Showing papers on "Variable-range hopping published in 2006"

Hopping conductivity in polymer matrix–metal particles composites

Abstract: Charge transport properties, such as the temperature dependent dc conductivity and the frequency dependent conductance, of polymer matrix–metal particles composites, are investigated in the present study. Dc and ac conductivity is examined with varying parameters the filler content, temperature and the frequency in the case of ac field. The examined systems, though they are characterized as dielectrics, exhibit considerable conductivity, which alters by several orders of magnitude with temperature and frequency. The temperature and frequency dependence of conductivity gives evidence for the charge carriers transport mechanism via the occurred agreement of experimental results with the employed hopping models (variable range hopping model and random free-energy barrier model).

Electrical properties of single-wall carbon nanotube-polymer composite films

Abstract: The electrical properties of single-walled carbon nanotubes (SWNTs) embedded in a poly(3-octylthiophene) matrix have been investigated as a function of SWNT concentration. The electrical conductivity and its temperature dependence were measured as a function of the SWNT concentration. As the nanotube concentration increased from 0to20wt%, the conductivity of the resulting films is dramatically increased by six orders of magnitude. The enhancement in conductivity can be explained by means of a three dimension simple percolation path theory, resulting in an estimated threshold of 4wt%. The temperature dependence of the SWNT conductivity mat obeys a three-dimensional variable range hopping. In contrast, the polymer-nanotube composite conductivity follows a fluctuation induced tunneling model. The main divergence is that in the polymer-nanotube composite, the nanotubes are coated with polymer, which acts a barrier in bundle to bundle hopping.

Complexity in charge transport for multiwalled carbon nanotube and poly(methyl methacrylate) composites

Abstract: We report on studies of the charge transport mechanism of the composites of multiwalled carbon nanotubes MWCNTs and poly methyl methacrylatePMMA. The MWCNTs were synthesized by a chemical vapor deposition method using Fe as a catalyst. The dc conductivity dc and its temperature dependence dcT were measured in a temperature range of 0.5– 300 K to study the charge transport mechanism of the composites. The dc of composites at room temperature increased as the MWCNT concentration increased, which shows typical percolation behavior with percolation threshold of pc at 0.4 wt % of the MWCNTs. The dcT of the MWCNT-PMMA composites were fitted to the combination of Sheng’s fluctuation induced tunneling FIT model and the one-dimensional variable range hopping 1D VRH model. The tunneling mechanism and the 1D VRH transport were attributed to the charge tunneling between MWCNT clusters and the charge hopping through 1D MWCNTs, respectively. Magnetoresistance MR of MWCNT-PMMA composites was measured up to 9 T of magnetic field. The results of MR and dcT showed that the FIT model was dominant in the low temperature region T 10 K, while the 1D VRH process became dominant as the temperature increased 10 KT 300 K. We observed unusually large negative MR in the composites at the lower temperatures T 4K due to FIT conduction. We related the parameters specifying charge transport to the percolation structures of MWCNT-PMMA composites based on the results of temperature dependence of dc and of the MR.

High temperature ferromagnetism in Ni-doped In2O3 and indium-tin oxide

Abstract: Observation of high temperature ferromagnetism in Ni-doped In2O3 and indium-tin-oxide (ITO) samples prepared by a solid state synthesis route is reported. Both Ni-doped compounds showed a clear ferromagnetism above 300K with the magnetic moments of 0.03–0.06μB∕Ni and 0.1μB∕Ni at 300 and 10K, respectively. Ni-doped In2O3 samples showed a typical semiconducting behavior with a room temperature resistivity of ρ∼2Ωcm, while Ni-doped ITO samples were metallic with ρ∼2×10−2Ωcm. Analysis of different conduction mechanisms suggested that variable range hopping model explains our ρ-T data for the Ni-doped In2O3 sample the best.

