We investigates exciton-exciton annihilation (EEA) in tungsten disulfide $\left(\mathrm{W}{\mathrm{S}}_2\right)$ 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 $\mathrm{W}{\mathrm{S}}_2$ monolayers $(\mathrm{W}{\mathrm{S}}_2/\mathrm{hBN})$. In contrast, the sample without the bottom hBN layer $(\mathrm{W}{\mathrm{S}}_2/\mathrm{Si}{\mathrm{O}}_2)$ 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 $\mathrm{W}{\mathrm{S}}_2/\mathrm{hBN}$ was determined as $(6.3\pm 1.7)\times {10}^{-3}\mathrm{c}{\mathrm{m}}^2{\mathrm{s}}^{-1}$, being about 2 orders of magnitude smaller than that of $\mathrm{W}{\mathrm{S}}_2/\mathrm{Si}{\mathrm{O}}_2$. Thus, the observed EEA rate reduction played a key role in enhancing luminescence intensity at high exciton densities in the $\mathrm{W}{\mathrm{S}}_2$ monolayer.

• Received 13 March 2017
• Revised 15 May 2017

DOI:https://doi.org/10.1103/PhysRevB.95.241403