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{{Nanotech}}
[[Image:Types of Carbon Nanotubes.png|thumb|250px|3D model of three types of single-walled carbon nanotubes.]]
[[Image:Kohlenstoffnanoroehre_Animation.gif|thumb|240px|This animation of a rotating Carbon nanotube shows its 3D structure.]]
'''Carbon nanotubes''' (CNTs) are [[allotropes of carbon|allotropes of carbon]]. A single wall carbon nanotube is a one-atom thick graphene sheet
of [[graphite]] (called [[graphene]]) rolled up into a seamless [[Cylinder (geometry)|cylinder]] with diameter of the order of a [[nanometer]].
This results in a nanostructure where the length-to-diameter ratio exceeds 10,000. Such cylindrical [[carbon]] [[molecule]]s have novel [[chemical property|properties]] that make them potentially useful in many applications in [[nanotechnology]], [[electronics]], [[optics]] and other fields of [[materials science]]. They exhibit extraordinary strength and unique [[electricity|electrical]] properties, and are efficient [[heat conduction|conductors of heat]]. [[Inorganic nanotube]]s have also been synthesized.
Nanotubes are members of the [[fullerene]] structural family, which also includes [[Buckyball#Buckminsterfullerene|buckyballs]]. Whereas buckyballs are [[sphere|spherical]] in shape, a nanotube is [[cylinder (geometry)|cylindrical]], with at least one end typically capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is in the order of a few [[nanometer]]s (approximately 50,000 times smaller than the width of a human hair), while they can be up to several millimeters in length. There are two main types of nanotubes: [[#Single-walled|single-walled nanotubes]] (SWNTs) and [[#Multi-walled|multi-walled nanotubes]] (MWNTs).
The nature of the bonding of a nanotube is described by applied [[quantum chemistry]], specifically, [[orbital hybridization]]. The [[chemical bonding]] of nanotubes are composed entirely of [[sp² bond|sp<sup>2</sup> bonds]], similar to those of [[graphite]]. This bonding structure, which is stronger than the [[Orbital hybridisation#sp3 hybrids|sp<sup>3</sub> bonds]] found in [[diamond]], provides the molecules with their unique strength. Nanotubes naturally align themselves into "ropes" held together by [[Van der Waals force]]s. Under high pressure, nanotubes can merge together, trading some sp<sup>2</sup> bonds for sp<sup>3</sup> bonds, giving great possibility for producing strong, unlimited-length wires through high-pressure nanotube linking.<ref>{{cite journal | first=T. | last=Yildirim | coauthors=''et al.'' | year=2000 | title=Pressure-induced interlinking of carbon nanotubes | journal=[[Physical Review]] B | volume=62 | pages=19}}</ref>
==Discovery==
{{seealso|Timeline of carbon nanotubes}}
A 2006 editorial written by Marc Monthioux and Vladimir Kuznetsov in the journal ''Carbon'' has described the interesting and often misstated origin of the carbon nanotube. A large percentage of academic and popular literature attributes the discovery of hollow, nanometer sized tubes composed of graphitic carbon to [[Sumio_Iijima|Sumio Iijima]] of [[NEC]] in 1991.<ref>[http://www.cemes.fr/fichpdf/GuestEditorial.pdf Who should be given the credit for the discovery of carbon nanotubes?]</ref>
In [[1952]] [[Radushkevich]] and [[Lukyanovich]] published clear images of 50 nanometer diameter tubes made of carbon in the Russian ''Journal of Physical Chemistry''<ref>http://carbon.phys.msu.ru/publications/1952-radushkevich-lukyanovich.pdf</ref>. This discovery was largely unnoticed, the article was published in the Russian language, and Western scientists' access to Russian 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]] allowed the 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 fibres with nanometer-scale diameters using a vapour-growth technique.<ref>A. Oberlin, M. Endo, and T. Koyama, J. Cryst. Growth, 1976, 32, 335.</ref> 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.<ref>Morinobu Endo Interview, Nagano, Japan, October 26, 2002 and http://web.mit.edu/tinytech/Nanostructures/Spring2003/MDresselhaus/i789.pdf</ref>
In 1981 a group of Ukrainian 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. Additionally, they speculated that during 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).<ref>Izvestiya Akademii Nauk SSSR, Metals. 1982, #3, p.12-17 [in Russian]</ref>
In 1987, Howard G. Tennent 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…."<ref>http://www.freepatentsonline.com/4663230.html</ref>.
