Single-Walled Carbon Nanotubes

What are Single-Walled Carbon Nanotubes?

SWNTs are sheets of graphene that have been rolled up to form a long hollow tube, with wall thickness of a single atom. The existence of thin, hollow carbon tubes has been known about since their first observations by L. V. Radushkevich and V. M. Lukyanovich in 1952, however, the first observations of SWNTs themselves were not until 1976 when M. Endo synthesised a series of hollow carbon tubes via chemical vapour-growth. Wider interest in these low-dimensional materials did not occur until 1991, when two articles were independently published by: i) S. Iijima on the fabrication of multi-walled carbon nanotubes via arc discharge, and ii) J. W. Mintire, B. I. Dunlap, and C. T. White on the predicted properties of SWNTs. The combination of a simple method for producing SWNTs and the potentially extraordinary properties they exhibit kick-started the growth of a wider research community into carbon nanotubes.

Much like graphene, SWNTs have properties that differ considerably to those of bulk carbon (e.g. graphite). The mechanical properties vary significantly depending upon the axis you are measuring with nanotubes having extremely high Youngs Moduli (Up to 1TPa) and tensile strength (Up to 100 GPa) along the longitudinal axis. Along the radial axis, these values are a few orders of magnitude lower.

The electrical properties of carbon nanotubes are dependent upon the orientation of the lattice. The lattice orientation is given by two parameters (n, m). The image to the right shows how the n and m orientations relate to the longitudinal axis of the nanotube and the rotational axis. There are typically three types of nanotubes that can form, these are: the armchair (where n = m), zig-zag (n=x, m=0), and chiral (n=x, m=y).

Carbon nanotubes can exhibit either metallic properties or semiconducting properties, depending upon the orientation of the lattice. Zig-zag and armchair carbon nanotubes exhibit metallic properties, whilst chiral nanotubes can be either metallic or semiconducting depending upon the difference between the n and m units. In addition to this ability to exhibit both metallic and semiconducting electronic structures carbon nanotubes offer exceptional charge carrier mobilities, this is due to the combination of the delocalisation of electrons across the lattice and the small dimensions in the radial axis constraining movement of charge carriers along the longitudinal axis of the tubes.

In addition to the electronic and mechanical properties of SWNTs, the thermal properties of these materials exhibit extreme anisotropy. Along the length of the tube, thermal conductivity can be up to 9 times higher than materials such as copper. However - across the radial axis, the thermal conductivity can be 250 times lower than that of copper. Much like its electrical and mechanical properties, SWNT's thermal properties can be severely affected by the presence of defects along the nanotube length. The presence of these defects lead to phonon scattering. When these defects interact with low frequency phonons, scattering can occur - reducing the thermal conductivity.

At the time being, there are limited commercial applications for SWNTs. They are used in composite materials as a method of improving mechanical strength. One of the current limiting factors in improving the range of applications of carbon nanotubes is the ordering of nanotube structure. Current commercial applications utilise disordered bundles of nanotubes, and these bundles have a significantly lower performance than that of individual nanotubes. Potential future uses for carbon nanotubes could be seen in areas such as transparent conducting layers for use in display technologies, conductive wires for nanoelectronics, electrodes in thin-film electronic devices, carbon nanotube yarns for ultra-strong fabrics, thermal management systems, advanced drug delivery systems and many other wide-ranging fields.