Ends Of Carbon Nanotubes Research Articles

Molecular Dynamics of Carbon Nanotubes Deposited on a Silicon Surface via Collision: Temperature Dependence

We investigated how temperature influences the structural and energetic dynamics of carbon nanotubes (CNTs) undergoing a high-speed impact with a Si (110) surface. By performing molecular dynamics simulations in the temperature range of 100 - 300 K, we found that a low temperature CNT ends up with a higher vibrational energy after collision than a high temperature CNT. The vibrational temperature of CNT increases by increasing the surface temperature. Overall, the structural and energy relaxation of low temperature CNTs are faster than those of high temperature CNTs.

First Principles Studies of the Effect of Nickel Carbide Catalyst Composition on Carbon Nanotube Growth

Density functional theory calculations were used to investigate the stability of single-walled carbon nanotubes (CNTs) attached to nanoparticles. The total energies and the adhesion energies between the CNTs and the nanoparticles were calculated for systems where the nanoparticles were either pure Ni or Ni carbide. It was found that the adhesion between the CNT and a pure Ni cluster is stronger than between the same CNT and a Ni carbide cluster although the energy difference was small compared to the total adhesion energies. This adhesion strength implies that CNTs are likely to remain attached to both pure Ni and Ni carbide clusters and that either pure Ni or Ni carbide clusters may be docked onto the open CNT ends to achieve continued growth or electronic contacts between CNTs and electrode materials. The system with a CNT attached to a pure Ni cluster was found to be energetically favored compared to a system containing the same CNT attached to a Ni carbide. The difference in total energy implies that a CNT should act as a sink for C atoms dissolved in the Ni carbide cluster, which means that the dissolved C atoms will be drained from the cluster, yielding a pure metal in the zero Kelvin thermodynamic limit. It is argued that this draining procedure is likely to occur even if carbon is added to the cluster at a proper rate, for example, during CNT growth.

Electroluminescent properties of CdS/CNT hybrid material

AbstractAligned multiwall carbon nanotube (CNT) arrays were used for deposition of cadmium sulfide (CdS) nanoparticles from an aqueous solution containing thiourea, CdCl2, and ammonia. Scanning electron microscopy (SEM) showed that used deposition technique does not destroy vertical orientation of CNTs to silicon substrate. The size of CdS nanoparticles formed on the side surface and ends of CNTs was estimated by transmission electron microscopy (TEM) to be about 30 nm. It was found that CdS nanoparticles became luminescent during field electron emission from CNTs. Different color of the luminescence image was related to different size of the nanoparticles.magnified imageScheme of appearance of electroluminescence from CdS/CNT hybrids and image of luminous cathode surface.

Low Temperature PROX Reaction of CO Catalyzed by Dual Functional Catalysis of the Pt Supported on CNT, CNF, Graphite, and Amorphous-C with Ni−MgO, Fe, and Fe-Al2O3: Oxidation of CO via HCOO Intermediate

Preferential oxidation (PROX) reaction of CO in H2 proceeded rapidly on Pt supported on carbon nanotube (CNT) at room temperature, whereas the oxidation of CO was very slow in the absence of H2. On the other hand, no low temperature PROX reaction proceeded on Pt supported on the CNT purified Ni−MgO (CNT-p). Similarly, Pt supported on carbon nanofiber (Pt/CNF) was extremely active for the PROX reaction of CO at room temperature, but the activity of Pt supported on purified CNF (CNF-p) was very low. Pt particles are on the wall of CNT and CNF, whereas Ni−MgO and Fe are at the end of CNT and CNF. When Ni−MgO or Fe−Al2O3 was doped to inactive Pt/CNT-p, Pt/CNF-p, Pt/graphite, and Pt/amorphous-C (a-C), the activity for the oxidation of CO in H2 was markedly improved but no effect on the oxidation of CO in the absence of H2. The kinetic feature of the oxidation of CO on Pt/CNT in the presence of H2 or H2O was very similar to that on the FeOx/Pt/TiO2, where the oxidation of CO in the presence of H2/D2 had an isot...

Molecular Dynamics Simulations for Molecular Linear Motor Inside Nanotube

We have investigated the trap and escape process of a nanocargo (NC) inside a multiwall carbon nanotube (CNT) using molecular dynamics simulation. It was revealed that the trapping and escaping motions of NC at the end of CNT are governed by the competition between thermal energy and the van der Waals (vdW) interaction between the NC and the CNT cap. The trapping and escaping motions of NC at a temperature higher than 1000 K showed neither chirality nor size dependence, because the thermal energy at 1000 K is much higher than the energy corrugation induced by the vdW interaction between the NC and the CNT sidewall.

