Control of the single-wall carbon nanotube mean diameter in sulphur promoted aerosol-assisted chemical vapour deposition

The influence of gas flow on nanotube diameter during the synthesis of high-purity, very long single-wall carbon nanotubes (SWCNT) via aerosol-assisted chemical vapour deposition is reported. The sample morphology, nanotube yield, defect concentration and amount of carbonaceous impurities, as well as the mean diameter and the diameter distribution of the SWCNTs were analysed by combined scanning- and transmission electron microscopy, Fourier Transform Raman spectroscopy and optical absorption spectroscopy. The results show that by using a solution of ferrocene and sulphur in m-xylene the addition of sulphur as a promoter was found to enhance the SWCNT growth and to increase the yield. A reduction of the mean diameter and a change in the diameter distribution are observed when the total gas flow is increased.     

1D-confinement of polyiodides inside single-wall carbon nanotubes

1D-confinement of polyiodides inside single-wall carbon nanotubes (SWCNT) is investigated. Structural arrangement of iodine species as a function of the SWCNT diameters is studied. Evidence for long range one dimensional ordering of the iodine species is shown by X-ray and electron diffraction experiments independently of the tube diameter. The structure of the confined polyiodides is investigated by X-ray absorption spectroscopy. The confinement influences the local arrangement of the chains. Below a critical diameter Φ_c of 1 nm, long linear polyiodides are evidenced leading to a weaker charge transfer than for nanotube diameter above Φ_c. A shortening of the polyiodides is exhibited with the increase of the nanotube diameter leading to a more efficient charge transfer. This point reflects the 1D-confinement of the polyiodides inside the nanotubes.     

A theoretical investigation of the effect of adsorbed NO2 molecules on electronic transport in semiconducting single-walled carbon nanotubes

We investigated the variation of electronic transport in a semiconducting single-walled carbon nanotube (SWCNT) due to NO₂ molecules adsorbed on different locations of the SWCNT surface through a theoretical means. Our results indicate that the electronic conductance of a semiconducting SWCNT strongly depends on the distance between the electrode and the NO₂ adsorption site. In some cases, the conductance variation can reach ten fold. Negative differential resistance behavior is identified in the current–voltage characteristics when a NO₂ molecule is adsorbed on the surface of a semiconducting SWCNT. These phenomena originate from the interaction between the electrode and the NO₂ due to the finite length of the SWCNT. A NO₂ sensing mechanism of an SWCNT-based chemical sensor at low NO₂ concentration was proposed. Several examples of this SWCNT-based sensor adsorbed with two NO₂ molecules were used to demonstrate the complicated conductivity variation when the multiple NO₂ adsorptions take place in an SWCNT-based NO₂ gas sensor.     

Crosstalk analysis of carbon nanotube bundle interconnects

ABSTRACT: Carbon nanotube (CNT) has been considered as an ideal interconnect material for replacing copper for future nanoscale IC technology due to its outstanding current carrying capability, thermal conductivity, and mechanical robustness. In this paper, crosstalk problems for single-walled carbon nanotube (SWCNT) bundle interconnects are investigated; the interconnect parameters for SWCNT bundle are calculated first, and then the equivalent circuit has been developed to perform the crosstalk analysis. Based on the simulation results using SPICE simulator, the voltage of the crosstalk-induced glitch can be reduced by decreasing the line length, increasing the spacing between adjacent lines, or increasing the diameter of SWCNT.     

Effect of supra-molecular microstructures on the adhesion of SWCNT fiber/iPP interface

Carbon nanotube (CNT) fibers are a novel type of fibrous materials that show potential in polymeric composite fields. In this study, we investigated the interfacial behavior of a single single-walled carbon nanotube (SWCNT) fiber embedded in isotactic polypropylene (iPP) matrix. The SWCNT fibers were found to be able to act as a heterogeneous nucleating agent which inducing the formation of transcrystals around the fiber surface. According to the theory of heterogeneous nucleation, the interfacial free energy difference Δσ of iPP on the SWCNT fibers was determined and compared with that on the conventional fibers. By carefully controlling the crystallizing conditions, three types of α-iPP supra-molecular microstructures with different optical birefringences were obtained. Raman spectra were utilized to investigate the influences of the supra-molecular microstructures of the transcrystalline (tc) layer on the strain transfer efficiency from the matrix to the fibers at a microscopic level. Conventional single-fiber pull-out tests were further employed to compare with the results derived from the Raman tests.     

