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Stanford Report, July 12, 2000

Researchers demonstrate the strange behavior of bent nanowires


Poking at a nanotube with a pointy rod can lead to intriguing nano-science.

Nanotubes, tiny carbon pipes, may someday play a pivotal role in bringing nanoscale technology into everyday use. Now researchers have discovered that prodding these tubes with a pointy tip can alter their ability to carry an electric current.

The results, the first to demonstrate how mechanical deformations can affect a molecular wire's electrical properties, were published in the June 15 issue of the journal Nature by Hongjie Dai, Stanford assistant professor of chemistry, and graduate student Thomas Tobler in collaboration with University of Kentucky theoretical physicists.

Hongjie Dai, assistant professor of chemistry, top, and, clockwise, graduate fellows Jing Kong and Thomas Tombler, study carbon nanotubes that are less than one-millionth the diameter of a human hair. photo: L.A. Cicero

The discovery could be used for making tiny electromechanical devices, such as transducers that convert mechanical movements into electrical signals. Other applications include creating high-frequency telephone lines to carry voice and data and making on/off switches for nanoscale computer chips.

Dai and his colleagues studied carbon nanotubes that are less than one-millionth the diameter of a human hair and just millionths of an inch long. Each tiny structure resembles a rolled-up graphitic sheet of carbon atoms arranged in a honeycomb pattern.

To prod the nanotube, the researchers used the sharp tip of an atomic force microscope (AFM). A real-space microscopy technique, the AFM makes images of surface topography by dragging a pointy tip over a structure's bumps and folds. The tip reads the shape like a blind person reads Braille. The textures are then translated into a visual image.

To conduct experiments on a single nanotube, Dai's group used a technique he perfected with AFM co-inventor Calvin Quate , professor of electrical engineering at Stanford, and graduate student Jing Kong. They placed an array of finely powdered metal nanoparticles on a silicon dioxide substrate, and then fed a carbon-containing gas (methane) over the substrate heated to a high temperature. The carbon infused into the metal particles, which acted as catalysts that converted carbon atoms into honeycomb-lattice nanotubes.

The researchers used the technique to grow a single nanotube across a silicon dioxide trench. They then attached an electrode to each end of the tube. They used the AFM tip to push the wire down into the trench, while measuring the wire's electrical conductance.

The group was initially surprised 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 predicted no significant change in the conductance of nanotubes due to mechanical deformation.

Dai hypothesized that a dent that formed near the AFM tip could be responsible for strongly affecting the electrical flow. To make sense of the results, Dai enlisted the help of theoretician Shi-yu Wu at the University of Kentucky. Wu and his colleagues used computer simulations to show that the AFM tip dented one wall toward the other, as when a garden hose gets kinked and the flow of water is stopped.

As one side of the tube is pushed closer to the other, carbon atoms form bonds across the inside of the tube. Normally, each carbon atom binds to three other carbons, leaving one electron free for use in conducting electricity. But when the walls of the tube come close together, each carbon binds to four rather than three carbons. The resulting decrease in the number of free electrons causes the electrical conductance to slow.

"The AFM tip squashes the tube, causing each atom to bond with more atoms," said Dai. "This causes the tube to turn 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 mechanical reversibility allows the full recovery of the nanotube's electrical property, Dai said.

"Dai's work is a very exciting experimental demonstration of what our theoretical work predicted," said Kyeongjae Cho, Stanford assistant professor of mechanical engineering, "namely that local nanotube deformation is a way to develop different functional components of nanotube transistors." SR