Optical properties and PL enhancement of anisotropic ReS2

Semiconducting transition metal dichalcogenides (TMDCs) is a class of materials exhibiting strong exciton binding energies of above 100 meV, which allows for novel ways of tuning the exciton resonances. Among the TMDCs, ReS2 is unique, as the anisotropic crystal structure causes multiple linearly polarized excitons to exist, which show high binding energies in bulk. However, contradicting reports exist on the number of excitons and their binding energies. Interestingly, the reported photoluminescence (PL) quantum yield (QY) of ReS2 of around 10^-4 suggests that the bandgap of ReS2 could be indirect. In this work, we use differential reflection spectroscopy to elucidate the number of resolvable exciton resonances and determine their exciton binding energies. In addition, we perform power-dependent and polarization resolved PL measurements to investigate the PL saturation behaviour of various features in the PL spectra. We show 2 differently polarized excitons in differential reflectance measurements, where some higher energy peaks can be resolved that, together with the ground-state excitons, are well described by Rydberg’s hydrogenic model. Finally, we attempt various strategies to enhance the PL(QY) of few-layer ReS2 and shed light on the exact nature of its bandgap.