All-thiolate-protected silver and silver-rich alloy nanoclusters with atomic precision: stable sizes, structural characterization and optical properties

Ultrasmall metal clusters (<2 nm), with dimensions between those of small molecules and plasmonic metal nanoparticles, have received significant attention due to their discrete electronic structure as well as unique physical and chemical properties. It remains to be a major dream for nanochemists to accomplish the controllable synthesis and structure analysis of clusters with atomic precision. This dream has been realized in gold clusters, especially all-thiolate (SR)-protected clusters. In contrast, despite the similarities between gold and silver, all-thiolate-protected silver clusters have been less reported due primarily to the low stability and synthetic bottlenecks. Nevertheless, there has been some success in Ag clusters recently. Since the total structure of [Ag44(SR)30]4− was solved by Bigioni's group and Zheng's group independently, a few more structures have been achieved including [Ag25(SR)18]− and [Ag29(SR)12(TPP)4]3− (TPP: triphenylphosphine) very recently. Moreover, some Ag-rich alloy clusters have also been reported, such as [MAg24(SR)18]2− (M = Pd, Pt), [AuAg24(SR)18]−, [Au12Ag32(SR)30]4− and [Ag46Au24(SR)32]2+. In this Highlight, some recent progress in developing strategies for the synthesis of high-quality all-thiolated Ag and Ag-rich alloy clusters will be emphasized. The stable sizes, structural characterization and optical properties will be discussed in detail. A brief outlook on the future development of Ag and Ag-rich alloy clusters from the viewpoint of controlled synthesis, stability enhancement and single-atom level manipulation will be given.