Carbon Nanotubes
Carbon, a group IV element, has two crystalline forms: diamond and graphite.
Carbon nanotubes (CNTs) are allotropes of carbon. These cylindrical carbon
molecules have novel properties that make them potentially useful in many
applications in nanotechnology, electronics, optics and other fields of materials
science, as well as potential uses in architectural fields. They exhibit extraordinary
strength and unique electrical properties, and are efficient conductors of heat. CNTs
are members of the fullerene structural family, which also includes the spherical
buckyballs 1. The ends of a CNT might be capped with a hemisphere of the buckyball
structure.
Carbon nanotubes are one of the most commonly mentioned building blocks
of nanotechnology. With one hundred times the tensile strength of steel, thermal
conductivity better than all but the purest diamond, and electrical conductivity
similar to copper, but with the ability to carry much higher currents, they seem to be
a wonder material. However, when we hear of some companies planning to produce
hundreds of tons per year, while others seem to have extreme difficulty in producing
grams, there is clearly more to this material than meets the eye.
Carbon nanotubes, long thin cylinders of carbon, were discovered in 1991
by Iijima’s. Carbon nanotubes (CNTs) are allotropes of carbon which are members
of the fullerene structural family, which also includes the spherical buckyballs.
These are large macromolecules which are unique for there size, shape and
remarkable physical properties.
The nature of the bonding of a nanotube is described by applied quantum
chemistry, specifically, orbital hybridization. The chemical bonding of nanotubes is
composed entirely of sp2 bonds, similar to those of graphite. This bonding
structure, which is stronger than the sp3 bonds found in diamond, provides the
molecules with their unique strength. Nanotubes naturally align themselves into
“ropes” held together by Van der Waals forces. Under high pressure, nanotubes
can merge together, trading some sp² bonds for sp³ bonds, giving the possibility
of producing strong, unlimited-length wires through high-pressure nanotube
linking.
CNTs are named on the basis of derived from their size, since the diameter of
a nanotube is on the order of a few nanometers, while they can be up to several
millimeters in length CNTs are categorized as single-walled nanotubes (SWNTs) and
multi-walled nanotubes (MWNTs) depending upon the number of walls. CNTs may
consist of one up to tens and hundreds of concentric shells of carbons with adjacent
shells separation of 0.34 nm i.e. (002). The carbon network of the shells is closely
related to the honeycomb arrangement of the carbon atoms in the graphite sheets.
The amazing mechanical and electronic properties of the nanotubes stem in their
quasi-one dimensional (1D) structure and the graphite-like arrangement of the
carbon atoms in the shells. Thus, the nanotubes have high Young’s modulus and
tensile strength, which makes them suitable for composite materials with improved
mechanical properties. The nanotubes can be metallic or semi conducting depending
on their structural parameters.
The term nanotube is normally used to refer to the carbon nanotube, which
has received enormous attention from researchers over the last few years and
promises, along with close relatives such as the nanohorn, a host of interesting
applications. There are many other types of nanotube, from various inorganic kinds,
such as those made from boron nitride, to organic ones, such as those made from
self assembling cyclic peptides (protein components) or from naturally-occurring
heat shock proteins (extracted from bacteria that thrive in extreme environments).
However, carbon nanotubes excite the most interest, promise the greatest variety of
applications, and currently appear to have by far the highest commercial potential.
Only carbon nanotubes will be covered in this white paper.
NANOTUBES are the most successful materials that are now attracting a
broad range of scientists and industries due to their fascinating physical and
chemical properties. In this review, we enlighten you about this material. We are
introducing here, the structure, synthesis and the most important applications of
carbon nanotubes in different fields. The session will feature technology that
exploits novel electronic, electro-mechanical, transistors, and electrical circuits,
optical and structural properties of a carbon nanotube for the solution of
engineering problems.
Nanotubes are members of the fullerene structural family, which also
includes the spherical buckyballs, and the ends of a nanotube may be capped with
a hemisphere of the buckyball structure. Their name is derived from their long,
hollow structure with the walls formed by one-atom-thick sheets of carbon, called
graphene.
Graphene:Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets
of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice,
the term graphene was coined as a combination of graphite and the suffix ene by Hanns-Peter Boehm who described single-layer carbon foils in 1962.
Graphene is most easily visualized as an atomic-scale chicken wire made of carbon
atoms and their bonds. The crystalline or "flake" form of graphite consists of
many graphene sheets stacked together.
The carbon-carbon bond length in graphene is about 0.142 nanometres. Graphene
sheets stack to form graphite with an interplanar spacing of 0.335 nm, which
means that a stack of three million sheets would be only one millimetre thick.
Graphene is the basic structural element of some carbon allotropes including
graphite, charcoal, carbon and fullerenes. It can also be considered as an
indefinitely large aromatic molecule, the limiting case of the family of
flat polycyclic aromatic hydrocarbons.
Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional
(2D) honeycomb lattice, and is a basic building block for graphitic materials of all
other dimensionalities. It can be wrapped up into 0D fullerenes, rolled into 1D
nanotube or stacked into 3D graphite.
Graphene is an isolated atomic plane of graphite. From this perspective, graphene
has been known since the invention of X-ray crystallography. Graphene planes
become even well separated in intercalated graphite compounds. In 2004
physicists at the University of Manchester and the Institute for Microelectronics
Technology, Chernogolovka, Russia, first isolated individual graphene planes by
using adhesive tape. They also measured electronic properties of the obtained
flakes and showed their unique properties.
The Nobel Prize in Physics for 2010 was awarded to Andre Geim and Konstantin
Novoselov”for groundbreaking experiments regarding the twodimensional material graphene".
The Royal Swedish Academy of Sciences has awarded the Nobel
Prize in Physics for 2010 to Andre Geim and Konstantin Novoselov, both
of the University of Manchester, "for groundbreaking experiments
regarding the two-dimensional material graphene."
A thin flake of ordinary carbon, just one atom thick, lies behind this year's
Nobel Prize in Physics. Geim and Novoselov have shown that carbon in such a flat
form has exceptional properties that originate from the remarkable world of
quantum physics.
Geim and Novoselov extracted the graphene from a piece of graphite such as is
found in ordinary pencils. Using regular adhesive tape they managed to obtain a
flake of carbon with a thickness of just one atom. This at a time when many
believed it was impossible for such thin crystalline materials to be stable.
However, with graphene, physicists can now study a new class of twodimensional materials with unique properties. Graphene makes experiments
possible that give new twists to the phenomena in quantum physics. Also a vast
variety of practical applications now appear possible including the creation of new
materials and the manufacture of innovative electronics. Graphene transistors are
predicted to be substantially faster than today's silicon transistors and result in
more efficient computers.
Since it is practically transparent and a good conductor, graphene is
suitable for producing transparent touch screens, light panels, and maybe even
solar cells.
When mixed into plastics, graphene can turn them into conductors of electricity
while making them more heat resistant and mechanically robust. This resilience
can be utilized in new super strong materials, which are also thin, elastic and
lightweight. In the future, satellites, airplanes, and cars could be manufactured
out of the new composite materials.
Graphene formation:The Manchester group obtained graphene by mechanical exfoliation of
graphite. They used cohesive tape to repeatedly split graphite crystals into
increasingly thinner pieces. The tape with attached optically transparent flakes
was dissolved in acetone, and, after a few further steps, the flakes including
monolayers were sedimented on a silicon wafer. Individual atomic planes were
then hunted in an optical microscope. A year later, the researchers simplified the
technique and started using dry deposition, avoiding the stage when graphene
floated in a liquid. Relatively large crystallites (first, only a few micrometers in size
but, eventually, larger than 1 mm and visible by a naked eye) were obtained by the
technique. It is often referred to as a scotch tape or drawing method. The latter
name appeared because the dry deposition resembles drawing with a piece of
graphite. The key for the success probably was the use of high-throughput visual
recognition of graphene on a properly chosen substrate, which provides a small
but noticeable optical contrast. The Optical properties section below has a
photograph of what graphene looks like.
There were a number of previous attempts to make atomically thin graphitic films
by using exfoliation techniques similar to the drawing method. Multilayer samples
down to 10 nm in thickness were obtained. These efforts were reviewed in 2007.
Furthermore, a couple of very old papers were recently unearthed in which
researchers tried to isolate graphene starting with intercalated compounds. These
papers reported the observation of very thin graphitic fragments (possibly
minelayers) by transmission electron microscopy. Neither of the earlier
