Investigation of exciton states of ReS2 by temperature- and polarization-dependent photoluminescence and oxygen plasma treatment

Rhenium disulfide (ReS2) is a 1T′ phase, direct band-gap layered semiconductor with in-plane anisotropy of optical and electrical properties. It has been researched extensively for polarization-sensitive optoelectronic applications in the energy range of visible light. However, the comprehensive excitonic states of ReS2, including the emission induced by incidental defects such as sulfur vacancies, have not been investigated fully. We performed temperature- and polarization-dependent photoluminescence (PL) measurements to study the exciton states of ReS2 with thicknesses varying from monolayer to multilayer. Notably, we firstly identified the PL peak at ∼1.4 eV (X0). It showed a gradual enhancement by the oxygen plasma treatment. The X0 peak showed an isotropic polarization of the PL. This was distinguished from the linearly polarized emission of the other PL peaks of ReS2. Density functunal theory (DFT) calculations predicted the narrowing of the bandgap by the formation of ReO bonds at the sulfur vacancy sites. This is consistent with the observed energy of X0. Our comprehensive investigation of the four excitonic transitions of ReS2 including X0 impurity state, expands the utility of anisotropic ReS2 as a practical optoelectronic device.