Fluctuation-induced Tunneling Research Articles

Impedance spectroscopy investigation of conjugated polymer coated core-shell nanoparticles

Poly(styrene) nanoparticles decorated at their surface with poly(styrene sulfonate) brushes and subsequently loaded with polypyrrole have been prepared as film-forming materials to serve as hole injection layers in organic light-emitting diodes. Thin compressed pellets of these nanoparticles have been studied by impedance spectroscopy. Measurements were carried out in the temperature range between 123.15 and 453.15 K and frequency range from 10−1 to 106 Hz. The polypyrrole volume fraction φPPy was varied as well. The film-forming nanoparticles exhibit a core-shell-type morphology with a core of electrically insulating poly(styrene) and a shell consisting of a corona of poly(styrene sulfonate) chains, which form the matrix in which the electrically conducting complex of polypyrrole and poly(styrene sulfonate) is embedded. This conducting complex exists in forms of domains with nanoscale dimensions. It is demonstrated that the charge transport in samples with φPPy [for the calculation of the polypyrrole volume fractions the poly(styrene) volume was excluded] lower than 0.52 is dominated by ionic conductivity. At higher volume fractions the charge transport is mainly electronic in nature. The fluctuation-induced tunneling model can be used to describe the temperature dependence of the dc conductivity. For a sample with φPPy of 0.52, a transition from insulating to conducting behavior at 385 K has been found. An electrical percolating behavior has been observed with a percolation threshold at φPPy of 0.65, suggesting that polypyrrole rich and poor domains are present in the poly(styrene sulfonate) matrix and indicating a “transition” from electronically insulating to conducting behavior. A detailed analysis of the impedance spectra suggests a lower limit of the conducting domains of 2.5 nm.

Electrical/dielectric properties and conduction mechanism in melt processed polyamide/multi-walled carbon nanotubes composites

Abstract The electrical and dielectric properties of polyamide 6 (PA6)/multi-walled carbon nanotubes (MWCNT) nanocomposites prepared by melt mixing were investigated by employing dielectric relaxation spectroscopy in broad frequency (10−2–106 Hz) and temperature ranges (from −150 to 150 °C). Transmission electron microscopy revealed a good state of CNT dispersion in the polymeric matrix. The percolation threshold (pc) was found to be 1.7 vol.% by using the dependence of both dc conductivity and critical frequency (fc) from dc to ac transition on vol.% concentration in MWCNT. The actual aspect ratio of the nanotubes in the nanocomposites was calculated using a theoretical model (proposed by Garboczi et al.) and the obtained value was correlated with the pc value according to the excluded volume theory. Additionally, the contact resistance (Rc) between the conductive nanotubes was found to be ∼105 Ω. Investigation of the temperature dependence of conductivity revealed a charge transport which is controlled by thermal fluctuation-induced tunneling for temperatures up to the glass transition. Finally, it was shown that the addition of nanotubes has no significant influence on the relaxation mechanisms of the PA6 matrix.

Current-voltage characteristics of individual conducting polymer nanotubes and nanowires

We report the current-voltage (I–V) characteristics of individual polypyrrole nanotubes and poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires in a temperature range from 300 K to 2 K. Considering the complex structures of such quasi-one-dimensional systems with an array of ordered conductive regions separated by disordered barriers, we use the extended fluctuation-induced tunneling (FIT) and thermal excitation model (Kaiser expression) to fit the temperature and electric-field dependent I–V curves. It is found that the I–V data measured at higher temperatures or higher voltages can be well fitted by the Kaiser expression. However, the low-temperature data around the zero bias clearly deviate from those obtained from this model. The deviation (or zero-bias conductance suppression) could be possibly ascribed to the occurrence of the Coulomb-gap in the density of states near the Femi level and/or the enhancement of electron-electron interaction resulting from nanosize effects, which have been revealed in the previous studies on low-temperature electronic transport in conducting polymer films, pellets and nanostructures. In addition, similar I–V characteristics and deviation are also observed in an isolated K0.27MnO2 nanowire.

