Carbon nanotubes coated by carbon nanoparticles of turbostratic stacked graphenes

Carbon nanotubes (CNTs) decorated by a high density of carbon nanoparticles of turbostratic graphene stacks have been fabricated by low energy hydrocarbon ion deposition at 700 °C. Transmission and scanning electron microscopy show that the carbon particles of turbostratic graphene stacks extend from the nanotube surface. The diameter of CNTs decreases with the increasing percentage of hydrogen in the gas phase. Raman spectroscopy indicates that the formation of carbon nanoparticles of turbostratic graphene stacks results from the high temperature used in the experiment. Meanwhile, Raman spectroscopy and high resolution transmission electron microscopy indicate that the carbon nanoparticle degree of crystallinity is lower with increasing hydrogen content in the gas phase due to the etching effect of hydrogen ions.     

Carbon nanoflake growth from carbon nanotubes by hot filament chemical vapor deposition

We report the growth of carbon nanoflakes (CNFs) on Si substrate by the hot filament chemical vapor deposition without the substrate bias or the catalyst. CNFs were grown using the single wall carbon nanotubes and the multiwall carbon nanotubes as the nucleation center, in the Ar-rich CH₄–H₂–Ar precursor gas mixture with 1% CH₄, at the chamber pressure and the substrate temperature of 7.5 Torr and 840 °C, respectively. In the H₂-rich condition, CNF synthesis failed due to severe etch-removal of carbon nanotubes (CNTs) while it was successful at the optimized Ar-rich condition. Other forms of carbon such as nano-diamond or mesoporous carbon failed to serve as the nucleation centers for the CNF growth. We proposed a mechanism of the CNF synthesis from the CNTs, which involved the initial unzipping of CNTs by atomic hydrogen and subsequent nucleation and growth of CNFs from the unzipped portion of the graphene layers.     

A plasma enhanced chemical vapor deposition process to achieve branched carbon nanotubes

Vertically aligned carbon nanotubes (CNTs) have been grown on silicon substrates using nickel as the catalyst layer, acetylene as the carbon source, and hydrogen as the carrier gas. The quality of the CNTs has been examined using transmission and scanning electron microscopy and a tip-growth mechanism with an inner tube diameter of 5–8 nm was observed. The effect of plasma hydrogenation as a post-growth treatment was shown to lead to total or partial removal of the nickel seeds from the CNT tips. Using sequential hydrogenation and growth, it has been possible to achieve tree-like nanostructures.     

Selective growth of horizontally-oriented carbon nanotube bridges on patterned silicon wafers by electroless plating Ni catalysts

The main purpose of this research was to develop an Integrated Circuit compatible process to grow the horizontally-oriented carbon nanotubes (CNTs) across the trenches of the patterned Si wafer, which was produced by conventional photolithography technique. The selectivity of the process is based on the difference in electrical conductivity between amorphous silicon (a:Si) and silicon nitride (Si₃N₄), where the catalyst can be much easier deposited by electroless plating on the a:Si part of the pattern. The selectivity is also based on greater chemical reactivity of the catalyst with a:Si to form silicides, instead of with Si₃N₄. Furthermore, the Si₃N₄ barrier layer of the pattern was designed on top of the a:Si layer to guide the growth of CNTs in horizontal direction to bridge the trenches of the pattern. The as-deposited catalysts were examined by Auger electron spectroscopy (AES). The catalyst-coated pattern was pretreated in hydrogen plasma and followed by CNT growth in a microwave plasma chemical vapor deposition (MPCVD) system. The CNT bridges were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and I–V measurements. Under the present deposition conditions, TEM and HRTEM examinations indicate that the deposited nanostructures are bamboo-like multiwalled carbon nanotubes (MWNTs) with a wall thickness of 2030 graphene layers. Electrical conductivity of the as-deposited MWNTs can be greatly improved by subjecting to 760 °C heat treatment under nitrogen atmosphere. The results demonstrate that the amounts of CNTs and bridges are tunable with the Ni catalyst plating time. Under the present experimental configuration and at a catalyst plating time of 20 s, countable numbers of bridges can be obtained, which are selectively and horizontally grown on the areas of the pattern with Ni catalyst. This process can be a step approaching the application of CNTs in electronic devices.     

