New stable 2D and 3D GeC2 crystal structures predicted by first-principles study

New crystal structures of GeC2 in monolayer, bilayer two-dimensional (2D) and three-dimensional (3D) formats were predicted using first-principles density functional theory calculations. The structures were proven dynamically, thermodynamically stable and energetically favorable. The 2D crystal structure of GeC2 is composed of tetragonal and hexagonal rings formed by germanium and carbon atoms in a buckled plane, isostructural to the recently reported tetrahex-C allotrope. Hybrid functional HSE06 predicts that both the monolayer and bilayer GeC2 are semiconductors with a direct band gap of 0.84 eV and 0.12 eV, respectively. Comparing with other isostructural materials, these GeC2 structures show promising potential applications in infrared photoelectric devices. Moreover, the monolayer GeC2 can be stacked to form 3D bulk structures, which were also proven to be energetically and dynamically stable. However, the 3D GeC2 shows a metallic property. The effective masses of charge carriers in the 2D GeC2 are found to be anisotropic. Stacking into a bilayer structure, the effective mass of electron doubles in all crystalline directions, or even increases by an order of magnitude. The effective mass of hole only increases in the y-direction and is nearly unchanged in the x- and s-directions. The band gap variation in the material and its behind mechanism were analyzed and discussed in an atomic level.