Germanene: a Novel Two-Dimensional Germanium Allotrope Akin to Graphene and Silicene

We have grown an atom-thin, ordered, two-dimensional multi-phase film in situ through germanium molecular beam epitaxy using a gold (111) surface as a substrate. Its growth is similar to the formation of silicene layers on silver (111) templates. One of the phases, forming large domains, as observed in scanning tunneling microscopy, shows a clear, nearly flat, honeycomb structure. Thanks to thorough synchrotron radiation core-level spectroscopy measurements and advanced density functional theory calculations we can identify it as a $\sqrt{3}$ × $\sqrt{3}$ $R$(30$^\circ$) germanene layer in conjunction with a $\sqrt{ 7 } \times \sqrt{ 7 } $R$(19.1$^{\circ}$ Au(111) supercell, presenting compelling evidence of the synthesis of the germaniumbased cousin of graphene on gold