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We present a theoretical study of the electronic and mechanical properties of graphyne-based nanotubes (GNTs). These semiconducting nanotubes result from the elongation of one-third of the covalent interconnections of graphite-based nanotubes by the introduction of yne groups. The effect of charge injection on the dimensions of GNTs was investigated using tight-binding calculations. Low amounts of electron injection are predicted to cause qualitatively different responses for armchair and zigzag graphyne nanotubes. Although the behavior is qualitatively similar to the usual carbon nanotubes, the charge-induced strains are predicted to be smaller for the GNTs than for ordinary single walled carbon nanotubes. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Nanotube sheets, or “bucky papers,” have been proposed for use in actuating, structural and electrochemical systems, based in part on their potential mechanical properties. Here, we present results of detailed simulations of networks of nanotubes/ropes, with special emphasis on the effect of joint morphology. We perform detailed simulations of three-dimensional joint deformation, and use the results to inform simulations of two-dimensional (2D) networks with intertube connections represented by torsion springs. Upper bounds are established on moduli of nanotube sheets, using the 2D Euler beam-network simulations. Comparisons of experimental and simulated response for HiPco-nanotube and laser-ablated nanotube sheets, indicate that ∼2–30-fold increases in moduli may be achievable in these materials. Increasing the numbers of interrope connections appears to be the best target for improving nanotube sheet stiffnesses in materials containing straight segments. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of conjugation length (L) upon electronic conductivity components of conducting polymers is derived using a generalized resistor network model. Results are obtained for polymers which contain a statistical distribution of defects which limit conjugation, as well as for regular copolymers which have a fixed phase relationship between interruptions in conjugation on neighboring chains. The short-conjugation-length limits of the derived equations are identical with those previously obtained by evaluating molecular aspects of charge transfer. More specifically, when interchain transport fully limits both chain-direction conductivity (σ1) and an orthogonal conductivity (σ2), the calculated electrical anisotropy is σ1/σ2=L2/6d2F, where d is the interchain separation in a hopping direction, and the product d2F is the mean square average projection of the interchain vector on the electric field direction. The present analysis extends predictive capabilities over the entire range from short conjugation lengths to infinite conjugation lengths. For long conjugation lengths terminated by effectively infinite barrier defects, σ1/σ1(∞) and σ2/σ2(∞) are calculated from the parameters which define polymer structure and, for the former ratio, the ratio of infinite chain conductivities parallel [σ1(∞)] and orthogonal [σ2(∞)] to the chains. A general relationship, appropriate for a still wider range of conjugation lengths, is derived between σ1/σ1(∞) and [σ2/σ1(∞)]1/2(L/d)/F1/2, where the geometrical parameter F is of order unity in directions of high σ2. Using this relationship for a polymer of known structure, the chain-direction electrical conductivity in the infinite-chain limit can be derived from measurements of σ1, σ2, and average conjugation length. Good agreement is obtained between the calculated and observed dependence of... [ABSTRACT FROM AUTHOR]

Structure, thermodynamics, and electronic properties are predicted for a new low energy phase of carbon which contains planar sheets equally occupied by sp2 and sp carbon atoms. The isolated planar sheets have the same planar symmetry as do the layers in graphite (p6m) and can be formally viewed as resulting from the replacement of one-third of the carbon–carbon bonds in graphite by –C 3/4 C– linkages. This material, called graphyne, is predicted to have a crystalline state formation energy of 12.4 kcal/mol carbon, which appears to be much lower than for any carbon phase which contains acetylenic groups as a major structural component. Based on the major structural reorganization required for graphitization and the observed high temperature stability of known model compounds, high temperature stability is predicted for graphyne. While graphyne will have similar mechanical properties as graphite, it is predicted to be a large bandgap semiconductor (Eg=1.2 eV) rather than a metal or semimetal. Based on this bandgap and the known behavior of related conjugated polymers having linear structures, interesting nonlinear optical properties (including a large third-order susceptibility) are expected. Property aspects are also predicted for other previously uninvestigated carbon phases which are structurally related to graphyne. Finally, structural features of alkali metal charge–transfer complexes of graphyne, which are expected to be metallic, and of related carbon phases are predicted. [ABSTRACT FROM AUTHOR]

