Engineering the electronic structure and optical properties of monolayer 1T-HfX2 using strain and electric field: A first principles study

By using first-principles calculations, we characterized the structural stability, electronic and optical properties of novel two-dimensional (2D) monolayer transition-metal dichalcogenide (TMDC) HfX2 (X = S, Se). In our study, the PBE method is mainly used to calculate the electronic and optical properties, and the HSE06 method is used to further modify the band gap and optical absorption edge. It is indicated that monolayer 1-T HfX2 and HfX2 present a stable indirect band gap semiconductor when the strain is 0%. The band gap of all monolayer 1-T HfX2 decreases significantly and the optical absorption edge shows an obvious red-shift trend with an increasing in-layer biaxial compressive strain. When the compressive strain is greater than - 8% in HfX2 and -6% in HfX2 respectively, the band gap decreases to zero, and so the semiconductor-metal transition was produced. Conversely, the band gap increases insignificantly with the increasing in-layer biaxial tensile strain from 0% to + 10% while the absorption edge reveals an inconspicuous blue-shift trend. The general rule for the change of the band gap and the absorption edge tuned by strain of monolayer HfX2 could be explained well according to the different changes of near-band-edge states. Moreover, with the vertical electric field (E-field) increases, there is the band gap decreases and the absorption edge red shifts in all monolayer HfX2 under different in-layer biaxial strains. Our study suggests that the strain and electric field (E-field) engineering are effective tunable approaches to alter the electronic and optical properties of monolayer HfX2. Namely, we suggest that the functions of TMD could be changed by setting up suitable strain and E-field on few-layer materials in the future.