Silver Halide Crystals Research Articles

The effect of doping [Fe(CN6]4- and [Ru(CN)6]4- on the photographic properties of the T-grain emulsion

The effects of two dopants, K4[Fe(CN)6] and K4[Ru(CN)6], on the photographic properties of the iodobromide T-grain emulsion were studied in this paper. It was found that both the doping amount and the location within the grain had significant effects on the photographic properties of the emulsion. The doping by either [Fe(CN)6]4- or [Ru(CN)6]4- could increase the photographic sensitivity of the silver halide emulsion, indicating that they could form shallow electron traps in the silver halide crystal, effectively protecting the electrons from immediate recombination with the holes. The extent of the improvement varied with different dopants and the doping conditions. The sensitivity increased more if the location of the dopant was near the grain surface for [Fe(CN)6]4- and either near the surface or in the deep interior of the grain for [Ru(CN)6]4-. This study also indicated that iodide had an influence on the efficiency of the doping process. The chemical sensitization time could be prolonged and the formation of fog could be retarded by doping.

New Processing Techniques for the Creation of Micro-Opto-Mechanical Machines and Photonic Devices Embedded in Glass

ABSTRACTThe use of lasers to create intricate three-dimensional and buried structures [1] in photostructural glass has been well demonstrated at The Aerospace Corporation over the past four years. In these instances the glass used (Foturan™, made by the Schott Group) forms a silver nucleation sites on exposure to intense UV laser light via a two-photon process. Subsequent annealing causes a localized crystal growth to form a meta-silicate phase which can be etched in dilute hydrofluoric acid at rates of 20 to 50 times that of the unprocessed glass. We are now in the process of experimenting with another formulation of photosensitive glass, also pioneered by Corning Glass Works, that behaves differently during the bake process. In the second case, a photoexposure and bake process creates a silver-halide crystal and forms an adjacent void in the glass. A second photoexposure and bake allows for the migration of more silver into the void creating patterned formations of silver nano-wires [2]. Recent experiments with this type of glass have shown that the manipulation of the size and density of the embedded nano-wires as well as the overall pattern of the clusters can be controlled using direct-write exposure to laser processing.

The study of waveguides made by diffusion of Br into AgCl substrates and the transmission of mid-IR radiation through these waveguides

Silver halide crystals of the form AgCl x Br 1− x can be grown from the melt in all ratios of Cl:Br and they are highly transparent in the middle infrared. The index of refraction of such mixed crystals increases monotonically with x, from that of AgCl ( x=1, n=1.98) to that of AgBr ( x=0, n=2.16). These properties suggested that they could be used to form graded index waveguides for the mid-IR. Cylindrical ingots of crystals of composition AgCl 0.9Br 0.1 and AgCl 0.1Br 0.9 were grown; discs from each were cut and polished and placed in optical contact at 410 ° C . Diffusion of Br ions from the Br-rich side to the Cl-rich side produced a graded index waveguides whose thickness was determined by the diffusion time. The diffusion depth was obtained by scanning electron microscopy and by optical methods. These studies provide information about the diffusion coefficients of ions in silver halide crystals and the optical properties of the diffused layers. These silver halide waveguides will be useful as integrated optical elements in the mid-IR.

The concept of “scaleable reactor” in the precipitation of silver halide photographic microcrystals

The various types of silver halide microcrystals used in the photographic industry are all obtained by precipitation techniques. Highly insoluble silver and halide salts are used and the high kinetics of the nucleation and growth phenomena involved in the precipitation process often limit their capability to be scaled without further costly and time-consuming reformulation. A new concept of “scaleable reactor” for the precipitation of silver halide microcrystals has been developed, which allows to greatly speed-up or even eliminate the translation work between scales. It is based on the simple evidence that, in highly nonlinear systems such as precipitation reactors, scale-up is better ensured when all the critical parts of the process are exactly the same at all scales. Therefore, the concept of scaling by replication instead of the conventional scaling by dimensional changes according to phenomenological laws was studied further. The system presented in this paper uses both batch reactors and auxiliary side-arm loops containing elementary cell(s) in which all the reactants are separately injected. The side-arm loops are connected to the batch reactor which acts as a holding tank and in which only slow kinetics reactions, such as Ostwald ripening, take place, so that the reaction zone remains separated from the bulk. Scaling is ensured by the exact replication of the side-arm loop, the number of replicates being equal to the scale-up factor, while the batch reactor volume varies accordingly to the scale-up factor. The design of the elementary cell, based on the principle of Tee-mixers, was carefully optimized. The batch reactor designs were also studied at several scales, and macromixing compared at different scales. A large range of experimental results are available, demonstrating the validity of the concept for several sizes and morphologies of silver halide crystals.

