Nano Tubes

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Carbon Nanotubes

Carbon, a group IV element, has two crystalline forms: diamondand graphite. Carbon nanotubes (CNTs) are allotropes of carbon.These cylindrical carbon molecules have novel properties that makethem potentially useful in many applications in nanotechnology,electronics, optics and other fields of materials science, as well aspotential uses in architectural fields. They exhibit extraordinarystrength and unique electrical properties, and are efficientconductors of heat. CNTs are members of the fullerene structuralfamily, which also includes the spherical buckyballs 1. The ends of aCNT might be capped with a hemisphere of the buckyball structure.

Carbon nanotubes are one of the most commonly mentionedbuilding blocks of nanotechnology. With one hundred times thetensile strength of steel, thermal conductivity better than all but thepurest diamond, and electrical conductivity similar to copper, butwith the ability to carry much higher currents, they seem to be awonder material. However, when we hear of some companiesplanning to produce hundreds of tons per year, while others seem tohave extreme difficulty in producing grams, there is clearly more tothis material than meets the eye.

Carbon nanotubes, long thin cylinders of carbon, werediscovered in 1991 by Iijimas. Carbon nanotubes (CNTs) areallotropes of carbon which are members of the fullerene structuralfamily, which also includes the spherical buckyballs. These are largemacromolecules which are unique for there size, shape andremarkable physical properties.

The nature of the bonding of a nanotube is described by appliedquantum chemistry, specifically, orbital hybridization. The chemicalbonding of nanotubes is composed entirely of sp2 bonds, similar to

those of graphite. This bonding structure, which is stronger than thesp3 bonds found in diamond, provides the molecules with their uniquestrength. Nanotubes naturally align themselves into ropes heldtogether by Van der Waals forces. Under high pressure, nanotubescan merge together, trading some sp bonds for sp bonds, giving thepossibility of producing strong, unlimited-length wires through high-pressure nanotube linking.

CNTs are named on the basis of derived from their size, sincethe 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-wallednanotubes (MWNTs) depending upon the number of walls. CNTs may


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consist of one up to tens and hundreds of concentric shells of carbonswith adjacent shells separation of 0.34 nm i.e. (002). The carbonnetwork of the shells is closely related to the honeycombarrangement of the carbon atoms in the graphite sheets. Theamazing mechanical and electronic properties of the nanotubes stem

in their quasi-one dimensional (1D) structure and the graphite-likearrangement of the carbon atoms in the shells. Thus, the nanotubeshave high Youngs modulus and tensile strength, which makes themsuitable for composite materials with improved mechanicalproperties. The nanotubes can be metallic or semi conductingdepending on their structural parameters.

The term nanotube is normally used to refer to the carbonnanotube, which has received enormous attention from researchersover the last few years and promises, along with close relatives suchas the nanohorn, a host of interesting applications. There are manyother types of nanotube, from various inorganic kinds, such as thosemade from boron nitride, to organic ones, such as those made fromself assembling cyclic peptides (protein components) or fromnaturally-occurring heat shock proteins (extracted from bacteria thatthrive in extreme environments). However, carbon nanotubes excitethe most interest, promise the greatest variety of applications, andcurrently 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 theirfascinating physical and chemical properties. In this review, we


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enlighten you about this material. We are introducing here, thestructure, synthesis and the most important applications of carbonnanotubes in different fields. The session will feature technology thatexploits novel electronic, electro-mechanical, transistors, andelectrical 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 ananotube may be capped with a hemisphere of the buckyballstructure. Their name is derived from their long, hollow structure withthe walls formed by one-atom-thick sheets of carbon, calledgraphene.


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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 coinedas 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 ofthree 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


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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 Novoselovfor groundbreaking experimentsregarding the two-dimensional material graphene".

The Royal Swedish Academy of Sciences has awardedthe Nobel Prize in Physics for 2010 to Andre Geim andKonstantin 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 behindthis year's Nobel Prize in Physics. Geim and Novoselov have shownthat carbon in such a flat form has exceptional properties that

originate from the remarkable world of quantum physics.


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Geim and Novoselov extracted the graphene from a piece of graphitesuch as is found in ordinary pencils. Using regular adhesive tape theymanaged to obtain a flake of carbon with a thickness of just oneatom. This at a time when many believed it was impossible for suchthin crystalline materials to be stable.

However, with graphene, physicists can now study a new class oftwo-dimensional materials with unique properties. Graphene makesexperiments possible that give new twists to the phenomena inquantum physics. Also a vast variety of practical applications nowappear possible including the creation of new materials and themanufacture of innovative electronics. Graphene transistors arepredicted to be substantially faster than today's silicon transistorsand result in more efficient computers.

Since it is practically transparent and a good conductor,graphene is suitable for producing transparent touch screens, lightpanels, and maybe even solar cells.

When mixed into plastics, graphene can turn them into conductors ofelectricity while making them more heat resistant and mechanicallyrobust. 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 compositematerials.


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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, theresearchers 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 thingraphitic films by using exfoliation techniques similar to the drawing


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method. Multilayer samples down to 10 nm in thickness wereobtained. These efforts were reviewed in 2007. Furthermore, a coupleof very old papers were recently unearthed in which researchers triedto isolate graphene starting with intercalated compounds. Thesepapers reported the observation of very thin graphitic fragments

(possibly minelayers) by transmission electron microscopy. Neither ofthe earlier observations was sufficient to "spark the graphene goldrush", until the Science paper did so by reporting not onlymacroscopic samples of extracted atomic planes but, importantly,their unusual properties such as the bipolar transistor effect, ballistictransport of charges, large quantum oscillations, etc. The discovery ofsuch interesting qualities intrinsic to graphene gave an immediateboost to further research and several groups quickly repeated theinitial result and moved further. These breakthroughs also helped toattract attention to other production techniques, such as epitaxialgrowth of ultra-thin graphitic films. In particular, it has later beenfound that graphene monolayers grown on SiC and Ir are weaklycoupled to these substrates and the graphene-substrate interactioncan be passivated further.The weak van der Waals force that provides the cohesion ofmultilayer graphene stacks does not always affect the electronicproperties of the individual graphene layers in the stack. That is,while the electronic properties of certain multilayered epitaxialgraphenes are identical to that of a single graphene layer, in othercases the properties are affected as they are for graphene layers inbulk graphite. This effect is theoretically well understood and is

related to the symmetry of the interlayer interactions.

Experimental methods for the production of graphene ribbonsare reported consisting of cutting open nanotubes. In one suchmethod multi walled carbon nanotubes are cut open in solution byaction of potassium permanganate and sulfuric acid. In anothermethod graphene nanoribbons are produced by plasma etching ofnanotubes partly embedded in a polymer film.


