A Story with a Twist

Doing the twist may have faded away along with other fads of the 1960s, but the twist promises to make a comeback in the world of tiny molecular structures. As described recently in the first issue of the new journal Nature Nanotechnology, Dr. Ernesto Joselevich and his team in the Weizmann Institute’s Materials and Interfaces Department have managed to manipulate the properties of carbon nanotubes by twisting.

 

Carbon nanotubes are strong, flexible molecular wires that are harder than diamond and conduct electricity better than copper. Thanks to their unusual characteristics, they are ideal candidates to be components of tiny electronic and mechanical devices. Since their discovery more than a decade ago, scientists have been exerting great efforts toward understanding their properties, mainly in order to use them for building all sorts of specialized, highly structured instruments.

 

One of the traits of carbon nanotubes is their “split personality” when it comes to electrical conductivity. They can behave either like a metal, which is characterized by excellent conductivity, or like a semiconductor – such as the silicon in electronic chips – which, under different conditions, can be  either a conductor or an insulator. Whether carbon nanotubes behave like conductors or semiconductors depends on their diameter and the way they form a spiral – a property known as chirality – the direction in which the carbon surface “rolls up” to create the tube. The chirality phenomenon generates molecules that have identical chemical compositions but differ from one another in their spatial structure, so that one molecule is a mirror image of the other – just as the right and left hand both resemble and differ from each other (hence the name: chiros means “hand” in Greek). Therefore, despite their identical chemical makeup, these molecules cannot overlap with one another, just as the right hand cannot overlap with the left.

 

Chirality had always been considered a basic property that could not be changed, but Joselevich asked himself whether it could be altered by twisting the nanotube – thereby turning a conductor into a semiconductor, or vice versa. The team, in addition to Joselevich, included research students Tzahi Cohen-Karni, Lior Segev and Onit Srur-Lavi, as well as Dr. Sidney Cohen of Chemical Research Support.

 

To twist nanotubes, the scientists created a unique device: a nanotube connected to two electrical contacts, with a pedal in the middle. Pressing the pedal with the tiny tip of an atomic force microscope caused the tube to twist, and the change in conductivity produced by the twisting was measured by the electrical contacts.

They found that gradual twisting of the nanotube led to periodic increases and decreases in conductivity. A check of the electron distribution in the nanotube during the twisting showed that what was taking place was indeed a periodic transition from a conductor to a semiconductor. The scientists then proposed a mathematical model that makes it possible to calculate and predict the oscillations in conductivity as a function of twisting.

 

These findings might in the future help in the design and production of tiny smart springs capable of measuring their own twisting by monitoring the changes in the electric current passing through them. Such springs could form the basis for a variety of nano-electromechanical devices, such as chemical or biological sensors, or gyroscopes for guiding miniature aircraft.