Photographic Sensitivity: Theory and Mechanisms

Photographic Sensitivity

OXFORD SERIES IN OPTICAL AND IMAGING SCIENCES Editors MARSHALL LAPP JUN-ICHI NISHIZAWA BENJAMIN B. SNAVELY HENRY STARK ANDREW C. TAM TONY WILSON

1. D. M. Lubman (ed.). Lasers and Mass Spectrometry 2. D. Sarid. Scanning Force Microscopy With Applications to Electric, Magnetic, and Atomic Forces 3. A. B. Schvartsburg. Non-linear Pulses in Integrated and Waveguide Optics 4. C. J. Chen. Introduction to Scanning Tunneling Microscopy 5. D. Sarid. Scanning Force Microscopy With Applications to Electric* Magnetic, and Atomic Forces, revised edition 6. S. Mukamel. Principles of Nonlinear Optical Spectroscopy 1. A. Hasegawa and Y. Kodama. Solitons in Optical Communications 8. T. Tani. Photographic Sensitivity: Theory and Mechanisms

Photographic Sensitivity Theory and Mechanisms

TADAAKI TANI Ashigara Research Laboratories Fuji Photo Film Company

New York Oxford OXFORD UNIVERSITY PRESS 1995

Oxford University Press Oxford New York Athens Auckland Bangkok Bombay Calcutta Cape Town Dar es Salaam Delhi Florence Hong Kong Istanbul Karachi Kuala Lumpur Madras Madrid Melbourne Mexico City Nairobi Paris Singapore Taipei Tokyo Toronto and associated companies in Berlin Ibadan

Copyright © 1995 by Oxford University Press, Inc. Published by Oxford University Press, Inc., 198 Madison Avenue, New York, New York 10016 Oxford is a registered trademark of Oxford University Press All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of Oxford University Press. Library of Congress Cataloging-in-Publication Data Tani, Tadaaki. Photographic sensitivity : theory and mechanisms / by Tadaaki Tani. p. cm.—(Oxford series in optical and imaging sciences) Includes bibliographical references and index. ISBN 0-19-507240-5 1. Silver halide crystals. 2. Photography—Films. I. Title. II. Series. TR280.T36 1995 772'.4—dc20 95-6100

135798642 Printed in the United States of America on acid-free paper

Preface

Photography plays many important roles in our life. In particular, silver halide photography is a leading method for picture taking. It is therefore quite natural that one would want to know how a picture can be taken by silver halide photography. Since this method of photography is unique it will undoubtedly be further improved with respect to sensitivity, image quality, and other capabilities for imaging systems, as predicted in this book, and will be further extended in the future to conventional photographic systems as well as playing an important role in forthcoming multimedia systems. Thus, many scientists and engineers are now involved in the field of silver halide photographic development and will, therefore, need to know the theory and mechanisms of photographic sensitivity upon which silver halide photography is founded. Various fields of fundamental sciences now include the theory and mechanisms of photographic sensitivity, and silver halide photography itself contains unique phenomena in which scientists and students in various fundamental fields are interested. Those include formation and characterization of well-controlled silver halide microcrystals, solid state physics of macrocrystals and microcrystals of silver halide, characterization and behavior of microclusters as latent image and sensitization centers, the relationship between electronic and molecular structures of sensitizing dyes, J-aggregates of sensitizing dyes, light-induced electron transfer from sensitizing dyes to silver halide, and electrochemistry in development (i.e., electron transfer from developing agents to latent image centers). Fundamental scientists and students, who are interested in those phenomena, would need to know the theory and mechanisms of photographic sensitivity. However, many years have passed since standard textbooks on photographic sensitivity were published, and significant progress has been made in various fields of silver halide photography. Thus, many scientists, engineers, and students need to have a standard textbook describing the latest developments in photographic sensitivity. This book provides such a standard textbook on the theory and mechanisms of photographic sensitivity. One of the characteristics of this book is that it was written by a single author who has been deeply involved in the science and technology of photographic sensitivity for more than 30 years both at The University of Tokyo and mainly at the Fuji Photo Film Co., Ltd.

VI

PREFACE

I selected, weighed, and synthesized the important components needed for understanding the theory and mechanisms of photographic sensitivity: the structure and preparation of silver halide grains, physical properties of silver halides, mechanisms of latent image formation, spectral sensitization, chemical sensitization and stabilization, and development. Thus, this book provides a unified analysis of the present status and future prospect of silver halide photography, which is fully based on the above mentioned components. It is a great pleasure for me to thank those individuals who have directed, guided, and supported my research career in this field, starting in 1963 as a graduate student under the guidance of Prof. S. Kikuchi, to whom I am particularly indebted. In addition, I would like to single out for thanks Prof. K. Honda, Mr. Y. Koseki, Mr. H. Ueda, Mr. M. Sonoda, Mr. Y. Hayakawa, and Mr. Y. Oishi. Acknowledgment should be made of the usefulness of The Theory of the Photographic Process (4th edition), which was edited by T. H. James and published in 1977 by Macmillan Publishing Co., Inc. Special acknowledgment is due the staff at Oxford University Press for overseeing this publication. T.T.

Contents

1 Silver Halide Photographic Materials, 3 1.1 1.2 1.3 1.4 1.5

Overview of Silver Halide Photographic Materials, 3 Characteristics of Silver Halide Photography, 8 Silver Halide Photography Among Various Imaging Systems, 11 Silver Halide and Electronic Photography, 15 Summary, 21

2 Structure and Preparation of Silver Halide Grains, 24 2.1 Structure of Silver Halide Grains, 24 2.2 Preparation of Silver Halide Grains, 32 3 Physical Properties of Silver Halides, 45 3.1 3.2 3.3 3.4

Light Absorption and Electronic Structure of Silver Halides, 45 Ionic Properties of Silver Halides, 48 Electronic Properties of Silver Halides, 60 Electron and Positive Hole Traps in Silver Halides, 65

4 Mechanism of Latent Imaging Formation, 81 4.1 Outline of Mechanism of Latent Image Formation, 81 4.2 Electronic and Ionic Processes in Relation to the Concentration Principle, 87 4.3 Silver Microclusters as Latent Image Centers, 91 5 Spectral Sensitization, 111 5.1 5.2 5.3 5.4

Introduction, 111 Sensitizing Dyes, 111 Adsorption of Sensitizing Dyes to Silver Halide Grains, 122 Electronic Energy Levels of Sensitizing Dyes Adsorbed on Silver Halide, 126 5.5 Quantum Yield of Light-Induced Electron Transfer, 132 5.6 Kinetics of Light-Induced Electron Transfer, 140 5.7 Back Reaction of Light-Induced Electron Transfer, 143

CONTENTS

Vlll

5.8 Mechanisms of Spectral Sensitization, Supersensitization, and Desensitization, 149 6 Chemical Sensitization and Stabilization, 165 6.1 6.2 6.3 6.4 6.5

Introduction, 165 Sulfur Sensitization, 167 Gold Sensitization and Gold Intensification, 176 Reduction Sensitization, 180 Stabilizers and Antifoggants, 183

7 Development-Amplification of Latent Image, 200 7.1 7.2 7.3 7.4 7.5

Photographic Development Process, 200 Developing Agents, 205 Rate of Development, 211 Photographic Sensitivity in Relation to Development, 219 Image Processing Through Development, 221

8 Future Prospect of Silver Halide Photography, 228 8.1 Improvement in Efficiency of Image Formation in Relation to Sensitivity and Image Quality, 228 8.2 Improvement in Photographic Processing, 236 8.3 Development of New Photographic Systems, 236 Index, 241

Photographic Sensitivity

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h Silver Halide Photographic Materials

1.1 Overview of Silver Halide Photographic Materials Photography has a long history.1"3 The oldest existing photograph was taken in 1826 by Nicephore Niepse in France by means of heliographie with asphalt (natural photopolymer) as a photosensitive material. J. L. Marignier, who is a member of J. Belloni' s laboratory in France, has recently succeeded in reproducing heliographie. He had to expose to sunlight a photosensitive plate of heliographie for 5 days in order to take a picture.4 It is generally accepted that silver halide photography was born in 1839 with the Daguerreotype invented by L. Jacques M. Daguerre in France and the Calotype by W. H. Fox Talbot in England. In Daguerrotype, a thin layer of Agl was the photosensitive material, and a latent image was revealed by a development process with mercury vapor. However, it still took many minutes to take a picture. Furthermore, it was very laborious and expensive to take a picture by Daguerreotype.1""3 Nevertheless, many people enthusiastically accepted photography. Such enthusiasm indicates the strong attachment of human beings to photography as a medium for the exact recording and display of visual images. Judging from the exposure time needed for taking a Daguerreotype picture, photographic sensitivity was increased by more than 106 times over the past 150 years and is still increasing, as shown in Fig. l.l.5 In addition, improvement in image quality and the invention of color photography have been also achieved. It is much easier and cheaper to take pictures now than in the early days of photography. Figure 1.2 shows a scanning electron micrograph of a current color negative film with a sensitivity of ISO 100. White spots in this figure are silver halide grains, which are separately and randomly distributed in gelatin layers. On a film base there are blue-, green-, and red-sensitive layers, which are coated one over the other. Moreover, each color-sensitive layer is composed of high-speed, medium-speed, and low-speed sublayers, which are also coated one over the other. A photosensitive layer in a typical color negative film is composed of about 14 layers with different functions and is about 20 /xm thick in total. There are 12 million silver halide grains per square millimeter and more than 100 kinds of functional organic compounds in a film. 3

Fig. 1.1. Change in sensitivity of silver halide photographic materials.5

Fig. 1.2. (Left) Scanning electron micrograph of a cross section of the photosensitive layers of a color negative film with an ISO sensitivity of 100. White spots are silver halide grains. (Right) SEM micrographs showing silver halide grains in high-speed, medium-speed, and low-speed sublayers in a green sensitive layer (Courtesy of S. Ikenoue.)

4

Silver Halide Photographic Materials

5

The remarkable progress in silver halide photography during these 150 years is evident when we compare the composition of a Daguerreotype with a current color film. The former is a black-and-white photosensitive material with a thin layer of Agl, while the latter is a color film with many well-designed silver halide grains and many functional organic compounds. Detailed descriptions are given in Chapter 2 of the structure and properties of silver halide grains, showing the sophistication in design and preparation of these grains. It is also remarkable that so many kinds of functional organic compounds are used in concert with each other to take color pictures. As shown in Figs. 1.1 and 1.2, silver halide photography has progressed enormously in capability, and there have been many changes in structure during its long history. The gap between the actual sensitivity of silver halide photographic materials and its ultimate limit is decreasing. However, this does not mean that only limited room remains for further progress in silver halide photographic materials. On the contrary, silver halide photographic systems have continuously expanded the variety of their capabilities, including sensitivity, image quality, color reproduction, easy handling, storage stability, environmental safety, and cost performance, along with their applications, which include color negative film, color slide film, color paper, diagnostic X-ray film, microfilm, and films for printing involving various kinds of technologies and functional materials.6 These technologies and functional materials have expanded the area for growth for silver photography. Figure 1.3 shows photographic processes schematically. When exposed to light, a silver halide grain in a photographic emulsion forms a latent image center composed of a silver microcluster on its surface. A silver halide grain with a latent image center on its surface is reduced to a mass of silver (i.e., developed silver) in a developer. A grain without a latent image center does not change in a developer and is dissolved by a fixing solution. Thus, exposure to light of an emulsion layer forms a black-and-white negative-working image composed of silver. Figure 1.4 shows an example of the chemical reactions that occur in the development of a color film, in which each color-sensitive layer with a corresponding color coupler (1) and silver halide grains is developed in a color developer with phenylenediamine compounds (PPD) as developing agents.7 A PPD molecule reduces two silver ions and is converted into a quinone diimine (QDI). A QDI then takes part in the formation of an image composed of a dye (3) through an intermediate compound (2) by reaction with a coupler. Figure 1.3 indicates that a set of layers of photographic emulsions is a multifunctional device that has the functions of sensing, memory, and display without the aid of electricity. It is unique compared with other devices, such as those in electronic imaging systems, which are composed of several monofunctional devices such as a charge coupled device (CCD), used as a sensor; a magnetic disc, used as a memory device; and a cathode ray tube (CRT) used as a display device, all of which need electricity to work. This characteristic makes silver halide photographic systems compact compared with other such systems. A silver halide grain with a latent image center on its surface can be discriminated from a grain without a latent image center through the development process. As

Fig. 1.3. Illustration showing imaging processes in silver halide photography and electronic photography. In the former, a and b show the changes of exposed and unexposed silver halide grains during the processes.

Fig. 1.4. Example of coupling reactions for color image formation in silver halide photographic materials, where PPD is a p-phenylenediamine developing agent, QDI is a quinonediimine, and (1), (2), and (3) are a magenta coupler, a leuco dye, and a magenta dye, respectively. 6

Silver Halide Photographic Materials

7

Fig. 1.5. Illustration of a cubic AgBr grain with an edge length of 1 /^m and a latent image center composed of four silver atoms, demonstrating the very large degree of amplification brought about by development. described in Chapter 4, the smallest latent image center is composed of three or four atoms. Figure 1.5 shows the case of a cubic AgBr grain with an edge length of 1 /nm in which there are 20 billion silver ions. The photolytic reduction and clustering of only four silver ions are needed to change the fate of 20 billion silver ions in a grain. The degree of amplification in this case is as large as five billion, and is one of the most important reasons why silver halide photographic materials achieve their high sensitivity.8

Fig. 1.6. Illustration showing the steps and threshold for latent image formation.

6

PHOTOGRAPHIC SENSITIVITY

Figure 1.6 shows the steps in the formation of a latent image center. A single silver atom, a dimer, and a tetramer of silver atoms are called the latent preimage, subimage, and image centers, respectively. A preimage center is unstable, and subimage and image centers are stable. A latent image center can initiate development, whereas a subimage center cannot. Since a latent subimage center is composed of two silver atoms, the rate of formation of subimage centers is proportional to the square of the number of photons absorbed per second and thus to the square of the light intensity. Thus, a threshold exists between a latent preimage center and a subimage center in steps for the formation of a latent image center in terms of light intensity. Thus, any illumination whose intensity is not strong enough to form a latent subimage center does not change silver halide grains, even when it is repeated many times over a long period. Thus, silver halide photographic materials are considered to be quite sensitive for capturing a light image with an intensity larger than the threshold, while they are insensitive to an image with an intensity weaker than the threshold. A threshold also exists in the steps for the formation of a fog center, and photographic emulsions are stable for a long time during both their production and storage due to the existence of this threshold. This is the reason why silver halide photographic materials can achieve both high sensitivity and stability.8

1.2 Characteristics of Silver Halide Photography The overview presented in the previous section explains the characeristics of silver halide photography, including the numerous capabilities and applications possible with a variety of silver halide emulsion grains and functional organic compounds, as well as the compact systems and fine imaging possible with thin multilayers of

Table 1.1. Composition and Function

Variety of AgX grains and functional organic compounds Thin emulsion layers with many fine AgX grains Catalytic action of a latent image center on an AgX grain Threshold for latent image formation

Characteristics of silver halide photography Advantages

Variety in capability and application Room for progress Fine imaging Compact system Large frame area Large amplification leading to high sensitivity Storage stability and high sensitivity for short exposure time

Disadvantages

Nonsilver Materials

Expensive materials

Xerography

Poor light absorbance Short dynamic range Limited resolving power Wet processing with chemicals Low sensitivity for long exposure time

Imaging plate

Photopolymer Xerography, CCD

CCD at low temperature

Silver Halide Photographic Materials

9

photographic emulsions containing many fine silver halide grains in a random and loosely packed arrangement, the high sensitivity resulting from the large amplification function of a latent image center on a silver halide grain, and the stability during production and storage due to the threshold inherent in the latent image formation process. Although these characteristics enable silver halide photographic materials to provide the best materials for taking pictures, they are not necessarily suitable for other purposes. The characteristics of silver halide photography are summarized in Table 1.1 and are discussed in some detail below.

1.2.1 Capabilities, Applications, and Future Progress in Emulsion Grains and Functional Organic Compounds Many kinds of technologies applied to a variety of silver halide emulsion grains and functional organic compounds have been used in silver halide photographic materials to achieve fine imaging over the long history of the field, resulting in expansion of its many capabilities and applications, and providing more room for future progress. However, these materials are likely to be expensive due to their complicated compositions and nonerasable images.

1.2.2 Compact and Fine Imaging Systems Caused by Thin Multilayers of Photographic Emulsions Containing Many Fine Silver Halide Grains with a Random and Loose Arrangement A large number of fine and well-designed silver halide grains are separately and randomly arranged in thin gelatin layers with a relatively low packing density to achieve fine imaging. However, this arrangement prevents grains in these layers from absorbing incident light efficiently. Although a random arrangement of grains in the layers is not necessarily suitable for fine imaging, it does not make photographic materials sensitive to various defects in the layers and enables the production of photographic materials with a large frame area, as discussed later. A photographic emulsion layer with multiple functions (i.e., sensing, memory, and display) provides a compact system for taking pictures. It should be noted, however, that designing photographic emulsion layers as multifunctional devices tends to prevent each of these functions from being optimized. For example, designing photographic emulsion layers to be displaying devices restricts their light absorbance and dynamic range. Although thick layers with dense photosensitive materials are suitable for capturing incident radiation and are used in many systems, as described in the next section, photographic emulsion layers have to be thin and contain silver halide grains with a low packing density to achieve fine imaging. The need to design photographic emulsion layers as displaying devices thus makes them less effective in capturing incident radiation and detective quantum efficiency. Since the gradation of a displayed image should be large enough to be properly detected by the human eye, the dynamic range of silver halide materials as sensors is restricted.

10

PHOTOGRAPHIC SENSITIVITY

1.2.3 High Sensitivity Caused by Efficient Formation and Unique Amplification Function of Latent Image Centers High sensitivity is achieved in silver halide materials by the efficient formation and unique amplification function of a small latent image center on a large silver halide grain. Howeyer, the resolving power of silver halide materials is limited and cannot exceed that of photosensitive materials composed of photosensitive molecules and polymers, since the size of silver halide grains cannot be smaller than molecules and polymers. Tfye large degree of amplification is achieved by catalytic reaction of a latent image;center in a developer solution with various chemical reagents, which makes it difficult for photographic processes to be easy to perform, inexpensive, and compatible with environmental safety requirements. 1.2.4 High Stability During Production and Storage Due to the Threshold for Latent Image Formation The presence of the threshold for latent image formation enables silver halide photographic materials to achieve both high sensitivity with short exposure times and stability on storage for a long period of time. This threshold characterizes silver halide photographic materials and makes them unsuitable for taking pictures of faint objects for long exposure times. 1.2.5 Highly Reliable and Low Cost Mass Production Using Mass Production Technologies Silver halide photographic materials with the above-mentioned characteristics are suitable for mass production technologies and result in high reliability at a low cost.9"11 Reliability is very important for silver halide photosensitive materials, since deficient products are discovered only after they are exposed and processed. Photographic materials with high capability need elaborate design and precise production of silver halide emulsion grains, which have been achieved by progress in crystallization production technology.10 Silver halide grains, which are barely soluble in water, are prepared by a double decomposition reaction between a watersoluble silver salt and halide salts. Extensive efforts have been made to devise the reactors for this purpose. The coating technology used with photographic materials is quite unique and is characterized by great precision, high uniformity, and high speed, as well as simultaneous multilayer coating in the dark. A precision and uniformity containing 1-2% variation is necessary to achieve the desired abilities of photographic materials. Production of photographic emulsion layers with a large area and high uniformity depends on the layers being composed of many silver halide grains randomly arranged in a gelatin and thus must not be sensitive to defects and dust in the layers, as discussed later. During the manufacturing process, silver halide grains receive various stimulations, such as from agitation at high temperature in the crystallization and digestion processes and from pressures during various manufacturing processes. Manufacture

Silver Halide Photographic Materials

11

of photographic materials without deteriorating the capability of silver halide grains is achieved because silver halide grains are insensitive to any stimulation that is weaker than the threshold for latent image formation, as described earlier. The apparatus used for crystallization of silver halide emulsion grains and coating the emulsions is in principle the same as for all photographic materials, enabling an efficient investment in this technology on a large scale. This is due to the simple and similar compositions of different photographic emulsions (i.e., silver halide grains suspended in a gelatin).

1.3 Silver Halide Photography Among Various Imaging Systems Many kinds of photosensitive materials have been proposed and examined for practical use.6'12 In 1970, Ooue summarized the relation between sensitivity and resolving power of various photosensitive materials that had been produced and studied by that time.13"34 This is illustrated in Fig. 1.7, in which sensitivity is expressed as ASA or the reciprocal of the exposure (erg/cm2) for imaging, and resolving power is expressed as the number of parallel line pairs per millimeter detected by photosensitive materials. Ooue derived the equation used in the preparation of this figure by making the sensitivity proportional to the reciprocal of the resolving power squared. Figure 1.8 summarizes the relation between sensitivity and resolving power of the photosensitive materials used at present. In some cases the values for materials and systems are described separately. Sensitivity is expressed by ISO or by the reciprocal of the exposure (erg/cm2) for imaging, and resolving power is, in principle, given by the number of parallel line pairs per millimeter detected. However, one line pair corresponds to two pixels in CCD. Thus, for comparison between CCD and color negative film, the resolving power of the former in Fig. 1.8 should be compared with the spatial frequency corresponding to \ in the modulation transfer function of the latter, which is nearly one half of the resolving power shown in Fig. 1.8. The point of sensitivity and resolving power for CCD is thus situated in the vicinity of that of a corresponding color negative film in Fig. 1.8, although the latter contains information for three colors in each line pair and the former contains information for only a single color. Comparison between Figs. 1.7 and 1.8 indicates characteristics of silver halide photographic materials and other photosensitive materials. As seen in Fig. 1.7, many photosensitive materials have been studied for commercial use that would compete with silver halide photographic materials, and most have been unsuccessful. Some nonsilver photosensitive materials have been proposed to be as sensitive as silver halide materials27 but were not proven to be useful because they were unstable in storage due to the lack of a threshold in their imaging processes. As judged from Figs. 1.7 and 1.8, silver halide photographic materials are the best photosensitive materials known from the viewpoint of sensitivity and resolving power, and form the boundary region, ranging from high sensitivity with low resolving power to low sensitivity with high resolving power. There are, however, several successful nonsilver photosensitive materials, including electrophotographic materials,35 photopolymers,36 CCD37'38 and imaging plates.39

Fig. 1.7. Relation between sensitivity and resolving power of various photosensitive materials produced and studied in 1970. Bi are thin Bi films,14 CaF2:La are CaF2 and SrTiO3 photochromic crystals,15 Cas is an electrophotographic system with cascade development,16 CTF is a charge-transfer frost process, DG is dichromated gelatin,17 DZ is a diazo film,18 EDS:EK are dry processing silver photographic materials,19 PR is thermoplastic holography,20 HPP is a photopolymer based on dye-sensitized polymerization of barium acrylate solutions,21 HR is a high resolution plate made by Eastman Kodak Co., IL is a Ilford's plate for holography,22 Kal is Kalver film,23 Liq is electrophotography with liquid development,16 LNB is a single crystal of lithium niobate,24 LSG is a photochromic glass,25 MN is a microfilm, MnBi is Curie-point writing of magnetic holograms on MnBi thin layers,26 NVC is photopolymerization imaging based on the reaction between 9-N-vinyl carbazole and CB4,27 N-100 is a 12

Silver Halide Photographic Materials

13

Some silver halide materials are behind the boundary region. Instant films40 and full-color silver halide photothermography41 lose resolving power and gain real time processing by the use of diffusion transfer processes. In diagnostic X-ray films, the X-ray image is efficiently captured by a flat rectangular sheet of light-emitting phosphor material, which is called a screen. The screen fluoresces when stimulated by X-rays, and the light emitted by the screen exposes the film that stores the image. Thus, the screen benefits the above-mentioned system in having high sensitivity to X-rays, although it deteriorates the resolving power of the system by scattering the emitted light.42 Photographic films for printing are required to provide very high gradation and image qualities that are unique to those films.43 Color papers especially need to provide special image quality, good processing suitability, and cost performance, in addition to high sensitivity and resolving power.7'44 It is important to recognize the reason that the above-mentioned nonsilver materials replace silver halide materials for some purposes in order to properly understand the characteristics of silver halide photography among various imaging systems. Electrophotography includes imaging systems that first create images in charge and then render these images into hard copy images in opaque particles called toners that are fused onto paper or film. Xerography, which is the most successful system in electrophotography, was invented by Chester Carlson for a real-time, dry-processed, and inexpensive office copy system, which replaced silver halide photographic materials in that area and became the premier office copy system, taking advantage of the above-mentioned characteristics by using plain paper for the final image, and automating and speeding up the process. In xerography, the photoconductor, in the form of a rotating belt or drum, is corona charged, and then is imaged with light to create a negative discharge image. The positive-charge image is treated with toner powder, leaving a positive image in toner on the photoconductor. The toner image is transferred to plain paper and thermally fused on the paper to make the final image. Sulfur was originally used as a photoconductor and was later replaced by selenium. Electrophotography with nonparticulate photoconductors (e.g., organic or amorphous) and liquid toner development yields a resolving power that is high enough for monochrome microfirms45(a) and is used for compact and inexpensive system.45(b) Although this system has low sensitivity to meet the demand for the microform, electrophotography with liquid toner development could provide a sensitivity of about 1 erg/cm2 and a resolving power of 250-500 line pairs/mm.35(b) In photopolymer and resist systems, photons of the exposing light change the color negative film, OPC is electrophotography with an organic photoconductor,28 OFF is an organic photochromic material,18 PbI2 is a PbI2 thin layer,29 PPMZ is a photopolymerization process,30 PS is a photosolubilization process, P47 is a Polaroid's film type 47, RS is an Itek RS process,31 SEN is Sro^Bao^sNbaOs, SSS is a high-speed black-and-white negative film, VESH is an evaporated silver halide layer, 3MDS is a dry silver film made by 3M Company, 8E, 10E, and 14C are high-resolution photographic emulsions made by Agfa-Gevaert Inc. (8E-70, 10E-70, and 14C-70, respectively),32 649F is a spectroscopic plate made by Eastman Kodak Company,33 and KOR, KTFR, KPR-2, and AZ-1360 are photoresists.34

Fig. 1.8. Relation between sensitivity and resolving power of the various photosensitive materials presently in use. O, color negative and reversal films; ©, color papers; ®, microfilms; ®, films for graphic arts; ®, color instant films; ®, full color silver halide photothermography, ®, black-and-white instant films; •, diagnostic x-ray films with the sensitivity of films and resolving power of film/screen systems; D, high-resolution plates; A, electrophotography with cascade development; A, electrophotography with nonparticulate photoconductor and liquid development; ©, photoresists with and without chemical amplification process. The beginning and end of each arrow indicates the resolving powers of the corresponding material and system.

14

Silver Halide Photographic Materials

15

solubility of polymers by various photochemical reactions. Since polymer molecules correspond to and are much smaller than developed silver grains and dye clouds in silver halide photographic materials, toner particles in electrophotography, pixels in CCD, and BaFBrL:Eu2+ grains in imaging plates, photopolymers, and resists are characterized by sensors with low sensitivity and high resolving power, as compared with the other materials already described. Since photopolymer systems do not usually have intrinsic color, they are toned, dyed, or used as resists in their application as printing plates, proofing materials, and etching resists for electronic fabrication. Usually, they are not sensitive to visible light and are exposed to ultraviolet or other high-energy radiation. Photopolymer systems are replacing silver halide photographic materials in the field of microfabrication, taking advantage of their extremely high resolving power.46 Recently, an erasable X-ray imaging plate, which is based on X-ray excitation of a phosphor layer, has been developed for diagnostic radiography,39 and also has applications for recording a transmission electron microscope image,47 an X-ray diffraction image, and in autoradiography.48 The imaging plate is approximately 0.5 mm thick and is composed of a flexible plastic plate coated with fine photostimulable phosphor grains (BaFBr:Eu2+) combined with an organic binder. The mechanism of its sensitivity is being studied.49 The photostimulable phosphor can store a fraction of the absorbed incident energy from irradiation with X-rays, ultraviolet light, or electrons by exciting an electron from Eu2+ to a bromide vacancy to form an F center and Eu3+. When later stimulated by visible or infrared radiation, it emits a photostimulated luminescence from Eu2+ in the excited state, which is formed by photo-induced electron transfer from the F center to Eu3+. An imaging plate is much more sensitive to radiation than a photographic film, because it can efficiently absorb incident radiation, which differs from a photographic film. The phosphor grains in an imaging plate can be dense and thick enough to efficiently capture incident radiation, since the plate is not used as a display device.

1.4 Silver Halide and Electronic Photography All nonsilver photosensitive materials described thus far are not suitable for taking pictures. Silver halide photography now finds itself in competition with electronic photography for taking pictures. In silver halide photography, the internal photoelectric effect of silver halide is used to catch an optical image, which is followed by chemical reactions to record and reproduce the image. In electronic photography, the internal photoelectric effect of silicon is used to catch an optical image, which is treated in an electric circuit as an image signal. Comparison between silver halide photography and electronic photography is therefore important for understanding the nature, as well as the future, of silver halide photography.50"55 The representative photosensitive device in electronic photography is a CCD, the function of which is illustrated in Fig. 1.9. Each pixel has a potential well that is electrically connected with others in series. Photoelectrons created by photons

16

Fig. 1.9. Illustration showing transmission of photoelectrons photo-created and stored in each potential well in a CCD.

incident to each pixel are stored in the potential well, transferred from well to well, and taken out one after another to give an image signal in time series. The efficiency of color films as well as CCDs as photoreceptors can be evaluated by detective quantum efficiency (DQE), defined as follows56'57:

where R and R0 are signal-to-noise ratios at the input and output, respectively; y is the slope of the D-logE plot; E is exposure in terms of photons per unitf area; and G is the Selwyn granularity constant. The quantum efficiency of a CCD could be as large as several tens of percentages at low temperature.38'58 The estimated value of DQE of photographic films clusters in the range of 1-2%.56>57 It is estimated from this result that the DQE value of a CCD at low temperature could be more than one order of magnitude larger than those of photographic films. Being cooled to depress thermal noise and to enable exposure for a long period of time, a CCD is therefore quite useful for taking pictures of faint objects, such as stars in outer space, for long exposure times. In spite of its merits, a CCD is not as useful for taking color pictures as silver halide photography. This is because the sensitivity of a still video camera with a mass-produced CCD is at most comparable to that of a color film at room temperature, and the number of pixels in a frame of a \ inch CCD (4 X 10s) is considerably

Silver Halide Photographic Materials

17

smaller than that of equivalent pixels in a frame of color film with a 135 format (2 X 107).52 The area of a frame with a 135 format (36 X 24 mm2) is 28 times larger than that of a pixel of a f inch CCD (6.4 X 4.8 mm2). A detailed analysis of the difference between the frame area of a CCD and a color film will follow later. For taking usual color pictures, a mass-produced CCD is used at room temperature, and its exposure time should be short enough to get a good signal-to-noise ratio due to the thermal noise in silicon at room temperature.50 Furthermore, color microfilters with restricted transmittance are used to divide pixels into three groups, which are sensitive to blue, green, and red light, respectively. In a colorfilm,ho ever, blue-sensitive, green-sensitive, and red-sensitive layers are coated one on top of another without any color microfilters, as illustrated in Fig. 1.2. More importantly, the limitation for a CCD to take high quality color pictures is the limited number of pixels in a frame. There are two ways to increase the number of pixels in a CCD. Continued progress in microfabrication has contributed to progress in microelectronic devices by decreasing line width and thus increasing the degree of integration. However, a decrease in pixel size by itself cannot be regarded as progress for CCDs, since it is associated with a decrease in the signal-to-noise ratio (S/N). The decrease in pixel size in CCD corresponds to a decrease in the size of emulsion grains in silver halide photography, which should always be associated with a decrease in sensitivity, as described in Chapter 4. To calculate the S/N of the output of a nonideal sensor, one sums the photoelectric shot noise due to the Poisson-distributed fluctuations in the photon flux and the noise generated by the sensor itself, Nn, in quadrature, as follows50'58:

where P is the average number of photons per unit area that are incident on the sensor, a is the area that receives incident photons in each pixel, and © is the ratio of the number of photogenerated electrons to that of incident photons. In modern CCD sensors, the sensor noise Nn may include various components: reset noise, noise associated with the built-in output amplifier, fluctuations in the thermally generated dark current, and transfer noise when the transfer efficiency is poor. A perfect sensor should be associated with noises arising only from the Poisson-distributed fluctuations in the photon flux and in the thermally generated dark current, giving the ideal result as follows:

where i is the expected number of dark-current electrons per unit area per unit exposure time. Equation (1.3) indicates that the decrease in pixel size will result in a decrease in S/N, even for a perfect sensor. The ratio of a to the total area of a pixel decreases with decreasing pixel size.51 In addition to the noises in Equation (1.3), a nonideal

Fig. 1.10. Frames in color films and CCDs with various formats. The frame areas of a film with a 135 format and a § inch CCD are 36 X 24 mm2 and 8.8 X 6.6 mm2, respectively. sensor is associated with various kinds of sensor noise and fixed pattern noise if its image area contains structural nonuniformities. Those noises tend to increase with decreasing pixel size in CCD. Therefore, the decrease in the size of each pixel cannot result in improvement in the limited number of pixels in a CCD frame. Figure 1.10 compares the frame area of color films and CCDs of various formats. The frame area of a color film with a 135 format is about 28 times larger than that of aCCD with a | inch format. Figure 1.11 shows the productive yield of CCDs as

a function of their frame area.51 A color film with any frame area can be produced with a good productive yield, whereas the productive yield of a CCD decreases sharply with an increase in its frame area. Accordingly, the number of pixels in a mass-produced CCD has remained constant (about 4 X 105) for many years. Figure 1.12 shows the present state of the photographic products for taking color pictures as exemplified by the number of equivalent pixels as functions of sensitivity and frame area of color negative films versus CCD. In contrast to CCD, color films can have a wide variety of areas dependent on their purpose, with variation of image quality (i.e., number of equivalent pixels) and sensitivity. As to the difference in frame area between a color film and CCD, it should be noted that a camera system for color films can be more compact than that for a CCD because the former is based on multifunctional silver halide emulsion layers and can work without electricity, as stated in the explanation of Fig. 1.3. In addition to the fact that a still video camera is larger and heavier than a color film camera, the former needs equipment to provide an electric supply (i.e., secondary cells and charging equipment) and for transportation of electric images. Thus, a camera system for color films can be compact for its frame area.

Silver Halide Photographic Materials

19

The reason for this difference between color films and CCD can be found by comparing their structures and functions. Figure 1.13 shows a top view of pixels in a CCD and silver halide grains used for a color film with an ISO sensitivity of 100. In a CCD, each pixel produces photoelectrons in a potential well on exposure to light and is electrically connected to others as an electric circuit in series. A color negative film with an ISO sensitivity of 100, however, has about 1.2 X 107 silver halide grains/mm2 and 2 X 107 equivalent pixels/135 format (36 X 24 mm2). Therefore, the area that corresponds to an equivalent pixel in this film is 43 A2 and contains as many as 500 silver halide grains. These grains are randomly distributed in emulsion layers, act independently as photoreceptors with an on-off action, and give rise to the total function of a pixel. A defective silver halide grain or a dust particle whose size is comparable to those of silver halide grains (around 1 ^m) hardly disturbs the function of an equivalent pixel in a color film, because it is only one among 500 grains that are randomly arranged in an equivalent pixel, and because it is quite unlikely that a single dust particle desensitizes or fogs as many as 500 grains that independently act as photosensors and are chemically treated in parallel. On the other hand, it is quite probable that even a dust particle as small as 1 ^m disturbs the function of a CCD pixel by increasing its thermal noise or decreasing its photosensitivity by capturing photo-

Fig. 1.11. Murphy-See's model showing productive yield as a function of the frame area of CCD in the presence of defects of 5 cm~'. 51

Fig. 1.12. Number of equivalent pixels as functions of sensitivity and frame area for color negative films (open circles and solid lines)52 and for a still video camera (broken line).

Fig. 1.13. Top view of pixels in a CCD and silver halide grains used for a color negative film with an ISO sensitivity of 100 (Courtesy of S. Ikenoue.) 20

Silver Halide Photographic Materials

21

electrons, or disturbs the transportation of image signals by bringing about a short circuit or wire break, since the function of a CCD depends on an electric circuit of entirely connecting pixels in series. There is also a big difference in image processing and transportation between color film and still video camera systems. Image processing in color films is chemical, extends over each silver halide grain and/or equivalent pixel in parallel during the process of color development, and has large variety due to the performance of silver halide grains and many functional organic compounds used for color development. However, image processing of color films has some limitation, and cannot be redone or reused. Image processing in color films is described in Chapter 7. In contrast, image processing in still video camera systems has many options, electrically extending over each pixel in time series, and can be redone. Image transformation in still video camera systems is also electric in time series. These analyses have made it clear that silver halide photography is inherently superior to electronic photography for taking color pictures in general due to its ability to achieve high sensitivity and image quality with reasonable feasibility, and have revealed the essential differences between the two technologies on which the superiority of color films with respect to CCD depends. Electronic photography is superior to silver halide photography in quantum efficiency at low temperature and in image processing and transformation, and is useful for purposes that do not require high sensitivity, image quality, and feasibility. The design of a new system in which the advantages of both silver halide photography and electronic photography are included would be promising.59

1.5 Summary It has been concluded from an analysis of sensitivity and image quality of photosensitive materials that silver halide photographic materials are and will continue to be the best technology for taking pictures in the immediate future, and that one of the largest areas for improvement in photographic materials will be in the efficiency of photographic sensitivity. In order to significantly improve the efficiency of photographic sensitivity, knowledge of the mechanisms of photographic processes, which are described in Chapters 2-7, must be expanded. Based on the analysis made in this chapter and detailed descriptions provided in Chapters 2-7, the future prospects for silver halide photography are finally examined in detail in Chapter 8.

References 1. J. M. Eder, History of Photography, translated by E. Epstein, Columbia University Press, New York, 1945. 2. H. Gernsheim, A. Gernsheim, The History of Photography, Oxford University Press, London, 1955. 3. E. Ostroff, ed., Pioneers of Photography. Their Achievements in Science and Technology, SPSE, The Society for Imaging Science and Technology, Springfield, VA, 1987.

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4. J. L. Marignier, Nature, 346, 115(1990). 5. Amendment of a figure in S. Ooue, Seimitsu Kikai, 32, 300(1966); also in Round Table: Conventional and Unconventional, SPSS News, 8(3), 4(1965). 6. J. Sturge, V. Walworth, A. Shepp, eds., Imaging Processes and Materials. Neblette's Eighth Edition, Van Nostrand Reinhold, New York, 1989. 7. (a) J. R. Thirtle, L. K. Tong, L. J. Fleckenstein, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, Chapter 12; (b) P. Klause, in Ref. 6, Chapter 4; (c) N. Miyasaka, J. Soc. Photogr. Sci. Technol. Jpn., 54, 156(1991). 8. T. Tani, Physics Today, September 1989, 36. 9. T. Sasaki, Kagaku Kogaku, 51, 236(1987). 10. H. Saito, Kagaku. Souchi, 34, 61(1992). 11. (a) T. A. Russell, Eastman Kodak Co., Method of Multiple Coating, U. S. Patent 2,761,791(1956); (b) D. J. Hughes, Eastman Kodak Co., Method for Simultaneously Applying a Plurality of Coated Layers by Forming a Stable Multilayer Free-Falling Vertical Curtain, U. S. Patent 3,508,947(1970); (c) K. Miyamoto, FujifilmRes. Dev., 35,40(1990). 12. J. Kosar, Light-Sensitive Systems: Chemistry and Application of Nonsilver Halide Photographic Processes, John Wiley & Sons, New York, 1965. 13. (a) S. Ooue, Seimitsu Kikai, 32, 300(1966); (b) S. Ooue, Oyo Butsuri, 39, 887(1970). 14. J. J. Amodei, R. S. Mezrich, Appl. Phys. Lett., 15, 45 (1969). 15. D. R. Bosomworth, H. J. Gerritsen, Appl. Optics, 7, 95(1968). 16. C. J. Claus, Photogr. Sci. Eng., 7, 5(1963). 17. T. A. Shankoff, Appl. Optics, 7, 2101(1968). 18. Round Table, SPSS News, 8, 4(1965). 19. J. Marchant, Preprint of SPSE Seminar on Novel Imaging System, 31(1969). 20. J. C. Urbach, R. W. Meier, Appl. Optics, 5, 666(1966). 21. J. D. Margerum, L. J. Miller, J. B. Rust, Photogr. Sci. Eng., 12, 177(1968). 22. (a) Y. Tomoda, J. Synth. Org. Chem. Jpn., 27, 1205(1969); (b) Y. Tomoda, Koubunshi Gakkai Kaishi, 19, 153(1970). 23. A. E. Sporer, Photogr. Sci. Eng., 8, 35(1964). 24. F. S. Chen, F. S. Chen, J. T. LaMacchia, D. B. Fraser, Appl Phys. Lett., 13, 223(1968). 25. J. P. Kirk, Appl. Optics, 5, 1684(1966). 26. R. S. Mezrich, Appl. Phys. Lett., 14, 132(1969). 27. N. J. Notley, Photogr. Sci. Eng., 14, 97(1970). 28. M. Hayashi, Preprint of SPSE Annual Conference, 1969. 29. M. R. Tubbs, J. Photogr. Sci. 17, 162(1969). 30. D. W. Woodward, V. C. Chambers, A. B. Cohen, Photogr. Sci. Eng., 7, 360(1963). 31. E. Berman, Photogr. Sci. Eng., 13, 50(1969). 32. D. Scultze, Laser Focus, 4, 23(1968). 33. A. A. Friesem, J. S. Zelenka, Appl. Optics, 6, 1755(1967). 34. M. S. Htoo, Photogr. Sci. Eng., 12, 169(1968). 35. (a) C. F. Carlson, in Xerography and Related Processes, J. H. Dessauer, H. E. Clark, eds., Focal Press, New York, 1965, Chapter 1; (b) M. E. Scharfe, D. M. Pai, R. J. Gruber, in Ref. 6, Chapter 5. 36. (a) A. B. Cohen, P. Walker, in Ref. 6, Chapter 7; (b) T. Kokubo, Fujifilm Research & Development, 34, 21(1989). 37. W. S. Boyle, G. E. Smith, Bell System Tech. J., 49, 587(1970). 38. I. S. Mclean, Electronic and Computer-Aided Astronomy: From Eyes to Electronic Sensors, Ellis Horwood Ltd., Chichester, England, 1989. 39. (a) M. Sonoda, M. Takano, J. Miyahara, H. Kato, Radiology, 148, 833(1983); (b) J. Miyahara, K. Takahashi, Y. Amemiya, N. Kamiya, Y. Satow, Nucl. Instrum. Mech.,

Silver Halide Photographic Materials

40. 41. 42. 43. 44. 45. 46. 47. 48. 49.

50. 51. 52. 53. 54. 55. 56. 57.

58. 59.

23

A246, 572(1986); (c) Y. Amemiya, K. Wakabayashi, H. Tanaka, Y. Ueno, J. Miyahara, Science, 237, 164(1987); (d) Y. Amemiya, J. Miyahara, Nature, 336, 89(1988). V. K. Walworth, S. H. Mervis, in Ref. 6, Chapter 6. (a) H. Kara, K. Sato, Fujifilm Res. Dev., 34, 10(1989), (b) T. Yokokawa, K. Nakamura, N. Matsumoto, Fujifilm Res. Dev., 37, 49(1992). (a) L. K. Wagner, P. Narayama, R. C. Murry, Jr., in Ref. 6, Chapter 17; (b) N. Iwasaki, Mi. Yamada, J. Ose, S. Ohno, Fujifilm Res. Dev., 35, 11(1990). (a) M. H. Bruno, in Ref. 6, Chapter 16; (b) K. Katou, S. Sasaoka, S. Hirano, J. Soc. Photogr. Sci. Technol. Jpn., 52, 390(1989). O. Takahashi, T. Ogawa, Fujifilm Res. Dev., 36, 7(1991). (a) M. R. V. Sahyun, P. J. Vogelgesang, in Ref. 6, Chapter 12; (b) S. Ohtsuka, K. Yamana, A. Yoda, H. Tachikawa, N. Suzuki, S. Kondou, Fujifilm Res. Dev., 33, 1(1988). J. M. Shaw, in Ref. 6, Chapter 18. T. Oikawa, N. Mori, N. Takano, M. Ohnishi, /. Electr. Microsc., 39, 437(1990). S. Okano, J. Miyahara, Fujifilm Res. Dev., 36, 39(1991). (a) K. Takashashi, J. Miyahara, Y. Shionoya, J. Electrochem. Soc., 132, 1492(1985); (b) M. Z. Su, X. P. Sun, Mater. Res. Bull., 32, 879(1987); (c) H. von Seggern, T. Voigt, /. Appl. Phys., 64, 1405(1988); (d) M. K. Crawford, L. H. Brixner, K. Somaiah, J. Appl. Phys., 66, 3758(1989); (e) H. von Seggern, A. Meijerink, T. Voight, A. Winnacker, J. Appl. Phys., 66, 4418(1989); (f) H. H. Rueter, H. von Seggern, R. Reininger, V. Saile, Phys. Rev. Lett., 65, 2438(1990); (g) T. Hangleiter, F. K. Koschniek, J. M. Spaeth, R. H. D. Nuttall, R. S. Eachus, /. Phys. Condens. Matter, 2, 6837(1990); (h) Y. Iwabuchi, C. Umemoto, K. Takashashi, S. Shionoya, J. Luminescence, 4S&49, 481(1991); (i) F. K. Koschniek, J. M. Spaeth, R. S. Eachus, W. G. McDugle, R. H. D. Nuttall, Phys. Rev. Lett., 67, 3571(1991). L. J. Thomas, "Photographic Systems Analysis," in the preprint book of Tokyo Symposium sponsored by Soc. Photogr. Sci. Tech. Jpn., July, 1980, Tokyo. M. Tabei, Y. Mizobuchi, J. Soc. Photogr. Sci. Tech. Jpn., 49, 125(1986). A. Kriss,/. Soc. Photogr. Sci. Tech. Jpn., 50, 357(1987). S. Ikenoue, M. Tabei, J. Imaging Sci., 34, 187(1990). T. Tani, Y. Ohishi, J. Soc. Photogr. Sci. Tech. Jpn., 52, 218(1989). T. Tani, J. Soc. Photogr. Sci. Tech. Jpn., 53, 87(1990). G. R. Bird, R. C. Jones, A. E. Ames, Appl. Optics, 8, 2389(1969). (a) P. Kowalski, A. E. Saunders, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, Chapter 22; (b) R. Shaw, Opt. Acta, 20, 749(1973); (c) H. E. Spencer, Photogr. Sci. Eng., 15, 468(1971); (d) M. R. V. Sahyun, Photogr. Sci. Eng., 19, 38(1975); (e) G. R. Bird, M. D. Cox, Photogr. Sci. Eng., 25, 246(1981). A. Rose, P. K. Weimer, Physics Today, September 1989, 24. Eastman Kodak Company, Kodak Photo CD System—A Planning Guide for Developers, Rochester, NY (1991); J. W. Meyer, "Images of the Future: The Convergence of Silver Halide and Electronics Technologies," The International East-West Symposium III on New Frontiers in Silver Halide Imaging, November, 1992, Kaanapali, Hawaii.

2 Structure and Preparation of Silver Halide Grains

The preparation of silver halide grains in photographic emulsions is one of the most important technologies for the development of new silver halide photographic materials with high capability, and significant progress has been made over the last decades to make silver halide grains controllable with respect to size, shape, halide distribution, and crystal defects. Monodispersed silver halide grains with sophisticated structures are being developed. In this chapter, descriptions are focused on the structure and mechanism of the formation of these grains.

2.1 Structure of Silver Halide Grains Silver halide grains used in photographic materials are composed of AgBr, AgCl, and mixed crystals of AgBr with Agl and/or AgCl and of AgCl with AgBr and/or Agl. The average size of the silver halide grains usually used is around 1 ^im, ranging from less than 0.1 fan to several micrometers. The crystal structure is face-centered cubic, belonging to the space group Fm3m, and regular grains can exist in seven forms, which are the cube, octahedron, rhombic dodecahedron, icositetrahedra, trisoctahedra, tetrahexahedra, and hexaoctahedra,1 as shown in Fig. 2.1. Only {100} and {111} faces are usually stable on the grain surface, and thus the cube and octahedron are the shapes of stable regular grains, as shown in Fig. 2.2. Regular grains other than the cube and octahedron could be prepared in the presence of crystal habit modifiers. The rhombic dodecahedron has been observed for AgCl by Claes2 and for AgBr by Berry3 and Nishiyama.4 In addition, Maskasky has recently prepared other regular grains.1 It is believed that each crystal-habit modifier stabilizes a specific face. Since the surface of silver halide grains is usually composed of {100} and/or {111} faces, the ratios of the area of the {100} face and {111} face to the total surface area (PIOo and P i n , respectively, and P100 + P l u = 1) are important. The determination of P100 and PI 11 by use of the electron microscope is possible for some silver halide grains with simple shapes5 and is difficult for most grains. On the basis of the applicability of the Kubelka-Munk equation to the photographic emulsion,6 24

Structure and Preparation of Silver Halide Grains

25

Fig. 2.1. Illustration of regular AgBr grains as classified by Maskasky.1 Tani has developed a method to determine the values of P100 and Pm by use of the facts that the absorption spectra of a sensitizing dye in a family of 9-methyl-thiacarbocyanine dyes, including 3,3'-bis(4-disulfobutyl)-9-methyl-thiacarbocyanine, on the {100} and {111} faces of silver halide grains and in the gelatin phase are considerably different, and that the dye is at first adsorbed to the {100} face, then begins to be adsorbed to the {111} face after the {100} face is saturated with the dye, and finally is released in the gelatin phase after the {111} face is saturated with the dye.7 In addition to three-dimensional grains, tabular silver halide emulsion grains have been prepared for a long time.8 Recently, much progress has been made in the preparation and sensitization of tabular grains, since the advantages of tabular grains for various photographic materials have been proposed and recognized.9 Figure 2.3 shows an electron micrograph of tabular silver bromide grains and an illustration of the structure of a tabular grain. As seen in this figure, a hexagonal tabular grain has two twin planes on {111} lattice planes in parallel to its main surfaces, and uneven side surfaces with the lines of the intersection of the twin planes with them. Figure 2.4 shows the arrangement of {111} planes around a twin plane, indicating that a twin plane is formed as a result of a stacking fault in the abovementioned arrangement. Berriman and Herz proved the existence of parallel twin planes by X-ray diffraction analysis.8 Hamilton and Brady estimated the position of twin planes by observing the protrusions on the main surface as a result of the formation of the photolytic silver particles at twin planes in a tabular silver bromide grain.10 Observing the orientation of epitaxially formed pyramids on the main surfaces of tabular grains on optical micrographs, Maskasky demonstrated that a large

26

PHOTOGRAPHIC SENSITIVITY

Fig. 2.2. An illustration of the (100) and (111) faces of face-centered cubic crystal and electron micrographs of regular AgBr grains with (100) and (111) faces on their surface (i.e., cubic and octahedral grains).

hexagonal tabular grain had an even number of parallel twin planes and a large triangular one had an odd number of parallel twin planes.11 Black et al. directly observed twin planes on an electron micrograph of a cross section of a tabular AgBr grain,12
Structure and Preparation of Silver Halide Grains

27

Fig. 2.3. Electron micrograph of tabular AgBr grains (courtesy of M. Saito61) and (inset) an illustration showing the structure of a tabular grain.

Brady have proposed a model that has a half-filled top layer of silver ions with a hexagonal configuration.14 The structure of the {100} and {111} surfaces of silver halide has been recently studied by means of surface extended X-ray absorption fine structure (SEXAFS) combined with theoretical calculations.15 As shown in Fig. 2.5, two reconstructed low-energy surfaces have been identified by computer modeling. The top layer atoms in these two models have either a hexagonal or alternate-row configuration. SEXAFS data have agreed more with the alternate-row model. The low-energy configuration computed for the AgBr {100} surface has been supported by the SEXAFS data and is shown in Fig. 2.6, indicating that relaxation occurs with the displacement of Br ions slightly above the normal surface plane and Ag ions below the surface place, thus yielding a "rumpled" effect. Recently, the atomic force microscope has been successfully used for observation of each bromide ion on the surface of AgBr.16 It is expected that these attempts will reveal the surface structure of silver halide on the atomic scale in the near future. Various types of distribution of halide ions are available for grains with mixed crystals of silver halides. In particular, grains with heterojunction of different silver halides have been studied extensively. One of them is the so-called double-structured

Fig. 2.4. An illustration showing the structure of twin planes in silver halide.

Fig. 2.5. Top views of the reconstructed AgBr {111} surface, showing (a) the hexagonal model and (b) the alternate-row model with ions on the top layer (•; can be either Br or Ag ions), ion vacancies on the top layer (B), and ions on the second layer (O; can be either Ag or Br ions).15 28

Structure and Preparation of Silver Halide Grains

29

Fig. 2.6. A side view of the relaxed AgBr {100} surface with the atomic displacements exaggerated. The Br ions (O) are displaced above, and the Ag ions (•) are displaced below the normal surface plane.15

grain.17 For example, Fig. 2.7 illustrates a double-structured grain with ArBrI in its core and AgBr in its shell, and an X-ray diffraction profile of the grains. Another example is an epitaxial grain.18 For example, Fig. 2.8 shows an electron micrograph of epitaxial grains with a cubic AgCl grain as a host crystal and epitaxial AgBr grains at its corners as guest crystals. Knowledge of the distribution of halide ions in silver halide grains is important

Fig. 2.7. An illustration of a double-structured silver iodobromide grain and an X-ray diffraction profile of the grains.17

Fig. 2.8. An electron micrograph of epitaxial grains, each of which is composed of a cubic AgCl grain as a host and eight (AgBr(Cl) grains as guests at the corners of the host grain (Courtesy of T. Shiozawa.)

Fig. 2.9. Temperature dependence of interatomic distances of Br-Ag and Cl-Ag in AgBrCl as determined by the measurement of EXAFS of AgBrCl emulsion grains after annealing.25 The solid straight lines indicate the average X(Br, Cl)-Ag distance predicted from Vegard's law. 30

Structure and Preparation of Silver Halide Grains

31

Fig. 2.10. Transmission electron micrographs of a thin tabular AgBr grain (right) and an AgBr09I0 |, showing the generation of dislocation lines that were perpendicular to its edges (left).30

and has been obtained by the use of various experimental techniques, including X-ray photoelectron spectroscopy (XPS),19 ion scattering spectroscopy (ISS),20 secondary ion mass spectroscopy (SIMS),21 backscattered electron imaging,22 and energy dispersive spectroscopy,22
32

PHOTOGRAPHIC SENSITIVITY

grains showing the absence of any dislocation in a pure AgBr tabular grain and the generation of dislocations at the interface between different silver halide phases in a tabular silver iodobromide grain.30 The formation and observation of dislocations in silver halide grains were reviewed by Mitchell.31 The analysis of X-ray diffraction profiles of silver halide emulsion grains makes it possible to determine the average size of crystallites, in which the crystal lattice is perfect. In the absence of crystal defect, the crystallite size is the same as the grain size, whereas the former is smaller than the latter in the presence of crystal defects in the grains. It was confirmed by this method that many regular silver halide grains in emulsions were crystallographically perfect, whereas there were also many silver halide emulsion grains that contained many crystal defects and were divided into small crystallites, forming a mosaic structure.32

2.2 Preparation of Silver Halide Grains Silver halide grains used for photography are usually suspended in an aqueous gelatin solution, forming a photographic emulsion. They are prepared as a result of reaction between a water-soluble silver salt and a water-soluble halide salt to form an insoluble silver halide salt in an aqueous gelatin solution. Typical methods for the preparation of photographic emulsions are single-jet and double-jet precipitations. In single-jet precipitations, an aqueous solution of water-soluble silver salt is poured into an agitated aqueous reaction solution, which contains gelatin and water-soluble halide salt(s). In double-jet precipitations, aqueous solutions of a water-soluble silver salt and a water-soluble halide salt(s) are simultaneously poured into an agitated aqueous gelatin reaction solution. A double-jet precipitation in which the concentration of silver ions in the reaction vessel can be controlled is called controlled double-jet precipitation, and is useful for the control of the uniformity of size and shape of emulsion grains.33'34 This technique is of great importance for understanding the physical properties and photographic behavior of silver halide grains and for the development of new emulsion technologies. Figure 2.11 shows an apparatus for controlled double-jet precipitation. In an aqueous solution that is saturated with the silver halide salt, the activities of silver ions [Ag+] and halide ions [X~] are related by the activity solubility product Ksp.

where X" is Cl~, Br~, or I~. The value of Ksp of a silver halide is independent of the nature and concentration of the other electrolytes present, and is approximately obtained by considering the concentration of the ions, instead of their activities, when the ionic strength of the medium is low. The solubility product is frequently employed in this approximate form. When it is defined that — log[Ag+] = pAg, -log[X~] = pX, and -logKsp = pKsp,

The solubility of silver halide is expressed by its Ksp.