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Properties of graphene:-

Atomic structure:-The atomic structure of isolated, single-layer graphene wasstudied by transmission electron microscopy (TEM) on sheets ofgraphene suspended between bars of a metallic grid. Electrondiffraction patterns showed the expected hexagonal lattice ofgraphene. Suspended graphene also showed "rippling" of the flatsheet, with amplitude of about one nanometer. These ripples may beintrinsic to graphene as a result of the instability of two-dimensionalcrystals or may be extrinsic, originating from the ubiquitous dirt seenin all TEM images of graphene. Atomic resolution real-space imagesof isolated, single-layer graphene on SiO2 substrates were obtainedby scanning tunneling microscopy. Graphene processed usinglithographic techniques is covered by photoresist residue, which mustbe cleaned to obtain atomic-resolution images.

Graphene sheets in solid form (density > 1 g/cm3) usually showevidence in diffraction for graphite's 0.34 nm (002) layering. This istrue even of some single-walled carbon nanostructures. Transmissionelectron microscope studies show faceting at defects in flat graphenesheets.

Electronic property:-Graphene differs from most conventional three-dimensional

materials. Intrinsic graphene is a semi-metal or zero-gap semiconductor. Understanding the electronic structure ofgraphene is the starting point for finding the band structure ofgraphite. It was realized as early as 1947 by P. R. Wallace that the E-krelation is linear for low energies near the six corners of the two-dimensional hexagonal Brillouin zone, leading to zero effective forelectrons and holes. Due to this linear (or conical") dispersionrelation at low energies, electrons and holes near these six points,two of which are inequivalent, behave like relativistic particlesdescribed by the Dirac equation for spin 1/2 particles. Hence, theelectrons and holes are called Dirac fermions, and the six corners ofthe Brillouin zone are called the Dirac points.

The equation describing the E-k relation is

The Fermi velocity vF~ 106m/s


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Thermal properties:-The near-room temperature thermal conductivity ofgraphene was recently measured to be between(4.840.44) 103 to (5.300.48) 103Wm1K1. Thesemeasurements, made by a non-contact opticaltechnique, are in excess of those measured for carbonnanotubes or diamond. It can be shown by usingthe Wiedemann-Franz law, that the thermal conductionis phonon-dominated. However, for a gated graphene

strip, an applied gate bias causing a Fermi energy shiftmuch larger than kBT can cause the electroniccontribution to increase and dominate overthe phonon contribution at low temperatures.

Mechanical properties:-Graphene appears to be one of the strongest materialsever tested. Measurements have shown that graphenehas 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 beenmeasured. Graphene sheets, held together by van derWaals forces, were suspended over SiO2 cavities wherean AFM tip was probed to test its mechanical properties.Its spring constant was in the range 15 N/m andthe Young's modulus was 0.5 TPa, which differs fromthat of the bulk graphite. These high values makegraphene very strong and rigid. These intrinsicproperties could lead to using graphene


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for NEMS applications such as pressure sensors andresonators.

HISTORY:

The history of carbon nanotubes is not entirely clear even for thosein the science therefore giving proper credit to the person that

invented the carbon nanotube has been the subject of several hightech debates among the scientific communities.

The initial history of nanotubes started in the 1970s. A preparation ofthe planned carbon filaments was completed by Morinobu Endo whowas earning his Ph.D. at the University of Orleans, France.

This was still a highly important development in the history of carbonnanotubes, but it just wasnt the right time to be considered the firstrecognized invention.Giving the proper credit to who invented carbon nanotubes would not

come along for another 20 years. In 1991 the true first invention ofnanotube was finally made. It seems as though there was a racebetween Russian nanotechnologists and Sumio Iijima of IBM.

The first observation of the multiwalled carbon nanotubes wascredited to Iijima.

There are some that hold the belief that in the 1950s there was aninitial discovery of what could have possibly been seen as the firstcarbon nanotubes had Roger Bacon had the high powered electronmicroscope that would have been necessary.


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He was credited with the first visual impression of the tubes of atomsthat roll up and are capped with fullerene molecules by manyscientists in the field. Some state that his discovery just wasnt takenvery seriously at the time because science did not know how thisdiscovery could impact scientific research.

It would be in 1993 that Iijima and Donald Bethune found singlewalled nanotubes known as buckytubes. This helped the scientificcommunity 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 nanotubeswas realized and Iijima and Bethune were ultimately credited withtheir discovery in their entirety. Russian nanotechnologists wereindependently discovering the same visual affirmation. They werejust a little bit later in their announcement and the potential affect ofthis discovery.

The continuation of research revealed a great deal about nanotubesand their place in scientific discovery. The research has indicated that

there are three basic types of nanotubes (zigzag, armchair, andchiral) as well as single walled and multiwalled nanotubes.

There are buckytubes, which are completely hollow molecules thatare crafted from pure carbon and are bonded together in a pattern ofspecific hexagon patterns. The multiwalled nanotubes are likely tosuffer from defects. These defects happen in more than half of allmultiwalled nanotubes.

The multiwalled nanotubes have already made appearances inpractical applications like creating tennis rackets that are stronger

than steel but are ultra light in weight. These nanotubes are alsoresponsible for creating sunscreen and other skin care products that


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are clear or able to be blended into the skin without leaving behindresidue as well as the creation of UV protective clothing.

As nanotechnologists continue to research nanotubes, there is still arace to discover something new within the science. Scientists are

researching the potential for life saving techniques as well as thepotential to create nanotubes that can be tailored toward specificdesignated jobs.

With the creation of specified nanotubes, the potential for their usewill become unlimited and there will be a nanotechnology world hardat work crafting all kinds of products from the convenient to the lifesaving.


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TYPES OF CARBON NANOTUBES:-

a) SINGLE-WALLED CNTs

These are the stars of the nanotube world, and somewhat reclusiveones 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 arenormally capped at the ends although the caps can be removed. Thecaps are made by mixing in some pentagons with the hexagons andare the reason that nanotubes are considered close cousins ofbuckminsterfullerene a roughly spherical molecule made of sixtycarbon atoms, that looks like a soccer ball

and is named after the architect Buckminster Fuller (the wordfullerene is used to refer to the variety of such molecular cages, some

with more carbon atoms than buckminsterfullerene, and some withfewer).


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The theoretical minimum diameter of a carbon nanotube is around0.4 nanometers, which is about as long as two silicon atoms side byside, and nanotubes this size have been made. Average diameterstend to be around the 1.2 nanometer mark, depending on the processused to create them.

SWNTs are more pliable than their multi-walled counterparts and canbe twisted, flattened and bent into small circles or around sharpbends 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 exhibitimportant electric properties that are not shared by the multi-walledcarbon nanotubes (MWNT) variants.

