First-Principles Investigation of the Electronic, Excitonic, Optical, and Vibrational Properties of α-InS Monolayer

Novel two-dimensional structural phases drive the development of ultracompact and highly efficient semiconductor technologies. Although indium sulfide is a well-known material, the intrinsic limits of the excitonic interactions, specifically within its 2D phase, are not yet fully established. In this work, we performed an ab initio characterization of the α-InS hexagonal monolayer, encompassing its structural stability, electronic, excitonic, optical, and vibrational properties. The structural stability was confirmed at the mechanical, dynamical, and thermodynamical levels, with this monolayer being an indirect semiconductor exhibiting an HSE06+SOC electronic band gap of 2.72 eV and a weak SOC effect. Of its 9 optical phonon modes, only 1 is active in the IR spectrum at 243.9 cm–1 and 3 in Raman, at 102.7, 243.9, and 280.4 cm–1. Its excitonic ground states are indirect, with 2.27 eV and an exciton binding energy of 449 meV; these quasiparticle effects result in a red shift of the optical absorption gap and a small anisotropy in its linear optical response.