Cathodoluminescence and phase-change functionality of metallic nanoparticles

Nanoscale resolution cathodoluminescence (CL) has been used to demonstrate and investigate the functionality of nanoparticle-based components for future nanophotonic phase-change memory and optical antenna applications. An integrated experimental system based on a scanning electron microscope was developed for the fabrication and in situ characterization of nanoparticles. It was equipped with an atomic beam source for gallium nanoparticle growth, a liquid-nitrogen-cooled cryostat to control substrate temperature in the range from 90 to 315 K and a spectroscopic CL detection system to enable the analysis of electron-beam-induced light emission from nanoparticles across the wavelength range from 350 to 1150 nm. A new technique of light-assisted, size-controlled growth of gallium nanoparticles from an atomic beam has been developed. Through coupling to surface plasmons in nanoparticles, infrared radiation controls the adsorption/desorption rate of gallium atoms on the particles' surface. The experiments revealed a decrease in mean particle diameter (from 68 to 45 nm) with increasing infrared excitation intensity (from 160 to 630 W·cm-2) during deposition, and the production of larger particles with a narrower size distribution for longer deposition times. Gallium nanoparticle phase-change memory provides an important possibility to achieve small element size and low energy consumption. For the first time, it has been shown that information can be written into the structural phase of bistable gallium nanoparticles by electron beam excitation and readout achieved via measurements of their CL emission. Change of up to 20 % in CL emission intensity was observed following low fluence (> 35 fJ/nm2) electron-beam-induced, solid-to-liquid phase switching of a monolayer of 60 nm particles. Selective electron beam addressing and CL readout of individual memory elements (comprising less than 50 particles each), within a gallium nanoparticle film, have been also demonstrated. Numerical modeling of CL emission from gallium nanoparticles, performed using the boundary element method, qualitatively reproduces the experimentally observed effects. Optical antennae are expected to become essential elements of future nanophotonic circuits. For the first time, it has been demonstrated that electron-beam-excited pairs of coupled gold nanorods can act as transmitting optical antennae; i.e. they can efficiently convert the energy from a nanoscale excitation (created by a focused 40 keV electron beam) into far-field visible radiation. Enhanced light emission was observed for electron beam injection points in the vicinity of the junction between coupled nanorods, illustrating the increased local density of electromagnetic states in such areas

An immersed boundary method for particles and bubbles in magnetohydrodynamic flows

This thesis presents a numerical method for the phase-resolving simulation of rigid particles and deformable bubbles in viscous, magnetohydrodynamic flows. The presented approach features solid robustness and high numerical efficiency. The implementation is three-dimensional and fully parallel suiting the needs of modern high-performance computing. In addition to the steps towards magnetohydrodynamics, the thesis covers method development with respect to the immersed boundary method which can be summarized in simple words by From rigid spherical particles to deformable bubbles. The development comprises the extension of an existing immersed boundary method to non-spherical particles and very low particle-to-fluid density ratios. A detailed study is dedicated to the complex interaction of particle shape, wake and particle dynamics. Furthermore, the representation of deformable bubble shapes, i.e. the coupling of the bubble shape to the fluid loads, is accounted for. The topic of bubble interaction is surveyed including bubble collision and coalescence and a new coalescence model is introduced. The thesis contains applications of the method to simulations of the rise of a single bubble and a bubble chain in liquid metal with and without magnetic field highlighting the major effects of the field on the bubble dynamics and the flow field. The effect of bubble coalescence is quantified for two closely adjacent bubble chains. A framework for large-scale simulations with many bubbles is provided to study complex multiphase phenomena like bubble-turbulence interaction in an efficient manner