Structure and Preparation of Silver Halide Grains

33

Fig. 2.11. Block diagram of an apparatus for controlled double-jet precipitation.

Figure 2.12 illustrates a typical pattern of the precipitation with the concentration of solutes as a function of time.35 In the case of silver halide, the product of the concentrations of silver and halide ions can be applied to the ordinate of Fig. 2.12. There are three important concentrations of solutes, that is, the concentrations corresponding to solubility (Cs), the critical supersaturation (C£in) above which the rapid spontaneous nucleation of silver halide takes place, and critical limiting supersaturation (C*M). As the concentration increases with time after the beginning of the addition of silver and halide ions by a double-jet precipitation, the spontaneous nucleation of

Fig. 2.12. Illustration showing the nucleation and growth of silver halide grains during precipitation, where Cs is solubility and C^in and C£ax are the minimum and maximum concentration for nucleation, respectively. Regions I, II, and III indicate the period before nucleation, nucleation, and growth, respectively.

34

PHOTOGRAPHIC SENSITIVITY

silver halide begins to take place rapidly when the concentration exceeds C£in, and then ceases shortly thereafter, since the concentration begins to decrease and becomes lower than C*in. In order to control the uniformity of size of emulsion grains, it is important to keep the concentration lower than C*in after the initial nucleation is finished in order to prevent the occurrence of further nucleation. Thus, all the reactants added after the initial nucleation are used for the growth of the nuclei formed in the initial nucleation stage. The growth rate at C£in is called the critical growth rate. There are two major nucleation processes: homogeneous nucleation and heterogeneous nucleation. In the former case, nucleation takes place spontaneously without the aid of any other substances when the concentration of solutes exceeds C£in. In the latter case, nucleation is initiated by some other substances or physical disturbance under the condition of a relatively low degree of supersaturation. By observing that the nucleation of AgBr could be enhanced neither by CuS grains with a radius of about 25 A nor by Agl grains with diameter of about 150 A, Berry and Skillman concluded that the heterogeneous nucleation process was negligible for the precipitation of silver halide.36 It has been proposed that small and unstable nuclei of silver halide are at first formed when silver and halide ions are encountered, and stable nuclei then grow at the expense of the unstable nuclei.37 Several investigators reported a C*in/Cs above which the spontaneous nucleation of silver halide takes place.38^42 Those values ranged from 1.075 through 1.67, depending upon the experimental conditions used. The sizes of the critical stable nuclei, which were reported by several investigators,43^15 ranged from 140 A through 600 A. Analyses were also made on the number of stable nuclei formed at the nucleation process,46 which determined the size of the grains thus formed. Several attempts have been made to get the information on the unstable nuclei formed during precipitation process.47 Figure 2.13 illustrates the distribution of the concentration of solutes near the surface of growing grains, where C and Q are the concentrations in the bulk and at the grain surface, respectively.48 The difference between C and Q is the driving force for diffusion, and the difference between C; and Cs is the driving force for reaction. Typical growth processes are diffusion controlled and reaction controlled. Diffusion-controlled grain growth is important for the preparing silver halide grains with a uniform size distribution (i.e., monodispersed grains). By applying Pick's diffusion law to a growing spherical grain (Fig. 2.13), the one-dimensional growth rate dr/dt is given as follows39'40'49:

where D is the diffusion constant of solutes, Vm is the volume of 1 mole of silver bromide, and 8 is the width of the diffusion layer. Equation (2.3) shows that the growth rate increases with decreasing r and d. The agitation of a reaction solution and the overlapping of the diffusion layer of the grains with each other bring about the decrease in 8, thus increasing dr/dt. When r/5 is nearly zero, it is regarded that

Structure and Preparation of Silver Halide Grains

35

Fig. 2.13. Schematic representation of concentration distribution of solute in the vicinity of the surface of a growing crystal, where C, Ci( and Cs are concentrations of solute ions in the bulk and at the surface, and solubility of the crystal, respectively.

dr/dt is proportional to the reciprocal of r; that is, dr/dt of a small grain is larger than dr/dt of a large grain under diffusion-limited grain growth. This tendency was experimentally confirmed by several investigators,41'42'50'51 as shown in Fig. 2.14. During grain growth in a diffusion-controlled condition, a small grain grows faster than a large grain, and the difference in r among initially formed grains decreases, finally bringing about monodispersed emulsion grains. This was experimentally confirmed by Wey and Strong.40 They mixed two kinds of AgBr emulsion grains with differences in edge length and observed the change in the difference during growth under different supersaturation conditions. The difference decreased under a high degree of supersaturation, whereas the difference increased under a low degree of supersaturation. The following two types of reaction-controlled grain growth mechanism have been proposed52: polynuclear-layer growth and mononuclear-layer growth. The ratedetermining step of the former is surface nucleation, which is required many times for each layer growth, and the rate-determining step of the latter is surface nucleation, which is followed by instantaneous spreading of the layer. It is believed that dr/dt is proportional to r2 for the mononuclear-layer mechanism and dr/dt to r° for the polynuclear-layer mechanism.53 It is therefore hard to prepare monodispersed silver halide grains by reaction-controlled grain growth, since a small grain cannot grow faster and grows even slower than a larger grain, and the difference in r among initially formed grains cannot be decreasing during their growth. Analyses have been made on the dependence of the grain growth upon various factors, including the crystal habit of silver halide, pAg in a reaction solution, the solubility of silver halide, and the population of growing grains.50'54 Jagannathan and Wey observed the increase in growth rate due to the increase in the population of growing grains under the diffusion-controlled condition. This phenomenon was

36

PHOTOGRAPHIC SENSITIVITY

Fig. 2.14. Size dependence of critical growth rates of cubic AgBr (•, Wei and Strong50; A, Lewis31), cubic AgCl (X, Strong and Wey41), and octahedral AgBr (O, Saitou42) grains. ascribed to the decrease in d in Equation (2.3) due to the overlap of the diffusion layers of the grains.54(a) Figure 2.15 shows the solubility of AgBr and the critical growth rates of the {100} and {111} faces as a function of pBr.54(c) As seen in this figure, both solubility and critical growth rates are steeply dependent upon pBr, and the larger the solubility is, the higher is the critical growth rate. In the range where the solubility is small, the growth rates of the {100} and {111} faces are small and are equal to each other, indicating that the growth of AgBr in this region is diffusion controlled.r In the range in which pBr is small and the solubility is high, the critical growth rates of the {100} and {111} faces are high and differ from each other. It was reported that the growth of cubic grains was diffusion controlled, whereas that of octahedral grains was reaction controlled plus diffusion controlled in this region.54(b),50t fine fact that the growth rate of AgBr grains greatly depends upon the solubility of AgBr indicates that the reaction rate at the grain surface is great enough for the supply rate of solute ions to contribute to the growth rate. Especially in the range where the solubility is low, the amount of solute ions supplied by the diffusion from the bulk of the solution to the grain surface is thought to be limited and to be readily incorporated onto the growing grains regardless of the structure of the grain surface, and the growth rate is thought to be determined essentially not by the reaction rate at the grain surface, but by the supply rate of solute ions by diffusion. fin the range in which the solubility of AgBr is high, the amount of solute ions supplied from the bulk is considered to be too large for the grain surface to readily incorporate them and the reaction rate can therefore contribute to the growth rate of the grains, depending upon the structure of the grain surface.

Structure and Preparation of Silver Halide Grains

37

Fig. 2.15. Critical growth rates of (100) and (111) faces (solid curves with O and •, respectively) and solubility (broken curve) of AgBr as a function of pBr in a reaction vessel.54(c)

The fact that the growth rate was steeply dependent upon the solubility of silver halide could indicate that the reaction rate at the grain surface was high enough for the supply rate of solute ions to contribute to the growth rate. It is important to increase the growth rate to form large silver halide grains for the development of highly sensitive photographic materials, since photographic sensitivity is, in principle, proportional to the volume of silver halide grains and the spectral sensitivity of dyes grains is proportional to their average surface area.55 The increase in the supply rate of solute ions can be achieved by increasing the addition rate of the watersoluble silver salt and halide salt in concert with an increase in the area of the grain surface during the precipitation56 and by increasing the solubility of silver halide in the presence of a silver-complexing agent.57 Among those agents, ammonia is most popular and has been used for the preparation of large silver halide grains for a long time.49 Many discussions have been made on the factors controlling the shape of silver halide emulsion grains.49 In order to control the shape of emulsion grains, it is essentially important to control the concentration of halide ions during precipitation of silver halide grains in the presence of a protective colloid (i.e., gelatin in most cases) in a reaction solution. Figure 2.16 schematically illustrates one of the most convincing models for the effect of pX on the shape of silver halide emulsion grains5-58 by showing the structure and growth of the {100} and {111} face of silver

38

PHOTOGRAPHIC SENSITIVITY

Fig. 2.16. Two-dimensional nuclei (shaded areas) on (100) and (111) faces and adsorbed bromide ions (O).

halide with nuclei (shaded square) and adsorbed halide ions (open circles). As schematically shown in Fig. 2.2, as well as in Fig. 2.16, it is regarded that the {100} face is flat, while the {111} face is rough in atomic scale. A nucleus on the {100} face is considered to be unstable and is stabilized by adsorbed halide ions, and the stabilization of the nuclei enhances the growth of the {100} face. On the other hand, it is thought that a nuclei can find a stable site on the {111} face and cannot be stabilized by adsorbed halide ions, since the {111} face is rough. Therefore, the increase in halide ion concentration in a reaction solution enhances the growth of the {100} face more strongly than that of the {111} face. This model provides an explanation for the fact that the precipitation of silver bromide grains under low pBr condition gives octahedral grains with a {111} face on their surfaces, and precipitation under high pBr condition gives cubic grains with a {100} face on their surfaces. Thus, monodispersed cubic and octahedral silver halide emulsion grains were prepared33'34 and have been refined for many years. Twin planes are frequently observed in silver halide emulsion grains, bringing about various shapes for the grains. Klein, Metz, and Moisar classified twinned grains by analyzing the number and orientation of twin planes in each grain.59 Among those grains, tabular grains are the most important for the present and in the future in photographic materials. By use of a low-temperature luminescence microscope, Maskasky detected the position of iodide ions introduced into those grains at various stages of their growth processes, and revealed the structures and growth processes of tabular grains and various twinned grains.11 Since it has been commonly observed that twinning takes places at high pAg, Berry et al. have considered that the accumulation of the complex ion AgBrf ~ onto a growing grain brings about the formation of a twin plane.60 Saito performed experiments on the formation of tabular grains and concluded that twinning took place af random among grains and could be enhanced by conditions with a high degree of supersaturation,61 including a high concentration of halide ion, a high addition rate of solute ions, low temperature, a high concentration of iodide ions, a low concen-

Structure and Preparation of Silver Halide Grains

39

tration of gelatin, and so on. The landing of a solute ion on an inappropriate site on a grain surface by mistake is considered to be the beginning, and is easily followed by the formation of a twin plane under such conditions with a high degree of supersaturation, which enhances grain growth. Jagannathan described similar results and ascribed the formation of twin planes to instability of the {111} face at a high degree of supersaturation.62 Since twin planes are formed by chance and are randomly distributed among grains, it is difficult to introduce parallel twin planes into each grain at the nucleation stage of formation of emulsion grains. It has long been known and was later described that a tabular grain with parallel twin planes can grow more rapidly than other grains, since its side surfaces have a rough structure with re-entrant corners, which are believed to be suitable places for incorporation of solute ions.36 Therefore, during the process of Ostwald ripening, the pK^ of tabular grains is larger than the pKsp of other grains, and only tabular grains can grow with the dissolution of other grains, and finally survive to give emulsions composed of only tabular grains with parallel twin planes.61 Some grains with more than two twin planes that are not parallel to each other might grow as rapidly as tabular grains. It is therefore important to control the degree of supersaturation at the nucleation stage to avoid the formation of a grain that has more than two twin planes in it. Since twining takes place by chance, depending upon the degree of supersaturation, it is possible to prepare a considerable number of silver halide grains with two parallel twin planes in an emulsion, together with grains containing less than two twin planes in the presence of only a small number of grains containing more than two twin planes by controlling the degree of supersaturation. Then, the emulsion thus formed could be subjected to Ostwald ripening to give an emulsion composed of almost only tabular grains with parallel twin planes.61 Sugimoto, and Karpinski and Wey, studied the rate and mechanism of growth of tabular silver halide emulsion grains under conditions with a small degree of supersaturation.63 The growth rate of tabular grains increased by increasing their size, indicating that the dispersity of grain size increased as the grains grew under this condition. By analyzing the change in thickness and projection area of tabular AgBr grains during growth at nearly critical supersaturation, Hosoya and Tani could obtain the lateral and vertical critical growth rates as a function of the equivalent circular diameter (Dc, the diameter of a circle whose area is equal to the projection area) and pBr of the reaction solution during precipitation (Fig. 2.17).64 At pBr 3.3, the lateral and vertical critical growth rates were small, equal to each other, and decreased with increasing Dc. The growth of tabular grains at pBr 3.3 was thus similar to that of octahedral and cubic grains at the same pBr, as seen in Fig. 2.15, and was diffusion controlled. The lateral critical growth rate became much larger than the vertical critical growth rate as the pBr decreased, indicating the peculiar growth behavior of tabular grains. The lateral critical growth rate at pBr 2.63 still decreased with increasing Dc, indicating that the dispersity of Dc of tabular grains decreased as the grains grew. However, the lateral critical growth rate at pBr 2.0 steeply increased with increasing Dc.

40

PHOTOGRAPHIC SENSITIVITY

Fig. 2.17. Lateral (O) and vertical (D) growth rates of tabular AgBr with thickness of 0.1 /am as a function of equivalent circular diameter (Dc, diameter of the circle whose area is equal to the projection area) and pBr of the reaction solution during the growth of the grains.64

The vertical critical growth rate of tabular AgBr grains was compared with the critical growth rate of octahedral AgBr grains as a function of pBr in Fig. 2.18. Although both the surface of octahedral grains and the main surface of tabular grains were the normal {111} face, the critical growth rate of the former increased with decreasing pBr in accord with the pBr dependence of the solubility of AgBr, whereas that of the latter decreased. The reduction of the vertical critical growth rate of tabular grains at low pBr as compared with that of octahedral grains, together with a steep increase in the lateral critical growth rate with increasing Dc at low pBr, has indicated that, as illustrated in Fig. 2.19, a large portion of the solute ions that diffused toward the main surfaces of tabular grains was forced to diffuse further toward the side surfaces due to the fact that the lateral growth rate was much larger than the vertical growth rate in the low pBr condition. This growth behavior was peculiar to tabular grains and was enhanced as Dc increased. In order to control the shape and size of silver halide grains, it is important to precipitate them in the presence of protective colloids—gelatin is used in most cases—although there are some synthetic polymers useful for the precipitation of controlled silver halide grains.65 In the absence of a protective colloid, monodispersed silver halide grains could be prepared only in a extremely diluted solution.66

Fig. 2.18. Critical growth rate of octahedral AgBr grains (D), vertical critical growth rate of tabular grains (O), and solubility of AgBr (solid curve) as a function of the pBr of the reaction solution during the growth of those grains.64

Fig. 2.19. Illustration showing the flow of solute ions for the growth of small and large tabular grains.64 41

42

PHOTOGRAPHIC SENSITIVITY

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12.

13. 14. 15.

16.

17.

18. 19. 20. 21. 22. 23.

J. E. Maskasky, J. Imaging ScL, 30, 247(1986). F. H. Claes, J. Libeer, W. Vanassche, J. Photogr. ScL, 21, 39(1973). C. R. Berry, Photogr. Sci. Eng., 19, 17(1975). S. Nishiyama, U. S. Patent 4,683,192. E. Moisar, E. Klein, Bunsenges. Phys. Chem., 67, 949(1963). A. H. Herz, R. Danner, G. Janusonis, in Adsorption from Aqueous Solution, American Chemical Society, Washington, DC, 1968, p. 173. T. Tani, J. Imaging ScL, 29, 165(1985). R. W. Berriman, R. H. Herz, Nature, 180, 293(1957). (a) Anonymous, Res. Disci. (Item 22534) 225, 20(Jan. 1983); (b) J. T. Kofron, R. E. Booms, J. Soc. Photogr. Sci. Technol. Jpn., 49, 15(1986); (c) A. F. Sowinski, P. J. Wightman, J. Imaging ScL, 31, 162(1987); (d) R. E. Jacobson, N. R. Axford, A. Ford, J. Photogr. ScL, 38, 140(1990). J. F. Hamilton, L. E. Brady, J. Appl. Phys., 35, 414(1964). J. E. Maskasky, J. Imaging ScL, 31, 15(1987). (a) D. L. Black, J. A. Timmons, G. L. Bottger, "Direct Observation of Twin Planes in Silver Halide Grains," in the preprint book of The International East-West Symposium II on Factors Influencing the Efficiency of Photographic Imaging, October/November, 1988, Kona, Hawaii, (b) D. L. Black, W. Lam, J. A. Timmsons. ' 'A Detailed Examination of Twin Planes in Silver Halide Grains,'' in the preprint book of a joint meeting of ICPS '94: The Physics and Chemistry of Imaging Systems and IS&T's 47th Annual Conference, May 1994, Rochester, NY, Soc. Imaging Sci. Technol., Vol. I, p. 26. A. Mignot, E. Francois, M. Catinat, J. Crystal Growth, 23, 207(1974). J. F. Hamilton, L. E. Brady, Surface ScL, 23, 389(1970). (a) R. C. Baetzold, Y. T. Tan, P. W. Tasker, Surface ScL, 195, 579(1988); (b) P. Tangyunyong, T. N. Rhodin, Y. T. Tan, K. J. Lushington, Surface ScL, 255, 259(1991), (c) Y. T. Tan, K. J. Lushington, P. Tangyunyong, T. N. Rhodin, J. Imaging ScL Tech., 36, 118(1992). (a) H. Haefke, U. D. Schwarz, H.-J. Guntherodt, H. Frob, G. Gerth, R. Steiger, J. Imaging Sci. Technol., 37, 545(1993); (b) G. Hegenbart, Th. Mussig, Surface ScL Lett., 275, L655(1992). (a) S. Bando, Y. Shibahara, S. Ishimaru, J. Imaging ScL, 29, 193(1985); (b) S. Takada, H. Ayato, S. Ishimaru, in Progress in Basic Principles of Imaging Systems, F. Granzer, E. Moisar eds., Friedr. Vieweg & Shon, Braunschweig/Wiesbaden, 1986, pp. 81-87. (a) J. E. Maskasky, Photogr. Sci. Eng., 25, 96(1981); (b) J. E. Maskasky, J. Imaging ScL, 32, 160(1988); (c) T. Sugimoto, K. Miyake, J. Colloid Interface ScL, 140, 335(1990); (d) T. Sugimoto, K. Miyake,/ Colloid Interface ScL, 140, 348(1990). T. M. Kelly, M. G. Mason, J. Appl. Phys., 47, 4721(1976). (a) Y. T. Tan, Surface ScL, 61, 1(1976); (b) L. M. Niedzwiecki, Y. T. Tan, J. Photogr. ScL, 35, 155(1987). (a) B. K. Furman, G. H. Morrison, V. I. Saunders, Y. T. Tan, Photogr. ScL Eng., 25, 121(1981); (b) T. J. Maternaghan, C. J. Falder, R. LeviSetti, J. M. Chabala, J. Imaging ScL, 34, 58(1990). (a) W. K. Lam, Y. T. Tan, C. Palmstrom, J. W. Mayer, J. Photogr. ScL, 33, 219(1985); (b) X. L. Gao, R. Gijbeis, B. Nlys, W. Jacob, Y. Gilliams, /. Imaging ScL, 33, 87(1989). (a) H. Chateau, Compt. Rend., 248, 1950(1959); (b) H. Chateau, M. C. Moncet, and J. Pouradier, in Wissenschaftliche Photographie (Ergebnisse der Intern. Konf. fur Wis-

Structure and Preparation of Silver Halide Grains

24. 25. 26. 27. 28. 29. 30. 31. 32.

33. 34.

35. 36. 37. 38. 39. 40. 41. 42.

43. 44. 45. 46.

47. 48.

43

senschaftliche Photographic, Koln, 1956), W. Eichler, H. Frieser, O. Helwich, eds., Verlag Dr. O. Helwich, Damstadt, 1958, p. 16; (c) H. Chateau, Compt. Rend., 249 1887(1959); (d) H. Chateau, M. C. Oncet, J. Pouradier, in ref. 5(2), p. 20. Y. T. Tan, K. J. Lushington, J. Phys. Chem. Solids, 54, 309(1993). T. Yokoyama, F. Takamatsu, K. Seki, K. Miyake, T. Tani, T. Ohta, Jpn. J. Appl. Phys., 29, L1486(1990). (a) J. M. Hedges, J. W. Mitchell, Phil. Mag., 44, 223(1953); (b) J. W. Mitchell, /. Soc. Photogr. Sci. Technol. Jpn., 54, 248(1991). G. C. Farnell, R. B. Flint, J. B. Chanter, J. Photogr. Sci., 13, 25(1965). J. F. Hamilton, Photogr. Sci. Eng., 11, 57(1967). T. Shiozawa, J. Soc. Photogr. Sci. Technol. Jpn., 34, 16(1971). K. Ohzeki, S. Urabe, T. Tani, /. Imaging Sci., 34, 136(1990). J. W. Mitchell, J. Soc. Photogr. Sci. Technol. Jpn., 54, 258(1991). (a) C. R. Berry, J. Appl. Phys., 27, 636(1956); (b) J. F. C. Boardman, R. H. Herz, G. C. Terry, Nature, 182, 1726(1958); (c) W. Waidelich, Z. Anges. Phys., 10, 525(1958); (d) E. Pitts, F. W. Willet, Acta Cryst., 14, 1302(1961); (e) F. W. Willets, Br. J. Appl. Phys., 16, 323(1965); (f) F. W. Willets, J. Photogr. Sci., 13, 240(1965); (g) F. W. Willets, J. Photogr. Sci., 14, 433(1966); (h) K. Miyake, T. Tani, O. Nittono, "A Study of Size and Crystal Defect of Silver Halide Emulsion Grains by Means of Power X-ray Diffraction Method," in the preprint book of Annual Conf. Soc. Photogr. Sci. Tech., Japan, May, 1994, Tokyo. C. R. Berry, D. C. Skillman, Photogr. Sci. Eng., 6, 159(1962). (a) E. Klein, E. Moisar, Photogr. Wiss., 11, 3(1962); (b) E. Klein, E. Moisar, Ber. Bunsenges, Phys. Chem., 67, 349(1963); (c) E. Moisar, E. Klein, Ber. Bunsenges. Phys. Chem., 67, 949(1963); (d) E. Moisar, /. Soc. Photogr. Sci. Technol. Jpn., 54, 273(1991). V. V. LaMer, R. Dinegar, /. Am. Chem. Soc., 72, 4847(1950). C. R. Berry, D. C. Skillman, J. Photogr. Sci., 16, 137(1968). (a) C. R. Berry, D. C. Skillman, J. Phys. Chem., 68,1138(1964); (b) C. R. Berry, Photogr. Sci. Eng., 20, 1(1976). R. W. Berriman,7. Photogr. Sci., 12, 121(1964). A. Hirata, S. Hohnishi, Bull. Soc. Sci. Photogr. Jpn., 16, 1(1966). J. S. Wey, R. W. Strong, Photogr. Sci. Eng., 21, 248(1977). R. W. Strong, J. S. Wey, Photogr. Sci. Eng., 6, 344(1979). M. Saito, "Rate Equation for Growth of Silver Halide Grains Under Fixed Degree of Supersaturation," Annual Conference of Soc. Photogr. Sci. Technol. Jpn., May, 1980, Tokyo. C. R. Berry, PSA Tech. Q., 149(1955). F. Claes, R. Berendsen, Photogr. Korr., 101, 37(1965). J. S. Wey, in Preparation and Properties of Solid State Materials, Vol. 5, W. R. Wilcox, ed., Marcel Dekker, New York, 1980. (a) C. W. Wagner, private communication to E. Klein and E. Moisar cited in E. Klein, E. Moisar, in Ref. 23 (b); (b) V. G. Losinov, N. B. Denisova, Zh. Nauchn. Prikl. Photogr., 20, 231(1975); (c) C. R. Berry, D. C. Skillman, /. Phys. Chem., 68, 1138(1964); (d) I. H. Leubner, R. Jagannathan, J. S. Wey, Photogr. Sci. Eng., 24, 268(1980); (e) I. H. Leubner, J. Imaging Sci., 29, 219(1985); (f) I. H. Leubner, J. Imaging Sci., 31, 145(1987). (a) C. W. Davies, A. L. Jones, Diss. Faraday Soc., 5, 103(1949); (b) D. H. Klein, L. Gordon, T. H. Walnut, Talanta, 3, 177(1959); (c) T. Tanaka, /. Info. Recording Mater., 18,249(1990). J. W. Mullin, Crystallization, Butterworths, London, 1961, p. 117.

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49. C. R. Berry, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, Chapter 3. 50. ). S. Wey, R. W. Strong, Photogr. Sci. Eng., 21, 14(1977). 51. J. D. Lewis, J. Photogr. Sci., 27, 24(1979). 52. A. E. Nielsen, Kinetics of Precipitation, Pergamon Press, New York, 1964. 53. C. R. Berry, Photogr. Sci. Eng., 18, 4(1974). 54. (a) R. Jagannathan, J. S. Wey, J. Crystal Growth, 51, 601(1981); (b) R. Jagannathan, J. Imaging Sci., 32, 100(1988); (c) T. Sugimoto, J. Colloid Interface Sci., 93, 461(1983). 55. (a) H. Zwicky, Z. Wiss. Photogr., 53, 68(1959); (b) T. Tani, /. Photogr. Sci. Eng., 29, 93(1985). 56. (a) K. Irie, K. Suzuki, U. S. Patent 3,650,757; (b) J. D. Lewis, U. S. Patent 4,067,739; (c) M. Saitou, U. S. Patent 4,242,445. 57. (a) M. J. Harding, J. Photogr. Sci., 27, 1(1979); (b) T. Tani, J. Imaging Sci., 32, 227(1988); (c) A. H. Herz, J. Imaging Sci. TechnoL, 39, 40(1995); (d) J. Pouradier, A. Pailliotet, C. R. Berry, in The Theory of the Photographic Process, 4th ed., T. H. James,, ed., Macmillan, New York, 1977, pp. 1-12. 58. (a) O. Knack, I. N. Stranski, Z. Elektrochem., 60, 816(1956); (b) I. N. Stranski, VDIBerichte, 20, 5(1957). 59. (a) E. Klein, H. J. Metz, E. Moisar, Photogr. Korr., 99, 99(1963); (b) E. Klein, H. J. Metz, E. Moisar, Photogr. Korr., 100, 57(1964). 60. C. R. Berry, S. J. Marino, C. F. Oster, Jr., Photogr. Sci. Eng., 5, 332(1961). 61. (a) M. Saito, U. S. Patent 4,797,354; (b) M. Saitou, U. S. Patent 4,945,027; (c) M. Saito, U. S. Patent 4,977,074; (d) M. Saitou, U. S. Patent 5,087,555. 62. (a) R. Jagannathan, J. Imaging Sci., 35, 104(1991); (b) R. Jagannathan, V. V. Gokhale, J. Imaging Sci., 35, 113(1991). 63. (a) T. Sugimoto, Photogr. Sci. Eng., 28, 137(1984); (b) P. H. Karpinski, J. S. Wey, J. Imaging Sci., 32, 34(1987). 64. Y. Hosoya, T. Tani, J. Soc. Photogr. Sci. Technol. Jpn., in press. 65. For example, T. Yano, Photogr. Korr., 104, 121(1968). 66. R. H. Ottewill, R. F. Woodridge, /. Colloid Sci., 16, 581(1961).

3 Physical Properties of Silver Halides

As described in detail in Chapter 4, photographic sensitivity is based on the formation of a latent image center on a silver halide emulsion grain as a result of its light absorption. A latent image center is a microcluster of silver atoms and is formed by the electronic and ionic processes that take place repeatedly, one after another, when photoelectrons are created as a result of the light absorption of an emulsion grain. The electronic process is the capture of a photoelectron by an electron trap, which is usually provided by chemical sensitization, as described in Chapter 6. The ionic process is the reaction of the trapped electron with an interstitial silver ion to form a silver atom in an emulsion grain. It is therefore considered that knowledge of the light absorption, ionic, and electronic properties of silver halide emulsion grains is essential for understanding the mechanism of photographic sensitivity. Extensive studies have been successfully made by solid state physicists on the physical properties of large silver halide crystals to get fundamental knowledge that includes the characteristics of light absorption and electronic structure, formation, and transport of interstitial silver ions; transport and traps of photoelectrons and positive holes. Big differences in ionic and electronic properties between large crystals and emulsion grains of silver halides have turned out to be an important factor for photographic sensitivity.

3.1 Light Absorption and Electronic Structure of Silver Halides Figure 3.1 shows absorption spectra of AgCl and AgBr at various temperatures.1 Each of these absorption spectra is composed of a low-energy, weak, and highly temperature-dependent light absorption band with an extended tail in the visible wavelength region and an intense exciton band in the UV region. In contrast to silver halides, each of the absorption spectra of alkali halides is composed of only an intense exciton band in the UV region.2 These absorption properties reflect the electronic structure of silver halide, as has been studied from various aspects.3"17 The band structures have been determined and are schematically shown in Fig. 3.2 for AgBr as a representative of silver halides 45

46

PHOTOGRAPHIC SENSITIVITY

Fig. 3.1. Absorption spectra of AgCl and AgBr.1

used for photographic materials and for KBr as a representative of the alkali halides.18 For convenience, Fig. 3.2 also shows the simplified band structure of AgBr as a function of distance. The band with an extended tail has been assigned experimentally3 and theoretically4'5 to the indirect exciton transition, whereas the intense exciton band in the absorption spectrum of AgBr in the UV region and the first exciton band in KBr are assigned to the direct exciton transition. The energies of the indirect exciton transitions of AgCl and AgBr are 3.245 eV19'20 and 2.684 eV,19 respectively.

Fig. 3.2. Schematic band structures of AgBr and KBr, where CB and VB are the conduction and valence bands, respectively. Arrows A and B are the indirect and direct transitions, respectively.

Physical Properties of Silver Halides

47

The studies on the transport of electrons in silver halides by Brown and his collaborators9'10 revealed that the minimum of the conduction band in AgBr and AgCl are located at the F point in momentum space. It has been established that the maximum of the valence band of AgBr is located along the [111] direction and at the L point.11 It is confirmed that the band structure of AgCl is qualitatively very similar to AgBr.4'5'7'15 Thus, the transition with the least energy and the longest wavelength is the indirect exciton transition from the L point to the F point, and has a small probability, since it should be accompanied with the change in momentum. On the other hand, the transition by the intense exciton band in the UV region is assigned to the direct exciton transition and has a large probability, since it is not accompanied by a change in momentum. The change in momentum is achieved by the interaction of the excitons with phonons or impurities. The interaction of the indirect excitons with phonons and impurities is clearly observed in the phonon structures of the absorption and fluorescence spectra of silver halides at low temperature, as have been extensively studied by Kanzaki and others.16 In accord with these two schemes for the change in momentum, the strength of the phonon-assisted indirect-exciton transition in pure AgBr was temperature dependent, whereas that of the transition of the indirect exciton band bound at an isoelectric iodine impurity was nearly independent of temperature.16 According to theoretical calculations,4'5'7'15 the minimum of the conduction band of silver halides is mostly composed of 5s orbitals of silver ions, whereas the valence band is composed of np orbitals of halide ions and 4d orbitals of silver ions. The maximum of the valence band at the L point exhibits strong p-d mixing, whereas the valence band at the F point exhibits little p-d mixing. The repulsive interaction between halide np and silver 4d states raises the valence band in the region away from the F point, bringing about the low-energy indirect exciton transition. On the other hand, the valence bands of alkali halides are essentially composed of the np orbitals of halogen ions.18 The absorption of visible light, which is one of the most important properties of silver halides as photosensitive elements in photographic materials, arises from this indirect transition. It is noted that this transition is brought about by the p-d mixing in the valence band of silver halides. Although the absorption is weak due to its small transition probability and is not necessarily favorable for capturing incident light, the small transition probability also predicts a small recombination probability, which is favorable for efficient latent image formation. It is therefore noted that the contribution of 4d orbitals of silver ions to the valence bands of silver halides is important for the application of silver halides to photography from the viewpoints of electronic structure and light absorption. The electronic structure of silver halides as a function of the distance from the surface of their crystals and emulsion grains is described in the next section as their space charge layers arising from the displacement of silver ions from the surface to the bulk. The comparison in electronic structure among different silver halides has been studied with respect to the electron transfer between silver halides and compounds adsorbed to them.21 The electronic structures of the interfaces between different silver halides have been extensively studied by Granzer and coworkers22 and Kawasaki and Hada23 as models of double-structured silver halide emulsion grains

48

PHOTOGRAPHIC SENSITIVITY

and epitaxial grains described in Chapter 2. The electronic structures of the interfaces between silver halide thin layers and sensitizing dyes have been studied by means of UV photoelectron spectroscopy by Seki and Tani24 in relation to the electron transfer process in spectral sensitization. Further studies are needed on the electronic structures of the surfaces of silver halides and the interfaces between different silver halides, and between silver halides and other compounds of photographic interests, for understanding the mechanism of photographic sensitivity.

3.2 Ionic Properties of Silver Halides There are two types of intrinsic point defects in ionic crystals: Frenkel defect and Schottky defect, as schematically shown in Fig. 3.3. It is determined that Frenkel defects composed of interstitial silver ions and silver ion vacancies are predominant in silver halide25"29 and are the charge carriers for the dark conductivity (i.e., ionic conductivity) of silver halides.30 In a large crystal, a silver ion at a lattice position jumps into an interstitial position to form an interstitial silver ion and a silver ion vacancy. The thermodynamic equilibrium is expressed by

where ni; nv, and N are the volume concentrations of interstitials, vacancies, and lattice positions, respectively, and AGF is the Gibbs' free energy of formation of a Frenkel pair. The equilibrium intrinsic value of n; and nv is designated as nF. The ionic conductivity o-; is expressed as follows:

Fig. 3.3. Pathways for formation of Frenkel and Schottky defects in an ionic crystal.

Physical Properties of Silver Halides

49

where /A, and ^tv are the mobilities of the interstitial silver ions and silver ion vacancies, respectively. The formation enthalpies of Frenkel defects in AgCl and AgBr are 1.49 eV and 1.163 eV.30'31 It is determined that, among various motions of interstitial silver ions, the dominant one is the collinear interstitialcy jump, as schematically shown in Fig. 3.4. In this process, an interstitial silver ion moves by \ (111) to a lattice position, and the lattice silver ion is similarly displaced by \ (111) to the adjacent interstitial position. The enthalpies for the collinear interstitialcy jump in AgCl and AgBr are as small as 0.018 eV and 0.042 eV, respectively.29'30 According to Equation (3.2), the activation energy of the ionic conductivity is the sum of the half of the formation enthalpy of Frenkel defects and the enthalpy for the collinear interstitialcy jump when interstitial silver ions are predominant as carriers for the ionic conductivity. Impurity ions can influence the concentrations of interstitial silver ions and silver ion vacancies.32 For example, a divalent metal ion, which substitutes a silver ion at a lattice position, is associated with a silver ion vacancy for the neutralization of the excess charge on the divalent metal ion. The ionic conductivity of silver halide grains in a photographic emulsion has been measured by various methods and has been found to be significantly larger than that of the corresponding large crystal. Following the experiments of Hayes and Schockley with large AgCl crystals,33 Hamilton and Brady applied synchronized pulses of electric field and light to a AgBr emulsion layer to observe the deviation of the location of latent image centers on a grain. The delay of the light pulse after the beginning of the field pulses has permitted the determination of the time for the internal field relaxation due to ionic conduction. The relaxation time, TI, is related to the ionic conductivity CTJ, by the equation, T; = elA-ira^ where e is the static

Fig. 3.4. A schematic representation of the collinear interstitialcy jump. The interstitial ion (I) moves along the solid arrow to the lattice position L, displacing that lattice ion to the interstitial position I'.

50

PHOTOGRAPHIC SENSITIVITY

dielectric constant of the medium. They found that the ionic conductivity of the emulsion grains was two to three orders of magnitude higher than that of a large AgBr crystal,34'35 and that the activation energy of the ionic conductivity of the emulsion grains was much smaller than that of the large crystal.36 Observing the decrease in the ionic conductivity of the emulsion grains by adding a strong silver complexing agent, l-phenyl-5-mercaptotetrazole,35 and divalent cadmium ion to the emulsion,37 they indicated the surface generation of interstitial silver ions in concentrations high enough to account for the formation of latent image centers on emulsion grains. On the basis of ionic conductivity data of large crystals, Mitchell had previously pointed out that there was less than one interstitial silver ion per • ^K grain. Van Biesen39 measured the frequency dependence of the dielectric loss of emulsion layers to determine the ionic conductivities of the emulsion grains and could confirm the result reported by Hamilton and Brady. The dielectric loss method has been used by Takada and many other scientists.40^42 By analyzing the decay of the Dember photovoltage in emulsion grains, Saito43 determined the ionic conductivity of the grains and could confirm the result reported by Hamilton and Brady. The

Fig. 3.5. Schematic representation of dielectric loss measurement of an emulsion layer and dielectric loss curve of an emulsion layer containing cubic AgBr grains with an edge length at room temperature.5 of 0.89

Physical Properties of Silver Halides

51

above-stated results with silver halide emulsion grains have been supported by conventional conductivity measurements on thin films of silver balides44^6 and by investigations of the surface potential or the potential distribution near the surface.47"51 In accord with the above-stated ionic conductivities, the activation enthalpies of the ionic conductivity of cubic AgCl grains, and cubic and octahedral AgBr grains, which are the sums of the formation enthalpies and the activation energies for the collinear interstitialcy jumps of interstitial silver ions in the corresponding grains, were 0.66, 0.40, and 0.36ev,40(d)'53 respectively, and much smaller than those of the ionic conductivities of the corresponding large crystals. The ionic conductivities of silver halide emulsion grains are usually measured by the dielectric loss method, according to which a maximum appears in the curve of dielectric loss as a function of frequency (Fig. 3.5). The observed frequency at which a maximum occurs (o>max) can be related to the conductivity of silver halide emulsion grains as follows:

where

where e{ is the dielectric constant of gelatin, s'2 and cr2 are the dielectric constant and conductivity of silver halide emulsion grains, respectively, and n is the shape factor of the grains. Therefore, the conductivity of the grains is proportional to wmax.* As shown in Fig. 3.6, the conductivity of cubic AgBr emulsion grains as measured by the dielectric loss method was proportional to the surface to volume ratio of the grains, indicating that the charge carriers were created from the grain surface.40 The identification of interstitial silver ions as charge carriers was also confirmed by analyses of several factors influencing ionic conductivity of emulsion grains. It was confirmed by the dielectric loss and Dember methods that the ionic conductivity was decreased by incorporating polyvalent metal ions into emulsion grains and by adsorption to the grains of silver-complexing agents such as 4-hydroxy-6-methyll,3,3a,7-tetraazaindene.39^3 Figure 3.7 shows the dielectric loss measurement by Takada and Tani, indicating the effect of various dyes on the ionic conductivity of

*When an alternating electric field is applied to an emulsion layer, charge carriers in emulsion grains (i.e., interstitial silver ions in most cases) are displaced to give dielectric polarization [i.e., the sum of the concentration (c) and distance of displacement (d = /xEt) of charge carriers, where p, is mobility, E is electric field, and t is time], following the applied field. When the frequency of the alternating field is increased, the delay of the dielectric polarization begins to take place due to the limited concentration and mobility of the carriers, giving rise to the loss of the electric energy by way of its conversion to thermal energy (i.e., dielectric loss). By increasing the frequency of the alternating field, the dielectric loss increases, then decreases after passing its maximum at ft)max, and finally disappears when the frequency becomes to be too high for charge carriers to follow. Thus, o>mall is proportional to the product of the concentration and mobility of charge carriers (i.e., the conductivity) in emulsion grains.

Fig. 3.6. Size dependence of the ionic conductivity of cubic AgBr grains in emulsion layers.40(a)

Fig. 3.7. Dependence of the ionic conductivity of cubic AgBr grains upon sensitizing dyes in emulsion layers.52(a)

52

Physical Properties of Silver Halides

53

AgBr emulsion grains.52 These results were explained by assuming that the adsorption of cyanine dyes with a positive charge on their chromophore to the grains increased the conductivity by repelling silver ions at surface sites into interstitial positions, and the adsorption of a dye with a negative charge on their chromophore and a merocyanine dye with affinity to a silver ion to the grains decreased the conductivity by stabilizing silver ions at surface sites, as schematically shown in Fig. 3.8. Figure 3.9 shows the dielectric loss measurement by Ohzeki, Urabe, and Tani to indicate the influence of the surface to volume ratio of various AgBr grains upon their conductivity with interstitial silver ions as charge carriers.53 Contrary to the fact that the conductivity of octahedral grains was proportional to the surface to volume ratio and was much higher than that of cubic AgBr grains, the conductivity of tabular grains with constant thickness was independent of the surface to volume ratio, and even smaller than that of cubic grains. In accord with the above-stated result, the activation enthalpy of the ionic conductivity of tabular AgBr grains was 0.58 eV and larger than those of cubic and octahedral AgBr grains. Hamilton has proposed that the stacking sequence at a twin boundary is not the minimum in energy and that the layer spacing perpendicular to the boundary is greater in the immediate vicinity of twin boundaries than that in the bulk.54 Supporting Hamilton's hypothesis, the above result could indicate that a twin plane could trap interstitial silver ions,

Fig. 3.8. Illustration showing the effect of sensitizing dyes on the formation process of interstitial silver ions through the surface of AgBr grains, where open and closed circles are Ag+ and Br^, respectively.

54

PHOTOGRAPHIC SENSITIVITY

Surface/volume ratio

(l///m)

Fig. 3.9. Schematic representation of the ionic conductivities of octahedral, cubic, and tabular AgBr grains as a function of their surface/volume ratio.53 since they would be stable at the immediate vicinity of the twin plane than in the bulk. It is therefore considered that twin planes decreased the concentration of interstitial silver ions, and formed the electric double layers by enhancing the displacement of silver ions from surfaces to twin planes in tabular grains,53 as schematically shown in Fig. 3.10. The dielectric loss versus frequency curve of silver halide grains with a (111) face on their surface usually give two peaks. The peak at lower frequency corresponds to the ionic conductivity with interstitial silver ions as charge carriers,53 and the peak at higher frequency is believed to correspond to the conductivity due to the movement of silver ions on the grain surface.53 Further studies are needed for the clarification of the nature and role of the conductivity of silver ions on the (111) surface of silver halide grains. As shown above, the energy level of a silver ion at a surface site is higher than that of a silver ion at a lattice position, being responsible for the formation of interstitial silver ions with high concentration in emulsion grains. Figure 3.11 illustrates the structure of the (100) face with a kink site. Although each silver ion is surrounded by six halide ions in the bulk, a silver ion at a kink site is surrounded by only three

Fig. 3.10. Electric double layer model showing the electronic structure of a tabular AgBr grain, where C.B. and V.B. indicate the bottom of the conduction band and the top of the valence band, respectively.53

Fig. 3.11. Illustration of a kink site on the (100) surface of a silver halide crystal, with the electric charges at various sites indicated. 55

56

PHOTOGRAPHIC SENSITIVITY

halide ions and has half a positive charge. It is noted that a silver ion at the corner of a cubic grain is in the same situation as a silver ion at a kink site from this viewpoint, although the number of the latter is overwhelmingly larger than that of the former. It is therefore considered that a silver ion at a kink site has the highest energy among silver ions at probable sites on the (100) surface, and is most probable as the site responsible for the formation of an interstitial silver ion through the grain surface. Figure 3.12 illustrates the processes of the formation of Frenkel defects in a large crystal, and in a small grain with (100) faces on its surface. In the latter case, a silver ion at a kink site on the surface can jump into an interstitial position to give an interstitial silver ion more easily than can a silver ion at a lattice position. Furthermore, a silver ion at a lattice position can jump to a kink site to leave a silver ion

Fig. 3.12. Illustration showing the formation of a Frenkel pair in the bulk (a), an interstitial silver ion (b), and a silver ion vacancy (c) through the surface of silver halide, in which closed circles and open circles are silver ions and halide ions, respectively.

Physical Properties of Silver Halides

57

vacancy more easily than to an interstitial position. The experiments cited above39^13 indicated that the formation energy of interstitial silver ions was smaller than that of silver ion vacancies, bringing about interstitial silver ions with high concentration in emulsion grains. As shown in Chapter 2, the structure of the {111} face of silver halides is still under examination on proposed models. However, there might be surface sites responsible for the formation of interstitial silver ions with high concentration as well as for silver ions mobile on the grain surface, since a silver ion on the top layer is surrounded only three halide ions. Further studies are needed for the clarification of the structure of the {111} and surface sites responsible for the ionic conduction in a silver halide grain with {111} faces on its surface. Figure 3.13 schematically shows the energy diagram of silver ions at various sites on the surface and in the bulk. In order to understand the movement of silver ions between the surface and interior of a silver halide grain, it is convenient to regard the middle point between the free energies of silver ions at an interstitial and lattice positions as the Fermi level of silver ions; that is, the free energy of silver ions at kink sites with respect to the middle point could be the driving force for them to move to interstitial positions, and the movement comes to an end to form a space charge layer in which the potential difference across the layer is equal to the difference of the free energy of silver ions at kink sites from the middle point. According to Frenkel's concept,56 the potential profile in the space charge layer was described by Kliewer and Koehler,57 and extended by Poeppel and Blakely,58 and Hoyen and Tan.59 Namely, the potential at the surface with respect to that in the interior <&s is given as follows:

where AGi and AGV are the free energies required to form an interstitial silver ion and a silver ion vacancy through a kink site, respectively, AHj and AHV are the

Fig. 3.13. Schematic representation of energy levels of silver ions at various sites, indicating the free energies of the formation of a Frenkel pair in the bulk (AGF), and of an interstitial silver ion (AGj) and a silver ion vacancy (AGV) through kink sites on the grain surface, as expressed by Equations (3.5)-(3.8).

58

PHOTOGRAPHIC SENSITIVITY

corresponding enthalpies, AS, and ASV are the corresponding entropy, and the term kT ln(2) conies from the fact that the number of interstitial positions is twice the number of silver ions in the bulk of a grain. From a simple thermodynamic cycle it follows that

where AGF is the free energy for formation of a Frenkel pair. The width of the space charge layer, that is, the Debye length A, is given as follows:

where n; is the concentration of interstitial silver ions and e and s0 are the dielectric constants of silver halide and the vacuum. Slifkin and coworkers have recently determined the potential energy profile and energy required to form an interstitial silver ion and a silver ion vacancy from the grain surface50-51; that is, they doped radioactive divalent metal ions (54Mn2+) into a silver halide crystal in an equilibrium distribution. Since a divalent metal ion bears a positive charge in silver halide, the more negative the potential of a region is, the higher is the concentration of divalent metal ions; that is, the concentration profile of 54Mn2+ is expressed as follows:

where €>(x) is the potential at position x. Figure 3.14 shows the potential energy profile in AgCl, as determined by Hudson, Farlow, and Slifkin.30 However,

Fig. 3.14. Distribution of 54Mn2+ in a AgCl (100) crystal annealed for 40 days at 449K, indicating the potential distribution in its space charge layer with a surface potential of s and a Debye length of A.50

Physical Properties of Silver Halides

59

Fig. 3.15. Illustration showing the band structure of silver halide with a positive space charge layer and the behavior of a photoelectron and a positive hole, where CB and VB are the conduction band and valence band, respectively, of silver halide.

defect formation parameters such as AGi,AGv, and 4?s are still under consideration_45(b),51

Since interstitial silver ions play an important role in latent image formation, the formation of interstitial silver ions with high concentration through surface kink sites is important for understanding the mechanism of photographic sensitivity. Figure 3.15 schematically shows the conduction and valence bands of silver halide as a function of the distance from the crystal surface. It is believed that the band bending in the space charge layer has some influence on photographic sensitivity. The space charge layer might reduce the recombination between a photoelectron and a positive hole, since a photoelectron and a positive hole are forced to move to opposite directions in the space charge layer. The band bending might also influence the electron transfer processes between silver halide emulsion grains and adsorbed compounds on the grain surface.

60

PHOTOGRAPHIC SENSITIVITY

3.3 Electronic Properties of Silver Halides As described previously in this chapter, indirect excitons are first formed on exposure of silver halide emulsion grains to light for taking photographs. Since the binding energies of indirect excitons are as small as 0.022 eV for AgBr and 0.04 eV (estimated value) for AgCl,16(a)'(d) excitons in silver halide emulsion grains freely dissociate into an electron and a positive hole at room temperature. The quantum yield for the photoconduction of electrons in AgBr at 300K is therefore greater than 0.960 and favorable for photographic sensitivity. It is considered that one of the reasons for those small binding energies of excitons is the large dielectric constants of silver halides. Figure 3.16 illustrates the behavior of photoelectrons in a silver halide crystal. Moving toward an anode under an applied electric field, a photoelectron is scattered many times by phonons, captured by a shallow electron trap, and thermally released from it. The important properties of photoelectrons are their Hall mobility OH)> drift mobility()U,D), and lifetime (T). The Hall mobility, which is equal to microscopic mobility in this case, is the velocity of photoelectrons per unit electric field when they are actually moving. The drift mobility, which is called macroscopic mobility, is their velocity per unit electric field averaged over the period in which electrons are moving and also the period in which electrons are at rest in shallow electron traps. Thus, the difference between /JLH and ju,D indicates the portion of the period in which photoelectrons are at rest in shallow traps. A similar situation and discussion are valid for the behavior of positive holes in silver halide. In terms of shallow

Fig. 3.16. Illustration showing the behavior of photoelectrons with Hall mobility (/XH) and drift mobility (/XD). In this figure, V and E are the velocity of photoelectrons and the applied electric field, and CB and VB are the conduction band and valence band of silver halide, respectively.

61

Physical Properties of Silver Halides

was interpreted by Brown and Kobayashi61

trapping, the deviation of /x,D from as follows:

where Nc is the effective density of states at the conduction edge, and N, and E are the density and depth of the shallow traps. Equation (3.11) gives the fraction of time period during which an electron is free in the conduction band. The electronic transport properties of AgCl and AgBr have been extensively studied and described.8'62"64 According to those studies, a conduction electron can be represented as a charged carrier in a single isotropic and parabolic potential valley, having an effective mass of 0.399 for AgCl and 0.282 for AgBr.65 Table 3.1 shows some electronic properties of silver halide crystals. At room temperature and above, the values of electron drift mobility fj,D, as given by timeof-flight measurements with high-quality single crystals of silver halides,66"68 are in agreement with values of Hall mobility ^ H obtained from the photo-Hall effect69'70 and are 50 cm 2 V~ 1 s^ 1 for AgCl and 60 cm2V"1s"1 for AgBr. The mobility is limited not by shallow electron traps, but by scattering on LO phonons, whose probability is proportional to the LO phonon density. The mobility thus increases with decreasing temperature, as shown for AgBr in Fig. 2.n.l4-6S'S5(b) The drift mobility of electrons in emulsion grains has been measured by several groups of workers,71"75 with values depending upon the methods and emulsion grains studied (Table 3.1). However, it is obvious that the drift mobility of electrons in emulsion grains is considerably smaller than that of electrons in a large crystal, indicating that there are many shallow traps in emulsion grains. Details of those traps are described in the next section. The low-temperature transport properties of holes in silver halides are quite different from those of electrons.76'77 In AgCl, positive holes are self-trapped at liquid Table 3.1. Carrier

Sample

Electron

Large crystal Emulsion grains

Positive hole

Large crystal Emulsion grains

Electronic properties of AgBr PD (cnf/V-sec)

60 0.2 0.6 ~ 0.8 >8 2.1

1.1

0.001 *Experimentally observed value.

T

l[Eq. (10)]

(/jLSec)

(fwi)

0.1 -10 3

4-40 1.2

0.1

2.3 0.4* 0.5*

0.1 - 10 15

0.5 - 5 0.33* 0.19 0.2*

Kef.

