FRACTAL MODEL OF SUPERSTRUCTURES ON AGBR(111)

It has recently been observed that thin film AgBr(111) may exhibit well-pronounced superstructures which are visualized by surface decoration, i.e., by deposition of a very small quantity of metal. Depending on the growth conditions of the AgBr films, these superstructures establish superlattices with [110] and [321] directions, respectively, of the AgBr lattice. They are thought to result from surface reconstruction of the otherwise highly polar surface. A fractal model of AgBr(111) superstructures is presented which for the first time, to our knowledge, enables us to assign the experimentally observed superlattices to surface configurations of proper dimensions and directions. By applying a simple procedure of covering the surface lattice sites so as to approach a half coverage, superstructures are formed which may stabilize the surface by coarsening on the atomic scale. The contribution of the subsurface space-charge layer, which is mainly due to mobile silver ion interstitials in a near surface region, as well as its possible interaction with neutral metal clusters formed at the surface during vapor deposition, are emphasized.