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{{Distinguish|Carbon fiber}}
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{{Nanomat}}
[[File:Kohlenstoffnanoroehre Animation.gif|right|frame|Rotating Carbon Nanotube]]
 
'''Carbon nanotubes''' ('''CNTs''') are [[allotropes of carbon]] with a [[cylindrical]] [[nanostructure]]. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1,<ref name="Longest">
{{cite journal
|last1=Wang |first1=X.
|year=2009
|title=Fabrication of Ultralong and Electrically Uniform Single-Walled Carbon Nanotubes on Clean Substrates
|journal=[[Nano Letters]]
|volume=9 |issue=9 |pages=3137–3141
|doi=10.1021/nl901260b
|pmid=19650638
|last2=Li
|first2=Qunqing
|last3=Xie
|first3=Jing
|last4=Jin
|first4=Zhong
|last5=Wang
|first5=Jinyong
|last6=Li
|first6=Yan
|last7=Jiang
|first7=Kaili
|last8=Fan
|first8=Shoushan
|bibcode = 2009NanoL...9.3137W }}</ref> significantly larger than for any other material. These cylindrical [[carbon]] [[molecule]]s have unusual properties, which are valuable for [[nanotechnology]], [[electronics]], [[optics]] and other fields of [[materials science]] and technology. In particular, owing to their extraordinary [[thermal conductivity]] and mechanical and [[electricity|electrical]] properties, carbon nanotubes find applications as additives to various structural materials. For instance, nanotubes form a tiny portion of the material(s) in some (primarily [[carbon fiber]]) baseball bats, golf clubs, or car parts.<ref>
{{cite journal
|last1=Gullapalli |first1=S.
|last2=Wong |first2=M.S.
|year=2011
|title=Nanotechnology: A Guide to Nano-Objects
|url=http://www.aiche.org/uploadedFiles/Publications/CEPMagazine/051128_public.pdf
|journal=Chemical Engineering Progress
|volume=107 |issue=5 |pages=28–32
}}</ref>
 
Nanotubes are members of the [[fullerene]] structural family. Their name is derived from their long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called [[graphene]]. These sheets are rolled at specific and discrete ("[[chirality|chiral]]") angles, and the combination of the rolling angle and radius decides the nanotube properties; for example, whether the individual nanotube shell is a [[metal]] or [[semiconductor]]. Nanotubes are categorized as [[#Single-walled|single-walled nanotubes]] (SWNTs) and [[#Multi-walled|multi-walled nanotubes]] (MWNTs). Individual nanotubes naturally align themselves into "ropes" held together by [[van der Waals force]]s, more specifically, pi-stacking.
 
Applied [[quantum chemistry]], specifically, [[orbital hybridization]] best describes chemical bonding in nanotubes. The [[chemical bonding]] of nanotubes is composed entirely of [[Sp2 bond|''sp''<sup>2</sup> bonds]], similar to those of [[graphite]]. These bonds, which are stronger than the [[Sp3 bond|''sp''<sup>3</sub> bonds]] found in [[alkane]]s and [[diamond]], provide nanotubes with their unique strength.
{{toclimit|3}}
 
==Types of carbon nanotubes and related structures==
 
===Terminology===
There is no consensus on some terms describing carbon nanotubes in scientific literature: both "-wall" and "-walled" are being used in combination with "single", "double", "triple" or "multi", and the letter C is often omitted in the abbreviation; for example, multi-walled carbon nanotube (MWNT).
 
===Single-walled===
<gallery perrow="4" widths=160px heights=120px>
File:Carbon nanotube armchair povray.PNG|Armchair (''n'',''n'') i.e.: m=n
File:Carbon nanorim armchair povray.PNG|The translation vector is bent, while the chiral vector stays straight
File:Carbon nanoribbon povray.PNG|Graphene nanoribbon
File:Carbon nanorim zigzag povray.PNG|The chiral vector is bent, while the translation vector stays straight
File:Carbon nanotube zigzag povray.PNG|Zigzag (''n'',0)
File:Carbon nanotube_chiral povray.PNG|Chiral (''n'',''m'')
File:Carbon nanorim chiral povray.PNG|''n'' and ''m'' can be counted at the end of the tube
File:Carbon nanoribbon chiral povray.PNG|Graphene nanoribbon
</gallery>
 
[[File:CNTnames.png|thumb|300px|left|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.]]
[[File:Chiraltube.gif|thumb|right|A [[scanning tunneling microscopy]] image of single-walled carbon nanotube]]
[[File:CntHAADF.jpg|thumb|left|A [[transmission electron microscopy]] image of a single-walled carbon nanotube]]
 
Most single-walled nanotubes (SWNT) have a diameter of close to 1&nbsp;nanometer, with a tube length that can be many millions 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''). The integers ''n'' and ''m'' denote the number of unit [[Vector (geometric)|vectors]] along two directions in the honeycomb [[crystal lattice]] of graphene. If ''m'' = 0, the nanotubes are called zigzag nanotubes, and if ''n'' = ''m'', the nanotubes are called armchair nanotubes. Otherwise, they are called chiral. The diameter of an ideal nanotube can be calculated from its (n,m) indices as follows
 
: <math> d = \frac{a}{\pi} \sqrt{(n^2 + nm + m^2)}=78.3 \sqrt{((n+m)^2-nm)}  \rm pm,</math>
 
where ''a'' = 0.246&nbsp;nm.
 
SWNTs are an important variety of carbon nanotube because most of their properties change significantly with the (''n'',''m'') values, and this dependence is non-monotonic (see [[Optical properties of carbon nanotubes#Kataura plot|Kataura plot]]). In particular, their [[band gap]] can vary from zero to about 2 eV and their electrical conductivity can show metallic or semiconducting behavior. Single-walled nanotubes are likely candidates for miniaturizing electronics. The most basic building block of these systems is the electric wire, and SWNTs with diameters of an order of a nanometer can be excellent conductors.<ref>
{{cite journal
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|last3=White |first3=C.T.
|year=1992
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{{cite journal
|last=Dekker |first=C.
|year=1999
|title=Carbon nanotubes as molecular quantum wires
|journal=[[Physics Today]]
|volume=52 |pages=22–28
|doi=10.1063/1.882658
|issue=5
|bibcode = 1999PhT....52e..22D }}</ref> One useful application of SWNTs is in the development of the first intermolecular [[field-effect transistor]]s (FET). The first intermolecular [[logic gate]] using SWCNT FETs was made in 2001.<ref>
{{cite journal
|last1=Martel |first1=R.
|year=2001
|title=Ambipolar Electrical Transport in Semiconducting Single-Wall Carbon Nanotubes
|journal=Phys. Rev. Lett.
|volume=87 |issue=25 |page=256805
|doi=10.1103/PhysRevLett.87.256805
|pmid=11736597
|last2=Derycke
|first2=V.
|last3=Lavoie
|first3=C.
|last4=Appenzeller
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|first5=K.
|last6=Tersoff
|first6=J.
|last7=Avouris
|first7=Ph.
|bibcode = 2001PhRvL..87y6805M }}</ref> A logic gate requires both a p-FET and an n-FET. Because SWNTs are p-FETs when exposed to oxygen and n-FETs otherwise, it is possible to protect half of an SWNT from oxygen exposure, while exposing the other half to oxygen. This results in a single SWNT that acts as a NOT logic gate with both p and n-type FETs within the same molecule.
 
Single-walled nanotubes are dropping precipitously in price, from around $1500 per gram as of 2000 to retail prices of around $50 per gram of as-produced 40–60% by weight SWNTs as of March 2010.{{citation needed|date=February 2012}}
 
===Multi-walled===
[[File:CNTSEM.JPG|thumb|upright|A [[scanning electron microscopy]] image of carbon nanotubes bundles]]
[[File:Multi-walled Carbon Nanotube.png|thumb|upright|Triple-walled armchair carbon nanotube]]
Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphene. There are two models that 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, e.g., a (0,8) single-walled nanotube (SWNT) within a larger (0,17) 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 newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.4 Å. The Russian Doll structure is observed more commonly. Its individual shells can be described as SWNTs, which can be metallic or semiconducting. Because of statistical probability and restrictions on the relative diameters of the individual tubes, one of the shells, and thus the whole MWNT, is usually a zero-gap metal.
 
Double-walled carbon nanotubes (DWNT) form a special class of nanotubes because their morphology and properties are similar to those of SWNT but their resistance to chemicals is significantly improved. This is especially important when [[Surface functionalization|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 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>
{{cite journal
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|year=2003
|title=Gram-Scale CCVD Synthesis of Double-Walled Carbon Nanotubes
|journal=[[Chemical Communications]]
|volume=12 |issue=12 |pages=1442–1443
|doi=10.1039/b301514a
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}}</ref> by the CCVD technique, from the selective reduction of oxide solutions in methane and hydrogen.
 
The telescopic motion ability of inner shells<ref>
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|year=2000
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|journal=Science
|volume=289 |issue=5479 |pages=602–604
|doi=10.1126/science.289.5479.602
|pmid=10915618
|bibcode = 2000Sci...289..602C }}</ref> and their unique mechanical properties<ref>
{{cite journal
|last=Treacy |first=M.M.J.
|last2=Ebbesen |first2=T.W.
|last3=Gibson |first3=J.M.
|year=1996
|title=Exceptionally high Young's modulus observed for individual carbon nanotubes
|journal=Nature
|volume=381 |issue=6584 |pages=678–680
|doi=10.1038/381678a0
|bibcode = 1996Natur.381..678T }}</ref> will permit the use of multi-walled nanotubes as main movable arms in coming nanomechanical devices. Retraction force that occurs to telescopic motion caused by the [[Lennard-Jones interaction]] between shells and its value is about 1.5&nbsp;nN.<ref>
{{cite journal
|last=Zavalniuk |first=V.
|last2=Marchenko |first2=S.
|year=2011
|title=Theoretical analysis of telescopic oscillations in multi-walled carbon nanotubes
|journal=Low Temperature Physics
|volume=37 |issue=4 |page=337
|arxiv=0903.2461
|doi=10.1063/1.3592692
|bibcode = 2011LTP....37..337Z }}</ref>
 
===Torus===
[[File:NanoBud.JPG|thumb|A stable [[Carbon nanobud|nanobud]] structure]]
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.<ref name="nanotori">
{{cite journal
|last1=Liu |first=L.
|year=2002
|title=Colossal Paramagnetic Moments in Metallic Carbon Nanotori
|journal=Phys. Rev. Lett.
|volume=88 |issue=21 |page=217206
|doi=10.1103/PhysRevLett.88.217206
|pmid=12059501
|last2=Guo
|first2=G.
|last3=Jayanthi
|first3=C.
|last4=Wu
|first4=S.
|bibcode = 2002PhRvL..88u7206L }}</ref> Properties such as [[magnetic moment]], thermal stability, etc. vary widely depending on radius of the torus and radius of the tube.<ref name="nanotori"/><ref>
{{cite journal
|last1=Huhtala |first1=M.
|last2=Kuronen |first2=A.
|last3=Kaski |first3=K.
|year=2002
|title=Carbon nanotube structures: Molecular dynamics simulation at realistic limit
|url=http://www.princeton.edu/~msammalk/publications/cpc146_02.pdf
|journal=[[Computer Physics Communications]]
|volume=146 |page=30
|doi=10.1016/S0010-4655(02)00432-0
|bibcode = 2002CoPhC.146...30H }}</ref>
 
===Nanobud===
[[Carbon nanobud]]s are a newly created material combining two previously discovered allotropes of carbon: carbon nanotubes and [[fullerene]]s. 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.
 
===Graphenated carbon nanotubes (g-CNTs)===
[[File:SEM of carbon nanotubes with graphene foliates of varying density, deposited via microwave plasma enhanced CVD.tif|thumb|SEM series of graphenated CNTs with varying foliate density]]
Graphenated CNTs are a relatively new hybrid that combines [[Graphite|graphitic]] foliates grown along the sidewalls of multiwalled or bamboo style CNTs. Yu ''et al.''<ref>{{cite journal|last=Yu|first=Kehan|coauthors=Ganhua Lu, Zheng Bo, Shun Mao, and Junhong Chen|title=Carbon Nanotube with Chemically Bonded Graphene Leaves for Electronic and Optoelectronic Applications|journal=J. Phys. Chem. Lett.|year=2011|volume=2|series=13|pages=1556–1562|doi=10.1021/jz200641c|issue=13}}</ref> reported on "chemically bonded graphene leaves" growing along the sidewalls of CNTs. Stoner ''et al.''<ref>{{cite journal|last=Stoner|first=Brian R.|coauthors= Akshay S. Raut, Billyde Brown, Charles B. Parker, and Jeffrey T. Glass|title=Graphenated carbon nanotubes for enhanced electrochemical double layer capacitor performance|journal=Appl. Phys. Lett.|year=2011|volume=99|series=18|page=183104 |doi=10.1063/1.3657514|issue=18|bibcode = 2011ApPhL..99r3104S }}</ref> described these structures as "graphenated CNTs" and reported in their use for enhanced [[supercapacitor]] performance. Hsu ''et al.'' further reported on similar structures formed on carbon fiber paper, also for use in [[supercapacitor]] applications.<ref>{{cite journal|last=Hsu|first=Hsin-Cheng|coauthors=Chen-Hao Wang, S.K. Nataraj, Hsin-Chih Huang, He-Yun Du, Sun-Tang Chang, Li-Chyong Chen, Kuei-Hsien Chen|title=Stand-up structure of graphene-like carbon nanowalls on CNT directly grown on polyacrylonitrile-based carbon fiber paper as supercapacitor|journal=Diamond and Related Materials|year=2012|volume=25|pages=176–9|doi=10.1016/j.diamond.2012.02.020}}</ref> The foliate density can vary as a function of deposition conditions (e.g. temperature and time) with their structure ranging from few layers of [[graphene]] (< 10) to thicker, more [[graphite]]-like.<ref>{{cite journal|last=Parker|first=Charles B.|coauthors= Akshay S. Raut, Billyde Brown, Brian R. Stoner, and Jeffrey T. Glass|title=Three-dimensional arrays of graphenated carbon nanotubes|journal=J. Mater. Res.|year=2012|volume=27|series=7|pages=1046–53 |doi=10.1557/jmr.2012.43|issue=7|bibcode = 2012JMatR..27.1046P }}</ref>
 
The fundamental advantage of an integrated [[graphene]]-CNT structure is the high surface area three-dimensional framework of the CNTs coupled with the high edge density of graphene. [[Graphene]] edges provide significantly higher charge density and reactivity than the basal plane, but they are difficult to arrange in a three-dimensional, high volume-density geometry. CNTs are readily aligned in a high density geometry (i.e., a vertically aligned forest)<ref>{{cite journal|last=Cui|first=Hong-tao|coauthors=O. Zhou, and B. R. Stoner|title=Deposition of aligned bamboo-like carbon nanotubes via microwave plasma enhanced chemical vapor deposition|journal=J. Appl. Phys.|year=2000|volume=88|pages=6072–4|doi=10.1063/1.1320024|issue=10|bibcode = 2000JAP....88.6072C }}</ref> but lack high charge density surfaces—the sidewalls of the CNTs are similar to the basal plane of graphene and exhibit low charge density except where edge defects exist. Depositing a high density of graphene foliates along the length of aligned CNTs can significantly increase the total [[Capacitance|charge capacity]] per unit of nominal area as compared to other carbon nanostructures.<ref>{{cite journal|last=Stoner|first=Brian R.|coauthors=Jeffrey T. Glass|title=Carbon nanostructures: a morphological classification for charge density optimization|journal=Diamond and Related Materials|year=2012|volume=23|pages=130–4|doi=10.1016/j.diamond.2012.01.034|bibcode = 2012DRM....23..130S }}</ref>
 
===Nitrogen Doped Carbon Nanotubes===
Nitrogen doped carbon nanotubes (N-CNT's), can be produced through 5 main methods, [[Chemical Vapor Deposition]],<ref>J. Kouvetakis, M. Todd, B. Wilkens, A. Bandari, N. Cave, Novel synthetic routes to carbon–nitrogen thin films, Chem. Mater. 6 (1994) 811–814.</ref><ref>Journal of Physics and Chemistry of Solids 71 (2010) 134–139</ref> high-temperature and high-pressure reactions, gas-solid reaction of amorphous carbon with NH<sub>3</sub> at high temperature,<ref>L.-W. Yin, Y. Bando, M.-S. Li, Y.-X. Liu, Y.-X. Qi, Unique single-crystalline beta carbon nitride nanorods, Adv. Mater. 15 (2003) 1840–1844.</ref> solid reaction,<ref>T. Oku, M. Kawaguchi, Microstructure analysis of CN-based nanocage materials by high-resolution electron microscopy, Diamond Relat. Mater. 9 (2000) 906–910.</ref> and solvothermal synthesis.<ref>Q.X. Guo, Y. Xie, X.J. Wang, S.Y. Zhang, T. Hou, S.C. Lv, Synthesis of carbon nitride nanotubes with the C3N4stoichiometry via a benzene-thermal process at low temperatures, Chem. Commun. 1 (2004) 26–27.</ref>
 