68-70 73 74.75 76 77 100

101

83.84 104 ' 73

• 105

62

PHOTOGRAPHIC SENSITIVITY

Fig. 3.17. Hall mobilities of electrons and holes in an AgBr large crystal in the range near room temperature.14

helium temperature. Electron spin resonance studies have revealed that a positive hole is located on a silver ion with displacement of the nearest-neighbor chloride ions (Fig. 3.18).76'78 The thermal ionization energy of the self-trapped hole was estimated as 0.12 eV.79 Although the self-trapped hole is unstable at temperature above about 50K, the self-trapped state severely limits the drift mobility of positive holes in AgCl at room temperature, which is about 10"1 cm2V"1s"1, as given by measurements of the diffusion effect,80 and much smaller than that of photoelectrons. Although it has been found from cyclotron resonance studies that positive holes are not self-trapped at liquid helium temperature in AgBr,11'81'82 the drift mobility83-84 and Hall mobility85 of positive holes in AgBr at room temperature are about 1 cm2V"1s"1 and are much smaller than that of photoelectrons, steeply decreasing with increasing temperature (Fig. 3.i7).14-85(b).88 These mobility characteristics of positive holes in AgBr have also been attributed to the self-trapped state, which is regarded as a metastable state in AgBr.86'87 The drift mobility of positive holes in a large AgBr crystal is a little smaller than the Hall mobility,83'84 indicating that the movement of a positive hole in AgBr is influenced by a trapping level, the depth of which is estimated as 0.44 eV according to equation (3.11). According to Kanzaki, a silver ion vacancy is a dominant hole center, and its binding energy was estimated as 0.4 eV from the luminescence mod-

Physical Properties of Silver Halides

63

Fig. 3.18. John-Teller distortion of the six chloride ions around the self-trapped hole (Ag24~) in AgCl. ulation spectra.14 Although the measured values of the drift mobility of positive holes in emulsion grains are scarce,71 it is also obvious that the drift mobility of positive holes is much smaller than the Hall mobility, indicating that there are many shallow traps. Details of those traps are described in the next section. The photoconductivity and electron lifetime in silver halide emulsion grains were measured by several methods. West and Carroll reported the results of classical DC photoconductivity measurements on emulsion coatings, demonstrating the coincidence of the spectral range of the photoconductivity with that of the photographic sensitivity of spectrally sensitized emulsions.88 Extending the Haynes-Shockley experiments33 by applying synchronized pulses of light and electric field to emulsion layers as described in the previous section, Hamilton and Brady measured the electron lifetime in various emulsion grains,35'36'71 and observed a striking correlation between ionic field decay times and electron lifetimes. The results have supported the absence of deep electron traps in the emulsions, as proposed by Mitchell89 and supported by other arguments.90 The correlation has indicated that the electron lifetime was determined by the ionic event at an otherwise shallow trapping center. The electron lifetimes in emulsion grains are now frequently measured by a microwave photoconductivity technique, which was first reported by Baranov and Akimov,91 and has been extended by Kellogg and coworkers,92 and followed by many groups of workers.72'73'75'93 Figure 3.19 shows a diagram of an apparatus for the microwave photoconductivity measurement. The exposure of a sample (an emulsion layer) in a tuned microwave cavity to a light pulse creates electronic charge carriers, which can absorb the microwave energy and detune the microwave cavity

64

PHOTOGRAPHIC SENSITIVITY

Fig. 3.19. Block diagram of an apparatus for microwave photoconductivity measurements. to give the signal in proportion to the concentration of electronic charge carriers. It is considered that only free electrons are usually responsive to the signal of microwave photoconductivity, since the mobility of photoelectrons is much larger than that of positive holes in silver halide, as shown in Table 3.1. The time resolution of microwave photoconductivity has been reduced to several nanoseconds.72'73'75'93(e) The Dember photovoltage in silver halide emulsion grains was first observed by Levy94 and used by several workers43(a)>95 to indicate the displacement of photoelectrons in the grains, although its decay was related to the ionic conductivity of the grains,43 Keevert used the four-capacitor bridge arrangement with a light flash and a delayed field pulse to measure electron trapping times as well as field decay time.96 For AgBr emulsion grains, he could confirm the correlation between electron lifetimes and field-decay times as reported by Hamilton and Brady.35'36'71 However, the field-decay time was as much as 40 times longer than the electron lifetime in AgCl emulsions. By means of the extended Haynes-Shockley experiment,71 Hamilton and Brady measured the lifetime of positive holes in tabular AgBr emulsion grains as 15 /tsec. Since positive holes are less mobile than photoelectrons, the detection of positive holes in silver halide emulsion grains could be realized when a photoconductivity apparatus with high sensitivity was applied to the emulsion layers, in which the number and/or mobility of photoelectrons were reduced. Kellogg, Hodes, and Muenter recently developed such a sensitive apparatus by using radiowaves instead of microwaves in the photoconductivity apparatus, and observed the lifetime of positive holes in 0.5 /jan octahedral AgBr emulsion grains as 26 /usec at room temperature.97 Kaneda, Ohshima, and Tani observed the decay of Dember photovoltage in AgBr emulsion grains, which could be attributed to the movement of positive holes, when the number and/or mobility of photoelectrons were reduced.98 Kaneda used time-offlight measurement to determine the drift mobilities of photoelectrons and positive holes in AgBr emulsion grains.74 The diffusion length of charge carriers 1 is important for the analysis of size dependence of photographic sensitivity and was estimated according to the following equation:

Physical Properties of Silver Halides

65

where /id and T are the drift mobility and lifetime of charge carriers, respectively. Equation (3.12) with 60 cm 2 V~ 1 s~ 1 for /td and 0.1-10 /Asec for r gives 4-40 /Am for 1 of photoelectrons in a large AgBr crystal. Hirano obtained 2.1 cm 2 V~ 1 s~ 1 for /Ad and 0.1 /AS for T, giving 2.3 /AIB for 1 of photoelectrons in AgBr emulsion grains.75 Detailed discussions were described by Mitchell on the distance to which photoelectrons could move in silver halide crystals." Farnell estimated that 1 was 0.4 yam for photoelectrons in tabular AgBr emulsion grains on the basis of the range where the formation of surface photolytic silver specks was prohibited due to the strong electron trapping by a internal photolytic silver speck.100 Matejec and Moisar estimated that 1 was 0.5 /AHI for photoelectrons in AgBr core/shell emulsion grains with electron trapping centers on the core by observing the thickness of the shell beyond which the electron-trapping centers on the core could not prohibit the formation of surface latent image centers.101 The deviation of the proportionality of sensitivity to grain size was usually observed when the grain volume exceeded about 1 /Am and could be attributed to the limited diffusion length of photoelectrons with respect to the size of emulsion grains.102'103>93(g) Equation (3.12) with 1.1 cnrV^s"1 for /Ad and 0.1-10 for T gives 0.5-5 tun for 1 of positive holes in a large AgBr crystal. Luckey estimated that 1 was f /«n for positive holes in a large AgBr crystal by observing the distance to which positive holes could escape from the bulk to the surface of the crystal.104 On the basis of photon-stimulated desorption of bromine molecules from a AgBr large crystal, Kanzaki and Mori estimated the diffusion length of positive holes as 1 .4 /Am.105 Equation (3.12) with 0.001 for /Ad and 15 /AS for r71 gives 0.19 /Am for 1 of positive holes in AgBr emulsion grains. Tani estimated that 1 was 0.2 /AHI for positive holes in AgBr emulsion grains by observing the size dependence of the bleaching of surface fog centers on the grain surface by positive holes and also the oxidation of sensitizing dyes on the grain surface by positive holes.106 However, Malinowski and Platikanova observed that in the absence of a field, positive holes could diffuse a distance much larger than that corresponding to the measured mobility and lifetime in a large AgBr crystal.107 Platikanova and Malinowski also found that, upon reaching the free surface of the crystal, the hole came to be in equilibrium with adsorbed halogen species, which could diffuse to bleach silver tenths of a millimeter away.108

3.4 Electron and Positive Hole Traps in Silver Halides 3.4.1 Intrinsic Traps for Electrons and Positive Holes Shallow electron levels in a large AgCl and AgBr crystals were first detected by Brandt and Brown as transient, exposure-induced absorption spectra in the far infrared at 9.3K.109 The spectrum had absorption peaks identified as the ls-2p transition and Is- oo, one of a hydrogenic center consisting of an electron loosely bound

66

PHOTOGRAPHIC SENSITIVITY

by the Coulomb potential of a charged defect. The energies of the 1 s-2p and 1 s- °° transitions were 33.7 and 36.2 meV, respectively, for AgCl, and 20.8 and 23.8 meV, respectively, for AgBr. For a hydrogenic system, neglecting central core corrections, the ls-2p transition energies would give binding energies of 45 meV for AgCl and 28 meV for AgBr, respectively. Determining the spectral dependence of actinicexposure-induced absorption and conductivity, Sakuragi and Kanzaki110'111 attributed the absorption to an electron center. Subsequent magnetic resonance studies have demonstrated that those traps are paramagnetic, consisting of an electron that is shallowly bound to a charged center.112"14 The energetics of a carrier bound to a charged center has received much attention from theorists.62'109'115 The estimated orbital radii are 15.1 A for AgCl and 23.9 A for AgBr. It is noted that predominant intrinsic electron traps in large AgCl and AgBr crystals are positively charged centers with a shallow potential well, with a radius of several lattice spacings and a binding energy of some 20-40 meV and have little influence upon the drift mobilities of photoelectrons in both AgCl and AgBr at room temperature or above. Silver halides with intrinsic shallow electron centers are in strong contrast to the alkali halides, in which stable color centers such as the F center with a binding energy of as large as 2-3 eV are efficiently produced. It is considered that one of the reasons for the small binding energy of the intrinsic electron centers is the large dielectric constants of silver halides. Some consideration has been made of similar shallow levels associated with the partially charged surface and internal kink and job sites.90 It seems that intrinsic electron traps on the surface of silver halide emulsion grains are deeper than the above-stated positively charged center in the bulk,93(g) as shown later. Descriptions are made on intrinsic traps for positive holes in AgCl and AgBr, including the self-trapped states and silver ion vacancies described earlier in this chapter. The microwave photoconductivity measurement of silver halide emulsion grains have reflected intrinsic electron traps in silver halide emulsion grains.72'73'75'92'93 One of the comprehensive observations and explanations for the decay of photoelectrons in AgCl and AgBr emulsion grains are described by Kaneda, Saeki, Oikawa, and Tani.93(s) The decay of photoelectrons in cubic AgBr emulsion grains with an edge length of 0.6 fjum was measured at room temperature by means of a microwave photoconductivity apparatus with a high time resolution, indicating two components as shown in Fig. 3.20. The decay probability (i.e., I/T, where r is the electron lifetime) of the first component was proportional to the surface/volume ratio (i.e., 1/d, where d is the edge length) of the grains, as shown in Fig. 3.21, indicating that photoelectrons were captured by intrinsic traps on the grain surface according to the reaction-controlling scheme and came in equilibrium between the conduction band and the traps. On the other hand, I/T of the second component was proportional to the square of the surface/volume of the grains (i.e., 1/d2), as seen in Fig. 3.21. As described in the previous section, the concentration of interstitial silver ions is proportional to the surface/volume ratio (i.e., 1/d) of the grains. In addition, I/T is proportional to the concentration of electron traps on the grain surface (i.e., proportional to the surface/ volume ratio of the grains), when the electron trapping was reaction controlling. It is therefore considered that I/T of the second component was proportional to the

Fig. 3.20. Signal of microwave photoconductivity measurement at room temperature for cubic AgBr emulsion grains with an edge length of 0.6 jam (a) and an alaysis of the decay kinetics of the above signal (b), indicating two components (T, and r2, respectively).93(s)

Fig. 3.21. Dependence of the electron lifetimes for the first and second components in the decay of photoconductivity (T, and T2, respectively) upon the edge length (d) of cubic AgBr emulsion grains at room temperature.93'6'

67

PHOTOGRAPHIC SENSITIVITY

product of the electron-trapping probability on the grain surface and the probability of the reaction of a trapped electron with an interstitial silver ion, indicating that the decay of photoelectrons in the second component was due to the electron trapping on the grain surface followed by the reaction of the trapped electrons with interstitial silver ions. This is in accord with the correlation between electron lifetimes and field-decay times observed by Hamilton and Brady,35'36'71 and the observation by Kaneda.93(b) This decay component is important, since it is the beginning of the formation of latent image centers. As shown in Fig. 3.22, the decay of photoelectrons in cubic AgCl emulsion grains with an edge length of 0.6 /jan also had two components. When the chloride content (x) in cubic AgClxBr!_x emulsion grains with an edge length of 0.6 /am was increased, as shown in Fig. 3.23, the second decay component, observed in the AgBr grains, disappeared when x exceeded about 50% and the first decay component, observed in the AgBr grains, corresponded to the second decay component in the AgCl grains. However, r (i.e., the reciprocal of the decay probability) in the second component in the AgCl grains was proportional to d2 of the grains, contrary to the case of the first component of the AgBr grains, in which T was proportional to d of the grains. According to reaction-controlling scheme, the electron-trapping probability is expressed as follows72'73:

Fig. 3.22. Signal of microwave photoconductivity measurement at room temperature for cubic AgCl emulsion grains with an edge length of 0.6 /xm (a) and an analysis of the decay kinetics of the above signal (b), indicating two components (T{ and r'2, respectively).93*6'

Physical Properties of Silver Halides

69

Fig. 3.23. Electron lifetimes in cubic AgClxBr, _-x emulsion grains with an edge length of 0.6 fjan and with a variation of x at room temperature.93(s)

where TB is the electron lifetime in the bulk and k is the rate constant. When I/TB is negligibly small, T is proportional to d of the grains. According to the diffusioncontrolling scheme, the electron-trapping probability is expressed as follows72'73:

where D is the diffusion coefficient. When I/TB is negligibly small, T is proportional to d2 of the grains. From Equations (3.13) and (3.14), the proportionality of T in the second component in the AgCl grains to d2 of the grains indicates that photoelectrons were trapped on the grain surface according to a diffusion-controlling scheme; that is, photoelectrons were immediately and irreversibly captured by electron traps on the surface of AgCl grains when they arrived on the grain surface. This was confirmed by the observation that the electron lifetime in the second decay component in the AgCl grains was independent of the presence of strong extrinsic electron traps (e.g., sulfur sensitization centers; see Chapter 6), contrary to the first decay component in the AgBr grains, in which T was significantly decreased by the extrinsic electron traps (Fig. 3.24). These results are consistent with Keevert's observation that the

70

PHOTOGRAPHIC SENSITIVITY

Fig. 3.24. Dependence of electron lifetimes in cubic AgClxBr,_x emulsion grains with a variation of x upon extrinsic electron traps (i.e., sulfur sensitization centers provided by the digestion of the emulsions in the presence of Na2S2O3 at 60°C for 30 min with the amount shown in the figure) .93
electron lifetime was much shorter than the field-decay time in AgCl emulsion grains.96 In the presence of a silver complexing agent, 4-hydroxy-6-methyll,3,3a,7-tetraazaindene (TAI), the electron trapping on the AgCl grains was reaction controlling. It is therefore considered that the above-stated electron traps on the surface of the AgCl grains are silver ions at some surface sites, presumably at kink sites on the grain surface. There is little knowledge on the fast component in the decay of photoelectrons in AgCl emulsion grains. The studies on physical properties and photographic behavior of AgCl emulsion grains are only beginning. Crystal defects in silver halide provide electron traps, decreasing the lifetime of photoelectrons. Those defects include dislocations and twin planes. Hayes and Scockley33(a) and Suptitz116 found that the motion of photoelectrons was impeded by a strained region. A strain-induced reduction of the photoconductive response

Physical Properties of Silver Halides

11

was also reported.117"120 Ozeki, Urabe, and Tani have confirmed that the electron lifetime in tabular AgBr grains that contained twin planes was shorter than those in cubic and octahedral AgBr grains that did not contain such crystal defects.53 3.4.2 Extrinsic Traps for Electrons and Positive Holes There are various kinds of extrinsic traps for electrons and holes. Among them are shallow isoelectronic (i.e., uncharged) traps for holes such as I~~ in AgBr, I" in AgCl, and Br~ in AgCl. According to Kanzaki and Sakuragi, the trapping of a positive hole at a residual iodide impurity was dominant in nominally pure AgBr at liquid helium temperature, and the localization energy of the hole trapped at iodide ion can be estimated as 0.04 eV for an isolated iodide ion and as 0.1 to 0.15 eV for an iodide ion pair.121>16(a) The deeper localization at an impurity pair compared to a single impurity has been observed commonly for isoelectric traps in compound semiconductors.121 There are various kinds of extrinsic charged centers. They can be classified into shallow and deep centers. In the former electron centers, an electron is bound to the positive charge of the center with hydrogenic shallow potential well. The latter electron centers have deep vacant orbitals, in addition to the shallow potential wells, to accept electrons. In the case of the series of divalent metal ions whose redox potential (M = M 2+ + 2e~) as the measure for the height of their vacant orbitals is on the order of Hg2+, Cu2+, Pb2+, Ni2+, Co2+, Cd2+, Fe2+, Zn2+, Ba2+, Sr2+, and Mn 2+ , only Hg2+ and Cu2+ could decrease the luminescence intensity and photographic sensitivity of a cubic AgBr emulsion by providing deep traps for electrons when they were added to the emulsion.123 There are several examples of extrinsic shallow charged centers in silver halides. In relation to the absorption for electrons bound to charged intrinsic centers in silver halides, Sakuragi and Kanzaki110'111 found similar absorptions for electrons bound to the Coulomb centers of doped closed-shell divalent cations such as Pb2+, Cd2+, andFe 2+ . There are various kinds of extrinsic deep charged traps for electrons and holes. Lax outlines the general scheme for trapping into any deep level,124 according to which a charge carrier is first captured into a shallow Coulombic level and then slowly relaxed into a deeper valence level associated with the trap.125 The overall crosssection of the deep charged trap is given by

where

where v is the rate of de-excitation from the shallow level to the deep and v-, is the ionization rate of the carrier from the shallow level (Fig. 3.25).125

72

PHOTOGRAPHIC SENSITIVITY

Fig. 3.25. Schematic diagram of the transitions in the trapping process at deep charged centers in silver halides.124-125

This trapping mechanism was observed and extended for Ir3+ in AgBr by Eachus and Olm114'126; namely, Ir3+ shortened the photoelectron lifetime in AgBr to the range of ps or ns72'75 by shallow trapping,

whereas the paramagnetic Ir2+ was formed to give the electron spin resonance (ESR) signal on a much longer time scale through the following electronic relaxation:

and ionic relaxation,

where V is a silver ion vacancy. The thermal trap depth for [(IrQ6)4~V]° has been measured as 0.46 eV by kinetic ESR spectroscopy. In AgBr, [(IrClg)4^]0 has a lifetime of about 20 msec at room temperature and decays by several competitive pathways, among which is the reduction of an interstitial silver ion to produce a silver atom at an adjacent site.127 Another important pathway is the thermally assisted excitation of the trapped electron to the conduction band. It is retrapped many times at other lr>+ sites, so that the time before the electron is permanently annihilated by silver formation at an image center is increased. It is therefore considered that the addition of Ir3+ reduces high-intensity reciprocity law failure of emulsions when this decay mechanism predominates.114

Physical Properties of Silver Halides

73

Although both Rh3+ and Ir3+ traps electrons to produce Rh2+ and Ir2+128, and to reduce the photoelectron lifetime to about 10~8 to 1(T9 sec,72'75 their effects are dramatically different. In the case of rhodium, the trapped electron state is so long lived that all of Rh3+ in the emulsion grain can be saturated. Kinetic ESR studies of the [(RhCle)4 ]° center in AgBr yield a value of about 0.8 eV for the thermal trap depth and a lifetime of >105 sec at 300K.129 The sensitivity of the emulsion is reduced as a result of Rh3+ doping, but the contrast is increased, making it suitable for applications in printing industries.114'130-131 The detailed structure of [(RhCl6)4~~]° has been studied by means of electron-nuclear double resonance (ENDOR) spectroscopy.132 These processes, as expressed by Equations (3.17)-(3.19) and exemplified by Ir3+ and Rh 3+ , could provide the general sequence of the trapping of electronic carriers by deep charged centers. The trapping processes are also influenced by the site charge and conformation of dopant-vacancy complexes. After trapping electrons, complexes with weakly bound vacancies will relax more efficiently than complexes with more tightly bound configurations, increasing their effectiveness as permanent electron traps.114 The effects of site charge on the trapping behavior of an impurity have been studied by ESR experiments on Pt2+, Pd2+, and Re3+ in silver halides. The behavior of Pt2+ and Pd2+ in AgBr and AgCl can be explained133'134 in terms of electron trapping by [M2+]+ and hole trapping by [M2+V]°. In the case of Re3+, the dopant is distributed among [M3+]2+, [M3+V]+, and [M3+2V]° sites, all of which trap photoelectrons with rates dependent upon their charge.135 Various transition-metal electron traps in silver halides have been characterized by kinetic ESR spectroscopy.114 Among them [Ru(NO)Cl5]2~ is new and unique. This complex deeply traps conduction band electrons, and the resultant [Ru(NO)Q5]3~ center has a lifetime of at least 1.6 X 109 sec and a trap depth in excess of 1.2 eV.136 Among various electron traps introduced onto silver halide emulsion grains, sulfur sensitization centers and sulfur-plus-gold sensitization centers are most important for efficient latent image formation, as described in detail in Chapter 6, and their electronic properties are being studied by many groups of workers. The original hypothesis of Gurney and Mott was that a silver sulfide speck has a vacant electronic energy level below the bottom of the conduction band of silver halides and acts as an electron trap.137 Electron-trapping by sulfur sensitization centers have been confirmed by several groups of workers with photoconductivity measurements_88,92(b),138,139

Quantitative knowledge of sulfur sensitization and sulfur-plus-gold sensitization centers have been reported recently. It is known that, being excited by red light, these centers can inject electrons into the conduction band of silver halide with the aid of thermal energy and thus give rise to red sensitivity. Hamilton, Harbison, and Jeanmaire estimated the depth of these electron-trapping centers on octahedral AgBr emulsion grains by analyzing the temperature coefficient of their red sensitivity140 and found the thermal depth of sulfur sensitization centers as 0.33 eV and of sulfurplus-gold sensitization centers as 0.19 eV. Using a thermally stimulated current technique, Kellogg and Hodes estimated the thermal depth of sulfur sensitization

74

PHOTOGRAPHIC SENSITIVITY

centers as 0.39 eV and of sulfur-plus-gold sensitization centers as 0.17 eV on octahedral AgBr emulsion grains.141 Kanzaki and Tadakuma have applied an excitation spectroscopy using luminescence modulation to the study of sulfur sensitization centers on AgBr emulsion grains. They filled sulfur sensitization centers with electrons by exciting the grains and excited the localized electrons by infrared radiation with varying excitation energy. According to their measurement, the electrons localized at sulfur sensitization centers gave a transient absorption band peaked at 0.5 eV, indicating that the optical depth of sulfur sensitization centers for electron trapping was 0.5 eV142 on AgBr emulsion grains. It is judged from the low energy threshold of the absorption band that the thermal trap depth of sulfur sensitization centers was about 0.3 eV. They also estimated that a sulfur sensitization center is composed of a dimer of silver sulfide, according to the observation of the relative number of the centers as a function of the amount of Ag2S formed. A similar conclusion was also presented by Keevert and Gokhale.143 According to the estimation by Tadakuma, Yoshida, and Kanzaki on the basis of a quantitative analysis of the amount of Ag2S and Smoluchouski's equation, the number of sulfur sensitization centers were as many as 2800/ /jim2 of the (100) face of AgBr emulsion grains when the grains were optimally sulfur sensitized.144 Spencer, Brady, and Hamilton have demonstrated that there are two types of silver clusters acting as electron traps or positive hole traps.145 On the basis of studies on the silver clusters formed during reduction sensitization, Tani has proposed that silver clusters acting as positive hole traps are located at electrically neutral sites, while silver clusters acting as electron traps are located at positively charged sites on silver halide grains.146 Hamilton and Baetzold have reported theoretical and experimental results to support this idea and named the electron-trapping center and the hole-trapping center as P and R centers, respectively.147 Decreases in electron lifetime by pre-exposure have indicated that the photolytically formed silver clusters, such as latent image centers, were electron traps.35>92(b)>148 As is shown in Chapters 4 and 6, Tani has confirmed that there are two types of reduction sensitization centers, both of which increase photographic sensitivity. One of them did not decrease the photoelectron lifetime and was ascribed to an R center, while the other decreased the photoelectron lifetime and was ascribed to a P center.149 For the trapping of a positive hole by an R center, Mitchell has proposed that the hole-trapping by an R center is followed by its release of a silver ion into an interstitial position, preventing the recombination between the trapped hole with a photoelectron.150 This proposal is also in accord with the trapping process of an electronic charge carrier by a charged deep center outlined by Lax (Fig. 3.26).124 Including this process, Lowe has proposed the following relaxation processes151:

Physical Properties of Silver Halides

75

where Agj+ is an interstitial silver ion; that is, the capture of a positive hole by an R center achieves not only the prohibition of the recombination, but also the creation of an extra free electron available for the formation of a latent image center. Tani has experimentally proven this hypothesis for the first time.152 By using positive holes slowly released from sensitizing dyes on the grain surface, he observed the growth of latent subimage centers to latent image centers with the aid of the electrons formed from the processes described in Equations (3.20)-(3.22). Trapping of positive holes and electrons by sensitizing dyes and desensitizing dyes are described in detail in Chapter 5. Berry153 outlined the depth (E) and resident time of electronic charge carriers at various traps (t) by the use of Marker's equation,154 as expressed below:

According his estimation, the values of t are 1 ^sec, 10 sec, and 10 years when the values of E are 0.3, 0.7, and 1.2 eV, respectively.

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78

72. 73. 74. 75.

76. 77. 78.

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R. J. Deri, J. P. Spoonhower, Appl. Phys. Lett., 43, 65(1983). R. J. Deri, J. P. Spoonhower, J. Appl. Phys., 57, 2806(1985). T. Kaneda, J. Imaging Sci., 35, 293(1991). A. Hirano, "Highly Time Resolved Microwave Photoconductivity Technique Applied to Measurements of Drift Mobility and Capture Cross Sections in AgBr Microcrystals," in the preprint book of The SPSE Annual Conference, May, 1990, Rochester, New York. (a) M. Hohne, M. Stasiw, Phys. Stat. Sol., 25, K55, (1968); (b) M. Hohne, M. Stasiw, Phys. Stat. Sol., 28, 247(1968). Y. Toyozawa, Prog. Theor. Phys., Kyoto, 26, 29(1961). (a) C. L. Marquart, R. T. Williams, M. N. Karler, Solid State Commun., 9, 2285(1971); (b) M. Fukui, Y. Hayashi, H. Kanzaki, S. Sakuragi, J. Phys. Soc. Jpn., 30, 1510(1971); (c) M. Fukui, Y. Hayashi, H. Yoshioka, J. Phys. Soc. Jpn., 34, 1226(1973). (a) E. Laredo, W. B. Paul, L. G. Rowan, L. Slifkin, Phys. Rev., B, 27, 2470(1983); (b) E. Laredo, W. B. Paul, S. E. Wang, L. G. Rowan, L. Slifkin, in The Physics of Latent Image Formation in Silver Halides, International Symposium, Trieste, 1983, A. Baldereschi, W. Czaja, E. Tosatti, M. Tosi, eds., World Scientific, Singapore, 1984, p. 35. P. Mueller, S. Spenke, J. Teltow, Phys. Stat. Sol, 41, 81(1970). (a) H. Tamura, T. Masumi, /. Phys. Soc. Jpn., 30, 897(1971); (b) H. Tamura, Solid State Commun., 10, 297(1972). (a) J. W. Hodby, Solid State Commun., 1, 811(1969); (b) J. W. Hodby, J. Phys. C, 4, L8(1971); (c) J. W. Hodby, J. G. Crowder, C. C. Bradley, J. Phys. C, 7, 3033(1974). R. K. Ahrenkiel, R. S. van Heyningen, Phys. Rev., 144, 576(1966). (a) M. Georgiew, J. Malinowski, J. Phys. Chem. Solids, 28, 931(1967); (b) A. Buroff, J. Malinowski, Phys. Stat. Sol, 26, 267(1968). (a) R. C. Hanson, F. C. Brown, J. Appl. Phys., 31, 210(1960); (b) R. C. Hanson, J. Phys. Chem., 66, 2376(1962). Y. Toyozawa, A. Sumi, Twelfth International Conference on Physics of Semiconductors, B. G. Teubner, Stuttgart, 1974, p. 179. H. Kanzaki, Semicond. Insulators, 3, 285(1978). (a) W. West, B. H. Carroll, J. Chem. Phys., 15, 529(1947); (b) W. West, in Fundamental Mechanisms of Photographic Sensitivity (Proc. Bristol ymp., 1950), J. W. Mitchell, ed., Butterworth Science Publications, Ltd., London, 1951, p. 99. J.. W. Mitchell, Sci. Ind. Photogr., (2)28, 493(1957). J. F. Hamilton, Photogr. Sci. Eng., 18, 493(1974). E. V. Baranov, I. A. Akimov, Dokl. Akad. Nauk. SSSR, 154, 7(1964). (a) L. M. Kellogg, N. B. Liebert, T. H. James, Photogr. Sci. Eng., 16, 115(1972); (b) L. M. Kellogg, Photogr. Sci. Eng., 18, 378(1974). (a) J. Beutel, Photogr. Sci. Eng., 19, 96(1975); (b) T. Kaneda, J. Imaging Sci., 33, 115(1989); (c) T. Harada, T. lijima, T. Koitabashi, Photogr. Sci. Eng., 26, 137(1982); (d) T. Nakayama, J. Soc. Photogr. Sci. Tech. Jpn., 50, 459(1987); (e) R. J. Deri, J. P. Spoonhower, J. F. Hamilton, / Appl. Phys., 57, 1968(1985); (f) A. Hasegawa, O. Koseki, T. Sakaguchi, J. Soc. Photogr. Sci. Tech. Jpn., 49, 509(1986); (g) T. Oikawa, N. Saeki, T. Kaneda, A. Hirano, T. Tani, /. Imaging Sci. Technol, in press. (a) B. Levy, Photogr. Sci. Eng., 15, 278(1971); (b) K. C. Chang, F. P. Chen, B. Levy, Photogr. Sci. Eng., 24, 55(1980); (c) K. C. Chang, F. P. Chen, B. Levy, Photogr. Sci. Eng., 25, 138(1981). (a) V. G. Vlasov, P. V. Meiklyar, Zh. Nauch. Prikl. Fotogr. Kinematogr., 13, 128(1968); (b) V. G. Vlasov, P. V. Meiklyar, Photogr. Sci. Engr., 17, 343(1973). J. E. Keevert, in the preprint book of The 28th Annual Conference and Seminar on

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Quality Control, Denver, 1975, Society Photogr. Sci. Eng., Washington, D.C., pp. 186-7. L. M. Kellogg, J. Hodes, J. Muenter, "Measurement of Photocarrier Decay Times in Silver Halide Emulsion Grains Using Radiofrequency Techniques,'' in the preprint book of The International East-West Symposium II on Factors Influencing The Efficiency of Photographic Imaging, Kona, Hawaii, October/November, 1988. T. Kaneda, N. Ohshima, T. Tani, J. Imaging Sci., 31, 50(1987). J. W. Mitchell, J. Imaging Sci., 34, 113(1990). G. C. Farnell, R. B. Flint, J. B. Chanter, /. Photogr. Sci., 13, 25(1965). R. Matejec, E. Moisar, Photogr. Sci. Eng., 12, 133(1968). G. C. Farnell, J. Photogr. Sci., 17, 116(1969). T. Tani, J. Imaging Sci., 29, 165(1985). (a) G. W. Luckey, J. Phys. Chem., 57, 791(1953); (b) G. W. Lukey, J. Chem. Phys., 23, 882(1955). H. Kanzaki, T. Mori, Semicond. Insul, 5, 401(1983). T. Tani, "Estimation of Diffusion Length of Positive Holes in Cubic AgBr Grains," in the preprint book of The Autumn Meeting of Soc. Photogr. Sci. Tech. Jpn., November, 1989, Kyoto. J. Malinowski, V. Platikanova, Phys. Status Solidi, 6, 885(1964). V. Platikanova, J. Malinowski, Phys. Status Solidi, 14, 205(1966). R. C. Brandt, F. C. Brown, Phys. Rev., 181, 1241(1969). S. Sakuragi, H. Kanzaki, Phys. Rev. Lett, 38, 1302(1977). H. Kanzaki, S. Sakuragi, Photogr. Sci. Eng., 17, 69(1973). J. Z. Brescia, R. S. Eachus, R. James, M. T. Olm, Cryst. Latt. Def. Amorph. Mater., 17, 165(1987). R. S. Eachus, R. James, M. T. Olm, "Papers Summaries," in Fifth Europhysical Topical Conference on Lattice Defect in Ionic Crystals, Madrid, 1986, p. 259. A. P. Marchetti, R. S. Eachus, in Advances in Photochemistry, Volume 17, D. Volman, G. Hammond, D. Neckers, eds., John Wiley & Sons, 1992, pp. 145-216. (a) K. K. Bajaj, T. D. Clark, Phys. Stat. Sol.(b), 52, 195(1972); (b) M. H. Eigineer, N. Tozar, Phys. Rev. B, 5, 3029(1972); (c) M. Brauwers, R. Evrard, E. Kartheuser, Solid State Commun., 19, 857(1976); (d) M. Grinberg, S. Legowski, H. Meczynska, Phys. Stat. Sol.(b), 52, 195(1972). P. Suptitz, Z. Elektrochem., 63, 400(1959). K. Hecht, Z. Physik., 77, 235(1932). W. Lehfeldt, Nachr. Ges. Wiss. Goettingen, Math.-Physik. Kl. Fachgruppe 2, l(N.S), 171(1935). K. Yamada, S. Oka, Naturwissenschaften, 43, 175(1956). (a) R. Matejec, Naturwissenschaften 43, 533(1956); (b) R. Matejec, Z. Physik, 148, 454(1957). H. Kanzaki, S. Sakuragi, J. Phys. Soc. Jpn., 27, 109(1969). D. G. Thomas, J. Hopfield, Phys. Rev., 150, 680(1965). (a) T. Tani, M. Saito, Nature Phys. Sci., 242, 47(1973); (b) T. Tani, M. Saito, Photogr. Sci. Eng., 17, 434(1973). M. Lax, Phys. Rev., 119, 1502(1960). R. M. Gibb, G. J. Rees, B. W. Thomas, B. L. H. Wilson, B. Hamilton, D. R. Wight, N. F. Mott, Phil. Mag., 36, 1021(1977). R. S. Eachus, M. T. Olm, Cryst. Latt. Def. Amorph. Mater., 18, 297(1989). R. S. Eachus, in The Physics of Latent Image Formation in Silver Halides, International

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Symposium, Trieste, 1983, A. Bladereschi, W. Czaja, E. Tosatti, M. Tosi, eds., World Scientific, Singapore, 1984, p. 249. J. W. Mitchell, /. Imaging Scl, 33, 103(1989). R. S. Eachus, R. E. Graves, /. Chem. Phys., 61, 2860(1974). (a) H. Welzel, Z. Wiss, Photogr. 58, 59(1964); (b) G. B. Patent 1,395,923(1972). J. Assa, E. Spasova, G. Danev, J. Signalaufz.-Mater., 12, 41(1984). M. T. Olm, J. R. Niklas, J.-M. Spaeth, M. C. Symons, Phys. Rev., B38, 4343(1988). R. S. Eachus, R. E. Graves, M. T. Olm, /. Chem. Phys., 69, 4580(1978). (a) R. S. Eachus, R. E. Graves, J. Chem. Phys., 65, 5445(1976); (b) R. S. Eachus, R. E. Graves, M. T. Olm, Phys. Status Solidi (a)57, 429(1980). J. R. Buntaine, R. S. Eachus, M. T. Olm, Radiation Effects and Defects in Solids, 119121,621(1991). W. G. McDugle, R. S. Eachus, M. T. Olm, in "Papers Summaries," in 44th Annual Conference of the SPSE, St. Paul, MN, 1991. (a) R. W. Gurney, N. F. Mott, Proc. R. Soc., Land. A, 164, 151(1938); (b) N. F. Mott, R. W. Gurney, Electronic Processes in Ionic Crystals, Clarendon Press, Oxford, 1948, Chap. VII. S. S. Collier, Photogr. Sci. Eng., 20, 43(1976). T. Tani, M. Saito, Photogr. Sci. Eng., 23, 323(1979). J. F. Hamilton, J. M. Harbison, D. L. Jeanmaire, J. Imaging Sci., 32, 17(1988). L. M. Kellogg, J. Hodes, "The Measurement of Electron Trap Depths for Sulfur and Sulfur-Plus-Gold Centers in AgBr Emulsions Using Thermally Stimulated Current (TSC) Techniques," in the preprint book of The 40th Annual Conference of SPSE, Rochester, New York, May, 1987. (a) H. Kanzaki, Y. Tadakuma, "Localized Electrons and Holes in Chemically Sensitized Emulsion Grains of Silver Bromide," in the preprint book of The International EastWest Symposium II. Factors Influencing the Efficiency of Photographic Imaging, October/November, 1988, Hawaii; (b) H. Kanzaki, Y. Tadakuma, J. Phys. Chem. Solids, 55,631(1994). J. E. Keevert, V. V. Gokhale, J. Imaging Sci., 31, 243(1987). Y. Tadakuma, Y. Yoshida, H. Kanzaki, "A Study on Aggregation Processes of Sulfur Sensitization Centers on AgBr Emulsion Grains," in the preprint book of The Annual Conference of Soc. Photogr. Sci. Technol. Jpn., May, 1990, Makuhari, Chiba. H. E. Spencer, L. E. Brady, J. F. Hamilton, J. Opt. Soc. Am., 57, 1020(1967). (a) T. Tani, Photogr. Sci. Eng., 15, 181(1971); (b) T. Tani, Photogr. Sci. Eng., 18, 569(1974). J. F. Hamilton, R. C. Baetzold, Photogr. Sci. Eng., 25, 189(1981). A. Hasegawa, T. Sakaguchi, J. Imaging Sci., 30, 13(1986). (a) T. Tani, M. Murofushi, J. Imaging Sci. Technol., 36, 1(1994). J. W. Mitchell, Rep. Prog. Phys., 20, 433(1957). J. M. Harbison, H. E. Spencer, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, p. 152. T. Tani, /. Imaging Sci., 30, 41(1986). C. R. Berry, J. Photogr. Sci., 18, 169(1970). R. E. Marker, J. Opt. Soc. Am., 44, 625(1954).

4 Mechanism of Latent Image Formation

4.1 Outline of Mechanism of Latent Image Formation 4.1.1 Framework of Current Mechanism of Latent Image Formation As shown in Fig. 3 in Chapter 1, the photographic process is composed of the capture of incident light by silver halide emulsion grains (sensing), formation of latent image centers on the silver halide grains (memory), and image formation by the catalytic action of the latent image centers for the reduction of silver halide grains to silver (display). It was first suggested by Sheppard, Trivelli, and Loveland1 in 1925 and is now accepted that a latent image center is an aggregate of silver atoms. Therefore, it is important that the mechanism of photographic sensitivity explain the concentration principle by which a silver aggregate is photolytically formed as a latent image center on a silver halide grain. Figure 4.1 schematically shows the steps in latent image formation on a AgBr emulsion grain. As described in Chapter 3, a silver halide emulsion grain contains many interstitial silver ions and shallow electron traps at which latent image centers can be formed. Latent image centers are efficiently formed on silver halide emulsion grains, with electron traps provided by sulfur sensitization and sulfur-plus-gold sensitization (i.e., sulfur sensitization centers and sulfur-plus-gold sensitization centers) as well as by positive hole traps provided by reduction sensitization (i.e., reduction sensitization centers). Details of those chemical sensitizations are also described in Chapter 6. Figure 4.1 also illustrates the framework of the current mechanism of photographic sensitivity,2 which is based on the proposals by many scientists, including Sheppard, Trivelli, and Loveland,1 Gurney and Mott,3 Burton and Berg,4 Mitchell,5 and Hamilton.6'7 As described in Chapter 3, a photoelectron and a positive hole are efficiently created as a result of the light absorption of a silver halide grain. A photoelectron is then trapped by one of the electron traps. This is called the electronic process for latent image formation. The trapped electron attracts and reacts with an interstitial silver ion to form a silver atom, which is called a latent preimage center. This is 81

82

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Fig. 4.1. Framework of the current mechanism for the formation of latent image centers on AgBr grains, where Ags+ is an interstitial silver ion, and CB and VB are the conduction and valence bands, respectively, of AgBr2. called the ionic process. The repetition of electronic and ionic processes one after another at the same center leads to the formation of a latent image center composed of a cluster of silver atoms. Although a latent preimage center is unstable, it is capable of trapping a free electron and then capturing a second interstitial silver ion to form Ag2, which is fully stable and is called a latent subimage center. The growth of a latent subimage center by the repetition of the electronic and ionic processes leads to the formation of a latent image center; namely, a latent image center forms in two distinct stages, which are termed nucleation and growth.4 The formation of a stable subimage center through the formation of an unstable preimage center completes the nucleation stage and provides the threshold for latent image formation, making it possible for silver halide photographic materials to achieve both high sensitivity and high stability, as stated in Chapter 1. The nucleation and growth model also proposes that the growth of the silver aggregate may continue indefinitely by alternate repetition of the electron-trapping and silver ion events in the same sequence. It is estimated that the

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smallest latent image center is composed of three atoms on sulfur-plus-gold-sensitized emulsion grains and of four to five atoms on sulfur-sensitized emulsion grains, as described in Section (4.3) in this chapter. It is considered that the electrostatic charge of an initial electron-trapping center suitable for efficient latent image formation has a partial, preferably +\ charge, as for the case of a surface kink site, and becomes a -| charge when the center traps an electron. Electron trapping by the center is followed by the ionic process, which makes the center become + \ charged and thus "reset" for trapping another electron. The concept of a partially charged electron trap results in an alternating sign of charge at each step in the process of latent image formation and minimizes the recombination probability by providing an electrostatic driving force for both the electronic and ionic processes of latent image formation. Reaction of a trapped electron with an interstitial silver ion comprises the ionic relaxation process, leading to latent image formation, and usually competes with recombination of the trapped electron with a positive hole. As described in Section (4.3), the most effective positive hole trap for prevention of recombination is a reduction sensitization center composed of Ag2. The trapping of a positive hole by a reduction sensitization center is not followed by recombination of the trapped hole with a free electron, but by ionic relaxation (i.e., release of a silver ion from the center into an interstitial position) and, further, by release of a free electron from the relaxed center. It is noted that ionic relaxation processes following electronic processes are very important for efficient formation of latent image centers on silver halide grains. The behavior of positive holes in the presence of reduction sensitization centers and the action of a reduction sensitization center, in contrast to that of a latent subimage center, has been clarified by many scientists, including Lowe, Mitchell, Spencer, Hamilton, Baetzold, and Tani, as described in Section (4.3) in this chapter as well as in Section (3.4) in Chapter 3. A latent image center on a silver halide emulsion grain has a deep electronaccepting level and initiates photographic development by receiving an electron from a developer and acting as a catalyst for reduction of the silver halide grain to silver. Photographic development is a unique amplification process that can amplify as much as >109, as shown in Chapter 1. Details of photographic development are described in Chapter 7. 4.1.2 Progress of Mechanism of Latent Image Formation Following Sheppard's recognition that Ag2S specks could increase photographic sensitivity, Sheppard, Trivelli, and Loveland1 first proposed in 1925 the "concentration-speck theory," which is the concept of aggregation of silver atoms at an Ag2S speck; namely, a silver atom is formed as a result of absorption of a photon by a silver halide grain, migrates, and coagulates with other photolytically formed silver atoms to form a silver aggregate at an Ag2S speck. Although the detailed mechanism is not accepted now, the proposed concept (i.e., the concentration principle) is of great importance for the mechanism of photographic sensitivity. In 1938, Gurney and Mott proposed the mechanism of photographic sensitivity3 on the basis of the progress of solid state physics. The most general concept, which

S4

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was termed the Gurney-Mott principle by Mitchell,8 was that the process of latent image formation consists of electronic and ionic stages; that is, a photoelectron is liberated as a result of the absorption of a photon by a silver halide grain. An Ag2S speck acting as a deep electron trap then captures photoelectrons to bear negative charges. This electronic process is followed by an ionic process: The negatively charged speck attracts and combines with interstitial silver ions to give silver atoms. They assumed that there were many interstitial silver ions, which were mobile and available for the latent image formation in a silver halide grain. It was also assumed that a positive hole was immobile and not susceptible to recombination with an electron. Another important feature in the Gurney-Mott mechanism was the concept of a stable and undevelopable latent subimage center, which was later proved and extended by Burton and Berg.4 They clearly showed, by means of double-exposure techniques, that a latent image center formed in two distinct stages, which they termed nucleation and growth. The inefficiency in the latent image formation for high-intensity exposure (i.e., high-intensity reciprocity law failure) could be eliminated by low-intensity postexposure. This result indicates the formation of many stable and undevelopable image centers that could grow further to latent image centers by postexposure. On the other hand, the inefficiency in latent image formation for low-intensity exposure (i.e., low-intensity reciprocity law failure) could be eliminated by high-intensity pre-exposure in order to form stable and undevelopable image centers. This result indicates the presence of an unstable image center prior to the formation of a stable and undevelopable image center. Taking into account the fact that the smallest latent image center is composed of three to five atoms, it is considered that the above-mentioned unstable image center is a single silver atom, and the stable and undevelopable image center is a dimer of silver atoms. Based on progress in understanding the solid-state physics of a large silver halide crystal, Mitchell5 pointed out that there were several problems in the Gurney-Mott mechanism that could not be supported by the properties of a large silver halide crystal. Since it turned out that positive holes in silver halide are more mobile than an interstitial silver ion, Mitchell pointed out that a positive hole should reach a negatively charged center with a trapped electron more rapidly than an interstitial silver ion. He also pointed out, by studying the ionic conductivity of a large silver bromide crystal at high temperature, that there would be less than one interstitial silver ion per emulsion grain at room temperature.8 Mitchell then proposed that a positive hole is captured by a trap, such as a bromide ion at a surface kink site, before a photoelectron is captured by a trap. Hole trapping is then followed by ionic relaxation, which makes it unlikely for the trapped hole to take part in recombination. Thus, a bromide ion at a kink site that captures a positive hole bears a partial positive charge and repels its adjacent silver ion into an interstitial position to create an interstitial silver ion available for latent image formation and to leave Br^", which cannot be a recombination center. One of the most important achievements made by Mitchell was the observation of dislocations. The observation was made with crystals of AgBr, and the dislocations were made visible by decoration with photolytic silver.9'10 This observation

Mechanism of Latent Image Formation

85

has indicated the importance of crystal defects as sites for latent image formation. The role of crystal defects was not mentioned in the Gurney-Mott mechanism. Mitchell also considered that deep electron traps should not be present before exposure to light; otherwise, they could initiate development to cause fog formation. According to Mitchell, a crystal defect is a shallow electron trap and can be a deep electron trap to capture a photoelectron in the presence of an interstitial silver ion in its vicinity. A photoelectron and an interstitial silver ion are then combined with each other at the defect to form a silver atom. As stated earlier, the Mitchell mechanism is based on the properties of a large silver halide crystal. It has turned out, as described in Chapter 3, that the ionic and electronic properties of silver halide emulsion grains are quite different from those of a large silver halide crystal. Hamilton and Brady11'12 recognized this difference in 1959, and it is included in the current mechanism of photographic sensitivity. One of the most important findings was the high ionic conductivity of silver halide emulsion grains as compared with that of a large crystal, as described in Chapter 3. It has been found that a large number of interstitial silver ions are present and available for latent image formation in an emulsion grain. It is quite probable that an interstitial silver ion can reach a trapped electron faster than a positive hole can, since it is judged that mobility multiplied by concentration (i.e., conductivity) of interstitial silver ions is larger than that of positive holes.13 Measurement of the lifetimes of electrons and positive holes in AgBr emulsion grains do not seem to support Mitchell's hypothesis that capture of a positive hole by a trap should be faster than that of a photoelectron.11'14 Several points that Mitchell has proposed are, however, important and are included in the current mechanism of photographic sensitivity. The participation of crystal defects in formation of latent image centers is important from the viewpoints of photographic science and technology.15 As he pointed out, the electron traps available for the latent image formation should be shallow, as discussed in more detail in the next section as well as in Section (3.4) in Chapter 3. Ionic relaxation following trapping of a positive hole is of great importance to prevent recombination of the trapped positive hole with an electron and is discussed in Section (4.3) in this chapter as well as in Section (3.4) in Chapter 3. The importance of the concept of the nucleation and growth processes was initiated by Burton and Berg,4 and has been extended with computer simulation for latent image formation by Hamilton, Hailstone, and coworkers.6'7'16'17 Following the proposal of Seitz18 that the specific trapping centers are jogs on edge dislocations, which are associated with an effective charge of half an electronic unit, Hamilton and Brady proposed the concept of the partially charged trap, which minimizes the recombination probability by bringing about an alternating sign of charge at each step in the process of latent image formation.13 Hamilton has also introduced the concept of lattice relaxation to explain the processes starting from a photoelectron bound to a shallow trap through the formation of a silver atom.7'19 Malinowski has proposed the migration of photolytically formed silver atoms on the surface of silver halide for the concentration principle.20 Moisar, Granzer, and coworkers have made a thermodynamic analysis of the processes for latent image formation.21

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4.1.3 Recent Progress on Properties of Silver Halides and Silver Microclusters As described in Chapter 3, recent progress on the properties of silver halides and silver microclusters is remarkable and has made great contributions to clarification of the mechanism of photographic sensitivity. From the viewpoint that latent image centers are composed of silver clusters and are formed through electronic and ionic processes, knowledge of electronic and ionic properties of silver halides, as well as properties of silver microclusters, is very important. The ionic process that works efficiently for latent image formation relies upon the small formation energy and large mobility of interstitial silver ions in silver halide emulsion grains. The formation energy of interstitial silver ions through the surface of silver halides is so small that sufficient interstitial silver ions are present for the ionic process in a silver halide emulsion grain, as was indicated by Hamilton and Brady12 and was recently refined by Slifkin and coworkers.22 The reason for the large mobility of interstitial silver ions was clarified by Friauf and coworkers by the mechanism of movement of interstitial silver ions in silver halides with a small activation energy (i.e., the collinear interstitialcy jump, as shown in Fig. 3.4 in Chapter 3).23 Measurement of ionic conductivity of various silver halide emulsion grains has been extensively made to give useful information on their ionic property by means of the dielectric loss method, which was developed by van Biesen.24 The electronic process that works efficiently for latent image formation relies upon the large formation yield and large mobility of photoelectrons. The former arises from the small binding energy of excitons, and the latter from the small depth of charged centers as well as from the small effective mass for electrons.25'26 Extensive studies on the self-trapped state for positive holes have provided another explanation for the small mobility of positive holes with respect to photoelectrons in silver halides.7'27 Detailed mechanisms of electron-trapping processes by shallow and deep charged centers have been studied by Eachus and others in doped metal ions in silver halides.27 This quantitative knowledge has recently been presented on the number and trap depth of sensitization centers on silver halide emulsion grains.28'29 It is noted that the estimated values for the trap depth of sulfur sensitization centers and sulfur-plus-gold-sensitized centers on AgBr emulsion grains were 0.33 and 0.19 eV, respectively, and that the number of sulfur sensitization centers per grain is very large (e.g., 2800//mi2 on cubic AgBr grains). This knowledge of electron-trapping processes and the nature of the centers should be taken into account for understanding the nucleation and growth processes for latent image formation. On the basis of recognition that the smallest latent image center is composed of three atoms on sulfur-plus-gold-sensitized emulsion grains and four to five atoms on sulfur-sensitized ones, as is described in Section (4.3) in this chapter, extensive studies have been made on the peculiar properties and behavior of these small silver clusters in relation to their photographic effects. On the basis of the electronic structure determined by molecular orbital calculation, redox potential, and the absorption spectra of sensitization and image centers, comprehensive knowledge has been accumulated recently on the dependence of the property and behavior of such small silver clusters upon their size (i.e., the quantum size effect) and upon the sites at

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which they are formed (i.e., the site effect), as described in Section (4.3) in this chapter. It has been proposed that the quantum size effect provides small and developable silver clusters as latent image centers and that the site effect provides Ag2 acting as an electron trap (i.e., P center) and Ag2 acting as a positive hole trap (i.e., R center), both of which play important roles in latent image formation. Along with progress in understanding the ionic and electronic properties of silver halides and silver microclusters, we have learned that dispersion of image centers and recombination between photoelectrons and positive holes are the major factors limiting photographic sensitivity,30 as described in Chapter 8. In the following sections, some detailed descriptions are given of the electronic and ionic processes of silver halide emulsion grains in relation to the dispersion of image centers, and of the properties and behavior of silver microclusters as sensitization and image centers in relation to recombination between photoelectrons and positive holes.

4.2 Electronic and Ionic Processes in Relation to the Concentration Principle As discussed in the previous section, the alternate occurrence of the electronic and ionic processes at the same site leads to the concentration principle. Understanding and optimization of the electronic and ionic processes in silver halide emulsion grains are of great importance in photographic science and engineering. As described in Chapter 3, crystal defects and electron-trapping sensitization centers, such as sulfur sensitization centers and sulfur-plus-gold sensitization centers, provide electron traps suitable for the formation of latent image centers on silver halide grains. Many electron-trapping sensitization centers are formed on a silver halide grain by sulfur sensitization and sulfur-plus-gold sensitization,29 as described in Section (3.4) in Chapter 3. In the presence of many electron-trapping sensitization centers, nucleation and growth processes are important for understanding the formation of latent image centers. Nucleation of latent image formation is completed by formation of a latent subimage center at an electron-trapping sensitization center. By taking into account the facts that a latent preimage center is unstable and that a latent subimage is stable and composed of a dimer of silver atoms, it is considered that the rate of the nucleation is proportional to the square of the concentration of photoelectrons (i.e., proportional to the square of light intensity), and is therefore larger for high intensity and short exposure than for low intensity and long exposure under the condition with constant exposure. For the efficient growth of the nucleus (i.e., a latent subimage center), the growing nucleus should capture subsequent free electrons in competition with many other sensitization centers. Although electron-trapping sensitization centers enhance the nucleation process, they disturb the growth process, decreasing the rate for a subsequent electron to reach the nucleus by decreasing the mobility and diffusion length of the electron and increasing the probability for the subsequent free electron to take part in the formation of another competing nucleus. Thus, optimization of the elec-

PHOTOGRAPHIC SENSITIVITY

Iron-trapping ability of sensitization centers might be highly desirable for efficient latent image formation. In accord with this consideration, it is usually observed that photographic sensitivity increases, reaches a maximum, and then decreases by increasing the number of electron-trapping sensitization centers. For example, Fig. 4.2 shows the surface and internal sensitivities of sulfur-sensitized tabular AgBr grains as a function of the amount of the sensitizer.31 It has been described by Farnell32 and is now widely observed that the sensitivity of silver halide emulsion grains to blue light (inherent sensitivity) is proportional to their volume only under the condition that the grain size does not exceed a certain value. Since the absorbance of a silver halide emulsion grain in the blue region is weak and nearly proportional to its volume, the inherent sensitivity of an emulsion grain should be proportional to its volume such that the diffusion length of photoelectrons is large enough to allow them to reach a growing nucleus in the grain. This value is thus thought to be related to the diffusion length of photoelectrons.32"34 Figure 4.3 shows the comparative size dependence of inherent sensitivities of chemically unsensitized and sulfur-plus-gold-sensitized octahedral AgBr grains.35 As seen here, deviation of the sensitivity from its proportionality to grain volume appeared in sulfur-plus-gold-sensitized grains at a smaller size than in unsensitized grains,

Fig. 4.2. Surface sensitivity (a) and internal sensitivity (b) of sulfur-sensitized tabular AgBr emulsion grains with a 10 sec exposure as a function of the amount of sodium thiosulfate used for sulfur sensitization.31 The diameter of the circle whose area was equal to the average projection area of the grains was 1.1 /am.

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Fig. 4.3. Size dependence of photographic sensitivities (S) of unsensitized (O) and sulfurplus-gold-sensitized (•) octahedral AgBr emulsion grains, where d is the diameter of the circle whose area was equal to the average projection area of the grains. (Courtesy of S. Nishiyama.35)

indicating that the diffusion length of photoelectrons was shorter in the former than in the latter. The deviation from the reciprocity law (i.e., reciprocity law failure) reflects the condition of electron-trapping in emulsion grains.36 Photographic sensitivity (S) is usually expressed by the reciprocal of the exposure required to make a certain proportion of emulsion grains developable, and exposure (E) is the product between light intensity (I) and exposure time (t). The reciprocity law means that S is solely given by 1/E, regardless of I and t. Failure of the reciprocity law is usually observed in the ranges of high I with short t [i.e., high-intensity reciprocity law failure (HIRF)] and/or low I with long t [i.e., low-intensity reciprocity law failure (LIRF)]. Studies on LIRF and HIRF have been made by many groups of workers.4'36 It is accepted that LIRF arises from inefficiency in formation of the nucleus due to instability of a latent preimage, and that HIRF arises from inefficiency of latent image formation due to dispersion of the nuclei (i.e., formation of more than one image center per grain). Hamilton and Hailstone have proposed that dispersed image centers are likely to enhance recombination of trapped electrons with positive holes.17(h)-37 Thus, it is generally observed that LIRF could be increased and HIRF decreased by increasing the number of electron-trapping sensitization centers. For example, increase in the number of sulfur sensitization centers usually results in improvement in LIRF and deterioration of HIRF.36 Farnell examined the photographic sensitivity of emulsion grains as a function of the density of crystal defects, and found that the grains with few crystal defects as well as those with too many crystal defects suffered from low efficiency for latent

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image formation, and grains with a medium number of defects showed the highest sensitivity.38 All the above-mentioned results indicate that the optimization of electron traps on emulsion grains is of great importance producing for the most efficient latent image formation. The ionic process is another important component of the concentration principle. As shown in Fig. 4.4, Takada prepared silver halide emulsion grains with variations of ionic conductivity (i.e., variation of concentration of interstitial silver ions) by changing their halide composition and crystal habit. He then sulfur-sensitized the grains and examined the degree of their HIRF.39 It was observed that the smallest degree of HIRF was observed in the grains with medium ionic conductivity, and that severe HIRF was observed in the grains with low ionic conductivity, such as AgCl grains, as well as in grains with high conductivity, such as octahedral AgBrI grains (Fig. 4.5). In the case of the grains with low ionic conductivity, it is considered that electron trapping by a sensitization center cannot be followed rapidly enough by the ionic process for the center to be reset for trapping a subsequent electron, which allows the formation of a new nucleus on exposure to high-intensity light. On the other hand, an excessive concentration of interstitial silver ions is thought to enhance the

Fig. 4.4. Dependence of ionic conductivity (ai) upon halide composition and crystal habit of silver halide emulsion grains.39 The diameter of the circle whose area was equal to the average projection area of the grains studied was 0.2 /am.

