A simple and scalable way to purify mixtures of carbon nanotubes has been developed by researchers in the US. The process uses ultraviolet light and air to produce purified semiconducting nanotubes, which could be valuable in developing the next generation of computer chips. 

Due to their unique shape and electrical properties, nanotubes are highly promising candidates for electronic components of the future. However, nanotubes can be either highly conductive (metallic) or semiconducting, depending on how their component carbon atoms are aligned. Up to a third of the nanotubes in a freshly synthesised sample can be the metallic form, and this blend is troublesome when trying to use nanotubes in electronic devices such as transistors that rely on semiconductance. The electrical current will just travel down the metallic tubes, as they are the path of least resistance. 

Previous attempts to overcome this problem have relied on trying to selectively separate out the metallic tubes. Now, a team led by Chongwu Zhou at the University of Southern California, Los Angeles, have found a much easier way to homogenise the tubes: using ultraviolet light to  oxidise them, a process which forms defects in the carbon structure of the metallic tubes, turning them semiconducting. 

At the correct wavelength, ultraviolet light will break up oxygen molecules in air, producing oxygen radicals and ozone. These can attack the carbon nanotubes, attaching oxygen groups onto the surface. Although this does not seriously affect the semiconducting tubes, it dramatically reduces the conductivity of the metallic tubes. 

'This conversion leaves the nanotubes with excellent transistor characteristics,' says David Carey, a carbon nanotube expert at the University of Surrey, UK. Semiconductor devices made from nanotubes have great potential, he explains. 'By removing the metallic nanotubes, they are increasing the transistor current on/off ratio - which means that devices can really be turned off.'

Importantly, the process promises to be scalable - so large quantities of material could be converted at once, or the conversion could be used as the last step in the manufacturing process. 'I think this really opens up the possibility of nanotube-based large-area nanoelectronics,' Carey says. 

Iris Nandhakumar at the University of Southampton, UK,  is more cautious about the results. 'To really show industrial scalability this will require further studies,' she says, 'but if the results can be reliably reproduced then it is an exciting breakthrough.'

Lewis Brindley

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