(212f) The Formation of Polytetrahedral Structures In Elongated Gold Nanowires

The observation of quantized conductance in single-atom wide metallic junctions has fueled much interest into understanding the behavior of nanowires undergoing tensile elongation [1].  Quantifying the structural changes that occur as a nanowire elongates is important to elucidating the origin of changes to nanowire properties, such as conductance or the opening of a bandgap, and has additional implications regarding the appropriateness of the assumed geometries used in high level theoretical calculations of nano- and molecular-electronics, e.g., the conductance through single atom wide chains or bridged single molecules.  

In this work, we use molecular simulation to study the structural evolution of Au nanowires undergoing tensile elongation.  While most of the previous work on gold nanowires has focused on the crystalline nature of the wire, here we focus on identifying the non-crystalline, "amorphous" structures that often form within the necks of the nanowires.  To study this under conditions matching experiment, we perform molecular dynamics simulations with two different interatomic Au potentials.  We use the tight-binding second-moment approximation (TB-SMA) potential, a common semi-emperical potential used to study Au nanowire elongation [2], as well as the more sophisticated ReaxFF bond-order potential with fitting parameters derived by Keith, et al. [3].  

We report the widespread formation of polytetrahedral Z12 (i.e., icosahedron) and Z14 structures with both potentials, quantifying their presence using a shape-matching order parameter based on spherical harmonics [4,5]. We combine these results with density functional theory (DFT) calculations to investigate the energetic description of the polytetrahedral structures observed with TB-SMA and ReaxFF, finding that ReaxFF better captures the energetics predicted by DFT.  While both potentials form polytetrahedra, our study suggests that TB-SMA based simulations over-predict their formation, most notably resulting in increased mechanical stability for nanowires with diameters less than 1.5 nm.  We further perform calculations of the conductance [6] of the elongated nanowires configurations generated using ReaxFF, which show close agreement with experimental measurements [1], further validating the structural evolution predicted by our simulation results.

[1] J.M. Krans, J.M. van Rutlenbeek, V.V. Fisun, I.K. Yanson, and L.J. de Jongh, Nature 375, 767 (1995).
[2] Q. Pu, Y.S. Leng, P.T. Cummings, Rate-Dependent Energy Release Mechanism of Gold Nanowires under Elongation, JACS 130, 52, (2008)
[3] J.A. Keith, D. Fantauzzi, T. Jacob, A.C.T. van Duin, Reactive Forcefield for Simulating Gold Surfaces and Nanoparticles, PRB, 81, 235404, (2010)
[4] C.R. Iacovella, A.S. Keys, M.A. Horsch, S.C. Glotzer, Icosahedral Packing of Polymer-tethered Nanospheres and Stabilization of the Gyroid Phase, PRE 75, 4, (2007)
[5] A.S. Keys, C.R. Iacovella, S.C. Glotzer, Characterizing Structure Through Shape Matching and Applications to Self-Assembly. Annual Review of Condensed Matter Physics, 2, (2011)
[6] K. Varga, S.T. Pantelides, Quantum Transport in Molecules and Nanotube Devices, PRL 98, 076804 (2007)