Fluctuation-induced Tunneling Research Articles

ChemInform Abstract: Cobalt‐Based Magnetic Nanocomposites. Fabrication, Fundamentals and Applications

AbstractReview: 172 refs.

Low temperature conductivity of carbon nanotube aggregates

We compare, over wide temperature ranges, the transport properties of single-wall carbon nanotubes arranged in the form of aligned arrays or in the form of fibres. The experimental data show that both the forms of aggregates present a crossover in the transport mechanism from three-dimensional hopping of the electrons between localized states at high temperature to fluctuation-induced tunnelling across potential barriers at low temperature. The role of the junctions formed between the bundles in the array and between the nanotubes inside the fibres is discussed on the basis of the experimental results.

Fluctuation-induced tunneling conduction through RuO2 nanowire contacts

A good understanding of the electronic conduction processes through nanocontacts is a crucial step for the implementation of functional nanoelectronic devices. We have studied the current- voltage (I-V) characteristics of nanocontacts between single metallic RuO2 nanowires and contacting Au electrodes, which were pre-patterned by simple photolithography. Both the temperature behavior of contact resistance in the low-bias voltage ohmic regime and the I-V curves in the high-bias voltage non-ohmic regime have been investigated. We found that the electronic conduction processes in the wide temperature interval 1-300 K can be well described by the fluctuation-induced tunneling (FIT) conduction theory. Taken together with our previous work [Lin et al., Nanotechnology 19, 365201 (2008)], where the nanocontacts were fabricated by delicate electron-beam lithography, our study demonstrates the general validity of the FIT model in characterizing electronic nanocontacts.

Fluctuation-Induced Tunneling Conductivity in Nanoporous TiO2 Thin Films

The electronic mechanisms responsible for dark conductivity in nanoporous TiO2 thin films remain only partially understood, although they control the efficiency of charge transport in a wide range of technological applications. Measurements in the 78–335 K temperature range show DC conductivity values spanning over 4 orders of magnitude, with a high-temperature Arrhenius dependence that gradually changes into a temperature-independent plateau at low temperatures. We show evidence that a fluctuation-induced tunneling conductivity (FITC) mechanism is fully consistent with the experimental data. Quantitative agreement is demonstrated for the entire temperature range (T = 78–335 K) with a FITC model parametrized according to atomistic models of nanoporous TiO2 and the characterization of the films by X-ray diffraction and scanning electron microscopy measurements. These findings suggest that dark DC conductivity in nanoporous TiO2 films depends strongly on the properties of the junctions linking the constituent nanoparticles.

Effects of the action of hydrogen peroxide on the electrical properties of polyaniline-aluminium composites

The effects of the action of hydrogen peroxide (H2O2) as an oxidizing agent on the electrical properties of polyaniline-aluminium (PANI-Al) composites are studied. Direct current (dc) electrical conductivity is investigated in the temperature range 303–453 K. A decrease in the conductivity with attack time is observed. The results are compatible with UV–visible and infrared absorption measurements and scanning electron microscopy analysis. The experimental data show a transition from a negative temperature coefficient of resistivity (TCR) to a positive TCR. This transition becomes more pronounced when the attack time increases. Depending on the H2O2 attack time, we found that the dc conductivity of the composites is described by two distinct models: the first is a series combination between the fluctuation-induced tunnelling (FIT) model with a metallic transport model, the second is a combination between Mott's three-dimensional variable range hopping with an intrinsic metallic model. The different FIT and Mott parameters are evaluated. The change in the conduction mechanism is attributed to the disorder and the localization effect caused by the action of H2O2 on the PANI-Al composite.

Mechanical and electrical properties of novel poly(ether ether ketone)/carbon nanotube/inorganic fullerene-like WS 2 hybrid nanocomposites: Experimental measurements and theoretical predictions

The mechanical and electrical properties of poly(ether ether ketone) (PEEK) based hybrid nanocomposites incorporating single-walled carbon nanotubes (SWCNTs) and inorganic fullerene-like tungsten disulfide (IF-WS 2) nanoparticles have been extensively investigated from both experimental and theoretical point of views. Dynamic mechanical studies revealed a remarkable increase in the storage modulus and glass transition temperature of the matrix by the inclusion of both nanofillers. Moreover, tensile and flexural tests indicated significant enhancements in stiffness and strength, attributed to synergistic reinforcement effects combined with strong PEEK–SWCNT interfacial interactions. The Young's modulus of these nanocomposites was fairly well predicted by simple theoretical models such as the rule of mixtures. The hybrids with SWCNT content equal or higher than 0.5 wt% exhibited semiconducting behaviour and the temperature dependence of their electrical conductivity followed a fluctuation-induced tunnelling model. Enhanced overall performance was found for composites prepared by a single-step melt-blending process compared to those manufactured in two stages. The addition of both nanoreinforcements opens up new opportunities for the development of high-performance multifunctional materials suitable for industrial applications.

