Higher-Order Surface Plasmon Modes in Encapsulated Silver Nanoparticles Probed with Electron Energy-Loss Spectroscopy

The optical properties of silver metal nanoparticles are dominated by the excitation of localized surface plasmons. Light incident on silver particles with spherical shape and diameters below 100 nm excites primarily the dipolar mode. The higher-order modes, i.e., modes with larger angular momentum l, such as the quadrupole mode, are strongly damped as their larger resonance energy tend to be positioned in the range of interband transitions in silver. By encapsulating silver nanoparticles in a high permittivity dielectric medium, in this work silicon nitride, we redshift all of the plasmon modes and thereby get access to the higher-order modes of silver nanoparticles. We map the dipolar and higher-order modes of silver nanoparticles with EELS in a state-of-the-art transmission electron microscope.

We acquire EELS maps and line scans from isolated silver nanoparticles. For impact parameters outside the particle, we find that the EELS signal is dominated by the dipole mode, which shows a resonance energy around 2.8 eV for particle radii 4 - 􀀀20 nm. For impact parameters close to the surface of the particle or inside the particle, an additional resonance at higher energy shows up due to the excitation of modes with larger angular momentum, i.e., l > 1. The resonance energy of the additional peak is around 3.2 eV for the same particle radius range. To our knowledge, this is the first experimental observation of higher-order modes in nanometer-sized silver nanoparticles. For impact parameters inside the nanoparticle, we also excite the bulk plasmon which has a resonance energy close to 3.8 eV and is clearly distinguished from the peak at 3.2 eV. We note that the measured EELS peak at 3.2 eV contains several modes with l > 1 as the energy spacing between these modes is lower than the resolution of our EELS setup. We also perform electron energy-loss simulations using the boundary element method (BEM) [1], which shows excellent agreement with our experimental observations.

niste@fotonik.dtu.dk