Good news for Rumpelstiltskin. Scientists have made another advance toward spinning the world’s strongest fibers out of molecule-thin nanotubes.
Last year, Jie Liu of Duke University in Durham, N.C., and his colleagues grew carbon nanotubes up to 4 millimeters in length (SN: 5/3/03, p. 275: Nanoscale Networks: Superlong nanotubes can form a grid). Now, he and his collaborators at Los Alamos National Laboratory in New Mexico have pushed the limits of nanotube growth by a factor of 10. In the October Nature Materials, they report making carbon nanotubes a full 4 centimeters long.
Many labs around the world are producing macroscopic fibers out of carbon nanotubes. Such fibers could find applications in products ranging from power-transmission cables to lightweight aircraft materials (SN: 6/5/04, p. 363: Available to subscribers at Nice Threads). However, the nanotubes in these fibers are short—stretching tens of microns, or about the diameter of a grain of sand. Longer nanotubes could twine into more continuous, and therefore stronger, fibers, says Liu.
Long carbon nanotubes could also be useful in producing nanoelectronic devices such as sensors and transistors. Liu says that rather than making one device at a time, manufacturers could chop a single, long nanotube into thousands of tiny devices, all with the same electronic properties.
To grow their nanotubes, Liu’s team deposits iron particles on the surface of a silicon wafer mounted inside a furnace. After ramping up the temperature to 900°C, the researchers pass a stream of gas containing ethanol molecules over the wafer. There, each iron particle catalyzes the growth of a nanotube from the ethanol’s carbon atoms.
The position of the iron catalysts vis-à-vis the growing structures permitted the nanotubes to attain their unprecedented length. In most other methods, nanotubes grow upward from catalytic particles lying on a surface. As new material enters the tube’s base, the tube gets pushed sideways along the surface. The force needed to push the growing nanotube eventually becomes too great, so growth stops while the nanotubes are still short.
In Liu’s technique, the catalyst sits at the tube’s tip rather than at its base. The catalytic tip waves freely, in the ethanol wind, like a low-flying kite, says Liu. Bit by bit during growth, the tube settles onto the silicon surface, much as a slack kite string would.
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Without any resistance from the silicon surface, the nanotube can grow as long as the length of the wafer.
To measure their longest carbon nanotube, the researchers used a scanning electron microscope to shoot 230 images and assembled them into a composite.
Matteo Pasquali of Rice University in Houston says, “The research is significant because it opens up new ways of making continuous, straight carbon nanotubes.” The trick now, he adds, will be to figure out how to make lots of the tubes and how to spin them into strong fibers.