Optical and Electrical Properties of Au- and Ag-Doped ReSe2

Single crystals of Au- and Ag-doped ReSe2 (ReSe2:Au and ReSe2:Ag) have been grown by chemical vapor transport (CVT) method using iodine as a transporting agent. The optical properties are studied by absorption and polarized-thermoreflectance (PTR) spectroscopy at different polarization angles in the temperature range between 12 and 300 K. The direct band-edge excitonic transitions (E1ex, E2ex, E3ex, and Esex) have been clearly revealed for the first time in the absorption spectra when the samples are made thin enough. The dominant E1ex and E2ex excitonic transitions gets strongest as the polarization is parallel and perpendicular to the b-axis, respectively. The parameters that describe the temperature dependence of the excitonic transition energy and the broadening function are extracted and discussed. We have also performed electrical conductivity and Hall measurements at different temperatures to derive the activation energy (∼95 and 50 meV for ReSe2:Au and ReSe2:Ag, respectively).