First-principles identifications of superstructures of germanene on Ag(111) surface and h-BN substrate.

Using first-principle calculations, we show that germanene can attach on Ag(111) surface forming germanene/Ag superstructures via electrostatic interactions. In all the optimized superstructures, we found a kind of epitaxially grown germanene is similar to the isolated low-buckled germanene. The adsorption energy of germanene on Ag(111) surface is about -464 meV to -428 meV per Ge atom, close to that of silicene on Ag(111) surface. Germanene on Ag(111) is a continuous layer and the p-d hybridization between Ag and Ge is revealed. These indicate Ag(111) surface is a good substrate for stabilizing germanene. The band structures of germanene are submerged in electronic states of metallic Ag substrate. To preserve the excellent electronic structures of germanene, we also considered another substrate hexagonal boron nitride (h-BN). We show that germanene can stably attach on h-BN substrate via Van der Waals (vdW) interactions, forming germanene/BN Moiré superstructures. At equilibrium state, a small band gap of about 50 meV is opened up in the Dirac point of germanene, whose value is insensitive to the rotation angle and the sliding between the two lattices, but can be effectively tuned by changing the interlayer distance. In these superstructures, the high carrier mobility of germanene is well preserved. These imply that h-BN can act as an ideal substrate material for germanene to achieve specific applications in nanoscale electronic devices.