Flat-top silver nanocrystals on the two polar faces of ZnO: An all angle x-ray scattering investigation

The growth of silver at room temperature on the two polar faces of ZnO, (0001) Zn terminated and (000 (1) over bar) O terminated, has been investigated in situ by using grazing incidence x-ray diffraction and grazing incidence small angle x-ray scattering (GISAXS). On both surfaces, silver forms nanoclusters with (111) top facets in hexagon-on-hexagon epitaxy with [10 (1) over bar ]Ag parallel to[100](ZnO). Despite the high lattice mismatch in the latter direction (-11%), the Ag islands adopt their bulk parameter even at the very beginning of the growth, a key feature assigned to a quasi-perfect coincidence of a 9 x 9 Ag cell on a 8 x 8 ZnO cell. The GISAXS patterns give evidence of triangular and hexagonal cluster shapes on the Zn and O surfaces, respectively, which reflect the topographies of the two surfaces. Quantitative analysis is performed to deduce the morphological parameters as a function of coverage. To represent highly covered surfaces involving large clusters with complex shapes, a specific GISAXS treatment in terms of holes in a continuous metal layer is proposed. In the O case, flat-top islands develop mostly laterally until an incompletely filled thick layer is formed. In the Zn case, the islands grow both in height and in diameter until the film can be represented by a layer with holes. The derived growth parameters attest that silver spreads significantly better on the O-terminated face than on the Zn termination.