Silicon's successor lurks in the lab
Published: 20 Oct 2003 16:05 BST
To the drawing board
While the benefits seem infinite, researchers are quick to point out that such results have been limited so far because mass manufacturing has yet to take place.
Today, carbon nanotubes are made in two ways. The first, known as the laser ablation method, was pioneered by CNI and involves blasting graphite with a laser. The second, the modified gas method, involves spraying a hydrocarbon gas like methane or CO2 over a molten metal catalyst.
Removing impurities, such as metallic catalyst particles, is a challenge in both. IBM and others are experimenting with new fabrication techniques, such as building silicon-carbon crystals and then evaporating the silicon, but no one has an answer yet.
Another major problem lies in controlling something called "chirality," a measure of the arrangement of the hexagons on the surface of a tube. If the carbon hexagons run in parallel vertical lines on the surface of the tube, they will act like a metal and can't be used in electronics. If the rows of tubes are slightly swirled (think of the cardboard on a paper towel roll), they will act like semiconductors and can be used as transistors.
Unfortunately, the factors behind tube formation remain something of a mystery.
"You leave the world of classic physical mechanics, and you enter the world of quantum mechanics," Appenzeller said. "The graphene sheet is the same. That's why it is so difficult to predict the chirality." Graphene sheets are made up of carbon hexagons.
Smalley, Dai and others are hoping to control both of these characteristics through "selective catalysts." "If you can control the seed (catalyst) well, you should be able to control the nanotube," Dai said. "Over the years, we have found that the catalyst controls everything."
The next challenge is arranging the nanotubes in products. Placing tubes in exact locations in products such as chemical sensors or flat panels isn't a problem, because they are painted in. Chips, however, will require that individual nanotubes be placed between specific contacts.
Scientists hope to grow the nanotubes on a wafer. Researchers at Duke University and Stanford have shown that it is technically possible to grow and position tubes, but many hurdles still need to be cleared.
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your efforts are appriciated to provide thew usefu... Moazzam Ali Kausar
