Carbon Nanotubes
Carbon Nanotubes, Electronics and Photonics
Jessica Josilevich, Rebekah Miller, Jason Surbrook
Summer Ventures in Science and Mathematics: Optical Engineering
Dr. Kasra Daneshvar
13 July 2010
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Carbon Nanotubes
Abstract
Carbon nanotubes are microscopic tubes made of carbon atoms arranged in a hexagonal shape.
They are known for their strength, conductivity, and thermal stability. Depending on how the
carbon nanotubes are rolled they can either be conductors, insulators, or semiconductors. There
are currently few areas in which carbon nanotubes are used because most applications are still
being studied and experimented with. The electronics industry could greatly benefit from the
high conductivity of carbon nanotubes and their minimal heat losses. Field effect transistors and
field emission are two potential uses for carbon nanotubes in electronics and photonics. Carbon
nanotubes are a great unknown in much of society, yet are full of potential for those who seek
advancement in industry.
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Carbon Nanotubes
Carbon Nanotubes, Electronics and Photonics
Introduction
A carbon nanotube is a microscopic tube made of carbon atoms arranged in a hexagonal
shape. In 1991 when carbon nanotubes were officially discovered by Sumio Iijima, endless
possibilities opened up. Carbon nanotubes are not only extremely small but their incredible
properties make them useful for many applications. Some of the unique properties of carbon
nanotubes include their great strength, density, extremely small size and ability to conduct heat.
Carbon nanotubes can be single walled or multi-walled. They appear to be either a single layer
or multiple layers of graphite rolled into tubes. Carbon nanotubes are already currently used in
high-end sports equipment and could, in the future, be used for hip replacements and bullet proof
clothing. In electronics the possibilities are even more incredible. The ways the tubes roll
determine whether or not they are conductors, insulators, or semiconductors. Semiconductor
nanotubes have properties similar to those in silicon and are therefore likely to be used in the
future as transistors in electronic devices. Two popular ways to create nanotubes are arcdischarge and chemical vapor deposition. Arc-discharge produces a small number of carbon
nanotubes and many other forms of carbon found along with the nanotubes. Unfortunately, only
some of the tubes are useful and it is difficult to separate the other forms of carbon from the
nanotubes. In the chemical vapor deposition process, the nanotubes are “grown” from metal
catalysts and their construction can be manipulated more easily; this process can ensure the
nanotubes are rolled in a way that facilitates the properties of a semiconductor. Their minuscule
size gives carbon nanotubes the ability to drastically shrink the electronics industry. Carbon
nanotubes that are semiconductors can, in theory, be attached to metal electrodes and used in
Field Effect Transistors. Field Effect Transistors can be used as switches in electrical devices
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Carbon Nanotubes
and when carbon nanotubes are used the heating losses are lowered due to the fantastic
conductivity of the tubes. Carbon nanotubes are also used in a process known as Field Emission.
In this process the tubes release electrons on to electroluminescent material and a range of colors
can be produced. However, there are some dangers involved in using carbon nanotubes. The
tubes are similar in structure to asbestos and could cause cancer if inhaled. As a general rule
though, the carbon nanotubes are used in more controlled situations and less likely to cause
cancer when woven together or used inside electronic devices. To fully understand the
properties, applications, and hazards of carbon nanotubes, knowledge of their early development
is necessary.
History
Carbon nanotubes, as we identify them today, began with the discovery of carbon 60. C60
was discovered by Richard Smalley, Robert Curl, and Harry Kroto in 1985. The scientists
vaporized carbon and by putting it in a mass spectrometer they were able to see the points at
which the carbon had the strongest peaks. The strongest peaks appeared for carbon 60. The C60
molecule has 60 carbon atom molecules and 32 faces, 12 are in the shape of a pentagon, and 20
are hexagonal in shape. The molecule is spherically shaped and seems to resemble a soccer ball.
Many physical manmade objects take on the shape of the C60 molecule. Some examples include
golf balls, soccer balls, architecture, and art. The ride at Epcot Center in Disney World also
resembles the shape of the C60 atom. The carbon 60 atom is referred to as
“buckminsterfullerene” or the “buckyball” after the architect Buckminster Fuller (Shapter, 2004).
In the 1970s, scientist Morinobu Endo noticed a carbon structure similar to the
“buckyball” but in the shape of a cylinder instead of a ball. He observed tubes produced by a gas
phase process (Shapter, 2004). In 1991 while observing carbon soot Sumio Iijima noticed
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Carbon Nanotubes
microscopic threads made of carbon and coined the name “carbon nanotubes” for the objects he
noticed. The tubes Iijima observed were mulitwalled tubes; two years later Iijima and Donald
Bethune of IBM created a single walled carbon nanotube made up of one layer of graphene
(Collins, & Avouris, 2000). One major obstacle currently deterring the wide spread use of
carbon nanotubes is the complexity of their manufacture.
