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P olarizedresonant Raman spectra for the G-band were obtained from a rope of aligned semiconducting SWNTs and from nonaligned semiconducting and metallic SWNTs. Based on group theory analysis and related theoretical predictions, we assign the symmetry for the modes in the G-band of both metallic and semiconducting SWNTs. he frequency shifts of the tangential G modes from the 2D graphite-like E2g2 frequency are discussed in terms of the nanotube geometry.
We studied the nucleation mechanism of carbon nanotubes based on the hypothesis that the starting nanotube seed can be nucleated by rolling a small fragment of a graphite sheet (graphene) under thermal fluctuations. The energy barriers for rolling a graphene along different crystallographic directions are calculated from a tight-binding model,. We then estimate the relative weight of the large-amplitude fluctuations corresponding to low-frequency vibrational modes of graphene sheets of increasing size. Direct molecular dynamics simulation of the high- temperature fluctuation of a pair of parallel graphenes demonstrates that a nanotube closed at one end can spontaneously form. We discuss the combined effects due to: (a) the decrease of the energy barriers against rolling with increasing nanotube radius, and (b) the increase of random fluctuations with increasing size of the graphene sheet. The superposition of such effects may lead to a preferential range of nanotube diameters which could nucleate more abundantly than others.
Aligned carbon nanofibers and hollow carbon nanofibers were grown by MW ECR-CVD method using methane and argon mixture gas at the temperature of 550••. Carbon nanofibers and hollow carbon nanofibers were deposited perpendicularly on Si substrate and on Si substrate coated with Ni catalyst, respectively. Raman spectra of aligned carbon nanofibers and hollow carbon nanofibers showed new bands of 1340 and 1612 cm-1 of the first-order Raman scattering and 2660, 2940 and 3220 cm-1 of the second-order Raman scattering. The second-order Raman scattering bands were assigned to two overtone and one combination bands on the basis of a similar assignment of micro crystal graphite. Combination bands are intense unusually. Field emitter characteristics of the well-aligned carbon nanofibers and hollow carbon nanofiberswere investigated and the current densities were 7.25 mA/cm2 and 0.69 mA/cm2at 12.5 V/μm, respectively.
New techniques for thermal management in ceramics at the nanoscale level have been investigated using low percentages of carbon nanotubes to reduce thermal conductivity of bulk ceramics. Samples of yttria-stabilized zirconia containing purified single-walled carbon nanotubes (SWNT) or vapor grown carbon fibers (VGCF) have been prepared by tape casting and analyzed using the laser flash method to evaluate reductions in thermal conductivity at high temperatures. New features in the samples due to the presence of carbon nanotubes have been characterized using Raman, SEM, TEM and, in the case of VGCFs, are related to significant reductions in thermal conductivity (>25%). The inclusion of a low percentage of nanoscale carbon fibers, the intimate relationship between the fibers and ceramic particles, and the indication that the fibers possess a crystalline overcoating, all contribute to the lowering of the thermal conductivity.
Multiwall carbon nanotubes have been obtained from carbon soot after hydrothermal treatment at 800°C and 100 MPa. High-resolution electron microscopy (HRTEM) study reveals multiwall carbon nanotubes and carbon nanoparticles made of a hollow core enclosed in well-ordered concentric graphitic layers after hydrothermal treatment. Condensed solid products are free of the amorphous phase. Micro-Raman spectroscopy reveals that the hydrothermal multiwall nanotubes have a characteristic perfectly closed graphitic lattice in the basal plane, without edges or plane terminations.
The work functions of multi- and single-walled carbon nanotubes are found to be 4.95eV and 5.10eV respectively. The measurements were carried out using the photoelectron emission (PEE) method, which allows easy and precise measurements to be made in air. We have found that the work function of the nanotubes is 0.1–0.2eV larger than that of highly oriented pyrolytic graphite (HOPG) of which the valence state is σ–Φ orthogonal. This result is ascribable to reflection of the σ–Φ⊏ mixed valence state in the case of carbon nanotubes. The experimental data were well reproduced in ab-initio calculations on planar and cylindrical conjugated states.
We study electronic properties of single-walled carbon nanotubes (SWNTs) upon gas molecules adsorption with first principles methods. The adsorption energy, charge transfer, and electronic structures are studied for various gas molecules (NO2, O2, NH3, N2, CO2, CO, H2O, H2, He). Most molecules adsorb weakly on SWNTs and can be either charge donor or accepter to the nanotubes. The electronic properties of nanotubes are sensitive to the adsorption of certain molecules such as NO2 and O2. Charge transfer and gas-induced charge fluctuations are found to significantly affect the transport properties of SWNTs. Our results are consistent with recent experiments.
