Self-assembly of single-walled carbon nanotubes into multiwalled carbon nanotubes in water: molecular dynamics simulations

We report discoveries from a series of molecular dynamics simulations that single-walled carbon nanotubes, with different diameters, lengths, and chiralities, can coaxially self-assemble into multiwalled carbon nanotubes in water via spontaneous insertion of smaller tubes into larger ones. The assembly process is tube-size-dependent, and the driving force is primarily the intertube van der Waals interactions. The simulations also suggest that a multiwalled carbon nanotube may be separated into single-walled carbon nanotubes under appropriate solvent conditions. This study suggests possible bottom-up self-assembly routes for the fabrication of novel nanodevices and systems.     

Studies on mechanism of single-walled carbon nanotube formation

We presented detailed studies of the formation of single-walled carbon nanotubes by an aerosol method based on the introduction of pre-formed catalyst particles into conditions leading to carbon nanotube synthesis. Carbon monoxide and iron nanoparticles were used as a carbon source and a catalyst, respectively. The vital role of etching agents such as CO2 and H2O in CNT formation was demonstrated on the basis of on-line Fourier-transform infrared spectroscopy measurements. Hydrogen was shown to participate in the reaction of carbon release and to prevent the oxidation of the catalyst particles and the hot wire. The addition of H2 and small amounts of CO2 and H2O led to an increase in the carbon nanotube lengths. The catalyst particle evaporation process inside the reactor was found to become significant at temperatures higher than 1100 degrees C. The carbon nanotube growth was found to occur at a temperature of around 900 degrees C in the heating section of the reactor by in situ sampling and the growth rate was calculated to exceed 1.1 microm/s. A detailed analysis of possible processes during carbon nanotube formation revealed heptagon transformation as a limiting stage. A mechanism for carbon nanotube formation was proposed.     

Investigation of non-covalent association of single-walled carbon nanotube with amylose by molecular dynamics simulation

Functionalization of carbon nanotubes (CNTs) with the assistance of biological molecules remarkably improves the solubility of nanotubes in aqueous or organic environment and, thus, facilitates the development of novel biotechnology, biomedicine and bioengineering. In the present study, molecular dynamics (MD) simulations are performed on the complex amylose-nanotube system to study the mode of interaction between the initially separated amylose and SWNT fragments, which can be either wrapping or encapsulation. It has been found that the van der Waals force is dominant and it always plays an important role in promoting non-covalent association. The influence of the size of nanotube on MD simulation is also studied. Our study illustrates that amylose molecules can be used to bind with nanotubes and, thus, favor non-covalent functionalization of carbon nanotubes.     

Generation of single-walled carbon nanotubes from alcohol and generation mechanism by molecular dynamics simulations

Recent advances in high-purity and high-yield catalytic chemical vapor deposition (CVD) generation of single-walled carbon nanotubes (SWNTs) from alcohol are comprehensively presented and discussed on the basis of results obtained from both experimental and numerical investigations. We have uniquely adopted alcohol as a carbon feedstock, and this has resulted in high-quality, low-temperature synthesis of SWNTs. This technique can produce SWNTs even at a very low temperature of 550 degrees C, which is about 300 degrees C lower than the conventional CVD methods in which methane or acetylene is typically used. We demonstrate the excellence of the proposed alcohol catalytic CVD method for high-yield production of SWNTs when Fe-Co on USY-zeolite powder was used as a catalyst. At optimum CVD conditions, a SWNT yield of more than 40 wt % was achieved over the weight of the catalytic powder within the reaction time of 120 min. In addition to the advantages for mass production, this method is also suitable for the direct synthesis of high-quality SWNTs on Si and quartz substrates when combined with the newly developed liquid-based "dip-coat" technique to mount catalytic metals on the surface of substrates. This method allows easy and costless loading of catalytic metals without the need for any support or underlayer materials that were usually required in previous studies for the generation of a sufficient quantity of SWNTs on an Si surface. Finally, the result of molecular dynamics simulation for the SWNT growth process is presented to obtain a fundamental insight into the initial growth mechanism on the catalytic particles.     

A fully tunable single-walled carbon nanotube diode

We demonstrate a fully tunable diode structure utilizing a fully suspended single-walled carbon nanotube. The diode's turn-on voltage under forward bias can be continuously tuned up to 4.3 V by controlling gate voltages, which is ～6 times the nanotube band gap energy. Furthermore, the same device design can be configured into a backward diode by tuning the band-to-band tunneling current with gate voltages. A nanotube backward diode is demonstrated for the first time with nonlinearity exceeding the ideal diode. These results suggest that a tunable nanotube diode can be a unique building block for developing next generation programmable nanoelectronic logic and integrated circuits.     

