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{{Short description|Allotropes of carbon with a cylindrical nanostructure}}
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[[File:Chiraltube.png|thumb|A [[scanning tunneling microscopy]] image of a single-walled carbon nanotube]]

[[File:Kohlenstoffnanoroehre Animation.gif|thumb|Rotating single-walled zigzag carbon nanotube]]
{{Nanomaterials}}

A '''carbon nanotube''' ('''CNT''') is a tube made of carbon with a diameter in the nanometre range ([[nanoscale]]). They are one of the [[allotropes of carbon]].  Two broad classes of carbon nanotubes are recognized:
* ''Single-walled carbon nanotubes'' (''SWCNTs'') have diameters around 0.5–2.0 [[nanometer|nanometres]], about 100,000 times smaller than the width of a human hair. They can be idealised as cutouts from a two-dimensional [[graphene]] sheet rolled up to form a hollow cylinder.
* ''Multi-walled carbon nanotubes'' (''MWCNTs'') consist of nested single-wall carbon nanotubes in a nested, tube-in-tube structure. Double- and triple-walled carbon nanotubes are special cases of MWCNT.

Carbon nanotubes can exhibit remarkable properties, such as exceptional [[tensile strength]] and [[thermal conductivity]] because of their [[nanostructure]] and [[bond strength|strength]] of the bonds between carbon atoms. Some SWCNT structures exhibit high [[electrical conductivity]] while others are [[semiconductor]]s. In addition, carbon nanotubes can be chemically modified. These properties are expected to be valuable in many areas of technology, such as [[electronics]], [[optics]], [[composite materials]] (replacing or complementing [[carbon fibers|carbon fibres]]), [[nanotechnology]] (including nanomedicine), and other applications of [[materials science]].

The predicted properties for SWCNTs were tantalising, but a path to synthesising them was lacking until 1993, when [[Sumio Iijima|Iijima]] and Ichihashi at [[NEC]], and Bethune and others at [[IBM]] independently discovered that co-vaporising carbon and transition metals such as iron and cobalt could specifically catalyse SWCNT formation.  These discoveries triggered research that succeeded in greatly increasing the efficiency of the catalytic production technique, and led to an explosion of work to characterise and find applications for SWCNTs.

== History ==
{{See also|Timeline of carbon nanotubes|Fullerene#History}}
The true identity of the discoverers of carbon nanotubes is a subject of some controversy.<ref name="carbon1">{{cite thesis|title=Carbon Nanotubes as Platforms for Biosensors with Electrochemical and Electronic Transduction| vauthors = Pacios Pujadó M |publisher=Springer Heidelberg|year=2012|pages=xx, 208|doi=10.1007/978-3-642-31421-6|isbn=978-3-642-31421-6|series=Springer Theses|hdl=10803/84001|s2cid=199491391 |url=http://ddd.uab.cat/record/101063|hdl-access=free}}</ref> A 2006 editorial written by Marc Monthioux and Vladimir Kuznetsov in the journal ''Carbon'' described the origin of the carbon nanotube.<ref name="carbon">{{cite journal |vauthors = Monthioux M, Kuznetsov VL |title=Who should be given the credit for the discovery of carbon nanotubes? |journal=Carbon |date=August 2006 |volume=44 |issue=9 |pages=1621–1623 |doi=10.1016/j.carbon.2006.03.019 |bibcode=2006Carbo..44.1621M |url=https://nanotube.msu.edu/HSS/2006/1/2006-1.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://nanotube.msu.edu/HSS/2006/1/2006-1.pdf |archive-date=2022-10-09 |url-status=live }}</ref> A large percentage of academic and popular literature attributes the discovery of hollow, nanometre-size tubes composed of graphitic carbon to [[Sumio Iijima]] of [[NEC]] in 1991. His paper initiated a flurry of excitement and could be credited with inspiring the many scientists now studying applications of carbon nanotubes. Though Iijima has been given much of the credit for discovering carbon nanotubes, it turns out that the timeline of carbon nanotubes goes back much further than 1991.<ref name="carbon1" />

In 1952, L. V. Radushkevich and V. M. Lukyanovich published clear images of 50-nanometre diameter tubes made of carbon in the ''Journal of Physical Chemistry Of Russia''.<ref name="CVD" /> This discovery was largely unnoticed, as the article was published in Russian, and Western scientists' access to Soviet press was limited during the [[Cold War]]. Monthioux and Kuznetsov mentioned in their ''Carbon'' editorial:<ref name="carbon" /> {{blockquote|text=The fact is, Radushkevich and Lukyanovich [...] should be credited for the discovery that carbon filaments could be hollow and have a nanometre-size diameter, that is to say for the discovery of carbon nanotubes.}}

In 1976, [[Morinobu Endo]] of [[Centre national de la recherche scientifique|CNRS]] observed hollow tubes of rolled up graphite sheets synthesised by a chemical vapour-growth technique.<ref name="endo1" /> The first specimens observed would later come to be known as single-walled carbon nanotubes (SWNTs).<ref name="carbon2">{{cite report|title=WTEC Panel Report on 'International Assessment of Research and Development of Carbon Nanotube Manufacturing and Applications' Final Report| vauthors = Eklund PC |publisher=World Technology Evaluation Center (WTEC)|year=2007|url=http://www.wtec.org/cnm/CNM_final_report.pdf|access-date=5 August 2015|archive-url=https://web.archive.org/web/20170311174000/http://www.wtec.org/cnm/CNM_final_report.pdf|archive-date=11 March 2017}}</ref> Endo, in his early review of vapor-phase-grown carbon fibers (VPCF), also reminded us that he had observed a hollow tube, linearly extended with parallel carbon layer faces near the fiber core.<ref>{{cite journal | vauthors = Oberlin A, Endo M, Koyama T |title=Filamentous growth of carbon through benzene decomposition |journal=Journal of Crystal Growth |date=March 1976 |volume=32 |issue=3 |pages=335–349 |doi=10.1016/0022-0248(76)90115-9 |bibcode=1976JCrGr..32..335O |url=http://www.kroto.info/wp-content/uploads/2015/10/1.M.ENdoJCrystalGrowth1976.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.kroto.info/wp-content/uploads/2015/10/1.M.ENdoJCrystalGrowth1976.pdf |archive-date=2022-10-09 |url-status=live }}</ref> This appears to be the observation of multi-walled carbon nanotubes at the center of the fiber.<ref name="carbon2" /> The mass-produced MWCNTs today are strongly related to the VPGCF developed by Endo.<ref name="carbon2" /> In fact, they call it the "Endo-process", out of respect for his early work and patents.<ref name="carbon2" /><ref>{{cite patent | inventor = Koyama T, Endo MT | gdate = 1983 | title = Method for Manufacturing Carbon Fibers by a Vapor Phase Process | country = JP | number = 1982-58,966 }}</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 | vauthors = Abrahamson J, Wiles PG, Rhoades BL |title=Structure of carbon fibres found on carbon arc anodes |journal=Carbon |date=January 1999 |volume=37 |issue=11 |pages=1873–1874 |doi=10.1016/S0008-6223(99)00199-2 |bibcode=1999Carbo..37.1873A }}</ref>

In 1981, a group of Soviet scientists published the results of chemical and structural characterization of [[carbon nanoparticles]] produced by a thermocatalytic 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 via this rolling, many different arrangements of graphene hexagonal nets are possible. They suggested two such possible arrangements: a circular arrangement (armchair nanotube); and a spiral, helical arrangement (chiral tube).<ref>{{cite journal | author = Missing | title = Missing | journal = Izvestiya Akademii Nauk SSSR Metally | trans-journal = Proceedings of the Academy of Sciences of the USSR. Metals | language = ru | date = 1982 | volume = 3 | pages = 12–17 }} {{full citation needed|missing title and author|date=October 2021}}</ref>

In 1987, Howard G. Tennent of Hyperion Catalysis was issued a U.S. patent for the production of "cylindrical discrete carbon fibrils" with a "constant diameter between about 3.5 and about 70&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| inventor = Tennent HG }}</ref>

Helping to create the initial excitement associated with carbon nanotubes were Iijima's 1991 discovery of multi-walled carbon nanotubes in the insoluble material of arc-burned graphite rods;<ref name=Iijima1 /> and Mintmire, Dunlap, and White's independent prediction that if single-walled carbon nanotubes could be made, they would exhibit remarkable conducting properties.<ref name=mintmire1 /> Nanotube research accelerated greatly following the independent discoveries<ref name=Iijima2 /><ref name=Bethune /> by Iijima and Ichihashi at NEC and Bethune ''et al.'' at IBM of methods to specifically produce ''single-walled'' carbon nanotubes by adding transition-metal catalysts to the carbon in an arc discharge. Thess et al.<ref name=Thess /> refined this catalytic method by vaporizing the carbon/transition-metal combination in a high-temperature furnace, which greatly improved the yield and purity of the SWNTs and made them widely available for characterization and application experiments. The arc discharge technique, well known to produce the famed [[Buckminsterfullerene]],<ref name="Kratschmer-C60">{{cite journal| vauthors = Krätschmer W, Lamb LD, Fostiropoulos KH, Huffman DR |year=1990|title=Solid C60: a new form of carbon|journal=Nature|volume=347|pages=354–358|doi=10.1038/347354a0 |issue=6291|bibcode = 1990Natur.347..354K|s2cid=4359360}}</ref>{{failed verification|date=October 2024}} thus played a role in the discoveries of both multi- and single-wall nanotubes, extending the run of serendipitous discoveries relating to fullerenes.  The discovery of nanotubes remains a contentious issue. Many believe that Iijima's report in 1991 is of particular importance because it brought carbon nanotubes into the awareness of the scientific community as a whole.<ref name="carbon1" /><ref name="carbon2" />

In 2020, during an archaeological excavation of [[Keezhadi]] in [[Tamil Nadu]], [[India]], ~2600-year-old pottery was discovered whose coatings appear to contain carbon nanotubes.  The robust mechanical properties of the nanotubes are partially why the coatings have lasted for so many years, say the scientists.<ref>{{cite journal | vauthors = Kokarneswaran M, Selvaraj P, Ashokan T, Perumal S, Sellappan P, Murugan KD, Ramalingam S, Mohan N, Chandrasekaran V | title = Discovery of carbon nanotubes in sixth century BC potteries from Keeladi, India | journal = Scientific Reports | volume = 10 | issue = 1 | page = 19786 | date = November 2020 | pmid = 33188244 | pmc = 7666134 | doi = 10.1038/s41598-020-76720-z | bibcode = 2020NatSR..1019786K }}</ref>

== Structure of SWCNTs ==
{{multiple image
| image1   = ZigzagCNT.png
| caption1 = Zigzag nanotube, configuration (8, 0)
| image2   = ArmchairCNT.png
| caption2 = Armchair nanotube, configuration (4, 4)
}}

=== Basic details ===
[[File:Nanotube strip +03 +01.pdf|thumb|A "sliced and unrolled" representation of a carbon nanotube as a strip of a graphene molecule, overlaid on a diagram of the full molecule (faint background). The arrow shows the gap ''A2'' where the atom ''A1'' on one edge of the strip would fit in the opposite edge, as the strip is rolled up]]
[[File:Nanotube strip master.pdf|thumb|left|The basis vectors '''''u''''' and '''''v''''' of the relevant sub-lattice, the (n,m) pairs that define non-isomorphic carbon nanotube structures (red dots), and the pairs that define the enantiomers of the chiral ones (blue dots)]]

The structure of an ideal (infinitely long) single-walled carbon nanotube is that of a regular hexagonal lattice drawn on an infinite [[cylinder|cylindrical]] surface, whose vertices are the positions of the carbon atoms. Since the length of the carbon-carbon bonds is fairly fixed, there are constraints on the diameter of the cylinder and the arrangement of the atoms on it.<ref name="sinnott">{{cite journal | vauthors = Sinnott SB, Andrews R |title=Carbon Nanotubes: Synthesis, Properties, and Applications |journal=Critical Reviews in Solid State and Materials Sciences |date=July 2001 |volume=26 |issue=3 |pages=145–249 |doi=10.1080/20014091104189 |bibcode=2001CRSSM..26..145S |s2cid=95444574 }}</ref>

In the study of nanotubes, one defines a zigzag path on a graphene-like lattice as a [[path (graph theory)|path]] that turns 60 degrees, alternating left and right, after stepping through each bond. It is also conventional to define an armchair path as one that makes two left turns of 60 degrees followed by two right turns every four steps. On some carbon nanotubes, there is a closed zigzag path that goes around the tube. One says that the tube is of the '''zigzag type''' or configuration, or simply is a '''zigzag nanotube'''. If the tube is instead encircled by a closed armchair path, it is said to be of the '''armchair type''', or an '''armchair nanotube'''. An infinite nanotube that is of one type consists entirely of closed paths of that type, connected to each other.

The zigzag and armchair configurations are not the only structures that a single-walled nanotube can have. To describe the structure of a general infinitely long tube, one should imagine it being sliced open by a cut parallel to its axis, that goes through some atom ''A'', and then unrolled flat on the plane, so that its atoms and bonds coincide with those of an imaginary graphene sheet—more precisely, with an infinitely long strip of that sheet. The two halves of the atom ''A'' will end up on opposite edges of the strip, over two atoms ''A1'' and ''A2'' of the graphene. The line from ''A1'' to ''A2'' will correspond to the circumference of the cylinder that went through the atom ''A'', and will be perpendicular to the edges of the strip. In the graphene lattice, the atoms can be split into two classes, depending on the directions of their three bonds. Half the atoms have their three bonds directed the same way, and half have their three bonds rotated 180 degrees relative to the first half. The atoms ''A1'' and ''A2'', which correspond to the same atom ''A'' on the cylinder, must be in the same class. It follows that the circumference of the tube and the angle of the strip are not arbitrary, because they are constrained to the lengths and directions of the lines that connect pairs of graphene atoms in the same class.

Let '''''u''''' and '''''v''''' be two [[linear dependency|linearly independent]] vectors that connect the graphene atom ''A1'' to two of its nearest atoms with the same bond directions. That is, if one numbers consecutive carbons around a graphene cell with C1 to C6, then '''''u''''' can be the vector from C1 to C3, and '''''v''''' be the vector from C1 to C5. Then, for any other atom ''A2'' with same class as ''A1'', the vector from ''A1'' to ''A2'' can be written as a [[linear combination]] ''n'' '''''u''''' + ''m'' '''''v''''', where ''n'' and ''m'' are integers. And, conversely, each pair of integers (''n'',''m'') defines a possible position for ''A2''.<ref name="sinnott" /> Given ''n'' and ''m'', one can reverse this theoretical operation by drawing the vector '''''w''''' on the graphene lattice, cutting a strip of the latter along lines perpendicular to '''''w''''' through its endpoints ''A1'' and ''A2'', and rolling the strip into a cylinder so as to bring those two points together. If this construction is applied to a pair (''k'',0), the result is a zigzag nanotube, with closed zigzag paths of 2''k'' atoms. If it is applied to a pair (''k'',''k''), one obtains an armchair tube, with closed armchair paths of 4''k'' atoms.

== Types ==
The structure of the nanotube is not changed if the strip is rotated by 60 degrees clockwise around ''A1'' before applying the hypothetical reconstruction above. Such a rotation changes the corresponding pair (''n'',''m'') to the pair (−2''m'',''n''+''m''). It follows that many possible positions of ''A2'' relative to ''A1'' — that is, many pairs (''n'',''m'') — correspond to the same arrangement of atoms on the nanotube. That is the case, for example, of the six pairs (1,2), (−2,3), (−3,1), (−1,−2), (2,−3), and (3,−1). In particular, the pairs (''k'',0) and (0,''k'') describe the same nanotube geometry. These redundancies can be avoided by considering only pairs (''n'',''m'') such that ''n'' > 0 and ''m'' ≥ 0; that is, where the direction of the vector '''''w''''' lies between those of '''''u''''' (inclusive) and '''''v''''' (exclusive). It can be verified that every nanotube has exactly one pair (''n'',''m'') that satisfies those conditions, which is called the tube's '''type'''. Conversely, for every type there is a hypothetical nanotube. In fact, two nanotubes have the same type if and only if one can be conceptually rotated and translated so as to match the other exactly. Instead of the type (''n'',''m''), the structure of a carbon nanotube can be specified by giving the length of the vector '''''w''''' (that is, the circumference of the nanotube), and the angle ''α'' between the directions of '''''u''''' and '''''w''''',
may range from 0 (inclusive) to 60 degrees clockwise (exclusive). If the diagram is drawn with '''''u''''' horizontal, the latter is the tilt of the strip away from the vertical.
{{multiple image
| caption           = Unrolled nanotube diagrams
| image1            = nanotube strip +03 +01.pdf
| caption1          = Chiral nanotube of the (3,1) type
| image2            = nanotube strip +01 +03.pdf
| caption2          = Chiral nanotube of the (1,3) type, mirror image of the (3,1) type
| image3            = nanotube strip +02 +02.pdf
| caption3          = Nanotube of the (2,2) type, the narrowest "armchair" one
| image4            = nanotube strip +03 000.pdf
| caption4          = Nanotube of the (3,0) type, the narrowest "zigzag" one
| align             = center
}}

=== Chirality and mirror symmetry ===

A nanotube is [[chirality|chiral]] if it has type (''n'',''m''), with ''m'' > 0 and ''m'' ≠ ''n''; then its [[enantiomer]] (mirror image) has type (''m'',''n''), which is different from (''n'',''m''). This operation corresponds to mirroring the unrolled strip about the line ''L'' through ''A1'' that makes an angle of 30 degrees clockwise from the direction of the '''''u''''' vector (that is, with the direction of the vector '''''u'''''+'''''v'''''). The only types of nanotubes that are [[achiral]] are the (''k'',0) "zigzag" tubes and the (''k'',''k'') "armchair" tubes. If two enantiomers are to be considered the same structure, then one may consider only types (''n'',''m'') with 0 ≤ ''m'' ≤ ''n'' and ''n'' > 0. Then the angle ''α'' between '''''u''''' and '''''w''''', which may range from 0 to 30 degrees (inclusive both), is called the "chiral angle" of the nanotube.

=== Circumference and diameter ===

From ''n'' and ''m'' one can also compute the circumference ''c'', which is the length of the vector '''''w''''', which turns out to be:

: <math> c = \left|\boldsymbol{u}\right| \sqrt{(n^2 + n m + m^2)} \approx 246 \sqrt{((n+m)^2-n m)} </math>

in [[picometre]]s. The diameter <math>d</math> of the tube is then <math>c/\pi</math>, that is

: <math>d \approx 78.3 \sqrt{((n+m)^2-n m)}</math>

also in picometres. (These formulas are only approximate, especially for small ''n'' and ''m'' where the bonds are strained; and they do not take into account the thickness of the wall.)

The tilt angle ''α'' between '''''u''''' and '''''w''''' and the circumference ''c'' are related to the type indices ''n'' and ''m'' by:

: <math> \alpha \;=\; \arg(n + m/2,\, m \sqrt{3}/2) \;=\; \mathop{\mathrm{arc}}\cos\frac{n + m/2}{c} </math>

where arg(''x'',''y'') is the clockwise angle between the ''X''-axis and the vector (''x'',''y''); a function that is available in many programming languages as <code>atan2</code>(''y'',''x''). Conversely, given ''c'' and ''α'', one can get the type (''n'',''m'') by the formulas:

: <math>m = \frac{2 c}{\sqrt{3}}\sin \alpha \quad\quad n = c \cos\alpha - \frac{m}{2}</math>
which must evaluate to integers.

== Physical limits ==

=== Narrowest examples ===
{{multiple image
| header   = Tube types that are "degenerate" for being too narrow
| image1   = nanotube strip +01 000.pdf
| caption1 = Degenerate "zigzag" tube type (1,0)
| image2   = nanotube strip +02 000.pdf
| caption2 = Degenerate "zigzag" tube type (2,0)
| image3   = nanotube strip +01 +01.pdf
| caption3 = Degenerate "armchair" tube type (1,1)
| image4   = nanotube strip +02 +01.pdf
| caption4 = Possibly degenerate chiral tube type (2,1)
}}
{{clear}}
If ''n'' and ''m'' are too small, the structure described by the pair (''n'',''m'') will describe a molecule that cannot be reasonably called a "tube", and may not even be stable. For example, the structure theoretically described by the pair (1,0) (the limiting "zigzag" type) would be just a chain of carbons. That is a real molecule, the [[Linear acetylenic carbon|carbyne]]; which has some characteristics of nanotubes (such as orbital hybridization, high tensile strength, etc.) — but has no hollow space, and may not be obtainable as a condensed phase. The pair (2,0) would theoretically yield a chain of fused 4-cycles; and (1,1), the limiting "armchair" structure, would yield a chain of bi-connected 4-rings. These structures may not be realizable.

