Abstract: We report the growth, structural, and electrical characterization of single-crystalline iron pyrite (FeS2) nanorods, nanobelts, and nanoplates synthesized via sulfidation reaction with iron dichloride (FeCl2) and iron dibromide (FeBr2). The as-synthesized products were confirmed to be single-crystal phase pure cubic iron pyrite using powder X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. An intermediate reaction temperature of 425 °C or a high sulfur vapor pressure under high temperatures was found to be critical for the formation of phase pure pyrite. Field effect transport measurements showed that these pyrite nanostructures appear to behave as a moderately p-doped semiconductor with an average resistivity of 2.19 ± 1.21 Ω·cm, an improved hole mobility of 0.2 cm2 V–1 s–1, and a lower carrier concentration on the order of 1018–1019 cm–3 compared with previous reported pyrite nanowires. Temperature-dependent electrical transport measurements reveal Mott variable range hopping ...

Abstract: The outstanding catalytic properties of cerium oxides and, consequently, the broad use in heterogeneous catalysis rely on the easy Ce3+ ↔ Ce4+ redox conversion. Within the two-state model of Marcus, the electron transfer associated with the redox process is governed by the electronic coupling matrix element VAB that accounts for the interaction between the diabatic electronic states at the crossing seam. Here we present a computational analysis based on ab initio quantum mechanics theory that allows for a characterization of negative polaron structures and intrinsic polaron hopping in bulk CeO2. The relevant parameters inherent to the model: reorganization energy, activation barrier, and electronic coupling for the 4f→ 4f electron hopping are estimated for several models. Our analysis predicts an activation barrier of 0.4 eV and a transmission coefficient κ = 0.81, confirming the earlier proposed adiabatic theory of small polaron and hopping conductivity in reduced bulk ceria.

Abstract: The electrical properties of ferrites are important to their applications at high frequencies. In this paper, we investigate the colossal permittivity behavior of Ni0.5Zn0.5Fe2O4 ceramic prepared from powders synthesized by the citric acid combustion method. Its bulk conductivity is attributed to the variable-range hopping of localized polarons. These polarons exhibit universal dielectric response with frequency. They are frozen at low temperature and activated at high temperature. As temperature decreases from 235 to 130 K, their hopping energy decreases from 223 to 113 meV, while the estimated hopping range increases from 3.5 to 4.1 nm. The direct correlation of polaron conduction and colossal permittivity of NZFO is definitely established by a modified Koops’ equivalent circuit based on the internal barrier layer model, which is well confirmed by the experimental data.

Abstract: Temperature dependent Hall effect measurements from 20 to 300 K have been performed on the quaternary compounds Cu2ZnSnS4 (CZTS) single crystals. The conductivity mechanisms can be described by a two-path system using Mott variable range hopping and typical thermal activation conduction. The center level of the acceptor band is 132 meV above the valence band maximum and is of width 40 meV. A correlation between the activation energy and acceptor concentration in CZTS is observed.

