Defective Nanotubes Research Articles

The Electronic and Magnetic Properties of Tetragonal Ultrathin BaTiO3 Nanotube

The unique structural, electronic and magnetic properties of intrinsic defect in tetragonal ultrathin BaTiO3 nanotube (u-BTONT) have been investigated by first-principle calculations. The zigzag (9,0) u-BTONTs with two different terminations (TiO2 and BaO) can be formed by rolling up one monolayer BTO along a certain crystallographic axis, and the BaO-terminated NT is more stable than TiO2 terminated due to its lower binding energy. The oxygen vacancies on the tube are more stable than cation vacancies, and their magnetic coupling is related not only to the kinds of oxygen vacancies but also to the distance of vacancies. Moreover, both Ba and Ti vacancies also can introduce the ferromagnetism in u-BTONT, which is different from the origin of magnetism in BTO bulk and (001) surface. It is indicated that one-dimensional structure with high surface area can make it easier to form more useful vacancies which prefer ferromagnetism. Our work offers a possible route to fabricate the multiferroic materials.

Synthesis and electrochemical performance of vertical carbon nanotubes on few-layer graphene as an anode material for Li-ion batteries

Bulk graphite is widely used in today's Li-ion batteries as an anode material due to its high cycling stability. However, graphite's low capacity and rate capability prevent its application in high power density batteries. Here, a nanostructured carbon, consisting of vertical carbon nanotubes on few-layer graphene, has been synthesized and tested as an anode material for high capacity Li-ion batteries. In this synthesis, a simple hydrothermal method was used to assemble homogeneous distributed Fe2O3 nanoparticles on few-layer graphene based on van der Waals interaction, and these were then used as a catalyst to grow carbon nanotubes further by means of a chemical vapor deposition method. The electrochemical properties of the nanocomposites were measured. A reversible capacity of 525 mAh g−1 after 1000 cycles at 500 mA g−1 with Coulombic efficiency in excess of 98% was accomplished. Voids between vertical carbon nanotubes provide a short diffusion path for both electron and Li-ion, and the defects in carbon nanotubes provide more storage sites for lithium. Three kinds of nanocomposites with different carbon nanotube diameters were synthesized and characterized. The influence of the carbon nanotube diameter and morphology on the lithium storage properties of these nanocomposites was also studied.

Mechanical properties and fracture behaviour of defective phosphorene nanotubes under uniaxial tension

The easy formation of vacancy defects and the asymmetry in the two sublayers of phosphorene nanotubes (PNTs) may result in brand new mechanical properties and failure behaviour. Herein, we investigate the mechanical properties and fracture behaviour of defective PNTs under uniaxial tension using molecular dynamics simulations. Our simulation results show that atomic vacancies cause local stress concentration and thus significantly reduce the fracture strength and fracture strain of PNTs. More specifically, a 1% defect concentration is able to reduce the fracture strength and fracture strain by as much as 50% and 66%, respectively. Interestingly, the reduction in the mechanical properties is found to depend on the defect location: a defect located in the outer sublayer has a stronger effect than one located in the inner layer, especially for PNTs with a small diameter. Temperature is also found to strongly influence the mechanical properties of both defect-free and defective PNTs. When the temperature is increased from 0 K to 400 K, the fracture strength and fracture strain of defective PNTs with a defect concentration of 1% are reduced further by 71% and 61%, respectively. These findings are of great importance for the structural design of PNTs as building blocks in nanodevices.

Radiation modification of Ni nanotubes by electrons

Electron irradiation of metal nanostructures is an effective tool for stimulating a controlled modification of the structural and conductive material properties. Use of the electron irradiation with energies less than 500 keV allows conducting controlled annealing of nanotube defects, which leads to the improvement of the conductive properties due to decreasing resistance. In this case, the use of radiation doses above 150 kGy induces the samples destruction, caused by the thermal heating of nanotubes, leading to the crystal lattice destruction and the sample amorphization.

Adsorption and Diffusion of Fluids in Defective Carbon Nanotubes: Insights from Molecular Simulations.