The thinnest carbon nanotube proper is the armchair structure with type (2,2), which has a diameter of 0.3&nbsp;nm. This nanotube was grown inside a multi-walled carbon nanotube. Assigning of the carbon nanotube type was done by a combination of [[high-resolution transmission electron microscopy]] (HRTEM), [[Raman spectroscopy]], and [[density functional theory]] (DFT) calculations.<ref>{{cite journal | vauthors = Zhao X, Liu Y, Inoue S, Suzuki T, Jones RO, Ando Y | title = Smallest carbon nanotube is 3 a in diameter | journal = Physical Review Letters | volume = 92 | issue = 12 | page = 125502 | date = March 2004 | pmid = 15089683 | doi = 10.1103/PhysRevLett.92.125502 | url = http://juser.fz-juelich.de/record/38262/files/48588.pdf | url-status = live | bibcode = 2004PhRvL..92l5502Z | archive-url = https://ghostarchive.org/archive/20221009/http://juser.fz-juelich.de/record/38262/files/48588.pdf | archive-date = 2022-10-09 }}</ref>

The thinnest ''freestanding'' single-walled carbon nanotube is about 0.43&nbsp;nm in diameter.<ref>{{cite journal | vauthors = Torres-Dias AC |title=From mesoscale to nanoscale mechanics in single-wall carbon nanotubes |journal=Carbon |date=2017 |volume=123 |pages=145–150 |doi=10.1016/j.carbon.2017.07.036 |bibcode=2017Carbo.123..145T |url= http://qmro.qmul.ac.uk/xmlui/handle/123456789/25311}}</ref> Researchers suggested that it can be either (5,1) or (4,2) SWCNT, but the exact type of the carbon nanotube remains questionable.<ref name="Smallest">{{cite journal | vauthors = Hayashi T, Kim YA, Matoba T, Esaka M, Nishimura K, Tsukada T, Endo M, Dresselhaus MS | year = 2003 | title = Smallest Freestanding Single-Walled Carbon Nanotube | journal = [[Nano Letters]] | volume = 3 |issue=7 |pages=887–889 | doi = 10.1021/nl034080r | author-link8 = Mildred Dresselhaus | bibcode = 2003NanoL...3..887H}}</ref> (3,3), (4,3), and (5,1) carbon nanotubes (all about 0.4&nbsp;nm in diameter) were unambiguously identified using aberration-corrected [[high-resolution transmission electron microscopy]] inside double-walled CNTs.<ref>{{cite journal | vauthors = Guan L, Suenaga K, Iijima S | title = Smallest carbon nanotube assigned with atomic resolution accuracy | journal = Nano Letters | volume = 8 | issue = 2 | pages = 459–462 | date = February 2008 | pmid = 18186659 | doi = 10.1021/nl072396j | author-link3 = Sumio Iijima | bibcode = 2008NanoL...8..459G }}</ref>

=== Length ===
[[File:Cycloparaphenylene.PNG|thumb|Cycloparaphenylene]]

The observation of the ''longest'' carbon nanotubes grown so far, around 0.5&nbsp;metre (550&nbsp;mm) long, was reported in 2013.<ref name="Zhang" /> These nanotubes were grown on [[silicon]] substrates using an improved [[chemical vapor deposition]] (CVD) method and represent electrically uniform arrays of single-walled carbon nanotubes.<ref name=wang2009long />

The ''shortest'' carbon nanotube can be considered to be the organic compound [[cycloparaphenylene]], which was synthesized in 2008 by [[Ramesh Jasti]].<ref>{{cite journal | vauthors = Jasti R, Bhattacharjee J, Neaton JB, Bertozzi CR | title = Synthesis, characterization, and theory of [9]-, [12]-, and [18]cycloparaphenylene: carbon nanohoop structures | journal = Journal of the American Chemical Society | volume = 130 | issue = 52 | pages = 17646–17647 | date = December 2008 | pmid = 19055403 | pmc = 2709987 | doi = 10.1021/ja807126u }}</ref> Other small molecule carbon nanotubes have been synthesized since.<ref>{{cite journal | vauthors = Cheung KY, Segawa Y, Itami K | title = Synthetic Strategies of Carbon Nanobelts and Related Belt-Shaped Polycyclic Aromatic Hydrocarbons | journal = Chemistry: A European Journal | volume = 26 | issue = 65 | pages = 14791–14801 | date = November 2020 | pmid = 32572996 | doi = 10.1002/chem.202002316 | s2cid = 219983922 }}</ref>

=== Density ===
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&nbsp;°C. The tubes averaged a height of 380&nbsp;nm and a mass density of 1.6 g cm<sup>−3</sup>. 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=27 September 2013}}</ref><ref>{{cite journal | vauthors = Sugime H, Esconjauregui S, Yang J, D'Arsié L, Oliver RA, Bhardwaj S, Cepek C, Robertson J |title=Low temperature growth of ultra-high mass density carbon nanotube forests on conductive supports |journal=Applied Physics Letters |date=12 August 2013 |volume=103 |issue=7 |page=073116 |doi=10.1063/1.4818619 |bibcode=2013ApPhL.103g3116S }}</ref>

== Variants ==
There is no consensus on some terms describing carbon nanotubes in the 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). The [[International Standards Organization]] typically uses "single-walled carbon nanotube (SWCNT)" or "multi-walled carbon nanotube (MWCNT)" in its documents.<ref>{{cite web |title=Nanotechnologies — Vocabulary — Part 3: Carbon nano-objects |url=https://www.iso.org/obp/ui/es/#iso:std:iso:ts:80004:-3:ed-2:v1:en |website=www.iso.org |access-date=22 October 2024}}</ref>

=== Multi-walled ===
[[File:Multi-walled Carbon Nanotube.png|thumb|Triple-walled armchair carbon nanotube]]

Multi-walled nanotubes (MWNTs) 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.<ref name=Hamada /><ref name=wildoer /> 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.<ref>{{cite journal | vauthors = Das S |title=A review on Carbon nano-tubes – A new era of nanotechnology |journal=International Journal of Emerging Technology and Advanced Engineering |volume=3 |issue=3 |date=March 2013 |pages=774–781 |url=https://ijetae.com/files/Volume3Issue3/IJETAE_0313_132.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://ijetae.com/files/Volume3Issue3/IJETAE_0313_132.pdf |archive-date=2022-10-09 |url-status=live |citeseerx=10.1.1.413.7576 }}</ref>

Double-walled carbon nanotubes (DWNTs) form a special class of nanotubes because their [[Morphology (biology)|morphology]] and properties are similar to those of SWNTs but they are more resistant to attacks by chemicals.<ref>{{cite journal | vauthors = Piao Y, Chen CF, Green AA, Kwon H, Hersam MC, Lee CS, Schatz GC, Wang Y |title=Optical and Electrical Properties of Inner Tubes in Outer Wall-Selectively Functionalized Double-Wall Carbon Nanotubes |journal=The Journal of Physical Chemistry Letters |date=7 July 2011 |volume=2 |issue=13 |pages=1577–1582 |doi=10.1021/jz200687u }}</ref> This is especially important when it is necessary to graft chemical functions to the surface of the nanotubes ([[Surface functionalization|functionalization]]) to add properties to the CNT. Covalent functionalization of SWNTs 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 DWNTs, only the outer wall is modified. DWNT synthesis on the gram-scale by the [[CCVD]] technique was first proposed in 2003<ref>{{cite journal | vauthors = Flahaut E, Bacsa R, Peigney A, Laurent C | title = Gram-scale CCVD synthesis of double-walled carbon nanotubes | journal = Chemical Communications | issue = 12 | pages = 1442–1443 | date = June 2003 | pmid = 12841282 | doi = 10.1039/b301514a | url = https://hal.archives-ouvertes.fr/hal-00926035/file/Flahaut_10551.pdf | url-status = live | s2cid = 30627446 | archive-url = https://ghostarchive.org/archive/20221009/https://hal.archives-ouvertes.fr/hal-00926035/file/Flahaut_10551.pdf | archive-date = 2022-10-09 }}</ref> from the selective reduction of oxide solutions in methane and hydrogen.

The telescopic motion ability of inner shells, allowing them to act as low-friction, low-wear nanobearings and nanosprings, may make them a desirable material in [[nanoelectromechanical systems]] (NEMS) .<ref>{{cite journal | vauthors = Cumings J, Zettl A | title = Low-friction nanoscale linear bearing realized from multiwall carbon nanotubes | journal = Science | volume = 289 | issue = 5479 | pages = 602–604 | date = July 2000 | pmid = 10915618 | doi = 10.1126/science.289.5479.602 | citeseerx = 10.1.1.859.7671 | bibcode = 2000Sci...289..602C }}</ref> The retraction force that occurs to telescopic motion is caused by the [[Lennard-Jones interaction]] between shells, and its value is about 1.5&nbsp;nN.<ref>{{cite journal | vauthors = Zavalniuk V, Marchenko S | year = 2011 | title = Theoretical analysis of telescopic oscillations in multi-walled carbon nanotubes | journal = Low Temperature Physics | volume = 37 | issue = 4 | pages = 337–342 | arxiv = 0903.2461 | doi = 10.1063/1.3592692 | bibcode = 2011LTP....37..337Z | s2cid = 51932307 | url = http://dspace.nbuv.gov.ua/xmlui/bitstream/123456789/118539/1/12-Zavalniuk.pdf | archive-url = https://ghostarchive.org/archive/20221009/http://dspace.nbuv.gov.ua/xmlui/bitstream/123456789/118539/1/12-Zavalniuk.pdf | archive-date = 2022-10-09 | url-status = live }}</ref>

=== Junctions and crosslinking ===

[[File:Nanotube junction.jpg|thumb|Transmission electron microscope image of carbon nanotube junction]] Junctions between two or more nanotubes have been widely discussed theoretically.<ref name=Chernozatonskii>{{cite journal | vauthors = Chernozatonskii LA |year=1992 |title=Carbon nanotube connectors and planar jungle gyms |journal=[[Physics Letters A]] | volume = 172 | issue = 3 | pages = 173–176 | doi=10.1016/0375-9601(92)90978-u| bibcode = 1992PhLA..172..173C}}</ref><ref name= Menon>{{cite journal | vauthors = Menon M, Srivastava D |title=Carbon Nanotube 'T Junctions': Nanoscale Metal-Semiconductor-Metal Contact Devices |journal=Physical Review Letters |date=1 December 1997 |volume=79 |issue=22 |pages=4453–4456 |doi=10.1103/physrevlett.79.4453 |bibcode=1997PhRvL..79.4453M }}</ref> Such junctions are quite frequently observed in samples prepared by [[electric arc|arc discharge]] as well as by [[chemical vapor deposition]]. The electronic properties of such junctions were first considered theoretically by Lambin et al.,<ref name=Lambin>{{cite journal | vauthors = Lambin P | year = 1996| title = Atomic structure and electronic properties of bent carbon nanotubes | journal = [[Synth. Met.]] | volume = 77 | issue = 1–3 | pages = 249–1254 | doi=10.1016/0379-6779(96)80097-x}}</ref> who pointed out that a connection between a metallic tube and a semiconducting one would represent a nanoscale heterojunction. Such a junction could therefore form a component of a nanotube-based electronic circuit. The adjacent image shows a junction between two multiwalled nanotubes.
Junctions between nanotubes and graphene have been considered theoretically<ref name=Ma>{{cite journal | vauthors = Ma KL | year = 2011| title = Electronic transport properties of junctions between carbon nanotubes and graphene nanoribbons| journal = [[European Physical Journal B]] | volume = 83 | issue = 4 |pages=487–492 | doi=10.1140/epjb/e2011-20313-9| bibcode = 2011EPJB...83..487M| s2cid = 119497542 }}</ref> and studied experimentally.<ref name=Harris>{{cite journal | vauthors = Harris PJ, Suarez-Martinez I, Marks NA | title = The structure of junctions between carbon nanotubes and graphene shells | journal = Nanoscale | volume = 8 | issue = 45 | pages = 18849–18854 | date = December 2016 | pmid = 27808332 | doi = 10.1039/c6nr06461b | s2cid = 42241359 | url = http://centaur.reading.ac.uk/67327/1/NANOSCALE%20FOR%20CENTAUR.pdf | url-status = live | archive-url = https://ghostarchive.org/archive/20221009/http://centaur.reading.ac.uk/67327/1/NANOSCALE%20FOR%20CENTAUR.pdf | archive-date = 2022-10-09 }}</ref> Nanotube-graphene junctions form the basis of [[pillared graphene]], in which parallel graphene sheets are separated by short nanotubes.<ref name= Dimitrakakis>{{cite journal | vauthors = Dimitrakakis GK, Tylianakis E, Froudakis GE | title = Pillared graphene: a new 3-D network nanostructure for enhanced hydrogen storage | journal = Nano Letters | volume = 8 | issue = 10 | pages = 3166–3170 | date = October 2008 | pmid = 18800853 | doi = 10.1021/nl801417w | bibcode = 2008NanoL...8.3166D }}</ref> Pillared graphene represents a class of [[#Three-dimensional carbon nanotube architectures|three-dimensional carbon nanotube architectures]].
[[File:3D carbon scaffolds.PNG|thumb|3D carbon scaffolds]]
Recently, several studies have highlighted the prospect of using carbon nanotubes as building blocks to fabricate three-dimensional macroscopic (>100&nbsp;nm in all three dimensions) all-carbon devices. Lalwani et al. have reported a novel radical-initiated thermal crosslinking method to fabricate macroscopic, free-standing, porous, all-carbon scaffolds using single- and multi-walled carbon nanotubes as building blocks.<ref name="PMID 23436939">{{cite journal | vauthors = Lalwani G, Kwaczala AT, Kanakia S, Patel SC, Judex S, Sitharaman B | title = Fabrication and Characterization of Three-Dimensional Macroscopic All-Carbon Scaffolds | journal = Carbon | volume = 53 | pages = 90–100 | date = March 2013 | pmid = 23436939 | pmc = 3578711 | doi = 10.1016/j.carbon.2012.10.035 }}</ref> These scaffolds possess macro-, micro-, and nano-structured pores, and the porosity can be tailored for specific applications. These 3D all-carbon scaffolds/architectures may be used for the fabrication of the next generation of energy storage, supercapacitors, field emission transistors, high-performance catalysis, photovoltaics, and biomedical devices, implants, and sensors.<ref name="PMID 25788440">{{cite journal | vauthors = Lalwani G, Gopalan A, D'Agati M, Sankaran JS, Judex S, Qin YX, Sitharaman B | title = Porous three-dimensional carbon nanotube scaffolds for tissue engineering | journal = Journal of Biomedical Materials Research. Part A | volume = 103 | issue = 10 | pages = 3212–3225 | date = October 2015 | pmid = 25788440 | pmc = 4552611 | doi = 10.1002/jbm.a.35449 }}</ref><ref name="noyce2">{{cite journal | vauthors = Noyce SG, Vanfleet RR, Craighead HG, Davis RC | title = High surface-area carbon microcantilevers | journal = Nanoscale Advances | volume = 1 | issue = 3 | pages = 1148–1154 | date = March 2019 | pmid = 36133213 | pmc = 9418787 | doi = 10.1039/C8NA00101D | doi-access = free | bibcode = 2019NanoA...1.1148N }}</ref>

=== Other morphologies ===
[[File:NanoBud.JPG|thumb|A stable [[Carbon nanobud|nanobud]] structure]]
[[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 electron emission|field emitters]].<ref name="Nasibulin">{{cite journal | vauthors = Nasibulin AG, Pikhitsa PV, Jiang H, Brown DP, Krasheninnikov AV, Anisimov AS, Queipo P, Moisala A, Gonzalez D, Lientschnig G, Hassanien A, Shandakov SD, Lolli G, Resasco DE, Choi M, Tománek D, Kauppinen EI | title = A novel hybrid carbon material | journal = Nature Nanotechnology | volume = 2 | issue = 3 | pages = 156–161 | date = March 2007 | pmid = 18654245 | doi = 10.1038/nnano.2007.37 | doi-access = free | bibcode = 2007NatNa...2..156N }}</ref> In [[composite material]]s, the attached fullerene molecules may function as molecular anchors preventing slipping of the nanotubes, thus improving the composite's mechanical properties.

A [[carbon peapod]]<ref>{{cite journal| vauthors = Smith BW, Monthioux M, Luzzi DE |year=1998 |title= Encapsulated C-60 in carbon nanotubes |journal=Nature |volume=396 |issue=6709 |pages=323–324 |bibcode=1998Natur.396R.323S|doi=10.1038/24521 |s2cid=30670931}}</ref><ref>{{cite journal| vauthors = Smith BW, Luzzi DE |year=2000|title=Formation mechanism of fullerene peapods and coaxial tubes: a path to large scale synthesis|journal=Chem. Phys. Lett.|volume=321|issue=1–2|pages=169–174|bibcode=2000CPL...321..169S|doi=10.1016/S0009-2614(00)00307-9 }}</ref> is a novel hybrid carbon material which traps fullerene inside a carbon nanotube. It can possess interesting magnetic properties with heating and irradiation. It can also be applied as an oscillator during theoretical investigations and predictions.<ref>{{cite journal | vauthors = Su H, Goddard WA, Zhao Y |year=2006 |title= Dynamic friction force in a carbon peapod oscillator |journal=Nanotechnology |volume=17 |issue=22 |pages=5691–5695 |arxiv=cond-mat/0611671|bibcode=2006Nanot..17.5691S|doi=10.1088/0957-4484/17/22/026 |s2cid=18165997|url=https://authors.library.caltech.edu/6289/1/SUHnanotech06.pdf |archive-url= https://ghostarchive.org/archive/20221009/https://authors.library.caltech.edu/6289/1/SUHnanotech06.pdf |archive-date=2022-10-09 |url-status=live}}</ref><ref>{{cite journal | vauthors = Wang M, Li CM | title = An oscillator in a carbon peapod controllable by an external electric field: a molecular dynamics study | journal = Nanotechnology | volume = 21 | issue = 3 | page = 035704 | date = January 2010 | pmid = 19966399 | doi = 10.1088/0957-4484/21/3/035704 | s2cid = 12358310 | bibcode = 2010Nanot..21c5704W }}</ref>

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 that previously expected for certain specific radii.<ref name="nanotori">{{cite journal | vauthors = Liu L, Guo GY, Jayanthi CS, Wu SY | title = Colossal paramagnetic moments in metallic carbon nanotori | journal = Physical Review Letters | volume = 88 | issue = 21 | page = 217206 | date = May 2002 | pmid = 12059501 | doi = 10.1103/PhysRevLett.88.217206 | bibcode = 2002PhRvL..88u7206L | url = http://ntur.lib.ntu.edu.tw//handle/246246/163841 }}</ref> Properties such as [[magnetic moment]], thermal stability, etc. vary widely depending on the radius of the torus and the radius of the tube.<ref name="nanotori" /><ref>{{cite journal| vauthors = Huhtala M, Kuronen A, Kaski 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|issue=1|pages=30–37 |bibcode=2002CoPhC.146...30H |doi=10.1016/S0010-4655(02)00432-0 |archive-url=https://web.archive.org/web/20080627183309/http://www.princeton.edu/~msammalk/publications/cpc146_02.pdf|archive-date=27 June 2008}}</ref>

[[Graphenated carbon nanotube]]s are a relatively new hybrid that combines [[Graphite|graphitic]] foliates grown along the sidewalls of multiwalled or bamboo-style CNTs. The foliate density can vary as a function of deposition conditions (e.g., temperature and time) with their structure ranging from a few layers of [[graphene]] (< 10) to thicker, more [[graphite]]-like.<ref>{{cite journal| vauthors = Parker CB, Raut AS, Brown B, Stoner BR, Glass JT |title=Three-dimensional arrays of graphenated carbon nanotubes|journal=J. Mater. Res.|year=2012|volume=27|series=7|pages=1046–1053 |doi=10.1557/jmr.2012.43|issue=7|bibcode = 2012JMatR..27.1046P|s2cid=137964473 }}</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. 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| vauthors = Stoner BR, Glass JT |title=Carbon nanostructures: a morphological classification for charge density optimization|journal=Diamond and Related Materials|year=2012|volume=23|pages=130–134|doi=10.1016/j.diamond.2012.01.034|bibcode = 2012DRM....23..130S}}</ref>

Cup-stacked carbon nanotubes (CSCNTs) differ from other quasi-1D carbon structures, which normally behave as quasi-metallic conductors of electrons. CSCNTs exhibit semiconducting behavior because of the stacking microstructure of graphene layers.<ref>{{cite journal| vauthors = Liu Q, Ren W, Chen ZG, Yin L, Li F, Cong H, Cheng HM |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 |bibcode=2009Carbo..47..731L |archive-url= https://web.archive.org/web/20150109002837/http://carbon.imr.ac.cn/file/Journal/2009/1249016438117.pdf |archive-date=2015-01-09}}</ref>

== Properties ==
Many properties of single-walled carbon nanotubes depend significantly on the (''n'',''m'') type, and this dependence is non-[[monotonic]] (see [[Optical properties of carbon nanotubes#Kataura plot|Kataura plot]]). In particular, the [[band gap]] can vary from zero to about 2 eV and the electrical conductivity can show metallic or semiconducting behavior.

