Capillary absorption of metal nanodroplets by single-wall carbon nanotubes

We present a simple model that demonstrates the possibility of capillary absorption of nonwetting liquid nanoparticles by carbon nanotubes (CNTs) assisted by the action of the Laplace pressure due to the droplet surface tension. We test this model with molecular dynamics simulation and find excellent agreement with the theory, which shows that for a given nanotube radius there is a critical size below which a metal droplet will be absorbed. The model also explains recent observations of capillary absorption of nonwetting Cu nanodroplets by carbon nanotubes. This finding has implications for our understanding of the growth of CNTs from metal catalyst particles and suggests new methods for fabricating composite metal-CNT materials.     

Ultralong single-wall carbon nanotubes

Since the discovery of carbon nanotubes in 1991 by Iijima, there has been great interest in creating long, continuous nanotubes for applications where their properties coupled with extended lengths will enable new technology developments. For example, ultralong nanotubes can be spun into fibres that are more than an order of magnitude stronger than any current structural material, allowing revolutionary advances in lightweight, high-strength applications. Long metallic nanotubes will enable new types of micro-electromechanical systems such as micro-electric motors, and can also act as a nanoconducting cable for wiring micro-electronic devices. Here we report the synthesis of 4-cm-long individual single-wall carbon nanotubes (SWNTs) at a high growth rate of 11 microm s(-1) by catalytic chemical vapour deposition. Our results suggest the possibility of growing SWNTs continuously without any apparent length limitation.     

Filling double-walled carbon nanotubes with WO3 and W nanowires via confined chemical reactions

Carbon nanotubes filled with metals and semiconductors have been regarded as one of the most promising materials for nanodevices. Here, we demonstrate a simple and effective method to produce tungsten trioxide (WO3) and tungsten (W) nanowires with diameters of below 4 nm inside double-walled carbon nanotubes (DWCNTs). First, the precursors, i.e., phosphotungstic acid (HPW, H3PW12O40) molecules, are successfully introduced into DWCNTs. Subsequent decomposition and reduction lead to the formation of WO3 and W nanowires inside DWCNTs. The products were carefully characterized by high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. FTIR spectra provide a direct proof that the HPW molecules enter the DWCNTs as an ionic state, i.e., PW12O40(3-) and H+, instead of the molecular state. HRTEM analysis shows that the diameter of the WO3 nanowires inside DWCNTs is 1.1-2.4 nm with the average length of 16-18 nm, and that for W nanowires is 1.2-3.4 nm with the average length of 15-17 nm. Meanwhile, DWCNTs are doped by the encapsulated WO3 and W nanowires. Tangential band shift in Raman spectra revealed the charge transfer between the nanowires and carbon nanotubes.     

Filling carbon nanotubes with particles

The filling of carbon nanotubes (CNTs) with fluorescent particles was studied experimentally and theoretically. The fluorescent signals emitted by the particles were visible through the walls of the nanotubes, and the particles inside the tubes were observable with an electron microscope. Taking advantage of the template-grown carbon nanotubes' transparency to fluorescent light, we measured the filling rate of the tubes with particles at room conditions. Liquids such as ethylene glycol, water, and ethylene glycol/water mixtures, laden with 50 nm diameter fluorescent particles, were brought into contact with 500 nm diameter CNTs. The liquid and the particles' transport were observed, respectively, with optical and fluorescence microscopy. The CNTs were filled controllably with particles by the complementary action of capillary forces and the evaporation of the liquid. The experimental results were compared and favorably agreed with theoretical predictions. This is the first report on fluorescence studies of particle transport in carbon nanotubes.     

Decarboxylation of oxidized single-wall carbon nanotubes

A classical protocol widely used in organic chemistry of aromatic and polyaromatic molecules has been successfully applied in this work for the decarboxylation of oxidized single-wall carbon nanotube (SWNT) to rend C-H SWNT derivatives. SWNT produced by arc discharge method have been oxidized during a purification process using strongly oxidant agents, such as hydrogen peroxide and nitric acid. The decarboxylation of oxidized SWNT has been conduced with copper(I) oxide in a 50:50 solution of N-methylpyrrolidone and quinoline. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and acid-base potentiometric titration analyses were carried out to characterize quali and quantitatively the changes in the chemical environment on the SWNT surface in each step of the purification and the decarboxylation process. Those techniques showed the appearance of mainly carboxylic and phenolic groups after the purification process and the disappearance of the carboxylic groups after the decarboxylation reaction. Fourier transform infrared spectroscopy analysis indicated also the formation of aliphatic and aromatic C-H groups. X-ray photoelectron spectroscopy and potentiometric titration results determined an efficiency higher than 90% for our decarboxylation procedure. The purity and structural quality of the SWNT sample used in the decarboxylation process were evaluated by thermogravimetry and Raman spectroscopy. Thermogravimetric analysis identified a purified sample with approximately 80 wt% of SWNT, in fractions distributed in highly structured SWNTs (25 wt%), with distribution in composition, length and structural quality (35 wt%) and with very defective and short tubes (25 wt%). The damages on the purified SWNT walls were characterized by the Raman scattering analysis.     