observations was sufficient to "spark the graphene gold rush", until the Science
paper did so by reporting not only macroscopic samples of extracted atomic
planes but, importantly, their unusual properties such as the bipolar transistor
effect, ballistic transport of charges, large quantum oscillations, etc. The
discovery of such interesting qualities intrinsic to graphene gave an immediate
boost to further research and several groups quickly repeated the initial result
and moved further. These breakthroughs also helped to attract attention to other
production techniques, such as epitaxial growth of ultra-thin graphitic films. In
particular, it has later been found that graphene monolayers grown on SiC and Ir
are weakly coupled to these substrates and the graphene-substrate interaction
can be passivated further.
The weak van der Waals force that provides the cohesion of multilayer graphene
stacks does not always affect the electronic properties of the individual graphene
layers in the stack. That is, while the electronic properties of certain multilayered
epitaxial graphenes are identical to that of a single graphene layer, in other cases
the properties are affected as they are for graphene layers in bulk graphite. This
effect is theoretically well understood and is related to the symmetry of the
interlayer interactions.
Experimental methods for the production of graphene ribbons are
reported consisting of cutting open nanotubes. In one such method multi walled
carbon nanotubes are cut open in solution by action of potassium
permanganate and sulfuric acid. In another method graphene nanoribbons are
produced by plasma etching of nanotubes partly embedded in a polymer film.
Properties of graphene:Atomic structure:The atomic structure of isolated, single-layer graphene was studied
by transmission electron microscopy (TEM) on sheets of graphene suspended
between bars of a metallic grid. Electron diffraction patterns showed the
expected hexagonal lattice of graphene. Suspended graphene also showed
"rippling" of the flat sheet, with amplitude of about one nanometer. These ripples
may be intrinsic to graphene as a result of the instability of two-dimensional
crystals or may be extrinsic, originating from the ubiquitous dirt seen in all TEM
images of graphene. Atomic resolution real-space images of isolated, single-layer
graphene on SiO2 substrates were obtained by scanning tunneling microscopy.
Graphene processed using lithographic techniques is covered
by photoresist residue, which must be cleaned to obtain atomic-resolution
images.
Graphene sheets in solid form (density > 1 g/cm3) usually show evidence in
diffraction for graphite's 0.34 nm (002) layering. This is true even of some singlewalled carbon nanostructures. Transmission electron microscope studies show
faceting at defects in flat graphene sheets.
Electronic property:Graphene differs from most conventional three-dimensional materials.
Intrinsic graphene is a semi-metal or zero-gap semiconductor. Understanding the
electronic structure of graphene is the starting point for finding the band
structure of graphite. It was realized as early as 1947 by P. R. Wallace that the E-k
relation is linear for low energies near the six corners of the two-dimensional
hexagonal Brillouin zone, leading to zero effective for electrons and holes. Due to
this linear (or “conical") dispersion relation at low energies, electrons and holes
near these six points, two of which are inequivalent, behave
like relativistic particles described by the Dirac equation for spin 1/2
particles. Hence, the electrons and holes are called Dirac fermions, and the six
corners of the Brillouin zone are called the Dirac points.
The equation describing the E-k relation is
The Fermi velocity vF ~ 106m/s
Thermal properties:The near-room temperature thermal conductivity of graphene was
recently measured to be between (4.84±0.44) ×103 to (5.30±0.48) ×103Wm−1K−1.
These measurements, made by a non-contact optical technique, are in excess of
those measured for carbon nanotubes or diamond. It can be shown by using
the Wiedemann-Franz law, that the thermal conduction is phonon-dominated.
However, for a gated graphene strip, an applied gate bias causing a Fermi
energy shift much larger than kBT can cause the electronic contribution to
increase and dominate over the phonon contribution at low temperatures.
Mechanical properties:Graphene appears to be one of the strongest materials ever tested.
Measurements have shown that graphene has a breaking strength 200 times
greater than steel, with a tensile strength of 130GPa (19,000,000 psi).
Using an atomic force microscope (AFM), the spring constant of suspended
graphene sheets has been measured. Graphene sheets, held together by van der
Waals forces, were suspended over SiO2 cavities where an AFM tip was probed to
test its mechanical properties. Its spring constant was in the range 1–5 N/m and
the Young's modulus was 0.5 TPa, which differs from that of the bulk graphite.
These high values make graphene very strong and rigid. These intrinsic properties
could lead to using graphene for NEMS applications such as pressure sensors and
resonators.
HISTORY:
The history of carbon nanotubes is not entirely clear even for those in the science
therefore giving proper credit to the person that invented the carbon nanotube
has been the subject of several high tech debates among the scientific
communities.
The initial history of nanotubes started in the 1970s. A preparation of the planned
carbon filaments was completed by Morinobu Endo who was earning his Ph.D. at
the University of Orleans, France.
This was still a highly important development in the history of carbon nanotubes,
but it just wasn’t the right time to be considered the first recognized invention.
Giving the proper credit to who invented carbon nanotubes would not come
along for another 20 years. In 1991 the true first invention of nanotube was finally
made. It seems as though there was a race between Russian nanotechnologists
and Sumio Iijima of IBM.
The first observation of the multiwalled carbon nanotubes was credited to Iijima.
There are some that hold the belief that in the 1950s there was an initial discovery
of what could have possibly been seen as the first carbon nanotubes had Roger
Bacon had the high powered electron microscope that would have been
necessary.
He was credited with the first visual impression of the tubes of atoms that roll up
and are capped with fullerene molecules by many scientists in the field. Some
state that his discovery just wasn’t taken very seriously at the time because
science did not know how this discovery could impact scientific research.
It would be in 1993 that Iijima and Donald Bethune found single walled nanotubes
known as buckytubes. This helped the scientific community make more sense out
of not only the potential for nanotube research, but the use and existence of
fullerenes.
With this information, the complete discovery of carbon nanotubes was realized
and Iijima and Bethune were ultimately credited with their discovery in their
entirety. Russian nanotechnologists were independently discovering the same
visual affirmation. They were just a little bit later in their announcement and the
potential affect of this discovery.
The continuation of research revealed a great deal about nanotubes and their
place in scientific discovery. The research has indicated that there are three basic
types of nanotubes (zigzag, armchair, and chiral) as well as single walled and
multiwalled nanotubes.
There are buckytubes, which are completely hollow molecules that are crafted
from pure carbon and are bonded together in a pattern of specific hexagon
patterns. The multiwalled nanotubes are likely to suffer from defects. These
defects happen in more than half of all multiwalled nanotubes.
The multiwalled nanotubes have already made appearances in practical
applications like creating tennis rackets that are stronger than steel but are ultra
light in weight. These nanotubes are also responsible for creating sunscreen and
other skin care products that are clear or able to be blended into the skin without
leaving behind residue as well as the creation of UV protective clothing.
As nanotechnologists continue to research nanotubes, there is still a race to
discover something new within the science. Scientists are researching the
potential for life saving techniques as well as the potential to create nanotubes
that can be tailored toward specific designated jobs.
With the creation of specified nanotubes, the potential for their use will become
unlimited and there will be a nanotechnology world hard at work crafting all kinds
of products from the convenient to the life saving.
TYPES OF CARBON NANOTUBES:a) SINGLE-WALLED CNT’s
These are the stars of the nanotube world, and somewhat reclusive ones at that,
being much harder to make than the multi-walled variety. The oft-quoted amazing
properties generally refer to SWNTs. As previously described, they are basically
tubes of graphite and are normally capped at the ends although the caps can be
removed. The caps are made by mixing in some pentagons with the hexagons and
are the reason that nanotubes are considered close cousins of
buckminsterfullerene a roughly spherical molecule made of sixty carbon atoms,
that looks like a soccer ball
and is named after the architect Buckminster Fuller (the word fullerene is used to
refer to the variety of such molecular cages, some with more carbon atoms than
buckminsterfullerene, and some with fewer).
The theoretical minimum diameter of a carbon nanotube is around 0.4
nanometers, which is about as long as two silicon atoms side by side, and
nanotubes this size have been made. Average diameters tend to be around the 1.2
nanometer mark, depending on the process used to create them.
SWNTs are more pliable than their multi-walled counterparts and can be twisted,
flattened and bent into small circles or around sharp bends without breaking.
Most single-walled nanotubes (SWNT) have a diameter close to 1nm, with a tube
length that can be many thousands of times longer. SWNTs are very important
carbon nanotube because they exhibit important electric properties that are not
shared by the multi-walled carbon nanotubes (MWNT) variants.
SWNTs can be excellent conductors and the most building block of SWNT system
