Electron-stimulated damage processes in oxides under ultra-high vacuum (UHV) conditions

Abstract

HREM has recently entered the surface science arena as a complementary technique for studies of electron-stimulated damage of oxide surfaces. The primary advantage offered is the ability to observe in-situ structural changes of the surface, as has been exemplified in the literature for NiO, WO3, V2O5, and TiO2, among other oxide crystals. One major drawback has been the inability to perform experiments under UHV conditions comparable to those used in conventional surface science techniques. Previous HREM reports have not considered the effect of the microscope environment in surface radiation damage.In this study, we compared the surface radiation damage effects of oxides under UHV conditions (l0-10 Torr in a Hitachi UHV-H9000 instrument) to damage effects observed in conventional instruments (operating at approximately 10-7 Torr). A number of oxides were investigated, including NiO, V2O5 , WO3 , MoO3 , TiO2, and Ta2O5. Our results indicate that although the same basic damage mechanisms are operative under both vacuum conditions, subsequent surface alteration due to reoxidation or reaction with surface contamination occurs under non-UHV conditions. WO3, for example, underwent surface-initiated reduction to metallic W under UHV conditions (Fig. 1, 2 ).