U. Diebold

Adsorption of sulfur on TiO2(110) studied with STM, LEED and XPS: temperature-dependent change of adsorption site combined with O–S exchange

We have studied the adsorption of elemental sulfur on TiO 2 (110) for exposure at room temperature and at 300°C. Depending on the substrate temperature we found diVerent adsorption sites with scanning tunneling microscopy (STM ). At room temperature sulfur adsorbs on the titanium rows with a high mobility along the [001] direction. At 300°C sulfur adsorbs at the position of the bridging oxygen rows and forms short chains along the [11:0] direction at low coverages. High coverages result in the formation of a (3◊1) superstructure. X-ray photoelectron spectroscopy ( XPS) shows a 2 eV shift of the sulfur 2p peak to lower binding energy concurrently with the change of adsorption site from the titanium to the oxygen rows. For the (3◊1) structure a distinct Ti3+ shoulder appears at the Ti 2p 3/2 peak. Based on these measurements we present a model where sulfur replaces every third bridging oxygen atom of the substrate and the other bridging oxygens are completely removed.

Novel electronic and magnetic properties of ultrathin chromium oxide films grown on Pt(111)

The growth of epitaxial metal–oxide films on lattice-mismatched metal substrates often results in the formation of unique overlayer structures. In particular, epitaxial chromium oxide films grown on Pt(111) exhibit a p(2×2) symmetry through the first two monolayers of growth which is followed by a (√3×√3)R30° phase that is attributed to the growth of a Cr2O3(0001) overlayer. Ultraviolet photoelectron spectroscopy measurements have been performed on the CrOx/Pt(111) system. The electronic structures of CrO2, Cr2O3, and Cr3O4 were calculated using the linear muffin-tin orbital method in the atomic sphere approximation. Comparison of the photoemission valence band spectra with the calculated density of states indicates that the CrOx initially grows in a cubic spinel Cr3O4 structure. Beyond ∼0.2 monolayers, the metallic behavior of the CrOx overlayer begins a transformation to an insulating state. The measured valence emission for the p(2×2) phase beyond ∼0.2 monolayers is more consistent with either a γ-Cr2O...