Protonation of carbon single-walled nanotubes studied using 13C and 1H-13C cross polarization nuclear magnetic resonance and Raman spectroscopies

Densification effect on field emission characteristics of CNT film emitters for electron emission devices

Field electron emission characteristics of the carbon nanotube (CNT) film emitters were investigated according to densification conditions such as nitric acid, acetic acid, and salicylic acid. The emission performance of the CNT film emitters was strongly affected by the densification conditions. Salicylic acid exhibits the best field electron emission properties of the CNT film emitters, followed by nitric acid and acetic acid. The efficient densification of the CNT film emitter by salicylic acid is caused by the role of polarity and p orbitals, nitric acid by hydrogen ions, and acetic acid by weak polarity. After the densification with salicylic acid, the turn-on field of the CNT film emitter decreases from 1.94 Vμm-1to 1.86 Vμm-1, the threshold field decreases from 3.41 Vμm-1to 2.95 Vμm-1, the emission current significantly increases from 20.92 mA to 43.98 mA, and the degradation rate from the long-term emission stability decreases from 49.9% to 21%. The improved emission characteristics are attributed to the increased emission sites at the CNT film and the increased electrical conductivity of the CNT film. The densification is a useful way to enhance the field electron emission properties of CNT film emitters.

Comprehensive Study of the Chemistry behind the Stability of Carboxylic SWCNT Dispersions in the Development of a Transparent Electrode

Single-walled carbon nanotubes (SWCNTs) are well-known for their excellent electrical conductivity. One promising application for SWCNT-based thin films is as transparent electrodes for uncooled mid-IR detectors (MIR). In this paper, a combination of computational and experimental studies were performed to understand the chemistry behind the stability of carboxylic SWCNTs (SWCNTs-COOH) dispersions in different solvents. A computational study based on the density functional tight-binding (DFTB) method was applied to understand the interactions of COOH-functionalized carbon nanotubes with selected solvents. Attention was focused on understanding how the protonation of COOH groups influences the binding energies between SWCNTs and different solvents. Thin film electrodes were prepared by alternately depositing PEI and SWCNT-COOH on soda lime glass substrates. To prepare a stable SWCNT dispersion, different solvents were tested, such as deionized (DI) water, ethanol and acetone. The SWCNT-COOH dispersion stability was tested in different solvents. Samples were prepared to study the relationship between the number of depositions, transparency in the MIR range (2.5–5 µm) and conductivity, looking for the optimal thickness that would satisfy the application. The MIR transparency of the electrode was reduced by 20% for the thickest SWCNT layers, whereas sheet resistance values were reduced to 150–200 kΩ/sq.

Digital printing of a novel electrode for stable flexible organic solar cells with a power conversion efficiency of 8.5

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) mixed with single-wall nanotubes (SWNTs) (10:1) and doped with (0.1 M) perchloric acid (HClO4) in a solution-processed film, working as an excellent thin transparent conducting film (TCF) in organic solar cells, was investigated. This new electrode structure can be an outstanding substitute for conventional indium tin oxide (ITO) for applications in flexible solar cells due to the potential of attaining high transparency with enhanced conductivity, good flexibility, and good durability via a low-cost process over a large area. In addition, solution-processed vanadium oxide (VOx) doped with a small amount of PEDOT-PSS(PH1000) can be applied as a hole transport layer (HTL) for achieving high efficiency and stability. From these viewpoints, we investigate the benefit of using printed SWNTs-PEDOT-PSS doped with HClO4 as a transparent conducting electrode in a flexible organic solar cell. Additionally, we applied a VOx-PEDOT-PSS thin film as a hole transporting layer and a blend of PTB7 (polythieno[3,4-b] thiophene/benzodithiophene): PC71BM (phenyl-C71-butyric acid methyl ester) as an active layer in devices. Zinc oxide (ZnO) nanoparticles were applied as an electron transport layer and Ag was used as the top electrode. The proposed solar cell structure showed an enhancement in short-circuit current, power conversion efficiency, and stability relative to a conventional cell based on ITO. This result suggests a great carrier injection throughout the interfacial layer, high conductivity and transparency, as well as firm adherence for the new electrode.

