Optical anisotropy of single walled carbon nanotubes investigated by spectroscopic ellipsometry

The optical properties of single walled carbon nanotube (SWCNT) films, produced by vacuum filtration, are correlated with their diameter and their in plane preferred orientation by coupling transmission and scanning electron microscopy to conventional ellipsometry. We focused on the optical anisotropy of this material and we demonstrated that it was originated from a breakdown in the selection rules.     

Characterization of single wall carbon nanotubes by nonane preadsorption

The preferential blocking of the interior adsorption sites of single walled carbon nanotubes (SWNTs) by n-nonane is demonstrated. Following adsorption of n-nonane and evacuation for 24 h at 323 K, it was found that interior sites with diameters less than ∼14 Å remained filled with n-nonane, blocking the physical adsorption of N₂ on these sites at 77.3 K. We demonstrate that “nonane blocking” is a very useful technique for nanotube porosity characterization.     

Liquid crystal behavior of single wall carbon nanotubes

Single wall carbon nanotubes are dispersed in water with the water-soluble polymer polyvinylpyrrolidone and the surfactant sodium dodecylbenzene sulfonate, and then deposited by evaporative deposition onto degeneratively-doped silicon wafer substrates. These deposits were examined by scanning electron microscopy, which revealed highly-ordered arrays of large single wall carbon nanotube bundles. Various solution concentrations were prepared and deposition conditions were varied to determine their affect on the single wall carbon nanotube arrays. These observations were related to existing lyotropic liquid crystal theory and theories explaining the behavior of carbon nanotubes in solution, which allowed for further development and interpretation of the phase diagram which describes the behavior of single wall carbon nanotubes in lyotropic liquid crystal systems, and how competing liquid crystal systems in the same solution directly affect the ordering of the single wall carbon nanotube arrays.     

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.     

Raman spectroscopy on isolated single wall carbon nanotubes

A review is presented on the resonance Raman spectra from one isolated single wall carbon nanotube. The reasons why it is possible to observe the spectrum from only one nanotube are given and the important structural information that is provided by single nanotube spectroscopy is discussed. Emphasis is given to the new physics revealed by the various phonon features found in the single nanotube spectra and their connection to spectra observed for single wall nanotube bundles. The implications of this work on single wall carbon nanotube research generally are also indicated.     

Oriented and exfoliated single wall carbon nanotubes in polyacrylonitrile

Polyacrylonitrile (PAN)/single wall carbon nanotubes (SWNT) fibers were gel spun at 0, 0.5, and 1 wt% SWNT content to a draw ratio of 51. Structure, morphology, and mechanical and dynamic mechanical properties of these fibers have been studied. PAN/SWNT composite exhibited much higher electron beam radiation resistance than PAN. As a result, PAN lattice images could be easily observed in the composite fiber by high resolution transmission electron microscopy. The PAN/SWNT composite fiber also exhibited higher solvent resistance than the control PAN fiber. UV-vis spectroscopy of highly drawn fiber exhibited van Hove transitions, suggesting SWNT exfoliation upon drawing. SWNT exfoliation was also confirmed by high resolution transmission electron microscopy (HRTEM). At 1 wt% SWNT loading, fiber storage modulus (at 1 Hz) increased by 13.9, 6.6, and 0.2 GPa at −75, 25, and 150 °C, respectively. This suggests that the load transfer ability and hence interfacial strength is increasing with decreasing temperature, even below the polymer's γ transition temperature.     

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.     

Investigation of ionomers as dispersants for single wall carbon nanotubes

A novel conjugated ionomer was prepared from a diamine and a bis(pyrylium salt). Single-walled carbon nanotubes (SWNT) were dispersed in solutions of the ionomer in N,N-dimethylacetamide resulting in homogenous suspensions or quasi-solutions. These suspensions were used to cast unoriented thin films. In addition, the ionomer/SWNT solutions were used to aid in the dispersal of SWNTs in a soluble, low color polyimide. The use of the ionomer as a dispersant enabled the nanotubes to be dispersed at loading levels up to 1 wt% in a polyimide solution without visual agglomeration. SWNTs were well dispersed in the thin films as evidenced by visual inspection, optical microscopy, and high resolution scanning electron microscopy. The films were further characterized for their electrical and mechanical properties.     

