  We directly monitor exciton propagation in freestanding and SiO2-supported
WS2 monolayers through spatially- and time-resolved micro-photoluminescence
under ambient conditions. We find highly nonlinear behavior with
characteristic, qualitative changes in the spatial profiles of the exciton
emission and an effective diffusion coefficient increasing from 0.3 to more
than 30 cm2/s, depending on the injected exciton density. Solving the diffusion
equation while accounting for Auger recombination allows us to identify and
quantitatively understand the main origin of the increase in the observed
diffusion coefficient. At elevated excitation densities, the initial Gaussian
distribution of the excitons evolves into long-lived halo shapes with
micrometer-scale diameter, indicating additional memory effects in the exciton
dynamics.