=== Mechanical ===
{{Main|Mechanical properties of carbon nanotubes}}
[[File:CNTSEM.JPG|thumb|A [[scanning electron microscopy]] image of carbon nanotube bundles]]
Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of [[tensile strength]] and [[elastic modulus]]. This strength results from the covalent sp<sup>2</sup> bonds formed between the individual carbon atoms. In 2000, a multiwalled carbon nanotube was tested to have a tensile strength of {{convert|63|GPa|abbr=on}}.<ref name="Strength and Breaking">{{cite journal | vauthors = Yu MF, Lourie O, Dyer MJ, Moloni K, Kelly TF, Ruoff RS | title = Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load | journal = Science | volume = 287 | issue = 5453 | pages = 637–640 | date = January 2000 | pmid = 10649994 | doi = 10.1126/science.287.5453.637 | bibcode = 2000Sci...287..637Y | s2cid = 10758240 }}</ref> (For illustration, this translates into the ability to endure tension of a weight equivalent to {{convert|6422|kgf}} on a cable with cross-section of {{convert|1|mm2|abbr=on}}). Further studies, such as one conducted in 2008, revealed that individual CNT shells have strengths of up to ≈{{convert|100|GPa|abbr=on}}, which is in agreement with quantum/atomistic models.<ref name="B. Peng">{{cite journal | vauthors = Peng B, Locascio M, Zapol P, Li S, Mielke SL, Schatz GC, Espinosa HD | 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 | date = October 2008 | pmid = 18839003 | doi = 10.1038/nnano.2008.211 }}</ref> Because 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 | vauthors = Collins PG, Avouris P | title = Nanotubes for electronics | journal = Scientific American | volume = 283 | issue = 6 | pages = 62–69 | date = December 2000 | pmid = 11103460 | doi = 10.1038/scientificamerican1200-62 | bibcode = 2000SciAm.283f..62C }}</ref> its [[specific strength]] of up to 48,000&nbsp;kN·m/kg is the best of known materials, compared to high-carbon steel's 154&nbsp;kN·m/kg.

Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes lead to significant reduction in the effective strength of multiwalled carbon nanotubes and carbon nanotube bundles down to only a few GPa.<ref name="T. Filleter, R. A 2011">{{cite journal | vauthors = Filleter T, Bernal R, Li S, Espinosa HD | title = Ultrahigh strength and stiffness in cross-linked hierarchical carbon nanotube bundles | journal = Advanced Materials | volume = 23 | issue = 25 | pages = 2855–2860 | date = July 2011 | pmid = 21538593 | doi = 10.1002/adma.201100547 | bibcode = 2011AdM....23.2855F | s2cid = 6363504 }}</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 multiwalled 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 | vauthors = Jensen K, Mickelson W, Kis A, Zettl A |title=Buckling and kinking force measurements on individual multiwalled carbon nanotubes |journal=Physical Review B |date=26 November 2007 |volume=76 |issue=19 |page=195436 |doi=10.1103/PhysRevB.76.195436 |bibcode=2007PhRvB..76s5436J }}</ref>

On the other hand, there is evidence that in the radial direction they are rather soft. The first [[Transmission electron microscopy|transmission electron microscope]] observation of radial elasticity suggested that even [[van der Waals force]]s can deform two adjacent nanotubes. Later, [[nanoindentation]]s with an [[atomic force microscope]] were performed by several groups to quantitatively measure the radial elasticity of multiwalled carbon nanotubes and tapping/contact mode [[atomic force microscopy]] was also performed on single-walled carbon nanotubes. Their high [[Young's modulus]] in the linear direction, of on the order of several GPa (and even up to an experimentally-measured 1.8 TPa, for nanotubes near 2.4 μm in length<ref>{{cite journal |last1=Treacy |first1=M. M. J. |last2=Ebbesen |first2=T. W. |last3=Gibson |first3=J. M. |title=Exceptionally high Young's modulus observed for individual carbon nanotubes |journal=Nature |date=June 1996 |volume=381 |issue=6584 |pages=678–680 |doi=10.1038/381678a0|bibcode=1996Natur.381..678T }}</ref>), further suggests they may be soft in the radial direction.

=== Electrical ===
[[File:Carbon nanotube bands.gif|thumb|Band structures computed using a tight binding approximation for (6,0) CNT (zigzag, metallic), (10,2) CNT (semiconducting) and (10,10) CNT (armchair, metallic)]]
Unlike graphene, which is a two-dimensional [[semimetal]], carbon nanotubes are either metallic<ref name=Tans /> or [[Semiconductor|semiconducting]] along the tubular axis. For a given (''n'',''m'') nanotube, if ''n'' = ''m'', the nanotube is metallic; if ''n'' − ''m'' is a multiple of 3 and n ≠ m, then the nanotube is quasi-metallic with a very small band gap, otherwise the nanotube is a moderate [[semiconductor]].<ref name="QuantumTransport">{{cite journal| vauthors = Laird EA, Kuemmeth F, Steele GA, Grove-Rasmussen K, Nygård J, Flensberg K, Kouwenhoven LP |title=Quantum Transport in Carbon Nanotubes|journal=[[Reviews of Modern Physics]] | volume=87|issue=3|pages=703–764|year=2015|doi=10.1103/RevModPhys.87.703|arxiv = 1403.6113 |bibcode = 2015RvMP...87..703L |s2cid=119208985|url=https://repository.tudelft.nl/islandora/object/uuid%3A6a686f0b-1314-42ad-b6c1-7f3cc5edddb8/datastream/OBJ/download}}</ref>
Thus, all armchair (''n'' = ''m'') nanotubes are metallic, and nanotubes (6,4), (9,1), etc. are semiconducting.<ref name="Curvature">{{cite journal | vauthors = Lu X, Chen Z | title = Curved pi-conjugation, aromaticity, and the related chemistry of small fullerenes (< C60) and single-walled carbon nanotubes | journal = Chemical Reviews | volume = 105 | issue = 10 | pages = 3643–3696 | date = October 2005 | pmid = 16218563 | doi = 10.1021/cr030093d }}</ref>
Carbon nanotubes are not semimetallic because the degenerate point (the point where the π [bonding] band meets the π* [anti-bonding] band, at which the energy goes to zero) is slightly shifted away from the ''K'' point in the Brillouin zone because of the curvature of the tube surface, causing hybridization between the σ* and π* anti-bonding bands, modifying the band dispersion.

The rule regarding metallic versus semiconductor behavior has exceptions because curvature effects in small-diameter tubes can strongly influence electrical properties. Thus, a (5,0) SWCNT that should be semiconducting in fact is metallic according to the calculations. Likewise, zigzag and chiral SWCNTs with small diameters that should be metallic have a 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 | vauthors = Hong S, Myung S | title = Nanotube electronics: a flexible approach to mobility | journal = Nature Nanotechnology | volume = 2 | issue = 4 | pages = 207–208 | date = April 2007 | pmid = 18654263 | doi = 10.1038/nnano.2007.89 | bibcode = 2007NatNa...2..207H }}</ref> where for [[copper interconnects]], current densities are limited by [[electromigration]]. Carbon nanotubes are thus being explored as [[Carbon nanotubes in interconnects|interconnects]] and conductivity-enhancing components in composite materials, and many groups are attempting to commercialize highly conducting electrical wire assembled from individual carbon nanotubes. There are significant challenges to be overcome however, such as undesired current saturation under voltage,<ref>{{cite journal| vauthors = Vasylenko A, Wynn J, Medeiros PV, Morris AJ, Sloan J, Quigley D |year=2017 |title=Encapsulated nanowires: Boosting electronic transport in carbon nanotubes |journal=Physical Review B |volume=95 |issue=12 |page=121408 |doi=10.1103/PhysRevB.95.121408 |arxiv=1611.04867|bibcode = 2017PhRvB..95l1408V |s2cid=59023024}}</ref> and the much more resistive nanotube-to-nanotube junctions and impurities, all of which lower the electrical conductivity of the macroscopic nanotube wires by orders of magnitude, as compared to the conductivity of the individual nanotubes.

Because of its nanoscale cross-section, electrons propagate only along the tube's axis. 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>{{cite journal | doi = 10.1103/RevModPhys.79.677| title = Electronic and transport properties of nanotubes| journal = Reviews of Modern Physics| volume = 79| issue = 2| pages = 677–732| year = 2007| vauthors = Charlier JC, Blase X, Roche S |bibcode = 2007RvMP...79..677C| url = https://hal.archives-ouvertes.fr/hal-03070873/file/RevModPhys.79.677.pdf}}</ref>

Because of the role of the π-electron system in determining the [[Graphene#Electronic|electronic properties of graphene]], [[Doping (semiconductor)|doping]] in carbon nanotubes differs from that of bulk crystalline semiconductors from the same group of the periodic table (e.g., silicon). Graphitic substitution of carbon atoms in the nanotube wall by boron or nitrogen [[dopant]]s leads to p-type and n-type behavior, respectively, as would be expected in silicon. However, some non-substitutional ([[Intercalation (chemistry)|intercalated]] or adsorbed) dopants introduced into a carbon nanotube, such as alkali metals and electron-rich [[metallocene]]s, result in n-type conduction because they donate electrons to the π-electron system of the nanotube. By contrast, π-electron acceptors such as FeCl<sub>3</sub> or electron-deficient metallocenes function as p-type dopants because they draw π-electrons away from the top of the valence band.

Intrinsic [[superconductivity]] has been reported,<ref>{{cite journal | vauthors = Tang ZK, Zhang L, Wang N, Zhang XX, Wen GH, Li GD, Wang JN, Chan CT, Sheng P | title = Superconductivity in 4 angstrom single-walled carbon nanotubes | journal = Science | volume = 292 | issue = 5526 | pages = 2462–2465 | date = June 2001 | pmid = 11431560 | doi = 10.1126/science.1060470 | s2cid = 44987798 | bibcode = 2001Sci...292.2462T }}</ref><ref>{{cite journal | vauthors = Takesue I, Haruyama J, Kobayashi N, Chiashi S, Maruyama S, Sugai T, Shinohara H | title = Superconductivity in entirely end-bonded multiwalled carbon nanotubes | journal = Physical Review Letters | volume = 96 | issue = 5 | page = 057001 | date = February 2006 | pmid = 16486971 | doi = 10.1103/PhysRevLett.96.057001 | arxiv = cond-mat/0509466 | s2cid = 119049151 | bibcode = 2006PhRvL..96e7001T }}</ref><ref>{{cite journal | vauthors = Lortz R, Zhang Q, Shi W, Ye JT, Ye JT, Qiu C, Wang Z, He H, Sheng P, Qian T, Tang Z, Wang N, Zhang X, Wang J, Chan CT | title = Superconducting characteristics of 4-A carbon nanotube-zeolite composite | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 106 | issue = 18 | pages = 7299–7303 | date = May 2009 | pmid = 19369206 | pmc = 2678622 | doi = 10.1073/pnas.0813162106 | doi-access = free }}</ref> although other experiments found no evidence of this, leaving the claim a subject of debate.<ref>{{cite journal | vauthors = Bockrath M  |title=The weakest link |journal=Nature Physics |date=1 March 2006 |volume=2 |issue=3 |pages=155–156 |doi=10.1038/nphys252 |s2cid=125902065 }}</ref>

In 2021, Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT, published department findings on the use of carbon nanotubes to create an electric current.<ref>{{cite journal | vauthors = Liu AT, Kunai Y, Cottrill AL, Kaplan A, Zhang G, Kim H, Mollah RS, Eatmon YL, Strano MS | title = Solvent-induced electrochemistry at an electrically asymmetric carbon Janus particle | journal = Nature Communications | volume = 12 | issue = 1 | page = 3415 | date = June 2021 | pmid = 34099639 | pmc = 8184849 | doi = 10.1038/s41467-021-23038-7 | s2cid = 235370395 | bibcode = 2021NatCo..12.3415L }}</ref> By immersing the structures in an organic solvent, the liquid drew electrons out of the carbon particles. Strano was quoted as saying, "This allows you to do [[electrochemistry]], but with no wires," and represents a significant breakthrough in the technology.<ref name=Trafton2021>{{cite web |url=https://scitechdaily.com/mit-engineers-have-discovered-a-completely-new-way-of-generating-electricity/ |title=MIT Engineers Have Discovered a Completely New Way of Generating Electricity | vauthors = Trafton A |date=June 7, 2021 |website=SciTechDaily |access-date=June 8, 2021}}</ref> Future applications include powering micro- or nanoscale robots, as well as driving alcohol oxidation reactions, which are important in the chemicals industry.<ref name=Trafton2021 />

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 metallic 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>

==== Electromechanical ====
Semiconducting carbon nanotubes have shown [[Piezoresistive effect|piezoresistive]] property when applying mechanical force. The structural deformation causes a change in the band gap which effects the conductance. This property has the potential to be used in strain sensors.<ref>{{Cite journal |last1=Yang |first1=Liu |last2=Anantram |first2=M. P. |last3=Han |first3=Jie |last4=Lu |first4=J. P. |date=1999-11-15 |title=Band-gap change of carbon nanotubes: Effect of small uniaxial and torsional strain |url=https://link.aps.org/doi/10.1103/PhysRevB.60.13874 |journal=Physical Review B |language=en |volume=60 |issue=19 |pages=13874–13878 |doi=10.1103/PhysRevB.60.13874 |arxiv=cond-mat/9811263 |bibcode=1999PhRvB..6013874Y |issn=0163-1829}}</ref><ref>{{Cite journal |last1=Obitayo |first1=Waris |last2=Liu |first2=Tao |date=2012 |title=A Review: Carbon Nanotube-Based Piezoresistive Strain Sensors |journal=Journal of Sensors |language=en |volume=2012 |pages=1–15 |doi=10.1155/2012/652438 |doi-access=free |issn=1687-725X}}</ref>

=== Optical ===
{{Main|Optical properties of carbon nanotubes}}Carbon nanotubes have useful [[Absorption spectroscopy|absorption]], [[photoluminescence]] ([[fluorescence]]), and [[Raman spectroscopy]] properties. Spectroscopic methods offer the possibility of quick and non-destructive characterization of relatively large amounts of carbon nanotubes. There is a strong demand for such characterization from the industrial point of view: numerous parameters of [[#Synthesis|nanotube synthesis]] can be changed, intentionally or unintentionally, to alter the nanotube quality, such as the non-tubular carbon content, structure (chirality) of the produced nanotubes, and structural defects. These features then determine nearly all other significant optical, mechanical, and electrical properties.

Carbon nanotube optical properties have been explored for use in applications such as for light-emitting diodes ([[LED]]s)<ref name="led1">{{cite journal | vauthors = Misewich JA, Martel R, Avouris P, Tsang JC, Heinze S, Tersoff J | title = Electrically induced optical emission from a carbon nanotube FET | journal = Science | volume = 300 | issue = 5620 | pages = 783–786 | date = May 2003 | pmid = 12730598 | doi = 10.1126/science.1081294 | bibcode = 2003Sci...300..783M | s2cid = 36336745 }}</ref><ref name="led2">{{cite journal | vauthors = Chen J, Perebeinos V, Freitag M, Tsang J, Fu Q, Liu J, Avouris P | title = Bright infrared emission from electrically induced excitons in carbon nanotubes | journal = Science | volume = 310 | issue = 5751 | pages = 1171–1174 | date = November 2005 | pmid = 16293757 | doi = 10.1126/science.1119177 | bibcode = 2005Sci...310.1171C | s2cid = 21960183 }}</ref> and [[Photodiode|photo-detectors]]<ref>{{cite journal | vauthors = Freitag M, Martin Y, Misewich JA, Martel R, Avouris P |year=2003 |title= Photoconductivity of Single Carbon Nanotubes|journal=[[Nano Letters]] | volume=3 |issue=8 |pages=1067–1071 |bibcode=2003NanoL...3.1067F |doi=10.1021/nl034313e }}</ref> based on a single nanotube have been produced in the lab. Their unique feature is not the efficiency, which is yet relatively low, but the narrow selectivity in the [[wavelength]] of emission and detection of light and the possibility of its fine-tuning through the nanotube structure. In addition, [[bolometer]]<ref>{{cite journal | vauthors = Itkis ME, Borondics F, Yu A, Haddon RC | title = Bolometric infrared photoresponse of suspended single-walled carbon nanotube films | journal = Science | volume = 312 | issue = 5772 | pages = 413–416 | date = April 2006 | pmid = 16627739 | doi = 10.1126/science.1125695 | bibcode = 2006Sci...312..413I | doi-access = free }}</ref> and optoelectronic memory<ref>{{cite journal|year=2004|title=Nanotube Optoelectronic Memory Devices|journal=[[Nano Letters]] | volume=4|issue=9|pages=1587–1591|bibcode=2004NanoL...4.1587S|doi=10.1021/nl049337f| vauthors = Star A, Lu Y, Bradley K, Grüner G |url=https://figshare.com/articles/Nanotube_Optoelectronic_Memory_Devices/3325894}}</ref> devices have been realised on ensembles of single-walled carbon nanotubes. Nanotube fluorescence has been investigated for the purposes of imaging and sensing in biomedical applications.<ref name="cherukuri">{{cite journal
 |author1=Paul Cherukuri |author2=Sergei M. Bachilo |author3=Silvio H. Litovsky |author4=R. Bruce Weisman  |title=Near-Infrared Fluorescence Microscopy of Single-Walled Carbon Nanotubes in Phagocytic Cells
 |journal=[[Journal of the American Chemical Society]]
 |volume=126 |issue=48 |pages=15638–15639
 |year=2004
 |doi=10.1021/ja0466311 |pmid=15571374}}</ref><ref name="welsher">{{cite journal
 |author1=Kevin Welsher |author2=Sarah P. Sherlock |author3=Hongjie Dai  |title=Deep-tissue anatomical imaging of mice using carbon nanotube fluorophores in the second near-infrared window
 |journal=[[Proceedings of the National Academy of Sciences]]
 |volume=108 |issue=22 |pages=8943–8948
 |year=2011
 |doi=10.1073/pnas.1014501108 |arxiv = 1105.3536 |bibcode = 2011PNAS..108.8943W |pmid=21576494 |pmc=3107273 |doi-access=free }}</ref><ref name="barone">{{cite journal
 |author1=Paul W. Barone |author2=Seunghyun Baik |author3=Daniel A. Heller|author-link4=Michael Strano |author4=Michael S. Strano  |title= Near-infrared optical sensors based on single-walled carbon nanotubes
 |journal=[[Nature Materials]]
 |volume=4 |issue=1 |pages=86–92
 |year=2005
 |doi=10.1038/nmat1276 |bibcode = 2005NatMa...4...86B |pmid=15592477 |s2cid=43558342 }}</ref>