Creation of Nanotubes
There are currently many different ways in which to create carbon nanotubes. The two
most popular methods are arc-discharge and chemical vapor deposition. For arc-discharge two
rods of carbon graphite are used; one rod is the anode and the other is the cathode. The rods are
placed inside a chamber where nonreactive gas is heated up. The rods are brought close to each
other and because of heated gas a spark is produced. The process continues and as the anode is
broken down by the constant sparking the carbon nanotubes, along with many other forms of
carbon, are produced. The cathode is removed after a certain amount of time and the carbon
inside of the chamber, a great deal of which is stuck to cathode, is removed. The carbon
nanotubes found on the cathode usually contain twenty to thirty walls. However, if a metal
catalyst is included in the anode before the process begins, single walled nanotubes can be
produced. Carbon nanotubes made using arc-discharge are often excellent in quality;
nonetheless, the number of tubes produced this way is minute. Other forms of carbon are
produced and are difficult to separate from the carbon nanotubes (Grobert, 2008).
The process of chemical vapor deposition is much more promising as far as the creation
of carbon nanotubes for industrial uses is concerned. The starting materials in this process
consist of a large chamber holding carbon based gas, tiny metal catalysts, and non reactive gas.
The minuscule metal catalysts act as seeds from which the carbon nanotubes grow. The diameter
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Carbon Nanotubes
of the carbon nanotubes is therefore determined by the size of the metal catalysts. The gases and
catalysts are heated to between 300 and 1150 degrees Celsius. The process can continue for as
little as five minute or even hours depending on how long the nanotubes are desired to be
(Grobert, 2008). If carbon nanotubes could be created uniformly and in mass, the everyday
applications of such objects would be extraordinary.
General Applications
Carbon nanotubes can be used for a variety of functions. Their unique properties are
serviceable in a wide range of functions including sporting and electronics. Carbon nanotubes
are currently used in high end sports equipment such as tennis racquets, golf clubs, baseball bats,
hockey sticks and even in bikes used during the Tour de France. Diamond cutting is another
function of the carbon nanotubes because of their incredible strength and density. Some
proposed applications of carbon nanotubes include bullet proof clothing, dent resistant vehicles,
buildings that can withstand earthquakes, microscopic robots, and even cables that could carry an
elevator to space. Still, the most interesting functions of carbon nanotubes are in the field of
electronics and photonics. The conductivity, durability, and minute size of semiconductor
nanotubes make them an ideal replacement for silicon in transistors.
Electrical Applications
Carbon nanotubes are usually tested to be used as super conductors, which carry
electricity very efficiently, or as semiconductors, similar to silicon (Avouris, 2009). Many
engineers are fascinated by the electrical possibilities of carbon nanotubes. Despite being a nonmetal, carbon nanotubes share some conductive properties with copper, aluminum, and other
metals. Engineers are specifically looking at the super conduciveness of carbon nanotubes, their
thermal properties and their potential for smaller transistors in electronics (Avouris, 2009).
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Carbon Nanotubes
Carbon nanotubes have an unbound electron in each carbon atom, giving it conductive
properties (Solin, 2000). The nanotubes are seamlessly rolled hexagonal sheets of graphene, a
single atom layer of graphite (Avouris, 2009). Since carbon has four valence electrons (electrons
in the outermost layer), the covalent bonding hexagonal arrangement of the atoms takes three of
those valence electrons and shares them with neighboring carbon atoms. The final of the four is
left in its own orbital, free of bonds, which is useful when carrying electricity. Near the ends of
each of the tubes, the sides fold in and make a rounded tip, all the while continuing the same
hexagonal shape throughout (IPE Nanotube Primer).
As mentioned before, the bonding of atoms in carbon nanotubes leave a free electron per
atom in the tube. When an electrical current is passed through the tube, these electrons repel each
other and move down the tube, continuing the flow of electricity. In a pure carbon nanotube,
there is almost no resistance, which makes carbon nanotubes great conductors. In multiwalled
carbon nanotubes, there are multiple sheets of graphene, meaning more atoms, and more free
electrons to carry electricity. The more layers in the carbon nanotube, the more electricity it will
conduct; however, for most cases a single walled carbon nanotube will be more than sufficient in
carrying electricity.