SWNTs can be excellent conductors and the most building blockof SWNT system is the electric wires. One useful application ofSWNTs is in the development of the first intramolecular field effecttransistors (FETs).

STRUCTURE:

The bonding in carbon nanotubes is sp, with each atom joinedto three neighbours, as in graphite. The tubes can therefore beconsidered as rolled-up graphene sheets (graphene is an individualgraphite layer). There are three distinct ways in which a graphenesheet can be rolled into a tube, as shown below.

The terms armchair and zig-zag refer to the arrangement ofhexagons around the circumference. The third class of tube, which inpractice is the most common, is known as chiral, meaning that it canexist in two mirror-related forms. An example of a chiral nanotube isas shown in fig. below.


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In the ideal case, a carbon nanotube consists of either one cylindricalgraphene sheet (single-wall nanotube, SWNT) or of several nestedcylinders with an interlayer spacing of 0.34 0.36 nm that is close tothe 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":


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A de Heer abacus: to realize a (n,m) tube, move n times a1and m times a2 from the origin to get to point (n,m) and roll-up thesheet 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 possibleto roll up the sheet in a direction that differs from a symmetry axis:one obtains a chiral nanotube, in which the equivalent atoms of eachunit cell are aligned on a spiral. Besides the chiral angle, thecircumference of the cylinder can also be varied. In general, thewhole family of nanotubes is classified as zigzag, armchair, and chiraltubes of different diameters:


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Models of different single wall nanotubes (generated withMathematica 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 SWNTsand MWNTs are usually well over 1 m and diameters range from~1 nm (for SWNTs) to ~50 nm (for MWNTs). Pristine SWNTs areusually closed at both ends by fullerene-like half spheres that containboth pentagons and hexagons. As shown in the electron microscopyimages below, a SWNT has a well-defined spherical tip, whereas theshape of a MWNT cap is more polyhedral than spherical. An openMWNT, as the name implies, doesn't have a cap and the ends of thegraphene layers and the internal cavity of the tube are exposed.

Transmission electron microscopy (TEM) pictures of the ends ofdifferent nanotubes. Each black line corresponds to one graphenesheet viewed edge-on.

Defects in the hexagonal lattice are usually present in the form of

pentagons and heptagons. Pentagons produce a positive curvature ofthe graphene layer and are mostly found at the cap. Heptagons giveraise to a negative curvature of the tube wall. Defects consisting ofseveral pentagons and/or heptagons have also been observed.


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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 CNTs:

Multi-walled nanotubes (MWNT) consist of multiple rolled in onthemselves to form a tube shape. There are two models which can beused to describe the structures of multi-walled nanotubes. In theRussian Doll model, sheets of graphite are arranged in concentriccylinders. In the Parchment model, a single sheet of graphite is rolledin around itself, resembling a scroll of parchment or a rolled upnewspaper. The interlayer distance in multi-walled nanotubes is closeto the distance between graphene layers in graphite, approximately0.33 nm.


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Although it is easier to produce significant quantities of MWNTs thanSWNTs, their structures are less well understood than single-wallnanotubes because of their greater complexity and variety.Multitudes of exotic shapes and arrangements, often with imaginativenames such as bamboo-trunks, sea urchins, necklaces or coils, have

also been observed under different processing conditions. The varietyof forms may be interesting but also has a negative sideMWNTsalways (so far) have more defects than SWNTs and these diminishtheir desirable properties.

Many of the nanotube applications now being considered or put intopractice involve multi-walled nanotubes, because they are easier toproduce in large quantities at a reasonable price and have beenavailable in decent amounts for much longer than SWNTs. In fact oneof the major manufacturers of MWNTs at the moment, HyperionCatalysis, does not even sell the nanotubes directly but only pre-mixed with polymers for composites applications. The tubes involvedtypically have 8 to 15 walls and are around 10 nanometres wide and10 micrometers long.

Companies are moving into this space, notably formidable playerslike Mitsui, with plans to produce similar types of MWNT in hundredsof tons a year, a quantity that is greater, but not hugely so, than thecurrent production of Hyperion Catalysis. This is an indication thateven these less impressive and exotic nanotubes hold promise ofrepresenting a sizable market in the near future.


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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 formaking large quantities.

CNTs have probably been around for a lot longer than was firstrealized, and may have been made during various carbon combustionand vapor deposition processes, but electron microscopy at that timewas not advanced enough to distinguish them from other types oftubes. The first method for producing CNTs and fullerenes in

reasonable quantities was by applying an electric current acrosstwo carbonaceous electrodes in an inert gas atmosphere. Thismethod is calledplasma arcing. It involves the evaporation of oneelectrode as cations followed by deposition at the other electrode.This plasma-based process is analogous to the more familiarelectroplating process in a liquid medium. Fullerenes and CNTs areformed by plasma arcing of carbonaceous materials, particularlygraphite. The fullerenes appear in the soot that is formed, while theCNTs 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. Asnoted above, the nanotube product is a compact cathode deposit ofrod like morphology. However when cobalt is added as a catalyst, thenature of the product changes to a web, with strands of 1mm or sothickness that stretch from the cathode to the walls of the reactionvessel. The mechanism by which cobalt changes this process isunclear, however one possibility is that such metals affect the localelectric fields and hence the formation of the five-membered rings.

Synthesis of carbon nanotubes can be done by differentmethods:-


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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 graphiteelectrodes during an arc discharge, by using a current of 100amperes that was intended to produce fullerenes.

The carbon arc discharge method, initially used for producing C60fullerenes, is the most common and perhaps easiest way to produce

CNTs, as it is rather simple. However, it is a technique that producesa complex mixture of components, and requires further purification -to separate the CNTs from the soot and the residual catalytic metalspresent in the crude product. This method creates CNTs through arc-vaporization of two carbon rods placed end to end, separated byapproximately 1mm, in an enclosure that is usually filled with inertgas at low pressure. Recent investigations have shown that it is alsopossible to create CNTs with the arc method in liquid nitrogen. Adirect current of 50 to 100 A, driven by a potential difference ofapproximately 20 V, creates a high temperature discharge between

the two electrodes.


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The discharge vaporizes the surface of one of the carbonelectrodes, and forms a small rod-shaped deposit on the otherelectrode. Producing CNTs in high yield depends on the uniformity ofthe plasma arc, and the temperature of the deposit forming on thecarbon electrode. The carbon contained in the negative electrode

sublimates because of the high temperatures caused by thedischarge. Because nanotubes were initially discovered using thistechnique, it has been the most widely used method of nanotubesynthesis.

The yield for this method is up to 30 percent by weight and itproduces both single- and multi-walled nanotubes with lengths of upto 50 micrometers.