Simultaneous Detection of Morphine and Codeine in the Presence of Ascorbic Acid and Uric Acid and in Human Plasma at Nafion Single-Walled Carbon Nanotube Thin-Film Electrode

In clinical settings, the dosing and differential diagnosis of the poisoning of morphine (MO) and codeine (CO) is challenging due to interindividual variations in metabolism. However, direct electrochemical detection of these analytes from biological matrices is inherently challenging due to interference from large concentrations of anions, such as ascorbic acid (AA) and uric acid (UA), as well as fouling of the electrode by proteins. In this work, a disposable Nafion-coated single-walled carbon nanotube network (SWCNT) electrode was developed. We show facile electron transfer and efficient charge separation between the interfering anions and positively charged MO and CO, as well as significantly reduced matrix effect in human plasma. The Nafion coating alters the voltammetric response of MO and CO, enabling simultaneous detection. With this SWCNT/Nafion electrode, two linear ranges of 0.05-1 and 1-10 μM were found for MO and one linear range of 0.1-50 μM for CO. Moreover, the selective and simultaneous detection of MO and CO was achieved in large excess of AA and UA, as well as, for the first time, in unprocessed human plasma. The favorable properties of this electrode enabled measurements in plasma with only mild dilution and without the precipitation of proteins.

CE with multi-walled carbon nanotubes (MWCNTs). Part I. Functionalized and SDS coated MWCNTs as pseudo-stationary phases in nanoparticle EKC - Studies on retention energetics

In this study, multi-walled carbon nanotubes (MWCNTs) in either unmodified, hydroxylated (MWCNT-OH), carboxylated (MWCNT-COOH) or sulfonated (MWCNT-SO₃H) forms were incorporated into the running electrolytes in capillary electrophoresis (CE) to play the role of pseudo-stationary phases (PSPs) and perform nanoparticle electrokinetic capillary chromatography (NPEKC). MWCNT-COOH and MWCNT-SO₃H were derived from MWCNTs via their treatment with concentrated strong acids. These functionalized MWCNTs were characterized by Raman and FTIR spectroscopies to demonstrate their covalent functionalization. The study of MWCNT-SO₃H and MWCNT-OH as PSPs were introduced in this research report for the first time in NPEKC. The results obtained with functionalized MWCNTs were compared to those obtained using unmodified MWCNTs for better understanding the electrophoretic behavior of these functionalized MWCNTs. While only MWCNT-COOH allowed the separation of some nucleic acid bases and nucleosides, neutral solutes such as alkylbenzenes (ABs), phenyl alkyl alcohols (PAAs) and aniline derivatives in neutral forms (i.e., at basic pH) were not resolved in the presence of neither MWCNT-COOH nor MWCNT-SO₃H in the running electrolytes, indicating that these functionalized MWCNTs do not have enough surface charge density to function as effective PSPs in NPEKC. This necessitated the coating of the functionalized MWCNTs under investigation with sodium dodecyl sulfate (SDS) to bring about the separation of neutral solutes by NPEKC. The SDS coated MWCNTs whether unmodified or functionalized were characterized with two homologous series namely ABs and PAAs in order to evaluate their relative retention energetics under the same electrolyte composition. The results showed that the systems pairs SDS-MWCNT-COOH/SDS-MWCNTs and SDS-MWCNT-OH/SDS-MWCNTs were homoenergetics (i.e., same energetics) while the system pair SDS-MWCNT-SO₃H/SDS-MWCNTs was homeoenergetics (i.e., similar energetics). On the other hand, all the systems pairs SDS coated MWCNTs/SDS were homeoenergetics. Homoenergetics means that the solute retention has an identical physico-chemical basis and the differences observed in the magnitude of solute retention on the various PSPs are attributed to differences in the nonpolar phase ratios of the PSPs under otherwise the same electrolyte composition. Conversely, homeoenergetics signifies that the solute retention has a similar physico-chemical basis in the PSPs systems under investigation, which also differ in their phase ratios.

Additive-free carbon nanotube dispersions, pastes, gels, and doughs in cresols

Cresols are a group of naturally occurring and massively produced methylphenols with broad use in the chemical industry. Here, we report that m-cresol and its liquid mixtures with other isomers are surprisingly good solvents for processing carbon nanotubes. They can disperse carbon nanotubes of various types at unprecedentedly high concentrations of tens of weight percent, without the need for any dispersing agent or additive. Cresols interact with carbon nanotubes by charge transfer through the phenolic hydroxyl proton and can be removed after processing by evaporation or washing, without altering the surface of carbon nanotubes. Cresol solvents render carbon nanotubes polymer-like rheological and viscoelastic properties and processability. As the concentration of nanotubes increases, a continuous transition of four states can be observed, including dilute dispersion, thick paste, free-standing gel, and eventually a kneadable, playdough-like material. As demonstrated with a few proofs of concept, cresols make powders of agglomerated carbon nanotubes immediately usable by a broad array of material-processing techniques to create desirable structures and form factors and make their polymer composites.

