Geometric and electronic structure of epitaxial NbxTi1-xO2 on TiO2(110)

We have investigated the detailed geometric and electronic structure of MBE-grown NbxTi1-xO2 on TiO2(110) by means of high-resolution transmission electron microscopy, X-ray photoelectron diffraction, ultraviolet and X-ray photoemission and electron energy loss spectroscopy. We find 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 epitaxial 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 in the lattice introduces an additional valence electron per atom. The resulting density of states falls in the valence band region, but no new 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 (parts per thousand), where Nb electrons occupy a shallow donor level near the conduction band minimum. Based on the electron counting rule, the extra Nb electrons form a non-bonding band which is degenerate with the valence band. The significance of these results for enhanced thermal and photochemistry on NbxTi1-xO2 surfaces vis a vis TiO2 is discussed.