Raman spectra obtained from a nanocrystalline CuO thin film are observed to exhibit significant variation in the peak position and peak line-shape as a function of spatial position within the film. We attribute this effect to variation in the degree of local heating beneath the focused spot of the Raman probe laser. To understand this, we have undertaken a detailed study of the temperature-dependence of the CuO Ag Raman peak. We observe a linear relationship between line-width and peak position, which persists over a wide temperature range, and is characteristic of a Raman process in which the temperature-dependence is dominated by anharmonic 3-phonon decay. We provide an analytical description of the Raman line-shape as a function of temperature and use this model to interpret the degree of laser heating observed within our sample. Using this relationship, we identify that the local micro-morphology of the CuO sample under study can dramatically affect the temperature achieved due to laser heating. We find that spectra collected from the surface of “micro-bubbles” within the CuO film studied can reach temperatures of >1000 K beneath the focused spot of our low power (5 mW) probe laser.

An experimental study of the low temperature magnetoresistance (MR) in a FeCl3-doped copolymer in which ethylenedioxythiophene and phenylene are alternatively linked by vinylene units is reported. The temperature dependence of conductivity followed characteristics of Coulomb gap variable range hopping (VRH) at low temperatures. The MR showed a crossover from negative to positive values. The crossover was explained by competition of negative and positive contributions which originated from quantum interference and shrinkage of wave function, respectively. (C) 2013 Elsevier B.V. All rights reserved.

We show from a bond valence sum correlation that very high superconducting $ T_{c} $ values should be found in optimally hole-doped infinite-layer ACuO$ _{2} $ cuprates - up to 160~K for A = Ba. The projected increase in $ T_{c} $ across the series arises from "internal pressure" effects as A runs from Mg to Ba. We then use density functional theory to investigate these pressure effects on the band structure in an attempt to understand this progressive increase in $ T_{c} $. Where these materials have been synthesised we find good agreement between our calculated structural parameters and the experimental ones. We find that internal pressure associated with increasing ion size does indeed enhance the superconducting energy gap, as observed, via modifications to the electronic dispersion. Furthermore, in our calculations, pressure alters the dispersion independently of how it is applied (internal or external) so that the superconducting energy gap correlates with the unit-cell volume and a Fermi-surface shape-parameter describing ratio of next-nearest-neighbor to nearest-neighbor hopping integrals. We infer an energy scale for the pairing interaction of the order of 1~eV, well above the magnetic energy scale.

Variable-range hopping (VRH) is an important conduction mechanism in disordered conductors. One example of such a disordered conductor is reduced graphene oxide in which VRH dominates the temperature dependence of electronic conduction. Electronic transport is generally by electric field-assisted, thermally-driven VRH, following the theory of Mott, and Pollak and Riess. However, with the increase of electric field and decrease of temperature, we identify a surprisingly smooth crossover to solely field-driven VRH described by the theory of Shklovskii. We give the analytic expression for the crossover field E C as a function of temperature and coefficients from thermally-driven and field-driven VRH. Besides reduced graphene oxide, we show in this work that our crossover scenario can also account for the experimentally measured conductivity data of three-dimensional (3D) carbon networks as well as that of quasi-1D highly-doped conducting polymers, illustrating the wide validity of our proposed physical scenario. Our crossover scenario has the advantage of combining two distinct regimes of VRH conduction yet remaining within the currently established theoretical framework.

To investigate the electronic transport mechanisms in multi-layered graphene oxide (MGO), the temperature-dependent electrical conductivity (σ(T)) has been measured as a function of the annealing temperature (Ta). An individual MGO flake was gradually reduced by thermal annealing at Ta from 88 to 300°C, with the reduction process confirmed at each stage by X-ray photoelectron spectroscopy. As Ta increases, the σ(T) of the MGO also increases. We found that the σ(T) is well interpreted by variable-range hopping in disordered regions in series through activated conduction across small barriers. We associate the localized states for hopping with the oxygen functional groups in GO, as well as the small activation barriers with the domain boundaries between the clustered oxygen functional groups and the graphitic region. Both the hopping and activation barrier resistances decrease systematically as the Ta increases.