Individualized p-Doped Carbon Nanohorns

A facile approach to individualize spherically aggregated pristine carbon nanohorns (pr-CNHs) was established. Specifically, we found that treatment of pr-CNHs with chlorosulfonic acid generates positively charged polarized species, which disintegrate toward individualized carbon nanohorns (in-CNHs). Interestingly, the isolated in-CNHs were revealed to be p-doped owing to the adsorption of chlorosulfonate units. The findings were confirmed by data derived from high-resolution transmission electron microscopy imaging, Raman and ultraviolet photoemission spectroscopy, and additionally supported by theoretical calculations and thermogravimetry.

π(+)-π interactions between (hetero)aromatic amine cations and the graphitic surfaces of pyrogenic carbonaceous materials

Many organic compounds of environmental concern contain amine groups that are positively charged at environmental pH. Here we present evidence that (hetero)aromatic amine cations can act as π acceptors in forming π(+)–π electron donor–acceptor (EDA) interactions with the π electron-rich, polyaromatic surface of pyrogenic carbonaceous materials (PCMs) (i.e., biochar, black carbon, and graphene). The π(+)–π EDA interactions combine a cation−π force with a π–π EDA force resulting from charge polarization of the ring’s quadrupole. Adsorption on a biochar and reference adsorbent graphite was conducted of triazine herbicides, substituted anilines, heterocyclic aromatic amines, and other amines whose charge is insulated from the aromatic ring. When normalized for the hydrophobic effect, the adsorption increased with decreasing pH as the amines became ionized, even on graphite that had no significant fixed or variable charge. The cationic π acceptor (quinolinium ion) was competitively displaced more effectively by the π acceptor 2,4-dinitrobenzene than by the π donor naphthalene. The maximum electrostatic potential of organocations computed with density functional theory was found to be a strong predictor of the π(+)–π EDA interaction. The π(+)–π EDA interaction was disfavored by electropositive alkyl substituents and by charge delocalization into additional rings. Amines whose charge was insulated from the ring fell far out of the correlation (more positive free energy of adsorption). Identifying and characterizing this novel π(+)–π EDA interaction on PCMs will help in predicting the fate of organocations in both natural and engineered systems.

Selective and reversible noncovalent functionalization of single-walled carbon nanotubes by a pH-responsive vinylogous tetrathiafulvalene-fluorene copolymer

A vinylogous tetrathiafulvalene (TTFV) monomer was prepared and copolymerized with fluorene to give a conformationally switchable conjugated copolymer. This copolymer was shown to undergo a conformational change upon protonation with trifluoroacetic acid (TFA). When mixed with single-walled carbon nanotubes (SWNTs), this TTFV-fluorene copolymer exhibited strong interactions with the SWNT surface, leading to stable, concentrated nanotube dispersions in toluene. Photoluminescence excitation mapping indicated that the copolymer selectively disperses low-diameter SWNTs, as would be expected from its ability to form a tightly coiled conformation on the nanotube surface. Addition of TFA to the copolymer-SWNT dispersion resulted in a rapid conformational change and desorption of the polymer from the SWNT surface, resulting in precipitation of pure SWNTs that were completely free of polymer. Importantly, the nanotubes isolated after dispersion and release by the TTFV-fluorene copolymer were more pure than the original SWNTs that were initially dispersed.

Effect of solvent polarity and electrophilicity on quantum yields and solvatochromic shifts of single-walled carbon nanotube photoluminescence

In this work, we investigate the impact of the solvation environment on single-walled carbon nanotube (SWCNT) photoluminescence quantum yield and optical transition energies (E(ii)) using a highly charged aryleneethynylene polymer. This novel surfactant produces dispersions in a variety of polar solvents having a wide range of dielectric constants (methanol, dimethyl sulfoxide, aqueous dimethylformamide, and deuterium oxide). Because a common surfactant can be used while maintaining a constant SWCNT-surfactant morphology, we are able to straightforwardly evaluate the impact of the solvation environment upon SWCNT optical properties. We find that (i) the SWCNT quantum yield is strongly dependent on both the polarity and electrophilicity of the solvent and (ii) solvatochromic shifts correlate with the extent of SWCNT solvation. These findings provide a deeper understanding of the environmental dependence of SWCNT excitonic properties and underscore that the solvent provides a tool with which to modulate SWCNT electronic and optical properties.

