Large-Scale Synthesis of Aligned Carbon Nanotubes

High Voltage Electrophoretic Deposition of Aligned Nanoforests for Scalable Nanomanufacturing of Electrochemical Energy Storage Devices

High voltage electrophoretic deposition (HVEPD) has been used to obtain forests of aligned multi-walled carbon nanotubes (MWCNTs) on long strips of flexible, conductive substrates. Successful design and integration of a continuous HVEPD setup has enabled scalable fabrication of electrodes for electrochemical energy storage. The mechanism of continuous HVEPD has been investigated to ensure appropriate alignment. Well-aligned forests of MWCNTs were obtained using a conductive holding layer which helped reduce internal resistance and enhance the electrochemical performance of the electrodes.

Catalyst poisoning by amorphous carbon during carbon nanotube growth: fact or fiction?

The influence of amorphous carbon on FePt catalyst particles under chemical vapor deposition conditions typically applied for CNT growth is examined through two routes. In the first, FePt catalyst particles supported on alumina are exposed to a well-established cyclohexane thermal CVD reaction at various temperatures. At higher temperatures where self-pyrolysis leads to copious amorphous carbon and carbon tar formation, carbon nanotubes are still able to form. In the second route, an amorphous carbon film is first deposited over the catalyst particles prior to the CVD reaction. Even for reactions where further amorphous carbon is deposited due to self-pyrolysis, graphitization is still demonstrated. Our findings reveal that the presence of amorphous carbon does not prevent catalytic hydrocarbon decomposition and graphitization processes. We also show an additional catalytic reaction to be present, catalytic hydrogenation, a process in which carbon in contact with the catalyst surface reacts with H(2) to form CH(4).

ALIGNMENT OF CARBON NANOTUBES USING MAGNETIC NANOPARTICLES

Carbon nanotubes are driving scientific research nowadays. This field has several important directions in basic research, including chemistry, electronic transport, mechanical, and field emission properties. The most eye-catching features of carbon nanotubes are their electronic, mechanical, optical, and chemical characteristics, which open a way to future applications. One of the most important applications of nanotubes based on their properties will be as reinforcements in composite materials. One of the biggest concerns to nanotube industry is the alignment problem which has limited the usage and utilizations of carbon nanotubes in composites. The ability to impose a preferred alignment of carbon nanotubes in a composite will increase the effectiveness of utilizing nanotubes in composite applications. The alignment of nanotubes will maximize the interfacial bonding across the nanotube matrix interface.

In this research, we developed a methodology and a process to align nanotubes in polymer nanocomposites by means of a magnetic field. By doing so, we will get a very strong nanocomposite that can be used in the composites industry. The proposed mechanism aims at aligning the carbon nanotubes by means of nanomagnetic particles that are adsorbed on the nanotube surfaces and by applying an external magnetic field. SEM analysis have shown that nanomagnetic particles with the assistance of the magnetic field were able to align the carbon nanotubes in the desired direction.

FLUID DYNAMICS ISSUES IN SYNTHESIS OF CARBON NANOTUBES

The majority of carbon nanotubes' synthesis processes occur in the presence of fluid (liquid, gas, plasma, or multi-phase flow) that may function as a carrier of catalyst particles, feedstock of carbon, and the heating or cooling agent. The fluid motion defines the temperature of catalyst particles and the local chemical composition of the fluid that determines the success of synthesis of high-purity nanotubes. In this review paper, the laser ablation process, high-pressure carbon oxide process, and chemical vapor deposition process are considered from the prospective of fluid dynamics modeling. The multi-model approach should be used for concurrent rendering of different areas of computational domain by different models and/or different time steps for the same model. For multiple plume ejection in laser ablation, the near-target area could be rendered by molecular dynamics approach whereas continuous gas dynamics algorithms should be employed to simulate plume dynamics of previously ejected plumes apart of the target. Such an approach combines continuous mechanics of multi-species flow of feedstock gas or plume; micro-fluidic flow model that is needed to find heat and mass transfer rate to catalysts in presence of individual nanotubes in close proximity to each other; and molecular dynamics of evaporation and ejection of plume in laser ablation.

