Vertically aligned carbon nanotubes grown on geometrically different types of surface

Aligned carbon nanotubes (CNTs) which were always perpendicular to the surface of substrate were synthesized on geometrically different surface through microwave plasma chemical vapour deposition (MWPCVD) at the low temperature of 550 °C. Growth was performed in a flowing mixture of CH₄ and H₂. Scanning electron microscopy (SEM) shows that the vertically aligned growth occurred under the effect of plasma. When the substrate was not contacted with plasma, only randomly entangled CNTs had grown on the substrate. The research results demonstrate that the electrical self-bias imposed on the surface is the primary mechanism responsible for the alignment of CNTs.     

Diameter-controlled and nitrogen-doped vertically aligned single-walled carbon nanotubes

Diameter controlled and vertically aligned single-walled carbon nanotubes were synthesized from pure and mixed ethanol/acetonitrile feedstock. With increasing acetonitrile concentration in the feedstock, nitrogen incorporation into the sp² carbon network increased until saturating at approximately one atomic percent. The incorporation of nitrogen correlates with a significant diameter reduction from a mean diameter of 2.1 nm down to 0.7 nm. Heteroatom-mediated diameter control is independent of catalyst preparation and represents a versatile tool for the direct synthesis of tailored single-walled carbon nanotubes.     

Covering vertically aligned carbon nanotubes with a multiferroic compound

This work highlights the possible use of vertically-aligned multiwall carbon nanotubes (VA-MWCNTs) as bottom electrodes for microelectronics, for example for memory applications. As a proof of concept BiFeO₃ (BFO) films were fabricated in-situ deposited on the surface of VA-MWCNTs by RF (radio frequency) magnetron sputtering. For in situ deposition temperature of 400 °C and deposition time up to 2 h, BFO films cover the MWCNTs and no damage occurs either in the film or MWCNTs. In spite of the macroscopic lossy polarization behaviour, the ferroelectric nature, domain structure and switching of these conformal BFO films was verified by piezo force microscopy. G type antiferromagnetic ordering with weak ferromagnetic ordering loop was proved for BFO films on VA-MWCNTs having a coercive field of 700 Oe.     

Fluffy carbon nanotubes produced by shearing vertically aligned carbon nanotube arrays

Fluffy carbon nanotubes (CNTs), which are cotton-like macroscopic structures, are obtained by simple high-speed shearing of vertically aligned CNT (VACNT) arrays. The fluffy CNTs are composed of CNT bundles with a diameter of several micrometers, and have an extremely low apparent density of 3-10 g/L. A requisite for their formation is the alignment of CNTs in the initial array. The shear between the rotor and the arrays tears the arrays along the axial direction and this results in their dispersion into low density fluffy CNTs.     

Synthesis of vertically aligned carbon nanotubes on metal deposited quartz plates

Without plasma aid, we have successfully synthesized vertically aligned carbon nanotubes (CNTs) on iron-, cobalt- or nickel-deposited quartz plates by chemical vapor deposition with ethylenediamine as a precursor. The amine serves as both etching reagent for the formation of metal nanoparticles and carbon source for the growth of aligned carbon nanotubes. The carbon nanotubes were vertically aligned in high density on a large area of the plain silica substrates. The density and diameter of CNTs is determined by the thickness of the deposited metal film and the length of the tubes can be controlled by varying the reaction time. High-resolution transmission electron microscopy analysis reveals that the synthesized CNTs are multiwalled with a bamboo-like structure. Energy dispersive X-ray spectra demonstrate that the CNTs are formed as tip growths. Raman spectrum provides definite evidence that the prepared CNTs are multiwalled graphitic structure.     

Oil sorption and recovery by using vertically aligned carbon nanotubes

We used carbon nanotubes as oil adsorbents and evaluated recycling performance by squeezing method. The sorption capacity of 3 mm long vertically aligned carbon nanotubes is almost 6.9 times higher than that of agglomerated carbon nanotubes due to the existence of large-sized macropores. Compared with exfoliated graphite (41 g/g), aligned carbon nanotubes exhibit higher sorption capacity (69 g/g) and better recycling performance due to their unique mechanical strength and excellent rebound resilience properties at high strains.     