Single-walled carbon nanotubes (SWNTs) have been shown from both simulations and experiments to have remarkably low resistance to gas and liquid transport. This has been attributed to the remarkably smooth interior surface of pristine SWNTs. However, real SWNTs are known to have various defects that depend on the synthesis method and procedure used to activate the SWNTs. In this paper, we study adsorption and transport properties of atomic and molecular fluids in SWNTs having vacancy point defects. We construct models of defective nanotubes that have either unrelaxed defects, where the overall structure of the SWNT is not changed, or reconstructed defects, where the bonding topology and therefore the shape of the SWNT is allowed to change. Furthermore, we include partial atomic charges on the SWNT carbon atoms due to the reconstructed defects. We consider adsorption and diffusion of Ar atoms and CO2 and H2O molecules as examples of a noble gas, a linear quadrupolar fluid, and a polar fluid. Adsorption isotherms were found to be fairly insensitive to the defects, even for the case of water in the charged, reconstructed SWNT. We have computed both the self-diffusivities and corrected diffusivities (which are directly related to the transport diffusivities) for each of these fluids. In general, we found that at zero loading that defects can dramatically reduce the self- and corrected diffusivities. However, at high, liquidlike loadings, the self-diffusion coefficients for pristine and defective nanotubes are very similar, indicating that fluid-fluid collisions dominate the dynamics over the fluid-SWNT collisions. In contrast, the corrected diffusion coefficients can be more than an order of magnitude lower for water in defective SWNTs. This dramatic decrease in the transport diffusion is due to the formation of an ordered structure of water, which forms around a local defect site. It is therefore important to properly characterize the level and types of defects when accurate transport diffusivities are needed.

Study of the quality of carbon nanotubes produced by chemical vapor deposition

Method for obtaining carbon nanotubes by the method of chemical vapor deposition with varied amount of catalyst, reaction duration, and temperature is considered. The synthesis of carbon nanotubes with various mode parameters was experimentally studied. The quality of the carbon nanotubes was examined by Raman spectroscopy. It was found that the defectiveness of the carbon nanotubes depends on the synthesis parameters.

A DFT study on the healing of N‐vacancy defects in boron nitride nanosheets and nanotubes by a methylene molecule

AbstractIn the present work, density functional theory calculations are used to investigate the healing mechanism of a N‐vacancy defect in boron nitride nanosheet (BNNS) or nanotube (BNNT) with a CH2 molecule. The healing process starts with the chemisorption of CH2 at the defect site, followed by its dehydrogenation over the surface. Next, a H2 molecule is produced which can be easily released from the surface due to its small adsorption energy. For the dehydrogenation of CH2 molecule over the defective BNNS or BNNT, the first CH bond dissociation is the rate determining step. Our results indicate that the dehydrogenation of CH2 over BNNS is both thermodynamically and kinetically more favorable than over BNNT. Besides, this study proposes a novel method for achieving C‐doped BNNSs and BNNTs. Given that the healing process proceeds without using a metal catalyst, therefore, no any purification is needed to remove the catalyst.

Modeling of tensile testing on perfect and defective graphenylene nanotubes using molecular dynamics simulations

Molecular dynamics simulations are employed here to study the mechanical properties of graphenylene nanotubes (NTs). The effects of different geometrical parameters, such as NT length and diameter, on the behavior of graphenylene NTs under tensile testing are investigated. Moreover, the tensile test is simulated at several temperatures, to obtain the stress–strain curves of both armchair and zigzag graphenylene NTs. It is shown that graphenylene NTs with larger diameter possess larger elastic moduli. The elastic modulus of graphenylene NTs is about one half that of carbon NTs. However, the maximum tolerable stress and strain of the graphenylene NTs decreases with increasing NT diameter. Investigating the effect of vacancy defects on the elastic properties of the graphenylene NTs, it is shown that Young’s modulus of armchair and zigzag graphenylene NTs decreases nonlinearly with increasing defect percentages.

Tunable room-temperature single-photon emission at telecom wavelengths from sp3 defects in carbon nanotubes

Generating quantum light emitters that operate at room temperature and at telecom wavelengths remains a significant materials challenge. To achieve this goal requires light sources that emit in the near-infrared wavelength region and that, ideally, are tunable to allow desired output wavelengths to be accessed in a controllable manner. Here, we show that exciton localization at covalently introduced aryl sp3 defect sites in single-walled carbon nanotubes provides a route to room-temperature single-photon emission with ultrahigh single-photon purity (99%) and enhanced emission stability approaching the shot-noise limit. Moreover, we demonstrate that the inherent optical tunability of single-walled carbon nanotubes, present in their structural diversity, allows us to generate room-temperature single-photon emission spanning the entire telecom band. Single-photon emission deep into the centre of the telecom C band (1.55 µm) is achieved at the largest nanotube diameters we explore (0.936 nm). Single-photon emission with 99% purity is generated from sp3 defects in carbon nanotubes (CNTs) by optical excitation at room temperature. By increasing the CNT diameter from 0.76 nm to 0.94 nm, the emission wavelength can be changed from 1,100 nm to 1,600 nm.