TOWARDS THE SINGLE-WALLED B- AND/OR N-DOPED CARBON NANOTUBES

Soon after the discovery of carbon nanotubes (CNTs) in the early 1990's, the B - and/or N -doped CNTs began to attract increasing interest owing to their modified structural, physical and chemical properties. In comparison with the multi-walled nanotubes and nanofibers, substitutional doping of the single-walled nanotubes (SWNTs) has proved to be much more difficult, and it is only in very recent years that some experimental studies concerning the B - and/or N -doped SWNTs are emerging. This paper intends to provide an up-to-date overview of current research on the doped SWNTs, with scopes covering both the binary system of CB x- and CN x-SWNTs and the ternary B x C y N z-SWNTs. A survey of the latest achievements in the syntheses of doped SWNTs through either the direct syntheses methods or the post-synthetic substitution reaction route is first presented; then the aspects concerning their structural features, elemental compositions, dopants bonding configuration and atomic distributions, as well as their physical and chemical properties are discussed.

SYNTHESIS OF 2D QUASI-ALIGNED MULTIWALLED CARBON NANOTUBES BY CATALYTIC CHEMICAL VAPOR DEPOSITION METHOD

Alignment or patterning of carbon nanotubes (CNTs) is particularly important for fabricating functional devices such as field emitters, nanophotonics, nanoelectronics, and ultrahydrophobic materials. This work reports on the synthesis of 2D quasi-aligned carbon nanotube bundles by catalytic chemical vapor deposition (CCVD) method using a series of catalysts and a study of their performance in a nutshell.

Low temperature thermocompression bonding between aligned carbon nanotubes and metallized substrate

Vertically aligned carbon nanotube (VACNT) turf is proposed for use as an electrical and thermal contact material. For these applications, one route for circumventing the high temperatures required for VACNT growth using chemical vapor deposition (CVD) is used to grow firstly VACNTs on one substrate and then transfer them to other substrates. In this work, a nano thermocompression bonding technique between VACNTs and a metallized substrate is developed to allow dry mechanical transfer of the VACNTs. Unlike the diffusion bonding between two bulk materials, nano metal clusters have a high surface energy and the atoms are very active to form alloy with the contacted bulk metal material even at much lower temperatures, so nano thermocompression bonding can decrease the bonding temperature (150 °C) and pressure (1 MPa) and greatly shorten the bonding time from hours to 20 min. A debonding experiment shows that the bonding strength between VACNTs and the metallized layer is so high that a break is less likely to occur at the bonding interface.

Enzymes immobilized on carbon nanotubes

Enzyme immobilizations on carbon nanotubes for fabrication of biosensors and biofuel cells and for preparation of biocatalysts are rapidly emerging as new research areas. Various immobilization methods have been developed, and in particular, specific attachment of enzymes on carbon nanotubes has been an important focus of attention. The method of immobilization has an effect on the preservation of the enzyme structure and retention of the native biological function of the enzyme. In this review, we focus on recent advances in methodology for enzyme immobilization on carbon nanotubes.

Carbon nanotube mass production: principles and processes

Our society requires new materials for a sustainable future, and carbon nanotubes (CNTs) are among the most important advanced materials. This Review describes the state-of-the-art of CNT synthesis, with a focus on their mass-production in industry. At the nanoscale, the production of CNTs involves the self-assembly of carbon atoms into a one-dimensional tubular structure. We describe how this synthesis can be achieved on the macroscopic scale in processes akin to the continuous tonne-scale mass production of chemical products in the modern chemical industry. Our overview includes discussions on processing methods for high-purity CNTs, and the handling of heat and mass transfer problems. Manufacturing strategies for agglomerated and aligned single-/multiwalled CNTs are used as examples of the engineering science of CNT production, which includes an understanding of their growth mechanism, agglomeration mechanism, reactor design, and process intensification. We aim to provide guidelines for the production and commercialization of CNTs. Although CNTs can now be produced on the tonne scale, knowledge of the growth mechanism at the atomic scale, the relationship between CNT structure and application, and scale-up of the production of CNTs with specific chirality are still inadequate. A multidisciplinary approach is a prerequisite for the sustainable development of the CNT industry.

Recent progress on the growth mechanism of carbon nanotubes: a review

Tremendous progress has been achieved during the past 20 years on not only improving the yields of carbon nanotubes and move progressively towards their mass production, but also on gaining a profound fundamental understanding of the nucleation and the growth processes. Parameters that influence the yield but also the quality (e.g., microstructure, homogeneity within a batch) are better understood. The influence of the carbon precursor, the reaction conditions, the presence of a catalyst, the chemical and physical status of the latter, and other factors have been extensively studied. The purpose of the present Review is not to list all the experiments reported in the literature, but rather to identify trends and provide a comprehensive summary on the role of selected parameters. The role of the catalyst occupies a central place in this Review as a careful control of the metal particle size, particle dispersion on the support, the metastable phase formed under reaction conditions, its possible reconstruction, and faceting strongly influence the diameter of the carbon nanotubes, their structure (number of walls, graphene sheet orientation, chirality), their alignment, and the yield. The identified trends will be compared with recent observations on the growth of graphene. Recent results on metal-free catalysts will be analyzed from a different perspective.