Structure-conductivity correlation in ferric chloride-doped poly(3-hexylthiophene)

Abstract: Poly(3-hexylthiophene) (P3HT) matrix has been chemically doped (redox doping) by ferric chloride (FeCl3) with different molar concentrations to get P3HT–FeCl3 charge-transfer complexes. The effect of redox doping on photo-physical, structural, and morphological properties and dc electrical conductivity of P3HT matrices has been examined. The dc conductivity has been measured on films of pristine P3HT and P3HT–FeCl3 charge-transfer complexes in the temperature range 6–300 K. Analysis of dc conductivity data reveals that in the temperature range 40–300 K, the dc conductivity is predominantly governed by Mott's 3-dimensional variable range hopping (3D-VRH); however, below 40 K tunnelling seems to dominate. A slight deviation from 3D-VRH to 1D-VRH is observed with an increase in doping level or precisely with an increase in the extent of P3HT–FeCl3 charge-transfer complexes. We attribute this deviation to the induced expansion in crystallographic lattices as revealed by x-ray diffraction data and formation of discrete conducting domains as observed by atomic force microscope imaging.

Hopping conduction in size-controlled Si nanocrystals

Abstract: We investigate the temperature dependence of conduction in size-controlled silicon nanocrystals. The nanocrystals are ∼8nm in diameter, covered by ∼1.5nm thick SiO2 shells. In 300nm thick films for temperatures T from 30to200K, the conductivity σ follows a ln(σ) vs 1∕T1∕2 dependence. This may be associated with either percolation-hopping conductance or Efros-Shklovskii variable range hopping. Assuming hopping sites only on the nanocrystals, the data agree well with the percolation model.

Electron transport properties of cobalt doped polyaniline

Abstract: Electrical transport properties of cobalt doped polyaniline in an aqueous ethanol medium were investigated in the temperature range 77 ≤ T ≤ 300 K, applying magnetic fields up to 1 T in the frequency range 20 Hz–1 MHz. The room temperature dc resistivity increases with increase in Co content. The dc resistivity and magnetoresistivity of these samples have been interpreted in terms of the variable range hopping theory. The frequency dependence of conductivity has been described by a power law σ(ω) ∝ ωS. The value of s is found to be temperature dependent, which shows a decreasing trend with temperature. The correlated barrier hopping model is the most likely mechanism for the electron transport. The different physical parameters were calculated from the experimental data.

Hopping conduction in boron doped amorphous carbon films

Abstract: The electrical transport properties of boron doped graphitelike amorphous carbon films have been studied. Variable range hopping (VRH) conduction is found to be the conduction mechanism in these carbon films. The conduction in high temperature regime follows the Mott VRH [J. Non-Cryst. Solids 1, 1 (1968)], whereas a crossover from the Mott [Electronic Process in Non-crystalline Materials (Clarendon, Oxford, 1971)] to the Efros-Shklovskii [J. Phys. C 8, L49 (1975)] VRH is observed at low temperatures. Additional support to this transition is evident from negative magnetoresistance in VRH regime when the sample is deep inside the insulating side of the metal insulator transition. Various calculated parameters such as localization length, density of states at the Fermi level, and Coulomb gap for insulating samples have been estimated from the experimental data. The density of states at the Fermi level shows a gradual shift with a corresponding variation in boron doping level, indicating a change in the density of conducting \pi electrons due to substitutional doping of boron in the carbon network.

Structural and electrical studies on tetrameric cobalt phthalocyanine and polyaniline composites

Abstract: Polyaniline and oligomeric cobalt phthalocyanine are blended in different proportions by chemical methods. These blends are characterised by spectroscopic methods and dielectric measurements. Dielectric studies on the conducting polymer blends are carried out in the frequency range of 100 kHz to 5 MHz from room temperature (300 K) to 373 K. Dielectric permittivity and dielectric loss of these blends are explained on the basis of interfacial polarisation. From the dielectric permittivity studies, ac conductivity of the samples were calculated and the results are correlated. In order to understand the exact conduction mechanism of the samples, dc electrical conductivity of the blends is carried out in the temperature range of 70–300 K. By applying Mott’s theory, it is found that the conducting polymer composites obey a 3D variable range hopping mechanism. The values of Mott’s temperature (T0), density of states at the Fermi energy (N(EF)), range of hopping (R) and hopping energy (W) for the composites are calculated and presented. © 2006 Elsevier B.V. All rights reserved.