Carbon nanotubes terminated with hard magnetic FePt nanomagnets

The advancement in carbon nanotube (CNT) technology includes significant interest in their functionalization to modify their chemical and physical properties. In particular, the selective functionalization of the CNT ends opens exciting opportunities to design nanoscale architectures and networks. The realization of hard-magnetically terminated CNT via plasma enhanced chemical vapor deposition from Fe–Pt thin films is reported. Although FePt is rarely used as a catalyst for CNT synthesis the said binary catalyst affords attractive hard magnetic properties when present in the chemically ordered L10 phase.

Contact mechanics and thermal conductance of carbon nanotube array interfaces

A model is developed in this work to predict the thermal contact resistance of carbon nanotube (CNT) array interfaces with CNT arrays synthesized directly on substrate surfaces. An analytical model for contact mechanics is first developed in conjunction with prior data from load–displacement experiments to predict the real contact area established in CNT array interfaces as a function of applied pressure. The contact mechanics model is utilized to develop a detailed thermal model that treats the multitude of individual CNT–substrate contacts as parallel resistors and considers the effects on phonon transport of the confined geometry that exist at such contacts. The influence of CNT array properties, e.g. diameter and density, are explicitly incorporated into the thermal model, which agrees well with experimental measurements of thermal resistances as a function of pressure for different types of interfaces. The model reveals that: (1) ballistic thermal resistance dominates at the CNT array interface; (2) the overall performance of CNT array interfaces is most strongly influenced by the thermal resistance at the contacts between free CNT ends and the opposing substrate surface (one-sided interface) or the opposing CNT array (two-sided interface); and (3) dense arrays with high mechanical compliance reduce the thermal contact resistance of CNT array interfaces by increasing the real contact area in the interface.

Catalyst effects of fabrication of carbon nanotubes synthesized by chemical vapor deposition

Catalytic effects of the fabrication of carbon nanotubes (CNTs) by chemical vapor deposition of methane were investigated by thermogravimetric analysis. More specifically, the total yield and thermal stability characteristics of the product were examined with respect to physicochemical characteristics of the catalyst. Three kinds of Ni/Al catalysts with 5 wt%, 10 wt% and 15 wt% Ni, respectively were employed to synthesize CNTs. It was determined that an optimal Ni content of the catalyst resulted in maximum yield and most stable product. With increasing the Ni content, the CNT yield increased but they became less stable during heat treatment in air. According to transmission electron microscopy observations, the defect sites along the walls and at the ends of the raw CNTs facilitated the thermal oxidative destruction of the CNTs.

Functionalization of Carbon Nanotubes Using Nitric Acid Oxidation and DBD Plasma

In this study, multiwall carbon nanotubes (MWNTs) were modified with nitric acid chemically and by dielectric barrier discharge (DBD) plasma in an oxygen-based atmosphere. Used carbon nanotubes (CNTs) were prepared by chemical vapour deposition (CVD) floating catalyst method. For removing amorphous carbon and metal catalyst, MWNTs were exposed to dry air and washed with hydrochloric acid. Heating purified CNTs under helium atmosphere caused elimination of acidic functional groups. Fourier transformed infrared spectroscopy (FTIR) shows formation of oxygen containing groups such as C=O and COOH. Brunauer, Emmett, Teller (BET) analysis revealed that functionalization causes generation of defects on the sidewalls and opening of the ends of CNTs. Results of temperature-programmed desorption (TPD) and gas chromatography(GC) indicate that nitric acid treatment create more acidic groups than plasma treatment. Keywords—Carbon nanotubes (CNTs), chemical treatment, functionalization, plasma.

