Helical Gold Nanotube Synthesized at 150 K

Gold nanowires were synthesized at 150 K by electron beam thinning of a gold thin foil in an UHV electron microscope. The gold nanowires were found to have a helical multishell structure (HMS). One particular nanowire, which was thinner than the 7-1 HMS nanowire, was found to have a tubular structure. The gold single wall nanotube is composed of five atomic rows that coil about the tube axis. The diameter was 0.4 nm and the pitch was 11 nm. The stability of the $(5,3)$ nanotube was discussed in terms of the shear deformation of the triangular network of gold atoms.

Figure 1

(a) A sheet of the triangular network of gold atoms that forms coaxial tubes of the HMS nanowire. Large circles show gold atoms. $\mathbf{a}$ and $\mathbf{b}$ indicate the basic vector of the triangular network. The gold atomic row has a nearest neighbor atom distance of 0.29 nm. The triangular network is deformed by shear strain $ϵ$ in the $\mathbf{b}$ direction. $\mathbf{C}(n,m)$ is the chiral vector, defined as $\mathbf{C}(n,m)=n\mathbf{a}+(m+nϵ/2)\mathbf{b}$. $\mathbf{C}(n,m)$ in the figure is $\mathbf{C}(5,3)$ for the gold $(5,3)$ nanotube. The diameter of the tube, $D$, is given by $D=|\mathbf{C}(n,m)|/\pi$. (b) The helical pitch, $L$, of the nanotube is given by $L=|\mathbf{C}(n,m)|/\tan \theta$, where $\theta$ is the angle between the atomic row (parallel to the $\mathbf{b}$ direction) and the tube axis. The atomic row rotates by $2\pi$ radians when it translates in the tube axis by the helical pitch, $L$.

Figure 2

TEM images of (a) 11-4 helical multishell (HMS) gold nanowire and (b) 7-1 HMS nanowire synthesized at 150 K. Images are reproduced from a video recording of the thinning process. The number of spotty dark lines is four in (a), and three in (b). The simulated image and the model (viewed from the HMS axis) are shown for each TEM image. The image simulation was performed by multislice calculation [31] for 200 kV (accelerating voltage), $C_s=0.7\mathrm{m}\mathrm{m}$ (spherical aberration), $C_c=1.2\mathrm{m}\mathrm{m}$ (chromatic aberration), $\Delta E={10}^{-6}$ (voltage and lens stability), $\Delta f=-55\mathrm{n}\mathrm{m}$ (under focus of the objective lens), and a beam divergence of 3.4 mrad. The convergent beam illumination (CBI) condition was used for the simulation program [32]. $L/n$ indicates the period of the wavy modulation; $L/11=5.9\mathrm{n}\mathrm{m}$ for (a) and $L/7=4\mathrm{n}\mathrm{m}$ for (b).

Figure 3

A series of TEM images of the thinning process of a gold nanowire at 150 K. (a) A newly observed gold nanowire showing two spotty dark lines, $\sim 0.25\mathrm{n}\mathrm{m}$ apart. (b) A single strand of 11 gold atoms. This will be described in detail elsewhere.

Figure 4

(a) Structure model for the gold $(5,3)$ nanotube; (b) side view of the tube with a diameter of $D=0.40\mathrm{n}\mathrm{m}$ and a helical pitch of $L=11\mathrm{n}\mathrm{m}$ (for $ϵ=0.005$); (c) simulated image; and (d) observed TEM image. In the TEM image, spatial frequencies above 0.16 nm have been filtered out. The local hexagonal patterns (marked by A and B) are shown by dots and lines in (b) and (c). The distance between pattern A and B is about 1.1 nm.