FACETING OF W(111) INDUCED BY ULTRATHIN PD FILMS

The surface stability of a model bimetallic system, Pd on W(111), has been studied by using ultrahigh vacuum surface science methods. Fractional monolayer Pd thin films are stable and remain on the surface at all temperatures below approximately 110 K, at which thermal desorption begins. Above a threshold coverage of about one monolayer, the Pd-covered W(111) surface is structurally unstable; upon annealing above 700 K the W atoms undergo massive rearrangement to expose [211] oriented facets. Multilayer uniform Pd thin films deposited at room temperature are also unstable upon heating to T > 500 K and agglomerate to form three-dimensional Pd clusters on top of the faceted W surface covered by a monolayer of Pd.