Growth and Surface Structure of Zinc Oxide Layers on a Pd(111) Surface

The growth and geometric structure of ultrathin zinc oxide films on Pd(111) has been studied by scanning tunneling microscopy, low-energy electron diffraction, and density functional theory calculations. For submonolayer coverages, depending on the oxygen pressure, two well-ordered zinc oxide phases with (4 x 4) and (6 x 6) coincidence structures form, which are attributed to H-terminated Zn6O5 and graphite-like Zn6O6 layers, respectively. The (6 x 6) phase exhibits a pronounced oxygen pressure dependence: at low p(O-2) a well-ordered (6 x 6) two-dimensional array of O vacancies develops, yielding a layer with a formal Zn25O24 stoichiometry, while at high p(O-2) the Zn6O6 monolayer transforms into bilayer islands. For oxide coverages up to 4 monolayers the graphite-like Zn6O6 structure is thermodynamically the most stable phase over a large range of oxygen chemical potentials, before it converges to the bulk-type wurtzite structure. Under oxygenpoor conditions a compressed overlayer of Zn adatoms can be stabilized on top of the Zn6O6 structure.