Electron Spin Resonance of K-Doped Single-Wall Carbon Nanohorns and Single-Wall Carbon Nanotubes

Abstract

K-doping into the aggregate of single-wall carbon nanohorns (SWNHs) and into the bundle of single-wall carbon nanotubes (SWNTs) are introduced in this study. Electron spin resonance (ESR) of the pristine SWNHs and SWNTs show that the pristine SWNHs are ESR active and the linewidth (ΔH) is susceptive to the partial pressure of O₂, while the pristine SWNTs are ESR silent. ΔH of K-doped SWNHs becomes wider by a factor of ?2 than that of the pristine one. For K-doped SWNTs, the Dysonian type ESR comes to be observed. Details of the ESR features are discussed.     

Structural phase transition in crystals of the molecular conductor α′-(BDH-TTP)<Subscript>6</Subscript>[Hg(SCN)<Subscript>3</Subscript>][Hg(SCN)<Subscript>4</Subscript>]

Single crystals of the organic conductor α′-(BDH-TTP)₆[Hg(SCN)₃][Hg(SCN)₄] are studied by low-temperature X-ray diffraction at 100–300 K. It is shown that, in the temperature range 250–200 K, the compound undergoes a phase transition, which is accompanied by the appearance of superstructure reflections and a rapid increase in their intensity due to the partial anion and cation ordering. The superstructure is observed down to a temperature of 100 K, at which the set of diffraction data is collected. The temperature dependence of the electrical resistance exhibits a minimum in the range of the structural transition temperature. Under a pressure of up to 10 kbar, the general behavior of the dependence remains unchanged and only the minimum is shifted toward the low-temperature range. The phase transition is completely reversible.     

Effect of Pressure on Antiferromagnetic Transition in Alkali-Electro-Sodalite

Abstract

Pressure (0–19 kbar) and temperature (4–300 K) dependent EPR study of Sodium-Electro-Sodalite (SES) is presented. SES, which consists of a bcc sub-lattice of F-centers supported by a zeolite-like framework, is known to be a Mott insulator at room temperature. On cooling, SES undergoes an AF transition at 48±2 K providing the first example of an s-electron antiferromagnet. We find that the width of the EPR resonance above T _N is influenced not only by a strong exchange interaction, but also by a fast spin-lattice relaxation. Also, with increasing pressure, T _N decreases linearly and extrapolates to 0 K at about 65 kbar. The reason for this seemingly unexpected behavior is briefly discussed.     

Crystal Chemistry of Nanotubes Lattices

Abstract

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.     

High Pressure Studies of the Phase Transition Enthalpies of the Liquid Crystal EBBA up to 2.5 kbar

The liquid crystal p-Ethoxy-benzylidee-p-n-butylaniline(EBBA) exhibits one nematic-liquid isotropic and two solid-nematic coexistence lines.

With a high pressure micro calorimeter based on differential scanning calorimetry (DSC) previously described the phase transition enthalpies of these three phase transitions have been measured in the pressure range 1 bar to 2.5 kbar. Numerical data are presented for the transition temperatures, the enthalpy changes, the entropy changes, and the volume changes for both solid-nematic transitions as well as for the nematic-liquid isotropic transition.

Additionally the effect of impurities on the nematic-liquid isotropic transition enthalpy at normal pressure is discussed.     

Pressure and temperature dependence of the electrical resistivity of bulk Al23 Te77 glass

Electrical resistivity of bulk amorphous Al₂₃T₇₇ samples has been studied as a function of pressure (up to 80 kbar) and temperature (down to 77 K). At atmospheric pressure the temperature dependence of resistivity obeys the relation ? = π₀ exp(δE/RT) with two activation energies. In the temperature range 300 K ? T > 234 K the activation energy is 0.58 eV and for 234 >T ? 185 K the value is δE = 0.30 ev. The activation energy has been measured as a function of pressure. The electrical resistivity decreases exponentially with the increase of pressure and at 70 kbar pressure the electrical behaviour of the sample shows a metallic nature with a positive temperature coefficient. The high pressure phase of the sample is found to be a crystalline hexagonal phase.     

Low-Temperature Electron Spin Resonance in (TMTSF)2PF6 in the High Pressure Metallic Phase

Abstract

The electron spin resonance of (TMTSF)₂PF₆ has been observed at low fields (H_o < 110 Oe) in the high pressure, metallic phase (p > 6.5 kbar) in the temperature range 1-4^?K. The anisotropy in the g value is similar to that observed at ambient pressure above the metal-insulator transition. The linewidth is very narrow and the spin susceptibility strongly decreases as the superconducting transition is approached from above. We interpret this as evidence for singlet-paired superconductivity. Superconductivity is observed at 1.1 K and the critical field has angular dependence in the be plane. These observations lead us to conclude that (TMTSF)₂PF₆ is a singlet paired superconductor.     

