Gold is a malleable metal, but just how far can it be stretched before breaking? In a diatomic gold molecule, the Au-Au bond length is 2.48 Å. On the other hand, for mechanically stressed, single atom-thick Au nanowires, Au-Au distances as large as 4.0 Å have been measured by transmission electron microscopy and cluster around 3.6 Å, yet calculations for pure Au wires have yielded a maximum of 3.1 Å.
Using total energy density functional theory, Novaes et al. calculated the effect of impurity atoms on stressed gold nanowires. Single H, B, C, N, O, or S atoms, which probably would not have been observable in micrographs of nanowires due to insufficient contrast, were inserted at various positions. The relaxed wires were then pulled until they ruptured, which always occurred at an Au-Au bond. The addition of a carbon atom led to a Au-C-Au distance of 3.7 Å for the unstressed wire, which increased to 3.9 Å just prior to breaking; wires containing B, N, and O displayed even larger distances when stretched. However, the addition of H produced a distance of 3.6 Å, consistent with the experimentally observed cluster, and an anomalously large distance of 4.8 Å could be explained as the separation between gold atoms in an Au-S-Au nanowire. — MSL
Phys. Rev. Lett.90, 036101 (2003).