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February 2, 2004
Volume 82, Number 05 CENEAR 82 05 pp. 26-31
ISSN 0009-2347
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THE MANY FACETS OF MAN-MADE DIAMONDS
Synthetic diamond makers are targeting the gem market first, but their product could transform many other industries, too
AMANDA YARNELL, C&EN WASHINGTON
Before the 1930s, the gems of choice for engagement rings included opals, rubies, and sapphires. But in the 1940s, De Beers--the
South African mining firm that controls the majority of the world's
diamond supply--introduced "A Diamond Is Forever." The success of this
campaign turned diamond into the symbol of eternal love and
dramatically increased demand for the gems.
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DIAMOND RING
Because of its optical transparency, high thermal conductivity, and
resistance to chemical attack, synthetic diamond is an attractive
material for making optical windows for instruments used in extreme
environments. ELEMENT SIX PHOTO
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Today,
two start-up companies are staking their futures on the lure of more
affordable, laboratory-grown diamond gemstones. But because of
diamond's remarkable optical, thermal, chemical, and electronic
properties, synthetic diamond promises to offer a lot more than just
beautiful jewelry.
In a warehouse in Sarasota, Fla., a company called Gemesis
is growing diamonds in two dozen or so high-pressure, high-temperature
crystal growth chambers, each the size of a washing machine. Within
each chamber, a tiny sliver of natural diamond is bathed in a molten
solution of graphite and a proprietary metal-based catalyst at
approximately 1,500 °C and 58,000 atm of pressure. Slowly, carbon
precipitates onto the diamond seed crystal. A gem-quality, 2.8-carat
rough yellow diamond grows in just under three-and-a-half days.
A
rough diamond of this size can be cut and polished to give a diamond
gem larger than 1.5 carats. (One-half carat is equal to 100 mg of
diamond and is roughly the size of a kernel of corn.) Just like
naturally occurring yellow diamonds, the yellow lab-grown stones get
their color from trace amounts of nitrogen impurities: Replacing fewer
than five out of each 100,000 carbon atoms in the diamond crystal
lattice with nitrogen atoms gives a yellow diamond.
Naturally
occurring fancy-colored diamonds--yellows, blues, pinks, and reds--are
very rare and thus very valuable. A Gemesis-created yellow
fancy-colored diamond--visibly indistinguishable from a natural one,
even to a trained gemologist--can be purchased for about $4,000 per
carat. That's about 30% less than the price of a natural diamond of
similar color and quality, according to Robert Chodelka, Gemesis' vice
president for technology.
SYNTHETIC DIAMONDS are
nothing new. Producing them has been a stable business for the past
half century. Today, more than 100 tons of the stones is produced
annually worldwide by firms like Diamond Innovations (previously part
of General Electric), Sumitomo Electric, and De Beers. Tiny synthetic
diamonds are used in saw blades for cutting asphalt and marble, in
drill bits for oil and gas drilling, and even as an exfoliant in
cosmetics.
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IN THE ROUGH
To grow its gem-quality yellow diamonds (a rough one is shown above),
Gemesis uses washing-machine-sized crystal-growing chambers to
reproduce the high pressures and high temperatures that nature relies
on. GEMESIS PHOTOS
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The
first synthetic diamonds (diamond grit) were produced in the early
1950s by researchers at the Allmanna Svenska Elektriska Aktiebolaget
Laboratory in Stockholm, Sweden. They did not immediately publish their
work. Soon thereafter, GE researchers reported their own successful
diamond synthesis in Nature. Like
Gemesis, both teams used conditions that mimic the pressures and
temperatures under which diamonds are thought to form naturally.
Prior
to Gemesis, GE, Sumitomo Electric, and De Beers had reported the
synthesis of large diamonds by similar processes. But these companies
marketed their synthetic stones as heat sinks for electronics or used
them solely for research purposes. Gemesis, on the other hand, is
growing diamonds for jewelry. And because Gemesis' yellow lab-grown
diamonds are visually indistinguishable from their mined counterparts,
some in the gem industry have expressed concern that the lab-grown
diamonds could be passed off as naturals.
