Carbon Nanotube Samples Research Articles

Critical Investigation of Defect Site Functionalization on Single-Walled Carbon Nanotubes

The presence of carboxylated carbonaceous material in nitric acid-treated single-walled carbon nanotube (SWNT) samples has recently brought renewed focus on the processes by which covalent functionalization of such materials are carried out. Using a widely reported 2-step purification/oxidation procedure, we have investigated the effect of basic treatment and solvent washing on the functionalization and final properties and behavior of SWNTs. We have demonstrated, using a number of spectroscopic techniques, that in the absence of NaOH treatment, COOH functionality is introduced directly onto SWNTs, and not only onto carbonaceous material present in the sample. Covalent functionalization of the oxidized materials was also investigated by attachment of a fluorescent probe, and ultimately, whether treated with base or solvent washed, the resulting materials are close to identical with respect to both their appearance and properties. In addition, we have demonstrated that using either of these purification/ox...

Pulmonary Toxicity in Mice Exposed to Low and Medium Doses of Water-Soluble Multi-Walled Carbon Nanotubes

Carbon nanotubes (CNTs) are a class of new allotrope of carbon. Different functionalized CNTs may vary from their physical and chemical properties to the biological property. In this study, the toxicity of water-soluble taurine multi-walled CNTs (tau-MWNTs), raw MWNTs and positive control crystalline silicon dioxide particles on mouse lungs via intratracheal instillation (i.t.) was investigated. The dosages we used were 0.125, 0.25, 0.5 or 1 mg/kg of tau-MWNTs and raw MWNTs, and 1 mg/kg of silicon dioxide particles; Serum and lungs were collected at 1, 7, 14 or 28 days postexposure. The biochemical and cellular parameters were assessed, which include the ratio of the lung weight and body weight (lung indices), lactate dehydrogenase (LDH), alkaline phosphatase (ALP) and angiotensin converting enzyme (ACE) in serum, and malondialdehyde (MDA), reduced glutathione (GSH), total sulfhydryl group (TSH) in lung tissue homogenates as well as the hydroxyproline in lungs. The characteristic recovery of the lung injury at 28 days postexposure was examined by the assessment of LDH, ALP, lung indices, and histopathology. ACE, MDA, GSH, TSH and histopathological changes showed that tau-MWNTs were less toxic than the raw MWNTs. Histopathological and ultrastructural investigation indicated that the acute pulmonary inflammation in lungs alleviated after 7d postexposure, and were greatly recovered within 28d. Meanwhile, the entrapment of tau-MWNTs reduced greatly by the 28d postexposure. Whereas the heavier pathologic changes induced by raw MWNTs lasted 7 days more than that of tau-MWNTs. Notably, no occurrence of granulomas and fibrosis were found in this study both in the two CNTs samples through 28d postexposure. Silicon dioxide particles, on the contrary, produced more severe damage to lungs than CNTs did in lung index, as well as other biochemical and cellular parameters. These findings indicate that water-soluble tau-MWNTs in low and medium doses induce slight and recoverable pulmonary inflammation in mice, and are less toxic than the insoluble raw MWNTs.

INVESTIGATING HYDROGEN STORAGE BEHAVIOR OF CARBON NANOTUBES AT AMBIENT TEMPERATURE AND ABOVE BY ION BEAM ANALYSIS

Hydrogen storage capacity of a carbon nanotube (CNT) sample is investigated using Elastic Recoil Detection Analysis (ERDA) at constant hydrogen uptake pressure of 5 bar and different adsorption temperatures within 30°C–500°C. The results of hydrogen concentration versus temperature revealed three distinct temperature intervals in which a certain adsorption or desorption mechanism is dominant. Moreover, the results showed that hydrogen storage capacity of CNTs at the applied conditions of pressure and temperature is about 0.1 wt.% which is well below the DOE requirements for a viable hydrogen storage system. The physidesorption activation energy is calculated using the Arrhenius plot to be 6 kJmol-1.

