Diamond Nanotechnology: Synthesis and Applications

Published by Pan Stanford Publishing Pte. Ltd. 5 Toh Tuck Link Singapore 596224 Distributed by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

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DIAMOND NANOTECHNOLOGY Syntheses and Applications Copyright © 2010 by Pan Stanford Publishing Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.

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ISBN-13 978-981-4241-41-0 ISBN-10 981-4241-41-5

Printed in Singapore.

Preface Carbon is uniquely capable to form bonds of polymers (sp1 ), metals (sp2 ), and ceramics (sp3 ). As a result, carbon becomes the backbone of organic materials, fast conductors, and superhard ceramics. Diamond is not only the prevalent superabrasives (1000 tons consumed every year), but also the supreme semiconductor with figure of merits many times of silicon. In fact, silicon, with larger atoms arranged in diamond structure, can be viewed as weak diamond material. Diamond possesses unsurpassed hardness thermal conductivity and sound speed. Consequently, diamond can be the best heat spreader for cooling integrated circuits and high power laser diodes, and the ideal vibrator for tweeter diaphragms and surface acoustic wave (SAW) filters. The evolution of human civilizations depends on the advancement of material technology. However, up to now, the industry has produced materials by easy access. This straight forward path will eventually lead to diamond, the dream material for fantasy applications. Diamond has the most extreme properties. Some of them have been exploited, but more potential applications are developed, such as the fastest computation possible with using quantum bits of diamond (e.g. NV). We are entering diamond age with numerous novel diamond gadgets to be unveiled in this century. Thus, we may encounter diamond SAW filters, diamond LED, diamond solar cells, diamond IR detectors, diamond front panel displays, etc. In this book, we introduced many facets of diamond, from its cosmic origin to high-pressure synthesis, from CVD diamond films to dynamite nanodiamonds. As diamond is also the most common solid in the universe at large, the cosmic presence and biological connections of nanodiamonds are proposed. We have also included amorphous diamond coating for tribological applications and gem diamond growth for jewelry industry. Except for Chapter 8 that was compiled by Dr. Jianping Lin, all the other chapters in the book were authored by me. We would like to thank Ms. Sammy Ho, Queenie Huang and Emily Sung for editing this book. James C. Sung

Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diamond in the Sky

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Chapter 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Nanodiamond Connection of Life and Consciousness

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Chapter 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dawn of the Diamond Age

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Chapter 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diamond Synthesis in Perspective

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Chapter 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Micron Fines and Nanodiamonds

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Chapter 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dynamite Diamond

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Chapter 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanodiamond Applications

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Chapter 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biological Applications of Diamond

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Chapter 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amorphous Diamond as Thermionic Energy Converters

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Chapter 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fluorinated DLC for Tribological Applications

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Chapter 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gem Diamond Growth

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Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter One

Diamond in the Sky

Twinkle, twinkle, little star; how I wonder what you are? Up above the world so high, like a diamond in the sky. Jane Taylor, 1806

1.1

DIAMOND STARS

When Ms. Jane Taylor composed the famous lyric “diamond in the sky” about 200 years ago, she could not have imagined that her figurative poetry would foretell the monumentous scientific discovery made in the 20th century. As it turns out, real diamonds are not only in the sky, but it may even be the most common solid in the universe. In addition, as will be introduced in Chapters 2 and 3, diamond is also a very unique and an essential material for a myriad of different applications. Thus, the most abundant material is also the most versatile. Diamond is truly the king of all jewels. Carbon is the fourth most abundant element in the universe, after hydrogen, helium, and oxygen. However, these more abundant elements are gases, so carbon becomes the most abundant solid in the universe. The majority of carbon is locked up deep inside of stars where the pressure is exceedingly high. Since diamond is the stable form of carbon under pressure, it is natural to assume that diamond is commonplace inside a large star. Stars are large nuclear reactors that generate heat by fusing nuclei together. The first step of nuclear fusion is to merge hydrogen atoms in order to form helium atoms. For an ordinary star, this process may take a long time to complete. For example, the Sun Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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Chapter 1 Diamond in the Sky

has been shining for 5 billion years and it will continue the process for another 5 billion years. During the sustaining period of hydrogen fusion, stars maintain its stability by balancing two opposing forces, an inward pulling gravity against the outward moving photons. However, when a star with a size comparable to our Sun exhausts its nuclear fuel, it will shrink to become a white dwarf. Inside such white dwarfs lie colossal mines of diamond and the more massive the dwarf, the more plentiful its core of diamonds. A typical white dwarf is about a million times more massive than that of Earth; hence, chances are that most white dwarfs contain a mine of diamond much larger than the entire Earth. In 1969, Saslaw and Gaustad proposed that diamond might exist in space. In 1987, Levis and others identified nanodiamonds 3–5 nm in size in meteorites. Such diamonds belongs to a rare clase known as lonsdaleite (hexagonal diamond). The most common polymorph of diamond from both natural and synthetic origins is the cubic diamond. Hexagonal diamond is often formed by shock wave transformation (e.g. the collision of two meteorites) of the common form of graphite — the hexagonal graphite. In contrast, cubic diamond must take time to form under sustained conditions of high pressure and high temperature, such as inside a large star. Diamonds have been found in the sky based on the spectra of interstellar light and the measured absorption spectra of nanodiamond. For example, the surface of a nanodiamond is often terminated by the absorption of hydrogen. These carbon–hydrogen (C–H) bonds may vibrate to emit or absorb electromagnetic radiation with a characteristic wavelength. Matching extraterrestrial light with lab measurements of diamond spectra provides clues to the existence of diamond in the sky (Figs. 1.1 and 1.2). Nanodiamond may also form inside a giant star that undergoes an explosion to form a supernova. However, more commonly, nanodiamond particles may be formed when carbon-containing molecules collide with one another in interstellar dusts. Most of these nanodiamond particles contain hydrogen terminated bonds on its surface. The C–H bonds vibrate at a characteristic frequency that can shed light on the size and structure of the interior nanodiamond. Starlight may also be compared with the frequency of diamond lattice resonance. There are only a handful of stars that lie within 20 light years from the Earth. In 1998, astronomer Steve Kawaler

Diamond Stars

Figure 1.1. The similarity between an emission spectrum from an interstellar object and the absorption spectrum from stretching C–H bonds on the surface of nanodiamond (top diagram). The bottom diagram demonstrates that the absorption spectrum is dependent on the size of the nanodiamond. Source: Data provided by Huan-Cheng Chang.

discovered that one of them was a diamond star (BPM37093) that emitted the characteristic resonance frequency. This star is located 17 light years from the Earth. It belongs to the Constellation Centauras which is visible only from the southern hemisphere. This diamond star is 12,800 km in diameter and is almost the same size as Earth. However, it is a white dwarf and is the ruminant of a

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Figure 1.2. Idealized nanodiamond structures that may be formed by collisions of carbon containing molecules in space as inferred from their emission spectra. Source: Data provided by Huan-Cheng Chang.

sun-like star after it exhausted its fuel for nuclear fusion. Such a cinder of dead star is very dense, and a teaspoonful of material may weigh over one ton. The surface of this diamond star reaches about 12,000◦ C twice that of our Sun. It emits a greenish blue light that reveals the characteristic absorption spectrum of diamond. It would appear that the diamond it contains might also be blue in color. As most diamonds found on Earth are either colorless or yellowish, blue diamonds are extremely scarce. The blue color is caused by the incorporation of minute amount of boron atoms that absorb the

Diamond Planets

yellow color. This could be formed in conjunction of carbon during nuclear synthesis. Boron-doped diamond is an electron-receiving (P-type) semiconductor due to the presence of electron deficient holes. Boron-doped diamond may out perform boron doped silicon or other P-type semiconductors in hole mobility and in electrical conductivity. The famous Hope diamond is the largest blue diamond known. It was originally embedded in the forehead of Rama-Sita in an Indian temple. The French adventurer J. J. Tavernier adventured to India many times, and at the age of 80, he made the last trip to India where he stole the blue diamond from Rama-Sita. Upon his return to France, he sold this 112 carat diamond to King Louis XIV. This blue diamond was eventually passed on to Louis XVI who was beheaded during the French Revolution. In 1792, the blue diamond was robbed away from the palace by a mob. When it reappeared in 1839, it was re-cut into a much smaller 44.5-carat gem. This blue diamond was said to carry a curse of Rama-Sita that led to tragedy for many of its owners, including the death of Louis XVI. Eventually, the blue diamond fell into the hands of the English banker Henry Hope and hence it bears his name since. In 1958, the American gem collector Harry Winston presented it to the Smithsonian Museum in Washington, D.C. The Hope diamond has become the most visited attraction for tourists who visit the museum (Fig. 1.3). Although the Hope diamond may be the most glamorous gem ever existed, it could easily be dwarfed by a chip from the blue diamond star of BMP37093.

1.2

DIAMOND PLANETS

In addition to stars that may harbor colossal mines of diamond deep inside, many planets may have diamond rain down from the sky above. This is because both carbon containing gases; methane (CH4 ) and carbon dioxides (CO2 ), are common constituents in planetary atmosphere. Methane and carbon dioxide can be decomposed at high temperatures to form diamond. This is routinely practiced nowadays by a technique called chemical vapor deposition (CVD) (Fig. 1.4). In addition to high temperature decomposition of methane, high pressure may also assist in the formation of diamond. It is amazing to think that diamond raining down in

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Figure 1.3. The Hope diamond is the largest blue diamond known. It contains boron atoms which make it a positive semiconductor.

Figure 1.4. Diamond grown from thermally decomposed methane heated by tungsten filaments (Diamonex’s sample) (left). Diamond formed from pyrolyzed carbon dioxide heated by laser beams (QQC’s sample) (right).

a planets’ atmosphere may be as common as water raining from Earth’s sky. In addition to diamonds that may found buried deep in stars, and those that may rain down from the atmosphere on planets, diamonds are also ubiquitous in other places. Since carbon atoms exist everywhere in the universe, the frequent impact of these atoms have the potential to form diamond inevitably.

Nuclear Synthesis and Carbon Formation

There are many ways that carbon atoms may be squeezed together to form a diamond. First, all stars and planets move relative to one another, so the act of collision between them may not uncommon. Such a collision will certainly convert the majority of carbon atoms into diamond. Second, each planet or satellite is constantly bombarded by asteroids or comets. This is why many satellites such as the moon, are covered with impact craters. Again, each meteorite impact can produce diamond dust. Indeed, nanosized diamond particles are present in meteorites as well as in impact craters. Third, billions of asteroids and hundreds of billions of comets are constantly banging against one another. Hence, microscopic diamond may be formed in the solar system all the time. Fourth, as giant stars explode after running out of nuclear fuel (e.g. supernovae), the explosion can spew diamond debris into space. Fifth, the universe is soaked with cosmic rays, star winds, gamma ray bursts, and X-ray beams. Such energetic particles and electromagnetic radiation can convert carbon atoms into diamond. This has been proved as the absorption spectrum of diamond has been used to find atomic clusters of diamond in stellar dusts. Coalmines could also be impregnated with diamond. For example, an old meteorite hit a coalmine in Brazil hundreds million years ago. Some of the coal was instantly melted and then quenched to form the radiant structure of amorphous diamond known as ballas. Other less hot coal was converted into a black aggregate of diamond known as carbonados. Certain uranium containing coals also contain sub-nano particles of diamond. These clusters, each containing hundreds of carbon atoms, were located along the fission tracks of disintegrated uranium nuclei. Apparently, the impact of nuclear debris against the carbon atoms in coal turned them into diamond. In addition to the various sources of diamond mentioned, the existence of diamond is known to be deep within Earth as well. This is why volcanic mantle plumes such as kimberlite had been found to bring natural diamond up to the surface here on Earth. 1.3

NUCLEAR SYNTHESIS AND CARBON FORMATION

According to the Big Bang theory, the orthodox teaching of the origin of the universe, we owe our existence to a proto universe

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10 cm across that was formed when the universe was about 10−32 sec in existence. This proto universe has expanded since to form Earth as we know it today about 15 billion years later. The proto universe was extremely hot (1027 K), but the expansion of space has decreased its temperature to only 2.7 K now. During the early stage of quenching, the energy in the proto universe condensed into elementary particles that eventually coalesced to form protons and electrons. After 300,000 years of ongoing expansion, the temperature cooled enough (1000 K) to allow the formation of atoms. However, the cooling of the universe was so fast that only hydrogen atoms (93 at.% or 79 wt.%) and helium atoms (6 at.% or 20 wt.%) could form in time. Hydrogen and helium atoms are less stable than heavier atoms, so they have a tendency to fuse together. But, this fusion may only take place at the high temperature that existed when the universe was still in infancy. Although the ambient temperature of Earth is now been too cold, the center of a star is hot enough to continue the fusion of light atoms. Stars are formed by accumulating sufficient amount of light atoms that attract one another by gravity. As soon as pressure and temperature reach the right threshold level, nuclear synthesis by fusing light atoms into heavier ones will begin. Initially, like our Sun, hydrogen atoms will fuse together to form helium atoms. If the star is sufficiently massive (e.g. 3 times more massive than our Sun) helium atoms will also combine to form heavier atoms, and the trend will continue. The binding energy of nuclei is the lowest for iron atoms (Fig. 1.5). Accordingly, nuclear synthesis may continue until iron atoms are formed. After that, any enlargement of nuclei will not release enough energy to sustain the reaction, but will absorb energy, such that the nuclear synthesis will halt. At high temperature, nucleons have enough energy to move around and so tend to form the most stable nuclei possible. Hence, four hydrogen atoms will fuse into one helium atom (two hydrogen atoms become two neutrons that join the nucleus, at the same time, two protons and two electrons from the other two atoms will combine in the same atom). Subsequently, helium atoms may also fuse into beryllium atoms. Beryllium atoms may combine with helium atoms to form carbon atoms. Carbon atoms may fuse with helium atoms to form oxygen atoms, and the cycle can continue in this fashion.

Nuclear Synthesis and Carbon Formation

Figure 1.5. Iron (N = 26) has the lowest nuclear binding energy, so any atoms lighter than iron may combine (fusion) into larger atoms; but any atoms heavier than iron may disintegrate (fission) to form smaller atoms.

If the binding energy of nucleons monotonously increase with the addition of nucleons, then we would expect that the cosmic abundance of these elements to decrease with the atomic size (atomic number N, proton number or electron number). But the stability of nucleus size differ greatly with the number of protons present. Hence, the amounts of atoms formed are also highly variable. Although the general trend shows a decrease in the abundance of elements with increasing atomic size, but there are many exceptions. For example, although iron atoms are more than 10 times heavier than beryllium atoms, they are millions of times more abundant. Beryllium nuclei formed by the impact of two helium nuclei is unusually unstable and has a half-life of only 10−17 sec. Hence, beryllium atoms are rarely found inside a star. However, beryllium atoms provide the source for making heavier atoms such as carbon and oxygen. These are the two of the most vital elements for us humans (e.g. our body may contain 12 at.% or 23 wt.% of carbon atoms, and we need to breath an air that contain about 20 wt.% oxygen). If there were no mechanism to boost the production of these essential elements, not only there would be no diamond stars, but also the universe would be lifeless.

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In order to explain the ubiquitous presence of carbon and oxygen atoms in the universe, the English astrophysicist Fred Hoyle proposed in 1953 that carbon nuclei could resonant at a frequency 4% higher than beryllium nuclei, so when the latter is hit by helium nuclei, the combined energy will make carbon nuclei tick. According to this hypothesis, although beryllium nuclei are extremely unstable, the chance for these nuclei to transmute into carbon nuclei is exceedingly high. As a result, the abundance of carbon increases at the expense of beryllium. Holye also proposed that the resonance frequency of oxygen nuclei is 1% less than that of carbon. Because of this, the collision of carbon and helium would not form oxygen readily. Hoyle’s two predictions were confirmed latter by experimentation, a triumph because he backwardly reasoned the cause based on its effect. Thus, it is the subtle balance of the vibration frequencies between carbon and oxygen that result in how abundant these two critical elements are. By having the right balance of carbon and oxygen, the miracle of life is possible. Carbon and oxygen atoms are much more abundant than any other heavy element. As a consequence, the four most abundant elements in the universe are H (93 at.%), He (6 at.%), O (0.06 at.%), and C (0.03 at.%) (Fig. 1.6). Among them, C is the only solid element. All the other heavy elements combined would account for only 0.1 at.% of the cosmic abundance, hence, carbon may actually account for 30% of all solid material in the universe. For example, the cosmic abundance of carbon is more than ten times that of silicon (a major semiconductor material and the essential constituent of rocks) or iron (a major industrial material and the core element of Earth). No wonder carbon is ubiquitous in space and therefore, diamond is everywhere in the cosmos. 1.4

SUPERNOVA EXPLOSION

Currently, our sun is fusing the nuclei of hydrogen together to form helium. However, the sun’s mass is not large enough to cause the synthesis of a significant amount of heavier elements, so when its nuclear fuel of hydrogen is exhausted in about 5 billion years, it will stop the production of heat and light and gradually cool down to form a white dwarf, eventually becoming a brown dwarf or even a dark dwarf. If a star’s mass is larger than 1.4 times that of the

Supernova Explosion

Figure 1.6. The cosmic abundance of elements. Note that carbon is the fourth-abundant element and the most abundant solid element.

Sun, its nuclear synthesis will proceed until it just begins to form iron. In this case, the dying star will contain concentric zones made of increasingly heavier elements (Fig. 1.7). Upon the exhaustion of fusible fuels, the star cannot produce enough energy to sustain its size so the outer layer will collapse instantly. This implosion may smash against the iron core with such a shocking pressure that most electrons surrounding protons would fuse together to form neutrons instantaneously. During nuclear fusion, a star generates energy by combining electrons and protons in light elements to form neutrons at a slower rate. Even so, the energy formed from the loss of mass can be a tremendous amount as calculated by Einstein’s equation,

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Figure 1.7. The onion-like structure of a large white dwarf. Note that carbon is buried deep inside and has the potential to form into diamond. The mass of a typical white dwarf is about one million times that of Earth, so the amount of diamond in such a star may easily overshadow the entire Earth.

E = mc2 . In the case of the implosion of a massive star when it uses up its nuclear fuels, the sudden surge of pressure and temperature could break most iron atoms. As a consequence outer electrons will be squeezed into the nucleus when they combine with protons to form neutrons almost simultaneously. The result would be the formation of a gigantic soup of neutrons known as a neutron star. A neutron star has a density as high as a nucleus (1014 g/cc), so if you weighed a piece the size of a grape it weighs over 100 million tons! The energy to create hydrogen fusion in a star is created by combining an electron with a proton to form a neutron one by one. The formation of a neutron star is likely to achieve nuclear fusion 100 billion times faster than an ordinary star. Hence, once a neutron star is formed, the amount of energy must be released in a hurry. The result is a gigantic explosion that expels more than 90% of its outer materials into space as stellar dusts. This spectacular astronomical phenomenon is known as a supernova explosion. After the explosion, the supernova can outshine for weeks an entire galaxy that contains about 100 billion stars. Supernova explosions occur periodically in each galaxy (hundreds of such cosmic glares have been recorded in human history).

Supernova Explosion

Figure 1.8. The Crab Nebula is the remnant of a supernova that exploded in 1054. This explosion was so violent that Chinese astronomers recorded it as “the guest star” that appeared in bright daylight like a full moon for more than a week. As diamond might be formed in a thick crust on this supernova, the shattered star could send ample nanodiamond dust into space.

The explosion will release carbon and oxygen that is otherwise locked up inside a supernova. The condensation of the expelled debris will make latter generation stars richer in heavy elements. Our sun was one such star formed on the ashes of a previous supernova. The entire Earth was also built on such cinders of cosmic violence. The diamond that was formed on the outside of heavier elements inside a supernova was shattered during the gigantic explosion to form nanodiamond dusts that scattered in the interstellar region. In our solar system, such nanodiamonds are found trapped

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inside asteroids or even comets. Some meteorites found on Earth do contain nanodiamonds that contain isotopes traceable to their pre-solar origins. Dusts expelled out from an exploded supernova may condense to form second generation stars and planets. Due to the recompression of already formed carbon atoms, diamond rich stars or planets may be omnipresent in regions where matters are dense, such as closer to the center of the Milky Way galaxy. 1.5

DIAMOND SHOWERS

If we exclude the noble gas He since it does not react with any other element, the three most abundant elements H, C, and O will form the most common molecules of water (H2 O), methane (CH4 ), and carbon dioxide (CO2 ) (Fig. 1.9). Thus, these three compounds are the common gases in planets that harbor an atmosphere. Both methane and carbon dioxide can decompose under high pressure, and the segregated carbon will form diamond. The carbon atom in methane is surrounded by four hydrogen atoms that form a tetrahedron. The carbon’s atom forms four bonds that extend to each of the four hydrogen atoms. This bonding structure (sp3 ) is exactly that of diamond except that each of the bond is slightly ionic due to the different bonding strength of electrons between the carbon and hydrogen atoms. Hence, we may view

Figure 1.9. Common atmospheric gases are derived from the three most abundant cosmic elements.

Diamond Showers

Figure 1.10. Methane is a gas composed of floating single atom diamonds. Each atom is confined in a tetrahedron cage of hydrogen atoms.

methane as a caged single atom of diamond (Fig. 1.10). Methane is the common ingredient in natural gas. It may be of interest to note that cooking is often done by burning floating diamonds. When methane is heated in the presence of hydrogen atoms, single atoms of diamond can be coerced to link together to form multiple atom diamonds. With time, micron diamonds may grow

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Figure 1.11. Diamond micron powder formed by CVD (left diagram) and the linking of micron sized diamond to form a diamond film (right diagram).

to microscopic size (Fig. 1.11), and eventually, they may link to form a continuous layer. This is a common CVD practice to produce diamond films. Methane can also decompose under high pressures to form diamond. Physicists have discovered that diamond can be formed by pressing the solid form of methane to 50 GPa, or by shooting it with a bullet traveling at a speed of 30 mach (10 km/sec). It is, therefore, conceivable that diamond snowflakes may shower from the sky in these giant planets. If the temperature is high enough (e.g. 4000◦ C), these snowflakes may melt to form a rain of diamonds. At a much higher pressure, this diamond rain may condense to form diamond ice or hail stones. At a great depth, diamond rain or diamond ice may accumulate to become a diamond ocean or diamond continent, depending on the temperature. Of all the gas giants in the solar system, Uranus and Neptune contain the most methane in their atmosphere. Moreover, if the methane is under a high enough pressure without overheating as in the case of the atmosphere on Saturn and Jupiter (Fig. 1.12), such an environment is conducive to form diamond. In 1981, the physicist Marvin Ross pointed out that Uranus and Neptune may have accumulated a thick layer of diamond ice around its rocky core where the pressure may be as high as 60 GPa and the temperature at 7000◦ C.

Diamond Showers

Figure 1.12. The pressure–temperature profile for Saturn, Jupiter, Uranus, Neptune and Earth. Note that it is possible for the methane on Uranus and Neptune to decompose to form diamond.

One place to look for a spectacular diamond storm is deep within Neptune. Neptune has a blue appearance because the presence of methane in its atmosphere, which absorbs yellow light. Astronomers have found that this blue gaseous planet could form its own energy as it radiates 2.6 times more heat than it absorbs from the Sun. However, the source of such energy is unknown. In 1999, Robin Benedetti from University of California at Berkeley squeezed methane in a pair of diamond anvils and heated it up with a laser to simulate the pressure and temperature conditions of about one third of that found in the center of Neptune. He found that methane decomposed to form carbon atoms and these atoms then bonded to one another to become diamond flakes. Hence, diamond snow may shower from the skies on Neptune (Fig. 1.13). The condensation of carbon to form diamond on Neptune could account for the mysterious heat source that has baffled astronomers. Not only can methane form diamond under high pressure, so can carbon dioxide. In 1995, metallurgist Pravin Mistry accidentally discovered that diamond could be formed by bombarding carbon dioxide with laser beams. Apparently, the

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Figure 1.13. The blue color of Neptune reflects the abundance of methane in its atmosphere. Inside this interior methane atmosphere, showers of diamond flakes may rain down onto its rocky core.

decomposed carbon atoms banged against one another and linked together to form a diamond lattice. Carbon dioxide is also common on gaseous giant planets. It is the major constituent in the atmosphere of Venus and is 100 times thicker than the atmosphere on Earth. Although the pressure (20 MPa) and temperature (400◦ C) there may not high enough to form diamond rapidly, given the long period of geological time, a thin layer of diamond may have veneered to the surface of Venus. 1.6

ARTIFICIAL CVD DIAMOND

Although diamond showers are formed routinely on giant gaseous planets in the solar system, humanity has learned the trick and made CVD diamond based on similar principles. Most CVD diamonds are formed by thermally decomposing a carbonaceous gas (e.g. methane) in the presence of an over abundance (e.g. 100X) of hydrogen. The diatomic hydrogen partially dissociates to form

Artificial CVD Diamond

Figure 1.14. Various sizes of CVD diamond films deposited by microwave plasma of hydrogen and methane gas mixture (Aixtron brochure). Note that the blue diamond film contains doped boron so it is semiconducting.

Figure 1.15. The range of transparency for various optical materials.

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hydrogen atoms that fly around. When carbon atoms are formed, the hydrogen atoms bombard the detached carbon atoms to maintain carbon’s diamond (sp3 ) bonds until they are joined by other carbon atoms. As a result, diamond is formed by linking dissociated atoms without forming the more stable form of carbon — graphite. Even if graphite is formed, graphitic (sp2 ) bonds can be converted into diamondoid bonds (sp3 ) by the continual bombardment of hydrogen atoms. Thus, diamond film can be formed metastably with the protection of hydrogen atoms as the catalyst.

Figure 1.16. A large transparent CVD diamond grown by De Beers. Similar diamond windows can be as large as 16 cm in diameter and more than 2 mm in thickness (e.g. Raytheon). The amount of absorbed energy is less than 1%. The absorption coefficient is less than 0.1 cm−1 , and can be as low as 0.027 cm−1 .

Artificial CVD Diamond

The more hydrogen atoms that are present during this process, the faster the growth of diamond without the formation of graphitic carbon or other defects. Diamond films with low concentration of defects can be excellent optical windows for shielding generators of X-ray (Fig. 1.14), visible light (Fig. 1.15), or infrared electromagnetic radiation. A diamond window can absorb so little of the electromagnetic radiation that even megawatts of microwave can pass through without warming it (Fig. 1.16). This makes diamond windows ideal for igniting hydrogen fusion with a megawatts microwave beam. Diamond windows may also survive high corrosive environments. For example, a natural diamond lens was used to explore the acidic atmosphere of Venus.

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Chapter Two

The Nanodiamond Connection of Life and Consciousness 2.1

DIAMOND AS LIFE GIVER

The origin of life has been a mystery to scientists for a long time. The abundance in water has allowed the life to prosper on Earth, but the sudden appearance of prokaryotes (e.g. bacteria) immediately after the solidification of Earth’s crust about 3.8 billion years ago was a surprise. Although no mechanism was given, the hypothesis of panspermia hinted that the precursor of prokaryotes might be formed outside the solar system before the Earth was formed. These building blocks of life might have showered the early sky when the atmosphere contained mostly carbon dioxide methane, and water vapor. Once the primordial broth on the Earth was taking shape with the precursors of life various forms of prokaryotes could be developed in a few hundred million years. James Ferris of Rensselaer Polytechnic Institute surmised that clay minerals (e.g. montmorillonite) might serve the function of a template for assembling biomaterials. However, clay minerals are not stoichiometric stable so they may not reproduce the results reliably. Recently, German scientists speculated that water wetted diamond could have jump-started the first life on Earth (http: news.yahoo.com/s/ livescience/20080726). Their model was based on electrical conductivity of hydrated diamond surface. A living organism requires both the blue print and duplicating mechanism for making and maintaining itself. The blue print Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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used most today is based on DNA. The manufacturing house and maintainence facilities are based on protein. These machineries are overly complicated for the first life to acquire naturally by chance. However, according to the hypothesis of RNA world, the primitive RNA could be the precursor of both DNA and protein. Microscopic RNA could duplicate itself although unreliably. A folded RNA molecule might also assemble amino acids for making proteins. In fact, RNA of virus size has been “lubricant” of cells. Without them to run around and to fix things, a cell will die of functional deterioration. For all complicated structures formed in nature, they have to be assembled from atoms and molecules. However, this bottom up approach works only if certain building blocks are preformed. In the case of RNA or protein, the building blocks themselves are rather complex. For example, all nucleotides contain aromatic carbon rings with conjugated sp2 bonds (e.g. benzene). Moreover, all proteins are polymers with a back bone made of complementary sp1, sp2, and sp3 bonds. If carbon derived molecules are formed, further additions or modifications of them can be easier. Thus, the precursor of life is logically made of carbon derived molecules as building blocks. Many scientists have been baffled with the question that the evolution of living organism have been able to defy the second law of thermodynamics. Lord Kevin once said that the second law was the law of laws so it could not be modified. In fact, the very reason of Big Bang is based on the entropy increase associated with space expansion. Consequently, the second law violating life evolution seems impossible. However, the second law may be overcome if the system is open. For example, a refrigerator is capable to pump heat uphill from a cold place a hotter environment. Living organism are open systems so they can excrete entropy by metabolism. However, the building blocks of life precursors cannot metabolize, so where its energy may come from? The answer lies in the potential energy of gravity. The flexible carbon bonding acts like a spring the provide the uphill driving force. The carbon spring was made by sintering in a supernova that was compressed by gravity. As discussed in the previous chapter, carbon is the most prevailing solid in the universe. The carbon element was made in a giant star with heat generated by the consolidating of gasses. In essence, diamond was formed at the core of stars.

Diamond as Life Giver

The diamond core could explode as supernova. The gigantic diamond was shattered as nanodiamonds. As the temperature was still hot during the rapid decompression process, most nanodiamond transformed into graphite with disintegrated carbon rings. Moreover, many carbon particles are forming onions, bucky balls, carbon nanotubes, carbon blacks, hydrocarbon molecules, CNO compounds, and carbon soots. Such carbon derivatives may be found in interstellar dusts. They may also be incorporated in comets and meteorites. Based on the above top down model, the potential energy of gravity was stored as chemical energy of carbon. The dexterity of carbon bonding (i.e. sp1 , sp2 , sp3 ) allows the release of carbon energy to facility the formation of proto RNA and protein. However, even with the availability of the carbon derived molecules, they are still difficult to be assembled in the form of biomaterials. But this problem is solved once again by the presence of diamond. As the back converted diamond can not only provide the raw materials for organic chemistry, it may also act as versatile mold for assembling biological molecules. Nanodiamond are clustered carbon atoms with both graphitic (sp2 ) and diamondoid (sp3 ) bonds. The two types of bonds can be interchangeable, for example, the stretched (1 1 1) face of diamond is a graphene plane. In reverse, the puckered graphene may become a diamond surface. This interchangeability allows nanodiamond particles to be flexible templates, particularly around the curved surface where electrons are unstable. If diamond played a role in coercing the formation of organic molecules, it could do so everywhere in the universe, even before the solar system began to condense about 5 billion years ago. Diamond could be the most abundant solid already when the first stars were formed more than ten billion years ago. The density of the universe was much higher then so massive stars were commonplace. These jumbo stars were active nuclear fusion bombs for synthesizing heavier atoms. Some of the big stars exploded as supernovae and they spewed diamond debris to mix with interstellar dusts. Most stars we see today were formed later on in the cinders of early stars. The new stars were not as massive, but many of them could synthesis carbon at core if the temperature reached 15 million degrees centigrade. After exhausting most lighter elements by

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nuclear synthesis, a star might collapse and it cooled down. Subsequently, carbon vapors in the interior of the star condensed to form liquid and it eventually solidified to become crystalline diamond. Hence, many dead stars known as white dwarfs may contain a diamond core. The colder dwarf often pulsates with a characteristic frequency that reveals diamond as the major internal constituent. In 2004, astronomers identified BPM37093 as such a diamond star. It is located only 17 light years away in Constellation Centaurs visible from the southern hemisphere of Earth. It was named Lucy after the Beatles’ song “Lucy in the Sky with Diamonds”. Our Sun may join Lucy as another diamond star when it runs out the fusion fuel about 10 billion years in the future. 2.2

DIAMOND AS THE CATALYST

All living organisms are based on organic materials, i.e. carbon containing molecules. Moreover, most carbon atoms are diamondlike with covalent bonds of tetrahedral (sp3 ) configuration. Since carbon is the fourth abundant elements in the universe, nanodiamond particles can be commonplace. Gaseous carbon molecules (e.g. methane) may also be permeating in stellar dusts. The interaction of diamond and various types of gasses could form building blocks of biological materials. For example, amino acids could be triggered by integrating methane (CH4 ), water (H2 O), carbon dioxide (CO2 ), and ammonia (NH3 ). Scientist have long established that certain reducing gasses could react to form amino acids at high temperature, for example, when they were heated by lightning. In addition to be omnipresent, diamond’s bonding structure can be highly flexible. Carbon is the only element that can form linear organic (sp1 ), planar metallic (sp2 ), and solid ceramic (sp3 ) bonds. Moreover, such bonds are highly stable due to the covalent nature of small carbon atoms. Hence, diversified carbon molecules can be very enduring. There are more organic (carbon containing) compounds than that formed by all other elements. The organic chemistry with its extension of biochemistry is the foundation of life science. Nanodiamond formed in space contains a lot of defects; particularly it was shattered during the explosion of giant stars. The carbon atoms on the surface and along the vicinity of defects are graphitic that allows electrically conducting. Moreover, the

Diamond as Synthesizer

Figure 2.1. Diamond surface may be terminated to form semiconductor that can boost the current and heat the surface to cause chemical reactions of attached radicals.

diamond surface terminated by hydrogen is P-type semiconductor; and by oxygen, N-type semiconductor. These semiconductors may concentrate electrical charge like capacitors. The stored electrons can avalanche to heat diamond surface. Due to the prevailing presence of nanodiamond with surfaces attached with various elements, the biochemical reactions triggered by capacitive discharge must be active. If the hydrogen terminated carbon layers are joined by oxygen terminated one,5 the composite film can be amphibious with both the hydrophobic hydrogen face and hydrophilic oxygen face (Fig. 2.1). This may become the precursor to derive a membrane that can regulate water to flow from one side toward the other, but not the reverse. Such a membrane is required to hide gene inside a cell (e.g. cellular boundaries made of phospholipid). 2.3

DIAMOND AS SYNTHESIZER

Diamond has the tightest lattice of all solids. The surface of diamond lattice is full of dangling bonds. The bonding of foreign atoms with the dangling electrons on diamond’s surface can force the alignment of different radicals, particularly those formed with light elements (e.g.) (Fig. 2.2). Hydrogen, oxygen and nitrogen are among the commonest gasses that could be anchored on the surface of nanodiamond. The gas-terminated nanodiamond could be floating around in space so it could be impinged frequently with other molecules. Due to the versatility of carbon bonding, nanodiamond could act as a catalyst to promote the formation of stable radicals, such as acidic

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Figure 2.2. The dangling electrons on diamond’s faces are capable to attach atoms of various elements’ atoms to form specific patterns.

