Controllable preparation and properties of single-/double-walled carbon nanotubes

In this paper, we discussed recent studies done in our laboratories with a floating catalyst chemical vapor deposition (CVD) method. We can grow single- or double-walled carbon nanotubes (SWNTs/DWNTs) with different kinds of catalysts. Single-walled carbon nanotubes without amorphous carbon coating were prepared by thermally decomposing acetylene (C₂H₂) at the temperature range of 750–1200 °C with ferrocene as catalyst. While with sulfur promoted ferrocene catalyst, double-walled carbon nanotubes were mass-produced by pyrolizing C₂H₂ at the temperature range of 900–1100 °C. Furthermore, tunable growth of DWNTs with different diameter was achieved in our experiment. It is found that DWNTs produced at lower carbon partial pressure have much smaller inner tubes, even DWNTs with the smallest inner diameter of 0.4 nm was found in here. As convenient and effective tool, radial breathing mode (RBM) of Raman scattering technique can be used to distinguish SWNTs from DWNTs. In further studies of Raman scattering with DWNTs, the possible match of the inner tubes and the outer tubes according to the RBM bands was assigned, and different chirality types were discussed according to the diameter and chirality dependence of resonant Raman vibration. We also investigated the temperature-dependent frequency shift of resonant Raman spectra of DWNTs in the range of 78–650 K. We found that different RBM peaks, which are relative to different tube diameters, have different temperature coefficient of frequency shift, and the larger diameter carbon nanotubes have more RBM frequency downshift with increasing temperature. It is ascribed to the RBM frequency variation to the temperature dependence of the stretching force constant of C–C bond. Besides, Polarized Raman spectra were preformed on well-aligned SWNTs structure fabricated through post-growth method and found that the angular dependence of Raman intensity is consistent well with the predictions of the resonance Raman theory.     

Double-walled carbon nanotubes under hydrostatic pressure: Raman experiments and simulations

We have used Raman spectroscopy to study the behavior of double-walled carbon nanotubes (DWNT) under hydrostatic pressure. We find that the rate of change of the tangential mode frequency with pressure is higher for the sample with traces of polymer compared to the pristine sample. We have performed classical molecular dynamics simulations to study the collapse of single (SWNT) and double-walled carbon nanotube bundles under hydrostatic pressure. The collapse pressure (pc) was found to vary as 1/R3, where R is the SWNT radius or the DWNT effective radius. The bundles showed approximately 30% hysteresis and the hexagonally close packed lattice was completely restored on decompression. The pc of a DWNT bundle was found to be close to the sum of its values for the inner and the outer tubes considered separately as SWNT bundles, demonstrating that the inner tube supports the outer tube and that the effective bending stiffness of DWNT, D(DWNT) - 2D(SWNT).     

Separation of single-walled carbon nanotubes by use of ionic liquid-aided capillary electrophoresis

This paper presents a simple, highly efficient method for analyzing single-walled carbon nanotube (SWNT) bundles based on (1) ultrasound-assisted solubilization/dispersion of SWNTs in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, (2) encapsulation of the nanotubes in sodium dodecyl sulfate micelles, and (3) analysis by capillary electrophoresis. The process by which SWNTs disperse in the ionic liquid was studied by Raman spectroscopy. No degradation of SWNTs was observed under mild sonication conditions. The shape and position changes observed in the Raman spectral bands for the nanotubes are ascribed to debundling and interaction with the ionic liquid. Separation of solubilized SWNTs was accomplished by using a 50 mM formic acid solution at pH 2.0 as background electrolyte and a potential of -10 kV. Under these conditions, separation was completed within only 4 min. Eighteen peaks for SWNTs were identified in the analysis of commercial SWNT bundles. The two types of bundles studied exhibited distinct, highly characteristic electrophoretic profiles which could be used to control SWNTs purity.     

