Electronic Conduction Mechanism Research Articles

Charge-trapping capability and AC conductivity at different humidities of poly(ethyleneimine)–TiO2–anthocyanin-modified cellulose fibres

Modified cellulose materials are finding increasing application in electronics, because of the need for more environmental-friendly electronic circuits. The papers prepared from poly(ethyleneimine)–TiO2–anthocyanin-modified cellulose fibres are completely environmentally friendly and can be applied to the construction of photocells or photo-/humidity sensors. To better understand the mechanisms of electrical conductivity of the said cellulose composites, the effect of humidity (RH = 9% → 100%) on its dielectric properties is presented in this article. The possibility of trapping the negative and positive charges in the composite under different humidity conditions is also examined. A large number of studies suggest that proton conductivity, stimulated by humidity, is the dominant mechanism of electrical conductivity in cellulose-based materials. The results presented in this paper indicate that the electronic conductivity mechanisms also play a significant role in papers prepared from poly(ethyleneimine)–TiO2–anthocyanin-modified cellulose fibres.

Electronic Conduction Mechanisms in Insulators

The current density–electric field ${(}{J}-\xi {)}$ characteristics of four insulators of dramatically different electrical qualities are assessed in terms of their operative electronic conduction mechanisms. Conduction in the two high-quality insulators is dominated by Ohmic conduction and Fowler–Nordheim tunneling, whereas conduction in the two low-quality insulators involves Ohmic conduction and space-charge limited current (SCLC). Ohmic conduction and SCLC are somewhat puzzling mechanisms for contributing to insulator leakage current since they require the existence of an Ohmic contact at the cathode. Our conventional understanding of an Ohmic contact makes it difficult to ascertain how an Ohmic contact could be formed to a wide bandgap insulator. This Ohmic contact dilemma is resolved by formulating an equivalent circuit appropriate for assessing the ${J}-\xi $ characteristics of an insulator and then recognizing that an insulator Ohmic contact is obtained when the injection-limited current density from the cathode electrode is greater than that of the operative bulk-limited current density, i.e., Ohmic or SCLC for the four insulators under consideration.

Doping Li-rich cathode material Li2MnO3 : Interplay between lattice site preference, electronic structure, and delithiation mechanism

We report a detailed first-principles study of doping in Li$_2$MnO$_3$, in both the dilute doping limit and heavy doping, using hybrid density-functional calculations. We find that Al, Fe, Mo, and Ru impurities are energetically most favorable when incorporated into Li$_2$MnO$_3$ at the Mn site, whereas Mg is most favorable when doped at the Li sites. Ni, on the other hand, can be incorporated at the Li site and/or the Mn site, and the distribution of Ni over the lattice sites can be tuned by tuning the materials preparation conditions. There is a strong interplay between the lattice site preference and charge and spin states of the dopant, the electronic structure of the doped material, and the delithiation mechanism. The calculated electronic structure and voltage profile indicate that, in Ni-, Mo-, or Ru-doped Li$_2$MnO$_3$, oxidation occurs on the electrochemically active transition-metal ion(s) before it does on oxygen during the delithiation process. The role of the dopants is to provide charge-compensation and bulk electronic conduction mechanisms in the initial stages of delithiation, hence enabling the oxidation of the lattice oxygen in the later stages. This work thus illustrates how the oxygen-oxidation mechanism can be used in combination with the conventional mechanism involving transition-metal cations in design of high-capacity battery cathode materials.

Electronic conduction mechanism and defect chemical model of LaNi0.4Fe0.6O3 − δ

In order to elucidate the electronic conduction mechanism and defect chemical model of LaNi0.4Fe0.6O3−δ at high temperatures, electrical conductivity (σ), Seebeck coefficient (S), and oxygen vacancy concentration (δ) of LaNi0.4Fe0.6O3−δ were measured as a function of oxygen partial pressure (p(O2)) and temperature. Relatively large σ and small positive S values are observed, which indicates the contribution of ionic conduction is negligibly small to thermoelectric power and the major electronic carrier is a hole. From the analysis of σ and S, it is expected that the LaNi0.4Fe0.6O3−δ has small polaron hopping conduction mechanism where an electron is localized on the Fe. The slope of δ vs p(O2) shows a minimum value near the stoichiometric composition and the δ increases as p(O2) reduces and temperature increases. The relationship between δ vs. log p(O2) is analyzed by a localized electron model and the behavior of the oxygen nonstoichiometry of LaNi0.4Fe0.6O3−δ can be explained.

