GROWTH AND PROPERTIES OF THIN AG FILMS ON PT(111) SURFACES

The growth of Ag films on Pt(111) surfaces is studied using low energy electron diffraction, thermal desorption spectroscopy, X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy, work function and CO adsorption methods. After completion of the first monolayer, Ag grows via a Stranski-Krastanov mechanism at 300 K. On annealing, however, the film morphology changes to a Frank-van der Merwe structure, at least within the first three monolayers coverage regime. The Ag layer growth is pseudomorphic. The binding energies of the Ag 3d 5/2, 3d 3/2, and 3p 3/2 core levels in the first Ag layer on PT(111) are about 0.35 eV lower than observed for bulk Ag. A model in which this is attributed to the difference of densities of states at the Fermi level of the overlayer and substrate metals gives a reasonable fit of the present and other data on core level binding energies of metal-on-metal systems. Ag atoms in the second and subsequent layers exhibit the bulk 3d 5/2 core level binding energy. The apparent shift of the respective XPS peak maximum with Ag coverage is a consequence of a superposition of signals from the first and consecutive layers. UPS reveals that annealed Ag submonolayers form 2-dimensional islands. CO chemisorption experiments with Ag-Pt(111) films suggest that adsorbed Ag atoms block the Pt(111) with respect to CO adsorption in a purely geometrical manner.