Defective Nanotubes Research Articles

Radiation damage to multi-walled carbon nanotubes and their Raman vibrational modes

Since their discovery in 1991 by Iijima, carbon nanotubes (CNTs) have been of great interest, both from a fundamental point of view and for future applications. As recent experimental and theoretical studies demonstrate, irradiation of CNTs with energetic particles can successfully be used for nano-engineering, e.g., for creating molecular junctions between the nanotubes, making nanotube-based quantum dots and composite materials with enhanced mechanical properties. In this work, the Raman spectroscopy investigation of nanotube defects created by irradiation of Ar+ ions with various energies and doses was presented. In order to obtain more informative data, scanning electron microscope and thermal gravimetric analysis were carried out. Sheet resistivity of samples was also measured with the standard four-point probe technique. From the results, it was evident that the intensity ratio of the ‘D band’ at around 1300 cm-1 to the intensity of the ‘G band’ at 1590 cm-1, relative intensity of the D to G bands increases with enhancement of the irradiation dose. Comparing the intensity ratio \(\boldsymbol {I}_{\mathbf {D^{\boldsymbol {\prime }}}}\boldsymbol {/}\boldsymbol {I}_{\mathbf {G } }\) (the D\(^{\boldsymbol {\prime }}\) band was assigned to the lattice defects which occur inside the graphene atomic layer and induces a break in the two-dimensional translational symmetry) with I D / I G tend to decide the D\(^{\boldsymbol {\prime }}\) band treated as a defect. Electrical measurements showed that resistance of samples increases with enhancement of irradiation dose, clearly due to creation of more defects.

Narrowing of band gap and effective charge carrier separation in oxygen deficient TiO2 nanotubes with improved visible light photocatalytic activity

Oxygen vacancies are introduced into hydrothermally processed TiO2 nanotube by vacuum calcination. Formation of oxygen vacancies modifies the local coordination in TiO2 as evident from Raman spectroscopy and secondary ion mass spectrometry (SIMS) results. The surface area is increased from 172.5m2/g in pure to 405.1m2/g in defective TiO2 nanotube. The mid-band gap electronic states created by oxygen vacancies are mostly responsible for the effective narrowing of band gap. Charge carrier separation is sufficiently prolonged as the charged oxygen defect states inhibit facile carrier recombination. With high surface area, narrowed band gap and separated charge carriers defective TiO2 nanotube is a suitable candidate in the photodegradation of methylene blue (MB) and phenol under visible light illumination. Photosensitized electron transfer from MB to the conduction band of TiO2 and the photodegradation of MB is facilitated in presence of high density of oxygen vacancies. Unlike MB, phenol absorbs in the UV region and does not easily excited under visible light. Phenol shows activity under visible light by forming charge transfer complex with TiO2. Defect trapped carriers become available at the phenol–TiO2 interface and finally interact with phenol molecule and degrade it.

A Review on Defects in Carbon Nanotubes

Various defects on the CNT wall have been reported, which are formed during the synthesizing process. CNTs have superior properties compared to the traditional engineering materials. However, these properties hold only for the ideal case of carbon nanotubes, where these are made of perfect hexagonal graphite honeycomb lattice of mono-atomic layer thickness. The advantages or disadvantages of the presence of defects in carbon nanotubes depend on their applications. Structural defects may increase the adhesion of other atoms and molecules to carbon nanotubes. It has also been found that the defects in CNT do cause a change in its resonant frequency as compared to that of a non-defective CNT. The defects that have been considered for the purpose of analysis in this research includes defects in the carbon nanotubes likewise Waviness, Vacancy Defect, Pinhole Defect, Fracture and Stone Wales Defect. It has been observed that with the increase in the number of defects in CNT, a reduction in the fundamental frequency is observed.

One-step preparation of multiwall carbon nanotube/silicon hybrids for solar energy conversion

The hybrid material consisting of a thin layer of multiwall carbon nanotubes (MWCNTs) on an n-doped silicon wafer was obtained in one step using an aerosol-assisted catalytic chemical vapor deposition. The MWCNTs were grown from a mixture of acetone and ethanol with ∼0.2 wt.% of iron polyoxomolybdate nanocluster of the keplerate-type structure. The samples produced at 800°C and 1050°C were tested as a solar energy converter. It was shown that photoresponse of the hybrid material significantly depends on the presence of structural defects in MWCNTs, being much higher in the case of more defective nanotubes. This is because defects lead to p-doping of nanotubes, whereas the p−n heterojunction between MWCNTs and silicon provides a high efficiency of the solar cell.

