Monolayer*

We report an excitation density-dependent visualization of excitonic energy diffusion in a h-BN encapsulated WSe₂monolayer. At low excitation densities, we observe a linear evolution of the mean-squared displacement of the exciton density with a diffusivity of 0.5 cm²/s and a mono-exponential decay (300ps) of time-resolved photoluminescence (TRPL). At high excitation densities, however, the TRPL splits into two regimes: an excitation density-dependent, short-lived regime (150ps to 80ps), and an excitation-independent, long-lived regime (300ps). This observation and the fact that the exciton density preserves its initially Gaussian profile suggest that the results are not due to exciton-exciton annihilation but due to the remaining density of unfilled trap states. We also observe an excitation density-dependent increase in exciton diffusivity in the short-lived regime that saturates at 3 cm²/s and eventually transitions into the long-lived regime with a diffusivity of 0.5 cm²/s. At the transition point, the exciton density corresponds to the trap density in the WSe₂monolayer which was measured to be 5*10¹¹/cm².

Z.L, D.C-L, S.J contributed equally to this work.