Dimensional Variable Range Hopping Research Articles

Dielectric and electrical studies on iron oxide (α-Fe2O3) nanoparticles synthesized by modified solution combustion reaction for microwave applications

Hematite (α-Fe2O3) nanoparticles were synthesized by modified solution combustion method using a mixture of hexamethylenetetramine (HMTA) and glycine fuels at fuel to oxidant molar ratio (ϕ = 1) and fuel (HMTA) to fuel (glycine) weight ratio (1:3). DC-conductivity and dielectric studies of (α-Fe2O3) nanostructures were reported as a function of temperature. Above room temperature (RT), the conductivity showed an increasing trend with temperature which confirmed the semiconducting behaviour of the material. The conductivity behaviour followed Motts law which confirmed the three dimensional variable range hopping (3D VRH) mechanism in the synthesized system. Dielectric studies were carried out at room temperature with frequency range (1000 Hz to 1.5 MHz) as well as varying temperature (100 °C to 400 °C) at different frequencies (0.1 MHz, 0.3 MHz, 1 MHz, 1.3 MHz, and 1.5 MHz).The dielectric permittivity (є’)and tangent loss (tan δ) was found to increase with increase in temperature. In this synthesised material low value of dielectric loss was observed which depicted these materials as potential candidates for microwave applications. More than that studied material was characterised by Raman spectroscopy which revealed that seven active Raman modes were present with different intensities, namely two “A1g” modes (225 and 498 cm−1) and five “Eg” modes (247, 293, 299, 412 and 613 cm−1). The most representative band present in the synthesised iron oxide nanoparticles was found at 225 cm−1.

ZnTe dispersed in RGO matrix: Investigation of electrical transport processes, magnetic properties and their synergistic effect

The electrical transport processes of zinc telluride (ZnTe) uniformly dispersed in reduced graphene oxide matrix (RGO-ZnTe composite) were investigated. It is observed from DC measurements that the charge transport process is consistent with Mott three dimensional variable range hopping (3D-VRH) in 85 K–280 K, confirmed by resistance curve derivative analysis (RCDA) method. AC conductivity indicates a Debye-like relaxation process caused by quantum mechanical tunneling (QMT) between localized sites in 90 K–473 K range. Magnetization data of RGO-ZnTe composite from SQUID and electron paramagnetic resonance (EPR) spectra show paramagnetic behaviour in 5 K–330 K despite the diamagnetism of pure RGO and ZnTe at room temperature. In photosensitivity, one order enhancement was observed in the RGO-ZnTe composite alongside three times higher reduction rate constant in photoreduction of aqueous pollutant 4-nitrophenol as compared to pure ZnTe. These significant increases in photocurrent and photoreduction efficiencies are attributed to dispersion of ZnTe on the 2D RGO surface indicating the synergy of ZnTe nanoparticle with RGO sheets.

Inter-cluster separation induced change in charge transport mechanism in Ni40Pd60 nanoclusters

Nanoclusters offer a fascinating possibility of studying the evolution of properties of a physical system by varying the number, size and inter-cluster separation of a given cluster to go from one limit to another. By systematically varying the inter-cluster separation in a nanocluster assembly of Ni40Pd60 alloy, that is known to be a metal in bulk, we observe an unusual and hitherto unreported, spatial dimension change as well as a change in the transport mechanism. In the nanocluster form, the temperature dependent resistance shows an activated behavior for virtually all inter-cluster separations, contrary to, the bulk metallic behaviour. At large average inter-cluster separation, the transport happens via three dimensional Efros-Shklovskii hopping, due to the opening of a Coulomb gap at the Fermi surface. With a reduction in the inter-cluster separation, the transport mechanism changes from three dimensional Efros-Shklovskii hopping to that of a three dimensional Mott variable range hopping (VRH) due to the closing up of the gap. With a further reduction in average inter-cluster separation, the three dimensional Mott VRH changes to that of a two dimensional Mott VRH with additional signatures of an insulator to a weak metal-like transition in this particular assembly. So, nanoclusters offer a paradigm for studying the important problem of evolution of charge transport in physical systems with the possibility of directly tuning the average inter-cluster separation enabling the system to go from insulating to metallic limit via intermediate changes in the charge transport mechanism.

