Well-aligned Multi-walled Carbon Nanotubes Research Articles

Effects of strong magnetic field on carbon nanotube formation using rf glow-discharge plasma

We have investigated the formation of uniform and well-aligned multi-walled carbon nanotubes (MWNTs) by controlling radio-frequency (rf) glow-discharge plasmas in weak and strong magnetic fields. The MWNTs growing directly on the rf electrode in the weak magnetic field (0.03 T) are deformed and combined with each other by the bombardment of the high energy ions generated by the large sheath electric field in front of the rf electrode. When the strong magnetic field (2 T) is applied, on the other hand, the MWNTs are observed to be well-aligned and not to be deformed. These results can be explained by the fact that the ion bombardment energy reduces in spite of the existence of the large sheath electric field, which is caused by the magnetization of the ions in the strong magnetic field.

Uniform-diameter, aligned carbon nanotubes from microwave plasma-enhanced chemical-vapor deposition

Well-aligned multiwalled carbon nanotubes (CNTs) were grown by microwave plasma-enhanced chemical-vapor deposition using N2 as the carrier gas and CH4 as the carbon source. Thin iron films with different thicknesses (0.5–5 nm) on silicon substrates acted as catalysts. Atomic force microscopy was used to investigate the relationship between the iron film thickness and the size of the iron clusters formed after the plasma treatment. Scanning and transmission electron microscopies were utilized to study the morphology, structure, and diameter of the as-grown CNTs. With the iron film thickness of 0.5 nm, the CNTs showed a remarkable structural uniformity in terms of diameter (standard deviation was 11.4% of the average diameter). Field-emission measurements revealed the turn-on field (at a current density of 10μA∕cm2) and threshold field (at a current density of 10mA∕cm2) to be 5–8 and 6–11.5V∕μm, respectively.

Synthesis of Well-Aligned Carbon Nanotubes Using a High-Density RF Inductive Coupling Plasma

Growth of vertically well-aligned carbon nanotubes was demonstrated utilizing high-density rf inductive coupling plasma-enhanced chemical vapor deposition of methane. A sufficient supply of CH4-related precursors as well as a rapid large number of dangling bonds at the top surface due to a negative substrate dc bias are essential for promoting the deposition rate of vertically well-aligned carbon nanotubes. Rapid deposition at 60 Å/s of vertically well-aligned multi-walled carbon nanotubes was achieved using pure methane (50 sccm) by adjusting the plasma conditions.

NEXAFS and X-ray scattering study of structure changes after post-annealing treatments of aligned MWNTs

The control of the crystallization degree is an important issue for carbon nanotube applications. In this context, we present the evolution of the structure after post-annealing treatment of multiwalled carbon nanotubes (MWNT), obtained by the aerosol pyrolysis process, involving liquid aerosols containing both hydrocarbon source and the metal catalyst precursor. The alignment and the degree of crystallinity of the raw and annealed carbon nanotubes have been quantified by X-ray diffraction measurements also used to analyze the metallic phases inside the nanotubes. The Near-Edge X-ray Absorption Fine Structure (NEXAFS) studies were performed at the C K-edge in order to follow the evolution of the electronic structure with the local structure and the degree of crystallinity of the nanotubes. We confirm that this chemical vapour deposition synthesis method enables to produce well-aligned multiwalled carbon nanotubes. The effect of post-annealing temperature both on local ordering and graphitization degree of nanotube walls are described. The results are compared to the previous observations of individual nanotubes by HRTEM concerning the structural changes occurring during annealing treatments.

Well-aligned Co-filled carbon nanotubes: preparation and magnetic properties

Carpet-like flakes (area ≤ 9mm2; thickness < 50μm) of well-aligned Co-filled multi-walled carbon nanotubes were grown by decomposition of cobaltocene. The nanotubes have outer diameters of 50–90nm and a metal core of 15–30nm diameter. They are discontinuously filled with f.c.c.-Co nanowires of up to a few micrometers in length. Magnetometry studies show a weak uniaxial magnetic anisotropy with the easy axis along the nanowires and a high coercivity of about 59mT. Electron holography provides information about the magnetic characteristics of individual nanowires.

