Showing papers on "Variable-range hopping published in 2014"

Electrical Transport Properties of Single-Layer WS2

Abstract: We report on the fabrication of field-effect transistors based on single layers and bilayers of the semiconductor WS2 and the investigation of their electronic transport properties. We find that the doping level strongly depends on the device environment and that long in situ annealing drastically improves the contact transparency, allowing four-terminal measurements to be performed and the pristine properties of the material to be recovered. Our devices show n-type behavior with a high room temperature on/off current ratio of similar to 10(6). They show clear metallic behavior at high charge carrier densities and mobilities as high as similar to 140 cm(2)/(V s) at low temperatures (above 300 cm(2)/(V s) in the case of bilayers). In the insulating regime, the devices exhibit variable range hopping, with a localization length of about 2 nm that starts to increase as the Fermi level enters the conduction band. The promising electronic properties of WS2, comparable to those of single layer MoS2 and WSe2, together with its strong spin-orbit coupling, make it interesting for future applications in electronic, optical, and valleytronic devices.

Large thermoelectricity via variable range hopping in chemical vapor deposition grown single-layer MoS2.

Abstract: Ultrathin layers of semiconducting molybdenum disulfide (MoS2) offer significant prospects in future electronic and optoelectronic applications. Although an increasing number of experiments bring light into the electronic transport properties of these crystals, their thermoelectric properties are much less known. In particular, thermoelectricity in chemical vapor deposition grown MoS2, which is more practical for wafer-scale applications, still remains unexplored. Here, for the first time, we investigate these properties in grown single layer MoS2. Microfabricated heaters and thermometers are used to measure both electrical conductivity and thermopower. Large values of up to ∼30 mV/K at room temperature are observed, which are much larger than those observed in other two-dimensional crystals and bulk MoS2. The thermopower is strongly dependent on temperature and applied gate voltage with a large enhancement at the vicinity of the conduction band edge. We also show that the Seebeck coefficient follows S ∼ T(1/3), suggesting a two-dimensional variable range hopping mechanism in the system, which is consistent with electrical transport measurements. Our results help to understand the physics behind the electrical and thermal transports in MoS2 and the high thermopower value is of interest to future thermoelectronic research and application.

Large Thermoelectricity via Variable Range Hopping in Chemical Vapor Deposition Grown Single-layer MoS2

Abstract: Ultrathin layers of semiconducting molybdenum disulfide (MoS2) offer significant prospects in future electronic and optoelectronic applications. Although an increasing number of experiments bring light into the electronic transport properties of these crystals, their thermoelectric properties are much less known. In particular, thermoelectricity in chemical vapor deposition grown MoS2, which is more practical for wafer-scale applications, still remains unexplored. Here, for the first time, we investigate these properties in grown single layer MoS2. Micro-fabricated heaters and thermometers are used to measure both electrical conductivity and thermopower. Large values of up to ~30 mV/K at room temperature are observed, which are much larger than those observed in other two dimensional crystals and bulk MoS2. The thermopower is strongly dependent on temperature and applied gate voltage with a large enhancement at the vicinity of the conduction band edge. We also show that the Seebeck coefficient follows S~T^1/3 suggesting a two-dimensional variable range hopping mechanism in the system, which is consistent with electrical transport measurements. Our results help to understand the physics behind the electrical and thermal transports in MoS2 and the high thermopower value is of interest to future thermoelectronic research and application.

Electrical and thermal conductivities of reduced graphene oxide/polystyrene composites

Abstract: The author reports an experimental study of electrical and thermal transport in reduced graphene oxide (RGO)/polystyrene (PS) composites. The electrical conductivity (σ) of RGO/PS composites with different RGO concentrations at room temperature shows a percolation behavior with the percolation threshold of ∼0.25 vol. %. Their temperature-dependent electrical conductivity follows Efros-Shklovskii variable range hopping conduction in the temperature range of 30–300 K. The thermal conductivity (κ) of composites is enhanced by ∼90% as the concentration is increased from 0 to 10 vol. %. The thermal conductivity of composites approximately linearly increases with increasing temperature from 150 to 300 K. Composites with a higher concentration show a stronger temperature dependence in the thermal conductivity.

