Synthesis and Ultrafast Optical Characterization of Solid State and Functional Metal Nanocluster

[摘要] Metal nanoclusters have been extensively researched in the past few decades due to their unusual optical and physicochemical properties. Optical and theoretical investigations on these quantum metal nanoclusters allowed researchers to understand the origin of their properties and the evolution of optical properties with size. Optical characterizations of metal nanoclusters have revealed a lot about the electronic structures of metal nanoclusters. However, most of these investigations were conducted in the solution phase, while many applications will require solid state materials. This dissertation is aimed at the investigation of the effect of cluster-cluster and cluster-chromophore coupling on the optical properties of solid state and assemblies of metal nanoclusters. Metal nanoclusters in the solid state were prepared by embedding the nanoclusters into a polymer matrix. The use of a polymer host allowed different nanocluster densities to be made, thus resulting in different inter-cluster distances. The nanocluster films were investigated by linear and non-linear optical spectroscopy. The fluorescence and two-photon absorption cross-sections of the nanocluster films are greatly enhanced compared to the solution phase nanoclusters. These results indicate that there is a strong dipole coupling between the nanoclusters due to the short inter-cluster distances, and possible energy transfer between the nanoclusters. Similarly, metal nanocluster assemblies and architectures have been explored for practical applications due to the possibility of observing collective properties. On the fundamental science’s point of view, the super-atom concept of metal nanoclusters means that they can add a third dimension to the periodic table, and used as building blocks for super-molecules. Presented in this dissertation is the optical characterization of a type of nanocluster assembly, metal nanoclusters linked with an organic linker.A chromophore was used as the linker to add functionality to the material.The correlation between the chromophore-Au25 nanocluster oligomer length and molecular geometry with their optical properties was analyzed. The chromophore-Au25 nanocluster oligomers showed larger transition dipole moment, which results in a two-photon absorption enhancement of up to 68 times. An analysis of the molecular geometry around the cluster-chromophore-cluster bonds reveals that the optical properties of the oligomers are very dependent on the molecular geometry. Therefore, solid state metal nanoclusters and nanocluster assembly materials are promising candidates for applications such as small molecular devices and protective coating for optical limiting.Finally, this dissertation also aims to demonstrate the use of metal nanoclusters for photodynamic therapy application. Due to the strong quantum confinement effects of metal nanoclusters, the existence of triplet excited states and strong absorption, the nanoclusters are good candidates for singlet oxygen photosensitization. The rate of singlet oxygen generation is compared against three different metal nanoclusters and the electronic structure analysis of the metal nanoclusters indicate that high absorption-to-volume ratio nanoclusters are a select group of metal nanoclusters that will show high singlet oxygen formation efficiency. Furthermore, photodynamic therapy by two-photon excitation (800 nm) is demonstrated to be more effective than one-photon excitation (400 nm) due to the large two-photon absorption cross sections of metal nanoclusters. Thus, metal nanoclusters can be used as a dual agent for effective photodynamic therapy treatment and high-resolution imaging.The dissertation is closed by an analysis of the non-linear optical properties of the highly fluorescent bimetallic Au@Ag nanoclusters and future directions regarding the study of these bimetallic nanoclusters and the fabrication and optical characterization of solid state metal nanoclusters-conjugated polymer films.

[发布机构] University of Michigan

[效力级别] solid state

[关键词] metal nanocluster;solid state;two photon absorption;Chemistry;Science;Chemistry