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{{Nanomat}}
A '''carbon nanothread''' (informally “diamond” nanothread) is a [[Orbital hybridisation#sp3|sp<sup>3</sup>-bonded]], one-dimensional carbon crystalline [[nanomaterial]]. The tetrahedral sp<sup>3</sup>-bonding of its carbon is similar to that of [[diamond]]. Nanothreads are only a few atoms across, more than 20,000 times thinner than a [[Hair|human hair]]. They consist of a stiff, strong carbon core surrounded by hydrogen atoms. [[Carbon nanotube| Carbon nanotubes]], although also one-dimensional nanomaterials, in contrast have [[Orbital hybridisation#sp2|sp<sup>2</sup>-carbon]] bonding as is found in [[graphite]]<nowiki/>.

== Synthesis ==

Nanothreads are synthesized by compressing liquid [[benzene]] to extreme pressure of 20 GPa (around 200,000 times the pressure at the surface of the [[Earth]]), and then slowly relieving that pressure.<ref name="tcfitz2014"/> The synthesis reaction can be considered a form of [[Organic chemistry| organic]] [[Solid-state chemistry| solid state]] chemistry. The benzene chains form extremely thin, tight rings of carbon that are structurally similar to diamonds.<ref>[https://finance.yahoo.com/news/scientists-might-accidentally-solved-hardest-155000475.html Scientists Might Have Accidentally Solved The Hardest Part Of Building Space Elevators], Business Insider, 13 October 2014, Ajai Raj</ref> Researchers at [[Cornell University|Cornell Universit]]<nowiki/>y have traced pathways from benzene to nanothreads, which may involve a series of organic [4+2] [[cycloaddition]] reactions along stacks of benzene molecules, followed by further reaction of unsaturated bonds.<ref name=":7">{{Cite journal|last=Chen|first=Bo|last2=Hoffmann|first2=Roald|last3=Ashcroft|first3=N. W.|last4=Badding|first4=John|last5=Xu|first5=Enshi|last6=Crespi|first6=Vincent|date=2015-11-18|title=Linearly Polymerized Benzene Arrays As Intermediates, Tracing Pathways to Carbon Nanothreads|url=http://dx.doi.org/10.1021/jacs.5b09053|journal=Journal of the American Chemical Society|volume=137|issue=45|pages=14373–14386|doi=10.1021/jacs.5b09053|issn=0002-7863}}</ref> Recently synthesis of macroscopic [[single crystal]] arrays of nanothreads hundreds of microns in size has been reported.<ref name=":1">{{Cite journal|last=Li|first=Xiang|last2=Baldini|first2=Maria|last3=Wang|first3=Tao|last4=Chen|first4=Bo|last5=Xu|first5=En-shi|last6=Vermilyea|first6=Brian|last7=Crespi|first7=Vincent H.|last8=Hoffmann|first8=Roald|last9=Molaison|first9=Jamie J.|date=2017-11-15|title=Mechanochemical Synthesis of Carbon Nanothread Single Crystals|url=http://dx.doi.org/10.1021/jacs.7b09311|journal=Journal of the American Chemical Society|volume=139|issue=45|pages=16343–16349|doi=10.1021/jacs.7b09311|issn=0002-7863}}</ref> The order and lack of grain boundaries in single crystals is often very desirable because it facilitates both applications and characterization. In contrast, [[Carbon nanotube|carbon nanotubes]] form only thin crystalline ropes. Control of the rate of compression and/or decompression appears to be important to the synthesis of polycrystalline and single crystal nanothreads.<ref name="tcfitz2014" /> <ref name=":1" /> Slow compression/decompression may favor low energy reaction pathway(s).<ref name=":1" /> The reaction, as is often found in organic chemistry, is under [[Thermodynamic versus kinetic reaction control| kinetic control]]<nowiki/>. If the synthesis pressure for nanothreads can be reduced to 5 to 6 GPa, which is the pressure used for [https://minerals.usgs.gov/minerals/pubs/commodity/diamond/mcs-2016-diamo.pdf synthesis of industrial diamond],  production on the large scale of >10<sup>6</sup> kg/yr would be possible. 