is the electric wires. One useful application of SWNTs is in the development of the
first intramolecular field effect transistors (FETs).
STRUCTURE:
The bonding in carbon nanotubes is sp², with each atom joined to three
neighbours, as in graphite. The tubes can therefore be considered as rolled-up
graphene sheets (graphene is an individual graphite layer). There are three
distinct ways in which a graphene sheet can be rolled into a tube, as shown
below.
The terms “armchair” and “zig-zag” refer to the arrangement of hexagons
around the circumference. The third class of tube, which in practice is the most
common, is known as chiral, meaning that it can exist in two mirror-related forms.
An example of a chiral nanotube is as shown in fig. below.
In the ideal case, a carbon nanotube consists of either one cylindrical graphene
sheet (single-wall nanotube, SWNT) or of several nested cylinders with an
interlayer spacing of 0.34 – 0.36 nm that is close to the typical spacing of
turbostratic graphite (multiwalled nanotube, MWNT).
There are many possibilities to form a cylinder with a graphene sheet: the simplest
way of visualizing this is to use a "de Heer abacus":
A “de Heer abacus”: to realize a (n,m) tube, move n times a1 and m times
a2 from the origin to get to point (n,m) and roll-up the sheet so that the two
points coincide...
Basically, one can roll up the sheet along one of the symmetry axis: this gives
either a zig-zag tube or an armchair tube. It is also possible to roll up the sheet in a
direction that differs from a symmetry axis: one obtains a chiral nanotube, in
which the equivalent atoms of each unit cell are aligned on a spiral. Besides the
chiral angle, the circumference of the cylinder can also be varied. In general, the
whole family of nanotubes is classified as zigzag, armchair, and chiral tubes of
different diameters:
Models of different single wall nanotubes (generated with Mathematica on the
left, and taken from Saito et al., APL 60, 2204 (1992) on the above).
This diversity of possible configurations is indeed found in practice, and no
particular type is preferentially formed. The lengths of SWNTs and MWNTs are
usually well over 1 µm and diameters range from ~1 nm (for SWNTs) to ~50 nm
(for MWNTs). Pristine SWNTs are usually closed at both ends by fullerene-like half
spheres that contain both pentagons and hexagons. As shown in the electron
microscopy images below, a SWNT has a well-defined spherical tip, whereas the
shape of a MWNT cap is more polyhedral than spherical. An open MWNT, as the
name implies, doesn't have a cap and the ends of the graphene layers and the
internal cavity of the tube are exposed.
Transmission electron microscopy (TEM) pictures of the ends of different
nanotubes. Each black line corresponds to one graphene sheet viewed edge-on.
Defects in the hexagonal lattice are usually present in the form of pentagons and
heptagons. Pentagons produce a positive curvature of the graphene layer and are
mostly found at the cap. Heptagons give raise to a negative curvature of the tube
wall. Defects consisting of several pentagons and/or heptagons have also been
observed.
A simple model indicates that the diameter and/or chirality of the tube are changed
from one side of the defect to the other. Such an arrangement forms therefore a link
between two different tubes and is accordingly called a junction.
b) MULTI-WALLED CNT’s:
Multi-walled nanotubes (MWNT) consist of multiple rolled in on themselves to
form a tube shape. There are two models which can be used to describe the
structures of multi-walled nanotubes. In the Russian Doll model, sheets of
graphite are arranged in concentric cylinders. In the Parchment model, a single
sheet of graphite is rolled in around itself, resembling a scroll of parchment or a
rolled up newspaper. The interlayer distance in multi-walled nanotubes is close to
the distance between graphene layers in graphite, approximately 0.33 nm.
Although it is easier to produce significant quantities of MWNTs than SWNTs,
their structures are less well understood than single-wall nanotubes because of
their greater complexity and variety. Multitudes of exotic shapes and
arrangements, often with imaginative names such as bamboo-trunks, sea urchins,
necklaces or coils, have also been observed under different processing conditions.
The variety of forms may be interesting but also has a negative side—MWNTs
always (so far) have more defects than SWNTs and these diminish their desirable
properties.
Many of the nanotube applications now being considered or put into practice
involve multi-walled nanotubes, because they are easier to produce in large
quantities at a reasonable price and have been available in decent amounts for
much longer than SWNTs. In fact one of the major manufacturers of MWNTs at
the moment, Hyperion Catalysis, does not even sell the nanotubes directly but
only pre-mixed with polymers for composites applications. The tubes involved
typically have 8 to 15 walls and are around 10 nanometres wide and 10
micrometers long.
Companies are moving into this space, notably formidable players like Mitsui, with
plans to produce similar types of MWNT in hundreds of tons a year, a quantity
that is greater, but not hugely so, than the current production of Hyperion
Catalysis. This is an indication that even these less impressive and exotic
nanotubes hold promise of representing a sizable market in the near future.
SYNTHESIS:There are a number of methods of making CNTs and fullerenes. Fullerenes were
first observed after vaporizing graphite with a short-pulse, high-power laser,
however this was not a practical method for making large quantities.
CNTs have probably been around for a lot longer than was first realized, and may
have been made during various carbon combustion and vapor deposition
processes, but electron microscopy at that time was not advanced enough to
distinguish them from other types of tubes. The first method for producing CNTs
and fullerenes in reasonable quantities – was by applying an electric current
across two carbonaceous electrodes in an inert gas atmosphere. This method is
called plasma arcing. It involves the evaporation of one electrode as cations
followed by deposition at the other electrode. This plasma-based process is
analogous to the more familiar electroplating process in a liquid medium.
Fullerenes and CNTs are formed by plasma arcing of carbonaceous materials,
particularly graphite. The fullerenes appear in the soot that is formed, while the
CNTs are deposited on the opposing electrode.
Another method of nanotube synthesis involves plasma arcing in the
presence of cobalt with a 3% or greater concentration. As noted above, the
nanotube product is a compact cathode deposit of rod like morphology. However
when cobalt is added as a catalyst, the nature of the product changes to a web,
with strands of 1mm or so thickness that stretch from the cathode to the walls of
the reaction vessel. The mechanism by which cobalt changes this process is
unclear, however one possibility is that such metals affect the local electric fields
and hence the formation of the five-membered rings.
Synthesis of carbon nanotubes can be done by different methods:-
1) Arc discharge method
2) Laser ablation method
3) Chemical vapour deposition method
a) plasma enhanced chemical vapour deposition
b) Thermal chemical vapour deposition
c) Vapour phase growth
a) ARC DISCHARGE METHOD
Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during
an arc discharge, by using a current of 100 amperes that was intended to produce
fullerenes.
The carbon arc discharge method, initially used for producing C60 fullerenes, is
the most common and perhaps easiest way to produce CNTs, as it is rather simple.
However, it is a technique that produces a complex mixture of components, and
requires further purification - to separate the CNTs from the soot and the residual
catalytic metals present in the crude product. This method creates CNTs through
arc-vaporization of two carbon rods placed end to end, separated by
approximately 1mm, in an enclosure that is usually filled with inert gas at low
pressure. Recent investigations have shown that it is also possible to create CNTs
with the arc method in liquid nitrogen. A direct current of 50 to 100 A, driven by a
potential difference of approximately 20 V, creates a high temperature discharge
between the two electrodes.
The discharge vaporizes the surface of one of the carbon electrodes, and
forms a small rod-shaped deposit on the other electrode. Producing CNTs in high
yield depends on the uniformity of the plasma arc, and the temperature of the
deposit forming on the carbon electrode. The carbon contained in the negative
electrode sublimates because of the high temperatures caused by the discharge.
Because nanotubes were initially discovered using this technique, it has been the
most widely used method of nanotube synthesis.
The yield for this method is up to 30 percent by weight and it produces both
single- and multi-walled nanotubes with lengths of up to 50 micrometers.
b) LASER ABLATION PROCESS
In the laser ablation process, a pulsed laser vaporizes a graphite target in a high
temperature reactor while an inert gas is bled into the chamber. The nanotubes
develop on the cooler surfaces of the reactor, as the vaporized carbon condenses.
A water-cooled surface may be included in the system to collect the nanotubes.
In 1996 CNTs were first synthesized using a dual-pulsed laser and achieved yields
of >70wt% purity. Samples were prepared by laser vaporization of graphite rods
with a 50:50 catalyst mixture of Cobalt and Nickel at 1200°C in flowing argon,
followed by heat treatment in a vacuum at 1000°C to remove the C60 and other
fullerenes. The initial laser vaporization pulse was followed by a second pulse, to
vaporize the target more uniformly. The use of two successive laser pulses
minimizes the amount of carbon deposited as soot. The second laser pulse breaks
up the larger particles ablated by the first one, and feeds them into the growing
nanotube structure.
The material produced by this method appears as a mat of “ropes”, 10-20nm in
diameter and up to 100µm or more in length. Each rope is found to consist
primarily of a bundle of single walled nanotubes, aligned along a common axis. By