When electricity is run through a material with high resistance, then moving electrons
lose energy and heat is given off, and lost from the device. The more heat given off, more fans
and/or larger circuit boards are needed to not let the heat lost due to electrical resistance reach
points where components of the device could melt. A carbon nanotube’s high conductivity
allows very minimal, almost not any, heat to be lost. This requires less electricity since less will
be lost to heat, as well as smaller fans can be used to cool the device. This makes the device
overall to be more energy efficient and cooler.
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Carbon Nanotubes
Carbon nanotubes not only create less heat, but are more resistant to heat as well. It is
estimated that carbon nanotubes can withstand temperatures of up to 2800˚C in a vacuum and
750˚C in atmospheric air (Collins, & Avouris, 2000). This will allow heat in electronic devices to
be less of an engineering issue when designing new technologies. With less heat emissions,
components may get closer together and the device smaller. One of today’s largest problems in
shrinking electronics is the size of silicon transistors. Transistors are electrical switches made of
three parts: a source, a gate, and a drain. The source and the drain carry electricity to and from
different parts of the device. The gate is an on/off switch that will act as a conductor or an
insulator, traditionally silicon. The problem is that each part must be built separately and
arranged together (Avouris, 2009). Another possibility for carbon nanotubes is in the area of
photonics.
Field Emission
Carbon nanotubes can be used in a process known as field emission, an aspect of
photonics. Field emission involves the extraction of electrons from a solid by tunneling through
the surface potential barrier. When voltage is passed through the carbon nanotubes the electrons
carried by the carbon nanotubes attempt to separate from each other. Because the electrons are
trying to escape each other, the electrons travel to the pointed ends of the carbon nanotubes.
Eventually the electrons are emitted out of the end of the nanotube. If an electroluminescent
material is placed opposite of the pointed carbon nanotubes, when the electrons are released they
can transfer energy to the material and cause color to be produced (Collins, & Avouris, 2000).
Unfortunately, carbon nanotubes are not necessarily safe to use for all purposes.
Hazards
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Carbon Nanotubes
Similar to the functions of carbon nanotubes, the risks carbon nanotubes pose to human
health are still being discovered. In the early 1900s many people thought that asbestos would
revolutionize the industrial world. As it turned out, asbestos is now known as one of the most
dangerous and deadly materials to have been mass produced for industrial purposes. Many
people cringe at the mere mention of asbestos as thoughts of lung cancer and death invade their
mind. Carbon nanotubes have been observed as having a similar structure to asbestos; therefore,
many scientists have been studying the effects inhaling carbon nanotubes could have on a
person’s lungs. The pointed ends of the carbon nanotubes make it difficult for a human’s lungs
to get rid of them. If left inside the lungs, the carbon nanotubes could form lesions and inflame
the lungs. Carbon nanotubes are used on a much smaller scale than asbestos and have yet to be
mass produced. The uses of carbon nanotubes will make their inhalation unlikely, all the same it
is crucial that the risk to those manufacturing and using carbon nanotubes be assessed
(Sanderson, 2008).
Our Ideas and Conclusion
As the future approaches the possibilities for carbon nanotubes are increasing by the
second. Our predictions for the uses of carbon nanotubes encompass ideas ranging from clean
energy to possible applications in medicine. In the future we believe that carbon nanotubes
could be used in prosthetics and for stronger bone reinforcements. Carbon nanotubes used in
combination with metals could create indestructible jewelry. Also, the unique properties of
carbon nanotubes that make them capable of transferring large amounts of energy with minimal
heat losses could lead to miniature nanotube generators powering small cities. Electronic
devices could become drastically smaller if carbon nanotube processers were manufactured for
the general public. While their potential is still being determined, the future for these tiny tubes
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is promising. As scientists continue to research the properties, hazards, and applications of
carbon nanotubes it becomes clear that these small devices could easily revolutionize the way we
live.
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Carbon Nanotubes
Works Cited
Avouris, P. (2009, January). Carbon nanotibe electronics and photonics. Physics Today, 34-40.
Collins, P.G., & Avouris, P. (2000, December). Nanotubes for electronics. Scientific American,
62-69.
Grobert, N. (Producer). (2008). Nanoseries 2/5: how are carbon nanotubes made? . [Web].
Retrieved from http://www.youtube.com/watch?v=B4VTfgaKLAM
Sanderson, K. (2008). Carbon nanotubes: the new asbestos?. naturenews, Retrieved from
http://www.nature.com/news/2008/080520/full/news.2008.845.html
Shapter, J. (2004, June 17). Carbon nanotubes-history and development of carbon nanotubes.
Retrieved from http://www.azonano.com/details.asp?ArticleID=982
Solin, S.A. (2004, July). Magnetic field nanosensors. Scientific American, 71-77.
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