Electrical resistivity, magnetic and magneto-caloric studies on perovskite manganites Nd1−xCdxMnO3 (x = 0 and 0.1) polycrystals

We report here on the temperature- and field- dependence of resistivity measurements of Nd1−xCdxMnO3 (x = 0 and 0.1) polycrystals and compare them with the magnetization measurements on the same set of samples reported earlier. A transition from an anti-ferromagnetic insulator to a ferromagnetic insulator is observed for x = 0.1. Magnetic entropy change of both the samples around semiconducting to insulating transitions have been estimated using magnetization and resistivity measurements for the first time. The values for x = 0 and 0.1 samples are 45 J·kg−1·K−1 and 63 J·kg−1·K−1 respectively for a magnetic field difference of 5 T indicating an enhancement as increases from x = 0 to 0.1. The activation energy (Ea) and density of states at the Fermi level [N(EF)] are estimated for both the samples using small polaron hopping and two dimensional variable range hopping models respectively. By Cd doping, Ea is reduced from 238.6 meV (for x = 0) to 224.04 meV (for x = 0.1) and N(EF) is increased from 1.47 × 1020 eV−1·cm−3 (for x = 0) to 1.89 × 1020 eV−1·cm−3 (for x = 0.1) thus confirming the enhancement of ferromagnetic and magneto caloric effect features. Such high magnetic entropy change values make these materials useful for magnetic refrigeration.

Electrical conduction mechanisms in graphene nanoplatelet/yttria tetragonal zirconia composites

Yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composites with 5, 10 and 20 vol% graphene nanoplatelets (GNPs) were prepared by spark plasma sintering (SPS) and their electrical conductivity as a function of temperature was characterized. The composites exhibit anisotropic microstructures so the electrical conductivity studies were carried out in two directions: perpendicular (σ⊥) and parallel (σ||) to the SPS pressing axis. The composites with 5 and 10 GNP vol% showed high electrical anisotropy, whereas the composite with 20 GNP vol% exhibited nearly isotropic electrical behavior. σ⊥ shows metallic-type behavior in the composites with 10 and 20 vol% GNP revealing that charge transport takes place through defect-free GNPs. For the composite with 5 vol% GNP the observed semiconductor-type behavior was explained by a two dimensional variable range hopping mechanism. σ|| shows metallic-type conductivity in the composite with 20 GNP vol% and positive dσ||/dT slope in the composites with 5 and 10 GNP vol%.

Temperature-dependent charge transport mechanisms in carbon sphere/polyaniline composite

Charge transport in the temperature range 80 K < T < 300 K was studied in a composite of carbon spheres (CS), prepared via hydrothermal carbonization of sucrose, and the conducting polymer polyaniline (PANi). PANi was synthesized via the oxidative polymerization of aniline with ammonium peroxydisulfate (APS) in acidic media. The CS/PANi composite was prepared by coating the spheres with a thin polyaniline (PANi) film doped with hydrochloric acid (HCl) in situ during the polymerization process. Temperature dependent conductivity measurements show that three dimensional variable range hopping of electrons between polymeric chains in PANi-filled gaps between CS is the predominant transport mechanism through CS/PANi composites. The high conductivity of the CS/PANi composite makes the material attractive for the fabrication of devices and sensors.

Dielectric Relaxation and Room Temperature Magnetoresistance Under Low Magnetic Field in Polypyrrole-BaTiO3 Hybrid Nanocomposites

BaTiO3 and polypyrrole (ppy)-BaTiO3 hybrid nanocomposites have been synthesized by chemical oxidative polymerization method. Microstructure and crystallinity of the hybrids are studied using field emission scanning electron microscope (FE-SEM), high resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD) technique. As prepared BaTiO3 are rod-like, while PPY-BaTiO3 nanocomposites indicate the formation of bulging agglomerates of spherical particles with various sizes (40–50 nm). Dielectric constants at room temperature of the composites have largely enhanced (up to 6000). The hybrid composite shows grain boundary relaxation in the frequency range (42 Hz-5 MHz). Three dimensional (3D) variable range hopping (VRH) with high localization of charge carriers (Mott temperature ≈ 8725658 K) is observed in the temperature dependent conductivity evaluation of composite system. Negative magnetorestance (MR ≈ 4.3%) has been measured at 1 T. The observed MR is explained with the help of forward interference model.

