Raman scattering study of the thermal conversion of bundled carbon nanotubes into graphitic nanoribbons

Raman scattering is used to study the temperature-driven structural transformations of bundled single-walled carbon nanotubes (SWCNTs) observed in HiPCO and ARC synthesis by electron microscopy, i.e., tube-tube coalescence ∼1300-1400 °C, coalesced tubes to multi-walled tubes (MWCNT) at ∼1600-1800 °C and finally (only ARC tubes) MWCNT to graphitic nanoribbons (GNRs) at ∼1800 °C. All these transformations occurred in vacuum. Here, we present the details of these transformations as seen through the “eyes” of Raman scattering via changes in the radial (R) SWCNT band, the G-band (and its substructure) and the relative intensity of the disorder-induced D- and D′-band scattering. The Raman spectrum of GNRs is also discussed in detail. For 514.5 nm laser excitation, five relatively broad GNR Raman bands are observed: 1350, 1580, 1620, 2702 and 3250 cm⁻¹. A Knight plot is used to estimate the GNR width and we find w ∼ 9 nm, which is in reasonable agreement with the estimate of 7.6 nm based on TEM and the model that a GNR is a collapsed MWCNT.     

The use of camphor-grown carbon nanotube array as an efficient field emitter

Three-dimensional growth of well-aligned high-purity multiwall carbon nanotubes (CNTs) is achieved on silicon, nickel-coated silicon and cobalt-coated silicon substrates by thermal decomposition of a botanical carbon source, camphor, with different catalyst concentrations. Field emission study of as-grown nanotubes in a parallel-plate diode configuration suggests them to be an efficient emitter with a turn-on field of ∼1 V/μm (for 10 μA/cm²) and a threshold field of ∼4 V/μm (for 10 mA/cm²). Maximum current density lies in a range of 20-30 mA/cm² at 5.6 V/μm with significant reversibility. Prolonged stability test of camphor-grown CNT emitters suggests a life time of ∼5 months under continuous operation. A new feature, metal-assisted electron emission from CNTs, has been addressed. Isolated nanotubes used as a cold cathode in a field emission microscope reveal the pentagonal emission sites and hence the atomic structure of the nanotube tips.     

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.     

Growth of high-quality carbon nanotubes on free-standing diamond substrates

Carbon nanotubes (CNTs) were grown on diamond-coated Si substrates and free-standing diamond wafers to develop efficient thermal interface materials for thermal management applications. High-quality, translucent, free-standing diamond substrates were processed in a 5 kW microwave plasma chemical vapor deposition (CVD) system using CH₄ as precursor. Ni and Ni-9%W-1.5%Fe catalyst islands were deposited to nucleate CNTs directly onto the diamond substrates. Randomly-oriented multi-walled CNTs forming a mat of ∼5 μm thickness and consisting of ∼20 nm diameter tubes were observed to grow in a thermal CVD system using C₂H₂ as precursor. Transmission electron microscopy and Raman analyses confirmed the presence of high-quality CNTs on diamond showing a D/G peak ratio of 0.2-0.3 in Raman spectra.     

Study of the stability of the molten zone and the stresses induced during the growth of Al2O3-Y3Al5O12 eutectic composite by the laser floating zone technique

A series of analytical calculations is used to study both the effect of the thermal gradients and the stability of the molten zone in the laser floating zone growth of Al₂O₃-Y₃Al₅O₁₂ eutectic composite. The thermal gradients in the solidification interface have been calculated and the axial gradient compared with the experimental one of 4.5 × 10⁵ K/m. For these calculations the coefficients of heat transfer from the molten zone to the ambient at the solid-melt interface have been previously obtained. The thermal stresses generated by the high thermal gradients can induce crack formation during the cooling depending on the rod diameter. The theory predicts that it is possible to grow rods free of cracks up to R = 1.7 mm, at low rates (10 mm/h) in close agreement with the experimental critical radius of 1.6 mm.The dependence of the zone length on the input laser power used to carry out the growth is shown. The study of the floating zone profile allows determining the maximum stable zone length, verifying the stability criterion established by some authors.     

