Mesogenicity drives fractionation in lyotropic aqueous suspensions of multiwall carbon nanotubes

Vertical Array of Graphite Oxide Liquid Crystal by Microwire Shearing for Highly Thermally Conductive Composites

Excellent through-plane thermally conductive composites are highly demanded for efficient heat dissipation. Giant sheets have large crystalline domain and significantly reduce interface phonon scattering, making them promising to build highly thermally conductive composites. However, realizing vertical orientation of giant sheets remains challenging due to their enormous mass and huge hydrodynamic drag force. Here, we achieve highly vertically ordered liquid crystals of giant graphite oxide (more than 100 µm in lateral dimension) by microwire shearing, which endows the composite with a recorded through-plane thermal conductivity of 94 W m^-1 K^-1 . Microscale shearing fields induced by vertical motion of microwires conquer huge hydrodynamic energy barrier and vertically reorient giant sheets. The resulting liquid crystals exhibit extremely retarded relaxation and impart large-scale vertical array with bidirectional ordering degree as high as 0.82. The graphite array-based composites demonstrate an ultrahigh thermal enhancement efficiency of over 35 times per unit volume. Furthermore, the composites improve cooling efficiency by 93% for thermal management tests compared to commercial thermal interface materials. This work offers a novel methodology to precisely manipulate the orientation of giant particles and promote large-scale fabrication of vertical array with advanced functionalities.

Aligned 2D carbon nanotube liquid crystals for wafer-scale electronics

Semiconducting carbon nanotubes promise faster performance and lower power consumption than Si in field-effect transistors (FETs) if they can be aligned in dense arrays. Here, we demonstrate that nanotubes collected at a liquid/liquid interface self-organize to form two-dimensional (2D) nematic liquid crystals that globally align with flow. The 2D liquid crystals are transferred onto substrates in a continuous process generating dense arrays of nanotubes aligned within ±6°, ideal for electronics. Nanotube ordering improves with increasing concentration and decreasing temperature due to the underlying liquid crystal phenomena. The excellent alignment and uniformity of the transferred assemblies enable FETs with exceptional on-state current density averaging 520 μA μm⁻¹at only −0.6 V, and variation of only 19%. FETs with ion gel top gates demonstrate subthreshold swing as low as 60 mV decade⁻¹. Deposition across a 10-cm substrate is achieved, evidencing the promise of 2D nanotube liquid crystals for commercial semiconductor electronics.

Chromatic Conductive Polymer Nanocomposites of Poly (p-Phenylene Ethynylene)s and Single-Walled Carbon Nanotubes

We report on dispersions and thin films of chromatic conductive nanocomposites of poly(p-phenylene ethynylene)s (PPEs) and single-walled carbon nanotubes (SWNTs) generated via solution mixing. The linear, conjugated PPEs with dialkyl- and dialkyloxy-side chain groups are shown to debundle and disperse high concentration (up to 2.5 mg/mL) SWNTs in various organic solvents. The solubilization of SWNTs and PPE wrapping is accompanied with the change in the solution color. Ultraviolet visible absorption spectra of nanocomposite solutions demonstrate a new absorption peak at a higher wavelength, supporting the observed chromatism. Fluorescence spectra of nanocomposite solutions display significant quenching of the fluorescence intensity and the Stern–Volmer model is used to analyze fluorescence quenching. Electron microscopy of the chromatic solid films of high mass fraction PPE/SWNT nanocomposites obtained by vacuum filtration reveals the debundled SWNTs in the PPE matrix. The tensile strength and Young’s modulus of these PPE/SWNT nanocomposite films are as high as 150 MPa and 15 GPa, respectively. The composite films exhibit remarkably high conductivities, ranging from ~1000 S/m to ~10,000 S/m for 10 wt% and 60 wt% SWNT nanocomposites, respectively.

Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience

The research field of liquid crystals and their applications is recently changing from being largely focused on display applications and optical shutter elements in various fields, to quite novel and diverse applications in the area of nanotechnology and nanoscience. Functional nanoparticles have recently been used to a significant extent to modify the physical properties of liquid crystals by the addition of ferroelectric and magnetic particles of different shapes, such as arbitrary and spherical, rods, wires and discs. Also, particles influencing optical properties are increasingly popular, such as quantum dots, plasmonic, semiconductors and metamaterials. The self-organization of liquid crystals is exploited to order templates and orient nanoparticles. Similarly, nanoparticles such as rods, nanotubes and graphene oxide are shown to form lyotropic liquid crystal phases in the presence of isotropic host solvents. These effects lead to a wealth of novel applications, many of which will be reviewed in this publication.

Liquid crystallinity of carbon nanotubes

In this review, we first briefly recapitulate the orientation characteristics of liquid crystalline carbon nanotubes (CNTs), emphasizing their inherent properties. Both the high Young's modulus and the strong attractive interaction between them make the liquid crystallinity apt to show splay deformations (splay defects). It is these defects that often produce apparent low-order structures for long and deformed nanotubes. However, the application of doping, shearing, magnetic or electric fields will be efficient routes toward highly ordered CNT assemblies from such defects. Then, we describe the electrical behavior of CNTs in the electric field, which combines desirable features of the CNTS with those of classical liquid crystals (LCs). An electric field will generate an induced dipole moment on CNTs and align them in the field direction, minimizing the dipolar energy. Finally, we review the potential application of CNTs in the area of liquid crystal displays (LCD). In the LC cell unit, CNTs as dopants in LC layers can have compatible stability with LCs, with the orientation consistent and with surprising complementary advantages. And also CNT films as nanostructured electrodes can substitute ITO electrodes in the LC cell unit, exhibiting a strong electrical anisotropy due to their excellent axial conductivity. Furthermore, CNT films as an alignment layer have the potential to replace the traditional PI film, aligning LC molecules effectively along the direction of the nanotubes. Besides, CNTs acting as polarizers can absorb or transmit incident light when the electric vector propagates parallel or perpendicular to the nanotube axis. All of these applications demonstrate that CNTs in LC ordering will effectively improve the performance of materials and their related devices. Thus, we should improve the ordering of CNT assemblies as far as possible, which is critical to make full use of their exceptional axial properties and further to develop novel materials and applications successfully.

A Mini Review on Nanocarbon-Based 1D Macroscopic Fibers: Assembly Strategies and Mechanical Properties

Nanocarbon-based materials, such as carbon nanotubes (CNTs) and graphene have been attached much attention by scientific and industrial community. As two representative nanocarbon materials, one-dimensional CNTs and two-dimensional graphene both possess remarkable mechanical properties. In the past years, a large amount of work have been done by using CNTs or graphene as building blocks for constructing novel, macroscopic, mechanically strong fibrous materials. In this review, we summarize the assembly approaches of CNT-based fibers and graphene-based fibers in chronological order, respectively. The mechanical performances of these fibrous materials are compared, and the critical influences on the mechanical properties are discussed. Personal perspectives on the fabrication methods of CNT- and graphene-based fibers are further presented.

Direct Imaging of Carbon Nanotube Liquid-Crystalline Phase Development in True Solutions

Using direct-imaging cryogenic transmission and scanning electron microscopy, we show different stages of liquid-crystalline phase development in progressively more concentrated solutions of carbon nanotubes in chlorosulfonic acid: a dilute phase of individually dissolved carbon nanotubes; semidilute and concentrated isotropic phases; coexisting concentrated isotropic and nematic phases in local equilibrium with each other; and a fully liquid-crystalline phase. Nanometric resolution of cryogenic electron microscopy reveals carbon nanotube self-assembly into liquid-crystalline domains of several nanometers in width at very early stages. We find significant differences in carbon nanotube liquid-crystalline domain morphology as a function of the carbon nanotube aspect ratio, diameter, and degree of purity.

Controlled self-assembly of biomolecular rods on structured substrates

We report on the evaporative self-assembly and orientational ordering of semi-flexible spherocylindrical M13 phages on asymmetric stranded webs of thin amorphous carbon films. Although the phages were dispersed with a low concentration in the isotropic phase, the substrate edges induced nematic ordering and bending of the phages. As revealed by transmission electron microscopy, phages were aligned parallel to the curved substrate edges. This two-dimensional self-assembly on structured substrates opens a new route to the design of structures of orientationally ordered semi-flexible biomacromolecules.

