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A carbon nanothread (informally “diamond” nanothread) is a sp3-bonded, one-dimensional carbon crystalline nanomaterial. The tetrahedral sp3-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 human hair. They consist of a stiff, strong carbon core surrounded by hydrogen atoms. Carbon nanotubes, although also one-dimensional nanomaterials, in contrast have sp2-carbon bonding as is found in graphite.

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. The synthesis reaction can be considered a form of organic  solid state chemistry. The benzene chains form extremely thin, tight rings of carbon that are structurally similar to diamonds. Researchers at Cornell Universit 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. Recently synthesis of macroscopic single crystal arrays of nanothreads hundreds of microns in size has been reported. The order and lack of grain boundaries in single crystals is often very desirable because it facilitates both applications and characterization. In contrast, 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. Slow compression/decompression may favor low energy reaction pathway(s). The reaction, as is often found in organic chemistry, is under kinetic control. If the synthesis pressure for nanothreads can be reduced to 5 to 6 GPa, which is the pressure used for synthesis of industrial diamond, production on the large scale of >106 kg/yr would be possible.

The formation of nanothread crystals appears to be guided by uniaxial stress (stress in a particular single direction), to which the nanothreads consistently align. Reaction to form the crystals is not topochemical, as it involves a major rearrangement from a lower symmetry monoclinic benzene crystal to a higher symmetry hexagonal 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 separations between them must shrink by 40% or more as the short, strong  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 polycrystalline benzene reacts to form macroscopic single crystal packings of nanothreads hundreds of microns across. 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. 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.

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 Penn State University and later by researchers at Cornell University. In 2014, researchers at Penn State University created the first sp3-carbon nanothreads. Prior to 2014, and despite a century of investigation, benzene was thought to produce only hydrogenated amorphous carbon when compressed. 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.

Structure
Since “diamond nanothreads” are sp3-bonded and one-dimensional they are unique in the matrix of hybridization (sp2/sp3) and dimensionality (0D/1D/2D/3D) for carbon nanomaterials.

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. Some of the more commonly discussed nanothread structures are known informally as polytwistane, tube (3,0), and Polymer I. Polytwistane is chiral. 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. Polymer I was predicted to form from benzene at high pressure.

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%) sp3-bonding, the atomic structure of nanothreads is still under investigation. Nanothreads have also been observed by transmission electron microscopy.

Nanothreads have also been classified by their degree of saturation. Fully saturated degree 6 nanothreads have no 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. 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 × 107 N·m/kg.

Much as graphite 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 that are held together with van der Waals forces. These fibers exhibit birefringence, as would be expected from their low dimensional character. 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 ratio. The thermal conductivity of nanothreads has been modeled.

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 elevators and would be useful in other applications related to transportation, aerospace, and sports equipment. They may uniquely combine, extreme strength, flexibility, and resilience. Chemically substituted nanothreads may facilitate load transfer between neighbors through covalent bonding to transfer their mechanical strength to a surrounding matrix. 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. Bonds to carbon nanotubes require their carbon to change from near planar sp2-bonding to tetrahedral sp3-bonding, thus disrupting their tubular geometry and possibly weakening them. Nanothreads may be less susceptible to loss of strength through defects than carbon nanotubes. 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.

Applications of carbon nanomaterials for bioimaging and therapy are emerging, 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, or possibly superconductors.