"Magic-Numbered Colloidal Silver Clusters"
Colloidal metal clusters lie at the boundary between molecular species and nanoparticles, and can provide insights into the origins of the optical, electronic and chemical properties of nanomaterials. Insight into the origins of why some nanostructures are more stable than others is of particular interest. For example, the atomic and electronic structures of gas-phase metal clusters have been shown to lead to the anomalous stability of certain discrete particles sizes, known as magic-number clusters.[1,2] Magic number theories have been used to explain the anomalous stability of a family of Au clusters,[3,4] but generalization of these theories requires their application to other materials systems. To this end, we have discovered that small Ag clusters can also be synthesized as a family of discrete sizes and structures. We used a bulky ligand (glutathione) to favor a small radius of curvature, which in turn favors magic number clusters due to the small number of metal atoms in the cluster cores. Mass spectrometry results show the discrete nature of the different cluster sizes and give insights into their stability. Optical measurements show that the electronic structure of the Ag clusters is quite different than the Au clusters. The Ag clusters have a much larger optical cross section and have fewer but more pronounced features compared with Au clusters. This appears to be related to the relative positions of the d-bands in bulk Ag and Au. A simple electronic model will be presented to attempt to explain the Ag cluster spectra and how they may evolve toward those of larger plasmonic nanoparticles.