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 (M-7, M=Au/Cu). The M-7 kernel is then enclosed by a second shell of homogold (Au-47), giving rise to a two-shelled M-54 (i.e. Au52Cu2) full decahedron. A comparison of the non-truncated M-54 decahedron with the truncated homogold Au-49 kernel in similar-sized gold nanoparticles provides for the first time an explanation for Marks decahedron truncation. Second, a Cu-70(SR)(55) exterior cage resembling a 3D Penrose tiling protects the M-54 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 M-54 kernel's high symmetry (D-5h). Third, interparticle interactions in the assembly are closely related to the symmetry of the particle, and a quadruple-gear-like interlocking pattern is observed. The formation of Marks truncated decahedra in nanoparticles is ubiquitous but the mechanism has not been fully understood. Here, the authors provide atomic-level insights by creating a non-truncated Au52Cu72(SR)(55) decahedral nanocluster and comparing it with the truncated homogold decahedra.