Germanene Reformation from Oxidized Germanene on Ag(111)/Ge(111) by Vacuum Annealing

For group 14 mono-elemental 2D materials, such as silicene, germanene, and stanene, oxidation is a severe problem that alters or degrades their physical properties. This study shows that the oxidized germanene on Ag(111)/Ge(111) can be reformed to germanene by simple heating ≈500 °C in a vacuum. The key reaction in reforming germanene is the desorption of GeO and GeO₂ during heating ≈350 °C. After removing surface oxygen, Ge further segregates to the surface, resulting in the reformation of germanene. The reformed germanene has the same crystal structure, a (7√7 × 7√7) R19.1° supercell with respect to Ag(111), and has equivalent high quality to that of as-grown germanene on Ag(111)/Ge(111). Even after air oxidation, germanene can be reformed by annealing in a vacuum. On the other hand, the desorption of GeO and GeO₂ at high temperatures is not suppressed in the O₂ backfilling atmosphere. This instability of oxidized germanene/Ag(111)/Ge(111) at high temperatures contributes to the ease of germanene reformation without residual oxygen. In other words, the present germanene reformation, as well as the segregation of germanene on Ag(111)/Ge(111), is a highly robust process to synthesize germanene.