Magnetic Order in Organic Superconductors

We have undertaken an electron spin resonance study at both low and high magnetic field on the superconducting phase of the ambient pressure superconductor (BEDT-TTF>₂I₃, (ET)₂I₃, over the temperature range 1-300 K and hydrostatic pressure range from 0 to 2 kbar. At ambient pressure the ESR results are consistent with the picture of (ET)₂I₃ as a metal. Superconductivity is observed at 1.6 K via low field ESR. Application of modest pressures strongly suppresses the superconducting transition temperature. At pressures above about 0.3 kbar the superconductivity is suppressed in favor of an as-yet-unidentified magnetic state whose onset is at 7K. Through an analysis of the microwave ESR lineshape we find that the microwave conductivity over the temperature range 5-50 K is in agreement with dc measurements.     

Electrochemical Doping of Single Wall Carbon Nanotubes with Lithium

Abstract

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.     

Superconductivity in the Organic Charge Transfer Salts: (TMTSF)2X and (TMTTF)2X

Abstract

We report pressure dependent studies of the a-axis resistivity as a function of temperature for several members of the isostructural families of organic charge transfer salts, (TMTSF)₂X and (TMTTF) ₂X. For a typical (TMTSF)₂X material the low temperature metal-insulator transition seen at 1 bar is suppressed above some critical pressure, P_c, where a superconducting transition is observed near 1 K. We find a correlation between P_c and the ambient pressure c lattice parameter which reflects the anion size. The (TMTTF) _cX salts exhibit very different ambient pressure behaviour but we find that with the application of sufficiently high pressures (～30 kbar) their behaviour resembles that seen in the (TMTSF)₂X family but at lower pressures. In particular we find evidence of a possible superconducting transition near 4 K in (TMTTF)₂Br at 25 kbar. At this pressure the conductivity near 4 K is extremely high with a value approaching 10⁶ (Ωcm)^?1 and the resistivity ratio is about 400.     

Chiral Gay–Berne model for molecular dynamics computer simulations

Abstract

We have extended the chiral Gay–Berne (GB) pair potential of Memmer and co–workers [Liq. Cryst. (15), 345 (1993)] to the general case of biaxial ellipsoids, and we provide explicit expressions for the gradient and the torques to be used in molecular dynamics (MD) codes. To test our results against published results we have performed MD simulations of samples formed by N?=?1024 uniaxial GB ellipsoids with constant volume, and we have studied how the thermotropic behavior is affected by particle chirality. We have run temperature scans starting from isotropic samples and found clear signatures of the spontaneous formation of both lower temperature blue and cholesteric phases.     

Thermal Expansion and Superconductivity in Alkali Metal- and Silver-doped Bi?Sr?Ca?Cu?O System

Samples with the nominal composition Bi₂Sr₂CaCu₂O_y with na doped at the Ca site and K doped at the Sr site are prepared by solid state reaction method. From the X-ray diffraction data it is found that all the the samples have exhibited a single phase 2212. The D.C. electrical resistivity data show that for Na-doped samples the T_c (zero) varies from 80 K to 85 K and for K-doped samples it is from 79 K to 82 K. The loss of oxygen from these samples around 400°C was confirmed by high temperature dilatometry. The variation of the thermal expansion coefficient (“α”) with temperature for different alkali dopings are discussed. Also the samples with the nominal composition Bi₄Sr₃Ca₃Cu_4−xAg_xO_y (with x = 0, 0.1, 0.2, 0.3) were studied.     

Electrical investigation of polythiophene–poly(vinyl acetate) composite films via VTF and impedance spectroscopy

Polythiophene (PTP) and poly (vinyl acetate) (PVAc) composite films were prepared by chemical oxidative polymerization method with FeCl₃ as an oxidant, in methanol at room temperature. Their dc conductivities as a function of temperature (313–358 K) were measured. An attempt has been made to investigate the effect of temperature and concentration of ferric chloride oxidant on the conductivity of polythiophene–poly(vinyl acetate) composite films. For fixed wt% of PVAc, the dc electrical conductivity of the film initially increases with the molar concentration of FeCl₃ and then decreases with the further increase in the concentration of FeCl₃. The temperature dependence of conductivity showed Arrhenius behavior. The conductivity isotherms show VTF behavior which is typical for ion conducting polymers. The Nyquist plot of Z′ vs. Z″ were plotted for frequency range 100 Hz–200 kHz and temperature range of 313–358 K. These plots consist of semicircles for higher temperatures. This suggests Debye type relaxation mechanism. The equivalent circuit consists of parallel combination of bulk resistance R_B and bulk capacitance C_B. The bulk resistance R_B of the samples decreased with increase in temperature.     

