Vertical alignment of single-walled carbon nanotube films formed by electrophoretic deposition

Films of chemically shortened and functionalized single-walled carbon nanotubes (SWNTs) have been formed on a gold electrode by electrophoretic deposition. Applying ultrasonic energy resulted in dramatic changes of the film morphology; the deposited SWNT bundles reassembled and oriented normal to the electrode. Oriented SWNT bundles with high density (more than 250 bundles/microm (2)) not only presented narrow size distributions, but uniformly spread on the electrode. We discuss the mechanism of SWNT orientation by analyzing the variation in the film morphology with ultrasonication time. In addition, we suggest that the 3D displays of AFM images can lead to misjudgment of nanotube alignment. The method for aligning SWNTs normal to the electrode may be competitive with chemical vapor deposition or screen printing, the predominant methods by which vertically aligned SWNT films have been fabricated to date.     

Aligned single‐wall carbon nanotube polymer composites using an electric field

While high shear alignment has been shown to improve the mechanical properties of single‐wall carbon nanotube (SWNT)‐polymer composites, this method does not allow for control over the electrical and dielectric properties of the composite and often results in degradation of these properties. Here, we report a novel method to actively align SWNTs in a polymer matrix, which permits control over the degree of alignment of the SWNTs without the side effects of shear alignment. In this process, SWNTs were aligned via AC field‐induced dipolar interactions among the nanotubes in a liquid matrix followed by immobilization by photopolymerization under continued application of the electric field. Alignment of SWNTs was controlled as a function of magnitude, frequency, and application time of the applied electric field. The degree of SWNT alignment was assessed using optical microscopy and polarized Raman spectroscopy, and the morphology of the aligned nanocomposites was investigated by high‐resolution scanning electron microscopy. The structure of the field induced aligned SWNTs was intrinsically different from that of shear aligned SWNTs. In the present work, SWNTs are not only aligned along the field, but also migrate laterally to form thick, aligned SWNT percolative columns between the electrodes. The actively aligned SWNTs amplify the electrical and dielectric properties of the composite. All of these properties of the aligned nanocomposites exhibited anisotropic characteristics, which were controllable by tuning the applied field parameters. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1751–1762, 2006     

Single-walled carbon nanotube combing during layer-by-layer assembly: from random adsorption to aligned composites

Oriented SWNTs in polymer composites have shown dramatic improvements in the physical properties of a composite because of the anisotropic shape and properties of SWNTs. Controlled alignment of SWNTs during composite fabrication implies better material function performance. This letter reports a new fabrication technique whereby aligned SWNTs and robust SWNT-polymer composites can be made using a fusion method of SWNT combing and layer-by-layer (LBL) assembly. As we previously reported, LBL assembly demonstrated exceptional processing ability in constructing the uniform distribution of a SWNT-polymer composite. Combined with this uniformity, this SWNT combing technique endows controlled alignment of single-stranded SWNTs in a SWNT-polymer composite system. SWNT combing employs air-water interfacial forces to change the molecular topography from the random adsorption state to the stretched alignment of SWNTs. More specifically, air-water interfacial forces are associated with an excess viscous drag force and an intrinsic dewetting rate along SWNTs. Moreover, the alignment efficiency of SWNTs is high enough to construct a multilayered LBL film with horizontal-linear weaving structures. This simple method also can be applied for aligning other nanowire materials because it utilizes simple geometric features of SWNTs.     

Coagulation method for preparing single-walled carbon nanotube/poly(methyl methacrylate) composites and their modulus,electrical conductivity,and thermal stability

A coagulation method providing a better dispersion of single-walled carbon nanotubes (SWNTs) in a polymer matrix was used to produce SWNT/poly(methyl methacrylate) (PMMA) composites. Optical microscopy and scanning electron microscopy showed an improved dispersion of SWNTs in the PMMA matrix, a key factor in composite performance. Aligned and unaligned composites were made with purified SWNTs with different SWNT loadings (0.1–7 wt %). Comprehensive testing showed improved elastic modulus, electrical conductivity, and thermal stability with the addition of SWNTs. The electrical conductivity of a 2 wt % SWNT composite decreased significantly (>10⁵) when the SWNTs were aligned, and this result was examined in terms of percolation. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3333–3338, 2003     

Recent progress in chemical detection with single-walled carbon nanotube networks

Single-walled carbon nanotubes (SWNTs) have had significant impact on the development of gas sensors in the last decade. However, useful applications of SWNTs are limited by the lack of manufacturable routes to device formation. This Highlight article chronicles recent progress in this area and demonstrates the great promise of a new room temperature deposition method for SWNT networks in gas sensing applications. This liquid deposition technique allows the deposition of pre-treated, highly aligned SWNT networks on a wide variety of substrates. A significant advantage of SWNT-network sensors is that fluctuations in the electrical response of individual SWNTs become less important as the size of the network increases. Therefore, device properties can be controlled by the overall density of the network rather than the physical properties of any individual SWNT. At densities where semiconducting pathways dominate, highly sensitive thin-film chemoresistive sensors can be fabricated. Such devices also have higher signal-to-noise ratios and are easier to fabricate than devices based on a single SWNT.     

