Growth of single-walled carbon nanotubes from size-selected catalytic metal particles

Single-walled carbon nanotubes (SWNTs) were synthesized using size-controlled catalyst nanoparticles created by the pulsed laser ablation method. Specifically, the alloy particles (Co/Mo or Co/Pt) were prepared by ablation of the target alloy materials in an inert gas atmosphere. Size selection was performed using a differential mobility analyzer (DMA). The obtained nanoparticles were deposited on a quartz substrate from which SWNTs were grown by the alcohol catalytic CVD (ACCVD) technique that was developed by the authors group. AFM and Raman scattering analysis revealed that SWNTs were successfully synthesized. It seems the Co/Mo alloy catalyst was more effective for the synthesis of SWNTs than the Co/Pt catalyst, though this is a preliminary result to be further investigated. PACS 36.40.-c; 61.46.+w; 65.80.+n; 78.30.Na; 81.07.de     

A facile and efficient gas phase process for purifying single-walled carbon nanotubes

Reproducible, low loss and high-yield purification process for single-walled carbon nanotubes (SWNTs) was developed by coupling of oxidation in wet air and treatment with gas phase nitric acid. Oxidation in wet air can burn out the carbonaceous particles and gas phase nitric acid treatment can etch away the catalytic metals. The as-synthesized SWNTs and the purified SWNTs were characterized by field emission scanning electron microscopy, high resolution transmission electron microscopy, Raman spectrophotometer and thermal gravimetric analysis. It was shown that such a facile and controllable gas phase process for purifying SWNTs can lead to the minimal losses and minimal destruction of SWNTs, thus highly purified SWNTs without amorphous carbon and with very low metal content were obtained.     

Controllable growth and characterization of isolated single-walled carbon nanotubes catalyzed by Co particles

Discrete Co catalytic nanoparticles with small diameters are obtained by pulsed vacuum arc evaporation on Si/SiO₂ substrates, which are used for the growth of isolated single-walled carbon nanotubes (SWNTs) by an ethanol chemical vapor deposition approach (CVD). The distributions of catalytic nanoparticles change with the number of arc pulses, which allows control of the nanotubes formation. We find that an increase of ethanol pressure during CVD growth can change SWNTs from isolated ones into bundles. A new growth mechanism which combines a tip and base model for SWNT growth has been tentatively proposed. It is suggested that the small size catalytic particles prepared by pulsed arc evaporation have a potential advantage for small diameter SWNT growth. PACS 78.67.Ch; 78.67.Bf; 78.67.-n; 81.07.De; 61.46.-w     

On the vibrations of single-walled carbon nanotubes

In this paper, a detailed numerical study on the free and forced vibrations of single walled carbon nanotubes is presented. A simple and straightforward method developed such that the proximity of the mathematical model to the actual atomic structure of the nanotube is significantly retained, is used for this purpose. Both zigzag and armchair chiralities of the carbon nanotubes for clamped-free and clamped-clamped boundary conditions are analyzed and their natural frequencies and corresponding mode shapes are obtained. Results pertaining to axial, bending, and torsional modes of vibration are reported with discussions. These modes of vibration appear in the eigen-values and eigen-vectors without any distinction. The direct integration method by Newmark is used extensively along with the fast Fourier transform to identify different types of vibrational modes. In the case of zigzag nanotubes, the axial, bending, and torsional modes appear to be decoupled, whereas the armchair nanotubes show coupling between such modes.     

金属铈催化剂对单壁纳米碳管生长和结构的影响

研究了一种利用新型催化剂制备单壁纳米碳管（SWNTs）的工艺方法.将金属铈的氧化物（Ce O₂）与石墨粉按一定比例混合，填充到已打好孔的石墨棒中制成复合石墨电极 ，以其为阳 极在高温氦电弧中实施电弧放电.放电后的生成物经高分辨透射电子显微镜（HRTEM）和拉曼 光谱(Raman)的观察及分析表明：生成物中含有大量呈束状存在的单壁纳米碳管，管径均匀 ，平均直径为120—132nm. 关键词： 单壁纳米碳管 电弧放电 透射电镜分析     

On the size of small single-walled carbon nanotubes

We derive a formula relative to the diameter/length ratio of a small single-walled carbon nanotube by means of calculations referring to Fermi energy. These calculations arise from the consideration of the Fermi's velocity for a multiwalled carbon nanotube.     

