Polarized Thermoreflectance and Reflectance Study of ReS2 and ReS2:Au Single Crystals

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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 ($E_{\text{g}\|}$, $E_{\text{g}\bot}$), and direct band-edge excitonic transitions ($E_{1}^{\text{ex}}$, $E_{2}^{\text{ex}}$, $E_{3}^{\text{ex}}$ and $E_{4}^{\text{ex}}$) at various polarization angles. The amplitudes of $E_{1}^{\text{ex}}$ and $E_{2}^{\text{ex}}$ 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 $E_{1}^{\text{ex}}$ and $E_{2}^{\text{ex}}$ transitions. Reflectance measurements at various polarizations have been performed to confirm our observations.
2011-04-25