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Fig. 4.5. Effect of ionic conductivity on the degree of HIRF as expressed by the difference between sensitivities with exposures for 10~4 sec [S(l(T4s)] and for 1 sec [S(ls)] for the emulsions described in Fig. 4.4.39 The degree of sulfur sensitization of each emulsion was optimized for a 1 sec exposure.

ionic process and thus enhance the nucleation process of latent image formation. Since the rate of nucleation is proportional to the square of light intensity, this enhancement of nucleation is more significant for high-intensity exposures than for the low-intensity exposures, bringing about dispersion of image centers on exposure to high-intensity light. As described in Chapter 3, adsorption of cyanine dyes to cubic silver halide emulsion grains increased the ionic conductivity and concentration of interstitial silver ions in the grains.40 Tani and Ihama found that cyanine dyes brought about desensitization due to enhancement of HIRF (i.e., dispersion of image centers) on sulfur-sensitized cubic AgBr grains, and attributed it to the increase in concentration of interstitial silver ions caused by adsorption of the dyes onto the grains.41 As stated above, various sources of knowledge have been accumulated to indicate that optimization of the electronic and ionic processes is of great importance for efficient latent image formation from the viewpoint of the concentration principle. 4.3 Silver Microclusters as Latent Image Centers 4.3.1 Size of Latent Image Centers Many attempts have been made to determine the size of the smallest latent image centers. Analysis of characteristic curves gives some indication of the size of the

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smallest latent image centers.42^*5 Hamilton and Logel, by analyzing the ability of evaporated metal clusters to initiate physical development, estimated that the smallest latent image center is composed of four silver atoms or two gold atoms.46 One of the most important attempts to estimate the size of the minimum latent image center was to determine and analyze the quantum sensitivity (i.e., the number of absorbed photons per grain required to make half of the grains developable). Farnell and Chanter were the first to measure the quantum sensitivity of various emulsion grains with high sensitivity.47 Marriage then analyzed their data from a statistical viewpoint.48 It was found that the most sensitive grains were made developable by three absorbed photons per grain. Hydrogen hypersensitization was invented by Babcock et al.49 and enabled the most efficient latent image formation thus far on fine AgBr grains in combination with sulfur-plus-gold sensitization. Babcock and James found that only two to three absorbed photons per grain were required to make more than half of the grains developable.50 By taking into account the fact that an absorbed photon can create at most two free electrons in a grain in the presence of hydrogen hypersensitization centers or reduction sensitization centers, as detailed later in this section and in Section (3.4) in Chapter 3, their data indicate that the smallest latent image center was composed of about three atoms of silver and gold. Computer simulation for latent image formation on the basis of sensitometric data was helpful for estimation of the size of the smallest latent image centers. Hailstone and Hamilton thus estimated that the smallest latent image center was composed of three atoms on sulfurplus-gold-sensitized AgBr grains and of four to five atoms on sulfur-sensitized grains.17(bMd) Recently, Fayet et al. measured the developability of silver cluster ions that were selected by size and deposited on AgBr grains in a mass spectrometer.51 According to their measurements, the smallest developable silver cluster ion was found to be Ag^J". There might, however, be some concern about the soft landing of these cluster ions on AgBr grains, and further experiments are desirable to confirm these results. Since latent image centers are composed of silver clusters, the properties of silver clusters are of great importance. Konstantinov et al. used vapor deposition on carbon to prepare silver clusters whose sizes were several tens of angstroms.52 They determined the size dependence of the redox potential of the clusters by keeping the clusters in equilibrium with redox buffer solutions, containing silver ions, and by observing the change in size distribution of the deposited clusters after treatment with the solutions in an electron microscope. They observed that the clusters whose size was in equilibrium with a redox buffer solution (i.e., the equilibrium size) was unchanged. The clusters that were smaller than the equilibrium size were oxidized and decreased in size by releasing silver ions to the solutions, and the clusters that were larger than the equilibrium size grew by reducing silver ions to silver atoms in the solutions and adsorbing them (i.e., by initiating physical development). This result indicated that the electronic energy levels of the clusters studied were continuously arranged; that is, the lowest unoccupied electronic energy level was situated just above the highest occupied level. The results they obtained are summarized in Fig. 4.6. Note that the smallest cluster studied still contained several hundreds of silver atoms. The redox potential of silver clusters was more negative than that of

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Fig. 4.6. Size dependence of the redox potential of silver clusters, where AE is the redox potential of silver clusters with respect to that of a large crystal of silver, and d is the diameter of the corresponding clusters.52 bulk silver, and its deviation (AE) was proportional to the reciprocal of the cluster diameter (d), obeying the Gibbs-Thompson equation. The values of AE extrapolated to the diameter of Ag4 and Ag7 are as negative as ca. -380 and -340 mV, respectively, and are comparable to the redox potentials of most active developers.53 This is not consistent with the fact that the smallest latent image center is composed of a cluster with four to five silver atoms, since the redox potentials of active developers should be much more negative than that of the redox potential of the smallest latent image center to produce rapid initiation of development. The values of AE for the oxidation (i.e., the oxidation potential) of the smallest latent image centers for chemical development were experimentally estimated to be -185 mV by Cramp and Hillson54 and around -100 mV by Hillson.53 The value of AE of the smallest latent image centers for physical development was determined to be 0 mV by Pontius et al.55 It is clear that the experimentally observed oxidation potential of the smallest latent image centers was considerably less negative than that of the smallest latent image centers estimated by extrapolation from clusters in Fig. 4.6 on the basis of the Gibbs-Thompson equation. It is thus clear that the size of latent image centers is too small to be treated by this method for large clusters, as shown in Fig. 4.6, and should be treated by other methods suitable for much smaller clusters. The molecular orbital method and the shell model are in particular considered to be useful, since the former has already

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been used successfully for study of the electronic structures of small metal clusters as well as for conjugated organic molecules, and the latter for study of the electronic structure of metal microclusters.56'57 Baetzold has made extensive studies of the electronic structures of metal clusters by molecular orbital methods, and has applied his results to the mechanism of latent image formation.58'59 In order to determine the general tendency of the electronic structure of small metal clusters, the electronic energy levels of linearly arrayed silver clusters were calculated by the Hueckel-approximation molecular orbital method and are shown in Fig. 4.7.2 The electronic energy levels of the clusters are discrete, and, in particular, the highest occupied molecular orbital (HOMO) of a dimer is lower in energy than larger clusters. It is known that for molecules of interest to photography the electronic energy level of HOMO and the lowest unoccupied molecular orbital (LUMO) (eHO and eLU, respectively) are related to oxidation and reduction potentials (Eox and ER, respectively) as follows:60

It is thus expected that Eox of Ag2 is more positive than those of larger silver clusters. Another feature of the clusters from the viewpoint of molecular orbital methods is that the electronic structure of a cluster with an even number of silver atoms is quite different from that of a cluster with an odd number of silver atoms, and this is experimentally observed as the odd-even alternation.61'62 A cluster with an odd number of atoms is considered to be reactive and less stable than a cluster with an even number of electrons. It seems, however, that the odd-even alternation is overemphasized in Fig. 4.7 in explaining the growth of image centers. Figure 4.8 illustrates the electronic energy levels of silver clusters as expressed by the shell model. According to this model, each cluster is spherical in shape, and has discrete electronic energy levels, such as the Is, Ip, and Id shells, and so on. According to this model, one of the most important and experimentally confirmed

Fig. 4.7. Schematic representation of electronic energy levels of silver clusters in a linear structure as calculated by the Huekel-approximation molecular orbital method.2

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Fig. 4.8. Schematic representation of the electronic energy levels of Ag3 as expressed by the shell model.

characteristics of metal microclusters is the magic number, which is the number of valence electrons at the closed shell configurations. Thus, the clusters with a closed shell, such as Ag2 with a closed Is shell; Ag8 with closed Is and lp shells; and Ag18 with closed Is, lp, and Id shells, are more stable and less reactive than corresponding clusters with an open shell. Experimental confirmation of the stability of metal clusters with a magic number of valence electrons and of the anomaly of ionization energy of the clusters with a magic number of silver atoms has been achieved.57'61'63 These models predict the characteristics of molecules or microclusters of silver atoms as image centers to be as follows: 1. Electronic energy levels of image centers are discrete. This feature contributes to the stability of small centers. 2. While a single silver atom is unstable, Ag2 is stable. This feature provides the threshold for latent image formation. A silver dimer can also contribute to photographic sensitivity in various ways. 3. It is suggested that clusters with an open shell or clusters with an odd number of electrons have deep electron-acceptinng levels suitable for the initiation of development. 4. Since image centers are very small, it is quite probable that their properties and behavior are influenced by the sites where they are formed.

Properties 1-3 are called the quantum-size effect, and property 4 is called the site effect in this book. 4.3.2 Quantum Size Effect and Site Effect in Relation to Recombination Processes Spencer and coworkers treated emulsion layers composed of reduction-sensitized and exposed AgBr grains with gold latensification and then with arrested development in order to observe, by an electron microscope, the silver specks grown from reduction sensitization centers and latent image centers.64 As shown in Fig. 4.9, they observed two kinds of specks: large and small. As shown in Fig. 4.10, only small

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Fig. 4.9. Electron micrograph of reduction-sensitized AgBr grains exposed for 4 sec and bathed in a solution for gold latensification before arrested development. Large and small spots are the silver specks that grew from latent image centers and reduction sensitization centers during arrested development.64 (Courtesy of Prof. H. E. Spencer.)

specks were present before exposure, and they disappeared during exposure to light. On the other hand, large clusters, which were absent before exposure, appeared during exposure to light. The large specks were ascribed to latent image centers, which grew by capturing photoelectrons during exposure to light. On the other hand, the small specks were ascribed to reduction sensitization centers and disappeared by capturing positive holes during exposure to light. Thus, there were two types of silver clusters: One of them trapped a photoelectron and grew, and the other trapped a positive hole, was bleached and disappeared during exposure to light. By observing the interaction of reduction sensitization centers with electrontrapping and hole-trapping dyes, Tani classified these centers into two groups: elec-

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Fig. 4.10. Variation with time of exposure of the average number of specks per grain of a reduction-sensitized emulsion bathed in a solution for gold latensification before arrested development.64

tron trapping and hole trapping.65 He observed that the electron-trapping centers increased the surface sensitivity of the AgBr grains, which was otherwise in competition with internal sensitivity, and that hole-trapping centers increased the surface sensitivity of the AgBr grains, which was otherwise reduced by recombination between photoelectrons and positive holes. Tani and Takada also confirmed that there were two kinds of reduction sensitization centers on AgBr emulsion grains: One decreased the photoconductivity of the grains (i.e., electron traps) and the other did not (i.e., positive hole traps).66 It was also confirmed by Moisar and coworkers that hole-trapping reduction sensitization centers increased both surface and internal sensitivities, while electron-trapping centers increased surface sensitivity and decreased internal sensitivity.67 Several proposals were made to explain the presence of the two kinds of silver clusters. Tani has proposed that an electron-trapping silver cluster is formed at a positively charged site, while a hole-trapping center is formed at an electrically neutral site,65(b) and he also obtained the evidence for this proposal. One piece of evidence was observation of the effect of charged dye molecules on the developability of the two kinds of centers.68 Spencer proposed that a hole-trapping silver cluster is formed at a negatively charged site.69 On the basis of a thermodynamic analysis of silver clusters, Moisar proposed that a small silver cluster is a holetrapping center, while a large cluster is an electron-trapping center.67 Following Tani's proposal and applying a molecular orbital method to models of Ag2 on AgBr, as shown in Fig. 4.11, Hamilton and Baetzold have proposed that Ag2 at a positively charged kink site is an electron trap, while Ag2 at a neutral kink site is a positive hole trap.70 Electron-trapping silver clusters, which are mostly formed during exposure to light, are called P centers, and hole-trapping silver clusters, which are usually formed by reduction sensitization, are called R centers. Tani has made further studies on the properties of silver clusters formed during

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Fig. 4.11. Models for the calculation of the electronic energy levels of Ag2 at a positively charged kink site (i.e., P center) and Ag2 at a neutral kink site (i.e., R center) on the surface of AgBr and a schematic representation of the electronic energy levels of the centers obtained with respect to that of AgBr.70

reduction sensitization.71 Figure 4.12 shows photoconductivity, photographic sensitivity, and fog density of reduction-sensitized AgBr grains as a function of the amount of a reduction sensitizer. There were three kinds of centers: reduction sensitization centers with and without decreasing photoconductivity of the grains, and fog centers. The centers that did not decrease the photoconductivity were thought to be R centers, and the centers that decreased the photoconductivity were thought to be chemically produced P centers. The absorption spectra of these silver clusters were measured and analyzed for the first time by Murofushi and Tani.72 Figure 4.13 shows diffuse reflection spectra of thick layers of the reduction-sensitized emulsions, indicating that the absorption bands peaked at 474 and 540 nm. Since the band at 474 nm appeared with the amount of reduction sensitizer that was required to form P centers, as seen in Figs. 4.12 and 4.13, it was assigned to P centers. Since the band at 540 nm appeared with the amount of reduction sensitizer that formed fog centers, it was assigned to fog centers. According to the Kubelka-Munk equation, (1 - R)2/2R is proportional to the absorption coefficient, that is, the product of the concentration and molar absorption coefficient of the centers specified, when R is the diffuse reflectance of a thick emulsion layer at the absorption band of the centers.73 As shown in Fig. 4.14, the absorption coefficient of P centers was proportional to the square of the amount of reduction sensitizer, indicating that a P center was Ag2 under the assumptions that the molar absorption coefficient of P centers was fixed and that the following equilibrium is realized:

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Fig. 4.12. Photographic sensitivity (O) with a 10 sec exposure, fog density (X), and photoconductivity (•) of reduction-sensitized octahedral AgBr grains with an average diameter of 0.2 /x,m as a function of the amount of the reduction sensitizer (DMAB, dimethylamine borane).71

It is considered from this condition that an R center was also Ag2, although the absorption band of R centers could not be observed. With a further increase in the amount of reduction sensitizer, the absorption coefficient of P centers was saturated and the absorption coefficient of fog centers increased in proportion to the amount of reduction sensitizer. A P center formed during reduction sensitization is considered to be identical to a latent subimage center, and a fog center is identical to or similar to a latent image center. Figure 4.15 shows the diffuse reflectance spectra of unsensitized emulsions with octahedral AgBr grains of 0.2 fjum diameter, which were exposed to a highpressure mercury lamp in a photochemical reactor for the time durations indicated in the figure. The observed absorption band was similar to that of the fog centers in Fig. 4.13. Each of these exposed emulsions was coated on a triacetyl cellulose film base, dried, and developed. Figure 4.16 shows the optical densities of the developed emulsion layers as a function of exposure time. It is clear that the absorption bands

Fig. 4.13. Diffuse reflection spectra of thick emulsion layers containing reduction-sensitized octahedral AgBr grains with a diameter of 0.2 /am.72 The numbers in this figure correspond to the numbers in Fig. 4.12.

Fig. 4.14. The absorption coefficient of centers as given by the Kubelka-Munk equation as a function of the amount of the reduction sensitizer (DMAB, dimethylamine borane), where R is the diffusive reflectance of a thick emulsion layer and (1 - R)2/2R at 474 and 540 nm give the relative absorption coefficients of P centers and fog centers, respectively.72 The numbers in this figure correspond to the numbers in Figs. 4.12 and 4.13. 100

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Fig. 4.15. Diffuse reflectance spectra of unsensitized emulsions with octahedral AgBr grains of 0.2 /am diameter that were exposed for the time duration indicated in this figure in a photochemical reactor.

in Fig. 4.15 are ascribed to the latent image centers formed in the gradation region of the characteristic curve of the unsensitized grains in Fig. 4.16. This conclusion, however, is judged from the low efficiency of formation (about 5000 photons/grain) and low concentration (I/grain) of the latent image centers on the grains. Further studies are being conducted on the absorption spectra of latent image centers, as well as fog centers and reduction sensitization centers on emulsion grains. These centers in emulsions were kept in equilibrium with redox buffer solutions. The oxidation potential (Eox) of these centers was given by the redox potential of the solution that eliminated half of the sensitizing effect of the centers and could be related to the HOMO energy level OHo) of the centers on the basis of Equation

Fig. 4.16. Optical densities of emulsion layers exposed for the time duration indicated on the abscissa in a photochemical reactor, coated on triacetyl cellulose film bases, developed, fixed, and washed.

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Fig. 4.17. The lower figure shows the residual proportions of P centers and R centers on the surface and R centers in the interior of octahedral AgBr grains with an average diameter of 0.2 /am after keeping them in redox buffer solutions with the redox potential indicated on the abscissa for 17 and 41 hours at 25°C. The residual proportion of the centers was expressed by the residual proportion of the logarithmic sensitivity increase caused by the centers. The oxidation potential of the centers was given by the midpoints of their curves. The upper figure shows the heights of HOMO levels of the centers thus obtained on the basis of Equation (4.1) with respect to the top of the valence band of AgBr (VB), and also schematically shows LUMO levels of the centers with respect to the bottom of the conduction band of AgBr (CB) by taking into account their electron-trapping ability on the grains.71 (4.1).72 As shown in Fig. 4.17, the oxidation potential of P centers was more positive by about 0.4 V than that of R centers, indicating that the HOMO level of P centers was about 0.4 eV lower than that of R centers. The oxidation potential of R centers in the interior of the grains was 0.05 V less positive than that of P centers. Since positive holes had to be injected from the solutions to the valence band of AgBr grains to bleach R centers in the interior of the grains, the oxidation potential of R centers in the interior of the grains is thought to indicate the position of the top of

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the valence band of AgBr grains. The electronic energy levels of those centers obtained are in good accord with those described in Fig. 4.11, supporting the site effect, due to which R centers are positive hole traps and P centers are electron traps. Figure 4.18 shows the electronic energy levels of latent image centers as well as those of reduction sensitization centers on the basis of their oxidation potentials and Equation (4.1). Since the oxidation potential of P centers was more positive than

Fig. 4.18. The lower figure shows the residual proportions of P centers formed during reduction sensitization (•), and latent image centers formed on unsensitized (U; O), sulfursensitized (S; ®), and sulfur-plus-gold-sensitized (S + Au; X) octahedral AgBr emulsion grains with an average diameter of 0.2 /urn after keeping them in redox buffer solutions with the redox potential indicated on the abscissa for 17 hours at 25°C. The residual proportion of the centers was expressed by the residual proportion of the logarithmic sensitivity increase caused by the centers (AS) and the residual density of the exposed and developed emulsion layer. The oxidation potentials of those centers were given by the midpoints of their curves. The upper figure schematically shows the height of HOMO levels of the centers thus estimated on the basis of Equation (4.1) and the height of LUMO levels of the centers with respect to the bottom of the conduction band (CB) of AgBr and the redox potential of a developer by taking into account the electron-trapping ability on the grains and their developability.71

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those of latent image centers, it is considered that the HOMO level of a P center (i.e., Ag2) was lower than that of latent image centers (i.e., Agn; n > 2) on the basis of Equation (4.1). The LUMO level of a P center is considered to be higher than those of latent image centers, since the former cannot initiate development. The electronic energy levels of P centers and latent image centers thus estimated could support the quantum size effect, as shown in Fig. 4.7. It has been found from these analyses that two kinds of silver dimers (R and P centers) contribute through different means to the processes of photographic sensitivity. As to the behavior of R centers, the proposals by Mitchell and Lowe are important. According to Mitchell, the mechanism for photographic sensitivity, holetrapping by an R center, is followed by ionic relaxation (i.e., release of a silver ion into an interstitial position), preventing recombination of the trapped hole at the R center with a photoelectron. Including the above-mentioned process, Lowe proposed the so-called Lowe's hypothesis for the behavior of positive holes in the presence of R centers,74 which is composed of the processes of light absorption of a silver halide emulsion grain,

trapping of a positive hole by an R center,

ionic relaxation of the positive hole-trapping,

and dissociation of a single silver atom and liberation of an extra free electron,

In total,

where Ag2(R) is an R center. Thus, in the presence of R centers, one absorbed photon can produce two free electrons available for latent image formation. Tani has confirmed Lowe's hypothesis for the first time by observing the growth of latent subimage centers after exposure of reduction-sensitized grains to light in the presence of hole-trapping dyes.75 Latent subimage and image centers are formed rapidly. When positive holes migrate on the grain surface in the presence of many sensitizing dyes acting as deep positive hole traps, it takes a long time (e.g., several minutes depending upon the dyes used) for positive holes to bleach latent image centers as well as to bleach R centers to bring about the liberation of extra free electrons available for the growth of latent subimage centers.75'76

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It was then observed that, in the presence of a sensitizing dye, the number of latent image centers decreased after exposure to light in the absence of R centers, while the number of latent image centers increased after exposure to light in the presence of R centers, as shown in Fig. 4.19. The time constants of these two reactions were similar to each other and to the time constant of the decay of Dye"1" as measured by means of an electron paramagnetic spectrometer.76 Thus, positive holes reacted with latent image centers with a time constant of several minute to bring about their fading in the absence of R centers, while positive holes mostly reacted with R centers with a similar time constant to create extra electrons, which were used for the growth of subimage centers to latent image centers. Other evidence for Lowe's hypothesis was given by Hailstone and coworkers,17® who observed that a large fraction of latent image centers could be formed by only

Fig. 4.19. Time-dependent characteristic curves of unsensitized (solid line) and hydrogenhypersensitized (broken line) octahedral AgBr emulsion grains (0.2 /jun) with 3.3'-disulfopropyl-5,5'-dichloro-9-ethyl-thiacarbocyanine adsorbed. The emulsions were exposed for 10"3 sec, were kept for t sec, were immersed in water to deactivate positive holes trapped by the dye, and were developed by MAA-1 surface developer.75'77

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two absorbed photons per grain on hydrogen-hypersensitized and sulfur-plus-gold-sensitized fine AgBr grains (Fig. 4.20), in spite of the fact that the smallest latent image center was estimated to be composed of three atoms. These could not be explained without taking into account the extra electrons created according to Lowe's hypothesis. Verification of Lowe's hypothesis also means verification of Mitchell's proposal as expressed by Equation (4.6). The ionic process in latent image formation and the ionic process following hole-trapping by an R center as proposed by Mitchell are symmetric, as shown in Fig. 4.21, and are important ionic relaxation processes following electronic processes in silver halide to prevent recombination between photoelectrons and positive holes. In summary, the quantum size effect and site effect of silver microclusters are important for silver halide photographic materials to achieve high sensitivity. The quantum size effect provides a stable silver dimer in spite of an unstable single silver atom, giving the threshold for latent image formation. This threshold is of great importance for the stability of silver halide photographic materials, as described in Chapter 1. The discrete energy levels could be the reason for the high stability of a small silver cluster. A cluster with an open shell or with an odd number of valence electrons has a deep electron-accepting level and might be the reason for high developability of a small silver cluster. The site effect provides two kinds of silver dimers: R and P centers. Being assisted by ionic relaxation processes, a P center (i.e., a latent subimage center) captures a photoelectron to grow during exposure to light, while an R center captures

Fig. 4.20. Observed D-log E curve of a hydrogen-hypersensitized and sulfur-plus-gold-sensitized octahedral AgBr emulsion (solid curve) and theoretical curves of the corresponding emulsions in which a latent image center is formed by two and three absorbed photons per grain (broken curves).17'0

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Fig. 4.21. Schematic representation of the mechanism of latent image formation with high efficiency through symmetric ionic relaxation processes, in which an open circle and a closed circle represent a silver ion and a silver atom, respectively. a positive hole to prevent recombination between a photoelectron and a positive hole, and to create an additional electron.

References 1. 2. 3. 4.

S. E. Sheppard, A. P. H. Trivelli, R. P. Loveland, J. Franklin Inst., 200, 51(1925). T. Tani, Phys. Today, September 1989, 36. R. W. Gurney, N. F. Mott, Proc. R. Soc. Land., Ser. A 164, 151(1938). (a) P. C. Burton, W. F. Berg, Photogr. J., 86B, 2(1946); (b) P. C. Burton, Photogr. J., 86B, 62(1946); (c) W. F. Berg, P. C. Burton, Photogr. J., 88B, 84(1948); (d) P. C. Burton, Photogr. J., 88B, 13(1948); (e) P. C. Burton, Photogr. J., 88B, 123(1948); (f) W. F. Berg, Rep. Prog. Phys., 11, 248(1948).

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5. (a) J. W. Mitchell, Recent Progr. Phys., 20,433(1957); (b) J. W. Mitchell, J. Phys. Chem., 66, 2359(1962); (c) J. W. Mitchell, Photogr. Sci. Eng., 22, 249(1978); (d) J. W. Mitchell, Photogr. Sci. Eng., 25, 170(1981). 6. J. F. Hamilton, in The Theory of the Photographic Process, 4th ed., T. H. James ed., Macmillan, New York (1977), pp. 105-132. 7. J. F. Hamilton, Adv. Phys., 37, 359(1988). 8. J. W. Mitchell, J. Photogr. Sci., 5, 49(1957). 9. J. M. Hedges, J. W. Mitchell, Phil. Mag., 44, 223(1953). 10. J. W. Mitchell, J. Soc. Photogr. Sci. Technol. Jpn., 54, 248(1991). 11. J. F. Hamilton, L. E. Brady, J. Appl. Phys., 30, 1893(1959). 12. J. F. Hamilton, L. E. Brady, J. Appl. Phys., 30, 1902(1959). 13. J. F. Hamilton, L. E. Brady, Photogr. Sci. Eng., 8, 189(1964). 14. L. M. Kellogg, J. Hodes, J. Muenter, "Measurement of Photocarrier Decay Times in Silver Halide Emulsion Grains Using Radiofrequency Techniques," in the preprint book of The International East-West Symposium II on Factors Influencing the Efficiency of Photographic Imaging, Kona, Hawaii, October/November, 1988. 15. J. W. Mitchell, J. Soc. Photogr. Sci. Technol. Jpn., 54, 258(1991). 16. (a) J. F. Hamilton, Photogr. Sci. Eng., 14, 102(1970); (b) J. F. Hamilton, Photogr. Sci. Eng., 18, 371(1974). 17. (a) R. K. Hailstone, N. B. Liebert, M. Levy, J. Imaging Sci., 34, 169(1980); (b) R. K. Hailstone, J. F. Hamilton, / Imaging Sci., 29, 125(1985); (c) R. K. Hailstone, N. B. Liebert, M. Levy, J. F. Hamilton, J. Imaging Sci., 31, 185(1987); (d) R. K. Hailstone, N. B. Liebert, M. Levy, J. F. Hamilton, J. Imaging Sci., 31,255(1987); (e) R. K. Hailstone, N. B. Liebert, N. B. Levy, J. F. Hamilton, J. Photogr. Sci., 36, 2(1988); (f) R. K. Hailstone, N. B. Leibert, M. Levy, R. T. McCleary, S. R. Gilolmo, D. L. Jeanmaire, C. R. Boda, J. Imaging Sci., 32, 113(1988); (g) R. K. Hailstone, N. B. Liebert, M. Levy, J. F. Hamilton, /. Imaging Sci., 32, 150(1988); (h) R. K. Hailstone, N. B. Liebert, M. Levy, J. Imaging Sci., 34, 169(1990). 18. F. Seitz, Rev. Mod. Phys., 23, 328(1951). 19. J. F. Hamilton, in The Physics of Latent Image Formation in Silver Halides, A. Balderaschi, W. Czaja, E. Tosatti, M. Tosi, eds., World Scientific, Singapore, 1984, p. 203. 20. J. Malinowski, Photogr. Sci. Eng., 18, 363(1974). 21. E. Moisar, F. Granzer, D. Dantrich, J. Photogr. Sci., 25, 12(1977). 22. (a) R. A. Hudson, G. C. Farlow, L. M. Slifkin, Phys. Rev. B, 36, 4651(1987); (b) R. A. Hudson, S. K. Wonnell, G. C. Farlow, L. M. Slifkin,/ Imaging Sci., 34, 101(1990), (c) S. K. Wonnel, L. M. Slifkin. Phys. Rev., B48, 78(1993). 23. R. J. Friauf, in The Physics of Latent Image Formation in Silver Halides, A. Baldereschi, W. Czaja, E. Tosatti, M. Tosi, eds., World Scientific, Singapore, 1984, p. 79. 24. J. van Biesen, J. Appl. Phys., 41(5) 1910(1970). 25. F. C. Brown, in Treatise on Solid State Chemistry, Vol. 4, B. Hannay, ed., Plenum, New York, 1973, Chapter 10. 26. M. Ueta, H. Kanzaki, K. Kobayashi, Y. Toyozawa, E. Hanamura, Exitonic Processes in Solids, Springer, 1986, Chapter 6. 27. A. P. Marchetti, R. S. Eachus, in Advances in Photochemistry, Vol. 17, D. Volman, G. Hammond, D. Neckers, eds., John Wiley & Sons, New York, 1992, pp. 145-216. 28. (a) J. F. Hamilton, J. M. Harbison, D. L. Jeanmaire, J. Imaging Sci., 32, 17(1988); (b) L. M. Kellogg, J. Hodes,' 'The Measurement of Electron Trap Depths for Sulfur and SulfurPlus-Gold Centers in AgBr Emulsions Using Thermally Stimulated Current (TSC) Techniques," in the preprint book of the 40th Annual Conference of SPSE, Rochester, New York, May, 1987.

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29. (a) H. Kanzaki, Y. Tadakuma, J. Phys Chem. Solids, 55, 631(1994); (b) J. E. Keevert, V. V. Gokhale, J. Imaging Sci., 31, 243(1987). 30. T. Tani, J. Imaging Sci., 29, 93(1985). 31. T. Tani, Photogr. Sci. Eng., 15, 28(1971). 32. G. C. Farnell, J. Photogr. Sci., 17, 117(1969). 33. G. C. Farnell, R. B. Flint, J. B. Chanter, J. Photogr. Sci., 13, 25(1965). 34. R. Matejec, E. Moisar, Photogr. Sci. Eng., 12, 133(1968). 35. T. Tani, J. Imaging Sci., 29, 93(1985). 36. J. F. Hamilton, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, pp. 133-148. 37. J. F. Hamilton, Photogr. Sci. Eng., 26, 263(1982). 38. G. C. Farnell, J. Photogr. Sci., 27, 160(1979). 39. S. Takada, J. Soc. Photogr. Sci. Technol. Jpn., 42, 112(1979). 40. (a) S. Takada, T. Tani, J. Appl. Phys., 45, 4764(1974); (b) T. Tani, S. Takada, Photogr. Sci. Eng., 18, 620(1974). 41. (a) M. Ihama, T. Tani, J. Imaging Sci., 31, 157(1987); (b) T. Tani, M. Ihama, in Progress in Basic Principles of Imaging Systems, Proceedings of the International Congress of Photographic Science, Koeln, 1986, F. Granzer, E. Moisar, eds., Friedr. Vieweg & Sons, Braunschweig/Wiesbaden, 1987, pp. 335-341. 42. (a) L. Silverstein, Phil. Mag., 44, 257, 956(1922); (b) L. Silverstein, Phil. Mag., 45, 1062(1923). 43. A. E. Ames, Photogr. Sci. Eng., 17, 154(1973). 44. R. Shaw, J. Imaging Sci., 21, 25(1973). 45. (a) M. Kawasaki, S. Fujiwara, H. Hada, Photogr. Sci. Eng., 22, 290(1978); (b) H. Hada, M. Kawasaki, H. Fujimoto, Photogr. Sci. Eng., 24, 232(1980); (c) H. Hada, M. Kawasaki, J. Imaging Sci., 29, 51(1985). 46. J. F. Hamilton, P. C. Logel, Photogr. Sci. Eng., 18, 507(1974). 47. G. C. Farnell, J. B. Chanter, J. Photogr. Sci., 9, 73(1961). 48. A. Marriage, J. Photogr. Sci., 9, 93(1961). 49. T. A. Babcock, P. M. Ferguson, W. C. Lewis, T. H. James, Photogr. Sci. Eng., 19, 49(1975). 50. T. A. Babcock, T. H. James, /. Photogr. Sci., 24, 19(1976). 51. P. Fayet, F. Granzer, G. Hegenbart, E. Moisar, B. Pischel, L. Woeste, Phys. Rev. Lett., 55, 3002(1985). 52. (a) I. Konstantinov, A. Panov, J. Malinowski, J. Photogr. Sci., 21, 250(1973); (b) I. Konstantinov, J. Malinowski, J. Photogr. Sci., 23, 1(1975); (c) I. Konstantinov, J. Malinowski, J. Photogr. Sci., 23, 45(1975). 53. P. J. Hillson, Photogr. Sci. Eng., 23, 38(1979). 54. J. H. W. Cramp, P. J. Hillson, J. Photogr. Sci., 24, 25(1976). 55. P. B. Pontius, C. R. Van Der Voorn, R. M. Cole, Photogr. Sci. Eng., 12, 102(1968). 56. M. G. Mayer, J. H. Jensen, Elementary Theory of Nuclear Shell Structure, Wiley, New York, 1955. 57. S. Sugano, Microcluster Physics, Springer-Verlag, Berlin, 1991. 58. R. C. Baetzold, J. Solid State Chem., 6, 352(1973). 59. R. C. Baetzold, Photogr. Sci. Eng., 17, 78(1973). 60. T. Tani, /. Imaging Sci., 34, 143(1990). 61. W. D. Knight, K. Clemenger, W. A. de Heer, W. A. Saunders, M. Y. Chou, M. L. Cohen, Phys. Rev. Lett., 52, 2141(1984). 62. M. Kawasaki, Y. Tsujimura, H. Hada, Phys. Rev. Lett., 57, 2796(1986). 63. (a) I. Katakuse, T. Ichikawa, Y. Fujita, T. Matsuo, T. Sakurai, H. Matsuda, Intern. J. Mass.

110

64. 65. 66. 67.

68. 69. 70. 71.

72.

73.

74. 75. 76. 77.

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Spectrom. Ion Proc., 67, 229(1985); (b) I. Katakuse, in Microdusters, S. Sugano, Y. Nishina, S. Ohnishi, eds., Springer Ser. Mat. Sci., Vol. 4, Springer, Berlin, 1987, p. 10. H. E. Spencer, L. E. Brady, J. F. Hamilton, /. Opt. Soc. Am., 57, 1020(1967). (a) T. Tani, Photogr. Sci. Eng., 15, 28(1971); (b) T. Tani, Photogr. Sci. Eng., 15, 181(1971); (c) T. Tani, Photogr. Sci. Eng., 16, 35(1972). T. Tani, S. Takada, Photogr. Sci. Eng., 26, 111(1982). (a) E. Moisar, F. Granzer, D. Dautrich, E. Palm, J. Photogr. Sci., 25, 12(1977); (b) E. Moisar, E. Palm, F. Granzer, D. Dautrich, J. Photogr. Sci., 25, 19(1977); (c) D. Dautrich, F. Granzer, E. Moisar, E. Palm, J. Photogr. Sci., 25,169(1977); (d) E. Moisar, F. Granzer, D. Dautrich, E. Palm, J. Photogr. Sci., 28, 71(1980). T. Tani, Photogr. Sci. Eng., 18, 569(1974). (a) H. E. Spencer, Photogr. Sci. Eng., 11, 352(1967); (b) H. E. Spencer, J. Photogr. Sci., 20, 143(1972). J. F. Hamilton, R. C. Baetzold, Photogr. Sci. Eng., 25, 189(1981). T. Tani, "Formation and Characterization of Reduction Sensitization Centers," in the preprint book of The Autumn Meeting of Soc. Photogr. Sci. Technol. Jpn., Nov., 1991, Kyoto. (a) M. Nakashima, T. Tani, "Observation and Analysis of Absorption Spectra of Reduction Sensitization Centers and Latent Image Centers in Photographic Emulsion," in the preprint book of The Annual Meeting of Soc. Photogr. Sci. Technol. Jpn., May, 1992, Tokyo; (b) T. Tani, M. Murofushi, J. Imaging Sci. Technol., 38, 1(1994). A. H. Herz, R. P. Danner, G. A. Janusonis, in Adsorption from Aqueous Solution (Advanced Chemistry), Series 79 (American Chemical Society, Washington, DC, 1968), p. 173. J. M. Harbison, H. E. Spencer, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, p. 152. T. Tani, /. Imaging Sci., 30, 41(1986). (a) T. Tani, Photogr. Sci. Eng., 19, 356(1975); (b) T. Tani, J. Appl. Phys., 62, 2456(1987); (c) T. Tani, Y. Sano, /. Appl. Phys., 69, 4391(1991). T. Tani, Photogr. Sci. Eng., 26, 213(1982).

5 Spectral Sensitization

5.1 Introduction As described in Chapter 1, the ability to form color images is one of the most important properties of photosensitive materials. In order to form color images, photosensitive materials are required to be sensitive to the three primary colors, that is, blue, green, and red. As described in Chapter 3, silver halide itself absorbs ultraviolet and blue light, and is thus sensitive only to blue light in the visible region. Spectral sensitization was discovered by Vogel1 in 1873 and is now widely used to render silver halides sensitive to wavelengths longer than that of blue light, such as green, red, and infrared, by use of sensitizing dyes that absorb light in these spectral regions.2 In this chapter the sensitivities of the excitation of silver halides and sensitizing dyes are called intrinsic sensitivity and spectral sensitivity, respectively. The former and the latter are also called blue sensitivity and minus-blue sensitivity, respectively, in the case of AgBr and AgBrl.

5.2 Sensitizing Dyes As described in Chapter 4, photoelectrons initiate latent image formation in silver halide grains. After many debates by many groups of workers over the electron transfer versus energy transfer mechanism,2 it is now accepted3 that spectral sensitization in silver halide photographic materials takes place according to the electron transfer mechanism proposed by Gurney and Mott4 (i.e., the transfer of electrons from optically excited dye molecules to the conduction band of silver halides). Sensitizing dyes are thus required to have the abilities to absorb light at a desired wavelength and to transfer electrons to the conduction band of silver halides from their excited states. Comprehensive reviews of sensitizing dyes have been published by several authors.5"7 The essential structure of sensitizing dyes is the polymethine chain, with an odd number of methine groups and hetero atoms at its ends. As shown in Fig. 5.1, there are three major systems, that is, amidinium, carboxyl ion, and amidic systems, which are exemplified by Groups (1), (2), and (3), respectively, in Table 111

Fig. 5.1. Three major chromophore systems available for photographic sensitizing dyes: (a) amidinium-ion system, (b) carboxyl-ion system, and (c) amidic system.

Table 5.1.

Absorptions of vinylogous dyes5

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113

g j 5,8-11 Qroup (1) js an exampie of cyanine dyes, among which the dyes with an n of 0, 1,2, and 3 are denoted cyanine, carbocyanine, dicarbocyanine, and tricarbocyanine dyes, respectively. Group (2) is an example of oxonol dyes, and Group (3) is an example of merocyanine dyes. The compounds in Groups (l)-(3) contrast with those of Groups (4) and (5), which are not sensitizing dyes, in that the absorption band of compounds in the former groups are stronger and appear at a longer wavelength than those of the latter groups in terms of their chain length. Table 5.2 shows the molecular structure of a variety of dyes studied in this chapter. Molecular structures of sensitizing dyes were studied by X-ray crystallography.5'12"18 Most typical sensitizing dyes were nearly planar, and the bond lengths agreed with those observed for simpler molecules and with theoretical bond orders. Steric effects were observed for some cyanine dyes. Highly twisted structures were observed by X-ray analysis of simple cyanine dyes including Dye I.17 The distance between the two nonbonded sulfur atoms in Dye 5 was short enough to suggest an attractive interaction between them.18 Several theoretical approaches can predict the absorption wavelengths of dyes,5 including the free electron model19 and the molecular orbital (MO) methods.20"22 The free electron model treats TT electrons in a dye molecule as free electrons in a one-dimensional potential well and successfully predicts the absorption wavelengths of cyanine dyes. The systematic examination, using the MO method, of the electronic transitions of various kinds of cyanine dyes was made by Tani and Kikuchi by means of the Hueckel molecular orbital (HMO) method.20 The wavelength of the absorption maximum (Amax) of a dye is related to the transition energy (ETR) from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), as described below:

where eHo and £LU are the energy levels of HOMO and LUMO, respectively. Figure 5.2 shows an example of the correlation between the calculated and observed values of ETR for thiacyanine and 2.2'-quinocyanine dyes with variation in polymethine chain length. The agreement between them is fairly good, except for monomethine dyes with steric effects,17'18 which will be analyzed in Section 5.4 in this chapter. Some MO methods with higher order approximations were also used to calculate the electronic structures of sensitizing dyes.5 Figure 5.3 shows the Tr-electron density distribution in the frontier molecular orbitals (i.e., HOMO and LUMO) of Dye 6 as calculated by the HMO method. As seen in this figure, it-electron density is delocalized within the polymethine chain with nitrogen atoms at the ends. Since the frontier orbitals predominate the electronic transition of sensitizing dyes, as described by Equations (5.1) and (5.2), the result in Fig. 5.3 predicts that the polymethine chain is the essential part of the molecular structure of sensitizing dyes. It is noted that a ir-electron in HOMO is localized on odd atoms from the ends of the polymethine chain and a 7r-electron in LUMO on

Table 5.2.

Dyes used in this chapter

114

Table 5.2.

Dyes used in this chapter (Continued)

Table 5.2.

Dyes used in this chapter (Continued)

116

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117

Fig. 5.2. Relation between observed and calculated transition energies (ETR) on the basis of Equations (5.1) and (5.2),21(* where p is the resonance integral between 2p orbitals of two adjacent carbon atoms in a conjugated system. The numbers in this figure refer to the dyes listed in Table 5.2, and • and X are 2.2'-quinocyanine and thiacyanine dye series, respectively.

even atoms. It is usual to attach substituents to sensitizing dye molecules to modify their properties. The change in eHO and £LU caused by substituents is proportional to the electron density of HOMO and LUMO on the atom to which a substituent is attached.23 For example, the attachment of an electron-withdrawing substituent to an odd atom in the polymethine chain mainly lowers eHO and thus increases ETR, and the attachment of an electron-withdrawing substituent to an even atom in the polymethine chain mainly lowers eLU and thus decreases ETR according to Equation (5.1).5'24 The value of ETR of a molecule is usually a function of both the electrostatic dielectric constant (D) and the optical dielectric constant (n2, where n is the refractive index) of its surrounding according to the theory of solvent effects of absorption spectra, as described by MacRae's equation.25 However, it was found that ETO of symmetrical cyanine dyes in solutions,26 and even on the surface and in the interior of silver halide,27 was a linear function of only (n2 - l)/(2n2 + 1) of solvents and silver halide. This result indicates that the shift of ETR resulted solely from electronic polarization of the solvents and silver halide for the excited dye molecules and that

118

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Fig. S.3. Electron clouds of LUMO and HOMO of Dye 6 as calculated by the HMO method. The area of the circle represents the electron density of the molecular orbital at each atom. The open and closed circles represent the difference in sign of each atomic orbital in LUMO and HOMO. the interaction of dye molecules with solvents and silver halide was based on the van der Waals force. For merocyanine dyes, this general shift was also expected, but it was usually superimposed on other, more powerful effects involving dyesolvent permanent dipoles and hydrogen bonding.5 Many cyanine dyes form aggregates in aqueous solutions, and especially in silver halide emulsions, producing large spectral shifts and distinct changes in band shape. Figure 5.4 shows the absorption spectrum of Dye 11 in methanol and those of Dye 11 in photographic emulsions composed of octahedral AgBr grains that were agitated at 40°C and 70°C for 20 min, respectively. The dye showed an M-band corresponding to its monomeric state in methanol and a sharp and bathochromatically shifted J-band corresponding to its J-aggregate.28'29 The J-band became sharper, indicating the increase in aggregate size with increasing agitation temperature. J-bands are especially important for color photography, since they make photographic emulsions sensitive to specialized narrow wavelength regions. It is also known that some cyanine dyes showed a hypsochromatically shifted H-band corresponding to their H-aggregates.5 Several structural models have been proposed for dye aggregates.5'13'30'31 However, it is generally accepted that the angle between the long axis of a dye molecule and that of an aggregate (a) is most important, and that J-aggregates have a small a, while H-aggregates have a large a. On the basis of the molecular exciton theory,32"34 a description is given of the transition energy of an aggregate composed of

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119

Fig. 5.4. Absorption spectra of Dye 11 in methanol (a) and in octahedral AgBr emulsions, which were agitated for 20 min at 40°C (b) and at 70°C (c). The diffuse reflectance spectra of emulsions were treated by Equation (5.9) to give the absorption spectra of J-aggregates of Dye 11.

molecules 1 and 2 (ETR) with respect to that of a monomer (E^)- The Hamiltonian for this system (H) is given as follows:

where H1; H2, and H12 are the Hamiltonians for molecules 1 and 2, and the interaction between them. The wavefunctions of the aggregate in the ground state (<J>G) and in the excited state (<&+_) are given as follows:

The energy of the aggregate in the ground state (EG) is then given as follows:

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PHOTOGRAPHIC SENSITIVITY

where the first term is the energy of molecules 1 and 2 in the ground state, and the second term is the energy of the interaction between them. The energies of the aggregate in the excited state (E + _) are given as follows:

where the first and second terms are the energies of molecules in the excited state and in the ground state, respectively, and the third and fourth terms are the energies of the interaction between molecules 1 and 2. The transition probability of the aggregate is determined by the transition moment (M+_) as follows:

Figure 5.5 shows the electronic transitions of a monomer and an aggregate as a function of a according to Equations (5.6)~(5.8). Since ($i$ 2 ^i2\*4>2 H ^ l ^ f ^ a ) are small and nearly equal to each other, ((p*(p2\U-i2\(t>i(l)2.) is the main cause for the shift of E + _ from EG. According to Equation (5.8), the transitions to the excited states of aggregates with transition dipoles in parallel and opposite directions give M and 0 as M+ and M_, respectively, and are therefore allowed and forbidden, respectively. The energy of the allowed transition (i.e.,

Fig. 5.5. Schematic representation of the electronic transitions in a monomer, H-aggregate, and J-aggregate on the basis of the interaction between molecular excitons in aggregates, where a is the slip angle between molecules in an aggregate, and ETR(M), E TR (H), and E TR (J) are the transition energies of a monomer, H-aggregate, and J-aggregate, respectively.

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121

Fig. 5.6. Absorption spectra of Dye 14 in the monomeric state in a gelatin layer (a) and in its aggregates with herringbone structure giving a double band in an emulsion layer containing cubic AgBr grains with an edge length of 0.65 /jan and Dye 14 of 0.23 mmole/mole AgBr (b). E+ - EG) is larger than s* for the aggregate with large a (i.e., H-aggregate), and smaller than e* for the aggregate with small a (i.e., J-aggregate). There is another important aggregate that gives a so-called double band with two absorption peaks,30'35 as shown in Fig. 5.6. As shown in Fig. 5.5, both J- and H-aggregates have a molecular arrangement with one molecule in a unit cell, and are composed of an allowed transition and a forbidden transition. On the other hand, the aggregate that gives the double band has a molecular arrangement with two molecules in a unit cell and is composed of two allowed transitions. A herringbone structure, as shown in Fig. 5.6, has been proposed for the molecular arrangement of the aggregate that gives the double band.30'35 It is known that 9-methyl-thiacarbocyanine dye derivatives, such as Dyes 14 and 15, adsorbed on the (100) face of silver halide exhibit the double band. Brooker classified the molecular structures of cyanine dyes into three groups, that is, loose, compact, and crowded, and found that compact cyanine dyes were the most likely to form J-aggregates.36 Several groups of workers have observed the

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PHOTOGRAPHIC SENSITIVITY

general tendency that J-aggregates are formed by cyanine dyes with large substituents, which prevents molecules from being piled up with a large slip angle.37 This observation supports the explanation of the relation between molecular arrangements and absorption spectra of aggregates on the basis of molecular exciton theory, as described in Fig. 5.5 and Equations (5.3)-(5.8). Direct evidence for the relation between the molecular arrangements and absorption spectra of dyes could be obtained by measurements of X-ray diffraction analyses and microscopic reflection spectra of single crystals of cyanine dyes.13'15'16 It was confirmed by Tanaka and others that a molecular arrangement with a small slip angle in a single crystal of Dye 39 gave a sharp J-band and that the direction of the transition moment of the J-band coincided with that of the transition moment of each molecule,38 in agreement with the molecular exciton theory.32"34 Maskasky formed large J-aggregates of cyanine dyes on tabular AgBr grains, and cubic AgBr and AgCl grains, in emulsions; directly observed them by means of polarized fluorescence microscopy; and determined the molecular arrangements of J-aggregates of many cyanine dyes on those grains.39 He also confirmed the herringbone structure in the aggregate of a 9-methyl-thiacarbocyanine dye on the (111) face of AgBr. Saijo, Shiojiri, and others are trying to observe each J-aggregate on silver halide grains in emulsions by means of analytical color fluorescence electron microscopy.40 Haefke and others have reported that they observed islands of a merocyanine dye evaporated on a thin silver halide layer,41(a) and merocyanine dye molecules preferentially adsorbed to the steps on a thin AgBr layer.41(b) Jiang et al. have reported that they observed each molecule in aggregates of cyanine dyes on the surface of graphite by means of scanning tunneling microscopy.42

5.3 Adsorption of Sensitizing Dyes to Silver Halide Grains Since spectral sensitization takes place as a result of electron transfer from excited dyes to the conduction band of silver halide, knowledge of the adsorption of sensitizing dyes to silver halide grains and the interaction between them is of great importance for understanding the mechanism of spectral sensitization. Adsorption isotherms and heats of adsorption have been measured for various sensitizing dyes and silver halide grains by many groups of workers.43'44 The amount of adsorbed dye and its adsorption isotherm to silver halide grains have been measured by a conventional centrifugal method and an optical method that has been developed by Herz and coworkers on the basis of diffuse reflectance spectroscopy of dyed emulsions with the aid of the Kubelka-Munk equation.45 Reflectance (R) is measured under the condition in which an emulsion is too thick for incident light to transmit, and is related to the concentration and molar absorption coefficient of the dye in a specified state (c and s, respectively) by the following equation:

where K and S are absorption and scatter coefficients of the substrate, respectively. This technique could be applied to both liquid emulsions and dried emulsion layers.46

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123

Although it is known that the adsorptivity of sensitizing dyes to silver halide grains varies to a large extent,43^*7 there are many sensitizing dyes that are adsorbed on grains, forming monomolecular layers with Langmuir-type adsorption isotherms, as shown in Fig. 5.7. The area occupied by a dye molecule on the grain surface, given by the saturated amount of the adsorbed dye, is usually in agreement with the product of the length and thickness of the dye molecule, indicating the edge-on orientation instead of flat-on and end-on orientations of the dye molecule to the grain surface.44"47 The heats of adsorption of sensitizing dyes to silver halide grains were measured by several methods and were ca. 10 kcal/mole. Although gelatin tends to reduce the rate and heat of dye adsorption,43'44'48'49 this influence of gelatin diminishes with increasing dye levels, and the saturated amount of adsorbed dyes was independent of the presence of gelatin for various cyanine dyes.43'44'50'51 At high concentrations of cyanine dyes with strong adsorption to AgX grains, all gelatin can ultimately be displaced from the surface of the grains. However, Tani and Suzumoto observed that competition of sensitizing dyes with gelatin for adsorption to AgX grains was more severe in dried emulsion layers than in liquid emulsions, and that the saturated amount of a cyanine dye adsorbed to AgBr grains was diminished by gelatin in dried emulsion layers.46 The adsorption behavior of Dye 14 in AgBrI emulsions was studied in detail.52 The heats of its adsorption to octahedral and cubic AgBrI grains, as measured by a microcalorimeter, were 9 and 11 kcal/mole, respectively.*1 The reversibility of the adsorption was confirmed by observing the movement of all the dye molecules on the octahedral grains to the cubic ones, when an emulsion with the undyed cubic grains was mixed with an emulsion with the dyed octahedral grains. The heat of the desorption of the dye from the octahedral grains was 26 kcal/mole. Thf potential energy profile for the dye in an aqueous gelatin solution, on (111) and (IOC) surfaces

Fig. 5.7. Adsorption isotherms of Dye 14 to cubic AgBr grains in an emulsion, as given by a method with diffuse reflectance spectroscopy and Equation (5.9) (O) and by a phase-separation method (•).46

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of AgBr, is illustrated in Fig. 5.8. Thus, the dye molecules were being rearranged on the grain surfaces through their repeated desorption and adsorption, and the ratedetermining step was their desorption from the grain surface. The desorption of the dye molecules in J-aggregates on the octahedral grains was a zero-order reaction, indicating that the J-aggregates were linear in structure and that only the molecules at the ends of the aggregates could be desorbed. The adsorption equilibrium (i.e., for the movement of the dye molecules from the octahedral grains to the cubic ones) took several hours at 40°C, around 10 min at 60°C, and less than several minutes at 70°C. Bird et al. have proposed that thiacyanine dyes are chemisorbed to silver halide, forming coordinate bonds between sulfur atoms in the heterocycles of the dyes and silver ions on the surface of silver halide.53 However, it is judged from the following experimental results that the interaction between cyanine dyes and silver halide grains is physical in nature and is based on the van der Waals and Coulombic forces. Infrared absorption spectra of sensitizing dyes have been studied by several groups of workers.54 The spectra of cyanine dyes were not subjected to any essential change when they were adsorbed to silver halide grains.54'55 This result indicates that the molecular structures of cyanine dyes were not changed by their adsorption to the grains. The wavelengths of the peaks of the visible absorption spectra of cyanine dyes in the monomeric state on silver halide grains were 2CM-0 nm longer than those of corresponding dyes in solution. As described earlier, this bathochromatic shift of absorption bands of cyanine dyes due to their adsorption to silver halide grains was within the range of the solvent effect on the absorption spectra of cyanine dyes based on the van der Waals forces between the dyes and grains.25"27'55 As stated in Chapter 3, the adsorption of cyanine dyes molecules with positively charged chromophores increased the concentration of interstitial silver ions in cubic AgBr grains by repelling silver ions at the surface kink sites on the grain surface into interstitial positions.56 In accord with this result, the adsorption of positively charged cyanine dyes to silver halide grains increased the concentration of free silver ions in a liquid emulsion by repelling silver ions at surface sites into solutions.57 In agreement with those results, the adsorption of cyanine dyes to silver halide grains

Fig. 5.8. Potential energy profile of Dye 14 on the (111) and (100) faces of AgBrI grains and in an aqueous gelatin solution with enthalpy differences in kcal/mole.52

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is enhanced by elevated pAg Values.43'4858~61 These phenomena indicate that Coulombic interaction, that is, the minimization of the electric energy of a positive charge at the chromophore of a cyanine dye molecule by the association of negative ions on the grain surface, is a necessary condition for adsorption of cyanine dyes to emulsion grains, and that cyanine dyes could modify the grain surface for their adsorption. Benzodithiolylium cyanine dyes with only sulfur atoms in their heterocycles (Dyes 40 and 41) increased the concentration of interstitial silver ions in cubic AgBr grains and were replaced by silver ions from the grains surface in emulsions. Sulfur atoms in heteronuclei are thus considered to have a positive charge and not to form a coordinate bond with silver ions on the grain surface.62 This examination was also conducted on cyanine dyes with a variation in the number of sulfopropyl groups attached to the dyes, and thus with a total electric charge of +1, 0, and -1 (i.e., Dyes 11, 12, and 13). The concentration of interstitial silver ions in cubic AgBr grains was similarly increased by these dyes.3'63 This result indicates that it was not the sulfopropyl groups but rather the positively charged chromophores in those dye molecules that were attached to the surface of the grains, and that the driving force of the adsorption of cyanine dyes to silver halide grains was given not only by a Coulombic force, but also by the van der Waals force between them, since it is thought that a chromophore with many polarizable electrons takes part in van der Waals' interaction with silver halide more strongly than sulfopropyl groups. Reversibility of adsorption and mobility of adsorbed molecules are thought to be a reflection of physical adsorption. As stated above, Tani has confirmed the reversibility of adsorption of Dye 14 to silver halide grains and the mobility of the dye molecules on the grain surface.3'52 Maskasky determined the epitaxial arrangement of various cyanine dye molecules in J-aggregates on the surface of silver halide emulsion grains and could not find the arrangement that indicated the chemical interaction between dye molecules and the grain surface.39 In the case of adsorption of merocyanine dyes to silver halide, there was some experimental evidence to suggest the presence of a chemical interaction of the dye molecules with the surface of silver halide. For merocyanine dyes with longer polymethine chains, the infrared absorption wavenumber corresponding to the stretching vibration of the C=O bond decreased on their adsorption to AgBr grains.64 West et al. found that merocyanine dyes with C=S groups were adsorbed to silver halide grains more strongly than those without C=S.43 Seki, Tani, and coworkers observed the IS-TT* transition in XANES (X-ray absorption near edge structure) spectra at K edge of sulfur in C=S groups in merocyanine dyes, which disappeared on their adsorption to AgCl grains. This result was attributed to the formation of coordinate bonding of a sulfur atom in a C=S group with a silver ion on the surface of AgCl.65 Adsorption of cyanine dyes to silver halide grains was strengthened when the dyes formed J-aggregates, indicating that a dye molecule was stabilized by its interaction with other dye molecules as well as with the grain surface.66 It is therefore considered that a dye molecule is more stable in a large aggregate than in a small aggregate, and that enhancement of the desorption and adsorption processes by increasing the temperature and time of the agitation of dyed emulsions brought about the growth of aggregates.3

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5.4 Electronic Energy Levels of Sensitizing Dyes Adsorbed on Silver Halide The light-induced electron transfer process in spectral sensitization is illustrated in Fig. 5.9. An electron is excited from HOMO to LUMO in a dye moelcule, and then transferred to the conduction band of silver halide. It is therefore considered that the electronic energy levels of LUMO and HOMO of a dye («LU and SHO, respectively) with respect to the bottom of the conduction band and the top of the valence band of silver halide is important for understanding photo-induced electron transfer in spectral sensitization. In general, eLU and sm are related to electron affinity (EA) and ionization energy (I), respectively, as follows:

I Fig. 5.9. (A) Schematic representation of the electron transfer process in spectral sensitization. Excitation of an electron from HOMO to LUMO to give a dye molecule in the lowest excited singlet state (Dye*) is followed by transfer of the excited electron to the conduction band of AgX(CB) with an efficiency of s, a rate constant of ks, and an energy gap of AE in competition with deactivation process with rate constant of kL. The reduction potential (ER) of a dye could be used as a measure for the height of LUMO and for AE on the basis of Equation (5.12). (B) Potential energy profiles of the initial state (a) and final state (b) of electron transfer, where A is the rearrangement energy of solvents as expressed in Equation (5.18).