Synthesis and Surface Tension Study of the Spacer Chain Length Effect on the Adsorption and Micellization Properties of a New Kind of Carboxylate Gemini Surfactant

A series of carboxylate gemini surfactants, which contain two hydrocarbon chains linked by amide groups, two carboxylate groups, a flexible alkane spacer were synthesized by three-step reactions and named alkylidene–bis-(N,N′-dodecyl-carboxypropylamides) (2C₁₂H₂₅CnAm; n = 2, 3, 4, 6, 8 is the number of methylene groups of the spacer), their structures were confirmed by FTIR,¹H NMR, and LC–MS/TOF, and their purity checked by HPLC. The micellar properties with increasing spacer chain length of these gemini surfactants were determined by surface tension methods. The critical micelle concentration (CMC) varies slightly with spacer chain length; surface tension at CMC(γ_CMC), the tendency of micellization versus adsorption, CMC/C₂₀, the minimum area per surfactant molecule at the air/solution interface (A_CMC), all decrease with increasing spacer chain length; surface reduction efficiency, pC₂₀, the surface excess at the air/solution interface (Г_CMC) increase with increasing spacer chain length. The results probably indicate that increasing spacer chain length of these carboxylate gemini surfactants will increase spacer incorporation into the double hydrophobic chain.     

Solution properties of polymacromonomers consisting of polystyrene

Summary Dynamic light scattering measurements have been made on 14 samples of a polymacromonomer consisting of polystyrene with 15 styrene side-chain units in cyclohexane at 34.5°C (the theta point) to determine the translational diffusion coefficient D as a function of molecular weight. The dependence of D on the main-chain length is analyzed on the basis of the wormlike chain by taking into account the end effect arising from side chains near the main-chain ends. The model parameters describing this dependence, i.e., the Kuhn segment length (11.5 ± 1.5 nm), the linear mass density (5600 ± 700 nm⁻¹), the diameter (5.2 ± 0.5 nm), and the end-effect parameter δ (2.5 ± 0.3 nm), are close to those determined previously from <S ²>_z (the z-average mean-square radius of gyration) and [η] (the intrinsic viscosity), leading to the conclusion that the wormlike chain model is capable of consistently explaining <S ²>_z, [η], and D of the polymacromonomer in the Θ solvent. Received: 8 February 2000/Accepted: 18 February 2000     

Fabrication and characterization of single-walled carbon nanotube fiber for electronics applications

We present a customized technique to spin fiber from unique single-wall carbon nanotube (SWCNT) films utilizing a motorized pulling/twisting stage. The manufactured SWCNT fibers’ diameter ranged from 30 to 130 μm. Electrical measurements show fusing current of 1–200 mA – depending on the spun fiber’s diameter – which is close to the values known for copper wires with similar diameter. These results reveal that the fiber spinning process could retain most of the advantageous electrical properties of original nanotubes, and also indicate a good possibility for using these low cost, sustainable SWCNT fibers in electrical wiring applications.     

Structure–property–processing relationships of single-wall carbon nanotube thin film piezoresistive sensors

In an investigation of structure–property–processing relationships for SWCNT thin film piezoresistive sensors, the gauge factor of the sensors for a small tensile deformation (less than 2% strain) was found to be close to unity and showed negligible dependence on the film thickness and SWCNT bundle length (L) and diameter (d). However, for a large tensile deformation (20–30% strain), the film thickness and the microstructure of SWCNTs had a compounding effect on the piezoresistive behavior. A gauge factor of ∼5 was obtained for the sensors fabricated with SWCNT bundles of short length and thin diameter (L = 549 nm and d = 3.7 nm) with thicker films. Furthermore, the gauge factor of the sensors was found inversely proportional to the excluded volume V_ex of SWCNT bundles (V_ex ∝ 1/L² d).     

