Wireless goes nano – the world’s smallest radio

Two competing groups of engineers and applied physicists recently reported on the development of radio receivers based on single carbon nanotubes. The development could lead to integrated nanoscale wireless systems with a number of industrial, medical and other applications.

Carbon nanotube used as a radio receiver (waves added for visual effect)

First came Chris Rutherglen and Peter Burke from the University of California, Irvine, with a paper in Nano Letters on 17 October describing a carbon nanotube demodulator. This is a core element of a radio receiver.

In their lab demonstration, the researchers incorporated their demodulator into a complete receiver system, and used this to transmit music from an MP3 player to a speaker positioned around a metre away from the player.

Demodulation arises from the nonlinear current-voltage characteristics of both metallic and semiconducting nanotubes, and this can be used to convert part of the radio signal to direct current. The demodulation effect can be maximised by biasing the nanotube so that the operating point is centred on the maximum nonlinear portion of the current-voltage curve.

“Our next steps are to miniaturise the radio even more, and investigate whether the nanotube itself can serve as an antenna,” says Burke. “We have projects funded by the US Army to work on this. We also are looking at making amplifiers out of this technology.”

It looks as if Burke and Rutherglen have been beaten to the line, as but two weeks after their announcement, there was a paper published online in the same journal detailing a complete radio system based on a single carbon nanotube.

A team of researchers led by Lawrence Berkeley National Laboratory and University of California at Berkeley physicist Alex Zettl has created the first fully functional nano-radio, in which the carbon nanotube serves as antenna, tuneable band-pass filter, amplifier and demodulator. Their system works with both AM and FM signals in the 40–400 MHz range used for broadcast radio, and also the >1 GHz frequencies of mobile telephone networks and satellite navigation.

“One of the most exciting features of our single-nanotube radio is that it is a fully functioning radio,” says Zettl. “It integrates all of these critical functions together into a single package that is again nanoscale; indeed, the entire package is of molecular scale.”

Unlike conventional radios, which are entirely electrical in operation, nano-radios exploit the vibration of a carbon nanotube tip when radio waves interact with it. Zettl’s colleague Kenneth Jensen, who carried out the actual design and construction of the nano-radio, describes nanotubes as being like tiny cats’ whiskers. Jensen’s work on the nano-radio followed the construction of a sensor capable of measuring forces of the order of attonewtons. “Within a few hours of figuring out that our force sensor was in fact a radio, we were playing music!”

Amplification and demodulation arise from the needle-point geometry of nanotubes, which gives them unique field emission properties. By concentrating the electric field of the DC bias applied across the electrodes, the radio produces a field-emission current which is sensitive to the mechanical vibrations of the nanotube. And since this field-emission current is generated by an external power source, amplification of the radio signal is possible.

As well as applications in communication systems, Zettl says that the nano-radio technology could prove especially useful in biological and medical applications. “The entire radio would easily fit inside a living cell, and this small size allows it to safely interact with biological systems,” he says. “One can envision interfaces with brain or muscle functions, or radio-controlled biological devices moving through the bloodstream.”

Further reading: Carbon Nanotube Radio, Chris Rutherglen and Peter Burke, Nano Lett. 7, 3296 (2007); Nanotube Radio, Jensen et al., Nano Lett. 7, 3508 (2007).

Figure: Nanometre scale organisation of molecular components on a copper surface demonstrates sorting of two sizes of molecules through molecular self-selection. The spacing between molecular rows is around 1 nm (source: Forschungszentrum Karlsruhe & Max-Planck-Institut für Festkörperforschung, Stuttgart).

Article first published in Nanomaterials News.