At Brookhaven, molecular nanowires millions of times smaller than a human hair have been developed. At Stanford, researchers have subsequently discovered that nudging these tubes with a sharp tip can alter their ability to carry an electric current.
The team employed the tip of an atomic force microscope (AFM) to poke the nanotube. As in real-space microscopy techniques, the AFM was used to make images of surface topography by dragging a sharp tip over the bumps and folds on the structure's surface.
An array of finely powdered metal nanoparticles was placed on a silicon dioxide substrate. A carbon-containing gas (methane) was then fed over the substrate, which was heated to a high temperature. The carbon infused into the metal particles, which acted as catalysts and converted carbon atoms into honeycomb-lattice nanotubes.
After an electrode had been attached to a single nanotube across a silicon dioxide trench, the AFM tip was used to push the wire down into the trench while measuring its electrical conductance. Researchers were amazed to observe that the flow of electricity dropped sharply as the nanotube bent.
When the AFM tip was removed, the tube straightened and the flow of electricity returned to normal. Previous theoretical studies had predicted no significant change in the conductance of nanotubes due to mechanical deformation.
As one side of the tube is pushed closer to the other during the experiment, carbon atoms form bonds across the inside of the tube. Normally, each carbon atom binds to three other carbons, leaving one electron free for conducting electricity. However, when the walls of the tube are forced closer together, each carbon atom binds to four rather than three other carbons. The resulting reduction in the number of free electrons decreases the electrical conductance.
As the AFM tip squashes the tube, causing each atom to bond with more atoms, the tube changes from an electrical conductor into an insulating structure similar to that found in diamonds. Remarkably, the dent disappears once the perturbing tip is removed. This high level of mechanical reversibility allows a full recovery of the nanotube's electrical property. Local nanotube deformation is a way to develop different functional components of nanotube transistors.
The discovery could be used for making tiny electromechanical devices, such as transducers for converting mechanical movements into electrical signals. Other possible applications might include high-frequency telephone lines for carrying voice and data, and on/off switches for nanoscale computer chips.
Brookhaven Bulletin/AIP