Fluctuation-induced Tunneling Research Articles

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 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.

Probing into the metal-graphene interface by electron transport measurements

Metal-graphene contact recently attracts much attention because of its effects on the performance and the operational speed of graphene field-effect transistor. Simple two-probe graphene devices on mechanically exfoliated graphene flakes are fabricated and the temperature behavior of resistance is measured from room temperature down to liquid helium temperature for the study of electron transport in the interface. Comparing experimental data with several different transport theories, it is confirmed that the model of fluctuation-induced tunneling conduction describes precisely the electron transport and indicates the existence of a thin insulating layer in the metal-graphene interface. Through the interface probing by electron transport measurements, the way to reduce the contact resistance is suggested.

Broad-band conductivity and dielectric spectroscopy of composites of multiwalled carbon nanotubes and poly(ethylene terephthalate) around their low percolation threshold

Composites of multiwalled carbon nanotubes with poly(ethylene terephthalate) (PET–MWCNT) with up to 3 vol% MWCNTs were prepared and characterized by broad-band AC conductivity and dielectric spectroscopy up to the infrared range using several techniques. A very low electrical percolation threshold of 0.07 vol% MWCNTs was revealed from the low-frequency conductivity plateau as well as from DC conductivity, whose values show the same critical power dependence on MWCNT concentration with the exponent t = 4.3. Above the plateau, the AC conductivity increases with frequency up to the THz range, where it becomes overlapped with the absorption of vibrational modes. The temperature dependence down to ∼5 K has shown semiconductor behaviour with a concentration-independent but weakly temperature-dependent small activation energy of ∼3 meV. The behaviour is compatible with the previously suggested fluctuation-induced tunnelling conductivity model through a thin (∼1 nm) polymer contact layer among the adjacent MWCNTs within percolated clusters. At higher frequencies, deviations from the simple universal conductivity behaviour are observed, indicating some distribution of energy barriers for an electron hopping mechanism.

Effects of cross-sectional area on the tunneling-junction array in octahedral PbSe colloidal-nanocrystal solids

Octahedral PbSe colloidal nanocrystals (NCs) are used to assemble a solid. Because of the special feature of the apexes of the octahedrons, the cross-sectional area of the inter-dot tunneling junctions is much smaller than that formed between spherical NCs. The inter-dot separation between NCs is easily adjusted by mild thermal treatment. Like a spherical NC-solid, the resistance of the octahedral NC-solid is exponentially dependent on the inter-dot separation. On the contrary, due to the difference in the cross-sectional area between the NCs, electron transport in the octahedral NC-solid does not follow the same model used for the explanation of electron transport in a spherical NC-solid. Through analyses of current-voltage and resistance-temperature behaviors, we have confirmed that the model of fluctuation-induced tunneling conduction fits very well with all of the data and explains the variation in the electrical properties of octahedral PbSe colloidal NC-solids after thermal annealing.

Nanocrystal shape and nanojunction effects on electron transport in nanocrystal-assembled bulks

Bulk nanostructured materials are made from the assembly of octahedral PbSe nanocrystals. After thermal annealing, the artificial bulk demonstrates a large difference in behavior depending on the temperature, and a large variation of room-temperature resistivity of up to seven orders of magnitude. This variation originates from the high-indexed sharp edges of the octahedral nanocrystals. As the nanocrystals are arranged in the edge-to-edge configuration, which was observed in scanning electron microscopy images, the inter-nanocrystal capacitance is small due to the small parallel area between the nanocrystals. The small capacitance results in a high thermal fluctuation voltage and drives electron transport. The temperature-dependent resistivity and the electric field-dependent current are highly in agreement with the model of fluctuation-induced tunneling conduction. Thermal annealing reduces the inter-nanocrystal separation distance, creating a large variation in the electrical properties. Specifically, octahedral-shaped PbSe nanocrystals are employed in tailoring the electron transport in bulk nanostructured materials.