Tip‐heating‐assisted Raman spectroscopy at elevated temperatures

AbstractWe demonstrate tip‐heating‐assisted enhanced Raman spectroscopy to investigate the temperature dependence of the carbon nanotube G‐band with nanoscale resolution. The controllable and nanoscale heat generated at the tip apex was used to thermally perturb and characterize a small volume in a carbon nanotube sample that is precisely positioned underneath the tip. The dependence of tip enhancement with temperature was also experimentally examined, which is in good agreement with the enhancement calculated from the electromagnetic model of isolated spheroids. The technique presented may open up opportunities in the study of controlled heat‐assisted biochemical reactions and physical transformations of nanostructures. It can also be used for thermal characterization of various materials requiring site‐selective and controllable nanoscale heat source and could enable the realization of new photothermal devices. Copyright © 2010 John Wiley & Sons, Ltd.

Curvature-induced symmetry lowering and anomalous dispersion of the G-band in carbon nanotubes

Here, we report rich and new resonant Raman and infrared (IR) spectral features for several sub-nanometer diameter single wall carbon nanotubes (sub-nm SWCNTs) samples grown using chemical vapor deposition technique operating at different temperatures. We find that the high curvature in sub-nm SWCNTs leads to (i) an unusual S-like dispersion of the G-band frequency due to perturbations caused by the strong electron–phonon coupling, (ii) an activation of diameter-selective intermediate frequency modes that are as intense as the radial breathing modes (RBMs), and (iii) a clear observation of the IR modes. Furthermore, an analytical approach which includes the effects of curvature into the overlap integral and the energy gap between the van Hove singularities is discussed. Lastly, we show that the phonon spectra for sub-nm SWCNTs obtained from the molecular dynamic simulations which employs a curvature-dependent force field concur with our experimental observations.

Determination of the Metallic/Semiconducting Ratio in Bulk Single-Wall Carbon Nanotube Samples by Cobalt Porphyrin Probe Electron Paramagnetic Resonance Spectroscopy

A simple and quantitative, self-calibrating spectroscopic technique for the determination of the ratio of metallic to semiconducting single-wall carbon nanotubes (SWCNTs) in a bulk sample is presented. The technique is based on the measurement of the electron paramagnetic resonance (EPR) spectrum of the SWCNT sample to which cobalt(II)octaethylporphyrin (CoOEP) probe molecules have been added. This yields signals from both CoOEP molecules on metallic and on semiconducting tubes, which are easily distinguished and accurately characterized in this work. By applying this technique to a variety of SWCNT samples produced by different synthesis methods, it is shown that these signals for metallic and semiconducting tubes are independent of other factors such as tube length, defect density, and diameter, allowing the intensities of both signals for arbitrary samples to be retrieved by a straightforward least-squares regression. The technique is self-calibrating in that the EPR intensity can be directly related to the number of spins (number of CoOEP probe molecules), and as the adsorption of the CoOEP molecules is itself found to be unbiased toward metallic or semiconducting tubes, the measured intensities can be directly related to the mass percentage of metallic and semiconducting tubes in the bulk SWCNT sample. With the use of this method it was found that for some samples the metallic/semiconducting ratios strongly differed from the usual 1:2 ratio.

The effects of carrier gas and liquid feed flow rates on longitudinal patterns of CNT growth

We report detailed longitudinal profiles of four samples of carbon nanotubes (CNT) arrays synthesized using ferrocene–xylene in 850 °C. Point-to-point analysis of the CNTs grown in “growth windows” (GWs) were performed using optical microscopy, Raman spectroscopy, FESEM, and high-resolution TEM techniques. All samples showed a maximum array height around the middle of their GWs. The height profiles comprised a secondary peak/shoulder, attributed to evolution of C 2 byproducts. The results showed that pyrolytic deposition of amorphous carbon as well as catalytic growth of CNTs strongly depend on the local temperature along the GW. It was also observed that the CNT grew longer at the higher flow rate of the carrier gas while the GWs became wider and extended towards the end of the reactor. On the other hand, the higher liquid flow rate reduced the average diameter and length of the CNTs. While residence time was revealed to be important, no significant analogy was found between samples with the same feed dilution. The termination of GWs was attributed to the inhibition of amorphous carbon as well as temperature drop below ∼815 °C.

