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Carbon nanotubes are molecular-scale tubes of graphitic carbon with amazing properties. They are among the stiffest and strongest fibres known, and have remarkable electronic properties and many other unique characteristics. For these reasons they have attracted huge academic and industrial interest.

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Carbon Nanotubes Overview

Posted By on August 20, 2010

Carbon nanotubes, also known as buckytubes, are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1 which is significantly larger than any other material. These cylindrical carbon molecules have novel properties that make them potentially useful in many applications in nanotechnology, electronics, optics, and other fields of materials science, as well as potential uses in architectural fields. They exhibit extraordinary strength and unique electrical properties, and are efficient thermal conductors. Nanotubes are members of the fullerene structural family, which also includes the spherical buckyballs. The ends of a nanotube might be capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is on the order of a few nanometers, while they can be up to 18 centimeters in length. Nanotubes are categorized as single-walled nanotubes, and multi-walled nanotubes. The nature of the bonding of a nanotube is described by applied quantum chemistry, specifically, orbital hybridization. The chemical bonding of nanotubes is composed entirely of sp2 bonds, similar to those of graphite. This bonding structure, which is stronger than the sp3 bonds found in diamonds, provides the molecules with their unique strength. Nanotubes naturally align themselves into “ropes” held together by Van der Waals forces.In 1952 L. V. Radushkevich and V. M. Lukyanovich published clear images of 50 nanometer diameter tubes made of carbon in the Soviet Journal of Physical Chemistry. This discovery was largely unnoticed, as the article was published in the Russian language, and Western scientists’ access to Soviet press was limited during the Cold War. It is likely that carbon nanotubes were produced before this date, but the invention of the transmission electron microscope (TEM) allowed direct visualization of these structures. Carbon nanotubes have been produced and observed under a variety of conditions prior to 1991. A paper by Oberlin, Endo, and Koyama published in 1976 clearly showed hollow carbon fibers with nanometer-scale diameters using a vapor-growth technique. Additionally, the authors show a TEM image of a nanotube consisting of a single wall of graphene. Later, Endo has referred to this image as a single-walled nanotube. In 1979 John Abrahamson presented evidence of carbon nanotubes at the 14th Biennial Conference of Carbon at Pennsylvania State University. The conference paper described carbon nanotubes as carbon fibers which were produced on carbon anodes during arc discharge. A characterization of these fibers was given as well as hypotheses for their growth in a nitrogen atmosphere at low pressures. In 1981 a group of Soviet scientists published the results of chemical and structural characterization of carbon nanoparticles produced by a thermocatalytical disproportionation of carbon monoxide. Using TEM images and XRD patterns, the authors suggested that their “carbon multi-layer tubular crystals” were formed by rolling graphene layers into cylinders. They speculated that by rolling graphene layers into a cylinder, many different arrangements of graphene hexagonal nets are possible. They suggested two possibilities of such arrangements: circular arrangement (armchair nanotube) and a spiral, helical arrangement (chiral tube). In 1987, Howard G. Tennett of Hyperion Catalysis was issued a U.S. patent for the production of “cylindrical discrete carbon fibrils” with a “constant diameter between about 3.5 and about 70 nanometers length 102 times the diameter, and an outer region of multiple essentially continuous layers of ordered carbon atoms and a distinct inner core.Iijima’s discovery of multi-walled carbon nanotubes in the insoluble material of arc-burned graphite rods in 1991 and Mintmire, Dunlap, and White’s independent prediction that if single-walled carbon nanotubes could be made, then they would exhibit remarkable conducting properties helped create the initial buzz that is now associated with carbon nanotubes. Nanotube research accelerated greatly following the independent discoveries by Bethune at IBM and Iijima at NEC of single-walled carbon nanotubes and methods to specifically produce them by adding transition-metal catalysts to the carbon in an arc discharge. The arc discharge technique was well-known to produce the famed Buckminster fullerene on a preparative scale, and these results appeared to extend the run of accidental discoveries relating to fullerenes. The original observation of fullerenes in mass spectrometry was not anticipated, and the first mass-production technique by Kratschmer and Huffman was used for several years before realizing that it produced fullerenes. The discovery of nanotubes remains a contentious issue. Many believe that Iijima’s report in 1991 is of particular importance because it brought carbon nanotubes