Composition and structure of ultrathin vanadium oxide layers deposited on SnO2()

Ultrathin layers of vanadium oxides on SnO 2 (1 1 0) were prepared by evaporation of metallic vanadium in oxygen atmosphere (partial pressure of O 2 : 1×10 −5 Pa) at room temperature (RT). Vanadium oxide layers were deposited on the (4×1) reconstructed surface of SnO 2 (1 1 0). The substrate surface was prepared by cycles of sputtering and annealing in vacuum. X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering (LEIS) results show that the growth mechanism of the oxide layers involves the migration of tin atoms from the substrate onto the surface of the film already at RT. Although no LEED pattern is observed, the X-ray photoelectron diffraction (XPD) measurements reveal that the vanadium oxide films grow epitaxially on the substrate surface. The similarity between the Sn 3d XPD pattern measured for the clean SnO 2 (1 1 0) surface and the V 2p pattern measured for the oxide layers suggests that these films have a structure close to that of the substrate (rutile-type lattice). The present results can be explained with a formation of a vanadium oxide of the series V n O 2 n −1 with 4⩽ n ⩽8. These so called Magneli phases of vanadium oxides have structures similar to that of rutile VO 2 . This interpretation was confirmed by comparison of multiple scattering calculations with the experimental XPD curves. The vanadium oxide films are stable and the formation of mixed vanadium–tin oxide does not occur after annealing in vacuum at ≈800 K. The XPD data seem to indicate that tin atoms are present at the surface as a two-dimensional phase. STM images collected for the annealed phases reveal a morphology consistent with the XPS and LEIS results.