As stated before, nanotubes generally have a length to diameter ratio of about 1000 so they can be considered as nearly one-dimensional structures.

Figure 1: Some SWNTs with different chiralities. The difference in structure is easily shown at the open end of the tubes. (a) Armchair structure (b) Zigzag structure (c) Chiral structure [Ref: 1]

A SWNT consists of two separate regions with different physical and chemical properties. The first is the sidewall of the tube and the second is the end cap of the tube. The end cap structure is similar to or derived from a smaller fullerene, such as C60. C-atoms placed in hexagons and pentagons form the end cap structures. It can be easily derived from Euler’s theorem that twelve pentagons are needed in order to obtain a closed cage structure which consists of only pentagons and hexagons. The combination of a pentagon and five surrounding hexagons results in the desired curvature of the surface to enclose a volume. A second rule is the isolated pentagon rule that states that the distance between pentagons on the fullerene shell is maximised in order to obtain a minimal local curvature and surface stress, resulting in a more stable structure. The smallest stable structure that can be made this way is C60 the one just larger is C70. Another property is that all fullerenes are composed of an even number of C-atoms because adding one hexagon to an existing structure means adding two C-atoms.

The other structure of which a SWNT is composed is a cylinder. It is generated when a graphene of a certain size that is wrapped in a certain direction. As the result is cylinder symmetric we can only roll in a discreet set of directions in order to form a closed cylinder (Figure 1). Two atoms in the graphene sheet are chosen, one of which servers the role as origin. The sheet is rolled until the two atoms coincide. The vector pointing from the first atom towards the other is called the chiral vector and its length is equal to the circumference of the nanotube (Figure 1). The direction of the nanotube axis is perpendicular to the chiral vector. SWNTs with different chiral vectors have dissimilar properties such as optical activity, mechanical strength and electrical conductivity.¹

Multi-walled:

Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphite. 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, e.g. a (0,8) single-walled nanotube (SWNT) within a larger (0,17) single-walled nanotube. In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.4 Å.

The special place of double-walled carbon nanotubes (DWNT) must be emphasized here because their morphology and properties are similar to SWNT but their resistance to chemicals is significantly improved. This is especially important when functionalization is required (this means grafting of chemical functions at the surface of the nanotubes) to add new properties to the CNT. In the case of SWNT, covalent functionalization will break some C=C double bonds, leaving "holes" in the structure on the nanotube and thus modifying both its mechanical and electrical properties. In the case of DWNT, only the outer wall is modified. DWNT synthesis on the gram-scale was first proposed in 2003 by the CCVD technique, from the selective reduction of oxide solutions in methane and hydrogen.

Torus:

In theory, a nanotorus is a carbon nanotube bent into a torus (doughnut shape). Nanotori are predicted to have many unique properties, such as magnetic moments 1000 times larger than previously expected for certain specific radii. Properties such as magnetic moment, thermal stability, etc. vary widely depending on radius of the torus and radius of the tube.

Nanobud:

Carbon nanobuds are a newly created material combining two previously discovered allotropes of carbon: carbon nanotubes and fullerenes. In this new material, fullerene-like "buds" are covalently bonded to the outer sidewalls of the underlying carbon nanotube. This hybrid material has useful properties of both fullerenes and carbon nanotubes. In particular, they have been found to be exceptionally good field emitters. In composite materials, the attached fullerene molecules may function as molecular anchors preventing slipping of the nanotubes, thus improving the composite’s mechanical properties.

Cup stacked carbon nanotubes:

Cup-stacked carbon nanotubes (CSCNTs) differ from other quasi-1D carbon structures, which normally behave as quasi-metallic conductors of electrons. CSCNTs exhibit semiconducting behaviors due to the stacking microstructure of graphene layers.

Extreme carbon nanotubes:

The observation of the longest carbon nanotubes (18.5 cm long) was reported in 2009. These nanotubes were grown on Si substrates using an improved chemical vapor deposition (CVD) method and represent electrically uniform arrays of single-walled carbon nanotubes. The shortest carbon nanotube is the organic compound cycloparaphenylene which was synthesized in early 2009. The thinnest carbon nanotube is armchair (2,2) CNT with a diameter of 3 Å. This nanotube was grown inside a multi-walled carbon nanotube. Assigning of carbon nanotube type was done by combination of high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and density functional theory (DFT) calculations. The thinnest freestanding single-walled carbon nanotube is about 4.3 Å in diameter. Researchers suggested that it can be either (5, 1) or (4, 2) SWCNT, but exact type of carbon nanotube remains questionable. (3,3), (4,3) and (5,1) carbon nanotubes (all about 4 Å in diameter) were unambiguously identified using more precise aberration-corrected high-resolution transmission electron microscopy. However, they were found inside of double-walled carbon nanotubes.

Special properties of carbon nanotubes:

Electronic, molecular and structural properties of carbon nanotubes are determined to a large extent by their nearly one dimensional structure. The most important properties of CNTs and their molecular background is stated below.

Chemical reactivity. The chemical reactivity of a CNT is, compared with a graphene sheet, Enhanced as a direct result of the curvature of the CNT surface. Carbon nanotube reactivity is directly related to the pi-orbital mismatch caused by an increased curvature. Therefore, a distinction must be made between the sidewall and the end caps of a nanotube. For the same reason, a smaller nanotube diameter results in increased reactivity. Covalent chemical modification of either sidewalls or end caps has shown to be possible. For example, the solubility of CNTs in different solvents can be controlled this way. Though, direct investigation of chemical modifications on nanotube behaviour is difficult as the crude nanotube samples are still not pure enough.

Electrical conductivity. Depending on their chiral vector, carbon nanotubes with a small diameter is either semi-conducting or metallic. The differences in conducting properties are caused by the molecular structure that results in a different band structure and thus a different band gap. The differences in conductivity can easily be derived from the graphene sheet properties. It was shown that a (n,m) nanotube is metallic as accounts that: n=m or (n-m) = 3i, where i is an integer and n and m are defining the nanotube. The resistance to conduction is determined by quantum mechanical aspects and was proved to be independent of the nanotube length.

Optical activity. Theoretical studies have revealed that the optical activity of chiral nanotubes disappears if the nanotubes become larger. Therefore, it is expected that other physical properties are influenced by these parameters too. Use of the optical activity might result in optical devices in which CNTs play an important role.

Mechanical strength. Carbon nanotubes have a very large Young modulus in their axial direction. The nanotube as a whole is very flexible because of the great length. Therefore, these compounds are potentially suitable for applications in composite materials that need anisotropic properties.