=== Thermal ===
{{Main|Thermal transport in nanostructures#Carbon nanotubes}}

All nanotubes are expected to be very good [[thermal conductor]]s along the tube,<ref name=Bever /><ref name=Kim /> exhibiting a property known as "[[ballistic conduction]]", but good insulators lateral to the tube axis. Measurements show that an individual 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 | vauthors = Pop E, Mann D, Wang Q, Goodson K, Dai H | title = Thermal conductance of an individual single-wall carbon nanotube above room temperature | journal = Nano Letters | volume = 6 | issue = 1 | pages = 96–100 | date = January 2006 | pmid = 16402794 | doi = 10.1021/nl052145f | arxiv = cond-mat/0512624 | s2cid = 14874373 | 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>. An individual SWNT has a room-temperature thermal conductivity lateral to its axis (in the radial direction) of about 1.52 W·m<sup>−1</sup>·K<sup>−1</sup>,<ref name="Sinha">{{cite journal | vauthors = Sinha S, Barjami S, Iannacchione G, Schwab A, Muench G | title = Off-axis thermal properties of carbon nanotube films | journal = Journal of Nanoparticle Research | date = 5 June 2005 | volume = 7 | issue = 6 | pages = 651–657 | doi = 10.1007/s11051-005-8382-9 | bibcode = 2005JNR.....7..651S | s2cid = 138479725  }}</ref> which is about as thermally conductive as soil. Macroscopic assemblies of nanotubes such as films or fibres have reached up to 1500 W·m<sup>−1</sup>·K<sup>−1</sup> so far.<ref>{{cite journal| vauthors = Koziol KK, Janas D, Brown E, Hao L |date=2017-04-01|title=Thermal properties of continuously spun carbon nanotube fibres|journal=Physica E|volume=88|pages=104–108|doi=10.1016/j.physe.2016.12.011|bibcode = 2017PhyE...88..104K}}</ref> Networks composed of nanotubes demonstrate different values of thermal conductivity, from the level of [[thermal insulation]] with the thermal conductivity of 0.1 W·m<sup>−1</sup>·K<sup>−1</sup> to such high values.<ref>{{cite journal| vauthors = Kumanek B, Janas D |date=May 2019|title=Thermal conductivity of carbon nanotube networks: a review|journal=Journal of Materials Science|volume=54|issue=10|pages=7397–7427|doi=10.1007/s10853-019-03368-0 |bibcode=2019JMatS..54.7397K|doi-access=free }}</ref> That is dependent on the amount of contribution to the thermal resistance of the system caused by the presence of impurities, misalignments and other factors. 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| vauthors = Thostenson E, Li C, Chou T |year=2005|title=Nanocomposites in context|journal=Composites Science and Technology |volume=65 |pages=491–51 |doi=10.1016/j.compscitech.2004.11.003 |issue=3–4  }}</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 [[phonon]]s. 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 the 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| vauthors = Mingo N, Stewart DA, Broido DA, Srivastava D |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|issue=3|bibcode = 2008PhRvB..77c3418M |hdl=1813/10898|hdl-access=free}}</ref>
[[File:Antibacterials CNTs.tif|thumb|Schematic of the antimicrobial mechanism of CNT reinforcement – Nanomaterials 2024, 14, 756. https://doi.org/10.3390/nano14090756 Open access icon]]
=== Antibacterial ===
Recently, carbon-nanotubes have been shown to have antibacterial properties. They disrupt normal bacterial function by causing physical/mechanical damage, facilitating oxidative stress or lipid extraction, inhibiting bacterial metabolism, and isolating functional sites via wrapping with CNM-containing nanomaterials.<ref>The Effectiveness Mechanisms of Carbon Nanotubes (CNTs) as Reinforcements for Magnesium-Based Composites for Biomedical Applications: A Review Nanomaterials 2024, 14, 756. https://doi.org/10.3390/nano14090756 {{Open access}}</ref>

== Synthesis ==
{{Main|Synthesis of carbon nanotubes}}

Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, [[chemical vapor deposition]] (CVD) and high-pressure carbon monoxide disproportionation (HiPCO). Among these arc discharge, laser ablation are batch by batch process, Chemical Vapor Deposition can be used both for batch by batch or continuous processes,<ref name="Endo – 2004">{{cite journal | vauthors = Endo M | title = Applications of carbon nanotubes in the twenty-first century | journal = Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences | volume = 362 | issue = 1823 | pages = 2223–2238 | date = October 2004 | doi = 10.1098/rsta.2004.1437 | pmid = 15370479 | s2cid = 20752554 }}</ref><ref name="Zhou – 2003">{{cite journal | vauthors = Zhou Z | title = Producing cleaner double-walled carbon nanotubes in a floating catalyst system | journal = Carbon | volume = 41 | issue = 13 | pages = 2607–2611 | date = January 2003 | doi = 10.1016/S0008-6223(03)00336-1 | bibcode = 2003Carbo..41.2607Z }}</ref> and HiPCO is gas phase continuous process.<ref name="Nikolaev-2004">{{cite journal | vauthors = Nikolaev P | title = Gas-phase production of single-walled carbon nanotubes from carbon monoxide: a review of the hipco process | journal = Journal of Nanoscience and Nanotechnology | volume = 4 | issue = 4 | pages = 307–316 | date = April 2004 | pmid = 15296221 | doi = 10.1166/jnn.2004.066 }}</ref> Most of these processes take place in a vacuum or with process gases. The CVD growth method is popular, as it yields high quantity and has a degree of control over diameter, length and morphology. Using particulate catalysts, large quantities of nanotubes can be synthesized by these methods, and industrialisation is well on its way, with several CNT and CNT fibers factory around the world. One problem of CVD processes is the high variability in the nanotube's characteristics <ref>{{cite book|url=https://books.google.com/books?id=L_zVa647-80C&pg=PA476 |title= Nanomedicine Design of Particles, Sensors, Motors, Implants, Robots, and Devices| vauthors = Schulz MJ, Shanov VN, Yun Y |date=2009 |publisher=Artech House|isbn=978-1-59693-280-7}}</ref> The HiPCO process advances in catalysis and continuous growth are making CNTs more commercially viable.<ref>{{cite journal | vauthors = Takeuchi K, Hayashi T, Kim YA, Fujisawa K, Endo M | title = The state-of-the-art science and applications of carbon nanotubes. | journal = Nanosystems: Physics, Chemistry, Mathematics | date = February 2014 | volume = 5 | issue = 1 | pages = 15–24 | url = http://nanojournal.ifmo.ru/en/articles-2/volume5/5-1/paper01/ }}</ref> The HiPCO process helps in producing high purity single-walled carbon nanotubes in higher quantity. The HiPCO reactor operates at high [[temperature]] 900–1100&nbsp;°C and high [[pressure]] ~30–50 bar.<ref>{{cite journal | vauthors = Bronikowski MJ, Willis PA, Colbert DT, Smith KA, Smalley RE |title=Gas-phase production of carbon single-walled nanotubes from carbon monoxide via the HiPco process: A parametric study |journal=Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films |date=July 2001 |volume=19 |issue=4 |pages=1800–1805 |doi=10.1116/1.1380721 |bibcode=2001JVSTA..19.1800B |s2cid=3846517 }}</ref> It uses [[carbon monoxide]] as the carbon source and [[iron pentacarbonyl]] or [[nickel tetracarbonyl]] as a catalyst. These catalysts provide a [[nucleation]] site for the nanotubes to grow,<ref name="Nikolaev-2004" /> while cheaper iron-based catalysts like Ferrocene can be used for CVD process.

[[Vertically aligned carbon nanotube arrays]] are also grown by thermal chemical vapor deposition. A substrate (quartz, silicon, stainless steel, carbon fibers, etc.) is coated with a catalytic metal (Fe, Co, Ni) layer. Typically that layer is iron and is deposited via sputtering to a thickness of 1–5&nbsp;nm. A 10–50&nbsp;nm underlayer of alumina is often also put down on the substrate first. This imparts controllable wetting and good interfacial properties.
When the substrate is heated to the growth temperature (~600 to 850&nbsp;°C), the continuous iron film breaks up into small islands with each island then nucleating a carbon nanotube. The sputtered thickness controls the island size and this in turn determines the nanotube diameter. Thinner iron layers drive down the diameter of the islands and drive down the diameter of the nanotubes grown. The amount of time the metal island can sit at the growth temperature is limited as they are mobile and can merge into larger (but fewer) islands. Annealing at the growth temperature reduces the site density (number of CNT/mm<sup>2</sup>) while increasing the catalyst diameter.

The as-prepared carbon nanotubes always have impurities such as other forms of carbon (amorphous carbon, fullerene, etc.) and non-carbonaceous impurities (metal used for catalyst).<ref>{{cite journal| vauthors = Itkis ME, Perea DE, Niyogi S, Rickard SM, Hamon MA, Hu H, Zhao B, Haddon RC |date=2003-03-01|title=Purity Evaluation of As-Prepared Single-Walled Carbon Nanotube Soot by Use of Solution-Phase Near-IR Spectroscopy|journal=Nano Letters|volume=3|issue=3|pages=309–314|doi=10.1021/nl025926e |bibcode = 2003NanoL...3..309I}}</ref><ref>{{cite journal | vauthors = Wang L, Pumera M | title = Residual metallic impurities within carbon nanotubes play a dominant role in supposedly "metal-free" oxygen reduction reactions | journal = Chemical Communications | volume = 50 | issue = 84 | pages = 12662–12664 | date = October 2014 | pmid = 25204561 | doi = 10.1039/C4CC03271C }}</ref> These impurities need to be removed to make use of the carbon nanotubes in applications.<ref>{{cite journal | vauthors = Eatemadi A, Daraee H, Karimkhanloo H, Kouhi M, Zarghami N, Akbarzadeh A, Abasi M, Hanifehpour Y, Joo SW | title = Carbon nanotubes: properties, synthesis, purification, and medical applications | journal = Nanoscale Research Letters | volume = 9 | issue = 1 | page = 393 | date = 2014-08-13 | pmid = 25170330 | pmc = 4141964 | doi = 10.1186/1556-276X-9-393 | bibcode = 2014NRL.....9..393E | doi-access = free }}</ref>

== Purification ==
As-synthesized carbon nanotubes typically contain impurities and most importantly different chiralities of carbon nanotubes. Therefore, multiple methods have been developed to purify them including polymer-assisted,<ref>{{Cite journal |last1=Chen |first1=Fuming |last2=Wang |first2=Bo |last3=Chen |first3=Yuan |last4=Li |first4=Lain-Jong |date=2007-10-01 |title=Toward the Extraction of Single Species of Single-Walled Carbon Nanotubes Using Fluorene-Based Polymers |url=https://pubs.acs.org/doi/10.1021/nl071349o |journal=Nano Letters |language=en |volume=7 |issue=10 |pages=3013–3017 |doi=10.1021/nl071349o |pmid=17867716 |bibcode=2007NanoL...7.3013C |issn=1530-6984}}</ref><ref>{{Cite journal |last1=Nish |first1=Adrian |last2=Hwang |first2=Jeong-Yuan |last3=Doig |first3=James |last4=Nicholas |first4=Robin J. |date=October 2007 |title=Highly selective dispersion of single-walled carbon nanotubes using aromatic polymers |url=https://www.nature.com/articles/nnano.2007.290 |journal=Nature Nanotechnology |language=en |volume=2 |issue=10 |pages=640–646 |doi=10.1038/nnano.2007.290 |pmid=18654390 |bibcode=2007NatNa...2..640N |issn=1748-3395}}</ref><ref>{{Cite journal |last1=Lemasson |first1=Fabien A. |last2=Strunk |first2=Timo |last3=Gerstel |first3=Peter |last4=Hennrich |first4=Frank |last5=Lebedkin |first5=Sergei |last6=Barner-Kowollik |first6=Christopher |last7=Wenzel |first7=Wolfgang |last8=Kappes |first8=Manfred M. |last9=Mayor |first9=Marcel |date=2011-02-02 |title=Selective Dispersion of Single-Walled Carbon Nanotubes with Specific Chiral Indices by Poly( N -decyl-2,7-carbazole) |url=https://pubs.acs.org/doi/10.1021/ja105722u |journal=Journal of the American Chemical Society |language=en |volume=133 |issue=4 |pages=652–655 |doi=10.1021/ja105722u |pmid=21171609 |s2cid=23209007 |issn=0002-7863}}</ref> density gradient ultracentrifugation (DGU),<ref>{{Cite journal |last1=Arnold |first1=Michael S. |last2=Stupp |first2=Samuel I. |last3=Hersam |first3=Mark C. |date=2005-04-01 |title=Enrichment of Single-Walled Carbon Nanotubes by Diameter in Density Gradients |url=https://pubs.acs.org/doi/10.1021/nl050133o |journal=Nano Letters |language=en |volume=5 |issue=4 |pages=713–718 |doi=10.1021/nl050133o |pmid=15826114 |bibcode=2005NanoL...5..713A |issn=1530-6984}}</ref><ref>{{Cite journal |last1=Green |first1=Alexander A. |last2=Hersam |first2=Mark C. |date=2011-05-17 |title=Nearly Single-Chirality Single-Walled Carbon Nanotubes Produced via Orthogonal Iterative Density Gradient Ultracentrifugation |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.201100034 |journal=Advanced Materials |language=en |volume=23 |issue=19 |pages=2185–2190 |doi=10.1002/adma.201100034 |pmid=21472798 |bibcode=2011AdM....23.2185G |s2cid=5375678 |issn=0935-9648}}</ref> chromatography <ref>{{Cite journal |last1=Flavel |first1=Benjamin S. |last2=Kappes |first2=Manfred M. |last3=Krupke |first3=Ralph |last4=Hennrich |first4=Frank |date=2013-04-23 |title=Separation of Single-Walled Carbon Nanotubes by 1-Dodecanol-Mediated Size-Exclusion Chromatography |url=https://pubs.acs.org/doi/10.1021/nn4004956 |journal=ACS Nano |language=en |volume=7 |issue=4 |pages=3557–3564 |doi=10.1021/nn4004956 |pmid=23540203 |issn=1936-0851}}</ref><ref>{{Cite journal |last1=Huang |first1=Xueying |last2=Mclean |first2=Robert S. |last3=Zheng |first3=Ming |date=2005-10-01 |title=High-Resolution Length Sorting and Purification of DNA-Wrapped Carbon Nanotubes by Size-Exclusion Chromatography |url=https://pubs.acs.org/doi/10.1021/ac0508954 |journal=Analytical Chemistry |language=en |volume=77 |issue=19 |pages=6225–6228 |doi=10.1021/ac0508954 |pmid=16194082 |issn=0003-2700}}</ref><ref>{{Cite journal |last1=Moore |first1=Katherine E. |last2=Pfohl |first2=Moritz |last3=Hennrich |first3=Frank |last4=Chakradhanula |first4=Venkata Sai K. |last5=Kuebel |first5=Christian |last6=Kappes |first6=Manfred M. |last7=Shapter |first7=Joe G. |last8=Krupke |first8=Ralph |last9=Flavel |first9=Benjamin S. |date=2014-07-22 |title=Separation of Double-Walled Carbon Nanotubes by Size Exclusion Column Chromatography |url=https://pubs.acs.org/doi/10.1021/nn500756a |journal=ACS Nano |language=en |volume=8 |issue=7 |pages=6756–6764 |doi=10.1021/nn500756a |pmid=24896840 |issn=1936-0851}}</ref> and aqueous two-phase extraction (ATPE).<ref>{{Cite journal |last1=Ao |first1=Geyou |last2=Khripin |first2=Constantine Y. |last3=Zheng |first3=Ming |date=2014-07-23 |title=DNA-Controlled Partition of Carbon Nanotubes in Polymer Aqueous Two-Phase Systems |url=https://pubs.acs.org/doi/10.1021/ja504078b |journal=Journal of the American Chemical Society |language=en |volume=136 |issue=29 |pages=10383–10392 |doi=10.1021/ja504078b |pmid=24976036 |issn=0002-7863}}</ref><ref>{{Cite journal |last1=Fagan |first1=Jeffrey A. |last2=Khripin |first2=Constantine Y. |last3=Silvera Batista |first3=Carlos A. |last4=Simpson |first4=Jeffrey R. |last5=Hároz |first5=Erik H. |last6=Hight Walker |first6=Angela R. |last7=Zheng |first7=Ming |date=May 2014 |title=Isolation of Specific Small-Diameter Single-Wall Carbon Nanotube Species via Aqueous Two-Phase Extraction |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.201304873 |journal=Advanced Materials |language=en |volume=26 |issue=18 |pages=2800–2804 |doi=10.1002/adma.201304873 |pmid=24448916 |bibcode=2014AdM....26.2800F |s2cid=205253171 |issn=0935-9648}}</ref><ref>{{Cite journal |last1=Lyu |first1=Min |last2=Meany |first2=Brendan |last3=Yang |first3=Juan |last4=Li |first4=Yan |last5=Zheng |first5=Ming |date=2019-12-26 |title=Toward Complete Resolution of DNA/Carbon Nanotube Hybrids by Aqueous Two-Phase Systems |url=https://pubs.acs.org/doi/10.1021/jacs.9b09953 |journal=Journal of the American Chemical Society |language=en |volume=141 |issue=51 |pages=20177–20186 |doi=10.1021/jacs.9b09953 |pmid=31783712 |s2cid=208498347 |issn=0002-7863}}</ref><ref>{{Cite journal |last1=Li |first1=Han |last2=Gordeev |first2=Georgy |last3=Garrity |first3=Oisin |last4=Reich |first4=Stephanie |last5=Flavel |first5=Benjamin S. |date=2019-01-28 |title=Separation of Small-Diameter Single-Walled Carbon Nanotubes in One to Three Steps with Aqueous Two-Phase Extraction |url=https://pubs.acs.org/doi/10.1021/acsnano.8b09579 |journal=ACS Nano |volume=13 |issue=2 |pages=2567–2578 |language=en |doi=10.1021/acsnano.8b09579 |pmid=30673278 |s2cid=59224819 |issn=1936-0851}}</ref> These methods have been reviewed in multiple articles.<ref>{{Cite journal |last1=Yang |first1=Feng |last2=Wang |first2=Meng |last3=Zhang |first3=Daqi |last4=Yang |first4=Juan |last5=Zheng |first5=Ming |last6=Li |first6=Yan |date=2020-03-11 |title=Chirality Pure Carbon Nanotubes: Growth, Sorting, and Characterization |url=https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00835 |journal=Chemical Reviews |language=en |volume=120 |issue=5 |pages=2693–2758 |doi=10.1021/acs.chemrev.9b00835 |pmid=32039585 |s2cid=211071215 |issn=0009-2665}}</ref><ref>{{Cite journal |last=Janas |first=Dawid |date=2017-12-21 |title=Towards monochiral carbon nanotubes: a review of progress in the sorting of single-walled carbon nanotubes |url=https://pubs.rsc.org/en/content/articlelanding/2018/qm/c7qm00427c |journal=Materials Chemistry Frontiers |language=en |volume=2 |issue=1 |pages=36–63 |doi=10.1039/C7QM00427C |issn=2052-1537}}</ref><ref>{{Cite journal |last1=Wei |first1=Xiaojun |last2=Li |first2=Shilong |last3=Wang |first3=Wenke |last4=Zhang |first4=Xiao |last5=Zhou |first5=Weiya |last6=Xie |first6=Sishen |last7=Liu |first7=Huaping |date=May 2022 |title=Recent Advances in Structure Separation of Single-Wall Carbon Nanotubes and Their Application in Optics, Electronics, and Optoelectronics |journal=Advanced Science |language=en |volume=9 |issue=14 |pages=e2200054 |doi=10.1002/advs.202200054 |issn=2198-3844 |pmc=9108629 |pmid=35293698}}</ref>

Certain polymers selectively disperse or wrap CNTs of a particular chirality, metallic character or diameter. For example, poly(phenylenevinylenes) disperses CNTs of specific diameters (0.75–0.84&nbsp;nm) and polyfluorenes are highly selective for semiconducting CNTs. It involves mainly two steps, sonicate the mixture (CNTs and polymers in solvent), centrifuge and the supernatant are desired CNTs.