Biodistribution of carbon single-wall carbon nanotubes in mice

Carbon nanotubes are promising for use in biomedical and pharmaceutical sciences. Therefore, it becomes imperative to know the basic biological properties of carbon nanotubes in vivo. We labeled the water-soluble hydroxylated carbon single-wall nanotubes with radioactive 125I atoms, and then the tracer was used to study the distribution of hydroxylated carbon single-wall nanotubes in mice. They moved easily among the compartments and tissues of the body, behaving as small active molecules though their apparent mean molecular weight is tremendously large. This study, for the first time, affords a quantitative analysis of carbon nanotubes accumulated in animal tissues.     

Interaction of molecular and atomic hydrogen with single-wall carbon nanotubes

Density functional calculations are performed to study the interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes. Molecular physisorption is predicted to be the most stable adsorption state, with the molecule at equilibrium at a distance of 5-6 a.u. from the nanotube wall. The physisorption energies outside the nanotubes are approximately 0.07 eV, and larger inside, reaching a value of 0.17 eV inside the (5,5) nanotube. Although these binding energies appear to be lower than the values required for an efficient adsorption/desorption operation at room temperature and normal pressures, the expectations are better for operation at lower temperatures and higher pressures, as found in many experimental studies. A chemisorption state with the molecule dissociated has also been found, with the H atoms much closer to the nanotube wall. However, this state is separated from the physisorption state by an activation barrier of 2 eV or more. The dissociative chemisorption weakens carbon-carbon bonds, and the concerted effect of many incoming molecules with sufficient kinetic energies can lead to the scission of the nanotube.     

Fabrication of metal nanowires in carbon nanotubes via versatile nano-template reaction

A nano-template reaction has been developed to fabricate metal nanowires using metallofullerene nanopeapods (i.e., carbon nanotubes (CNTs) encapsulating endohedral metallofullerenes) as starting materials. In this nanometer-scale reaction, the structure of resulting products should possess specific low-dimensional structures, because of their uniform starting orientation of metallofullerene molecules and the rigid restriction of reaction space by the presence of walls of CNTs. Using the nano-template reaction, we have fabricated various Gd nanowires, including a single Gd atomic chain, a one-dimensional alignment of Gd squares, and Gd nanowires that correspond to a one-dimensional segment of the bulk close-packed structure. The same reaction, in principle, can be applied to fabricate more than 20 different types of metal nanowires in the CNTs, which simply are dependent on the use of the corresponding different types of metallofullerenes as encapsulates in the CNTs. The present novel reaction will provide a wide variety of unusual low-dimensional nanowires and nanomaterials in the CNTs, which have not been synthesized via the fabrication techniques that have been reported so far.     

Water-filled single-wall carbon nanotubes as molecular nanovalves

It is known that at low temperature, water inside single-wall carbon nanotubes (water-SWNTs) undergoes a structural transition to form tube-like solid structures. The resulting ice NTs are hollow cylinders with diameters comparable to those of typical gas molecules. Hence, the gas-adsorption properties of ice- and water-SWNTs are of interest. Here, we carry out the first systematic investigation into the stability of water-SWNTs in various gas atmospheres below 0.1 MPa by means of electrical resistance, X-ray diffraction, NMR measurements and molecular dynamics calculations. It is found that the resistivity of water-SWNTs exhibits a significant increase in gas atmospheres below a critical temperature Tc, at which a particular type of atmospheric gas molecule enters the SWNTs in an on-off fashion. On the basis of this phenomenon, it is proposed that water-SWNTs can be used to fabricate a new type of molecular nanovalve.     

Surface-enhanced Raman scattering on single-wall carbon nanotubes

Exploiting the effect of surface-enhanced Raman scattering (SERS), the Raman signal of single-wall carbon nanotubes (SWNTs) can be enhanced by up to 14 orders of magnitude when the tubes are in contact with silver or gold nanostructures and Raman scattering takes place predominantly in the enhanced local optical fields of the nanostructures. Such a level of enhancement offers exciting opportunities for ultrasensitive Raman studies on SWNTs and allows resonant and non-resonant Raman experiments to be done on single SWNTs at relatively high signal levels. Since the optical fields are highly localized within so-called "hot spots" on fractal silver colloidal clusters, lateral confinement of the Raman scattering can be as small as 5 nm, allowing spectroscopic selection of a single nanotube from a larger population. Moreover, since SWNTs are very stable "artificial molecules" with a high aspect ratio and a strong electron-phonon coupling, they are unique "test molecules" for investigating the SERS effect itself and for probing the "electromagnetic field contribution" and "charge transfer contribution" to the effect. SERS is also a powerful tool for monitoring the "chemical" interaction between the nanotube and the metal nanostructure.     

