Step mediated surface alloy formation of Pt/Cu(1Â 1Â 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.