Abstract
This paper reports co-crystallization of two atomically precise, different-size ligand-stabilized nanoclusters, a spherical (AuAg)267(SR)80 and a smaller trigonal-prismatic (AuAg)45(SR)27(PPh3)6 in 1:1 ratio, characterized fully by X-ray crystallographic analysis (SR = 2,4-SPhMe2). The larger cluster has a four concentric-shell icosahedral structure of Ag@M12@M42@M92@Ag120(SR)80 (M = Au or Ag) with the inner-core M147 icosahedron observed here for metal nanoparticles. The cluster has an open electron shell of 187 delocalized electrons, fully metallic, plasmonic behavior, and a zero HOMO-LUMO energy gap. The smaller cluster has an 18-electron shell closing, a notable HOMO-LUMO energy gap and a molecule-like optical spectrum. This is the first direct demonstration of the simultaneous presence of competing effects (closing of atom vs. electron shells) in nanocluster synthesis and growth, working together to form a co-crystal of different-sized clusters. This observation suggests a strategy that may be helpful in the design of other nanocluster systems via co-crystallization.
Highlights
This paper reports co-crystallization of two atomically precise, different-size ligand-stabilized nanoclusters, a spherical (AuAg)267(SR)[80] and a smaller trigonal-prismatic (AuAg)45(SR)27(PPh3)[6] in 1:1 ratio, characterized fully by X-ray crystallographic analysis (SR = 2,4-SPhMe2)
Precise metal nanoparticles stabilized by organic ligands are of interest due to their intermediate size that bridges atoms and bulk solids, inducing distinct physico-chemical properties in the quantum-size regime[1,2,3,4]
Achieving a high degree of control over their size and atomic composition is generally a complex process that is affected by several factors during synthesis such as the metal: ligand ratio, reaction temperature, rate of reduction, postprocessing methods such as size focusing, and purification
Summary
Three factors are commonly considered to play important roles in the formation of magic cluster sizes and compositions: (i) Favorable surface chemical structure (the ability to protect the metal core from the environment); (ii) Favorable atomic packing, often seen as spherical structures of concentric polyhedral shells; (iii) Closed-shell electronic structure stabilizing smaller, molecule-like nanoclusters through opening of a large energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the so-called HOMO-LUMO gap. A detailed analysis revealed that (AuAg)[267] nanoparticle exhibits a remarkable high-symmetry three-layer Mackay icosahedron (MIC) molecular structure with efficient fcc atom packing (Fig. 2a, b), with the fourth layer in anti-Mackay configuration.
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