SURFACE STATES ASSOCIATED WITH CHEMISORBED SPECIES ON ZINC OXIDE

This paper introduces a new method for measuring electronic energy levels (surface states) associated with foreign species on zinc oxide and describes early results. The method is based on electrostatic charging from the gas phase. The surface is shifted far from electronic equilibrium by depositing negative ions on the surface, created by a corona discharge in oxygen, and the surface potential is subsequently monitored as electrons are injected from the surface states to restore equilibrium. Measurements of the rate of injection as a function of temperature can be interpreted in terms of the surface state energy. Various behavior patterns have been observed for the zinc oxide surface with and without surface additives. With no intentionally added surface states, the return to equilibrium seems to be dominated by donor diffusion and by surface decomposition of the ZnO, rather than injection from surface states. With many additives, e.g., methanol, the injection from the surface states seems to follow theory, and the surface state energies are discussed in terms of the chemical and physical properties of the species. With some additives (e.g., H2 and H2O) an interaction between surface states may be involved, with the product providing the active surface state. © 1969.