Growth of aligned single-walled carbon nanotubes under ac electric fields through floating catalyst chemical vapour deposition

Through floating catalyst chemical vapour deposition(CVD) method,well-aligned isolated single-walled carbon nanotubes (SWCNTs) and their bundles were deposited on the metal electrodes patterned on the SiO₂/Si surface under ac electric fields at relatively low temperature(280℃). It was indicated that SWCNTs were effectively aligned under ac electric fields after they had just grown in the furnace.The time for a SWCNT to be aligned in the electric field and the effect of gas flow were estimated. Polarized Raman scattering was performed to characterize the aligned structure of SWCNTs. This method would be very useful for the controlled fabrication and preparation of SWCNTs in practical applications.     

Production of individual suspended single-walled carbon nanotubes using the ac electrophoresis technique

We have developed a new method for aligning individual suspended single-walled carbon nanotubes (SWNTs) using a combination of the ac electrophoresis technique and electron beam lithography. A poly(methyl methacrylate) (PMMA) underpinning was used in the region between the Au electrodes to prevent the carbon nanotube (CNT) from falling down on to the substrate. O₂ plasma ashing was used to control the height of the underpinning PMMA layer so that it was same as that of the pre-defined electrodes. The biggest advantage of this method is that one can easily align the suspended nanostructure in a controllable manner. This method can also be applied to making suspended structures of organic materials that are sensitive to acid treatment. We measured the temperature-dependent I–V characteristics of the suspended SWNTs and found that most of the aligned SWNTs were metallic. PACS 61.46.+w; 73.63.Fg; 81.07.De; 81.16.Rf; 85.35.Kt     

Synthesis and characterization of carbon nanotubes on carbon microfibers by floating catalyst method

In this paper, carbon nanotubes were synthesized on carbon microfibers by floating catalyst method with the pretreatment of carbon microfibers at the temperature of 1023 K, using C₂H₂ as carbon source and N₂ as carrier gas. The morphology and microstructure of carbon nanotubes were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The composition of carbon nanotubes was determined by energy dispersive X-ray spectroscopy (EDX). The results showed that the surface of treated carbon microfibers was thickly covered by carbon nanotubes with diameters of about 50 nm. EDX image indicated that the composition of carbon nanotubes was carbon. In comparison with the sample grown on untreated carbon microfibers surface, it was found that after carbon microfibers were boiled in the solution of sulfur acid and nitric acid (VH₂SO₄:VHNO₃ = 1:3) and immersed in the solution of iron nitrate and xylene, carbon nanotubes with uniform density can be grown on carbon microfibers surface. Based on the results, we concluded that the pretreatment of carbon microfibers had great effect on the growth of carbon nanotubes by floating catalyst method.     

Encapsulation of floating carbon nanotubes in SiO(2)

In many applications of carbon nanotubes (CNT), it is desirable to have them embedded in a dielectric such as SiO(2), without significantly impacting their electronic properties. Here we investigate the CNT-SiO(2) interface of an embedded CNT using first-principles calculations. We show that strong Si-O-C bonds form, suggesting the feasibility of SiO(2) deposition on CNTs. We further show that subsequent hydrogenation eliminates all the Si-O-C bonds, leading to floating CNTs with electronic properties very close to those of pristine CNTs in vacuum.     