Growth behavior and interfacial reaction between carbon nanotubes and Si substrate

The effect of the interfacial reaction of Co catalyst with a Si substrate on growth of carbon nanotubes (CNTs) was investigated. Well-aligned multiwall CNTs (MWCNTs) were synthesized and applied barrier layers by microwave plasma-enhanced chemical vapor deposition (MPECVD). Growth proceeded in a flowing mixture of H2, CH4, and N2 as precursors at a temperature of 600 °C and a −200 V substrate bias; Co was sputtered as the catalytic material. Transmission electron microscopy (TEM) and x-ray diffraction were employed to examine the growth behavior of CNTs on Si (100) substrates on which Co had been deposited by MPECVD. The TEM results indicate that discrete conical CoSi2 layers with {111} and (100) faceted interfaces were formed on a Si (100) substrate during CNTs growth. Direct evidence that the growth is by tip growth and base growth is presented. The results show that well-aligned CNTs exhibit a significant emission current. The field emission characteristics of CNTs are contributed to the relationship between the application of different barrier layers and the growth mode of CNTs.

Structure control of carbon nanotubes using radio-frequency plasma enhanced chemical vapor deposition

Carbon nanotube structures such as tube diameter, growth site, and formation density are controlled using radio-frequency (RF, 13.56 MHz) plasma enhanced chemical vapor deposition (RF-PECVD) method. We have produced uniformly well-aligned multi-walled carbon nanotubes (MWNTs) grown over the large scale area and linearly arrayed MWNTs grown in a selected area without any highly-sophisticated patterning process. In our RF-PECVD experiment, furthermore, individually grown single-walled carbon nanotubes (SWNTs) or their thin bundles are synthesized for the first time within the scope of the PECVD methods. These results indicate that PECVD method provides the high potential for the further development of nano-technology.

Nonenzymatic glucose detection using multi-walled carbon nanotube electrodes

The electrocatalytic oxidation of glucose in alkaline medium directly at well-aligned multi-wall carbon nanotubes (MWNTs) electrodes has been investigated in the present study. Compared to glassy carbon electrode, a substantial (+400 mV) decrease in the overvoltage of the glucose oxidation reaction was observed at MWNTs electrodes with oxidation starting at ca. +0.10 V (vs. 3 M KCl–Ag|AgCl). The electrocatalytic effect is mainly attributed to carbon nanotubes with possible minor contributions from the Co catalysts present on the Ta substrate. At an applied potential of +0.20 V, MWNTs electrodes give a high and reproducible sensitivity of 4.36 μA cm −2 mM −1 in the presence of high concentration of chloride ion. The well-aligned MWNTs electrode thus allows highly sensitive, low-potential, stable, and fast amperometric sensing of glucose, promising for the development of nonenzymatic glucose sensors.

Controlled Growth of Composite Nanowires Based on Coating Ni on Carbon Nanotubes by Electrochemical Deposition Method

A novel electrochemical deposition method has been used for coating Ni onto well-aligned multiwall carbon nanotubes. Both synthesis of carbon nanotubes and deposition of Ni are processed in porous anodic aluminum oxide (AAO) templates. The design of the AAO pores allows us easily to control the size of nanotubes and the thickness of the Ni coating. Without any surface treatment to the carbon nanotubes, a continuous and smooth Ni layer is achieved.