Facile synthesis and thermoelectric properties of aluminum doped zinc oxide/polyaniline (AZO/PANI) hybrid

Now-a-days metal oxide-conducting polymer composites are used as promising candidates for light-weight, low-cost and non-toxic thermoelectric applications. In the present work, aluminum (Al) doped zinc oxide/polyaniline-AZO/PANI (AZP) hybrid are successfully synthesized by combining sol-gel and in-situ oxidative polymerization method. The materials are structurally characterized by X-ray diffraction pattern (XRD), transmission electron microscopic (TEM) and field emission scanning electron microscopic (FESEM) images. Measurement of thermoelectric properties of AZO/PANI hybrids reveal a simultaneous improvement in electrical conductivity which is attributed to carrier mobility and Seebeck coefficient due to scattering of low-energy carriers via energy-filtering effect. Carrier concentrations have been evaluated from Hall-Effect measurements. The mechanism of charge carrier transport follows a two dimensional variable-range hopping. Inclusion of AZO on the layers of PANI reduces the thermal conductivity to a great extend compared to pure AZO, leading to enhance the room temperature dimensionless figure of merit (ZT=0.0035). The probable interaction of formation of the hybrid of AZO and PANI validates the enhancement of material property.

Hybrid Gas Sensing and Transport Properties of Few-Walled CNTs Decorated with Discrete SnO₂ Nanoparticles.

CVD grown, few walled carbon nanotubes (FWCNTs) were quasi decorated with SnO₂nanoparticles (FWCNTs-SnO₂) and its gas sensing properties were analyzed with ammonia and ethanol. At room temperature FWCNTs-SnO₂show enhanced 'p type' gas sensing response than FWCNTs. Activation of SnO₂at high temperatures led to systematic changes in the sensing behavior towards 'n type' response. Temperature dependent transport behavior was found to be a one dimensional variable range hopping mechanism (1 D-VRH) for the FWCNTs and a 3D-VRH mechanism for the FWCNTs-SnO2. These temperature dependent gas transport and sensing properties elucidate the hybrid nature of the nanocomposite with novel characteristics. This also implies its importance as a potential gas sensor material.

Effect of loading titanium dioxide on structural, electrical and mechanical properties of polyaniline nanocomposites

Nanocomposites based on Polyaniline Titanium dioxide (PANI-TiO2) were prepared by in situ chemical polymerization. The structure was confirmed by X-rays diffraction studies. The interesting behavior of the XRD pattern around 10% TiO2 content was marked as critical concentration of the dopant. The crystallite size was found to increase for PANI-TiO2 nanocomposites. Charge transport mechanism was investigated by temperature dependent dc electrical conductivity measurements in the temperature range of 293 K–353 K. An increase in dc conductivity was found on addition of TiO2 at all temperatures and three dimensional variable range hopping (3D VRH) model was the most probable model of charge transport. Activation energy, density of states and hopping length were calculated and found to be influenced by TiO2 incorporation. Structural morphology was studied by scanning electron microscopy (SEM). The granular structure and grain size increased and was confirmed by Transmission Electron Micrographs. The tensile modulus as a function of TiO2 loading in PANI showed enhancement in mechanical properties.

Observation of negative magneto-resistance in SrFe1−xTixO3−δ (x = 0 to 0.3) systems

Resistance and magneto-resistance of nanocrystalline SrFe1−xTixO3−δ (x = 0 to 0.3) systems are investigated. Substitution of non magnetic Ti4+ stabilizes the single phase tetragonal crystal structure with I4/mmm space group and Ti introduces bond disorder as well as site disorder. With increase in Ti content, the disorder increase leading to localization and large change in resistivity is observed in all the systems. In undoped system, the increase in resistivity is associated with the magnetic transition. All the Ti doped samples show large change in resistivity much above the magnetic ordering temperature and three dimensional Variable Range Hopping mechanism seems to explain the transport and magneto-transport behavior in these systems.

