Observation of a surface alloying-to-dealloying transition during growth of Bi on Ag(111)

Abstract

The atomic structures that develop as a function of coverage during deposition of Bi on Ag(111) have been studied using low-temperature scanning tunneling microscopy, low-energy electron diffraction, and ab initio calculations. The growth process involves two sequential stages. At low coverage, Bi atoms are incorporated into the topmost layer of Ag(111), resulting in the formation of an Ag${}_{2}$Bi alloy confined to the surface and ordered ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$)$R$30\ifmmode^\circ\else\textdegree\fi{} Ag${}_{2}$Bi islands supported on Ag(111). This mode of accommodation of Bi was found to be energetically favorable based on ab initio total-energy calculations. At coverage above a critical value of 0.55 monolayers, the Ag${}_{2}$Bi alloy phase gradually converts into an ordered Bi ($p\ifmmode\times\else\texttimes\fi{}\sqrt{3}$) overlayer structure supported on Ag(111). We postulate that the dealloying transition is likely driven by compressive strain induced by incorporation of large-size Bi atoms into Ag at a high coverage and the subsequent lack of miscibility of Ag and Bi bulk phases. After completion of the dealloying process, Bi(110) thin films can be grown epitaxially on top of Ag(111) with a chemically abrupt interface.