Hydrogen adsorption on single-walled carbon nanotubes studied by core-level photoelectron spectroscopy and Raman spectroscopy

We have investigated the adsorption of atomic hydrogen on vertically aligned carbon nanotube (CNT) films using in situ synchrotron-radiation-based core-level (CL) photoelectron spectroscopy and Raman spectroscopy. From C 1s CL spectra, we identified a CL peak component due to C-H bonds of carbon atoms in single-walled carbon nanotubes (SWCNTs). We also found the suppression of π-plasmon excitation, indicating that the hydrogen adsorption deforms the bonding structure. Raman spectra of the SWCNT film indicated that the radial-breathing-mode intensities of SWCNTs decreased due to the adsorption-induced bonding-structure deformation. Moreover, the decrease for small-diameter SWCNTs was more severe than that for large-diameter SWCNTs. Our results strongly suggest that the hydrogen adsorption, which induces the structure deformation from sp² to sp³-like bonding, depends on the diameter of SWCNTs.     

Controlling the thickness of carbon nanotube random network films by the estimation of the absorption coefficient

Here, we investigate the thickness of single-walled (SWCNT) and multi-walled carbon nanotube (MWCNT) random network films by angle-resolved X-ray photoemission spectroscopy. Furthermore, we estimate the absorption coefficient of carbon nanotube (CNT) films through the Lambert–Beer law, by measuring film optical spectra. Moreover, the knowledge of the absorption coefficient provides an easier, reliable, and faster method of investigation for generic CNT film thickness. In addition, the absorption coefficient leads to the information of the absorption length for SWCNT and MWCNT films, which is a physical quantity of fundamental interest for optoelectronic applications, such as light emitting diodes, photovoltaics, and in general light absorbers.     

Using oxidation to increase the electrical conductivity of carbon nanotube electrodes

Recent studies have demonstrated that significantly low sheet resistance (Rs) (<100 Ω/sq; comparable to ITO) were achieved in single-walled carbon nanotube (SWCNT) films treated with HNO₃ followed by thionyl chloride. Here we show that H₂SO₄ can effectively reduce the Rs of SWCNT electrodes. H₂SO₄ treatment generates defects (COOH and SO₃H functionalities) on SWCNTs and the produced chemical functionalities are beneficial for enhancing the electrical conductivity in SWCNT electrodes. It is plausible that the H₂SO₄p-dopes the SWCNTs and the attachment of chemical functionalities helps to stabilize p-doping owing to their electron-deficient property.     

Synthesis,characterization, and photophysical properties of covalent-linked ferrocene–porphyrin–single-walled carbon nanotube triad hybrid

The ferrocene–porphyrin–single-walled carbon nanotube (Fc–H₂P–SWCNT) triad hybrid was prepared by amidation reaction between carboxylated SWCNT and aminoporphyrin bearing an appended ferrocenyl substituent. The hybrid described here was fully characterized by a combination of analytical techniques such as Fourier transform infrared spectroscopy, Raman, absorption and emission spectroscopy, atomic force and scanning electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The steady emission characteristics revealed the existence of the effective photoinduced electron transfer among ferrocene, excited porphyrin moiety and SWCNT, which was further confirmed by the results of time-resolved transient absorption spectra. The final lifetime of charge-separation state was observed to be 62.9 μs in N,N-dimethylformamide, which was significant increased compared to the reference nanohybrid porphyrin–SWCNT and the reported ferrocene–porphyrin–fullerene triad. Therefore, Fc–H₂P–SWCNT triad hybrid constructed by amidation is rationally expected to be an improved photon-to-electron conversion system.     

