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     

Hydrogen storage in sonicated carbon materials

The hydrogen storage in purified single-wall carbon nanotubes (SWNTs), graphite and diamond powder was investigated at room temperature and ambient pressure. The samples were sonicated in 5 M HNO₃ for various periods of time using an ultrasonic probe of the alloy Ti-6Al-4V. The goal of this treatment was to open the carbon nanotubes. The maximum value of overall hydrogen storage was found to be 1.5 wt %, as determined by thermal desorption spectroscopy. The storage capacity increases with sonication time. The sonication treatment introduces particles of the Ti alloy into the samples, as shown by X-ray diffraction, transmission electron microscopy, and chemical analysis. All of the hydrogen uptake can be explained by the assumption that the hydrogen is only stored in the Ti-alloy particles. The presence of Ti-alloy particles does not allow the determination of whether a small amount of hydrogen possibly is stored in the SWNTs themselves, and the fraction of nanotubes opened by the sonication treatment is unknown. Received: 18 December 2000 / Accepted: 18 December 2000 / Published online: 9 February 2001     

Growth of single-wall carbon nanotubes dependent on laser power density and ambient gas pressure during room-temperature CO2 laser vaporization

Single-wall carbon nanotubes (SWNTs) were synthesized by the irradiation of 20-ms CO₂ laser pulses onto a graphite–Co/Ni target at room temperature. We investigated the effect of laser power density (10–150 kW/cm²) and ambient Ar gas pressure (150–760 Torr) on the abundance of SWNTs with lengths of up to about 200 nm in soot-like carbonaceous deposits. For a constant power density (30 kW/cm²), depending on the Ar gas pressure, SWNTs with diameters of 1.2–1.4 nm were synthesized. Expansion behavior and temperature-fall rates of clusters and/or particles in laser plumes were also analyzed by high-speed video imaging and temporally and spatially resolved emission spectroscopy. The temperature-fall rates were estimated to be 171–427 K/ms. The SWNT growth on the time scale of a few milliseconds appeared to be related to some features of condensing clusters and/or particles, including resident densities, collision frequencies and temperatures. Received: 16 July 2001 / Accepted: 23 July 2001 / Published online: 30 August 2001     

Causes of different catalytic activities of metals in formation of single-wall carbon nanotubes

When single-wall carbon nanotubes (SWNTs) were formed by pulsed Nd:YAG laser ablation or arc discharge, the yield depended on the metal catalyst: NiCo> Ni∼NiFe≫Co∼Fe>Pd∼Pt. It appears that an effective catalyst for SWNT growth must satisfy three conditions: it must be a good graphitization catalyst, have low solubility in carbon, and have a stable crystallographic orientation on graphite. NiCo, Ni, and NiFe satisfy these three conditions. The poor catalytic activities of Co, Fe, Pd, and Pt for SWNT formation would be explained by the ineffectiveness of Pt and Pd as graphitization catalysts, crystallographic orientation instability of Co crystals on graphite, and high solubility of Fe in graphite. Received: 29 October 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

Relaxed GexSi1−x layer formation with reduced dislocation density grown by ultrahigh vacuum chemical vapor deposition

A mixture of as-grown single-wall carbon nanotubes (SWNTs) and a monochlorobenzene (MCB) solution of polymethylmethacrylate (PMMA) was sonicated and homogenized. As a result, SWNTs were separated from carbonaceous impurities and metal particles, which enabled us to purify the SWNTs by filtration. We also found that the number of short (about 1-μm) SWNTs and thin bundles of SWNTs increased. The thin bundles contained one to three SWNTs. These short, thin SWNTs suspended in the MCB solution of PMMA were spin-coated onto a Si wafer, and could be dispersed on it. Received: 18 July 2000 / Accepted: 20 July 2000 / Published online: 6 September 2000     

碳纳米管晶格振动模及拉曼光谱的研究进展

本文介绍了碳纳米管的结构特征和晶格振动模的理论研究 ,综述了不同方法生长的多壁碳纳米管和单壁碳纳米管拉曼光谱的研究进展。另外 ,还简单描述了单壁碳纳米管的应用前景     

