Local-barrier-height images ofTiO2(110)surfaces

Abstract

Local barrier height (LBH) and constant current mode (CCM) images of nonreconstructed (110) single-crystalline titanium dioxide surfaces are determined under ultrahigh vacuum conditions. The barrier height measurements are performed with the tip-oscillation technique in the constant current mode (CCM) of a beetle-type scanning tunneling microscope with tunneling currents between 1 and $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{nA}$. The mean apparent height $\ensuremath{\phi}$ of the surface is derived as a function of the tunneling gap width. The data $\ensuremath{\phi}$ are in accordance with the results from $I\ensuremath{-}z$ spectroscopy. A simple electrostatic space-charge layer model explains the observed decrease of the apparent height for small tunneling gaps. The LBH technique is applied to investigate the structures of point defects on $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ and their effects on the work function on an atomic scale. We find small concentrations of vacancies of the protruding bridging oxygen atoms, giving rise to bright atomic-scale spots between the Ti rows. The barrier height images indicate a localized and reduced apparent barrier height $\ensuremath{\phi}$ at these sites, in correspondence to the overall decrease in the work function $\ensuremath{\Phi}$ that has been observed earlier in ultraviolet photoelectron spectra of ion-sputtered and reduced $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ surfaces. Besides the oxygen vacancies, the surface reveals the presence of distinct other types of atomic-scale features, both in topographic and barrier height images. Along the Ti rows, depressions with a concentration of approximately 0.02 monolayers can be identified in the constant current as well as in the barrier height images extending over one and (to a significantly lower extent) two atomic distances along [001]. They are attributed to the so-called type-B defects and they exhibit a local increase of the work function $\ensuremath{\Phi}$.