Fluctuation-induced tunneling conduction through nanoconstrictions

We propose a generalization of the well-known fluctuation-induced tunneling conduction mechanism based on electron tunneling through finite segment(s) of nanoconstrictions whose transverse dimension is less than half the Fermi wavelength. By considering the effects of thermally induced voltage fluctuations across the nanoconstrictions, a temperature-dependent conductivity behavior is obtained which is in good agreement with the experiments, with reasonable parameter values. In the limit of high applied voltage the present model predicts interesting electronic Fabry-Perot behavior manifesting as peaks in differential conductance with linear variation in their voltage separations.

Current–Voltage Characteristics in Individual Polypyrrole Nanotube, Poly(3,4-ethylenedioxythiophene) Nanowire, Polyaniline Nanotube, and CdS Nanorope

In this paper, we focus on current–voltage (I–V) characteristics in several kinds of quasi-one-dimensional (quasi-1D) nanofibers to investigate their electronic transport properties covering a wide temperature range from 300 down to 2 K. Since the complex structures composed of ordered conductive regions in series with disordered barriers in conducting polymer nanotubes/wires and CdS nanowires, all measured nonlinearI–Vcharacteristics show temperature and field-dependent features and are well fitted to the extended fluctuation-induced tunneling and thermal excitation model (Kaiser expression). However, we find that there are surprisingly similar deviations emerged between theI–Vdata and fitting curves at the low bias voltages and low temperatures, which can be possibly ascribed to the electron–electron interaction in such quasi-1D systems with inhomogeneous nanostructures.

Thermal fluctuation-induced tunneling conduction through metal nanowire contacts

The temperature behavior of how electrons propagate through an insulating electroniccontact formed at the interface between a submicron Cr/Au electrode and a metallicRuO2 nanowire (NW) has been studied between 300 and 1 K. The NWs are typically of∼70 nm in diameter and a few microns long. The submicron electrodeswere fabricated by the standard electron-beam lithography technique.By employing the two-probe method, the electronic contact resistances,Rc(T), have been determined. We found that, in general,Rc increases rapidly with decreasing temperature but eventually saturates atliquid-helium temperatures. Such a temperature behavior can be well describedby a thermal fluctuation-induced tunneling (FIT) conduction process whichconsiders the crossover feature from thermal activation conduction at hightemperatures to simple elastic tunneling conduction at low temperatures. The wideapplicability of this FIT model has further been established by employing metallicIrO2 andSn-doped In2O3−x NWs. This work demonstrates that the underlying physics for the charge transportproperties of an insulating electronic contact can be well understood.

Giant magnetoresistance in Co–Al 2O 3 granular films prepared by self-organized growth

Co–Al 2O 3 granular films with a narrow distribution in cluster size of Co clusters embedded in Al 2O 3 matrix were prepared by sequential deposition based on self-organized growth. Resistivity dependence of giant magnetoresistance (GMR) was studied. The GMR takes a maximum of 5.2% at room temperature and 9.4% at 13 K and 5700 Gs when the resistivity of the sample is 4×10 5–7×10 5 μΩ cm. The temperature dependence of resistivities and GMR were discussed especially. A temperature dependence of conductance ρ∼exp[ T 1/( T+ T 0)] was found, which indicates the dominant conduction mechanism is fluctuation-induced tunneling. A linear relationship of GMR versus T was observed, GMR= a– kT, in applied magnetic field 5700 Gs. The remarkable character of temperature dependence of GMR should be due to the special microstructure that the clusters are monodispersed in the films.

Charge transport mechanisms in microcrystalline silicon

A heterogeneous charge transport model for microcrystalline silicon based on fluctuation-induced tunneling is presented that fits the low-temperature saturation observed in dark conductivity measurements and accounts for the film microstructure. Excellent agreement is found when the model is applied to data reported in the literature, particularly for highly crystalline samples, which produce the highest performance transistors. Values obtained for the three fitting parameters are consistent with typical measurements of microcrystalline silicon film morphology and the conduction band offset between amorphous and crystalline silicons.

