In this article, we present the optical study of undoped and Au-doped rhenium disulphide (ReS 2 and ReS 2 :Au) using temperature-and polarization-dependent thermoreflectance (PTR) and reflectance (R) measurements. We have observed the indirect band gaps (E gk , E g? ), and direct band-edge excitonic transitions (E ex 1 , E ex 2 , E ex 3 and E ex 4 ) at various polarization angles. The amplitudes of E ex 1 and E ex 2 transitions have an orthogonal characteristic and follow the Malus rule. A small blue-shift of indirect band gaps has been observed for the ReS 2 :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 ex 1 and E ex 2 transitions. Reflectance measurements at various polarizations have been performed to confirm our observations.
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