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.

Highly organized two- and three-dimensional single-walled carbon nanotube-polymer hybrid architectures

Single-walled carbon nanotube (SWCNT) network architectures combined with flexible mediums (especially polymers) are strong candidates for functional flexible devices and composite structures requiring the combination of unique electronic, optical, and/or mechanical properties of SWCNTs and polymer materials. However, to build functional flexible devices with SWCNTs, it is required to have abilities to assemble and incorporate SWCNTs in desired locations, orientations, and dimensions on/inside polymer substrates. Here, we present unique two- and three-dimensional SWCNT network-polymer hybrid architectures by combining unprecedented control over growth, assembly, and transfer processes of SWCNTs. Several SWCNT architectures have been built on polymer materials ranging from two-dimensional suspended SWCNT microlines on PDMS microchannels to three-dimensional "PDMS-vertically aligned SWCNTs-PDMS" sandwich structures. Also a combined lateral SWCNT microline and vertically aligned SWCNT flexible device was demonstrated with good electrical conductivity and low junction resistance. The results reported here open the pathway for the development of SWCNT-based functional systems in various flexible device applications.

Studies towards synthesis, evolution and alignment characteristics of dense, millimeter long multiwalled carbon nanotube arrays

We report the synthesis of aligned arrays of millimeter long carbon nanotubes (CNTs), from benzene and ferrocene as the molecular precursor and catalyst respectively, by a one-step chemical vapor deposition technique. The length of the grown CNTs depends on the reaction temperature and increases from ~85 µm to ~1.4 mm when the synthesis temperature is raised from 650 to 1100 °C, while the tube diameter is almost independent of the preparation temperature and is ~80 nm. The parallel arrangement of the CNTs, as well as their tube diameter can be verified spectroscopically by small angle X-ray scattering (SAXS) studies. Based on electron diffraction scattering (EDS) studies of the top and the base of the CNT films, a root growth process can be deduced.

Macroscopic carbon nanotube assemblies: preparation, properties, and potential applications

As classical 1D nanoscale structures, carbon nanotubes (CNTs) possess remarkable mechanical, electrical, thermal, and optical properties. In the past several years, considerable attention has been paid to the use of CNTs as building blocks for novel high-performance materials. In this way, the production of macroscopic architectures based on assembled CNTs with controlled orientation and configurations is an important step towards their application. So far, various forms of macroscale CNT assemblies have been produced, such as 1D CNT fibers, 2D CNT films/sheets, and 3D aligned CNT arrays or foams. These macroarchitectures, depending on the manner in which they are assembled, display a variety of fascinating features that cannot be achieved using conventional materials. This review provides an overview of various macroscopic CNT assemblies, with a focus on their preparation and mechanical properties as well as their potential applications in practical fields.

Cold-atom scanning probe microscopy

Scanning probe microscopes are widely used to study surfaces with atomic resolution in many areas of nanoscience. Ultracold atomic gases trapped in electromagnetic potentials can be used to study electromagnetic interactions between the atoms and nearby surfaces in chip-based systems. Here we demonstrate a new type of scanning probe microscope that combines these two areas of research by using an ultracold gas as the tip in a scanning probe microscope. This cold-atom scanning probe microscope offers a large scanning volume, an ultrasoft tip of well-defined shape and high purity, and sensitivity to electromagnetic forces (including dispersion forces near nanostructured surfaces). We use the cold-atom scanning probe microscope to non-destructively measure the position and height of carbon nanotube structures and individual free-standing nanotubes. Cooling the atoms in the gas to form a Bose-Einstein condensate increases the resolution of the device.

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 Production of Carbon Nanotube Transistors: A Generic Platform for Chemical Sensors

We report our work on the fabrication of nanotube-based field effect transistors (NTFET). Nanotubes were grown by chemical vapor deposition using various approaches, including a new formulation of nanotube growth catalysts that were directly patterned using UV lithography. We also report NTFETs based on randomly oriented nanotube networks that have a modulation of one. Finally, we report that a systematical and statistical characterization of millions of devices has led to the development of a robust process that may be useful in large scale production of reproducible, nanotube-based FETs, which, in turn, can be used as a generic platform for chemical sensors.

