). In Eq. (3.13) lC represents the so called coherence length: SH radiation generated at a point x gets 180◦ out of phase with SH radiation generated at x ± j 2lC , being j an integer. Coherence length is given in terms of refractive indices nω and n2ω respectively at ω and 2ω frequency and of the wavelength λ of the fundamental radiation lC =

λ 4 nω cos ϕω − n2ω cos ϕ2ω

(3.15)

3.2.2 Nonlinear Ellipsometry; Teng – Man Technique When the frequency () of one of the applied electric fields is much lower than the other (<<ω), one often speaks of linear electro-optic (EO) or Pockels effect. An EO coefficient can be introduced by means of the expression:

1 2 n

= i

3

rij Ej

(3.16)

j=1

where rij is connected to the real part of the second-order susceptibility via the (2) relation rij = −2χiij ( − ω,ω,0)/n4 [24]. Nonlinear ellipsometry is based on a single wavelength reflection configuration proposed, independently, by Teng and Man [25] and Schildkraut [26] in 1990. The Teng and Man Technique (TMT) is sensitive to a number of contributions to the measured signals, that can lead to the evaluation of several nonlinear parameters [27, 28]. It has been widely used for the determination of the electro-optic susceptibility of organic polymer films. The TMT permits the measurement of both the χ(2) and of χ(3) complex susceptibilities. Moreover, it is sensitive to space distributions of charges trapped in the polymer films [29].

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Macromolecular Systems with Nonlinear Optical Properties

127

The TMT operates on samples in which the polymer film is sandwiched between two electrodes. The two electrodes are used both to pole the film and to apply a modulating voltage on the polymer film itself in order to measure the χ(2) and χ(3) values. The TMT is based on the measurement of the electric field induced change of the optical phase difference experienced by the s- (perpendicular to the incidence plane) and p- (parallel to the incidence plane) polarization components of a laser which propagates through the polymer film. Therefore, at least one (in reflection configuration) of the electrodes must be transparent to the probe light. Usually, a semiconductive transparent oxide thin layer, deposited on a glass substrate, is used as transparent electrode on which a polymer film is deposited by spin coating. A metal electrode is finally deposited by thermal evaporation or sputtered. Alternatively the polymer film can be sandwiched between two transparent electrodes and the experiments performed in a transmittance configuration [30]; in this case the polymer films are spin coated onto pairs of oxide coated glass slides and then sandwiched face to face; elimination of the air gap is guaranteed by a baking process at glass transition temperature under mechanical pressure. The experimental set-up is sketched in Fig. 3.4 [31]. An elliptically polarized laser beam is incident at an angle α on the back surface of the sample. A beam stop is used to eliminate the reflections from the air-substrate interface. After reflection at the polymer-metal interface the beam is analyzed by a second polarizer, that is crossed with respect to the input one. First a dc voltage Vp is applied between the two electrodes to pole the sample; then a voltage V(t), modulated at a frequency << ω the optical frequency, is applied in order to measure the electro-optic coefficient: V(t) = Vs + Vm cos ( t)

(3.17)

that gives rise to an internal electric field: E(z,t) = Es (z) + Em (z) cos ( t)

(3.18)

where z is the direction orthogonal to the film plane. The Vs is a static offset voltage. The optical power transmitted by the whole system is modulated, due to the nonlinear response of the polymer film, both at and 2 through the linear χ(2) and quadratic χ(3) electro-optic response [32]. The power measured by the photodi ) and 2 (V 2 ) effective ode is sent to a lock-in amplifier, which extracts the (Vac ac components of the modulated signal at the two frequency; while the Vdc voltage, measured by means of a digital multimeter, gives a measurement of the average optical power Pdc . A measurements of the r33 component of the electro-optic coefficient is obtained in a simple and fast way. The phase difference between the s- and p- component of the beam is tuned operating on the phase compensator. In correspondence of one of the two values corresponding to the maximum power modulation, the Pockels electro-optic coefficient can be estimated by the following relation [32]:

128

P. Prosposito and F. De Matteis L

P1

P2

C

S

PD

2α

+

Substrate Transparent electrode Polymer Metal

–

Fig. 3.4 TMT experimental set-up. L cw-laser, P1-P2 linear polarizers (45◦ with respect to the incidence plane), C phase compensating device for s-p polarization, S beam stop, PD photodiode

r33

3 √ λ 1 Vac = 2 4 π Vm Vdc

n2 − sin2 α n2 sin2 α

(3.19)

3.3 Poling Techniques The search for materials having large macroscopic EO activities (r33 ) has produced, on one side, the synthesis of a large mass of different NLO chromophores with high molecular nonlinearities (μβ), where μ is the chromophore dipole moment and β is the first molecular hyperpolarizability. On the other side, one of the major challenges concerns the effective translation of such molecular activity into a macroscopic nonlinearity to be used in an optical device. The goal is to obtain large values of EO coefficient and long-term alignment stability. The first issue has been extensively described in Chapter 2 of this book. In the following sections we will summarize the main achievements in the techniques to obtain macroscopic systems and EO devices. In general, as-deposited organic materials are centrosymmetric on a macroscopic scale and they are not endowed of second order nonlinear properties. Poling, i.e. the orientation of the microscopic molecular dipoles, is required in order to break this symmetry. The polymer is heated close to its glass transition temperature (Tg ), so as to increase the molecular mobility, while application of a DC electric field results in statistical polar orientation of the molecular dipoles along the field direction. Then the polar orientation is frozen in by cooling down to room temperature, while keeping the orientating field on [33, 34]. The main problems are related to effective amount of molecules that are aligned and the orientational time stability. Thermal- or photo-cross-linking can increase temporal stability of achieved induced orientation. Photo-assisted poling (PAP) uses the mobility energy which is provided locally to the chromophores due to optical pumping, at room temperature, instead of thermal energy in the normal procedure [35, 36]. The model systems for PAP are the

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Macromolecular Systems with Nonlinear Optical Properties

129

azo-dyes in polymers where trans-cis photoisomerization of azobenzene groups is considered to play a key role. Upon irradiation within their absorption band, azobenzene chromophores can transform reversibly from the trans- to the cis-isomer form, which is more compact and therefore more mobile. This process is very fast in liquids and is significantly slower in solids (about 150 ns) [37]. Relaxation back into the trans-isomer goes through non-radiative channels and is very slow (seconds in solids). Molecular reorientation can lead to the previous configuration, but the system will be again subjected to excitation. The only stable configuration will be the one with the dipole moment perpendicular to the incident light polarization. Provided that the light polarization is perpendicular to the poling electric field, an increase of the electrooptic coefficient is observed [38].

3.3.1 Corona Poling Corona poling is surely the most used electric field poling technique. Intense electric fields, obtained by depositing on the film surface the charges created by ionization of the surrounding atmosphere can be achieved with such technique. A typical set-up for corona poling of polymer films is shown in Fig. 3.5. The gas ionization is caused by the strong electric field at a sharp needle, a wire or a grid placed at high potential (kilovolts) with respect to the ground. Due to the small dimension (curvature) of the electrode surface (be it a needle or a wire), the extremely high local field is able to ionize molecules of the gas which are then accelerated toward and deposited on the film. A constant voltage grid is often interposed between electrode and film to control more accurately the poling field [39]. The efficiency of corona poling depends upon several factors: voltage, distance of electrode from film surface (typically 1÷2 cm), temperature [33], type of atmosphere [40]. Depending on the polarity of electrode, so called negative or positive poling can be achieved [40, 41]. In positive corona poling, HV electrodes are NITROGEN ATMOSPHERE BOX

ω

ELECTRODE (Cu)

MATRIX

+ + + + + + + +++++++++++++++ +

FILM

SUBSTRATE HEATING STAGE

Fig. 3.5 Set-up for corona poling of polymer films and in-situ SHG measurements

PLANAR ELECTRODE

CHROMOPHORES

2 2ω

HIGH VOLTAGE (~ 10 kV)

130 1,0 SHG Intensity (A. U.)

Fig. 3.6 The effect of atmosphere on corona poling. Intensity of second harmonic signal is shown as a function of time for a guest-host material (DR1 in PMMA) poled in air and in dry nitrogen atmosphere

P. Prosposito and F. De Matteis

Dry Nitrogen

0,5 Air 0

20

40

60

Time (min)

directly responsible for the ionization of molecules or atoms of the gas. In negative corona poling, the HV electrode is involved in sustaining the discharge avalanche, providing a source of electrons by secondary emission, ion impact or photoelectric effect: the result is that negative corona poling is less stable and very dependent on the chemical composition of surrounding atmosphere, as it has been demonstrated experimentally [41]. Both growth and decay of polar order in corona poling depend on surrounding atmosphere. In Fig. 3.6 is shown the comparison of intensity of SHG for the same guest-host material, Disperse Red 1 (DR1) molecules in poly(methyl(methacrylate)) (PMMA) matrix, in air and in a nitrogen atmosphere [42]. The decay of the SHG intensity is due to two different, but related, mechanism: first, the decay of the residual electric field due to the neutralization of the charges deposited on the film surface, and second the polar order decay due to the rotational relaxation of molecules in the film. Corona discharge induced charge penetration, which may occur at high temperature, leads to a space charge layer at the film surface. As the poling field is removed, the surface charge progressively discharges with characteristic times which depends on the environment. Typical relaxation times are of the order of tens of minutes in air [40] and, as such, it is of no practical interest in view of possible exploitation of the macroscopic nonlinearity of the materials. Moreover, poling atmosphere can influence the orientation efficiency even for chemical reasons, e.g. O2 molecules and OH- ions in the atmosphere can cause a degradation of active molecules or film surfaces. The dc electric field (typically up to 100÷200 V/μm) is applied to the material at a temperature close to the polymer glass transition temperature (Tg ) during the poling procedure. At such temperatures the molecule dipoles acquire a sufficient mobility to be oriented under the electric field effect. The achieved orientation is then frozen in by cooling the polymer to room temperature. Thermal- or photo-cross-linking can be used to increase the temporal stability of achieved induced orientation. In this case, more elaborate poling schemes have to be considered in order to arrive at the final matrix in a series of steps which permits an initial good solubility and a low glass transition in order to start efficiently the poling procedure [43]. The hardening process in such a thermosetting scheme occurs during all processing stages (spin-casting, poling and lattice hardening). Indeed,

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Macromolecular Systems with Nonlinear Optical Properties

131

an initial hardening (pre-curing) step is typically used to increase viscosity to an appropriate level for spin-casting optical quality thin (1–2 μm) films. The films are then heated to a temperature near the Tg temperature and poled. Because the Tg is increasing as the lattice hardening proceeds, poling is typically carried out in a series of temperature steps for best results. Since hardened materials are able to withstand greater voltages without damage, applied voltage is also often increased in a stepwise fashion. Because the polar order and the thermal stability of the matrix are determined by the kinetics of the hardening reactions and chromophore molecular dynamics in various lattice structures, exact results will depend strongly upon the precise processing protocols used. Recently, a new three steps procedure (swelling–poling–deswelling) to produce a stable alignment of second order NLO-phores covalently attached to a cross-linked polymeric network has been reported [44]. The film has been cross-linked before poling and the mobility of the NLO-phores has simply been induced by swelling the polymer network with a suitable solvent, to allow their alignment under poling conditions even at room temperature. A gentle heating has been used to speed up poling and, afterwards, to dry the matrix, freeze the NLO-phores orientation and stabilize the SHG signal.

3.3.2 Contact Poling When the electric poling field is created between two conductive electrodes, one speaks of contact poling. The advantage of such poling configuration relies on the homogeneity and the direct control on the poling field. Moreover the same electrodes can be eventually used for electro-optic applications. Electrodes configuration allows for new poling procedures to be used for electrooptic waveguides in the context of polarization controlling devices. Coplanar electrodes are used when one wants to obtain orientation in the plane of the film [45], while a sandwich (electrode-film-electrode) configuration (parallel plate electrodes) can be used to achieve perpendicular poling (Fig. 3.7). A limitation to poling voltage used in electrode poling is cast by dielectric breakdown [46]. That can occur both across the film (mostly in sandwich configuration) and in surrounding atmosphere (coplanar configuration). In the latter configuration the process is sometimes performed in a high vacuum chamber or an insulating cap layer is deposited on top of the device in order to avoid it. Poling with coplanar electrodes can be limited also by charge injection processes near sharp electrode edges, so a careful design of electrodes shape is necessary. Charge injection can also occur Fig. 3.7 Sketch of two configuration for contact poling. Left coplanar waveguide geometry (CPW), right sandwich configuration (parallel plate electrodes)

Cladding Core Electrode Substrate

132

P. Prosposito and F. De Matteis

when parallel plate electrodes are used. For such reason also careful attention to material purity and dust-free film processing must be paid. In order to prevent breakdown of the electrodes to ground when they are exposed to air and voltages are applied, an insulating polymer cap layer is often employed on top of the electro-optic polymer/electrode layer. Notwithstanding, during poling, bleaching of the NLO functional group near the electrode-polymer interface can occur mostly due to strong electric field inhomogeneousity (tips or sharp corners of electrodes). This poling induced bleaching of the electro-optic polymer significantly increases the optical losses [47]. A bleaching effect is also related to the exposure to oxygen during electrode poling which, together with the strong electric field, can induce oxidation-reduction reaction at the chromophore. During a poling in air, oxygen can readily diffuse through the polymer cap layer and through the cladding and active layers. This poling induced damage to electrooptic chromophores is often avoided performing the poling process in an oxygen free environment. Typically, the EO polymers with large EO coefficients r33 have also relatively high dc conductivity [48] which makes them difficult to be poled. Therefore poling these EO polymers becomes a crucial issue and a challenge [49, 50]. When a conventional three optical layer waveguide is concerned, if the conductivity of EO core material is higher than those of cladding polymers, it is difficult to get the required sufficient dc poling field into the EO core with vertical top and bottom electrodes. The different conductivities of the three layers produce different voltage drops across the layers (voltage partition). The effective poling voltage across the EO core decreases as the ratio of the conductivities and the efficiency of poling becomes relatively low. Indeed, the conductivity of passive polymers, which are commercially available and often used as cladding materials, is of the order of 10−9 –10−11 S/m [51, 52] compared to the conductivity of EO polymers, which can reach order of 10−7 S/m or higher. Recently poling and modulation of a polymeric Mach-Zehnder modulator with conductivity dependency free, overcoming the latter poling issues, has been accomplished by using an in-plane coplanar waveguide (CPW) structure.[48] Furthermore, the use of buried electrode can improve overlap factor to enhance poling efficiency. A MZ modulators using the buried CPW showed the Vπ ’s of 6.7 V [53]. An optical structure containing a poled EO polymer on a sol-gel cladding layer has been recently patented [54]. Making the optical structure provides a highly efficient method of poling EO polymers. The optical structure containing a poled EO polymer on a sol-gel cladding layer can be used to fabricate a variety of EO devices. The reported procedure inhibits the onset of dielectric breakdown when poling EO materials by placing the EO materials in contact with a sol-gel cladding material. The sol-gel cladding layer increases the maximum Pockel’s coefficient r33 that can be achieved in the poled EO material and offers a simple way to reduce the operating voltage of EO devices by more than 200%. Optical structures includes, in order, a substrate, a transparent electrode, a cladding layer comprising an organically modified sol-gel, a poled electro-optic polymer layer and a second metallic electrode in direct contact with the poled electro-optic polymer. In addition to increase r33 , the

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Macromolecular Systems with Nonlinear Optical Properties

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sol-gel cladding layer provides refractive index tunability and relatively low optical loss. Moreover the organically modified sol-gel cladding layer can act as a resist and be directly patterned lithographically.

3.3.3 All-Optical Poling (AOP) All-optical poling technique permits the orientation of molecules and the induction of large quadratic susceptibilities by purely optical means [55, 56]. The first optically induced orientation of chromophores in a grafted DR1-doped PMMA polymer has been observed in a four-wave mixing geometry (see Fig. 3.8) with two picosecond pump beams at 1,064 nm and a probe beam at double frequency. The observed signal at 532 nm rose slowly with time up to a saturation value, and was observed also after switching off the probe beam. Significantly larger χ(2) value was obtained in seeding geometry using two collinear picosecond beams at 1,064 and 532 nm [57]. Indeed, the interference between a light wave and its coherent SH leads to a polar optical field able to write a sort of holographic periodically poled material. As a matter of fact, the thickness of the polymeric films is much lower than the coherence length for SHG and less than a quarter of period is recorded. Let’s shortly sketch the interference process which is responsible for the AOP. When a medium is exposed to a sum of mutually coherent optical fields, the fundamental Eω and the second harmonic (SH) E2ω , the probability P(θ ) of excitation of a given molecule is determined by the following three contributions: a1 |E2ω |2 cos2 θ associated with one-photon absorption of SH, a2 |Eω |4 cos4 θ associated with two-photon absorption of fundamental and an interference term a3 |E2 ω E∗ 2ω |cos(Δkz+(ϕ 2ω -2ϕ ω ))Xcos3 θ . Coefficients a1 , a2 , a3 depend on the transition dipole moment and the difference between the dipole moments in the excited and ground states. θ is the angle between polar axis and the dipole moment of the molecule, z is the propagation direction, Δk = k2ω –2 kω is a wave vector mismatch and ϕ ω , ϕ 2ω are initial phases for the fundamental and SH waves, respectively. The last term possesses a polar asymmetry and is responsible for creation of the noncentrosymmetry in an originally isotropic medium. Under polar-asymmetrical optical field Eω +E2ω (fundamental beam plus coherent SH), the molecules with dipole moments nearly parallel to the polar axis experience trans-cis isomerization, that results in the Angular Hole Burning (AHB) and

I4

Thin Film

Substrate

I3

Fig. 3.8 Four wave mixing geometry. Pump beams I1 and I2 are at ω while probe I3 and signal I4 beams at 2 ω

I1

I2

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short-lived χ(2) in first few seconds of poling process. However, the subsequent behavior of χ(2) depends on whether the thermal cis-trans relaxation exists or not. In azo-systems, the reverse cis-trans transition results in molecular reorientation and, consequently, in the further increase of χ(2) to some saturated value, comparable with that obtained with the corona method. This is because, under optimized poling conditions, there is no considerable decrease of the number of trans molecules, but there is only an angular redistribution. If the trans-cis isomerization is almost irreversible, the further pumping reduces the total number of trans molecules without their reorientation, so the decrease of χ(2) occurs during the rest of time. In the following model, a simplified four-level scheme of the molecular photoisomerization (Fig. 3.9) is applied. We suppose that the χ(2) is contributed only by trans molecules and the first hyperpolarizability of trans molecule has only one nonzero component β = β ΔΔΔ , where Δ is a given direction in the (x, y) plane of the film. If the interaction between molecules is neglected, the general expression for χ(2) ΔΔΔ is then

(2)

χ = f 3 β

NT () cos3 θ d

(3.20)

where f 3 = f 2 ω f2ω is a collection of the local field factors, NT (Ω) is a number density of trans molecules oriented in the direction Ω and θ is the angle between the polar axis of the poling optical field and the molecular direction Ω. In the absence of thermal cis-trans relaxation and the molecular reorientation, the dynamics of the trans population is described by the following equation dNT () = −NT ()ϕTC PT (θ ) + NC ()ϕCT PC (θ ) dt

(3.21)

where NT (Ω) and NC (Ω) are the number densities of trans and cis molecules oriented in the Ω direction. PT (θ ) and PC (θ ) are the excitation probabilities for trans and cis molecule oriented at the angle θ , and ϕ TC and ϕ CT are quantum efficiencies for the trans-cis and cis-trans transitions, respectively. Taking into account that in

*

*

σC σT ϕ CT

ϕ TC CIS

TRANS

Fig. 3.9 Simplified scheme of trans-cis and cis-trans photoisomerization in the absence of thermal cis to trans relaxation. Here σT and σC are the absorption cross section of trans and cis molecules and ϕTC and ϕCT are the quantum yields for trans-cis and cis-trans transitions, respectively

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the absence of molecular rotation NT (Ω)+NC (Ω) = N/(4π ), where N is the number density of dye molecules in original isotropic distribution, one has the following NT (θ , t) = NT (θ , ∞) +

t 1 N sin θ − NT (θ , ∞) · e τ (θ) 2

(3.22)

with the equilibrium number density is NT (θ ,∞)=(1/2) Nsinθ [1+ϕ TC PT (θ )/ ϕ CT PC (θ )]–1 and the relaxation time is τ (θ )=[ϕ CT PC (θ )+ϕ TC PT (θ )]–1 . Substituting Eq. (3.22) into Eq. (3.20), one has the temporal dependence of χ(2) ΔΔΔ . The AOP technique opens prospects that are not permitted with corona poling. In particular, orientation of molecules without a permanent dipole moment in the ground state, such as octupolar molecules has been demonstrated [58]. An experimental setup for all-optical poling is presented in Fig. 3.10 [59, 60]. The beam source is a Q-switched Nd:YAG laser delivering 7 ns pulses at 1,064 nm with 10 Hz repetition rate. A type II KDP crystal is used as frequency doubler. An half-wave plate is used to rotate the polarization of the fundamental beam. A polarizer assured linear and parallel polarization of the two beams. The experiment consisted in two alternating phases: seeding (writing) and probing (reading). In the writing phase, both beams were co-propagating and weakly focused by a lens (L) onto the sample at a spot of 3.5 mm diameter. A polarizer (P) was used to ensure parallel polarization of the two beams. The photoinduced second-order susceptibility χ(2) was probed by SHG inside the sample and was detected by a photomultiplier tube (PMT). During the readout phase a red-glass filter is inserted just after the laser output to block the 532 nm light. An interferential filter was placed in front of the photomultiplier tube to stop the infrared beam. A set of calibrated filters was used to ensure

OSC PMT

KDP Nd:YAG

1064 nm

P

S

L

KG3

1064 nm spatial filtering

532 nm λ/2 @ 1064 nm

sample on a prism RG 630

Fig. 3.10 Experimental setup for AOP. P-polarizer, λ/2 for 1,064 nm-half wave plate at 1,064 nm, RG 630-Schott green blocking filter, 3 mm thick, KG3-Schott heat blocking filter, shutter (S) was synchronized with the insertion of the red filter (Shott RG 630) which cut the 2ω beam during χ(2) readout process, F spatial filter for 532 nm. The signal was detected by a photomultiplier tube (PMT) and registered on a digital oscilloscope (d-OSC)

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the correct scaling of the SH signal. The signal from the photomultiplier tube was measured with a digital oscilloscope (OSC). Moreover, an electronically controlled shutter was placed in front of the PMT in order to avoid the possible saturation due to the strong green seeding beam during seeding step. The fundamental beam served both as seeding and probing beam, while the SH beam was used as a seeding beam only. The influence on the efficiency of AOP of such seeding parameters as the relative phase between writing beams, their relative intensities and the beam polarization states as well as the temperature dependence has been studied by several groups [61, 62]. The relative phase is an important factor, particularly when the writing beams are linearly polarized. Efficient SHG requires phase matching, so the harmonic fields generated in different regions along the material interfere constructively at the output. Experimentally, the phase difference can be tuned placing the samples on an isosceles glass prism [63]. Recently, a polymer thin film doped with a chiral almost centrosymmetric bisazomolecule has been demonstrated to exhibit second harmonic generation after alloptical poling [64].

3.4 Orientation Stability The time evolution of orientation distribution of dipoles G(θ ,t) is generally described using the Smoluchowski equation for rotational diffusion. [65] The use of this model to describe the relaxation implies that all chromophores have the same rotational diffusion coefficient. In amorphous polymers that is very often not the case. As a result, one does not necessarily expect to observe a single exponential decay as predicted by Smoluchowski model. A variety of functional decay forms have been used to describe time-dependent process observed in disordered systems, like stretched exponential [66] or two exponential functions [33]. In many cases, however, it is difficult to choose between these two phenomenological models only on the basis of fitting of experimental data. While stretched exponential function accounts for the dishomogeneity of the chromophore environment with a minimum number of fitting parameters, a model based on a bi-exponential fit function has the merit of yielding a clear picture of prominent applicative interest. If one is able to distinguish two clear-cut time regime in the NLO activity decay, as is often the case, different relaxation mechanisms can be assumed, each with a characteristic rotational diffusion constant. A poled polymer can be treated in analogy to dielectric relaxation processes [67] where α-relaxations (occurring at the higher temperatures) are generally assigned to relative motions of polymer chains while β-relaxations are frequently associated with local motions of side groups. The faster decay (τ1 ≈ h) can be related to the rotation of the dopant molecules in the polymer free volume while the slower one (τ2 ≈ years) to thermal relaxation of the system as a whole [68]. For device applications, the slow process is much more important than the fast one. This picture implies a tight correlation between polymer glass transition temperature Tg and second order NLO activity decay time τ2 . This behavior has been

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recently observed [69]. A measure of the orientation stability can be carried out monitoring the SHG intensity as a function of time, starting just after the poling field is switched off. Comparison of the absorption spectrum of the poled sample with that of a twin unpoled one, kept at the same conditions, prevents to incorrectly attribute a decrease in SH signal to order relaxation while that is rather due to a decomposition of the chromophore. If the test is performed at fixed temperature (e.g. RT) this procedure is called isothermal decay method [40]. However, for materials with decay times of the order of years, this direct procedure does not give results with sufficient accuracy in a feasible observation time. Therefore accelerated ageing procedures have to be developed, provided that degrading agents are recognized. [70] In case of NLO systems based on chromophores in polymers, where the decrease of functionality can be correlated to dipoles reorientation, temperature is the main driving force. The accelerated ageing procedure consists in studying isothermal decays in a temperature range where they are sufficiently fast. Then the behaviour at RT can be extrapolated applying a suitable model. In order to illustrate the method, we can refer to the measurements of the isothermal decay of nonlinear d33, proportional to the square root of the SHG intensity, of DR19 in a polyimide films [71]. Polyimides are good candidates for NLO devices since they offer high thermal and good chemical stability. The second order nonlinear coefficient d33 have been fitted with a double exponential function: d33 (t) ∝ a · e(−t/τ1 ) + (1 − a) · e(−t/τ 2) d33 (0)

(3.23)

The first decay component (τ1 ) contributes to the short time regime (it is over in a hour). The second component accounts for the thermal relaxation of the polymeric matrix and the parameter τ2 shows a well defined dependence on temperature. The following Arrhenius relationship between the decay probability 1/τ2 and the inverse temperature 1/T can be assumed: 1 E = Ae − τ2 kT

(3.24)

where A is the frequency factor, E is the activation energy of the process, and k is the Boltzmann constant. A τ2 value of 31 years could be extrapolated for the nonlinear coefficient d33 decay at RT (Fig. 3.11). The value at 35◦ C, reported in Figure as a cross, is a lower limit for the decay time at that temperature and was not taken into account in the fit. The high Tg , value (196◦ C) of the matrix and the large size of the monomer units determine the good stability performance of this system. Moreover the formation of hydrogen bonds between the two hydroxyethyl functionalities of DR19 and 1,3-phenylenedioxy moieties of the polymer chain have suggested to play a role in maintaining the alignment.

138

–1

Log101/ τ

Fig. 3.11 Temperature dependence of the relaxation time τ2 of the second order nonlinear coefficient d33

P. Prosposito and F. De Matteis

–2

31 years @ 25°C

–3 –4 2,8

3,0 3,2 1000/ T[K–1]

3,4

3.5 Device Fabrication Techniques Starting point for device fabrication are planar waveguides. They can be synthesized in different ways such as sputtering, evaporation, molecular beam epitaxy, etc. However, one of the most simple method is the spin coating technique which allows deposition of polymeric and hybrid materials quite simply and with very good homogeneity even on large substrates. The most commonly employed substrate is silicon in order to integrate devices on common boards. Buffer layers of low refractive index (SiO2 or undoped polymer or hybrid materials) and adequate thickness are commonly used as optical insulators from the high refractive index of silicon. In planar waveguides the light is confined vertically by the thickness of the guiding material which is typically from hundreds of nanometers to few micrometers. For applicative purpose, light has to be guided through the board to the devices and from devices to the external environment by optical fibers, hence an horizontal confinement is necessary, too. Optical waveguides shaped as channels are used to route the light and the related information from one point to another one. Moreover, such channel waveguides are commonly integrated with electrodes to realize optical devices that are used to switch and modulate the travelling light. Channel waveguides are usually covered and surrounded by a cladding material having refractive index slightly lower than the lightwave channel with the aim to protect the device from the outer environment. Therefore light in the channel waveguide is transversely confined in the buried lightwave channel. Guiding layer is composed by the polymer or hybrid material with NLO chromophores dispersed or functionalized to the matrix backbone. A schematic representation of an optical planar waveguide with buffer and cladding layers and with two metallic electrodes to apply an external voltage to the overall structure is shown in Fig. 3.12a. The lateral confinement can be attained tailoring the desired shapes directly on the guiding layer. Different techniques are employed ranging from conventional lithographic processes to direct writing, from thermal or UV-imprinting to soft lithographic methods, from laser etching to spatially selective poling [72]. A schematic representation of a waveguide confined both in vertical and horizontal direction is reported in Fig. 3.12b.

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Macromolecular Systems with Nonlinear Optical Properties

Fig. 3.12 Schematic picture of a generic device. (a) A planar structure, (b) a laterally confined structure

139

Metallic electrode

Metallic electrode

Cladding layer

Cladding layer

Guiding layer with NLO chromophores

Guiding layer

Buffer layer

Buffer layer

Metallic electrode

Metallic electrode

Si substrate

Si substrate

(a)

(b)

In the following we will describe shortly some of these techniques and in the next section some devices based on NLO chromophores focussing the attention mainly on switches and modulators.

3.5.1 Photolithography The most common method to fabricate bi-dimensional waveguide structures is by optical lithography [73–76]. This method is employed for the fabrication of circuits on wafers and it requires a process where specific patterns of various materials can be deposited on or removed from the wafer’s surface. Lithography uses photoresist materials to cover homogeneously a large area of a substrate (wafer). This is deposited on the substrate by spin coating producing a planar layer of hundreds of nanometers depending on the spinning conditions and photoresist type. Photoresist is then prebaked for times ranging from tens of seconds to few minutes, in an oven or on a hotplate to eliminate the excess of solvents. The substrate with photoresist is subsequently exposed to ultraviolet (UV) light through a photomask which is a metallic (typically chrome) pattern realized on a quartz or glass substrate that is used to transfer the pattern on the photoresist layer. The illumination takes place in an exposure system (mask aligner). Light, which shines on the entire mask, filters the open spaces and is blocked by the metallic structures of the mask. Exposure systems can be classified as contact or proximity printer. In the first case the system puts a photomask in direct contact with the substrate covered by the photoresist and exposes it to a uniform light. In the proximity printer a small gap between the photomask and wafer is present. Contact printing can damage both the mask and the wafer due to the contact but in this case the optical resolution is better than in the proximity method since the gap distance is approximately zero (the resolution is approximately the square root of the product of the wavelength and the gap distance). A post-exposure baking is performed before developing, typically to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light through the openings of the photomasks. After the

140 Fig. 3.13 Representation of lithographic process for channel waveguide fabrication

P. Prosposito and F. De Matteis

1. Photoresist deposition and UV Lighting

2. Photoresist development

3. Aluminium film deposition and lift-off film

4. RIE attack

5. Chemical etching of aluminium film

6. Channel Wave guide

post-bake procedure a chemical bath is used to remove some parts of the exposed resists. The most common type of photoresist (called positive) becomes soluble in a specific developer when exposed to UV light. Alternatively negative photoresists become insoluble in the (organic) developer when exposed. After this step a hard baking is commonly employed in order to solidify the remaining photoresist which has now the desired patterns and to make a more durable protecting layer for the next steps. At this stage the features defined in the photoresist have to be transferred to the substrate and for this purpose ion milling, wet chemical etching or reactive ion etching processes can be applied [73, 74]. Two strategies can be pursued: the first one is the direct use of the densified photoresists features exploiting the difference in the erosion rate of the dry or wet etching procedures between substrate material and densified photoresist. The second one, sketched in Fig. 3.13, consists in a further deposition of an aluminium layer on the developed photoresist and a lift off process consisting in a chemical bath removing the densified photoresist and leaving only the aluminium features to be transferred to the substrate. The process is then accomplished by a reactive ion etching process which allows the transfer of the features to the substrate and a final wet etching to remove the residual aluminium film [73, 74].

3.5.2 Direct Patterning Conventional microfabrication technologies use a multi-step process that includes exposure of a photoresist film through a photomask, development, and wet or dry

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etching transfer of the optical structures into the substrate material as described previously. A simple fabrication method of micro-structured surfaces is based on the direct exploitation of photosensitive materials. In this case refractive index and/or geometrical modulation are produced by a direct light exposure. Optical patterning with refractive index modulation induced by light illumination has been seen in many photo-polymers [77]. Cross-linking induced by light in the molecular structure of a photo-polymer makes it resistive to etching, so that the developing process removes the unexposed parts and allows the definition of micro structures. Typical examples of photosensitive materials are photoresist and in general photosensitive polymers, such as azo-polymers, that can create directly a micro-structured surface of refractive index changes and topological modulation upon light illumination [78–81]. Recently, also sol-gel derived inorganic-organic hybrids have been shown to be very photosensitive and thus suitable for fabrication of micro-optics by photopatterning [82–84]. In typical photosensitive sol-gel hybrids, selective photo-induced cross-linking was obtained, and micro-optical structures were created, by etching the unexposed area of film. A simple scheme of the direct exposition of a photopolymerizable materials through a mask having some apertures and the following development process in a solvent is illustrated in Fig. 3.14. However, the development step which involves the use of organic solvents may potentially decrease the surface quality of the optical elements introducing high losses in the waveguide. In the last years, it has been demonstrated that it is possible to fabricate directly optical structures in sol-gel hybrids by using only the exposure step [82, 84, 85]. This direct photo-imprinting enabled the simple and cheap

(a)

Fig. 3.14 Wet etching process. (a) UV exposition of the photopolymerizable material trough a metallic mask; (b) development of the unexposed film by wet etching (bath in a chemical solution); (c) resulting channel waveguide

(b)

(c)

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Fig. 3.15 Direct patterning of a waveguide without development process

1. Film deposition and UV Lighting

2. Channel Waveguide

fabrication of micro-optical structures with improved surface quality and low optical losses. The process of direct exposition of a photo-patternable material through a mask by an UV illumination is reported in Fig. 3.15. In this case the difference in the refractive index of the material (increased density of the material in the light exposed parts) can be used for the micro-structured features fabrication. However, micro- and nano-structuring of photosensitive materials can be achieved also by direct writing without masks just using light from lasers [84, 86–88] and even one- and bi-dimensional patterns can be fabricated using for example interference patterns of laser light [89, 90]. Direct patterning processes are extremely interesting for many aspects such as low costs, ease of fabrication, good quality, etc. and surely they will receive an even stronger attention in the next future.

