The optical signature of few-layer ReSe2

Optical properties of thin layers of rhenium diselenide (ReSe2) with thicknesses ranging from mono- (1 ML) to nona-layer (9 MLs) are demonstrated. The photoluminescence (PL) and Raman scattering were measured at low (T=5K) and room (T=300K) temperature, respectively. The PL spectra of ReSe2 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 120cm−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 ReSe2 flakes in the few-layer limit.