into the awareness of the scientific community as a whole. The bonding in carbon nanotubes is sp², with each atom joined to three neighbours, as in graphite. The tubes can therefore be considered as rolled-up graphene sheets. There are three distinct ways in which a graphene sheet can be rolled into a tube. The first two of these, known as “armchair” and “zig-zag” have a high degree of symmetry. The terms “armchair” and “zig-zag” refer to the arrangement of hexagons around the circumference. The third class of tube, which in practice is the most common, is known as chiral, meaning that it can exist in two mirror-related forms.The structure of a nanotube can be specified by a vector, which defines how the graphene sheet is rolled up. To produce a nanotube, the sheet is rolled up so that the atom is superimposed.. The arc-evaporation method, which produces the best quality nanotubes, involves passing a current of about 50 amps between two graphite electrodes in an atmosphere of helium. This causes the graphite to vaporise, some of it condensing on the walls of the reaction vessel and some of it on the cathode. It is the deposit on the cathode which contains the carbon nanotubes. Single-walled nanotubes are produced when Co and Ni or some other metal is added to the anode. It has been known since the 1950s, if not earlier, that carbon nanotubes can also be made by passing a carbon-containing gas, such as a hydrocarbon, over a catalyst. The catalyst consists of nano-sized particles of metal, usually Fe, Co or Ni. These particles catalyse the breakdown of the gaseous molecules into carbon, and a tube then begins to grow with a metal particle at the tip. It was shown in 1996 that single-walled nanotubes can also be produced catalytically. The perfection of carbon nanotubes produced in this way has generally been poorer than those made by arc-evaporation, but great improvements in the technique have been made in recent years. The big advantage of catalytic synthesis over arc-evaporation is that it can be scaled up for volume production. The third important method for making carbon nanotubes involves using a powerful laser to vaporise a metal-graphite target. This can be used to produce single-walled tubes with high yield. The strength of the sp² carbon-carbon bonds gives carbon nanotubes amazing mechanical properties. The stiffness of a material is measured in terms of its Young’s modulus, the rate of change of stress with applied strain. The Young’s modulus of the best nanotubes can be as high as 1000 GPa which is approximately five times higher than steel. The tensile strength, or breaking strain of nanotubes can be up to 63 GPa, around fifty times higher than. These properties coupled with the lightness of carbon nanotubes, gives them great potential in applications such as aerospace. It has even been suggested that nanotubes could be used in the “space elevator”, an Earth-to-space cable first proposed by Arthur C. Clarke. The electronic properties of carbon nanotubes are also extraordinary. Especially notable is the fact that nanotubes can be metallic or semiconducting depending on their structure. Thus, some nanotubes have conductivities higher than that of copper, while others behave more like silicon. There is great interest in the possibility of constructing nanoscale electronic devices from nanotubes, and some progress is being made in this area. However, in order to construct a useful device it is required to arrange many thousands of nanotubes in a defined pattern, and scientists do not yet have the degree of control necessary to achieve this. There are several areas of technology where carbon nanotubes are already being used. These include flat-panel displays, scanning probe microscopes and sensing devices. The unique properties of carbon nanotubes will undoubtedly lead to many more applications. Carbon-carbon bond strength is one of the highest and as a result any structure based on aligned carbon-carbon bonds will have the ultimate strength. Nanotubes are therefore the ultimate high strength carbon fibres. The Young’s modulus of nanotubes measured is 1.8 TPa18. Theoretical prediction is in the range of 1-5 TPA, which may be compared to the in-plane graphite value of 1 TPA. Measurements on individual nanotubes are difficult. The problem with multi walled nanotubes is that the individual cylinder can slide away giving lower estimate for Young’s modulus. It is also possible that individual single walled nanotubes can slip from a bundle, again reducing the experimentally measured Young’s T. Measurements based on vibration spectroscopy, AFM and transmission electron microscopy can be used in determining estimates, and all of them come up with nearly the same numbers.One of the important properties of nanotubes is the ability to withstand extreme strain in tension that is up to 40%. The tubes can recover from severe structural distortions. The resilience of graphite sheet is manifested in this property, it is due to the ability of carbon atoms to rehybridise. Any distortion of a tube will change the bonding of nearby carbon atoms and to come back to the planar structure the atoms have to reverse to sp2 hybridisation. If the tube is subjected to elastic stretching, beyond a