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. ((c) 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)