Electron spectroscopy of metal oxides

The validity of the classical dielectric theory of HREELS is investigated. Group theory is employed to obtain a tabulation of SO phonon modes expected to appear strongly in the spectra of given faces of many common crystal structures. The effects of crystalline anisotropy and surface defects are considered in detail. The theoretical investigation is used in conjunction with experiment to obtain a more detailed understanding of the HREELS of rutile (110), (100) and (001) surfaces than has been obtained previously. XPS, UPS and HREELS are used to investigate the surface composition and electronic structure of Sn-doped In2O3 ceramics (containing 1-6 at.% Sn) and thin films. XPS of the well-equilibrated ceramics reveals substantial tin enrichment in the surface atomic layer, with a heat of segregation of ~-20kJmol-1, and provides evidence for a sub-surface region partly depleted in tin. UPS and HREELS results are consistent with a free-carrier concentration close to the surface considerably below the bulk nominal value. XPS of the thin films reveals considerably less surface tin segregation, suggesting that thermal equilibrium is not attained during film production. Vacuum annealing dramatically increases the free carrier concentration, as shown by the shift in the surface plasmon frequency in HREELS. There is a substantial discrepancy between the bulk plasmon frequency predicted from HREELS, and that measured directly from optical transmission. The shift and attenuation of the HREELS plasmon is compared with a model where the surface layer is completely depleted of free carriers. Surface depletion layers have been created on rutile (001), and Sb-doped SnO2 ceramics containing 0.1 and 1 at.% Sb, by adsorption of Cl2 and NO2. The surface coverage of adsorbate is monitored by XPS. Shifts in work function and valence band edge are measured by UPS. HREELS of Cl2-dosed Sb-doped SnO2 show changes consistent with a depletion layer model.