Density gradient ultracentrifugation is a method based on the density difference of CNTs, so that different components are layered in centrifuge tubes under centrifugal force. Chromatography-based methods include size exclusion (SEC), ion-exchange (IEX) and gel chromatography. For SEC, CNTs are separated due to the difference in size using a stationary phase with different pore size. As for IEX, the separation is achieved based on their differential adsorption and [[desorption]] onto chemically functionalized resins packed in an IEX column, so understanding the interaction between CNTs mixtures and resins is important. The first IEX is reported to separate DNA-SWCNTs.<ref>{{Cite journal |last1=Zheng |first1=Ming |last2=Semke |first2=Ellen D. |date=2007-05-01 |title=Enrichment of Single Chirality Carbon Nanotubes |url=https://pubs.acs.org/doi/10.1021/ja071577k |journal=Journal of the American Chemical Society |language=en |volume=129 |issue=19 |pages=6084–6085 |doi=10.1021/ja071577k |pmid=17458969 |issn=0002-7863}}</ref> Gel chromatography is based on the partition of CNTs between stationary and mobile phase, it's found semiconducting CNTs are more strongly attracted by gel than metallic CNTs.<ref>{{Cite journal |last1=Liu |first1=Huaping |last2=Nishide |first2=Daisuke |last3=Tanaka |first3=Takeshi |last4=Kataura |first4=Hiromichi |date=2011-05-10 |title=Large-scale single-chirality separation of single-wall carbon nanotubes by simple gel chromatography |journal=Nature Communications |language=en |volume=2 |issue=1 |pages=309 |doi=10.1038/ncomms1313 |issn=2041-1723 |pmc=3113293 |pmid=21556063|bibcode=2011NatCo...2..309L }}</ref><ref>{{Cite journal |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 |last10=Kataura |first10=Hiromichi |date=2009-04-08 |title=Simple and Scalable Gel-Based Separation of Metallic and Semiconducting Carbon Nanotubes |url=https://pubs.acs.org/doi/10.1021/nl8034866 |journal=Nano Letters |language=en |volume=9 |issue=4 |pages=1497–1500 |doi=10.1021/nl8034866 |pmid=19243112 |bibcode=2009NanoL...9.1497T |issn=1530-6984}}</ref> While it shows potential, the current application is limited to the separation of semiconducting (n,m) species.

ATPE uses two water-soluble polymers such as polyethylene glycol (PEG) and [[dextran]]. When mixed, two [[Miscibility|immiscible]] aqueous phases form spontaneously, and each of the two phases shows a different affinity to CNTs. Partition depends on the solvation energy difference between two similar phases of microscale volumes. By changing the separation system or temperatures, and adding strong oxidants, reductants, or salts, the partition of CNTs species into the two phases can be adjusted.

Despite the progress that has been made to separate and purify CNTs, many challenges remain, such as the growth of chirality-controlled CNTs, so that no further purification is needed, or large-scale purification.

=== Advantages of monochiral CNTs ===
Monochiral CNTs have the advantage that they do contain less or no impurities, well-defined non-congested optical spectra. This allows to create for example CNT-based biosensors with higher sensitivity and selectivity.<ref name="Ackermann-2022" /> For example, monochiral SWCNTs are necessary for multiplexed and ratiometric sensing schemes,<ref>{{Cite journal |last1=Nißler |first1=Robert |last2=Kurth |first2=Larissa |last3=Li |first3=Han |last4=Spreinat |first4=Alexander |last5=Kuhlemann |first5=Ilyas |last6=Flavel |first6=Benjamin S. |last7=Kruss |first7=Sebastian |date=2021-04-27 |title=Sensing with Chirality-Pure Near-Infrared Fluorescent Carbon Nanotubes |url=https://pubs.acs.org/doi/10.1021/acs.analchem.1c00168 |journal=Analytical Chemistry |language=en |volume=93 |issue=16 |pages=6446–6455 |doi=10.1021/acs.analchem.1c00168 |pmid=33830740 |issn=0003-2700}}</ref><ref>{{Cite journal |last1=Nißler |first1=Robert |last2=Ackermann |first2=Julia |last3=Ma |first3=Chen |last4=Kruss |first4=Sebastian |date=2022-07-19 |title=Prospects of Fluorescent Single-Chirality Carbon Nanotube-Based Biosensors |url=https://pubs.acs.org/doi/10.1021/acs.analchem.2c01321 |journal=Analytical Chemistry |language=en |volume=94 |issue=28 |pages=9941–9951 |doi=10.1021/acs.analchem.2c01321 |pmid=35786856 |s2cid=250283972 |issn=0003-2700}}</ref> enhanced sensitivity <ref name="Ackermann-2023" /> of [[biocompatibility]].<ref>{{Cite journal |last1=Nadeem |first1=Aceer |last2=Kindopp |first2=Aidan |last3=Wyllie |first3=Ian |last4=Hubert |first4=Lauren |last5=Joubert |first5=James |last6=Lucente |first6=Sophie |last7=Randall |first7=Ewelina |last8=Jena |first8=Prakrit V. |last9=Roxbury |first9=Daniel |date=2023-07-26 |title=Enhancing Intracellular Optical Performance and Stability of Engineered Nanomaterials via Aqueous Two-Phase Purification |journal=Nano Letters |language=en |volume=23 |issue=14 |pages=6588–6595 |doi=10.1021/acs.nanolett.3c01727 |pmid=37410951 |bibcode=2023NanoL..23.6588N |s2cid=259356687 |issn=1530-6984|pmc=11068083 }}</ref>

== Functionalization ==
{{Main|Carbon nanotube chemistry|Selective chemistry of single-walled nanotubes}}
Carbon nanotubes can be [[functionalized]] to attain desired properties that can be used in a wide variety of applications.<ref>{{cite journal| last1 = Bekyarova| first1 = E.| last2 = Davis | first2 = M.| last3 = Burch | first3 = T.| last4 = Itkis | first4 = M. E.| last5 = Zhao | first5 = B.| last6 = Sunshine | first6 = S.| last7 = Haddon | first7 = R. C.| title = Chemically Functionalized Single-Walled Carbon Nanotubes as Ammonia Sensors| journal = J. Phys. Chem. B| volume = 108| issue = 51| pages = 19717–19720| publisher = ACS Publications| location = Washington, D.C.| date = October 9, 2004| url = https://www.sensigent.com/img/pdf/2004%20Bekyarova%20et%20al%20J.%20JPhys%20Chem%20Chemically%20Functionalized%20Single-Walled%20Carbon%20Nanotubes%20as%20Ammonia%20Sensors.pdf| issn = 1520-5207| doi = 10.1021/jp0471857 | s2cid = 96173424}}</ref> The two main methods of carbon nanotube functionalization are covalent and non-covalent modifications. Because of their apparent hydrophobic nature,<ref>{{cite journal | vauthors = Stando G, Łukawski D, Lisiecki F, Janas D |date=January 2019 |title=Intrinsic hydrophilic character of carbon nanotube networks|journal=Applied Surface Science |volume=463 |pages=227–233 |doi=10.1016/j.apsusc.2018.08.206 |bibcode=2019ApSS..463..227S|s2cid=105024629 |url=https://depot.ceon.pl/handle/123456789/19888 }}</ref> carbon nanotubes tend to agglomerate hindering their dispersion in solvents or viscous polymer melts. The resulting nanotube bundles or aggregates reduce the mechanical performance of the final composite. The surface of the carbon nanotubes can be modified to reduce the [[Hydrophobe|hydrophobicity]] and improve interfacial [[adhesion]] to a bulk [[polymer]] through chemical attachment.<ref name="Karousis-2010">{{cite journal | vauthors = Karousis N, Tagmatarchis N, Tasis D | title = Current progress on the chemical modification of carbon nanotubes | journal = Chemical Reviews | volume = 110 | issue = 9 | pages = 5366–5397 | date = September 2010 | pmid = 20545303 | doi = 10.1021/cr100018g }}</ref>

Chemical routes such as covalent functionalization have been studied extensively, which involves the oxidation of CNTs via strong acids (e.g. [[sulfuric acid]], nitric acid, or a mixture of both) in order to set the carboxylic groups onto the surface of the CNTs as the final product or for further modification by esterification or amination. Free radical grafting is a promising technique among covalent functionalization methods, in which alkyl or aryl peroxides, substituted anilines, and diazonium salts are used as the starting agents.

Functionalization can improve CNTs characteristically weak dispersibility in many solvents, such as water - a consequence of their strong intermolecular p–p interactions. This can enhance the processing and manipulation of insoluble CNTs, rendering them useful for synthesizing innovative CNT [[nanofluid]]s with impressive properties that are tunable for a wide range of applications.

Free radical grafting of macromolecules (as the functional group) onto the surface of CNTs can improve the solubility of CNTs compared to common acid treatments which involve the attachment of small molecules such as hydroxyl onto the surface of CNTs. The solubility of CNTs can be improved significantly by free-radical grafting because the large functional molecules facilitate the dispersion of CNTs in a variety of solvents even at a low degree of functionalization. Recently an innovative environmentally friendly approach has been developed for the covalent functionalization of multi-walled carbon nanotubes (MWCNTs) using clove buds. This approach is innovative and green because it does not use toxic and hazardous acids which are typically used in common carbon nanomaterial functionalization procedures. The MWCNTs are functionalized in one pot using a free radical grafting reaction. The clove-functionalized MWCNTs are then dispersed in water producing a highly stable multi-walled carbon nanotube aqueous suspension (nanofluids).<ref>{{cite journal | vauthors = Sadri R, Hosseini M, Kazi SN, Bagheri S, Zubir N, Solangi KH, Zaharinie T, Badarudin A | title = A bio-based, facile approach for the preparation of covalently functionalized carbon nanotubes aqueous suspensions and their potential as heat transfer fluids | journal = Journal of Colloid and Interface Science | volume = 504 | pages = 115–123 | date = October 2017 | pmid = 28531649 | doi = 10.1016/j.jcis.2017.03.051 | bibcode = 2017JCIS..504..115S }}</ref>

The surface of carbon nanotubes can be chemically modified by coating spinel [[nanoparticle]]s by hydrothermal synthesis<ref>{{cite journal | vauthors = Sahoo P, Shrestha RG, Shrestha LK, Hill JP, Takei T, Ariga K |title=Surface Oxidized Carbon Nanotubes Uniformly Coated with Nickel Ferrite Nanoparticles |journal=Journal of Inorganic and Organometallic Polymers and Materials |date=November 2016 |volume=26 |issue=6 |pages=1301–1308 |doi=10.1007/s10904-016-0365-z |s2cid=101287773 }}</ref> and can be used for water oxidation purposes.<ref>{{cite journal | vauthors = Sahoo P, Tan JB, Zhang ZM, Singh SK, Lu TB |title=Engineering the Surface Structure of Binary/Ternary Ferrite Nanoparticles as High-Performance Electrocatalysts for the Oxygen Evolution Reaction |journal=ChemCatChem |date=7 March 2018 |volume=10 |issue=5 |pages=1075–1083 |doi=10.1002/cctc.201701790 |s2cid=104164617 }}</ref>

In addition, the surface of carbon nanotubes can be [[Halogenation|fluorinated]] or halofluorinated by heating while in contact with a fluoroorganic substance, thereby forming partially fluorinated carbons (so-called Fluocar materials) with grafted (halo)fluoroalkyl functionality.<ref>{{cite patent|country=US|number=10000382|title=Method for carbon materials surface modification by the fluorocarbons and derivatives|url=http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/PTO/srchnum.htm&r=1&f=G&l=50&s1=10000382.PN.&OS=PN/10000382&RS=PN/10000382|issue-date=June 19, 2018|inventor=Zaderko A, Vasyl UA}} {{Webarchive|url=https://web.archive.org/web/20180917143354/http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=10000382.PN.&OS=PN/10000382&RS=PN/10000382 |date=17 September 2018 }}</ref><ref>{{cite web|url=https://patentscope.wipo.int/search/en/detail.jsf?docId=WO16072959|title=WO16072959 Method for Carbon Materials Surface Modification by the Fluorocarbons and Derivatives|website=patentscope.wipo.int|access-date=2018-09-17}}</ref>

== Modeling ==
[[File:Computer simulated microstructure of nanocomposite.jpg|thumb|Computer simulated microstructures with agglomeration regions]]
Carbon nanotubes are modelled in a similar manner as traditional composites in which a reinforcement phase is surrounded by a matrix phase. Ideal models such as cylindrical, hexagonal and square models are common. The size of the [[micromechanics]] model is highly function of the studied mechanical properties. The concept of representative volume element (RVE) is used to determine the appropriate size and configuration of the computer model to replicate the actual behavior of the CNT-reinforced nanocomposite. Depending on the material property of interest (thermal, electrical, modulus, creep), one RVE might predict the property better than the alternatives. While the implementation of the ideal model is computationally efficient, they do not represent microstructural features observed in scanning electron microscopy of actual nanocomposites. To incorporate realistic modeling, computer models are also generated to incorporate variability such as waviness, orientation and agglomeration of multiwall or single-wall carbon nanotubes.<ref>{{cite journal |doi=10.1115/1.4043410 |title=Effect of Nanocomposite Microstructure on Stochastic Elastic Properties: An Finite Element Analysis Study |journal=ASCE-ASME Journal of Risk and Uncertainty in Engineering Systems, Part B: Mechanical Engineering |volume=5 |issue=3 |page=030903 |year=2019 | vauthors = Sanei SH, Doles R, Ekaitis T |s2cid=140766023 }}</ref>

== Metrology ==
There are many [[metrology]] standards and [[reference material]]s available for carbon nanotubes.<ref name="Stefaniak-2017">{{cite book|title=Metrology and Standardization of Nanotechnology| vauthors = Stefaniak AB |date=2017|publisher=Wiley-VCH Verlag|isbn=978-3-527-80030-8| veditors = Mansfield E, Kaiser DL, Fujita D, Van de Voorde M |pages=151–174|chapter=Principal Metrics and Instrumentation for Characterization of Engineered Nanomaterials|doi=10.1002/9783527800308.ch8}}</ref>

For single-wall carbon nanotubes, [[International Organization for Standardization|ISO]]/TS 10868 describes a measurement method for the diameter, purity, and fraction of metallic nanotubes through [[Optical radiation|optical]] [[absorption spectroscopy]],<ref>{{cite web|url=https://www.iso.org/standard/69547.html|title=ISO/TS 10868:2017 – Nanotechnologies – Characterization of single-wall carbon nanotubes using ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectroscopy|website=[[International Organization for Standardization]] | archive-url=https://web.archive.org/web/20170907032921/https://www.iso.org/standard/69547.html|archive-date=2017-09-07|access-date=2017-09-06}}</ref> while ISO/TS 10797 and ISO/TS 10798 establish methods to characterize the morphology and elemental composition of single-wall carbon nanotubes, using [[transmission electron microscopy]] and [[Scanning electron microscope|scanning electron microscopy]] respectively, coupled with energy dispersive [[X-ray spectroscopy|X-ray spectrometry]] analysis.<ref>{{cite web|url=https://www.iso.org/standard/46127.html|title=ISO/TS 10797:2012 – Nanotechnologies – Characterization of single-wall carbon nanotubes using transmission electron microscopy|website=International Organization for Standardization|archive-url=https://web.archive.org/web/20170907033223/https://www.iso.org/standard/46127.html|archive-date=2017-09-07|access-date=2017-09-06}}</ref><ref name="ISO">{{cite web|url=https://www.iso.org/standard/46128.html|title=ISO/TS 10798:2011 – Nanotechnologies – Characterization of single-wall carbon nanotubes using scanning electron microscopy and energy dispersive X-ray spectrometry analysis|website=International Organization for Standardization|archive-url=https://web.archive.org/web/20170907033356/https://www.iso.org/standard/46128.html|archive-date=2017-09-07|access-date=2017-09-06}}</ref>

[[National Institute of Standards and Technology|NIST]] SRM 2483 is a soot of single-wall carbon nanotubes used as a reference material for [[elemental analysis]], and was characterized using [[thermogravimetric analysis]], [[prompt gamma activation analysis]], induced [[neutron activation analysis]], [[Inductively coupled plasma mass spectrometry|inductively coupled plasma mass spectroscopy]], resonant [[Raman scattering]], [[Optical radiation|UV-visible-near infrared]] [[fluorescence spectroscopy]] and absorption spectroscopy, scanning electron microscopy, and transmission electron microscopy.<ref name="Fagan-2009">{{cite news|url=https://www.nist.gov/programs-projects/carbon-nanotube-reference-materials|title=Carbon Nanotube Reference Materials| vauthors = Fagan J |date=2009-03-05|work=U.S. [[National Institute of Standards and Technology]] | access-date=2017-09-06}}</ref><ref>{{cite web|url=https://www-s.nist.gov/srmors/view_detail.cfm?srm=2483|title=SRM 2483 – Single-Wall Carbon Nanotubes (Raw Soot)|website=U.S. National Institute of Standards and Technology|archive-url=https://web.archive.org/web/20130218233137/https://www-s.nist.gov/srmors/view_detail.cfm?srm=2483|archive-date=2013-02-18|access-date=2017-09-06}}</ref> The [[National Research Council (Canada)|Canadian National Research Council]] also offers a certified reference material SWCNT-1 for elemental analysis using neutron activation analysis and inductively coupled plasma mass spectroscopy.<ref name="Stefaniak-2017" /><ref>{{cite web|url=https://www.nrc-cnrc.gc.ca/eng/solutions/advisory/crm/certificates/swcnt_1.html|title=SWCNT-1: Single-Wall Carbon Nanotube Certified Reference Material – National Research Council Canada|date=2014-11-07|website=Canadian [[National Research Council (Canada)|National Research Council]] | access-date=2017-09-06}}</ref> NIST RM 8281 is a mixture of three lengths of single-wall carbon nanotube.<ref name="Fagan-2009" /><ref>{{cite web|url=https://www-s.nist.gov/srmors/view_detail.cfm?srm=8281|title=RM 8281 – Single-Wall Carbon Nanotubes (Dispersed, Three Length-Resolved Populations)|website=U.S. National Institute of Standards and Technology|archive-url=https://web.archive.org/web/20150401143054/https://www-s.nist.gov/srmors/view_detail.cfm?srm=8281|archive-date=2015-04-01|access-date=2017-09-06}}</ref>

For multiwall carbon nanotubes, ISO/TR 10929 identifies the basic properties and the content of impurities,<ref>{{cite web|url=https://www.iso.org/standard/46424.html|title=ISO/TR 10929:2012 – Nanotechnologies – Characterization of multiwall carbon nanotube (MWCNT) samples|website=International Organization for Standardization|archive-url=https://web.archive.org/web/20170907034138/https://www.iso.org/standard/46424.html|archive-date=2017-09-07|access-date=2017-09-06}}</ref> while ISO/TS 11888 describes morphology using scanning electron microscopy, transmission electron microscopy, [[Viscometer|viscometry]], and [[light scattering]] analysis.<ref>{{cite web|url=https://www.iso.org/standard/69549.html|title=ISO/TS 11888:2017 –Nanotechnologies – Characterization of multiwall carbon nanotubes – Mesoscopic shape factors|website=International Organization for Standardization|archive-url=https://web.archive.org/web/20170907032855/https://www.iso.org/standard/69549.html|archive-date=2017-09-07|access-date=2017-09-06}}</ref> ISO/TS 10798 is also valid for multiwall carbon nanotubes.<ref name="ISO" />

== Safety and health ==
{{Main|Health and safety hazards of nanomaterials|Toxicology of carbon nanomaterials}}

[[File:Carbon nanotubes penetrating lung cell.tif|alt=A greyscale microscope image showing a rigid rod extending from both sides of a mottled cellular mass|thumb|A [[scanning electron microscope]] image of bundles of multiwalled carbon nanotube piercing an [[Alveolar cells|alveolar epithelial cell]].]]