Temperature dependence of electrical conductivity in double-wall and multi-wall carbon nanotube/polyester nanocomposites

The aim of this study is to investigate temperature dependence of electrical conductivity of carbon nanotube (CNT)/polyester nanocomposites from room temperature to 77 K using four-point probe test method. To produce nanocomposites, various types and amounts of CNTs (0.1, 0.3 and 0.5 wt.%) were dispersed via 3-roll mill technique within a specially formulized resin blend of thermoset polyesters. CNTs used in the study include multi walled carbon nanotubes (MWCNT) and double-walled carbon nanotubes (DWCNT) with and without amine functional groups (–NH2). It was observed that the incorporation of carbon nanotubes into resin blend yields electrically percolating networks and electrical conductivity of the resulting nanocomposites increases with increasing amount of nanotubes. However, nanocomposites containing amino functionalized carbon nanotubes exhibit relatively lower electrical conductivity compared to those with non-functionalized carbon nanotubes. To get better interpretation of the mechanism leading to conductive network via CNTs with and without amine functional groups, the experimental results were fitted to fluctuation-induced tunneling through the barriers between the metallic regions model. It was found that the results are in good agreement with prediction of proposed model.

Fluctuation-induced tunneling in TiO2-derived nanotube pellets

The frequency-dependent conductivity of TiO2-derived nanotubes pressed to a pellet was measured over the temperature range 100 to 390 K. The temperature dependence of the electrical conductivity measured at 100 Hz indicates a three-dimensional variable range hopping mechanism at higher temperatures and fluctuation-induced tunneling conduction below 300 K. From the frequency dependence of the conductivity it is possible to conclude that the conductivity is governed by two parallel channels. The first channel, dominating at higher temperatures, is characterized by the three-dimensional variable range hopping mechanism and the second channel, which takes over at lower temperatures, by the fluctuation-induced tunneling mechanism. Such a two-channel mechanism may be responsible for the similar temperature dependence of the electrical conductivity observed in some other three-dimensional systems.

Fluctuation-induced tunneling conductance and enhanced magnetoresistance in polycrystallineCrO2and its composites

Intergranular conduction in half metallic ${\mathrm{CrO}}_{2}$ is known to occur through a combination of spin dependent tunneling [driven by Coulomb blockade (CB) effects] together with certain spin independent (SI) hopping processes. We present evidence that in polycrystalline ${\mathrm{CrO}}_{2}$ with enhanced grain size, both these process (CB effect and SI hopping) are suppressed and the functional form of conductance is best described by fluctuation-induced tunneling (FIT) below $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In this temperature range, FIT is seen to be retained when pure ${\mathrm{CrO}}_{2}$ is diluted by two different insulators ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Cr}}_{2}{\mathrm{O}}_{5}$. Above $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a step in conductance is observed in pure ${\mathrm{CrO}}_{2}$ which is seen to be modulated by the magnetic state of the grain boundary as well as applied magnetic field. In the vicinity of this step, a significant enhancement of magnetoresistance is observed. Overall, magnetotransport studies in these compounds reveal that a systematic variation in crystallographic microstructure in granular ${\mathrm{CrO}}_{2}$ leads to a different functional form of conductance which is intimately coupled to the enhanced magnetoresistive properties as observed here.

Electrical conductivity of Mo 6S 3I 6 and Mo 6S 4.5I 4.5 nanowires

Electrical conductivity measurements on novel materials composed of bundles of Mo 6S 3I 6 and Mo 6S 4.5I 4.5 nanowires pressed in a sheet form, in the frequency range from 10 Hz to 1 MHz for temperatures between 4 K and 330 K, are reported. Temperature dependence of the quasistatic electrical conductivity show similar dependence at higher temperatures as expected for three-dimensional variable range hopping mechanism with decreasing electrical conductivity as temperature decreases. Below 50 K the electrical conductivity becomes completely temperature independent. It seems that at lower temperatures fluctuation-induced tunnelling mechanism dominates in these nanostructured materials.