Liquid crystal graphene oxide with different layers: fabrication, characterization and applications

Graphene oxide liquid crystals with different layers were successfully preparedviasimple centrifugation. The self-assembled film demonstrates radial schlieren textures (in the background) and has a high electrical conductivity.

Nematic ordering of SWNT in meso-structured thin liquid films of polystyrenesulfonate

The formation of nematic-like islands of single-walled carbon nanotubes (SWNT) in polystyrenesulfonate (PSS) dispersions confined into nanometrically thin films is reported. The SWNT are observed to assemble into orientationally ordered phases, where the intertube distance, as measured via transmission electron microscopy at cryogenic temperatures, matches the polyelectrolyte's bulk correlation length deduced from X-ray scattering. The micrometers-long islands of orientationally ordered carbon nanotubes are observed in both SWNT and double-walled carbon nanotubes (DWNT) but not in specimens prepared from similar dispersions of multiwalled carbon nanotubes (MWNT). These observations, together with relaxation and rheological experiments, suggest that the orientational ordering may result from coupling between confinement of the polymer-wrapped SWNT and DWNT and the microstructure of the solvated polyelectrolyte.

Orientation of charged clay nanotubes in evaporating droplet meniscus

During drying, an aqueous suspension of strongly charged halloysite clay nanotubes concentrates at the edge of the droplet ("coffee-ring" effect) which provides alignment of the tubes along the liquid-substrate contact line. First, the surface charge of the nanotubes was enhanced by polyanion adsorption inside of the lumen to compensate for the internal positive charges. This increased the magnitude of the ξ-potential of the tubes from -36 to -81 mV and stabilized the colloids. Then, colloidal halloysite was dropped onto the substrate, dried at 65 °C and after a concentration of ∼0.05 mg mL(-1) was reached, the alignment of nanotubes occurred starting from the droplet edges. The process was described with Onsager's theory, in which longer nanorods, which have higher surface charge, give better ordering after a critical concentration is reached. This study indicates a new application of halloysite clay nanotubes in polymeric composites with anisotropic properties, microchannel orientation, and production of coatings with aligned nanotubes.

Liquid crystal size selection of large-size graphene oxide for size-dependent N-doping and oxygen reduction catalysis

Graphene oxide (GO) is aqueous-dispersible oxygenated graphene, which shows colloidal discotic liquid crystallinity. Many properties of GO-based materials, including electrical conductivity and mechanical properties, are limited by the small flake size of GO. Unfortunately, typical sonochemical exfoliation of GO from graphite generally leads to a broad size and shape distribution. Here, we introduce a facile size selection of large-size GO exploiting liquid crystallinity and investigate the size-dependent N-doping and oxygen reduction catalysis. In the biphasic GO dispersion where both isotropic and liquid crystalline phases are equilibrated, large-size GO flakes (>20 μm) are spontaneously concentrated within the liquid crystalline phase. N-Doping and reduction of the size-selected GO exhibit that N-dopant type is highly dependent on GO flake size. Large-size GO demonstrates quaternary dominant N-doping and the lowest onset potential (-0.08 V) for oxygen reduction catalysis, signifying that quaternary N-dopants serve as principal catalytic sites in N-doped graphene.

Vacuum filtration based formation of liquid crystal films of semiconducting carbon nanotubes and high performance transistor devices