Catalytic activity of Fe, Co and Fe-Co-MCM-41 for the growth of carbon nanotubes by chemical vapour deposition method

Iron, cobalt and a mixture of iron and cobalt incorporated mesoporous MCM-41 molecular sieves were synthesised by hydrothermal method and used to investigate the rules governing their nanotube producing activity. The catalysts were characterised by XRD and N₂ sorption studies. The effect of the catalysts has been investigated for the production of carbon nanotubes at an optimised temperature 750 °C with flow rate of N₂ and C₂H₂ is 140 and 60 ml/min, respectively for a reaction time 10 min. Fe-Co-MCM-41 catalyst was selective for carbon nanotubes with low amount of amorphous carbon with increase in single-walled carbon nanotubes (SWNTs) yield at 750 °C. Formation of nanotubes was studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Transmission electron microscope and Raman spectrum was used to follow the quality and nature of carbon nanotubes formed and the graphitic layers and disordered band, which shows the clear evidence for the formation of SWNTs, respectively. The result propose that the diameter of the nanotubes in the range of 0.78-1.35 nm. Using our optimised conditions for this system, Fe-Co-MCM-41 showed the best results for selective SWNTs with high yield when compared with Fe-MCM-41 and Co-MCM-41.     

Quality of horizontally aligned single-walled carbon nanotubes: Is methane as carbon source better than ethanol?

Chemical vapor deposition (CVD) growth of horizontally aligned single-walled carbon nanotubes (SWNTs) was studied using two representative carbon source sources: ethanol and methane. The resulting SWNTs were compared for similar reaction conditions which were based on the formation of Ni metal nanoparticles selective electrochemical deposition (SED) on the defect sites of SWNTs. The products were analyzed by Raman spectroscopy and SEM. The results demonstrate that methane was much better carbon source for growing high quality horizontal alignment of SWNTs than ethanol due to the etching effects of OH radicals on the SWNTs.     

单层碳纳米管的磁输运特性

依据磁场中Boltzmann输运方程及单层磁纳米管(SWNTs)的能量色散关系,对单个SWNTs中轴向磁场诱发的低温磁阻进行了数值计算.分析表明:当电子以低能输运时,SWNTs的磁阻有明显的Aharonov-Bohn(A-B)效应,与并SWNTs的能隙相对应.     

Photoluminescence from single-walled carbon nanotubes: a comparison between suspended and micelle-encapsulated nanotubes

Single-walled carbon nanotubes (SWNTs) are luminescent. Up to now, two preparation methods, both of which isolate individual SWNTs, have enabled the detection of nanotube bandgap photoluminescence (PL): encapsulation of individual SWNTs into surfactant micelles and direct growth of individual SWNTs suspended in air between pillars. This paper compares the PL obtained from suspended SWNTs to published PL data obtained from encapsulated SWNTs. We find that emission peaks are blueshifted by 28 meV on average for the suspended nanotubes as compared to the encapsulated nanotubes. Similarly, the resonant absorption peaks at the second set of van Hove singularities are blueshifted on average by 16 meV. Both shifts depend weakly on the particular chirality and diameter of the SWNT. PACS 78.67.Ch; 78.55.-m     

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.     

Effects of carbon nanotube structures on mechanical properties

This paper describes a structural mechanics approach to modelling the mechanical properties of carbon nanotubes (CNTs). Based on a model of truss structures linked by inter-atomic potentials, a closed-form elastic solution is obtained to predict the mechanical properties of single-walled carbon nanotubes (SWNTs). Moreover, the elastic modulus of multi-walled carbon nanotubes (MWNTs) is also predicted for a group of the above mentioned SWNTs with uniform interval spacing. Following the structural mechanics approach, the elastic modulus, Poissons ratio, and the deformation behaviors of SWNTs were investigated as a function of the nanotube size and structure. Poissons ratio of SWNTs shows a chirality dependence, while the elastic modulus is insensitive to the chirality. The disposition of the strain energy of bonds shows quite a difference between the zigzag and armchair tubes subjected to axial loading. A zigzag tube is predicted to have a lower elongation property than an armchair tube. PACS 62.20-x; 62.20.Dc; 62.25+g     