Chodelka
tells C&EN that Gemesis is "committed to disclosure," noting that
all of the firm's diamonds are laser inscribed. In addition, he says
trace amounts of nickel left in the diamond from the metal catalyst
cause a short-lived phosphorescence after exposure to intense
ultraviolet light--a characteristic not shared by most natural
diamonds. He also points out that differences in the spatial
distribution of nitrogen defects between natural and Gemesis-grown
diamonds can be detected by Fourier transform infrared spectroscopy and
X-ray absorption spectroscopy.
But
Gemesis' business plan only begins with gems. Diamond has an
extraordinary range of materials properties: It is the hardest and
stiffest material known; is an excellent electrical insulator; has the
highest thermal conductivity of any material yet barely expands when
heated; is transparent to UV, visible, and infrared light; and is
chemically inert to nearly all acids and bases.
Diamond's
superlative properties are fine-tuned by impurities found in the carbon
lattice--the same impurities that produce colors in naturally occurring
diamond. Diamonds having a perfect carbon crystal lattice without
defects or substitutions are colorless. Such diamond has a large band
gap--meaning that the energy required to free an electron so it can
move through the diamond lattice is high--and therefore is an excellent
electrical insulator. But replacing some of the carbon atoms in the
diamond lattice with boron--an impurity that produces the pretty blue
color in some rare diamonds, including the famed Hope Diamond--transforms
diamond into a p-type semiconductor. That's because boron has only
three outer-shell electrons and can make only three of four bonds that
carbon normally does in the diamond lattice. The result is a missing
electron or "hole" that can move freely through the crystal, allowing
the diamond to conduct positive charge.
For
materials applications that take advantage of these remarkable
properties, natural diamonds have obvious flaws: They are prohibitively
expensive and limited in size. "Plus, with natural diamonds, you can't
control the type or placement of dopants," notes James E. Butler, who
is spearheading attempts to study, grow, and use diamond at the U.S.
Naval Research Laboratory. As a consequence, Gemesis and many others
are eager to create large synthetic diamonds with carefully selected
impurities--for instance, boron-doped semiconducting diamonds that
could be used to fabricate diamond-based electronic devices that could
stand up to heat and chemical attack.
But
high-pressure, high-temperature methods of synthesizing diamond like
Gemesis' offer limited control of impurities and produce diamonds of
limited size, Butler says. Apollo Diamond,
a start-up company in Boston, thinks that a low-pressure technique
called chemical vapor deposition (CVD) could be the answer. Butler
agrees. "As interesting and as important as the high-pressure,
high-temperature method is, it won't have the technological impact of
diamond growth by chemical vapor deposition," he tells C&EN.
Apollo
is using CVD to grow single-crystal diamond wafers big enough to be cut
into diamond gemstones of a carat of more. Apollo's method can grow
larger diamonds and is less expensive than high-pressure,
high-temperature methods, notes Robert C. Linares, Apollo's founder and
chairman.
CVD allows finer
control of impurities than do high-pressure, high-temperature methods,
Linares says. This enables Apollo to produce a wider variety of colored
diamonds--including colorless, pink, blue, honey brown, and even black.
Like Gemesis, Apollo inscribes its larger lab-grown gems to aid
detection. A combination of spectroscopic methods--including infrared
spectroscopy and photoluminescence spectroscopy--can normally be used
to distinguish Apollo gems from naturally occurring ones, according to
Wuyi Wang, a research scientist at the Gemological Institute of America in New York City [Gems & Gemol., 39, 268 (2004)].
A
slow, tedious version of the low-pressure CVD process was first
documented in 1952 by William G. Eversole of Union Carbide. Back then,
"there was a great deal of skepticism that one could grow diamond at
low pressures because diamond is thermodynamically unstable with
respect to graphite," recalls John C. Angus,
professor of chemical engineering at Case Western Reserve University,
Cleveland. "Many people said that growth of diamond at low pressure
violated the second law of thermodynamics. You were thought to be a
fool or a fraud if you proposed this," he says.
