573 research outputs found
Towards Plasmon-Band Engineering in Ordered Plasmonic Nanostructures
In this thesis, the hybridization of localized surface plasmons to generate continuous plasmonic excitation bands was investigated. Localized surface plasmons are quasi-particles corresponding to collective oscillations of charge carriers (for example, conduction electrons in metals). They arise at interfaces between nanoparticles and their surroundings if the signs of the dielectric functions of the facing materials are opposite. If the spatial extension of the plasmon is confined to the nanoparticle, standing plasmon waves (so-called plasmon modes) with localized and amplified electromagnetic fields emerge. The field enhancement is confined to the boundaries of the nanoparticle, with material-dependent evanescent damping (decrease to 1/e in the range of several 10 nm).
The penetration of the fields into the environment allows interaction of plasmon modes of different individual nanoparticles arranged in assemblies with interparticle distances of several nanometers. This interaction can lead to hybridization, i.e., spectral splitting of the coupled plasmons into bonding and anti-bonding modes, which is the basis for the emergence of continuous plasmonic excitation bands. In analogy to electronic band structures, which arise by multiple hybridization of atomic orbitals in periodic lattices, plasmonic band structures can be created in large periodic arrays of plasmonic nanoparticles. Also, tuning of the bands to generate desired band properties is possible in principle by periodically varying either the individual building blocks of the arrangement (shape, size, material) or the coupling strength (distance, dielectric spacer).
In the present dissertation, the spectral and spatial characteristics (excitation energy, localization, etc.) of different arrangements of plasmonic nanostructures and their plasmonic response were investigated. Using a focused electron beam (probe) in a conventional transmission electron microscope, the plasmons were excited by the evanescent electromagnetic fields of the fast beam electrons (around half of the speed of light). By analyzing the energy loss of the beam electrons (which is caused by plasmon excitation) at different probe positions on the sample, the plasmons were characterized in terms of excitation energies and spatial localization. In addition, the measured data were supported by numerical simulations to verify the experiments. To gain a theoretical understanding, appropriate models were adapted to the present experiments. For example, the classical Mie theory (which describes the plasmonic response of a sphere to transverse electromagnetic waves) was generalized to the inhomogeneous case corresponding to the plasmon excitation by the evanescent field of the beam electrons. Furthermore, surface effects (the so-called axion mixing of magnetic and electric field components), for example, present in topological insulators, were taken into account in the generalization of the Mie theory. As a first step towards plasmon band engineering, the plasmonic response of gold nanoparticles of different shapes was studied to get a comprehensive understanding of the plasmonic behavior of isolated single nanoparticles, which can later be arranged into coupled plasmonic nanostructures.
In the next step, gold nanospheres were arranged into chains of different lengths to observe the formation of plasmonic band structures. By examining the hybridization as a function of chain length, the formation of a quasi-continuous plasmonic band with strong dispersion was observed.
To create more complex band structures with band gaps or crossings, gold and silver nanospheres were assembled to heterogeneous chains. Focusing on plasmon hybridization in coupled nanoparticles of different kinds, all possible permutations of four coupled gold and silver nanospheres were analyzed. Considering first pure gold and silver tetra chains, similar hybridized plasmon modes, differing only in a spectral red shift in the case of gold were observed. The mixed chains also show similar hybridized modes with intermediate spectral positions depending on the number of gold and silver spheres in the chains, which proves hybridization in heterogeneous arrangements. In addition, it was found that in particular silver nanoparticles degrade in air, resulting in a bad and undefined plasmonic response. The latter hampers the use of silver for plasmonic band engineering, although it has relatively low dielectric loss.
To address the degradation and to deliberately tune the distance between the coupled nanoparticles, the use of a silicon dioxide shell as a dielectric spacer and protection layer was elicited. Silver nanocubes were encapsulated in silica shells of various controllable thicknesses and investigated in terms of the plasmonic properties. It was found that the coating significantly reduces both degradation and influence of the substrate, resulting in a highly predictable and reproducible plasmonic response. The dielectric silica shell can additionally sustain Mie type resonances, which may couple to plasmons and thus mediate effective plasmonic coupling over relatively large distances (about a factor of two compared to the coupling of uncoated nanoparticles).
In contrast to the delocalized quasi-continuous plasmon bands in periodic nanostructures, localized (spectral and spatial) plasmonic modes can occur in disordered geometries. This effect can hamper the formation of plasmonic bands if the plasmons localize at imperfections (shape, size, or distance deviation) of the coupled nanoparticles. Related to this, the effect of plasmon localization in randomly disordered 2-dimensional gold webs was studied. Stronger localization with increasing plasmon excitation energy was found here. Finally, a geometry-dependent spectral threshold of vanishing localized plasmon modes was observed.
