Atomistic Simulations of Rare Gas Transport through Breathable Single-wall Nanotubes with Constrictions and Knees

We present the results of molecular dynamics (MD) computer simulations of rare gas diffusion through breathable nanotubes with pentagon–heptagon pair defects resulting in constrictions and knees. Diffusion involves interrupted high speed “choppy” motion with intermittent reversal in velocity direction. Single atoms exhibit a spiral-like path, in contrast to atoms traveling in groups. Considerable resistance to flow appears to reside in the upstream section of the nanotube where density gradients are small, prior to the constriction. Subsequently, considerable density gradients are present and speeds increase, becoming greatest at the tube exit. For the nanotubes examined, Kr and Xe diffusion was too hindered to provide reliable results. Diffusion of He through the nanotubes with knees occurs in a single-file fashion nearly along the center of the tube and the knee has no detectable effect on the diffusion kinetics. Transport diffusion coefficients are in the order of 10^-4–10^-2?cm²/s.     

An ultrathin carbon nanoribbon study as a component of nanoelectromechanical devices

We investigated a carbon nanoribbon (CNR) using atomistic simulations based on Tersoff–Brenner potential function. The CNR was obtained from a compressed (5,5) carbon nanotube (CNT). The obtained CNR had a cross-sectional view as a binocular telescope structure composed of both sp² and sp³ bonds. One carbon atom per ten carbon atoms had sp³ bond. For the optimized structures, the residual forces on the CNR were 3-order higher than that on the CNR and the lattice constant of the CNR was higher 0.0624?Å than that of the CNT along the tube axis. The Young's modulus of the CNR was the same as that of the CNT whereas the critical strain of the CNR was significantly lower than that of the CNT because the residual stresses on the CNR was very higher than those on the CNT. The tensile force curve vs. the strain of the CNT was slightly higher than that of the CNR.     

Molecular Dynamics Simulations of Carbon Nanotube Rolling and Sliding on Graphite

Abstract

Molecular dynamics simulations were carried out to investigate the origin of friction for carbon nanotubes on graphite substrates. In an initial simulation, a (10,10) nanotube was placed in an ‘in-registry’ starting position where the hexagonal lattice of the substrate matched that of the nanotube. In a second simulation, the substrate was oriented 90 degrees to the nanotube. A uniform force was applied to the nanotubes for 500 fs to set them into motion. The simulation was then run until the nanotubes stopped moving relative to the substrate. Only sliding was observed in the out-of-registry simulation, while periodic sliding and rolling was observed in the in-registry simulation. The latter is a result of the relatively larger surface corrugation for the in-registry case and occurs to avoid direct atomic collisions between nanotube and substrate atoms as the nanotube is moved along the substrate. Analysis of the kinetic energy suggests that the transition between sliding and rolling contributes to enhanced energy dissipation and higher net friction. These results are consistent with preliminary experimental observations by Superfine and coworkers.     

Atomistic simulation of dislocation interactions in a model crystal subjected to shear

The interaction of edge dislocations in a two-dimensional (2D) model crystal subjected to “simple shear” is studied using molecular statics simulations. An initial point defect is introduced in the model to trigger the dislocation activities in a controlled manner. We consider dislocations gliding towards one another on parallel slip planes separated by various distances. The overall load-displacement response of the crystal is obtained from the simulations, which can be correlated with the nano-scale atomistic mechanisms. Although the crystal is inherently anisotropic, the incipient dislocation plasticity is such that slip is parallel to the primary shear direction as clearly demonstrated in this work. It is also illustrated that dislocation annihilation, as well as dislocation encounter which leaves behind a point defect, can be unambiguously modeled. Throughout the deformation history, more dislocations capable of gliding in the crystal tend to generate a weaker mechanical response and more pronounced plasticity. The present study also offers mechanistic insight into experimentally observed small-scale crystal plasticity.     

Atomistic simulations on interwall sliding behaviour of double-walled carbon nanotube: effects of structural defects

In this study, we investigated the interwall sliding behaviours of double-wall carbon nanotubes (DWCNTs) using molecular dynamics (MD) simulations, focusing on the effects of different structural defects including the vacancy, adsorbed atom (Adatom) and Stone-Wales (SW) defects. The simulation results showed that structural defects, especially the Adatom ones, caused large fluctuations and decreased the overall pull-out force. Stick-slip motions were observed in the interwall sliding processes of DWCNTs containing multiple structural defects. Among three types of structural defects, the Adatom defects most significantly weaken the interwall load transferring capability and degrade the interface shear strength (IFSS). This work provides useful information for promoting DWCNTs’ applications in Micro/Nano Electro-Mechanical Systems (M/NEMS).     

