ARCHIVED - Perfecting the recipe for nanotubes
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September 02, 2008— Ottawa, Ontario
What's stronger than steel, carries heat better than aluminium and conducts electricity better than copper? If you said carbon nanotubes, then you've heard some of the hype that surrounds these superheroes of the nano world. At 1/10,000th the width of a human hair, carbon nanotubes have unusual physical and optical properties that make them an exciting new material in diverse areas such as mechanics, electronics and optoelectronics.
Nanotubes are typically grown from carbon-containing gases in a hot furnace, but the actual growth process is something of a mystery.
"People don't really understand a lot of the basic molecular processes going on," says Dr. Paul Finnie of the NRC Institute for Microstructural Sciences (NRC-IMS). Understanding how nanotubes form, and controlling that process, will be a big step towards realizing their huge potential.
|This scanning electron microscope image shows nanotubes grown in a hot furnace. The close-ups at right and bottom are bundles of individual nanotubes. NRC's new Global Raman Imaging method will be able to photograph individual nanotubes as they form.|
In a world's first, researchers at NRC-IMS have photographed single nanotubes as they form in a red hot furnace. Using an advanced optical microscopy technique called Global Raman Imaging (GRI), researchers have watched carbon nanotubes growing at temperatures of up to 900 degrees Celsius.
"At that temperature, everything is glowing, so that makes it hard to use certain imaging techniques," says Dr. Finnie. "But it's not a problem for Raman imaging – we can do it at a wavelength that makes the glowing irrelevant."
Observing materials in situ – as they're forming – is a well established method for improving synthesis processes. Carbon nanotubes, which at the atomic scale look much like tubes made of chicken wire, are grown from nanometre-sized metal particles. The particles absorb carbon until they become overloaded and begin to grow a nanotube of carbon atoms. "That's a dynamic process – if you don't do in situ measurements, you would just see the final result," says Dr. Finnie.
NRC's researchers are developing in situ imaging methods to observe the growth process as it happens, all the way down to the single nanotube level. The GRI method allows researchers to photograph growing carbon nanotubes in intervals of a fraction of a second, making a digital movie. "So you're watching it at every stage and seeing dynamically what's happening," says Dr. Finnie. Unlike other observation methods such as electron microscopy, GRI doesn't damage the nanotubes or affect how they form.
Choose your nanotube
Carbon nanotubes come in many varieties, including single-walled (a single tube) or multi-walled (multiple tubes). They can be long or short (on a nano-scale), and suspended in space versus touching a surface.
Technologies are being developed at NRC and elsewhere to make nanotubes in large quantities. The next challenge is fine tuning that process to select the qualities that make them desirable. "You don't just want any carbon nanotube, you want lots of really good, controlled nanotubes," says Dr. Finnie.
To reach this goal, NRC researchers are using GRI and other in situ imaging methods to understand why the nanotubes form the way they do. By adjusting factors such as the temperature, flow rate and mixture of gas, and the catalyst used to start the nanotubes growing, researchers eventually hope to get the recipe just right to create nanotubes that suit particular applications.
"For optical devices, suspended nanotubes are particularly good," says Dr. Finnie. Single-walled nanotubes are also of great interest, since they are strong and light and have the best optical properties. Long nanotubes are interesting for many applications, including light emission.
GRI can show how adjusting any of a multitude of factors will influence the final product. "The ultimate goal is to have a rational synthesis process where, for example, you know that if you add 5 percent more methane into the mixture, you get nanotubes that are 10 percent less defective."
This research was funded as part of an international project with the Japan Science and Technology Agency. Right now, the high-quality nanotubes needed for advanced applications are very expensive, but as knowledge of them improves, Dr. Finnie expects that situation to change. "Eventually the price will come down, the production capacity will come up, and our ability to control factors such as size and diameter will improve," he says. "As nanotube samples continue to improve, more and more applications will come online, including ones that rely on their optoelectronic properties, which are especially interesting to us at NRC-IMS."
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