Growth of carbon nanotubes using nanocrystalline carbon catalyst

The basic growth of carbon nanotubes (CNTs) involves dissociation of hydrocarbon molecules over a metal layer as a catalyst. Generally, the metals used for the catalyst include nickel, cobalt, gold, iron, platinum, and palladium. However, the metal catalyst used with CNTs could have a harmful influence on the electrical properties of electronic devices. Therefore, we propose the use of nanocrystalline carbon (nc-C) as the catalyst for the growth of CNTs. We used a nc-C catalyst layer deposited by the closed-field unbalanced magnetron (CFUBM) sputtering method, and CNTs were grown by the hot filament plasma-enhanced chemical vapor deposition (HF-PECVD) method with ammonia (NH₃) as a pretreatment and acetylene gas (C₂H₂) as a carbon source. The CNTs were grown on the nc-C layers pretreated with a variation of the pretreatment time. The characteristics of the pretreated nc-C layers and the grown CNTs were investigated by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) measurements. Also, the structural variation of the pretreated nc-C layers was investigated by Raman measurement. We used the nc-C catalyst without metal, and we confirmed that our CNTs were composed with only carbon elements through an EDS measurement. Also, the pretreatment time was attributed to the growth of CNTs.     

AC Stark effect in toroidal carbon nanotubes threaded with an ac magnetic flux

The ac Stark effect is investigated in the toroidal carbon nanotube system threaded with an ac magnetic flux. The Floquet theory is employed to deal with the time-dependent quantum problems. The time-averaged energy of the system is derived and is found to exhibit a strong relationship with an external field, and the modified energy gap has been presented. The ac flux enhances energy gaps to cause metal-semiconductor transition. The steady current has been obtained by employing the free energy approach, and the persistent current is a special case as the magnitude of the ac flux approaches zero. The photon-assisted current is quite different from the persistent current due to the absorption and emission of photons. The local density of states is obtained by calculating the Green's function in the Floquet state, and photon-resonant structures are observed. All of the novel features are associated with the ac Stark effect, which is caused by the modification of energy levels. Received 20 November 2002 / Received in final form 7 February 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: zhaohonk@yahoo.com     

Diameter-dependent thermal-oxidative stability of single-walled carbon nanotubes synthesized by a floating catalytic chemical vapor deposition method

In this paper, purified single-walled carbon naotubes (SWCNTs) with three different diameters were synthesized using a floating catalytic chemical vapor deposition method with ethanol as carbon feedstock, ferrocene as catalyst, and thiophene as growth promoter. The thermal-oxidative stability of different-diameter SWCNTs was studied by using thermal analysis (TG, DTA), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis. The results indicate that small diameter SWCNTs (∼1 nm) are less stable and burn at lower temperature (610 °C), however, the larger diameter SWCNTs (∼5 nm) survive after burning at higher temperature (685 °C), the oxidation rate varies inversely with the tube diameter of SWCNTs, which may be concluded that the higher oxidation-resistant temperature of larger diameter SWCNTs can be attributed to the lower curvature-induced strain by rolling the planar graphene sheet for the larger diameter, so small tubes will become thermodynamically unstable.     

Chemisorption of hydrogen by carbon nanotubes

Chemisorption of hydrogen by carbon nanotubes (CNTs) is studied by thermodynamics and kinetics methods. Expressions are derived for the adsorption isotherm and desorption kinetics. Methods for determining chemisorption parameters are developed. The partial free energy of binding of hydrogen with a CNT (3.6 eV) is determined. It is shown that residual products of synthesis are removed from CNTs as a result of prolonged annealing at high temperatures. The capacity of a CNT relative to chemisorbed hydrogen is esti mated at 4 mass %.     

Mono-dispersed single-walled carbon nanotubes made by using arc-burning method in nitrogen atmosphere

Single-walled carbon nanotubes made by using arc-burning technique in nitrogen atmosphere were dispersed in sodium cholate (SC) solution, and the diameter and chirality distribution of semiconductive SWNTs was investigated by UV-VIS-NIR and Raman spectroscopy, and photoluminescence mapping technique. In the typical formation condition, the diameter distribution of them is found to be relatively narrow (1.2 nm–1.4 nm in diameter), less chirality dependent, almost the same as the diameter distribution of SWNTs obtained by using laser-furnace technique.     