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b) LASER ABLATION PROCESS

In the laser ablation process, a pulsed laser vaporizes a graphitetarget in a high temperature reactor while an inert gas is bled into thechamber. The nanotubes develop on the cooler surfaces of thereactor, as the vaporized carbon condenses. A water-cooled surfacemay be included in the system to collect the nanotubes.

In 1996 CNTs were first synthesized using a dual-pulsed laser andachieved yields of >70wt% purity. Samples were prepared by laservaporization of graphite rods with a 50:50 catalyst mixture of Cobaltand Nickel at 1200C in flowing argon, followed by heat treatment ina vacuum at 1000C to remove the C60 and other fullerenes. The

initial laser vaporization pulse was followed by a second pulse, tovaporize the target more uniformly. The use of two successive laserpulses minimizes the amount of carbon deposited as soot. Thesecond laser pulse breaks up the larger particles ablated by the firstone, 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 100m 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,


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the catalyst composition, and other process parameters, the averagenanotube diameter and size distribution can be varied. Arc-dischargeand laser vaporization are currently the principal methods forobtaining small quantities of high quality CNTs. However, bothmethods suffer from drawbacks. The first is that both methods

involve evaporating the carbon source, so it has been unclear how toscale up production to the industrial level using these approaches.The second issue relates to the fact that vaporization methods growCNTs in highly tangled forms, mixed with unwanted forms of carbonand/or metal species. The CNTs thus produced are difficult to purify,manipulate, and assemble for building nanotube-device architecturesfor practical applications

This method has a yield of around 70% and produces primarily single-walled carbon nanotubes with a controllable diameter determined bythe reaction temperature.


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c) CHEMICAL VAPOUR DEPOSITIONChemical vapor deposition of hydrocarbons over a metal catalyst is aclassical method that has been used to produce various carbonmaterials such as carbon fibers and filaments. For over twenty years.Large amounts of CNTs can be formed by catalytic CVD of acetyleneover Cobalt and iron catalysts supported on silica or zeolite. Thecarbon deposition activity seems to relate to the cobalt content of thecatalyst, whereas the CNTs selectivity seems to be a function of thepH in catalyst preparation.

Fullerenes and bundles of single walled nanotubes were also foundamong the multi walled nanotubes produced on the carbon/zeolitecatalyst. Some researchers are experimenting with the formation ofCNTs from ethylene. Supported catalysts such as iron, cobalt, andnickel, containing either a single metal or a mixture of metals, seemto induce the growth of isolated single walled nanotubes or singlewalled nanotubes bundles in the ethylene atmosphere. Theproduction of single walled nanotubes, as well as double-walled CNTs,on molybdenum and molybdenum-iron alloy catalysts has also beendemonstrated. 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 545C for Nickel-catalyzed CVD, and 900C for an uncatalyzed process. The resultantcarbon nanostructures have open ends, with no caps. Methane hasalso been used as a carbon source. In particular it has been used toobtain nanotube chips containing isolated single walled nanotubesat controlled locations. High yields of single walled nanotubes havebeen obtained by catalytic decomposition of an H2/CH4 mixture overwell-dispersed metal particles such as Cobalt, Nickel, and Iron onmagnesium oxide at 1000C. It has been reported that the synthesisof composite powders containing well-dispersed CNTs can be

achieved by selective reduction in an H2/CH4 atmosphere of oxidesolid solutions between a non-reducible oxide such as Al2O3 or


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MgAl2O4 and one or more transition metal oxides. The reductionproduces very small transition metal particles at a temperature ofusually >800C. The decomposition of CH4 over the freshly formednanoparticles prevents their further growth, and thus results in a veryhigh proportion of single walled nanotubes and fewer multi walled

nanotubes.

These are the basic principles of the CVD process. In the lastdecennia, different techniques for the carbon nanotubes synthesiswith 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 differenttechniques will be explained more detailed in this chapter.

Using CVD, a substrate is prepared with a layer of metal catalystparticles, most commonly nickel, cobalt, iron, or a combination. Thediameters of the nanotubes that are to be grown are related to thesize of the metal particles. This can be controlled bypatterned deposition of the metal, annealing, or by plasma etching ofa metal layer. The substrate is heated to approximately 700C. To

initiate the growth of nanotubes, two gases are bled into the reactor:a process gas (such as ammonia, nitrogen, hydrogen, etc.) and acarbon-containing gas (such as acetylene, ethylene, ethanol,methane, etc.). Nanotubes grow at the sites of the metal catalyst; thecarbon-containing gas is broken apart at the surface of the catalystparticle, and the carbon is transported to the edges of the particle.

CVD is a common method for the commercial production of carbonnanotubes.. For this purpose, the metal nanoparticles will be carefullymixed with a catalyst support (e.g., MgO, Al2O3, etc) to increase the

specific surface area for higher yield of the catalytic reaction of thecarbon feedstock with the metal particles. One issue in this synthesis


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route is the removal of the catalyst support via an acid treatment,which sometimes could destroy the original structure of the carbonnanotubes. However, alternative catalyst supports that are soluble inwater have been shown to be effective for nanotube growth. Ifplasma is generated by the application of a strong electric field

during the growth process (plasma enhanced chemical vapordeposition), then the nanotube growth will follow the direction of theelectric field. By properly adjusting the geometry of the reactor it ispossible 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 toboth electrodes.

Figure shows a schematic diagram of a typical plasma CVD apparatuswith a parallel plate electrode structure.

Figure: Schematic diagram of plasma CVDapparatus.

A substrate is placed on the grounded electrode. In order to form auniform 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. Afternanoscopic fine metal particles are formed, carbon nanotubes will begrown on the metal particles on the substrate by glow dischargegenerated from high frequency power. A carbon containing reactiongas, such as C2H2, CH4, C2H4, C2H6, CO is supplied to the chamberduring the discharge.


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The catalyst has a strong effect on the nanotube diameter, growthrate, wall thickness, morphology and microstructure. Ni seems to bethe most suitable pure-metal catalyst for the growth of aligned multi-walled carbon nanotubes (MWNTs)36. The diameter of the MWNTs isapproximately 15 nm. The highest yield of carbon nanotubes

achieved was about 50% and was obtained at relatively lowtemperatures (below 330o C).

2)Thermal chemical vapour deposition

In this method Fe, Ni, Co or an alloy of the three catalytic metals isinitially deposited on a substrate. After the substrate is etched in adiluted HF solution with distilled water, the specimen is placed in a

quartz boat. The boat is positioned in a CVD reaction furnace, andnanometer-sized catalytic metal particles are formed after anadditional etching of the catalytic metal film using NH3 gas at atemperature of 750 to 1050o C. As carbon nanotubes are grown onthese fine catalytic metal particles in CVD synthesis, forming thesefine catalytic metal particles is the most important process.

