Germanene Epitaxial Growth by Segregation through Ag(111) Thin Films on Ge(111)

Large-scale two-dimensional sheets of graphene-like germanium, namely, germanene, have been epitaxially prepared on Ag(111) thin films grown on Ge(111), using a segregation method, differing from molecular beam epitaxy used in previous reports. From the scanning tunneling microscopy (STM) images, the surface is completely covered with an atom-thin layer showing a highly ordered long-range superstructure in wide scale. Two types of protrusions, named hexagon and line, form a (7√7 × 7√7) R19.1° supercell with respect to Ag(111), with a very large periodicity of 5.35 nm. Auger electron spectroscopy and high-resolution synchrotron radiation photoemission spectroscopy demonstrate that Ge atoms are segregated on the Ag(111) surface as an overlayer. Low-energy electron diffraction clearly shows incommensurate "(1.35 × 1.35)" R30° spots, corresponding to a lattice constant of 0.39 nm, in perfect accord with close-up STM images, which clearly reveal an internal honeycomb arrangement with corresponding parameter and low buckling within 0.01 nm. As this 0.39 nm value is in good agreement with the theoretical lattice constant of free-standing germanene, conclusively, the segregated Ge atoms with trivalent bonding in honeycomb configuration form a characteristic two-dimensional germanene-like structure.