Molecular dynamics study of carbon nanotube oscillator on gold surface

We investigated the substrate effect of carbon nanotube (CNT) oscillators using classical molecular dynamics simulations. Double-walled CNT oscillators on {100} gold surface were considered. The nanotube–gold interactions induced the compressive deformations of the outer nanotube and affected the transitional velocity and the energy dissipation of the nanotube oscillator. When the inner nanotube was extruded from the outer nanotube, the central regions of the outer nanotube were compressed by the nanotube–gold interactions and then, these compressive forces pushed out the inner nanotube and finally, the transitional velocity of the inner nanotube was slightly increased at the edges regions. Since the energy dissipation of the nanotube oscillator on gold surface was higher than that in vapor, the decrease of the transitional velocity for the nanotube oscillator on gold surface was greater than that for the nanotube oscillator in vapor.     

Atomistic behavior analysis of Cu nanowire under uniaxial tension with maximum local stress method

This study analyzes the atomistic behaviors of a Cu nanowire (NW) during uniaxial tensile deformation by molecular dynamics simulation. In this work, the maximum local stress calculated method (MLS) is proposed to validly elucidate the plastic behaviors of the Cu NW. Analysis results demonstrate that the pre-tension stress is caused by the intense surface tension, which is an important factor for dislocation emission from surface. The motion of Shockley partials that interact to produce a stair-rod dislocation is determined. Following the dislocation mechanism, deformation twinning is the primary mechanism that dominates the plastic deformation at such a high strain rate. Immediately before fracture, the stress increases markedly since the primary failure mode is atomic bond breakage.     

Simultaneous Synthesis of Single-walled Carbon Nanotubes and Graphene in a Magnetically-enhanced Arc Plasma

Carbon nanostructures such as single-walled carbon nanotubes (SWCNT) and graphene attract a deluge of interest of scholars nowadays due to their very promising application for molecular sensors, field effect transistor and super thin and flexible electronic devices^1-4. Anodic arc discharge supported by the erosion of the anode material is one of the most practical and efficient methods, which can provide specific non-equilibrium processes and a high influx of carbon material to the developing structures at relatively higher temperature, and consequently the as-synthesized products have few structural defects and better crystallinity.To further improve the controllability and flexibility of the synthesis of carbon nanostructures in arc discharge, magnetic fields can be applied during the synthesis process according to the strong magnetic responses of arc plasmas. It was demonstrated that the magnetically-enhanced arc discharge can increase the average length of SWCNT ⁵, narrow the diameter distribution of metallic catalyst particles and carbon nanotubes ⁶, and change the ratio of metallic and semiconducting carbon nanotubes ⁷, as well as lead to graphene synthesis ⁸. Furthermore, it is worthwhile to remark that when we introduce a non-uniform magnetic field with the component normal to the current in arc, the Lorentz force along the J×B direction can generate the plasmas jet and make effective delivery of carbon ion particles and heat flux to samples. As a result, large-scale graphene flakes and high-purity single-walled carbon nanotubes were simultaneously generated by such new magnetically-enhanced anodic arc method. Arc imaging, scanning electron microscope (SEM), transmission electron microscope (TEM) and Raman spectroscopy were employed to analyze the characterization of carbon nanostructures. These findings indicate a wide spectrum of opportunities to manipulate with the properties of nanostructures produced in plasmas by means of controlling the arc conditions.     

Fabrication,Densification, and Replica Molding of 3D Carbon Nanotube Microstructures

