Cutting single-walled carbon nanotubes

A two-step process is utilized for cutting single-walled carbon nanotubes (SWNTs). The first step requires the breakage of carbon-carbon bonds in the lattice while the second step is aimed at etching at these damage sites to create short, cut nanotubes. To achieve monodisperse lengths from any cutting strategy requires control of both steps. Room-temperature piranha and ammonium persulfate solutions have shown the ability to exploit the damage sites and etch SWNTs in a controlled manner. Despite the aggressive nature of these oxidizing solutions, the etch rate for SWNTs is relatively slow and almost no new sidewall damage is introduced. Carbon-carbon bond breakage can be introduced through fluorination to ～C(2)F, and subsequent etching using piranha solutions has been shown to be very effective in cutting nanotubes. The final average length of the nanotubes is approximately?100?nm with carbon yields as high as 70-80%.     

Role of catalysts in the surface synthesis of single-walled carbon nanotubes

We demonstrate the role of catalysts in the surface growth of single-walled carbon nanotubes (SWNTs) by reviewing recent progress in the surface synthesis of SWNTs. Three effects of catalysts on surface synthesis are studied: type of catalyst, the relationship between the size of catalyst particles and carbon feeding rates, and interactions between catalysts and substrates. Understanding of the role of catalysts will contribute to our ability to control the synthesis of SWNTs on various substrates and facilitate the fabrication of nanotube-based devices.      

Diameter controlled chemical vapor deposition synthesis of single-walled carbon nanotubes

In this study, we systematically investigated the influence of catalyst preparation procedures on the mean diameter of single-walled carbon nanotubes (SWNTs) synthesized by the alcohol catalytic chemical vapor deposition (ACCVD) process. It was found that the SWNT diameter is dependent upon both reduction temperature and time, with lower reduction temperature and/or shorter reduction time resulting in smaller diameter SWNTs. The morphology of the SWNTs also changed from vertically aligned to randomly oriented when the reduction temperature was below 500 degrees C. We also found that introducing a small amount of water during the catalyst reduction stage significantly decreased the mean diameter of the SWNTs. Lastly, we report on the use of a new binary catalyst system in which rhodium was combined with cobalt. This new Co/Rh combination produced SWNTs of smaller diameter than the conventional Co/Mo catalyst.     

Formylation of single-walled carbon nanotubes

We report an improved, elegant method for the covalent formylation of single-wall carbon nanotubes (SWNTs) via formyl transfer from N-formylpiperidine, which could potentially open the gateway for more versatile chemical modification of carbon nanotube (CNT) walls than is possible via other reported functionalisation methods. The formylation reaction does not inflict damage upon the pristine CNT structure, unlike the currently commonly used carboxylation route, and involves much fewer steps, and takes considerably less time, than most other reported routes. The modified SWNTs have been characterised by Raman spectroscopy, ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy and "covalent tagging" with derivatising groups followed by thermogravimetric analysis-mass spectroscopy (TGA-MS). UV-vis-NIR spectroscopy shows that there is only limited disruption of the intrinsic electronic structure of the SWNTs. This is confirmed from estimates of the extent of functionalisation from TGA-MS, which suggest that it may be as low as 2 atomic per cent.     

Purification of single-walled carbon nanotubes

The discovery of carbon nanotubes (CNTs) created much excitement and stimulated extensive research into the properties of nanometer-scale cylindrical networks. From then on, various methods for the synthesis and characterization of aligned CNTs-both single-walled (SWCNTs) and multi-walled (MWCNTs) by different methods have been hotly pursued. Unfortunately, most methods currently in use produce raw multi component solid products, only a small fraction of which contains carbon nanotubes. The balance of the material is composed of residual catalyst particles (some of which are encased in concentric graphitic shells), fullerenes, other graphitic materials and amorphous carbon. These impurities cause a serious impediment for their detailed characterization and applications. If the carbon nanotube is ever to fulfill its promise as an engineering material, large, high quality aliquots will be required. A number of purification methods involving elimination processes such as physical separation, gas phase and liquid phase oxidation in combination with chemical treatments have been developed for nanotube materials. Though the quantitative determination of purity remains controversial, reported yields are best regarded with an appropriate level of skepticism on the method of assay. In this article, a review is given on the past and recent advances in purification of SWCNTs.     

Bound-state solitons in a linear-cavity fiber laser mode-locked by single-walled carbon nanotubes

The bound-state solitons generated from a nanotube-mode-locked linear-cavity fiber laser are experimentally observed and numerically simulated for the first time to author’s best knowledge. Two chirped fiber Bragg gratings are exploited as the mirrors for the proposed laser. The pulse duration of solitons is about 6.3?ps and the pulse-to-pulse separation is about 23?ps in the experimental observations. Numerical simulations confirm the experimental results and agree well with the experimental observations. Both experimental and numerical results show that the bound-state solitons are strongly stable.     

