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.