The [[National Institute for Occupational Safety and Health]] (NIOSH) is the leading United States federal agency conducting research and providing guidance on the occupational safety and health implications and applications of [[nanomaterials]]. Early scientific studies have indicated that nanoscale particles may pose a greater health risk than bulk materials due to a relative increase in surface area per unit mass. Increase in length and diameter of CNT is correlated to increased toxicity<ref>{{cite journal | vauthors = Fraser K, Kodali V, Yanamala N, Birch ME, Cena L, Casuccio G, Bunker K, Lersch TL, Evans DE, Stefaniak A, Hammer MA, Kashon ML, Boots T, Eye T, Hubczak J, Friend SA, Dahm M, Schubauer-Berigan MK, Siegrist K, Lowry D, Bauer AK, Sargent LM, Erdely A | title = Physicochemical characterization and genotoxicity of the broad class of carbon nanotubes and nanofibers used or produced in U.S. facilities | journal = Particle and Fibre Toxicology | volume = 17 | issue = 1 | page = 62 | date = December 2020 | pmid = 33287860 | pmc = 7720492 | doi = 10.1186/s12989-020-00392-w | doi-access = free | bibcode = 2020PFTox..17...62F }}</ref> and pathological alterations in lung.<ref>{{cite journal | vauthors = Fraser K, Hubbs A, Yanamala N, Mercer RR, Stueckle TA, Jensen J, Eye T, Battelli L, Clingerman S, Fluharty K, Dodd T, Casuccio G, Bunker K, Lersch TL, Kashon ML, Orandle M, Dahm M, Schubauer-Berigan MK, Kodali V, Erdely A | title = Histopathology of the broad class of carbon nanotubes and nanofibers used or produced in U.S. facilities in a murine model | journal = Particle and Fibre Toxicology | volume = 18 | issue = 1 | page = 47 | date = December 2021 | pmid = 34923995 | pmc = 8686255 | doi = 10.1186/s12989-021-00440-z | doi-access = free | bibcode = 2021PFTox..18...47F }}</ref> The biological interactions of nanotubes are not well understood, and the field is open to continued toxicological studies. It is often difficult to separate confounding factors, and since carbon is relatively biologically inert, some of the toxicity attributed to carbon nanotubes may be instead due to residual metal catalyst contamination. In previous studies, only Mitsui-7 was reliably demonstrated to be carcinogenic, although for unclear/unknown reasons.<ref>{{cite journal | vauthors = Barbarino M, Giordano A | title = Assessment of the Carcinogenicity of Carbon Nanotubes in the Respiratory System | journal = Cancers | volume = 13 | issue = 6 | page = 1318 | date = March 2021 | pmid = 33804168 | pmc = 7998467 | doi = 10.3390/cancers13061318 | doi-access = free }}</ref> Unlike many common mineral fibers (such as asbestos), most SWCNTs and MWCNTs do not fit the size and aspect-ratio criteria to be classified as respirable fibers. In 2013, given that the long-term health effects have not yet been measured, NIOSH published a Current Intelligence Bulletin<ref>{{Cite web |title=CDC – NIOSH Numbered Publications: Current Intelligence Bulletins (CIB) – Sorted By Date, Descending Order Without Publication Numbers |url=https://www.cdc.gov/niosh/pubs/cib_date_desc_nopubnumbers.html |access-date=2022-11-09 |website=www.cdc.gov}}</ref> detailing the potential hazards and recommended exposure limit for carbon nanotubes and fibers.<ref>{{cite periodical |vauthors=Howard J |periodical=Current Intelligence Bulletin |number=65 | title = Occupational exposure to carbon nanotubes and nanofibers. | publisher = DHHS (NIOSH) Publication | date = April 2013 |doi=10.26616/NIOSHPUB2013145 | doi-access = free }}</ref> The U.S. [[National Institute for Occupational Safety and Health]] has determined non-regulatory [[recommended exposure limit]]s (RELs) of 1 μg/m<sup>3</sup> for carbon nanotubes and [[carbon nanofiber]]s as background-corrected elemental carbon as an 8-hour time-weighted average (TWA) respirable mass concentration.<ref>{{Cite periodical |date=2020-07-14 |periodical=Current Intelligence Bulletin |number=65 |title=ccupational exposure to carbon nanotubes and nanofibers |url=https://www.cdc.gov/niosh/docs/2013-145/default.html |language=en-us |doi=10.26616/NIOSHPUB2013145|doi-access=free }}</ref> Although CNT caused pulmonary inflammation and toxicity in mice, exposure to aerosols generated from sanding of composites containing polymer-coated MWCNTs, representative of the actual end-product, did not exert such toxicity.<ref>{{Cite journal |last1=Bishop |first1=Lindsey |last2=Cena |first2=Lorenzo |last3=Orandle |first3=Marlene |last4=Yanamala |first4=Naveena |last5=Dahm |first5=Matthew M. |last6=Birch |first6=M. Eileen |last7=Evans |first7=Douglas E. |last8=Kodali |first8=Vamsi K. |last9=Eye |first9=Tracy |last10=Battelli |first10=Lori |last11=Zeidler-Erdely |first11=Patti C. |last12=Casuccio |first12=Gary |last13=Bunker |first13=Kristin |last14=Lupoi |first14=Jason S. |last15=Lersch |first15=Traci L. |date=2017-09-26 |title=In Vivo Toxicity Assessment of Occupational Components of the Carbon Nanotube Life Cycle To Provide Context to Potential Health Effects |url=https://pubs.acs.org/doi/10.1021/acsnano.7b03038 |journal=ACS Nano |language=en |volume=11 |issue=9 |pages=8849–8863 |doi=10.1021/acsnano.7b03038 |pmid=28759202 |issn=1936-0851}}</ref>

As of October 2016, single-wall carbon nanotubes have been registered through the European Union's [[Registration, Evaluation, Authorization and Restriction of Chemicals]] (REACH) regulations, based on evaluation of the potentially hazardous properties of SWCNT. Based on this registration, SWCNT commercialization is allowed in the EU up to 100 metric tons.<ref>{{Cite web |title=ECHA CHEM |url=https://chem.echa.europa.eu/100.242.364/dossier-list/reach/dossiers/active?pageIndex=1&pageSize=100&searchText=943-098-9 |access-date=2024-06-10 |website=chem.echa.europa.eu}}</ref> Currently, the type of SWCNT registered through REACH is limited to the specific type of single-wall carbon nanotubes manufactured by [[OCSiAl]], which submitted the application.<ref name="reach">{{cite web|url=http://www.pcimag.com/articles/102634-reach-registration-completed-for-single-wall-carbon-nanotubes|title=REACH Registration Completed for Single-Wall Carbon Nanotubes|date=16 October 2016|website=pcimag.com|publisher=PCI Mag|access-date=24 November 2016|archive-url=https://web.archive.org/web/20161124221024/http://www.pcimag.com/articles/102634-reach-registration-completed-for-single-wall-carbon-nanotubes|archive-date=24 November 2016}}</ref>

== Applications ==

[[File:Nano tape.jpg|thumb|right|Nano tape]]
Carbon nanotubes are currently used in multiple industrial and consumer applications. These include battery components, [[polymer]] composites, to improve the mechanical, thermal and electrical properties of the bulk product, and as a highly absorptive black paint. Many other applications are under development, including field effect transistors for electronics, high-strength fabrics, biosensors for biomedical and agricultural applications, and many others.

=== Biomedical Applications ===
Because of their relatively large surface area, CNTs are capable of interacting with a wide variety of therapeutic and diagnostic agents (drugs, genes, vaccines, antibodies, biosensors, etc.). This can be utilized to assist in drug delivery directly into cells. <ref>{{cite journal |last1=He |first1=Hua |last2=Pham-Huy |first2=Lien Ai |last3=Dramou |first3=Pierre |last4=Xiao |first4=Deli |last5=Zuo |first5=Pengli |last6=Pham-Huy |first6=Chuong |title=Carbon Nanotubes: Applications in Pharmacy and Medicine |journal=BioMed Research International |date=2013 |volume=2013 |pages=1–12 |doi=10.1155/2013/578290|doi-access=free |pmid=24195076 |pmc=3806157 }}</ref> In addition, CNTs have recently been used as reinforcements in implants and scaffolds due to their suitable reaction area, high elastic modulus, and load transfer capability.<ref>The Effectiveness Mechanisms of Carbon Nanotubes (CNTs) as Reinforcements for Magnesium-Based Composites for Biomedical Applications: A Review Nanomaterials 2024, 14, 756. https://doi.org/10.3390/nano14090756{{Open access}}  PMCID: PMC11085746  {{Open access}} PMID 38727350 </ref><ref>Carbon Nanotube (CNT) Encapsulated Magnesium-Based Nanocomposites to Improve Mechanical, Degradation and Antibacterial Performances for Biomedical Device Applications Coatings 2022, 12, 1589. https://doi.org/10.3390/coatings12101589 {{Open access}}</ref>

[[File:Applications of CNT. jpg.jpg|thumb|Schematic of carbon nanotubes applications in biomedicine Nanomaterials 2024, 14, 756. https://doi.org/10.3390/nano14090756{{Open access}}  PMCID: PMC11085746  {{Open access}} ]]

CNTs have been shown to increase the effectiveness of bioactive coatings for the attachment, proliferation, and differentiation of [[osteoblasts]], and has been used as a bone substitution material.<ref>{{cite journal |last1=Zhao |first1=Meng |last2=Su |first2=Rongjun |last3=Ji |first3=Lingyu |last4=Zhang |first4=Ying |last5=Wu |first5=Huirong |last6=Wen |first6=Zhaohui |last7=Dai |first7=Changsong |title=The Biocompatibility and Antibacterial Properties of the CNTs-Doped Magnesium Based Composite Implants in a Long-Term Biodegradation Process |journal=Journal of Nanomaterials |date=22 January 2023 |volume=2023 |issue=1 |pages=1–17 |doi=10.1155/2023/5012576|doi-access=free }}</ref>

CNTs may be used as reinforcing materials for [[chitosan]]-containing coatings used on implants and medical [[Tissue_engineering#Scaffolds|scaffolds]].<ref>{{cite journal |last1=Francis |first1=A. A. |last2=Abdel-Gawad |first2=S. A. |last3=Shoeib |first3=M. A. |title=Toward CNT-reinforced chitosan-based ceramic composite coatings on biodegradable magnesium for surgical implants |journal=Journal of Coatings Technology and Research |date=July 2021 |volume=18 |issue=4 |pages=971–988 |doi=10.1007/s11998-021-00468-y}}</ref>

=== Biosensing ===
SWCNTs have nanoscale dimensions that fit to the size of biological species. Due to this size compatibility and their large surface-to-volume ratio, they are sensitive to changes in their chemical environment.<ref>{{Cite journal |last1=Cognet |first1=Laurent |last2=Tsyboulski |first2=Dmitri A. |last3=Rocha |first3=John-David R. |last4=Doyle |first4=Condell D. |last5=Tour |first5=James M. |last6=Weisman |first6=R. Bruce |date=2007-06-08 |title=Stepwise Quenching of Exciton Fluorescence in Carbon Nanotubes by Single-Molecule Reactions |url=https://www.science.org/doi/10.1126/science.1141316 |journal=Science |language=en |volume=316 |issue=5830 |pages=1465–1468 |doi=10.1126/science.1141316 |pmid=17556581 |arxiv=0707.3246 |bibcode=2007Sci...316.1465C |s2cid=7476534 |issn=0036-8075}}</ref><ref name="Heller-2008">{{Cite journal |last1=Heller |first1=Iddo |last2=Janssens |first2=Anne M. |last3=Männik |first3=Jaan |last4=Minot |first4=Ethan D. |last5=Lemay |first5=Serge G. |last6=Dekker |first6=Cees |date=2008-02-01 |title=Identifying the Mechanism of Biosensing with Carbon Nanotube Transistors |url=https://pubs.acs.org/doi/10.1021/nl072996i |journal=Nano Letters |language=en |volume=8 |issue=2 |pages=591–595 |doi=10.1021/nl072996i |pmid=18162002 |bibcode=2008NanoL...8..591H |issn=1530-6984}}</ref> Through covalent and non-covalent surface functionalization, SWCNTs can be precisely tailored for selective molecular interactions with a target analyte.<ref name="Ackermann-2022">{{Cite journal |last1=Ackermann |first1=Julia |last2=Metternich |first2=Justus T. |last3=Herbertz |first3=Svenja |last4=Kruss |first4=Sebastian |date=2022-04-25 |title=Biosensing with Fluorescent Carbon Nanotubes |journal=Angewandte Chemie International Edition |language=en |volume=61 |issue=18 |pages=e202112372 |doi=10.1002/anie.202112372 |issn=1433-7851 |pmc=9313876 |pmid=34978752}}</ref> The SWCNT represents the transduction unit that converts the interaction into a signal change (optical or electrical). Due to continuous progress in the development of detection strategies, there are numerous examples of the use of SWCNTs as highly sensitive nanosensors (even down to the single molecule level<ref>{{Cite journal |last1=Zhang |first1=Jingqing |last2=Boghossian |first2=Ardemis A. |last3=Barone |first3=Paul W. |last4=Rwei |first4=Alina |last5=Kim |first5=Jong-Ho |last6=Lin |first6=Dahua |last7=Heller |first7=Daniel A. |last8=Hilmer |first8=Andrew J. |last9=Nair |first9=Nitish |last10=Reuel |first10=Nigel F. |last11=Strano |first11=Michael S. |date=2011-01-26 |title=Single Molecule Detection of Nitric Oxide Enabled by d(AT) 15 DNA Adsorbed to Near Infrared Fluorescent Single-Walled Carbon Nanotubes |url=https://pubs.acs.org/doi/10.1021/ja1084942 |journal=Journal of the American Chemical Society |language=en |volume=133 |issue=3 |pages=567–581 |doi=10.1021/ja1084942 |pmid=21142158 |issn=0002-7863}}</ref><ref>{{Cite journal |last1=Jin |first1=Hong |last2=Heller |first2=Daniel A. |last3=Kalbacova |first3=Marie |last4=Kim |first4=Jong-Ho |last5=Zhang |first5=Jingqing |last6=Boghossian |first6=Ardemis A. |last7=Maheshri |first7=Narendra |last8=Strano |first8=Michael S. |date=April 2010 |title=Detection of single-molecule H2O2 signalling from epidermal growth factor receptor using fluorescent single-walled carbon nanotubes |journal=Nature Nanotechnology |language=en |volume=5 |issue=4 |pages=302–309 |doi=10.1038/nnano.2010.24 |issn=1748-3387 |pmc=6438196 |pmid=20208549}}</ref><ref name="Kruss-2017">{{Cite journal |last1=Kruss |first1=Sebastian |last2=Salem |first2=Daniel P. |last3=Vuković |first3=Lela |last4=Lima |first4=Barbara |last5=Vander Ende |first5=Emma |last6=Boyden |first6=Edward S. |last7=Strano |first7=Michael S. |date=2017-02-21 |title=High-resolution imaging of cellular dopamine efflux using a fluorescent nanosensor array |journal=Proceedings of the National Academy of Sciences |language=en |volume=114 |issue=8 |pages=1789–1794 |doi=10.1073/pnas.1613541114 |doi-access=free |issn=0027-8424 |pmc=5338365 |pmid=28179565|bibcode=2017PNAS..114.1789K }}</ref>) for a variety of important biomolecules. 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|pmc=7570893 |pmid=32937986|bibcode=2020Senso..20.5247S }}</ref> as well as bacteria.<ref name="Nißler-2020">{{Cite journal |last1=Nißler |first1=Robert |last2=Bader |first2=Oliver |last3=Dohmen |first3=Maria |last4=Walter |first4=Sebastian G. |last5=Noll |first5=Christine |last6=Selvaggio |first6=Gabriele |last7=Groß |first7=Uwe |last8=Kruss |first8=Sebastian |date=2020-11-25 |title=Remote near infrared identification of pathogens with multiplexed nanosensors |journal=Nature Communications |language=en |volume=11 |issue=1 |page=5995 |doi=10.1038/s41467-020-19718-5 |issn=2041-1723 |pmc=7689463 |pmid=33239609|bibcode=2020NatCo..11.5995N }}</ref>

