CARBON NANO TUBES

Computer Engineering electronics Engineering Civil Engineering

Carbon nanotubes are wires of pure carbon with nanometer diameters and lengths of many microns. A single-walled carbon nanotube (SWNT) may be thought of as a single atomic layer thick sheet of graphite (called graphene) rolled into a seamless cylinder. Multi-walled carbon nanotubes (MWNT) consist of several concentric nanotube shells. Understanding the electronic properties of the graphene sheet helps to understand the electronic properties of carbon nanotubes. Graphene is a zero-gap semiconductor; for most directions in the graphene sheet, there is a bandgap, and electrons are not free to flow along those directions unless they are given extra energy. However, in certain special directions graphene is metallic, and electrons flow easily along those directions. This property is not obvious in bulk graphite, since there is always a conducting metallic path which can connect any two points, and hence graphite conducts electricity. However, when graphene is rolled up to make the nanotube, a special direction is selected, the direction along the axis of the nanotube. Sometimes this is a metallic direction, and sometimes it is semiconducting, so some nanotubes are metals, and others are semiconductors. Since both metals and semiconductors can be made from the same all-carbon system, nano tubes are ideal candidates for molecular electronics technologies. In addition to their interesting electronic structure, nanotubes have a number of other useful properties. Nanotubes are incredibly stiff and tough mechanically - the world's strongest fibers. Nanotubes conduct heat as well as diamond at room temperature. Nanotubes are very sharp, and thus can be used as probe tips for scanning-probe microscopes, and field-emission electron sources for lamps and displays. History The current huge interest in carbon nanotubes is a direct consequence of the synthesis of buckminsterfullerene, C60, and other fullerenes, in 1985. The discovery that carbon could form stable, ordered structures other than graphite and diamond stimulated researchers worldwide to search for other new forms of carbon. The search was given new impetus when it was shown in 1990 that C60 could be produced in a simple arc-evaporation apparatus readily available in all laboratories. It was using such an evaporator that the Japanese scientist Sumio Iijima discovered fullerene-related carbon nano tubes in 1991. The tubes contained at least two layers, often many more, and ranged in outer diameter from about 3 nm to 30 nm. They were invariably closed at both A transmission electron micrograph of some multiwalled nanotubes is shown in the figure (left). In 1993, a new class of carbon nano tube was discovered, with just a single layer. These single-walled nanotubes are generally narrower than the multiwalled tubes, with diameters typically in the range 1-2 nm, and tend to be curved rather than straight. The image on the right shows some typical single-walled tubes It was soon established that these new fibres had a range of exceptional properties (see below), and this sparked off an explosion of research into carbon nanotubes. It is important to note, however, that nanoscale tubes of carbon, produced catalytically, had been known for many years before Iijima loading..