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The effect of film thickness on water uptake and structure in ultra-thin Nafion is probed via in situ neutron reflectometry for a series of 10 films with thicknesses ranging from 5 to 153 nm. Observed interfacial lamellae are used to understand anomalous transport limitations in such films. Results show three distinct thickness regimes: (i) in the truncated regime (

Polycrystalline GaN fibers have been produced by the polymer-derived-ceramic (PDC) technique. The wurtzite-polymorphic fibers appear to emerge from complex nucleation and grain growth mechanisms, being mostly unconstrained during initial polymer to ceramic conversion. The importance of carrier polymer architecture and alignment is highlighted towards controlled microstructure.

We describe the temperature-dependent characterization of resistive switching behavior in bulk silver nanowire−polystyrene composites between 10 and 300 K. We propose that the resistive switching behavior is caused by the electroformation of silver filaments between adjacent nanowire clusters, resulting in an extension of the electrical percolation network in the on state. This process is reversible above 200 K, and irreversible below 100 K. The switching fields are shown to depend strongly on sample composition (i.e., proximity to the electrical percolation threshold), as well as measurement temperature.

The design and preparation of isotropic silver nanowire-polystyrene composites is described, in which the nanowires have fi nite L/D ( < 35) and narrow L/D distribution. These model composites allow the L/D dependence of the electrical percolation threshold, φ c , to be isolated for fi nite- L/D particles. Experimental φ c values decrease with increasing L/D , as predicted qualitatively by analytical percolation models. However, quantitative agreement between experimental data and both soft-core and core–shell analytical models is not achieved, because both models are strictly accurate only in the infi nite- L/D limit. To address this analytical limitation, a soft-core simulation method to calculate φ c and network conductivity for cylinders with fi nite L/D are developed. Our simulated φ c results agree strongly with our experimental data, suggesting i) that the infi nite-aspect-ratio assumption cannot safely be made for experimental networks of particles with L/D < 35 and ii) in predicting φ c , the soft-core model makes a less signifi cant assumption than the infi nite- L/D models do. The demonstrated capability of the simulations to predict φ c in the fi nite- L/D regime will allow researchers to optimize the electrical properties of polymer nanocomposites of fiL/D particles.

Carbon-based cryosorbers are generally synthesized in the form of powders, which compromises the volumetric capacity in gas storage applications. Here we report the synthesis of monolithic carbide-derived carbon (CDC) by chlorination of fully-dense ceramic titanium carbide plates. Volume change is minimal, consistent with conformal transformation from TiC to CDC, and the weight loss is consistent with nearly 100% conversion. The resulting materials have a microporous carbon structure with little or no macroporosity and exhibit enhanced volumetric gas storage capacity compared to powder equivalents. Optimized volume uptakes are 35 g L−1 at −196 °C and 60 bar for H2, 193 V(STP) v−1 at 35 bar and 219 V(STP) v−1 at 60 bar (25 °C) for CH4. Monolithic CDCs thus offer potential as gas storage media for on-board fuel-cells and other applications.

Development of high-capacity hydrogen-storage systems utilizing physisorption at high pressure and low temperature is hindered by poor understanding of the pore size/shape requirements for achieving the maximum hydrogen uptake. Tuning the carbon structure and pore size of carbide-derived carbons (CDCs) with high accuracy by using different starting carbides, chlorination temperatures and activation temperatures allows rational design of carbon materials with increased hydrogen-storage capacity. Systematic experimental investigation of a large number of CDCs with controlled pore size distributions and specific surface area (SSA) shows that pores larger than ∼1.5 nm contribute little to hydrogen storage. It has been experimentally demonstrated that, just as at ambient pressure, pores of 0.6–0.7 nm in diameter provide the largest H 2 uptake per unit SSA at elevated pressures and liquid nitrogen temperatures. The effect of pore size was stronger than the effect of surface chemistry on the hydrogen uptake.

Carbide-derived carbons (CDC) allow a precise control over the pore size through the selection of the carbide precursor and varying of the synthesis conditions. However, their pore volume is limited by the carbide stoichiometry. While activation of carbons derived from various organic precursors has been widely studied, this process may similarly be able to increase the pore volume and specific surface area of CDC. Oxidation of carbide-derived carbon in air and CO2 at different temperatures and times allows for significant increase in pore volume and specific surface area as well as control over average pore size with subnanometer accuracy. The effect of activation and associated changes in the pore volume and surface area on the hydrogen uptake are also discussed.

