Electrochemical characterization of carbon nanotube forests grown on copper foil using transition metal catalysts

Growth of vertical, multiwalled carbon nanotubes (CNTs) on bulk copper foil substrates can be achieved by sputtering either Ni or Inconel thin films on Cu substrates followed by thermal chemical vapor deposition using a xylene and ferrocene mixture. During CVD growth, Fe nanoparticles from the ferrocene act as a vapor phase delivered catalyst in addition to the transition metal thin film, which breaks up into islands. Both the thin film and iron are needed for dense and uniform growth of CNTs on the copper substrates. The benefits of this relatively simple and cost effective method of directly integrating CNTs with highly conductive copper substrates are the resulting high density of nanotubes that do not require the use of additional binders and the potential for low contact resistance between the nanotubes and the substrate. This method is therefore of interest for charge storage applications such as double layer capacitors. Inconel thin films in conjunction with Fe from ferrocene appear to work better in comparison to Ni thin films in terms of CNT density and charge storage capability. We report here the power density and specific capacitance values of the double layer capacitors developed from the CNTs grown directly on copper substrates.     

Robust and Aligned Carbon Nanotube/Titania Core/Shell Films for Flexible TCO‐Free Photoelectrodes

Carbon nanotube (CNT)/semiconducting oxide hybrids are an ideal architecture for light‐harvesting devices, in which the CNTs are expected to not only act as a scaffold but also provide fast transport paths for photogenerated charges in the oxide. However, the current potential of CNTs for charge transport is largely suppressed due to the nanotubes not being interconnected but isolated by the low conductive oxide coatings. Herein, a flexible and conductive CNT/TiO₂ core/shell heterostructure film is reported, with aligned and interconnected CNTs wrapped in a continuous TiO₂ coating. Without using additional transparent conducting oxide (TCO) substrates, this unique feature of the film boosts the incident photon‐to‐electron conversion efficiency to 32%, outperforming TiO₂ nanoparticle electrodes fabricated on TCO substrates. Moreover, the film shows high structural stability and can generate a stable photocurrent even after being bent hundreds of times.     

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.     

Flexible High‐Conductivity Carbon‐Nanotube Interconnects Made by Rolling and Printing

Applications of carbon nanotubes (CNTs) in flexible and complementary metal‐oxide‐semiconductor (CMOS)‐based electronic and energy devices are impeded due to typically low CNT areal densities, growth temperatures that are incompatible with device substrates, and challenges in large‐area alignment and interconnection. A scalable method for continuous fabrication and transfer printing of dense horizontally aligned CNT (HA‐CNT) ribbon interconnects is presented. The process combines vertically aligned CNT (VA‐CNT) growth by thermal chemical vapor deposition, a novel mechanical rolling process to transform the VA‐CNTs to HA‐CNTs, and adhesion‐controlled transfer printing without needing a carrier film. The rolling force determines the HA‐CNT packing fraction and the HA‐CNTs are processed by conventional lithography. An electrical resistivity of 2 mΩ · cm is measured for ribbons having 800‐nm thickness, while the resistivity of copper is 100 times lower, a value that exceeds most CNT assemblies made to date, and significant improvements can be made in CNT structural quality. This rolling and printing process could be scaled to full wafer areas and more complex architectures such as continuous CNT sheets and multidirectional patterns could be achieved by straightforward design of the CNT growth process and/or multiple rolling and printing sequences.     

Transferable thin films of pristine carbon nanotubes

We describe here a simple and low-cost method to prepare ultra-thin, homogeneous, and transferable films of pristine carbon nanotubes (CNTs). The highly efficient chemical vapor deposition (CVD) growth method involves silica supported catalysts and alcohol vapor as gaseous carbon source. By varying the amount of catalysts, the thickness of synthesized films can be easily tuned from 20 nm (sub-monolayer) to 150 nm in a controlled fashion. High-resolution transmission electron microscopy (HRTEM) revealed that the films are composed primarily of single-walled and a small fraction of double-walled CNTs. A nonlinear relationship between film conductivity and thickness was observed. Our sub-monolayer ( 20 nm) film, which is noticeably thinner than conductive CNT films synthesized using other methods (typically > 50 nm and up to 100 microm), shows the highest conductivity of 400 mho x cm(-1) with 90% transparency in the visible range and close to 100% transparency in the infrared range. This ultra-thin film can also be transferred carrier-film free to a wide range of substrates including low-cost plastics for flexible electronics. Compared to CNT films prepared by filtration techniques, our films demonstrated superior stability against mechanical bending.     