Enhanced visible light photocurrent generation at surface-modified TiO2 nanotubes

Photoelectrodes consisting of TiO₂ nanotube layers with different thicknesses (0.5 μm, 1.7 μm, 3 μm, 6 μm, 9 μm, and 18 μm) were prepared by anodization of titanium substrates and subsequent surface modification by a heat treatment at 400 °C in the presence of urea pyrolysis products. In contrast to unmodified TiO₂ nanotubes, the modified photoelectrodes exhibit photocurrents under visible light irradiation down to 750 nm. Photocurrent transients indicate enhanced recombination unless a suitable hole-scavenger, like iodide, is present since the photogenerated holes do not oxidize water efficiently. In the visible light the photoconversion efficiency increases significantly with nanotube length. The maximum incident photon-to-current efficiency (IPCE) was observed for tubes with the length of 6-9 μm (IPCE ∼4.5% and 1.4% at 450 nm and 550 nm, respectively) and the photocurrent enhancement with increasing tube length is found to be stronger at longer irradiations wavelengths.     

Enhanced conductivity in polybenzoxazoles doped with carboxylated multi-walled carbon nanotubes

Two representative polybenzazoles, poly(p-phenylene benzobisoxazole) (PBO) and poly(2,5-benzoxazole) (ABPBO), have been used as matrix materials for fabricating electrically conducting nanocomposite films. In this strategy, pristine multi-walled carbon nanotubes (MWCNTs) were first treated with nitric acid to form carboxylated multi-walled carbon nanotubes (MWCNTs-COOH). Subsequently, MWCNTs-COOH were dispersed efficiently in the methanesulfonic acid (MSA) solution of polybenzazole, sonicated, and then processed into thin films. MWCNTs-COOH in MSA formed an isotropic regime at the concentration of ∼0.1 wt.%. Nanotubes could form net like structures and conductive channels in the polymer matrix to improve electrical conductivity, mechanical properties, and thermal stability. At the MWCNT-COOH composition of 5 wt.%, polybenzazole/MWCNT-COOH composite films exhibited a dramatic enhancement in electrical conductivity by 8 orders of magnitude from ∼10⁻¹² to 1.6 × 10⁻⁴ S cm⁻¹ without significantly sacrificing optical transparency.     

Efficient microwave energy absorption by carbon nanotubes

The absorption of energy from microwave frequency electromagnetic fields by carbon nanotubes is investigated by measuring the heating rate of dispersions of carbon nanotubes in silicone oil. Microwave absorbance of silicone oil is enhanced by 500 times with the addition of as little as 0.04 wt% carbon nanotubes. The removal of residual iron catalyst is found to have little effect on the microwave absorbance; an acid based surface modification however causes the enhancement to be lost, with the performance little better than carbon black. As-synthesized tubes showed a percolation threshold of ∼0.025 wt% for both conductivity and microwave absorbance. The efficient energy absorption is explained by a mechanism of conduction loss in the samples.     

Synthesis of nitrogen-doped horn-shaped carbon nanotubes by reduction of pentachloropyridine with metallic sodium

Nitrogen-doped horn-shaped carbon nanotubes (CNTs) have successfully been prepared by reducing pentachloropyridine with metallic sodium at 350 °C. A typical CNT has an open-end diameter of ∼2 μm, a close-end diameter of ∼0.3 μm, a wall thickness of ∼30 nm, and a length up to 8 μm. TEM observation indicates that the CNTs account for ∼30% of the products, and the rest is solid and hollow carbon nanospheres (CNSs) with a diameter of about 50-290 nm. Elemental analysis shows that the N/C atomic ratio of the carbon nanostructures is about 0.0208. XRD and HRTEM measurements reveal that the CNTs are amorphous. To understand the growth process and refine the growth condition, various control experiments have been finished. At last, a sodium-catalysis-reduction solid-liquid-solid growth mechanism of the CNTs has been suggested on the basis of the experiments.     

Controlling the diameter distribution of carbon nanotubes grown from camphor on a zeolite support

Single-wall and multi-wall carbon nanotubes (SWNTs and MWNTs, respectively) of controlled diameter distribution were selectively grown by thermal decomposition of a botanical hydrocarbon, camphor, on a high-silica zeolite support impregnated with Fe-Co catalyst. Effects of catalyst concentration, growth temperature and camphor vapor pressure were investigated in wide ranges, and diameter distribution statistics of as-grown nanotubes was analyzed. High yields of metal-free MWNTs of fairly uniform diameter (∼10 nm) were grown at 600-700 °C, whereas significant amounts (∼30%) of SWNTs were formed at 850-900 °C within a narrow diameter range of 0.86-1.23 nm. Transmission electron microscopy and micro-Raman spectroscopy reveal that camphor-grown nanotubes are highly graphitized as compared to those grown from conventional CNT precursors used in chemical vapor deposition.     