Iron nanoparticles in aligned arrays of pure and nitrogen-doped carbon nanotubes

Arrays of aligned carbon nanotubes (CNTs) and nitrogen-doped carbon (CN_x) nanotubes have been grown on silicon substrates as the result of thermolysis of ferrocene/toluene and ferrocene/acetonitrile mixture. The microstructure of materials was studied by transmission and scanning electron microscopy, and X-ray diffraction was used to control the carbon and iron forms. The composition and properties of iron nanoparticles developed in the CNT and CN_x nanotube samples were determined from Mössbauer spectroscopy data. The total iron content in CN_x nanotubes was found to be considerably higher than that in CNTs. Three forms of iron nanoparticles α-Fe, γ-Fe, and Fe₃C were detected in CNTs and only two last of them in CN_x nanotubes. In the interior of CNT channels the α-Fe and Fe₃C nanoparticles were observed to be coupled by a strong exchange interaction and to exhibit magnetic behavior at room temperature.     

Effects of CF4 plasma on the field emission properties of aligned multi-wall carbon nanotube films

We present a simple method to functionalize the surface and to modify the structures of aligned multi-wall carbon nanotube (CNT) arrays grown on silicon substrates using CF₄ plasma produced by reactive ion etching (RIE). Field emission (FE) measurements showed that after 2 min of plasma treatment, the emission currents were enhanced compared with as-grown CNTs; however, extended treatment over 2 min was found to degrade the FE properties of the film. Scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy have been employed to investigate the mechanism behind the modified FE properties of the CNT film. The FE enhancement after 2 min of etching could be attributed to favorable surface morphologies, open-ended structures and a large number of defects in the aligned CNT films. On the other hand, deposition of an amorphous layer comprising carbon and fluorine during extended CF₄ plasma treatment may hamper the field emission of CNT films.     

Graphene nanoribbons produced by the oxidative unzipping of single-wall carbon nanotubes

Graphene nanoribbons were synthesized by oxidative unzipping of single-wall carbon nanotubes (SWCNTs). The nanoribbons produced from SWCNTs were characterized using FT-IR, Raman and X-ray photoelectron spectroscopy. For the morphological study of the product obtained from the SWCNT unzipping reaction, transmission electron microscopy and atomic force microscopy were used, confirming the typical graphene nanoribbon structure.     

A composite material made of carbon nanotubes partially embedded in a nanocrystalline diamond film

A composite material, made of carbon nanotubes (CNTs) partially embedded in a nanocrystalline diamond film was produced. The diamond film was first decorated with palladium or nickel nanoparticles. An array of nanopores was drilled in the film in a hot filament CVD (HFCVD) reactor thanks to the anisotropic etching that takes place under the nanoparticles. During this etching process, the metallic particles penetrate the diamond film to a controlled depth, thus remaining at the bottom of the nanopores. The buried nanoparticles remain catalytically active and are used to grow a multiwall carbon nanotube forest using HFCVD in the same reactor without breaking the vacuum. The quality of the CNTs was assessed by scanning electron microscopy and Raman spectroscopy. The interface between the carbon nanotubes and the diamond was characterized by ultrasonication, lateral force microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. As a result of these characterizations, we demonstrate that the buried carbon nanotubes exhibit higher mechanical stability and improved electrical behavior compared to CNTs directly grown on the diamond surface.     

Chemical-free graphene by unzipping carbon nanotubes using cryo-milling

Current paper reports synthesis of chemical free graphene by unzipping of the carbon nanotubes (CNTs) using high strain rate deformation at 150 K. A specially designed cryomill operating at 150 K was used for the experiments. The mechanism of unzipping was further explored using molecular dynamics (MD) simulations. Both experimental and simulation results reveal two modes of unzipping through radial and shear loading.     

Fabrication of optical magnetic mirrors using bent and mushroom-like carbon nanotubes

Fabrication of an optical magnetic mirror using carbon nanotube-based 3-dimensional nano-structures is reported. Carbon nanostructures were grown in a plasma enhanced chemical vapor deposition reactor on Ni catalyst islands using a mixture of C₂H₂ and H₂ gases. Bent carbon nanotubes (CNTs) with a controllable bending angle were obtained by changing the direction of the electrical field of the applied plasma. Mushroom-like carbon nanostructures with a high-impedance surface were obtained by deposition of a gold layer on vertically grown CNTs using a sputtering system. Phase shift measurements of the fabricated nanostructured surfaces were carried out by an interferometer setup. Phase differences between the incident beam on the as-prepared surfaces and the reflected beam from them were found to be almost zero, indicating that the direction of the electric field had not been changed upon reflection. Scanning electron microscopy and atomic force microscopy were used for morphological study of the prepared samples.     