Transformation optics: a tool to reveal and make use of symmetries in plasmonics

Symmetries are omnipresent in physics. From classical mechanics, via solid state physics to particle physics, symmetries provide a means for classification and often lead to deep physical insight. In this thesis, we study symmetries in plasmonics using Transformation optics. We show how Transformation optics can be used to reveal, study and make us of symmetries in practical calculations, by studying a range of plasmonic systems. First, we show that an ellipse and spheroids possess a `hidden' rotational symmetry that becomes apparent when transforming them to a rotationally symmetric structure. Next, we investigate plasmonic gratings and show that a whole class of plasmonic gratings (and other periodic structures) can be related to a translationally invariant slab, thereby inheriting all the slabs spectral properties. In studying the plasmonic grating, we extend the Transformation optics approach to treat periodic systems with extent larger than the wavelength in one direction. Finally, we use Transformation optics to study electron energy loss spectroscopy and cathodoluminescence problems in plasmonics, by mapping the plasmonic nanoparticles under investigation to more symmetric ones. Thus, again using the symmetry of the transformed structures to derive analytical solutions to the problem at hand.Open Acces

SOL-GEL ROUTES TO MESOPOROUS TUNGSTEN OXIDES WITH MIXED ELECTRON/PROTON CONDUCTIVITY

The present thesis is focused on the development of novel, straightforward sol-gel techniques for the synthesis of highly mesoporous, mixed-conducting tungsten oxide monoliths and powders. Such materials are extremely interesting in view of potential applications for a variety of emerging electrochemical technologies, including electrode design in Polymer-Electrolyte-Membrane Fuel Cells. Both hydrolytic and non-hydrolytic methods are set up. The hydrolytic route is based on a proper steam-treatment as an effective way to control the supply of water molecules to the gelling phase and thus also the oxide formation rate, which is crucial in determining mesoporous features. The non-hydrolytic route is based on a metal halide/alcohol system and affords a variety of mesoporous frameworks. An extended investigation is carried out in order to establish a correlation between alcohol molecular structure and physical properties of final oxide materials. All samples are systematically characterized as to mesoporous properties, chemical composition and electrical properties. Mesoporosity is mainly investigated by means of nitrogen adsorption/desorption analysis, which allows determination of surface area and pore volume/size as well as surface fractal dimension. In particular, the fractal dimension is shown to be a fundamental parameter in controlling and tayloring the mesoporous properties. Additional structural information is obtained from Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD). Chemical composition (non-stoichiometry) plays a key role in electron conduction and is studied by X-Ray Photoelectron Spectroscopy (XPS). Finally, electrical properties are subjected to a detailed quantitative inspection by means of Electrical Impedance Spectroscopy (EIS). Electron Conductivity is discussed in terms of hopping-transport models. Proton conductivity takes place in humid conditions according to the Grotthuss mechanism and can be extracted from EIS data by fitting with a proper equivalent circuit. Fractal dimension has a deep influence on proton dynamics and two well-distinct transport regimes are observed for rough and smooth oxide matrices. Based on preparation and processing conditions, the following important values can be achieved: surface area up to 184 m2/g, pore volume up to 0.56 cm3/g, fairly monodisperse pore diameter in the range 3 ÷ 20 nm, electron conductivity up to 20 S/cm and proton conductivity up to 47 mS/cm

On the drying of polymer-plate-like particle-composites : Numerical study and experimental validation