Union
Carbide subsequently abandoned the project. But a small band of Russian
and American scientists, including Angus, pushed forward. By the late
1960s, Angus managed to prove that diamond growth by CVD was indeed
feasible. The method was further refined into a viable commercial
process in the 1980s by scientists at the National Institute for
Research in Inorganic Materials in Tsukuba, Japan.
Hydrogen
is the key to growing diamond and not graphite under these conditions,
Angus' early work showed. At the surface, the carbon lattice of diamond
is decorated with "dangling bonds" that can potentially cross-link to
reorganize the surface into more stable graphite. Capping these bonds
with hydrogen prevents graphite formation and generates reactive
surface sites for attachment of carbon radicals.
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A CUT ABOVE
Apollo uses chemical vapor deposition to grow plates of very pure
diamond (left) that can be cut and polished into beautiful gems
(right). APOLLO DIAMOND PHOTO
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In
Apollo's CVD reactor, hydrogen gas and methane are flowed through a
chamber containing a diamond seed crystal (often a highly polished
synthetic one produced by high-pressure, high-temperature methods). The
hydrogen gas is split into atomic hydrogen by the action of a hot
filament or a microwave-generated plasma. The atomic hydrogen thus
generated reacts with methane to give methyl radical and hydrogen gas.
The carbon-containing radical species eventually deposit onto the
diamond seed, forming new diamond carbon-carbon bonds. But the surface
chemistry of how carbon atoms actually attach to the diamond lattice
still remains murky, Linares notes.
Apollo's
CVD method produces single-crystal diamond, just as nature does. But
until relatively recently, most of the diamond grown by CVD methods was
polycrystalline, not single-crystal. Polycrystalline diamond is a
patchwork of minuscule diamond crystals (and sometimes tiny crystals of
graphite). Because it retains many of naturally occurring
single-crystal diamond's excellent properties, polycrystalline diamond
has been targeted for a number of uses.
For instance, chemistry professor Robert J. Hamers
of the University of Wisconsin, Madison, has developed a photochemical
method for covalently linking DNA via an organic tether to the surface
of polycrystalline boron-doped diamond films made by CVD. Recently, he
and graduate student Wensha Yang found that the binding of
complementary DNA strands to the DNA-labeled diamond surface can be
detected directly by measuring the change in electrical properties of
the diamond film. The direct electrical detection allowed by diamond
eliminates the need for labor- and time-intensive labeling steps
required by other biosensing methods.
And
because semiconducting diamond can generate a wider range of potentials
than other electrode materials, electrodes made of this material can be
used to study redox reactions that can't be studied with conventional
electrodes, notes assistant professor of chemical engineering Heidi B. Martin of Case Western. That and the many other excellent properties of diamond have led chemistry professor Greg M. Swain
of Michigan State University and many other scientists to use CVD to
grow polycrystalline boron-doped diamond electrodes that can
detect--and in some cases degrade--redox-reactive organic contaminants
in water supplies. In addition, Martin is using CVD to grow highly
conductive boron-doped polycrystalline diamond microelectrodes that
could directly sense a variety of redox-active neurotransmitters during
neurotransmission. The diamond microelectrodes should be more
sensitive, stable, and versatile than ones made of other materials,
Martin says.
U.K.-based Element Six,
formerly known as De Beers Industrial Diamonds, is already selling
CVD-grown polycrystalline diamond films for various applications, notes
Steven E. Coe, the firm's R&D manager. The company markets its
polycrystalline diamond for use as heat spreaders in high-power
electronic devices. It also uses the material to fashion surgical
blades that are resistant to dulling and optical windows for
high-powered CO2 lasers.
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LET IT RAIN
To make diamond by chemical vapor deposition, hydrogen gas and methane
are flowed through a chamber containing a substrate. Heat or a
microwave-generated plasma is used to split hydrogen gas into atomic
hydrogen, which then reacts with methane to give methyl radical and
hydrogen gas. The carbon-containing radical species eventually deposit
as diamond onto the substrate.
