Impact on the photothermal emission from single wall nanotubes by some alkali halide salts

We demonstrate that alkali-halide salts, particularly potassium bromide, can reduce the photothermal emission (PTE) from single walled carbon nanotubes (SWNT). PTE is a prominent spectral feature in Raman spectroscopy when a near infrared laser is used to analyze a dark colored sample. We subsequently show that trapping salts inside SWNT and coating SWNT with the salt has a more pronounced impact on not only reducing PTE, but also enhancing the intensity of the Raman spectral features. The effect, which we have called nanotube enhanced Raman spectroscopy (NERS), has differences and similarities to the widely studied surface enhanced Raman spectroscopy (SERS).     

Electrochemical tuning of high energy phonon branches of double wall carbon nanotubes

Double wall carbon nanotubes (DWCNT) were studied by Raman spectroscopy and by in situ Raman spectroelectrochemistry. Two components of the G′-mode were assigned to the signals from inner and outer tubes. The dependence of the G′-mode intensity and frequency on the excitation laser energy was different for the inner and outer tubes. The changes in G′-mode caused by the applied electrochemical potential were discussed in detail. The red shifted component of the G′-mode exhibits delayed response to the electrochemical charging. Moreover, bleaching of the red shifted component was less symmetrical than that of the blue shifted component, if the applied electrode potential swept in anodic and cathodic directions. Hence, the Raman G′-mode has favorable properties for detailed characterization of DWCNT, both in the pristine state and upon electrochemical charging.     

Hydrogen adsorption studies on single wall carbon nanotubes

Hydrogen adsorption data on as-grown and heat-treated single walled carbon nanotubes (SWNTs) obtained by a volumetric procedure using a Quantachrome Autosorb-1 equipment are presented. The amounts of hydrogen adsorbed at atmospheric pressure reach approximately 0.01 wt.% at 298 K and 1 wt.% at 77 K. The isosteric heat of adsorption has been calculated for both samples from H₂ equilibrium adsorption data at three temperatures, having initial values of 7.42 and 7.75 kJ mol⁻¹. Studies in porous structure by N₂ adsorption and density measurements in helium pycnometer are reported.     

Complete elimination of metal catalysts from single wall carbon nanotubes

The complete removal of entrapped metallic impurities (i.e. Ni and Co) incorporated within single wall carbon nanotubes (SWNTs) has been a long-standing issue. A sonication-mediated treatment of as-obtained SWNT soot in a 1:1 mixture of aqueous hydrofluoric and nitric acids resulted in the complete elimination of these impurities as shown by energy dispersive X-ray analysis (EDAX). Contact angle measurements indicated that the wetting of SWNTs is enhanced in the presence of HF. The presence of HNO₃ and surfactant was found essential in removing the catalyst due to SWNT etching of end-caps/defects and providing better dispersion, respectively. Moreover, Raman spectroscopy indicated that the structural purity of the SWNTs is not compromised by the HF/HNO₃ purification treatment.     

Raman spectroscopy study of HM carbon fibres: effect of plasma treatment on the interfacial properties of single fibre/epoxy composites

The effect of an oxygen plasma treatment upon the structural and morphological properties of high-modulus carbon fibres has been studied by means of several characterisation techniques. Scanning electron microscopy showed that there were only minor changes of the morphology of the fibres following treatment. X-ray diffraction traces revealed that there were differences in structural parameters between the untreated fibres but no further modifications in the crystalline structure were detected after the plasma oxidation. Raman spectroscopy was used to follow the changes on the fibre surface structure following treatment. The peak positions and widths of the four main Raman bands (D, G, D′ and G′) were determined, with no significant changes observed after the surface treatment. A relationship between the width of the G band and the crystal parameter d₀₀₂ was found, with the magnitudes of both decreasing as the fibre modulus increased. A reference order parameter I_D/(I_D+I_G) ratio was calculated from the intensities of D and G bands. The treated fibres exhibited a more highly disordered surface structure that the untreated ones, as revealed by the increase of I_D/(I_D+I_G) after the plasma oxidation.     

