UC Riverside

Noble metal nanostructures with tunable localized surface plasmon resonance (LSPR) frequency have attracted significant attention due to their broad applications in chemical sensing, photothermal therapy, and energy conversion. Because the resonance is sensitive to the particle geometry, a significant amount of efforts have been made to develop non-spherical metal nanostructures which provide access to many interesting plasmonic absorption and scattering phenomena that are not seen in spherical ones. Although conventional wet chemical methods have been proved to be powerful for the synthesis of non-spherical nanostructures, it still has limitations of being only applicable to specific materials and lacking effective mechanisms for controlling the orientation of the nanostructures collectively. In this dissertation, we discuss our unconventional methods for the synthesis of non-spherical plasmonic nanostructures.

Firstly, we developed a seed-mediated growth for creating secondary structures of Au on spherical Au seeds. The formation of nanoislands is induced by modifying the seeds with Pt to create a mismatched surface for metal deposition. The number of the islands on each seed can be controlled by adjusting the reaction parameters from three aspects: the strain energy, the degree of oxidative ripening and the diffusion of metal precursors on the surface of the metal seed. It is worth noting that ligand-assisted ripening, should be considered as an important factor to tune the structure of the nanoparticle. By integrating all the important parameters, Au satellite structure, dimers, trimers and tetramers were obtained. It is worth noting that the Au dimers synthesized by using this method is of controllable particle size, high purity, well-defined structures and unique optical property, which has not been reported previously.

Through the study of the optical properties of dimers, we found out that the plasmonic resonance of dimers could be separated into two modes: a transverse mode and a longitudinal mode, which can be excited by light polarized along the short side and the long side of the dimers, respectively. The two modes correspond with light absorption at different wavelengths, which have never been carefully studied and utilized before. The key to split the resonant mode of dimers is to control their orientation under polarized light. To control the orientation of the dimer structures, we developed a stepwise seeded growth method for the synthesis of self-registered anisotropic plasmonic nanostructures. The synthesis requires using colloidal nanoparticle as substrates for the assembly of the metal seeds, and precise control over the exposure of the metal seeds by surface passivation. The anisotropic seeded growth of metal nanostructure can be induced by creating a physical barrier on the seeds, which partially passivates the exposed area of the seeds. Au dimers, linear trimers and Au-Ag dimers with distinguishable plasmonic excitation bands have been synthesized. This method is very versatile for the synthesis of other metals and semiconductors, for example, Pd and Cu2O.

This synthesis method results in rod-like nanostructures perpendicularly registered to the substrates. Taking advantages of this unique feature, we report dynamic tuning of the plasmonic excitation of Au-Au and Au-Ag dimers by controlling their orientation relative to the incident light. Such tuning is enabled by switching the substrate to anisotropic magnetic nanoparticles, whose orientation can be magnetically controlled. By tuning the direction of the magnetic field, we are able to control the excitation of plasmonic modes of the Au-Au dimers and Au-Ag dimers under the incidence of polarized light. The optical switching of Au-Au and Au-Ag dimers exhibits bright colors with high contrast. The nanostructures with orthogonal orientations are fixed in the hydrogel to obtain anti-counterfeiting patterns.