Coupling and stacking order of ReS[subscript 2] atomic layers revealed by ultralow-frequency Raman spectroscopy

Abstract

We investigate the ultralow-frequency Raman response of atomically thin ReS[subscript 2], a special type of two-dimensional (2D) semiconductors with unique distorted 1T structure. Bilayer and few-layer ReS[subscript 2] exhibit rich Raman spectra at frequencies below 50 cm[superscript –1], where a panoply of interlayer shear and breathing modes are observed. The emergence of these interlayer phonon modes indicate that the ReS[subscript 2] layers are coupled and orderly stacked. Whereas the interlayer breathing modes behave similarly to those in other 2D layered crystals, the shear modes exhibit distinctive behavior due to the in-plane lattice distortion. In particular, the two shear modes in bilayer ReS[subscript 2] are nondegenerate and clearly resolved in the Raman spectrum, in contrast to the doubly degenerate shear modes in other 2D materials. By carrying out comprehensive first-principles calculations, we can account for the frequency and Raman intensity of the interlayer modes and determine the stacking order in bilayer ReS[subscript 2].