Wave propagation in double walled carbon nanotubes by using doublet mechanics theory

Flexural and axial wave propagation in double walled carbon nanotubes embedded in an elastic medium and axial wave propagation in single walled carbon nanotubes are investigated. A length scale dependent theory which is called doublet mechanics is used in the analysis. Governing equations are obtained by using Hamilton principle. Doublet mechanics results are compared with classical elasticity and other size dependent continuum theories such as strain gradient theory, nonlocal theory and lattice dynamics. In addition, experimental wave frequencies of graphite are compared with the doublet mechanics theory. It is obtained that doublet mechanics gives accurate results for flexural and axial wave propagation in nanotubes. Thus, doublet mechanics can be used for the design of electro-mechanical nano-devices such as nanomotors, nanosensors and oscillators.     

单壁碳纳米管吸附对三联苯的研究

利用对三联苯对单壁碳纳米管进行了化学修饰,并利用透射电镜、紫外可见吸收光谱、拉曼光谱对修饰后的单壁碳纳米管进行了表征分析.通过对比吸附前后的紫外可见吸收光谱发现,吸附后的光谱强度大约下降63.1%,说明单壁碳纳米管吸附上了对三联苯.通过拉曼光谱分析发现,吸附后单壁碳纳米管的拉曼光谱中主要峰的位置向长波方向移动了6～7 cm-1,认为拉曼光谱发生移动的原因是单壁碳纳米管吸附对三联苯前后状态的改变导致的.     

All-fiber polarization locked vector soliton laser using carbon nanotubes

We report an all-fiber mode-locked erbium-doped fiber laser (EDFL) employing carbon nanotube (CNT) polymer composite film. By using only standard telecom grade components, without any complex polarization control elements in the laser cavity, we have demonstrated polarization locked vector solitons generation with duration of ~583 fs, average power of ~3 mW (pulse energy of 118 pJ) at the repetition rate of ~25.7 MHz.     

High-performance electric double-layer capacitors using mass-produced multi-walled carbon nanotubes

Mass-produced multi-walled carbon nanotubes (MWNTs, which have the trademark VGNF®) have been investigated for their potential for use in electric double-layer capacitors (EDLCs). The variation aspects of these MWNTs by KOH activation showed quite interesting features. The gravimetric capacitance enhancement and specific surface area on KOH activation increased linearly. However, the capacitance per unit surface area has a maximum at 200 wt.?% of KOH addition. The VGNF-KOH 500 sample exhibits a capacitance enhancement as much as 13 times greater (28.3 F/g) than that of the as-grown materials (2.2 F/g), under the conditions of charging up to 3.5 V and discharging at a current density of 10 mA/cm². Interestingly, for this MWNT (VGNF®), selective attack on its amorphous carbon impurity has also been observed, as demonstrated from both scanning electron microscopy observations and Raman spectra. Consequently, the results of this study will provide insight into the potentiality of using MWNTs for EDLC electrodes, which would enable the cheapest production cost among the various types of carbon nanotubes.     

Catalytic production of carbon nanotubes by vapor phase growth method using tungsten-containing organic compound

We report the production of carbon nanotubes of high purity by a vapor phase growth method using a catalytic reaction of a tungsten-containing organic compound (C₁₄H₁₀O₇W) and acetylene mixture. The products were multi-walled carbon nanotubes with hollow cores. Transmission electron microscopy investigation revealed that the graphitic layers at the inner region were highly crystallized but the graphitic layers at the outer region had a wavy structure. We have demonstrated the effective production of carbon nanotubes using a tungsten-based catalyst. PACS 81.07.De; 61.46.+w; 68.37.-d     

Synthesis of single wall carbon nanotubes by using arc discharge technique in nitrogen atmosphere

Single wall carbon nanotubes (SWNTs) were prepared with arc discharge technique using Ni/Co carbon composite rod in He, Ar, and N₂ atmosphere, respectively. The yield and the diameter distribution of them were compared with each other. The results show that N₂ atmosphere at low pressure gives the highest yield for the formation of SWNTs, almost comparable to that obtained with laser furnace technique. It also declares that He atmosphere seems to make SWNTs having smaller diameter distribution than those obtained in N₂ and Ar atmosphere. These findings were summarized and used for the discussion related to the formation mechanism of SWNTs.     