N-CNTs can also be prepared by a CVD method of pyrolysizing [[melamine]] under Ar at elevated temperatures of 800<sup>o</sup>C - 980<sup>o</sup>C. However synthesis via CVD and melamine results in the formation of bamboo structured CNTs. XPS spectra of grown N-CNT's reveals nitrogen in five main components, pyridinic nitrogen, pyrrolic nitrogen, quaternary nitrogen, and nitrogen oxides. Furthermore synthesis temperature affects the type of nitrogen configuration.<ref>Y. Zhong et al. / Journal of Physics and Chemistry of Solids 71 (2010) 134–139</ref>
 
[[Nitrogen]] doping plays a pivotal role in [[Lithium]] storage. N-doping provides defects in the walls of CNT's allowing for Li ions to diffuse into interwall space. It also increases capacity by providing more favorable bind of N-doped sites. N-CNT's are also much more reactive to [[metal oxide]] nanoparticle deposition which can further enhance storage capacity, especially in anode materials for [[Lithium-ion battery|Li-ion batteries]].<ref>Nitrogen-Doped Multiwall Carbon Nanotubes for Lithium Storage with Extremely High Capacity
Weon Ho Shin, Hyung Mo Jeong, Byung Gon Kim, Jeung Ku Kang, and Jang Wook Choi
Nano Letters 2012 12 (5), 2283-2288</ref> However [[Boron]] doped nanotubes have been shown to make batteries with triple capacity.<ref>[http://www.theregister.co.uk/2013/02/14/doped_nanotubes_lithium_battery/ "Doped nanotubes boost lithium battery power three-fold."]</ref>
 
===Peapod===
A [[Carbon peapod]]<ref>
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===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.<ref>
{{cite journal
|last1=Liu |first1=Q.
|year=2009
|title=Semiconducting properties of cup-stacked carbon nanotubes
|url=http://carbon.imr.ac.cn/file/Journal/2009/1249016438117.pdf
|journal=Carbon
|volume=47 |issue=3 |pages=731–736
|doi=10.1016/j.carbon.2008.11.005
|last2=Ren
|first2=Wencai
|last3=Chen
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|last5=Li
|first5=Feng
|last6=Cong
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|last7=Cheng
|first7=Hui-Ming
}}</ref>
 
===Extreme carbon nanotubes===
[[File:Cycloparaphenylene.PNG|thumb|Cycloparaphenylene]]
 
The observation of the ''longest'' carbon nanotubes (18.5&nbsp;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.<ref name="Longest"/>
 
The ''shortest'' carbon nanotube is the organic compound cycloparaphenylene, which was synthesized in early 2009.<ref>
{{cite web
|date=January 5, 2009
|title=A Better Way to Make Nanotubes
|url=http://newscenter.lbl.gov/feature-stories/2009/01/05/better-way-to-make-nanotubes/
|publisher=[[Lawrence Berkeley National Laboratory]]
}}</ref><ref>
{{cite web
|last1=Bertozzi |first1=C.
|year=2009
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|url=http://www.lbl.gov/msd/assets/docs/highlights/09-Bertozzi.pdf
|publisher=[[Lawrence Berkeley National Laboratory]]
}}</ref>
 
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.<ref name="Smallest_at_MWCNT">
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|last2=Liu
|first2=Y.
|last3=Inoue
|first3=S.
|last4=Suzuki
|first4=T.
|last5=Jones
|first5=R.
|last6=Ando
|first6=Y.
|bibcode = 2004PhRvL..92l5502Z }}</ref>
 
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.<ref name="Smallest">
{{cite journal
|last1=Hayashi |first1=Takuya
|year=2003
|title=Smallest Freestanding Single-Walled Carbon Nanotube
|journal=[[Nano Letters]]
|volume=3 |issue=7 |pages=887–889
|doi=10.1021/nl034080r
|last2=Kim
|first2=Yoong Ahm
|last3=Matoba
|first3=Toshiharu
|last4=Esaka
|first4=Masaya
|last5=Nishimura
|first5=Kunio
|last6=Tsukada
|first6=Takayuki
|last7=Endo
|first7=Morinobu
|last8=Dresselhaus
|first8=Mildred S.
|bibcode = 2003NanoL...3..887H }}</ref> (3,3), (4,3) and (5,1) carbon nanotubes (all about 4 Å in diameter) were unambiguously identified using aberration-corrected [[high-resolution transmission electron microscopy]] inside double-walled CNTs.<ref name="Smallest_at_DWCNT">
{{cite journal
|last1=Guan |first1=L.
|last2=Suenaga |first2=K.
|last3=Iijima |first3=S.
|year=2008
|title=Smallest Carbon Nanotube Assigned with Atomic Resolution Accuracy
|journal=[[Nano Letters]]
|volume=8 |issue=2 |pages=459–462
|doi=10.1021/nl072396j
|pmid=18186659
|bibcode = 2008NanoL...8..459G }}</ref>
 
The ''highest density'' of CNTs was achieved in 2013, grown on a conductive titanium-coated copper surface that was coated with co-catalysts cobalt and molybdenum at lower than typical temperatures of 450 °C. The tubes averaged a height of 0.38 μm and a mass density of 1.6 g cm−3. The material showed ohmic conductivity (lowest resistance ∼22 kΩ).<ref name=kurz>{{cite web|url=http://www.kurzweilai.net/densest-array-of-carbon-nanotubes-grown-to-date |title=Densest array of carbon nanotubes grown to date |publisher=KurzweilAI |date=2013-09-27 |accessdate=2013-10-03}}</ref><ref>{{cite doi|10.1063/1.4818619}}</ref>
 
==Properties==
 
===Strength===
{{See also|Mechanical properties of carbon nanotubes}}
 
Carbon nanotubes are the strongest and stiffest materials yet discovered 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&nbsp;[[Pascal (unit)|gigapascals]] (GPa).<ref name="Strength and Breaking">
{{cite journal
|last=Yu |first=M.-F.
|year=2000
|title=Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load
|journal=Science
|volume=287 |issue=5453 |pages=637–640
|doi=10.1126/science.287.5453.637
|pmid=10649994
|last2=Lourie
|first2=O
|last3=Dyer
|first3=MJ
|last4=Moloni
|first4=K
|last5=Kelly
|first5=TF
|last6=Ruoff
|first6=RS
|bibcode = 2000Sci...287..637Y }}</ref> (For illustration, this translates into the ability to endure tension of a weight equivalent to 6422&nbsp;kg (14,158&nbsp;lbs) on a cable with cross-section of 1&nbsp;mm<sup>2</sup>.) Further studies, such as one conducted in 2008, revealed that individual CNT shells have strengths of up to ~100 GPa, which is in agreement with quantum/atomistic models.<ref name="B. Peng">
{{cite journal
|last1=Peng |first1=B.
|year=2008
|title=Measurements of near-ultimate strength for multiwalled carbon nanotubes and irradiation-induced crosslinking improvements
|journal=[[Nature Nanotechnology]]
|volume=3 |issue= 10|pages=626–631
|doi=10.1038/nnano.2008.211
|pmid=
|last2=Locascio
|first2=Mark
|last3=Zapol
|first3=Peter
|last4=Li
|first4=Shuyou
|last5=Mielke
|first5=Steven L.
|last6=Schatz
|first6=George C.
|last7=Espinosa
|first7=Horacio D.
}}</ref> Since carbon nanotubes have a low density for a solid of 1.3 to 1.4&nbsp;g/cm<sup>3</sup>,<ref name="nanotubes for electronics">
{{cite journal
|last=Collins |first=P.G.
|year=2000
|title=Nanotubes for Electronics
|url=http://www.crhc.uiuc.edu/ece497nc/fall01/papers/NTs_SciAm_2000.pdf
|journal=[[Scientific American]]
|pages=67–69
}}</ref> its [[specific strength]] of up to 48,000&nbsp;kN·m·kg<sup>−1</sup> is the best of known materials, compared to high-carbon steel's 154&nbsp;kN·m·kg<sup>−1</sup>.
 
Under excessive tensile strain, the tubes will undergo [[plastic deformation]], which means the deformation is permanent. This deformation begins at strains of approximately 5% and can increase the maximum strain the tubes undergo before fracture by releasing strain energy.
 
Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes leads to significant reductions in the effective strength of multi-walled carbon nanotubes and carbon nanotube bundles down to only a few GPa’s.<ref name="T. Filleter, R. A 2011">
{{cite journal
|last1=Filleter |first1=T.
|year=2011
|title=Ultrahigh Strength and Stiffness in Cross-Linked Hierarchical Carbon Nanotube Bundles
|journal=[[Advanced Materials]]
|volume= 23|issue= 25|page=2855
|doi=10.1002/adma.201100547
|last2=Bernal
|first2=R.
|last3=Li
|first3=S.
|last4=Espinosa
|first4=H.D.
}}</ref> This limitation has been recently addressed by applying high-energy electron irradiation, which crosslinks inner shells and tubes, and effectively increases the strength of these materials to ~60 GPa for multi-walled carbon nanotubes<ref name="B. Peng"/> and ~17 GPa for double-walled carbon nanotube bundles.<ref name="T. Filleter, R. A 2011"/>
 
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.<ref>
{{cite journal
|last1=Jensen |first1=K.
|year=2007
|title=Buckling and kinking force measurements on individual multiwalled carbon nanotubes
|journal=[[Physical Review B]]
|volume=76 |issue=19 |page=195436
|doi=10.1103/PhysRevB.76.195436
|last2=Mickelson
|first2=W.
|last3=Kis
|first3=A.
|last4=Zettl
|first4=A.
|bibcode = 2007PhRvB..76s5436J }}</ref>
 
{| style="font-size:95%; text-align:center;" class="wikitable"
|+ Comparison of mechanical properties<ref>
{{cite journal
|last1=Belluci |first1=S.
|year=2005
|title=Carbon nanotubes: Physics and applications
|journal=[[Physica Status Solidi (c)]]
|volume=2 |issue=1 |pages=34–47
|doi=10.1002/pssc.200460105
|bibcode = 2005PSSCR...2...34B }}</ref><ref>
{{cite journal
|last1=Chae |first1=H.G.
|last2=Kumar |first2=S.
|year=2006
|title=Rigid Rod Polymeric Fibers
|journal=[[Journal of Applied Polymer Science]]
|volume=100 |issue=1 |pages=791–802
|doi=10.1002/app.22680
}}</ref><ref>
{{cite journal
|last=Meo |first=M.
|last2=Rossi |first2=M.
|year=2006
|title=Prediction of Young's modulus of single wall carbon nanotubes by molecular-mechanics-based finite element modelling
|journal=Composites Science and Technology
|volume=66 |issue=11–12 |pages=1597–1605
|doi=10.1016/j.compscitech.2005.11.015
}}</ref><ref>
{{cite journal
|last1=Sinnott |first1=S.B.
|last2=Andrews |first2=R.
|year=2001
|title=Carbon Nanotubes: Synthesis, Properties, and Applications
|journal=Critical Reviews in Solid State and Materials Sciences
|volume=26 |issue=3 |pages=145–249
|doi=10.1080/20014091104189
|bibcode = 2001CRSSM..26..145S }}</ref>
|-
! Material
! [[Young's modulus]] (TPa)
! [[Tensile strength]] (GPa)
! Elongation at break (%)
|-
| SWNT<sup>E</sup>
| ~1 (from 1 to 5)
| 13–53
| 16
|-
| Armchair SWNT<sup>T</sup>
| 0.94
| 126.2
| 23.1
|-
| Zigzag SWNT<sup>T</sup>
| 0.94
| 94.5
| 15.6–17.5
|-
| Chiral SWNT
| 0.92
|
|
|-
| MWNT<sup>E</sup>
| 0.2<ref name="Strength and Breaking"/>–0.8<ref name="Demczyk">
{{cite journal
|last1=Demczyk |first1=B.G.
|year=2002
|title=Direct mechanical measurement of the tensile strength and elastic modulus of multiwalled carbon nanotubes
|journal=Materials Science and Engineering A
|volume=334 |issue=1–2 |pages=173–178
|doi=10.1016/S0921-5093(01)01807-X
|last2=Wang
|first2=Y.M
|last3=Cumings
|first3=J
|last4=Hetman
|first4=M
|last5=Han
|first5=W
|last6=Zettl
|first6=A
|last7=Ritchie
|first7=R.O
}}</ref>–0.95<ref name="Strength and Breaking"/>
| 11<ref name="Strength and Breaking"/>–63<ref name="Strength and Breaking"/>–150<ref name="Demczyk"/>
|
|-
| [[Stainless steel]]<sup>E</sup>
| 0.186<ref name="ASSDAStainlessSteel">
{{cite web
|url=http://www.assda.asn.au/index.php?option=com_content&task=view&id=61&Itemid=87
|title=Properties of Stainless Steel
|publisher=Australian Stainless Steel Development Association
}}</ref>–0.214<ref name="GoodfellowStainlessSteel-17-7PH">
{{cite web
|title=Stainless Steel – 17-7PH (Fe/Cr17/Ni 7) Material Information
|url=http://www.goodfellow.com/E/StainlessSteel-17-7PH.html
}}</ref>
| 0.38<ref name="ASSDAStainlessSteel"/>–1.55<ref name="GoodfellowStainlessSteel-17-7PH"/><!-- 3 ??? Can anyone find a direct/specific reference for such a high value for stainless steel?-->
| 15–50
|-
| [[Kevlar]]–29&149<sup>E</sup>
| 0.06–0.18<ref name="Kevlar">
{{cite book
|last1=Wagner |first1=H.D.
|year=2002
|chapter=Reinforcement
|title=Encyclopedia of Polymer Science and Technology
|publisher=[[John Wiley & Sons]]
|doi=10.1002/0471440264.pst317
}}</ref>
| 3.6–3.8<ref name="Kevlar"/>
| ~2
|}
 
<sup>E</sup>Experimental observation; <sup>T</sup>Theoretical prediction
 
The above discussion referred to axial properties of the nanotube, whereas simple geometrical considerations suggest that carbon nanotubes should be much softer in the radial direction than along the tube axis. Indeed, [[Transmission electron microscopy|TEM]] observation of radial elasticity suggested that even the van der Waals forces can deform two adjacent nanotubes.<ref>
{{cite journal
|last1=Ruoff |first1=R.S.
|year=1993
|title=Radial deformation of carbon nanotubes by van der Waals forces
|journal=Nature
|volume=364 |issue=6437 |page=514
|doi=10.1038/364514a0
|last2=Tersoff
|first2=J.
|last3=Lorents
|first3=Donald C.
|last4=Subramoney
|first4=Shekhar
|last5=Chan
|first5=Bryan
|bibcode = 1993Natur.364..514R }}</ref> Nanoindentation experiments, performed by several groups on multiwalled carbon nanotubes<ref>
{{cite journal
|last1=Palaci |first1=I.
|year=2005
|title=Radial Elasticity of Multiwalled Carbon Nanotubes
|journal=Phys. Rev. Lett.
|volume=94 |issue=17 |page=175502
|doi=10.1103/PhysRevLett.94.175502
|last2=Fedrigo
|first2=S.
|last3=Brune
|first3=H.
|last4=Klinke
|first4=C.
|last5=Chen
|first5=M.
|last6=Riedo
|first6=E.
|arxiv = 1201.5501
|bibcode = 2005PhRvL..94q5502P }}</ref><ref>
{{cite journal
|last1=Yu |first1=M.-F.
|year=2000
|title=Investigation of the Radial Deformability of Individual Carbon Nanotubes under Controlled Indentation Force
|journal=Phys. Rev. Lett.
|volume=85 |issue= 7|pages=1456–1459
|doi=10.1103/PhysRevLett.85.1456
|pmid=10970528
|last2=Kowalewski
|first2=Tomasz
|last3=Ruoff
|first3=Rodney
|bibcode = 2000PhRvL..85.1456Y }}</ref> and tapping/contact mode [[atomic force microscope]] measurement performed on single-walled carbon nanotube,<ref>
{{cite journal
|last1=Yang |first1=Y.H.
|year=2011
|title=Radial elasticity of single-walled carbon nanotube measured by atomic force microscopy
|journal=[[Applied Physics Letters]]
|volume=98 |page=041901
|doi=10.1063/ApplPhysLett.98.041901
}}</ref> indicated Young's modulus of the order of several GPa confirming that CNTs are indeed rather soft in the radial direction.
 
===Hardness===
Standard single-walled carbon nanotubes can withstand a pressure up to 25 GPa without deformation. They then undergo a transformation to superhard phase nanotubes. Maximum pressures measured using current experimental techniques are around 55 GPa. 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).<ref>
{{cite journal
|last1=Popov |first1=M.
|year=2002
|title=Superhard phase composed of single-wall carbon nanotubes
|url=http://www.ssl.physics.ncsu.edu/publication/browse/getFileAction?fileref=2003-02-27+12:53:01&dbfilename=2002-PRB65-033408.pdf
|journal=Phys. Rev. B
|volume=65 |issue=3 |page=033408
|doi=10.1103/PhysRevB.65.033408
|last2=Kyotani
|first2=M.
|last3=Nemanich
|first3=R.
|last4=Koga
|first4=Y.
|bibcode = 2002PhRvB..65c3408P }}</ref>
 
===Kinetic properties===
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 [[Synthetic molecular motors|motor]].<ref>
{{cite press
|last1=Sanders |first1=R.
|date=March 23, 2003
|title=Physicists build world's smallest motor using nanotubes and etched silicon
|url=http://www.berkeley.edu/news/media/releases/2003/07/23_motor.shtml
|publisher=[[UC Berkeley]]
}}</ref> Future applications such as a gigahertz mechanical oscillator are also envisioned.
 
