Control of one-dimensional transport in multi-wall carbon nanotubes by wall destruction

We have investigated the damage in multi-wall carbon nanotubes (MWNTs) destroyed by electrical breakdown and focused ion beam bombardment (FIBB). The transport properties of a MWNT destroyed by electrical breakdown have been compared with those of a MWNT destroyed by FIBB. Also the Tomonaga–Luttinger transport (TLT) model has been applied to each type of destroyed MWNT. The MWNT destroyed by FIBB showed TLT behavior because of the weak destruction of the remaining walls. However, in the case of MWNTs destroyed by electrical breakdown, three-dimensional variable-range hopping (VRH) was observed in the low temperature transport. This suggests that the local damage has been caused by strong breakdown. There exists a clear difference between the effects of electrical breakdown and FIBB. Wall destruction by FIBB could be applied to control the one-dimensional transport of MWNTs.     

Lateral growth of aligned mutilwalled carbon nanotubes under electric field

In this work we report laterally aligned multi-walled carbon nanotube (MWNT) by an electric field during growth. The MWNTs were selectively grown between lateral sides of the catalytic metals on predefined electrodes by chemical-vapor deposition. The electric field distribution for various geometries was simulated using Maxwell 2D simulation in order to realize better alignment of laterally grown carbon nanotubes (CNTs). The experimental results show that the electric field direction at the vicinity of catalyst and nanotubes-substrate interactions are principal factor in aligning CNTs laterally.     

Functional multi-walled carbon nanotube/polyaniline composite films as supports of platinum for formic acid electrooxidation

Embedding of carbon nanotubes in conducting polymeric matrices for various nanocomposites material is now a popular area. In this article, a concise chemical method has been described for the preparation of homogeneous nanocomposite of multi-walled carbon nanotube (MWNT)/polyaniline (PANI) by electrochemical codeposition. For this we functionalized the MWNTs via the diazotization reaction. This helped to disperse the nanotubes in aniline. The composite films were dispersed Pt by electrodeposition technique. The presence of MWNTs and platinum in the composite films was confirmed by XRD analysis and transmission electron microscopy (TEM). Four-point probe investigations revealed that the MWNT/PANI composite films exhibited a good conductivity. Cyclic voltammograms (CV) showed that Pt-modified MWNT/PANI composite films perform higher electrocatalytic activity and better long-term stability than Pt-modified pure PANI film toward formic acid oxidation. The results imply that the MWNT/PANI composite films as a promising support material improves the electrocatalytic activity for formic acid oxidation greatly.     

Electrophoresis and orientation of multiple wall carbon nanotubes in polymer solution

This paper contains an in-depth analysis of the electrophoresis of multi-wall carbon nanotubes (MWNTs) in liquid epoxy where electrophoresis experiments under DC and AC fields were carried out for five different types of multi-wall carbon nanotubes (MWNTs). DC electrophoresis and particle image velocimetry were used to determine the electrophoretic particle mobility and zeta potential, where the MWNTs with the largest outer diameter and length led to the highest mobility values. The orientation and agglomeration of MWNTs into “striation” lines under AC electrophoresis were investigated by analysing the hue, saturation and intensity of the transmitted polarised light under microscope, following a schedule of step-wise applied voltage in the range of 0 to 100 V. Plots of hue and saturation as a function of the applied voltage were used to assess the degree of orientation and density of orientated MWNT structures, respectively, and to determine an optimum AC electric field value for the orientation of a specific MWNT type by electrophoresis.     

Effects on the field emission properties by variation in surface morphology of patterned photosensitive carbon nanotube paste using organic solvent

Photosensitive carbon nanotube (CNT) paste was prepared by 3-roll milling of multi-walled carbon nanotubes (MWNTs), UV-sensitive binder solution, and Ag as filler additives. Arrays of MWNT dots with a diode structure were fabricated by a combination of screen printing method and photolithography using these paste, and acetone utilized as the developer. The MWNT dots were well-defined and the organic binder materials in the dots were partially removed. The MWNT film without a heat treatment showed a high current density of 1.35 mA/cm² at 3.25 V/μm and low turn-on field of 2.2 V/μm at 100 μA/cm². Acetone can be used as an efficient developer to form patterns and to remove the organic residues in patterns, simultaneously.     