(–COOH), methyl (–CH3 ), amine (–NH2 ), hydroxyl (–OH) and others. Upon further heating, these radicals might assemble to form components of amino acids. The ability of the carbon atoms to join in linear (polymeric), planar (graphitic) and tetrahedral (diamondoid) structures made methane or its derivative active reactants. Moreover, nanodiamonds were readily available as the mold for assembly. The heat required to trigger the chemical reaction may come from collisions of molecules or dusts. Consequently, diamond facilitated synthesis of amino acids in the proto universe could be inevitable and widespread. Due to the presence of the vast amount of nanodiamond particles, amino acids could be synthesized and attached to the surface of some larger particles (Fig. 2.3). If the right sequence of amino acids was present, these molecules could combine and fold themselves into proteinoids, the component bits and pieces of protein (Fig. 2.4). 2.4

DIAMOND AS TEMPLATE

The octahedral (1 1 1) faces of nanodiamond could also form polygon-shaped organic molecules, such as the hexagonal benzenoids and the pentagon fructose. These ring molecules are building blocks of complicated amino acids of proteins (Fig. 2.5). They are also necessary for assembling into the base components of DNA. During the supernova’s explosion, the surface of most nanodiamond might be converted to graphitic layers, among them were fullerenes and carbon onions. The curved surface atoms may form

Diamond as Template

Figure 2.3. Amino acids could be derived from molecules of methane and other simple compounds. The integration of these components could be facilitated by surface reactions of nanodiamond particles.

Figure 2.4. Nanodiamond in the early universe might anchor different types of amino acids that could be flaked off to form platinoids, the precursor of proteins.

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Figure 2.5. The hexagon spiral of diamond’s (1 1 1) face could be stretched or molted to cultivate benzene rings. The nitrogen-terminated diamond face might shed the nitride derivatives of benzinoids.

the mold or they flake off to become hexagonal and pentagonal molecules (Fig. 2.6). The absorbent on hexagons might assemble to form benzinoids. The ribose could be imaged from fructose. If phosphate molecules were around in the primordial broth of the Earth, the primitive RNA might be assembled by chance.

2.5

DIAMOND AS PLAYGROUND

As nanodiamond was capable to form various components of bases, it was possible that these bases could be arranged on the diamond surface to form components of transfer RNA (t-RNA) or ATP, the omnipresent messengers in cells. They may be synthesized by attaching bases with molecules of sugar and phosphate. As sugar and phosphate molecules were likely present in the primitive ocean of the still warm Earth, t-RNA and ATP (ADP, AMP) could be present widespread as precursors of life, immediately after the solidification of primitive rocks (Fig. 2.7). The attachment of amino acids or functional bases on diamond might also be indirect with cushions in between. Such springy anchorages could prompt the reactions of diversified organic radicals on diamond surface. The bouncing of acids and bases on a common template might have allowed flexible assembly of larger components, such as protinoids and t-RNA (Fig. 2.8).

Diamond as Playground

Figure 2.6. A nanodiamond shed fullerene with illustration of molding and molting pentagonal fructose and hexagonal benzene. Note that this process of molding and subsequent molting can result in the succession of base pairs that are separated by 3.35 Å, the distance of graphene like carbon rings held by van der Waals force, just like graphenes stacked in graphite. The base pairs formed by consecutive molting can be attached by a RNA rail in sequential order. In the diagram, the backbone of RNA is depicted by combining ribose with phosphate. Phosphate could be derived form phosphorous permeated silica (quartz) present in hot springs of volcanic origin.

Figure 2.7. The surface of nanodiamond particle could act as the primitive backbone to fasten the bases to be attached to ribose floating around.

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Figure 2.8. The floating anchorage of biological components on diamond surface could allow flexible assembly of larger biological structures.

Once t-RNA molecules were formed, nanodiamond could serve the function of ribosome to manufacture proteins, although their yields could be poor. However, ribosome molecules are simply snug fitting of several very small protinoids. Consequently, it was possible that the stumbling of protinoids cast by nanodiamond could also form ribosome-like proteins by chance (Fig. 2.9).

2.6

DIAMOND AS GENE PRECURSOR

As the function of cells need both the blue print, i.e. DNA, and the manufacturing facility, i.e. proteins, the first lives must acquire both traits simultaneously. Although scientists have been debating whether chicken or egg came first, the universal presence of nanodiamond could provide building blocks of both proteins and DNA. The shower of such building blocks and nanodiamond in the primordial broth of Earth might have allowed the assembly of ample nanometers RNA molecules by incorporating phosphates from volcanic fluids. RNA was likely the precursor of the most primitive forms of life. In fact, RNA molecules are the enabler of living cells. For example, they play a vital role to coordinate the functions of brain neurons.

Diamond as Gene Precursor

Figure 2.9. The effective protein manufacture by biological ribosome (top) and the chancy protein formation on diamond surface (bottom).

RNA can splice easily to form the analogue version of the digital DNA. Due to the flexibility of RNA, the engagements of biological functions can be fussy and forgiving. Although such engagements may be imprecise, their signals can be recognized. Evolution from a polymer to an organism required fast turnarounds of trials and errors. The fuzzy evolution by analogue RNA were much faster compared to the slow speciation with digital DNA. This difference is contrasted between rapid mutations of viruses compared to that of relative stable cells (Fig. 2.10). According to the above hypothesis, nanodiamond might mold RNA molecules in the primordial broth by incorporating phosphate to form a backbone. Nanodiamond could further provide RNA with proteinoids to become the most primitive viruses. The retroviruses latter acquired more sugar-phosphate in “the little

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Figure 2.10. The flexible fitting of RNA on diamond surface might allow the first assembling of viruses by shrouding it with proteinoids.

warm ponds” to make the simplest DNA with double helices. From there, bacteria began to flourish. Latter, photosynthesis was invented by blue green algae. The release of oxygen by photosynthesis promoted the formation of eukaryotes with cored DNA (Fig. 2.11). After then, life could have radiated as we know today. The nanodiamond origin of life may explain the sudden radiation of life on Earth as soon as it was cooled to form the first crust. According to this model, the omnipresence of nanodiamond in interstellar dusts could facilitate the formation of amino acids and component bases in space. These building blocks were assembled in primordial broth of the Earth to form viruses that latter evolve to form prokaryote bacteria. The symbiosis of lives in ocean allowed the integration of bacteria colony to become first eukaryote cells.

Diamond as Gene Precursor

Figure 2.11. The structure of DNA and the role of hydrogen bonds. The stability of DNA is provided by the snug fitting of coiled double helices. Note that the separation of nucleotides is exactly the distance between graphene planes that could be formed by stretching (1 1 1) faces of nanodiamond.

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2.7

DIAMOND AS QUANTUM COMPUTER

The trend of human civilization has been to provide the comfort of living conditions. Thus, brick buildings have replaced wooden shelters; and polymeric clothes have replaced animal hides. Moreover, the Earth seemed shrunk in size due to the quickened pace of technical advancements. Therefore, automobiles have shortened the distance of travel and Internets have closed the gap of communications. However, the most startling of innovations will be the man-made evolution toward the construction of supermen. For examples, nanotechnology (e.g. NEM) may boost the functionality of the human body, and gene engineering (e.g. DNA enrichment) can cultivate post-Homo sapiens species. In the diamond age of human civilization, not only diamond semiconductors are prevailing computation tools with routine speeds reaching teraflops in a single chip, but also quantum computing based on quantum bits (qubits) supported by rigid diamond lattice may even obsolete digital computers altogether (Fig. 2.12). Diamond has the most stable lattice of all materials, consequently, it can provide the quantum-computing environment that is isolated totally from thermal and optical interference of the external world, at least in a brief moment to allow the completion of computation. 2.8

DIAMOND AS DREAM CHIP

In addition to quantum computing, nanodiamond may also be used to build a superbrain. Scientists are just beginning to learn that neuron cells may be cultivated on a computing chip. Moreover, these cells can be coerced to grow axons and dendrites so that the network can follow a logic pattern. The thinking neurons outside the brain can perform useful functions (e.g. flight simulation). The artificial network of neuron cells can be much smarter than even the champion of human experts in a particular field. Thus, dream chips may be fabricated to think things that are beyond the comprehension of human brains (Fig. 2.13). 2.9

DIAMOND AS SUPERBRAIN

Although the above dream chips may perform complicated duties that overwhelm even the super computers (e.g. to predict the weather pattern), multiple layers of neurons are needed to enhance

Diamond as Superbrain

Figure 2.12. A group of quantum computing qubits made of nitrogenvacancy can be isolated long enough to sustain superfast computation by communicating among themselves with photons triggered by a laser. The same lattice can serve as a perfect photonic crystal for X-ray computation at the speed of light. The synergistic integration of quanta and photons will trigger the omni-understanding of the reality, i.e., the mind of God.

their capabilities further. Nanodiamond particles may anchor neurons in three dimensions to form thinking tanks. This is analogous histone proteins in holding DNA strands together to form chromatins. With such emulations, trillions of neurons may be consolidated by nanodiamond in lumps of a superbrain, akin to chromatins inside the core of a cell (Fig. 2.14). The nanodiamond itself can act as photonic quantum computer capable to answer complicated questions on spot. The integration of living brain and material world may reveal the mind of God. Unlike human brains that can be lazy to think, and tired of learning, the nanodiamond supported superbrain loves to excel itself in knowing everything limited only by itself. As consciousness is dependent on neuron functions, such a superbrain will be aware of itself completely, i.e. a superconscious may emerge. If the superbrain continues to augment its capacity, for example, in space

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Figure 2.13. Carbon nanotubes are formed at the crisscrossed array of electrodes as the input and output terminals. The panels are filled with neuron cells soaked in nutrients. The neurons in each panel are taught with logical signals to stimulate the connection of axons and dendrites. The brain network so formed will be self-learning after the initial trigger from outside (reproduced from James C. Sung, Kinik Journal, 2008).

where gravity is not destructive, the superconscious could reach the level of knowing everything that is relevant to its existence, including the history that created itself. Once the ultimate brain is constructed, it would know all things from the beginning of the

Diamond as Superbrain

Figure 2.14. Just like various sized histones may hold genes together in each cell, so do various sized nanodiamond can anchor neurons inside a superbrain.

universe to the ending of human civilization. At this last stage, the ultimate brain has acquired the omniconscious. Quantum mechanics describes the world with multiple histories, but we can witness only one of them. Scientists are debating why the observable universe must be so unique as to allow humans to evolve. One theory is that humans’ collective consciousness creates the world in front of us by “collapsing” the quantum waves of multiple universes. To form Statistical studies of certain chancy events revealed that the concentration of humans’ collective will could change the environment, although the influence was trivial due to the weakness of humans consciousness. In addition to will things to happen, scientists also found that brains could actuate machinery (e.g. moving computer cursor) without using muscles. If properly connected to a trigger mechanism, human’s will is capable of doing things without using hands. The ultimate brain to be built in future will be able to do more than just thinking. It could cultivate the good and eliminate the evil. The omniconscious will be the God itself in the future. In fact, we may be going through its memory as the way to transform fleshy consciousness to spiritual omniconscious. In summary, the omnipresent nanodiamond could have played a vital role in initiating the building blocks of life, such as

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amino acids and proteinoids. Moreover, the bucky onion shrouded nanodiamond may have molded bases in succession and molt them to form RNA rungs. The nanodiamond in the future may also be fabricated to become photonic crystal that may be operated by confined X-ray. Moreover, due to the superhard rigidity of diamond lattice, the N-V defects in nanodiamond can be stable quantum bits for parallel processing in real time. The photonic access and quantum computing would make nanodiamond the fastest and the most powerful computer possible. Furthermore, neurons can be cultivated to network with such nandiamond photo-quantum computers. Consequently, human consciousness can be extended to become superconscious that is capable to understand the meaning of everything including itself.

Chapter Three

Dawn of the Diamond Age

3.1

THE MIRACLE OF DIAMOND

In the animal kingdom, humans (Homo sapiens) have the most of a species in quantity, and they are also highest of intelligence in quality. Hence, humans are the undisputable king of all animals. In the material world, diamonds also enjoy the same royal status, as this material is the most abundant in the universe and also have the most useful properties. Carbon ranks number 4 in the universal abundances of elements, after the three gas elements: hydrogen, helium, and oxygen. Hence, it is the most abundant solid in the universe. Carbon forms the backbone of organic compounds — compounds that are 10 times more diverse than all other materials combined. Moreover, carbon is the essence of the biological cells (e.g. DNA and protein) that have sparked the life in the otherwise unanimated world. In the entire carbon family, diamond stands out as the supreme ruler, as it has the most extreme properties of all materials. Although diamonds may be rare on Earth, they may be found everywhere in the universe. Diamond has been detected from the spectra of interstellar dusts. Nano-sized diamond particles were also recovered from plunging meteorites. Moreover, scientists have speculated that the interiors of giant planets (e.g. Uranus or Neptune) are composed primarily of diamond formed by the decomposition of ubiquitous methane under tremendous pressure. Diamond may also form in many colossal stars as the high pressure phase of carbon that is one of the terminal elements of nuclear synthesis. The deep buried diamond in these gigantic stars may have a mass millions of times greater than that of Earth. Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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Miraculously, this universe’s omnipresent solid is also omnipotent. The omnipotent diamond exhibits many extreme and useful properties. They include aesthetical brilliance, mechanical strength, abrasion resistance, edge sharpness, thermal conductivity, thermal expansion, heat capacity, wave propagation, electrical insulation, hole mobility, chemical inertness, optical transparency, radiation hardness, electron emission, surface smoothness, friction coefficient, etc. These supreme properties, coupled with other unique attributes, such as extremely low thermal expansion coefficient, have distinguished diamond from all other materials as a totally unique substance (Fig. 3.1). Thus, the Queen of Gems as it has been known in the past will become the King of Materials in the future. In the past, industrial diamonds have been limited by size and geometry. Therefore, the primary uses for industrial diamonds have been in the mechanical industry as ultrahigh pressure anvils, surgical knives, and most commonly, as superabrasives (e.g. used in grinding wheels). But with the emerging availability of large sized diamond films and complicated diamond-like carbon coatings many other functional applications of diamond and its related materials are now either commercially available or under development. Thus, the application of diamonds have led to the development of the fastest heat spreader available (e.g. for packaging

Figure 3.1. Diamond possesses properties that are a cut above all other materials.

The Miracle of Diamond

200 watts computer CPU or for bonding millions of semiconductor chips); the highest frequency resonator attainable (as 100 kHz tweeter diaphragm or 10 GHz surface acoustic wave filter); the most powerful semiconductor allowable (e.g. recent Japanese initiative for developing ultracomputer with 20 times speed of silicon chips); the most transparent radiation window achievable (e.g. for megawatts microwave transmission, or as six mach missile radome); the most inert chemical barrier invented (e.g. for probing the corrosive atmosphere at high temperature); the most energetic particle detector possible (e.g. for monitoring cosmic rays), the most efficient cold cathode known (e.g. for field emission display). And this is just the tip of the iceberg. In addition, diamond-like carbon (DLC) coatings have become a must for protecting computer hard drives and other chemicalmechanical parts during such applications as thin film deposition on silicon wafers or acid slurry polishing of integrated circuitry. DLC coated cassette disks and razor blades are also commonplace consumer products nowadays. In the medical device industry, DLC coatings are also critical for sustained reliability of medical components such as heart valves, artificial joints, and expandable stents. With the availability of a greater variety and more complex shapes and sizes at affordable prices, applications for the use of diamonds have increased steadily (Fig. 3.2). Diamond has also been

Figure 3.2. Diamond has been used in almost every sector of industrial products. In many such applications, the functionality that diamond provides cannot be substituted by other materials.

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applied constantly for new frontier applications and most of them become the enabling function with no alternative substitute possible. With the increasing commercial development underway, the arrival of the ultimate diamond age in the future seems inevitable. 3.2

DIAMOND AS THE KING OF MATERIALS

How did diamond acquire its royal quality above all materials? The secret lies in its ability to form 4 covalent bonds that defines the simplest three-dimensional volume — a tetrahedron. Each atom in a crystal lattice can be surrounded by 0, 1, 2, 3, 4, 6, 8, and 12 neighbors. The increasing trend of this “kissing number” (coordination number) marks the gain in order (enthalpy) of a pile of atoms at the expense of the chaos (entropy). There is only one type of extreme order and one type of extreme chaos, so the compromise between them would allow diversification and complexity. The diversification of the tetrahedral bonding in diamond has helped enrich the field of organic chemistry, whereas its complexity has prompted the birth of biology. A kissing number less than four define a point (e.g. gas molecules), line (e.g. polymer molecules), or plane (e.g. foliated molecules) relationship. This relationship lacks either a structure or it defines a loosely attached structure. A low kissing number will highlight the local relationship and hence its exhibits insulating properties. On the other hand, with a larger kissing number beyond 4, the structure of the solid will become tighter so that the global property of metal becomes dominating. The delicate balance between a loose and tight structures lies exactly at the kissing number of 4. The flexibility of the tetrahedral bonding in diamond reflects this well. This intermediate structure makes diamond’s property fall between an insulator and a metal, and therefore, allows diamond to be a semiconductor. Moreover, the packing between a loose and tight structure can allow for flexibility of atomic positions that are not possible in insulators or metals. Carbon atoms with the capability to form tetrahedral diamond bonding (e.g. methane and its polymers) and planar graphitic bonding (e.g. benzene and its derivatives) have become the corner stone of the organic chemistry. The open tetrahedral bonding also serves as the building block of many versatile semiconductors that power indispensable consumer products

Diamond as the King of Materials

Figure 3.3. Locally connected atoms with a kissing number less than 4 cannot form an integral solid. On the other hand, the globally dominated structures with a kissing number greater than 4 do not allow for many variations in structure. The compromise between local and global allows a versatile tetrahedral bonding that makes complex structures possible. This complexity has prompted the formation of organic compounds and the evolution of living organisms. Tetrahedral bonding has also enabled semiconductors for the making of integrated circuitry.

such as computers, lasers, and other electronic devices. Alternatively, the versatility of tetrahedral bonding can be coupled with closer packed metal atoms to form advanced composite materials for micro electro-mechanical devices (Fig. 3.3). Although tetrahedral bonding is the common feature of all semiconductors, diamond has the smallest atoms and therefore it possesses the highest atomic density (176 atoms/nm3 ) of all the semiconductors, and in fact, of all materials. The combination of the highest atomic density with the largest number of covalent bonds (4) has lead to diamond’s concentrated bond energy (7.4 eV). This strong bonding of atoms has imparted diamond with its unique properties that single this material out as the King of Materials. Although diamond’s “royal status” stems from its ability to form tetrahedral bonds, this ability is actually the result of carbon’s “crown position” in the periodic table of elements (Fig. 3.4). Carbon is located at the center column (Group IV) and top period

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Li

Be

B

C

N

O

F

Na

MG

Al

Si

P

S

Cl

K

Ca

Ga

Ge

As

Se

Br

Rb

Sr

In

Sn

Sb

Te

I

Cs

Ba

Tl

Pb

Bi

Po

At

Figure 3.4. Periodic table showing solid materials with atomic bonding. This table excludes hydrogen and the noble gases that form solids only with weak metallic bonds (e.g. metallic H) or with polarized (van der Waals) electrostatic force (e.g. cryogenic He). The transitional metals have inner filling elements of d and f orbitals so they are hidden in the parental Group II. This periodic table highlights carbon’s crown position at the center column and top row. The center column gives carbon the highest number of covalent bonds per atom (4). The top position makes its atoms the smallest among all center column elements. The combination of these two attributes imparts diamond with the highest concentration of bond energy, and thus lends it unsurpassed hardness and other useful properties.

of all elements capable of forming chemically bonded solids. At part of the center column, carbon is impartial between valence electron deficient metals (kissing number less than 4) on the left side of the periodic table, and valence electron surplus insulators (kissing number greater than 4) on the right side. The atoms of the left side elements (Groups I, II, and III) have many electron vacancies so they can attract more neighbors (kissing number = 6, 8, 12) to share their few valence electrons. The consequence of sharing few valence electrons by many neighbors is the formation of isotropic metallic bonds. On the other hand, the atoms of the right side elements (Group V–VII) possess many valence electrons that repel one another. As a result, they can allow for only a few neighbors (kissing number = 1, 2, and 3) to share their valence electrons. The consequence of sharing many by few is the formation of directional covalent bonds. Compounds formed between indiscriminate metallic elements and selective covalent elements possess intermediate ionic bonds. Covalent bonds have the most concentrated electron cloud between atoms so they are much stronger than metallic bonds or ionic bonds. The center column of elements of diamond and silicon

The Expansion of The Diamond Market

possess four covalent bonds, the most before these bonds become metallic with further increases of kissing number. As a result of this high density of covalent bonds, carbon or silicon possesses the most rigid crystal lattice among all structures formed by the same or higher period elements. Carbon sits at the top of the middle column of elements so its atoms are the smallest. Hence, diamond, which is comprised of carbon, has the highest bond energy density of all materials. Diamond is the miracle material with so many extreme properties that it overshadows all the other materials. 3.3

THE EXPANSION OF THE DIAMOND MARKET

With the availability of diamond in more sizes and shapes, applications using this versatile material have increased steadily. The reduction of diamond prices also prompted the growth of its existing market. Diamond sales have been increasing for more than four decades, a feat unmatched by the growth in sale of any other material. The upward trend for all other industrial commodities will inevitably plateau in less than a decade (e.g. steel topped the production at around 800 million tons after World War II). This monotonous increase of industrial diamond sales implies that its market is far from being saturated. It would appear that sales for industrial diamond would continue to grow almost without limit due to the constant discoveries of novel diamond applications, provided that the price of diamond materials will continue to decrease due to the improvements in manufacturing efficiency. Human civilization has been built on the foundation of useful materials available. Thus, wood and stone were the first materials available for prehistoric men. They were used to make tools and shelter for hunters and gatherers as well as agriculture settlers. Then ancient societies found the benefits of extractive metals such as copper, iron, and aluminum. These metals were made into swords and spears, as well as constructed into structures. At the dawn of the industrial revolution, metals were superceded by engineered materials such as plastics. Soon thereafter, utensils and textiles made of polymers and plastics became commonplace. In this modern time, civilization has entered into the silicon age. Computers and electronics are now part of our daily life. However, sometimes these materials are chosen due to current limitation in

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Figure 3.5. The relative importance of materials as the foundation of human civilization. Note the rise of diamond in this century.

manufacturability or technical knowledge. Many times they are not the ultimate material that will optimize performance. As diamond has the most extreme properties of all materials, its dominance in functional devices will be inevitable especially with the continual decline in price due to more efficient manufacturability and increase in availability. The Age of Diamond has begun (Fig. 3.5). The rise in diamond useage as the dominant building blocks of human civilization is not dependent on its abundance as with the other dominant materials in the past. The contribution of diamond to the value of life lies in its prestige and quality. Diamond’s dominance as the key element for functions is more due to its enabling capability than to its cost effectiveness. For example, diamond’s performance in superabrasives, heat spreaders, radiation windows, acoustic filters, and semiconductor circuitry cannot be substituted by alternative competing materials. The value and quantity of industrial materials often form an inverse relationship that is representable by a product pyramid. Since diamond is the king of industrial materials, its will become the icing on the cake for the product pyramid. Although diamond’s dominance in the materials world is just beginning, diamond superabrasives has already occupied the tip of the representative corner stones of industrial materials (Fig. 3.6).

The Ultimate Semiconductor

Figure 3.6. The quality–quantity pyramid for standard industrial materials. The three civilization enabling materials that rule in the past, present, and future (iron, silicon, and diamond, respectively) lie exactly on a straight line on the logarithmic scale of a time pyramid (2004).

3.4

THE ULTIMATE SEMICONDUCTOR

Among many of diamond’s beneficial properties are its semiconductor capabilities. The structure of diamond forms the basis of almost all semiconductors including the most prevailing material today — silicon. However, with its high hole mobility and high temperature stability of all possible semiconductor materials, diamond can be the dream CPU chip for future supercomputers (Fig. 3.7). Moreover, by possessing unmatched thermal conductivity, diamond will be the only semiconductor that can dissipate heat effectively by itself. In each of the major industries, computer, communication, energy, there are bottlenecks that can be cleared by using diamond devices. For the computer industry this is the heat spreader. As the computing material, diamond has another trump property — negative electron affinity. Diamond containing material can be the most efficient cold cathode capable of emitting electrons at extremely low voltage (less than 10 V). With an array of miniature electron guns, diamond can revive the vacuum tube, the antique computing unit that was replaced by silicon transistors in 1950s. Thus, the ultimate computing machines in the future will be comprised between diamond transistors and diamond emitters. In

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Figure 3.7. The incremental evolution of semiconductor’s performance can be achieved by optimizing circuitry design, but a quantum revolution may only be attained by utilizing a more effective material. The history of semiconductor progression is moving upward along group of tetrahedrally bonded elements (Group IV) located at the center column of periodic table. It has progressed from Ge to Si. It will gradually advance to SiC, a half diamond semiconductor. Eventually, the advancement of semiconductor chips require the use of full diamond, the terminal material for all semiconductors.

either case, diamond will win the race as the champion above all other computing materials. Although the computers today are electronic machines, they may also be run by other mechanisms, such as by chemical potential (e.g. DNA computers) or electromagnetic waves (e.g. optical computers). In particular, photons have been envisioned as a much more efficient carrier of information denoted as bits of 0 or 1. Not only can photons travel at light speed that is much faster than electrons moving along either a conductor in the case of a diamond semiconductor, or through vacuum in the case of a diamond emitter, their motion also consumes less energy and therefore minimal wasted heat will be generated. Current semiconducing supercomputers can reach the speed of teraflops (trillions floating point operations per second, a float point operation is equivalent to one arithmetic operation). New hypercomputers using diamond electron emitters may boost the speed 1000 fold to petaflops (quadrillions floating point operations per second). Even

The Diamond Family

Figure 3.8. The atomic sized optical sieve of diamond when looked through its octahedral (111) surface. The hexagonal helical openings will change to square spiral holes when looked through the (100) face. Only electromagnetic radiations with wavelength shorter than twice the opening may pass through this atomic sieve. Consequently, diamond lattice can serve as the ultimate photonic computer capable to calculate at the speed of light by bouncing inside with X-rays.

so, it may eventually be superceded by photonic ultracomputers (Fig. 3.8) that are capable of running at an astonishing speed 1000 times faster than the futuristic hypercomputer. But even so, diamond still comes ahead as it is the most efficient optical conduit nature has provided. Diamond has the smallest openings of all crystal lattice; its (111) face covers myriad hexagonal helical openings of only 2.4 Å across. These atom-sized openings can filter gramma rays like an optical sieve that blocks all wavelengths longer than X-ray. Diamond may be used to construct a photonic ultracomputer with clock speed unimaginable (1018 Hz). This capability, coupled with other superior diamond properties such as fast heat dissipation will make diamond the ultimate computing material. 3.5

THE DIAMOND FAMILY

Diamond and graphite are polymorphs of the same carbon elements. The two structures are inter-related in such a way that by stretching its (111) planes a diamond can transform into graphite.

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In reverse, by puckering the basal planes, graphite will transform into diamond. Hence, we may call diamond a puckered graphite; and graphite, a stretched diamond. The above illustration indicates that the two polymorphs may convert entirely from one phase to the other, and vice versa. In addition, diamond and graphite may transform into each other only partially. In this case, the intermediate structure may contain both diamond’s (sp3 ) and graphite’s (sp2 ) bonds. In fact, there is no pure diamond structure as it would require that perfect alignment of the lattice infinitely in all directions, so all diamonds are inevitably contain a certain amount of graphitic bonds. There are several ways that graphitic bonds can be incorporated in a diamond structure. The most common source of graphite bonds are located on the surface of a diamond crystal where at least one of the diamond bonds of the carbon atom is terminated to form an unpaired single electron, i.e., a dangling bond. Because a grain boundary is the source of graphitic bonds, the smaller the grain, the more graphitic bonds are possible. In fact, a nano-crystalline diamond with about 1000 atoms (10 atoms on each edge in average) will have about the same number of graphitic atoms located on the surface as diamond atoms located in the body, so this nano particle should be renamed graphitic diamond. Another way to introduce graphitic bonds is to drill holes through diamond (e.g. by making an optical sieve) or to induce defects in the crystal. In this case, the situation is reversed, instead of graphitic bonds surrounding diamond bonds, graphitic bonds are enclosed in between diamond bonds. Another way to form graphitic diamond is to mix the two types of bonds in the same volume. This is the structure of amorphous diamond, a diamond-like carbon that contains no foreign elements (e.g. hydrogen free). A third way to mix the two types of bonds is to introduce a slight diamond character into each of the atom in graphite. The net result is to curl up the basal planes, and depending on the distribution of the curled atoms, the structure may form either a ball or a tube, the former is known as bucky balls; the latter, carbon nano-tubes. These curled graphitic layers may also be viewed as hallow diamond balls or tubes as they may also be derived by eliminating the interior atoms from a solid diamond. Keeping the above relationship between graphitic and diamond bonds in mind, DLC, bucky balls, nano-tubes, and other

Diamond is Forever

Figure 3.9. Carbon family with the diamond taste. This diagram shows the connection of many carbon materials that exhibits various degrees of diamond characters. In the above diagram, the nano-cubicles represent a horde of hypothetical structures that may be formed by assembling carbon nanotubes in various forms like using steel beams for constructing houses. This molecular manipulation of carbon nano-tubes may be developed in the future.

structures that contain partial characteristics of sp3 bond may be considered as diamondoids (partial diamond) materials (Fig. 3.9). This terminology is relevant as the unique shapes and properties of bucky balls and nano-tubes are indeed derived from this partial diamond character. Because of this diamond connection, bucky balls and nano-tubes have phenomenal properties and they have been envisaged to be the enabling material for many exotic applications (e.g. hydrogen storage, field emission, drug carrier, molecule scale, nano-transistor, etc.). 3.6

DIAMOND IS FOREVER

With the trend of diversifying the product folio and the reduction of manufacturing costs, numerous new applications will be made debut. For example, diamond is the best component for micro electro mechanical devices (Fig. 3.10). When diamond products become commonplace items, humans’ material civilization will enter its final dynasty — the

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Figure 3.10. Images of a laser cut micro diamond electro mechanical components of perpetual gears (left) and the strongest screw (right) possible. Both of them can last indefinitely even in a highly acidic environment.

Diamond Age (Fig. 3.11). As there is no element that is above carbon on periodic table, diamond will never be succumbed by any other material in the future. Thus, although diamond is chemically metastable and it can be easily shattered, “diamond is forever!”

Figure 3.11. Materials civilization is succeeded by implementing more advanced materials from time to time. The conventional materials are not replaced in wholesale; rather, they are supplemented by a new champion material that can better meet with demand.

Diamond is Forever

may not be just a figurative expression of desire, but a literal prediction for the ultimate stage of human civilization. The Scottish mathematical physicist James Clerk Maxwell derived the entire theory of electromagnetism in his head in 1861. He was amazed that this pure theoretical theory could explain the nature of ordinary light then unknown to scientists. In fact, the theory actually predicts the speed of light for the first time. Moreover, this theory predicted the existence of other invisible lights that might transmitted with different frequencies. Maxwell envisaged that these then undetectable waves could be used to transmit messages over a great distance without making direct contacts. This foresight prompted Heinrich Rudolf Hertz to discover molecular energy radio waves in 1886; Guglielmo Roentgen to discover atomic energy X-rays in 1895; and Paul Villard to discover nuclear energy gamma radiations in 1900. In 1901, Guglielmo Marconi received the first trans-Atlantic radio message. This feat confirmed Maxwell’s vision of acting at a distance without attaching a string. Maxwell was so amazed that something that is everywhere could be so beautiful and perfect, legend states that he exclaimed, “Nothing can be too wonderful to be true.” Indeed, without such a possibility, life would be seriously flawed. Fortunately, we do have diamond, the omnipresent material that is also omnipotent. Light and diamonds make our world a much more interesting and worthwhile to live. In fact, it is light that gives diamonds its brilliant sparkle. Diamond will also become the ideal optical window that ushers light into numerous wonderful applications in the future.

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Diamond Synthesis in Perspective Graphite is nature’s best spring. Percy Bridgman (1944)

4.1

HISTORICAL MILESTONES OF DIAMOND SYNTHESIS

Previously it was believed that diamond was never synthesized before the 20th century. However, in the light of the discovery of CVD methods for depositing diamond, it is possible that diamond could be formed in regions where the burning of hydrocarbon gas is incomplete. Minute diamond-like particles could have been made inadvertently when Cro-Magnon made wall paintings by brushing torches against a cave wall some 35 thousand years ago (DeVries, 1995). If this was the case, then diamond synthesis may have occurred before the modern man Homo sapiens first appeared in Africa approximately 150 thousand years ago. For example, Homo erectus who is known to be the first makers of fire, could have left behind nano-sized diamond-like particles within their half burnt wood more than one and half million years ago. Modern attempts in the quest for diamond did not actually begin until after the renaissance. The two most famous historical cases for creating artificial diamonds were by Hannay and Moisson. However, their methods were not able to produce stable forms of diamond and therefore their claims for diamond making

Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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was long discounted by scientists. Hannay’s experiments involved sealing hydrocarbons in the form of bone oil and lithium in steel tubes that were then heated until it was red-hot. Although it was discredited that he actually produced diamond, in hindsight, he could have done so by reducing the hydrocarbons with Li. It has been since demonstrated that diamonds do in fact form when CCl4 is heated at low pressures with Na. It is also conceivable that the hydrocarbon in Hannay’s experiment may have decomposed and the catalytic action of iron could have generated hydrogen. Although Hannay may have tried to make diamonds under pressure, the possibility that he accidentally deposited some crystallites via a vapor deposition route is a possibility. Some of the tiny crystals formed via Hannay’s method show the unusual birefringence of natural diamond. Such characteristics resemble the modern diamonds that have been synthesized by chemical vapor deposition (CVD).

Table 4.1.

Historical milestones of diamond synthesis

When

Who

How

1.5 MYA 35 KYA 1880 1904 1905 1911 1936 1952 1953 1954 1968 1970 1971 1976 1981 1988 1989

Homo Erectus Crog-Magnon Hannay Moisson Burton Von Bolton Leipunskii Eversole ASEA Hall Hibsman Wentorf Angus Derjaquin Setaka Hirose Norton

Incomplete burning of wood in fires Painting cave walls using fire Reduction of paraffin by Li Exsolution of C from Fe Exsolution of C from Pb/Ca with H2 O Reduction of C2 H2 by Na P/T for exsolution of C from Fe Thermal decomposition of CH4 High pressure synthesis with Fe High pressure synthesis with 12 metals Hint of using H atoms for CVDD Gem diamond growth CVDD by hot filament (no H2 addition) CVDD by glow discharge CVDD by microwave CVDD by flame CVDD production by DC arc

Source: Abbreviations: MYA = million years ago, KYA = thousand years ago, CVDD = chemical vapor deposition diamond.

