Controlled assembly of metal nanoparticles with enhanced optical properties.

摘要

Controlled synthesis of metal nanoparticles is an important area of study because of the unique size and shape dependent properties of such nanoparticles. For example, gold and silver nanoparticles show distinct surface Plasmon resonance (SPR) bands varying with the size, shape and surface morphology of the nanoparticles. The assembly of these nanoparticles usually exhibit interesting collective optical properties different from their individual components. However, controlled synthesis and assembly of metal nanoparticles with high yield and enhanced optical properties is still challenging. In this thesis, I presented the synthesis of ultrathin gold/silver nanowires and sub-50 nm gold triangular nanoprisms by adopting silver seeds and benzyl dimethyl hexadecyl ammonium chloride (BDAC) in a seed-mediated growth method. Furthermore, I developed a templated surfactant-assisted seed growth method to synthesize gold nanoshells with varying surface morphologies by changing the type of surfactants and ions in the growth solution. The optical properties of the nanoshells could be controlled over a wide wavelength range by varying the surface morphology. By using a combination of BDAC surfactant and template surfactant-assisted seed growth method, isotropic shell-type gold nanoparticle clusters, or raspberry-like meta-molecules (raspberry-MMs), were successfully synthesized. The raspberry-MMs exhibited interesting far field and near field optical properties. The raspberry-MMs showed unusually strong magnetic resonances, yielding broad SPR bands in the visible and near-IR region. Both experimental data and finite-difference time-domain (FDTD) simulations showed that the magnetic dipole can be even larger than that of the electric dipole resonance in large raspberry-MMs. Moreover, I utilized Raman spectroscopy as a tool to probe the near field optical properties of the raspberry-MMs. Due to the existence of a large number of hotspots at the gaps among gold beads within individual raspberry-MMs, individual raspberry-MMs proved to be efficient Raman substrate. Importantly, the contribution from hotspots created at the gap between two adjacent raspberry-MMs is negligible compared to that from a large number of hotspots on individual raspberry-MMs. Consequently, the Raman signal intensity of the analyte molecules on the raspberry-MM dimers exhibited quite narrow distribution and is weakly dependent on the distance between the two raspberry-MMs. This paved the way for fabricating large-area raspberry-MM films which can serve as macroscopic efficient and reproducible Raman substrate. The robustness and tunability of the synthetic method presented in this thesis, the strong magnetic responses of the raspberry-MMs and the efficient Raman enhancement from single raspberry-MM, raspberry-MM dimer and films can lead to large-scale manufacture and wide applications of magnetic metamaterials as well as commercialization of uniform Raman substrate.