13X分子筛为载体制备单壁碳纳米管研究

采用流化床法,以Fe/13X分子筛作为催化剂载体,催化裂解正己烷制备出定向排列的、较纯的单壁碳纳米管.利用TEM、HRTEM、TG和Raman对产物进行了表征,对不同浸泡时间Fe/13X分子筛制成的单壁碳管的含量和分子筛的负载量进行了分析,研究了催化剂铁负载量对单壁碳纳米管的产量和直径的影响.结果表明,单壁碳纳米管产量受催化剂含量和活性的共同影响,且在一个特定催化剂负载量下碳管产量可以达到最高,而其直径变化不大,且不受催化剂负载量的影响.     

Nuclear magnetic resonance of molecular hydrogen trapped in single-walled carbon nanotube bundles

Molecular dynamics of hydrogen trapped in single-walled carbon nanotube bundles was analyzed by nuclear magnetic resonance. The chemical shift of hydrogen was about 5.1 ppm at 293 K, which is similar to that of water. The relaxation time, T1, was about 0.1-0.2 s. Values in this work are comparable to those for hydrogen loaded in silica and a-Si.     

The bending-kinking analysis of a single-walled carbon nanotube—a combined molecular dynamics and continuum mechanics technique

This paper investigates the deformation mechanism of a single-walled carbon nanotube in pure bending. The molecular dynamics analysis and continuum mechanics characterisation were used together to achieve a deeper understanding. It was found that at a bending angle of 24° the nanotube buckles locally, forming a kink in the middle of the nanotube. As the bending angle increases, the kink progresses along the nanotube and varies its shape in both longitudinal and circumferential directions. The kink formation can be considered as the result of rotations of planes/surfaces about the moving and stationary hinge lines. It was also found that the kink deformation influences the load bearing capacity of the nanotube.     

Photon-drag effect in single-walled carbon nanotube films

We observed an interaction of single-walled carbon nanotube films with obliquely incident nanosecond laser radiation in visible and infrared regions generating unipolar voltage pulses replicating the shape of the laser pulses. The photoelectric signal significantly depends on the laser polarization and has maximum value at the laser beam incidence angle of ±65° and at the film thickness of 350 nm. The results are explained in the framework of the photon-drag effect.     

Mechanical properties of carbon nanotube by molecular dynamics simulation

The mechanical properties of single-walled carbon nanotube (SWCNT) are computed and simulated by using molecular dynamics (MD) in this paper. From the MD simulation for an armchair SWCNT whose diameter is 1.2 nm and length is 4.7 nm, we get that its Young modulus is 3.62 TPa, and tensile strength is 9.6 GPa. It is shown that the Young modulus and tensile strength of armchair SWCNTs are 12 order higher than those of ordinary metal materials. Therefore we can draw a conclusion that carbon nanotubes (CNT) belong to a particular material with excellent mechanical properties.     

Molecular dynamics study of dipalmitoylphosphatidylcholine lipid layer self-assembly onto a single-walled carbon nanotube

Single-walled carbon nanotubes (SWNTs) are possible nano-injectors and delivery vehicles of molecular probes and drugs into cells. In order to explore the interaction between lipid membranes and carbon nanotubes, we investigate the binding mechanism of dipalmitoylphosphatidylcholine (DPPC) with SWNTs by molecular dynamics. In low concentration range simulations, the DPPC molecules form a supramolecular two-layered cylindrical structure wrapped around the carbon nanotube surface. The hydrophobic part of DPPC is adsorbed on the surface of the nanotube, and the hydrophilic top is oriented towards the aqueous phase. For higher concentration ranges, the DPPC molecules are found to form a supramolecular multi-layered structure wrapped around the carbon nanotube surface. At the saturation point a membrane-like structure is self-assembled with a width of 41.4 Å, which is slightly larger than the width of a cell membrane. Our study sheds light on the existing conflicting simulation data on adsorption of single-chained phospholipids.      

Tribological properties of single-walled carbon nanotube solids

Binder-free single-walled carbon nanotube (SWCNT) solids were evaluated for solid lubrication applications. The steady-state friction coefficients (mu) for the SWCNT solids were found to reach values as low as 0.22-0.24, according to unidirectional sliding friction tests using Si3N4 counterparts in air. The values were slightly higher than that of bulk graphite material (mu = 0.20). SEM and Raman analyses showed that most SWCNTs that existed in the friction surface transformed into SWCNT-derived transferred film made up of amorphous carbon during sliding. The resultant friction behavior may be related to the smearing of transferred film over the contact area, which was expected to permit easy shear and then help to achieve a lubricating effect during sliding.     