Rheological, thermal, and mechanical characterizations of polystyrene/single-walled carbon nanotube composites

For preparation of polystyrene (PS) composites, a polymeric dispersant, pyrene-capped polystyrene (PyPS), was applied for noncovalent functionalization of single-walled carbon nanotubes (SWNTs) to improve both dispersion quality and PS–SWNT interfacial interactions. To demonstrate the critical role of PyPS, the composites with the absence of PyPS (PS/SWNT) were also prepared for comparison. Rheological studies suggest that addition of SWNTs, particularly of PyPS-functionalized SWNTs, suppresses significantly large-scale relaxation of PS chains but has little effect on their short-range dynamics. Relative to PS, moderately improved thermal and mechanical properties took place on the composites with either pristine or PyPS-functionalized SWNTs. The PS/PyPS/SWNT composite usually presents better performance than the PS/SWNT one at a fixed SWNT content.     

Crystal plane dependent growth of aligned single-walled carbon nanotubes on sapphire

On single-crystal substrates, such as sapphire (alpha-Al 2O 3) and quartz (SiO 2), single-walled carbon nanotubes (SWNTs) align along specific crystallographic axes of the crystal, indicating that the SWNT growth is influenced by the crystal surface. Here, we show that not only the orientation, but also the diameter and chirality of SWNTs are affected by the crystal plane of the sapphire substrate. The aligned SWNTs grown on the A- and R-planes of sapphire have narrower diameter distributions than randomly oriented tubes produced on the C-plane sapphire and amorphous SiO 2. Photoluminescence measurements reveal a striking difference between the aligned SWNTs: near-zigzag tubes are observed on the A-plane and near-armchair tubes on the R-plane. This study shows the route for the diameter and chirality control of SWNTs by surface atomic arrangements of a single-crystal substrate.     

A biomimetic "polysoap" for single-walled carbon nanotube dispersion

As-synthesized single-walled carbon nanotubes (SWNTs) are bundled mixtures of different species. The current challenge in the field of carbon nanotube research lies in the processing and separation of SWNTs, which first require efficient dispersion of individual SWNTs in solvents. We report DNA-mimicking polysoap surfactants that disperse SWNTs in aqueous solutions more effectively than DNA. The polysoaps are synthesized by functionalizing the side chain of poly(styrene-alt-maleic acid) with aminopyrene. The synthetic nature of the polysoap opens a new approach to further optimization of not only SWNT dispersion efficiency but also multi-functional SWNT dispersing surfactant.     

Antenna chemistry with metallic single-walled carbon nanotubes

We show that, when subjected to microwave fields, surfactant-stabilized single-walled carbon nanotubes (SWNTs) develop polarization potentials at their extremities that readily drive electrochemical reactions. In the presence of transition metal salts with high oxidation potential (e.g., FeCl3), SWNTs drive reductive condensation to metallic nanoparticles with essentially diffusion-limited kinetics in a laboratory microwave reactor. Using HAuCl4, metallic particles and sheaths deposit regioselectively at the SWNT tips, yielding novel SWNT-metal composite nanostructures. This process is shown to activate exclusively metallic SWNTs; a degree of diameter selectivity is observed using acceptors with different oxidation potentials. The reaction mechanism is shown to involve Fowler-Nordheim field emission in solution, where electric fields concentrate at the SWNT tips (attaining approximately 10(9) V/m) due to the SWNT high aspect ratio (approximately 1000) and gradient compression in the insulating surfactant monolayer. Nanotube antenna chemistry is remarkably simple and should be useful in SWNT separation and fractionation processes, while the unusual nanostructures produced could impact nanomedicine, energy harvesting, and synthetic applications.     

Electric-field-enhanced assembly of single-walled carbon nanotubes on a solid surface

We report a novel electric-field-enhanced chemical assembly approach for fabricating highly aligned SWNT arrays on a solid surface with remarkably improved efficiency and packing density, which is very important for the real applications of carbon nanotube arrays. With the enhancement of the electric field, the assembling kinetics of SWNTs is remarkably speeded up to effectively decrease the assembling time, and the packing density can even exceed the saturated density of conventional assembly method by four times within only half an hour. The molecular dynamics simulation results illustrated the alignment of SWNTs with their long axes along the electric flux in solution, leading to the increase of packing density and efficiency through overcoming the steric hindrance of the "giant" carbon nanotubes.     