Crystal surfaces and crystal growth mechanisms: application to crystals having the NaCl structure and especially silver halide crystals

Results of computer simulation studies of crystal surfaces and related crystal growth mechanisms as well as statistical mechanical roughening transition theories of these surfaces are reviewed. The implications of these theories for crystal growth mechanisms, such as the spiral growth mechanism and the substep mechanism, recently described by Ming and co-workers, are treated briefly. Relations between roughening theories and recent developments in the crystallographic morphological theory of Hartman and Perdok are introduced. It is shown how this integrated theory can be applied to crystals having an NaCl structure, such as AgBr and AgCl crystals. The most stable faces are the {100} and {111} faces respectively. The {111} faces will be stable only if the high dipole moment can be removed by reconstruction or interaction with the solvent. Supersaturation is the key parameter that determines the growth morphology. At low supersaturations, only {100} faces are present. At higher supersaturations, {111} faces will become dominant. On these faces, formation of twins will occur at even higher supersaturations, leading, as a result of the substep mechanism, to tabular and needle-like crystals. The detailed understanding of all kinds of tabular and needle-like AgBr and AgCl crystals will be the subject of a subsequent paper.

Twin formation and growth mechanism of tabular silver halide crystals

In this study the growth mechanism of {111} tabular silver halide crystals will be examined. The side-face structure and aspect ratio of tabular silver bromide crystals grown from different concentrations of silver bromide dissolved in dimethylsulphoxide (DMSO) were established in situ and ex situ. For tabular silver bromide crystals grown by the industrial doublejet precipitation method, transmission electron microscopy images of ultramicrotomed sections of the side faces were analysed. Also for tabular silver bromide and tabular silver chloride crystals grown from the vapour phase the relation between the aspect ratio and side-face structure was studied. Using the result of the different crystallization techniques it is shown that there is a strong relation between the aspect ratio and the side-face structure. For tabular crystals grown from DMSO, by industrial double-jet precipitation and from the vapour phase the same relation between the aspect ratio and side-face structure is observed. This provides evidence to support the idea that the growth mechanism of {111} tabular crystals is universal and independent of the crystal growth technique or crystallization medium. The preferential lateral growth of the tabular crystals can be explained by the substep mechanism proposed by Ming and co-workers in an earlier study. This mechanism explains the increase in growth rate of a {111} side face linked via a twin plane to a faster growing {100} side face.

New method for the production of silver halide tabular crystals

A novel method for the production of silver halide crystals with a large application for the study of the mechanisms of nucleation and growth and the physical properties of these crystals is presented. The method consists on the precipitation of silver halide crystals from silver halide complex solution in polar aprotic solvent by addition of water. Isolated crystals with various shapes and morphologies can be selectively produced with this method in the absence of any additive including gelatin. Silver bromide tabular crystals with a length to thickness ratio higher than 100 and sizes up to 1 cm were produced with this method. Studies on many important aspects of silver halide crystallization that bore great experimental difficulties in the past can be easily undertaken owing to this method. The method has been successfully applied in preliminary studies on the atomic structure of silver halide crystal surface by AFM, tabular crystal side face structure, twin plane disposition, “in situ” observation of crystal nucleation and growth, etc. Crystals obtained with this method can also be used for a direct measurement of crystal physical properties, like the ionic conductivity.

