July 1, 2005

Diamonds are a chemist's best friend

Better diamonds through chemistry.

By using a chemical vapor deposition (CVD), researchers can now produce a 10-carat, half-inch thick single-crystal diamond at a growth rate of 100 micrometers per hour.

That size is five times that of commercially available diamonds produced by the standard high-pressure/high-temperature (HPHT) method and other CVD techniques, said researchers at the Carnegie Institution's Geophysical Laboratory. In addition, the team has made colorless single-crystal diamonds, transparent from the ultraviolet to infrared wavelengths.

"High-quality crystals over three carats are very difficult to produce using the conventional approach," said Dr. Russell Hemley who leads the diamond group at Carnegie. "Several groups have begun to grow diamond single crystals by CVD, but large, colorless, and flawless ones remain a challenge. Our fabrication of 10-carat, half-inch, CVD diamonds is a major breakthrough."

Most HPHT synthetic diamonds are yellow, and most CVD diamonds are brown, limiting their optical applications. Colorless diamonds are costly to produce, and so far those reported are small. This situation limits general applications of these diamonds as gems, in optics and in scientific research. Last year, the Carnegie researchers found HPHT annealing enhances not only the optical properties of some CVD diamond, but also the hardness. Using new techniques, the Carnegie scientists have now produced a transparent diamond using a CVD method without HPHT annealing.

To further increase the size of the crystals, the Carnegie researchers grew gem-quality diamonds sequentially on the six faces of a substrate diamond plate with the CVD process. By this method, three-dimensional growth of colorless single-crystal diamond in the inch-range (~300 carat) is achievable.

Researchers can now make different shapes with the blocks of the CVD single crystals.

The standard growth rate is 100 micrometers per hour for the Carnegie process, but they have reached growth rates in excess of 300 micrometers per hour. Researcher said a growth rate of 1 millimeter per hour may be possible. With the colorless diamond produced at higher growth rates and low cost, large blocks of diamonds should be available for a variety of applications.

"The diamond age is upon us," Hemley said.