The effect of nitric acid doping on the optical properties of carbon nanotube films

AbstractTreatment of carbon nanotubes by oxidizing acids is extensively used to dissolve metal residues from raw nanotube samples. The effect of acid treatment on the tubes themselves is twofold: it introduces uniform carboxylic sidegroups on the tube ends and defects, and it increases the overall charge concentration on the nanotube walls, thereby p‐doping the material. Wide‐range optical spectroscopy is an ideal tool to characterize both effects from one type of measurement: vibrational signatures indicate the presence of the sidegroups, the increase in far‐infrared absorption signals the presence of free carriers, and the disappearance of interband transitions is the consequence of the depletion of Van Hove singularities when introducing holes. We have subjected carbon nanotube samples to two kinds of acid treatment: mild (in HNO3 vapor at room temperature) and aggressive (high‐temperature reflux). We found that on mild treatment, electronic doping occurs without defect oxidation to carboxylic groups, while the aggressive procedure leads to both doping and defect oxidation. Our spectroscopic data unambiguously prove that the changes in electrical conductivity on acid doping are intrinsic, and not just a result of improving the contact between nanotubes.

Electron spin resonance on single‐walled carbon nanotubes obtained from different sources

AbstractWe used electron spin resonance (ESR) to analyze several kinds of single‐walled carbon nanotubes (SWNTs) synthesized by different methods. We observed that ESR is a sensitive probe that can be used to observe major differences between samples of carbon nanotubes (CNTs), in relation to their proportion of magnetic catalysts, impurities or defects. We analyzed the ESR spectra of CNTs in the usual way, by three overlapping lines. We then, implemented a function to fit the spectra, which gave us the intensity, line width (ΔHPP), and resonance line position (g factor) of each component line. The asymmetric line, which is assigned to the conduction electrons, could be used as a fingerprint of CNT purity. However, this asymmetric line is not always observed, since the three ESR lines often overlap, particularly when there is a large amount of paramagnetic ions. We studied the ESR spectra for different samples and estimated their purity by comparing the resulting parameters with data obtained by Raman spectroscopy on the same samples.

Evaluation of defect concentration in doped SWCNT

AbstractIn the present study we analyze and discus Raman spectra of doped SWCNTs. We focus on the development of the integral area of the D mode (AD), the TG mode (ATG) and the G′ mode (AG′). It is shown that area of the D band is significantly attenuated in doped carbon nanotubes samples for both semiconducting and metallic tubes. A similar dependence on the doping as in the case of the area of the D mode was found also for the area of the TG and the G′ mode. The area of the G′ mode was found to be more significantly dependent on the electrode potential than the area of the TG mode. However, this difference is only small, hence the AD/AG′ and AD/ATG ratios give only slightly different results in evaluation of the amount of defects in doped SWCNT samples.

Synthesis and study of photoluminescence characteristics of carbon nanotube/ZnS hybrid nanostructures

Single-walled carbon nanotube (SWCNT) samples were rigorously purified by high temperature oxidation, acid treatment, ultrasonication, neutralisation and filtration process. By a very simple wet chemical route, SWCNT bundles were hybridised with freshly prepared ZnS nanocrystals of average size 2.00 nm. Characterisation of the prepared samples was done by scanning electron microscopy, energy-dispersive X-ray spectroscopy, high resolution transmission electron microscopy, Fourier transform infrared ray spectroscopy, Raman spectroscopy and UV-Vis spectroscopy. Photoluminescence (PL) study of the SWCNT/ZnS hybrid structure showed two distinct types of emission patterns depending upon the excitation wavelength, one in the ultraviolet region wavelengths and the other, for the higher excitation wavelength range, when the PL spectrum mostly covered the visible region.

Characterization, electrical percolation and magnetization studies of polystyrene/multiwall carbon nanotube composite films

Polystyrene/multiwall carbon nanotube composite films are prepared with loading up to 7 weight percent (wt%) of multiwall carbon nanotubes by solution processing and casting technique. In the formation of these composite films, iron filled carbon nanotubes with high aspect ratio (∼4000) were used. Scanning electron microscopy study shows that the nanotubes are uniformly dispersed within the polymer matrix. At high magnification, bending of carbon nanotubes is noticed which can be attributed to their elastic properties. The electrical conductivity measurements show that the percolation threshold is rather low at 0.21 wt%. Hysteresis loop measurements on the bulk multiwall carbon nanotube and composite samples are done at 10, 150 and 300 K and the coercivity values are found to be largest at all the temperatures, for 1 wt% composite sample.

