by Mark Schrope
Berkeley - February 21, 2000 - The squeaky-clean image of buckytubes has taken a beating. Ever since their discovery, these carbon nanotubes have dazzled scientists with their near-ideal combination of strength, heat conduction and electrical properties.
But now the truth is out: research published this week shows they could be one of the noisiest components ever considered for use in electronics.
A team from the University of California at Berkeley and the Lawrence Berkeley National Laboratory, led by Alex Zettl, has reported on the first extensive study of noise in nanotubes.
Noise is the name given to voltage fluctuations that can occur when a constant electric current is injected into a device.
Zettl found that buckytubes were millions of times noisier than conventional conductors and semiconductors.
This unususpected flaw in their character might rule them out for some applications.
"It's good work," says Hongjie Dai, a chemist at Stanford University in Palo Alto, California.
"If you talk about using nanotubes as transistors in computer chips, this kind of noise level could be a problem.
" Zettl says: "Some people interpreting my results have said, 'Boy, you just threw cold water on this entire field. It's dead.' But for me it's exactly opposite.
" He takes the view that the phenomenal noise is just another fascinating and unexplored characteristic of nanotubes.
"If you want to solve the problem you better find the origin first," says Dai. But Zettl has a theory about what is happening.
In conventional conductors, noise can be produced when atoms of a contaminant move around, disrupting the flow of electrons.
Zettl says some researchers assumed nanotubes wouldn't have this problem because their carbon atoms are tightly bound, leaving no path for disruption.
He believes the noise arises because nanotubes are built up from rolled sheets of carbon just one atom thick.
Every electron passing through "sees" the surface of the sheet, and this makes the nanotube vulnerable to the effects of anything striking the surface.
In contrast, in a conventional conductor such as a copper wire, most electrons flow through the centre of the wire so the surface is relatively unimportant.
According to Zettl, gas molecules from the environment could be scurrying about like gnats on the outside of nanotubes, disrupting electron flow.
If so, it might be possible to solve the problem by building nanotube-based electronics in clean rooms, sealed off from outside air, like those used by chip manufacturers.
Zettl's group is also studying multiwalled nanotubes--groups of concentric tubes resembling closed telescoping antennas.
He says there may be a way to send electric current through the inner tubes to avoid noise.
Dai and Zettl agree that in some applications noise won't matter.
For instance, when nanotubes are used as chemical sensors (New Scientist, 25 December 1999, p 12), the target substances produces such a strong signal that the noise can't drown it out.
"I don't want to pretend this noise is not a big problem," Zettl says. "But I don't think it's at all a show stopper.
This article appeared in the February 19 issue of New Scientist New Scientist. Copyright 1999 - All rights reserved. The material on this page is provided by New Scientist and may not be published, broadcast, rewritten or redistributed without written authorization from New Scientist.
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