Epoxy/MWCNT Composite as Temperature Sensor and Electrical Heating Element

An epoxy/carbon nanotubes (CNTs) composite material with a low concentration of multiwalled CNTs (0.5 wt%) has been shown to be applicable in a wide temperature range (up to 160°C) as heating and temperature-sensing element. It can be prepared in any type of geometry allowing a simple application to all kinds of surfaces that have to be sensed and heated. The composite material itself and the electric contacts have demonstrated excellent stability even under extreme ambient conditions. The electrical resistivity of the composite has shown a temperature dependence consistent with the fluctuation-induced tunneling model. This model assumes that the electrical resistance of the nanotube network is dominated by the interconnections between the individual nanotubes rather than by the nanotube resistance itself.

Fluctuation-induced tunneling dominated electrical transport in multi-layered single-walled carbon nanotube films

Low temperature measurements may give some insight into the transport mechanism of single-walled carbon nanotube (SWCNT) films, which could lead to an optimal SWCNT film with designed photoelectric properties. Despite intense research efforts on the low temperature transport in SWCNT films, it is still an open question for the low temperature transport in multi-layered SWCNT films. In this work, the multi-layered SWCNT films were prepared with a layer by layer vacuum filtration. It suggests that the space between different layers of the multi-layered SWCNT can be ignored. For deposition of different-layered SWCNT films using the same total amount of SWCNT suspension, the increase of the layer numbers can reduce the density of the resulting films, which may account for the low temperature transport. The effect of thermal annealing and subsequent nitric acid treatment on the electrical properties of the SWCNT films has also been investigated. At the temperature range of 80–300 K, the transport of the multi-layered SWCNT films can be explained by a fluctuation-induced tunneling model. Our results could build a bridge connecting measured temperature coefficient of resistance and the microscopic tunneling barrier.

Structural, electrical, and magnetic properties of polycrystalline Fe3−xPtxO4 (0 ≤ x ≤ 0.10) films

Polycrystalline Fe3-xPtxO4 films have been prepared by cosputtering at room-temperature. The composition, magnetization and Hall effect measurements indicate that Pt ions have been doped at B-sites. The resistivity is dominated by fluctuation-induced tunneling and decreases with increasing x. The absolute magnetoresistance at room-temperature is above 7% for x≤0.07. The carrier concentration is lower than that of the single-crystal and epitaxial films. A scaling relation of σxy∝σxxn between the Hall and longitudinal conductivities is obtained for all samples, which fits well with the recent developed universal scaling theory. The decreasing trend of the exponent n from 1.72 to 1.57 with increasing x could be qualitatively ascribed to the influence of the Pt2+ ions on the magnetic scattering center concentration of the B-sites Fe2+ ions. These findings make the Fe3-xPtxO4 film a useful candidate for spintronic or extraordinary Hall effect devices applications.

The Luttinger-liquid behavior in single-walled carbon nanotube networks

Changes in the carrier transport properties of individually dispersed single-walled carbon nanotube (SWNT) random networks caused by changes in the network structure through the removal of the surfactant and electrical breakdown of highly conductive paths were investigated. Electrical breakdown of SWNT networks caused the Luttinger-liquid-like transport to emerge from the fluctuation-induced tunneling transport before the breakdown. The power-law exponent of the dependence of conductance on both temperature and bias voltage was increased as the breakdown proceeded. A possible mechanism of the phenomenon is discussed on the basis of the relationship between the exponent and the potential barrier between the SWNTs and metal contacts.