A Micromechanics Model for the Thermal Conductivity of Nanotube-Polymer Nanocomposites

A micromechanics approach for assessing the impact of an interfacial thermal resistance, also known as the Kapitza resistance, on the effective thermal conductivity of carbon nanotube-polymer nanocomposites is applied, which includes both the effects of the presence of the hollow region of the carbon nanotube (CNT) and the effects of the interactions amongst the various orientations of CNTs in a random distribution. The interfacial thermal resistance is a nanoscale effect introduced in the form of an interphase layer between the CNT and the polymer matrix in a nanoscale composite cylinder representative volume element to account for the thermal resistance in the radial direction along the length of the nanotube. The end effects of the interfacial thermal resistance are accounted for in a similar manner through the use of an interphase layer between the polymer and the CNT ends. Resulting micromechanics predictions for the effective thermal conductivity of polymer nanocomposites with randomly oriented CNTs, which incorporate input from molecular dynamics for the interfacial thermal resistance, demonstrate the importance of including the hollow region in addition to the interfacial thermal resistance, and compare well with experimental data.

Effects of size constraint on water filling process in nanotube

Molecular dynamics (MD) simulation and the potential of mean force (PMF) analysis are used to investigate the structural properties of water molecules near the end of nanotube for the whole process from the initial water filling up to the configuration stabilization inside the carbon nanotubes (CNTs). Numerical simulations showed that when a small-sized nanotube is immersed into the water bath, the size constraint will induce a prevailing orientation for the water molecule to diffuse into the tube and this effect can persist approximately 3.3 angstroms from the end of CNT. As the structure within the CNTs stabilizes, the ambient structural properties can indirectly reflect their corresponding properties inside the nanotube. Our results also showed that there exists a close correlation between the PMF analysis and the results of MD simulations, and the properties at nanometer scale are closely related to the size-constraint effect.

Grinding a Nanotube

Field evaporation phenomena, the removal of atoms from positively charged surfaces at strong field of tens of volts per nanometer, are of tremendous scientific and technological importance. The technique of field-ion microscopy (FIM) has been developed based on this phenomenon, and is historically the first technique to have achieved atomic resolution. The atom probe, a variation of FIM, came later and is generally used to identify the species of atoms that are being individually evaporated from the surface of a FIM tip. Recently, atom-probe tomography (APT) has been developed as a powerful analytical characterization technique that ties compositional information to structure. This is the only approach that is capable of determining the 3D location and elemental identity of atoms in a sample with near atomic precision. In principle, the field evaporation phenomenon can be utilized not only for materials characterization, but also for materials processing and morphology control with extremely high precision because of its unique atom-by-atom removal capability. However, FIM and atom probe investigations only provide access to structural information on the tip surface or the evaporated segments of the specimen, and it is not possible to directly observe the structural evolution of the sample. This limitation has greatly restricted the potential applications of field evaporation as a materials-processing tool. Here, we have investigated the positive field evaporation of nanomaterials by transmission electron microscopy (TEM). Direct TEM observations of the details of the structural evolution during field evaporation have been obtained for the first time. In previous work, the shortening of individual positively biased carbon nanotubes (CNTs) has been observed by scanning electron microscopy (SEM), but the observed phenomenon may perhaps arise from the electron ablation effect because of electrons emitted from the counter CNT electrodes. Detailed information about the end caps of CNTs is difficult to obtain because of the limited spatial resolution of SEM. On the other hand, structural changes in individual CNTs have also been observed in previous in situ field-emission measurements. However, it is likely that the observed structural changes do not arise from “pure” field evaporation phenomena. The reason for this is that when CNTs are negatively charged, the large field-emission current can induce very irregular tip structures and can even damage the emitters. In contrast, as will be demonstrated here, positive field evaporation is much more controllable. Indeed, we demonstrate that positive field evaporation in combination with in situ TEM may actually provide a very simple and effective means for controlling the morphology of nanomaterials with atomic precision. All the in situ experiments have been carried out using a 200 kV Tecnai G20 transmission electron microscope with a vacuum level of about 1×10 Pa. The manipulations and measurements have been made using a Nanofactory single-tilt sample holder. All CNTs used in this work are multiwalled CNTs (MWCNTs) prepared via chemical vapor deposition. In our experiment, a single CNT protruding from the edge of a Pt wire has been selected using a W tip. By running a high current, the CNT is broken into two segments with the bottom section attached to the W tip. As shown in Figure 1a, the W tip is then moved to a different location where the counter (top) Pt electrode is microscopically flat. The distance between the CNT tip and the counter electrode is usually several tens to hundreds of nanometers. We have used electronbeam-induced deposition to deposit amorphous carbon (a-carbon) on the contact area, which effectively prevents the removal of the CNT from the W tip even under extremely strong fields. A maximum voltage of ±140 V can be applied between the Pt electrode and the grounded counter W tip. The attached CNT can be positively charged for field evaporation experiments by applying a negative voltage to the Pt electrode, or negatively charged for field-emission experiments by applying a positive bias to the Pt electrode. During the field-emission or field evaporation measurements, the electron-beam (e-beam) has been blanked out. The CNT has been positively charged with a constant bias of 140 V, which may produce enough of a local field to cause field evaporation. As the CNT end is progressively shortened and becomes flat, field evaporation slows down and eventually stops. Figure 1b and c show the end cap of the CNT before and after the evaporation experiment, respectively; the corresponding field-emission current–voltage (I–V) curves are shown in Figure 1d. Here, field-emission measurements of the CNT have been conducted to provide quantitative estimates of the field conversion factor and threshold field for positive field evaporation. C O M M U N IC A TI O N