In this paper, we report ultra-thin liquid crystal films of semiconducting carbon nanotubes using a simple vacuum filtration process. Vacuum filtration of nanotubes in aqueous surfactant solution formed nematic domains on the filter membrane surface and exhibited local ordering. A 2D fast Fourier transform was used to calculate the order parameters from scanning electron microscopy images. The order parameter was observed to be sensitive to the filtration time demonstrating different regions of transformation namely nucleation of nematic domains, nanotube accumulation and large domain growth.Transmittance versus sheet resistance measurements of such films resulted in optical to dc conductivity of σ(opt)/σ(dc) = 9.01 indicative of purely semiconducting nanotube liquid crystal network.Thin films of nanotube liquid crystals with order parameters ranging from S = 0.1-0.5 were patterned into conducting channels of transistor devices which showed high I(on)/I(off) ratios from 10-19,800 and electron mobility values μ(e) = 0.3-78.8 cm(2) (V-s)(-1), hole mobility values μ(h) = 0.4-287 cm(2) (V-s)(-1). High I on/I off ratios were observed at low order parameters and film mass. A Schottky barrier transistor model is consistent with the observed transistor characteristics. Electron and hole mobilities were seen to increase with order parameters and carbon nanotube mass fractions. A fundamental tradeoff between decreasing on/off ratio and increasing mobility with increasing nanotube film mass and order parameter is therefore concluded. Increase in order parameters of nanotubes liquid crystals improved the electronic transport properties as witnessed by the increase in σ(dc)/σ(opt) values on macroscopic films and high mobilities in microscopic transistors. Liquid crystal networks of semiconducting nanotubes as demonstrated here are simple to fabricate, transparent, scalable and could find wide ranging device applications.

Organic solvent-based graphene oxide liquid crystals: a facile route toward the next generation of self-assembled layer-by-layer multifunctional 3D architectures

We introduce soft self-assembly of ultralarge liquid crystalline (LC) graphene oxide (GO) sheets in a wide range of organic solvents overcoming the practical limitations imposed on LC GO processing in water. This expands the number of known solvents which can support amphiphilic self-assembly to ethanol, acetone, tetrahydrofuran, N-dimethylformamide, N-cyclohexyl-2-pyrrolidone, and a number of other organic solvents, many of which were not known to afford solvophobic self-assembly prior to this report. The LC behavior of the as-prepared GO sheets in organic solvents has enabled us to disperse and organize substantial amounts of aggregate-free single-walled carbon nanotubes (SWNTs, up to 10 wt %) without compromise in LC properties. The as-prepared LC GO-SWNT dispersions were employed to achieve self-assembled layer-by-layer multifunctional 3D hybrid architectures comprising SWNTs and GO with unrivalled superior mechanical properties (Young's modulus in excess of 50 GPa and tensile strength of more than 500 MPa).

Increased solubility, liquid-crystalline phase, and selective functionalization of single-walled carbon nanotube polyelectrolyte dispersions

The solubility of single-walled carbon nanotube (SWCNT) polyelectrolytes [K(THF)]nSWCNT in dimethyl sulfoxide (DMSO) was determined by a combination of centrifugation, UV-vis spectral properties, and solution extraction. The SWCNT formed a liquid crystal at a concentration above 3.8 mg/mL. Also, crown ether 18-crown-6 was found to increase the solubility of the SWCNT polyelectrolytes in DMSO. Raman spectroscopy and near-infrared (NIR) fluorescence analyses were applied to study the functionalization of SWCNTs. Small-diameter SWCNTs were found to be preferentially functionalized when the SWCNT polyelectrolytes were dispersed in DMSO.

Liquid crystals of carbon nanotubes and graphene

Liquid crystal ordering is an opportunity to develop novel materials and applications with spontaneously aligned nanotubes or graphene particles. Nevertheless, achieving high orientational order parameter and large monodomains remains a challenge. In addition, our restricted knowledge of the structure of the currently available materials is a limitation for fundamental studies and future applications. This paper presents recent methodologies that have been developed to achieve large monodomains of nematic liquid crystals. These allow quantification and increase of their order parameters. Nematic ordering provides an efficient way to prepare conductive films that exhibit anisotropic properties. In particular, it is shown how the electrical conductivity anisotropy increases with the order parameter of the nematic liquid crystal. The order parameter can be tuned by controlling the length and entanglement of the nanotubes. In the second part of the paper, recent results on graphene liquid crystals are reported. The possibility to obtain water-based liquid crystals stabilized by surfactant molecules is demonstrated. Structural and thermodynamic characterizations provide indirect but statistical information on the dimensions of the graphene flakes. From a general point of view, this work presents experimental approaches to optimize the use of nanocarbons as liquid crystals and provides new methodologies for the still challenging characterization of such materials.