Optical properties of fullerene and non-fullerene peapods

Single-wall carbon nanotubes (SWNTs) encapsulating fullerenes, so-called fullerene peapods, were synthesized in high yield by using diameter-selected nanotubes as pods. Transmission electron microscopy revealed high-density fullerene chains inside the nanotubes. X-ray-diffraction measurements indicate 85% filling for C₆₀ and 72% filling for C₇₀ molecules as a total yield. Interestingly, C₆₀ peas do not show any thermal expansion while C₇₀ peas show normal behavior. Room-temperature Raman spectra show one-dimensional photopolymerization of C₆₀ inside nanotubes by blue-laser irradiation, suggesting molecular rotation inside them. In C₇₀ peapods, no photopolymerization was observed but the relative Raman intensity of each peak is different from the C₇₀ 3D crystal. This is probably caused by mixing of two different crystal structures in C₇₀ peas. Furthermore, we synthesized Zn-diphenylporphyrin peapods. Optical absorption and Raman spectra suggest that the encapsulated molecules are deformed by interaction with the SWNT. Received: 12 November 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Synthesis, integration, and electrical properties of individual single-walled carbon nanotubes

High-quality single-walled carbon nanotubes (SWNTs) are synthesized by chemical vapor deposition (CVD) of methane on silicon-dioxide substrates at controlled locations using patterned catalytic islands. With the synthesized nanotube chips, microfabrication techniques are used to reliably contact individual SWNTs and obtain low contact resistance. The combined chemical synthesis and microfabrication approaches enable systematic characterization of electron transport properties of a large number of individual SWNTs. Results of electrical properties of representative semiconducting and metallic SWNTs are presented. The lowest two-terminal resistance for individual metallic SWNTs (≈5 μm long) is ≈16.5 kΩ measured at 4.2 K. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 14 July 1999     

Thermal properties of carbon nanotubes and nanotube-based materials

The thermal properties of carbon nanotubes are directly related to their unique structure and small size. Because of these properties, nanotubes may prove to be an ideal material for the study of low-dimensional phonon physics, and for thermal management, both on the macro- and the micro-scale. We have begun to explore the thermal properties of nanotubes by measuring the specific heat and thermal conductivity of bulk SWNT samples. In addition, we have synthesized nanotube-based composite materials and measured their thermal conductivity. The measured specific heat of single-walled nanotubes differs from that of both 2D graphene and 3D graphite, especially at low temperatures, where 1D quantization of the phonon bandstructure is observed. The measured specific heat shows only weak effects of intertube coupling in nanotube bundling, suggesting that this coupling is weaker than expected. The thermal conductivity of nanotubes is large, even in bulk samples: aligned bundles of SWNTs show a thermal conductivity of >200 W/m K at room temperature. A linear K(T) up to approximately 40 K may be due to 1D quantization; measurement of K(T) of samples with different average nanotube diameters supports this interpretation. Nanotube–epoxy blends show significantly enhanced thermal conductivity, showing that nanotube-based composites may be useful not only for their potentially high strength, but also for their potentially high thermal conductivity. Received: 17 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Field emission from single-walled carbon nanotubes aligned on a gold plate using a self-assembly monolayer

Field emission from single-walled carbon nanotubes (SWNTs) aligned on a patterned gold surface is reported. The SWNT emitters were prepared at room temperature by a self-assembly monolayer technique. SWNTs were cut into sub-micron lengths by sonication in an acidic solution. Cut SWNTs were attached to the gold surface by the reaction between the thiol groups and the gold surface. The field-emission measurements showed that the turn-on field was 4.8 V/μm at an emission current density of 10 μA/cm². The current density was 0.5 mA/cm² at 6.6 V/μm. This approach provides a novel route for fabricating CNT-based field-emission displays. Received: 3 May 2002 / Accepted: 6 May 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +82-54/279-8298, E-mail: ce20047@postech.ac.kr     

