Multilayer Germanenes Formed in Zintl-Phase CaGe_2 by Fluoride Diffusion

摘要

When reacted with ionic liquid [BMIM][BF_4], Ge monolayer networks (germanene) in Zintl-phase CaGe_2 crystals transform into multilayer (bi- or tri-layer) germanenes in CaGe_2F_X (0 ≤ X ≤ 1.8) compounds. The structures of these germanenes are discussed with the aid of high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images. One of the bilayer germanenes is a new allotrope comprising four-, five-, and six-membered Ge rings, which is of the same structure as previously reported bilayer silicene. ~[1] The newly obtained structures of trilayer germanene are not of the cubic diamond type; however, they are described with combinations of hexagonal stacking, similar to the high-pressure bulk phases. Their differences, as compared with multilayer silicene, suggest that the structures of multilayer germanene and silicene depend on the misfit between the layers of CaF_2 and Ge (Si). We anticipate that these results will contribute to the design of materials with attractive properties using tetrahedral coordina- tion and/or by controlling the segregation and reconstruction of group-IV elements.