Controlling conductivity in carbon nanotubes

A South African PhD student has found ways to control the spin transport in networks of the smallest electrical conductor known to man — the carbon nanotube. Her work has been published in the journal Scientific Reports.

Discovered in Japan in 1993, carbon nanotubes are the thinnest tubes in the universe, consisting of a cylinder of single carbon atoms. At the time of their discovery, these revolutionary materials were expected to replace silicon in electronic circuits, such as microchips and computer hard drives.

“Carbon nanotubes are known for their ability to carry a high amount of electrical current and they are very strong,” said Siphephile Ncube, from the University of the Witwatersrand (Wits University). “They are very thin but electrons can move very fast in them, with speeds of up to gigahertz or terahertz, and when coupled to nanomagnets they greatly extend the functionality of the carbon nanotubes, which is required to advance modern technology through the development of high-speed spintronic devices.”

Collaborating with other researchers from Wits University, the University of Johannesburg and Paul Sabatier University, Ncube chemically attached nanoparticles of the rare earth element gadolinium to the surface of carbon nanotubes to test whether the magnetism increases or inhibits the transfer of electrons through the system. The measurements to interrogate the effect of magnetic nanoparticles on a network of multiwalled carbon nanotubes were carried out at Wits’ Nanoscale Transport Physics Laboratory (NSTPL).

The researchers found that the electrical conductivity in the nanotubes can be increased by incorporating the spin properties of the gadolinium, which arises from its magnetic nature. The presence of a magnet in an electron transfer media introduces another degree of freedom that enhances the electron transfer, but only if tailored precisely.

“We found that the effect of the magnetic nanoparticles is read off in the electronic transport of the nanotubes,” said Ncube. “Due to the presence of the magnet, the electrons become spin polarised and the charge transfer is dependent on the magnetic state of the gadolinium.

“When the overall magnetic poles of the gadolinium are oppositely aligned, it causes higher resistance in the nanotubes and slows down the flows of electrons. When the magnetic poles are misaligned, it has a low resistance, and assists the electron transport.”

This phenomenon is known as the spin-valve effect, and has wide application in the development of hard disk drives used for data storage.

“Ncube’s research established the great potential of carbon nanotubes for ultrafast switching device and magnetic memory applications, a realisation we have been working towards since the establishment of the NSTPL facility in 2009,” said Ncube’s PhD supervisor, Professor Somnath Bhattacharyya. “To date, modified nanotubes have demonstrated good spin transport for devices made from individual nanotubes. For the first time we have demonstrated spin-mediated electron transport in a network of nanotubes without incorporation of magnetic leads.”

Image caption: Siphephile Ncube. Image credit: Wits University.