Defective Single-walled Carbon Nanotubes Research Articles

Electron-stimulated defect formation in single-walled carbon nanotubes studied by hydrogen thermal desorption spectroscopy

Defects in single-walled carbon nanotubes introduced by low-energy electron irradiation at 8 K were sensitively detected by cryogenic thermal desorption of hydrogen molecules H 2 in the temperature range of 10–40 K. The thermal desorption spectrum was found to change significantly with sample annealing at temperatures as low as 40–70 K. Experimental results suggest that the H 2 physisorption sites responsible for the ‘defect’ peak at 28 K are interstitial channel space between nanotubes closely packed in bundles which becomes more easily accessible on damage. It is also suggested that the disordering provides groove sites for H 2 physisorption with smaller binding energy causing the damage-induced spectral component around 16 K, slightly lower than the desorption peak at 20 K that is observed in undamaged samples. The spectral change at 40–70 K could be interpreted by migration of adatoms at the low temperatures.

The influence of SWCNT–metallic nanoparticle mixtures on the desorption properties of milledMgH2 powders

We have examined the effect of single-walled carbon nanotube(SWCNT)–metallic nanoparticle additions on the hydrogen desorption behavior ofMgH2 after high-energy co-milling. The metallic nanoparticles were the catalysts used for theSWCNT growth. The co-milling consisted of high-energy planetary milling in an inertargon environment of the hydride powder mixed with the SWCNTs. Identically milled pureMgH2 powders were used as a baseline. The composites were tested using a combined differentialscanning calorimeter and thermogravimetric analyzer, while the microstructures wereexamined using a variety of techniques including x-ray diffraction and transmission electronmicroscopy (TEM). We found that the SWCNT–nanoparticle additions do have aninfluence on the desorption kinetics. However, the degree to which they are effectivedepends on the composite’s final state. The optimum microstructure for sorption, obtainedafter 1 h of co-milling, consists of highly defective SWCNTs in intimate contact withmetallic nanoparticles and with the hydride. This microstructure is optimum,presumably because of the dense and uniform coverage of the defective SWCNTs on theMgH2 surface. Prolonged co-milling of 7 h destroys the SWCNT structure and reduces theenhancement. Even after 72 h of co-milling, when the SWCNTs are completelydestroyed, the metallic nanoparticles remain dispersed on the hydride surfaces.This indicates that the metallic nanoparticles alone are not responsible for theenhanced sorption and that there is indeed something catalytically unique about adefective SWCNT–metal combination. Cryo-stage TEM analysis of the hydridepowders revealed that they are nanocrystalline and in some cases multiplytwinned. To our knowledge this is the first study where the structure of milledα-MgH2 has been directly imaged. Since defects are an integral component of hydride-to-metalphase transformations, such analysis sheds new insight regarding the fundamentalmicrostructural origins of the sorption enhancement due to mechanical milling.

Electrical and magnetic interaction along a defective single-walled carbon nanotube channel

We study the electrical and magnetic interaction in a low conduction regime for laterally as-grown defective single-walled carbon nanotube channel as functions of bias and gate voltage, combined with direct current and magnetic phase image by current-atomic force microscopy and magnetic force microscopy. For the SWNT field effect transistor at very low bias direct imaging of the current flow on the 0.9 and 2 nm diameter single-walled nanotube (SWNT) devices revealed that locally conducting islands occur along the SWNT channel, and become an origin of electrical behavior in the stage of minimal conduction. In contrast, the homogenous magnetic interaction along the as-laterally-grown individual SWNT channel observed by magnetic force measurements suggest a hint that as-grown single SWNT channel may be a reliable candidate for use in magnetoelectronics, regardless of whether clean or defective SWNT.

Is It Possible to Dope Single‐Walled Carbon Nanotubes and Graphene with Sulfur?

Herein, we investigate sulfur substitutional defects in single-walled carbon nanotubes (SWCNTs) and graphene by using first-principles calculations. The estimated formation energies for the (3,3), (5,5), and (10,0) SWCNTs and graphene lie between 0.9 and 3.8 eV, at sulfur concentrations of 1.7-4 atom %. Thus, from a thermodynamic standpoint, sulfur doping is not difficult. Indeed, these values can be compared with that of 0.7 eV obtained for a nitrogen-doped (5,5) SWCNT. We suggest that it may be possible to introduce sulfur into the SWCNT framework by employing sulfur-containing heterocycles. Our simulations indicate that sulfur doping can modify the electronic structure of the SWCNTs and graphene, depending on the sulfur content. In the case of graphene, sulfur doping can induce different effects: the doped sheet can be a small-band-gap semiconductor, or it can have better metallic properties than the pristine sheet. Thus, S-doped graphene may be a smart choice for constructing nanoelectronic devices, since it is possible to modulate the electronic properties of the sheet by adjusting the amount of sulfur introduced. Different synthetic routes to produce sulfur-doped graphene are discussed.

