An XPS Study of the Effect of Ion Bombardment on Transition Metal Oxides

Abstract

The effect of ion bombardment on various transition metal oxides has been investigated by means of XPS. These oxides are commercially obtained powders of FeO, Fe304, Fe2O3, CoO, Co304, TiO2, ZrO, Nb2O5, Ta2O5 and WO3. In order to establish the beam induced effects, it is necessary to characterize the unbombarded oxide surface. For each oxide this has been carried out by determination of the binding energies associated with the core-electron levels of the metal and oxygen in various chemical states. In addition, the bulk-purity of the oxides were investigated using X-ray powder diffraction technique. Ion bombardment induced surface compositional changes of all oxides have been observed through a chemical reduction to the lower oxidation states and eventually to the metallic state. The results found have been quantified to yield the extent of the surface compositional change. This quantification is given in terms of the (metal to oxygen)-surface to bulk ratio, which enables comparisons to be made with predictions of the linear cascade theory of preferential sputtering. Theoretical understanding of the ion bombardment induced compositional changes in oxides is currently insufficient, mainly due to a lack of quantified experimental evidence. The results found here thus provide an extensive basis of quantitative information. In addition to the effect of ion bombardment on the surface chemistry of the oxides, investigations involving the influence of the primary beam energy, current density , dose, mass and charge of the projectile on the extent of the chemical damage are also carried out. The results have been compared with recent theories of preferential removal of oxygen.