Carbon nanotubes (CNTs) are tubules made of carbon with diameters typically measured in nanometers. The name carbon nanotubes often refers to single-wall carbon nanotubes (SWCNTs). Although not made this way, these tubes can be thought of as cutouts from a two-dimensional hexagonal lattice of carbon atoms rolled up along one of the Bravais lattice vectors of the hexagonal lattice to form a hollow cylinder. This construction, described by Iijima, yields a lattice with helical symmetry of correctly bonded carbon atoms on the cylinder surface. Single-wall carbon nanotubes are one of the allotropes of carbon, intermediate between fullerene cages and flat graphene. Besides single-wall carbon nanotubes, carbon nanotubes can also refer to multi-wall carbon nanotubes (MWCNTs) consisting of nested single-wall carbon nanotubes, which are essentially Oberlin, Endo and Koyama's long straight and parallel carbon layers cylindrically rolled around a hollow tube. Individual nanotubes naturally align themselves into "ropes" held together by relatively weak van der Waals forces. While nanotubes of other compositions exist, most research has been focused on the carbon ones. Therefore, the "carbon" qualifier is often left implicit in the acronym, and the names are abbreviated NT, SWNT, and MWNT.

The majority of studies and applications of these materials have focused on tubes whose circumferences range from a few graphene cells to a few hundred cells, meaning diameters of about 0.25 to 25 nm. The common production methods are such that the length of a carbon nanotube is often indeterminate, but much larger than its diameter. Nanotubes half a meter long have been created, with a length-to-diameter ratio of more than 100,000,000:1. For many purposes, the length of carbon nanotubes can be assumed to be infinite.

Carbon nanotubes can exhibit remarkable electrical conductivity. They also have exceptional tensile strength and thermal conductivity, because of their nanostructure and strength of the bonds between atoms. In addition, they can be chemically modified. These properties are expected to be valuable in many areas of technology, such as electronics, optics, composite materials (replacing or complementing carbon fibers), nanotechnology, and other applications of materials science.

The properties of a long and narrow carbon nanotube (for example, whether it is a metal or semiconductor) are largely determined by its diameter and by the "rolling" angle between the main directions of the graphene lattice and the axis of the cylinder. These parameters are constrained so that the type of nanotube can be described by two small integers. Most nanotube types are chiral, meaning that a tube cannot be rotated and translated to match its mirror image. Apart from that, carbon nanotubes are highly symmetrical: every atom in an infinite nanotube is equivalent to any other atom.

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