Synthesis and crystallinity of carbon nanotubes produced by a vapor-phase growth method

We have studied the effect of temperature on the growth and crystallinity of carbon nanotubes (CNTs), synthesized by a vapor-phase growth method using a catalytic reaction of iron pentacarbonyl (Fe(CO)₅) and acetylene (C₂H₂) gas. By increasing the growth temperature from 750 °C to 950 °C, both the growth rate and the diameter of the CNTs increase. Moreover, the crystallinity of the graphite sheets improves progressively with increasing growth temperature. Adjustment of the growth temperature gives potential for controlled growth of CNTs in a large-scale synthesis of CNTs. PACS 61.46.+w; 68.37.-d; 81.07.De     

Evaluation of imaging in planar anisotropic and isotropic dendritic left-handed materials

Carbon nanotubes (CNTs) with totally hollow channels and/or totally filled copper nanowires have been fabricated by methane decomposition using copper microgrid as a catalyst at 1173 K. The formation mechanism of CNTs with totally hollow channels is carbon precipitation at carbon-metal interface via the preferable surface diffusion mode of carbon. The selectivity of these CNTs can be improved by increasing the purity of copper catalysts and adding hydrogen in the feed gas. To form long and continuous copper nanowires up to 8–10 μm the filling of copper in the CNT channel requires the liquid or quasi-liquid state capillary adsorption of nanosized copper at 1173 K under the thermal driving force. The filling volume ratio of copper to total nano-channel of the CNTs is firstly increased to about 50%. The copper inside the CNTs is of single crystalline form and face centered cubic (fcc) structure. The method is useful for further controlled synthesis of CNTs with totally hollow channels and/or totally copper filled nanowires. PACS 81.07.De; 82.33.Ya     

Synthesis of well-aligned carbon nanotubes by inductively coupled plasma chemical vapor deposition

Uniform and well-aligned carbon nanotubes (CNTs) have been grown using a high density inductively coupled plasma chemical vapor deposition (ICP-CVD) system. A gas mixture of methane-hydrogen was used as the source and Ni as the catalyst for the CNT growth. The effect of process parameters, such as inductive RF power, DC bias voltage and CH₄/H₂ ratio, on the growth characteristics of CNTs was investigated. It was found that both plasma intensity and ion flux to the substrate, as controlled by the inductive RF power and DC bias voltage, respectively, can greatly affect the growth of CNTs. The relative importance of the generation of ions and the subsequent transport of ions to the substrate as serial process steps are considered as the two underlying factors in determining the growth characteristics of CNTs. PACS 81.05.Uw; 81.07.De; 81.15.Gh     

Contact improvement of carbon nanotubes via electroless nickel deposition

Individual multi-walled carbon nanotubes (CNTs) were deposited onto microelectrodes and embedded in nickel to achieve low-ohmic contact resistances. Electroless deposition of nickel onto gold/iron, palladium, and cobalt microelectrodes was used to form electrically stable bonds at the interfaces between the electrodes and CNTs. Resistance measurements showed that the contact resistances of the CNTs on gold/iron and palladium were significantly improved by nickel embedding, whereas no further improvement was found for the CNTs on cobalt. Electroless metal deposition is a parallel process providing stable electrical and mechanical contacts between CNTs and metallic microelectrodes. PACS 81.07.De     

Detailed profiling of CNTs arrays along the growth window in a floating catalyst reactor

We report a detailed longitudinal and depth profiles of multi-wall carbon nanotubes (CNTs) arrays synthesized using xylene and ferrocene in a floating catalyst reactor. Point to point analyses of the CNTs grown in a “growth window” with CNTs arrays longer than 0.5 mm were performed using optical microscopy, Raman spectroscopy, FESEM, high-resolution TGA/DTA, and TEM techniques. The heights of the CNTs arrays show a maximum at a mid point of the growth window, while a reverse trend of minimum is observed for iron-to-CNTs atomic ratios. The ratio of amorphous carbon to CNTs sharply increases along the growth window and from the bottom to top of CNTs arrays. The CNTs diameter also increases along the growth window, due to deposition of the amorphous carbon, which can be almost removed by temperature programmed oxidation up to around 500 °C. A base growth mechanism, the variations of catalyst content, residence time and temperature profile along the growth window, the adsorption and decomposition of polycyclic aromatic hydrocarbons to amorphous carbon, and a limited diffusion of hydrocarbon species through the arrays covered by excessive amorphous carbon may explain the results.     