Carbide-derived carbons (CDCs) produced by chlorination of carbides offer great potential for precise pore size control at the atomic level, making them attractive candidates for energy storage media. CDCs activated with CO 2 or KOH possess distinct improvements in porosity, displaying specific surface areas above 3000 m 2 g −1 and pore volumes above 1.3 cm 3 g −1 . These correspond to gravimetric methane uptake of 16 wt% at 35 bar and 25 °C, close to the currently best reported material PCN-14, a metal-organic framework (MOF), at 35 bar and 17 °C or KOH activated anthracite at 35 bar and 25 °C. The best excess gravimetric methane uptake is obtained with a TiC-derived CDC activated with CO 2 at 975 °C for 2 h, namely a very large surface area of 3360 m 2 g −1 resulting in 18.5 wt% at 25 °C and 60 bar. To obtain realistic volumetric methane capacity, the packing density of completely dried CDC was measured, from which we obtain excess capacity of 145 v(STP) v −1 from CDC activated with CO 2 at 875 °C for 8 h, 81% of the DOE target (180 v(STP) v −1 ) at 35 bar and 25 °C. From small-angle X-ray scattering (SAXS) measurements, pore radii of gyration ( R g ) between 0.5 nm and 1 nm are determined. Temperature-dependent methane isotherms show that the isosteric heat of adsorption reaches 24 kJ mol −1 at the initial stage of low loading.

Physisorption of hydrogen in microporous molybdenum carbide (Mo 2 C)-derived carbons has been studied as a function of synthesis conditions. Changes in local structure induced by varying the chlorination temperature afford controllable variations in average pore size and specific surface area. Optimal hydrogen storage capacity of 4.3 wt%, measured at −196 °C and 35 bar pressure, is obtained from a sample chlorinated at 660 °C for 3 h. This optimum correlates with the largest fraction of total pore volume having average pore sizes in the 0.6–0.8 nm range.

Hydrogen storage measurements on peels formed from powdered carbide-derived carbons (CDCs) mixed with 3–5 wt% PTFE binder are presented. Polymer interactions with adsorption sites are explored by examining CDCs derived from different precursors. The observed trends in loss of gravimetric capacity suggest that the detrimental effect of the polymer is greatest for large (>1.5 nm) pores and small ( 2 /L, respectively, at 77 K and 4 MPa hydrogen pressure.

Laser flash and self-heating 3 ω techniques were employed to determine the anisotropic thermal conductivity and thermal diffusivity of a highly oriented, free-standing multiwalled carbon nanotube (MWCNT) sheet and a yarn drawn from a sidewall of the MWCNT forest grown by chemical-vapor deposition. Normalized to ideal high density structure the thermal conductivity and the thermal diffusivity along the alignment are 50 ± 5 W/m K and 45 ± 5 mm 2 /s, respectively, and are mostly limited by dangling terminals of bundles, intrinsic defects of individual nanotubes and phonon scattering within the bundles, which form the supporting matrix of the MWCNT sheet. The high degree of tube–tube overlap substantially decreases the electrical and thermal interconnection resistance, which usually dominates in randomly deposited mat-like nanotube assemblies. The extremely high surface area of the MWCNT sheet leads to excessive radial heat radiation that does not allow transferring the heat energy along the MWCNT sheet by means of phonons to distances >2 mm. On the other hand, the high surface area and negligible heat capacitance make it a perfect material for bolometric sensing ( r = 38 V/W) and heat dissipation.

Due to their exceptional thermal properties, single-wall carbon nanotubes (SWNT) are considered as very promising filler materials for improving the thermal conductivity of conventional polymers. We carefully investigated the thermal conductivity of SWNT/PMMA nanocomposites with SWNT loading in the range up to10 wt% using the comparative technique. The samples were prepared by coagulation method. We demonstrated moderate improvement in the composites’ thermal conductivity of about 250% at 10wt%. The experimental results were analyzed using the versatile Nielsen model, which takes into account many important factors, like the fillers aspect ratio and maximum packing fraction. The aspect ratio of SWNT material used to prepare our composites was determined by AFM and careful image analysis in order to use it as an input parameter in the Nielsen model. We obtained good agreement between our experimental results and the predictions of the Nielsen model. Based on our analysis we concluded that higher aspect ratio of filler material was needed to achieve better improvement in the composites thermal conductivities. One should also take steps in order to improve the thermal contact between the SWNT network and the matrix material.

