Linearly Polarized Excitons in Single- and Few-Layer ReS₂ Crystals

Rhenium disulfide (ReS₂), a layered group VII transition metal dichalcogenide, has been studied by optical spectroscopy. We demonstrate that the reduced crystal symmetry, as compared to the molybdenum and tungsten dichalcogenides, leads to anisotropic optical properties that persist from the bulk down to the monolayer limit. We find that the direct optical gap blueshifts from 1.47 eV in the bulk to 1.61 eV in the monolayer limit. In the ultrathin limit, we observe polarization-dependent absorption and polarized emission from the band-edge optical transitions. We thus establish ultrathin ReS₂ as a birefringent material with strongly polarized direct optical transitions that vary in energy and orientation with sample thickness