High-pressure Raman study of debundled single-walled carbon nanotubes

We report the pressure dependence for the radial (omega R) and tangential (omega T) band frequencies in debundled single-walled carbon nanotubes (SWNTs) derived from laser-synthesized SWNT bundles. As previously described, a chemical procedure was used to prepare debundled SWNTs from as-prepared, large SWNT bundles. The normalized pressure coefficient for omega R in the debundled sample was compared with the corresponding value in the bundled sample to quantify the strength of van der Waals interactions between tubes in these nanotube materials. Furthermore, the pressure dependences for the radial (omega R) and tangential (omega T) band frequencies in debundled tubes were also compared with corresponding dependences predicted for isolated SWNTs, obtained with generalized tight binding molecular dynamic (GTBMD) simulations described in our previous work. The results presented here collectively suggest that the van der Waals interaction is still strong in the debundled sample studied here, which contained predominantly small bundles of SWNTs rather than isolated tubes.     

Selective tuning of the electronic properties of coaxial nanocables through exohedral doping

The electronic properties of exohedrally doped double-walled carbon nanotubes (DWNTs) have been investigated using density functional theory and resonance Raman spectroscopy (RRS) measurements. First-principles calculations elucidate the effects of exohedral doping on the M@S and S@M systems, where a metallic (M) tube is either inside or outside a semiconducting (S) one. The results demonstrate that metallic nanotubes are extremely sensitive to doping even when they are inner tubes, in sharp contrast to semiconducting nanotubes, which are not affected by doping when the outer shell is a metallic nanotube (screening effects). The theoretical predictions are in agreement with RRS data on Br2- and H2SO4-doped DWNTs. These results pave the way to novel nanoscale electronics via exohedral doping.     

Synthesis and characterization of new polyaniline/nanotube composites

New polyaniline/nanotube (PANI/NT) composites have been synthesized by “in situ” polymerization processes using both multi-wall carbon nanotubes (MWNTs) and single-wall carbon nanotubes (SWNTs) in concentrations ranging from 2 to 50 wt.%. Although no structural changes are observed using MWNTs above a concentration of 20 wt.%, the in situ synthesis results in electronic interactions between nanotubes and the quinoid ring of PANI leading to enhanced electronic properties and thus to the formation of a genuine PANI/MWNT composite material. On the other hand, using SWNTs favors the formation of inhomogeneous mixtures rather than of a homogeneous composite materials, independent of the SWNT concentration. X-ray diffraction, Raman and transport measurements show the different behavior of both classes of nanotubes in PANI/NT materials. The difficulties in the formation of a true PANI/SWNT composite are related to the far more complex structure of the SWNT material itself, i.e. to the presence of entangled bundles of SWNTs, amorphous carbon and even catalytic metal particles.     

Employing Raman spectroscopy to qualitatively evaluate the purity of carbon single-wall nanotube materials

Carbon single-wall nanotubes (SWNTs) have highly unique electronic, mechanical and adsorption properties, making them interesting for a variety of applications. Raman spectroscopy has been demonstrated to be one of the most important methods for characterizing SWNTs. For example, Raman spectroscopy may be employed to differentiate between metallic and semi-conducting nanotubes, and may also be employed to determine SWNT diameters and even the nanotube chirality. Single-wall carbon nanotubes are generated in a variety of ways, including arc-discharge, laser vaporization and various chemical vapor deposition (CVD) techniques. In all of these methods, a metal catalyst must be employed to observe SWNT formation. Also, all of the current synthesis techniques generate various non-nanotube carbon impurities, including amorphous carbon, fullerenes, multi-wall nanotubes (MWNTs) and nano-crystalline graphite, as well as larger micro-sized particles of graphite. For any of the potential nanotube applications to be realized, it is, therefore, necessary that purification techniques resulting in the recovery of predominantly SWNTs at high-yields be developed. It is, of course, equally important that a method for determining nanotube wt.% purity levels be developed and standardized. Moreover, a rapid method for qualitatively measuring nanotube purity could facilitate many laboratory research efforts. This review article discusses the application of Raman spectroscopy to rapidly determine if large quantities of carbon impurities are present in nanotube materials. Raman spectra of crude SWNT materials reveal tangential bands between 1500-1600 cm(-1), as well as a broad band at approximately 1350 cm(-1), attributed to a convolution of the disorder-induced band (D-band) of carbon impurities and the D-band of the SWNTs themselves. Since the full-width-at-half-maximum (FWHM) intensity of the various carbon impurity D-bands is generally much broader than that of the nanotube D-band, an indication of the SWNT purity level may be obtained by simply examining the line-width of the D-band. We also briefly discuss the effect of nanotube bundling on SWNT Raman spectra. Finally, sections on employing Raman spectroscopy, and Raman spectroscopy coupled with additional techniques, to identify the separation and possible isolation of a specific nanotube within purified SWNT materials is provided. Every SWNT can be considered to be a unique molecule, with different physical properties, depending on its (n, m) indices. The production of phase-pure (n, m) SWNTs may be essential for some nanotube applications.     