Cryoadsorption is a promising method of enhancing gravimetric and volumetric onboard H2 storage capacity for future trans- portation needs. Inexpensive carbide-derived carbons (CDCs), produced by chlorination of metal carbides, have up to 80% open-pore volume with tunable pore size and specific surface area (SSA). Tuning the carbon structure and pore size with high sensitivity by using different starting carbides and chlorination temperatures allows rational design of carbon materials with en- hanced C-H2 interaction and thus increased H2 storage capacity. A systematic experimental investigation of a large number of CDCs with controlled pore size distributions and SSAs shows how smaller pores increase both the heat of adsorption and the total volume of adsorbed H2. It has been demonstrated that increasing the average heat of H2 adsorption above 6.6 kJmol -1 substantially enhances H2 uptake at 1 atm (1 atm=101325 Pa) and -196°C. The heats of adsorption up to 11 kJmol -1 exceed values reported for metal-organic framework compounds and carbon nanotubes.

High surface area nanoporous carbon has been prepared by thermo-chemical etching of titanium carbide TiC in chlorine in the temperature range 200–1200 °C. Structural analysis showed that this carbide-derived carbon (CDC) was highly disordered at all synthesis temperatures. Higher temperature resulted in increasing ordering and formation of bent graphene sheets or thin graphitic ribbons. Soft X-ray absorption near-edge structure spectroscopy demonstrated that CDC consisted mostly of sp 2 bonded carbon. Small-angle X-ray scattering and argon sorption measurements showed that the uniform carbon-carbon distance in cubic TiC resulted in the formation of small pores with a narrow size distribution at low synthesis temperatures; synthesis temperatures above 800 °C resulted in larger pores. CDC produced at 600–800 °C show great potential for energy-related applications. Hydrogen sorption experiments at −195.8 °C and atmospheric pressure showed a maximum gravimetric capacity of ∼330 cm 3 /g (3.0 wt.%). Methane sorption at 25 °C demonstrated a maximum capacity above 46 cm 3 /g (45 vol/vol or 3.1 wt.%) at atmospheric pressure. When tested as electrodes for supercapacitors with an organic electrolyte, the hydrogen-treated CDC showed specific capacitance up to 130 F/g with no degradation after 10 000 cycles.

Porous carbons have received much attention recently for potential applications in energy generation and storage, molecular sieving, and environmental remediation. Property optimization for specific applications rests largely on controlling the volume, size, and shape of the pores at the synthetic level. Direct atom-scale experiments which might accurately and reliably measure these quantities are problematic, so indirect methods such as gas sorption are generally employed. Here we apply a second indirect method, small-angle X-ray scattering (SAXS), to study porosity in carbide-derived carbons (CDC). The results qualitatively confirm and reinforce model-dependent conclusions drawn from gas sorption isotherms. In particular, both techniques indicate the onset of broad polydispersity under the same processing conditions for particular porous carbon materials.

We review our current progress on semiconductor nanowires of β-Ga2O3, Si and GaN. These nanowires were grown using both vapor–solid (VS) and vapor–liquid–solid (VLS) mechanisms. Using transmission electron microscopy (TEM) we studied their morphological, compositional and structural characteristics. Here we survey the general morphologies, growth directions and a variety of defect structures found in our samples. We also outline a method to determine the nanowire growth direction using TEM, and present an overview of device fabrication and assembly methods developed using these nanowires.

High-resolution transmission electron microscopy (HRTEM) and cross-sectional transmission electron microscopy (XTEM) are used to characterize common defects in wurtzite GaN nanowires grown via a vapor-liquid-solid (VLS) mechanism. HRTEM shows that these nanowires contain numerous (001) stacking defects interspersed with small cubic GaN regions. Using XTEM, bicrystalline nanowires are discovered with twofold rotational twin axes along their growth directions, and are found to grow along high-index directions or vicinal to low-index planes. We propose a defect-mediated VLS growth model to qualitatively account for the prevalence of these extended defects, and discuss the implications of these defects for nanowire growth kinetics and device behavior.