===Electrical properties===
[[File:Carbon nanotube bands.gif|thumb|Band structures computed using tight binding approximation for (6,0) CNT (zigzag, metallic) (10,2) CNT (semiconducting) and (10,10) CNT (armchair, metallic).]]
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'', the nanotube is metallic; if ''n'' − ''m'' is a multiple of 3, then the nanotube is semiconducting with a very small band gap, otherwise the nanotube is a moderate [[semiconductor]]. Thus all armchair (''n'' = ''m'') nanotubes are metallic, and nanotubes (6,4), (9,1), etc. are semiconducting.<ref name="Curvature">{{cite journal|first=X.|last=Lu|first2=Z.|title=Curved Pi-Conjugation, Aromaticity, and the Related Chemistry of Small Fullerenes (C<sub>60</sub>) and Single-Walled Carbon Nanotubes|journal=[[Chemical Reviews]]|volume=105|pages=3643–3696|year=2005|doi=10.1021/cr030093d|issue=10|last2=Chen|pmid=16218563}}</ref>
 
However, this rule has exceptions, because curvature effects in small diameter carbon nanotubes can strongly influence electrical properties. Thus, a (5,0) SWCNT that should be semiconducting in fact is metallic according to the calculations. Likewise, ''vice versa''—zigzag and chiral SWCNTs with small diameters that should be metallic have finite gap (armchair nanotubes remain metallic).<ref name="Curvature"/> In theory, metallic nanotubes can carry an electric current density of 4 × 10<sup>9</sup> A/cm<sup>2</sup>, which is more than 1,000 times greater than those of metals such as [[copper]],<ref>{{Cite journal|first=Seunghun|last=Hong|year=2007|title=Nanotube Electronics: A flexible approach to mobility|journal=Nature Nanotechnology|volume=2|pages=207–208|doi=10.1038/nnano.2007.89|pmid=18654263|last2=Myung|first2=S|issue=4|bibcode = 2007NatNa...2..207H }}</ref> where for copper interconnects current densities are limited by [[electromigration]].
 
Because of their nanoscale cross-section, electrons propagate only along the tube's axis and electron transport involves quantum effects. As a result, carbon nanotubes are frequently referred to as one-dimensional conductors. The maximum [[electrical conductance]] of a single-walled carbon nanotube is 2''G''<sub>0</sub>, where ''G''<sub>0</sub> = 2''e''<sup>2</sup>/''h'' is the [[Conductance quantum|conductance of a single ballistic quantum channel]].<ref>J.-C. Charlier, X. Blase, and S. Roche, "Electronic and transport properties of nanotubes," ''Rev. Mod. Phys.'' '''79''', 677-732 (2007), {{doi|10.1103/RevModPhys.79.677}}</ref>
 
There have been reports of intrinsic [[superconductivity]] in carbon nanotubes.<ref>{{cite journal |journal = Science|volume = 292|year = 2001| doi = 10.1126/science.1060470| title = Superconductivity in 4 Angstrom Single-Walled Carbon Nanotubes|pmid=11431560|issue=5526 |last1 = Tang |first1 = Z. K. |last2 = Zhang |first2 = L |last3 = Wang |first3 = N |last4 = Zhang |first4 = XX |last5 = Wen |first5 = GH |last6 = Li |first6 = GD |last7 = Wang |first7 = JN |last8 = Chan |first8 = CT |last9 = Sheng |first9 = P |pages = 2462–5|bibcode = 2001Sci...292.2462T }}</ref><ref>{{cite journal|title=Superconductivity in Entirely End-Bonded Multiwalled Carbon Nanotubes|journal=Phys. Rev. Lett.|volume=96|page=057001|year=2006|doi=10.1103/PhysRevLett.96.057001|url=http://www.ee.aoyama.ac.jp/Labs/j-haru-www/paper/Haruyama%20SuperCNT%20PRL%20publication.pdf |pmid=16486971|issue=5|arxiv = cond-mat/0509466|last1=Takesue|first1=I.|last2=Haruyama|first2=J.|last3=Kobayashi|first3=N.|last4=Chiashi|first4=S.|last5=Maruyama|first5=S.|last6=Sugai|first6=T.|last7=Shinohara|first7=H.|bibcode = 2006PhRvL..96e7001T }}</ref><ref>{{cite journal |journal = Proceedings of the National Academy of Sciences|volume = 106|year = 2009| doi = 10.1073/pnas.0813162106| title =Superconducting characteristics of 4-A carbon nanotube–zeolite composite|url=http://www.pnas.org/content/106/18/7299.short|issue=18 |last1 = Lortz |first1 = R. |last2 = Zhang |first2 = Q |last3 = Shi |first3 = W |last4 = Ye |first4 = J. T. |last5 = Qiu |first5 = C. Y. |last6 = Wang |first6 = Z. |last7 = He |first7 = H. T. |last8 = Sheng |first8 = P |last9 = Qian |first9 = T. Z. |last10 = Tang |first10 = Z. K. |last11 = Wang |first11 = N. |last12 = Zhang |first12 = X. X. |last13 = Wang |first13 = J |last14 = Chan |first14 = C. T. |pages = 7299–7303|bibcode = 2009PNAS..106.7299L }}</ref> Many other experiments, however, found no evidence of superconductivity, and the validity of these claims of intrinsic superconductivity remains a subject of debate.<ref>{{cite journal| author = M. Bockrath|journal = Nature Physics|volume = 2|year = 2006|page = 155|doi = 10.1038/nphys252|title = Carbon nanotubes: The weakest link| issue = 3|bibcode = 2006NatPh...2..155B }}</ref>
 
===Optical properties===
{{Main|Optical properties of carbon nanotubes}}
 
===Thermal properties===
{{Main|Thermal properties of nanostructures}}
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. Measurements show that a SWNT has a room-temperature thermal conductivity along its axis of about 3500 W·m<sup>−1</sup>·K<sup>−1</sup>;<ref name="Pop">
{{cite journal
|title = Thermal conductance of an individual single-wall carbon nanotube above room temperature
|journal=Nano Letters
|date=2005-12-22
|volume=6
|issue=1
|pages= 96–100
|doi =10.1021/nl052145f
|pmid = 16402794|arxiv = cond-mat/0512624
|last1 = Pop
|first1 = Eric
|last2 = Mann
|first2 = David
|last3 = Wang
|first3 = Qian
|last4 = Goodson
|first4 = Kenneth
|last5 = Dai
|first5 = Hongjie |bibcode = 2006NanoL...6...96P }}</ref> compare this to copper, a metal well known for its good [[thermal conductivity]], which transmits 385 W·m<sup>−1</sup>·K<sup>−1</sup>. A SWNT has a room-temperature thermal conductivity across its axis (in the radial direction) of about 1.52 W·m<sup>−1</sup>·K<sup>−1</sup>,<ref name="Sinha">
{{cite journal
|title = Off-axis thermal properties of carbon nanotube films
|journal=Journal of Nanoparticle Research
|date=2005-06-05
|volume=7
|issue=6
|pages= 651–657
|doi =10.1007/s11051-005-8382-9
|last1 = Sinha
|first1 = Saion
|last2 = Barjami
|first2 = Saimir
|last3 = Iannacchione
|first3 = Germano
|last4 = Schwab
|first4 = Alexander
|last5 = Muench
|first5 = George
}}</ref> which is about as thermally conductive as soil. The temperature stability of carbon nanotubes is estimated to be up to 2800&nbsp;°C in [[vacuum]] and about 750&nbsp;°C in air.<ref>{{Cite journal|first=Erik|last=Thostenson|year=2005|title=Nanocomposites in context|journal=Composites Science and Technology|volume=65|pages=491–516|doi=10.1016/j.compscitech.2004.11.003|last2=Li|first2=C|last3=Chou|first3=T|issue=3–4}}</ref>
 
===Defects===
As with any material, the existence of a [[crystallographic defect]] affects the material properties. Defects can occur in the form of atomic [[Vacancy defect|vacancies]]. High levels of such defects can lower the tensile strength by up to 85%. An important example is 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 its weakest segment in a similar manner to a chain, where the strength of the weakest link becomes the maximum strength of the chain.
 
Crystallographic defects also affect the tube's electrical properties. A common result is lowered conductivity through the defective region of the tube. A defect in armchair-type tubes (which can conduct electricity) can cause the surrounding region to become semiconducting, and single monatomic vacancies induce magnetic properties.<ref>''Carbon-Based Magnetism: An Overview of the Magnetism of Metal Free Carbon-based Compounds and Materials'', Tatiana Makarova and Fernando Palacio (eds.), Elsevier, 2006</ref>
 
Crystallographic defects strongly affect the tube's thermal properties. 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. Phonon transport simulations indicate that substitutional defects such as nitrogen or boron will primarily lead to scattering of high-frequency optical phonons. However, larger-scale defects such as [[Stone Wales defect]]s cause phonon scattering over a wide range of frequencies, leading to a greater reduction in thermal conductivity.<ref>{{Cite journal|first=N.|last=Mingo|year=2008|title=Phonon transmission through defects in carbon nanotubes from first principles|journal=Phys. Rev. B|volume=77|page=033418|doi=10.1103/PhysRevB.77.033418|last2=Stewart|first2=D. A.|last3=Broido|first3=D. A.|last4=Srivastava|first4=D.|issue=3|bibcode = 2008PhRvB..77c3418M }}</ref>
 
===Toxicity===
{{see also|Fullerene#Safety and toxicity}}
The toxicity of carbon nanotubes has been an important question in nanotechnology. As of 2007, 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 [[Frequency distribution|distribution]], [[surface area]], surface chemistry, [[surface charge]], and [[agglomeration]] state as well as purity of the samples, have considerable impact on the [[Reactivity (chemistry)|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.<ref name=tox1>{{cite journal |author=Kolosnjaj J, Szwarc H, Moussa F |title=Toxicity studies of carbon nanotubes |journal=Adv Exp Med Biol. |volume=620 |pages=181–204 |year=2007 |pmid=18217344 |doi=10.1007/978-0-387-76713-0_14 |series=Advances in Experimental Medicine and Biology |isbn=978-0-387-76712-3}}</ref>
 
Under certain conditions CNTs can enter human cells and accumulate in the [[cytoplasm]], causing cell death.<ref name="tox2">{{Cite journal|last=Porter|first=Alexandra|title=Direct imaging of single-walled carbon nanotubes in cells|journal=Nature Nanotechnology|year=2007|volume=2|doi=10.1038/nnano.2007.347|last2=Gass|first2=Mhairi|last3=Muller|first3=Karin|last4=Skepper|first4=Jeremy N.|last5=Midgley|first5=Paul A.|last6=Welland|first6=Mark|pmid=18654411|issue=11|pages=713–7|bibcode = 2007NatNa...2..713P }}</ref>
 
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]], [[Granuloma|epithelioid granulomas]] (microscopic nodules), [[fibrosis]], and biochemical/toxicological changes in the lungs.<ref>Zumwalde, Ralph and Laura Hodson (March 2009). [http://www.cdc.gov/niosh/docs/2009-125/ "Approaches to Safe Nanotechnology: Managing the Health and Safety Concerns Associated with Engineered Nanomaterials"]. National Institute for Occupational Safety and Health. NIOSH (DHHS) Publication 2009-125.</ref> 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.<ref name=tox3>{{cite journal |author=Lam CW, James JT, McCluskey R, Arepalli S, Hunter RL |title=A review of carbon nanotube toxicity and assessment of potential occupational and environmental health risks |journal=Crit Rev Toxicol.|volume=36|pages=189–217 |year=2006 |pmid=16686422 |doi=10.1080/10408440600570233 |issue=3}}</ref>
 
Carbon nano tubes deposit in the [[alveolar ducts]] by aligning length wise with the airways; the nano tubes will often combine with metals.<ref>{{cite journal|url=http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2322933/|title=The significance of nano particles in particle-induced pulmonary fibrosis|author1=James D Byrne|author2=John A Baugh|journal=McGill Journal of Medicine|year=2008|volume=11|pages=43–50}}</ref> The needle-like fiber shape of CNTs is similar to [[asbestos fibers]]. This raises the idea that widespread use of carbon nanotubes may lead to [[pleural]] mesothelioma, a cancer of the lining of the lungs or [[peritoneal mesothelioma]], a cancer of the lining of the abdomen (both caused by exposure to asbestos). A recently published pilot study supports this prediction.<ref name="tox5"/> Scientists exposed the [[mesothelial]] lining of the body cavity of mice to long multiwalled carbon nanotubes and observed asbestos-like, length-dependent, pathogenic behavior that included inflammation and formation of lesions known as [[granulomas]].
Authors of the study conclude:
{{quote|This is of considerable importance, because research and business communities continue to invest heavily in carbon nanotubes for a wide range of products under the assumption that they are no more hazardous than graphite. Our results suggest the need for further research and great caution before introducing such products into the market if long-term harm is to be avoided.<ref name="tox5">{{Cite journal|author=Poland, CA|title=Carbon nanotubes introduced into the abdominal cavity of mice show asbestos-like pathogenicity in a pilot study|journal=Nature Nanotechnology|year=2008|volume=3|doi=10.1038/nnano.2008.111|pmid=18654567|issue=7|last2=Duffin|first2=Rodger|last3=Kinloch|first3=Ian|last4=Maynard|first4=Andrew|last5=Wallace|first5=William A. H.|last6=Seaton|first6=Anthony|last7=Stone|first7=Vicki|last8=Brown|first8=Simon|last9=MacNee|first9=William|pages=423–8}}</ref>}}
 
Although further research is required, the available data suggests that under certain conditions, especially those involving chronic exposure, carbon nanotubes can pose a serious risk to human health.<ref name=tox1/><ref name=tox2/><ref name=tox3/><ref name=tox5/>
 
==Synthesis==
[[File:Mutr-nanotubes1.jpg|thumb|Powder of carbon nanotubes]]
Techniques have been developed to produce nanotubes in sizable quantities, including arc discharge, laser ablation, high-pressure carbon monoxide disproportionation, and [[chemical vapor deposition]] (CVD). Most of these processes take place in vacuum or with process gases. CVD growth of CNTs can occur in vacuum or at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth are making CNTs more commercially viable.
 
===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>{{Cite journal|first=Sumio|last=Iijima|year=1991|title=Helical microtubules of graphitic carbon|journal=Nature|volume=354|pages=56–58|doi=10.1038/354056a0|issue=6348|bibcode = 1991Natur.354...56I }}</ref> However the first [[macroscopic]] production of carbon nanotubes was made in 1992 by two researchers at [[NEC]]'s Fundamental Research Laboratory.<ref>{{Cite journal|first=T. W.|last=Ebbesen|year=1992|title=Large-scale synthesis of carbon nanotubes|journal=Nature|volume=358|pages=220–222|doi=10.1038/358220a0|last2=Ajayan|first2=P. M.|issue=6383|bibcode = 1992Natur.358..220E }}</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-discharge temperatures. 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% by weight and it produces both single- and multi-walled nanotubes with lengths of up to 50 micrometers with few structural defects.<ref name="nanotubes for electronics" />
 
===Laser ablation===
In laser ablation, a [[pulsed laser]] vaporizes a graphite target in a high-temperature reactor while an [[inert gas]] is bled into the chamber. 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.
 
This process was developed by Dr. [[Richard Smalley]] and co-workers at [[Rice University]], who at the time of the discovery of carbon nanotubes, were blasting metals with a laser to produce various metal molecules. When they heard of the existence of nanotubes they replaced the metals with graphite to create multi-walled carbon nanotubes.<ref>{{Cite journal|last=Guo|first=Ting|title=Self-Assembly of Tubular Fullerenes|url=http://www.pa.msu.edu/cmp/csc/eprint/DT085.pdf|year=1995|journal=J. Phys. Chem.|volume=99|pages=10694–10697|doi=10.1021/j100027a002|last2=Nikolaev|first2=Pavel|last3=Rinzler|first3=Andrew G.|last4=Tomanek|first4=David|last5=Colbert|first5=Daniel T.|last6=Smalley|first6=Richard E.|issue=27}}</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 synthesize single-walled carbon nanotubes.<ref>{{Cite journal|last=Guo|first=Ting|title=Catalytic growth of single-walled nanotubes by laser vaporization|journal=Chem. Phys. Lett.|year=1995|volume=243|pages=49–54|url=http://www.orgchem.science.ru.nl/molmat/mm-web/education/caput-college/ChemPhysLett-1995-243-49.pdf |format=PDF| doi=10.1016/0009-2614(95)00825-O|last2=Nikolaev|first2=P|last3=Thess|first3=A|last4=Colbert|first4=D|last5=Smalley|first5=R|bibcode = 1995CPL...243...49B }}</ref>
 
The laser ablation method yields 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 name="nanotubes for electronics" />
 
===Plasma torch===
Single-walled carbon nanotubes can be synthesized by the induction thermal plasma method, discovered in 2005 by groups from the University of Sherbrooke and the National Research Council of Canada.<ref>{{Cite journal|last=Kim|first=K.S.|title=Large-scale production of single-wall carbon nanotubes by induction thermal plasma|year=2007|journal= Journal of Physics D: Applied Physics|volume=40|page=2375|last2=Cota-Sanchez|first2=German|last3=Kingston|first3=Chris|last4=Imris|first4=M.|last5=Simard|first5=Benoît|last6=Soucy|first6=Gervais|bibcode = 2007JPhD...40.2375K |doi = 10.1088/0022-3727/40/8/S17 }}</ref> The method is similar to arc-discharge in that both use ionized gas to reach the high temperature necessary to vaporize carbon-containing substances and the metal catalysts necessary for the ensuing nanotube growth. The thermal plasma is induced by high frequency oscillating currents in a coil, and is maintained in flowing inert gas. Typically, a feedstock of carbon black and metal catalyst particles is fed into the plasma, and then cooled down to form single-walled carbon nanotubes. Different single-wall carbon nanotube diameter distributions can be synthesized.
 
