Anisotropy study of phonon modes in ReS2 flakes by polarized temperature-dependent Raman spectroscopy

With present in-plane anisotropic optical, electrical, and optoelectronic properties, atomically thin Two-dimensional (2D) rhenium disulfide (ReS2) is of considerable interest for its wide-range intriguing applications in novel devices. However, although the research field related to ReS2 has made growing progress, the axis-dependent thermal properties are still unknown. Herein, temperature-dependent polarized Raman spectroscopy is carried out to systematically investigate the anisotropic thermal properties of ReS2 flakes with layers (L) of 1, 7, and 15 with temperature ranging from 110 K to 590 K. The polarization of the incident laser is tuned parallel to the crystal axis of the ReS2 flakes. It is discovered that Raman modes of ReS2 redshift with an increase of temperature and the first-order temperature coefficients are obtained along different crystal axes whose values are decreased with an increase in number of layers. The different temperature coefficients along different axis may be originated from the different group phonon velocities. In addition, the full width at half maximum (FWHM) of Raman modes of ReS2 flakes along different axes are also quantitatively studied. It is found that the broadening of phonon modes is originated from phonon decay. This work will promote the study of thermal anisotropy in two-dimensional anisotropic materials.