In summary, several fundamental aspects of plasmonic band engineering were investigated, providing a basis for the specific design of plasmonic nanostructures with desired properties.:Abstract
Acronyms
List of Symbols
List of Figures
List of Tables
Contents
1 Introduction
1.1 Synthesis of Plasmonic Systems
1.2 State of the Art
1.3 Outline
2 Theory
2.1 Surface Plasmons at Planar Interfaces
2.2 Modeling Dielectric Functions - the Drude Model
2.3 Axion Electrodynamics of Topological Insulators
2.4 Surface Plasmons at Spherical Geometries - Mie Theory and Generalization to Topological Insulators
2.4.1 Vector Spherical Harmonic Expansion
2.4.2 Axion Boundary Conditions
2.4.3 Homogeneous Case
2.4.4 Inhomogeneous Case
2.5 Complex Geometries and Coupled Nanoparticles
2.5.1 Plasmon Mode Hybridization
2.5.2 Numerical Solvers
2.5.3 Discrete Dipole Model
2.6 Plasmon Mode Classification
2.7 Plasmonics in the Transmission Electron Microscope
2.7.1 Electron Energy-Loss Probability
3 Methods
3.1 Experimental Setup
3.1.1 Energy Filter
3.1.2 Spectroscopy Mode - Direct Imaging of the Energy-Dispersive Plane
3.1.3 Imaging Mode - Energy-filtered Imaging of the Filter Entrance Plane
3.1.4 Alternative Modes
3.1.5 High-Angle Annular Dark Field-Detector
3.2 Data Post-Processing
3.2.1 Zero-Loss Peak Subtraction and Deconvolution
3.2.2 Correction of the Scattering Absorption
3.2.3 Enhancement of the Signal-to-Noise Ratio
3.3 Uncertainties of the Measurement
3.4 Plasma Cleaning of the Sample
4 Results
4.1 Interplay of the Nanoparticle’s Shape and Plasmonic Response
4.2 Self-Assembly of Spherical Nanoparticles to Homogeneous Chains
4.3 Self-Assembly of Spherical Nanoparticles to Heterogeneous Chains
4.4 Silica Encapsulation of Air Sensitive Nanoparticles
4.5 Localization of Surface Plasmon Modes in Disordered 2-Dimensional Webs
5 Summary and Outlook
5.1 Summary
5.2 Outlook
5.2.1 Measurement of the Plasmon Band Dispersion
5.2.2 Generalization of Anderson Localization to Plasmons
5.2.3 Measurement of the Axion Contribution in TIs
5.2.4 Non-Local Measurements
Bibliography
List of Publications
Danksagung
Erklärun
De geometria acvstica nec non de ratione 0:0 cev basi calcvuli differentialis Dissertatio II. Qvam pro loco professionis matheseos ordinariae secvndvm statvta academica rite sibi vndicando pvblice tvebitvr Ioannes Schvltz ... Respondente Ioanne Beniamin Iachmann ... Opponentibvs Ioanne Frider. Gensichen ... Friderico Wolff ... Christ. Gottl. Zimmermann ... Anno MDCCXXXVII die XV. Aprilis horis locoqve solitis
https://www.ester.ee/record=b4113239*es
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Sodium is known as a soft metal that can easily change its particle morphology. It can form outstretched and rolled fibers with plastic or brittle behavior, and cubes. In Na-batteries, metallic Na anodes demonstrate a high reactivity towards the majority of electrolyte solutions, volume change and a random deposition process from the electrolyte, accompanied by dendrite formation.
In order to smooth the electrochemical Na deposition, we propose NaOH as a simple artificial protective layer for sodium, formed by its exposure to ambient conditions for a certain period of time. The formed NaOH layer on top of the metallic sodium suppresses the volume change and dendrite growth on the sodium surface. Additionally, the protected sodium does not change its morphology after a prolonged contact with carbonate-based electrolytes. In symmetric Na-batteries, the NaOH layer increases the lifetime of the electrochemical cell by eight times in comparison to non-protected Na.
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Tailoring Plasmonics of Au@Ag Nanoparticles by Silica Encapsulation
Hybrid metallic nanoparticles encapsulated in oxide shells are currently
intensely studied for plasmonic applications in sensing, medicine, catalysis,
and photovoltaics. Here, we introduce a method for the synthesis of
Au@Ag@SiO cubes with a uniform silica shell of variable and adjustable
thickness in the nanometer range; and we demonstrate their excellent, highly
reproducible, and tunable optical response. Varying the silica shell thickness,
we could tune the excitation energies of the single nanoparticle plasmon modes
in a broad spectral range between 2.55 and 3.25\,eV. Most importantly, we
reveal a strong coherent coupling of the surface plasmons at the silver-silica
interface with the whispering gallery resonance at the silica-vacuum interface
leading to a significant field enhancement at the encapsulated nanoparticle
surface in the range of 100\,\% at shell thicknesses 20\,nm.
Consequently, the synthesis method and the field enhancement open pathways to a
widespread use of silver nanoparticles in plasmonic applications including
photonic crystals and may be transferred to other non-precious metals
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