Figure shows a schematic diagram of thermal CVD apparatus in thesynthesis of carbon nanotubes.

Figure: Schematic diagram of thermal CVDapparatus.

When growing carbon nanotubes on a Fe catalytic film by thermalCVD, the diameter range of the carbon nanotubes depends on thethickness of the catalytic film. By using a thickness of 13 nm, thediameter distribution lies between 30 and 40 nm. When a thickness of27 nm is used, the diameter range is between 100 and 200 nm. Thecarbon nanotubes formed are multiwalled.


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3)Vapour phase growth

Vapour phase growth is a synthesis method of carbon nanotubes,directly supplying reaction gas and catalytic metal in the chamberwithout a substrate 39.

Figure shows a schematic diagram of a vapour phase growthapparatus. Two furnaces are placed in the reaction chamber.Ferrocene is used as catalyst. In the first furnace, vaporization ofcatalytic carbon is maintained at a relatively low temperature. Finecatalytic particles are formed and when they reach the secondfurnace, decomposed carbons are absorbed and diffused to thecatalytic metal particles. Here, they are synthesized as carbonnanotubes. The diameter of the carbon nanotubes by using vapourphase growth are in the range of 2 - 4 nm for SWNTs40 and between

70 and 100 nm for MWNTs.

MethodArc dischargemethod

Chemical

vapourdeposition

Laser ablation(vaporization)

WhoEbbesen andAjayan, NEC, Japan1992 15

Endo, ShinshuUniversity,Nagano, Japan 53

Smalley, Rice, 199514

How

Connect twographite rods to apower supply,place them a fewmillimeters apart,and throw theswitch. At 100amps, carbonvaporizes andforms hot plasma.

Place substrate inoven, heat to600 oC, and slowlyadd a carbon-bearing gas suchas methane. Asgas decomposes itfrees up carbonatoms, whichrecombine in theform of NTs

Blast graphite withintense laser pulses;use the laser pulsesrather than electricityto generate carbongas from which theNTs form; try variousconditions until hit onone that producesprodigious amountsof SWNTs

Typicalyield

30 to 90% 20 to 100 % Up to 70%

SWNT Short tubes withdiameters of 0.6 -Long tubes withdiameters rangingLong bundles oftubes (5-20 microns),


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MethodArc dischargemethod

Chemical

vapourdeposition

Laser ablation(vaporization)

1.4 nm from 0.6-4 nm with individual

diameter from 1-2nm.

M-WNT

Short tubes withinner diameter of1-3 nm and outerdiameter of approximately 10nm

Long tubes withdiameter rangingfrom 10-240 nm

Not very muchinterest in thistechnique, as it is tooexpensive, but MWNTsynthesis is possible.

Pro

Can easily produceSWNT, MWNTs.

SWNTs have fewstructural defects;MWNTs withoutcatalyst, not tooexpensive, openair synthesispossible

Easiest to scale upto industrial

production; longlength, simpleprocess, SWNTdiametercontrollable, quitepure

Primarily SWNTs, withgood diameter

control and fewdefects. The reactionproduct is quite pure.

Con

Tubes tend to beshort with randomsizes anddirections; often

needs a lot of purification

NTs are usuallyMWNTs and oftenriddled withdefects

Costly technique,because it requiresexpensive lasers andhigh power

requirement, but isimproving

Table 2-2: A summary of the major production methodsand their efficiency

Characterization of carbon

nanotubes:-The experimental techniques used for growth and characterization ofcarbon nanotubes are discussed. Plasma enhanced chemical vapordeposition (PECVD) method was used for the deposition of thesefilms. Scanning electron microscopy (SEM), Transmission electronmicroscopy (TEM), Energy dispersive X-ray spectroscopy (EDS),Raman spectroscopy and X-ray diffraction were used for thecharacterization of carbon nanostructures and catalyst nanoparticles.

SEM:-


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A scanning electron microscope (SEM) is a type ofelectron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern. The electronsinteract with the atoms that make up the sample producing signalsthat 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 anelectron gun fitted with a tungsten filament cathode. Tungsten isnormally used in thermionic electron guns because it has the highestmelting point and lowest vapour pressure of all metals, therebyallowing it to be heated for electron emission, and because of its lowcost. Other types of electron emitters include lanthanum hexaboride(LaB6) cathodes, which can be used in a standard tungsten filamentSEM if the vacuum system is upgraded and field emission guns (FEG),which may be of the cold-cathode type using tungsten single crystalemitters or the thermally-assisted Schottky type, using emitters ofzirconium oxide.

The electron beam, which typically has an energy ranging from 0.5keV to 40 keV, is focused by one or two condenser lenses to a spotabout 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 thexandyaxesso that it scans in a raster fashion over a rectangular area of thesample surface.

When the primary electron beam interacts with the sample, theelectrons lose energy by repeated random scattering and absorptionwithin a teardrop-shaped volume of the specimen known as theinteraction volume, which extends from less than 100 nm to around5 m into the surface. The size of the interaction volume depends onthe electron's landing energy, the atomic number of the specimenand the specimen's density. The energy exchange between the


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electron beam and the sample results in the reflection of high-energyelectrons by elastic scattering, emission of secondary electrons byinelastic scattering and the emission of electromagnetic radiation,each of which can be detected by specialized detectors. The beamcurrent absorbed by the specimen can also be detected and used to

create images of the distribution of specimen current. Electronicamplifiers of various types are used to amplify the signals which aredisplayed as variations in brightness on a cathode ray tube. Theraster scanning of the CRT display is synchronized with that of thebeam on the specimen in the microscope, and the resulting image istherefore a distribution map of the intensity of the signal beingemitted from the scanned area of the specimen. The image may becaptured by photography from a high resolution cathode ray tube,but in modern machines is digitally captured and displayed on acomputer 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 anultra thin specimen, interacting with the specimen as it passes


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through. An image is formed from the interaction of the electronstransmitted through the specimen; the image is magnified andfocused onto an imaging device, such as a fluorescent screen, on alayer of photographic film, or to be detected by a sensor such as aCCD camera.

TEMs are capable of imaging at a significantly higher resolution thanlight microscopes, owing to the small de Broglie wavelength ofelectrons. This enables the instrument's user to examine fine detaileven as small as a single column of atoms, which is tens of thousandstimes smaller than the smallest resolvable object in a lightmicroscope. TEM forms a major analysis method in a range ofscientific fields, in both physical and biological sciences. TEMs findapplication in cancer research, virology, materials science as well as

pollution, nanotechnology, and semiconductor research.

XRD:-


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X-ray scattering techniques are a family of non-destructiveanalytical techniques which reveal information about thecrystallographic structure, chemical composition, and physicalproperties of materials and thin films. These techniques are based onobserving the scattered intensity of an X-ray beam hitting a sample

as a function of incident and scattered angle, polarization, andwavelength or energy.