Freestanding few layer graphene platelet/polyvinylidene fluoride composites have been synthesized, resulting in flexible, light weight, and durable electrically conducting films. These composites exhibit reasonable thermoelectric properties including a maximum electrical conductivity of 2005 S m(-1), Seebeck coefficient of 18.3 mu V K-1, and power factor of 0.52 mu W m(-1) K-2. The temperature dependent behavior of these properties is also investigated. The electrical conductivity of the composites exhibits thermal fluctuation-assisted tunneling behavior coupled with a high energy phonon scattering term, while the thermoelectric power is characterized by electron-phonon enhanced metallic diffusion thermopower plus a phonon drag term. This results in a characteristic change in majority charge carrier type from hole to electron as the temperature decreases below 60 K. These composites have the potential to be used in low power applications where sufficient waste heat is available. (c) 2013 Published by Elsevier B.V.

Multiwalled carbon nanotubes (MWNTs) have been grown using a standard chemical vapor deposition method, except for varying the growth temperature. Nanotubes grown below 770 °C exhibit typical positive thermoelectric powers, while those grown above have negative values. This behavior is attributed to the larger nanotube diameters observed at higher growth temperatures. Below 770 °C, the average nanotube diameter is about 50 nm, while above, nanotubes reach diameters of 300 nm. This increase in diameter and number of inner shells leads to the intrinsic negative thermoelectric power of the inner nanotube shells becoming larger than the positive thermoelectric power due to oxygen doping on the outer surface of the nanotube. The overall negative thermopower (about −6 μV/K, compared to +7 μV/K for smaller diameter nanotubes) can be understood in terms of a parallel conduction model. Our large-diameter multiwalled carbon nanotubes allow the intrinsic negative thermopower of MWNTs to be accessed without requiring specific deoxygenation treatment.

Conduction in thin random networks of single‐walled carbon nanotubes (SWNTs) is typically dominated by metallic SWNT segments and limited by variable‐range hopping (VRH) in disordered junction regions. However, in our surfactant‐free networks, we show that in parallel with VRH there is another mode of conduction involving both semiconducting and metallic SWNTs. This second process showing activated behavior makes a substantial contribution to conductance at higher temperatures, with similar activation energies in different samples despite a large variation in overall magnitude of the conductivity. From the magnitude of the activation energy (0.160.05\,eV), we ascribe this additional term to activation across Schottky barriers between metallic and semiconducting SWNTs in conducting paths involving a small number of semiconducting SWNTs. The Mott parameters for VRH are also independent of the overall conductivity, indicating that the local structure of intertube contacts between metallic SWNTs in the most conductive paths is also similar in samples of different overall conductivity.

We use density functional theory to investigate external-pressure and effects in the infinite-layer cuprate ACuO$_2$ for A={Mg, Ca, Sr, Ba}, where internal-pressure is induced by ion-size substitution. Where these materials have been synthesised we find good agreement between our calculated structural parameters and the experimental ones. We find that these non-hydrostatic pressure-effects can have a significant effect on the superconducting energy gap via modifications to the electronic dispersion. Furthermore, pressure alters the dispersion independently of how it is applied (internal or external) so that the superconducting energy gap correlates with the unit-cell volume.

We report an analysis of low-temperature measurements of the conductance of partially disordered reduced graphene oxide, finding that the data follow a simple crossover scenario. At room temperature, the conductance is dominated by two-dimensional (2D) electric field-assisted, thermally driven (Pollak–Riess) variable-range hopping (VRH) through highly disordered regions. However, at lower temperatures T, we find a smooth crossover to follow the exp(−E0/E)1/3 field-driven (Shklovskii) 2D VRH conductance behaviour when the electric field E exceeds a specific crossover value E C ( T ) 2D = ( E a E 0 1 / 3 / 3 ) 3 / 4 ?> determined by the scale factors E0 and Ea for the high-field and intermediate-field regimes respectively. Our crossover scenario also accounts well for experimental data reported by other authors for three-dimensional disordered carbon networks, suggesting wide applicability.

We suggest, and demonstrate, a systematic approach to the study of cuprate superconductors, namely, progressive change of ion size in order to systematically alter the interaction strength and other key parameters. R(Ba,Sr)2Cu3Oy (R={La,…,Lu,Y}) is such a system where potentially obscuring structural changes are minimal. We thereby systematically alter both dielectric and magnetic properties. Dielectric fluctuation is characterized by ionic polarizability while magnetic fluctuation is characterized by exchange interactions measurable by Raman scattering. The range of transition temperatures is 70-107 K, and we find that these correlate only with the dielectric properties, a behavior which persists with external pressure. The ultimate significance may remain to be proven, but it highlights the role of dielectric screening in the cuprates and adds support to a previously proposed novel pairing mechanism involving exchange of quantized waves of electronic polarization.