High Pressure Origin of Diamond

On the other hand, Moisson could not have possibly made diamond using his method of quenching molten iron. Even if he did, the diamond formed would have completely converted to amorphous carbon by the catalytic action of molten iron at high temperature. The historical milestones in diamond synthesis are listed in Table 4.1. Of particular importance is the discovery of catalysts, notably iron for high pressure synthesis and hydrogen atoms for CVD deposition. 4.2

HIGH PRESSURE ORIGIN OF DIAMOND

As early as 1694, Florentine academicians suspected that diamond was made of carbon when they found that the precious stone could be burned completely in air. In 1772, this speculation was confirmed by the French chemist Antoine Lavoisier who discovered that the gas released from a burnt diamond was indeed carbon dioxide. In 1797, Smithson Tennant proved beyond any doubt that diamond is made of carbon by burning a diamond in pure oxygen. He measured the amount of carbon dioxide released and found that the carbon content matched exactly the original weight of gasified diamond (Mellor, 1924). As soon as this precious gem was known to be no more than ordinary carbon, the quest for synthesizing diamond began. But success did not come until one and a half century later. Since the density of diamond is 3.52 g/cm3 and 56% higher than the next densest form of carbon (graphite), it was soon realized that high pressure is essential in order to squeeze carbon into diamond. Another clue that diamond could be formed under high pressure comes from the occurrence of natural diamond. The very first natural diamond was found on a riverbed in India, but its source was eventually traced back to Brazil. South Africa confirmed that a special volcanic pipe called kimberlite was found on this continent, one that contained natural diamonds. Kimberlite contains a mineral assemblage that includes garnet and biotite, both of which are formed under high pressure. Volcanic eruptions are typically derived from shallow origins. According to plate tectonics, the surface of earth is covered by several large plates that can move horizontally. These plates are driven by the convection flow of mantle that supports these plates.

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When the plates rub against one another along the borderline, rocks from these plates may melt to form a magma that may erupt as volcanoes. However, magma with shallow roots will solidify to form mostly granite and basalt that contain no diamond. On the other hand, with stable continental shields that are far away from tectonic active areas, kimberlite from deep within the Earth may extrude to the surface. A geotherm underneath a continental shield is milder than those found underneath tectonic active regions. The colder geotherm may intersect the phase boundary of graphite and diamond at a depth greater than 130 km (Fig. 4.1). Kimberlites are derived from ascending mantle plumes that rise to release heat. While they move upward, they may melt due to the reduction of pressure. The volume of magma will rapidly increase so it may extrude to the surface. If the rising kimberlite flows across the region where diamonds are located, it will carry them up to the surface (Fig. 4.2). Kimberlites contain a very low abundance of diamond, typically about 0.05 PPM. Less than 10% of these diamonds can be polished to become gems. Most kimberlite pipes were extruded about 100 million years ago, but the diamonds that they carry could have been formed even 30 billion years earlier.

Figure 4.1. The continental geotherm intersects the phase boundary of graphite and diamond at a depth of 130 km.

Historical High Pressure Synthesisof Diamond

Figure 4.2. The occurrence of a kimberlite pipe in South Africa. Mining shafts were used to recover the diamond from these pipes. The funnel shape of the pipe was formed by the expansion of magma as it decompresses near the surface.

4.3

HISTORICAL HIGH PRESSURE SYNTHESIS OF DIAMOND

Numerous methods using high pressure technology have been invented to synthesize diamond in the past. In fact, high pressure technology was developed largely because of the need to synthesize diamond.

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Hannay developed sealed steel tubes that could contain organic volatiles at high temperatures to about 0.2 GPa (1880). Parsons built a piston-cylinder apparatus that could bring pressures up to 1 GPa (1888). The Nobel laureate Moissan quenched molten steel to attain a pressure of about 0.5 GPa (1894). However, it was the other Nobel laureate, P. W. Bridgman, who invented truncated anvils that could penetrate the barrier to reach the ultrahigh pressure of 5 GPa (1946). An extensive review of high pressure technologies, in particular those for diamond synthesis, is presented elsewhere (Sung, 1997). Although Hannay and Moissan claimed success at synthesizing diamond, no one has since been able to repeat their process. Most scientists believe that their diamonds were either carbides or other compounds, as in the case of Hannay; or seeded natural diamonds, as in the case of Moissan. Although Bridgman routinely squeezed graphite to 20 GPa, a pressure well within the stability field of diamond, and heated the sample to over 1000◦ C, this brutal force approach still did not yield any diamond. This lead to Bridgman’s (1955) lament, “graphite is nature’s best spring.” Diamond synthesis is one of few areas where chemists succeeded while physicists have failed. Although there were discussions of using catalysts to accelerate the formation of diamond (e.g. Russians already mentioned using iron as the carbon solvent for diamond synthesis), Bridgman was a physicist, so he did not pay much attention to this chemical aspect. It would take some chemists a decade later to apply catalysts and collapse that “graphite spring” to form diamond at a much lower pressure than what Bridgman attempted. The first artificial diamond was created by ASEA(General Electric Company of Sweden) engineers on February 15, 1953 (Fig. 4.3). The diamond was formed in a high pressure apparatus designed by Baltzar von Platen (Liander, 1955). This apparatus was composed of a large cubic press that contained six anvils arranged in a shape of a split-sphere. The sample volume was over 40 cm3 , an enormous size at that time. The first diamond was produced from a mixture of cementite (Fe3 C) and graphite. The charge was compressed to a pressure of about 75 Kb and heated to a temperature of over 1500◦ C by a thermistic reaction. After more than 3 min of heating, several dark diamond crystals hundreds of microns in size were formed.

Historical High Pressure Synthesisof Diamond

Figure 4.3. ASEA’s split sphere high pressure assembly designed by von Platen (top left). The actual apparatus revealed the inside structure (top right). The assembly drawing of the apparatus (bottom left). The center cell assembly (top right), and the diamond ASEAclaimed that was synthesized in 1953 (bottom right).

On December 16, 1954, Tracy Hall of General Electric Company in the United States also was successful in synthesizing diamond. He used a much simpler belt apparatus of his own design (Hall, 1970). The sample generated had a volume of less than 0.1 cm3 . It contained a mixture of troilite (FeS) and graphite. The sample was first compressed to a pressure of about 70 Kb and then heated by passing through an electric current to a temperature of about 1600◦ C. After heating for two minutes, several minute diamond

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crystals were formed. The diamond did not grow in the sample mixture as originally intended. Instead, it was embedded in a solid end cap made of tantalum. The end cap was used to lead the electric current to the sample. Subsequent research from General Electric scientists led to the discovery that diamonds could be formed at high pressure by the catalytic action of molten metal (Fig. 4.4). According to these researches, this metal must contain one or more elements selected

Figure 4.4. GE’s belt apparatus as invented by Hall (top left), and the first diamonds that was synthesized in 1954 using this apparatus (top right). The early GE press used at “Diamond Mine” of corporate R&D (bottom).

Historical High Pressure Synthesisof Diamond

from a list that contained eight Group VIIIB elements (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt) and three other transitional metals (Mn, Cr, and Ta) (Bundy et al., 1955). The success in synthesizing diamond in 1950s represented a major feat of science and technology that, although in a more minuscule scale, may be compared to that of the landing of men on the moon in 1969. This success came from contributions of more than one group (including the Swedish scientists) and it is difficult to attribute the contribution to just a few individuals. Perhaps it was for this reason that the Nobel Prize was not awarded for such an epoch making event. In 1961, diamond was converted directly from graphite without the use of any catalyst by the scientists DeCarIi and Jamieson from DuPont. The conversion was triggered by shock compression from an explosion that created a momentary (a few microseconds in length) pressure of ∼350 kb and a temperature of ∼770◦ C. In 1963, direct graphite to diamond conversion was also achieved by Francis Bundy of the General Electric Company. This time the transition took place under a static pressure of about 120 kb and a transient (a few milliseconds in length) temperature of about 3000◦ C. The temperature of the sample was raised by releasing an electric flash from a capacitor at high voltage (Bundy, 1963). The possible mechanisms of such direct graphite to diamond transitions are discussed elsewhere (Sung and Tai, 1997b). In 1972, another milestone was reached when the General Electric scientists Strong and Wentorf announced their success in making gem diamonds (Fig. 4.5). These diamonds were grown under controlled pressures that deviated within 0.1 GPa, and temperatures that fluctuated by less than 10◦ C. The time of synthesis lasted up to 1 week. In order to avoid the pressure decay caused by the volume reduction (about one-third) associated with the graphite to diamond transition, the carbon nutrient used was in the form of minute crystals of diamond itself. These crystals were dissolved in the hot zone of a molten metal. The dissolved carbon atoms then diffused toward a cold region (about 50◦ C) where they precipitated onto a diamond seed. This temperature gradient method can grow diamonds several milligrams an hour. Today, the largest single crystal diamond synthesized in this fashion is over 25 carats (over 30 carats for imperfect crystals) as demonstrated by De Beers’ scientists.

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Figure 4.5. The cell design of the temperature gradient method for growing gem diamond (upper diagram). The polished carat-sized diamond synthesized in the 1970s by GE (bottom left) and the 25 carat diamond crystal grown by De Beers in the 1990s (bottom right).

4.4

THE CATALYTIC MECHANISM

Saw grits are synthesized in the stability of diamond. According to common understanding, graphite will dissolve into molten metal and diamond will precipitate out as it has a lower solubility. However, this simple solution model cannot explain why the graphite microstructure can drastically affect the type of diamond formed. In fact, it has been observed that non-graphitized carbon can only form diamond, if any, under a much higher pressure. Moreover, amorphous glassy carbon must first crystallize to form graphite before transforming into diamond (Hirano et al., 1982). Furthermore, the higher the degree of graphitization in the carbon

The Catalytic Mechanism

source, the faster the diamond can nucleate and grow (Hirano et al., 1984). The solution mechanism also cannot explain why certain metals such as Cu or Zn cannot form diamond at all, although they may dissolve a minute amount of carbon. The apparent dependency of diamond formation from graphitic structures and metal chemistry can be attributed to the catalytic action. This catalytic mechanism was revealed by Gou in 1972 based on the electronic interaction between the empty 3d orbitals of transition metals and the unbonded (π bond) 2p electron of carbon. According to Gou’s model, when the catalyst metal melts, its atoms can assume a configuration of pseudo-closest packing in short ranges, resembling the (111) face of a CCP (FCC) lattice. If the metal atoms can cover roughly every other carbon atom on the graphite basal plane, then their empty 3d orbitals may attract the dangling 2p electrons of carbon. This attraction could pull every other carbon atoms toward the metal atoms. As a result, the graphite hexagon may pucker in such a way that the unmatched carbon atoms would move toward the opposite direction, i.e. toward the next layer of the graphite basal plane. This puckering can deform a plane hexagon of graphite to a “chair” of diamond. If graphite has the right stacking sequence of hexagon layers, the away moving carbon atoms may interact with matching carbon atoms in the next layer to form diamond bonds. This puckering action may sweep through the graphite lattice like a wave (Fig. 4.6). As a consequence of this domino effect, an entire grain of graphite may transform into diamond. The above-mentioned model explains the catalytic mechanism of the graphite–diamond transition. In order for this mechanism to operate, carbon must first form the graphite structure, and a catalyst must possess empty d orbitals. Both requirements are consistent with the empirical observations. For example, glassy carbon cannot form diamond, nor can d-orbital filled transition metals (e.g. Cu and Zn). In order to facilitate the formation of diamond, the graphite structure must be well developed and the catalyst must contain empty d-orbitals. However, the above model falls in two deficiencies. First, the common graphite is predominantly 2H type, i.e. the stacking sequence is ABAB, etc. As such, only half of the atoms can find

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matching ones in its adjacent layers, and therefore the puckering cannot transmit onto the next layer. In order to allow the puckering to sweep through the structure, the graphite must be either the rhombohedral 3C type with an ABCABC, etc. sequence, or a hexagonal IH type with the AAA, etc. sequence. 3C type graphite can pucker into cubic diamond; and IH type graphite into hexagonal diamond (lonsdaleite) (Fig. 4.6). As a diamond synthesized under high pressure is exclusively cubic in structure, it follows that ordinary graphite must first slide into the rhombohedral form before puckering into diamond. Graphite basal plans are held together by weak van der Waals bonding. The activation energy for shuffling these planes is 0.076 eV, much less than 2% of the energy required to break a graphite bond (4.8 eV). This activation energy is easily overcome at a temperature of 883◦ C. Moreover, the sliding of basal planes can be conveniently accomplished when graphite recrystallizes under the influence of a molten catalyst, a common phenomenon taking place during the formation of diamond. The more empty d-orbitals a catalyst contains, the stronger its attraction toward carbon. As a result, when the deficiency of d-orbitals is too high, there is a tendency for the transition metal to form carbide. It has been observed that strong carbide former such as Ti or V cannot catalyze diamond formation. This observation must be factored in with the above catalytic model. In order for a catalyst to pucker the graphite net, it must do so without sticking to it permanently. Thus, the golden rule for a diamond catalyst is “touch and go.” The best manifestation of this phenomenon is the dissolution of carbon in a catalyst. When carbon atoms are dissolved as solute, they “touch and go” without forming carbide. The ability of the catalyst to moderate its interaction with graphite is also revealed in its solubility of carbon. Hence, the higher the solubility of a catalyst, the stronger its interaction with carbon without the formation of a compound. In other words, the solubility of carbon in a transition metal is the indicator of its catalytic power for converting graphite into diamond. This power may be manifested in the thickness of a graphite flake that it can pucker into a diamond nucleus.

The Catalytic Mechanism

Figure 4.6. Schematic showing the puckering of graphite into diamond. An effective metal catalyst has the right-sized atoms (large circles) that match every other carbon atoms (small circles) on graphite’s basal plane (001) (top diagram). The metal atoms will pull half of the carbon atoms (lightly dotted) toward them. The result is an accordion effect that puckers the graphite structure into that of diamond (lower diagram). The middle diagram illustrates the catalytic conversion (all black atoms moving upward; and white ones, downward) of a rhombohedral graphite (ABC type) into a cubic diamond. The lower diagram shows a similar catalytic conversion (all white atoms moving upward; and black ones, downward) of a hexagonal graphite (AAA type) into lonsdaleite (hexagonal diamond) (Sung et al., 1996).

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4.5

RANKING OF CATALYSTS FOR DIAMOND SYNTHESIS

Based on the above model of “atomic matching” and “touch and go”, the relative catalytic power of transition metals can be calculated for their ability to nucleate, grow, and form diamond. Moreover, if the catalyst has a low melting point, it can also reduce the pressure of diamond synthesis. A lower synthesis pressure can allow a larger reaction volume for a higher output, or a longer carbide life for a lower cost. Hence, a low melting point is another merit to be considered for selecting the proper catalyst for diamond synthesis (Fig. 4.7).

Figure 4.7. The relative catalytic power of transition metals for graphite to diamond transition under high pressure. N = Nucleation Index, G = Growth Index, T = Transformation Index, and A = All-merits Index. Note the dome-shaped curves that indicate transition metals with about half full d-orbitals; these metals are both efficient solvents of carbon atoms and effective catalysts for diamond formation.

Ranking of Catalysts for Diamond Synthesis

Figure 4.8. The ranking (numbers shown in the lower diagram) of catalyst merits of transition metals for graphite to diamond transition. In the figure, heavily stippled elements are the 12 original catalysts identified by General Electric scientists (Bovenkerk et al., 1959). Lightly stippled elements are other likely catalysts (Sung and Tai, 1997).

Figure 4.8 shows the ranking of transition metals according to the above merit scale as catalysts for graphite to diamond conversion. Based on the above ranking scale, the most powerful catalysts are listed in the decreasing order of: Fe, Co, Ni, Mn, and Cr. If low melting points are factored in, the order is changed to: Co, Fe, Mn, Ni, and Cr (Fig. 4.8). These five transition metals are indeed the most commonly used catalysts for diamond synthesis in the industry. The catalyst composition can be analyzed from the metal inclusions trapped inside diamond during its synthesis. For example, General Electric’s saw diamonds were formed from a catalyst of FeNiCr. The saw diamonds formed by De Beers used the catalysts FeCo or FeNi. Russian scientists formed diamond using NiMn as catalysts. Chinese diamond makers used mainly NiMnCo. The agreement of theoretical predictions with empirical experience supports the validity of the above described catalytic mechanism for graphite to diamond transition.

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4.6

SEQUENCE OF EVENTS

According to the solution model, diamond nuclei ought to form inside the original volume of molten catalyst. In an experiment performed in 1977, the author observed that the first diamond nuclei were actually precipitated out inside the original volume of graphite where molten metal intruded. Moreover, macroscopic graphite flakes were recrystallized in regions where diamond had not yet nucleated. However, as soon as a diamond nucleus was formed, it served as the carbon sink and sucked up microscopic flakes suspended in molten catalyst. As a result, microscopic flakes could not be accumulated to form macroscopic flakes. Once a diamond nucleus reaches a critical size, it will continue to grow in the graphite volume. However, there is always a thin film (<0.1 mm) of molten catalyst that separates diamond from graphite. This thin film will wrap around the diamond and it grows continually along with the diamond. Meanwhile, diamond is feeding on microscopic flakes of recrystallized graphite that drifts across the strait of thin film. It would appear that the sequence of events for the diamond synthesis under high pressure is as follows: (1) Molten metal invades into graphite in structural weak regions. (2) Graphite disintegrates to form microscopic flakes. (3) Catalyst atoms penetrate microscopic flakes by intercalation, and then shuffle the stacking sequence to become rhombohedral graphite. (4) Rhombohedral flakes are puckered under the influence of molten catalyst and they stick together to form a diamond nucleus. (5) The diamond nucleus is growing by continually feeding on microscopic flakes. (6) As the diamond grows in size, molten catalyst is pulled in by the capillary effect, so the thin metal envelope can expand continually around the growing diamond. The above model suggests that diamond is not nucleated from supersaturated solution of carbon atoms, but by puckering suspended graphite flakes (Fig. 4.9). The growth of diamond is also

Sequence of Events

Figure 4.9. Diamond is wrapped in a metal film that is impregnated with suspended graphite flakes. These flakes are continually disintegrated form the surrounding graphite disk. The immersed flakes are reshuffled in stacking sequence to form rhombohedral graphite. When these recrystallized flakes reach the diamond surface, they have been puckered and may attach to the growing diamond in chunks measured in micrometer. The catalyst side of the diamond crystal is growing slowly by the attachment of dissolved carbon atoms. The unequal rate of growth is often reflected in the asymmetrical appearance of the diamond crystal. Note that there is a temperature gradient across the catalyst envelope. Graphite, with electrical resistance 10,000 times higher than that of molten metal, is hotter than the catalyst metal in the corner regions where they are incorporated to form rays of inclusions.

dominated by adding microscopic chunks of flakes instead of discrete solutes. This conclusion is consistent with the observation that diamond facing graphite is growing nearly twice faster than that facing catalyst (growing predominantly by adding atoms). Moreover, when microscopic diamond is dissolved in molten catalyst, diamond may not nucleate at all. This situation is evidenced when gem diamond is grown with a seeded crystal using temperature gradient method. In this case, the growth rate achieved by feeding dissolved diamond atoms are comparable to that of growing saw

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diamond in the catalyst side. This growth rate is about half of that in graphite side.

4.7

DIAMOND SYNTHESIS ROUTES

Diamond is everywhere; inside supernovae, with star dusts, in meteorites, around impact craters, and under ground, consequently, there are numerous ways that can form diamond. However, all these synthesis routes employ a common strategy, namely, by squeezing loosely held carbon atoms surrounded by contact neighbors of 0 (sp0 , e.g. C), 1 (sp0.5 , e.g. C2 ), 2 (sp1 , e.g. carbyne, =C=C=), 3 (sp2 , e.g. graphite), and 4 (sp3 , e.g. methane) to make them more compact by forming a continuous network of tetrahedral (sp3 ) bonds. The difference between non-diamond carbon atoms and diamond carbon atoms are that the former contain either dangling valence electrons, or they are bonded to non-carbon atoms. In the case of diamond, there are no foreign atoms nor unbonded electrons or multiple bonded electrons, as all four valence electrons are involved in single covalent bonding. There are two general ways to subject carbon atoms under pressure, by squeezing them simultaneously, or by squeezing them in turn. In order to squeeze carbon atoms together, the raw material should be solid. Such a solid may either form linear structures with sp1 bonding (e.g. carbyne or its hydrogenated form such as polyethane), or most commonly, it possesses a layered structures of sp2 bonding (graphite). The solid carbon atoms can be squeezed instantaneously, as in the case of explosive synthesis of diamond, or they may be squeezed continually as in the case of static high pressure synthesis of diamond. In either case, high temperature must be accompanied to facilitate the transformation of loose carbon atoms into compact diamond. However, the temperature of the dynamic process is transient, whereas it is steady in the static process. Hydrocarbon molecules, such as methane, may also be dissociated at high temperature. The dissociated carbon atoms may be influenced by surrounding hydrogen atoms to maintain the tetrahedral bonding and they connect one-by-one to form a continuous diamond film. This process is known as CVD.

The Activation Energies of Diamond Formation

Table 4.2. Major diamond synthesis methods State

Material

Pressure

Mechanism

Catalyst

S

Graphite

High

Puckering

None

L

Graphite

High C solute

Puckering Low

Fe group Exsolution metal

G

Methane Plasma

Low C ions

CVD Low

H PVD none

Alternatively, ionized carbon atoms or hydrocarbon molecules may be smashed one-by-one onto a substrate, where they are accumulated to form a thin coating of diamond-like carbon with distorted tetrahedral bonding. This process is known as PVD. Although high temperature is also required in CVD or PVD process, the temperature is uniform in the deposited film in the former case, but it is restricted locally in the latter case. In both cases, the ambient pressure is lower than atmospheric pressure. In the case of PVD process, the ambient temperature may also be low (e.g. <100◦ C). Diamond may also be formed by other methods. For example, it could be produced by laser irradiation, accelerated quenching, ultrasonic booming, reduction pyrolysis, or hydrothermal reaction. Major diamond synthesis methods are listed in Table 4.2. The commercial diamond production has been using primarily methods of static high pressure, dynamic explosion, CVD synthesis, and PVD deposition. Although the synthesis methods may differ, many share the common diamond formation route as shown in Fig. 4.10. Thus, it is the focus of this review to compare the mechanisms and kinetics of major diamond synthesis routes, with the examples based on commercial methods of production.

4.8

THE ACTIVATION ENERGIES OF DIAMOND FORMATION

Carbon can form free atoms with no bond (sp0 ), linear chains (e.g. carbyne) with two bonds (sp1 ), planar structure (e.g. graphite) with three bonds (sp2 ), or volumetric framework (e.g. diamond) with four bonds (sp3 ). Thus, the three-dimensional diamond can be

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Figure 4.10. Diamond synthesis routes from various bonding statuses of carbon atoms and the possible use of catalysts. Abbreviations: S = solid, L = liquid, G = gas, HP = high pressure, M = catalyst metal, H = hydrogen atoms. The energy source can be chemical, electrical, heat, electromagnetic, plasma, sound, etc.

derived from zero-, one-, two- or three-dimensional carbon structures as precursors. If the source material is free carbon atoms, they can form diamond by hitting one another. The diamond bond has an energy of 3.7 eV. However, the activation energy above the diamond energy for joining two carbon atoms to form a diamond bond is only with the magnitude of about 0.33 eV. This energy barrier is reached when two carbon atoms are approaching each other at a distance of about 1.8 Å (Fig. 4.11). Carbon atoms may also be formed by detonating a TNT with a high carbon content. In this case, the carbon containing molecules in the TNT themselves are smashed together to form nano-crystalline diamond (e.g. 4 nm). This direct explosion process is in contrast to the indirect process that explosion is used to create shock waves. It is the latter that puckers graphite into diamond. In the case of shock synthesis, the diamond crystals formed are

The Activation Energies of Diamond Formation

Figure 4.11. The activation energy of two approaching carbon atoms. The highest energy (0.33 eV) exists at an inter-atomic distance of 1.8 Å that is somewhere between a graphitic van der Waals bond of 3.35 Å and a diamond’s covalent bond of 1.54 Å.

microns in size. The activation energy (2.5 eV) for smashing linear carbon structures together to form imperfect diamond is intermediate between that for bombarding with carbon atoms (0.33 eV) and for puckering graphite (0.17 eV). Diamond may also be generated by dissociating carbonaceous gases in an atmosphere that does not contain hydrogen atoms. For example, if methane is mixed with argon or nitrogen and the gas is energized by microwave agitation, the carbonaceous gases may form dimers of carbon atoms (C2 ) analogous of nitrogen (N2 ), oxygen (O2 ) or fluorine (F2 ). Unlike these dimer molecules of gases that hold two atoms by single (F2 ), double (O2 ), or triple (N2 ) covalent bonds, the C2 dimer is held together by quadruple covalent bonds that is metastable. At high temperature, C2 dimers will decompose to form graphitic carbon. But at low temperature (e.g. 700◦ C), their covalent bonds may reconnect to form diamond if they have sufficient concentration. However, because the temperature is not high enough to allow long range diffusion of diamond atoms, so the diamond grains formed are nano-crystalline. The diamond film so formed is typically very thin (e.g. 1 µm). Because the grains are so tiny, a significant proportion of atoms is actually located on grain boundary rather than inside the crystal.

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Another unusual CVD process capable of depositing diamond film continually on a steel substrate at ambient condition involve using multiple lasers. This method known as QQC (Mistry et al., 1996) has been used to grow diamond or diamond-like materials by bombarding CO2 and N2 gases with three lasers (CO2 , YAG, and Excimer) simultaneously. Although the mechanism of diamond formation is unclear, but it is possible that C2 dimers are also involved as the nutrient for diamond growth. Apparently, the reaction induced by laser ablation is so violent that the dimers can collide among themselves at a rate fast enough to sustain the diamond growth to a significant size (e.g. 100 µm). Subsequently, the quenching rate from the high temperature of laser ablation is sufficiently high so the diamond formed is preserved without much oxidation or back conversion. 4.9

THE HIGH PRESSURE COLLISION OF ATOMS

Although PVD coating of DLC, and CVD deposition of diamond films are preformed in partial vacuum, the action of diamond’s sp3 bond formation is actually taking place at very high pressure and temperature. In the case of PVD process, carbon atoms are smashed together to form sp3 bonds. Locally, the pressure and temperature are higher than that for conventional high pressure synthesis of diamond (Fig. 4.12). Hence, the vacuum environment and low temperature condition refer to the carbon atoms that have already been quenched from the extreme conditions of deposition. When a carbon containing ion smashes on a group of carbon atoms that already bonded to form diamond, the kinetic energy will be transmitted to many atoms. Hence, the smashing atoms must possess an excess amount of energy so they can overcome the activation energy for the formation of sp3 bonds even after losing the energy to other atoms as heat. However, if the kinetic energy is too high, the vibrations of atoms are excessive so even diamond’s sp3 bonds are formed instantaneously, they will immediately be back converted to graphitic sp2 bonds at high temperature. Hence, the kinetic energy of the impinging ions must be optimized to maximize the diamond content of a DLC coating. In most cases, the optimized energy is around 100 eV. At such an energy, new sp3 bonds will be formed with the joining of the carbon cation to the existing DLC coating, at the same time the surrounding carbon

The High Pressure Collision of Atoms

Figure 4.12. The extreme conditions of PVD deposition of DLC. The distributions are pressures (top) and temperatures (bottom) of carbon atoms near the vicinity of collision by a methane cation that traveled at a kinetic energy of 100 eV. Note that although the coating is performed in vacuum and low temperature, locally, sp3 bonds are actually formed at pressures and temperatures way higher than that for conventional diamond synthesis under ultra high pressure.

atoms will absorb the extra energy by vibrations (i.e. heated up), but the shaking is not strong enough as to break the bonds already formed. Free carbon ions can be generated by several ways. For example, positively charged carbon ions may be derived from vaporized graphite cathode that is bombarded by an electric arc. If these carbon ions are accelerated toward a target in an electric field, they may acquire enough kinetic energy and attach to the target to form

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nano-crystalline (amorphous) diamond or DLC. This method of making diamond is known as cathodic arc. Cathodic arc is a popular PVD method for depositing nanocrystalline diamond that does not contain hydrogen. During the ion bombardment, the ambient pressure is kept in a vacuum (e.g. 10−5 torr), and the substrate temperature is relatively low (e.g. 100◦ C). Hence, such a PVD process is viewed to be a metastable synthesis of imperfect diamond crystallites. However, at the site where carbon atoms are smashed to form diamond bond, the local pressure may exceed 15 GPa, and temperature, more than 1000◦ C. Hence, such PVD processes are actually microscopically under high pressure, but macroscopically at low pressure. In contrast, a CVD process starts with a carbon atom that is already compressed by four hydrogen atoms to yield sp3 configuration. The strategy is then to link this single atomic diamond to form a molecular diamond without being decompressed to form sp2 bonds. The C–H bond has an energy that is equivalent to a pressure of 200 GPa; and the C–C diamond bond, at a somewhat less pressure. Hence, during the CVD growth of diamond, carbon atoms are always under high pressure throughout the entire synthesis route. Thus, it is a misnomer to call a CVD process the metastable growth of diamond. Although gas molecules are under partial vacuum during the growth of CVD diamond, carbon atoms are always in the diamond stability field. It would appear that all diamond synthesis routes are intrinsically high pressure methods. With this new insight in mind, the major commercial diamond synthesis routes can be classified according to the durations of pressure and temperature applied as depicted in Table 4.3.

Table 4.3.

P/T duration’s for diamond synthesis

Process

C-Atoms

Pressure

Temperature

High Pressure Explosion PVD CVD

All All Few Few

Steady Instant Instant Steady

Steady Instant Instant Steady

The Threshold Conditions of Diamond Synthesis

4.10

THE THRESHOLD CONDITIONS OF DIAMOND SYNTHESIS

Diamond may be converted directly from a non-diamond carbon source in microseconds by a displacive movement of atoms without diffusion, or it can be synthesized catalytically in minutes by reconstructive means with the transport of atoms. The diamond formation pressure and temperature are dependent on the time scale and the structure of the carbon source. The instant conversion may occur with zero-dimensional carbon (point-like sp0 atoms) by PVD, one-dimensional carbon (line-like sp1 chains) by dynamite detonation, or two-dimensional carbon (plane-like sp2 surface) by shock wave compaction. The timely synthesis may proceed with two-dimensional (sp2 ) graphite (e.g. by puckering recrystallized flakes in a molten catalyst), or with three-dimensional (sp3 ) diamond-like molecules such as solute atoms in a molten catalyst (e.g. by temperature gradient method), or caged atoms in methane molecules (e.g. by CVD). The approximate activation energies (E) for the major diamond synthesis methods may be estimated from their reaction routes. The threshold pressure and temperature for each of these synthesis routines may be calculated based on the estimated activation energy. The estimated values are summarized in Table 4.4. The pressure and temperature for diamond synthesis may be determined from the activation energy associated with the mechanism of the transition. For each activation energy, the higher the pressure is available, the lower the temperature is necessary. Hence, the inverse function of the P–T trace is an U-shaped curve. The position of such a curve is dependent on the time scale of

Table 4.4. Threshold conditions for diamond formation C Source

State Method

Route

Time

E (eV) P (GPa) T (◦ )

Plasma Dynamite Graphite Graphite Solute Methane

sp0 sp1 sp2 sp2 sp3 sp3

Direct Direct Direct Catalytic Catalytic Catalytic

Instant Instant Instant Timely Timely Timely

0.33 0.25 0.17 0.13 0.13 0.09

PVD Explosion Shock wave Puckering Precipitation Adjoining

16 12 8.1 6.2 6.2 4.3

3500 2700 1800 1250 1250 800

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synthesis (Sung and Tai, 1996). For example, if the time for the synthesis is short, as in the case of explosion synthesis, the P/T trace for the transition would be displaced toward high values. The activation energy for diamond synthesis depends on the original form of carbon and the route it takes to become diamond. In the case of direct graphite–diamond transition by the puckering mechanism, such as that may happen in the explosion synthesis, the activation energy is approximately 0.17 eV. If the puckering is assisted by the catalytic action of molten metal, this activation energy may be further decreased to about 0.13 eV, so the P/T requirements for the synthesis are greatly reduced. The longer durations for static high pressure processes may further depress this P/T regime required for the synthesis. In the above example of PVD synthesis of diamond, the starting carbon is in the form of carbon atom. The activation energy for forming diamond is about 0.33 eV. This amount of high energy barrier, coupled with its extremely short duration of time in forming diamond bonds would shift the P/T curve significantly to the high side. For the CVD process, the starting carbon is already in the diamond-like configuration. The activation energy is then equal to joining such diamond-like carbon atoms together. This activation energy (0.09 eV) is the lowest for all diamond synthesis processes. Table 4.4 summarizes the activation energies of major diamond synthesis routes and the representative pressure and temperature regimes for these processes to occur. 4.11

THE KINETICS OF DIAMOND FORMATION

Based on the above activation energies, the kinetic diagram for diamond synthesis can then be constructed as shown in Fig. 4.13. Figure 4.13 indicates that all diamond syntheses are performed under high pressure, and the P/T regimes for their syntheses are primarily determined by the energy barrier and the conversion time. In summary, there are numerous ways to synthesize diamond. They differ in the selection of starting material and the duration of the synthesis. The starting material will affect the activation energy for the synthesis. This energy barrier and the duration of synthesis may determine the pressure and temperature for the synthesis.

The Kinetics of Diamond Formation

Figure 4.13. The kinetic diagram of diamond synthesis that shows the pressure and temperature conditions of in situ at real time for the formation of diamond by taking different transition routes. The actual pressure and temperature required for the actual synthesis will increase with increasing activation energy and they will decrease with the increasing of synthesis time.

If the starting material contains carbon atoms/ions that are generated by electric arc, lasers, or dynamite explosion, the activation energy is high (0.33 eV), and the synthesis duration can be very short (microseconds). As a result, the pressure and temperature for the synthesis are high. If the starting material is graphite that is puckered instantly by either shock wave or flash heating, the activation energy is moderate (0.17 eV), so are pressure and temperature for the synthesis. If the starting material is graphite that is disintegrated into flakes or dissolved atoms, the synthesis time is long. The activation energy is relatively low (0.13 eV), so are pressure and temperature for the synthesis. If the starting material is diamond-like carbon as in the case of ionized methane gas, the synthesis time is also relatively long.

