Surface physics of silver halides

Abstract

Since the inception of photography over a century ago, the silver halide emulsion grain has remained central to the photographic process. Much effort has been devoted to the improvement of the silver halide grain in terms of increased sensitivity, spectral response and development processes. The ability to use silver halides effectively in commercial products was established long before an actual understanding of the physical process was achieved. In the 1960s, research progressed on the bulk characteristics of silver halide single crystals. The electron and hole mobilities, the mechanism of ionic conductivity and the optical properties were among the major areas where mechanistic conclusions were drawn. Much of this work has recently been reviewed by Brown. ~1) An understanding of the bulk proved only a first step towards a fuller knowledge of the photographic process. Since an emulsion grain may range in size from a few tenths of a micrometer to a few micrometers, many of its properties must be dominated by its surface structure. A concerted research effort on silver halide surfaces did not develop until the late sixties. Some of the delay in studying the surface properties was caused by the nature of the surface itself. Silver bromide crystals, for instance, could not be easily cleaved, even at very low temperatures. Moreover, owing to its inherent softness, at tempts to obtain a smooth surface by polishing usually resulted in such surface disruption that no Laue X-ray diffraction pattern was seen. This fact, combined with the photosensitivity of the silver halides, made annealing in a bromine atmosphere almost a prerequisite to any sui'face study. It was only with the advent of sheet crystals and evaporated epitaxial films that well-defined surfaces could be obtained without the necessity of laborious surface treatment. Hence, this review will concentrate on some of the recent developments in experimental and theoretical investigations of silver halide surfaces. In particular, the discussion will be focused on the silver hal ide-vacuum surface since the knowledge of this interface is more advanced than that of silver halide-solution interfaces. The physical structure of the surface as well as the subsurface region will be examined in detail. Other aspects of silver halide research have been discussed in recent reviews by Hamilton on the photographic process