Impact of Interconnections, Dynamic Conductivity, Pore Size on the Performance of V2O5 Cathode for Lithium Ion Batteries

It is well known that a nanostructured electrodes have superior performance over their bulk counter parts. It has been pursued by the battery community to explore different nanostructures to discover critical parameters for designing next generation batteries. Here, we describe a systematic study from 1D array electrode to 3D electrode enabled by nanostructured scaffolds and atomic layer deposition. Nanostructured scaffolds have been used as templates for the fabrication of nanostructured materials for wide range of applications. One of the commonly used scaffolding material is porous anodized aluminum oxide films (AAO). AAO are used for their hexagonally ordered nanoporous structures and the ease of controlling the structural parameters such as the pore diameter and the pore length by altering the anodization conditions. Here we use voltage control to fabricate not only the traditional straight porous AAO templates, for 1D nanotube array, but also interconnected porous AAO templates, 3D interconnected nanotube array. From previous work, by deviating from a constant voltage, which results in straight pores, to lower voltage and back to the initial voltage cause the pores to deviate from their straight structure to interconnecting layer and back to their straight structure allowing controllable insertion of the interconnecting layers throughout the film. Using atomic layer deposition, V2O5 was conformally coated along the walls of these high aspect ratio AAO with sputtering of gold current collector at the top. Using AAO as a template which can be tuned systematically, the resulting nanostructured V2O5 electrodes were examined for Li ion batteries as it is one of the well characterized cathode material. V2O5 can reversibly accommodate up to two Li ions. From the previous study, when V2O5 was cycled within the one Li insertion window, the 3D interconnected nanotubular structure showed superior power capabilities over the 1D nanotubular array. This was due to the interconnections allowed more of the materials closer to the top current collector. As faradaic reactions are much more sensitive to the ohmic loss, this close proximity to the current collector allowed more efficient utilization of the overall electrode. With the two Li ion insertion window, an opposite trend was observed, with interconnected electrode performing worse. As the structure between the one and two Li insertion windows were kept constant, we hypothesized two possibilities. One, because we have more material within the interconnected electrodes, it will require more Li ions than the straight electrodes. However the pore opening is the same which could lead to ion starvation in the interconnected electrodes because the diffusion of the Li ions into these interconnected electrodes could not keep up with the demand. Two, V2O5 with the second lithiation causes a dramatic change the electrochemical properties more severely in the interconnected electrode. To test these hypothesis, different pore diameter electrodes were used. With the larger pores, the electrodes overall performed better but the difference between the interconnected and the straight electrodes could not be explained. Literatures show the electronic conductivity of V2O5 varies dramatically depending on the degree of lithiation. Up to one lithiation, the conductivity of the material slightly increases whereas with the second lithiation the conductivity dramatically drops by two orders of magnitude due to the disruption in the electron conduction mechanism. This could explain the performance difference between the straight and the interconnected electrodes. When the current density is low, the electrons have enough time to travel through the electrode regardless of the structure. When the current density is high, now the conductivity matters. In the interconnected electrode, with one Li insertion, the conductivity of the electrode near the current collector increases with the degree of lithiation allowing better usage of the material farther away from the current collector due to lower ohmic loss of the overpotential needed. However, with two Li insertion the conductivity drops near the current collector which prevents the utilization of the material farther away resulting in lower power performance. Comsol simulations are in progress to better understand the overpotentials and ion depletion in these straight and interconnected electrodes. This work is able to show the need to match the properties of the electroactive material with the properties of the nanostructure for high performance electrodes.