Optimal synthesis of mesostructured hollow titania nanotubes templated on CaCO3 nanoparticles

Approximate 10 µm length of mesostructured hollow titania nanotubes with intact configuration was successfully prepared by using needle-like calcium carbonate and octadecylamine as double templates at room temperature in nonaqueous system. During the whole preparation, two parameters i.e. tetrabutoxytitanium/calcium carbonate molar ratio and annealing temperature, were optimized to obtain titania nanotubes with well-defined tubular morphology and mesoporous structure in tube walls. The as-prepared samples were characterized by scanning electron microscopy, transmission electron microscopy, broad-angle X-ray diffraction, pore size distribution and Brunauer-Emmett-Teller. The results showed that under optimal experimental conditions i.e. tetrabutoxytitanium/calcium carbonate molar ratio (50 wt.%) and annealing temperature (773 K), the tube materials exhibited uniform tubular structure with a length of 8-15 µm and an inner diameter of ∼ 400 nm, a wall thickness of ∼ 40 nm, a surface area of 112.2 m²/g and a pore volume of 0.18 cm³/g. The optimized titania nanotubes were utilized as a carrier for the immobilized of ibuprofen via a simple adsorption method. It was found that the loaded drug presented good sustained release property in three release media, i.e. simulated body fluid, normal saline and pure water.     

Improved mechanical strength and electrical conductivity of organogels containing carbon nanotubes

We have fabricated the first organogel/carbon nanotube composites using 12-hydroxystearic acid (HSA) as the gelator molecule, multi-wall carbon nanotubes as the nanofillers, and 1,2-dichlorobenzene as the organic solvent. We have achieved significant improvements in the mechanical and electrical properties of the organogels by incorporating pristine or carboxylated carbon nanotubes. For example, the linear viscoelastic regime of the HSA organogel, an indicator of the strength of the gel, extends by a factor of four with the incorporation of 0.2 wt% of the carboxylated nanotubes. Also, the carbon nanotubes (specially the pristine tubes) improve the electrical conductivity of the organogels, e.g. six orders of magnitude enhancement in electrical conductivity with 0.2 wt% of pristine tubes. Differential scanning calorimetry experiments indicate that the nanotubes do not affect the thermoreversibility of the organogels.     

Direct synthesis of carbon nanotubes decorated with size-controllable Fe nanoparticles encapsulated by graphitic layers

A simple method has been developed for direct synthesis of magnetic multi-walled carbon nanotubes (MWCNTs) homogeneously decorated with size-controllable Fe nanoparticles (Fe-NPs) encapsulated by graphitic layers on the MWCNT surface by pyrolysis of ferrocene. These composites have similar C/Fe atomic ratio of ∼10 and exhibit sufficiently high saturation magnetization for magnetic separation in a liquid phase. Moreover, with 0, ∼1, ∼2 wt% sulfur as growth promoter, the size of Fe-NPs can be controlled with an average diameter of ∼5, ∼22 and ∼42 nm, respectively. When compared to time-consuming wet-chemical methods, the simplicity of this method should allow easy large-scale production of these magnetically functionalized MWCNTs, which can be used as catalyst supports with high stability for effective magnetic separation in liquid-phase reactions, especially under acid/basic conditions.     

Viscoelasticity and high buckling stress of dense carbon nanotube brushes

We report on the mechanical behavior of a dense brush of small-diameter (1-3 nm) non-catalytic multiwall (2-4 walls) carbon nanotubes (CNTs), with ∼10 times higher density than CNT brushes produced by other methods. Under compression with spherical indenters of different radii, these highly dense CNT brushes exhibit a higher modulus (∼17-20 GPa) and orders of magnitude higher resistance to buckling than vapor phase deposited CNT brushes or carbon walls. We also demonstrate the viscoelastic behavior, caused by the increased influence of the van der Waals’ forces in these highly dense CNT brushes, showing their promise for energy-absorbing coatings.     