The temporal and spatial evolution of emitting carbon nanoparticles were investigated using a laser furnace apparatus combined with a high-speed video camera. An apparent increase in the blackbody emission intensity at Δt > 400 [.proportional]sec after laser vaporization of a graphite rod was clearly recognized. Also, it was found that this increasing tendency corresponds well to that of the fullerene yield, where fullerene species was obtained as sublimed carbon material using in situ sublimation method. These findings suggest that a certain exothermic process related to the formation of C60, other higher fullerenes, and carbon nanotubes should occur at Δt > 400 νsec inside the furnace.
Gas dynamic and time resolved imaging studies have been performed on the growth of single-walled carbon nanotubes (SWNTs) in the laser ablation process. SWNTs were synthesized by laser ablation of Ni-Co catalyzed graphite targets at 1200°C under argon flow. The effects of the temperature gradient near the target and the gas flow rate were studied in order to understand the effect of gas dynamics over the diameter distribution of SWNTs. The gas flow rate affects the diameter distribution of SWNTs especially when the growth species flow through a large temperature gradient. Scattering images from the growth species at different flow rates was recorded by high-speed video imaging. The results indicate that the velocities of these species are dependent on the gas flow rate but this dependence is evident 30 ms after the laser ablation. These findings are used to estimate the time period for the nucleation and the growth of SWNTs.
We present a resonant Raman study of the tangential G-band in metallic SWNTs. By measuring the Raman spectra for isolated SWNTs, we show that the two different lineshapes observed for semiconducting and metallic SWNTs in bundles also occur for isolated SWNTs. A lineshape analysis of the tangential G-band feature for metallic SWNT bundles is presented, showing that only two components are present, the higher frequency component having a Lorentzian lineshape, and the lower one having a Breit–Wigner–Fano (BWF) line-shape. Through comparisons of the Raman tangential G-band spectra from three different diameter distributions of carbon nanotubes, we find that both the frequency and linewidth of the BWF component are diameter dependent.
We report on hydrogen adsorption onto carbon single-walled nanotube (SWNT) material as measured by the volumetric technique. This method determines the amount adsorbed by monitoring the pressure in a known volume at a known temperature with a given number of moles of hydrogen introduced. The raw material for the samples is produced by pulsed laser vaporization of a graphite target containing Ni (0.6 at%) and Co (0.6 at%) dopants. This is followed with a purification procedure and then by a cutting procedure. Finally, the sample is degassed before the hydrogen adsorption is measured. The cutting procedure introduces a titanium alloy which participates in the overall hydrogen adsorption of the SWNT material. Importantly though, the amount of hydrogen adsorbed in the SWNT material cannot be explained by absorption from the alloy alone. A description of the apparatus that has allowed us to measure these samples is given and we discuss crucial experimental procedures needed to activate the SWNT samples for hydrogen adsorption.
In this paper, we present results obtained by Surface Enhanced Raman Scattering (SERS) on SWNTs films with different thicknesses. We show that the “D” band associated to disorder in graphitic compounds can also be due to intrinsic defects in SWNTs. In addition, we put in evidence strong interfacial reactions at the SWNTs/metallic support interface when NMP is used as solvent to disperse the tubes. Our observations suggest that these reactions can induce a breaking of the SWNTs. Presumably a reconstruction of compounds close to C60 molecules can occur.
Recent experimental measurements on transport through carbon nanotubes in the Colomb Blockade regime showedm at zero bias and zero magnetic field, an alternation in the Coulomb peak heights as electrons are added to the nanotube. This surprsing peak-height alternation is shown to be related to the odd/even parity of the number of electrons in sider the nanotube. We invesitiaged this phenmenon theoretically with a new many-body apporach. and find that the anomalous behaviour may be attributed to the spin degenerate states. We alos show that asymmetric current saturation as a function of bias voltage polarity, also observed experimentally, is due to eigenstate relaxtions within the nanotube.
Field emission of electrons from multiwall carbon nanotubes (MWCNTs) has been investigated by field emission microscopy (FEM) in an ultra-high vacuum chamber. An MWCNT whose tip is capped by curved graphite layers gives a FEM pattern consisting of 6 bright pentagons when the surface of the nanotube tip is clean. Even in the ultra-high vacuum with a base pressure of about 10-10 Torr, residual gas molecules, attracted by polarization forces, adsorb on the nanotube tips. The adsorbed molecules reside preferentially on the pentagonal sites, giving bright spots in the FEM pattern. A flash heating the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers the original clean surfaces. The adsorption and desorption of gas molecules are responsible for stepwise increases and decreases in the emission current, respectively.
