Coherent phonon anisotropy in aligned single-walled carbon nanotubes

By time-resolved reflectivity measurements with sub-10 fs laser pulses at 395 nm, the coherent phonons of aligned bundles of single-walled carbon nanotubes are observed for various polarization directions of the pump and probe pulses. In the isotropic reflectivity measurement, we observe the radial breathing modes, G, and even D modes, while in the anisotropic reflectivity mode, only the G mode appears. A complex polarization dependence of the G band phonon amplitude in the isotropic reflectivity is explained by the superposition of G band phonons with different symmetries.     

Controlling nonequilibrium phonon populations in single-walled carbon nanotubes

We studied spatially isolated single-walled carbon nanotubes (SWNTs) immobilized in a quasi-planar optical lambda/2-microresonator using confocal microscopy and spectroscopy. The modified photonic mode density within the resonator is used to selectively enhance or inhibit different Raman transitions of SWNTs. Experimental spectra are presented that exhibit single Raman bands only. Calculations of the relative change in the Raman scattering cross sections underline the potential of our microresonator for the optical control of nonequilibrium phonon populations in SWNT.     

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.     

Bound excitons in metallic single-walled carbon nanotubes

We extend previous ab initio calculations on excitonic effects in metallic single-walled carbon nanotubes to more experimentally realizable larger diameter tubes. Our calculations predict bound exciton states in both the (10,10) and (12,0) tubes with binding energies of approximately 50 meV providing experimentally verifiable changes to the absorption line shape in each case. The second and third van Hove singularities in the joint density of states also give rise to a single optically active bound or resonant excitonic state.     

Optical transitions in metallic single-walled carbon nanotubes

We report vertical electronic transitions of 20 metallic single-walled carbon nanotubes calculated as band energy differences from Kohn-Sham density functional theory. Our first-order transitions (E11) calculated with hybrid functionals (containing a portion of exact exchange) are in very good agreement with available experimental data. Recently, we have reported similar agreement between experiment and theory for semiconducting tubes. We find that the trigonal warping splitting in the band structure of metallic tubes is about 1.5 to 2 times larger than that reported previously.     

Reversible metal-insulator transitions in metallic single-walled carbon nanotubes

We report on reversible metal to insulator transitions in metallic single-walled carbon nanotube devices induced by repeated electron irradiation of a nanotube segment. The transition from a low-resistive, metallic state to a high-resistive, insulating state by 3 orders of magnitude was monitored by electron transport measurements. Application of a large voltage bias leads to a transition back to the original metallic state. Both states are stable in time, and transitions are fully reversible and reproducible. The data is evidence for a local perturbation of the nanotube electronic system by removable trapped charges in the underneath substrate and excludes structural damage of the nanotube. The result has implications for using electron-beam lithography in nanotube device fabrication.     

金属型和半导体型单壁碳纳米管的分离研究进展

单壁碳纳米管(SWNTs)有优良的电学性能,但是目前制备的单壁碳纳米管都是金属型(met-)和半导体型(sem-)SWNTs的混合物,极大地限制了SWNTs进一步的应用。以物理法和化学法概述了近年来met-和sem-SWNTs的分离方法,并对各方法进行了机理分析和优劣比较。     

Highly selective dispersion of single-walled carbon nanotubes using aromatic polymers

Solubilizing and purifying carbon nanotubes remains one of the foremost technological hurdles in their investigation and application. We report a dramatic improvement in the preparation of single-walled carbon nanotube solutions based on the ability of specific aromatic polymers to efficiently disperse certain nanotube species with a high degree of selectivity. Evidence of this is provided by optical absorbance and photoluminescence excitation spectra, which show suspensions corresponding to up to approximately 60% relative concentration of a single species of isolated nanotubes with fluorescence quantum yields of up to 1.5%. Different polymers show the ability to discriminate between nanotube species in terms of either diameter or chiral angle. Modelling suggests that rigid-backbone polymers form ordered molecular structures surrounding the nanotubes with n-fold symmetry determined by the tube diameter.     