Coating of carbon nanotubes with amorphous carbon nitride thin films and characterization of long-term emission stability

The effects of amorphous carbon nitride (CN) thin films that were coated on carbon nanotubes (CNTs) and their thermal treatment were investigated, in terms of the chemical bonding and morphologies of the CNTs and their field emission properties. CNTs were directly grown on conical tip-type tungsten substrates via the inductively coupled plasma-chemical vapor deposition (ICP-CVD) system, and the CNTs were coated with CN films using the RF magnetron sputtering system. The CN-coated CNTs were thermally treated using the rapid thermal annealing (RTA) system by varying the temperature (300-700 °C). The morphologies, microstructures, and chemical compositions of the CN-coated CNTs were analyzed as a function of the thickness of the CN layers and the RTA temperatures. The field emission properties of the CN/CNT hetero-structured emitters, and the fluctuation and long-term stability of the emission currents were measured and compared with those of the conventional non-coated CNT-emitter. The results showed that the electron emission capability of CNT was noticeably improved by coating a thin CN layer on the surface of the CNT. This was attributed to the low work function and negative electron affinity nature of the CN film. The CN-coated CNT-emitter had a more stable emission characteristic than that of the non-coated one. In addition, the long-term emission stability of the CN-coated emitter was further enhanced by thermal treatment, which was verified by x-ray photoelectron spectroscopy (XPS) analysis.     

Etching processes of transparent carbon nanotube thin films using laser technologies

Carbon nanotubes (CNTs) have potential as a transparent conductive material with good mechanical and electrical properties. However, carbon nanotube thin film deposition and etching processes are very difficult to pattern the electrode. In this study, transparent CNT film with a binder is coated on a PET flexible substrate. The transmittance and sheet resistance of carbon nanotube film are 84% and 1000 Ω/□, respectively. The etching process of carbon nanotube film on flexible substrates was investigated using 355 nm and 1064 nm laser sources. Experimental results show that carbon nanotube film can be ablated using laser technology. With the 355 nm UV laser, the minimum etched line width was 20 μm with a low amount of recast material of the ablated sections. The optimal conditions of laser ablation were determined for carbon nanotube film.     

Highly conductive and transparent carbon nanotube composite thin films deposited on polyethylene terephthalate solution dipping

In this letter we present highly conductive and transparent thin films of single-walled carbon nanotubes (SWCNT) and conductive polymer composite deposited on polyethylene terephthalate film substrates by solution dipping. The initial results show that 66 Ω/? sheet resistance can be achieved with 80% transmission at the wavelength of 550 nm. This result is much superior to the performances of the pure SWCNT thin films deposited using the same technique. The improvement is attributed to the increase of effective electric conductive tube-tube junctions in the CNT network.     

Formicary-like carbon nanotube/copper hybrid nanostructures for carbon fiber-reinforced composites by electrophoretic deposition

An electrophoretic deposition process has been applied to produce unique carbon nanotube (CNT)/copper nanostructures on the carbon fiber surfaces. During the deposition process, ionized copper and positively charged CNTs are accelerated towards the carbon fiber under applied electric fields. An interconnected formicary-like network of nanotubes and nanoparticles is formed where copper nucleation and growth occurs predominantly at nanotube crossing and edge-contact locations. When embedded in a structural composite the CNT/copper structures create a highly conductive and strongly bonded network shown by significant enhancements in both electrical conductivity and interlaminar shear strength as compared to composites without the CNT/copper nanostructures.     

Field emission properties of RuO2 thin film coated on carbon nanotubes

Carbon nanotubes (CNTs) have been widely applied in field emission (FE) due to their high geometric aspect ratio and low work function. More recently, researchers have introduced ruthenium dioxide (RuO₂) as a field emitter because of its excellent chemical and thermal stability due to its oxide nature. This study used the surface morphology of CNTs and the field emission (FE) stability of RuO₂ to improve FE characteristics. Since the work functions of CNTs and RuO₂ are very close, this study combined these two elements by applying a thin film of RuO₂ on the CNT surface. In the process of covering the tips of CNTs with a thin film of RuO₂ eventually obtained the best matching of these two elements. The study not only enhanced the FE performance of CNTs but also extended FE lifetime by applying a thin film of RuO₂ on the CNT tips.     

Field emission properties of RuO2 thin film coated on carbon nanotubes

Carbon nanotubes (CNTs) have been widely applied in field emission (FE) due to their high geometric aspect ratio and low work function. More recently, researchers have introduced ruthenium dioxide (RuO₂) as a field emitter because of its excellent chemical and thermal stability due to its oxide nature. This study used the surface morphology of CNTs and the field emission (FE) stability of RuO₂ to improve FE characteristics. Since the work functions of CNTs and RuO₂ are very close, this study combined these two elements by applying a thin film of RuO₂ on the CNT surface. In the process of covering the tips of CNTs with a thin film of RuO₂ eventually obtained the best matching of these two elements. The study not only enhanced the FE performance of CNTs but also extended FE lifetime by applying a thin film of RuO₂ on the CNT tips.     

