Electronic structure of Au-Ag bimetallics: Surface alloying on Ru(001)

Abstract

We report a study of the electronic structure of Au-Ag overlayers (total coverage varies from 2 to 9 ML) supported on Ru(001). Photoemission spectroscopy with synchrotron radiation and a Mg $K\ensuremath{\alpha}$ x-ray source has been used to study the valence band and the Au $4f$ and Ag $3d$ core levels of these systems. It is found that, in most cases, two-dimensional alloy formation occurs upon deposition at room temperature. The extent of surface alloying depends on the order of deposition, composition, thickness, and annealing temperature. By comparing the valence band and binding energy shifts for bulk and surface alloys, we found that both systems exhibit the same trend: that is, that relative to the d band of pure Au, the alloy d band narrows and the Au $4f$ shifts to higher binding energy upon decreasing Au concentration. However, the surface alloy d band is consistently narrower than that of the bulk with the same stoichiometry. Similar surface bulk behavior is also seen in the Au $4f$ binding energy shift and the Au d-band doublets. This behavior is attributed to the two-dimensional nature (reduction in coordination number of like nearest neighbors on average) of the surface alloy.