Exciton diffusion in monolayer semiconductors with suppressed disorder

TL;DR

This study demonstrates highly efficient exciton diffusion in monolayer semiconductors encapsulated in hexagonal boron nitride, revealing insights into exciton dynamics, interactions, and the influence of dark states at room temperature.

Contribution

It provides the first direct measurement of exciton diffusion in encapsulated monolayer semiconductors under ambient conditions, highlighting the role of dark states and many-particle interactions.

Findings

Diffusion coefficients up to 10 cm²/s at room temperature

Effective mobilities as high as 400 cm²/Vs

Observation of exciton-exciton interactions and suppressed Auger processes

Abstract

Tightly bound excitons in monolayer semiconductors represent a versatile platform to study two-dimensional propagation of neutral quasiparticles. Their intrinsic properties, however, can be severely obscured by spatial energy fluctuations due to a high sensitivity to the immediate environment. Here, we take advantage of the encapsulation of individual layers in hexagonal boron nitride to strongly suppress environmental disorder. Diffusion of excitons is then directly monitored using time- and spatially-resolved emission microscopy at ambient conditions. We consistently find very efficient propagation with linear diffusion coefficients up to 10\,cm$^2$/s, corresponding to room temperature effective mobilities as high as 400\,cm$^2$/Vs as well as a correlation between rapid diffusion and short population lifetime. At elevated densities we detect distinct signatures of many-particle interactions and consequences of strongly suppressed Auger-like exciton-exciton annihilation. A combination of analytical and numerical theoretical approaches is employed to provide pathways towards comprehensive understanding of the observed linear and non-linear propagation phenomena. We emphasize the role of dark exciton states and present a mechanism for diffusion facilitated by free electron hole plasma from entropy-ionized excitons.