Interface tailoring to enhance mechanical properties of carbon nanotube reinforced magnesium composites

This study highlights the use of a metallic coating of nanoscale thickness on carbon nanotube to enhance the interfacial characteristics in carbon nanotube reinforced magnesium (Mg) composites. Comparisons between two reinforcements were targeted: (a) pristine carbon nanotubes (CNTs) and (b) nickel-coated carbon nanotubes (Ni–CNTs). It is demonstrated that clustering adversely affects the bonding of pristine CNTs with Mg particles. However, the presence of nickel coating on the CNT results in the formation of Mg₂Ni intermetallics at the interface which improved the adhesion between Mg/Ni–CNT particulates. The presence of grain size refinement and improved dispersion of the Ni–CNT reinforcements in the Mg matrix were also observed. These result in simultaneous enhancements of the micro-hardness, ultimate tensile strength and 0.2% yield strength by 41%, 39% and 64% respectively for the Mg/Ni–CNT composites in comparison with that of the monolithic Mg.     

Preconcentration of volatile organics on self-assembled, carbon nanotubes in a microtrap

This paper reports the self-assembly of carbon nanotubes (CNTs) on the inside wall of a steel capillary to fabricate a microtrap for the adsorption/desorption of trace organics. The microtrap functioned as a nanoconcentrator and an injector for gas chromatography (GC). The CNTs were deposited as a thin film by catalytic chemical vapor deposition from either CO or C2H4 as the precursor. The sorbent film synthesized from C2H4-CVD (CVD = chemical vapor deposition) had higher CNT density and thus was a stronger sorbent. In general, the CNT microtraps showed high-capacity adsorption and fast quantitative desorption, and the process showed excellent precision. This study demonstrates that CNT films can be deposited quite easily in a steel capillary for use in different analytical applications, and CNT films can perform as efficiently as packed-bed carbon sorbents.     

Direct growth of aligned carbon nanotubes on bulk metals

There are several advantages of growing carbon nanotubes (CNTs) directly on bulk metals, for example in the formation of robust CNT-metal contacts during growth. Usually, aligned CNTs are grown either by using thin catalyst layers predeposited on substrates or through vapour-phase catalyst delivery. The latter method, although flexible, is unsuitable for growing CNTs directly on metallic substrates. Here we report on the growth of aligned multiwalled CNTs on a metallic alloy, Inconel 600 (Inconel), using vapour-phase catalyst delivery. The CNTs are well anchored to the substrate and show excellent electrical contact with it. These CNT-metal structures were then used to fabricate double-layer capacitors and field-emitter devices, which demonstrated improved performance over previously designed CNT structures. Inconel coatings can also be used to grow CNTs on other metallic substrates. This finding overcomes the substrate limitation for nanotube growth which should assist the development of future CNT-related technologies.     

Single Carbon Fibers with a Macroscopic‐Thickness, 3D Highly Porous Carbon Nanotube Coating

Carbon fiber (CF) grafted with a layer of carbon nanotubes (CNTs) plays an important role in composite materials and other fields; to date, the applications of CNTs@CF multiscale fibers are severely hindered by the limited amount of CNTs grafted on individual CFs and the weak interfacial binding force. Here, monolithic CNTs@CF fibers consisting of a 3D highly porous CNT sponge layer with macroscopic‐thickness (up to several millimeters), which is directly grown on a single CF, are fabricated. Mechanical tests reveal high sponge–CF interfacial strength owing to the presence of a thin transitional layer, which completely inhibits the CF slippage from the matrix upon fracture in CNTs@CF fiber–epoxy composites. The porous conductive CNTs@CF hybrid fibers also act as a template for introducing active materials (pseudopolymers and oxides), and a solid‐state fiber‐shaped supercapacitor and a fiber‐type lithium‐ion battery with high performances are demonstrated. These CNTs@CF fibers with macroscopic CNT layer thickness have many potential applications in areas such as hierarchically reinforced composites and flexible energy‐storage textiles.     

Tailoring Thickness of Conformal Conducting Polymer Decorated Aligned Carbon Nanotube Electrodes for Energy Storage

Aligned Carbon nanotubes (A‐CNT) based electrodes have emerged as high‐performance elements in electric energy storage and conversion devices. Morphological tailoring of conformal coatings of poly(ethylenedioxythiophene) (PEDOT) conductive polymer (CP) on the A‐CNT scaffold is demonstrated by controlling CP thickness at the nm scale. Results show that the CP nano‐films dominate the electrode capacitance in a supercapacitor application, contributing as much as 10x (pseudo)capacitance over the electric double layer of pristine A‐CNT due to volumetric vs. surface charge storage. Comparison to theoretical ion mobilities shows that the conformal CP films have active sites at ∼30% doping, indicating the CP quality is similar to thin films on flat substrates and that all these sites are accessed at all CP thickness values (up to 10 nm PEDOT thickness) and do not limit the rate of ion transport in and out of the CP film volume. Supercapacitor electrodes fabricated from these novel morphology‐controlled nanostructured composites provide a new route towards high‐performance next generation energy storage devices.     