Chemical engineering of the single-walled carbon nanotube-nylon 6 interface

We report an approach to the chemical engineering of the single-walled carbon nanotube (SWNT)-polymer interfacial interaction in a nylon 6 graft copolymer composite which is based on the degree of SWNT functionality. Continuous fibers are drawn from composites fabricated from the in situ polymerization of caprolactam with SWNTs possessing a range of carboxylic acid (SWNT-COOH) and amide (SWNT-CONH(2)) functionalities. Mechanical performance evaluation of the composite fibers shows that a high concentration of the carboxylic acid functional groups leads to a stronger SWNT-nylon interfacial interaction, as reflected in greater values of the Young's modulus and mechanical strength. Replacement of the COOH group by CONH(2) in the SWNT starting material changes the grafting polymerization chemistry, thereby leading to the covalent attachment of longer graft copolymer chains to the SWNTs, and alters the composite morphology while increasing the composite flexibility and toughness.     

Light-controlled single-walled carbon nanotube dispersions in aqueous solution

We have succeeded in dispersing single-walled carbon nanotubes (SWNTs) into an aqueous solution of poly(ethylene glycol)-terminated malachite green derivative (PEG-MG) through simple sonication. It was found that UV exposure caused reaggregation of these predispersed SWNTs in the same aqueous medium, as adsorbed PEG-MG photochromic chains could be effectively photocleavaged from the nanotube surface. The observed light-controlled dispersion and reaggragation of SWNTs in the aqueous solution should facilitate the development of SWNT dispersions with a controllable dispersity for potential applications.     

Electrochemical templating of metal nanoparticles and nanowires on single-walled carbon nanotube networks

The use of single-walled carbon nanotube (SWNT) networks as templates for the electrodeposition of metal (Ag and Pt) nanostructures is described. Pristine SWNTs, grown on insulating SiO2 surfaces using catalyzed chemical vapor deposition, served as the working electrode. In the simplest case, electrical contact was made by depositing a gold strip on the SWNT substrate (device 1). Deposition of Ag and Pt over extensive periods (30 s) resulted in a high density of particles on the SWNTs, with almost contiguous nanowire formation from the Au/SWNT boundary moving to isolated nanoparticles at further distances from the contact. For direct electrochemical studies of Ag and Pt nucleation, the assembly was coated in a resist layer and a small window opened up to expose only the electrically connected SWNTs to solution (device 2). In this case, the electrochemical signature in voltammetric and amperometric studies of metal deposition was due solely to processes at the SWNTs. Coupled with high-resolution microscopy measurements (atomic force microscopy and field emission scanning electron microscopy), this approach provided detail on the nucleation and growth mechanisms of Ag and Pt on SWNTs under electrochemical control. In particular, Ag growth was found to be rapid and progressive with an increasing nanoparticle density with time, whereas Pt deposition was characterized by lower nucleation densities and slower growth rates with a tendency for larger particles to be produced over long times.     

Controlled confinement and release of gases in single-walled carbon nanotube bundles

A simple procedure is described that locks small quantities of SF6, CO2, and 13CO2 into opened single-walled carbon nanotube (SWNT) bundles and keeps the gas in the SWNTs above the desorption temperature of these molecules. The technique involves opening the SWNTs with ozonolysis at 300 K followed by vacuum-annealing at 700 K. Gases are then cryogenically adsorbed into the opened SWNTs and locked into the SWNT pores by functionalizing the sample with a low-temperature ozone treatment. The low-temperature ozone treatment functionalizes the entry ports into the SWNT pores, which in turn create a physical barrier for gases trying to desorb through these functionalized ports. The samples are stable under vacuum for periods of at least 24 h, and the trapped gases can be released by vacuum-heating to 700 K. Reduced quantities of the trapped gases remain in the SWNTs even after exposure to room air. Fourier transform infrared spectroscopy is used to monitor the functionalities resulting from the ozone treatment and to detect the trapped gas species.     

Preparation of horizontally aligned single-walled carbon nanotubes with floating catalyst

A strategy to prepare horizontally aligned single-walled carbon nanotubes(SWNTs) at moderate temperatures(≤600 ℃) were developed.Using ferocene as the catalyst precursor,Fe nanoparticles are formed in the gaseous phase and catalyze the nucleation and growth of SWNTs in situ.Then the resultant SWNTs are deposited onto the substrates downstream and aligned by the surface lattice of the ST-cut single crystal quartz.The preparation of SWNT arrays at moderate temperatures is important for combining the tube growth with device fabrication.     