varying the growth temperature, the catalyst composition, and other process
parameters, the average nanotube diameter and size distribution can be varied.
Arc-discharge and laser vaporization are currently the principal methods for
obtaining small quantities of high quality CNTs. However, both methods suffer
from drawbacks. The first is that both methods involve evaporating the carbon
source, so it has been unclear how to scale up production to the industrial level
using these approaches. The second issue relates to the fact that vaporization
methods grow CNTs in highly tangled forms, mixed with unwanted forms of
carbon and/or metal species. The CNTs thus produced are difficult to purify,
manipulate, and assemble for building nanotube-device architectures for practical
applications
This method has a yield of around 70% and produces primarily single-walled
carbon nanotubes with a controllable diameter determined by the reaction
temperature.
c) CHEMICAL VAPOUR DEPOSITION
Chemical vapor deposition of hydrocarbons over a metal catalyst is a classical
method that has been used to produce various carbon materials such as carbon
fibers and filaments. For over twenty years. Large amounts of CNTs can be
formed by catalytic CVD of acetylene over Cobalt and iron catalysts supported on
silica or zeolite. The carbon deposition activity seems to relate to the cobalt
content of the catalyst, whereas the CNTs’ selectivity seems to be a function of
the pH in catalyst preparation.
Fullerenes and bundles of single walled nanotubes were also found among the
multi walled nanotubes produced on the carbon/zeolite catalyst. Some
researchers are experimenting with the formation of CNTs from ethylene.
Supported catalysts such as iron, cobalt, and nickel, containing either a single
metal or a mixture of metals, seem to induce the growth of isolated single walled
nanotubes or single walled nanotubes bundles in the ethylene atmosphere. The
production of single walled nanotubes, as well as double-walled CNTs, on
molybdenum and molybdenum-iron alloy catalysts has also been demonstrated.
CVD of carbon within the pores of a thin alumina template with or without a
Nickel catalyst has been achieved. Ethylene was used with reaction temperatures
of 545°C for Nickel-catalyzed CVD, and 900°C for an uncatalyzed process. The
resultant carbon nanostructures have open ends, with no caps. Methane has also
been used as a carbon source. In particular it has been used to obtain ‘nanotube
chips’ containing isolated single walled nanotubes at controlled locations. High
yields of single walled nanotubes have been obtained by catalytic decomposition
of an H2/CH4 mixture over well-dispersed metal particles such as Cobalt, Nickel,
and Iron on magnesium oxide at 1000°C. It has been reported that the synthesis of
composite powders containing well-dispersed CNTs can be achieved by selective
reduction in an H2/CH4 atmosphere of oxide solid solutions between a nonreducible oxide such as Al2O3 or MgAl2O4 and one or more transition metal
oxides. The reduction produces very small transition metal particles at a
temperature of usually >800°C. The decomposition of CH4 over the freshly formed
nanoparticles prevents their further growth, and thus results in a very high
proportion of single walled nanotubes and fewer multi walled nanotubes.
These are the basic principles of the CVD process. In the last decennia, different
techniques for the carbon nanotubes synthesis with CVD have been developed,
such as plasma enhanced CVD, thermal chemical CVD, alcohol catalytic CVD,
vapour phase growth, aero gel-supported CVD and laser-assisted CVD. These
different techniques will be explained more detailed in this chapter.
Using CVD, a substrate is prepared with a layer of metal catalyst particles, most
commonly nickel, cobalt, iron, or a combination. The diameters of the nanotubes
that are to be grown are related to the size of the metal particles. This can be
controlled by patterned deposition of the metal, annealing, or by plasma etching
of a metal layer. The substrate is heated to approximately 700°C. To initiate the
growth of nanotubes, two gases are bled into the reactor: a process gas (such as
ammonia, nitrogen, hydrogen, etc.) and a carbon-containing gas (such as
acetylene, ethylene, ethanol, methane, etc.). Nanotubes grow at the sites of the
metal catalyst; the carbon-containing gas is broken apart at the surface of the
catalyst particle, and the carbon is transported to the edges of the particle.
CVD is a common method for the commercial production of carbon nanotubes..
For this purpose, the metal nanoparticles will be carefully mixed with a catalyst
support (e.g., MgO, Al2O3, etc) to increase the specific surface area for higher
yield of the catalytic reaction of the carbon feedstock with the metal particles.
One issue in this synthesis route is the removal of the catalyst support via an acid
treatment, which sometimes could destroy the original structure of the carbon
nanotubes. However, alternative catalyst supports that are soluble in water have
been shown to be effective for nanotube growth. If plasma is generated by the
application of a strong electric field during the growth process (plasma enhanced
chemical vapor deposition), then the nanotube growth will follow the direction of
the electric field. By properly adjusting the geometry of the reactor it is possible
to synthesize vertically aligned carbon nanotubes.
1) Plasma enhanced chemical vapour deposition
The plasma enhanced CVD method generates a glow discharge in a chamber or a
reaction furnace by a high frequency voltage applied to both electrodes.
Figure shows a schematic diagram of a typical plasma CVD apparatus with a
parallel plate electrode structure.
Figure: Schematic diagram of plasma CVD apparatus.
A substrate is placed on the grounded electrode. In order to form a uniform film,
the reaction gas is supplied from the opposite plate. Catalytic metal, such as Fe, Ni
and Co are used on for example a Si, SiO2, or glass substrate using thermal CVD or
sputtering. After nanoscopic fine metal particles are formed, carbon nanotubes
will be grown on the metal particles on the substrate by glow discharge
generated from high frequency power. A carbon containing reaction gas, such as
C2H2, CH4, C2H4, C2H6, CO is supplied to the chamber during the discharge.
The catalyst has a strong effect on the nanotube diameter, growth rate, wall
thickness, morphology and microstructure. Ni seems to be the most suitable puremetal catalyst for the growth of aligned multi-walled carbon nanotubes
(MWNTs)36. The diameter of the MWNTs is approximately 15 nm. The highest yield
of carbon nanotubes achieved was about 50% and was obtained at relatively low
temperatures (below 330o C).
2) Thermal chemical vapour deposition
In this method Fe, Ni, Co or an alloy of the three catalytic metals is initially
deposited on a substrate. After the substrate is etched in a diluted HF solution
with distilled water, the specimen is placed in a quartz boat. The boat is
positioned in a CVD reaction furnace, and nanometer-sized catalytic metal
particles are formed after an additional etching of the catalytic metal film using
NH3 gas at a temperature of 750 to 1050o C. As carbon nanotubes are grown on
these fine catalytic metal particles in CVD synthesis, forming these fine catalytic
metal particles is the most important process.
Figure shows a schematic diagram of thermal CVD apparatus in the synthesis of
carbon nanotubes.
Figure: Schematic diagram of thermal CVD apparatus.
When growing carbon nanotubes on a Fe catalytic film by thermal CVD, the
diameter range of the carbon nanotubes depends on the thickness of the catalytic
film. By using a thickness of 13 nm, the diameter distribution lies between 30 and
40 nm. When a thickness of 27 nm is used, the diameter range is between 100 and
200 nm. The carbon nanotubes formed are multiwalled.
3) Vapour phase growth
Vapour phase growth is a synthesis method of carbon nanotubes, directly
supplying reaction gas and catalytic metal in the chamber without a substrate 39.
Figure shows a schematic diagram of a vapour phase growth apparatus. Two
furnaces are placed in the reaction chamber. Ferrocene is used as catalyst. In the
first furnace, vaporization of catalytic carbon is maintained at a relatively low
temperature. Fine catalytic particles are formed and when they reach the second
furnace, decomposed carbons are absorbed and diffused to the catalytic metal
particles. Here, they are synthesized as carbon nanotubes. The diameter of the
carbon nanotubes by using vapour phase growth are in the range of 2 - 4 nm for
SWNTs40 and between 70 and 100 nm for MWNTs.
Method
Who
How
Typical
yield
SWNT
M-WNT
Pro
Con
Arc
method
discharge
Chemical
Laser
(vaporization)
ablation
vapour deposition
Ebbesen and Ajayan, Endo,
Shinshu Smalley, Rice, 199514
15
NEC, Japan 1992
University, Nagano,
Japan 53
Connect two graphite Place substrate in Blast
graphite
with
o
rods to a power oven, heat to 600 C, intense laser pulses; use
supply, place them a and slowly add a the laser pulses rather
few millimeters apart, carbon-bearing gas than
electricity
to
and throw the switch. such as methane. As generate carbon gas
At 100 amps, carbon gas decomposes it from which the NTs
vaporizes and forms frees up carbon form;
try
various
hot plasma.
atoms,
which conditions until hit on
recombine in the one
that
produces
form of NTs
prodigious amounts of
SWNTs
30 to 90%
20 to 100 %
Up to 70%
Short tubes with Long tubes with Long bundles of tubes
diameters of 0.6 - 1.4 diameters
ranging (5-20 microns), with
nm
from 0.6-4 nm
individual diameter from
1-2 nm.
Short tubes with Long tubes with Not very much interest in
inner diameter of 1-3 diameter
ranging this technique, as it is
nm
and
outer from 10-240 nm
too
expensive,
but
diameter
of
MWNT
synthesis
is
approximately 10 nm
possible.
Can easily produce Easiest to scale up to Primarily SWNTs, with
SWNT,
MWNTs. industrial production; good diameter control
SWNTs have few long length, simple and few defects. The
structural
defects; process,
SWNT reaction product is quite
MWNTs
without diameter
pure.
catalyst, not too controllable,
quite
expensive, open air pure
synthesis possible
Tubes tend to be NTs
are
usually Costly
