Observation of anisotropic interlayer Raman modes in few-layer ReS₂

ReS₂ has recently emerged as a new member in the rapidly growing family of two-dimensional materials. Unlike MoS₂ or WSe₂, the optical and electrical properties of ReS₂ are not isotropic due to the reduced symmetry of the crystal. Here, we present layer-dependent Raman measurements of ReS₂ samples ranging from monolayers to ten layers in the ultralow frequency regime. We observe layer breathing and shear modes which allow for easy assignment of the number of layers. Polarization-dependent measurements give further insight into the crystal structure and reveal an energetic shift of the shear mode which stems from the in-plane anisotropy of the shear modulus in this material. (copyright 2016 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)