The induction thermal plasma method can produce up to 2 grams of nanotube material per minute, which is higher than the arc-discharge or the laser ablation methods.
 
===Chemical vapor deposition (CVD)===
[[File:PICT0111.JPG|thumb|Nanotubes being grown by plasma enhanced [[chemical vapor deposition]]]]
The catalytic vapor phase deposition of carbon was reported in 1952<ref name="CVD">{{cite journal|last=Радушкевич|first=Л. В.|year=1952|title=О Структуре Углерода, Образующегося При Термическом Разложении Окиси Углерода На Железном Контакте|journal=Журнал Физической Химии|volume=26|pages=88–95|url=http://nanotube.msu.edu/HSS/2006/4/2006-4.pdf|format=PDF|language=Russian}}</ref> and 1959,<ref>{{Cite journal|first=P. L.|last=Walker Jr.|journal=J. Phys. Chem.|volume=63|page=133|year=1959|title=Carbon Formation from Carbon Monoxide-Hydrogen Mixtures over Iron Catalysts. I. Properties of Carbon Formed|doi=10.1021/j150572a002|last2=Rakszawski|first2=J. F.|last3=Imperial|first3=G. R.|issue=2}}</ref> but it was not until 1993<ref>{{Cite journal|first=M.|last=José-Yacamán|title=Catalytic growth of carbon microtubules with fullerene structure|journal=Appl. Phys. Lett.|volume=62|page=657|year=1993|doi=10.1063/1.108857|last2=Miki-Yoshida|first2=M.|last3=Rendón|first3=L.|last4=Santiesteban|first4=J. G.|issue=6|bibcode = 1993ApPhL..62..657J }}</ref> that carbon nanotubes were formed by this process. In 2007, researchers at the [[University of Cincinnati]] (UC) developed a process to grow aligned carbon nanotube arrays of 18&nbsp;mm length on a FirstNano ET3000 carbon nanotube growth system.<ref>{{Cite news|title=UC Researchers Shatter World Records with Length of Carbon Nanotube Arrays|date=2007-04-27|last=Beckman|first=Wendy|publisher=University of Cincinnati|url=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,<ref>
{{cite journal
|title=Synthesis-condition dependence of carbon nanotube growth by alcohol catalytic chemical vapor deposition method
|url=http://iopscience.iop.org/1468-6996/8/4/A11/pdf/1468-6996_8_4_A11.pdf
|journal=Sci. Technol. Adv. Mater.
|volume=8
|year=2007
|page=292
|doi=10.1016/j.stam.2007.02.009
|format=PDF
|last1=Inami
|first1=Nobuhito
|last2=Ambri Mohamed
|first2=Mohd
|last3=Shikoh
|first3=Eiji
|last4=Fujiwara
|first4=Akihiko
|issue=4
|accessdate=2013-03-27
|bibcode = 2007STAdM...8..292I }}</ref> [[iron]], or a combination.<ref>{{cite journal|author=N. Ishigami |title=Crystal Plane Dependent Growth of Aligned Single-Walled Carbon Nanotubes on Sapphire|doi=10.1021/ja8024752|journal=J. Am. Chem. Soc.
|volume=130|pages=9918–9924|year=2008|pmid=18597459|last2=Ago|first2=H|last3=Imamoto|first3=K|last4=Tsuji|first4=M|last5=Iakoubovskii|first5=K|last6=Minami|first6=N|issue=30}}</ref> 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]] or [[hydrogen]]) and a carbon-containing gas (such as [[acetylene]], [[ethylene]], [[ethanol]] or [[methane]]). 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 being studied.<ref>{{cite journal|author=Naha, Sayangdev, and Ishwar K. Puri|title=A model for catalytic growth of carbon nanotubes|journal=Journal of Physics D: Applied Physics|volume= 41|year=2008|doi=10.1088/0022-3727/41/6/065304|page= 065304|bibcode = 2008JPhD...41f5304N }}</ref> The catalyst particles can stay at the tips of the growing nanotube during growth, or remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate.<ref>{{cite journal|author=Banerjee, Soumik, Naha, Sayangdev, and Ishwar K. Puri|title=Molecular simulation of the carbon nanotube growth mode during catalytic synthesis|journal=Applied Physics Letters|volume= 92|year=2008|doi=10.1063/1.2945798|page= 233121|bibcode = 2008ApPhL..92w3121B }}</ref> Thermal catalytic decomposition of hydrocarbon has become an active area of research and can be a promising route for the bulk production of CNTs. Fluidised bed reactor is the most widely used reactor for CNT preparation. Scale-up of the reactor is the major challenge.<ref>{{cite journal|last1 = Pinilla|first1 = JL|last2 = Moliner|first2 = R| year = 2007|last3 = Suelves|first3 = I|last4 = Lazaro|first4 = M|last5 = Echegoyen|first5 = Y|last6 = Palacios|first6 = J|title = Production of hydrogen and carbon nanofibers by thermal decomposition of methane using metal catalysts in a fluidized bed reactor|journal = International Journal of Hydrogen Energy|volume = 32|issue = 18| page = 4821|doi = 10.1016/j.ijhydene.2007.08.013}}</ref><ref>{{cite journal|last1 = Muradov|first1 = N|year = 2001|title = Hydrogen via methane decomposition: an application for decarbonization of fossil fuels|journal = International Journal of Hydrogen Energy|volume = 26|issue = 11|pages = 1165–1175| doi= 10.1016/S0360-3199(01)00073-8}}</ref>
 
CVD is a common method for the commercial production of carbon nanotubes. For this purpose, the metal nanoparticles are mixed with a catalyst support such as MgO or Al<sub>2</sub>O<sub>3</sub> to increase the 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 proven effective for nanotube growth.<ref>{{Cite journal|first=A.|last=Eftekhari|title=High-yield synthesis of carbon nanotubes using a water-soluble catalyst support in catalytic chemical vapor deposition|doi=10.1016/j.carbon.2005.12.006|journal=Carbon|volume=44|page=1343|year=2006|last2=Jafarkhani|first2=P|last3=Moztarzadeh|first3=F|issue=7}}</ref>
 
If a [[plasma (physics)|plasma]] is generated by the application of a strong electric field during growth (plasma-enhanced chemical vapor deposition), then the nanotube growth will follow the direction of the electric field.<ref>{{Cite journal| title=Synthesis of Large Arrays of Well-Aligned Carbon Nanotubes on Glass|journal=Science|volume=282|year=1998|doi=10.1126/science.282.5391.1105|pmid=9804545|issue=5391|pages=1105–7| last1=Ren| first1=Z. F.| last2=Huang| first2=ZP| last3=Xu| first3=JW| last4=Wang| first4=JH| last5=Bush| first5=P| last6=Siegal| first6=MP| last7=Provencio| first7=PN|bibcode = 1998Sci...282.1105R }}</ref> By adjusting the geometry of the reactor it is possible to synthesize vertically aligned carbon nanotubes<ref>[http://www.nano-lab.com/imagegallery.html SEM images & TEM images of carbon nanotubes, aligned carbon nanotube arrays, and nanoparticles]. Nano-lab.com. Retrieved on 2012-06-06.</ref> (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. 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, because of its price/unit ratio, and because 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.<ref>{{Cite journal|title=Synthesis and field emission properties of vertically aligned carbon nanotube arrays on copper|journal=Carbon|volume=50|year=2012|doi=10.1016/j.carbon.2012.02.024|issue=7|pages=2641–50| last1=Neupane| first1=Suman| last2=Lastres| first2=Mauricio| last3=Chiarella| first3=M| last4=Li| first4=W.Z.| last5=Su| first5=Q| last6=Du| first6=G.H.}}</ref> In 2007, a team from [[Meijo University]] demonstrated a high-efficiency CVD technique for growing carbon nanotubes from [[camphor]].<ref>{{Cite journal|title=Carbon Nanotubes from Camphor: An Environment-Friendly Nanotechnology|journal=Journal of Physics: Conference Series|volume=61|year=2007| page=643|url=http://www.iop.org/EJ/article/1742-6596/61/1/129/jpconf7_61_129.pdf|doi = 10.1088/1742-6596/61/1/129|last1=Kumar|first1=Mukul|last2=Ando|first2=Yoshinori|bibcode = 2007JPhCS..61..643K }}</ref> Researchers at Rice University, until recently led by the late [[Richard Smalley]], have 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.<ref name="SmalleyLi2006">{{cite journal|last1=Smalley|first1=Richard E.|last2=Li|first2=Yubao|last3=Moore|first3=Valerie C.|last4=Price|first4=B. Katherine|last5=Colorado|first5=Ramon|last6=Schmidt|first6=Howard K.|last7=Hauge|first7=Robert H.|last8=Barron|first8=Andrew R.|last9=Tour|first9=James M.|title=Single Wall Carbon Nanotube Amplification:  En Route to a Type-Specific Growth Mechanism|journal=Journal of the American Chemical Society|volume=128|issue=49|year=2006|pages=15824–15829|issn=0002-7863|doi=10.1021/ja065767r}}</ref>
 
====Super-growth CVD====
Super-growth CVD (water-assisted chemical vapor deposition) was developed by Kenji Hata, [[Sumio Iijima]] and co-workers at [[National Institute of Advanced Industrial Science and Technology|AIST]], Japan.<ref>
{{cite journal
|title=Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes
|journal=Science
|volume=306
|pages=1362–1365
|year=2004
|doi=10.1126/science.1104962 |pmid=15550668 |issue=5700
|last1=Hata
|first1=K.
|last2=Futaba
|first2=DN
|last3=Mizuno
|first3=K
|last4=Namai
|first4=T
|last5=Yumura
|first5=M
|last6=Iijima
|first6=S
|bibcode = 2004Sci...306.1362H }}</ref> In this process, the activity and lifetime of the catalyst are enhanced by addition of water into the CVD reactor. Dense millimeter-tall nanotube "forests", aligned normal to the substrate, were produced. The forests growth rate could be expressed, as
 
: <math> H(t) = {\beta}{\tau}_o ({1 - e^{-t / {\tau}_o}}) .</math>
 
In this equation, β is the initial growth rate and <math> {\tau}_o </math> is the characteristic catalyst lifetime.<ref>
{{cite journal
|title=Kinetics of Water-Assisted Single-Walled Carbon Nanotube Synthesis Revealed by a Time-Evolution Analysis
|journal=Phys. Rev. Lett.
|volume=95
|page=056104
|year=2005
|doi=10.1103/PhysRevLett.95.056104 |issue=5
|last1=Futaba
|first1=Don
|last2=Hata
|first2=Kenji
|last3=Yamada
|first3=Takeo
|last4=Mizuno
|first4=Kohei
|last5=Yumura
|first5=Motoo
|last6=Iijima
|first6=Sumio
|bibcode = 2005PhRvL..95e6104F }}</ref>
 
Their specific surface exceeds 1,000 m<sup>2</sup>/g (capped) or 2,200 m<sup>2</sup>/g (uncapped),<ref>{{cite journal
|title=Compact and light supercapacitors from a surface-only solid by opened carbon nanotubes with 2,200 m<sup>2</sup>/g
|journal=[[Advanced Functional Materials]]
|volume=20
|pages=422–428
|year=2009
|doi=10.1002/adfm.200901927 |issue=3
|last1=Hiraoka
|first1=Tatsuki
|last2=Izadi-Najafabadi
|first2=Ali
|last3=Yamada
|first3=Takeo
|last4=Futaba
|first4=Don N.
|last5=Yasuda
|first5=Satoshi
|last6=Tanaike
|first6=Osamu
|last7=Hatori
|first7=Hiroaki
|last8=Yumura
|first8=Motoo
|last9=Iijima
|first9=Sumio
}}</ref> surpassing the value of 400–1,000 m<sup>2</sup>/g for HiPco samples. The synthesis efficiency is about 100 times higher than for the [[laser ablation]] method. The time required to make SWNT forests of the height of 2.5&nbsp;mm by this method was 10 minutes in 2004. Those SWNT forests can be easily separated from the catalyst, yielding clean SWNT material (purity >99.98%) without further purification. For comparison, the as-grown HiPco CNTs contain about 5–35%<ref>{{cite web|title=Unidym product sheet SWNT|url=http://www.unidym.com/files/Unidym_Product_Sheet_SWNT.pdf}}</ref> of metal impurities; it is therefore purified through dispersion and centrifugation that damages the nanotubes.  Super-growth avoids this problem. Patterned highly organized single-walled nanotube structures were successfully fabricated using the super-growth technique.
 
The [[mass density]] of super-growth CNTs is about 0.037 g/cm<sup>3</sup>.<ref>{{cite web|title=Characteristic of Carbon nanotubes by super-growth method|language=japanese|url=http://www.nanocarbon.jp/sg/002.html}}</ref><ref name="K.Hata">{{cite web|author=K.Hata|title=From Highly Efficient Impurity-Free CNT Synthesis to DWNT forests, CNTsolids and Super-Capacitors|url=http://www.nanocarbon.jp/english/research/image/review.pdf}}</ref> It is much lower than that of conventional CNT powders (~1.34 g/cm<sup>3</sup>), probably because the latter contain metals and [[amorphous carbon]].
 
The super-growth method is basically a variation of CVD. Therefore, it is possible to grow material containing SWNT, DWNTs and MWNTs, and to alter their ratios by tuning the growth conditions.<ref>
{{cite journal
|title=Size-selective growth of double-walled carbon nanotube forests from engineered iron catalysts
|journal=[[Nature Nanotechnology]]
|volume=1
|pages=131–136
|year=2006
|doi=10.1038/nnano.2006.95 |pmid=18654165 |issue=2
|last1=Yamada
|first1=Takeo
|last2=Namai
|first2=Tatsunori
|last3=Hata
|first3=Kenji
|last4=Futaba
|first4=Don N.
|last5=Mizuno
|first5=Kohei
|last6=Fan
|first6=Jing
|last7=Yudasaka
|first7=Masako
|last8=Yumura
|first8=Motoo
|last9=Iijima
|first9=Sumio
|bibcode = 2006NatNa...1..131Y }}</ref> Their ratios change by the thinness of the catalyst. Many MWNTs are included so that the diameter of the tube is wide.<ref name="K.Hata"/>
 
The vertically aligned nanotube forests originate from a "zipping effect" when they are immersed in a solvent and dried. The zipping effect is caused by the surface tension of the solvent and the van der Waals forces between the carbon nanotubes. It aligns the nanotubes into a dense material, which can be formed in various shapes, such as sheets and bars, by applying weak compression during the process. Densification increases the [[Vickers hardness]] by about 70 times and density is 0.55 g/cm<sup>3</sup>. The packed carbon nanotubes are more than 1&nbsp;mm long and have a carbon purity of 99.9% or higher; they also retain the desirable alignment properties of the nanotubes forest.<ref>
{{cite journal
|title=Shape-engineerable and highly densely packed single-walled carbon nanotubes and their application as super-capacitor electrodes
|journal=[[Nature Materials]]
|volume=5
|pages=987–994
|year=2006
|doi=10.1038/nmat1782 |pmid=17128258 |issue=12
|last1=Futaba
|first1=Don N.
|last2=Hata
|first2=Kenji
|last3=Yamada
|first3=Takeo
|last4=Hiraoka
|first4=Tatsuki
|last5=Hayamizu
|first5=Yuhei
|last6=Kakudate
|first6=Yozo
|last7=Tanaike
|first7=Osamu
|last8=Hatori
|first8=Hiroaki
|last9=Yumura
|first9=Motoo|bibcode = 2006NatMa...5..987F }}</ref>
 
