Atom-by-Atom Evolution of the Same Ligand-Protected Au₂₁, Au₂₂, Au₂₂Cd₁, and Au₂₄ Nanocluster Series

Atom-by-atom manipulation on metal nanoclusters (NCs) has long been desired, as the resulting series of NCs can provide insightful understanding of how a single atom affects the structure and properties as well as the evolution with size. Here, we report crystallizations of Au₂₂(SAdm)₁₆ and Au₂₂Cd₁(SAdm)₁₆ (SAdm = adamantanethiolate) which link up with Au₂₁(SAdm)₁₅ and Au₂₄(SAdm)₁₆ NCs and form an atom-by-atom evolving series protected by the same ligand. Structurally, Au₂₂(SAdm)₁₆ has an Au₃(SAdm)₄ surface motif which is longer than the Au₂(SAdm)₃ on Au₂₁(SAdm)₁₅, whereas Au₂₂Cd₁(SAdm)₁₆ lacks one staple Au atom compared to Au₂₄(SAdm)₁₆ and thus the surface structure is reconstructed. A single Cd atom triggers the structural transition from Au₂₂ with a 10-atom bioctahedral kernel to Au₂₂Cd₁ with a 13-atom cuboctahedral kernel, and correspondingly, the optical properties are dramatically changed. The photoexcited carrier lifetime demonstrates that the optical properties and excited state relaxation are highly sensitive at the single atom level. By contrast, little change in both ionization potential and electron affinity is found in this series of NCs by theoretical calculations, indicating the electronic properties are independent of adding a single atom in this series. The work provides a paradigm that the NCs with continuous metal atom numbers are accessible and crystallizable when meticulously designed, and the optical properties are more affected at the single atom level than the electronic properties.