The crystallization kinetics and resulting morphology of polyethylene (PE) in the presence of single wall carbon nanotubes (SWNT) are investigated in isotropic and aligned composites. A hot coagulation method was developed to incorporate SWNT loadings as high as 30 wt % with uniform distribution. Thermal analysis interpreted using the Avrami equation showed that the nanotubes provide nucleation sites to PE and accelerate the PE crystal growth rate while reducing the crystal dimensionality from spherulitic to disk-shaped. Nucleating on SWNT bundles significantly increases the shish density during melt fiber spinning, so that the PE microstructure is shish kebab with straight (not twisted) lamellae in 1 wt % SWNT-HDPE composite fibers. By comparing the orientations of SWNT and PE produced by various processing conditions, we show that SWNT bundles template the growth of PE crystals by imposing a growth direction perpendicular to the SWNT. This provides a new route toward controlling the SWNT-polymer interface and thereby the physical properties of nanocomposites.

An interfacial polymerization method for nylon 6,6 was adapted to produce nanocomposites with single wall carbon nanotubes (SWNT) via in situ polymerization. SWNT were incorporated in purified, functionalized or surfactant stabilized forms. The functionalization of SWNT was characterized by FTIR, Raman spectroscopy and TGA and the SWNT dispersion was characterized by optical microscopy before and after the in situ polymerization. SWNT functionalization and surfactant stabilization improved the nanotube dispersion in solvents but only functionalized SWNT showed a good dispersion in composites, whereas purified and surfactant stabilized SWNT resulted in poor dispersion and nanotube agglomeration. Weak shear flow induced SWNT flocculation in these nanocomposites. The electrical and mechanical properties of the SWNT/nylon nanocomposites are briefly discussed in terms of SWNT loading, dispersion, length and type of functionalization.

GaN is a technologically important semiconductor with a wide direct bandgap (3.39 eV), and boasts strong light emission in the blue and UV regions of the electromagnetic spectrum. It finds extensive commercial applications in lasers and light-emitting diodes. Because of its high melting temperature, high breakdown field, and high saturation drift velocity, it is a prime candidate for high-temperature, high-voltage, and high-power optoelectronic-device applications. In recent years, research interest in GaN nanowires has increased significantly because, for sufficiently thin nanowires, quantumconfinement effects may be observed, which may lead to novel behavior and applications. GaN nanowires appear to be especially attractive as low-dimensional high-power blue and UV laser light sources, because it is anticipated that high optical gains and low lasing thresholds will be achievable when the nanowire diameter is smaller than the exciton radius. It has been demonstrated that GaN nanowires possess great potential for photonic-, optoelectronic-, and electronic-device applications. For practical device applications, nanowires have to be controllably assembled, precisely located, and individually contacted in order to build device architectures. This continues to be a formidable technical challenge. Traditional lithographic approaches are hindered by the need to register contacts to individual nanowires, although new methodologies to circumvent this limitation have been proposed. Recently, direct writing techniques using focused ion beams (FIBs) have been used to form interconnects to individual nanowires. To do this, an organoplatinum precursor gas flow was directed at the surface of the sample while the target region was irradiated with an energetic particle beam, which decomposed the gas and deposited Pt over the irradiated region. Decomposition of the precursor and Pt deposition can be achieved using focused beams of ions (IBID-Pt, where IBID = ion-beaminduced deposition) or electrons (EBID-Pt, where EBID = electron-beam-induced deposition). The unusually low resistance or ohmic contacts on n-type GaN nanowires suggest that IBID-Pt contacts are different from conventional thin-film Pt contacts, which generally exhibit a large Schottky barrier on n-GaN. Here we present a complete investigation of the structural and chemical character of FIB-deposited metal contacts on GaN nanowires, in order to understand how composition and microstructure affect the electrical performance. Contact structures have been traditionally studied using cross-sectional transmission electron microscopy (TEM). TEM enables imaging, diffraction, and spectroscopy at nearatomic spatial resolution on the same region of interest, providing a wealth of morphological, structural, and compositional information, so that even the most complex contact structures can be readily understood. Although many preparation techniques are capable of creating cross-sections suitable for TEM imaging, these generally offer little control over the region exposed for TEM observation and are, therefore, not useful for preparing cross-sections of our nanowire circuits. The so-called lift-out technique based on FIB machining gives high positional specificity, so we employed it to prepare cross-sections of individual IBID-Pt contacts for cross-sectional TEM imaging (see Experimental for details). Figures 1a,b are a typical pair of cross-sectional images obtained from the pristine nanowire segment away from the C O M M U N IC A TI O N S

