High-conductance State Research Articles

Investigation of an effect of electric field applied to ferroelectric film upon conductivity of quasi-two dimensional electron gas at the Ba0.8Sr0.2TiO3/LaMnO3 interface

We investigate the effect of electric field on the properties of quasi-two-dimensional electron gas at the interface between ferroelectric oxide and insulating oxide in Ba0.8Sr0.2TiO3/LaMnO3 (BSTO/LMO) heterostructure. Interface conductivity measurements were performed by a four-point probe method before and after the effect of electric field on Ba0.8Sr0.2TiO3 ferroelectric film. We have shown that when a non-uniform electric field is applied to a ferroelectric film, we can turn off the state with high conductivity. Whereas under the influence of a uniform electric field on the ferroelectric film, we can turn on the interface state of high conductivity.

Organic ferroelectric field‐effect transistor memories with poly(vinylidene fluoride) gate insulators and conjugated semiconductor channels: a review

AbstractOrganic electronics have attracted considerable interest because of their low temperature solution processability, versatile material synthesis and compatibility with a wide range of traditional fabrication methods such as rod‐coating, inkjet and roll‐to‐roll printing techniques. Organic ferroelectric field‐effect transistors (OFeFETs) with a three‐terminal device architecture have ferroelectric thin films as gate insulator layers and semiconductor films as charge transport channel layers. OFeFETs have been demonstrated with non‐volatile memory functions that can retain their high or low conductive states in the semiconductor channel when the power is turned off. The reversible polarization switching in the ferroelectric gate insulator layer between two stable polarized states provides the basis for binary coded memory functions. In this review, we summarize recent development of OFeFETs based on ferroelectric poly(vinylidene fluoride) dielectrics and organic semiconductor channels. © 2020 Society of Chemical Industry

Redox Control of Charge Transport in Vertical Ferrocene Molecular Tunnel Junctions

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Emergent Elements of Inspiratory Rhythmogenesis: Network Synchronization and Synchrony Propagation

We assessed the mechanism of mammalian breathing rhythmogenesis in the preBötzinger complex (preBötC) invitro, where experimental tests remain inconsistent with hypotheses of canonical rhythmogenic cellular or synaptic mechanisms, i.e., pacemaker neurons or inhibition. Under rhythmic conditions, in each cycle, an inspiratory burst emerges as (presumptive) preBötC rhythmogenic neurons transition from aperiodic uncorrelated population spike activity to become increasingly synchronized during preinspiration (for ∼50-500ms), which can trigger inspiratory bursts that propagate to motoneurons. In nonrhythmic conditions, antagonizing GABAA receptors can initiate this synchronization while inducing a higher conductance state in nonrhythmogenic preBötC output neurons. Our analyses uncover salient features of preBötC network dynamics where inspiratory bursts arise when and only when the preBötC rhythmogenic subpopulation strongly synchronizes to drive output neurons. Furthermore, downstream propagation of preBötC network activity, ultimately to motoneurons, is dependent on the strength of input synchrony onto preBötC output neurons exemplifying synchronous propagation of network activity.

Mechanism of Modulation of AMPA Receptors by TARPγ8

Fast excitatory synaptic transmission in the mammalian central nervous system is mediated by glutamate activated AMPA receptors (AMPARs). The core GluA subunits of AMPARs in neurons co-assemble with auxiliary subunits, including transmembrane AMPAR regulatory proteins (TARPs), several of which modifiy AMPA receptor-channel gating kinetics. In the case of AMPARs containing four copies of TARP γ8, achieved by expression of the tandem GluA2Q∗TARPγ8 subunit developed in the lab, AMPARs exhibit resensitization, or partial recovery from desensitization in continued presence of glutamate. We have recorded single channel currents in cell-attached patch recordings and show that under resensitizing conditions, these AMPARs show transitions primarily from closed to higher conductance levels, similar to activation of homomeric GluA receptors in the presence of cyclothiazide. To study the conformation associated with these states we used single molecule FRET and show that this high conductance state produced by GluA2Q∗TARPγ8 channels exhibits tighter coupling between subunits in the extracellular parts of the receptor.

Selective formation of molecular junctions with high and low conductance states by tuning the velocity of electrode displacement.

A single-molecule junction of 1,4-di(4-pyridyl)benzene (DPB) was prepared in a nano-gap between two Au electrodes using the scanning tunnelling microscopy-based break junction method (STM-BJ). Electric conductance and current versus bias voltage (I-V) measurements during the pulling and pushing processes of DPB single-molecule junctions revealed that high (H) and low (L) conductance states formed in both the pulling and pushing processes. Analysis of the I-V curves based on a single-level model indicated that the difference in conductivity of the H and L states mainly arises from high and low metal-molecule electric coupling in the junction. We demonstrated the controllable formation of H and L conductance states by simply tuning the velocity of electrode displacement in the pushing process. In the pulling process, both H and L states formed regardless of the velocity (v) of electrode displacement, while in the pushing process, H and L states could be selectively fabricated by using low (v = 16 nm s-1) and high (v = 64 nm s-1) velocities of displacement, respectively. This study provides a simple approach to selectively fabricate high and low conductance states by fine tuning of the electrode displacement.