Improving the effectiveness of interfacial trapping in removing single-walled carbon nanotube bundles

Single-walled carbon nanotube (SWNT) bundles are selectively removed from an aqueous dispersion containing individually suspended carbon nanotubes coated with gum Arabic via interfacial trapping. The suspensions are characterized with absorbance, fluorescence, and Raman spectroscopy as well as atomic force microscopy (AFM) and rheology. The resulting aqueous suspensions have better dispersion quality after interfacial trapping and can be further improved by altering the processing conditions. A two-step extraction process offers a simple and fast approach to preparing high-quality dispersions of individual SWNTs comparable to ultracentrifugation. Partitioning of SWNTs to the liquid-liquid interface is described by free energy changes. SWNT bundles prefer to reside at the interface over individually suspended SWNTs because of greater free energy changes.     

Free-standing highly conductive transparent ultrathin single-walled carbon nanotube films

Transparent and conductive single-walled carbon nanotube (SWNT) films are of great importance to a number of applications such as optical and electronic devices. Here, we describe a simple approach for preparing free-standing highly conductive transparent SWNT films with a 20-150 nm thickness by spray coating from surfactant-dispersed aqueous solutions of SWNTs synthesized by an improved floating-catalyst growth method. After the HNO(3) treatment, dipping the SWNT films supporting on glass substrates in water resulted in a quick and nondestructive self-release to form free-standing ultrathin SWNT films on the water surface. The obtained films have sufficiently high transmittance (i.e., 95%), a very low sheet resistance (i.e., ～120 Ω/sq), and a small average surface roughness (i.e., ～3.5 nm for a displayed 10 × 10 μm area). Furthermore, the floating SWNT films on the water surface were easily transferred to any substrates of interest, without intense mechanical and chemical treatments, to preserve their original sizes and network structures. For example, the transferred SWNT films on poly(ethylene terephthalate) films are mechanically flexible, which is a great advantage over conventional indium-tin oxide (ITO) and therefore strongly promise to be "post ITO" for many applications.     

Noncovalent functionalization of single-walled carbon nanotubes with water-soluble porphyrins

We have employed water-soluble porphyrin molecules [meso-(tetrakis-4-sulfonatophenyl) porphine dihydrochloride] to solubilize single-walled carbon nanotubes (SWNTs), resulting in aqueous solutions that are stable for several weeks. The porphyrin-nanotube complexes have been characterized with absorption and fluorescence spectroscopy and with AFM. We find that the porphyrin/SWNT interaction is selective for the free base form, and that this interaction stabilizes the free base against protonation to the diacid. Under mildly acidic conditions nanotube-mediated J-aggregates form, which are unstable in solution and result in precipitation of the nanotubes over the course of a few days. Porphyrin-coated SWNTs can be precisely aligned on hydrophilic poly(dimethylsiloxane) (PDMS) surfaces by combing SWNT solution along a desired direction and then transferred to silicon substrates by stamping. Parallel SWNT patterns have been fabricated in this manner.     

Atomic force microscopy measurements of peptide-wrapped single-walled carbon nanotube diameters.

With a vertical resolution of 0.1 nm, atomic force microscopy (AFM) height measurements can be used to determine accurately the diameter of single-walled carbon nanotubes (SWNT) with the assumption that they have circular cross sections. The aim of this article is to draw attention to the need to optimize operating parameters in tapping mode for quantitative AFM height (diameter) analysis of SWNTs. Using silicon tip/cantilever assemblies with force constants ranging from 0.9 to 40 N m(-1), we examined the effect of applied force on the apparent diameter of SWNT wrapped with a 29-residue amphiphilic alpha-helical peptide. A decrease in apparent height (SWNT diameter) with increasing applied force was observed for the higher force constant cantilevers. Cantilevers having force constants of 0.9 and 3 N m(-1) demonstrated minimal vertical sample compression with increasing applied force. The effects of AFM image pixel density and scan speed on the measured height (diameter) of SWNTs were also assessed.     

Optical properties of ultrashort semiconducting single-walled carbon nanotube capsules down to sub-10 nm

Single-walled carbon nanotubes (SWNTs) are typically long (greater than or approximately equal 100 nm) and have been well established as novel quasi one-dimensional systems with interesting electrical, mechanical, and optical properties. Here, quasi zero-dimensional SWNTs with finite lengths down to the molecular scale (7.5 nm in average) were obtained by length separation using a density gradient ultracentrifugation method. Different sedimentation rates of nanotubes with different lengths in a density gradient were taken advantage of to sort SWNTs according to length. Optical experiments on the SWNT fractions revealed that the UV-vis-NIR absorption and photoluminescence peaks of the ultrashort SWNTs blue-shift up to approximately 30 meV compared to long nanotubes, owing to quantum confinement effects along the length of ultrashort SWNTs. These nanotube capsules essentially correspond to SWNT quantum dots.