Recent studies of SWNT/polymer nanocomposites identify the large interfacial thermal resistance at nanotube/nanotube junctions as a primary cause for the only modest increases in thermal conductivity relative to the polymer matrix. To reduce this interfacial thermal resistance, we prepared a freestanding nanotube framework by removing the polymer matrix from a 1 wt % SWNT/PMMA composite by nitrogen gasification and then infiltrated it with epoxy resin and cured. The SWNT/epoxy composite made by this infiltration method has a micron-scale, bicontinuous morphology and much improved thermal conductivity (220% relative to epoxy) due to the more effective heat transfer within the nanotube-rich phase. By applying a linear mixing rule to the bicontinuous composite, we conclude that even at high loadings the nanotube framework more effectively transports phonons than well-dispersed SWNT bundles. Contrary to the widely accepted approaches, these findings suggest that better thermal and electrical conductivities can be accomplished via heterogeneous distributions of SWNT in polymer matrices. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1513–1519, 2006

We report structure-property correlations in single-wall carbon nanotube (SWNT) fibers, among electrical, thermal, and chemical parameters with respect to stretch-induced preferential SWNT alignment along the fiber axis. Purified HiPco (high-pressure CO) conversion tubes are dispersed with the aid of an anionic surfactant and coagulated in the co-flowing stream of an adsorbing polymer. The fibers are then dried, rewetted under tensile load, and redried to improve the alignment. Complete removal of the polymer was assured by annealing in hydrogen at 1000°C. The degree of alignment was determined by x-ray scattering from individual fibers using a two-dimensional detector. The half width at half maximum describing the axially symmetric distribution of SWNT axes decreases linearly from 27.5° in the initial extruded fiber to 14.5° after stretching by 80%. The electrical resistivity ρ at 300K decreases overall by a factor ∼4 with stretching, for both as-spun composite and polymer-free annealed fibers. However, ...

Single-walled carbon nanotube (SWNT)/poly(methyl methacrylate) (PMMA) nanocomposites were prepared via our coagulation method providing uniform dispersion of the nanotubes in the polymer matrix. Optical microscopy, Raman imaging, and SEM were employed to determine the dispersion of nanotube at different length scales. The linear viscoelastic behavior and electrical conductivity of these nanocomposites were investigated. At low frequencies, G‘ becomes almost independent of the frequency as nanotube loading increases, suggesting an onset of solidlike behavior in these nanocomposites. By plotting G‘ vs nanotube loading and fitting with a power law function, the rheological threshold of these nanocomposites is ∼0.12 wt %. This rheological threshold is smaller than the percolation threshold of electrical conductivity, ∼0.39 wt %. This difference in the percolation threshold is understood in terms of the smaller nanotube−nanotube distance required for electrical conductivity as compared to that required to impe...

Well-aligned macroscopic fibers composed solely of single-walled carbon nanotubes (SWNTs) were produced by conventional spinning. Fuming sulfuric acid charges SWNTs and promotes their ordering into an aligned phase of individual mobile SWNTs surrounded by acid anions. This ordered dispersion was extruded via solution spinning into continuous lengths of macroscopic neat SWNT fibers. Such fibers possess interesting structural composition and physical properties.

We have investigated the dispersion of single-walled carbon nanotubes (SWNTs) in heavy water with the surfactant octyl-phenol-ethoxylate (Triton X-100) using small angle neutron scattering. The results indicate an optimal surfactant concentration for dispersion, which we suggest results from competition between maximization of surfactant adsorption onto SWNT surfaces and a depletion interaction between SWNT bundles mediated by surfactant micelles. The latter effect drives SWNT reaggregation above a critical volume fraction of micelles. These behaviors could be general in dispersing SWNTs using amphiphilic surfactant. The data also reveal significant incoherent scattering from hydrogen in SWNTs, most likely due to acid and water residues from the purification process.

