Crystal-field effects at the TiO2-SiO2 interface as observed by X-ray absorption spectroscopy

The electronic structure of the TiO2-SiO2 interface has been investigated u sing X-ray absorption spectroscopy (XAS). TiO2 overlayers have been grown o n two substrates, amorphous SiO2 and highly oriented pyrolytic graphite (fo r comparison), by evaporation of Ti in an oxygen atmosphere at room tempera ture. The evaporation rate was low enough to allow a detailed study of the early stages of growth, i.e., the submonolayer regime (theta < 1). The Ti 2 p XAS spectra of the TiO2 overlayers have been measured for different cover ages. The spectra corresponding to the submonolayer regime show an unusual shape not reported up to now. A comparison with existing atomic multiplet c alculations indicates a significant decrease of the crystal field (approxim ate to 0.7 eV) as compared with bulk TiO2 (approximate to 1.7 eV) due to st rong electronic interactions at the interface. The presence of the SiO2 sub strate and the covalent character of the Si-O bonds lowers the crystal fiel d of the TiO2 overlayer at the interface. In contrast, the graphite substra te inhibits the total oxidation of the overlayer forming Ti2O3. XAS has pro ved to be a useful experimental tool for the study of the electronic struct ure of interfaces.