Metallic to hopping conduction transition in Ta2O5−x/TaOy resistive switching device

Abstract: In this Letter, a comprehensive analysis of how the metallic behavior transition to hopping was presented by studying the transport mechanisms of low resistance states (LRS) in Ta2O5−x/TaOy resistive switching devices at very low temperatures. Three types of conduction behaviors were reported through temperature-dependent measurements ranging from 5 K to 250 K. Memory cells at low LRS show metallic behavior due to the formation of metallic filament. The temperature dependence of resistance at medium LRS exhibits an interesting phenomenon that a positive temperature coefficient transfers into a negative one at temperature of 20 K. Detailed analysis reveals that this phenomenon is caused by the coexistence of extended and localized states, with metallic conduction at higher temperatures and variable-range hopping at lower temperatures. Carrier transport at high LRS is dominated by electrons hopping conduction with nearest-neighboring hopping conduction changing into variable-range hopping as temperature dec...

Temperature dependent electrical transport of disordered reduced graphene oxide

Abstract: We report on the simple route for the synthesis of chemically reduced graphene oxide (rGO) using ascorbic acid (a green chemical) as a reducing agent. Temperature-dependent electrical transport properties of rGO thin films have been studied in a wide range (50 K T 400 K) of temperature. Electrical conduction in rGO thin films was displayed in two different temperature regimes. At higher temperatures, Arrhenius-like temperature dependence of resistance was observed indicating a band gap dominating transport behavior. At lower temperatures, the rGO sample showed a conduction mechanism consistent with Mottʼs two-dimensional variable range hopping (2D-VRH). An unsaturated negative magnetoresistance (MR) was observed up to 3 T field. A decrease in negative MR at high temperatures is attributed to the phonon scattering of charge carriers.

Variable Range Hopping Conduction in BaTiO3 Ceramics Exhibiting Colossal Permittivity

Abstract: Fast-fired barium titanate ceramics have been shown to exhibit room temperature colossal permittivity (er′ ≈ 105, tan δ ≈ 0.05 at 1 kHz) when synthesized via the spark plasma sintering technique. Broadband dielectric spectroscopy analysis is here utilized to identify the active conduction mechanisms in the compound. Analysis of the temperature-dependent bulk dc conductivity reveals that the bulk conduction in fast-fired barium titanate is the result of variable-range hopping with relatively short hopping distance (∼0.5 nm) compared to other colossal permittivity materials. A common equivalent circuit that can model the contributions of the different polarization mechanisms to both the dc conductivity and colossal permittivity is established.

Theory of charge hopping along a disordered polymer chain

Abstract: We built a model of charge transport in a single disordered polymer chain starting from a model Hamiltonian of the system. The parameters entering the Hamiltonian determine both the density of states (DOS) and the hopping rate unlike the most common modelling strategies of transport in polymeric materials that parametrize both the DOS and the hopping rate from the outset. This model incorporates the effect of variable delocalization of one-electron states and is designed to link atomistic calculations of polymeric systems with full device models in multiscale modelling protocols. The initial and final states for the hopping process are determined by static disorder and further stabilized by polaronic effects. The coupling between these states is due to the residual (and much smaller) dynamic disorder. We find that, at lower static disorder, long-distance hopping events become more frequent, i.e. the hopping range and disorder are not unrelated parameters, as commonly assumed. The availability of low energy relatively delocalized states promotes long range displacement of charge and it can be at the origin of the high mobility observed in some polymers. The description of the hopping rates from the model Hamiltonian also allows the identification of the breakdown of the incoherent transport limit.

Dielectric and Magnetodielectric Properties of R2NiMnO6 (R = Nd, Eu, Gd, Dy, and Y)

Abstract: We have prepared polycrystalline R2NiMnO6 (R = Nd, Eu, Gd, Dy, and Y) samples by conventional solid-state reaction and all the samples have shown monoclinic structure with P21/n space group. With the decrease in rare-earth ionic size ( ), the bond angle decreases, correspondingly a decrease in ferromagnetic (FM) Curie temperature is noticed. In the dielectric measurement, the dielectric anomaly shifts to high temperature with the decrease in the and shows no correlation with the FM Curie temperature and hints the absence of apparent magnetodielectric (MD) coupling. Appearance of multiple relaxations in the dielectric study suggests the electrical heterogeneity of the system. The dielectric/impedance analysis has revealed a close correlation between dc conductivity and the dielectrics; in fact, both dc resistivity and the grain relaxations follow the variable range hopping mechanism. The thermal activation of charge carriers at the grain boundary led to Maxwell–Wagner interfacial polarization. Finally, dielectric study under magnetic field showed no effect, it implies that not only the intrinsic MD is absent, but also the extrinsic MD due to the lack of magnetoresistance.

