High resolution photoemission study of growth, alloying, and intermixing of ultrathin ruthenium films on W(111) and W(211)

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 x 10(15) atoms/cm(2)). 1ncremental 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. (C) 2003 Elsevier B.V. All rights reserved.