A SEARCH FOR SURFACE ALLOY FORMATION IN FACETING INDUCED BY MONOLAYER METAL-FILMS - PD W(111) AND NI W(111)

A monolayer of Pd on a W(111) surface will cause faceting of the surface upon annealing to T> 700 K, with the formation of (211) oriented microfacets. In order to learn whether surface alloy formation or intermixing of surface layers occurs during faceting we have employed low energy ion scattering (LEIS), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) to investigate the Pd/W(111) system. We find that 3.0 Angstrom Pd deposited at room temperature almost completely covers the substrate W. In the temperature range from room temperature to similar to 1125 K (the onset of Pd desorption) there is no intermixing of Pd atoms with W atoms: Pd atoms stay on top of the planar surface as well as the faceted surface. In comparison with the Pd/W(111) system, we have also studied the Ni/W(111) system which does not exhibit faceting behavior. Within experimental uncertainty we find that there is similar to 10% intermixing of Ni atoms with W atoms in the topmost surface layer, after annealing at 1175 K.