Synthetic diamonds for science and industry

Are natural diamonds really a girl’s best friend? Physicists, regardless of their gender, tend to prefer artificial alternatives synthesized in a laboratory. Those who need exceptionally pure gems for their experiments tend to call up Element Six , headquartered in Luxembourg and part of the De Beers Group of diamond companies.

‘Element Six is the most prominent and successful company growing diamonds with very high purities,’ says Ronald Walsworth, a physicist at Harvard University.

On 16 July the synthetic diamond veteran opened a new research center in the UK. Costing €20 million (about $26.5 million), the facility consolidates Element Six’s research labs around the world in the town of Harwell, just outside Oxford. Its 114 employees will tinker with different recipes for diamond in the hopes of improving its properties.

Their top priority will be polycrystalline diamond grown by fusing diamond powder at high temperatures and pressures, says Steve Coe, executive director of group innovation at Element Six. The exceptional hardness and toughness of this material can be customized for different applications by adjusting the sizes and geometries of the grains that form within it. Enhancing diamond properties could be a boon for the oil and gas industry, which uses diamond drill bits to cut through rock. Polycrystalline diamonds also improve the durability of road construction equipment used to grind up asphalt.

Though industrial polycrystalline diamonds account for most of Element Six’s business, the new research center will also explore other types of diamond more relevant to the scientific community. A method for growing especially pure gems called chemical vapor deposition (CVD) will be central to those efforts. First patented in the 1950s, this technique starts with methane gas as a carbon source. Hot plasma cooks a mixture of methane and hydrogen gas in a specially designed chamber, stacking layers of carbon on top of each other.

Thanks to improvements in every step of this process over the years, researchers now count on their diamonds to have a purity of about 99.99% or greater. CVD diamonds have come to play important roles both in scientific equipment and in basic research.

Peek under the hood of a modern particle accelerator, for instance, and you’ll find all sorts of diamond components. Sensors made from Element Six diamonds monitor beams at CERN’s Large Hadron Collider near Geneva. The world’s most powerful synchrotron machines, which generate beams of radiation by accelerating electrons, also rely on CVD diamond, which has in many places replaced traditional materials (like silicon) used in electronics..

‘The power of the beams has gotten to the point where silicon doesn’t work very well,’ says Joe Tabeling, president of Applied Diamond in Wilmington, Delaware. A smaller company than Element Six, Applied Diamond has been filling orders for diamond parts for the National Synchrotron Light Source II at Brookhaven National Laboratory in Upton, New York; the Advanced Photon Source at Argonne National Laboratory in Lamont, Illinois; and other facilities in the United States and abroad. Unlike silicon, diamond neither degrades over time when exposed to the intense radiation generated at those facilities nor distorts a beam passing through it.

‘As we get better at being able to engineer the diamonds, we open up opportunities for new applications,’ says Coe. Another frontier of interest to Element Six is lasers. Coe highlighted a trend in recent years toward increasing the power and beam widths of carbon dioxide lasers. Better carbon dioxide lasers could allow the semiconductor industry to etch ever-smaller components for computers and other pieces of electronic equipment.

Already the highest-powered lasers in existence, carbon dioxide lasers generate a beam of infrared light from a cloud of gas and pass that beam though a clear diamond window. Larger diamonds that absorb less infrared radiation could thus improve performance. But achieving a uniform crystal structure over a large diamond isn’t easy. It requires better control over the steps of the CVD process and better control over the unruly plasma that cooks up the diamond. ‘It’s relatively easy to grow a small piece of diamond,’ says Coe. ‘Controlling the growth rate and quality over a large area is very challenging.’

Toward the atomic scale

Physicists are working with Element Six to push that control of the diamond-making process down to the scale of individual atoms. Diamonds made for the Delft University of Technology in the Netherlands, for instance, contain deliberately introduced defects that consist of a hole in the crystal structure flanked by a single nitrogen atom. Such nitrogen vacancy (NV) centers are implanted into otherwise pure diamonds after the diamonds are grown.

The defects have unusual quantum properties. The Dutch team created the first long-distance quantum link, or entanglement, between defects in two different diamonds, potentially useful for a diamond-based quantum network for relaying information analogous to the Internet. NV centers can store quantum information, which could be useful for the development of a quantum computer.

Harvard physicist Amir Yacoby is making incredibly sensitive detectors out of diamonds embedded with NV centers, which respond to magnetic fields. To improve the performance of these miniature MRIs, he needs to precisely place the impurities as close as possible to their targets—ideally, only a few nanometers from a diamond’s surface. That’s a huge challenge for the diamond construction process. NV centers can typically be placed only at greater depths and with an accuracy of no better than 10 nm.

Another obstacle to the construction of accurate NV center sensors is the diamond’s surface itself. Unlike the crystal’s well-ordered interior, the surface tends to be a jungle of dangling bonds, unbound electrons that generate their own magnetic signals. ‘The quality of nitrogen vacancy centers very close to the surface is not very good right now,’ says Yacoby.

A potential solution for the surface problem recently came out of David Awschalom’s lab at the University of California, Santa Barbara, one of the few universities capable of growing state-of-the-art CVD diamonds. Instead of implanting impurities in diamonds, his team slows down the diamond’s growth. Nitrogen is worked into the final layers as they form, a technique called delta doping.

Element Six is working with Yacoby to explore other solutions and plans to continue collaborating with other university groups. Still in early stages, the basic research coming out of such partnerships is considered a long-term investment by the company. ‘The ideas university groups are coming up with could be the next generation of opportunities for us,’ says Coe.

Devin Powell is a freelance science writer based in Washington, DC. His stories have appeared in Science News, Wired, US News & World Report, and other outlets.