Selective decoration of silver nanoparticles on the defect sites of single-walled carbon nanotubes

Selective decoration of Ag nanoparticles has been achieved on the defect sites of single-walled carbon nanotubes (SWCNTs) without causing damage to their tubular morphology. SWCNTs were purified using HCl treatment and thermal annealing process, which induced little functionalized or oxidized sites. The decoration of Ag nanoparticles was performed through η 2-coordination using Sn 2+ ions and reduction of Ag + ions on the defects of SWCNTs. Ag nanoparticles were deposited selectively and heterogeneously on the SWCNT bundles with a mean diameter of 4 nm. It is suggested that the defects of SWCNTs play a role of electron acceptor for the landing Sn 2+ ions.

Magneto-mechanical coupling behavior of defective single-walled carbon nanotubes

Spin-polarized density functional theory is employed to investigate the mechanical andmagnetic properties of a (5, 5) carbon nanotube (CN) with a mono-vacancy. It isfound that the magnetic properties of a defective CN can be modified by applieduniaxial tensile strain. The effects of uniaxial strains on the magnetic properties arerelated to the stretched bonds in the defective CN, and the effect of a mixture ofsp2 and sp3 hybridization on the magnetism is investigated. Furthermore, because of the strongmagneto-mechanical coupling between the defective CN and metal, the mechanicalproperties of the defective (5, 5) CN are found to be significantly alternated by thesubstitution of metal atoms to its vacancy sites. Such strong coupling among magneticmoment, spin-dependent transport, and mechanical deformation in a defective CN could bebeneficial in some potential applications of CNs in spintronic devices and metal matrixcomposites.

Buckling of defective single-walled and double-walled carbon nanotubes under axial compression by molecular dynamics simulation

Buckling of defective single-walled and double-walled carbon nanotubes (SWCNTs and DWCNTs, respectively) due to axial compressive loads has been studied by molecular dynamics simulations, and results compared with those of the perfect structures. It is found that single vacancy defect greatly weakens the carrying capacity of SWCNTs and DWCNTs, though it does slight harm to the effective elastic modulus of the tubes. The influence of defects on the buckling properties of nanotubes is related to the density of the defects, and the relative position of defects also plays an important role in buckling of DWCNTs. The van der Waals force among atoms in the inner and the outer tubes of short defective DWCNTs makes the critical buckling strain of DWCNTs greater than that of the inner tube.

First-principles study of a hybrid carbon material: Imperfect fullerenes covalently bonded to defective single-walled carbon nanotubes

The electronic properties of a novel hybrid carbon material, the NanoBud, is investigated by using a first-principles method. Our results suggest that NanoBuds of zigzag carbon nanotubes are semiconductors, no matter whether the zigzag nanotubes are metallic or semiconducting. Local states appear in the band gap of the NanoBuds near the Fermi level. NanoBuds of armchair carbon nanotubes remain metallic. These properties may have great potential applications in future nanodevice design.

Influences of atom vacancies on buckling properties of armchair single-walled carbon nanotubes shown by molecular dynamics simulation

Molecular dynamics simulations are employed to simulate the axial compression of both perfect and defective armchair single-walled carbon nanotubes (SWCNTs). Three temperature conditions are considered. It is revealed that the vacancy defects in SWCNTs have remarkable influences on their buckling properties, especially at low temperature. It is noticed that the single vacancy defect leads to excessive stress concentration, as a result of which the local buckling occurs easily at the defective position. Thus, the (7, 7) tube with single vacancies seems inferior to that with double vacancies.