Combining the quantum chemical valence effective Hamiltonian (VEH) technique and crystal packing methods, we have investigated the electronic properties and crystal structure of poly(perinaphthalene), PPN. The goal is to understand the origin of high conductivities observed for a pyrolysis product which contains PPN chains. VEH band structure calculations for an isolated PPN chain predict a band gap which is small, 0.44 eV. The highest occupied and lowest unoccupied (HOMO and LUMO) bands have a combined width of about 9 eV. Analogous calculations for poly(perianthracene), PPA, yield a band gap (2.26 eV) which is much larger than that calculated for PPN. The symmetries of the HOMO and LUMO bands provide an explanation for the band gap difference between PPN and PPA. Crystal packing analysis predicts the existence of two phases for PPN, a phase in which PPN molecules are arranged as overlapping dimer pairs and a nondimeric phase in which there is little intermolecular overlap. The observed diffraction patterns for PPN suggests the packing of overlapping molecules in a third type of structure, possibly stabilized by a degree of irregular reaction during synthesis. Quantum chemical calculations indicate that interchain overlap decreases the band gap of PPN, from the 0.44 eV isolated chain value, to 0.29 eV. [ABSTRACT FROM AUTHOR]

We discuss the formation of gold metal colloids in a variety of surfactant/solvent systems. Static and dynamic light scattering, small angle x-ray and neutron scattering, TEM analysis, and UV-visible absorbance are used to characterize the kinetics of formation and final colloid stability. These gold colloids exhibit a dramatic blueshift and broadening of the plasmon resonance with decreasing colloid size. Several types of reduction method are discussed and differences between micelle (water-free) or microemulsions as reaction media are compared. Use of inverse micelles allows smaller clusters to be formed with greater long-term stability. [ABSTRACT FROM AUTHOR]

We confirm the existence of a 15 Å period in iodine-doped polyacetylene and provide a new interpretation for this key feature as part of a general model for structural changes during iodine doping. The observed diffraction intensities for different samples suggest the existence of structures with two different types of dopant-containing layers: layers obtained by complete replacement of polyacetylene chains by iodine columns (F layers) and layers obtained by replacement of every other polyacetylene chain by an iodine column (P layers). The F layers in the heavily doped complex alternate with dopant-free layers of polyacetylene chains (U layers), corresponding to a (UF)n stacking sequence. The phase obtained at a lower dopant concentration, which provides the 15 Å spacing, is attributed to a (UPUF)n stacking sequence. At still lower dopant concentrations, one obtains a (UP)n stacking sequence. This model, along with published Raman, Mössbauer, and photoelectron spectroscopy data, suggests that the ratio of I-5 to I-3 increases in going from P layers to F layers. Intense and monotonically decreasing, diffuse x-ray scattering suggests that vacancies of size ∼3 Å are present, probably in iodine columns. A diffuse reflection at 3.1 Å, observed in all iodine-doped samples, is due to an average iodine–iodine distance in disordered columnar arrays. On the other hand, ordered arrays of iodine columns in oriented samples give rise to sharp meridional reflections. All ten observed reflections (down to 1.17 Å) in one sample could be indexed based on a 33.8 Å repeat corresponding to (–I-3–I-5–I-3–)n arrays. The observed diffraction pattern was calculated from this model without using any freely adjustable parameters. [ABSTRACT FROM AUTHOR]

Poly(carbon diselenide), (CSe2)x, is obtained as a black powder either by thermal polymerization of CSe2 solutions at high pressure, slow thermal polymerization under ambient conditions of neat CSe2, or photopolymerization of CSe2 solutions. X-ray diffraction measurements indicate that the as-prepared polymer is highly disordered and does not contain free crystalline or amorphous selenium. This result contrasts with the extensive formation of free selenium in the high pressure, high temperature polymerization of neat CSe2. The electrical conductivity is below 10-6 S cm-1, which is consistent with a band gap of ∼2 eV obtained by diffuse reflection spectroscopy. Details of the thermal properties, and the optical absorption, electron spin resonance, infrared absorption, Raman scattering, and extended x-ray absorption fine structure (EXAFS) spectra of the polymer are discussed. The EXAFS data clearly indicate that Se–Se bonds (but not Se chains) are present. The resonance-enhanced Raman and infrared absorption spectra are consistent with the presence of conjugated carbon–carbon and C=Se bonds, respectively. Based on these results, the structure indicated for (CSe2)x is the head-to-head polymer Se || [–Se–Se–C–C–]n, || Se rather than the head-to-tail polymer Se Se || || [–C–Se–C–Se–]n, proposed by previous workers. This polymer thermally decomposes above about 130 °C to form trigonal selenium as a by-product. [ABSTRACT FROM AUTHOR]

A single-crystal charge–transfer complex of a phenyl-end-capped tetramer of polyaniline has been synthesized and studied along with a similar dimer of polyaniline. Structural, optical, and electrochemical studies of these oligomers in various oxidation states provide detailed information which has been used to model the structure of polyaniline and its evolution during electronic doping. These studies of the polymer and its oligomers suggest that the emeraldine salt form of the polymer (50% doping per nitrogen) is a preferred low-energy structure. The preference for this structure leads to phase segregation in doped compositions having average doping levels less than 50%. [ABSTRACT FROM AUTHOR]

