Browsing by Author "Strohfeldt, Nikolai"

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Open Access
Hydrogen in metal nanoparticles : understanding and applying thermodynamic properties of metal-hydrogen nanostructures
(2017) Strohfeldt, Nikolai; Giessen, Harald (Prof. Dr.)
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.
Open Access
Long-term stability of capped and buffered palladium-nickel thin films and nanostructures for plasmonic hydrogen sensing applications
(2013) Strohfeldt, Nikolai; Tittl, Andreas; Giessen, Harald
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.