Point defects in metal oxides such as TiO 2 are key to their applications in numerous technologies. The investigation of thermally induced nonstoichiometry in TiO 2 is complicated by the difficulties in preparing and determining a desired degree of nonstoichiometry. We study controlled self-doping of TiO 2 by adsorption of 1/8 and 1/16 monolayer Ti at the ͑110͒ surface using a combination of experimental and computational approaches to unravel the details of the adsorption process and the oxidation state of Ti. Upon adsorption of Ti, x-ray and ultraviolet photoemission spectroscopy ͑XPS and UPS͒ show formation of reduced Ti. Comparison of pure density functional theory ͑DFT͒ with experiment shows that pure DFT provides an inconsistent description of the electronic structure. To surmount this difficulty, we apply DFT corrected for on-site Coulomb interaction ͑DFT+ U͒ to describe reduced Ti ions. The optimal value of U is 3 eV, determined from comparison of the computed Ti 3d electronic density of states with the UPS data. DFT+ U and UPS show the appearance of a Ti 3d adsorbate-induced state at 1.3 eV above the valence band and 1.0 eV below the conduction band. The computations show that the adsorbed Ti atom is oxidized to Ti 2+ and a fivefold coordinated surface Ti atom is reduced to Ti 3+ , while the remaining electron is distributed among other surface Ti atoms. The UPS data are best fitted with reduced Ti 2+ and Ti 3+ ions. These results demonstrate that the complexity of doped metal oxides is best understood with a combination of experiment and appropriate computations.

Epitaxial ultrathin titanium dioxide films of 0.3 to approximately 7 nm thickness on a metal single crystal substrate have been investigated by high resolution vibrational and electron spectroscopies. The data complement previous morphological data provided by scanned probe microscopy and low energy electron diffraction to provide very complete characterization of this system. The thicker films display electronic structure consistent with a stoichiometric TiO(2) phase. The thinner films appear nonstoichiometric due to band bending and charge transfer from the metal substrate, while work function measurements also show a marked thickness dependence. The vibrational spectroscopy shows three clear phonon bands at 368, 438, and 829 cm(-1) (at 273 K), which confirms a rutile structure. The phonon band intensity scales linearly with film thickness and shift slightly to lower frequencies with increasing temperature, in accord with results for single crystals.

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The rutile TiO2(110) surface has been doped with sub-monolayer metallic Cr, which oxidises and donates charge to specific surface Ti ions. X-Ray and ultra violet photoemission spectroscopy and first principles density functional theory with Hubbard U are used to assign the oxidation states of Cr and surface Ti and we find that Cr2+ forms on bridging oxygen ions and a 5-fold coordinated surface Ti atom is reduced to Ti3+ and the Cr ions readily react with oxygen (to Cr3+), which leads to depletion of surface Ti3+ 3d electrons.

In this paper, we report the surprising formation of square-based facetted islands with linear dimension of the order of 500 nm upon dewetting of a Cr multilayer on W(100). We show that these square islands are composed of inclined facets surrounding a depressed center such that the facet slopes inward with the outer edges of the islands thicker than the centers. The islands' shapes do not represent traditional equilibrium crystal shapes as expected for a Wulf construction. In situ UV and x-ray photoelectron emission microscopy allied to spatially resolved spectroscopy throws considerable light on the nature of the dewetting and shows that the metal surface between the islands remains covered by a thin pseudomorphic wetting layer of ∼1 ML. Low-energy electron diffraction and scanning tunneling and atomic force microscopies allow quantification of facet slopes, and we identify a predominance of tilted Cr(100) facets ± 5 • off of the substrate normal bound by (210) planes at ∼26 • . The epitaxial Cr islands adopt the bulk Cr lattice constant but are tilted with respect to the surface normal. We suggest that the Cr crystallite tilting creates a vicinal-like interface structure that determines the island morphology.

. (2009) . Higher temperature processing leads to a loss of Cr from the surface region -indicative of its substitution into the bulk. IntroductionReducible transition metal oxides play a key role as active materials in the development of

The authors have studied the adsorption of Cu II (hfac) 2 on the surface of a model oxide system, TiO 2 (110), and probed the molecular stability with respect to thermal cycling, using atomic scale imaging by scanning tunneling microscopy supported by x-ray photoemission spectroscopy. They find that at 473 K, the adsorbed metal-organic molecules begin to dissociate and release Cu atoms which aggregate and form Cu nanoparticles. These Cu nanoparticles ripen over time and the size (height) distribution develops into a bimodal distribution. Unlike other organometallic systems, which show a bimodal distribution due to enhanced nucleation or growth at surface step edges, the nanoparticles do not preferentially form at steps. The reduced mobility of the Cu islands may be related to the co-adsorbed ligands that remain in very small clusters on the surface.