Subterahertz electromagnetic wave generation in a randomly networked single-walled carbon nanotubes photoconductive switch

We demonstrated an electromagnetic (EM) wave generation that reaches up to 250 GHz in the photoconductive switch based on randomly networked single-walled carbon nanotubes (SWNTs). Furthermore, we investigated the bias dependence of the electromagnetic wave amplitudes. This subterahertz radiation is generated by the acceleration of photogenerated carriers through fluctuation-induced tunneling in single-walled carbon nanotube bundles. Below the bias field of 20 kV/cm, the signal was enhanced with an increase in the bias field. However, the signal amplitudes decreased above 20 kV/cm due to emerging space-charge accumulation and scattering effect occurring at the defects and contact points.

Interfacial effects in the electrical conductivity and viscous deformation of multiwall carbon nanotube–epoxy composites prepared by sonication

Multiwall carbon nanotube composites with epoxy matrix were prepared by sonication followed by long curing. The electrical conductivity of the composite samples was measured and found to follow percolation behavior with low threshold mass fraction of multiwall carbon nanotubes and smaller value of critical exponent. The lower values of threshold and exponent are associated with higher purity and better interfacial contact between the multiwall carbon nanotubes and with the epoxy. The temperature dependence of electrical conductivity showed that both variable range hopping and fluctuation-induced tunneling mechanisms are followed. The dependence of conductivity on the width of the epoxy barrier film between multiwall carbon nanotubes is used to establish that fluctuation-induced tunneling through interfacial regions is the appropriate mechanism of conduction in the composites. The activation energy for viscous deformation of the composites was evaluated using dynamic mechanical analysis and found to increase for higher mass fraction of multiwall carbon nanotubes in the composite. These results indicate that interfacial interaction is significant in the molecular movement in the epoxy.

Thin-Film Transistors with a Graphene Oxide Nanocomposite Channel

Graphene oxide (GO) and graphene oxide-zinc oxide nanocomposites (GO-ZnO) were used as channel materials on SiO(2)/Si to fabricate thin-film transistors (TFT) with an aluminum source and drain. Pure GO-based TFT showed poor field-effect characteristics. However, GO-ZnO-nanocomposite-based TFT showed better field-effect performance because of the anchoring of ZnO nanostructures in the GO matrix, which causes a partial reduction in GO as is found from X-ray photoelectron spectroscopic data. The field-effect mobility of charge carriers at a drain voltage of 1 V was found to be 1.94 cm(2)/(V s). The transport of charge carriers in GO-ZnO was explained by a fluctuation-induced tunneling mechanism.

Transport properties in MoS2 selective morphology system

Transport properties are studied in the MoS2 selective morphology system. Fluctuation-induced tunneling dominates in pressed pellets at temperatures up to about 90 K. At higher temperatures, variable range hopping (VRH) transport mechanism prevails, with diverse dimensionality and temperature range. In Mo6S2I8 nanowires, 1D VRH is shown, in MoS2 plate-like crystals 2D VRH and in MoS2 coaxial nanotubes and MoS2 size-controlled nanotubes 3D VRH is found. Decreasing of dimensionality is accompanied by an increase of the conductivity in MoS2 nanotubes, compared to the plate-like crystals. Transport properties are discussed in relation to the nature and density of defects, giving rise to changes in the electronic structure and influencing the scattering mechanism governing the charge transport.

Features of Transport in Ultrathin Gold Nanowire Structures

The origin of the interface formation appearing due to the realization of contacts to ultrathin gold nanowire devices is revealed. Such interfaces play an important role in transport mechanisms in nanowire structures and can determine the electrical and operating parameters of a nanodevice. Based on experimental results, the specific electrical properties of bundles of ultrathin gold nanowires fabricated by wet chemical synthesis and subsequently assembled and contacted with gold electrodes are reported. It is demonstrated that these properties are strongly affected by the monolayers of organic molecules inevitably present on the surface of the nanowires due to synthetic conditions. In particular, such layers form a potential barrier to tunneling of the electrons from contacts to the nanowires. The electric transport behavior of the investigated nanowire structures in the temperature range from 500 mK to 300 K obeys the model of thermal fluctuation-induced tunneling conduction through the nanowire-metal electrode molecular junction. Application of this model allows calculation of the parameters of the molecular potential barrier. The formation of such a molecular barrier is verified by scanning tunneling microscope (STM) and transmission electron microscope (TEM) measurements performed using a supporting graphene layer. These findings are important for designing novel nanodevices for molecular electronics on the basis of ultrathin nanowires.