Exploring electronic properties and NO gas sensitivity of Si-doped SW-BNNTs under axial tensile strain

Continuously tuning electronic and magnetic properties of nanomaterials specially by applying an axial tensile strain is a promising route for construction of impending electronic and optoelectronic nanodevices. In the present work, Si doping and axial tensile strain were simultaneously utilized in exploring the structural and electronic properties of single-walled (6,0) SiN, SiB and SiN,B-doped Stone–Wales defective boron nitride nanotubes at M05-2X/6–31+G(d) level. Our findings demonstrate that the Si doping of SW-BNNT destroys the hexagonal BN network and alters the insulating feature of the SW-BNNT. Binding energies of Si-doped SW-BNNTs are estimated to be smaller than un-doped SW-BNNT and decrease continuously upon axial tensile strain. It can be estimated that the Si-doped SW–BNNTs and, in turn, their axial strained forms are more suitable than SW-BNNT one for photoconductivity applications. The unstrained SiN,B has a lower band gap than unstrained SiN and SiB. The results show that the axial tensile strain is not a suitable strategy to improve the conductivity of SiN,B, contrary to those found in SiN and SiB. In the second part of this work, sensitivity of strained and unstrained Si-doped SW-BNNTs toward NO gas is evaluated. The results show that the chemical adsorption of NO is thermodynamically favored in both strained and unstrained forms. Among the Si-doped SW-BNNT–NO complexes, SiN,B-ON1 and SiB-NO2 complexes with adsorption energy of −32.7 and −33.3 kcal mol−1, respectively, are thermodynamically more stable than other complexes. In addition, dispersion-corrected adsorption energies were evaluated at M05-2X-D3/6-31++G(d,p)//M05-2X/6–31+G(d) level of theory. The greatest charge transfer value and change in the band gap upon adsorption was predicted in all complexes. Thus, it is expected that Si-doped SW-BNNT could be a favorable NT for removing and sensing the NO gas.

Laser-induced graphitic healing of carbon nanotubes aligned in a sheet

A laser scanning method was developed to heal the graphitic defects of carbon nanotubes (CNTs). The laser beam was controlled to longitudinally scan CNTs aligned in the freestanding CNT sheets. The localized heating zone generated by the laser beam moved along the CNT axes and exhibited the extreme thermal conditions of fast-heating and fast-cooling. This unique laser-CNT interaction was beneficial to quickly heal the CNT defects and limited the undesired structural transformation of CNTs. The effectiveness of the introduced laser method in healing CNTs was confirmed by comparing with the conventional furnace annealing method and also verified by treating the plasma-bombarded CNTs. The generated localized heating zone and the scanning pattern controllability of the laser irradiation method are favorable to thermally treat the 1D-structured nanomaterials having the dominant longitudinal thermal conductivities such as CNTs or nanowires.

Controlled Addition of Carbon Nanotube Defects and Their Role in Limiting Solar Cell Performance

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have attracted significant attention as a photoactive component in thin film photovoltaics due to their strong absorptivity and high charge mobility. However, the external quantum efficiency (EQE) of s-SWCNT/C60 heterojunction solar cells is limited by poor exciton diffusion to the heterointerface. Exciton diffusion is believed to be strongly dependent on the length between defects, which quench excitons. Here, we systematically study the influence of defects on s-SWCNT/C60 planar heterojunction photovoltaic devices. We use covalent sidewall functionalization via diazonium chemistry to introduce sp3 defects to the s-SWCNTs at varying concentrations. We generate a defect-adding model to describe the position of defects on the surface of the nanotube, and use it to successfully reproduce the time-resolved exciton decay rate measured with transient absorption spectroscopy. We fabricate s-SWCNT/C60 heterojunction photovoltaic cells and characterize the EQE as a function of defect concentration. We develop a diffusion limited contact quenching Monte Carlo model to reproduce the EQE trends we observe. The model allows us to predict the behavior of theoretical s-SWCNT devices with varying length distribution and defect concentration, toward the limit of long length and zero defects. This work guides the field of nanotube photovoltaics and illuminates the promise and limitations of s-SWCNT solar cell devices.

