Unlike silicene, for which the demonstration of its existence has been done through numerous independent studies, the possibility of growing epitaxial germanene remains highly controversial. It has been recently shown by scanning tunneling microscopy that the (3 × 3) surface reconstruction formed upon Ge deposition on Al(111) presents a honeycomb structure, and it was assigned to a pure germanene monolayer. Using quantitative measurements by surface X-ray diffraction compared to density functional theory calculations, we demonstrate that this Ge/Al­(111) (3 × 3) reconstruction corresponds, in fact, to a mixed Ge–Al honeycomb layer on top of an alloyed interfacial layer. The model of a germanene monolayer on top of the Al(111) surface can be completely excluded.

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 Ag2Ge 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 × √3) Au(111) reconstruction.