SYNTHESIS AND POLYMER-MEDIATED REGIOSELECTIVE SELF-ASSEMBLY OF SHAPED PLASMONIC NANOPARTICLES

Abstract

Plasmonic nanoparticles with collective excitations of the conduction-band electrons have many potential applications in surface­enhanced spectroscopies, photocatalysis, photovoltaics, and biomedicine. The plasmonic properties are highly tunable via the size, shape, chemical composition, and surrounding media of individual nanoparticles, as well as by the interactions with other nanoparticles in close proximity. Fabricating plasmonic nanostructures with precisely controlled size, shape, and interparticle distance is critical for harnessing desirable properties. Although top-down lithographic techniques are widely used for fabricating shaped plasmonic nanoparticles and ensembles, the products are limited to 2D planar structures and the cost can be prohibitive. As low-cost and versatile alternatives, colloidal synthesis and self-assembly have been explored to fabricate complex plasmonic nanostructures.In this dissertation, wet-chemistry synthetic and self-assembly approaches are explored for fabricating well-defined plasmonic nanostructures with high structural complexity. Chapter 2 describes a facile synthetic method for circular and triangular gold nanorings with tunable diameters, ring thicknesses, surface roughness, and hence the plasmonic response. The gold nanorings with rough surface show 100-fold higher enhancement factor than solid gold nanoparticles as substrate for surface-enhanced Raman spectroscopy. In chapter 3, we demonstrate regioselective bonding between nanospheres and nanoplates originating from the steric hindrance of polymeric ligand brushes and the anisotropy of nanoparticles. The regioselectivity enables a self-assembly system with precise control over the relative orientation of Au nanospheres on Ag nanoplates and the stoichiometry of reactive groups of copolymeric ligands dictates the number of nanoparticles in one nanocluster. The yield of each assembly was ~70% without further purification. Optical study reveals that different bonding modes affect the plasmonic coupling of assembled structures. In chapter 4, the regioselective bonding is applied to fabricate complex plasmonic nanocluster, nanoflowers and nanobuds, with distinct bonding modes. Compared with nanobuds, nanoflowers with the same number of petals show stronger electric field enhancement and further localized surface plasmon resonance peak shifts. The synthetic and self-assembly methods demonstrated in this dissertation have great potentials and versatility in designing not only plasmonic nanoclusters, but also other inorganic nanoparticle-based functional structures with high complexity.