Reversible hole engineering for single-wall carbon nanotubes

Experimental results are provided for reversible generation of holes on single-wall carbon nanotubes and their closing by temperature treatment. The generation of the holes was analyzed by checking the amount of C60 fullerenes that can be filled into the tubes and subsequently transformed to an inner-shell tube. The concentration of the latter was determined from the Raman response of the radial breathing mode. The tube opening process was performed by exposure of the tubes to air at elevated temperatures. This process was found to be independent from the tube diameters. In contrast, the tube closing process was found to depend strongly of the tube diameter. For large diameter tubes (d = 1.8 nm) the activation energy was 1.7 eV whereas for the small diameter tubes this energy was only 0.33 eV. Optimum conditions for tube closing were found to be one hour at 800 degrees C or 10 minutes at 1000 degrees C. From the almost identical Raman spectra for the tubes before and after engineering, a predominant generation of the holes at the tube ends is concluded.     

Understanding catalysed growth of single-wall carbon nanotubes

Classical molecular dynamics simulations using a reactive force field, which allows simulation of bond-breaking and bond-forming, are carried out to investigate the several stages of a catalysed synthesis process of single-wall carbon nanotubes. The simulations assume instantaneous catalysis of a precursor gas on the surface of metallic nanoclusters, illustrating how carbon atoms dissolve in the metal cluster and then precipitate on its surface, evolving into various carbon structures, finally forming a cap which eventually grows to a single-wall nanotube. The results are discussed in the context of experimental synthesis results.     

Selective etching of metallic single-wall carbon nanotubes with hydrogen plasma

We present Raman scattering and scanning tunnelling microscopy (STM) measurements on hydrogen plasma etched single-wall carbon nanotubes (SWNTs). Interestingly, both the STM and Raman spectroscopy show that the metallic SWNTs are dramatically altered and highly defected by the plasma treatment. In addition, structural characterizations show that metal catalysts are detached from the ends of the SWNT bundles. For semiconducting SWNTs we observe no feature of defects or etching along the nanotubes. Raman spectra in the radial breathing mode region of plasma-treated SWNT material show that most of the tubes are semiconducting. These results show that hydrogen plasma treatment favours etching of metallic nanotubes over semiconducting ones and therefore could be used to tailor the electronic properties of SWNT raw materials.     

单壁碳纳米管制备研究新进展

碳纳米管由于具有独特的结构和奇异的物理、化学性质而倍受人们的青睐，并逐渐展示出巨大的应用前景。简要介绍了碳纳米管的结构、性质和应用，并详细评述了单壁碳纳米管制备的新进展。     

Analysis of localized failure of single-wall carbon nanotubes

A method is developed to determine the conditions for the onset of localized failure of carbon nanotubes. Examples of failure modes include ductile necking under tension or localized crushing under compression. A nanoscale continuum theory for carbon nanotube is adapted. The onset of localized failure is identified by the singularity point of the acoustic tensor derived from continuum energy function based on Tersoff–Brenner potential. The analysis predicts 35–44% of breaking strains for tension and 18–25% compressive strain for plastic collapse. The results are in agreement with molecular dynamics simulations and experimental estimations reported in the literature.     

Microwave-assisted functionalization of single-wall carbon nanotubes through diazonium

Single-wall carbon nanotubes (SWNTs) have been functionalized by a diazonium method through both a classical thermal reaction and a microwave-assisted reaction. The functionalized SWNTs have been characterized by nIR-Vis-UV absorption spectroscopy, Raman spectroscopy, and thermal gravimetric analysis. The results show that SWNTs are covalently functionalized through both reactions and that the microwave-assisted reaction is more rapid. Moreover, optimal choice of the reaction time can prevent the microwave irradiation from the adverse effect of subsequently removing the functional groups on the SWNT surface.     

Translocation of single-wall carbon nanotubes through solid-state nanopores

We report the translocation of individual single-wall carbon nanotubes (SWNTs) through solid-state nanopores. Single-strand DNA oligomers are used to both disperse the SWNTs in aqueous solution and to provide them with a net charge, allowing them to be driven through the nanopores by an applied electric field. The resulting temporary interruptions in the measured nanopore conductance provide quantitative information on the diameter and length of the translocated nanotubes at a single-molecule level. Furthermore, we demonstrate that the technique can be utilized to monitor bundling of SWNT in solution by using complementary nucleotides to induce tube-tube agglomeration.     

Room temperature growth of single-wall coiled carbon nanotubes and Y-branches

Straight carbon nanotubes, carbon nanotube “knees,” Y-branches of carbon nanotubes and coiled carbon nanotubes were grown on a graphite substrate held at room temperature by the decomposition of fullerene under moderate heating (450 °C) in the presence of 200-nm Ni particles. The grown structures were investigated without any further manipulation by STM. The growth and the chemical stability of the carbon nanostructures containing nonhexagonal rings are discussed.     

Resonant Raman spectroscopy of individual strained single-wall carbon nanotubes

Resonance Raman spectra of individual strained ultralong single-wall carbon nanotubes (SWNTs) are studied. Torsional and uniaxial strains are introduced by atomic force microscopy manipulation. Torsional strain strongly affects the Raman spectra, inducing a large downshift in the E2 symmetry mode in the G+ band, but a slight upshift for the rest of the G modes and also an upshift in the radial breathing mode (RBM). Whereas uniaxial strain has no effect on the frequency of either the E2 symmetry mode in the G+ band or the RBM, it downshifts the rest of the G modes. The Raman intensity change reflects the effect of these strains on the SWNT electronic band structure.