Surface structures of ultrathin vanadium oxide films on Pd(1 1 1)

The growth and the atomic structure of epitaxial vanadium oxide thin films on Pd(1 1 1) 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(1 1 1) with the corundum structure, which is typical of the bulk-type V2O3. Two stable V2O3(0 0 0 1) 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.