STUDIES OF ELECTRON-IRRADIATION AND ANNEALING EFFECTS ON TIO2 SURFACES IN ULTRAHIGH-VACUUM USING HIGH-RESOLUTION ELECTRON-MICROSCOPY

Electron-beam-induced surface damage and subsequent annealing processes in rutile crystals have been investigated with a 300 keV high-resolution electron microscope which has been modified for ultrahigh vacuum and equipped with a specimen heating holder. Room temperature irradiation produced a reduction sequence identical to that observed previously in conventional microscope vacuum, except for some evidence of enhanced sputtering. Irradiation at higher temperatures (200-400-degrees-C) increased the damage rate resulting in approximately the same amount of the surface TiO phase but producing larger areas of the intermediate TiO2-II phase. Sputter pits in both rutile and TiO2-II phases became markedly more facetted as the temperature was increased. Irradiation at temperatures greater than 500-degrees-C resulted in the formation of well-facetted holes in the rutile, but no accumulation of crystalline reduced phases was observed. Annealing of the damaged areas in the absence of the electron beam, as shown by the reformation of crystalline rutile, revealed a temperature dependence for the restoration of the stoichiometry of the reduced areas, while complete removal of the sometimes extensive sputter pits was observed at temperatures in excess of 200-degrees-C.