Enhanced Radiative Exciton Recombination in Monolayer WS2 on the hBN Substrate Competing with Nonradiative Exciton-Exciton Annihilation

The photoluminescence quantum yield (PLQY) is a critical factor of monolayer transition metal dichalcogenides (1L-TMDs) for optoelectronic applications. However, the PLAY of 1L-TMD is severely affected by strong exciton-exciton annihilation (EEA) in two dimensions as the exciton density increases. Therefore, it is very important to suppress EEA or to improve radiative exciton recombination for ensuring a high PLQY. In this work we performed a comparative study on the PLQYs of five specimens of 1L-WS2 prepared on two different substrates of (i) multilayer hBN and (ii) SiO2 in order to investigate the impact of dielectric environment on the PLQY. Although the PL lifetime was observed to notably decrease in each of the 1L-WS2 on hBN, the specimens on both substrate types displayed similar PLQYs at low exciton densities, indicating that the lifetime reduction factors are quite similar for the radiative and nonradiative lifetimes of excitons. In our sample geometry with no encapsulation, we confirmed that the EEA rate constant does not alter for the two different substrates. Intriguingly, however, the 1L-WS2 on hBN exhibited a much-suppressed PLQY drop with increasing exciton density. The observed correlation between the reduced PL lifetime and the persisting PLQY under high excitation indicates that dense excitons bypass EEA via fast radiative recombination. Our results demonstrate that the intrinsic PLQY drop in 1L-TMDs at high exciton densities can be significantly suppressed by simple dielectric engineering with hBN, thereby promoting their practical functionality toward highly luminescent two-dimensional light-emitting devices.