===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>{{Cite journal|first=J.M.|last=Singer|title=Carbon formation in very rich hydrocarbon-air flames. I. Studies of chemical content, temperature, ionization and particulate matter|journal=Seventh Symposium (International) on Combustion|year=1959}}</ref> produced by burning methane,<ref>{{cite journal|last=Yuan|first=Liming|year=2001|title=Nanotubes from methane flames|journal=Chemical physics letters|volume=340|pages=237–241|doi=10.1016/S0009-2614(01)00435-3|issue=3–4|last2=Saito|first2=Kozo|last3=Pan|first3=Chunxu|last4=Williams|first4=F.A|last5=Gordon|first5=A.S|bibcode = 2001CPL...340..237Y }}</ref> ethylene,<ref>{{cite journal|last=Yuan|first=Liming|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|last2=Saito|first2=Kozo|last3=Hu|first3=Wenchong|last4=Chen|first4=Zhi|bibcode = 2001CPL...346...23Y }}</ref> and benzene,<ref>{{cite journal|last=Duan|first=H. M.|year=1994|title=Nanoclusters Produced in Flames|journal=Journal of Physical Chemistry|volume=98|pages=12815–12818|doi=10.1021/j100100a001|last2=McKinnon|first2=J. T.|issue=49}}</ref> and they have been found in [[soot]] from both indoor and outdoor air.<ref>{{cite journal|last=Murr|first=L. E.|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|last2=Bang|first2=J.J.|last3=Esquivel|first3=E.V.|last4=Guerrero|first4=P.A.|last5=Lopez|first5=D.A.|issue=2/3}}</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 satisfy the many needs of both research and industry. Recent efforts have focused on producing more uniform carbon nanotubes in controlled flame environments.<ref>{{Cite journal|first=R.L.|last=Vander Wal|title=Fe-catalyzed single-walled carbon nanotube synthesis within a flame environment|journal=Combust. Flame|volume=130|pages=37–47|year=2002|doi=10.1016/S0010-2180(02)00360-7}}</ref><ref>{{Cite journal|first=A.V.|last=Saveliev|title=Metal catalyzed synthesis of carbon nanostructures in an opposed flow methane oxygen flame|doi=10.1016/S0010-2180(03)00142-1|journal=Combust. Flame|volume=135|pages=27–33|year=2003|last2=Merchan-Merchan|first2=Wilson|last3=Kennedy|first3=Lawrence A.}}</ref><ref>{{Cite journal|first=M.J.|last=Height|title=Flame synthesis of single-walled carbon nanotubes|doi=10.1016/j.carbon.2004.05.010|journal=Carbon|volume=42|pages=2295–2307|year=2004|issue=11|last2=Howard|first2=Jack B.|last3=Tester|first3=Jefferson W.|last4=Vander Sande|first4=John B.}}</ref><ref>{{Cite journal|first=S.|last=Sen|title=Flame synthesis of carbon nanofibers and nanofibers composites containing encapsulated metal particles|journal=Nanotechnology|volume=15|pages=264–268|year=2004|doi=10.1088/0957-4484/15/3/005|last2=Puri|first2=Ishwar K|issue=3|bibcode = 2004Nanot..15..264S }}</ref> Such methods have promise for large-scale, low-cost nanotube synthesis based on theoretical models,<ref>{{Cite journal|author=Naha, Sayangdev, Sen, Swarnendu, De, Anindya K., and Puri, Ishwar K.|title=A detailed model for the Flame synthesis of carbon nanotubes and nanofibers|journal=Proceedings of The Combustion Institute|volume=31|pages=1821–29|doi=10.1016/j.proci.2006.07.224|year=2007}}</ref> though they must compete with rapidly developing large scale CVD production.
 
===Removal of catalysts===
Nanoscale metal [[Catalysis|catalysts]] are important ingredients for fixed- and [[Fluidized bed|fluidized-bed]] [[Chemical vapor deposition|CVD]] synthesis of CNTs. They allow increasing the growth efficiency of CNTs and may give control over their structure and chirality.<ref>{{Cite journal|author=Yamada T, Namai T, Hata K, Futaba DN, Mizuno K, Fan J, et al.|title=Size-selective growth of double-walled carbon nanotube forests from engineered iron catalysts|journal=Nature Nanotechnology|volume=1|pages=131–136|doi=10.1038/nnano.2006.95|year=2006|bibcode = 2006NatNa...1..131Y|pmid=18654165}}</ref> During synthesis, catalysts can convert carbon [[Precursor (chemistry)|precursors]] into tubular carbon structures but can also form encapsulating carbon overcoats. Together with metal oxide supports they may therefore attach to or become incorporated into the CNT product.<ref>{{Cite journal|author=MacKenzie KJ, Dunens OM, Harris AT|title=An updated review of synthesis parameters and growth mechanisms for carbon nanotubes in fluidized beds|journal=Industrial & Engineering Chemical Research|volume=49|pages=5323–38|doi=10.1021/ie9019787|year=2010}}</ref> The presence of metal impurities can be problematic for many applications. Especially catalyst metals like [[nickel]], [[cobalt]] or [[yttrium]] may be of toxicological concern.<ref>{{Cite journal|author=Jakubek LM, Marangoudakis S, Raingo J, Liu X, Lipscombe D, Hurt RH|title=The inhibition of neuronal calcium ion channels by trace levels of yttrium released from carbon nanotubes|journal=Biomaterials|volume=30|pages=6351–6357|doi=10.1016/j.biomaterials.2009.08.009|year=2009}}</ref> While un-encapsulated catalyst metals may be readily removable by acid washing, encapsulated ones require oxidative treatment for opening their carbon shell.<ref>{{Cite journal|author=Hou P-X, Liu C, Cheng H-M|title=Purification of carbon nanotubes|journal=Carbon|volume=46|pages=2003–2025|doi=10.1016/j.carbon.2008.09.009|year=2008}}</ref> The effective removal of catalysts, especially of encapsulated ones, while preserving the CNT structure is a challenge and has been addressed in many studies.<ref>{{Cite journal|author=Ebbesen TW, Ajayan PM, Hiura H, Tanigaki K|title=Purification of nanotubes|journal=Nature|volume=367|page=519|doi=10.1038/367519a0|year=1994|bibcode = 1994Natur.367..519E|issue=6463}}</ref><ref>{{Cite journal|author=Xu Y-Q, Peng H, Hauge RH, Smalley RE|title=Controlled multistep purification of single-walled carbon nanotubes|journal=Nano Letters|volume=5|pages=163–168|doi=10.1021/nl048300s|year=2005|bibcode = 2005NanoL...5..163X }}</ref> A new approach to break carbonaceaous catalyst encapsulations is based on rapid thermal annealing.<ref>{{Cite journal|author=Meyer-Plath A, Orts-Gil G, Petrov S et al.|title=Plasma-thermal purification and annealing of carbon nanotubes|journal=Carbon|volume=50|pages=3934–3942|doi=10.1016/j.carbon.2012.04.049|year=2012}}</ref>
 
===Application-related issues===
[[File:Cnt diameter effect.jpg|thumb|upright|Centrifuge tube with a solution of carbon nanotubes, which were sorted by diameter using density-gradient [[Differential centrifugation|ultracentrifugation]].<ref name=grad>{{cite journal|doi=10.1038/nnano.2006.52|title=Sorting carbon nanotubes by electronic structure using density differentiation|year=2006|last1=Arnold|first1=Michael S.|last2=Green|first2=Alexander A.|last3=Hulvat|first3=James F.|last4=Stupp|first4=Samuel I.|last5=Hersam|first5=Mark C.|journal=Nature Nanotechnology|volume=1|pages=60–5|pmid=18654143|issue=1|bibcode = 2006NatNa...1...60A }}</ref>]]
Many electronic applications of carbon nanotubes crucially rely on techniques of selectively producing either semiconducting or metallic CNTs, preferably of a certain chirality. Several methods of separating semiconducting and metallic CNTs are known, but most of them are not yet suitable for large-scale technological processes. The most efficient method relies on density-gradient ultracentrifugation, which separates surfactant-wrapped nanotubes by the minute difference in their density. This density difference often translates into difference in the nanotube diameter and (semi)conducting properties.<ref name=grad/> Another method of separation uses a sequence of freezing, thawing, and compression of SWNTs embedded in [[agarose]] gel. This process results in a solution containing 70% metallic SWNTs and leaves a gel containing 95% semiconducting SWNTs. The diluted solutions separated by this method show various colors.<ref>{{cite journal |title=Simple and Scalable Gel-Based Separation of Metallic and Semiconducting Carbon Nanotubes|journal=[[Nano Letters]]|volume=9|pages=1497–1500|year=2009|doi=10.1021/nl8034866 |pmid=19243112 |issue=4 |last1=Tanaka |first1=Takeshi |last2=Jin |first2=Hehua |last3=Miyata |first3=Yasumitsu |last4=Fujii |first4=Shunjiro |last5=Suga |first5=Hiroshi |last6=Naitoh |first6=Yasuhisa |last7=Minari |first7=Takeo |last8=Miyadera |first8=Tetsuhiko |last9=Tsukagoshi |first9=Kazuhito|bibcode = 2009NanoL...9.1497T }}</ref><ref>{{cite web|author=T.Tanaka|title=New, Simple Method for Separation of Metallic and Semiconducting Carbon Nanotubes|url=http://www.aist.go.jp/aist_e/latest_research/2009/20090406/20090406.html}}</ref> The separated carbon nanotubes using this method have been applied to electrodes, e.g. electric double-layer capacitor.<ref name="yamada">{{cite doi|10.1016/j.carbon.2011.09.062}}</ref> Moreover, SWNTs can be separated by the [[column chromatography]] method. Yield is 95% in semiconductor type SWNT and 90% in metallic type SWNT.<ref>
{{cite journal |title=Continuous Separation of Metallic and Semiconducting Carbon Nanotubes Using Agarose Gel|journal=Applied Physics Express|volume=2|page=125002|year=2009|doi=10.1143/APEX.2.125002 |issue=12 |last1=Tanaka |first1=Takeshi |last2=Urabe |first2=Yasuko |last3=Nishide |first3=Daisuke |last4=Kataura |first4=Hiromichi|bibcode = 2009APExp...2l5002T }}</ref>
 
In addition to separation of semiconducting and metallic SWNTs, it is possible to sort SWNTs by length, diameter, and chirality. The highest resolution length sorting, with length variation of <10%, has thus far been achieved by size exclusion chromatography (SEC) of DNA-dispersed carbon nanotubes (DNA-SWNT).<ref>{{cite journal | title=High-Resolution Length Sorting and Purification of DNA-Wrapped Carbon Nanotubes by Size-Exclusion Chromatography|journal=[[Anal. Chem.]]|volume=77|pages=6225–6228|year=2005|doi=10.1021/ac0508954 |pmid=16194082 |issue=19 | last1=Huang | first1=Xueying | last2=McLean | first2=Robert S. | last3=Zheng | first3=Ming}}</ref> SWNT diameter separation has been achieved by density-gradient ultracentrifugation (DGU)<ref>{{cite journal |author=Mark C Hersam |title=Progress towards monodisperse single-walled carbon nanotubes|journal=Nature Nanotechnology|volume=3|pages=387–394|year=2008|doi=10.1038/nnano.2008.135 |pmid=18654561 |issue=7|bibcode = 2008NatNa...3..387H }}</ref> using surfactant-dispersed SWNTs and by ion-exchange chromatography (IEC) for DNA-SWNT.<ref>{{cite journal | title=Structure-Based Carbon Nanotube Sorting by Sequence-Dependent DNA Assembly|journal=Science|volume=302|pages=1545–1548|year=2003|doi=10.1126/science.1091911 |pmid=14645843 |issue=5650 | last1=Zheng | first1=M. | last2=Jagota | first2=A | last3=Strano | first3=MS | last4=Santos | first4=AP | last5=Barone | first5=P | last6=Chou | first6=SG | last7=Diner | first7=BA | last8=Dresselhaus | first8=MS | last9=McLean | first9=RS|bibcode = 2003Sci...302.1545Z }}</ref> Purification of individual chiralities has also been demonstrated with IEC of DNA-SWNT: specific short DNA oligomers can be used to isolate individual SWNT chiralities. Thus far, 12 chiralities have been isolated at purities ranging from 70% for (8,3) and (9,5) SWNTs to 90% for (6,5), (7,5) and (10,5)SWNTs.<ref>{{cite journal |title=DNA sequence motifs for structure-specific recognition and separation of carbon nanotubes|journal=Nature|volume=460|pages=250–253|year=2009|doi=10.1038/nature08116 |pmid=19587767 |issue=7252 |last1=Tu |first1=Xiaomin |last2=Manohar |first2=Suresh |last3=Jagota |first3=Anand |last4=Zheng |first4=Ming|bibcode = 2009Natur.460..250T }}</ref> There have been successful efforts to integrate these purified nanotubes into devices, e. g. FETs.<ref>{{cite journal| title=Optical characterizations and electronic devices of nearly pure (10,5) single-walled carbon nanotubes|journal=[[J Am Chem Soc]]|volume=131|pages=2454–2455|year=2009|doi=10.1021/ja8096674 |pmid=19193007 |issue=7| last1=Zhang| first1=Li| last2=Tu| first2=Xiaomin| last3=Welsher| first3=Kevin| last4=Wang| first4=Xinran| last5=Zheng| first5=Ming| last6=Dai| first6=Hongjie}}</ref>
 
An alternative to separation is development of a selective growth of semiconducting or metallic CNTs. Recently, a new CVD recipe that involves a combination of ethanol and methanol gases and quartz substrates resulting in horizontally aligned arrays of 95–98% semiconducting nanotubes was announced.<ref>{{cite journal|doi=10.1021/nl803496s|title=Selective Growth of Well-Aligned Semiconducting Single-Walled Carbon Nanotubes|year=2009| journal=Nano Letters|volume=9|pages=800–5|pmid=19159186|issue=2|last1=Ding|first1=Lei|last2=Tselev|first2=Alexander|last3=Wang|first3=Jinyong|last4=Yuan|first4=Dongning|last5=Chu|first5=Haibin|last6=McNicholas|first6=Thomas P.|last7=Li|first7=Yan|last8=Liu|first8=Jie|bibcode = 2009NanoL...9..800D }}</ref>
 
Nanotubes are usually grown on nanoparticles of magnetic metal (Fe, Co), which facilitates production of electronic ([[spintronic]]) devices. In particular, control of current through a field-effect transistor by magnetic field has been demonstrated in such a single-tube nanostructure.<ref>{{cite journal |author=M.A. Mohamed|title=Fabrication of spintronics device by direct synthesis of single-walled carbon nanotubes from ferromagnetic electrodes|journal=Sci. Technol. Adv. Mater.|volume=8|year=2007|page=292|doi=10.1016/j.stam.2007.02.009|last2=Ambri Mohamed|first2=Mohd|last3=Shikoh|first3=Eiji|last4=Fujiwara|first4=Akihiko |issue=4|bibcode = 2007STAdM...8..292I }}</ref>
 
==Current applications==
Current use and application of nanotubes has mostly been limited to the use of bulk nanotubes, which is a mass of rather unorganized fragments of nanotubes. 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. Bulk carbon nanotubes have already been used as composite fibers in [[polymers]] to improve the mechanical, thermal and electrical properties of the bulk product.
*[[Easton-Bell Sports, Inc.]] have been in partnership with [[Zyvex Performance Materials]], using CNT technology in a number of their [[bicycle]] components—including flat and riser handlebars, cranks, forks, seatposts, stems and aero bars.
*[[Zyvex Technologies]] has also built a 54' maritime vessel, the [[Piranha Unmanned Surface Vessel]], as a technology demonstrator for what is possible using CNT technology. CNTs help improve the structural performance of the vessel, resulting in a lightweight 8,000&nbsp;lb boat that can carry a payload of 15,000&nbsp;lb over a range of 2,500 miles.<ref>{{cite web |url=http://www.reinforcedplastics.com/view/7467/pirahna-usv-built-using-nanoenhanced-carbon-prepreg/ |title=Pirahna USV built using nano-enhanced carbon prepreg |date=19 February December 2009 |work=ReinforcedPlastics.com |accessdate=25 February 2010}}</ref>
*[[Amroy Europe Oy]] manufactures [[Hybtonite]] carbon nanoepoxy resins where carbon nanotubes have been chemically activated to bond to epoxy, resulting in a composite material that is 20% to 30% stronger than other composite materials. It has been used for wind turbines, marine paints and variety of sports gear such as skis, ice hockey sticks, baseball bats, hunting arrows, and surfboards.<ref name="Pagni">{{cite web
| last = Pagni
| first = John
| title = Amroy aims to become nano-leader
| publisher = European Plastics News
| date = 5 March 2010
| url = http://www.europeanplasticsnews.com/subscriber/headlines2.html?cat=1&id=1267781513
| accessdate = 2010-11-21}}</ref>
 
Other current applications include:
*tips for [[Atomic Force Microscopy|atomic force microscope]] probes<ref>{{cite web|title=Nanotube Tips|url=http://www.nanoscience.com/store/pc/viewCategories.asp?idCategory=9|publisher=nanoScince instruments}}</ref>
*in tissue engineering, carbon nanotubes can act as scaffolding for bone growth<ref name=bone/>
 