Influence of substrate temperature on structural, electrical, and optical properties of transparent conductive hydrogen and vanadium co-doped ZnO films fabricated by radiofrequency magnetron sputtering

In this study, we fabricate hydrogen and vanadium co-doped ZnO (HVZO) films using radio frequency magnetron sputtering from a ceramic target with various substrate temperatures (STs) at a fixed H2 flow rate. The results reveal that the HVZO films exhibit a (002) preferred orientation with the c-axis perpendicular to the substrate, and the morphology changes from small and loose grains to large blocks and then to small grains. A minimum resistivity of 1.15×10−3Ωcm, along with a high average transmittance greater than 80% at a wavelength of 400–1200nm, is obtained at an ST of 150°C. We apply theoretical calculations based on density functional theory to analyze the electronic structure and conduction mechanism. The configuration of HVZO-1, where the optical band gap is widening and the charge carrier predominantly originates from the orbits of V 3d, O 2p, and H 1s, is found to be the most stable.

Impedance spectroscopy study of Bi 0.5 (Na 0.74 K 0.16 Li 0.10 ) 0.5 TiO 3 -Ba(Zr 0.05 Ti 0.95 )O 3 ceramics prepared via combustion technique

Abstract Lead free (1- x )Bi 0.5 (Na 0.74 K 0.16 Li 0.10 ) 0.5 TiO 3 - x Ba(Zr 0.05 Ti 0.95 )O 3 or BNKLT-100 x BZT, 0.025 ≤ x ≤ 0.125, ceramics with single-phase perovskite structure were prepared via a combustion technique. Impedance spectroscopy was conducted over a wide temperature and frequency range in order to investigate the influences of BZT on electrical properties and conduction mechanisms. Analysis using impedance complex plane plots combined with spectroscopic plots of the imaginary component of the impedance and electric modulus revealed an electrically homogeneous microstructure with increasing BZT content. With the increase in BZT content, the resistivity increased from ~10 6 Ω-cm at x = 0.025 to ~10 7 Ω-cm at x = 0.125. The activation energy determined from an Arrhenius-type plot of conductivity was also found to increase from 0.97 eV ( x = 0.025) to 1.62 eV ( x = 0.125), which is close to half of the optical bandgap, suggesting a change in conduction mechanism towards an intrinsic electronic conduction mechanism. IS measurement under different oxygen partial pressure was also performed on selected sample. The possible defect chemistry conditions responsible for the observed electrical properties will also be discussed.

Evidence of spin-glass like ordering and exchange bias effect in antisite-disordered nanometric La1.5Ca0.5CoMnO6 double perovskite

Single-phase polycrystalline La1.5Ca0.5CoMnO6 double perovskite nanoparticles (∼25nm) have been synthesized by chemical sol-gel method. We report here the structural, magnetic and transport properties using X-ray diffraction, dc magnetization, ac susceptibility, exchange bias and dc resistivity measurements. The Rietveld refinement of X-ray diffraction pattern reveals that the La1.5Ca0.5CoMnO6 (LCCMO) system crystallizes in orthorhombic structure with pbnm space group. Mn and Co ions are not completely ordered on the B sites due to the presence of about 30% antisite-disorder in the system. The ordering of Co2+ and Mn4+ gives rise to the ferromagnetism below 145K. A spin glass like ground state has also been observed near 37.6(4)K, arising mainly due to the presence of competing magnetic interactions and antisite-disorder in the LCCMO nanoparticles. The frequency dependence peak shift of the Ac-susceptibility peak in the glassy state follows the critical slowing down model. The observed memory effect in ac susceptibility data reveals the existence of interacting clusters in a competing magnetic interactions state. The presence of noticeable exchange bias effect can be best explained on the basis of uncompensated interface (ferromagnetic/spin-glass) spins of antisite-disordered LCCMO system. This anti-site disordered nanocompound exhibits semiconducting behavior with variable range hopping kind of electronic conduction mechanism in the temperature range of 200–300K. We have also observed large negative magnetoresistance (−30% at 100K and 60kOe) mainly due to the spin-polarized transport across the grain boundaries.

The Role of Field Electron Emission in Polypropylene/Aluminum Nanodielectrics Under High Electric Fields.