Phase diagrams of binary mixtures of liquid crystals and rodlike polymers in the presence of an external field

We theoretically study phase separations in mixtures of a low molecular-weight-liquid crystalline molecule (LC) and a rigid-rodlike polymer (rod) under an external field, such as magnetic or electric fields. By taking into account two orientational order parameters of the rod and the LC, we define four nematic phases (N(0), N(1), N(2), N(3)) on the temperature-concentration plane. Depending on the sign of the dielectric anisotropy Δε(i) of the rod (i = 1) and LC(i = 2), we examine the phase behavior of rod/LC mixtures in the case of Δε(1) > 0, Δε(2) > 0 (a), Δε(1) < 0, Δε(2) > 0 (b), Δε(1) > 0, Δε(2) < 0 (c), and Δε(1) < 0, Δε(2) < 0 (d). We predict a variety of phase separations induced by an external field.

Controlled orientation and alignment in films of single-walled carbon nanotubes using inkjet printing

An inkjet printing procedure for depositing films of carbon nanotubes (CNTs) that exhibit a very high degree of long-range mutual alignment as well as a controlled orientation with respect to the printed geometry is presented. CNT self-assembly was induced by the intrinsic lyotropic liquid crystallinity of CNT suspensions. Sufficient concentrations are reached by matching the inkjet deposition rate to the numerically modeled local evaporation rate of the printed feature and enable the CNT suspension to be printed using standard inkjet printing. Surface alignment was verified using scanning electron microscopy (SEM) and polarized light microscopy. In addition, the bulk morphology was investigated and found to be composed of stacked planar layers that did not necessarily have the same long-range orientation found on the surface. The bulk morphology was characterized by removing layers through an elastomeric peeling process and by observing cross sections of the films using SEM. CNT concentration and length were spanned experimentally, and it was found that very short and very long CNTs as well as low concentration suspensions did not yield long-range alignment.

Liquid crystalline order and magnetocrystalline anisotropy in magnetically doped semiconducting ZnO nanowires

Controlled alignment of nanomaterials over large length scales (>1 cm) presents a challenge in the utilization of low-cost solution processing techniques in emerging nanotechnologies. Here, we report on the lyotropic liquid crystalline behavior of transition-metal-doped zinc oxide nanowires and their facile alignment over large length scales under external fields. High aspect ratio Co- and Mn-doped ZnO nanowires were prepared by solvothermal synthesis with uniform incorporation of dopant ions into the ZnO wurtzite crystal lattice. The resulting nanowires exhibited characteristic paramagnetic behavior. Suspensions of surface-functionalized doped nanowires spontaneously formed stable homogeneous nematic liquid crystalline phases in organic solvent above a critical concentration. Large-area uniaxially aligned thin films of doped nanowires were obtained from the lyotropic phase by applying mechanical shear and, in the case of Co-doped nanowires, magnetic fields. Application of shear produced thin films in which the nanowire long axes were aligned parallel to the flow direction. Conversely, the nanowires were found to orient perpendicular to the direction of the applied magnetic fields. This indicates that the doped ZnO possesses magnetocrystalline anisotropy sufficient in magnitude to overcome the parallel alignment which would be predicted based solely on the anisotropic demagnetizing field associated with the high aspect ratio of the nanowires. We use a combination of magnetic property measurements and basic magnetostatics to provide a lower-bound estimate for the magnetocrystalline anisotropy.

Lyotropic self-assembly of high-aspect-ratio semiconductor nanowires of single-crystal ZnO

Lyotropic nanowire dispersions are attractive precursors for semiconductor device fabrication because they permit the alignment control of active nanomaterials. The reliable production of nanowire-based mesophases, however, is very challenging in practice. We show that appropriately functionalized high-aspect-ratio nanowires of single-crystal ZnO spontaneously form nematic phases in organic and aqueous media. These systems show isotropic, biphasic, and nematic phases on increasing concentration, in reasonable agreement with Onsager's theory for rigid rods interacting via excluded volume. Suspensions were readily processed to produce films with large-area monodomains of aligned nanowires. Imprints of the director field in quiescently dried films display a propensity for bend deformation in the organic mesophase versus splay deformation in the aqueous case, suggesting that system elasticity may be tuned via surface functionalization. These results provide critical insight for the utilization of semiconductor nanowires as novel mesogens and further enable the use of solution-based routes for fabricating optoelectronic devices.

