Fluctuation-induced Tunneling Model Research Articles

Electrical conduction and photoresponses of gamma-ray-irradiated single-stranded DNA/single-walled carbon nanotube composite systems

Effects of gamma-ray irradiation on the electrical conductivity and photoresponse have been studied for single-stranded DNA (ssDNA)/single-walled carbon nanotube (SWNT) composite films. The temperature-dependent electrical conductivity of the ssDNA/SWNT composite films, well described by a fluctuation-induced tunneling model, indicated modification of the barrier for thermally activated conduction by the gamma-ray irradiation. Besides, the photoresponse measurements indicated modified photoexcited charge carrier generation and oxygen photodesorption in the composite systems due to the gamma-ray irradiation.

Impact of carbon nanotube length on electron transport in aligned carbon nanotube networks

Here, we quantify the electron transport properties of aligned carbon nanotube (CNT) networks as a function of the CNT length, where the electrical conductivities may be tuned by up to 10× with anisotropies exceeding 40%. Testing at elevated temperatures demonstrates that the aligned CNT networks have a negative temperature coefficient of resistance, and application of the fluctuation induced tunneling model leads to an activation energy of ≈14 meV for electron tunneling at the CNT-CNT junctions. Since the tunneling activation energy is shown to be independent of both CNT length and orientation, the variation in electron transport is attributed to the number of CNT-CNT junctions an electron must tunnel through during its percolated path, which is proportional to the morphology of the aligned CNT network.

Temperature dependent current-voltage characteristics of zinc oxide nanowire/polypyrrole nanocomposite

Temperature dependent current-voltage (I-V) measurements of electrochemically prepared zinc oxide nanowire/polypyrrole (ZnONW/PPy) nanocomposite yielded non-linear I-V characteristics at temperatures between 300 and 4.5 K. The low-field conductance (G) of the ZnONW/PPy film exhibits pronounced temperature dependence with room temperature conductance (G300K) ∼ 10−3 S and a conductance ratio (G300K/G4.5K) of ∼104, indicating dominance of significant temperature dependent charge transport processes. The conduction mechanism of the film is satisfactorily understood by extended fluctuation induced tunneling (FIT) model as the non-linear I-V characteristics fit fairly well to the extended FIT model. Further, the temperature dependence of Ĝ obtained from fitting followed Sheng's model also.

Synthesis and magnetotransport studies of CrO2–Al2O3 composite films

CrO2–Al2O3 composite films were deposited on Si(100) substrates by the atmospheric pressure chemical vapor deposition method using CrO3 and AlBr3 as precursors. The CrO2–Al2O3 composite films consist of large CrO2 grains and Al2O3 nanoparticles. The composition of the films was controlled by adjusting the temperature of CrO3 during deposition. The CrO2–Al2O3 composite film with Al2O3 content of 0.38 (molar fraction) shows insulator–metal transition. Below 120K, it is insulator and the temperature dependence of its resistance can be well explained by the fluctuation-induced tunneling model; above 175K, it is metallic. It shows large intergrain tunneling magnetoresistance at low temperature, which reaches −15.2% at 5K and 50kOe. While at high temperature the intergrain tunneling magnetoresistance in the film nearly disappears, and the film shows a small linear magnetoresistance, which should be due to the double-exchange mechanism in CrO2.

Magnetic and magnetotransport properties of half-metallic CrO2-SnO2 composites

Half-metallic (CrO2)1−x-(SnO2)x composites were prepared under high temperature and high pressure conditions. The composites are composed of large rod-like CrO2 grains and small SnO2 nanoparticles. The CrO2 in the composites is very pure and its saturation magnetization is very close to the theoretical value. The composition dependence of magnetic and magnetotransport properties of the composites was studied. The coercive force (Hc) and remanence ratio (Mr/Ms) of the composites increase dramatically with increasing SnO2 content x for x > 0.6. This should be due to that the CrO2 grains have been well separated by SnO2 nanoparticles and the magnetic interactions among CrO2 grains become weak when x > 0.6. The resistivity and magnetoresistance at 5 K of the composites increase with increasing x, and the increase quickens up at x = 0.5. When x ≥ 0.5, the (CrO2)1−x-(SnO2)x composites show insulator behavior, and the temperature dependence of the resistivity can be well described by fluctuation-induced tunneling model. But when x ≤ 0.4, the (CrO2)1−x-(SnO2)x composites show insulator-metal transitions, and the transition temperature increases with increasing SnO2 concentration. Below the transition temperature, their resistivity can also be explained by fluctuation-induced tunneling model. The (CrO2)1−x-(SnO2)x composites show greater magnetoresistance than pure CrO2 at low temperature, which is attributed to enhancement of tunneling magnetoresistance by adding of SnO2.