Aligned Carbon Nanotubes Via Microwave Plasma Enhanced Chemical Vapor Deposition

Aligned multi-wall carbon nanotubes have been grown on silicon substrates by microwave plasma enhanced chemical vapor deposition using methane/ammonia mixtures. The concentration ratio of methane/ammonia in addition to substrate temperature was varied. The morphology, structure and alignment of carbon nanotubes were studied by scanning electron microscopy and transmission electron microscopy. Both concentric hollow and bamboo-type multi-wall carbon nanotubes were observed. Growth rate, size distribution, alignment, morphology, and structure of carbon nanotubes changed with methane/ammonia ratio and growth temperature. Preliminary results on field emission properties are also presented.

Electrical Switching Using Compliant Metal Infiltrated Multi-Wall Nanotube Arrays

The topology of conventional noble-metal-coated switch counterfaces creates modes of switch failure via fouling, arcing, and local melting when impacted. To avoid these failure phenomena, compliant conductive contact surfaces of vertically aligned multi-wall nanotube arrays grown on conductive substrates have been fabricated. Infiltration of the array by noble metal results in a robust compliant switch contact surface. Cyclic hot-switch testing of the nanotube based switch, via modified nano-indentation, results in performance surpassing conventional designs with stable resistance of 0.4Ω over 3000 cycles. Investigating the physical performance of the array shows the array is compacted less than 3% over the first 500 cycles with no observable compaction through the remaining cycles. The improvement in performance of the nanotube based switch is attributed to the ability for the compliant contact surface to conform to the probe tip geometry, increasing the effective contact surface area.

CHARACTERIZATION AND ELECTRICAL PROPERTIES OF ALIGNED CARBON NANOTUBES WITH HIGH ASPECT RATIO

High-quality multiwall carbon nanotubes (MWNTs) were produced by a simple one-step technique. The production of MWNTs was based on thermal decomposition of the mixture of a liquid phase organic compound and ferrocene. High degree of alignment was noticed by scanning electron microscopy. The aspect ratio of as-synthesized MWNTs was quite high (more than 4500). Transmission electron microscopy analysis showed the presence of the catalytic iron nanorods at various lengths of MWNTs. Raman spectroscopy was used to know the quality of MWNTs. The ratio of intensity of the G-peak to the D-peak was very high which revealed high quality of MWNTs. Magnetotransport studies were carried out at low temperature and a negative MR was noticed.

High-voltage electrophoretic deposition for vertically aligned forests of one-dimensional nanoparticles

Deposition of aligned forests of 1D nanoparticles (carbon nanotubes and MnO(2) nanorods) on conductive, including flexible and transparent, substrates has been achieved at room temperature. The process, named high-voltage electrophoretic deposition (HVEPD), has been enabled by three key elements: high deposition voltage for alignment, low dispersion concentration of the nanoparticles to avoid aggregation, and simultaneous formation of a holding layer by electrodeposition. The effects of key parameters are investigated. The alignment on the vertical direction has been revealed by scanning electron microscopy of the samples, their superhydrophobicity, electrochemical performance, and capability to electrically connect two separated electrodes. Compared with their randomly oriented counterparts, the aligned nanoforests showed higher electrochemical capacitance, lower electrical resistance, and the capability to achieve superhydrophobicity, implicating their potential in a broad range of applications.

Scalable synthesis of aligned carbon nanotubes bundles using green natural precursor: neem oil

Practical application of aligned carbon nanotubes (ACNTs) would have to be determined by a matter of its economical and large-scale preparation. In this study, neem oil (also named Margoaa oil, extracted from the seeds of the neem--Azadirachta indica) was used as carbon source to fabricate the bundles of ACNTs. ACNTs have been synthesized by spray pyrolysis of neem oil and ferrocene mixture at 825°C. The major components of neem oil are hydrocarbon with less amount of oxygen, which provided the precursor species in spray pyrolysis growth of CNTs. The bundles of ACNTs have been grown directly inside the quartz tube. The as-grown ACNTs have been characterized through Raman spectroscopy, scanning and transmission electron microscopic (SEM/TEM) techniques. SEM images reveal that the bundles of ACNTs are densely packed and are of several microns in length. High-resolution TEM analysis reveals these nanotubes to be multi-walled CNTs. These multi-walled CNTs were found to have inner diameter between 15 and 30 nm. It was found that present technique gives high yield with high density of bundles of ACNTs.

