Excitonic Transport and Intervalley Scattering Dynamics in Large-Size Exfoliated MoSe2 Monolayer Investigated by Heterodyned Transient Grating Spectroscopy

Exciton intervalley scattering, annihilation, relaxation dynamics, and diffusive transport in monolayer transition metal dichalcogenides are central to the functionality of devices based on them. Here, these properties in a large-size exfoliated high-quality monolayer MoSe2 are addressed directly using heterodyned transient grating spectroscopy at room temperature. While the free exciton population is found to be long-lived (approximate to 230 ps), an extremely fast intervalley scattering (<= 170 fs) is observed, leading to a negligible valley polarization, consistent with steady state photoluminescence measurements and theoretical calculation. The exciton population decay shows an appreciable contribution from the exciton-exciton annihilation, with an annihilation rate of approximate to 0.01 cm(2) s(-1). The annihilation process also leads to a significant distortion of the transient grating evolution. Taking this distortion into account, consistent exciton diffusion constants D approximate to 1.4 cm(2) s(-1) are found by a model simulation in the excitation density range of 10(11)-10(12) cm(-2). The presented results highlight the importance of correctly considering the many-body annihilation processes to obtain a pronounced understanding of the excitonic properties of monolayer transition metal dichalcogenides.