SCANNING-TUNNELING-MICROSCOPY AND SPECTROSCOPY OF OXIDE SURFACES

Considerable progress has been made using scanning tunneling microscop y (STM) and tunneling spectroscopy (STS) to examine the atomic structu res and properties of transition metal oxide surfaces. The surfaces ar e found to be very sensitive to thermochemical history; consequently a large variety of surface structures have been observed. This paper re views the results to date of STM on single crystal transition metal ox ides (excluding superconductors) and, while mentioning ambient analyse s, will emphasize ultra high vacuum and atomic scale structural inform ation of single crystal surfaces. First, a summary of salient features of oxide bulk and surface geometric and electronic structures is prov ided as a framework within which to discuss the STM results. The princ iples of STM and STS are reviewed with extensive discussion of special considerations for analyses of oxides, including effects of band bend ing and image interpretation. Variations of surface stoichiometry are illustrated with results on TiO2 and SrTiO3, for which the most data e xist. The yet unresolved controversy regarding the basis of contrast i n STM images of oxides is introduced explicitly into discussion of the se results. Recent work on vanadyls and tungstate bronzes is also pres ented. First observations of the structures of local defects on surfac es, including vacancies, dopants, steps, and domain boundaries, are il lustrated with results from ZnO, NiO, SrTiO3, and TiO2. Finally, the f ew studies of surface reactions are considered, including those of oxi des with metals, with reducing gases, and with organic molecules.