Zinc oxide catalyzed growth of single-walled carbon nanotubes

We demonstrate that zinc oxide can catalyze the growth of single-walled carbon nanotubes (SWNTs) with high efficiency by a chemical vapor deposition process. The zinc oxide nanocatalysts, prepared using a diblock copolymer templating method and characterized by atomic force microscopy (AFM), were uniformly spaced over a large deposition area with an average diameter of 1.7 nm and narrow size distribution. Dense and uniform SWNTs films with high quality were obtained by using a zinc oxide catalyst, as characterized by scanning electron microscopy (SEM), Raman spectroscopy, AFM, and high-resolution transmission electron microscopy (HRTEM).     

Electric field aligned growth of single-walled carbon nanotubes

Electric field aligned, single-walled carbon nanotubes are grown between electrodes using thermal chemical vapour deposition (CVD) of methane. The growth occurs on a thin film layered catalyst of aluminium, iron and molybdenum patterned on top of electrodes. The nanotubes bridge 10 μm sized electrode gaps and have a typical diameter of less than 2 nm as measured by Raman spectroscopy and atomic force microscopy. We present electrical transport measurements on a directly grown nanotube which shows p-type semiconducting behaviour.     

Effect of metal oxide and oxygen on the growth of single-walled carbon nanotubes by electric arc discharge

The effect of oxygen on the growth of single-walled carbon nanotubes was studied with Ni–Co alloy powder as catalyst under helium atmosphere of 500 Torr by electric arc discharge. The oxygen included in nickel or (and) cobalt oxides was added in catalyst. The content of oxygen in atmosphere was controlled by changing vacuum degree inside furnace before inputting buffer gas. The examinations of TEM and Raman scattering showed that oxygen in metal oxide as catalyst promotes the nucleation of SWCNT by taking effect on the metal catalyst particles. However, O₂ in atmosphere has the role of oxidizing amorphous particles along with nanotubes. When its molar proportion is higher than 0.22 ppm (Parts per million), the carbon nanotubes produced are oxidized and their purity decreases. The diameter of single-walled carbon nanotube obtained under different condition has a narrow distribution around 1.28 nm.     

Surface growth of single-walled carbon nanotubes from ruthenium nanoparticles

In this paper, we report that ruthenium is an active and efficient catalyst for growth of single-walled carbon nanotubes (SWNTs) by a chemical vapor deposition (CVD) process for the first time. High density random and horizontally superlong well-oriented SWNTs on substrate can be fabricated via CH₄ or EtOH as carbon source under suitable conditions. Scanning and transition electron microscopy investigations, Raman spectroscopy and atomic force microscopy measurements show the tubular structure, the high crystallinity, and the properties of the grown nanotubes. The results show that the SWNTs from ruthenium have better structural uniformity with less defects and provides an alternative catalyst for SWNTs growth. The successful growth of SWNTs by Ru catalyst provides new experimental information for understanding the growth mechanism of SWNTs, which may be helpful for their controllable synthesis.     

Reduced carbon solubility in Fe nanoclusters and implications for the growth of single-walled carbon nanotubes

Fe nanoclusters are becoming the standard catalysts for growing single-walled carbon nanotubes via chemical vapor decomposition. Contrary to the Gibbs-Thompson model, we find that the reduction of the catalyst size requires an increase of the minimum temperature necessary for the growth. We address this phenomenon in terms of solubility of C in Fe nanoclusters and, by using first-principles calculations, we devise a simple model to predict the behavior of the phases competing for stability in Fe-C nanoclusters at low temperature. We show that, as a function of particle size, there are three scenarios compatible with steady state growth, limited growth, and no growth of single-walled carbon nanotubes, corresponding to unaffected, reduced, and no solubility of C in the particles.     

On the electronic band structure of zigzag-type single-walled carbon nanotubes

The electronic band structure of a zigzag-type carbon nanotube has been computed by using the tight-binding approximation method in the framework of SSH Model Hamiltonian modified by the inclusion of two Lagrange multipliers instead of one. This modification yielded an electronic band structure consistent with the experimental reports that an infinite (3,0) zigzag-type single-walled carbon nanotubes displays a metallic behaviour.     