Bipolar diffusion charging characteristics of single-wall carbon nanotube aerosol particles

Bipolar diffusion charging characteristics of airborne single-wall carbon nanotube (SWCNT) agglomerates were investigated in the mobility diameter range of 100–1000 nm. Neutral fractions of three types of SWCNT aerosols following bipolar charge equilibrium in a radioactive source were experimentally measured to infer their electrical charging characteristics. Significant deviation from Boltzmann and Fuchs stationary charge equilibrium was observed, with neutral fractions of SWCNT particles lower by 30–53% compared to that of spherical particles of the same mobility. Particles with mobility diameter larger than 400 nm showed high electrical charging efficiencies compared to that of mobility-equivalent spherical particles. Higher charging efficiencies of SWCNT particles were attributed to their higher electrical capacitance resulting from complex nonspherical morphologies. Numerical calculations using idealized fiber geometries confirmed the qualitative trend in the experimental data. The electrical capacitance of nanotubes particles deduced from experimentally measured neutral fractions were also found to be higher by a factor ranging from 1.6 to 4.6 compared to that of mobility-equivalent spherical particles, indicating high charge carrying capacity. The charging-equivalent diameters of nanotube particles were computed and were found to be higher than their mobility diameter by a factor of 2.85–4.34.     

Advanced carbon nanotube/polymer composite infrared sensors

We demonstrate that both single-walled carbon nanotube (SWCNT) types and nanotube-matrix polymer-nanotube (CNT-P-CNT) junctions have profound impact on electro-optical properties of SWCNT/polymer composites. Composite IR sensors based on CoMoCAT^®-produced SWCNTs (SWCNTs_CoMoCAT) significantly outperform those based on HiPco^®-produced SWCNTs (SWCNTs_HiPco). Higher semiconducting nanotube concentration in a SWCNT material is critical to enhance the photo effect of IR light on SWCNT/polymer nanocomposites, whereas CNT-P-CNT junctions play a dominant role in the thermal effect of IR light on supported SWCNT/polymer composite films.     

“动态超分子”假说预测烷烃吸收CO2相平衡

采用“动态超分子假说”对烷烃吸收CO₂相平衡进行了研究,并结合最小二乘法和试算对烷烃数据进行拟合,得到相关参数,拟合值和文献实验值的线性相关度均大于0.95,符合良好。研究表明,烷烃分子之间形成的同种“动态超分子”对吸收过程影响较大,远远大于CO₂同种超分子和溶剂与CO₂之间形成的异种超分子,而且其随着烷烃碳链增加,“动态超分子”形成概率和稳定性均降低,吸收能力逐渐增强。文中还研究了模型参数对拟合结果的影响,及其最佳取值范围。     

Low electrical conductivity threshold and crystalline morphology of single-walled carbon nanotubes - high density polyethylene nanocomposites characterized by SEM, Raman spectroscopy and AFM

The electrical conductivities (σ) of nanocomposites of single-walled carbon nanotubes (SWCNTs) and high density polyethylene (HDPE) have been studied for a large number of nanocomposites prepared in a SWCNT concentration range between 0.02 and 8 wt%. The values of σ obey a percolation power law with an SWCNT concentration threshold, p_c = 0.13 wt%, the lowest yet obtained for any kind of carbon-polyethylene nanocomposites. Improved electrical conductivities attest to an effective dispersion of SWCNT in the polyethylene matrix, enabled by the fast quenching crystallization process used in the preparation of these nanocomposites. Characterization by scanning electron microscopy (SEM) and Raman spectroscopy consistently points to a uniform dispersion of separate small SWCNT bundles at concentrations near p_c and increased nanotube clustering at higher concentrations. Near p_c, high activation energies and geometries of long isolated rods suggest that electron transport occurs by activated electron hopping between nanotubes that are close to each other but still geometrically separate. The degree of SWCNT clustering given by Raman spectroscopy and the barrier energy for electrical conductivity are highly correlated. The nanotubes act as nucleants in the crystallization of the polyethylene matrix, and change the type of supermolecular aggregates from spherulites to axialitic-like objects. The size of crystal aggregates decreases with SWCNT loading, however, in reference to the unfilled polyethylene, the three-dimensional growth geometry extracted from the Avrami exponents remains unchanged up to 2 wt%. Consistency between SEM, Raman and electrical transport behavior suggests that the electrical conductivity is dominated by dispersion and the geometry of the SWCNT in the nanocomposites and not by changes or lack thereof in the HDPE semicrystalline structure.     

