Growth of germanene at interfaces between van der Waals materials and Ag(111)

ファンデルワールス材料とAg(111)界面でのゲルマネン成長

鈴木 誠也   

Suzuki, Seiya

Germanene is a two-dimensional (2D) allotrope of germanium (Ge), with a honeycomb lattice similar to that of graphene. Theoretical calculations have predicted that germanene is a 2D topological insulator with a moderate spin-orbit bandgap (~24 meV), which is desirable for next-generation electronic devices. The growth of germanene has been reported on single crystal metal surfaces, such as Au, Al, and Ag in ultra-high vacuum. However, there are no reports of electronic devices using germanene. One of the main reasons for this is the chemical instability of germanene, which hinders its device fabrication. To overcome the instability of germanene, we present the direct growth of germanene at the graphene/Ag(111) and hexagonal boron nitride (h-BN)/Ag(111) interfaces. Our growth process consists of transfer a cap layer (graphene or h-BN) on Ag(111)/Ge(111) followed by heating in N at ambient pressure. The grown germanene is stable in air, so that Raman spectra of germanene can be recorded using an ordinary ex situ Raman microscope. The Raman spectra of the samples after heating up to 550 C, we found two new Raman peaks at 155 and 255 cm. With density functional theory calculations, we assigned these Raman peaks are the out-of-plane (155 cm) and in-plane (255 cm) vibrational modes of germanene. It was also found that van der Waals materials as the cap layer are the key to germanene growth, since germanene was not grown with a cap layer of AlO, which is an ordinary metal oxide insulator.