technique,
short with random MWNTs and often because
it
requires
sizes and directions; riddled with defects expensive lasers and
often needs a lot of
high power requirement,
purification
but is improving
Table 2-2: A summary of the major production methods
and their efficiency
Characterization of carbon nanotubes:The experimental techniques used for growth and characterization of carbon
nanotubes are discussed. Plasma enhanced chemical vapor deposition (PECVD)
method was used for the deposition of these films. Scanning electron microscopy
(SEM), Transmission electron microscopy (TEM), Energy dispersive X-ray
spectroscopy (EDS), Raman spectroscopy and X-ray diffraction were used for the
characterization of carbon nanostructures and catalyst nanoparticles.
SEM:A scanning electron microscope (SEM) is a type of electron microscope that
images a sample by scanning it with a high-energy beam of electrons in a raster
scan pattern. The electrons interact with the atoms that make up the sample
producing signals that contain information about the sample's surface
topography, composition, and other properties such as electrical conductivity.
In a typical SEM, an electron beam is thermionically emitted from an electron gun
fitted with a tungsten filament cathode. Tungsten is normally used in thermionic
electron guns because it has the highest melting point and lowest vapour
pressure of all metals, thereby allowing it to be heated for electron emission, and
because of its low cost. Other types of electron emitters include lanthanum
hexaboride (LaB6) cathodes, which can be used in a standard tungsten filament
SEM if the vacuum system is upgraded and field emission guns (FEG), which may
be of the cold-cathode type using tungsten single crystal emitters or the
thermally-assisted Schottky type, using emitters of zirconium oxide.
The electron beam, which typically has an energy ranging from 0.5 keV to 40 keV,
is focused by one or two condenser lenses to a spot about 0.4 nm to 5 nm in
diameter. The beam passes through pairs of scanning coils or pairs of deflector
plates in the electron column, typically in the final lens, which deflect the beam in
the x and y axes so that it scans in a raster fashion over a rectangular area of the
sample surface.
When the primary electron beam interacts with the sample, the electrons lose
energy by repeated random scattering and absorption within a teardrop-shaped
volume of the specimen known as the interaction volume, which extends from
less than 100 nm to around 5 µm into the surface. The size of the interaction
volume depends on the electron's landing energy, the atomic number of the
specimen and the specimen's density. The energy exchange between the electron
beam and the sample results in the reflection of high-energy electrons by elastic
scattering, emission of secondary electrons by inelastic scattering and the
emission of electromagnetic radiation, each of which can be detected by
specialized detectors. The beam current absorbed by the specimen can also be
detected and used to create images of the distribution of specimen current.
Electronic amplifiers of various types are used to amplify the signals which are
displayed as variations in brightness on a cathode ray tube. The raster scanning of
the CRT display is synchronized with that of the beam on the specimen in the
microscope, and the resulting image is therefore a distribution map of the
intensity of the signal being emitted from the scanned area of the specimen. The
image may be captured by photography from a high resolution cathode ray tube,
but in modern machines is digitally captured and displayed on a computer monitor
and saved to a computer's hard disk.
TEM:Transmission electron microscopy (TEM) is a microscopy technique
whereby a beam of electrons is transmitted through an ultra thin specimen,
interacting with the specimen as it passes through. An image is formed from the
interaction of the electrons transmitted through the specimen; the image is
magnified and focused onto an imaging device, such as a fluorescent screen, on a
layer of photographic film, or to be detected by a sensor such as a CCD camera.
TEMs are capable of imaging at a significantly higher resolution than light
microscopes, owing to the small de Broglie wavelength of electrons. This enables
the instrument's user to examine fine detail—even as small as a single column of
atoms, which is tens of thousands times smaller than the smallest resolvable
object in a light microscope. TEM forms a major analysis method in a range of
scientific fields, in both physical and biological sciences. TEMs find application in
cancer research, virology, materials science as well as pollution, nanotechnology,
and semiconductor research.
XRD:X-ray scattering techniques are a family of non-destructive analytical
techniques which reveal information about the crystallographic structure,
chemical composition, and physical properties of materials and thin films. These
techniques are based on observing the scattered intensity of an X-ray beam
hitting a sample as a function of incident and scattered angle, polarization, and
wavelength or energy.
In an X-ray diffraction measurement, a crystal is mounted on a goniometer and
gradually rotated while being bombarded with X-rays, producing a diffraction
pattern of regularly spaced spots known as reflections. The two-dimensional
images taken at different rotations are converted into a three-dimensional model
of the density of electrons within the crystal using the mathematical method of
Fourier transforms, combined with chemical data known for the sample. Poor
resolution (fuzziness) or even errors may result if the crystals are too small, or not
uniform enough in their internal makeup.
X-ray crystallography is related to several other methods for determining atomic
structures. Similar diffraction patterns can be produced by scattering electrons or
neutrons, which are likewise interpreted as a Fourier transform. If single crystals
of sufficient size cannot be obtained, various other X-ray methods can be applied
to obtain less detailed information; such methods include fiber diffraction,
powder diffraction and small-angle X-ray scattering (SAXS). If the material under
investigation is only available in the form of nano-crystalline powders or suffers
from poor crystallinity, the methods of electron crystallography can be applied for
determining the atomic structure.
RAMAN SPECTROSCOPY:Raman spectroscopy (named after C. V. Raman) is spectroscopic technique used
to study vibrational, rotational, and other low-frequency modes in a system. It
relies on inelastic scattering, or Raman scattering, of monochromatic light, usually
from a laser in the visible, near infrared, or near ultraviolet range. The laser light
interacts with molecular vibrations, phonons or other excitations in the system,
resulting in the energy of the laser photons being shifted up or down. The shift in
energy gives information about the vibrational modes in the system. Infrared
spectroscopy yields similar, but complementary, information.
Typically, a sample is illuminated with a laser beam. Light from the illuminated
spot is collected with a lens and sent through a monochromatic. Wavelengths
close to the laser line, due to elastic Rayleigh scattering, are filtered out while the
rest of the collected light is dispersed onto a detector.
Spontaneous Raman scattering is typically very weak, and as a result the main
difficulty of Raman spectroscopy is separating the weak inelastically scattered
light from the intense Rayleigh scattered laser light.
Raman spectra are typically expressed in wave numbers, which have units of
inverse length. In order to convert between spectral wavelength and wave
numbers of shift in the Raman spectrum, the following formula can be used:
Where Δw is the Raman shift expressed in wave number, λ0 is the excitation
wavelength, and λ1 is the Raman spectrum wavelength. Most commonly, the units
chosen for expressing wave number in Raman spectra is inverse centimeters
(cm−1). Since wavelength is often expressed in units of nanometers (nm), the
formula above can scale for this units conversion explicitly, giving
PROPERTIES:The most important properties of CNTs are:-
a) Strength and elasticity:
CNTs are the strongest and stiffest materials on earth, in terms of tensile strength
and elastic modulus respectively. This strength results from the covalent sp²
bonds formed between individual carbon atoms.
The carbon atoms of a single sheet of graphite form a planar honeycomb
lattice, in which each atom is connected via a strong chemical bond to three
neighboring atoms. Because of these strong bonds, the basal plane elastic
modulus of graphite is one of the largest of any known material. For this reason,
CNTs are expected to be the ultimate high-strength fibers. Single walled
nanotubes are stiffer than steel, and are very resistant to damage from physical
forces. Pressing on the tip of a nanotube will cause it to bend, but without
damage to the tip. When the force is removed, the nanotube returns to its original
state. This property makes CNTs very useful as probe tips for very high-resolution
scanning probe microscopy. Quantifying these effects has been rather difficult,
and an exact numerical value has not been agreed upon.
Using atomic force microscopy, the unanchored ends of a freestanding nanotube
can be pushed out of their equilibrium position, and the force required to push
the nanotube can be measured. The current Young’s modulus value of single
walled nanotubes is about 1 TeraPascal, but this value has been widely disputed,
and a value as high as 1.8 Tpa has been reported. Other values significantly higher
than that have also been reported. The differences probably arise through
different experimental measurement techniques. Others have shown
theoretically that the Young’s modulus depends on the size and chirality of the
single walled nanotubes, ranging from 1.22 Tpa to 1.26 Tpa. They have calculated a
value of 1.09 Tpa for a generic nanotube. However, when working with different
multi walled nanotubes, others have noted that the modulus measurements of
multi walled nanotubes using AFM techniques do not strongly depend on the
diameter. Instead, they argue that the modulus of the multi walled nanotubes