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Figure 4.14. Different precursors of carbon may transform into diamond with various activation energies. The term “flake” refers to the recrystalized graphite micro particles that are suspended in a molten metal catalyst. These flakes may be rhombohedral graphite that possesses the layer configuration easily puckerable into diamond structure.

In this case, the activation energy is the lowest (0.09 eV), so are pressure and temperature for the synthesis. The above four major diamond formation routes and their associated activation energies are shown schematically in Fig. 4.14. 4.12

COMMERCIAL PRODUCTION OF DIAMOND SUPERABRASIVES

Natural diamond was the only superabrasive capable to machine tungsten carbide tools before 1957. During the World War I, the consumption of natural diamond surged to 0.4 ton a year. When World War II broke out, this demand increased drastically to 4 tons per year. At then only South Africa can export natural diamond, so US has stocked piled strategic supply of natural diamond that was released only after 2000. In 1941, Percy Bridgman was supported by General Electric, Norton, and Carborandum to explore the possible means of making diamond. This attempted was interrupted by World War II. After the war, Norton continued the trial but without success. In 1951, GE renewed the attempt and it eventually succeeded in making the first diamond in 1954. In 1957, GE introduced the first synthetic diamond superabrasive at its Carboloy department that mass manufactured cemented tungsten carbide. At then only

Commercial Production of Diamond Superabrasives

imperfectly grown diamond was used for grinding cemented tungsten carbide. GE soon perfected the diamond growth technology and it continually introduces larger and better grown diamond for making grinding wheels and saw blades. Because natural diamond was formed billions years ago, its properties are fixed, the only modification can be made is to sort the crystals by size and shape. However, different industrial applications may require diamond to break down in different mode that cannot be satisfied by natural diamond. On the other hand, synthetic diamond can be grown at different rates so their strengths vary. This variation can allow the adjustment of break down mode. Consequently, synthetic diamond became more desirable for most industrial application. Hence, in 1970, the world consumption of synthetic diamond eventually exceeded natural diamond. Today, less than 5% industrial diamond is still served by natural diamond. In 2002, more than 400 tons of diamond grits (up to 1 mm in size) were made as superabrasives, such as used in circular saws, drill bits, or grinding wheels. Almost all such diamonds are synthesized under high pressure (up to 6 GPa) by converting graphite using a molten metal catalyst (e.g. an iron alloy). Diamond grits may also be sintered at high pressure to form plates of polycrystalline diamond (PCD). PCD products are widely used as cutting tools, drill bits, and wire dies. Large single crystals can also be synthesized under high pressure. Such crystals (up to 1/3 of a carat) are commercially available

Figure 4.15. High pressure synthesized sub-carat single crystals manufactured by Element Six (Monocrystals) (left) and Sumitomo Electric (Sumidia) (right).

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as cutting tools (e.g. De Beers’ Monocrystals) or heat spreaders (e.g. Sumitomo’s Sumidia) (Fig. 4.15). Some 20 countries are engaged in high pressure syntheses of diamond on commercial scale. They include United States, South Africa, Ireland, Sweden, Germany, France, Greece, United Kingdom, Russia, Ukraine, Romania, Poland, Ctsechslovikia, Almenia, Japan, China, North Korea, South Korea, Malaysia, and India. In the production history of synthetic diamonds, the followings are major milestones. In 1957, GE starts commercial production, before then natural diamond (4 tons) was the only choice of industrial diamond. In 1970, synthetic diamond (20 tons) outweighs natural diamond as the dominant industrial diamond. In

Figure 4.16. Three views of De Beers production plants for diamond synthesis. These pictures disclose the evolutionary trend of press types and plant layouts for the commercial production of industrial diamonds. They were taken at three geographical locations in the last three decades. Top, early small presses used by De Beers in Springs, South Africa; middle, 10000 T presses in Robersfors, Sweden; Bottom, 10000 T presses in Shannon, Ireland (closed in 2001).

Commercial Production of Diamond Superabrasives

1993, China became the world leading producer (40 tons) of synthetic diamond in quantity. China produces the most of synthetic diamond under high pressure (about two/third of the world’s total output), but mainly in low-grade materials (about one-tenth of the world’s value of superabrasives). Ireland has been the largest exporter of industrial diamonds due to the presence of both General Electric Company of USA, and De Beers Company of South Africa (Fig. 4.16), the two most prominent manufacturers of industrial diamonds. The major diamond makers are listed in Table 4.5 along with their starting year of research and manufacturing. Although there are over 400 companies (China has more than 400 of them) that are engaged in commercial production of diamond superabrasives, only General Electric and De Beers were capable of making premium saw diamond that was typically more than twice as valuable as other diamond grits (e.g. for making grinding wheels). However, in 1989 the author designed a manufacturing plant for Iljin Diamond and transferred manufacturing

Table 4.5. History and scale of high pressure diamond synthesis Country

Research success

Production implementation

China U.S.A. South Africa Sweden Russia/Ukraine South Korea Japan Germany Ireland

1962 1954 1959 1953 1960 1987 1961 1964 1980

1966 1957 1961 1962 1962 1989 1963 1967 1984

Total

2007 Quantity manufactured (tons) 800 60 60 40 40 40 20 20 20 1100

Source: Notes: De Beers’ manufacturing sites were in South Africa’s (Springs), Germany (Winter, discontinued in 2001), Sweden (Scantidiamont), and Ireland (Shannon, discontinued in 2001). General Electric’s output included that manufactured in USA (Worthington) and Ireland (Dublin).

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technology of making premium saw grit. Iljin Diamond has since become the third largest manufacturer of saw diamond in the world.

4.13

THE DIAMOND’S BIG BANG IN CHINA

Although China was a latecomer for diamond synthesis, and their technology has been crude, however, they have evolved as the major production country of industrial diamond grits. In 2007, China produced 800 tons (4 billion carats) of diamond grits that account for more than 80% of the world production. The reason that Chinese could become the powerhouse of diamond grits production was actually triggered by their inability to make large dies for making ultrahigh pressure apparatus (the belt system). The large dies are made of cemented tungsten carbide that often breaks if the sintering is not homogeneous. In order to circumvent this problem, Chinese were forced to use cubic presses that use anvils to substitute the enclosure of the die (Fig. 4.17). Although such technology was judged rudimentary, Chinese were able to scale up cubic presses for mass production of diamond grits. It was fortuitous that die-free cubic press could cost less for manufacturing because large dies are expensive consumable parts. As a result of low cost production, leading diamond companies were forced to reorganize. For example, De Beers Industrial Diamonds was restructured as Element Six. GE Superabrasives were sold twice as Diamond Innovations (now under Sandvik). The Chinese taking over of the diamond grits production is now upgrading with a new technology that allow the random nucleation of diamond by patterned diamond seeding (Fig. 4.18). This technology was invented by the author (US Patent Nos. 6,159,286; 7,172,745; and 7,323,049).

4.14

CVD DIAMOND FILM AND PVD DIAMOND-LIKE CARBON (DLC) COATING

Diamond is also grown metastably in partial vacuum to form either freestanding films or adhered coatings. Diamond films of up to 1 mm in thickness, and up to 30 cm wide, can be precipitated by a variety of CVD methods (e.g. hot filament, microwave plasma,

CVD Diamond Film and PVD Diamond-Like Carbon (DLC) Coating

Figure 4.17. The production cubic presses in Huanghe (the largest diamond plant in the world), China that can manufacture over 3 million carats a day.

Figure 4.18. High pressure diamond synthesis technology advancements. The original production methods were based on the alternation of graphite layers and catalyst layers (invar composition). Most products made today are using mixture of graphite and catalyst powder, some with micron diamond seeds mixed in randomly. The latest innovation is to grow diamond seeds that are arranged to maximize the volume utilization (Sung et al., 2006). Such a new technology is now in pilot manufacturing in China.

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Figure 4.19. Former Norton Diamond Film’s DC arc CVD reactor (the author helped acquire the Technion technology in 1988) and its produced diamond films for brazing on cutting tools.

Figure 4.20. Former Norton Diamond Film’s microwave CVD reactor (the author helped acquire the Wavemat technology in 1987) and its produced coated cutting inserts.

CVD Diamond Film and PVD Diamond-Like Carbon (DLC) Coating

Figure 4.21. Kinik Company’s 30 cm × 40 cm hot filament CVD reactor (the author helped acquire the sp3 , Inc. system in 1998) with its diamond film products displayed from top to bottom: coated cutting inserts, coated silicon wafers, free standing diamond tweeter, and diamond pyramids formed by inverse casting of silicon.

oxyacetylene flame, DC arc, etc.). Thin layers (up to 3 µm) of nanocrystalline diamond or DLC may be coated over a substrate of up to 1 m in length. They are deposited commercially by many PVD techniques (e.g. cathodic arc, methane ions, argon sputtering, laser ablation, etc). Both CVD and PVD methods are emerging technologies for depositing diamond (CVDD) and DLC (PVDD), respectively. These new diamond products can greatly expand the scope of diamond applications to non-superabrasive regimes, such as heat spreaders, optical windows, tweeter diaphragms, surface acoustic waver (SAW) filters, particle detectors, semiconductor devices for CVD diamond films; field emitters, solar cells, electron radiators, electrical capacitors, tribological surfaces, chemical barriers for DLC coatings. The author had introduced several large production systems for CVD growth of diamond films and PVD coating of DLC. These systems were installed at former Norton Diamond Film (then world’s largest diamond film maker) and Kinik Company (now one of world’s largest DLC coaters). Norton’s systems include

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Figure 4.22. Kinik Company’s cathodic arc PVD coater of amorphous diamond (the author helped acquire the Multi Arc system in 1997) in action and its products of DLC electro-luminescence (DEL) and LED heat spreader.

CVD Diamond Film and PVD Diamond-Like Carbon (DLC) Coating

Figure 4.23. Kinik Company’s RF CVD coater of DLC (the author helped acquire the Ion Bond system in 2001) in appearance.

Technion’s DC arc reactor (Fig. 4.19) and Wavemat’s microwave reactor (Fig. 4.20); and Kinik’s systems include sp3 s hot filament reactor (Figs. 4.21 and 4.22) and Ion Bond’s cathodic arc and radio frequency plasma CVD (Fig. 4.23).

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Chapter Five

Micron Fines and Nanodiamonds

5.1

DIAMOND PULVERIZATION

Most micron diamonds are produced by breaking larger diamond grains of poor quality. There are various ways to pulverize diamond, each with its range of size applicability (Fig. 5.1). The breaking of diamond can be accomplished by stamping, compression, impact, shear, and attrition. Stamping and compression may be suitable for objects (e.g. stone) much larger (e.g. >1 cm) than diamond. Impact and shear may be used to pulverize diamond, but often the produced grains are irregular in shape (e.g. platy or slender). Attritors are effective in forming fine powders so they are capable to make micron diamond. A more common technique is to mill diamond with balls (e.g. made of steel) in a rotating cylinder. The turning of the mill is rather fast so the balls can be raised high to smash diamond more effectively. In order to further increase the dropping height of the balls so to accelerate the production of diamond fines, the cylinder may be rotated around the center of cylinder instead of around the center of axis (Fig. 5.2). Because the reduction ratio of diamond should be kept low to increase the efficiency of milling, so several ball mills may be used in stages to pulverize diamond to very fine sizes. In each stage, diamond is sized so the feedstock contains diamond of similar sizes. Typically, the load of ball mill is half full. The size of steel balls is about 1 cm. The total weight of balls can be four times higher than

Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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Chapter 5 Micron Fines and Nanodiamonds

Figure 5.1. Typical crushers and grinders with their ranges of intake sizes and output sizes.

Figure 5.2. The rotation mode of a ball mill can be around the axis (left) or perpendicular to it (right).

Diamond Pulverization

Figure 5.3. Jet mill can bring two streams of air suspended micron diamonds in head on collision. In this way, irregular shaped diamond will chip off corners to become more blocky in shape.

diamond. Water may be added to the charge to prevent powder from clinging on the wall. Ball milling may be effective to reduce the size of diamond, but the broken diamonds are irregular in shape. The sharp corners may be chipped off by using a jet mill that force two streams of micron diamonds to collide each other (Fig. 5.3). Jet milling may not be suitable to reduce diamond in size, but it is effective to smooth the surface by removing weak protrusions. After jet milling, the chipped diamond is mixed in with much debris and impurities derived from the interior of the milling machine. Most impurities can be dissolved in warm acids (e.g. HNO3 or HCl) or bases (e.g. KOH or NaNO3 ). The siliceous impurities can be etched away by HF or KF solution.

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5.2

THE CLASSIFICATION OF MICRON DIAMOND

Micron diamonds are too small to be sized by sieving (it can plug the hole in no time). Hence, fluid separation is used to separate them. The fluid used can be either water or air. Typically, sedimentation is performed in water, and in certain cases (e.g. for separating submicron particles) centripetal force may be applied to accelerate the settling. The sedimentation method is based on Stokes’ Law for particle settling in any fluid. If the particle is near spherical in shape, its terminal velocity may be expressed as follows: Vt =

(ρd − ρw) 2 d 18η

(5.1)

where Vt is the particle’s terminal velocity, g is the acceleration of gravity or centripetal force, ρd is diamond’s density (i.e. 3.52 gm/cc), ρw is the density of the fluid (i.e. 1 gm/cc for water), η is the viscosity of the fluid (i.e. 0.01 gm/sec-cm for water), and d is the diameter of diamond particle. For diamond particles settling in water, the following equation may be used to approximate the terminal velocity: Vt = 13665 d2 cm/sec

(5.2)

Based on this formula, the settling distance and time for diamond particles of various sizes can be computed as shown in Table 5.1. For a typical settling distance of 20 cm, the diamond fine with size of 1 µm may take 40 h that is extremely slow. By putting water Table 5.1.

The settling distance and time of diamond particles.

Size (µm)

Velocity (cm/sec)

Settling Distance (cm/hr)

Settling Time/cm

40 20 10 5 1 0.5

0.22 0.055 0.014 0.0034 0.00014 0.000051

787 197 49 12 0.5 0.1

4 sec 18 sec 1 min 5 min 2h 6h

The Classification of Micron Diamond

in a centripetal machine, the above settling time may be reduced to only minutes. Stokes’ Law is applicable only to spherical particles. As diamond particles are not spherical, so their settling time are longer than that predicted Eq. (5.2). In the case of irregular shaped diamond, the increase of settling time may be excessive. Hence, large irregular shaped diamond may settle at the similar speed as small round ones. Sometimes the size difference can be as large as 3 times. Hence, the more irregular the shape, the broader the diamond size distribution. The size distribution is critical for precision polishing, so more blocky shapes are preferred. In this way, the chance to cause scratch to work piece is less because less sharp protrusion is associated with blocky shape. A typical procedure for sedimentation is described as follows: (1) Mixing upgraded diamond fines with deionized water to form a homogeneous suspension of diamond fines. (2) Pour the liquid in a sedimentation tank to allow diamond fines to settle. (3) After a certain time, siphon out water from a fixed depth. In this case all particles still suspended are smaller than a predetermined size. (4) The water in the bottom of the tank contains mostly larger particles, but there are also small particles that began to settle from close to the bottom. (5) Fill deionized water in the tank to reach the same water level and thoroughly stir the settled diamond fine to make them suspended again. (6) Repeat the same process several times to separate the smaller diamond fines over and over until the solution contains almost all large particles. (7) Combine the solution with smaller diamond fines and repeat the sedimentation to separate even smaller particles. The above sedimentation process can be processed by dedicated machines with computer control. During the sedimentation process, in order to allow free settling of diamond fines, the concentration of particles must be limited. Based on experience, if the volume content of the diamond fines is less than 1%, the interference of settling is minimal. Hence,

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the threshold level of concentration may be determined to be: Diamond weight/water volume = 0.01 × diamond density = 35 g/l

(5.3)

Based on the above concentration and assuming that the distance for diamond fines to settle is 20 cm, then the amount of processed diamond in each batch is about 2.2 R2 g, where R is the radius of the water tank in centimeters. In most cases, if the grading of diamond fines is not critical, the above diamond fines concentration and also the batch size may be increased by 7 times. Hence, for a settling tank with a diameter of 50 cm, about 10 kg diamond fines may be treated each time. But because the settling time is rather long, only 15 batches may be processed each year with a throughput of about 150 kg per tank. During the sedimentation, diamond fines may agglomerate due to the accumulation of electrical charges. In this case, the enlarged diamond clusters may settle much faster and they can mix in with smaller particles. In order to prevent this from happening, a proper sulfactant may be added to the solution. The sulfactant contains cations that may surround diamond fines that are often charged with electrons. In this case, the repulsion of sulfactant cations can keep diamond particles separated so they may settle more freely (Fig. 5.4). The common sulfactants include sodium silicate, Arabian balsam, sodium phosphate, olive oil, oxalate acid, etc. One example is to add about 0.04 wt.% of the sulfactant in solution. After sedimentation, the graded diamond fines are fully cleaned. For special applications, such as that for making electroplated tools, diamond surface must free of any metal contamination (e.g. iron from the catalyst inclusion) lest nickel will

Figure 5.4. The agglomerated micron diamonds (left) and their dispersed individual particles (right).

The Polishing by Micron Diamond

Figure 5.5. The size distribution of well-graded diamond fines produced by Tomei.

be deposited following the electrical conducting path. In this case, diamond’s surface can be etched by oxidants such as by molten salts (e.g. KNO3 ). The etched diamond must again be thoroughly rinsed in deionized water to remove any salt contamination. The graded diamond fines must be characterized for their size distribution and particle shape. The characterization may be done by using laser projection instrument, scanning electron microscopy, optical electrical measurement, etc. For example, the cross section of diamond particles can be projected and magnified by 3000 times to achieve a resolution of 0.1 µm (Fig. 5.5). 5.3

THE POLISHING BY MICRON DIAMOND

Diamond micron powder is mainly used in polishing of various industrial materials, although there are also other applications such

Figure 5.6. Fast pulverization can form shard, plates, and other irregular shapes of micron diamonds (GE Superabrasives’ brochure).

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as using micron diamond as feedstock for sintering polycrystalline diamond (PCD). Because single crystal micron diamonds are formed by pulverizing larger diamonds of poor quality, so their shapes resemble broken glass. If the breaking force is too concentrated, very irregular shapes may form (Fig. 5.6). Such shapes contain ample sharp protrusions, so they can easily scratch work surface. The shape of a micron diamond can be defined by aspect ratio or other projected features. Such features can be measured automatically by laser optical means. For most applications, the requirements for micron diamond are the same, which is the particle should be as blocky as possible. The blocky shape can assure tighter size distribution. In this way, sharply protruded particles and oversized particles can be avoided, so is the possibility to scratch work surface, the nightmare for all precision polishing workers (Fig. 5.7). Moreover, tight grading may

Figure 5.7. Larger micron diamonds will receive higher force for polishing and hence they are likely to scratch work surface (top). More uniform size distribution of micron diamonds can assure more even force distribution and the less chance to scratch processed material. Source: GE Superabrasives brochure.

The Heat Treatment of Mono Diamond Fines

Figure 5.8. The specific surface area for micron diamond of various sizes. Note that the irregular shaped micron diamonds have a much larger surface area than blocky ones (GE Superabrasives brochure).

also prevent under sized particles that can do no work, and hence they are wasted. In addition, blocky shape can minimize surface areas and it is easy to clean, hence, micron diamond with blocky shape can be made very clean (Fig. 5.8). Surface cleanliness is an important attribute to avoid overgrowth of nickel during electro deposition for making plated diamond tools, or to prevent defects after sintering for making polycrystalline diamond (PCD). The surface contaminants can slow or stop the cobalt sweep across the grain boundaries, an essential requirement to form diamond bridges among individual diamond grains at high pressure and high temperature.

5.4

THE HEAT TREATMENT OF MONO DIAMOND FINES

In general, a diamond fine that can achieve high removal rate may also cause scratches to the super smooth surface. Tomei solved this problem by carbonization of diamond’s surface by heat

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treatment under vacuum (e.g. PM series fines). The carbonization consumes all sharp points on the diamond particle. Moreover, the superhard core is now enveloped by a more benign coating. In addition, due to the spherization of the diamond fines, the sizing can be more precise. Tomei’s PM series fines are uniquely capable of policing read/write heads (e.g. GMR, TMR) fast without causing many scratches. Although dynamite fines are scratch free, but their removal rates are slow. Hence, nonodiamond fines with carbonization still dominate the head polishing business. The diamond fines may be formed in slurry. If organic solvent is used as the medium (e.g. for polishing heads), the diamond surface should by hydrophobic. On the other hand, if water suspension is needed (e.g. for texturing hard drives), the diamond surface must be hydrophilic. The hydrophobic surface can be made by terminating surface carbon atoms with unpolarized absorbents, such as hydrogen or fluorine. For example, by heating diamond under hydrogen at 600◦ C for 2 h. The nanodiamond’s surface may

Figure 5.9. The carbonization of micron fines (left) and its effect on size distribution after heat treatment at 1250◦ C (right). Source: Ryuji Ohshima, Hiroshi Yamanaka, Satoru Hosomi, Carbon, TANSO, 2001, No. 197, 88–91.

Conventional Micron Diamonds

become hydrophilic by adding polarized radicals, such as ions of hydroxide, surfate, nitrate, and chlorate. For example, by soaking diamond in a solution of H2 SO4 and HNO3 at 250◦ C for 1 h (Figs. 5.10–5.13). The pulverized micron diamonds are relatively perfect crystals with little defects or impurities. This is in contrast to other forms of nanodiamond that may be synthesized by explosion (see later chapters). 5.5

CONVENTIONAL MICRON DIAMONDS

Micron diamonds are indispensable for polishing industrial products including metals, ceramics, and plastics. There are several types of micron diamond fines. Most micron diamond fines are

Figure 5.10. The turning of hydrophilic diamond to hydrophobic by heating in hydrogen atmosphere (left). The conversion of hydrophilic diamond from hydrophobic one by heating in nitrogen atmosphere (right). Source: Satoru Hosomi, Hiroshi Yamanaka, Rev. High Pressure Science and Technology, 2003, V13, No. 1, 31–35.

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Figure 5.11. The size effect on the mass loss and specific surface area by heat treatment at 800◦ C under hydrogen atmosphere (left). The emission of gas by thermaographic analysis of micron diamond (Tomei Dia’s MD150) in air (right). Source: Hiroshi Yamanaka, Satoru Hosomi, J. Soc. Mater. Sci., Japan, 2003, V52, No. 6, 592–595.

Figure 5.12. The comparison of X-ray diffraction patterns of different diamond origins. Source: Satoru Hosomi, Hiroshi Yamanaka, Rev. High Pressure Science and Technology, 2003, V13, No. 1, 31–35.

Conventional Micron Diamonds

Figure 5.13. The comparison of FTIR spectra of different diamond origins. Source: Satoru Hosomi, Hiroshi Yamanaka, Rev. High Pressure Science and Technology, 2003, V13, No. 1, 31–35.

produced by crushing and pulverizing low valued larger diamonds. Such diamonds are formed due to the gradients of pressure and temperature existed in the high pressure cell. Other micron diamond is formed by shock wave compression of graphite, or by CVD nucleation of diamond. A typical pulverized micron diamond powder contains angular corners that may cause scratch to the smooth surfaces. Hence, it is critical to remove irregular shaped micron particles (Figs. 5.14 and 5.15).

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Figure 5.14. Micron diamond of different origin including pulverized natural diamond (top left), pulverized synthetic diamond (top right), shock wave compressed diamond (bottom left), and CVD micron diamond (bottom right).

There have been no micron diamond that show euhedral morphology, i.e. with grown crystallographic faces. However, recently, the author developed a CVD process that can grow near perfect diamond crystals in high concentration (Fig. 5.16). Such euhedral micron diamond has the lowest amount of specific surface area of all micron diamonds, so they are the cleanest. Moreover, the high symmetry can allow them to be sorted in very tight size distribution. Moreover, because all solid angles are obtuse, these micron jewels will not scratch the work piece even when the polishing is performed under high pressure to achieve a high removal rate of the work piece.

Conventional Micron Diamonds

Figure 5.15. The typical morphology of conventional micron diamond made by pulverizing unwanted large diamond.

Figure 5.16. Euhedral micron diamond grown by CVD method with different magnification. The top left figure shows more diamond crystals (1011 ) than stars in the entire Milky Way galaxy.

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5.6

CVD DIAMOND PARTICLES

Most CVD diamond film is formed by initial nucleation that is followed by the growth of nuclei. However, Argonne National Lab pioneered to avoid the growth of nuclei, so the nuclei are continually added to join together to form nano-crystalline diamond film. The way to avoid nuclei to grow is to replace hydrogen gas by argon gas. Normally, hydrogen molecules are dissociated at CVD to form hydrogen atoms. The presence of hydrogen atoms can stabilize the sp3 bonds of carbon atoms and hence they allow diamond nuclei to grow. The unavailability of hydrogen atoms prompt nuclei to join one another. According to Argonne National Labs’ analysis, C2 molecules were formed by pyrolysis of either CH4 or C2 H2 (Fig. 5.17). The C2 molecules then join together to form nano-grained diamond. Another way to deposit nanodiamond is to increase the rate of diamond nucleation. The nucleation rate can be boosted by seeding of the substrate (e.g. a silicon wafer) with nanodiamond particles. Alternatively, the nucleation rate can be promoted by applying a negative bias (e.g. 200 V) that allows the increase of the kinetic energy of impinging carbon ions. The nucleation rate can also be enhanced by increasing the methane-nitrogen ratio (e.g. from 1% to 5%). The growth rate of diamond can be suppressed by reducing the substrate temperature (e.g. from 900◦ C to 600◦ C). Micron diamond particles may also be bonded by nanodiamond film. For example, Diamonex has a product with micron diamond grains bonded to silicon wafer by a thin layer of diamond film with submicron grains (Fig. 5.18). 5.7

SHOCK WAVE COMPRESSED DIAMOND

Instead of breaking large diamond grains to form micron-sized particles, shock wave may be applied to collapse graphite. Due to the instantaneous rises of pressure and temperature diamond crystallites are formed. However, as the high pressure is maintained for only a few microseconds, such crystallites are virus sized (e.g. 50 nm). But due to the slow cooling of the charge nano particles are sticking together to form sintered polygrits up to 50 microns (Fig. 5.19). The direct transformation of graphite to diamond by shock wave compaction was commercialized by Du Pont in 1970s. The

Shock Wave Compressed Diamond

Figure 5.17. Argonne National Labs nanodiamond film structure (top) and the HRTEM image of nanodiamond particles (bottom).

process requires the packing of dynamite around a steel tube. Inside the steel tube are intimately mixed graphite powder and copper grains (e.g. 92 wt.%, about 1 mm in size). The charge was first compressed to increase the density (e.g. by cold isostatic pressing). The presence of high porosity in the charge can dampen the shock wave so the desired pressure cannot be attained. Moreover, the collapse of large amount of pores may also increase the temperature that can back convert diamond formed.

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Figure 5.18. The submicron grained diamond film bonded micron diamond (e.g. 50 µm).

The charge could be round bars of about 20 cm in diameter by about 5 m long. They were then sealed in steel tubes with a hallow wall that contained vacuum. Many of such steel tubes were packed inside a cylindrical culvert along with several tons of dynamite. The culvert was located in a deep shaft of an abandoned mine. As the explosive detonated, it progressively collapsed the outer (driver) tube onto the inner (product) wall, subjecting the graphite to enormous pressure, from 8 GPa up to as high as 40 GPa. The propagation of the shock wave would compress graphite from one end of the steel tube to the other. As a result, microscopic polycrystalline diamond may form. Such diamonds may have 98% of the theoretical density of diamond (3.52 gm/cc). Because the pressure was excessive, nucleation rate of diamond was extremely fast

Shock Wave Compressed Diamond

Figure 5.19. Shock wave compacted diamond conglomerates of several microns in size (Former Du Pont’s Mypolex product).

with the result of forming crystallites of up to 60 nm. However unlike the dynamite transformed nanodiamond, the shock waves allowed time for millions of crystallites to sinter into micron-sized aggregates. During the shocking process, temperature might reach 2000◦ C. Such a high temperature could convert diamond just formed back to graphite. The charge contained copper particles as heat sink that could quench the hot diamond as soon as it was formed. As a result, several kilograms of diamond may be preserved in each explosion (Fig. 5.20). Shock wave compressed micron diamonds are embedded in unconverted graphite along with metal powder. The latter could be dissolved in acids. Graphite might be oxidized by roasting the acid treated residue with PbO (e.g. at 400). Another method was to exfoliate graphite by soaking the charge in warm concentrate sulfuric acid. The intercalated graphite and subsequently vibrate the puffed graphite away in water with agitation. Shock wave compressed diamond is formed by direct transition from graphite without using catalyst. Moreover, the transition time is so short (microseconds) that long-range diffusion of carbon atoms are prohibited. Consequently, atoms in graphite must collapse directly into diamond structure.

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Figure 5.20. Polycrystalline aggregates of diamond fines may be produced by shock compression of graphite during explosion.

Graphite contains hexagonal planes that are stacked in sequence. Each graphene plane may be offset to assume one of the three positions relative to the adjacent plane. In the case of 1 H polytype (hexagonal), the position of all graphene planes is matched so the sequence becomes AAA . . .. Alternatively, graphene planes may stack up with offset in alternation as in the case of 2 H polytype (hexagonal), so the sequence now reads ABA . . .. Yet, there is one more polytype of graphite that is designated as 3C (rhombohedral). This time the sequence has a triple period in the form of ABC . . .. The common graphite contains most (e.g. 85%) 2 H polytype that may mixed in with some (e.g. 15%) graphene sequence in 3C. The 1 H graphite is unstable because of the repulsion of dangling electrons that is lined up in the same position, so it does not exist in normal graphite. It turns out that the carbon atoms in both 1 H

Shock Wave Compressed Diamond

and 3C graphite has matching atoms in adjacent layer so they can readily transform into diamond under extreme pressure, as in the case of shock wave compression. In the case of 1H graphite, it can form hexagonal diamond (lonsdaleite) that exists rarely (e.g. in meteorite or impact crater). On the other hand, 3C graphite can transform directly into a cubic diamond, the common form of diamond of both natural and synthetic origins (Fig. 5.21). It would be interesting to note that half of carbon atoms in 2H graphite do not have matching atoms in adjacent planes, so the common 2H graphite cannot form diamond directly. In the case of diamond synthesis that utilizes molten metal catalyst, 2H

Figure 5.21. The puckering of hexagonal graphene planes in rhombohedral (3C) graphite into cubic diamond (top). The buckling of hexagonal graphene planes in 1H graphite into hexagonal diamond (bottom).

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graphite, with the intercalation of liquid catalyst, has ample time to shuffle into 3C sequence before it transforms into cubic diamond. However, under the shock wave compression, no such time is allowed for shuffling graphene planes, hence, 2H graphite under compression will bounce right back upon decompression. This was why Bridgman failed in making diamond and hence his lament “graphite is nature’s best spring”. The inability of 2H graphite to transform directly into diamond has also limited the yield of diamond formed by shock wave compression. As the graphite used in shock wave compression since the time Du Pont invented this method contains only about 15% rhombohedral sequence, this has become the yield of polycrystals formed each time the dynamite exploded. Had the process use rhombohedral graphite as the carbon source, the diamond yield would have been increased dramatically (Sung, US Application No. 2006/0251567). One way to shuffle hexagonal graphite to rhombohedral graphite is to mill graphite with high shear force (e.g. by attritor). The shearing of graphite can slide graphene planes.

5.8

MICRON DIAMOND APPLICATIONS

Because of the poor diamond yield and the excessive steps required to recover and purify diamond, shock wave compressed micron diamonds are intrinsically expensive (e.g. four times more than pulverized micron diamond of the same size), hence, their market size is limited. The annual production of shock wave compressed micron diamonds is less than 1 ton with the sales value of about $10 million that is less than 1/10 of conventional micron diamond. Hence, they are mainly used in premium applications, such as for the texture grinding of hard drives (Fig. 5.22) and super smooth polishing of gemstones. Shock wave compressed micron diamond has a specific surface area that is about three times higher than pulverized single crystals. Although the abundance of surface area may be the source of contamination, it may actually eliminate the sharp protrusion of corners. Sharp protrusion has been the single most troublesome problem associated with pulverized single crystal micron diamond. The sharp protrusion can easily scratch the work surface

Micron Diamond Applications

Figure 5.22. Shock wave compressed micron diamond used to carve out concentric marks of a hard drive (left) and super smooth polishing of read head ferrite (right).

that needs to be polished by using micron diamonds (Figs. 5.23 and 5.24). In addition, during the polishing of a heterogeneous material that contains two or more phases with different hardness such as hard particles embedded in a soft matrix (e.g. cobalt cemented WC), the polycrystalline nature of shock wave compressed micron diamond can moderate the hardness difference and sweep through without causing too much variation in height. In contrast, the pulverized single crystal can easily gouge up soft spots and leave pits behind (Fig. 5.25). Another advantage of polishing with polycrystalline micron diamond is that it can form microchips along weak interfaces between constituent grains. In essence, the polycrystals can constantly sharpen itself for continual polishing. On the other

Figure 5.23. Texturing comparison of various diamond types.

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Figure 5.24. The potato shaped shocked wave compressed micron diamond particle does not possess sharp corners so it will not damage the work surface much (left). On the other hand, the conventional pulverized single crystal micron diamond particle resemble the shape of broken glass. It has many sharp corners so the scratching of the work surface is inevitable (right).

Figure 5.25. Single crystal micron diamond can easily pick up soft spots from work surface and leave many scars behind (top). Polycrystalline micron diamond can polish composite materials evenly without causing depressed regions (bottom).

hand, single crystal micron diamond may either break in half or move away without micro chipping (Fig. 5.26). The ability to self-sharpen has allowed polycrystals to maintain the aggressive polishing speed (Fig. 5.27). Hence, large polycrystals can polish fast without scratching work piece. Although shock wave compressed polygrits of nanodiamond have replaced micron diamond as the preferred texturing diamond

Micron Diamond Applications

Figure 5.26. The integral single crystal will break in large chunks along cleavage planes and lose the polishing ability (left). The homogeneous polycrystal may chip off individual grains and retain much of its original size for subsequent work (right).

