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Friday, January 23, 2009

Clear Carbon nanotube Films


Nanotube shrink-wrap: A small sample of the carbon-nanotube-coated plastic film that could be used as the see-through electrodes in touch screens, roll-up displays, and thin-film solar cells.
Credit: Unidym

The first electronic product using carbon nanotubes is slated to hit the market this year. Unidym, a startup based in Menlo Park, CA, plans to start selling rolls of its carbon-nanotube-coated plastic films in the second half of 2009.

The transparent, conductive films could make manufacturing LCD screens faster and cheaper. They could enhance the life of touch panels used in ATM screens and supermarket kiosks. They might also pave the way for flexible thin-film solar cells and bright, roll-up color displays. The displays could be used in cell phones, billboards, and electronic books and magazines.

In all of these applications, the nanotube sheets would replace the indium tin oxide (ITO) coatings that are currently used as transparent electrodes. ITO cracks easily and is a more expensive material. "The cost of indium has gone up by 100 times in the last 10 years," says Peter Harrop, chairman of IDTechEx, a research and consulting firm based in Cambridge, U.K.

Sean Olson, vice president of business development at Unidym, says that touch panels--which are particularly susceptible to the brittleness of ITO--will be the first market that the company will target. He says that Unidym is already working with leading touch-panel makers.

Many display manufacturers are working on products using the new films. In October, Samsung demonstrated the first prototype of a 14.3-inch color electronic paper device made with the films. Earlier last year, at the Society for Information Display symposium, Unidym demonstrated a color LCD prototype in collaboration with Silicon Display Technology, based in Seoul, Korea. Unidym is also working with Japanese chemical company Nippon Kayaku to make thin-film solar cells.

Using nanotube films instead of ITO coatings would bring multiple advantages to display manufacturers. Carbon is a cheap, abundant material. Carbon nanotubes are stronger and more flexible than ITO. Most important, the nanotube films are easier to deposit on plastic and glass substrates.

"The big benefit LCD guys are looking at is not materials cost," Olson says. "It's going from ITO, which [requires] vacuum deposition, to something that is more easily coated." That would increase the company's yields and bring down production costs.

The new film is a tangled mat of carbon nanotubes on plastic. Unidym's method to manufacture nanotubes is a key technology breakthrough. Electronic products with carbon nanotubes have been kept at bay mainly because of the difficulty in making pure batches of high-performance conducting nanotubes at reasonable cost. Batches of the material contain tubes that are both conducting and semiconducting. Nanotubes' properties also depend on other factors, such as length, diameter, and the number of walls that they have.

Unidym uses a chemical vapor deposition method to grow the material. A mixture of carbon-containing gas and a metal-catalyst-coated substrate is heated at a high temperature. The carbon atoms from the gas attach to the substrate and form nanotubes. Then the company uses purification processes, including oxidation and acid treatment, to remove unwanted nanotubes and contaminants, such as other forms of carbon.

Unidym has also patented a process to disperse the nanotubes in a liquid. It uses a roll-to-roll technique akin to printing on paper to deposit the nanotube ink on plastic. The company can print at speeds up to 50 meters a minute. Olson won't divulge any more information but says that the company has "optimized these techniques to get the best transparent, conductive film performance."

At least one other ITO replacement is already being sold. Fujitsu is using a transparent, conducting organic polymer to make touch screens. However, the polymer degrades over time when exposed to heat or light, and its conductivity is not high enough for use in LCDs or electronic-paper displays.

Other potential ITO replacements are currently being worked on in various labs. Many research groups are making advances with the carbon material graphene. University of Michigan electrical-engineering and computer-science professor L. Jay Guo has made grids of extremely thin metal wires. He says that these would be more suitable than carbon nanotubes for making electrodes on thin-film solar cells because they would enhance light absorption. "Think of it as concentrating incoming sunlight energy into a very thin layer," Guo says.

But Unidym's carbon-nanotube films could be the first viable ITO replacement for touch screens, flexible displays, and thin-film solar cells. Unidym is also developing printable thin-film transistors and fuel-cell electrodes using carbon nanotubes.

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