Layer parity dependent Raman-active modes and crystal symmetry in ReS2

ReS2, a group VII transition metal dichalcogenide (TMD), bears immense prospect in optoelectronic, thermoelectric, catalytic, and energy storage applications. Its distorted structure and significant in-plane anisotropy introduce extra degree of freedom but, on the other hand, make it challenging to determine the absolute characteristics of the material. Here, through experimental and theoretical analysis, we present additional phonon modes in the Raman spectrum of layered ReS2 which, in literature, are regarded as ambiguous modes arising due to either double resonance or defects. However, we demonstrate that these additional phonon modes arise from the crystal symmetry. This report proves the most followed notion of a layer-independent monolayer (ML)-based unit cell of ReS2 to be misleading. We further demonstrate that the observed additional phonon modes are driven by unique layer-dependent variations in the crystal symmetry. Moreover, layer-parity-dependent splitting of Eg1 mode and inversion symmetry breaking are discussed in detail. Such layer-dependent features have remained largely ignored in the literature. The results presented here may open new avenues for the application of this material and resolve several contradictions in its properties including the significance of stacking order, layer-dependent crystal symmetry, and shifting of Raman modes.