Abstract
Monolayers of transition metal dichalcogenides 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 (WS2) 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 WS2 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.
Highlights
Q NvQ(r, t) [34]
We find that the diffusion becomes faster or slower with strain in a non-trivial and non-monotonic way
This is a result of the interplay between lattice-distortions and the remarkable multi-valley excitonic landscape in TMDs
Summary
Q NvQ(r, t) [34]. At the spatial and temporal scales considered here, NvQ(r, t) can be directly interpreted as probability of finding excitons with momentum. We predict non-trivial dependence of the diffusion on strain, showing a non-monotonic behaviour, where the overall diffusion is either dominated by specific dark excitons or determined by intervalley scattering. We investigate the strain-dependence of the stationary diffusion coefficient covering a larger range of compressive to tensile biaxial strain values, cf figure 3(a).
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