3.5.3 Soft Lithography Research on alternative processes for fabrication of devices and structures with respect to conventional lithography has received a considerable development in the last years [91–93]. Alternative techniques to cost-intensive or limited-access fabrication methods with nanometer resolution are necessary for a massive and complete exploitation of the potentiality of the nanometric devices. Alternative lithographies include micro contact printing, UV imprinting, nanoimprint lithography, micromolding in capillaries, etc. All these techniques can be included under the general name of soft lithography and their main advantages are the low cost and the high throughput with good reproducibility [91]. Many of them are based on molding elements (i.e. solid objects with structures of relief on their surfaces) to direct the flow (as in the case of micromolding) or softened solids into the desired shapes. [94–96] The basic one is the embossing technique also referred as nanoimprint lithography (NIL). In this case the mold, made of hard or soft material with specific negative features, is directly pressed against the

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film cast on a substrate in order to transfer the patterning into the film [97, 98]. If the mold is made of hard material (silicon, rigid polymers, etc.) a pressure on the mold can be applied by means of a precision press in order to transfer the patterning in the film surface. In the case of soft mold (e.g. polydimethylsiloxane (PDMS)) a conformal contact between mold and film surface has to be accomplished. At the end the stamp is detached from the printed film and as a result the negative structures of the stamp are impressed in the film. After embossing an etching step can be performed to remove the residual layers between the imprinted features. This process can be performed many times using the same mold on different films and this makes this technique extremely flexible, simple and low-cost. The quality of the structures is very good and features of few nanometers have been obtained [99]. A schematic picture of the embossing process is reported in Fig. 3.16. Several variations of this method have been tested. One of the most exploited is the UV nanoimprint lithography, where the PDMS stamp after conformal contact with the film surface is illuminated by an UV light that densifies the underneath film during the contact with the mold. This process is possible thanks to the transparency of the PDMS material at the UV light [100–102]. Another alternative method is the roll-to-roll nanoimprint lithography [103] which uses substrates of large dimensions and makes possible imprinting of large area with very good reproducibility and

Mold Film

(a)

Substrate Fig. 3.16 Embossing or nanoimprint lithography procedure. A hard or soft mold is in contact with surface of a film (a) A pressure is accomplished on the hard mold or a conformal contact is realized for the soft one in order to transfer the features of the mold on the film surface (b) sometimes at this stage a thermal treatment is necessary in order to soften the film and allow a better transfer of the patterning. Mold is removed (c) A final etching process can be performed to reduce the residual layers between the transferred structures (d)

Mold Film

(b)

Substrate

Film Substrate

(c)

Substrate

(d)

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in single step. Other implementations of the NIL method are the Step-and-Stamp Imprint Lithography which involves a sequential transfer of the stamp to different positions of the substrate and the Step and Flash Imprint Lithography which uses ultraviolet light instead of temperature for polymer curing [104–106]. In general, as shortly summarized in this section, a great interest on the alternative lithographies for their wide potentialities in the field of integrated photonics is currently present and probably important results will be pursued in the next future for the low costs, the high throughput and the easy of fabrication.

3.6 Devices 3.6.1 Modulators Light modulators are key devices for telecommunication in order to send a signal from one to another place. They modulate the phase or the intensity of the electromagnetic wave. Many of them are based on NLO materials and some of the main characteristics of such devices will be described in this paragraph. The most studied NLO devices are the light intensity or phase modulators. Basic modulators are the reflective-type ones such as the attenuated-total-reflection (ATR) and resonant grating waveguide modulators. They are based on prism or grating to couple the energy of the incident light into the waveguide. [107] In the device presented by Katchalski et al. [108] the waveguide layer is composed of a 30% molar ratio DR1 chromophore side chain methyl methacrilate polymer, while the layer where the grating is inscribed is a photoresist layer having a period of 960 nm and a depth of about 400 nm. Varying the external voltage applied between the two electrodes the refractive index of the waveguide layer can be modulated and the reflected intensity can be modulated with a modulation bandwidth of about 1 MHz. A schematic picture of such device is represented in Fig. 3.17.

Light

Modulated reflected light

Electrode Grating

ITO Electrode

Cladding layer Polymer waveguide with poled DR1 molecules Buffer layer

V

Fig. 3.17 Reflective-type resonant grating waveguide modulator operating in free space

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Recently Deng et al. [109] reported on an electro-optic polymer modulator employing a symmetrical metal-cladding waveguide structure based on PMMA containing DR1. The device operates on the free-space coupling technique with no prism, grating or other coupling components. The applied electric field modulates the reflected intensity by changing the energy coupling efficiency of the incident light into the waveguide. A 8.2% modulation depth with a 10 V (Vp–p ) driving voltage at 1 MHz has been attained. However, the most common modulators are those integrated on substrates which work not in free space but in waveguide geometry and in particular the most studied are the Mach-Zehnder interferometers. They are constituted by a single channel where the light beam travels. The single channel is splitted by an Y-arm into two branches for a length of few centimeters and then the two branches converge again, by a reverse Y-splitter, in a single channel. They are based on the electro-optic effect which acts on one or both branches of the modulator. On this purpose electrodes are deposited on the sides or on the top and bottom of the arms and allow the application of a voltage across the active material. The sinusoidal voltage induces a periodic variation of the refractive index of the material by Pockels effect inducing a difference in the travelling time of the light in the two branches. When the two beams recombine in the output channel such an effect can be used to modulate the light intensity by constructive and destructive interference. In Fig. 3.18 two schematic representation of Mach-Zehnder modulators are reported. In the first case only one arm is modulated in the second one inverse voltages are applied on both branches. Usually the electrodes are present on a single arm so that applying an electric field a phase shift of the travelling optical beam can be attained. The voltage required for a 180◦ phase shift, indicated as Vπ , is given by: Vπ = dλ/Ln0 3 r33

IN

OUT

a)

IN

OUT

b)

Fig. 3.18 (a) Mach-Zehnder modulator where only a single arm is modulated with a sinusoidal voltage (grey zone on the lower arm). (b) A push-pull Mach-Zehnder modulator where both arms are modulated by a sinusoidal voltage but with opposite phases

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Where d is the gap between the electrodes, λ is the light wavelength, n0 is the ordinary refractive index of the material, L is the interaction length and r33 is the electro-optical coefficient of the material. The increasing interest in the materials used for these devices derives from the need for high-speed (wide bandwidth) and low drive-voltage (low Vπ ) electro-optical modulators [110]. The most common modulators are based on lithium niobate and they operate with halfwave voltage of about 5 V. This value is quite high and represents a drawback for gain and noise levels. Lower drive voltages are desirable since they make the supporting electronics cheaper and vastly reduce the amount of power which is consumed by the modulator. Moreover, the on-chip power dissipation for electro-optic modulators is an important limitation toward achieving massive integration [111]. In the case of pushpull polymeric electro-optic Mach-Zehnder modulators based on electrodes on both modulator arms acting with opposite phases, the driving voltage Vπ can be reduced to the half [49]. Lower drive voltage can be attained also varying other parameters such as the electrode gap, the interaction length or the electro-optical coefficient of the material. However, very often changing these parameters other problems arise such as high optical losses or low modulation frequencies. Current efforts are concentrated on getting drive-voltage lower than 1 V [14, 110]. On this regard research on innovative materials having low losses and high EO coefficient is very attractive, in particular on polymeric and hybrid materials that offer a huge field of variability from the chemical point of view. There are some advantages in the use of organic NLO materials for device fabrication with respect to the inorganic ones. The main are: – Low dielectric constants of the organic materials reduce the resistor – capacitor delay time constant, increase the device speed and reduce crosstalk so that electrodes can be spaced more closely together; – Fast electro-optical response time of the order of few femtoseconds [2]; – High electro-optic coefficient (r33 up to 300 pm/V have been measured for EO polymers) [112]; – Low variation (almost constant value) of the refractive index of polymeric and hybrid materials in the range from infrared to microwave region ensures a proper work operation of the modulator over the whole spectral range; – Easy integration on silicon substrates and therefore an integration with semiconductor electronics can be easily accomplished; – Refractive index of the NLO polymers and of the cladding materials can be easily tuned to match determined conditions and restrictions; – Easy and cheap processing methods with respect to the inorganic materials [43]; – Good thermal, temporal and mechanical stability and low optical losses; – Low power consumption and capability to work at high frequency (higher than litium niobate crystals based devices). In Table 3.1 is reported a comparison between lithium niobate and EO polymers based devices. The requirements of high nonlinear coefficients are related both to microscopic properties of the NLO chromophores (high β and μ values) and to

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Table 3.1 Main properties of devices based on LiNbO3 and electro-optic polymers

EO coefficient (pm/V) Working frequency (GHz) Driving voltage (V) Device workability Integration Costs

LiNbO3

Electro-optic Polymers

32 40 5 Difficult Difficult High

Up to 300 Greater than 100 Lower than 1 Easy Easy Low

macroscopic properties (high r33 and χ(2) ). The latter depend on the orientation of the molecules and also the total number of chromophores present in the material. However the total number can’t be too high since a high concentration can produce a strong intermolecular interaction and can reduce the transparency of the material. For these reasons in the device fabrication a trade-off between high second order materials and low optical losses has to be pursued. Polymer electro-optic materials have the necessary properties to function in photonic devices beyond the 40-GHz bandwidth which is typical for lithium niobate based devices. In particular, Lee et al. [2] reported on an EO light modulator with a bandwidth of 150–200 GHz and producing detectable modulation signal even at 1.6 THz. Even if the geometry with top and bottom electrodes is the most used for MachZehnder modulators, some examples of coplanar and coplanar buried waveguide electrodes geometry have been recently presented [48, 53]. In this case the electric and the optical fields are aligned and the overlap factor is enhanced. Thus, the poling efficiency can increase and Vπ decreases. With the in-plane poling technique the conductivity dependency of core and claddings results eliminated. Vπ of 7.7 V with a single-arm poled Mach-Zehnder or 3.8 V for push-pull operation has been achieved [48]. Moreover, a 20% improvement in the overlap x poling efficiency by using the buried electrode has been measured [53]. The problem of the optical losses in optical modulators is very important and many efforts have been accomplished in order to minimize them. For example Yuan et al. [113] reported on a simple low-loss interconnection structure between electrooptic and passive polymer waveguides. The structure is simple and easy to fabricate and the excess loss was lower than 0.3 dB. In order to minimize the coupling and scattering losses a new device design has been recently presented [114]. In this case a high numerical aperture passive waveguide coupled with a strip loaded active waveguide by refractive index tapers has been fabricated. Organically modified solgels have been chosen as the passive materials while amorphous polycarbonate with AJLS102 chromophore (25% wt) has been chosen as EO polymer. The configuration of the modulator was a Mach-Zehnder showing a fiber-to-lens insertion loss of 5.7 dB at 1,550 nm and a half-wave voltage Vπ of 2.8 V. This insertion loss value is one of the lowest never been measured for a non evanescent EO polymer waveguide modulator having a 1.5 cm or greater active length [114].

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The screening of EO polymer by cladding resistive polymeric layers is a key factor to efficiently develop better material systems and devices having broadband frequency, low driving voltages and low insertion losses. An efficient method to select an EO polymer material for a device having the desired characteristics is the measurement of the signal from the second harmonic generation and leak through current during the poling procedure as reported recently by Chen et al. [115]. Some authors [116] reported on hybrid polymer/sol-gel waveguide modulators where larger EO coefficient r33 can be attained when the NLO polymer is surrounded by a low resistance cladding layer such as organically modified sol-gel material. They found a value of Vπ less than 4 V when a poling voltage of 1 kV was used for contact poling in the hybrid modulator. Moreover, the presence of cladding layer of hybrid sol-gel material improves also the coupling losses if specific design are realized. In particular, coupling losses down to 1 dB per end face have been measured [117]. Recently, Enami and co-workers [118] presented a hybrid cross-linked EO polymer/sol-gel Mach-Zehnder waveguide modulator with Vπ of 0.65 V at 1,550 nm. Such a low value of half wave voltage, which is one of the best results obtained in literature, is related to the combination of a very high electro-optic effect and to the device design that combines physical vertical tapers in the sol-gel core, index tapers in the EO polymer, a suitable electrode structure and a top buffer layer designed for cross-linked EO polymer systems. In such a device also the optical losses result to be reduced with respect to a more traditional configurations. Baher-Jones and Hochberg reported on the combination of nonlinear optical polymers and silicon waveguides to provide a hybrid platform with unique advantages [119]. NLO polymers can provide a flexible source of large second- and third-order optical nonlinearities and the ability to engineer material properties for specific applications. At the same time silicon can provide tight optical confinement due to its high index of refraction, to build nanoscale features and to massively concentrate optical and electric fields. Moreover, fabrication technology for silicon is quite advanced for the significant investments made by the silicon microelectronics industry. They reported on different nonlinear polymer based modulators with low Vπ value (lower than 1 V) and they proved also that using the best NLO polymers and working on the device design and scaling down the dimension of the silicon waveguides even values of tens of milliwatts can be attained [120]. Recently all-optical modulators built in silicon and exploiting NLO polymers have been the subject of increased interest. Such devices are usually built based on Mach-Zehnder geometry where both arms of the modulator are covered by NLO polymers. An optical signal goes through both arms and a gate signal is introduced into one of the arms. A nonlinear refractive index shift in one of the arms can then lead to all-optical modulation whereby the gate optical mode can switch the signal optical mode. Depending on the nonlinear mechanism the operating bandwidth can be as high as the terahertz [119, 121]. In analogy with the case of electrooptic modulator where a Vπ value is defined, in the case of all-optical modulators it is possible to characterize the response in terms of Pπ which represents the instantaneous power required to obtain an induced phase shift of π radians in one arm.

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Lower values of Pπ are desirable as they correspond to more responsive modulators that require less gate power. Values of Pπ of about 2 W have been obtained [121] and it is likely that values of less than 1 W will eventually be possible even without resonant enhancement. Another configuration for electro-optical modulators is that based on microring resonators. Microring resonators are gaining increasing importance in high speed applications such as switches, modulators [122–124] and filters [124–126] and are promising candidates for very large scale integration photonics. A schematic draw of a microring resonator is presented in Fig. 3.19. The resonator consists of a ring and of a straight waveguide. The gap between the two structures is very important for the coupling of the electromagnetic wave travelling into the waveguide with the ring. Due to the gap a small fraction of the light travelling into the waveguide is coupled to the ring. The light travelling into the ring after a roundtrip will interfere with the light travelling in the waveguide. If the phases of the light entering the ring and the light in the ring are equal or opposite constructive or destructive interference will occur. In principle the ring and the channel waveguide are made of NLO polymer but also a passive waveguide can be employed and/or alternatively the EO polymer can be used as top cladding material for the entire structure. A bottom and a top electrodes together with optical insulating cladding layers are deposited in correspondence of the minimum gap between channel waveguide and ring (sometimes the gap is vertical and not horizontal as the one represented in figure) so to periodically change the optical mode effective index and to impart an index change of the NLO polymer that modulates the intensity of the outgoing light (on/off resonance condition). Typical dimension of the curvature radii of the ring is of few hundreds of micrometers since smaller radii can induce high bending losses, therefore such feature limits miniaturization of the modulators [124, 127]. Typical EO polymer modulators have an electrode spacing of about 10 μm. Gap between straight waveguide and ring is of fundamental importance for the correct functioning of the device. Although very small gaps of about 50 nm are possible by using expensive electron beam lithography [128] and nano-imprint techniques [129], usually larger gaps (hundreds of nanometers) are used. In travelling wave Mach-Zehnder high speed modulators the bandwidth is limited by the loss of the several centimeter long micro-strip line [130]. On the other

Fig. 3.19 Microring resonator modulator

Light input

Electrodes

Light output

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hand in microring modulators, since the device is small the electrode loss can be much smaller and as the microwave index of refraction of polymers is around 1.5 the device’s high speed behaviour is mainly capacitive. Hence electrode loss is not an issue and the bandwidth is set by the electrode capacitance. When used as modulators the properties of the rings can be changed slightly by an electric signal, causing the rings to go in and off resonance. Because of the narrow resonance peaks this change in the ring can be very small and still have a large modulation. A very sensitive modulator can be fabricated using these rings. Ring resonators made of EO polymers have some advantages over inorganic materials such as the low microwave/optical velocity mismatch allowing simple design of high speed modulation devices.[14] A laterally coupled EO polymer microring resonator was recently designed and fabricated by Balakrishnan et al. [131, 132]. They investigated microrings made of PMMA/DR1 defined by RIE and SU8 fabricated by photodefinition (exposition to UV light). However, their final aim is to fabricate a device based on SU8 and TCVDPA (tricyanovinylidenediphenylaminobenzene) a NLO chromophore having a high photochemical stability. In particular, they studied the behaviour of the transmitted light as a function of the wavelength varying the racetrack length and the gap distance between ring and channel waveguide. They found a passive ring resonance behaviour at around 1,550 nm with a maximum extinction ratio of 12 dB for 160 μm racetrack length showing a possible use of such devices for electro-optic modulator applications. Resonant modulators based on EO polymeric material (CLD1/APC) acting at 1,300 and 1,550 nm have been fabricated by Rabiei et al. [130] The resonance wavelength voltage tunes at the rate of 0.82 GHz/V and the modulators have a bandwidth larger than 2 GHz. Very recently, Block et al. [133] reported on optical modulators with ring diameters smaller than 50 μm in a silicon nitride based waveguide system on silicon oxide with a top cladding of an electro-optic polymer, namely AJTB141 chromophore 28% wt into an amorphous polycarbonate polymer. Using Cu electrodes they attained a high frequency modulator with modulation up to 10 GHz with low drive voltage (2.7 Vpp ). Microring resonators can be used also as tunable filters. In this case they can be employed to take a signal of a certain wavelength from the main waveguide. The resonance peaks of a ring resonator are very narrow and the wavelength selectivity is very high. This aspect will be described in more details in the next paragraph.

3.6.2 Switches, Filters and Other Devices Many optoelectronic devices based on NLO properties of materials have been fabricated in the last years and their importance in integrated optics is growing more and more. For example they can be used in dense wavelength division multiplexers (WDM), laser cavities, spectrometers, reflectors and filters in optical interconnects. In particular, electro-optic and all-optical switches are important for high bit rate optical communication and optical computing systems.

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An all-optical switch is a device that allows one optical signal travelling into some material to control its own position changing the intensity or another optical signal, i.e. control of light by light. All-optical switches have some advantages with respect to electronic switches as the small size, the high speed and low heating of substrates and junctions. One of the basic all-optical switch is the one where a light beam passes through an NLO material and exits in a certain direction. Then a second beam can pass through the NLO material following the same path as the first one and changes the direction of the first beam. So doing the exiting beams in the two conditions are spatially separated [134]. For some isotropic nonlinear materials the refractive index, due to the existence of a nonlinearity, can be expressed as: n = n0 + n2 I where n0 is the constant linear refractive index and n2 is the nonlinear correction term and I is the light intensity passing through the nonlinear material. If the light intensity increases, the refractive index of the NLO material also increases linearly. This property can be used to implement an all-optical switch as described in Fig. 3.20. The incoming beam light is refracted by the linear material. At the interface with the NLO material the beam light will be refracted again and will leave the NLO material at the point A going toward the direction of channel 1 (CH1). If the light intensity is higher than in the first case the refractive index of the NLO material will be higher (as reported by the previous equation) and as consequence the angle of refraction (angle between the normal at the interface and the refracted beam) will be smaller and the outgoing beam will exit the NLO material from the point B and will go toward the channel 2 (CH2). In this way the NLO material can be used as an optical switch where the change of the outcoming channel of the light signal from the NLO material depends on its intensity level [135]. Birefringence effect can be exploited for all-optical switch devices. A simple alloptical switch based on photinduced birefringence effect has been demonstrated in azo dye (DR1) doped polymer (PMMA) films. The all-optical switching effect has

Linear material NLO material B

Fig. 3.20 Schematic draw of an all-optical switch

A

CH2

CH1

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been studied at different control beam power and different temperature of the sample. With a control power of about 31 mW and a temperature of 56◦ C the response time of the switching was less than 5 ms and the depth of the modulation was about 80% [136]. An all-optical switching polymer thin film with azobenzene dye ethyl orange as the guest material and polyvinyl alcohol (PVA) as the host material was recently presented [137]. The all-optical switching characteristics of the samples were measured at different intensities and modulation frequencies of the pump beam (532 nm, CW) and the influence of doping concentration on the all-optical switching effect of the films was studied. It has been shown that, under room temperature conditions and with a low pump power of 6 mW, the all-optical switch has a response time of about 2 ms and a modulation depth of 45%, and the maximal modulation depth reaches 90%. Samples with higher doping concentration show a stronger all-optical switching effect but a larger background signal. Guo et al. [138] reported on an optical switching device based on resonant grating waveguide structure using nanoimprinted DR1-doped PMMA polymer grating. In this case electrodes are not necessary and the process is based on a change in the refractive index of the material which depends on the light intensity. As a result the resonant condition is simultaneously modified and a shift in the resonance reflection peak is detected. They found a relatively low threshold intensity for switching at wavelength of 570 nm of about 10 MW/cm2 . In the same work they proposed also a Mach-Zehnder modulator where the NLO molecules in the EO polymer are self assembled by using a nanopatterned dielectric template instead of using the more common electrical poling. In particular, by using a dielectric grating EO materials can be self-assembled onto this grating template. In this way a periodic EO polymeric structure with long-term stability and a thick waveguiding layer can be attained for integrated electro-optic devices fabrication. Photonic switching in various photosensitive molecular structures, in a very simple pump-probe setup, based on nonlinear absorption, can be efficiently performed by exploiting their photochromic behaviour [139]. Recently a photoinduced birefringence with large optical nonlinearity in a bacteriorhodopsin/polymer composite film has been observed [140]. A high refractive index change has been reached even with low intensity. When the linearly polarized pumping beam strikes the film the photoinduced birefringence is generated and the probing beam is modulated and the transmittance of the probing beam is changed accordingly. Based on such system a broadband all-optical photonic switch has been realized with an optical controlling switch system. Moreover, recently a theoretical investigation on the enhancement of the speed of digital operation in bacteriorhodopsin-based photonic from earlier reported kHz to MHz has been published [141]. Also all-optical Mach-Zehnder waveguide interferometer devices working as switches have been presented in the last years [142]. In this case the change of the refractive index in the arms of the modulator is produced by the light inducing a local optical nonlinearity. Recently, Wu et al. [143] reported on an all optical switch by using the position shift of the spatial solitons controlled by phase modulation

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created in the local non linear Mach-Zehnder interferometer with a straight control waveguide. The most studied switches are those based on the application of electric field to accomplish the switching process. Some of these operate on the deflection of the beam light, such devices are based on the change of the propagation direction of the light beam induced by the changing of the refractive index in an EO material when an electric field is applied [144–146]. Experimental results of deflection angles versus various applied electric fields in a prism shape optical beam deflector based on a thermoplastic electro-optic polymer have been reported recently by Wang et al. [145]. The light beam propagating within the planar waveguide formed by the polymer layers will thus have its direction of propagation modified in a manner similar to that of a beam passing through a set of physical prisms. For such a device a deflection efficiency of 28 mrad/kV and maximum deflection angle of ± 8.4 mrad at ±300 V has been found [144]. Glebov et al. [146] reported on a fast electro optic deflector switch and on the enhancement of the deflection angles by using a glass volume Bragg gratings structures. Using such features the beam deflection angle can be increased by more than a factor of 5 for the same switching voltages. Electro-optic switches based on Mach-Zehnder, microrings and resonant grating configurations have been extensively studied in the last years. A polymeric electro-optic modulator based on a 1 × 2 Y-fed directional waveguide codirectional coupler has been recently reported [147]. The EO polymer showed an EO coefficient of 25 pm/V and the switching voltage of such device was only 3.4 V with an extinction ratio of about 26 dB. Ahn and co-workers reported on an all-polymer wavelength selector based on a 16-channel electrooptic polymer switch array between two polymer arrayed waveguide gratings [148]. The EO polymer material was PI-DAIDC having an EO coefficient of about 30 pm/V and the switch array was based on a Mach-Zehnder interferometer structure. Switching voltages of about 10 V with extinction ratio of more than 18 dB was attained. Sarailou et al. [149] presented an optical switch that is capable of tuning of about 0.2 nm in an applied field of about 116 V/μm. They inscribed a depth grating having 474 nm period into a NLO polymer functionalized with azo chromophores by using an interferometric set-up based on a continuous wave laser with frequency in the absorption band of the NLO polymer. On the basis of Pockels effect the refractive index of the NLO polymer can be changed by an applied external electric field and a change in the transmission spectrum as a function of the wavelength can be measured. This configuration can act like a modulator, a tuneable filter, a Bragg reflector and ultimately also as an electro-optic switch. Ultra-fast optical switches based on one dimensional-polymeric photonic crystals doped with nonlinear-optical dyes have been recently reported by Katouf et al. [150]. They presented optical switches operating at 1,064 nm wavelength, that can be controlled either by an applied electric-field voltage or by a pump light by use of two different optical-configurations. The response time of the electro-optic switch and the all-optical switch are limited by the applied voltage and the laser used,

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respectively. The polymeric photonic crystals can be easily fabricated with low cost. For electro optic device PVK has been used as the high refractive index layer and NLO Disperse Red 1-doped PAA as the low refractive index layer and stacked 39 layers at a wavelength of 1,064 nm. The entire structure is sandwiched between two indium tin oxide (ITO) glasses. For the all-optical device the light-intensity dependent change of the refractive index through the third-order optical nonlinearity has been used employing as NLO material Styryl 9M having a large third-order NLO susceptibility. Even the concept of molecular switch is an important topic from a variety of perspectives ranging from the modelling of biological processes to the design of devices for molecular electronics. The idea of switching directly the NLO properties has recently been envisioned [151]. Lamere et al. have reported on the synthesis and characterization of a boronate chromophore built up from two push-pull NLO sub-units in a quasi-free rotation around a chemical axis. They discussed on the possibility to use such a system as a molecular NLO switch induced by an electric field [152]. Other devices, based on NLO materials, essential for data transmission applications, are the integrated wavelength filters in guided-wave optics. In particular, channel dropping filters that access one channel of a wavelength division multiplexed WDM signal, and do not disturb the other channels, are extremely useful elements for WDM communications. Such type of filters are essentially of two types those based on microring resonators and those based on gratings. In the first case the geometry is similar to that showed in Fig. 3.19 where a ring is optically coupled with a straight waveguide. The principle is similar to the microring resonator modulators since depending on parameters such as the gap between the two structures (ring and channel waveguide), the coupling length, the material, the curvature radii and so on a resonant wavelength with a very narrow linewidth of the light passing through the channel waveguide can be attained. Applying an electric field between the two electrodes the coupling parameters can be changed and as a result the resonant wavelength can be slightly modified realizing tuneable filters. High-Q polymer microring resonators in amorphous polycarbonate doped with a nonlinear electro-optic chromophore CLD-1 have been fabricated by Poon and coworkers [153] using soft-lithography replica molding. They obtained a wide tunability of the filter of 8.73 nm exploiting the photobleaching of CLD-1 chromophores with visible light. The trimming rate and range can be controlled by the concentration of chromophores, the size of the exposure area and the optical intensity. The maximum extinction ratio obtained for their resonator filters was -34 dB. Sun et al. [154] fabricated EO polymer microring resonators with feature sizes of the order of 100 nm using a scanning electron microscopy-based e-beam lithography system. Extinction ratio of more than 16 dB and quality factor of the order 103 –104 were achieved. Material employed was PMMA doped with YL124 as nonlinear chromophore in a 20% wt concentration. The coupling was adjusted by changing the size of the coupling gap, coupling length and the refractive index of the top cladding. Results changing gap in the range 0–500 nm and length in the range 40–100 μm are reported.

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Microring wavelength filters working at 1,300 and 1,550 nm using EO polymer based on amorphous polycarbonate and CLD1 as nonlinear chromophore have been fabricated by Rabiei and co-workers [130]. Rings, made exploiting optical lithography, were integrated with vertically coupled input and output waveguides. Ring radii larger than 300 μm for 1,550 nm and 200 μm for 1,300 nm were fabricated to achieve negligible bending losses. Resonators with a quality factor Q of the order of 105 at 1,300 nm was demonstrated. Moreover, filters can be temperature tuned [130, 155]. A shift of the resonance wavelength of about 5 nm was measured when the temperature was changed from 20 to 60◦ C [130]. Laterally coupled electro-optic polymer microring resonator was designed and fabricated by Balakrishnan et al. [131] exploiting PMMA and DR1 as NLO chromophores. The design consists of microrings of 200 μm radius with racetrack structures varying from 20 to 240 μm and gap from 1 to 1.5 μm to improve the coupling between the bus waveguide and the ring. Two different fabrication schemes namely reactive ion etching (RIE) and photodefinition are presented. A maximum extinction of about 12 dB and a finesse of 6 were reported. Filters based on gratings are similar to the structure already described for switch configurations schematically represented in Fig. 3.17. When surface-relief gratings are inscribed on waveguides, the grating-waveguides can act as filters to select particular signals from many arriving signals. The desired characteristics of the filter can be achieved by the selection of parameters of the waveguide and the grating such as the period, the depth, the refractive index of the material, etc. For optical filter applications, high-resolution and high aspect ratio of the gratings are important for their impact on the filtering characteristics and on the size of the devices. Typical techniques for patterning gratings on polymer films include holographic lithography, [156, 157] electron-beam (e-beam) lithography [158], laser beam direct writing [159] and phase mask lithography [160, 161]. Katchalski et al. [108] fabricated resonant grating waveguide structures based on active polymer with resonance wavelength of 1,565 nm and resonance bandwidth of about 2 nm. A very narrow-band 0.03-nm (5 GHz) phase shifted Bragg grating filter at 1,290 nm on a LD-3 polymer ridge waveguide with a blocking band of transmission of about –12 dB has been recently presented by Wang and co-workers [162]. They performed a study of the electrode effect showing that a shift of about 1 nm is possible when an electric field is applied to change the effective index. More recently, another group [163] reported on a novel fast tuneable electrooptic polymer waveguide grating with resonance wavelength that can be linearly tuned via the first-order EO effect with a high sensitivity of 6.1 pm/V. Sarailou et al. [149] inscribed a depth grating into a polyimide polymer functionalized with NLO chromophores using a continuous wave laser within the absorption bands of the NLO polymer. The resonance wavelength was around 1,560 nm with an attenuation of more than 99.5% and with a full width at half maximum (FWHM) of about 1.4 nm. Exploiting the Pockels effect the refractive index of the poled polymer was changed and a shift up to 0.17 nm in the resonance peak was measured when voltages from 0 to about 100 V/μm were applied.

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Kang et al. reported on fabrication of diffraction gratings in hybrid sol gel materials by a simple two beam interference technique with possibility to insert NLO chromophores also in hybrid organic/inorganic materials [164]. Fabry-Perot etalon devices based on materials sensitive to electric fields have been studied as tuneable optical filters. Gan et al. [165] presented in their work a hybrid organic/inorganic sol-gel material based on 3-methacryloxypropyl trimethoxysilane as precursor and TCBD (15% mol) as NLO chromophore that was covalently incorporated into the silica network. They demonstrated an optical filter having large tunability and high finesse. The EO sol-gel film was corona poled with 5 kV/cm at 170◦ C for 30 min. The entire structure is reported schematically in Fig. 3.21. The distributed Bragg reflector (DBR) is designed to have a reflectivity higher than 99% at 1,550 nm. A collimated light is normally incident on the structure and the transmitted light is analyzed through a single mode fiber. If an electrical field is applied to the EO solgel material its refractive index changes and the resonant wavelength of the etalon device shifts to a new resonant condition. A wide tuneable range (>50 nm) centered at 1,550 nm has been obtained applying a voltage between ±30 V. The FWHM of the peaks was about 2 nm. Bragg gratings directly inscribed on channel waveguides are possible in a configuration similar to that proposed schematically in Fig. 3.22. In this way a direct integration, with lower losses and a more effective coupling with channel waveguides and other devices on substrate is more reliable. Gratings can be transferred to tunnel waveguides by different methods such as reactive ion etching, electron beam direct writing and nanoimprint technique. For example, the electron-beam direct-writing method has been used to inscribe the polymeric ridge waveguide with a corrugated sidewall Bragg grating by Zhu et al. [166]. They fabricated a multiple channel band pass filter in a polymer waveguide with phase-shifted corrugated sidewall Bragg gratings. The impact of signal polarization and upper waveguide cladding on the performance of the designed filter has been investigated. A tuneable long period waveguide grating filter based on a new hybrid sol-gel material has been presented by Moujoud and co-workers [167]. This filter has an attenuation of about 22 dB and a high temperature sensitivity of around 3.3 nm/◦ C. Glass ITO layer DBR V

E-O hybrid sol-gel DBR ITO layer

Fig. 3.21 Fabry-Perot etalon device structure

Glass

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Outcoming filtered light Bragg grating inscribed in NLO channel waveguide

Incoming light

Substrate, electrode and buffer layer

Fig. 3.22 Channel waveguide made of EO polymer with a Bragg grating. A lower and an upper electrode (together with a top cladding layer) not shown in figure allow to shift the resonance wavelength of the filter

Kim et al. [168] fabricated a tuneable wavelength filter by using the ultraviolet nanoimprint technique. It consists of a Bragg grating in polymer waveguides and a heating electrode. Fabrication of the grating was accomplished by using a smart imprint stamp containing a waveguide pattern integrated with the grating pattern. The center wavelength of the filter was successfully tuned by taking advantage of the thermo optic effect in polymers. A transmission dip of about 15 dB and a 3 dB bandwidth of 0.8 nm were obtained at the Bragg wavelength of about 1,560 nm. The achieved thermo optic tuning rate was about 0.28 nm/mW, while the center wavelength was shifted from 1,560 to 1,558 nm with the electrical power consumption of 7 mW. Gratings on OG 146 polymer using e-beam direct writing and stamp transfer techniques have been recently fabricated by Kocabas and co-workers [169]. Subsequently, a BCB polymeric ridge waveguide was fabricated on the grating using reactive ion etching technique. Resonance wavelength was around 1,528 nm with a bandwidth of about 1.5 nm with a reflectivity of about 20 dB for a 1.5 cm grating length. A polymeric waveguide-type wavelength filter based on a Bragg grating has been fabricated using a simple nanoimprint technique by Ahn et al. [170] They obtained a maximum reflection of 25 dB at 1,569 nm and the bandwidth at 3 dB was about 0.8 nm for a device length of 1.5 cm. A channel waveguide with grating was fabricated in an NLO polymer thin film by means of simultaneous embossing and photobleaching [171]. A combined structure of mask/master consisting of a photomask part and a grating die part made of polyimide was fabricated. Then the mask/master was set on the NLO polymer films and both embossing and photobleaching were performed simultaneously. Profiles of the replica grating and the channel waveguide were estimated.

