Researchers at the Carnegie
Institution have developed a new technique for improving the properties
of diamonds-not only adding sparkle to gemstones, but also simplifying the process
of making high-quality diamond for scalpel blades, electronic components, even
quantum computers. The results are published in the October 27-31 online edition
of the Proceedings of the National Academies of Science.
Diamonds such as these grown in the laboratory using a chemical vapor deposition process can be treated by a new high temperature, low pressure method to improve their color and optical clarity. Credit: Carnegie Institution for Science.
A diamond may be forever, but the very qualities that make it a superior material
for many purposes—its hardness, optical clarity, and resistance to chemicals,
radiation, and electrical fields? can also make it a difficult substance with
which to work. Defects can be purged by a heating process called annealing,
but this can turn diamond to graphite, the soft, grey form of carbon used in
pencil leads. To prevent graphitization, diamond treatments have previously
required high pressures (up to 60,000 times atmospheric pressure) during annealing,
but high pressure/high temperature annealing is expensive and there are limits
on the size and quantities of diamonds that can be treated.
Yu-fei Meng, Chih-shiue Yan, Joseph Lai, Szczesny Krasnicki, Haiyun Shu, Thomas
Yu, Qi Liang, Ho-kwang Mao, and Russell Hemley of the Carnegie Institution's
Geophysical Laboratory used a method called chemical vapor deposition (CVD)
to grow synthetic diamonds for their experiments. Unlike other methods, which
mimic the high pressures deep within the earth where natural diamonds are formed,
the CVD method produces single-crystal diamonds at low pressure. The resulting
diamonds, which can be grown very rapidly, have precisely controlled compositions
and comparatively few defects.
The Carnegie team then annealed the diamonds at temperatures up to 2000°
C using a microwave plasma at pressures below atmospheric pressure. The crystals,
which are originally yellow-brown if produced at very high growth rates, turned
colorless or light pink. Despite the absence of stabilizing pressure there was
minimal graphitization. Using analytical methods such as photoluminescence and
absorption spectroscopy, the researchers were also able to identify the specific
crystal defects that caused the color changes. In particular, the rosy pink
color is produced by structures called nitrogen-vacancy (NV) centers, where
a nitrogen atom takes the place of a carbon atom at a position in the crystal
lattice next to a vacant site..
"This low-pressure/high-temperature annealing enhances the optical properties
of this rapid-grown CVD single crystal diamond." says Meng. "We see
a significant decrease in the amount of light absorbed across the spectrum from
ultraviolet to visible and infrared. We were also able to determine that the
decrease arises from the changes in defect structure associated with hydrogen
atoms incorporated in the crystal lattice during CVD growth."
"It is striking to see brown CVD diamonds transformed by this cost-efficient
method into clear, pink-tinted crystals," says Yan. And because the researchers
pinpointed the cause of the color changes in their diamonds, "Our work
may also help the gem industry to distinguish natural from synthetic diamond."
"The most exciting aspect of this new annealing process is the unlimited
size of the crystals that can be treated. The breakthrough will allow us to
push to kilocarat diamonds of high optical quality" says coauthor Ho-kwang
Mao. Because the method does not require a high pressure press, it promises
faster processing of diamonds and more types of diamonds to be de-colored than
current high-pressure annealing methods. There is also no restriction on the
size of crystals or the number of crystals, because the method is not limited
by the chamber size of a high pressure press. The microwave unit is also significantly
less expensive than a large high-pressure apparatus.
"The optimized process will produce better diamond for new-generation
high pressure devices and window materials with improved optical properties
in the ultraviolet to infrared range." concludes laboratory director Russell
Hemley. "It has the advantage of being applicable in CVD reactors as a
subsequent treatment after growth."
The high-quality, single crystal diamond made possible by the new process has
a wide variety of applications in science and technology, such as the use of
diamond crystals as anvils in high-pressure research and in optical applications
that take advantage of diamond's exceptional transparency. Among the more exotic
future applications of the pink diamonds made in this way is quantum computing,
which could use the diamonds' NV centers for storing quantum information.