Polymer-Partikel-Komposite sind zunehmend in den Fokus der angewandten Materialforschung gerückt. Durch die Einbettung von kolloidalen Nanopartikeln in eine Polymermatrix werden solchen Polymer-Partikel Kompositen neue Eigenschaften verliehen. Hierzu zählen u. a. Beeinflussung der mechanischen Eigenschaften, der elektrischen Leitfähigkeit, der thermischen Eigenschaften, der optischen Transparenz oder auch der Biokompatibilität. Die Funktionalität von Polymer-Partikel-Kompositen hängt in erster Linie von der Materialauswahl und deren Zusammensetzung, aber auch maßgeblich sowohl von der Verteilung als auch der räumlichen Ausrichtung dieser ab. Die meisten Kompositbeschichtungen werden durch Beschichten einer Dispersion mit anschließender Trocknung hergestellt. Die Komponentenverteilung in der Schicht selbst, die dabei entsteht; hängt vor allem von der Partikel-größe, -material, -geometrie und von der Trocknungsrate ab. Letztere wird von den Wärme- und Stofftransportprozessen bei den gegebenen Randbedingungen bestimmt. Die Partikelausrichtung wird auch durch unterschiedliche Scherung während des Beschichtungsprozesses z.B. einer Couette-Strömung in einem Beschichtungsspalt beeinflusst. Der Einfluss der Trocknungsrandbedingungen auf die Komponentenverteilung von sphärischen Partikel-Polymer-Kompositen wurde in vorherigen Arbeiten (Baesch at al., Cardinal et al. and Routh at al.) untersucht. Für sphärische, monomodale Partikel in einer Polymerlösung wurden die Mechanismen der Stoffübertragung von kolloidalen und nicht-kolloidalen Komponenten identifiziert. Auch zeigen Ergebnisse, dass die Partikelgeometrie einen größeren Einfluss auf die Komponentenverteilung hat. Man findet erste Ergebnisse zum Einfluss der Partikelverteilung bei sehr flachen, plättchenförmigen Partikelsystemen. In den bisherigen Arbeiten war das Ziel, den Einfluss der Abflachung der Geometrie auf die Partikelverteilung während der Trocknung zu untersuchen. Es konnte evaluiert werden, wie die Simulationsroutine und die Modellierung für sphärische Partikel auch bei plättchenförmigen Partikeln weiter-verwendet werden können. Diese neuen Geometrien wurden angepasst und durch die Untersuchung alternativer Stoffsysteme experimentell validiert. Zu diesem Zweck kann die Partikelverteilung im trockenen Film dreidimensional mittels Mess- und Auswerteroutinen mit Hilfe von in 3-D aufgenommen Ramanspektren bestimmt werden. Die Abhängigkeit zwischen Partikelform und Partikeldynamik wird mittels Lichtinterferometrie in einer Zentrifuge bestimmt. Dabei kann die Trocknungsrate, die Partikelgröße und die Anfangskonzentration variiert werden. Bei den 3-D Raman-Aufnahmen in den trockenen Filmen wurden sehr unterschiedliche Partikelverteilungen beobachtet: eine Partikelansammlung an der Unterseite, eine homogene Partikelverteilung über der Filmhöhe oder eine Partikelansammlung an der Oberfläche des Filmes. Beim Vergleich von plättchen zur sphärischen Form zeigte sich, dass die Bewegung von plättchenförmigen Partikeln in einer Polymerlösung durch die An-wesenheit viskoser Widerstände deutlich langsamer erfolgt. Dies konnte bei der Weiterentwicklung der Simulationsmodelle auch berücksichtigt und die entsprechenden Simulationsparameter konnten für plättchenförmige Partikel angepasst werden. Die Ansätze der Simulation beschreiben die Partikeldiffusion, -sedimentation, Polymerdiffusion und Lösemittelverdunstung. Für den Fall, dass die Plättchen durch ein typisches Beschichtungsver-fahren wie z.B. Rakeln und Schlitzguß senkrecht ausgerichtet werden, wurde ein neues Modell entwickelt und verifiziert. Die Einflussparameter dieser Simulation lassen sich analog zu den sphärischen Partikeln zusammenfassen: zum einen als Verhältnis vom Produkt der Schichtdicke und der anfänglichen Trocknungsrate zum Diffusionskoeffizienten der einzelnen Partikel und zum anderen als das Verhältnis von Trocknungsrate zum Sedimentationskoeffizienten der einzelnen Partikel auftragen. Ein dimensionsloses Seitenverhältnis kommt hier zur Péclet-Zahl und Sedimentationszahl hinzu. Durch die systematische Variation dieser entdimensionierten Größen ist es möglich die Komponentenverteilung graphisch in einer dimensionslosen Trocknungskarte darzustellen. Die so erstellten Trocknungskarten ermöglichen es die Partikelverteilung im trockenen Film bei unterschiedlichen Randbedingungen vorherzusagen. Die Ergebnisse dieser Arbeit zeigen eine klare Abweichung von sphärischen Partikeln durch die Trocknungskarten in Abhängigkeit der Partikelform

