Detailed Study of the Raman Response of Mono- and Few-Layer ReS$_{\mathrm{2}}$

ReS$_{\mathrm{2}}$ is an exciting 2-Dimensional (2-D) material due to its strong in-plane anisotropy, offering an additional physical knob to tune its properties for a wide variety of applications. In addition, ReS$_{\mathrm{2}}$ has been shown to be a direct-gap semiconductor for few-layer thicknesses, which is a major advantage in optoelectronics. Raman spectroscopy serves as the most useful, facile, and non-destructive method to characterize ReS$_{\mathrm{2}}$. Due to its lower symmetry, the Raman spectrum of ReS$_{\mathrm{2}}$ is significantly more complicated than its Mo or W counterparts, displaying 18 first-order modes. We will discuss the effects on the Raman spectrum under various experimental conditions, including polarization-dependent, layer-dependent, and resonant Raman, and outline the many aspects that need to be considered when using Raman spectroscopy to characterize ReS$_{\mathrm{2}}$ and other anisotropic 2-D materials. Comparisons between experiments and DFT calculations will also be analyzed. Furthermore, we will demonstrate the importance of correctly calculating thin film interference effects for Raman of ReS$_{\mathrm{2}}$ on SiO$_{\mathrm{2}}$/Si substrates.