The world’s smallest radio, made of a single carbon nanotube (CNT), was recently developed by researchers at the University of California at Berkley. The CNT radio has the four main components of a radio – an antenna, an adjustable tuner, an amplifier and a demodulator – all approximately in the size of a molecule. The CNT radio is “functionally equivalent to a standard AM or FM radio receiver,” says Professor Alex Zettl, head of the Zettl Research Group in the Department of Physics at the University of California at Berkley.
Despite functional equivalence, the CNT radio doesn’t work quite the same way as a conventional radio, which uses radio waves to impinge on a wire and induce an electrical current. The CNT radio starts vibrating mechanically when in tune with a radio signal. The CNT then uses a small voltage and electron field emission to push electrons from the tip of the nanotube to a nearby electrode. The signal is then amplified and has its information stripped from the radio carrier wave through demodulation.
“The vibration is sort of fixed mechanically,” says Zettl, “but when we put this voltage across it to make it field emit, that voltage pulls on the nanotube and tensions it.” The higher tension results in a higher frequency and makes the radio externally adjustable by controlling the amount of voltage applied to the CNT.
The original application intended for CNT radios was for data communication in sensors. “We wanted to build distributed sensors that are measuring air and water quality in a larger region, like over a city,” says Zettl. According to Zettl, these CNT radios can also be useful for any applications that require very small, low power radio devices, including biological systems.
“The radio is so small that it would easily fit inside a living cell and not perturb it very much,” says Zettl. Another potential use he identified was receiving and stimulating biological vision commands to help restore vision for the blind.
“The bottom line is this thing works over a huge frequency band, we can tune it, not just with tension, but we can chop the nanotube off and make it shorter and make it work in the FM range, the AM range or in the microwave range,” says Zettl.
Listen to Eric Clapton's "Layla" as received by the CNT radio. There is a significant amount of static noise, but according to the University of California at Berkeley it was accomplished with none of the external circuitry to filter or process the signal typically found in macroscopic radios.