Mechanical and morphological characterization of polymer-carbon nanocomposites from functionalized carbon nanotubes

Carbon nanotubes were functionalized with poly(vinyl alcohol) (PVA). The water-soluble PVA-functionalized carbon nanotubes were then embedded into PVA matrix via a wet-casting method, resulting in polymer-carbon nanocomposite films with homogeneous nanotube dispersion. Composites with pristine and functionalized nanotubes were tested in tension. It was found that the mechanical properties of these nanocomposite films were significantly improved compared to the neat polymer film. Functionalization allowed good distribution of the nanotubes in the matrix, leading to higher film strength. Scanning electron microscopy shows an apparent good wetting of the nanotubes by the PVA matrix. These results are supportive of good interfacial bonding between the functionalized carbon nanotubes and the hosting polymer matrix.     

Optical absorption matrix elements in single-wall carbon nanotubes

The optical absorption matrix element as a function of one-dimensional (1D) wave vector k, and subband index μ of a single wall carbon nanotube is given analytically for linearly polarized light with polarization parallel to the nanotube axis. For armchair nanotubes, it is found that the optical transitions for non-degenerate A symmetry bands are forbidden over the whole 1D k region and the transitions for all other bands are also forbidden at the k = 0 point. Near the Fermi level, the absorption for all metallic nanotubes is found to be approximately zero. For both metallic and semiconducting nanotubes, it is found that the absorption matrix element has a maximum absolute value at the van Hove singularity (vHS) k point around the Fermi energy for each band. The absorption dependence on diameter and chiral angle is also presented for semiconducting nanotubes. For light polarization perpendicular to the nanotube axis, on the other hand, the absorption for nanotubes is generally weak near a vHS.     

Phonons in single wall carbon nanotube bundles

We review recent and original results on the vibrational properties of single wall carbon nanotubes (SWNT). We especially focus on calculations and experiments performed on nanotube bundles. So far, the main technique for probing the dynamics has been Raman spectroscopy. Here, we discuss: (i) the relation between frequency of the A_1g radial breathing mode and nanotube diameter, (ii) the origin of resonance and the consequences on the profile and intensity of the Raman lines, and (iii) the assignment and resonant behaviour of the Raman lines between 700 and 1000 cm⁻¹. Recently, inelastic neutron scattering techniques (INS) were shown to be effective tools to probe the vibrational density of states of SWNT. We review the INS results and focus on the study of low frequency excitations, especially libration-twist modes and acoustic modes. Both Raman and INS results are analysed in the light of calculations performed in a valence force field model taking into account van der Waals intertubes interactions in the bundles.     

Texture of PAN- and pitch-based carbon fibers

The new technique of scanning microbeam X-ray diffraction is used to get information about the axial as well as the cross-sectional crystallographic texture of single PAN- and mesophase-pitch-based carbon fibers. The resulting preferred orientation in the axial direction is considerably higher than the values obtained from conventional X-ray diffraction measurements on fiber bundles. A change in azimuthal width of the 002 reflection was observed across some of the fibers, which can be attributed to a radial folded cross-sectional texture for pitch-based fibers, and to a different preferred orientation of skin and core layers for PAN-based fibers.     

Characterization of multiwall carbon nanotubes and influence of surfactant in the nanocomposite processing

Carbon nanotubes prepared by a classical CVD method with a nickel catalyst have been characterized, then used as conducting anisometric objects dispersed into a polymeric matrix. In a first part, these nanotubes are structurally characterized before and after heat treatments (HTT=1500, 2000, 2500 °C). Diffusion Raman experiments and diamagnetic susceptibility experiments demonstrated their limited graphitized structures.Then, in a second step, a well defined processing way to prepare nanocomposites with a standard epoxy resin is presented. In particular, the use or not of a non-ionic surfactant (Tergitol) to disperse these nanotubes is analyzed. The influence of nanotube contents is examined on the bulk nanocomposite density, the glass transition temperature of the nanocomposites, and the d.c. electrical conductivity behavior. These results demonstrated that the interfacial properties are playing a fundamental role. On one hand, the glass transition temperature is increasing with the nanotube content, and on the other hand, the percolation threshold is found for a rather high critical volumic concentration. Finally, it is demonstrated that a pure geometrical model is not sufficient to explain these behaviors and that a wrapping effect of the organic matrix around the nanotubes has to be considered.     

Reduction of solubilized multi-walled carbon nanotubes

In this paper, the chemical reduction of solubilized multi-walled carbon nanotubes by LiAlH₄ was investigated. The amide groups on the nanotubes could be reduced to hydroxyl groups, which was confirmed by FTIR and XPS studies. The Raman spectroscopic investigation showed that the morphology of the nanotubes did not change after the reduction.     