Ultraviolet laser treatment of multiwall carbon nanotubes grown at low temperature

Simple laser irradiation of well-aligned multiwall carbon nanotubes (MWCNTs) was performed to intentionally modify structural defects and to ablate possible contamination of the MWCNTs. Scanning electron microscopy and transmission electron microscopy confirmed the clear presence of the MWCNTs with open tips. A Raman spectra exhibited a decrease in an intensity ratio (ID/IG) of 1352 cm−1 (D band) over 1583 cm−1 (G band) peaks by significantly reducing the amorphous carbon phases of D band peaks. The structural improvement in the MWCNTs after optimum laser exposure resulted in a reduction of the turn-on voltage from 1.0 to 0.6 V/μm and an increase in the emission current.

Fabrication and Characterization of Pure and Well-Aligned Carbon Nanotubes Using Methane/Nitrogen−Ammonia Plasma

Well-aligned multiwalled carbon nanotubes (CNTs) were grown by microwave plasma-enhanced chemical vapor deposition using N2 and NH3 as the carrier gases and CH4 as the carbon source. Iron films with 1−5 nm thickness on silicon substrates acted as catalysts. Scanning electron microscopy revealed that the CNTs grew via the base growth mechanism at a rate ∼100 nm/s. Transmission electron microscopy showed that multiwalled CNTs had a “bamboo” structure, and the smallest CNTs of ∼6 nm in diameter were acquired on 1 nm Fe film. These “smallest” CNTs were comprised of amorphous structure, due to the formation of sp3 C−H bonds as proven by Fourier transform infrared spectroscopy and electron energy loss spectroscopy, suggesting hydrogen incorporation during growth of CNTs. Without N2 gas, no CNTs could be grown, while curly CNTs with poor alignment were grown with no NH3. In both cases, high-purity CNTs with no CNx impurities were obtained. Field electron emission revealed that the lowest turn-on and threshold fields were obtained from the CNTs grown on the 1 nm thick iron films on silicon substrate, i.e., 3.5 V/μm and 4.5 V/μm, respectively. Under an applied field of 7.5 V/μm, emission current density of 0.63 A/cm2 can be obtained.

Non-reactive rf treatment of multiwall carbon nanotube with inert argon plasma for enhanced field emission

External rf plasma irradiation of well-aligned multiwall carbon nanotubes (MWCTs) was demonstrated as a simple purification process. The plasma exposure to argon (Ar) gas was performed in situ in order to modify any structural defects and to vaporize contamination of the MWCTs grown at the extremely low temperature of 400 °C. Scanning electron microscopy and transmission electron microscopy confirmed the catalytic particle removal from the MWCT edges. In addition, Fourier transformer infrared spectroscopy and energy dispersive spectrometry showed that Ar plasma atoms were not chemically terminated by carbon atoms because of the inertness of the Ar atoms. The structural enhancement in the MWCTs resulted in a reduction of the turn-on field from 1.65 V/μm to 0.93 V/μm and an increase in the field emission current.

Carbon Nanotubes and Spheres Produced by Modified Ferrocene Pyrolysis.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Carbon Nanotubes and Spheres Produced by Modified Ferrocene Pyrolysis

Single-wall carbon nanotubes (SWCNTs) and well-aligned thin multiwall carbon nanotubes (MWCNTs) were successfully synthesized via pyrolysis of a powder mixture of ferrocene and anthracene or 9,10-dibromoanthracene. It is shown that the size of the MWCNTs can be tuned just by changing the ratio of ferrocene/anthracene in the mixture. A lower ratio resulted in low-diameter MWCNTs containing up to five graphene layers only; a high ratio has led to thick MWCNTs with more than 25 layers. Furthermore, there are two extreme cases, the pyrolysis of powders at a low ratio of 1:7 that gives rise to carbon nanospheres and the pyrolysis of pure ferrocene in a H2 flow that mainly gives rise to metal nanoparticles. When powder mixtures of ferrocene and 9,10-dibromoanthracene are pyrolyzed, both SWCNTs and spherical carbon-coated iron nanoparticles are obtained.