Spin-dependent variable range hopping and magnetoresistance in Ti(1-x)Co(x)O(2) and Zn(1-x)Co(x)O magnetic semiconductor films.

Abstract: Magnetic transport properties in Ti(1-x)Co(x)O(2) and Zn(1-x)Co(x)O magnetic semiconductors have been studied experimentally and theoretically. A linear relation of lnρ versus T(-1/2) (ρ is sheet resistance and T is temperature), which shows different slopes and intersections at different magnetic fields, was observed experimentally in the low temperature range. The spin-dependent variable range hopping model has been proposed by taking into account the electron-electron Coulomb interaction and the spin-spin exchange interaction in the same frame, which can well describe the observed magnetic transport properties in Ti(1-x)Co(x)O(2) and Zn(1-x)Co(x)O magnetic semiconductors.

Structure and magnetic properties of hollandite Ba1.2Mn8O16

Abstract: The synthesis, crystal structure and magnetic properties of Ba1.2Mn8O16, obtained using a KCl mineralizer, are reported. This material crystallizes in the monoclinic hollandite structure, containing Mn–O double chains (space group: I 2/m, a = 10.052(1) A, b = 2.8579(2) A, c = 9.7627(10) A, β = 89.96(1)°), and undergoes a magnetic transition at 40 K with the magnetization having a small ferromagnetic component. The ordering temperature is nearly 10 times lower than expected from the Weiss temperature of −385 K, evidencing magnetic frustration. Preliminary neutron powder diffraction studies suggest a complex spin arrangement within the ac-plane (perpendicular to the chain axis). The resistivity is consistent with one-dimensional variable range hopping.

Electrical conduction mechanisms in thermally evaporated tungsten trioxide(WO3) thin films

Abstract: Thin films of amorphous tungsten trioxide, a-WO3, have been thermally evaporated onto glass substrate held at 350K. Annealing at 723K caused the formation of polycrystalline tungsten trioxide, c-WO3, with a monoclinic structure. The dark DC electrical conductivity of both a-WO 3 and c-WO3 was studied over a temperature range from 298 to 625K in two environmental conditions (air and vacuum). A simple Arrhenius law, a polaron model and a variable range hopping model have been used to explain the conduction mechanism for a-WO3 films. Using the variable range hopping model, the density of localized states at the Fermi level, N(EF), was found to be 1.08 × 1019eV -1cm-3. The mechanism of electrical conduction in c-WO3 films is explained by means of the Seto model. The Seto model parameters were determined as the energy barrier (Eb ≤ 0.15eV), the energy of trapping states with respect to the Fermi level (Et ≤ 0.9eV) and the impurity concentration (ND ≤ 4.05 × 1015eV-1cm-3). The thickness dependence of resistivity of c-WO3 films has been found to decrease markedly with increasing film thickness, which is explained on the basis of the effective mean free path model. Using this model, the mean free path of electrons in c-WO 3 films was evaluated. The temperature dependence of the thermoelectric power for a-WO3 films reveals that our samples are n-type semiconductors.

Microstructure, ferromagnetism, and magnetic transport of Ti1-xCoxO2 amorphous magnetic semiconductor

Abstract: TiO2-based magnetic semiconductors with high Co doping concentrations (Ti1-xCoxO2) were synthesized under thermal nonequilibrium condition by sputtering machine Microstructure and composition analysis by transmission electron microscopy, x-ray photoelectron spectroscopy, and electron energy-loss spectroscopy indicated that Co element was incorporated into TiO2 to form Ti1-xCoxO2 compound The direct evidence for the compositional inhomogeneity of the Ti1-xCoxO2 compound was given Room temperature ferromagnetism with high magnetization was obtained, which could be attributed to the intrinsic properties of the amorphous magnetic semiconductor The electrical transport in a low temperature range was explained by spin-dependent Efros's variable range hopping, and correspondingly an exponential function of the magnetoresistance versus T-1/2 was found (c) 2006 American Institute of Physics

Breakup of the Fermi surface near the mott transition in low-dimensional systems.

