Structure of Pt overlayers on ZnO(0001) and ZnO(000(1)over-bar) surfaces

The growth of Pt films on the Zn- and O-terminated polar surfaces of ZnO has been studied. Clean surfaces, prepared by sputtering and annealing, were dosed with Pt by vapor deposition. Properties of the film were examined by low-energy alkali ion scattering and low-energy electron diffraction. It is confirmed that, near room temperature, Pt grows on both surfaces as a single hexagonal layer which aligns with the substrate low index directions. The alignment is better for the Zn-terminated than for the O-terminated surface. A Pt monolayer grows on the Zn-terminated surface until the ZnO surface is essentially covered with no evidence of a second Pt layer. Upon annealing above 675 K, Pt agglomerates into mostly bilayer islands, but maintains or improves its alignment with the substrate. Features associated with scattering from bi- or multilayer Pt indicate that the Pt forms oriented (Ill)rafts. A coincidence LEED pattern indicates that the Pt lattice may be expanded by 0.5% to form a 6:7 coincidence overlayer with the ZnO lattice.