THE STABILITY OF ULTRATHIN METAL-FILMS ON W(110) AND W(111)

Ultrathin films of platinum on tungsten display a rich variety of phenomena related to epitaxial growth. Pt grows on W(110) in a layer-by-layer mode at 300 K; upon annealing multilayers of Pt to T > 800 K, clusters of three-dimensional crystallites form on top of a pseudo-morphic Pt monolayer. When oxygen is present on the surface, the Pt monolayer is no longer stable. Annealing coadsorbed Pt and O on W(110) causes nearly all the Pt to agglomerate into clusters as the oxygen largely replaces the monolayer on the surface. Annealing Pt on W(111) in the range 800 < T < 1500 K leads to a different behavior: microscopic facets of W having {211} orientation are formed. The faceting appears to be driven by a Pt enhanced anisotropy in the surface free energy. Results based on LEED, Auger, and scanning tunneling microscopy (STM) are presented.