Abstract: The mechanism and magnitude of the in-plane conductivity of poly(3,4-ethy-lenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films is determined using temperature dependent conductivity measurements for various PEDOT:PSS weight ratios with and without a high boiling solvent (HBS). Without the HBS the in-plane conductivity of PEDOT:PSS is lower and for all studied weight ratios well described by the relation σ = σexp-(T0T) 0.5] with T0 a characteristic temperature. The exponent 0.5 indicates quasi-one dimensional (quasi-1D) variable range hopping (VRH). The conductivity prefactor σ0 varies over three orders of magnitudes and follows a power law σ0a∝c 3.5 PEDOT with cPEDOT the weight fraction of PEDOT in PEDOT:PSS. The field dependent conductivity is consistent with quasi-1D VRH. Combined, these observations suggest that conductance takes place via a percolating network of quasi-1D filaments. Using transmission electron microscopy (TEM) filamentary structures are observed in vitrified dispersions and dried films. For PEDOT:PSS films with HBS, the conductivity also exhibits quasi-1D VRH behavior when the temperature is less than 200 K. The low characteristic temperature T0 indicates that HBS-treated films are close to the critical regime between a metal and an insulator. In this case, the conductivity prefactor scales linearly with cPEDOT, indicating the conduction is no longer limited by a percolation of filaments. The lack of observable changes in TEM upon processing with the HBS suggests that the changes in conductivity are due to a smaller spread in the conductivities of individual filaments, or a higher probability for neighboring filaments to be connected rather than being caused by major morphological modification of the material. In-plane conductivity in PEDOT:PSS takes place via filaments. This conclusion is based on charge transport measurements that indicate quasi-one dimensional variable range hopping and is supported by transmission electron microscopy. At higher PEDOT to PSS ratios the number of connected filaments drastically increases, causing an orders of magnitude increase in conductivity, in good agreement with percolation theory. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract: We report on the characteristics of amorphous indium-gallium-zinc-oxide thin-film transistors (TFTs) in the temperature range of 10–300 K. In the range of 80–300 K, the transfer characteristics are consistent with thermally activated band conduction. Below 80 K, the drain current vs. temperature behavior follows Mott's law, exp(−B/T−1/4), with constant B, indicating variable range hopping. The subthreshold swing of the TFT remains unchanged in the band conduction region, but it increases rapidly with decreasing temperature below 80 K. With decreasing temperature, the hopping activation energy decreases and hopping distance increases, and are 16.8 meV and ∼11.6 nm, respectively, at 60 K.

Abstract: Highly transparent CuCr1−x Mg x O2 (0 ≤ x ≤ 12%) films were prepared on (001) sapphire substrates by sol-gel method The microstructure, phonon modes, optical band gap, and electrical transport properties have been systematically discussed It was found that Mg-doping improved the crystal quality and enhanced the (00l) preferred orientation The spectral transmittance of films approaches about 70%–75% in the visible-near-infrared wavelength region With increasing Mg-composition, the optical band gap first declines and climbs up due to the band gap renormalization and Burstein-Moss effect The direct and indirect band gaps of CuCr094 Mg 006O2 film are 300 and 256 eV, respectively In addition, it shows a crossover from the thermal activation behavior to that of three-dimensional variable range hopping from temperature-dependent electrical conductivity The crossover temperature decreases with increasing Mg-doping composition, which can be ascribed to the change of spin-charge coupling between the hole and the local spin at Cr site It should be noted that the electrical conductivity of CuCr1− x Mg x O2 films becomes larger with increasing x value The highest electrical conductivity of 385 S cm−1 at room temperature for x = 12% is four-order magnitude larger than that (881 × 10−4 S cm−1) for pure CuCrO2 film The high spectral transmittance and larger conductivity indicate that Mg-doped CuCrO2 films are promising for optoelectronic device applications

Abstract: Phosphoric acid doped conductive polyaniline (PANI) polymer nanocomposites (PNCs) reinforced with silicon nanopowders have been successfully synthesized using a facile surface initiated polymerization (SIP) method. The chemical structures of the nanocomposites are characterized using Fourier transform infrared (FT-IR) spectroscopy. The enhanced thermal stability of the silicon–PANI PNCs compared with pure PANI is obtained using thermogravimetric analysis (TGA). The obtained optical band gap of the PNCs using Ultraviolet–visible diffuse reflectance spectroscopy (UV-vis DRS) decreases with increasing silicon loading. The dielectric properties of the PNCs are strongly related to the silicon loading level. Temperature dependent resistivity analysis reveals a quasi 3-D variable range hopping (VRH) electrical conduction mechanism for the synthesized PNCs. Room temperature giant magnetoresistance (GMR) is observed in the synthesized non-magnetic nanocomposites and analyzed using the wave-function shrinkage model.

Abstract: Monolayer graphene synthesized by chemical vapor deposition was subjected to controlled and sequential hydrogenation using RF plasma while monitoring its electrical properties in situ. Low-temperature transport properties, namely, electrical resistance (R), thermopower (S), Hall mobility (μ), and magnetoresistance (MR), were measured for each sample and correlated with ex situ Raman scattering and X-ray photoemission (XPS) characteristics. For weak hydrogenation, the transport is seen to be governed by electron diffusion, and low-temperature transport properties show metallic behavior (conductance G remains nonzero as T → 0). For strong hydrogenation, the transport is found to be describable by variable range hopping (VRH) and the low T conductance shows insulating behavior (G → 0 as T → 0). Weak localization (WL) behavior is seen with a negative MR for weakly hydrogenated graphene, and these WL effects are seen to diminish as the hydrogenation progresses. A clear transition to strong localization (SL) is...

