Thermal Fluctuation-induced Tunneling Research Articles

Sensing mechanisms of nanomodified Portland cement composites

Mortar sensors were fabricated as beams incorporating different amounts of carbon nanofibers (CNFs) synthesized in-situ on cement particles. Changes in electrical resistivity were measured and compared to recorded changes in compressive stress, temperature, and humidity. Sensing mechanisms and corresponding models were developed. The findings of the study indicate that the piezoresistive effect is influenced by the critical concentration of CNFs inside the composite matrix and the tunneling effect. In addition, water absorption and desorption, as well as the amount of chemically bound water played an important role in humidity sensing. Thermal fluctuation-induced tunneling conduction was dominant for the temperature sensitivity.

Thermo-resistive and thermo-piezoresistive sensitivity of carbon nanostructure engineered thermoplastic composites processed via additive manufacturing

We experimentally examine the thermo-resistive and thermo-piezoresistive sensitivity of multiwall carbon nanotube (MWCNT)/polypropylene random copolymer (PPR) nanocomposites processed via fused filament fabrication (FFF) process. The filament feedstocks were fabricated by melt blending of neat PPR with a predetermined amount of MWCNTs (either 4, 6 or 8 wt%) using a twin-screw extruder. Thermo-resistive characteristics of MWCNT/PPR composites were measured under both constrained and unconstrained heating from approximately 30-100 °C. For all MWCNT concentrations considered here, negative temperature coefficients of resistivity (TCR) were observed for both constrained and unconstrained heating, as a consequence of thermal fluctuation-induced tunneling at MWCNT junctions. The highest thermo-resistive sensitivity was measured for the composite with the lowest MWCNT concentration (4 wt%) under unconstrained conditions, reporting a TCR of −12,800 × 10−6/°C, which is higher in magnitude than that of other polymer nanocomposites reported in the literature. Moreover, the MWCNT/PPR composites exhibit strong thermo-piezoresistive response under tensile loading. For 4 wt% MWCNT loading, the gauge factor (measured over 0–20% strain range) of the composite increased from 27.8 to 52.3 when the temperature was raised from 30 °C to 60 °C. Our results further evince higher thermo-piezoresistive sensitivity i.e., a gauge factor as high as 395 at 60 °C. The electron tunneling and hopping, both thermally-assisted and activated by mechanical deformation of the PPR matrix, significantly increase the thermo-piezoresistance with the increase in temperature in this range. The excellent thermo-resistive and thermo-piezoresistive characteristics of MWCNT/PPR composites reported in this study would enable the development of smart nanocomposites for self-sensing both temperature and strain/damage state.

Fabrication of Graphene-Polyimide Nanocomposites with Superior Electrical Conductivity.

We report on the fabrication of a novel class of lightweight materials, polyimide-graphene nanocomposites (0.01-5 vol %), with tunable electrical conductivity. The graphene-polyimide nanocomposites exhibit an ultra-low graphene percolation threshold of 0.03 vol % and maximum dc conductivity of 0.94 S/cm, which we attribute to excellent dispersion, extraordinary electron transport in the well-dispersed graphene, high number density of graphene nanosheets, and the π-π interactions between the aromatic moieties of the polyimide and the carbon rings in graphene. The dc conductivity data are shown to follow the power-law dependence on the graphene volume fraction near the percolation threshold. The ac conductivity of the nanocomposites is accurately represented by the extended pair-approximation model. The exponent s of the approximation is estimated to be 0.45-0.61, indicating anomalous diffusion of charge particles and a fractal structure for the conducting phase, lending support to the percolation model. Low-temperature dc conductivity of the nanocomposites is well-approximated by the thermal fluctuation-induced tunneling. Wide-angle X-ray scattering and transmission electron microscopy were utilized to correlate the morphology with the electrical conductivity. The lack of maxima in X-ray indicates the loss of structural registry and short-range ordering.

Average gap distance between adjacent conductive fillers in polyimide matrix calculated using impedance extrapolated to zero frequency in terms of a thermal fluctuation-induced tunneling effect