Solution-Processed Poly(3,4-ethylenedioxythiophene) Thin Films as Transparent Conductors: Effect of p-Toluenesulfonic Acid in Dimethyl Sulfoxide

Abstract: Conductivity enhancement of thin transparent films based on poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) by a solution-processed route involving mixture of an organic acid and organic solvent is reported. The combined effect of p-toluenesulfonic acid and dimethyl sulfoxide on spin-coated films of PEDOT-PSS on glass substrates, prepared from its commercially available aqueous dispersion, was found to increase the conductivity of the PEDOT-PSS film to ∼3500 S·cm(-1) with a high transparency of at least 94%. Apart from conductivity and transparency measurements, the films were characterized by Raman, infrared, and X-ray photoelectron spectroscopy along with atomic force microscopy and secondary ion mass spectrometry. Combined results showed that the conductivity enhancement was due to doping, rearrangement of PEDOT particles owing to phase separation, and removal of PSS matrix throughout the depth of the film. The temperature dependence of the resistance for the treated films was found to be in accordance with one-dimensional variable range hopping, showing that treatment is effective in reducing energy barrier for interchain and interdomain charge hopping. Moreover, the treatment was found to be compatible with flexible poly(ethylene terephthalate) (PET) substrates as well. Apart from being potential candidates to replace inorganic transparent conducting oxide materials, the films exhibited stand-alone catalytic activity toward I(-)/I3(-) redox couple as well and successfully replaced platinum and fluorinated tin oxide as counter electrode in dye-sensitized solar cells.

Crossover of conduction mechanism in Sr2IrO4 epitaxial thin films

Abstract: High quality epitaxial Sr2IrO4 thin films with various thicknesses (9–300 nm) have been grown on SrTiO3 (001) substrates and their electric transport properties have been investigated All samples showed the expected insulating behavior with a strong resistivity dependence on film thickness, which can be as large as three orders of magnitude at low temperature A close examination of the transport data revealed interesting crossover behaviors for the conduction mechanism upon variation of thickness and temperature While Mott variable range hopping (VRH) dominated the transport for films thinner than 85 nm, high temperature (>200 K) thermal activation behavior was observed for films with large thickness (≥85 nm), which was followed by a crossover from Mott to Efros-Shklovskii (ES) VRH in the low temperature range This low temperature crossover from Mott to ES VRH indicates the presence of a Coulomb gap (∼3 meV) Our results demonstrate the competing and tunable conduction in Sr2IrO4 thin films, which in turn would be helpful for understanding the insulating nature related to strong spin-orbital-coupling of the 5d iridates

Temperature evolution of magnetic and transport behavior in 5\textit{d} Mott insulator Sr$_2$IrO$_4$: Significance of magneto-structural coupling

Abstract: We have investigated the temperature evolution of magnetism and its interrelation with structural parameters in perovskite-based layered compound Sr$_2$IrO$_4$, which is believed to be a $J_{eff}$ = 1/2 Mott insulator. The structural distortion plays an important role in this material which induces a weak ferromagnetism in otherwise antiferromagnetically ordered magnetic state with transition temperature around 240 K. Interestingly, at low temperature below around 100 K, a change in magnetic moment has been observed. Temperature dependent x-ray diffraction measurements show sudden changes in structural parameters around 100 K are responsible for this. Resistivity measurements show insulating behavior throughout the temperature range across the magnetic phase transition. The electronic transport can be described with Mott's two-dimensional variable range hopping (VRH) mechanism, however, three different temperature ranges are found for VRH, which is a result of varying localization length with temperature. A negative magnetoresistance (MR) has been observed at all temperatures in contrast to positive behavior generally observed in strongly spin-orbit coupled materials. The quadratic field dependence of MR implies a relevance of a quantum interference effect.

Enhanced magnetic and electrical properties of Y and Mn co-doped BiFeO3 nanoparticles

Abstract: Multiferroic (Y, Mn) substituted BiFeO3 had been synthesized by a facial sol–gel method. The single phase polycrystalline nature of samples was confirmed from X-ray diffraction pattern. The average particle size was estimated to be around 30–32 nm from transmission electron microscopy. The magnetic properties of codoped nanoparticles had been studied using Bloch equation and the estimated value of the Bloch constant was found to be much larger than usual ferromagnetic materials. Coercivity values for different temperature is used to calculate the blocking temperature and found to lie above room temperature. The dc electrical transport properties were studied in the temperature range 298– 523 K and explained using a Motts 3D variable range hopping model and the density of states was estimated near the Fermi level. The ac electrical data were found to follow the correlated barrier hopping model. Well-developed PE hysteresis loops were observed in codoped nanoparticles, which were attributed to a decrease in oxygen vacancies, bismuth volatisation due to doping and an increase of the effective potential barrier height for charge carriers.

