Dynamic evolution of interacting carbon nanotubes suspended in a fluid using a dielectrophoretic framework

A theoretical investigation of the dynamic response of interacting carbon nanotubes (CNTs) dispersed in a liquid medium under alternating current electric fields is presented. The proposed modeling strategy is based on the dielectrophoretic theory and classical electrodynamics of rigid bodies, and considers the coupled rotation-translation motion of interacting CNTs represented as electrical dipoles. Based on experimental evidence, the parameters which are expected to cause a major contribution to the CNTs' motion are investigated for different initial configurations of CNTs. It is predicted that high electric field frequencies, long CNTs, high values of electrical permittivity and conductivity of the CNTs immersed in solvents of high polarity promote faster equilibrium conditions, achieved by CNT tip-to-tip contact and alignment along the electric field direction. For the majority of the scenarios, CNT alignment along the field direction is predicted as the first event, followed by the translation of aligned CNTs until the tip-to-tip contact condition is reached. For systems with interacting CNTs with different lengths, equilibrium of the shorter CNT is achieved faster. Predictions also show that the initial rotation angles and initial location of CNTs have a paramount influence on the evolution of the system towards the equilibrium configuration.     

Structure of Lennard–Jones nanowires encapsulated by carbon nanotubes

Molecular dynamics simulations have been performed to investigate the structures of Lennard–Jones(LJ) nanowires(NWs) encapsulated in carbon nanotubes(CNTs). We find that the structures of NWs in a small CNT only adopt multi-shell motifs, while the structures of NWs in a larger CNT tend to adopt various motifs. Among these structures, three of them have not been reported previously. The phase boundaries among these structures are obtained regarding filling fractions, as well as the interaction between NWs and CNTs.     

Individuals, grasses, and forests of single- and multi-walled carbon nanotubes grown by supported Co catalysts of different nominal thicknesses

The relationships among the nominal thickness of Co catalyst, the structure of the catalyst particles, and the structure of carbon nanotubes (CNTs) growing from the catalyst during chemical vapor deposition were investigated. Various morphologies of CNTs such as individuals, random networks parallel to the surface of the substrate (‘grasses’), and vertically aligned forests of single- and multi-walled carbon nanotubes were grown by only varying the nominal thickness of catalyst under the same reaction condition. These different morphologies at the same growth time were due to the different areal density rather than to the length of CNTs. With increasing nominal thickness of catalyst, the catalyst particles changed in diameter while their areal density remained relatively almost constant. The change in diameter possibly affected the number ratio of active catalyst particles to the whole particles, which in turn affected the areal density of CNTs and yielded the various morphologies. Longer growth time increased the CNT length, which caused further change in CNT morphologies from individuals to grasses and grasses to forests.     

Aligned carbon nanotube from catalytic chemical vapor deposition technique for energy storage device: a review

Carbon nanomaterial especially carbon nanotube (CNT) possesses remarkably significant achievements towards the development of sustainable energy storage applications. This article reviews aligned CNTs grown from chemical vapor deposition (CVD) technique as electrode material in batteries and electrochemical capacitors. As compared to the entangled CNTs, aligned or well-organized CNTs have advantages in specific surface area and ion accessibility in which more electrolyte ions can access to CNT surfaces for better charge storage performance. CVD known as the most popular technique to produce CNTs enables the use of various substrates and CNT can grow in a variety of forms, such as powder, films, aligned or entangled. Also, CVD is a simple and economic technique, and has good controllability of direction and CNT dimension. High purity of as-grown CNTs is also another beauty of the CVD technique. The current trend and performance of devices utilizing CNTs as electrode material is also extensively discussed.     

Studies on structural defects in carbon nanotubes

Structural defects in carbon nanotubes (CNTs) have been paid much attention, because they influence the properties of the CNTs to some extent. Among various defects in CNTs, both single vacancies and Stone-Wales (SW) defects are the simple and common ones. In this paper, we review the progress of research in these two kinds of defects in CNTs. For single vacancies, we first address their different structural features in both zigzag and armchair CNTs, and their stabilities in CNTs with different sizes and different symmetries systematically. The presence of the single vacancies in CNTs not only influences the electronic structures of the systems, but also affects the vibrational properties of the tubes. Nevertheless, being active chemically, the single vacancies in the tubes prefer to interact with adsorbates nearby, of which the interaction of the defects with hydrogen atom, hydrogen molecule and some small hydrocarbon radicals (-CH, -CH₂ and -CH₃) are discussed. The former is associated with H storage and the latter is of merit to improve the local structure of the defect in a CNT. For the Stone-Wales defect, we mainly focus on its stability in various CNTs. The influence of the SW defects on the conductance of CNTs and the identification of such a defect in CNT is described in brief.      

