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

Abstract

We investigates exciton-exciton annihilation (EEA) in tungsten disulfide $(\mathrm{W}{\mathrm{S}}_{2})$ 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 $(6.3\ifmmode\pm\else\textpm\fi{}1.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, 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.