Direct Growth of Single Walled Carbon Nanotubes for the Characterization of Structural and Electronic Properties

ABSTRACTFabricating horizontally aligned single wall carbon nanotubes (CNTs) with controlled properties has been one of the significant challenges for field-effect transistor (FET) applications. This report demonstrates a novel procedure for the fabrication of horizontally aligned single walled CNTs using the focused ion beam (FIB) and chemical vapor deposition (CVD). This method allows the morphologies, internal structures, and elemental compositions of CNTs to be directly analyzed in the scanning electron microscope (SEM) and transmission electron microscope (TEM) and avoids any sample preparation procedures that might alter the structure of the CNTs. The techniques of electron beam and ion beam induced deposition (EBID and IBID) of Pt electrodes to the CNT ends were compared and both were found to produce metal contamination around the target area. The fabrication of large area electrodes to assist in testing the CNT's electronic properties, including contact resistance and I-V characteristics was investigated. Using this fabrication technique we were able to perform an I-V sweep on a CNT circuit as well as detect the metal contamination on the CNTs which occurred as a result of electrode deposition.

Thermal Radiation from Carbon Nanotubes in the Terahertz Range

The thermal radiation from an isolated finite-length carbon nanotube (CNT) is theoretically investigated both in near- and far-field zones. The formation of the discrete spectrum in metallic CNTs in the terahertz range is demonstrated due to the reflection of strongly slowed-down surface-plasmon modes from CNT ends. The effect does not appear in semiconductor CNTs. The concept of a CNT as a thermal nanoantenna is proposed.

Application of homogenization method on the evaluation and analysis of the effective stiffness for noncontinuous carbon nanotube/polymer composites

AbstractThe effective stiffness of noncontinuous carbon nano‐tubes (CNTs)/polymer composites are predicted by the homogenization method with exact periodic boundary conditions. Numerical calculations for regular and staggered array models are performed by the macro–microscopic finite element methods. The results are compared with those from Mori–Tanaka and Halpin–Tsai theories, which show that the stiffness of CNTs composites are not only related to aspect ratio and CNTs volume fraction as described in the earlier theories, but also sensitive to the space between CNTs ends, the distribution of CNTs within the selected representative volume element, and the geometrical shape of CNTs. The results from the two empirical approaches are included in the present results with special spacing ratios of horizontal and vertical fiber ends (Tf/Hf). It is proved that the homogenization method is an efficient method to predict the effective stiffness of CNTs/polymer composites with periodic microstructure. POLYM. COMPOS., 28:688–695, 2007. © 2007 Society of Plastics Engineers

Synthesis of silicon carbide nanorods without defects by direct heating method

High quality silicon carbide single crystal nanorods were successfully synthesized through gas–solid reaction between silicon and multiwall carbon nanotubes (CNTS) by direct heating methods. The X-ray diffraction (XRD) analysis showed that the reaction product of Si vapor with CNTS is β-SiC. SEM and HRTEM images suggested that the SiC nanorods are 3C-SiC single crystals almost free of defects. Based on these results, it is presumed that the reaction first took place at an end of CNTS giving β-SiC nuclei, one of which with its [111] parallel to the tube direction could grow consecutively along this direction so that a straight and solid SiC single crystal rod was formed without defects.