In an X-ray diffraction measurement, a crystal is mounted on agoniometer and gradually rotated while being bombarded with X-rays, producing a diffraction pattern of regularly spaced spots knownas reflections. The two-dimensional images taken at differentrotations are converted into a three-dimensional model of the densityof electrons within the crystal using the mathematical method ofFourier transforms, combined with chemical data known for thesample. Poor resolution (fuzziness) or even errors may result if thecrystals are too small, or not uniform enough in their internalmakeup.

X-ray crystallography is related to several other methods fordetermining atomic structures. Similar diffraction patterns can beproduced by scattering electrons or neutrons, which are likewiseinterpreted as a Fourier transform. If single crystals of sufficient sizecannot be obtained, various other X-ray methods can be applied toobtain less detailed information; such methods include fiberdiffraction, powder diffraction and small-angle X-ray scattering

(SAXS). If the material under investigation is only available in theform of nano-crystalline powders or suffers from poor crystallinity, themethods of electron crystallography can be applied for determiningthe atomic structure.


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RAMAN SPECTROSCOPY:-

Raman spectroscopy (named after C. V. Raman) isspectroscopic 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 weakinelastically 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


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wavelength and wave numbers of shift in the Raman spectrum, the

following formula can be used:

Where w is the Raman shift expressed in wave number, 0 is the

excitation wavelength, and 1 is the Raman spectrum wavelength.

Most commonly, the units chosen for expressing wave number in

Raman spectra is inverse centimeters (cm1). Since wavelength is

often expressed in units of nanometers (nm), the formula above can

scale for this units conversion explicitly, giving


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PROPERTIES:-

The most important properties of CNTs are:-

a) Strength and elasticity:

CNTs are the strongest and stiffest materials on earth, in terms oftensile strength and elastic modulus respectively. This strengthresults from the covalent sp bonds formed between individualcarbon atoms.

The carbon atoms of a single sheet of graphite form a planarhoneycomb lattice, in which each atom is connected via a strongchemical bond to three neighboring atoms. Because of these strongbonds, the basal plane elastic modulus of graphite is one of thelargest of any known material. For this reason, CNTs are expected tobe the ultimate high-strength fibers. Single walled nanotubes arestiffer than steel, and are very resistant to damage from physicalforces. Pressing on the tip of a nanotube will cause it to bend, butwithout damage to the tip. When the force is removed, the nanotubereturns 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 exactnumerical value has not been agreed upon.Using atomic force microscopy, the unanchored ends of afreestanding nanotube can be pushed out of their equilibriumposition, and the force required to push the nanotube can bemeasured. The current Youngs modulus value of single wallednanotubes is about 1 TeraPascal, but this value has been widelydisputed, and a value as high as 1.8 Tpa has been reported. Othervalues significantly higher than that have also been reported. Thedifferences probably arise through different experimentalmeasurement techniques. Others have shown theoretically that theYoungs modulus depends on the size and chirality of the singlewalled nanotubes, ranging from 1.22 Tpa to 1.26 Tpa. They havecalculated a value of 1.09 Tpa for a generic nanotube. However, whenworking with different multi walled nanotubes, others have noted thatthe modulus measurements of multi walled nanotubes using AFMtechniques do not strongly depend on the diameter. Instead, theyargue that the modulus of the multi walled nanotubes correlates tothe amount of disorder in the nanotube walls. Not surprisingly, whenmulti walled nanotubes break, the outermost layers break first.


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CNTs are not nearly as strong under compression. Because of theirhollow structure and high aspect ratio, they tend to undergobuckling when placed under compressive, tensional or bendingstress.

b) Thermal conductivity and expansion:

All nanotubes are expected to be very good thermal conductorsalong the tube, exhibiting a property known as ballistic conduction,but good insulators laterally to the tube axis. The temperaturestability of carbon nanotubes is established to be up to 2800 degreesCelsius in vacuum and about 750 degrees Celsius in air.

CNTs have been shown to exhibit superconductivity below 20K(aprox. 253C). Research suggests that these exotic strands, alreadyheralded for their unparalleled strength and unique ability to adoptthe electrical properties of either semiconductors or perfect metals,may someday also find applications as miniature heat conduits in ahost of devices and materials. The strong in-plane graphitic carbon -carbon bonds make them exceptionally strong and stiff against axialstrains. The almost zero in-plane thermal expansion but large inter-plane expansion of single walled nanotubes implies strong in-planecoupling and high flexibility against non-axial strains.Many applications of CNTs, such as in nanoscale molecular

electronics, sensing and actuating devices, or as reinforcing additivefibers in functional composite materials, have been proposed. Reportsof several recent experiments on the preparation and mechanicalcharacterization of CNT-polymer composites have also appeared.These measurements suggest modest enhancements in strengthcharacteristics of CNT-embedded matrixes as compared to barepolymer matrixes. Preliminary experiments and simulation studies onthe thermal properties of CNTs show very high thermal conductivity.It is expected, therefore, that nanotube reinforcements in polymericmaterials 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 additivefor plastics of all types. Their high aspect ratio means that a lowerloading of CNTs is needed compared to other conductive additives toachieve the same electrical conductivity. This low loading preservesmore of the polymer resins toughness, especially at lowtemperatures, as well as maintaining other key performance

properties of the matrix resin. CNTs have proven to be an excellentadditive to impart electrical conductivity in plastics. Their high aspect


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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 smalldiameter 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 eithermetallic or semi-conducting in their electrical behavior. Conductivityin MWNTs is quite complex. Some types of armchair-structuredCNTs appear to conduct better than other metallic CNTs.Furthermore, interwall reactions within multi walled nanotubes havebeen found to redistribute the current over individual tubes non-uniformly. However, there is no change in current across differentparts of metallic single-walled nanotubes. The behavior of the ropes

of semi-conducting single walled nanotubes is different, in that thetransport current changes abruptly at various positions on the CNTs.The conductivity and resistivity of ropes of single walled nanotubeshas been measured by placing electrodes at different parts of theCNTs. The resistivity of the single walled nanotubes ropes was of theorder of 104 ohm-cm at 27C. This means that single wallednanotube ropes are the most conductive carbon fibers known. Thecurrent density that was possible to achieve was 10-7 A/cm2,however in theory the single walled nanotube ropes should be able tosustain 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 wallednanotubes to act as transistors. Likewise, joining CNTs together mayform transistor-like devices. A nanotube with a natural junction(where a straight metallic section is joined to a chiral semi conductingsection) behaves as a rectifying diode that is, a half-transistor in asingle molecule. It has also recently been reported that single wallednanotubes can route electrical signals at speeds up to 10 GHz whenused as interconnects on semi-conducting devices.