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Other devices that can be fabricated with NLO polymers are for example sensors for electric field. A number of sensors fabricated with EO polymers to detect electric field signals and map the electric field distribution have been implemented in the last years based on Mach-Zehnder intensity and polarization modulators. Other sensors are based on asymmetric Fabry-Perot microcavities to convert phase modulation into amplitude modulation and enhance the sampling signals. Very recently Sun et al. [172] presented a novel broadband electrooptic electric field sensor fabricated with an EO polymer microring resonator coupled to the core of a side-polished optical fiber. A sensitivity of 100 mV/m has been achieved at frequencies up to 550 MHz. Polymer microring resonators were demonstrated also for sensing biomolecules without using fluorescent labels. For example recently, microring devices, have been fabricated by direct imprinting technique and have Q factors of about 2 × 104 . This feature provides high sensitivity and a low detection limit for biochemical sensing applications. The devices were used to detect and quantify the biomolecules present either in a homogeneous solution that surrounds the microring waveguide (homogeneous sensing) or specifically bound on the waveguide surface (surface sensing). In the former sensing mechanism, the current devices can detect an effective index change of 10−7 refractive index units; in the latter, they can reach a detection limit of 250 pg/mm2 of biomolecular coverage on the microring surface [173]. Very recently, another important application of EO polymers was based on the exploitation of such materials for emission and detection of terahertz radiation. Poled polymer films of 10–100 μm thickness and having electro-optic coefficients up to 160 pm/V at 1,300 nm have been employed in terahertz emission and detection using telecommunication wavelength femtosecond pulses. A comparison with ZnTe, which is the most used material for sensors and emitters of THz radiation, showed the optimal performances of the EO polymers [174, 175]. In particular, polymers containing chromophores having an electron donor and an electron acceptor connected by a charge-transfer bridge of conjugated π-orbital electrons have been studied as sensors for trace of explosives [176]. In this case waveguide microring resonator and fiber Bragg grating structures were used to enhance the detection sensitivity. Many other applications and devices based on NLO materials are possible. We tried to give only an overview of the current state of the art in such a field but we are sure that not all the topics have been covered both because the literature in the field is huge and it would have required a dedicated book for a comprehensive review.

3.7 Conclusions Exploitation of polymeric materials for optoelectronic devices requires a deep understanding of several properties and behaviour of these materials. Many scientists from different disciplines such as Chemistry, Physics, Material Science and Engineering are involved on one side in the study of stable host materials and

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chromophores having high nonlinear coefficients and on the other side on the development of new device geometry to improve the optoelectronic performances. Some of the results cited in this review were unexpected only few years ago nevertheless we expect still extensive progress for the research in the field during the next few years. Many research groups all over the world are active in the field of NLO materials and device fabrication based on such materials. Since the amount of literature is very wide it is possible that in a completely unwanted way some of the groups involved in this subject have not been cited in this review. We apologize if this occurrence happened.

References 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.

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Chapter 4

Functional and Nanostructured Materials Investigated by XPS and NEXAFS Spectroscopies Giovanni Polzonetti and Chiara Battocchio

Abstract Surface properties of materials play an important role in a number of applications of technologically relevant materials. In the framework of the interest to the soft matter nanostructured systems it emerges the relevance of the task by surface and interfaces, providing the effectiveness of adhesion, presence of molecular organization at nanoscale and surface active sites. In this chapter the information gained about surface study of functional and nanostructured materials have been reviewed as concerning two main investigation methodologies: NEXAFS (Near Edge X-ray Absorption Fine Structure) and XPS (X-ray Photoelectron Spectroscopy). A rather extensive, even if not exhaustive, description of the fundamental basis of the two selected spectroscopies has been illustrated, developing all concepts from an elementary level. Special attention, concerning the investigation methods, has been dedicated to the use of synchrotron plants as sources of radiation with all the peculiarity deriving from this. Several selected, state-of-the-art, significant investigations have been taken as major examples and the achievable information have been discussed, ranging from simple to complex molecular systems.

4.1 Introduction The first chapter introduced us to the significance of soft matter in nanostructured systems, focusing on the meaning of self-assembly as a route to creating highly controlled and ordered structures on the nanoscale. In this framework, the properties of organic and organometallic materials can be exploited to make functional nanoscale devices. In this chapter we will confine ourselves to a general description of two main characterization methods as X-ray photoelectron spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) and their G. Polzonetti (B) Laboratory of Materials Chemistry, Department of Physics, University “Roma Tre”, Via della Vasca Navale 79, Rome 00146, Italy e-mail: [email protected]

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application to the analysis of systems so far discussed in the chapters of this book. We review the studies of the electronic and molecular structure of materials that are of interest to bioactivity and sensors technology, as well as to NLO, and examples of suitable systems are provided. In such a broad subject there are inevitably omissions. The exploitation of macromolecules at interfaces and the interaction between adsorbed molecule–substrate will be highlighted because it is a fundamental concern of nanostructured materials for functional devices. Self-organization of soft materials can be used to create nanostructures for diverse applications. The richness of structures results from the weak ordering due to non-covalent interaction between molecules. The nanostructured systems can be conveniently used in a number of applications, including the preparation of nanoparticle, the templating of nanostructures, nanomotor design, and exploitation of biomineralization and the development of functionalized delivery vectors. Functional macromolecular materials, such as those considered in this book are suitable for being investigated by X-ray photoelectron spectroscopy (XPS) also induced by synchrotron radiation and near edge X-ray absorption spectroscopy (NEXAFS). Synchrotron radiation offers several advantages for probing the electronic structure relative to traditional sources of radiation as for instance tunability, intensity and polarization. The tunability of the radiation energy is probably one of the most useful, allowing for the absorption spectroscopy and in addition can be used for selecting the surface sensitivity. The excitation of photoelectrons above the ionization threshold at 30-60 eV Ek produces the minimization of the escape depth as low as 5 Å. Another advantage of the synchrotron radiation is its polarization. Linear polarization can be conveniently used, coupled with absorption spectroscopy such as NEXAFS, allowing for determination of the orientation of valence orbitals and therefore the bond direction at surface and interfaces. Using the linear polarization of synchrotron radiation is fundamental to describe unoccupied states in molecular inner-shell spectroscopy. Because the core level electron is localized on a definite atom, spectroscopy based on core excitation into unoccupied valence MOs appear simplified with respect to valence electrons excitation spectroscopy. In both methodologies NEXAFS and XPS here considered, samples are taken under UHV and irradiated with monochromatic radiation. For the XPS, scanning of the kinetic energy of the photoemitted electrons is performed, at fixed radiation energy or after selection of the most appropriate synchrotron radiation photon energy, for instance as for enhancing or minimizing the surface sensitivity or maximizing the peak intensity. As for the NEXAFS analysis, upon selection of the appropriate energy region covering the photoemission threshold for the selected core level of the light element under investigation, the photon energy is varied and the absorption spectrum is measured in the soft X-ray region. The treatment in this chapter is relatively non-mathematical and the emphasis is on breadth rather than depth, aimed also at graduate students enrolled on PhD programmes.

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4.2 Principles of NEXAFS Spectroscopy 4.2.1 General Aspects NEXAFS spectroscopy measures the photoabsorption cross-section associated to excitation or emission of a core level electron. Incoming photons of suitable energy range, hν, impinging on the sample surface transfer to this their energy and as a consequence, ground state core level electrons are excited below or above the vacuum level (VL) to the continuum, into π∗ , Rydberg or σ∗ excited state molecular orbitals. Therefore, the NEXAFS spectral features are associated to the core electron transition into unoccupied valence levels. Because the initial state is highly localized, NEXAFS reflects the electronic structure around the absorbing atom. This aspect can provide information around the chemical state of the probed element. The NEXAFS spectrum ranges usually from few eV below the photoionization threshold up to few tenth of eV (∼30 eV) above. Analysis of the spectral features, within the selected photon energy range so far scanned, as for instance position, sharpness or broadening allows for the accomplishment of data relative to the electronic structure of the empty state for the analyzed system. A strong elemental specificity characterize the NEXAFS spectra because each of them possesses a typical core binding energy as for instance C1s: ∼285 eV, N1s: ∼400 eV, O1s: ∼530 eV and therefore each of them produce an absorption edge specific of the element. In this chapter we are mostly focusing the attention to the K absorption edges of light elements as for instance C, N, and O as measured for oligomers, polymers and biomaterials molecular systems self organized onto solid surfaces. Among these, also small molecules suitable as model systems are taken into account because of the peculiarity of low-Z molecules as for example, strong directionality of covalent bond between light atoms, short bond length and the dependence on the hybridization of the bond coupled with the polarized nature of the synchrotron radiation, gives rise to K-shell spectra with foremost, structure-sensitive resonances in the hν range starting from slightly below the threshold up to nearly above 30 eV. NEXAFS resonances are associated to core electron excitation from mostly 1s initial state to Rydberg or empty molecular orbitals (MOs) final state (see Fig. 4.1). These latter orbitals are denoted by means of their symmetry as π∗ and σ∗ ; the lowest unoccupied MO, LUMO, in molecules is usually a π∗ orbital and lies below the vacuum level, while the σ∗ occurs well above in the continuum states region. Differences observed among the two main kinds of resonances can be defined as a function of their energy position and additionally by their shape. π∗ resonances are associated to core electron transition into energy localized MO, usually denoted as 1s→π∗ transition, and appear pretty sharp, while σ∗ resonances (1s→ σ∗ ) are rather broad features. This aspect is clearly connected with the lifetime of the excited state and the reason which explains the broadening for the 1s→ σ∗ resonances is the presence of increased probability of decay to continuum states. Therefore, suitable

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Fig. 4.1 Schematic potentials (bottom) and K-shell spectra (top) of atoms and diatomic molecules. Resonances in K-shell spectra arise from electronic transitions from a 1s initial state to Rydberg or unfilled-MO final states. At the IP, corresponding to the threshold for transitions to continuum states, a step-like increase in X-ray absorption is expected. These effects lead to the characteristic spectra schematically shown in the upper part of the figure. In addiction to these “one electron” features other structures arising from “multielectron” transitions may be observed (Reprinted from Stöhr [2], with kind permission of Springer Science (2009))

parameters for at first glance classification between π∗ and σ∗ resonances are both position and shape of the spectral features. As will be described later, this report is also concerned to NEXAFS measurements performed in the angular dependence mode by means of linearly polarized synchrotron radiation. NEXAFS spectroscopy is associated to detector that either count electrons or photons; in the two cases we are dealing with the relaxation process after core excitation into empty antibonding MO. As for the most diffuse method the electron yield detection, both partial and total yield, this gives the opportunity for highlighting the surface sensitivity of NEXAFS. X-ray absorption leads to the creation of a core hole with emission of photoelectrons or Auger electrons. These electrons are originated inside the surface sample, at deepness which is a function of the X-ray energy, and on their travel to the surface undergo inelastic scattering by electron–electron, electron–plasmon and electron–phonon interaction. These events are of course influenced by the nature of the material whether metallic, inorganic or organic. The electron scattering length, or mean free path, follows the universal curve reported in

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Fig. 4.2 (a) Photoabsorption and electron production in a sample consisting of substrate atoms B and an adsorbate layer A. Only electrons created within a depth L from the surface contribute to the measured electron yield signal. (b) Electron mean free path in solids as function of the electron kinetic energy above the Fermi level. The shaded area represents the distribution tipically found for different materials (Reprinted from Stöhr [2], with kind permission of Springer Science (2009))

Fig. 4.2 as a function of kinetic energy and indicates that among the great amount of electrons produced within the sample, only those electrons that reaches the surface with sufficient energy to rise above the surface barrier can escape into the vacuum. From the above description it comes out a strong surface sensitivity of the NEXAFS spectroscopy.

4.2.2 Selection Rules A relevant aspect of the NEXAFS process derives from the selection rules and from the character of the orbitals involved in the core transition, which make the absorption strongly anisotropic. The anisotropy in absorption spectroscopy was firstly noted by Seki et al. [1] for poly-butadiene. However, the first main contribution to the use of NEXAFS absorption spectroscopy for soft matter samples at low energy edges was given by Stöhr. Stöhr and co-workers carried on a systematic investigation on adsorbed organic molecules onto metallic surface producing a bunch of data regarding the molecular orientation, the chemical interaction at the interface and the bond length [2]. The master work described by Stöhr is a reference for the spectral assignment and for the building block approaches that is attractive for interpreting and assigning the resonances to specific molecular functional groups. NEXAFS applied to organic or organometallic materials produces several useful information. First of all gives a qualitative indication about the presence of a specific chemical element in the material under examination. Secondly, by means of one of

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the synchrotron radiation peculiarity as the linear polarization, NEXAFS behave as a probe for the molecular orientation of the system under analysis, achieved from the analysis of resonances associated to specific chemical functional groups. Additionally produces information about the empty electronic structure. Surface sensitivity is quite high due to the low energy electrons originating from the relaxation process either Auger or secondary electrons usually taken as a reference source for the detector. NEXAFS and XPS are two surface sensitive spectroscopic techniques that provide similar but complementary information. XPS gives important information regarding the elemental quantitative composition and fundamental indication about the chemical state of the element under investigation. NEXAFS offers one advantage compared to X-ray photoelectron spectroscopy correlated to the lower damaging effect due to the usually lower photon energy used. When the photon energy is swept through a core level ionization potential, the absorption profile has resonance structure superimposed to the atomic profile. This structure is due to dipole transitions from the core level to unfilled molecular orbitals. Because the features are caused by dipole transitions between states of defined symmetry, the polarization dependence of these structures can be used to determine the orientation of transition dipoles with respect to the substrate. Therefore, NEXAFS spectroscopy can be used to determine the orientation of aligned molecules with or without the requirement of long range order. In the dipole selection rule, we are dealing with the change of the quantum number of the angular momentum, i.e., l= ±1. Considering a K-edge, for a 1s core level initial state, the final state MO must have a p component for an allowed electron transition, circumstances convenient for organic molecules containing mostly C, N and O with 1s core orbitals and the p orbitals involved in the bond between atoms produce MOs empty at low energy. For the case of excitation of L-edges, transitions to p states are forbidden while are allowed those to s and d containing final states MOs. Selection rule for core electron excitation into an empty state MO consider also the electric field vector E orientation, respect to the axis direction of the p orbital component in the MO final state: the transition is allowed in the case of these two are parallel and forbidden when are at normal angle. Then, electron transition into antibonding MOs are strongly influenced by the σ∗ or π∗ character of the antibonding orbitals.

4.2.3 Angular Dependent Measurements The well defined polarization peculiarity of synchrotron radiation is a valuable benefit for several spectroscopic investigations and specifically for NEXAFS measurements. Following the general subdivision of the final state molecular orbitals as vector type orbitals σ∗ , or plane type orbitals π∗ , because of their geometry and spatial orientation, these give rise to absorption which is dependent on the angle between the impinging X-rays and the sample surface, or more specifically between the electrical

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field vector associated to the radiation E and the transition moment vector, ϕ i | E · r |ϕ f . The electric field vector E of synchrotron radiation, as derived by bending magnets and the majority of undulators devices, is linearly polarized in the plane of the ring orbit. The transition intensity or the transition probability can be expressed as follows: I ∝ | ϕi |E · r| ϕf |2 = |E ϕi |r| ϕf |2 = A · cos2 δ

(1)

Where ϕ i and ϕ f are the initial state and the final state one-electron wavefunctions. δ being the angle between the direction of the electric dipole vector of the final state orbital r and the direction of the electric field vector E, A is the proportional constant. The feature intensity is strongest when the electric field vector of the incoming radiation is aligned with the transition moment vector (E · r = 1). If we operate with an experimental set up with the E vector at an angle θ comprised between parallel and normal respect to the surface normal, the intensity I is given by the equation: I(θ ) = A(cos2 θ cos2 α+1/2sin2 θ sin2 α) = A/3[1 + 1/2(3cos2 θ − 1)(3cos2 α−1)] (2) This equation indicates that the intensity I of a specific resonance can be expressed as a function of θ giving rise to the evaluation of the polarization dependence which in turns allows calculating the angle of the transition moment from the surface normal α. Given that the electric dipole vectors of the final state orbital for the two most common σ∗ and π∗ transitions are directed parallel and respectively normal to the chemical bond direction therefore, following the spectral feature assignment, polarization dependence of the peak intensity gives the direction of the transition moment providing then the information of bond direction and favourably the molecular orientation. In the previous equation the second term vanishes for cos2 θ = 1/3 meaning that for θ = 54, 7◦ the peak intensity is independent on the angular incidence; this is the so called magic angle. From an analysis of the σ∗ and π∗ symmetry it comes out that allowed transitions associated to these orbital final states have opposite polarization dependence. σ∗ MOs associated to σ bonds have a linear shape along the bond direction, while π∗ orbitals associated with π bonds are strictly connected with p-type orbitals perpendicular to the bond direction. Therefore, by applying the proper equations accounting for the angle of incident radiation on the sample surface and the resonance intensity, the angular arrangement of the final state orbital can be achieved, thus allowing for the determination of the orientation of specific functional chemical groups and afterwards the whole molecule under investigation. In fact, excitations to an antibonding σ∗ orbital is allowed for the E parallel to the bond direction, whilst excitation into a π∗ empty orbital is allowed for E perpendicular to the bond direction. Resonances intensity is therefore, strongly dependent

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Fig. 4.3 Polarization dependent NEXAFS spectra of benzene chemisorbed on Ag(110), illustrating the capability to determine molecular orientations (Reprinted with permission from Stöhr [2])

on the geometry of X-ray impinging on the sample surface and for a molecular ordered arrangement it is expected the maximization of the resonance intensity or conversely, vanishing of the resonance for the two cases, as shown in Fig. 4.3. In the presence of samples under analysis showing molecular order arrangement, the NEXAFS spectral features intensity will be perturbed by performing angular resolved measurements, and the features will exhibit clear evidence of intensity dependence. This dependence can be conveniently used for the determination of molecular orientation in the sample. Experimentally the angular dependence can be achieved by performing NEXAFS measurements on the same photon energy range at different angle of incidence of the radiation.

4.3 Experimental Methods NEXAFS spectroscopy basically does not require the most sophisticated apparatus to be performed but a source of tunable radiation as that dispensed by a photon factory or synchrotron plant. The experimental station for the study of macromolecular materials requires a UHV system and a detector apparatus for counting the emitted electrons. The primary process in NEXAFS is the core electron excitation into an appropriate final state empty molecular orbital. After excitation, the whole system undergoes relaxation and this can occur through two main decay processes; secondary or Auger electron emission and fluorescence emission. Mostly, the detector for NEXAFS uses a simple channeltron tuned for a specific Auger energy or tuned to collect the whole secondary electrons resulting from the relaxation process; fluorescence detector are also relatively common; alternatively, for sample insulator the measurement of the drain current from the conductor sample holder is often measured; examples are displayed in Fig. 4.4. Measurements can be performed on gas, solid and recently liquid state [3].

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Fig. 4.4 Two methods of recording X-ray absorption spectra: transmission and electron yield. The transmission technique requires thin foils while the electron yield technique, often called total electron yield (TEY) detection, can be used for conventional samples. The absorbed X-ray intensity is not measured directly in TEY measurements, but rather the photoelectrons that are created by the absorbed X-rays (Reprinted with permission from Stöhr [2])

4.4 Spectral Assignment Resonances characteristic in K-shell NEXAFS spectra are associated to the representative electronic level originating from the sequence in energy which gives first the π∗ levels, occurring below the ionization threshold, then the Rydberg series orbitals, still below ionization energy, while above the ionization potential (IP) resonances are associated to transitions into σ∗ antibonding orbitals. This simple approach can help in subdivide the spectral region between π∗ and σ∗ resonances. Let us consider in addition, that in a molecule the antibonding molecular orbital involved in the excitation process is delocalized above two, three or more atoms in the molecule itself. Therefore, a specific resonance observed in the K-edge NEXAFS spectrum measured for one atom must be detected also for the spectrum of the partner atom in the molecule. For organic-like molecular systems, which is the case treated in this chapter, there have been many studies about the K-edge spectra for light elements. However, the simplest method follows the building block approach that allows for the assignment of the spectral features to the correct resonances. Furthermore, angular dependence supply additional information connected to the specific character of the observed resonance. This method allows for unambiguous assignment to π∗ or σ∗ resonances when molecular order occurs at the sample surface because these resonances have opposite angular dependence and

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therefore, in the whole spectrum all the π∗ resonances behave in the same way, conversely σ∗ resonances follow the same trend but in an opposite way. By this scheme it is also feasible to detect spectral features that are not associable to π∗ or σ∗ , which most often originate from multi-electron processes. As a general approach, for complex molecular systems it is quite common to refer to a building block approach which consists in expecting specific spectral features associated to specific molecular functional groups, as for instance: >C=O, , –Ph, –O–H, –C≡N, etc, and therefore, assign the spectral –C≡C–, > C=C<, features as a function of the molecular structure. This rule has been proposed by Stöhr [2] on the basis of a systematic study on simple molecules containing the same structural base and introducing successive functional groups with increased complexity, thus providing a large variety of correlation between functional groups and NEXAFS resonances energy. In the following we report some example for the building block modelling: H3 C–CH3 , H3 C–C≡CH and HC≡CH; H3 C–C≡N and H–C≡N; molecules with only single, also double and triple carbon–carbon bond as for instance: H3 C–CH2 –CH2 –OH, H2 C=CH–CH2 –OH, H–C≡C–CH2 –OH. Stöhr has shown the general goodness of the building block approach, as well as the restriction of the model in the case of presence of conjugation or extended delocalization involving a number of π orbitals all over a number of atoms and the presence of π bond–bond interaction; in these cases the effect is the augmented spectral complexity by an increased number of features. In these cases the spectral assignment can be doubtful and therefore theoretical calculations are required to fully account for the experimental features.

4.5 Application to Molecular Systems 4.5.1 Nanostructured Systems The construction and organization of surface structures on a nanoscale level is fundamental for the growth of surface with novel and valuable properties. Using self-assembly of macromolecules to form surface nanopatterns is rising as a promising and flexible technique [4–11]. One of the main advantages of this process is that large surface areas can be customized promptly and at relatively low cost. The advance toward increasingly smaller sized electronics has turned interest toward organic molecules as possible circuit element/components as for instance conductor, rectifier, transistor, logic gates. In this framework, molecules with extended π-conjugation have received particular interest. Independently from the molecular functional peculiarity, as for instance electrical conduction or terminal sensing chemical groups, the required electrical contact connecting the molecular system and the device through grafting to a metal, oxide or semiconductor surface is usually accomplished by means of chemical bonding between the headgroup of the molecule and the surface. Molecular systems can then grow in layers and give rise to spontaneous organization over a surface therefore producing self-assembled monolayers (SAMs). Evidently, besides to the chemical

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affinity involving molecules and substrate, the SAM structure is governed by the interplay between intramolecular forces and the interaction between surface and head groups. Angular dependent NEXAFS measurements performed on nanostructured Oligo(phenyleneethynylene) derivatives SAMs deposited on gold have been analyzed also as a function of laterally extended π-system [12]. Molecular structures of the investigated systems are reported in the following Fig. 4.5. The measurements were performed at the C K-edge, with the incidence angle of the linearly polarized synchrotron radiation varied from 90º to 20º. Through the analysis of the spectral features with special attention to the C1s→π∗ sharp resonance at nearly 285 eV photon energy, assigned to aromatic ring carbon atoms, the authors observed a splitting of this peak allowing for distinction between benzene ring carbon of the main chain and benzene-like carbon of the side chain (naphthalene and anthracene) energy shifted to 285.6 eV and associated to the presence of symmetry-independent carbon atoms [13–16] (see spectra in Fig. 4.6). By the analysis of the transition dipole moment orientation relative to the polarization of the incident radiation the authors evidence the tilting of the molecular axis from 30º for the basic system, to 40º for naphthalene and respectively to 45º for the anthracene side groups, as shown in the following Fig. 4.7. Comparison between two thiol (-SH) terminated molecules as the 1,1 ;4 ,1’’terphenyl-4-thiol (TPT) and the analogous molecule after substitution of carbon with nitrogen in the ring, 6-(5-pyridin-2,yl-pyrazin-2-yl)pyridine-3-thiol (PPPT) have been investigated by NEXAFS spectroscopy [17] (Fig. 4.8).

Fig. 4.5 OPEs with benzene (1), naphthalene (2), and anthracene (3) as the central aromatic moiety combined with the coordinate system used for the molecules (Reprinted with permission from Nilsson et al. [12]. Copyright (2009) American Chemical Society)

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Fig. 4.6 Angular dependent C K-edge NEXAFS spectra collected on a nanostructured Oligo(phenyleneethynylene) derivative SAM deposited on gold; the spectrum difference Normal(90º)-Grazing(20º) is also reported (Reprinted in part with permission from Nilsson et al. [12]. Copyright (2009) American Chemical Society)

SAMs of PPPT growth on Au and Ag as substrates have shown by NEXAFS to be well-ordered and densely packed. Attachment to the substrate occurred grafting via the thiolate head group, with the pyridine-pyrazine backbone showing an upward orientation with tilt angles of 32º and 35º for PPPT/Au and PPPT/Ag respectively. Strong similarity is detected among the two substrates contrarily to what expected

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Fig. 4.7 (a) Drawing defining the angles that determine the SAM molecules orientation with respect to the Au(111) surface and the direction of the incident X-ray radiation; (b) drawing showing how changing the aryl moiety from benzene to naphthalene to anthracene results in larger molecular inclination (Reprinted in part with permission from Nilsson et al. [12]. Copyright (2009) American Chemical Society)

from literature [18–20]. NEXAFS data allowed the authors to explain the behaviour of the material with nitrogen and the observed reduced conductivity. Differences between TPT and the nitrogen substituted homologue (PPPT) is observed in the lower packing density and less orientational order, in addition the nitrogens in the aryl backbone give rise also to an electron withdrawing effect that stabilizes

Fig. 4.8 (a) The molecular structure of thioles PPPT and TPT; (b) C K-edge NEXAFS spectra of PPPT/Au collected at X-ray incidence angles of 90◦ , 55◦ and 20◦ , along with the respective grazing-normal incidence difference spectrum (Reproduced with permission from Grave et al. [17]. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA)

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the valence molecular electronic structure and pull negative charge from the thiol sulphur; the conclusion about the nitrogen atoms effect was the distortion of intermolecular interaction within the film and a weakening of the thiolate bond to the metal surface.

4.5.2 Organometallic Macromolecules In order to search for materials with specific properties in terms of conjugation and tendency to organize in a structurally convenient mode, molecular system made mainly by organic aromatic backbone structure can be coupled with a suitable metal complex. The purpose is to achieve both properties as from the organic and the inorganic components. Among these organometallic materials, only systems composed by an organic backbone alternating σ and/or π-bonded metal atoms within a molecular chain or inside a macrocycle will be reviewed in this chapter. Organometallic macromolecules such as metalloporphyrins and phtalocyanines are of considerably current interest, particularly motivated by their usefulness as building blocks in molecular materials for electronic applications. These organometallic complexes are ubiquitous in nature and are functional in a wide variety of roles ranging from oxygen transport, electron transfer, and oxidation catalysis to photosynthesis. Due to their photochemical (energy and exciton transfer), redox (electron transfer, catalysis) and coordination (metal and axial ligand binding) properties, these are among the most important fine chemicals in industry and are utilized in an ever-expanding array of applications ranging from their use as molecular optoelectronic gates, molecular wires, photoinducible energy or electron transfer systems, light-harvesting arrays, unidimensional conductors and semiconductors, nonlinear optics and sensors devices [21, 22]. Two examples are displayed in Fig. 4.9. NEXAFS spectroscopy is ideally suited for the study of these systems for several reasons; as a start, it probes the local unoccupied electronic states of the atoms absorbing X-rays, providing information on the chemical states. Moreover, it is capable of giving insight about the spatial orientation of the probed states, e. g. for molecules, the molecular orientation. What is more, as a synchrotron-based spectroscopy, NEXAFS effectively probes the molecular system in a light-induced state, which is crucial for all electro-optics applications mentioned above [23].

Fig. 4.9 Molecular structure of organometallic macromolecules; (a) Ni phtalocyanine; (b) Zn diethinyl porphyrin (ZnPf)

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In particular, 3d transition metal phthalocyanines (Pc) and porphyrins (Pf) are widely used in many technological applications due to their unique electronic structure, which has been intensely investigated during the last decade [24, 25]. The central part of these complexes, including the 3d-atom and its nearest neighbours, is known to be their most reactive component defining the most important applications of these compounds. The main responsible for the interesting properties of these compounds are the occupied and empty 3d electron states of the metal atom, wich are located near the Fermi level and are involved in chemical bonding. High resolution X-ray absorption spectroscopy, due to its site and symmetry selectivity, is a powerful tool to investigate the 3d-related features of chemical bonding; more specifically, metal atom 2p absorption spectra are of particular interest because they probe directly the unoccupied electron states with the transition metal 3d character [26]. In fact, in this case the dipole-allowed transitions are 2p-3d and 2p-4s, but transitions to 3d states dominate over those to 4s states. Focusing on this topic, Krasnikov et al. [27] reported an X-ray absorption investigation aimed at revealing the chemical bonding features of Ni phthalocyanines and Ni porphyrins with different peripheral substituents; in this work, the authors focus their attention on the Ni atom and its nitrogen-type coordination sphere, by acquiring and discussing Ni L-edge NEXAFS spectra. Absorption spectra were taken for Ni(II) – Pf, Ni(II) – Pc and Ni metal as well. Ni2p spectra of the two organometallic macromolecules are quite similar, as shown in the following Fig. 4.10, with a main feature A accompanied by a high-energy lines A2, C and D, resulting from the Ni2p3/2 – 3d,

Fig. 4.10 Ni2p NEXAFS spectra of NiPc and Ni porphyrins compared to Ni2p absorption spectra of the Ni metal. The spectra are normalized to the same intensity of band A (Reprinted from Krasnikov et al. [27] Copyright (2009) with permission from Elsevier)

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4s transitions and observed over the 10–12 eV energy range above the Ni2p3/2 absorption onset. The absorption spectrum of Ni metal shows only one main line A, accompanied by a satellite A∗ (attributed as due to the two-hole bound state [28] or to multiple scattering from the environment at intermediate range [29], depending on the interpretation). On the basis of comparison with the Ni L-edge NEXAFS spectrum of Ni metal and the following comparative analysis of the Ni L-edge and N K-edge spectra of the NiPc and NiOEP, the main line A can be associated with transitions of Ni2p3/2 electrons to the empty electron states with nearly pure Ni3d character, while the structures A2, C, C∗ and D, deal with transitions to the empty states with hybridized Ni3d–N2p character. The high-energy shift of the Ni2p3/2 absorption in going from Ni metal to NiPc (1.4 eV) and NiPf (1.2 eV) is a result of a decrease in the effective number of 3d electrons on the Ni atom due to the strong Ni3d–N2p covalent bonding; this effect is well known as the back-donation [30], and gives rise to a delocalization of the 3d states, decreasing the 3d electron density at the Ni atom then producing a lowering of the screening of the Ni2p–3d electron transitions and an increase in their energy (high-energy shift). This metal-to-ligand charge transfer in the case of NiPc and Ni porphyrins occurs from the occupied Ni3dxy,xz π (e.g.) orbitals to the unoccupied N2pπ∗ (eg) orbitals, which is confirmed by DFT calculations [31], resulting in the corresponding absorption bands (A2, A3) in the Ni2p NEXAFS spectra. The experimentally observed energy shift of band A can be used as a qualitative, and possibly as a quantitative, characterization of back-donation strength in similar nickel compounds. In the field of electronic devices, Pf and Pc are currently used as active organic overlayers to manufacture organic light-emitting diodes (OLEDs), organic fieldeffect transistors (OFETs) and organic solar cells [23]. W. Chen et al. [32] reported a NEXAFS investigation performed on CuPc/SAM/Au(111) systems obtained depositing by sublimation a Cu(II)Pc layer onto substrates of Au(111)/mica previously functionalised with SAMs. As SAMs the Authors choose two functionalized aromatic thiols, i.e., 4-trifluoromethyl-benzenethiol and 4-methyl-benzenethiol, and a charge transfer from CuPc to the SAM containing the strong electron-withdrawal group CF3 was assessed by SR-XPS spectroscopy (see Section 7.2 of this chapter). NEXAFS measurements were carried on in angular dependent geometry to characterize the molecular orientation of CuPc on SAM/Au, aiming at elucidating the effect of the charge transfer occurring at the CuPc/CF3 -SAM interface on the CuPc supramolecular organization. NEXAFS spectra were collected at the N K-edge as a function of the synchrotron light incidence angle ; the first three sharp resonances observed are assigned to N1s→π∗ transitions, and the broader peaks at higher photon energy to transitions to σ∗ states. Since for the disklike CuPc molecules the σ∗ and π∗ orbitals are directed essentially in-plane and respectively out-of-plane, from the intensity ratio between the sharper π∗ resonances collected at = 20◦ and = 90◦ it was possible to estimate the tilt angle α between the molecule plane and the substrate, resulting in an essentially standing-up configuration for CuPc/SAM/Au systems. Therefore, the charge transfer occurring at the CuPc/CF3 SAM interface has negligible influence on the CuPc molecular orientation. Planar π conjugated molecules such as pentacene and CuPc usually lay flat on gold electrodes

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and stand up on SiO2 gate dielectric channels [33, 34]; such orientation transition results in the formation of disconnected grains across the channel/gold interface in bottom-contact OFETs [32, 35]. By modifying gold electrodes with SAMs it is possible to force CuPc to the standing up geometry, eliminating the orientation transition as well as the disconnected grains across the channel/gold interface, thereby reducing the contact resistance and improving the device performance in bottom-contact OFETs. Due to their electron donor character, metalloporphyrins have been fruitfully coupled with electron acceptor macromolecules as C60 derivatives for the synthesis of supramolecular systems that can mime the photosynthetic energy conversion process, leading to a promising route to obtain cheap, mass produced solar-cells [36]. On this subject, Arima et al. [23] reported a NEXAFS and XPS investigation performed on cobalt tetra-butyl-phenyl porphyrins (CoTBPPf) immobilized on gold surfaces via ligation to self-assembled monolayers of aromatic aminothiophenols (4-ATP), both in their native state, and with ligated fulleropyrrolidines N-methyl-2(p-pyridyl)-3,4-fulleropyrrolidine (Py-C60 ), forming charge-separation complexes which may have applications in solar cells (see the following Fig. 4.11 for the assembly molecular structure). Photoemission spectra, that will be discussed in the following Section 7.2 of this chapter, appeared dominated by the individual CoTBPPf and Py-C60 components thus suggesting that the charge distribution arising in the complex CoTBPPf/Py-C60 assembly does not influence the initial electronic states. NEXAFS measurements, on

Fig. 4.11 (a) Molecular structure of N-methyl-2-(p-pyridyl)-3,4-fulleropyrrolidine (Py-C60 ); b) 4-ATP-CoTBPPf-Py-C60 system (Reprinted from Arima et al. [23], Copyright (2009), with permission from Elsevier)

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Fig. 4.12 (a) N K-edge NEXAFS spectra of 4-ATP (incidence angle of 50◦ ), 4-ATP-CoTBPP (incidence angle of 80◦ ) and 4-ATP-CoTBPPf-Py-C60 (incidence angle of 0◦ ); (b) C K-edge NEXAFS spectra of 4-ATP-CoTBPPf-Py-C60 and Py-C60 , both taken at 0◦ (Reprinted from Arima et al. [23], Copyright (2009), with permission from Elsevier)

the other hand, gave evidence of a charge redistribution involving the empty electronic states. N K-edge and C K-edge NEXAFS spectra collected on the investigated systems are reported in Fig. 4.12. The N K-edge spectrum of the metallo porphyrin shows features that are photon energy shifted by more than 1 eV to lower values in the CoTBPPf/Py-C60 assembly with respect to the pristine CoTBPPf. Since N1s initial states measured by XPS are unchanged, this effect suggests that the Py-C60 ligation strongly modifies the energies of the excited states of the CoTBPPf, due to the inherent dipole in the excited state of the donor–acceptor complex. In addition, the two main peaks observed in the C K-edge spectrum of the CoTBPPf/Py-C60 assembly, when compared with the spectrum of the metalloporphyrin, present two non equivalent energy shifts towards higher energies (0.28 eV for the low energy feature and 1.2 eV for the other), reflecting the different interactions in the possible donor-acceptor excited state complex. Supramolecular assemblies of metalloporphyrins and fullerenes have been also investigated by some of the authors of this book [37]. In an extensive work concerning the NEXAFS and XPS investigation of zinc diethinyl porphyrins vapour deposited on a Cu(111) surface and its interaction with increasing amounts of C60 , a tilting of the macrocycle on the substrate surface, from flat geometry to standing

Fig. 4.13 Drawing showing the molecular arrangement of fullerenes and ZnPf, with the diffusion of C60 spheres throughout the ZnPf layers

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up position as a function of the substrate coverage, was observed together with a charge transfer occurring from the porphyrin macrocycle to the fullerene species accompanied by a diffusion of the C60 spheres throughout the ZnPf layers (see Fig. 4.13).