One-dimensional transport:

Because of the nanoscale dimensions, electrons propagate only along the tube's axis and electron transport involves many quantum effects. Because of this, carbon nanotubes are frequently referred to as “one-dimensional”.

Hardness:

Standard single walled carbon nanotubes can withstand a pressure up to 24GPa without deformation. They then undergo a transformation to superhard phase nanotubes. Maximum pressures measured using current experimental techniques are around 55GPa. However, these new superhard phase nanotubes collapse at an even higher, albeit unknown, pressure. The bulk modulus of superhard phase nanotubes is 462 to 546 GPa, even higher than that of diamond (420 GPa for single diamond crystal).

Kinetic:

Multi-walled nanotubes are multiple concentric nanotubes precisely nested within one another. These exhibit a striking telescoping property whereby an inner nanotube core may slide, almost without friction, within its outer nanotube shell, thus creating an atomically perfect linear or rotational bearing. This is one of the first true examples of molecular nanotechnology, the precise positioning of atoms to create useful machines. Already, this property has been utilized to create the world's smallest rotational motor. Future applications such as a gigahertz mechanical oscillator are also envisaged.

Toxicity:

The toxicity of carbon nanotubes has been an important question in nanotechnology. Such research has just begun. The data are still fragmentary and subject to criticism. Preliminary results highlight the difficulties in evaluating the toxicity of this heterogeneous material. Parameters such as structure, size distribution, surface area, surface chemistry, surface charge, and agglomeration state as well as purity of the samples, have considerable impact on the reactivity of carbon nanotubes. However, available data clearly show that, under some conditions, nanotubes can cross membrane barriers, which suggests that if raw materials reach the organs they can induce harmful effects such as inflammatory and fibrotic reactions. Results of rodent studies collectively show that regardless of the process by which CNTs were synthesized and the types and amounts of metals they contained, CNTs were capable of producing inflammation, epithelioid granulomas (microscopic nodules), fibrosis, and biochemical/toxicological changes in the lungs. Comparative toxicity studies in which mice were given equal weights of test materials showed that SWCNTs were more toxic than quartz, which is considered a serious occupational health hazard when chronically inhaled. As a control, ultrafine carbon black was shown to produce minimal lung responses.²

Synthesis:

Introduction:

In this section, different techniques for nanotube synthesis and their current status are briefly explained. First, the growth mechanism is explained, as it is almost general for all techniques. However, typical conditions are stated at the sections of all the different techniques. The largest interest is in the newest methods for each technique and the possibilities of scaling up. Carbon nanotubes are generally produced by three main techniques, (1) arc discharge, (2) laser ablation and (3) chemical vapour deposition. Though scientists are researching more economic ways to produce these structures. In arc discharge, a vapour is created by an arc discharge between two carbon electrodes with or without catalyst. Nanotubes self-assemble from the resulting carbon vapour. In the laser ablation technique, a high-power laser beam impinges on a volume of carbon –containing feedstock gas (methane or carbon monoxide). At the moment, laser ablation produces a small amount of clean nanotubes, whereas arc discharge methods generally produce large quantities of impure material. In general, chemical vapour deposition (CVD) results in MWNTs or poor quality SWNTs. The SWNTs produced with CVD have a large diameter range, which can be poorly controlled.³

Growth mechanism:

The way in which nanotubes are formed is not exactly known. The growth mechanism is still a subject of controversy, and more than one mechanism might be operative during the formation of CNTs. One of the mechanisms consists out of three steps. First a precursor to the formation of nanotubes and fullerenes, C2, is formed on the surface of the metal catalyst particle. From this metastable carbide particle, a rodlike carbon is formed rapidly. Secondly there is a slow graphitisation of its wall. This mechanism is based on in-situ TEM observations. The exact atmospheric conditions depend on the technique used, later on; these will be explained for each technique as they are specific for a technique. The actual growth of the nanotube seems to be the same for all techniques mentioned.

Figure 2: Visualisation of a possible carbon nanotube growth mechanism [Ref: 4]

There are several theories on the exact growth mechanism for nanotubes. One theory postulates that metal catalyst particles are floating or are supported on graphite or another substrate. It presumes that the catalyst particles are spherical or pear-shaped, in which case the deposition will take place on only one half of the surface (this is the lower curvature side for the pear shaped particles). The carbon diffuses along the concentration gradient and precipitates on the opposite half, around and below the bisecting diameter. However, it does not precipitate from the apex of the hemisphere, which accounts for the hollow core that is characteristic of these filaments. For supported metals, filaments can form either by ‘extrusion (also known as base growth)’ in which the nanotube grows upwards from the metal particles that remain attached to the substrate, or the particles detach and move at the head of the growing nanotube, labelled ‘tip-growth’. Depending on the size of the catalyst particles, SWNT or MWNT are grown. In arc discharge, if no catalyst is present in the graphite, MWNT will be grown on the C2-particles that are formed in the plasma.⁴

Arc discharge Method:

The carbon arc discharge method, initially used for producing C60 fullerenes, is the most common and perhaps easiest way to produce carbon nanotubes as it is simple. However, it is a technique that produces a mixture of components and requires separating nanotubes from the soot and the catalytic metals present in the crude product. This method creates nanotubes through arc-vaporisation of two carbon rods placed end to end, separated by approximately 1mm, in an enclosure that is usually filled with inert gas (helium, argon) at low pressure (between 50 and 700 mbar).⁵ Recent investigations have shown that it is also possible to create nanotubes with the arc method in liquid nitrogen. A direct current of 50 to 100 A driven by approximately 20 V creates a high temperature discharge between the two electrodes. The discharge vaporises one of the carbon rods and forms a small rod shaped deposit on the other rod. Producing nanotubes in high yield depends on the uniformity of the plasma arc and the temperature of the deposit form on the carbon electrode. Insight in the growth mechanism is increasing and measurements have shown that different diameter distributions have been found depending on the mixture of helium and argon. These mixtures have different diffusions coefficients and thermal conductivities. These properties affect the speed with which the carbon and catalyst molecules diffuse and cool. Affecting nanotube diameter in the arc process. This implies that single-layer tubules nucleate and grow on metal particles in different sizes depending on the quenching rate in the plasma and it suggests that temperature and carbon and metal catalyst densities affect the diameter distribution of nanotubes.⁶

Figure 3: Experimental set-up of an arc discharge apparatus [Ref: 6]

Synthesis of SWNT:

If SWNTs are preferable, the anode has to be doped with metal catalyst, such as Fe, Co, Ni, Y or Mo. A lot of elements and mixtures of elements have been tested by various authors and it is noted that the results vary, even though they use the same elements. The quantity and quality of the nanotubes obtained depend on various parameters such as the metal concentration, inert gas pressure, kind of gas, the current and system geometry.Usually the diameter is in the range of 1.2 to 1.4 nm. A couple of ways to improve the process of arc discharge are stated below.

