Tensile properties of ultrathin double-walled carbon nanotube membranes

Direct tensile tests of double walled carbon nanotube (DWCNT) membranes with thickness of 40–80 nm were performed using a micro-stress-strain puller. The tensile strength and Young’s modulus are 4.8E2–8.4E2 MPa and 4.4–8.8 GPa, respectively. The deformation and fracture processes were analyzed using the stress vs. strain curves, and SEM observations of the fracture surface of a membrane. The membrane experienced elastic strain and plastic strain during tensile-loading to fracture, and the plastic process is due to the real plastic deformation of the membrane and the slippage between the DWCNT bundles. Cracks occur and spread during the tensile test which causes the membrane to be mangled. With these excellent mechanical properties, the DWCNT membranes can be used in nanotube-reinforced composites.     

Modeling and characterization of the electrical conductivity of carbon nanotube-based polymer composites

In this research, we investigate both analytically and experimentally the electrical properties of carbon nanotube (CNT)-based polymer composites. An analytical model was developed to predict the electrical conductivity of CNT-based composites. The micro/nanoscale structures of the nanocomposites and the electrical tunneling effect due to the matrix material between CNTs were incorporated within the model. Electrical conductivity measurements were also performed on CNT/polycarbonate composites to identify the dependence of their electrical transport characteristics on the nanotube content. The analytical predictions were compared with the experimental data, and a good correlation was obtained between the predicted and measured results. In addition, the effect of nanotube geometry on the nanocomposite electrical properties at the macroscale was examined.     

Memory effects of carbon nanotube-based field effect transistors

In this paper, we report the study on the non-volatile memory effects of carbon nanotube-based field effect transistors (CNTFETs), in which semiconducting single-wall carbon nanotubes (SWNTs) bridge the gold electrodes and the doped silicon substrate acts as the back gate. We find that our CNTFETs exhibit good performance with on/off ratio of more than 10⁴ and they also show strong memory effects. Hysteretic behaviors of the drain current as a function of the gate voltage are clearly observed at room temperature. The threshold voltage shift increases with increasing the sweeping range of the gate voltage. The CNTFET memory effects show good charge retention capability with the data storage time of around 7 days at ambient condition. Besides, the threshold voltage shift of the as-prepared CNTFETs is found to decrease with time and saturate after around 3 days. Water and alcohol molecules adsorbed on the carbon nanotube are suggested to be the origin of the phenomena. It is also observed that the threshold voltage shift in “top-contact” structures is larger than those in “bottom-contact” structures at the same gate voltage sweeping range.     

Structure and energetics of hydrogenated and dehydrogenated carbon tori

It is shown that large-diameter, finite and extended, hydrogenated, single-walled carbon nanotubes are energetically and structurally unstable. The instability of short, large-diameter, finite hydrogenated tubes leads to the formation of hydrogenated circular carbon tori of small tubular cross-section. It is also shown that carbon toroidal structures, formed by desorption of hydrogen from the hydrogenated tori, are more energetically stable than the corresponding open-ended, finite single-walled carbon nanotubes.     

Structural and electronic properties of carbon nanowires made of linear carbon chains enclosed inside zigzag carbon nanotubes

This article presents ab initio self-consistent-field crystal orbital calculations on the structural and electronic properties for recently-discovered carbon nanowires (CNWs) made of linear carbon chains inserted inside zigzag carbon nanotubes using density functional theory. The studies focus on the change of geometric structures and electronic properties upon the encapsulation. It is found that the carbon nanotubes can stabilize the encapsulated carbon chain which prefers a dimerized structure in the tube with larger diameters. The interaction between the tube and the chain becomes more obvious when the tube size decreases, leading to the change of structures and the energy bands upon encapsulation. All the CNWs we calculated are metals with zero band gap. The encapsulation of the carbon chain may modulate the electronic properties for the CNWs depending on the tube size and the filling density of carbon atoms. Therefore, it is expected that CNWs’s electronic properties can be controlled artificially by filling carbon chains with various densities of atoms into the nanotubes.     