Polymerization of carbon diselenide, CSe2, at 5 kbar and ∼100 °C has been reported to give a metallic (CSe2)n ladder polymer that superconducts near 6 K at 220 kbar pressure. Using a variety of techniques we find that the material synthesized (showing essentially the same x-ray diffraction pattern as previously published) is not (CSe2)n but instead consists of a mixture of free, trigonal Se and an amorphous C–Se polymeric composition consistent with the formula (CSe0.5)n. Interestingly, possibly as a consequence of staged reactions during synthesis, the Se phase consists of a mixture of large, aggregated crystallites (∼600 Å in diameter) melting at ∼220 °C and nonaggregated small crystallites (∼150–250 Å in diameter) showing a broad melting transition with an endothermic maximum at ∼180 °C. Percolation of the C–Se polymeric composition probably provides the high observed electrical conductivity [σ(300 K)≊10–20 S cm-1 and σ(300 K)/σ (8 K)≊2.5] in the presently available samples. The previously reported high pressure superconductivity is probably associated with the free Se phase in the samples, since selenium is a known superconductor in the 6 K and 220 kbar range. [ABSTRACT FROM AUTHOR]

Utilizes rapid thermal processing to sinter and anneal phase-pure and copper oxide-rich YBa[sub2]Cu[sub3]O[sub7] fibers and woven fibers formed by melt spinning a powder. Analysis of the optimum temperature for sintering the powders; Obstacle to the utilization of polycrystalline high-temperature superconductors; Significance of rapid thermal processing.

Describes a study on a technique to monitor the adsorption of mercury vapor on optically thin gold films. Experimental apparatus and methods; Detection of mercury vapor; Behavior of the reflectivity of semitransparent metal films as a function of thickness.

Monodisperse L10-FePt nanoparticles with size ranging from 3 to 8 nm were prepared by the salt-matrix annealing method. It was observed that the annealed particles have high thermal stability—no sintering occurred even when the particles were heated at 1100 °C for an hour. This high thermal stability resulted from carbon coating of the particles during salt-matrix annealing as consequence of decomposition of surfactants. [ABSTRACT FROM AUTHOR]

Twisted laminar composite structures for high power and large-stroke actuators based on coiled Multi Wall Carbon Nanotube (MWNT) composite yarns were crafted by integrating high-density Nanoenergetic Gas Generators (NGGs) into carbon nanotube sheets. The linear actuation force, resulting from the pneumatic force caused by expanding gases confined within the pores of laminar structures and twisted carbon nanotube yarns, can be further amplified by increasing NGG loading and yarns twist density, as well as selecting NGG compositions with high energy density and large-volume gas generation. Moreover, the actuation force and power can be tuned by the surrounding environment, such as to increase the actuation by combustion in ambient air. A single 300-μm-diameter integrated MWNT/NGG coiled yarn produced 0.7 MPa stress and a contractile specific work power of up to 4.7 kW/kg, while combustion front propagated along the yarn at a velocity up to 10 m/s. Such powerful yarn actuators can also be operated in a vacuum, enabling their potential use for deploying heavy loads in outer space, such as to unfold solar panels and solar sails. [ABSTRACT FROM AUTHOR]

X-ray diffraction measurements on all-trans-polyacetylene are consistent with a chain-axis length elongation upon donor doping (+0.026 Å for lithium and +0.04 Å for potassium) and a chain-axis length contraction upon acceptor doping (-0.010 Å for iodine), where the changes refer to the length L of a C2H2 unit (2.457 Å in the undoped polymer). These new experimentally derived results for heavily doped compositions, which ignore possible corrections for cell nonorthogonality in the lithium and iodine complexes, are similar to experimental results for graphite intercalation complexes and are consistent with theoretical predictions for doped polyacetylene. The meridional diffraction lines observed at L and L/2 for potassium-doped polyacetylene indicate that there is no lattice symmetry element which includes a translation operation of L/2 in the chain-axis direction. The observations are consistent with a structural model in which alkali–metal ions with an intracolumn spacing of 4.96 Å are commensurate with the polymer chains for the composition (CHM0.125)x. The likely polymer chain-axis repeat length is 2L (i.e., C4H4) and a lattice symmetry element which includes a translation of L is expected. [ABSTRACT FROM AUTHOR]

