Excitonic Absorption Signatures of Twisted Bilayer WSe$_{2}$ by Electron Energy-Loss Spectroscopy

TL;DR

This study combines electron energy-loss spectroscopy and first-principles calculations to analyze how twist angle affects excitonic absorption in bilayer WSe2, revealing a significant blueshift and providing insights into interlayer coupling.

Contribution

It presents the first combined experimental and theoretical analysis of excitonic absorption signatures in twisted bilayer WSe2 across various twist angles.

Findings

Blueshift of up to 200 meV in excitonic peak C with increasing twist angle.

Detailed spectral features explained by first-principles dielectric response calculations.

Unfolded DFT calculations show minimal change in valence band, significant uplift of conduction band near Q point.

Abstract

Moir\'{e} 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 moir\'{e} 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.