A carbon nanothread (also called diamond nanothread) is a sp³-bonded, one-dimensional carbon crystalline

nanomaterial * Nanomaterials describe, in principle, materials of which a single unit is sized (in at least one dimension) between 1 and 100 nm (the usual definition of nanoscale). Nanomaterials research takes a materials science-based approach to nan ...

. The tetrahedral sp³-bonding of its carbon is similar to that of

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...

. Nanothreads are only a few atoms across, more than 20,000 times thinner than a

human hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine ...

. 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 Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...

. The smallest carbon nanothread has a diameter of only 0.2 nanometer, much smaller than the diameter of a single-wall carbon nanotube.

Synthesis

Nanothreads are synthesized by compressing liquid

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...

to extreme pressure of 20 GPa (around 200,000 times the air pressure at the surface of the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

), and then slowly relieving that pressure. The mechanochemical 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 University Cornell University is a private statutory land-grant research university based in Ithaca, New York. It is a member of the Ivy League. Founded in 1865 by Ezra Cornell and Andrew Dickson White, Cornell was founded with the intention to tea ...

have traced pathways from benzene to nanothreads, which may involve a series of organic +2

cycloaddition In organic chemistry, a cycloaddition is a chemical reaction in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity". T ...

reactions along stacks of benzene molecules, followed by further reaction of unsaturated bonds. Recently synthesis of macroscopic

single crystal In materials science, a single crystal (or single-crystal solid or monocrystalline solid) is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries.RIWD. "Re ...

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 pathways. If the synthesis pressure for nanothreads can be reduced to 5 to 6 GPa, which is the pressure used fo
synthesis of industrial diamond
production on the large scale of >10⁶ kg/yr would be possible. Recent advance on using strained cage-like molecules such as

cubane Cubane () is a synthetic hydrocarbon compound that consists of eight carbon atoms arranged at the corners of a cube, with one hydrogen atom attached to each carbon atom. A solid crystalline substance, cubane is one of the Platonic hydrocarbons an ...

as a precursor has successfully brought down the synthesis pressure to 12 GPa. Expanding the precursor library to non-aromatic, strained molecules offers new avenues to explore scalable production of carbon nanothreads. The formation of nanothread crystals appears to be guided by uniaxial stress (mechanical 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 In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

benzene crystal to a higher symmetry

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A '' regular hexagon'' has ...

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 A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...

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 A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...

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. Organic synthesis efforts towards polytwistane nanothreads have been reported.

History

In popular culture, diamond threads were first described by Arthur C. Clarke in his sci-fi novel ''

The Fountains of Paradise ''The Fountains of Paradise'' is a 1979 science fiction novel by British writer Arthur C. Clarke. Set in the 22nd century, it describes the construction of a space elevator. This "orbital tower" is a giant structure rising from the ground ...

'' set in the 22nd century, written in 1979. Nanothreads were first investigated theoretically in 2001 by researchers at Penn State University and later by researchers at

. In 2014, researchers at Penn State University created the first sp³-carbon nanothreads in collaboration with

Oak Ridge National Laboratory Oak Ridge National Laboratory (ORNL) is a U.S. multiprogram science and technology national laboratory sponsored by the U.S. Department of Energy (DOE) and administered, managed, and operated by UT–Battelle as a federally funded research an ...

and the

Carnegie Institution for Science The Carnegie Institution of Washington (the organization's legal name), known also for public purposes as the Carnegie Institution for Science (CIS), is an organization in the United States established to fund and perform scientific research. Th ...

.T.C.Fitzgibbons et al.
Benzene-derived carbon nanothreads

Nature Materials ''Nature Materials'', is a peer-reviewed scientific journal published by Nature Publishing Group. It was launched in September 2002. Vincent Dusastre is the launching and current chief editor. Aims and scope ''Nature Materials'' is focused on all ...

