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.

We present a new approach to quantify the mass and 3D shape of nanoparticles on supports, using size-selected nanoclusters as mass standards in scanning transmission electron microscope. Through quantitative image intensity analysis, we show that the integrated high angle annular dark field intensities of size-selected gold clusters soft-landed on graphite display a monotonic dependence on the cluster size as far as $\sim 6500$ atoms. We applied this mass standard to study gold nanoparticles prepared by thermal vapor deposition and by colloidal wet chemistry, and from which we deduced the shapes of these two types of nanoparticles as expected.

Size-selected gold and nickel nanoclusters are of interest from an electronic, catalytic, and biological point of view. These applications require the deposition of the clusters on a surface, and a key challenge is to retain the cluster size. Here controlled energy impact is used to immobilize the size-selected clusters on the graphite surface at room temperature. The threshold energy for pinning of ionized ${\mathrm{Au}}_N$ $(N=20–100)$ and ${\mathrm{Ni}}_N$ $(N=10–300)$ clusters, over the impact energy range $350–2000\mathrm{eV}$, is shown by scanning tunneling microscopy to scale with the cluster mass. This behavior is consistent with a previous study of silver clusters and demonstrates the more general applicability of the cluster pinning model.