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

We investigates exciton-exciton annihilation (EEA) in tungsten disulfide (W S₂) 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 W S₂ monolayers (W S₂/hBN ) . In contrast, the sample without the bottom hBN layer (W S₂/Si O₂) 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 W S₂/hBN was determined as (6.3 ±1.7 ) ×10^-3c m²s^-1 , being about 2 orders of magnitude smaller than that of W S₂/Si O₂ . Thus, the observed EEA rate reduction played a key role in enhancing luminescence intensity at high exciton densities in the W S₂ monolayer.