Nanoscale radios & solar cells pave the way to smart dust

Smart dust refers to a network of wireless, autonomously-acting microscopic devices. Built with microelectromechanical systems (MEMS) and using molecular manufacturing processes, these devices would act as sensors, detecting anything from light and vibrations to chemicals and pathogens, and communicating the information over long distances.

Microscopic devices such as this are still hypothetical, and the only place smart dust can be found is within the pages of science fiction novels. (For example, Michael Crichton's Prey, published in 2002, is based on the idea of the emergence of organised behaviour in swarms of self-replicating nanomachines which undergo random and rapid mutations.)

With the rapid pace of technological advances, the development of such devices is getting closer. This week, two separate groups report devices which would prove very useful for the development of smart dust: researchers from the University of California, Irvine, report that they have developed a radio using carbon nanotubes, and a team of chemists from Harvard report that they have used nanotubes as solar-powered cells.

Chris Rutherglen and Peter Burke describe their carbon nanotube radio in the journal Nano Letters. Strictly speaking, the device is not a radio. It is an amplitude-modulated (AM) demodulator, a device which detects and receives electromagnetic radiation and alters its amplitude such that it can be transmitted.

The process of AM demodulation is crucial for the functioning of any radio system. Rutherglen and Burke incorporated their carbon nanotube-based demodulator into a radio system, which was then used to receive radio waves  so that music could be transmitted from an iPod to a speaker located several metres away.

The receiver is nearly 1,000 times smaller than currently available radio technology. Although the demodulator does not constitute a complete radio, the researchers believe it is conceivable that all the components of a radio could in the future be manufactured on the nanscale.

Meanwhile, Charles Lieber and his colleagues at Harvard have developed solar-powered cells based on silicon nanowires. Using the cells on their own or as interconnected groups, Leiber's team were able to provide power for nanoelectric sensors and logic gates (the latter being an electronic device, consiting of transistors or diodes, which are a basic component of integrated circuits).

There are numerous applications for microscopic radio technology. DARPA, the research and development wing of the Pentagon, proposed in 1999 that thousands of tiny wireless sensors could be sprinkled on a battlefield to monitor enemy troop movements, and there are dozens of research groups trying to develop such sensors.

Tiny radios also have potential medical applications, such as monitoring patients' vital signs, or aiding diagnostics by detecting and measuring levels of pathogens, plasma proteins or toxins. They could also be used to monitor environmental conditions, and meteoroligical or geophysical activity.

Like the carbon nanotube radio receiver, the nanowire photovoltaic cells have a wide range of applications. As well as being used for self-powering integrated circuits and other electronics that require tiny amounts of power, multiple stacks of nanowires could be used in nanoelectric, photonic and biological sensing devices. The authors also note that they will aid research into artificial photosynthesis as a clean and renewable energy source. 

References:

Rutherglen C. & Burke, P. (2007). Carbon nanotube radio. Nano Lett. doi: 10.1021/nl0714839. [Abstract]

Tian, B., et al. (2007). Coaxial silicon nanowires as solar cells and nanoelectric power sources. Nature 449: 885-889. [Abstract]