Measurement of multi-wall carbon nanotube penetration through a screen filter and single-fiber analysis

In this study, we carried out experiments to study penetration of airborne carbon nanotubes (CNTs) through a screen filter. An electrospray system was employed to aerosolize suspensions of multi-wall CNTs. The generated airborne CNTs were characterized by electron microscopy, and the length and diameter were measured. In the filtration experiments, the challenging CNTs are classified by a differential mobility analyzer. Monodisperse CNTs with the same electrical mobility were then employed to challenge the screen filter. Penetration was measured for CNTs in the range of 100–400 nm mobility diameters. The results showed that the CNT penetration was less than the penetration for a sphere with the same mobility diameter, which was mainly due to the larger interception length of the CNTs. We compared the modeling results using single-fiber filtration efficiency theories with the experimental data, and found that the effective interception length can be approximated by the CNT aerodynamic diameter multiplying a scaling factor. A hypothesis is proposed to understand the observation.     

Large scale intercalated growth of short aligned carbon nanotubes among vermiculite layers in a fluidized bed reactor

Short aligned carbon nanotubes (CNTs) were intercalated grown among vermiculite layers from ethylene using a simple fluidized bed chemical vapor deposition (CVD) process. The length of CNTs ranged from 0.5 to 1.5 μm after a synthesizing duration of 1-5 min at 650 °C. The as-grown CNTs vertically aligned to the vermiculite layers were with the mean outer and inner diameter of 6.7 and 3.7 nm, respectively. A CNT yield of 0.22 g/g_cat was obtained for a 5-min growth. Those indicated that the fluidized bed CVD was an effective way for mass production of short CNTs.     

Structural dependence of nonlinear elastic properties for carbon nanotubes using a continuum analysis

Based on molecular mechanics coupled with the atomistic-based continuum theory, a structural mechanics approach is presented to examine the nonlinear elastic properties of carbon nanotubes (CNTs) subjected to large axial deformations. According to molecular mechanics, the interaction force between atoms is modeled using the Morse potential. The nanoscale continuum theory is established to directly incorporate the Morse potential function into the constitutive model of CNTs. In this paper, we simulate and examine the influence of CNT structures on the stress–strain response. The linear elastic property of CNTs is independent of the helicity of the hexagonal carbon lattice along the tubes, while their nonlinear elastic behavior shows a larger chirality dependence. The present theoretical approach supplies a set of very simple formulas and is able to serve as a good approximation of the mechanical properties of CNTs. PACS 62.20.-x; 62.20.Dc; 62.25.+g     

A New Theoretical Model of a Carbon Nanotube Strain Sensor

Carbon nanotubes （CNTs） are potential strain sensors due to their excellent mechanical and spectral properties. A new theoretical model of a CNT strain sensor is obtained by applying the polarized Raman properties of CNTs, which calculates the synthetic contributions of Raman spectra from the CNTs in random directions. By using this theoretical model, the analytic relationship between planar strain components and the Raman shift increment of uniformly dispersed CNTs is obtained, which is applicable for accurately characterizing the strain in random directions on the surface of a measured microsystem.     

Synthesis of a composite consisting of carbon nanotubes and graphite shell-encapsulated cobalt nanoparticles using plasma-enhanced chemical vapor deposition

We report a simple and effective one-step synthesis route for synthesizing a composite consisted of carbon nanotubes (CNTs) and graphite shell-encapsulated cobalt nanoparticles using plasma-enhanced chemical vapor deposition on Si (1 0 0) substrate covered with catalyst Co particles, discharging a mixture of H₂ and CH₄ gas, and characterize the obtained composite by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscope, and X-ray photoelectron spectroscopy. The results show that CNTs align perpendicularly to the substrate and graphite shell-encapsulated Co nanoparticles clung to the external surfaces of aligned CNTs. The diameter of the graphite shell-encapsulated Co nanoparticles increases with increasing the H₂ content in H₂ and CH₄ carbonaceous gas. A possible growth mechanism of the CNTs and graphite shell-encapsulated cobalt nanoparticles composite has been explored.     