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip devices, and their applications. In particular, capabilities for cost-effective fabrication of polymer microstructures were transformed by the advent of soft lithography and other micromolding techniques ^{1, 2}, and this led a revolution in applications of microfabrication to biomedical engineering and biology. Nevertheless, it remains challenging to fabricate microstructures with well-defined nanoscale surface textures, and to fabricate arbitrary 3D shapes at the micro-scale. Robustness of master molds and maintenance of shape integrity is especially important to achieve high fidelity replication of complex structures and preserving their nanoscale surface texture. The combination of hierarchical textures, and heterogeneous shapes, is a profound challenge to existing microfabrication methods that largely rely upon top-down etching using fixed mask templates. On the other hand, the bottom-up synthesis of nanostructures such as nanotubes and nanowires can offer new capabilities to microfabrication, in particular by taking advantage of the collective self-organization of nanostructures, and local control of their growth behavior with respect to microfabricated patterns. Our goal is to introduce vertically aligned carbon nanotubes (CNTs), which we refer to as CNT "forests", as a new microfabrication material. We present details of a suite of related methods recently developed by our group: fabrication of CNT forest microstructures by thermal CVD from lithographically patterned catalyst thin films; self-directed elastocapillary densification of CNT microstructures; and replica molding of polymer microstructures using CNT composite master molds. In particular, our work shows that self-directed capillary densification ("capillary forming"), which is performed by condensation of a solvent onto the substrate with CNT microstructures, significantly increases the packing density of CNTs. This process enables directed transformation of vertical CNT microstructures into straight, inclined, and twisted shapes, which have robust mechanical properties exceeding those of typical microfabrication polymers. This in turn enables formation of nanocomposite CNT master molds by capillary-driven infiltration of polymers. The replica structures exhibit the anisotropic nanoscale texture of the aligned CNTs, and can have walls with sub-micron thickness and aspect ratios exceeding 50:1. Integration of CNT microstructures in fabrication offers further opportunity to exploit the electrical and thermal properties of CNTs, and diverse capabilities for chemical and biochemical functionalization ³.     

Thermal Properties of Supercritical Carbon Dioxide by Monte Carlo Simulations

We present simulation results for the volume expansivity, isothermal compressibility, isobaric heat capacity, Joule-Thomson coefficient and speed of sound for carbon dioxide (CO 2 ) in the supercritical region, using the fluctuation method based on Monte Carlo simulations in the isothermal-isobaric ensemble. We model CO 2 as a quadrupolar two-center Lennard-Jones fluid with potential parameters reported in the literature, derived from vapor-liquid equilibria (VLE) of CO 2 . We compare simulation results with an equation of state (EOS) for the two-center Lennard-Jones plus point quadrupole (2CLJQ) fluid and with a multiparametric EOS adjusted to represent CO 2 experimental data. It is concluded that the VLE-based parameters used to model CO 2 as a quadrupolar two-center Lennard-Jones fluid (both simulations and EOS) can be used with confidence for the prediction of thermodynamic properties, including those of industrial interest such as the speed of sound or Joule-Thomson coefficient, for CO 2 in the supercritical region, except in the extended critical region.     

Transport of Surface-modified Carbon Nanotubes through a Soil Column

Carbon nanotubes (CNTs) are widely manufactured nanoparticles, which are being utilized in a number of consumer products, such as sporting goods, electronics and biomedical applications. Due to their accelerating production and use, CNTs constitute a potential environmental risk if they are released to soil and groundwater systems. It is therefore essential to improve the current understanding of environmental fate and transport of CNTs. The transport and retention of CNTs in both natural and artificial media have been reported in literature, but the findings widely vary and are thus not conclusive. There are a number of physical and chemical parameters responsible for variation in retention and transport. In this study, a complete procedure of selected multiwalled carbon nanotubes (MWCNTs) is presented starting from their surface modification to a complete set of laboratory column experiments at critical physical and chemical scenarios. Results indicate that the stability of the commercially available MWCNTs are critical with their attached surface functional group which can also influence the transport and retention of MWCNT through the surrounding medium.     

Restrictions on the Production of Multi-Wall Carbon Nanotubes and Nanofibers by Gallionella sp.

The enzymatic oxidization of dissolved Fe(II) to Fe(III) by neutrophilic Fe-oxidizing bacteria plays a significant role in biological cycling of iron by inducing the precipitation of Fe(III) oxyhydroxide in aqueous environments. Among the diverse neutrophilic Fe-oxidizing bacteria, the genus Gallionella has received wide attention for its production of unique twisted extracellular stalks. Hallberg and Tai (2014 Hallberg R, Tai CW. 2014. Multi-wall carbon nanotubes and nanofibers in Gallionella. Geomicrobiol J 31(9):764–768.[Taylor & Francis Online], [Web of Science ®] , [Google Scholar]) recently reported the detection of multi-wall carbon nanotubes on the twisted-stalks, and they viewed those carbon nanotubes as being biologically produced by Gallionella. We scrutinized Gallionella-produced biofilms collected from natural environments by scanning electron microscopy and high-resolution transmission electron microscopy. Ferrihydrite and lepidocrocite were the only nano-scaled minerals observed on the stalk, while there were nanometer-sized sheet-like graphitic contaminants on the grid in the vicinity of the sample which showed the same morphology as Hallberg and Tai (2014 Hallberg R, Tai CW. 2014. Multi-wall carbon nanotubes and nanofibers in Gallionella. Geomicrobiol J 31(9):764–768.[Taylor & Francis Online], [Web of Science ®] , [Google Scholar]) observed. Moreover, similar materials on an empty grid and a grid loaded with randomly selected synthesized materials were also observed. Based on the current knowledge of carbon nanotube syntheses, none of the three known synthesizing methods including root-growth, rolling-up and bottom-up could be biochemically produced by any life because of the significant kinetic and energy obstacles. The carbon nanomaterials reported by Hallberg and Tai (2014 Hallberg R, Tai CW. 2014. Multi-wall carbon nanotubes and nanofibers in Gallionella. Geomicrobiol J 31(9):764–768.[Taylor & Francis Online], [Web of Science ®] , [Google Scholar]) were clearly contaminations from amorphous carbon film on the grids for holding samples for transmission electron microscopic observations.     

