Probing Plasmonic Oscillations in 2D Moiré Nanocrystal Superlattices by Low-Loss EELS

TL;DR

This work uses low-loss EELS to probe plasmonic excitations in 2D moiré Au nanocrystal superlattices, revealing twist-induced symmetry breaking and a blue shift of plasmon modes relative to single-layer configurations. A driven coupled-dipole framework with a Drude gold dielectric and Clausius-Mossotti polarizability is constructed to model the nanocrystal lattice and its interaction with a STEM electron beam, with lattice sums encoded via a dyadic Green's function. An eigenmode solver in the quasistatic limit reduces the problem to a det( Xi - lambda I ) eigenvalue problem, producing complex eigenfrequencies that predict out-of-plane polarization modes excited by the electron beam. The study explains discrepancies between EELS and optical spectroscopy and demonstrates that EELS provides complementary, high-spatial-resolution insight into how nanoscale geometry and twist control collective plasmonic properties, informing the design of plasmonic metamaterials.

Abstract

Electron energy loss spectroscopy (EELS) has been established as a powerful analytical technique for investigating the oxidation state, band structure, and dielectric properties of materials with exceptional spatial resolution. Inspired by twisted 2D materials, we utilize low-loss EELS to examine the plasmonic excitations in 2D moiré Au nanocrystal superlattices (NCSLs) formed by liquid-air interface self-assembly using a double-dipping method. This approach produces stacked hexagonal layers that can be twisted, forming moiré patterns in NCSLs whose twist angles are precisely measured via scanning transmission electron microscopy (STEM). Low-loss EELS effectively mitigates challenges arising from fabrication-induced non-uniformity and reveals a blue shift in plasmonic excitation when comparing single-layer, double-layer, and twisted configurations. This sharply contrasts with the optical spectroscopy measurements, which show an overall red shift relative to the EELS data. The high spatial resolution of STEM-EELS further demonstrates that twist-induced symmetry breaking strongly influences plasmonic behavior. Coupled dipole modeling explains the observed discrepancies: the electron beam excites out-of-plane polarization modes unavailable to optical probes, while optical measurements average over ensembles. Our findings highlight that EELS provides complementary information to optical spectroscopy for understanding how structural arrangements at the nanoscale influence collective electronic properties, advancing the design of plasmonic metamaterials.

Figures (9)

• Figure S1: HAADF-STEM image of our 2D NCSL of 5 nm Au NPs showing different domains.
• Figure S2: Identification of regions with different moire orientations in our 2D NCSL sample. In blue, moire pattern from twisting of the bilayers; in green, moire pattern from translation between the bilayers; in yellow, monolayer NCSL.
• Figure S3: (a) Identification of regions of monolayer and bilayer NCSLs with different moire configurations by low magnification TEM imaging. (b), (c), (d) are representative high magnification TEM images of color-coded bilayer regions with different moire configurations. (e) represents high magnification TEM image of a monolayer region. Insets in (b-e) are corresponding higher magnification TEM images.
• Figure S4: The top row (left to right) shows how the signal-to-noise ratio of the raw EELS spectrum changes with the dimension of the selected region of interest. The bottom row shows how the curve fitting improves with increasing signal-to-noise ratio (left to right).
• Figure S5: (a) The plasmonic extinction map with ROI of 3 by 3 pixels, showing well resolved peak shifts. (b) The plasmonic extinction map with ROI of 5 by 5 pixels. Even with higher signal-to-noise ratio for better curve fitting, the peak shifts are poorly resolved.