Silver halides hole trapping

Two main mechanisms were proposed for the supersensitization effect. One is the hole-trapping mechanism in which the electron from SS fills the hole in the highest occupied molecular orbital (HOMO) of the excited sensitizing dye, since the HUMO level of SS is chosen to be higher than that of the sensitizer (Fig. 5) [2,10,11]. The resultant ionic state gives up an electron to the conduction band of silver halide with much higher quantum yield. 

The majority of inorganic systems reported to exhibit photochromism are solids, examples being alkali and alkaline earth halides and oxides, titanates, mercuric chloride and silver halides.184 185 The coloration is generally believed to result from the trapping of electrons or holes by crystal lattice defects. Alternatively, if the sample crystal is doped with an impurity capable of existing in variable oxidation states (i.e. iron or molybdenum), an electron transfer mechanism is possible. 

In summary, there is evidence that the multitalented sulfur sensitization product can trap electrons, can trap holes to reduce recombination, can stabilize photolytic silver atoms, and can accelerate reduction sensitization. Conceivably, each of these properties could be of importance for latent image formation under at least some conditions. The silver sulfide centers are not of uniform size, they probably are not uniformly related energetically to the silver halide matrix, and they may differ in chemical consititution. 

Rhodium, incorporated in the silver halide grains, decreases sensitivity and increases contrast. This action has been attributed to depression of latent image formation because of deep electron trapping by the trivalent rhodium ion (183-185). Eachus and Graves (184) showed that rhodium, probably as a complex, acts as a deep trap for electrons at room temperature. Weiss and associates (186) concluded that the rhodium salts introduce deep traps for both electrons and holes. Monte Carlo simulation showed that the photographic properties could be accounted for in this way over a wide range of exposure times. 

Some excited dye molecules can inject holes into the valence band of the silver halide. Photohole injection has been demonstrated experimentally by the photobleaching of chemically produced fog silver (248) to produce direct positive photographs. This action depends on the favorable location of the SQ ground state of the dye relative to the valence band of the silver halide. The injection is accomplished by the transfer of an electron from the valence band to the vacant SQ level of the excited dye molecule, leaving a mobile hole in the valence band which can oxidize a silver atom. Conversely, if the SQ level of a dye molecule is located sufficiently above the top of the valence band, a mobile hole in the silver halide can become trapped by the nonexcited dye. Hole-trapping by dyes has been detected by ESR signals (249,250). 

In addition to the question of reversibility, there remains the question of the extent to which the potentials measured in solution are modified by adsorption and aggregation at the silver halide surface. No measurements are available for the potentials of the adsorbed molecules or aggregates. The adsorbed reduced form of the dye molecule would correspond to the radical formed when an electron from the conduction band is trapped by the ground state dye, or when the excited dye molecule injects a hole into the valence band. In each, the dye molecule acquires an additional electron. 

For Class 1 dyes, sensitization occurs by transfer of the electron from the state of the excited dye to the conduction band of the silver halide. The hole created by the transfer remains in the dye molecule. If the hole is not too deeply trapped, it may eventually escape into the valence band with the aid of thermal energy. These dyes provide few, if any, electron traps and desensitization by oxygen/moisture in their presence would equal that for the undyed emulsion. 

Figures 49(c) and 49(d) show two other important processes which can occur when the silver halide is excited directly in the presence of adsorbed dyes. In these cases an electron is transferred from the VB to the CB upon excitation, and the holes in the VB may be filled by electron transfer from the HOMO of the adsorbed dye. The product of this process in Figure 49(c) is the same as that from the electron-injection dye sensitization in Figure 49(a), i.e., a dye radical cation and a conduction band electron which may be trapped and contribute to latent image formation. Illustrated in Figure 49(d) is the consequence of excitation of silver halide in the presence of a dye in which the energy of the LUMO is lower than that of the CB. In this case, direct excitation of the silver halide results in a conduction band electron which can be transferred to the LUMO of the dye. Subsequent electron transfer of an electron from the HOMO of what would then be a dye radical anion results in effective deactivation of the band-gap excitation, and overall reduced photographic sensitivity of the silver halide toward direct excitation due to the presence of the dye. This process is known as dye desensitization.

Figure 52. Hole-trapping mechanism for supersensitization. The super-sensitizer, SS, transfers an electron to the excited state of the dye to form the radical anion of the dye, before electron transfer occurs to the silver halide. The radical anion subsequently injects the electron to the CB [207].

Figure 1. Mechanism of latent-image growth in silver halide. Key e, electrons Ag, interstitial silver ion h , holes T, traps and S, special sites.

Holes. Compared to the situation for photoelectrons in the silver halides, the properties of holes are less well understood. In the effective mass approximation, the binding energy is moderated by the background dielectric constant. When a mobile carrier is close to a trapping center, it does not experience the full dielectric constant of the perfect lattice. If the effective mass is sufficiently large or the dielectric constant sufficiently small, the... 

In the silver halides Mott and Gurney suggested a mechanism for the formation of colloidal Ag [167]. A conduction-band electron produced by irradiation is first trapped at a lattice imperfection which may be a silver atom or ion, a chemical impurity, or a trapping site along a dislocation. The trapped electron then attracts a Ag interstitial ion to form a Ag atom. Following this, electrons and Ag " interstitials are trapped at the site in proper sequence to cause the buildup of a colloidal silver particle. This mechanism requires the presence and mobility of silver ions, and it is further required that the hole motion be sufficiently small that trapped electrons are not annihilated by electron-hole recombinations. 

Decomposition models for silver azide are similar to those proposed for the silver halides. McLaren and Rogers [95] suggested that band to band transitions give rise to electrons and holes in accord with photoconductivity data. Trapped electrons attract interstitial silver atoms which eventually form colloids, and holes lead to the formation of the nitrogen through a less-clearly determined process, possibly a bimolecular reaction of neutral azide molecules near the surface. Such a process requires the presence of discrete band-gap acceptor states, such as cation vacancies, that would serve as reaction sites for holes. The details of the process, however, remain undetermined. 

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