Carbon nanotube-assisted enhancement of surface plasmon resonance signal

We describe a method of amplifying the biosensing signal in surface plasmon resonance (SPR)-based immunoassays using an antibody–carbon nanotube (CNT) conjugate. As a model system, human erythropoietin (EPO) and human granulocyte macrophage colony-stimulating factor (GM–CSF) were detected by sandwich-type immunoassays using an SPR biosensor. For the amplification of the SPR signal, the CNT was conjugated with a polyclonal antibody, and then the conjugates were reacted with antibodies coupled with the target proteins. This amplification strategy increases the dynamic range of the immunoassays and enhances the detection sensitivity. The SPR immunoassays, combined with the CNT-assisted signal amplification method, provided a wide dynamic range over four orders of magnitude for both EPO and GM–CSF (0.1–1000 ng/ml). The CNT amplification method is expected to realize the detection of picogram levels and a wide dynamic detection range of multiple proteins, enabling it to offer a robust analysis tool for the development of biopharmaceutical production.     

Tensile loading characteristics of hydrogen stored carbon nanotubes in PEM fuel cell operating conditions using molecular dynamics simulation

Mechanical characteristics of hydrogen stored single walled carbon nanotube (SWCNT) in proton exchange membrane fuel cell (PEMFC) operating conditions are analysed in this work using molecular dynamics simulation method. The investigation of mechanical characteristics of hydrogen stored SWCNT is critical in determining the lifetime and stability of SWCNT-based membranes used in PEMFC. The study provides a comprehensive analysis on the effects of geometry, vacancy defects and PEMFC operating temperature on the mechanical properties of hydrogen stored SWCNT. The findings show that the mechanical strength of the hydrogen stored SWCNT can be enhanced by deploying a bigger armchair SWCNT. Furthermore, increase in operating temperature of PEMFC reduces the mechanical resistance of hydrogen stored SWCNT, which however can be overcome by suitably introducing vacancy defects in the SWCNT geometry. This has provided potential way of increasing the hydrogen storage capacity of SWCNT which is very useful for onboard application of PEMFC. It is anticipated that the findings obtained from this paper will have a paramount importance in the field of hydrogen energy fuel cell technology and further compliment the potential applications of SWCNTs as promising candidates for applications in fuel cells and energy storage devices.     

Probing the Interaction of Mutiwalled Carbon Nanotubes and Catalase: Mutispectroscopic Approach

In this study, the mechanism of the interaction between multiwalled carbon nanotubes (MWCNTs) and catalase was investigated by fluorescence, UV–vis, and circular dichroism (CD) spectroscopy under physiological conditions. The fluorescence quenching mechanism of catalase by MWCNTs was shown to be a static quenching procedure and was a result of the formation of a catalase–MWCNT complex. The secondary structure and conformation of the catalase adsorbed on MWCNTs was determined by CD and UV‐vis spectroscopy, and the results indicate that the catalase in this complex is partially unfolded with its lost in α‐helical content and obtainment in β‐sheet content. Moreover, binding of MWCNTs to catalase inhibited the enzymatic activity, which may trigger some toxic effects and undesirable physiological consequences. © 2012 Wiley Periodicals, Inc. J Biochem Mol Toxicol 26:493‐498, 2012;Viewthis article online at wileyonlinelibrary.com . DOI 10.1002/jbt.21454     

Molecular simulation of carbon nanotubes as sorptive materials: sorption effects towards retene,perylene and cholesterol to 100 degrees Celsius and above