Thermal properties of continuously spun carbon nanotube fibres

As indicated by theory and experimental measurements individual carbon nanotubes (CNTs) have very high values of thermal conductivity. One of the challenges is to achieve high thermal conductivity in macroscopic assemblies of CNTs such as fibres, films and composites, paving the way to a wide range of applications. CNT fibres have tremendous potential in succeeding as the future materials for a variety of applications when properties at the nanoscale are translated to their macroscopic assemblies. In this paper we report the measurements of thermal conductivity of continuously spun CNT fibres and its dependence on temperature. Thermal conductivity measurements were performed using in-house built temperature sensing microscope probe. Specific thermal conductivity of CNT fibres showed an order of magnitude advantage over the traditional materials used for heat dissipation.     

Transparent carbon nanotube coatings

Thin networks of carbon nanotubes (CNTs) are sprayed onto glass or plastic substrates in order to obtain conductive transparent coatings. Transparency and conductivity at room temperature of different CNT material are evaluated. CNT coatings maintain their properties under mechanical stress, even after folding the substrate. At a transparency of 90% for visible light we observe a surface resistivity of 1 kΩ/sq which is already a promising value for various applications.     

Hot filament effects on CVD of carbon nanotubes

We grew vertically aligned CNTs via HFCVD using mixtures of methane and hydrogen as feedstock, and investigated the dependence of CNT growth on feedstock composition, filament temperature, and filament types. At the filament temperature of 2050 °C tungsten filaments were more efficient for CNT growth than tantalum ones, and higher CNT growth rates were observed when tungsten filaments were operated at 1900 °C. Regardless of filament temperatures and types, monotonic increase in growth rate of vertically aligned CNTs was observed as we increased the methane concentration in the feedstock. In‐situ investigation of feedstock dissociation revealed the generation of various radical species, and, moreover, a strong correlation between CNT growth rates and relative mole fractions of single‐carbon radicals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Aligned growth and alignment mechanism of carbon nanotubes by hot filament chemical vapor deposition

Carbon nanotubes (CNTs) growth on Inconel sheets was carried out using hot filament chemical vapor deposition (HFCVD) in a gas mixture of methane and hydrogen. Scanning electron microscopy, transmission electron microscopy and field electron emission (FEE) measurement were applied to study the structure and FEE properties of the deposited CNTs. The effect of bias voltage and substrate surface roughness on the growth of vertically aligned carbon nanotubes was investigated. Well-aligned CNTs were synthesized by bias enhanced HFCVD. The results show that a bias of −500 V generates the best alignment. It has been observed that at the early growth stage, aligned and non-aligned CNTs are growing simultaneously on the unscratched sheets, whereas only aligned CNTs are growing on the scratched sheets. The results indicate that tip growth is not necessary for the electric field to align the CNTs, and larger catalyst particles created by scratching before the heat treatment can induce alignment of CNTs at the early growth stage. In addition, tree-like CNTs bundles grown on the scratched substrates exhibit better FEE performances than dense carbon nanotube forest grown on the unscratched substrates due to the reduced screen effect.     

Surface‐enhanced Raman scattering of pyridine‐functionalized multi‐walled carbon nanotubes

Carbon nanotubes (CNTs) have attracted great attention for their potential use in many applications because of their intrinsic properties. The importance other than the impact of the application of CNT‐embedded membranes in the area of water technology development is immense. In this context, the identification and quantification of CNTs in aqueous resources during relevant water purification processes can be proven of high significance. Surface‐enhanced Raman scattering (SERS) potentially has the sensitivity required for trace analysis and has been previously used for CNT identification on solid substrates. A thorough study for the identification and quantification of small concentrations of multi‐walled CNTs (MWCNTs) in water suspensions via SERS has been performed. The functionalization of MWCNTs with pyridine groups seems to favor the surface enhancement of relevant Raman signal. This study constitutes the first step of a work in progress for the characterization of CNTs at quite low concentration range by SERS in any water suspension. It is based on an ex ante functionalization of the CNTs by pyridine, demonstrating the potential of the method. Our long‐term aim is its general application built, however, in an ex post relevant functionalization of the CNTs in any aqueous solution. Copyright © 2014 John Wiley & Sons, Ltd.     

