The optical signature of few-layer ReSe2

Optical properties of thin layers of rhenium diselenide ( ReSe 2) with thicknesses ranging from mono- (1ML) to nona-layer (9MLs) 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 120meV when the layer thickness decreases from 9MLs 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.