==Potential applications==
{{Main|Potential applications of carbon nanotubes}}
 
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 of an individual multi-walled carbon nanotube has been tested to be is 63&nbsp;[[GPa]].<ref name="Strength and Breaking"/> Carbon nanotubes were found in [[Damascus steel]] from the 17th century, possibly helping to account for the legendary strength of the swords made of it.<ref>{{cite journal| author=K. Sanderson|title=Sharpest cut from nanotube sword|journal=Nature News| doi=10.1038/news061113-11| year=2006}}</ref><ref>{{cite journal|first=M.|last=Reibold|date= November 16, 2006|title=Materials:Carbon nanotubes in an ancient Damascus sabre|journal=Nature|volume=444|page=286|doi =10.1038/444286a|pmid=17108950|last2=Paufler|first2=P|last3=Levin|first3=AA|last4=Kochmann|first4=W|last5=Pätzke|first5=N|last6=Meyer|first6=DC|issue=7117|bibcode = 2006Natur.444..286R }}</ref>
 
===Structural===
Because of the carbon nanotube's superior mechanical properties, many structures have been proposed ranging from everyday items like clothes and sports gear to combat jackets and [[space elevator]]s.<ref>{{Cite book|title=The Space Elevator|first=Brad C.|last=Edwards|publisher=BC Edwards|year=2003|isbn=0-9746517-1-0}}</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 name="nanotubes for electronics" />
 
For perspective, outstanding breakthroughs have already been made. Pioneering work led by Ray H. Baughman at the NanoTech Institute has shown that single and multi-walled nanotubes can produce materials with toughness unmatched in the man-made and natural worlds.<ref>{{Cite journal| title=Strong, Transparent, Multifunctional, Carbon Nanotube Sheets|journal=Science|year=2005|volume=309|pages=1215–1219|doi=10.1126/science.1115311|pmid=16109875|issue=5738| last1=Zhang| first1=M.| last2=Fang| first2=S| last3=Zakhidov| first3=AA| last4=Lee| first4=SB| last5=Aliev| first5=AE| last6=Williams| first6=CD| last7=Atkinson| first7=KR| last8=Baughman| first8=RH|bibcode = 2005Sci...309.1215Z }}</ref><ref>{{Cite journal| title=Super-tough carbon-nanotube fibres|doi=10.1038/423703a|journal=Nature|year=2003|volume=423|page=703 |last1=Dalton| first1=Alan B.| last2=Collins| first2=Steve| last3=Muñoz| first3=Edgar| last4=Razal| first4=Joselito M.| last5=Ebron| first5=Von Howard| last6=Ferraris| first6=John P.| last7=Coleman| first7=Jonathan N.| last8=Kim| first8=Bog G.| last9=Baughman| first9=Ray H.|bibcode = 2003Natur.423..703D }}</ref>
 
Carbon nanotubes are also a promising material as building blocks in bio-mimetic hierarchical composite materials given their exceptional mechanical properties (~1 TPa in modulus, and ~100 GPa in strength). Initial attempts to incorporate CNTs into hierarchical structures led to mechanical properties that were significantly lower than these achievable limits. Windle ''et al.'' have used an ''in situ'' chemical vapor deposition (CVD) spinning method to produce continuous CNT yarns from CVD grown CNT aerogels.<ref>{{cite journal| doi=10.1021/nl051419w|title=Hot-Drawing of Single and Multiwall Carbon Nanotube Fibers for High Toughness and Alignment|year=2005|journal=Nano Letters|volume=5|issue=11|pages=2212–2215|pmid=16277455| last1=Miaudet| first1=P.| last2=Badaire| first2=S.| last3=Maugey| first3=M.| last4=Derré| first4=A.| last5=Pichot| first5=V.| last6=Launois| first6=P.| last7=Poulin| first7=P.| last8=Zakri| first8=C.|bibcode = 2005NanoL...5.2212M }}</ref><ref>{{cite journal|doi=10.1126/science.1094982|title=Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis|year=2004|journal=Science|volume=304|issue=5668|pages=276–278|pmid=15016960|last1=Li|first1=Y.-L.|last2=Kinloch|first2=IA|last3=Windle|first3=AH|bibcode = 2004Sci...304..276L }}</ref> With this technology, they fabricated CNT yarns with strengths as high as ~9 GPa at small gage lengths of ~1&nbsp;mm, however, defects resulted in a reduction of specific strength to ~1 GPa at 20&nbsp;mm gage length.<ref>{{cite journal| doi=10.1002/adma.200700516|title=High Performance Fibres from 'Dog Bone' Carbon Nanotubes|year=2007|journal=Advanced Materials|volume=19|issue=21|pages=3721–3726| last1=Motta| first1=M.| last2=Moisala| first2=A.| last3=Kinloch| first3=I. A.| last4=Windle| first4=Alan H.}}</ref><ref>{{cite journal| doi=10.1126/science.1147635|title=High-Performance Carbon Nanotube Fiber|year=2007|journal=Science|volume=318|issue=5858|pages=1892–1895|pmid=18006708| last1=Koziol| first1=K.| last2=Vilatela| first2=J.| last3=Moisala| first3=A.| last4=Motta| first4=M.| last5=Cunniff| first5=P.| last6=Sennett| first6=M.| last7=Windle| first7=A.|bibcode = 2007Sci...318.1892K }}</ref> Espinosa ''et al.'' developed high performance DWNT-polymer composite yarns by twisting and stretching ribbons of randomly oriented bundles of DWNTs thinly coated with polymeric organic compounds. These DWNT-polymer yarns exhibited unusually high energy to failure of ~100 J·g<sup>−1</sup> (comparable to one of the toughest natural materials – spider silk<ref>{{cite journal| doi=10.1002/adma.200400344|title=Toughness of Spider Silk at High and Low Temperatures|year=2005|journal=Advanced Materials|volume=17|pages=84–88| last1=Yang| first1=Y.| last2=Chen| first2=X.| last3=Shao| first3=Z.| last4=Zhou| first4=P.| last5=Porter| first5=D.| last6=Knight| first6=D. P.| last7=Vollrath| first7=F.}}</ref>), and strength as high as ~1.4 GPa.<ref>{{cite journal| doi=10.1021/nn101404u|title=A Multiscale Study of High Performance Double-Walled Nanotube−Polymer Fibers|year=2010|journal=ACS Nano|volume=4|issue=11|pages=6463–6476|pmid=20977259| last1=Naraghi| first1=Mohammad| last2=Filleter| first2=Tobin| last3=Moravsky| first3=Alexander| last4=Locascio| first4=Mark| last5=Loutfy| first5=Raouf O.| last6=Espinosa| first6=Horacio D.}}</ref> Effort is ongoing to produce CNT composites that incorporate tougher matrix materials, such as [[Kevlar]], to further improve on the mechanical properties toward those of individual CNTs.
 
Because of the high mechanical strength of carbon nanotubes, research is being made into weaving them into clothes to create stab-proof and bulletproof clothing. The nanotubes would effectively stop the bullet from penetrating the body, although the bullet's kinetic energy would likely cause broken bones and internal bleeding.<ref>{{cite journal|first=T.|last=Yildirim|year=2000|title=Pressure-induced interlinking of carbon nanotubes|journal=Phys. Rev. B|volume=62|page=19|doi=10.1103/PhysRevB.62.12648|last2=Gülseren|first2=O.|last3=Kılıç|first3=Ç.|last4=Ciraci|first4=S.|issue=19|arxiv = cond-mat/0008476|bibcode = 2000PhRvB..6212648Y }}</ref>
 
===Electrical circuits===
Nanotube-based [[transistor]]s, also known as [[carbon nanotube field-effect transistor]]s (CNTFETs), have been made that operate at room temperature and that are capable of digital switching using a single electron.<ref>{{Cite journal|last=Postma|first=Henk W. Ch.|year=2001|title=Carbon Nanotube Single-Electron Transistors at Room temperature|journal=Science|volume=293|doi=10.1126/science.1061797|pmid=11441175|last2=Teepen|first2=T|last3=Yao|first3=Z|last4=Grifoni|first4=M|last5=Dekker|first5=C|issue=5527|pages=76–9|bibcode = 2001Sci...293...76P }}</ref> However, one major obstacle to realization of nanotubes has been the lack of technology for mass production. In 2001 IBM researchers demonstrated how metallic nanotubes can be destroyed, leaving semiconducting ones behind for use as transistors. Their process is called "constructive destruction," which includes the automatic destruction of defective nanotubes on the [[wafer (electronics)|wafer]].<ref>{{Cite journal|first=Philip G.|last=Collins|title=Engineering Carbon Nanotubes and Nanotube Circuits Using Electrical Breakdown|journal=Science|volume=292|year=2001|pages=706–709|doi=10.1126/science.1058782|pmid=11326094|last2=Arnold|first2=MS|last3=Avouris|first3=P|issue=5517|bibcode = 2001Sci...292..706C }}</ref> This process, however, only gives control over the electrical properties on a statistical scale.
 
The potential of carbon nanotubes was demonstrated in 2003 when room-temperature ballistic transistors with ohmic metal contacts and [[High-k dielectric|high-k gate]] dielectric were reported, showing 20–30x higher ON current than state-of-the-art Si [[MOSFET]]s. This presented an important advance in the field as CNT was shown to potentially outperform Si. At the time, a major challenge was ohmic metal contact formation. In this regard, [[palladium]], which is a high-[[work function]] metal was shown to exhibit [[Schottky barrier]]-free contacts to semiconducting nanotubes with diameters >1.7&nbsp;nm.<ref>{{Cite journal|first=Ali |last=Javey|title=Ballistic Carbon Nanotube Transistors|journal=Nature|volume=424|year=2003|pages=654–657|doi=10.1038/nature01797|last2=Guo|first2=J|last3=Wang|first3=Q|last4=Lundstrom|first4=M|last5=Dai|first5=H|author5-link=Hongjie Dai|pmid=12904787|issue=6949|bibcode = 2003Natur.424..654J }}</ref><ref>{{Cite journal| title=Self-aligned ballistic molecular transistors and electrically parallel nanotube arrays|journal=Nano Letters|volume=4|year=2004|pages=1319–1322|doi=10.1021/nl049222b|arxiv = cond-mat/0406494 |issue=7| last1=Javey| first1=Ali| last2=Guo| first2=Jing| last3=Farmer| first3=Damon B.| last4=Wang| first4=Qian| last5=Yenilmez| first5=Erhan| last6=Gordon| first6=Roy G.| last7=Lundstrom| first7=Mark| last8=Dai| first8=Hongjie|bibcode = 2004NanoL...4.1319J }}</ref>
 
The first nanotube 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>{{Cite journal| title=Monolithic Integration of Carbon Nanotube Devices with Silicon MOS Technology|journal=Nano Letters|volume=4|year=2004|pages=123–127|doi=10.1021/nl0349707| last1=Tseng| first1=Yu-Chih| last2=Xuan| first2=Peiqi| last3=Javey| first3=Ali| last4=Malloy| first4=Ryan| last5=Wang| first5=Qian| last6=Bokor| first6=Jeffrey| last7=Dai| first7=Hongjie|bibcode = 2004NanoL...4..123T }}</ref>
 
Another way to make carbon nanotube transistors has been to use random networks of them. By doing so one averages all of their electrical differences and one can produce devices in large scale at the wafer level.<ref>{{Cite journal|last=Gabriel| first=Jean-Christophe P.| title=Large Scale Production of Carbon Nanotube Transistors: A Generic Platforms for Chemical Sensors| journal=Mat. Res. Soc. Symp. Proc.|volume=762|year=2003|pages=Q.12.7.1| url=http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=2606&DID=110422&action=detail}}</ref> This approach was first patented by Nanomix Inc.<ref>[http://www.nano.com Nanōmix – Breakthrough Detection Solutions with the Nanoelectronic Sensation Technology]. Nano.com. Retrieved on 2012-06-06.</ref> (date of original application June 2002<ref>{{Cite journal|last=Gabriel| first=Jean-Christophe P. |title=Dispersed Growth Of Nanotubes on a substrate|journal=Patent WO 2004040671A2|url=http://www.freepatentsonline.com/EP1560958.html}}</ref>). It was first published in the academic literature by the [[United States Naval Research Laboratory]] in 2003 through independent research work. This approach also enabled Nanomix to make the first transistor on a flexible and transparent substrate.<ref>{{Cite journal|last=Bradley| first=Keith|title=Flexible nanotube transistors| journal=Nano Letters|volume=3|year=2003|pages=1353–1355|doi=10.1021/nl0344864|last2=Gabriel|first2=Jean-Christophe P.|last3=Grüner|first3=George|issue=10|bibcode = 2003NanoL...3.1353B }}</ref><ref>{{cite journal|last=Armitage| first=Peter N. |title=Flexible nanostructure electronic devices|journal=United States Patent 20050184641 A1|url=http://www.freshpatents.com/Flexible-nanostructure-electronic-devices-dt20050825ptan20050184641.php}}</ref>
 
Large structures of carbon nanotubes can be used for thermal management of electronic circuits. An approximately 1&nbsp;mm–thick carbon nanotube layer was used as a special material to fabricate coolers, this material has very low density, ~20 times lower weight than a similar copper structure, while the cooling properties are similar for the two materials.<ref>{{cite journal| title=Chip cooling with integrated carbon nanotube microfin architectures|journal=Appl. Phys. Lett.|volume=90|page=123105|year=2007|doi=10.1063/1.2714281|issue=12| last1=Kordás| first1=K.| last2=TóTh| first2=G.| last3=Moilanen| first3=P.| last4=KumpumäKi| first4=M.| last5=VäHäKangas| first5=J.| last6=UusimäKi| first6=A.| last7=Vajtai| first7=R.| last8=Ajayan| first8=P. M.|bibcode = 2007ApPhL..90l3105K }}</ref>
 
In 2013, researchers demonstrated a Turing-complete prototype micrometer-scale computer.<ref>Lee, Robert. (2002-10-03) [http://online.wsj.com/article/SB10001424052702304795804579097201829598522.html Scientists Build First Nanotube Computer - WSJ.com]. Online.wsj.com. Retrieved on 2013-09-28.</ref><ref>Hsu, Jeremy. (2013-09-24) [http://www.scientificamerican.com/article.cfm?id=carbon-nanotube-computer-hints-at-future-beyond-silicon Carbon Nanotube Computer Hints at Future Beyond Silicon Semiconductors]. Scientific American. Retrieved on 2013-09-28.</ref><ref>[http://www.bbc.co.uk/news/science-environment-24232896 BBC News - First computer made of carbon nanotubes is unveiled]. Bbc.co.uk. Retrieved on 2013-09-28.</ref> [[Carbon nanotube field-effect transistor|Carbon nanotube transistors]] as logic-gate circuits with densities comparable to modern CMOS technology has not yet been demonstrated.
 
===Electrical cables and wires===
Wires for carrying electrical current may be fabricated from pure nanotubes and nanotube-polymer composites. Recently small wires have been fabricated with specific conductivity exceeding copper and aluminum;<ref>{{cite news|url=http://www.youtube.com/watch?v=oJw6JmZmdqE|title=Nanocables light way to the future|publisher=YouTube|date =September 9, 2011}}</ref><ref>{{Cite journal |title=Iodine doped carbon nanotube cables exceeding specific electrical conductivity of metals|date =September 6, 2011|doi=10.1038/srep00083 |last1=Zhao |first1=Yao |last2=Wei |first2=Jinquan |last3=Vajtai |first3=Robert |last4=Ajayan |first4=Pulickel M. |last5=Barrera |first5=Enrique V. |journal=Scientific Reports (Nature)|volume=1|bibcode = 2011NatSR...1E..83Z }}</ref> these cables are the highest conductivity carbon nanotube and also highest conductivity non-metal cables.
Recently, composite of carbon nanotube and copper have been shown to exhibit nearly one hundred times higher current-carrying-capacity than pure copper or gold.<ref>Subramaniam, C., Yamada, T., Kobashi, K., Sekiguchi, A., Futaba, D.N., Yumura, M., Hata, K., One hundred fold increase in current carrying capacity in a carbon nanotube-copper composite, '''Nature Communications''', 2013, 4, 2202. http://www.nature.com/ncomms/2013/130723/ncomms3202/full/ncomms3202.html</ref>  Significantly, the electrical conductivity of such a composite is similar to pure Cu.  Thus, this Carbon nanotube-copper (CNT-Cu) composite possesses the highest observed current-carrying capacity among electrical conductors. Thus for a given cross-section of electrical conductor, the CNT-Cu composite can withstand and transport one hundred times higher current compared to metals such as copper and gold.
 