Photoluminescence from inner walls in double-walled carbon nanotubes: some do, some do not

Double-walled carbon nanotubes (DWNTs) have recently been recognized as important members in the carbon nanotube family because they are expected to have certain unique properties. For example, DWNTs are expected to replace single-walled carbon nanotubes (SWNTs) in biomarker applications and optoelectronics if the observed luminescence from DWNTs can be verified. However, due to unavoidable byproducts, such as SWNTs, optical properties of DWNTs still remain controversial. There is an ongoing debate concerning the ability of DWNTs to exhibit photoluminescence (PL). In this report, we aim to clearly resolve this debate through the study of carefully separated DWNTs. DWNTs were successfully separated from SWNTs using density gradient ultracentrifugation. Here we clearly show that light is emitted from the inner wall of DWNTs; however, the intensity of the emission is significantly quenched. Interestingly, it was found that a very narrow range of diameters of the inner walls of DWNTs is required for PL to be observable. All other diameters led to complete PL quenching in DWNTs. In short, we have shown that both sides of the debate are correct under certain situations. The real answer to the question is that some DWNTs do emit light but most DWNTs do not.     

Transparent boron-doped carbon nanotube films

We report results of studies on the sheet resistance and optical transmission of thin films of boron-doped single-walled carbon nanotubes (SWNTs). Boron doping was carried out by exposure of SWNTs to B 2O 3 and NH 3 at 900 degrees C and 1-3 atom % boron was found in the SWNT bundles via electron energy loss spectroscopy (EELS). Boron doping was found to downshift the positions of the optical absorption bands associated with the van Hove singularities (E 11 (s) E 22 (s) and E 11 (m)) by approximately 30 meV relative to their positions in acid-treated and annealed SWNTs. Raman spectroscopy, EELS, and optical data are consistent with the picture that a few atom % boron has been substituted for carbon in the sp (2) framework of SWNTs. Finally, our results show that boron doping does not significantly affect the optical transmittance in the visible region. However, boron doping lowers the sheet resistance by approximately 30% relative to pristine SWNT films from the same batch. Boron-doped SWNT may provide a better approach to touch-screen technology.     

Selective etching of metallic single-wall carbon nanotubes with hydrogen plasma

We present Raman scattering and scanning tunnelling microscopy (STM) measurements on hydrogen plasma etched single-wall carbon nanotubes (SWNTs). Interestingly, both the STM and Raman spectroscopy show that the metallic SWNTs are dramatically altered and highly defected by the plasma treatment. In addition, structural characterizations show that metal catalysts are detached from the ends of the SWNT bundles. For semiconducting SWNTs we observe no feature of defects or etching along the nanotubes. Raman spectra in the radial breathing mode region of plasma-treated SWNT material show that most of the tubes are semiconducting. These results show that hydrogen plasma treatment favours etching of metallic nanotubes over semiconducting ones and therefore could be used to tailor the electronic properties of SWNT raw materials.     

Aggregation behavior of chemically attached poly(ethylene glycol) to single-walled carbon nanotubes (SWNTs) ropes

The aggregation behavior of poly(ethylene glycol) (PEG)-grafted single-walled carbon nanotubes (SWNTs) was investigated by Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), and high-resolution transmission electron microscopy (HR-TEM). The hybrid functional material was synthesized by producing carboxylic acid groups at nanotube ends and side-wall defect sites, which provide reaction sites for various guest species, followed by a reaction with hydroxyl-terminated PEG in various solvents. FTIR and TGA analyses confirm that the PEG chains were covalently attached to the functionalized SWNTs ropes. Transmission electron microscopy (TEM) images of the PEG-grafted SWNTs revealed two different morphologies, depending on solvent quality: PEG and SWNT segments self-organize into ring-like structure in which the aggregated PEG core is surrounded by SWNT bundles, when freshly prepared PEG-grafted SWNTs are cast from benzene/tetrahydrofuran solvent mixture. Instead, the hybrid polymers form a highly dispersed morphology in selective solvents for SWNTs. The novel aggregation mode originates from strong association of polymer chains with nanotubes, as in the aggregation behavior of micro-phase separated copolymers in dilute solution.     

