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Tiny molecules called nanotubes have scientists dreaming big

Monday, October 11, 1999

By Byron Spice, Science Editor, Post-Gazette

In this computer-driven world, a new technology that might make computers even smaller and more powerful than today's will always garner attention. But in the case of nanotubes -- tiny, tube-shaped carbon molecules discovered in 1991 -- startling electronic properties are only one source of wonderment.

Karl Johnson, for instance, knows he can amaze his chemical engineering students at the University of Pittsburgh by discussing nanotubes' hydrogen storage capabilities. Imagine, he tells them, that you pump as much hydrogen gas into a container as it will hold. How can you get more hydrogen in? By adding a bunch of nanotubes.

The idea of making space for more hydrogen by adding solid material is counterintuitive, Johnson admits. But you have to understand that hydrogen gas is very dilute and that nanotubes have an uncanny ability to adsorb hydrogen -- that is, to collect hydrogen on their surfaces. Adsorbed hydrogen, he explained, can be more densely packed than is possible by compressing hydrogen gas.

Nanotubes thus might find use as a storage medium for cars powered by hydrogen fuel cells. Others have suggested that these light, elastic, but strong carbon molecules might be used to make new structural materials. Some say they could be used to build analytical chemistry laboratories the size of a button.

But this speculation is just that, maintains Ken Jordan, a Pitt chemist. Nanotubes seem quite simple -- each is basically a single-atom-thick sheet of carbon rolled into the shape of a tube -- but their properties in many ways remain as mysterious as they are amazing.

"Our goal is to try to understand at a microscopic level how these things are functioning," said Jordan, who directs Pitt's Laboratory of Molecular Simulations and Material Sciences.

That effort recently got a major boost with the installation of a $900,000 cluster of 25 IBM RS/6000 computers, obtained through grants from IBM and the National Science Foundation, along with university support. Linked by a high-speed switching unit, the computers can work together to run complex simulations of the performance of nanotubes and other molecules.

Nanotubes, like other examples of "nanotechnology," get their name from the nanometer, a unit of measurement equal to one-billionth of a meter. Some nanotubes measure just a nanometer in diameter.

They are in many ways similar to another recently discovered carbon molecule, called a fullerene. The carbon atoms in fullerenes are bonded together in hexagons and pentagons, connecting together like a soccer ball to form a sphere.

Nanotubes can be considered elongated fullerenes with their ends cut off. Rather than the combination of hexagons and pentagons necessary to form a ball, all the atoms in the straight-sided nanotubes can be arranged in sheets of hexagons.

One aspect of nanotubes that has fascinated researchers is the way their properties can change simply by varying the way the sheets are rolled.

If the sheet is rolled so that its hexagons line up straight along the tube's axis, the nanotube acts as an electrical conductor, like a metal. If the sheet is rolled on the diagonal, so that the hexagons spiral along the axis, the nanotube becomes a semiconductor, the type of material used to make transistors, diodes and many other electronic components.

Phaedon Avouris, who heads the nanoscale science group at IBM's T.J. Watson Research Center in Yorktown Heights, N.Y., said nanotubes have been used to make transistors and other devices and might well enable designers to cram more transistors onto faster, more powerful computer chips. The first uses, he added, are likely to be in specialized applications, such as on spacecraft, where light weight and durability are bigger concerns than cost.

When nanotubes might find their way into mainstream electronics is unclear. Silicon-based electronics is a trillion-dollar-a-year business, Avouris noted, "and nobody's going to drop that overnight and go into carbon nanotubes."

Hybrid devices are most likely; nanotube conductors are more durable than metal and thus might be used to interconnect silicon circuits.

Avouris' group has developed ways of manipulating nanotubes so that electronic devices can be constructed, but a number of puzzles remain regarding the properties of nanotubes. Recently, for instance, Stanford University scientists observed that nanotube semiconductors which conducted electricity in one manner under a vacuum suddenly changed conductive properties when exposed to oxygen. In response, Avouris and Jordan have begun experiments to explore how oxygen exposure affects nanotubes.

Also, Jordan is studying how contact with a surface, such as a computer chip, can change the nanotube's shape, which in turn changes its electrical properties. And he is exploring what role defects in nanotube structure might have on nanotube properties. Computer models, he explained, always show perfect structures but it's possible that nanotubes routinely have defects that might account for many of their properties.

Another Pitt chemist, Gilbert Walker, hopes to use nanotubes to construct "bioreactors." The nanotubes would be used as little test tubes, holding reagents used to perform chemical analyses. Other nanoscale machines, such as pumps, might be used to squirt the chemicals where needed.

"The whole laboratory would be on a chip," he said.

The military might use such a device as a sensor that each soldier might carry to detect poison gas or biological weapons. In the environmental field, water, soil or air samples that otherwise might need to be sent to a lab for analysis could be monitored in the field, or routinely checked by sensors mounted in smokestacks or effluent pipes.

But again, this remains only a promise for now. Walker said it appears nanotubes may be susceptible to the very reagents he hopes they might store, so more study is necessary to see how the tubes might be strengthened or made less reactive.

Johnson, the chemical engineer, has found that nanotubes might serve as a "quantum sieve" that can separate common hydrogen from its radioactive isotope, called tritium. Thanks to quantum mechanics, he explained, nanotubes can be designed to selectively suck up tritium.

Both the common isotope of hydrogen and tritium are roughly the same physical size, though tritium is more massive, Johnson said. Both isotopes might be squeezed into a properly sized nanotube, but the laws of quantum mechanics would cause the common hydrogen to feel confined and wiggle out of the tube.

This sort of nanotube thus could be used as a filter to separate the two isotopes, though Johnson acknowledged that outside of the nuclear weapons program, which produces tritium for nuclear warheads, it might not have an immediate use.

More likely is the use of nanotubes for storing hydrogen, a fuel that might be used to power automobiles someday. Johnson said he began studying the ability of nanotubes to adsorb hydrogen several years ago, when he worked at the Naval Research Laboratory, and has continued since joining Pitt four years ago.

But even in this area, questions about nanotubes' practicality abound. Despite their prodigious ability to adsorb hydrogen, computer simulations suggest they can't store enough hydrogen to provide the 200-mile range considered essential for public acceptance of a fuel-cell-powered car, Johnson said.

Some experiments, however, show that nanotubes seem to be adsorbing hydrogen at greater densities than are predicted by theory.

"That's a bit of a mystery that we're pursuing now," he added. "That's what makes this job interesting."

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