Working with Innovation Plasma Systems, Dieter Gruen (left), John Carlisle (center) and Orlando Auciello developed the first affordable large-area diamond film coating. The system was named one of the top developments in technology by R&D Magazine in 2003.

In diamonds as in all things: size matters

When it comes to diamonds, size is crucial. For emerging industrial applications, researchers have discovered, smaller is better.

Research into ultrananocrystalline diamond (UNCD) films, whose grains are measured in billionths of a meter, and the technology for creating them has opened up an array of opportunities not possible using old diamond film technologies that produce grains 1,000 times larger. Potential applications range from low-friction coatings to diamond electronics and biosensors.

Using a new patented plasma-enhanced chemical vapor deposition process, which won an R&D 100 award, Argonne researchers can deposit thin films of carbon atoms in a true diamond crystalline structure at relatively low temperatures — 400 degrees Celsius (750 degrees Fahrenheit) or lower. Earlier methods required 800°C (1,470°F) or higher. The resulting thin film has the same properties of hardness, conductivity, etc. as single crystal diamonds.

Because of the lower deposition temperatures, the UNCD films can be applied to a greater variety of substrates, including those containing microelectronic devices, than can other diamond films. The process reduces thermal stress at the interface and deposits continuous, uniform diamond films over large areas at rates of about one micrometer (millionth of a meter) per hour.

Continuous UNCD films can be grown to thicknesses from 100 nanometers (billionths of a meter) to 30 micrometers. They are very smooth as deposited, with low surface adhesion and a small coefficient of friction.

Structurally, the film’s grains are only 3 to 5 nanometers in size, setting UNCD apart from rougher thin films. These properties are being applied in low-friction coatings to decrease energy use in mechanical pumps and other rotating equipment. Wear tests conducted on UNCD-coated silicon carbide pump seals showed no detectable wear, while standard seals need replacing.

By changing the plasma chemistry, the electrical conductivity of UNCD can be controlled, ranging from insulating to highly conductive. This is done by introducing nitrogen gas into the plasma. UNCD films show high electron emission at low voltage and can be used to make a new generation of field-emission devices for such applications as flat-panel displays for televisions and computers.

UNCD is chemically inert and hydrophobic, or water resistant. It is also compatible with biological tissues. This has led to progress toward developing artificial retinas to correct some forms of blindness (Diamond films may help restore sight).

Films are flexible
Protective UNCD films can be applied in smooth layers that conform to the shape of a substrate, such as a computer chip with embedded circuitry. The films can be patterned using traditional integrated manufacturing methods developed for making microcircuits, providing a basis for new microelectromechanical systems, known as MEMS. UNCD permits MEMS to be more reliable, thermally stable, and resistant to corrosion; to be built on many substrates; and to have superior wear capabilities and low friction.

More recent work has shown that UNCD films can be used to make a biosensor sensitive enough to detect the presence of a single molecule. This is possible because the surface chemistry of UNCD can be finely controlled, allowing selective absorption of biomolecules such as DNA fragments and proteins. This has important implications for national security.

Initial technology, patented in 1991 by Dieter Gruen, an Argonne senior scientist in materials science, led to synthesis, characterization and development of UNCD film. Follow-up development by the research team in the Materials Science Division has been supported by the Department of Energy’s Basic Energy Sciences, the Office of Industrial Technology, the Office of Biological and Environmental Research, and Defense Advanced Research Projects Agency.

For more information, please contact Richard Greb.

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