Effect of carbon nanotubes dispersed in binder on properties of epoxy nanocomposite

Effect of quality parameters of starting raw materials, native carbon nanotubes (degree of defectiveness, thermal stability, morphology) on the properties of carboxylated and amidated nanotubes produced from these raw materials and on the homogeneity of a dispersion of amidated nanotubes in an epoxy oligomer was studied. The physicomechanical properties of epoxy nanocomposites produced from these dispersions were examined. It was found experimentally that an increase in the defectiveness of native nanotubes leads to a rise in the size of numerous aggregates in dispersions composed of an epoxy resin and amidated carbon nanotubes and, as a result, to deterioration of the physicomechanical characteristics of epoxy nanocomposites based on these dispersions.

Charge modulation of magnetization in X-doped MgO nanotube clusters (X=C, N)

First-principles calculations based on density functional theory are performed to study the magnetic and electronic properties of X-doped 8×7 MgO nanotube clusters (X=C, N). The N dopant easily occupies the O-site at the edge of MgO nanotube, embracing neutral or charged defect state, and induces notable magnetization in N-doped MgO tubular cluster. More important, this p-electron magnetization can be significantly modulated as the charged state of the defect changes. Regarding C doping, impurity atom readily substitute the Mg atom located at the edge of MgO nanotube to form neutral defect, and net magnetization is found to be zero. The calculated electron densities of states show that the O-site N doping at the edge greatly narrows or even destroys band-gap, while it enlarges somewhat for the Mg-site C doping at the edge. The results are likely to stimulate a promising class of materials for various applications ranging from spintronics to magneto-optics.

Effects of Microwave Absorption on Long and Short Single-Walled Carbon Nanotubes at 10-6 Torr

Carbon nanotubes have been observed to emit ultraviolet, visible and infrared radiation when exposed to microwaves. We have performed experiments in which both short (0.5 μm–2 μm) and long (5 μm–30 μm) single and double-walled carbon nanotubes were exposed to 2.46 GHz microwaves at a pressure of ~ 10-6 Torr. Structural modifications of the carbon nanotubes due to microwave absorption have been studied using the Raman spectroscopy G-band and D-band intensities, which suggest that microwave irradiation at relatively low pressure results in an increase in nanotube defects, especially in the case of the long nanotubes. Furthermore, a comparison of the spectra of the radiation emitted from the nanotubes suggests that the longer nanotubes emitted radiation of much greater intensity than the shorter nanotubes. Based on the results of the experiments and results described in previous reports, the ultraviolet, visible and infrared radiation emitted as the result of microwave absorption by carbon nanotubes seems to be primarily blackbody radiation emitted due to Joule heating. However, the presence of several broad photopeaks in the spectra of the emitted radiation (which do not seem to be related to gases absorbed by the nanotubes or the presence of catalyst particles) suggest that emissions are not the result of Joule heating alone.

Fast atom bombardment onto vertically aligned multi-walled carbon nanotube bumps to achieve low interconnect resistance with Au layer

Bump-shaped vertically aligned multi-walled carbon nanotubes (MWNTs) were bonded to a Au substrate by a surface activated bonding method using Ar fast atom bombardment (FAB). After carrying out Ar-FAB for 1200s at a bonding load of 0.74MPa, the interconnect resistance reduced to 1/1000 of the original resistance. The lowest resistance of 5.2Ω was achieved for the bonded specimens. Torn graphite layers were observed in the MWNT that was processed by Ar-FAB for 2400s, and the ratio of the G-band to the D-band in the Raman spectra indicated that the peaks attributable to the defects in the carbon nanotubes increased in response to the Ar-FAB process. These findings indicate that the torn graphite layers were exposed and were in contact with the Au layer, which caused the conductivity path between MWNTs and Au substrates to increase.

Carbon Nanotube Degradation in Macrophages: Live Nanoscale Monitoring and Understanding of Biological Pathway.