Microstructure, mechanical properties, and in vitro biocompatibility of spark plasma sintered hydroxyapatite–aluminum oxide–carbon nanotube composite

In the present work, HA reinforced with Al 2O 3 and multiwalled carbon nanotubes (CNTs) is processed using spark plasma sintering (SPS). Vickers micro indentation and nanoindentation of the samples revealed contrary mechanical properties (hardness of 4.0, 6.1, and 4.4 GPa of HA, HA–Al 2O 3 and HA–Al 2O 3–CNT samples at bulk scale, while that of 8.0, 9.0, and 7.0 GPa respectively at nanoscale), owing to the difference in the interaction of the indenter with the material at two different length scales. The addition of Al 2O 3 reinforcement has been shown to enhance the indentation fracture toughness of HA matrix from 1.18 MPa m 1/2 to 2.07 MPa m 1/2. Further CNT reinforcement has increased the fracture toughness to 2.3 times (2.72 MPa m 1/2). In vitro biocompatibility of CNT reinforced HA–Al 2O 3 composite has been evaluated using MTT assay on mouse fibroblast L929 cell line. Cell adhesion and proliferation have been characterized using scanning electron microscopy (SEM), and have been quantified using UV spectrophotometer. The combination of cell viability data as well as microscopic observations of cultured surfaces suggests that SPS sintered HA–Al 2O 3–CNT composites exhibit the ability to promote cell adhesion and proliferation on their surface and prove to be promising new biocompatible materials.

High performance resonance Raman spectroscopy using volume Bragg gratings as tunable light filters

We designed a near infrared tunable resonance Raman spectroscopy system based on a tandem of thick volume Bragg gratings (VBGs). VBGs are here the constituents of two light filtering units: a tunable laser line filter (LLF) and a tunable notch filter (NF). When adapted in a micro-Raman setup with a single stage monochromator (1800 gr/mm grating), the tandem of LLF and NF allowed measurements of Raman signals down to +/-20 cm(-1). The good performance and fast tunability of the VBG Raman system was demonstrated on a sulfur powder and on a bulk single-walled carbon nanotube sample through a series of 22 Stokes and anti-Stokes spectra recorded at excitation wavelengths between 800 and 990 nm. The main drawbacks of the setup are the limited spectral range to the near infrared and the small angular acceptance of the filters (approximately 1 mrad), which causes mainly attenuation problems with the NF. The impact of the main limitations is discussed and solutions are provided.

Chirality dependence of theK-momentum dark excitons in carbon nanotubes

Using a collection of twelve semiconducting carbon nanotube samples, each highly enriched in a single chirality, we study the chirality dependence of the $K$-momentum dark singlet exciton using phonon sideband optical spectroscopy. Measurements of bright absorptive and emissive sidebands of this finite momentum exciton identify its energy as 20 - 38 meV above the bright singlet exciton, a separation that exhibits systematic dependencies on tube diameter, $2n+m$ family, and semiconducting type. We present calculations that explain how chiral angle dependence in this energy separation relates to the Coulomb exchange interaction, and elaborate the dominance of the $K_{A_1'}$ phonon sidebands over the zone-center phonon sidebands over a wide range of chiralities. The Kataura plot arising from these data is qualitatively well described by theory, but the energy separation between the sidebands shows a larger chiral dependence than predicted. This latter observation may indicate a larger dispersion for the associated phonon near the $K$ point than expected from finite distance force modeling.

Structural Modifications of Multiwalled Carbon Nanotubes by Swift Heavy Ions Irradiation

Thin film samples of multi-walled carbon nanotubes (MWCNTs) were irradiated with 120 MeV gold ions. Transmission electron microscopy (TEM) images of the pristine and irradiated samples were obtained. TEM pictures show that in the irradiated sample, the CNTs are in general shorter and some have their inner cores filled, unlike in the pristine sample. We also find from these images that average inner and outer tube diameters change as a result of ion irradiation. The films were also characterized using Raman spectrometry. Modifications of the disorder mode (D mode) and the tangential mode (G mode) under different irradiation fluences were studied in detail. As fluence increases, the MWCNTs first show damage, then healing under somewhat higher fluences and again amorphization under still higher fluence of ion irradiation.

