Superhard electrical crystal carbon film

Jan. 25, 2002 at 2:41 PM
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MITAKA, Japan, Jan. 25 (UPI) -- Japanese scientists have made crystalline films of carbon, nearly as hard as diamond but many times more electrically conductive, which they hope will have practical uses in electronics.

The researchers hope these films -- thinner than the visible wavelengths of light -- may soon find use in electronics the size of nanometers, or billionths of a meter.

"The film is easy to manipulate in order to develop device applications," said lead researcher Shigeru Hirono, a scientist at the NTT Afty Corp. in Mitaka, Japan. "The prospects for this technology are good."

Carbon is an element that can form a wide variety of molecular bonds, included the strongest ones possible -- the so-called "sp3" bonds found in diamonds. The diamond bonds derive their strength from carbon atoms exchanging all their free electrons with each other.

No one expected carbon films as hard as diamond to possess electrical conductivity -- a property that depends on electron flow.

"However, we have found that such apparently contradictory mechanical and electrical properties can co-exist in carbon films," the researchers said in their report in the scientific journal Applied Physics Letters.

In the past, scientists have made carbon films that have high concentrations of the diamond bonds. These films, however, were never diamond-hard -- the atoms in the films were arranged in a disordered fashion. The atoms in a diamond, on the other hand, are aligned into crystalline structures.

The researchers in Japan created a form of carbon possessing both diamond bonds and weaker, sp2 bonds -- the kind found in the soft, black graphite used in pencil lead. This hybrid film is an intricate lattice made up of crystalline sheets whose edges rise up from a surface like folders in a file cabinet.

These fine nanocrystalline layers are made up of carbon atoms linked to each other in graphite bonds, and rise only 45 nanometers high -- more than 20,000 times thinner than a human hair. In turn, these sheets are glued together with diamond bonds.

"The carbon film is almost as hard as diamond, but its electrical conductivity is 19 orders of magnitude larger than diamond's," Hirono said. This makes the films about as electrically conductive as the chemically modified semiconductors used in microchips.

While other carbon films require a lot of heat to form, the process through which the scientists deposit their nanocrystalline films works at room temperature.

"The fact that they can grow these things in room temperature means they might be able to grow on heat-sensitive substrates like plastics," noted carbon films expert Tom Friedmann, a physicist at Sandia National Laboratories in Albuquerque, N.M. "That would all depend on how well materials adhere to these films, however."

Both Hirono and Friedmann agree that many tests still need to be run on the properties of the films, in terms of conductivity, hardness, thickness and rate of growth.

"This is a significant result," Friedmann said in an interview with United Press International. "I think that it just needs a little more work to tell me scientifically what they've done."

(Reported by Charles Choi in New York.)

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