Band gap anomaly in single-layer Nb2SiTe4-based compounds

Two-dimensional (2D) niobium silicon telluride (Nb2SiTe4) with good stability, a narrow band gap of 0.39 eV, high carrier mobility and superior photoresponsivity, is highly desired for applications as mid-infrared (MIR) detections, ambipolar transistors. Intensive investigations on its ferroelasticity, anisotropic carrier transport, anisotropic thermoelectric property, etc., have also been reported recently. Motivated by the above prominent properties and promising applications, we have systematically studied the electronic properties of single-layer (SL) A2BX4 analogues (A=V, Nb, Ta, B=Si, Ge, Sn, X=S, Se, Te) and found a band-gap anomaly with respect to anions change, which differs from conventional 2D metal chalcogenides. In conventional binary chalcogenides, when cations keep fixed, the bandgap tends to decrease when the atomic numbers of anions in the same group increase. However, in SL A2BX4, when atomic numbers of anions increase, their bandgaps tend to increase with cations kept fixed. In order to find the underlying mechanism of such abnormal bandgap, using first-principles calculations, we have thoroughly investigated the electronic structures of Nb2SiX4 (X=S、Se、Te) as an example. It is found that the valance band maximum (VBM) and conduction band minimum (CBM) are mainly derived from the bonding and antibonding coupling between Nb 4d states. The bandwidth of Nb 4d states determines the relative value of the band gap in Nb2SiX4. We demonstrate that the band gap is largely influenced by the competition effect between Nb-Nb and Nb-X interactions in Nb2SiX4. When the anion atomic number increases, the Nb-Nb bond length also increases, yielding increased bandwidths of Nb 4d states as well as a smaller bandgap of Nb2SiX4. Meanwhile, when Nb-X bond length increases, the bandwidth of Nb 4d however decreases, yielding a larger bandgap. The interaction between Nb and X should be dominant and responsible for the overall bandgap increase of Nb2SiX4 compared with Nb-Nb interaction.