Parameter setting on growth of carbon nanotubes over transition metal/alumina catalysts in a fluidized bed reactor

This work investigates the synthesis of multilayered carbon nanotubes (CNTs) using the catalytic decomposition of acetylene at 700-850 °C over Fe- and Ni-supported Al₂O₃ catalysts in a fluidized bed reactor. Thermogravimetric analysis showed that the CNTs grown in a fluidized bed reactor have better thermal stability and higher production yield, compared to that in a fixed bed reactor. The CNT production yield increased with the growth temperature, and Fe-catalyst exhibited greater activity than Ni-catalyst in the formation of CNTs. According to Arrhenius plots, the apparent activation energies for the growth of CNTs were estimated to be 25.6 kJ/mol for Fe-catalyst and 65.6 kJ/mol for Ni-catalyst. The as-grown CNT products were characterized by high-resolution transmission electron spectroscopy, N₂ physisorption, Raman spectroscopy, and X-ray diffraction. After purification, the CNT products were of the multilayered type, which were composed of perfect graphene layers. The results of this study demonstrate that the fluidized bed technology favors the large-scale production of CNTs with uniformity and at low cost.     

Growth of carbon nanotubes on carbon fibers using the combustion flame oxy-acetylene method

Multi-walled carbon nanotubes (MWCNTs) were directly grown on carbon fibers (CFs) using the combustion flame oxy-acetylene method. Ferrocene deposited on the fiber surface acts as a catalyst for carbon nanotubes (CNTs) growth. The effects of ferrocene concentration on the morphology of the CNTs coating were investigated. Growth temperature ranges from 500 to 650 °C at atmospheric pressure, while growth surface is a continuous 10 × 1000 mm² tape. CNTs are produced with a dense entanglement, covering the CFs uniformly. Tube outer diameters are in the range of 20–40 nm. Tube length is quite long (about 4–5 μm) and uniform. Particularly, growth times are very short: about 0.3–0.6 s. Growth morphology and other characteristics of the as-grown tubes were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Energy dispersive X-ray (EDX) and by Raman spectroscopy.     

Synthesis and field emission properties of carbon nanotubes/nanofibers grown on pyrolyzed polyaniline–SiO2 substrates

Pyrolyzed polyaniline–SiO₂ substrates with the rough surface containing some holes were prepared by the pyrolysis of polyaniline–SiO₂ composites at temperature of 900 °C. Carbon nanotubes/nanofibers (CNTs/CNFs) were grown on the rough surface and inside the holes using a CVD method with a xylene–ferrocene mixture as a carbon and catalyst precursor source. The structural and morphological properties of CNTs were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicated that the SiO₂ content of the substrates was responsible to the diameter and electron field emission properties of CNTs.     

Experiments on C nanotubes synthesis by Fe-assisted ethane decomposition

We performed experiments on the synthesis of carbon nanotubes (CNTs) by iron-catalyzed chemical vapor deposition (CVD) in C₂H₆ + H₂ atmosphere. We varied flow-rates of reactant gases (ethane: 30–120 sccm, hydrogen: 0–120 sccm), as well as their ratio, in order to study the evolution of the growth kinetics. We used scanning electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy to investigate the morphologies, dimensions and crystalline structure of the samples. Our results demonstrate the crucial role played by H₂ in the enhancement of C diffusion-rate and in the consequent development of ordered and smooth graphene layers. A faster growth-rate is achieved by the increase of C₂H₆ flow-rate. However, if H₂ flow-rate is not adequately enhanced, the improvement is only apparent. The excess of C supplied gives rise to deposition of amorphous carbon onto the CNT walls, and to the co-production of different nanostructures. A substantial agreement is found with results reported for CVD growth of CNTs by the use of different catalysts, reactants and gas-flowing setups.     

Synthesis of carbon nanotubes on graphene quantum dot surface by catalyst free chemical vapor deposition

The growth of carbon nanotubes (CNTs) on graphene quantum dot surface has been explored using acetylene as the carbon source in a catalyst free chemical vapor deposition process. Dynamic studies were conducted to observe the CNT growth. The obtained nanotubes have a diameter distribution of 10–30 nm and show medium graphitic quality. Transmission electron microscopy observations and dynamic studies indicate that the formation of CNTs follows a different mechanism from traditional growth models, in which a wire-to-tube process and self-assembling of CNTs are involved. On the basis of these observations, a tentative continuous growth model is proposed for the CNT growth.     