Scalable fabrication of multifunctional freestanding carbon nanotube/polymer composite thin films for energy conversion

Translating the unique properties of individual single-walled carbon nanotubes (SWNTs) to the macroscale while simultaneously incorporating additional functionalities into composites has been stymied by inadequate assembly methods. Here we describe a technique for developing multifunctional SWNT/polymer composite thin films that provides a fundamental engineering basis to bridge the gap between their nano- and macroscale properties. Selected polymers are infiltrated into a Mayer rod coated conductive SWNT network to fabricate solar cell transparent conductive electrodes (TCEs), fuel cell membrane electrode assemblies (MEAs), and lithium ion battery electrodes. Our TCEs have an outstanding optoelectronic figure of merit σ(dc)/σ(ac) of 19.4 and roughness of 3.8 nm yet are also mechanically robust enough to withstand delamination, a step toward scratch resistance necessary for flexible electronics. Our MEAs show platinum utilization as high as 1550 mW/mg(Pt), demonstrating our technique's ability to integrate ionic conductivity of the polymer with electrical conductivity of the SWNTs at the Pt surface. Our battery anodes, which show reversible capacity of ∼850 mAh/g after 15 cycles, demonstrate the integration of electrode and separator to simplify device architecture and decrease overall weight. Each of these applications demonstrates our technique's ability to maintain the conductivity of SWNT networks and their dispersion within a polymer matrix while concurrently optimizing key complementary properties of the composite. Here, we lay the foundation for the assembly of nanotubes and nanostructured components (rods, wires, particles, etc.) into macroscopic multifunctional materials using a low-cost and scalable solution-based processing technique.

n-Type transparent conducting films of small molecule and polymer amine doped single-walled carbon nanotubes

In this report, we investigate the electrical and optical properties of thin conducting films of SWNTs after treatment with small molecule and polymeric amines. Among those tested, we find hydrazine to be the most effective n-type dopant. We use absorbance, Raman, X-ray photoelectron, and nuclear magnetic resonance spectroscopies on thin conducting films and opaque buckypapers treated with hydrazine to study fundamental properties and spectroscopic signatures of n-type SWNTs and compare them to SWNTs treated with nitric acid, a well-characterized p-type dopant. We find that hydrazine physisorbs to the surface of semiconducting and metallic SWNTs and injects large electron concentrations, raising the Fermi level as much as 0.7 eV above that of intrinsic SWNTs. Hydrazine-treated transparent SWNT films display sheet resistances nearly as low as p-type nitric-acid-treated films at similar optical transmittances, demonstrating their potential for use in photovoltaic devices as low work function transparent electron-collecting electrodes.

Solid-state 13C NMR assignment of carbon resonances on metallic and semiconducting single-walled carbon nanotubes

Solid-state (13)C NMR spectroscopy was used to investigate the chemical shift of nanotube carbons on m- and s-SWNTs (metallic and semiconducting single-walled nanotubes) for samples with widely varying s-SWNT content, including samples highly enriched with nearly 100% m- and s-SWNTs. High-resolution (13)C NMR was found to be a sensitive probe for m- and s-SWNTs in mixed SWNT samples with diameters of approximately 1.3 nm. The two highly enriched m- and s-SWNT samples clearly exhibited features for m- and s-SNWT (13)C nuclei (approximately 123 and 122 ppm, respectively) and were successfully fit with a single Gaussian, while five mixed samples required two Gaussians for a satisfactory fit.

Base and acid treatment of SWCNT-RNA transparent conductive films

RNA was used to exfoliate single-walled carbon nanotubes (SWCNTs) in aqueous solution, and the ratio of it was optimized to obtain the best dispersion state. The obtained homogeneous SWCNT solution with small bundle size was used to prepare flexible transparent conductive films by filtration method. Sodium hydroxide treatment combining short-time acid treatment was used to remove the RNA molecules. After treatment, the sheet resistance of the films decreased significantly, while the change on the transmittance was negligible. Besides, the polyethylene terephthalate substrate would not turn brittle through this treatment process. Flexible films with outstanding performance (190 Omega/sq, 85%) and good stability were obtained after treatment. X-ray photoelectron spectroscopy and scanning electron microscope were used to analyze the role of base and acid treatment in detail.

