Finite-Size Effects in Periodic Silver Nanoparticle Arrays Probed by Electron Energy-Loss Spectroscopy Simulations

Abstract Optical spectra of finite plasmonic nanoparticle arrays have been shown to converge rapidly toward those of infinite lattices, suggesting that finite-size effects play a minor role. Here, we show that this conclusion breaks down under localized excitation. Using electron energy-loss spectroscopy (EELS) simulations and plasmon hybridization theory, we investigate finite two-dimensional silver nanoparticle arrays containing between 2 × 2 and 12 × 12 nanoparticles. Localized electron-beam excitation reveals a rich modal landscape composed of collective, edge-, corner-, and symmetry-selected resonances that are largely hidden from optical measurements. We show that, unlike far-field optical spectra, which become relatively insensitive to further increases in array size, the EELS response retains strong sensitivity to array boundaries and reveals persistent edge- and corner-localized modes even in 12 × 12 nanoparticle arrays. EELS maps and surface charge distributions further reveal the coexistence of delocalized collective plasmons and strongly localized boundary modes whose excitation strongly depends on the beam position. These results demonstrate that finite plasmonic arrays retain a complex mode structure beyond that accessible through optical spectroscopy and establish EELS as a powerful tool for probing finite-size plasmonic phenomena.