Exciton Polariton Modes in Nanostructures

In this thesis, original theoretical and numerical investigations into the interaction of light with excitonic nanostructures are presented, in a bid to demonstrate that excitonic nanostructures are viable alternatives to the use of plasmonic nanostructures where electric field enhancement and confinement are sought. In particular, the field enhancement and confinement around excitonic nanostructures on resonance is shown to be comparable if not in excess of that around noble metal nanoparticles such as gold and silver. These excitonic modes, when set in the context of a core-shell geometry, are shown to offer tunability through nanoparticle design and through the index of the environment. In addition, hybrid `hyperbolic' and `plexcitonic' modes are shown to offer similar properties in metallic-excitonic nanostructures. Altogether, these excitonic and hybrid excitonic modes are shown to have potential in nanophotonic applications.Engineering and Physical Sciences Research Counci

Optimization of Plasmon Decay Through Scattering and Hot Electron Transfer

Light incident on metal nanoparticles induce localized surface oscillations of conductive electrons, called plasmons, which is a means to control and manipulate light. Excited plasmons decay as either thermal energy as absorbed phonons or electromagnetic energy as scattered photons. An additional decay pathway for plasmons can exist for gold nanoparticles situated on graphene. Excited plasmons can decay directly to the graphene as through hot electron transfer. This dissertation begins by computational analysis of plasmon resonance energy and bandwidth as a function of particle size, shape, and dielectric environment in addition to diffractive coupled in lattices creating a Fano resonance. With this knowledge, plasmon resonance was probed with incident electrons using electron energy loss spectroscopy in a transmission electron microscope. Nanoparticles were fabricated using electron beam lithography on 50 nanometer thick silicon nitride with some particles fabricated with a graphene layer between the silicon nitride and metal structure. Plasmon resonance was compared between ellipses on and off graphene to characterize hot electron transfer as a means of plasmon decay. It was observed that the presence of graphene caused plasmon energy to decrease by as much as 9.8% and bandwidth to increase by 25%. Assuming the increased bandwidth was solely from electron transfer as an additional plasmon decay route, a 20% efficiency of plasmon decay to graphene was calculated for the particular ellipses analyzed

Optically resonant structures for the enhancement of polycrystalline PbSe photoconductors

The mid-wave infrared (MWIR) regime of the electromagnetic spectrum is attractive for long-range imaging systems due to the atmospheric window between 3 and 5 [mu]m. Due to ambient thermal background, it is often necessary to operate sensor systems below room temperature to achieve an adequate signal-to-noise ratio (SNR). This cooling requirement adds size, weight, and complexity to systems in which these parameters are at a premium. In this work I investigated two methods for optically enhancing the absorptive properties of lead selenide (PbSe) photoconductive films to increase the operating temperature up to 290 K, thereby mitigating system cooling requirements. By employing surface plasmon resonant (SPR) and embedded reflective structures, we were able to demonstrate enhanced responsivity and raise the operating temperature to room-temperature. Sensitivity was observed to increase by a factor of three for SPR enhanced detectors, and up to two-times at room temperature in detectors with an embedded Pt back reflector. Moreover, PbSe detectors with SPR discs operating at room temperature were observed to have responsivity comparable to reference detectors at 230 K. Photoconductors with the embedded Pt back reflector had a performance at room temperature that was similar to the reference detector at 250 K. Herein, I discuss my design process, as well as the fabrication of these resonant structures. Also discussed are the measurement and test results I obtained from surface plasmon and embedded reflector enhanced PbSe detectors. In this dissertation, I present results that demonstrate the viability of SPR and interference structures as mechanisms for increasing the operating temperature of PbSe MWIR photodetectors up to 290 KIncludes bibliographical reference