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FEATURE ARTICLE

Fullerene Nanotubes: C1,000,000 and Beyond

Some unusual new molecules—long, hollow fibers with tantalizing electronic and mechanical properties—have joined diamonds and graphite in the carbon family

Boris Yakobson, Richard Smalley

Growing Nanotubes

Nanotubes owe their discovery to Sumio Iijima of NEC Corporation. Iijima's samples were created by a direct-current arc discharge between carbon electrodes immersed in a noble gas. A similar apparatus had been used by Wolfgang Kratschmer of the Max Planck Institute for Nuclear Physics and Donald Huffman of the University of Arizona to mass-produce fullerenes.

Figure 2. Sumio Iijima of NEC CorporationClick to Enlarge Image

The electric arc was used by Roger Bacon in the early 1960s to make "thick" carbon whiskers, and one can speculate that the nanotube discovery was a matter of looking more closely at the smallest products hidden in the soot. Iijima himself suggests that nanotubes may have been formed in those old experiments, but Bacon lacked the high-power microscope required to see them. Although various fullerenes can be produced by different ways of vaporizing carbon, followed by condensation in tiny clusters, the presence of an electric field in the arc discharge seems to promote the growth of the long tubules. Indeed, the nanotubes form only where the current flows, on the larger negative electrode. The typical rate of deposit is about a millimeter per minute at a current and voltage in the range of 100 amperes and 20 volts respectively, which maintains a high temperature of 2,000-3,000 degrees Celsius.

A year later, quite by chance, Thomas Ebbesen and P. M. Ajayan at NEC found a way to produce nanotubes in higher yields and make them available for studies by different techniques. Subsequently they found a way to purify them. An addition of a small amount of transition-metal powder (cobalt, nickel or iron) favors the growth of so-called single-walled nanotubes, a fact independently noticed by Donald Bethune at IBM Corporation and Iijima.

Figure 3. Single-walled nanotubesClick to Enlarge Image

Metal clearly serves as a catalyst, preventing the growing tubular structure from wrapping around and closing into a smaller fullerene cage. The presence of a catalyst also allows one to lower the temperature. Without such cooling the arc is just too hot a place: Nanotubes coalesce and merge like the foam bubbles in a glass of beer. Condensation of a laser-vaporized carbon-catalyst mixture at a lower temperature (1,200 degrees Celsius in an oven-heated quartz tube) allowed a recent breakthrough at Rice University. Single-walled nanotubes now can be produced in yield proportions of more than 70 percent. Moreover, these nanotubes self-organize into bundles--ropes more than one-tenth of a millimeter long that look very promising for engineering applications. It is in this context, connecting dreams with the strides of progress, that we discuss the properties of nanotubes.




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