Iijima's discovery of carbon nanotubes in the insoluble material of arc-burned graphite rods<ref>Sumio Iijima (1991), ''Helical microtubules of graphitic carbon'', Nature 354, 56 - 58</ref> created the buzz that is now associated with carbon nanotubes. Nanotube research accelerated greatly following the independent discoveries<ref>D. S. Bethune ''et al.'' (1993), ''Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls'', Nature 363, 605 - 607</ref><ref>Sumio Iijama (1993, ''Single-shell carbon nanotubes of 1-nm diameter'', Nature 363, 603 - 605</ref> by Bethune at IBM<ref>http://www.almaden.ibm.com/st/nanoscale_science/past/nanotubes/</ref> 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,<ref>W. Krätschmer ''et al.'' (1990), ''Solid C60: a new form of carbon'', Nature 347, 354 - 358</ref> 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,<ref>H. W. Kroto ''et al.'' (1985), ''C60: Buckminsterfullerene'', Nature 318, 162-163</ref> and the first mass-production technique by Krätschmer and Huffman was used for several years before realising that it produced fullerenes.<ref>W. Krätschmer ''et al.'' (1990),''Solid C60: a new form of carbon'', Nature 347, 354-358</ref>
The discovery of nanotubes remains a contentious issue, especially because several scientists involved in the research could be likely candidates for the Nobel Prize. 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. See the reference for a review of the history of the discovery of carbon nanotubes.<ref>Carbon 44, 1621, 2006</ref>
==Types of carbon nanotubes==
===Single-walled===
[[Image:CNTnames.png|thumb|300px|The (''n'',''m'') nanotube naming scheme can be thought of as a vector ('''C'''<sub>h</sub>) in an infinite graphene sheet that describes how to "roll up" the graphene sheet to make the nanotube. '''T''' denotes the tube axis, and '''a'''<sub>1</sub> and '''a'''<sub>2</sub> are the unit vectors of graphene in real space.]]
Most single-walled nanotubes (SWNT) have a diameter of close to 1 nanometer, with a tube length that can be many thousands of times longer. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of [[graphite]] called [[graphene]] into a seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices (''n'',''m'') called the chiral vector. The integers ''n'' and ''m'' denote the number of unit [[vector (spatial)|vector]]s along two directions in the honeycomb [[crystal lattice]] of graphene. If ''m''=0, the nanotubes are called "zigzag". If ''n''=''m'', the nanotubes are called "armchair". Otherwise, they are called "chiral".
Single-walled nanotubes are a very important variety of carbon nanotube because they exhibit important electric properties that are not shared by the multi-walled carbon nanotube (MWNT) variants. Single-walled nanotubes are the most likely candidate for miniaturizing electronics past the micro electromechanical scale that is currently the basis of modern electronics. The most basic building block of these systems is the electric wire, and SWNTs can be excellent conductors<ref>Dekker, ''et al.'', (1999)</ref>. One useful application of SWNTs is in the development of the first intramolecular [[field effect transistors]] (FETs). The production of the first intramolecular [[logic gate]] using SWNT FETs has recently become possible as well<ref>Derycke, ''et al.'', (2001)</ref>. To create a logic gate you must have both a p-FET and an n-FET. Because SWNTs are p-FETs when exposed to oxygen and n-FETs when unexposed to oxygen, they were able to protect half of a SWNT from oxygen exposure, while exposing the other half to oxygen. The result was a single SWNT that acted as a NOT logic gate with both p and n-type FETs within the same molecule.
Single-walled nanotubes are still very expensive to produce, around $1500 per gram as of 2000, and the development of more affordable synthesis techniques is vital to the future of carbon nanotechnology. If cheaper means of synthesis cannot be discovered, it would make it financially impossible to apply this technology to commercial-scale applications.<ref>Philip G. Collins and Phaedon Avouris (2000), ''Nanotubes for Electronics'' - Scientific American December 2000, page 67</ref> Several suppliers offer as-produced arc discharge SWNTs for ~$50-100 per gram as of 2007.<ref>http://www.carbonsolution.com</ref><ref>http://carbolex.com</ref>
===Multi-walled===
Multi-walled nanotubes (MWNT) consist of multiple layers of graphite rolled in on themselves to form a tube shape. There are two models which can be used to describe the structures of multi-walled nanotubes. In the ''[[Matryoshka doll|Russian Doll]]'' model, sheets of graphite are arranged in concentric cylinders, eg a (0,8) single-walled nanotube (SWNT) within a larger (0,10) single-walled nanotube. In the ''[[Scroll (parchment)|Parchment]]'' model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled up newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.3 Å.
The special place of double-walled Carbon Nanotubes (DWNT) must be emphasized here because they combine very similar morphology and properties as compared to SWNT, while improving significantly their resistance to chemicals. This is especially important when functionalisation 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 functionalisation will break some C=C [[double bond]]s, 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<ref>Flahaut et. al (2003), ''Gram-Scale CCVD Synthesis of Double-Walled Carbon Nanotubes'', ''Chemical Communications'', 1442-1443</ref> by the CCVD technique, from the selective reduction of oxides solid solutions in methane and hydrogen.