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

Porous solids are of great technological importance due to their ability to interact with gases and liquids not only at the surface, but throughout their bulk. Although large pores can be produced and well controlled in a variety of materials, nanopores in the range of 2 nm and below (micropores, according to IUPAC classification) are usually achieved only in carbons or zeolites. To date, major efforts in the field of porous materials have been directed towards control of the size, shape and uniformity of the pores. Here we demonstrate that porosity of carbide-derived carbons (CDCs) can be tuned with subångström accuracy in a wide range by controlling the chlorination temperature. CDC produced from Ti3SiC2 has a narrower pore-size distribution than single-wall carbon nanotubes or activated carbons; its pore-size distribution is comparable to that of zeolites. CDCs are produced at temperatures from 200-1,200 degrees C as a powder, a coating, a membrane or parts with near-final shapes, with or without mesopores. They can find applications in molecular sieves, gas storage, catalysts, adsorbents, battery electrodes, supercapacitors, water/air filters and medical devices.

Gallium phosphide nanowires with a most probable diameter of approximately 20.0 nm and more than 10 microns in length have been synthesized by pulsed laser vaporization of a heated GaP/5% Au target. The morphology and microstructure of GaP nanowires have been investigated by scanning electron microscopy and transmission electron microscopy. Twins have been observed along the crystalline nanowires, which have a growth direction of [111]. Raman scattering shows a 4 cm-1 downshift of the longitudinal optical phonon peak in the nanowire with respect to the bulk; the transverse optical phonon frequency and line width are, however, the same as in the bulk. The quantum confinement model first proposed by Richter et al. cannot explain the observed behavior of the Raman modes.

We demonstrate herein the possibility to prepare carbon anodes for Li-ion batteries using simple carbonized polymeric precursors such as cotton and phenolic cloths. Activation by controlled oxidation forms highly porous carbons whose electrochemical activity in Li salt solutions is mostly an irreversible reduction of solution species and double layer charging. Treating these porous carbons by chemical vapor deposition (CVD) of carbon on their surfaces, closes the pores in a way that they can insert Li-ions, but not solution species. These general carbon engineering processes form new carbons with nanoscopic, selectively closed pores, which can serve as highly reversible anode materials for Li-ion batteries, with relatively low irreversible capacity. The capacity of these electrodes depends on the nature of the carbon CVD process. This paper describes the scheme for carbon engineering, gas adsorption measurements that demonstrate the impact of the carbon CVD process, and the relevant changes in the structure of the pores and some preliminary electrochemical measurements in non-aqueous Li salt solutions.

Thick films of single wall carbon nanotubes (SWNT) exhibiting in-plane preferred orientation have been produced by filter deposition from suspension in strong magnetic fields. We characterize the field-induced alignment with x-ray fiber diagrams and polarized Raman scattering, using a model which includes a completely unaligned fraction. We correlate the texture parameters with resistivity and thermal conductivity measured parallel and perpendicular to the alignment direction. Results obtained with 7 and 26 T fields are compared. We find no significant field dependence of the distribution width, while the aligned fraction is slightly greater at the higher field. Anisotropy in both transport properties is modest, with ratios in the range 5–9, consistent with the measured texture parameters assuming a simple model of rigid rod conductors. We suggest that further enhancements in anisotropic properties will require optimizing the filter deposition process rather than larger magnetic fields. We show that both ...

Single-wall carbon nanotubes can be doped, or intercalated, with electron donors or acceptors, similar to graphite and some conjugated polymers. The resulting materials show many of the same features: enhanced electrical conductivity, conduction electron paramagnetism, partial or complete reversibility, and cyclability. Reactions may be carried out in vapor or liquid phase, or electrochemically. Structural information is sketchy at best, due to the limited quality of currently available materials and solvent-related effects. Recent developments in coagulation-based fiber extrusion and partially aligned materials offer new opportunities for novel material modifications by chemical doping.

Inelastic neutron-scattering spectra of oriented nylon-6 were measured using the filter- analyzer and time-of-flight techniques. The samples studied were semicrystalline fibers with the crystalline fraction either in the R or the A form, and selectively deuterated polymer films. Strong features observed in the inelastic spectra correspond to vibrations of methylene groups, however several of the typical amide modes are also identified. The frequencies of observed amide V, VI and VII modes depend on the type of crystal structure. The lowest amide VII mode is located at 27 meV in the R phase, and at 32 meV in the A phase. Broad boson peaks are observed in both types of fibers, and attributed to their amorphous fractions. The boson peak of the A fiber is found at a slightly higher frequency, which we explain by a higher cohesive energy density due to stronger hydrogen bonds or their higher density in the A amorphous phase. Density functional calculations, using the B3LYP functional, were employed to study the vibrational modes of smaller model molecules, including H-bonded couples. These calculations show that hydrogen bonding significantly modifies not just the frequencies of out-of-plane amide vibrations, but the character of mode polarization vectors as well.

