Horizontally aligned single-walled carbon nanotube field emitters fabricated on vertically aligned multi-walled carbon nanotube electrode arrays

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays were fabricated on an anodic aluminum oxide membrane bonded to a Si wafer. After obtaining a protruding tip for the MWCNTs by etching away some oxide, they were used as electrodes in the fabrication of carbon nanotube field emitters. Long single-walled carbon nanotubes (SWCNTs) were spin coated on the MWCNT arrays of uniform height. Clean SWCNTs were suspended by attaching them to the tips of the vertically aligned MWCNT arrays. The spin coated SWCNTs function as emitters, while the MWCNT arrays function as electrodes. The field emission was greatly improved by coating gold on the MWCNT arrays and annealing at 400 °C. Our field emitter exhibits good field emission properties such as a low turn-on field (1.4 V/μm), high current density (122 mA/cm²), and good stability (55 h for 10% degradation of current density from 400 μA/cm²).     

Alcohol-assisted rapid growth of vertically aligned carbon nanotube arrays

Millimeter-to-centimeter scale vertically aligned carbon nanotube (VACNT) arrays are widely studied because of their immense potential in a range of applications. Catalyst control during chemical vapor deposition (CVD) is key to maintain the sustained growth of VACNT arrays. Herein, we achieved ultrafast growth of VACNT arrays using Fe/Al₂O₃ catalysts by ethanol-assisted two-zone CVD. One zone was set at temperatures above 850 °C to pyrolyze the carbon source and the other zone was set at 760 °C for VACNT deposition. By tuning synthesis parameters, up to 7 mm long VACNT arrays could be grown within 45 min, with a maximal growth rate of ∼280 μm/min. Our study indicates that the introduction of alcohol vapor and separation of growth zones from the carbon decomposition zone help reduce catalyst particle deactivation and accelerate the carbon source pyrolysis, leading to the promotion of VACNT array growth. We also observed that the catalyst film thickness did not significantly affect the CNT growth rate and microstructures under the conditions of our study. Additionally, the ultralong CNTs showed better processability with less structural deformation when exposed to solvent and polymer solutions. Our results demonstrate significant progress towards commercial production and application of VACNT arrays.     

A simple combinatorial method to discover Co-Mo binary catalysts that grow vertically aligned single-walled carbon nanotubes

A simple yet versatile combinatorial method to discover binary metal nanoparticle catalysts was developed. In this method, the nominal thickness of component metals can be independently screened for a wide range by simply setting a mask with a slit above a substrate during sputter-deposition. Using this method, we prepared a catalyst library with Mo (0.2-4 nm) and Co (0.2-8 nm) thickness profiles on a SiO₂/Si wafer and discovered active catalysts that grow vertically aligned single-walled carbon nanotubes by alcohol catalytic chemical vapor deposition.     

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.     

Tribological properties of vertically aligned carbon nanotube arrays

Carbon nanotubes (CNTs) are a promising material for the fabrication of biomimetic dry adhesives. The dimensions of single CNTs are in the range of those of terminal elements of biological dry hairy adhesion systems, such as the setal branches on the toe of the gecko. Here, the tribological properties of densely packed arrays of vertically aligned and up to 1.1 mm long multi-walled CNTs (VACNTs) synthesized by chemical vapor deposition are examined. The coefficient of friction μ is as high as 5–6 at the first sliding cycle, and decreases down to stable values between 2 and 3 at the fourth to fifth sliding cycles. Such high values of μ can only be explained by the strong contribution of adhesion induced by applied shear force. After the tests, wear-induced deformations of the VACNT surface are observed, which strongly depend on the amount of normal force applied during the friction experiments. Interestingly, the plastic deformation of the VACNTs does not significantly affect μ after a preconditioning by a few sliding cycles. However, a strong decrease of μ during the initial wear cycles has to be taken into account for the development of applications, such as non-slip surfaces and pick-and-place techniques for manufacturing.     

Frictional behaviour of vertically aligned carbon nanotube films

Vertically aligned CNT films were grown on polycrystalline β-SiC wafers by the surface decomposition method. Their frictional behaviours were investigated by AFM at the nanometer scales. Compared with DLC film and silicon wafers, they demonstrate an extremely low friction coefficient at the nanometer scale about 0.03-0.04. The effect of the surface topography on the friction coefficient is obvious for the aligned CNT film sliding at the nanometer scale. This implies that the excellent tribological properties of the vertically aligned CNT films, combined with their small dimensions and structural perfection, might lead to significant improvement of the performance of nano-devices.     