===Fullerite===
[[Fullerite]]s are the solid-state manifestation of fullerenes and related compounds and materials. Being highly [[Physical compression|incompressible]] nanotube forms, [[polymerized]] single-walled nanotubes (P-SWNT) are a class of fullerites and are comparable to [[diamond]] in terms of [[Hardness (materials science)|hardness]]. However, due to the way that nanotubes intertwine, P-SWNTs don't have the corresponding crystal lattice that makes it possible to cut diamonds neatly. This same structure results in a less [[brittle]] material, as any impact that the structure sustains is spread out throughout the material.
===Torus===
A nanotorus is a theoretically described carbon nanotube bent into a [[torus]] (donut shape). Nanotori have many unique properties, such as magnetic moments 1000 times larger than previously expected for certain specific radii.<ref>Liu et al 2002 Phys. Rev. Lett. 88 217206)</ref> Properties such as magnetic moment, thermal stability, etc. vary widely depending on radius of the torus and radius of the tube.<ref>Previous paper plus Computer Physics Communications 146 (2002), Maria Huhtala, Antti Kuronen, Kimmo Kaski</ref>
===Nanobud===
[[Nanobud]]s are a newly discovered material combining carbon nanotubes with fullerenes wherein the fullerenes are covalently bonded to the outer sidewalls of the underlying nanotube. This new materials exhibits properties of both carbon nanotubes and fullerenes and is expected{{Who|date=June 2007}} to replace both materials in many commercial applications.
==
===Strength===
Carbon nanotubes are one of the strongest and stiffest materials known, in terms of [[tensile strength]] and [[elastic modulus]] respectively. This strength results from the covalent sp<sup>2</sup> bonds formed between the individual carbon atoms. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 [[GPa]].<ref>Min-Feng Yu et. al (2000), ''Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load'', Science 287, 637-640</ref> In comparison, high-carbon steel has a tensile strength of approximately 1.2 GPa. CNTs have very high [[elastic moduli]], on the order of 1 TPa.<ref>http://ipn2.epfl.ch/CHBU/papers/ourpapers/Forro_NT99.pdf</ref> Since carbon nanotubes have a low density for a solid of 1.3-1.4 g/cm³<ref>Philip G. Collins and Phaedon Avouris (2000), ''Nanotubes for Electronics'' - Scientific American December 2000, 69</ref>, its [[specific strength]] of up to 48,462 kN·m/kg is the best of known materials, compared to high-carbon steel's 154 kN·m/kg.
Under excessive tensile strain, the tubes will undergo [[Deformation|plastic deformation]], which means the deformation is permanent. This deformation begins at strains of approximately 5% <ref>Qian et al (2002)</ref> and can increase the maximum strain the tube undergoes before fracture by releasing strain energy.
CNTs are not nearly as strong under compression. Because of their hollow structure and high aspect ratio, they tend to undergo [[buckling]] when placed under compressive, torsional or bending stress.
===Kinetic===
Multi-walled carbon nanotubes, multiple concentric nanotubes precisely nested within one another, 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.<ref>http://dx.doi.org/10.1126/science.289.5479.505e</ref><ref>John Curnings ''et al.'' (2000), ''Low-Friction Nanoscale Linear Bearing Realized from Multiwall Carbon Nanotubes'', Science 289, 602-604</ref> 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 [[Synthetic molecular motors|motor]]<ref>A. M. Fennimore ''et al.'' (2003), ''Rotational actuators based on carbon nanotubes'', Nature 424, 408-410</ref> and a [[rheostat|nanorheostat]].<ref>John Curnings et.al. (2004), ''Localization and Nonlinear Resistance in Telescopically Extended Nanotubes'', Physical Review Letters 93</ref> Future applications such as a gigahertz mechanical oscillator are also envisaged.<ref>John Curnings et.al. (2000), ''Nanotubes in the Fast Lane'', Physical Review Letters 88</ref>
===Electrical===
:''See also: [[Fermi point (nanotech)|Fermi point]]''
Because of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given (''n'',''m'') nanotube, if ''n'' - ''m'' is a multiple of 3, then the nanotube is [[metallic]], otherwise the nanotube is a [[semiconductor]]. Thus all armchair (''n''=''m'') nanotubes are metallic, and nanotubes (5,0), (6,4), (9,1), etc. are semiconducting. In theory, metallic nanotubes can have an electrical current density more than 1,000 times greater than metals such as [[silver]] and [[copper]]<ref>Philip G. Collins and Phaedon Avouris (2000), ''Nanotubes for Electronics'' - Scientific American December 2000, 68</ref>.
