Researchers have fashioned the world’s tiniest radio out of a carbon nanotube. The nanotube, placed between two electrodes, combines the roles of all the major electrical components in a radio, including the tuner and amplifier. It can tune in to a radio signal and play the audio through an external speaker.
While the practical application of the radio is uncertain, it could be used in biological and environmental sensors. Researchers are now developing microelectromechanical (MEMS) sensors to measure blood sugar levels or cancer markers in the body. Instead of researchers using a stamp-size radio-frequency identification tag, a nanotube radio could be packaged with the MEMS-based sensor and injected directly into the bloodstream, says Alex Zettl, an experimental physicist at the University of California, Berkeley, who is leading the development of the nanotube radio. Once in the body, the radio could provide wireless communication between the tiny biological sensors and an external monitor. To do that, however, the nanotube radio would have to work as a transmitter. Right now, it is only configured as a receiver, but Zettl says that “the same physics would work as a transmitter.”
The nanotube radio works differently than a conventional radio does. Conventional radios have four main functional parts: antenna, tuner, amplifier, and demodulator. Radio waves falling on a radio antenna create electric currents at different frequencies. When someone selects a radio station, the tuner filters out all but one of the frequencies. Transistors amplify the signal, while a demodulator, typically a rectifier or a diode, separates the data–the music or other audio–that has been encoded on a “carrier” electromagnetic wave.
Zettl’s team used one carbon nanotube for all these functions. Because of their unique electrical properties, carbon nanotubes have been previously used to make electronic components such as diodes, transistors, and rectifiers. “It was a revelation that all of this could be built into the same [nanotube],” Zettl says.
The nanotube is grown sticking out from a tungsten surface, which acts as a negative electrode. The tip of the carbon nanotube is also negatively charged. A vacuum separates the nanotube from a positive copper electrode. The researchers use an external battery to apply a voltage between the two electrodes. Electrons jump out from the negative nanotube tip to the positive electrode, creating what is called a field emission current.
Zettl explains that the “nanotube does not act as an antenna in the conventional sense.” That is, instead of picking up electromagnetic waves electrically, it picks them up mechanically. This happens because of the nanotube’s natural resonance frequency. As soon as it encounters radio waves that match the frequency, the nanotube starts vibrating in step with the waves, effectively tuning in only to that radio signal. The nanotube’s vibrations change the field emission current, and the mechanical vibrations are converted into an electrical signal. An external battery powers the field emission current and amplifies the radio signal. The field emission is naturally asymmetrical–it allows current to flow only in one direction, just like the diodes and rectifiers used in demodulators. So the nanotube also acts as a demodulator and detects the music encoded onto the carrier wave.
To tune to a different radio station, the researchers change the resonance frequency of the nanotube. They do this by changing the voltage applied across the electrodes. “It’s like tuning a guitar string,” Zettl says. “The electric field pulls on the nanotube.” With the same nanotube, the researchers can cover the entire FM radio band.
Cees Dekker, a nanotube researcher at the Delft University of Technology, in the Netherlands, calls the new radio “an appealing demonstration that very simple devices can be used for everyday [tools].” Whether or not the device is used for sensors remains to be seen, he says, but for now, the simple demonstration is a good start.