Atomically resolved Au52Cu72(SR)55 nanoalloy reveals Marks decahedron truncation and Penrose tiling surface.

Gold-copper alloys have rich forms. Here we report an atomically resolved [Au52Cu72(p-MBT)55]+Cl- nanoalloy (p-MBT = SPh-p-CH3). This nanoalloy exhibits unusual structural patterns. First, two Cu atoms are located in the inner 7-atom decahedral kernel (M7, M = Au/Cu). The M7 kernel is then enclosed by a second shell of homogold (Au47), giving rise to a two-shelled M54 (i.e. Au52Cu2) full decahedron. A comparison of the non-truncated M54 decahedron with the truncated homogold Au49 kernel in similar-sized gold nanoparticles provides for the first time an explanation for Marks decahedron truncation. Second, a Cu70(SR)55 exterior cage resembling a 3D Penrose tiling protects the M54 decahedral kernel. Compared to the discrete staple motifs in gold:thiolate nanoparticles, the Cu-thiolate surface of Au52Cu72 forms an extended cage. The Cu-SR Penrose tiling retains the M54 kernel's high symmetry (D5h). Third, interparticle interactions in the assembly are closely related to the symmetry of the particle, and a "quadruple-gear-like" interlocking pattern is observed.