New water purification technologies are being developed as the world’s water sources are increasingly being polluted and experience a dramatic consumption with the increasing world population. In this context, the emerging era of nanotechnology has introduced a series of innovations and materials with promising potential as sorptive materials for water decontamination. The application of nanomaterials for the purification of ground/surface water introduces nevertheless a series of important challenges, such as health and safety, cost, process-efficiency and chemical sorption properties. In this study, we consider the latter class and present a study of the sorptive properties of carbon nanotubes (CNTs) as potential water decontaminating materials. Molecular dynamics simulations are used and three molecular candidates of the water contaminants, cholesterol, perylene and retene were selected for interaction study with CNTs at different diameters. The results show that CNTs form densely packed clusters with retene, perylene and cholesterol, binding each strongly to their tubular surfaces, as well as in their hollow tubular spaces. Cholesterol and perylene bind more strongly than retene, accounting for the calculated binding energies in vacuo, however the planar geometries of polycyclics may in general favour binding to CNTs over semi-polar molecules and can require further studies. Our studies show furthermore that the CNTs retain the adsorbed molecules also at 100 degrees Celsius, and require therefore additional steps of separation for eventual recycling and reusing the nanomaterials for additional decontamination. This study is important in providing data for initiating studies and developments of water purification approaches based on using CNTs.     

Electrochemical behavior of caffeic acid at single-walled carbon nanotube:graphite-based electrode

The performance of a single-walled carbon nanotube:graphite-based electrode, prepared by mixing single-walled carbon nanotubes (SWCNTs) and graphite powder, is described. The resulting electrode shows an excellent behavior for the redox of caffeic acid (CA), an important biological molecule. Due to the existing resemblance between electrochemical and biological reactions, it can be assumed that the oxidation mechanisms on the electrode and in the body share similar principles. SWCNT:graphite-based electrode presents a significant decrease in the overvoltage for the CA oxidation as well as a dramatic improvement in the reversibility of the CA redox behavior in comparison with the graphite-based and glassy carbon (GC) electrodes.     

Localization and Relative Quantification of Carbon Nanotubes in Cells with Multispectral Imaging Flow Cytometry

Carbon-based nanomaterials, like carbon nanotubes (CNTs), belong to this type of nanoparticles which are very difficult to discriminate from carbon-rich cell structures and de facto there is still no quantitative method to assess their distribution at cell and tissue levels. What we propose here is an innovative method allowing the detection and quantification of CNTs in cells using a multispectral imaging flow cytometer (ImageStream, Amnis). This newly developed device integrates both a high-throughput of cells and high resolution imaging, providing thus images for each cell directly in flow and therefore statistically relevant image analysis. Each cell image is acquired on bright-field (BF), dark-field (DF), and fluorescent channels, giving access respectively to the level and the distribution of light absorption, light scattered and fluorescence for each cell. The analysis consists then in a pixel-by-pixel comparison of each image, of the 7,000-10,000 cells acquired for each condition of the experiment. Localization and quantification of CNTs is made possible thanks to some particular intrinsic properties of CNTs: strong light absorbance and scattering; indeed CNTs appear as strongly absorbed dark spots on BF and bright spots on DF with a precise colocalization.This methodology could have a considerable impact on studies about interactions between nanomaterials and cells given that this protocol is applicable for a large range of nanomaterials, insofar as they are capable of absorbing (and/or scattering) strongly enough the light.     

Preparation and Evaluation of Hybrid Composites of Chemical Fuel and Multi-walled Carbon Nanotubes in the Study of Thermopower Waves

When a chemical fuel at a certain position in a hybrid composite of the fuel and a micro/nanostructured material is ignited, chemical combustion occurs along the interface between the fuel and core materials. Simultaneously, dynamic changes in thermal and chemical potentials across the micro/nanostructured materials result in concomitant electrical energy generation induced by charge transfer in the form of a high-output voltage pulse. We demonstrate the entire procedure of a thermopower wave experiment, from synthesis to evaluation. Thermal chemical vapor deposition and the wet impregnation process are respectively employed for the synthesis of a multi-walled carbon nanotube array and a hybrid composite of picric acid/sodium azide/multi-walled carbon nanotubes. The prepared hybrid composites are used to fabricate a thermopower wave generator with connecting electrodes. The combustion of the hybrid composite is initiated by laser heating or Joule-heating, and the corresponding combustion propagation, direct electrical energy generation, and real-time temperature changes are measured using a high-speed microscopy system, an oscilloscope, and an optical pyrometer, respectively. Furthermore, the crucial strategies to be adopted in the synthesis of hybrid composite and initiation of their combustion that enhance the overall thermopower wave energy transfer are proposed.