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and have been studied by several methods, including photoelectron spectroscopy, polarography, and molecular orbital calculations. Sheppard first recognized the photographic significance of the polarographic potentials of dyes by showing that the degree of decrease in intrinsic sensitivity of silver halide grains by dyes increased with decreasing negative values of their reduction potentials (ER).67 The measurement of ER and Eox of sensitizing dyes was made by Stanienda68 and then by several groups of workers, including Daehne,69 Tamura and Hada,70 and Honda and coworkers.71 The systematic examination of the relationship of «LU and eHO with ER and Eox of various kinds of sensitizing dyes was studied by Tani and Kikuchi,20'71'72 according to the following relationships:

Figure 5.10 shows an example of the linear relationships between eLU and ER, and between SHO and Eox, of cyanine dyes with a variation of the polymethine chains.21(d)

Fig. 5.10. (a) Relations of ER and E0x with calculated electronic energy levels of quinocyanine dyes (Dyes \-$) and thiacyanine dyes (Dyes 5-8), ER and E0x are expressed in V with respect to the standard redox potential of a saturated calomel electrode (SCE), where ^LU and XHO are (eLU — a)//3 and (eHO — a)//3, respectively, and a and j8 are Coulomb and resonance integrals of 2p atomic orbitals of carbons in 7r-conjugated hydrocarbons, respectively.21(d) (b) Electronic energy levels of the dyes are on the basis of Equations (5.12) and (5.13).

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On the basis of Equations (5.12) and (5.13), the electronic energy levels of various dyes with respect to the conduction and valence bands of silver halides were estimated and used for study of the mechanism of spectral sensitization by several groups of workers, including Daehne,69 Tani and Kikuchi,20 Berriman and Oilman,73 and James.74 Linear relationships between polarographic potentials and electronic energy levels as given by HMO method were so good that deviations of the values of monomethine dyes (i.e., Dyes 1 and 5) from the linear relationships could characterize the steric effects in their molecules.21(d) The interaction between two sulfur atoms in a molecule of Dye 5, as suggested by X-ray crystallography,18 is expected to stabilize its HOMO and to destabilize its LUMO, and actually made Eox more positive, ER more negative, and ETR larger according to Equations (5.1), (5.12), and (5.13). The twisted structure of Dye 1, as indicated by X-ray crystallography,17 is expected to destabilize bonding orbitals such as HOMO and stabilize antibonding orbitals such

Fig. 5.11. Relation of ER and Eox to Hammett's ap of the substituents at the 5,5'-position of 9-ethyl-thiacarbocyanine dyes.79 The numbers in this figure refer to the dyes listed in Table 5.2.

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as LUMO, and actually made Eox less positive, ER less negative, and ETR smaller according to Equations (5.1), (5.12), and (5.13). It is usual to attach substituents to sensitizing dye molecules to modify their eLU and SHO. The modification depends upon the nature of a substituent and the position to which it is attached. Hammett's cris a good measure of the nature of a substituent, as seen in Fig. 5.11. In general, an electron-withdrawing substituent lowers eLU and eHO, and an electron-donating substituent raises their values. The degree of change in SLU and SHO is proportional to electron densities in LUMO and HOMO on the atom to which a substituent is attached. The electron density distribution in LUMO and HOMO is unique in the polymethine chain of sensitizing dye molecules, as shown in Fig. 5.3. Thus, it is possible to predict the degree of the effect of a substituent upon £LU and eHO of a sensitizing dye.5'24 As seen in Table 5.1, heterocycles are important parts and have a large influence on the electronic energy levels of sensitizing dyes. Brooker and Kiprianov independently found that the absorption wavelengths of unsymmetrical dyes were shorter than the mean wavelengths for the related symmetrical dyes, depending upon the kind of heterocycles in the dyes.5 This deviation, studied by Brooker for many

Fig. 5.12. Electronic energy levels of LUMO and HOMO of symmetrical monomethine cyanine dyes with heteronuclei as indicated in this figure, where XLU and ^H0 are OLU - a)//3 and (eHO — a)//3, respectively, and a and ft are Coulomb and resonance integrals of 2p atomic orbitals of carbons in 7r-conjugated hydrocarbons, respectively.62

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Fig. 5.13. Relation of E0x as given by phase-selective second-harmonic voltammetry with ionization energy (I) given by UV photoelectron spectroscopy for thiacyanine dyes with different methine chain lengths (Dyes 5-8 in order of I).79

sensitizing dyes, related the electron-donating abilities (i.e., basicities) of the heterocycles,75 and is called Brooker's deviation. Link76 found that the Brooker's deviation was in linear relationship with Eox for many sensitizing dyes and was useful for the estimation of electronic energy levels of sensitizing dyes according to Equations (5.12), (5.13), and (5.1). Analyzing benzodithiolylium cyanine dyes in comparison with conventional cyanine dye, Tani et al. suggested that the heights of LUMO and HOMO of cyanine dyes decreased with size (i.e., bond length) as well as with the electronegativity of heteroatoms in heterocycles,62 as shown in Fig. 5.12. The accuracy of the values of Eox and ER as measured by conventional polarography is deteriorated by secondary reactions of the oxidized and reduced dyes, respectively.77 Recently, phase-selective second-harmonic voltammetry has been introduced in this field by Lenhard,78 and has been confirmed to give more accurate values of ER and Eox of sensitizing dyes.78'79 Figure 5.11 shows the relations of ER and EOX as measured by phase-selective second-harmonic voltammetry with Hammett's a values of the substituents at the 5,5'-positions of 9-ethyl-thiacarbocyanine dyes.79 The result shown in this figure is an example of the close relation between the electronic and molecular structures of sensitizing dyes. Although the values of I and EA of sensitizing dyes in solid state and on silver halide have been determined by several groups of workers with the techniques of

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solid state physics, such as UV photoelectron spectroscopy,80'81 they could not be related to the molecular structures and photographic capabilities of the dyes. Nielsen has critically reviewed the theory of energy levels and their experimental determinations.82 Recently, Seki, Tani, and others determined the ionization energy and Fermi level of cyanine and merocyanine dyes by UV photoelectron spectroscopy using the retardation potential technique, and confirmed the close relation of ionization energy with the molecular structure of sensitizing dyes and with their Eox and eH0.79'83 Figure 5.13 shows the relation between Eox and I of Dyes 5-8. It is noted that the linear relationship between them was observed for the dyes, including Dye 5, with a steric effect, as seen in Figs. 5.2 and 5.10. Figure 5.14 represents the electronic structures of merocyanine dyes with a variation of the polymethine chain length on evaporated AgBr layers as determined by the above method.65(b)>84 The energy levels of LUMO of merocyanine dyes with n = 0 and 1 (Dyes 42 and 43, respectively) were above the bottom of the conduction band of AgBr, and those of merocyanine dyes with n = 2 and 3 (Dyes 44 and 45, respectively) were below it. This result was consistent with the spectrally sensitizing abilities of these dyes. It is noted that the vacuum level of each dye was considerably shifted to match its Fermi level with that of AgBr when they were in contact with each other. It is not necessarily clear how the Fermi levels of the two were matched. As stated above, it has been confirmed that the electronic energy levels of sensitizing dyes are situated at levels that allow the transfer of an electron excited in a

Fig. 5.14. Electronic energy level diagram with respect to the Fermi level of bulk silver (EF) for the interfaces between AgBr and merocyanine dyes as determined by a UV photoelectron spectrometer, where VL is the vacuum level of materials, and CB and VB are the bottom of the conduction band and the top of the valence band of AgBr, respectively.84 The numbers in this figure refer to the dyes listed in Table 5.2.

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sensitizing dye molecule to the conduction band of AgBr. The heights of HOMO and LUMO of sensitizing dyes can be estimated by means of oxidation and reduction potentials, molecular orbital calculation, and photoelectron spectroscopy. At present, the oxidation and reduction potentials, as given by phase-selective second-harmonic voltammetry, are convenient and appropriate for estimation of the heights of HOMO and LUMO for a wide variety of dyes, although measurements are made on dye molecules in monomeric state in solutions. Photoelectron spectroscopy would be promising to give realistic knowledge about aggregated sensitizing dyes on the surface of silver halide.

5.5 Quantum Yield of Light-Induced Electron Transfer The quantum yield of spectral sensitization is usually given by the ratio of quantum yield of the latent image formation on the excitation of sensitizing dyes to that on the excitation of silver halide grains (i.e., the ratio of the number of quanta absorbed only by the silver halide necessary to produce a specified developed density to the number absorbed only by the dye at a wavelength A within its absorption band required to produce the same density), and is called the relative quantum yield of spectral sensitization (
where E400 is the incident radiation energy at 400 nm required to produce the specific density, A400 is the fraction of this radiation absorbed by the emulsion coating, and EA and AA are the corresponding quantities for wavelength A. The value of <£r is related to the efficiency of the electron transfer in spectral sensitization (<£s), the quantum yields of the latent image formation by free electrons produced by the incident radiation of 400 and A nm (u>4oo an(i $LI,A> respectively), the rate constants of the electron transfer and competing deactivation channels (kj and kL, respectively), and the rate constants of radiative and nonradiative recombination processes (kr and knr, respectively) as follows:

Thus, 4>r = 4>s only when U,A = Li,4oo- It is noted that all positive holes are first trapped by sensitizing dyes on excitation of the dyes, whereas all positive holes are first in the valence band of silver halide on excitation of the silver halide. Therefore, <pr = 4>s under the condition where all the positive holes appeared in the valence band of the silver halide are then trapped by the dyes, or positive holes are rapidly

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released from the dyes and equilibrated between the dyes and the valence band of silver halide. Realization of the condition where s was realized in the following two cases90: 1. In emulsion grains with reduction sensitization centers (Ag2), it is considered that $ L j A was smaller than <j>u,4oo for those dyes, since some positive holes produced in the valence band of the grains by the excitation at 400 nm could react with Ag2 before they were captured by the dye molecules, whereas positive holes produced on the dyes by the excitation at A nm could not. 2. In emulsion grains that are free from reduction sensitization centers and too large for all the positive holes in the interior to reach the grain surfaces, Li,4oo was smaller than ^>L,iA for those dyes, since some positive holes produced in the interior were subjected to rapid recombination with photoelectrons in the interior of the grains. Some examples of these cases are shown below. Figure 5.15 shows an example of case 1 by comparing the values of r of dyes on cubic AgBr emulsion grains (edge length of 0.6 ^im) with and without Ag2. In

Fig. 5.15. Relation between (f>T in the presence and absence of Ag2 for several sensitizing dyes90 on cubic AgBr grains (0.2 /xm). The numbers in this figure refer to the dyes listed in Table 5.2.

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the cases of Dyes 11 and 27, whose HOMOs are moderate (Eox = +0.89 V vs. SCE) and low (Eox = +0.97 V), respectively, cf>r in the presence of Ag2 was nearly the same as <^>r in the absence of Ag2, whereas r in the presence of Ag2 was considerably smaller than that in the absence of Ag2 in the cases of Dyes 35 (Eox = +0.575 V) and 26 (Eox = +0.70 V), whose HOMO are extremely high. Figure 5.16 shows an example of case 2, with r of Dye 35 as a function of edge length of cubic AgBr emulsion grains in the absence of Ag2. The value of (j)T increased and even exceeded unity with increasing edge length. In order to analyze cf>s by measuring 4>i, it is desirable to use only dyes whose HOMOs are not extremely high and/or to use small emulsion grains without reduction sensitization centers. Determination of (j)T on the basis of photoconductivity measurements should be free from this problem and was done for large crystals91 and emulsions.90 Figure 5.9 shows the electronic energy levels of a sensitizing dye in the excited state with respect to those of silver halide and the potential energy profile for electron transfer in spectral sensitization. The energy gap for electron transfer is shown by

Fig. 5.16. Dependence of r of Dye 35 upon the edge length of cubic AgBr grains free from reduction sensitization.90 The grains were prepared at pH 2.5, and the surface coverage by the dye was 0.1.

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AE in this figure and is related to the reduction potential (ER) of the dye studied. In the potential energy profile, curves a and b indicate the state before and after electron transfer, and A is the rearrangement energy of solvents. The energy gap dependence of spectral sensitization has been studied by several groups of workers,5'73'92'93 and systematically by Tani, Suzumoto, and Ohzeki with r and ER, as given by phase-selective second-harmonic voltammetry.94 The energy levels of HOMO of almost all dyes used were moderate and low according to the classification in Fig. 5.15. As seen in Fig. 5.17, $ r greatly depended on the ER of the dyes, supporting the electron transfer mechanism. The steep threshold of energy gap dependence of r in Fig. 5.17 agrees with electronic energy levels of dyes that are defined sharply rather than distributed widely. Nelson et al. have proposed that quantities such as ionization energy and electron affinity of adsorbed dye layers are not sharply defined but are distributed over a band of energy values much greater than that associated with thermal fluctuations on the basis of external photoemission from adsorbed dye layers on substrates other than silver halide in a vacuum. The spread of ionization energies and electron affinities was attributed to perturbing interactions of adsorbed dye molecules with randomly distributed positively and negatively charged crystal defects and impurities on the surface of the substrate.81(h)i(k)'(1) Nelson's proposal was followed by Sturmer et al.,92 Penner and Oilman,95 and James.74 It is considered that the distribution of electronic energy levels of cyanine dyes on silver halide grains in emulsions

Fig. 5.17. Energy gap dependence of
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is inclined to be sharper than on other substrates in a vacuum. The positive charge of the chromophore in a cyanine dye molecule is thus surrounded by negative ions on the surface of AgX, since the dye molecules can smooth the surface of silver halide by repelling silver ions at kink sites into interstitial positions, as described in Section 5.3, and are surrounded by gelatin containing water and ions in high concentration. Marcus theory,96 which is well accepted for explanation of electron transfer phenomena in solutions, has been applied to electron transfer in photographic emulsions97'94 by taking into account the potential energy profile in Fig. 5.9(b) and Equation (5.16). Marcus theory is expressed by the following equations:

where k° is the maximum rate constant of the electron transfer, AG* is the activation energy for electron transfer, A is the rearrangement energy of solvents, and ER is the reduction potential of the dye whose AE is zero. Thus, the electron transfer from an excited dye molecule to the conduction band of silver halide takes place through the tunnel effect at the crossing points between curve a (the potential energy of the state before the electron transfer) and curve b (that of the state after electron transfer), and AG is needed to raise the potential energy of state a to the crossing point through rearrangement of the surroundings of the molecule. The solid curve in Fig. 5.17 is derived from these equations with k° of 1011 sec"1 and a small rearrangement energy (A = 0.05 eV), indicating that the region where r steeply increases with AE is followed by the region where r steeply decreases with AE (i.e., the inverted region). The experimental points were plotted along the curve in the first region, and apart from it in the inverted region. The energy-gap dependence of r in the present system was thus characterized by a small rearrangement energy and absence of the inverted region. The electron transfer processes in solutions were generally associated with the inverted region and A, which is more than 0.3 eV.98 The presence of the inverted region is explained by the potential energy profiles before and after electron transfer (curves a and b, respectively), with variation of AE as shown in Fig. 5.18. With increasing AE, the rate constant of electron transfer increases to go uphill, reaches the maximum when the crossing points between curves a and b are at the bottom of curve a, and then decreases to go downhill, since the crossing point is above the bottom of curve a. With increasing AE, the activation energy of electron transfer decreases, becomes zero at the point with the maximum rate constant, and then increases. In the case of electron transfer in the present system, the electron acceptor is the conduction band of silver halide composed of many electronic energy levels. Thus, the state after electron transfer could be represented by many curves, as shown in Fig. 5.18. Even in the downhill region, there is always some curve in the state after electron transfer that crosses at the bottom of curve a. This is considered to be the

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Fig. 5.18. Relation between ks and AE and the corresponding potential energy profiles for the initial (a) and final (b) state of electron transfer with variation of AE. The solid curves are for electron transfer processes between molecules in solutions, and the broken curves are for electron transfer from excited dyes to the conduction band of silver halide.

reason for the absence of an inverted region in electron transfer in spectral sensitization of the photographic process. As expected from the situation shown in Figs. 5.9,5.17, and 5.18, the temperature dependence for ks decreased with increasing AE, and became nearly zero with increasing AE.99'100 The importance of the temperature dependence of the efficiency

Fig. 5.19. Schematic diagram showing two types of thresholds for electron transfer processes in spectral sensitization. Case A shows the transfer from Dye* to the conduction band (CB) of silver halide (AgX) to bring about spectral sensitization. Case B shows transfer from CB of AgX to Dye to bring about electron-trapping desensitization. Dye, Dye*, and Dye" are dye molecules in the ground state, in the lowest excited singlet state, and in the one-electronsurplus state, respectively, and CB and VB are the conduction and valence bands of AgX, respectively.

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of spectral sensitization was already pointed out by Frieser and coworkers, according to whom the efficiency of spectral sensitization was usually reduced at low temperature, while it was maintained at low temperature in emulsions containing a dyesupersensitizer combination (Dyes 1 and 46).101 Tani et al. confirmed through Dember photovoltage measurement, as well as sensitometric results, that the dyesupersensitizer combination could not only increase the efficiency of the spectral sensitization, but also extinguish its temperature dependence.102 Tani also observed that the temperature coefficient of (/>r decreased and became nearly zero with in-

Fig. 5.20. (a) Intrinsic sensitivity (O) of dyed emulsions with respect to that of an undyed emulsion and 4>r (^) as a function of the ER of the dyes. The thresholds for spectral sensitization (ER) and for electron-trapping desensitization (ER) were -1.42 and — 1.25 eV, respectively, as shown in this figure.94 (b) Schematic representation of the electronic energy levels of Dye* and Dye" on the basis of the values of ER and ER and Equations (5.12) and (5.13), where Dye* and Dye" are dyes in the lowest excited singlet state and in the one-electronsurplus state. The numbers in this figure refer to the dyes listed in Table 5.2.

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Fig. 5.21. Electronic energy level diagram showing hole-trapping supersensitization of Dye 1 (Dye) by a supersensitizer (SS; Dye 46). Dye* and Dye9 are Dye 1 in the lowest excited singlet state and in one-electron-surplus state, respectively, and CB and VB are the conduction and the valence bands of AgBr, respectively. creasing AE,99'100 although it contained the temperature dependence of kL as well as that of ks according to Equation (5.16). As seen in Fig. 5.19, there are two kinds of electron transfer processes associated with spectral sensitization. One is electron transfer from the excited dye molecule (Dye*) to the conduction band of silver halide, which brings about spectral sensitization. The other is transfer of a free electron in the conduction band of silver halide to the LUMO level of a dye molecule in the ground state (Dye) to produce dye in the one-electron-surplus state (Dye"), bringing about a decrease in the intrinsic sensitivity of silver halide grains (i.e., electron-trapping desensitization).2'3 As shown in Fig. 5.20, the degree of the desensitization by dyes exhibited a steep energy gap dependence in AgBr emulsions.94 The threshold of the energy gap dependence of electron transfer from AgBr to Dye was by about 0.2 V less negative in ER than that of the energy gap dependence of electron transfer from Dye* to AgBr. This difference in the threshold of the energy gap dependence indicates that the energy level of an electron excited to LUMO in Dye* was about 0.2 eV lower than that of an electron injected to LUMO in Dye", as shown schematically in Fig. 5.20. This result is the first experimental evidence of the difference in the electronic energy level between Dye* and Dye", and favors the deduction by Sturmer et al.,92 and not that of James.74 This provides an explanation for the improvement of r of dyes with insufficient AE by hole-trapping supersensitization,103 as shown for Dye 1 (Dye) with a supersensitizer (SS; Dye 46) in Fig. 5.21. Thus, ks and (f>r of the dye alone was small, since the excited electron in Dye* was situated below the bottom of the conduction band of AgBr. When Dye was excited to give Dye* in the presence of SS, electron transfer took place first from SS to Dye* to give Dye~. Since the energy level of

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the excited electron in Dye* was raised by about 0.2 eV by electron injection from SS, the electron could be situated above the bottom of the conduction band of AgBr, and transferred with high efficiency to AgBr.94 The long lifetime of Dye" as compared with Dye* could be another cause for the efficient electron transfer from Dye~ to the conduction band of AgBr.103 However, the fact that the temperature dependence of the efficiency of spectral sensitization by Dye 1 was extinguished by Dye 46, as shown in Section 5.7, could be explained not in terms of the lifetime, but in terms of the electronic energy level of Dye" as compared with Dye*.

5.6 Kinetics of Light-Induced Electron Transfer Referring to the processes in Fig. 5.9 and Equation (5.16), the rate constants of electron transfer and competing deactivation processes (ks and kL, respectively) can be given by the following equations:

where rF is the fluorescence lifetime and r is the quantum yield of electron transfer. The measurement of TF of sensitizing dyes is a key experiment for study of the kinetics of the electron transfer process in spectral sensitization and has been carried out by several groups with subnanosecond- and picosecond-spectroscopic methods, such as a streak camera system and a photon-counting system.104"9 The deterioration of measured values due to sample damage caused by excitation light pulses should be diminished for those measurements. A streak camera system with a single strong light pulse for the excitation of samples always gave two components in the decay curves of the fluorescence of J-aggregated cyanine dyes.104>105(bx(c) It was found that the light pulse was so strong that almost all the dye molecules present in each sample were excited.106 Brumbaugh and coworkers have made it clear that the first component of the fluorescence decay arose from so-called singlet-singlet exciton annihilation.105(b)>(0)

where S0, S1( and Sn are a dye in an aggregate in the ground state, and the first and n* excited singlet states. The second component is thought to have given the fluorescence lifetime of J-aggregated cyanine dyes in emulsion layers. Contrary to a streak camera system, in a photon-counting system each sample is excited by many weak light pulses. Thus, the singlet-singlet exciton annihilation described earlier was absent in the fluorescence decays of J-aggregated cyanine dye molecules measured by a photon-counting system.105(b)'(c)'107"9 However, each sample was excited so many times, and the accumulated exposure was so large, that the measured fluorescence lifetimes were still deteriorated due to sample damage caused

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by the light pulses. Suzumoto and coworkers solved this problem by moving each sample such that the position that was excited by light pulses for the measurement of the fluorescence lifetime was continuously changed. The observed value changed and converged to a certain value by increasing the rate of movement of the sample. Thus, they decreased the exposure per unit sample area and thus the sample damage.108 Picosecond spectroscopy was then applied to the study of spectral sensitization by J-aggregates of cyanine dyes with a variation in aggregate size.106"9 As stated in Section 5.3, the growth of J-aggregates of Dye 11 was enhanced by digesting the dyed emulsions at an elevated temperature.3 Figure 5.22 shows the ks and kL of Dye 11 as a function of
where k r (J) and k r (M) are the rate constants of the radiative recombination processes of dyes in the J-aggregate and monomeric states, respectively; N is the number of dye molecules in a J-aggregate; and a is the angle between the long axes of a J-aggregate and of a dye molecule in it. The increase in k r with increasing aggregate

Fig. 5.22. Relation of ks and kL with <£r for the spectral sensitization of octahedral AgBr grains by Dye 11 with variation in aggregate size.107'108 The increase in the agitation temperature of dyed emulsions resulted in the growth of J-aggregates of the dye and in the decrease in c/>r. In this figure, ks, kL, kr, and k nr are the rate constants of the electron transfer, deactivation channels, and radiative and nonradiative recombinations, respectively, as shown in Fig. 5.9 and Equations (5.16), (5.20), and (5.21).

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size is thus thought to have contributed to the increase in kL, which caused the decrease in r with increasing aggregate size. The decrease in ks with increasing aggregate sizeloe^8 is consistent with the mechanism of exciton-trapping supersensitization proposed by West and Carroll (Fig. 5.23),114 according to which the dye molecules at the ends of a J-aggregate are the traps for an exciton and are suitable places to achieve the charge separation necessary to inject an electron into the conduction band of a silver halide grain. Muenter and others investigated the effect of aggregate size on the kinetics of spectral sensitization by Dye 1, varying the average physical size of the aggregate by diluting it with a close structural analog.109 Among processes that might compete with spectral sensitization, they have pointed out the importance of energy transfer to defects in the aggregates, which was enhanced by the increase in aggregate size. They have recognized the increase in ks with increasing aggregate size. Picosecond spectroscopy was also applied to analysis of hole-trapping supersensitization for Dye I,105 as shown in Fig. 5.21. Figure 5.24 shows the relation of ks and kL as estimated by Equations (5.20) and (5.21) with r for dye with and without a supersensitizer, as well as for dye with a variation of the adsorption temperature. It is noted that ks increased with increasing r, while kL was independent of r. Thus, the increase in <pr brought about by hole-trapping supersensitization was solely caused by the increase in ks, in accordance with the mechanism of hole-trapping supersensitization described in Fig. 5.21. It is also noted from these results that the rate constant of electron transfer from an excited dye molecule to the conduction band of a silver halide grain is on the order of 1(T10 to 1CT11 sec. It is judged from the results shown in Figs. 5.22 and

Fig. 5.23. Illustration showing exciton-trapping supersensitization, in which supersensitizer molecules divide large aggregates into smaller ones, and thus increase the number of exciton traps (i.e., the molecules at the ends of aggregates) that enhance charge separation."4

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Fig. 5.24. Relation of ks and kL to 4>r for spectral sensitization of Dye I.3 The experimental points are ks for the dyes on octahedral AgBr grains with variation of aggregate size (•), ks for the dye on cubic AgBr grains with and without (D) a supersensitizer, and kL (X) for the dyes in the corresponding state.

5.24 that the values of ks and kL are widely variable, depending on the condition of spectral sensitization.

5.7 Back Reaction of Light-Induced Electron Transfer Electron transfer from an excited dye molecule to a silver halide grain leaves a positive hole in the dye phase, which is called a dye positive hole (Dye+). It is likely that dye positive holes are subjected to recombination through the following reactions:

where h + is a positive hole and Agn is a latent image center. Process (5.24) is rapid and process (5.25) is generally slow. Equation (5.27) describes the reaction of Dye+ with Agn when positive holes can move within the aggregates and reach Agn.

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Following the results with gelatin-free AgBr grains at liquid nitrogen temperature by Needier, et al.,115 Tani found that Dye"1" is paramagnetic and gives rise to an electron spin resonance (ESR) signal in an emulsion layer at room temperature.116"19 Thus, the light-induced ESR signal arising from Dye+ in photographic emulsions is useful for analysis of recombination through Dye+ in spectral sensitization. Lenhard and coworkers spectroscopically detected and analyzed Dye+ that was photochemically and electrochemically formed on the grain surface of silver halide in emulsions and solutions.120 The light-induced ESR signal is structureless and sharp, as seen in Fig. 5.25. The signal was remarkable only with dyes in the J-aggregated state, and its halfwidth decreased with increasing aggregate size, as seen in this figure, and by increasing the temperature for ESR measurement.116 These results indicated that some kind of motional narrowing by movement of positive holes in J-aggregates gives rise to such a sharp and structureless signal. This light-induced ESR signal could be observed only with dyes whose HOMOs were situated significantly (>0.3 eV) higher than the top of the valence band of octahedral AgBr grains. The signal decayed slowly with second-order kinetics. The rate constant of the decay increased by decreasing the height of HOMO. These results indicate that a positive hole was trapped by the HOMO of a dye to produce Dye + , which gave rise to the ESR signal. By using dyes that could not trap photoelectrons in their ground state, it was found that desensitization was significant only for the dyes that showed the lightinduced ESR signal in the emulsions studied (Fig. 5.26).116"18 As shown in Fig. 5.27, the latent image centers also decayed in the presence of dyes that gave rise to the

Fig. 5.25. (a) Electron spin resonance spectra of the emulsion layer that contained octahedral AgBr grains with an edge length of 0.88 tun and 0.16 /amole of Dye 11/m2 of the grain surface."6 The emulsions were agitated at 40°C (dotted curve) and 70°C (solid curve) for 60 min for the adsorption of dye to the grains. Only the dye was excited during ESR measurement, (b) Decay curves of light-induced ESR signals of these emulsion layers, which were subjected to minus-blue exposure and ceased at t = 0.

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Fig. 5.26. (a) Intensity of light-induced ESR signal and (b) degree of desensitization (SB/SB) by dyes in octahedral AgBr emulsions as a function of the energy level of HOMO (sHo) of dyes with reference to the top of the valence band of AgBr, where S£ and SB are blue sensitivities of the emulsion without and with dyes, respectively.116'0' The numbers in this figure refer to the dyes listed in Table 5.2.

light-induced ESR signal.116417'121 The decay of the latent image centers obeyed second-order kinetics. The rate constant and activation energy of decay of the latent image centers in the presence of dyes was comparable to those of the decay of the light-induced ESR signal of the dye in the emulsions.118'119 These results indicate that one mode of desensitization caused by Dye+ takes place through processes (5.25)-(5.27) and Fig. 5.28. An excited dye molecule injects an electron into the conduction band of a silver halide grain, leaving Dye+ in the dye phase. Injected electrons rapidly take part in the formation of latent image centers. A positive hole is freed from Dye+, and then trapped again by a dye molecule. Positive holes move slowly through many repetitions of these trapping and detrapping processes by dye molecules, and destroy latent image centers on arrival at these

Fig. 5.27. Time-dependent D-log E curves showing the decay of latent image centers in emulsion layers that contained octahedral AgBr grains (0.24 /xm) and Dye n.116'117 The amount of dye was 5.5 X 10~4 mole/mole AgBr. An emulsion layer was exposed for 10~3 sec to (a) blue and (b) minus-blue light, kept for t sec, immersed in water to terminate the decay of latent image centers, and developed by a MAA-1 surface developer.

Fig. 5.28. Schematic representation of behavior of electrons and positive holes on a AgBr surface, with the presence of Dye 11 leading to the formation and decay of latent image centers and dye positive holes, where £LU and sao are the energy levels of LUMO and HOMO of dye molecules, and CB and VB are the conduction band and valence band, respectively, of AgBr.117

146

Spectral Sensitization

147

centers. Thus, both the light-induced ESR signal and latent image centers decayed by the same second-order kinetics with comparable rate constants. Time-dependent sensitivity was analyzed for fine octahedral AgBr grains in the presence of Dye 11. In the presence of the dye with the 40°C agitation, the intrinsic sensitivities just after exposure were nearly the same in emulsions with and without a hydrogen hypersensitization treatment. This result indicates that all the positive holes that appeared in the interior of the grains were first trapped by the dye just after exposure.89 The intrinsic sensitivity of hydrogen-hypersensitized emulsion with the dye gradually increased with time and reached the sensitivity of the emulsion without the dye, indicating that recombination (5.24) did not take place in this case.119'121 Figure 5.29 shows the time-dependent sensitivities of the dyed emulsions at the 40° and 70° agitation.118'119 The sensitivity just after exposure to light of the dyed emulsion, which was subjected to desensitization through recombination (5.24), markedly decreased with increasing agitation temperature, and thus with increasing aggregate size. This result indicated that desensitization through recombination (5.24), which was nearly absent with small aggregates formed by 40°C digestion, increased with increasing aggregate size. On the other hand, the degree of desensi-

Fig. 5.29. Photographic sensitivity of emulsion layers, which contained octahedral AgBr grains with an edge length of 0.25 /am and 0.35 yumole of Dye 11/m2 of the grain surface. The emulsions were agitated at 40°C (O) and 70°C (X) for 60 min.118'119 Each emulsion layer was exposed for 10~3 sec through blue (a) and minus-blue (b) filters, kept for t sec, immersed in water to terminate the decay of latent image centers, and developed by a MAA-1 surface developer.

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Fig. 5.30. Arrhenius plot of the rate constant of decay of dye positive holes (k2) of emulsion layers that contained octahedral AgBr grains with an edge length of 0.25 fjan and 0.57 /amole of Dye 11/m2 of the grain surface, and were agitated at 40°C (a), 50°C (b), 60°C (c), and 70°C (d) for 20 min."9 The activation enthalpies given by the slopes of the straight lines in this figure were 0.53 eV (a), 0.51 eV (b), 0.53 eV (c), and 0.52 eV (d).

tization through (5.25) and (5.26), which was represented by the decrease in sensitivity with time after the exposure, decreased with increasing aggregate size. As seen in Fig. 5.25, decay of the ESR signal of the aggregate formed at 70°C digestion was slower than that at 40°C digestion. As seen in Fig. 5.30, the rate constants of second-order decays of the ESR signals of J-aggregates formed at the 40°, 50°, 60°, and 70°C agitation were in a linear relationship with the reciprocal of the temperature at which the ESR measurements were made. The rate constant decreased with increasing digestion temperature and with increasing aggregate size. It was thus found that the free energy of positive holes in the aggregates decreased with increasing aggregate size. However, the slopes of the straight lines that gave the activation enthalpy of the escape of positive holes from the J-aggregates were nearly constant regardless of the aggregate size. These results indicated that the entropy of positive holes in J-aggregates increased with increasing aggregate size, while the enthalpy was rather independent of aggregate size.119 The increase in entropy of positive holes would be a reflection of movement of positive holes in the J-aggregates, which might be responsible for the enhancement of the recombination of dye positive holes with free electrons caused by the increase in aggregate size.

Spectral Sensitization

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5.8 Mechanisms of Spectral Sensitization, Supersensitization, and Desensitization As stated above, the electron transfer mechanism is accepted as the explanation of spectral Sensitization and its details are now being intensively studied from various viewpoints by means of new techniques.3 In the past, two major mechanisms (i.e., the electron transfer mechanism4 and the energy transfer mechanism122) were proposed for spectral Sensitization in silver halide photography.2 In the latter, the energy in the excited dye molecule is transferred to silver halide to create an electron in its conduction band by a Foerster-type long-range resonance transfer123 or a Dextertype short-range energy exchange.124 Jiuhn and coworkers observed and analyzed the spectral Sensitization of silver halide as a function of the dye-silver halide separation from van der Waals contact to greater distances in steps of 27 A by use of a Langmuir-Blodgett membrane of fatty acids of long chain length.125 After several discussions of the validity of their results,126 they provided evidence that under some conditions the energy transfer mechanism operated in spectral Sensitization of silver halide; however, its rate was considerably smaller than that of the electron transfer mechanism, even for the dyes that were in contact with silver halide.125(f) Several groups of investigators, including those of Meier127 and Levy,128 proposed the semiconductor junction model for spectral Sensitization. According to this model, a macropotential difference across the junction between a p-type dye and an n-type AgBr is developed by electron flow from AgBr to the dye because of equalization of Fermi levels. On exposure to light absorbed by the dye, excited electrons readily move across the junction from the dye to AgBr. On the basis of this model, Levy and coworkers proposed the system in which AgX was spectrally sensitized

Fig. 5.31. Schematic representation of the modified electron transfer mechanism for spectral Sensitization, where Dyes a and b denote dyes with and without spectrally sensitizing ability, respectively, in the excited state; £LU and eHO denote the respective heights of LUMO and HOMO of a dye in the ground state; Ec and E v denote the bottom of the conduction band (CB) and the top of the valence band (VB) of AgX, respectively; and arrows 1 and 2 denote the respective electron and positive hole injections from an excited dye molecule to AgX.71

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by inorganic compounds such as PbO. However, a recent study of the electronic structures of silver halide and sensitizing dyes by ultraviolet photoelectron spectroscopy has indicated that the Fermi level of the former was lower than that of the latter before their contact with each other.65(b)>84 It has been found that positive hole transfer,73'129'130'131 as well as electron transfer, takes place from dyes in the excited state to silver halide. The photographic effects of electron transfer and hole transfer by dyes have been studied and found to be most useful for the modified electron transfer mechanism proposal for spectral sensitization.71'89'132 Figure 5.31 illustrates the electronic structures and transfers of electrons and positive holes to silver halide from sensitizing dyes (Dye a) and desensitizing dyes (Dye b) in the excited state. In this scheme, electron transfer brings about the formation of latent image centers, and hole transfer carried out the destruction of latent image centers in a silver halide grain. Spectral sensitization can be observed only when the formation of latent image centers due to electron transfer overcomes the destruction of centers due to hole transfer. On the basis of Equations (5.12), (5.13), (5.17) and (5.19), and of the linear free energy relation, the rate constants of electron transfers of Dyes a and b (ks la and k s lb, respectively), and of the hole transfers of Dyes a and b (k s2a and k s2b , respectively), can be expressed as follows:

where /31 and /32 are constants. When /3j = /32, the following equations are obtained:

Equations (5.28)-(5.30) have been experimentally confirmed by observing the destruction of development centers by positive holes that were injected into silver halide grains from excited dyes with variation of electronic energy levels.89'132 The terms (eLU + sHO)/2 and (ER + Eox)/2 are named quasi-Fermi level and the hyperredox potential.71'132 The significance of Equation (5.29) in spectral sensitization was verified by Loutfy and Sharp.133 For practical sensitizing dyes, k s j/k s 2 » 1 and eLU, and therefore k s , actually determines the rate and efficiency of spectral sensitization, as seen in Fig. 5.17. There are, however, many cases in which the competition between k s r and k s>2 determines the appearance of spectral sensitization. The difference in sensitizing capability between infrared-sensitizing dyes and desensitizing dyes comes not from the difference in eLU, but from the difference in (£LU + eHO)/2 between them.132<e) Even desensitizing dyes could spectrally sensitize the formation of latent image centers when k s j was enhanced and/or k s 2 was depressed.132(d)

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Supersensitization is of great importance from the viewpoints of photographic science and engineering,2'103*0' and is denned in this chapter as phenomena in which 0 r is increased on addition of a second substance. There are two major mechanisms that are discussed in this chapter for Supersensitization in photography. One of them is hole-trapping Supersensitization, as described in Fig. 5.21, and the other is aggregate-partitioning Supersensitization, as illustrated in Fig. 5.23. The mechanisms of these two kinds of supersensitizations were described in Sections (5.4) and (5.5). In contrast, antisensitization is denned as phenomena in which r is decreased on addition of a second substance (i.e., an antisensitizer).2 Aggregate-partitioning Supersensitization is based on exciton-trapping by dye molecules at the ends of J-aggregates, as proposed by West and Carroll114 and described in Fig. 5.23. Bird et al. proposed another explanation.134 They have consid-

Fig. 5.32. Temperature dependence of the ratios of minus-blue to blue sensitivity [S(—B)/ S(B)] (O, A) and of minus-blue to blue Dember photovoltage [V(-B)/V(B)J (•, A) of emulsion layers composed of octahedral AgBr grains (0.7 /j,m) with Dye 1 [O, •] and with Dye 1 and a supersensitizer (Dye 46) (A, A).102 The amounts of the dye and supersensitizer were 1.2 X 10"4 and 3 X 10~5 mole/mole AgBr, respectively. The exposure was for 10 sec.

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ered that 4>r of larger J-aggregates of a sensitizing dye is smaller, since the larger aggregates are likely to incorporate more antisensitizers, and that aggregate-partitioning supersensitizers isolate such antisensitizers by dividing larger aggregates into smaller ones. The mechanism of hole-trapping supersensitization was proposed by Oilman,103 and has been extensively studied for a combination between Dye 1 and Dye 46 and analogs.102'103 The temperature dependences of the ratio of minus-blue to blue sensitivities and of the ratios of minus-blue to blue Dember photovoltage102 are shown in Fig. 5.32, indicating that the efficiency of spectral sensitization of cubic AgBr grains for Dye 1 was enhanced by Dye 46, both for the appearance of free electrons and for latent image formation. Such results were already observed by West and Carroll with d.c. photoconductivity measurement114 and by Collier with microwave photoconductivity measurements.135 Furthermore, the temperature dependence of the efficiency of spectral sensitization by Dye 1 for both cases was extinguished by supersensitization, as seen in this figure, in accord with the difference in electronic energy levels between Dye" and Dye* shown in Figs. 5.20 and 5.21. Table 5.3 shows the reduction and oxidation potentials as measures for £LU and eHO, and the supersensitizing abilities of various compounds for Dye 1 that Oilman studied.103 It is clear that only the compounds with SHO much higher than that of Dye 1 could supersensitize. Although Dye 1 on AgBr grains did not exhibit any ESR signal under illumination,116"19 Dye 1 and the compound that could supersensitize on the grains exhibited the ESR signal.102 The action spectrum of the ESR signal coincided with the absorption spectrum of Dye I.102 These results supported the notion that the ESR signal came from positive holes trapped by the supersensiti/er, and that light absorption of the dye was followed by transfer of positive holes from the excited dyes to the supersensitizers, as shown in Fig. 5.21. According to the result shown in Fig. 5.20, dyes can be classified into three groups: Dye I with ER more negative than -1.42 V, Dye II with ER less negative than -1.42 V and more negative than —1.25 V, and Dye III with ER less negative than -1.25 V. Hole-trapping supersensitization is not necessary for Dye I, whose r should already be high from the viewpoint of its energy gap dependence; can Table 5.3.

Redox potentials, ESR signals, and supersensitization of various compounds for dye 1

Compounds

EK

EOX

4>2

ESR signal

Dye 36 (Dye 1) Dye 27 Dye 37 Dye 6 Dye 46 Dye 25 Dye 38

-1.32 -1.03 -1.26 -1.06 -1.00 -1.54 -1.12 -1.35

1.00 0.99

0.04 0.03 0.03 0.07 0.39 0.82 0.54 0.76

+ + + +

0.94 0.88 0.78 0.64 0.62 0.54

ER, EQX = polarographic reduction and oxidation potentials (V vs. Ag/AgCl electrode)103(c); 4>y ~ relative quantum yield for spectral sensitization by Dye 1 in the presence of various compoundsI03(c); ESR = + and — represent the appearance and absence of the light-induced ESR signal of emulsion layers with each compound.l02

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improve the r of Dye II; and can barely improve the 0 r of Dye III. Self-supersensitization was proposed to explain the efficient spectral sensitization by an analog of Dye 35,136 which, however, is classified as Dye I. Desensitization is the decrease in the intrinsic sensitivity caused by dyes, and often provides severe problems to be solved for the development of silver halide photographic materials. There are the following three major mechanisms for desensitization: 1. Electron-trapping desensitization 2. Hole-trapping desensitization 3. Desensitization due to dispersion of image centers caused by dyes

Fig. 5.33. Intrinsic sensitivity and photoconductivity of emulsion layers containing cubic AgBr grains (0.82 /am) and Dye 11 as a function of the amount of dye.90 Photoconductivity was given by the peak height of the first component of the photoconductivity of each emulsion layer with a time constant of several tens of nanoseconds, as measured by the 35 GHz microwave photoconductivity signal of each emulsion layer at room temperature.

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Electron-trapping desensitization is described in Figs. 5.19 and 5.20 in Section (5.5), and the hole-trapping desensitization in Section (5.7). Some aspects of these two types of desensitizations are compared later.90 Figure 5.33 shows the intrinsic sensitivity and photoconductivity in the several tens of nanoseconds region (corresponding to the first component in Fig. 3.20) of emulsion layers containing cubic AgBr grains (0.82 ^tm) and Dye 11. By increasing the amount of dye, the sensitivity decreased very slowly, and then sharply, when the surface coverage of the grains by the dye approached unity. The effect of the dye on photoconductivity was not significant. Attention was paid to two regions, A and B, as shown in Fig. 5.33. Figure 5.34 shows the results of similar experiments with Dye 8. Both intrinsic sensitivity and photoconductivity were significantly reduced by the dye, even at region A. The effect of dyes upon the intrinsic sensitivity at region A is shown in Fig. 5.35 as well as in Fig. 5.20. Similar effects of dyes were observed on the photoconductivity of the grains in the several tens of nanoseconds

Fig. 5.34. Conditions were the same as in Fig. 5.33, except Dye 8 was used.90

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Fig. 5.35. Intrinsic sensitivity and photoconductivity of dyed emulsions with respect to undyed emulsions at Region A as defined in Figs. 5.33 and 5.34.90 The abscissa indicates the reduction potential (measured by phase-selective second-harmonic volutammetry in V vs. SCE) of the dyes studied. The numbers in this figure refer to the dyes listed in Table 5.2.

region, as seen in Fig. 5.35. Thus, both intrinsic sensitivity and photoconductivity were little affected by the dyes whose ER was more negative than -1.25 V, and were sharply decreased with increasing ER when the ER of the dye was less negative than -1.25 V. This result indicates that the desensitization seen in Figs. 5.20 and 5.35 was due to electron trapping by LUMO of the dyes and took place as rapidly as several tens of nanoseconds. The correlation between desensitization and strong electron-accepting properties of dyes, which have been already observed,2'67'69'114137 suggests that the first step in desensitization is trapping by a dye molecule of a free electron in a silver halide grain in competition with the trapping process conductive to latent image formation, although the decrease in intrinsic photoconductivity of emulsion grains by desensi-

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tizing dyes could not be confirmed.2'11400'1370*138-139 However, recent microwave photoconductivity measurements could confirm this, as seen in Fig. 5.35.90'140'141 Studies of the motion of photoelectrons in large crystals of AgBr in an applied electric field yielded direct evidence for electron trapping by desensitizing dyes.142 Lewis and James observed a decrease in electron-trapping desensitization by evacuation, indicating that the transfer of electrons trapped by dyes to oxygen was the main cause for the desensitization.137<e>>143 James confirmed that hydrogen hypersensitization eliminated the residual desensitization after evacuation, indicating that recombination of electrons trapped by the dyes with positive holes was another cause for desensitization.137(e) Electron-trapping desensitization in room air was not observed on emulsion grains with strong electron traps (i.e., Ir 3+ ) in the interior.144 The degree of desensitization at region B was dependent not upon ew, but upon eHO of the dyes, as seen in Fig. 5.26. This result indicates that the desensitization shown in this figure was due to recombination of dye positive holes with photoelectrons (i.e., hole-trapping desensitization). However, the photoconductivity of this region was not correlated with the degree of the desensitization, indicating that holetrapping desensitization was not so rapid as to be detected by photoconductivity in several tens of nanoseconds. The trapping of positive holes by sensitizing dyes was confirmed by studies of the motion of positive holes in an applied electric field142 and detection of dye positive holes on silver halide emulsion grains by ESR and spectroscopy (see Section 5.7), and was supported by evaluation of the eHO of the dyes with respect to the top of the valence band of silver halides (see Section 5.4). The desensitization was

Fig. 5.36. Reciprocity-law curves of sulfur-sensitized emulsions containing cubic AgBr grains with an edge length of 0.58 /am and Dye 14 for a minus-blue exposure.151 The numbers in this figure refer to the amounts of dye added (mmole/mole AgBr).

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157

brought about by sensitizing dyes2'143'145 whose eLU would not capture photoelectrons in silver halides according to the estimated electronic energy levels of the dyes (see Section 5.4). Equation (5.24) was suggested by Carroll.146 Referring to Oilman's mechanism for the luminescence of dyes on silver halides,147 Tani proposed the participation of dyes in the triplet state in the mechanism of the desensitization shown below:

Evidence for processes (5.25)-(5.27) was obtained from analysis of the decay of dye positive holes and latent image centers by ESR and delayed development (see Section 5.7). Internally sulfur-plus-gold-sensitized emulsion grains, in which latent image centers are formed at the sensitization centers in the interior, were prepared148'^ and used for analysis of the mechanisms of desensitizations.148(bMd) The absence of hole-trapping desensitization on those grains148(b)'(c) was proposed as evidence for processes (5.25)-(5.27). However, favorable competition of internal sensitization centers with dye positive holes for electron-trapping might be another candidate for explanation of the absence of hole-trapping desensitization on these grains. As described in Section (6.5) in Chapter 6, hole-trapping desensitization by various cyanine dyes in sulfur-sensitized emulsions could be improved by a stabilizer, 4-hydroxy-6-methyl-l,3,3a,7-tetraazaindene (TAI).149 It was confirmed that TAI could enhance the trapping of photoelectrons by sulfur sensitization centers on the grain surface by observing that TAI increased the surface sensitivity and decreased the internal sensitivity of the sulfur-sensitized emulsions (Fig. 6.6), and increased

Fig. 5.37. Average number of development centers per grain as a function of exposure of the emulsions described in Fig. 5.36.151 The development centers were revealed by arrested development (O, A), arrested development after gold latensification (•, A) of the grains without any dye (•, O), and of the grains with Dye 14 (A, A).

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the fraction of the photoelectrons captured by sulfur sensitization centers (see Fig. 6.17).150 It is thus suggested that enhancement of the trapping of photoelectrons by sensitization centers to favorably compete with the recombination of dye positive holes with photoelectrons is a method to depress hole-trapping desensitization. As described in Chapter 4, the rate of photolytic silver formation can be enhanced by introduction of electron-trapping chemical sensitization centers and increase in the concentration of interstitial silver ions and should be optimized for achieving high sensitivity. Thus, an excessively enhanced silver formation process brings about the dispersion of image centers. It was described in Section 5.3 that cyanine dyes increase the concentration of interstitial silver ions in silver halide grains. It is therefore anticipated that the dispersion of image centers is enhanced by cyanine dyes in chemically sensitized AgBr and AgBrI emulsions. Tani and Ihama actually observed that cyanine dyes brought about desensitization due to the dispersion of image centers on sulfur-sensitized AgBr grains.151 The dye accelerated the high-intensity reciprocity-law failure (Fig. 5.36) and increased the number of undevelopable versus developable image centers per grain (Fig. 5.37).

References 1. H. Vogel, Ber., 6, 1302(1873). 2. W. West, P. B. Oilman, in The Theory of Photographic Sensitivity, 4th ed., T. H. James, ed., Macmillan, New York, 1977, Chapter 10. 3. (a) T. Tani, J. Imaging ScL, 34, 143(1990); (b) T. Tani, J. Chem. Soc. Jpn., Chem. & Chem. Ind., 985(1992). 4. R. W. Gurney, N. F. Mott, Proc. R. Soc. London, Ser. A, 146, 151(1938). 5. D. M. Sturmer, D. W. Heseltine, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, Chapter 8. 6. S. Daehne, Science, 199, 1163(1978). 7. N. Tyutyulkov, J. Fabian, A. Mehlhorn, F. Dietz, A. Tadjer, Polymethine Dyes, St. Kliment Ohridski University Press, Sofia, 1991. 8. L. G. S. Brooker, R. H. Sprague, C. P. Smyth, G. L. Lewis, J. Am. Chem. Soc., 63, 1116(1940). 9. L. G. S. Brooker, G. H. Keyes, R. H. Sprague, R. H. Van Dyke, E. Van Lare, G. Van Zandt, F. L. White, H. W. J. Cressman, S. G. Dent, Jr., /. Am. Chem. Soc., 73, 5332(1951). 10. M. V. Deichmeister, I. I. Levkov, E. B. Lifshits, Zh. Obshch. Khim., 23, 1529(1953). 11. K. W. Hausser, R. Kuhn, G. Seitz, Z. Physik. Chem., B, 29, 391(1935). 12. (a) P. J. Wheatly, /. Chem. Soc., 3245(1959); (b) P. J. Wheatly, /. Chem. Soc., 4096(1959). 13. D. L. Smith, Photogr. Sci. Eng., 18, 309(1974). 14. (a) K. Nakatsu, H. Yoshioka, S. Nishigaki, Kwansei Gakuin Univ. Annual Studies, XXIX, 213(1980); (b) K. Nakatsu, H. Yoshioka, S. Nishigaki, J. Soc. Photogr. Sci. Technol. Jpn., 46, 89(1983). 15. (a) A. P. Marchetti, C. D. Salzberg, E. I. P. Walker, Photogr. Sci. Eng., 20, 107(1976); (b) J. Delaney, M. Morrow, C. J. Eckhardt, Chem. Phys. Lett., 122, 347(1985). 16. (a) J. Tanaka, M. Tanaka, M. Hayakawa, Bull. Chem. Soc. Jpn., 53, 3109(1980), (b) N. Sano, J. Tanaka, Bull. Chem. Soc. Jpn., 59, 843(1986).

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139. P. V. Meiklyar, M. D. Mirmil'Shtein-Eberman, A. A. Sadykova, Zh. Nauch. Prikl. Fotogr. Kinematogr., 10, 410(1965). 140. L. M. Kellogg, Photogr. Sci. Eng., 18, 378(1974). 141. A. Hirano, "Capture Cross Section of Electron Traps of AgBr Emulsion Grains," in the preprint book of Annual Meeting of Soc. Photogr. Sci. Technol. Jpn., May, 1993, Tokyo, p. 80. 142. V. I. Saunders, R. W. Tyler, W. West, J. Chem. Phys., 46, 199(1967). 143. W. C. Lewis, T. H. James, Photogr. Sci. Eng., 13, 54(1969). 144. P. B. Oilman, Jr., J. Photogr. Sci., 31, 185(1983). 145. E. B. Gutoff, W. J. Timson, Photogr. Sci. Eng., 13, 134(1969). 146. B. H. Carroll, Photogr. Sci. Eng., 5, 65(1961). 147. (a) P. B. Oilman, Jr., Photogr. Sci. Eng., 12, 23(1968); (b) P. B. Oilman, Jr., inMolecular Luminescence (An International Confernece), E. C. Lim, ed., W. A. Benjamin, Inc., New York, 1969, p. 879; (c) P. B. Oilman, Jr., Photogr. Sci. Eng., 11, 222(1967). 148. (a) D. M. Sturmer, L. N. Blackburn, Photogr. Sci. Eng., 19, 352(1935); (b) F. J. Evans, P. B. Oilman, Jr., Photogr. Sci. Eng., 19, 333(1975); (c) J. M. Simson, W. S. Gaugh, Photogr. Sci. Eng., 19, 339(1975); (d) D. M. Sturmer, W. S. Gaugh, Photogr. Sci. Eng., 19, 345(1975). 149. T. Tani, Photogr. Sci. Eng., 25, 230(1981). 150. T. Tani, M. Saitou, Photogr. Sci. Eng., 23, 323(1979). 151. (a) M. Ihama, T. Tani, / Imaging Sci., 31, 157(1987); (b) T. Tani, M. Ihama, in Progress in Basic Principles of Imaging Systems, Proceedings of the Intern. Cong. Photogr. Sci., Koeln (Cologne), 1986, F. Granzer, E. Moisar, eds., Friedr. Vieweg & Soh, Braunschweig/Wiesbaden, 1987, p. 335-41.