Low T oxidation followed by brief sonication in hot HCl has been optimized to remove Fe catalyst residues from HiPco material with minimal loss of tubes. One pass reduces Fe from 6–10 to 0.6–1.6 at.% with 60% yield, most of the loss being Fe. Raman scattering reveals a broad diameter distribution with a mean of 1.0–1.1 nm. After purification we find excellent correlation between oxidation temperature and Raman spectra whereby the oxidation rate varies inversely with tube diameter. This can be explained by the larger strain associated with greater curvature of small tubes.

This work relates to a rigorous study of the correlation among surface chemistry (FTIR, XPS), 3D structure (X-ray and neutron scattering), morphology (SEM, AFM), and electrochemical and impedance behavior of lithiated carbon electrodes in commonly used liquid electrolyte solutions. Four different types of carbons were explored in a single study. These included, for comparison, two types of disordered carbons, single-wall carbon nanotubes (SWNT), and synthetic graphite powder as a reference system. All four types of carbons develop a similar surface chemistry in alkyl carbonate solutions which is dominated by solvent reduction. The differences in the 3D structure of these carbons leads to pronounced differences in the mechanisms of Li-insertion into them. The effect of the carbons’ 3D structure on the solid-state diffusion of Li-ions is demonstrated and discussed.

Electrochemical doping and in situ Raman scattering were used to study charge transfer in K- and Li-doped single wall carbon nanotubes (SWNT) as a function of alkali concentration. An 8 cm ˇ1 downshift was observed for the tangential phonon mode of SWNT doped to stoichiometries of KC24 and Li1:25C6. The shift in both systems is reversible upon de-doping despite an irreversible loss of crystallinity. These results indicate that the tangential mode shifts result from electron transfer from alkali dopants to the SWNT, and that these modes are only weakly aAected by long-range order within the ropes. ” 2001 Elsevier Science B.V. All rights reserved.

Polarized resonant Raman and optical spectroscopy of aligned single-wall carbon nanotubes show that the optical transitions are strongly polarized along the nanotubes axis. This behavior is consistent with recent electronic structure calculations.

Electrochemical impedance spectroscopy (EIS) was used to study the kinetics and insertion mechanism of alkali metals in single wall carbon nanotubes (SWNT). Three distinct processes with very different time constants were identified: charge transfer across the macroscopic electrolyte/electrode interface, diffusion through the mesoscale porosity of the SWNT electrode, and nanoscale diffusion in individual SWNT ropes. Electrode resistance and capacitance from EIS compare well with direct dc measurements. We propose that alkalis decorate the internal and external surfaces of the SWNT ropes.

This Letter describes the production of single-wall carbon nanotube (SWNT) – polymer composites with enhanced mechanical and electrical properties and exceptional nanotube alignment. A combination of solvent casting and melt mixing was used to disperse SWNT materials in poly(methyl methacrylate) (PMMA). Composite films showed higher conductivity along the flow direction than perpendicular to it. Composite fibers were melt spun to achieve draw ratios between 20 and 3600. The elastic modulus and yield strength of SWNT–PMMA composite fibers increased with nanotube loading and draw ratio. Polarized resonant Raman spectroscopy indicates that the nanotubes in the fibers are well aligned, with mosaic distribution FWHMs as small as 4°.

We report inelastic neutron scattering results on hydrogen adsorbed onto samples containing single-wall carbon nanotubes. These materials have attracted considerable interest recently due to reports of high density hydrogen storage at room temperature. Inelastic neutron scattering clearly shows the ortho‐para conversion of physisorbed hydrogen in a nanotube containing soot loaded with hydrogen. From the rotational Ja 0! 1 transition, no indication of a significant barrier to quantum rotation is seen. ” 2000 Elsevier Science B.V. All rights reserved.

X-ray diffraction profiles of pristine and hypothetical “intercalated” crystalline bundles of single-wall carbon nanotubes are computed and compared with experiment. The pristine case is complicated by finite size effects, tube diameter dispersion and the cylindrical form factor for uncorrelated tube rotations. Experimental profiles of “doped” samples are not in agreement with simulations based on 2-D ordered sublattices except at very low doping levels.

