Hydrogen in metal nanoparticles : understanding and applying thermodynamic properties of metal-hydrogen nanostructures

The mobility sector is undergoing a fundamental change from fossil fuels through electricity to hydrogen. However, for hydrogen technology to be successful, the storage devices need to be pushed forward. Currently, the most promising path is to employ nanotechnology in metal hydride storage systems. This thesis presents different methods and material systems exploring the interaction of metallic nanoparticles and hydrogen. It aims to expand the limited literature knowledge about size dependent effects on thermodynamic and optical properties at the nanoscale. Several analytical and numerical models are developed and compared to own experimental data as well as existing literature. The experimentally investigated structures are palladium square patches, palladium-gold disk stacks, and yttrium nanorods. All structures throughout the thesis are characterized using plasmonic extinction spectroscopy, an optical measurement technique employing localized oscillations of the conduction electrons as a sensitive tool for structural and electronic changes in nanoparticles. The palladium square-patch investigations show a hydrogen loading pressure that is increasing with nanoparticle size, whereas the hydrogen induced in-plane expansion is decreasing with size. In the yttrium rod antenna studies, a drastic but reversible hydrogen induced elimination of the plasmonic resonance is observed, rendering the structure a highly interesting plasmonic switch. A sensitive plasmonic gas sensor is realized combining palladium nanoparticles with gold antennas. Through palladium-gold disk nanostacks that plasmonically behave as one superstructure, large hydrogen induced peak shifts of comparatively narrow resonances are demonstrated. Complementing the experimental findings, analytical models are developed for the isotherms of palladium nanoparticles and the plasmonic resonances of square nanopatches. The isotherm model reveals a coherent loading mechanism of palladium nanoparticles. In contrast, the unloading mechanism and the general bulk behavior follow incoherent transitions with a reduced hysteresis. The developed plasmon resonance model illustrates a method for obtaining broadband dielectric data of nanoparticles without prior knowledge of any material properties besides the particle geometry and the plasmon resonance wavelength. The findings presented in this thesis will be helpful to develop more efficient energy storage systems and powerful hydrogen sensors through well designed nanostructured devices.Mit der Verdrängung klassischer Verbrennungsmotoren durch elektrische Antriebe mit Batterie- oder Brennstoffzellentechnologie ist ein weltweiter fundamentaler Umbruch im Automobilsektor im Gange. Um die Brennstoffzellentechnologie jedoch zu einem langfristigen Erfolg zu führen, sind Innovationen in der Wasserstoffspeicherung notwendig. Eine Schlüsselrolle sollen dabei neue hocheffiziente Wasserstoffspeicher auf Metall-Hydrid Basis spielen. In dieser Dissertation werden metallische Nanopartikel und ihre Interaktion mit Wasserstoff mit Hilfe unterschiedlichster Methoden und auf Basis mehrerer Materialsysteme untersucht. Das Ziel ist dabei das begrenztes Wissen über größenabhängige Effekte thermodynamischer und optischer Eigenschaften auf der Nanoskala zu erweitern. Zu diesem Zweck werden unter anderem mehrere analytische und numerische Modelle entwickelt und mit eigenen experimentellen Daten sowie Literaturdaten verglichen. Experimentelle Studien, durchgeführt an Palladium Nanoquadraten, Palladium-Gold Nanoscheiben-Stapeln und Yttrium Nanostäben mit Hilfe von plasmonischer Extinktions-Spektroskopie, zeigen optische, elektronische und strukturelle Eigenschaften der jeweiligen Systeme und die dazugehörige wasserstoffabhängige Dynamik. So zeigt sich bei den untersuchten Palladium Nanoquadraten, dass der Wasserstoff Ladedruck mit zunehmender Größe der Teilchen zunimmt, während die wasserstoffinduzierte laterale Größenzunahme mit der Teilchengröße abnimmt. Die Untersuchungen an Yttrium Stabantennen zeigen ein drastisches aber reversibles wasserstoffabhängiges Ausschalten der plasmonischen Resonanz, was die Strukturen zu einem interessanten Kandidaten für einen plasmonischen Schalter machen. Des Weiteren wurde ein plasmonischer Wasserstoffsensors bestehend aus einer Kombination von Palladium und Gold Nanoscheiben realisiert und charakterisiert. Dabei zeigten sich abhängig von der Anordnung relativ große wasserstoffinduzierte Resonanzverschiebungen und gutes dynamisches Verhalten. Komplementär zu den durchgeführten experimentellen Studien wurden analytische Modelle für die Isothermen von Palladium Nanopartikeln, die Wasserstoffdiffusion in Yttrium Nanostäben und die plasmonischen Resonanzen in Palladium Nanoquadraten entwickelt. Das Isothermen Model offenbart, dass Palladium Nanopartikel bis zu einer gewissen Größe während der Wasserstoffabsorption einem kohärenten Phasenübergang folgen, während die Desorption von einem inkohärenten Phasenübergang begleitet wird, wie es bei großflächigem Palladium üblich ist. Das neuentwickelte Modell für plasmonische Resonanzen von Nanoquadraten kann dazu verwendet werden die dielektrische Funktion von Nanostrukturen über einen breiten Wellenlängenbereich experimentell zu bestimmen - nur durch Kenntnis der Geometrie und plasmonischen Resonanz der Strukturen aber ohne spezielle Annahmen über das Material zu machen. All die Erkenntnisse dieser Thesis können als kleine aber erkenntnisreiche Schritte zur Entwicklung effizienterer Energiespeicher und leistungsstarker Wasserstoffsensoren auf Nanostrukturbasis betrachtet werden