External-grid induced well-aligned carbon nanotubes grown on corning glass at extremely low temperature of about 400°C

Well-aligned multi-wall carbon nanotubes (CNTs) have been grown on both corning glass and silicon substrates at about 400°C by a modified plasma-enhanced chemical vapor deposition system. External SUS-grid was inserted between showerhead and anode plate. This grid was found to be the key parameter for low-temperature deposition of CNTs in our case. The Ni-coated substrates were at first pre-treated by an ammonia gas at 400°C, and then followed by the CNT deposition with a mixture of acetylene and hydrogen gases. The CNT was about 2–4 μm in length and 30–60 nm in diameter for 5 min deposition time. CNT with short length of <2 μm was easily well aligned on the glass substrates at 400°C.

Growth and characterization of aligned carbon nanotubes from patterned nickel nanodots and uniform thin films

Microstructures of well-aligned multiwall carbon nanotubes grown on patterned nickel nanodots and uniform thin films by plasma-enhanced chemical vapor deposition have been studied by electron microscopy. It was found that growth of carbon nanotubes on patterned nickel nanodots and uniform thin films is different. During growth of carbon nanotubes, a nickel particle sits at the tip of each nanotube, and its [220] is preferentially oriented along the plasma direction, which can be explained by a channeling effect of ions coming into nickel particles in plasma. The alignment of nanotubes is induced by the electrical field direction relative to substrate surface.

Ethylene flame synthesis of well-aligned multi-walled carbon nanotubes

A stainless steel grid baked by a propane–air premixed flame had iron, chromium and nickel oxide deposits on the grid surface. With this grid, entangled and curved shape multi-walled carbon nanotubes (MWNTs) were harvested from an ethylene–air diffusion flame with yield rate of 3 mg/min. Nitrogen addition to the flame was found to straighten the entangled tubes probably by lowering the flame temperature. A cobalt-electrodeposited stainless steel grid was finally applied to the nitrogen-diluted ethylene diffusion flame; well-aligned and well-graphitized carbon nanotubes consisting of 20 nm diameter and 10 μm long element tubes were obtained.

Growth and emission characteristics of vertically well-aligned carbon nanotubes grown on glass substrate by hot filament plasma-enhanced chemical vapor deposition

Vertically well-aligned multiwall carbon nanotubes (MWNTs) were grown on nickel-coated glass substrates by plasma-enhanced chemical vapor deposition at low temperatures, below 600 °C, with and without hot filament. Acetylene and ammonia gas were used as the carbon source and a catalyst. Effects of growth parameters, such as plasma intensity, filament current, and substrate temperature, on the growth characteristics of MWNTs were investigated. Plasma intensity was found to be the most critical parameter controlling the growth of MWNTs. Field emission from the MWNTs was obtained using a phosphor anode with an onset electric field of 1.5 V/μm.

Substrate-site selective growth of aligned carbon nanotubes

We report highly substrate-site selective growth of carbon nanotubes by chemical vapor deposition from precursors of ferrocene and xylene mixtures. The technique allows us to grow well-aligned multiwalled carbon nanotubes preferentially on the SiO2 regions of patterned SiO2/Si substrates prepared by conventional lithography. This eliminates the catalyst predeposition step in the fabrication process. This simple approach may also be applied to build large-scale networks of organized nanotubes on planar substrates.

Growth of well-aligned carbon nanotubes on a large area of Co–Ni co-deposited silicon oxide substrate by thermal chemical vapor deposition

We have grown vertically well-aligned multiwalled carbon nanotubes (CNT) on a large area of cobalt–nickel (Co–Ni) co-deposited silicon oxide substrate by thermal chemical vapor deposition using C 2H 2 gas, at 950°C. The diameter of CNTs is in the range of 50–120 nm and the length is about 130 μm. The grown CNTs have a bamboo structure and closed tip with no catalytic particles inside. As the particle size of Co–Ni catalyst decreases, the vertical alignment is enhanced. The CNTs exhibits a low turn-on voltage of 0.8 V/μm with an emission current density of 0.1 μA cm −2.