e) Electronic properties:


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The electronic properties of SWNTs have been studied in a largenumber of theoretical works. All models show that theelectronic properties vary in a predictable way from metallic tosemi conducting with diameter and chirality. This is due to thevery 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 arelativistic Dirac electron with vanishing rest mass. This causesthe appearance of a nontrivial Berrys phase under 2 rotationin wave-vector space, leading to the absence of backscatteringand in the metallic carbon nanotube resulting in perfectconduction even in the presence of scatterers. The energybands in carbon nanotubes are determined by periodicboundary conditions with a fictitious Aharonov-Bohm fluxdetermined uniquely by the circumferential chiral vector. Ananotube becomes metallic when the flux vanishes andsemiconducting when the flux is nonzero. The conductivity ofgraphene is essentially independent of the Fermi energy andthe electron concentration as long as variations in effectivescattering strength are neglected, and therefore grapheneshould be regarded as a metal rather than a zero-gapsemiconductor. Various schemes are now being proposed andtested 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 ofthe graphene sheet, while all other tubes are semi-conducting with aband gap that scales approximately with the inverse of the tuberadius. Bandgaps of 0.4 1 eV can be expected for SWNTs(corresponding to diameters between 0.6 and 1.6 nm).


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On the left: band structure of the conduction band of graphene, whichcrosses 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 confirmedin 1998 by scanning tunneling spectroscopy. Numerous conductivityexperiments on SWNTs and MWNTs allowed gaining additional

informations. At low temperatures, SWNTs behave as coherentquantum wires where the conduction occurs through discrete


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electron states over large distances. Transport measurementsrevealed that metallic SWNTs show extremely long coherencelengths. MWNTs show also these effects in spite of their largerdiameter 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 ofgraphite sheets. Although a graphite sheet has a 2D symmetry,carbon nanotubes by geometry have different properties in axial andradial directions. It has been shown that CNTs are very strong in theaxial direction. Young's modulus on the order of 270 - 950 GPa andtensile strength of 11 - 63 GPa were obtained.

On the other hand, there was evidence that in the radial directionthey are rather soft. The first transmission electron microscopeobservation of radial elasticity suggested that even the van der Waalsforces can deform two adjacent nanotubes. Later, nanoindentationswith atomic force microscope were performed by several groups toquantitatively measure radial elasticity of multiwalled carbonnanotubes and tapping/contact mode atomic force microscopy wasrecently performed on single-walled carbon nanotubes. Young'smodulus of on the order of several GPa showed that CNTs are in factvery soft in the radial direction.

Radial direction elasticity of CNTs is important especially for carbonnanotube composites where the embedded tubes are subjected tolarge deformation in the transverse direction under the applied loadon the composite structure.

One of the main problems in characterizing the radial elasticity ofCNTs is the knowledge about the internal radius of the CNT; carbonnanotubes with identical outer diameter may have different internaldiameter (or the number of walls). Recently a method using atomicforce microscope was introduced to find the exact number of layers

and hence the internal diameter of the CNT. In this way, mechanicalcharacterization is more accurate.

Comparison of mechanical properties

Materi

al

Young's

modulus (T

Tensile

strength (G

Elongation at

break (%)


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Pa) Pa)

SWNT ~1 (from 1 to 5) 1353 16

ArmchairSWNT

0.94 126.2 23.1

ZigzagSWNT

0.94 94.5 15.617.5

ChiralSWNT

0.92 NA NA

MWNTE 0.20.80.95 1163150 NA

Stainlesssteel

0.1860.214 0.381.55 1550

Kevlar29&149

0.060.18 3.63.8] ~2

APPLICATIONS:-
http://en.wikipedia.org/wiki/Carbon_nanotube#cite_note-Kevlar-31http://en.wikipedia.org/wiki/Carbon_nanotube#cite_note-Kevlar-31


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Carbon nanotubes (Buckytubes) have extraordinary electricalconductivity, heat conductivity and mechanical properties. They areprobably the best electron field-emitter possible. They are polymersof pure carbon and can be reacted and manipulated using thetremendously rich chemistry of carbon. This provides opportunity to

modify the structure and to optimize solubility and dispersion.

Very significantly, buckytubes are molecularly perfect, which meansthat they are free of property-degrading flaws in the nanotubestructure. Their material properties can therefore approach closelythe very high levels intrinsic to them.

These extraordinary characteristics give buckytubes potential innumerous 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.


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b) ELECTROMAGNETIC

Buckypaper: It is a thin sheet made from nanotubes thatare 250 times stronger than steel and 10 times lighter thatcould be used as a heat sink for chipboards, a backlight for LCD

screens or as a faraday cage to protect electrical devices/aeroplanes.

Chemical nanowires: Carbon nanotubes additionally canalso be used to produce nanowires of other chemicals, such asgold or zinc oxide. These nanowires in turn can be used to castnanotubes of other chemicals, such as gallium nitride. Thesecan have very different properties from CNTs for example,gallium nitride nanotubes are hydrophilic, while CNTs arehydrophobic, giving them possible uses in organic chemistrythat CNTs could not be used for.

Computer circuits: A nanotube formed by joiningnanotubes of two different diameters end to end can act as adiode, suggesting the possibility of constructing electroniccomputer circuits entirely out of nanotubes. Because of theirgood thermal properties, CNTs can also be used to dissipate

heat from tiny computer chips.

Conductive films: CNTs are also introduced in developingtransparent, electrically conductive films to replace indium tinoxide(ITO).CNT films are substantially more mechanicallyrobust then ITO films ,making them ideal for more reliabilitytouch screens and flexible displays. Printable water based inksof carbon nanotubes are desired to enable the production ofthese 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 nanotubeshave been used for several years in brushes for commercialelectric motors.. The nanotubes improve electrical and thermalconductivity because they stretch through the plastic matrix ofthe brush. This permits the carbon filler to be reduced from30% down to 3.6%, so that more matrixes are present in the

brush. Nanotube composite motor brushes are better-lubricated(from the matrix), cooler-running (both from better lubrication


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and superior thermal conductivity), less brittle (more matrix,and fiber reinforcement), stronger and more accuratelymoldable (more matrix). Since brushes are a critical failurepoint in electric motors, and also dont need much material,they became economical before almost any other application.

Light bulb filament: Alternative to tungsten filaments inincandescent lamps.

Solar cells: Organic photovoltaic devices (OPVs) arefabricated from thin films of organic semiconductors, such aspolymers and small-molecule compounds, and are typically onthe order of 100 nm thick. Because polymer based OPVs can bemade using a coating process such as spin coating or inkjetprinting, they are an attractive option for inexpensivelycovering large areas as well as flexible plastic surfaces. Apromising low cost alternative to silicon solar cells, there is alarge amount of research being dedicated throughout industryand academia towards developing OPVs and increasing theirpower conversion efficiency. GEs carbon nanotube diode has aphotovoltaic effect. Nanotubes can replace ITO (Indium tinoxide) in some solar cells to act as a transparent conductive

film in solar cells to allow light to pass to the active layers andgenerate photocurrent.