Figure 5.27. Micron diamond polycrystals can polish at a much faster speed than monocrystals.

when hard drives became Gb per disk in storing density. However, as the storage medium approaches 100 Gb per disk, even the polygrits were too abrasive as to cause scratches. The dynamite nanodiamond is used instead. Due to the increased number of particles per carat with the downsizing of the nanodiamond, the weight used for dynamite diamond was actually reduced. For

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Figure 5.28. Specific surface area (ordinate) as a function of the average grain size (abscissa). Note that pulverized diamond fines are much more contamination prong than euhedral diamond fines produced in this research. Explosive (shock wave) and dynamite diamond fines are even worse as they can never be maintained clean.

example, when the hard drives were still in Mb densities, typical slurry for texturing may contain 10 carats per liter of micron diamond. This loading was reduced to 1 carat per liter when shock wave nanodiamond was used. The loading was further reduced by using dynamite nanodiamonds that have thousands of times more particles per carat. Surface areas of explosive nano and micron diamonds are much higher than conventional micron diamonds that are produced by pulverizing unwanted diamond scraps (e.g. diamond grits with irregular shapes and high levels of inclusion, also natural diamond boarts) (Fig. 5.28).

Chapter Six

Dynamite Diamond

Diamond may be formed by direct conversion of graphite with explosion-produced shock waves. Alternatively, the explosive can itself provide the carbon source (e.g. by dynamite detonation) to form diamond. In this case, the dynamite may decompose to form carbon and compressed gas. Due to the presence of instantaneous pressure and temperature, the carbon network in dynamite may rearrange to form diamond. However, such diamond particles, being extremely small, are immediately liquefied by the high temperature. The “diamond shower” is soon quenched to form nanodiamond particles. This process was originally developed by Russian in 1980s (e.g. by Russian Alta I). The explosive may also be used to generate strong shock waves that can collapse graphite to form microscopic conglomerates of nano-crystalline diamond particles (see the other chapter). This process was pioneered by du Pont scientists in 1960s. Dynamite derived graphite is mostly 3–10 nm in size (Fig. 6.1) that often form clusters with much larger sizes (e.g. one order of magnitude larger). Dynamite formed diamond is not completely transformed. The nano-sized diamond core may be covered by non-diamond carbon residues that may resemble bucky balls or carbon onions (Fig. 6.2). The surface may also be adhered with carbon soot that may shed the light of the dynamite origin. The dynamite diamonds contain various impurities (Figs. 6.3 and 6.4) so its density (e.g. 2.8–3.1 g/cc) is lower than true diamond (3.5 g/cc). One example is with 87 wt.% of carbon, 10 wt.% of oxygen, 2 wt.% of nitrogen. Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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Figure 6.1. A 3.5 nm nanodiamond containing about 4000 carbon atoms.

Figure 6.2. The onion rings of nanodiamond. The core may account for 1/2–3/4 by weight.

Figure 6.3. The HRTEM resolution of dynamite nanodiamond. Source: Courtesy of David Hsu.

Nanodiamond Characteristics

Figure 6.4. Dispersed dynamite nanodiamond. Source: Courtesy of YuFen Tseng.

6.1

NANODIAMOND CHARACTERISTICS

Nanodiamond particles may form clusters or agglomerates to become much larger collectives. The former may be due to the surface charges that are opposite to one another (Fig. 6.5). The latter can be produced by milling of shock wave compacted aggregates.

6.2

HIGH SURFACE AREAS

The nanodiamond particles derived from dynamite have very high specific surface areas. In fact, due to the small size of carbon atoms, nanodiamond particles possesses the highest proportion of surface atoms of all materials with the similar size (Table 6.1). Because nanodiamond can be extremely small, the number of particles per carat may be enormous. For example, if diamond is about 3 µm in size, each carat may have hundred thousands (105 ) particles. If the diamond size is reduced to about 100 nm, the same carat may contain hundreds trillions (1014 ) particles.

6.3

THE EXPLOSION PROCESS

The convert of dynamite to form nanodiamond was pioneered by Russians in 1980s. The dynamite used to make nanodiamond particles must contain high carbon/oxygen ratio (e.g. oxygen content less than 6 wt.%). It is typically a mixture of TNT with a pressure enhancer such as RDX (e.g. one to one ratio). RDX may be replaced

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Figure 6.5. Nanodiamond clusters (left) and nanodiamond agglomerates (right). The bottom diagrams compare the X-ray diffraction patterns of the two origins of nanodiamond. Note that the dynamite derived nanodiamonds contained much of non-diamond materials. However, the shock wave formed aggregates included some amount of hexagonal diamond (lonsdaleite).

by more explosive HMX, PETN, or even BTF (oxygen free dynamite). During the explosion, pressure may reach 20 GPa for a duration of a fraction of one micro (10−6 ) second; and temperature could exceed 3000◦ C. Although carbon (graphite and diamond included) will not melt at this temperature, but the decomposed carbon atoms are highly active and their combination of non-carbon atoms (e.g. H, O, N) greatly reduced the melting point. Consequently, these atoms are attracting one another to form atomic scaled liquid.

The Explosion Process

Table 6.1. Surface atoms of small diamond particles. Size

1 nm 2 nm 5 nm 10 nm 50 nm 100 nm 500 nm 1 µm 10 µm 50 µm 100 µm 500 µm 1 mm

Atom Surface Atom (Number) (%) 143 1147 1.8×104 1.4×105 1.8×107 1.4×108 1.8×1010 1.4×1011 1.4×1014 1.8×1016 1.4×1017 1.8×1019 1.4×1020

92% 23% 3.7% 0.9% 3.7×10−2 % 9.2×10−3 % 3.7×10−4 % 9.2×10−5 % 9.2×10−7 % 3.7×10−8 % 9.2×10−9 % 3.7×10−10 % 9.2×10−11 %

Weight (gm) 1.8×10−21 1.5×10−20 2.3×10−19 1.8×10−18 2.3×10−16 1.8×10−15 2.3×10−13 1.8×10−12 1.8×10−9 2.3×10−7 1.8×10−6 2.3×10−4 1.8×10−3

Particle Surface Area (#/Carat) (m2 /gm) 1.1×1020 1.4×1019 8.7×1017 1.1×1017 8.7×1014 1.1×1014 8.7×1011 1.1×1011 1.1×108 8.7×105 1.1×105 8.7×102 1.1×102

1705 852 341 170 34 17 3 2 0.2 3.4×10−2 1.7×10−2 3.4×10−3 1.7×10−3

These liquid clusters can be quenched by pouring in water, CO2 , or N2 to preserve diamond-like nano-particles. Because the quenching effect is more pronounced with water, the diamond yield is higher than using gas as coolant (Fig. 6.6). During the process of detonation, the nanodiamond particles so formed have a melting point that is actually below the detonation temperature. As a result, nanodiamond formed droplets of liquid that are soon frozen by super cooling to become defect ridden diamond particles (Figs. 6.7 and 6.8). Due to the rapid vitrification of the super cooled liquid of diamond, the particle size distribution is rather tight for detonated nanodiamond (Fig. 6.9). A design sketch of the explosion assembly is shown in Fig. 6.10. During the detonation, the pressure may surge momentarily to 30 GPa, and temperature can rise above 2000◦ C. During the few microseconds when pressure and temperature are the highest, non-oxidized carbon atoms are squeezed to form nanodiamond. The region where non-carbon atoms are concentrating will form carbon soots. One example of detonation is to mix TNT with solids composed of C,H,N,O atoms and subject the charge to detonation. The detonation produces N2 , H2 O, CO2 and solid carbon at a pressure

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Figure 6.6. Various dynamite molecular structures that may form nanodiamond after detonation in oxygen deficient.

Figure 6.7. The diamond yield as a function of dynamite composition.

The Explosion Process

Figure 6.8. The detonation pressures and temperatures of dynamite and the melting point of diamond as a function of temperature.

Figure 6.9. The nucleation and growth rate of nanodiamond condensed from super cooled liquid of diamond.

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Figure 6.10. Some examples of detonation device.

of about 30 GPa, and about 3000◦ C. The explosion is immediately quenched with gas (e.g. CO2 or N2 ) or water. Even so, the surface of nanodiamond remain hot enough (>1600)◦ C that it immediately back converted to form carbon onions. In addition, the carbon on the surface that has not converted to diamond could be packed around the carbon onions to form carbon black, bucky balls, and carbon nano tubes. The nanodiamond so formed is centered at 5 nm with a size spread that may be as tight as 5 Å. The soot can be purified by boiling in benzene to remove organic materials. Subsequent acid treatment can eliminate metal contaminants (e.g. boiling in 18% HCl for 1 h). The cleaned residue may be dipped in a 200◦ C solution of 6 HClO4 and 1 HNO3 and stir for 2 h. Finally, the product is washed and rinsed in distilled water. The nanodiamond so recovered may still contain a significant amount of non-carbon elements. For example, the following amounts of impurities were found, 10 wt.% of oxygen, 2 wt.% of nitrogen, and 1 wt.% of hydrogen.

6.4

NANODIAMOND PROPERTIES

Due to the presence of diamond structure, nanodiamond is superhard that may penetrate hard materials such as ceramics (Fig. 6.11). The hardness of diamond may be compared with other materials in relative and absolute scales as follows.

Nanodiamond Applications

Figure 6.11. The schematic that contrasts the atomic sizes and their structures of various materials. Diamond is superhard due to its small carbon atoms with strong covalent bonds.

It is interesting to note that superhard diamond may be as slippery as the soft Teflon. This is because while Teflon’s softness is due to its weak bonds between molecules, the slippery of diamond does not come from the yielding of chemical bonds, but the lack of reaction on smooth surface of diamond (Figs. 6.12 and 6.13).

6.5

NANODIAMOND APPLICATIONS

The properties of dynamite formed diamond-like carbon particles are compared with other diamond micron powder as listed in Table 6.2. One example of explosive derived nanodiamond is shown in Table 6.3.

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Figure 6.12. The scale of hardness for various materials.

Figure 6.13. The frictional coefficient as a function of hardness.

Nanodiamond Applications

Table 6.2. Properties of diamond micron powder Making Method Size Structure

Density (g/cm3 ) Packing Density (g/cm3 ) Purity (C%) Specific Surface (m2 /gm)

Crushed Grain Microns Single Crystal

Shock Wave Microns Polycrystals

Explosive Derivative Nanometers Diamond-like Carbon

3.5 0.7

3.4 0.5

3.2 0.3

100 <10

99 30

90 >300

Table 6.3. Nanodiamond characteristics Application

Polishing

Lubrication

4–6 nm

4–6 nm

Composition

96% diamond 2–3% ash (Fe, Si)

56% diamond, 41% carbon 3% ash (Fe, Si)

Bulk density Surface area Oxidation threshold (air) Graphitization in vacuum

0.7 gm/cm3 350–390 m2 /gm 450

Size

1100

The explosive derived nanodiamond can be used for ultra polishing of eye glasses, contact glasses, hard drive (nickel layer), magnetic ferrite, optical and laser components, diamond knives, gem stones, etc. The surface finish may attain a super smoothness of Ra < 0.2 nm. Another major application is in lubrication coating on engine components, such as piston-cylinder, crane shaft, gears box, pumping components, etc. For the application of engine oil additive, 0.02% addition may reduce 30% of surface friction; and 0.1%, more than 50%. Normally, an abrasive particle can scratch the moving parts of an engine is the size of the particle is in the same magnitude

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Table 6.4. The abrasion resistance enhancement by plating with nanodiamond Coating

Increase in Wear-Resistance, N-Fold where N

Cr Ni Cu Au Ag Sn AI

2–12 5–9 9–10 2–6 4–12 2–4 10–13

of the gap between two moving parts, but for nanodiamond particles, their sizes are much smaller than the gap, so they will not scratch either part. Instead, these particles may be embedded on the wall of the moving part and become the hard facing component. As a consequence, they can protect the engine parts from further mechanical wear or chemical erosion. In addition, the opposing moving parts are now sliding against the hard facing that has a much lower frictional coefficient than any metal component. As the motion of the engine part will receive much less drag, less energy (e.g. gasoline) is needed to power the engine. Another application of nanodiamond is to be the ingredient of composite hard facing for metal parts. Thus, by dispersing nanodiamond in an ordinary electrolyte, the electro deposited metal will incorporate nanodiamond in its matrix. Hence, nanodiamond may be co-deposited by electroplating of gold, nickel or chromium. The coated surface will be much harder and it is highly resistant to chemical corrosion. For example, by dispersing nano-diamond in an electrolyte (e.g. 2.5 g/liter) for Ni plating, 5–9 times increase of wear resistance may be expected. For gold plating with nanodiamond, the thickness may be halved to retain the same appealing luster, but it may come with a 10-fold increase of wear resistance (Table 6.4). Alternatively, nanodiamond may be blended in with copper plating to form a highly thermal conductive heat spreader (e.g. for CPU or laser diode applications). Other applications of nanodiamond include as mechanical reinforcement fillers (e.g. polymer/rubber matrix elastomers,

Chinese Dynamite Diamond

PTFE Teflon matrix lubricant, ceramic matrix composites); hydrogen storage medium for fuel cell battery; nucleation seeds for CVD deposition of diamond; feedstock for high pressure sintering of polycrystalline diamond, protein absorption, separation, etc. 6.6

CHINESE DYNAMITE DIAMOND

Although the West could develop and make advancements in technologies earlier, the Chinese have been notorious in overtaking the manufacturing capacity after learning the secret. Thus, the static high pressure processes for making mesh sized diamonds were perfected by GE Superabrasives and De Beers Industrial Diamond for decades, China became the largest diamond maker at the turn of this century. GE was forced to sell the superabrasives business that became Diamond Innovations. De Beers was compelled to reorganize the diamond business as Element Six. Despite these survival damage controls, Chinese diamond capacity continued to surge. Thus, in 2007, China made more than 80% of the world’s industrial diamonds totaled about 1000 tons (five billion carats). The dynamite diamond may not be immuned to Chinese glutted production. Although the technology was pioneered by Russians in 1960s, Chinese cracked the code in 1990s. Thus, several entities have been engaged in pilot production of dynamite diamonds (e.g. Lanzhou Institute of Chemical Physics of Chinese Academy of Sciences, Beijing Institute of Technology, The Second Artillery College, Southwest Institute of Fluid Physics, Northwest Instutte of Nuclear Technology). It is expected that China will dominate dynamite diamond production in the next decade. The following are summarized Chinese technologies that was presented by Annie Hui and Wang Guangzu at the Fifth Zhenzhou International Conference on Superhard Materials, September 5, 2008 (Table 6.5). The process design parameters include ratios of dynamites, their geometry (size and shape), and loading methods (injection or pressing). The amount (about 10%) of detonation soots appears to be maximized with TNT=RDX. TNT provides the transformed nanodiamond while RDX generated formation pressure that controls the yield, and temperature that affects the size. Larger charges and premixing of nanodiamond seeds tend to increase the yield.

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Table 6.5. Main attributes of dynamites Properties Scientific Name Colour Molecular Formula Molecular Weight

RDX

TNT

Cyclotrimethylenetrinitramine

Trinitrotoluene

White Powder

Yellow Squamose Crystals

C3 H6 N6 O6

C7 H5 N3 O6

222.1

227.13

−0.216

−0.740

Crystal Density/g/cm3

1.816

1.654

Detonation Velocity m/s

8661 (p = 1.765 g/cm3 )

6928 (p = 1.634 g/cm3 )

Detonation Pressure/Gpa

32.6 (p = 1.765g/cm3 )

19.1 (p = 1.634g/cm3 )

3700

3000

5620 (p = 1.77g/cm3)

3000 (p = 1.62g/cm3 )

Performance Ability Calculated/kJ/g

1.81

1.03

Observed/kJ/g

1.716

1.06

908

690

900

790

Oxygen Balance

Detonation Temperature/K Detonation Heat/kJ/kg

Gas Products Calculated/cm3 /g Measured

478

353.9

Weight of Carbon Content/%

Melting Point/K

16.22

37.00

Price/rmb/ton

23000

9000

The nanodiamond yield (Fig. 6.14) is also influenced by the type of protective atmosphere in the reaction chamber. CO2 seems to produce more nanodiamond than N2 , Ar or He. Water or ice may also be used to quench the explosive products so the conversion to graphitic carbon may be minimized. The sealed metal container is the source of contamination so coating the inner side with a refractory metal or ceramics may reduce the amount of vaporized contaminants.

Chinese Dynamite Diamond

Figure 6.14. Particle size distribution of nanodiamond from detonation soot. Table 6.6. Product characteristics Item Specific Surface Area (m2 /g) Particle Size (nm) Shape Dust Colour Nanodiamond Content (%) Density (g/cm3 ) Pore Volume (cm3 /g) Surface Functional Groups Initial Oxidation Temperature (K) Hydrophilic Degree (Mj/mol. G)

Black Soot

Grey Soot

360 ∼ 420 4 ∼ 15 Spherical or Banded Black 52 ∼ 85

278 ∼ 335 3.2 Spherical Grey > 95 3.05 ∼ 3.3 1.314 –OH, –C=O, –CN –COOH –C –O– 803 −3100

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Nanodiamond Applications

Being extremely small and with high amount of surface atoms, nanodiamond has diversified applications. Some applications involve using the superhard properties of diamond. The others may adapt low frictional coefficient of diamond (Table 7.1). 7.1

LUBRICATION OF ENGINE OIL AND MACHINE GREASE

Nanodiamond can reduce significantly the frictional coefficient by coating on the sliding surface. The nanodiamond coating reduces the contact area. Moreover, the inertness of the diamond surface reduces the atomic drag force across the interface of the two relatively moving surfaces (Fig. 7.1). Nanodiamond used as engine oil additive have demonstrated reduced gasoline consumption and increased engine life (Fig. 7.2). These benefits are attributed to the coating of nanodiamond on the walls of the cylinder. As a result, the frictional coefficient is reduced. The electroplating of metal coating using electrolyte with nanodiamond dispersion allows the impregnation of nanodiamond. However, the incorporation may be small (e.g. 5 V%) due to the Brownian motion of the nanodiamond in the liquid medium. The incorporation of nanodiamond can be boosted by electrophoresis with charged nanodiamond in electrolyte. More than 20 V% may be attained after the metal coating. The impregnation of nanodiamond in metal coating can increase the hardness and significantly improve the wear resistance. Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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Table 7.1. Mechanical applications of nanodiamond Super smooth polishing of gems, ceramics, glass, silicon wafers, and surgical knives Impregnation in coatings of gold, silver, copper, and chromium for hard facing Reinforcement of rubber, resins, plastic, PTFE, and metals (Cu, Al)

Figure 7.1. The pin-on-disc sliding experiments and their results in improvements of the tribological properties of nanodiamond coated surfaces.

An application of nanodiamond impregnated coating is for gilding the watch wands or other ornaments. Because the gold is soft, the impregnation of nanodiamond can prevent the scratching of gold (Fig. 7.3). Due to the dramatically increased hardness and strength, the gilded thickness can be reduced in half so the overall cost is comparable with coating with thicker gold. For conventional nickel coating, nanodiamond addition may also improve the wear resistance and corrosion resistance (Table 7.2

Sintering of Micron and Nanodiamonds

Figure 7.2. The schematic illustration of nanodiamond lubrication of two moving parts.

and 7.3). The reduced frictional coefficient is also obvious. Moreover, nanodiamond is conductive. Due to its conductivity, nickel may deposit much quicker and with a finer finish. Such quality may also aid the value of nanodiamond impregnated nickel hardfacing. 7.2

SINTERING OF MICRON AND NANODIAMONDS

Most micron diamonds are formed by pulverizing larger diamond formed under high pressure. There are tons of micron diamonds that are pressed again, this time to sinter them together to form polycrystalline diamond (PCD). There are also tons of mesh or micron diamonds that are used to grind or polish the PCD formed. For sintering to proceed relatively fast and to eliminate most porosity present in the original powder mix, the temperature

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Figure 7.3. The increase of gold hardness with nanodiamond impregnation.

ought to be about 3/4 of the melt point of the powder. Because diamond will not melt below 4000◦ C, so it means the sintering may have to be performed above 3000◦ C. However, if diamond were heated to such a high temperature, it would have been graphitized so the PCD cannot be made unless the pressure is applied to reach diamond’s stability field. The pressure for reaching the diamond stability field increases with the increasing temperature, at a temperature of about 3000◦ C, the pressure ought be about 10 GPa to keep diamond stable. Such a high pressure would make industrial sintering impractical as most ultrahigh pressure apparatuses used today to synthesis diamond can reach a pressure of about 5 GPa. In order to sinter diamond at a pressure of about 5 GPa, molten cobalt is used to infiltrate micron diamond. In this case, cobalt can dissolve diamond at the contact points of micron diamond where the pressure is high and it then precipitate dissolved carbon atoms in regions of original pores. As a result, micron diamond grains are

Sintering of Micron and Nanodiamonds

Table 7.2. The properties enhancement of nanodiamond dispersed nickel coating Electric Plating

Prush Plating Micro hardness (Mpa) Dynamic friction coefficient Static friction coefficient Wearing rate (MM3/Nm) Wearing depth (µm) Adhesive strength (N)

Brush Plating

Nickel– Nickel– black Nickel graphite powder (3–5 µm) (3–5 µm) (0.2–0.5 µm) Nickel

Nickel– graphite

Nickel– black powder

3180

2304

5123

1291

4785

0.75–0.90 0.14–0.16 0.18–0.20

2293

0.22–0.30 0.15–0.20 0.21–0.27

0.25–0.37 0.20–0.25 0.25–0.31

18.53 5

4

0

30

30

30

11.06

10.61

gradually merging together with new diamond forming bridges across all voids. Such a PCD contains inter-grown micron diamond particles. Normally, cobalt may come from the melting of a cemented tungsten carbide substrate that is placed adjacent to the volume of micron diamond. When the temperature becomes higher than the melting point of cobalt, it will melt and infiltrate into micron diamonds where sintering is taking place. This method is ideal in making PCD/cWC compact that is widely used as cutting tools, drill bit cutters, and wire drawing dies. The composite will allow the hard diamond to cut but it is supported by a tough cWC that does not break. Alternatively, cobalt may be mixed with the diamond powder beforehand. In this case, the path of infiltration is shorter. This is particularly useful for sintering smaller diamond grains, such as for making PCD with submicron grain size. An important consideration of high pressure sintering of PCD is grain size. Normally, the larger the size of the diamond grains, the easier for cobalt to infiltrate. But for precision cutting applications

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Table 7.3.

The effects of doping chromium coating with nanodiamond

Tool

Material being processed

Efficiency increased by (times)


8 ∼ 20 drill

Glass reinforced plastic, copper, stainless steel Steel, aluminum glass

1.9–20.0

reinforced plastic, brass steel

1.6–2.40, 2.0–4.0 1.4–1.8

Pig iron powder, stainless

9–15

Steel powder, ceramic Powder for radio, bakelite

4–5 2–3

Stamping die, coldextruded, sheared, extended Cold stamping die

Saw blade, needle file Internal combustion engine cylinder

1.6–1.80, 2.0–3.0

4.0–8.0 2–3

and wire drawing practices, the finer the diamond grains, the better finish of the work surface. Hence, a choice of grain size has to be made for making PCD for various applications. The presence of cobalt in PCD, although useful for many mechanical applications, has severely limited the applicability of PCD. As cobalt is the catalyst for making diamond, it can also catalyze the back conversion unless the PCD is pressed in the stability field of diamond. So for all applications performed at low pressures, there is threshold temperature at about δ that may not cross, otherwise, diamond will be back converted to amorphous carbon. Such back conversion cannot only soften the diamond cutter, but also, the sudden volume expansion (up to 50%) may easily crack the PCD, or cause delamination of the compact. Most applications would expose PCD to temperature above 700◦ C. For example, the cutter’s tip will be hotter than this temperature if the cutting speed is high or if the work material is abrasive. Alternatively, the process of making PCD tools, e.g. brazing it to join the shank, will also expose PCD above the threshold

Sintering of Micron and Nanodiamonds

temperature. Hence, the industry has been scrambling on the best way to avoid such a problem of thermal degradation. So far the approaches have been to eliminate (e.g. by acid dissolution) or replace cobalt (e.g. by silicon) as the sintering aid of PCD. But these so called thermally stable PCD have their own weaknesses. For one thing they are not as tough and finer grained PCD cannot be produced. Hence, cobalt sintered PCD is still dominates the market. However, the sintering is a process of powder consolidation by eliminating surface area or grain boundaries. So the smaller the grain size, the faster the powder can sinter. As a consequence, a lower temperature is needed to sinter a finer grained powder. In fact, by reducing the size from 10 to 1 µm, the sintering temperature may decrease by 200◦ C. However, as the grain size become smaller, more dramatic decrease of sintering temperature may be realized. Hence, by using nano-grained diamond, the sintering temperature may be lowered to about 1500◦ C. In this case, no sintering aid is necessary because the increase of diamond surface area itself can drive the sintering (Fig. 7.4). Commercial PCD contain grains of from 50 µm down to about 2 µm. Even with the grain as small as 1 µm, its surface area is only about 0.02 m2 /gm. On the other hand, nanodiamond (e.g. 5 nm) may have a surface area of 400 m2 /gm. The sintering of nanodiamond with such a high surface area may require a much lower temperature and so the sintering may be performed by using conventional ultrahigh pressure apparatuses. The direct sintered nano-PCD has many advantages. For example, it has a much higher thermal stability, most likely >1200◦ C. Moreover, its hardness and wear resistance must be much higher than conventional PCD so the tool life can be dramatically increased. In addition, with its nano grains in random orientation, the isotropic wear pattern should allow it to produce a mirror finish without requiring further polishing. There are other possible novel properties of this nano-PCD. For example, it may be highly transparent in a wide spectrum of electromagnetic radiations. In fact, with proper sintering and to allow enough grain coarsening, optical transparent PCD may be made. Such an optical transparent PCD can be used as window materials for electromagnetic radiation of different energies. Another possibility is that nano PCD may become the best heat spreader ever invented. With the trend of increasing power of

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many electronic devices (e.g. CPU, laser diode), the semiconductor chip can no longer be cooled by conventional copper heat spreader. Although CVD diamond may be an excellent heat spreader, but the cost of deposition may be prohibitive. Moreover, the larger sized electronic devices now require a heat spreader thicker than 2 mm. No CVD diamond film today has been able to penetrate this thickness barrier. Hence, nano PCD with its thickness that can be designed will be the heat spreader for the future. The cost of nanodiamond used to be more than $10/carat, so their price would be prohibitive for making nano PCD. But today the commercial products of nanodiamond is down to less than $1/carat that is on par with a high grade pulverized micron diamond. Hence, the time to make nano PCD has come. In order to make good use of a cemented tungsten substrate to reinforce the brittle nano PCD, cobalt infiltration may still apply. But this time the micron PCD is used to support nano PCD. For example, micron PCD may form an interlayer between nano PCD and cWC. There are many possibilities to make products based on this three-layer concept.

7.3

ADAMANTANE MOLECULES

A simple carbon structure that may be formed with high stability is adamantane that contains 10 carbon atoms in diamondoid structure. In fact, the very word “diamond” was derived from the Greek admas that means invincible or unbreakable. Adamantane or its derivatives may be found in petroleum residue. The first artificial synthesis of adamantine was accomplished by V. Prelog in 1941 (Prelog, V., Seiwerth, R., 1941, Uber die Synthese des Adamantane, Berichte 74, 1769–1772). Adamantane melts at 270◦ C that is a small fraction of about 4000◦ C for diamond. This shows the power of suppressing melting point by moving carbon atoms to the surface with hydrogen termination (Fig. 7.5). Adamantane and its derivatives (e.g. amantadine, remantadine, memantine) have been used as drugs for killing influenza viruses and for treating brain diseases (e.g. Alzheimer, Parkinson). It has been noticed that such drugs are not poisonous to human body.

Adamantane Molecules

Figure 7.4. Novel designs of future nano-PCD with the interlayer of micron PCD that is supported by conventional cWC. Such designs can combine the longest tool life with the most smooth surface finish.

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Properties Molecular formula Molar mass Appearance Density Melting point Solubility in water Solubility in other solvents

C10 H16 136.23 g/mol White to off-white powder 1.07 g/cm3 (20◦ C), solid 270◦ C (543 K) Poorly soluble Soluble in hydrocarbons

Figure 7.5. The molecular structures and properties of adamantane Source: Wikipedia.

Figure 7.6. Adamantane moiety used as photoresist.

Adamantane molecules may be further bonded to absorbents (e.g. H, OH, NH2 ) or organic radicals (e.g. CH3 , NH3 Cl). Being diamondoid, the compound is strong and it can be transparent. Such features allow diamondoid derivatives to be used as optical lenses. For example, adamantane polymers are used as UV photoresist materials for ArF excimer laser with a wavelength of 193 nm (Fig. 7.6). Additionally, such derived molecules may be polymerized to form strong threads that can be weaved into fabrics (Fig. 7.7). Such

Other Diamondoids

Figure 7.7. Adamantane derivatives (top) and the polymerization of diamantine derivatives (bottom). Source: Industrial Diamond Review, 2008, 1, 20–22.

nanodiamond-impregnated fabrics are superhard cloths that are scratch free and water repelling. Moreover, they can be stacked up to make bulletproof vests that are lighter and softer than Kevlar or Nomex. Nanodiamond impregnated fabrics may also be used as armor protected seats for helicopters. With further additions and repetitions, diamondoid structure can be augmented to take forms of real diamond. Diamondoid molecules are stable to body fluids and they are benign to human body. An ideal application is to use diamondoid structure as the anchor for the delivery of amino acids, proteins, genes, and drugs. 7.4

OTHER DIAMONDOIDS

A methane (CH4 ) molecule contains a carbon atom with diamondoid bonds (sp3 ). It is the simplest molecule with carbon caged by hydrogen termination. More complicated cage structures may be derived from the collisions of methane molecules. In addition, bucky ball structures or non-hydrocarbons may be incorporated as well (Fig. 7.8).

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Figure 7.8. Additional diamondoid structures. Source: Industrial Diamond Review, 2008, 1, 20–22.

7.5

NANODIAMOND COSMETICS

Nanodiamond is the best hardener for polymers. The wetting and dispersing of diamond by organic materials are excellent, particularly with diamond surfaces terminated by hydrogen or fluorine. One example of nanodiamond-dispersed polymer is diamondimpregnated Teflon (DiT). DiT can be used to coat on metals or glass to render scratch free surface that is also water repelling. Another example is nanodiamond-impregnated paint (DiP) that may be applied on automobile’s exterior. Again, the paint can be scratch free and rain dry. As nanodiamond is non-poisonous, a big area of application is nanodiamond-impregnated cosmetics.

Nanodiamond Cosmetics

Figure 7.9. The diamondoid carrier with DNA strands anchoring on its surface.

Each carbon atom on the surface of nanodiamond has at least one dangling electron that may bond to a light element, such as H, N, or O. As biological materials are made of carbon compounds, almost all life sustaining chemicals can be absorbed by nanodiamond. Thus, nanodiamond is an excellent absorbent for amino acids, proteins, platelets, and DNA (Fig. 7.9), to name a few example. Due to the smallness of dynamite nanodiamond compared to even a virus, nanodiamond may be used as a drug to exterminate virus. Human body contains about 1/4 of carbon by weight. Nanodiamond as carbon is non-poisonous (Fig. 7.10). Moreover, it is not only cancer inactive, but also a catalyst for promoting drug effectiveness. For examples, nanodiamond has been used to treat burning skin infections, food poisons, and intestine malfunctions with good results.

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Chapter 7 Nanodiamond Applications

Figure 7.10. Various carbon structures that may be derived by the collision of simple molecules of carbon compounds.

Nanodiamond Cosmetics

Figure 7.11. The relative scale of nanodiamond compared to microbes and cells.

In comparison to tiny nanodiamond particles, the human cells appear to be colossal (Fig. 7.11). Much larger than microbes, nanodiamond cannot harm normal cells. Nanodiamond cannot penetrate the cell’s membrane. One the contrary, nanodiamond can stick to the NDA of bacteria or RNA of viruses. For example, nanodiamond has been used to cure gum diseases. It is conceivable that nanodiamond may also be effective in attaching genes and hence it is capable to kill drug resistant viruses (e.g. HIV, SARS). Nanodiamond is perceived to be able to inhibit the growth of malignant, or to kill cancer cells. It has also been reported that nanodiamond may relieve mental stresses and body pains. Additional remedies include the restoration of digestion disorders, improvements of blood circulations, the enhancement of immune systems, or even the extension of patient’s lives. Nanodiamond surfaces may be modified by the termination of various chemical radicals (Fig. 7.12). If the termination is nonpolar (e.g. H or F), the surface is hydrophobic. On the other hand, if hydrogen bonds may be formed on the absorbents (e.g. O or S), the surface is hydrophilic. The water repelling or wetting behavior is important for dispersion in liquid. The surface modification also allows the attachment of organic molecules, such as amine, carboxyl, carbonyl, hydroxyl, amide, nitrile, sulfide, epoxyl, phosphryl, sulfate, imide, etc. For nanodiamond just formed, the surface contain ample C–O, C=O, C–N, C=N, and OH. Although these radicals are wettable by water, they tend to agglomerate. After boiling in sulfuric acid, the dispersion in water improved, so is the dispersion. However, if an organic polymer is used to as the matrix material, nanodiamond should be heat treated beforehand in hydrogen, fluorine or chlorine to render the surface hydrophobic.

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Chapter 7 Nanodiamond Applications

Figure 7.12. The attachment of surface absorbents on nanodiamond (top diagram) and the major surfactant radicals (bottom diagram).