6 Chemical Sensitization and Stabilization

6.1 Introduction In order to achieve latent image formation in photographic emulsions with high efficiency and stability, many attempts are made to modify the silver halide grains in emulsions. While these attempts include chemical sensitization, hypersensitization, and latensification, chemical sensitizations are most effective and are widely used in practical emulsions.1 Chemical sensitization is a method to place a small amount of chemical species on the surface or in the interior of silver halide grains by chemical reactions taking place during the ripening of emulsions (i.e., agitation of emulsions at an elevated temperature for a certain time period). Hypersensitization is a method to treat coated emulsion layers before exposure to light for improvement of the efficiency of latent image formation. Latensification is a method to treat coated emulsion layers after exposure to light but before development in order to increase the developability of latent subimage centers and small latent image centers. There are three major chemical sensitizations, that is, sulfur sensitization, gold sensitization, and reduction sensitization. In this book, chemical species that are provided by chemical sensitization and are effective for improvement in the efficiency of latent image formation are called chemical sensitization centers. Figure 6.1 illustrates the mechanism of latent image formation with emphasis on the roles of chemical sensitizations. Sulfur sensitization centers and sulfur-plus-gold sensitization centers provide shallow electron traps where photoelectrons are preferentially trapped and silver clusters (latent image centers) are thereby formed. An important role of gold sensitization centers is to increase the developability of small silver clusters by incorporating gold atoms into the clusters. The role of reduction sensitization centers is to prevent recombination between photoelectrons and positive holes by trapping and deactivating positive holes. Table 6.1 summarizes the origin of typical sensitizers and sensitization centers, and the functions of sulfur, gold, and reduction sensitizations. As seen in Table 6.1 and Fig. 6.1, the functions of three chemical sensitizations differ and should cooperate to achieve latent image formation with high efficiency. Sulfur-plus-gold sensitization is particularly effective and is widely used in practical 165

166

PHOTOGRAPHIC SENSITIVITY

Fig. 6.1. Illustration of the mechanism for formation of a latent image center on a AgBr grain, with emphasis on the roles of chemical sensitization centers, where e~ and h + are a photoelectron and a positive hole, respectively, and CB and VB are the conduction and valence bands of AgBr, respectively. An electron-trapping sensitization center is composed of Ag2S and/or Au2S, while Ag2 is a hole-trapping reduction sensitization center (i.e., R center), and the smallest latent image center is composed of three atoms containing a gold atom.

photographic materials. Progress in sulfur-plus-gold sensitization is reviewed in the section on gold sensitization in this chapter. Sulfur sensitization and reduction sensitization can effectively cooperate with each other. However, there is some difficulty with cooperation between gold sensitization and reduction sensitization, since reduction sensitization centers are inclined to be converted into development centers (i.e., fog centers) by gold sensitization, as described in the section on gold sensitization. Stabilizers and antifoggants are also important, since chemical sensitizations tend to make emulsions unstable and to bring about fog formation during storage and development. Some detailed descriptions of sulfur sensitization, gold sensitization, reduction sensitization, hypersensitization, Intensification, properties and photo-

167

Chemical Sensitization and Stabilization Table 6.1.

Summary of chemical sensitization Sensitization Center

Function

Type

Origin

Sensitizer

Sulfur sensitization

1871; introduction of gelatin (Maddox) 1925; discovery (Sheppard) 1936; discovery (Koslowsky)

Sodium thiosulfate, allylthiourea, etc.

(Ag2S)2 in solid solution type

Shallow electron trap for preferential formation of a latent image center

KAuCL, + KSCN, Au(SCN)J, Au(S2O3)2", etc.

Au+

1933; sensitization by thiosulfite (Carroll) 1951; concept (Lowe, Roberts, & Jones)

SnCl2, thiourea dioxide, hydrazin deriv., etc.

Ag2 at a neutral site (R center) Ag2 at a positive kink site (P center)

Increase in developability of image centers; modification of sulfur sensitization center R center: hole trap to prevent recombination and provide an extra electron P center: electron trap as a precursor of a latent image center

Gold sensitization

Reduction sensitization

graphic effects of stabilizers, and antifoggants for the modification of silver halide grains in emulsions for efficient latent image formation are provided in this chapter.

6.2 Sulfur Sensitization Since gelatin inherently contains some sulfur compounds available for sulfur sensitization, it is believed that sulfur sensitization was introduced into silver halide photographic materials when Maddox used gelatin for these materials in 1871. Focusing on the fact that photographic sensitivity depended remarkably upon the kind of gelatin used, Sheppard analyzed the chemical contents of gelatin and discovered in 1925 that sulfur compounds could increase photographic sensitivity.2'3 Among many sulfur sensitizers, thiosulfates and allylthiourea are well known and widely used for sulfur sensitization. Sheppard found the formation of sulfur compounds such as allylthiourea during the procedure of producing gelatin.2'3 The presence of thiosulfates in gelatin was reported by Steigmann4 and was confirmed by several investigators.5"7 Sheppard proposed that Ag2S specks were sulfur sensitization centers, and that Ag2Se and Ag2Te specks should function similarly. It is now agreed that sulfur sensitization centers are composed of Ag2S. The formation of Ag2S in sulfur sensitization has been studied by means of several methods, including potentiometry,8'9 observation and identification of Ag2S specks by electron microscopy10"13 and diffraction, quantitative analysis of silver sulfide by use of radioactive tracer of 35s,14~20 calorimetry,17 and analysis of light absorption of Ag2S specks.21"23 Several important analyses have been made by means of a tracer technique with

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35

S, since it is most sensitive and applicable to quantitative analysis, of a very small amount of Ag2S that forms on silver halide emulsion grains during sulfur sensitization under practical conditions. Frieser and Ranz provided a method to determine the amount of Ag2S formed during sulfur sensitization by the use of thiosulfate ion with 35S in its outer S. Their analysis was based on the following results14: 1. Thiosulfate ions in the gelatin layer could be removed by washing with water. 2. Thiosulfate ions adsorbed on silver halide grains could not be removed by washing with water, but could be removed by washing with an aqueous solution of KBr of 10"' mole/1. 3. Ag2S formed on silver halide grains could be removed neither by washing with water nor by washing with an aqueous solution of KBr of 10"' mol/1.

They found that only outer S in thiosulfate ion was incorporated into Ag2S formed on silver halide grains during sulfur sensitization. By the use of a tracer technique with 35S, Spracklen15 found that the adsorption of thiosulfate ions to silver halide grains was reversible and showed a Langmuir-type isotherm, and that the decomposition of thiosulfate ions was a first-order reaction with an activation energy of 22 kcal/mol. Tracer analysis with 35S was extended to several other sulfur sensitizers, such as triethyl thiourea, 3-ethyl-5-benzylidene rhodanine,20 and 1,1'-diphenyl-2-thiourea.19(d) It was found that the rate of Ag2S formation with thiosulfate ions was larger on octahedral silver halide grains than on cubic grains, while the rate with triethylthiourea was larger on cubic grains than on octahedral grains.20 The mechanism and rate of Ag2S formation depend upon several factors, including the kind of sulfur sensitizers and the crystal habit of emulsion grains, as stated above, and have not yet been fully explained. However, it seems worthwhile considering the mechanism of Ag2S formation according to the proposal by Chateau and Pouradier,24 which is shown below:

where Ag+(crys) is an interstitial silver ion in a silver halide emulsion grain. By use of thiosulfate ions in an excessive amount as compared with that used for practical sulfur sensitization, Peelaers and Claes observed weblike specks on the (111) surface of AgBr and dotlike specks on the (100) surface by the electron microscope, and identified those specks as Ag2S by electron diffraction. As shown in Fig. 6.2, Farnell et al.12 and Shiozawa et al.13 observed a large number of Ag2S specks on AgBr grains by digesting emulsions in the presence of sodium thiosulfate, the amount of which was much larger than that for optimum sulfur sensitization. The absorption spectra of Ag2S specks formed on silver halide grains in emulsions could be observed when the emulsions were digested in the presence of an excess of sulfur sensitizers. Moisar studied the formation of Ag2S on silver halide grains by analyzing its absorption band and reported that the formation of Ag2S was a first-order reaction on the (111) surface of the grains and a self-catalyzed reaction

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Fig. 6.2. Electron micrograph of Ag2S specks formed on tabular AgBr grains in an emulsion, on which fog centers are formed during digestion in the presence of sodium thiosulfate.13 (Courtesy of T. Shiozawa.)

on the (100) surface of the grains.21'23 Takiguchi and Morikawa supported the selfcatalyzed reaction by 35S analysis of Ag2S.19(c) Since it was repeatedly observed that photographic sensitivity was nearly independent of the amount of Ag2S formed during sulfur sensitization, keen attention has been paid to the formation and state of Ag2S centers that are effective for increase in photographic sensitivity.25 The formation of Ag2S centers was explained on the basis of the cluster mechanism, according to which they were formed by preferential adsorption of sulfur sensitizers and their reaction at some special sites.15'26'27 However, this mechanism has become unrealistic in light of the following observations and analyses. On the basis of the heat of adsorption of sulfur sensitizers to silver halide and Ag2S grains and sensitometric results, Cash proposed a two-step mechanism, according to which the production of sulfide ions on the grain surface was followed by their rearrangement into specks.28"30 Contrary to the fact that the activation energy of the formation of Ag2S during sulfur sensitization with sodium thiosulfate as a

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PHOTOGRAPHIC SENSITIVITY

sensitizer was 22 kcal/mol,15 Ridgeway and Hillson found, by use of a sensitometric method, that the activation energy of the sulfur sensitization process with a variety of sulfur sensitizers was nearly independent of the kind of sensitizers and was 32-35 kcal/mol.31 This result seems to support the two-step mechanism in that the second step (i.e., rearrangement of Ag2S) was the rate-determining step for sulfur sensitization, and had a larger activation energy than the first step (i.e., formation of Ag2S). Corbin and others observed that immediate formation of Ag2S on addition of Na2S to emulsions did not bring about sulfur sensitization until the emulsion was heated, indicating that sulfur sensitization is the result of redistribution of Ag2S after its formation. The sulfur sensitization process after the formation of Ag2S was prevented by the presence of a stabilizer, 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene.32 Roth and Simpson observed the stepwise increase in sensitivity of a photographic emulsion as a result of sulfur sensitization, and ascribed this phenomenon to evidence for the two-step mechanism.33 Yoshida gave evidence for the two-step mechanism by observing the sensitivity increase during digestion even after the Ag2S formation was completed.20 The function of Ag2S centers formed during sulfur sensitization (i.e., sulfur sensitization centers) has long been studied by many groups of investigators, as described in Chapter 4. Gurney and Mott proposed that sulfur sensitization centers were deep electron traps.34 Mitchell proposed that sulfur sensitization centers were positive hole traps,35 emphasizing that deep electron traps on silver halide grains should be fog centers before their exposure to light. As stated in Chapter 4, it is now accepted that sulfur sensitization centers are shallow electron traps, where latent image centers are preferably formed. Some evidence for that idea are as follows. Saunders et al. observed sensitization by Ag2S on a sheet crystal of AgBr in the absence of positive holes.36 As shown in Fig. 4.2 in Chapter 4,37 it is generally observed that sulfur sensitization enhances latent image formation on the grain surface, while it depresses latent image formation in the interior of the grains. Moisar incorporated sulfur sensitization centers in the interior as well as on the surface of emulsion grains, and found that latent images were formed at the place where sulfur sensitization centers were incorporated.38'39 Spencer observed and analyzed the enhancement of dispersion of latent image centers caused by sulfur sensitization.40 Kellogg and others observed a decrease in the lifetime of photoelectrons in silver halide grains by sulfur sensitization.41 Farnell's observation that latent image centers were preferentially formed at the points where dislocation lines crossed the surface of emulsion grains42 indicated that crystal defects could play an important role in the formation of sulfur sensitization centers or that sulfur sensitization centers formed at crystal defects played an important role in the formation of latent image centers. Remarkable progress has been made recently on the characterization of sulfur sensitization centers. Hamilton and Harbison focused on the fact that sulfur sensitization centers make silver halide emulsion grains sensitive to red light due to their spectral sensitization [i.e., the electron transfer from the center to the grain when an electron was excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied one (LUMO) in the center as a result of its absorption of red light]. They regarded the activation energy of the red sensitivity (i.e., 0.33 eV) as

Chemical Sensitization and Stabilization

171

the depth of the electron traps of sulfur sensitization centers on AgBr emulsion grains (i.e., the electronic energy difference between LUMO of the center and the top of the conduction band of AgBr).43 Using a thermally stimulated current technique, Kellogg and Hodes estimated the thermal depth of 0.39 eV for sulfur sensitization centers on octahedral AgBr emulsion grains.44 They filled LUMO of sulfur sensitization centers with photoelectrons at low temperature and detected the current arising from electrons that were thermally released from the centers to the conduction band of the grains when the temperature of the grains was gradually increased. Keevert analyzed the rearrangement process of Ag2S on the basis of Smoluchouski's equation45 and concluded that sulfur sensitization centers were dimers of Ag2S, which were as many as 1000//u,m2. Kanzaki and Tadakuma studied sulfur sensitization centers by means of luminescence-moduration spectroscopy; that is, photoelectrons and positive holes were created on excitation of silver halide grains at liquid helium temperature, and were captured by sulfur sensitization centers and luminescence centers, respectively. The electrons captured by the sulfur sensitization centers were then reexcited by infrared radiation to the conduction band to give the luminescence-modulated absorption spectrum of the captured electrons. Measuring the wavelengths of the peak and threshold of the spectrum, they determined that the optical and thermal depths of sulfur sensitization centers were 0.5 and 0.3 eV, respectively. Since the number of sulfur sensitization centers was proportional to the squared amount of Ag2S formed, they considered sulfur sensitization centers to be dimers of Ag2S.46 By applying Smoluchouski' s equation to the results obtained, Tadakuma, Yoshida, and Kanzaki estimated that there were 2800 dimers/jum2 on the surface of cubic AgBr grains (i.e., 11,000 dimers on the surface of a cubic AgBr grain with edge length of 0.8 )Ltm).47 Figure 6.3 shows the surface sensitivity and fog density of the sulfur-sensitized emulsions composed of octahedral AgBr grains with an average diameter of 0.2 /Am.48 Sulfur sensitization was carried out by digesting the emulsions at 60°C for 60 min in the presence of Na2S2O3, the amount of which is shown on the abscissa. The amount of Na2S2O3 for the formation of fog centers was two orders of magnitude larger than that for maximum sensitivity, and large enough for the formation of Ag2S specks, which are observable by means of electron microscopy, as shown for tabular AgBr grains in Fig. 6.2. Figure 6.4 shows the surface sensitivity, fog density, and photoconductivity of the same octahedral AgBr emulsions, which were, however, sulfur-sensitized by agitating the emulsions at 40°C for 15 min and at 60°C for 60 min in the presence of uniformly distributed Na2S.48 By increasing the amount of Na2S, the sensitivity increased through two steps for emulsions digested at 40°C for 15 min. While the second step was associated with the decrease in photoconductivity of the emulsion grains, the first step was not. Digestion of the emulsions at 60°C for 60 min made discrimination between the first and second steps uncertain. It is therefore considered that there are two types of surface sensitization centers, electron traps and positive hole traps, and the sensitivity increase in the first step was brought about by the centers acting as positive hole traps and the sensitivity increase in the second step by the centers acting as electron traps. The centers acting as positive hole traps were formed by a smaller amount of

172

PHOTOGRAPHIC SENSITIVITY

Fig. 6.3. Surface sensitivity (S; O) and fog density (•) of the sulfur-sensitized emulsions composed of octahedral AgBr grains (0.2 /urn) with digestion at 60°C for 60 min in the presence of Na2S2O3 with the amount shown in the abscissa.48 The emulsion coatings were exposed for 10 sec to a tungsten lamp (color temperature: 2854K) and developed by the surface developer MAA-1. Na2S than those acting as electron traps, and the digestion converted the former into the latter. Thus centers acting as positive hole traps and electron traps were considered to be monomers and dimers of Ag2S, respectively. For comparison with the results shown in Figs. 4.17 and 4.18, the oxidation potentials of sulfur sensitization centers and fog centers formed during sulfur sensitization of octahedral and cubic AgBr grains with a diameter of 0.2 /an were measured by the use of Frei's redox buffer solutions48 and are shown in Fig. 6.5 and Table 6.2. The difference in oxidation potential between sulfur sensitization centers and reduction sensitization centers (R centers) favors the idea that sulfur sensitization centers are composed of Ag2S, and not silver.49 Similar, but less quantitative, results have already been reported by several groups of workers.50~52 The oxidation potential of sulfur sensitization centers was much more positive than that of R centers, as seen in Fig. 6.5. In the light of Equation (4.1) in Chapter 4, the results shown in Fig. 6.5 and Table 6.2 indicate that the highest occupied electronic energy level of the former is lower than that of the latter, as shown schematically in Fig. 6.5. This result is in good agreement with the fact that the ability of sulfur sensitization centers to be positive hole traps or halogen acceptors was weak.17'53'54 It is noted that the oxidation potential of sulfur sensitization centers acting as electron traps (i.e., dimers of Ag2S) was only slightly less positive than that of sulfur

Chemical Sensitization and Stabilization

173

Fig. 6.4. Surface sensitivity and fog density, and photoconductivity of sulfur-sensitized emulsions composed of octahedral AgBr grains (0.2 /nm) that were digested at 40°C for 15 min (O) and at 60°C for 60 min (•) in the presence of uniformly distributed Na2S with the amount shown on the abscissa.48 The emulsion layers were exposed for 10 sec to a tungsten lamp (color temperature: 2854 K) and developed by the surface developer MAA-1. The photoconductivity was measured at room temperature by means of 35 GHz microwave photoconductivity equipment, and the peak height of the signal was used as the photoconductivity of each sample.

sensitization centers acting as hole traps (i.e., monomers of Ag2S).48 The splitting of HOMO levels of Ag2S due to the formation of its dimer was thus considered to be small, while the splitting of the LUMO level was large, as judged by the distinct difference in electron-trapping ability between a monomer and a dimer of Ag2S. Fog centers of sulfur sensitization, as seen in Fig. 6.3, could be oxidized, not by Frei's redox buffer solutions, but by much stronger oxidizing solutions such as bromine water.55 The oxidability of the fog centers was similar to that of Ag2S powder. The oxidation potential of the fog centers was thus judged to be much more positive than those of sulfur sensitization centers and to be similar to that of Ag2S powder.

PHOTOGRAPHIC SENSITIVITY

174

Fig. 6.5. The lower figure shows the residual proportions of sulfur sensitization centers [(Ag2S)2] and reduction sensitization centers (R centers and P centers) on octahedral AgBr grains (0.2 jam) in emulsion layers after keeping them in Frei's redox buffer solutions, with redox potential (vs. SCE) indicated on the abscissa and with a pBr of 3 at 25°C for 17 and 41 hours. The residual proportion of the centers was expressed by the residual proportion of the sensitivity increase (AS) in logarithm caused by the centers. The oxidation potentials of the centers were given by the midpoints of their curves.48 The upper figure shows the electronic energy level of HOMO of the centers estimated on the basis of their oxidation potentials and Equation (4.1) in Chapter 4. Thus, fog centers formed during sulfur sensitization are quite different in condition from sulfur sensitization centers and are similar to Ag2S powder and specks, as seen

in Fig. 6.2. On the basis of these results, a model has been proposed for the structure and electronic energy levels of sulfur sensitization centers and fog centers.48 In Fig. Table 6.2. Oxidation potentials of sulfur sensitization and fog centers AgBr grains

Ag2S centers Function

Assignment

Cubic

Octahedral

Sensitization centers as hole traps Sensitization centers as electron traps Fog centers

(Ag2S)! (solid solution) (Ag2S)2 (solid solution) Ag2S (cluster)

280 mV 240 mV >400 mV

360 mV 345 mV >400 mV

Chemical Sensitization and Stabilization

175

6.6(a), two types are proposed for the condition of Ag2S on AgBr grains, that is, a solid solution and a cluster type. In the former case, a sulfide ion replaces a bromide ion at a lattice site and is associated with an interstitial silver ion for charge compensation. In the latter case, Ag2S clusters are formed on the surface of AgBr grains, being isolated from the lattice of AgBr. An interstitial silver ion, which a Ag2S monomer in the solid solution type carries, provides a very shallow trap with a Coulombic orbital for a free electron in AgBr. Two Coulombic orbitals, which a Ag2S dimer in the solid solution type carries, interact with each other to give bonding and nonbonding orbitals. The bonding orbital provides a deeper trap level for a free electron. Effective sulfur sensitization centers and fog centers are assigned to Ag2S dimers in the solid solution type and Ag2S clusters, respectively, as shown in Fig. 6.6. These results indicate that tremendous numbers of sulfur sensitization centers

Fig. 6.6. (a) Proposed configurations of Ag2S on the surface of AgBr; solid solution type with S (©), interstitial silver ions (Ags+; +), and electronic orbitals associated with Agj+ (broken curves) and cluster type with a Ag2S cluster on the surface of AgBr. (b) Electronic energy levels of HOMO [3d orbitals of S""] and LUMO [the electronic orbital associated with Agi+ as shown in (a)] of a monomer and a dimer of Ag2S in a solid solution type on the surface of AgBr.48

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PHOTOGRAPHIC SENSITIVITY

are formed during sulfur sensitization and act as shallow electron traps. As indicated in Chapter 4, the optimization of the number and depth of shallow electron traps provided by sulfur sensitization centers is of great importance for efficient formation of latent image centers.

6.3 Gold Sensitization and Gold Intensification In collaboration with Mueller in 1936,57"59 Koslowsky discovered gold sensitization by the use of chloroauric acid and thiocyanate. Considerable efforts have been made to clarify the chemistry of gold sensitization by many investigators. It has been discovered that Au + rather than Au3+ is a gold sensitization center. Attention was then paid to the reduction of Au3+ to Au + and the incorporation of Au + into silver halide grains. Steigmann60'61 and Faelens and Tavernier62 found that gelatin could effectively reduce Au3+ to Au + . Oishi63 and Tavernier and Faelens64 found that thiocyanate ions could also reduce Au3+ to Au(SCN)^, and that an excessive amount of thiocyanate ions was needed to stabilize Au(SCN)^. Several groups of workers then pointed out that the strong bonding of Au+ with gelatin prevented Au + from being incorporated into silver halide grains.62'65'66 Ions of Pd and Pt were also found to bond strongly with gelatin,67 and the bonding of Pd ions with gelatin was so strong as to release Au+ from gelatin and make it possible for Au + to be incorporated into silver halide grains.68 Au + and other noble metal ions were also found to sensitize silver halide grains in the absence of gelatin.67'69 In terms of the release of Au + from gelatin, it is important to examine aurous complex ions for gold sensitization. Among them, Au(SCN)J, Au(S2O3)2~, and Au(CN)^ are of photographic interest. Contrary to the fact that neither Au + nor Au(CN)^" are sensitizers for photographic emulsions, Au(SCN)J and Au(S2O3)|~ are good sensitizers. It is useful to consider the stability constant (/?) of aurous complex ions for understanding their behavior in gold sensitization:

where L is a ligand. The values of /3 are on the order of Au(SCN)2~ (9.6 X 1016)70, Au(S2O3)i~ (1026),71 and Au(CN);T (0.2 X 1039).72 Thus, it is thought that Au + cannot reach the surface of silver halide grains due to its strong bonding with gelatin, and that Au(CN)^ cannot release Au + into silver halide grains due to its strong stability. On the contrary, SCN~ and S2O?r can favorably compete with gelatin for Au + to form aurous complex ions, which are not too stable to prevent Au + from being released from them and incorporated into silver halide grains. As shown in Fig. 6.7, S2Ol" was more suitable than SCN~ for releasing Au + from gelatin. Considerable efforts have been made to clarify the interaction of Au + with thiosulfate ions and other sulfur sensitizers.9'52
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177

Fig. 6.7. Amount of gold remaining in gelatin layers after a number of washes.65

gold sensitization centers composed of both silver sulfide and gold sulfide during digestion of emulsions with chloroauric acid and sodium thiosulfate. It is now accepted that gold atoms are incorporated in latent image centers and decrease the size of the smallest latent image centers in gold-sensitized emulsions, as proposed by Larson.74 The oxidation potential of latent image centers formed in gold-sensitized emulsions was more positive than that of latent image centers in unsensitized, sulfur-sensitized, and reduction-sensitized emulsions,55'75"77 as seen in Fig. 4.18. In the light of the fact that the electron affinity of gold (2.4 eV) is larger than that of silver (1.1 eV),65 this result clearly indicates that gold atoms are incorporated into latent image centers formed in gold-sensitized emulsions. Analyzing the developability of evaporated silver and gold clusters, Hamilton and Logel estimated that the size of the smallest clusters with developability was composed of four atoms for silver and two atoms for gold.89 This mechanism of enhancement of the developability of latent image centers by gold sensitization is consistent with James' observation of the improvement in high-intensity reciprocity law failure with gold latensification78 and Spencer's observation of a narrower distribution of rate of development of latent image centers formed in gold-plus-sulfur-sensitized emulsions compared with sulfur-sensitized emulsions.79 Several authors considered that this enhancement of the developability of latent image centers was adequate to explain the mechanism of gold sensitization. Bathing of exposed emulsion layers in an aqueous solution containing Au(SCN)^ (i.e., gold latensification) could improve their high-intensity reciprocity failure.78 It was reported by Larson74 and Karajov et al.80 that gold sensitization could not influence internal sensitivity. However, several experimental results indicate that gold sensitization in collaboration with sulfur sensitization provides sensitization centers (i.e., sulfur-plus-gold sensitization centers) that are more suitable for the electron-trapping and cluster formation processes for latent image formation. Contrary to the paper by Larson and Karajov et al., it was also reported that sulfur-plus-gold sensitization centers on the grain surface decreased internal sensitivity more effectively than sulfur sensitization centers.81'82 Harbison and Hamilton confirmed that the sensitivity of

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sulfur-plus-gold-sensitized emulsion layers became the same as that of the corresponding sulfur-sensitized emulsion layers when Au + was eliminated by a solution of KCN. They introduced Au + into emulsion layers by bathing them in an aqueous solution of Au(SCN)J before exposure (pre-exposure gold treatment) and after exposure (gold latensification). The sensitivity of the sulfur-sensitized emulsion with pre-exposure gold treatment was the same as that of the sulfur-plus-gold-sensitized emulsion, and higher than that of the gold-latensined, sulfur-sensitized emulsion.83 They also estimated that the depth of the electron traps of sulfur-plus-gold sensitization centers (0.19 eV) differed from that of sulfur sensitization centers (0.33 eV), on the basis of the activation energy of the sensitivity at wavelengths longer than 620 nm, which were created by those sensitizations.43 Kellogg and Hodes reported similar results by means of a thermally stimulated current technique.44 From these results it seems reasonable to consider that there are two functions for gold sensitization centers: One of them is to decrease the size of the smallest latent image centers by increasing the developability of image centers, and the other is to provide sulfur-plus-gold sensitization centers, which are different from, and can be superior in electron-trapping and cluster-forming capability to, sulfur sensitization centers. Comparison of photographic capability was made among emulsions with a variation of crystal habits and chemical sensitization.85 As stated earlier, gold latensification is usually used to incorporate gold atoms into latent image and subimage centers by bathing exposed emulsion layers in an aqueous solution containing Au(SCN)J. Spencer has recently considered two possibilities for the mechanism of gold latensification: One is the replacement of silver atoms by gold atoms, which converts Agn into Agn_mAum. The other is the plating of latent image centers with gold atoms, which convert Agn into AgnAum. He could not observe any decrease in developability of latent image centers when exposed emulsion layers were subjected to gold latensification and then to immersion in an aqueous solution of KCN to eliminate gold from the emulsion layers, and concluded that the mechanism of latensification was the plating of latent image and subimage centers with gold atoms.85 This description of sulfur-plus-gold sensitization is well understood on the basis of the model shown in Fig. 6.6; that is, a sulfur-plus-gold sensitization center is composed of a dimer of sulfide ions in a solid solution type, where at least one of the two interstitial silver ions for the charge compensation of a sulfur sensitization center (i.e., a dimer of Ag2S in a solid solution type) is replaced by Au + . The incorporation of Au + in a sulfur-plus-gold sensitization center is consistent with enhancement of the incorporation of Au + into silver halide grains by sulfur sensitization, as stated in the previous section, and with the incorporation of gold atoms in latent image centers, as stated in this section. Since Au + is larger than Ag+, the distance between interstitial ions in a sulfurplus-gold sensitization center is considered to be larger than that in a sulfur sensitization center, and the degree of splitting of two Coulombic orbitals in the former is thought to be smaller than in the latter. Thus, the model shown in Fig. 6.6 predicts that the electron trap provided by a sulfur-plus-gold sensitization center is shallower than that provided by a sulfur sensitization center, and is therefore consistent with

Chemical Sensitization and Stabilization

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the trap depths of those centers estimated by Harbison and Hamilton43 and by Kellogg and Hodes.44 Since it was found that there were many sulfur sensitization centers on an emulsion grain according to the analyses by Kanzaki and Tadakuma,46'47 and by Keevert and Gokhale,45 there are also thought to be many sulfur-plus-gold sensitization centers on an emulsion grain. Hailstone made a simulation of latent image formation to predict the quantum sensitivity (i.e., the number of absorbed photons per grain required for the formation of a latent image center) as a function of the number and depth of electron traps, and indicated that the number of electron traps per grain needed to achieve the maximum quantum sensitivity should increase by decreasing their trap depths, and the maximum quantum sensitivity increased with decreasing the trap depth within the conditions examined.86 It is expected that the electron traps provided by sulfur-plus-gold sensitization centers are better for latent image formation than those provided by sulfur sensitization centers, since the depth of the former is shallower than that of the latter and the number of the former can be larger than the latter. Since Au + is subjected to the following disproportionation reaction:

it has been pointed out that metallic gold is not formed during the chemical sensitization procedure when Au+ is prevented from being involved in the disproportionation by appropriate ligands.87 However, the idea that metallic gold is a gold sensitization center52(b)'58'88 does not seem to be realistic in light of the following results. Phenomena associated with gold sensitization are well understood on the basis of the model in Fig. 6.6, with Au+ as a gold sensitization center. The smallest, stable gold center (i.e., Au2) is considered to be a development center (i.e., a fog center). Faelens and Van Veelen took the stability of sensitization centers in redox buffers to indicate that the centers formed from aurous thiocyanate have an ionic character,91 in agreement with the conclusions of Bourdon and Bonnerot.76'77 There might be several candidates for fog centers formed during gold sensitization, and metallic gold would be among them.62(c) The application of gold sensitization to emulsion grains in the presence of reduction sensitization centers could produce fog centers composed of silver and gold atoms according to the procedure that took place during gold latensification. In the model in Fig. 6.6, fog centers formed by excessive sulfur sensitization treatment are Ag2S clusters. It is therefore considered that a cluster composed of Au2S or a mixture of Au2S and Ag2S is another candidate for a fog center formed during the gold sensitization treatment.52'95'96

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PHOTOGRAPHIC SENSITIVITY

6.4 Reduction Sensitization Carroll and Hubbard observed chemical sensitization using sodium sulfite as early as 1933.97 Paying attention to its difference from sulfur sensitization, the concept of reduction sensitization was introduced by Lowe et al. in 1951.50 Although both sulfur and reduction sensitization centers could be removed by careful bleaching with bromine water, a milder oxidizing agent, ferricyanide, could not remove sulfur sensitization centers, whereas it could remove reduction sensitization centers. This observation could be confirmed by measurement of the oxidation potentials of reduction sensitization centers98 and sulfur sensitization centers.48 Many kinds of reducing compounds and methods have been proposed as reduction sensitizers, including stannous chloride,50 dimethylamineborane," aminoiminomethanesulfinic acid (thiourea dioxide),52(b)'100'101 and hydrazine derivatives.102'103 It is known that the digestion of emulsions at low pAg (silver digestion),51'101'104"6 and in the presence of hydrogen gas,106 could also produce reduction sensitization centers. In terms of the functions of silver clusters on silver halide grains, the mechanism of reduction sensitization was described in detail in Chapter 4. It is now known that reduction sensitization centers are small silver clusters without developability. It is well established that gold latensification makes developable, small silver clusters formed during exposure.78 Similarly, reduction sensitization centers can be made developable by gold latensification.107"9 Based on the facts that the smallest latent image centers are composed of three or four atoms and that a silver dimer is stable,90 as stated in Chapter 4, it seems to be most reasonable to consider that reduction sensitization centers are silver dimers on the surface or in the interior of silver halide grains. It is also obvious that one of the most important functions of reduction sensitization centers is the retardation of recombination between photoelectrons and positive holes by trapping and reacting with positive holes in silver halide grains. It was confirmed that internal sensitivity as well as surface sensitivity were increased by reduction sensitization.39>50'52(b) In particular, the increase in internal sensitivity due to the presence of reduction sensitization centers on the grain surface could support the idea that they prevented recombination. By using gold treatment, it was observed that reduction sensitization centers were oxidized during exposure.107"9 Using electron microscopy, Spencer et al.107 demonstrated, as shown in Figs. 4.9 and 4.10, that reduction sensitization centers were oxidized during exposure and that the sites of the latent image centers were different from those of reduction sensitization centers. It is obvious that latent subimage centers are small silver clusters (presumably dimers) without developability and grow during exposure by trapping photoelectrons. It is therefore considered that there are two types of silver clusters on silver halide grains: One of them grows during exposure by trapping a photoelectron, and the other is oxidized by a positive hole during exposure. One of the proposals to explain this difference was to ascribe it to the difference in size between them: Larger ones could function as electron traps.39'106 It was also proposed that small clusters are negatively charged and trap holes, whereas large centers are positively charged and trap electrons.107'110 Mitchell also suggested that all silver centers of three atoms

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or more would have adsorbed an additional silver ion to acquire a positive charge."' However, it seems that latent subimage centers are too small to accept this proposal. Tani confirmed the presence of reduction sensitization centers acting as positive hole traps and electron traps under the idea that sensitization by hole-trapping centers should be disturbed by hole-trapping dyes and sensitization by electron-trapping centers should be disturbed by electron-trapping dyes.112~15 He also observed two types of fog centers, positively charged and neutral. The developability of the former was larger with a negatively charged developing agent than with a neutral agent, while the developability of the latter was not dependent upon the charge of the developing agent. Neutral fog centers are formed more easily on cubic grains than on octahedral grains.114 As judged from their influence on the photoelectron lifetime in the grains (Figs. 6.8 and 6.9), reduction sensitization centers formed on octahedral AgBr grains contained more electron-trapping centers than those formed on cubic grains.116 On the basis of these results, Tani has proposed that silver clusters acting as positive hole traps and electron traps are formed at neutral and positively charged sites, respectively. Latent subimage centers and image centers belong to the latter, while reduction sensitization centers contain both. Hamilton and Baetzold studied the growth kinetics of development centers formed at gold-treated reduction sensitization centers and at small photolytic centers to show that these two types of centers differed in some property other than size. They called the former R centers and the latter P centers. As shown in Fig. 4.11, their molecular orbital calculation, together with the above-mentioned observation, supports Tani's proposal that reduction sensitization centers usually form at uncharged sites, whereas photolytic silver separates at partially charged kinks and jogs.117 One of the most effective methods to identify R and P centers is to examine their influence on the photographic sensitivity and photoconductivity of silver halide

Fig. 6.8. Photographic sensitivity (S; O) and fog density (•) of emulsions composed of cubic AgBr grains (0.2 /j,m) (a) and octahedral grains (0.2 yum) (b) as a function of the amount of SnCl2 used for reduction sensitization.116

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Fig. 6.9. Photoelectron lifetimes in the reduction-sensitized cubic (O) and octahedral (•) AgBr grains in the emulsion layers described in Fig. 6.8.116 grains.116 As described in Chapter 3, the photoconductivity of silver halide grains is proportional to the concentration of photoelectrons, which depends predominantly not upon recombination between photoelectrons and positive holes, but upon combination of photoelectrons with interstitial silver ions. It is thus expected that R centers increase sensitivity without an influence on photoconductivity, whereas P centers increase sensitivity with decreasing photoconductivity. An example is illustrated in Fig. 4.12.98 By increasing the amount of a reduction sensitizer, the sensitizing region of R centers, without any change in photoconductivity, was followed by the sensitizing region of P centers with a decrease in photoconductivity. Figure 4.17 shows that HOMO of R centers was higher than that of P centers, and also was higher than the top of the valence band of AgBr, as judged by their oxidation potential. Moreover, the absorption band of P centers could be observed and analyzed to conclude that the P centers were silver dimers. The above result supports the idea that P centers are formed at positively charged sites, whereas R centers are formed at neutral sites. The function of R centers is especially important for efficient formation of latent image centers, and has been described in detail as Lowe's hypothesis,118 followed by experimental verification by Tani119 and Hailstone et al.,120 in Chapter 4. As shown in Fig. 6.10, James and colleagues showed that hydrogen gas is an effective sensitizer for coated emulsion layers, providing a unique sensitization method called hydrogen hypersensitization.121"25 It seems that there is some difference between reduction sensitization centers and hydrogen hypersensitization centers. Its influence upon photoconductivity of silver halide grains indicates that hy-

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183

Fig. 6.10. Characteristic curves of emulsion layers composed of 0.15 /am cubic AgBr grains with optimum sulfur-plus-gold sensitization. The emulsion layers were kept in room air, in a vacuum, and in hydrogen gas at 23°C for 6 hours, and exposed to 400 nm light for 100 sec.124

drogen hypersensitization hardly forms P centers.126 The rate of hydrogen hypersensitization decreased markedly with increasing relative humidity.125 Hydrogen hypersensitization is effective for sulfur-plus-gold-sensitized emulsions without bringing about severe fog formation.124'125 Chemical reaction of hydrogen hypersensitization takes place in dry emulsion layers, and might be simply expressed as follows:

It is quite likely that H + ions leave the reaction site more easily than X ions, and that Ag2 is formed at negatively charged sites with remaining X~ ions in the dried emulsion layers.127 This hypothesis seems to explain the difference between reduction sensitization and hydrogen hypersensitization centers.

6.5 Stabilizers and Antifoggants The increase in sensitivity caused by chemical sensitizations is more or less associated with the decrease in stability of photographic materials during storage and development. Thus, chemically sensitized emulsion layers tend to suffer from fog formation and/or a change in sensitivity during storage and development. For this

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Table 6.3. 4-hydroxy-tetraazaindenes*

Compound

Structure

R

K'

PKa

PKV

1 2 3 4 5

A

CH3 H Ph CF3 t-Bu

H H H H H

6.27

10.5

5.86

10.0

5.32 3.38 6.41 6.10 6.53

10.2 10.8

6 7

A A A A A B

—(CH2)3—

9.3 — —

*Although the structures and names of the agents are expressed in the traditional way in this table, it is known that a dissociative proton is attached to one of the nitrogen atoms in each agent.

reason, most photographic materials contain stabilizers and/or antifoggants to enhance stabilization. Stabilizers are agents that decrease the changes in developable fog and/or other sensitometric characteristics of emulsion layers that occur during storage. Antifoggants are agents that decrease the rate of fog density growth during development to a larger degree than they decrease the rate of development of latent images. Tremendous numbers of stabilizers and antifoggants have been described and studied.128-31 Among the many stabilizers and antifoggants, some detailed descriptions have been provided of the fundamental properties of the following three families; 4-hydroxy-tetraazaindenes (Table 6.3), benzotriazoles (Table 6.4), and mercaptoazoles (Table 6.5). Among stabilizers, the most extensive studies have been made on 4-hydroxy-6-methyl-l,3,3a,7-tetraazaindene (TAI).128^139 Among antifoggants, the most extensive studies have been made on l-phenyl-5-mercaptoazole (PMT).128"30 These families range from agents with weak affinity to silver ions (4-hydroxy-tetraazainTable 6.4.

Benzotriazoles

Compound

R

PKa

PK,P

8 9 10 11 12

B2 Cl H CH3 NO2

7.49 7.54 8.30 8.50 6.40

12.8 12.9 13.6 13.7 11.9

185

Chemical Sensitization and Stabilization Table 6.5.

Mercaptoazoles

Compound

Structure

Z

pKa

pKv

13 14 15 16 17 18 19

A A A B C C D

N CCH3 CH

2.95 7.68 7.28 4.81 6.94 10.07 4.31

16.1 16.1 15.9 16.5 16.5 16.5 16.8

S NH

denes) to agents with very strong affinity to silver ions (mercaptoazoles). It is therefore expected that these three families are good representatives of stabilizers and antifoggants. It is known that these agents (except compounds 4 and 5) form silver salts whose solubility products are very low in water, and that they are adsorbed to silver halide grains, forming their silver salts. The presence of a NH stretching vibration band in the infrared absorption spectrum indicates that a dissociative proton is attached not to a sulfur atom, but to a nitrogen atom in a mercaptoazole,140"42 being in accord with X-ray analysis;143 and not to a oxygen atom, but to a nitrogen atom in an 4-hydroxytetraazaindenes.140'142'145 The NH stretching vibration band was absent in the infrared absorption spectra of all agents (except compounds 4 and 5, which did not have adsorptivity to AgX) in Tables 6.3, 6.4, and 6.5, when they formed their silver salts and were adsorbed to silver halide grains.140'141'146 This result indicates that the dissociative proton in each agent was replaced by a silver ion to form its silver salt and to be adsorbed on the grain surface, in accord with X-ray analysis.144 It was found from XPS spectra of these agents that a silver ion was attached not to a nitrogen atom, but to a sulfur atom in a mercaptoa/ole,141'142 whereas a silver ion was attached to a nitrogen atom in a 4-hydroxy-tetraazaindene on silver halide grains as well as in silver salts.144'147 X-ray analyses143'144 indicate that each H + in those compounds and Ag+ in their silver salts interacts with two adjacent molecules, forming three-dimensional networks. This networks in the silver salts might be the reason for their low solubility in water. Silver salt formation is thus expressed for 4-hydroxytetraazaindenes and benzotriazoles as follows:

and for mercaptoazoles as follows:

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PHOTOGRAPHIC SENSITIVITY

and their important properties are the solubility product of silver salt (Ksp; pKsp = -log K^p), as expressed by the following equation:

where [Ag+] and [L^] are the concentrations of silver ions and anions of a stabilizer or an antifoggant (LH), and the dissociation constant (Ka; pKa = -log Ka) of those agents (LH), as expressed by the following equations:

and for their acid-base equilibrium:

The values of pKa and pKsp of stabilizers and antifoggants are important for studies of their photographic effects and have been measured by several groups of investigators.128'139'145'146'148-55 Tables 6.3, 6.4, and 6.5 show the values of pKa and pKsp of typical compounds belonging to 4-hydroxy-tetraazaindene,145 benzotriazole,146 and mercaptoazole families,155 respectively. Figure 6.11 shows the relation between pKa and Hammett's cr of the substituents to 4-hydroxy-tetraazaindenes and benzotriazoles. The observed linear relationships between them for those two families indicate that pKa could be a good measure of the electron density at the nitrogen atom of each agent, to which H + should be attached, and that the electrostatic force plays an important role for binding between L^ and H + . The pKa value of a benzotriazole is larger than that of a 4-hydroxy-tetraazaindene due to the fact that the number of electron-withdrawing nitrogen atoms in a benzotriazole is smaller than that in a 4-hydroxy-tetraazaindene, and results in the pKsp of a benzotriazole being larger than that of a 4-hydroxy-tetraazaindene. The slope of the straight line for benzotriazoles in this figure was smaller than that for 4-hydroxy-tetraazaindenes because the distance between a substituent and a nitrogen with H + attached is longer for benzotriazoles than for 4-hydroxytetraazaindenes. Figure 6.12 shows the relation between pKsp and pKa of those three families.155 Upon comparison with pKa, the value of pKsp was on the order of the 4-hydroxytetraazaindene, benzotriazole, and mercaptoazole families. Reflecting the similarity between Equations (6.8) and (6.9), linear relationships with a slope of nearly unity were observed between pKsp and pKa for the 4-hydroxytetraazaindene and benzotriazole families. This result indicates that the electron density at the nitrogen atom was the origin for its binding with Ag+ as well as that for its binding with H + . It is therefore considered that the interaction of Ag+ with L~ was similar to that of H + with LT, and electrostatic in nature for 4-hydroxy-tetraazaindene and benzotriazole families. However, the values of pKsp of mercaptoazoles were very large and independent of their pKa values. This result reflects the strong influence of mercaptoazoles on the properties of silver halide grains and their function as antifoggants, discriminating them from those of 4-hydroxytetraazaindenes and benzotriazoles. It

Chemical Sensitization and Stabilization

187

Fig. 6.11. Relation between pKa and Hammett's
is noted that H + is attached to a nitrogen atom, whereas Ag+ is attached to a sulfur atom in a mercaptoazole.141'142 In view of the fact that those agents are adsorbed from their aqueous solutions to silver halide grains forming their silver salts on the grain surface,140'141'145'146'155'156 the description of the equilibrium condition of silver salts of those agents in water could be a good model for their adsorption behavior with silver halide grains. In equilibrium, a silver salt dissolves into water according to equations (6.8) and (6.9). Then,

Putting (8) and (9) into (11),

Equation (6.12) is expressed in Fig. 6.13 for those agents listed in Tables 6.3-6.5. In this figure, the ordinate represents the reciprocal of the square of the concentration

188

PHOTOGRAPHIC SENSITIVITY

Fig. 6.12. Relation between pK5p and pKa for tetraazaindenes (O), benzotriazoles (•), and mercaptoazoles (X). 155 Numbers refer to the compounds listed in Tables 6.3-6.5.

of dissolved silver salt. Figure 6.13 can therefore be used to predict the adsorption behavior of an agent to silver halide grains at certain pH values in such a way that the larger the value of the ordinate is, the larger would be the amount of the agent adsorbed to the grains. It is expected from Fig. 6.13 that the stabilizers and antifoggants in Tables 6.3-6.5 could cover a fairly large variety of adsorptivity to silver halide grains. Figure 6.13 predicts that the adsorptivity of a nitrobenzotriazole (compound 12) to AgBr grains is considerably lower than those of other benzotriazoles (compounds 8-11) at pH 10, whereas the adsorptivity of the former is higher than those of the latter at pH 5. This prediction could be confirmed as shown in Fig. 6.14.146 At pH 10, all the benzotriazoles examined were dissociated, and the adsorptivity increased with decreasing Hammett's a values of the substituents, thus increasing pKa and the electron density at the nitrogen atom to which Ag+ should be attached. At pH 5, competition between Ag+ and H + for binding with a benzotriazole anion was easier for adsorption of a nitrobenzotriazole to grains than those of other benzotriazoles. As predicted by Fig. 6.13, the adsorptivity to silver halide grains is in the order of mercaptoazoles, benzotriazoles, and 4-hydroxy-tetraazaindenes. It is noted, however, that Fig. 6.13 gives only a principal tendency. For instance, a mercaptoazole with a hydrophilic substituent forms water-soluble silver salt and shows only weak adsorptivity to silver halide grains. The influence of stabilizers and antifoggants on silver halide grains can be clearly seen by observing their influence upon the ionic conductivity and concentration of interstitial silver ions in silver halide grains in emulsions. As stated in Chapter 3,

Chemical Sensitization and Stabilization

189

Fig. 6.13. Values of equation (6.12) as a function of pH on the basis of the measured values of pKa and pJQp of the agents listed in Tables 6.3-6.5.155

the ionic conductivity of the grains in emulsion layers is proportional to the concentration of interstitial silver ions, and can be given by the frequency of the applied electric field that brings about maximum dielectric loss in the grains. All agents studied in this chapter decreased the ionic conductivity of silver halide grains. 145,147,157-63 For example, Fig. 6.15 shows the dependence of the ionic conductivity of cubic AgBr grains in emulsion layers upon the amount of benzotriazole added to the emulsions.146 Among those compounds, the capability to decrease the ionic conductivity coincided well with the adsorptivity to AgBr emulsion grains, as seen in Fig. 6.14. As stated in Chapter 3, interstitial silver ions are in equilibrium with silver ions at kink sites on the grain surface:

It is thus considered that the bonding of anions of those agents with silver ions at kink sites decreased the concentration of interstitial silver ions in the grains according to Equation (6.13), and that those agents could influence photographic properties of silver halide grains in emulsions through these processes. Numerous investigations have been made on the photographic effects of stabi-

190

PHOTOGRAPHIC SENSITIVITY

Fig. 6.14. Adsorption isotherms of benzotriazoles to AgBr grains from their aqueous solutions at pH of 5, 7, and 10 at 30°C, where c is their equilibrium concentration and the specific surface area is 0.271 m2/g.146 Numbers in this figure refer to the compounds listed in Table 6.4. lizers. However, their effects have not yet been fully understood, since several actions causing changes in photographic properties are taking place during storage. Mueller has listed the possible actions of stabilizers and antifoggants.130'131 The former includes the prevention of the formation of an excessive number of sensitization centers by blocking reactive sites on the grain surface,164 the retardation of the growth of sensitization centers or fog centers by becoming adsorbed on their respective surfaces,165"68 the formation of a silver complex salt in the gelatin matrix that will react chemically with sulfur and reduction sensitizers,169 thus decreasing the amount of sensitizer available for reaction at the grain during storage; the immobilization of silver ions by salt formation;170 the elimination or reduction of the size of fog centers by oxidation171 or by dislodging them from the grain surface172; the displacement of other sensitizing complexes from the grain surface165'166; and the interaction with other emulsion ingredients and/or protection of the silver halide from their detrimental influences. In view of the fact that unstable conditions are usually brought about by chemical sensitizations, it is now reasonable to consider the mechanisms of photographic stabilization in relation to the mechanisms of chemical sensitizations reviewed in Sections 6.2-6.4. In relation to the mechanism of sulfur sensitization, one of the most probable mechanisms for stabilization is prevention of the formation of sensitization centers and fog centers during storage. As proposed in Section 6.2, sulfur sensitization centers are dimers of Ag2S, which are formed through two-step mechanism,28"33'48 and fog centers are Ag2S clusters, which are formed with accumulation of sulfur sensitization.48 It was also demonstrated that TAI could effectively retard

Chemical Sensitization and Stabilization

191

Fig. 6.15. Ionic conductivity of cubic AgBr grains (0.5 /am) in emulsion layers as a function of the amount of benzotriazoles added to the emulsions.146 The conductivity was given by the decay of Dember photovoltage of the grains. The numbers in this figure refer to the compounds listed in Table 6.4.

the second step, that is, the aggregation of the monomers for the formation of the dimers.32 It is thus expected that stabilizers retard the formation of sulfur sensitization centers and fog centers by preventing the aggregate formation of Ag2S during storage. In relation to reduction sensitization, silver digestion suggests that gelatin can reduce silver halide grains to form reduction sensitization centers on the grain surface in the presence of high concentrations of silver ions. It is thus probable that reduction sensitization centers are also formed on silver halide grains in the presence of gelatin and silver ions during storage over a long period of time, which become fog centers, especially when they interact with gold sensitizers and/or gold sensitization centers in emulsions. It is therefore expected that the decrease caused by stabilizers in the concentration of silver ions available for the formation of reduction sensitization centers stabilizes the photographic properties of emulsions by preventing the formation of reduction sensitization centers during storage. It has generally been believed that stabilizers and antifoggants decrease photographic sensitivity,173 or at most do not change it. Tani has described a new photographic phenomenon of stabilizers and antifoggants: They actually can increase photographic sensitivity of emulsions with and without spectral sensitization.145^17'156'162'174 It seems reasonable that there are several probable mechanisms for the sensitivity increase caused by these agents.

192

PHOTOGRAPHIC SENSITIVITY

Figure 6.16 shows that TAI increased surface sensitivities and decreased internal sensitivities of sulfur-sensitized octahedral and cubic AgBr grains in emulsions.162 The photoelectron lifetime of sulfur-sensitized grains [T(S)] was shorter than that of unsensitized grains [T(U)], and both T(S) and T(U) increased by increasing the added amount of TAI.162 The trapping probabilities in sulfur-sensitized grains [!/T(S)], in primitive grains [l/r(u)], and in sulfur sensitization centers [l/r(Ag2S)] are given as follows:

Fig. 6.16. Characteristic curves of a (a) weakly and (b) moderately sulfur-sensitized emulsion layers composed of octahedral AgBr grains (1.2 /urn) [A] and cubic grains (0.7 fim) [B]. TAI with the amount described in this figure was added to the emulsion. The coated emulsion layers were exposed for 1 sec, and subjected to surface development (D19) and internal development [D19 + KI(0.5 g/1)] after bleaching the latent image centers on the grain surface.

Chemical Sensitization and Stabilization

193

Fig. 6.17. (a) Electron lifetimes of unsensitized (TU; O) and sulfur-sensitized (TS; •) emulsions composed of octahedral AgBr grains (1.2 /urn), (b) Trapping probability in unsensitized grains (1/v, O), by sulfur sensitization centers (l/TAg2s; •), and fraction of photoelectrons captured by sulfur sensitization centers (Fs; X) as a function Of TAI added to the emulsions.

As seen in Fig. 6.17, the fraction of photoelectrons captured by sulfur sensitization centers, which was given by the ratio of [l/r(Ag2S)] to I/T(S), increased by increasing the amount of added TAI. This result provides one of the mechanisms by which TAI could increase photographic sensitivity by increasing the fraction of photoelectrons captured by sulfur sensitization centers on the grain surface. By decreasing the concentration of interstitial silver ions, TAI brought about an increase in the Dember photovoltage of cubic and octahedral AgBr grains in emulsion layers (Fig. 6.18), and therefore an increase in the mobility of photoelectrons.162 Thus, TAI could increase the fraction of photoelectrons captured by sulfur sensitization centers by decreasing the concentration of electron traps that are in competition with sulfur sensitization centers. Another probable action for TAI is to decrease the degree of band bending in the space charge layers in emulsion grains, as shown in Fig. 3.15.

194

PHOTOGRAPHIC SENSITIVITY

Fig. 6.18. Dember photovoltage (O) and ionic conductivity (•) of octahedral AgBr grains (1.2 /xm) [a] and cubic grains (0.7 /urn) [b] in emulsion layers as a function of the amount of TAI added to the emulsions.

TAI could also decrease the hole-trapping desensitization caused by cyanine dyes in sulfur-sensitized emulsions.174 Since hole-trapping desensitization is caused by recombination of dye positive holes (i.e., positive holes trapped by dyes) with photoelectron or photolytic silver, the trapping of photoelectrons by sulfur sensitization could also be enhanced by TAI and favorably compete with recombination of photoelectrons with dye positive holes. As stated in Chapter 4, the degree of dispersion of latent image centers increased with increasing the concentration of interstitial silver ions in AgBr and AgBrI grains in emulsions. It was shown that sensitization by TAI was pronounced with highintensity exposures and almost disappeared after gold latensification. However, cyanine dyes enhanced the dispersion of latent image centers by increasing the concentration of interstitial silver ions in AgBr grains in emulsions.176'177 Sensitization by stabilizers and antifoggants was pronounced for emulsions with added cyanine dyes.174 Thus, another mechanism for sensitization by stabilizers and antifoggants might be retardation of the dispersion of latent image centers through a decrease in the concentration of interstitial silver ions in the grains by those agents.

References 1. J. M. Harbison, H. E. Spencer, in The Theory of the Photographic Process, 4th ed., T. H. James ed., Macmillan, New York, 1977, pp. 149-160.

Chemical Sensitization and Stabilization 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

20.

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71. J. Pouradier, M.-C. Gadet, /. Chim. Phys. Phys.-Chim. Biol, 66, 109(1969). 72. K. B. Yatsimirskii, V. P. Yasil'ev, Instability Constants of Complex Compounds, D. A. Paterson transl., Pergamon Press, Oxford, 1960, p. 108. 73. A. Steigmann, Sci. Ind. Photogr., (2)24, 179(1953). 74. E. T. Larson, PSA J (Photogr. Sci. Tech.), 17B, 19(1951). 75. P. J. Hillson, J. Photogr. Sci., 6, 97(1958). 76. J. Bourdon, A. Bonnerot, Sci. Ind. Photogr., (2)30, 205(1959). 77. J. Bourdon, A. Bonnerot, Sci. Ind. Photogr., (2)32, 369(1961). 78. T. H. James, W. Vanselow, R. F. Quirk, PSA J., 14, 349(1948). 79. H. E. Spencer, R. E. Atwell, J. Opt. Soc. Am., 54, 498(1964). 80. G. Karadjov, J. Malinowski, Photogr. Korr., 104, 68(1968). 81. G. C. Farnell, L. R. Solman, J. Photogr. Sci., 24, 303(1976). 82. G. C. Farnell, L. R. Solman, J. Photogr. Sci., 28, 185(1980). 83. J. M. Harbison, J. F. Hamilton, Photogr. Sci. Eng., 19, 322(1975). 84. A. P. Marchetti, S. S. Collier, N. P. Crews, Photogr. Sci. Eng., 28, 146(1984). 85. H. E. Spencer, C. A. DeCann, R. T. McCleary,•]. Imaging Sci., 31, 8(1987). 86. R. Hailstone, in IS&T Proceedings. Symposium on Electronics and Ionic Properties of Silver Halides-Common Trends with Photocatalysis. Part of the 44th Annual Conference, B. Levy, ed., Soc. Imaging Sci. Techno!., 1991, p. 102. 87. J. Pouradier, M. C. Gadet, H. Chateau, /. Chim. Phys., 62, 203(1965). 88. F. L. Broun, K. V. Chibisov, Z. Wiss. Photogr., 58, 134(1964). 89. J. F. Hamilton, P. C. Logel, Photogr. Sci. Eng., 18, 507(1974). 90. (a) J. W. Mitchell, Photogr. Sci. Eng., 22, 1(1978); (b) J. W. Mitchell, Photogr. Sci. Eng., 22, 249(1978). 91. P. A. Faelens, G. F. Van Veelen, Photogr. Korresp., 99, 185(1963). 92. D. C. Shuman, T. H. James, Photogr. Sci. Eng., 15, 42(1971). 93. J. W. Mitchell, Photogr. Korresp., 1, Sonderheft(1957). 94. P. A. Faelens, B. H. Tavernier, J. Phys. Chem., 66, 2411(1962). 95. R. A. Bassett, H. O. Dickinson, J. Photogr. Sci., 11, 239(1962). 96. W. Bahnmuller, Photogr. Korresp., 105, 149(1969). 97. B. H. Carroll, H. Hubbard, /. Res. Nat. Bur. Stand., 11, 743(1933). 98. (a) T. Tani, "Formation and Characterization of Reduction Sensitization Centers," in the preprint book of The Autumn Meeting of Soc. Photogr. Sci. Technol. Jpn., November, 1991, Kyoto; (b) T, Tani, M. Murofushi, J. Imaging Sci., 38, 1(1994). 99. S. S. Collier, Photogr. Sci. Eng., 26, 98(1982). 100. E. Moisar, Photogr. Korresp., 106, 149(1970). 101. W. Vanassche, H. Borginon, H. Pattyn, W. Peelaers, E. Moisar, / Photogr. Sci., 22, 121(1974). 102. K. V. Chibisov, J. Photogr. Sci., 1, 41(1959). 103. Yu. Sh. Mushkovskii, 7h. Nauk. Prikl. Fotogr. Kinematogr., 3, 51(1958). 104. H. W. Wood, J. Photogr. Sci., 6, 33(1958). 105. T. Tani, Photogr. Sci. Eng., 15, 181(1971). 106. E. Moisar, B. Schmitt, F. Granzer, J. Imaging Sci., 29, 233(1985). 107. H. E. Spencer, L. E. Brady, J. F. Hamilton, J. Opt. Soc. Am., 57, 1020(1967). 108. H. E. Spencer, Photogr. Sci. Eng., 11, 352(1967). 109. H. E. Spencer, R. E. Atwell, J. Opt. Soc. Am., 58, 1131(1968). 110. H. E. Spencer, /. Photogr. Sci., 20, 143(1972). 111. J. W. Mitchell, Photogr. Sci. Eng., 22, 249(1978). 112. T. Tani, Photogr. Sci. Eng., 15, 28(1971). 113. T. Tani, Photogr. Sci. Eng., 15, 181(1971). 114. T. Tani, Photogr. Sci. Eng., 16, 35(1972).