Synthesis of iron-filled carbon nanotubes with a great excess of ferrocene and their magnetic properties

Iron-filled carbon nanotubes (CNTs) were synthesized using a floating catalyst chemical vapor deposition method with a great excess of ferrocene (more than 30 mg/min of vaporization rate) at 800 C. The amount of ferrocene is more than that in previous reports. The ferrocene was employed as both catalyst precursor and an iron source. Our observations indicate that the CNTs were more than 10 micrometer in length, with an outer diameter of 20–100 nm and inner diameter of 10–30 nm. The inner cavity of the CNTs was partial filled with iron nanowires. Magnetic property measurements reveal that the iron-filled CNTs exhibit an average coercivity of about 257.05 G.     

Field emission properties of carbon nanotube film using a spray method

We fabricated carbon nanotube (CNT) emitters by a spray method using a CNT suspension with ethanol. Indium with a low melting pointing metal or indium tin oxide (ITO) was deposited on the glass substrate. The CNTs were sprayed on these layers and thermally annealed. The sprayed CNTs on an ITO were obtained a high emission current density, field enhancement factor, and a uniform emission pattern than the sprayed CNTs on an ITO layer. We found that the sprayed emitters on the indium layer had good field emission characteristics because of the strong adherence between the metal layer and CNTs.     

Synthesis and field emission properties of carbon nanotubes grown in ethanol flame based on a photoresist-assisted catalyst annealing process

Carbon nanotubes (CNTs) have been grown directly on a Si substrate without a diffusion barrier in ethanol diffusion flame using Ni as the catalyst after a photoresist-assisted catalyst annealing process. The growth mechanism of as-synthesized CNTs is confirmed by scanning electron microscopy, high resolution transmission-electron microscopy and energy-dispersive spectroscopy. The photoresist is the key for the formation of active catalyst particles during annealing process, which then result in the growth of CNTs. The catalyst annealing temperature has been found to affect the morphologies and field electron emission properties of CNTs significantly. The field emission properties of as-grown CNTs are investigated with a diode structure and the obtained CNTs exhibit enhanced characteristics. This technique will be applicable to a low-cost fabrication process of electron-emitter arrays.     

Dynamic evolution of interacting carbon nanotubes suspended in a fluid using a dielectrophoretic framework

A theoretical investigation of the dynamic response of interacting carbon nanotubes (CNTs) dispersed in a liquid medium under alternating current electric fields is presented. The proposed modeling strategy is based on the dielectrophoretic theory and classical electrodynamics of rigid bodies, and considers the coupled rotation-translation motion of interacting CNTs represented as electrical dipoles. Based on experimental evidence, the parameters which are expected to cause a major contribution to the CNTs' motion are investigated for different initial configurations of CNTs. It is predicted that high electric field frequencies, long CNTs, high values of electrical permittivity and conductivity of the CNTs immersed in solvents of high polarity promote faster equilibrium conditions, achieved by CNT tip-to-tip contact and alignment along the electric field direction. For the majority of the scenarios, CNT alignment along the field direction is predicted as the first event, followed by the translation of aligned CNTs until the tip-to-tip contact condition is reached. For systems with interacting CNTs with different lengths, equilibrium of the shorter CNT is achieved faster. Predictions also show that the initial rotation angles and initial location of CNTs have a paramount influence on the evolution of the system towards the equilibrium configuration.     