Despite numerous applications, the cellular-clearance mechanism of multiwalled carbon nanotubes (MWCNTs) has not been clearly established yet. Previous in vitro studies showed the ability of oxidative enzymes to induce nanotube degradation. Interestingly, these enzymes have the common capacity to produce reactive oxygen species (ROS). Here, we combined material and life science approaches for revealing an intracellular way taken by macrophages to degrade carbon nanotubes. We report the in situ monitoring of ROS-mediated MWCNT degradation by liquid-cell transmission electron microscopy. Two degradation mechanisms induced by hydroxyl radicals were extracted from these unseen dynamic nanoscale investigations: a non-site-specific thinning process of the walls and a site-specific transversal drilling process on pre-existing defects of nanotubes. Remarkably, similar ROS-induced structural injuries were observed on MWCNTs after aging into macrophages from 1 to 7 days. Beside unraveling oxidative transformations of MWCNT structure, we elucidated an important, albeit not exclusive, biological pathway for MWCNT degradation in macrophages, involving NOX2 complex activation, superoxide production, and hydroxyl radical attack, which highlights the critical role of oxidative stress in cellular processing of MWCNTs.

Effects of graphene imperfections on the structure of self-assembled pentacene films

The quality of pentacene films in pentacene-based devices significantly affects their performance. In this report, the effects of various defects in graphene on a pentacene film were studied with scanning tunneling microscopy. The two most common defects found in the epitaxial graphene grown on SiC(0 0 0 1) substrates were subsurface carbon nanotube (CNT) defects and step edges. The most significant perturbation of the pentacene films was induced by step edges between single-layer and bilayer graphene domains, while the effect of step edges between single-layer domains was marginal. The subsurface CNT defects slightly distorted the structure of the single-layer pentacene, but the influence of such defects decreased as the thickness of the pentacene film increased. These results suggest that the uniformity of the graphene layer is the most important parameter in the growth of high-quality pentacene films on graphene.

C-, N-, S-, and Fe-Doped TiO2 and SrTiO3 Nanotubes for Visible-Light-Driven Photocatalytic Water Splitting: Prediction from First Principles

The ground state electronic structure and the formation energies of both TiO2 and SrTiO3 nanotubes (NTs) containing CO, NO, SO, and FeTi substitutional impurities are studied using first-principles calculations. We observe that N and S dopants in TiO2 NTs lead to an enhancement of their visible-light-driven photocatalytic response, thereby increasing their ability to split H2O molecules. The differences between the highest occupied and lowest unoccupied impurity levels inside the band gap (HOIL and LUIL, respectively) are reduced in these defective nanotubes down to 2.4 and 2.5 eV for N and S doping, respectively. The band gap of an NO+SO codoped titania nanotube is narrowed down to 2.2 eV (while preserving the proper disposition of the gap edges relatively to the reduction and oxidation potentials, so that eHOIL < ϵO2/H2O < ϵH+/H2 < ϵLUIL), thus decreasing the photon energy required for splitting of H2O molecule. For C- and Fe-doped TiO2 NTs, some impurity levels lie in the interval between both redox po...

Role of Defect Density in Optimizing MWCNT-Polymer Composite Sensor Performance: Tuning of Its Acceptable Limit by Acid Treatment Condition

Surface defects of carbon nanotubes (CNTs) are always a debatable issue about its role in sensing properties and a systematic study is presented in this paper for its exploration. Intentional defect sites of various types were produced at different dose by liquid phase oxidation technique followed by acid reflux for different durations; and its effect on the toxic gas sensing performances were experimented in a rigorous manner to understand the actual sensing mechanism in the presence of increasing dose of defects. Interestingly, the sensor response was found to increase to a certain limit, dependent on the critical acid treatment time, beyond which the performance of the sensor goes reverse. And in depth analysis has been put up for the first time to optimize the acid treatment condition in order to achieve the maximum sensitivity of the sensor. Transmission electron microscopy (TEM) results confirm the introduction of many new types of defects on Multi-walled carbon nanotubes (MWCNTs) surface as acid treatment was intensified. Its ultimate consequence is that the controllably introduction of desired density of defects on nanotubes, by tuning acid treatment condition, plays another major role to improve the gas sensor sensitivity apart from the conventional techniques where increase in surface area by stable homogenous dispersion of CNTs is prime concern.

Atom Vacancies on a Carbon Nanotube: To What Extent Can We Simulate their Effects?