(a) Inert gas:

The most common problems with SWNT synthesis are that the product contains a lot of metal catalyst, SWNTs have defects and purification is hard to perform. On the other hand, an advantage is that the diameter can slightly be controlled by changing thermal transfer and diffusion, and hence condensation of atomic carbon and metals between the plasma and the vicinity of the cathode can control nanotube diameter in the arc process. This was shown in an experiment in which different mixtures of inert gases were used. It appeared that argon, with a lower thermal conductivity and diffusion coefficient, gave SWNTs with a smaller diameter of approximately 1.2 nm. A linear fit of the average nanotube diameter showed a 0.2 nm diameter decrease per 10 % increase in argon helium ratio, when nickel/yttrium was used (C/Ni/Y was 94.8:4.2:1) as catalyst.

(b) Optical plasma control:

A second way of control is plasma control by changing the anode to cathode distance (ACD). The ACD is adjusted in order to obtain strong visible vortices around the cathode. This enhances anode vaporisation, which improves nanotubes formation. Combined with controlling the argon-helium mixture, one can simultaneously control the macroscopic and microscopic parameters of the nanotubes formed. With a nickel and yttrium catalyst (C/Ni/Y is 94.8:4.2:1) the optimum nanotube yield was found at a pressure of 660 mbar for pure helium and 100 mbar for pure argon. The nanotube diameter ranges from 1.27 to 1.37 nanometre.

(c) Catalyst:

Chemical vapour deposition (CVD) could give SWNTs with a diameter of 0.6–1.2 nm, researchers tried the same catalyst as used in CVD on arc discharge. Not all of the catalysts used appeared to result in nanotubes for both methods. But there seemed to be a correlation of diameter of SWNTs synthesised by CVD and arc discharge. As a result, the diameter can be controllably lowered to a range of 0.6-1.2 nm with arc-discharge. Using a mixture of Co and Mo in high concentrations as catalyst resulted in this result. These diameters are considerably smaller than 1.2-1.4 nm, which is the usual size gained from arcdischarge.⁷

Open air synthesis with welding arc torch:

Only a couple of years ago, researchers discovered that it was possible to form MWNTs in open air. A welding arc torch was operated in open air and the process was shielded with an argon gas flow. The anode and cathode were made of graphite containing Ni and Y (Ni/Y is 4.2:1 at. %).

Figure 4: Experimental set-up of the torch arc method in open air [Ref: 8]

This method was modified for preparing SWNTs. a plate target made of graphite containing metal catalyst Ni and Y (Ni/Y is 3.6:0.8 at. per cent), was fixed at the sidewall of a water–cooled, steel based electrode. The torch arc aimed at the edge of the target and the soot was deposited on the substrate behind the target (Figure 4). The arc was operated at a direct current of 100 A. and shielding argon gas flowed through the torch, enhancing the arc jet formation beyond the target. In the soot, carbon nanohorns (CNHs) and bundles of SWNT with an average diameter of 1.32 nm were found. There are two reasons for this fact. At first, because of the open air, the lighter soot will escape into the atmosphere. Secondly, the carbon vapour might be oxidised and emitted as carbon dioxide gas. In order to improve the yield in this method, contrivances for collecting soot and development of an appropriate target are required. This method promises to be convenient and inexpensive once the conditions for higher yield are optimised. With a Ni/Y catalyst (Ni/Y is 3.6:0.8), SWNT bundles and CNHs are formed. In this case the SWNTs have a diameter of approximately 1.32 nm.⁸

Synthesis of MWNT:

If both electrodes are graphite, the main product will be MWNTs. But next to MWNproducts are formed such as fullerenes, amorphous carbon, and some graphite sheets. Purifying the MWNTs, means loss of structure and disorders the walls. However scientistr are developing ways to gain pure MWNTs in a large-scale process without purification. Typical sizes for MWNTs are an inner diameter of 1-3 nm and an outer diameter of approximately 10 nm. Because no catalyst is involved in this process, there is no need for a heavy acidic purification step. This means, the MWNT, can be synthesised with a low amount of defects.⁹

(a) Synthesis in liquid nitrogen:

A first, possibly economical route to highly crystalline MWNTs is the arc-discharge method in liquid Nitrogen, with this route mass production is also possible. For this option low pressures and expensive inert gasses are not needed.¹⁰

Figure 5: Schematic drawings of the arc discharge apparatus in liquid nitrogen [Ref: 11]

The content of the MWNTs can be as high as 70 % of the reaction product. Analysis with Augerspectroscopym revealed that no nitrogen was incorporated in the MWNTs. There is a strong possibility that SWNTs can be produced with the same apparatus under different conditions.¹¹

(b) Magnetic field synthesis:

Synthesis of MWNTs in a magnetic field gives defect-free and high purity MWNTs that can be applied as nanosized electric wires for device fabrication. In this case, the arc discharge synthesis was controlled by a magnetic field around the arc plasma. Extremely pure graphite rods (purity > 99.999 %) were used as electrodes. Highly pure MWNTs (purity > 95 %) were obtained without further purification, which disorders walls of MWNTs.^12,13

Figure 6: Schematic diagram of the synthesis system for MWNTs in a magnetic field [Ref: 13]

(c) Plasma rotating arc discharge:

A second possibly economical route to mass production of MWNTs is synthesis by plasma rotating arc discharge technique. The centrifugal force caused by the rotation generates turbulence and accelerates the carbon vapour perpendicular to the anode. In addition, the rotation distributes the micro discharges uniformly and generates stable plasma. Consequently, it increases the plasma volume and raises the plasma temperature.^14,15

Figure 7: Schematic diagram of plasma rotating electrode system [Ref: 15]

Laser ablation:

