Vibrational surface EELS probes confined Fuchs-Kliewer modes

TL;DR

This paper introduces a formalism based on confined Fuchs-Kliewer modes to interpret vibrational EELS experiments on nanoparticles, explaining discrepancies and enabling detailed surface charge mapping.

Contribution

It presents a new theoretical framework for understanding vibrational EELS signals through confined modes, bridging gaps between different experimental observations.

Findings

Confined Fuchs-Kliewer modes explain EELS signal variations.

The formalism predicts environment-induced energy shifts.

Enables 3D mapping of surface charge densities.

Abstract

Recently, two reports have demonstrated the amazing possibility to probe vibrational excitations from nanoparticles with a spatial resolution much smaller than the corresponding free-space phonon wavelength using electron energy loss spectroscopy (EELS). While Lagos et al. evidenced a strong spatial and spectral modulation of the EELS signal over a nanoparticle, Krivanek et al. did not. Here, we show that discrepancies among different EELS experiments as well as their relation to optical near- and far-field optical experiments can be understood by introducing the concept of confined bright and dark Fuchs-Kliewer modes, whose density of states is probed by EELS. Such a concise formalism is the vibrational counterpart of the broadly used formalism for localized surface plasmons; it makes it straightforward to predict or interpret phenomena already known for localized surface plasmons such as environment-related energy shifts or the possibility of 3D mapping of the related surface charge densities.