Room-temperature diffusion of excitons in WS₂ monolayers with suppressed disorder

Excitons are found to dominate the electro-optical properties of single layers of semiconducting transition metal dichalcogenides (TMDCs) and their heterostructures. In addition they are also able to move freely across the two-dimensional plane, motivating detailed studies of the exciton transport in these materials. However, their intrinsic properties can be often obscured by inhomogeneities such as the fluctuations of the dielectric environment [1]. In our study, we address the topic of exciton propagation by using WS₂ monolayers encapsulated in high-quality boron nitride, where long-range disorder effects are strongly suppressed [2]. Monitoring the spatial behavior of excitons through spatially- and time-resolved photoluminescence microscopy we find strongly enhanced efficient diffusion at low excitation densities in contrast to pristine, as-exfoliated flakes. In addition, we find a pronounced non-monotonous density-dependence of the effective diffusivity at elevated densities. We discuss our findings in view of the complex multi-valley structure and propose a mechanism for diffusion facilitated by free-electron hole plasma from entropy-ionized excitons.

[1] A. Raja et al., Nat. Nanotech. 14, 832-837 (2019).
[2] J. Zipfel et al., Phys. Rev. B 101, 115430 (2020).