Near infrared fluorescence and enhanced electrical conductivity of single walled carbon nanotube-lead silicate glass composite

Enhanced electrical conductivity of SWCNT-Lead silicate glass composite was observed in the temperature range of 308K–473K. Current–Voltage characteristics of the composite was found to be ohmic in nature, wherein the electrical conductivity was observed to increase significantly with the increase in temperature. Optical properties of the composite through UV-VIS-NIR absorption and NIR fluorescence spectroscopy were ascertained wherein one can observe broad fluorescence between 861 and 1904nm at room temperature. Micro structural analysis by HRTEM and FESEM shows that single walled carbon nanotubes were randomly distributed inside the glass matrix in the form of bundles. Finally the electrical and optical properties of the composite were explained by fluctuation induced tunneling model, exciton–exciton resonance and excitonic energy transfer.

Effect of PANI rate percentage on morphology, structure and charge transport mechanism in PANI–PVDF composites above percolation threshold

Polyaniline–Poly(vinylidene) fluoride (PANI–PVDF) composites were prepared by adding PANI to the PVDF by different weight percentages p % (p = 0, 5, 10, 20, … until 100%). The dc and ac electrical conductivity were studied as a function of PANI percentage in the temperature range 303–453 K. The percolation threshold was found to be equal to 2.95%. When the amount of PANI varies from 5 to 30%, the charge transport mechanism was found to be governed by Mott's three-dimensional variable range hopping model and the dc conductivity decreases within this range. For p > 30%, the conductivity increases and the charge transport mechanism are better fitted by a fluctuation induced tunnelling model (FIT). By calculating the distance ‘s’ between two successive clusters (the distance between two active imines centres (=N+H–) of PANI) from the FIT model, we deduce that electron charge transfer is done by inter-chain hopping for the range [p = 40 to 60%] and by intra-chain hopping for p = 70 to 90%. Some insights about the contribution of the ionic charge transport for PANI concentrations in the interval 5% < p < 30% were obtained using impedance measurements at different frequencies. X-ray diffraction measurements, Fourier transform infrared spectroscopy and scanning electron microscopy were used to investigate the effect of PANI on the structure and morphology of composites.

Charge transport and photoresponses in a single-stranded DNA/single-walled carbon nanotube composite film

Electrical conductivity and photoresponse measurements have been carried out on a single-stranded DNA (ssDNA)/single-walled carbon nanotube (SWNT) composite film in comparison to those of a SWNT film. While the temperature-dependent electrical conductivity of the pristine SWNT film was described well by the combined mechanism of a three-dimensional variable-range hopping and hopping conduction, that of the ssDNA/SWNT composite film followed a fluctuation-induced tunneling model. Besides, competition of photoexcited charge carrier generation and oxygen adsorption/photodesorption in the photoresponses of the films was observed and discussed in view of the role of the DNA wrapping. Thus, the biopolymer coating of the SWNTs is shown to play a significant role in modifying the charge dynamics of the composite system.

Comparison of Current-Voltage Characteristics of Bulk Polyaniline and Nano Dimensional Polyaniline Particles in Nanoporous Dielectric Matrix of MIL-101

We investigated current-voltage (I-V) characteristics of bulk polyaniline and aniline polymerized inside nanopores of chromium terephthalate dielectric matrix MIL-101. The temperature dependence of electrical conductivity σ (T) of these materials are described by the fluctuation-induced tunneling model (FIT), which means that the main contribution to a net conductivity is caused by contacts between particles of the polyaniline. The comparison of I-V for these two types of materials shown that I-V characteristics of bulk polyaniline are described by the quasi-1D VRH model while for aniline polymerized inside nanopores of chromium terephthalate dielectric matrix MIL-101 by extended FIT model.

Electrical conductivity of single-wall carbon nanotube films in strong electric field

Carrier transport features in single-wall carbon nanotube (SWCNT) films under strong electric fields (up to 105 V/cm) are presented. Application of electrical pulses of nanosecond duration allowed to minimize Joule heating and resolve intrinsic nonlinearities with the electric field. Investigations within a wide range of temperatures—4.2–300 K—indicated that carrier localization as well as tunneling through the insulating barriers between conducting regions takes place in SWCNT films. Crossover from semiconducting behavior to metallic behavior in strong electric field is described using the fluctuation induced tunneling model and assuming that the conducting regions demonstrate characteristic metallic conductivity.

