Understanding Plasmonic Properties in Metallic Nanostructures by Correlating Photonic and Electronic Excitations

Abstract

A large number of optical phenomena rely on the excitation of localized surface plasmon resonances (LSPR) in metallic nanostructures. Electron-energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has emerged as a technique capable of mapping plasmonic properties on length scales 100 times smaller than optical wavelengths. While this technique is promising, the connection between electron-driven plasmons, encountered in EELS, and photon-driven plasmons, encountered in plasmonic devices, is not well understood. This Perspective highlights some of the contributions that have been made in correlating optical scattering and STEM/EELS from the exact same nanostructures. The experimental observations are further elucidated by comparison with theoretical calculations obtained from the electron-driven discrete dipole approximation, which provides a method to calculate EEL spectra for nanoparticles of arbitrary shape. Applications of plasmon mapping to the electromagnetic hot-spots encountered in single-molecule surface-enhanced Raman scattering and electron beam induced damage in silver nanocubes are discussed. It is anticipated that the complementarity of both techniques will address issues in fundamental and applied plasmonics going forward.

Probing plasmonic nanostructures by photons and electrons

“…49,50,52 The inverse relation between EELS maps and the local field distribution has been observed independently on specific geometries of nanoaggregates. 26,34,51 Measurements on dimers with gap sizes between 12 nm and 1 nm (Fig. 4b) show a direct, almost linear relation between the EELS signal measured in the gap and the gap width.…”

Mapping Bright and Dark Modes in Gold Nanoparticle Chains using Electron Energy Loss Spectroscopy.

“…[49][50][51][52] The l = 3 mode has a net dipole moment and is therefore a bright sub-radiant mode. We observe this mode for particle chain with N = 3 and N = 5 (Figure 2 and Figure 4 respectively), although it is either absent or has too low loss probability to be observed for the N = 4 chain, either in the EELS experiments or modelled data.…”

“…48 In contrast, studies on coupled systems that incorporate theoretical simulations along with experimental results suggest that the energy-loss map is representative of the electric field strength. [49][50][51][52] Energyloss maps of the super-radiant mode in nanosphere dimers display the highest loss probability at the ends of the nanoparticles, furthest from the gaps. 53,54 This result is also obvious for larger aggregates.…”

The Substrate Effect in Electron Energy-Loss Spectroscopy of Localized Surface Plasmons in Gold and Silver Nanoparticles

“…3 accessible, allowing LSPs to be probed and studied with sub-nanometer spatial resolution 6,8,[22][23][24][25][26] . TEM requires specimens thin enough to be electron transparent (typically below 100 nm) and therefore, plasmonic nanoparticles studied by EELS are typically supported on thin membranes 6,8,27 or buried in a thin embedding material 28 .…”