Surface properties of vertically aligned carbon nanotube arrays

This study focuses on the structural changes of vertically aligned carbon nanotube (CNT) arrays while measuring their adhesive properties and wetting behaviour. CNT forests grown by chemical vapor deposition with a height of ~ 100 µm, an outer CNT diameter of ~ 10 nm and a density of the order of ~ 10¹⁰ CNTs/cm² show an average adhesion of 4 N/cm² when pressed against a glass surface. The applied forces lead to the collapse of the regular CNT arrays which limits their reusability as functional dry adhesives. Goniometric water contact angle (CA) measurements on CNT forests show a systematic decrease from an initial value of ~ 126° to a final CA similar to highly orientated graphite. Environmental scanning electron microscopy shows that this loss of hydrophobicity is due to an evaporation induced compaction of CNTs together with the loss of their vertical alignment. We observe the formation of cellular patterns for controlled drying.     

On-substrate growth of single-walled carbon nanotube networks by an “all-laser” processing route

We report on the process latitude of an “all-laser” approach for the controlled growth of single-walled-carbon-nanotube (SWCNT) mats at predefined locations on silicon substrates. Unlike the conventional laser ablation methods where the SWCNTs are produced in the soot form, from the concomitant ablation of a graphite target loaded with metal catalyst, the “all-laser” process proceeds in two consecutive and independent steps. Indeed, the same KrF pulsed laser is first used to deposit at room temperature, the Co/Ni catalyst nanoparticles (NPs) onto the substrates – of which size and surface density can be controlled by adjusting the number of laser ablation pulses – and subsequently to grow SWCNTs onto the Co/Ni NPs sites, from the laser ablation of a pure graphite target. The grown SWCNT networks are shown to be fairly controllable by choosing the appropriate ratio of “graphite to Co/Ni-NPs” laser ablation pulses. The Co/Ni NPs and the grown SWCNTs were systematically characterized by atomic force microscopy, scanning/tunnelling electron microscopy, Raman spectroscopy and thermogravimetry analysis, and their potential as an active material in thin film transistor was evaluated. Obtained characterization data have led to identify key growth parameters of this novel approach, and to propose growth mechanism models that best describe our observations.     

Radial growth of vertically aligned carbon nanotube arrays from ethylene on ceramic spheres

Vertically aligned carbon nanotube (VACNT) arrays grown on ceramic spheres are obtained from ethylene using a floating catalysis process. The exhaust gas mainly contains light gaseous hydrocarbons, which decreases the contamination at the outlet of the reactor. Linear synchronous growth of the VACNT arrays is demonstrated and the morphology evolution of VACNT array grown on spheres is shown. The VACNT arrays on the spheres crack radially into a flower-like structure when the length of CNT is above 400 μm. The VACNT arrays grown on spheres still possess good flowability even when the length of the array reaches 1100 μm after a 2-h growth at 800 °C. The arrays on the spheres show good alignment, high purity and good graphitization. Meanwhile, with a decrease in temperature, the diameter of CNTs in the array correspondingly decreases, the distribution becomes narrower, and the growth rate decreases. The apparent activation energy is 180 ± 8 kJ/mol, indicating that ethylene is a good carbon source for fast and continuous radial growth of millimeter VACNT arrays on ceramic spheres.     

Molecular dynamics simulation of physical vapor deposition of metals onto a vertically aligned single-walled carbon nanotube surface

Behaviors of nickel and gold clusters deposited onto a vertically aligned single-walled carbon nanotube film (VA-SWCNT) were investigated by molecular dynamics simulation methods. Brenner potential was applied for carbon–carbon, and bond-order potential was applied for metal–metal and carbon–metal interactions. Their parameters of gold–gold and carbon–gold were derived by density functional theory calculations. After metal clusters were fully annealed, they were deposited onto the flat nanotube surface with small evaporation energy. Nickel clusters spread over the surface; on the other hand, the gold clusters sometimes formed a grain-like structure, especially when they formed a large cluster before arriving at the surface. When another CNT was placed on the vertically aligned surface, nickel clusters spread and formed a smooth surface, as in the case of a VA-SWCNT film; however, gold clusters could not spread but formed a grain-like structure. This result is in good agreement with the experimental and indicates the mechanism for forming a grain-like structure on a VA-SWCNT. To avoid the formation of a grain-like structure on the surface of a SWCNT, the deposition should be carried out under a high vacuum condition and at a low deposition rate for preventing clustering as it obstructs formation of a flat surface.     

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.     

Development of microreactors made of vertically aligned carbon nanotube films

Microreactors consisting of multiwalled carbon nanotube (CNT) microchannels have been developed. Vertically aligned CNT films with negative pattern shapes of microchannels are grown on silicon oxide films, providing CNT microchannels. Polymethyl methacrylate plates are placed on the CNT microchannels for the flow experiments. Since CNTs are hydrophobic and the silicon oxide film is hydrophilic, fluids can flow in the silicon oxide regions in the CNT microreactors. Fermat’s spiral and Y-junction type multiwalled CNT microreactors were synthesized.     

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.     