===Thermal===
All nanotubes are expected to be very good [[thermal conductor]]s along the tube, exhibiting a property known as "[[ballistic conduction]]," but good insulators laterally to the tube axis. It is predicted that carbon nanotubes will be able to transmit up to 6000 [[watt]]s per meter per [[kelvin]] at room temperature; compare this to [[copper]], a metal well-known for its good [[thermal conductivity]], which only transmits 385 W/m/K. The temperature stability of carbon nanotubes is estimated to be up to 2800 degrees Celsius in [[vacuum]] and about 750 degrees Celsius in air.<ref>Philip G. Collins and Phaedon Avouris (2000), ''Nanotubes for Electronics'' - Scientific American December 2000, 69</ref>
===Defects===
As with any material, the existence of defects affects the material properties. Defects can occur in the form of atomic vacancies. High levels of such defects can lower the tensile strength by up to 85%.<ref name=APS_Paper>M. Sammalkorpi ''et al.'' (2004), ''Mechanical properties of carbon nanotubes with vacancies and related defects'', Physical Review B</ref> Another well-known form of defect that occurs in carbon nanotubes is known as the [[Stone Wales defect]], which creates a pentagon and heptagon pair by rearrangement of the bonds. Because of the very small structure of CNTs, the tensile strength of the tube is dependent on the weakest segment of it in a similar manner to a chain, where a defect in a single link diminishes the strength of the entire chain.<ref>Philip G. Collins and Phaedon Avouris (2000), ''Nanotubes for Electronics'' - Scientific American December 2000, page 67</ref>
The tube's electrical properties are also affected by the presence of defects. A common result is the lowered conductivity through the defective region of the tube. Some defect formation in armchair-type tubes (which can conduct electricity) can cause the region surrounding that defect to become semiconducting. Furthermore single monoatomic vacancies induce magnetic properties.
The tube's thermal properties are heavily affected by defects. Such defects lead to [[phonon]] scattering, which in turn increases the relaxation rate of the phonons. This reduces the [[mean free path]], and reduces the [[thermal conductivity]] of nanotube structures.
===One-Dimensional Transport===
Due to their nanoscale dimensions, electron transport in carbon nanotubes will take place through quantum effects and will only propagate along the axis of the tube.<ref>Dresselhaus, M.S. Carbon nanotubes. Retrieved July 12, 2007, from Physics Web Web site: http://physicsweb.org/articles/world/11/1/9</ref> This behavior is the same as that of a [[quantum wire]].<ref>Harris, Peter J.F. (1999). Carbon Nanotubes and Related Structures. Cambridge, UK: Cambridge University Press.</ref> Because of this special transport property, carbon nanotubes are frequently referred to as “one-dimensional” in scientific articles.
==
Techniques have been developed to produce nanotubes in sizeable quantities, including [[Electric arc|arc discharge]], [[laser ablation]], high pressure carbon monoxide ([[HiPco]]), and [[chemical vapor deposition]] (CVD). Most of these processes take place in vacuum or with process gases. CVD growth of CNTs can take place in vacuum or at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth processes are making CNTs more commercially viable.
It is now thought by some that the catalysts or methods involved in forging [[damascus steel]] (a forging technique lost to time) may provide vital hints for manufacturing nanotubes cheaply, after they were recently discovered to be a component of that ancient sword metal<ref>{{Cite news | title = Legendary Swords' Sharpness, Strength From Nanotubes, Study Says | last = Inman | first = Mason | date = November 16, 2006 | publisher = National Geographic | url = http://news.nationalgeographic.com/news/2006/11/061116-nanotech-swords.html | accessdate = 2007-05-26}}</ref> <ref>[http://www.newscientisttech.com/channel/tech/nanotechnology/mg19225780.151 Secret's out for Saracen sabres]</ref>.
===Arc discharge===
Nanotubes were observed in [[1991]] in the carbon soot of graphite [[electrode]]s during an arc discharge, by using a current of 100 [[ampere|amps]], that was intended to produce fullerenes.<ref>Sumio Iijima (1991), ''Helical microtubules of graphitic carbon'', Nature '''354''', 56 - 58</ref>. However the first [[macroscopic]] production of carbon nanotubes was made in [[1992]] by two researchers at [[NEC]]'s Fundamental Research Laboratory.<ref>T. W. Ebbesen, and P. M. Ajayan (1992), ''Large-scale synthesis of carbon nanotubes'', Nature '''358''', 220-222</ref> The method used was the same as in 1991. During this process, the carbon contained in the negative electrode sublimates because of the high temperatures caused by the discharge. Because nanotubes were initially discovered using this technique, it has been the most widely used method of nanotube synthesis.