7.6

COSMETIC APPLICATIONS OF NANODIAMOND

A remedial healthcare nanodiamond composition can include a biologically acceptable carrier and a plurality of nanodiamond particles dispersed in the carrier. Depending on the carrier, an optional

Cosmetic Applications of Nanodiamond

dispersant can be used. The remedial healthcare composition can be formulated as a dental filling, lotion, deodorant, toothpaste, shampoo, antibiotic, dermal strip, skin cleanser, or exfoliant. Other similar compositions can also be formulated to incorporate nanodiamond particles given the disclosure herein. Of course, the particular biologically acceptable carriers and other components may vary depending on the specific formulation. However, the following discussion illustrates several currently preferred nanodiamond compositions and associated benefits. Nanodiamond particles typically carry an electrical charge that leads to aggregation and flocculation of particles. In most cases, this aggregation of nanodiamond particles is undesirable. Therefore, an optional dispersant can be included which improves the uniformity of nanodiamond distribution. In this way, a colloidal suspension can be formed in which the nanodiamond particles remain substantially uniformly dispersed over an extended period of time, e.g. typically months or years. Preferably, the nanodiamond particles remain dispersed during the useful shelf life of the particular composition. The dispersant can be provided in the form of a specific compound separate from the carrier in a liquid nanodiamond composition. However, for highly viscous compositions the carrier can also be the dispersant. Thus, in some embodiments such as a solid deodorant, toothpaste, soaps, viscous nail polish, and the like, the carrier can provide sufficient viscous support to prevent agglomeration and/or settling of the nanodiamond particles. Any suitable dispersant can be used which is compatible with a particular carrier. However, several non-limiting examples of dispersants include anionic surfactants, electrolytes, alcohols, metal chlorides and nitrates such as Al, Na, Ca, and Fe chlorides and nitrates, and the like. Other suitable nanodiamond dispersants include isopropyl triisosteroyl titanate, polyethylene-oxides, and other anionic surfactants. One specific suitable surfactant that can be used is stearalkonium hectorite. The dispersant can also provide other properties to a composition such as pH control. Further, the amount of dispersant can depend on the amount of nanodiamond present and the viscosity of the composition. However, as a general guideline, the remedial healthcare composition can include from 1 wt.% to about 30 wt.% dispersant. Suitable nanodiamond particles can have an average size of from about 0.5 nm to about 50 nm. In some embodiments the

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plurality of nanodiamond particles can have an average size from 1 nm to about 10 nm, preferably from about 4 nm to about 8 nm, and most preferably about 5 nm. The concentration of nanodiamond particles will vary depending on the composition and the desired effect, as discussed in more detail below. As a practical matter, the plurality of nanodiamond particles is typically about 1 wt.% to about 80 wt.% of the composition. Nanodiamond particles can be formed using a number of known techniques such as shock wave synthesis, CVD, and the like. Currently preferred nanodiamond particles are produced by shock wave synthesis. 7.7

THE DENTAL USE OF NANODIAMOND

The remedial healthcare composition may be formulated as a dental material. The dental material can be formulated for use as a filling, veneer, reconstruction, and the like. The dental material can include an acceptable carrier and a plurality of nanodiamond particles. Acceptable carriers are known in the art and can include, for example, composite resins, polymeric resins, ceramics, and other known carriers. In addition, the dental material can include additives such as colorants, fillers, etc. Although dental compositions can include colorants or additives to provide whiteness for cosmetic purposes, the primary purpose of the dental material is remedying a defect in a tooth and preventing future decay. Such dental materials are known and a more detailed description can be found in US Patent Nos. 6,020,395; 6,121,344; and 6,593,395, which are each incorporated herein by reference in their respective entireties. Dental compositions include a plurality of nanodiamond particles. The nanodiamond particles can provide additional mechanical strength, as well as an appearance that approximates natural enamel when dry. The nanodiamond particles can be present in the composition at from about 1 wt.% to about 60 wt.%, and preferably from about 10 wt.% to about 40 wt.%. In addition to mechanical strength, introduction of nanodiamond particles to a composition can provide a number of beneficial properties. One of such beneficial properties is an impressive ability of nanodiamonds to absorb oil and other organic materials. Carbon atoms are very small (about 1.5 Å); thus, various forms of carbon can pack to form a high atomic concentration. In fact, diamond has the highest atomic concentration (176 atom/nm3 ) of all

The Dental Use of Nanodiamond

known materials. This high atomic concentration contributes to the exceptional hardness of diamond. As a result, any given surface area of a nanodiamond particle can include many more atoms than other nanoparticles of the same size. Diamond is among the most inert materials known. Specifically, at temperatures below about 500◦ C, diamond typically does not react with other materials. Further, diamond is compatible with most biological systems. This is due, at least in part, to the sp3 bonding of diamond and the similar bonding of most biological materials containing roughly around 25% carbon in sp3 bonding. As such, diamond is ideal for use in medical applications, e.g. artificial replacements (joint coatings, heart valves, etc.), and will not deteriorate over time. Although diamond is highly stable, if the nanodiamond surface is free of adsorbent or absorbent, i.e. clean, it is thought that carbon atoms on the surface contain unpaired electrons that are highly reactive. As a result, nanodiamond particles can readily bond to and effectively absorb a variety of atomic species. For example, small atoms such as H, B, C, N, O, and F can be readily adsorbed on the nanodiamond surface, although other atoms can also be absorbed. Hence, nanodiamond particles, with their vast number of surface atoms, can hold a large amount of such adsorbed atoms. For example, nanodiamond particles are capable of absorbing almost as many hydrogen atoms as the number of carbon atoms. Thus, nanodiamond particles can be used as storage sites for hydrogen. In addition, those small atoms are building blocks, e.g. H, CO, OH, COOH, N, CN and NO, of organic materials including biological molecules. Consequently, nanodiamond particles can readily attach to amino acids, proteins, cells, DNA, RNA, and other biological materials, and nanodiamond particles can be used to remove skin oils, facial oils, compounds that result in body odor, bacteria, etc. Further, nanodiamonds are typically smaller than most viruses (10–100 nm) and bacteria (10–100 µm). Therefore, nanodiamond can be used to penetrate the outer layers of viruses and bacteria and then attach to RNA, DNA or other groups within the organism to prevent the virus or bacteria from functioning. Similarly, nanodiamond can be used in conjunction with known drug delivery mechanisms to treat cancer or acquired immune deficiency syndrome.

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7.8

NANODIAMOND SKIN LOTION

A method of binding biological molecules can include formulating a nanodiamond composition containing a plurality of nanodiamond particles. The nanodiamond particles can be dispersed in a biologically acceptable carrier. The nanodiamond composition can then be contacted with a biological material such that at least a portion of the biological material is bonded to the nanodiamond composition. Examples of biological materials include organic oils, sebum, bacteria, epithelial cells, amino acids, proteins, DNA, and combinations thereof. Once the biological material is bonded to nanodiamond, the nanodiamond composition can be removed from the surface or environment. The nanodiamond composition can then be discarded or further treated to identify or otherwise utilize the absorbed biological material. In one embodiment, the nanodiamond composition can be formulated as any of the following products: deodorant, toothpaste, shampoo, antibiotic, dermal strip, DNA test strip, or skin cleanser. Similarly, nanodiamond compositions can be remedial healthcare compositions formulated for skin care. Examples of skin care formulations include lotions, facial tissue lotion, deodorant, dermal strip, skin cleanser, soap, antibiotic, and exfoliant. Alternatively, the remedial healthcare composition can be formulated as toothpaste, shampoo, or other similar product. The remedial healthcare composition can be formulated as a lotion. The lotion can include an acceptable carrier and a plurality of nanodiamond particles. Acceptable carriers are known in the literature, and can include, for example, glycerin, alcohols, water, gels, combinations of these materials, and other known carriers. In addition, the lotion can include additives such as fragrance, colorants, vitamin E, herbal supplements, antibiotics, UV absorbers, sun-block agents, and the like. A more detailed description of various lotions can be found in US Patent Nos. 6,207,175 and 6,248,339, which are each incorporated herein by reference in their respective entireties. The nanodiamond particles can be present in the composition at from about 1 wt.% to about 40 wt.%, and preferably from about 2 wt.% to about 15 wt.%. In a similar embodiment, the remedial healthcare composition can be formulated as a lotion for application in a facial tissue. The facial tissue lotion can include an acceptable carrier and a plurality

Nanodiamond Skin Lotion

of nanodiamond particles. Acceptable carriers are known in the art and can include, for example, glycerin, alcohols, water, gels, combinations of these materials, and other known carriers. In addition, the lotion can include additives such as fragrance, colorants, vitamin E, herbal supplements, antibiotics, UV absorbers, sun-block agents, and the like. A more detailed description of facial lotion formulations can be found in US Patent No. 6,428,794, which is incorporated herein by reference in its entirety. The presence of nanodiamond particles can improve absorption of oils and undesirable deposits from the skin without abrasiveness associated with larger diamond particles. The nanodiamonds can be present in the facial tissue lotion composition at from about 1 wt.% to about 30 wt.%, and preferably from about 2 wt.% to about 15 wt.%. The remedial healthcare composition can be formulated as a deodorant. The deodorant can include an acceptable carrier and a plurality of nanodiamond particles. Acceptable carriers can vary considerably depending on the specific formulation. For example, deodorants can be formulated as a solid, gel, cream or the like. Suitable carriers can include, but are not limited to, dimethicones, silicone fluids (e.g. siloxanes), glycerin, alcohols, water, gels, sorbitols, and other known carriers. In addition, the deodorant can include additives such as fragrance, stabilizing agents, pH or buffer agents, solvents, antiperspirant agents, and the like. A more detailed description of various deodorant formulations can be found in US Patent Nos. 5,968,490; 6,358,499; and 6,503,488, which are each incorporated herein by reference in their respective entireties. When included in deodorant compositions, the nanodiamond particles can be present in the composition at from about 1 wt.% to about 40 wt.%, and preferably from about 2 wt.% to about 20 wt.%. The remedial healthcare composition can be formulated as a dermal strip. Dermal strips are typically formed having a backing substrate with an oil or sebum absorbing composition coated thereon or within the substrate. The dermal strip can include an acceptable carrier and a plurality of nanodiamond particles on the substrate. Acceptable carriers are known in the literature and can include, for example, hemp, pulp papers, porous polymeric thermoplastics, and other known carriers. Additives such as herbal extracts, vitamins, antibiotics, anti-inflammatories, fragrance, and the like can also be included. The nanodiamond particles can be

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present in the composition at from about 1 wt.% to about 40 wt.%, and preferably from about 2 wt.% to about 15 wt.%. The remedial healthcare composition can be formulated as a skin cleanser. The skin cleanser can include an acceptable carrier and a plurality of nanodiamond particles. Acceptable carriers are known in the literature and can include, for example, glycerin, alcohols, collagen, elastin, gels, copolymeric materials, and other known carriers. In addition, the skin cleanser can include additives such as fragrance, colorants, vitamin E, herbal supplements, antibiotics, UV absorbers, hydrating agents, sun-block agents, exfoliating agents, and the like. A more detailed description of various skin cleansers can be found in US Patent Nos. 3,944,506; 4,048,123; 4,737,307; and 6,518,228, which are each incorporated herein by reference in their respective entireties. The nanodiamond particles can be present in the skin cleanser composition at from about 1 wt.% to about 50 wt.%, and preferably from about 5 wt.% to about 30 wt.%. The remedial healthcare composition can be formulated as an antibiotic composition. Such antibiotic compositions can be formed as a skin cleanser, lotion, wound dressing, and the like similar to the other compositions described herein. Nanodiamonds in antibiotic and lotion compositions can also increase healing of skin and removal of damaged skin such as with sunburns and scar tissue. 7.9

TOOTHPASTE AND NAIL POLISH

Alternatively, the remedial healthcare composition can be formulated as toothpaste including an acceptable carrier and a plurality of nanodiamond particles. Basic formulation of toothpastes is known in the literature. Common acceptable carriers can include, for example, glycerin, sorbitol, silicas (e.g. amorphous, hydrated, etc.), thickening agents such as carrageenan and salts of cellulose ethers, alcohols, water, gels, combinations of these materials, and other known carriers. In addition, the toothpaste can include additives such as sodium fluoride, fragrance, flavors, colorants, herbal supplements, and the like. The nanodiamond particles can be present in the composition at from about 1 wt.% to about 40 wt.%, and preferably from about 2 wt.% to about 15 wt.%. Nanodiamond added toothpaste has another advantage, as nanodiamond is known to cure gum disease.

Toothpaste and Nail Polish

The remedial healthcare compositions can also be formulated as a shampoo. The shampoo can include an acceptable carrier and a plurality of nanodiamond particles. Acceptable carriers are known in the literature and can include, for example, surfactants, alcohols, water, glycerin, gels, combinations of these materials, and other known carriers. In addition, the shampoo can include additives such as fragrance, colorants, vitamin E, herbal supplements, and the like. The nanodiamond particles can be present in the composition at from about 1 wt.% to about 40 wt.%, and preferably from about 2 wt.% to about 15 wt.%. Optional bubbling agents can also be added to the nanodiamond compositions. Suitable bubbling agents can be included to increase contact of unsaturated nanodiamonds with a biological material. For example, over time, nanodiamond particles near a surface can become saturated with biological or other material. The presence of vapor bubbles can improve the rate at which such saturated nanodiamonds are removed from a surface. This can be advantageous in maximizing the effect of nanodiamonds in skin cleansers, deodorants, shampoos, soaps, toothpaste, and the like. Another cosmetic nanodiamond composition can be formulated including a cosmetically acceptable carrier and a plurality of nanodiamond particles dispersed in the carrier with a dispersant. For example, the cosmetic composition can be formulated as a nail polish, eyeliner, lip-gloss, or exfoliant (Fig. 7.13). Preferably, the cosmetic nanodiamond composition can be formulated as a nail polish. A nanodiamond nail polish composition can include a cosmetically acceptable carrier and a plurality of nanodiamonds dispersed therein. Additives can also be included such as, but not limited to, dispersant, pigment, plasticizer, bubbling agent, solvent, stabilizer, UV stabilizer, moisturizers, fragrances, and combinations thereof. Additional considerations and materials for nail enamel compositions generally are discussed in US Patent Application No. 2003/0064086 and U.S. Patent Nos. 5,725,866; 5,882,636; and 6,352,687, which are incorporated herein by reference in their entireties. In one specific embodiment, the nanodiamond nail composition can include a polymeric resin, plasticizer, pigment, nanodiamonds, dispersant, solvent, and a UV stabilizer. Suitable cosmetically acceptable carriers can include, but are not limited to, polymeric resins such as nitrocellulose resins,

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cellulose acetate resins, vinyl resins, acrylate resins, polyester resins, aldehyde derivatives such as tosylamide/formaldehyde resins, and other similar polymeric resins. Other resins can also be used which provide mechanical strength to the nail composition upon drying. Typically, such carriers can comprise from about 5 wt.% to about 60 wt.% of the nanodiamond nail polish composition. Many of the above listed cosmetically acceptable carriers are somewhat rigid. Thus, softer resins can be combined with more rigid resins in order to provide mechanically sound nail enamel with some degree of flexibility. Additionally, optional plasticizers can be added to further increase the flexibility of the nail enamel upon drying. Addition of such softer resins and plasticizers can reduce premature cracking and chipping. Examples of suitable plasticizers can include benzoates, stearates, phosphates such as tricresyl phosphate, phthalates such as dibutyl phthalate and dioctyl phthalate, camphor, and the like. The nanodiamond nail compositions typically include a solvent that provides a fluid, or spreadable composition that is suitable for application to a nail. The solvent then evaporates once applied to provide a durable hardened film on the nail, wherein the resin acts as a binder for the remaining components, e.g. pigments, nanodiamonds, etc. Non-limiting examples of common solvents that are suitable include acetates such as butyl acetate and ethyl acetate, alcohols such as isopropyl alcohol, ketones, toluene, xylene, and combinations of these solvents. One of the primary purposes of nail compositions can be to provide an aesthetically pleasing appearance. Specifically, various additives can be included which provide a wide range of colors and/or effects to the applied nail composition. For example, pigments can be included which provide a specific color to the applied nail composition. Organic pigments are most common; however, inorganic pigments can also be used. Such pigments are well known in the literature and can be chosen accordingly to provide a desired color and consistency. Optional particulate materials such as mica, metal oxides, diamonds and the like can be added to provide a sparkle or other effects. For example, larger particulates create a sparkle appearance, while progressively smaller particulates can create a shimmer, or even pearlescent appearance.

Toothpaste and Nail Polish

The cosmetic nanodiamond composition can also include a dispersant such as those discussed above. One specific suitable dispersant for nanodiamond nail compositions is stearalkonium hectorite. In addition to the above-recited advantages of including nanodiamondparticles in a nail formulation, the nanodiamond particles can also improve the durability of the applied nail compositions. Specifically, nanodiamonds can provide increased resistance to chipping and wear, e.g. typically a nanodiamond nail polish can last from about three to ten times longer than typical nail lacquer formulations. The nanodiamonds can be included in the cosmetic nanodiamond compositions at about 1 wt.% to about 50 wt.% of the composition, and preferably from about 2 wt.% to about 30 wt.%. One example of nail polish composition is prepared including 20 wt.% nitrocellulose resin, 20 wt.% ethyl acetate, 25 wt.% toluene and 10 wt.% isopropyl alcohol solvents, 9 wt.% dibutyl phthalate plasticizer, 5 wt.% stearalkonium hectorite, and 3 wt.% benzophenone UV stabilizer. The remaining weight percent includes an aqueous aluminum chloride suspension of 60 wt.% nanodiamond particles.

Figure 7.13. Nanodiamond cosmetic products and the Chinese product instruction.

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Biological Applications of Diamond 8.1

PROPERTIES OF DIAMOND FOR BIOLOGICAL APPLICATIONS

The following superior properties of diamond make it a special and promising material that can be widely applied to biological fields: (A) Diamond is a type of superhard material. Table 8.1 shows some physical properties of diamond compared to Titanium and stainless steel. The hardness of diamond is about 50 times of Titanium and stainless steel. The toughness of diamond makes it suitable in applications in biomedical fields such as implant, cutting tools for surgeries, etc. (B) Chemical inertness is an important factor for diamond to be applied in biology, since the biological environment is corrosive. Azevedo and coworkers’ corrosion tests (Fig. 8.1) of diamond films grown on Ti6Al4V alloy show that the diamond films have a very good chemical resistance to the corrosive liquid.2 (C) Biocompatibility cannot be ignored when diamond is applied to biology. Yu and coworkers investigated the biocompatibility of fluorescent nanodiamond (FND) powder with size of 100 nm in cell culture and found low cytotoxicity in kidney cells.3 Further, Schrand and coworkers4 showed that nanodiamond (ND) with small size of 2–10 nm are not toxic to a variety of cells through mitochondrial function (MIT) and luminescent ATP production assays. Figure 8.2 shows that after the Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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Table 8.1. Comparison of properties of chemical vapor deposited (CVD) diamond, titanium and 316 stainless steel Properties

Hardness (kg mm−2 ) Young’s modulus (GPa) Bulk modulus (GPa) Thermal conductivity 0–100◦ C (Wm−1 K−1 ) Thermal expansion (×10−8 K−1 )

CVD Diamond

Titanium

316 Stainless Steel

10000 1000 442 20

230 120.2 108.6 0.21

210 215.3 166 0.16

1.1

8.8

17.2

Source: From Ref. 1 and references therein.

Figure 8.1. Scanning electron microscopy (SEM) images of diamond films deposited on Ti6Al4V alloy after (a) 1-month immersion in an isotonic NaCl solution, (b) 1 month immersion in a Ringer’s physiological solution, and (c) 2 months in the 2% HCl. Source: From Ref. 2.

Properties of Diamond for Biological Applications

Figure 8.2. Incubation of cells with two types of ND after 24 h viewed by light microscopy: (A) control; (B) 100 µg/ml ND-raw; (C) 100 µg/ml ND-COOH. Scale bars are 20 µm. Source: From Ref. 4.

incubation of cells with NDs, cell morphology is unaffected by the presence of NDs while NDs are seen surrounding the cell borders and attached to neurite extensions. (D) Excellent optical property is necessary for diamond to be applied as a biomarker or a biolabel. There are impurity sites within core, defects in the diamond or sp2 clusters on the ND surface. With the light excitation, the ND will emit light with different frequency due to different type of impurity sites. Figure 8.3 shows some of these processes.5 Raman spectrum of diamond also exhibits a sharp peak located at 1332 cm−1 for phonon mode of the sp3 bonding carbons.19 (E) Chemical modification of diamond surface is essential for diamond to be applied as potential biosensor or biochip,7 or a

Figure 8.3. Schematic of energy levels within the diamond band gap capable of undergoing excitation and photoluminescence. Grey wave arrows represent light absorption or emission; black wave arrows is for non-radiative energy loss. Source: From Ref. 5.

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substrate to immobilize biological molecules. Diamond surface can be hydrogen-terminated by exposing the surface to a 13.5-MHz inductively coupled hydrogen plasma (15 torr) at 800◦ C.6 With the hydrogen-terminated nanocrystalline diamond, Yang and coworkers successfully designed a chemical procedure to attach DNA onto the diamond surface.8 Figure 8.4 shows the schematic diagram of the attachment. Recently, Chang and coworkers9 carboxylated the ND with the size of 5–100 nm in diameter using the following method: The diamonds were heated in a 9:1 mixture of concentrated H2 SO4 and HNO3 at 75◦ C for 3 days, and subsequently in 0.1 M NaOH aqueous solution at 90◦ C for 2 h, and finally in 0.1 M HCl aqueous solution at 90◦ C. In Fig. 8.5, Holt showed various surface modifications of ND starting from carboxylated nanodiamond.5 Chemically modified diamond has good physical aborption properties including hydrophobic and hydrophilic interaction, which can be used to immobilize biomolecules (see, e.g. Section 7.3). Figure 8.6 shows the physical properties on hydrogen- and oxygen-terminated diamond.26 The following sections will introduce the applications of diamond including diamond film, nanodiamond, nanocrystalline diamond film, and diamond-like carbon in biological fields.

8.2

DIAMOND IN BIOMEDICAL APPLICATIONS

Due to its hardness, chemical intertness, thermal conductivity, and low cytotoxicity, diamond could be applied as coating materials of implants, other surgery tools, etc. in biomedical fields. In 1995, for the first time, Zolynski ´ and coworkers implanted orthopedic screws, coated with nanocrystelline diamond film (NCD) to a patient with a complex fracture of femoral bone.10 Figure 8.7 shows the implanted screws in an X-ray radiogram.11 After surgery, no ejection was observed, whereas the standard metal implants were rejected twice. Another implantation application of diamond is that an endoprothesis of hip joint, coated with NCD film was successfully implanted to a living organism (Fig. 8.8),11 after positive results

Diamond in Biomedical Applications

Figure 8.4. Sequential steps in DNA attachment to diamond thin films. Source: From Ref. 8.

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Figure 8.5. Schemes for the chemical modification of carboxylated nanodiamond ND. Source: From Ref. 5.

Figure 8.6. Schematic diagram of the H- and O-terminated (0 0 1) diamond surfaces and their physical properties. Source: From Ref. 26.

Diamond in Biomedical Applications

Figure 8.7. An X-ray radiogram taken after an implantation of orthopedic screws, coated with NCD film, into a human organism. Source: From Ref. 11.

Figure 8.8. Endoprothesis of a hip joint, coated with NCD film. Source: From Ref. 11.

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of preliminary surface studies of it, carried out in Lyon and in Bratislava. With the development of CVD, diamond coating is an attractive method to improve the cutting performance and tool life. Jackson and coworkers12 used hot filament CVD (HFCVD) technique to coat tungsten carbide (WC-Co) dental burs with diamond film and examined the cutting performance of these dental burs by drilling materials such as borosilicate glass, acrylic teeth, and natural human teeth. Figure 8.9 shows the dental bur drilling machine. Figures 8.10 show that diamond film is uniformly coated onto the cylindrical substrate surface after HFCVD. After drilling human tooth materials, the tooth materials such as dentine clog interstices on the bur, reducing its abrasive performance. The life of the burs was measured by comparing the amount of flank wear of dental bur. Figure 8.10(C) shows that HFCVD diamond bur has the best performance. These experiments show that HFCVD diamond coated dental bur gives longer bur life and a much better quality of drilling and machining. Catheter ablation is an invasive treatment method for cardiac arrhythmia by thermal destruction of the structures causing arrhythmia. Figure 8.11 shows the sketch of conventional ablation catheter setup and the temperature sensor.13 To improve the accurate measurement and controllability of the temperature so as to control the size of the lesion, Müller and coworkers designed a diamond-based heater system shown in Fig. 8.12.13 Figure 8.13

Figure 8.9. Dental bur drilling machine and air-operated spindle unit attached to the clamping device and driving unit attached to the teeth. Source: From Ref. 12.

Diamond in Biomedical Applications

Figure 8.10. (A) (1 1 1) faceted CVD diamond-coated dental bur. (B) HFCVD diamond-coated dental bur after drilling human tooth materials. (C) Flank wear of burs matching human tooth materials. Source: From Ref. 12.

Figure 8.11. (a) Sketch of a conventional ablation catheter setup. (b) Tip of a conventional ablation catheter with four electrodes and one temperature sensor. Source: From Ref. 13.

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Figure 8.12. (a) Cross-section of the designed heater. (b) Photograph of the test structure. Source: From Ref. 13.

Figure 8.13. (A), (B) dead pig myocardium tissue samples treated with the diamond heater. (C), (D) dead pig myocardium tissue samples treated with a conventional RF ablation catheter. The scales are 0.5 mm/div. Source: From Ref. 13.

shows that the diamond heater causes nearly perfect circular lesions with homogeneous depths to the tissue, whereas the conventional catheter (radio frequency (RF) catheter) results in large lesions, which are difficult to control.

Immobilization of Biomolecules

8.3

IMMOBILIZATION OF BIOMOLECULES

The chemical modification and physical absorption of diamond surface hold promises for diamond to be applied in immobilization of protein and DNA for purification, separation, and further analysis. Using detonation ND, Bondar and coworkers14 successfully separated recombinant Ca2+ -activated photoprotein apoobelin and recombinant luciferase from bacterial cells of Escherichia coli through physical absorption of proteins on ND. For traditional purification by chromatographic means, it usually takes several days. Figure 8.14 shows the procedures using ND. The cells were disrupted through ultrasound and the cell debris was removed by centrifugations. Then suspended detonation ND was added and then centrifuge to separate ND with protein absorbed on it. When treated by a selective block of SH group such as dithiothreitol,

Figure 8.14. Scheme of separation and purification of the recombinant protein from E. coli cells using adsorption of protein onto detonation ND. Source: From Ref. 5.

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protein was desorbed from ND. The whole process took 30–40 min with a yield of 35–60%. Kong and corworker15,16 applied the same principle to capture proteins and DNA for the matrix-assisted laser desorption/ionization (MALDI) time of flight (TOF) mass spectrometry (MS). Carboxylated ND (∼100 nm) exhibits high affinity to proteins and polypeptides through hydrophobic and hydrophilic forces. Proteins in very dilute solution can be easily captured by ND and separated and directly analyzed by MALDI-TOF-MS. The sensitivity is enhanced by more than two orders of magnitude. Carboxylated ND coated with poly-L-lysine was used to form stable complex with DNAoligonucleotides for MALDI-TOF-MS analysis. No-pre-separation of ploy-L-lysine or DNA was necessary prior to MS analysis. Puzyr’ and cowoekers designed a luminescent biochip with nanodiamonds and bacterial luciferase.40 Figure 8.15 shows the structure. It is demonstrated that the enzyme in this structure retains the catalytic activity from recording the luminescent signal. The luminescent intensity is sufficient high for this biochip to be used in bioluminescent analysis.



 Figure 8.15. A hypothetical “aluminum oxide film-adhesive layernanodiamond-luciferase” supermoleculer structure of the biochip for the components arranged in monolayers. Source: From Ref. 40.

Drug Delivery Vehicles

8.4

DRUG DELIVERY VEHICLES

Recently, the uptake of ND by living cell found in the biological ND research facilitated the use of NDs as drug carriers and delivery vehicles. In 1995, Kossovsky and coworkers used ND coated with cellobiose,17 a disaccharide to immobilize mussel adhesive protein (MAP) antigen. Figure 8.16 shows schematic diagram of the structure of diamond–callbiose–MAP. Then the complex was injected into New Zealand white rabbits which have their specificity against MAP. The delivery of antigen caused a strong and specific antibody response. Further experiments showed that ND immobilization

Error!

Figure 8.16. Schematic representation of the structure of the ∼300 nm diamond–cellobiose-MAP antigen delivery system. Source: From Ref. 17.

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Figure 8.17. Schematic diagram of NaCl-mediated loading and releasing of DOX with NDs. Source: From Ref. 18.

resulted in maintaining the protein conformations allowing better antibody binding and hence a strong immune response. For the first time, Huang and coworkers18 demonstrated that NDs serve as efficient chemotherapeutic drug carriers. Doxorubicin hydrochloride (DOX), an apoptosis-inducing drug used in chemotherapy, was coated on the NDs or embedded into the intervals of ND aggregates and introduced into living cell. NDs moved quickly inside cell and served as a stabilization matrix for DOX to preserve drug functionality. First, the concentration of NaCl was found to be able to control the absorption/desorption of DOX onto ND as shown in Fig. 8.17. Then, the interaction between DOX/ND and cell was studied. NDs were also coated with fluorescent poly-L-lysine. Figure 8.18 shows that the NDs are attached to cell instantly and interact with the living cells dynamically. Within 10 h, NDs are internalized in the cell.

8.5

BIOMARKERS AND BIOLABELING

As in Fig. 8.3, very strong fluorescence at 700 nm is emitted after excited at 560 nm20 due to the nitrogen-vacancy (N-V)− center within diamonds. This is advantageous for imaging in biological cells, as the background fluorescence in cell is 300–400 nm.5 Fu and coworkers found that under the same excitation conditions,21 the

Biomarkers and Biolabeling

Figure 8.18. (a–e) Confocal images of macrophage RAW cells with addition of NDs (20 µg/ml) coated with fluorescent poly-L-lysine (FTTCPL/ND ∼10 wt%). (a,b) Taken from the cells after the addition of NDs without and with incubation (37◦ C for 10 h), respectively. The excitation wavelength is 488 nm. (d,e) are bright-field images corresponding to (a), (b), respectively. (c) From the same cells as in (b) but was stained with DNA-binding dye TOTO-1 and excited with 514 nm. The nucleus of the cell can be clearly identified. (f) TEM image showing NDs in macrophage cytoplasm. The image was taken after 3 h incubation of the cells with addition of DOX coated NDs (10 wt%). The cells are dehydrated, fixed, and sliced for TEM observation. The scale is 20 nm. Source: From Ref. 18.

fluorescence of a single 35 nm diamond is significantly brighter than that of a single dye molecule such as Alexa Fluor 546. Fu and coworkers21 used fluorescent nanodiamond (FND) coated with ploy-L-lysine (PL) to study the interaction between DNA and FND on an amine-terminated glass substrate. The PL was used to facilitate the binding of DNA (fluorescently labeled with TOTO-1 dye molecule) to FND. Due to the specific strong fluorescence at 700 nm of FND, Fig. 8.19 clearly shows that the DNA molecule is wrapped around the PL-coated FND particle. Fu and coworkers21 also demonstrated that it is possible to conduct a single particle tracking for a 35-nm FND in the cytoplasm of a live Hela cell. Figure 8.20 shows the diffusion of FND in the Hela cells. The bright red dot in (A) is the FND, whereas (B) shows the trajectory of FND starting from the origin (0 0). This tracking method

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Figure 8.19. Observation of a single FND particle bound with a single T4 DNA molecule on an amine-terminated glass substrate. Dual-view fluorescence images of a single DNA/FND complex. An overlay (Right) of the images (545–605 nm) and longer (675–685 nm) wavelength channel reveals that Ta DNA was wrapped around the 100 nm FND particle and stretched to a V-shape configuration. Source: From Ref. 21.

Figure 8.20. Tracking of a single FND in a live Hela cell. (A) Overlay of bright-field and epifluorescence images of a live Hela cell after an uptake of 35-nm FNDs. (B) A 100-step (139 ms per step) trajectory of a single FND, indicated by the yellow in A, moving in the cytoplasm of the Hela cell. Source: From Ref. 21.

could be applied to drug delivery system of ND, where the interaction between ND and cell could be monitored using fluorescence microscopy. Raman spectrum of diamond exhibits a sharp peak at 1332 cm−1 . And the peak is isolated and the Raman absorption cross section is large.19 This peak can be used as an indicator of the location of nanodiamond. Cheng and co-workers22 tracked growth hormone factor in one single cancer cell using nanodiamondgrowth hormone complex (cND-rEaGH) as a specific probe. The

Electrochemical Applications

Figure 8.21. (A) Confocal Raman mapping image of A549 cell and 100 cND. (B) Color Raman mapping of A549 cell and 100 cND-rEaGH carboxylated nanodiamond complexes. The images shown are at different z position scans: (a) at z = 10 µm with diamond collected in 1320– 1340 cm−1 and cell collected in 1432–1472 cm−1 ; (b) at z = 0 µm position; (c) at z = −10 µm. Source: From Ref. 25.

growth hormone factor of A549 human lung epithelial cell can recognize the growth hormone. Using Raman mapping, the endocytosis of cND and cND-rEaGH were observed (shown in Fig. 8.21). From (A), cND can penetrate inside cells, whereas from (B), cNDrEaGH resides only on the surface of the cell. This observation is consistent with the fact that hormone-binding domain of growth hormone receptor is extracellular domain23 and the GH/GHR complex forming is on the extracellular part of membrane. Using the same method, Chao studied the interaction of E. Coli with cND– lysozyme complex.24 The lysozyme can be attached to a carboxylated ND through physical absorption.25 Figure 8.22 shows that the cND can be used as a marker to identify the location of the interacting protein lysozyme with E. Coli using Raman mapping.24

8.6

ELECTROCHEMICAL APPLICATIONS

Due to its physicochemical stability, large electrochemical potential winow, and chemical sensitivity,27 diamond is an excellent candidate for electrochemical applications. Diamond electrodes show the most stable response among electrodes by far, and do

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Figure 8.22. The interaction of E. Coli with cND–lysozyme complex as viewed with a conventional optical microscope (objective 100×) and confocal Raman spectrometer. The location of the cND is indicated in yellow. Arrow points to the location of cND. Source: From Ref. 24.

not require extensive pretreatment to regenerate the electroactive surface.28 Diamond electrodes/microelectrodes have been applied to biological system as biosensors.29 Rubio-Retama studied the nanocrystalline diamond (NCD) electrodes immobilized with horseradish peroxidase (HRP),30 which reacts with H2 O2 through reduction/oxidation (redox) reaction, and found that using the modified electrode for hydrogen peroxide determination shows a linear response in the 0.1–45 mM H2 O2 range. Using the same immobilization method as Yang,8 HRP was immobilized to the NCD electrode surface. Figure 8.23 shows the model of modified NCD structure.30 The modified NCD electrode as biosensor showed a linear response in the 0.1–45 mM H2 O2 range, at +0.05 V vs Ag/AgCl, for hydrogen peroxide determination. The enzymatic activity was constant during the stability study

Electrochemical Applications

Figure 8.23. Schematic representation of the two-layer structure of the modified NCD surface. Source: From Ref. 30.

on the modified NCD for 15 days. This modified NCD biosensor can be potentially used for in vivo applications or for clinical methodologies that are based on the measurement of hydrogen peroxide produced during the analysis. Halpern and coworkers studied neurodynamics in an animal model, Aplysia calfornica, using diamond microelectrodes.28 The microelectrodes (about 30 µm diameter) were made using hotfilament-assisted CVD to deposit boron-doped diamond onto a tungsten microelectrode (Fig. 8.24). The microelectrode was used to study the extracellular stimulation of the single neuron, B4 or B5 (Fig. 8.25(A)). And recording activity and stimulating of the cells were observed by the diamond electrode through fast scan cyclic voltammetry (FSCV). Further, the same method was able to quantitatively record increased levels of serotonin at the I2 muscle upon specific stimulation of MCC (Fig. 8.25(B) shows the experimental setup).

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Figure 8.24. SEM images of (a,b) selective diamond onto a (c) tungsten microelectrode (quartz insulator); final tip diameter, 35 µm. Source: From Ref. 28.