Temperature gradient chemical vapor deposition of vertically aligned carbon nanotubes

We present temperature gradient chemical vapor deposition (TG CVD) for producing vertically aligned (VA-) carbon nanotubes (CNTs). Independent heaters on the gas inlet and catalyst substrate sides of a cold-wall, vertical CVD reactor can modulate the gas temperature gradient to lead to controlled thermal histories of acetylene precursor. Our growth results reveal that such a precursor thermal history can play a significant role in the growth and structural features of the resultant VA-CNTs. We find several gas thermal zones particularly important to the VA-CNT growth by evaluating the precursor dwell time in different zones. Thermal treatment of the acetylene precursor at 600–700 °C is found crucial for the synthesis of VA-CNTs. When this thermal zone is conjoined in particular with a zone >700 °C, efficient growths of single-walled and double-walled VA-CNTs can be achieved. These gas thermal zones can contribute to VA-CNT growths by mixing various secondary hydrocarbons with acetylene, corroborated by the results of our reacting flow simulation. Our findings emphasize the influence of gas-phase reactions on the VA-CNT growth and suggest that our TG CVD approach can be practically utilized to modulate complex gas-phase phenomena for the controlled growth of VA-CNTs.     

Thick vertically aligned carbon nanotube/carbon composite electrodes for electrical double-layer capacitors

We present a new method for synthesis of thick, self-standing porous carbon electrodes with improved physicochemical properties and unique porous structure. The synthesis is based on the use of vertically aligned carbon nanotubes (VACNT) as templates for polymer-based activated carbon materials. The VACNT template enables the production of 1 mm thick, binder-free electrodes with high capacity values even at high rates (>160 Fg⁻¹ at more than 1 Ag⁻¹ for 1 mm thick electrode), and very good stability upon cycling. The electrochemical performance after more than 50,000 cycles, the pore characterization by adsorption isotherms, and the structural analysis of the composite electrode are also reported.     

Patterned growth and field emission properties of vertically aligned carbon nanotubes

Vertically aligned carbon nanotubes were grown selectively on patterned Ni thin films by microwave plasma-enhanced chemical vapor deposition and their field emission properties were investigated using a diode-structure. Ni thin films patterned with a form of dot-arrays were prepared using a shadow mask having an array of holes. The nanotubes were found to be well-graphitized with multiwalled structures. The measurements of field emission properties revealed that the carbon nanotube tips emitted high current density at low macroscopic electric field. The Fowler–Nordheim (F–N) plot clearly showed two characteristic regions where the current saturates at the high electric field region. It was found that the saturation behavior was caused by the adsorbates-enhanced field emission mechanism. Eliminating the adsorbates resulted in no saturation behavior, increasing turn-on field, decreasing current, and increasing field enhancement factor. Using ZnS/Cu,Al phosphor, very bright and uniform emission patterns were obtained.     

Electrical properties of annealed MPCVD grown vertically aligned carbon nanotube films

Vertically aligned multiwalled carbon nanotubes (MWNTs) were grown on silicon substrate at a low temperature (<520 °C) using microwave plasma-enhanced chemical vapor deposition (MPCVD). From the Raman spectra, it was found that the I_D/I_G ratio of MWNTs decreased after annealing, indicating that more graphenes were formed by the annealing process. Nevertheless, a strong Si signal was found in Raman spectra after annealing at a high-temperature (600 °C). From X-ray photoelectron spectroscopy (XPS) analysis it was observed that the ratio of the oxygen to carbon (O/C) signal intensity was from 0.15 to 1.88 for the increasing annealed temperatures of MWNTs, and a Si signal was found nearby the surface of MWNTs after annealing at 600 °C. Moreover, from the I–V measurement, the less symmetric I–V characteristic was found for the metal/MWNTs/metal (MIM) sandwich structure of unannealed MWNTs. After 300 °C annealing process, the positive current was increase and the negative current was decrease. It was conjectured that the MWNTs could obtain more graphenes structure by the 300 °C annealing process. Moreover, the I–V trace of the sample annealed by 600 °C exhibited rapid current descent, indicating the oxygenated and partly silicided phenomena might cover outer graphite layer of MWNTs. The equivalent circuit for the MIM sandwich structure could be represented as two Schottky barrier diodes in a back-to-back configuration. From the data fitting, it was found that the Schottky barrier height (_B0) decreased and the current density (J) increased from unannealing to 300 °C annealing temperature. However, the Schottky barrier height (_B0) was increased from 300 to 600 °C annealing temperature. Comparison with the XPS, this may due to the oxygenated and partly silicided phenomenon on the surface of the MWNTs.     

