High resolution photoemission study of growth, alloying, and intermixing of ultrathin ruthenium films on W(1 1 1) and W(2 1 1)

The alloying, intermixing, growth and faceting of ruthenium overlayers on tungsten single crystals have been studied with high resolution soft X-ray photoelectron spectroscopy, SXPS, (using synchrotron radiation) and low energy electron diffraction (LEED). W4f core-level photoemission spectra and W valence band spectra have been measured at various photon energies, and at normal and grazing emission angles, for W(1 1 1) and W(2 1 1). These two surfaces were chosen for this study because it has been shown previously that W(1 1 1) covered with monolayer films of several different 4d and 5d transition metals develop nanoscale pyramidal facets with (2 1 1) faces, upon annealing. The present work extends the measurement to another catalytically active overlayer metal, Ru. The growth and evolution for dosing and annealing on both W surfaces are investigated for coverages from 0 to greater than 3 monolayers of Ru (1 ML=1.7 × 1015 atoms/cm2). Incremental dosing of Ru causes intermixing of the Ru and W atoms at the interface, even at fractional monolayer coverages. Annealing of surfaces with Ru coverages > 1 ML produces a complex set of SXPS results that indicate the formation of surface Ru/W alloys. Faceting of Ru/W(1 1 1) is observed by LEED for Ru coverages > 1 ML, after annealing at temperatures between 700 and 1000 K. Upon annealing to temperatures higher than 1000 K, the SXPS data indicate that clustering of the Ru–W surface alloys may occur.