Grain electron trapping

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. 

In an unsensitized grain, shallow trap states provided by crystal imperfections are important in the trapping of both electrons and holes. Hamilton assumes that the fraction of holes trapped is approximately 1, that is, the concentration of mobile holes is near 0. Nucleation to form silver is inefficient, and a high level of free-electron/trapped-hole recombination occurs. There is a certain probability, however, that a trapped electron will unite with a silver ion to form an atom which may either dissociate back into electron and silver ion or trap another electron and, with a second Ag, form a silver atom pair. This pair is relatively stable and can grow by... 

Electron hole pairs generated by photon absorption enable oxygen to desorb from the surface (bottom of Fig. 3). The oxygen desorption annihilates some of the holes, thereby decreasing the surface, so that electrons are now able to move from one ZnO grain to another. Thus, photoconductivity of the layer is produced. In the dark period which follows, the photoconductivity of the layer is preserved for some time due to the large number of shallow electron traps. 

Fig. 2.11. Linear row of grains of identical length L, doping A, and with grain barriers of height b caused by a continuous distribution of electron trap states of density At [142]. Two different transport paths for electrons are indicated TE, thermionic emission across the barrier T, tunneling through the barrier...

Elwtronic trap + electronic traps - Grain-boundaries - Groin boumtario... 

In a recent series of experiments, values for electron trapping cross-section, have been estimated for Ir3+ and Rh3+ centers in cubic AgBr emulsion grains from high-frequency microwave photoconductivity data [196]. These weighted averages for the various... 

Intense research efforts have been devoted to characterize the electronic trap states in single crystal and colloidal Ti02 [173-176], although it is still unclear whether these states originate from defects in the bulk and surface regions, from the grain boundaries of the particles, from Coulomb trapping due to interactions of electrons with the cations of the electrolyte, or from a combination of all these factors. 

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