A quarter of a century after their discovery the mechanism that pairs carriers in the cuprate high-Tc superconductors (HTS) still remains uncertain. Despite this the general consensus is that it is probably magnetic in origin [1] so that the energy scale for the pairing boson is governed by J, the antiferromagnetic exchange interaction. Recent studies using resonant inelastic X-ray scattering strongly support these ideas [2]. Here as a further test we vary J (as measured by two-magnon Raman scattering) by more than 60 changing ion sizes in the model HTS system LnA2Cu3O7-δ where A=(Ba,Sr) and Ln=(La, Nd, Sm, Eu, Gd, Dy, Yb, Lu). Such changes are often referred to as "internal" pressure. Surprisingly, we find Tcmax anticorrelates with J where internal pressure is the implicit variable. This is the opposite to the effect of external pressure and suggests that J is not the dominant energy scale governing Tcmax.

Nanocrystalline lms of cupric oxide (CuO) produced by thermal oxidation havebeen characterised using x-ray analysis, SEM image analysis and temperature-dependent con-duction measurements.We describe in detail the x-ray di ractometer calibration, paying partic-ular attention to a function- tting procedure which enables accurate subtraction of instrumentalcontributions to the sample di ractograms. The Scherrer and Williamson-Hall models are usedto calculate crystallite size and sample strain and also give some indication of spatial inhomo-geneity. Image analysis techniques which can discern individual `grains' (the circular Houghtransform and the ImageJ particle analyser) were used to evaluate the grain size distributionfrom SEM images. An average crystallite diameter of 30 nm - determined by the ImageJ par-ticle analyser - closely agrees with the various XRD analysis approaches. Electronic conductionin our samples is found to proceed via thermally activated transport, which we attribute tothe presence of a well-de ned trap state that lies 0.2 eV from the valence band edge. Thevoltage-dependence of the activation energy additionally shows that the activation is a bulke ect and not due to Schottky barriers between the sample and the metal contact.

The dependencies of electrical conductivity on the electrical field and temperature of high-resistivity polystyrene/carbon nanotube (CNT) composites were investigated. The electrical conductivity of the samples and their thermal activation energy were systematically dependent on the CNT doping level. Several unusual phenomena were observed which could be explained by the inhomogeneous distribution of CNTs in the polystyrene matrix. At higher levels of CNT doping, an interconnected conducting network is formed. If the amount of CNT is equal to or less than 0.6%, "islands" of CNT become separated by the layers of polystyrene, which limit charge transport. At low voltages, there is a sharper decrease in conductance as the voltage decreases.

Thermoelectrics are materials capable of the solid-state conversion between thermal and electrical energy. Carbon nanotube/polymer composite thin films are known to exhibit thermoelectric effects, however, have a low figure of merit (ZT) of 0.02. In this work, we demonstrate individual composite films of multiwalled carbon nanotubes (MWNT)/polyvinylidene fluoride (PVDF) that are layered into multiple element modules that resemble a felt fabric. The thermoelectric voltage generated by these fabrics is the sum of contributions from each layer, resulting in increased power output. Since these fabrics have the potential to be cheaper, lighter, and more easily processed than the commonly used thermoelectric bismuth telluride, the overall performance of the fabric shows promise as a realistic alternative in a number of applications such as portable lightweight electronics.

We prepared three graphene devices with different levels of disorder in the graphene structure by 1. exfoliation from highly oriented pyrolytic graphite (HOPG), 2. chemical vapour deposition (CVD), using Cu foil as a catalyst, and by 3. CVD, using a Ni layer evaporated on a Si/SiO2 substrate as a catalyst. The gate voltage characteristics, quantum Hall effect (QHE) and temperature (T) dependences of the conductivity of these samples were studied. A comparison of the transport features indicates that, unlike the highly ordered crystal of exfoliated graphene, the graphene sample produced by the CVD method on the Ni substrate shows a significant degradation of the QHE signature with a resistivity value three orders of magnitude higher than that in a graphene crystal. In the exfoliated material phonon scattering dominates the electronic transport at high carrier density, thus reducing the conductivity at high T. Thermal assistance plays an important role in the electronic transport of our graphene grown on Ni with more disordered structure, which causes conductivity to increase with rising T. As to graphene grown on Cu foil, it displays many features similar to those of a perfect graphene crystal. This is good news for possible applications of CVD-grown graphene. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim