Don’t squash the nanotubes

Researchers at the University of Surrey in Guildford, England, have found that the electronic behaviour of double-walled carbon nanotubes can change drastically when they are squashed or twisted. Among other things, his will have serious implications for the use of nanotubes as interconnects in microchips.

Schematic of a collapsed and twisted double-walled carbon nanotube

Under normal circumstances double-walled nanotubes are metallic, and engineers are looking to use them as chip interconnects and in nano- and micro-electromechanical systems. But the research results obtained by Cristina Giusca and her Advanced Technology Institute (ATI) colleagues reveal that an electronic band gap can open when the tiny carbon cylinders are deformed, in which case they lose their conductivity.

“Our results reveal that a metallic double-walled carbon nanotube behaves as a semiconductor upon severe squashing and twisting,” says Giusca. “This study clearly highlights the role of structural defects at the atomic scale, and the importance of carbon nanotubes’ structural integrity for their use as essential components in the design of practical applications.”

Deformations that could lead to such conductivity changes can result from nanotube growth and processing, where control electrodes are placed on top of nanotubes, and when nanotubes are embedded in other structures. While in many situations this could be a serious problem, the effect of controlled deformations could actually be exploited to tune the material’s electronic characteristics for particular applications.

“These findings will be of relevance to those examining the future integration of carbon nanotubes with conventional existing electronic technologies,” says ATI director Ravi Silva. “And especially for their use as interconnects for the billion dollar semiconductor industry.”

Further reading: “Evidence for Metal-Semiconductor Transitions in Twisted and Collapsed Double-Walled Carbon Nanotubes by Scanning Tunneling Microscopy”, Giusca et al., Nano Lett. 8, 3350 (2008).

Figure: Schematic of a collapsed and twisted double-walled carbon nanotube. Deformed in this way, double-walled nanotubes – which are normally metallic – could lose their ability to function as interconnects in microchips and other electronic devices (source: Cristina Giusca/University of Surrey).

This is a revised version of an article first published in Nanomaterials World.