Step mediated surface alloy formation of Pt/Cu(1 1 1)

Medium energy ion scattering has been used to probe the early stages of the development of a surface alloy of platinum deposited on Cu(1 1 1). Our experiments reveal the presence of a novel two-layer “precursor” Pt–Cu alloy at low temperatures prior to formation of the expected Cu3Pt surface alloy at near 550 K. Comparison of the alloying behaviour of one monolayer of platinum on a nominally flat copper surface with that on a vicinal copper surface, Cu(12 12 11), demonstrates that the formation of this precursor alloy is mediated by steps on the surface. A model is proposed for the mechanism of formation of this precursor alloy, which occurs at much lower temperatures than previously observed for surface alloying of the platinum–copper system.