Effect of catalyst pattern geometry on the growth of vertically aligned carbon nanotube arrays

We report the effect of catalyst pattern geometry on the growth behaviour of carbon nanotube (CNT) vertical arrays. Larger patterns are seen to produce longer CNT arrays. We show that this is predominantly related to the pattern size dependence of the number of walls and relate this to the local availability of carbon feedstock species. In addition, the vertical alignment of CNT pillar arrays is seen to depend on the pattern design, in particular the relationship between the pillar dimension and the inter-pillar spacing.     

Millimeter-tall single-walled carbon nanotube forests grown from ethanol

Millimeter-tall vertically-aligned carbon nanotubes (VA-CNTs) were grown from ethanol under ambient pressure by Co-catalyzed chemical vapor deposition (CVD), with systematic optimization of the CVD temperature and catalytic conditions using combinatorial catalyst libraries. We investigated the use of both aluminum oxide and silicon oxide as underlayers for the Co catalyst and found that VA-CNTs grew to millimeter heights in 15-30 min when the pyrolysis of ethanol was carried out at high temperatures (?850 °C) and long residence times (?10 s). Thick Co catalytic layers (?1.3 nm) produced (sub)millimeter-tall multi-walled VA-CNTs on both the aluminum oxide and silicon oxide underlayers. However, thin Co catalytic layers (0.62-1.0 nm) produced (sub)millimeter-tall VA-CNTs, which consisted mainly of single-walled CNTs, only on the aluminum oxide underlayers. Stripe patterns were found in the VA-CNTs near the substrate on both aluminum oxide and silicon oxide, indicating some instability prior to growth termination. The possible roles of aluminum oxide in growing millimeter-tall single-walled VA-CNTs were discussed.     

Terahertz time-domain spectroscopy characterization of vertically aligned carbon nanotube films

Terahertz time-domain spectroscopy measurements were made for vertically aligned multi-walled carbon nanotube (VACNT) films. We obtained the frequency dependent complex permittivity and conductivity (on the assumption that permeability μ = 1) of several samples exhibiting Drude behaviour for lossy metals. The obtained material properties of VACNT films provide information for potential microwave and terahertz applications.     

Strain engineering for thermal conductivity of single-walled carbon nanotube forests

We perform classical molecular dynamics simulations to investigate the mechanical compression effect on the thermal conductivity of the single-walled carbon nanotube (SWCNT) forest, in which SWCNTs are closely aligned and parallel with each other. We find that the thermal conductivity can be linearly enhanced by increasing compression before the buckling of SWCNT forests, but the thermal conductivity decreases quickly with further increasing compression after the forest is buckled. Our phonon mode analysis reveals that, before buckling, the smoothness of the inter-tube interface is maintained during compression, and the inter-tube van der Waals interaction is strengthened by the compression. Consequently, the twisting-like mode (good heat carrier) is well preserved and its group velocity is increased by increasing compression, resulting in the enhancement of the thermal conductivity. The buckling phenomenon changes the circular cross section of the SWCNT into ellipse, which causes effective roughness at the inter-tube interface for the twisting motion. As a result, in ellipse SWCNTs, the radial breathing mode (poor heat carrier) becomes the most favorable motion instead of the twisting-like mode and the group velocity of the twisting-like mode drops considerably, both of which lead to the quick decrease of the thermal conductivity with further increasing compression after buckling.     

Polarization-dependent optical reflection from vertically aligned multiwalled carbon nanotube arrays

Optical reflection from an array of vertically aligned multiwalled carbon nanotubes (MWCNTs) acting as a black body absorber was investigated. We present the novel results of the experimental analysis of specular reflection involving the different light polarization configurations and wavelengths from visible and near infrared range. It was shown that with the increasing incidence angle, reflectance rises dramatically and is highly dependent upon the polarization state of the incoming light. Our results indicate that popular antenna model commonly used for explaining optical properties of MWCNTs must be applied with care when referenced to nanotube arrays.     

Growth termination mechanism of vertically aligned centimeter long carbon nanotube arrays

The mechanism of abrupt growth termination for growing vertically aligned centimeter long CNT arrays in chemical vapor deposition process was studied. We found that the growth length increases linearly with increasing growth time and the CNT growth terminates abruptly for all the experimental conditions applied. We investigated the change of particle size distribution, particle number density and the catalyst composition on the surface of the substrate with the exposure time and temperature in the reactor. For this study X-ray Photoelectron Spectroscopy (XPS), High Resolution Transmission Electron Microscopy (HRTEM) and Atomic Force Microscopy (AFM) were employed. Focused Ion Beam (FIB) was used for preparation of the STEM samples. The experimental results demonstrated that the formed catalyst nanoparticles undergo compositional and morphological changes during the heating step of the substrate, when the temperature of the reactor is rising from ambient to the growth temperature, and during annealing at the set growth temperature. Most of the changes in the catalyst composition and morphology were related to metal diffusion into neighboring substrate layers during heating up to the growth temperature. Based on the results, a new scenario of abrupt termination mechanism for growing centimeter long CNT arrays using an iron–gadolinium binary catalyst was proposed.     

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.