Effects of Frequency, Percolation, and Axisymmetric Microstructure on the Electrical Response of Hot-Pressed Alumina-Silicon Carbide Whisker Composites

The electrical and dielectric properties of hot-pressed composites containing alumina and silicon carbide (SiC) whiskers were characterized over a wide frequency range (0.1 Hz–1.8 GHz). The results were correlated to the average distances between SiC inclusions which were measured by stereology as a function of orientation and composition. Percolation of the whiskers caused a drastic increase in the dc conductivity and the prominence of a dc-conductivity tail associated with a high-frequency Maxwell–Wagner interfacial polarization. In percolated samples, the tail obscured the dielectric loss peak and there was evidence for the fluctuation-induced tunneling mechanism of conduction. In nonpercolated samples, the loss peak was observed and the complex permittivity data were fit with a modified Maxwell–Wagner equation to account for a distribution of relaxation times. The frequency–dispersion magnitudes, fitting exponents, and central relaxation times were orientation-dependent. Also, a damped resonance was observed between 1.4 and 1.7 GHz. The influence of the results on the microwave-heating application is discussed.

Effect of concentration on low-frequency noise of multiwall carbon nanotubes in high-density polyethylene matrix

Transport and noise measurements of multiwall carbon nanotubes in high-density polyethylene matrix are reported. In these composites current transport occurs through a random tunnel junctions network, formed by adjacent carbon nanotubes. Low-frequency noise investigations reveal a 1/f behavior induced by resistance fluctuations. An unusual temperature dependence in samples with different nanotube concentration is found. This can be explained by a transition from a fluctuation-induced tunneling mechanism to a thermally activated regime, occurring at increasing nanotube concentration and resulting in a decrease in the overall noise.

Synthesis and electrical properties of a single walled carbon nanotube–borosilicate glass composite

Single walled carbon nanotube–borosilicate glass composite has been fabricated in a controlled atmosphere furnace by a melt-quench technique. Current–voltage characteristics were analyzed at room temperature and found to be non-ohmic in behavior. The temperature dependence of the current–voltage characteristics was ascertained and it was observed that the electrical conductivity of the composite increased substantially with temperature. Analysis by scanning and transmission electron microscopy clearly demonstrates that the carbon nanotubes are randomly distributed in the form of bundles throughout the glass. The charge conduction mechanism of the composite was found to be well explainable by the fluctuation induced tunneling (FIT) model.

Graphene oxide as dyestuffs for the creation of electrically conductive fabrics

Graphene oxide (GO) was immobilized on the surfaces of acrylic yarns through a conventional dyeing approach. The GO dyed yarns and/or the fabric were immersed in an aqueous sodium hydrosulfite solution at around 363 K for 30 min, which converted the GO into graphene. The graphene created a graphitic-coloured and electrically conductive thin layer over each yarn in the fabric. Data on the electrical conductance of the yarns versus temperature (30–300 K) fit well with the so-called fluctuation-induced tunneling model, which suggests that the graphene layer belongs to a continuously interconnected network. Values of the electrical resistivity ranged from 10 2 to 10 10 Ohm/cm, as verified by the content of graphene in the conductive layer.

Temperature-dependent transport and1/fnoise mechanisms in single-walled carbon nanotube films

The temperature dependence of $1/f$ noise and resistivity in single-walled carbon nanotube (CNT) films are studied. We find that as the temperature decreases, resistivity monotonically increases whereas $1/f$ noise amplitude first decreases, then increases, reaching a minimum at around 40 K. At temperatures considerably smaller than 40 K, the temperature dependence of both resistivity and $1/f$ noise amplitude can be explained by three-dimensional Mott variable-range hopping, which is due to localization effects that result in an insulating behavior in CNT films. At higher temperatures, on the other hand, the dependence of resistivity on temperature can be explained by fluctuation-induced tunneling. In this high-temperature regime, we analyze the temperature dependence of the noise amplitude to extract the density of fluctuators as a function of their energy. Our results show a characteristic peak between 0.3 and 0.6 eV that is responsible for the majority of $1/f$ noise. We also find that, due to its correlation with the number of carriers, the noise amplitude is very sensitive to CNT film device dimensions, especially near the percolation threshold where the resistivity increases. These results not only provide fundamental physical insights about transport and $1/f$ noise mechanisms in CNT films at different temperatures but also help assess the suitability of these films for device applications.

Electrical conductivity of carbon-nanotube/cellulose composite paper

We fabricated multiwalled carbon-nanotube/cellulose composite papers and measured their temperature dependences of electrical conductivity. The dependences were described with the Sheng’s fluctuation-induced tunneling (FIT) model. A possible mechanism of the electrical conduction in the composite paper was discussed in the context of the FIT model.