Polymer/metallic particle nanocomposites or nanodielectrics can exhibit colossal dielectric constants with a relatively low dissipation factor under low electric fields and thus seem to be promising for high-energy density dielectric capacitors. To study this possibility, this work focused on the dielectric performance and loss mechanisms in polypropylene (PP)/aluminum nanoparticle (nAl NP) composites under high electric fields. Phosphonic acid-terminated poly(ethylene-co-1-butene) was grafted to the Al2O3 surface layer on the nAl NPs in order to achieve reasonable dispersion in the PP matrix. The dielectric breakdown study showed that the breakdown strength decreased to nearly 1/20 that of the neat PP film as the nAl content increased to 25.0 vol %. The leakage current study revealed three electronic conduction mechanisms in the PP/100 nm nAl nanocomposites, namely, ohmic conduction at low fields, hopping conduction at intermediate fields, and Fowler-Nordheim (FN) field electron emission above a critical field, depending on the filler content. Compared to the 100 nm nAl NPs, smaller (e.g., 18 nm) nAl NPs needed a much higher electric field to exhibit FN field electron emission. It was the FN electron tunneling that induced a substantial reduction in breakdown strength for the PP/nAl nanocomposites. Meanwhile, electron-tunneling injected space charges (electrons) from nAl NPs into the PP matrix, and internal electronic conduction led to significant dielectric nonlinearity at high poling fields. Although polymer/metallic NP composites are not suitable for high-field electric applications, they can be good candidates for electrical switches and quantum tunneling composites operated at relatively low electric fields.

Electron Localization and Conduction Mechanism of Icosahedral Quasicrystal Al-Pd-Ru

We have argued the temperature dependence of conductivities of the icosahedral Al-Pd-Ru quasicrystals from the standpoint of the quasicrystal-like system with sp-d hybridization, using the thermal Green’s function technique.

Geobacter sulfurreducens pili support ohmic electronic conduction in aqueous solution.

The bacterium Geobacter sulfurreducens is a model biological catalyst in microbial electrochemical devices. G. sulfurreducens forms electrically conductive, electrode-associated biofilms, but the biological structures mediating electrical conduction from cells to the electrodes are a matter of debate. Bacteria in these communities produce a network of fiber-like Type IV pili, which have been proposed to act either as inherent, protein-based electronic conductors, or as electronically inert scaffolds for cytochromes mediating long-range charge transport. Previous studies have examined pilus conduction mechanisms under vacuum and in dry conditions, but their conduction mechanism under physiologically relevant conditions has yet to be characterized. In this work, we isolate G. sulfurreducens pili, and compare the electronic conduction mechanism of both live biofilms and purified pili networks under dry and aqueous conditions. Solid-state I-V characteristics indicate that electronic transport in films of purified pili is representative of conduction in a fiber percolation network. Electrochemical gating measurements in a bipotentiostat device configuration confirm previous results suggesting redox currents dominate live biofilm conduction. Purified pili films, however, exhibit non-redox electronic conduction under aqueous, buffered conditions, and their conductivity increases with decreasing temperature. These findings show that isolated pili possess inherent, non-redox-mediated conductivity consistent with a metallic-like model of charge carrier transport. The results demonstrate an experimental platform for studying electronic transport in biomaterials and suggest that pili serve as an exemplary model for designing bioelectronic interfaces.

Electric-field-induced insulator to Coulomb glass transition via oxygen-vacancy migration in Ca-dopedBiFeO3

The multiferroic $\mathrm{BiFe}{\mathrm{O}}_{3}$ (BFO) as a charge-transfer-type insulator is an interesting system in which to explore correlated electronic conduction. Here, we substitute divalent Ca ions into the parent BFO and apply an external electric field at elevated temperatures to spatially redistribute spontaneously created oxygen vacancies, thereby generating hole carriers in regions of less dense oxygen-vacancy concentrations. X-ray diffraction and photoemission spectroscopic measurement are employed to quantify a large variation of local oxygen-vacancy concentration, as much as $\ensuremath{\sim}{10}^{21}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$, and explore the consequent evolution of electronic band structure. We find that a nonrigid polaronic band is created by hole doping as a result of a strong electron-lattice coupling. We also show strong evidence for the disorder-driven formation of a Coulomb-glass state through electronic transport measurements on a quantitative level. These spectroscopic and transport results can be combined and understood in the framework of intrinsic spatial inhomogeneity of the polaronic charge density. The finding not only offers a promising platform and methodology for examining the interplay of functional defects and correlated electronic behaviors, but also suggests a unique electronic conduction mechanism applicable to systems with coexistence of strong electron correlation, electron-lattice interaction, and randomness beyond the Coulomb-glass physics in semiconductors.

