Electron Energy-Loss Spectroscopy Theory and Simulation Applied to Nanoparticle Plasmonics

Electron Energy-Loss Spectroscopy Theory and Simulation Applied to Nanoparticle Plasmonics Nicholas Walker Bigelow Chair of the Supervisory Committee: Assistant Professor David J. Masiello Department of Chemistry The vast array of potential applications for plasmons has laid bare the need for a detailed understanding of the complex interactions that occur between multiple plasmons and between plasmons and near-field probes. In this dissertation, the capacity of electron energy-loss spectroscopy (EELS) to probe plasmons is examined in detail. EELS is shown to be able to detect both electric hot spots and Fano resonances in contrast to the prevailing knowledge prior to this work. The most detailed examination of magnetoplasmonic resonances in multiring structures to date and the utility of electron tomography to computational plasmonics is explored, and a new tomographic method for the reconstruction of a target is introduced. Since the observation of single-molecule surface-enhanced Raman scattering (SMSERS) in 1997, questions regarding the nature of the electromagnetic hot spots responsible for such observations still persist. A computational analysis of the electronand photon-driven surfaceplasmon resonances of monomer and dimer metal nanorods is presented to elucidate the differences and similarities between the two excitation mechanisms in a system with well understood optical properties. By correlating the nanostructure’s simulated electron energy loss spectrum and loss-probability maps with its induced polarization and scattered electric field we discern how certain plasmon modes are selectively excited and how they funnel energy from the excitation source into the nearand far-field. Using a fully retarded electron-scattering theory capable of describing arbitrary three-dimensional nanoparticle geometries, aggregation schemes, and material compositions, we find that electron energy-loss spectroscopy (EELS) is able to indirectly probe the same electromagnetic hot spots that are generated by an optical excitation source. EELS is then employed in a scanning transmission electron microscope (STEM) to obtain maps of the localized surface plasmon modes of SMSERS-active nanostructures, which are resolved in both space and energy. Singlemolecule character is confirmed by the bianalyte approach using two isotopologues of Rhodamine 6G. The origins of this observation are explored using a fully three-dimensional electrodynamics simulation of both the electron energy loss probability and the near-electric field enhancements. The calculations suggest that electron beam excitation of the hot spot is possible, but only when the electron beam is located outside of the junction region, and further that the location of the hot spot can be inferred from the node in the loss probability in the junction along with the high loss probability on the edges away from the junction. v The optical-frequency magnetic and electric properties of cyclic aromatic plasmon-supporting metal nanoparticle oligomers are explored through a combination of STEM/EELS simulation and first-principles theory. A tight-binding type model is introduced to explore the rich hybridization physics in these plasmonic systems and tested with full-wave numerical electrodynamics simulations of the STEM electron probe. Building from a microscopic electric model, connection is made at the macroscopic level between the hybridization of localized magnetic moments into delocalized magnetic plasmons of controllable magnetic order and the mixing of atomic pz orbitals into delocalized π molecular orbitals of varying nodal structure spanning the molecule. It is found that the STEM electrons are uniquely capable of exciting all of the different hybridized eigenmodes of the nanoparticle assembly—including multipolar closed-loop ferromagnetic and antiferromagnetic plasmons, giant electric dipole resonances, and radial breathing modes—by raster scanning the beam to the appropriate position. Comparison to plane wave light scattering and cathodoluminescence (CL) spectroscopy is made. The presented work provides a unified understanding of the complete plasmon eigenstructure of such oligomer systems as well as of the excitation conditions necessary to probe each mode. Through numerical simulation, we predict the existence of the Fano interference effect in the EELS and CL of symmetry-broken nanorod dimers that are heterogeneous in material composition and asymmetric in length. The differing selection rules of the electron probe in comparison to the photon of a plane wave allow for the simultaneous excitation of both optically bright and dark plasmons of each monomer unit, suggesting that Fano resonances will not arise in EELS and CL. Yet, interferences are manifested in the dimer’s scattered nearvi and far-fields and are evident in EELS and CL due to the rapid π-phase offset in the polarizations between super-radiant and sub-radiant hybridized plasmon modes of the dimer as a function of the energy loss suffered by the impinging electron. Depending upon the location of the electron beam, we demonstrate the conditions under which Fano interferences will be present in both optical and electron spectroscopies (EELS and CL) as well as a new class of Fano interferences that are uniquely electron-driven and are absent in the optical response. Among other things, the knowledge gained from this work bears impact upon the design of some of the world’s most sensitive sensors, which are currently based upon Fano resonances. The Fano interference phenomenon between localized surface plasmon resonances (LSPRs) of individual silver nanocubes is then investigated experimentally using dark-field optical microscopy and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). By computing the polarization induced by the electron beam, we show that the hybridized modes responsible for this Fano interference are the same as those present in the resonance-Rayleigh scattering spectrum of an individual nanocube on a substrate. Finally, a group of five semi-collinear nanoparticles are modeled both by making a guess as to the third dimension from a single top-down electron micrograph and also through electron tomography. The former technique is the conventional modeling method most often employed in creating computational models of plasmonic targets, though it is akin to modeling a sky scraper from a satellite picture of the roof. Electron tomography offers a way to reconstruct the particles fully, with a minimal amount of guesswork. The degree of similarity between the computed properties of the target built through the two different methods is examined, vii as are the targets themselves. It is shown that purely far-field properties, such as the optical scattering are largely unaffected, but near-field properties, which are highly dependent on the fine-scale structure of the targets, differ considerably depending on which modeling method is employed. This work suggests that caution should be used by the theoretician when attempting to model the near-field of a physical structure, and that electron tomography offers an attractive way to overcome the problem of the third dimension.