| Noble metal nanocrystals have attracted much interest due to their rich optical properties, which arise from the localized surface plasmon resonances, the collective oscillations of free electrons confined on the nanoscale. Under resonant excitation by light, noble metal nanocrystals exhibit extremely large light scattering and absorption, as well as large near-field enhancements. These fascinating properties bring about a variety of applications, including plasmonic sensing, plasmonic waveguiding, surface-enhanced Raman scattering, plasmon-enhanced fluorescence, photothermal cancer therapy, and plasmonic-enhanced energy harvesting. Among various noble metal nanocrystals, gold nanocrystals exhibit high chemical stability and large biological compatibility. Moreover, their plasmon resonance wavelengths can be synthetically tuned from the visible to near infrared spectral regions. In this thesis, a systematic study on the localized surface plasmon resonances of gold nanocrystals is presented, both experimentally and theoretically.;The refractive index sensitivity of gold nanocrystals is a key factor in their practical sensing applications. I will first introduce the systematic studies on the dependence of the index sensitivity on the shapes and sizes of gold nanocrystals that have varying plasmon resonance wavelengths. The index sensitivity has been found to generally increase as the plasmon resonance wavelength for a fixed nanocrystal shape becomes longer and as the curvature of the nanocrystals gets larger. I have further studied the dependence of the index sensitivity on the different shapes of gold nanocrystals that have the same longitudinal plasmon resonance wavelength. The refractive index sensitivities have been found to vary with the nanocrystal shape. Finite-difference time-domain (FDTD) calculations have been performed on these nanocrystals to reveal the origin of this dependence. A linear relationship is found between the index sensitivity and the product of the electric polarizability with the curvature. On the basis of these studies, a novel plasmonic optical fiber device has further been fabricated to detect small changes in the local dielectric environment.;Assembly of noble metal nanocrystals gives rise to extraordinary plasmonic properties that are distinct from those of isolated ones. We have prepared clusters that are composed of two-dimensionally-ordered gold nanocubes on flat substrates and investigated their plasmonic properties. It is found that the plasmon resonances of the nanocube clusters are highly dependent on both the number and ordering of the nanocubes in the clusters. FDTD calculations reveal that the rich plasmon modes in the clusters originate from the interparticle couplings in the cluster and the couplings between the entire clusters and the substrate.;When fabricating plasmonic devices, such as waveguides, optical switches, plasmonic sensors, and plasmon-enhanced solar cells, one needs to attach metal nanocrystals onto different substrates. The interactions between gold nanocrystals and the substrates can strongly modify the plasmonic responses of the nanocrystals and therefore need to be taken into account when designing of various plasmonic devices. We have further investigated the coupling between gold nanocrystals and substrates with different dielectric properties, including insulating, semiconducting, and metallic ones. It is found that the substrates play an important role in both the scattering patterns and scattering spectra of the supported gold nanocrystals. Specifically, Fano-type resonances can be observed for large nanocrystals sitting on silicon substrates that have a large dielectric constant.;Photothermal conversion of gold nanocrystals can be applied in the areas of photothermal polymerization, photothermal imaging, drug release from capsules, and photothermal therapy. We have investigated the photothermal conversion properties of different gold nanocrystals and their composites. The studies show that the plasmon wavelength, particle volume, shell coating, and assembly of gold nanocrystals all play important roles in their photothermal conversion efficiency.;I believe that my research work has provided an in-depth fundamental understanding of the localized surface plasmon resonances of gold nanocrystals and will have a number of implications for the applications of metallic nanostructures in optics, optoelectronics, and biotechnology. |
