Multipole plasmons and their disappearance in few-nanometer silver nanoparticles

TL;DR

This study uses electron energy-loss spectroscopy to observe multiple surface plasmon resonances in silver nanoparticles as small as 4 nm, revealing the disappearance of higher-order modes below this size and highlighting differences from optical spectra.

Contribution

It provides the first experimental evidence of multipole plasmons in nanoparticles as small as 4 nm and compares observations with GNOR theory predictions.

Findings

Higher-order surface plasmon modes are observed in 4 nm silver nanoparticles.

Higher-order modes disappear in particles smaller than 4 nm, consistent with GNOR theory.

The dipole resonance blueshift exceeds GNOR predictions in ultra-small particles.

Abstract

In electron energy-loss spectroscopy (EELS) of individual silver nanoparticles encapsulated in silicon nitride, we observe besides the usual dipole resonance an additional surface plasmon (SP) resonance corresponding to higher angular momenta. We even observe both resonances for nanoparticle radii as small as 4 nm, where previously only the dipole resonance was assumed to play a role. Electron beams positioned outside of the particles mostly excite the dipole mode, but the higher-order resonance can even dominate the dipole peak when exciting at the particle surface, the usual choice for maximal EELS signal. This allows us to study the radius dependence of both resonances separately. For particles smaller than 4 nm, the higher-order SP mode disappears, in agreement with generalized nonlocal optical response (GNOR) theory, while the dipole resonance blueshift exceeds GNOR predictions. Unlike in optical spectra, multipole surface plasmons are important in EELS spectra even of ultra-small metallic nanoparticles.