Abstract: We investigate the Mott transition in weakly coupled one-dimensional (1D) fermionic chains. Using a generalization of dynamical mean field theory, we show that the Mott gap is suppressed at some critical hopping ${t}_{\ensuremath{\perp}}^{c2}$. The transition from the 1D insulator to a 2D metal proceeds through an intermediate phase where the Fermi surface is broken into electron and hole pockets. The quasiparticle spectral weight is strongly anisotropic along the Fermi surface, both in the intermediate and metallic phases. We argue that such pockets would look like ``arcs'' in photoemission experiments.

Influence of strain relaxation on magnetotransport properties of epitaxial La0.7Ca0.3MnO3 films

Abstract: In this paper, we study the effect of strain relaxation on the magnetotransport properties of La0.7Ca0.3MnO3 (LCMO) epitaxial films (200 nm thick), which were deposited by pulsed laser deposition technique under identical conditions. All the films are epitaxial and have a cubic unit cell. The amount of strain relaxation has been varied by taking three different single crystal substrates of SrTiO3, LaAlO3 and MgO. It has been found that for thicker films the strain becomes relaxed and produces a variable amount of disorder depending on the strength of strain relaxation. The magnitude of lattice relaxation has been found to be 0.384%, 3.057% and 6.411% for film deposited on SrTiO3, LaAlO3 and MgO respectively. The films on LaAlO3 and SrTiO3 show higher TIM (insulator–metal transition temperature) of ~243 and 217 K respectively as compared to TIM of ~191 K for the film on MgO. Similarly the Curie temperature TC of the films on SrTiO3 and LaAlO3 is sharper and has values of ~245 and 220 K respectively, whereas the TC of the film on MgO is ~186 K. A higher degree of relaxation creates more defects and hence the TIM (TC) of the film on MgO is significantly lower than those of SrTiO3 and LaAlO3. We have adopted a different approach to correlate the effect of strain relaxation on the magnetotransport properties of LCMO films by evaluating the resistivity variation through Mott's variable range hopping (VRH) model. The variable presence of disorder in these thick films due to lattice relaxation, which have been analysed through Mott's VRH model, provides strong additional evidence that the strength of lattice relaxation produces disorder dominantly by an increase in density of defects such as stacking faults and dislocations, which affect the magnetotransport properties of thick epitaxial La0.7Ca0.3MnO3 films.

Transport properties of Mo6S3I6 nanowire networks

Abstract: We present measurements of resistivity on Mo6S3I6 nanowire networks under different conditions. The room-temperature values of conductivity of as-grown nanowire networks are on the order of σ300K=0.04S∕m and show linear current-voltage characteristics, indicating that—in agreement with band structure calculations—there is a finite density of states at the Fermi level. The conductivity of pristine samples is thermally activated, closely following three-dimensional variable range hopping (VRH) behavior of the form σ=σ0exp−(T0∕T)β, where β=1∕4. Removal of interstitial iodine from the network by annealing in a vacuum gives rise to a cross over to one-dimensional VRH with β=1∕2 and a concurrent increase in room-temperature conductivity. The introduction of water vapor leads to a decrease in conductivity and reveals that the resistivity of the network is sensitive to interstitial water molecules.

Large Negative Magnetoresistance and Strong Localization in Highly Disordered Electrospun Pregraphitic Carbon Nanofiber

Abstract: A highly disordered pregraphitic carbon nanofiber with the product of its quasi-Fermi wave vector and mean free path close to 1 was fabricated using electrospinning technique. Strong localization made the conductivity vary with temperature as lnσ∝T−1∕2 from 300to5K, suggesting variable range hopping as the conductivity mechanism, and resulted in a large negative magnetoresistance from 300K down to 1.9K that can still be quantitatively described using weak localization and electron interaction models.

