Alloy Plasmonics - Fundamentals and Applications

Sammanfattning: Alloys have for a long time been important in the development of our society; from the Bronze Age, where man learned how to alloy copper with tin, to today, where many products are made of steel and aluminum alloys. Similarly, but maybe not as generally well known, alloys have lately been proposed as a new paradigm in nanophotonics, to tailor optical properties of nanomaterials that find applications within telecommunication, sensing, or biotechnology. Furthermore, alloys are explored in heterogeneous catalysis to develop solutions to increase activity and selectivity of chemical processes. Nanophotonics and catalysis, separately and in combination, are the focus of this thesis. Specifically, we have compiled a library of alloy complex dielectric functions for the late transition metals by utilizing time-dependent density-functional theory. The calculated dielectric functions were benchmarked by (i) nanofabricating series of alloy nanoparticle arrays with systematically varying composition, (ii) measuring their plasmonic properties, and (iii) comparing these properties with electrodynamic simulations of alloy nanoparticles, using the dielectric function library as the input. These dielectric functions allowed us further to screen the absorption efficiency of nanoparticles of multiple combination of size and composition to show the superior performance of alloys compared to their neat constituents. The second theme in this thesis is plasmon-enhanced catalysis. In this field, there is a continuous discussion regarding the reaction enhancing mechanisms when noble metal catalyst nanoparticles are irradiated with visible light during a catalytic reaction. Here we investigated the role of photothermal enhancement of reactions by tailoring the catalytic activity of nanofabricated particles without radiation by means of alloying Pd with Au, while keeping the optical absorption cross section constant, as confirmed by electrodynamics simulations using our dielectric function library as the input. Temperature is a crucial parameter during catalysis in general and photocatalysis in specifically. However, it is intrinsically difficult to measure the temperature of nanoparticles with traditional methods. Therefore, we presented a hydrogen nanothermometry method that allows measuring nanoparticle temperature directly and noninvasively via the temperature dependent phase transition during Pd-hydride formation. We showed that the Pd particle temperature during light-induced heating can be measured with a resolution of 1 °C.

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