CNTs in dye-sensitized solar cells:-

Due to the simple fabrication process,low production cost, and high efficiency, there issignificant interest in dye-sensitized solar cells (DSSCs).

Thus, improving DSSC efficiency has been the subject ofa variety of research investigations because it has the


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potential to be manufactured economically enough tocompete with other solar cell technologies. Titaniumdioxide nanoparticles have been widely used as aworking electrode for DSSCs because they provide a highefficiency, more than any other metal oxide

semiconductor investigated. Yet the highest conversionefficiency 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 enhanceefficiency has not produced any major results. Thetransport of electrons across the particle network hasbeen a key problem in achieving higher photo conversionefficiency in nanostructured electrodes. Becauseelectrons encounter many grain boundaries during thetransit and experience a random path, the probability oftheir recombination with oxidized sensitizer is increased.Therefore, it is not adequate to enlarge the oxideelectrode surface area to increase efficiency becausephoto-generated charge recombination should beprevented. Promoting electron transfer through filmelectrodes and blocking interface states lying below theedge of the conduction band are some of the non-CNTbased strategies to enhance efficiency that have beenemployed.

With recent progress in CNT development and

fabrication, there is promise to use various CNT basednanocomposites and nanostructures to direct the flow ofphoto generated electrons and assist in charge injectionand extraction. To assist the electron transport to thecollecting electrode surface in a DSSC, a popular conceptis to utilize CNT networks as support to anchor lightharvesting semiconductor particles.

"Efficiencies reaching 4.4% have already been achievedand hopefully 10-15% efficiencies are feasible in the near-futureupon further optimization" says Kymakis. "Once this obstacle is

tackled, the lifetime issue, which is directly related to the celltemperatures, can be explored. A working environment combiningthe strengths of scientists and business leaders may soon result inrapid commercialization of this technology."

Superconductor: Nanotubes have been shown to besuperconducting at low temperatures.


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Ultra capacitors: Nanotubes, when bound to plates ofcapacitors increase the surface area and thus increase energystorage ability.

Displays: One use for nanotubes that has already beendeveloped is as extremely fine electron guns, which could beused as miniature cathode ray tubes in thin high-brightnesslow-energy low-weight displays. This type of display wouldconsist of a group of many tiny CRTs, each providing theelectrons to hit the phosphor of one pixel, instead of having onegiant CRT whose electrons are aimed using electric andmagnetic fields. These displays are known as field emissiondisplays (FEDs).

Others: Artificial muscles, magnets, optical ignition etc.

c) CHEMICAL

Air pollution filter: Future applications of nanotubemembranes include filtering carbon dioxide from power plantemissions.

Biotech container: Nanotubes can be opened and filledwith materials such as biological molecules, raising thepossibility of applications in biotechnology.

Hydrogen storage: Research is currently beingundertaken into the potential use of carbon nanotubes forhydrogen storage. They have the potential to store between 4.2and 65% hydrogen by weight. This is an important area ofresearch, since if they can be mass produced economicallythere is potential to contain the same quantity of energy as a50L gasoline tank in 13.2L of nanotubes. See also, HydrogenEconomy.


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Water filter: Recently nanotube membranes have beendeveloped for use in filtration. This technique can purportedlyreduce desalination costs by 75%. The tubes are so thin thatsmall 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 ofmagnitude 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 theirapplicability as Very-large-scale integration (VLSI) interconnects ofthe future because of their desirable properties of high thermalstability, high thermal conductivity and large current carrying

capacity. An isolated CNT can carry current densities in excess of1000 MA/sq-cm without any signs of damage even at an elevatedtemperature of 250 degrees C, thereby eliminating electromigrationreliability concerns that plague Cu interconnects. Recent modelingwork comparing the performance, power dissipation andthermal/reliability aspects of CNT interconnect to scaled copperinterconnects have shown that CNT bundle interconnects canpotentially offer more advantages over copper.

f) TRANSISTORS

Smaller silicon based integrated circuits result in both a higher speedand device density. As a result, downscaling of these devices has


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been very important since their first implementation. However, at themoment it is generally accepted that silicon devices will reachfundamental scaling limits within a decade or so. This limit is causedby the minimum wavelength of light used in lithographic techniquesused for integrated circuit production nowadays. For this reason a

quest for alternative, integrated circuits with smaller dimensions hasstarted. A major step in downscaling would be the application ofsingle molecules in electronic devices. Carbon nanotubes havealready shown promising results in single molecular transistors. Forsuccessful implementation of molecular transistors in large andcomplex logic systems, they must show signal amplification. Signalamplification makes it possible to reference separate signals along achain of logical operations. In addition, noise caused by thermalfluctuations and environmental disturbances is also reduced. Threeterminal nanotransistors, in special, field-effect-transistors showamplifying behavior and have recently been investigated for thisreason.

g) Electrical circuits

Carbon nanotubes have many propertiesfrom their unique

dimensions to an unusual current conduction mechanismthat 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 sofar 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


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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 orbiological 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 nanotubesmetallic, semiconducting, single-walled, multi-

walledproduced. 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 electromechanicalsystems, including mechanical memory elements.

CNTs have also been proposed as a possible gene deliveryvehicle and for use in combination in radio frequency fields todestroy cancer cells.

Nanomix Inc was the first to put on the market an electronicdevice that integrated carbon nanotubes on a silicon platform,in may 2005. It was a hydrogen sensor. Since then nanomix hasbeen patenting many such sensor applications such as in thefield of Carbon-di-oxide, Nitrous Oxide, glucose, DNAdetection etc.

As a container for drug delivery: Because of the versatilestructure of the CNT, it can be used for a variety of tasks inand around body. Often in the cancer related incidents, the CNT

is often used as a container for transporting drugs into thebody. Here drugs can actually be placed inside the nanotubesor can be attached to the side or trailed behind. Both of thesemethods are effective for the delivery and distribution of druginside out of the body.

CNTs can be used as light emitting semiconductors.


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CONCLUSION

Rise in demand and production, and ease of accessibility of carbonnanotubes would lead to the extensive use of carbon nanotubes in awide variety of applications. The use of nanotechnology for humanwill become common need in 21st century. As world is suffering fromserious pollution problems, hydrogen will becoming need of 21st

century & carbon nanotubes provide better solution for hydrogenstorage.

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 thisand fore coming generation will be fulfilled by carbon nanotubes.

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Nano Tubes