We report in situ measurements of four-probe dc resistance (R) of K-doped purified single wall carbon nanotube (SWNT) buckypaper as a function of quasi-hydrostatic pressure. Doped samples show completely different behavior compared to that of pristine nanotubes in the pressure range up to 90 khar. The characteristic increase in the resistance of pristine huckypaper above 10 kbar, associated with the formation of kinks or/and twists of tubes, is not observed in K-doped samples. This may originate from I ) a substantial change in electronic hand structure of donor intercalated nanotubes, 2) completely different transport properties of defect structures, or 3) higher stiffness of doped SWNT's which prevents formation of kinks and twists in this pressure range. On deintercalation, the pristine behavior of R(P) is restored, establishing the reversibility of potassium vapor-transport doping.

Reversible insertion of Li into purified single wall carbon nanotubes was achieved electrochemically. Galvanostatic charge-discharge and cyclic voltammetry indicated that there is no well-defined redox potential for Li insertion or removal in the nanotube lattice. The Li reversible capacity was found to be 460 mA.h/g, significantly higher than the theoretical value for graphite. In-situ X-ray diffraction revealed an irreversible loss of the 2-D triangular lattice upon doping. In-situ resistivity measurements presented a 20-fold decrease in resistance upon doping, reversible upon undoping.

Disordered carbons offer promise of performance gains relative to graphite as the anode host material in Li ion batteries. They also present major challenges in determining structure-property relationships, in particular the location, density and properties of the site (or sites) at which reversible Li uptake occurs. Our understanding of graphite anodes is built on 35 years of research on the crystal compound LiC6. Here we review our efforts to obtain similar understanding of disordered carbons, using local structure determination via radial distribution function analysis, quantum chemical simulations and neutron vibrational spectroscopy. The ultimate goals are to understand the origin of the very high capacity for Li uptake exhibited by some of these materials, and to establish a scientific basis for optimizing their performance in real batteries. Two illustrative examples will be discussed in detail: the effect of residual hydrogen and ‘edge sites’ on Li binding, and the detailed evolution from long-range to short-range order in ball-milled graphite.

Spherical objects (presumably C60 molecules) have been observed inside ∼14 A diameter single-wall carbon nanotubes. We applied acid etching to access the interior of the tubes, and leaching in toluene to dissolve liberated C60. Quantitative UV–Vis of the 335 nm C60 transition establishes beyond doubt that these are indeed C60, and that their relative abundance in a macroscopic sample can be as high as 5.4% (expressed as the filled fraction of total tube length). Variations in abundance with growth and processing conditions provide clues as to how they form.

The structures of the lamellar selenodiphosphates CuMP2Se6 (M=Cr, In) are reexamined using powder diffraction and auxiliary techniques. Antiferromagnetic behavior observed in CuCrP2Se6 below 40 K is shown to be consistent with the existence of separate, triangular Cu and Cr sublattices rather than a random cation distribution. Analyses of neutron powder diffraction patterns recorded at various temperatures for CuCrP2Se6 confirm this as well as reveal partial occupation by copper of equivalent sites shifted away from the layer midplane. Diffraction evidence is also given for site disorder involving off-center positions in the copper sublattice of CuInP2Se6 at room temperature. The deduced off-center shift for copper in CuCrP2Se6 is less than in CuInP2Se6; moreover, these displacements are significantly smaller than in the thiophosphates CuMP2S6 (M=Cr, In). No indication of dipole ordering within the copper sublattice of CuCrP2Se6 was found down to T=10 K, in contrast with the appearance of antipolar copper order in CuCrP2S6 below 150 K. On the other hand, calorimetry detected a transition in CuInP2Se6 between T=200 and 240 K, a range much lower and broader than for the ferro-paraelectric transition in CuInP2S6 (Tc=315 K). The possible nature of the low T-phase of CuInP2Se6 is discussed based on preliminary diffraction data and the known thermal behavior of CuInP2S6.

Single-wall carbon nanotubes exhibit many properties analogous to quantum dots and wires at very low temperatures: Coulomb blockade and single-electron charging. These and other phenomena may be exploited in constructing active electronic devices of unprecedentedly small size. Bulk material may be chemically doped to yield mass-normalized electrical conductivity higher than that of copper.