Characteristics of electrodeposited single-walled carbon nanotube films

Thin films of chemically-functionalized single walled carbon nanotubes (SWNTs) were fabricated by using a direct current (DC) electrodeposition method. SWNTs were shortened and then functionalized with acid chloride group to combine with amine group-terminated gold substrate. The electrodeposited SWNT films were characterized by using Raman spectroscopy, attenuated total reflectance infrared (ATR/IR) spectrometry and atomic force microscopy. We demonstrated that the SWNT film was well distributed on an electrode with robust adhesion.     

Electrochemical hydrogen storage in single-walled carbon nanotube paper

Single-walled carbon nanotube (SWNT) papers were successfully prepared by dispersing SWNTs in Triton X-100 solution, then filtered by PVDF membrane (0.22 microm pore size). The electrochemical behavior and the reversible hydrogen storage capacity of single-walled carbon nanotube (SWNT) papers have been investigated in alkaline electrolytic solutions (6 N KOH) by cyclic voltammetry, linear micropolarization, and constant current charge/discharge measurements. The effect of thickness and the addition of carbon black on hydrogen adsorption/desorption were also investigated. It was found that the electrochemical charge-discharge mechanism occurring in SWNT paper electrodes is somewhere between that of carbon nanotubes (physical process) and that of metal hydride electrodes (chemical process), and consists of a charge-transfer reaction (Reduction/Oxidation) and a diffusion step (Diffusion).     

Phase diffusion in single-walled carbon nanotube Josephson transistors

We investigate electronic transport in Josephson junctions formed by individual single-walled carbon nanotubes coupled to superconducting electrodes. We observe enhanced zero-bias conductance (up to 10e ²/h) and pronounced sub-harmonic gap structures in differential conductance, which arise from the multiple Andreev reflections at superconductor/nanotube interfaces. The voltage-current characteristics of these junctions display abrupt switching from the supercurrent branch to the resistive branch, with a gate-tunable switching current ranging from 65 pA to 2.5 nA. The finite resistance observed on the supercurrent branch and the magnitude of the switching current are in good agreement with the classical phase diffusion model for resistively and capacitively shunted junctions.     

A self-sensing nanomechanical resonator built on a single-walled carbon nanotube

We present observations of resonance behavior in a torsional nanoelectromechanical device built with an individual single-walled carbon nanotube. The effect of applied torsional strain on the transport properties of the nanotube provides an electrical signal transducer and hence a means of measuring oscillation amplitude, resonance frequency, and quality factor. The mechanical resonance is confirmed by imaging and the electromechanical signal is compared to quasi-static measurements.     

Role of defects in single-walled carbon nanotube chemical sensors

We explore the electronic response of single-walled carbon nanotubes (SWNT) to trace levels of chemical vapors. We find adsorption at defect sites produces a large electronic response that dominates the SWNT capacitance and conductance sensitivity. This large response results from increased adsorbate binding energy and charge transfer at defect sites. Finally, we demonstrate controlled introduction of oxidation defects can be used to enhance sensitivity of a SWNT network sensor to a variety of chemical vapors.     

Efficient spectrofluorimetric analysis of single-walled carbon nanotube samples

A new method and instrumentation are described for rapid compositional analysis of single-walled carbon nanotube (SWCNT) samples. The customized optical system uses multiple fixed-wavelength lasers to excite NIR fluorescence from SWCNTs individualized in aqueous suspensions. The emission spectra are efficiently captured by a NIR spectrometer with InGaAs multichannel detector and then analyzed by a computer program that consults a database of SWCNT spectral parameters. The identities and relative abundances of semiconducting SWCNTs species are quickly deduced and displayed in graphs and tables. Results are found to be consistent with those based on manual interpretation of full excitation-emission scans from a conventional spectrofluorometer. The new instrument also measures absorption spectra using a broadband lamp and multichannel spectrometers, allowing samples to be automatically characterized by their emission efficiencies. The system provides rapid data acquisition and is sensitive enough to detect the fluorescence of a few picograms of SWCNTs in ~50 μL sample volumes.     

Noise characteristics of single-walled carbon nanotube network transistors

The noise characteristics of randomly networked single-walled carbon nanotubes grown directly by plasma enhanced chemical vapor deposition (PECVD) are studied with field effect transistors (FETs). Due to the geometrical complexity of nanotube networks in the channel area and the large number of tube-tube/tube-metal junctions, the inverse frequency, 1/f, dependence of the noise shows a similar level to that of a single single-walled carbon nanotube transistor. Detailed analysis is performed with the parameters of number of mobile carriers and mobility in the different environment. This shows that the change in the number of mobile carriers resulting in the mobility change due to adsorption and desorption of gas molecules (mostly oxygen molecules) to the tube surface is a key factor in the 1/f noise level for carbon nanotube network transistors.