Synthesis, Shape Control, and Preservation of Novel Noble Metal Plasmonic Nanostructures

Metallic structures at the nanoscale exhibit numerous properties which enable fascinating uses. Plasmonic responses, electromagnetic field generation, chemical sensing and catalytic capabilities are facilitated by the appropriate implementation of metallic nanoparticles. These properties can be enhanced significantly by strategic morphology and placement control; namely selective faceting, and nanogap formation. While colloidal syntheses have been explored in great detail throughout the literature, providing a vast selection of available morphologies which enable facet selection, controllable placement of the structures remains challenging for colloidally synthesized particles. Conversely, substrate-immobilized structures such as those achieved through thin film dewetting can provide excellent placement capabilities, through lithography processes, though the ability to control morphology remains underdeveloped. Even provided acceptable syntheses, the challenge of preserving such structures remains, as material properties of nano-dimensioned structures differ significantly from our understanding of those in bulk materials. Thermal degradation, as well as oxidation and other chemical reactions tend to alter the nanostructures much more significantly, rendering many materials unfeasible for the applications for which they may otherwise be selected. This dissertation provides means by which each of these challenges can be addressed.

It is demonstrated that the faceting of dewetted metallic structures can be enhanced by subjecting them to a liquid-phase chemical environment in which metal ions are reduced and deposited on the nanostructure surface in a manner that leads to facet formation. The resultant structures exhibit sharper corners which far surpass those which can be produced by thermal routes alone. The faceting procedure, which can be carried out in minutes, is also shown to be amenable to a templated dewetting approach in which arrays of lithographically defined nanostructures are formed. The work has the potential to increase the functionality of dewetted nanostructures by enabling facet dependent chemical reactivity and enhanced plasmonic hot spots.The functionality of such well-tailored nanomaterials can only be retained if they are robust to the environmental factors in which they operate. Herein, it is demonstrated that atomic layer deposition can be used as a pliable technique for the application of oxide coatings to substrate-based metallic nanostructures. Emphasis is placed on Cu, which is typically rejected as a feasible material despite having favorable plasmonic and electronic properties at low cost. It is shown that suitably protected structures become robust to oxidation, high temperatures, and aqueous, acidic, and alkaline solutions without unduly influencing important plasmonic properties. Moreover, strategies are presented for maximizing plasmonic near-fields and allowing for the transport of hot electrons while maintaining coating integrity. Within the scope of the investigation, alumina, hafnia, titania, and combinations thereof are all shown to be effective under certain conditions, but where hafnia shows the greatest durability in extreme pH environments, enabling LSPR preservation for over 100 days in 1 M HNO3 or NaOH. Alumina-hafnia laminates provide optimal thermal protection against oxidation and morphology altering surface diffusion, to temperatures as high as 600 °C.

Strategic application and removal of ALD alumina is then used to facilitate the synthesis of advanced nanogap structures. The fabrication method enables an easily tunable nanogap width, at angstrom level precision. The resultant trimer structures form parallel to the substrate and show preferred alignment, which enables optimal coupling under surface-normal light, and polarization dependent LSPR spectra, which are of importance to various optical sensing modalities. The trimer geometry enables high density of nanogaps on the substrate, at 4.9 billion gaps per square centimeter, a feature of great importance to photocatalysis and SERS. The superior crystallinity of these trimer structures is also notable, as many other nanogap syntheses produce only polycrystalline structures.

The combined scientific achievements demonstrated in this dissertation serve to further the capabilities of nanoparticles for real-world applications: Greater morphology control over substrate-immobilized nanostructures, preservation of the shape-engineered structures, and advanced synthesis methods are developed, each of which may serve independently or in tandem to greatly advance the capabilities of applied nanotechnology.