Increasing the octane number of gasoline using functionalized carbon nanotubes

The octane number is one of the characteristics of spark-ignition fuels such as gasoline. Octane number of fuels can be improved by addition of oxygenates such as ethanol, MTBE (methyl tert-butyl ether), TBF (tertiary butyl formate) and TBA (tertiary butyl alcohol) as well as their blends with gasoline that reduce the cost impact of fuels. Carbon nanotubes (CNTs) are as useful additives for increasing the octane number. Functionalized carbon nanotubes containing amide groups have a high reactivity and can react with many chemicals. These compounds can be solubilized in gasoline to increase the octane number. In this study, using octadecylamine and dodecylamine, CNTs were amidated and the amino-functionalized carbon nanotubes were added to gasoline. Research octane number analysis showed that these additives increase octane number of the desired samples. X-ray diffraction (XRD), Fourier transforms infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and thermal gravimetry analyses (TGA) were used for characterization of the prepared functionalized carbon nanotubes.     

Structure of single-wall carbon nanotubes purified and cut using polymer

Following on from our previous report that a monochlorobenzene solution of polymethylmethacrylate is useful for purifying and cutting single-wall carbon nanotubes (SWNTs) and thinning SWNT bundles, we show in this report that polymer and residual amorphous carbon can be removed by burning in oxygen gas. The SWNTs thus obtained had many holes (giving them a worm-eaten look) and were thermally unstable. Such severe damage caused by oxidation is unusual for SWNTs; we think that they were chemically damaged during ultrasonication in the monochlorobenzene solution of polymethylmethacrylate. Received: 28 March 2001 / Accepted: 2 August 2001 / Published online: 17 October 2001     

Manipulation and soldering of carbon nanotubes using atomic force microscope

Manipulation of carbon nanotubes (CNTs) by an atomic force microscope (AFM) and soldering of CNTs using Fe oxide nanoparticles are described. We succeeded to separate a CNT bundle into two CNTs or CNT bundles, to move the separated CNT to a desirable position, and to bind it to another bundle. For the accurate manipulation, load of the AFM cantilever and frequency of the scan were carefully selected. We soldered two CNTs using an Fe oxide nanoparticle prepared from a ferritin molecule. The adhesion forces between the soldered CNTs were examined by an AFM and it was found that the CNTs were bound, though the binding force was not strong.     

Molecular junctions by joining single-walled carbon nanotubes

Crossing single-walled carbon nanotubes can be joined by electron beam welding to form molecular junctions. Stable junctions of various geometries are created in situ in a transmission electron microscope. Electron beam exposure at high temperatures induces structural defects which promote the joining of tubes via cross-linking of dangling bonds. The observations are supported by molecular dynamics simulations which show that the creation of vacancies and interstitials induces the formation of junctions involving seven- or eight-membered carbon rings at the surface between the tubes.     

Liquid dimethyl sulphoxide confined by carbon nanotubes

Here, we report the molecular dynamics simulation on liquid dimethyl sulphoxide (DMSO) confined by single-walled carbon nanotubes (SWCNTs) in comparison with DMSO in the bulk phase at 298 K. The local order of DMSO, analysed in terms of radial distribution functions is similar to that in the bulk except the case with the SWCNT (8, 8) where the anomalous structure pattern is realized. Meanwhile, the translational self-diffusion coefficients of DMSO in confinements are much lower then in the bulk phase (by a factor of 2–3) and correlate with a value of the SWCNT internal diameter. Using cylindrical distribution functions of DMSO atoms we elucidate that the slowdown of self-diffusion coefficient of DMSO confined in the SWCNTs is reduced by the first layer of DMSO molecules close to the SWCNT wall.