Wilson, Daniel John; (2005) Defect and surface properties of the silver halides. Doctoral thesis , UCL (University College London).

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Abstract

In this thesis, state-of-the-art density functional theory calculations have been performed to study a number of properties of the rocksalt-structured silver halides which can broadly be defined as photographically-relevant. These involve point defects and their interaction with free electrons and holes, created upon excitation by actinic light. In our initial calculations, we studied the primary intrinsic point defects within the bulk of the material, in both charged and neutral forms, using the supercell technique. We have correctly predicted the dominance of the Frenkel defect in both AgCl and AgBr, and have found that the lowest energy configuration for the interstitial cation defect in both materials involved a second cation, forming a Ag22+ split-interstitial species, orientated in a 111 direction. We then extended this work by applying a hybrid QM/MM embedding technique to model two systems: the first represented the ideal (100) surface, while the second represented a 'realistic' finite cluster. With the first, we have calculated defect structures and formation energies on the flat surface, and have examined the well-known space-charge layer. With our finite cluster, we studied extended surface defects (steps, kinks etc.) and their interaction with individual point defects. We have also calculated trap depths, and the localisation of holes and electrons at each of the sites. We have determined that, regardless of their location, an electron diffusely localises around the interstitial cation and strongly localises on the anion vacancy, while the corresponding hole becomes trapped around the cation vacancy on a nearest-neighbour cation. However, overall, we consider the positive kink on the surface to be the most likely electron trap, due to its relative abundance and its position within the band gap. Finally, we have investigated the properties of the latent pre-image centre, which plays a key role in the photographic process.

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