Park and coworkers studied the neurotransmitter,31 endogenous norepinephrine (NE) release with the evoked contractile response of a mesenteric artery from a healthy Sprague Dawley rat using continuous amperometry with a diamond microelectrode and video microscopy. Figure 8.26 A–D shows the diagram of the diamond microelectrode insulated with polypropylene. Figure 8.26 F–G show the video micrographs of a mesenteric artery (F) before and (G) after a 20-Hz electrical stimulation. The experiments demonstrated recording of NE released from sympathetic nerves at the surface of a mesenteric artery and the evoked response using the instrument shown in Fig. 8.26.

Electrochemical Applications

Figure 8.25. (A) Schematic of the in vitro, extracellular recording/stimulation apparatus. The diamond electrode is pressed into the insulation sheath over a neural cell of interest to specifically record or stimulate that cell. (B) Experimental setup of in vitro detection of serotonin at the I2 muscle of the Aplysia — the cerebral ganglion is desheathed in a Petri dish with inlet and outlet flow of high divalent cationic solution. An intracellular electrode is placed inside the metacerebral cell (MCC) and a diamond electrode is placed within the muscle fibers of the I2 muscle. Source: From Ref. 28.

Figure 8.26. (A) Diagram of the conically shaped diamond microelectrode insulated with polypropylene. SEM images of the polypropyleneinsulated diamond at (B) lower and (C) higher magnification. Top-view SEM images of the diamond film morphology without (D) and with (E) the polypropylene insulation layer. Video micrographs showing a mesenteric artery (F) before and (G) after a 20-Hz electrical stimulation. Source: From Ref. 31.

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8.7

BIOLOGICAL APPLICATIONS OF DIAMOND-LIKE CARBONS

Diamond-like carbons (DLC) is a carbon material, which in fact is not like crystalline diamond, not as hard, and is virtually amorphous. Because its microstructure allows the incorporation of other species such as hydrogen, nitrogen, silicon, sulfur, tungsten, titanium, and silver, the properties of DLC can be tailored far more readily than those of diamond.32 DLC is also a potential material for biological and biomedical applications due to its high hardness, low friction coefficient, high wear and corrosion resistance, chemical inertness, high electrical resistivity, infraredtransparency, high refractive index, excellent smoothness, and good biocompatibility.33 DLC can be produced by a number of techniques such as radio frequency plasma enhanced chemical vapor deposition,34 ion plating,35 pulsed laser deposition,36 magnetron sputtering.37 For biomedical application, DLC-coated materials have been shown successfully to be applied for metal and plastic joint replacements. Figure 8.27 (Roy, 2007) shows the DLC coated ultrahigh

Figure 8.27. A C:H coated head and UHMWPE acetabular surface of a hip joint by radio frequency plasma chemical vapor deposition. Source: From Ref. 33.

Biological Applications of Diamond-Like Carbons

molecular weight polyethylene (UHMWPE) surface of a hip joint. Figure 8.28 shows the Ankle joint coated with DLC.38 Another application of DLC is to make DLC-coated stents. A stent is a metal tube that is inserted permanently into an artery, which helps to open an artery so that blood can flow through it. Figure 8.29 from shows a biodiamond stent of Plasmachem.33 They used a DLC coating made by plasma-induced cold deposition technique to coat the inside and outside of their stainless 316L stents. Recently, Fu and coworkers designed a TiNi/DLC microcage.39 The fingers of the microcage closed/opened through the shape memory effect depending on temperature. Figure 8.30 shows the schematic drawing of the bimorph TiNi/DLC microfinger structures. Figure 8.31 shows the closing of a five-finger

Figure 8.28. Ankle joint with both parts coated with DLC (talar component left picture, tibial component right picture). Source: Pictures from M. I. L. SA. From Ref. 38.

Figure 8.29. A biodiamond stent of Plasmachem. Source: From Ref. 33.

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Figure 8.30. Schematic drawing of bimorph TiNi/DLC microfinger structures: (a) top view; (b) cross-section view after bending up. (c) illustration of bending angular and displacement. Source: From Ref. 39.

Figure 8.31. Optical microscopy images showing the closing of a fivefinger microcage during heating (during cooling the process is reversed; beam length: 150 µm): (a) 20◦ C, (b) 55◦ C, (c) 65◦ C, and (d) 80◦ C. Source: From Ref. 39.

microcage during heating using optical microscopy. Figure 8.32 shows SEM picture of a microcage capturing a micro-polymer ball. Further cell culture experiments showed that there is no cytotoxicity of TiNi/DLC. This microcage can be used as microgrippers for biological applications such as biopsy, tissue

Conclusions

Figure 8.32. SEM picture of a microcage capturing a micro-polymer ball. Source: From Ref. 39.

sampling, cell manipulation, nerve repair, and minimal-invasive surgery.39 8.8

CONCLUSIONS

Diamond has superior physical properties such as high hardness, high wear and corrosion resistance, unique optical properties, chemical inertness, good biocompatibility. Diamond also can be chemically functionalized. Hence, diamond can be applied in a lot of biological fields such as immobilization of proteins for purification and separation or applied in MALDI analysis, biomarker, biolabeling or biochips, electrochemical application for neurotransmitter, biomedical field including implant, cutting tool. There are still some factors that limit the application of diamond. First, by now, the size of detonation ND is limited to 2–100 nm. If the size of ND can be made even small, in future, it is possible for ND to be delivered into cells by membrane transduction instead of endocytosis through certain chemical modifications such as making ND amphiphilic. This will improve dramatically the efficiency of ND as a potential drug delivery tool. Second, the adhesive effect between materials and coated diamond film need

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to be improved. Finally, the cytotoxicity of ND need to be further investigated, especially the long-term effects on cell or an animal.

REFERENCES 1.

2.

3.

4.

5.

6.

7.

8.

9. 10.

11.

12.

L. Tang, C. Tsai, W. W. Gerberich, L. Kruckeberg, and D. R. Kania, “Biocompatibility of chemical-vapour-deposited diamond,” Biomater, 16, 483–488 (1995). A. F. Azevedo, E. J. Corat, N. G. Ferreira, and V. J. Trava-Airoldi, “Wettability and corrosion tests of diamond films grown on Ti6Al4V alloy,” Surf. Coatings Tech., 194, 271–275 (2005). S.-J. Yu, M.-W. Kang, H.-C. Chang, K.-M. Chen, and Y.-C. Yu, “Bright fluorescent nanodiamonds: No photobleaching and low cytotoxicity,” J. Am. Chem. Soc., 127, 17604–17605 (2005). ¯ A. M. Schrand, H. Huang, C. Carlson, J. J. Schlager, E. Osawa, S. M. Hussain, and L. Dai, “Are diamond nanoparticles cytotoxic?,” J. Phys. Chem. B, 111, 2–7 (2007). K. B. Holt, “Diamond at the nanoscale: Applications of diamond nanoparticles from cellular biomarkers to quantum computing,” Phil. Trans. R. Soc. A, 365, 2845–2861 (2007). B. D. Thoms, M. S. Owens, and J. E. Butler, “Production and characterization of smooth, hydrogen-terminated diamond C(100),” Appl. Phys. Lett., 65, 2957–2959 (1994). B. Guan, L. Z. Wu, B. Ren, and J. F. Zhi, “An easy method for attaching nanodiamond particles to amine glass-like carbon,” Carbon, 44, 2858–2860 (2006). W. Yang, O. Auciello, J. E. Butler, W. Cai, J. A. Carlisle, J. E. Gerbi, D. M. Gruen, T. Knickerbocker, T. L. Lasseter, J. N. Russell, J. R. Lloyd, M. Smith, and R. J. Hamers, “DNA-modified nanocrystalline diamond thin films as stable, biologically active substrates,” Nature Mater., 1, 253–257 (2002). H.-C. Chang, and C.-C. Han, “Diamond crystallites for biotechnological applications,” USA patent, US 2006/0154259 A1 (2006). K. Zolynski, ´ W. Pitkowski, A. Kaluzny, Z. Has, P. Niedzielski, S. Mitura, “Implants with hard carbon layers for application in: pseudoarthretis feroris sin, ortisis post fracturam apertam olin factam,” J. Chem. Vapor Deposit., 4, 253–258 (1996). S. Mitura, K. Mitura, P. Niedzielski, P. Louda, and V. Danilenko, “Nanocrystalline diamond, its, synthesis, properties, and applications,” Journal of Archievements in Materials and Manufacturing Engineering, 16, 9–16 (2006). M. J. Jackson, L. J. Hyde, W. Ahmed, H. Sein, and R. P. Flaxman,

Conclusions

13.

14.

15.

16.

17.

18.

19. 20.

21.

22.

23.

“Diamond-coated cutting tools for biomedical applications,” Journal of Materials Engineering and Performance, 13, 421–430 (2004). R. Müller, M. Adamschik, D. Steidl, E. Kohn, S. Thamasett, S. Stiller, H. Hanke, and V. Hombach, “Application of CVD-diamond for catheter ablation in the heart,” Diamond and Related Materials, 13, 1080–1083 (2004). V. S. Bondar, I. O. Pozdnyakova, and A. P. Puzyr, “Applications of Nanodiamonds for separation and purification of proteins,” Physics of the Solid States, 46, 737–739 (2004). X. L. Kong, L. C. L. Huang, C.-M. Hsu, W.-H. Chen, C.-C. Han, and H.-C. Chang, “High-affinity capture of proteins by diamond nanoparticles for mass spectrometric analysis,” Anal. Chem., 77, 259–265 (2005a). X. L. Kong, L. C. L. Huang, S.-C.V. Liau, C.-C. Han, H,-C. Chang, “Polylysine-coated diamond nanocrystals for MALDI-TOF mass analysis of DNA oligonucleotides,” Anal. Chem., 77, 4273–4277 (2005b). N. Kossovsky, A. Gelman, H. J. Hnatyszyn, S. Rajguru, R. L. Garrell, S. Torbati, S. S. F. Freitas, and G.-M. Chow, “Surface-modified diamond nanoparticles as antigen delivery vehicles,” Bioconjugate Chem., 6, 507–511 (1995). H. Huang, E. Pierstorff, E. Osawa, and D. Ho, “Active nanodiamond hydrogels for chemotherapeutic delivery,” Nano Lett., 7, 3305–3314 (2007). D. S. Knight, and W. B. White, “Characterization of diamond films by Raman spectroscopy,” J. Mater. Res., 4, 385–393 (1989). S. J. Yu, M. W. Kang, H. C. Chang, K. M. Chen, and Y. C. Yu, “Bright fluorescent nanodiamonds: No photobleaching and low cytotoxicity,” J. Am. Chem. Soc., 127, 17604–17605 (2005). C.-C. Fu, H.-Y. Lee, K. Chen, T.-S. Lim, H.-Y. Wu, P.-K. Lin, P.-K. Wei, P.-H. Tsao, H.-C. Chang, and W. Fann, “Charaterization and application of single fluorescent nandiamonds as cellular biomarkers,” Proc. Natl. Acad. Soc., 104, 727–732 (2007). C.-Y. Cheng, E. Perevedentseva, J.-S. Tu, P.-H. Chung, C.-L. Cheng, K.-K. Liu, J.-I. Chao, P.-H. Chen, and C.-C. Chang, “Direct and in vitro observation of growth hormone receptor molecules in A549 human lung epithelial cells by nanodiamond labeling,” Appl. Phys. Chem., 90, 163903-1–163903-3 (2007). P. Colosi, K. Wong, S. R. Leong, and W. Wood, “Mutational analysis of the intracellular domain of the human growth hormone receptor,” 268, 12617–12623 (1993).

189

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24.

25.

26.

27.

28.

29. 30.

31.

32.

33.

34.

35.

36.

J.-I. Chao, E. Perevedentseva, P.-H. Chung, K.-K. Liu, C.-Y. Cheng, C.-C. Chang, and C.-L. Cheng, “Nanometer-sized diamond particle as a probe for biolabeling,” Biophys. J., 93, 2199–2208 (2007). P.-H. Chung, E. Perevedentseva, J.-S. Tu, C. C. Chang, and C. L. Chang, “Spectroscopic study of bio-functionalized nanodiamond,” Diamond Relat. Mater., 15, 622–625 (2006). M. Tachiki, Y. Sumikawa, M. Shigeno, T. Banno, K. S. Song, H. Umezawa, and H. Kawarada, “Diamond nanofabrication and characterization for biosensing application,” Phys. Stat. Sol., 199, 39–43 (2003). C. Prado, G.-U. Flechsig, P. Gründler, J. S. Foord, F. Marken, and R. G. Compton, “Electrochemical analysis of nucleic acids at borondoped diamond electrodes,” Analyst, 127, 329–332 (2002). J. M. Halpern, S. Xie, G. P. Sutton, B. T. Higashikubo, C. A. Chestek, H. Liu, H. J. Chiel, and H. B. Martin, “Diamond electrodes for neurodynamics studies in Aplysia californica,” Diamond Relat. Mater., 15, 183–187 (2006). C. A. Martinez-Huitle, “Diamond microelectrodes and their applications in biological studies,” Small, 3, 1474–1476 (2007). J. Rubio-Retama, J. Hernando, B. López-Ruiz, A. Härtl, D. Steinmüller, M. Stutzmann, E. López-Cabarcos, and T. A. Garrido, “Synthetic nanocrystalline diamond as a third-generation biosensor support,” Langmuir, 22, 5837–5842 (2006). J. Park, J. J. Galligan, G. D. Fink, and G. M. Swain, “In vitro continuous amperometry with a diamond microelectrode coupled with video microscopy for norepinephrine and its effect on the contractile response of a rat mesenteric artery,” Anal. Chem., 78, 6756–6764 (2006). G. Dearnaley, and J. H. Arps, “Biomedical applications of diamondlike carbon (DLC) coatings: A review,” Surf. Coatings Tech., 200, 2518–2524 (2005). R. K. Roy, and K.-R. Lee, “Biomedical applications of diamond-like carbon coatings: A review,” J. Biomed. Meater. Res. Part B: Applied Biomaterials, DOI 10.1002, 72–84 (2007). J. C. Sánchez-López, C. Donnet, J. Fontaine, M. Belin, A. Grill, V. Patel, and C. Jahnes, “Diamond-like carbon prepared by high density plasma,” Diamond Relat. Mater., 9, 638–642 (2000). Y. S. Zou, W. Wang, G. H. Song, H. Du, J. Dong, R. F. Huang, and L. S. Wen, “Influence of the gas atmosphere on the microstructure and mechanical properties of diamond-like carbon films by arc ion plating,” Mater. Lett., 58, 3271–3275 (2004). K. S. Shim, S. M. Kim, S. H. Bae, S. Y. Lee, H. S. Jung, H. H. Park, “Fabrication and characterization of diamond-like carbon thin

Conclusions

37.

38. 39.

40.

films by pulsed laser deposition,” Appl. Surf. Sci., 154/155, 482–484 (2000). N. A. Sánchez, C. Rincón, G. Zambrano, H. Galindo, and P. Prieto, “Characterization of diamond-like carbon (DLC) thin films prepared by r.f. magnetron sputtering,” Thin Solid Films, 373, 247–250 (2000). R. Hauert, “A review of modified DLC coatings for biological applications,” Diamond Relat. Mater., 12, 583–589 (2003). Y. Q. Fu, J. K. Luo, S. E. Ong, S. Zhang, A. J. Flewitt, and W. I. Milne, “A shape memory microcage of TiNi/DLC films for biological applications,” J. Micromech. Microeng., 18, 035026 (1-8) (2008). A. P. Puzyr’, I. O. Pozdnyakova, and V. S. Bondar’, “Design of a luminescent biochip with nanodiamonds and bacterial luciferase,” Phys. Solid State, 46, 761–763 (2004).

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Amorphous Diamond as Thermionic Energy Converters

Amorphous diamond may be viewed to be sintered subnano (i.e. angstroms sized) diamond grains. In the following descriptions, amorphous diamond is portrayed as a micron thin coating material. However, in many cases, thicker coatings made by nanodiamond impregnated metal matrix may also substitute amorphous diamond coating to achieve similar applications, such as for field emitters, or even as solar cells. Amorphous diamond is essentially a chaotic carbon mixture with distorted sp2 and sp3 bonds. As such it possesses both metallic character of conductive graphite and semiconductor character of insulating diamond. Moreover, as each carbon atom is unique in its electronic state that is determined by the degree of distortion of its bonds, amorphous diamond contains numerous discrete potential energies for electrons. In fact, amorphous diamond may have the highest density of atoms (1.8 × 1023 per cubic centimeter) that is several times higher than ordinary materials (e.g. about four times of iron atoms or silicon atoms). Thus, amorphous diamond has the highest configuration entropy for both atoms and valence electrons. Due to the distribution of discrete electronic energies with high density, amorphous diamond is uniquely capable of generating electricity and emit radiation. It has been demonstrated that amorphous diamond can be made as silicon free solar cells, front panel display field emission source, sensitive thermal sensing by IR detection, and perfect black body for energy conversion. Various Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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amorphous diamond devices are being fabricated to exploit the superb properties of amorphous diamond.

9.1

THE POLLUTED SOLAR CELLS MADE OF CRYSTALLINE SILICON

The mainstream solar cells are made of crystalline silicon. Although the cost for generating one KwH is 5–10 times of conventional fuel fossil power plants, crystalline silicon solar cells are thought to be environmentally friendly. However, crystalline silicon is typically produced from highly pure quartz that may be extracted from beach sand. White beach sand can only be produced after weathering for tens of million years due to the fact that quartz is likely the most stable common mineral. Consequently, it would require very high temperature (e.g. 1700◦ C) to reduce quartz to form metallurgical grade silicon. While silicon is formed from quartz, 1.5 time of carbon dioxide is released that contributes to the green house effect. Moreover, the metallurgical grade silicon must be purified, again at high temperature. Eventually, the pure silicon is melted above 1450◦ C to form either ingot for single crystal applications or to cast in polycrystal blocks. In all these high temperature process steps, significant amount of electricity may be needed. The crystalline silicon for making solar cells may have a price tag as expensive as $100/Kg. For making solar cells, about 0.5 mm thick silicon including cutting kerf, is needed, so 1 kg of silicon may produce about 1 m2 of exposed area under the sun. As the solar constant is about 1 Kw/m2 , and the average efficiency of silicon solar cells as measured around the clock is less than 10%, we may expect that the power generated from one kilogram silicon material is about 0.1 Kw. Assuming that the power cost is about $0.05/KwH, the above 0.1 Kw solar panel would require about 2000 KwH or energy to produce the silicon material for constructing the panel. To pay back the bill, the silicon solar cells may have to run 20,000 hours or about 2.3 years! Governments have to subsidize of the electricity produced by as much as three times of the ordinary cost of electricity derived from power plants. Consequently, crystalline silicon solar cells are neither clean, nor cheap. It is desirable to constructing solar cells by using more environmental friendly materials. One of such materials is amorphous diamond.

Super-Entropic Material

9.2

SUPER-ENTROPIC MATERIAL

Amorphous diamond appears to be contradictory term, like liquid crystal or glassy metal. Amorphous means non-crystalline and diamond implies crystalline. However, this terminology is meaningful because unlike silicon that forms only sp3 bonds, i.e. diamond structure, carbon may form either sp2 (graphitic) or sp3 (diamond) bond. Although there is one form of amorphous silicon, there can be at least two forms of amorphous carbon, so amorphous diamond can be distinguished from amorphous graphite, and together they are amorphous carbon. Amorphous diamond is formally known as tetrahedral amorphous carbon (tac), it is really a diamond-like carbon (DLC) that contains no non-carbon impurities (e.g. H). Amorphous diamond is essentially a chaotic carbon mixture with distorted sp2 and sp3 bonds. As such it possesses both metallic character of conductive graphite and semiconductor character of insulating diamond. Moreover, as each carbon atom is unique in its electronic state that is determined by the degree of distortion of its bonds. Hence, amorphous diamond contains numerous discrete potential energy for electrons. In fact, amorphous diamond may have the highest density of atomic occupancy (1.8 × 1023 per cubic centimeter) that is several times higher than ordinary materials (e.g. about four times of iron atoms or silicon atoms). Thus, amorphous diamond has the highest configuration entropy for both atoms and valence electrons (Fig. 9.1). Amorphous diamond can be conveniently deposited by PVD methods, such as by sputtering or arc depositions. Due to the low temperature (< 150◦ C) of deposition, amorphous diamond can be coated on most materials including metal, semiconductor, or even polymers. This flexibility makes amorphous diamond useful for many applications (Fig. 9.2). Due to such high configuration entropy of valence electrons, amorphous diamond is capable to advance electron energy by absorbing small increments of energy, such as by converting thermal energy (lattice vibration) to potential energy (electron state). If amorphous diamond is exposed in high vacuum (e.g. 10−6 torr), the energy state may be higher than vacuum state so amorphous diamond my emit electrons simply by heating. Because amorphous diamond has the highest discrete electronic states, it is the most thermionic material known.

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Figure 9.1. The high atomic density and the unique way of distorting bonds for each atom makes amorphous diamond the material with the highest configurational entropy. As a result, amorphous diamond has the densest electron states that are discrete. This is in contrast of all materials that have either overlapped electron orbitals, as in the case of metal, or few discrete electron states, as in the case of semiconductors or insulators.

Low Effective Work Function

Figure 9.2. Amorphous diamond can be coated on substrates by cathodic arc process. In this case, the surface smoothness can be adjusted by the arc current; and the material properties (e.g. sp3 /sp2 ratio of the carbon atoms), by the bias on the substrate.

9.3

LOW EFFECTIVE WORK FUNCTION

In general, materials fall in three camps, conductor, semiconductor and insulator, amorphous diamond is an atomistic mixture of all, so electrons can pass through it and be emitted in vacuum. By contrast, all other materials will stop electrons either inside the crystal lattice (e.g. an insulator) or on the surface (e.g. a conductor). The unique ability for amorphous diamond to emit electrons in vacuum by receiving low levels of energy makes it the excellent of field emitter with very low apparent work function. Amorphous diamond can be coated on metallic substrate by cathodic arc process to become a useful field emitter in vacuum (Fig. 9.3).

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Figure 9.3. The dramatic enhancement of emission current by coating amorphous diamond on aluminum cathode with bumps.

9.4

THERMIONIC EMISSION

Even without high vacuum, amorphous diamond coated nickel electrodes of cold cathode fluorescent lamps (CCFL) used for back lighting can reduce significantly the turn-on voltage (Fig. 9.4). Due to its exceptional ability to increase the potential energy of electrons by absorbing heat, amorphous diamond coated metal is highly thermionic (Fig. 9.5). Based on the above thermionic effect, the effective work function, i.e. the activation energy for electron emission in vacuum, can be lower than 1 eV. This is the lowest of all materials that have effective work function higher than 2 eV. Due to this unique thermionic character, amorphous diamond can emit more current than even

Thermionic Emission

Figure 9.4. The reduction of ignition voltage of CCFL by coating nickel electrodes with amorphous diamond.

Figure 9.5. The great enhancement of emitted current from amorphous diamond coated nickel electrode in CCFL by modest heating.

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carbon nanotubes (CNT) that have a high work function, but with a nanometer radius to enhance the electrical field (Sung, ChienMin, US Patent No. 7,352,559). Moreover, as amorphous diamond is solid in content, it can emit electrons at a much lower temperature than CNT that will concentrate electricity on the skin of the hallow structure. In fact, the skin of each CNT will burn out when the current exceeds 20 µA. As a result, CNT devices are not reliable (e.g. Samsung’s CNT front panel display or Iljin’s CNT backlight). In contrast, amorphous diamond field emission can be highly robust. This is particularly suited for display or backlight applications. 9.5

FIELD EMISSION DISPLAY

The sensitive field emission and thermionic enhancement of amorphous diamond would make it an ideal coating material for Spindt (metal spikes) array that may be used for front panel displays. Such field emission displays will have the lowest operational power and panel temperature (Fig. 9.6). By coating amorphous diamond on arrays of Spindt cones that are controlled by the cross bars of electrodes, the emission of a particular addressed pixel can be triggered to excite phosphor of RBG colors (Fig. 9.7). Such a device can be used as field emission display for TV and other devices. 9.6

BLACK BODY RADIATOR

Materials fall under two camps: electrical conductors (metals) can conduct heat (phonon), but not emit heat (IR), and electrical insulators (ceramics) are just the opposite. However, amorphous diamond is both the thermal conductor and thermal emitter. Amorphous diamond is not only thermionic; it is also the perfect black body. Typically a metal has a low emissivity (e.g. 2%), but an insulator has a poor thermal conductance (e.g. 10 W/mK), so both of them cannot sustain the emission far infrared from a warm surface. However, amorphous diamond has a thermal conductivity (about 500 W/mK) that is even higher than the best metal (420 W/mK for silver), and its emissivity is nearly 100%. It was measured that at a temperature as low as 70◦ C, the sustained heat emission was 0.088 W/cm2 , this equals to what predicted by Stefan–Boltzmann’s equation (5.67 × 10−8 T[K]4/m2 ) for black body. This implies that

Black Body Radiator

Figure 9.6. The field emission of amorphous diamond coated nickel alloy cones can be enhanced by either sharpening the cone tips (upper) or by modest heating (lower).

amorphous diamond has an emissivity of about 100% and the emission is not limited by its thermal conductivity. The exceptional ability to emit heat makes amorphous diamond an excellent thermal radiator for cooling high-powered LED (Figs. 9.8 and 9.9). As the black body is reversible, amorphous diamond can be used as the heat absorber for applications related to advanced thermal imaging of infrared source or mundane water heating by absorbing sunlight.

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Figure 9.7. The schematic of front panel display using amorphous diamond coated micro tips of metal cones.

9.7

DLC ELECTRO-LUMINESCENCE (DEL)

Amorphous diamond coated PET, replacing silver grease, was used as the electrode for the light panel based on electro-luminescence. The result indicated that the brightness of the phosphors increased substantially than conventional EL. Moreover, the decay rate of the illumination was greatly reduced (Sea-Fue Wang, personal communications). In a previous study, the brightness of DEL was shown to increase linearly with the increase of the applied voltage, frequency of the alternative current, and the increase of temperature. The improvement of DEL over EL was like due to the increased voltage in amorphous diamond compared to metal conductor under the same externally applied electrical field. Because amorphous diamond electrons are bound loosely, they can be free with the input of low amount of energy (e.g. external filed). Consequently, under the external field of alternative current, higher concentration of electrons could be generated momentarily. The fast accumulation of free electrons in amorphous diamond was demonstrated in the rapid saturation of electrons when an external bias of direct current was applied. This phenomenon made amorphous diamond highly useful as the energy storage device (e.g. a high density capacitor). The enhanced capacitance

DLC Electro-Luminescence (DEL)

Figure 9.8. The cooling of LED junction temperature by coating amorphous diamond on the surface of its aluminum substrate heat spreader.

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Figure 9.9. The dramatic cooling effect of high bright LED that was operated at 350 mA by replacing the oxidized layer of the silicon submount with DLC coating.

on amorphous diamond apparently made the DEL brighter than EL that used metal conductors. Because the electrical current was not increased in DEL, the likelihood of thermal damage due to the local current concentration of electricity on phosphor particles can be reduced. This explained why DEL’s light faded away much slower than conventional EL devices. EL is a low cost illuminator that can be applied to large area. Unfortunately, its brightness has been low (e.g. < 3000 cd/m2 ), and the service life has been low (e.g. 2000 h). DEL can overcome the limitation of EL, and hence it can be an alternative to backlighting or for generally illumination (Fig. 9.10). 9.8

AMORPHOUS DIAMOND SOLAR CELL

The merit of amorphous diamond to convert either light or heat to electricity can be applied to solar cell panels or thermal electrical generators. For example, amorphous diamond was over covered coated on indium tin oxide (ITO), the transparent electrical conductor that was coated on a glass substrate. This panel was separated from another ITO coated glass by glass bead spacer. The gap was sealed around and the space was pumped down to high vacuum (10−6 torr). This panel was exposed to a xenon light that irradiated

Amorphous Diamond Solar Cell

Figure 9.10. The schematic of DEL design and the brightness of DEL panel (A4 size).

a spectrum with an energy output similar to solar constant (AM1.0 or 0.1 W/cm2 ). An external bias was applied and the electric current was monitored. It was demonstrated that the current increased substantially when light shone through or when amorphous diamond was heated up (Fig. 9.11). When the applied bias was gradually reduced to zero, the current enhanced by xenon lamp was not dependent on the bias. Hence, the field emission could be triggered by sunshine directly without adding an external bias. However, the current density was too low to be useful as a solar panel unless the vacuum gap could be reduced further from 7 µ. When the vacuum was back filled with iodine, the current could be generated noticeably without applying the external field. This current was further increased by sensitizing amorphous diamond with a light absorbent dye. But even so, it was still low with the conversion efficiency (Fig. 9.12).

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Figure 9.11. Amorphous diamond can enhance electron emission in vacuum by light absorption and by thermal agitation.

Figure 9.12. The field emission became spontaneous when the external bias was reduced to zero.

Amorphous Diamond Solar Cell

Figure 9.13. The photo-electric effect of amorphous diamond when exposed to a xenon lamp (AM1.0) of about 0.1 W/cm2 . In the experiment, the vacuum gap was back filled with liquid electrolyte of iodine.

The amorphous diamond solar cell was also constructed with a silicon layer in a hybrid design (Fig. 9.13). In this case, no vacuum was needed. In one example, the nitrogen doped amorphous diamond was coated on boron doped silicon substrate. This hybrid design showed a dramatic increase in photo electricity, much higher than using a vacuum gap or back filed with a liquid electrolyte (Fig. 9.14). When a monochrometer was used to filter the broad spectrum of the xenon lamp and the photocurrents of amorphous diamond coated silicon and silicon solar cell were measured and compared, the former exhibited a much higher value. Upon cooling the semiconductors to a cryogenic temperature of liquid nitrogen (70 K), the electrical current generated by light increased. Moreover, the increase was higher with shorter wavelengths (i.e. with higher energy). However, amorphous diamond coated silicon showed much higher cooling enhancement and also the blue shift of the peak wavelength. The above observation demonstrated that amorphous diamond could absorb light and generate electricity more effectively than silicon. This is particularly attractive as amorphous diamond

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Figure 9.14. The photo electric effect of nitrogen doped amorphous diamond coated on boron doped silicon layer.

is radiation superhard and it would not be susceptible to UV damage. Amorphous silicon solar cells have the advantages of thin film and low cost, but the aging problem of UV damage makes it less useful so more costly crystalline silicon plates are used as solar panels. It would appear that amorphous diamond coating of amorphous silicon solar cells could boost both the energy conversion efficiency and the longevity of the service. Due to the high electric resistance of amorphous diamond, the electric current it generated was dissipated as heat so the final output of electricity was significantly reduced. The dampening effect was greatly reduced by cooling the device at liquid nitrogen temperature. However, alternative methods by channeling out electricity rapidly once it is formed may also be effective to preserve the electricity generated. One example is to coat amorphous diamond on amorphous silicon layers and stack them together. Due to the thinness (e.g. 100 nm) of the amorphous diamond and amorphous silicon, the light absorbed by each layer can generate electricity independently. The electricity can then be channeled out readily due to the short distance of travel to reach the electrode. The combined electricity would retain most of the energy derived from sunlight (Fig. 9.15).

Amorphous Diamond Solar Cell

Figure 9.15. The photo electrical current as a function of optical wavelength. Note that amorphous diamond could convert more current with IR irradiation than pure silicon. Moreover, the cooling enhancement of both energy and intensity was more obvious.

In summary, amorphous diamond has the highest density of discrete electronic states. This unique feature makes amorphous diamond particularly useful as energy converters, such as field emitters, solar cells, thermal generators, radiation coolers, and heat absorbers.

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Amorphous diamond is highly complimentary to thin film solar cells (Fig. 9.16). There are two major contenders for solar cells without using expensive crystalline silicon. One of them is the coating of amorphous silicon. This solar cell typically incorporate boron-doped silicon as the P-type. However, boron atoms are much smaller than silicon, so the substitutional boron atoms in silicon pseudo lattice is limited. As a result, the whole concentration may not be sufficient. In contrast, boron atoms are only slightly larger carbon atoms, so the solid solution can be much more extensive than boron-doped silicon. The p-n junction made of phosphorus doped silicon and boron doped carbon can be more comparable for a higher efficiency output as solar cells. Another complimentary design is to couple nitrogen-doped amorphous diamond as N-type with CIGS as P-type. CIGS is rapidly rising as the leading solar cell without using silicon. CIGS stands for copper-indium-gallium-selenide (sulfide), it has the highest absorption energy of all thin films, except amorphous diamond. CIGS may also be coated on flexible substrate (e.g. aluminum foil) so it can be mounted on cars. The other flexible

Figure 9.16. The solar absorbed energy of CIGS is the highest among various thin films (shown in figure) with a theoretical conversion efficiency of 25% (top diagram). One example of CIGS solar panel (bottom diagram) is depicted with CdS replaceable by amorphous diamond to boost the overall performance including reliability.

Amorphous Diamond Solar Cell

panel made of dye sensitized titanium dioxide has a reliability issue as it contains unstable organic materials. Although CIGS is rising rapidly as commercial solar panels (e.g. NanoSolar venture, IBM/TOK joint development), it has an Achilles heel. This is because CIGS uses CdS as the N-type coating to furnish electrons. CdS is not only poisonous but emits damaging as well. If nitrogen-doped amorphous diamond replaces CdS, then the above mentioned weakness of CIGS is reinforced. In addition, amorphous diamond can also serve as the antireflaction layer. Furthermore, amorphous diamond is electrically-conductive so it could avoid using transparent cathode (e.g. indium tin oxide). Alternatively, amorphous diamond may also support thinner coating of aluminum/silver as electrode.

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Chapter Ten

Fluorinated DLC for Tribological Applications

Diamond-like carbon (DLC) can be slippery with a frictional coefficient on par with Teflon (e.g. 0.1). Hydrogen terminated DLC may be more slippery than pure carbon DLC, but fluorinated DLC can be super slippery with frictional coefficient less than engine oil (0.01). The fluorinated diamond is actually superhard Teflon that may be named dialon. Dialon is not only virtually frictionless, it is also super hydrophobic, more so than even lotus leaves. In fact, if dialon is coated onto heart valves, it will not accumulate platelets as other artificially made part will do.

10.1

INTRODUCTION

Diamond has the most extreme properties of all materials. Among them are superhard tribological properties and wear resistance, super fast thermal conductivity and acoustic transmission, super inert chemical properties and corrosion resistance. DLC retains major properties of diamond and it is applicable as coating materials for most substrates. DLC has been used widely in industry including hard drives coating for computers, razor blades coating for consumables, and artificial joints coating for biomaterials. DLC can be doped with none carbon elements for improvements of adherence at the expense of hardness. Hydrogenated DLC is typically used for this purpose, but fluorinated DLC can further enhance the bonding strength to the substrate. Moreover, F-DLC combines the superlative properties of diamond and teflon Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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becomes the most suitable coating material for tribological applications with super slippery and super hydrophobic attributes.