Dielectric relaxations in La2NiMnO6 with signatures of Griffiths phase

Abstract: Polycrystalline La2NiMnO6 prepared by sol–gel auto combustion technique is found to possess dual crystallographic phases R-3c (71 vol. %) and Pbnm (29 vol. %), with respective ferromagnetic transitions at 277 K and 160 K, respectively. The impedance data are analysed via Havriliak-Negami equivalent for impedance (HNEI). Using HNEI equation, the resistance “R,” relaxation time “τ,” and symmetric “α” and asymmetric “β” broadening parameters, for the two crystallographic phases are obtained. The dielectric relaxations for the two crystallographic phases are found to be independent of each other. The activation energy of relaxation for the R-3c and Pbnm phases is found to be 147 meV and 88 meV, respectively. Strong correlations of dielectric relaxations with the magnetic phases are seen. Before the onset of ferromagnetic transition, i.e., above 160 K for Pbnm and 277 K for R-3c phase, relaxations are dominantly non-Debye type. Once the ferromagnetic state is achieved, relaxations attain Debye nature. Hopping type conduction mechanism is confirmed from electrical transport measurements: nearest neighbour hopping near room temperature and gradual transition to variable range hopping (VRH) at low temperature. Signature of short range magnetic ordering, reminiscent to Griffiths like phase, is seen in the temperature range 295 K > T > 277 K.

Conduction mechanism in bismuth silicate glasses containing titanium

Abstract: Bismuth silicate glasses mixed with different concentrations of titanium dioxide having compositions xTiO2–(60−x)Bi2O3–40SiO2 with x=0, 5, 10, 15 and 20 were prepared by the normal melt quench technique. The frequency dependence of the ac electrical conductivity of different compositions of titanium bismuth silicate glasses has been studied in the frequency range 10−1 Hz to 10 MHz and in the temperature range 623–703 K. The temperature and frequency dependent conductivity is found to obey Jonscher׳s universal power law for all the compositions of titanium bismuth silicate glass system. The dc conductivity (σdc), so called crossover frequency (ωH), and frequency exponent (s) have been estimated from the fitting of experimental data of ac conductivity with Jonscher׳s universal power law. Enthalpy to dissociate the cation from its original site next to a charge compensating center (Hf) and enthalpy of migration (Hm) have also been estimated. The conductivity data have been analyzed in terms of different theoretical models to determine the possible conduction mechanism. Analysis of the conductivity data and the frequency exponent shows that the correlated barrier hopping of electrons between Ti3+ and Ti4+ ions in the glasses is the most favorable mechanism for ac conduction. The temperature dependent dc conductivity has been analyzed in the framework of theoretical variable range hopping model (VRH) proposed by Mott which describe the hopping conduction in disordered semiconducting systems. The various polaron hopping parameters have also been deduced. Mott׳s VRH model is found to be in good agreement with experimental data and the values of inverse localization length of s-like wave function (α) obtained by this model with modifications suggested by Punia et al. are close to the ones reported for a number of oxide glasses.

Dielectric Constant and Transport Mechanism of Percolated Polyaniline Nanoclay Composites.

Abstract: We report the dielectric constant and transport mechanism of intercalated nanoclay–polyaniline composite, an industrially ready to use novel nanocomposite, which is prepared by a simple mechanochemical method. The effects of clay concentration on structure and structure variations on properties were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and impedance spectroscopy (20 Hz–1 MHz and temperature range from 300–380 K). The phenomenon of percolation was observed in these composites. The values of Mott’s temperature, density of states at the Fermi level, hopping distance, and barrier height for polyaniline–nanoclay (PANC) composites were calculated. By applying Mott’s theory, it is found that the PANC composites obey the one-dimensional variable range hopping mechanism. This type of percolated sample can be used as a nanocapacitor in many devices because of its enhanced transport pro...

Electrically induced insulator to metal transition in epitaxial SmNiO3 thin films

Abstract: We report on the electrically induced insulator to metal transition (IMT) in SmNiO3 thin films grown on (001) LaAlO3 by pulsed laser deposition. The behavior of the resistivity as a function of temperature suggests that the primary transport mechanism in the SmNiO3 insulating state is dominated by Efros-Shklovskii variable range hopping (ES-VRH). Additionally, the magnetic transition in the insulating state of SmNiO3 modifies the characteristics of the ES-VRH transport. Systematic DC and pulsed current-voltage measurements indicate that current-induced joule heating is the fundamental mechanism driving the electrically induced IMT in SmNiO3. These transport properties are explained in context of the IMT in SmNiO3 being related to the strong electron-lattice coupling.

