Two-dimensional hexagonal M3C2 (M = Zn, Cd and Hg) monolayers: novel quantum spin Hall insulators and Dirac cone materials

The intriguing Dirac cones in honeycomb graphene have motivated the search for novel twodimensional (2D) Dirac materials. Based on density functional theory and the global particle-swarm optimization method, herein, we predict a new family of 2D materials in honeycomb transition-metal carbides M3C2 (M = Zn, Cd and Hg) with intrinsic Dirac cones. The M3C2 monolayer is a kinetically stable state with a linear geometry (CQMQC), which to date has not been observed in other transition-metal-based 2D materials. The intrinsic Dirac cones in the Zn3C2, Cd3C2 and Hg3C2 monolayers arise from p-d band hybridizations. Importantly, the Hg3C2 monolayer is a room-temperature 2D topological insulator with a sizable energy gap of 44.3 meV. When an external strain is applied, additional phases with node-line semimetal states emerge in the M3C2 monolayer. These novel stable transition-metal-carbon-framework materials hold great promise for 2D electronic device applications.