| This thesis describes the establishment of a new, universal " size focusing" methodology for the synthesis of atomically precise gold nanoparticles with strict stoichiometries (denoted as Au n(SR)m, where SR refers to thiolate). A major principle discovered in the thesis work lies in that the different sized Aun(SR)m nanoclusters exhibit vastly different stability, that is, certain stoichiometries (sometimes called magic numbers) of Aun(SR) m, possess extraordinary stability compared to other sizes. This new discovery has led to the development of a size focusing synthetic methodology, in which the robustest nanoparticle is picked out through the size focusing process. By adjusting the reaction parameters (e.g. solvent, reaction temperature, growth kinetics, and ligand bulkiness, etc.), the size distribution of the starting material (i.e. size mixed nanoclusters) prior to size focusing can be controlled in a proper range, which is critical for size focusing into one-size of robust nanoclusters. Using this methodology, a number of gold nanoclusters with strict stoichiometries have been successfully prepared with size spanning from n=25 to 333 (equivalent diameter 1.0 to 2.2 nm). In the thesis, Chapter 1 first gives a general introduction of the synthesis, characterization and properties of thiolate-protected gold nanoparticles. In Chapter 2, the Au25 nanorods (∼1 nm x 0.5 nm) and nanospheres (∼1 nm) were synthesized from phosphine capped, polydisperse Au nanoparticles (1--3 nm) via one phase and two phase routes, respectively. Chapter 3 focuses on the synthesis of palladium and platinum doped 25-atom and 38-atom gold nanoclusters, which allows fine tuning and deeper understanding of the electronic and optical properties of such nanoclusters. In Chapter 4, the "size focusing" methodology was used to prepare the Au 33(SC2H4Ph)24 nanoclusters with molecular purity, which has led to successful crystallization and structure determination. Surprisingly, a pair of enantiomeric nanoclusters (R- Au 38(SR)24 and L- Au38(SR) 24) was found in the unit cell. In Chapter 5, two new gold-thiolate nanoclusters Au40(SC2H4Ph)24 and Au55(SC2H4Ph)31, were successfully isolated via size exclusion chromatography (SEC). Chapter 6 discusses the synthesis and properties of an even larger Au144(SR)60 nanocluster, which possesses a diminishing HOMO-LUMO gap and non-fcc structure, but the Au144(SR)60 nanocluster is still not large enough to exhibit plasmonic behavior. The first example of plasmonic gold nanocrystal molecules with a formula of Au333(SR)79 is presented in Chapter 7. This nanocrystal molecule exhibits fcc-crystallinity and surface plasmon resonance at ∼520 nm in the optical spectrum. Overall, in this thesis a successful "size focusing" methodology has been established for preparing a series of "perfect" Aun(SR) m nanoparticles with strict stoichiometries. The various sizes attained thus far, including Au25(SR)18, Au38(SR) 24, Au40(SR)24, Au55(SR)31, Au144(SR)60, and Au333(SR)79, have explicitly demonstrated the evolution of the structural, optical and electronic properties with the number of gold atoms in the particle. These well defined nanoparticles hold great promise in many applications such as catalysis. |