Raman spectroscopy and field-emission properties of CVD-grown carbon-nanotube films

Carbon-nanotube films are very efficient cathodes for field-emission devices. This study presents a comprehensive comparison between structural, spectroscopic and field-emission properties of films of aligned and non-aligned multi-wall nanotubes (MWNTs) which are grown by thermal chemical vapour deposition. Three types of films are investigated: vertically aligned MWNTs with clean and coated nanotube side walls as well as non-aligned MWNT films. Raman spectra taken on the aligned MWNT films consist of many lines of first-, second- and third-order signals. Several lines are reported here for the first time for MWNTs. The presence of the surface coating leads to a decrease and broadening of the higher-order signals as well as an increase in the disorder-induced contributions in the first-order regime. The aligned MWNT films have excellent field-emission properties with very high emission current densities and low turn-on and threshold fields. The presence of a surface coating has no impact on the efficiency of the field-emission process. Films of non-aligned MWNTs show considerably reduced electron-emission current densities and larger critical fields. Received: 25 April 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Fabrication and Properties of Carbon Nanotube and Poly(vinyl alcohol) Composites

Dispersion of carbon nanotubes in a polymer matrix is one of the most critical issues in carbon nanotube/polymer composites. In this paper we discuss the considerable improvement in the dispersion of multiwalled carbon nanotubes (MWNTs) in poly(vinyl alcohol) (PVA) matrix that was attained through gum arabic treatment. The mechanical properties of these MWNT/PVA composites show that only 2 wt% nanotube loading increases the tensile modulus by more than 130%.     

Supercritical fluid-assisted synthesis of a carbon nanotubes-grafted biocompatible polymer composite

A nanocomposite consisting of multi-wall nanotubes (MWNTs) grafted with a biocompatible polymer poly(2-hydroxyethyl methacrylate) was prepared by in situ polymerization in supercritical carbon dioxide. The surface of the MWNTs was first surface modified with hydroxyl groups in the solution of KMnO₄ and a phase-transfer catalyst. MWNT-OH was then functionalized with vinyl groups using a silane coupling agent, γ-methacryloxypropyltrimethoxysilane. The silane groups can improve the dipersion of MWNTs in supercritical carbon dioxide, while the terminal vinyl groups help fabricate polymer chains on the MWNT surface. The as-synthesized products were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermo-gravimetric analysis. The SEM and TEM images showed that the nanotubes were well coated with the polymer shell. The composite had higher thermal stability than the pure polymer and dispersed well in methanol. This biocompatible polymer composite was prepared using a green method and is expected to be useful as a biomaterial composite with potential applications in the biological field.     

Preparation and Characterization of Linear Low Density Polyethylene/Carbon Nanotube Nanocomposites

Linear low‐density polyethylene (LLDPE)/multiwalled carbon nanotube (MWNT) nanocomposites were prepared via melt blending. The morphology and degree of dispersion of nanotubes in the polyethylene matrix were investigated using scanning electron microscopy (SEM). Both individual and agglomerates of MWNTs were evident. The rheological behavior and mechanical and electrical properties of the nanocomposites were studied using a capillary rheometer, tensile tester, and Tera ohm‐meter, respectively. Both polyethylene and its nanocomposites showed non‐Newtonian behavior in almost the whole range of shear rate. Addition of carbon nanotubes increased shear stress and shear viscosity. It was also found that the materials experience a fluid‐solid transition below 1 wt% MWNT. Flow activation energy for the nanocomposites was calculated using an Arrhenius type equation. With increasing nanotube content, the activation energy of flow increases. A decrease of about 7 orders of magnitude was obtained in surface and volume resistivity upon addition of 5 wt% MWNT. In addition, a difference between electrical and rheological percolation thresholds was observed. The results confirm the expected nucleant effect of nanotubes on the crystallization process of polyethylene. A slight increase in Young's modulus was also observed with increasing MWNT content.     

Static dielectric properties of carbon nanotubes from first principles

We characterize the response of isolated single-wall (SWNT) and multiwall (MWNT) carbon nanotubes and nanotube bundles to static electric fields using first-principles calculations and density-functional theory. The longitudinal polarizability of SWNTs scales as the inverse square of the band gap, while in MWNTs and bundles it is given by the sum of the polarizabilities of the constituent tubes. The transverse polarizability of SWNTs is insensitive to band gaps and chiralities and is proportional to the square of the effective radius; in MWNTs, the outer layers dominate the response. The transverse response is intermediate between metallic and insulating, and a simple electrostatic model based on a scale-invariance relation captures accurately the first-principles results. The dielectric response of nonchiral SWNTs in both directions remains linear up to very high values of applied field.     