Enhanced yield of photoinduced electrons in doped silver halide crystals

The conventional photographic process1,2,3 involves several steps: the photogeneration of electron–hole pairs in crystals of a silver halide; the reduction of silver cations to atoms by some fraction of these electrons; the subsequent build up of atoms to give clusters (the ‘latent image’); and the complete reduction by a developer of crystallites having more than a critical number of silver atoms per cluster. The effective quantum yield, Φeff, of photoinduced electron–hole pairs produced per photon absorbed is less than the theoretical limit (Φtheory = 1), because of the fast recombination of some fraction of the pairs1,2,3,4,5,6. Here we describe an approach for enhancing the yield of useful photogenerated electrons, in which the silver halide is doped with formate ions, HCO-2. The dopant ions act as hole scavengers, thus enhancing the escape of electrons from pair recombination. Moreover, the resulting CO˙-2 radical can itself transfer an electron to another silver cation, so raising the theoretical yield to two silver atoms per photon absorbed. This photoinduced bielectronic transfer mechanism is strictly proportional to the light quanta absorbed—the dopant ions do not induce spontaneous reduction of silver cations in the dark—and appears to be close to the theoretical limit of efficiency. The efficiency is constant at all illumination levels and applies to both dye-sensitized and unsensitized crystals. We suggest that this approach is a promising route for improving the performance of photographic emulsions7.

Quantitative secondary ion mass spectrometry depth profiling of surface layers of cubic silver halide microcrystals

The present article describes depth-profiling studies on populations of cubic silver halide microcrystals (typical size between 400 and 600 nm). These crystals consist of mixed halides (Cl, Br, I) and are characterized by internal halide distributions such as core-shell structures. Determination of the spatial distribution of the different halides in the microcrystals offers valuable information for the optimization of the crystal design of new photographic materials. The first part describes the calibration of the sputtered depth, which is more complicated than in the one-dimensional case of flat surfaces. In a second part, three different quantification methods [halide intensity ratios, XCs 2 + detection (X = halide) and the infinite velocity method] are used to determine the mean composition of the surface layer as a function of sputtered depth in the silver halide crystals. Although particles of submicrometer size are obviously not the ideal samples for depth profiling, a good correlation with values obtained by scanning electron microscopy energy dispersive X-ray microanalysis and wavelength dispersive X-ray fluorescence bulk analysis could be achieved.

Femtosecond transient absorption and luminescence decay studies of spectrally sensitized photographic emulsions

Fast electronic processes taking place immediately after excitation of sensitized silver halide emulsions were studied with picosecond time-correlated single-photon counting (SPC) and femtosecond transient absorption (TA). The fluorescence decays of the J aggregate obtained by SPC measurements were also analyzed as a sum of three exponentials. For the J aggregates of a benchmark dye, a dimethyl-μ-ethyl-thiacarbocyanine, a component with a 20 ps decay time, which was absent in samples containing no AgBr crystals, was attributed to J aggregates of the sensitizer adsorbed on the AgBr microcrystals. The fluorescence decay results were compared with photographic quantum yields in an attempt to show the relevance of the spectroscopic measurements. The second component (150 ps) is a combination of fluorescence decays of the unaggregated sensitizer molecules not adsorbed on the AgBr microcrystals and monomers of the dye on AgBr surface. The fluorescence decay was found to be independent of the crystal coverage. Prolonged illumination bleached the fluorescence of the fast component, which is consistent with depletion of the J aggregates by electron injection. The decay time of this fast component shows strong dependence on both the structure of the dye and the composition of the silver halide. Excitation in the narrow absorption band of the J aggregates with a femtosecond pump pulse resulted in ground state bleaching followed by a recovery that was analyzed as a sum of three exponentials. The fast (170 fs and 2 ps) components of the recovery are related to the relaxation of bi- or multiexciton states and to singlet-singlet annihilation processes. The remaining component (1 ns) can be attributed to the radiative relaxation of dye molecules not adsorbed on the silver halide crystals. The observed bleaching recovery demonstrated a complex dependence on both the excitation flux and the total illumination dose.

A Study on the Mechanism of Nucleation and Growth of Twin Tabular AgBr Crystals

On the mechanism of nucleation and growth of (111) tabular silver halide crystals, it is proposed that twinning (nucleation) occurs by stacking fault and that the anisotropic lateral growth is brought about by the ridge trough structure on tabular grain side faces. Although the “coalescence” model for nucleation and the “rough-smooth” side face structure for anisotopic growth were proposed recently, they have not yet been supported by direct experimental evidence. We analyzed the nucleation process by a combination of light scattering and X-ray diffraction and examined the coalescence of untwinned fine grains after decomposition of gelatin. These experimental results provided evidence to support the coalescence model for twinnning. We observed by TEM imaging of ultramicrotomed sections of side faces of tabular grains that have relatively thick spacings between parallel twin planes and confirmed that on the side faces of tabular grains, both rough-smooth and ridge-trough structures exist.

