| Nobel metal nanocrystals with various sizes and shapes are of great interest because of their unique plasmonic, catalytic, and magnetic properties. In particular, gold and silver nanomaterials have been the nearly exclusive subjects in plasmonics research because they support extremely high-quality plasmon resonances at optical frequencies, which is partially owing to the high densities of conduction-band electrons in them. Gold is more stable, chemically and physically, than silver, while silver possesses better plasmonic properties than gold. Both of them have been increasingly employed in diverse technological applications ranging from chemical/biochemical sensing, imaging to solar energy harvesting. The plasmon resonance energy of gold and silver nanomaterials can be easily tuned by changing their size, shape and composition. In this thesis, I provide systematic studies, experimentally and theoretically, on the fabrication of metallic nanocrystals and the investigation of their plasmonic properties.;Au nanobipyramids are nearly monodisperse and therefore their inhomogeneous spectral broadening is suppressed. In addition, the electric field enhancement exhibited by Au nanobipyramids is much stronger than that of Au nanorods with equivalent sizes owing to the much sharper tips. Such strong field enhancements make Au nanobipyramids highly preferable as building units to design structures and devices for various high-performance plasmonic applications. However, it has been a great challenge to produce Au nanobipyramids in high yields. The number yields of Au nanobipyramids are typically limited below ∼50%. This low yield prevents their full utilization in various plasmonic applications. Herein, I developed a method for the production of Au nanobipyramids with yields up to 100%. Moreover, their longitudinal plasmon wavelengths can be tuned from ∼650 nm to ∼1350 nm by combining seed-mediated growth, Ag overgrowth into longer nanorods, and depletion force-induced purification.;Compared with monometallic nanocrystals, bimetallic nanocrystals can not only introduce better optical properties than single-component ones, but also bring about new functions, such as nanoscale bar-codes and magneto-plasmon effects. Controlling the morphology of bimetallic nanocrystals are therefore of importance for their further applications. The growth behaviors of bimetallic nanocrystals on Au nanocrystals with various crystalline structures are systematically investigated. The results show that both Ag and Pd preferentially grow on the side surfaces of single-crystalline Au seeds while they prefer coat on the end surfaces of multi-twinned Au seeds. The results indeed help in understanding the growth mechanism and thereafter paving the way for finely controlling the shape of multi-metallic nanocrystals.;According to the previous works, Au/Ag heteronanorods with uniform morphology and high yield are obtained using high-purity Au nanobipyramids as seeds. As far as I know, there have been no works showing such nearly monodispere Ag nanorods. These nanocrystals can form highly assembled structures which are useful to the fabrication of optical devices. Multipolar resonances possessed by these heteronanorods are found to sustain asymmetric Fano lineshapes. The linewidths of these higher-order plasmon resonances are much narrower than the dipolar plamson resonance on one spectrum. The excellent multipolar plasmon resonance properties make these heteronanorods function as better candidates for single-particle plasmonic sensors and reduced-threshold nanolasers.;Although the frontier of plasmonics is far beyond what I have studied, the experimental investigations are believed to be rather helpful for fundamentally understanding the localized plasmon resonances exhibited by metal nanocrystals, and providing more possibilities for applications in biotechnology, photovoltaic devices, as well as advanced photonic circuits. |
