Temperature Dependence of Phonon Modes, Optical Constants, and Optical Band Gap in Two-Dimensional ReS ₂ Films

Effects of temperature on the optical properties of large-area ReS ₂ films (10 layers), which are prepared by chemical vapor deposition, have been investigated by Raman and reflectance spectra. The phonon frequencies of 18 Raman modes red-shift about 3 cm ^-1 by increasing the temperature from 140 to 320 K. The optical constants (n and k) at a photon energy region of 0.46-6.52 eV are obtained, and the values blue-shift with increasing temperature. Four interband transitions (E _p1 , E _p2 , E _p3 , and E _p4 ) are observed at 1.53, 2.98, 4.25, and 5.37 eV at 303 K, respectively, and the values increase with increasing the temperature. The physical origins have been assigned to the different band-to-band direct electronic transitions. The optical band gap of the ReS ₂ films increases from 1.36 eV at 303 K to 1.38 eV at 383 K. Based on the first-principles calculation results, the band gap increases from 1.32 eV at a normal lattice constant to 1.40 eV at 1.1 times lattice constant. This is because the energy levels present the tendencies of degeneracy, due to which the coupling between the Re 5d orbital and S 3p orbital is weaker and the energy level splitting is smaller with increasing temperature.