What is a Carbon Nanotube?
Single-walled carbon nanotubes (SWNT) can be imagined as rolled-up rectangular strips of hexagonal graphite monolayers. The short side of the rectangle becomes the tube diameter and therefore is "quantized" by the requirement that the rolled-up tube must have a continuous lattice structure. Similarly, the rectangle must be properly oriented with respect to the flat hexagonal lattice, which allows only a finite number of roll-up choices. Two of these correspond to high symmetry SWNT's; in "zigzag" tubes (left, top), some of the C-C bonds lie parallel to the tube axis, while in "armchair" tubes (left, bottom), some bonds are perpendicular to the axis. Chiral tubes have a left- or right-handed screw axis, like DNA.
The allowed electron wave functions are no longer those of an infinite two-dimensional system. The rolling operation imposes periodic boundary conditions for propagation around the circumference, which have different consequences on the band structure for different symmetries. As a consequence, SWNT's can be either metallic or insulating, the bandgaps in the latter ranging from a few milli-electron volts to about one electron volt.
To the right are some real-world nanotube materials, produced by laser ablation of a graphite target containing metal catalyst additives. On top is an atomic force microscopy image of a chiral tube with a diameter of 1.3 nanometers (Technical University, Delft).
Shown next is a high resolution transmission electron microscopy image of a crystalline nanotube bundle, consisting of many SWNT's with similar diameters which self-organize during growth into a triangular lattice. Bulk material consists of a porous mat of intertwined bundles, shown on the bottom in a scanning electron microscope image (last two frames from Rice University).
SWNT bundles are the latest example of carbon-based materials into which heteroatoms or molecules can be inserted and removed. The proper choice of "invader" (alkali metals, halogen or acid molecules) can transform an insulating polymeric host into a doped semiconductor or even a metal, an example being sodium-doped polyacetylene, top right. The insulating molecular fullerene solid becomes superconducting upon addition of 3 alkali ions per molecule (bottom right). Reversible insertion in graphite (top left) and SWNT bundles (bottom left) can be exploited for energy storage applications such as rechargeable batteries and "containers" for hydrogen-burning vehicles. Lithium-graphite compounds are used as the anode material in lithium-ion batteries, common in laptops and cellphones. Many researchers believe that Li-doped SWNT bundles will perform better than the graphite or polyacetylene analogs.