Polygonal assembly of atomically precise Au₂₅ superatoms through mixed-ligand shell engineering

Atomically precise metal nanoclusters have emerged as a distinct class of superatomic building blocks with unique electronic structures. Although their organization into higher-order architectures is expected to generate emergent collective properties, rational control over assembly morphology while preserving atomic precision remains elusive. Here, we demonstrate that mixed-ligand shell engineering provides an effective strategy for directing the self-assembly of thiolate-protected Au₂₅ superatoms into polygonal nanocrystals. A Au₂₅ nanocluster protected by a chemically heterogeneous mixed ligand shell composed of PEG (M_n = 550)-functionalized and non-functionalized alkanethiolate ligands (Au₂₅(PEGT)_x(SC6)_18-x; PEGT = PEGylated alkanethiolate, SC6 = S(CH₂)₅CH₃; x_ave = 12.3) spontaneously assembled in aqueous THF to afford well-faceted polygonal nanocrystals preferentially with the retention of the atomic precision of the constituent clusters. Systematic variation of the ligand-shell composition revealed a pronounced dependence of the morphology on the degree of PEG functionalization: clusters with a low PEG content (x_ave = 3.6) predominantly formed triangular nanocrystals, whereas increasing the PEG content (x_ave = 16.3) favoured polygons with four or more sides. In contrast, non-PEGylated Au₂₅(SC6)₁₈ formed only ill-defined aggregates under identical conditions, while nearly fully PEGylated clusters (x_ave = 17.5) remained predominantly monomeric, highlighting the importance of a mixed ligand shell for polygonal assembly. Time-dependent TEM analyses revealed that initially formed ill-defined aggregates progressively evolved into well-faceted polygonal nanocrystals. These findings demonstrate that mixed-ligand shell engineering provides a strategy for directing the assembly and morphological evolution of atomically precise superatoms.