Simultaneous synthesis of vertically aligned carbon nanotubes and amorphous carbon thin films on stainless steel

Carbonaceous nanostructures such as carbon nanotubes, graphene and transparent carbon-based thin films are envisioned to be part of the next generation of electronic devices, mechanical structures, and energy-storage systems. To synthesize these nanostructures on a large scale by chemical vapor deposition, large-area, flexible substrates are needed. Here, we studied the role of a metallic foil, stainless steel, as a self-catalytic substrate for carbon nanostructure synthesis. As a result, vertically aligned carbon nanotubes and amorphous carbon thin films were simultaneously obtained. We further showed that the evolution of the stainless steel foil during the different steps of the process played a critical role in carbon nanotubes and carbon thin film growth. A better understanding of how the growth of these carbon nanostructures is affected by stainless steel evolution under chemical vapor deposition conditions will enable the synthesis of hybrid carbon nanotubes/amorphous carbon nanostructures and pave the way to scale-up of their low-cost production.     

High-rate flame synthesis of vertically aligned carbon nanotubes using electric field control

The electric field controlled synthesis of carbon nanomaterials on a Ni-based catalytic support positioned at the fuel side of the opposed flow oxy-flame is studied experimentally. Carbon nanomaterials formed on the probe surface are comparatively analyzed for two characteristic operational modes: a grounded probe mode and a floating probe mode. In a grounded mode a number of various carbon nanostructures are formed depending on the probe location in flame. Observed nanoforms include multi-walled carbon nanotubes (MWNTs), MWNT bundles, helically coiled tubular nanofibers, and ribbon-like coiled nanofibers with rectangular cross-section. The presence of various carbon nanoforms is attributed to the space variation of flame parameters, namely flame temperature and concentration of chemical species. It is found that the presence of an electric potential (floating mode operation) provides the ability to control the nanostructure morphology and synthesis rate. A thick layer (35-40 μm) of vertically aligned carbon nanotubes (VACNTs) is found to be formed on the probe surface in the floating potential mode. This layer is characterized by high uniformity and narrow distribution of nanotube diameters. Overall, the electric field control method demonstrates stabilization of the structure in a wide flame region while growth rate remains dependent on flame location.     

Semi-quantitative study on the fabrication of densely packed and vertically aligned single-walled carbon nanotubes

This paper presents, for the first time, a semi-quantitative study on the production of densely packed and vertically aligned (DPVA) single-walled carbon nanotubes (SWNTs) from ultra-thin catalytic films. An up-to-date highest volume density (60-70 kg m⁻³) and the corresponding high surface density on the order of 10¹⁶ m⁻² of DPVA-SWNTs have been achieved by point-arc microwave plasma chemical vapor deposition. The precise thickness control of the sandwich-like catalytic nanostructure of 0.5 nm Al₂O₃/0.5 nm Fe/>5 nm Al₂O₃, developed by the authors, and a short-time (5 min) heat pretreatment of substrates at a temperature as low as 600 °C play the very key role in the process of fabricating DPVA-SWNTs.     

Synthesis of vertically aligned carbon nanotubes on submicron-sized dot-catalyst array using plasma CVD method

Vertically aligned carbon nanotubes were synthesized on submicron-sized dot-catalyst array which have a dot size of 400 nm and dot intervals varying from 0.3 μm to 10 μm, using direct current plasma enhanced chemical vapour deposition(PECVD) method. The dot-catalyst array was fabricated by lift-off process using electron beam lithography. Field emission characteristics were measured and compared for different dot intervals. Experimental results of field emission characteristics and the Fowler–Nordheim plots suggest that spatial control of dotted-catalyst is very important to improve the electric field emission characteristics. The results show that the highest field enhancement factor β was obtained when the dot interval was two times longer than the length of CNTs.     