We studied angle-dependent reflectivity spectra of opal photonic crystals infiltrated with cyanine dye aggregates, which are highly polarizable media with very large Rabi frequency. We show that, at resonance condition between the exciton-polariton of the dye aggregate and the Bragg gap, the Bragg stop band decomposes into two reflectivity bands with a semitransparent spectral range in between, that is due to propagation of braggoritons inside the gap. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have measured for the first time the temperature-dependent anisotropic resistivity of several Tl2Ca2Ba2Cu3Ox single crystals. The resistivity ratio between the c axis and the a-b plane orientations in the best crystals is approximately 60 at room temperature. Resistivity data indicate substantial differences in both Tc and the transition width of nominal Tl2Ca2Ba2Cu3Ox plates grown from two different melt compositions. A Tl-O and CuO-rich flux yielded approximately stoichiometric plates with sharp transitions beginning near 113 K, while a Tl-O-rich flux produced plates containing more Tl and less Ba with broad transitions starting near 103 K. These data demonstrate the extreme sensitivity of the superconductivity to cation site disorder in the Tl-Ca-Ba-Cu-O system. [ABSTRACT FROM AUTHOR]

Flux creep data are reported which clearly demonstrate a dramatic enhancement in the pinning potential for magnetic vortices (fluxoids) in a Tl2Ca2Ba2Cu3Oy single crystal upon irradiation with 4.5 MeV protons. The creep rate is markedly reduced between 5 and 80 K, and the effective pinning potential is quadrupled from ≊100 to ≊400 meV at 20 K in a field of 50 mT. This stronger pinning enhances the magnetization hysteresis and hence the inferred low-field critical current density by a factor of 5 at 40 K. [ABSTRACT FROM AUTHOR]

The in-plane structure of single-crystal samples of Tl2Ba2CaCu2O8 has been imaged at room temperature using a scanning tunneling microscope. Atomic-resolution surface images exhibit areas in which the structure has tetragonal symmetry (peak spacing 2.5±0.2 Å) and regions in which the structure is distorted from tetragonal symmetry. The lattice spacing indicates that the observed structure corresponds to the in-plane thallium and oxygen positions. The observation of both sites also suggests that the Tl-O band makes a significant contribution to the density of states near the Fermi level. In addition, a weak one-dimensional superlattice (period 10±0.5 Å) which shows short-range order has been observed. [ABSTRACT FROM AUTHOR]

Thin unoriented polycrystalline films in the Tl-Ca-Ba-Cu-O system were prepared by sequential electron beam evaporation of multiple Tl, Ca, Ba, and Cu layers and a two-stage anneal under controlled Tl and oxygen overpressures. These films show zero resistance as high as 97 K and transport critical current densities at 76 K up to 110 000 A/cm2 with typical values of 50 000 A/cm2. The critical currents exhibit a modest magnetic field dependence. Meissner effect data show an onset at 110 K and a superconducting fraction of 75%. Compositional and structural studies indicate that the films, although polyphase, are predominantly Tl2Ca1Ba2Cu2Oy. There is no evidence of other superconducting phases. [ABSTRACT FROM AUTHOR]

Optical Faraday rotation has been measured as a function of the frequency of an applied magnetic field in two compositions of the dilute magnetic semiconductor CdMnTe. A roll-off in response is observed with the roll-off frequency increasing with Mn concentration. The frequency dependence is fit well by a simple Debye relaxation model. This dependence is attributed to relaxation of the magnetization through interactions between the Zeeman and spin-spin energy reservoirs. [ABSTRACT FROM AUTHOR]

A novel electrochemical process has been developed for the formation of superconducting films. Using this process, superconducting films of Bi2Sr2Ca1Cu2O8 and (Pb,Bi)2Sr2Ca1Cu2O8 have been formed. The process consists of simultaneously depositing the metallic constituents of the superconductor from a single electrolyte, and thermally oxidizing the resulting precursor film to form the superconducting phase. Application of -4 to -5 V vs Ag/Ag+ to a conductive cathode substrate, immersed in an electrolyte containing salts of all of the metals, reduces the metal cations causing them to deposit on the cathode as a metallic film precursor. Precursor films having desired stoichiometries were obtained by regulating the electrolyte bath composition. [ABSTRACT FROM AUTHOR]

Methods for obtaining sample modulated Raman (SMR) spectra and frequency modulated visible light are described. The sample modulated Raman method is one in which the frequency of the Raman scattered light is modulated by the sample which is responding to an externally applied periodic stress. The Raman active modes of the sample which are due to the vibrational motion of those molecular-scale units that change in response to the external stress are modulated and appear in the frequency-locked phase sensitively detected Raman spectrum. Those modes due to units that do not respond significantly to the stress do not appear. In principle the SMR method permits determination of those molecular units of a complex substance that are responsible for the substance's response to an external stress. The Raman scattered light is frequency modulated, has the wave form nu=nu(0)-nu(i)-(deltanu) sin(2pi ft), where nu(0) is the incident monochromatic light, nu(i) is the vibrational frequency of the statically stressed material, and (deltanu) is the magnitude of the frequency shift caused by the maximum stress applied at the frequency f.