The effect of bundling on the G' Raman band of single-walled carbon nanotubes

The behaviour of the Raman G' band of solubilized SWCNTs in aqueous solution is investigated with respect to ultrasonication time, surfactant type and centrifugation. The linewidth of the high frequency component of the G' band is shown to correlate with the degree of bundling in the solution. In particular, the linewidths of all dispersions decrease as a consequence of subsequent centrifugation, reaching values characteristic of individual SWCNTs (30 cm(-1)). These results correlate well with the observed changes of the radial breathing modes. Further, using this approach an evaluation of the extent of debundling SWCNTs using surfactant solutions (sodium dodecylbenzenesulfonate and sodium dodecyl sulfate) and deoxyribonucleic acids with respect to surfactant concentration, ultrasonication time and centrifugation was performed.

Why single-walled carbon nanotubes can be dispersed in imidazolium-based ionic liquids

Spectroscopic and molecular modeling studies were performed to investigate the underlying dispersion mechanism of single-walled carbon nanotubes (SWCNTs) in imidazolium-based ionic liquids. Both the experimental and the simulation evidence indicate that the ionic liquids interact with SWCNTs through weak van der Waals interaction other than the previous assumed "cation-pi" interaction. Therefore the electronic structure of SWCNTs in the dispersions can be kept intrinsically. The SWCNTs do not significantly influence the local structure of the imidazolium cations, though the local environment of anions adjacent to SWCNTs is somewhat perturbed because of the interfacial effect. The ionic liquids basically keep their overall bulk phase organization. A pi-pi interaction-shielding model is proposed to account for the dispersion of SWCNTs in the ionic liquids. The ionic liquids, which possess very high dielectric constants, can effectively shield the strong pi-pi stacking interaction among SWCNTs and thus evidently disperse the SWCNTs. The retaining of SWCNTs' intrinsic property and the higher SWCNT content make the ionic liquids ideal media for the study and application of SWCNTs.

Protonation effects on the branching ratio in photoexcited single-walled carbon nanotube dispersions

The ensemble PL quantum yield for raw single-walled carbon nanotubes (SWNTs) dispersed in sodium cholate (SC) is approximately 5 times greater than that for the same raw SWNTs dispersed in sodium dodecyl sulfate (SDS) and approximately 10 times greater than the quantum yield of purified SWNTs dispersed in SC. Absorbance and Raman spectra indicate that purified SC-dispersed SWNTs and raw SDS-dispersed SWNTs are hole-doped by protonation. Experiments comparing PL emission efficiency using E2 and E1 excitation show that protonation significantly affects the E2 --> E1 relaxation process, which has typically been assumed to occur with unit efficiency. The E2 --> E 1 relaxation is 5 times more efficient in producing E 1 PL when SWNTs are unprotonated and protected by the SC surfactant. The results provide clear evidence that extrinsic factors, such as residual acids and the specific nature of SWNT-surfactant and SWNT-solvent interactions, can significantly affect measured SWNT luminescence quantum yields.

A Density Functional Study of the 13C NMR Chemical Shifts in Functionalized Single-Walled Carbon Nanotubes

The 13C NMR chemical shifts for functionalized (7,0), (8,0), (9,0), and (10,0) single-walled carbon nanotubes (SWNTs) have been studied computationally using gauge-including projector-augmented plane-wave (GIPAW) density functional theory (DFT). The functional groups NH, NCH3, NCH2OH, and CH2NHCH2 have been considered, and different sites where covalent addition or substitution may occur have been examined. The shifts of the carbons directly attached to the group are sensitive to the bond which has been functionalized and may, therefore, be used to identify whether the group has reacted with a parallel or a diagonal C-C bond. The addition of NH to a parallel bond renders the functionalized carbons formally sp3-hybridized, yielding shifts of around 44 ppm, independent of the SWNT radius. Reaction with a diagonal bond retains the formal sp2 hybridization of the substituted carbons, and their shifts are slightly lower or higher than those of the unsubstituted carbon atoms. The calculated 1H NMR shifts of protons in the functional groups are also dependent upon the SWNT-group interaction. Upon decreasing the degree of functionalization for the systems where the group is added to a parallel bond, the average chemical shift of the unfunctionalized carbons approaches that of the pristine tube. At the same time, the shifts of the functionalized carbons remain independent upon the degree of functionalization. For the SWNTs where N-R attaches to a parallel bond, the average shift of the sp2 carbons was found to be insensitive to the substituent R. Moreover, the shifts of the functionalized sp3 carbons, as well as of the carbons within the group itself, are independent of the SWNT radius. The results indicate that a wealth of knowledge may be obtained from the 13C NMR of functionalized SWNTs.

