Low-temperature exciton transport in strained atomically thin semiconductors

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Semiconducting monolayers of transition metal dichalcogenides (TMDs) are atomically thin, flexible, and exhibit considerable light emission and ultrafast non-equilibrium dynamics. They are considered as promising candidates for next-generation optoelectronic devices. A strong Coulomb interaction in these materials gives rise to the formation of bound electron-hole pairs. TMDs show a rich exciton landscape, including regular bright excitons, as well as optically inaccessible dark exciton states. Having large binding energies, excitons dominate the optical response, dynamics and transport in TMDs even at room temperature. Transport of charge carriers is crucial for nanoelectronics. In conventional materials, electronic transport can be conveniently controlled by external electric fields. However, TMDs are governed by tightly bound excitons, which as neutral particles are only weakly affected by electrical fields. Recently, strain engineering has been introduced to manipulate the propagation of excitons in TMDs. Strain-induced energy gradients give rise to exciton funneling up to a micrometer range. The funneling usually occurs towards spatial regions with the highest strain, where the exciton energy is minimal. Very recently, we have demonstrated exactly the opposite behavior, i.e. excitons surprisingly funnel in the opposite direction under certain conditions. The main goal of this joint theory-experiment proposal is to provide a microscopic understanding and control of the exciton transport in atomically thin semiconductors under the influence of homogeneous and inhomogeneous strain at low temperature. We will perform spatiotemporal photoluminescence measurements in strained TMDs at cryogenic temperatures and combine them with microscopic many-particle theory, which will allow us to track the way of excitons in time, energy, and space. We will resolve all relevant many-particle processes including exciton formation, thermalization, decay, as well as exciton transport. In particular, we will investigate (i) low-temperature exciton diffusion in homogeneously strained TMDs, (ii) low-temperature exciton funnelling and anti-funneling in TMDs under inhomogeneous strain, focusing on phonon- and defect-induced activation of dark excitons, and (iii) optical tuning of exciton transport by varying the excitation conditions. Our combined theory-experiment study will provide a major advance for a microscopic understanding of exciton transport, which is key for the realization of novel optoelectronic devices based on atomically thin semiconductors.