Exciton Dynamics on Suspended and Substrate-supported MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ Monolayers.

MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ monolayers are promising atomic-scale platform for novel light emission devices, however, despite perfect surface passivation and strong exciton binding energy, their luminescence efficiencies are very low. Here, through the observation of exciton dynamics by ultrafast transient reflection, we revealed that the substrate can affect the exciton dynamics on MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ monolayers, by facilitating the non-radiative recombination pathways, thus reducing the luminescence efficiency. Furthermore, strong many-body interactions such as exciton-exciton annihilation, are enhanced on suspended MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ monolayers, whereas the defects in substrates may efficiently quench excitons thus mitigate those effects.