Conductivity of single Mo6S9?xIx molecular nanowire bundles

Abstract: Longitudinal resistivity measurements on single Mo6S9−xIx (x = 4.5, 6 and 7) molecular nanowire bundles ranging in diameter from d = 7 nm to 1 µm are performed to investigate the longitudinal transport properties of individual bundles. Different contacting methods are used to study diverse nanocircuit manufacturing technologies that can be used for interconnects based on Mo6S9−xIx. The measurements show ubiquitously linear I–V characteristics with Pd, Au, Ag and Ti contact metals. The highest room-temperature conductivity achieved is σ0~10 S m−1 using Ag contacts. The critical current densities typically achieved are Jc~104 A cm−2. The observed metallic behaviour at room temperature is consistent with the band structure calculated using density functional theory (DFT). At low temperatures, the conductivity is found to decrease, following variable range hopping (VRH) behaviour of the form σ = σ0exp−(T0/T)β reasonably well, but the exponent β changes upon annealing. From fits to the temperature dependence of the conductivity, a change from β~1/4 to β~1/2 is observed, which may be explained by a change in dimensionality from 3D-like VRH to 1D-like VRH following the removal of intra-bundle interstitial iodine.

Pulsed versus dc I-V characteristics of resistive manganites

Abstract: We report on pulsed and dc I-V characteristics of polycrystalline samples of three charge-ordered manganites, Pr2∕3Ca1∕3MnO3, Pr1∕2Ca1∕2MnO3, and Bi1∕2Sr1∕2MnO3, and of a double perovskite, Sr2MnReO6, in a temperature range where their Ohmic resistivity obeys the Efros-Shklovskii variable range hopping relation [J Phys C 8, L49 (1975)] For all samples, the dc I(V) exhibits at high currents negative differential resistance and hysteresis, which mask a perfectly Ohmic or a moderately non-Ohmic conductivity obtained by pulsed measurements This demonstrates that the widely used dc I-V measurements are usually misleading

Structural variations in Gd5Si4-xSnx: size vs. electronic effects

Abstract: B 12 icosahedra cluster-containing rare earth borosilicides REB 44 Si 2 (RE = Tb, Dy, Ho, Er, Tm, Yb) were grown by the floating zone method and their physical properties were investigated. REB 44 Si 2 compounds are isostructural to REB 50 and they are unique among the newly discovered boron-rich phases in that it is possible to grow large crystals with dimensions exceeding 10 mm. The rare earth atoms form a coupled chain structure like a ladder along the c-axis which is also the axis along which there is a B 12 icosahedral chain. Magnetic properties, resistivity, and specific heat of the compounds were investigated. The conductivity follows the 3D variable range hopping mechanism and it was determined that localization lengths ξ are extremely short, actually close to the distances of the atoms themselves. Magnetic transitions above 4 K are indicated for all the compounds except for TmB 44 Si 2 and it is found that specific heat measurement is the expedient probe of such compounds in which the low temperature susceptibility is influenced by external conditions such as crystal growth rate. Despite dimer-like features to the magnetic transition a spin gap is not observed. The magnetic coupling in these systems is indicated to be different from the conventional mechanisms in f-electron systems of dipole-dipole coupling or RKKY interaction.

Transformation of electrical transport from variable range hopping to hard gap resistance in Zn1−xFexO1−v magnetic semiconductor films

Abstract: Transformation of the electrical transport from the Efros and Shklovskii [J. Phys. C 8, L49 (1975)] variable range hopping to the “hard gap” resistance was experimentally observed in a low temperature range as the Fe compositions in Zn1−xFexO1−v ferromagnetic semiconductor films increase. A universal form of the resistance versus temperature, i.e., ρ∝exp[TH∕T+(TES∕T)1∕2], was theoretically established to describe the experimental transport phenomena by taking into account the electron-electron Coulomb interaction, spin-spin exchange interaction, and hard gap energy. The spin polarization ratio, hard gap energy, and ratio of exchange interaction to Coulomb interaction were obtained by fitting the theoretical model to the experimental results. Moreover, the experimental magnetoresistance was also explained by the electrical transport model.

