It’s Raining Gems:
"Not only were the diamonds so hard they broke the measuring equipment, we were able to grow gem-size crystals in about a day,” stated Chih-Shiue Yan of Carnegie’s Geophysical Laboratory (GL).Yan is the lead author of a study published in the February 20, 2004, online Physica Status Solidi, which reported on his group’s production of gem-size diamonds that are harder than any other crystals.The diamonds were grown directly from a gas mixture by the chemical vapor deposition (CVD) process at a rate that is up to 100 times faster than other methods used to date.

Producing a material that is harder than natural diamond has been a goal of materials science for decades.Now the GL group has figured out how to do it as a by-product of its principal goal—to create the next generation of high pressure devices based on large single-crystal diamond anvils grown by CVD. “Making diamonds has not been the primary purpose of our research,” explained Russell Hemley of GL’s high-pressure group.“Our team is interested in the behavior of materials at extreme pressures and temperatures.We need large, perfectly faceted diamond crystals to create new devices and therefore decided to explore whether we could make these crystals by CVD processes.” The group’s first single crystal diamonds were produced in 1998. Now the crystals can be used to exert extreme pressures (pressures at least as high as those exerted by natural diamonds, so far up to 200 GPa—almost 2 million times the pressure at sea level).The small team leads the world in producing large single-crystal diamonds by CVD.

More than seven years ago, interest in the CVD process led Hemley and GL’s Dave Mao to join forces with Professor Yogesh Vohra at the University of Alabama, Birmingham (UAB), where Yan was a graduate student.Yan subsequently came to Carnegie in 2002 as a postdoctoral fellow. Yu-Chun (Brad) Chen, a Ph.D. student at Auburn University interested in using the diamonds for engineering purposes, also joined the GL diamond-making project after meeting Yan at a conference in 2001.

The group’s recently acquired ability to make these ultrahard crystals very fast has opened up an entirely new way of producing single-crystal diamonds for a variety of applications. Beyond romance, diamonds are appealing for their extraordinary strength, transparency, insulating, and thermal powers. Different types of diamonds are sought after to use as electronics semiconductors, cutting tools, and for other purposes.The team’s work will likely benefit many such applications.

Two Roads to Synthetic Diamonds
The traditional means for making the hardest of gems is to mimic nature by subjecting carbon to high pressures and temperatures.This approach has been around for decades and in fact has been used at GL, but it is limited to producing generally very small diamonds.Very special processes are required to create single crystals as large as several carats. It is the goal of Carnegie’s CVD group to produce diamonds in the 10-carat range, and ultimately as large as 100 carats, for scientific and technological applications—most immediately for high-pressure experiments. Producing diamonds this big can only be done by chemical vapor deposition. Besides producing larger diamonds, the CVD process has other advantages. It is highly flexible and can more easily control impurities in the crystal. Impurities in diamonds are important. They are responsible for their color, strength, and other properties.The strongest diamonds are transparent and impurity-free. But a diamond needed for semiconducting, for example, needs a dash of boron, or another so-called dopant, to transform the material from an insulator into a semiconductor.The CVD process also permits the production of diamonds with a variety of shapes, with electrical or magnetic circuits embedded in them, and micromechanical devices made of the material.

Sow Seed, Add Gas, and Heat
The Carnegie researchers grow the crystals using a special apparatus and process they developed.The apparatus has several parts—a cylindrical chamber where a flat, synthetic seed diamond, typically 4 millimeters (mm) x 4 mm x 1.5 mm, is placed; a standard microwave; and an infrared detector for determining the temperature inside the growing chamber. The device has two unique features—a special holder for the seed diamond and the infrared temperature gauge. Patent applications have been filed in the U.S. and in a number of other countries for this one-of-a-kind diamond maker.

Once the seed is placed in the chamber, the microwave is turned on and a mixture of methane, nitrogen, and hydrogen gas flows into the unit.The gas is bombarded with charged particles, or plasma, generated by the microwave, which prompts a complex chemical reaction resulting in a “carbon rain” that falls on the seed. The carbon atoms then arrange themselves in the same crystalline structure as the seed. Using this method, the group has grown single crystals of diamonds up to 8 millimeters across and up to 4.5 millimeters thick. Soon it will also start growing its own seed stock.

The crystals are then subjected to high temperatures and pressures, which further harden the material. Although this annealing process has been used on synthetic diamonds made by high pressure, high-temperature processes, the Carnegie scientists are the first to anneal single-crystal diamonds made by CVD. The diamonds, annealed off-site, are heated to 2000°C and put under pressures of between 50,000 and 70,000 times atmospheric pressure (5-7 GPa) for 10 minutes.This final process renders the diamonds at least 50% harder than conventional crystals, as measured by collaborators at Los Alamos National Laboratory. “These diamonds are real diamonds made of carbon and identical in structure to those formed in nature and by high-pressure and high-temperature methods,” Yan emphasized.

Cut ’n’ Polish
The group uses several methods to cut and polish the diamonds. One method is for diamonds needed for the diamond-anvil cell. Another is for making diamond plates or windows. Diamonds used for characterization studies also require cutting and polishing.

Crystals destined for the diamond-anvil cell have to be more precisely cut and polished (e.g., faceted) than diamonds intended for jewelry so that they can withstand maximum pressures at the tips without fracturing during experiments. Appropriately, these gems are sent to a variety of highly skilled diamond cutters in New York and Europe. It takes much longer to cut and polish a CVD single crystal than a mined diamond because CVD gems are much tougher.Yan and Chen polish crystals that are used for analysis and characterization at GL.

World Standing
This small cadre of GL scientists, which is smaller than any other such group, leads the world in the development of the biggest, strongest, and fastest growing CVD single-crystal diamonds yet produced. Its accomplishments have come from systematic hard work and Carnegie’s long-term commitment to the project. Not surprisingly, word of its results has prompted a flurry of interest in the diamond industry. But despite such distractions, the team is tightly focused on its goal: to produce very large single-crystal diamonds for science and technology.This effort will allow the investigators to maintain their leadership in high-pressure, high-temperature research in materials science, planetary science, and fundamental physics.
The CVD diamond-making team at GL stands with its one-of-a-kind diamond maker. Staff member Russell Hemley (left), Brad Chen (middle), and Chih-Shiue Yan (right) are examining the chamber where the diamonds grow.
This CVD diamond is fresh out of the growth chamber; it has not yet been polished. It is about 5 x 5 x 2.7 mm3. The original seed, approximately 1.5 mm thick, is underneath. The crystal was grown in about 12 hours.
To polish a diamond destined for characterization studies, a new crystal is placed in the gem holder and diamond powder is sprinkled onto the polishing wheel. The diamond is maneuvered to a specific position for the desired surface and the wheel is turned on. The researcher checks the progress about every 30 minutes by examining the diamond with a jeweler’s loupe. When the surface looks good, the crystal is further examined under a microscope to determine its smoothness.
This synthetic brilliant-cut single-crystal diamond, grown by chemical vapor deposition (CVD), is 2.5 mm high and was made in about one day at Carnegie.
(Image courtesy Physica Status Solidi http://www.pss-rapid.com/.)
This research was supported by the National Science Foundation, the U.S. Department of Energy through the Carnegie/DOE Alliance Center (CDAC), the W. M. Keck Foundation, and the G. Unger Vetlesen Foundation. It was conducted in collaboration with researchers at the Phoenix Crystal Corporation and Los Alamos National Laboratory.