Aligned multi-wall carbon nanotubes (MWNTs) were successfully grown on a Si substrate based on a thermal chemical vapor deposition (CVD) method. We employed Co metal nanoparticles as the catalyst for nanotube growth, which were prepared by a reverse micelle method. The reverse micelle method provides nanoparticles covered with surfactants so that they are dispersed in organic solvent and, thus highly processible. The present MWNT arrays are promising for application in field emission displays, because of much lower nanotube density compared with the previously reported arrays.
Closed-end multi-wall carbon nanotubes, which contain an encapsulated aqueous multi-phase fluidunder high pressure, have been produced by hydrothermal synthesis. These nanotubes are leak-tight by virtue of holding the fluid at the high vacuum of a transmission electron microscope (TEM) and can be used as a testplatform for unique in-situ nanofluidic experiments in TEM. They form an experimental apparatus, which is at least two orders of magnitude smaller than the smallest capillaries used in fluidic experiments so far. Excellent wettability of the carbon tube walls by the liquid and a dynamic behavior similar to that in micro-capillaries demonstrates the possibility of use of nanoscale (<100 nm) tubes in nanofluidic devices.However, complex interface behavior that can potentially create hurdles to fluid transport is also demonstratedherein.
Using two different approaches: Monte Carlo simulations with Tersoff empirical potential and first principles calculations, the energetics and the structural properties of double-walled carbon and silicon nanotubes are investigated. Through Tersoff potential, the changes on cohesive energies for the Si and C systems are determined for several outer tubules for a fixed inner tube. Adopting first principles calculations, based on density functional theory, the trends, in terms of the cohesive energies, are compared with the corresponding obtained results using Tersoff empirical potential. The structures, specially of the most stable double-walled nanotubes, are discussed.
We study the phase behavior of single walled carbon nanotubes in aqueous solutions of surfactant molecules or amphiphilic polymers. Homogeneous dispersions can be obtained by using sodium dodecyl sulfate (SDS) in a well-defined concentration range. In contrast, polyvinyl alcohol (PVA) is not efficient at stabilizing the tubes. Carbon nanotubes stick with each other when PVA is added to homogeneous dispersions initially stabilized by SDS. This behavior is the basis of a simple method that we developed to assemble single walled carbon nanotubes into indefinitely long ribbons and fibers. The processing consists of dispersing the nanotubes in SDS solutions, re-condensing the nanotubes in the flow of a PVA solution to form a nanotube mesh, and then collating this mesh to a nanotube fiber. Flow induced alignment may lead to a preferential orientation of the nanotubes in the mesh that has the form of a ribbon. Unlike classical carbon fibers, the nanotube fibers can be strongly bent without breaking. Their obtained elastic modulus is 10 times higher than the modulus of high-quality bucky paper.
The crystal growth behaviour and crystallography of a variety of metal halides incorporated within single walled carbon nanotubes (SWNTs) as determined by high resolution electron microscopy (HRTEM) is described. Simple packed structures, such as the alkali halides, form related structures within SWNTs that are found to be integral atomic layers in terms of their thickness as a function of the encapsulating SWNT diameter. An enhanced HRTEM image restoration technique reveals precise data concerning lattice distortions present in these crystals. More complex structures, such as those derived from 3D complex, layered and chain halides form related crystal structures within SWNTs. In narrow SWNTs (i.e. with diameters less than ca. 1.6 nm), structures consisting of individual 1D polyhedral chains (1D-PHCs) were obtained that were derived from the corresponding bulk halides structures. In the case of infinite 3D network and layered halides, the 1D polyhedral chains form with lower co-ordinations than in the bulk. Molecular halides also intercalate into SWNTs but these do not readily form organised structures within SWNTs.
We have described a novel experimental technique to separate nanotubes from other unwanted carbon species in arc generated carbon soot. A conjugated polymer was used to bind to nanotubes in solution. The resultant hybrid was soluble while extraneous carbon material formed a sediment at the bottom of the sample bottle. This process was monitored using electron paramagnetic resonance (EPR) which showed that 63% of nanotubes were kept in solution while 98.1% of impurities were rejected. This allowed the calculation of the nanotube content in the carbon soot using EPR and thermo-gravitational analysis (TGA) yielding a purity value of 34% for the soot used in this study. This is compatible with estimates made using electron microscopy.