4.5.3 Biomolecular Systems Among the large variety of molecular systems possessing properties connected to the interface formation between organic system and cellular growth, avoiding any reject reaction, are inorganic base as well as organic base substrates. NEXAFS spectroscopy with the basic variation in the absorption fine structure of peptides and proteins supplies a suitable tool to identify and map these in different biological milieux. Providentially, the spectra of large molecules often look like a sum of the functional groups present allowing for the building block assumption [2]. Although this approach is usually applied to small molecules as for instance benzene and alanine [38] systems with increased complexity as peptides built by two or three monomer units have been used to demonstrate the applicability of the building block approach [39]: the spectra of the peptides are similar to the sum of the spectra of the amino acid components [40]. The first X-ray absorption investigation of amino acids and small peptides attempting to find a correlation between experimental spectral features and the composition of peptides and proteins was reported by Boese et al. [41] (see the following Fig. 4.14). In this work the authors tried to state that if the effect of the peptide

Fig. 4.14 NEXAFS C K-edge spectra and chemical structures of the six amino acids glicyne (Gly), phenilalanine (Phe), histidine (His), tyrosine (Tyr), tryptophane (Trp) and arginine (Arg). (Reprinted from Boese et al. [41], Copyright (2009), with permission from Elsevier)

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bond is weak, then the spectrum of a protein will be just the weighted sum of the constituent amino acid spectra. An accurate work focused on selecting between the modifications occurring upon peptide bond formations and the change in the zwitter-ions/neutral molecules ratio within the sample [42], has been reported for glycine and glycine-based oligopeptides as diglycine, triglycine and 2,5-diketopiperazine. Through the analysis of the spectral features in the C, N and O K-edge spectra and correlation between spectral changes, as for instance the main π∗ -resonance shift in the carbon spectrum or an increased intensity of the analogous feature in the nitrogen spectrum, these authors came to the conclusion that the changes are not exclusively related to the peptide bond formation but significantly influenced by changes occurring to the zwitter ion/neutral molecules ratio. High quality spectra at the C, N and O K-edge for the most common 20 amino acids in their zwitterionic/neutral form allows the prediction of any kind of peptide or protein by means of a weighted sum of the spectra of the amino acids units [43]. It has been shown [43, 44] that in principle, carrying out NEXAFS measurements in conditions of satisfactorily accuracy and good resolution, peptides built by less than 50 aminoacidic units as well as protein possessing atypical sequences (e.g., rich in cysteine and methionine), should be differentiated. This expectation comes from the experimental evidence that among the most diffused amino acids, each of them produces a separate NEXAFS spectrum. Hitchcock and his group have made an extensive work about this topic and recently have developed a software tool for quantitatively predicting the C1s, N1s and O1s NEXAFS spectra of peptides and proteins from their amino acid sequences, within a customized building block approach [45]; the approach considers the summation, based on the sequence of amino acids, and furthermore take into account an alteration which mimics the spectral modification arising from the structural changes correlated with peptide bond formation [46] (see Fig. 4.15). In this study these authors presented a successful quantitative investigation of a spatially non-uniform peptide-protein blend by scanning transmission Xray microscopy (STXM) [47–49] performed at the STXM [50] beamline at the Advanced Light Source (ALS), using the fundamental C1s spectral diversities; sample preparation was made by depositing a few microliters of deionized water solution onto a Si3 N4 window and dried in air. Actually, STXM is a version of NEXAFS spectroscopy based on microscopy, which brings together excellent energy resolution (<0,1 eV) with a spatial resolution ( ∼40 nm) comprised between the values usually achieved by optical techniques and electron or scanning probe microscopy. NEXAFS spectra simulation of proteins, by analyzing the complete 3D structure of a ribonuclease, has also been done in order to visualize the polarization dependence for the N1s→π∗ amide and S1s→σ∗ s−s excitations in the NEXAFS spectra of an oriented sample. By this approach, Liu et al. [51] have shown that polarization dependent absorption spectra could be used to classify oriented and unoriented protein molecule as ribonuclease. A merely computational investigation by the STEX approximation has been reported by Carravetta et al. for large amino acids such as glycine, phenylalanine,

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Fig. 4.15 Illustration of the NEXAFS building block model with a tetrapeptide (IWRK-NH2 ): (a) C1s spectra of the individual amino acids; (b) ball and stick model of IWRK-NH2 ; (c) simple sum of C1s spectra of the four amino acids, compared to the experimental C1s spectrum of human serum albumin (Reprinted with permission from Stewart-Ornstein et al. [45]. Copyright (2009) American Chemical Society)

histidine, tyrosine and tryptophan [52]. The simulated NEXAFS spectra resulting from this study have been compared with experiment. By this way these authors claim that the different subunits can be clearly identified and that their spectral features remain largely unperturbed in the different molecules. Measurements at the carbon K-edge has been reported for the most commonly occurring in nature 20 amino acids [44] and comparison has been performed with extensive ab initio calculations with the static-exchange approximation (STEX) [53]. The conceptual approach named building block principle is limited in complex molecular structures by the presence of delocalization of electronic charge across multiple functional groups. However, also in this case the authors have approached the spectral features analysis attempting to highlight the contributions of individual units and substitutional groups in order to survey their fingerprinting character using the building block model. Among the features occurring in the NEXAFS spectra of the investigated amino acids, the clear identification of the pronounced feature due to the carboxyl group C1s→π∗ C=O at 288,6 eV and to the modified phenol ring C1s→π∗ C=C with a sharp structure around 285 eV have been assessed with the aid of calculations. The identification of other building blocks such as the CNHn group and the CH, CC, CO, CN pair bonds has also been achieved although their spectral features are less pronounced in the C K-edge spectrum. Considering the peptides and proteins adsorption on solid surfaces we must remark that this is a quite complex route influenced by several factors as for instance the state of hydratation of both biomolecule and surface, the molecular structure, the intermolecular interaction and the ambient conditions [54]. In several biomaterials using a key attempt is taking place to organize protein adsorption, by structuring and/or chemical patterning the surface at a sub-micrometer scale [55–57]. The absorption of human serum albumine (HAS) on the surface of polystyrene/poly(methyl methacrilate) (PS/PMMA) mix together as a function of

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concentration, exposure time, pH, temperature, ionic strength and competitive adsorption has been studied by the Hitchcock group by STXM and X-PEEM (Xray photoemission electron microscopy) [58]. X-PEEM is a modification of the total electron yield method, which strongly highlights low kinetic energy secondary electrons [59] and has proved to be suitable for mapping the polymer surface at good spatial resolution [60] allowing for identifying the favoured sites of fibrinogen grafting at submonolayer levels onto a PS/PMMA surface, as shown in the following Fig. 4.16.

Fig. 4.16 (a) Color-coded component map (left top, rescaled) for the (0.005 mg/mL, 20 min) albumin-covered PS/PMMA blend sample. The color wheel which allows the viewer to determine the composition in mixed regions. (b) Masks used to extract spectra of specific regions. [red = PS > 4 nm, green = PMMA > 4 nm, blue = PS/PMMA interface (all pixels not identified in the masks of the PS and PMMA domains)]. (c–e) Curve fits to the average C1s spectra extracted from the masked regions (data, points; fit, thick solid line; components, thin lines) (Reprinted with permission from Li et al. [58]. Copyright (2009) American Chemical Society)

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The application of a phenomenological building-block model to the NEXAFS spectra achieved on two-dimensional bacterial surface protein layers (S layer) of Bacillus sphaericus NCTC 9602 has also been made [61] (see Fig. 4.17). This system is commonly used as a protein template for the bottom-up production of advanced metallic and hybrid nanostructure. Each unit cell of this S-layer lattice is composed of four identical protein subunits and each subunit consists of 1050 amino acids. Polarization dependent NEXAFS investigation has been performed on samples deposited onto oxidized Si(100) surface from a suspension of protein in MgCl2 solution at pH 7,4; the achieved results at the C1s, N1s and O1s adsorption edges were useful allowing for determining that the crystallographic axes of the deposited S-layer sheets are statistically oriented [46].

Fig. 4.17 Angular dependent NEXAFS spectra of S layer of B. sphaericus NCTC 9602 collected at N K-edge and O K-edge (Reprinted with permission from Vyalikh et al. [61]. Copyright (2009))

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4.5.4 NLO Molecules Many oligomers as well as polymers by the appropriate combination of a pushpull electronic activity, originating from functional groups opportunely linked at the organic base system, produce materials suitable for NLO activity.

Fig. 4.18 Idealized schematic diagram of a polar Zr-P multilayer. The bracketed arrows represent polar chromophores (Reprinted with permission from Katz et al. [69]. Copyright (2009))

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There is a great scientific and technological interest in the synthesis and study of the properties of molecule-based materials having large second-order optical non linearities due to their potential applications in second harmonic generation (SHG), electro-optic and photorefractive devices [62, 63]. Molecule-based second-order NLO materials offer many attractions such as non-resonant response, ultrafast response time, low dielectric constants and intrinsic architectural tailorability [64, 65]. The essential requirement for efficient bulk second-order NLO materials is a noncentrosymmetric organization of the high molecular hyperpolarizability constituent chromophores, and the construction of such acentric supramolecular assemblies is a difficult task for conventional synthetic methodologies [66], among which the most commonly employed are the poled-polymer [48, 49] and the Langmuir-Blodgett [67] film transfer approaches. The exploitation of molecular self assembling properties is an attractive alternative approach to second-order NLO materials since it targets the construction of covalently linked, intrinsically acentric superlattices containing molecular chromophoric subunits [68, 69]. In Fig. 4.18 is displayed a schematic diagram of a SAM multilayer that, functionalized with appropriate chromophores, would show second-order NLO properties.

Fig. 4.19 Chemical structure of a PAMAM dendrimer. Arrows A point to selected tertiary nitrogen atoms from two neighboring dendrite branches; dashed arrows B point to carbonyl oxygen atoms from two neighboring dendrite branches. The twodimensional representation is shown for schematic purposes only (Reproduced with permission from Bubeck et al. [70]. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA)

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As already introduced in Section 2.4, angular dependent NEXAFS spectroscopy is a powerful tool in the study of self assembled systems, and was successfully applied to the investigation of self assembled monolayers and multilayers of molecular-based NLO materials. Among others, Bubeck and co-workers [70] investigated the interaction of Cu2+ with C, N and O moieties in nanocomposites containing copper ions complexed in poly(amidoamine-organosilicon), PAMAMOS (dendrimer molecular structure is shown in Fig. 4.19). The dendrimer networks were investigated using near edge X-ray absorption fine structure (NEXAFS) at both C and N K-edge (see Fig. 4.20). The effects on the 1s→π∗ and 1s→σ ∗ orbital transitions were determined at the C, N and O K-edges. The influence of Cu2+ on the transitions of 1s → π∗ (N–H) at 401 eV, and 1s→σ ∗ (N–C) type at 412 eV, within the N K-edge increased with increasing Cu2+ content up to the theoretical limit of the copper-accommodating capacity of the used PAMAMOS dendrimer. As ligands, the carbonyls were found to saturate at much lower Cu2+ concentrations compared with those associated with N.

Fig. 4.20 (a) NEXAFS C K-edge spectra of PAMAMOS [4,1]DMOMS dendrimer networks for three wt.-% Cu2+ contents (CuCl2 ) plotted above “0” and their difference spectra plotted below “0”; b) NEXAFS N-K edge spectra for PAMAMOS [4,1]DMOMS dendrimer networks for six wt.-% Cu2+ contents (CuCl2 ) plotted above “0” and their difference spectra plotted below “0” (Reproduced with permission from Bubeck et al. [70]. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA)

4.6 Principles of XPS Spectroscopy 4.6.1 Background Among the number of surface sensitive techniques, photoelectron spectroscopy is one of the most used either by chemists, physicists and material scientists. XPS infact provides information from the sample surface and relative to the presence,

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the occurrence and the chemical state of the elements constituents of the material under analysis. Incoming photons of appropriate energy, hν, impinging on the sample surface transfer to this their energy and as a consequence, core and valence level electrons are excited above the vacuum level (VL) to the continuum and leave the atom in the sample with a residual kinetic energy (Ek). Analysis of the Ek distribution of the photo-emitted electrons allows for the achievement of the information relative to the chemical element under investigation and the calculation of the binding energy (BE) of the selected electron (see Fig. 4.21). Given that the calculated BEs of core level electrons are all well known for all the chemical elements in their natural state (for example, carbon in graphite), the identification of each element can be easily accomplished by simply verifying in the spectrum the presence of contributions at the expected energy (es. 285 eV BE for the C1s core level and consequently carbon atoms in the sample), and this can be done for all the elements except for hydrogen. Respecting the principle of energy conservation for the whole process, we can write:

BE = hv − Ek −

(3)

where through the balance of the energy transferred from the excitation source, hν, the experimentally measured kinetic energy Ek, and accounting also for the analyzer work function , the initial state binding energy BE of the photoemitted electron can be achieved.

Fig. 4.21 (a) Schematic diagram of photoemission process from a core level; (b) Ti2p core level spectrum of TiO2

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4.6.2 Experimental Methods In order to carry on an XPS experiment and therefore accomplish the measurement of the kinetic energy of photoelectrons emitted from a sample, there are some requirements that must be fulfilled regarding the experimental apparatus. First of all, the instrumentation must include an X-ray source of radiation that can be either discrete and for instance the most commonly used are Al Kα and Mg Kα or alternatively have access to a synchrotron radiation plant. In the first case aluminium or magnesium metallic anodic surface positively polarized up to several thousands of V, typically 12–15 kV, is bombarded by electrons extracted from a powered filament; the result is an energy transfer to the core level electrons of Al or Mg giving rise to excited electronic states that subsequently undergo relaxation by means of X-ray emission. The radiation in this way emitted exhibits a discrete energy and appear sufficiently monochromatic; for the Al Kα1,2 primary emission line the corresponding photon energy is hν =1486,6 eV while for Mg Kα1,2 we have hν=1253,6 eV. Photons originating from these sources are not properly monochromatic for two main reasons, as the primary relaxation process 2p3/2,1/2 → 1s involves a transition between an excited state core level doublet and a single final state ( Kα1,2 ), and in addition contains several secondary lines as the K α , K α 3 , K α 4 , . . ., kβ emitted by means of transitions from multiply ionized states→ 1s or from Valence Band→ 1s transition (Bremsstrahlung continuum). These conventional X-ray sources suffer two main limitations regarding the photon flux, strictly connected to the energy of the incident electrons and the accelerating voltage, which is limited by the cooling system of the source, and the non monocromaticity of the radiation. Selection of an individual X-ray line from the unresolved Kα1,2 doublet, elimination of satellites and removal of Bremsstrahlung continuum can be achieved by monochromatization [71]. This technique depends on dispersion of X-ray energy diffraction in a crystal, according to the Bragg relation: nλ = 2d sin θ

(4)

Where n is the diffraction order; λ is the X-ray wavelength; d is the crystal spacing and θ is the Bragg angle. For a fixed line source the photoionization cross-section of a core level is also fixed, then for a group of core levels the cross-section might be high while for another group might be low. What is needed is a source of adequate intensity capable of being tuned continuously over the required energy range, then allowing for variation of the cross-section for a selected core level. Synchrotron plants offer several advantages producing a tunable source of radiation and a considerable increase of the photon flux, by several orders of magnitude with respect to the traditional sources. From the above discussion it is clear that in order to perform XPS measurements without additional and non wanted complications, the whole system requires ultra high vacuum conditions. In addition, after inducing the emission of core level

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photoelectrons and allowing for preserving the elasticity of the photoemission process and then avoiding inelastic process due to loss in energy of the photoelectrons, most important is the analysis of the kinetic energy by means of a suitable electron analyzer, nowadays exclusively hemispherical electrostatic. The role played by the analyzer is of primary importance because discriminating among photoelectrons with differences in Ek allows playing with the resolution conditions and then separation of peak features. Nowadays there is a commercially available Scienta 200 or 300 mm mean radius hemispherical electron analyzer, which exhibits the best performances and therefore is most diffuse all around the world. In sequence, in the experimental apparatus, there will be the electron detector which counts the photoelectrons after their travelling and discrimination throughout the analyzer. A good performances analyzer necessitate a good detector together with a high intensity photons source; all this allow for selecting the experimental conditions which give rise to an excellent spectral resolution accompanied by a first-rate intensity.

4.6.3 The X-ray Photoelectron Spectrum A series of peaks are observed on a background generally increasing with the binding energy increase (or kinetic energy decrease), as reported in Fig. 4.22. The core level structure detected from the spectrum is a direct reflection of the electron structure of the metal under analysis. It is immediately clear that differences in intensity characterize the different levels as well as the FWHM (full width at half maximum), and that s-type core level give rise to single photoelectron line while non-s levels (i.e., p, d, f) are all characterized by doublets. Differences in intensity arise from the relative cross-section values, differences in FWHM derives from the lifetime of the ion state remaining after ionization by the uncertainty principle: = h/τ

(5)

Where G is the line width, h is the Planck’s constant and τ is the lifetime expressed in second, if h is expressed in eV · seconds (4,1 · 10−15 ) then G is given

Fig. 4.22 The wide scan spectrum of pure gold is displayed, together with the core levels Binding Energies; Auger peaks are also reported

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in eV. Contribution to FWHM comes also from the intrinsic amplitude of the Xray line (0,85 eV for Al Kα and 0,7 eV for Mg Kα and specific values for the selected energy photons of the synchrotron radiation (SR) and from the contribution originating from the electron analyzer and function of the selected entrance pass energy; narrowing of the pass energy (PE) produces an increase in resolution together with a reduction in intensity, conversely enlargement of the PE gives rise to decrease of resolution and consequently signal broadening together with an intensity increase. As for the presence of doublets associated to signal originating from p, d and f core levels, an electron is a charged particle and therefore its orbit around the nucleus induces a magnetic field whose intensity and direction depend on the electron velocity and on the orbit radius respectively. Orbit radius and velocity are characterized by an angular momentum, the orbital angular momentum that is of course quantized. The characteristic quantum number is l, and l can assume the values 0, 1, 2, 3, 4 . . . Electrons are also characterized by the spin, positive or negative, which also induces an inherent magnetic field; the spin has associated a spin momentum, characterized by a spin quantum number s that can take the values ±1/2. Therefore, the total electronic angular momentum is a combination of the orbital angular and spin momenta, simply the vector sum of the two momenta. To achieve the total angular momentum for the whole atom, the summation must be done for all the electrons J = j. The vector summation can be performed in two ways, j–j coupling and L–S coupling. For a selected electron the total angular momentum is then characterized by the quantum number j, where j = l + s and therefore j can have values of 1/2, 3/2, 5/2, 7/2. . . Following the historical X-ray notation, states with n = 1, 2, 3, 4, . . ., by means of various combinations of l = 0, 1, 2, 3, . . . and s = ±1/2, the resulting total angular momentum values are j = 1/2, 3/2, 5/2, 7/2,. . . For the j–j coupling the spectroscopic nomenclature of the states associated to l = 0, 1, 2, 3,.. are designed s, p, d, f. . . . respectively, and the j values are reported as suffixes. Thus a state characterized by n = 2, l = 1 and j = 3/2 is written as 2p3/2 however, in such a case we must also take into account that the summation between the l value =1 and the two possible values for s (±1/2) produce two electronic levels p, e.g., 2p3/2 and 2p1/2 that in the XPS spectrum are characterized by a doublet. The difference in energy between the two states reflects the parallel or antiparallel nature of the spin and orbital angular momentum vectors of the electrons left after photoionization, the energy separation between lines, or spin-orbit splitting, is proportional to the spin-orbit coupling constant [71]. The relative intensity of the doublet peaks are correlated to their respective degeneracies (2j + 1) and for instance for the p level with j = 1/2, 3/2 the corresponding area ratio results 1:2, while for the d level with j =3/2, 5/2 the area ratio is 2:3 and for the f level with j =5/2, 7/2 we obtain 3:4 (see Fig. 4.23). Some times useful information can be achieved by the analysis of the spin-orbit energy separation.

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Fig. 4.23 Spin orbital splitting and peak notations

4.6.4 Electronic Structure and Chemical State The core level binding energies are relative to the atomic state level energies nevertheless, changes occurring at valence level as derived for instance by bond formation giving rise to the formation of a molecule, affect the potential at the core level. Non-equivalent atoms of the same element in a solid give rise to core level peaks with noticeably differences in BEs therefore, with a chemical shift. Non-equivalent atoms can arise in a number of ways as for instance differences in formal oxidation state, differences in molecular environment, differences in lattice sites and more. Depending on the chemical element affinity, there could be not great change at the valence levels, or the charge density can be strongly perturbed in decreasing or increasing depending whether the partner atoms have more electronegative character or, vice versa electropositive tendency. In both the last two cases the formal oxidation state of the element under analysis is subject to significant modification and as a consequence this modifies the electric field around the atom. Examples of different chemical shifts for carbon C1s signal are shown in Fig. 4.24. In the framework of the simple charge potential model, considering as a model the atom basically as a hollow sphere with the valence charge q localized on the surface, then the classical potential inside the sphere is the same at all points and corresponds to the ratio q/r , where r is the average valence orbital radius. A variation in the charge density in the valence region as high as q induces a change in the potential within the whole sphere equal to q/r. It is noticeable from this that the BE for all core levels will change by the same extent q/r. This effect is called core

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Fig. 4.24 Examples of chemical shifts in C1s spectra

level chemical shift and can vary among few tenth of one eV up to values of nearly 5 eV, as detected for the most electroactractive element F as a partner of bonding in a molecule. Important for a correct evaluation of the core level chemical shift the choice of a suitable reference level that, while for gas samples corresponds directly to the vacuum level (VL ), for solids samples is taken as the Fermi level (EF ). Conducting samples in good electrical contact with the spectrometer align their EF for a correct calibration to the spectrometer EF as constant energy reference then taken as BE = 0, semiconductors essentially do not give problems on aligning their EF while problems arise for non conducting samples that in addition undergo charging effect under X-ray excitation because depletion of negative charge. Several methods have been advanced and used with success as for instance internal reference, extensive research work has been done in the past about this aspect. Analysis of the BE for a specific chemical element allows for detection the perturbations to the valence levels by the vicinity of partner atoms in one molecule thus allowing for sensing the charge distribution or electronic structure at valence levels and therefore the chemical state of the atom under analysis. The XPS spectrum for a specific element is mostly composed by feature associate to the primary event as the core electron emission, but quite often provides features associated to secondary events giving secondary or satellite structures as shakeup and shake-off. Electronic excitations are commonly a multi-electron process because they influence the whole electronic configuration of the atom or molecule under study. The secondary structures are associated to indirect excitation of the so called passive electrons. The core level electron emission produces a core hole that can give rise to the indirect excitation of spectator electrons into high energy level empty orbitals producing excited states. In the case the excited states fall below the Fermi level, the secondary spectral structure is named shake-up, while for the case of excitation above the vacuum level in the continuum we have the shake-off. Considering that the energy needed for both primary emission and secondary excitation comes from the incoming X-rays, then in XPS the satellite structure occurs at lower kinetic energy than the primary line.

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From the analysis of such features additional information can be achieved for instance relative to the presence of π-electron or paramagnetic and diamagnetic electronic arrangement.

4.6.5 Surface Sensitivivity Additionally, excited core level electrons travelling within the surface of a solid material are subject to elastic and inelastic collision therefore they can loose kinetic energy respect to the expected EK . Travelling without loosing any energy requires that the distance be equal to the mean free path λ of the specific electron (1s, 2p, 3d) in a specific material (organic, inorganic, metallic). The mean free path is strongly dependent on energy EK (λ ∼ EK 1/2 ) of the excited and then emitted photoelectron, in a less extent depends on the kind of solid material. This dependence of λ on the photoelectron kinetic energy can be favourably and suitably used to tune the sensitivity to the surface by the photoelectron spectroscopy. Synchrotron radiation is a tunable source of radiation and therefore a specific core level electron can be analyzed by means of different energy photons producing therefore, photoelectrons possessing different kinetic energy EK . With reference to the mean free path dependence on the EK , the surface sensitivity can be opportunely tuned and maximized for the appropriate kinetic energy producing the minimum value for λ [71]. There have been reports for which the extremely enhanced surface sensitivity has produced response from surface layer and bulk layer well resolved in energy; a kind of sensitivity of this nature can only be resolved for well organized and ordered metallic surface [71]. Atoms at the first layer differ considerably from the atoms underneath and bulk-like, due to their lower coordination and as a result core levels of the surface atoms are shifted respect to the core level of bulk atoms usually by few tenths of an eV.

Fig. 4.25 Scheme representing the surface sensitivity enhancement by variation of the photoelectron “take-off” angle

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Surface sensitivity can be improved in XPS by traditional X-ray source by opportunely using the angular effect on the photoemission process arising from the geometry and relative orientation of X-rays, sample surface and analyzer entrance. Photoelectrons with λ as a value of inelastic mean free path or attenuation length, emerge from a dept as high as 3 times λ, and then the thickness of the sample involved in the photoelectron production corresponds to d = 3λsinα where α is the take-off angle for photoelectron relative to the sample surface (see Fig. 4.25). This layer thickness has a maximum value for take-off angle corresponding to 90◦ . By performing measurements at decreasing values of α, enhancement of surface sensitivity is achieved.

4.7 Applications to Molecular Systems 4.7.1 Nanostructured Systems In the field of materials science, as in nature, it has become apparent that the macroscopic properties of a system are strictly related to its microscopic structure. Among the large variety of nanostructured materials developed and investigated in the literature, in the present chapter we will deal only with molecular systems organized at nanometer-sized level onto suitable substrates, commonly inorganic. In an effort to understand these relationship, methodologies have been evolved which allow structural control over the deposition of materials on a variety of substrates. These molecular film growths give rise to organic-inorganic hybrid thin films systems. The hybrid materials provide both the established and efficient optical, electrical and magnetic properties of inorganic components and the structural flexibility of organic components. Such materials are particularly attractive because they can provide resources for combining the best properties of heterogeneous material systems, leading to enhanced performances or new valuable properties. Furthermore, hybrid well organized materials exhibits distinctive optical and electrical properties which differ from their constituents [72–74]. New synthetic methods have produced molecules capable of spontaneous organization in 3D, resulting in many remarkable supramolecular assemblies. Structurally defined thin films have been fabricated by Langmuir-Blodgett (LB) [75] technique and successively by self-assembly (SA) [76]. The construction and control of surface structures on a nanoscale level is fundamental for the development of surface with novel and valuable properties. Using self-assembly of macromolecules to form surface nanopatterns is growing as a capable and flexible technique [5–11]. Technological application for nanopatterned macromolecular devices started to be reported as for instance in flash memory and metal oxide semiconductor capacitors [77–80]. For several potential purposes the dimension of the nanostructures requires to be optimized experimentally. For thin films obtained by microphase separation, the regulation of size and morphology of the nanopatterned system has been reported as achievable by changing the molecular weight [81] or similarly

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swelling the microphases by admixture of homopolymers corresponding to one or both of the polymer block [82–86]. From block copolymers dissolved in a selective solvent, nanostructured systems are obtained in the form of spheres, wormlike or cylindrical, with one dimension falling within 10–200 nm [87]. Deposition of micelles like these, onto surfaces in the monolayer regime gives rise to chemical and topographical nanopatterns [88–91]. It has also been reported that systems like those above mentioned show a marked tendency to behave as nanocontainers for inducing the molecular organization of nanoparticles, biomolecules or other functional units [92]. Moulds, as these here described, commonly confirm a quick to answer smart behaviour and can also be replicated in other materials [93–95]. In the framework of nanostructured molecular systems, both organic and inorganic as well as organometallic systems have been largely investigated. The commonly followed approach in the building of nanostructures, starts from the requirements of functionalization for a surface of a suitable material; depending on the required specificity, attempt to obtain nanowires, nanotubes, etc is searched for. Several methods have been developed and are in progress for the preparation of nanostructured systems. A molecular layer deposition (MLD) process has been recently discussed [96] which describes the preparation of a high quality organicinorganic multilayer, accounting for factors such as interfacial irregularity and interdiffusion between adjacent layers (see Fig. 4.26). The growth of alkylsiloxane self-assembled multilayers (SAMs) has been performed by sequential growth under vacuum of C=C terminated alkylsilane and titanium hydroxide with ozone activation (see Fig. 4.27). The XPS investigation

Fig. 4.26 Example of self assembling structure for alkylsiloxane self-assembled multilayers (SAMs) obtained throughout the sequential growth under vacuum of C=C terminated alkylsilane and titanium hydroxide with ozone activation (Reprinted with permission from Lee et al. [96]. Copyright (2009) American Chemical Society)

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Fig. 4.27 Schematic showing a proposed mechanism for the reaction of a C=C-Thiole self assembling monolayer anchored on gold with ozone (Reprinted with permission from Fiegland et al. [98]. Copyright (2009) American Chemical Society)

has evidenced the formation of a carboxylic chemical group through the reaction between C=C terminated SAM and ozone [97, 98]. A method that has become quite common for the anchoring of organic as well as organometallic molecules onto a support surface, considers the functionalization of the terminal group with sulphur, as for instance with the –SH thiole function, and the substrate as gold, by using the well known affinity between sulphur and gold [99] (see Fig. 4.28). In this framework, several investigations by XPS have been reported.

Fig. 4.28 (a) Schematic representation of nonanedithiol grafted on gold surface; (b) High resolution XPS spectrum and decomposition into simple spin-orbit 1/2, 3/2 components for the S2p binding energy region (Reproduced with permission from Kazzi et al. [99]. Copyright 2009 Wiley-Blackwell)

Molecular systems as the oligo(phenyleneethynylene) (OPE) possessing properties of electrical conductivity and therefore interesting in the field of molecular electronic have been investigated after functionalization with sulphur and assembling on gold [12, 100]. Attempt to evidence the influence of intermolecular interaction and molecule-substrate interaction has been performed with attention to the C1s and S2p core levels (Fig. 4.29) leading the authors to the conclusive assignment of the molecular structure among all the possible ones. There are also examples in the literature of molecular systems organized in nanostructures that use as substrate nanometer-sized metal clusters [101]. These

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Fig. 4.29 (a) Summary of chemical structures and notations of SAM precursor molecules (b) The XPS C1s region spectra of monolayers of the 1-, 2-, and 3-SAMs, shown as curves a, b, and c, respectively. Spectra were collected with a photoelectron take-off angle of 45◦ and a pass energy of 40 eV (Reprinted with permission from Stapleton et al. [100]. Copyright (2009) American Chemical Society)

substrates show properties considerably changed from the bulk metal. Metal clusters have gained increasing interest in several field of nanoscale material science as promising candidates for elementary units of electronic devices, catalysts, sensors etc. Alkanethiolate monolayer on palladium clusters has been investigated both about the structure and stability.

4.7.2 Organometallic Macromolecules As for this classification of molecular systems, only systems composed by an organic backbone alternating σ and/or π-bonded metal atoms within a molecular chain or inside a macrocycle, will be reviewed in this chapter. Quite often such systems appear particularly appealing for their peculiar properties in electronics. Investigations of electrical transport in molecules are infact stimulated by the finding of fascinating transport phenomena as for instance Coulomb blockades [102–106], Kondo resonance [107, 108], current rectification [109, 110], switching [111, 112] and negative differential resistance [113–115]. In the field of molecular electronic strong interest is devoted to highly conjugated molecules self organized and assembled between metal contacts in order to produce a molecular wire allowing for a large current transport. Redox-active molecular wires as formed by CN terminated SAM of ruthenium (II) containing bis(σ-arylacetylide) complexes, have been interfaced across two gold wires, as reported in Fig. 4.30 [116].

