The features in conventional microelectronic circuits are getting smaller and smaller and will soon reach the limit imposed by the fundamental properties of silicon. Scientists hope that carbon nanotubes - which are essentially rolled up sheets of graphite, just nanometres in diameter, with excellent electronic and mechanical properties - might one day be used to replace silicon in electronic circuits.

Large quantities of single-walled nanotubes can be produced by the high-pressure decomposition of carbon monoxide (HiPCO) method. However, nanotubes grown by this technique usually contain large amounts of carbon-based impurities that degrade the properties of nanotube devices.

The purification method developed by Alan Johnson and colleagues at the University of Pennsylvania begins by heating nanotubes produced by the HiPCO method in wet air in the presence of hydrogen peroxide, followed by a gentle acid treatment. Next, magnetic fields are used to separate the nanotubes from the impurities. The semiconductor chips are then dipped into a solution containing the nanotubes to create circuits. "Ultimately we can make it so the nanotubes only stick where we want them to in order to form a circuit," explains lead author Danvers Johnston.

The UPenn team has already made field-effect transistors from the purified nanotubes and shown that they have superior properties compared with devices made from non-purified HiPCO material. Moreover, they have shown that they can determine the energy gap of individual semiconducting nanotubes by measuring the current in their circuits and varying the temperature and gate voltage.