===Actuators===
The exceptional electrical and mechanical properties of carbon nanotubes have made them alternatives to the traditional electrical actuators for both microscopic and macroscopic applications. Carbon nanotubes are very good conductors of both electricity and heat, and they are also very strong and elastic molecules in certain directions.<ref>[http://nanofluid.ir]</ref>
 
===Paper batteries===
A [[paper battery]] is a [[Battery (electricity)|battery]] engineered to use a paper-thin sheet of [[cellulose]] (which is the major constituent of regular paper, among other things) infused with aligned carbon nanotubes.<ref>{{cite news|url=http://www.eurekalert.org/pub_releases/2007-08/rpi-bbs080907.php|title=Beyond Batteries: Storing Power in a Sheet of Paper|publisher=Eurekalert.org|date =August 13, 2007|accessdate=2008-09-15}}</ref> The nanotubes act as [[electrodes]]; allowing the storage devices to conduct electricity. The battery, which functions as both a lithium-ion battery and a [[supercapacitor]], can provide a long, steady power output comparable to a conventional battery, as well as a supercapacitor’s quick burst of high power—and while a conventional battery contains a number of separate components, the paper battery integrates all of the battery components in a single structure, making it more energy efficient.{{Citation needed|date=February 2010}}
 
===Solar cells===
One of the promising applications of single-walled carbon nanotubes (SWNTs) is their use in solar panels, due to their strong UV/Vis-NIR absorption characteristics. Research has shown that they can provide a sizeable increase in efficiency, even at their current unoptimized state. Solar cells developed at the [[New Jersey Institute of Technology]] use a carbon nanotube complex, formed by a mixture of carbon nanotubes and carbon [[buckyball]]s (known as [[fullerene]]s) to form snake-like structures. Buckyballs trap electrons, but they can't make electrons flow. Add sunlight to excite the polymers, and the buckyballs will grab the electrons. Nanotubes, behaving like copper wires, will then be able to make the electrons or current flow.<ref>{{cite news|url=http://www.sciencedaily.com/releases/2007/07/070719011151.htm|title=New Flexible Plastic Solar Panels Are Inexpensive And Easy To Make|publisher=ScienceDaily|date=July 19, 2007}}</ref>
 
Additional research has been conducted on creating SWNT hybrid solar panels to increase the efficiency further. These hybrids are created by combining SWNT's with photexcitable electron donors to increase the number of electrons generated. It has been found that the interaction between the photoexcited porphrin and SWNT generates electro-hole pairs at the SWNT surfaces. This phenomenon has been observed experimentally, and contributes practically to an increase in efficiency up to 8.5%.<ref>{{cite journal|author=Guldi, Dirk M., G.M.A. Rahman, Maurizio Prato, Norbert Jux, Shubui Qin, and Warren Ford|title=Single-Wall Carbon Nanotubes as Integrative Building Blocks for Solar-Energy Conversion|journal=Angewandte Chemie|pmid=15724261|volume= 117 |year=2005|pages=2051–2054|doi=10.1002/ange.200462416|issue=13}}</ref>
{{Further|Carbon nanotubes in photovoltaics}}
 
===Hydrogen storage===
In addition to being able to store electrical energy, there has been some research in using carbon nanotubes to store hydrogen to be used as a fuel source. By taking advantage of the capillary effects of the small carbon nanotubes, it is possible to condense gases in high density inside single-walled nanotubes. This allows for gases, most notably hydrogen (H<sub>2</sub>), to be stored at high densities without being condensed into a liquid. Potentially, this storage method could be used on vehicles in place of gas fuel tanks for a hydrogen-powered car. A current issue regarding hydrogen-powered vehicles is the onboard storage of the fuel. Current storage methods involve cooling and condensing the H<sub>2</sub> gas to a liquid state for storage which causes a loss of potential energy (25–45%) when compared to the energy associated with the gaseous state. Storage using SWNTs would allow one to keep the H2 in its gaseous state, thereby increasing the storage effciency. This method allows for a volume to energy ratio slightly smaller to that of current gas powered vehicles, allowing for a slightly lower but comparable range.<ref>{{cite journal|author=Dillon, A. C., K. M. Jones, T. A. Bekkedahl, C. H. Klang, D. S. Bethune, and M. J. Heben|title=Storage of hydrogen in single-walled carbon nanotubes|journal=Nature|volume= 386|year=1997|doi=10.1038/386377a0|pages=377–379|issue=6623|bibcode = 1997Natur.386..377D }}</ref>
 
An area of controversy and frequent experimentation regarding the storage of hydrogen by adsorption in carbon nanotubes is the efficiency by which this process occurs. The effectiveness of hydrogen storage is integral to its use as a primary fuel source since hydrogen only contains about one fourth the energy per unit volume as gasoline.
 
====Experimental capacity====
One experiment<ref>{{cite journal|author=Safa, S., Mojtahedzadeh Larijani, M., Fathollahi, V., Kakuee, O. R.|title=Investigating Hydrogen Storage Behavior of Carbon Nanotubes at Ambient Temperature and Above by Ion Beam Analysis|journal=NANO|doi=10.1142/S1793292010002256|volume= 5|issue=6|year=2010|pages=341–347}}</ref> sought to determine the amount of hydrogen stored in CNTs by utilizing [[elastic recoil detection analysis]] (ERDA). CNTs (primarily SWNTs) were synthesized via [[chemical vapor disposition]] (CVD) and subjected to a two-stage purification process including air oxidation and acid treatment, then formed into flat, uniform discs and exposed to pure, pressurized hydrogen at various temperatures. When the data was analyzed, it was found that the ability of CNTs to store hydrogen decreased as temperature increased. Moreover, the highest hydrogen concentration measured was ~0.18%; significantly lower than commercially viable hydrogen storage needs to be. A separate experimental work performed by using a gravimetric method also revealed the maximum hydrogen uptake capacity of CNTs to be as low as 0.2%.<ref>{{cite journal|author=SH Barghi, TT Tsotsis, M Sahimi|title=Chemisorption, physisorption and hysteresis during hydrogen storage in carbon nanotubes|journal=International Journal of Hydrogen Energy|volume= 39|issue=3|year=2014|pages=1390–1397}}</ref>
 
In another experiment,<ref>{{cite journal|author=Yuca, N., Karatepe, N.|title=Hydrogen Storage in Single-Walled Carbon Nanotubes Purified by Microwave Digestion Method|journal=World Academy of Science, Engineering and Technology|volume= 79|year=2011|pages=605–610}}</ref> CNTs were synthesized via CVD and their structure was characterized using [[Raman spectroscopy]]. Utilizing [[microwave digestion]], the samples were exposed to different acid concentrations and different temperatures for various amounts of time in an attempt to find the optimum purification method for SWNTs of the diameter determined earlier. The purified samples were then exposed to hydrogen gas at various high pressures, and their [[adsorption]] by weight percent was plotted. The data showed that hydrogen adsorption levels of up to 3.7% are possible with a very pure sample and under the proper conditions. It is thought that microwave digestion helps improve the hydrogen adsorption capacity of the CNTs by opening up the ends, allowing access to the inner cavities of the nanotubes.
 
====Limitations on efficient hydrogen adsorption====
The biggest obstacle to efficient hydrogen storage using CNTs is the purity of the nanotubes. To achieve maximum hydrogen adsorption, there must be minimum [[graphene]], amorphous carbon, and metallic deposits in the nanotube sample. Current methods of CNT synthesis require a purification step. However, even with pure nanotubes, the adsorption capacity is only maximized under high pressures, which are undesirable in commercial fuel tanks.
 
===Supercapacitor===
{{main|Supercapacitor}}
MIT Research Laboratory of Electronics uses nanotubes to improve [[supercapacitors]]. The activated charcoal used in conventional ultracapacitors has many small hollow spaces of various size, which create together a large surface to store electric charge. But as charge is quantized into elementary charges, i.e. electrons, and each such elementary charge needs a minimum space, a significant fraction of the electrode surface is not available for storage because the hollow spaces are not compatible with the charge's requirements. With a nanotube electrode the spaces may be tailored to size—few too large or too small—and consequently the capacity should be increased considerably.<ref name="MIT">Halber, Deborah. [http://lees.mit.edu/lees/battery_001.htm MIT LEES on Batteries]. Lees.mit.edu. Retrieved on 2012-06-06.</ref>
 
===Radar absorption===
Radars work in the microwave frequency range, which can be absorbed by MWNTs. Applying the MWNTs to the aircraft would cause the radar to be absorbed and therefore seem to have a smaller signature. One such application could be to paint the nanotubes onto the plane. Recently there has been some work done at the [[University of Michigan]] regarding carbon nanotubes usefulness as stealth technology on aircraft. It has been found that in addition to the radar absorbing properties, the nanotubes neither reflect nor scatter visible light, making it essentially invisible at night, much like painting current stealth aircraft black except much more effective. Current limitations in manufacturing, however, mean that current production of nanotube-coated aircraft is not possible. One theory to overcome these current limitations is to cover small particles with the nanotubes and suspend the nanotube-covered particles in a medium such as paint, which can then be applied to a surface, like a stealth aircraft.<ref>Bourzac, Katherine. [http://www.technologyreview.com/article/39238/?mod=chfeatured "Nano Paint Could Make Airplanes Invisible to Radar."] Technology Review. MIT, 5 December 2011.</ref>
 
===Medical===
In the [[Kanzius RF Therapy#Nanoparticles|Kanzius cancer therapy]], single-walled carbon nanotubes are inserted around cancerous cells, then excited with radio waves, which causes them to heat up and kill the surrounding cells.
 
Researchers at Rice University, Radboud University Nijmegen Medical Centre and University of California, Riverside have shown that carbon nanotubes and their polymer nanocomposites are suitable scaffold materials for bone cell proliferation<ref name=bone>{{cite journal|last=Haddon|first=Robert C.|coauthors=Laura P. Zanello, Bin Zhao, Hui Hu|title=Bone Cell Proliferation on Carbon Nanotubes|journal=Nano Letters|date=16|year=2006|volume=6|pmid=16522063|issue=3|pages=562–567|doi=10.1021/nl051861e|bibcode = 2006NanoL...6..562Z }}</ref><ref>{{Cite journal|year=2007|title=Fabrication of porous ultra-short single-walled carbon nanotubenanocomposite scaffolds for bone tissue engineering|doi=10.1016/j.biomaterials.2007.05.033|journal= Biomaterials|volume=28|pages=4078–4090|issue=28|pmid=17576009|pmc=3163100|last1=Shi|first1=Xinfeng|last2=Sitharaman|first2=Balaji|last3=Pham|first3=Quynh P.|last4=Liang|first4=Feng|last5=Wu|first5=Katherine|last6=Edward Billups|first6=W.|last7=Wilson|first7=Lon J.|last8=Mikos|first8=Antonios G.}}</ref> and bone formation.<ref>{{Cite journal| year=2008|title=In vivo biocompatibility of ultra-short single-walled carbon nanotube/biodegradable polymer nanocomposites for bone tissue engineering|doi= 10.1016/j.bone.2008.04.013|journal=Bone|volume=43|pages=362–370|issue=2|pmid= 18541467| last1=Sitharaman| first1=Balaji| last2=Shi| first2=Xinfeng| last3=Walboomers| first3=X. Frank| last4=Liao| first4=Hongbing| last5=Cuijpers| first5=Vincent| last6=Wilson| first6=Lon J.| last7=Mikos| first7=Antonios G.| last8=Jansen| first8=John A.}}</ref><ref>Dalton, Aaron. [http://www.wired.com/medtech/health/news/2005/08/68512 Nanotubes May Heal Broken Bones]. Wired.com (2005-08-15). Retrieved on 2012-06-06.</ref>
 
In November 2012 researchers at the American [[National Institute of Standards and Technology]] (NIST) proved that single-wall carbon nanotubes may help protect DNA molecules from damage by [[oxidation]]. [http://phys.org/news/2012-11-njit-professor-material-millennium-autoclave.html (PhysOrg)] {{doi-inline|10/1002/smll.201201217|(''Small'')}}
 
===Textile===
The previous studies on the use of CNTs for textile functionalization were focused on fiber spinning for improving physical and mechanical properties.<ref>{{cite journal|last=Pötschke|first=P.|coauthors=T. Andres, T. Villmow, S. Pegel, H. Brünig, K. Kobashi, D. Fischer, L. Häussler,|title=Liquid sensing properties of fibres prepared by melt spinning from poly(lactic acid) containing multi-walled carbon nanotubes|journal=Composites Science and Technology|year=2010|volume=70|pages=343–349}}</ref><ref>{{cite journal|last=Chen|first=P.|coauthors=H.S. Kim, S.M. Kwon, Y. S. Yun, H. J. Jin|title=Regenerated bacterial cellulose/multi-walled carbon nanotubes composite fibers prepared by wet-spinning|journal=Current Applied Physics|year=2009|volume=9|pages=96–99 |doi = 10.1016/j.cap.2008.12.038 |bibcode = 2009CAP.....9...96C }}</ref><ref>{{cite journal|last=Coleman|first=J. N.|coauthors=U. Khan, W. J. Blau, Y. K. Gunko|title=, Small but strong: A review of the mechanical properties of carbon nanotube–polymer composites|journal=Carbon|year=2006|volume=44|pages=1624–1652}}</ref> Recently a great deal of attention has been focused on coating CNTs on textile fabrics. Various methods have been employed for modifying fabrics using CNTs. Shim et al. produced intelligent e-textiles for Human Biomonitoring using a polyelectrolyte-based coating with CNTs.<ref>{{cite journal|last=Shim|first=B. S.|coauthors=W. Chen, C. Doty, C. Xu, N. A. Kotov|title=Smart Electronic Yarns and Wearable Fabrics for Human Biomonitoring made by Carbon Nanotube Coating with Polyelectrolytes|journal=Nano Letters|year=2008|volume=8|pages=4151–4157|doi=10.1021/nl801495p|bibcode = 2008NanoL...8.4151S }}</ref> Additionally, Panhuis et al. dyed textile material by immersion in either a poly (2-methoxy aniline-5-sulfonic acid) PMAS polymer solution or PMAS-SWNT dispersion with enhanced conductivity and capacitance with a durable behavior.<ref>{{cite journal|last=Panhuis|first=M.|coauthors=J. Wu, S.A. Ashraf, G.G. Wallace|title=Conducting textiles from single-walled carbon nanotubes|journal=Synthetic Metals|year=2007|volume=157|pages=358–362}}</ref> In another study, Hu and coworkers coated single-walled carbon nanotubes with a simple “dipping and drying” process for wearable electronics and energy storage applications.<ref>{{cite journal|last=Hu|first=L.|coauthors=M. Pasta, F. La Mantia, L.F. Cui, S. Jeong, H.D. Deshazer, J.W. Choi, S.M. Han, Y. Cui|title=Porous, and Conductive Energy Textiles|journal=Nano Letter|year=2010|volume=10|pages=708–714}}</ref> CNTs have an aligned nanotube structure and a negative surface charge. Therefore, they have similar structures to direct dyes, so the exhaustion method is applied for coating and absorbing CNTs on the fiber surface for preparing multifunctional fabric including antibacterial, electric conductive, flame retardant and electromagnetic absorbance properties.<ref>F. Alimohammadi, M. Parvinzadeh, A. shamei, Carbon Nanotube Embedded Textiles, US-Patent 2011, 0171413.</ref><ref>{{cite journal|last=Alimohammadi|first=Farbod|coauthors=Mazeyar Parvinzadeh Gashti, Ali Shamei|title=Functional cellulose fibers via polycarboxylic acid/carbon nanotube composite coating|journal=, Journal of Coatings Technology and Research|doi=10.1007/s11998-012-9429-3}}</ref><ref>{{cite journal|last=Alimohammadi|first=Farbod|coauthors=M. Parvinzadeh Gashti, A. Shamei|title=A novel method for coating of carbon nanotube on cellulose fiber using 1,2,3,4-butanetetracarboxylic acid as a cross-linking agent|journal=Progress in Organic Coatings|year=2012|volume=74|pages=470– 478}}</ref>
 
===Optical power detectors===
 
A spray-on mixture of carbon nanotubes and ceramic demonstrates unprecedented ability to resist damage while absorbing laser light. Such coatings that absorb as the energy of high-powered lasers without breaking down are essential for optical power detectors that measure the output of such lasers. These are used, for example, in military equipment for defusing unexploded mines. The composite consists of multiwall carbon nanotubes and a ceramic made of silicon, carbon and nitrogen. Including boron boosts the breakdown temperature. The nanotubes and graphene-like carbon transmit heat well, while the oxidation-resistant ceramic boosts damage resistance.
Creating the coating involves dispersing he nanotubes in [[toluene]], to which a clear liquid polymer containing boron was added. The mixture was heated to {{convert|1100| C|F}}. The result is crushed into a fine powder, dispersed again in toluene and sprayed in a thin coat on a copper surface.
The coating absorbed 97.5 percent of the light from a far-infrared laser and tolerated 15 kilowatts per square centimeter for 10 seconds. Damage tolerance is about 50 percent higher than for similar coatings, e.g., nanotubes alone and carbon paint.<ref>{{cite web|url=http://www.kurzweilai.net/super-nanotubes-remarkable-spray-on-coating-combines-carbon-nanotubes-with-ceramic?utm_source=KurzweilAI+Daily+Newsletter&utm_campaign=055a5d11ed-UA-946742-1&utm_medium=email |title=Super-nanotubes: ‘remarkable’ spray-on coating combines carbon nanotubes with ceramic |publisher=KurzweilAI |date= |accessdate=2013-04-20}}</ref><ref>{{cite doi|10.1021/am302755x}}</ref>
 
===Loud speaker and earphone===
Carbon nanotubes have also been applied in the acoustics(such as loudspeaker and earphone). In 2008 it was shown that a sheet of nanotubes can operate as a loudspeaker if an alternating current is applied. The sound is not produced through vibration but [[thermoacoustics|thermoacoustically]].<ref>{{cite journal|last=Lin Xiao|coauthors=Zhuo Chen, Chen Feng, Liang Liu, Zai-Qiao Bai, Yang Wang, Li Qian, Yuying Zhang, Qunqing Li, Kaili Jiang and Shoushan Fan|title=Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers|journal=Nano Letters|year=2008|volume=8|issue=12|pages=4539–4545|doi=10.1021/nl802750z|url=http://pubs.acs.org/doi/abs/10.1021/nl802750z}}</ref><ref>[http://technology.newscientist.com/article/dn15098-hot-nanotube-sheets-produce-music-on-demand.html Hot nanotube sheets produce music on demand], ''New Scientists News'', 31 October 2008</ref>
In 2013, a carbon nanotube (CNT) thin yarn thermoacoustic earphone together with CNT thin yarn thermoacoustic chip was demonstrated by a research group of Tsinghua-Foxconn Nanotechnology Research Center in Tsinghua University,<ref>{{cite journal|last=Yang Wei|coauthors=Xiaoyang Lin, Kaili Jiang, Peng Liu, Qunqing Li, and Shoushan Fan|title=Thermoacoustic Chips with Carbon Nanotube Thin Yarn Arrays|journal=Nano Letters|year=2013|doi=10.1021/nl402408j|url=http://pubs.acs.org/doi/abs/10.1021/nl402408j}}</ref> using a Si-based semi-conducting technology compatible fabrication process.
 