Effect of nanostructuring of ZnO for gas sensing of nitrogen dioxide

Nitrogen dioxide (NO2) is a toxic gas that contributes to photochemical pollution and can cause adverse effects to human and environmental health. Monitoring this gas is therefore important to warn of potential exposure. Nanostructures of zinc oxide (ZnO) have been studied for the sensing of NO2 gas, however, the effect of nanostructure morphology on the gas-sensor reaction mechanism is not understood. We examine the NO2 sensing mechanism for three different ZnO nanostructures, namely a nanowire, a facetted nanotube and a single walled (8,0) nanotube using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. The effect of gas concentration and oxygen vacancies is also determined. It was shown that NO2 adsorbs on all nanostructures in multiple configurations, with the bonding being strongest on the nanowire and weakest on the single walled nanotube. NO2 behaves as a charge acceptor, consistent with adsorption on the thin film and desorbs intact from the nanostructures at simulation temperatures of 300K and 700K, providing new surface sites for detection of other molecules. The presence of surface oxygen vacancies was found to enhance the binding, with NO2 chemisorbing on all 3 nanostructures via two bonds to the surface. The AIMD simulations showed that at a simulation temperature of 500K, NO2 dissociates on the nanowire and facetted nanotube defect surfaces resulting in a surface bound oxygen atom which fills the defect site, with the remaining NO desorbing from the surface. The single walled nanotube was shown to be unstable at these simulation temperatures. These findings provide important information for the experimental development of ZnO nanostructure-based gas sensors.

Unzipping of multi-wall carbon nanotubes with different diameter distributions: Effect on few-layer graphene oxide obtention

Few-layer graphene oxide (FLGO) was obtained by chemical unzipping of multi-wall carbon nanotubes (MWCNT) of different diameter distributions. MWCNT were synthesized by catalytic decomposition of methane using Fe‐Mo/MgO catalysts. The variation in the Fe/Mo ratio (1, 2 and 5) was very influential in MWCNT diameter distribution and type of MWCNT obtained, including textural, chemical, structural and morphological characteristics. MWCNT diameter distribution and surface defects content had a profound impact on the characteristics of the resulting FLGO. Thus, MWCNT obtained with the catalyst with a Fe/Mo: 5 and presenting a narrow diameter distribution centered at 8.6±3.3nm led to FLGO maintaining non-oxidized graphite stacking (according to XRD analysis), lower specific surface area and higher thermostability as compared to FLGO obtained from MWCNT showing wider diameter distributions. The presence of more oxygen-containing functionalities and structural defects in large diameter nanotubes promotes the intercalation of species towards the inner layers of the nanotube, resulting in an enhanced MWCNT oxidation and opening into FLGO, what improves both micro- and mesoporosity.

Relationship between local buckling and atomic elastic stiffness in multi-walled carbon nanotubes under compression and bending deformations

This paper discusses unified criteria for the local buckling of multi-walled carbon nanotubes in both compression and bending deformations from a standpoint of atomic simulations. Defective and non-defective five-walled carbon nanotubes are subjected to compression and bending deformation by the molecular dynamics method using the adaptive intermolecular reactive empirical bond order potential. The atomic elastic stiffness of each atom, Bijα, is then evaluated during the deformations to discuss the onset of local buckling. The Bijα corresponds to the second-order derivatives of the atomic energy, i.e., the gradient of the local stress–strain surfaces in six-dimensional strain space. If Bijα has a negative eigenvalue, a local unstable path will exist in the direction of the strain. Under compression, the smallest eigenvalues, λ(1)α, of the Bijα for all atoms changes to a negative value long before buckling occurs, while the second-smallest eigenvalues, λ(2)α, of the Bijα of some atoms change to a negative value just prior to buckling. Under bending deformation, the change in the positivity of λ(2)α corresponds to a rippling deformation on the side of compressive bending stresses. A variation in the location of defects in the carbon nanotubes affects the peak stresses under compression and the peak moments under a bending deformation. However, for all models, local buckling occurs from the dense region of atoms whose λ(2)α<0 in some outer layers. This suggests that the atomic elastic stiffness is capable of acting as an evaluation criterion for local buckling in multi-walled carbon nanotubes.

Impact of carbon nanotube defects on fracture mechanisms in ceramic nanocomposites

Carbon nanotubes (CNTs) can improve the fracture toughness of ceramic composites. It is widely believed that defects in the CNTs will alter key properties that ultimately dictate composite toughening. However, to date there have been no studies that directly correlate these specific properties to controlled defect levels in CNTs. This type of investigation is presented here, using ceramic nanocomposites with a polymer derived ceramic (PDC) matrix, reinforced with multiwalled CNTs. This work includes basic fracture measurements on full composite films, combined with careful testing of individual CNTs embedded in the same PDC matrix. The defect levels in these CNTs were controlled with high energy carbon ions. By using this approach, it was possible to directly correlate defect levels with the interfacial shear strength (IFSS) of the CNT/PDC interface, the CNT fracture strengths, and the pull-out lengths observed after fracture of the full composites. The radiation induced defects led to substantial increases in the IFSS, and only a marginal decrease is observed in the measured fracture strength. Based on this, the shorter pull-out lengths that occurred with higher defect levels were primarily attributed to stronger bonding at the CNT/PDC interface.