correlates to the amount of disorder in the nanotube walls. Not surprisingly, when
multi walled nanotubes break, the outermost layers break first.
CNTs are not nearly as strong under compression. Because of their hollow
structure and high aspect ratio, they tend to undergo buckling when placed
under compressive, tensional or bending stress.
b) Thermal conductivity and expansion:
All nanotubes are expected to be very good thermal conductors along the tube,
exhibiting a property known as ballistic conduction, but good insulators laterally
to the tube axis. The temperature stability of carbon nanotubes is established to
be up to 2800 degrees Celsius in vacuum and about 750 degrees Celsius in air.
CNTs have been shown to exhibit superconductivity below 20°K (aprox.
253°C). Research suggests that these exotic strands, already heralded for their
unparalleled strength and unique ability to adopt the electrical properties of
either semiconductors or perfect metals, may someday also find applications as
miniature heat conduits in a host of devices and materials. The strong in-plane
graphitic carbon - carbon bonds make them exceptionally strong and stiff against
axial strains. The almost zero in-plane thermal expansion but large inter-plane
expansion of single walled nanotubes implies strong in-plane coupling and high
flexibility against non-axial strains.
Many applications of CNTs, such as in nanoscale molecular electronics, sensing
and actuating devices, or as reinforcing additive fibers in functional composite
materials, have been proposed. Reports of several recent experiments on the
preparation and mechanical characterization of CNT-polymer composites have
also appeared. These measurements suggest modest enhancements in strength
characteristics of CNT-embedded matrixes as compared to bare polymer
matrixes. Preliminary experiments and simulation studies on the thermal
properties of CNTs show very high thermal conductivity. It is expected, therefore,
that nanotube reinforcements in polymeric materials may also significantly
improve the thermal and thermo mechanical properties of the composites.
c) High aspect ratio:
CNTs represent a very small, high aspect ratio conductive additive for plastics of
all types. Their high aspect ratio means that a lower loading of CNTs is needed
compared to other conductive additives to achieve the same electrical
conductivity. This low loading preserves more of the polymer resins’ toughness,
especially at low temperatures, as well as maintaining other key performance
properties of the matrix resin. CNTs have proven to be an excellent additive to
impart electrical conductivity in plastics. Their high aspect ratio, about 1000:1
imparts electrical conductivity at lower loadings, compared to conventional
additive materials such as carbon black, chopped carbon fiber, or stainless steel
fiber.
d) Electrical Conductivity:
Depending on their chiral vector, carbon nanotubes with a small diameter are
either semi-conducting or metallic.
CNTs can be highly conducting, and hence can be said to be metallic. Their
conductivity has been shown to be a function of their chirality, the degree of twist
as well as their diameter. CNTs can be either metallic or semi-conducting in their
electrical behavior. Conductivity in MWNTs is quite complex. Some types of
“armchair”-structured CNTs appear to conduct better than other metallic CNTs.
Furthermore, interwall reactions within multi walled nanotubes have been found
to redistribute the current over individual tubes non-uniformly. However, there is
no change in current across different parts of metallic single-walled nanotubes.
The behavior of the ropes of semi-conducting single walled nanotubes is different,
in that the transport current changes abruptly at various positions on the CNTs.
The conductivity and resistivity of ropes of single walled nanotubes has been
measured by placing electrodes at different parts of the CNTs. The resistivity of
the single walled nanotubes ropes was of the order of 10–4 ohm-cm at 27°C. This
means that single walled nanotube ropes are the most conductive carbon fibers
known. The current density that was possible to achieve was 10-7 A/cm2, however
in theory the single walled nanotube ropes should be able to sustain much higher
stable current densities, as high as 10-13 A/cm2. It has been reported that
individual single walled nanotubes may contain defects. Fortuitously, these
defects allow the single walled nanotubes to act as transistors. Likewise, joining
CNTs together may form transistor-like devices. A nanotube with a natural
junction (where a straight metallic section is joined to a chiral semi conducting
section) behaves as a rectifying diode – that is, a half-transistor in a single
molecule. It has also recently been reported that single walled nanotubes can
route electrical signals at speeds up to 10 GHz when used as interconnects on
semi-conducting devices.
e) Electronic properties:
The electronic properties of SWNTs have been studied in a large number of
theoretical works. All models show that the electronic properties vary in a
predictable way from metallic to semi conducting with diameter and chirality. This
is due to the very peculiar band structure of graphene and is absent in systems
that can be described with usual free electron theory.
Electron motion in graphene is equivalent to that of a neutrino or a relativistic
Dirac electron with vanishing rest mass. This causes the appearance of a nontrivial
Berry’s phase under 2π rotation in wave-vector space, leading to the absence of
backscattering and in the metallic carbon nanotube resulting in perfect
conduction even in the presence of scatterers. The energy bands in carbon
nanotubes are determined by periodic boundary conditions with a fictitious
Aharonov-Bohm flux determined uniquely by the circumferential chiral vector. A
nanotube becomes metallic when the flux vanishes and semiconducting when the
flux is nonzero. The conductivity of graphene is essentially independent of the
Fermi energy and the electron concentration as long as variations in effective
scattering strength are neglected, and therefore graphene should be regarded as
a metal rather than a zero-gap semiconductor. Various schemes are now being
proposed and tested for the purpose of opening the band gap in graphene.
Basically, all armchair tubes are metallic. One out of three zigzag and chiral tubes
show a small very small band gap due to the curvature of the graphene sheet,
while all other tubes are semi-conducting with a band gap that scales
approximately with the inverse of the tube radius. Bandgaps of 0.4 – 1 eV can be
expected for SWNTs (corresponding to diameters between 0.6 and 1.6 nm).
On the left: band structure of the conduction band of graphene, which crosses
the Fermi level at the edges of the Brillouin zone.
On the right: predicted band-gap as a function of SWNT radius, reproduced
from Kane and Mele, PRL 78, 1932 (1997).
These theoretical predictions made in 1992 were actually confirmed in 1998 by
scanning tunneling spectroscopy. Numerous conductivity experiments on SWNTs
and MWNTs allowed gaining additional information’s. At low temperatures,
SWNTs behave as coherent quantum wires where the conduction occurs through
discrete electron states over large distances. Transport measurements revealed
that metallic SWNTs show extremely long coherence lengths. MWNTs show also
these effects in spite of their larger diameter and multiple shells.
f) Mechanical properties:
Carbon nanotube is the one of the strongest materials in nature. Carbon
nanotubes (CNTs) are basically long hollow cylinders of graphite sheets. Although
a graphite sheet has a 2D symmetry, carbon nanotubes by geometry have
different properties in axial and radial directions. It has been shown that CNTs are
very strong in the axial direction. Young's modulus on the order of 270 - 950 GPa
and tensile strength of 11 - 63 GPa were obtained.
On the other hand, there was evidence that in the radial direction they are rather
soft. The first transmission electron microscope observation of radial elasticity
suggested that even the van der Waals forces can deform two adjacent
nanotubes. Later, nanoindentations with atomic force microscope were
performed by several groups to quantitatively measure radial elasticity of
multiwalled carbon nanotubes and tapping/contact mode atomic force
microscopy was recently performed on single-walled carbon nanotubes. Young's
modulus of on the order of several GPa showed that CNTs are in fact very soft in
the radial direction.
Radial direction elasticity of CNTs is important especially for carbon nanotube
composites where the embedded tubes are subjected to large deformation in the
transverse direction under the applied load on the composite structure.
One of the main problems in characterizing the radial elasticity of CNTs is the
knowledge about the internal radius of the CNT; carbon nanotubes with identical
outer diameter may have different internal diameter (or the number of walls).
Recently a method using atomic force microscope was introduced to find the
exact number of layers and hence the internal diameter of the CNT. In this way,
mechanical characterization is more accurate.
Comparison of mechanical properties
Material
Young's
Tensile
modulus (TPa) strength (GPa)
Elongation at
break (%)
SWNT
~1 (from 1 to 5)
13–53
16
Armchair
SWNT
0.94
126.2
23.1
Zigzag
SWNT
0.94
94.5
15.6–17.5
Chiral
SWNT
0.92
NA
NA
MWNTE
0.2–0.8–0.95
11–63–150
NA
Stainless
steel
0.186–0.214
0.38–1.55
15–50
Kevlar–
29&149
0.06–0.18
3.6–3.8]
~2
APPLICATIONS:Carbon nanotubes (Buckytubes) have extraordinary electrical conductivity, heat
conductivity and mechanical properties. They are probably the best electron fieldemitter possible. They are polymers of pure carbon and can be reacted and
manipulated using the tremendously rich chemistry of carbon. This provides
opportunity to modify the structure and to optimize solubility and dispersion.
Very significantly, buckytubes are molecularly perfect, which means that they are
free of property-degrading flaws in the nanotube structure. Their material
properties can therefore approach closely the very high levels intrinsic to them.
These extraordinary characteristics give buckytubes potential in numerous
applications.
a) Structural