The Mechanism of Cyanine Sensitizing Dye Adsorption on AgBr Crystals

Time evolution of the adsorption of cyanine sensitizing dyes onto silver halide crystals was investigated. Observing the adsorption process in this method was found to be useful in determining dye adsorption mechanisms and their rate determining steps, in revealing how the N-substituents of cyanine dyes change the adsorption, and in exploring the relationship between rate determining steps and the states of the dyes adsorbed.

Electron microscope characterization of AgBr heterojunctions with silver carboxylates and their influence on the morphology of developed silver particles in thermally developed photomaterials

Silver halide crystals formed during in situ treatment of silver stearate crystals with various halidizing agents are observed by scanning and transmission electron microscopy to form on the lateral edges of the silver carboxylate crystals. The location of the silver halide phase on the crystal edge is dictated by the anisotropic structure of the silver stearate crystal lattice, specifically, the layered structure in which silver ion layers are separated by long-chain hydrocarbon groups. The formation of AgBr on the lateral faces of these crystals is proposed to be typical not only of the formation of silver halide on silver stearate but also for all silver carboxylates of the general formula [AgCnH2n-1O2]2 when the crystals of these silver carboxylates have anisotropic, layered structures. The silver bromide/silver carboxylate heterojunction in an in situ system has been clearly observed by transmission electron microscopy. The heterojunction is comprised of a distorted silver carboxylate lattice, which accommodates the misalignment between the AgBr and [Ag(O2CR)]2 crystal lattices. The nature of heterojunction between the AgBr and the silver carboxylate when the AgBr is prepared separately from the preparation of the silver carboxylate differs from the in situ heterojunction. In this case, a layered compound, proposed to have a Ag1-xNaxSt composition, forms between the AgBr and the silver stearate which is a unique feature of this interface. The differences in the structure of interfaces formed between the silver halide and the silver fatty acid complex result in different silver particle morphologies during thermal development of exposed photothermographic films. The developed silver is generally filamentary when the photothermographic material contains silver halide prepared by the in situ exchange reaction between silver carboxylate and a brominating agent. If the photothermographic material is prepared from previously synthesized silver halide crystals, the preformed AgBr route, the developed silver generally crystallizes as dendritic crystals. Microsc. Res. Tech. 42:152–172, 1998. © 1998 Wiley-Liss, Inc.

Electron microscope characterization of AgBr heterojunctions with silver carboxylates and their influence on the morphology of developed silver particles in thermally developed photomaterials.

Silver halide crystals formed during in situ treatment of silver stearate crystals with various halidizing agents are observed by scanning and transmission electron microscopy to form on the lateral edges of the silver carboxylate crystals. The location of the silver halide phase on the crystal edge is dictated by the anisotropic structure of the silver stearate crystal lattice, specifically, the layered structure in which silver ion layers are separated by long-chain hydrocarbon groups. The formation of AgBr on the lateral faces of these crystals is proposed to be typical not only of the formation of silver halide on silver stearate but also for all silver carboxylates of the general formula [AgCnH2(n-1)O2]2 when the crystals of these silver carboxylates have anisotropic, layered structures. The silver bromide/silver carboxylate heterojunction in an in situ system has been clearly observed by transmission electron microscopy. The heterojunction is comprised of a distorted silver carboxylate lattice, which accommodates the misalignment between the AgBr and [Ag(O2CR)]2 crystal lattices. The nature of heterojunction between the AgBr and the silver carboxylate when the AgBr is prepared separately from the preparation of the silver carboxylate differs from the in situ heterojunction. In this case, a layered compound, proposed to have a Ag1-xNaxSt composition, forms between the AgBr and the silver stearate which is a unique feature of this interface. The differences in the structure of interfaces formed between the silver halide and the silver fatty acid complex result in different silver particle morphologies during thermal development of exposed photothermographic films. The developed silver is generally filamentary when the photothermographic material contains silver halide prepared by the in situ exchange reaction between silver carboxylate and a brominating agent. If the photothermographic material is prepared from previously synthesized silver halide crystals, the preformed AgBr route, the developed silver generally crystallizes as dendritic crystals.

