'I think people have avoided what dispersing the nanotubes does to them,' says Kruse. 'We assumed that all the chemical procedures in nanotube preparation were well-studied. But this whole doping issue is not something people have picked up on.'
Kruse and Moonoosawmy came across the doping effect whilst trying to understand how carbon nanotubes are functionalised. They noticed unexplained peak shifts in the Raman spectra of nanotubes that had been sonicated in chlorinated solvents. Sonication, usually in an ultrasonic bath, is a way of agitating nanotubes to prevent them clumping together and is one of the most common procedures carried out prior to bulk processing.
As the team suspected, the peaks shifts indicated doping. The effect is caused by iron contaminants reacting with decomposing solvents to produce iron chloride, a known dopant. Iron is often used as a catalyst for nanotube growth, but it is difficult to eliminate entirely from later processing. According to Kruse, sonication can cause chlorinated solvents such as ortho-dichlorobenzene (ODCB) and dichloroethane to decompose, releasing chloride ions and chlorine gas that react with the iron impurities.
Getting the dope
To prove their theory, the researchers showed that adding chlorine gas and hydrogen chloride to ODCB caused doping without sonicating, and confirmed iron chloride was the dopant using X-ray photoelectron spectroscopy. Like other 'p-type', electrophilic dopants - as opposed to 'n-type', or nucleophilic, dopants - iron chloride draws electrons from nanotubes, increasing the conductivity of a material. But, says Kruse, whether it sits inside the nanotube, clings to the outside, or just gets stuck in the spaces between tubes is not entirely clear.
Kruse thinks the research will help engineers pin down the source of unintentional doping and achieve greater control over the electronic properties of devices that use nanotubes, such as transistors. 'You can use it to avoid doping or you can use it to intentionally dope. But to get control of doping is important because in electronics you don't just need conductors, you also need insulators,' he says.
But John Robertson, an electrical engineer at the University of Cambridge, says the finding is by no means revolutionary and thinks the applications of nanotubes in electronics are still limited. 'Nanotubes, like a lot of organic conductors, aren't wonderful in the sense of doping. You're not going to use this so-called chemical doping for something like a field-effect transistor, because the dopants can just come out.'