Direct production of carbon nanotubes decorated with Cu2O by thermal chemical vapor deposition on Ni catalyst electroplated on a copper substrate

Carbon nanotubes (CNTs) decorated with Cu₂O particles were grown on a Ni catalyst layer deposited on a Cu substrate by thermal chemical vapor deposition from liquid petroleum gas. Ni catalyst nanoparticles with different sizes were produced in an electroplating system at 45 °C using the corrosive effect of H₂SO₄ which was added to solution. These nanoparticles provide the nucleation sites for CNT growth avoiding the need for a buffer layer. The surface morphology of the Ni catalyst films and CNT growth over this catalyst was studied by scanning electron microscopy (SEM). High temperature surface segregation of the Cu substrate into the Ni catalyst layer and its exposition to O₂ at atmospheric environment, during the CNTs growth, lead to the production of CNTs decorated with about 6 nm Cu₂O nanoparticles. We used SEM to study the surface characteristics of Ni catalyst films and characteristic of grown CNTs. Raman spectroscopy, transmission electron microscopy (TEM), electron diffraction (EDX), X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) revealed the formation of CNTs. The selected area electron diffraction pattern, EDX, and XPS studies show that these CNTs were decorated with Cu₂O nanoparticles. This way of fabrication is the easiest and lowest cost method.     

Field emission characteristics of screen-printed carbon nanotubes cold cathode by hydrogen plasma treatment

Selective plasma etching and hydrogen plasma treatment were introduced in turn to improve field emission characteristics of screen-printed carbon nanotubes (CNTs) cold cathode, which was prepared by using slurry of mixture of multi-wall CNTs, organic vehicles and inorganic binder, i.e. silicon dioxide sol. The results show that selective plasma etching process could effectively remove parts of surface inorganic vehicle (SiO₂) layer and expose more smooth and clean CNTs on cathode surface, which could significantly decrease the operating field of CNTs cathode. There are some nanoparticles emerging on the out of CNTs wall after hydrogen plasma treatment, which are equivalent to increase field emission point of cathode. At the same time, these nanoparticles can increase the local electric field of CNTs, which can decrease operating voltage of CNTs cathode and improve uniformity field emission.     

Field emission properties of electrophoretic deposition carbon nanotubes film

The field emission properties of electrophoretic deposition(EPD) carbon nanotubes (CNTs) film have been improved by depositing CNTs onto the titanium (Ti)-coated Si substrate, followed by vacuum annealing at 900 °C for 2 h, and the enhanced emission mechanism has been studied using X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy. Field emission measurements showed that the threshold electric field was decreased and the emission current stability was improved compared to that of EPD CNTs film on bare Si substrate. XRD and Raman spectroscopy investigations revealed that vacuum annealing treatment not only decreased the structural defects of CNTs but made a titanium carbide interfacial layer formed between CNTs and substrate. The field emission enhancement could be attributed to the improved graphitization of CNTs and the improved contact properties between CNTs and substrate including electrical conductivity and adhesive strength due to the formed conductive titanium carbide.     

Growth of carbon nanotubes on stainless steel substrates by DC-PECVD

We report on the fabrication of carbon nanotubes (CNTs) on Ni-coated stainless steel (SUS) substrates by using dc plasma enhanced chemical vapor deposition. The synthesized CNTs have the diameter of about 30 nm and the length of about 1.2 μm. To verify the effects of SUS substrates on the growth of CNTs, CNTs had also been grown on Ni-coated Si substrates. CNTs grown on the SUS substrates were more uniform compared with those grown on the Si substrates. Field emission properties of the CNT films were measured in the diode configuration, and the turn-on electric field of 3.87 V/μm and field enhancement factor β of about 1737 were obtained from the synthesized CNTs at the gap of 500 μm between the SUS substrate and the anode. These results have not only clarified the effects of the substrate on the growth of CNTs, but also shown the potential of CNTs in field emission applications, especially CNT-based cold-cathode X-ray tubes.     

Carbon nanotube synthesis from alcohols by a novel aerosol method

Single- and multiwalled carbon nanotubes (CNTs) were synthesised by a novel aerosol method using alcohols, namely ethanol and octanol, as carbon precursors. Preformed iron and nickel aerosol nanoparticles, produced by evaporation from resistively heated metal wire, were used as catalysts. Multiwalled CNTs were initiated by 10 nm sized catalyst particles and produced in the presence of ethanol vapour with the partial pressure of 7072 Pa, while combination of 2.4 nm particles and decreased alcohol vapour pressure (123 Pa) resulted in the formation of mainly single-walled and a small fraction of double-walled CNTs. The effect of a promoter (thiophene) in the system was found to be very important for the synthesis of multiwalled CNTs, while only a 30% number concentration increase was found for the single-walled CNT production.