Temperature Dependence of Phonon Modes, Optical Constants, and Optical Band Gap in Two-Dimensional ReS₂ Films

Abstract

Effects of temperature on the optical properties of large-area ReS2 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 ReS2 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.

Temperature-dependent optical and vibrational properties of PtSe2 thin films

“…The linearly fitted result indicated that the temperature coefficient was 1.75 × 10 −4 eV/K. This value is lower than those of other TMDs such as ReS 2 48 . Figures 7 and 8 illustrate the peak energy, damping, and normalized intensity of 2.22-and 3.75-eV optical transitions for bilayer PtSe 2 thin film and 1.51-and 3.39-eV optical transitions for multilayer PtSe 2 thin film as a function of temperature.…”

Two-dimensional ReS2 : Solution to the unresolved queries on its structure and inter-layer coupling leading to potential optical applications

“…For ReS 2 , unlike the other TMDC systems, the lack of symmetry, intra-mode couplings and the interactions with acoustic modes incite 36 vibrational modes altogether. Keeping apart the 15 infrared active and three zero-frequency modes, the remaining 18 Raman active ones constitute the A g -like and E g -like vibrational modes, originating from the out-of-plane and in-plane vibrations of the Re atoms, respectively [54]. Figures 6(a)-6(c) depict the Raman intensity maps with the associated color scales, after considering the most intense A g -like (149 cm -1 ) peak, having much less sensitivity toward polarization.…”

Anisotropy and Modal Hybridization in Infrared Nanophotonics Using Low-Symmetry Materials

“…18 Understanding the role that the off-diagonal elements of the permittivity tensor play in driving polariton losses, and propagation can provide significant insights into what additional opportunities they can afford. Notably, several 2D materials offer such low symmetry crystal structures, while still maintaining the potential for exfoliating thin films, such as ReS 2 , which is a triclinic, van der Waals crystal with low-energy phonon modes 167 and the expectation of a series of broad bandwidth resonances in the far-infrared.…”