Positive and negative magnetoresistance phenomena observed in magnetic electrospun polyacrylonitrile-based carbon nanocomposite fibers

Abstract: Unique temperature dependent resistance behavior (positive/negative temperature coefficient switching at 240 K), and tunable negative and positive magnetoresistance switching phenomena were observed in the magnetic carbon nanocomposite fibers prepared from heat treatment of the nitrate soaked oxidation stabilized polyacrylonitrile (PAN) nanofibers. The physical mechanisms were revealed by considering the variable range hopping electron transportation model, electron quantum interference and spin-dependent scattering effects.

Effect of anionic surfactant concentration on the variable range hopping conduction in polypyrrole nanoparticles

Abstract: The mechanism of charge transport in polypyrrole (PPy) nanoparticles prepared with different concentrations (5 to 30 mM) of anionic surfactant (sodium dodecyl sulfate) is reported. Transmission electron microscopy technique confirms the formation of PPy nanoparticles of sizes ∼52 to 28 nm under surfactant directed approach. The room temperature electrical conductivity of the prepared nanoparticles found to increase from 3 to 22 S/cm with surfactant concentration. The temperature dependent activation energy rules out the possibility of band conduction mechanism in the prepared PPy nanoparticles and thus the synthesized nanoparticles are analyzed under variable range hopping (VRH) model for conduction mechanism. The PPy nanoparticles, reduced with liquid ammonia, hold 3D VRH conduction mechanism for the charge transport. However, in the doped samples, some deviation from 3D VRH conduction behavior at higher temperatures (>150 K) has been observed. This may be attributed to the presence of anionic surfactant...

Charge carrier transport mechanisms in perovskite CdTiO3 fibers

Abstract: Electrical transport properties of electrospun cadmium titanate (CdTiO3) fibers have been investigated using ac and dc measurements. Air annealing of as spun fibers at 1000 °C yielded the single phase perovskite fibers having diameter ∼600 nm - 800 nm. Both the ac and dc electrical measurements were carried out at temperatures from 200 K – 420 K. The complex impedance plane plots revealed a single semicircular arc which indicates the interfacial effect due to grain boundaries of fibers. The dielectric properties obey the Maxwell-Wagner theory of interfacial polarization. In dc transport study at low voltages, data show Ohmic like behavior followed by space charge limited current (SCLC) with traps at higher voltages at all temperatures (200 K – 420 K). Trap density in our fibers system is Nt = 6.27 × 1017 /cm3. Conduction mechanism in the sample is governed by 3-D variable range hopping (VRH) from 200 K – 300 K. The localized density of states were found to be N(EF) = 5.51 × 1021 eV−1 cm−3 at 2 V. Other VR...

Changes in electrical transport and density of states of phase change materials upon resistance drift

Abstract: Phase-change memory technology has become more mature in recent years. But some fundamental problems linked to the electrical transport properties in the amorphous phase of phase-change materials still need to be solved. The increase of resistance over time, called resistance drift, for example, poses a major challenge for the implementation of multilevel storage, which will eventually be necessary to remain competitive in terms of high storage densities. To link structural properties with electrical transport, a broader knowledge of (i) changes in the density of states (DoS) upon structural relaxation and (ii) the influence of defects on electrical transport is required. In this paper, we present temperaturedependent conductivity and photo-conductivity measurements on the archetype phase change material GeTe. It is shown that trap-limited band transport at high temperatures (above 165 K) and variable range hopping at low temperatures are the predominating transport mechanism. Based on measurements of the temperature dependence of the optical band gap, modulated photo-conductivity and photo-thermal deflection spectroscopy, a DoS model for GeTe was proposed. Using this DoS, the temperature dependence of conductivity and photo-conductivity has been simulated. Our work shows how changes in the DoS (band gap and defect distributions) will affect the electrical transport before and after

Gate-modulated thermopower of disordered nanowires: II. Variable-range hopping regime

Abstract: We study the thermopower of a disordered nanowire in the field effect transistor configuration. After a first paper devoted to the elastic coherent regime (Bosisio, Fleury and Pichard 2014 New J. Phys. 16 035004), we consider here the inelastic activated regime taking place at higher temperatures. In the case where the charge transport is thermally assisted by phonons (Mott Variable Range Hopping regime), we use the Miller?Abrahams random resistor network model as recently adapted by Jiang et al for thermoelectric transport. This approach, previously used to study the bulk of the nanowire impurity band, is extended for studying its edges. In this limit, we show that the typical thermopower is largely enhanced, attaining values larger than and exhibiting a non-trivial behaviour as a function of the temperature. A percolation theory by Zvyagin extended to disordered nanowires allows us to account for the main observed edge behaviours of the thermopower.