Conductivity of Poly(pyrrole)‐Poly(styrene sulfonate) Core–Shell Nanoparticles

The dielectric properties of poly(styrene) nanoparticles decorated at their surfaces with poly(styrene sulfonate) [PSS] brushes and subsequently loaded with polypyrrole (PPy) were studied. These film-forming materials which may serve as hole-injection layers in organic light-emitting diodes, exhibit a core-shell-type morphology with a core of electrically insulating poly(styrene) and a shell consisting of a corona of PSS chains which form the matrix in which the electrically conducting complex of PPy and PSS is embedded. This conducting complex exists in form of domains of nanoscale dimensions. Thin compressed pellets of these nanoparticles were studied using mainly impedance spectroscopy. Measurements were carried out in the temperature range between 123 and 453 K and frequency range from 10(-1) to 10(6) Hz. While earlier studies were centered around the effect of polypyrrole volume fraction on the conductivity films and pellets composed of these nanoparticles, the present study reveals in which way the conductivity can be modified by exchange of the mobile inorganic counter ions of PSS. Besides the free-acid form (H(+)), the Li(+)-, Na(+)- and Cs(+)-salts of PSS were investigated. The PPy volume fraction was the same for all PPy/PSS core-shell nanoparticles. The distance for phonon-assisted hopping between next-neighbor polypyrrolium chains is influenced by the presence of these inorganic cations. For all samples containing PPy, a transition from insulating to conducting behavior in the range of 300-350 K was found. Using the fluctuation-induced tunneling model, the average tunneling distance, as well as the potential energy barrier separating neighboring conducting grains was estimated. Finally, a detailed analysis of the dielectric spectra suggests the localization length of the charge carriers to be about 0.33 nm.

Synthesis and electrical conductivity of perchlorate-doped TTF–diamide nanofibers with double and triple helix structures

We synthesized 4,5-bis(octadecylthio)-4′,5′-bis(ethylcarbamoyl)tetrathiafulvalene and measured the electrical conductivity of its perchlorate (ClO4−)-doped nanofibers, which have double and triple helix structures. The nanostructure of the bis(octadecylthio)-TTF–diamide and its ClO4−-doped fibers in a 3 : 1 ratio was deduced in relation to the X-ray crystal structure of bis(methylthio)-TTF–diamide. The doubly coiled nanofibers form when initially formed spiral ribbons of lamellarly arranged TTF–diamide perchlorate split in the middle, and a further split of the double helix produces the triplex structure. Temperature-dependent conductance and current–voltage (I–V) characteristics of the coiled fibers were measured in the temperature (T) range of 70–300 K. The conductance decreased with a decrease in T, and the I–V characteristics were nonlinear over the entire T range. The results were analyzed by using a modified fluctuation-induced tunneling conduction model, where the barrier height and width were linearly dependent on the electric field.

Evaluation of the film formation and the charge transport mechanism of indium tin oxide nanoparticle films

The structure formation and charge transfer of thin nanoparticulate indium tin oxide (ITO) films prepared by dip-coating was studied as a function of stabilizer before and after annealing at different temperatures. The analysis of the film structure by optical methods revealed that it is a function of the stability. Suspensions containing an optimum stabilizer concentration of 0.1 mol/l resulted in densely packed films with a peak specific conductivity of 8.3 S cm − 1 after annealing at 550 °C for 1 h in air and 121 S cm − 1 after annealing in forming gas at 250 °C for 1 h, respectively. Furthermore, for the densely packed films fluctuation-induced tunnelling was found to be the dominant charge transport mechanism, whereas for the low density films a thermally activated charge transport was observed. That the films of maximum density showed a metallic charge transport behaviour at temperatures above 300 K indicated the optimal contact between ITO particles had been achieved.

Electronic transport through crossed conducting polymer nanowires

In order to study the electronic properties of conjugated polymer nanowire junctions, we have fabricated two devices consisting of two crossed poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires with platinum microleads attached to each end of each nanowire. We find that the junction resistance of the crossed nanowires is much larger than the intrinsic resistance of the individual PEDOT nanowire, and increases with decreasing temperature, which can be described by a thermal fluctuation-induced tunneling conduction model. In addition, the crossed junctions show linear current-voltage characteristics at room temperature.