Noise boosts nanotube antennas
By Eric Smalley, Technology Research NewsResearchers at the University of Southern California have shown that the right amount of noise can enable carbon nanotube transistors to detect weak electrical signals. This is the same effect -- stochastic resonance -- that neurons use to communicate in biological brains.
Systems that exhibit this effect have thresholds, meaning signals must exceed a minimum strength in order for a transistor or neuron to detect them. Adding noise raises the overall energy level, boosting signals that are otherwise too weak to cross the threshold. Too little noise has no effect; too much swamps the signal.
The effect had been predicted in carbon nanotubes because transistors made from the tubes have a threshold. The researchers confirmed that nanotube transistors can detect subthreshold electrical signals under electrically noisy conditions.
The effect makes nanotubes useful as microscopic antennas in communications devices, including cell phones.
Antennas made from arrays of nanotubes would be particularly useful for spread spectrum communications, which involve distributing communications signals across a broad range of frequencies in order to improve reception and make eavesdropping more difficult. "Each tube can act as a dedicated antenna or signal detector for a specific signal," said Bart Kosko, a professor of electrical engineering at the University of Southern California.
In theory, billions of nanotubes could be packed into individual chips. "One scheme would allow each tube to decode for a given frequency," said Kosko.
A major challenge in carrying out the scheme is finding efficient ways to tune the large numbers of nanotubes, said Kosko. A carbon nanotube's resonant frequency depends on its length.
The stochastic resonance effect also makes carbon nanotubes good candidates for processing pixel-based image data. A nanotube transistor could drive each pixel in a display, and noise could help the transistors pick up subthreshold signals, making the transistors more sensitive to input, which would improve image quality.
Nanotube transistor arrays could also be used to sense chemical and biological substances. Carbon nanotube transistors work in saline solution, which means the devices could work inside the body, said Kosko. "A more speculative application would be inserting nanotube arrays into damage nerve tissue to detect nerve signal spikes," he said.
The researchers' experiment involved sending subthreshold signals to carbon nanotube transistors and then adding three different types of noise to the transistor's input. They measured the output to determine how much of the input signal the transistor detected. Adding noise increased nanotubes sensitivity in all three cases.
The experiment showed that stochastic resonance in carbon nanotubes could be extremely useful in signal detection and processing under electrically noisy conditions, said Deepak Srivastava, a senior scientist and task lead at NASA Ames Research Center. "Electrically noisy environments can arise in a variety of conditions such as vast parallel arrays of densely packed nanotube-based devices, broadband communications systems and chemical detection in liquid or bio[logical] environments," he said.
The most exciting aspect of the stochastic resonance behavior is its similarity to biological neurons, said Srivastava. "This shows that it may be feasible to conceptualize and implement signal pulse-train type neural networks on carbon nanotube-based branching networks," he said.
The effect should also apply to non-carbon nanotubes, said Kosko.
Practical applications could be possible in five to ten years, according to Kosko. Researchers will need to improve nanotube transistor performance, and develop techniques for a physically and/or chemically modifying individual tubes in arrays of millions of tubes, according to Kosko.
Kosko's research colleagues were Ian Y. Lee and Chongwu Zhou. The work appeared in the December 2003 issue of Nano Letters. The research was funded by the National Science Foundation (NSF).
Timeline: 5-10 years
Funding: Government
TRN Categories: Nanotechnology; Signal Processing
Story Type: News
Related Elements: Technical paper, "Nanosignal Processing: Stochastic Resonance in Carbon Nanotubes That Detect Subthreshold Signals," Nano Letters, December 2003
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February 11/18, 2004
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