The formation of nanothread crystals appears to be guided by [[Stress–strain analysis#Uniaxial stress| uniaxial stress]] (stress in a particular single direction), to which the nanothreads consistently align.<ref name=":1" /> Reaction to form the crystals is not topochemical,<ref>{{Cite journal|last=Lauher|first=Joseph W.|last2=Fowler|first2=Frank W.|last3=Goroff|first3=Nancy S.|date=2008-09-16|title=Single-Crystal-to-Single-Crystal Topochemical Polymerizations by Design|url=http://dx.doi.org/10.1021/ar8001427|journal=Accounts of Chemical Research|volume=41|issue=9|pages=1215–1229|doi=10.1021/ar8001427|issn=0001-4842}}</ref> as it involves a major rearrangement from a lower symmetry [[Monoclinic crystal system| monoclinic]]<nowiki/> benzene crystal to a higher symmetry[[Hexagonal crystal family| hexagonal]]<nowiki/> nanothread crystal.  Topochemical reactions generally require commensuration between the periodicities and interatomic distances between reactant and product. The distances between benzene molecules with [[Van der Waals force| van der Waals]] separations between them must shrink by 40% or more as the short, strong [[Covalent bond| covalent]] carbon-carbon bonds between them form during the nanothread synthesis reaction. Such large changes in geometry usual break up crystal order, but the nanothread reaction instead creates it.  Even [[Crystallite| polycrystalline]] benzene reacts to form macroscopic single crystal packings of nanothreads hundreds of microns across.<ref name=":1" />  Topochemical solid state reactions such as the formation of single crystal [[polydiacetylenes]] from diacetylenes usually require a single crystal reactant to form a single crystal product. 

The impetus for the formation of a hexagonal crystal appears to be the packing of circular cross section threads.<ref name=":1" />  The details of how it is possible to transform from a monoclinic benzene crystal to a hexagonal nanothread crystal are not yet fully understood. Further development of the theory of the effect of pressure on reactions may help.<ref>{{Cite journal|last=Chen|first=Bo|last2=Hoffmann|first2=Roald|last3=Cammi|first3=Roberto|date=2017-09-04|title=The Effect of Pressure on Organic Reactions in Fluids—a New Theoretical Perspective|url=http://onlinelibrary.wiley.com/doi/10.1002/anie.201705427/abstract|journal=Angewandte Chemie International Edition|language=en|volume=56|issue=37|pages=11126–11142|doi=10.1002/anie.201705427|issn=1521-3773}}</ref> 
[[File:Polytwistane.gif|thumb|left|Rotating polytwistane, a prototypical  nanothread structure.<ref name=":5" /><ref name=":4" /> Black atoms are carbon.  Light grey atoms are hydrogen.]]
[[File:Polytwistane Crystal.png|thumb|Polytwistane crystal viewed down its hexagonal c axis.  Black atoms are carbon and pink atoms are hydrogen.  The length of the threads is going into the page, showing their circular cross section and hexagonal packing that (experimentally) extends over hundreds of microns in crystals. The outline of the hexagonal [[Crystal_structure#Unit_cell| unit cell]] is shown in blue.  These crystals exfoliate into bundles of nanothreads.<ref name=":1" />]]

== History ==
Diamond threads were described by Arthur C. Clarke in his novel [[The Fountains of Paradise]] in 1979. Nanothreads were first investigated theoretically in 2001 by researchers at [[Pennsylvania State University|Penn State University]]<ref name=":2">{{Cite journal|last=Stojkovic|first=Dragan|date=2001|title=Smallest Nanotube: Breaking the Symmetry of|url=https://link.aps.org/doi/10.1103/PhysRevLett.87.125502|journal=Physical Review Letters|volume=87|issue=12|doi=10.1103/physrevlett.87.125502}}</ref> and later by researchers at [[Cornell University]].<ref name=":3">{{Cite journal|last=Wen|first=Xiao-Dong|last2=Hoffmann|first2=Roald|last3=Ashcroft|first3=N. W.|date=2011-06-15|title=Benzene under High Pressure: a Story of Molecular Crystals Transforming to Saturated Networks, with a Possible Intermediate Metallic Phase|url=http://dx.doi.org/10.1021/ja201786y|journal=Journal of the American Chemical Society|volume=133|issue=23|pages=9023–9035|doi=10.1021/ja201786y|issn=0002-7863}}</ref> In 2014, researchers at [[Penn State University]] created the first sp<sup>3</sup>-carbon nanothreads.<ref name="tcfitz2014">T.C.Fitzgibbons et al., [http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4088.html Benzene-derived carbon nanothreads], [[Nature Materials]], September 21, 2014</ref> Prior to 2014, and despite a century of investigation, benzene was thought to produce only hydrogenated amorphous carbon when compressed.<ref>{{Cite journal|last=Ciabini|first=Lucia|last2=Santoro|first2=Mario|last3=Gorelli|first3=Federico A.|last4=Bini|first4=Roberto|last5=Schettino|first5=Vincenzo|last6=Raugei|first6=Simone|date=2007/01|title=Triggering dynamics of the high-pressure benzene amorphization|url=http://www.nature.com/articles/nmat1803|journal=Nature Materials|language=En|volume=6|issue=1|pages=39–43|doi=10.1038/nmat1803|issn=1476-4660}}</ref> As of 2015, fibers 90 nanometers in length had been created (compared to .5 meters for CNTs). Also in 2015, a simulation indicated that the strength of the material was not a function of its length.<ref name=":0" />