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Fig. 4.30 Molecular structures (a) and schematic representations of the CP-AFM (b) and the X-wire (c) junction test structures. In both test structures, the top Au electrode was brought into contact with a SAM of ruthenium complexes formed on the bottom Au electrode. I–V traces were obtained over ± 1.0 V (Reprinted with permission from Kim et al. [116]. Copyright (2009) American Chemical Society)

XPS analysis [117] allowed for determining the length of the molecular wire through the evaluation of the ruthenium/nitrogen atomic ratio and for evaluation of the film thickness from which the authors claim the tilting of the molecules. Chemical interaction between surface-oxidized multiwall carbon nanotube (o-MWCNTs) and tetrasulfonate copper phtalocyanine (TS-CuPc) molecular semiconductor has been investigated [117] (see Fig. 4.31a for a representation of the assembly). The examined system shows evidence of spontaneous tendency to form nanostructures upon rapid drying. By analysis of the N1s, S2p and Cu2p core levels

Fig. 4.31 (a) Pictorial representation of cofacially extended TS-CuPc aggregates adsorbed onto the outer wall of an o-MWCNT; (b) XPS spectra of N1s (top), Cu2p3/2 (middle), and S2p (bottom) levels in TS-CuPc. For each set of core level spectra the upper spectra pertain to the composite with o-MWCNTs and the lower spectra to TS-CuPc only. Experimental data are shown as filled shapes, and the dashed/solid lines are synthetic fitting curves (Reprinted with permission from Hatton et al. [117]. Copyright (2009) American Chemical Society)

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the authors have the evidence for a charge transfer interaction from o-MWCNT towards the TS-CuPc molecules oriented with the molecular plan parallel to the nanotube surface. Chemical shift for all the measured core levels has been detected and in addition the nitrogen signal external to the phtalocyanine macrocycle vanishes upon interaction (as shown in Fig. 4.31b) Fullerenes and metallo-macromolecules as phtalocyanines and porphyrins are strategic materials for the fabrication of nanoscale molecular devices for optoelectronics, as for example solar cells. Synchrotron radiation-based photoelectron spectroscopy is one of the most suited techniques for the study of organic chargeseparation complexes as C60 -Pf and C60 -Pc, since it effectively probes the system in a light-induced excitation state. In a work partially discussed in the previous Section 5.2 [23], Arima and co-workers report Photoemission and NEXAFS spectra collected at the ELETTRA storage ring, beamline BEAR, on a CoTBPPf anchored to gold substrates previously functionalised with 4-ATP SAMs and on the same system bonding Py-C60 through Co-N coordination (see Fig. 4.11 in Section 5.2 for the molecular structure). Here, SR-XPS spectroscopy was used to investigate the electronic modifications in the self-assembled monolayers at different immobilization steps, up to the final step where charge transfer phenomena can occur. N1s and C1s core level spectra confirm the presence of both Co-TBPPf and Py-C60 macromolecules, and the spectra collected on the 4-ATP-CoTBPPf-Py-C60 complex appear completely similar to those of the single molecules, thus suggesting that the charge redistribution confirmed by NEXAFS measurements does not influence the core levels. Chen et al. [32] used both NEXAFS and SR-XPS spectroscopy to investigate CuPc anchored on CH3 -SAMs/Au and respectively CF3 -SAM/Au. The NEXAFS results have been extensively discussed in Section 5.2. SR-XPS spectroscopy was performed at the valence band and core levels, with the aim to characterize the charge transfer at CuPc/SAMs interfaces. From HOMO peaks shifts observed in the VB spectra and the corresponding C1s and N1s BE shifts, an electron transfer from CuPc to CF3 -SAM was deduced, leaving an electron accumulation layer in the CF3 -SAM and a depletion layer in CuPc (p-type doping of the CuPc interface layer). In contrast, no electron transfer was observed when CH3 -SAM was used. Furthermore, the spectra were collected for different coverages, showing that the charge transfer induced electron depletion region in CuPc is mainly situated in the fist 3nm of the film (see Fig. 4.32). SR induced photoelectron spectroscopy was also successfully applied to the investigation of ZnPf/C60 assemblies (Pf: diethynilporphyrin) by some of the authors of this book [37]. Both VB, N1s and C1s core level spectra were acquired while increasing C60 coverage on a ZnPf multilayer previously vapour deposited on Cu(111), showing that C60 grows on the Zn-porphyrin sample and undergoes diffusion in the multilayer substrate, leading to a composite assembly with C60 spheres probably sitting in between two ZnPf planes, as reported in the following Fig. 4.33. The interaction at the interface between substrate and overlayer produces a perturbation of the electronic structure for both C60 and ZnPf. A charge transfer, that is reflected mainly on the N1s core level and on the C1s level of the C60 species, occurs from C60 to ZnPf. The enhancement of the N1s shake-up satellite structures

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Fig. 4.32 (a) The evolution of work functions of SAM/Au(111) as a function of the CuPc coverage. (b) Binding energy shift of the N1s core level and HOMO peak maximum relative to 1.5 nm CuPc/SAM/Au(111) as a function of the CuPc coverage. Solid circles and triangles represent CuPc/CF3 SAM and CuPc/CH3 -SAM, respectively. (Reproduced with permission from Chen et al. [32]. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA)

Fig. 4.33 Picture of the interaction between Zn-Pf and C60 , upon deposition on Cu(111) substrate

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as a function of the increasing of dopant, reflects the lowering of the HOMO–LUMO energy gap and the orbital mixing. This trend is observed also in the VB region with an orbital mixing of C60 and ZnPf.

4.7.3 Biomolecular Systems The study of biomolecules by photoelectron spectroscopy, compared with the large extension received by other methodologies as for instance NEXAFS spectroscopy, appears somehow less widespread. The scientific reports appeared about this topic are mostly related with single aminoacids and small peptides analyzed in bulk or after adsorption on suitable solid surfaces. Adsorption of biomolecules on solids can be of interest in several fields ranging from medicine to nanotechnology [118] and this derives by the application in the construction of scaffolds suitable in implantology [119, 120] in which case the substrate can be either inorganic, as for example TiO2 , or organic [121] Generally, the assembling of hybrid bio-organic/inorganic in the case of aminoacids can be much useful and serve as model system then used as a reference structure. For these simple systems the sample preparation can be pursuit by two main routes as for instance deposition from aqueous solutions or by means of vapour deposition in ultra high vacuum (UHV) conditions. Early study of aminoacids on various surfaces (silicon, graphite, noble metals etc) used the way of deposition by diluted water solutions [122–125]. Aminoacids deposited thin films onto metallic substrates in UHV conditions have also been studied [126–133]. The last deposition method causes some serious problems connected with the stability of these molecules giving rise to situations in which the saturation vapour pressure of several aminoacids is particularly low even at temperatures close to their crash temperature. This aspect originates from the zwitterionic nature of aminoacids in solid state afterwards producing a strong intermolecular interaction, resembling in some aspect organic polymers and as a result the melting temperature for most aminoacids occurs above 250ºC with decomposition. Therefore, this is the reason why only few investigations about aminoacids film have been performed on vacuum deposited samples. In this framework, histidine was investigated upon adsorption on polycrystalline gold substrate in monolayer and multilayer regimes [134], by means of high resolution synchrotron induced photoelectron spectroscopy (see Fig. 4.34 for the scheme of the interaction and evolution of the XPS analyzed core levels). Zubavichus et al. [135] succeeded in observing the formation of strong ionic-covalent bonds between istidine and the gold substrate atoms with charge depletion at the gold sites, as histidinate anions, for adsorption in monolayer regime. Three istidine sites are involved in the interaction as the −NH2 amino groups, the −COOH carboxylate groups and the pyridine-like N3 nitrogen in the imidazole ring; the carboxylate was considered as bonding with a single oxygen atom. As for the multilayer adsorption regime, these authors demonstrated that histidine molecules are mostly zwitterionic in two forms, protonated amino groups and protonated imidazole rings. The molecules present a strong intermolecular interaction producing 3D aggregates and softly interact with the gold substrate.

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Fig. 4.34 (a) Models for the adsorption of histidine on polycrystalline gold in the monolayer (bottom) and multilayer (top) regimes; (b) Top row (from left to right): Au4f, O1s, N1s, and C1s core-level spectra (hν) 610 eV) for histidine films 1–3 and freshly sputtered gold (Au4f) or histidine powder (O1s, N1s, and C1s), normalized to the maximum intensity. Dotted lines show peak-fitting results. Bottom row: identical spectra but normalized to the storage ring current (Reprinted with permission from Zubavichus et al. [134]. Copyright (2009) American Chemical Society)

Adsorption of glycine on Cu(110) single crystal has been studied also by X-ray photoelectron spectroscopy and compared to the formate/ and acetate/Cu(110) adsorption structure [135] (see Fig. 4.35). Glycine or α-amino acetic acid was taken because of the presence of several chemical functionalities and a low symmetry. Investigation at the C1s, O1s and N1s core level regions gave indications for differences originating from significant chemical shift as high as 1,59 eV for the CH3 group (C1s occurring at 284, 64 eV for acetate and 286, 23 eV for glycine) and for the carboxylic group the shift is 0, 74 eV (C1s at 287, 51 eV for acetate and 288, 25 eV for glycine). A chemical shift of the same significance (0,72 eV) was detected for the O1s core level (O1s core level at 530, 85 eV for acetate and 531, 57 eV for glycine). The origin of the chemical shift has been then analyzed and the conclusions were that the chemical shifts could either indicate a different local geometric environment for the carboxylic group in glycine compared to acetate or could simply be due to the presence of the amino group. With the help of others acquired data, the conclusion was that in both cases the adsorption occurs with bond formation through two equivalent oxygen atoms and for glycine the adsorption geometry is consistent with the molecule bent down on the surface with a second bond occurring through the amino group of the molecule and a copper atom. Carravetta and coworkers [136] reported a theoretical and experimental investigation on the adsorption on TiO2 surface of two dipeptides AE (l-alanine-l-glutamic acid) and AK (l-alanine–l-lysine), that are “building blocks” of the more complex oligopeptide EAK16. Classical molecular dynamics simulations have been used to study the adsorption of H-Ala-Glu-NH2 and H-Ala-Lys-NH2 dipeptides onto a rutile

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Fig. 4.35 Cls and Ols XPS spectra for glycine (bottom) and acetate (top) adsorbed on Cu(110). All spectra were recorded at a photon energy of 700 eV with the electron analyzer normal to the surface (Reprinted from Hasselström et al. [135]. Copyright (1998), with permission from Elsevier)

TiO2 (110) surface in water solution, considering simultaneously several peptide conformers upon the surface, as to identify the most probable contact points between the molecules and the surface (the final molecule/substrate interaction scheme is reported in Fig. 4.36). In fact, carbonyl oxygens as well as nitrogen atoms are possible Ti coordination points, and local effects are responsible for adsorption and desorption events.

Fig. 4.36 Final arrangement of AEzw(1) and AEzw(2) structures (stick model) bound to the TiO2 (110) surface. Surface atoms are represented as gray (titanium) and red (oxygen) balls. Titanium atoms in direct contact with peptide oxygen and nitrogen atoms are green. Peptide carbons, oxygens, nitrogens, and hydrogens are gray, red, blue, and cyan, respectively (Reprinted with permission from Monti et al. [136]. Copyright (2009) American Chemical Society)

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The chemical structure and composition of thin films of the two dipeptides on TiO2 were experimentally investigated by XPS at both O and N K-edges. Theoretical ab initio calculations (SCF) were also performed to simulate the spectra, allowing for a direct comparison between experiment and theory (see Fig. 4.37).

Fig. 4.37 N1s and O1s XPS spectra of AK and AE; curve-fitting components are superimposed to the experimental spectrum (Reprinted with permission from Monti et al. [136]. Copyright (2009) American Chemical Society)

4.7.4 NLO Molecules In the field of the synthesis of molecule-based second order nonlinear optical materials, the self assembly approach has attracted much attention in recent years because it leads to obtain functional and structurally stable molecular films, as a consequence of the intrinsic versatility of molecular chemistry and mild deposition conditions possible [75, 137, 138]. Although the most detailed investigations of self-assembly have focused on long chain alkanethiol and alkylsilane derivatives, the potential for formation of relatively ordered, densely packed arrays of electronically functional molecules is of great scientific interest; the chance to modify surfaces with self-assembled layers terminated with reactive

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groups susceptible of further modification has indeed been used to achieve, among other functionalised materials, NLO – active multilayers [139–141]. For this purpose, the ability to assembly heteromultilayer structures by sequential chemisorption of different molecular species is of fundamental relevance [142, 143]. In the characterization of these artificially – structured thin – film materials X-ray Photoelectron Spectroscopy plays a fundamental role, probing the chemical composition of the SAMs and allowing to gain informations on the monolayers surface chemistry. In this context, Marks and co-workers [144] obtained functional multilayer films by the initial deposition of a benzyl halide – terminated trichlorosilane, followed by a nucleophilic attack of the benzyl position by a chromophore precursor, the 4-aminostilbazole, as to form an anchored, oriented, cationic stilbazolium layer (see Fig. 4.38); the electron deficient center accomplished through this quaternarization reaction also represents the last step in the synthesis of a conjugated donor-acceptor NLO-active chromophore of large hyperpolarizability (βzzz calcd = 946 · 10−30 cm5 esu−1 at λ0 = 1064 nm) using the reliable ZI N DO/SOS formalism [145]. In this synthetic procedure, the structural restrictions imposed by a surface reaction are advantageously exploited, inhibiting the formation of electrostatically favoured antiparallel arrangements of the molecular dipoles; instead, the NLO-active dipoles are necessarily organized acentrically in a nearly parallel arrangement, which is optimal for efficient bulk optical secondharmonic generation (SHG) and electro – optical response. Besides, monolayers prepared in this way display remarkably high bulk second-order nonlinearity (χ(2) = 4 − 6 · 10−7 esu, at λ0 = 1064 nm). Information on the monolayer surface chemistry has been achieved by XPS spectroscopy, that proved itself particularly useful for the specific procedure, involving the displacement of a benzilic chlorine to yield a chloride anion; indeed, XPS spectra of films displayed no chlorine signals, indicating that all Si–Cl functionalities are consumed in the self – assembly process (see Fig. 4.39).

Fig. 4.38 Schematic depiction of the deposition chemistry for the layer-by-layer assembly of chromophoric multilayer films, (Reprinted with permission from Roscoe et al. [144]. Copyright (2009) American Chemical Society)

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Fig. 4.39 (a) Chlorine 2p region of the XPS spectra of the bare coupling agent film (step i, Scheme 1) fit with two component peaks as described in the Experimental Section. The data at the bottom are the Cl2p region of a monolayer of the p-methyl analog. (b) Chlorine 2p region of the coupling agent film after being treated with (dimethylamino)pyridine (step ii, Scheme 11), fit with two pairs of component peaks (Reprinted with permission from Roscoe et al. [144]. Copyright (2009) American Chemical Society)

Furthermore, the success of the quaternarization reaction was demonstrated by the appearance of a N1s peak and the shift at lower BE values of the chlorine signal (from 199.9 to 199.0 eV), consistent with the formation of a charge – compensating chloride anion. In the same field, Lin and co-workers synthesised [146] and characterised [147] some siloxane-based self-assembled stilbazolium multilayers that are intrinsecally acentric and exhibit very large second order nonlinear optical response (see Fig. 4.40 for the detailed multilayer construction). The assembly of these superlattices involved sequential deposition of a sylane coupling layer, a stilbazolium chromophore layer, and an octachlorotrisiloxane

Fig. 4.40 Construction of self-assembled 4-[[4[Bis(hydroxylethyl)amino]phenyl azopyridinium multilayers (Reprinted with permission from Lin et al. [146]. Copyright (2009) American Chemical Society)

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capping layer. This approach permits to incorporate different chromophore modules and to accommodate different counteranions, due to the salt like structure of the assemblies. Since a number of recent experimental and theoretical studies indicate that the NLO responses of self-assembled stilbazolium structures are strongly dependent on the number and location of counteranions [148], the determination of the relative amount and chemical state of halides in the material is mandatory. In [147] X-ray Photoelectron Spectroscopy of Br3d core level led to assess the occurrence of Br anions upon quaternarization. In the precursor sample, Br3d signal at 70.5 eV is consistent with an alkyl bromide moiety [149]. Br3d BE of the quaternarized compound was energy shifted to 68.1 eV, and such a shift to lower binding energy values is consistent with the formation of Br anions [147]. These results were also supported by N1s core level spectra: while the coupling monolayer sample did not show N1s signal, the bilayer structure showed a couple of equal-intensity N1s peaks at 399.5 and 401.6 eV, corresponding to the amino and pyridinium nitrogen centers respectively [147] (in Fig. 4.41 XPS N1s, I3d and Br3d spectra are reported).

Fig. 4.41 X-ray photoelectron spectra of self-assembled coupling and stilbazolium chromophore monolayers. (a) Br3d XPS spectra of self assembled monolayers with 3-BrC3 H6 SiCl3 as the coupling agent. Bottom spectrum: after coupling agent deposition; top spectrum: after coupling agent and chromophore deposition. (b) I3d XPS spectra of self-assembled monolayers with 3-BrC3 H6 SiCl3 as the coupling agent. Bottom spectrum: after coupling agent deposition; top spectrum: after coupling agent and chromophore deposition. (c) N1s XPS spectra of a self-assembled stilbazolium monolayer. Bottom spectrum: after coupling agent deposition; top spectrum: after coupling agent and chromophore deposition (Reprinted with permission from Lin et al. [147]. Copyright (2009) American Chemical Society)

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4.8 Perspective and Future Applications In this chapter macromolecular systems have been considered as for nanostructured systems, organometallic macromolecules, biomolecular systems and NLO molecules. A range of applications has been explored according to the topics treated in the other chapters of this book such us bioactivity, sensor and NLO. Particular attention has been dedicated to the study of surfaces and properties of macromolecules at interfaces which is a fascinating interaction between fundamental surface study and applied chemistry. We focused our attention to the study of the described systems by two selected analysis methodologies as X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Adsorption Fine Structure spectroscopy (NEXAFS). A key theme is that self-assembly in soft materials, both synthetic and biological, can be used to template nanostructures in inorganic matter, either in bulk or at a surface. Many developments are under way to exploit nanostructures of soft materials in nanotechnology. Concerning to the experimental apparatus for XPS and NEXAFS analysis, as the time goes by the general development of apparatus never stops. As for traditional X-ray source using instrumentations, the development of analyzer with higher luminosity and high resolution received a strong impulse in the early nineties when the Scienta 200 and 300 analyzers were developed. So far, since several years the most scientifically advanced laboratories around the world have adopted this analyzer used in conjunction with the formal instrumentation or better taken as a part for a custom or similar instrument. The majority of the beamlines at synchrotron radiation plants around the world have adopted such analyzer. Research development is however still working on analyzer, detector and insertion devices at synchrotron plants.

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Chapter 5

Hybrid Systems Biomolecule-Polymeric Nanoparticle: Synthesis, Properties and Biotechnological Applications Cleofe Palocci and Laura Chronopoulou

Abstract The role of nanomaterials and nanotechnology in life sciences is yet to be fully understood, as it has resulted in the constantly developing field of nanobiotechnology. In fact, nanomaterials can interact with biomacromolecules as well as with living systems and these interactions can be used to develop new materials and technologies which are foreseen to revolutionize our understanding of biological phenomena. This chapter deals with the synthesis of biologically functionalized nanoparticles and their interface properties as well as with their innovative applications, in particular in the biomedical field, in imaging and therapy.

5.1 Introduction Substantial segments of the scientific community are confident that nanoscience and nanotechnology will revolutionize research and applications in the areas of biology and medicine. In particular nanobiotechnology, as part of nanotechnology, has gained increasing importance during the last 10 years. This area of research opens up new perspectives in medicine and pharmacology e.g., in analytics and therapy. Nanobiotechnology is an interdisciplinary field of research based on the cooperative work of chemists, physicists, biologists, medical doctors and engineers. At the interface between biotechnology and nanotechnology, nanobiotechnologists carry out research on the phenomena of self-assembly or self-organisation of biomolecules such as cell membranes, virus particles, proteins or antibodies in order to adapt these principles to the technical production of nanostructures. This review will discuss as a challenge for the future newer and more sophisticated nanotechnologies that can address problems associated with the diagnosis, treatment and management of multigene diseases (e.g., cancer, cardiovascular diseases, environmental diseases, etc.) and structural disorders that aging population C. Palocci (B) Department of Chemistry, SAPIENZA University of Rome, Piazzale Aldo Moro 5, Rome 00185, Italy e-mail: [email protected]

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will experience over the next century. In this field the evolution of novel imaging nanotechnologies will be discussed in order to demonstrate their importance to probe nanoscale physiological processes occurring in human organs whether we try to diagnose cancer or any other disease at an early stage or whether the goal is to treat diseased tissues more effectively. In the field of nanotechnology the science of bio-nanohybrid materials constitutes an emerging interdisciplinary field on the frontier between Life Sciences, Material Sciences and Nanotechnology. Special attention is being devoted to bionanohybrids due to their incidence in significant areas from regenerative medicine to the production of new materials showing improved functional and structural properties. This chapter will be also focused on some selected works showing recent research on the synthesis and properties of bio-hybrid nanomaterials, which are based on the assembly at the nanometric scale of compounds derived from natural sources (proteins, antibodies, nucleic acids, drugs) with different polymeric nanostructured materials. Among these nanostructured materials, the structural and functional bio-hybrids resulting from the combination of natural polymers, such as polysaccharides, polyesters, RNA and DNA, polypeptides, fibrous and globular proteins, enzymes, inorganic substrates, such as silica and phyllosilicates, layered double hydroxides (LDHs), phosphates and metal oxides. These are significant examples illustrating new insights in this multidisciplinary area in which the stability of the achieved bioconjugates is increased allowing future applications as catalysts, membranes or energy conversion devices, biosensors and tissue engineering. Advances in the field of drug delivery and intelligent therapeutics will also be discussed.

5.2 Synthesis of Biomolecule-Functionalized Nanoparticles Bioactive molecules have been immobilized on polymeric matrices and inorganic supports through a variety of techniques, including physical adsorption, electrostatic binding, specific recognition and covalent coupling [1–5]. These supports, which are modified with biological molecules such as proteins/enzymes, antigens/antibodies, and DNA/oligonucleotides, have been used for numberous biotechnological applications: affinity separations, biosensing, bioreactors, and the construction of biofuel cells. Recently, these immobilization techniques developed for the functionalization of macrosize supports, have been applied in order to associate biomolecules with nanoparticles.

5.2.1 Functionalization of Nanoparticles with Biomolecules Through Electrostatic Adsorption The physical adsorption of biological macromolecules on NPs has frequently been performed and studied for biomolecules, which range from low-molecular-weight

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Fig. 5.1 Schematic illustration of the (EG3-S-)/GSH mixed monolayer protected nanoparticle chemically bonded with a biotin molecule and its interaction with a streptavidin molecule (Reprinted with permission from Zheng and Huang [6]. Copyright 2009 American Chemical Society)

organic substances (e.g., vitamin C) to large protein/enzyme molecules (Fig. 5.1) [6–10]. In the case of NPs that are stabilized by anionic ligands such as carboxylic acid derivatives (citrate, tartrate, lipoic acid), the adsorption of positively charged proteins can originate from electrostatic interactions [11–15]. For example, gold and silver NPs, produced by citrate reduction [16], were functionalized with immunoglobulin G (IgG) molecules at pH values that lie slightly above the isoelectric point of the citrate ligand. This allowed effective binding between the positively charged amino acid side chains of the protein and the negatively charged citrate groups of the colloids. Other examples of protein coating through electrostatic interactions include the direct adsorption of heme-containing redox enzymes on citrate-stabilized Ag NPs and the binding of basic leucine zipper proteins to lipoic acid stabilized semiconductor CdSecore/ZnSshell particles [14]. The electrostatic deposition of biomolecules, particularly proteins or enzymes, can also be extended to multilayer-level assemblies [17]. In fact proteins that are electrostatically attracted to the charged nanoparticles can provide an interface for the further deposition of an oppositely charged polyelectrolyte polymer, which again allows the deposition of a secondary protein layer. Multilayer films of BSA [18], IgG [18], b-glucosidase [19], glucose oxidase [20], urease [21] and horseradish peroxidase [22], have been assembled on polystyrene NPs by the alternate deposition of the proteins and an oppositely charged synthetic polyelectrolyte as linker (e.g., poly (diallyldimethylammonium chloride) or poly(sodium 4-styrenesulfonate) were used as positively or negatively charged polyelectrolytes, respectively). The protein/polymer multilayer shell thickness could be varied from a few nanometers to hundreds of nanometers. This strategy permits the preparation of functional films on nanoparticles with a high density of enzyme molecules. An increase in the loading of NPs with organic materials upon layer-by-layer deposition results in the enhancement of the sensitivity of the analytical protocol to which the NPs are applied.

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5.2.2 Functionalization of Nanoparticles with Biomolecules by Chemisorption of Thiol Derivatives and by Covalent Binding Through Bifunctional Linkers The primary binding of functionalized thiolated molecules, such as oligopeptides, to Au NPs could provide a means for the covalent tethering of biomolecules to nanoparticles. Au NPs have been functionalized with L-cysteine through the thiol groups. Subsequent oligomerization of the cysteine moieties in an aqueous solution resulted in the binding of oligopeptides to the Au NPs [23]. Elsewhere, CdS NPs were capped with glutathione through the strong binding of its thiol groups to Cd2+ ionic sites on the surface of the nanoparticles. In some cases, tight chemisorption of proteins on Au NPs can originate from the binding of thiol groups from cysteine residues present in the proteins (e.g., serum albumin) to the Au surface. If no thiolated residues are available in the native proteins, thiol groups can be incorporated by chemical means, for example, with 2-iminothiolane or through genetic engineering [24–27]. Some proteins and enzymes preserve their native structures and activities when they are physically adsorbed on nanoparticles. With the covalent attachment of proteins to nanoparticle surfaces, problems of instability and inactivation can be overcome [17]. Low-molecular weight bifunctional linkers, which have anchor groups for their attachment to NP surfaces and functional groups for their further covalent coupling to the target biomolecules, are extensively used in the generation of covalent-tethered conjugates of biomolecules with various NPs. Anchor groups such as thiols, disulfides, or phosphine ligands are often used for the binding of the bifunctional linkers to Au, Ag, CdS and CdSe NPs. These anchor groups may readily substitute weakly adsorbed molecules to stabilize the NPs or may be incorporated in the NP synthesis to yield a functionalized surface available for further reactions. Alkoxy- or halosilane groups are used for the covalent attachment of bifunctional linkers to the surfaces of NPs coated with SiO2 and other oxides. Au NPs covered with polysiloxane shells and functionalized with a variety of functional groups for covalent binding to biomolecules have been synthesized [23]. A wide variety of terminal functional groups are available in different bifunctional linkers. The most common amine, active ester, and maleimide groups are also used to couple biological compounds covalently by means of carbodiimidemediated esterification and amidation reactions or through reactions with thiol groups.

5.2.3 Functionalization of Nanoparticles with Biomolecules by Specific Affinity Interactions Nanoparticles functionalized with groups that provide affinity sites for the binding of biomolecules have been used for the specific attachment of proteins and oligonucleotides. For example, streptavidin-functionalized Au NPs have been used for the affinity binding of biotinylated proteins (e.g., immunoglobulins and serum

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albumins) or biotinylated oligonucleotides. Also, NP-antibody conjugates have been used for affinity binding of their respective antigens [28, 29]. As an example stable aqueous dispersions consisting of CdS nanoparticles having modal diameters, ranging between 2 and 8 nm, were prepared with aminoderivatized polysaccharides (aminodextrans, hence abbreviated as Amdex) as the stabilizing agents [29]. It was also shown that the Amdex-CdS NP complexes could be activated and conjugated to antibodies by conventional means (see Fig. 5.2) This may be advantageous because upon association with their respective antigens, NP conjugated antibodies can demonstrate association constants that are even higher than those of the free antibody [30]. Recently, metal NPs have been used to tether a variety of carbohydrate ligands [31–38]. Such 3D multivalent ligands provide a globular structure on which clustering and orientation effects may be studied.

Fig. 5.2 Schematic illustration of T4-5X-Amdex-CdS conjugate and its binding to T4+ white blood cells (Reprinted with permission from Sondi et al. [29]. Copyright 2009 American Chemical Society)

5.2.4 Synthesis of Nanoparticles Through Biochemical and Microbial Systems The use of bacteria as a novel biotechnology to facilitate the production of NPs is in its infancy. In contrast to purely chemical procedures for the manufacture of NPs, biological and, particularly, microbial reactions, characterized by high selectivity and precision for NP formation [39] provide environmentally friendly technological processes [40].

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In most bioprocesses, it is assumed that highly specific structures such as enzymes and proteins exist on the bacterial membrane which, in turn, drive highly specific interactions with the culture medium. These biological processes could rapidly produce copious amounts of nanoparticles. Many details of the underlying biology remain unanswered, but from an electrochemical point of view, bacteria may be thought of as electrodes that operate at a relatively fixed potential value.The range of inorganic nanosized products observed is consistent with this model. The NPs produced by bacteria could be isolated with the parent biomaterial shells to demonstrate the use of bacteria as living reactors for the preparation of nanomaterials. Microorganisms, particularly prokaryotic bacteria, are often exposed to extreme environmental conditions that force them to resort to specific defense mechanisms to quell stresses such as the toxicity of foreign metal ions or metals. The toxicity of metal ions is reduced or eliminated by a change in the redox state of the metal ions and/or precipitation of the metals intracellularly. This forms the basis of many important applications of microorganisms such as bioleaching, bioremediation, microbial corrosion, as well as the synthesis of nanoparticles. A number of biological systems have developed highly orchestrated detoxification mechanisms towards the bioreduction and mineralization of noble metals. These systems incorporate small peptides and proteins as nucleation sites to bind metals and stabilize NPs. For example, Ag NPs in the 2–5-nm range were synthesized extracellularly by a silver-tolerant yeast strain MKY3 when challenged with Ag+ ions (1 mm) in the log phase of growth [41]. Live fungus and plants could also be used to generate metallic and semiconductive NPs [42]. Intrinsic properties of biomolecules can be changed upon their association with metal nanoparticles. For example, vibrations of the biologically active prosthetic

Fig. 5.3 Recognition of flavin by diaminopyridinefunctionalized Au NPs (Reprinted with permission from Boal and Rotello [44]. Copyright 2009 American Chemical Society)

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heme groups of myoglobin or hemoglobin are selectively enhanced by adsorption of the proteins on metal NPs relative to the vibrational modes characteristic of the protein backbone as recently shown in a SERR spectroscopic study [43]. Alterations by external signals (e.g., electrical, optical) of the chemical properties of the biomolecules or the biomaterial analogues that modify nanoparticles can be used to control the interactions of the modified nanoparticles with the environment. Thereby, the binding properties of the modifier or the aggregation of the nanoparticles can be controlled. For example, the formation of a complex between a diaminopyridine derivative and a flavin derivative was studied at the surface of a Au NP (Fig. 5.3) [44]. It was shown that a more-stable hydrogen bonding was formed when the flavin derivative was electrochemically reduced. This allowed the electrochemically controlled switching of the binding of the bioorganic molecules to the organic-functionalized shell of the nanoparticles.

5.3 Biomolecule-Nanoparticle Interface Properties The recognition of biomacromolecule surfaces is challenging because of their size and complexity. As reported in Fig. 5.4 there are two key basic challenges to be overcome for specific/selective NP-biomacromolecule surface recognition to be realized. First, a large surface area is required or high-affinity binding, because of the convex shape of the binding surface coupled with the solvent-exposed protein surface. Studies of protein–protein interactions reveal that surface areas >6 nm2

Fig. 5.4 Challenges for surface recognition of biomacromolecules. (a) A large surface area is required. (b) A preorganized yet flexible receptor (Reprinted with permission from You et al. [50]. Copyright 2009 Elsevier)

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per protein are typically buried in these interactions [45–47]. A second issue is preorganization, where a structurally well-defined surface is required for efficient and selective interaction. Scaffolds with large surfaces are of particular utility for biomolecular recognition. Monolayer-protected clusters (MPCs) and mixed MPCs (MMPCs), where polyhedral-shaped metal cores are immediately surrounded by a self-assembled monolayer, are promising materials for the creation of biomacromolecular receptors. NPs can be readily fabricated with sizes from 1.5 to >10 nm, comparable to proteins and other biomacromolecules providing large surface areas for interaction with biomacromolecules [48, 49]. Second, the self-assembled monolayer on the surface of the particle imparts preorganization to the appended recognition elements. Additionally, the array of metal and semiconductor core materials that are readily available provides access to fascinating optical, electronic, and magnetic properties [51–54]. Applying these attributes to the selective recognition of biomacromolecules, however, requires careful tailoring of surface functionality, a goal that can be achieved only by using synthetic methods.

5.4 Biomolecule-Functionalized Nanoparticles for Controlled Chemical Reactivity The interactions of functionalized nanoparticles with biomaterials or within biomaterial structures could permit to control the chemical reactivity of the biomolecules. Alternatively, these interactions can report on the state of the reaction or reactants and allow the reactivity to be controlled externally. It is well known that small molecules and polymers can affect the chemical reactivity of biomolecules. If there are several possible parallel reactions, the effect produced by a promoter/inhibitor on a specific chemical reaction can change the effective chemical path of the whole process to result in the regulation of the biochemical system. As an example, molecular labels such as fluorescent dyes incorporated into biomolecules can report on the state of the biomolecule through transduction of the molecular transformations into an output signal: this is used, for example, in biomaterial-based diagnostics. Functionalized NPs can operate in the same way, demonstrating properties of a biopromoter/bioinhibitor or a reporter with some advantages over their molecularsized counterparts owing to the unique photophysical and electronic properties of the NPs. Nanomechanical devices based on NPs functionalized with biomolecules (particularly DNA) are also feasible [55, 56].