Synthesis and characterization of thermotropic liquid crystalline polyester/multi-walled carbon nanotube nanocomposites

Thermotropic liquid crystalline polyester (TLCP) was synthesized via low-temperature solution polycondensation from 1,4-Bis(4-Hydroxybenzoyloxy)butane and terephthaloyl dichloride. Polymer nanocomposites based on a small quantity of multi-walled carbon nanotubes (MWNTs) were prepared by in situ polymerization method. The wide-angle X-ray diffraction (WAXD) results suggested that the addition of MWNTs to TLCP matrix did not significantly change the crystal structure of TLCP. The interactions between the molecules of the TLCP host phase and the carbon nanotubes were investigated through Raman spectroscopy investigations. We detected a distinct wave number shift of the radial breathing modes, confirming the carbon nanotubes interacted with the surrounding liquid crystal molecules, most likely through aromatic interactions (π-stacking). The interactions between liquid crystal host and nanotube guests were also evident from a polarizing microscopy (POM) study of the liquid crystal-isotropic phase transition in the proximity of nanotubes. The thermal properties and the morphological properties of the TLCP/MWNTs nanocomposites were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). TGA data demonstrated the addition of a small amount of MWNTs into TLCP matrix could improve the thermal stability of TLCP matrix. DSC results revealed that melt transition temperatures and isotropic transition temperatures of the hybrids were enhanced.     

Synthesis of multiwall carbon nanotubes by chemical vapor deposition of ferrocene alone

Multiwall carbon nanotubes (MWNTs) filled with Fe nanoparticles (NPs) have been synthesized by thermal chemical vapor deposition of ferrocene alone as the precursor. The MWNTs were grown at different temperatures: 980 and 800 ^°C. Characterization of as-prepared MWNTs was done by scanning and transmission electron microscopy, and X-ray diffraction. The transmission electron microscopy study revealed that Fe NPs encapsulated in MWNTs grown at 980 and 800 ^°C are spherical and rod shaped, respectively. Room-temperature vibrating sample magnetometer studies were done on the two samples up to a field of 1 T. The magnetization versus magnetic field loop reveals that the saturation magnetization for the two samples varies considerably, almost by a factor of 4.6. This indicates that Fe is present in different amounts in the MWNTs grown at the two different temperatures.     

Polyurethane‐Carbon Nanotube Nanocomposites Prepared by In‐Situ Polymerization with Electroactive Shape Memory

In‐situ polymerization was employed to achieve well‐dispersed carbon nanotube‐reinforced polyurethane composites. In‐situ polymerization showed predominant as primarily dispersal of carbon nanotubes in the matrix polymer according to scanning electron microscopy (SEM) observation and atomic force microscopy (AFM) images. Differential scanning calorimetry (DSC) results suggested that the addition of multi walled nanotubes (MWNTs) into polyurethane increased the rate of crystallization, this effect being more significant in polyurethane (PU)‐MWNT composite, which was prepared by an in‐situ polymerization process. The composites obtained by in‐situ polymerization showed enhanced mechanical properties as well as good electroactive shape memory. The original shape of the sample was almost recovered with bending mode when an electric field of 50 V was applied.     

浓硝酸处理前后多壁碳纳米管的荧光特性研究

对浓硝酸处理前后多壁碳纳米管的荧光特性进行了研究,结果发现多壁碳纳米管在浓硝酸处理前后都能产生荧光;与浓硝酸处理前相比,浓硝酸处理后的多壁碳纳米管的荧光有所增强,且荧光谱峰向短波长方向发生蓝移。碳纳米管产生的荧光与碳纳米管在其缺陷处捕获激发光的能量以及最低空轨道和最高占有轨道之间的能隙有关。多壁碳纳米管在浓硝酸处理前后都有缺陷,因而都能在其缺陷处捕获激发光能量而产生荧光;浓硝酸处理引起多壁碳纳米管的缺陷增多、捕获激发光能量增加因而荧光也随之增强。碳纳米管的最低空轨道和最高占有轨道之间的能隙随其长度减小而增大,浓硝酸处理引起多壁碳纳米管被削短导致其最低空轨道和最高占有轨道之间的能隙增大,因而引起多壁碳纳米管的荧光谱峰向短波方向蓝移。     

Tomonaga-Luttinger liquid and Coulomb blockade in multiwall carbon nanotubes under pressure

We report that the conductance of macroscopic multiwall nanotube (MWNT) bundles under pressure shows power laws in temperature and voltage, as corresponding to a network of bulk-bulk connected Tomonaga-Luttinger liquids (LLs). Contrary to individual MWNTs, where the observed power laws are attributed to Coulomb blockade, the measured ratio for the end and bulk obtained exponents, approximately 2.4, can be accounted for only by LL theory. At temperatures characteristic of interband separation, it increases due to thermal population of the conducting sheets unoccupied bands.     

