Strain-dependent exciton diffusion in transition metal dichalcogenides

TL;DR

This study combines theory and experiments to explore how strain affects exciton diffusion in monolayer WS₂, revealing a non-monotonic relationship with a significant speed-up at specific strain levels.

Contribution

It provides the first detailed joint theoretical and experimental analysis of strain-dependent exciton diffusion in TMD monolayers, highlighting non-trivial effects of lattice distortions.

Findings

Minimal diffusion coefficient in unstrained WS₂

Speed-up of exciton diffusion by a factor of 3 at 0.6% tensile strain

Strain alters excitonic landscape and scattering channels

Abstract

Monolayers of transition metal dichalcogenides (TMDs) have a remarkable excitonic landscape with deeply bound bright and dark exciton states. Their properties are strongly affected by lattice distortions that can be created in a controlled way via strain. Here, we perform a joint theory-experiment study investigating exciton diffusion in strained tungsten disulfide (WS$_2$) monolayers. We reveal a non-trivial and non-monotonic influence of strain. Lattice deformations give rise to different energy shifts for bright and dark excitons changing the excitonic landscape, the efficiency of intervalley scattering channels, and the weight of single exciton species to the overall exciton diffusion. We predict a minimal diffusion coefficient in unstrained WS$_2$ followed by a steep speed-up by a factor of 3 for tensile biaxial strain at about 0.6\% strain - in excellent agreement with our experiments. The obtained microscopic insights on the impact of strain on exciton diffusion are applicable to a broad class of multi-valley 2D materials.