Mathematical modeling of resistive states and dynamic switching of a metal oxide memristor

A mathematical model of resistive states and dynamic switching of a memristor from a low-conductive to a highly-conductive state is presented. The model based on a physical model of charge transport without taking into account the heat transfer process in the metal-oxide-metal structure with the dominant transport mechanism of electron tunneling through oxygen vacancies migrating under the influence of an inhomogeneous self-consistent electric field. An analytical solution for the oxygen vacancies distribution over the oxide layer was found by the approximation of a constant electric field. The memristor model with inhomogeneous electric field is implemented as a specialized program based on the finite difference method for solving a stationary nonlinear first-order differential equation. This model well describes the physical effect of reduction in the conductivity growth of a thin dielectric layer under the dominant transport mechanism of electron tunneling through oxygen vacancies, which arises as a result of an increase in the concentration of trapped electrons with increasing voltage at the electrodes near the threshold switching voltage. Numerical modeling of discrete resistive states and dynamic resistive switching of a memristor has been carried out. The obtained current-voltage characteristic of the memristor with the help of numerical simulation is in better agreement with the experimental data compared to the analytical simulation. The numerical model can be used in the research and development of memristors with given electrical characteristics. A simple analytical memristor model, which does not require a large amount of computation, is applicable for modeling basic processes such as write operation, signal summation, and associative self-learning that occur in super-large memory and logic matrices of a biomorphic neuroprocessor when memristors are used as synapses of neurons.

Hydrothermal Synthesis of Three-Dimensional Perovskite NiMnO3 Oxide and Application in Supercapacitor Electrode

Supercapacitors are attractive as a major energy storage device due to their high coulombic efficiency and semi-permanent life cycle. Transition metal oxides are used as electrode material in supercapacitors due to their high conductivity, capacitance, and multiple oxidation states. Nanopowder transition metal oxides exhibit low specific surface area, ion diffusion, electrical conductivity, and structural stability compared with the three-dimensional (3D) structure. Furthermore, unstable performance during long-term testing can occur via structural transition. Therefore, it is necessary to synthesize a transition metal oxide with a high specific surface area and a stable structure for supercapacitor application. Transition metal oxides with a perovskite structure control structural transition and improve conductivity. In this study, a NiMnO3 perovskite oxide with a high specific surface area and electrochemical properties was obtained via hydrothermal synthesis at low temperature. Hydrothermal synthesis was used to fabricate materials with an aqueous solution under high temperature and pressure. The shape and composition were regulated by controlling the hydrothermal synthesis reaction temperature and time. The synthesis of NiMnO3 was controlled by the reaction time to alter the specific surface area and morphology. The prepared perovskite NiMnO3 oxide with a three-dimensional structure can be used as an active electrode material for supercapacitors and electrochemical catalysts. The prepared NiMnO3 perovskite oxide showed a high specific capacitance of 99.03 F·g−1 and excellent cycle stability with a coulombic efficiency of 77% even after 7000 cycles.

Spontaneous Transitions to High-Conductivity States in Polyvinylchloride Composite Films

Experimental results on anomalous conductivity in specifically synthesized copolymer films are analyzed. Polyacetylene molecular fragments with variable concentration are introduced into polyvinylchloride macromolecules. It is shown that such samples exhibit spontaneous and stimulated conductivity jumps by 13 orders of magnitude and the lifetime of such states may range from several minutes to a day. A qualitative model is proposed to describe the anomalous behavior of the polyvinylchloride composite including effects that provide stabilization of the high-conductivity states, conditions for development of instability in the transitions between the states, and the reasons for long-lived high-conductivity state even in the absence of external voltage. Simple numerical estimations that prove the proposed effects are considered.

A Highly Conductive Layer at an Interface between Ferroelectric BaSrTiO3 and Ferromagnetic LaMnO3

Properties of the Ba0.8Sr0.2TiO3/LaMnO3 heterostructure are investigated for when LaMnO3 and Ba0.8Sr0.2TiO3 are deposited on a surface SrTiO3. No state of high conductivity is observed. The effect a magnetic field has on the temperature behavior of resistivity is determined for samples of the Ba0.8Sr0.2TiO3/LaMnO3 heterostructure in its “normal” architecture when a Ba0.8Sr0.2TiO3 film is deposited on a LaMnO3 single crystal.

High temperature superconductivity at the interface Ba0.8Sr0.2TiO3/La2CuO4

We present the results of the investigations of high conducting area and superconductivity at the interfaces between ferroelectric oxide and insulating oxide in heterostructures, isostructural to BaTiO3/La2CuO4. The performed numerical simulations of BaTiO3/La2CuO4 heterostructure show the possibility of high conductivity state at the interface. The temperature dependence of the measured electrical resistance of the Ba0.8Sr0.2TiO3/La2CuO4 heterostructure interface have been studied and the superconducting behaviour with transition temperature Tc about 30K has been found. Therefore, the transition to the state with 2DEG at the interface is demonstrated. The results offer the possibility to design novel electronic devices.