Polymer Solar Cells Using Single-Wall Carbon Nanotubes Modified with Thiophene Pedant Groups

Single-wall carbon nanotubes (SWCNT) have attracted great interest for applications in a variety of research areas, including electronics and functional materials. However, a good dispersion of these materials in polymer matrices is a demanding factor in order to obtain more homogeneous and less bundled films for constructing optoelectronic and electrochemical devices. In this report we describe a covalent modification with thiophene groups located at the edges and at defects in SWCNT to modify the interaction with the polymer matrix aiming at its application in solar cells. The modified SWCNT and its composites with poly(3-octylthiophene) were characterized by Raman, infrared, UV−vis, and luminescence spectroscopies and cyclic voltammetry. The best bulk heterojunction solar cell was obtained using 5 wt % of the modified carbon nanotube (SWCNT-THIOP) and shows open circuit voltage (VOC), short-circuit current (Isc), and efficiency (η) of 0.75 V, 9.5 μA cm-2, 0.184%, respectively.

Novel Method to Evaluate the Carbon Network of Single-Walled Carbon Nanotubes by Hydrogen Physisorption

Physical and chemical properties of single-walled carbon nanotubes (SWCNTs) are sensitive to the defects in the tubular graphene. To date, however, the method to characterize the defect of SWCNTs has been limited to transmission electron microscopy (TEM) and atomic force microscopy (AFM), which can only be used to observe the local structure of SWCNTs. In an effort to explore the evaluation method of SWCNTs, physisorption of H2 on high-quality SWCNTs was investigated by cryogenic thermal-desorption spectroscopy (cryo-TDS), which revealed particular sensitivity to the defects of SWCNTs. TDS showed a sharp desorption peak at 20 K (α) for as-purified SWCNTs, and additional desorption signals were observed at 27 and ∼24 K (β and γ) for air-oxidized SWCNTs. The α, β, and γ peaks were attributed to H2 at the groove site and interstitial channel of a SWCNT bundle and inside of SWCNT, respectively. The sharpness of the α peak and the ratio of α to β and γ were found to depend strongly on the sample preparation condition. With this cryo-TDS technique, the perfectibility of the carbon network of SWCNTs can be reliably and sensitively evaluated.

A photoelectron spectroscopy study of ion-irradiation induced defects in single-wall carbon nanotubes

Valence band and core level photoemission spectroscopies were used to study the changes brought about by irradiation of a single-wall carbon nanotube (SWCNT) film by 3 keV Ar + ions at room temperature. At low ion doses (low defect density) an increase in spectral intensity near the Fermi level ( E F) is observed, associated with formation of localized defect-related states. These states are acceptor-like as evidenced by a shift to lower binding energy for both valence band features and the C1s core level. For large ion doses (high defect density) the spectral intensity near E F decreases, valence band features associated with delocalized π bonding disappear, and a core level component associated with sp 3 bound carbon appears. This behaviour is attributed to amorphisation of the SWCNT films and occurs at ion doses consistent with those theoretically predicted.

Buckling and axially compressive properties of perfect and defective single-walled carbon nanotubes

Molecular dynamics simulation is employed for the axial compression of both perfect and defective single-walled carbon nanotubes (SWCNTs). Morse potential, harmonic angle potential, a proper dihedral potential and the Lennard-Jones potential are used to simulate the interactions among carbon atoms. It is revealed that the buckling and axially compressive properties of SWCNTs obviously lie on the length, the chirality, the temperature and the initial structural defects of the tube. Especially at normal temperature, the Euler formula could be adopted to predict the axially critical buckling loads of SWCNTs with large aspect ratio.

Self-healing in defective carbon nanotubes: a molecular dynamics study

The self-healing phenomenon of defective single-walled carbon nanotubes (SWCNTs) isobserved at the atomic level from a molecular dynamics (MD) simulation test. The idealnetwork of carbon nanotubes is unable to avoid damage under destabilizing loads at hightemperature, leading to unforeseen patterns in bond breakages and vacancy defects on thewall. We observe that (10, 10) and (17, 0) carbon nanotubes containing such vacancies areenergetically unstable. In the situation of unloading or increasing temperature, the localstructures around the vacancies reconstruct through dangling bond saturation,forming non-hexagonal rings, 5–7–7–5 defects or an ideal graphite network. We findthat a defective carbon nanotube with large vacancies is re-mendable, and theStone–Wales (SW) construction is energetically preferred in self-healing processes.

Mechanical Properties of Single-Walled (5,5) Carbon Nanotubeswith Vacancy Defects

First-principles simulation is used to investigate the structural andmechanical properties of vacancy defective single-walled (5,5) carbonnanotubes. The relations of the defect concentration, distribution andcharacteristic of defects to Young's modulus of nanotubes arequantitatively studied. It is found that each dangling-bond structure(per supercell) decreases Young's modulus of nanotube by 6.1% forsymmetrical distribution cases. However the concentrative vacancystructure with saturated atoms has less influence on carbon nanotubes.It is suggested that the mechanical properties of carbon nanotubesdepend strongly upon the structure and relative position of vacancies ina certain defect concentration.

