Excitonic Absorption Signatures of Twisted Bilayer WSe${}_2$ by Electron Energy-Loss Spectroscopy

Abstract

Moiré twist angle underpins the interlayer interaction of excitons in twisted van der Waals hetero- and homo-structures. The influence of twist angle on the excitonic absorption of twisted bilayer tungsten diselenide (WSe${}_2$) has been investigated using electron energy-loss spectroscopy. Atomic-resolution imaging by scanning transmission electron microscopy was used to determine key structural parameters, including the nanoscale measurement of the relative twist angle and stacking order. Detailed spectral analysis revealed a pronounced blueshift in the high-energy excitonic peak C with increasing twist angle, up to 200 meV when compared to the AA${}^{\prime }$ stacking. The experimental findings have been discussed relative to first-principle calculations of the dielectric response of the AA${}^{\prime }$ stacked bilayer WSe${}_2$ as compared to monolayer WSe${}_2$ by employing the \textit{GW} plus Bethe-Salpeter equation (BSE) approaches, resolving the origin of higher energy spectral features from ensembles of excitonic transitions, and thus any discrepancies between previous calculations. Furthermore, the electronic structure of moiré supercells spanning twist angles of $\sim$9.5-46.5${}^{\circ }$ calculated by density functional theory (DFT) were unfolded, showing an uplifting of the conduction band minimum near the $Q$ point and minimal change in the upper valence band concurrently. The combined experiment/theory investigation provides valuable insight into the physical origins of high-energy absorption resonances in twisted bilayers, which enables to track the evolution of interlayer coupling from tuning of the exciton C transitions by absorption spectroscopy.

References in corpus (14)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Observation of Moiré Excitons in WSe2/WS2 Heterostructure Superlattices
• Evolution of Interlayer Coupling in Twisted MoS2 Bilayers
• Many-body perturbation theory calculations using the yambo code
• Unfolding first-principles band structures
• Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSe${}_2$/MoSe${}_2$ bilayers
• Exciton mapping at subwavelength scales in two-dimensional materials
• Hybridized intervalley moiré excitons and flat bands in twisted WSe${}_2$ bilayers
• Band-unfolding approach to Moirè-induced band-gap opening and Fermi-level-velocity reduction in twisted bilayer graphene
• Spinorial formulation of the GW-BSE equations and spin properties of excitons in 2D Transition Metal Dichalcogenides
• Substrate influence on transition metal dichalcogenide monolayer exciton absorption linewidth broadening
• Unfolding energy spectra of multi-periodicity materials
• Interlayer excitonic spectra of vertically stacked MoSe${}_2$/WSe${}_2$ heterobilayers
• Band alignment and interlayer hybridisation in transition metal dichalcogenide/hexagonal boron nitride heterostructures

Cited by in corpus (4)

• Nano-optics of transition metal dichalcogenides and their van der Waals heterostructures with electron spectroscopies
• Universal Moiré Buckling of Freestanding 2D Bilayers
• Quantum Confined Luminescence in Two dimensions
• Excitons in epitaxially grown WS2 on Graphene: a nanometer-resolved EELS and DFT study