Formation Mechanism and Aggregation of Atomic Clusters for Optical Applications

Metal nanoclusters (NCs), an important class of nanoparticles (NPs), are extremely small in size and possess quasi-molecular properties, exhibiting unique photophysical and chemical properties. Due to accurate stoichiometry of constituent atoms and ligands, NCs have strong structure-property relationship. However, the synthetic procedure of metal NCs is highly convoluted. Reactive ligands can convert metal salts to complexes, actual precursors to metal NCs. This process results in various metal–ligand complexes, which have different coordinations of metal with ligand molecules. Those various metal species have different reactivity and fraction depending on synthetic conditions. It can alter their degree of participation in NC synthesis and the homogeneity of final products. Indeed, NC synthesis occurs through sequential transformations into a series of intermediate NCs before the formation of the desired NCs. We investigate the effects of complex formation on the entire NC synthesis and important roles of ligands in the overall synthesis of NCs. We find that the mechanistic understanding for the formation of the representative NCs (Au NCs) can be universally applied to synthesize Ag, Pt, Pd, and Rh NCs. Furthermore, NCs are prone to the guided aggregation by additional chelating ions. Ligand molecules, again, play an important role as centers for linking many NCs for the formation of aggregated structures. Aggregated NCs often show distinct optical properties. As an important example, we demonstrate that the aggregated Au NCs exhibit strong and sharp blue emission.