GEOMETRIC AND ELECTRONIC-STRUCTURE OF EPITAXIAL NBXTI1-XO2 ON TIO2(110)

We have investigated the detailed geometric and electronic structure o f MBE-grown NbxTi1-xO2 on TiO2(110) by means of high-resolution transm ission electron microscopy, X-ray photoelectron diffraction, ultraviol et and X-ray photoemission and electron energy loss spectroscopy. We f ind no measurable change in the Nb-O bond length relative to that for Ti-O bonds in TiO2 in the dilute limit (x = 0.05), and that the epitax ial layers remain strained and coherent with the substrate for x less than or equal to approximate to 0.3. However, significant dislocation generation occurs for x > approximate to 0.3. Nb substitution for Ti i n the lattice introduces an additional valence electron per atom. The resulting density of states falls in the valence band region, but no n ew state density occurs in the either the band gap or conduction band. This result is in contrast to what occurs in the very dilute limit (p arts per thousand), where Nb electrons occupy a shallow donor level ne ar the conduction band minimum. Based on the electron counting rule, t he extra Nb electrons form a non-bonding band which is degenerate with the valence band. The significance of these results for enhanced ther mal and photochemistry on NbxTi1-xO2 surfaces vis a vis TiO2 is discus sed.