Strain-induced indirect-to-direct band-gap transition in bulk SnS2

While SnS2 is an earth-abundant large-band-gap semiconductor material, the indirect nature of the band gap limits its applications in light harvesting or detection devices. Here, using density functional theory in combination with the many-body perturbation theory, we report indirect-to-direct band-gap transition in bulk SnS2 under moderate, 2.98% uniform biaxial tensile (BT) strain. Further enhancement of the BT strain up to 9.75% leads to a semiconductor-to-metal transition. The strain-induced weakening of the interaction of the in-plane orbitals modifies the dispersion as well as the character of the valence-and the conduction-band edges, leading to the transition. A quasiparticle direct band gap of 2.17 eV at the Gamma point is obtained at 2.98% BT strain. By solving the Bethe-Salpeter equation to include excitonic effects on top of the partially self-consistent GW(0) calculation, we study the dielectric functions, optical oscillator strength, and exciton binding energy as a function of the applied strain. At 2.98% BT strain, our calculations show the relatively high exciton binding energy of 170 meV, implying strongly coupled excitons in SnS2. The effect of strain on vibrational properties, including Raman spectra, is also investigated. The Raman shift of both in-plane (E-2g(1)) and out-of plane (A(1g)) modes decreases with the applied BT strain, which can be probed experimentally. Furthermore, SnS2 remains dynamically stable up to 9.75% BT strain, at which it becomes metallic. A strong coupling between the applied strain and the electronic and optical properties of SnS2 can significantly broaden the applications of this material in strain-detection and optoelectronic devices.