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Structurally, two basic types of carbon nanotubes (CNTs) exist — single-walled nanotubes (SWNT) and multi-walled nanotubes (MWNT) — but the arrangement of carbon atom groups in these structures also varies. Carbon nanotubes are essentially rolled-up sheets of graphite, which are built upon a series of interlocking, hexagonal, six-carbon atom bonds. These bonds can be arranged in one of three configurations: zig-zag, where they alternate in a linear pattern down the length of the cylindrical nanotube wall; armchair, where the structure is a collection of straight lines of bonds; and chiral, where the bonds drift in a linear fashion to a left or right angle down the length of the tube.
Within this fundamental class of structures, carbon nanotubes also vary by being straight cylinders, or distorted in some manner such as coiled or branched. Additional forms that have been created include the nanotube with a carbon buckyball sphere attached to it, known as a nanobud, and cup-stacked nanotubes, which are a series of concave, disc-shaped structures aligned in tube form. Torus, or donut-shaped, nanotube structures have also been made and have high magnetic moment properties that would make them useful as powerful sensors.
The structure of carbon nanotubes also determines their physical and chemical properties, where armchair nanotubes are always metallic in terms of electrical conductivity, and zig-zag and chiral forms are semiconducting. The six carbon bonds that make up the basic hexagonal structure of a carbon nanotube are spaced around 0.14 nanometers from each other in strong molecular, covalent bonds. These rolled sheets of graphite are then bound to each other in multi-walled nanotubes, which are essentially cylinders within cylinders, by weak van der Waals forces, at a distance of about 0.34 nanometers between cylinder walls. This weak molecular bond allows the graphite sheet structures to slip against each other, which makes it easy to rub off graphite in applications such as when a pencil is pressed against paper.
Other types of carbon nanotubes include extreme carbon nanotubes, which are simply variations on the natural design where they are very long, short, or thin. They have applications in the building of cable 20 to 100 times stronger than steel for such things as a space elevator, and for artificial muscles which can operate in a temperature range from -321° to 2,800° Fahrenheit (-196° to 1,538° Celsius). Some extreme nanotube films are also capable of capturing infrared wavelengths of light known as black body radiation or heat radiation. This would make them useful in solar cells that could capture this heat emitted by the Earth into space at night, which would allow for around-the-clock energy generation at an efficiency level of over 35%, which is two to five times better than that of conventional solar cells.