, September 21, 2014 Prior to 2014, and despite a century of investigation, benzene was thought to produce only hydrogenated amorphous carbon when compressed. As of 2015, threads at least 90 nanometers in length had been created (compared to .5 meters for CNTs).

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, transmission electron diffraction, and

solid-state nuclear magnetic resonance Solid-state NMR (ssNMR) spectroscopy is a technique for characterizing atomic level structure in solid materials e.g. powders, single crystals and amorphous samples and tissues using nuclear magnetic resonance (NMR) spectroscopy. The anisotropic pa ...

(NMR) 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 Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...

. Individual threads have been observed to pack in hexagonal crystals and layer-lines indicative of order along their length have been observed. Nanothreads have also been classified by their degree of saturation. Fully

saturated Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds ** Saturated and unsaturated compounds **Degree of unsaturation ** Saturated fat or fatty ac ...

degree 6 nanothreads have no

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...

s 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. NMR has revealed that nanothread crystals consist of both degree 6 and degree 4 threads. Moreover, spin diffusion experiments show that the sections of the threads that are fully saturated degree 6 must be at least 2.5 nm long, if not longer. NMR also shows that no second hydrocarbon or carbon phase is present in nanothread crystals. Thus all of the sp² carbon is either in degree 4 nanothreads or small amounts of aromatic linker molecules, or even smaller amounts of C=O groups. NMR provides the chemical structural information necessary to refine syntheses towards pure degree 6 nanothreads, which are stronger than the partially saturated ones.

Carbon nitride nanothreads

Pyridine compressed slowly under pressure forms carbon nitride C₅H₅N nanothread crystals. They exhibit the six-fold diffraction "signature" of nanothread formation. NMR, chemical analysis and infrared spectroscopy provide further evidence for the synthesis of nanothreads from pyridine. Pyridine nanothreads incorporate significant amounts of nitrogen directly into their backbone. In contrast sp2 carbon nanotubes can only be doped with a small amount of nitrogen. A wide range of other functionalized nanothreads may be possible, as well as nanothreads from polycyclic aromatic hydrocarbon molecules.

Smallest nanothreads

Extending the ability to design and create nanothread architecture from a non-aromatic, saturated molecule has been a recent interest in order to achieve an entirely sp3-bonded nanothread structure. Hypothetical nanothread architectures built from the smallest

diamondoids In chemistry, diamondoids are variants of the carbon cage molecule known as adamantane (C10H16), the smallest unit cage structure of the diamond crystal lattice. Diamondoids also known as nanodiamonds or condensed adamantanes may include one or mor ...

(

adamantane Adamantane is an organic compound with a formula C10H16 or, more descriptively, (CH)4(CH2)6. Adamantane molecules can be described as the fusion of three cyclohexane rings. The molecule is both rigid and virtually stress-free. Adamantane is the ...

) have been proposed to have higher mechanical strength than

nanothreads. The first experimental synthesis of a novel purely sp3 bonded one-dimensional carbon nanomaterial is realized via an endogenous solid-state polymerization of

. Pre-arranged

monomers in the bulk crystal undergo diradical polymerization guided by applied uniaxial stress, similar to

, produce a single-crystalline carbon nanomaterial. The

-derived nanothread exhibits a linear

structure with subnanometre-diameter of 0.2 nm, which is considered as the smallest member in the carbon nanothread family; thus, they promise to form the stiffest one-dimensional system known.

Properties

Every type of nanothread has a very high

Young's modulus Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied le ...

(stiffness). The value for the strongest type of nanothread is around 900

GPa Grading in education is the process of applying standardized measurements for varying levels of achievements in a course. Grades can be assigned as letters (usually A through F), as a range (for example, 1 to 6), as a percentage, or as a numbe ...

compared to steel at 200 GPa and diamond at over 1,200 GPa. The strength of carbon nanothreads may rival or exceed that of

carbon nanotubes A scanning tunneling microscopy image of a single-walled carbon nanotube Rotating single-walled zigzag carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers. ''Single-wall carbon na ...

