Helical multi-shell structures in gold nanobridge and suspending nanowire

Phase transformations of ultra-thin (<2 nm) 〈 110 〉 gold nanobridges and nanowires of 11-4, 12-5, 13-6, 14-7-1, and 15-8-1 helical multi-shell (HMS) structures were investigated by accelerated molecular dynamics at room temperature. The ultra-thin nanowires with face-centered-cubic (FCC) structure can easily transform into the HMS structure by shear-like transformation and dissociation of the inner and outer shells of the nanowire. The latter leads to twist deformation and the formation of the helical structure. It is noted that the shear-like transformation leads to the formation of quasi-hexagonal atomic packing on the surface. Owing to the constrained atoms at both ends of the nanobridge, collective rearrangements of atoms at the nanobridge/bulk crystal junction were observed in the formation of HMS nanobridges. Except for the 15-8-1 structure, configurations of the HMS structures in nanobridges and nanowires are similar. The helical angles of different types of HMS nanowires are measured: 11-4, 13-6, and 15-8-1 nanowire are 4, 2.5, and 2°, respectively, whereas the 12-5 and 14-7-1 nanobridges with even number of outer shell atoms have zero helical angle. The characteristic angles between atoms in HMS gold structures have an extra 75, 105, 135, 150, and 165° when compared with bulk FCC crystal. The result of the pristine stress distribution shows that the tensile stresses extend over the interior of the nanowires that consist of only two atomic shells.