The structural and chemisorptive properties of ultrathin cobalt overlayers on W(110) and W(100)

Abstract

The structure, stability, surface electronic properties, and chemisorptive properties of vapordeposited cobalt overlayers (0–4 ML) on W(110) and W(100) have been studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), work function changes (ΔΦ), and temperature programmed desorption (TPD) of cobalt, hydrogen, and carbon monoxide. The first layer of cobalt grows pseudomorphically with respect to the tungsten substrate, and is thermally stable to 1300 K. Second and subsequent layers grow layer-by-layer at 100 K, but form three-dimensional clusters above 400–500 K. The relative work functions of these surfaces are strongly dependent on temperature, cobalt coverage, and substrate geometry. The annealed pseudomorphic monolayer of Co/W(100) has a positive ΔΦ value (+155 mV), indicating a net electronic charge transfer from the tungsten to the cobalt. The chemisorptive properties of the cobalt overlayers are quite different from those of bulk cobalt surfaces, the former having new TPD features for hydrogen chemisorption and TPD features consistent with carbon monoxide dissociation. CO apparently dissociates on cobalt-tungsten interfacial sites and on the pseudomorphic monolayer of Co/W(100).