Understanding the Plasmonic Properties of Metallic Nanostructures with Correlated Photon- and Electron-Driven Excitations

The collective oscillation of the conduction band electrons in metal nanostructures, known as plasmons, can be used to manipulate light on length scales that are smaller than the diffraction limit of visible light. In this dissertation, a correlated approach is used to probe localized surface plasmon resonances (LSPRs) in metallic nanostructures, and their application to surface-enhanced spectroscopy. This correlated approach involves the measurement of LSPRs with dark-field optical microscopy (resonance-Rayleigh scattering), and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Structural parameters of the exact same nanostructures obtained from the STEM are subsequently used in performing fully three-dimensional continuum electrodynamics simulations to support the experimental observables.

The first part of this work utilizes the correlated approach with theoretically calculated near-electric field enhancements, in exploring the LSPRs of silver nanorods with varying aspect-ratios. Multivariate statistical analysis (MVSA) is used to extract the experimentally measured plasmon modes obtained from STEM/EELS, with a spatial resolution on the length scale of the plasmon itself. These results demonstrate the ability of the correlated approach to yield complementary information not accessible from either technique on its own. In the second study, the electromagnetic hot spots responsible for single-molecule surface-enhanced Raman scattering (SMSERS) are investigated with the correlated approach and theoretical simulations. The results suggest the possibility of exciting a hot spot with an electron beam, and inducing Raman scattering from a single molecule when the beam is positioned antisymmetrically with respect to the hot spot. The third and final part of this work investigates Fano resonances in silver nanocubes with STEM/EELS, and the changes that occur in the LSPR spectra of nanocubes after exposure to the electron beam. The results from this study suggest that the hybridized modes responsible for Fano interference in STEM/EELS are the same as those present in optical spectroscopy.