Synthesis of PEGylated single wall carbon nanotubes by a photoinitiated graft from polymerization

A considerable amount of research has been devoted to carbon nanotubes because of their unique electrical, mechanical, optical, and chemical properties. Here, in this report, we introduce a novel, simple ultraviolet initiated “graft from” polymerization method to synthesize PEGylated carbon nanotubes. This grafting procedure significantly enhanced nanotube aqueous dispersibility and long term stability in solution. Mass of grafted polymer chains was easily modulated by adjusting polymerization reaction time, and nanomaterials containing up to 80% polymer by weight were synthesized. Nanotube morphology was characterized by SEM, TEM before and after the functionalization. In addition, the covalent bonding of polymer chains to the nanotubes structure was elucidated by Raman, ATR‐FTIR, and XPS spectroscopy. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Raman characterization of single wall carbon nanotubes prepared by the solar energy route

A Raman scattering characterization is reported that confirms the preparation of single wall carbon nanotubes (SWNT) by the solar energy route. The results are presented for samples synthesized with various catalysts—mixtures of Ni and Co (Y, La)—and compared to those obtained from electric arc discharge or laser ablation. In the light of the calculations of the vibrational spectra of SWNT by Eklund et al. (Carbon, 1995, 33, 959) it is shown that both the diameter and structure of the nanotubes depend strongly on the synthesis conditions. For the first time the presence of nanotubes with “zigzag” or “chiral” helical pitches for some of the samples are shown as well as a large distribution of tube diameters.     

Synthesis,characterization and field emission of single wall carbon nanotubes

In this work, high-quality SWCNTs were synthesized by pyrolysis of ethanol at 900 °C in a tubular furnace. The catalyst was manufactured by depositing Fe and Mo metal on the surface of sol–gel MgO nanopowder. The SWCNTs produced by sol–gel MgO nanopowder have better yield, higher purity, and smaller diameters than those produced by commercial milling MgO powder. The lengths of SWCNTs are up to several microns with diameters in the range of 0.73–1.40 nm. The extremely small turn-on field E_to (0.01 V/μm) and threshold field E_th (0.07 V/μm) show that the SWCNTs have potential applications in field emission displays (FEDs).     

羧基化高纯单壁碳纳米管修饰电极对乙酰甲喹的高灵敏检测

采用羧基化高纯单壁碳纳米管(c SWCNTs)固载于金电极表面来构筑乙酰甲喹电化学传感器。运用循环伏安法(CV)、差分脉冲伏安法(DPV)和电化学交流阻抗法(EIS)测定传感器的性能。结果显示,该传感器对乙酰甲喹的电化学还原具有显著的催化性能,修饰电极的峰电流达到-128.40μA,是裸电极峰电流的1167倍。通过对传感器性能影响因素的考察,得到最优制备条件为cSWCNTs涂覆量为4μL(质量分数0.10%分散液),搅拌速度1000 r/min,pH=7.0的PBS浓度为0.2 mol/L,-0.5V电压下富集30 min。在该条件下制备的传感器峰电流与乙酰甲喹在1～500nmol/L范围内呈良好的线性关系,相关系数为0.998,方法的检出限为0.76nmol/L。该传感器被成功地应用于实际样品检测,加标回收率在81.7%～124.0%。     

Matrix mediated alignment of single wall carbon nanotubes in polymer composite films

Alignment of single wall carbon nanotubes (SWNT) in liquid crystalline (LC) polymer matrix imparting orientation to the nanotubes along the nematic director was studied by atomic force microscopy, measurements of electrical conductivity and Raman spectroscopy of the composite in the directions parallel and perpendicular to the nematic director. The composites were prepared through dispersion of SWNT with LC monomer in a common solvent, their alignment in nematic monomer and consequent UV polymerization of the monomer. The anisotropy of electrical and optical properties of the system depends strongly on the concentration of the nanotubes in the range of 1-10% SWNT being especially strong for smaller concentrations and negligible at higher loads. A simple semi-quantitative model is suggested to account for the orientational behavior of nanotubes in nematic matrices. It successfully describes the observed anisotropy of physical properties at microscale (up to 200 μm) in terms of anchoring of the polymer chains to the nanotubes surface and adjustment of the nanotubes orientation to the nematic direction due to such coupling. The increasing disorientation of the nematic domains at higher nanotubes loads is explained as a development of larger number of LC defects induced by the nanotubes in the nematic matrix due to their intrinsic nature of aggregation. The anisotropy of physical properties at macro scale (several millimeters) is much smaller and less dependable on SWNT concentration because differently oriented LC domains effectively wash out the anisotropy.     

Electronic and magnetic properties of deformed and defective single wall carbon nanotubes

The combined effect of radial deformation and defects on the properties of semiconducting single wall carbon nanotubes are studied using density functional theory. A Stone-Thrower-Wales defect, a substitutional nitrogen impurity, and a mono-vacancy at the highest curvature side of a radially strained nanotube are considered. The energies characterizing the deformation and defect formation, the band gap energies, and various bond lengths are calculated. We find that there is magneto-mechanical coupling behavior in the nanotube properties which can be tailored by the degree of radial deformation and the type of defect. The carbon nanotube energetics and magnetism are also explained in terms of electronic structure changes as a function of deformation and types of defects present in the structure.     