Atom vacancies are intrinsic defects of carbon nanotubes. Using a zigzag nanotube as reference, this paper focuses on the comparison of calculations performed within density functional theory and a number of classical force fields widely used for carbon systems. The results refer to single and double vacancies and, in particular, to the induced structural changes, the formation energies, and the energy barriers relative to elementary processes such as reconstruction, migration, and coalescence. Characterization of these processes is remarkably different in the different approaches. These findings are meant to contribute to the construction of DFT-based classical schemes for carbon nanostructures.

(Invited) New Solar and Chemical Energy Conversion Mechanisms Enabled By Nanocarbon Systems

Our laboratory at MIT has been interested in how nanocarbon systems, such as carbon nanotubes and graphene, can enable new energy conversion mechanisms. This presentation will highlight two such conversion schemes: chemical energy conversion via reactivity waves and solar energy harvesting using nanotube based photovoltaics. Towards the former, spatially propagating reaction waves are already central to a variety of energy applications, such as high temperature solid phase or combustion synthesis, and thermopower waves. In this work, we identify and study a previously unreported property of such waves, specifically that they can generate temperatures far in excess of the adiabatic limit. We show that this superadiabaticity occurs when a reaction wave in either one dimension (1D) or two dimensions (2D) impinges upon an adiabatic boundary under specific reaction and heat transfer conditions. This property is studied analytically and computationally for a series of 1D and 2D example systems, producing an estimate of the upper bound for excess temperature rise as high as 1.8 times the adiabatic limit, translating to temperatures approaching 2000 K for some practical materials. We show that superadiabaticity may enable several new types of energy conversion mechanisms, including thermophotovoltaic wave harvesting, which we analyze for efficiency and power density. Towards the second application, photovoltaics using single-walled carbon nanotubes (SWNTs) as near-infrared photo-absorbers have seen rapid recent progress, offering promise for long-wavelength light harvesting devices. Despite this interest the fundamental design questions remain, such as optimal device thickness, nanotube orientation, density, and the impact of impurities. To address this challenge, we develop a deterministic model of SWNT PVs derived directly from SWNT photophysics using photon, exciton, and charge carrier population balances. The model accounts for arbitrary distributions of nanotube chiralities, lengths, orientations, defect types and concentrations, bundle fraction and size, and density. We show that feasible devices can achieve external quantum efficiencies above 60%. We reveal a sharply optimal device thickness that is a function of nanotube density, orientation, and quenching site concentration. This thickness stems from a tradeoff between exciton generation and diffusion to the electrodes, and is at a minimum at the limit of close-packed nanotube density. We show that this minimum characterizes a given device design and scales with mean nanotube length to exponent 0.4. The normalized difference between optimal thickness and this close-packed limit scales inversely with density to the 0.24 power. Practically, in-plane aligned nanotube configurations yield optimal thicknesses less than 10 nm, increasing to a range of 50 to 200 nm for vertical alignment. Due to weak inter-SWNT exciton transport relative to exceptional intra-SWNT diffusion, vertically-aligned films are unambiguously favored at densities above 3% of the close-packed limit; at lower densities however an optimum emerges at an intermediate angle to compensate for weaker light absorption of vertical nanotubes. Comparison to published experimental devices displays the model’s utility for device design.

Diameter-dependent annealing kinetics of X-ray-induced defects in single-walled carbon nanotubes

Diameter-dependent annealing effects of X-ray-induced defects in single-walled carbon nanotubes (SWNTs) were studied by Raman scattering spectroscopy. We found that X- ray-induced defects were formed in thin SWNTs at higher density than that in thick SWNTs. The X-ray-induced defects, distant pairs of interstitials and vacancies (I-V), were eliminated by thermal annealing. The recovery temperature of the X-ray-induced defects in the thin SWNTs were higher than that of the thick ones, indicating the thermal stability of the defects in thinner SWNTs are higher. In order to simulate the annealing behaviours of the X-ray-induced defects in SWNTs with diameters of ∼0.9 and ∼1.4 nm, we suggested the competitive reactions of diffusion of interstitials, formation of close I-V pairs and their recombination. The simulated results showed that the reaction-rate constant of elimination of close I-V pairs is dependent on tube diameter, which is presumably derived from nano-structure effects of SWNTs.