Thermal Stability Concern of Metal-Insulator-Semiconductor Contact: A Case Study of Ti/TiO2/n-Si Contact

This work discusses the thermal stability of metal-insulator-semiconductor (MIS) contacts. A case study is performed on a typical low-Schottky barrier height ( $q\varphi _{b})$ MIS contact: Ti/TiO2/n-Si. By incorporating different levels of donor concentration in n-Si, we perform a systematic Ti/TiO2/n-Si thermal stability study under different electron conduction mechanisms. We find that both $q\varphi _{b}$ and contact resistivity ( $\rho _{c})$ of the Ti/TiO2/n-Si MIS contacts vary dramatically after mere 300 °C–500 °C 1-min rapid thermal treatments. The variations in $q\varphi _{b}$ and $\rho _{c}$ are related to the thermally driven TiO2 decomposition. This thermal stability study of Ti/TiO2/n-Si reveals a general concern for the MIS contact application: since the MIS contacts on n-type semiconductor generally utilize a reactive low-work function metal and an ultrathin insulator, it is difficult to maintain their interface quality considering the thermal budget in standard manufacturing of integrated circuits. Possible solutions to this MIS thermal stability issue are discussed.

Physiochemical Characterization of tubulin from Arachis hypogaea

The physiochemical properties such as (i) electrical conductivity in solution phase, (ii) response to electrochemical perturbations, (iii) functional group finger print in Fourier Transform InfraRed (FTIR), Ultra Violet–Visible (UV–vis) spectroscopy and (iv) surface morphology employing Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS) were studied for α,β-tubulin dimer (microtubules) extracted from the plant source, Arachis Hypogaea. To understand the difference in basic mechanism of electronic conduction, a comparison of physiochemical properties of microtubules, Graphite Encapsulated Iron Cobalt nanoparticles (GE-FeCo NPs) and Multiwalled Carbon Nanotubes (MWCNTs) dispersed in the buffer solution were also studied. The electrical conductivity of the microtubules suspended in buffer solution is calculated as 0.056S/m±0.005S/m and the calculated value is approximately 37 times higher than the value of 0.0015S/m reported in the literature.

Molecular machines and devices.

