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This is a nanoscopic structure made of carbon atoms in the shape of a hollow cylinder. The cylinders are typically closed at their ends by semi-fullerene-like structures. There are three types of carbon nanotubes: armchair, zig-zag and Chiral (helical) nanotubes. These differ in their symmetry. Namely, the carbon nanotubes can be thought of as graphene planes `rolled up` in a cylinder (the closing ends of carbon nanotubes cannot be obtained in this way). Depending on how the graphene plane is `cut` before rolled up, the three types of carbon nanotubes are obtained. Within a particular type, carbon nanotubes with many different radii can be found (depending on how large is the graphene area that is folded onto a cylinder). These tubes can be extremely long (several hundreds of nanometers and more). Some consider them as special cases of fullerenes. When produced in materials, carbon nanotubes pack either in bundles (one next to another within a triangular lattice) - single-walled carbon nanotubes, or one of smaller radius inside others of larger radii - multi-walled carbon nanotubes. Carbon nanotubes have already found several technological applications, including their application in high-field emission displays. Carbon nanotubes were discovered by Sumio Ijima in 1991.