An Electron Spin Resonance Study of Internally Sensitive Emulsions

The election spin resonance spectra of two kinds of internally sensitive core–shell emulsions, with chemical structure imperfections and with physical lattice imperfections respectively, were studied in this paper. It was found thai both the internal sensitivity and the intensity of the rlecl rou spin resonance (ESR) signal increased with an increase of the chemical sensitization time to a maximum and then decreased. the ESR signal intensity decreased with a decrease of the core–shell ratio and an increase of the prefogging level. Different desensitizing dyes hud different effects on the internal photographic property and the intensity of the ESR signal because of their desensitizing mechanisms. The results tun be explained on the basis of the behaviour of the positive hole within the silver halide crystal.

The Progress of Photosensitive Silver Halide Crystals Supporting High Sensitivity and High-Image Quality of Photosensitive Materials

The Progress of Photosensitive Silver Halide Crystals Supporting High Sensitivity and High-Image Quality of Photosensitive Materials

Small-Angle X-ray Scattering Study on Growth of AgCl Crystallites in Photochromic Glasses

Reversible changes in the optical properties of photochromic glasses are observed owing to the presence of small silver halide crystals inside the glassy matrix. These crystals grow during the glass heat-treatment processing. Samples with molar composition of 40SiO2.10Al2O3.16.1K20.33.9B2O3, doped with AgCl and CuO, were produced and submitted to different heat treatments: (i) for 0.5 h at temperatures from 753 to 893 K and (ii) at 873 K for periods of time from 0.25 to 1.25 h. Small-angle X-ray scattering (SAXS) was used to characterize the samples. The samples heat treated between 843 and 893 K presented an increasing growth rate of the Guinier radius (Rg ). Samples heat treated at a fixed temperature of 873 K and different time t showed a law Rg 3 = kt + c. Variations in the optical absorbance at 280 nm and the additional absorbance spectra of samples exposed to light showed correlation with the SAXS results.

Fogging Treatment in Preparing Direct Positive Emulsions

This study focuses on behavior of light-induced electrons, and electrons provided by fogging agents, and the effects of silver halide solvent and gold deposition on fog centers of direct positive emulsions. When an emulsion is photofogged, fog centers form preferably in the interior of the grain, indicating that light-induced electrons can be transferred to the conduction band of the silver halide crystal directly. Whereas, when a fogging agent is used, the internal fog centers form only where a sufficient amount of fogging agent exists and only after formation of the surface fog centers. In the above two cases, the ways electrons enter the conduction band are different. The results suggest that when emulsions are fogged, the addition of gold salt might reduce the dispersity of the fog centers. Plating of gold is the main reaction when enough fogging agent exists in the emulsion, and the replacement of silver atoms by gold atoms becomes important when the fogging agent is insufficient. When a direct positive emulsion is fogged, the plating of gold atoms on the fog centers is the main reaction, because of the large quantity of fogging agent.

Calculated electronic structure of silver halide crystals

Ab initio electronic structure calculations are presented for five photographically relevant silver halide crystals: AgCl, AgBr, $\ensuremath{\alpha}$-AgI, $\ensuremath{\beta}$-AgI, and $\ensuremath{\gamma}$-AgI. The full-potential linear augmented-Slater-type-orbital implementation of the local-density approximation is employed. Theoretical predictions for most observed features, such as bandwidths and the location of band gaps, are in good agreement with experiment. Although band-gap magnitudes are underestimated, the relative ordering of the direct gaps is correctly predicted for the five crystals.

Defect models in silver halides

Silver halide crystals AgCl and AgBr exhibit all the three types of intrinsic point defects, namely, cationic Frenkel disorder, Schottky disorder and the bound vacancy pair disorder. The ionic conductivity anomaly at high temperatures, the unusual behaviour in respect of self- and impurity diffusion and the contrasting features in the anion self-diffusion between AgCl and AgBr do not all satisfactorily fit into the prevailing models of ionic transport. We review these features critically. We also discuss the potential and polarization models used in the theoretical evaluation of the enthalpies of formation of the intrinsic defects in these crystals. Results of our recent investigations on these questions will be presented to highlight their implications to the defect physics of these materials.