[[File:Biosensing.png|thumb|614x614px|Optical biosensors with SWCNTs. The functionalization of SWCNTs with (bio)polymers leads to nanosensors for various molecules. The interaction with these molecules influences the NIR fluorescence of the SWCNTs.]]
The signal change manifests itself in an increase or decrease in the current (electrical)<ref name="Heller-2008" /> or in a change in the intensity or wavelength of the fluorescence emission (optical).<ref name="Ackermann-2022" /> Depending on the type of application, both electrical or optical signal transmission can be advantageous.<ref>{{Cite journal |last1=Clément |first1=P. |last2=Ackermann |first2=J. |last3=Sahin-Solmaz |first3=N. |last4=Herbertz |first4=S. |last5=Boero |first5=G. |last6=Kruss |first6=S. |last7=Brugger |first7=J. |date=November 2022 |title=Comparison of electrical and optical transduction modes of DNA-wrapped SWCNT nanosensors for the reversible detection of neurotransmitters. |url=https://linkinghub.elsevier.com/retrieve/pii/S0956566322006820 |journal=Biosensors and Bioelectronics |language=en |volume=216 |pages=114642 |doi=10.1016/j.bios.2022.114642|pmid=36055131 }}</ref> For sensitive measurement of electronic changes, [[field-effect transistor]]s (FET) are often used in which the flow of charges within the SWCNTs is measured. The FET structures allow easy on-chip integration and can be parallelized to detect multiple target analytes simultaneously.<ref name="Xu2018" /> However, such sensors are more invasive for in vivo applications, as the entire device has to be inserted into the body. Optical detection with semiconducting SWCNTs is based on the radiative recombination of excitons in the near-infrared (NIR) by prior optical (fluorescence<ref>{{Cite journal |last1=O'Connell |first1=Michael J. |last2=Bachilo |first2=Sergei M. |last3=Huffman |first3=Chad B. |last4=Moore |first4=Valerie C. |last5=Strano |first5=Michael S. |last6=Haroz |first6=Erik H. |last7=Rialon |first7=Kristy L. |last8=Boul |first8=Peter J. |last9=Noon |first9=William H. |last10=Kittrell |first10=Carter |last11=Ma |first11=Jianpeng |last12=Hauge |first12=Robert H. |last13=Weisman |first13=R. Bruce |last14=Smalley |first14=Richard E. |date=2002-07-26 |title=Band Gap Fluorescence from Individual Single-Walled Carbon Nanotubes |url=https://www.science.org/doi/10.1126/science.1072631 |journal=Science |language=en |volume=297 |issue=5581 |pages=593–596 |doi=10.1126/science.1072631 |pmid=12142535 |bibcode=2002Sci...297..593O |s2cid=22623119 |issn=0036-8075}}</ref>) or electrical excitation (electroluminescence<ref>{{Cite journal |last1=Xu |first1=Beibei |last2=Wu |first2=Xiaojian |last3=Kim |first3=Mijin |last4=Wang |first4=Peng |last5=Wang |first5=YuHuang |date=2021-01-28 |title=Electroluminescence from 4-nitroaryl organic color centers in semiconducting single-wall carbon nanotubes |url=https://pubs.aip.org/jap/article/129/4/044305/1079034/Electroluminescence-from-4-nitroaryl-organic-color |journal=Journal of Applied Physics |language=en |volume=129 |issue=4 |page=044305 |doi=10.1063/5.0039047 |bibcode=2021JAP...129d4305X |issn=0021-8979|doi-access=free }}</ref><ref>{{Cite journal |last1=Li |first1=Min-Ken |last2=Riaz |first2=Adnan |last3=Wederhake |first3=Martina |last4=Fink |first4=Karin |last5=Saha |first5=Avishek |last6=Dehm |first6=Simone |last7=He |first7=Xiaowei |last8=Schöppler |first8=Friedrich |last9=Kappes |first9=Manfred M. |last10=Htoon |first10=Han |last11=Popov |first11=Valentin N. |last12=Doorn |first12=Stephen K. |last13=Hertel |first13=Tobias |last14=Hennrich |first14=Frank |last15=Krupke |first15=Ralph |date=2022-08-23 |title=Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects |url=https://pubs.acs.org/doi/10.1021/acsnano.2c03083 |journal=ACS Nano |language=en |volume=16 |issue=8 |pages=11742–11754 |doi=10.1021/acsnano.2c03083 |pmid=35732039 |osti=1879407 |s2cid=249956650 |issn=1936-0851}}</ref>). The emission in the NIR enables detection in the biological transparency window, where optical sensor applications benefit from reduced scattering and autofluorescence of biological samples and consequently a high signal-to-noise ratio.<ref>{{Cite journal |last1=Hong |first1=Guosong |last2=Antaris |first2=Alexander L. |last3=Dai |first3=Hongjie |date=2017-01-10 |title=Near-infrared fluorophores for biomedical imaging |url=https://www.nature.com/articles/s41551-016-0010 |journal=Nature Biomedical Engineering |language=en |volume=1 |issue=1 |doi=10.1038/s41551-016-0010 |s2cid=78795936 |issn=2157-846X}}</ref> Compared to optical sensors in the [[UV]] or [[Visible spectrum|visible]] range, the penetration depth in biological tissue is also increased. In addition to the advantage of a contactless readout SWCNTs have excellent photostability,<ref>{{Cite journal |last1=Hong |first1=Guosong |last2=Diao |first2=Shuo |last3=Antaris |first3=Alexander L. |last4=Dai |first4=Hongjie |date=2015-10-14 |title=Carbon Nanomaterials for Biological Imaging and Nanomedicinal Therapy |url=https://pubs.acs.org/doi/10.1021/acs.chemrev.5b00008 |journal=Chemical Reviews |language=en |volume=115 |issue=19 |pages=10816–10906 |doi=10.1021/acs.chemrev.5b00008 |pmid=25997028 |issn=0009-2665}}</ref> which enables long-term sensor applications. Furthermore, the nanoscale size of SWCNTs allows dense coating of surfaces which enables chemical imaging, e.g. of cellular release processes with high spatial and temporal resolution.<ref name="Kruss-2017" /><ref name="Ackermann-2023" /> Detection of several target analytes is possible by the spatial arrangement of different SWCNT sensors in arrays<ref name="Nißler-2020" /><ref>{{Cite journal |last1=Dong |first1=Juyao |last2=Salem |first2=Daniel P. |last3=Sun |first3=Jessica H. |last4=Strano |first4=Michael S. |date=2018-04-24 |title=Analysis of Multiplexed Nanosensor Arrays Based on Near-Infrared Fluorescent Single-Walled Carbon Nanotubes |url=https://pubs.acs.org/doi/10.1021/acsnano.8b00980 |journal=ACS Nano |language=en |volume=12 |issue=4 |pages=3769–3779 |doi=10.1021/acsnano.8b00980 |pmid=29614219 |issn=1936-0851}}</ref><ref>{{Cite journal |last1=Salem |first1=Daniel P. |last2=Gong |first2=Xun |last3=Liu |first3=Albert Tianxiang |last4=Akombi |first4=Kayla |last5=Strano |first5=Michael S. |date=2020-01-07 |title=Immobilization and Function of nIR-Fluorescent Carbon Nanotube Sensors on Paper Substrates for Fluidic Manipulation |url=https://pubs.acs.org/doi/10.1021/acs.analchem.9b03756 |journal=Analytical Chemistry |language=en |volume=92 |issue=1 |pages=916–923 |doi=10.1021/acs.analchem.9b03756 |pmid=31829619 |s2cid=209340238 |issn=0003-2700}}</ref> or by hyperspectral detection<ref name="Nißler-2020" /><ref>{{Cite journal |last1=Roxbury |first1=Daniel |last2=Jena |first2=Prakrit V. |last3=Williams |first3=Ryan M. |last4=Enyedi |first4=Balázs |last5=Niethammer |first5=Philipp |last6=Marcet |first6=Stéphane |last7=Verhaegen |first7=Marc |last8=Blais-Ouellette |first8=Sébastien |last9=Heller |first9=Daniel A. |date=2015-09-21 |title=Hyperspectral Microscopy of Near-Infrared Fluorescence Enables 17-Chirality Carbon Nanotube Imaging |journal=Scientific Reports |language=en |volume=5 |issue=1 |page=14167 |doi=10.1038/srep14167 |issn=2045-2322 |pmc=4585673 |pmid=26387482|bibcode=2015NatSR...514167R }}</ref> based on monochiral SWCNT sensors that emit at different emission wavelengths. For fluorescence applications, however, optical filters to distinguish between excitation and emission and a NIR-sensitive detector must be used. Standard silicon detectors can also be used if monochiral SWCNTs (extractable by special purification processes) emitting closer to the visible range (800 – 900&nbsp;nm) are used.<ref name="Ackermann-2023" /><ref>{{Cite journal |last1=Wei |first1=Xiaojun |last2=Tanaka |first2=Takeshi |last3=Akizuki |first3=Naoto |last4=Miyauchi |first4=Yuhei |last5=Matsuda |first5=Kazunari |last6=Ohfuchi |first6=Mari |last7=Kataura |first7=Hiromichi |date=2016-05-19 |title=Single-Chirality Separation and Optical Properties of (5,4) Single-Wall Carbon Nanotubes |url=https://pubs.acs.org/doi/10.1021/acs.jpcc.6b03257 |journal=The Journal of Physical Chemistry C |language=en |volume=120 |issue=19 |pages=10705–10710 |doi=10.1021/acs.jpcc.6b03257 |issn=1932-7447}}</ref> In order to avoid susceptibility of optical sensors to fluctuating ambient light, internal references such as SWCNTs that are modified to be non-responsive or stable NIR emitters<ref name="Nißler-2020" /><ref>{{Cite journal |last1=Selvaggio |first1=Gabriele |last2=Chizhik |first2=Alexey |last3=Nißler |first3=Robert |last4=Kuhlemann |first4=llyas |last5=Meyer |first5=Daniel |last6=Vuong |first6=Loan |last7=Preiß |first7=Helen |last8=Herrmann |first8=Niklas |last9=Mann |first9=Florian A. |last10=Lv |first10=Zhiyi |last11=Oswald |first11=Tabea A. |last12=Spreinat |first12=Alexander |last13=Erpenbeck |first13=Luise |last14=Großhans |first14=Jörg |last15=Karius |first15=Volker |date=2020-03-20 |title=Exfoliated near infrared fluorescent silicate nanosheets for (bio)photonics |journal=Nature Communications |language=en |volume=11 |issue=1 |page=1495 |doi=10.1038/s41467-020-15299-5 |issn=2041-1723 |pmc=7083911 |pmid=32198383|bibcode=2020NatCo..11.1495S }}</ref> can be used. An alternative is to measure fluorescence lifetimes<ref>{{Cite journal |last1=Sistemich |first1=Linda |last2=Galonska |first2=Phillip |last3=Stegemann |first3=Jan |last4=Ackermann |first4=Julia |last5=Kruss |first5=Sebastian |date=2023-06-12 |title=Near-Infrared Fluorescence Lifetime Imaging of Biomolecules with Carbon Nanotubes** |journal=Angewandte Chemie International Edition |language=en |volume=62 |issue=24 |pages=e202300682 |doi=10.1002/anie.202300682 |pmid=36891826 |issn=1433-7851|doi-access=free }}</ref> instead of fluorescence intensities. Overall, SWCNTs therefore have great potential as building blocks for various biosensors.
To render SWCNTs suitable for biosensing, their surface needs to be modified to ensure colloidal stability and provide a handle for biological recognition. Therefore, biosensing and surface modifications (functionalization) are closely related.<ref name="Ackermann-2022" /><ref>{{Cite journal |last1=Kruss |first1=Sebastian |last2=Hilmer |first2=Andrew J. |last3=Zhang |first3=Jingqing |last4=Reuel |first4=Nigel F. |last5=Mu |first5=Bin |last6=Strano |first6=Michael S. |date=December 2013 |title=Carbon nanotubes as optical biomedical sensors |url=https://linkinghub.elsevier.com/retrieve/pii/S0169409X13001695 |journal=Advanced Drug Delivery Reviews |language=en |volume=65 |issue=15 |pages=1933–1950 |doi=10.1016/j.addr.2013.07.015|pmid=23906934 }}</ref><ref>{{Cite journal |last1=Boghossian  |first1=Ardemis A. |last2=Zhang  |first2=Jingqing |last3=Barone |first3=Paul W. |last4=Reuel |first4=Nigel F. |last5=Kim |first5=Jong-Ho |last6=Heller |first6=Daniel A. |last7=Ahn |first7=Jin-Ho |last8=Hilmer |first8=Andrew J. |last9=Rwei |first9=Alina |last10=Arkalgud |first10=Jyoti R. |last11=Zhang |first11=Cathy T. |last12=Strano |first12=Michael S. |date=2011-07-18 |title=Near-Infrared Fluorescent Sensors based on Single-Walled Carbon Nanotubes for Life Sciences Applications |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cssc.201100070 |journal=ChemSusChem |language=en |volume=4 |issue=7 |pages=848–863 |doi=10.1002/cssc.201100070 |pmid=21751417 |bibcode=2011ChSCh...4..848B |issn=1864-5631}}</ref>

Potential future applications include biomedical and environmental applications such as monitoring plant health in agriculture,<ref name="Wu-2020" /><ref name="Lew-2020" /><ref>{{Cite journal |last1=Nißler |first1=Robert |last2=Müller |first2=Andrea T. |last3=Dohrman |first3=Frederike |last4=Kurth |first4=Larissa |last5=Li |first5=Han |last6=Cosio |first6=Eric G. |last7=Flavel |first7=Benjamin S. |last8=Giraldo |first8=Juan Pablo |last9=Mithöfer |first9=Axel |last10=Kruss |first10=Sebastian |date=2022-01-10 |title=Detection and Imaging of the Plant Pathogen Response by Near-Infrared Fluorescent Polyphenol Sensors |journal=Angewandte Chemie International Edition |language=en |volume=61 |issue=2 |pages=e202108373 |doi=10.1002/anie.202108373 |issn=1433-7851 |pmc=9298901 |pmid=34608727}}</ref> standoff process control in bioreactors, research/diagnostics of neuronal communication<ref>{{Cite journal |last1=Elizarova |first1=Sofia |last2=Chouaib |first2=Abed Alrahman |last3=Shaib |first3=Ali |last4=Hill |first4=Björn |last5=Mann |first5=Florian |last6=Brose |first6=Nils |last7=Kruss |first7=Sebastian |last8=Daniel |first8=James A. |date=2022-05-31 |title=A fluorescent nanosensor paint detects dopamine release at axonal varicosities with high spatiotemporal resolution |journal=Proceedings of the National Academy of Sciences |language=en |volume=119 |issue=22 |pages=e2202842119 |doi=10.1073/pnas.2202842119 |doi-access=free |pmid=35613050 |pmc=9295782 |bibcode=2022PNAS..11902842E |issn=0027-8424}}</ref> and numerous diseases such as coagulation disorders,<ref>{{Cite journal |last1=Gerstman |first1=Efrat |last2=Hendler-Neumark |first2=Adi |last3=Wulf |first3=Verena |last4=Bisker |first4=Gili |date=2023-05-10 |title=Monitoring the Formation of Fibrin Clots as Part of the Coagulation Cascade Using Fluorescent Single-Walled Carbon Nanotubes |journal=ACS Applied Materials & Interfaces |language=en |volume=15 |issue=18 |pages=21866–21876 |doi=10.1021/acsami.3c00828 |issn=1944-8244 |pmc=10176323 |pmid=37128896}}</ref> diabetes,<ref name="Zubkovs2022" /><ref>{{Cite journal |last1=Ehrlich |first1=Roni |last2=Hendler-Neumark |first2=Adi |last3=Wulf |first3=Verena |last4=Amir |first4=Dean |last5=Bisker |first5=Gili |date=July 2021 |title=Optical Nanosensors for Real-Time Feedback on Insulin Secretion by β-Cells |url=https://onlinelibrary.wiley.com/doi/10.1002/smll.202101660 |journal=Small |language=en |volume=17 |issue=30 |pages=e2101660 |doi=10.1002/smll.202101660 |pmid=34197026 |issn=1613-6810}}</ref> cancer,<ref>{{Cite journal |last1=Kim |first1=Mijin |last2=Chen |first2=Chen |last3=Wang |first3=Peng |last4=Mulvey |first4=Joseph J. |last5=Yang |first5=Yoona |last6=Wun |first6=Christopher |last7=Antman-Passig |first7=Merav |last8=Luo |first8=Hong-Bin |last9=Cho |first9=Sun |last10=Long-Roche |first10=Kara |last11=Ramanathan |first11=Lakshmi V. |last12=Jagota |first12=Anand |last13=Zheng |first13=Ming |last14=Wang |first14=YuHuang |last15=Heller |first15=Daniel A. |date=2022-03-17 |title=Detection of ovarian cancer via the spectral fingerprinting of quantum-defect-modified carbon nanotubes in serum by machine learning |journal=Nature Biomedical Engineering |language=en |volume=6 |issue=3 |pages=267–275 |doi=10.1038/s41551-022-00860-y |issn=2157-846X |pmc=9108893 |pmid=35301449}}</ref> microbial and viral infections,<ref name="Nißler-2020" /><ref>{{Cite journal |last1=Metternich |first1=Justus T. |last2=Wartmann |first2=Janus A. C. |last3=Sistemich |first3=Linda |last4=Nißler |first4=Robert |last5=Herbertz |first5=Svenja |last6=Kruss |first6=Sebastian |date=2023-07-12 |title=Near-Infrared Fluorescent Biosensors Based on Covalent DNA Anchors |url=https://pubs.acs.org/doi/10.1021/jacs.3c03336 |journal=Journal of the American Chemical Society |language=en |volume=145 |issue=27 |pages=14776–14783 |doi=10.1021/jacs.3c03336 |pmid=37367958 |s2cid=259261621 |issn=0002-7863}}</ref> testing the efficacy of pharmaceuticals<ref>{{Cite journal |last1=Ackermann |first1=Julia |last2=Reger |first2=Eline |last3=Jung |first3=Sebastian |last4=Mohr |first4=Jennifer |last5=Herbertz |first5=Svenja |last6=Seidl |first6=Karsten |last7=Kruss |first7=Sebastian |date=February 2024 |title=Smart Slides for Optical Monitoring of Cellular Processes |journal=Advanced Functional Materials |language=en |volume=34 |issue=6 |doi=10.1002/adfm.202309064 |issn=1616-301X|doi-access=free }}</ref> or infection monitoring using smart implants. In industry, SWCNTs are already used as sensors in the detection of gases and odors in the form of an electronic nose<ref>{{Cite web |title=Smart Nanotubes – Gas sensor development |url=https://smart-nanotubes.com/ |access-date=2024-02-09 |website=Smart Nanotubes |language=en-US}}</ref> or in enzyme screening.<ref>{{Cite web |title=Welcome |url=https://www.zymosense.com/ |access-date=2024-02-09 |website=www.zymosense.com}}</ref>

=== Other current applications ===

* [[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.
* [[Amroy Europe Oy]] manufactures [[Hybtonite]] carbon nano-epoxy 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 a variety of sports gear such as skis, ice hockey sticks, baseball bats, hunting arrows, and surfboards.<ref name="Pagni">{{cite web| vauthors = Pagni J |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 |archive-url=https://web.archive.org/web/20110710204854/http://www.europeanplasticsnews.com/subscriber/headlines2.html?cat=1&id=1267781513 |archive-date=10 July 2011}}</ref>
* [[Surrey NanoSystems]] synthesizes carbon nanotubes to create [[vantablack]] ultra-absorptive black paint.
* "Gecko tape" (also called "[[nano tape]]") is often commercially sold as double-sided [[adhesive tape]]. It can be used to hang lightweight items such as pictures and decorative items on smooth walls without punching holes in the wall. The carbon nanotube arrays comprising the [[synthetic setae]] leave no residue after removal and can stay sticky in extreme temperatures.<ref>{{cite news|url=https://www.nanowerk.com/nanotechnology-news2/newsid=53150.php|title=Carbon nanotube tape stays sticky in extreme temperatures|date=Jul 10, 2019|newspaper=Nanowerk Newsletter|publisher=American Chemical Society}}</ref>
* 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=nanoScience instruments |archive-url=https://web.archive.org/web/20111027064700/http://www.nanoscience.com/store/pc/viewCategories.asp?idCategory=9 |archive-date=27 October 2011}}</ref>

=== Applications under development ===

Applications of nanotubes in development in academia and industry include:

* Medical devices: Using single wall carbon nanotubes in medical devices results in no skin contamination, high flexibility, and softness, which are crucial for healthcare applications.<ref>{{Cite web |date=2022-09-21 |title=Nanotube solution for medical devices approved to enter EU market |url=https://www.med-technews.com/news/medtech-materials-and-assembly-news/nanotube-solution-for-medical-devices-approved-to-enter-eu-m/ |access-date=2024-09-06 |website=Med-Tech Innovation |language=en-gb}}</ref>
* Wearable electronics and 5G/6G communication: Electrodes with single wall carbon nanotubes (SWCNTs) exhibit excellent electrochemical properties and flexibility.<ref>{{Cite journal |last1=Kim |first1=Mi-Jeong |last2=Kim |first2=Hyeonbeom |last3=Kim |first3=Jungmo |last4=Lee |first4=Yeo Jin |last5=Lee |first5=Wonki |last6=Hwang |first6=Jun Yeon |last7=Kim |first7=Ki Kang |last8=Jeong |first8=Hee Jin |date=2024-06-28 |title=Molecular-level hybridization of single-walled carbon nanotubes and a copper complex with counterbalanced electrostatic interactions |journal=Communications Materials |language=en |volume=5 |issue=1 |page=111 |doi=10.1038/s43246-024-00548-7 |issn=2662-4443|doi-access=free |bibcode=2024CoMat...5..111K }}</ref><ref>{{Cite journal |last1=Paleo |first1=Antonio J. |last2=Martinez-Rubi |first2=Yadienka |last3=Krause |first3=Beate |last4=Pötschke |first4=Petra |last5=Jakubinek |first5=Michael B. |last6=Ashrafi |first6=Behnam |last7=Kingston |first7=Christopher |date=2023-10-13 |title=Carbon Nanotube–Polyurethane Composite Sheets for Flexible Thermoelectric Materials |journal=ACS Applied Nano Materials |language=en |volume=6 |issue=19 |pages=17986–17995 |doi=10.1021/acsanm.3c03247 |issn=2574-0970 |pmc=10580240 |pmid=37854856}}</ref>
* Bitumen and asphalt: The world's first test section of road pavement with single wall carbon nanotubes (SWCNTs) showed a 67% increase in resistance to cracks and ruts, increasing the lifespan of the materials.<ref>{{Cite journal |last=Tiza |first=Michael Toryila |date=December 6, 2022 |title=Applications of nanomaterials in highway pavements |url=https://dergipark.org.tr/en/download/article-file/2643879 |journal=NanoEra |issue=2(2), 23-29 |via=Atatürk University Press}}</ref>
* Nanocomposites for aviation, automotive, and renewable energy markets: Modifying resin with just 0.02% single wall carbon nanotubes (SWCNTs) increases electrical conductivity by 276% without compromising the mechanical properties of fiber-reinforced polymers, also improving flexural properties and delaying thermal degradation.<ref>{{Cite journal |last1=Demski |first1=Szymon |last2=Misiak |first2=Michał |last3=Majchrowicz |first3=Kamil |last4=Komorowska |first4=Gabriela |last5=Lipkowski |first5=Adrian |last6=Stankiewicz |first6=Karolina |last7=Dydek |first7=Kamil |last8=Waśniewski |first8=Bartłomiej |last9=Boczkowska |first9=Anna |last10=Ehrlich |first10=Hermann |date=2024-05-21 |title=Mechanical recycling of CFRPs based on thermoplastic acrylic resin with the addition of carbon nanotubes |journal=Scientific Reports |language=en |volume=14 |issue=1 |pages=11550 |doi=10.1038/s41598-024-62594-y |pmid=38773242 |issn=2045-2322|pmc=11109235 |bibcode=2024NatSR..1411550D }}</ref>
* Additive manufacturing: single wall carbon nanotubes (SWCNTs) are mixed with a suitable printing medium or used as a filler material in the printing process, creating complex structures with enhanced mechanical and electrical properties.<ref>{{Cite journal |last1=Nechausov |first1=Sergey |last2=Miriyev |first2=Aslan |date=2024-09-15 |title=3D-Printable high-mixed-conductivity ionogel composites for soft multifunctional devices |journal=Chemical Engineering Journal |volume=496 |pages=153759 |doi=10.1016/j.cej.2024.153759 |issn=1385-8947|doi-access=free |bibcode=2024ChEnJ.49653759N }}</ref>
* Utilizing carbon nanotubes as the channel material of [[carbon nanotube field-effect transistor]]s.<ref name="noyce1">{{cite journal | vauthors = Noyce SG, Doherty JL, Cheng Z, Han H, Bowen S, Franklin AD | title = Electronic Stability of Carbon Nanotube Transistors Under Long-Term Bias Stress | journal = Nano Letters | volume = 19 | issue = 3 | pages = 1460–1466 | date = March 2019 | pmid = 30720283 | doi = 10.1021/acs.nanolett.8b03986 | s2cid = 73450707 | bibcode = 2019NanoL..19.1460N }}</ref>
* Using carbon nanotubes as a scaffold for diverse microfabrication techniques.<ref>{{cite web |url=http://nano.byu.edu/publications |title=Publications on carbon nanotube applications including scaffold microfabrication |date=27 May 2014 |website=nano.byu.edu}}</ref>
* Energy dissipation in self-organized nanostructures under the influence of an electric field.<ref>{{cite journal | vauthors = Belkin A, Hubler A, Bezryadin A | title = Self-assembled wiggling nano-structures and the principle of maximum entropy production | journal = Scientific Reports | volume = 5 | page = 8323 | date = February 2015 | pmid = 25662746 | pmc = 4321171 | doi = 10.1038/srep08323 | bibcode = 2015NatSR...5.8323B }}</ref>
* Using carbon nanotubes for environmental monitoring due to their active surface area and their ability to absorb gases.<ref>{{cite journal | vauthors = Tan CW, Tan KH, Ong YT, Mohamed AR, Zein SH, Tan SH |title=Energy and environmental applications of carbon nanotubes |journal=Environmental Chemistry Letters |date=September 2012 |volume=10 |issue=3 |pages=265–273 |doi=10.1007/s10311-012-0356-4 |bibcode=2012EnvCL..10..265T |s2cid=95369378 }}</ref>
* Jack Andraka used carbon nanotubes in his pancreatic cancer test. His method of testing won the Intel International Science and Engineering Fair Gordon E. Moore Award in the spring of 2012.<ref>{{Cite web| vauthors = Tucker A |title=Jack Andraka, the Teen Prodigy of Pancreatic Cancer|url=https://www.smithsonianmag.com/science-nature/jack-andraka-the-teen-prodigy-of-pancreatic-cancer-135925809/|access-date=2021-03-02|website=Smithsonian Magazine|language=en}}</ref>
* [[The Boeing Company]] has patented the use of carbon nanotubes for structural health monitoring<ref>[https://patents.google.com/patent/US9329021]{{cite patent|country=US|number=9329021|title=System and methods for use in monitoring a structure|pubdate=May 3, 2016|inventor=DeLuca MJ, Felker CJ, Heider D|url=http://www.google.com/patents/US9329021}}</ref> of composites used in aircraft structures. This technology is hoped to greatly reduce the risk of an in-flight failure caused by structural degradation of aircraft.
* [[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 2009 |publisher=ReinforcedPlastics.com |archive-url=https://web.archive.org/web/20120303141747/http://www.reinforcedplastics.com/view/7467/pirahna-usv-built-using-nanoenhanced-carbon-prepreg/ |archive-date=3 March 2012 |df=dmy}}</ref>
* [[IMEC]] is using carbon nanotubes for [[Pellicle mirror|pellicles]] in semiconductor lithography.<ref>{{Cite web |last=LaPedus |first=Mark |date=2021-03-22 |title=EUV Pellicles Finally Ready |url=https://semiengineering.com/euv-pellicles-finally-ready/ |access-date=2022-11-13 |website=Semiconductor Engineering |language=en-US}}</ref>
* In [[tissue engineering]], carbon nanotubes have been used as scaffolding for bone growth.<ref name="bone">{{cite journal | vauthors = Zanello LP, Zhao B, Hu H, Haddon RC | title = Bone cell proliferation on carbon nanotubes | journal = Nano Letters | volume = 6 | issue = 3 | pages = 562–567 | date = March 2006 | pmid = 16522063 | doi = 10.1021/nl051861e | bibcode = 2006NanoL...6..562Z }}</ref>

