Two-dimensional Pb–Sn alloy monolayer films on Ag(1 1 1)

Abstract

Single and binary two-dimensional (2D) films of Pb and Sn on Ag(111) prepared at room temperature have been investigated using low-energy electron diffraction (LEED) and scanning tunneling microscopy. (√28×√28)-Pb is observed in addition to (√3×√3)-Pb at coverages higher than 0.35ML. The nominal coverages for the (√28×√28)-Pb and (√3×√3)-Pb structures are determined to be 0.68 and 1/3ML, respectively. The (1×1)-Sn structure is formed at coverages less than 1ML. Both Pb and Sn films form a hexagonal close-packed structure on Ag(111). When the Sn coverage increases to more than 1ML, excess Sn atoms form a (√3×√3) structure on the (1×1)-Sn surface. The 2D binary films exhibit an incommensurate structure close to (√13×√13) at Pb and Sn coverages of 0.5 and 0.25ML, respectively. Atomic-resolution STM images exhibit a hexagonal close-packed structure. From the DFT total energy calculations, it is concluded that the Pb and Sn atoms of the (“√13×√13″) structure do not form an ordered alloy but, rather, form a solid solution alloy. From these results, it is concluded that the binary 2D films also follow the Hume-Rothery rule.