Buckling and postbuckling analysis of single-walled carbon nanotubes in thermal environments via molecular dynamics simulation

Buckling and postbuckling analysis of single-walled carbon nanotubes (SWCNTs) with (n, n)- and (n, 0)-helicity, when acted upon by the destabilizing loads of axial compression, torsion and external pressure, is presented by using molecular dynamics simulation. Based on the interatomic interactions given by Brenner and Lennard-Jones potentials, the molecular dynamics method is used to determine the postbuckling equilibrium paths as well as the variation of strain energy. Temperature changes and van der Waals interaction forces between the opposite walls of SWCNTs are both taken into account. Comprehensive numerical results for armchair (12, 12)- and zigzag (21, 0)-tubes are presented. The results reveal that the effect of van der Waals interactions on the postbuckling behavior of SWCNTs under axial compression can be negligible, while the additional van der Waals forces will affect the postbuckling equilibrium paths of SWCNTs under torsion and external pressure when the deformation of the tube is sufficiently large. The results also show that the temperature change has a significant effect on the postbuckling response of SWCNTs under axial compression, but it has a small effect in the loading case of torsion. In contrast, it only has a less effect on the postbuckling response of SWCNTs under external pressure.     

Electronic properties of double-walled carbon nanotube films

The electronic properties of as-prepared and purified DWNT films were tested using the four-probe method. The resistivity of the purified DWNT films is about 1 mΩ cm at room temperature and has a positive dρ/dT above 55 K, which shows a good metallic property. The electrical resistivities of the purified DWNT films are quite similar to those of the SWNT bundles and acid-treated SWNTs. Our results are correspondent with the early theoretical calculation on the band structure of DWNTs.     

Transformation of fullerene peapods to double-walled carbon nanotubes induced by UV radiation

Double-walled carbon nanotubes were prepared by XeCl-laser irradiation of fullerene (C₆₀ or C₇₀) peapods. Raman spectroscopy evidences less defect structure of outer tubes, as compared to those in double-walled carbon nanotubes grown by thermal treatment of peapods. The diameter distribution also differs from that of the thermally prepared nanotubes. At the given laser fluence, the conversion of C₇₀@SWCNT into double-walled carbon nanotubes was more efficient than the corresponding conversion of C₆₀@SWCNT.     

Electronic and optical properties of double-walled armchair carbon nanotubes

Magnetoelectronic structures of double-walled armchair carbon nanotubes are calculated according to the tight-binding model. Their features are dominated by the intertube interactions, the symmetric configurations, the magnetic flux, and the Zeeman splitting. The drastic changes of the low energy states, such as energy dispersion, wave function, and Fermi level, which also rely on the different symmetries, are caused by the intertube interactions. The magnetic flux could change linear bands into parabolic bands, destroy state degeneracy, open an energy gap, and shift Fermi level. The magnetic flux and the intertube interactions, however, compete with each other in the metallic or semiconducting behavior. The Zeeman splitting would suppress the metal–semiconductor transition while the opposite is true of the magnetic flux. The main characteristics of energy bands are directly reflected in the magneto-optical absorption spectra. The different symmetric configurations can be distinguished by the absorption peaks, and the threshold absorption frequency is not identical with the energy gap.     

Producing cleaner double-walled carbon nanotubes in a floating catalyst system

We demonstrate that Fe impurities in double-walled carbon nantoubes (DWNTs) may be greatly depressed by improving the experimental setup in a floating catalyst CVD method. In the paper, the effect of different experimental parameters on sample purity has been systematically studied. The possible reasons for the decrease of impurity in the DWNT samples prepared with the improved apparatus are discussed. The process should be helpful for preparing high quality single- or double-walled carbon nanotubes in scale-up applications.     

Structural and thermal stability of narrow and short carbon nanotubes and nanostrips

We have studied the relative stability of narrow-finite length Carbon nanotubes and nanostrips as a function of their length. We find that the critical radius (∼2 Å) for tube stability is independent of the length of the tubes and equals the critical radius of infinite length tubes. The independence of the critical radius on chirality is also established. We have also investigated the thermal stability of short nanotubes and nanostrips by performing extensive molecular dynamics simulations. The strips exhibit a higher thermal stability than the tubes even in those cases when they are structurally less stable. The thermal decomposition temperature (∼1000 K) of the tubes comes out in good agreement with the experiments.     