In 1995, Smalley's group at Rice University reported the synthesis of carbon nanotubes by laser vaporisation. A pulsed or continuous laser is used to vaporise a graphite target in an oven at 1200 °C. Themain difference between continuous and pulsed laser, is that the pulsed laser demands a much higher light intensity (100 kW/cm2 compared with 12 kW/cm2). The oven is filled with helium or argon gas in order to keep the pressure at 500 Torr. A very hot vapour plume forms, then expands and cools rapidly. As the vaporised species cool, small carbon molecules and atoms quickly condense to form larger clusters, possibly including fullerenes. The catalysts also begin to condense, but more slowly at first, and attach to carbon clusters and prevent their closing into cage structures. Catalysts may even open cage structures when they attach to them. From these initial clusters, tubular molecules grow into single-wall carbon nanotubes until the catalyst particles become too large, or until conditions have cooled sufficiently that carbon no longer can diffuse through or over the surface of the catalyst particles. It is also possible that the particles become that much coated with a carbon layer that they cannot absorb more and the nanotube stops growing. The SWNTs formed in this case are bundled together by van der Waals forces.^16,17

Figure 8: Schematic drawings of a laser ablation apparatus [Ref: 17]

Large scale synthesis of SWNT:

Because of the good quality of nanotubes produced by this method, scientists are trying to scale up laser ablation two of the newest developments on large-scale synthesis of SWNTs will be discussed.

1) Ultra fast Pulses from a free electron laser method,

2) Continuous wave laser-powder’ method.

1) Ultra fast Pulses from a free electron laser (FEL) method:

In this FEL system the pulse width is ~400 fs. The repetition rate of the pulse is enormously increased from 10 Hz to 75 MHz. To give the beam the same amount of energy as the pulse in an Nd:YAG system, the pulse has to be focused. The intensity of the laser bundle behind the lens reaches ~5 x 10¹¹W/cm2, which is about 1000 times greater than in Nd:YAG system. A jet of preheated (1000 °C) argon through a nozzle tip is situated close to the rotating graphite target, which contains the catalyst. The argon gas deflects the ablation plume approximately 90° away from the incident FEL beam direction, clearing away the carbon vapour from the region in front of the target. The produced SWNT soot is collected in a cold finger. The yield at this moment is 1,5 g/h, which is at 20 % of the maximum power of the not yet upgraded FEL. If the FEL is upgraded to full power and is working at 100 % power, a yield of 45 g/hcould is reached since the yield was not limited by the laser power.¹⁸

Figure 9: Schematic drawings of the ultra fast-pulsed laser ablation apparatus [Ref: 19]

Continuous wave laser-powder method:

This method is a novel continuous, highly productive laser-powder method of SWNT synthesis based on the laser ablation of mixed graphite and metallic catalyst powders by a 2-kW continuous wave CO2 laser in an argon or nitrogen stream. Because of the introduction of micron-size particle powders, thermal conductivity losses are significantly decreased compared with laser heating of the bulk solid targets in known laser techniques. As a result, more effective utilisation of the absorbed laser power for material evaporation was achieved.^19,20 The set-up of the laser apparatus is shown in Figure 9, 10.

Figure 10: (Left) The principle scheme of the set-up for carbon SWNT production by continuous wave laser-powder method (Right) HRTEM of a SWNT-bundle cross-section [Ref: 20]

Chemical vapour deposition:

Chemical vapour deposition (CVD) synthesis is achieved by putting a carbon source in the gas phase and using an energy source, such as plasma or a resistively heated coil, to transfer energy to a gaseous carbon molecule. Commonly used gaseous carbon sources include methane, carbon monoxide and acetylene. The energy source is used to “crack” the molecule into reactive atomic carbon. Then, the carbon diffuses towards the substrate, which is heated and coated with a catalyst (usually a first row transition metal such as Ni, Fe or Co) where it will bind. Carbon nanotubes will be formed if the proper parameters are maintained. Excellent alignment, as well as positional control on nanometer scale, can be achieved by using CVD. Control over the diameter, as well as the growth rate of the nanotubes can also be maintained. The appropriate metal catalyst can preferentially grow single rather than multi-walled nanotubes. CVD carbon nanotube synthesis is essentially a two-step process consisting of a catalyst preparation step followed by the actual synthesis of the nanotube. The catalyst is generally prepared by sputtering a transition metal onto a substrate and then using either chemical etching or thermal annealing to induce catalyst particle nucleation. Thermal annealing results in cluster formation on the substrate, from which the nanotubes will grow. Ammonia may be used as the etchant. The temperatures for the synthesis of nanotubes by CVD are generally within the 650–900°C range. 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.

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 11 shows a schematic diagram of a typical plasma CVD apparatus with a parallel plate electrode structure.

Figure 11: Schematic diagram of plasma CVD apparatus [Ref: 22]

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 pure-metal catalyst for the growth of aligned multiwalled carbon nanotubes (MWNTs). The diameter of the MWNTs is approximately 15 nm.^21,22

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 nanometre-sized catalytic metal particles are formed after an additional etching of the catalytic metal film using NH₃ gas at a temperature of 75° to 1050°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 12 shows a schematic diagram of thermal CVD apparatus in the synthesis of carbon nanotubes.

Figure 12: Schematic diagram of thermal CVD apparatus [Ref: 23]

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.²³

Alcohol catalytic chemical vapour deposition:

Alcohol catalytic CVD (ACCVD) is a technique that is being intensively developed for the possibility of large-scale production of high quality single wall nanotubes SWNTs at low cost. In this technique, evaporated alcohols, methanol and ethanol, are being utilised over iron and cobalt catalytic metal Particles supported with zeolite. Generation is possible is possible at a relatively low minimum temperature of about 550^oC. It seems that hydroxyl radicals, who come from reacting alcohol on Catalytic metal particles, remove carbon atoms with dangling bonds, which are obstacles in creating high-purity SWNTs. The diameter of the SWNTs is about 1 nm. Figure 13 shows the ACCVD experimental apparatus.

Figure 13: ACCVD experimental apparatus [Ref: 24]

The lower reaction temperature and the high-purity features of this ACCVD technique guarantee an easy possibility to scale production up at low cost. Furthermore, the reaction temperature, which is lower than 600°C, ensures that this technique is easily applicable for the direct growth of SWNTs on semiconductor devices already patterned with aluminium.²⁴

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. Figure 14 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, vaporisation 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 synthesised as carbon nanotubes. The diameter of the carbon nanotubes by using vapour phase growth are in the range of 2 – 4 nm for SWNTs and between 70 and 100 nm for MWNTs.²⁵

Figure 14: Schematic diagram of a vapour phase growth apparatus [Ref: 25]

Aero gel-supported chemical vapour deposition:

In this method SWNTs are synthesised by disintegration of carbon monoxide on an aero gel-supported Fe/Mo catalyst. There are many important factors that affect the yield and quality of SWNTs, including the surface area of the supporting material, reaction temperature and feeding gas. Because of the high surface area, the porosity and ultra-light density of the aero gels, the productivity of the catalyst is much higher than in other methods. After a simple acidic treatment and a oxidation process high purity (>99%) SWNTs can be obtained. When using CO as feeding gas the yield of the nanotubes is lower but the overall purity of the materials is very good. The diameter distribution of de nanotubes is between 1, 0 nm and 1, 5 nm. The optimal reaction temperature is approximately 860°C.

