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

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