Carbon Nanotube Electronics Powers Up
Chemical & Engineering News
Volume 83, Number 01, January 3, 2005
Materials Research Society's Annual Fall Meeting
Electronics and semiconductor makers have been salivating over the potential of single-walled carbon nanotubes (SWNTs) since the earliest reports of the structures' electronic properties. However, there are still no SWNT-based electronic devices on the market.
That's about to change, according to Nanomix scientist Jean-Christophe P. Gabriel. The company, based in Emeryville, Calif., plans to begin marketing a SWNT-based hydrogen sensor early this year.
The main stumbling block to using SWNTs in a manufactured device has been that of SWNT chirality. The tubes are usually produced as a mixture of semiconducting and metallic SWNTs. While semiconducting tubes have the desirable electronic properties for making field-effect transistors, the metallic nanotubes do not.
Gabriel said that Nanomix was able to get around this problem of SWNT chirality by using a random network of nanotubes grown directly onto the transistor wafer, as opposed to just a few nanotubes specifically grown from patterned catalysts. This way, Gabriel explained, the chirality issue is moot so long as one can avoid metallic pathways between the electrodes by increasing the ratio of semiconducting SWNTs to metallic SWNTs. The chemical vapor deposition process that Nanomix uses to grow the SWNT network produces more than twice as many semiconducting nanotubes as it does metallic nanotubes.
"Increasing the number of nanotubes allows you to gain robustness and increase the signal-to-noise ratio without increasing size," Gabriel added.
The current wireless version of the hydrogen sensor that Nanomix plans to introduce is a little larger than a matchbook, although Gabriel reckoned that the device could be much smaller given that the battery and integrated circuit take up most of the space. The size of the hydrogen-sensing core, he noted, is in the submillimeter range, and the device requires only a few microwatts of power.
The mechanism of hydrogen sensing is different from hydrogen sensors that rely on the burning of hydrogen, Gabriel noted. Unlike those devices, the Nanomix sensor doesn't require high temperatures. Instead, a proprietary noble metal alloy in the sensor generates hydrogen radicals at room temperature. Gabriel said that the Nanomix scientists aren't precisely certain what the underlying molecular mechanism is, but they think that the hydrogen radicals react with oxygen that's adsorbed onto the SWNT surface. This reaction changes the conductance in the nanotubes, and that change, he explained, is what the device senses.
Gabriel also touted the system's versatility. It can be used in transparent, flexible substrates, he said [Nano Lett., 3, 1353 (2003)]. Furthermore, by functionalizing the nanotubes, it's possible to use the platform to sense for a range of different gases, liquids, and even light. The company recently used the SWNT transistor in a carbon dioxide monitor they hope to market as a portable, disposable breathing monitor for paramedics and first responders [Adv. Mater., 16, 2049 (2004)].
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