Anomalous interlayer exciton diffusion in WS2/WSe2 moiré heterostructure

Abstract

Stacking van der Waals crystals allows for on-demand creation of a periodic potential landscape to tailor the transport of quasiparticle excitations. We investigate the diffusion of photoexcited electron-hole pairs or excitons at the interface of WS2/WSe2 Van der Waals heterostructure over a wide range of temperatures. We observe the appearance of distinct interlayer excitons for parallel and anti-parallel stacking, and track their diffusion through spatially and temporally resolved photoluminescence spectroscopy from 30 K to 250 K. While the measured exciton diffusivity decreases with temperature, it surprisingly plateaus below 90K. Our observations cannot be explained by classical models like hopping in the moiré potential. Using a combination of ab-initio theory and molecular dynamics simulations, we demonstrate that low energy moiré phonons, also known as phasons, play a key role in describing and understanding this anomalous behavior of exciton diffusion. In particular, we show that the moiré potential landscape is dynamic down to very low temperatures. Our observations show that the phason modes arising from the mismatched lattices of a moiré heterostructures can enable surprisingly efficient transport of energy in the form of excitons, even at low temperatures.

Twist-Driven Deep-Ultraviolet-Wavelength Exciton Funnel Effect in Bilayer Boron Nitride

“…Therefore, by exploiting the periodic quasiparticle band gap variation and exciton funnel effect in moiré superlattices of bilayer h -BN, we can realize arrays of DUV QDs formed by bright exciton states. Finally, it is worth mentioning that the exciton diffusion length reported in 2D materials is typically on the order of ∼1 μm, , which is much larger than the moiré period discussed in this work. This validates the proposed exciton funnel effect in the moiré superlattices before the excitons recombine.…”

Pseudo-heterostructure and condensation of 1D moiré excitons in twisted phosphorene bilayers

“…For example, since the low-frequency shear modes can be populated at low temperatures, they could lead to translations of the moiré potential, thus diffusion of moiré excitons trapped at the potential. This could explain anomalous exciton diffusion at low temperatures in moiré heterostructures (25).…”

Electrons surf phason waves in moiré bilayers

“…We have also discussed the impact of the substrate and disorder on free surfing. Our findings are relevant for the design of transport devices based on moiré materials [42].…”