The optical signature of few-layer ReSe$_2$

TL;DR

This study investigates the optical properties of few-layer ReSe$_2$, revealing layer-dependent photoluminescence and Raman features, and proposes a Raman-based method for determining layer thickness in the few-layer regime.

Contribution

It provides detailed optical characterization of ReSe$_2$ layers from monolayer to nine layers and introduces a Raman mode energy difference as a reliable thickness indicator.

Findings

PL emission blueshifts with decreasing layer number

Raman modes show opposite evolution with layer thickness

Raman mode energy difference correlates with layer thickness

Abstract

Optical properties of thin layers of rhenium diselenide (ReSe$_2$) with thickness ranging from mono- (1 ML) to nona-layer (9 MLs) are demonstrated. The photoluminescence (PL) and Raman scattering were measured at low ($T$=5 K) and room ($T$=300 K) temperature, respectively. The PL spectra of ReSe$_2$ layers display two well-resolved emission lines, which blueshift by about 120 meV when the layer thickness decreases from 9 MLs to a monolayer. A rich structure of the observed low-energy Raman scattering modes can be explained within a linear chain model. The two phonon modes of intralayer vibrations, observed in Raman scattering spectra at about 120 cm$^{-1}$, exhibit very sensitive and opposite evolution as a function of layer thickness. It is shown that their energy difference can serve as a convenient and reliable tool to determine the thickness of ReSe$_2$ flakes in the few-layer limit.