Wafer scale synthesis of dense aligned arrays of single-walled carbon nanotubes

Here we present an easy one-step approach to pattern uniform catalyst lines for the growth of dense, aligned parallel arrays of single-walled carbon nanotubes (SWNTs) on quartz wafers by using photolithography or polydimethylsiloxane (PDMS) stamp microcontact printing (μCP). By directly doping an FeCl₃/methanol solution into Shipley 1827 photoresist or polyvinylpyrrolidone (PVP), various catalyst lines can be well-patterned on a wafer scale. In addition, during the chemical vapor deposition (CVD) growth of SWNTs the polymer layers play a very important role in the formation of mono-dispersed nanoparticles. This universal and efficient method for the patterning growth of SWNTs arrays on a surface is compatible with the microelectronics industry, thus enabling of the fabrication highly integrated circuits of SWNTs.     

Optically active single-walled carbon nanotubes

The optical, electrical and mechanical properties of single-walled carbon nanotubes (SWNTs) are largely determined by their structures, and bulk availability of uniform materials is vital for extending their technological applications. Since they were first prepared, much effort has been directed toward selective synthesis and separation of SWNTs with specific structures. As-prepared samples of chiral SWNTs contain equal amounts of left- and right-handed helical structures, but little attention has been paid to the separation of these non-superimposable mirror image forms, known as optical isomers. Here, we show that optically active SWNT samples can be obtained by preferentially extracting either right- or left-handed SWNTs from a commercial sample. Chiral 'gable-type' diporphyrin molecules bind with different affinities to the left- and right-handed helical nanotube isomers to form complexes with unequal stabilities that can be readily separated. Significantly, the diporphyrins can be liberated from the complexes afterwards, to provide optically enriched SWNTs.     

Three dimensional single-walled carbon nanotubes

We report a simple fabrication method of creating a three-dimensional single-walled carbon nanotube (CNT) architecture in which suspended CNTs are aligned parallel to each other along the conventionally unused third dimension at lithographically defined locations. Combining top-down lithography with the bottom-up block copolymer self-assembly technique and utilizing the excellent film forming capability of polymeric materials, highly uniform catalyst nanoparticles with an average size of 2.0 nm have been deposited on sidewalls for generating CNTs with 1 nm diameter. This three-dimensional platform is useful for fundamental studies as well as technological exploration. The fabrication method described herein is applicable for the synthesis of other very small 1D nanomaterials using the catalytic vapor deposition technique.     

Raman study on single-walled carbon nanotubes with different laser excitation energies

The industrial use of carbon nanotubes is increasing day by day; therefore, it is very important to identify the nature of carbon nanotubes in a bundle. In this study, we have used the Raman spectroscopic analysis on vertically aligned single-walled carbon nanotubes (SWCNTs) grown by the chemical vapour deposition (CVD) technique. The grown sample is excited with two laser excitation wavelengths, 633 nm from He-Ne laser and 514·5 nm from Ar⁺ laser. Raman spectrum in the backscattering geometry provides the characteristic spectra of SWCNTs with its radial breathing mode (RBM), defect-induced disorder mode (D band), and high-energy modes (G and M bands). The Raman signal positions of the spectra in RBM, G and M bands confirm the grown sample to be of semiconducting type in nature.     

Charge-induced strains in single-walled carbon nanotubes

This paper investigates the electromechanical coupling in single-walled carbon nanotubes. In the model system, the extra electric charge of the nanotube is assumed to be uniformly distributed on carbon atoms. The electrostatic interactions between charged carbon atoms are calculated using the Coulomb law. The deformation of the charged nanotube is obtained by using the molecular structural mechanics method and considering the electrostatic interactions as an external loading acting on carbon atoms. The axial strain is found to be a symmetric function of applied charge, and our predictions are in very good agreement with those from ab initio calculations. The present results indicate that the nanotube aspect ratio has a strong effect on the axial strain when the ratio is less than 10 and the general trend is that the strain increases with the aspect ratio. The peak axial and radial strains occur at nanotube diameters of around 1.2-1.5?nm.     

单壁碳纳米管束的排布

流体排布法是实现碳纳米管定向排列的一种简单的方法。采用流体排布法在具有浸润性图案化的基底上成功地对单壁碳纳米管(SWNTs)束进行了水平方向上的排布。将SWNTs悬浮液滴入光刻胶制成的微通道中,在流体剪切力作用下,弯曲的SWNTs在一定程度上会被拉伸并且平行地排列在纳米级宽度的微通道中。将排列好的SWNTs阵列转移到一些不同间距的金电极对上面,制作成碳纳米管场效应晶体管(CNTFET)。CNTFET的电性能测试结果表明,制备的SWNTs束可以制造出不同电极间距同时具有良好电性能的CNTFET。     