Separation of metallic and semiconducting single-walled carbon nanotubes through fluorous chemistry

Separation of metallic from semiconducting single-walled carbon nanotubes has been a major challenge for some time and some previous efforts have resulted in partial success. We have accomplished the separation effectively by employing fluorous chemistry wherein the diazonium salt of 4-heptadecafluorooc tylaniline selectively reacts with the metallic nanotubes present in the mixture of nanotubes. The resulting fluoroderivative was extracted in perfluorohexane leaving the semiconducting nanotubes in the aqueous layer. The products have been characterized by both Raman and electronic absorption spectroscopy. The method avoids the cumbersome centrifugation step required by some other procedures. This article is published with open access at Springerlink.com     

Colored semitransparent conductive coatings consisting of monodisperse metallic single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are promising materials for transparent conduction as a result of their exceptional electrical, optical, mechanical, and chemical properties. However, since current synthetic methods yield polydisperse mixtures of SWNTs, the performance of SWNT transparent conductive films has previously been hindered by semiconducting species. Here, we describe the performance of transparent conductors produced using predominantly metallic SWNTs. Compared with unsorted material, films enriched in metallic SWNTs can enhance conductivity by factors of over 5.6 in the visible and 10 in the infrared. Moreover, by using monodisperse metallic SWNTs sorted with angstrom-level resolution in diameter, semitransparent conductive coatings with tunable optical transmittance can be produced.     

Gel-based separation of single-walled carbon nanotubes for metallic and semiconducting fractions

Common technique for biomaterials recovery in genetics is freeze-squeeze procedure. However, this method found a new application in carbon nanotubes field in a selective separation of metallic and semiconducting nanotubes. None-commercial agarose gel acts as a selective absorbent for semiconducting nanotubes and allows to separate them from metallic type of nanotubes. In this work we point out the great potential of freeze-squeeze technique in the field of separation of nanotubes and prove that the post-separation purification procedure is crucial to perform the quality and quantity estimation of the fractionated samples. Furthermore, the detailed quantitative analysis of the efficiency of this process is shown. Additionally, we emphasize that this technique can be used for high-scale separation of metallic counterparts of single-walled carbon nanotubes due to its simplicity and low cost.     

Why semiconducting single-walled carbon nanotubes are separated from their metallic counterparts

The separation of single-walled carbon nanotubes (SWNTs) according to their electronic structure has attracted much recent attention. In many cases, metallic SWNTs are separated from semiconducting SWNTs and enriched in the supernatant due to stronger interaction between metallic SWNTs and adsorbates. However, the inverse separation of semiconducting from metallic SWNTs is often observed. In this computational study, the underlying mechanism is elucidated by density functional theory. We show that the shape of an aromatic molecule, the degree of hybridization between a molecule and a SWNT, and the oxidative state of SWNTs can affect the type of enriched SWNTs. In principle, one can control the type of enriched SWNTs by selecting a structurally compatible aromatic molecule or changing the hole concentration of the SWNTs.     

Water-assisted self-sustained burning of metallic single-walled carbon nanotubes for scalable transistor fabrication

Nano Research - Although aligned arrays of semiconducting single-walled carbon nanotubes (s-SWNTs) are promising for use in next-generation electronics owing to their ultrathin bodies and ideal...     

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%.     

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.     

Direct identification of metallic and semiconducting single-walled carbon nanotubes in scanning electron microscopy

Because of their excellent electrical and optical properties, carbon nanotubes have been regarded as extremely promising candidates for high-performance electronic and optoelectronic applications. However, effective and efficient distinction and separation of metallic and semiconducting single-walled carbon nanotubes are always challenges for their practical applications. Here we show that metallic and semiconducting single-walled carbon nanotubes on SiO(2) can have obviously different contrast in scanning electron microscopy due to their conductivity difference and thus can be effectively and efficiently identified. The correlation between conductivity and contrast difference has been confirmed by using voltage-contrast scanning electron microcopy, peak force tunneling atom force microscopy, and field effect transistor testing. This phenomenon can be understood via a proposed mechanism involving the e-beam-induced surface potential of insulators and the conductivity difference between metallic and semiconducting SWCNTs. This method demonstrates great promise to achieve rapid and large-scale distinguishing between metallic and semiconducting single-walled carbon nanotubes, adding a new function to conventional SEM.     

Effect of cutting and functionalization of single-walled carbon nanotubes on their dispersion behaviors

Cutting of single-walled carbon nanotubes (SWCNTs) and their modification with poly (ethylene glycol) and polystyrene were successfully performed through a treatment of the SWCNTs with piranha solutions and monoamine terminated polymers. The shortening and functionalization of SWCNTs were confirmed through size exclusion chromatography and Raman spectroscopy observations. The shortened SWCNTs exhibited more aggregated morphology than as-prepared SWCNTs in scanning electron microscope and atomic force microscope observations. Cutting and functionalization of SWCNTs with different polymer chains effectively tailored their dispersion behaviors in solvents. Polystyrene composite films exhibited improved transparency employing shortened SWCNTs, suggesting that the size of SWCNTs are critical factors in controlling the transparency of polymer composite films.