Lyotropic liquid crystal formation of polystyrene polymacromonomers in dichloromethane

Liquid crystallinity of dichloromethane (DCM) solutions of five samples of polymacromonomer F65 consisting of 65 styrene residues in each side chain was studied by birefringence observation and phase separation experiments at different temperatures in a molecular weight range from 9.4 × 10⁵ to 4.1 × 10⁶. Dilute-solution characterization was also made by light scattering and viscometry in DCM at 20 °C. The polymer concentration c_I on the phase boundary between the isotropic and biphasic regions was lower than the previously determined c_I for a polymacromonomer with a shorter side-chain length of 33 styrene residues at the same molecular weight, reflecting the higher chain stiffness and larger diameter of the F65 polymer. The molecular weight dependence of c_I for the two polymacromonomers (at 20 °C) was explained almost quantitatively by the scaled-particle theory for worm-like cylinders with the model parameters (the Kuhn length, the linear mass density, and the chain diameter) describing gyration radius and intrinsic viscosity data in DCM but without chain-end effect. It was concluded that this theory is capable of predicting the phase boundary concentration of brush-like polymers with essentially the same degree of accuracy as that known for linear, stiff chains.     

Single-walled carbon nanotube/silicone rubber composites for compliant electrodes

Single-walled carbon nanotube (SWCNT)/silicone rubber composites that can be used in fabricating compliant electrodes are prepared by spraying a mixed solution of ionic-liquid-based SWCNT gel and silicone rubber onto an elastic substrate. Subsequently, the composites are exposed to nitric acid vapor. Scanning electron microscopy and atomic force microscopy images of the composites show that the SWCNTs are finely dispersed in the polymer matrix due to the addition of the ionic liquid. Doping of the SWCNTs by nitric acid can significantly lower the sheet resistance (R_s) of the composites; samples with 4 wt% of SWCNT content exhibit the lowest R_s value (50 Ω sq^?1). This sheet resistance corresponds to a conductivity value of 63 S cm^?1. In addition, the composites retain a high conductivity after several tensile strains are applied. Stretching the composite sample to 300% of the original length increased the R_s value to 320 Ω sq^?1 (19 S cm^?1). Even after 20th stretch/release/stretch cycle, the conductivity remains constant at a value of 18 S cm^?1. These results provide a scalable route for preparing highly stretchable and conductive SWCNT composites with relatively low SWCNT concentrations.     

Thermodynamic parameters of poly(lactic acid) solutions in dialkyl phthalate

Some of thermodynamic parameters for poly(lactic acid) (PLA)/dialkyl phthalate systems have been investigated. Both poly(dl-lactic acid) (PDLLA) and poly(l-lactic acid) (PLLA), which are stereoisomers of PLA, were used and a series of 1,2-dialkyl phthalates with different alkyl chain, from methyl to hexyl, was adopted as a solvent. Theta temperatures of PLA/dialkyl phthalate system were determined and subsequently the second virial coefficient and the interaction parameter were evaluated. Theta temperature was determined by the extrapolation of the highest liquid-liquid phase separation temperatures and Zimm plots were constructed by static light scattering to obtain second virial coefficient and z-average radius of gyration. Second virial coefficient and z-average radius of gyration was examined quantitatively as a function of temperature and solvent. Thermodynamic parameters that could not be obtained experimentally were calculated based on the Flory-Huggins theory.