In the previous work, the present author proposed a comprehensive model for sulfur sensitization according to which monomers and dimers of substitutional sulfide ions on silver halides are sulfur sensitization centers acting as positive hole traps and electron traps, respectively, while silver sulfide clusters are fog centers. The model was supported by characterizing those centers on octahedral and cubic silver bromide emulsion grains with equivalent circular diameter of 0.2 μm. The difference in oxidation potential and absorption spectrum between sulfur sensitization centers and fog centers was distinctive enough to be ascribed not merely to the difference in size, but also to the difference in the state of the sulfide ions. As many as 500 silver sulfide clusters (i.e, fog centers)/grain (i.e, 2400/μm2) were observed by an electron microscope in slightly fogged emulsions, indicating that the developability of each cluster was very low. The above concentration of fog centers was independent of the amount of silver sulfide formed among variously fogged emulsions, suggesting that sulfur sensitization centers as well as fog centers were formed at surface sites, where sulfide ions were stable and liable to be coagulated. The belief is that low developability of silver sulfide clusters makes it possible to achieve high sensitivity and stability without fog formation in sulfur-sensitized emulsions.
Tadaaki Tani, "Comprehensive Model for Sulfur Sensitization (2): Characterization of Sulfur Sensitization Centers and Fog Centers" in Journal of Imaging Science and Technology, 1998, pp 135 - 143, https://doi.org/10.2352/J.ImagingSci.Technol.1998.42.2.art00007