Electrical and magnetic properties of ε-Fe 3N nanoparticles synthesized by chemical vapor condensation process

ε-Fe 3N nanoparticles synthesized by chemical vapor condensation (CVC) are covered with shells of disordered Fe 3O 4 phase, as observed by a transmission electron microscopy. The zero-field cooling and field cooling temperature dependence of magnetization, ac susceptibility as a function of frequency, magnetic hysteresis loops, and the temperature dependence of resistivity of the ε-Fe 3N nanoparticles are systematically studied. The results indicate the existence of complex magnetic properties, such as superparamagnetic behavior, exchange bias, magnetic dipole interaction, and the possible coexistence of ferromagnetic and spin-glass-like states and/or disordered surface spins of the shells at low temperatures. The temperature dependence of resistivity ρ( T) for compacted ε-Fe 3N nanoparticles in a temperature range of 110 K< T< 300 K can be well described by the mechanism of fluctuation-induced tunneling conduction, while that below 110 K can be ascribed to conducting electrons scattered by localized magnetic moments and impurity as well as the influence of freezing of spin-glass-like moments and/or disordered surface spins of the shells.

Characterization of facing-target reactive sputtered polycrystalline Fe3O4 films

Polycrystalline half-metallic Fe3O4 films fabricated by facing-target reactive sputtering were investigated systemically. Structural results reveal that typical Fe3O4 grains were well isolated by grain boundaries and grew with columnar structure. The room-temperature magnetization of Fe3O4 films at 50kOe field is much lower than that of bulk Fe3O4 (471emu∕cm3), due to the presence of strong antiferromagnetic coupling within grain boundary and the amorphous bottom and top layers. The insaturation magnetization and loop shift in high field region at low temperature also stem from the strong antiferromagnetic coupling. The resistivity increases with the decreasing temperature and is consistent with fluctuation-induced tunneling mechanism in a wide temperature ranging from 50to300K. The negative magnetoresistances up to −9.7% at 150K and −6.4% at 300K under 90kOe magnetic field perpendicular to the film plane were observed and discussed.

Dc transport properties and resistance fluctuation processes inSr2FeMoO6polycrystalline thin films

We performed transport measurements and voltage noise analysis on ${\mathrm{Sr}}_{2}\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_{6}$ thin films, in zero applied magnetic field as a function of temperature $T$, down to $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The samples, grown by pulsed laser deposition on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ substrates, showed a negative resistivity thermal coefficient and, at $T&lt;50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the observed temperature dependence of resistivity and the $I\text{\ensuremath{-}}V$ curves were correctly described by the fluctuation-induced tunneling model which assumes the presence of intergranular tunneling of charge carriers. Moreover, the analysis of the low temperature resistance fluctuation processes, pointed out the presence of a voltage noise component independent of the frequency within the whole investigated bandwidth with a spectral density directly proportional to the square of the bias current. We interpreted this unusual behavior as a possible consequence of tunneling intergranular processes.

Two-channel electrical conduction in air-stable monodispersed Mo6S3I6 nanowire sheets

The quasistatic and frequency-dependent electrical conductivities below 1MHz were measured in the temperature range of 4–330K for air-stable monodispersed bundles of Mo6S3I6 nanowires pressed in the form of a sheet. The orientation of the bundles of nanowires within the sheet was approximately isotropic. The observed weak nonlinearity of the current-voltage (I-V) curves could be attributed to the electric-field-induced increase in the number of charge carriers. The temperature dependence of the quasistatic electrical conductivity shows a crossover from a three-dimensional variable-range-hopping mechanism, which dominates at higher temperatures, to a fluctuation-induced tunneling conduction below 50K. In fact, the electrical conductivity becomes almost temperature independent below 50K. The frequency dependence of the electrical conductivity obtained in the frequency range of 10−3–106Hz reveals that this crossover in the temperature dependence is a consequence of the crossover between two parallel conductivity channels. The first channel, which is governed by the three-dimensional variable-range-hopping mechanism, dominates at higher temperatures, while the second channel, governed by the tunneling mechanism, takes over at lower temperatures.