Carbon nanotubes: engineering biomedical applications

Carbon nanotubes (CNTs) are cylinder-shaped allotropic forms of carbon, most widely produced under chemical vapor deposition. They possess astounding chemical, electronic, mechanical, and optical properties. Being among the most promising materials in nanotechnology, they are also likely to revolutionize medicine. Among other biomedical applications, after proper functionalization carbon nanotubes can be transformed into sophisticated biosensing and biocompatible drug-delivery systems, for specific targeting and elimination of tumor cells. This chapter provides an introduction to the chemical and electronic structure and properties of single-walled carbon nanotubes, followed by a description of the main synthesis and post-synthesis methods. These sections allow the reader to become familiar with the specific characteristics of these materials and the manner in which these properties may be dependent on the specific synthesis and post-synthesis processes. The chapter ends with a review of the current biomedical applications of carbon nanotubes, highlighting successes and challenges.

Plugging into proteins: poisoning protein function by a hydrophobic nanoparticle

Nanoscale particles have become promising materials in many fields, such as cancer therapeutics, diagnosis, imaging, drug delivery, catalysis, as well as biosensors. In order to stimulate and facilitate these applications, there is an urgent need for the understanding of the nanoparticle toxicity and other risks involved with these nanoparticles to human health. In this study, we use large-scale molecular dynamics simulations to study the interaction between several proteins (WW domains) and carbon nanotubes (one form of hydrophobic nanoparticles). We have found that the carbon nanotube can plug into the hydrophobic core of proteins to form stable complexes. This plugging of nanotubes disrupts and blocks the active sites of WW domains from binding to the corresponding ligands, thus leading to the loss of the original function of the proteins. The key to this observation is the hydrophobic interaction between the nanoparticle and the hydrophobic residues, particularly tryptophans, in the core of the domain. We believe that these findings might provide a novel route to the nanoparticle toxicity on the molecular level for the hydrophobic nanoparticles.

Fabrication, Characterisation and Application of Carbon Nanotube Polymer Composites

Carbon nanotubes (CNTs) have been used in making composites because CNTs have high strength, large aspect-ratio and excellent thermal and electrical conductivity. However, to realize the wide applications of CNT composites, further R&D must be carried out. This review will discuss some fabrication, characterisation and application issues of CNT polymer composites. Aspects to outline are purification, dispersion, alignment, stress transfer, interface bonding, wear and friction and rheological properties. Some research challenges will be briefly highlighted as well, including the mass production of long and aligned CNTs at low cost, and the optimization of the microstructures, properties and functioning features of CNT composites.

Aligned Si(3)N(4)@SiO(2) coaxial nanocables derived from a polymeric precursor

Well-aligned coaxial nanocables, composed of a crystalline alpha-Si(3)N(4) inner core and amorphous SiO(2) outer shell, were prepared on silicon substrates by pyrolysis of a preceramic polymer (perhydropolysilazane) with iron as catalyst. The nanocables have high density, and the longest nanocable can be up to millimeters. Photoluminescence measurement reveals a strong ultraviolet emission band centered at 360 nm and a weaker visible-light emission at 625 nm. The growth mechanism of the nanocables is discussed in detail.

Electrochemical patterning of transparent single-walled carbon nanotube films on plastic substrates

We report a new patterning method for single-walled carbon nanotubes (SWCNTs) films on flexible, transparent poly(ethylene terephthalate) using electrochemical etching in an aqueous electrolyte solution. Electrochemical etching of the SWCNT films patterned with photoresist polymer was accomplished in a three-electrode system, and the electrochemically patterned SWCNT films were then characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The voltammetry curve showed that SWCNTs underwent drastic oxidation above an applied potential of 1.315 V with the generation of gas bubbles, and the oxidation current became constant above 2.6 V due to the mass transfer limit. SEM images showed that the networks of SWCNTs in the area protected with the photoresist polymer had no damage and vivid connections were obvious, while the connections and shapes of SWCNTs in the area exposed to electrochemical etching were indistinct and slightly damaged. In the Raman spectra of the area protected with the photoresist polymer and the exposed SWCNT area, the intensity ratio of the D-line to the G-line increased from 0.077 to 1.136, which indicated that the ordered carbons of the SWCNT film gradually became amorphous carbons due to electrochemical etching. For optimal patterning, the electrochemical etchings of SWCNT films were performed under various conditions (the applied potential, pH of the electrolyte solution, and electrolyte concentration). An applied potential of 3.0 V in 0.1 M NaCl electrolyte solution (pH 7.0) was optimal for homogeneous electrochemical patterning of SWCNT films. In an electrochemiluminescence reaction, the SWCNT films patterned by this technique could be used successfully as flexible and transparent electrodes.