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 sub-monolayer coverages, depending on the oxygen pressure, two well-ordered zinc oxide phases with (4 × 4) and (6 × 6) coincidence structures form, which are attributed to H-terminated Zn 6 O 5 and graphite-like Zn 6 O 6 layers, respectively. The (6 × 6) phase exhibits a pronounced oxygen pressure dependence: at low p (O 2 ) a well-ordered (6 × 6) two-dimensional array of O vacancies develops, yielding a layer with a formal Zn 25 O 24 stoichiometry, while at high p (O 2 ) the Zn 6 O 6 monolayer transforms into bilayer islands. For oxide coverages up to 4 monolayers the graphite-like Zn 6 O 6 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 oxygen-poor conditions a compressed overlayer of Zn adatoms can be stabilized on top of the Zn 6 O 6 structure.