CHARACTERIZATION OF OXIDE FILM ON TITANIUM BY SCANNING TUNNELING MICROSCOPY SPECTROSCOPY - INFLUENCE OF THE TIP COMPOSITION

Atomic level structural and electronic analysis of titanium passivated by an oxide film is crucial for understanding the biocompatible properties of this transition metal. Scanning tunneling microscope (STM) images of electropolished titanium samples show a rather smooth surface in the nanometer range with structure attributed to surface defects induced by the electropolishing technique. I-V spectra were performed with the STM using W, PtIr, and Au tips. These spectra completed by normalized conductivities spectra are compared and discussed in order to determine surface electronic properties of the oxide film and to estimate the influence of the STM tip. The surface band gap of this amorphous thin film mainly composed of TiO2 is shifted to positive tip voltage and is similar to the one of an n-type semiconductor with band bending bringing the valence band closer to the Fermi level. The surface band gap extends from - 0.5 to + 0.8 eV for a Au tip and from - 0.1 to 0.7 eV for a W tip, which shows that W induces states in the band gap. Reproducible peaks in the local density of states of our Ti oxide surface appear both for Au and W tips at the same energies and are clearly apparent in the normalized conductivity curves.