In 2000, we reported on new research on buckyballs, spherical forms of carbon that were first discovered in 1985. In 1991, physicist Sumio Iijima discovered related structures called nanotubes. These tubular forms of carbon seem to have even greater potential as conductors, superconductors, semiconductors, reinforcing fibers in building materials, and other uses. In fact, Richard E. Smalley, codiscoverer of buckyballs, is now concentrating his research on nanotubes.

Carbon exhibits very different properties, depending on how the carbon atoms bond to one another. In the diamond structure, carbon atoms bond to four other atoms, while in the graphite structure they bond to just three. With graphite, carbon forms six-member rings, or hexagons, much like the benzene (C₆H₆) ring but without bonds to hydrogen. All the bonds are equivalent in this arrangement and each has some single bond character and some double bond character. One way of showing this ring is to use the hexagon with a circle in center of it. These rings join together to form large, planar sheets, which in turn make up the graphite molecule.(2)

Nanotubes are formed by heating graphite to about 1200°C in a helium gas atmosphere. After the vapor condenses, the various components are separated. The atoms at the edge of the graphite sheets bond together, causing the sheets to curl into microscopic tubes that look much like folded chicken wire fencing (see photo). Their name comes from their size?about 1 nanometer (1 billionth of a meter) in diameter.

Carbon’s double bond character in this structure accounts for the conductivity of the nanotubes. There are electrons above and below the planes of carbon atoms, which are freer to move through the structure. When the hexagons in these tubes are aligned along the long axis of the tube, they act as conductors much like metals. If the tube is twisted into a spiral, it acts as a semiconductor, meaning it will not conduct electricity unless a certain minimum voltage is applied. (3) Nanotubes can also be made into superconductors at room temperature?at least over very short distances. Other substances must be cooled to very low temperatures before they become superconducting.

In addition to their semiconducting and superconducting potential, nanotubes have unusual mechanical properties? They have a tensile strength greater than steel but are as light as plastics. With this combination of properties, nanotubes might find a use as reinforcing fibers in building materials. In imagers like the scanning tunneling microscope, they could be attached to the end of a silicon tip; with their great stiffness, they could give sharper images of surfaces at the atomic level. They also have the potential to make transistors at the molecular level, replacing silicon computer chips. Another possible application is to hold hydrogen fuel for a fuel cell, replacing the large, highly pressurized tanks currently used. A modified nanotube, open at one end, can hold hydrogen gas at low pressures and slightly below room temperature. Simply raising the temperature slightly can force the gas out.

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