Electron Energy-Loss Spectroscopy: Analytical Theory and Numerical Simulations of Individual Nanoparticles and Nanostructures

Abstract

The field of nanotechnology has experienced rapid growth over the past three decades which can largely be attributed to advances in technologies and experimental techniques. One technology in particular, the electron microscope, has been instrumental in nanoscale exploration allowing one to avoid ensemble measurements and investigate single nanoparticles and individual nanostructures. The ability to probe the fundamental behavior of such systems is paramount to the design of nanoscale devices. This dissertation focuses on spectroscopic methods mediated by the electron microscope, mainly electron energy-loss spectroscopy (EELS), and the capacity of such techniques to explore nanoscale behavior in aggregates of plasmon supporting metal nanoparticles (MNPs) and semiconducting nanoparticles. We present a theoretical framework to describe the EELS experiment in general, as well as for the MNPs in the quasistatic limit that lends itself to the simple treatment of MNP dimers and aggregates by mapping MNPs onto harmonic oscillators. The spatial dependence of EELS is explored for MNP aggregates, focusing on the ability of the electron to unevenly force a nanostructure. An analytic treatment is presented to describe Fano resonances in MNP dimers driven by the electron beam where the evanescent nature of the electron’s electric field is exploited to investigate a broader class of Fano resonances than those available in optical spectroscopies. We further explored the spatial dependence of EELS to image hybridized normal modes of MNP nanostructures. Analysis of nodal structure in EEL maps is shown to provide a rubric for determining the relative phase of charge oscillations within a nanostructure. This rubric is further applied to plasmon oligomer dimers effectively mapping magnetic modes using numerical simulations and experimental measurements. We further observe signatures of magnetic field localization in symmetry broken oligomer dimers. This interference effect is shown to occur at two distinct energies that controls the location of magnetic field by the energy of incident far-field radiation. We move away from the oscillator model when considering phenomena that occur in semiconducting nanoparticles that hold a high index of refraction. Such systems are experi- mentally shown to exhibit band gap peaks which do no correspond to electron excitations within the material. Through numerical simulations and an EELS Mie theory analysis these unusual peaks are shown to correspond to cavity modes of the nanoparticle and are analogous to geometric scattering.