Direct and Indirect Exciton Dynamics in Few-Layered ReS2 Revealed by Photoluminescence and Pump-Probe Spectroscopy

Atomically thin-layered ReS2 with a distorted 1T structure has attracted attention because of its intriguing optical and electronic properties. Here, the direct and indirect exciton dynamics of a three-layered ReS2 is investigated by polarization-resolved transient photoluminescence (PL) and ultrafast pump-probe spectroscopy. The various time scales of the decay signals of the time-resolved PL (<10 ps), with monitoring of the populations of electron-hole pairs (exciton), and the transient differential reflectance (approximate to 1 and 100 ps), with monitoring of the populations of electrons and/or holes in the excited states, are observed. These results reveal the characteristic exciton dynamics: rapid relaxation of direct excitons (electron-hole pairs) and slow relaxation of the momentum-mismatched indirect excitons accompanied by a one-phonon emission process. These findings provide important information regarding the indirect bandgap nature of few-layered ReS2 and its characteristic exciton dynamics, boosting the understanding of the novel electronic and optical properties of atomically thin-layered ReS2.