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NEVERTHELESS, using
single-crystal diamond instead of polycrystalline diamond in such
applications has advantages. Because the C—C bonds that hold its
patchwork of tiny crystals together are weaker than C—C bonds in
single-crystal diamond, polycrystalline diamond isn't quite as
thermally conductive, as optically transparent, or as strong as
single-crystal diamond. In fact, for some applications--particularly
those such as electronics that require the highest carrier
mobility--only single-crystal diamond will do, Linares tells C&EN.
For diamond to live up to its promise as an alternative to silicon for
fabricating electronic devices, "what's required is high-quality,
single-crystal CVD diamond in usable sizes," Coe adds.
Coe and his colleagues at Element Six proved this was possible just over a year ago [Science, 297, 1670
(2002)] and now can grow high-quality, single-crystal diamond wafers
that are 5 mm square. Linares tells C&EN that Apollo currently can
grow high-quality, single-crystal diamond wafers that are about double
that size. He predicts that within the next four years the company will
be cranking out 4-inch square wafers.
Both
Coe and Linares suggest that, thanks to its high thermal conductivity
and electrical carrier mobility, single-crystal semiconducting diamond
will be the ultimate material for fashioning high-powered electronic
devices. Element Six is already making some simple prototype devices,
such as switches, from p-type semiconducting diamonds, Coe says. But
most devices will require both hole-conducting (p-type) and
electron-conducting (n-type) diamond semiconductors. The former is
easy: Both Element Six and Apollo report that they can use their CVD
methods to make boron-doped single-crystal diamond wafers that are
excellent p-type semiconductors. Producing n-type semiconducting
diamond has proven more challenging, however.
A number of potential n-type dopants have been investigated, most notably phosphorus. A group led by Hisao Kanda of Japan's National Institute for Materials Science has
shown that doping diamond with phosphorus gives n-type semiconducting
diamond. The team has gone on to show that phosphorus-doped and
boron-doped diamond can be combined to make a simple electrical device
called a p-n junction.
But so
far neither phosphorus nor any other n-type dopant has demonstrated
exactly the right electrical properties, according to Butler. Butler,
Jacques Chevallier of the Laboratoire de Physique des Solides et de
Cristallogénèse, in Meudon, France, and their colleagues recently
reported that impregnating boron-doped CVD diamond with deuterium
yields n-type semiconducting diamond [Nat. Mater., 2, 482
(2003)]. Despite this promising development, Angus--whose own lab is
doping CVD diamond with a combination of boron and sulfur to get n-type
semiconductivity--comments that "all of the n-type work, including
ours, is interesting in a scientific sense but not yet practical for
devices."
The payoff for such
work is potentially huge: Today's microchips are running hotter and
hotter because more and more transistors are being crammed onto them.
If the trend continues, silicon may not be able to take the heat.
Diamond could be the perfect solution.
Despite
its superior combination of electrical, optical, thermal, and chemical
properties, though, diamond may never totally replace silicon for two
reasons: Silicon is both cheap and firmly entrenched in the computer
industry. Still, Reza Abbaschian, a professor of materials science and
engineering at the University of Florida, Gainesville, whose lab helped
to perfect Gemesis' diamond-growing method, believes that "for certain
specialized applications, such as devices that run at high power or
high temperature, diamond may be just the ticket."
COVER STORY
THE MANY FACETS OF MAN-MADE DIAMONDS
Synthetic diamond makers are targeting the gem market first, but their
product could transform many other industries, too
IMPROVING ON NATURE
Heat Treatment And Chemical Additives Make More Mundane Stones Look Like Rare Gems
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WHODUNIT
First Diamond Synthesis: 50 Years Later, A Murky Picture Of Who Deserves Credit
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Chemical & Engineering News
Copyright © 2004 American Chemical Society
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COVER STORY
|
THE MANY FACETS OF MAN-MADE DIAMONDS
Synthetic diamond makers are targeting the gem market first, but their
product could transform many other industries, too
IMPROVING ON NATURE
Heat Treatment And Chemical Additives Make More Mundane Stones Look Like Rare Gems
|
WHODUNIT
First Diamond Synthesis: 50 Years Later, A Murky Picture Of Who Deserves Credit
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