Fabrication of interfaces between carbon nanotubes and catalytic palladium using dielectrophoresis and its application to hydrogen gas sensor

The authors have developed a novel fabrication method of a carbon nanotube (CNT) gas sensor using positive dielectrophoresis (DEP). One advantage of the DEP technique is that one can readily build up interfaces between electrodes and CNT, or between CNT and other nanomaterials. In this paper, the DEP technique was employed to fabricate interfaces between CNT and catalytic palladium (Pd) to realize a CNT-based hydrogen (H2) gas sensor. Two types of CNT/Pd interfaces were proposed and tested. For one type, CNTs were trapped onto a microelectrode made of Pd so that the CNT/Pd interfaces were formed at both ends of CNTs lying over the Pd electrode surface. The other type of gas sensor was fabricated by simultaneously trapping CNTs and Pd nanoparticles under action of the positive DEP. It was found that both types of the CNT/Pd gas sensors could respond to hydrogen, while the CNT sensor without the Pd functionalization could not. For the hydrogen gas diluted with dry air, the electrical resistance of the CNT/Pd sensors increased at the moment of hydrogen exposure, and then turned to decrease in the later stage. The resistance increase just after the H2 exposure was probably due to reduction of CNTs by H atoms, which were produced by dissociative adsorption of H2 molecules on the catalytic Pd surface. In the later stage, the dissociated H atoms might react with dissociated oxygen atoms to create H2O molecules accompanying heat generation.

The Oscillatory Characteristics of a 2C60/CNT Oscillator System

The authors have studied, using molecular dynamic (MD) simulations, the oscillatory characteristics of a 2C60/CNT oscillator system, in which two C60 fullerenes oscillate inside a single walled carbon nanotube (CNT) in two basic modes, i.e., the symmetric and non-symmetric motions. In the symmetric mode, with each oscillation the two fullerenes move symmetrically from the CNT ends towards the CNT center where they bounce off each other and head back towards the ends. In the non-symmetric mode, the two fullerenes move back and forth inside the CNT crossing the center point of the CNT together with each oscillation. The simulations show that the non-symmetric oscillation mode is stable for the prescribed initial (maximum) velocities up to 300 m/s, while the symmetric oscillation mode however, experiences dynamic instabilities for a prescribed initial (maximum) velocity larger than 250 m/s. The instability takes place as a result of the transfer of energy from the translational to the rotational motion of the fullerenes. This characteristic differentiates 2C60/CNT oscillators from double-walled CNT oscillators. The rotation is primarily caused by the inter-colliding of the two fullerenes, which subjects the fullerenes to large van der Waals repelling forces. These repelling forces are not necessarily aligned perfectly along the CNT axis nor precisely pointing towards the mass centers of the fullerenes. These misalignments cause the fullerenes to rock around the CNT's axis, while their offsets from the mass centers cause the fullerenes to rotate. The rocking motion, being severely confined by the CNT, does not gain much energy itself, but instead, channels energy from translational to rotational motion. The energy channeling is found to be reversed in some very short time intervals, but the rotational motion always gains energies from the translational motion over a time interval that is long enough at the MD time scale. This feature, contrary to our experiences in the macroscopic world, appears to be unique for such nanoscopic mechanical systems.

Interfacial stress transfer of fiber pullout for carbon nanotubes with a composite coating

An analytical approach has been established to evaluate the interfacial stress transfer characteristics of single- and multi-walled carbon nanotubes (CNTs) with composite coatings by means of fiber pullout model. According to the present model, the effects of several parameters such as coating thickness, layer numbers and dimension of CNTs on interfacial stress transfers were investigated and analyzed. The results suggested that the maximum interfacial shear stress occurred at the pullout end of CNTs and decreased with increasing coating thickness as well as CNT wall thickness (layer numbers). Moreover, the distribution of the interfacial shear and coating axial stress along the CNT length was found to be largely affected by the friction coefficient in the interface between the CNT and the coating layer.

Investigation on Site Density of Carbon Nanotube Forests

AbstractWe report on a method for direct measurement of site density of vertically-aligned carbon nanotubes (CNTs). Site density is an important parameter of vertically-aligned carbon nanotube forests for various applications. By freezing the CNT forests in a polymer matrix and exposing the CNT ends, we obtained the site density of vertically aligned multi-walled CNTs through SEM observation and particle counting. Site densities of multi-walled CNTs grown by two different CVD processes, ferrocene/xylene process and Fe-Al/ethylene process, were measured to be ∼10 tubes/Ým2 and ∼53 tubes/Ým2, respectively. The results of site density distributions indicate non-uniform growth of carbon nanotubes at the micrometer scale in both processes.