Shanju Zhang

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.

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.

Microwave makes carbon nanotubes less defective.

An ultrafast microwave annealing process has been developed to reduce the defect density in vertically aligned carbon nanotubes (CNTs). Raman and thermogravimetric analyses have shown a distinct defect reduction in the CNTs annealed in microwave for 3 min. Fibers spun from the as-annealed CNTs, in comparison with those from the pristine CNTs, show increases of approximately 35% and approximately 65%, respectively, in tensile strength ( approximately 0.8 GPa) and modulus (approximately 90 GPa) during tensile testing; an approximately 20% improvement in electrical conductivity (approximately 80000 S m(-1)) was also reported. The mechanism of the microwave response of CNTs was discussed.

Nanocomposites of carbon nanotube fibers prepared by polymer crystallization.

Nanocomposites of carbon nanotube fibers have been prepared using controlled polymer crystallization confined in nanotube aerogel fibers. The polyethylene nanocomposites have been investigated by means of polarized optical microscopy (POM), scanning electron microscopy (SEM) and wide-angle X-ray diffraction (WAXD). The individual nanotubes are periodically decorated with polyethylene nanocrystals, forming aligned hybrid shish-kebab nanostructures. After melting and recrystallization, transcrystalline lamellae connecting the adjacent aligned nanotubes develop. Microstructural analysis shows that the nanotubes can nucleate the growth of both orthorhombic and monoclinic crystals of polyethylene in the quiescent state. The tensile strength, modulus, and axial electrical conductivity of these polyethylene/CNT composite fibers are as high as 600 MPa, 60 GPa, and 5000 S/m, respectively.

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.

Directed self-assembly of hybrid oxide/polymer core/shell nanowires with transport optimized morphology for photovoltaics

Hybrid organic-inorganic solar cells are promising for the development of next generation low-cost, high efficiency photovoltaics (PVs). They combine the facile solution processability, large optical extinction coefficients, and good hole mobility of conjugated polymers with the high electron affinity and electron mobility of inorganic nanoparticles.[1] The most common configuration for effectively generating photocurrent with these materials is the bulk heterojunction (BHJ) device where intimate mixing of the polymer with inorganic nanoparticles generates a random bicontinuous morphology with nanometerscale dimensions. The morphology and ultimately the performance of this photoactive layer depend on a complex interplay of materials parameters and processing conditions such as temperature, polymer-solvent and particle-solvent interactions, solvent evaporation rate, solution composition (polymer volume fraction), and post-deposition treatments.[2] In particular, the role of polymer–particle miscibility has been well highlighted by recent reports.[3] Although there has been steady progress in improving device efficiency, the inherently disordered and kinetically-dictated structure of BHJ devices is suboptimal, particularly in terms of charge transport, but also in exciton utilization.[1a,4] An ideal hybrid device is one in which donor and acceptor materials are arranged in a densely packed vertical array. Nanometer-scale periodicity would minimize radiative decay of excitons, and the vertical alignment of the materials ensures a non-tortuous path for charge transport.[1a,4a,5] Such an ordered BHJ motif (OBHJ) has thus been the focus of recent interest,[6] but its realization, particularly in a manner compatible with low-cost fabrication of devices, remains elusive. A further refinement of the OBHJ entails atomic and molecularscale control of the donor and acceptor materials such that their

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 Onsagers 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.

In situ study of dynamic conformational transitions of a water-soluble poly(3-hexylthiophene) derivative by surfactant complexation.

Transitions in the backbone conformation of polythiophenes (PTs) in organic solvents, measurable spectroscopically, have been widely observed to influence thin-film morphology; however, such conformational transitions of water-soluble PT derivatives, with respect to their intramolecular versus intermolecular origin, remain largely obscure. Here, we report on dynamic conformational transitions of poly(3-potassium hexanoate thiophene) in aqueous cetyltrimethylammonium bromide investigated by means of Fourier transform infrared spectroscopy, differential scanning calorimetry, polarizing optical microscopy, and ultraviolet-visible absorption and fluorescence spectroscopy. As-prepared complexes exist as stable hydrogels. Upon dilution, a significant time-dependent chromism occurs spontaneously. A coil-to-rod conformational transition is identified in this mechanism. Study into the corresponding kinetics demonstrates an inverse first-order rate law. It is found that the conformational transition is thermally reversible and concentration-independent. The critical transition temperature is largely dependent on the surfactant architecture. A theoretical model is presented to explain this new phenomenon and the mechanisms behind its influence on the optoelectronic properties.

Graphene-Induced Oriented Interfacial Microstructures in Single Fiber Polymer Composites

Interfacial interactions between the polymer and graphene are pivotal in determining the reinforcement efficiency in the graphene-enhanced polymer nanocomposites. Here, we report on the dynamic process of graphene-induced oriented interfacial crystals of isotactic polypropylene (iPP) in the single fiber polymer composites by means of polarized optical microscopy (POM) and scanning electron microscopy (SEM). The graphene fibers are obtained by chemical reduction of graphene oxide fibers, and the latter is produced from the liquid crystalline dispersion of graphene oxide via a wet coagulation route. The lamellar crystals of iPP grow perpendicular to the fiber axis, forming an oriented transcrystalline (TC) interphase surrounding the graphene fiber. Various factors including the diameter of graphene fibers, crystallization temperature, and time are investigated. The dynamic process of polymer transcrystallization surrounding the graphene fiber is studied in the temperature range 124-132 °C. The Lauritzen-Hoffman theory of heterogeneous nucleation is applied to analyze the transcrystallization process, and the fold surface free energy is determined. Study into microstructures demonstrates a cross-hatched lamellar morphology of the TC interphase and the strong interfacial adhesion between the iPP and graphene. Under appropriate conditions, the β-form transcrystals occur whereas the α-form transcrystals are predominant surrounding the graphene fibers.