Length matters for carbon nanotubes

Researchers at the University of California, Irvine, US, have managed to create electrically-conducting nanotubes up to 70mm in length. These are by far the longest electrically-conducting nanotubes ever produced and could lead to a range of new technologies, including ultra-dense memory storage devices for computers and high-strength composite materials.

The team, led by Peter Burke, has developed a unique method for producing carbon nanotubes. This involves creating a triple layer catalyst pad consisting of thin films of chromium and gold, onto which the researchers deposit iron-laden alumina nanoparticles. When methane is passed over the catalyst pad at high temperatures, the nanoparticles catalyse the growth of carbon nanotubes. Previously nanotubes of a few millimetres in length were grown but by extending growth time to 70 minutes they could be grown much longer.

The gold film is the secret ingredient allowing the tubes to grow to centimetre lengths. Furthermore, the gold film provides a convenient way to hook the nanotubes up to an electricity supply which can be quite tricky. Burke and his team got around this problem by growing nanotubes between two catalyst pads spaced 40mm apart. The gold in the pads melts during the growth process and flows over the end of the nanotubes, which allows the pads to be used as electrodes.

The nanotubes conduct electricity efficiently, comparable to the best conductivities measured for both metallic and semiconducting nanotubes. ’Our research has taken a significant step forward by showing we can pass electricity through these long nanotubes,’ says Burke. ’Significantly, we have found that our nanotubes have electrical properties superior to copper. This clearly shows for the first time that long nanotubes have outstanding electrical properties.’

According to the researchers, there is no reason why even longer nanotubes cannot be grown, because they only stopped growing either because they hit an obstruction or because the flow of methane was turned off.

Jon Evans