Tuning the electronic properties of transition-metal trichalcogenides via tensile strain

A comprehensive study of the effect of tensile strain (ε = 0% to 8%) on the electronic structures of two-dimensional (2D) transition-metal trichalcogenide (TMTC) monolayers MX₃ (M = Ti, Zr, Hf, Nb; X = S, Se Te) is performed on the basis of density functional theory (DFT) computation. The unstrained TiS₃, ZrS₃, ZrSe₃, HfS₃, HfSe₃ and NbS₃ monolayers are predicted to be semiconductors with their bandgap ranging from 0.80 to 1.94 eV. Our DFT computations show that the biaxial and uniaxial tensile strains can effectively modify the bandgap of many TMTC monolayers. In particular, we find that ZrS₃ and HfS₃ monolayers undergo an indirect-to-direct bandgap transition with increasing tensile strain. The indirect bandgaps of ZrSe₃ and HfSe₃ monolayers also increase with the tensile strain. Both ZrTe₃ and HfTe₃ monolayers are predicted to be metals, but can be transformed into indirect bandgap semiconductors at ε = 4% and ε = 6%, respectively. Importantly, the TiS₃ monolayer can retain its direct-bandgap feature over a range of biaxial or uniaxial tensile strains (up to 8%). The highly tunable direct bandgaps of MS₃ (M = Hf, Ti, and Zr) by strain and the availability of metallic and semiconducting properties of MTe₃ (M = Hf and Zr) provide exciting opportunities for designing artificial layered structures for applications in optoelectronics and flexible electronics.