Temperature dependence of conductivity of polymer composites was analyzed using the DC component (frequency → 0 Hz) of AC conductivity in terms of thermal fluctuation-induced tunneling through thin barriers to evaluate the average gap distance (D) between adjacent vapor-grown carbon fibers (VGCFs) in a polyimide (PI) matrix. This approach was confirmed to be reasonable in comparison with direct calculation of the D value via the DC measurement. The direct DC measurement provides the average conductivity of the system, whereas the DC component (frequency → 0 Hz) of AC conductivity provides two or three types of phase lag mechanisms, the interface between the composites and electrode, the contact region between adjacent VGCFs and the impedance of VGCFs themselves. The best fit between experimental and theoretical frequency dependence of impedance was realized by the equivalent circuit models, which were classified into several phase lags. The D values calculated by the phase lag due to electron charge on adjacent VGCF surfaces were 1.20 and 1.00 nm for the composites with 3.11 and 6.28 VGCF vol% contents, respectively. The D values are independent of temperature from 25 up to 160 °C, which indicates high thermal resistivity of PI. Equivalent circuit with two units for the 3.11 vol% PI/VGCF composite and three units for the 6.28 vol% PI/VGCF composite was proposed to study the complicated relaxation behavior. The first unit was: interface between bulk and electrode; the second unit was: interface between adjacent VGCFs inserting PI; the third unit was: carrier movement within VGCFs.

Electrical transport properties in Co nanocluster-assembled granular film

A Co nanocluster-assembled granular film with three-dimensional cross-connection paralleled conductive paths was fabricated by using the plasma-gas-condensation method in a vacuum environment. The temperature-dependent longitudinal resistivity and anomalous Hall effect of this new type granular film were systematically studied. The longitudinal resistivity of the Co nanocluster-assembled granular film first decreased and then increased with increasing measuring temperature, revealing a minimum value at certain temperature, Tmin. In a low temperature region (T<Tmin), the barrier between adjacent nanoclusters governed the electrical transport process, and the temperature coefficient of resistance (TCR) showed an insulator-type behavior. The thermal fluctuation-induced tunneling conduction progressively increased with increasing temperature, which led to a decrease in the longitudinal resistivity. In a high temperature region, the TCR showed a metallic-type behavior, which was primarily attributed to the temperature-dependent scattering. Different from the longitudinal resistivity behavior, the saturated anomalous Hall resistivity increased monotonically with increasing measuring temperature. The value of the anomalous Hall coefficient (RS) reached 2.3 × 10−9 (Ω cm)/G at 300 K, which was about three orders of magnitude larger than previously reported in blocky single-crystal Co [E. N. Kondorskii, Sov. Phys. JETP 38, 977 (1974)]. Interestingly, the scaling relation (ρxyA∝ρxxγ) between saturated anomalous Hall resistivity (ρxyA) and longitudinal resistivity (ρxx) was divided into two regions by Tmin. However, after excluding the contribution of tunneling, the scaling relation followed the same rule. The corresponding physical mechanism was also proposed to explain these phenomena.

Transport and field emission properties of buckypapers obtained from aligned carbon nanotubes

We produce 120-µm-thick buckypapers from aligned carbon nanotubes. Transport characteristics evidence ohmic behavior in a wide temperature range nonlinearity appearing in the current–voltage curves only close to 4.2 K. The temperature dependence of the conductance shows that transport is mostly due to thermal fluctuation-induced tunneling, although to explain the whole temperature range from 4.2 to 430 K a further linear contribution is necessary. The field emission properties are measured by means of a nano-controlled metallic tip acting as collector electrode to access local information about buckypaper properties from areas as small as 1 µm2. Emitted current up to 10−5 A and turn-on field of about 140 V/µm are recorded. Long operation, stability and robustness of emitters have been probed by field emission intensity monitoring for more than 12 h at pressure of 10−6 mbar. Finally, no tuning of the emitted current was observed for in-plane applied currents in the buckypaper.

Electrical transport properties of polycrystalline TaN1- films

Tantalum nitride with a face-centered cubic structure (TaN1-) has received much attention due to its high hardness, good wear resistance, chemical inertness, thermodynamic stability, and low temperature coefficients of resistivity. First-principles calculations have indicated that cubic-TaN possesses metallic energy band structure, and the experimental results show that the carrier concentration in TaN1- films are comparable to that of normal metals. However, semiconductor-like temperature behavior of resistivity is often observed in polycrystalline TaN1- film. In the present paper, we systematically study the crystal structures and electrical transport properties of a series of TaN1- thin films, deposited on quartz glass substrates at different temperatures by the rf sputtering method. Both X-ray diffraction patterns and scanning electron microscope images indicate that the films are polycrystalline and have face-centered cubic structure. It is also found that the mean grain sizes of the films gradually increase with increasing depositing temperature. The temperature dependence of resistivity is measured from 350 K down to 2 K. The films with large grain sizes have a superconductor-insulator transition below ～ 5 K, while the films with small grain sizes retain the semiconductor characteristics down to the minimum measuring temperature, 2 K. These phenomena are similar to that observed in superconductor-insulator granular composites. Above 5 K, the temperature coefficients of the resistivities of the films are all negative. In the temperature range between 10 and 30 K, the films show hopping transport properties which are often seen in metal-insulator granular systems, i. e. the logarithm of the resistivity (log ) varies linearly with T-1/2, where T represents the measured temperature. The thermal fluctuation-induced tunneling conductive mechanism dominates the temperature behaviors of resistivities from 70 K up to 350 K. It can be seen that the thermal fluctuation induced tunneling conductive mechanism is also the main conductive mechanism in metal-insulator granular systems in the higher temperature regions. Our results indicate that the electrical transport properties of the polycrystalline TaN1- films are similar to that of metal-insulator granular films with different volume fractions of metal, where the metal possesses superconductivity at low temperatures. Hence the high resistivity and negative temperature coefficient of resistivity of TaN1- polycrystalline film can be reasonably ascribed to the similarity in microstructures between TaN1- polycrystalline film and metal-insulator granular film.