Pressure Dependence of the Metal-Insulator and Superconducting Phase Transitions in (TMTSF)2 Reo4

Measurements of the pressure and temperature dependence of the a-axis resistivity of (TMTSF) ₂ReO₄, are presented. The metal-insulator transition seen in this material at the remarkably high temperature of ～180 K at ambient pressure and which is associated with an ordering of the ReO₄, anions is suppressed under pressure. For pressures above ～9.5 kbar we observe a superconducting transition near 1.3 K. There is a narrow intermediate pressure regime about 2.5 kbar in width in which both superconductivity and effects of anion ordering are observed. In this regime (i) a superconducting transition is seen near 1.3 K even though ρ just above the transition can be up to 10-100 times greater than ρ(300 K), and (ii) there is an extraordinarily large hysteresis in p below ～ 100 K with the possibility of varying the resistance of the low temperature state by several orders of magnitude by appropriate temperature cycling. These results establish the first order character of the transition. We suggest that at high pressures the anions remain frozen in a metastable disordered state to low temperatures.     

Pressure and Temperature Induced Frequency Shifts of Raman-Active Phonons in the MX Solid Pt2Br6(NH3)4

Abstract

We report the dependencies on hydrostatic pressure up to 100 kbar, and on temperature from 10 K to 300 K of the frequencies of Raman-active stretching modes in the single crystal MX chain solid Pt₂Br₆(NH₃)₄ (PtBrn). The data show that a predicted pressure-induced metallization does not occur, and suggests instead that either a 3-D structural distortion or a transition to a broken-symmetry ground state different from the charge-density wave (CDW) takes place.     

High Pressure Studies on Mesomorphic and Polymesomorphic Transitions

The paper presents the following results: (1) pressure induced mesomorphism in p-p'-n-butylazobenzene and biphenyl p-ethyl benzoate. (2) the monotropic nematic-isotropic transition in trans-p-n-propoxy-x-methyl cyanophenyl cinnamate becoming enantiotropic under pressure, (3) suppression of the cholesteric mesophase in cholesteryl nonanoate and the location of a solid-cholesteric-isotropic triple point at ～ 2.85 kbar. confirming an earlier observation of Keyes. Weston and Daniels on the same compound, (4) a measurement of dT/dp for the solid-smectic A, smectic A-cholesteric and cholesteric-isotropic transitions in cholesteryl myristate and a comparison with the values evaluated from the Clausius-Clapeyron equation using the known latent heat and volumetric data.     

The effect of high pressure on crystallization of splat-cooled V2O5

Crystallization of splat-cooled V₂O₅ under high pressure was examined. By X-ray diffraction and thermal analysis, it is shown that the non-crystalline V₂O₅ crystallizes dramatically up to about 6 kb. Crystallization temperature increases with increasing pressure under this pressure range.At higher pressure (20–60 kbar), crystallization occurs gradually and the crystallization temperature has a slight tendency to decrease with increasing pressure.     

High-pressure Raman study of 6H-PbI2

High-pressure Raman spectra of two solid phases of 6H-PbI₂ have been investigated under hydrostatic pressure up to 14 kbar at ambient temperature. The results are quite reproducible on different samples, when covering the same pressure cycle. A consistent Raman spectrum is obtained after the sample is compressed at least twice. The effect of hysteresis is reduced after several pressure treatments.     

Effects of Lattice Vacancy in Carbon Nanotubes Conductance Quantization

Abstract

Conductance of carbon nanotubes (CN's) with a lattice vacancy or strong and short-range impurity potential is studied within a tight-binding model. The conductance is quantized into zero, one, or two times the conductance quantum e ²/π? depending on the type of the vacancy if its size is much smaller than the circumference of CN.     

Poly(3,4-ethylenedioxythiophene)/carbon-based nanocomposite for gas sensing

Abstract

In this study, we have manufactured sensory films based on poly(3,4-ethylenedioxythiophene)/carbon nanotubes/reduced graphene oxide nanocomposite. The charge transport in obtained films was analyzed using impedance spectroscopy and temperature dependence of conductivity in the 90–325?K temperature range. The activation mechanism of charge transfer was established and the activation energy of conductivity was determined. The percolation effect in the nanocomposite was found with increasing the content of carbon nanoparticles. A decrease in resistance and an increase in the capacity of hybrid films due to the adsorption of water and ammonia molecules were revealed. Found features of the charge transport processes can expand the prospects of application of nanocomposite films for sensor electronics.