Carbon nanotubes and carbon nanofibers fabricated on tubular porous Al2O3 substrates

Carbon nanotubes and carbon nanofibers were grown at different temperatures on porous ceramic Al₂O₃ substrates with single channel geometry by means of a chemical vapor deposition technique using methane as carbon source and palladium as catalyst. Time-resolved in-situ Fourier transformed infrared spectroscopy was used for the investigation of methane decomposition for characterizing the catalyst’s performance. With increasing synthesis temperature, a structural transition from carbon nanofibers to carbon nanotubes was observed. At a synthesis temperature of 700 °C, solely carbon nanofibers were found, whereas at 800 °C a mixture of two types of bamboo-shaped carbon nanofibers were obtained, suggesting a structural transition. A synthesis temperature to 850 °C results in bamboo-shaped multi-walled carbon nanofibers and multi-walled carbon nanotubes. The carbon products and the observed structural transition were characterized by means of field emission scanning electron microscopy, high-resolution transmission electron microscopy, thermal gravimetric analysis, and Raman spectroscopy.     

CVD growth and field electron emission of aligned carbon nanotubes on oxidized Inconel plates without addition of catalyst

Direct growth of carbon nanotubes (CNTs) on Inconel 600 sheets was investigated using plasma enhanced hot filament chemical vapor deposition in a gas mixture of methane and hydrogen. The Inconel 600 sheets were oxidized at different temperatures (800 °C, 900 °C, 1000 °C, and 1100 °C) before CNT deposition. The structure and surface morphology of the pre-treated substrate sheets and the deposited CNTs were studied by scanning electron microscopy (SEM) and X-ray diffraction. The field electron emission (FEE) properties of the CNTs were also tested. The SEM results show that well aligned CNTs have been grown on the pre-treated Inconel sheets without addition of any catalysts and the higher treatment temperature resulted in CNTs with better uniformity, indicating that the oxidation pre-treatment of the substrate is effective to enhance the CNT growth. FEE testing shows that CNTs with better height uniformity exhibit better FEE characteristics.     

Characterization and hydrogen storage in multi-walled carbon nanotubes grown by aerosol-assisted CVD method

The characterization and hydrogen storage capacity of multi-walled carbon nanotubes (MWCNTs) have been studied in the present work. MWCNTs with high purity and bulk yield were achieved from a mixture of camphor/alcohol on a Ni/zeolite support by aerosol-assisted chemical vapor deposition (AACVD). The morphology, surface quality and structure of MWCNTs were characterized by transmission electron microscopy (TEM). Crystallinity and defects of the MWCNTs were studied by Raman spectroscopy and thermo gravimetric analysis (TGA). Hydrogen storage properties of MWCNTs were investigated using a quartz crystal microbalance (QCM). Values between 1.2 and 2.0 wt.% of adsorbed H₂ were reached depending on the exposure pressure. The results also showed that the remaining zeolite present in the as-prepared MWCNT powder adsorbs hydrogen, allowing better adsorption performance of the CNT12 and CNT13 samples. The hydrogen adsorption behavior of CNTs is significantly affected by their structural and morphological characteristics.     

Carbon nanotubes coated with diamond nanocrystals and silicon carbide by hot-filament chemical vapor deposition below 200 °C substrate temperature

Multi-walled carbon nanotubes (MWCNTs) dispersed onto a silicon substrate have been coated with diamond nanocrystals (DNC) and silicon carbide (SiC) from solid carbon and silicon sources exposed to H₂ activated by hot filament chemical vapor deposition (HFCVD) at around 190 °C substrate temperature. MWCNT coating by DNC initiates during filament carburization process at 80 °C substrate temperature under conventional HFCVD conditions. The hybrid nanocarbon material was analyzed by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, electron energy loss spectroscopy, selected area electron diffraction, X-ray diffraction and Raman spectroscopy. The structure of the MWCNTs is preserved during coating and the smooth DNC/SiC coating is highly conformal. The average grain size is below 10 nm. The growth mechanism of DNC and SiC onto MWCNT surface is discussed.     

Synthesis of vertically-aligned carbon nanotubes without a catalyst by hydrogen arc discharge

Vertically-aligned carbon nanotubes (CNTs) were prepared by hydrogen arc discharge, using pure graphite powder as carbon source without catalysts added. Scanning and transmission electron microscopy characterizations show that the aligned CNTs have a bamboo-like structure, and their lengths and diameters are about 30 μm and 40–60 nm, respectively. No metallic impurities can be detected in the samples by careful X-ray photoelectron spectroscopy detection. The activation of hydrogen radicals, the heating effect of the arc, and the electric field surrounding the arc column area are considered to play important roles for the non-catalyst growth of the CNTs.