limit some bonds are broken. The defect is redistributed along the tube surface.Nanotubes have high strength to weight ratio, multi walled nanotubes have density of 1.8g/cm3 and single walled nanotubes have density of 0.8g/cm3. This is indeed useful for light weight applications. This value is about 100 times that of steel and over twice that of conventional carbon fibres. Nanotubes are highly resistant to chemical attack. It is difficult to oxidize it and onset of oxidation is 100 degrees higher than carbon fibres. As a result in practical applications of nanotubes, temperature is not a limitation. Surface area of nanotubes is of the order of 10-20 m2/g, higher than graphite but lower than mesoporous carbon used as catalytic supports where the value is of the order of 1000 m2/g. Nanotubes are expected to have high thermal conductivity but the value has not been measured on individual nanotubes. Thermal conductivity of nanofluids containing nanotubes is shown to be much larger, with smaller weight percent of nanotubes. A basic layer of these consists of a sheet of carbon atoms wrapped into a cylindrical shape. The SWNTle walled nanotubes or sing containing only one of these and the MWNT or multi walled nanotubes consisting of several separated at a distance equal to that of carbon atoms in a graphite block.Whereas mechanical, electrical and electrochemical properties of the carbon nanotubes are well established and have immediate applications, the practical use of optical properties is yet unclear. The aforementioned tunability of properties is potentially useful in optics and photonics. In particular, light-emitting diodesor LEDs and photo-detectors based on a single nanotube have been produced in the lab. Their unique feature is not the efficiency, which is yet relatively low, but the narrow selectivity in the wavelength of emission and detection of light and the possibility of its fine tuning through the nanotube structure. In addition, bolometer and optoelectronic memory devices have been realised on ensembles of single-walled carbon nanotubes.Single-walled carbon cones with morphologies similar to those of nanotube caps were first prepared by Peter Harris, Edman Tsang and colleagues in 1994. They were produced by high temperature heat treatments of fullerene soot. Sumio Iijima’s group subsequently showed that they could also be produced by laser ablation of graphite, and gave them the name “nanohorns”. This group has demonstrated that nanohorns have remarkable adsorptive and catalytic properties, and that they can be used as components of a new generation of fuel cells. Nanotubes can be opened and filled with materials such as biological molecules, raising the possibility of applications in biotechnology. They can be used to dissipate heat from tiny computer chips. The strength and flexibility of carbon nanotubes makes them of potential use in controlling other nanoscale structures, which suggests they will have an important role in nanotechnology engineering. The highest tensile strength an individual single walled nanotube has been tested to is 63 GPa. In Earth’s upper atmosphere, atomic oxygen erodes the carbon nanotubes, but other applications of carbon nanotubes rarely need the surface to be protected. Though it is debatable if nanotube materials can ever be made with a tensile strength approaching that of individual tubes, composites may still yield incredible strengths potentially sufficient to allow the building of such things as space elevators, artificial muscles, ultrahigh-speed flywheels, and more. MIT is working on combat jackets utilizing carbon nanotubes for ultrastrong fibers and for monitoring its wearer’s condition. Carbon nanotubes additionally can also be used to produce nanowires of other chemicals, such as gold or zinc oxide. These nanowires in turn can be used to cast nanotubes of other chemicals, such as gallium nitride. These can have very different properties from carbon nanotubes for example, gallium nitride nanotubes are hydrophilic, while carbon nanotubes are hydrophobic, giving them possible uses in organic chemistry that carbon nanotubes could not be used for. One use for nanotubes that has already been developed is as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness low-energy low-weight displays. This type of display would consist of a group of many tiny cathode ray tubes, each providing the electrons to hit the phosphor of one pixel, instead of having one giant cathode ray tube whose electrons are aimed using electric and magnetic fields. These displays are known as Field Emission Displays or FEDs. A nanotube formed by joining nanotubes of two different diameters end to end can act as a diode, suggesting the possibility of constructing electronic computer circuits entirely out of nanotubes. Scanning tunneling spectroscopy has shown that the band gaps of the nanotubes vary from 0.2 to 1.2 eV16. The gap varies along the tube body and reaches a minimum value at the tube ends. This is due to the presence of localized defects at the ends due to the extra states. Measurements on single walled nanotubes showed the helicity and size dependent changes in the electronic structure. The advantages of nanotubes are many one of the major advantage is