title: Multipole plasmons and their disappearance in few-nanometre silver nanoparticles.
abstract: Electron energy-loss spectroscopy can be used for detailed spatial and spectral characterization of optical excitations in metal nanoparticles. In previous electron energy-loss experiments on silver nanoparticles with radii smaller than 20 nm, only the dipolar surface plasmon resonance was assumed to play a role. Here, applying electron energy-loss spectroscopy to individual silver nanoparticles encapsulated in silicon nitride, we observe besides the usual dipole resonance an additional surface plasmon resonance corresponding to higher angular momenta for nanoparticle radii as small as 4 nm. We study the radius and electron beam impact position dependence of both resonances separately. For particles smaller than 4 nm in radius the higher-order surface plasmon mode disappears, in agreement with generalized non-local optical response theory, while the dipole resonance blueshift exceeds our theoretical predictions. Unlike in optical spectra, multipole surface plasmons are important in electron energy-loss spectra even of ultrasmall metallic nanoparticles.
authors.affiliation: Department of Photonics Engineering, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.|Center for Nanostructured Graphene (CNG), Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.|Department of Micro- and Nanotechnology, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.|Center for Electron Nanoscopy, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.
authors.forename: Søren
authors.initials: S
authors.lastname: Raza
authors.affiliation: Center for Electron Nanoscopy, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.
authors.forename: Shima
authors.initials: S
authors.lastname: Kadkhodazadeh
authors.affiliation: Department of Photonics Engineering, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.|Center for Nanostructured Graphene (CNG), Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.
authors.forename: Thomas
authors.identifier: http://orcid.org/0000-0002-9131-2717
authors.initials: T
authors.lastname: Christensen
authors.affiliation: Nanophotonics-Physics of Devices, Debye Institute for Nanomaterials Science, Utrecht University, P O Box 80000, Utrecht 3508 TA, The Netherlands.
authors.forename: Marcel
authors.initials: M
authors.lastname: Di Vece
authors.affiliation: Department of Photonics Engineering, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.|Center for Nanostructured Graphene (CNG), Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.
authors.forename: Martijn
authors.identifier: http://orcid.org/0000-0002-8286-7825
authors.initials: M
authors.lastname: Wubs
authors.affiliation: Department of Photonics Engineering, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.|Center for Nanostructured Graphene (CNG), Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.
authors.forename: N Asger
authors.initials: NA
authors.lastname: Mortensen
authors.affiliation: Department of Photonics Engineering, Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.|Center for Nanostructured Graphene (CNG), Technical University of Denmark, Kgs. Lyngby DK-2800, Denmark.
authors.forename: Nicolas
authors.identifier: http://orcid.org/0000-0002-6808-9211
authors.initials: N
authors.lastname: Stenger