5.4.1 Nanoparticles and Catalysis Catalysts are widely used in the large-scale manufacture of chemicals and in the production of fine chemicals and pharmaceuticals. Fuel processing is a good example: the gasoline that we use in our cars requires at least ten different catalysts during

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its transformation from crude oil. Environmental technologies also rely heavily on catalysts; the best known example being the catalytic converter in the exhaust of every car. It is estimated that more than 20% of the gross national product (GNP) of industrial countries relies in one way or another on catalysis [57]. In heterogeneous catalysis, the reacting molecules adsorb on the catalytically active solid surface. Chemical bonds are broken and formed on the surface and eventually the products are released back into the liquid or gas phase. Many of the heterogeneous catalysts used in industry today consist of small particles of a catalytically active material, typically with a diameter of 1–10 nm, anchored on a porous support. The use of NPs results in a large contact area between the active material of the catalyst and the surrounding gas or liquid phase. This ensures that the catalytic material is used effectively. One of the interesting scientific and technological challenges associated with the use of NPs as catalysts is the understanding of how the composition and atomic-scale structure of NPs produce the best catalytic activity. The second challenge is to synthesize catalyst particles with maximum control over their composition and structure. Modern nanotechnological methods clearly offer great potential for future developments in both characterization and synthesis of heterogeneous catalysts based on supported NPs. Maximizing the surface area is not the only reason for using NPs as heterogeneous catalysts. Au is usually considered chemically inert but in 1987, Haruta et al. [58–61] showed that nanosized (<5 nm) Au particles can be very effective catalysts. This indicates that the catalytic properties of a particular material can be dramatically influenced by the particle size. The fact that Au particles with diameters of about 5 nm or less have unique catalytic properties has initiated a search both for an explanation of this quite unexpected effect and for chemical reactions that are catalyzed by Au. In some cases, catalysts based on nanosized Au particles allow a significantly lower reaction temperature than the one used in existing processes, which is promising for the development of energy efficient processes [62–66].

5.4.2 Protein Adsorption on Solid Surfaces: Conformational Effects and Stability Protein adsorption on solid surfaces often induces structural changes that may affect the entire molecule [67–74]. This is a frequently observed phenomenon, and the resulting changes in structure, and function, can have profound consequences in various fields, such as biology, medicine, biotechnology, and food processing [75, 76]. Therefore, an understanding of the conformational behaviour of proteins at solidsolution interfaces is desirable for a variety of reasons. For example, a detailed mapping of conformational changes is necessary for understanding the mechanism of protein adsorption and can help identify optimal conditions to preserve functionality following protein immobilization. Even though major scientific contributions have been made to our understanding of proteins on solid surfaces in recent years

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[76–80] many questions concerning the adsorption process and the structural alterations that occur in the adsorbed proteins still need to be clarified. For example, in some cases structural changes may be induced in the protein by the physicochemical nature of the solid surface, while in others structural changes may occur due to intrinsic properties of the protein, essentially independently of the surface chemistry, but often the relative strength and nature of the extrinsic and intrinsic forces involved are poorly understood. The adsorption of proteins on solid surfaces and the conformational changes they undergo when adsorbed to the surface have been thoroughly characterized and monitored using a variety of different methods such as total internal reflectance fluorescence [81], CD [82], IR spectroscopy [83], AFM [84], MS [85], and NMR [86, 87]. As an example, Billsten et al. [68, 82] have shown that the protein’s structural stability is a key factor in determining the rate of conformational change for different stable human carbonic anhydrase II (HCAII) variants. More recently, Karlsson et al. [72] used CD and fluorescence spectroscopy to further explore the adsorption process for different variants of HCAII to evaluate the dependence of protein stability on the rate and extent of conformational changes induced by surface adsorption. Their results show that under their test conditions HCAII first rapidly binds to the particle surface and then undergoes a series of conformational changes in a stepwise manner, the active site rupturing before the rest of the tertiary structure. They also observed that the kinetics of the conformational changes are affected by the stability of the HCAII variants, and the conformation of the final state is not affected by protein stability; i.e., in all cases a molten-globule-like state is adopted after prolonged incubation with particles. To obtain residue-specific structural information regarding the protein, a technique such as NMR spectroscopy is required, and indeed localized conformational information has been obtained for some adsorbed peptides using solid-state NMR. HCAI has many advantages as a model for studying the processes involved in protein adsorption to interfaces, since several of the conformational states it adopts have been characterized in extensive studies of its stability and dynamic properties using NMR in conjunction with H/Dexchange experiments [88]. HCAI is a monomeric enzyme, consisting of 260 amino acid residues [89], with a molecular weight of 28,700 Da and pI of 6.6 [90], and the charged side chains are rather evenly dispersed over the surface. Therefore, no obvious interaction site is easily identified with the possible exception of a cluster of positive charges in the N-terminal region. It is largely composed of â-sheets and is bisected by 10 strands that span the entire molecule [91]. The unfolding of the protein has been demonstrated to be a three-state process that includes formation of a stable equilibrium intermediate of the molten-globule type which is formed at moderate concentrations of denaturant [88–92]. These characterized states can be used as references when the conformations induced by adsorption are compared. In this study reported by Karlsson et al. [72] the authors mainly used HCAI, and also variants of HCAII which are described elsewhere. In the cited study, analytical ultracentrifugation and gel permeation chromatography were used to investigate changes in the amounts of free and bound protein present in the silica particle suspension over time [72, 82]. Both of these analytical methodologies can easily separate unbound protein from the

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particle and particle-protein fractions. In addition, CD was used to follow changes in the secondary and tertiary structures over time after the protein was mixed with silica particles. HCAI is more stable than HCAII and establishes a dynamic equilibrium between bound and unbound protein following mixture with silica particles. At equilibrium the exchange between the two states is slow on the NMR time scale. This allows to study the effects of the interaction with the solid surface on the protein structure in more detail than it would be possible for a process with faster kinetics. That is, the effects on the protein structure from the interaction with the solid surface lingers in the unbound protein and can therefore be inferred from monitoring the unbound state with high-resolution solution NMR. This study shows that differing degrees of particle curvature strongly influence the amount of the protein’s secondary structure that is perturbed. In contrast, the effects on the tertiary structure seem to be independent of the particles’ curvature. Figure 5.5 shows the size of the different silica particles compared to that of HCAI. From the schematic illustration in Fig. 5.5 it can be seen that the curvature (particle size) affects the particle surface area that is readily available for interaction with the protein, in the absence of structural rearrangements or loss of native conformation. Side-on adsorption of the slightly ellipsoidal protein to the particles would give 240, 270, and 310 Å2 interaction areas for 6, 9, and 15 nm silica particles at a limit distance of 3 Å between the protein and the particles, and the corresponding values for end-on adsorption are 230, 265, and 300 Å2 . Interfacing proteins with CNTs, with retention of their biological activity, is critical in a number of emerging applications including biosensing, biorecognition, delivery, and the development of functional composites/coatings [93–96]. As a result, a variety of methods, both involving covalent and non covalent binding, have been explored in the past for the attachment of proteins [93, 94, 97–103] including enzymes, antibodies, and lectins onto SWCNTs. The carbodiimide-activated amidation of carboxylic acids generated by the sidewall oxidation of SWCNTs has been used for the covalent attachment of proteins onto SWCNTs [95, 97]. A variety of non covalent methods including direct physical adsorption [98, 103], immobilization using surfactants [99, 100] or adsorbed polymer layers [94] have also been explored. Such conjugates have been shown to be useful in a variety of novel

Fig. 5.5 To the left of the arrow are shown the differences in size between HCAI (represented by a ribbon structure) and the different particles (represented by gray spheres) in a 2D plane. To the right of the arrow is a schematic illustration of the effect of the particle curvatures on the protein’s secondary structure

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applications. Specifically, Chen et al. have used Tween-20 for the selective immobilization of protein antigens to prepare biosensors capable of selectively detecting monoclonal antibodies in solution [94]. Barone et al. have also recently reported the design of near-infrared optical sensors based on SWCNT-glucose oxidase conjugates [93]. Furthermore, SWCNTs functionalized with proteins have been shown to enter human promyelocytic leukemia cells and human T cells [95], providing the basis of a new tool for protein and gene delivery. The majority of these studies, however, has been driven by the desire to enhance nanomaterial properties by conferring a specific biological function, and in contrast, very little is known about the ability of these nanoscale materials to enhance protein function. However, the ability of SWCNTs to enhance protein stability to a greater extent than conventional flat supports in harsh environments was also reported [95]. The enhanced stability of proteins adsorbed on nanotubes will be particularly useful in applications that also make use of the other attractive features of SWCNTs (e.g., high surface area per unit weight, electrical conductivity, and high mechanical strength). Examples include biocatalytic films [104], biosensors [93, 94], and composites [96]. The experimental results reported by Asuri et al. [105] and the accompanying theoretical analysis also suggest that the observed enhancements in protein stability are not unique to nanotubes and should also be obtained with other nanomaterials. Consistently with this prediction, the authors have observed an enhancement in the stability of proteins in harsh environments on other nanoscale supports, including gold nanoparticles. The ability to enhance protein stability in harsh environments by interfacing them with nanomaterials may therefore be a widely applicable strategy. Also, many enzymes, e.g., lipases, improve their activity upon the adsorption onto nanostructured carrier materials [106]. Nanostructured polystyrene (PS) and polymethylmethacrylate (PMMA) were used as carriers for the preparation of bioconjugates with lipolytic enzymes, such as Candida rugosa lipase (CRL) and Pseudomonas cepacia lipase (PCL). Simple addition of the lipase solution to the polymeric nanoparticles under protein-friendly conditions (pH 7.6) led to the formation of polymer-enzyme bioconjugates. Energy filtered-transmission electron microscopy (EF-TEM) performed on immuno-gold labeled samples revealed that the enzyme preferentially binds to the polymer NPs and that the binding does not affect the nanostructured features of the carriers. The studies performed on the activity of the bioconjugates pointed out that lipolytic enzymes adsorbed onto polymeric NPs show an improved performance in terms of activity and selectivity with respect to those shown by lipases adsorbed on the same non-nanostructured carriers. The residual activities of CRL and PCL immobilized on nanostructured PMMA and PS reached 60 and 74%, respectively. Moreover, enantioselectivity and pH and thermal stability increased upon immobilization. On the basis of the chemical structuresof the selected polymers and the slopes of the adsorption isotherms, the author suggested an hydrophobic binding model for lipase/nanostructured polymers. In fact the data point out that both enzymes increased their activity after adsorption as a consequence of the strong hydrophobic interactions of the protein with the carrier, which seems to prevent, to some extent,

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the possible surface-induced inactivation. Moreover, a remarkable feature observed for both enzymes is that the activity of the adsorbed lipases onto the nanostructured polymers is substantially higher than that of free lipases in solution, with conversion values that from 10% (free CRL) rise up to 50% for the immobilized one in n-hexane. As far as CRL bioconjugates are concerned, the enantioselectivity also seems to be influenced by the nature of the support. A comparison of the enantiomeric excess (ee) values, for the free and bioconjugated CRL preparations, shows that there is an increase of the selectivity upon adsorption onto the polymeric nanostructured carriers. These results highlight the possibility that CRL molecules have retained their catalytic activity also in the organic medium and that it is also possible to obtain and stabilize new protein conformers. Recent advances in nanoscale materials offer a new pathway for regulating enzyme behaviour through surface interactions, providing a promising alternative to small molecule inhibition. Nanomaterials possess at least two attributes not possessed by small molecule ligands: first, they may provide large surface areas for efficient protein binding, as typically >6 nm of buried surface are involved in forming protein–protein interfaces in nature [107]. Also, multivalent functionalities can be grafted on the materials to meet the structural complexity of proteins. Among diverse nanoscale entities, monolayer-protected NPs are extremely attractive due to their ease of synthesis and structural diversity and have found successful applications in the interaction with proteins [108]. As a representative serine protease, R-chymotrypsin (ChT) provides an excellent enzyme model for studying the modulation of activity due to its well-defined structure and extensively characterized enzymatic properties [109]. As illustrated in Fig. 5.6a, the active pocket of ChT is surrounded by a ring of positively charged residues. Meanwhile, a number of hydrophobic “hot spots” are distributed on the

Fig. 5.6 (a) Molecular structure of R-chymotrypsin. (b) Chemical structure of amino-acid-functionalized gold nanoparticles and SPNA-derived substrates. (c) Schematic representation of monolayer-controlled diffusion of the substrate into and the product away from the active pocket of nanoparticlebound ChT (Reprinted with permission from You et al. [110]. Copyright 2009 American Chemical Society)

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surface. Such structural features allow the electrostatic as well as hydrophobic interaction with receptors possessing complementary surfaces. You et al. [110] have systematically investigated the enzymatic kinetics of ChT upon binding to amino-acid-functionalized gold NPs towards different substrates and demonstrated that the complex formation provides a powerful tool to tune enzyme specificity. The association of ChT with anionic NPs leads to the increase of specificity towards a positively charged substrate and the decrease of specificity towards a negatively charged one. Such enhanced substrate selectivity originates from the electrostatic interaction as well as the steric repulsion of the substrates with the NP monolayer. Significantly, the presence of NPs can also affect the catalytic constants of the enzyme towards different substrates. The underlying mechanism is attributed to the electrostatic interactions and controlled diffusion of the hydrolyzed product. These results revealed that the inhibition of the enzymes by NPs is by far different from that of conventional competitive or non competitive mechanisms. Monolayer-functionalized nanoparticles provide a potent scaffold for the creation of an enzyme modulator based on surface recognition. The key role of protein–nanoparticle interactions in nanomedicine and nanotoxicity has begun to emerge recently with the development of the idea of the nanoparticle-protein “corona”. This dynamic layer of proteins (and other biomolecules) adsorbs on NP surfaces immediately upon contact with living systems [111]. It is well-known that adsorption of proteins on interfaces often induces substantial alterations to the protein structure. The interaction between human adult hemoglobin (Hb) and bare CdS QDs has been investigated by fluorescence, synchronous fluorescence, CD, and Raman spectroscopic techniques under physiological pH 7.43. CdS QDs dramatically alter the conformation of Hb, quenching the intrinsic fluorescence of Hb and decreasing the α-helix content of the secondary structure from 72.5 to 60.8%. Raman spectroscopy results indicate that the sulfur atoms of the cysteine residues form direct chemical bonds on the surface of the CdS QDs [112]. Geo-inspired synthetic chrysotile is an asbestos reference standard, and has been used to investigate homomolecular exchange of BSA between the adsorbed and dissolved state at the interface between asbestos fibers and the biological medium. Fourier transform infrared spectroscopy (FTIR) and CD spectroscopy show that, in the solid state, BSA modifications are driven by surface interactions with the substrate. Once BSA is desorbed back into solution its structure rearranges, although some of the modifications occurred, with respect to the native species, are irreversible [113]. Similar effects have been observed with polystyrene NPs – adsorption and subsequent desorption from polystyrene particles causes irreversible changes in the stability and secondary structure of BSA [114]. The α-helix content is reduced, while the β-turn (a region of the protein involving four consecutive residues where the polypeptide chain folds back on itself by nearly 180◦ ) [115] fraction is increased in the exchanged molecules. The irreversible surface-induced conformational change may be related to the aggregation of BSA molecules after exposure to a hydrophobic surface.

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Fig. 5.7 Schematic representation of lysozyme adsorption on silica particles with different sizes. Stronger protein–particle interactions exist in the case of larger nanoparticles, resulting in more protein unfolding and less enzymatic activity (Reprinted with permission from Vertegel et al. [116]. Copyright 2009 American Chemical Society)

Adsorption of chicken egg lysozyme on silica NPs of various diameters has been studied. Special attention has been paid to the effect of NP size on the structure and function of the adsorbed protein molecules. Lysozyme structure and function upon adsorption onto silica NPs is strongly dependent upon the size of the NPs (see Fig. 5.7). A less significant perturbation of lysozyme’s secondary structure is observed when the protein is adsorbed onto smaller NPs under otherwise similar conditions. Furthermore, the structural results correlate nicely with retention of more native-like enzyme activity for adsorption on smaller NPs. The results reported provide evidence that smaller NPs, perhaps because of their greater surface curvature, promote the retention of more native-like protein structure and function when compared to their larger (and hence less curved) particle counterparts. The influence of surface curvature is not entirely unexpected. Nature is replete with examples of nanoscale surfaces that are highly curved, for example the molecular components of subcellular organelles and membranes. These curved surfaces may result in the stabilization of proteins, nucleic acids, and other biological macromolecules with significant secondary and tertiary structure [116]. While, in general, the loss of secondary structure and consequent changes in the activity of proteins upon binding to NPs can be seen as a drawback or a potential source of Np toxicity, there is a potential positive outcome too. Promising uses of NPs include increasing protein stability towards enzyme degradation and increasing the activity of enzymes via immobilization on surfaces. Enzymes such as Candida rugosa lipase (CRL) and Pseudomonas cepacia lipase (PCL) have been adsorbed on nanostructured polystyrene (PS) and polymethylmethacrylate (PMMA) by simple addition of the lipase solution to the polymeric nanoparticles under protein-friendly conditions (pH 7.6). Adsorption leads to improved performance in terms of activity and selectivity with respect to that shown by lipases adsorbed on the same nonnanostructured carriers, as well as increased enantioselectivity and pH and thermal stability [106]. Karajanagi et al. have also examined the structure and function of two enzymes, R-chymotrypsin (CT) and soybean peroxidase (SBP), adsorbed onto single-walled carbon nanotubes (SWNTs). SBP retained up to 30% of its native activity upon adsorption, while the adsorbed CT retained only 1% of its native activity. Analysis of the secondary structure of the proteins via FT-IR spectroscopy

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revealed that both enzymes undergo structural changes upon adsorption, with a substantial secondary structural perturbation observed for CT. FT-IR spectroscopy provided clear evidence of secondary structural perturbations as a result of the protein interaction with the nanotube surface. The substantial structural perturbation for CT is consistent with its nearly complete loss in catalytic activity. SBP, however, clearly retains its native shape and a large fraction of its native secondary structure and is highly active on the hydrophobic nanomaterial. Although it is unclear why the two enzymes behave so differently on the surface of the SWCNTs, this study highlights the complexity of protein–material interactions at the nanoscale. Investigating the structure and function of proteins immobilized onto SWCNTs, as shown in this study, will be critical for developing a better understanding of how proteins interact with SWCNTs and eventually for the design of functional carbon nanotube-protein systems [117]. To date, extensive R&D efforts have been conducted to optimize the carrier materials’ structure to obtain more efficient biocatalysts [118]. In this regard, nanostructured materials will provide the upper limits in terms of balancing the contradictory issues including surface area, mass transfer resistance, and effective enzyme loading [119]. The reported works in this area have revealed the great potential for the use of nanoporous [120], nanofibrous [119], and nanoparticle [106] materials as new classes of carriers for biocatalysts. The effective enzyme loading on nanomaterials can be very high (for example, it can reach over 10 wt% with particles smaller than 100 nm [122], and a large surface area per unit mass is also provided to facilitate reaction kinetics). An interesting question arises considering the unique physical behaviors of NPs. Unlike large-size solid materials, NPs dispersed in a solution are mobile in form of Brownian motion. In that sense, the enzymes attached to nanoparticles are not “immobilized.” On the other hand, according to the StokesEinstein equation, the mobility or diffusivity of the NPs has to be smaller than those of native free enzymes due to their relatively larger sizes. This mobility difference may point to a transitional region between the homogeneous catalysis with free enzymes and the heterogeneous catalysis with immobilized enzymes. The question that follows that assumption is “Will the mobility of the nano-sized biocatalysts, in addition to other factors such as structural stress, impact the intrinsic activities of the attached enzymes?” In other words, does the size of the particles affect enzymatic activity? Many researchers address these questions through experimental observations and theoretical modelling of the catalytic behaviors of NPs. Among them Jia et al. have demonstrated the validity of a simple model developed based on StokesEinstein equation and collision theory for the prediction of catalytic behaviours of NP biocatalysts [119]. The model was proven feasible in correlating the effects of particle size and viscosity on the catalytic kinetics of the particle-attached enzyme. These observations suggest that the mobility of the catalysts is an important factor in determining their activity, and thus providing an explanation, among other considerations, for the high activities usually observed for enzymes attached to NPs. It was also demonstrated that the loss of catalyst mobility, in addition to other factors such as protein conformational changes, also contribute to the loss of intrinsic activities of immobilized enzymes.

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As far as enzyme immobilization is concerned, the biocompatibility of support is another important requirement [120–123], as the biocompatible surface can reduce some non-biospecific enzyme-support interactions, create a specific microenvironment for the enzymes and thus provide substantial benefits to the enzyme activity [124]. To increase the biocompatibility of the support, various surface modification protocols have often been used to introduce a biofriendly interface on the support surface, such as coating, adsorption, self-assembly and graft polymerization. Among these methods, it is relatively easy and effective to directly tether natural macromolecules on the support surface to form a biomimetic layer for enzyme immobilization. In fact, this protocol has been used in tissue engineering recently [125–127]. Chitosan (D-glucosamine), which is present in variety of sources and commercially obtained from wastes generated by fishing industry, is considered to be a suitable support for enzyme immobilization since it is nontoxic and user-friendly, and has protein affinity [128, 129]. Gelatin (a protein) with characteristics of easily available and low in cost is derived from collagen [130]. It shows biological properties that is almost identical with those of collagen. These features make gelatin widely used in a variety of biomedical applications. Since these two biomaterials containing reactive groups (amino groups) have been successfully used as enzyme immobilization supports, it is reasonable to choose them for the formation of biomimeticlayers on the PANCMA nanofibrous membrane surface for enzyme immobilization. Lipase, one kind of the most utility enzyme [131–134], was immobilized on these nanofibrous membranes with GA and 1-ethyl-3-(dimethylaminopropyl) carbodiimide hydrochloride (EDC)/N-hydroxyl succinimide (NHS) as coupling agent, respectively (see Figs. 5.8 and 5.9). The protocol established by the authors demanstrate that there is an increase of the activity retention of the immobilized lipase on the gelatine modified nanofiber membrane and on the chitosan-modified nanofiber membrane, compared to that on the nascent nanofiber membrane. The kinetic parameter Km values of the immobilized lipase on the nanofiber membranes are lower than that on the hollow fiber membrane. In comparison with the chitosan-modified membrane, there is a decrease of the Km value for the immobilized enzyme on the gelatine modified membrane. After immobilization, the pH, thermal and reuse stabilities of the immobilized

Fig. 5.8 Schematic representation of the preparation of the support and enzyme immobilization (Reprinted with permission from Ye et al. [135]. Copyright 2009 Elsevier)

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Fig. 5.9 SEM photographs of the electrospun PANCMA fibrous membranes. Voltages are (a) 8 kV, (b) 10 kV, (c) 12 kV and (d) 14 kV (Reprinted with permission from Ye et al. [135]. Copyright 2009 Elsevier)

enzyme can be enhanced. Thus, the method described in this work indicates that the nanofiber membrane modified with chitosan and gelatin could be potential supports in the enzyme immobilization technology for industrial applications.

5.4.3 Immobilization of Whole Cells on Nanostructured Polymers Impressive advances are being made also in the synthesis of chemically functionalized and patterned biocompatible surfaces for the immobilization of whole cells of microorganisms such as bacteria and yeast [136–139]. Such surfaces have important application wherein immobilized bacterial and fungal cells (genetically engineered and otherwise) may be used as “factories” for the production of industrially and medically important enzymes and metabolites [140]. The repair or replacement of damaged tissues using in vitro strategies has focused on manipulation of the cell environment by modulation of cell–extracellular matrix interactions and cell–cell interactions. These methodologies have potential applications in tissue engineering and implant biology [141]. Moreover, entrapment, immobilization, and spatial control over cell patterns on different surfaces represents an important tool for fundamental studies in cell biology [142, 143], biosensing [144, 145], and studies of cell interactions with different materials [146, 147]. Control over cell shape and

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functions is generally achieved by the immobilization of the cells on spatially controlled (preferably on a submicron to micron scale) designed surfaces of varying “adsorptivity” of the biological components. Such patterned surfaces for immobilization of cells have been obtained using microcontact printing (í-CP) on reactive [142], mixed self-assembled monolayers (SAMs) [148], SAMs made up of alkanethiolates [149], alkylsilanes [150], by the sol-gel technique [151], nanometer controlled laser ablation [152], and using lastomeric membranes [153]. Recently organic monolayers and subsequent polymer functionalization has been used to develop patterns in the seeding of bacterial cells [154]. Patterned copolymers on chitosan substrates have been used to form two-dimensional patterns that limit cell attachment and spreading within the unprinted chitosan regions [155]. Sumant Phadtare et al. have focused their work on assembly of specific cells on surfaces from the point of view of using the cells as sources of enzymes for biotransformations and synthesis of new materials. This research group has recently shown that Yarrowia lipolitica [156] and Candida bombicola yeast cells [157] can be immobilized on patterned hydrophobic regions of thermally evaporated fatty lipid films. In the latter case, the enzyme cytochrome P450 present in the yeast cells was used to catalyze in situ ö and ö-1 hydroxylation of arachidonic acid (AA). As part of our search for newer and more versatile materials with tailorable surfaces for cell immobilization, they describe herein the synthesis of a free-standing gold nanoparticles membrane whose surface may readily be modified to render it compatible for a variety of applications. More specifically, they demonstrate the formation of a self-supporting gold nanoparticle membrane at a liquid interface by the spontaneous reduction of aqueouschloroaurate ions by bis(2-(4-aminophenoxy)ethyl)ether (DAEE) present in chloroform [158]. The gold nanoparticle membrane is extremely stable, robust, easily handled, and malleable and can be grown over large areas and thickness by suitably varying the experimental conditions. The surface chemical properties of the gold nanoparticles embedded in the polymeric membrane are easily modified using different molecules such as amino acids [159], alkanethiols, etc. Simple immersion of the organic-gold nanoparticle membrane in a solution of octadecylamine (ODA) in ethanol leads to complexation nof ODA molecules with gold nanoparticles [160] and, thereby, hydrophobization of the membrane. The gold nanoparticles embedded in the polymeric membrane are capped with octadecylamine (ODA) molecules, thereby rendering the nanoparticles membrane hydrophobic. Immersion of this hydrophobized membrane in an aqueous dispersion of C. bombicola cells results in their facile assembly on the membrane (Fig. 5.10, step 2). Enzymes of the cytochrome P450 monooxygenase family are known to catalyze hydroxylation of arachidonic acid. Candida bombicola cells possess cytochrome P450 in their cell membrane (this enzyme is highly unstable outside the cells) that is capable of catalyzing hydroxylation of arachidonic acid to form sophorolipids. Thereafter, acid hydrolysis of these sophorolipids yields 20HETE. In cytochrome P450 dependent arachidonic acid monooxygenase reactions the catalytic activity turnover of this enzyme was nicotinamide adenine dinucleotide phosphate (NADPH) dependent [161]. Since the yeast cells are used the cofactors such as NADPH are not required and are readily supplied by the cells along with

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Fig. 5.10 (a) Organic fluorophores (red) are embedded in a calcium phosphate matrix (gray), and the resulting composite particles are stabilized by PEG molecules (light blue) on their surfaces. (b) Polyelectrolyte capsules comprise a multilayer wall (gray) in which magnetic particles (green) can be incorporated for magnetic targeting. The cavity of the capsules can be loaded with analytesensitive fluorophores (red). Though most practical capsule systems thus far have diameters of 1.5 μm, their size can be reduced to 100–200 nm. The capsule assembly technology also enables immobilizing ligands for active targeting (dark blue) and stabilization (light blue) on the capsule surface. (c) Magnetic NPs (green) are embedded in a dextran matrix (gray). Through magnetic targeting, an anticancer drug (red), which is adsorbed on the particles, can be delivered to the tumor tissue. (d) Gold NPs (yellow) can be integrated in the wall of polyelectrolyte capsules (gray), which surrounds an anticancer drug (red) inside the cavity. Light-induced heating of the gold particles locally disintegrates the capsule wall and releases the anticancer drug. (e) A photosensitizer (red) can be excited via energy transfer from a quantum dot NP (yellow) to produce a radical oxygen species. In this case, the particle is also the carrier matrix. The NP is stabilized by ligands on the particle surface (light blue) (Reprinted with permission from Rivera and Parak [199]. Copyright 2009 American Chemical Society)

the primary enzyme. This is a major advantage in using the whole cells rather than using enzyme. The yeast cells bind strongly to the organic-gold nanoparticle membrane, thus providing an excellent, highly reusable bioconjugate material for the above biotransformation.

5.4.4 Nanoparticles and Vaccines Delivery As it is well known oral vaccination offers a large number of advantages compared to other routes of administration. It is a non-invasive, user-friendly method avoiding the need of sterile injection by qualified personnel. Nevertheless, oral administration of free vaccine often results in a very low immune response, which is mainly due to the premature degradation of the antigen in the gastrointestinal (GI) tract. Encapsulation of the vaccine in particulate systems became widely employed to solve this problem. It has been demonstrated that oral immunization with antigenloaded microparticles induces mucosal IgA and systemic IgG antibodies responses, providing a complete immune response. Besides protecting the antigen against the harsh environment of the GI tract, nanocarriers are efficiently taken up by M cells, key players of the mucosal immunity induction. In addition, biodegradable particles allow a sustained release of the antigen, increasing the duration of the contact between antigen and immune cells thus favouring an effective immune response [162, 163].

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The study proposed by Garinot et al. [164] was to target M cells with a specific ligand grafted on antigen-loaded PEGylated PLGA-based nanoparticles. Nanoparticles are known to be better taken up by M cells than microparticles [165–168]. Poly(lactide-co-glycolide) (PLGA) was chosen for its biodegradability properties, its biocompatibility and its approval by FDA. Poly(ε-caprolactonecoethylene glycol) (PCL–PEG), an amphiphilic copolymer, was added to the formulations (i) to take advantage of PEG repulsive properties to provide a higher stability of nanoparticles in biological fluids [169, 170], (ii) to promote uptake by M cells in vitro [171], and (iii) to allow the grafting of a molecule targetingM cells on the surface of the nanoparticles. Three non-targeted formulations were first developed, composed of various percentages of PLGA, PLGA–PEG and PCL–PEG and encapsulating ovalbumin, as model antigen. Formulations were physicochemically characterized and their stability in biological media was evaluated. Their transport by M cells in vitro was studied and compared to enterocyte transport. Formulations were then targeted to β1 integrins expressed at the apical side of M cells [172, 173] via an RGD peptide. A novel photografting method was developed to graft the peptide on PEG chain of PCL–PEG. The transport of targeted and non-targeted nanoparticles across M cells model was measured in vitro and they also evaluated the immune response induced by their oral delivery potential. Gullberg et al. [173] demonstrated that the use of a RGD ligand at the nanoparticle surface to target M cells provoked a slight increase in the number of mice producing IgG after immunization, thus confirming the importance of targeting the carriers. Nevertheless, no increase of the IgG production in serum was observed with the use of the RGD ligand. This could be due to a partial degradation of the peptide during its trafficking in the gastrointestinal tract. To avoid the ligand degradation in the stomach or in the gut, non-peptidic β1 integrin ligand could be used and grafted on the nanoparticles in the place of RGD peptides.

5.5 Applications of Nanomaterials to Molecular Imaging The identification and characterization of intracellular events and protein–receptor interactions are of great importance for basic medical studies and therapeutic applications. Optical analysis based on fluorescence labeling has been extensively used to study these interactions. Detection and in vivo imaging of a variety of biological samples, such as living cells and tissues, can be carried out in this way. Of the many fluorescent labeling reagents, organic fluorophores are used most often in these cases. The fluorophores usually have high quantum yield and can be easily used in many different applications. However, they usually suffer from limited sensitivity and photostability. Moreover, most fluorophores present a certain level of toxicity that hinders their application in in vivo cellular studies and imaging. In the last 10 years or so, significant advances have led to a large variety of labeling reagents based on nanomaterials with controlled size and shape. These nanomaterials represent an exciting and often more effective alternative to the use of organic fluorophores. The superior properties of such nanomaterials provide multiple possibilities for

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highly sensitive detection of various targets under different conditions, from the single-molecule level to human body applications, from in vitro diagnosis to in vivo real-time imaging [174–176]. Loo and coworkeers describe the use of a novel class of contrast agents based on nanoshell bioconjugates for molecular imaging in living cells. Nanoshells offer significant advantages over conventional imaging probes including continuous and broad wavelength tunability, far greater scattering and absorption coefficients, increased chemical stability, and improved biocompatibility. Nanoshell bioconjugates can be used to effectively target and image human epidermal growth factor receptor 2 (HER2), a clinically relevant biomarker, in live human breast carcinoma cells [177]. Gold NPs are already in commercial products. One well-known example is the lateral flow strip developed for fast pathogen detection and pointof- care diagnosis [178, 179] The unique optical properties of QDs (high quantum yield, high molar extinction coefficients (∼10–100) that of organic dyes, broad absorption with narrow, symmetric photoluminescence (PL) spectra (full-width at half-maximum ∼25–40 nm) spanning the UV to near-infrared, large effective Stokes shifts, high resistance to photobleaching and exceptional resistance to photo- and chemical degradation) make them appealing as in vivo and in vitro fl uorophores in a variety of biological investigations. QD spectroscopic properties can be exploited to achieve somewhat deeper penetration thanthe available nearinfrared dyes [180]. This was demonstrated by synthesizing near-infrared emitting QDs (840–860 nm) and applying them to sentinel lymph-node mapping in cancer surgery of animals. Using only 5 mW cm−2 excitation, they imaged lymph nodes 1 cm deep in tissue, where lymphatic vessels were clearly visualized draining QD solutions into the sentinel nodes [181]. As an added bonus, the location of QD accumulation in the excised nodes may be the most likely place for the pathologist to find metastatic cells. Interestingly, QDs have been demonstrated to remain fluorescent in tissues in vivo for 4 months [182]. In addition to the properties described above, QDs have large two-photon cross-sectional efficiency with a two-photon fluorescence process 100–1,000. that of organic dyes. This makes them suitable for in vivo deep-tissue imaging using two-photon excitation at low intensities [183]. Using this technique to excite green emitting QDs in the near infrared allowed imaging of mice capillaries hundreds of micrometres deep and subcellular resolution of mouse brain [184]. QDs may also be useful for tracking cancer cells in vivo during metastasis A multifunctional QD probe has been developed linked with tumour-targeting antibodies [185]. In vivo studies in mice expressing human cancer showed that these QD probes accumulated at the tumour sites. Using a slightly different approach, tumour cells were labelled with QDs, injected into mice and then tracked with multiphoton microscopy as they invaded lung tissue. In both studies, spectral imaging and autofl uorescent subtraction allowed multicolour in vivo visualization of cells and tissues. The ability to track cells in vivo without continuously sacrifi cing animals represents a substantial improvement over current techniques. Although QDs are clearly superior to dyes for these purposes, the question of whether the large QD probe mirrors true in vivo physiology remains unanswered. NPs with superparamagnetic properties are ideal media for the manipulation of biological materials, targeting delivery of therapeutic compounds, and hyperthermia treatment. Because of their

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superparamagnetism, magnetic NPs can also act as a contrast reagent in magnetic resonance imaging (MRI) for diagnosis.

