This discovery, reported in the April 26 issue of the journal Science, could mean that SWCNs might be used in light sensors or to remotely trigger explosives and combustion reactions, although researchers say that more testing needs to be done to realize these possibilities.

Pulickel Ajayan, associate professor, and Ganapathiraman Ramanath, assistant professor, both of materials science, explain that the loud popping sound heard after the flash is actually a well-known phenomenon called the photo-acoustic effect, known since Alexander Graham Bell's time. This phenomenon had not previously been associated with carbon nanotubes. It occurs when porous black objects, such as nanotubes, absorb a large amount of light, which results in the expansion and contraction of the gas surrounding them, releasing sound (see movie and images at http://www.rpi.edu/dept/NewsComm/sub/Pressimgs/nanoflash/).

What surprised the researchers even more was the fact that the nanotubes then spontaneously ignited and burned upon photographic flash exposure.

"The single-walled carbon nanotube samples in this situation were just a jumble of tubes. They were not laid out in any pattern, and because of that, the heat generated from the flash could not dissipate, so the nanotubes just burned," explained Ajayan.

The discovery was initially noted by Andres de la Guardia while he took flash photographs of the nanotubes. De la Guardia, currently an international graduate student from Panama in operations research and statistics at Rensselaer, was a first-semester freshman at the time of the discovery. De la Guardia's interest in carbon nanotubes was seeded by taking "Chemistry of Materials" classes taught by Ajayan and Ramanath, and led to an Undergraduate Research Project, which gives undergraduates hands-on, real-world research experience.

"I was just in the right place at the right time," de la Guardia admitted. "I wasn't even a materials sciences major, but I was interested in carbon nanotubes and was asked to join the research team. I'm glad I had the foresight to bring my observations to Prof. Ajayan's attention," he added.

Since the discovery, the researchers have conducted a variety of experiments to test how light exposure affects the nantoubes. They found that while the tubes burn only when oxygen is present, their atomic structure is altered even in inert gas environments when exposed to the flash.

"While the initial surprise is that the nanotubes will ignite upon exposure to a camera flash, perhaps most exciting is that fact that the nanotubes are transformed into new carbon structures in the absence of oxygen. It is an illustration of how new behavior is observed at the nanometer scale, which justifies the interest in nanosciences," commented T.W. Ebbesen, one of the co-authors of the paper, from the Laboratoire des Nanostructures, Université Louis Pasteur.

"To the best of our knowledge, no other material emits such a loud sound and ignites spontaneously when exposed to unfocused low-power light; this adds to the long list of unique properties of carbon nanotubes," said Ramanath. "From an applications perspective, our work opens up exciting possibilities of using low-power light sources to create new forms of nanomaterials, and will serve as a starting point for developing nanotube-based actuators and sensors that rely on remote activation and triggering," he added.

The research is a collaborative effort between Rensselaer, a French group headed by T.W. Ebbesen, and researchers in Mexico and Germany.

Rensselaer Polytechnic Institute, founded in 1824, is the nation's oldest technological university. The school offers degrees in engineering, the sciences, information technology, architecture, management, and the humanities and social sciences. Institute programs serve undergraduates, graduate students, and working professionals around the world. Rensselaer faculty are known for pre-eminence in research and teaching. The Institute is especially well known for its success in the transfer of technology from the laboratory to the marketplace so that new discoveries and inventions benefit human life, protect the environment, and strengthen economic development.

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