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T. Tani, Photogr. Sci. Eng., 18, 569(1974). T. Tani, S. Takada, Photogr. Sci. Eng., 26, 111(1982). J. F. Hamilton, R. C. Baetzold, Photogr. Sci. Eng., 25, 189(1981). J. M. Harbison, H. E. Spencer, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, p. 152. T. Tani, /. Imaging Sci., 30, 41(1986). R. K. Hailstone, N. B. Liebert, M. Levy, R. T. McCleary, S. R. Girolmo, D. L. Jeanmaire, C. R. Boda, J. Imaging Sci., 32, 113(1988). T. A. Babcock, M. H. Sewell, W. C. Lewis, T. H. James, Astron. J., 79, 1479(1974). T. A. Babcock, P. M. Ferguson, W. C. Lewis, T. H. James, Photogr. Sci. Eng., 19, 49(1975). T. A. Babcock, P. M. Ferguson, W. C. Lewis, T. H. James, Photogr. Sci. Eng., 19, 211(1975). T. A. Babcock, T. H. James, J. Photogr. Sci., 24, 19(1976). T. H. James, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, pp. 164-165. T. Tani, J. Imaging Sci., 29, 93(1985). T. H. James, private communication. E. J. Birr, Stabilization of Photographic Silver Halide Emulsions, Focal Press, London, 1974. T. H. James, The Theory of the Photographic Processes, 4th ed., T. H. James, ed., Macmillan, New York, 1977, pp. 396-399. F. W. H. Mueller, in The Photographic Image (Invited papers, International Congress of Photographic Science, Tokyo, 1967), S. Kikuchi, ed., Focal Press, London, 1970, p. 91. G. Fischer, J. Inf. Rec. Mater., 16, 91(1988). E. J. Birr, Z. Wiss. Photogr., 47, 72(1952). E. J. Birr, Z. Wiss. Photogr., 48, 103(1953). E. J. Birr, Z. Wiss. Photogr., 49, 1(1954). E. J. Birr, Z. Wiss. Photogr., 49, 261(1954). E. J. Birr, Z Wiss. Photogr., 50, 107(1955). E. J. Birr, Z. Wiss. Photogr., 50, 124(1955). E. J. Birr, M. Gandino, P. M. Tacconi, F. Pocchiari, Photogr. Korresp., 99, 163(1963). E. J. Birr, C. Bertolotto, G. Cibrario, M. Gandino, J. Photogr. Sci., 17, 91(1969). M. Tamura, H. Hada, J. Noguchi, S. Hayashi, /. Phys. Chem., 66, 559(1962). T. Tani, Photogr. Sci. Eng., 21, 317(1977). (a) L. J. Gerenser, M. G. Mason, P. J. Trotter, J. Phys. Chem., 66, 559(1962); (b) P. J. Trotter, M. G. Mason, L. J. Gerenser, J. Phys. Chem., 81, 1325(1977). J. P. Chesick, J. Donohue, Acta Cryst, B27, 1441(1971). D. L. Smith, H. R. Luss, Photogr. Sci. Eng., 20, 184(1976). T. Tani, Photogr. Sci. Eng., 21, 37(1977). T. Tani, Photogr. Sci. Eng., 25, 230(1981). T. Tani, Y. Sano, S. Takada, M. Saito, J. Photogr. Sci., 28, 28(1980). J. Pouradier, A. Pailliotet, C. R. Berry, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, p. 8. (a) G. P. Faerman, J. Photogr. Sci., 15, 22(1967); (b) E. Klein, Z Wiss. Photogr., 52, 157(1958). C. J. Battaglia, Photogr. Sci. Eng., 14, 275(1970). J. Havir, Collect. Czech. Chem. Commun., 32, 130(1967). Z. C. H. Tan, R. C. Vought, R. B. Pontius, Anal. Chem., 44, 411(1972).

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7 Development-Amplification of Latent Image

Photographic development is a process that amplifies a latent image formed according to the mechanisms of photographic sensitivity described in the previous chapters, and is important in relation to photographic sensitivity and image quality from the following viewpoints. 1. As described in Chapter 1, photographic development is a unique and effective amplification process that makes it possible for silver halide photographic materials to achieve very high sensitivity and image quality. In general, an increase in the degree of the amplification brings about an increase in sensitivity and the deterioration of image quality. 2. The initiation of development of a latent image involves electron transfer from developing agents to latent image centers and should thus determine photographic sensitivity, since the development condition might determine the size of the smallest detectable latent image centers, which is one of the most important factors influencing sensitivity. 3. The development process is based on a redox reaction that involves silver halide grains and developing agents with the aid of the catalyzing effect of latent image centers, and thus provides the opportunities for a variety of image processing techniques for silver halide photographic materials.

Based on these conditions, descriptions are made on the principles concerning the mechanism of development, developing agents, rate of development, photographic sensitivity in relation to development, and image processing during development.

7.1 Photographic Development Process The centers that can initiate development by themselves are called development centers and are latent image centers and fog centers. A development center grows to a mass of silver during photographic development, being repeatedly supplied with an electron and a silver ion one after the other. There are two major types of development, that is, chemical and physical. Silver ions in a silver halide grain are the source of silver ions for chemical development, while silver ions in a developer are 200

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201

the source of silver ions for physical development. In this chapter, chemical development is mainly described since development for practical photographic materials is essentially chemical development. Figure 7.1 schematically illustrates the processes of chemical development of a silver bromide grain bearing a latent image center on its surface. In comparison with Fig. 4.1, it is understood that the development process is regarded as an extension of the formation and growth of silver clusters on silver halide grains during exposure to light. In both cases, silver clusters grow through capture of an electron and an interstitial silver ion one after the other. In the case of growth of silver clusters during exposure, electrons are provided by silver halide grains as a result of light absorption of grains or sensitizing dyes adsorbed by the grains. In the case of development, electrons are provided by developers, since a latent image center is such a deep electron trap as to readily accept an electron from a developing agent in a developer solution. In the first step of the development processes, a development center on a silver halide grain receives an electron from a developing agent in a developer (i.e., an

Fig. 7.1. Schematic illustration of the development process on a silver bromide grain, where Agn is a latent image center, and CB and VB are the conduction and valence bands of silver bromide, respectively.

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electronic process). In the next step, the center attracts and combines with an interstitial silver ion (i.e., an ionic process). The repetition of the above-stated electronic and ionic processes finally reduces all the silver ions in a grain to silver atoms, releasing halide ions into a developer solution. Interstitial silver ions initially present in a grain are limited in number and should be rapidly supplied during development through a process such as that shown in Fig. 3.12(b). The development process is usually expressed in an electrochemical way.1'2 The general reversible reaction of the development can be expressed as follows:

where Red is a developing agent, Ox is an oxidized form of the developing agent, n is the number of electrons given up by a molecule of the developing agent, and m is the number of protons produced. The general cell Equation (7.1) can be separated into two half-cells, that is, the silver half-cell (EAg),

and the developer half-cell (EDev),

The cell potential Ecell, for (7.1) is the difference between these half-cell potentials,

When EceU is positive, reaction (7.1) proceeds to the right, and when Ecell is negative, reaction (7.1) proceeds to the left. For the silver half-cell reaction,

For the typical developer half-cell reaction,

There are three species of reductant whose sum is given by

Development-Amplification of Latent Image

203

The ionization constants are given by

Then,

Under the condition where [Ox] = [R], EDev is approximately equal to the polarographic half-wave potential E,/2,

From an electrochemical viewpoint,2 the rate of silver formation is given by the net rate of transfer of electrons from the developing agent to the silver according to Equation (7.1),

where kox and kR are the specific rates of the forward and backward reactions in Equation (7.1). When kR is sufficiently small, or when the concentration of the oxidized form of a developing agent is small, as is often the case in practice, the net rate of development is approximately the rate of oxidation of the developing agent, and

where a is the partition coefficient,3 17 is the overpotential that is equal to EAg EDev in the simplest case, and k^x is the rate of the oxidation in the absence of the overpotential and is dependent upon several factors, including the surface area of development centers and the diffusion of the chemical species involved in development. A quantum-chemical expression is also useful to understand photographic development by regarding its initiation as electron injection from the highest occupied

204

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molecular orbital (HOMO) of a developing agent to the lowest unoccupied molecular orbital (LUMO) of a development center. It is known that Ip of an organic molecule is related to the electronic energy level of its HOMO (£Ho) and oxidation potential (Eox). which is related to Eoev by Equation (7.11) under the condition that the oxidation reaction at the electrode is reversible.4'5

Since, as described in Chapter 4, silver microclusters as latent image centers are so small that their electronic structures can be treated by molecular orbital methods,6 it is considered that Equation (7.14) is also valid for eHO, IP, and Eox of initial development centers, and that EA and the electronic energy level of LUMO (SLU) of those centers are related to their reduction potential (ER) by the following equation:

It is considered that the difference between SLV and suo, and the difference between ER and EOXl decrease with increasing the size of the centers and are nearly vanishing when the centers become as large as the Ag clusters whose redox potential obeyed the Gibbs-Thomson effect,7 as shown in Fig. 4.6. Some corrections are needed on both EAg and EDev when they are applied to real development processes. The silver half-cell potential was dependent on the size of silver clusters,8 as shown in Fig. 4.6, where the potential became more negative, obeying the Gibbs-Thomson effect, as the diameter of silver clusters decreased to be several tens of angstroms. Therefore, the correction should be made on EAg on the basis of the dependence of EAg upon the size of the silver clusters according to the Gibbs-Thomson effect when the size of development centers is in the range described in the figure. It is also noted that the property and behavior of silver clusters that are as small as image centers and sensitization centers are considerably different from those extrapolated from the Gibbs-Thomson effect,7 as described in Chapter 4. The electronic structures of those centers could well be treated as molecules or microclusters, and their deviation from the Gibbs-Thomson effect could be analyzed as the quantum size effect and site effect.7'9 It is thus considered that SHO and SLV of those centers are related to Eox and ER, respectively, according to Equations (7.14) and (7.15), and that eHO and £LU as well as Eox and ER are considerably separated from each other, since the electronic energy levels of those centers are discrete. On the contrary, eHO and £LU of silver clusters that obey the Gibbs-Thomson effects are nearly the same as each other and are related to their redox potential (EAg). In the developer half-cell reaction, a developing agent molecule gives up two electrons to the electrode. A latent image center, however, is thought to be too small to receive two electrons at the same time from a developing agent molecule, in contrast to an electrode, which has enough capacity to receive two electrons at the same time from a developing agent. This view was verified by the results reported

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Fig. 7.2. Dependence of relative development rates of various hydroquinones (k) for chemical development at pH 9 upon E, and E1/2.n

by Tong et al. Using semiquinone formation constants,10 they obtained the potential for the one-electron oxidation reaction of a developing agent (Ej)11 as follows:

in addition to the potential for the two-electron oxidation reaction, as given by their polarographic half-wave potential (E1/2) according to Equation (7.11). As shown in Figure 7.2, they could confirm that the rate of development by various hydroquinones and p-phenylenediamines was correlated more closely with their Et than with their El/2-

7.2 Developing Agents Since photographic development is initiated by electron injection from developing agents to latent image centers, SHO of a developing agent should be high enough for electron injection. General rules have been proposed by many investigators,1'12 including Lumieres,13 Anderson,14'15 Kendall,16 Pelz,17 Abribat,18 and Umberger,19'20 for the molecular structure of developing agents. It is thus helpful to examine the relation between molecular and electronic structures of developing agents to better understand the mechanism of development. Among all these rules, Kendall's and Pelz's rules are simple and cover most developing agents. According to Kendall's rule, substances are developing agents if they have the following structure:

Fig. 7.3. Schematic configuration of ir-electrons in benzene derivatives of Group I, which are developing agents when the substituents are —OH, —NH2, —NHR, and —NR2. Dotted lines represent the height of a 2p atomic orbital of a carbon atom. The atomic orbitals are alternately starred. Zs and Zu are the numbers of starred and nonstarred atomic orbitals, respectively, and n is the number of electrons that take part in the ir-electronic conjugated systems.21

Fig. 7.4. Same as Fig. 7.3, except benzene derivatives of Group II, which are not developing agents, are configured.21

206

Development-Amplification of Latent Image

207

where a and a' are —OH, —NH2, —NHR, —NRiR2 and m = 0 or a whole number. According to Pelz's rule, substances that have the following structure are also developing agents:

For example, benzene derivatives with these substituents are classified into two groups: one gives the substances that obey Kendall's rule and are developing agents (Fig. 7.3), and the other gives the substances that do not obey it and are not developing agents (Fig. 7.4).21 The examination in terms of the alternate nature of 7r-electron conjugated systems in molecular orbital theory was useful to correlate the molecular structure with electronic one of developing agents. Conjugated systems can be divided into two classes: alternant and nonalternant.21"26 In the former case, atomic orbitals that take part in the conjugated system can be starred alternately. The substances that obey Kendall's and Pelz's rules and the above-mentioned benzene derivatives are the model substances for the examination of Kendall's and Pelz's rules and belong to the alternant system. In the case of an alternant system in which the numbers of the starred and nonstarred atomic orbitals are Zs and Zu (Zs > Zu), respectively, the numbers of bonding, nonbonding, and antibonding molecular orbitals in the system are Zu, Zs - Zu, and Zu, respectively,21~30 according to molecular orbital theory. The ir-electron configurations of these benzene derivatives are constructed according to the alternant nature of the conjugated system and are shown schematically in Figs. 7.3 and 7.4. The configurations in these figures clearly indicate that HOMO of the benzene derivatives that obey Kendall's rule are antibonding in nature, whereas the HOMO of the benzene derivatives that do not obey Kendall's rule are nonbonding or bonding, and it is obvious that the electronic energy level of HOMO of the former is higher than that of the latter, since the electronic energy level of an antibonding orbital is higher, and that of a bonding orbital is lower, than that of a nonbonding orbital.21 The result shown in Figs. 7.3 and 7.4 also indicates that the benzene derivatives that obey Kendall's rule satisfy the condition given by the following equation:

and the benzene derivatives that do not obey Kendall's rule satisfy the following equation:

where n is the number of electrons that take part in the conjugated system. Since n = 2m + 4 and Zs = m + 1 in Kendall's and Pelz's rules, the compound that satisfies Kendall's or Pelz's rule also satisfies Equation (7.19). It is therefore understood from this examination that the HOMO of the compound that satisfies Kendall's or Pelz's rule is antibonding in nature and higher than that of the compound that does not satisfy it.21

PHOTOGRAPHIC SENSITIVITY

208

Quantum-chemical examination, as stated earlier, provides an explanation for Kendall's and Pelz's rules, and proposes the following general rules for developing agents21: 1. The highest occupied electronic energy levels of developing agents are higher than those of nondeveloping agents in accordance with Equation (7.14). 2. HOMO of developing agents are antibonding in nature, while HOMO of nondeveloping agents are nonbonding or bonding. 3. Developing agents satisfy Equation (7.19).

Examples of developing agents are shown with Zs and n in Table 7.1.21 More examples and detailed chemical knowledge of developing agents are given in the literature.1'12

7.3 Rate of Development The rate of development is expressed by Equation (7.13), depending upon several factors, including the overpotential of the development reaction (7.1), the concentration of developing agents, the surface area of development centers, and diffusion of the chemical species involved. Kinetic studies of photographic development have been made by many investigators from various viewpoints, as exemplified by references 31-46. The most important process for initiation of development is injection of an electron from a developing agent to a development center on a silver halide grain, the rate of which depends upon the overpotential of development reaction (7.1), that is, C-Ag ~~ ^Dev

The value of EDev is related to the polarographic half-wave oxidation potential (E1/2) of a developing agent by Equations (7.10) and (7.11). Although it has been demonstrated by Tong et al. that El is more closely related to the rate of development than E1/2, E]/2 is still useful for study of the rate of development among developing agents, whose molecular structures are closely related to each other. The most useful developing agents for color photography are found among 4-amino-N-dialkylanilines, and therefore have been studied extensively from various viewpoints. Figure 7.5 shows the result of the extensive analysis of 4-amino-N-dialkylanilines31 as developing agents by Bent et al., indicating the relation between their development rates (1/t') and E1/2, where t' = t - t0, t0 was the induction period and t was the time required to give an optical density of 0.2 above fog. The linear relationship between the logarithm of the development rate and E1/2 indicates that E1/2 is approximately proportional to the activation energy of the rate of development by those compounds. Compounds with potentials more negative than about +170 mV give development rates lower than would be expected. The authors considered that the rate of development by these compounds was so high that diffusion rather than the chemical reaction became the rate-determining step. As stated before, EAg is dependent upon the size of development centers, which in turn depends upon the Gibbs-Thomson effect8 for centers with a diameter of several tens of angstroms and upon discrete electronic energy levels in order for such small centers as to be treated by molecular orbital methods. According to the

Table 7.1. Classification

Examples of developing agents

Examples Examples

Z

Z,

10

8

Alternant 1,2-Substituted benzenes

1,4-Substituted benzenes

209

n

8

4

10

8

4

10

8

4

10

8

4

10

8

4

10

8

4

10

8

4

10

8

4

10

8

4

10

Table 7.1. Classification

Examples of developing agents (Continued) ued)

Examples

1,2,3-Substituted benzenes

1,2,4-Substituted benzenes

Other benzene derivatives

210

Z

Z,

n

9

5

12

9

5

12

9

5

12

9

5

12

9

5

12

8

4

10

14

7

16

15

8

18

16

8

20

Table 7.1. Classification

Examples of developing agents (Continued)

Examples

Other benzene derivatives (cont.)

211

Z

Z,

n

14

7

16

14

7

16

14

7

16

14

7

16

16

8

18

15

8

18

11

6

14

9

5

12

12

6

14

212

PHOTOGRAPHIC SENSITIVITY Table 7.1.

Classification

Examples of developing agents (Continued)

Examples

Non-aromatic compounds

Z

Z,

n

2

I

4

2

1

4

2

1

4

5

3

8

4

2

6

7

4

10

4

2

6

Z = Number of atomic orbitals that take part in the conjugated systems; Z, = number of starred atomic orbitals in the conjugated systems when the atomic orbitals are starred alternately; « = number of electrons that take part in the conjugated systems.

Fig. 7.5. Relation between development rates and half-wave oxidation potentials of 4-aminoN-dialkylanilines, where t' is development time less induction period.31

Development-Amplification of Latent Image

213

Gibbs-Thomson effect, EAg becomes more positive as the size of the centers increases, as seen in Fig. 4.6. According to molecular orbital methods, eLU becomes lower and ER becomes more positive as the size of the centers increases, as seen in Fig. 4.7. Thus, both the Gibbs-Thomson effect and molecular orbital methods predict that the overpotential in Equation (7.13) increases, and thus the rate of development increases by increasing the size of development centers when the rate-determining step is electron transfer from developing agents to the centers. The size of latent image centers generally decreases with increasing degrees of dispersion of image centers by high-intensity exposure and/or by introducing electron-trapping sensitization centers. The fact that the rate of development is lower over the portion of high-intensity and short exposure in the reciprocity curve due to the increased dispersity and smaller size of latent image centers is known as the Cabannes-Hoffmann effect.32"35 As a consequence of this effect, high-intensity reciprocity law failure is greatest for short or less active development and is markedly decreased by prolonged development.36 Some details of the rate of development as a function of the degree of dispersity of image centers are described later. The Gibbs-Thomson effect was examined by Hillson for the development of silver clusters on emulsion grains.37 According to the electrochemical model expressed by Equation (7.13), the rate of development is proportional to the surface area of developing silver clusters. Pontius and coworkers measured the rate of growth of silver specks on emulsion grains during development by an electron microscope and confirmed this prediction,38 supporting the electrochemical model for photographic development. The shape of silver specks was initially spherical and then became filamentary. The rate of growth of silver specks was proportional to their surface area during development regardless of their shape, and thus abruptly increased when the silver specks began to grow in a filamentary shape, since the specific surface area of filamentary specks was larger than that of spherical ones. They attributed the induction period of development to the time required for the silver specks to begin to grow into a filamentary shape. Their observation is consistent with the growth mechanism for filamentary silver proposed by Berry.47 Berry noticed that the diameter of filaments formed during development in the absence of solvents for silver halide was always about 200 A, and considered a silver atom to be formed at the interface of a silver speck and a silver halide grain, and to migrate on the surface of the silver speck in order to keep its shape spherical. Since the diffusion length of a silver atom is limited (about 200 A), the shape of silver specks could be spherical when their diameter is less than 200 A, and became filamentary when their diameter exceeds this value. Studies have been made not only on the dependence of the rate of development on the concentration of a developing agent in the bulk of a developer, but also on the conditions that influence the concentration of a developing agent in the vicinity of a development center. In the latter, there have been discussions on the adsorptivity of a developing agent to a silver halide grain with development centers and on the charge on the grain surface that influences the approach of a developing agent in ionized form.2 James has proposed the charge barrier theory,39"44 according to which, under usual developing conditions, a silver halide grain bears negative charges on its surface, which inhibit the approach of negatively charged developing agents. Although there have been several discussions on the validity of the charge barrier theory, he

214

PHOTOGRAPHIC SENSITIVITY

Fig. 7.6. Relation of the induction period to the charge of developing agents.41^4 proved it by demonstrating that the induction period increased by increasing the number of negative charges per molecule or ion of a developing agent under practical conditions, as shown in Fig. 7.6, and that the adsorption of positively charged compounds to silver halide grains decreased the negative charge barrier on their surface and could decrease the induction period of the development. Although it is believed that the sources of silver ions for the growth of development centers during chemical development are mobile silver ions in silver halide grains, such as interstitial silver ions, it seems that there is no direct evidence to indicate that the concentration of interstitial silver ions in grains could thus influence the rate of development. For example, the concentration of interstitial silver ions in emulsion grains is silver iodobromide > silver bromide > silver bromochloride > silver chloride, but the rate of development does not seem to be in the same order. However, this kind of observation does not necessarily deny the contribution of

Fig. 7.7. Two extreme types for the increase in the amount of developed silver as development proceeds, for parallel development (A) and granular development (B).57

Development-Amplification

of Latent Image

215

Fig. 7.8. Changes in optical density of dye image (in the lower figure) and the frequencies of undeveloped, developing, and fully developed grains (in the upper figure) in emulsion layers composed of sulfur-sensitized cubic silver bromide emulsion grains as a function of time of development by CP-20. The exposure time was 0.1 sec.57

interstitial silver ions to chemical development as the source of silver ions, since other factors, including the orders of EAg in the presence of silver halide and the solubility of silver halide, might explain the order of the rates of development among various silver halides. One of the most important phenomena affecting the increase in the rate of development is the rate acceleration that occurs when two developing agents act synergistically, which is called superadditivity.48 Most popular combinations for superadditivity are metol-hydroquinone and l-phenyl-3-pyrazolidone (Phenidon)hydroquinone. The mechanism of superadditivity has been studied from various viewpoints. Levenson proposed a regeneration mechanism,49"51 according to which superadditivity can occur when the reduced form of developing agent A reduces silver halide and developing agent B is able to reduce the oxidized form of A. This regeneration should be effective when an oxidized form of A inhibits development and/or when B by itself develops more slowly than A. It has also been pointed out that the degree of superadditivity in many combinations is much larger than expected by a simple regeneration mechanism and that the increase in the concentration of developing agents in the vicinity of development centers due to the charge barrier

216

PHOTOGRAPHIC SENSITIVITY

Fig. 7.9. Same as Fig. 7.8, except development is by D72.57 effect and adsorption of developing agents to silver halide grains contributes to superadditivity. Some factors influencing the rate of development depend upon the type of progress of development. There are two extreme types that determine the progress of development,2'52"57 as shown in Fig. 7.7. The amount of developed silver is proportional to the proportion of the developed silver in each grain in one extreme type (A) and to the number of fully developed grains in the other extreme type (B). Types A and B are here denoted parallel and granular developments, respectively. Several examples have been reported for the development of types A and B. It has been demonstrated that for silver bromide emulsions, development by color developer CP-20 with a 4-amino-N-dialkylaniline derivative as a developing agent belongs to type A, and development by the black-and-white developer D72 with metol-hydroquinone as the developing agents belongs to type B.57 Figure 7.8 shows the changes in optical density and in the fractions of undeveloped, developing, and fully developed grains as development proceeds for emulsion layers composed of sulfur-sensitized cubic AgBr emulsion grains (0.86 /urn) that were exposed and developed by CP-20. As seen here, the presence of a large fraction of developing grains at any stage of development was characteristic for development by CP-20. Analysis of this observation indicated that it took about 2 min for each grain to be fully developed and that the increase in optical density came from the increase in the proportion of developed silver in each grain as the development proceeded. Figure

Development-Amplification of Latent Image

217

7.9 shows the results with similar experiments on development by D72. In this case, developing grains were hardly observed at any stage of development. The analysis of the above observation indicates that it took about 20 sec for each grain to be fully developed and that the increase in optical density came mostly from an increase in the number of fully developed grains. Figure 7.10 shows the sensitivity and rate of development of emulsion layers composed of sulfur-sensitized cubic AgBr emulsion grains that were exposed and developed by D72. As seen here, an increase in the degree of sulfur sensitization and an increase in light intensity, both of which generally make image centers dispersed, significantly decreased the rate of development. It was also confirmed that the rate of development was nearly independent of the size of emulsion grains. By taking into account the fact that latent image dispersion generally brings about the formation of many small latent image centers, whose ability to accept electrons from developing agents is small, the rate of the granular development was determined not

Fig. 7.10. Rate of development (in the upper figure), sensitivity (in the middle figure), and fog density (in the lower figure) in emulsion layers of sulfur-sensitized cubic silver bromide emulsion grains as a function of the amount of sodium thiosulfate used for sulfur sensitization.57 The exposure time was 0.1 sec (•) and 10~4 sec (O), and development was done by D72.

Fig. 7.11. Same as Fig. 7.10, except the exposure time was 0.1 sec and development was done by CP-20.57

Fig. 7.12. Relation between rate of development by CP-20 and reciprocal of grain size for octahedral AgBr grains that were exposed for 0.1 sec (•), cubic AgBr grains that were exposed for 0.1 sec (•), and cubic AgBr grains that were exposed for 10~4 sec (D).57

218

Development-Amplification of Latent Image

219

by the rate of growth of developed silver, but by the first electron injection from a developing agent to a latent image center, and thus decreased by decreasing the size of development centers. Figure 7.11 shows similar experiments with development by CP-20. As seen here, the rate of development was nearly independent of the degree of sulfur sensitization. Figure 7.12 shows that the rate of development by CP-20 was nearly proportional to the reciprocal of the size of the emulsion grains (i.e., the surface to volume ratio) for various grains with variation of the light intensity. The rate of parallel development is thus considered to be determined not by the rate of the first electron transfer from a developing agent to a development center, but by the electron transfer from developing agents to the surface of developed silver at its growth stage. In granular development, the rate-determining step is its initiation with the first electron injection from a developing agent to a development center, and its rate is sensitive to the developability of development centers. On the other hand, the ratedetermining step of parallel development is the growth of developed silver, and this rate is nearly proportional to the surface-to-volume ratio of emulsion grains. 7.4 Photographic Sensitivity in Relation to Development Photographic sensitivity should depend upon development conditions. However, it has been confirmed by several groups of workers that photographic sensitivity did not depend upon the kind of developers when each developer was used under its optimum conditions,2'58"61 Thus, each developer under its optimum conditions could detect the same smallest latent image centers on emulsion grains, while it could hardly detect latent subimage centers. As stated in Chapter 6, gold sensitization decreases the size of the smallest latent image centers by incorporating gold atoms into image centers. As stated in Chapters 4 and 6, it is believed that the smallest latent image center formed in sulfur-plusgold-sensitized emulsions is composed of three atoms, while the smallest latent image center formed in sulfur-sensitized emulsions is composed of four or five atoms. This is the major contribution of gold sensitization to the increase in photographic sensitivity. It is known that latent subimage centers are stable and become developable after latensification treatment64'65 or the enforced development condition is applied.64"67 These facts indicate there is some possibility for a further increase in photographic sensitivity if one could find the development condition that could detect and discriminate image centers composed of two atoms (i.e., a latent subimage center under usual development condition) from fog formation. Thus, discriminating between the development of image centers and fog formation might be one of the key technologies to produce further increases in sensitivity. There are two types of fog, emulsion fog and developer fog.2 Emulsion fog originates from development of definite development centers (i.e., fog centers) composed of clusters of silver, silver sulfide, gold sulfide, and so on, while developer fog does not. Developer fog is thus thought to result from the formation of development centers (i.e., fog centers) during development. When emulsion fog centers

220

PHOTOGRAPHIC SENSITIVITY

are composed of silver, they should be removed by an oxidizing agent such as ferricyanide. As shown in Fig. 7.13, development of emulsion fog proceeds rapidly. On the other hand, the rate of developer fogging is determined by its initiation (i.e., the formation of development centers through electron injection from developing agents to the surface of emulsion grains), and therefore proceeds slowly.66 As stated in Chapter 6, sulfur sensitization centers and sulfur-plus-gold sensitization centers act as electron traps. Emulsion grains with these chemical sensitization centers can thus accept electrons from developing agents to form fog centers during development more easily than emulsion grains without these centers, although these are not emulsion fog centers. It is desirable to form chemical sensitization centers without forming fog centers, and to form centers that greatly enhance the efficiency of the image formation and barely cause developer fog. In order to detect latent subimage centers without causing fog formation, it is necessary to reduce developer fog that arises from electron injection from developing agents to chemical sensitization centers. Hillson proposed a method to add antifoggants to developers under an accelerated development condition, expecting that antifoggants were effective for discrimination between image and fog development at the initial state of development of each grain, and did not reduce the development of image centers at the growth stage, and that the reduction in the development at the initial stage could be recovered by introducing an acceleration condition.67

Fig. 7.13. Growth of fog density in a metol-hydroquinone developer for aged emulsion (A), the same emulsion as in A bleached for 2 min in an acid 0.1 % potassium ferricyanide solution before immersion in the developer (B), and emulsion with a low degree of emulsion fog (C).66

Development-Amplification of Latent Image

221

7.5 Image Processing Through Development The development process is based on a redox reaction that involves silver halide grains and developing agents, with the aid of the catalyzing effect of development centers. In addition, there are various chemical interactions among silver halide grains during development. Development processes thus provide various image processing techniques for silver halide photographic materials. One of the most popular chemical interactions between silver halide grains during development is the chemical adjacency effect, which is based on the finite diffusion and mass transfer rates of development byproducts and developer.68 On development of a knife-edge exposure, the concentration of developer is higher and the concentration of development inhibiting byproducts is lower in the edge region than in the remainder of the high-exposure region, and density develops more rapidly (the border effect). The development byproducts that flow into the low-exposure region of the knife-edge exposure can curtail development close to the edge, resulting in a low-density region (the fringe effect). Chemical adjacency effects can take place between emulsion layers in multilayer films, as well as within a single emulsion layer, and can be controlled to improve image quality. Another type of chemical interaction between silver halide grains during development is based on the release of development byproducts that initiate the development of neighboring grains that otherwise would not develop (i.e., infectious de-

Fig. 7.14. Characteristic curves obtained on the basis of the simulation of development of a grain without any adjacent effect (c = 1), with release of development byproducts that fog neighboring grains to bring about infectious development (c > 1), and with release of development inhibitors (c < I).69

222

PHOTOGRAPHIC SENSITIVITY

velopment), or inhibit the development of neighboring exposed grains. Figure 7.14 shows the characteristic curves that Anderson simulated by producing a model in which the adjacency effect was involved.69 In this figure, the curve with c = 1 corresponds to the condition without an adjacency effect, the curves with c larger than 1 result from infectious development, and the curves with c smaller than 1 result from inhibition of development by development byproducts. As seen here, infectious development can be used to get increased sensitivity and gamma at the expense of increased graininess and decreased sharpness. This development inhibition provides an enhanced adjacency effect, and can be used to improve graininess and sharpness at the expense of sensitivity and gamma. As seen in Figure 7.14, infectious development provides photographic materials with high contrast and a short toe. It is believed that semiquinone, which is formed by the reaction of quinone with hydroquinone in the vicinity of a developing grain, is so active that is initiates the development of a neighboring grain bearing a latent subimage center and produces materials with high contrast and a short toe in the presence of a very low concentration of free sulfite and a suitable amount of a soluble bromide. This is called lith development and is used to provide materials for the graphic arts.62'70"72 The reaction of oxidation products from the development reaction

Fig. 7.15. An example of the reaction of a colored coupler (A), the absorption spectra of a conventional magenta dye (B), and a magenta dye formed from a colored coupler with increasing amounts (C). The absorption spectrum of a yellow-colored magenta coupler is also shown by a dotted curve in C.

Development-Amplification of Latent Image

223

with a hydrazine gives a substance that can fog a neighboring grain,73 providing photographic materials with high contrast and a short toe, which are used to produce materials for the graphic arts.74~78 Figure 1.4 shows an example of a color-forming chemistry that takes place during color development. During development, a developing agent (PPD) oxidizes silver ions in emulsion grains to produce silver atoms and a quinonediimine (QDI), and QDI reacts with a coupler to produce a dye molecule. There are several requirements for couplers. Among them are the stoichiometry and coupling rate of the reaction, and the hue, fastness, and immobilization of formed dye molecules. This colorforming chemistry has expanded the opportunity for image processing of silver halide photographic materials by providing various functional couplers. One of them is a colored coupler,79'80 which is shown in Fig. 7.15. In the absorption spectra of conventional magenta dyes, main bands in the green region are always associated with sub-bands in the blue region, which make the color of the dyes impure. On color development, a yellow-colored magenta coupler loses its absorption band in the blue region, which is then compensated for by the sub-band in the same region of the formed magenta dye. As a result, the difference in the light absorption of the above coupler before and after color development appears only in the green region and can provide pure magenta color. Other examples are development-inhibitor-releasing (DIR) couplers81 and development-accelerator-releasing (DAR) couplers.82'83 With regard to color development in the presence of DIR coupler molecules, oxidation products of developing agents react with DIR coupler molecules to give dye molecules and development inhibitor molecules in the vicinity of a developing grain. The inhibitor molecules then depress the development of the grain, causing the formation of fine dye clouds

Fig. 7.16. Schematic illustration of image processing in series and parallel.8

224

PHOTOGRAPHIC SENSITIVITY

around it. Thus, it is expected that DIR couplers provide fine images at the expense of sensitivity and gamma. On the other hand, DAR coupler molecules react with oxidation products of developing agents to produce dye molecules and development accelerator molecules in the vicinity of a developing grain, and provide high sensitivity at the expense of graininess. In addition, polymeric couplers,84 in which coupler molecules are combined with a polymer chain, made it possible to put many coupler molecules in a small volume and to form a fine dye cloud. Diffusible-dye-forming couplers could improve graininess by adjusting the diffusibility of dye molecules, and thus the shape of the absorption spectra of dye clouds. Two types of image processing are illustrated in Fig. 7.16. One of them is image processing in series as adopted in electronic imaging systems. Each signal in each picture element is taken out one after another, subjected to image processing, and put back into its original place. The other is parallel image processing, as is achieved during the development process of silver halide photographic materials; each signal in each picture element is simultaneously subjected to image processing in a parallel fashion. Although image processing in photographic materials is not yet satisfactory at the present, it has advantages on the basis of the characteristics of parallel image processing. For example, parallel image processing is especially suitable for images with a large number of picture elements, such as photographic images, since it does not suffer from the troubles resulting from transportation and processing of signals in many picture elements one after another. In addition, development in photographic materials provides a wide variety of image processing based on chemical reactions taking place during development.85

References 1. W. E. Lee, E. R. Brown, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, Chapter 11. 2. T. H. James, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, Chapter 13. 3. (a) K. Vetter, Electrochemical Kinetics, Academic Press, New York, London, 1967, Chap. 2A; (b) J. O'M. Bockris, D. M. Drazic, Electro-Chemical Science, Barnes and Noble Books, New York, 1972, Chapter 3. 4. T. Tani, S. Kikuchi, Photogr. Sci. Eng., 11, 129(1967). 5. T. Tani, K. Ohzeki, K. Seki, /. Electrochem. Soc., 138, 1411(1991). 6. (a) R. C. Baetzold, Photogr. Sci. Eng., 17, 78(1973); (b) R. C. Baetzold, Photogr. Sci. Eng., 19, 11(1975); (c) R. C. Baetzold, J. Photogr. Sci., 28, 15(1980). 7. T. Tani, Phys. Today, September 1989, p. 36. 8. (a) I. Konstantinov, A. Panov, J. Malinowski, J. Photogr. Sci., 21, 250(1973); (b) I. Konstantinov, J. Malinowski, /. Photogr. Sci., 23, 1(1975); (c) I. Konstantinov, J. Malinowski, J. Photogr. Sci., 23, 45(1975). 9. T. Tani, M. Murofushi, J. Imaging Sci. Technol, 38, 1(1994). 10. L. K. J. Tong, M. Carolyn Glesmann, Photogr. Sci. Eng., 8, 319(1964). 11. L. K. J. Tong, C. A. Bishop, M. Carolyn Glesmann, Photogr. Sci. Eng., 8, 326(1964).

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12. J. Eggers, in Die Grundlagen der Photographischen Prozesse mit Silberhalogeniden, H. Frieser, G. Haase, E. Klein, eds., Akademische Verlag, Frankfurt, 1968, p. 813. 13. A. Lumier, L. Lumier, Bull. Soc. Franc. Photogr., 1, 310(1891). 14. M. Anderson, Photogr. Mitt., 28, 286(1891). 15. M. Anderson, Photogr. Mitt., 28 296(1892). 16. J. D. Kendall, IX Congress Intern, de Photographic Scientifique etAppliquee, Paris, 1935, L. P. Clere, ed., Edition Rev. d'Optique, Paris, 1936, p. 277. 17. W. Pelz, Angew. Chem., 66, 231(1954). 18. M. Abribat, Bull. Soc. Chim. France, 29, 265(1921). 19. J. Q. Umberger, Photogr. Sci. Eng., 3, 18(1959). 20. J. Q. Umberger, Photogr. Sci. Eng., 10, 8(1966). 21. T. Tani, Photogr. Sci. Eng., 15, 374(1971). 22. A. Streitwieser, Jr., J. Am. Chem. Soc., 82, 4123(1960). 23. N. Muller, L. W. Pickett, R. S. Mulliken, J. Am. Chem. Soc., 76, 4770(1954). 24. N. Muller, R. S. Mulliken, /. Am. Chem. Soc., 80, 3489(1958). 25. A. Streitwieser, Jr., P. V. Nair, Tetrahedron, 5, 149(1959). 26. O. Chalvet, R. Daudel, J. Chim. Phys., 49, 79(1952). 27. C. A. Coulson, H. C. Longet-Higgins, Proc. Ry. Soc. Land., A192, 16(1947). 28. C. A. Coulson, G S. Rushbrooke, Proc. Cambridge Phil. Soc., 36, 193(1940). 29. A. Streitwieser, Jr., Molecular Orbital Theory for Organic Chemists, John Wiley & Sons, New York, 1961, Chapter 2. 30. R. Daudel, R. Lefevre, C. Moser, Quantum Chemistry-Methods and Applications, Interscience Publishers, New York, 1959, Chapter 4. 31. R. L. Bent, J. C. Dessloch, F. C. Duennebier, D. W. Fassett, D. B. Glass, T. H. James, D. B. Julian, W. R. Ruby, J. M. Snell, J. H. Sterner, J. R. Thirtle, P. W. Vittum, A. Weissberger, J. Am. Chem. Soc., 73, 3100(1951). 32. J. Cabannes, Y. Rocard, La Diffusion Moleculaire de la Lumiere, Les Presses Universite de France, Paris, 1929, p. 83. 33. P. O. Hoffmann, Phys. Z, 36, 650(1935). 34. (a) T. H. James, Photogr. Sci. Tech., (2)2, 153(1955); (b) T. H. James,Sc;ewce and Applications of Photography (Proc. Int. Centenary Conf. London, 1953), R. S. Schultze, ed., Royal Photographic Society, London, 1955, p. 155. 35. (a) A. L. Kartuzanskii, Zh. Nauch. Prikl. Fotogr. Kinematogr., 1, 183(1956); (b) A. L. Kartuzanskii, Zh. Nauch. Prikl. Fotogr. Kinematogr., 4, 417(1959). 36. H. E. Spencer, R. E. Atwell, J. Opt. Soc. Am., 54, 498(1964). 37. (a) P. J. Hillson, / Photogr. Sci., 22, 31(1974); (b) J. H. W. Cramp, P. J. Hillson, J. Photogr. Sci., 24, 30(1976); (c) J. H. W. Cramp, P. J. Hillson, J. Photogr. Sci., 24, 25(1976). 38. (a) R. B. Pontius, R. G. Willis, R. J. Newmiller, Photogr. Sci. Eng., 16, 406(1972); (b) R. B. Pontius, R. G. Willis, Photogr. Sci. Eng., 17, 21(1973); (c) R. B. Pontius, R. G. Willis, Photogr. Sci. Eng., 17, 326(1973). 39. T. H. James, J. Phys. Chem., 43, 701(1939). 40. T. H. James, J. Phys. Chem., 44, 42(1940). 41. T. H. James, J. Franklin Inst., 240, 15(1945). 42. T. H. James, J. Franklin Inst., 240, 83(1945). 43. T. H. James, J. Franklin Inst., 240, 229(1945). 44. T. H. James, J. Franklin Inst., 240, 327(1945). 45. E. B. Brown, L. K. J. Tong, Photogr. Sci. Eng., 19, 314(1975). 46. (a) D. D. F. Shiao, E. L. Dedio, Photogr. Sci. Eng., 25, 145(1981); (b) D. D. F. Shiao,

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47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75.

76.

77. 78.

79. 80. 81. 82.

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J. Imaging Sci., 31, 64(1987); (c) D. D. F. Shiao, J. Imaging Sci., 31, 70(1987); (d) D. D. F. Shiao, T. Bistrovich, J. Imaging Sci., 35, 279(1991). C. R. Berry, Photogr. Sci. Eng., 13, 65(1969). W. E. Lee, in The Theory of the Photographic Process, 4th ed., T. H. James, Ed., Macraillan, New York, 1977, pp. 432-435. G. I. P. Levenson, Photogr. J., 88B, 102(1948). G. I. P. Levenson, Photogr. J., 89B, 2(1949). G. I. P. Levenson, Photogr. J., 92B, 109(1952). T. H. James, J. Phys. Chem., 66, 2416(1962). T. H. James, Photogr. Sci. Eng., 14, 371(1970). T. H. James, / Photogr. Sci., 16, 121(1971). G. C. Farnell, R. L. Jenkins, J. Photogr. Sci., 29, 39(1981). S.-X. Ji, F.-Y. Ma, X.-M. Ren, / Photogr. Set, 39, 28(1991). K. Miyake, T. Tani, J. Imaging Sci. TechnoL, in press (1995). V. I. Sheberstov, Yu. I. Bukin, Kino-Foto Prom., 1, 101(1932). T. H. James, J. Franklin Inst., 239, 41(1945). V. I. Sheberstov, Usp. Nauch. Fotogr., 4, 210(1955). P. J. Hillson, J. Photogr. Sci., 22, 31(1974). T. H. James, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, Chapter 14. T. H. James, W. Vanselow, R. F. Quirk, PSA J., 14, 349(1948). H. E. Spencer, J. Imaging Sci., 32, 40(1988). M. Kawasaki, H. Hada, J. Imaging Sci., 35, 129(1991). R. W. Swenson, Photogr. Sci. Eng., 1, 119(1958). P. J. Hillson, Photogr. Sci. Eng., 23, 38(1979). M. A. Kriss, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, Chapter 21. W. J. Anderson, Photogr. Sci. Eng., 21, 1(1977). A. C. Yule, J. Franklin Institute, 239, 221(1945). R. L. Childers, Photogr. Sci. Eng., 15, 480(1971). M. J. Austin, J. Photogr. Sci., 22, 293(1974). R. E. Stauffer, W. F. Smith, A. P. H. Trivelli, J. Franklin Inst., 238, 291(1944). H. C. Daly, R. A. Ramirez, G. L. Tice, J. C. Loblaw, J. Photogr. Sci., 32, 170(1984). (a) S. Moriuchi, J. Soc. Printing Sci. TechnoL, 24, 299(1987); (b) K. Katou, S. Sasaoka, S. Hirano, J. Soc. Photogr. Sci. TechnoL Jpn., 52, 390(1989); (c) K. Katou, M. Yagihara, J. Soc. Photogr. Sci. TechnoL, 53, 495(1990); (d) H. Okamura, J. Soc. Photogr. Sci. TechnoL, 55, 211(1992). (a) J. P. Kitchin, K. P. Hall, A. W. Mott, C. Marchesano, R. Bowman, J. Photogr. Sci., 35, 162(1987); (b) J. P. Kitchin, K. P. Hall, A. W. Mott, C. Marchesano, R. Bowman, J. Imaging TechnoL, 15, 282(1989). K. Shinohara, E. Bayer, J. Photogr. Sci., 35, 181(1987). D. L. Kerr, "An Environmentally Improved Nucleation Process for Graphic Arts Imaging,' ' in the preprint book of' The International East-West Symposium III. New Frontiers in Silver Halide Imaging, November 8-13, 1992, Kaanapali, Hawaii", co-sponsored by Soc. Imaging Sc. TechnoL and Soc. Photogr. Sci. TechnoL Jpn., E-l. W. T. Hanson, Jr., P. W. Vittum, J. Photogr. Soc. Am., 13, 94(1947). W. T. Hanson, Jr., J. Opt. Soc. Am., 40, 171(1950). C. R. Barr, J. R. Thirtle, P. W. Vittum, Photogr. Sci. Eng., 13, 74(1969). H. Kobayashi, K. Mihayashi, SPSE Annual Conference, Atlantic City, May 13-14, 1985.

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83. J. R. Jarvis, P. J. Twist, "The Effect of Development Accelerator Releasing Coupler on Color Negative Imaging Efficiency," The International East-West Symposium II, Factors Influencing the Efficiency of Photographic Imaging, October/November 4, 1988, Kona, Hawaii. 84. L. K. J. Tong, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, pp. 339-353. 85. T. Tani, /. Imaging Sci. TechnoL, 39, 31(1995).

8 Future Prospect of Silver Halide Photography

As seen in Figs. 1.7 and 1.8 and Table 1.1, the strongest point of silver halide photography as compared with other photosensitive materials is its capability to achieve both high sensitivity and high image quality (i.e., resolving power). However, the most serious problems to be solved for the future of silver halide photography are in its processing. There are strong demands for improvement in photographic processing in order for silver halide photography to meet rapid access demands and environmental protection requirements. It is also noted that photographic science and technology are making progress, and new attractive products and systems to which they can be applied are needed. In this chapter comments are presented in some detail on the future prospects of the efficiency of image formation in relation to sensitivity and image quality, and briefly, to processing and systems of silver halide photography.

8.1 Improvement in Efficiency of Image Formation in Relation to Sensitivity and Image Quality In order to predict future prospects for sensitivity with reference to the image quality of silver halide photography, it would be useful to measure and analyze quantum sensitivity (see Chapter 4) and detective quantum efficiency (DQE; see Chapter 1). Bird et al. measured, for the first time, the DQE of photographic materials and found that the DQE of a high-speed photographic film for taking pictures was as low as about 1%. According to their analysis, the DQE value was reduced to 42% by incomplete light absorption, to 64% (total 27%) by the four-atom threshold for latent image formation, to 16% (total 4.3%) by recombination losses in latent image formation, to 36% (total 1.5%) by the distributions of grain size and sensitivity, and to 72% (total 1.1%) by random grain arrangement.1 The DQE values of photographic films have been improved to a limited degree since that time.2 There is thus considerable room for improvement in the efficiency of photographic sensitivity. In this chapter analyses are given of the possibility of improving the efficiency of photographic sensitivity for taking pictures. Since photosensitive materials of 228

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silver halide photography are photographic emulsion layers with silver halide grains suspended in gelatin layers, analyses are made on problems with an individual silver halide grain and with assembly of the grains. 8.1.1 Analyses of Problems with Individual Silver Halide Grains Light absorption The light absorbance of a photographic film is quite low. According to Bird et al.,1 the percent absorbance of a high-sensitive photographic film is as low as in the intrinsic (blue) region and 10-20% in the spectrally sensitized region. These incomplete light absorptions of photographic film arise from the poor light absorbance of silver halide grains (see Chapter 3) as well as from the arrangement of grains in thin layers with a low packing density (see Chapter 1). In color films, blue light is absorbed by silver halide grains, and minus-blue light is absorbed by sensitizing dyes on the grain surface. As stated in Chapter 3, the absorption of blue light by silver halide is ascribed to its indirect transition, which is associated with the change in momentum, and thus has a small probability. The absorbance of blue light by a silver halide grain is thus small,3 and the number of photons absorbed by a grain is nearly proportional to its volume.4 The percent absorptions of a cubic AgBr grain with an edge length of 1 /urn at 420 and 460 nm are about 10% and 1%, respectively. One of the most effective methods for increasing the light absorbance of silver bromoiodide grains is to increase their silver iodide content. The incorporation of 3 mol percent of silver iodide into silver bromide increase the percent absorption at 460 nm by five times.3 As shown in Fig. 2.7, the double-structured grains have been designed to increase the light absorbance of the grains by incorporating silver bromoiodide with a high concentration of silver iodide into the core, which is then covered with silver bromide to inhibit the appearance of harmful photographic effects of iodide ions such as retardation of development.5 The problem of incomplete light absorption of silver halide grains could be improved by double- structured grains without introducing harmful photographic effects, although light absorption of the grains in the blue region is still far from completion. Another method to improve poor light absorption of emulsion grains in the blue light region is to spectrally sensitize the grains with sensitizing dyes for blue light. Since sensitizing dye molecules are adsorbed to each grain surface with a monomolecular layer, the amount of the dye that can be adsorbed by a grain is limited by its surface area. The number of photons absorbed by sensitizing dye molecules per grain is therefore proportional to its surface area,4(b)?6 and is quite poor. Extensive efforts have been made to improve the poor light absorbance of sensitizing dyes on an emulsion grain. One of these has been to use emulsion grains with a large specific area, as exemplified by tabular grains,7 which can greatly improve the absorbance of color film.8 Another method to improve poor light absorption by sensitizing dyes is to use sensitizing dyes whose molar absorption coefficients are large and are concentrated in the desired wavelength range. From this viewpoint, J-aggregates giving very sharp and intense absorption bands, as described in Chapter 5, are indispensable, especially

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for color photographic materials. The light absorbance of J-aggregates of a panchromatic sensitizing dye (5,5'-dichloro-9-ethyl-thiacarbocyanine), which covered 65% of the surface of an evaporated AgBr layer, was 0.09 at its peak, and the halfwidth of the absorption band was 283 cm"1, as shown in Fig. 8.1.9 Taking into account the similarity of an evaporated layer to a tabular grain, the absorbance of these dye molecules on a tabular grain is too weak to absorb all incident photons in the panchromatic range (i.e., 600-700 nm). Because sensitizing dyes with a monomolecular layer on a grain surface are used for spectral sensitization of the grain, the fraction of incident photons absorbed by the grain is probably far from unity. Furthermore, the degree of desensitization and stain of films caused by sensitizing dyes steeply increases when the amount of adsorbed dye molecules approaches the value for saturation of the grain surface.10 Thus, it is important to develop technologies to reduce the desensitization and stain caused by dyes. Several ideas have been proposed to achieve spectral sensitization by sensitizing dye molecules that are not in contact with the grains, although they have not nec-

Fig. 8.1. Absorption spectrum of J-aggregates of 3,3'-disulfopropyl-9-ethyl-thiacarbocyanine that covered 65% of the surface of an evaporated AgBr layer (i.e., 1.3 X 10~6 mol/m2).

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essarily been successful up until the present time. Oilman proposed the dye multilayer technique, according to which a silver halide grain is covered with multilayers composed of a first monomolecular layer of positively charged dye molecules, a second monomolecular layer of negatively charged dye molecules, and some supersensitizer molecules.11 The electronic structures of the dyes need to be adjusted to meet the condition whereby the positively charged dye could be supersensitized by the negatively charged dye, and the positively charged dye does not hinder electron transfer from the negatively charged dye in the excited state to the grain. Using antenna chlorophyll in photosynthesis as a model, some proposals have been made for light-harvesting systems, according to which energy transfer takes place to sensitizing dyes on the surface of silver halide grains from excited dye molecules that are not in contact with the grains.12 There have been some reports on spectral sensitization of silver halide grains by grains of sensitizing dyes and colored inorganic compounds, that are in contact with the silver halide grains.13 Recombination between photoelectrons and positive holes As described in Chapters 3-6, recombination between photoelectrons and positive holes takes place through various processes, and is the main cause for the low efficiency of latent image formation in silver halide photographic emulsions. As described in Chapters 4 and 6, the most effective way to prevent recombination is to introduce R centers of reduction sensitization (Ag2) on the surface and/or interior of silver halide grains.14 The essential processes for the retardation of recombination by R centers are described by Equations (4.5)-(4.7) in Chapter 4 and are composed of efficient trapping of a positive hole by an R center, an ionic relaxation process to prevent the trapped positive hole from recombining with a photoelectron, and liberation of an extra free electron available for latent image formation. Reduction sensitization or hydrogen hypersensitization introduces R centers on silver halide emulsion grains. The fact that latent image formation with an efficiency at the theoretical limit could be nearly achieved on sulfur-plus-gold-sensitized hydrogenhypersensitized fine AgBr grains indicated that R centers could be effective in the prevention of recombination.15 There are several difficulties in practice with using R centers. As implied by the fact that reduction sensitization centers are, at least in part, easily altered to fog centers by gold latensification,16 application of reduction sensitization together with gold sensitization to emulsion grains leads to the formation of fog centers. Incorporating reduction sensitization centers into the interior of the grains17 should be effective for sensitization of photographic emulsions in the intrinsic region. However, the reduction sensitization centers in the interior of the grains cannot be used for preventing recombination of dye positive holes present on the grain surface with photoelectrons or image centers unless positive holes can be released from dyes and moved to the interior of the grains. Berriman and Oilman showed that positive holes could not move to the interior of silver halide emulsion grains from sensitizing dyes whose HOMO (i.e., the highest occupied molecular orbital) levels are high and considerably apart from the top of the valence band of the silver halide.18 In spectrally sensitized silver halide grains, it is not easy to prevent electron-hole

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recombination by reduction sensitization centers, even when they are present on the surface of grains. Electron transfer from excited dye molecules to grains leaves positive holes on the dyes, which then move on the grain surface with low mobility through repetitive detrapping and trapping by dye molecules.19 Thus, it is not easy for positive holes to reach reduction sensitization centers on the grain surface in the presence of sensitizing dyes when their HOMO levels are high. It is desirable to find other species that can prevent recombination of positive holes with photoelectrons and image centers as well as to develop technologies for application of reduction sensitization to practical emulsions.