High pressure CO disproportionation process:

The High pressure CO disproportionation process (HiPco) is a technique for catalytic production of SWNTs in a continuous-flow gas phase using CO as the carbon feedstock and Fe (CO)₅ as the iron containing catalyst precursor. SWNTs are produced by flowing CO, mixed with a small amount of Fe(CO)₅ through a heated reactor. Size and diameter distribution of the nanotubes can be roughly selected by controlling the pressure of CO. This process is promising for bulk production of carbon nanotubes. Nanotubes as small as 0.7 nm in diameter, which are expected to be the smallest achievable chemically stable SWNTs, have been produced by this method. The average diameter of HiPco SWNTs is approximately 1.1 nm. The yield that could be achieved is approximately 70%. The highest yields and narrowest tubes can be produced at the highest accessible temperature and pressure SWNT material with 97% purity can be produced at rates of up to 450 mg/h with this process.

Flame synthesis:

This method is based on the synthesis of SWNTs in a controlled flame environment that produces the temperature, forms the carbon atoms from the inexpensive hydrocarbon fuels and forms small aerosol metal catalyst islands. SWNTs are grown on these metal islands in the same manner as in laser ablation and arc discharge. These metal catalyst islands can be made in three ways. The metal catalyst (cobalt) can either be coated on a mesh, on which metal islands resembling droplets were formed by physical vapour deposition. These small islands become aerosol after exposure to a flame. The second way is to create aerosol small metal particles by burning a filter paper that is rinsed with a metal-ion (e.g. iron nitrate) solution. The third way is the thermal evaporating technique in which metal powder (e.g. Fe or Ni) is inserted in a trough and heated.

Figure 15 Meshes on which the metal catalyst is coated, used in flame synthesis [Ref: 26]

In a controlled way a fuel gas is partially burned to gain the right temperature of ~800 °C and the carbon atoms for SWNT production. On the small metal particles the SWNTs are than formed.²⁶

Purification:

Introduction:

SWNT soot contains a lot of impurities. The main impurities in the soot are graphite (wrapped up) sheets, amorphous carbon, metal catalyst and the smaller fullerenes. These impurities will interfere with most of the desired properties of the SWNTs. The common industrial techniques use strong oxidation and acid refluxing techniques, which have an effect on the structure of the tubes. Several purification techniques of the SWNT will be discussed. Basically, these techniques can be divided into two mainstreams, 1) structure selective and 2) size selective separations. The first one will separate the SWNTs from the impurities; the second one will give a more homogeneous diameter or size distribution. The techniques that will be discussed are oxidation, acid treatment, annealing, ultrasonication, micro filtration, ferromagnetic separation, cutting, and functionalisation and chromatography techniques.

Techniques:

Oxidation:

Oxidative treatment of the SWNTs is a good way to remove carbonaceous impurities clear the metal surface. The main disadvantages of oxidation are that not only the impurities are oxidised, and the SWNTs. These impurities have relatively more defects or a more open structure. Another reason why impurity oxidation is preferred, is that these impurities are most commonly attached to the metal catalyst, which also acts as oxidizing catalyst. Altogether, the efficiency and the yield of the procedure are highly dependable on a lot of factors, such as metal content, oxidation time, environment, oxidising agent and temperature. The fact that metal acts as oxidising catalyst, the metal content should certainly be taken into consideration, when looking at the oxidising time. Most commonly, the metal catalyst stays intact during these processes, but when oxygen is used in a wet atmosphere, the outer layer of the metal will be oxidized. Then, the density of this surface increases and the surface covering carbon layer ruptures. Not only is the carbon impurities oxidized but also the metal is partially oxidised and exposed.

Acid treatment:

In general the acid treatment will remove the metal catalyst. First of all, the surface of the metal must be exposed by oxidation or sonication. The metal catalyst is then exposed to acid and solvated. The SWNTs remain in suspended form. When using a treatment in HNO₃, the acid only has an effect on the metal catalyst. It has no effect on the SWNTs and other carbon particals. If a treatment in HCl is used, the acid has also a little effect on the SWNTs and other carbon particals. The mild acid treatment (4 M HCl reflux) is basically the same as the HNO₃ reflux, but here the metal has to be totally exposed to the acid to solvate it.

Annealing:

Due to high temperatures (873 – 1873 K) the nanotubes will be rearranged and defects will be Consumed. The high temperature also causes the graphitic carbon and the short fullerenes to pyrolyse. When using high temperature vacuum treatment (1873 K) the metal will be melted and can also be removed.

Ultrasonication:

In this technique particles are separated due to ultrasonic vibrations. Agglomerates of different nanoparticles will be forced to vibrate and will become more dispersed. The separation of the particles is highly dependable on the surfactant, solvent and reagent used. The solvent influences the stability of the dispersed tubes in the system. In poor solvents the SWNTs are more stable if they are still attached to the metal. But in some solvents, such as alcohols, monodispersed particles are relatively stable. When an acid is used, the purity of the SWNTs depends on the exposure time. When the tubes are exposed to the acid for a short time, only the metal solvates, but for a longer exposure time, the tubes will also be chemically cut.

Magnetic Purification:

In this method ferromagnetic (catalytic) particles are mechanically removed from their graphitic Shells. The SWNTs suspension is mixed with inorganic nanoparticles (mainly ZrO₂ or CaCO₃) in an ultrasonic bath to remove the ferromagnetic particles. Then, the particles are trapped with permanent magnetic poles. After a subsequent chemical treatment, a high purity SWNT material will be obtained. This process does not require large equipment and enables the production of laboratory-sized quantities of SWNTs containing no magnetic impurities.