Pulsed laser CVD investigations of single-wall carbon nanotube growth dynamics

The nucleation and rapid growth of single-wall carbon nanotubes (SWNTs) were explored by pulsed-laser assisted chemical vapor deposition (PLA-CVD). A special high-power, Nd:YAG laser system with tunable pulse width (>0.5 ms) was implemented to rapidly heat (>3×10⁴°C/s) metal catalyst-covered substrates to different growth temperatures for very brief (sub-second) and controlled time periods as measured by in situ optical pyrometry. Utilizing growth directly on transmission electron microscopy grids, exclusively SWNTs were found to grow under rapid heating conditions, with a minimum nucleation time of >0.1 s. By measuring the length of nanotubes grown by single laser pulses, extremely fast growth rates (up to 100 microns/s) were found to result from the rapid heating and cooling induced by the laser treatment. Subsequent laser pulses were found not to incrementally continue the growth of these nanotubes, but instead activate previously inactive catalyst nanoparticles to grow new nanotubes. Localized growth of nanotubes with variable density was demonstrated through this process and was applied for the reliable direct-write synthesis of SWNTs onto pre-patterned, catalyst-covered metal electrodes for the synthesis of SWNT field-effect transistors.     

Formation of single-wall carbon nanotubes in Ar and nitrogen gas atmosphere by using laser furnace technique

The formation of single-wall carbon nanotubes (SWNTs) by using laser vaporization technique in different ambient gas atmosphere was investigated. SWNTs were prepared with Rh/Pd (1.2/1.2 atom%)-carbon composite rod in Ar and nitrogen gas atmosphere, respectively. Raman spectra of raw carbon materials including SWNTs and photoluminescence mapping of dispersed SWNTs in a surfactant solution demonstrate that the diameter distribution of SWNTs prepared in Ar atmosphere is narrower than those obtained by using CVD technique (e.g. HiPco nanotube), even when the ambient temperature is as high as 1150 ^○C. It was also found that nitrogen atmosphere gives wider diameter distribution of SWNTs than that obtained with Ar atmosphere. Furthermore, the relative yield of fullerenes (obtained as byproducts) is investigated by using HPLC (high-performance liquid chromatography) technique. It was found that the relative yield of higher fullerenes becomes lower, when nitrogen is used as an ambient gas atmosphere. Based on these experimental findings, a plausible formation mechanism of SWNTs is discussed.     

Effect of purification on the electron structure and field emission characteristics of a carbonaceous material containing single-wall carbon nanotubes

A carbonaceous material containing single-wall carbon nanotubes (SWNTs) has been synthesized by arc-discharge evaporation of graphite with a catalytic additive of nickel and cobalt powders. The synthesized SWNTs were purified from an amorphous carbon component (soot) and the catalyst particles by boiling in nitric acid. A comparison of the X-ray fluorescence spectra measured before and after this treatment showed that acid etching significantly decreased the content of soot in the material. The material enriched with SWNTs is characterized by a reduced threshold for the appearance of the field emission current, which is explained by a decrease in the screening effect of soot. The current-voltage characteristics of SWNTs exhibit a hysteresis, which is suggested to be due to the adsorption of molecules and radicals on the surface and at the ends of carbon nanotubes.     

Single-wall carbon nanotube chemical attachment at platinum electrodes

Self-assembled monolayer (SAM) techniques were used to adsorb 4-aminothiophenol (4-ATP) on platinum electrodes in order to obtain an amino-terminated SAM as the base for the chemical attachment of single-wall carbon nanotubes (SWCNTs). A physico-chemical, morphological and electrochemical characterizations of SWCNTs attached onto the modified Pt electrodes was done by using reflection-absorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and cyclic voltammetry (CV) techniques. The SWNTs/4-ATP/Pt surface had regions of small, medium, and large thickness of carbon nanotubes with heights of 100-200 nm, 700 nm to 1.5 μm, and 1.0-3.0 μm, respectively. Cyclic voltammetries (CVs) in sulfuric acid demonstrated that attachment of SWNTs on 4-ATP/Pt is markedly stable, even after 30 potential cycles. CV in ruthenium hexamine was similar to bare Pt electrodes, suggesting that SWNTs assembly is similar to a closely packed microelectrode array.