Electronic properties of ultrathin Cu and Fe films onTiO2(110) studied by photoemission and inverse photoemission

We have studied the electronic properties of the Cu/${\mathrm{TiO}}_2$(110) and Fe/${\mathrm{TiO}}_2$(110) interfaces using in situ ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPS). The chemical interaction between these two transition metals and the substrate was monitored by examining O 2p and band-gap emission in UPS, and the unoccupied Ti 3d band in IPS. For Cu film thicknesses ranging from 0.1 to 80 Å, there is no sign of overlayer oxidation or substrate reduction in either spectroscopy indicating that there is no significant charge transfer between Cu and the substrate. In contrast, strong interaction between Fe and ${\mathrm{TiO}}_2$(110) has been found, especially at low coverages. Charge transfer between the Fe overlayer and the substrate takes place at the very initial stage of deposition resulting in oxidation of the adsorbed Fe atoms and reduction of the Ti ions on the surface. Although the band-gap emission due to the formation of Ti 3d states is usually obscured by the Fe 3d band in UPS, IPS clearly shows a ${\mathrm{Ti}}^{3+}$ induced state at ∼3 eV above the Fermi level as seen on defective ${\mathrm{TiO}}_2$(110). The UPS spectrum from an oxygen-exposed Fe overlayer indicates formation of an iron-oxide layer composed of FeO and ${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$ phases.