(CNTs).

Molecular dynamics Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamic "evolution" of t ...

and

Density functional theory Density-functional theory (DFT) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (principally the ground state) of many-body ...

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.Roman, R. Kwan, K., and Cranford, S.W.
Mechanical Properties and Defect Sensitivity of Diamond Nanothreads

Nano Letters ''Nano Letters'' is a monthly peer-reviewed scientific journal published by the American Chemical Society. It was established in January 2001. The editor-in-chief is Teri W. Odom (Northwestern University). The journal covers all aspects of nanosci ...

, Feb. 18, 2015, 15 (3), pp 1585–1590 Much as graphite exfoliates into sheets and ultimately

graphene Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure.

, 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 Crystallization of polymers is a process associated with partial alignment of their molecular chains. These chains fold together and form ordered regions called lamellae, which compose larger spheroidal structures named spherulites. Polymers can c ...

) rather than forming into crystals that readily exfoliate. Modeling suggests certain nanothreads may be auxetic, with a negative

Poisson ratio In materials science and solid mechanics, Poisson's ratio \nu ( nu) is a measure of the Poisson effect, the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. The value of Poi ...

. The

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

of nanothreads has been modeled. Modeling indicates their Bandgaps are tunable with strain over a wide range. The electrical conductivity of fully saturated nanothreads, driven by topology, may be much higher than expected.

Potential applications

Nanothreads can be thought of essentially as "flexible diamond". The extremely high

specific strength The specific strength is a material's (or muscle's) strength (force per unit area at failure) divided by its density. It is also known as the strength-to-weight ratio or strength/weight ratio or strength-to-mass ratio. In fiber or textile applic ...

predicted for them by modeling has attracted attention for applications such as

space elevator A space elevator, also referred to as a space bridge, star ladder, and orbital lift, is a proposed type of planet-to-space transportation system, often depicted in science fiction. The main component would be a cable (also called a space tethe ...

s 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. Modeling also suggests that the kinks associated with Stone-Wales transformations in nanothreads may facilitate interfacial load transfer to a surrounding matrix, making them useful for high strength composites. 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. Nanothreads may thus be thought of as "hybrids" that are both hydrocarbon molecules and carbon nanomaterials. 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. Theory indicates that "caged saturated hydrocarbons offering multiple σ-conductance channels (such as nanothreads) afford transmission far beyond what could be expected based upon conventional superposition laws, particularly if these pathways are composed entirely from quaternary carbon atoms." The carbon core of nanothreads is very stiff relative to the backbone of conventional polymers. They should thus be able to precisely orient molecular functions attached along their length (by substitution of hydrogen) relative to each other and to heteroatoms or unsaturated bonds in their backbone. These features may enable biological applications, for example. Defects, functional groups, and/or

heteroatom In chemistry, a heteroatom () is, strictly, any atom that is not carbon or hydrogen. Organic chemistry In practice, the term is usually used more specifically to indicate that non-carbon atoms have replaced carbon in the backbone of the molecula ...

s incorporated either into or exterior to the backbone of nanothreads with controlled orientation and distance between them may allow for robust, well controlled fluorescence. 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. Modeling suggests carbon nanothread resonators exhibit low dissipation and may be useful as chemical sensors that can detect very small mass changes.

Energy storage

Simulations show some achiral nanothread bundles may have specific energy density (when twisted) higher than lithium batteries.''High density mechanical energy storage with carbon nanothread bundle''
/ref>

External links

Forget Graphene and Carbon Nanotubes, Get Ready for Diamond Nanothread
technologyreview.com
Synthesizing Carbon Nanothreads from Benzene
spie.org
Liquid Benzene Squeezed to Form Diamond Nanothreads
Scientific American
Center for Nanothread Chemistry

References

{{DEFAULTSORT:Carbon Nanotube Carbon nanoparticles Emerging technologies Transparent electrodes Refractory materials Substances discovered in the 2000s