Excitonic Effect Drives Ultrafast Dynamics in van der Waals Heterostructures

Abstract

Recent experiments revealed stacking-configuration-independent and ultrafast charge transfer in transition metal dichalcogenides van der Waals (vdW) heterostructures, which is surprising given strong exciton binding energies and large momentum mismatch across the heterojunctions. Previous theories failed to provide a comprehensive physical picture for the charge transfer mechanisms. To address this challenge, we developed a first-principles framework which can capture exciton−phonon interaction in extended systems. We find that excitonic effect does not impede, but actually drives ultrafast charge transfer in vdW heterostructures. The many-body electron−hole interaction affords cooperation among the electrons, which relaxes the constraint on momentum conservation and reduces energy gaps for charge transfer. We uncover a two-step process in exciton dynamics: ultrafast hole transfer followed by much longer relaxation of intermediate "hot" excitons. This work establishes that many-body excitonic effect is crucial to the ultrafast dynamics and provides a basis to understand relevant phenomena in vdW heterostructures.

Real-time GW -BSE investigations on spin-valley exciton dynamics in monolayer transition metal dichalcogenide

“…GW + BSE calculations were proposed to be prohibitively expensive to combine with NAMD simulations (40). In this work, we introduce rigid dielectric function approximation to overcome this difficulty and realize the GW + rtBSE-NAMD simulation for TMD systems.…”

“…Here, the e-ph is treated in a similar way, yet the contributions of both electron and hole are included on equal footing by explicitly accounting for exciton-phonon interaction. Comparing with LR-TDDFT-NAMD developed by Liu et al (40), LR-TDDFT-NAMD has the advantage in computational cost yet the GW + rtBSE-NAMD is believed to be able to treat the many-body e-h interaction more accurately in extended systems. For liquids and molecular systems, if the rigid dielectric function approximation does not work, then LR-TDDFT-NAMD is a good choice to include the e-h interaction.…”

Interlayer exciton formation, relaxation, and transport in TMD van der Waals heterostructures

“…A number of studies have been carried out to explore the intrinsic mechanism for efficient charge transfer in TMD vdW heterostructures 38,40,[64][65][66][67][68][69][70][71][72] . In TMD monolayers, one may find that the exciton binding energy (0.5-1 eV) is comparable to the typical Frenkel exciton [73][74][75][76][77] , while its wavefunction favors a Wannier-Mott type with electron-hole separation extending over several tens of unit cells (the exciton Bohr radius was calculated to bẽ 1-3 nm) 73,78,79 .…”

Shedding light on moiré excitons: A first-principles perspective

“…To determine the energies and the many-body wave functions of excitons in MoS 2 /WS 2 heterostructures, we use a recently developed first-principles approach based on the linear-response TDDFT (LR-TDDFT) (41,42), with an optimally tuned, screened, and range-separated hybrid XC functional (OT-SRSH) (43)(44)(45)(46). The method has been implemented in conjunction with plane waves and pseudopotentials to study excitonic properties in semiconductors, including graphene fluoride, phosphorene, and 2D perovskites, and TMD heterostructures (36)(37)(38)(39)(40)64). The OT-SRSH involves the partition of the Coulombic interaction into a short-range and a long-range contribution based on the following expression (65)…”