Identification of twist-angle-dependent excitons in WS2/WSe2 heterobilayers

Abstract

Stacking atomically thin films enables artificial construction of van der Waals heterostructures with exotic functionalities such as superconductivity, the quantum Hall effect, and engineered light-matter interactions. In particular, heterobilayers composed of monolayer transition metal dichalcogenides have attracted significant interest due to their controllable interlayer coupling and trapped valley excitons in moiré superlattices. However, the identification of twist-angle-modulated optical transitions in heterobilayers is sometimes controversial since both momentum-direct (K-K) and -indirect excitons reside on the low energy side of the bright exciton in the monolayer constituents. Here, we attribute the optical transition at approximately 1.35 eV in the WS2/WSe2 heterobilayer to an indirect Γ-K transition based on a systematic analysis and comparison of experimental PL spectra with theoretical calculations. The exciton wavefunction obtained by the state-of-the-art GW-Bethe-Salpeter equation (GW-BSE) approach indicates that both the electron and hole of the exciton are contributed by the WS2 layer. Polarization-resolved k-space imaging further confirms that the transition dipole moment of this optical transition is dominantly in-plane and is independent of the twist angle. The calculated absorption spectrum predicts that the usually called interlayer exciton peak coming from the K-K transition is located at 1.06 eV, but with a much weaker amplitude. Our work provides new insights into understanding the steady-state and dynamic properties of twist-angle-dependent excitons in van der Waals heterostructures.

Boosting Exciton Transport in WSe2 by Engineering Its Photonic Substrate

“…Therefore, aligning the transition dipole moment and the evanescent field is favored to enter the strong coupling regime. The layered WSe 2 supports the bright exciton resonance with the in-plane dipole moment at room temperature; therefore, the BSW is designed to be transversely polarized along the interface of 1DPC, which can also avoid any influence from the out-of-plane dark or interlayer excitonic states in our experiment . The configuration under study is schematically depicted, as shown in Figure a, where the 1DPC with a defect top layer can support the BSW with the optical field concentrated near the layered WSe 2 .…”

Pristine Interlayer Coupling for Strain Engineering of WS2/WSe2 Nanosheet-Based van der Waals Heterostructures

“…Recently, the observation of intense interlayer excitons in the WS 2 /WSe 2 HS has extended the study scope and depth of novel optical transitions, such as the discovery of double indirect interlayer excitons, the fabrication of long-lived interlayer excitons, and the twist-angle dependence of excitons . These studies present profound physical significance; however, the interlayer coupling, a crucial degree of freedom, has rarely been mentioned in these WS 2 /WSe 2 HSs.…”

“…Recently, the observation of intense interlayer excitons in the WS 2 /WSe 2 HS has extended the study scope and depth of novel optical transitions, such as the discovery of double indirect interlayer excitons, 35 the fabrication of long-lived interlayer excitons, 36 and the twist-angle dependence of excitons. 37 These studies present profound physical significance; however, the interlayer coupling, a crucial degree of freedom, has rarely been mentioned in these WS 2 /WSe 2 HSs. Herein, we elaborately investigate the evolution of the interlayer excitons in the WS 2 /WSe 2 HS by simultaneously considering tensile strain and interlayer coupling, which greatly enriches the modulation strategy of the interlayer exciton.…”

Complex Strain Scapes in Reconstructed Transition-Metal Dichalcogenide Moiré Superlattices

“…Parzefall et al reported on the correlation between the interlayer shear mode observed in the low-frequency Raman spectrum of MoSe 2 /WSe 2 heterostructures and the atomic reconstruction taking place in the system, which essentially indicates a near-0° alignment. The dependence of the interlayer exciton emission (both energy and intensity) with the twist angle has also been observed for different TMDC heterobilayers. , …”

“…The dependence of the interlayer exciton emission (both energy and intensity) with the twist angle has also been observed for different TMDC heterobilayers. 43,44 In the present work, we focus on MoS 2 /MoSe 2 heterobilayers. The ensuing intralayer strain states in reconstructed MoS 2 /MoSe 2 heterobilayers are examined via polarized Raman spectroscopy, AFM measurements, and molecular dynamics (MD) simulations.…”