Band theory of single-walled carbon nanotubes

In this paper, a curvilinear coordinate system is used in space and in k-space to study the energy band of single-walled carbon nanotubes wrapped at a helical angle. Using this method, a general function of the bandgap associated with the radius of the tube and the helical angle is derived based on the tight-binding theory. The three-dimensional hexagonal Brillouin zone of the tube is on the surface of cylinder in the k-space. For two tubes with different diameters, there is a distance between the cylindrical Brillouin zones in the radial direction. The Brillouin zone varies with the radius of the tube and the number of cells on the circumference. For the metallic zigzag tubes, the bandgaps decrease discretely to zero at the corners of the Brillouin zones, and those corners are singular points of zero gaps. With the transformation of coordinates, the metallic zigzag type is proven to be equivalent to an armchair configuration. Electrical characteristics of the chiral effects are briefly highlighted.     

Length-dependent extraction of single-walled carbon nanotubes

A two-phase liquid-liquid extraction process is presented which is capable of extracting water-soluble single-walled carbon nanotubes into an organic phase. The extraction utilizes electrostatic interactions between a common phase transfer agent and the sidewall functional groups on the nanotubes. Large length-dependent van der Waals forces for nanotubes allow the ability to control the length of nanotubes extracted into the organic phase as demonstrated by atomic force microscopy.     

Continued growth of single-walled carbon nanotubes

We demonstrate the continued growth of single-walled carbon nanotubes (SWNTs) from ordered arrays of open-ended SWNTs in a way analogous to epitaxy. Nanometer-sized metal catalysts were docked to the SWNT open ends and subsequently activated to restart growth. SWNTs thus grown inherit the diameters and chirality from the seeded SWNTs, as indicated by the closely matched frequencies of Raman radial breathing modes before and after the growth.     

Ozonolysis of functionalized single-walled carbon nanotubes

Room temperature ozonolysis of fluorinated SWNT and phenyl-sulfonated SWNT have been studied in perfluoropolyether (PFPE) solvents. Etching at the end caps (approximately 70 nm/hour for fluorinated SWNT/PFPE suspension with 1 g/l concentration) has been demonstrated to be the dominating effect during the ozonolysis of fluorinated SWNT. Base on characterization by AFM analysis, X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy, fluorination along the SWNT sidewalls protects F-SWNT from extensive functionalization by ozonolysis. An ozone reaction with fluorinated SWNT has been found to improve its solubility in 96% sulfuric acid. This allows oxidative cutting by ammonium peroxydisulfate without defluorination. In comparison to fluorinated SWNT, phenyl-sulfonated SWNT was found to be effectively and homogeneous cut by ozonolysis in a water suspension.     

RF response of single-walled carbon nanotubes

We present for the first time an in-depth study of the RF response of a single-walled carbon nanotube (SWCNT) rope. Our novel electrode design, based on a tapered coplanar approach, allows for single tube measurements well into the GHz regime, minimizing substrate-related parasitics. From the analysis of the S-parameters, the ac transport mechanism in the range 30 kHz to 6 GHz is established. This work is an essential prerequisite for the fabrication of high-speed devices based on bundles of nanowires or low-dimensional structures.     

Co-synthesis of single-walled carbon nanotubes and carbon fibers

A new vertical floating catalytic technique is developed and used to prepare both single-walled carbon nanotubes (SWNTs) and carbon fibers (CFs). Scanning electron microscopy (SEM) observation shows a clear separation of these two materials. Thin films of SWNTs can be peeled easily from the CF substrate which just acts as a catalyst support for the SWNT growth. The production process is also semicontinuous, resulting in a yield of ∼1.0 g h⁻¹ of SWNTs film with high purity. Structure and vibrational properties of these materials are investigated by electron microscopy and Raman spectroscopy, respectively.     

Microwave transport in metallic single-walled carbon nanotubes

The dynamical conductance of electrically contacted single-walled carbon nanotubes is measured from dc to 10 GHz as a function of source-drain voltage in both the low-field and high-field limits. The ac conductance of the nanotube itself is found to be equal to the dc conductance over the frequency range studied for tubes in both the ballistic and diffusive limit. This clearly demonstrates that nanotubes can carry high-frequency currents at least as well as dc currents over a wide range of operating conditions. Although a detailed theoretical explanation is still lacking, we present a phenomenological model of the ac impedance of a carbon nanotube in the presence of scattering that is consistent with these results.     

Long-lived charged states in single-walled carbon nanotubes

We present continuous wave photoinduced absorption spectroscopy of single-walled carbon nanotubes dispersed in a polymer matrix. The spectrum is dominated by a modulation of the absorption line shape, predominantly of large diameter tubes, that we assigned to electroabsorption caused by local electric fields arising from trapped photoinduced charges. The lack of selectivity in the excitation points to an efficient migration of the photoexcited states, either the singlet excitons or the charges resulting from their dissociation.