Phase boundary potentials of adsorbed films on metals.—Part III.—The examination of the interaction of copper and iodine vapour by the method of surface potentials

With both platinum and gold, measurements by the method of surface potentials revealed the fact that at low temperatures iodine is reversibly adsorbed on the surface of these metals on which a film of chemi-adsorbed oxygen was present, and does not penetrate into or undergo chemical reaction with the metal substrate. With less noble metals it is known from the work of Kohlschütter and Krähenbtihl, of Tammann and especially Evans, that in spite of the presence of a thin layer of oxide which is invariably present, iodine readily attacks the metal, forming an iodide. The rate of attack is governed by a diffusion process through an ever-increasing thickness of solid, and the process is of the type designated as activated, in that the temperature coefficient of the diffusion process is exponential in character. After the exposure of a sheet of copper to the vapour of iodine we might anticipate that the phase boundary potential would be modified both by an adsorbed film of iodine on cuprous iodide, which adsorbed film should be readily volatile, as well as by a layer of cuprous iodide on the metal substrate. Further, the cuprous iodide is unstable (pseudomorphous form), and should in course of time become converted into a true cuprous iodide lattice (idiomorphous form), which later may undergo a process of sintering or crystallization. Since the iodide of copper is a semi-conductor it might be anticipated, from analogy with cuprous oxide and from the theory of formation of rectifying contacts suggested by Schottky and by Wilson that the full contact potential between the iodide and metal would not come into existence until the layer of pure insulating iodide attains a sufficient and critical thickness. Finally, if the idiomorphous as well as the pseudomorphous iodide form rectifying contacts with copper the conversion of the one into the other will affect the contact potential as well as the critical thickness required. It was thought desirable to examine the possibility of following the growth of a semi-conductor, formed by chemical attack at a metal surface, by the measurement of the phase boundary potentials.