Probing Plasmonic Oscillations in 2D Moir\'e Nanocrystal Superlattices by Low-Loss EELS

TL;DR

This study uses low-loss EELS to investigate plasmonic excitations in twisted 2D gold nanocrystal superlattices, revealing how moiré patterns and symmetry breaking influence their electronic properties with high spatial resolution.

Contribution

It demonstrates the effectiveness of low-loss EELS in probing plasmonic behavior in twisted 2D nanocrystal superlattices and explains discrepancies with optical spectroscopy through coupled dipole modeling.

Findings

EELS detects a blue shift in plasmonic excitations with twisting.

Optical spectroscopy shows a red shift, contrasting EELS results.

Twist-induced symmetry breaking significantly affects plasmonic behavior.

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 moir\'e 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 moir\'e 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.