Band structure and absorption spectrum of double-walled zigzag carbon nanotubes in an electric field

The electronic structure of the (9, 0)-(18, 0) double-walled zigzag carbon nanotubes in the presence of a uniform transverse electric field is studied by the tight-binding model. The electric field could induce the semiconductor-metal transition, change the direct gap into the indirect gap, alter the subband curvatures, destroy the double degeneracy, produce the new band-edge states, make more subbands group around the Fermi level, and widen the π-band width. Such effects are directly reflected in density of states and optical excitation spectra. The absorption spectra exhibit a lot of prominent peaks, mainly owing to the rich one-dimensional energy subbands. The intensity, the number, and the frequency of absorption peaks are strongly modulated by the electric field. The modulation of electronic and optical properties is amplified by the parallel magnetic field. The predicted electronic and optical properties can be, respectively, verified by the conductance measurements and the optical spectroscopy.     

Tensile properties of long aligned double-walled carbon nanotube strands

The mechanical properties of well-aligned double-walled carbon nanotube (DWNT) strands with diameters of 3-20 μm and lengths of ∼10 mm were measured using a stress-strain puller. The average tensile strength and Young’s modulus of the tested strands are 1.2 GPa and 16 GPa, respectively. Deformation and fracture processes of these samples are discussed. The tensile strength and Young’s modulus of an individual DWNT bundle were estimated, with values comparable to those of SWNT bundles. The superior mechanical strengths of our as-prepared DWNT strands are expected to give them potential as a high-strength material and a reinforcement in composites.     

Role of polymers in the design of 3D carbon nanotube-based scaffolds for biomedical applications

Since pioneer works by Iijima in 1991, carbon nanotubes (CNTs) have received a great deal of attention as confirmed by the increasing number of papers in the topic. Their unique and attractive properties have made them extensively demanded materials for a wide variety of technological applications, including their promising use as scaffolds in tissue engineering. In this review, we focus on the role that polymers (both natural and synthetic) play on the fabrication of three-dimensional (3D) CNT-based scaffolds for biomedical applications, with emphasis on biocompatible fabrication strategies such as freeze-casting, electrospinning and gel formation. These 3D matrices may be an interesting and alternative platform to circumvent structural limitations and toxicity problems of bare CNTs by the use of biocompatible dispersant polymers that allow the preparation of substrates better resembling native extracellular matrices. In any case, due to the relevance of CNT toxicity in this context, we also discuss significant works concerning cell and tissue responses to CNTs in dispersion, highlighting: (1) the asbestos-like behavior of CNTs, (2) surface functionalization as a tool to reduce CNT toxicity and (3) CNT biodistribution from the blood stream and posterior excretion. In this sense, literature revision has evidenced major toxicity issues related to: (a) the inherent insolubility and tendency to aggregate of pristine CNTs, (b) the rigidity of their structures that makes them resemble asbestos, (c) the presence of residual metal impurities or amorphous carbon from their synthesis, and (d) the depletion of culture media components due to the adsorptive properties of CNTs. Nevertheless, as expected for almost any material, we also illustrate how dose plays a key role in the biological responses induced. Overall, this critic review is expected to help research community working on polymers and CNTs, as well as other carbon nanomaterials such as graphene, to identify useful guidelines that help advancing the use of 3D CNT-based scaffolds in biomedical applications.     

Microwave-assisted synthesis and pulse electrodeposition of palladium nanocatalysts on carbon nanotube-based electrodes

The deposition of Pd nanoparticles prepared by microwave-assisted synthesis (MS) and pulse electrodeposition (PE) on networks of multiwall carbon nanotubes (CNTs) was investigated. The CNTs were grown directly on microscaled carbon paper using catalytic chemical vapor deposition. Both MS and PE methods enabled the quick formation of nanosized Pd particles over a CNT surface without any additional thermal reduction. Cyclic voltammetry and electrochemical impedance spectroscopy were used to examine the electrochemical behavior of the Pd catalysts. The Pd catalyst prepared with the MS method not only offers a higher active coverage for adsorption/desorption of hydrogen but also a more stable durability toward acid electrolytes when compared with that of the catalyst prepared with the PE method. The electrochemical surface area of the Pd catalyst was approximately 1.38 times than that of the Pt catalyst, which was also prepared with MS method. The equivalent series resistance for all the catalyst electrodes was kept between 2.07 and 2.25 Ω after potential cycling. Based on the results, the Pd catalyst is found to be a feasible alternative to the Pt catalyst because of its low cost, durability, and high catalytic activity.