Surface alloying of Pd and Cu on Ru(0 0 0 1)

Abstract

Codeposition of Pd and Cu on Ru(0001) with total coverages varying from submonolayer to nm thickness has been carried out at temperatures of 300 and 550 K. The effects of heating the overlayers up to 1250 K were studied. Besides the characterization of the overlayer composition as a function of temperature and observation of possible ordering, a series of questions is addressed. This includes conditions for alloying, possible effects related to dimensionality (2- or 3-dimensions), and the significance of the order of deposition. Photoelectron spectroscopy (PES) based on application of synchrotron radiation and the Mg Kα X-rays (XPS) was used to obtain information on the electronic structure of the bimetallic overlayers via the Ru 3d, Pd 3d core levels (PES) and Cu 2p core levels (XPS). Furthermore, the valence band (VB) electron energy distribution curves (EDC's) were measured at a photon energy of 40 eV which is the Cooper minimum of Cu 3d electrons. LEED was used to establish the order of the overlayers.The main findings of the bimetallic studies are that (i) the deposited Cux and Pd1−x for 0⩽x⩽1 with a total coverage of 1 ML, are distributed in one layer with temperature dependent intermixing, (ii) the valence bands are very characteristic with the Cu 3d band separated from the Pd 4d band and alloying gives rise to new features with binding energies (BE's) between these bands, (iii) the Cu 2p core level shifts (CLS) are correlated linearly with the composition of the films in 2- and 3-dimensions, an effect which is attributed to the different coordination numbers between Pd and Cu in the different dimensions, (iv) the structure of the overlayers depends only slightly on the order of deposition of the two coadsorbates for T>660 K and (v) (1×1) LEED patterns are obtained for 1 ML Cu+2 ML Pd at room temperature and for 2 ML Cu+2 ML Pd after annealing at 660 K indicating, in combination with the pronounced hybridization of Cu 3d and Pd 4d orbitals, formation of a surface alloy.