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.