Simple silicon trick controls nanotubes


TROY, N.Y., April 3 (UPI) -- An innovative approach to placing patterns on silicon chips could be the key to making molecule-sized tubes of carbon an economical building block for products such as materials, video equipment and filters, scientists said Wednesday.

A team of materials science researchers at Rensselaer Polytechnic Institute, led by Professors Pulickel Ajayan and Ganapathiraman Ramanath, describe in the journal Nature their method for reliably controlling the growth of carbon nanotubes.


The team's work hinges on a simple fact of chemistry -- the carbon tubes will not grow on pure silicon but assemble quite well on silicon dioxide. As long as there are distinct edges to the oxide areas on a silicon chip, the tubes can grow in several directions at once, Ajayan told United Press International.

"Playing around with the topography of the (silicon/oxide) pattern is actually easier to do than the existing processes where they pattern the metal catalysts on the (silicon)," Ajayan said. "What we've achieved is getting rid of a major step in the process. The whole thing has become as simple as patterning silicon dioxide on silicon, which microelectronics people know how to do very well."


The hollow nanotubes, structures with walls only one atom thick, a few atoms across and hundreds of atoms long, are seen as the building blocks for stronger, lighter materials of all kinds, but usually they grow in a random way when scientists create the proper conditions. The RPI team's method has so far coaxed the tubes into creating cones, well-ordered and densely packed squares and even 3-D shapes resembling flowers.

The method is so predictable that even complex solids, such as pyramids, could be patterned to grow nanotubes on each face of the silicon dioxide object, Ajayan told United Press International.

This should help bridge the gap between computer models of nanotubes and their real-world counterparts, he said, since researchers will be able to more accurately duplicate their models in experiments.

The most immediate use for this discovery, Ajayan said, could be to "weave" nanotubes into a membrane with pores only a few atoms across. Such a filter could be dozens of times more effective than the best products available today.

Such nanofilters are likely a first step in engineering nanostructures, said Mikhail Roco, a physicist who is the National Science Foundation's chief adviser on nanotechnologies. The RPI work results in a process easy to apply on a larger scale, Roco told UPI, adding he thought such a development would have taken much longer to achieve.


"I won't say this is a breakthrough is science, it's more of an innovation," Roco said. "This is part of a trend we've observed in the past few months ... you find several groups working on creating (nano)systems instead of components."

The process could be vital to attempts to create computer circuits out of nanotubes, Roco said. Researchers now should be able to grow the tubes in the positions they choose instead of trying to manipulate them after they are grown, he said. The method will become even more useful when scientists figure out how to apply it to other materials, he said.

As tools for patterning silicon improve, it could soon be possible to grow single nanotubes in a regular pattern, Ajayan said. This could be very useful for electronics makers looking to make next-generation video displays with field emitter arrays, which could use nanotubes to transform electric current into radiated energy such as light.

Roco said the RPI method would make nanotubes more affordable for such uses. Other challenges remain, he noted, such as how to properly control each tube or group of tubes to create an individual dot on the screen.

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