Determination of nanotubes properties by Raman spectroscopy

The basic concepts and characteristics of Raman spectra from single-wall carbon nanotubes (SWNTs, both isolated and bundled) are presented. The physical properties of the SWNTs are introduced, followed by the conceptual framework and characteristics of their Raman spectra. Each Raman feature, namely the radial breathing mode, the tangential G band, combination modes and disorder-induced bands are discussed, addressing their physical origin, as well as their capability for characterizing SWNT properties.     

Mechanical Peeling of Free‐Standing Single‐Walled Carbon‐Nanotube Bundles

An in situ electron microscopy study is presented of adhesion interactions between single‐walled carbon nanotubes (SWNTs) by mechanically peeling thin free‐standing SWNT bundles using in situ nanomanipulation techniques inside a high‐resolution scanning electron microscope. The in situ measurements clearly reveal the process of delaminating one SWNT bundle from its originally bound SWNT bundle in a controlled‐displacement manner and capture the deformation curvature of the delaminated SWNT bundle during the peeling process. A theoretical model based on nonlinear elastica theory is employed to interpret the measured deformation curvatures of the SWNTs and to quantitatively evaluate the peeling force and the adhesion strength between bundled SWNTs. The estimated adhesion energy per unit length for each pair of neighboring tubes in the peeling interface based on our peeling experiments agrees reasonably well with the theoretical value. This in situ peeling technique provides a potential new method for separating bundled SWNTs without compromising their material properties. The combined peeling experiments and modeling presented in this paper will be very useful to the study of the adhesion interactions between SWNTs and their nonlinear mechanical behaviors in the large‐displacement regime.     

Raman Spectroscopy and SEM Study of SWNTs in Aqueous Solution and Films with Surfactant or Polymer Surroundings

Raman spectra of single-walled carbon nanotubes (SWNTs) in aqueous solutions with sodium dodecylsulfate (SDS) or fragmented single-stranded DNA (ss-DNA) and films obtained from these solutions have been studied. Scanning electron microscope (SEM) film study shows that micelles formed by SDS molecules around SWNT in solution do not keep individual nanotubes from sticking together in bundles during drying out the film. DNA wrapped around SWNT precludes the full nanotubes sticking in the film that facilitates the following splitting of these bundles.     

Raman spectroscopy study of isolated double-walled carbon nanotubes with different metallic and semiconducting configurations

A double-walled carbon nanotube (DWNT) provides the simplest system to study the interaction between concentric layers in carbon nanotubes. The inner and outer walls of a DWNT can be metallic (M) or semiconducting (S), and each of the four possible configurations (M@M, M@S, S@S, S@M) has different electronic properties. Here we report, for the first time, detailed Raman spectroscopy experiments carried out on individual DWNTs, where both concentric tubes are measured under resonance conditions, in order to understand the dependence of their electronic and optical properties according to their configuration. Interestingly, for the three DWNTs that were studied, the inner-outer tube distance (e.g., 0.31-0.33 nm) was less than the interlayer spacing in graphite. We believe these results have important implications in the fabrication of electronic devices using different types of S and M tubular interconnects.     

Supramolecular discrimination of carbon nanotubes according to their helicity

Adsorption of specifically designed and geometrically constrained polyaromatic amphiphiles on single-walled carbon nanotubes (SWNTs) was found to be selective of the nanotube helicity angle. Starting from the same SWNT mixture, photoluminescence and resonant Raman spectroscopies show that a pentacenic-based amphiphile leads to the solubilization of armchair SWNTs and that a quaterrylene-based amphiphile leads to the solubilization of zigzag SWNTs. The results were predicted by the design of the two amphiphiles and are consistent with a supramolecular recognition of the nanotube graphene-type atomic structure by the aromatic part of the molecules through optimized pi-pi-stacking interactions.     

