Functionalization of gold-coated carbon nanotubes with self-assembled monolayers of thiolates

Multi-walled carbon nanotubes (CNT) were synthesized by chemical vapor deposition using Co–Fe as a catalyst and ethylene as a carbon source. Afterward, a simple method combining wet-chemistry and chemical reduction was used to prepare carbon nanotube/gold material (CNT/Au). Pristine nanotubes and CNT/Au were characterized by transmission electron microscopy micrographs. It appeared that gold formed nanoparticles on CNTs endings and their sidewalls. Further functionalization was carried out by using thiols of different chemical properties and molecule sizes. Thiols formed self-assembled monolayer on gold surface that led to formation of CNT/gold/thiol-functionalized material. The amounts of chemisorbed thiols were measured by elemental analysis and thermogravimetry.     

Effect of carbon nanotubes on the interfacial shear strength of T650 carbon fiber in an epoxy matrix

The interfacial shear strength of carbon nanotube coated carbon fibers in epoxy was studied using the single-fiber composite fragmentation test. The carbon fibers were coated with carbon nanotubes (CNT) on the fiber surface using thermal chemical vapor deposition (CVD). The CVD process was adjusted to produce two CNT morphologies for the study: radially aligned and randomly oriented. The purpose of the CNT coating was to potentially produce a multifunctional structural composite. Results of the single-fiber fragmentation tests indicate an improvement in interfacial shear strength with the addition of a nanotube coating. This improvement can most likely be attributed to an increase in the interphase yield strength as well as an improvement in interfacial adhesion due to the presence of the nanotubes.     

Growth of horizontally aligned dense carbon nanotubes from trench sidewalls

Horizontally aligned, dense carbon nanotubes (HADCNTs) in the form of CNT cantilevers/bridges were grown from selected trench sidewalls in silicon substrate by chemical vapor deposition (CVD). The as-grown CNT cantilevers/bridges are packed with multiwalled carbon nanotubes (MWCNTs) with a linear density of about 10 CNTs μm(-1). The excellent horizontal alignment of these CNTs is mainly ascribed to the van der Waals interactions within the dense CNT bundles. What is more, the Raman intensity ratio I(G)/I(D) shows a gradual increase from the CNT roots to tips, indicating a defect gradient along CNTs generated during their growth. These results will inspire further efforts to explore the fundamentals and applications of HADCNTs.     

ZnO-based FBAR resonators with carbon nanotube electrodes

Film bulk acoustic resonator (FBAR) devices with carbon nanotube (CNT) electrodes directly grown on a ZnO film by thermal chemical vapor deposition have been fabricated. CNT electrodes possess a very low density and high acoustic impedance, which reduces the intrinsic mass loading effect resulting from the electrodes? weight and better confines the longitudinal acoustic standing waves inside the resonator, in turn providing a resonator with a higher quality factor. The influence of the CNTs on the frequency response of the FBAR devices was studied by comparing two identical sets of devices; one set comprised FBARs fabricated with chromium/ gold bilayer electrodes, and the second set comprised FBARs fabricated with CNT electrodes. It was found that the CNTs had a significant effect on attenuating traveling waves at the surface of the FBARs' membranes because of their high elastic stiffness. Three-dimensional finite element analysis of the devices fabricated was carried out, and the numerical simulations were consistent with the experimental results obtained.     

Enhanced field emission characteristics of nitrogen-doped carbon nanotube films grown by microwave plasma enhanced chemical vapor deposition process

Nitrogen-doped carbon nanotube (CNT) films have been synthesized by simple microwave plasma enhanced chemical vapor deposition technique. The morphology and structures were investigated by scanning electron microscopy and high resolution transmission electron microscopy. Morphology of the films was found to be greatly affected by the nature of the substrates. Vertically aligned CNTs were observed on mirror polished Si substrates. On the other hand, randomly oriented flower like morphology of CNTs was found on mechanically polished ones. All the CNTs were found to have bamboo structure with very sharp tips. These films showed very good field emission characteristics with threshold field in the range of 2.65-3.55 V/μm. CNT film with flower like morphology showed lower threshold field as compared to vertically aligned structures. Open graphite edges on the side surface of the bamboo-shaped CNT are suggested to enhance the field emission characteristics which may act as additional emission sites.     

