By Sunil Bhardwaj

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Carbon nanotubes (CNTs; also known as buckytubes) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been synthesised with length-to-diameter ratio of up to 132,000,000:1, which is significantly larger than any other materials known. They possess extraordinary strength and electrical and thermal properties. These novel properties make them useful in many nano electronics, optics and other areas of materials science.

Types of Carbon Nanotubes

1. Single-walled nanotubes (SWNTs) have a diameter of approx 1nm, with a tube length many million times longer. The structure of a SWNT can be imagined to be a wrapping of a one-atom-thick layer of graphite (graphene) into a seamless cylinder. The way the graphene sheet is wrapped is denoted by(n,m) called the chiral vector. If m=0, the nanotubes are called zigzag. If n=m, the nanotubes are called armchair.

2. Multi walled nanotubes (MWNTs) consist of multiple rolled layers or otherwise concentric tubes of graphite. There are two models that describe the structures of multiwalled nanotubes. In the Russian Doll model, sheets of graphite are arranged in concentric cylinders, that is, single-walled nanotube with in a larger single walled nanotube. In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a rolled newspaper. The interlayer distance in multiwalled nanotubes is approximately equal to the distance between graphene layers in graphite which is 3.4 Ao.

3. The morphology and properties of double-walled carbon nanotubes (DWNT) are similar to SWNTs but their resistance to chemicals is extremely good. This chemical resistant property is very important when new chemical properties are to be added to the CNTs (called functionalization). In the case of SWNTs covalent functionalization will break C=C double bonds, leaving "holes" in the structure of the nanotube, hence modifying its mechanical and electrical properties. However in the case of DWNTs, only the outer wall gets modified.

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