Resolving the structure of the striped Ge layer on Ag(111): Ag$_2$Ge surface alloy with alternate fcc and hcp domains

TL;DR

This study reveals that the striped Ge layer on Ag(111) is actually an Ag$_2$Ge surface alloy with alternating fcc and hcp domains, not germanene, using surface X-ray diffraction and DFT calculations.

Contribution

It provides the first precise structural characterization of the Ge/Ag(111) interface, clarifying its true alloy nature and atomic arrangement.

Findings

The striped layer is an Ag$_2$Ge surface alloy, not germanene.

The structure features alternating fcc and hcp domains similar to Au(111) reconstruction.

The atomic density is 6.45% higher than Ag(111).

Abstract

Two-dimensional (2D) honeycomb lattices beyond graphene, such as germanene, promise new physical properties such as quantum spin Hall effect. While there have been many claims of growth of germanene, the lack of precise structural characterization of the epitaxial layers synthesized hinders further research. The striped layer formed by Ge deposition on Ag(111) has been recently ascribed as a stretched germanene layer. Using surface X-ray diffraction and density functional theory calculations, we demonstrate that it corresponds in fact to a Ag$_2$Ge surface alloy with an atomic density 6.45% higher than the Ag(111) atomic density. The overall structure is formed by stripes associated with a face-centered cubic top-layer alignment, alternating with stripes associated with an hexagonal-close-packed top-layer alignment, in great analogy with the ($22\times\sqrt3$) Au(111) reconstruction.