The creation of microscopic surface structures by interfacial diffusion of Au and Ag on Ag(110): A XPS and STM study

Experiments on single crystal Au/Ag alloy surfaces may provide insight into the physical and chemical phenomena that determine the reactivity of complex alloy surfaces such as nanoporous gold or bimetallic nanoparticles. In this study, we report the highly unexpected observation that as soon as interfacial diffusion is feasible (400 K) thin gold films drastically restructure Ag(110) surfaces on the micrometer scale and create regular islands in a pattern which resembles the result of a sputter-etching process; bulk diffusion plays apparently no role during this phenomenon. Scanning tunneling microscopy (STM) reveals that the deposition of monolayer quantities of gold on the surface creates elongated islands, typically with a length of 1000 nm, a width of ~ 50 nm, and a height of ~ 30 lattice planes. The islands are predominantly elongated along the [1–10] direction and have a mutual distance of about 50 nm. Approximately, one monolayer of gold causes the relocation of ~ 11 ML of material. The islands are thermally unstable as further heating between 450 K and 600 K reduces the island structure significantly. The ‘etching’ of gold into the Ag(110) bulk material and the associated island formation can be rationalized with a simple kinetic model involving the diffusion of surface atoms only, illustrated with Monte Carlo simulations. A kinetic interpretation is also supported by the fact that those structures diminish after further annealing to higher temperatures. The observed large-scale reorganization underlines that even seemingly simple and well-defined surfaces may evolve in complex ways and that massive restructuring of surfaces can occur at temperatures well below the onset of bulk diffusion.