Electrical Conductivity of Nanodimensional Polyaniline Particles in Nanoporous Dielectric Matrix of MIL-101

We investigated an electrical conductivity of aniline polymerized inside nanopores of chromium terephthalate dielectric matrix MIL-101. We found that temperature dependences of conductivity (T) are described by the fluctuation-induced tunneling model, which means that the main contribution to a net conductivity is caused by contacts between particles of the polyaniline. We also found that the nanoporous matrix affects the composite conductivity via the size reduction of the conducting polyaniline phase encapsulated in the pores. Dependence of conductivity from a current was thoroughly investigated. This dependence is resulted from a reduction of energy gap for current carriers which is, in turn, induced by an increasing electric field applied.

Electrical Properties and Scaling Behavior of MWCNT–Soda Lime Silica Glass

Multiwall carbon nanotube (MWCNT)–soda lime silica glass composites were prepared by the direct mixing method. The dielectric properties of the composites were studied to explore the effect of MWCNT content on the conduction and relaxation mechanisms in such composites. A gradual increase in the direct-current (dc) conductivity σ dc was observed up to 7 wt.% MWCNT, with a sharp increase in σ dc for the 10 wt.% sample. Such behavior was related to the increase of internanotube connections. The correlation between σ dc and the nanotube loading p followed the fluctuation-induced tunneling (FIT) model, which can be described by the equation, lnσ dc ∝ p −1/3. The alternating-current (ac) conductivity exhibited two distinct regimes: (i) a low-frequency plateau and (ii) a high-frequency dispersion regime. The switchover frequency between the two regimes indicated the conductivity relaxation. The onset frequency shifted to higher frequencies with increasing MWCNT content, which was related to connectivity improvement. Investigating the universality of the ac conductivity of these composites, it was found that the data obtained followed a Rolling scaling model. The obtained master curve revealed that the conductivity relaxation can be considered a temperature-independent process. The frequency dependence of the ac conductivity dielectric constant followed the intercluster polarization model.

Synthesis and Magnetotransport Properties of CrO2-TiO2 Composites

CrO2-TiO2 composites are synthesized by using a high temperature and high pressure method using CrO3 and H2TiO3 as precursors. The composites consist of large rod-like CrO2 crystals separated by small TiO2 grains. The CrO2 in the composites is very pure and its saturation magnetization is very close to the theoretical value (i.e., 2μB per formula unit). The composites exhibit a large negative magnetoresistance (MR) at 5K. The MR in CrO2-TiO2 composites is mainly attributed to spin-polarized tunneling between CrO2 crystals. The conductivity of the composites is best described by a fluctuation-induced tunneling model below 230K.

Enhanced magnetoresistance in half-metallic CrO2–TiO2 composites

CrO2–TiO2 composites were synthesized by a high temperature and high pressure method (HTHP). The CrO2–TiO2 composites are composed of large rod-like CrO2 crystals separated by TiO2 nanoparticles. The saturation magnetization of the CrO2 in the composites is very close to the theoretical value. The CrO2–TiO2 composites show greatly enhanced magnetoresistance than that of pure CrO2. This is mainly attributed to spin-dependent tunneling between adjacent CrO2 grains enhanced by the addition of TiO2. The tunneling mechanism in the composites can be best described by the fluctuation-induced tunneling model as convinced by the temperature dependence of the conductivity of the CrO2–TiO2 composites at low temperature.

Design and synthesis of the polyaniline interface for polyamide 66/multi-walled carbon nanotube electrically conductive composites

A polyaniline interface had been designed and built between multi-walled carbon nanotubes (MWCNTs) and polyamide 66 (PA66) in order to help in the dispersion of MWCNTs in PA66 and improve the interfacial combination between them. Transmission electron microscopy characterizations indicated that functionalized MWCNTs (f-MWCNTs) could be well-distributed in PA66 matrix and the interfacial boundary between them was indiscernible. The mixing conditions, such as f-MWCNT content, temperature, and mixing speed, played important roles in determining the formation of the conductive network and the electrical conductivities of PA66/f-MWCNT composites synthesized. A continuous conductive network was formed at 10 wt% f-MWCNT content, and the corresponding PA66/f-MWCNT composite exhibited an electrical conductivity of 8 orders of magnitude higher than pure PA66. The conducting mechanism agreed well with a thermal fluctuation-induced tunneling model.

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

AbstractReview: 172 refs.

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.

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.