Surface structures of ultrathin vanadium oxide films on Pd()

The growth and the atomic structure of epitaxial vanadium oxide thin films on Pd(111) have been investigated by scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS), combined with ab initio density-functional theory (DFT) calculations. At submonolayer coverage a well ordered (4×4) oxide overlayer forms, which transforms into a porous oxide network with an internal (2×2) periodicity upon exposure to H2 at room temperature. The (2×2) phase represents an interface-stabilised surface–V2O3 layer, which becomes compact upon mild annealing in vacuum exhibiting a (2×2) honeycomb structure. Between 0.5 and 1 monolayer equivalents (MLE) the growth of oxide islands with a zigzag stripe structure is observed along with the (2×2) layer. At 1 MLE two distinct VO2-like phases are coexistent at the surface in the form of islands with rectangular and hexagonal structures, interestingly they are clearly different from the known bulk-type rutile VO2 lattice. The detailed atomic structure and energetic stability of these monolayer V-oxide phases have been revealed by the DFT calculations. Above 2 MLE three-dimensional crystallites grow epitaxially on Pd(111) with the corundum structure, which is typical of the bulk-type V2O3. Two stable V2O3(0001) terminations have been found in the STM images, which are due to bulk-type oxygen planes and terminal vanadyl species, as suggested by the DFT calculations and confirmed by high-resolution electron energy loss phonon spectra.