Exciton binding energy and hydrogenic Rydberg series in layered ReS₂

Unlike monolayers of transition metal dichalcogenides such as MoS_2, which possess high in-plane symmetry, layered ReS₂ exhibits reduced in-plane crystal symmetry with a distorted 1 T structure. This unique symmetry leads to anisotropic optical properties, very promising for light polarization devices. Here, we report on low temperature polarization-resolved emission and absorption measurements of excitons in ReS₂ from bulk to monolayer. In photoluminescence and reflectivity contrast spectra we distinguish two strongly polarized excitons X₁ and X₂ with dipole vectors along different crystal directions, which persist from bulk down to monolayer. Basing on the PL and RC spectra of bulk crystals we determine the energy of the ground and first four excited states of both excitons, which follow the usual hydrogenic Rydberg series of energy levels of 3D excitonic states (E_n = Ry^*/n²). From the numerical fit we estimate that the energy gap is direct and equal to 1671.7 meV and binding energy of X₁ and X₂ is equal to 117.5 and 86.6 meV, respectively. In magneto-PL spectra of bulk ReS₂ up to B = 10 T, the energy shift of all the states is below 2 meV. On reducing the crystal thickness from bulk to monolayer the ground state experience a strong blue shift.