Nanotubes Fall into Line

For a decade, researchers have explored the strange and fascinating molecules called carbon nanotubes. But so far, no one has solved the field’s biggest hurdle: growing the tubes they want, where they want them. In a landmark paper published April 5 in Science Express, a team from the University of Cambridge reports success at exactly that.

Picture a tangled plate of cooked pasta next to a neat bundle of raw spaghetti. That’s the difference between ordinary nanotubes and the nanocrystal arrays produced by the Cambridge team. Their technique for growing nanocrystals yields perfectly aligned, dense groves of single-wall nanotubes-and controls exactly where the crystals are deposited.

“It’s a very big breakthrough,” says Tom Theis, research director at IBM’s Watson Research Center. “The indication is that under certain conditions they were able to get bundles of nanotubes of a single type to grow, all aligned and all of the same type.”

The Cambridge discovery may open the way to that wide range of practical applications researchers envision because it puts in their hands for the first time the ability to produce carbon nanotubes as a bulk material.

Happy Accident

Until now, nanotechnology researchers and commercial companies haven’t had an easy method for producing large numbers of uniform nanotubes with predictable properties (see A Bucketful of Buckytransistors). Tube production techniques that use lasers, carbon arcs or gas-phase deposition generate clumps of nanotubes in stringy, random heaps. The result is a disordered mix of single-wall and multi-wall tubes and nanostructures of varying sizes-a molecular pasta salad.

The work done by Mark Welland’s team at Cambridge’s Nanoscale Science Laboratory apparently solves the tube production problem.

But as with many scientific innovations, the discovery was made almost by accident.

Working with UCLA chemist James Gimzewski and Maria Seo and Reto Schlittler at IBM’s Zurich Research Laboratory, Welland’s group had been planning to produce multi-walled nanotubes, hoping to connect them to other molecules to form a nanocircuit.

What they discovered was far more interesting, and a little hard to believe. Indeed, the team spent six months verifying what they had created-the first-ever perfectly ordered nanotube arrays.

“Our findings were totally unexpected,” says Colm Durkan, a lecturer in Cambridge’s engineering department. “We can now fabricate nanotubes with the electrical and mechanical properties we require and position them where we want.”

From Buckyball to Tube Crystal

To grow the nanotube crystals, the team created pillars of buckyballs, then cooked them in a vacuum. (Buckyballs are molecules made of 60 carbon atoms, shaped like a soccer ball; they were named for Buckminster Fuller.)

The pillars were created by depositing alternating layers of buckyballs and a nickel catalyst onto a substrate, patterning them with a ceramic mask attached to an atomic force microscope. The screen-like mask was perforated with holes 300 nanometers in diameter; the microscope allowed the scientists to position the pillars with one-nanometer precision. (One nanometer is one-billionth of a meter, about the width of five atoms.)

The researchers then heated the pillars to 900 °C in the presence of a magnetic field. The result was a pattern of carbon nanotube crystals. Although nanotube type varied from crystal to crystal, each crystal contained millions of uniform and well-ordered nanotubes.

“We’re beginning to understand how the crystals self-assemble, but there’s still some strange things going on,” says Welland. In some cases the crystals didn’t grow at all. In other circumstances the crystals sprouted in sizes as large as 10 microns.

Welland says the group is investigating how to produce a continuous film of the nanotube crystals. “We’re also aiming to grow crystals of semiconducting tubes so that we can build nanotransistors with them,” he says.

As the principal sponsor of the research, IBM will have the first opportunity to commercialize the nanotube crystal process.