== Structure ==
Since “diamond nanothreads” are sp<sup>3</sup>-bonded and one-dimensional they are unique in the matrix of [[orbital hybridisation| hybridization]] (sp<sup>2</sup>/sp<sup>3</sup>) and dimensionality (0D/1D/2D/3D) for carbon nanomaterials.<ref>{{Cite journal|last=Badding|first=John V.|last2=Crespi|first2=Vincent H.|title=Synthesizing carbon nanothreads from benzene|url=http://www.spie.org/x112253.xml|journal=SPIE Newsroom|doi=10.1117/2.1201501.005713}}</ref>

Assuming a topological unit cell of one or two benzene rings with at least two bonds interconnecting each adjacent pair of rings, 50 topologically distinct nanothreads have been enumerated.  15 of these are within 80 meV/carbon atom of the most stable member.<ref name=":4">{{Cite journal|last=Xu|first=En-shi|last2=Lammert|first2=Paul E.|last3=Crespi|first3=Vincent H.|date=2015-08-12|title=Systematic Enumeration of sp3 Nanothreads|url=http://dx.doi.org/10.1021/acs.nanolett.5b01343|journal=Nano Letters|volume=15|issue=8|pages=5124–5130|doi=10.1021/acs.nanolett.5b01343|issn=1530-6984}}</ref> Some of the more commonly discussed nanothread structures are known informally as polytwistane, tube (3,0), and Polymer I.  Polytwistane is chiral.<ref name=":4" /><ref name=":5">{{Cite journal|last=Barua|first=Shiblee R.|last2=Quanz|first2=Henrik|last3=Olbrich|first3=Martin|last4=Schreiner|first4=Peter R.|last5=Trauner|first5=Dirk|last6=Allen|first6=Wesley D.|date=2014-02-03|title=Polytwistane|url=http://onlinelibrary.wiley.com/doi/10.1002/chem.201303081/abstract|journal=Chemistry – A European Journal|language=en|volume=20|issue=6|pages=1638–1645|doi=10.1002/chem.201303081|issn=1521-3765}}</ref> Tube (3,0) can be thought of as the thinnest possible thread that can be carved out of the diamond structure, consisting of stacked cyclohexane rings.<ref name=":2" />  Polymer I was predicted to form from benzene at high pressure.<ref name=":3" />

Although there is compelling evidence from two dimensional X-ray diffraction patterns and transmission electron diffraction for a structure consisting of hexagonally packed crystals of 6.5 Angstrom diameter nanothreads with largely (75 to 80%) sp<sup>3</sup>-bonding,<ref name="tcfitz2014" /><ref name=":1" /> the atomic structure of nanothreads is still under investigation. Nanothreads have also been observed by [[Transmission electron microscopy| transmission electron microscopy]]<nowiki/>.<ref name="tcfitz2014" />

Nanothreads have also been classified by their degree of saturation.<ref name=":7" /> Fully [[Saturation_(chemistry)#Organic_chemistry| saturated]] degree 6 nanothreads have no [[Double bond|double bonds]] remaining. Three bonds form between each pair of benzene molecules. Degree 4 nanothreads have a double bond remaining from benzene and thus only two bonds formed between each pair of benzene molecules.  Degree 2 have two double bonds remaining. Unless otherwise specified the term nanothread is assumed to refer to a degree six structure.

