Abstract
Transport of charge carriers is at the heart of current nanoelectronics. In conventional materials, electronic transport can be controlled by applying electric fields. Atomically thin semiconductors, however, are governed by excitons, which are neutral electron-hole pairs and as such cannot be controlled by electrical fields. Recently, strain engineering has been introduced to manipulate exciton propagation. Strain-induced energy gradients give rise to exciton funneling up to a micrometer range. Here, we combine spatiotemporal photoluminescence measurements with microscopic theory to track the way of excitons in time, space and energy. We find that excitons surprisingly move away from high-strain regions. This anti-funneling behavior can be ascribed to dark excitons which possess an opposite strain-induced energy variation compared to bright excitons. Our findings open new possibilities to control transport in exciton-dominated materials. Overall, our work represents a major advance in understanding exciton transport that is crucial for technological applications of atomically thin materials.
Highlights
Transport of charge carriers is at the heart of current nanoelectronics
Recent studies have shown that besides the regular bright excitons accessible in optical spectra, dark excitonic states play a significant role for understanding the optical response and non-equilibrium dynamics in TMDs5–8
Flexible, and exhibiting strong absorption and ultrafast non-equilibrium dynamics, transition metal dichalcogenides (TMDs) have been considered as promising candidates for next-generation optoelectronic applications including light-emitting, detecting, and harvesting devices[10]
Summary
Transport of charge carriers is at the heart of current nanoelectronics. In conventional materials, electronic transport can be controlled by applying electric fields. Tungsten-based TMD monolayers (WS2, WSe2) are indirect semiconductors, where dark excitons are the energetically lowest states[9,25] These excitons are not directly accessible in optical spectra, they have been demonstrated to crucially influence optics, dynamics, and even diffusion in TMDs3,4,26. Their role on exciton funneling has still remained in the dark. The energy of momentum-dark KΛ excitons increases with strain, and this type of excitons funnels opposite to the strain gradient toward spatial regions with minimal strain (Fig. 1e). Our joint experiment-theory study represents a major They funnel towards lower-strain regions (opposite to conventional funneling), when the excitation spot is subject to a considerably large strain gradient (spot B and C). The supplementary material includes PL movies further illustrating this surprising anti-funneling behavior
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