Highly Controlled Synthesis of Au-Ag Head-Body Nanostructures and Study of Their Geometry-dependent Optical Properties

Abstract

Noble metal nanoparticles (Au, Ag or Cu) have been extensively exploited due to their unique applications in various fields that range from optics, electronics, energy conversion to biomedicine. Most of fascinating properties of noble metal nanoparticles come from localized surface plasmon resonances (LSPR) which is a coherent collective oscillation of electrons in a conduction band excited by the oscillating electric field of the<br/> incident light. The resonance frequency LSPs are highly dependent on the size, shape, composition of nanoparticles, small changes in interparticle distance and dielectric environment. Therefore, designing and synthetic strategies of plasmonic nanostructures allow us to manipulate, enhance and utilize the various properties. Recently, the full potential of plasmonic nanoparticles has been expanded from single<br/> component which has monotonic and confined properties to bio or multiple component in forming and utilizing its structure and applications. Compared to single component nanoparticles, bimetallic nanoparticles can generate more diverse and new properties, emerging from integration and synergistic interactions of physical and chemical properties of individual component in combined system. Especially, bimetallic<br/> nanoparticles can enhance the optical signals such as Raman scattering or fluorescence depending on the internal structures in a similar way to the pair of nanoparticles separated in a nanometer distance. The important geometric factor to enhance such an optical signal is internal nanocrevices or deliberately<br/> formed nanogap structures by amplifying electromagnetic fields at hot-spot regions. Particularly, formation of sharp nanocrevice or narrow gap around 1 nm is significant to facilitate and enhance the effective plasmonic coupling. Among the various kinds of bimetallic nanostructures, plasmonic structures, asymmetrically connected through a conductive junction can support diverse plasmon modes which can be explained by plasmon hybridization model. The symmetry breaking in size or composition of the nanoparticle and following incomplete cancellation of charges support exceptional antibonding plasmon mode, accompanying a onding plasmon mode. However, it is highly challenging to synthesize plasmonic bimetallic nanostructures with a delicate structural control, and high reproducibility as the energy level is not preferred in a thermodynamics point of view. Especially, controlling the sharpness, thickness and width of conductive junction in bimetallic nanostructures enables us to control the optical, and electrical characteristics of bimetallic material, so that<br/> fundamental understanding of synthetic mechanism and developing the controlling methods should be emphasized. In this thesis, various nanostructures possessing a conductive junction has been designed and synthesized by utilizing DNA-based synthetic strategies. Especially, Au-Ag head-body structures with highly controllable conductive junctions and resulting plasmonic properties depending on their geometries will be introduced. In the synthesis, DNA based-kinetic control enabled the anisotropic growth of secondary metal (Ag) on the Au surface under the appropriate concentration of reducing agent, precursor, salt and pH value in a combinatory manner. Hence, these kinetics factors tailor the final morphologies of Ag body structures in Au-Ag head-body into a sphere, plate, truncated right bipyramid or bar, accompanying diverse junction<br/> geometries with different length and thickness. Through the fine control of the conductive junction, we could investigate the relationships in structural changes, EM field enhancement and signal amplification. This highly precise and controllable synthesis is expected to open revenues and provide new insights for plasmonic nanostructures which can be applied in variety of fields.