Fabrication and field emission property studies of vertically aligned multiwalled carbon nanotubes grown by double plasma chemical vapour deposition technique

The field emissive carbon nanotubes (CNTs) with nickel tips were grown by a novel technique, double plasma hot filament chemical vapor deposition technique. Surface morphologies and structure of the sample were studied by a field emission scanning electron microscope, high resolution transmission electron microscope, Fourier transform infrared and Raman spectroscopy. Field emission (FE) of the sample was observed with a threshold field of ~ 3.4 V/μm. The high enhancement factor β derived from the slope of Fowler-Nordheim is attributed to the isolated nature of CNTs and nickel present at the CNT tip.     

Fabrication of vertically aligned single-walled carbon nanotubes in atmospheric pressure non-thermal plasma CVD

A fabrication technique of high-purity vertically aligned single-walled carbon nanotubes (VA-SWCNTs) using atmospheric pressure plasma enhanced chemical vapor deposition is presented. Although densely mono-dispersed Fe-Co catalysts of a few nanometers is primarily responsible for VA-SWCNT growth, carbon precipitation was virtually absent in the thermal CVD regime at 700 °C. On the other hand, high-purity VA-SWCNTs without measurable defects were grown at 4 μm min⁻¹ by applying atmospheric pressure radio-frequency discharge (APRFD) which has been previously developed for this purpose. The results proved that cathodic ion sheath adjacent to the substrates, where a large potential drop exists, also plays an essential role for the controlled growth of SWCNTs, while ion damage to the VA-SWCNTs is inherently avoided due to high collision frequency among molecules in atmospheric pressure. Operation regime of APRFD and tentative reaction mechanisms for VA-SWCNT growth are discussed along with optical emission spectroscopy of near substrate region.     

Local probing of the field emission stability of vertically aligned multi-walled carbon nanotubes

Metallic cantilever in high vacuum atomic force microscope has been used as anode for field emission experiments from densely packed vertically aligned multi-walled carbon nanotubes. The high spatial resolution provided by the scanning probe technique allowed precise setting of the tip-sample distance in the submicron region. The dimension of the probe (curvature radius below 50 nm) allowed to measure current contribution from sample areas smaller than 1 μm². The study of long-term stability evidenced that on these small areas the field emission current remains stable (within 10% fluctuations) several hours (at least up to 72 h) at current intensities between 10⁻⁵ and 10⁻⁸ A. Improvement of the current stability has been observed after performing long-time conditioning process to remove possible adsorbates on the nanotubes.     

Synthesis of vertically aligned, double-walled carbon nanotubes from highly active Fe-V-O nanoparticles

Monodispersed Fe-V-O nanoparticles were prepared by a liquid-phase synthesis to be used as catalysts for carbon nanotube (CNT) growth. Vertically aligned, dense CNTs have been grown from the highly active Fe-V-O nanoparticles by chemical vapor deposition. Diameter distribution of CNTs (3.7 ± 0.6 nm) was consistent with that of the original nanoparticles (3.1 ± 0.5 nm), and the value was smaller than those of other reported vertically aligned CNTs from as-prepared nanoparticles. TEM study showed that the CNTs consisted mainly of double-walled CNTs (single: 14%, double: 74%, and triple: 12%). The CNT diameter increased to 4.4 ± 0.8 nm as the growth temperature was increased from 810 to 870 °C. Energy dispersive X-ray spectroscopy of nanoparticles before and after the CNT growth revealed that the V content decreased from 7.2 to 2.7 at.%, suggesting that the segregation of Fe and V played an important role for the high activity of the Fe-V-O nanoparticles.     

Direct growth of vertically aligned carbon nanotubes on a planar carbon substrate by thermal chemical vapour deposition

Uniform, vertically aligned multiwalled carbon nanotube arrays (VACNTs) were grown on glassy carbon-like thin films by thermal chemical vapour deposition (CVD). Thin (5 nm) aluminum and iron catalyst layers were pre-deposited by evaporation on the carbon substrates and VACNTs were grown at 750 °C by water-assisted CVD using ethylene as the carbon source. The aluminum layer was shown to be essential for aligned nanotube growth. VACNT arrays adhered strongly to the carbon film with low contact resistance between the VACNTs and the substrate. The VACNT arrays grown directly on the planar conducting carbon substrate have attractive properties for use as electrodes. Excellent voltammetric characteristics are demonstrated after insulating the arrays with a dielectric material.