The yield for this method is up to 30 percent by weight and it produces both single- and multi-walled nanotubes with lengths of up to 50 microns.<ref>Philip G. Collins and Phaedon Avouris (2000), ''Nanotubes for Electronics'' - Scientific American December 2000, page 67</ref>
===Laser ablation===
In the laser ablation process, a [[pulsed laser]] vaporizes a graphite target in a high temperature reactor while an inert gas is bled into the chamber. The nanotubes develop on the cooler surfaces of the reactor, as the vaporized carbon condenses. A water-cooled surface may be included in the system to collect the nanotubes.
It was invented by [[Richard Smalley]] and co-workers at [[Rice University]], who at the time of the discovery of carbon nanotubes, were blasting metals with the laser to produce various metal molecules. When they heard of the discovery they substituted the metals with graphite to create multi-walled carbon nanotubes.<ref>Guo et al, ''J. Phys. Chem.'', '''99''', 10694-10697</ref>. Later that year the team used a composite of graphite and metal catalyst particles (the best yield was from a [[cobalt]] and [[nickel]] mixture) to synthesise single-walled carbon nanotubes.<ref>Guo et al, ''Chem. Phys. Lett.'', '''243''', 49-54</ref>
This method has a yield of around 70% and produces primarily single-walled carbon nanotubes with a controllable diameter determined by the reaction [[temperature]]. However, it is more expensive than either arc discharge or chemical vapor deposition.<ref>Philip G. Collins and Phaedon Avouris (2000), ''Nanotubes for Electronics'' - Scientific American December 2000, page 67</ref>
===Chemical vapor deposition (CVD)===
[[Image:PICT0111.JPG|thumb|Nanotubes being grown by plasma enhanced [[chemical vapor deposition]]]]
The catalytic vapor phase deposition of carbon was first reported in 1959,<ref>P. L. Walker Jr. ''et al.'', J. Phys. Chem. 63, 133 (1959).</ref> but it was not until 1993<ref>M. José-Yacamán ''et al.'', Appl. Phys. Lett. 62, 657 (1993).</ref> that carbon nanotubes could be formed by this process. In 2007, researchers at the University of Cincinnati (UC) developed a process to grow 18 mm long aligned carbon nanotube arrays <ref>http://www.uc.edu/news/NR.asp?id=5700</ref>.
During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly [[nickel]], [[cobalt]], [[iron]], or a combination. The metal nanoparticles can also be produced by other ways, including reduction of oxides or oxides solid solutions. The diameters of the nanotubes that are to be grown are related to the size of the metal particles. This can be controlled by patterned (or masked) deposition of the metal, annealing, or by plasma etching of a metal layer. The substrate is heated to approximately 700°C. To initiate the growth of nanotubes, two gases are bled into the reactor: a process gas (such as [[ammonia]], [[nitrogen]], [[hydrogen]], etc.) and a carbon-containing gas (such as [[acetylene]], [[ethylene]], [[ethanol]], [[methane]], etc.). Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes. This mechanism is still under discussion. The catalyst particles can stay at the tips of the growing nanotube during the growth process, or remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate.
CVD is a common method for the commercial production of carbon nanotubes. For this purpose, the metal nanoparticles will be carefully mixed with a catalyst support (e.g., MgO, Al2O3, etc) to increase the specific surface area for higher yield of the catalytic reaction of the carbon feedstock with the metal particles. One issue in this synthesis route is the removal of the catalyst support via an acid treatment, which sometimes could destroy the original structure of the carbon nanotubes. However, alternative catalyst supports that are soluble in water have been shown to be effective for nanotube growth.<ref>A. Eftekhari ''et al.'', Carbon 44, 1343 (2006).</ref>
If a [[plasma (physics)|plasma]] is generated by the application of a strong electric field during the growth process (plasma enhanced [[chemical vapor deposition]]), then the nanotube growth will follow the direction of the electric field.<ref>Z. F. Ren ''et al.'', Science 282, 1105 (1998).</ref> By properly adjusting the geometry of the reactor it is possible to synthesize vertically aligned carbon nanotubes[http://www.nano-lab.com/imagegallery.html](i.e., perpendicular to the substrate), a morphology that has been of interest to researchers interested in the electron emission from nanotubes. Without the plasma, the resulting nanotubes are often randomly oriented, resembling a bowl of [[spaghetti]]. Under certain reaction conditions, even in the absence of a plasma, closely spaced nanotubes will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest.
