Electron spectroscopy of transition metal oxide surfaces

Abstract

A review of XPS and AES studies performed on clean transition metal oxide (TMO) surfaces is presented. A discussion of the main aspects of these spectra and the difficulties encountered in their interpretation is given. Recent developments in the calculation of binding energies and Auger kinetic energies are described and used to discuss interpretations of chemical shifts in XPS and AES. A possible correlation between extra-atomic relaxation and the ionicity of the compound, for the zinc chalcogenides and zinc halides, is put into perspective. Multiplet splitting is briefly discussed. The influence of the number of unpaired spins and of the nature of the chemical bond, is emphasized. Spectra of the oxides MnO, CoO and ZnO are shown and discussed together with the data of the metals Co, Ni, Cu and compounds of Cr, Mn, Fe. Multiple excitations are shown to be linked to the paramagnetic character of the compound. This is illustrated by the CuO, Cu 2O, NiO, CoO and MnO spectra. Multiple effects in the corresponding Auger spectra are also discussed. It is shown that in some compounds the doubly excited state is sufficiently long lived to give rise to shake-up satellites in Auger spectra (NiO), but not in others (CuO and CoO). Valence band studies using XPS and AES are described. XPS-valence band spectra have been interpreted on the basis of band structure calculations (ZnO and ReO 3) or crystal field theory (NiO and Cr 2O 3). For ReO 3 cross-section modulation effects are emphasized. The valence band structures of V 2O 5 and of lower oxides (V 4O 9 and V 6O 13), present on V 2O 5 after different reducing treatments, are discussed. A discussion of the influence of valence band structure in Auger spectra, based on the comparison of the O (KLL) spectra of different oxides, is presented. Finally, broadening effects, mainly lifetime broadening, of both photolines and Auger transitions are discussed. For the Zn chalcogenides the width of certain Zn lines in different chemical environments, is apparently linked to other properties, such as the ionicity of the bond.