Coupling and stacking order of ReS2 atomic layers revealed by ultralow-frequency Raman spectroscopy

TL;DR

This study uses ultralow-frequency Raman spectroscopy and first-principles calculations to reveal the coupling, stacking order, and unique interlayer phonon modes in atomically thin ReS2, challenging previous assumptions of layer decoupling.

Contribution

It demonstrates the presence of coupled and orderly stacked ReS2 layers through interlayer phonon modes, and characterizes their unique shear mode behavior and stacking order.

Findings

ReS2 layers are coupled and stacked orderly.

Distinct non-degenerate shear modes in bilayer ReS2.

First-principles calculations match observed interlayer phonon frequencies.

Abstract

We investigate the ultralow-frequency Raman response of atomically thin ReS2, a special type of two-dimensional (2D) semiconductors with unique distorted 1T structure. Bilayer and few-layer ReS2 exhibit rich Raman spectra at frequencies below 50 cm-1, where a panoply of interlayer shear and breathing modes are observed. The emergence of these interlayer phonon modes indicate that the ReS2 layers are coupled and stacked orderly, in contrast to the general belief that the ReS2 layers are decoupled from one another. While the interlayer breathing modes can be described by a linear chain model as 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 ReS2 are non-degenerate and well separated 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 ReS2.