TROY, N.Y. In an accidental discovery at Rensselaer Polytechnic Institute, materials researchers have found that carbon nanotubes can be restructured using light.
The effect was discovered when a laboratory assistant, undergraduate Andres de la Guardia, was photographing a batch of synthesized single-walled carbon nanotubes. His photo showed the nanotubes igniting. Intrigued, project leader Pulickel Ajayan exposed the tubes in a neutral atmosphere of helium and also in a vacuum, which eliminated the oxygen catalyzing the burning.
He found that the light restructured the tubes into horn-shaped configurations, surprising since carbon bonds need temperatures between 1,500°C and 2,000°C to break and reform. Also, multiple-walled carbon nanotubes, carbon soot and samples of Buckminsterfullerene (C60) were not affected by the light, which came from an ordinary photographic flash lamp. Ignition in the presence of oxygen required a local temperature of 600°C to 700°C, which could be achieved at a threshold illumination of 100 milliwatts/cm2.
One clue to how the heat-induced restructuring might be controlled was discovered by studying samples exposed to a flash lamp in different chemically neutral atmospheres.
When heated in a helium atmosphere, for example, the tubes tended to break down into much smaller graphene components. But in argon, many more horn-shaped structures were created. The researchers speculate that the difference could result from argon's being a better heat conductor than helium, which has a longer time scale available for carbon bond restructuring.
Carbon nanotubes, discovered in the early 1990s, emulate a variety of semiconductors, operating as either conducting or superconducting wires. The tubes can grow in a nested form with tubes inside of tubes, and researchers have also found a way to insert C60 buckyballs into the tubes. Several experiments have also shown that carbon nanotubes can act as the channel in tiny FET designs.
The Rensselaer group plans to look at possible applications of the effect in restructuring nanotubes with light pulses. The effect might have some direct applications as a microscopic fuel source for MEMS and the ability to ignite material with a remote pulse of light, which could be used to catalyze novel chemical reactions.
The experiments were reported in a recent issue of the journal Science.