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Exciton intervalley scattering, annihilation and relaxation dynamics, anddiffusive transport in a monolayer transition metal dichalcogenides are centralto the functionality of devices based on them. This motivated us to investigatethese properties in exfoliated high-quality monolayer MoSe2 using heterodynednonlinear four-wave-mixing transient grating spectroscopy. While free excitonexcitations are found to be long-lived (~230 ps), an extremely fast intervalleyscattering ( less than 120 fs) is observed leading to a negligible valleypolarization, consistent with steady state photoluminescence measurements. Theexciton population decay shows an appreciable contribution from exciton-excitonannihilation reactions with an annihilation constant of ~ 0.01 cm2s-1, which inaddition leads to an extra contribution to the transient grating response. Theunderlying excitonic dynamics were numerically modeled by includingexciton-exciton annihilation, also in the diffusion equation, which allowsextraction of the diffusion constant, D ~1.4 cm2s-1. Our results provide amethod that allows for the disentanglement of the intricate dynamics involvingmany-body annihilation processes and a detailed characterization of theexcitonic properties of monolayer MoSe2

Excitonic Transport and Intervalley Scattering in Exfoliated MoSe2 Monolayer Revealed by Four-Wave-Mixing Transient Grating Spectroscopy