5.6 Applications of Nanostructured Materials in Drug Delivery and Cancer Therapy Cancer was once considered an incurable disease, but today most patients diagnosed with an early stage disease will survive their illness. Advances in cancer diagnostics and therapeutics over the last few decades are largely responsible for this dramatic improvement. This is shown by the 5-year survival rate for all cancers diagnosed between 1996 and 2002, which is 66%, up from 51% in 1975–1977. Moreover, for the second year in a row – and for only the second time ever – the actual number of cancer deaths in the US fell in 2006. Despite these advances, cancer remains the second leading cause of death in the US, exceeded only by heart disease, and accounts for one in four deaths [186]. Chemotherapy has become an integral component of cancer treatment for most cancers. Despite the last 30 years of efforts in oncology drug discovery, conventional chemotherapeutic agents still exhibit poor specificity in reaching tumor tissue and are often restricted by dose-limiting toxicity. The combination of developing controlled release technologies and targeted drug delivery may provide a more efficient and less harmful solution to overcome the limitations found in conventional chemotherapy. Recent interest has been focused on developing nanoscale delivery vehicles capable of controlling the release of chemotherapeutic agents directly inside cancer cells [187, 188]. Controlled release occurs when a natural or synthetic polymer is combined with a drug in such a way that the drug is encapsulated within the polymer system for subsequent release in a predetermined manner. Polymeric drug delivery vehicles that are designed as particles can range in size from 50 nm to over 10 μm and can release encapsulated drugs through surface or bulk erosion, diffusion or swelling, followed by diffusion, in a time- or condition-dependent manner. The release of the active agent may be constant over a long period, or it may be triggered by the environment or other external events [189]. In general, controlledrelease polymer systems can provide drug levels in the optimum range over a longer period of time than other drug delivery methods, thus increasing the efficacy of the drug and maximizing patient compliance. The primary consideration of drug delivery is to achieve more effective therapies while eliminating the potential for both under- and over-dosing. Other advantages of using controlled release delivery systems can include the maintenance of drug levels within a desired range, the need for fewer administrations, optimal use of the drug in question, and increased patient compliance. Strategies on delivering drug-encapsulated NPs to cancerous tissue have been focused on passive and active targeting. The former approach uses the unique properties of the tumor microenvironment, most notably: (i) leaky tumor vasculature, which is highly permeable to macromolecules relative to normal tissue; and (ii) a dysfunctional lymphatic drainage system, which results in enhanced fluid

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retention in the tumor interstitial space [190]. Active targeting can be achieved by the functionalization of NPs with ligands such as antibodies, peptides, nucleic acid aptamers, carbohydrates, and small molecules. As discussed in the next section of this review, some of these therapeutic conjugates are now under clinical development or in clinical practice today. However, the success has been largely limited to ligand-drug conjugates and attempts have been made to enhance these systems by encapsulating the therapeutic agents in NPs. Several classes of materials have been developed for targeted NPs, including biodegradable polymers, dendrimers, nanoshells, nucleic-acid-based NPs, and liposomes. Biodegradable polymer NPs have been extensively investigated for cancer therapy [191]. Polymeric NPs typically have a prolonged systemic circulating half-life by grafting, conjugating, or adsorbing sterically amphiphilic polymers such as polyethylene glycol (PEG) to the particle surface [192]. Polymeric NPs can be formulated to encapsulate hydrophilic or hydrophobic small drug molecules, and macromolecules such as proteins and nucleic acids [193]. These NPs can be used to release the encapsulated drugs at a controlled rate via surface or bulk erosion, diffusion, or swelling followed by diffusion, in a time- or condition-dependent manner. The rate of drug release can be controlled by modification of the polymer side chain, development of novel polymers, or synthesis of copolymers [194].

5.6.1 Nanoparticles and Gene Delivery Small positively charged particles are formed by the self assembly of cationic, hydrolytically degradable poly(-amino esters) and plasmid DNA. These nanoparticles are biodegradable and suitable for nonviral gene transfer to stem cells; moreover the end group of the polymer can be modified leading to improvement of the gene delivery efficacy [195]. Nowadays polymer-DNA complexes (polyplexes), polymer-drug conjugates, and polymer micelles bearing hydrophobic drugs, have received increasing attention for their ability to improve the efficacy of cancer therapeutics. The polymer therapeutic agents, due to their nano-size and biocompatibility, show striking properties such as capability to reach the target site, to specifically bind receptors that are overexpressed in cancer cells and then to improve the efficacy of therapy [196]. A deep investigation on the assembly into multilayer thin films of DNA-PNIPAM (poly(N-isopropylacrylamide) micelles, and subsequent poly(ethyleneglycol) (PEG) functionalization to produce DNA_PNIPAM microcapsules is described [197]. The authors highlight the engineering of oligonucleotide-grafted poly(Nisopropylacrylamide) (PNIPAM) micelles, bearing cDNA sequences assembled with the layer-by-layer (LBL) method, into stable films/capsules, giving evidence for the microcapsules being considerably less permeable than single-component DNA capsules. These engineered systems envisage their applications in the controlled delivery of therapeutics. The role of composite multifunctional nanoparticles (including optical excitation of functionalized gold nanoparticles) in cancer detection and treatment are clearly depicted in papers that describe the perspectives and advances of this important

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research worldwide [198]. A comprehensive example of these complex systems is schematically shown in the Fig. 5.10.

5.6.2 Targeting Molecules for the Development of Targeted NPs Over the past decade, a wide variety of antibody-based targeting molecules have been assessed for their potential application in cancer therapy [200]. Monoclonal antibodies (mAb) were the first and are still the preferred class of targeting molecules. Current developments of antibodies have been focused on chimeric, humanized, and fully humanized derivatives to decrease their immunogenicity. Some of these antibody-based drugs have already undergone clinical development and have been successfully translated into the clinical environment. Such examples R trastuzumab (Herceptin), R cetuximab (Erbitux), R include rituximab (Rituxan), R Rituximab was approved by the FDA for treating and bevacizumab (Avastin). B-cell lymphoma in 1997. A novel class of molecules, referred to as nucleic acid ligands (aptamers), has been developed that may rival antibodies in its potential for therapeutic and diagnostic applications [80, 81]. Aptamers are DNA or RNA oligonucleotides or modified DNA or RNA oligonucleotides that fold by intramolecular interaction into unique conformations with ligand-binding characteristics [201]. Like antibodies, aptamers can be prepared to bind target antigens with high specificity and affinity. The use of aptamers as targeting molecules has several potential advantages over antibodies. Aptamers with high affinity for a target can be prepared through in vitro selection; a process called systemic evolution of ligands by exponential enrichment (SELEX). SELEX is essentially reiterative rounds of in vitro selection and amplification to enrich aptamers present in a library of ∼1,015 random oligonucleotides [202]. Peptides have gained a lot of attention as a potent targeting ligand. The recent success is partially a result of the development of peptide phage libraries (∼1,011 different peptide sequences), a bacterial peptide display library, a plasmid peptide library, and new screening technologies. Combinatorial libraries have led to the discovery of short peptides (10–15 amino acids) that are able to bind to targeted proteins, cells, or tissues specifically [203]. One of the most extensively studied small molecule targeting moieties for drug delivery is folic acid (folate). The high-affinity vitamin is a commonly used ligand for cancer targeting because folate receptors (FRs) are frequently over-expressed in a range of tumor cells [204]. Folate specifically binds to FRs with a high affinity (KD = ∼10–9 M), enabling a variety of folate derivatives and conjugates to deliver molecular complexes to cancer cells without causing harm to normal cells.

5.6.3 Microfluidics – An Emerging Technology for Targeted Drug Delivery NPs Microfluidics is the science and technology of manipulating nanoliter volumes in microscale fluidic channels [205]. Manz et al. [206], have shown that several

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labor-intensive and time-consuming steps such as sample preparation, purification, mixing, reactions, separations, and detection could be performed on a single monolithic microfabricated device. Miniaturization in conjunction with integration of multiple functionalities, which harness unique microscale phenomena, has led to microfluidic systems that perform better than macroscale systems, reduce labor input, and have the potential for low-cost mass production [205]. Since then, the field of microfluidics has blossomed and is now poised to impact areas as diverse as chemical synthesis, biological analysis, optics, information technology, forensics, and environmental monitoring. The ability of microfluidic systems to mix reagents rapidly and provide homogeneous reaction environments, continuously vary reaction conditions, enable rapid temperature control, and allow addition of reagents at precise time intervals during the progress of a reaction are some of the key features that have made microfluidic systems useful for the synthesis of NPs [207]. For example, millisecond timescale control of reaction conditions in a droplet-based microfluidic device has enabled the synthesis of highly monodisperse CdS NPs and CdS/CdSe core-shell NPs. Microreactors enable screening through a variety of reaction conditions by systematically varying flow rates, temperature, and concentrations. The optimal reaction conditions for CdSe NP synthesis have been identified in this way using very small amounts of reagents [208]. Microfluidic devices have since then been used for the synthesis of high-quality CdS, CdSe, Ag, Pd, Cu, and CdSe/ZnS core-shell NPs [209] While it is doubtful that microfluidics will enable synthesis of NPs from start to finish at least in the near future, there are still critical processing steps where microfluidic synthesis will be of advantage – most notably, NP precipitation. Biological applications of carbon nanotubes (CNTs) have started to emerge [210]. This opening has been favored by the development of novel methods of solubilisation either by covalent or supramolecular functionalisation [211]. Within the covalent strategy, the organic functionalisation of single- and multi-walled carbon nanotubes (SWCNTs, MWCNTs) by 1,3-dipolar cycloaddition of azomethine ylides can produce highly water soluble derivatives, ready for further modifications [212]. The authors also have recently demonstrated that it is possible to prepare amino acid and peptide functionalised CNTs. In particular, peptide–SWCNT conjugates elicit strong antipeptide antibody responses in mice with no detectable crossreactivity to the carbon nanotube.1b In this study it was demonstrated that water soluble SWCNT derivatives, modified with a fluorescent probe, translocate across cell membranes. Davide Pantarotto et al. showed that a peptide responsible for the G protein function, when covalently linked to SWCNTs, penetrates into the cell. G proteins are an important family of proteins involved in signal transduction. Since the functionalised CNTs appear to be non toxic for the cells (videinfra), they can be considered as a new tool to deliver peptides, peptidomimetics, or small organic molecules into the cells [213]. To determine whether CNTs can be used to deliver active molecules into the cell, the authors have prepared the FITC-labeled single walled carbon nanotube derivative and the peptide–SWCNT conjugate (FITC, fluorescein isothiocyanate). From this study, it clearly appears that CNTs are a very promising carrier system for future

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applications in drug delivery and targeting therapy. For this purpose, the authors are currently exploring the use of CNTs as nanovehicles and evaluating the biological functions of the covalently linked molecules aftercellular uptake. Therefore, these systems can help to solve transport problems for pharmacologically relevant compounds that need to be internalised, and may have potential therapeutic applications including vaccine delivery [214].

5.7 Drug Delivery and Magnetic NPs Different organic materials (polymeric NPs, liposomes, micelles) have been investigated as drug delivery nanovectors using passive targeting, active targeting with a recognition moiety (e.g., antibody), or active targeting by a physical stimulus (e.g., magnetism in magnetoliposomes). However, these organic systems still present limited chemical and mechanic stability, swelling, susceptibility to microbiological attack, inadequate control over the drug release rate, and high cost. Polymer NPs also suffer from the problem of high polydispersity. Synthesis produces particles with a broad size distribution and irregular branching, which could lead to heterogeneous pharmacological properties. One alternative is to use dendrimers, which have a monodisperse character and globular architecture resulting from their stepwise synthesis and can be purified at each step of growth [215]. Visualization of dendrimers requires tagging with a specific moiety (i.e., a fluorophore or metal). A major drawback of dendrimers and dendritic polymers, however, is their high cost. The preparation of dendritic polymers that circulate in the blood long enough to accumulate at target sites but that can also be removed from the body at a reasonable rate to avoid long-term accumulation also remains a challenge. Preparation methods for NPs generally fall into the category of so-called “bottom-up” methods, where nanomaterials are fabricated from atoms or molecules in a controlled manner that is thermodynamically regulated such as self-assembly [216]. Some biomedical applications require core-shell magnetic NPs. They consist of a metal or metallic oxide core, encapsulated in an inorganic or a polymeric coating that renders the particles biocompatible, stable, and may serve as a support for biomolecules. Their magnetic properties enable these particles to be used in numerous applications, belonging to one or more of the following groups: (i) Magnetic contrast agents in magnetic resonance imaging (MRI); (ii) Hyperthermia agents, where the magnetic particles are heated selectively by application of a high frequency magnetic field. (e.g., in thermal ablation/hyperthermia of tumors); and (iii) Magnetic vectors that can be directed by means of a magnetic field gradient towards a certain location, such as in the case of the targeted drug delivery [217]. The scientific community is seeking to exploit the intrinsic properties of magnetic NPs to obtain medical breakthroughs in diagnosis, and drug delivery. Perhaps the

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most promising applications relate to cancer therapy. The potential of drug delivery systems based on the use of nano and microparticles stems from significant advantages such as: (i) the ability to target specific locations in the body; (ii) the reduction of the quantity of drug needed to attain a particular concentration in the vicinity of the target; and (iii) the reduction of the concentration of the drug at nontarget sites minimizing severe side effects. All these benefits justify the exponential growth in the number of publications dealing with NPs for drug delivery applications. NPs can act at the tissular or cellular level. The latter implies that they can be endocytosed or phagocytosed (i.e., by dendritic cells, macrophages) resulting in internalization of the NP. In this process, the NP can reach beyond the cytoplasmatic membrane and, in some cases, also beyond the nuclear membrane (i.e., transfection applications). Tumor targeting with magnetic NPs may use passive or active strategies. Passive targeting occurs as a result of extravasation of the NPs at the diseased site (tumor) where the microvasculature is hyperpermeable and leaky, a process aided by tumor-limited lymphatic drainage. Combined, these factors lead to the selective accumulation of NPs in tumor tissue, a phenomenon known as enhanced permeation and retention (EPR). The majority of solid tumors exhibit a vascular pore cut-off size between 380 and 780 nm, although vasculature organization may differ depending on the tumor type, its growth rate, and microenvironment [218]. Apart from tumors, size-dependent removal of NPs is a common occurrence in healthy capillaries.

5.7.1 The Development of Magnetic Drug Delivery Any overview on drug delivery should start with the deserved recognition of Paul Ehrlich (1854–1915), who proposed that if an agent could selectively target a disease-causing organism, then a toxin for that organism could be delivered along with the agent of selectivity. Hence, a “magic bullet” would be created able to kill the targeted organism exclusively. Ehrlich received the 1908 Nobel Prize in Medicine for his work in the field of immunity, and the magic bullet idea was even used as a script for the 1940 movie Dr. Ehrlich’s Magic Bullet. Since then, various strategies have been proposed to deliver a drug to the vicinity of a tumor including, as mentioned above, the use ofvectors sensitive to physical stimuli and tumor-recognition moieties conjugated to a drug. The use of magnetic micro- and NPs for the delivery of chemotherapeutics has evolved since the 1970s. Zimmermann and Pilwat [219] in 1976 used magnetic erythrocytes for the delivery of cytotoxic drugs. Widder et al. [220] described the targeting of magnetic albumin microspheres encapsulating an anticancer drug (doxorubicin) in animal models. In the 1980s, several authors developed this strategy to deliver different drugs using magnetic microcapsules and microspheres [221]. In 1994, Häfeli et al. [222] prepared biodegradable poly(lactic acid) microspheres that incorporated magnetite and the beta-emitter 90Y for targeted radiotherapy, and successfully applied them to subcutaneous tumors. However, all these initial approaches were microsized. Magnetic NPs were used for the first time in

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animal models by Lübbe et al. [223]. In 1996, the first Phase I clinical trial was carried out by the same group in patients with advanced and unsuccessfully pretreated cancers using magnetic NPs loaded with epirubicin. However, in that first trial, more than 50% of the NPs ended up in the liver. Since then, a number of groups around the world have synthesized magnetic vectors and shown potential applications. Different start-ups now manufacture magnetic micro- and NPs, which are used in MRI, magnetic fsensing, enzyme immobilization, immunoassays, and gene transfection and detection systems. FeRx, Inc. (founded in 1997) produced doxorubicin-loaded magnetic NPs consisting of metallic Fe ground together with activated carbon [224]. A Phase II clinical study in patients with primary liver cancer was conducted using these NPs, although the trial was not successful. Chemicell GmbH currently commercializes TargetMAGdoxorubicin NPs involving a multidomain magnetite core and a cross-linked starch matrix with terminal cations that can be reversibly exchanged by the positively charged doxorubicin. The particles have a hydrodynamic diameter of 50 nm and are coated with 3 mg/ml doxorubicin [225]. These NPs loaded with mitoxantrone have already been used in animal R models with successful results [226]. Chemicell also commercializes FluidMAG R from Alnis for drug delivery applications. Magnetic NP hydro-gel (MagNaGel) Biosciences, Inc. is a material comprising chemotherapeutic agents, Fe oxide colloids, and targeting ligands [227]. The main advantages of magnetic (organic or inorganic) NPs are that they can be: (i) visualized (superparamagnetic NPs are used in MRI); (ii) guided or held in place

Fig. 5.11 High resolution transmission electron microscopy (HRTEM) images of (a) magnetite NP encapsulated in silica; (b) magnetite NPs embedded in a zeolitic (aluminosilicate) matrix; (c) Fe NPs encapsulated in silica (energyfiltering TEM, EFTEM, color map); (d) magnetite NP encapsulated in graphite (Reprinted with permission from Arruebo et al. [228]. Copyright 2009 Elsevier)

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Fig. 5.12 Temporal evolution in the number of scientific papers published involving drug delivery using NPs (Source: ISI Web of Knowledge © The Thomson Corporation. Search terms: “drug delivery” and “nanoparticles”. Date of search: December 2006, Reprinted with permission from Arruebo et al. [228]. Copyright 2009 Elsevier)

by means of a magnetic field; and (iii) heated in a magnetic field to trigger drug release or to produce hyperthermia/ablation of tissue. It is important to point out that the latter capability is not restricted to magnetic NPs, but also to other particles capable of absorbing near-infrared, microwave, and ultrasound radiation. Depending on the synthesis procedure, magnetic NPs or nanocapsules can be obtained. We refer to NPs when the drug is covalently attached to the surface or entrapped or adsorbed within the pores of the magnetic carrier (polymer, mesoporous silica, etc.). Nanocapsules (“reservoirs”) designate magnetic vesicular systems wherether drug is confined to an aqueous or oily cavity, usually prepared by the reverse micelle procedure, and surrounded by an organic membrane (magnetoliposomes) or encapsulated within a hollow inorganic capsule (Figs. 5.11 and 5.12).

5.8 Nanostructured Materials and Tissue Engineering Tissue engineering represents a promising approach to regenerate and repair a wide variety of damaged human tissues, avoiding the problems involved in transplantation.

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Different natural and synthetic materials have been investigated to produce tissue engineering scaffolds, i.e., usually porous implants acting as 3-D templates for cell adhesion, growth and proliferation [229, 230]. For both tissue development and regeneration, numberous factors are essential for the growth and differentiation of cells to form tissues. These factors must be presented on the biomaterial matrices or scaffolds that are employed for tissue engineering, in a manner whereby they are accessible to the cells. Nanotechnologies can be useful in developing strategies for immobilizing bioactive ligands on the scaffold or delivering biomolecules in a sustained fashion. So far, a plethora of techniques for the fabrication of scaffolds have been devised. A few of them have reached a sophisticated degree of control over scaffold morphological characteristics such as textile technology, solid free-form fabrication or three dimensional printing [231–233]. These technologies require the use of sophisticated and expensive apparatus and are limited mainly to the use of synthetic polymers such as alpha-hydoxyacids (e.g., poly(lactic) and poly(glycolic) acids) while are less suitable for the processing of biopolymers. In general, the techniques for the creation of scaffolds by employing biopolymers as the components have not been developed to the same extent as those involving synthetic polymers. The techniques most used in conjunction with biopolymers are particulate leaching or freeze drying which allows the creation of scaffolds characterised by an irregular and scarcely interconnected morphology [234–236]. Electrospinning has emerged to be a simple, elegant and scalable technique to fabricate polymeric nanofibers. Pure polymers as well as blends and composites of both natural and synthetics have been successfully electrospun into nanofiber matrices. Physiochemical properties of nanofiber matrices can be controlled by manipulating electrospinning parameters to meet the requirements of a specific application. Such efforts include the fabrication of fiber matrices containing nanofibers, microfibers, combination of nano-microfibers and also different fiber orientation/alignments. Polymeric nanofiber matrices have been extensively investigated for diversified uses such as filtration, barrier fabrics, wipes, personal care, biomedical and pharmaceutical applications. Recently electrospun nanofiber matrices have gained a lot of attention, and are being explored as scaffolds in tissue engineering due to their properties that can modulate cellular behavior. Electrospun nanofiber matrices show morphological similarities to the natural extra-cellular matrix (ECM), characterized by ultrafine continuous fibers, high surface-to-volume ratio, high porosity and variable pore-size distribution. Efforts have been made to modify nanofiber surfaces with several bioactive molecules to provide cells with the necessary chemical cues and a more in vivo like environment [237–239].

5.8.1 Electrospun Nanofibers in Tissue Engineering Applications The use of electrospun fibers and fiber meshes in tissue engineering applications often involves several considerations, including choice of material, fiber orientation, porosity, surface modification and tissue application [240–243]. Choices in materials include both natural and synthetic materials, as well as hybrid blends of

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the two, which can provide an optimal combination of mechanical and biomimetic properties. As an example aligned nanofibrous scaffolds based on poly(D, L-lactideco-glycolide) (PLGA) and nano-hydroxyapatite (nano-HA) were synthesized by electrospinning for bone tissue engineering [244]. Morphological characterization using scanning electron microscopy showed that the addition of different amounts of nano-HA (1, 5, 10 and 20 wt%) increased the average fiber diameter from 300 nm (neat PLGA) to 700 nm (20% nano-HA) (Fig. 5.13). By varying processing and solution parameters fiber orientation and porosity/pore size of the electrospun scaffold can be controlled and optimized for each

Fig. 5.13 SEM micrographs of nanocomposite scaffolds: (a) neat PLGA (b) PLGA + 1% (c) PLGA + 5% H, (d) PLGA + 10% HA and (e) PLGA + 10% HA. The arrows indicate the orientation direction and the circles indicate broken fiber (Reprinted with permission from Jose et al. [244]. Copyright 2009 Elsevier)

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application. After fabrication the surface of the scaffold can be modified with a high density of bioactive molecules due to the relatively high scaffold surface area. Due to the flexibility in material selection as well as the ability to control the scaffold properties, electrospun scaffolds have been employed in a number of different tissue applications including: vascular, bone, neural, and tendon/ligament. Electrospun fibres and scaffolds may provide beneficial results due to their high fraction of available surface, which aids in cell attachment. Further enhancement of cell attachment and interaction with the scaffold can be achieved through surface modification and inclusion of natural materials. He et al. [245] examined the effects of electrospinning a collagen-blended poly(L-lactic acid)-co-poly(e-caprolactone) (PLLA-CL), (70:30) nanofiber scaffold on human coronary artery endothelial cell (HCAEC) viability and attachment [240]. In an in vitro study they demonstrated that incorporation of collagen was able to enhance HCAEC viability, spreading, and attachment, while also preserving the endothelial cell phenotype. Another tissue for which electrospun nanofiber meshes have emerged as potential scaffolds is bone. Thomas et al. [246] examined the mechanical properties of aligned PCL electrospun nanofiber meshes collected at different rotation speeds (0, 3,000, and 6,000 RPM) [241]. They observed that the nanofibers became more aligned as the rotation speed increased. They also observed that increasing the rotation speed had a significant effect on the mechanical properties of both the individual nanofibers and the bulk scaffold. The hardness and Young’s modulus of individual fibers were found to decrease with increasing rotation speeds. The authors attribute this decrease in mechanical properties to a decrease in fiber crystallinity at higher rotation speeds. However, due to the increased fiber alignment seen at higher rotations speeds, the tensile strength and modulus of the bulk scaffold along the axis of aligned fibers increased with increasing rotation speed. The ability to electrospin nanofiber scaffolds that contain aligned fibers has demonstrated great potential for guidingneurite outgrowth. Yang et al. [247] examined the performance of both aligned and random PLLA electrospun scaffolds for neural tissue engineering applications [242]. By varying the polymer concentration from 1 to 5% w/w they were also able to produce both nanofibers and microfibers, allowing them to examine the effects of both fiber alignment and fiber diameter on the morphology, differentiation, and neurite outgrowth of neural stem cells (NSCs) in vitro. The authors observed that NSCs oriented parallel to the aligned fibers, giving directed neurite outgrowth. However, the NSCs cultured on random PLLA scaffolds did not show a directed orientation. NSCs cultured on aligned nanofibrous scaffolds demonstrated the largest neurite outgrowth with neurites as long as 100 mm after 2 days of culture. These studies demonstrate potential applications in which it is desirable to develop a nanofibrous scaffold containing aligned fibers (with a high degree of anisotropy). An electrospun nanofiber scaffold can provide superior endothelial cell attachment due to the large fraction of surface available for interacting with cells. Additionally, due to its small pores the same scaffold can prevent smooth muscle cell migration into the lumen of the vessel, while still allowing sufficient transport for nutrients and waste removal. Pore size can also be modified to allow for selective

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filtration of macromolecules, which can be used to control transport of nutrients and waste products. However, small pores are not advantageous for all applications. In more 3-dimensional scaffolds the cells must be able to infiltrate deep into the scaffold, which requires pores of adequate size (approximately 10 mm) to allow for cell migration. The pore size of an electrospun scaffold will essentially dictate whether it is viewed as a 2-dimensional mat or a 3-dimensional scaffold by cells. Depending on the application either might be desirable. Additionally, the ability to engineer scaffolds with a desired pore size distribution may allow for the use of nanofibrous scaffolds without limiting cell infiltration, combining the advantages of both microand nanofibrous scaffolds.

5.9 Conclusions and Perspectives There is something ultimate about Nanotechnology: matter is manipulated at its most elementary level, the atom. Nanotechnologies are a logical step, unavoidable in the course of human progress. More than just progress in a narrow realm of technology, this represents the birth process of a new “age” as we harness Nanotechnology’s potential. The areas of potential applications are multiple; from powerful UV-blocking sunscreens to nano-robots designed to repair damages at the cellular level. Below is presented a non-exhaustive list of the principal domains which will be affected by developments in Nanotechnology: – Materials: new materials, harder, more durable and resistant, lighter and less expensive. – Electronics: electronic components will become smaller and smaller, allowing the design of more powerful computers. – Energy: a vast increase in the potential of solar energy generation is envisioned, for example. – Health and nanobiotechnologies: great expectations are held in the areas of prevention, diagnostics, and treatment. For example, nanoscopic probes could be put in place to measure our state of health around the clock, new tools could be developed to fight genetic disease at the level of the gene, and markers could be created to detect and, one by one, destroy cancerous cells, just to name a few of the many possibilities. Developments in these domains would impact a broad range of industries, such as cosmetics, pharmaceuticals, consumer appliances, hygienics, construction, communication, security and safety, and space exploration. Our environment will benefit as well, in terms of clean, economical energy production, and the use of more environmentally friendly materials. In brief, many areas of our daily lives will be affected in one way or another by the development of Nanotechnologies, because Nanotechnologies will permit us to do better, with less.

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Appendix Principal Characterization Techniques of Nanostructured Macromolecules

Many experimental techniques for the characterization of bulk macromolecules are known and widely used, but a few techniques with sufficient lateral resolution are available to characterize nanostructured macromolecules. Among microscopy techniques, the most popular characterization methods for polymeric nanostructures are Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM), together with Near-Field Optical Microscopy (SNOM), Scanning Tunneling Microscopy (STM) and Transmission Electron Microscopy (TEM). Other spectroscopic techniques that offer chemical information with sub-100 nm lateral resolution are Infrared Reflection Absorption Spectroscopy (IRRAS) and RAMAN based techniques, i.e. micro RAMAN (μRAMAN) and Surface Enhanced Raman Spectroscopy (SERS). Dynamic Light Scattering (DLS) is also a well suited method for the assessment of the size of nanoparticles. A short overview on these techniques is outlined here with some literature hints.

Atomic Force Microscopy (AFM) A key feature of AFM is its ability to provide topographic and height maps of “soft”, organic surfaces, with nanometer lateral resolution and without causing damage [1, 2]. In general, AFM provides imaging capability in gaseous and liquid environment and is based on the use of a proper tip that acts as a mechanical probe on the surface. AFM was invented in 1985 by Binnig, Quate, and Gerber and one of its advantages is to overcame the need of using conducting or semiconducting surfaces, opening to almost any type of surfaces, including polymers and biological samples. Schematically, AFM consists of a tip on a cantilever contacted to the surface with the interatomic van der Waals forces that provides the interaction mechanism with the sample. AFM tips are usually microfabricated from Si or Si3 N4 , with tip radius of about tens of nm. The forces between the tip and sample are calculated by measuring the deflection of the lever, by applying the Hook’s law. AFM modes of operation can be modified for specific application requirements, ranging for example from contact mode (where the tip is in hard contact with the surface and is raster-scanned across the surface being deflected), to non contact mode (where tip is quite close to the M.V. Russo (ed.), Advances in Macromolecules, DOI 10.1007/978-90-481-3192-1, C Springer Science+Business Media B.V. 2010

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sample, but not touching it), or using lateral forces (measuring frictional forces on a surface) or tapping mode (intermittent-contact).

Scanning Electron Microscopy (SEM) The scanning electron microscopy is based on a high-energy beam of electrons that scans a sample surface; the interaction between the electrons with the atoms at or near the surface produces signals that contain information on topography, composition and properties of the sample’s surface [3]. In the most common detection mode i.e. secondary electron imaging, this microscopy allows to gain very high-resolution images revealing details of about 1–5 nm in size. In general, a wide range of magnifications is achievable (from about × 25 to × 250,000). Back-scattered electrons, reflected from the sample by elastic scattering can be used in analytical applications by using the spectra made from the characteristic x-rays emitted when the electron beam removes an inner shell electron from the sample, causing a higher energy electron to fill the shell and release energy. These characteristic x-rays can be used to identify the composition of the sample.

Near-Field Optical Microscopy (SNOM) SNOM microscopy can image polymeric surfaces below the diffraction limit with high spatial, spectral and temporal resolving power [4, 5]. This technique is particularly suitable for nanostructure investigation and it is based upon the passage into a small orifice of intense and nearly planar light behind an opaque and thin metal film. A scanning tip, a detector, a focused laser light source and a piezoelectric substrate are primary components of the SNOM. By placing the detector very close (<< λ) to the specimen surface, lateral resolution of 20 nm and vertical resolution of 2–5 nm can be achieved.

Scanning Tunneling Microscopy (STM) STM is a powerful microscopy for viewing surfaces at the atomic level, firstly developed by Binning and Rohrer in 1981. It is based on the probe of the density of states of a material using tunneling currents. Lateral resolution of about 0.1 nm and depth resolution of 0.01 nm can be obtained. The concept of quantum tunneling is at the basis of STM microscopy, i.e. when a conducting tip is brought very close to a metallic or semiconducting surface, a bias between the tip and the surface allows the electrons to tunnel through. The variations of current as the probe crosses the surface are translated into an image [6].

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263

Transmission Electron Microscopy (TEM) Transmission electron microscopy is a technique based on an electron beam, transmitted through an ultra thin specimen, that interacts with the sample as it passes through [7]. The interaction of the electrons transmitted through the specimen produces an image with a significantly higher resolution than optical microscopes, due to the small de Broglie wavelength of the electrons, enabling the instrument to resolve fine details as small as single atoms. TEM images are due to the phenomenon of absorption of electrons in the material which is in turn related to the thickness and composition of the material.

Infrared Reflection Absorption Spectroscopy (IRRAS) Infra red beam can be resolved into polarized components in which the electric vector oscillation is parallel and perpendicular to the plane of incidence, respectively. When one polarized component hits a metal surface, a stationary wave is generated, resulting from the interference between incident and reflected beams. The spectral band features in IRRAS mode, peak positions, band shapes and intensities will considerably differ from the transmission spectra of the material. In IRRAS spectra, if the film thickness is thin enough in comparison to the wavelength of the incident IR light, the normalized reflectivity change increases in proportion to the film thickness d (up to nanometer dimension) and quantitative information on the thickness can be obtained [8].

Micro Raman (μRAMAN) The new generation of Raman microscopy offers a powerful non-destructive and non-contact method of sample analysis. In general, Raman technique enables the study of selected vibrations in molecules and solids through the interaction of light [9, 10]. It relies on inelastic scattering of monochromatic light, in visible, near infrared and near ultraviolet spectral regions. Micro-Raman spectroscopy allows the study of small sample regions down to the micron scale The spatial distribution of the Raman intensity can be studied giving a 2D map of the sample and depth profiles [11].

Surface Enhanced Raman Spectroscopy (SERS) The surface enhanced Raman spectroscopy is a surface sensitive technique with a greatly enhanced signal by molecules adsorbed on rough metal surfaces; the enhancement factor can be as much as 1014 –1015 , which permits this technique to be sensitive enough to detect single molecules. In most experiments the SERS

264

Appendix:Principal Characterization Techniques of Nanostructured Macromolecules

spectra are very similar to the non-surface enhanced spectra, but often differences in the number of detected modes present can be observed. When molecules are adsorbed to a surface, the symmetry of the system can change, slightly modifying the symmetry of the molecule, which can lead to differences in mode selection. This technique results selective for the study of adsorption on surfaces and of the orientation in which the molecule is attached, with resolving power up to monolayers [12].