Clothes: waterproof tear-resistant cloth fibers

Combat jackets: MIT is working on combat jackets that use carbon
nanotubes as ultra strong fibers and to monitor the condition of the
wearer.

Concrete: In concrete, they increase the tensile strength, and halt crack
propagation.

Polyethylene: Researchers have found that adding them to polyethylene
increases the polymer's elastic modulus by 30%.

Sports equipment: Stronger and lighter tennis rackets, bike parts, golf
balls, golf clubs, golf shaft and baseball bats.

Space elevator: This will be possible only if tensile strengths of more than
about 70 GPa can be achieved. Monoatomic oxygen in the Earth's upper
atmosphere would erode carbon nanotubes at some altitudes, so a space
elevator constructed of nanotubes would need to be protected (by some
kind of coating). Carbon nanotubes in other applications would generally
not need such surface protection.

Ultrahigh-speed flywheels: The high strength/weight ratio enables very
high speeds to be achieved.
b) ELECTROMAGNETIC

Buckypaper:
It is a thin sheet made from nanotubes that are 250 times
stronger than steel and 10 times lighter that could be used as a heat sink
for chipboards, a backlight for LCD screens or as a faraday cage to protect
electrical devices/aero planes.

Chemical nanowires: Carbon nanotubes additionally can also be used to
produce nanowires of other chemicals, such as gold or zinc oxide. These
nanowires in turn can be used to cast nanotubes of other chemicals, such
as gallium nitride. These can have very different properties from CNTs – for
example, gallium nitride nanotubes are hydrophilic, while CNTs are
hydrophobic, giving them possible uses in organic chemistry that CNTs
could not be used for.

Computer circuits:
A nanotube formed by joining nanotubes of two
different diameters end to end can act as a diode, suggesting the
possibility of constructing electronic computer circuits entirely out of
nanotubes. Because of their good thermal properties, CNTs can also be
used to dissipate heat from tiny computer chips.

Conductive films: CNTs are also introduced in developing transparent,
electrically conductive films to replace indium tin oxide(ITO).CNT films are
substantially more mechanically robust then ITO films ,making them ideal
for more reliability touch screens and flexible displays. Printable water
based inks of carbon nanotubes are desired to enable the production of
these films to replace the ITO. Nanotube films show promise for use in
displays for cell phones, computers, PDAs, and ATMs.

Electric motor brushes:
Conductive carbon nanotubes have been
used for several years in brushes for commercial electric motors.. The
nanotubes improve electrical and thermal conductivity because they
stretch through the plastic matrix of the brush. This permits the carbon
filler to be reduced from 30% down to 3.6%, so that more matrixes are
present in the brush. Nanotube composite motor brushes are betterlubricated (from the matrix), cooler-running (both from better lubrication
and superior thermal conductivity), less brittle (more matrix, and fiber
reinforcement), stronger and more accurately moldable (more matrix).
Since brushes are a critical failure point in electric motors, and also don’t
need much material, they became economical before almost any other
application.

Light bulb filament:
lamps.

