Ultrathin Gold Nanowires with the Polytetrahedral Structure of Bulk Manganese
Despite the intensive interest in thin gold nanowires for a variety of technologically important applications, key details of the mechanism of their formation and atomic-scale structure remain unknown. In this work we synthesize highly uniform, very long, and ultrathin gold nanowires in a liquid-phase environment and study their nucleation and growth using in situ high-energy synchrotron X-ray diffraction. By controlling the type of solvents, reducing agents, and gold precursor concentration, it is shown that the nucleation and growth of gold nanowires involve the emergence and self-assembly of transient linear gold complexes, respectively. In sharp contrast with the face-centered-cubic bulk gold, the evolved nanowires are found to possess a tetrahedrally close packed structure incorporating distorted icosahedra and larger size coordination polyhedra of the type observed with the room-temperature phase of bulk manganese. We relate the complexes to synergistic effects between the selected precursor and reducing agents that become appreciable over a narrow range of their molar ratios. We attribute the unusual structural state of gold nanowires to geometrical frustration effects arising from the conflicting tendencies of assemblies of metal atoms to evolve toward attaining high atomic packing density while keeping the atomic-level stresses low, ultimately favoring the growth of cylindrical nanowires with a well-defined diameter and atomically smooth surface. Our work provides a roadmap for comprehensive characterization and, hence, better understanding of 1D metallic nanostructures with an unusual atomic arrangement and may have important implications for their synthesis and performance in practical applications.