Two-dimensional distribution of carbon nanotubes in copper flake powders

We report an approach of flake powder metallurgy to the uniform, two-dimensional (2D) distribution of carbon nanotubes (CNTs) in Cu flake powders. It consists of the preparation of Cu flakes by ball milling in an imidazoline derivative (IMD) aqueous solution, surface modification of Cu flakes with polyvinyl alcohol (PVA) hydrosol and adsorption of CNTs from a CNT aqueous suspension. During ball milling, a hydrophobic monolayer of IMD is adsorbed on the surface of the Cu flakes, on top of which a hydrophilic PVA film is adsorbed subsequently. This PVA film could further interact with the carboxyl-group functionalized CNTs and act to lock the CNTs onto the surfaces of the Cu flakes. The CNT volume fraction is controlled easily by adjusting the concentration/volume of CNT aqueous suspension and Cu flake thickness. The as-prepared CNT/Cu composite flakes will serve as suitable building blocks for the self-assembly of CNT/Cu laminated composites that enable the full potential of 2D distributed CNTs to achieve high thermal conductivity.     

Investigation of the effective parameters in the growth of carbon nanotubes by thermal chemical vapour deposition method

Synthesis and growth of carbon nanotubes (CNTs) from C₂H₂ by thermal chemical vapour deposition (TCVD) using a mixture of different gases were investigated. A thin film of nickel was coated as catalyst on silicon substrates by ion beam sputtering technique. Various parameters such as thickness of oxide layer and time, as well as reduction temperature were investigated in view of obtaining the best conditions for CNTs growth. C₂H₂ was very effective as carbon feedstock and NH₃ pretreatments were crucial steps towards obtaining a high density of nucleation sites for CNTs growth by inhibiting amorphous carbon generation in the initial stage of the synthesis. The substrate oxide layer was analysed by secondary ion mass spectrometry. The prepared CNTs were confirmed by Raman spectroscopy and were further characterised using scanning electron microscopy and transmission electron microscopy.     

Growth and characterization of bamboo-shaped carbon nanotubes using nanocluster-assembled ZnO:Co thin films as catalyst

Bamboo-shaped carbon nanotubes (CNTs) had been successfully fabricated by a plasma enhanced chemical vapor deposition method, in which nanocluster-assembled ZnO:Co thin film was used as catalyst. It was found that bamboo-shaped CNTs were generally grown in a direction perpendicularly to the substrate surface with the tops of CNTs dominated by the droplet-like catalyst covered by the carbon layer. The diameter of CNTs was ranged from 20-50 nm. High resolution of TEM image showed that the typical CNT had a multi-walled structure with an inner core presented. The ordered graphite layers were inclined to an axis of CNT about 18 degrees and the interlayer space of a CNT was about 0.35 nm. Two peaks in Raman spectrum at 1586 cm(-1) and 1372 cm(-1) were identified as G-band and D-band for graphite, respectively. The results showed that catalyst based on ZnO:Co thin films could be used for the growth of CNTs with bamboo-shaped structure.     

Templated growth of carbon nanotubes with controlled diameters using organic-organometallic block copolymers with tailored block lengths

Block copolymer thin films fabricated from polystyrene-polyferrocenylsilane (PS-b-PFS) block copolymers on silicon substrates were used as precursors of well-ordered, nanosized growth catalysts for carbon nanotubes (CNTs). The size of the catalytic domains was tuned by changing the molecular weight of the block copolymer, enabling control of the diameter of the CNTs grown from these substrates. CNT growth on catalytic substrates with larger organometallic domain sizes, using acetylene as a carbon source, resulted in enhanced amounts of CNT deposition compared to smaller PFS domains, which exhibited low catalytic activity. The inner and outer diameters of the multi-walled CNTs obtained were typically 8 and 16 nm, respectively, and were not influenced by the catalytic domain sizes. Various annealing strategies in inert or in hydrogen atmosphere were investigated. The use acetylene with an additional hydrogen flow as gas feed resulted in a significant increase in deposition on all PS-b-PFS decorated substrates. Under these conditions, the CNT diameters could be controlled by the catalyst domain sizes, resulting in decreasing diameters with decreasing domain sizes. Multiwalled CNTs with inner and outer diameters of 4 and 7 nm, respectively, and a narrow diameter distribution were obtained.