Probing Plasmonic Nanostructures with Electron Energy - Loss Spectroscopy

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Abstract

This thesis presents theoretical and experimental results on plasmonic phenomena in nanosized metallic structures. The theoretical aspect concerns the extension of the local-response approximation, which leads to a description of metals based on the classical dielectric function, to account for nonlocal response. The experimental work comprises the use of electron energy-loss spectroscopy (EELS) to excite and study both localized and propagating surface plasmons in metal structures.
Following a short introduction, we present the theoretical foundation to describe nonlocal response in Maxwell’s equations for arbitrary geometries. We show that the key quantity which is modified by nonlocality is the induced charge in the metal. In particular, the induced surface charge is smeared over an Ångstrom length scale in contrast to the delta-function induced charge distribution in the local-response approximation. Irrespective of the microscopic origin, we find that nonlocal response modifies the electromagnetic wave equation by an additional Laplacian term. The hydrodynamic model, which includes nonlocal response through the Thomas–Fermi pressure of a free-electron gas, is discussed. We present also the generalized nonlocal optical response model, which expands the hydrodynamic model by taking into account the diffusion of free electrons in metals through Fick’s law. We go on to consider the implications of these two nonlocal models in the following plasmonic geometries: metal-insulator interface, nanosphere, dimer with nanometer-sized gaps, core-shell nanowire with ultrathin metal shell, and a thin metal film. In all cases we compare the nonlocal models with the local-response approximation. Below the plasma frequency, we find that the distance between the induced positive and negative surface charges is the main indication for the importance of nonlocal response. Specifically, the mentioned distance in nanospheres translates into a size-dependent resonance energy and linewidth broadening of the surface plasmons, while in the dimer a gap-dependent resonance energy and linewidth broadening is observed. Above the plasma frequency, resonant excitations are supported by nonlocal theory due to the inclusion of curl-free waves. The application of EELS to study surface plasmons in nanosized metallic systems is then presented. In particular, we discuss that EELS can provide important information on the optical response of plasmonic structures. We perform two separate EELS experiments and discuss their theoretical interpretations. The first experiment concerns the study of localized surface plasmon resonances of chemically prepared silver nanoparticles with diameter sizes down to 3.5 nm dispersed on a thin substrate. The second experiment is devoted to the investigation of propagating gap surfaceplasmon modes in gold nanogrooves, which are experimentally observed to subsist in gaps of only 5 nm.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages203
Publication statusPublished - 2014

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