Thermal relaxation kinetics of defects in single-wall carbon nanotubes

The defects in single-wall carbon nanotubes irradiated with a 248 nm pulsed excimer laser were studied using Raman spectroscopy. The thermal relaxation kinetics of the laser-induced defects was examined at sample temperatures from 296 to 698 K. Two relaxation processes are revealed; one is the fast process with an activation energy of 0.4 eV and the other is the slow process with an activation energy of 0.7 eV. These two processes can correspond to vacancy-interstitial recombination and vacancy migration along the tube axis, respectively.

Healing of Low-Energy Irradiation-Induced Defects in Single-Walled Carbon Nanotubes at Room Temperature

Single-walled carbon nanotubes are damaged by low-energy electron and photon irradiation, depending strongly on the diameter. In this study, the formation and healing of the irradiation-induced defects were found to be in competition during irradiation even at room temperature or below. The diameter dependence of the damage can be mainly ascribed to a diameter dependence of the activation energy of the defect healing. The activation energy was estimated to be about 1 eV.

Si-coated single-walled carbon nanotubes under axial loads: An atomistic simulation study

The mechanical properties of the Si-coated imperfect (5, 5) single-walled carbon nanotube (SWCNT), the imperfect (5, 5) SWCNT and several perfect armchair SWCNTs under axial loads were investigated using molecular dynamics simulation. The interactions between atoms were modeled using the empirical Tersoff potential and the Tersoff–Brenner potential coupled with the Lennard–Jones potential. We get Young's modulus of the defective (5, 5) nanotube with and without the Si coating under axial tension 1107.92 and 1076.02 GPa, respectively. The results also show that the structure failure of the Si-coated imperfect (5, 5) SWCNT under axial compression occurs at a slightly higher strain than for the perfect (5, 5) SWCNT. Therefore, we can confirm the protective effect of Si as a coating material for defective SWCNTs. We also obtain the critical buckling strains of perfect SWCNTs.

Characterization of single-walled carbon nanotubes containing defects from their local vibrational densities of states

The vibrational properties of defects in single-wall carbon nanotubes are investigated with an empirical dynamical model based on force constants fitted to graphite [Al-Jishi R, Venkataraman L, Dresselhaus MS, Dresselhaus G. Phonon modes in carbon nanotubules. Chem Phys Lett 1993;209:77–82]. The defected structures are: an isotopic substitution impurity in a chiral tube, a Stone–Wales defect in a series of (10, m) ( m = 4, … , 10) nanotubes and in armchair tubes, and an intramolecular junction between (12, 0) and (9, 0) nanotubes. The study emphasizes differences in local vibrational densities of states (lVDOS) between perfect and imperfect nanotubes. lVDOS are computed with the recursion method. This is a first step towards a more complete approach for characterizing defects in carbon nanotube structures based on the recognition of localized vibrational fingerprint.

Surface-enhanced Raman scattering of single-walled carbon nanotubes on silver-coated and gold-coated filter paper

Surface-enhanced Raman scattering (SERS) spectra of single-walled carbon nanotubes (SWCNTs) on metal-coated filter paper are reported for the first time. Experimental results show that the metal-coated filter paper is very effective and active. The SERS spectrum not only shows that all Raman bands of SWCNTs in normal Raman scattering have been generally enhanced, but also shows many new bands, which characterize the structure of SWCNTs and the interaction between SWCNTs and silver/gold nanoparticles, arising from symmetry lowering and selection rule relaxing of SWCNTs induced by the silver/gold surface. In our case, it is difficult to separate the contributions of the electromagnetic and chemical mechanisms to the great enhancement of the Raman signal. The analysis shows that the SERS spectra of SWCNTs on the metal-coated filter paper provide convenience for probing the sample molecules with fine structures related to defects of SWCNTs, the diameter of SWCNTs, and the SERS mechanism of SWCNTs deposited on metal-coated filter paper. Moreover, this can be used as a probe technique for monitoring the synthesis quality of SWCNTs with significant higher sensitivity than other methods, which has promise of being a new technique for monitoring synthesis quality of SWCNTs.