Measurement of the conduction band spin-orbit splitting in WSe2 and WS2 monolayers

We have investigated charge tunable devices based on WSe2 and WS2 monolayers encapsulated in hexag-onal boron nitride. In addition to the well-known radiative recombination of neutral and charged excitons, stationary photoluminescence measurements highlight a weaker-intensity optical transition that appears when the monolayer is electrostatically doped with electrons. We have carried out a detailed characterization of this photoluminescence line based on its doping dependence, its variation as a function of the optical excitation power, its polarization characteristics, and its g factor measured under longitudinal magnetic field. We interpret this luminescence peak as an impurity-assisted radiative recombination of the intervalley negatively charged exciton (triplet trion). In addition to the standard direct recombination, the triplet trion has a second recom-bination channel triggered by elastic scattering off the short-range impurity potential. The energy difference between the emitted photons from these two possible recombination processes of the same triplet trion is simply the single-particle conduction band spin-orbit splitting Ac. We measure Ac = 12 & PLUSMN; 0.5 meV for the WSe2 monolayer and Ac = 12 & PLUSMN; 1 meV for the WS2 monolayer. These results demonstrate the importance of second-order exciton recombination processes in transition-metal dichalcogenide structures.