== Properties ==
Every type of nanothread has a very high [[Young's modulus]] (stiffness). The value for the strongest type of nanothread is around 900 [[GPa]] compared to steel at 200 GPa and diamond at over 1,200 GPa.<ref name=":0">{{Cite web|title = Super-Strong Diamond Nanothread Has People Dreaming Of A Space Elevator|url = http://www.iflscience.com/technology/super-strong-diamond-nano-thread-could-make-your-carbon-nanotubes-dreams-reality|website = IFLScience|accessdate = 2015-11-29|date = November 28, 2015|first = Alfredo|last = Carpineti}}</ref> The strength carbon nanothreads may rival or exceed that of [[carbon nanotubes]] (CNTs). [[Molecular dynamics]] simulations have indicated a stiffness on the order of carbon nanotubes (approx. 850 GPa) and a specific strength of approx. 4 × 10<sup>7</sup> N·m/kg.<ref name=":6">Roman, R. Kwan, K., and Cranford, S.W., [http://pubs.acs.org/doi/full/10.1021/nl5041012 Mechanical Properties and Defect Sensitivity of Diamond Nanothreads], [[Nano Letters]], Feb. 18, 2015, 15 (3), pp 1585–1590</ref>

Much as graphite [[Intercalation (chemistry)#Exfoliation| exfoliates]] into sheets and ultimately [[graphene]], nanothread crystals exfoliate into fibers, consistent with their structure consisting of stiff, straight threads with a persistence length of ~100 nm<ref name=":6" /> that are held together with van der Waals forces. These fibers exhibit [[birefringence]], as would be expected from their low dimensional character.<ref name=":1" /> In contrast, most polymers are much more flexible and often fold into crystalline lamella (see [[Crystallization of polymers]]) rather than forming into crystals that readily exfoliate.

Modeling suggests certain nanothreads may be auxetic, with a negative [[Poisson's ratio|Poisson ratio]].<ref>{{Cite journal|last=Saha|first=Biswajit|last2=Pratik|first2=Saied Md.|last3=Datta|first3=Ayan|date=2017-09-18|title=Coexistence of Normal and Auxetic Behavior in a Thermally and Chemically Stable sp3 Nanothread: Poly[5]asterane|url=http://onlinelibrary.wiley.com/doi/10.1002/chem.201702775/abstract|journal=Chemistry – A European Journal|language=en|volume=23|issue=52|pages=12917–12923|doi=10.1002/chem.201702775|issn=1521-3765}}</ref> The thermal conductivity of nanothreads has been modeled.<ref name=":8">{{Cite book|url=https://doi.org/10.1016/B978-0-32-346240-2.00007-8|title=Thermal Conductivity of Diamond Nanothread|last=Zhan|first=Haifei|last2=Gu|first2=Yuantong|pages=185–204|doi=10.1016/b978-0-32-346240-2.00007-8}}</ref><ref>{{Cite journal|last=Zhan|first=Haifei|last2=Zhang|first2=Gang|last3=Zhang|first3=Yingyan|last4=Tan|first4=V.B.C.|last5=Bell|first5=John M.|last6=Gu|first6=Yuantong|title=Thermal conductivity of a new carbon nanotube analog: The diamond nanothread|url=https://doi.org/10.1016/j.carbon.2015.11.012|journal=Carbon|volume=98|pages=232–237|doi=10.1016/j.carbon.2015.11.012}}</ref><ref>{{Cite journal|last=Zhu|first=Taishan|last2=Ertekin|first2=Elif|date=2016-04-11|title=Generalized Debye-Peierls/Allen-Feldman model for the lattice thermal conductivity of low-dimensional and disordered materials|url=https://link.aps.org/doi/10.1103/PhysRevB.93.155414|journal=Physical Review B|volume=93|issue=15|pages=155414|doi=10.1103/PhysRevB.93.155414}}</ref>