Realization, characterization and functionalization of lipidic wrapped carbon nanotubes

Mass-produced carbon nanotubes (CNTs) are strongly aggregated and highly hydrophobic, and processes to make them water soluble are required for biological applications. Both covalent and non-covalent strategies are pursued for obtaining stable, highly concentrated CNT aqueous dispersions. Covalent functionalization has the great disadvantage of producing an irreversible chemical modification of nanotubes, thus alterating their mechanical, chemical and electric properties. On the other hand, non-covalent functionalization is often obtained by employing surfactants that sensibly affect cell viability. Moreover, derivatization with biological moieties is often impossible through non-covalent CNT dispersion. This paper proposes a non-covalent dispersion of multi-wall CNT based on a lipidic mixture that can guarantee high concentration and high stability as well as high cytocompatibility. Moreover, CNTs wrapped with a lipid membrane are realized to demonstrate that the proposed CNTs can be functionalised with a dodecapeptide that specifically recognizes activated platelets without chemical modification of the nanotube itself.     

Aligned Ni nanoparticle arrays encapsulated in carbon nanotubes

Using the carbon nanotube (CNT) arrays embedded in anodic aluminum oxide (AAO) template as an electrode, large amounts of Ni nanoparticles have been encapsulated into the CNTs by an alternating current (AC) electrodepostion technique. As deposited Ni nanoparticles with a typical size of 50–60 nm randomly nucleated on the CNT walls, thus inhomogeneously distributed in the CNTs. After annealing at 600 ^°C, the nanoparticles transformed into quasi-spherical structures with the diameter increasing to 60–80 nm. The quasi-spherical nanoparticles were aligned in orderly rows along the axis of the CNT channels. Magnetic hysteresis measured at 5 K showed that the coercivity was 450 Oe for the as-deposited sample and 385 Oe for annealed sample, with the applied magnetic field parallel with the CNT’s axis. The structures and magnetic properties were discussed for both as-deposited and annealed samples.     

锯齿型碳纳米管的结构衍生及电子特性

利用密度泛函理论研究锯齿型单、双壁碳纳米管从核到管状团簇直至纳米管的逐层结构衍生.研究结果表明五边形结构在管状团簇生长中发挥关键作用.此外,基于管状团簇的研究,运用周期性边界条件得到锯齿型单、双壁碳纳米管,并通过计算能带和态密度研究其电子特性.对单壁(n,0)和双壁(n,0)@(2n,0)碳纳米管,当n=3q(q为整数)时,具有金属或窄带隙半导体特性;n?=3q时,具有较宽带隙半导体特性,且带隙随管径的增加而减小.然而,小管径碳纳米管受曲率效应的明显影响,n?=3q的(4,0),(4,0)@(8,0)和(5,0)@(10,0)均呈现金属性;n=3q的(6,0)@(12,0)则表现出明显的半导体特性.     

A comparison between the mechanical and thermal properties of single-walled carbon nanotubes and boron nitride nanotubes

Carbon nanotubes (CNTs) are semimetallic while boron nitride nanotubes (BNNTs) are wide band gap insulators. Despite the discrepancy in their electrical properties, a comparison between the mechanical and thermal properties of CNTs and BNNTs has a significant research value for their potential applications. In this work, molecular dynamics simulations are performed to systematically investigate the mechanical and thermal properties of CNTs and BNNTs. The calculated Young’s modulus is about 1.1 TPa for CNTs and 0.72 TPa for BNNTs under axial compressions. The critical bucking strain and maximum stress are inversely proportional to both diameter and length-diameter ratio and CNTs are identified axially stiffer than BNNTs. Thermal conductivities of (10, 0) CNTs and (10, 0) BNNTs follow similar trends with respect to length and temperature and are lower than that of their two-dimensional counterparts, graphene nanoribbons (GNRs) and BN nanoribbons (BNNRs), respectively. As the temperature falls below 200 K (130 K) the thermal conductivity of BNNTs (BNNRs) is larger than that of CNTs (GNRs), while at higher temperature it is lower than the latter. In addition, thermal conductivities of a (10, 0) CNT and a (10, 0) BNNT are further studied and analyzed under various axial compressive strains. Low-frequency phonons which mainly come from flexure modes are believed to make dominant contribution to the thermal conductivity of CNTs and BNNTs.     