Micro filtration:

Micro filtration is based on size or particle separation. SWNTs and a small amount of carbon nanoparticles are trapped in a filter. The other nanoparticles (catalyst metal, fullerenes and carbon nanoparticles) are passing through the filter. One way of separating fullerenes from the SWNTs by micro filtration is to soak the as-produced SWNTs first in a CS₂ solution. The CS₂ insolubles are then trapped in a filter. The fullerenes which are solvated in the CS₂, pass through the filter. A special form of filtration is cross flow filtration. In cross flow filtration the membrane is a hollow fibre. The membrane is permeable to the solution. The filtrate is pumped down the bore of the fibre at some head pressure from a reservoir and the major fraction of the fast flowing solution which does not permeate out the sides of the fibre is fed back into the same reservoir to be cycled through the fibre repeatedly. A fast hydrodynamic flow down the fibre bore (cross flow) sweeps the membrane surface preventing the build-up of a filter cake.

Cutting:

Cutting of the SWNTs can either be induced chemically, mechanically or as a combination of these. SWNTs can be chemically cut by partially functionalising the tubes, for example with fluor. Then, the fluorated carbon will be driven off the sidewall with pyrolisation in the form of CF₄or COF₂. This will leave behind the chemically cut nanotubes. Mechanical cutting of the nanotubes can be induced by ball-milling. Here, the bonds will break due to the high friction between the nanoparticles and the nanotubes will be disordered. A combination of mechanical and chemical cutting of the nanotubes is ultrasonical induced cutting in an acid solution. In this way the ultrasonic vibration will give the nanotubes sufficient energy to leave the catalyst surface. Then, in combination with acid the nanotubes will rupture at the defect sites.

Functionalisation:

Functionalisation is based on making SWNTs more soluble than the impurities by attaching other groups to the tubes. Now it is easy to separate them from insoluble impurities, such as metal, with filtration. Another functionalisation technique also leaves the SWNT structure intact and makes them soluble for chromatographic size separation. For recovery of the purified SWNTs, the functional groups can be simply removed by thermal treatment, such as annealing.

Chromatography:

This technique is mainly used to separate small quantities of SWNTs into fractions with small length and diameter distribution. The SWNTs are run over a column with a porous material, through which the SWNTs will flow. The columns used are GPC (Gel Permeation Chromatography) and HPLC-SEC (High Performance Liquid Chromatography - Size Exclusion Chromatography) columns. The number of pores the SWNTs will flow through depends on their size. This means that, the smaller the molecule, the longer the pathway to the end of the column will be and that the larger molecules will come off first. The pore size will control what size distribution can be separated. However, a problem is that the SWNTs have to be either dispersedor solvated.

Determination of single nanotube properties:

Synthesis and purification methods are still not successful enough to synthesise carbon nanotubes with all similar structures. However, engineering molecular electronics requires a very detailed understanding of physical properties of the molecules. To gain a better understanding, numerous theoretical and practical investigations on electronic, mechanical and molecular properties of carbon nanotubes have been performed until now. At the moment, insufficient knowledge of handling single nanotubes and performing measurements on them complicates the practical investigation of their physical properties. Therefore, most literature on single nanotube properties focuses on modelling. Though, determination of mechanic properties and electron conductivity has shown to be practically feasible. For investigation of only a single carbon nanotube, the different nanotubes have to be well separated in the sample. Otherwise, different nanotubes will influence each other’s physical properties. It is not straightforward to isolate single walled nanotubes since these try to bundle. This section starts with growing separated single walled nanotubes on a substrate and handling single carbon nanotubes by AFM (Atomic Force Microscope) techniques. Then, identification of a single walled nanotube structure by Raman spectroscopy and the investigation of conductional and mechanical properties follow. Eventually, theoretical studies on charging and discharging effects of single nanotubes will be treated.

Catalytic growth on a support:

Catalytic growth of isolated single walled nanotubes with a diameter from 1 to ~3 nm on a silicium wafer has already been performed. First, a wafer that contains nanometre sized iron particles, that fulfil the role of catalyst, is prepared. Hereafter, CVD is applied and isolated SWNTs are formed on the substrate. Finally, the silicium support is partially oxidised in order to prevent charge transfer from the nanotube to support. Confirmation by AFM and TEM measurements proved that isolated SWNTs were present on the wafer.

Positioning by AFM techniques:

In order to perform measurements on a single nanotube, it must be isolated on a measuring site, which is, most of the times, called a Micro-electromechanical System (MEMS). These systems have predefined sites for placing a nanotube. First, CNTs are picked up from a cartridge with an AFM tip by attraction due to van der Waals forces. When a single CNT is picked up the AFM tip moves the CNT towards the MEMS. Near the MEMS, the real positioning starts. In four steps the CNT is placed across the gap and is welded to the system by Electron Beam Deposition. This is a technique that uses a focused SEM electron beam to dissociate organic species in a specific area and deposit the residual ionised organic gas molecules on the junction of the CNT with the measuring system. It takes approximately 15 minutes to weld a single side of a CNT. After positioning and welding, electric and/or mechanic measurements can be carried out. Currents can be measured as well as stress strain relationships using the AFM tip to apply a strain to the CNT.

Electronic property measurements of single SWNTs:

Studying the electronic properties of SWNTs, scientists have been able to calculate models for energy bands and Density of States (DOS) in single walled carbon nanotubes. If these models have to be experimentally confirmed, a way has to be found in which atomic structures and electron densities can be visualised. Scanning Tunnelling Microscopy (STM) can do this job as it can display atomic structures as well as measure the DOS. Chirality can clearly be determined from STM measurements. Combining this fact with the ability to measure electronic properties allows studying the effect of nanotube chirality on electronic properties. The spectroscopic image shows the density of states as measured by a STM. As these two images show, this technique can also be used to determine properties of intra-molecular SWNT junctions. In addition, it can it can also be used for measuring influence of symmetry, defects, doping, electronic contacts and so on.

Identification of single nanotubes by Raman spectroscopy:

Determination of the structure of a single carbon nanotube by Raman spectroscopy is possible as the density of electronic states is very large for some energy ranges in single walled carbon nanotubes. The density of states is an indication for the number of energy states, ΔN, per energy difference, ΔE. Every different nanotube geometry, i.e. a different (n,m) pair, results a unique pattern for the density distribution of states which can also be calculated theoretically. If the photon energy is (almost) equal to the energy needed for the valence to conduction band transition, an intense Raman signal is found as a direct result of the strong coupling between the electrons and phonons of a nanotube under resonance conditions. In the identification of the different tube geometries the radial breathing mode (RBM), which is a certain type of vibration, plays an important role. An inversely linear dependence of the RBM feature intensity on tube diameter exists if one dimensional physical behaviour is assumed. After a theoretical calculation for certain geometry is performed, the RBM measured can be linked to individual nanotube geometry. However, small differences in intensity and RBM frequency are caused by difference in nanotube length and incompleteness of the theoretical models used. Additional differences arise for signals with almost the same RBM mode frequency if nanotube chirality is different.