Thin porous indium tin oxide nanoparticle films: effects of annealing in vacuum and air

Electrical and optical properties were investigated in porous thin films consisting of In2O3:Sn (indium tin oxide; ITO) nanoparticles. The temperature-dependent resistivity was successfully described by a fluctuation-induced tunneling model, indicating a sample morphology dominated by clusters of ITO nanoparticles separated by insulating barriers. An effective-medium model, including the effect of ionized impurity scattering, was successfully fitted to measured reflectance and transmittance. Post-deposition treatments were carried out at 773 K for 2 h in both air and vacuum. It is shown that vacuum annealing increases either the barrier width or the area between two conducting clusters in the samples and, furthermore, an extra optical absorption occurs close to the band gap. A subsequent air annealing then reduces the effect of the barriers on the electrical properties and diminishes the absorption close to the band gap.

Inter-grain tunneling conductivity of Sr2FeMoO6: effects of doping and grain-boundary modification

Abstract The inter-grain conductivity ( σ GB ) of polycrystalline and granular Sr 2 FeMoO 6 (SFMO) increases linearly with temperature between liquid He to room temperature at least. This very unusual behavior motivated the new investigations reported here. It was found that σ GB ( T ) is linear also for granular samples of La-doped SFMO and for sintered SFMO with modified grain boundaries (GBs), but it is nonlinear for granular samples of Ag- and Na-doped SFMO. The fluctuation-induced tunneling (FIT) model was successfully applied in the interpretation of both types of behavior. Current–voltage measurements were carried out on a resistive sample with oxygenated GBs, using pulsed high currents. The interpretation of the effect of a high electric field on transport, in terms of the FIT model, is discussed.

Low-Temperature Iodine Heat Treatment of Quasi-One-Dimensional KFeS2 Crystals

Low-temperature iodine heat treatment of quasi-one-dimensional KFeS2 crystals was carried out. The resistivity along the c-axis is seen to decrease rapidly with increasing iodine-heat-treatment temperature and the nonmetallic temperature dependence was found to become weaker. The fluctuation-induced tunneling model was used to explain the reduction of the potential barrier at grain boundaries caused by the iodine heat treatment. In particular, samples subjected to iodine heat treatment at 100°C for 2 days exhibited a low room-temperature resistivity of approximately 10-1 Ωcm and a corresponding positive temperature gradient, which are evidence of the first observation of metallic behavior. Furthermore, the room-temperature Seebeck coefficient of these iodine-heat-treated samples was small, a value of +5.8 µV/K, and exhibited an almost linear temperature dependence that showed the samples to be metallic. Finally, although KFeS2 normally exhibits metallic conduction, the presence of grain boundaries formed during crystallization from the molten state is thought to have led to the nonmetallic conductivity observed in our experiments.

Structures and transport properties of polycrystallineFe3O4 films

Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature.A transmission electron microscope image shows that the films consist ofFe3O4 grains well separated by grain boundaries with long-range and atomic scale disorders. Thewidth of the long-range disordered grain boundaries observed by high resolutiontransmission electron microscopy is about , which is consistent with the boundary dimension derived from the resistivityand magnetization measurements. The temperature dependence of resistivityindicates that the transport properties of the films are dominated by themechanism of fluctuation-induced tunnelling of electrons across the grainboundaries. Ordinary and extraordinary Hall constants were measured to be−8.22 × 10−11 and , which are two and three orders of magnitude larger than those of iron, respectively.