that they have high electrical and thermal conductivity. Nanotubes are also known for having very high tensile strength. When it comes to elasticity, nanotubes are highly flexible and elastic their elongation ranges approximately 18% before they completely fail. Other advantages of nanotubes include; high aspect ratios and good field emission. Some of the disadvantages associated with nanotubes are also explained here. The biggest disadvantage is that as it is a new kind of technology so not much testing is done with regards to nanotubes. The lifetime of nanotubes is less only 1750 hours. Another disadvantage is that higher is required for field emission as the tubes are not so well localised. Carbon nanotubes have the potential for use across biomedical, energy, industrial adhesives and textile sectors. One of the biomedical applications of nanotube is its use as a drug delivery vehicle. Researchers at the University of London are investigating the carbon nanotube penetration of cells membranes and their interaction with different cell types. Carbon nanotubes in energy applications have been demonstrated in fuel cells as backing material for the electrodes in the membrane electrode assembly and also for the storage of hydrogen. In solar cell applications, Georgia Tech Research Institute have used carbon nanotubes as supports for arrays of photovoltaic material and also serve to connect them to the silicon wafers. University of Akron has demonstrated an adhesive application with 200 times the gripping power of gecko’s foot. It is expected to be used as a dry adhesive in microelectronics, robotics, space and other fields. Carbon nanotubes just 10-50nm in size spun in a yarn have great potential for use within textiles. Such yarns are strong, durable, flexible and retain the electrical properties of the nanotubes. Emerging application examples of the carbon nanotubes have been observed in vibration isolation applications and are being considered in the design of NASA morphing wing of gliders. The nanomatteress developed at the Nanyang Technological University, consists of a layer of hosting reviews diamond-like carbon over a layer of aligned carbon nanotubes. The composite material has excellent mechanical properties and can provide vibration isolation and wear resistance applications in harsh environments. Researchers at University of California have demonstrated a nanoscale radio where the main component of the circuit consists of a single carbon nanotube (pthg.org). This is expected to be used in mobile phones and environment sensors.The carbon nanotubes market can be viewed as a market of the material and applications. Other applications for nanotubes that are currently being researched include high tensile strength fibers. Two methods are currently being tested for the manufacture of such fibres. A French team has developed a liquid spun system that involves pulling a fibre of nanotubes from a bath which yields a product that is approximately 60% nanotubes. The other method, which is simpler but produces weaker fibres and uses traditional melt-drawn polymer fibre techniques with nanotubes mixed in the polymer. After drawing, the fibres can have the polymer burned out of them to make them purely nanotube or they can be left as they are. Computer storage devices using nanotubes are currently in the prototype stages. Both high speed non-volatile memory which can be used to replace nearly all solid state memory in computers today, and high density storage that may replace hard drives, are being developed. Major limiting factors in development include orienting the nanotubes, which tend to tangle because of their length, and their price. Scientists working at the University of Texas at Dallas produced the current toughest material known in mid-2003 by spinning fibres of single wall carbon nanotubes with polyvinyl alcohol. Beating the previous contender, spider silk, by a factor of four, the fibres require 600J/g to break. In comparison, the bullet-resistant fibre Kevlar is 27-33J/g. On September 19, 2003, NEC Corporation, Japan, announced stable fabrication technology of carbon nanotube transistors. In June 2004 scientists from China’s Tsinghua University and Louisiana State University demonstrated the use of nanotubes in incandescent lamps, replacing a tungsten filament in a lightbulb with a carbon nanotube one. Research led by the University of Edinburgh focused on carbon nanotubes cylindrical molecules that are 1/50,000th as wide as a human hair, which are a key product of nanotechnology industries. The researchers found that when the carbon nanotube fibres were short they appeared harmless. However, the body’s scavenger cells were unable to deal with the longer fibres, which provoked inflammation and disease in sensitive tissue surrounding organs in the body including the lungs. The reaction is similar to asbestos, where longer fibres are also more harmful and can cause mesothelioma also known as lung cancer.There are two variations of carbon nanotubes, open ended and closed ended. The closed tips were more efficient in field emission compared to the open end ones, which is opposite as to what was expected. The open-ended tubes have a smaller effective curvature therefore a