Mechanism of direct current electrical charge conduction in p-toluenesulfonate doped polypyrrole/carbon composites

Polypyrrole/carbon (PPy/C) composites have been synthesized using varying concentration of p-toluenesulfonate (pTS) dopant by surface initiated in-situ chemical oxidative polymerization. The synthesis and influence of pTS on the structure of the PPy/C composites are confirmed by Fourier transform infrared studies and the morphological features have been examined by scanning electron microscopy. X-ray photoelectron spectroscopy, employed to examine the surface composition and doping level of these composites, confirms the anionic doping into the polymer backbone. Electron spin resonance measurement has been carried out on these samples to identify the nature of the charge carriers and their concentration at different doping levels. The dc electrical conductivity of these composites has been measured in the temperature range ∼10–305 K. The observed results have been analyzed in the framework of existing theoretical models. Different Mott's parameters, such as characteristic temperature (T0), density of states at the Fermi level {N(EF)}, average hopping distance (R), and average hopping energy (W), evaluated from dc conductivity data supports the applicability of Mott's three dimensional variable range hopping mechanism in this system.

Aromatic bi-, tri- and tetracarboxylic acid doped polyaniline nanotubes: effect on morphologies and electrical transport properties

Polyaniline nanotubes have been prepared in a facile manner via the chemical polymerization of aniline in an aqueous medium using ammonium persulphate (APS) and ten different dopant acids. The formation of PANI in the composite was confirmed using 1H-NMR, UV-Vis and FTIR spectroscopic studies. An intensive FESEM investigation indicated that the dopant acid plays an important role in the formation of the PANI nanotubes. In particular, the symmetrical positioning of –COOH groups in the aromatic dopant acid is key for forming PANI nanotubes. Symmetrical BTA-1 produces PANI nanotubes, whereas BTA-2 and BTA-3 only generate granular aggregates of PANI. The diameter of the observed nanotubes varies with the molar ratio of BTA-1 to aniline and the diameter of the nanotubes increases with the increasing number of –COOH groups in dopant acids that have the same central core. A comparative study with other tricarboxylic acids shows that the outer diameter (OD) of the PANI nanotubes depends on the size of the dopant under identical reaction conditions and follows the order: ODTPCA > ODBPCA > ODBTA-1. Low temperature charge transport in these composites mainly follows a three dimensional variable range hopping mechanism.

Enhanced capacitance and stability of p-toluenesulfonate doped polypyrrole/carbon composite for electrode application in electrochemical capacitors

Polypyrrole/carbon (PPy/C) composites have been synthesized using varying concentration of p-toluenesulfonate (pTS) dopant by surface initiated in-situ chemical oxidative polymerization with the purpose to develop an electrode material for supercapacitors. The influence of pTS on the structure of the composite is observed through Fourier transform infrared (FT-IR) and Raman spectroscopy while the morphological features have been examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The dc conductivity shows direct correlation with pTS concentration and follows Mott's three dimensional variable range hopping (3D-VRH) charge transport. The performance of PPy/C composite electrode for charge storage has been analyzed using electrochemical tools such as cyclic voltammetry and electrochemical impedance spectroscopy. The maximum specific capacitance ∼395 F g−1 in 0.5 M Na2SO4 aqueous solution with significant stability (∼500 cycles) is obtained for the material synthesized using equimolar concentration (0.1 M) of pTS to pyrrole.

Quasi‐One Dimensional in‐Plane Conductivity in Filamentary Films of PEDOT:PSS

AbstractThe 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 $ \sigma = \sigma _0 {\rm exp}[- \left({{{{T_0}}\over{T}}} \right)^{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 σ0∝c3.5PEDOT 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.

Band structure, magnetic, and transport properties of two dimensional compounds Sr2−xGdxCoO4

The layered perovskite compound Sr2−xGdxCoO4 has not yet been subjected to detailed study so far. In this report, structures, transport, magnetic properties, and first principle calculations will be reported for the two dimensional compounds Sr2−xGdxCoO4 (x = 0.5, 0.75, 1, 1.25). Rietveld refinement revealed that these compounds are crystallized in K2NiF4-type structures with space group I4/mmm. It was found that the lattice parameter c decreases as x increases. Through the Curies Weiss fitting of the temperature dependent magnetization, it was found that the Sr1.25Gd0.75CoO4 sample exhibits a weak ferromagnetic to paramagnetic transition at about 62 K, with a Curie constant of 0.113 emu K/mol. Band structure calculations indicated that electrons of these compounds are spin polarized at the Fermi level. The 2-D Variable Range Hopping model fitting indicated that the two dimensional variable range hopping mechanism could be used to account for the conducting mechanism for these samples.

