Structural and morphological changes during reduction of ultrathin vanadium oxide films on Pd(111)

Full text: High-resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM) have been used to study the structural and morphological modifications of ultra-thin vanadium oxide films on Pd(111), which are induced by the exposure to hydrogen and annealing in vacuum. A well-ordered (4 x 4) structure forms at submonolayer oxide coverages (up to 0.5 MLE) which is observed in STM and LEED. The phonon spectrum, as measured in specular HREELS, exhibits features at 60, 68, 105, 115 and 126 meV. After exposure to hydrogen the (4 x 4) phase transforms into a porous (2 x 2) oxide network, as detected by STM. The phonons at 105 and 115 meV have disappeared at this stage which relates them to the (4 x 4) oxide phase. The loss feature at 126 meV is due to a vanadium oxide islands exhibiting a zig-zag stripe structure in STM. As seen in both STM and HREELS this latter phase is not significantly affected by H2 exposure. The porous (2 x 2) network layer becomes compact upon annealing to 570 K. This transition is also detected in HREELS, where a broad band at 63 meV transforms into a sharp single loss peak at 68 meV. While the latter is known to be the Γ-point dipole-active optical phonon mode of a surface-V2O3 oxide phase, the former can be explained by the presence of boundaries effects in the porous (2 x 2) network where the oxygen atoms are situated in lower coordination sites. (author)