The suggestion to start this Thematic Series was first made at the symposium held on September 16–18, 2014 in Potsdam, Germany, under the same title, “Molecular Machines and Devices.” The format of this Beilstein Nanotechnology Symposium is intended to bring a relatively small group of specialists together for a few days to discuss progress and open challenges. The range of topics is chosen such that it is not too wide in order to have enough concrete overlap of interests and backgrounds, but at the same time, wide enough to bring researchers together from fields that are somewhat disjoint. The three main topics in the 2014 Potsdam symposium were: molecular electronics, one-dimensional conductors, and synthetic molecular machines. Molecular electronics is currently being developed mostly at the interface between organic chemistry and nanophysics, leaning strongly towards the fundamental understanding of electron transport at the smallest scale and applications in nanoelectronics. The second topic, one-dimensional conductors, has been studied both theoretically and experimentally in the field of mesoscopic physics. More recently, truly one-dimensional wires, which are assembled from molecular building blocks into conductive oligomeric chains, have started to be investigated. This development provides a natural overlap between these two subfields of research. The third topic, synthetic molecular motors, has seen spectacular development in recent years, mostly in the research field of organic chemistry. In parallel, new theoretical ideas have been put forward by Tchavdar Todorov, Daniel Dundas, Mads Brandbyge, Per Hedegard, and others, which open the way for driving nanoscale motion by a dc current. The Potsdam meeting provided one of the first opportunities for researchers in each of these exciting but somewhat disconnected fields to interact and share ideas. The format worked out beautifully. The number of participants, the setting at the shore of the beautiful Griebnitzsee, the diversity of research themes, the combination of theory and experiment, physics and chemistry, and the quality of the oral contributions all contributed to an atmosphere of enthusiasm and creativity. This can be judged from the fact that discussions continued over lunch and dinner and until late evening between participants with widely different backgrounds. One of the intriguing problems discussed included magnetic effects such as the giant, low field magnetoresistance observed at room temperature in organic nanowires presented by Wilfred van der Wiel [1]. The spin selectivity of transport through a helical organic wire presented by Ron Naaman [2] provided ample food for debate and led to new ideas for experiments and theoretical interpretation. There is also an interesting link between the theme of the symposium to recent observations in biology. Spiros Skourtis introduced the audience to the remarkable discovery of electron transport along micrometer long biomolecular wires, so-called pilli, growing as extracellular appendages from the surface of certain bacteria. The present understanding of the electron-conduction mechanism is described in terms of a hopping mechanism having remarkable properties [3]. These and many more topics are represented by the contributions to this Thematic Series. This series presents the current research developments for many of these scientific problems and challenges. The works illustrate the interesting combination of theoretical questions, dedicated instrumentation development, synthetic chemistry, and experimental observations. Concepts for electrically actuated, molecular scale motion are discussed in many of the contributions, where switches, pumps, or motors are considered, and first experimental evidence is presented. The progress in theory and experiment on single-molecule electron transport can be judged from many contributions in this volume. On the other hand, some subjects are not as well-covered, including electron transport in biomolecules and one-dimensional conductors. This is merely the result of fortuitous fluctuations in having the proper material timely available for this volume, rather than a bias of interest. Indeed, important developments can be anticipated at the interface with biomolecular sciences. In parallel, the first applications of molecular electronics are appearing on the market in the form of molecular layers. The top-contact problem, which has hindered applications for a long time, is now at least partially resolved, so that we may anticipate more applications to follow. These will initially be in the form of layers of single molecules. The path to single-molecule devices is still long, but there is a lot of interesting material to be discovered on the way. Jan van Ruitenbeek Leiden, February 2016

Ambient Pressure XPS Study of Mixed Conducting Perovskite-Type SOFC Cathode and Anode Materials under Well-Defined Electrochemical Polarization.

The oxygen exchange activity of mixed conducting oxide surfaces has been widely investigated, but a detailed understanding of the corresponding reaction mechanisms and the rate-limiting steps is largely still missing. Combined in situ investigation of electrochemically polarized model electrode surfaces under realistic temperature and pressure conditions by near-ambient pressure (NAP) XPS and impedance spectroscopy enables very surface-sensitive chemical analysis and may detect species that are involved in the rate-limiting step. In the present study, acceptor-doped perovskite-type La0.6Sr0.4CoO3-δ (LSC), La0.6Sr0.4FeO3-δ (LSF), and SrTi0.7Fe0.3O3-δ (STF) thin film model electrodes were investigated under well-defined electrochemical polarization as cathodes in oxidizing (O2) and as anodes in reducing (H2/H2O) atmospheres. In oxidizing atmosphere all materials exhibit additional surface species of strontium and oxygen. The polaron-type electronic conduction mechanism of LSF and STF and the metal-like mechanism of LSC are reflected by distinct differences in the valence band spectra. Switching between oxidizing and reducing atmosphere as well as electrochemical polarization cause reversible shifts in the measured binding energy. This can be correlated to a Fermi level shift due to variations in the chemical potential of oxygen. Changes of oxidation states were detected on Fe, which appears as FeIII in oxidizing atmosphere and as mixed FeII/III in H2/H2O. Cathodic polarization in reducing atmosphere leads to the reversible formation of a catalytically active Fe0 phase.