Potential applications of CNTs:

Energy storage:

Graphite, carbonaceous materials and carbon fibre electrodes are commonly used in fuel cells, batteries and other electrochemical applications. Advantages of considering nanotubes for energy storage are their small dimensions, smooth surface topology and perfect surface specificity. The efficiency of fuel cells is determined by the electron transfer rate at the carbon electrodes, which is the fastest on nanotubes following ideal Nernstian behaviour. Electrochemical energy storage and gas phase intercalation will be described more thoroughly in the following chapters of the report.

Hydrogen storage:

The advantage of hydrogen as energy source is that its combustion product is water. In addition, hydrogen can be easily regenerated. For this reason, a suitable hydrogen storage system is necessary, satisfying a combination of both volume and weight limitations. The two commonly used means to store hydrogen are gas phase and electrochemical adsorption. Because of their cylindrical and hollow geometry, and nanometre-scale diameters, it has been predicted that carbon nanotubes can store a liquid or a gas in the inner cores through a capillary effect. As a threshold for economical storage, the Department of Energy has set storage requirements of 6.5% by weight as the minimum level for hydrogen fuel cells. It is reported that SWNTs were able to meet and sometimes exceed this level by using gas phase adsorption (physisorption). Yet, most experimental reports of high storage capacities are rather controversial so that it is difficult to assess the applications potential. What lacks, is a detailed understanding of the hydrogen storage mechanism and the effect of materials processing on this mechanism. Another possibility for hydrogen storage is electrochemical storage. In this case not a hydrogen molecule but an H atom is adsorbed. This is called chemisorption.

Lithium intercalation:

The basic principle of rechargeable lithium batteries is electrochemical intercalation and deintercalation of lithium in both electrodes. An ideal battery has a high-energy capacity, fast chargin time and a long cycle time. The capacity is determined by the lithium saturation concentration of the electrode materials. For Li, this is the highest in nanotubes if all the interstitial sites (inter-shell van der Waals spaces, inter-tube channels and inner cores) are accessible for Li intercalation. SWNTs have shown to possess both highly reversible and irreversible capacities. Because of the large observed voltage hysteresis, Li-intercalation in nanotubes is still unsuitable for battery application. This feature can potentially be reduced or eliminated by processing, i.e. cutting, the nanotubes to short segments.

Electrochemical supercapacitors:

Supercapacitors have a high capacitance and potentially applicable in electronic devices. Typically, they are comprised two electrodes separated by an insulating material that is ionically conducting in electrochemical devices. The capacity of an electrochemical supercap inversely depends on the separation between the charge on the electrode and the counter charge in the electrolyte. Because this separation is about a nanometre for nanotubes in electrodes, very large capacities result from the high nanotube surface area accessible to the electrolyte. In this way, a large amount of charge injection occurs if only a small voltage is applied. This charge injection is used for energy storage in nanotube Supercapacitors. Generally speaking, there is most interest in the double-layer supercapacitors and redox supercapacitors with different charge-storage modes.

Molecular electronics with CNTs:

Field emitting devices:

If a solid is subjected to a sufficiently high electric field, electrons near the Fermi level can be extracted from the solid by tunnelling through the surface potential barrier. This emission current depends on the strength of the local electric field at the emission surface and its work. The applied electric field must be very high in order to extract an electron. This condition is fulfilled for carbon nanotubes, because their elongated shape ensures very large field amplification. For technological applications, the emissive material should have a low threshold emission field and large stability at high current density. Furthermore, an ideal emitter is required to have a nanometer size diameter, a structural integrity, a high electrical conductivity, a small energy spread and a large chemical stability. Carbon nanotubes possess all these properties. However, a bottleneck in the use of nanotubes for applications is the dependence of the conductivity and emission stability of the nanotubes on the fabrication process and synthesis conditions. Examples of potential applications for nanotubes as field emitting devices are flat panel displays, gasdischarge tubes in telecom networks, electron guns for electron microscopes, AFM tips and microwave amplifiers.

Transistors:

The field-effect transistor – a three-terminal switching device – can be constructed of only one semiconducting SWNT. By applying a voltage to a gate electrode, the nanotube can be switched from a conducting to an insulating state. Such carbon nanotube transistors can be coupled together, working as a logical switch, which is the basic component of computers.

Nanoprobes and sensors:

Because of their flexibility, nanotubes can also be used in scanning probe instruments. Since MWNTtips are conducting, they can be used in STM and AFM instruments Advantages are the improved resolution in comparison with conventional Si or metal tips and the tips do not suffer from crashes with the surfaces because of their high elasticity. However, nanotube vibration, due to their large length, will remain an important issue until shorter nanotubes can be grown controllably. Other applications are the following: A pair of nanotubes can be used as tweezers to move nanoscale structures on surfaces. Sheets of SWNTs can be used as electromechanical actuators, mimicking the actuator mechanism present in natural muscles. SWNTs may be used as miniaturised chemical sensors. On exposure to environments, which contain NO₂, NH₃ or O₂, the electrical resistance changes?

Composite materials Because of the stiffness of carbon nanotubes, they are ideal candidates for structural applications. For example, they may be used as reinforcements in high strength, low weight, and high performance composites. Theoretically, SWNTs could have a Young’s Modulus of 1 TPa. MWNTs are weaker because the individual cylinders slide with respect to each other. Ropes of SWNTs are also less strong. The individual tubes can pull out by shearing and at last the whole rope will break. This happens at stresses far below the tensile strength of individual nanotubes. Nanotubes also sustain large strains in tension without showing signs of fracture. In other directions, nanotubes are highly flexible. One of the most important applications of nanotubes based on their properties will be as reinforcements in composite materials. However, there have not been many successful experiments that show that nanotubes are better fillers than the traditionally used carbon fibres. The main problem is to create a good interface between nanotubes and the polymer matrix, as nanotubes are very smooth and have a small diameter, which is nearly the same as that of a polymer chain. Secondly, nanotube aggregates, which are very common, behave different to loads than individual nanotubes do. Limiting factors for good load transfer could be sliding of cylinders in MWNTs and shearing of tubes in SWNT ropes. To solve this problem the aggregates need to be broken up and dispersed or cross-linked to prevent slippage. A main advantage of using nanotubes for structural polymer composites is that nanotube reinforcements will increase the toughness of the composites by absorbing energy during their highly flexible elastic behaviour. Other advantages are the low density of the nanotubes, an increased electrical conduction and better performance during compressive load. Another possibility, which is an example of a non-structural application, is filling of photoactive polymers with nanotubes. PPV (Poly-p-phenylenevinylene) filled with MWNTs and SWNTs is a composite, which has been used for several experiments. These composites show a large increase in conductivity with only a little loss in photoluminescence and electro-luminescence yields. Another benefit is that the composite is more robust than the pure polymer. Of course, nanotube-polymer composites could be used also in other areas. For instance, they could be used in the biochemical field as membranes for molecular separations or for osteointegration (growth of bone cells). However, these areas are less explored. The most important thing we have to know about nanotubes for efficient use of them as reinforcing fibres is knowledge on how to manipulate the surfaces chemically to enhance interfacial behaviour between the individual nanotubes and the matrix material.