A method for creating reliable and low-resistance contacts between carbon nanotubes and microelectrodes

An ultrasonic bonding technique has been developed for bonding single wall carbon nanotubes (SWNTs) onto metal microelectrodes. The bonding was formed by pressing SWNTs against the electrodes with a vibrating press at an ultrasonic frequency. With this technology, low-resistance contacts are achieved between both metallic and semiconducting SWNTs and electrodes. After bonding, the effective Schottky barrier height between semiconducting SWNT and Ti electrode is as low as ∼6.6 meV in the ON-state and the barrier width is ∼0.9 nm at V_g = 0. The performance of carbon nanotube field-effect transistors (FETs) fabricated by this ultrasonic bonding technique is also significantly improved, with a transconductance as high as 3.4 μS for solid-state back-gate individual nanotube FETs.     

The development and characterisation of polyaniline—single walled carbon nanotube composite fibres using 2-acrylamido-2 methyl-1-propane sulfonic acid (AMPSA) through one step wet spinning process

High strength, flexible and conductive polyaniline (PANi)-carbon nanotube (SWNT) composite fibres have been produced using wet spinning. The use of dichloroacetic acid (DCAA) containing 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPSA) has been shown to act as an excellent dispersing medium for carbon nanotubes and for dissolution of polyaniline. The viscosity of DCAA-AMPSA solution undergoes a transition from Newtonian to non-Newtonian viscoelastic behaviour upon addition of carbon nanotubes. The ultimate tensile strength and elastic modulus of PANi-AMPSA fibres were increased by 50 and 120%, respectively, upon addition of 0.76% (w/w) carbon nanotubes. The elongation at break decreased from 11 to 4% upon addition of carbon nanotubes, however, reasonable flexibility was retained. An electronic conductivity percolation threshold of ∼0.3% (w/w) carbon nanotubes was determined with fibres possessing electronic conductivity up to ∼750 S cm⁻¹. Raman spectroscopic evidence confirmed the presence of carbon nanotubes in the polyaniline and also the interaction of the quinoid ring with the nanotubes to provide a doping effect.     

A simple route to short cup-stacked carbon nanotubes by sonication

A simple but effective way of producing short cup-stacked carbon nanotubes with high quality is demonstrated using sonication in an ethanol/distilled water mixture. The substantial removal of amorphous carbon deposited on the truncated conical layer-stacked nanotubes by controlled air oxidation was a critical precondition to cut the nanotubes without substantial structural degradation. The short nanotubes (below 1 μm) prepared by a sonication-generated shear force exhibited a narrow length distribution (median length = 0.2 μm) and chemically active edges in the outer surface, thereby resulting in high dispersibility in an aqueous solution.     

Buckling of functionalized single-walled nanotubes under axial compression

Buckling characteristics of functionalized single-walled carbon nanotubes under axial compression are investigated by molecular mechanics simulation. The influences of the content, the distribution density and the location of the sp³-hybridized carbon atoms as well as the chirality on the critical buckling strains of functionalized single-walled carbon nanotubes are carefully studied. The results indicate that the chirality and the distribution density have dominant effect on the critical buckling strains. The critical buckling strains of present armchair (5, 5) and zigzag (10, 0) carbon nanotube are degraded by about 43% and 70%, respectively, due to the dense distribution of the sp³-hybridized carbon atoms. The reduction amplitude of the critical strain increases with increasing the tubule radius of an armchair or zigzag single-wall carbon nanotube. The dramatic reduction of the critical strain could cause a great loss of reinforcing role of carbon nanotubes in composites.     

Thermally activated model for tensile yielding of pristine single-walled carbon nanotubes with nonlinear elastic deformation

A thermally activated model for evaluating the tensile yield strain of pristine single-walled carbon nanotubes (SWCNTs) is established. Using a parabolic function to accurately describe the dependence of stress on strain, we derive a yield function relating the yield strain to temperature, strain rate and tube length. The activation energy and activation area are then determined from the MD results of the tensile yield strain as a function of temperature. We find that the activation energies for armchair SWCNTs range from 7.18 to 11.94 eV, depending on the radius of SWCNTs. Analyses of activation area and MD results reveal that the nucleation of a critical defect, which leads to the failure of SWCNTs, grows from a single bond size at 300 K to almost twice the size at 2100 K. On the basis of activation parameters, our model can be used to predict the yield strain of SWCNTs under experimental conditions.