Growth of carbon nanotubes at low powers by impedance-matched microwave plasma enhanced chemical vapor deposition method

A solo carbon nanotube (CNT) was successfully grown on nickel electrodes by a microwave plasma enhanced chemical vapor deposition (MPECVD) method equipped with an impedance-matched substrate holder with the reaction gases composed of hydrogen (H2), carbon dioxide (CO2), and methane (CH4) mixtures. An introduction of carbon dioxide gas before CNTs growth, the substrate temperature can easily be reached above 610 degrees C even heated at a low microwave power. This can be enunciated from fact that carbon dioxide inherits with higher bond energy for molecular dissociation, lower thermal conductivity, and higher heat capacity in comparing to other gases. The electron field emissions for randomly aligned CNTs and well-aligned CNTs grown by MPECVD and by radio frequency assisted hot-filament methods, respectively, are measured and compared. The higher field emission characteristic of the randomly aligned CNTs is presumed to be due to the protruded CNTs, which inheriting with less screening effect and manifesting with defects are crucial to play the effective emission sites.     

Synthesis and characterization of vertically aligned carbon nanotube forest for solid state fiber spinning

Continuous carbon nanotubes (CNT) fibers were directly spun from a vertically aligned CNT forest grown by a plasma-enhanced chemical vapor deposition (PECVD) process. The correlation of the CNT structure with Fe catalyst coarsening, reaction time, and the CNTs bundling phenomenon was investigated. We controlled the diameters and walls of the CNTs and minimized the amorphous carbon deposition on the CNTs for favorable bundling and spinning of the CNT fibers. The CNT fibers were fabricated with an as-grown vertically aligned CNT forest by a PECVD process using nanocatalyst an Al2O3 buffer layer, followed by a dry spinning process. Well-aligned CNT fibers were successfully manufactured using a dry spinning process and a surface tension-based densification process by ethanol. The mechanical properties were characterized for the CNT fibers spun from different lengths of a vertically aligned CNT forest. Highly oriented CNT fibers from the dry spinning process were characterized with high strength, high modulus, and high electrical as well as thermal conductivities for possible application as ultralight, highly strong structural materials. Examples of structural materials include space elevator cables, artificial muscle, and armor material, while multifunctional materials include E-textile, touch panels, biosensors, and super capacitors.     

Synthesis of high aspect-ratio carbon nanotube "flying carpets" from nanostructured flake substrates

We present a robust method for synthesis of aligned, single-walled carbon nanotube (CNT) "flying carpets" from nanostructured alumina flakes. Roll-to-roll e-beam deposition is utilized to produce the flakes, and hot filament chemical vapor deposition is utilized to grow dense, aligned carbon nanotubes from the flakes with remarkably high CNT yields. The flakes are captured inside a mesh cage and freely suspended in the gas flow during growth. Optical characterization indicates the presence of high quality, small diameter single-walled carbon nanotubes.     

Effects of feed gas composition and catalyst thickness on carbon nanotube and nanofiber synthesis by plasma enhanced chemical vapor deposition

Many engineering applications require carbon nanotubes with specific characteristics such as wall structure, chirality and alignment. However, precise control of nanotube properties grown to application specifications remains a significant challenge. Plasma-enhanced chemical vapor deposition (PECVD) offers a variety of advantages in the synthesis of carbon nanotubes in that several important synthesis parameters can be controlled independently. This paper reports an experimental study of the effects of reacting gas composition (percentage methane in hydrogen) and catalyst film thickness on carbon nanotube (CNT) growth and a computational study of gas-phase composition for the inlet conditions of experimentally observed carbon nanotube growth using different chemical reaction mechanisms. The simulations seek to explain the observed effects of reacting gas composition and to identify the precursors for CNT formation. The experimental results indicate that gas-phase composition significantly affects the synthesized material, which is shown to be randomly aligned nanotube and nanofiber mats for relatively methane-rich inlet gas mixtures and non-tubular carbon for methane-lean incoming mixtures. The simulation results suggest that inlet methane-hydrogen mixture coverts to an acetylene-methane-hydrogen mixture with minor amounts of ethylene, hydrogen atom, and methyl radical. Acetylene appears to be the indicator species for solid carbon formation. The simulations also show that inlet methane-hydrogen mixture does not produce enough gas-phase precursors needed to form quality CNTs below 5% CH4 concentrations in the inlet stream.     

