Large-Gap Quantum Spin Hall State in MXenes: d‑Band Topological Order in a Triangular Lattice

MXenes are a large family of two-dimensional (2D) early transition metal carbides that have shown great potential for a host of applications ranging from electrodes in supercapacitors and batteries to sensors to reinforcements in polymers. Here, on the basis of first-principles calculations, we predict that Mo₂MC₂O₂ (M = Ti, Zr, or Hf), belonging to a recently discovered new class of MXenes with double transition metal elements in an ordered structure, are robust quantum spin Hall (QSH) insulators. A tight-binding (TB) model based on the d_z²-, d_xy-, and d_x²–y²-orbital basis in a triangular lattice is also constructed to describe the QSH states in Mo₂MC₂O₂. It shows that the atomic spin–orbit coupling (SOC) strength of M totally contributes to the topological gap at the Γ point, a useful feature advantageous over the usual cases where the topological gap is much smaller than the atomic SOC strength based on the classic Kane–Mele (KM) or Bernevig–Hughes–Zhang (BHZ) model. Consequently, Mo₂MC₂O₂ show sizable gaps from 0.1 to 0.2 eV with different M atoms, sufficiently large for realizing room-temperature QSH effects. Another advantage of Mo₂MC₂O₂ MXenes lies in their oxygen-covered surfaces which make them antioxidative and stable upon exposure to air.