Coalescence of single-walled carbon nanotubes and formation of multi-walled carbon nanotubes under high-temperature treatments

Electric arc-discharge single-wall carbon nanotubes are annealed between 1600 and 2800 °C under argon flow. Their stability and evolution are studied by coupling TEM, X-ray diffraction and Raman spectroscopy. The first modifications appear at 1800 °C with a significant decrease of the crystalline order. It is due to SWNTs coalescence leading to smaller bundles but with an increase of the tube diameters from 2 to 4 nm. From 2200 °C, SWNTs progressively disappear to the benefit of MWNTs having at first two to three carbon layers then reaching 7 nm external diameter. The possible mechanisms responsible for the SWNTs coalescence and instability and their transformation in MWNTs are discussed.     

Transfer-matrix simulations of electronic transport in single-wall and multi-wall carbon nanotubes

We present simulations of electronic transport in single-wall and multi-wall carbon nanotubes, which are placed between two metallic contacts. We consider situations where the electrons first encounter a singe-wall nanotube (corresponding to either the inner or the outer shell of the (10, 10)@(15, 15)@(20, 20) and (10, 10)@(20, 10)@(20, 20) nanotubes), before encountering the multi-wall structures. The role of this two-step procedure is to enforce the electrons to enter a single shell of the multi-wall nanotubes, and we study how from that point they get redistributed amongst the other tubes. Because of reflections at the metallic contacts, the conductance of finite armchair nanotubes is found to depend on the length of the tubes, with values that alternate between three separate functions. Regarding the transport in multi-wall nanotubes, it is found that the electrons keep essentially propagating in the shell in which they are initially injected, with transfers to the other tubes hardly exceeding one percent of the whole current. In the case where the three tubes are conducting, these transfers are already completed after four nanometers. The conductance and repartition of the current present then oscillations, which are traced to the band structure of the nanotube. The transfers between the shells and the amplitude of these oscillations are significantly reduced when the intermediate tube is semiconducting.     

Elastic moduli of nanocrystallites in carbon fibers measured by in-situ X-ray microbeam diffraction

The in-plane Young’s modulus and the shear modulus of carbon nanocrystallites were investigated during in-situ tension tests of single carbon fibers by X-ray diffraction using the shift of the 10 band in the meridional direction and the change in the azimuthal width of the 002 reflection. The limiting value for the Young’s modulus was found to be 1140 GPa, which is higher than the value for graphite obtained from macroscopic specimens, but coincides with recent measurements on nanotubes. Furthermore, the shear modulus was evaluated using a uniform stress approach and was found to increase with increasing misorientation of the crystallites. It turns out that both the in-plane Young’s modulus and the shear modulus are not constant, but dependent on the orientation parameter.     

XRD and TEM study of high pressure treated single-walled carbon nanotubes and C60-peapods

In situ synchrotron X-ray diffraction measurements of single-walled carbon nanotube and C₆₀-peapod samples under high pressures up to 13 GPa and at high temperature were carried out. Anisotropical shrinkages of their bundle two-dimensional lattices by compression at room temperature were observed. It was found that the lattices recover original forms reversibly upon pressure release. It was also found that irreversible phase transformations occur by raising temperature at the highest pressure. The high pressure and high temperature treated samples were examined by X-ray diffraction, transmission electron microscope, and Raman measurements. It was indicated by transmission electron microscope observation that hexagonal diamond is able to be synthesized by high pressure and high temperature treatment of C₆₀-peapods.     

Equilibrium and dynamics of acetylene sorption in multiwalled carbon nanotubes

Multiwalled carbon nanotubes (MWCNT) were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and by adsorption of N₂ and acetylene. The dynamics of acetylene sorption was studied by frequency response spectroscopy. The average tube dimension and the shell number were determined from a statistical evaluation of the TEM images. This value agreed with that obtained from XRD via the Scherrer equation only if a shape factor of 0.49 was used. The diffusion of acetylene in the nanotubes of the MWCNT sample was found to be the rate-controlling step of the sorption process. Relationships between the tube dimensions and the equilibrium and dynamic sorption properties were demonstrated.     

Structure of carbon nanotubes probed by local and global probes

A review is proposed of different techniques available today for the characterization of the atomic structure of carbon nanotubes. This review covers the electron microscopies, various diffraction techniques, scanning probe microscopies, and optical spectroscopies, including Raman scattering. The advantages and limitations of the characterization techniques are discussed.