Orientation dictates resistance

A new study from the University of North Carolina at Chapel Hill shows that electrical resistance between nanotubes and graphite surfaces that support them varies according to how the tubes are oriented.

The discovery, which could be important to telecommunications and other electronics industries, indicates it’s possible to alter the resistance by changing the tubes’ position on a flat surface.

Resistance peaks six times, as the end of a nanotube is rotated 360 degrees, the scientists found. This, they say, is because atoms in the nanotube and graphite are arranged in hexagons.

Richard Superfine, associate professor of physics and astronomy believes the discovery is important on two counts.

‘First, it is the most direct measurement that electrons in a material travel in particular directions and that those favoured directions need to be matched as you go from one material to another where they touch,’ he said. ‘Second, this effect is pronounced in carbon nanotubes, threadlike molecules that conduct electricity and have the potential to be used for ultra-small circuits.’

Researchers need to be sure when making such devices that the preferred directions are aligned when the devices are assembled, said Superfine.

Conversely, the effect can be used to make sensors that measure the rotation of nanometer-scale objects.

Nanotubes are created by arcing electricity between two sticks of carbon. They measure 10 to 30 nanometers in diameter and about one to five millionths of a meter long.

‘Tuneable resistance in nanotubes may be useful in molecular scale machinery where you have moving, sliding and rotating parts,’ said Superfine. ‘You need to be able to sense the motion of those parts in an indirect way, such as through the measured current, because in an assembled device you will not be able to look directly at the part.’

Earlier research by the UNC-CH team showed that carbon nanotubes roll across a surface rather than slide when the nanotube is put on graphite.

Rolling occurs because the atoms in the outermost layer of the nanotube interlock with the atoms on the graphite surface. When the atoms interlock, the nanotube rolls, and when the atoms are not enmeshed, the nanotube slides.

This means that the atoms are acting like gear teeth. Together with findings on the electrical properties of these atomic scale contacts, the UNC-CH researchers are creating the foundation of ultra-small scale engineering of machines.

The continuing experiments involve recording mechanical and electrical properties of carbon nanotubes with a nanoManipulator; a unique device invented by the UNC-CH researchers.

The nanoManipulator combines a commercially available atomic force microscope with a force-feedback virtual reality system.

The former employs an atomically small, gold-tipped probe capable of bending and otherwise manipulating molecule-sized particles. The latter allows scientists to see and feel a representation of the surface a million times bigger than its actual size

‘This is one potentially very important piece of that puzzle – how do really small contacts conduct electricity?’ said Mike Falvo, research assistant professor of physics. We’ve shown that unlike in large contacts, in very small ones their relative orientation can have a profound effect on current flowing through them. Knowing this could be critical to building the tiniest electromechanical switches.’