Computational study of interaction of alkali metals with C3N nanotubes

Interaction of the alkali metals (AMs) like lithium (Li), sodium (Na), and potassium (K) with defective and non-defective (8,0) C3N nanotubes (C3NNT) have been investigated using the first-principles study. In addition to structural properties, we have also studied the electronic properties, charge transfer, and work function of the AM-C3NNT complexes. AMs are adsorbed on hollow sites, regardless of the initial positions. Upon the adsorption of AMs, the structures exhibit semiconducting behavior. Furthermore, interaction of Li atom can be explained by Dewar model, whereas for the other atoms there are different explanations. For all metal adsorbates, the direction of the charge transfer is from adsorbate to adsorbent, because of their high surface reactivity. The results showed that the nanotube with carbon vacancy is the most favorite adsorbent. Our findings also indicated that the enhancement in absolute adsorption energy is in order of Li > K > Na. It is noteworthy that clustering of AM atoms on the nanotubes with and without defects is not expected. It is worthy that C3NNT is a better adsorbent for AM atoms than CNT, graphene, C60, and B80.

Structure of boron nitride nanotubes

The crystallographic structure of boron nitride nanotubes has been investigated. Various defects that may arise during nanotube synthesis are revealed by electron microscopy. Nanotubes with different numbers of walls and different diameters are modeled by molecular dynamics methods. Structural features of single-wall nanotubes are demonstrated. The causes of certain defects in multiwall nanotubes are indicated.

A computational study of platinum adsorption on defective and non-defective silicon carbide nanotubes

Platinum adsorption on the pristine, Stone–Wales defect, and vacancy defects sites in (8,0) zigzag silicon carbide nanotubes are studied based on the spin-polarized density functional theory. The formation of the Stone–Wales defects with the axial bond rotation is more favorable than the circumferential one. In addition, the vacancy of the carbon atom is more desirable than the silicon atom. The stable adsorption sites and their binding energies on different defect types are analyzed and compared to those on the perfect side wall. It is determined that the adsorption of Pt atom on nine-membered ring in carbon vacancy defect is the most exothermic site. Thus, the presence of intrinsic defects can enhance the reactivity of silicon carbide nanotubes toward Pt atom. Furthermore, the dangling bonds are the main driving force in preventing Pt atom from clustering. It is noticeable that the systems with Pt atom remained semiconductor with direct band gaps. Pt atom on pristine and vacancy-defective silicon carbide nanotubes were positively charged, whereas on Stone–Wales structures, Pt atom gained some charge. In addition, only silicon vacancy structure as structure without Pt atom showed ferromagnetic ordering, while all the systems in presence of Pt atom exhibited non-magnetic moment.

Density Functional Theory Calculations of Atomic Hydrogen Adsorption on (3, 3) Single-Wall Carbon Nanotubes with Vacancy Defects

In this paper, we have employed density functional theory (DFT) to investigate the adsorption mechanisms of atomic hydrogens on the sidewalls of (3, 3) single-wall carbon nanotubes (CNTs) which have vacancy defects. All the calculations were performed using the generalized gradient approximation (GGA) with the Perdew, Burke and Ernzerhof (PBE) correlation functional.Our results show that hydrogen atoms can chemically adsorb on the defective nanotube. Bonding energy of per hydrogen atom decreases with the number of adsorbed hydrogen atoms. The hydrogen atoms will enhance the electrical conductivity of the (3, 3) nanotube. Besides one hydrogen atom adsorbing on the nanotube with a vacancy defect (MVD), hydrogen atoms move towards the MVD of the nanotube.

Growth of carbon nanotubes decorated with Fe nanoparticles encapsulated by graphitic layers on diamond film on cemented carbide substrate

MWCNTs decorated with Fe-NPs have been obtained on diamond film on cemented carbide substrate (MWCNTs/Fe-NPs/diamond/cemented carbide) by the pyrolysis of FePC at 850°C. The microstructure and elemental compositions of the prepared samples are studied with SEM, TEM and EDS. MWCNTs/Fe-NPs/diamond/cemented carbide is also analyzed by Raman spectroscopy. The results show that all MWCNTs randomly lay down and are curvy on the diamond film with short lengths. High density of structural defects in carbon nanotubes are formed in the pyrolysis process at a relative low temperature. The Fe-NPs are encapsulated with graphitic layers and anchored on the MWCNT surface. The growth of these MWCNT/Fe-NPs composites is well explained using the concepts of the vapor–liquid–solid (VLS) model.