Studies of the near-surface ionic space charge in silver halide crystals†

Two ongoing experiments dealing with ionic surface charge effects in AgCl and AgBr are described. The first consists of a mapping of the depth profile of the electrostatic potential in macroscopic single crystals, using as a probe the distribution of trace levels of radioactive, divalent 54Mn. The crystals are sectioned chemically after aging, with a depth resolution of 10 Å, using the radiotracer 36C1 as a depth gauge. The resulting potential profiles agree with theoretical expectation. The effects of temperature, dopants, and surface orientation are now being investigated. The second experiment is a study of the effects of adsorbents (such as photographic dyes) and surface treatments (e.g., transient exposure to water or methanol) on the sub-surface space charge, as manifested through changes in the longitudinal conductance of evaporated thin films (about 4000 Å) of AgBr. Rather large effects have been observed, sometimes corresponding to a change in the space charge contribution to the conductance of about a factor of ten, thus reflecting the change in the distribution of charged jogs at the surface.