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

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NaOH protective layer for a stable sodium metal anode in liquid electrolytes

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. In the full-cell with a layered sodium oxide cathode, the NaOH-protected sodium anode also leads to a high cycling stability, providing 81 % of the initial cell capacity after 500 cycles with a 1C current rate. In contrast, batteries with a non-protected Na-anode reach only 20 % of their initial capacity under the same conditions. Therefore, the main benefits of the NaOH artificial layer are the chemical compatibility with the carbonate-based electrolytes, the protection of Na metal against reaction with the electrolyte solution, the rapid Na-ion diffusion through the layer and the formation of a mechanical barrier, mitigating Na-dendrite growth. This work presents an easily scalable method to protect sodium without any additional chemicals or a special environment for this reaction

Großwetterlagen und das raumzeitliche Muster des Jahrringwachstums in Mitteleuropa : ein neuer methodischer Analyseansatz

Für das Verständnis des Klimasystems ist es unerlässlich, lange Zeiträume zu betrachten. Dieses Vorgehen ermöglicht es, die Prozesse und Rückkopplungen im Klimasystem zu beschreiben, auf ihre Periodizität hin zu untersuchen und zu verstehen. Dieses Wissen ist unter anderem von Bedeutung für die Abschätzung des anthropogenen Anteils an den beobachteten und prognostizierten Klimaänderungen (IPCC 2007). Aufgrund des kurzen Zeitfensters, für das meteorologische Messdaten in ausreichender Zahl, Güte und räumlicher Abdeckung zur Verfügung stehen, werden vergangene Klimabedingungen durch historische Quellen oder Proxidaten wie z. B. Eisbohrkerne, Warven oder Jahrringe rekonstruiert (Glaser 2008). Zahlreiche Studien konnten die Potenziale von Jahrringen für die Klimarekonstruktion belegen; in diesem Zusammenhang seien exemplarisch die Arbeiten von Esper et al. (2012, 2002), Büntgen et al. (2011), Cook et al. 2010 und Treydte et al. (2006) genannt. Die vorliegende Arbeit hat sich zum Ziel gesetzt, durch die Einführung eines neuen, auf Wetterlagen basierenden Ansatzes das Verständnis von Klima / Wachstums-Beziehungen (vgl. Neuwirth 2010, 2011) zu verbessern. Im Gegensatz zur gängigen Praxis in der Dendroklimatologie wird nicht der Einfluss einzelner Klimaelemente auf das Wachstum, sondern der Einfluss von Wetterlagen, die sich ihrerseits auf die lokalen klimatischen Bedingung auswirken, untersucht. Teile dieser Arbeit, insbesondere die Beschreibung des neuen Ansatzes bzw. der Prozedur, sind in Agricultural and Forest Meteorology (Schultz & Neuwirth 2012) publiziert.Weather types and the spatiotemporal patterns of tree-ring growth in Central Europe – A new methodological approach For the understanding of the climate system, it is essential to consider long periods. This approach makes it possible to describe the processes and feedbacks in the climate system, to examine their periodicity and to understand them. This knowledge is essential, inter alia, for the estimation of anthropogenic proportion of the observed and projected climate change (IPCC 2007). Due to the short time window, for which meteorological data in a sufficient number, quality and spatial coverage are available, past climate conditions are often reconstructed by using historical sources or proxy data such as ice cores, tree-rings or varves (Glaser 2008). Numerous studies have proven the potential of tree-rings in climate reconstruction, see Esper et al. (2012, 2002), Büntgen et al. (2011), Cook et al. (2010) Treydte et al. (2006). The target of the thesis is to improve the understanding of climate-growth relationships (see Neuwirth 2010, 2011) by introducing a new approach based on weather types. In contrast to the common practice in dendroclimatology the influence of weather types, which are the dominate factor influencing local climate conditions, on growth are investigated and not the influence of single climate elements. Parts of this thesis, in particular the description of the new approach and the procedure are published in Agricultural and Forest Meteorology (Schultz & Neuwirth, 2012)

Enabling secure subsurface storage in future energy systems: An introduction

Geological structures in the subsurface have been used for the storage of energy and waste products for over a century. Depleted oil and gas fields, saline aquifers or engineered caverns in salt or crystalline rocks are used worldwide to store energy fluids intended to provide demand buffers and sustained energy supply. The transition of our energy system into a clean, renewable-based system will most likely require an expansion of these subsurface storage activities, to host a wide variety of energy products (e.g. natural gas, hydrogen, heat or waste energy products, like CO2) to balance the inherent intermittence of the renewable energy supply. Ensuring the safety and effectiveness of these subsurface storage operations is therefore crucial to achieve the sought-after renewable energy transition while ensuring energy security

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$_2$ 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 $t\,$$\simeq\,$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

Axion Mie theory of electron energy loss spectroscopy in topological insulators

Electronic topological states of matter exhibit novel types of responses to electromagnetic fields. The response of strong topological insulators, for instance, is characterized by a so-called axion term in the electromagnetic Lagrangian which is ultimately due to the presence of topological surface states. Here we develop the axion Mie theory for the electromagnetic response of spherical particles including arbitrary sources of fields, i.e., charge and current distributions. We derive an axion induced mixing of transverse magnetic and transverse electric modes which are experimentally detectable through small induced rotations of the field vectors. Our results extend upon previous analyses of the problem. Our main focus is on the experimentally relevant problem of electron energy loss spectroscopy in topological insulators, a technique that has so far not yet been used to detect the axion electromagnetic response in these materials

Ca2+-controlled competitive diacylglycerol binding of protein kinase C isoenzymes in living cells

The cellular decoding of receptor-induced signaling is based in part on the spatiotemporal activation pattern of PKC isoforms. Because classical and novel PKC isoforms contain diacylglycerol (DAG)-binding C1 domains, they may compete for DAG binding. We reasoned that a Ca2+-induced membrane association of classical PKCs may accelerate the DAG binding and thereby prevent translocation of novel PKCs. Simultaneous imaging of fluorescent PKC fusion proteins revealed that during receptor stimulation, PKCα accumulated in the plasma membrane with a diffusion-limited kinetic, whereas translocation of PKCɛ was delayed and attenuated. In BAPTA-loaded cells, however, a selective translocation of PKCɛ, but not of coexpressed PKCα, was evident. A membrane-permeable DAG analogue displayed a higher binding affinity for PKCɛ than for PKCα. Subsequent photolysis of caged Ca2+ immediately recruited PKCα to the membrane, and DAG-bound PKCɛ was displaced. At low expression levels of PKCɛ, PKCα concentration dependently prevented the PKCɛ translocation with half-maximal effects at equimolar coexpression. Furthermore, translocation of endogenous PKCs in vascular smooth muscle cells corroborated the model that a competition between PKC isoforms for DAG binding occurs at native expression levels. We conclude that Ca2+-controlled competitive DAG binding contributes to the selective recruitment of PKC isoforms after receptor activation