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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

Originally published in the July-August 1997 issue of American Scientist.

Imagine holding in your hand a wand that is a single molecule. Such a wand would be so thin that it could not be seen under an ordinary microscope; it would be a nanotube, just a few atoms in circumference. But it could be very long. Such a tubule in fact exists in the laboratory today, and it has even been used there as a probe, poking down into the world of molecules. Called the carbon nanotube, it is elongated like a fiber, and yet it is hollow and inherits the perfection of atomic arrangement made famous by its predecessor, the buckyball—the remarkable closed cage of 60 symmetrically arranged carbon atoms that was recognized as a new form of carbon when it was discovered a decade ago. The buckyball's molecular family, the fullerenes, has expanded since that discovery; the nanotube is a new and very useful addition.

Figure 1. Carbon fibersClick to Enlarge Image

Carbon nanotubes are, effectively, buckyball structures played out as long strands rather than spheres. Their length can be millions of times greater than their tiny diameter. Their properties as a new material are remarkable—a fact that was evident almost as soon as they were first spotted in 1991, turning up in the soot and dirt piles that fill chambers where scientists produce fullerenes, large geometric carbon molecules.

A chemist might think of a carbon nanotube as a monoelemental polymer. (Most polymers, such as polyethylene, are carbon chains with other elements attached.) The buckyball is designated C60; a carbon nanotube might be C1,000,000. In physics terms it can be described as a single crystal in one direction, with a unit cell that keeps on propagating and repeating. This periodic pattern has the symmetry of a helix, not as complex as the double helix responsible for life itself, but possessing a special beauty in its monotonous order—a molecular incarnation of Ravel's Bolero. A mathematician will be delighted by the symmetry and rigor of these structures and how nicely they obey Euler's rule for polyhedra.

Likewise the nanotube has what a civil engineer would recognize as a beam-and-truss construction, a billion times smaller than such structures built to human scale. To satisfy the standard chemical requirements of carbon, every beam ties two carbon atoms by a strong covalent bond, and every atom accommodates exactly three neighbors. A nanotube, no matter how long, must thus ultimately be sealed on the ends to leave no dangling "unhappy" chemical bonds. The strength of these bonds and their clever organization makes nanotubes so highly resistant to tension that (with apologies to high-energy physicists) they could justly be called superstrings. Furthermore, electrons move easily along some carbon tubules, although their minuscule cross section permits electrical conductance only in a quantized fashion.

Several analogies, some of them famous, have been invented by science teachers to help their students and themselves comprehend the smallness of the small, and these can help us imagine nanotubes. One difficult dimension to imagine is the nanometer, a billionth of a meter. One could stare at a chip of graphite as Richard Feynman once stared at a drop of water. After magnifying our chip a billion times, creating a metal-gray rocky landscape about the size of Texas, we might spot a three-foot-diameter pipeline stretching from horizon to horizon. This is the nanotube. Actually, a one-nanometer-wide pipeline occupies almost no space even over a substantial length. In fact, nanotubes sufficient to span the 250,000 miles between the earth to the moon at perigee could be loosely rolled into a ball the size of a poppyseed. Together, the smallness of the nanotubes and the chemical properties of carbon atoms packed along their walls in a honeycomb pattern are responsible for their fascinating and useful qualities.





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