Nanoparticle Plasmons in Classic and Novel Materials - Fundamentals and Hydrogen Sensing

Doctoral thesis, 2009

The interaction of light with sub-wavelength noble metal nanoparticles and nanostructures has developed into one of the most vibrant themes of nanoscience during the past decade. The optical properties of metal nanoparticles are dominated by so-called nanoparticle plasmons or localized surface plasmon resonances (LSPR). These are coherent collective oscillations of the free electrons giving rise to strong absorption and scattering of light as well as strongly enhanced plasmonic fields in the particle nano-environment. Summarized under the name “plasmonics” their fundamental physical aspects are being studied and a plethora of possible applications is explored, like chemical and biological sensors, metamaterials, cancer treatments, waveguides and more efficient solar cells. In this thesis a step away was taken from the “classic plasmonic materials” gold and silver towards other metals formerly not recognized to exhibit tunable LSPR excitations. The motivation of this work was the prospect of combining interesting intrinsic material properties, like catalytic activity and metal hydride or surface oxide formation, with the presence of a strong plasmonic excitation. Experimental and theoretical studies of the plasmonic properties of nanodisks made from the heterogeneous catalysts platinum and palladium as well as from aluminum were carried out. Similarities and differences in terms of size-dependent spectral response and plasmon decay between novel and classic materials were addressed. In the later part of the thesis, two novel nanoplasmonic hydrogen sensing schemes were developed and applied to characterize nano-sized hydrogen storage materials. The sensing schemes are generic and can be applied to other sensing situations. The first scheme – direct plasmonic sensing – is based on that structural and/or electronic changes in a plasmonic nanoparticle, induced e.g. by hydrogen absorption, could be detected through an altered optical response of that nanoparticle. Thus, during a direct sensing event, the nanoparticle acted at the same time as sensor and as entity modified in the process to be detected. The second sensing scheme – indirect plasmonic sensing – employs LSPRs of gold nanodisks as “the sensor” to characterize another nanosized hydrogen storage material deposited onto the sensor. The hydrogen-induced physical change in the storage material was detected through the coupling of the Au nanodisk LSPR-field with the probed hydrogen absorbing material. A thin dielectric spacer layer separated the sensor nanodisks from the probed storing nanomaterial and the hydrogen environment. Thus the sensor itself did not interact with the hydrogen gas. Both platforms have been successfully applied to study kinetics and thermodynamics of metal hydride formation in nanoparticle systems over a wide size range (1-500 nm) and with various particle geometries, using palladium as the model system. A patent application has been filed.