Carbon nanotubes are tiny strips of graphite sheet rolled into tubes a few nanometers in diameter and up to hundreds of micrometers (microns) long. The graphite has a network of hexagonal rings, leaving it with many unpaired electrons. Carbon nanotubes were first observed by Iijima196 while studying the surface of carbon electrodes used in an electric arc-discharge apparatus which had been used to make fullerenes. The large scale synthesis of nanotubes opened the door for their widespread study by Ebbesen and Ajayan197. Generically two distinct types of nanotube exist, depending on whether the tube walls are made of one layer (graphene tubes or single walled nanotubes or SWNT) or more than one (graphitic tubes or multiwalled nanotubes or MWNT). The MWNT consists of two or more concentric cylindrical shells of graphene sheets coaxially arranged around a central hollow with a constant separation between the layers (0.34nm)198,199. These have diameters ranging from 2 to 25 nm and lengths reaching up to several microns. The SWNT consists of tubes made of single layers of graphene cylinders with a very narrow distribution in size range (1-2nm) and lengths extending up to several microns200-201. The carbon nanotubes conduct electricity and heat amazingly well, so they are being considered for use as wires for nanosized electronic devices in future computers, charge-storage devices in batteries, and electron guns for semiconductor chip etching and flat-screen televisions and computer monitors. Electrons flow through a nanotube without scattering off any atoms, so they encounter hardly any resistance and lose virtually no energy. In fact, a superconducting current was recently induced in a carbon nanotube at low temperatures." Carbon nanotubes can carry an electric current as high as 109 amps/cm2. The conductive nanotubes are excellent for field emission due to high curvature of the tips.