Of the various means for nanotube synthesis, CVD shows the most promise for industrial scale deposition in terms of its price/unit ratio. There are additional advantages to the CVD synthesis of nanotubes. Unlike the above methods, CVD is capable of growing nanotubes directly on a desired substrate, whereas the nanotubes must be collected in the other growth techniques. The growth sites are controllable by careful deposition of the catalyst. Additionally, no other growth methods have been developed to produce vertically aligned nanotubes.<ref>Philip G. Collins and Phaedon Avouris (2000), ''Nanotubes for Electronics'' - Scientific American December 2000, page 67</ref>
Recently, this area of synthesis has been advanced by a team of researchers at Rice University. The team, until recently led by the late Dr. Richard Smalley, has concentrated upon finding methods to produce large, pure amounts of particular types of nanotubes. Their approach grows long fibers from many small seeds cut from a single nanotube; all of the resulting fibers were found to be of the same diameter as the original nanotube and are expected to be of the same type as the original nanotube. Further characterization of the resulting nanotubes and improvements in yield and length of grown tubes are needed.<ref> http://www.media.rice.edu/media/NewsBot.asp?MODE=VIEW&ID=9070 </ref>
CVD growth of multi-walled nanotubes is used by several companies to produce materials on the tonne scale, including NanoLab, [[Bayer]], [[Arkema]], [[Nanocyl]], [http://www.nanothinx.com Nanothinx] , [[Hyperion Catalysis]], [[Mitsui]], and [[Showa Denko]].
===Natural, incidental, and controlled flame environments===
[[Fullerene]]s and carbon nanotubes are not necessarily products of high-tech laboratories; they are commonly formed in such mundane places as ordinary [[flame]]s,<ref>J.M. Singer, J. Grumer, Proc. Combust. Inst. 7, 559 (1959).</ref> produced by burning methane,<ref>{{cite journal | last = Yuan | first = Liming | coauthors = Kozo Saito, Chunxu Pan, F.A. Williams, and A.S. Gordon | year = 2001 | title = Nanotubes from methane flames | journal = Chemical physics letters | volume = 340 | pages = 237–241 | doi = 10.1016/S0009-2614(01)00435-3}}</ref> ethylene,<ref>{{cite journal | last = Yuan | first = Liming | coauthors = Kozo Saito, Wenchong Hu, and Zhi Chen | year = 2001 | title = Ethylene flame synthesis of well-aligned multi-walled carbon nanotubes | journal = Chemical physics letters | volume = 346 | pages = 23–28 | doi = 10.1016/S0009-2614(01)00959-9}}</ref> and benzene,<ref>{{cite journal | last = Duan | first = H. M. | coauthors = and J. T. McKinnon | year = 1994 | title = Nanoclusters Produced in Flames | journal = Journal of Physical Chemistry | volume = 98 | issue = 49 | pages = 12815–12818|doi = 10.1021/j100100a001}}</ref> and they have been found in [[soot]] from both indoor and outdoor air.<ref>{{cite journal | last = Murr | first = L. E. | coauthors = J.J. Bang, E.V. Esquivel, P.A. Guerrero, and D.A. Lopez | year = 2004 | title = Carbon nanotubes, nanocrystal forms, and complex nanoparticle aggregates in common fuel-gas combustion sources and the ambient air | journal = Journal of Nanoparticle Research | volume = 6 | pages = 241–251 | doi = 10.1023/B:NANO.0000034651.91325.40}}</ref> However, these naturally occurring varieties can be highly irregular in size and quality because the environment in which they are produced is often highly uncontrolled. Thus, although they can be used in some applications, they can lack in the high degree of uniformity necessary to meet many needs of both research and industry. Recent efforts have focused on producing more uniform carbon nanotubes in controlled flame environments.<ref>R.L. Vander Wal, Combust. Flame 130 37-47 (2002).</ref><ref>A.V. Saveliev, W. Merchan-Merchan, L.A. Kennedy, Combust. Flame 135, 27-33 (2003).</ref><ref>M.J. Height, J.B. Howard, J.W. Tester, J.B. Vander Sande, Carbon 42, 2295-2307 (2004).</ref><ref>S. Sen, I.K. Puri, Nanotechnology 15, 264-268 (2004).</ref> Nano-C, Inc <ref>http://www.nano-c.com/</ref> of Westwood, Massachusetts, is producing flame synthesized single-walled carbon nanotubes. This method has promise for large scale, low cost nanotube synthesis, though it must compete with rapidly developing large scale CVD production.
==Potential and Current Applications==
{{main|Potential applications of carbon nanotubes}}
:''See also, for last current applications: [[Timeline of carbon nanotubes]]''
[[Image:Louie nanotube.jpg|thumb||330px|The joining of two carbon nanotubes with different electrical properties to form a [[diode]] has been proposed.]]