High temperature rearrangement of disordered nanoporous carbon at the interface with single wall carbon nanotubes

Composites of nanoporous carbon and single wall carbon nanotubes were heat treated in vacuum at temperatures ranging from 1200 to 2000 °C. The resultant interface between the two allotropes of carbon was characterized using high resolution transmission electron microscopy and Raman spectroscopy. At the interface between the nanoporous carbon and the nanotube, the nanotube served as a template for ordering and orientation of the normally disordered nanoporous carbon along the nanotube axis during high temperature treatment. When annealed at 2000 °C, the nanoporous carbon transformed to graphitic nanoribbon which in turn crushed the nanotube to form a nanoscale carbon “bulb”. This result is interesting since at these temperatures, the native nanoporous carbon is well known to resist ordering and is therefore referred to as being a “non-graphitizing” carbon. That the nanotube should act as a template for the incipient graphitization suggests that bonding and strength for load transfer may be developed at these interfaces.     

Morphology of composite particles of single wall carbon nanotubes/biodegradable polyhydroxyalkanoates prepared by spray drying

Spray drying was investigated as a strategy for producing single wall carbon nanotube (SWCNT)/polymer composites. The spray-drying method produced SWCNT/poly(3-hydroxybutyrate) and SWCNT/poly(3-hydroxyoctanoate) composite particles in which the SWCNTs have been trapped in a well-dispersed state throughout the polymer matrix. Increasing SWCNT content in the composite led to a change in particle morphology from spherical and smooth to rosette shape with angular distortions. The technique shows potential for bulk carbon composite fabrication.     

Effect of plasma modification of single wall carbon nanotubes on ethanol vapor sensing

The oxygen and fluorine modified single wall carbon nanotubes (SWCNTs) were manufactured by microwave plasma enhanced chemical vapor deposition (MPECVD), and were developed as novel gas sensor materials. The sensor characteristic has shown a p-type response with resistance enhancement upon exposure to 100 ppm ethanol at room temperature. Oxygen plasma modification can increase the sensor response from 1.13 to 1.74 on process duration of 30 s due to the apparent elimination of amorphous carbon, as demonstrated by FESEM and Raman results. However, oxygen plasma modification has no effective assistance in decreasing the response and recovery time. By applying fluorine plasma modification, the sensor response only increases from 1.13 to 1.51 on process duration of 60 s, but the response and recovery time can decrease apparently from 178 to 54 s and 364 to 97 s due to the existence of numerous fluorine-included functional groups, as demonstrated by XPS and AES results. Therefore, the plasma modified SWCNT can elevate the sensitivity and reactivity for room temperature ethanol sensing.     

Density functional study of the metallization of a linear carbon chain inside single wall carbon nanotubes

By means of the density functional theory, we studied the relaxed structure and electronic properties of new one-dimensional carbon nanostructures conformed by a linear carbon chain (LCC) inside (5,5) and (8,0) single-walled carbon nanotubes (SWCNTs). The calculations were performed with a linear combination of atomic orbitals method using pseudopotentials and the generalized gradient approximation for the exchange-correlation potential. We analyzed the atomic structure, band structure, and the local density of states. We found that, despite the fact that LCC and (8,0) SWCNT have a band gap, the system LCC@(8,0) shows a metallic character. This metallic behavior is provided by the electronic states from the LCC exclusively, due to charge transfer from carbon nanotube to the LCC. However, the electronic characters of the nanotubes in LCC@SWCNT are the same as that of isolated SWCNTs.     

Deposition of functionalized single wall carbon nanotubes through matrix assisted pulsed laser evaporation

Single-wall carbon nanotubes (SWCNTs) functionalized with oxygen-containing groups were deposited onto glass substrates by matrix assisted pulsed laser evaporation (MAPLE). The experiments were performed by subjecting ultraviolet laser pulses (KrF¹ excimer laser, 248 nm wavelength) to frozen SWCNT-toluene targets placed in a parallel plane a few cm in front of the substrate. The morphology, structure, and chemical composition of the deposited materials were studied through atomic force microscopy, high resolution transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The influence of the laser fluence on the material structure was investigated. The results indicate that the functionalized SWCNTs can be transferred by MAPLE at low laser fluences without the alteration of the structure of the initial material used as targets in MAPLE experiments. An increase of the fluence leads to the decomposition of the functional groups, mainly carboxylic acid groups, without degradation of the SWCNT structure whereas, at the highest fluences, the amorphization and even coalescence of the carbon nanotubes takes place.