Polydomain growth at isotropic-nematic transitions in liquid crystalline polymers

We studied the dynamics of isotropic-nematic transitions in liquid crystalline polymers by integrating time-dependent Ginzburg-Landau equations. In a concentrated solution of rodlike polymers, the rotational diffusion constant D(r) of the polymer is severely suppressed by the geometrical constraints of the surrounding polymers so that the rodlike molecules diffuse only along their rod directions. In the early stage of phase transition, the rodlike polymers with nearly parallel orientations assemble to form a nematic polydomain. This polydomain pattern, with characteristic length ℓ, grows with self-similarity in three dimensions over time with an ℓ~t(1/4) scaling law. In the late stage, the rotational diffusion becomes significant, leading to a crossover of the growth exponent from 1/4 to 1/2. This crossover time is estimated to be on the order of t~D(r)(-1). We also examined the time evolution of a pair of disclinations placed in a confined system by solving the same time-dependent Ginzburg-Landau equations in two dimensions. If the initial distance between the disclinations is shorter than some critical length, they approach and annihilate each other; however, at larger initial separations, they are stabilized.

Strategies for post-synthesis alignment and immobilization of carbon nanotubes

Carbon nanotubes (CNTs) have developed into a standard material used as a building block for nanotechnological developments. Based on the unique properties that make CNTs useful for many different applications in nanotechnology, optics, electronics, and material science, there has been a rapid development of this research area and many different applications have emerged in the past few years. Frequently, the alignment and immobilization of CNTs play an important role for many applications and different strategies, in particular post-synthesis approaches, can be applied. Recent developments of different techniques to immobilize and align carbon nanotubes are discussed and classified into three main categories: chemical immobilization and alignment, physical immobilization and alignment, and the use of external fields for these purposes. Many of the techniques involve multiple steps and may also cross these rather crudely defined boundaries. As such, the techniques are classified according to their most important or unique step.

Large-scale aligned carbon nanotubes from their purified, highly concentrated suspension

Large-scale aligned single-walled carbon nanotube (SWCNT) composite membranes have been successfully prepared from highly concentrated purified SWCNT suspensions. Biopolymer dispersant gellan gum was used to achieve aqueous dispersion of highly concentrated SWCNTs, which can be used to form the SWCNT liquid crystal phase. To achieve alignment of SWCNTs, purification of SWCNTs is found to be very important. Purification was achieved by a facile and nondestructive physical method that can prepare large volumes of SWCNTs in high yield for experimental use. Composite membranes of aligned SWCNTs could be obtained by simple evaporation SWCNT liquid crystal. The orientation direction of aligned SWCNTs was controlled by mechanical shearing of SWCNT liquid crystal. The aligned SWCNTs in the biopolymer matrix were observed by electron microscopy, and the anisotropic electrical performance of this composite thin film has been characterized.

Ordering in a droplet of an aqueous suspension of single-wall carbon nanotubes on a solid substrate

We report on a series of experiments on the aqueous, nematic liquid crystalline phase of single-wall carbon nanotubes (SWNTs) and their ordered assemblies on the solid substrates. The nanotubes were dispersed at a low concentration of isotropic phase, and the concentration was gradually increased by the controlled evaporation of water. In-situ isotropic-to-liquid crystalline phase transition via a biphasic region was observed during water evaporation. Drying on a substrate demonstrated the effect of surface fields on the order and alignment of SWNTs in the liquid suspension and the influence on the structure of the deposited nanotubes after evaporation.