Fractionation of SWNT/nucleic acid complexes by agarose gel electrophoresis

We show that aqueous dispersions of single-walled carbon nanotubes (SWNTs), prepared with the aid of nucleic acids (NAs) such as RNA or DNA, can be separated into fractions using agarose gel electrophoresis. In a DC electric field, SWNT/NA complexes migrate in the gel in the direction of positive potential to form well-defined bands. Raman spectroscopy as a function of band position shows that nanotubes having different spectroscopic properties possess different electrophoretic mobilities. The migration patterns for SWNT/RNA and SWNT/DNA complexes differ. Parallel elution of the SWNT/NA complexes from the gel during electrophoresis and subsequent characterization by AFM reveals differences in nanotube diameter, length and curvature. The results suggest that fractionation of nanotubes can be achieved by this procedure. We discuss factors affecting the mobility of the nanotube complexes and propose analytical applications of this technique.     

Electrical conductivity and Raman imaging of double wall carbon nanotubes in a polymer matrix

Raman spectroscopy is used to access the dispersion state of DWNTs in a PEEK polymer matrix. The interaction of the outer tube with the matrix can be determined from the line shape of the Raman G band. This allows us to distinguish regions where the nanotubes are well dispersed and regions where the nanotubes are agglomerated. The percolation threshold of the electrical conductivity of the double wall carbon nanotubes (DWNTs)/PEEK nanocomposites is found to be at 0.2-0.3 wt%. We find a maximum electrical conductivity of 3 × 10⁻² S cm⁻¹ at 2 wt% loading. We detect nanotube weight concentrations as low as 0.16 wt% by Raman spectroscopy using a yellow excitation wavelength. We compare the Raman images with transmission electron microscopy images and electrical conductivity measurements. A statistical method is used to find a quantitative measure of the DWNTs dispersion in the polymer matrix from the Raman images.     

Dual Raman features of double coaxial carbon nanotubes with N-doped and B-doped multiwalls

Double coaxial carbon nanotubes with nitrogen (N)-doped and boron (B)-doped multiwalls possess composite Raman characteristics, originating not only from the outer N-doped but also from inner B-doped layers. Both N and B dopings result in substantial shifts of the characteristic D band and G band of sp(2) carbon constituting nanotube walls but in different ways. The downshift of the G band is correlated with the decreases of electrical resistivity of carbon nanotubes regardless of N or B doping.     

Experimental-computational study of shear interactions within double-walled carbon nanotube bundles

The mechanical behavior of carbon nanotube (CNT)-based fibers and nanocomposites depends intimately on the shear interactions between adjacent tubes. We have applied an experimental-computational approach to investigate the shear interactions between adjacent CNTs within individual double-walled nanotube (DWNT) bundles. The force required to pull out an inner bundle of DWNTs from an outer shell of DWNTs was measured using in situ scanning electron microscopy methods. The normalized force per CNT-CNT interaction (1.7 ± 1.0 nN) was found to be considerably higher than molecular mechanics (MM)-based predictions for bare CNTs (0.3 nN). This MM result is similar to the force that results from exposure of newly formed CNT surfaces, indicating that the observed pullout force arises from factors beyond what arise from potential energy effects associated with bare CNTs. Through further theoretical considerations we show that the experimentally measured pullout force may include small contributions from carbonyl functional groups terminating the free ends of the CNTs, corrugation of the CNT-CNT interactions, and polygonization of the nanotubes due to their mutual interactions. In addition, surface functional groups, such as hydroxyl groups, that may exist between the nanotubes are found to play an unimportant role. All of these potential energy effects account for less than half of the ~1.7 nN force. However, partially pulled-out inner bundles are found not to pull back into the outer shell after the outer shell is broken, suggesting that dissipation is responsible for more than half of the pullout force. The sum of force contributions from potential energy and dissipation effects are found to agree with the experimental pullout force within the experimental error.