Dynamic Light Scattering (DLS) Dynamic light scattering is a non-invasive, well-established technique used for measuring the size of molecules and particles typically in the submicron region and, with the latest technology, for the investigation of sizes smaller than 1 nm. When the light hits nanoparticles, it scatters in all directions (Rayleigh scattering); if the light source is a laser a time-dependent fluctuation in the scattering intensity can be observed, due to the Brownian motion in solution. The dynamic information of the particles can be used to determine the size distribution profile of small particles in solution [13]. The diameter measured by DLS is called the hydrodynamic diameter and refers to how a particle diffuses within a fluid.

References

Books and Web Sites 1. West P, Introduction to atomic force microscopy: theory, practice and applications – www. AFMUniversity.org 2. Morris VJ, Kirby AR, Gunning AP (1999) Atomic force microscopy for biologists. Imperial College Press, London 3. Goldstein GI, Newbury DE, Echlin P, Joy DC, Fiori C, Lifshin E (1981) Scanning electron microscopy and x-ray microanalysis. Plenum Press, New York 4. Hecht B, Sick B, Wild UP, Deckert V, Zenobi R, Martin OJF, Dieter DW (2000) Scanning near-field optical microscopy with aperture probes: fundamentals and application. J Chem Phys 18:112 5. Kaupp G (2006) Atomic force microscopy, scanning nearfield optical microscopy and nanoscratching: application to rough and natural surfaces. Springer, Heidelberg 6. Bonnell DA, Huey BD (2001) In: Bonnell DA (ed) Scanning probe microscopy and spectroscopy: theory, techniques, and applications, 2nd edn. Wiley-VCH, Inc, New York 7. Egerton R (2005) Physical principles of electron microscopy. Springer, Heidelberg 8. Trenary M (2000) Ann Rev Phys Chem 51:381–403 9. Gardiner DJ (1989) Practical Raman spectroscopy. Springer Verlag, Berlin 10. Weber WH, Merlin R (eds) (2000) Raman scattering in materials science, Springer series in materials science, vol 42. Springer Verlag, Berlin 11. Baia L, Gigant K, Posset U, Schottner G, Kiefer W, Popp J (2002) Appl Spectrosc 56:409–548 12. Nie S, Emory SR (1997) Science 275:1102–1106 13. Chu B (1992) Laser light scattering: basic principles and practice, 2nd edn. Academic Press, Elsevier, London

Index

A AFM, see Atomic force microscopy (AFM) Aggregation equilibria, 5 AHB, see Angular hole burning (AHB) All-optical modulators, 148 See also Devices All-optical poling (AOP) centrosymmetric bisazomolecule, 136 experimental setup for, 135 four-level scheme of, 134 interference process, 133 photomultiplier tube, 135–136 trans-cis and cis-trans transitions, 134–135 See also Poling techniques Amino-acid-functionalized gold nanoparticles and SPNA-derived substrates chemical structure of, 231 Amphiphilic dendrimers, vectors for gene delivery, 12 Amphiphilic graft polyphosphazenes self-assembly, 8 Angular hole burning (AHB), 133–134 Anodic aluminum oxide (AAO) template, FESEM images of, 21 AOP, see All-optical poling (AOP) Arachidonic acid (AA) in situ o¨ and o¨ -1 hydroxylation of, 237 Atmospheric plasma (AP) study, 30–31 Atomic force microscopy (AFM), 26–27 Hook’s law, 261 operation, modes, 261–262 Atom transfer radical polymerization (ATRP), 8, 28, 33 mechanism for, 34 methods for conducting, 35 reverse, drawback of, 35 simultaneous normal and reverse initiation (SR&NI) ATRP, 36 TEM micrographs after, 34

4-ATP-CoTBPPf-Py-C60 system, 181–182 ATR-FTIR spectroscopy, see Attenuated total reflection–Fourier transform infrared spectroscopy (ATR-FTIR spectroscopy) ATRP, see Atom transfer radical polymerization (ATRP) Attenuated total reflection–Fourier transform infrared spectroscopy (ATR-FTIR spectroscopy), 26–27 Auger electrons, 168, 170, 172 R Avastin , see Bevacizumab Azobenzenesulfonic acid, amphiphilic micelle, 22 B Bacillus sphaericus NCTC 9602, NEXAFS spectra, 187 Bevacizumab, 243 Bifunctional linkers, 222 Biomolecule-functionalized nanoparticles for controlled chemical reactivity catalysts fuel processing, 226–227 and heterogeneous catalysts, use, 227 nanoparticles and vaccines delivery GI tract, 238 PLGA/PLGA–PEG and PCL–PEG, 239 RGD ligand, 239 protein adsorption on solid surfaces, 227 chitosan (D-glucosamine), 235 CRL bioconjugates, 230–231 FTIR and CD spectroscopy, 232 geo-inspired synthetic chrysotile, 232 HCAII, 228–229 immobilized lipase, kinetic parameter Km values, 235–236 interfacing with CNTs, 229–230

265

266 lysozyme adsorption on silica particles, 233 NMR spectroscopy used, 228 PANCMA fibrous membranes, 236 protein’s secondary structure, effect of particle curvatures on, 229 R-chymotrypsin (ChT), 231–232 support and enzyme immobilization, preparation, 235 SWCNTs, oxidation of, 229–230 whole cells, immobilization damaged tissues, repair/replacement, 236 gold NPs, 238 ´ı-CP and SAMs, 237 magnetic NPs in dextran matrix, 238 NADPH, 237–238 organic fluorophores, 238 photosensitizer, 238 Yarrowia lipolitica and Candida bombicola, 237 Biomolecule-functionalized nanoparticles synthesis electrostatic adsorption, functionalization, 220 NPs, with immunoglobulin G (IgG) molecules, 221 streptavidin/biotin/(EG3-S-)/GSH, interaction with, 221 and microbial systems bacteria, use of, 223 flavin recognition by diaminopyridine functionalized Au NPs, 224 intrinsic properties, 224–225 specific affinity interactions, 222 T4-5X-Amdex-CdS conjugate and its binding, 223 thiol derivatives, chemisorption anchor groups, 222 Biomolecule-nanoparticle interface properties biomacromolecules, surface recognition challenges for, 225 MPCs and MMPCs, 226 Bionanofabrication process, 64 Biosensors based on nanomaterials, 67 Biphasic electropolymerization, 46 Block copolymers self-assembly, 8–9 2-(4 -hydroxybenzeneato) benzoic acid, 10 labyrinth-like patterns of, 10 moir`e-type superstructures, 10 polystyrene-block-poly (4-vinylpiridine), 10

Index Bottom-up approaches, 2 Bragg equation, 192 Bremsstrahlung continuum, 192 Brillouin scattering, 90 Building block approach, for spectral assignment, 173–174 C Candida rugosa lipase (CRL), 230, 233 Carbon nanotubes (CNTs) biological applications, 244 use, 244–245 Cationic linear poly(ethyleneimine) layer-by layer assembly, 6 Cetuximab, 243 Cetyltrimethylammonium bromide (CTAB), 20 Chitosan (D-glucosamine), 3, 235 R-Chymotrypsin (ChT), 233 molecular structure of, 231 monolayer-controlled diffusion of, 231 CNTs, see Carbon nanotubes (CNTs) Coarse-grained lattice gas model, 6 Coaxial electrospinning, 64 Cobalt tetra-butyl-phenyl porphyrins (CoTBPPf), 181–182 Colloidal nanolithography, 24 Comb-shaped supramolecules self-assembly, 8 Conductive polymer nanotubes, controlled electrochemical synthesis, 47 π-Conjugated polymers self-assembly polyacetylenes and polyynes, 17–18 polyaniline (PANI), 13–15 polypyrrole (PPy), 15 polythiophene (PTh), 16–17 Contact poling, 131–133 air and voltages, 132 bleaching effect, 132 configuration for, 131 coplanar electrodes, 131 EO core material, 132 MZ modulators, 132 See also Poling techniques Controlled-living radical polymerization (CRP) methods, 33 Coplanar waveguide (CPW) structure, 132 Core level chemical shift, 195–196 Corona poling, 129–131 effect of atmosphere on, 130 growth and decay of polar order in, 130 set-up for, 129 swelling–poling–deswelling procedure, 131

Index thermal-or photo-cross-linking, 130 See also Poling techniques CRL, see Candida rugosa lipase (CRL) CRP, see Controlled-living radical polymerization (CRP) methods CTAB, see Cetyltrimethylammonium bromide (CTAB) CuPc/SAM/Au systems, 180 Cyclodextrin (CyD), 33 D DBSA, see Dodecil-benzenic sulphonic acid (DBSA) dopant Deep silicon etching, 24 Dendrimers macroinitiators, 8 networks and NEXAFS, 190 self-assembly, 11 applications in neurodegenerative diseases, 13 box-nanocrystals, 12 CdSe dendron stabilized nanoclusters, 12 chain entanglement of linear polymers, 13 gold cluster superstructures, 13 liquid crystalline materials, 13 L-lysine building-blocks, 12 metal coordination chemistry and, 12 nanoclusters and nanotubes, 12 PAMAM-DNA complex, 12 Devices all-optical modulators, 148 birefringence effect, 151 Bragg grating filter, 155 channel waveguide of EO polymer with Bragg grating, 157 charge-transfer bridge, 158 distributed Bragg reflector (DBR), 156 electro-optic and all-optical switches, 150–151 modulators, 149 switches, 152 fabrication techniques channel waveguides, 138 direct patterning, 140–142 lateral confinement, 138 photolithography, 139–140 representation of, 138–139 soft lithography, 142–144 Fabry-Perot etalon devices, 156 high-Q polymer microring resonators, 154 light modulators

267 microring resonator modulator, 149–150 ring and straight waveguide, 149 Mach-Zehnder waveguide modulator, 145–148 microring wavelength filters, 155 molecular switch, 154 organic NLO materials for, 146 photonic switching, 151 properties of, 147 reflective-type resonant grating waveguide modulator, 144 surface-relief gratings, 155 tuneable wavelength filter, 157 ultra-fast optical switches, 153–154 Dextran, 3 DFT, see Fluids density functional theory (DFT) Diblock copolymer amphiphilic LC-coil, TEM micrographs of, 34 drying-mediated self-assembly, 6 Diffusion processes, 6 Dimethylaminoindoaniline (DIA), 93 Direct laser interference micro-nanopatterning (DLIP), 24 Direct patterning processes, 140–142 Dissipative particle dynamics (DPD) method, 10 DLIP, see Direct laser interference micro-nanopatterning (DLIP) DLS, see Dynamic light scattering (DLS) Dodecil-benzenic sulphonic acid (DBSA) dopant, 15 DPD, see Dissipative particle dynamics (DPD) method Drug delivery and cancer therapy, nanostructured materials chemotherapy, 241 (PEG), 242 polymeric NPs, 242 and magnetic NPs, 245 advantages, 247–248 development of, 246–248 EPR, 246 R FluidMAG , 247 HRTEM images, 247 R MagNaGel , 247 microfluidics, 243 CNTs, 244–245 nanoparticles and gene delivery composite multifunctional nanoparticles, role, 242–243

268 LBL method, 242 PNIPAM and PEG, 242 targeting molecules monoclonal antibodies (mAb), 243 SELEX and FRs, 243 Drying kinetics, 6 Drying-mediated self-assembly, 6 Dynamic light scattering (DLS), 261, 264 E EDOT, see 3,4-Ethylenedioxythiophene (EDOT) E-DPN, see Electrochemical dip-pen nanolithography (E-DPN) EFISH, see Electric field induced second harmonic (EFISH) technique EF-TEM, see Energy filtered-transmission electron microscopy (EF-TEM) Electric field induced second harmonic (EFISH) technique, 89 Electroauxiliary (EA), 38 Electrochemical dip-pen nanolithography (E-DPN), 24 Electrochemical methods, 3 aligned nanostructures, 45–46 coaxial nanowires and nanotubes, 46–48 electron-transfer process, 38 electrosynthesis, 38 features, 36 nanowires and nanotubes, 42–45 reaction media, role of, 40–42 Electrochemical template method, 42 Electro-optical absorption (EOA) spectroscopy, 90 Electro-optic modulators (EOM), 119, 149 See also Devices Electrospinning technology, 2–3 coaxial, core-shell nanofibers and nanotubes, 64–65 electrospun fiber properties, 65 fibers produced by, 57 overview, 55 parameters affecting, 58 applied voltage, 59 capillary tip and collector, distance between, 60 choice of solvent, 60–61 nozzle configuration, 62–63 polymer concentration, 61 polymeric fibers, 60 solution conductivity, 61–62 process and mechanism, 57–58 representation of, 56

Index Electrospun nanofiber matrices, 249 Electrospun poly(vinyl alcohol) nanofibers, 64 Electrosynthesis, 37 Emulsion polymerization, 2–3 critical parameters for, 53–54 features, 48–50 kinetics process in, 50 theoretical overview interval I, 50 interval II and III, 51 rate of polymerization, 52 Energy filtered-transmission electron microscopy (EF-TEM), 230 Enhanced permeation and retention (EPR), 246 EOM, see Electro-optic modulators (EOM) EPR, see Enhanced permeation and retention (EPR) R Erbitux , see Cetuximab ETFE, see Poly(ethylene-alttetrafluorethylene) (ETFE) 3,4-Ethylenedioxythiophene (EDOT), 16 Evaporation procedures, 6 F Fabry-Perot etalon devices, 156 Ferromagnetic Co nanoparticles self-assembly, 6 Flash welding, 67 R FluidMAG , 247 Fluids density functional theory (DFT), 5 Folate receptors (FRs), 243 Folded accordion polymers, 107 Fourier transform infrared spectroscopy (FTIR), 232 Four wave mixing geometry, 133 Free-radical graft polymerization, 29 FRs, see Folate receptors (FRs) FTIR, see Fourier transform infrared spectroscopy (FTIR) Fullerenes and ZnPf, molecular arrangement of, 182–183 G Gas-phase silylation, 29 Gastrointestinal (GI) tract, 238 Gel-phase assemblies of dendritic molecules, 11 Glycidyl methacrylate (GMA), 33 Glycine adsorption, study by XPS, 206 GMA, see Glycidyl methacrylate (GMA) Gold cluster superstructures, 13 See also Dendrimers

Index Graft polymerization, 2 atom transfer radical polymerization, 33–36 characterization, 26 features of, 25–28 free radicals, 28–30 plasma surface treatment and plasmainduced graft polymerization, 30–33 and polymer grafting, 25 H HCAEC, see Human coronary artery endothelial cell (HCAEC) HCAII, see Human carbonic anhydrase II (HCAII) HER2, see Human epidermal growth factor receptor 2 (HER2) R Herceptin , see Trastuzumab Hierarchical self-assembly approach, 7–8 Highest occupied molecular orbital (HOMO) of monomer, 38 Highly ordered pyrolytic graphite (HOPG) electrode, 39 High-Q polymer microring resonators, 154 See also Devices High resolution transmission electron microscopy (HRTEM) images, 247 Homogeneous electrolysis processes, effect of ultrasounds on, 39 Host-guest systems, 9 HRTEM, see High resolution transmission electron microscopy (HRTEM) Human carbonic anhydrase II (HCAII), 228 Human coronary artery endothelial cell (HCAEC), 251 Human epidermal growth factor receptor 2 (HER2), 240 Human serum albumine (HAS), 185–186 Hyaluronic acid derivatives, 3 Hybrid (Au-PPy-Au) nanowire arrays, 20 Hybrid systems, 3 Hydrodynamic effects, 6 Hydrophobic drug, 7 Hyper-Rayleigh scattering (HRS), 90 I Infrared reflection absorption spectroscopy (IRRAS), 261, 263 Ionic liquids, chemical structures of cations and anions, 40 IRRAS, see Infrared reflection absorption spectroscopy (IRRAS)

269 Isocyanate end-capped prepolimer, 110 Isothermal decay method, 137 J Joint experimental-theoretical approach, 7 K Kleinman symmetry, 82 Kolbe electrolysis, 39 L Lamellae-within-cylinders structure, 8 Langmuir-Blodgett technique, 2, 21 Laser micro/nanopatterning, 2 Layer-by-layer (LBL) method, 6, 242 Layered double hydroxides (LDHs), 220 LDHs, see Layered double hydroxides (LDHs) Light modulators microring resonator modulator, 149–150 ring and straight waveguide, 149 See also Devices Lipid tubules, 22 Liquid crystalline assemblies of dendritic molecules, 11 Liquid-phase silylation, 29 Lithium Niobate (LiNbO3 ), 119 devices, growth and patterning, 120 Living radical polymerization, 33 Lotus effect in macromolecules, 66 M mAb, see Monoclonal antibodies (mAb) Mach-Zehnder waveguide modulator, 145–148 interferometer, 119–120 See also Devices R MagNaGel , 247 Mass-transport process, 39 MBE, see Molecular beam epitaxy (MBE) Metal clusters, 200–201 Metal ion coordination polymers self-assembly, 10 Metalloporphyrins, 178, 181–182 and synthesis of supramolecular systems, 181 Metal organic chemical vapour deposition (MOCVD), 120 Microcontact printing (´ı-CP), 237 Microphase separation, 24 Micro Raman (μRAMAN), 261, 263 Microscopy techniques, 26–27 Mixed MPCs (MMPCs), 226 MOCVD, see Metal organic chemical vapour deposition (MOCVD) Molecular beam epitaxy (MBE), 120

270 Molecular imaging, nanomaterials applications in fluorophores, 239 human epidermal growth factor receptor 2 (HER2), 240 nanomaterials, superior properties, 239–240 QD, 240 Molecular layer deposition (MLD) process, 199 Molecular self-assembly, 4 Molecular template-assisted electrosynthesis, 45 Monoclonal antibodies (mAb), 243 Monolayer-protected clusters (MPCs), 226 Monte Carlo simulation techniques, 6 MPCs, see Monolayer-protected clusters (MPCs) μRAMAN, see Micro Raman (μRAMAN) Multi walled carbon nanotubes (MWCNTs), 244 MWCNTs, see Multi walled carbon nanotubes (MWCNTs) N Na-DNA/POMA-ES system, 14 NADPH, see Nicotinamide adenine dinucleotide phosphate (NADPH) Nanobiotechnology, 219 bio-nanohybrid materials, science, 220 nano-HA, see Nano-hydroxyapatite (nano-HA) Nano-hydroxyapatite (nano-HA), 250 Nanoimprint lithography (NIL), 142–144 Nanomolding, 24 Nanoparticles (NPs), 222–239 with immunoglobulin G (IgG) molecules, 221 Nanostructured macromolecules applications and perspectives, 65–69 self-assembly aggregation, 4 block copolymers, 8–10 2D and 3D structures, 4 dendrimers, 11–12 drying-mediated, 6 evaporation procedures and, 6 features, 4 hierarchical based on LbL, 6–8 hollow hydrophilic metal functionalized nanostructures, 9 porphyrins, 8 spheres and wire-like threads, 8

Index theoretical approach, 5 top-down and bottom-up approach, 5 systems and NEXAFS, 174–178 use of, 166 XPS, investigations by, 199–200 Near edge X-ray absorption fine structure spectroscopy (NEXAFS), 165–166 apparatus for, 172 application to molecular systems biomolecular, 183–187 nanostructured, 174–178 NLO molecules, 188–190 organometallic macromolecules, 178–183 principles of angular dependent measurements, 170–172 selection rules, 169–170 spectral features, 167–168 surface sensitivity of, 168–169 resonances, 167–168 spectral assignment, method for, 173–174 Near-field optical microscopy (NOM), 261–262 NEXAFS, see Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) Nicotinamide adenine dinucleotide phosphate (NADPH), 237–238 Ni phtalocyanine, 178–180 NLO, see Nonlinear optics (NLO) NLO macromolecular systems and device fabrication techniques, 138–139 direct patterning, 140–142 photolithography, 139–140 soft lithography, 142–143 devices based on filters, 154–157 microring resonators, 154 modulators, 144–150 other applications and devices, 158 sensors for electric field, 158 switches, 150–154 optical characterization of, 121–123 nonlinear ellipsometry, 126–128 second harmonic generation, 124–126 orientation stability, 136–138 poling techniques, 128–129 all-optical poling technique, 133–136 contact poling, 131–133 corona poling, 129–131

Index N-Methyl-2-pyrrolidone (NMP), graft polymerization in, 26–27, 181 NOM, see Near-field optical microscopy (NOM) Non centrosymmetric crystals, 119 Nonlinear ellipsometry single wavelength reflection configuration, 126 Nonlinear optical (NLO) properties of organic materials, 119 See also NLO macromolecular systems Nonlinear optics (NLO), 79, 84 chromophores, 92 2 D, 94 DIA, limiting resonance formulae for, 93 forming acentric crystals, 96 molecular nonlinearity of, 93 zwitterionic, limiting resonance forms for, 94 molecules, 188–190, 208–211 polymers, 98 electric poling for, 99 SHG NLO data, 99–100 Nonporous alumina templates, 7 NPs, see Nanoparticels (NPs) O Octadecylamine (ODA) Candida bombicola, aqueous dispersion, 237 OLEDs/PLEDs, see Organic/polymer lightemitting diodes (OLEDs/PLEDs) Oligo(phenyleneethynylene) (OPE), 200 Optical characterization, 121 birefringence of systems, 123 extraordinary index, 123 induced electric dipole, 123 isotropic distribution of molecular orientation, 123 linear contribution, 122 macroscopic linear polarization, 123 microscopic hyperpolarizability, relationship between, 122 molecular and laboratory axes, 122 molecule-based coordinate system, 122–123 polarizability tensor, 122 quasi-one-dimensional molecules, 122 refractive index, 123 Optical nonlinearity in materials derivatives, 81 electric dipole moment and Taylor series, 81–82

271 electro-optic coefficients, 85 experimental techniques, 89–91 Kleinman symmetry, 82 material property and external perturbation tensor, 80–81 molecular optical nonlinearities, 85 dipole moment operator, 88 explicit form of components, 86 Gaussian units, 87 HOMO-LUMO transition, 88–89 intrinsic permutation and, 86 polarizability and hyperpolarizability tensors, 86 reference system, 87 second order perturbation theory, 87 two-level model, 88 optimizing, 91–95 Pockels effect, 84–85 pump-waves of frequency, 82–83 SHG process, 84 susceptibility tensor, 84 two-index and three-index in SHG coefficients, 84 Organic materials, requirements for commercial use of, 121 See also NLO macromolecular systems Organic optoelectronics, in communication technology, 120 Organic/polymer light-emitting diodes (OLEDs/PLEDs), 16 Organometallic macromolecules, and NEXAFS spectroscopy, 178–183 Orientation stability activation energy of, 137 double exponential function, 137 first decay component, 137 polymer glass transition temperature, 136–137 second order NLO activity decay time, 136–137 SHG intensity, 137 Smoluchowski equation, 136 time evolution, 136 Oriented gas model, 122 Osmosis, 2 Oxidative polymerization, 15 P PANANA, see Poly(aniline-co-anthranilic acid) (PANANA) PANI, see Polyaniline (PANI) PCL, see Pseudomonas cepacia lipase (PCL)

272 PCL–PEG, see Poly(ε-caprolactonecoethylene glycol) (PCL–PEG) PDDA, see Poly(diallyldimethylammonium chloride) (PDDA) PDMS, see Poly(dimethylsiloxane) (PDMS) stamps PDPSA, see 3-Pentadecyl phenol-4-sulphonic acid (PDPSA) PEDOT, see Poly(3,4-ethylenedioxythiophene) (PEDOT) PEG, see Polyethylene glycol (PEG) 3-Pentadecyl phenol-4-sulphonic acid (PDPSA), 22 pGMA, see Poly(glycidyl methacrylate) (pGMA) Photo-assisted poling (PAP), 128–129 Photolithography, 139–140 Phthalocyanines, 179 PIGP, see Plasma-induced graft polymerization (PIGP) Plasma-induced graft polymerization (PIGP), 28, 30–31 multistep process of, 32 Plasma surface treatment, 30–33 Plasmid DNAcopolymers (dendritic poly L-lysine and linear PEG blocks) self-assembly, 12 PLGA, see Poly(lactide-co-glycolide) (PLGA) PLLA-CL, see Poly(L-lactic acid)-co-poly(ecaprolactone) (PLLA-CL) PMMA, see Polymethylmethacrylate (PMMA) PNIPAM, see Poly(N-isopropylacrylamide) PNIPAM Pockels effect, 84–85 Poling techniques, 128 all-optical poling centrosymmetric bisazomolecule, 136 experimental setup for, 135 four-level scheme of, 134 interference process, 133 photomultiplier tube, 135–136 trans-cis and cis-trans transitions, 134–135 contact poling air and voltages, 132 bleaching effect, 132 configuration for, 131 coplanar electrodes, 131 EO core material, 132 MZ modulators, 132 corona poling effect of atmosphere on, 130 growth and decay of polar order in, 130

Index set-up for, 129 swelling–poling–deswelling procedure, 131 thermal-or photo-cross-linking, 130 Polyacetylenes and polyynes, 17–18 See also π-Conjugated polymers self-assembly Polyamidoamine (PAMAM) dendrimers and DNA transport, 12 Poly(aniline-co-anthranilic acid) (PANANA), 14 Polyaniline (PANI), 13 arrays, 14 DLIP by, 24 nanowires-gold nanoparticles hybrid networks, 44 oriented nanowires, 45 self assembly copolymers, 14 dispersion polymerization in PVA matrix and formation, 14 emeraldine base (EB) and salt (ES) forms of POMA, 14 morphology of, 23 PANI/acid concentration ratio, 14 solid state properties of, 22 synthesis of, 22 tetrachloroaurate use, 14 See also π-Conjugated polymers self-assembly Polydiacetylene (PDA) films and nanotubes, self assembly, 7 Poly(diallyldimethylammonium chloride) (PDDA), 22 Poly(dimethylsiloxane) (PDMS) stamps, 30 Poly(ε-caprolactonecoethylene glycol) (PCL–PEG), 239 Poly(ethylene-alt-tetrafluorethylene) (ETFE), 25 Poly(3,4-ethylenedioxythiophene) (PEDOT), 16 electrochromic devices based on, 68 electrodeposition, 47 growth mechanism of, 47 oxidative chemical polymerizations of, 20 Polyethylene glycol (PEG), 242 Polyferrocenylsilane cores and polyisoprene coronas self-assembly, 9–10 Poly(glycidyl methacrylate) (pGMA), 25 Poly(lactide-co-glycolide) (PLGA), 239, 250 Poly(L-lactic acid)-co-poly(e-caprolactone) (PLLA-CL), 251 Polymer grafting, 2

Index Polymeric NPs, 242 Polymethylmethacrylate (PMMA), 3, 230 Polymethylmethacrylate (PMMA) diblock copolymers (PS-b-PMMA), 10 Poly(N-isopropylacrylamide) PNIPAM, 242 Poly(N-methylpyrrole) (PNMPy), oxidative chemical polymerizations, 20 Poly(N,N-dimethylpropargylamine) derivatives, 3 Poly(o-methoxyaniline) (POMA), 14 Polyphenylacetylene (PPA), 17 Polypyrrole (PPy) self assembly aligned PPy by CV electropolymerizations, 46 dopant, 15 by FeCl3 induced oxidative polymerization, 15 films SEM images, 41 inverse opal patterns, 23 morphology of, 23 nanowire arrays, 20 oriented nanofibers, 45 oxidative chemical polymerizations of, 20 TEM images of, 15 templated chemical polymerization of, 21 template oxidative polymerization of, 23 tubules of, 45–46 two-step method of preparing nanowires, 44 See also π-Conjugated polymers self-assembly Poly(sodium-4-styrenesulfonate) (PSS), 22 Polystyrene/poly(methyl methacrilate) (PS/PMMA), 185–186 Polystyrene (PS), 10 nanospheres, 44 nanostructured as carriers for, 230 and polyphenylacetylene, 3 templating particles, 22 Polythiophene (PTh), 16–17 oxidative chemical polymerizations of, 20 See also π-Conjugated polymers self-assembly POMA, see Poly(o-methoxyaniline) (POMA) Porphyrin molecules arrays, 7 self-assembly, 8 Post-bake procedure, 139–140 PPA, see Polyphenylacetylene (PPA) Prussian Blue nanoparticles layer-by layer assembly, 6–7 Pseudomonas cepacia lipase (PCL), 230, 233

273 PSS, see Poly(sodium-4-styrenesulfonate) (PSS) Pt-polymetallayne, 3 6-(5-Pyridin-2,yl-pyrazin-2-yl)pyridine-3-thiol (PPPT), 175, 177 R RAFT, see Reversible addition-fragmentation chain transfer (RAFT) technique Reflective-type resonant grating waveguide modulator, 144 See also Devices Regioregular polyester, 108 Reprecipitation method, 8 Resonances, in K-shell spectra, 167–168, 173–174 Reversible addition-fragmentation chain transfer (RAFT) technique, 25, 52–53 Ribonuclease structure, by NEXAFS spectra, 184 Ring opening polymerization (ROP), 8 R , see Rituximab Rituxan Rituximab, 243 Rod-like polymers self-assembly, 8 ROP, see Ring opening polymerization (ROP) Rotaxane/dibenzo-24-crown-8 macrocycle, 12 S SAMs, see Self-assembled monolayers (SAMs) SBP, see Soybean peroxidase (SBP) Scanning electron microscopy (SEM), 26–27, 261 back-scattered electrons, 262 Scanning transmission X-ray microscopy (STXM), 184 Scanning tunneling microscopy (STM), 261 quantum tunneling, concept, 262 Scienta analyzer, 193, 212 Second-harmonic generation (SHG), 83 coefficient and nonlinear susceptibility, 124 coherence length, 126 Kleinmann’s symmetry condition, 124 measurement for poled materials, 125 microscopic and macroscopic properties, 124–125 nonlinear polarization term for, 124 tensors, 124 Second order nonlinear optically active materials autoassembling systems, 97 chemical strategies, 96

274 cross-linked systems and organic molecular glasses, 109 depoling behaviour, 111 multi-component systems, 110 one-component cross-linkable systems, 110 guest-host systems, 100 electro-optic activity of polymers, 101 main-chain NLO polymers class of, 106 folded accordion polymers, 107 polyurethane PUY3, 108 regioregular P-type, 108 transverse (T-type)/parallel (P-type), 106 T-type polyurethane based on Y-shaped chromophore, 107 NLO dendrimers, 112–113 NLO polymers, 98 electric poling for, 99 poling apparatus, 99 pseudo-centrosymmetric antiparallel orientation, 95 self-assembled multilayers, construction of, 97 side chain NLO polymer, 101 chromophores of, 102, 104 Diels-Alder reaction, 104 Knoevenagel condensation, 103 Mitsunobu condensation, 103 one step synthesis of, 104 polyurethane (type II) from diol chromophore and 2,4tolylendiisocianate, synthesis of, 105 post-functionalization by azo coupling, 103 synthetic procedure for, 102 time stability and, 103 type I, 103 SELEX, see Systemic evolution of ligands by exponential enrichment (SELEX) Self-assembled ferromagnetic PSCoNPs (DPS-CoNPs) TEM images, 7 Self-assembled monolayers (SAMs), 174–176, 237 Self-assembly procedure, 2 SEM, see Scanning electron microscopy (SEM) Semibatch/continuous reaction systems, 49 SERS, see Surface enhanced Raman spectroscopy (SERS) SFG, see Sum-frequency generation (SFG)

Index Shake-off and shake-up, in XPS, 196 SHG, see Second-harmonic generation (SHG) Simultaneous normal and reverse initiation (SR&NI) ATRP, 36 Single-walled carbon nanotubes (SWNTs), 233 Smoluchowski model, 136 Solar cells, 181, 203 Soybean peroxidase (SBP), 233–234 Spectator electrons, 196 Static-exchange approximation (STEX), for amino acids, 184–185 Step and flash imprint lithography, 144 Stilbazolium multilayers, construction of self-assembled, 210 STM, see Scanning tunneling microscopy (STM) Structure-property relationship of conjugated nanostructures, 7 Sum-frequency generation (SFG), 83 Supramolecular self-assembly approach, 7 Surface enhanced Raman spectroscopy (SERS), 261, 263–264 SWCNTs, see Single-walled carbon nanotubes (SWNTs) Synchrotron radiation, 166 for NEXAFS measurements, 170–172 Synchrotron radiation-based photoelectron spectroscopy, 203 Synthetic polymers, SEM images of morphologies, 3 Systemic evolution of ligands by exponential enrichment (SELEX), 243 T Taylor cone, 59–63 TEM, see Transmission electron microscopy (TEM) Template assisted electropolymerization, 44 endo and exotemplates, 19 features of, 18 nanopatterning of polymers, 23–24 polymeric, 20 “soap-bubble” use of, 20 surfactant, morphologically controlled nanostructures, 23 technique electrochemical methods, 18 grafting polymerization, 18 materials in, 22–23 templated assembly of dendrons, 11 “templateless” synthesis, 45

Index Teng and Man Technique (TMT), 126 electro-optic coefficient, 127 experimental set-up, 127–128 polymer film between electrodes, 127 Terphenyl-4-thiol (TPT), 175, 177 Tethered polymer phases, 2 Thiolate β-cyclodextrin self-assembled monolayers, 45 Thiol derivatives chemisorption covalent binding through bifunctional linkers, 222 Thiophenes, electropolymerization in, 37–38 Tissue engineering, 248 electrospinning, 249 electrospun nanofiber matrices, 249 electrospun nanofibers scaffold fabrication techniques, 249 nanocomposite scaffolds, SEM micrographs, 250 PLGA and nano-HA, 250 PLLA-CL and HCAEC, 251 pore size distribution, 252 use, 249–250 Top-down approaches, 2 Total electron yield (TEY) detection, 173 Transmission electron microscopy (TEM), 26–27, 261, 263 Trastuzumab, 243 Triethanolamine, 110 Tri-L-(glutamic diethyl esther)-1,3,5benzenetricarboxamide supramolecular assembly, 21

275 U Untemplated assembly of dendrons, 11 UV nanoimprint lithography, 143 UV-switch able framework, 12 V Vapor-phase plasma polymerization, 30 1-Vinyl-2-pyrrolidone (VP) graft polymerization, 32 W Wavelength division multiplexers (WDM), 150–151 Wet etching process, 141 See also Devices X X-ray photoelectron spectroscopy (XPS), 165 applications to molecular systems biomolecular systems, 205–208 nanostructured systems, 198–201 NLO molecules, 208–211 organometallic macromolecules, 201–205 principles of, 190–198 electronic structure and chemical state, 195–197 experimental methods, 192–193 surface sensitivity, 197–198 X-ray photoelectron spectrum, 193–195 X-ray photoemission electron microscopy (X-PEEM), 186 Z Zn diethinyl porphyrin (ZnPf), 178