The optical signature of few-layer ReSe$_2$.

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 60~meV when the layer thickness decreases from 9~MLs to a bilayer. 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.