Long-term stability of capped and buffered palladium-nickel thin films and nanostructures for plasmonic hydrogen sensing applications

One of the main challenges in optical hydrogen sensing is the stability of the sensor material. We found and studied an optimized material combination for fast and reliable optical palladium-based hydrogen sensing devices. It consists of a palladium-nickel alloy that is buffered by calcium fluoride and capped with a very thin layer of platinum. Our system shows response times below 10 s and almost no short-term aging effects. Furthermore, we successfully incorporated this optimized material system into plasmonic nanostructures, laying the foundation for a stable and sensitive hydrogen detector

Large-Area Low-Cost Plasmonic Perfect Absorber Chemical Sensor Fabricated by Laser Interference Lithography

We employ laser interference lithography as a reliable and low-cost fabrication method to create nanowire and nanosquare arrays in photopolymers for manufacturing plasmonic perfect absorber sensors over homogeneous areas as large as 5.7 cm(2). Subsequently, we transfer the fabricated patterns into a palladium layer by using argon ion beam etching. Geometry and periodicity of our large-area metallic nanostructures are precisely controlled by adjusting the interference conditions during single- and double-exposure processes, resulting in active nanostructures over large areas with spectrally selective perfect absorption of light from the visible to the near-infrared wavelength range. In addition, we demonstrate the method's applicability for hydrogen detection schemes by measuring the hydrogen sensing performance of our polarization independent palladium-based perfect absorbers. Since palladium changes its optical and structural properties reversibly upon hydrogenation, exposure of the sample to hydrogen causes distinct and reversible changes within seconds in the absorption of light, which are easily measured by standard microscopic tools. The fabricated large-area perfect absorber sensors provide nearly perfect absorption of light at 730 and 950 nm, respectively, and absolute reflectance changes from below 1% to above 4% in the presence of hydrogen. This translates to a relative signal change of almost 400%. The large-area and fast manufacturing process makes our approach highly attractive for simple and low-cost sensor fabrication, and therefore, suitable for industrial production of plasmonic devices in the near future

Yttrium Hydride Nanoantennas for Active Plasmonics

A key challenge for the development of active plasmonic nanodevices is the lack of materials with fully controllable plasmonic properties. In this work, we demonstrate that a plasmonic resonance in top-down nanofabricated yttrium antennas can be completely and reversibly turned on and off using hydrogen exposure. We fabricate arrays of yttrium nanorods and optically observe, in extinction spectra, the hydrogen-induced phase transition between the metallic yttrium dihydride and the insulating trihydride. Whereas the yttrium dihydride nanostructures exhibit a pronounced particle plasmon resonance, the transition to yttrium trihydride leads to a complete vanishing of the resonant behavior. The plasmonic resonance in the dihydride state can be tuned over a wide wavelength range by simply varying the size of the nanostructures. Furthermore, we develop an analytical diffusion model to explain the temporal behavior of the hydrogen loading and unloading trajectories observed in our experiments and gain information about the thermodynamics of our device. Thus, our nanorod system serves as a versatile basic building block for active plasmonic devices ranging from switchable perfect absorbers to active local heating control elements

Niobium as Alternative Material for Refractory and Active Plasmonics

The development of stable compounds for durable optics is crucial for the future of plasmonic applications. Even though niobium is mainly known as a superconducting material, it can qualify as an alternative material for high-temperature and active plasmonic applications. We utilize electron beam lithography combined with plasma etching techniques to fabricate nanoantenna arrays of niobium. Tailoring the niobium antenna geometry enables precise tuning of the plasmon resonances from the near- to the mid-infrared spectral range. Additionally, the hydrogen absorptivity as well as the high-temperature stability of the antennas have been investigated. Further advantages of niobium such as superconductivity make niobium highly attractive for a multitude of plasmonic devices ranging from active and refractory perfect absorbers/emitters to plasmon-based single photon detectors

Nonlinear Refractory Plasmonics with Titanium Nitride Nanoantennas

Titanium nitride (TiN) is a novel refractory plasmonic material which can sustain high temperatures and exhibits large optical nonlinearities, potentially opening the door for high-power nonlinear plasmonic applications. We fabricate TiN nanoantenna arrays with plasmonic resonances tunable in the range of about 950–1050 nm by changing the antenna length. We present second-harmonic (SH) spectroscopy of TiN nanoantenna arrays, which is analyzed using a nonlinear oscillator model with a wavelength-dependent second-order response from the material itself. Furthermore, characterization of the robustness upon strong laser illumination confirms that the TiN antennas are able to endure laser irradiation with high peak intensity up to 15 GW/cm² without changing their optical properties and their physical appearance. They outperform gold antennas by one order of magnitude regarding laser power sustainability. Thus, TiN nanoantennas could serve as promising candidates for high-power/high-temperature applications such as coherent nonlinear converters and local heat sources on the nanoscale