Studies on the transport properties of RuSr2RECu2Oy, RuSr2(RE)2Cu2Oy(RE = Gd, Nd) and RuSr2Gd1.4Ce0.6Cu2O10: Possibility of variable range hopping

We study synthesis, characterization and transport properties of several superconducting and nonsuperconducting ruthenocuprate materials by using different routes. RuSr2(RE)Cu2Oy (Ru-1212) and RuSr2(RE)2Cu2Oy (Ru-1222) have been synthesized using two different rare earths, RE=Gd or Nd. A similar route has been followed for the synthesis of all these four compounds. All ruthenocuprate samples are characterized by using X-ray diffraction (XRD), Scanning Electron Micrograph (SEM) and Energy Dispersive X-ray (EDX). The lattice constants have been extracted and so are the grain sizes. Stoichiometries of several ruthenocuprates have been determined by using EDX spectrum. The resistivities of ruthenocuprates annealed in different conditions have been compared. The resistivity in Gd based ruthenocuprate varies in order of magnitude in comparison to that of ruthenocuprates with Nd. We have studied how the annealing temperature also affects transport properties strongly. Using a different route we have synthesized few RuSr2Gd1.4Ce0.6Cu2O10 which shows insulating to superconductor transition. Three dimensional variable range hopping (3DVRH) may be considered as a possible conduction mechanism in the normal state of superconducting RuSr2Gd1.4Ce0.6Cu2O10.

Structural and Electrical Properties of Fluorine Doped (Bi, Pb)-1212 Ceramics

In the aim to test the effect of fluorine doping on structural and electrical properties of semiconducting Bi(Pb)-1212 phase, bulk samples with nominal composition of Bi0.4Pb0.35Sr2Y0.8Ca0.2Cu2.05O y F x with x=0,0.2,0.4 and 0.6 are prepared in air by solid state reaction method and characterized. Similarly to other Bi-based cuprates, the scanning electron microscopy shows that fluorine doping increases the grain size of the samples. The obtained X-ray diffraction patterns are indexed in P4/mmm space group. A slight increase of cell volume by doping is revealed, a result which is in line with that seen in Y-123 compounds. Even if the resistivity decreases with fluorine content, all compounds exhibit a semiconducting behavior. The conduction mechanism obeys well to the two dimensional variable range hopping (2D-VRH) model from room temperature down to approximately 40 K. The analysis of the fitting results suggests that the improvement of electrical conduction could be due to a delocalization of charge carriers.

Effect of conducting polypyrrole on the transport properties of carbon nanotube yarn

Experiments were conducted to measure the electrical conductivity in three types of pristine and carbon nanotube-polypyrrole (CNT-PPy) composite yarns and its dependence on over a wide temperature range. The experimental results fit well with the analytical models developed. The effective energy separation between localized states of the pristine CNT yarn is larger than that for both the electrochemically and chemically prepared CNT-PPy yarns. It was found that all samples are in the critical regime in the insulator–metal transition, or close to the metallic regime at low temperature. The electrical conductivity results are in good agreement with a Three Dimensional Variable Range Hopping model at low temperatures, which provides a strong indication that electron hopping is the main means of current transfer in CNT yarns at T<100K. We found that the two shell model accurately describes the electronic properties of CNT and CNT-PPy composite yarns in the temperature range of 5–350K.

Graphene nanoplatelet/silicon nitride composites with high electrical conductivity

Silicon nitride (Si3N4) processed with up to 25vol.% of graphene nanoplatelets (GNPs) gives conductive composites with the highest electrical conductivity (40Scm−1) reported for these ceramics with added conductive particles. During compaction and pressure-assisted densification of the composites in the spark plasma sintering (SPS), a preferred orientation of GNPs occurs. Consequently, the electrical conductivity measured along the direction perpendicular to the SPS pressing axis is more than one order of magnitude higher than the one measured along the parallel direction.Percolation in the composites is observed for 7–9 vol.% of GNPs, depending on the measuring direction, perpendicular or parallel to the pressing axis. Different conduction mechanisms are apparent for the two orthogonal orientations. Charge transport along the direction defined by the graphene ab-plane (perpendicular direction) may be explained by a two dimensional variable range hopping mechanism, whereas conduction in the parallel direction shows a more complex behavior, with a metallic-type transition (dσ/dT<0) for high GNP contents. A thin amorphous layer was identified at the Si3N4/GNPs interface that may affect the conduction for the parallel configuration.