Carbon nanotubes can serve 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, car parts, or [[damascus steel]].<ref>{{cite web|url=http://news.nationalgeographic.com/news/2006/11/061116-nanotech-swords.html|archive-url=https://web.archive.org/web/20061118060943/http://news.nationalgeographic.com/news/2006/11/061116-nanotech-swords.html|url-status=dead|archive-date=18 November 2006|title=Legendary Swords' Sharpness, Strength From Nanotubes, Study Says|website=news.nationalgeographic.com}}</ref><ref>{{cite journal | vauthors = Gullapalli S, Wong MS | 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 | access-date = 24 November 2011 | archive-url = https://web.archive.org/web/20120813180046/http://www.aiche.org/uploadedFiles/Publications/CEPMagazine/051128_public.pdf | archive-date = 13 August 2012 }}</ref>

IBM expected carbon nanotube transistors to be used on Integrated Circuits by 2020.<ref>{{Cite web|url=https://www.technologyreview.com/s/528601/ibm-commercial-nanotube-transistors-are-coming-soon/|title=IBM Expects Nanotube Transistor Computer Chips Ready Soon After 2020| vauthors = Simonite T |website=MIT Technology Review}}</ref>

SWCNTs have found use in long lasting, faster charged [[Lithium-ion battery|lithium ion batteries]];<ref>{{Cite web |last=Billington |first=James |date=2022-08-04 |title=Faster charging and more powerful EV batteries boosted by graphene nanotube production |url=https://www.electrichybridvehicletechnology.com/news/battery-technology/faster-charging-and-more-powerful-ev-batteries-boosted-by-graphene-nanotube-production.html |access-date=2024-08-07 |website=Electric & Hybrid Vehicle Technology International |language=en-US}}</ref> [[polyamide]] car parts for e-painting;<ref>{{Cite news |last=Moore |first=Stephen |date=2020-08-31 |title=Graphene Nanotubes Make Polyamide Paintable |url=https://www.plasticstoday.com/automotive-and-mobility/graphene-nanotubes-make-polyamide-paintable |access-date=2024-08-07 |work=Plastics Today}}</ref> automotive primers for cost benefits and better aesthetics of topcoats;<ref>{{Cite web |title=Clear Skies Coatings Presented a Waterborne Conductive Primer & Adhesion Promoter with Graphene Nanotubes – IPCM |url=https://www.ipcm.it/en/post/clear-skies-coatings-waterborne-conductive-primer.aspx |access-date=2024-08-07 |website=www.ipcm.it}}</ref> [[Antistatic device|ESD floors]];<ref>{{Cite web |title=Overcoming ESD-Control Flooring Challenges: A Comprehensive Guide to ANSI/ESD S20.20-2021 {{!}} FLOOR Trends & Installation |url=https://www.floortrendsmag.com/articles/110984-overcoming-esd-control-flooring-challenges-a-comprehensive-guide-to-ansi-esd-s2020-2021 |access-date=2024-08-07 |website=www.floortrendsmag.com |language=en}}</ref><ref>{{Cite journal |last1=Soto Beobide |first1=Amaia |last2=Bieri |first2=Rudolf |last3=Szakács |first3=Zoltán |last4=Sparwasser |first4=Kevin |last5=Kaitsa |first5=Ioanna G. |last6=Georgiopoulos |first6=Ilias |last7=Andrikopoulos |first7=Konstantinos S. |last8=Van Kerckhove |first8=Gunther |last9=Voyiatzis |first9=George A. |date=January 2024 |title=Raman Spectroscopy Unfolds the Fate and Transformation of SWCNTs after Abrasive Wear of Epoxy Floor Coatings |journal=Nanomaterials |language=en |volume=14 |issue=1 |pages=120 |doi=10.3390/nano14010120 |doi-access=free |pmid=38202575 |issn=2079-4991|pmc=10780583 }}</ref> electrically conductive lining coatings for tanks and pipes;<ref>{{Cite web |date=2024-07-26 |title=Graphene goes industrial without a bang |url=https://www.compositesworld.com/articles/graphene-goes-industrial-without-a-bang- |access-date=2024-08-07 |website=www.compositesworld.com |language=en}}</ref> [[Rubber pants|rubber parts]] with improved heat and oil aging stability;<ref>{{Cite web |title=OCSiAl, Daikin improve fluoropolymer resistance to extreme conditions {{!}} European Rubber Journal |url=https://www.european-rubber-journal.com/article/2090872/ocsial-daikin-improve-fluoropolymer-resistance-to-extreme-conditions |access-date=2024-08-07 |website=www.european-rubber-journal.com}}</ref><ref>{{Cite journal |last1=Liu |first1=Guangyong |last2=Wang |first2=Huiyu |last3=Ren |first3=Tianli |last4=Chen |first4=Yuwei |last5=Liu |first5=Susu |date=January 2024 |title=Systematic Investigation of the Degradation Properties of Nitrile-Butadiene Rubber/Polyamide Elastomer/Single-Walled Carbon Nanotube Composites in Thermo-Oxidative and Hot Oil Environments |journal=Polymers |language=en |volume=16 |issue=2 |pages=226 |doi=10.3390/polym16020226 |doi-access=free |pmid=38257025 |pmc=10820770 |issn=2073-4360}}</ref> conductive [[gelcoat]]s for ATEX requirements and tooling conductive gelcoats for increased safety and efficiency;<ref>{{Cite web |date=2024-07-26 |title=BÜFA releases line of novel conductive gelcoats |url=https://www.compositesworld.com/products/bufa-releases-line-of-novel-conductive-gelcoats |access-date=2024-08-07 |website=www.compositesworld.com |language=en}}</ref> and heating fiber coatings for infrastructure elements.<ref>{{Cite web |date=2022-02-21 |title=Polymer fibers with graphene nanotubes make it possible to heat hard-to-reach, complex-shaped items – Modern Plastics India |url=https://modernplasticsindia.in/polymer-fibers-with-graphene-nanotubes/ |access-date=2024-08-07 |language=en-US}}</ref>

=== Potential/Future 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.<ref>{{cite journal | vauthors = Thomas DJ |title=Ultrafine graphitised MWCNT nanostructured yarn for the manufacture of electrically conductive fabric |journal=The International Journal of Advanced Manufacturing Technology |date=June 2018 |volume=96 |issue=9–12 |pages=3805–3808 |doi=10.1007/s00170-017-1320-z |s2cid=115751858 }}</ref> The highest tensile strength of an individual multi-walled carbon nanotube has been tested to be 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 | vauthors = Sanderson K |title=Sharpest cut from nanotube sword |journal=Nature News |doi=10.1038/news061113-11| year=2006|s2cid=136774602 |doi-access=free }}</ref><ref>{{cite journal | vauthors = Reibold M, Paufler P, Levin AA, Kochmann W, Pätzke N, Meyer DC | title = Materials: carbon nanotubes in an ancient Damascus sabre | journal = Nature | volume = 444 | issue = 7117 | page = 286 | date = November 2006 | pmid = 17108950 | doi = 10.1038/444286a | s2cid = 4431079 | bibcode = 2006Natur.444..286R | doi-access = free }}</ref> Recently, several studies have highlighted the prospect of using carbon nanotubes as building blocks to fabricate three-dimensional macroscopic (>1mm in all three dimensions) all-carbon devices. Lalwani et al. have reported a novel radical initiated thermal crosslinking method to fabricated macroscopic, free-standing, porous, all-carbon scaffolds using single- and multi-walled carbon nanotubes as building blocks.<ref name="PMID 23436939" /> These scaffolds possess macro-, micro-, and nano- structured pores and the porosity can be tailored for specific applications. These 3D all-carbon scaffolds/architectures may be used for the fabrication of the next generation of energy storage, supercapacitors, field emission transistors, high-performance catalysis,<ref>{{cite journal | vauthors = Valenti G, Boni A, Melchionna M, Cargnello M, Nasi L, Bertoni G, Gorte RJ, Marcaccio M, Rapino S, Bonchio M, Fornasiero P, Prato M, Paolucci F | title = Co-axial heterostructures integrating palladium/titanium dioxide with carbon nanotubes for efficient electrocatalytic hydrogen evolution | journal = Nature Communications | volume = 7 | page = 13549 | date = December 2016 | pmid = 27941752 | pmc = 5159813 | doi = 10.1038/ncomms13549 | bibcode = 2016NatCo...713549V }}</ref> photovoltaics, and biomedical devices and implants.

CNTs are potential candidates for future via and wire material in nano-scale VLSI circuits. Eliminating [[electromigration]] reliability concerns that plague today's [[Cu interconnects]], isolated (single and multi-wall) CNTs can carry current densities in excess of 1000&nbsp;MA/cm<sup>2</sup> without electromigration damage.<ref name="EM_book">{{cite book| vauthors = Lienig J, Thiele M |title= Fundamentals of Electromigration-Aware Integrated Circuit Design|pages=138–140 |chapter=Mitigating Electromigration in Physical Design |publisher=Springer |date=2018 |doi=10.1007/978-3-319-73558-0 |isbn=978-3-319-73557-3}}</ref>

Single-walled nanotubes are likely candidates for miniaturizing electronics. The most basic building block of these systems is an electric wire, and SWNTs with diameters of an order of a nanometre can be excellent conductors.<ref name="mintmire1" /><ref>{{cite journal | vauthors = Dekker C |title=Carbon Nanotubes as Molecular Quantum Wires |journal=Physics Today |date=May 1999 |volume=52 |issue=5 |pages=22–28 |doi=10.1063/1.882658 |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 | vauthors = Martel R, Derycke V, Lavoie C, Appenzeller J, Chan KK, Tersoff J, Avouris P | title = Ambipolar electrical transport in semiconducting single-wall carbon nanotubes | journal = Physical Review Letters | volume = 87 | issue = 25 | page = 256805 | date = December 2001 | pmid = 11736597 | doi = 10.1103/PhysRevLett.87.256805 | 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 expose half of an SWNT to oxygen and protect the other half from it. The resulting SWNT acts as a ''not'' logic gate with both p- and n-type FETs in the same molecule.

Large quantities of pure CNTs can be made into a freestanding sheet or film by surface-engineered tape-casting (SETC) fabrication technique which is a scalable method to fabricate flexible and foldable sheets with superior properties.<ref>{{cite journal | vauthors = Susantyoko RA, Karam Z, Alkhoori S, Mustafa I, Wu CH, Almheiri S |title=A surface-engineered tape-casting fabrication technique toward the commercialisation of freestanding carbon nanotube sheets |journal=Journal of Materials Chemistry A |date=2017 |volume=5 |issue=36 |pages=19255–19266 |doi=10.1039/c7ta04999d }}</ref><ref>{{cite journal | vauthors = Karam Z, Susantyoko RA, Alhammadi A, Mustafa I, Wu CH, Almheiri S |title=Development of Surface-Engineered Tape-Casting Method for Fabricating Freestanding Carbon Nanotube Sheets Containing Fe<sub>2</sub>O<sub>3</sub> Nanoparticles for Flexible Batteries |journal=Advanced Engineering Materials |date=June 2018 |volume=20 |issue=6 |page=1701019 |doi=10.1002/adem.201701019 |s2cid=139283096 }}</ref> Another reported form factor is CNT fiber (a.k.a. filament) by wet [[Spinning (polymers)|spinning]].<ref>{{cite journal | vauthors = Behabtu N, Young CC, Tsentalovich DE, Kleinerman O, Wang X, Ma AW, Bengio EA, ter Waarbeek RF, de Jong JJ, Hoogerwerf RE, Fairchild SB, Ferguson JB, Maruyama B, Kono J, Talmon Y, Cohen Y, Otto MJ, Pasquali M | title = Strong, light, multifunctional fibers of carbon nanotubes with ultrahigh conductivity | journal = Science | volume = 339 | issue = 6116 | pages = 182–186 | date = January 2013 | pmid = 23307737 | doi = 10.1126/science.1228061 | hdl-access = free | s2cid = 10843825 | bibcode = 2013Sci...339..182B | hdl = 1911/70792 }}</ref> The fiber is either directly spun from the synthesis pot or spun from pre-made dissolved CNTs. Individual fibers can be turned into a [[yarn]]. Apart from its strength and flexibility, the main advantage is making an [[electrically conducting yarn]]. The electronic properties of individual CNT fibers (i.e. bundle of individual CNT) are governed by the two-dimensional structure of CNTs. The fibers were measured to have a [[Electrical resistivity and conductivity|resistivity]] only one order of magnitude higher than metallic conductors at {{Convert|300|K|abbr=on}}. By further optimizing the CNTs and CNT fibers, CNT fibers with improved electrical properties could be developed.<ref name="EM_book" /><ref>{{cite journal | vauthors = Piraux L, Araujo FA, Bui TN, Otto MJ, Issi JP |title=Two-dimensional quantum transport in highly conductive carbon nanotube fibers |journal=Physical Review B |date=26 August 2015 |volume=92 |issue=8 |page=085428 |doi=10.1103/PhysRevB.92.085428 |bibcode=2015PhRvB..92h5428P }}</ref>

CNT-based yarns are suitable for applications in energy and electrochemical water treatment when coated with an [[ion-exchange membrane]].<ref>{{cite journal | vauthors = Liu F, Wagterveld RM, Gebben B, Otto MJ, Biesheuvel PM, Hamelers HV |title=Carbon nanotube yarns as strong flexible conductive capacitive electrodes |journal=Colloid and Interface Science Communications |date=November 2014 |volume=3 |pages=9–12 |doi=10.1016/j.colcom.2015.02.001 |doi-access=free }}</ref> Also, CNT-based yarns could replace copper as a [[Electromagnetic coil|winding]] material. Pyrhönen et al. (2015) have built a motor using CNT winding.<ref>{{cite journal | vauthors = Pyrhönen J, Montonen J, Lindh P, Vauterin J, Otto M |title=Replacing Copper with New Carbon Nanomaterials in Electrical Machine Windings |journal=International Review of Electrical Engineering |date=28 February 2015 |volume=10 |issue=1 |page=12 |doi=10.15866/iree.v10i1.5253 |citeseerx=10.1.1.1005.8294 }}</ref><ref>[https://www.youtube.com/watch?v=fg_QZuVKWws Carbon Nanotube Yarn Rotates Electric Motors at LUT]. YouTube</ref>

== See also ==
{{div col|colwidth=20em}}
* [[Buckypaper]]
* [[Carbide-derived carbon]]
* [[Carbon nanocone]]
* [[Carbon nanofibers]]
* [[Carbon nanoscrolls]]
* [[Carbon nanotube computer]]
* [[Carbon nanotubes in photovoltaics]]
* [[Colossal carbon tube]]
* [[Diamond nanothread]]
* [[Filamentous carbon]]
* [[Molecular modelling]]
* [[Nanoflower]]
* [[Nano-I-beam]]
* [[Ninithi]] (nanotube modelling software)
* [[Optical properties of carbon nanotubes]]
* [[Organic semiconductor]]
{{div col end}}

== References ==
''This article incorporates public domain text from the National Institute of Environmental Health Sciences (NIEHS) as quoted.''
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}}

== External links ==
{{Commons}}
{{Scholia}}

* [https://www.wecanfigurethisout.org/VL/Nanocarbon.htm Nanocarbon: From Graphene to Buckyballs]. Interactive 3D models of cyclohexane, benzene, graphene, graphite, chiral & non-chiral nanotubes, and C60 Buckyballs – WeCanFigureThisOut.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://nanohub.org/resources/2762 Learning module for Bandstructure of Carbon Nanotubes and Nanoribbons]
* [http://stacks.iop.org/1367-2630/5/i=1/a=E04 Selection of free-download articles on carbon nanotubes]
* [https://demonstrations.wolfram.com/ElectronicBandStructureOfASingleWalledCarbonNanotubeByTheZon/ WOLFRAM Demonstrations Project: Electronic Band Structure of a Single-Walled Carbon Nanotube by the Zone-Folding Method]
* [http://www.demonstrations.wolfram.com/ElectronicStructureOfASingleWalledCarbonNanotubeInTightBindi/ WOLFRAM Demonstrations Project: Electronic Structure of a Single-Walled Carbon Nanotube in Tight-Binding Wannier Representation]

{{Allotropes of carbon}}
{{emerging technologies|topics=yes|robotics=yes|manufacture=yes|materials=yes}}
{{Space elevator}}

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[[Category:Allotropes of carbon]]
[[Category:Carbon nanotubes| ]]
[[Category:Transparent electrodes]]
[[Category:Refractory materials]]
[[Category:Space elevator]]
[[Category:Discovery and invention controversies]]
[[Category:Nanomaterials]]