Neural stimulation with a carbon nanotube microelectrode array

We present a novel prototype neural interface using vertically aligned multiwalled carbon nanotube (CNT) pillars as microelectrodes. Functionalized hydrophilic CNT microelectrodes offer a high charge injection limit (1-1.6 mC/cm2) without faradic reactions. The first repeated in vitro stimulation of hippocampal neurons with CNT electrodes is demonstrated. These results suggest that CNTs are capable of providing far safer and more efficacious solutions for neural prostheses than previous metal electrode approaches.     

Horizontally oriented carbon nanotubes coated with nanocrystalline carbon

Single-walled carbon nanotubes (CNTs) and multi-walled CNTs of length 2-5 mm were grown from Fe/Mo nanoparticles and Fe thin film catalyst, respectively, by thermal chemical vapor deposition. Following CNT growth, the CNTs were in-situ coated with nanocrystalline carbon shells of thickness 100-1500 nm. Horizontally oriented CNTs with carbon shells in the direction of the feeding gas were visible under a regular optical microscope. They were easily manipulated by optical manipulators, and CNT probes can thus be fabricated.     

Multi-branching carbon nanotubes via self-seeded catalysts

A novel multi-branching carbon nanotube (CNT) structure is synthesized by direct current plasma enhanced chemical vapor deposition. The structure consists of aligned CNTs which have branches of smaller diameters growing aligned along a direction perpendicular to the original CNT. The mechanism of branching is explained in terms of a self-seeding of Ni catalyst which is transferred by sputtering from the original catalyst particles in the backbone CNTs to the walls of those CNTs. It is also shown that the branching induced a large increase in surface area and total nanotube length and can be beneficial in supporting very fine Pt nanoparticles for fuel cell and other catalytic applications. Such an array of Y-junction nanostructures could be useful for the fabrication of a high-density array of nanoelectronics switches and transistors.     

Large area growth of aligned CNT arrays on spheres: Cost performance and product control

We present a systematic optimization of the ferrocene chemical vapor deposition process for the low cost mass production of millimeter aligned carbon nanotubes (CNT), and also a successful control on the structure of the CNT product in this process. An overall carbon efficiency of 50%, almost 50 times higher than the conventional procedure, was achieved. The length and average diameter of the CNTs can be controlled in a wide range. CNTs with narrow diameter distribution were also obtained from nickelocene based catalyst.     

Carbon nanotubes grown on stainless steel to form plate and probe electrodes for chemical/biological sensing

This paper describes the fabrication and evaluation of carbon nanotube (CNT) electrodes grown on stainless steel (SS) plate and wire for electrochemical sensor applications. Multi-wall carbon nanotubes with different diameters were grown on the SS plate and wire by chemical vapor deposition from an ethylene precursor. The SS provides a good electrical and mechanical connection to the CNT, and the SS is a tough substrate. The SS part of the electrode was electrically insulated from the analyte so that only the CNT were active in sensing. Cyclic voltammetry for the reduction of 6 mM K3Fe(CN)6 in a 1.0 M KNO3 supporting electrolyte was performed to examine the redox behavior of the CNT-SS electrode. The cyclic voltammograms showed sigmoidal-like shapes, indicating that mass transport around the electrodes is dominated by radial diffusion. Based on the cyclic voltammograms, the effective area of the CNT-SS electrodes and the number of individual CNTs were estimated. These results indicate that the CNT-SS plate and wire electrodes are good candidates to develop practical in vivo biosensors.     

Effect of fibre coating and geometry on the tensile properties of hybrid carbon nanotube coated carbon fibre reinforced composite

Hierarchically structured hybrid composites are ideal engineered materials to carry loads and stresses due to their high in-plane specific mechanical properties. Growing carbon nanotubes (CNTs) on the surface of high performance carbon fibres (CFs) provides a means to tailor the mechanical properties of the fibre–resin interface of a composite. The growth of CNT on CF was conducted via floating catalyst chemical vapor deposition (CVD). The mechanical properties of the resultant fibres, carbon nanotube (CNT) density and alignment morphology were shown to depend on the CNT growth temperature, growth time, carrier gas flow rate, catalyst amount, and atmospheric conditions within the CVD chamber. Carbon nanotube coated carbon fibre reinforced polypropylene (CNT-CF/PP) composites were fabricated and characterized. A combination of Halpin–Tsai equations, Voigt–Reuss model, rule of mixture and Krenchel approach were used in hierarchy to predict the mechanical properties of randomly oriented short fibre reinforced composite. A fractographic analysis was carried out in which the fibre orientation distribution has been analyzed on the composite fracture surfaces with Scanning Electron Microscope (SEM) and image processing software. Finally, the discrepancies between the predicted and experimental values are explained.     

