Twist Angle-Dependent Interlayer Exciton Lifetimes in van der Waals Heterostructures

Abstract

In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dichalcogenides, multiple exciton resonances with highly tunable properties are formed and subject to both vertical and lateral confinement. We investigate how a unique control knob, the twist angle between the two monolayers, can be used to control the exciton dynamics. We observe that the interlayer exciton lifetimes in MoSe 2 =WSe 2 twisted bilayers (TBLs) change by one order of magnitude when the twist angle is varied from 1°to 3.5°. Using a low-energy continuum model, we theoretically separate two leading mechanisms that influence interlayer exciton radiative lifetimes. The shift to indirect transitions in the momentum space with an increasing twist angle and the energy modulation from the moiré potential both have a significant impact on interlayer exciton lifetimes. We further predict distinct temperature dependence of interlayer exciton lifetimes in TBLs with different twist angles, which is partially validated by experiments. While many recent studies have highlighted how the twist angle in a vdW TBL can be used to engineer the ground states and quantum phases due to many-body interaction, our studies explore its role in controlling the dynamics of optically excited states, thus, expanding the conceptual applications of "twistronics".

Moiré-Bose-Hubbard model for interlayer excitons in twisted transition metal dichalcogenide heterostructures

“…Electron-hole pairs generated in the heterostructure quickly separate due to the type-II band alignment and fast carrier scattering [33][34][35], leaving electrons residing in the conduction band of the MoS 2 layer and holes in the valence band of the WS 2 layer. In the presence of the strong Coulomb interaction, momentum-indirect IXs form that possess long radiative lifetimes [36], allowing them to thermalize before recombining. Furthermore, with charge carriers separated to different layers, the IX dipole moments align perpendicular to the in-plane direction, an arrangement that is in close analogy to spatiallyindirect excitons in coupled quantum wells, and which results in a pronounced dipolar repulsion [37][38][39].…”

Site-controlled interlayer coupling in WSe2/2D perovskite heterostructure

“…However, properties of IXs in TMD heterostructures highly depend on the twist angle between adjacent layers due to the momentum mismatch, making it difficult to fabricate samples with consistent performance. The formation of moiré potential in vdW heterostructures will change the dynamics and transport properties of IXs significantly [14,15]. Alternative to TMD vdW heterostructures, TMDs can also be combined with other layered materials to achieve interlayer coupling [16][17][18][19].…”

Leaky exciton condensates in transition metal dichalcogenide moiré bilayers

“…The effects of stacking and twisting compound twice: First, the spatial separation of electrons and holes leads to long exciton lifetimes. The interlayer twist then rotates electron bands in momentum space [33], resulting in an indirect bandgap and lifetimes longer still [7,34]. Second, the misaligned layers form a large-scale moiré superlattice, with a spatiallymodulated bandgap [14,[35][36][37][38][39] that traps excitons in localized orbitals [40][41][42][43][44].…”

“…We have incorporated the exciton momentum mismatch, and made the rotating-wave approximation [62]. The coupling constants g pσ are obtained from electronic interband transition dipole matrix elements, and depend on the exciton pairing wavefunction [34] etc., but are largely inconsequential to the conclusions we present here. The recombination rate Γ k of each k mode inside the light cone can be computed [34] using Fermi's Golden Rule [63].…”

“…The coupling constants g pσ are obtained from electronic interband transition dipole matrix elements, and depend on the exciton pairing wavefunction [34] etc., but are largely inconsequential to the conclusions we present here. The recombination rate Γ k of each k mode inside the light cone can be computed [34] using Fermi's Golden Rule [63]. This defines a natural timescale, the lifetime of a localized exciton in a single moiré site, τ −1 loc = 1 N k∈LC Γ k .…”