Splitting of interlayer shear modes and photon energy dependent anisotropic Raman response in $N$-layer ReSe$_2$ and ReS$_2$

TL;DR

This study uses ultralow-frequency micro-Raman spectroscopy to analyze interlayer phonon modes in $N$-layer ReSe$_2$ and ReS$_2$, revealing anisotropic splitting, interlayer force constants, and photon energy-dependent Raman responses due to exciton-phonon coupling.

Contribution

It provides the first detailed analysis of interlayer shear and breathing modes in anisotropic ReSe$_2$ and ReS$_2$, including their angular and photon energy dependence, and models interlayer interactions with a linear chain approach.

Findings

Interlayer shear modes exhibit significant splitting due to in-plane anisotropy.

Interlayer force constants are smaller than in other TMDs but comparable to graphite.

Raman response angular dependence varies with photon energy, indicating exciton-phonon coupling.

Abstract

We investigate the interlayer phonon modes in $N$-layer rhenium diselenide (ReSe$_2$) and rhenium disulfide (ReS$_2$) by means of ultralow-frequency micro-Raman spectroscopy. These transition metal dichalcogenides exhibit a stable distorted octahedral (1T') phase with significant in-plane anisotropy, leading to sizable splitting of the (in-plane) layer shear modes. The fan-diagrams associated with the measured frequencies of the interlayer shear modes and the (out-of-plane) interlayer breathing modes are perfectly described by a finite linear chain model and allow the determination of the interlayer force constants. Nearly identical values are found for ReSe$_2$ and ReS$_2$. The latter are appreciably smaller than but on the same order of magnitude as the interlayer force constants reported in graphite and in trigonal prismatic (2Hc) transition metal dichalcogenides (such as MoS$_2$, MoSe$_2$, MoTe$_2$, WS$_2$, WSe$_2$), demonstrating the importance of van der Waals interactions in $N$-layer ReSe$_2$ and ReS$_2$. In-plane anisotropy results in a complex angular dependence of the intensity of all Raman modes, which can be empirically utilized to determine the crystal orientation. However, we also demonstrate that the angular dependence of the Raman response drastically depends on the incoming photon energy, shedding light on the importance of resonant exciton-phonon coupling in ReSe$_2$ and ReS$_2$.