Preparation and characterization of multiwall carbon nanotube/polypyrrole coaxial fibrils

Multiwall carbon nanotube (MWNT)/polypyrrole (PPy) fibrils were fabricated by template-free in situ electrochemical deposition of PPy over MWNTs, and characterized by electron microscopy and electrical measurements. Scanning and transmission electron microscopy studies reveal that PPy coating on the surface of nanotube is quite uniform throughout the length, with the possibility of forming unique Y-junctions. Current (I)-voltage (V) characteristics at various temperatures show nonlinearity due to tunneling and hopping contributions to transport across the barriers. AC conductivity measurements (300-4.2 K) show that the onset frequency scales with temperature, and the nanoscale connectivity in MWNT/PPy fibrils decreases with the lowering of temperature.     

Off-axis Thermal Properties of Carbon Nanotube Films

Theoretical calculations have predicted that individual Single-Walled Carbon Nanotubes (SWNT) have extremely high thermal conductivity (around 6.6 × 10⁴ W/m-K). The feasibility of constructing practical devices using the above mentioned properties, is critically dependent on the ability to synthesize high-thermal-conducting films. Highly conducting films would be of great use as heat sinks for the next generation of integrated chips. Excessive heating is currently a very serious problem in the endeavor for achieving faster and smaller chips. Since it is still not possible to perfectly align SWNT in the macroscopic scale, the thermal properties of the nano-films are therefore expected to have a statistical effect and thus lower than the intrinsic thermal conductivity of a single nanotube. Also the thermal conductivity perpendicular to the tube direction is more significant from a practical point of view. Multi-Walled Carbon Nanotubes (MWNT) were synthesized by Chemical Vapor Deposition (CVD) technique and subsequently characterized. The thin MWNT films were deposited by a solution casting technique over a metallic substrate. The off-axis thermal properties of these nano-films were studied by AC-calorimetry studies. In this method, the sample is heated by an AC source and the measurement of the relaxation rate is used to determine the thermal properties. This technique is well established for studying the thermal properties of complex fluids. Our results are contrasted with other thermal conductivity measurements intrinsic and bulk carbon nanotube samples. We have also measured off-axis thermal properties of nano-films synthesized from more crystalline SWNT samples and have compared this result with that of the MWNT-film. A model to explain the thermal conduction for our system is proposed. George Muench: Presently at the Department of Physics, University of New Haven, West Haven, CT-06516, USA     

Critical Exponents of Gelation and Conductivity in Polyacrylamide Gels Doped by Multiwalled Carbon Nanotubes

Polyacrylamide (PAM) doped by multiwalled carbon nanotube (MWNT) gels were prepared with different amounts of MWNTs varying in the range between 0.1 and 15 wt%. The PAM–MWNT composite gels were characterized by the steady state fluorescence technique (SSF). The alternative electrical conductivity (AC) of PAM–MWNT composite gels was measured by the dielectric spectroscopy technique. Observations around the gel point, t _gel for PAM–MWNTs composite gels showed that the gel fraction exponent β obeyed the percolation result. The critical exponent r of AC electrical conductivity for the composite PAM–MWNT gel was also measured and found to be about 2.0, which agrees with a random resistor network.     

The origin of polarized blackbody radiation from resistively heated multiwalled carbon nanotubes

We observed very pronounced polarization of light emitted by highly aligned free-standing multiwall carbon nanotube (MWNT) sheet in axial direction which is turned to the perpendicular polarization when a number of layers are increased. The radiation spectrum of resistively heated MWNT sheet closely follows to the Plank's blackbody radiation distribution. The obtained polarization features can be described by a classical dielectric cylindrical shell model, taking into consideration the contribution of delocalized π-electrons (π surface plasmons). In absorption (emission) the optical transverse polarizability, which is much smaller than longitudinal one, is substantially suppressed by depolarization effect due to screening by induced charges. This phenomenon suggests very simple and precise method to estimate the alignment of nanotubes in bundles or large assemblies.     

Nonlinear transport and heat dissipation in metallic carbon nanotubes

We show that the local temperature dependence of thermalized electron and phonon populations along metallic carbon nanotubes is the main reason behind the nonlinear transport characteristics in the high bias regime. Our model is based on the solution of the Boltzmann transport equation considering both optical and zone boundary phonon emission as well as absorption by charge carriers. It also assumes a local temperature along the nanotube, determined self-consistently with the heat transport equation. By using realistic transport parameters, our results not only reproduce experimental data for electronic transport but also provide a coherent interpretation of thermal breakdown under electric stress. In particular, electron and phonon thermalization prohibits ballistic transport in short nanotubes.