higher field enhancement factor. However after testing it is now thought that other molecules such as oxygen atoms attach to the free dangling bonds at the end of the open tube, therefore forming a localised electron state with a state less than that of the Fermi-energy hence not emitting electrons. The challenges of each application vary at each stage of the development cycle and technology adoption. Carbon nanotubes have three main variations based on the diameter of the tubes these variations are; single walled carbon nanotubes, double walled and multi-walled carbon nanotubes. The production capacity of individual companies remains in hundreds of tonnes. No accurate estimate of production capacity is available though it remains in few thousands of tonnes In April of 2001, IBM announced it had developed a technique for automatically developing pure semiconductor surfaces from nanotubes. While there would seem to be little risk to consumers using products containing nanotubes, toxicologists are concerned that there might be a health risk to involved in the manufacture of carbon nanotubes and to those who make products containing them. The study, published in the journal Nature Nanotechnology, follows concerns that carbon nanotubes could pose a health threat due to their similarity in shape to asbestos. The authors urge more research to investigate the potential risks during the manufacture of carbon nanotubes. These would include looking at how long the fibres are, how much is in the air, the likelihood of them being breathed in and the determination of safe exposure levels. There should be minimal risk in handling items made of carbon nanotubes because the fibres are so embedded. Scientists are more concerned that there may be higher exposure of the workers involved in production of items containing nanotubes. With the global market for carbon nanotubes predicted to exceed £1 billion by 2010 more research is needed. The optical properties of carbon nanotubes refer specifically to the absorption, photoluminescence, and Raman spectroscopy of carbon nanotubes. Spectroscopic methods offer the possibility of quick and non-destructive characterization of relatively large amounts of carbon nanotubes. There is a strong demand for such characterization from the industrial point of view: numerous parameters of the nanotube synthesis can be changed, intentionally or unintentionally, to alter the nanotube quality. Optical absorption, photoluminescence and Raman spectroscopies allow quick and reliable characterization of this “nanotube quality” in terms of non-tubular carbon content, structure of the produced nanotubes, and structural defects. Those features determine nearly any other properties such as optical, mechanical, and electrical properties. Carbon nanotubes are unique “one dimensional systems” which can be envisioned as rolled single sheets of graphite (or more precisely graphene). This rolling can be done at different angles and curvatures resulting in different nanotube properties. The diameter typically varies in the range 0.4–40 nm but the length can vary 10,000 times reaching 4 cm. Thus the nanotube aspect ratio, or the length-to-diameter ratio, can be as high as 132,000,000:1, which is unequalled by any other material. Consequently, all the properties of the carbon nanotubes relative to those of typical semiconductors are extremely anisotropic and tunable.Carbon nanotubes are very prevalent in today’s world of medical research and are being highly researched in the fields of efficient drug delivery and bio-sensing methods for disease treatment and health monitoring. Carbon nanotube technology has shown to have the potential to alter drug delivery and bio-sensing methods for the better, and thus, carbon nanotubes have recently garnered interest in the field of medicine. The use of carbon nanotubes in drug delivery and bio-sensing technology has the potential to revolutionize medicine. Functional aspect of SWNTs or single walled nanotubes has proven to enhance solubility and allow for efficient tumour targeting. It prevents SWNTs from being cytotoxic and altering the function of immune cells. Cancer, a group of diseases in which cells grow and divide abnormally, is one of the primary diseases being looked at with regards to how it responds to carbon nanotube drug delivery. Current cancer therapy primarily involves surgery, radiation therapy, and chemotherapy. These methods of treatment are usually painful and kill normal cells in addition to producing adverse side effects. Carbon nanotubes as drug delivery vehicles have shown potential in targeting specific cancer cells with a dosage lower than conventional drugs used, that is just as effective in killing the cells, however does not harm healthy cells and significantly reduces side effects. Current blood glucose monitoring methods by patients suffering from diabetes are normally invasive and often painful. The high electrochemically accessible surface area, high electrical conductivity and useful structural properties have demonstrated the potential use of single-walled nanotubes or SWNTs and multi-walled nanotubes or MWNTs in highly sensitive non-invasive glucose detectors.

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