Abstract: Epitaxial SrTi1-xVxO3 thin films with thicknesses of ~16 nm were grown on (001)-oriented LSAT substrates using the pulsed electron-beam deposition technique. The transport study revealed a temperature driven metal-insulator transition (MIT) at 95 K for the film with x = 0.67. The films with higher vanadium concentration (x > 0.67) were metallic, and the electrical resistivity followed the T^2 law corresponding to a Fermi liquid system. In the insulating region of x < 0.67, the temperature dependence of electrical resistivity for the x = 0.5 and 0.33 films can be scaled with the variable range hopping model. The possible mechanisms behind the observed MIT were discussed, including the effects of electron correlation, lattice distortion and Anderson localization.

Abstract: The dielectric and transport properties of Ba(Fe0.5Nb0.5)O3 ceramics have been investigated in a temperature range of 140–300 K and a frequency range of 1 Hz–10 MHz. The temperature dependence of bulk dc conductivity does not feature an Arrhenius behavior, but indicates a variable-range-hopping mechanism. The observed low temperature relaxation can be perfectly described by a polaronic model, which indicates that the dielectric relaxation is intimately related to the hopping motion caused by localized charge carriers.

Abstract: Polyaniline–Poly(vinylidene) fluoride (PANI–PVDF) composites were prepared by adding PANI to the PVDF by different weight percentages p % (p = 0, 5, 10, 20, ... until 100%). The dc and ac electrical conductivity were studied as a function of PANI percentage in the temperature range 303–453 K. The percolation threshold was found to be equal to 2.95%. When the amount of PANI varies from 5 to 30%, the charge transport mechanism was found to be governed by Mott's three-dimensional variable range hopping model and the dc conductivity decreases within this range. For p > 30%, the conductivity increases and the charge transport mechanism are better fitted by a fluctuation induced tunnelling model (FIT). By calculating the distance 's' between two successive clusters (the distance between two active imines centres (=N+H–) of PANI) from the FIT model, we deduce that electron charge transfer is done by inter-chain hopping for the range [p = 40 to 60%] and by intra-chain hopping for p = 70 to 90%. Some insights about the contribution of the ionic charge transport for PANI concentrations in the interval 5% < p < 30% were obtained using impedance measurements at different frequencies. X-ray diffraction measurements, Fourier transform infrared spectroscopy and scanning electron microscopy were used to investigate the effect of PANI on the structure and morphology of composites.

Abstract: We have investigated the structural, magnetic, and electrical transport properties of a series of ABO3-type perovskite compounds, La0.67Sr0.33Mn1−x V x O3 (0≤x≤0.15). The samples were characterized by X-ray diffraction and data were analyzed using Rietveld refinement technique, it has been concluded that these materials have the rhombohedral structure with $\mathrm{R}\overline{3}\mathrm{C}$ space group. The magnetization and resistivity measurements versus temperature proved that all our samples exhibit a ferromagnetic to paramagnetic transition and a metallic to semiconductor one when the temperature increases. Both the Curie temperature T C and the resistivity transition temperature T P of the composites decrease, while the resistance increases as the V content increases. It has been concluded that the electrical conduction mechanism in the metallic regime at low temperatures (TT P).

Abstract: The structures, optical and electrical transport properties of SnO2 films, fabricated by rf sputtering method at different oxygen partial pressures, were systematically investigated. It has been found that preferred growth orientation of SnO2 film is strongly related to the oxygen partial pressure during deposition, which provides an effective way to tune the surface texture of SnO2 film. All films reveal relatively high transparency in the visible range, and both the transmittance and optical band gap increase with increasing oxygen partial pressure. The temperature dependence of resisitivities was measured from 380 K down to liquid helium temperatures. At temperature above K, besides the nearest-neighbor-hopping process, thermal activation processes related to two donor levels ( and 100 meV below the conduction band minimum) of oxygen vacancies are responsible for the charge transport properties. Below K, Mott variable-range hopping conduction process governs the charge transport properties at higher temperatures, while Efros–Shklovskii (ES) variable-range-hopping conduction process dominates the transport properties at lower temperatures. Distinct crossover from Mott type to ES type variable-range-hopping conduction process at several to a few tens kelvin are observed for all SnO2 films.

