This review discusses how low-energy, valence excitations created by swift electrons can render information on the optical response of structured materials with unmatched spatial resolution. Electron microscopes are capable of focusing electron beams on sub-nanometer spots and probing the target response either by analyzing electron energy losses or by detecting emitted radiation. Theoretical frameworks suited to calculate the probability of energy loss and light emission (cathodoluminescence) are revisited and compared with experimental results. More precisely, a quantum-mechanical description of the interaction between the electrons and the sample is discussed, followed by a powerful classical dielectric approach that can be in practice applied to more complex systems. We assess the conditions under which classical and quantum-mechanical formulations are equivalent. The excitation of collective modes such as plasmons is studied in bulk materials, planar surfaces, and nanoparticles. Light emission induced by the electrons is shown to constitute an excellent probe of plasmons, combining sub-nanometer resolution in the position of the electron beam with nanometer resolution in the emitted wavelength. Both electron energy-loss and cathodoluminescence spectroscopies performed in a scanning mode of operation yield snap shots of plasmon modes in nanostructures with fine spatial detail as compared to other existing imaging techniques, thus providing an ideal tool for nanophotonics studies.

/pdf/optical-excitations-in-electron-microscopy-3d2rfnfkst.pdf

Optical excitations in electron microscopy

Electron energy loss spectroscopy (EELS) performed in a scanning transmission electron microscope (STEM) has demonstrated unprecedented power in the characterization of surface plasmons. The subangstrom spatial resolution achieved in EELS and its capability of exciting the full set of localized surface plasmon resonance (LSPR) modes supported by a metallic nanostructure makes STEM/EELS an ideal tool in the study of LSPRs. The plasmonic properties characterized using EELS can be associated with geometric or structural features collected simultaneously in a STEM to achieve a deeper understanding of the plasmonic response. In this review, we provide the reader a thorough experimental description of EELS as a LSPR characterization tool and summarize the exciting recent progress in the field of STEM/EELS plasmon characterization.

Probing Nanoparticle Plasmons with Electron Energy Loss Spectroscopy

Cathodoluminescence in the scanning transmission electron microscope

Low-cost layered oxides free of Ni and Co are considered to be the most promising cathode materials for future sodium-ion batteries. Biphasic Na0.78 Cu0.27 Zn0.06 Mn0.67 O2 obtained via superficial atomic-scale P3 intergrowth with P2 phase induced by Zn doping, consisting of inexpensive transition metals, is a promising cathode for sodium-ion batteries. The P3 phase as a covering layer in this composite shows not only in excellent electrochemical performance but also its tolerance to moisture. The results indicate that partial Zn substitutes can effectively control biphase formation for improving the structural/electrochemical stability as well as the ionic diffusion coefficient. Based on in situ synchrotron X-ray diffraction coupled with electron-energy-loss spectroscopy, a possible Cu2+/3+ redox reaction mechanism has now been revealed.

A Hydrostable Cathode Material Based on the Layered P2@P3 Composite that Shows Redox Behavior for Copper in High-Rate and Long-Cycling Sodium-Ion Batteries.

It is demonstrated that metallic horseshoe-shaped (also referred to as u-shaped) nanostructures can exhibit a magnetic resonance in the optical spectral range. This magnetic plasmon resonance is distinct from the purely geometric $LC$ resonance occurring in perfectly conducting split rings because the plasmonic nature of the metal plays the dominant role. Similarly to the electrical surface plasmon resonance, the magnetic plasmon resonance is determined primarily by the metal properties and nanostructure geometry rather than by the ratio of the wavelength and the structure's size. Magnetic plasmon resonance occurs in nanostructures much smaller in size than the optical wavelength. Electromagnetic properties of periodically assembled horseshoe-shaped nanostructures are investigated, and the close proximity of the electrical and magnetic plasmon resonances is exploited in designing a negative index metamaterial. Close to the magnetic plasmon resonance frequency both magnetic permeability $\ensuremath{\mu}$ and electric permittivity $\ensuremath{\epsilon}$ can become negative, paving the way for the development of subwavelength negative index materials in the optical range.

/pdf/magnetic-plasmon-resonance-1qynwl78dz.pdf

Magnetic plasmon resonance

Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

Mapping Surface Plasmons on a Single Mmetallic Nanoparticle using Sub-nm Resolved EELS Spectrum-Imaging

Plasmon modes of the exact same individual gold nanoprisms are investigated through combined nanometer-resolved electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) measurements. We show that CL only probes the radiative modes, in contrast to EELS, which additionally reveals dark modes. The combination of both techniques on the same particles thus provides complementary information and also demonstrates that although the radiative modes give rise to very similar spatial distributions when probed by EELS or CL, their resonant energies appear to be different. We trace this phenomenon back to plasmon dissipation, which affects in different ways the plasmon signatures probed by these techniques. Our experiments are in agreement with electromagnetic numerical simulations and can be further interpreted within the framework of a quasistatic analytical model. We therefore demonstrate that CL and EELS are closely related to optical scattering and extinction, respectively, with the addition of nanometer spatial resolution.

Advances in Scanning Transmission Electron Microscope Cathodoluminescence

A short review of electron-energy-loss spectroscopy (EELS) experiments of carbon nanotubes and onions is presented. The dielectric response function of these nanostructures is derived from electrodynamics. Loss spectra computed with the dielectric theory are compared with spatially-resolved experimental spectra. The main features of the loss spectra obtained with non-penetrating electrons can be attributed to surface plasmon excitations (π plasmon at 6 eV and π + σ plasmon at 15 and 17-18 eV).

Electrodynamic response of carbon nanostructures in electron-energy-loss spectroscopy

Plasmon modes in individual noble metal nanoparticles

In this work, the rich structure of SP modes localized in individual noble-metal nanoparticles prepared via lithography or colloidal chemistry is explored by optical spectroscopy, spatially resolved electron energy-loss spectroscopy (EELS) performed in a scanning transmission electron microscope (STEM), and by electron beam-induced radiation emission performed in a scanning electron microscope. Spectral features and spatially-resolved maps of SP modes collected for individual nanoparticles of different morphologies (rods, decahedra, prisms, and split-ring resonators) are shown to be in good agreement with theoretical STEM-EELS and optical excitation calculations obtained using the boundary element method based upon rigorous solution of Maxwell's equations.

Mapping localized plasmon modes in metal nanoparticles via electron energy loss spectroscopy and cathodoluminescence

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