Humidity sensing by composites of glass ceramics containing silver nanoparticles and their conduction mechanism

Abstract: Silver nanoparticles of diameters ranging from 6 to 10 nm were grown in glass–ceramic samples containing barium titanate, lithium niobate and zinc orthosilicate. These nanocomposites exhibited a four order of magnitude decrease in dc resistivity as the relative humidity was increased from 35 to 95%. The resistivity under different humidity conditions could be measured over a temperature range 300 to 330 K. The resistivity was shown to arise from a variable range hopping mechanism. The ac conductivity of the samples was also measured at different temperatures and frequencies ranging from 10 kHz to 1 MHz. The ac conductivity was explained on the basis of a small polaron tunneling mechanism. The density of states near Fermi level was calculated from both the dc and the ac conductivity data. The density of states estimated from dc data gave lower values than those from the ac result. The nature of the crystalline phase, ferroelectric or not, appeared to change the humidity sensing behavior in that the rate of change of resistivity showed two different values in two ranges of humidity. This difference was reduced in the case of the glass–ceramics containing non-ferroelectric additives.

Temperature dependent magnetization in Cr-doped CdTe crystals

Abstract: We report magnetic, structural, and electrical properties of Cr-doped CdTe crystals grown by the vertical solidification method. High-resolution x-ray diffraction showed a single phase with a zinc-blende structure of lattice constant ∼6.481A. It exhibited ferromagnetism with temperature-dependent magnetization. The ferromagnetic moment increased as the applied magnetic field increased in the field range of 100–2000Oe. The temperature dependence of resistance exhibited Efros-Shklovskii-type variable range hopping proportional to 1∕T1∕2. The appearance of ferromagnetism with a Curie temperature of ∼395K is explained by ferromagnetic interaction of Cr atoms dissolved in the CdTe matrix rather than intermetallic impurity phases.

Electrical conductivity and dielectric properties of some vanadium–strontium–iron unconventional oxide glasses

Abstract: An unconventional oxide glass system of the composition (80 − x)%V2O5·x%SrO·20%FeO (0 ≤ x ≤ 40) was prepared by the press-quenching technique. The electrical conductivity (σdc and σac) was measured in the temperature range from 308 to 588 K. σdc was found to follow a mixed conduction of both small polaron hopping (SPH) and variable range hopping (VRH) mechanisms up to 418 K. Above this temperature σdc follows SPH conduction mechanism only. σac was determined at four frequencies (102, 103, 104, 105 Hz). It was found that the correlated barrier hopping (CBH) was the applicable mechanism, and the exponent factor (s) varied from 0.1 to 0.83. The theoretical fitting between the proposed models and the experimental data showed good agreement. σdc and σac were found to decrease with the increase of SrO content, and both the dielectric constant ɛ′ and loss ɛ″ were found to increase with temperature and decrease with the frequency.

Temperature-dependent activation energy and variable range hopping in semi-insulating GaAs

Abstract: We measured resistivity in the range of 30–390 K on four semi-insulating low-temperature grown molecular-beam epitaxy GaAs samples. The growth temperature range was from 215 °C to 315 °C. Arrhenius fittings with T−1 and hopping fitting with T−1/4 do not permit us the definition of the temperature ranges controlled by band and hopping conduction, respectively. This leads to major errors in the calculation of both activation energies and hopping parameters. We have used the differential activation energy in order to clearly identify the temperature range for the different transport mechanisms. Hopping dominates at low temperatures and band conduction at high temperatures. In-between, a mixed conduction regime is observed. We introduce a criterion to clearly define the temperature range of hopping, band and mixed conduction. The lower temperature at which mixed conduction is identified decreases for samples with increasing growth temperature. Only the sample grown at 215 °C presents both forms of hopping conduction before entering the mixed conduction regime. Hopping parameters were obtained from the fittings of the differential activation energy and the values are in good agreement with the usual method of calculating them if the correct temperature range is used.