Probing Evolution of Twist-Angle-Dependent Interlayer Excitons in MoSe₂/WSe₂ van der Waals Heterostructures

Abstract

Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe/WSe vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe and WSe layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe and WSe.

Interlayer Excitons in Transition‐Metal Dichalcogenide Heterobilayers

“…By contrast, the K–K transition is only direct in momentum space for 0° and 60° twist angles, so that a change in the transition character from direct for aligned to indirect for twisted structures is expected. In fact, this is observed in experiments on the MoSe 2 –WSe 2 heterobilayer system and manifests itself as a drop in the PL intensity for twisted structures …”

“…Thus, electron–hole pairs that are optically generated in either of the constituent layers become spatially separated within a few tens of femtoseconds . These spatially separated charge carriers can form interlayer excitons (ILEs) on ultrafast timescales, which have been observed directly in photoluminescence (PL) for several TMDC heterobilayer combinations . Such ILEs have already been studied in conventional semiconductor heterostructures, e.g., coupled quantum wells.…”

Light–matter interaction in van der Waals hetero-structures

“…An increase of the absolute energy of the valence band at the Γ point of up to 120 meV for twist angles that deviate significantly from 0 • or 60 • in artificially stacked MoS 2 homo-bilayers has been determines from ARPES [157]. This trend is reflected in the twist angle dependence of the transition energy, the intensity and the decay time of interlayer excitons in hetero-bilayers [99,134,155,158]. In addition, slightly modified IX transition energies for AA and AB stacking order in hetero-bilayers have been reported theoretically [156] and experimentally [159].…”

“…In this regard, such structures have to be thought in terms of 'novel artificial vdW solids', more than just a sum of two 2D materials. Exact energies of the valley specific electronic states as well as hybridization of bands have been reported to depend crucially on the material combination, their rotational alignment and their dielectric environment [56,99,134]. The crystal alignment not only determines the energy of the electronic bands, but also the location of the different valleys in momentum space [149].…”

Two-Dimensional Halide Perovskites for Emerging New- Generation Photodetectors

“…Vapor-deposition methods including chemical vapor deposition (CVD), van der Waals (VDW) epitaxy and other related methods are effective approaches to the preparation of high quality 2D nanocrystals especially for conventional semiconductors [46,47]. These methods have now been attempted for the synthesis of 2D perovskites.…”