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The spectroscopic signature of oxygen vacancies on TiO2
Dr. C. L. Pang (London Centre for Nanotechnology and Department of Chemistry,University College London,)
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Spectroscopic measurements have been made from rutile TiO2 samples well-characterized with scanning tunneling microscopy (STM). Scanning tunneling spectroscopy (STS) measurements have been made simultaneously with STM on the (110) surface, revealing that the excess electronic charge associated with surface oxygen vacancies is delocalized over a number of Ti sites surrounding the vacancy, contrary to previous calculations. A defect state, which appears in the band gap at about 0.8 eV is also observed in the STS spectra. This band-gap state has long been attributed to surface oxygen vacancies.1 However, recently an alternative origin has been suggested, namely sub-surface interstitial Ti species.2 In order to determine the origin of the band-gap state, we monitored it using ultraviolet photoelectron spectroscopy (UPS) following a number of different surface treatments. Our results show that surface oxygen vacancies do indeed make the dominant contribution to the photoemission peak. Furthermore, the magnitude of this peak is found to be directly proportional to the surface oxygen vacancy density. We applied a similar methodology to the (011) 2x1 surface, where little is known about the band-gap state. The presence of surface oxygen vacancies was again shown to correlate with the appearance of a band-gap state in UPS.
1 C. L. Pang, R. Lindsay, and G. Thornton, Chem. Soc. Rev. 37, 2328 (2008)
2 S. Wendt, P. T. Sprunger, E. Lira, G. K. H. Madsen, Z. Li, J. O. Hansen, J. Matthiesen, A.?Blekinge-Rasmussen, E. Lagsgaard, B.Hammer, and F. Besenbacher, Science 320, 1755 (2008) .