Conductivity of single-walled carbon nanotubes

In a system of N interacting single-level quantum dots (QDs), we study the relaxation dynamics and the current–voltage characteristics determined by symmetry properties of the QD arrangement. Different numbers of dots, initial charge configurations, and various coupling regimes to reservoirs are considered. We reveal that effective charge trapping occurs for particular regimes of coupling to the reservoir when more than two dots form a ring structure with the C_N spatial symmetry. We reveal that the effective charge trapping caused by the C_N spatial symmetry of N coupled QDs depends on the number of dots and the way of coupling to the reservoirs. We demonstrate that the charge trapping effect is directly connected with the formation of dark states, which are not coupled to reservoirs due to the system spatial symmetry C_N. We also reveal the symmetry blockade of the tunneling current caused by the presence of dark states.     

Nucleation of single-walled carbon nanotubes

The nucleation pathway for single-wall carbon nanotubes on a metal surface is demonstrated by a series of total energy calculations using density functional theory. Incorporation of pentagons at an early stage of nucleation is energetically favorable as they reduce the number of dangling bonds and facilitate curvature of the structure and bonding to the metal. In the presence of the metal surface, nucleation of a closed cap or a capped single-wall carbon nanotube is overwhelmingly favored compared to any structure with dangling bonds or to a fullerene.     

Hydrogenation of single-walled carbon nanotubes

Towards the development of a useful mechanism for hydrogen storage, we have studied the hydrogenation of single-walled carbon nanotubes with atomic hydrogen using core-level photoelectron spectroscopy and x-ray absorption spectroscopy. We find that atomic hydrogen creates C-H bonds with the carbon atoms in the nanotube walls, and such C-H bonds can be completely broken by heating to 600 degrees C. We demonstrate approximately 65 +/- 15 at % hydrogenation of carbon atoms in the single-walled carbon nanotubes, which is equivalent to 5.1 +/- 1.2 wt % hydrogen capacity. We also show that the hydrogenation is a reversible process.     

Controlling growth and Raman spectra of individual suspended single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) were prepared with ethanol chemical vapor deposition (CVD) on SiO₂ flat and pillar-patterned Si substrates. The effect of CVD temperatures from 600 to 800 °C on SWNTs yields was investigated. By virtue of its unperturbed environment, an individual suspended SWNT grown between two different SiO₂ pillars provides a possibility to study the phonon band structure of SWNT itself at a single-nanotube level. Raman spectra of individual SWNTs grown between pillars were investigated systematically.     

Effects of gas adsorption and collisions on electrical transport in single-walled carbon nanotubes

Degassing of bundles of single-walled carbon nanotubes in vacuum at 500 K is found to drive the thermoelectricpower (TEP) strongly negative, indicating that the degassed metallic tubes in a bundle are n type. The magnitude of the negative TEP indicates that important asymmetry in the electronic carbon pi bands exists near the Fermi energy. Easily measurable increases in the TEP ( approximately 5-10 &mgr;V/K) and resistivity ( 2%-10%) are observed at 500 K upon exposure to N2 and He, suggesting that even gas collisions with the nanotube wall can contribute significantly to the transport properties.     

Piezoresistive effect in single-walled carbon nanotubes

This paper reports on the results of the theoretical investigation of the piezoresistive effect in single-walled carbon nanotubes of two structural modifications: arm-chair type and zig-zag type. The variation in the band gap of semiconducting nanotubes under the influence of the compressive and tensile deformations has been analyzed. The main quantitative characteristic of the piezoresistive effect—the longitudinal component of the elastic conductivity tensor—has been calculated, and its dependence on the diameter of semiconducting nanotubes has been shown. The variants of practical implementation of the effect under study have been proposed.     

Axial-strain-induced torsion in single-walled carbon nanotubes

Using classical molecular dynamics and empirical potentials, we show that the axial deformation of single-walled carbon nanotubes is coupled to their torsion. The axial-strain-induced torsion is limited to chiral nanotubes-graphite sheets rolled around an axis that breaks its symmetry. Small strain behavior is consistent with chirality and curvature-induced elastic anisotropy (CCIEA)-carbon nanotube rotation is equal and opposite in tension and compression, and decreases with curvature and chirality. The large-strain compressive response is remarkably different. The coupling progressively decreases, in contrast to the tensile case, and changes its sign at a critical compressive strain. Thereafter, it untwists with increasing axial strain and then rotates in the opposite direction, i.e., the same sense as under tension. This suggests that the response is now dictated by a combination of nonlinear elasticity and CCIEA.