== Potential Applications==
Nanothreads can be thought of essentially as "flexible diamond". The extremely high specific strength predicted for them by modeling has attracted attention for applications such as [[Space elevator| space elevators]] and would be useful in other applications related to transportation, aerospace, and sports equipment.  They may uniquely combine, extreme strength, flexibility, and resilience.<ref name=":6" /><ref>{{Cite journal|last=Zhan|first=Haifei|last2=Zhang|first2=Gang|last3=Tan|first3=Vincent B. C.|last4=Cheng|first4=Yuan|last5=Bell|first5=John M.|last6=Zhang|first6=Yong-Wei|last7=Gu|first7=Yuantong|date=2016-05-26|title=From brittle to ductile: a structure dependent ductility of diamond nanothread|url=http://xlink.rsc.org/?DOI=C6NR02414A|journal=Nanoscale|language=en|volume=8|issue=21|pages=11177–11184|doi=10.1039/c6nr02414a|issn=2040-3372}}</ref><ref name=":8" /> Chemically substituted nanothreads may facilitate load transfer between neighbors through covalent bonding to transfer their mechanical strength to a surrounding matrix.<ref name="tcfitz2014" /><ref name=":8" /> In contrast to carbon nanotubes, bonds to the exterior of nanothreads need not disrupt their carbon core because only three of the four tetrahedral bonds are needed form it. The “extra” bond usually formed to hydrogen could be instead be linked to another nanothread or another molecule or atom.<ref name="tcfitz2014" />  Bonds to carbon nanotubes require their carbon to change from near planar sp<sup>2</sup>-bonding to tetrahedral sp<sup>3</sup>-bonding, thus disrupting their tubular geometry and possibly weakening them.  Nanothreads may be less susceptible to loss of strength through defects than carbon nanotubes.<ref name=":6" /> Thus far the extreme strength predicted for carbon nanotubes has largely not been realized in practical applications because of issues with load transfer to the surroundings and defects at various length scales from that of atoms on up.   

Exfoliation into individual nanothreads may be possible, facilitating further functionalization and assembly into functional materials.<ref name=":1" />   

Applications of carbon nanomaterials for bioimaging and therapy are emerging,<ref>{{Cite journal|last=Hong|first=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=http://dx.doi.org/10.1021/acs.chemrev.5b00008|journal=Chemical Reviews|volume=115|issue=19|pages=10816–10906|doi=10.1021/acs.chemrev.5b00008|issn=0009-2665}}</ref> and the stiffness, precisely defined geometry, and availability of chemical bonds on the exterior for functionalization without disrupting the carbon core may enable biological applications of nanothreads. Defects and heteroatoms incorporated into their length may allow for robust fluorescent properties. Doping and incorporation of heteroatoms such as nitrogen or boron into the nanothread backbone may allow for enhanced conducting or semiconducting properties of nanothreads that allow for application as photocatalysts, electron emitters,<ref name="tcfitz2014" /> or possibly superconductors.

==See also==
{{Portal|Nanotechnology}}
{{colbegin|3}}
* [[Carbon nanotube]]
* [[Boron nitride#Boron nitride nanotubes|Boron nitride nanotube]]
* [[Buckypaper]]
* [[Carbide-derived carbon]]
* [[Carbon nanocone]]
* [[Carbon nanofibers]]
* [[Carbon nanoparticles]]
* [[Carbon nanoscrolls]]
* [[Carbon nanotube chemistry]]
* [[Colossal carbon tube]]
* [[Filamentous carbon]]
* [[Graphene oxide paper]]
* [[List of software for nanostructures modeling]]
* [[Molecular modelling]]
* [[Nanoflower]]
* [[Diamondoid]]
* [[Graphene]]
* [[Graphane]]
* [[Ninithi]] (nanotube modelling software)
* [[Organic semiconductor]]
* [[Selective chemistry of single-walled nanotubes]]
* [[Silicon nanotubes]]
* [[Timeline of carbon nanotubes]]
* [[Vantablack]], a substance produced in 2014; the blackest substance known
{{colend}}

== External links ==
* [http://www.technologyreview.com/view/543631/forget-graphene-and-carbon-nanotubes-get-ready-for-diamond-nanothread/ Forget Graphene and Carbon Nanotubes, Get Ready for Diamond Nanothread] technologyreview.com
* [http://www.spie.org/newsroom/5713-synthesizing-carbon-nanothreads-from-benzene?SSO=1/ Synthesizing Carbon Nanothreads from Benzene] spie.org
* [https://www.scientificamerican.com/article/liquid-benzene-squeezed-to-form-diamond-nanothreads/ Liquid Benzene Squeezed to Form Diamond Nanothreads] Scientific American
* [https://bg.chem.psu.edu/nanothreads.html Carbon Nanothread Bibliography]

==References==
{{Reflist|30em}}
{{Allotropes of carbon}}
{{Emerging technologies}}

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