POSSIBLE SEMICONDUCTING PROPERTIES OF NOBLE-METAL FILMS

Energy states in thin metallic films are considered. For very thin films, these are confined to two-dimensional sheets in k-space. Within a sheet, it is possible for an energy gap to exist between successive zones. Under certain circumstances, this energy gap can persist in three dimensions, provided that the film is exactly two atomic planes thick. In a monovalent metal film that is just two atom planes thick, there are just sufficient electrons to occupy the first zone in the first sheet of states. Therefore, if the three-dimensional gap exists, the metal will be a semiconductor or an insulator. This possibility has been investigated for the noble metals copper, silver and gold. The outcome of the calculations depends on the assumptions made concerning the effective thickness and the depth of the potential well associated with the film. However the conclusion is reached that such a film of a noble metal will probably be semiconducting, provided it is oriented in the (111) direction. It seems unlikely that the required films can be produced with present experimental techniques. However, it should be possible to product noble LUCS (or superlattices). Whether or not these are semiconducting cannot be predicted on a simple model.