定向碳纳米管薄膜的制备研究

采用化学气相沉积法制备了定向碳纳米管薄膜,研究了向反应室载入水、氨水对定向碳纳米管薄膜的影响,并用SEM、TEM、XRD对碳纳米管进行了表征。结果表明：向反应室载入水的量增大,定向碳纳米管的长度先增加后减小。载入水的氩气流量为400ml／min,定向碳纳米管的长度1750μm;向反应室载入氨水,得到疏密相间排列的定向碳纳米管薄膜,氨水量增加,定向碳纳米管薄膜的厚度减小。     

Wafer-Scale Growth and Transfer of Aligned Single-Walled Carbon Nanotubes

Experimental demonstration of wafer-scale growth of well-aligned, dense, single-walled carbon nanotubes on 4" ST-cut quartz wafers is presented. We developed a new carbon nanotube (CNT) wafer-scale growth process. This process allows quartz wafers to be heated to the CNT growth temperature of 865degC through the alpha-beta phase transformation temperature of quartz (573degC) without wafer fracture. We also demonstrate wafer-scale CNT transfer to transfer these aligned CNTs from quartz wafers to silicon wafers. The CNT transfer process preserves CNT density and alignment. Carbon nanotube FETs fabricated using these transferred CNTs exhibit high yield. Wafer-scale growth and wafer-scale transfer of aligned CNTs enable carbon nanotube very large-scale integration circuits and their large-scale integration with silicon CMOS.     

On the role of activation mode in the plasma- and hot filaments-enhanced catalytic chemical vapour deposition of vertically aligned carbon nanotubes

Catalytic chemical vapor deposition (CCVD) with different activation modes (thermal; hot filaments-enhanced; direct current plasma-enhanced and both hot filament and direct current plasma-enhanced) are achieved in order to grow vertically aligned carbon nanotubes (VA CNTs). By widely varying the power of the different activation sources of the gas (plasma, hot filaments, substrate heating) while keeping identical the substrate temperature (973 K) and the catalyst preparation, the results point out the important role of ions in the nucleation of carbon nanotubes (CNTs), as well as the etching behaviour of highly activated radicals such as H˙ in the selective growth of vertically aligned films of CNTs. Moreover, it is demonstrated that, within the deposition conditions (temperature, pressure, flow rate) used in this study, oriented carbon nanotubes can be grown only when both ions, mainly generated by the gas discharge plasma, and highly reactive radicals, mainly formed by the hot filaments, are produced in the gas phase. We propose that highly energetic ions are needed to nucleate the carbon nanotubes by increasing the carbon concentration gradient whereas the highly reactive radicals allow the selective growth of vertically aligned CNTs by preventing carbon deposition on the whole surface through chemical etching of edge carbons in graphene sheets.     

Air-assisted growth of ultra-long carbon nanotube bundles

We report an air-assisted chemical vapor deposition (CVD) method for the synthesis of super-long carbon nanotube (CNT) bundles. By mixing a small amount of air in the vapor phase catalyst CVD process, the catalyst lifetime can be dramatically increased, and extremely long dense and aligned CNT bundles up to 1.5?cm can be achieved. Electron microscopy characterization shows that the injection of air does not damage the CNT structures. Further, we have estimated that individual ultra-long CNTs can carry moderate current densities ～10(5)?A?cm(-2), indicating their possible use in nanoelectronic devices.     

The effects of carbon nanotube orientation and aggregation on vibrational behavior of functionally graded nanocomposite cylinders by a mesh-free method

In this paper, axisymmetric natural frequencies of nanocomposite cylinders reinforced by straight single-walled carbon nanotubes are presented based on a mesh-free method. The straight carbon nanotubes (CNTs) are oriented, aligned or randomly or locally aggregated into some clusters. Volume fractions of the CNTs and clusters are assumed to be functionally graded along the thickness, so material properties of the carbon nanotube reinforced composite cylinders are variable and are estimated based on the Eshelby–Mori–Tanaka approach. In the mesh-free analysis, moving least squares shape functions are used for an approximation of the displacement field in the weak form of motion equation, and the transformation method is used for the imposition of essential boundary conditions. The effects of orientation and aggregation of the functionally graded CNT are studied. It is observed that kind of distributions, aggregation or even randomly orientations of CNTs has significant effect on the effective stiffness and frequency parameter.