Identification of crystallographic structure of ReS₂ by polarized Raman spectroscopy

ReS₂ is attracting interest recently as a 2-dimensional semiconducting material. Unlike MoX₂ and WX₂(X=S, Se), ReS₂ has a direct bandgap from single-layer to bulk. In addition, due to the structural anisotropy, the physical properties are different depending on the orientation, like black phosphorus. Since the front and back sides are also distinct, a method for determining not only the in-plane direction but also vertical orientation of a ReS₂ is required. We performed polarized Raman spectroscopy to investigate the polarization dependence on crystallographic orientation of ReS₂. From the measurements, the polarization dependence of ReS₂ is different on the front and back side due to the anisotropic structure. In addition, we confirmed the lattice structure of the sample by high-resolution scanning transmission electron microscopy(HRSTEM) measurements. From the correlation between polarized Raman measurements and HRSTEM results, we developed a method to determine the crystallographic orientation of ReS₂ both the in-plane direction and the front and back side.