Two-Stage Formation Model and Helicity of Gold Nanowires

A model for the formation of helical multishell gold nanowires is proposed and is confirmed with quantum mechanical molecular dynamics simulations. The model can explain the magic number of the helical gold nanowires in the multishell structure. The reconstruction from ideal nonhelical to realistic helical nanowires consists of two stages: dissociations of atoms on the outermost shell from atoms on the inner shell and slip deformations of atom rows generating (111)-like structure on the outermost shell. The elementary processes are governed by competition between energy loss and gain by $s$ and $d$ electrons together with the width of the $d$ band. The possibility for the helical nanowires of platinum, silver, and copper is discussed.

Figure 1

(a) The ideal [110] fcc nanowire is intrinsically a nonhelical multishell nanowire. (b) Expanded a surface of the 7-1 multishell nanowire before (upper right) and after (lower right) surface reconstruction. Dashed lines connect the same atoms at the right and the left ends.

Figure 2

The sections of gold nanowires: (a) 7-1 at the initial (left), 400 (middle), and 5000 MD steps (right); (b) 11-4 at the initial (left), 400 (middle), and 6000 MD steps (right); (c) 12-6-1 at the initial (left), 7500 (middle), and 13 000 MD steps (right); (d) 13-6-1, at the initial (left), 750 (middle), and 9000 MD steps (right); (e) 14-7-1, at the initial (left), 400 (middle), and 7000 MD steps (right); and (f) 15-8-1 at the initial (left), 400 (middle), and 5000 MD steps (right). An atom row slips on the outermost shell and helicity is introduced. Temperatures in the simulations are 600 K in (a), (b), (e), and (f) and 900 K in (c) and (d).

Figure 3

Change of the total energy and the atomic configuration during the formation process of the 11-4 nanowire at 600 K. Red, blue, purple, and green atoms are named $A$, $B$, $C$, and $D$, respectively. Within 500 MD steps, the atom $A$ was disassociated from inner shell. Between 2000 and 5000 MD steps, the slip between the atom $B$ and $D$ brought the surface reconstruction, and (001) face becomes (111) face.

Figure 4

Partial densities of states and local energies in orthogonalized basis. Partial densities of states of (a) the atom $A$, (b) the atom $C$, and (c) the atom $B$. Red and blue lines are at the initial state and at 500 MD steps, respectively, in (a) and (b), and at 500 MD steps and 5000 MD steps in (c). Upper, middle, and lower figures are the partial densities of states of $s$ orbital, $xy$ orbital, and the total density of states, respectively. The energy changes of the densities of states are written at the upper right of each figure. The local coordinate system for each atom is written in text. Notice that, in all cases, the asymmetry of LDOS of $xy$ orbital is enhanced.