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 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 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). 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 (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 benzene crystal to a higher symmetryhexagonal
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.
Organic synthesis efforts towards polytwistane nanothreads have been reported.
Rotating polytwistane, a prototypical nanothread structure. Black atoms are carbon. Light grey atoms are hydrogen.
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 unit cell is shown in blue. These crystals exfoliate into bundles of nanothreads.
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 in collaboration with Oak Ridge National Laboratory and the Carnegie Institution for Science.
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
(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.
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.
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.
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.
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 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. Modeling indicates their Bandgaps are tunable with strain over a wide range.
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. 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.
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 heteoatoms or unsaturated
bonds in their backbone. These features may enable biological
applications, for example. Defects, functional groups, and/or heteroatoms
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.
