Adsorbate-induced structural changes of metal thin films: Cobalt-oxygen and cobalt-sulfur overlayers on Mo(110)

The oxygen- and sulfur-induced aggregation of cobalt in mixed Co-oxygen and Co-sulfur overlayers on Mo(110) have been studied using X-ray photoelectron and temperature-programmed desorption spectroscopies and low-energy electron diffraction. X-ray photoelectron data indicate that deposition of oxygen on 1 monolayer of Co results in the formation of Co oxide, which begins to decompose above 400 K. Only minor amounts of Co oxide are formed when 1 ML of Co is deposited on an oxygen overlayer. After annealing to 965 K for 60 s, the attenuation of the Co X-ray photoelectron signal and the temperature-programmed desorption data provide evidence for migration of oxygen to Mo and formation of three-dimensional Co clusters. Sulfur in mixed Co-S overlayers also induces Co island formation as sulfur bonds to Mo, although to a lesser extent than oxygen. The Co aggregation is explained in terms of the thermodynamic preference for oxygen and sulfur to bond to Mo over Co. These studies provide a means of constructing phases that can be used to investigate structural effects in surface reactions.