Scanning tunneling microscopy and spectroscopy of oxide surfaces

Considerable progress has been made using scanning tunneling microscopy (STM) and tunneling spectroscopy (STS) to examine the atomic structures and properties of transition metal oxide surfaces. The surfaces are found to be very sensitive to thermochemical history; consequently a large variety of surface structures have been observed. This paper reviews the results to date of STM on single crystal transition metal oxides (excluding superconductors) and, while mentioning ambient analyses, will emphasize ultra high vacuum and atomic scale structural information of single crystal surfaces. First, a summary of salient features of oxide bulk and surface geometric and electronic structures is provided as a framework within which to discuss the STM results. The principles of STM and STS are reviewed with extensive discussion of special considerations for analyses of oxides, including effects of band bending and image interpretation. Variations of surface stoichiometry are illustrated with results on TiO2 and SrTiO3, for which the most data exist. The yet unresolved controversy regarding the basis of contrast in STM images of oxides is introduced explicitly into discussion of these results. Recent work on vanadyls and tungstate bronzes is also presented. First observations of the structures of local defects on surfaces, including vacancies, dopants, steps, and domain boundaries, are illustrated with results from ZnO, NiO, SrTiO3, and TiO2. Finally, the few studies of surface reactions are considered, including those of oxides with metals, with reducing gases, and with organic molecules.