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Preparing Tiny Nanotubes for Big Role in TV

November 09, 1998|LEE DYE

Tiny tubes of pure carbon, microscopic in scale, have bedazzled scientists around the world ever since they were discovered seven years ago by Japanese electron microscopist Sumio Iijima.

These exotic tubes, called "nanotubes," are 100 times stronger than steel, can conduct electricity without releasing heat and flawlessly transmit optical signals--characteristics that have electrified the world of material science. The potential applications are enormous, but every time scientists probe deeper into this mysterious world the picture gets a little more complicated.

It isn't difficult to "grow" the tubes in a laboratory, but having them come out just the way they are supposed to has been a difficult challenge.

Now, scientists at the University of Buffalo claim to have made a major breakthrough that could mean those flat-panel television screens featured at high-tech shows could become much cheaper and much more efficient. The current generation of flat panel screens use liquid crystal displays that are expensive to build and have poor resolution unless viewed from a precise angle.

The scientists, who describe their work in Friday's issue of the journal Science, say they have solved some of the most difficult problems in growing nanotubes for commercial uses.

Nanotubes, measuring only a few billionths of a meter in diameter, are created by heating ordinary carbon until it vaporizes, then allowing it to condense in a vacuum or an inert gas. The carbon forms a series of hexagons that curl and connect into hollow tubes.

But when viewed through an electron microscope the tubes usually look like a bowl of spaghetti.

Physicist Zhifeng Ren of the University of Buffalo figured that if he could grow the tubes so that they were perfectly aligned, he would be a lot closer to using them for such things as flat panel television screens. In conventional television sets, a high voltage electron gun scans the screen, bombarding the phosphorous on the screen and producing the picture.

But the gun has to be some distance from the screen, thus requiring the depth seen in modern television sets. Ren believes millions of nanotubes could act as separate electron guns, and because they are so short, the set could be thin enough to hang on a wall.

The tubes also need to be grown on a substrate of glass--the best and cheapest material to use for a screen--but until now that has not been possible. The temperature required for the tubes to form was so high that the glass began to melt.

So Ren and his colleagues substituted ammonia for the nitrogen that is normally used as the inert gas, and the tubes formed well below the melting temperature of glass.

And they came out perfectly aligned.

The field is so new, Ren said in an interview, "that we don't fully understand it yet." But the results suggest that major progress in creating useful nanotubes has been made.

For example, the tubes grow from a thin coat of nickel that Ren's team applied to the glass. They found that by varying the thickness of the nickel, they can control the diameter of the tubes.

"If you want thin ones, we can grow thin ones," he said. "If you want thick ones, we can grow thick ones."

That is "pretty significant," he said, because "for different applications, you require different diameters."

Nanotubes of varying thickness could be used for everything from extremely strong cables to electrical or optical transmission devices.

Scientists at the Georgia Institute of Technology earlier this year discovered that electrons could pass through a nanotube without heating it, possibly paving the way for ever smaller electronic devices.

"This shows that you can constrain current flows to narrow areas without heating up the electronics," said Walt de Heer, a professor of physics at Georgia Tech.

That could pave the way for even smaller computers, but De Heer believes that may be decades away. The reason, he said, is that carbon materials are incompatible with the silicon that is the basis of current integrated circuits, and that will require a "revolution" in electronic design.

"You couldn't combine the two because they are from different worlds," he said. But, he added, we seem to have moved a step in that direction.

Meanwhile, scientists at the University of North Carolina in Chapel Hill have demonstrated the remarkable strength and durability of carbon nanotubes.

"What we have found is that under large strains, they have the extraordinary property of being one of the stiffest materials known, while also being able to bend without breaking and then be bent back into their original shape," UNC physicist Michael R. Falvo said. "This is unique."

And way out on the fringes, scientists at the Lawrence Berkeley National Laboratory have found that carbon nanotubes--50,000 times more narrow than a human hair--are actually very complicated, forming atom-sized "electronic devices" on their walls as they grow.

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