A key step was the discovery that nanotubes vibrate when they pick up a radio signal.
The scientists could then dial in precise stations by running electricity through a copper wire near the nanotube, making it more or less difficult for the nanotube to wiggle.
"It's like a guitar string," Zettl said. When you pluck the string, it vibrates with certain note, and if you tighten or loosen the string, it makes the note rise or fall.
To do more rough tuning—say, to change a nanotube that picks up AM stations into one that catches FM signals—the researchers had to shorten the nanotubes.
They did this by running a lot of electricity through the tubes, which made carbons atoms shoot off the tip.
The shorter nanotubes then resonated at a higher frequency, picking up a different range of radio signals.
"We can cover the full range [of FM or AM], but not with one nanotube," Zettl said.
The researchers received their first FM broadcast last year: Derek & The Dominos' "Layla" and the Beach Boys' "Good Vibrations" were transmitted from across the room.
(See a photo of a high-powered microscope for spotting nanoparticles.)
"This breakthrough is a perfect example of how the unique behavior of matter in the nanoworld enables startling new technologies," said Bruce Kramer, a senior advisor for engineering at the National Science Foundation, which funded the work.
Other research groups are also been hot on the trail of a nanotube radio.
In work to be published soon, John Rogers and colleagues at the University of Illinois at Urbana-Champaign, have made an array of six carbon nanotubes that together perform all the functions of a radio.
Whereas Zettl's nanotube radio needs some additional amplifying to produce a signal that could feed into headphones, Rogers said that with his setup, "the headphones literally plug into a nanotube device."
The nanotube radios could also work with tiny sensors in the environment that monitor air and water quality, Zettl said. (Related news: "'Smart Dust' Sensors to Be Used for Eco Detection" [November 14, 2006].)
"Having sensors collecting and relaying information is power intensive," Zettl said. But with the nanotube radio, they could drastically cut the amount of power needed.
For these applications, the nanotubes would not only have to pick up radio signals, but also broadcast them.
This works on the same physical principles, Zettl said, so the researchers "could reconfigure our radio to broadcast."
For most of these applications, nanotube radios also need a power source, such as a tiny battery, he added.
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