Carbon nanotubes

(CNTs) are tubular cylinders of carbon atoms exhibiting multiple forms, varying in diameter (0.5 to about 10 nm), length (few nanometers to tens of microns), and in the tendency of to form ropes and bundles of tubes. Carbon nanotubes (fullerene nanotubes) are part of the fullerene family of carbon materials which include single-wall carbon nanotubes (SWNTs), and two or more tubular fullerenes nested inside another (to form endohedral or endotopic) SWNTs, Each tubular fullerene is a huge carbon molecule, often having millions of carbon atoms bonded together to form a tiny tube.

Carbon readily bonds with itself to form extended sheets of atoms comprising linked hexagonal rings by covalently bonding to its three nearest neighbors leading to a unique sheet structure is called graphene. Solid graphite is made up of layers of graphene stacked together. This carbon-carbon bond is the strongest among chemical bonds. Some of the electrons in the carbon-carbon bonds are free to move about the entire graphene sheet, rather than stay home with their donor atoms, giving the structure good electrical conductivity. The tight coupling between atoms in the carbon-carbon bond provides an intrinsic thermal conductivity that exceeds almost all other materials. The structure of a fullerene nanotube is that of a sheet of graphene, wrapped into a tube and bonded seamlessly to it. This is a true molecule with every atom in its place and very few defects: an example of molecular perfection on a relatively large scale.

The special nature of the bonded carbon sheet, the molecular perfection of carbon nanotubes, and their long tubular shape endow them with physical and chemical properties that are unlike those of any other material. These properties include high surface area, excellent electrical and thermal conductivity, and tremendous tensile strength, stiffness, and toughness. Carbon nanotubes have extraordinary electrical, mechanical, optical, thermal, and chemical properties. Individual carbon nanotubes can conduct electricity better than copper, possess higher tensile strength than steel, and conduct heat better than diamond. In electronic applications, carbon nanotubes can possess higher mobilities than single crystal silicon. Depending on the orientation of the graphene sheet forming the tube's wall, the tube can be either metallic or highly conducting; other forms are semi conducting, and can form the basis of electronic switches. The metallic tubes conduct electricity just as metals do and the semi conducting ones have great promise as the basic elements of a new paradigm for electronic circuitry at the molecular level.

In a single tube, every atom is on two surfaces - the inside and the outside, and a single gram of nanotubes has over 2400 m2 of surface area. The nature of the carbon bonding gives the tubes their great tensile strength and electrical and thermal conductivity. The carbon nanotubes stiffness and toughness derives from their molecular perfection. In most materials the actual observed stiffness and toughness are degraded very substantially by the occurrence of defects in their structure. For example, high strength steel typically fails at about 1% of its theoretical breaking strength. Carbon nanotubes, however, achieve values very close to their theoretical limits because of their perfection of structure - there are no structural defects where mechanical failures can begin. It is, however, the tubular geometry of carbon nanotubes that gives them their most exotic properties.