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.

Processing and electrical characterization of a unidirectional CFRP composite filled with double walled carbon nanotubes

Carbon nanotubes represent new emergent multifunctional materials that have potential applications for structural and electrically conductive composites. In the current paper we present a suitable technique for the integration of Double Walled Carbon Nanotubes (DWCNTs) in a unidirectional Carbon Fiber Reinforced Polymer (CFRP) with high volume content of carbon fiber. We showed that the electrical conductivity of the laminates versus temperature follows a non-linear variation which can be well described by the Fluctuation-Induced Tunneling Conduction (FITC) model. The parameters of this model for CFRP/ DWCNTs and CFRP without DWCNTs were determined using best fit curves of the experimental data. This study has shown that DWCNTs have strong influence in the conductivity through laminate thickness. However, there are no significant effects on the electrical conductivity measured in the other two principle directions of the composite laminate. Furthermore, it was found that electron conduction mechanism of carbon fibers is dominated by the FITC.

Observation of fluctuation-induced tunneling conduction in micrometer-sized tunnel junctions

Micrometer-sized Al/AlOx/Y tunnel junctions were fabricated by the electron-beam lithography technique. The thin (≈ 1.5–2 nm thickness) insulating AlOx layer was grown on top of the Al base electrode by O2 glow discharge. The zero-bias conductances G(T) and the current-voltage characteristics of the junctions were measured in a wide temperature range 1.5–300 K. In addition to the direct tunneling conduction mechanism observed in low-G junctions, high-G junctions reveal a distinct charge transport process which manifests the thermally fluctuation-induced tunneling conduction (FITC) through short nanoconstrictions. We ascribe the experimental realization of the FITC mechanism to originating from the formations of “hot spots” (incomplete pinholes) in the AlOx layer owing to large junction-barrier interfacial roughness.

Electronic conduction and microstructure in polymer composites filled with carbonaceous particles

Physical and physico-chemical properties of polymer filled with carbon black (CB) particles, namely, the microstructure dependence of these properties, are not only interesting on their own but are particularly important for electronic applications as they can impose limits on the sensitivity of a device. With this purpose, we report on an experimental study of the structural and electrical properties of semi-crystalline ethylene-co-butyl acrylate polymer filled with conductive CB nano-particles. We found that the value of the direct current conductivity exhibits a jump of 12 orders of magnitude over a small change in CB concentration and is due to a percolation-like behavior. To assess the temperature evolution of supercolative samples, we present measurements of the conductivity as function of temperature. Above the glass transition temperature of the polymer, the CB network restricts the motions of the polymer chains. This behavior was ascribed to the change in CB mesostructure in the polymer matrix as probed by scanning electron microscopy and atomic force microscopy as well as to the difference in the thermal expansion between the two phases. In addition to the observed conductivity increase, the effect of adding CB particles in the polymer matrix is to increase the thermal stability as is probed by thermogravimetric analysis tests. The room temperature alternating current conductivity, studied over the frequency range from 100 Hz to 15 MHz, is interpreted as arising mainly from inter-aggregate polarization effects. By considering carefully the CB content of the alternating current conductivity, we found that our experimental data agree well with the Sheng’s model of fluctuation-induced tunnelling of charge carriers over nanometric gaps between adjacent CB aggregates. For studying the filler content dependence of the effective permittivity, several mixing laws and effective medium theories have been used. The observed discrepancies between our experimental data and these theoretical predictions may be occur partly because these analysis contain an inaccurate knowledge of the physicochemical properties of the carbonaceous phase, give a poor description of the interfaces in these complex heterostructures, or both. As part of the present investigation, present results are compared to transport properties of polystyrene-cobutyl acrylate latex and epoxy resin matrices filled with different loadings of multiwalled carbon nanotubes (MWCNT) and over wide temperature and frequency ranges. It is remarkable that the MWCNT’s anisotropy (length-to-diameter ratio close to 100) manifests itself in percolation-like behavior with lower threshold volume fraction and different mesostructure than that evidenced for CB filled samples.

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

AbstractReview: 172 refs.