=== Environmental remediation ===
 
A CNT nano-structured sponge (nanosponge) containing sulfur and iron is more effective at soaking up water contaminants such as oil, fertilizers, pesticides and pharmaceuticals. Their magnetic properties make them easier to retrieve once the clean-up job is done. The sulfur and iron increases sponge size to around {{convert|2|cm}}. It also increases porosity due to beneficial defects, creating buoyancy and reusability. Iron, in the form of [[ferrocene]] makes the structure easier to control and enables recovery using magnets. Such nanosponges increase the absorption of the toxic [[organic solvent]] [[dichlorobenzene]] from water by 3.5 times. The sponges can [[vegetable oil]] up to 150 times their initial weight and can absorb [[engine oil]] as well.<ref>[http://www.gizmag.com/carbon-nanotube-sponges/30511]</ref><ref>{{cite doi|10.1088/0957-4484/25/6/065701}}</ref>
 
Earlier, a magnetic boron-doped MWNT nanosponge  that could absorb oil from water. The sponge was grown as a forest on a substrate via chemical vapor disposition. Boron puts kinks and elbows into the tubes as they grow and promotes the formation of [[covalent bond]]s. The nanosponges retain their elastic property after 10,000 compressions in the lab. The sponges are both [[superhydrophobic]], forcing them to remain at the water's surface and oleophilic, drawing oil to them.<ref>[http://www.gizmag.com/oil-absorbing-nanosponge/22180/]</ref><ref>{{cite doi|10.1038/srep00363}}</ref>
 
=== Other applications ===
 
Carbon nanotubes have been implemented in nanoelectromechanical systems, including mechanical memory elements ([[Nano-RAM|NRAM]] being developed by [[Nantero|Nantero Inc.]]) and nanoscale electric motors (see [[Nanomotor]] or [[Nanotube nanomotor]]).
 
In May 2005, Nanomix Inc. placed on the market a hydrogen sensor that integrated carbon nanotubes on a silicon platform. Since then, Nanomix has been patenting many such sensor applications, such as in the field of carbon dioxide, nitrous oxide, glucose, DNA detection, etc.
 
[[Eikos|Eikos Inc]] of [[Franklin, Massachusetts|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 [[touchscreen]]s and flexible displays. Printable water-based inks of carbon nanotubes are desired to enable the production of these films to replace ITO.<ref>{{cite journal |last=Simmons |first=Trevor |year=2007 |title=Large Area-Aligned Arrays from Direct Deposition of Single-Wall Carbon Nanotubes |journal=J. Am. Chem. Soc.|volume=129|pages=10088–10089|doi=10.1021/ja073745e |pmid=17663555 |last2=Hashim |first2=D |last3=Vajtai |first3=R |last4=Ajayan |first4=PM |issue=33}}</ref> Nanotube films show promise for use in displays for computers, cell phones, [[Personal digital assistant|PDAs]], and [[Automated teller machine|ATMs]].
 
A [[nanoradio]], a radio receiver consisting of a single nanotube, was demonstrated in 2007.
 
A [[flywheel]] made of carbon nanotubes could be spun at extremely high velocity on a floating magnetic axis in a vacuum, and potentially store energy at a [[Energy density|density]] approaching that of conventional fossil fuels. Since energy can be added to and removed from flywheels very efficiently in the form of electricity, this might offer a way of [[Grid energy storage|storing electricity]], making the electrical grid more efficient and variable power suppliers (like wind turbines) more useful in meeting energy needs. The practicality of this depends heavily upon the cost of making massive, unbroken nanotube structures, and their failure rate under stress.
 
[[Carbon nanotube springs]] have the potential to indefinitely store elastic potential energy at ten times the density of lithium-ion batteries with flexible charge and discharge rates and extremely high cycling durability.
 
Ultra-short SWNTs (US-tubes) have been used as nanoscaled capsules for delivering MRI contrast agents in vivo.<ref>{{cite journal|title=Nanotechnology and MRI contrast enhancement|doi=10.4155/fmc.10.3|journal= Future Medicinal Chemistry|year= 2010|volume= 2|issue= 3|pages =491–502|last1=Matson|first1=Michael L|last2=Wilson|first2=Lon J|pmid=21426177}}</ref>
 
Carbon nanotubes provide a certain potential for [[carbocatalysis|metal-free catalysis]] of inorganic and organic reactions. For instance, oxygen groups attached to the surface of carbon nanotubes have the potential to catalyze oxidative [[dehydrogenation]]s<ref name=zhang>{{cite journal|last1 = Zhang|first1 = J.|last2 = Liu|first2 = X.|last3 = Blume|first3 = R.|last4 = Zhang|first4 = A.|last5 = Schlögl|first5 = R.|last6 = Su|first6 = D. S.|year = 2008|title = Surface-Modified Carbon Nanotubes Catalyze Oxidative Dehydrogenation of n-Butane|journal = Science|volume = 322|issue = 5898|pages = 73–77|doi = 10.1126/science.1161916 |pmid = 18832641|bibcode = 2008Sci...322...73Z }}</ref> or selective [[oxidations]].<ref name=frank>{{cite journal|last1 = Frank|first1 = B.|last2 = Blume|first2 = R.|last3 = Rinaldi|first3 = A.|last4 = Trunschke|first4 = A.|last5 = Schlögl|first5 = R.|year = 2011|title = Oxygen Insertion Catalysis by sp<sup>2</sup> Carbon| journal = Angew. Chem. Int. Ed.|volume = 50|issue = 43|pages = 10226–10230|doi = 10.1002/anie.201103340}}</ref> Nitrogen-doped carbon nanotubes may replace [[platinum]] catalysts used to reduce oxygen in [[fuel cell]]s. A forest of vertically aligned nanotubes can reduce oxygen in alkaline solution more effectively than platinum, which has been used in such applications since the 1960s. Here, the nanotubes have the added benefit of not being subject to carbon monoxide poisoning.<ref>{{cite journal|doi=10.1021/cen-v087n006.p007a|journal=Chemical & Engineering News|date=9 February 2009|title=Nanotube Catalysts|page=7|last1=Halford|first1=Bethany|volume=87|issue=6}}</ref>
 
[[Wake Forest University]] engineers are using multiwalled carbon nanotubes to enhance the brightness of [[field-induced polymer electroluminescent]] technology, potentially offering a step forward in the search for safe, pleasing, high-efficiency lighting. In this technology, moldable polymer matrix emits light when exposed to an electrical current. It could eventually yield high-efficiency lights without the mercury vapor of [[compact fluorescent lamps]] or the bluish tint of some fluorescents and LEDs, which has been linked with circadian rhythm disruption.<ref>{{cite journal |last=Dooley |first= Erin E.|last2= |first2= |date=February 2013 |title= The Beat, A New Lighting Alternative?|url= http://ehp.niehs.nih.gov/121-a47/|journal= Environmental Health Perspectives|publisher=National Institute of Environmental Health Sciences (NIEHS)  |volume=121 |issue=2 |pages= A47|doi= |accessdate=2013-12-01}}</ref>
 
[[Candida albicans]] has been used in combination with carbon nanotubes (CNT) to produce stable electrically conductive bio-nano-composite tissue materials that have been used as temperature sensing elements.<ref>http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6409466</ref>
 
==Discovery==
{{See also|Timeline of carbon nanotubes}}
A 2006 editorial written by Marc Monthioux and Vladimir Kuznetsov in the journal ''Carbon'' 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-size tubes composed of graphitic carbon to [[Sumio Iijima]] of [[NEC]] in 1991.<ref name="carbon">{{Cite journal|title=Who should be given the credit for the discovery of carbon nanotubes?|doi=10.1016/j.carbon.2006.03.019|first=Marc|last=Monthioux|journal=Carbon|volume=44|year=2006|url=http://www.cemes.fr/fichpdf/GuestEditorial.pdf |format=PDF|page=1621|last2=Kuznetsov|first2=V|issue=9}}</ref>
 
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''.<ref name="CVD"/> 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.<ref>{{Cite journal|title=Filamentous growth of carbon through benzene decomposition|doi=10.1016/0022-0248(76)90115-9|first=A.|last=Oberlin|year=1976|volume=32|pages=335–349|journal=Journal of Crystal Growth|issue=3|last2=Endo|first2=M.|last3=Koyama|first3=T.|bibcode = 1976JCrGr..32..335O }}</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>{{cite web|title=Carbon Fibers and Carbon Nanotubes (Interview, Nagano, Japan)| last=Endo|first=Morinobu|last2=Dresselhaus|first2=M. S.|date=October 26, 2002|url=http://web.mit.edu/tinytech/Nanostructures/Spring2003/MDresselhaus/i789.pdf |format=PDF}}</ref>
 
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 that 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.<ref>{{Cite journal|title=Structure of Carbon Fibers Found on Carbon Arc Anodes|journal=Carbon|volume=37|year=1999|page=1873|last=Abrahamson|first=John|last2=Wiles|first2=Peter G.|last3=Rhoades|first3=Brian L.|doi=10.1016/S0008-6223(99)00199-2|issue=11}}</ref>
 
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 [[X-ray scattering techniques|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).<ref>Izvestiya Akademii Nauk SSSR, Metals. 1982, #3, pp.12–17 (in Russian)</ref>
 
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&nbsp;nanometers..., length 10<sup>2</sup> times the diameter, and an outer region of multiple essentially continuous layers of ordered carbon atoms and a distinct inner core...."<ref>{{Ref patent|country=US|number=4663230|title=Carbon fibrils, method for producing same and compositions containing same|gdate=1987-05-05|fdate=1984-12-06|invent1=Tennent, Howard G.}}</ref>
 
Iijima's discovery of multi-walled carbon nanotubes in the insoluble material of arc-burned graphite rods in 1991<ref>{{Cite journal|first=Sumio|last=Iijima|title=Helical microtubules of graphitic carbon|journal=Nature|volume=354|pages=56–58|date=7 November 1991|doi=10.1038/354056a0|issue=6348|bibcode = 1991Natur.354...56I }}</ref> and Mintmire, Dunlap, and White's independent prediction that if single-walled carbon nanotubes could be made, then they would exhibit remarkable conducting properties<ref>{{cite journal|first=J.W.|last=Mintmire|title=Are Fullerene Tubules Metallic?|journal=Phys. Rev. Lett.|volume=68|pages=631–634|year=1992|doi=10.1103/PhysRevLett.68.631|pmid=10045950|last2=Dunlap|first2=BI|last3=White|first3=CT|issue=5|bibcode = 1992PhRvL..68..631M }}</ref> helped create the initial buzz that is now associated with carbon nanotubes. Nanotube research accelerated greatly following the independent discoveries<ref>{{cite journal| title=Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layer walls|journal=Nature|volume=363|pages=605–607|year=1993|doi=10.1038/363605a0|issue=6430| last1=Bethune| first1=D. S.| last2=Kiang| first2=C. H.| last3=De Vries| first3=M. S.| last4=Gorman| first4=G.| last5=Savoy| first5=R.| last6=Vazquez| first6=J.| last7=Beyers| first7=R.|bibcode = 1993Natur.363..605B }}</ref><ref>{{Cite journal|first=Sumio|last=Iijima|year=1993|title=Single-shell carbon nanotubes of 1-nm diameter|journal=Nature|volume=363|pages=603–605|doi=10.1038/363603a0|last2=Ichihashi|first2=Toshinari|issue=6430|bibcode = 1993Natur.363..603I }}</ref> by Bethune at IBM<ref>{{cite web|title=The Discovery of Single-Wall Carbon Nanotubes at IBM|publisher=IBM|url=http://reswat4.research.ibm.com/projects/earlyswnt/earlyswnt.nsf/main/toppage/}}</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 name="Kratschmer-C60">{{Cite journal|first=W.|last=Krätschmer|year=1990|title=Solid C60: a new form of carbon|journal=Nature|volume=347|pages=354–358|doi=10.1038/347354a0|last2=Lamb|first2=Lowell D.|last3=Fostiropoulos|first3=K.|last4=Huffman|first4=Donald R.|issue=6291|bibcode = 1990Natur.347..354K }}</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>{{Cite journal|first=H. W.|last=Kroto|year=1985|title=C60: Buckminsterfullerene|doi=10.1038/318162a0|journal=Nature|volume=318|pages=162–163|last2=Heath|first2=J. R.|last3=O'Brien|first3=S. C.|last4=Curl|first4=R. F.|last5=Smalley|first5=R. E.|issue=6042|bibcode = 1985Natur.318..162K }}</ref> and the first mass-production technique by Krätschmer and Huffman was used for several years before realizing that it produced fullerenes.<ref name="Kratschmer-C60" />
 
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.<ref name="carbon"/>
 
==See also==
{{Portal|Nanotechnology}}
{{colbegin|3}}
*[[Boron nitride#Boron nitride nanotubes|Boron nitride nanotube]]
*[[Buckypaper]]
*[[Carbide-derived carbon]]
*[[Carbon nanocone]]
*[[Carbon nanofibers]]
*[[Carbon nanoparticles]]
*[[Carbon nanotube chemistry]]
*[[Colossal carbon tube]]
*[[Filamentous carbon]]
*[[Graphene oxide paper]]
*[[List of software for nanostructures modeling]]
*[[Molecular modelling]]
*[[Nanoflower]]
*[[Ninithi]] (nanotube modelling software)
*[[Organic semiconductor]]
*[[Selective chemistry of single-walled nanotubes]]
*[[Silicon nanotubes]]
{{colend}}
 
==References==
''This article incorporates public domain text from National Institute of Environmental Health Sciences (NIEHS) as quoted.''
{{Reflist|35em}}
 
==External links==
{{Commons|Carbon nanotube}}
*[http://www.nanohedron.com Nanohedron.com] image gallery with carbon nanotubes
*[http://www.nanofluid.ir]
*[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/nanotube.html The Nanotube site]. Last updated 2009.05.03
*[http://www.carbio.eu EU Marie Curie Network CARBIO: Multifunctional carbon nanotubes for biomedical applications]
*[http://xstructure.inr.ac.ru/x-bin/theme3.py?level=2&index1=127236 Carbon nanotube on arxiv.org]
*[http://www.vega.org.uk/video/programme/223 C<sub>60</sub> and Carbon Nanotubes a short video explaining how nanotubes can be made from modified graphite sheets and the three different types of nanotubes that are formed]
*[http://education.mrsec.wisc.edu/143.htm Carbon Nanotubes & Buckyballs].
*[http://students.chem.tue.nl/ifp03/Wondrous%20World%20of%20Carbon%20Nanotubes_Final.pdf The Wondrous World of Carbon Nanotubes]
*Learning module for [http://nanohub.org/resources/2762 Bandstructure of Carbon Nanotubes and Nanoribbons]
*[http://www.safeworkaustralia.gov.au/AboutSafeWorkAustralia/WhatWeDo/Publications/Pages/DurabilityCarbonNanotubes.aspx Durability of carbon nanotubes and their potential to cause inflammation] by Dr Megan Osmond and others. (SafeWork Australia, May 2011). This was a collaboration between the [[Institute of Occupational Medicine]], Edinburgh University and CSIRO in Australia.
*[http://nanotube.msu.edu/nt06/ NT06 Seventh International Conference  on the Science and Application of Nanotubes]
*[http://nanotube.msu.edu/nt05/ NT05 Sixth International Conference  on the Science and Application of Nanotubes]
*[http://stacks.iop.org/1367-2630/5/i=1/a=E04 Selection of free-download articles on carbon nanotubes]
*[http://www.bbc.co.uk/news/science-environment-24232896 First computer made of carbon nanotubes is unveiled ([[BBC News]])] 2013-09-25
 
{{Allotropes of carbon}}
{{Emerging technologies}}
 
{{DEFAULTSORT:Carbon Nanotube}}
[[Category:Carbon nanotubes| ]]
[[Category:Emerging technologies]]
[[Category:Transparent electrodes]]
[[Category:Refractory materials]]
 
{{Link FA|ar}}

Revision as of 23:15, 28 February 2014

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