Solar cells:
Organic photovoltaic devices (OPVs) are fabricated from
thin films of organic semiconductors, such as polymers and small-molecule
compounds, and are typically on the order of 100 nm thick. Because
polymer based OPVs can be made using a coating process such as spin
coating or inkjet printing, they are an attractive option for inexpensively
covering large areas as well as flexible plastic surfaces. A promising low
cost alternative to silicon solar cells, there is a large amount of research
being dedicated throughout industry and academia towards developing
OPVs and increasing their power conversion efficiency. GE’s carbon
nanotube diode has a photovoltaic effect. Nanotubes can replace ITO
(Indium tin oxide) in some solar cells to act as a transparent conductive
film in solar cells to allow light to pass to the active layers and generate
photocurrent.
Alternative to tungsten filaments in incandescent
CNTs in dye-sensitized solar cells:Due to the simple fabrication process, low
production cost, and high efficiency, there is significant interest in
dye-sensitized solar cells (DSSCs). Thus, improving DSSC efficiency
has been the subject of a variety of research investigations because
it has the potential to be manufactured economically enough to
compete with other solar cell technologies. Titanium dioxide
nanoparticles have been widely used as a working electrode for
DSSCs because they provide a high efficiency, more than any other
metal oxide semiconductor investigated. Yet the highest
conversion efficiency under air mass (AM) 1.5 (100 mW/cm2)
irradiation reported for this device to date is about 11%. Despite this
initial success, the effort to further enhance efficiency has not
produced any major results. The transport of electrons across the
particle network has been a key problem in achieving higher photo
conversion efficiency in nanostructured electrodes. Because
electrons encounter many grain boundaries during the transit and
experience a random path, the probability of their recombination
with oxidized sensitizer is increased. Therefore, it is not adequate
to enlarge the oxide electrode surface area to increase efficiency
because photo-generated charge recombination should be
prevented. Promoting electron transfer through film electrodes
and blocking interface states lying below the edge of the
conduction band are some of the non-CNT based strategies to
enhance efficiency that have been employed.
With recent progress in CNT development and fabrication, there is
promise to use various CNT based nanocomposites and
nanostructures to direct the flow of photo generated electrons and
assist in charge injection and extraction. To assist the electron
transport to the collecting electrode surface in a DSSC, a popular
concept is to utilize CNT networks as support to anchor light
harvesting semiconductor particles.
"Efficiencies reaching 4.4% have already been achieved and
hopefully 10-15% efficiencies are feasible in the near-future upon further
optimization" says Kymakis. "Once this obstacle is tackled, the lifetime issue,
which is directly related to the cell temperatures, can be explored. A working
environment combining the strengths of scientists and business leaders may
soon result in rapid commercialization of this technology."

Superconductor:
Nanotubes have been shown to be superconducting
at low temperatures.

Ultra capacitors:
Nanotubes, when bound to plates of capacitors
increase the surface area and thus increase energy storage ability.

Displays:
One use for nanotubes that has already been developed is as
extremely fine electron guns, which could be used as miniature cathode
ray tubes in thin high-brightness low-energy low-weight displays. This type
of display would consist of a group of many tiny CRTs, each providing the
electrons to hit the phosphor of one pixel, instead of having one giant CRT
whose electrons are aimed using electric and magnetic fields. These
displays are known as field emission displays (FEDs).

Others:
Artificial muscles, magnets, optical ignition etc.
c) CHEMICAL

Air pollution filter:
Future applications of nanotube membranes
include filtering carbon dioxide from power plant emissions.

Biotech container:
Nanotubes can be opened and filled with materials
such as biological molecules, raising the possibility of applications in
biotechnology.

Hydrogen storage:
Research is currently being undertaken into the
potential use of carbon nanotubes for hydrogen storage. They have the
potential to store between 4.2 and 65% hydrogen by weight. This is an
important area of research, since if they can be mass produced
economically there is potential to contain the same quantity of energy as a
50L gasoline tank in 13.2L of nanotubes. See also, Hydrogen Economy.

Water filter:
Recently nanotube membranes have been developed for
use in filtration. This technique can purportedly reduce desalination costs
by 75%. The tubes are so thin that small particles (like water molecules) can
pass through them, while larger particles (such as the chloride ions in salt)
are blocked.

Oscillator:
Fastest known oscillators (> 50 GHz).

Nanotube membrane:
Liquid flows up to five orders of magnitude
faster than predicted by classical fluid dynamics.

Smooth surface:
Smoother than Teflon and waterproof.
d) MECHANICAL

Oscillator: fastest known oscillators (> 50 GHz).

Liquid flow array: Liquid flows up to five orders of magnitude faster than
predicted through array.

Slick surface: slicker than Teflon and waterproof.
e) CARBON NANOTUBE INTERCONNECTS
Metallic CNTs have aroused a lot of research interest in their applicability as Verylarge-scale integration (VLSI) interconnects of the future because of their
desirable properties of high thermal stability, high thermal conductivity and large
current carrying capacity. An isolated CNT can carry current densities in excess of
1000 MA/sq-cm without any signs of damage even at an elevated temperature of
250 degrees C, thereby eliminating electromigration reliability concerns that
plague Cu interconnects. Recent modeling work comparing the performance,
power dissipation and thermal/reliability aspects of CNT interconnect to scaled
copper interconnects have shown that CNT bundle interconnects can potentially
offer more advantages over copper.
f) TRANSISTORS
Smaller silicon based integrated circuits result in both a higher speed and device
density. As a result, downscaling of these devices has been very important since
their first implementation. However, at the moment it is generally accepted that
silicon devices will reach fundamental scaling limits within a decade or so. This
limit is caused by the minimum wavelength of light used in lithographic
techniques used for integrated circuit production nowadays. For this reason a
quest for alternative, integrated circuits with smaller dimensions has started. A
major step in downscaling would be the application of single molecules in
electronic devices. Carbon nanotubes have already shown promising results in
single molecular transistors. For successful implementation of molecular
transistors in large and complex logic systems, they must show signal
amplification. Signal amplification makes it possible to reference separate signals
along a chain of logical operations. In addition, noise caused by thermal
fluctuations and environmental disturbances is also reduced. Three terminal
nanotransistors, in special, field-effect-transistors show amplifying behavior and
have recently been investigated for this reason.
g) Electrical circuits
Carbon nanotubes have many properties—from their unique dimensions to an
unusual current conduction mechanism—that make them ideal components of
electrical circuits. Currently, there is no reliable way to arrange carbon nanotubes
into a circuit.
The major hurdles that must be jumped for carbon nanotubes to find prominent
places in circuits relate to fabrication difficulties. The productions of electrical
circuits with carbon nanotubes are very different from the traditional IC
fabrication process. The IC fabrication process is somewhat like sculpture - films
are deposited onto a wafer and pattern-etched away. Because carbon nanotubes
are fundamentally different from films, carbon nanotube circuits can so far not be
mass produced.
Researchers sometimes resort to manipulating nanotubes one-by-one with the tip
of an atomic force microscope in a painstaking, time-consuming process. Perhaps
the best hope is that carbon nanotubes can be grown through a chemical vapor
deposition process from patterned catalyst material on a wafer, which serve as
growth sites and allow designers to position one end of the nanotube. During the
deposition process, an electric field can be applied to direct the growth of the
nanotubes, which tend to grow along the field lines from negative to positive
polarity. Another way for the self assembly of the carbon nanotube transistors
consist in using chemical or biological techniques to place the nanotubes from
solution to determinate place on a substrate.
Even if nanotubes could be precisely positioned, there remains the problem that,
to this date, engineers have been unable to control the types of nanotubes—
metallic, semiconducting, single-walled, multi-walled—produced. A chemical
engineering solution is needed if nanotubes are to become feasible for
commercial circuits.
h) OTHER APPLICATIONS





CNTs have also been implemented in nano electromechanical systems,
including mechanical memory elements.
CNTs have also been proposed as a possible gene delivery vehicle and for
use in combination in radio frequency fields to destroy cancer cells.
Nanomix Inc was the first to put on the market an electronic device that
integrated carbon nanotubes on a silicon platform, in may 2005. It was a
hydrogen sensor. Since then nanomix has been patenting many such
sensor applications such as in the field of Carbon-di-oxide, Nitrous Oxide,
glucose, DNA detection etc.
As a container for drug delivery: Because of the versatile structure of the
CNT, it can be used for a variety of tasks in and around body. Often in the
cancer related incidents, the CNT is often used as a container for
transporting drugs into the body. Here drugs can actually be placed inside
the nanotubes or can be attached to the side or trailed behind. Both of
these methods are effective for the delivery and distribution of drug inside
out of the body.
CNTs can be used as light emitting semiconductors.
CONCLUSION
Rise in demand and production, and ease of accessibility of carbon nanotubes
would lead to the extensive use of carbon nanotubes in a wide variety of
applications. The use of nanotechnology for human will become common need in
21st century. As world is suffering from serious pollution problems, hydrogen will
becoming need of 21st century & carbon nanotubes provide better solution for
hydrogen storage.
Nanotubes market, which was growing at a moderate rate till 2006-2007, is
expected to rise at a skyrocketing pace in the coming years. Hence we can
conclude that most of the demands of human, in this and fore coming
generation will be fulfilled by carbon nanotubes.