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 multi-walled carbon nanotube has been tested to be is 63 [[GPa]].<ref>Min-Feng Yu ''et al.'' (2000), ''Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load'', Science 287, 637-640</ref> Bulk nanotube materials may never achieve a tensile strength similar to that of individual tubes, but such composites may nevertheless yield strengths sufficient for many applications. Carbon nanotubes have already been used as composite fibers in [[polymers]] to improve the mechanical, thermal and electrical properties of the bulk product. A 2006 study published in ''[[Nature (journal)|Nature]]'' determined that some carbon nanotubes are present in [[damascus steel]], possibly helping to account for the legendary strength of the (almost ancient) swords made of it<ref>http://news.nationalgeographic.com/news/2006/11/061116-nanotech-swords.html</ref> <ref>[http://www.newscientisttech.com/channel/tech/nanotechnology/mg19225780.151 Secret's out for Saracen sabres]</ref>.
===Structural===
Because of the great mechanical properties of the carbon nanotubule, a variety of structures has been proposed ranging from everyday items like clothes and sports gear to combat jackets and space elevators <ref>The Space Elevator, by Brad C. Edwards, NASA</ref>. However, the [[space elevator]] will require further efforts in refining carbon nanotube technology, as the practical tensile strength of carbon nanotubes can still be greatly improved.<ref>Philip G. Collins and Phaedon Avouris (2000), ''Nanotubes for Electronics'', Scientific American (2000)</ref>
For perspective, outstanding breakthroughs have already been made. Pioneering work lead by Ray H. Baughman at the NanoTech Institute has shown that single and multi-walled nanotubes can produce materials with toughness un-matched in the man-made and natural worlds.<ref> Zhang ''et al.'' Science (2005), 309(5738), 1215. and Dalton ''et al.'' Nature (2003), 423(6941), 703. </ref>
A good example of a practical use for the carbon nanotubules is the bicycle [[Floyd Landis]] used at the [[2006 Tour de France]], the SLC 01 from BMC, a Swiss bike manufacturer. Carbon nanotubes were used to enhance the strength of the carbon fiber frame and made it possible to make a bicycle's frame weighing only one kilogram.<ref>http://news.com.com/Carbon+nanotubes+enter+Tour+de+France/2100-11395_3-6091347.html?tag=fd_carsl Visited 10-15-2006</ref>
Recent research by James D. Iverson and Brad C. Edwards has revealed the possibility of cross-linking CNT molecules prior to incorporation in a polymer matrix to form a super high strength composite supermaterial. This CNT composite will have a tensile strength on the order of 20 million psi (138 GPa, for 106 MN·m/kg), revolutionizing many aspects of engineering design where low weight and high strength is required.{{Fact|date=June 2007}}
===In electrical circuits===
Carbon nanotubes have many properties—from their unique dimensions to an unusual current [[electrical conduction|conduction]] mechanism—that make them ideal components of electrical circuits.
Nanotube based [[transistor]]s have been made that operate at room temperature and that are capable of digital switching using a single electron. <ref>Dekker, Postma et al (2001), ''Carbon Nanotube Single-Electron Transistors at Room Temperature'' - Science 293.5527 (July 6, 2001)</ref>
One major obstacle to realization of nanotubes has been the lack of technology for mass production. However, in 2001 IBM researchers demonstrated how nanotube transistors can be grown in bulk, not very differently from silicon transistors. The process they used is called "constructive destruction" which includes the automatic destruction of defective nanotubes on the [[wafer (electronics)|wafer]].<ref>Avouris, Arnold, Collins ''Engineering Carbon Nanotubes and Nanotube Circuits Using Electrical Breakdown'' - Science 292.5517 (April 27, 2001):706-9</ref>
This has since then been developed further and single-chip wafers with over ten billion correctly aligned nanotube junctions have been created. In addition it has been demonstrated that incorrectly aligned nanotubes can be removed automatically using standard [[lithography]] equipment.<ref>Kalaugher ''Scalable Interconnection and Integration of Nanowire Devices Without Registration'' Nano Letters 4.5 (2004):915-19</ref>
The first nanotube made integrated memory circuit was made in 2004. One of the main challenges has been regulating the conductivity of nanotubes. Depending on subtle surface features a nanotube may act as a plain [[Electrical conductor|conductor]] or as a [[semiconductor]]. A fully automated method has however been developed to remove non-semiconductor tubes. <ref>Tesng et al''Monolithic Integration of Carbon Nanotube Devices with Silicon MOS Technology'' Nano Letters 4.1 (2004):123-127</ref>
===Other applications===
Carbon nanotubes have also been implemented in nanoelectromechanical systems, including mechanical memory elements ([[NRAM]] being developed by [[Nantero Inc.]]) and nanoscale electric motors (see [[Nanomotor]]).