Abstract: The present article demonstrates an intensive study upon the temperature dependent current density (J)-voltage (V) characteristics of moderately doped polypyrrole nanostructure and its silver nanoparticles incorporated nanocomposites. Analysis of the measured J–V characteristics of different synthesized nano-structured samples within a wide temperature range revealed that the electrical conduction behavior followed a trapped charge-limited conduction and a transition of charge transport mechanism from deep exponential trap limited conduction to shallow traps limited conduction had been occurred due to the incorporation of silver nanoparticles within the polypyrrole matrix. A direct evaluation of carrier mobility as a function of electric field and temperature from the measured J–V characteristics illustrates that the incorporation of silver nanoparticles within the polypyrrole matrix enhances the carrier mobility at a large extent by reducing the concentration of traps within the polypyrrole matrix. The calculated mobility is consistent with the Poole-Frenkel form for the electrical field up to a certain temperature range. The nonlinear low temperature dependency of mobility of all the nanostructured samples was explained by Mott variable range hopping conduction mechanisms. Quantitative information regarding the charge transport parameters obtained from the above study would help to extend optimization strategies for the fabrication of new organic semiconducting nano-structured devices.

Abstract: The canted G-type antiferromagnet PrFe0.5Mn0.5O2.95 shows an enhanced Jahn-Teller (JT) distortion below 150 K (T*). The resistivity of the grains can be described by variable range hopping between the localized states, and there is a dominant grain boundary contribution to dc resistivity, below T*. Above T*, the total dc resistivity follows small polaron hopping (SPH) conduction. A giant dielectric response is observed, and it can be ascribed to Maxwell-Wagner polarization and SPH mechanism. Despite the low concentration of JT active Mn3+ ions, our result indicates an important role of JT effect on physical properties of PrFe0.5Mn0.5O2.95.

Abstract: A unified set of parameters and dynamic equations have been developed to describe the time-dependent surface voltage and currents measured for a broad range of electron transport experiments conducted in parallel plate geometry with a dielectric slab above a grounded electrode and with either a floating or fixed voltage upper surface. The framework can model measurements of constant voltage, time-of-flight and AC conductivity; radiation induced conductivity; surface voltage accumulation and decay; electrostatic discharge; electron emission and electron-induced luminescence. The broad applications of the theoretical framework are outlined in terms a comprehensive classification of the ways in which charge is injected into or excited within a material; these classifications include surface deposition, bulk deposition and penetrating radiation for pulsed, stepped and periodic applied voltages/charge from either surface electrodes or electron beams. A set of equations are developed to model evolving electron transport and related phenomena in highly disordered insulating materials over large ranges of time, electric field, temperature, absorbed dose, and adsorbed dose rate. These analytic equations derived from physics-based theories predict the equilibrium and time-dependent accumulation, dissipation and transport of charge carriers; these basic equations are (i) Gauss’ law, (ii) a 1D electron continuity equation with Ohm’s law and source terms, (iii) a 1D continuity equation for holes with source terms, and (iv) the sum of currents due to various conduction mechanisms (including contributions from drift, diffusion, dispersion, polarization, and radiation-induced processes). The total conductivity is modeled as the sum of contributions from three independent conductivity mechanisms: thermally activated hopping, variable range hopping, and radiation-induced conductivity using a concise, unified set of independent fitting parameters. At a microscopic level, modeling and understanding these conduction mechanisms in disordered insulating materials is fundamentally based on a detailed knowledge of the distribution and occupation of the density of states (DOS) of nearly-free and trapped charged carriers. The conduction is controlled by transitions between extended valence and conduction band states, between localized trap states and the extended valence and conduction band states, and hopping between localized states; constant, linear, power law, exponential and Gaussian localized DOS are considered. By analyzing the observed temperature, field, dose rate and time dependent conductivities that result from both extended and localized trap state conduction, this theoretical framework provides new insight into the role of the localized trap state DOS in myriad ground-based materials testing methods.