pH-dependent electron-transport properties of carbon nanotubes

Carbon nanotube electrochemical transistors integrated with microfluidic channels are utilized to examine the effects of aqueous electrolyte solutions on the electron-transport properties of single isolated carbon nanotubes. In particular, pH and concentration of supporting inert electrolytes are examined. A systematic threshold voltage shift with pH is observed while the transconductance and subthreshold swing remain independent of pH and concentration. Decreasing pH leads to a negative shift of the threshold voltage, indicating that protonation does not lead to hole doping. Changing the type of contact metal does not alter the observed pH response. The pH-dependent charging of SiO2 substrate is ruled out as the origin based on measurements with suspended nanotube transistors. Increasing the ionic strength leads to reduced pH response. Contributions from possible surface chargeable chemical groups are considered.

Effects of Surfactant and Boron Doping on the BWF Feature in the Raman Spectrum of Single-Wall Carbon Nanotube Aqueous Dispersions

We examine the Breit-Wigner-Fano (BWF) line shape in the Raman spectra of carbon single-wall nanotubes (SWNTs) dispersed in aqueous suspensions. Bundling and electronic effects are studied by comparing undoped SWNTs (C-SWNTs) to boron-doped nanotubes (B-SWNTs) in a variety of different surfactant solutions. For SWNTs dispersed with nonionic surfactants that are less effective in debundling than ionic surfactants, the Raman spectra retain a large BWF feature. However, we demonstrate that even for SWNTs dispersed as isolated nanotubes by ionic surfactants the BWF feature may be present and that the intensity of the BWF is highly sensitive to the specific surfactant. In particular, surfactants with electron-donating groups tend to enhance the BWF feature. Also, modification of the SWNT electronic properties by boron doping leads to enhanced surfactant dispersion relative to undoped C-SWNTs and also to modification of the BWF feature. These observations are in agreement with reports demonstrating an enhancement of the BWF by bundling but also agree with reports that suggest electron donation can enhance the BWF feature even for isolated SWNTs. Importantly, these results serve to caution against using the lack or presence of a BWF feature as an independent measure of SWNT aggregation in surfactant dispersions.

Density Functional Study of the 13C NMR Chemical Shifts in Small-to-Medium-Diameter Infinite Single-Walled Carbon Nanotubes

NMR chemical shifts were calculated for semiconducting (n,0) single-walled carbon nanotubes (SWNTs) with n ranging from 7 to 17. Infinite isolated SWNTs were calculated using a gauge-including projector-augmented plane-wave (GIPAW) approach with periodic boundary conditions and density functional theory (DFT). In order to minimize intertube interactions in the GIPAW computations, an intertube distance of 8 A was chosen. For the infinite tubes, we found a chemical shift range of over 20 ppm for the systems considered here. The SWNT family with lambda = mod(n, 3) = 0 has much smaller chemical shifts compared to the other two families with lambda = 1 and lambda = 2. For all three families, the chemical shifts decrease roughly inversely proportional to the tube's diameter. The results were compared to calculations of finite capped SWNT fragments using a gauge-including atomic orbital (GIAO) basis. Direct comparison of the two types of calculations could be made if benzene was used as the internal (computational) reference. The NMR chemical shifts of finite SWNTs were found to converge very slowly, if at all, to the infinite limit, indicating that capping has a strong effect (at least for the (9,0) tubes) on the calculated properties. Our results suggest that (13)C NMR has the potential for becoming a useful tool in characterizing SWNT samples.

Photoactuation from a Carbon Nanotube−Nafion Bilayer Composite

A bilayer composite of single walled carbon nanotubes (SWNTs) deposited onto Nafion exhibits substantial mechanical motion upon exposure to visible or near-infrared light. The magnitude of the actuation parallels the absorption spectrum of the SWNTs in the near-infrared, but the actuation diminishes in the visible and disappears in the UV portions of the spectrum. In the near-infrared region, the photoactuation is linear with respect to the light intensity. The photoactuation also appears to be associated with a photocurrent across the nanotube/Nafion interface. The proposed mechanism for the actuation is that band bending of the semiconducting SWNTs induces polarization of mobile hydrogen ions at the Nafion interface, which then causes swelling of the polymer.