Considerable different frequency dependence of dynamic tensile modulus between self-heating (Joule heat) and external heating for polymer--nickel-coated carbon fiber composites.

Dynamic tensile moduli of polyethylene--nickel-coated carbon fiber (NiCF) composites with 10 and 4 vol % NiCF contents under electrical field were measured by a homemade instrument in the frequency range of 100--0.01 Hz. The drastic descent of the storage modulus of the composite with 10 vol % was verified in lower frequency range with elevating surface temperature (T(s)) by self-heating (Joule heat). The composite was cut when T(s) was beyond 108 °C. On the other hand, the measurement of the composite with 4 vol % beyond 88 °C was impossible, since T(s) did not elevate because of the disruption of current networks. Incidentally, the dynamic tensile moduli by external heating could be measured up to 130 and 115 °C for 10 and 4 vol %, respectively, but the two composites could be elongated beyond the above temperatures. Such different properties were analyzed in terms of crystal dispersions, electrical treeing, and thermal fluctuation-induced tunneling effect.

Joule heat dependence of dynamic tensile modulus of polyimide-vapor grown carbon fiber nanocomposites under applied electric field evaluated in terms of thermal fluctuation-induced tunneling effect

To study mechanical and structural stabilities of polyimide (PI) and carbon filler composites by Joule heat, PI-vapor grown carbon fiber (VGCF) composites were fabricated by in situ polymerization to realize excellent dispersion of VGCFs in PI matrix. Adoption of VGCFs similar to rigid carbon fibers is the strategy to investigate the conductive mechanism theoretically in terms of thermal fluctuation-induced tunneling conduction. Dynamic tensile modulus and X-ray intensity under applied electric field were measured to investigate frequency-temperature dependence of the modulus by Joule heat in relation to the electron transfer mechanisms. Such simultaneous measurements for the mechanical and structural properties under electric field provided important information about the characteristic of polymer-filler composites. As the results, good heat resistance of PI-VGCF composites was clarified to be attributed to very few thermal fluctuation of PI chain arrangement in spite of collision between electrons flowed out from the VGCF gaps and atoms of PI chains.

Grain boundary magnetism and conductivity in polycrystalline Ti1−xMnxN films by reactive sputtering

Polycrystalline Ti1−xMnxN films with different Mn atomic fractions (x) were grown by reactive facing-target sputtering. The preferred lattice orientation changes from (200) to (111) with the increase of x, which makes the surface morphology evolve from spherical to triangular-pyramid-like islands. Weak grain-boundary ferromagnetism with a Curie temperature of higher than 305K has been observed. The saturation magnetization (Ms) and coercivity do not show a monotonous change with x. Asymmetric M–H curves can be observed at low temperatures, and the shift of M–H curves decreases with the increase of temperature because of the relaxation of the low-temperature pinned moments. All of the films show semiconducting-like behavior with a mechanism of thermal fluctuation-induced tunneling. Magnetoresistance (MR) is negative, but is very small. The small Ms and MR can be ascribed to the weakly ferromagnetic interaction of moments at grain boundaries and low spin polarization.

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.

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.

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.

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.

Temperature Dependence of the Electrical Conductivity of Poly(Benzo[1,2-b:4,5-b'] Dithiophene-4,8-Diyl Vinylene) and Poly(Dodecylthiophene)

AbstractWe present results for the electrical conductivity (σ) of thin films of poly(benzo[1,2-b:4,5- b']dithiophene-4,8-diyl vinylene) (PBDV) and poly (dodecylthiophene) (PDDT) as a function of temperature in the range 15-295K. The polymers were doped with FeC13 and PF6 which resulted in electrical conductivities differing by two orders of magnitude at room temperature. We examine three sets of σ(T)-data by using the variable-range hopping (VRH) model that predicts a linear relationship between ln(T1/2σ) and T1/4. We observe a change in the slope of the ln(T1/2σ) vs T14 relationship in all three samples at low temperatures. We also analyze the temperature dependence of the resistivity of PBDV by using the thermal fluctuation-induced tunneling model.