碳纳米管和天鹅绒阴极强流发射特性的对比研究

采用电泳沉积法、碳纳米管纸和化学气相沉积直接生长法制备了三种碳纳米管阴极. 从强流发射性能、阴极等离子体膨胀、阴极起动、发射均匀性、工作稳定性以及脉冲放气特性等多个方面, 对比研究了碳纳米管阴极和化纤天鹅绒阴极的强流发射特性, 研究表明碳纳米管阵列和碳纳米管纸阴极发射性能明显优于普通化纤天鹅绒, 碳纳米管阴极发射性能与碳纳米管取向无关, 管壁的缺陷发射对无序碳纳米管阴极强流发射具有重要贡献. 碳纳米管阴极的起动场强约为普通化纤天鹅绒的2/3, 电场上升率相同时碳纳米管阴极比化纤天鹅绒阴极起动时间短12–17 ns. 碳纳米管阴极发射均匀性优于化纤天鹅绒, 尤其是碳纳米管阵列, 整个阴极表面等离子体光斑致密且均匀. 在二极管本底气压为6×10^-3 Pa时, 碳纳米管纸阴极对应的二极管峰值气压不到0.3 Pa, 约为普通化纤天鹅绒阴极的1/5, 碳纳米管阵列阴极放气量在三种阴极中最少, 仅为0.042 Pa. 结果表明, 碳纳米管阴极在强流电子束源和相关高功率微波器件领域具有潜在的应用价值.     

Memristor effect on bundles of vertically aligned carbon nanotubes tested by scanning tunnel microscopy

We report on the results of experimental study of an array of vertically aligned carbon nanotubes (VA CNTs) by scanning tunnel microscopy (STM). It is shown that upon the application of an external electric field to the STM probe/VA CNT system, individual VA CNTs are combined into bundles whose diameter depends on the radius of the tip of the STM probe. The memristor effect in VA CNTs is detected. For the VA CNT array under investigation, the resistivity ratio in the low- and high-resistance states at a voltage of 180 mV is 28. The results can be used in the development of structures and technological processes for designing nanoelectronics devices based on VA CNT arrays, including elements of ultrahigh-access memory cells for vacuum microelectronics devices.     

Dynamic operation of liquid crystal cell with inherently nanogroove-featured aligned carbon nanotube sheets

The use of a highly aligned carbon nanotube (CNT) sheet as a multifunctional constituent for liquid crystal (LC) displays and electro-optic LC applications is assessed. The CNT sheet can perform a dual function: one is an alignment layer for LCs, replacing the commonly used rubbed polyimide film, and the other is a transparent conductive layer, taking the place of indium tin oxide. The hydrophobic treatment improved the adhesion quality between aligned CNT bundles and the glass substrate, which helps to preserve the inherently aligned nanogroove morphology of transparent CNT sheets. The test LC display cells, comprising 4-cyano-4′-pentylbiphenyl molecules sandwiched between CNT-sheet-on-glass substrates, demonstrate the operation characteristics comparable to that of the conventional cell under temperature variation and ac electric field of 1?kHz. The results offer a possibility of deploying multifunctional CNT-sheet alignment layers in LC-based devices, especially in the future flexible display applications.     

Nanoelectronic Devices Based on Carbon Nanotubes

We review the state-of-the-art in the carbon nanotube (CNT) electronics. The emphasis is made on actually created devices. The history of discovery of fullerenes is outlined and their properties are considered. Experimental discovery of nanotubes and nanotube synthesis technologies are reviewed. The CNT conductivity dependence on the geometrical structure of nanotubes is discussed. Various nanoelectronic CNT devices, such as nanowires, heterojunctions, diodes, and field-effect transistors are presented. Quantum properties of CNTs at low temperatures are discussed. CNT-based mechanical devices, memory elements, and switches are considered. Field emission properties of CNTs are analyzed. The data on the developed CNT-based light-emitting elements and the manufactured pre-production models of CNT flat-panel displays are given.     

Rapid microwave-assisted synthesis and electrochemical characterization of gold/carbon nanotube composites

Hybrid nanostructures composed of gold nanoparticles (NPs) and carbon nanotubes (CNTs) have been prepared by a microwave-assisted method in the mixed solvents of oleylamine and oleic. The morphology, structure and composition of as-obtained Au/CNT composites are characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD). The composites show characteristic plasmon absorption of Au NPs in the Ultraviolet–visual spectrum. Fourier transform infrared spectrum shows the successful introduction of functional groups on the surface of CNTs, which are crucial factors to assist the nucleation in situ of Au NPs on the surface of CNTs. Electrochemical measurements show the enhancement electrochemical response for the gold electrode modified with Au/CNT composites.     

电场诱导碳纳米管阵列准直生长的研究

在传统的热化学气相沉积(CVD)的基础上,引入针尖电场,开展了电场对碳纳米管阵列准直性改善的研究。利用扫描电子显微镜(SEM)和Raman光谱两种表征手段,研究了加电场与不加电场两种情况下得到的碳纳米管(CNT)阵列的准直性,证明了电场对碳纳米管阵列准直生长的有效性。文章还对电场诱导碳纳米管阵列准直生长的机理进行了初步的探讨,认为电场使碳纳米管极化是CNT阵列准直生长的主要原因。