Photoluminescence kinetics of dark and bright excitons in atomically thin MoS_2.

The fine structure of the exciton spectrum, containing optically allowed (bright) and forbidden (dark) exciton states, determines the radiation efficiency in atomically thin transition metal dichalcogenides. We report on a study of the time-resolved micro-photoluminescence in monolayers and bilayers of MoS_2, both unstrained and compressively strained, carried out in a wide temperature range (10-300 K) with a period between excitation pulses increased to 25 ns. This makes it possible to estimate decay times characteristic for the spin- and momentum-forbidden exciton states, as well as their contributions to the total radiation intensity. The observed temperature dependencies either increase or decrease at certain temperature due to a change in the thermalized population of the upper state. Our results unambiguously show that, in an unstrained film, the spin-allowed state is the lowest for the A exciton series (1.9 eV), while a dark state is about 2 meV higher, and that this splitting energy can increase several times upon compression. In contrast, in the indirect exciton series in bilayers (1.5 eV), the spin-forbidden state is the lowest, being ~ 4 meV below the bright one. The strong effect of strain on the fine exciton spectrum can explain the large scatter between the published data and must be taken into account to realize the desired optical properties of 2D MoS_2.