Synthesis, Characterisation and Applications of Coiled Carbon Nanotubes

Coiled carbon nanotubes are helical carbon structures formed when heptagonal and pentagonal rings are inserted into the hexagonal backbone of a 'straight' nanotube. Coiled carbon nanotubes have been reported with both regular and irregular helical structures. In this work the structure, growth mechanism(s), synthesis, properties and potential applications of coiled carbon nanotubes are reviewed. Published data suggests that coiled carbon nanotube synthesis occurs due to nonuniform extrusion of carbon from a catalyst surface. To date, coiled carbon nanotubes have been synthesised using catalyst modification techniques including: (i) the addition of S or P containing compounds during synthesis; (ii) the use of binary or ternary metal catalysts; (iii) the use of microwaves to create a local temperature gradient around individual catalyst particles and; (iv) the use of pH control during catalyst preparation. In most instances coiled carbon nanotubes are produced as a by-product; high yield and/or large-scale synthesis of coiled carbon nanotubes remains problematic. The qualitative analysis of coiled carbon nanotubes is currently hindered by the absence of specific characterisation data in the literature, e.g., oxidation profiles measured by thermogravimetric analysis and Raman spectra of pure coiled carbon nanotube samples.

Comment on “The Effect of Stress Transfer Within Double‐Walled Carbon Nanotubes Upon Their Ability to Reinforce Composites”

The effect of stress transfer within double-walled carbon nanotubes upon their ability to reinforce composites has been reported. The level to which stress can be transferred between the different layers or walls in multiwalled carbon nanotubes (MWNT) is of profound significance for their ability to reinforce a matrix. The double multiwalled carbon nanotubes (DWNT) samples were prepared starting from commercial SWNT materials prepared via the electric-arc method (NANOCARBLAB). It has been found that the stress transfer from the outer to the inner walls in DWNT in composites is poor, such that the inner walls are virtually unstressed during both tensile and compressive deformation. The implication of this finding is that the effective Young's Modulus MWNT in composites will be relatively low. The pressurization involves only the hydrostatic component of the stress tensor, which deforms both the inner and outer walls equally.

Enrichment of Armchair Carbon Nanotubes via Density Gradient Ultracentrifugation: Raman Spectroscopy Evidence

We have used resonant Raman scattering spectroscopy to fully analyze the relative abundances of different (n,m) species in single-walled carbon nanotube samples that are metallically enriched by density gradient ultracentrifugation. Strikingly, the data clearly show that our density gradient ultracentrifugation process enriches the metallic fractions in armchair and near-armchair species. We observe that armchair carbon nanotubes constitute more than 50% of each (2n + m) family.

Design and Test of Carbon Nanotube Biwick Structure for High-Heat-Flux Phase Change Heat Transfer

With the increase in power consumption in compact electronic devices, passive heat transfer cooling technologies with high-heat-flux characteristics are highly desired in microelectronic industries. Carbon nanotube (CNT) clusters have high thermal conductivity, nanopore size, and large porosity and can be used as wick structure in a heat pipe heatspreader to provide high capillary force for high-heat-flux thermal management. This paper reports investigations of high-heat-flux cooling of the CNT biwick structure, associated with the development of a reliable thermometer and high performance heater. The thermometer/heater is a 100-nm-thick and 600 μm wide Z-shaped platinum wire resistor, fabricated on a thermally oxidized silicon substrate of a CNT sample to heat a 2×2 mm2 wick area. As a heater, it provides a direct heating effect without a thermal interface and is capable of high-temperature operation over 800°C. As a thermometer, reliable temperature measurement is achieved by calibrating the resistance variation versus temperature after the annealing process is applied. The thermally oxidized layer on the silicon substrate is around 1-μm-thick and pinhole-free, which ensures the platinum thermometer/heater from the severe CNT growth environments without any electrical leakage. For high-heat-flux cooling, the CNT biwick structure is composed of 250 μm tall and 100 μm wide stripelike CNT clusters with 50 μm stripe-spacers. Using 1×1 cm2 CNT biwick samples, experiments are completed in both open and saturated environments. Experimental results demonstrate 600 W/cm2 heat transfer capacity and good thermal and mass transport characteristics in the nanolevel porous media.