True solutions of single-walled carbon nanotubes for assembly into macroscopic materials

Translating the unique characteristics of individual single-walled carbon nanotubes into macroscopic materials such as fibres and sheets has been hindered by ineffective assembly. Fluid-phase assembly is particularly attractive, but the ability to dissolve nanotubes in solvents has eluded researchers for over a decade. Here, we show that single-walled nanotubes form true thermodynamic solutions in superacids, and report the full phase diagram, allowing the rational design of fluid-phase assembly processes. Single-walled nanotubes dissolve spontaneously in chlorosulphonic acid at weight concentrations of up to 0.5 wt%, 1,000 times higher than previously reported in other acids. At higher concentrations, they form liquid-crystal phases that can be readily processed into fibres and sheets of controlled morphology. These results lay the foundation for bottom-up assembly of nanotubes and nanorods into functional materials.

Multicomponent solubility parameters for single-walled carbon nanotube-solvent mixtures

We have measured the dispersibility of single-walled carbon nanotubes in a range of solvents, observing values as high as 3.5 mg/mL. By plotting the nanotube dispersibility as a function of the Hansen solubility parameters of the solvents, we have confirmed that successful solvents occupy a well-defined range of Hansen parameter space. The level of dispersibility is more sensitive to the dispersive Hansen parameter than the polar or H-bonding Hansen parameter. We estimate the dispersion, polar, and hydrogen bonding Hansen parameter for the nanotubes to be <delta(D)> = 17.8 MPa(1/2), <delta(P)> = 7.5 MPa(1/2), and <delta(H)> = 7.6 MPa(1/2). We find that the nanotube dispersibility in good solvents decays smoothly with the distance in Hansen space from solvent to nanotube solubility parameters. Finally, we propose that neither Hildebrand nor Hansen solubility parameters are fundamental quantities when it comes to nanotube-solvent interactions. We show that the previously calculated dependence of nanotube Hildebrand parameter on nanotube diameter can be reproduced by deriving a simple expression based on the nanotube surface energy. We show that solubility parameters based on surface energy give equivalent results to Hansen solubility parameters. However, we note that, contrary to solubility theory, a number of nonsolvents for nanotubes have both Hansen and surface energy solubility parameters similar to those calculated for nanotubes. The nature of the distinction between solvents and nonsolvents remains to be fully understood.

Carbon nanotubes as liquid crystals

Carbon nanotubes are the best of known materials with a combination of excellent mechanical, electronic, and thermal properties. To fully exploit individual nanotube properties for various applications, the grand challenge is to fabricate macroscopic ordered nanotube assemblies. Liquid-crystalline behavior of the nanotubes provides a unique opportunity toward reaching this challenge. In this Review, the recent developments in this area are critically reviewed by discussing the strategies for fabricating liquid-crystalline phases, addressing the solution properties of liquid-crystalline suspensions, and exploiting the practical techniques of liquid-crystal routes to prepare macroscopic nanotube fibers and films.

Macroscopic fibers of well-aligned carbon nanotubes by wet spinning

A simple process to spin fibers consisting of multi-walled carbon nanotubes (CNTs) directly from their lyotropic liquid-crystalline phase is reported. Ethylene glycol is used as the lyotropic solvent, enabling a wider range of CNT types to be spun than previously. Fibers spun with CNTs and nitrogen-doped CNTs are compared. X-ray analysis reveals that nitrogen-doped CNTs have a misalignment of only +/-7.8 degrees to the fiber axis. The tensile strength of the CNT and nitrogen-doped CNT fibers is comparable but the modulus and electrical conductivity of the are lower. The electrical conductivity of both types of CNT fibers is found to be highly anisotropic. The results are discussed in context of the microstructure of the CNTs and fibers.

Liquid-crystalline processing of highly oriented carbon nanotube arrays for thin-film transistors

We introduce a simple solution-based method for the fabrication of highly oriented carbon nanotube (CNT) arrays to be used for thin-film transistors. We exploit the liquid-crystalline behavior of a CNT solution near the receding contact line during tilted-drop casting and produced long-range nematic-like ordering of carbon nanotube stripes caused by confined micropatterned geometry. We further demonstrate that the performance of thin-film transistors based on these densely packed and uniformly oriented CNT arrays is largely improved compared to random CNTs. This approach has great potential in low-cost, large-scale processing of high-performance electronic devices based on high-density oriented CNT films with record electrical characteristics such as high conductance, low resistivity, and high career mobility.