Healing of a carbon-vacancy defect in silicon carbide nanotubes by CO molecules: A DFT study

DFT calculations are performed to investigate the healing process of C-vacancy defective silicon carbide nanotubes (SiCNTs) by CO molecules. The healing process consists of the CO-vacancy recombination, followed by extra O removal by another CO molecule. It is found that the removal of the extra O atom on SiCNTs is the rate-limiting step, for which the energy barrier decreases with an increase of tube diameter. Nevertheless, the healing of SiCNTs is nearly independent on the tube chirality. Our results can be useful not only in the purification of SiCNTs, as well as in the removal of CO gas.

Single Nanotube Spectral Imaging To Determine Molar Concentrations of Isolated Carbon Nanotube Species.

Electronic and biological applications of carbon nanotubes can be highly dependent on the species (chirality) of nanotube, purity, and concentration. Existing bulk methods, such as absorbance spectroscopy, can quantify sp2 carbon based on spectral bands, but nanotube length distribution, defects, and carbonaceous impurities can complicate quantification of individual particles. We present a general method to relate the optical density of a photoluminescent nanotube sample to the number of individual nanotubes. By acquiring 3-dimensional images of nanotubes embedded in a gel matrix with a reducing environment, we quantified all emissive nanotubes in a volume. Via spectral imaging, we assessed structural impurities and precisely determined molar concentrations of the (8,6) and (9,4) nanotube species. We developed an approach to obtain the molarity of any structurally enriched semiconducting single-walled carbon nanotube preparation on a per-nanotube basis.

多層カーボンナノチューブの強度・破壊特性評価と構造欠陥の影響に関する研究

In this study, we report the relationship between the mechanical properties and structural defects of multi-walled carbon nanotubes (MWCNTs) synthesized by a chemical vapor deposition (CVD) method. The tensile strength, Young's modulus and Weibull modulus of the individual MWCNTs were determined by conducting uniaxial tensile tests using a manipulator tool operated inside a scanning electron microscope. In addition, the structural defects which induced the failure of the MWCNTs were observed by a transmission electron microscope (TEM). TEM observations revealed that the MWCNTs exhibited several types of structural defects: discontinuous flaws such as holes; kinks and bends; impurities i.e., catalysts even though highly crystalline layers were almost perfectly aligned with the MWCNT axis. The tensile-loading experiments demonstrated that the average tensile strength, Young's modulus and Weibull modulus of the 23 MWCNTs were 5.2 ± 2.1 GPa, 210 ± 140 GPa and 2.7, respectively. The MWCNTs underwent failure leaving either a clean break or a very short sword and sheath failure, suggesting that a significant interwall load transfer might be facilitated by the irregular wall structures as mentioned above. These mechanical characteristics and fracture modes were reasonably consistent with those of previously reported CVD-grown MWCNTs. In order to identify the structural defects controlling the fracture of the MWCNTs, structural analyses were conducted by comparing TEM images captured before and after their breaking. The TEM images of the individual MWCNTs revealed that the defects described above induced the nanotube failure. These suggest that the tensile strength of the CVD-grown MWCNTs used in this study is dominated by the structural defects in particular discontinuous flaws and kinks and bends. The results reported here indicate that improvement and optimization of synthesis methods are needed to prepare stronger MWCNTs with less structural defects reported here.

Inducing structural defects in multi-walled carbon nanotubes by biological oxidation

In this study, carbon-nanotubes (CNTs) resistant soil bacteria were utilized to induce biotransformation in engineered multiwalled-carbon nanotubes (MWCNTs). It was observed that the CNTs-resistant bacteria upon interaction with engineered MWCNTs lead to their partial degradation. Raman spectra of bio-transformed-MWCNTs revealed increased intensity ratio of ID/IG with subsequent formation of C−O−H, C=O and COOH groups on the outer walls of nanotubes. Our results demonstrate a low-cost modification of MWCNTs by resistant bacteria to induce structural defects that potentially useful for industrial applications.