A new use for carbon nanotubes is as a possible gene delivery vehicle.<ref>{{cite journal |last=Singh |first=Ravi |coauthors=Et al. |year=2005 |title=Binding and condensation of plasmid DNA onto functionalized carbon nanotubes : Toward the construction of nanotube-based gene delivery vectors |journal=J. Am. Chem. Soc. |volume=127 |issue=12 |pages=9}}</ref>
[[Eikos|Eikos Inc]] of Franklin, Massachusetts and [[Unidym]] Inc. of Silicon Valley, California are developing transparent, electrically conductive films of carbon nanotubes to replace [[indium tin oxide]] (ITO). Carbon nanotube films are substantially more mechanically robust than ITO films, making them ideal for high reliability touch screens and flexible displays. Nanotube films show promise for use in displays for computers, cell phones, [[Personal digital assistant|PDA]]s, and [[Automated teller machine|ATM]]s.
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==Molecular Modelling Software of Carbon Nanotubes==
* [[CoNTub| CoNTub v1.0]]
* [http://www.nanorex.com Nanorex]
* [http://www.photon.t.u-tokyo.ac.jp/~maruyama/wrapping3/wrapping.html Wrapping]
* [http://www.jcrystal.com Nanotube Modeller]
* [http://k.1asphost.com/tubeasp/tubeasp.asp TubeASP]
* [http://turin.nss.udel.edu/research/tubegenonline.html Tubegen]
==Molecular Models of Carbon Nanotubes==
[http://www.indigo.com/models/carbon-nanotube-molecular-model-kits.html Carbon Nanotube Models]
== References ==
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==External links==
{{linkfarm}}
{{Commons|Carbon nanotube}}
*[http://www.brightsurf.com/search/r-a/Carbon_Nanotubes/1/Carbon_Nanotubes_news.html Carbon Nanotubes News]
*[http://www.newscientisttech.com/channel/tech/nanotechnology New Scientist Special Report] - a collection of nanotechnology articles, most on nanotubes
*[http://www.nanotube-suppliers.com/ Nanotube suppliers] - International List of nanotubes suppliers
*[http://news.com.com/The+stuff+of+dreams/2009-1008_3-5091267.html?tag=nl The stuff of dreams] - [[CNET]]
*[http://www.pa.msu.edu/cmp/csc/NTSite/nanopage.html The Nanotube site] - Last updated 2006.09.17
*[http://www.chimica.unipd.it/enzo.menna/pubblica/nanobookmark.html Nanotechnologies and nanotubes]
*[http://sinnott.mse.ufl.edu/Movies/29x0-swnt_deflex_Ar10eV.mpg Animation of a (29,0) being struck by 10 sets of 9 Argon atoms at 10 eV each] (opens in media player)
*[http://students.chem.tue.nl/ifp03/Wondrous%20World%20of%20Carbon%20Nanotubes_Final.pdf The wonderous World of Carbon Nanotubes] (In .pdf format, good introduction to nanotube)
*[http://www.nanomaterialdatabase.org/ Nanowerk Nanotechnology Portal] Introduction to nanmoaterials and nanotubes
*[http://nanotechweb.org nanotechweb.org] nanotube and nanotechnology news and information
*[http://www.forskning.no/Artikler/2006/juni/1149432180.36 Carbon - Super Stuff] Educational interactive with narration and 3D-models of nanotube, diamond, graphite and coal.
*[http://xstructure.inr.ac.ru/x-bin/theme2.py?arxiv=cond-mat&level=2&index1=43 Carbon nanotube on arxiv.org]
*[http://hielscher.com/ultrasonics/nano_03.htm Untangling and Dispersing of Carbon Nanotubes using Ultrasonics]
*[http://www.raymor.com They are trying to reduce the cost of SWNT production with a high capability production plant]
*[http://www.loima.fmns.rug.nl/Naphod.html Photoactive molecules inside nanotubes]
*[http://www.nature.com/news/2006/061113/full/061113-11.html Carbon nanotech may have given swords of Damascus their edge - Nature 2006]
*[http://students.chem.tue.nl/ifp03/ Interdisciplinary student project giving an excellent overview of literature on synthesis and purification]
*[http://www.carbio.eu EU Marie Curie Network CARBIO - Multifunctional carbon nanotubes for biomedical applications]
*[http://nanosatyadhar.webs.io/ Site on Single Walled Carbon Nanotube, physics and electronics of CNT-SWNT.]
[[Category:Nanomaterials]]
[[Category:Nanotechnology]]
[[Category:Carbon forms]]
[[Category:Exotic matter]]
[[Category:Antistatic agents]]
[[Category:Electrical conductors]]
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