配位プログラミングによる電子・光機能分子システムの創製

In this review, a new research area on highly integrated metal-containing p-conjugated and/or redox-active systems constructed by coordination programming, which is to utilize the advantages of coordination chemistry to control the chemical bonds and arrangements of metal atoms and ions for creation of superstructures, has been reported. The topics involved are 1) interfacial synthesis of p-conjugated redox complex oligomer/polymer wires on electrodes and analysis of their electron conduction mechanism and kinetics, 2) creation of coordination nanosheets (CONASHs), 3) synthesis of p-conjugated redox metal complex oligomers, polymers and clusters and analysis of their electronic properties, 4) synthesis of metal complexes responsive to external-stimuli and analysis of their unique properties and functions, 5) metal nanoparticles coordinated with functional molecules, and 6) development of bio-photosensors using combination of cyanobacterial photosystem I and molecular wire. These research results indicate that coordination programming is a promising approach to construct intelligent hetero-molecular conjugated systems useful for development of chemical devices.

Electronic transport in molybdenum dioxide thin films

Molybdenum dioxide (MoO2) is a mixed ionic electronic conductor with potential applications in energy storage and conversion. There is some ambiguity about the nature of the electronic conduction mechanism and its temperature dependence. Conductivity data as a function of temperature were obtained and explained within the framework of the band structure to support the description of MoO2 as a semi-metal. AC Hall effect measurements found low electron mobilities as expected for d band conduction. Collectively the data supported a combined conduction model including residual conductivity, low temperature hopping and impurity band conduction from Mo 4d bands degenerate with the conduction band.

Semicrystalline Structure-Dielectric Property Relationship and Electrical Conduction in a Biaxially Oriented Poly(vinylidene fluoride) Film under High Electric Fields and High Temperatures.

Poly(vinylidene fluoride) (PVDF)-based homopolymers and copolymers are attractive for a broad range of electroactive applications because of their high dielectric constants. Especially, biaxially oriented PVDF (BOPVDF) films exhibit a DC breakdown strength as high as that for biaxially oriented polypropylene films. In this work, we revealed the molecular origin of the high dielectric constant via study of a commercial BOPVDF film. By determination of the dielectric constant for the amorphous phase in BOPVDF, a high value of ca. 21-22 at 25 °C was obtained, and a three-phase (i.e., lamellar crystal/oriented interphase/amorphous region) semicrystalline model was proposed to explain this result. Meanwhile, electronic conduction mechanisms in BOPVDF under high electric fields and elevated temperatures were investigated by thermally stimulated depolarization current (TSDC) spectroscopy and leakage current studies. Space charge injection from metal electrodes was identified as a major factor for electronic conduction when BOPVDF was poled above 75 °C and 20 MV/m. In addition, when silver or aluminum were used as electrodes, new ions were generated from electrochemical reactions under high fields. Due to the electrochemical reactions between PVDF and the metal electrode, a question is raised for practical electrical applications using PVDF and its copolymers under high-field and high-temperature conditions. A potential method to prevent electrochemical degradation of PVDF is proposed in this study.

Electrical conductivity in Langmuir-Blodgett films of n-alkyl cyanobiphenyls using current sensing atomic force microscope

We report our studies on the nanoscale electrical conductivity in monolayers of n-alkyl cyanobiphenyl materials deposited on solid surface. Initially, the 8CB, 9CB, and 10CB monolayer films were prepared by the Langmuir technique at air-water interface and characterized by surface manometry and Brewster angle microscopy. The monolayer films were transferred on to solid substrates by the Langmuir-Blodgett (L-B) technique. The 8CB, 9CB, and 10CB monolayer L-B films were deposited on freshly cleaved mica and studied by atomic force microscope (AFM), thereby measuring the film thickness as ∼1.5 nm. The electrical conductivity measurements were carried out on 9CB and 10CB monolayer L-B films deposited onto highly ordered pyrolytic graphite using current sensing AFM. The nanoscale current-voltage (I-V) measurements show a non-linear variation. The nature of the curve indicates electron tunneling to be the mechanism for electrical conduction. Furthermore, analysis of the I-V curve reveals a transition in the electron conduction mechanism from direct tunneling to injection tunneling. From the transition voltage, we have estimated the values of barrier height for 9CB and 10CB to be 0.71 eV and 0.37 eV, respectively. For both 9CB and 10CB, the effective mass of electron was calculated to be 0.021 me and 0.065 me, respectively. These parameters are important in the design of molecular electronic devices.