Titanium dioxide TiO2 is one of the most widely studied oxides because of its specific surface properties making it a good candidate for photo-catalysis of water, for reducing gas pollutants for air and water treatment and for its photo-induced hydrophilic properties. Thin films of TiO2 are also subject of interest for photo-voltaic applications or resistive switching memory device. TiO2 is present in nature in three different polymorphs: rutile, anatase and brookite. Although the most stable form in ambient condition is rutile, the quasi-metastable anatase TiO2 is considered potentially more relevant for energy-related applications.
We report the study of anatase TiO2 (001)-oriented thin films grown by pulsed laser deposition on LaAlO 3 (001). A combination of in-situ and ex-situ methods have been used to address both the origin of the Ti3+ localized states and their relationship with structural and electronic properties at surface and subsurface. Thin films growth has been performed in the new UHV facility NFFA-APE at IOMCNR and Elettra (Trieste). Advanced electronic characterization by angular-resolved photo-emission spectroscopy (ARPES) and X-ray photo-emission spectroscopy (XPS) and has been performed at the NFFA-APE beamline end stations receiving undulator synchrotron radiation from the ELETTRA storage ring. All of the surface-related problems have been circumvented by being the characterization chambers UHV connected with the PLD growth apparatus.
Localized in-gap states are analyzed by resonant x-ray photo-electron spectroscopy and related to Ti3+ electronic configuration, homogeneously distributed over the entire film thickness. We find that an increase of the oxygen pressure corresponds to an increase of Ti 3+ only in a well-defined range of deposition pressure; outside this range Ti3+, as well as the strength of the in-gap states, are reduced. In addition to the localized states, a two-dimensional electron gas (2DEG) has been also observed by ARPES experiments and tentatively correlated to the (4x1)/(1x4) surface reconstruction. In particular, we have observed the stabilization of clear dispersing states close to the Fermi energy not induced by beam irradiation, thus arising questions on the nature of such a metallic state and suggesting a different origin with respect to the 2DEG observed in other oxide systems, such as SrTiO3.
Our results refine the understanding of the defects at the origin of the in-gap electronic state and open the way to a better control of the specific surface properties of TiO2 anatase through its growth conditions.