Dispersion of image centers The formation of more than one image center on a grain wastes photoelectrons and is called dispersion of image centers. As described in Chapters 4 and 6, sulfur sensitization centers and sulfur-plus-gold sensitization centers are formed in excess on a grain20'21 and provide shallow electron traps in which deep trapping latent image centers are formed. For example, sulfur sensitization centers are dimers of Ag2S and as many as 2800//zm2 exist on the (100) face of AgBr under optimum conditions.21 Thus, one latent image center must be formed on a grain in the presence of a large number of sensitization centers acting as electron traps. Thus, once the nucleation of an image center is achieved, only the growth of the image center should proceed, preventing the occurrence of further nucleation. Although these sensitization centers acting as electron traps are necessary for nucleation of the formation of an image center, they tend to disturb the growth of an image center, since they further enhance nucleation by decreasing the mobility and diffusion length of photoelectrons, which should reach the growing image centers. It is therefore important to look for the optimum condition for electron trapping of these sensitization centers to meet the demand for high efficiency in latent image formation. Factors influencing the rate of the nucleation of image centers include those associated with the electron traps provided by those sensitization centers and interstitial silver ions. The former includes the number, cross section, and depth of the electron traps, which depend upon the concentrations and kinds of sensitization centers and crystal defects. As seen in Fig. 4.2, the sensitivity of sulfur-sensitized AgBr grains increased and then decreased after passing its maximum by increasing the amount of sulfur sensitizer.22 Sensitivity also increased and then decreased by increasing the concentration of crystal defects.23 As seen in Fig. 4.5, the degree of dispersion of latent image centers decreased and then increased by increasing the concentration of interstitial silver ions in silver halide emulsion grains.24 It is important to control the rate of nucleation of image centers by adjusting the electronic and ionic processes to achieve latent image formation with high efficiency.

8.1.2 Analyses of Problems with Assembly of Silver Halide Grains Photographic phenomena appear as a result of the accumulated photographic behavior of many silver halide grains. In this section analyses are made of the arrangement

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of silver halide grains in photographic emulsion layers, the distribution of size and sensitivity among the grains, and the interactions between grains during processing. Arrangement of silver halide grains in photographic emulsion layers Since emulsion layers are also used for display, they are thin, contain silver halide grains with a low packing density, and have only a weak ability to absorb incident light, as described in Section 1.2 in Chapter 1. When the packing density is constant, the fraction of incident light absorbed by the layers increases, and the image quality decreases with increasing thickness of the layers. It is necessary to develop a technology to improve both the light absorbance and image quality of these layers. It has been demonstrated that the fraction of incident light absorbed by silver halide grains and sensitizing dyes on the grains increased to a considerable degree when a high-speed color negative film was designed using tabular grains instead of conventional grains,8 although there is still considerable room for improvement. As stated in Chapter 1, the random arrangement of large numbers of silver halide grains in emulsion layers brings about the insensibility of photographic materials to small numbers of inferior silver halide grains and dust particles introduced into the layers, and enables the mass production of photographic materials with a large area. This advantage is liable to deteriorate when grains are regularly arranged in order to improve the image quality and DQE of photographic materials. Taking into account the facts that the regular arrangement of the grains has a poor prospect of success at present and would increase DQE by at most 40% according to the analysis by Bird et al., it seems that the regular arrangement of silver halide grains in emulsion layers is not necessarily promising. Distribution of size and sensitivity among silver halide grains Most photographic materials need appropriate gradation and exposure latitude, and therefore photographic emulsion layers contain silver halide grains whose sensitivity is distributed over appropriate range. As stated in Section 8.1, the sensitivity of a photographic emulsion is dependent upon the average size of silver halide grains and is usually controlled by adjusting the grain size. When the distribution of sensitivity among grains is expanded, the DQE value becomes lower at the maximum and more wide ranging in the plot of DQE versus exposure. Bird took the integrated value of the function of DQE versus exposure as the efficiency of photographic films to take pictures and indicated that the integrated value was largest when the smallest number of monodispersed emulsions was used to make pictures with the desired gradation.25 Since the most sensitive grains should be the position to capture incident light in advance of light absorption by other grains, the emulsion with highest sensitivity should be at the top, followed by emulsion layers in the order of sensitivity.22 It is therefore important to prepare monodispersed emulsions with a sharp distribution of sensitivity among the grains, as described in Chapter 2. Although various monodispersed model emulsions have been prepared, the grain sizes of emulsions used for products are still widely distributed and leave much room for future improvement.

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Interaction among grains The representative phenomena resulting from interactions between silver halide grains are adjacency and interlayer effects taking place during development.26 These phenomena are used as methods for image processing in silver halide photographic materials.27 Retardation of development of a developing grain and its adjacent grains by substances released from development reactions, as described in Section 7.5 in Chapter 7, could improve image quality at the expense of sensitivity and gradation to some extent. However, enhancement of development of a developing grain and its adjacent grains by substances released from development reactions (i.e., infectious development) could increase the apparent sensitivity at the expense of image quality,28 as shown in Fig. 7.4. In the latter case, an idea to restrict the range to which the infectious development starting from a grain could reach was proposed to decrease the degree of the deterioration of the image quality by the infectious development.90 8.1.3 Are Silver Halide Grains Necessary? Silver halide photographic materials starting from a thin Agl layer (Chapter 1) have undergone significant improvement, and several functions originally brought about by silver halides have been achieved by other substances. Analyses are made in this section on the need for silver halide grains to capture incident light, to form latent image centers, and display an image. Capture of incident light Silver halide grains are not strong capturers of incident light and can be expected to be replaced by other materials for this purpose. Already, green and red light incident to photographic materials are captured by sensitizing dyes on silver halide grains. Proposals have been made for light-harvesting systems using dyes that are not in contact with silver halide grains and using silver halide grains spectrally sensitized by sensitizing dye grains and inorganic grains. Although these are not necessarily successful for practical use at present, they indicate the possibility of replacing silver halides with other substances to capture incident light. Latent image formation There is still a significant degree of inefficiency in latent image formation. However, cooperation between electronic and ionic processes in silver halides could provide the unique processes that brought about the concentration principle and prevention of recombination, and led to efficient formation and significant amplification of latent images centers. The stability and catalytic action of such small silver clusters as latent image centers are other aspects of the unique properties of silver halides. These functions have not be found in other materials. Furthermore, there is no evident prospect of finding substances in which these functions take place efficiently. Thus, silver halide grains will probably not be replaced by other substances for taking

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pictures in the future. When capture of incident light by substances other than silver halides and electron transfer from the substances to silver halide grains is efficiently achieved, only small silver halide grains will be needed and will be most suitable for latent image formation at a high efficiency.15 Display of images Although amplification of latent images leads to the display of images in silver halide photography, dependent upon the unique property of latent image centers on silver halide grains, silver halides could be successfully replaced by other materials for the purpose of image display. In color photography images are displayed by dyes formed by the reaction of oxidized products of color developing agents with couplers. These reactions are designed to improve the image quality of color films by the use of functional couplers.27 A new proposal has been made for a system in which an analog image in a color film is transferred to a digital electric image, subjected to image processing, and displayed by various devices.30 Thus, silver halides are in principle needed only for the formation and amplification of latent image centers and are expected to be replaced by other compounds for other purposes. The unique capability of silver halide photographic materials for taking pictures depends upon the functions of silver halide grains to efficiently form small silver clusters as a latent image center, which then achieves a large degree of amplification. 8.1.4 Possibility of Improving the Efficiency of Image Formation in the Near Future Although important, it is difficult to estimate the possibilities for improving the efficiency of image formation in silver halide photographic materials in the near future, since there have been few publications concerning the properties of light absorbance and the quantum yield of photographic products for practical emulsion layers.31 In 1985, Tani determined that photographic sensitivity is dependent upon light absorption, the quantum yield of latent image formation, and the size of the smallest latent image center.4(b) The fraction of incident photons absorbed by photographic films for taking pictures was thought to be at most one third and could thus be increased by about three times in the future. On the basis of the fact that the quantum sensitivity (i.e., the number of absorbed photons/grain needed for formation of a latent image center) of photographic films for taking pictures was around 10 and the theoretical limit is two, the quantum sensitivity was expected to increase by about five times in the future. In total, it was predicted that there would be room for an increase in sensitivity of about one order of magnitude in the future. Since then, remarkable progress has been made in improving the capabilities of photographic films for taking pictures.31"33 The light absorption of color films was improved by the use of tabular grains,8 and the possibility for increase in light absorption in the future is about two times. The average volume of silver halide grains in color films was greatly reduced without changing their sensitivity, indi-

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eating the achievement of an increase in quantum sensitivity of the grains.33 It is therefore reasonable to consider that there will be room for a several times improvement in the efficiency of image formation in the future.31 In 1989 Bird predicted that silver halide photographic systems would advance from the DQE value of 1-2% at that time to about 10% and that any such improvement would be reflected in all silver halide systems.34 The expected improvement in the efficiency of image formation in the future will be used to achieve either an increase in image quality by reducing the size of emulsion grains without changing their sensitivity or an increase in sensitivity without changing image quality in order to meet demands for photographic materials.

8.2 Improvement in Photographic Processing Photographic processing with solutions containing numerous chemicals poses great problems for silver halide photography to meet demands such as the need for rapid processing and environmental protection. Extensive efforts are being made to develop dry-processing silver halide photographic materials as well as to improve the processing of conventional photographic materials. Remarkable progress is being made in dry processing by developing photothermographic materials based on silver halide photography35'36 and in conventional processing, as seen in Fig. 8.2, especially by developing minilab printer-processors for rapid processing that meet environmental protection standards.37 These efforts will continue to bring progress to the field of photographic processing.

8.3 Development of New Photographic Systems From the analyses in Chapters 1 and 8, it is expected that silver halide photographic materials will be the leading imaging system and will not be replaced by other photosensitive materials for taking pictures.31'38 Silver halide photographic materials are founded on numerous technologies, including the core technologies of photographic emulsions and color forming chemistry. Photographic emulsion technology is considerably rich and has produced many fruitful results (i.e., seeds) that have accumulated over its long history and await new products and systems (i.e., needs) to which they can be applied. New seeds in emulsion technology will be continuously developed and accumulated in the future. New seeds seek new needs and new needs stimulate the development of new seeds. It is therefore important to search for new needs, that is, new products and systems. Among recent epoch-making events in silver halide photography were the development of the mini-lab printer-processor37 and lens-incorporated films (singleshot cameras).39 Neither was predicted before their appearance. Even when they appeared, it was not easy for even specialists in charge of development of conventional photographic systems to accurately predict the growth of these systems. These circumstances indicate that it is difficult to predict the future of photographic products and systems. In order to do this more precisely, proposals not only from the

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Fig. 8.2. Changes in processing time (a), replenishment rate of processing solutions (b), and processing temperature (c) for color negative films (O) and color papers (•). The number in (c) refers to the number of processing steps. specialists in charge of the development of conventional photographic products, but also from people throughout the industry have to be considered for examination.

References 1. G. R. Bird, R. C. Jones, A. E. Ames, Appl. Optics, 8, 2389(1969). 2. (a) P. Kowalski, A. E. Saunders, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, Chapter 22; (b) R. Shaw, Opt. Act., 20, 749(1973); (c) H. E. Spencer, Photogr. Sci. Eng., 15, 468(1971); (d) M. R. V. Sahyun, Photogr. Sci. Eng., 19, 38(1975); (e) G. R. Bird, M. D. Cox, Photogr. Sci. Eng., 25, 246(1981). 3. F. Moser, R. K. Ahrenkiel, in Ref. 2(a), pp. 37-50.

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4. (a) G. C. Farnell, J. Photogr. ScL, 17, 116(1969); (b) T. Tani, J. Imaging ScL, 29, 93(1985). 5. (a) S. Bando, Y. Shibahara, S. Ishimaru, J. Imaging ScL, 29, 193(1985); (b) S. Takada, H. Ayato, S. Ishimaru, in Progress in Basic Principles of Imaging Systems, F. Granzer, E. Moisar, eds., Friedr. Vieweg & Sohn, Braunschweig & Wiesbaden, 1987, pp. 81-87. 6. H. Zwicky, Z. Wiss. Photogr., 53, 68(1959). 7. (a) Anonymous, Res. Disc. (Item 22534) 225, 20(Jan. 1983); (b) A. F. Sowinski, P. J. Wightman, J. Imaging ScL, 31, 162(1987); (c) R. E. Jacobson, N. R. Axford, A. Ford, J. Photogr. ScL, 38, 140(1990). 8. J. C. Marchant, New Opportunities for Improved Image Recording Systems, Int. Congr. Photogr. Sci., Cologne, 1986; J. C. Marchant in Progress in Basic Principles of Imaging Systems, F. Granzer, E. Moisar, eds., Friedr. Vieweg & Sohn, Braunschweig/Wiesbaden, 1986, p. 9. 9. S. Watanabe, T. Tani, J. Imaging ScL Technol, 39, 81(1995). 10. (a) T. Tani, Photogr. Sci. Eng., 19, 356(1975); (b) T. Tani, J. Imaging ScL, 33, 17(1989); (c) T. Tani, T. Suzumoto, J. Appl. Phys., 70, 3626(1991). 11. T. L. Penner, P. B. Oilman, Photogr. Sci. Eng., 20, 97(1976). 12. (a) G. R. Bird, Photogr. Sci. Eng., 18, 562(1974); (b) R. Steiger, J. F. Reder, Photogr. ScL Eng., 21, 59(1983). 13. (a) G. House, in Ref. 62(b), p. 348; (b) B. Levy, M. Lindsey, Photogr. ScL Eng., 17, 135(1973). 14. T. Tani, Phys. Today, September 1989, 36. 15. T. A. Babcock, T. H. James, J. Photogr. ScL, 24, 19(1976). 16. (a) T. H. James, W. Vanselow, R. F. Quirk, PSA J., 14, 349(1948), (b) H. E. Spencer, L. E. Brady, J. F. Hamilton, J. Opt. Soc. Am., 57, 1020(1967). 17. E. Moisar, E. Palm, F. Granzer, D. Dautrich, J. Photogr. ScL, 25, 19(1977). 18. R. W. Berriman, P. B. Oilman, Jr., Photogr. ScL Eng., 17, 235(1973). 19. T. Tani, / Appl. Phys., 62, 2456(1987). 20. (a) T. Shiozawa, T. Kobayashi, Phys. Stat. Sol., 101, 375(1988); (b) T. Shiozawa, T. Kobayashi, Phys. Stat. Sol., (a) 116, 513(1989). 21. Y. Tadakuma, Y. Yoshida, H. Kanzaki, "A Study on Aggregation Process of Sulfur Sensitization Centers on AgBr Emulsion Grains," in the preprint book of the Annual Meeting of Soc. Photogr. Sci. Technol. Jpn., May, 1990, Makuhari. 22. T. Tani, Photogr. ScL Eng., 15, 28(1971). 23. G. C. Farnell, J. Photogr. ScL, 27, 160(1979). 24. S. Takada,/ Soc. Photogr. Sci. Technol. Jpn., 42, 112(1979). 25. G. R. Bird, "Optimizing the Entire Photographic Systems for Information Capture," in the preprint book of Tokyo Symposium sponsored by Soc. Photogr. Sci. Tech. Japan, July, 1980, Tokyo. 26. M. A. Kriss, in The Theory of the Photographic Process, 4th ed., T. H. James, ed., Macmillan, New York, 1977, Chapter 21. 27. (a) W. T. Hanson, Jr., P. W. Vittum, J. Photogr. Soc. Am., 13,94(1947); (b) W. T. Hanson, Jr., J. Opt. Soc. Am., 40, 171(1950); (c) C. R. Barr, J. R. Thirtle, P. W. Vittum, Photogr. ScL Eng., 13, 74(1969); (d) H. Kobayashi, K. Mihayashi, "Enhancement of Photographic Image by Image-Amplifier Releasing Coupler," in the preprint book of SPSE Annual Conference, Atlantic, May, 1985; (e) J. R. Jarvis, P. Twist, "The Effect of Development Accelerator Releasing Coupler on Color Negative Imaging Efficiency," in the preprint book of The International East-West Symposium II. ' 'Factors Influencing The Efficiency of Photographic Imaging", October/November, 1988, Kona, Hawaii; (f) S. Ichijima, Fuji Res. Dev., 35, 27(1990).

Future Prospect of Silver Halide Photography

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28. W. J. Anderson, Photogr. Sci. Eng., 21, 1(1977). 29. K. E. Whitmore, W080/01614. 30. (a) Eastman Kodak Company, Kodak Photo CD System—A Planning Guide for Developers, Rochester, NY, 1991; (b) J. W. Meyer, Images of the Future: The Convergence of Silver Halide and Electronics Technologies, The International East-West Symposium III on New Frontiers in Silver Halide Imaging, November, 1992, Kaanapali, Hawaii. 31. T. Tani, J. Imaging Sci. Technol, 39, 31(1995). 32. S. Ishimaru, H. Ikeda, K. Sakanoue, K. Miyazaki, S. Hirano, J. Tamano, Fujifilm Res. Dev., 37, 1(1992). 33. N. Sasaki, Fujifilm Res. Dev., 39, 1(1994). 34. G. R. Bird, in Imaging Processes and Materials. Neblette's Eight Edition, J. Sturge, V. Walworth, A. Shepp, eds., Van Nostrand Reinhold, New York, 1989, Chapter 19. 35. D. H. Klosterboer, in Imaging Processes and Materials. Neblette's Eight Edition, J. Sturge, V. Walworth, A. Shepp, eds., Van Nostrand Reinhold, New York, 1989, Chapter 9. 36. (a) H. Kara, K. Sato, Fuji Res. Dev., 34, 10(1989); (b) T. Yokokawa, K. Nakamura, N. Matsumoto, Fuji Res. Dev., 37, 49(1992). 37. (a) K. Tokuda, K. Uenaka, J. Nakajima, T. Terasaki, T. Kimura, Sci. Pub. Fuji Photo Film Co., Ltd., 32, 1(1986); (b) K. Suzuki, H. Sato, Y. Ozawa, T. Terashita, K. Tokuda, Fuji Res. Dev., 38, 1(1993). 38. (a) L. J. Thomas, "Photographic Systems Analysis," in the preprint book of Tokyo Symposium sponsored by Soc. Photogr. Sci. Tech. Jpn., July, 1980, Tokyo; (b) M. Tabei, Y. Mizobuchi, J. Soc. Photogr. Sci. Tech. Jpn., 49, 125(1986); (c) A. Kriss, J. Soc. Photogr. Sci. Tech. Jpn., 50, 357(1987); (d) S. Ikenoue, M. Tabei, J. Imaging Sci., 34, 187(1990); (e) T. Tani, Y. Ohishi, J. Soc. Photogr. Sci. Tech. Jpn., 52, 218(1989). 39. M. Mochida, H. Ohmura, H. Takei, Sci. Pub. Fuji Photo Film Co. Ltd., 33, 15(1988).

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Index absorption coefficient of dyes, 119, 121 absorption spectra cyanine dyes, 113, 122 double band, 121 fog centers of reduction sensitization, 98100, 122 H-band, 118 J-band, 118-119 latent image centers, 99-101 M-band, 118-119 P centers of reduction sensitization, 181, 122 silver bromide, 45 silver chloride, 45 silver sulfide specks, 168 wavelength maxima of vinylogous dyes, 112-113 activation energy collinear interstitialcy jump, 51 decay of dye positive holes, 144, 148 decay of latent image centers, 145 decomposition of thiosulfate ion, 168 electron transfer in spectral sensitization, 136-138, 151-152 formation of sulfur sensitization centers, 169-170 ionic conductivity, 49, 51, 53 rate of development, 208, 212 red sensitivity, 170, 178 adjacency effect. See chemical adjacency effect adsorption by silver halides antifoggant, 188 chemisorption of dyes, 124-125 developing agent, 125 edge-on orientation of dyes, 123 end-on orientation of dyes, 123

epitaxial arrangement of dyes, 125 flat-on orientation of dyes, 123 isotherms antifoggants, 188-190 Langmuir-type, 123 sensitizing dye, 122-123 potential energy profile, 123-124 phase-separation method, 123 physical adsorption of dyes, 124-125 reversibility, 123, 125 sensitizing dyes, 122-125 stabilizer, 188 thiosulfate, 168 aggregates of dyes H-aggregates. See H-aggregates of dyes herringbone structure, 121-122 J-aggregates. See J-aggregates of dyes molecular exciton theory, 118-122 slip angle, 118-122, 141 alternate-row model for {111} surface of silver halides, 28 analytical color fluorescence electron microscopy, 122 antenna chlorophyll, 230 antifoggants acid-base equilibrium, 186 adsorption by silver halides, 188-190 benzotriazoles, 184, 186-191 chemical structure, 184-185 definition, 166-167 dissociation constant (pKa), 186-189 Hammett'sr, 186-187 for improvement of discrimination, 220 mechanism, 183-194 mercaptoazoles, 184-188 l-phenyl-5-mercaptotetrazole (PMT), 50, 184

241

242

INDEX

antifoggants (continued) solubility product (pKsp) of silver salts, 185-186, 188-189 antisensitization, 151-152 atomic force microscope, 27 backscattered electron imaging, 31 Brooker's deviation, 130 Cabannes-Hoffman effect, 213 calorimetry, 167 Calotype, 3 carboxyl ion system, 111-112 CCDE. See charge coupled device characteristic curve, 91, 101, 105-106, 146, 183, 192, 221-222 charge barrier theory in development, 213, 215 charge coupled device, 5-6, 8, 11-21 charge-transfer frost process, 12 chemical adjacency effect border effect, 221 fringe effect, 221 interlayer effect, 234 chemical development, 93, 200-201, 205, 214 cluster mechanism for sulfur sensitization, 169 color image chemistry, 5-6, 222-223, 236 dry and/or rapid processing for, 236 image processing for, 222-223 color negative film, 3, 11-14, 237 color paper, 12-14, 237 color reversal film, 14 concentration principle, 81, 83, 85, 87, 90, 234 concentration-speck theory, 83 conduction band of silver halide behavior of photoelectrons in, 60-61, 66 definition, 46^-7 in space charge layer, 69 of tabular grains, 55 controlled double-jet precipitation, 32 core/shell grains, 65 couplers, 222-224, 235 crystal defects dislocations. See dislocations in silver halides

effect on sensitivity, 89 effect on sulfur sensitization, 170 Frenkel, 48-49, 56-58. See also interstitial silver ions; silver ion vacancies role in mechanism of latent image formation, 85, 87 Schottky, 48 stacking fault, 25-31 twin planes, 25, 28, 31, 38, 54, 70-71 crystal habit, 24-26, 178, 190 critical growth rate. See silver halide grains critical supersaturation of silver halides, 33, 39 cubic grains of silver halides adsorption of dyes on, 121-125 desensitization on, 152-158, 194 growth rate, 35-37 high-intensity reciprocity law failure, 9091 ionic conductivity, 51-54, 90 mechanism of formation, 38 progress of development, 215-218 reduction-sensitized, 181-182 sensitization of TAI, 191-192 structure, 24-27, 29 cyanine dyes benzodithiolylium, 125, 130 bond order, 113 Brooker's deviation, 130 chemical structure, 114-116 effect on ionic conductivity of silver halides, 53, 91, 158 effect on dispersion of image centers, 91, 158, 194 electron density distribution, 113, 118, 129 electronic energy levels, 126-131, 134135 H-aggregates. See H-aggregates of dyes J-aggregates. See J-aggregates of dyes loose, compact, and crowded, 121 9-methyl-thiacarbocyanines on, 25, 121122 oxidation potentials, 127-132, 134 polymethine chain, 112-113, 117-118, 127 quinocyanine, 113, 127 reduction potentials, 126-130, 135-136, 138-139

Index steric effect, 113, 128, 131 thiacyanine, 113, 124, 127, 130 transition energy, 117-118, 128-129 wavelength maxima, 112 cyclotron resonance, 62 Daguerreotype, 3 defects in crystal. See crystal defects Dember effect for ionic conductivity measurement, 5051, 191 by photoelectrons, 193-194 by positive holes, 64 for supersensitization for, 138, 151-152 delayed development for time-dependent sensitivity, 105, 144-147 desensitization by dispersion of image centers, 91, 153154, 157-158 effect of evacuation on, 156 by electron-trapping, 137, 139, 153-156 by hole-trapping, 144-147, 153, 156158, 194, 230 desensitizing dyes, 149-150, 156 desorption of dyes from silver halide, 124125 detective quantum efficiency (DQE), 9, 16, 228, 233, 236 developability, 92, 97, 106, 165, 167, 177181,219 developed silver after arrested development, 96, 213 filamentary, 213 growth, 216-220 developer black-and-white, 216 color, 216, 218-219 in development process, 200-202 electrochemistry, 202-203 fog, 219-220 half-cell reaction, 202-204, 208 internal, 192 overpotential, 203, 208-213 redox potentials, 93, 103 surface, 105, 146, 172-173, 192 developing agents adsorption by silver halides, 215 4-amino-N-dialkylanilines, 208, 216 analyzed by molecular orbital method, 205-207

243

chemical structure, 208-212 for color image formation, 6, 222 dissociation constant (pKa), 203 effect of charge, 181,213-214 electrochemistry of, 202-203 general rules, 205-208 highest-occupied molecular orbital, 203, 205 hydroquinones, 205, 215, 222 ionization energy, 204 Kendall-Perz rule, 205-208 one-electron oxidation potentials, 205 oxidation potentials, 204, 208 phenylenediamines, 5-6, 205 1 -pheny 1-3 -py razolidone(phenidon) ,215 superadditive, 215, 216 two-electron oxidation potentials, 205 development accelerator-releasing (DAR) coupler, 223-224 amplification by, 7-10, 200-227 arrested, 95-97, 157 centers. See also latent image centers and fog centers effects of charge, 181 size, 212-213, 219 charge barrier theory, 213, 215 chemical, 93, 200-201, 205, 214 chemical adjacency effect, 221, 234 color, 223 dry, 236 electrochemical model, 213 granular, 216-217, 219 induction period, 208, 213 infectious, 221-222, 234 inhibitor, 221, 223 inhibitor-releasing (DIR) coupler, 223224 initiation, 212, 219, 221 internal, 192 ionic process for, 202 lith, lithographic, 222 parallel, 216, 219 physical, 92-93, 200-201 progress of, 214-216 rate activation energy, 208 dependence on charge, 213-214 dependence on emulsion factors, 214219

244

INDEX

development (continued) rate (continued) dependence on oxidation potentials, 205, 208 diffusion-controlled, 208 distribution, 177 equation, 203 reaction-controlled, 208 simulation, 221 superadditivity, 215 surface, 105, 146, 172-173, 192 Dexter-type energy exchange, 149 diagnostic X-ray film, 12-14 diazo film, 12 dichromated gelatin, 12 dielectric loss method, 50, 51, 53, 86, 189 diffuse reflectance spectrum, 98-101, 119, 122-123 diffusion transfer process, 12 dislocations in silver halides, 31-32, 70, 84-86, 170 dispersion of image centers, 87-91, 153, 157-158, 170, 194, 213, 217, 232 display of images, 5-6, 233-235 double-structured grain, 27, 29, 47, 229 double-jet precipitation, 32 dry processing, 236 dry silver film, 12-13 dyes. See sensitizing dyes dye positive hole characterization, 143-148 inhole-trapping desensitization, 143-148, 156-157 lifetime, 144, 148 in Lowe's hypothesis, 104-105 in sensitization by stabilizers and antifoggants, 194 electric double layer model for tabular grains, 55 electrochemical model of development, 213 electrons. See photoelectrons electron microscopy developed silver after arrested development, 96, 180, 213 silver clusters, 92-93 silver sulfide clusters, 167-168 electron-nuclear double resonance, ENDOR, 73 electron paramagnetic resonance. See electron spin resonance

electron spin resonance (ESR) dye positive holes, 144-148 metal ions in silver halides, 72-73 self-trapped holes in AgCl, 62 electron transfer, 59, 232. See also lightinduced electron transfer electron transfer mechanism for spectral sensitization, 111, 135, 149 electron traps in silver halides charged centers, 65-66, 71-73 depth. See trap depth effect on dispersion of image centers, 232 effect on drift mobility of photoelectrons, 60-61 extrinsic, 70-75 intrinsic, 66-71 metal ions, 71-73 reduction sensitization centers, 97, 180 role in mechanism of latent image formation, 81-85 sulfur-plus-gold sensitization centers, 73, 87 sulfur sensitization centers, 73, 87, 171172, 178 electronic photography, 6, 15-21 electrophotography, 11-14 energy dispersive spectroscopy, 31 energy gap dependence desensitization, 139 spectral sensitization, 126, 134-138, 152 energy gap dependence efficiency, 132-141, 151-153 energy transfer mechanism for spectral sensitization, 111, 149 environmental protection, 228, 236 evaporated silver halide layers, 12-13, 122, 131, 230 excitation spectroscopy, 74 excitons in sensitizing dyes, 142 in silver halides, 60, 86 exciton trap mechanism for supersensitization, 142 extended X-ray absorption fine structure (EXAFS), 30, 31 Fermi level dyes, 131, 140, 150 silver halide, 131, 149-150 silver ions, 57

Index field decay time of silver halides, 40, 64, 68,70 fixing solutions, 5 fog bleaching, 65 centers molar absorption coefficient, 98 oxidation potential, 101 developer, 219-220 emulsion, 219-220 formation during storage, 166, 183-184, 190-191 by gold sensitization, 179 in mechanism of latent image formation, 85 by reduction sensitization, 98-101, 181 by reduction-plus-gold sensitization, 231 by sulfur sensitization, 169-170, 172-175 Foerster-type resonance transfer, 149, 230 Frenkel defects. See Frenkel defects in crystal defects gelatin effect on dye adsorption, 123 as protective colloid for silver halide grains, 32, 37-38, 40 in relation to gold sensitization, 176 in relation to reduction sensitization, 191 in relation to sulfur sensitization, 167 in relation to stabilization, 190-191 Gibbs-Thompson effect for silver clusters, 93, 204, 208-213 gold clusters, 177 gold latensification, 95-97, 157, 176-179, 180, 194, 231 gold sensitization analysis by potassium cyanate, 178 centers, 165, 167, 179, 191 effect on minimum size of latent image centers, 219 for efficient latent image formation, 165167, 231 mechanism, 176-179 in relation to stabilization, 191 role of potassium thiocyanate in, 176-179 sensitizer chloroauric acid, 176-177 disproportionation, 179 ligands for aurous ions, 176, 179 stability constant for aurous complexes, 176

245

with sulfur sensitization. See sulfur-plusgold sensitization gold sulfide, 177, 179, 219 Gurney-Mott mechanism, 84-85 Gurney-Mott principle, 84 H-aggregates of dyes, 118, 120-121 H-band, 118 halogen acceptor, 172 Haynes-Shockley experiment, 64 heliographie, 3 hexagonal model for {111} surface of silver halides, 28 hybrid system, 235 high resolution plate, 12, 14 highest occupied molecular orbital (HOMO) developing agents, 203-208 in relation to dye positive holes, 144146, 231-232 in relation to mechanism of spectral sensitization, 149 in relation to RQE, 132-134 reduction sensitization centers, 102-104, 174 sensitizing dyes, 113, 117-118, 126-130 silver clusters, 92, 94 sulfur sensitization centers, 170, 173, 175 hydrogen hypersensitization center, 92, 156, 182 definition, 182-183 for efficient latent image formation, 92, 231 for Lowe's hypothesis, 105-106, 147 hydrazines, 180, 223 hypersensitization, 165-166 image processing comparison between color film and still video camera, 21 through development process, 221-223, 234 for hybrid system, 235 parallel, 224 in series, 224 image quality contrast, 13, 73, 222, 224 graininess, 222, 224 modulation transfer function, 11 in relation to amplification of development, 200

246

INDEX

image quality (continued) in relation to arrangement of silver halide grains, 233-236 in relation to sensitivity, 228 resolving power, 10, 11-14, 228 sharpness, 222, 224 imaging plate, 8, 11, 15 infrared absorption spectra sensitizing dyes, 124-125 stabilizers and antifoggants, 185 instant film, 12-14 intensifying screen, 13 interstitial silver ion in silver halide collinear interstitialcy jump, 49, 51, 86 in development process, 201-202, 214 effect of antifoggants on formation, 188189, 191 effect of dyes on formation, 52-53, 124125 effect of emulsion factors on formation, 53-54, 90-91 effect of stabilizers on formation, 51, 188, 193-194 enhancement of dispersion of image centers by, 90-91, 158 formation mechanism, 48-51, 56-59 in ionic relaxation of electron-trapping, 66-68, 72, 182 in latent image formation, 81-86 in Lowe's hypothesis, 74-75, 104-105 mobility, 49, 85 in model of sulfur-plus-gold sensitization centers, 178 in model of sulfur sensitization centers, 175 role in formation of Ag2S in sulfur sensitization, 168 transport, 48-51 ion scattering spectroscopy, ISS, 31 ionic conductivity of silver halides cubic AgBr grains, 51, 53, 54, 91 effect on dispersion of image centers, 9091 effect of metal ions, 49, 58 effect of sensitizing dyes, 51-52, 91, 156-157, 194 effect of stabilizers and antifoggants, 188-191, 194 energetics, 57-59 measured by decay of Dember effect, 50, 64

measured by dielectric loss measurement, 48-51 measured by Hayes and Schockley experiment, 49-50 octahedral grains, 54 role in latent image formation, 84-86 tabular grains, 54 J-aggregates of dyes absorbance, 229-230 growth mechanism, 124-125 light-induced electron transfer, 140-144 molecular exciton theory, 118-122 size, 141-144, 147-148, 152 slip angle, 118-122, 141 J-band, 118-122,229-230 jogs in silver halides, 85, 181 Kalver film, 12 kink sites on silver halides, 54-57, 59, 8384, 97, 181, 189 Kubelka-Munk equation, 24, 98, 100, 122 Langmuir-Blodgett membrance of sensitizing dyes, 149 latensification, 165-166, 219 latent image centers absorption spectra, 99-101, 101 amplification by catalytic action, 7, 10, 200-201, 204-205, 235 catalytic action, 10, 200-201, 204-205 decay, 105, 143-147, 157 discrimination from fog, 219 dispersion. See dispersion of image centers formation. See latent image formation growth, 82, 84-87, 232 HOMO level, 103-104 intensification by Lowe's hypothesis, 103 LUMO level, 103-104 minimum size, 7, 86-87, 91-93, 166, 177-178, 200, 219, 235 nucleation, 82, 84-87, 91, 232 as P centers, 74-75 oxidation potential, 93, 103, 176-177 size, 91-95, 213, 217 latent image formation concentration principle, 81-91, 234 concentration-speck theory, 83 contribution of Lowe's hypothesis, 104105

Index efficiency, 165, 228, 232, 235-236 electronic process, 45, 81, 83, 86-87, 107 Gurney-Mott mechanism, 84-85 Gurney-Mott principle, 84 ionic process, 45, 82-84, 86-91, 106107 lattice relaxation model, 85 mechanism, 81-85, 87, 94, 107, 165166 nucleation and growth model, 82, 85. See also nucleation and growth in latent image centers role of interstitial silver ions, 59, 81-86 in silver halide photography, 6-8 simulation, 92, 179 thermodynamic model, 85 threshold for, 95, 106 latent preimage center, 8, 81-82, 87, 89 latent subimage center, 8, 82, 84, 87, 104107, 165, 178, 219 lattice relaxation model for latent image formation, 85 lens-incorporated film, 237 light absorption of emulsion grains, 229, 235 light-harvesting system, 230, 234 light-induced electron spin resonance. See also electron spin resonance dye positive hole, 105, 144-148, 152, 156-157 metal ions in silver halides, 72-73 light-induced electron transfer from dyes to silver halides activation energy, 136 back reaction, 143-148 deactivation process, 140-141 dependence on energy levels. See energy gap dependence efficiency Marcus theory, 135-137 in mechanism of spectral sensitization, 111, 135, 149-151 potential energy profile, 126, 134—136 rate constant, 136-137, 139-143, 150 in supersensitization, 139, 152 threshold, 135-138 tunnel effect, 136 light-induced positive hole transfer, 133, 149-150 low-intensity reciprocity law failure (LIRF), 84,89

247

Lowe's hypothesis, 74, 104-107, 182 low-temperature luminescence microscope, 38 luminescence modulation spectra, 63, 171 MacRae's equation for solvent effects, 117 Marcus theory for light-induced electron transfer, 135-137 mass spectroscopy of silver clusters, 92 memory of images, 5-9 merocyanine dyes absorption spectra, 118 adsorption by silver halides, 122-123, 125 chemical structure, 112, 116 effect on ionic conductivity of silver halides, 53 electronic energy levels, 131 UV photoelectron spectroscopy of, 130132 metal ions in silver halides as analyzed by ESR, 72-73 effect on electron-trapping in silver halides, 71 effect on ionic conductivity of silver halides, 49, 58 Ir3* in AgBr, 72-73 Rh 3+ , 73 [Ru(NO)Cl5]3+, 73 metal clusters, 92, 94-95 microcalorimeter for dye adsorption, 123 microfilm, 12-14 microscopic reflection spectrum of dye crystals, 122 microwave photoconductivity. See photoconductivity mini-lab printer-processor, 236-237 modified electron transfer mechanism, 149150 molecular orbital method as applied to silver clusters, 86, 93-94, 97-98, 181 as applied to developing agents, 204, 206-211,208,212 as applied to dyes, 113, 118, 127, 129, 132 Hueckel-approximation, 94, 113, 117118, 127-129 monomeric state, monomer dyes, 118-121, 132, 141

248

INDEX

monomeric state, monomer (continued) silver, 7-8, 81-82, 87, 89, 94, 98, 104105, 107 silver sulfide, 172-173 nonsilver photosensitive materials, 11-14 nucleation and growth model for latent image formation, 82, 85 octahedron, octahedral grains of silver halides adsorption of dyes by, 123-124 dye positive holes on, 143-148 grain growth, 39-40 ionic conductivity, 51, 53-54 J-aggregates of cyanine dyes on, 118-119 light-induced electron transfer from dyes, 141-143 mechanism of formation, 38 rate of development, 218 reduction-sensitized, 99-103, 181-182 as regular grains, 24 sensitization by stabilizers and antifoggants, 192-194 size dependence of sensitivity, 89 sulfur-sensitized, 168, 172-174 supersensitization on, 151 Ostwald ripening, 39 oxidation potentials developing agents, 204, 208 fog centers, 172 latent image centers, 93, 103, 177 reduction sensitization centers, 101-104, 172, 174, 180, 182 sensitizing dyes, 127-131, 134 silver cluster, 93-94 sulfur sensitization centers, 172, 174, 180 supersensitizers, 152 oxidizing agents, 220 P centers of reduction sensitization definition, 74, 87, 97-99, 181-182 effect on photoconductivity of silver halide grains, 181-183 HOMO level, 102-104, 174 molar absorption coefficient, 98 oxidation potential, 102-103, 174 role in latent image formation, 104, 106107, 167 phase-selective second-harmonic voltammetry, 130, 132, 135, 155

phonon in silver halides, 47, 61 photo-Hall effect of silver halides, 61 photochromic materials, 12 photoconduction of silver bromide, 60 photoconductivity of silver halides Dember effect, 193-194 effect of desensitizing dyes, 153-156 effect of reduction sensitization centers, 97-99, 181-182 effect of sulfur sensitization centers, 171173 microwave photoconductivity method, 63-64, 152-156 silver bromide emulsion grains, 66-70 silver chloride emulsion grains, 66-70 spectral sensitization for, 134, 151-152 supersensitization for, 151-152 photoelectrons in silver halides diffusion length, 65, 87-89, 232 drift mobility, 60-61, 64-66, 86-87, 193, 232 effect of space charge layer, 59 effective mass, 61, 86 formation mechanism, 45^7 formation yield, 86 Hall mobility, 60-62 lifetime, 60-61, 63-72, 85, 170, 181182, 193 macroscopic mobility. See drift mobility microscopic mobility. See Hall mobility in reduction-sensitized grains, 180-181 role in latent image formation, 84-87, 106-107 in sulfur-plus-gold-sensitized grains, 165-166 in sulfur-sensitized grains, 165-166, 171 trapping diffusion-controlled, 69 for latent image formation, 83 as measured by microwave photoconductivity, 66-70 reaction-controlled, 66, 68 by reduction sensitization centers, 9899, 181-182 by sulfur sensitization centers, 192193 traps. See electron traps in silver halides photographic sensitivity. See latent image formation

Index photo-induced electron transfer. See lightinduced electron transfer photolytic silver, 25, 31, 65, 84, 158, 181, 194 photopolymers, 8, 11-15 photoresists. See also photopolymers, 12-14 photo-stimulated desorption from silver bromide, 65 photosolubilization process, 12-13 photothermography, 12-14, 236 picosecond spectroscopy, 140-141 polarized fluorescence microscopy, 122 polarographic half-wave potentials, 127128, 130, 152, 203-205, 208 positive holes in silver halides in chemically sensitized emulsion, 165166 diffusion length, 65 drift mobility in AgBr, 60-62, 64-65 effect of space charge layer, 59 effect on fa, 132-134 lifetime, 64, 65, 85 in Lowe's hypothesis, 104-107 in mechanism of latent image formation, 45, 81, 84-85, 165-166 reaction with reduction sensitization centers, 96-97, 102, 180 recombination with photoelectrons, 89, 180, 231-232 role in high-intensity reciprocity law failure, 89 self-trapped analyzed by ESR, 62 effect on mobility of positive holes, 61-63, 66, 86 Jahn-Teller distortion with, 63 trapped by dyes. See dye positive holes positive hole traps, 65, 81, 83, 133, 171 potentiometry for analysis of Ag2S formation, 167 protective colloids for silver halide grains, 37,40 quantum sensitivity, 92, 179, 228, 235-236 quantum size effect for silver clusters, 8687, 95, 104, 106, 204 R centers definition, 87, 97-99, 181-182 in Lowe's hypothesis, 74-75, 104-106 oxidation potential, 102, 172, 174

249

role in latent image formation, 166-167, 231 radiowave photoconductivity, 64 reciprocity law, 89, 156 recombination in silver halides through dye positive holes, 143, 144, 147, 156, 158, 194 effect on photographic sensitivity, 228, 231-232, 234 for high-intensity reciprocity law failure, 89 in latent image formation process, 83-85, 87, 106-107 in relation to indirect transition, 47 retardation by R centers, 74, 97, 104, 165, 167, 180-182, 231 in space charge layer, 59 redox buffer solution, 92, 101-103, 172174 reduction potentials sensitizing dyes, 126-130, 132, 135-136, 138-139, 152, 155 silver clusters, 94 reduction sensitization centers absorption spectra, 98-100 HOMO level, 101-104, 174 LUMO level, 97-98, 102-103 oxidation potentials, 101-103, 172, 174 reaction with positive holes, 96-97, 133 definition, 74, 97-98, 180-183 effect on latent image formation, 81-83, 92-93, 165-167, 231-232 effect on photoconductivity, 98-99, 181182 with gold sensitization, 177, 179 sensitizers dimethylamineborane, 98-100 hydrazines, 180 sodium suifite, 180 stannous chloride, 180-182 thiourea dioxide, 167, 180 silver digestion, 191 relative quantum yield of spectral sensitization (RQE) definition, 132-134 energy gap dependence, 135-140 improved by supersensitization, 151-153

250

INDEX

relative quantum yield of spectral sensitization (RQE) (continued) in relation to kinetics of spectral sensitization, 141-142 temperature dependence, 137-139, 151152 resident time of electronic carriers at traps, 74 scanning tunneling microscopy, 122 secondary ion mass spectroscopy, SIMS, 31 semiquinones formation constant, 205 for infectious development, 222 sensitivity analysis of factors, 228-232 blue, 88, 111, 133, 145, 151-152 future prospect, 235-236 grain size dependence, 65, 88-89, 236 historical change, 3-4 inherent, intrinsic, 88, 111, 127, 138139, 147, 153-155 internal, 88, 97, 157, 177, 180, 192 minus-blue, 111, 151-152 quantum, 92, 179, 228, 235-236 red, 73 in relation to development, 219-220, 221-222 silver halide photography vs electronic photography, 15-21 spectral, 111 surface, 88, 97, 157, 171-173, 180, 192 time-dependent, 105, 146-147 sensitizing dyes adsorption by silver halides, 122-124 benzodithiolylium cyanine dyes, 125, 130 Brooker's deviation, 130 chemical structure, 112, 114-116 chromophore system, 111-113 cyanine. See cyanine dyes effect on dispersion of image centers, 91, 158 effect on ionic conductivity of silver halides, 52-53, 91, 158 electronic energy levels. See HOMO and LUMO electron affinity, 126-127, 130, 135 Fermi level, 131, 140, 150 first excited singlet state, 140 fluorescence lifetime, 140-141

free electron model, 113 frontier molecular orbitals, 113 ground state, 137, 139-140, 144, 149 H-aggregates, 118, 120-121 Hammett's a, 129-130, 186-187 heat of adsorption, 123-124 HOMO definition and analysis, 113, 126-130 electron density distribution, 117-118 in relation to dye positive holes, 144146, 231-232 in relation to mechanism of spectral sensitization, 149 in relation to RQE, 132-134 hyper-redox potential, 150 infrared, 150 infrared absorption spectra, 124-125, 185 ionization energy, 126-127, 130, 135 J-aggregates. See J-aggregates of dyes Langmuir-Blodgett membrance, 149 light absorption, 229-231 lowest excited singlet state, 137 LUMO definition and analysis, 113, 126-131 electron density distribution, 117-118 in relation to desensitization, 155 in relation to photoconductivity of silver halides, 155 in relation to RQE, 135-138 in relation to supersensitization, 139 in spectral sensitization, 126, 146, 149 merocyanine. See merocyanine dyes mobility of adsorbed dye molecules, 125 monomer, 118-121, 132, 141 oxidation, 65 oxidation potential, 127-131, 134 oxonol, 113 polymethine chains, 111-113, 117, 125, 127, 129-131 quasi-Fermi level, 150 steric effect in, 113, 128, 131 transition energies, 113, 117-118, 120, 128-129 triplet state, 157 UV photoelectron spectroscopy, 130131, 150 vacuum level, 131 vinylogous, 111-113 X-ray crystallography, 113, 128

Index sensitization by stabilizers and antifoggants, 194 silver bromide absorption spectra, 45-46, 229 adsorption of antifoggants, 190-191 adsorption of dyes, 123-125 band bending, 193 band structure of, 45-47, 131 charged centers in, 66, 72 crystal structure, 24 cyclotron resonance in, 62 development process on, 201, 215-219 diffusion length of electrons, 61, 64-65 diffusion length of positive holes, 61, 65 direct exciton transition of, 46^1-7 dislocations in, 31-32, 84-85 dispersion of image centers on, 91, 156158 drift mobility of electrons, 61 drift mobility of positive holes, 61 dye aggregates on, 119, 121-122 dye positive holes on, 144-148 effective mass of electrons, 61 energy gap dependence of desensitization on, 138 energy gap dependence of spectral sensitization on, 135, 138 evaporated layers of, 122, 131, 230 excitons in, 60, 66 EXAFS, 30-31 Fermi level, 131 formation enthalpy of Frenkel defects in, 49 grain growth, 36^4-1 Hall mobility of electrons, 61-62 Hall mobility of positive holes, 61-62 high-intensity reciprocity law failure, 91 hydrogen hypersensitization of, 92, 105106, 182-183 indirect exciton transition in, 46-47, 229 internal field relaxation in, 49 ionic conductivity of emulsion grains, 50-55, 90, 191 ionic relaxation in, 72 isoelectric traps in, 71 kink sites on, 97-98 latent image centers on, 101, 107 latent image formation on, 81-82, 165166 lifetime of electrons in, 61, 67-71

251

lifetime of positive holes in, 61 kinetics of light-induced electron transfer from dyes, 141-143 metal ions in, 49-50, 72-73 microwave photoconductivity, 67-71, 153-156 reduction sensitization centers on, 96103, 107, 181-182 RQEofdyeson, 133-135 sensitization by stabilizers and antifoggants on, 191-194 silver clusters on, 92 size dependence of sensitivity, 89 solubility, 37, 40-41, 214 sulfur-plus-gold sensitization centers on, 106 sulfur sensitization centers on, 74, 88, 168-175, 217-218 supersensitization on, 139, 151-152 transient absorption spectra, 65 X-ray diffraction, 30-31 silver bromoiodide, 29, 31, 90, 124, 229 silver chlorobromide epitaxial grains, 30 EXAFS of, 30-31 high-intensity reciprocity law failure, 91 ionic conductivity, 90-91 microwave photoconductivity, 68-70 X-ray diffraction, 30-31 silver chloride absorption spectra, 45-46 band bending, 58 band structure, 45-47 charged centers in, 66 crystal structure, 24 direct exciton transition in, 46-47 drift mobility of electrons, 61 drift mobility of positive holes, 62 EXAFS, 30-31 excitons in, 60 grain growth, 36 Hall mobility of electrons, 61 high-intensity reciprocity failure, 91 indirect exciton transition in, 46-41 ionic conductivity, 90-91 isoelectric traps in, 71 metal ions in, 58 microwave photoconductivity, 68-71 self-trapped holes in. See positive holes in silver halides

252

INDEX

silver chloride (continued) space charge layer in, 58-59 surface potential, 58 transient absorption spectra, 65 X-ray diffraction, 30-31 silver clusters closed shell configuration, 93 developed silver, 201, 213, 219 electron affinity, 177 electronic energy levels, 94-95 HOMO level, 92 latent image centers, 45, 85-87, 91-95, 235 LUMO level, 92, 94, 204 magic number, 95 mass spectroscopy, 92 odd-even alternation, 94 open shell configuration, 95, 106 P centers. See reduction sensitization centers quantum size effect, 86-87, 95, 104, 106, 204 R centers. See reduction sensitization center redox potential, 92-93 reduction sensitization centers, 74, 8687, 96-98, 180 shell model, 93-95 site effect, 87, 95, 103, 106, 204 silver-complexing agent, 37, 50, 51, 70 silver ion vacancy, 48, 49, 56-58, 62, 66, 72,81 silver halide emulsion grains apparatus for preparation of, 33 arrangement of, 233 assembly of, 232-234 controlled double jet precipitation, 32 core/shell, 65 crystal habit, 24-26 crystal habit modifier, 24 crystallite, 32 crystallization production technology, 10 cubic. See cubic grains of silver halides double-jet precipitation, 32 epitaxial, 29, 48 growth accelerators, 37 diffusion layer for, 34, 36, 41 Pick's diffusion law for, 34 mechanism, 33-35

mononuclear-layer, 35 polynuclear-layer, 35 rate. See critical growth rate of silver halide grains reaction-limited, 34-35 supersaturation, 33-35, 38-39 light absorption, 229-230 monodispersed, 24, 34, 35, 233 nucleation critical supersaturation, 33, 39 formation of twin planes at, 39 mechanism, 33-34 unstable nuclei and critical stable nuclei, 34 octahedral. See octahedral grains of silver halides in photographic materials, 3-9, 19, 234235 preparation, 32-41 regular, 24, 26, 32 in relation to development process, 214219, 221 single-jet precipitation, 32 size, 36, 39-41, 52, 54, 89, 218-229 structure, 24-32 surface-to-volume ratio, 51, 53, 66, 219 tabular. See tabular grains of silver halides silver half-cell potential, 202, 204, 208, 214 silver halide photosensitive materials, 3-11, 11-21,82,228-229,236 silver iodide, 3, 24, 34 silver microclusters. See silver cluster silver selenide, 167 silver sulfide specks electron diffraction, 168 electron microscopy, 167-169 fog centers, 167-176, 219 with gold sensitization, 176-179 in mechanism of latent image formation, 83-84 in mechanism of stabilization, 190-191 oxidation potentials, 173 rearrangement, 170-171 sulfur sensitization centers, 73, 166-176 tracer analysis, 167 silver telluride, 167 single-jet precipitation, 32 site effect of silver clusters, 87, 95, 103, 106, 204

Index solubility of silver halides, 32-33, 35, 36, 37, 40, 214 solubility product (pKsp) silver halide, 32, 39 silver salts of stabilizers and antifoggants, 185-186, 188-189 solvent effect of absorption spectrum, 117, 124 space charge layer in silver halides Debye length, 58-59 influenced by TAI, 193 potential profile, 58-59 spectral sensitization. See also light-induced electron transfer by colored inorganic compound, 231 dye multilayer technique, 230 electron transfer mechanism, 111, 135, 149 energy gap dependence, 126, 134-138, 152 energy transfer mechanism, 111, 149 by grain of sensitizing dyes, 321 light-harvesting system for, 230, 234 modified electron transfer mechanism, 149-150 relative quantum yield (RQE), 132-140, 151-152 by sulfur sensitization centers, 170 spectroscopic plate, 12-13 stabilization of photographic materials, 8, 165, 190 stabilizers acid-base equilibrium, 186 adsorption by silver halides, 188 chemical structure, 183 dissociation constant (pKa), 186-189 Hammett's cr, 185-188, 190 4-hydroxy-6-methyl-l,3,3a,7tetraazaindene (TAI) effect on electron trapping in AgCl grains, 70 effect on ionic conductivity of silver halide grains, 51 effect on sulfur sensitization centers, 170, 192-194 retardation of dye desensitization by, 157 sensitization by, 192-194 solubility product of silver salts, 186 still video camera, 16-21

253

sulfur sensitization centers activation energy for formation, 170 analysis, 167 cluster mechanism for formation, 169 concentration, 74 HOMO and LUMO levels, 170, 173, 175 mechanism of formation, 168-172, 190 model for, 178 oxidation potential, 174-177 trap depth, 73-74 two-step mechanism, 169-170, 179180 effect on dispersion of image centers and HIRF, 89-91, 156-158, 232 effect on photoconductivity of silver halides, 70, 171-173 effect on rate of development, 215-219 with gold sensitization, 177-180 in relation to developer fog, 220 in relation to stabilization, 190 role in latent image formation, 81, 86-87 role in sensitization by stabilizers and antifoggants, 191-194 sensitizers allylthiourea, 167 3-ethyl-5-benzylidene rhodamine, 168 heat of adsorption, 169 sodium sulfide, 170-173 sodium thiosulfate, 167-169, 172, 176-177, 217 sulfur-plus-gold sensitization centers model, 178 trap depth, 171-173, 178-179, 232 with hydrogen hypersensitization, 183 internal, 157 mechanism, 176-179 in relation to concentration principle, 8788 in relation to developer fog, 219-220 role in latent image formation, 81, 92, 106, 165-166, 183, 231-232 superadditivity, 215 supersensitization aggregate-partitioning, 142, 151-152 energy-gap dependence, 152 exciton-trapping, 142 hole-trapping, 139, 142, 151-152

254

INDEX

supersensitization (continued) self-supersensitization, 153 temperature-dependence, 138, 151-152 supersensitizer, 138-139, 142-143, 151152, 168, 231 surface extended X-ray absorption fine structure (SEXAFS), 27 surface potential of silver halides, 51, 58 tabular grains of silver halides electric double layer model, 55 ionic conductivity, 53-54 J-aggregates on, 122 in relation to light absorption of emulsions, 229, 235 sulfur-sensitized, 88, 169 twin planes, 25, 28, 31, 38, 54, 70-71 thermally stimulated current technique, 73, 171, 178 thermoplastic holography, 12 time-of-flight method, 61, 64 tracer analysis, 167-168 transient absorption spectra, 65, 74 twin planes in silver halides, 25, 28, 31, 38, 54, 70-71

two-step mechanism for sulfur sensitization, 169-170, 190 UV photoelectron spectroscopy (UPS), 48, 130-132, 150 valence band of silver halide definition, 46-47 p-d mixing in, 47 role in latent image formation, 82, 166 in space charge layer, 59 tabular grains, 55 vs HOMO level of dyes, 128, 131-133, 137, 144-146, 149, 156, 231 vs HOMO level of reduction sensitization centers, 102-103, 182 Vegard's law, 31 xerography, 8, 12-14 X-ray absorption near edge structure (XANES), 125 X-ray crystallography, 113, 128, 185 X-ray diffraction, 25, 29, 31, 32, 122 X-ray photoelectron spectroscopy (XPS), 31, 127, 185