Plasmonic Nanostructures for Optical Absorption Engineering and Hydrogen Sensing

Doctoral thesis, 2015

Nanoplasmonics concerns the interactions between light and metal nanoparticles. In this thesis nanoplasmonics is used in two different ways: i) to control and enhance light absorption, and ii) for optical hydrogen sensing. The light absorption engineering track is motivated by the idea to optically "drive" catalytic reactions via plasmon induced hot electrons in metal nanoparticles, created upon light absorption. Initial experimental efforts indeed report on the observation of this effect, but a deeper understanding of the process is still lacking. Furthermore, these studies have so far been limited to classical plasmonic materials such as Au, Ag, or Cu; limiting widespread application in heterogeneous catalysis. Therefore, in this thesis a different approach is used, namely making hybrid structures consisting of a single or several catalytic Pd nanoparticle(s) and a Ag or Au plasmonic nanoantenna, harvesting light and channeling the excitation into the catalyst particles to maximize light absorption. These studies are conducted through a combination of theoretical modeling and experimental observations of light absorption in the various nanostructures. The second part of the thesis uses plasmonic resonances to investigate interactions between hydrogen and metal nanoparticles, with a special focus on metals and metal alloys capable of forming metal hydrides. This is done both to reach deeper understanding of the mechanisms of the hydride formation in nanoparticles; e.g. what is the influence of nanoparticle size, shape, and chemical composition; but also with more practical applications in mind. Hydrogen sensors are of vital importance for secure use of hydrogen in, for instance, chemical processing. Moreover, with the prospect of a hydrogen economy recently boosted by the market introduction of hydrogen fuel cell cars, this kind of sensor is attracting even more attention. Here the use of plasmonic metal hydride nanostructures as optical hydrogen sensors is explored. A general topic of the thesis is the fabrication of advanced nanostructures, used both within the optical absorption engineering and the hydrogen sensing. These efforts result in the introduction of "shrinking-hole colloidal lithography", a new nanofabrication technique (based on colloidal lithography) for complex nanostructures consisting of multiple elements and materials.