Suppression of exciton-exciton annihilation in tungsten disulfide monolayers encapsulated by hexagonal boron nitrides

We investigates exciton-exciton annihilation (EEA) in tungsten disulfide (WS2) monolayers encapsulated by hexagonal boron nitride (hBN). It is revealed that decay signals observed by time-resolved photoluminescence (PL) are not strongly dependent on the exciton densities of hBN-encapsulated WS2 monolayers (WS2/hBN). In contrast, the sample without the bottom hBN layer (WS2/SiO2) exhibits a drastic decrease of decay time with increasing exciton density due to the appearance of a rapid PL decay component, signifying nonradiative EEA-mediated recombination. Furthermore, the EEA rate constant of WS2/hBN was determined as (6.3 +/- 1.7) x 10(-3) cm(2) s(-1), being about 2 orders of magnitude smaller than that of WS2/SiO2. Thus, the observed EEA rate reduction played a key role in enhancing luminescence intensity at high exciton densities in the WS2 monolayer.