Templates:

Because of the small channels, strong capillary forces exist in nanotubes. These forces are strong enough to hold gases and fluids in nanotubes. In this way, it may be possible to fill the cavities of the nanotubes to create nanowires. The critical issue here is the wetting characteristic of nanotubes.

Because of their smaller pore sizes, filling of SWNTs is more difficult than filling of MWNTs. If it becomes possible to keep fluids inside nanotubes, it could also be possible to perform chemical reactions inside their cavities. Especially organic solvents wet nanotubes easily. In this case we could speak of a nanoreactor. One of the problems in these cases is that nanotubes are normally closed. For the latter applications we have to open the nanotubes. This is possible through a simple chemical reaction: oxidation. The pentagons in the end cap of the nanotubes are more reactive than the sidewall. So, during oxidation, the caps are easily removed while the sidewall stays intact.

Energy storage:

Two elements that can be electrochemically stored in CNTs are hydrogen and lithium. Hydrogen can also be stored in CNTs by gas phase intercalation. Three units are commonly used to describe the hydrogen and lithium contents of storage materials with:

1. [H/C] ([Li/C]) as the ratio of hydrogen (lithium) atoms per atom of storage material, in this case carbon;

2. [wt%] as the ratio of the mass of hydrogen (lithium) to the mass of storage material (the gravimetric density);

3. [kgH2m-3] as the ratio of the mass of molecular hydrogen to the volume of storage material (volumetric density).

REFERENCES:

1. S. Iijima, Nature (London) 354, 56 (1991).

2. C. Dekker, Physics Today 52, 22 (1999).

3. S. J. Tans, A. R. M. Verschueren, and C. Dekker, Nature (London) 393, 49 (1998).

4. A. Javey, H. Kim, M. Brink, Q. Wang, A. Ural, J. Guo, P. McIntyre, P. McEuen, M. Lundstrom, H. J. Dai, Nat. Mater. 1, 241 (2002).

5. A. Javey, J. Guo, Q. Wang, M. Lundstrom, H. J. Dai, Nature 424, 654 (2003).

6. S. Rosenblatt, Y. Yaish, J. Park, J. Gore, V. Sazonova, and P. L. McEuen, Nano Lett. 2, 869 (2002).

7. C. Lu, Q. Fu, S. Huang, and J. Liu, Nano Lett. 4, 623 (2004).

8. S. Huang, M. Woodson, R. Smalley, G. P. Siddons, D. Merchin, J. H. Back, J. K. Jeong, and M. Shim, Nano Lett. 4, 927 (2004).

9. S. J. Tan, M. H. Devoret, H. J. Dai, A. Thess, R. E. Smalley, L. J. Geerligs, and C. Dekker, Nature (London) 386, 474 (1997).

10. M. Bockrath, D. H. Cobden, and P. L. McEuen, Science 290, 1552 (2000).

11. H. J. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert, and R. E. Smalley, Nature (London) 384, 147 (1996).

12. W. A. de Heer, A. Chatelain, and D. Ugarte, Science 270, 1179 (1995).

13. A. G. Rinzler, J. H. Hafner, P. Nikolaev, L. Lou, S. G. Kim, D. Tomanek, P. Nordlander, D. T. Colbert, and R. E. Smalley, Science 269, 1550 (1995).

14. P. G. Collins, K. Bradley, M. Ishigami, and A. Zettle, Science 287, 1801 (2000).

15. J. Kong, N. R. Franklin, C. Zhou, M. G. Chapline, S. Peng, K. Cho, and H. Dai, Science 287, 622 (2000).

16. R. J. Chen, S. Bangsaruntip, K. A. Drouvalakis, N. W. S. Kam, M. Shim, Y. M. Li, W. Kim, P. J. Utz, and H. J. Dai, P. Natl. Acad. Sci. USA 100, 4984 (2003).

17. A. Star, J. C. P. Gabriel, K. Bradley, and G. Gruner, Nano Lett. 3, 459 (2003).

18. Niyogi, S., Hamon, M. A., Hu, H., Zhao, B., Bhowmik, P., Sen, R., Itkis, M. E., and Haddon, R. C., Accounts of Chemical Research 35, 12 (2002).

19. Avouris, P., Chemical Physics, 281, (2-3), 429-445 (2002).

20. Tans, Sander J., Devoret, Michel H., Dal, Hongjie, Thess, Andreas, Smalley, Richard E., Geerligs, L. J.,and Dekker, Cees, Nature (London), 386, 6624 (1997).

21. Ajayan, P. M. and Zhou, O. Z., Carbon Nanotubes, 80, 391-425 (2001).

22. Damnjanovic, M., Milosevic, I., Vukovic, T., and Sredanovic, R., Physical Review B, 60, (4), 2728-2739 (1999).

23. Yasuda, Ayumu, Kawase, Noboru, and Mizutani, Wataru, Journal of Physical Chemistry, 106, 51 (2002).

24. Sinnot, S. B., Andrews, R., Qian, D., Rao, A. M., Mao, Z., Dickey, E. C., and Derbyshire, F., Chem. Phys.Lett., 315, 25-30 (1999).

25. Ebbesen, T. W. and Ajayan, P. M., Nature, 358, 220-222 (1992).

26. Huang, H. J., Marie, J., Kajiura, H., and Ata, M., Nano Letters, 2, (10), 1117-1119 (2002).

Received on 19.03.2011

Modified on 28.03.2011

Accepted on 15.04.2011

Research J. Science and Tech. 3(3): May-June. 2011: 137-150