Measurement of surface state energy levels of one-equivalent absorbates on ZnO

Abstract

Surface state energy levels associated with selected one-equivalent chemical redox couples deposited on zinc oxide are determined by two experimental methods. The results are compared with an earlier quasi-theoretical estimation based on the chemical redox potential. The experimental methods include, (1) a previously reported method involving a study of the electron injection rate from such surface states as monitored by a surface potential measurement, and (2) a new method based on control of the intergranular contact resistance in a pressed ZnO pellet by nonvolatile surface states. It is found that all three methods of surface state energy determination lead to the same energy level for ions with known and stable ligand shells, such as iron cyanide (energy ~0.1 eV below the conduction band) and iridium hexachloride (energy ~ 0.7 eV below the conduction band). For ions where a stronger adsorbate/surface interaction can be expected, the methods lead to differing energy values, which can be reconciled in terms of such an interaction. The chromium, manganese, and oxygen species (O 2 −) are in this category. For these species the energy level of the surface state when occupied (surface potential measurement) is found to be about 1.1, 0.8, and 0.9 eV below the conduction band, respectively. The activation energy for electron injection from O − is about 0.4 eV.