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

Abstract

While ${\mathrm{SnS}}_{2}$ 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 ${\mathrm{SnS}}_{2}$ 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 $\mathrm{\ensuremath{\Gamma}}$ 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 ${\mathrm{GW}}_{0}$ 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 ${\mathrm{SnS}}_{2}$. 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, ${\mathrm{SnS}}_{2}$ 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 ${\mathrm{SnS}}_{2}$ can significantly broaden the applications of this material in strain-detection and optoelectronic devices.