Polarized thermoreflectance and reflectance study of ReS₂ and ReS₂:Au single crystals

In this article, we present the optical study of undoped and Au-doped rhenium disulphide (ReS₂ and ReS₂:Au) using temperature- and polarization-dependent thermoreflectance (PTR) and reflectance (R) measurements. We have observed the indirect band gaps (E_g||, E _g⊥), and direct band-edge excitonic transitions (E ^ex₁, E^ex₂, E^ex ₃ and E^ex₄) at various polarization angles. The amplitudes of E^ex₁ and E^ex₂ transitions have an orthogonal characteristic and follow the Malus rule. A small blue-shift of indirect band gaps has been observed for the ReS₂:Au sample, we believe that this result is contributed from the doping effects. We also measured the temperature-dependent PTR spectra in the temperature range from 55 to 300 K, and determined the temperature dependences of the transition energies and broadening parameters of E^ex₁ and E ^ex₂ transitions. Reflectance measurements at various polarizations have been performed to confirm our observations.