10.2

DIAMOND-LIKE CARBON

Diamond-like carbon can be coated on a variety of substrates including metals (e.g. steel), polymers (e.g. teflon), and ceramics (e.g. glass). The coating provides certain diamond attributes, such as high wear resistance, low frictional coefficient, and extreme chemical inertness. There are different methods that may produce DLC, each with specific characteristics. Figure 10.1 shows the classification of DLC types based on the diamond-like bonding (sp3 ) and graphitic bonding (sp2 ) for carbon atoms, and also the content of hydrogen atoms. A major compromise of DLC property is between hardness and adherence. In general, the more diamond properties (e.g. hardness) retained by DLC, the weaker is its adherence to the substrate. Figure 10.2 shows such a trade off (Sung, 2000).

Figure 10.1. DLC’s atomic structure and composition based on hydrogen content. a-D is amorphous diamond or tetrahedral amorphous carbon (taC). a-C is amorphous carbon or amorphous graphite.

Diamond-Like Carbon

Figure 10.2. The inverse relationship between diamond properties of DLC and its coating merits.

Figure 10.3. The ranking of various coating materials for hardness (a) and slipperiness (b).

DLC can be engineered to exhibit the unusual combination of hardness (wear resistance) and smoothness (slipperiness) (Sung, 2003). Figure 10.3 shows the comparisons of wear resistance and frictional coefficient for various coating materials. Due to the exceptional properties of high wear resistance and low sliding resistance, DLC has been used for many tribological applications. Figure 10.4 shows some examples.

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(a)

(b)

(c)

Figure 10.4. DLC coated products of razor blades (a), CD molds (b), and golf club heads (c).

10.3

MICRODRILL’S DRY LUBRICANT

Microdrills are used to make holes on printed circuit boards (PCB). Due to the small diameter (e.g. 0.3 mm) and the high aspect ratio of the hole, debris may be trapped inside that can grip the drill and break it. In order to lubricate the flutes of the drills, DLC was coated on the surface. The coating on the cutting tips may be rubbed off quickly during the drilling of the PCB, but the cutting debris can be extruded readily along the slipper surface of DLC. As a result, the microdrills can continue to penetrate the board without been overly twisted. The breakage of microdrills due to twisting has been the limiting factor for the holes drilled from the same tool. Figure 10.5 shows the sample of DLC coated microdrills. Printed circuit boards were drilled by DLC coated microdrills. Table 10.1 summarizes the testing conditions.

Figure 10.5. The appearance micro drills with hydrogenated DLC coating (left) and Fluorinated DLC coating (right).

Microdrill’s Dry Lubricant

Table 10.1. The drilling parameters of DLC coated microdrills Size Speed In Feed Temperature Pressure PCB Circuitry Thickness Board

(a)

0.3 mm 140K RPM 49 mm/s 34◦ C 1400 mm Aq FR-4 2 Layers 1.6 mm 2 Plates

(b)

Figure 10.6. Examples of the good holes with surface roughness less than 25 µm (a); and the bad ones, more than 25 µm (b).

The quality of the drilled holes is in PCB is typically judged by the roughness of the hole wall. In particular, the copper laminate must not smear to cause excessive relief on the wall. The following photos show the examples of the hole profiles in PCB. Figure 10.6 illustrates the hole quality based on the smoothness of the wall as examined by polishing the cross section. Normally, the microdrills can sustain a drilling life of about 2000 holes through multiple boards (e.g. 2), but DLC coated ones demonstrated the capability of passing 50,000 holes as shown in Fig. 10.7 (Sung et al., 2006).

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Figure 10.7. Hydrogenated DLC coated microdrills showed that the wall smoothness could be maintained until reaching 50,000 holes that are about 25 times the life (about 2000) of uncoated microdrills.

10.4

FLUORINATED DLC

DLC’s tribological properties can be further enhanced by fluorination. Fluorinated DLC may become more inert and more slippery than other types of DLC, such as silicon doped one. Miyake et al. (1991) found the fluorinated DLC can be more adherent to the substrate. Moreover, the scratch caused by an indenter on the coating surface can be minimized. Figure 10.8 shows the dramatic effect of avoiding deep scratch by adding fluorine atoms to DLC. 10.5

DIALON COATING

The fluorinated DLC is akin to teflon that is essentially fluorine coated diamond atoms that form a single file. In fact, this structure may be named dialon for diamond–teflon for future reference. Figure 10.9 is a schematic diagram of teflon compound. The fluorination cannot only lubricate the DLC, but also enhance its adherence to the substrate, particularly on the surface of polymers. Moreover, the fluorine atoms attached to DLC can repel water or other polarized molecules so the surface is highly hydrophobic. The dialon-coated surface has the super lotus effect that can clean itself spotlessly. Figure 10.10 illustrate the electrostatic repulsive effect of hydrogen and fluorine terminated DLC. Dialon is virtually frictionless (frictional coefficient can be less than 0.001 if rubbing against itself). If Dialon were coated on bearings, it would be the most slippery surface of all materials. Dialon

Dialon Coating

Figure 10.8. The scratch line on DLC can be dramatically reduced when its surface is fluorine terminated (Source: Redrawn from Miyake et al. (1991).

Figure 10.9. A teflon (poly Tetra Fluoro Ethylene) molecule is made by replacing hydrogen with fluorine on diamond-like thread of ethylene.

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Figure 10.10. Hydrogen terminated DLC may repel each other (left), but fluorine terminated DLC can be levitating without contact (right).

may also be applied to biological objects, such as vein stents or heart valves. In this case, dialon can repel blood platelets or body fluid so the artificial implants may last long or even survive the life of its host (Fig. 10.11). 10.6

DIALON APPLICATIONS

Dialon may have numerous applications. For example, it can be coated on plastic material to improve the air tightness. For example,

Dialon Applications

Figure 10.11. Frictional coefficient between steel ball and DLC coated high speed steel plate.

the interior of PET bottles can be coated with dialon by inserting the container with an electrode that echoes with the exterior electrode in radio frequency (e.g. 15.6 MHz). The decomposed gas (e.g. acetylene) will coat the plastic to form DLC. The DLC sealed PET would not react with almost any liquid. Dialon would be a much better coating than ordinary DLC so the coated container can be used to store beverages or beers. Figure 10.12 shows the effectiveness of sealing a permeable wall by coating with DLC.

Figure 10.12. Air permeability as a function of time demonstrated that dialon is the ideal sealer for plastic materials.

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Table 10.2. Adherence of platelets on different materials Substrate

Density (No/cm2 )

Silicon DLC coating Dialon coating

500 millions 100 millions 5 millions

Another example is to coat dialon on biological implants. The fluorine atoms will repel water so the body fluid cannot stain the surface of the implant. Dialon coated heart valves and vein stents are the least likely to adhere bood platelets that may hinder their functions. Table 10.2 compares the densities of platelets that are adhered on various substrates immersed in blood. Dialon may also be coated on the exterior surface of piston or the interior surface of cylinder in an engine. In this case the hydrophobic surface is highly oil hungry (lipophilic) so it can be

Figure 10.13. The schematic of a running engine with oil firmly attached to dialon that is coated on the interior of the cylinder. The engine oil may also contain suspended nanodiamond made by the detonation of dynamic of further reduce the frictional coefficient.

Dialon Applications

firmly attached with a thin film of non-polar oil. This in situ lubricant can prevent frictional rubbing between the moving parts. As a result, not only the engine life may be extended, but also the running efficiency (e.g. higher gas mileage may be expected from an automobile). Figure 10.13 illustrates the oil retention effect of fluorine containing DLC.

Figure 10.14. The atomic distance of (111) faces in diamond (top). The stacking of (111) planes actually forms a sieve with hole sizes measured in angstroms (bottom). These planes are the smoothest surface possible of all materials, including other faces of diamond.

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Figure 10.15. The ideal storage mechanism of digital information by coding the diamond’s (111) face with F and H atoms. The density of such bits (0 and 1) can be easily million times higher than the current designs based on virus-sized magnets (hard drive) or capacitors (DRAM).

10.7

THE ULTIMATE STORAGE MEDIUM

The octahedral (111) face of diamond is the tightest atomic plane (2.71 atoms/nm2 ) possible of all materials, including other faces of diamond. Moreover, it can be the smoothest surface achievable (Ra = 0.04 nm). Figure 10.14 shows the atomic configuration of (111) face of a diamond lattice. Diamond atoms are very small, but fluorine atoms are even smaller. On the other hand, hydrogen atoms are the smallest of all elements. The atoms of fluorine and hydrogen may be attached to the extreme smooth diamond’s (111) face to exhibit opposite polarities, viz. negative for electron shrouded fluorine atoms and positive for proton exposed hydrogen atoms. Such polarities are very stable, up to 400◦ C, so it can be the robust storage of bit information

Summary

(0 for fluorine and 1 for hydrogen) for computer CPU. A single walled carbon nano tube (SWNT) can be used to traverse the surface. SWNT is flexible and conductive, it can detect the polarity of attached atoms. Such a device can be the densest storage medium ever invented (Sung, 2006). Figure 10.15 shows schematically the reading of an atomic storage plane based on diamond’s surface. 10.8

SUMMARY

Dialon combines the uncompromising attributes of diamond’s hardness and teflon’s slipperiness. Its coating is also more thermally stable and more chemically inert than conventional DLC coatings. Dialon’s surface can be used as robust DNA chips and compact information storage. Dialon coatings can be widely applied to bearing surfaces and hard facing. Such applications can reduce significantly the wear loss that may account to one tenth of total energy loss due to friction and heating. Furthermore, the reliability and longevity of mechanical engagement can be vastly improved.

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Chapter Eleven

Gem Diamond Growth

11.1

DIAMOND GEM MARKET

Diamond is the queen of jewelry. Due to the well-known slogans, such as “diamond is girl’s best friend”, diamond has become the symbol of love, particularly for wedding ceremonies (Fig. 11.1). In 2007, about $14B rough diamonds were sold to the polishers that were eventually supplied to the retail gem market of about $70 billion sales worldwide. Among the polished gems, about $100 million were synthesized under ultrahigh pressure and temperature by Gemesis and others (Figs. 11.2–11.4). 11.2

ULTRAHIGH PRESSURE TECHNOLOGY

In 1970s GE pioneered the gem diamond growth by using belt apparatuses that were capable to sustain a pressure of 5.5 GPa (55,000 time of atmospheric pressure) and a temperature of 1300◦ C. The diamond was formed by feeding micron diamond fines to a molten catalyst of iron nickel alloy (Invar). The diamond fines were dissolved in the liquid and precipitated onto a diamond seed (Fig. 11.5). GE diamonds were 1–2 carats after polishing. They could be colored or colorless depending on the dopant used in the catalyst. For example, the dopant free catalyst proceeded yellow diamond due to the incorporation of nitrogen from air. By adding a few PPM (part per million) of a nitrogen getter (e.g. titanium or zirconium), the diamond turned colorless (Fig. 11.6). GE’s gem diamond growth was costly and the process very slow. In 1980s, Sumitomo Electric used the similar technique to Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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Figure 11.1. The brilliant cut of a gem diamond.

Figure 11.2. Rough diamond supply chain and its ecosystem.

Russian Gems

Figure 11.3. The rough cut of natural diamond in 2006. The diamond gem market size was $63B that was compared to the ultimate jewelry sales of $143B.

Figure 11.4. The breakdown of diamond polishers and the segments of diamond gem market.

grow larger crystals. In 1990, De Beers grew gem diamond larger to more than 10 carats per crystal (Figs. 11.7 and 11.8). 11.3

RUSSIAN GEMS

Although GE, Sumitomo, and De Beers led the gem diamond growth for decades, the belt apparatus equipment they used was

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Figure 11.5. The GE process and its gem diamond grown in 1970s.

Figure 11.6. GE’s grown diamond and polished gems made in 1970s.

oversized. Due to the presence of a large temperature gradient, multiple diamond growth in belt apparatuses was not successful. Russians, on the other hand, began to develop a washer sized ultrahigh pressure technology using split sphere technology. In 1995, retired Korean war general Carter Clarke visited Leningrad and saw how such equipment operated. He ordered three machines with $170,000 and shipped them to University of

The Split Die Technology

Figure 11.7. The Sumitomo diamonds (left) and De Beers diamonds (center and right).

Florida in Sarasota. These machines were redesigned with computer control of pressure and temperature profiles. In 2005, Clarke formed Gemesis with 23 equipment that were capable to produce about 200 crystals per month (Fig. 11.9). Gemesis began to sell the polished gems under the trade name of Cultured Diamond. The diamonds were in rich yellow color due to the presence of ample nitrogen impurities. Gemesis priced the yellow diamond at about $4000 per carat that was about 1/3 of natural gem of the same size. In 1999, Chatham Created Gems of San Francisco displayed Russian diamonds at the world expo of gems in Las Vagas that made the world know about Russians’ coming (Figs. 11.10 and 11.11). Both Gemesis and Russians are increasing their production capacity in Sarasota and in St. Petersburg, respectively. It is estimated that the market is now taking in at least 1000 synthetic diamond gems per month (Fig. 11.12). 11.4

THE SPLIT DIE TECHNOLOGY

The belt apparatus is too bulky for making gem diamonds on one hand, and the split sphere is too complicated for making one crystal at a time. The solution is to extend the split chamber to form a longitudinal chamber that allow multiple crystal to grow simultaneously (Sung, Chien-Min, US Patent No. 7,128,547). The growth conditions of each diamond crystal can be precisely controlled by

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Figure 11.8. Polished artificial diamond gems with different colors.

its own temperature monitor with real time adjustment. Because all diamond growth chambers are coaxial, their pressures are uniform. The split chamber makes the use of a conventional design known as cubic press. The cubic presses have been used for massive production of diamond grits. Chinese produced most of such industrial grades with several thousands press in operation (Fig. 11.13).

The Split Die Technology

Figure 11.9. The Gemesis process with split sphere. Only one crystal per cycle could be grown. The lower diagram showed the crystal size and weight as a function of growth time.

The major feature of the split chamber design is that the major diamond growth directions are perpendicular to gravity so the convection in the molten catalyst would not disturb the growth rate. Consequently, all crystals will grow at the same pace to the same size with the same quality (Fig. 11.14). The cubic press has six anvils that are moving toward the common center of the high pressure assembly. By extending four of the six anvils, an enclosure can be formed where gem diamond seeds are placed inside. The other two anvils, lying horizontally, can be

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Figure 11.10. Gemsis diamonds in the rough and after polishing.

used to stroke the cavity to achieve the ultrahigh pressure required for gem diamond growth. As the diamond becomes larger, volume is reduced in the assembly so the two anvils can follow through to maintain the pressure for growth. The split sphere of Russian design has a complicate enclosure that involves two layers of multiple anvils with complex geometries. Any component of the multiple anvils could be misaligned with the consequence of pressure and temperature instability. Moreover, the expensive components could be damaged easily

The Split Die Technology

Figure 11.11. The Russian diamonds at the display.

Figure 11.12. Natural diamond gems (left) and synthetic ones (right) are indistinguishable unless very sophisticated instruments (e.g. DiamondSureTM or DiamondViewTM ) are used to reveal characteristic fluorescence spectra.

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Figure 11.13. The Chinese cubic presses used for the production of industrial diamond grits. Each year more than four billions carats (800 metric tons) of mesh sized (less than half of a millimeter) grits are produced for sawing concrete and stones.

with routing operations. In contrast, the split die takes the advantage of rapid withdraw of four anvils to expose the cylindrical cavity where diamond growth chambers may be arranged and thermocouple for temperature monitoring can be attached.

The Split Die Technology

Figure 11.14. The split sphere design of the diamond growth chamber (Sung, Chien-Min, US Patent No. 7,128,547).

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Consequently, the diamond production system is robust and efficient. 11.5

CVD DIAMOND FORMATION

Although graphite is stable at low pressures but this refers to large structures of more than hundreds atoms. If the clusters of atoms are less than several hundreds, diamond, bucky balls, or carbon nano tubes may still be more stable. This is because the edges of two-dimensional graphene planes are more energetic than threedimensional diamond surfaces, bucky balls with no ends, and rolled graphene planes with less ends. Hence, diamond, bucky balls and carbon nanotubes may form in smaller clusters of carbon atoms. Thus, in terms of thermodynamics, small-sized diamond is still more stable than small graphite even at low pressures (Fig. 11.15). In addition, the atomic distance (1.54 Å) in diamond lattice is much smaller than inter planar distance (3.35 Å) between graphene planes, hence, if the decomposed carbon atoms have a long mean free path, it can attach to a diamond nucleus much quicker than starting a new graphene plane. Consequently, diamond may grow prior the formation of graphite. Thus, in terms of kinetics, diamond can grow faster than graphite. Based on the above consideration, diamond may be preferentially grown from energetic carbon atoms formed from gas if the deposition rate is kept so slow that graphite is difficult to form. In 1952, William Eversole of Union Carbide was attempting to grow diamond this way by pyrolysis of methane at high temperature. In order to promote the diamond formation, he used seeded diamond as the precursor. Eversole might have found the window of diamond growth by decomposing methane gas at very slow rate. If so, he would have made diamond about a year before ASEA’s first artificial diamond made under high pressure; and it was 2 years before the widely publicized GE’s synthesis of diamond by converting graphite with metal catalyst. However, Eversole’s pyrolysis process could grow diamond only a few nanometers per hour, as any further rush of the process would yield deposition of only amorphous carbon. The characterization of nano-sized diamond was not possible in 1950s, particularly, it was a thin coating that covered a diamond seed,

The Role of Hydrogen Atoms

Figure 11.15. The nucleation of diamond is the easiest because of its three-dimensional structure. The nucleation of bucky ball (C60 ) is the second easiest because it is endless. The nucleation of carbon nanotube is still easier than graphite with further increases of size. The nucleation of graphite is the most difficult because the edge atoms are highly unstable with the presence of more than one dangling electrons. But with larger sizes of graphene planes, graphite becomes the most stable at low pressures. In the above diagram, the zero free energy refers to decomposed carbon atoms from a gas phase.

so Eversole was unable to confirm that he made any diamond at all. 11.6

THE ROLE OF HYDROGEN ATOMS

In 1968, Henry Hibshman suggested that hydrogen may be used to stabilize diamond growth during the pyrolysis of carbonaceous gases. In 1971, John Angus began to duplicate Eversole’s process using a hot filament process. This time he tried adding hydrogen

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gas to speed up the diamond growth. In 1976, Derjaquin starting experimenting the CVD growth of diamond by glow discharge. Again, he found that hydrogen addition was beneficial to his process. 1n 1981, Setaka first introduced microwave plasma for diamond growth using a high proportion of hydrogen gas. In 1988, Hirose found that acetylene flame could grow diamond film in air. This was the first time that vacuum chamber was not used to grow CVD diamond. 1n 1988, Norton acquired Technion’s arc jet technology and began to depositing thick diamond films (the author was responsible for this acquisition). The modern CVD processes for growing diamond films are dependent on using hydrogen atom as catalyst. Hydrogen atoms can be obtained by dissociation of hydrogen gas at high temperature. Because the dissociation efficiency is low, most CVD processes rely on diluting methane gas (or other carbonaceous gas) by adding an overwhelming proportion of hydrogen gas (e.g., 99%). The roles of hydrogen atoms are two folds. First, they can maintain the four valence electrons of carbon atoms in pseudo sp3 configurations so when they join together, they will form tetrahedral bonds of diamond. Second, they can gasify graphite to form methane if graphite is formed. In other words, hydrogen atoms can promote diamond formation and at the same time they may prevent graphite from being formed (Fig. 11.16). The ability for hydrogen atoms to catalyze the diamond formation lies in the same principal of “touch and go” as described above for the case of high-pressure synthesis. Just as carbon atoms must loosely attach to molten metal atoms so they can freely regroup to form diamond, so are carbons atoms must loosely attach to hydrogen atoms so they can readily link to deposit diamond. In either case, carbon atoms can maintain their sp3 configuration and attach easily to the growing diamond because of the constant arouse of metal solvent atoms (e.g. high pressure synthesis) or hydrogen solvent atoms (CVD growth). Hydrogen is more effective to catalyze carbon to form diamond than molten transition metals. Hence, the threshold temperature of diamond growth for the CVD process is lower than that for the high-pressure method. For example, CVD diamond is typically deposited at a temperature of about 900◦ C, whereas high-pressure diamond is synthesized around 1300◦ C. This difference in synthesis temperatures reflects the difference in activation energies. The

The Role of Hydrogen Atoms

Figure 11.16. The carbon atoms formed by decomposing sp3 configured methane may be coerced to maintain such configuration if abundant hydrogen atoms can be nearby to act like touch-and-go. When sp3 configured carbon atoms encounter one another, they will become part of diamond lattice. On the other hand, if graphite is formed, then hydrogen atoms can gasify it by combined graphite atoms to form methane again.

activation energies for hydrogen-assisted CVD processes are about 0.09 eV; and for metal catalyzed processes, 0.13 eV. In 1980s, Japanese invented several CVD processes that may grow metastable diamond at high deposition rates. Specifically, hydrogen molecules are decomposed by various means that heat reaction gases to high temperatures. The heating could be achieved by hot filament, microwave discharge, electric arc, or acetylene flame. It was found that the growth rate of diamond films is proportional to the square of the concentration of hydrogen atoms (Goodwin, 1993). As the concentration of hydrogen atoms tends to increase with increasing temperature of the gaseous phase, so is the growth rate of the diamond deposition and the quality of film formed. During a CVD process, the concentration of hydrogen atoms determines the relative growth rate between diamond and graphite (Fig. 11.17). Based on the above description, hydrogen atoms play a key role by keeping a dynamic balance between methane and decomposed carbon atoms. With the touch-and-go relationship, carbon

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Figure 11.17. The relative deposition rate of diamond and graphite is controlled by the concentration of hydrogen atoms in the gas mixture.

atoms are fooled to maintain its pseudo sp3 configuration with nearby hydrogen atoms until they join together to form diamond. In order to maintain this dynamic balance, it is critical that the carbon atoms must hit hydrogen atoms all the time and in no cases that carbon atoms should hit themselves, in particular, they must not run into one another consecutively. Hence, the mean free path of hydrogen atoms must be much shorter than the collision distance of carbon–carbon atoms. Consequently, for growing diamond films, there is a maximum carbon concentration that can be supported by the presence of hydrogen atoms; in other words, there is an optimal ratio of CH4 /H2 in the original gas mixture (Fig. 11.18). Thus, the higher the concentration of hydrogen atoms, the faster the diamond film can grow. Alternatively, if the deposition rate is maintained at optimal, then the higher the concentration of hydrogen atoms, the better the quality of the deposited film. The dependence of diamond’s growth rate (G) and the film’s defect

The Role of Hydrogen Atoms

Figure 11.18. Diamond formation rate and its conversion rate as a function of CH4 /H2 ratio in the original gas mixture.

concentration (D) on the concentration [H] of hydrogen atoms may be expressed by: G/D = K[H]

(11.1)

where K is dependent on the deposition conditions other than the concentration of hydrogen atoms. Thus, for a given concentration of hydrogen atoms in the reaction chamber, an optical grade diamond film must be grown very slowly, whereas a tool grade diamond film can be deposited at a higher rate. As both the growth rate and the quality of diamond film are dependent on the concentration of hydrogen atoms, hence, all CVD processes for growing diamond are designed to enhance the dissociation of hydrogen molecules in the gas mixture to form hydrogen atoms. The dissociation of hydrogen molecules are highly dependent on temperature and pressure (Table 11.1), hence, most CVD

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Table 11.1. The concentration of hydrogen atoms as a function of temperature. Temperature (◦ C)

[H] (kg mol/m3 ) P = 20 torr

[H] (kg mol/m3 ) P = 200 torr

1600 1800 2000 2200 2400 2600 2800 3000

7.62 × 10−8 4.43 × 10−7 1.78 × 10−6 5.47 × 10−6 1.35 × 10−5 2.75 × 10−5 4.69 × 10−5 6.65 × 10−5

2.41 × 10−7 1.40 × 10−6 5.66 × 10−6 1.75 × 10−5 4.43 × 10−5 9.50 × 10−5 1.77 × 10−4 2.92 × 10−4

processes are aimed to heat the gas mixture more effectively at a higher pressure. A common gas mixture for CVD growth of diamond film contains CH4 that is fully diluted in H2 . The dissociation of this gas mixture will produce various species of C and H radicals; all of them are temperature dependent (Fig. 11.19). 11.7

LIQUID PHASE GROWTH OF DIAMOND AT LOW PRESSURE

Gem diamond crystals have been grown by CVD methods at partial vacuum, but as graphite is the stable phase, the growth rate of diamond must be kept low lest graphitic carbons are formed. On the other hand, the growth of gem diamonds in molten alloy (e.g. invar composition) under ultrahigh pressure can be increased, but the volume of the apparatus is severely limited. It would be ideal if diamond can grow in liquid phase at atmospheric pressure. In this case, not only the growth rate can be high, but also there is no limitation in number of crystals growing simultaneously. During the growth of CVD diamond, hydrogen atoms must be around to stabilize diamondoid bonds. If a carbon solvent is also the catalyst for generating hydrogen atoms, diamond can grow without the accumulation of graphite at low pressures. The liquid phase is also a diamond catalyst (e.g. Fe, Co, Ni, Mn, Pd or its alloys) that may be alloyed with rare earth elements (La, Ce) to suppress its melting point (e.g. to 400–800◦ C). Such alloys have

Liquid Phase Growth of Diamond at Low Pressure

Figure 11.19. A literature example of possible gas species in an original mixture of CH4 (0.5%) and H2 . The plot is based on the gas pressure at 20 torr.

Figure 11.20. The phase diagrams of cerium alloy with either Co or Ni indicated that the eutectic compositions can be melted below 600◦ C.

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Chapter 11 Gem Diamond Growth

high solubilities of both carbon atoms and hydrogen atoms. Pd or Pt may be also be used as the liquid phase to enhance the catalytic absorption of hydrogen atoms. In the molten alloy, both carbon and hydrogen atoms hidden in voids of the metal atoms. The dissolved carbon atoms are surrounded by catalytic metal atoms, at the same time hydrogen atoms are nearby so the carbon atoms may maintain the diamondoid structure until they are deposited on the diamond seed (Fig. 11.20). With the presence of a liquid catalyst that can dissolve both carbon atoms and hydrogen atoms, diamond seeds (e.g. nanodiamonds) may be mixed with a diamondoid nutrient (e.g. adamantine) in such a way that diamond seeds are growing at the expense of diamondoid nutrient. An example of the reactor is shown below (Fig. 11.21). The above idea of diamond growth was tested by Prof. ChhiuTsu Lin and his Ph.D. candidate Chien-Chung Teng at Northern Illinois University. The following photographs represented the data taken from the initial experiments (Figs. 11.22 and 11.23).

Figure 11.21. The growth of diamond seeds in a diamond cooker with a molten catalyst and diamond nutrient thoroughly mixed by convection current. The left shows the concept; and the right, the schematic of an experimental reactor.

Liquid Phase Growth of Diamond at Low Pressure

Figure 11.22. The raw materials used for cooking diamond included nanodiamond (left diagram) as seeds, nano nickel as catalyst (middle diagram), and adamantine nano particles (right diagram) as nutrient.

Figure 11.23. The Raman spectra of the materials mixture before and after the cooking experiment (800◦ C for 10 hours). Note that the pronounced increase of diamond peak at 1332 cm−1 .

The above illustration indicated that multiple diamond seeds (loose or fixed) could be grown in a liquid system at ambient pressure. Such a novel idea, if implemented on a production scale, may revolutionize the way diamond is synthesized by either CVD or ultrahigh pressure. The ample availability of large diamond crystals can pave the road for the rapid advancement of diamond devices, including diamond semiconductors. The widespread use of diamond devices will mark the arrival of diamond age.

247

Index adamantine, 144, 146-148, 150 amorphous diamond, 7, 52, 92, 193-211, 214, 215 ankle joint, 185 Argonne National Labs, 110, 111 ASEA, 58, 62, 63, 238 atomic density, 45, 196 biocompatibility, 163, 184, 187 biomarker, 165, 176, 187 biomedical application, 166, 184 catalyst, 20, 26, 27, 49, 62, 65, 67-76, 81, 84, 85, 89, 100, 113, 115, 116, 142, 149, 227, 233, 238, 240, 244, 246, 247 catalytic mechanism, 66, 67, 71 China (Chinese), 13, 71, 86– 89, 133, 161, 232, 236 CNT (SWNT), 151, 200, 224, 225 cold cathode fluorescent lamp (CCFL), 198, 199 cosmetic, 148, 152, 154, 159– 161 cosmic abundance, 9–11 CVD, 5, 16, 18–20, 57–59, 74, 75, 78, 80–82, 88, 90, 91, 93, 107–110, 133,

134, 154, 164, 170, 171, 181, 215, 238, 240, 241, 243, 244, 247 CVD diamond (CVDD), 18– 20, 58, 80, 88, 91, 110, 144, 164, 170, 171, 238, 240, 244 dental, 153, 154, 170, 171 dermal strip, 153, 156, 157 detonation (explosion), 81, 121, 125–128, 133– 135, 173, 187, 222 dialon, 213, 218, 220–222, 225 diamond age, 36, 41, 44, 54, 247 diamond fine, 95, 98–101, 103–105, 114, 120, 227 diamond gem, 227, 229, 231, 232, 235 diamond-like carbon (DLC), 42, 43, 52, 53, 75, 78– 80, 82, 83, 88, 91–93, 129, 131, 166, 184– 186, 195, 202, 204, 213–223, 225 diamond market, 47 diamondoid, 20, 25, 28, 53, 144, 146–149, 244, 246 diamond synthesis, 57–59, 62, 70–72, 74–76, 79– 83, 86–89, 115 dispersant, 153, 159, 161

Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin Copyright © 2009 by Pan Stanford Publishing Pte Ltd www.panstanford.com 978-981-4241-36-6

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Index

display, 43, 91, 193, 200, 202, 231, 235 DLC electro-luminescence (DEL), 92, 202, 204, 205 DNA attachment, 167 drug delivery, 155, 175, 178, 187 dynamite diamond (dynamite formed diamond, dynamite nanodiamond, explosive detonated), 106, 107, 112, 119–123, 129, 133, 149 electrode, 38, 171, 179–183, 198–200, 202, 208, 211, 221 electroplating, 132, 137 Element Six, 85, 88, 133 emission spectrum, 3 endoprothesis, 166, 169 euhedral diamond, 120 explosive diamond, 120 field emission, 43, 53, 193, 200, 201, 205, 206 fluorinated DLC, 213, 216, 218, 219 friction coefficient, 42, 141 Gemesis, 227, 231, 233 General Electric (GE), 62–66, 71, 84–88, 101–103, 133, 227, 229, 230, 238 growth, 21, 47, 58, 70, 72–74, 78, 80, 85, 91, 110, 127, 151, 178, 179,

227, 229–234, 236– 244, 246 Hannay, 57, 58, 62 hardness (superhard), 40, 42, 46, 104, 117, 128– 130, 133, 137, 138, 140, 141, 143, 147, 155, 163, 164, 166, 184, 187, 208, 213– 215, 225 healthcare, 152–154, 156–159 heat spreader, 42, 48, 49, 86, 91, 92, 132, 143, 144, 203 hexagonal diamond, 2, 68, 69, 115, 124 hope diamond, 5, 6 hydrogen atom, 1, 8, 14, 15, 20, 21, 59, 74, 76, 77, 80, 110, 155, 214, 224, 239–244, 246 hydrophobic, 27, 104, 105, 151, 166, 174, 213, 214, 218, 222 immobilization, 173, 175, 180, 187 kimberlite, 7, 59–61 kinetic, 75, 78, 79, 82, 83, 110, 238 Kinik Company, 91–93 LED, 92, 201, 203, 204 liquid phase, 244, 246 lubrication, 131, 139 methane, 5, 6, 14–19, 23, 26, 28, 29, 41, 44, 74, 77,

INDEX

79, 81, 83, 91, 110, 147, 238, 240, 241 microdrill, 216–218 microelectrode, 180–183 micron fine (micron diamond), 15, 89, 95, 97, 100–110, 112, 113, 116–118, 120, 139–141, 144, 227 Moisson, 57–59 mono diamond (monocrystalline, single crystal), 65, 85, 102, 103, 106, 107, 116–119, 131, 194 mypolex, 113, 118, 119 nail polish, 153, 158–161 nanocrystelline diamond (NCD), 166, 169, 180, 181 nanodiamond, 2–4, 13, 14, 23, 25–37, 39, 40, 95, 104, 105, 110, 111, 113, 118–129, 131– 135, 137–145, 147– 149, 151–161, 163, 166, 168, 173, 174, 176–179, 193, 222, 246, 247 nanodiamond agglomerate, 124 nanodiamond cosmetics, 148, 161 nanodiamond structure, 4 Neptune, 16–18, 41 nitrogen-vacancy (N-V), 40, 176 nuclear binding energy, 9

nucleation, 70, 88, 107, 110, 112, 127, 133, 239 polishing, 43, 99, 101, 102, 104, 105, 108, 116– 119, 131, 138, 143, 217, 227, 234 polycrystal, 116–119, 131, 194 polycrystalline diamond (PCD), 85, 102, 103, 112, 130, 133, 139–145 printed circuit board (PCB), 216, 217 pulverized diamond, 120 PVD, 75, 78–82, 88, 91, 92, 195 Raman (Raman absorption), 165, 178–180, 247 RDX, 123, 126, 133, 134 rhombohedral, 68, 69, 72, 73, 84, 114–116 self sharp, 118 semiconductor, 5, 6, 10, 27, 36, 43–45, 48–50, 91, 144, 193, 195–197, 207, 247 shock (shock wave, shock wave diamond), 2, 11, 65, 76, 81, 83, 106–108, 110–118, 120, 121, 123, 124, 131, 154 sintering, 24, 88, 102, 103, 133, 139–141 skin lotion, 156 solar cell, 91, 193, 194, 204, 207–210 South Africa, 59, 61, 84, 86, 87 South Korea, 86, 87

251

252

Index

split die, 231, 236 stent, 43, 185, 220, 222 storage, 53, 119, 133, 155, 202, 224, 225 Sumitomo Electric, 85, 227 super-entropic (super entropy), 195 supernova explosion, 10, 12 Sweden, 62, 86, 87

thermionic (thermionic emission), 193, 195, 198, 200 TNT, 76, 123, 125, 126, 133, 134 toothpaste, 153, 156, 158, 159 Tracy Hall, 63 tungsten filament, 6

tetrahedral amorphous carbon (tac), 195, 214

white dwarf, 2, 3, 10, 12, 26 work function, 197, 198, 200

ultracomputer, 3, 51