Mechanism of modulation of AMPA receptors by TARP-γ8.

Fast excitatory synaptic transmission in the mammalian central nervous system is mediated by glutamate-activated α-amino-5-methyl-3-hydroxy-4-isoxazole propionate (AMPA) receptors. In neurons, AMPA receptors coassemble with transmembrane AMPA receptor regulatory proteins (TARPs). Assembly with TARP γ8 alters the biophysical properties of the receptor, producing resensitization currents in the continued presence of glutamate. Using single-channel recordings, we show that under resensitizing conditions, GluA2 AMPA receptors primarily transition to higher conductance levels, similar to activation of the receptors in the presence of cyclothiazide, which stabilizes the open state. To study the conformation associated with these states, we have used single-molecule FRET and show that this high-conductance state exhibits tighter coupling between subunits in the extracellular parts of the receptor. Furthermore, the dwell times for the transition from the tightly coupled state to the decoupled states correlate to longer open durations of the channels, thus correlating conformation and function at the single-molecule level.

Appearance of glassy ferromagnetic behavior in [formula omitted] ([formula omitted]) thin films: A revised phase diagram

We report on results of electrical transport properties of Gd1-xCaxMnO3 (0⩽x⩽1) thin films fabricated on top of SrTiO3 substrates by pulsed laser deposition. The lowest studied doping level (x<0.1) was enough to trigger a transition from the antiferromagnetic state present in the parent compound (x = 0) to a ferrimagnetic state. Up to x = 0.3, the system exhibits a high temperature insulator paramagnetic state and a low temperature insulator ferrimagnetic one. At higher concentrations, the electrical and magnetical properties of the films differ drastically from the bulk form which shows a strong antiferromagnetic behavior. At mid Ca substitution (0.4⩽x⩽0.7), samples show a high temperature charge ordered state with ordering temperatures around room temperature and a low temperature glassy state which was not observed in their bulk counterparts. In contrast, high x samples show a high temperature conductive paramagnetic state and a low temperature insulator antiferromagnetic phase. More interestingly, sample x = 0.9 shows a distinct minimum in the resistivity versus temperature plots and a region of thermal hysteresis which indicate a first order transition. Magnetoresistance measurements display a strong irreversibility which we attributed to phase separation effect of a kinetically arrested ferromagnetic metallic phase and a stable antiferromagnetic insulator phase. Based on the above observations and previous magnetic results, we have reconstructed a phase diagram and characterized the properties of each phase.

Ligand memory effect in purple quantum dot LEDs

We report a ligand-dependent, trap state memory effect in a purple quantum dot light emitting diode. The measured, bistable current-voltage characteristics show a high conductance state and a low conductance state. Thereby, a memory window with an ON state and an OFF state is achieved, which can be reversed by applying a negative bias. The height of the memory window depends strongly on the length of the ligand passivating the quantum dot (QD). Together with a 5 nm thick aluminum film under the hole transport layer, charges accumulate at the charge transport/quantum dot ligand interface, leading to an increase in conductance during the forward voltage sweep. During a reverse voltage sweep, previously injected charge carriers recombine until the active layer is free of charge carriers. Voltage-capacitance measurements confirm the changes in the charge carrier population within the active layer and demonstrate that they depend on both the direction of the bias sweep and ligands surrounding the QD.

Emergent dynamics of neuromorphic nanowire networks

Neuromorphic networks are formed by random self-assembly of silver nanowires. Silver nanowires are coated with a polymer layer after synthesis in which junctions between two nanowires act as resistive switches, often compared with neurosynapses. We analyze the role of single junction switching in the dynamical properties of the neuromorphic network. Network transitions to a high-conductance state under the application of a voltage bias higher than a threshold value. The stability and permanence of this state is studied by shifting the voltage bias in order to activate or deactivate the network. A model of the electrical network with atomic switches reproduces the relation between individual nanowire junctions switching events with current pathway formation or destruction. This relation is further manifested in changes in 1/f power-law scaling of the spectral distribution of current. The current fluctuations involved in this scaling shift are considered to arise from an essential equilibrium between formation, stochastic-mediated breakdown of individual nanowire-nanowire junctions and the onset of different current pathways that optimize power dissipation. This emergent dynamics shown by polymer-coated Ag nanowire networks places this system in the class of optimal transport networks, from which new fundamental parallels with neural dynamics and natural computing problem-solving can be drawn.

Dissociative electron attachment to 3-benzelidenephthalide and phenolphthalein molecules.

Electron attachment to the 3-benzelidenephthalide and phenolphthalein molecules and decay channels of their molecular anions were investigated by means of dissociative electron attachment spectroscopy and electron transmission spectroscopy. Interpretations of these experimental data were supported with UV-spectroscopy and density functional theory calculations. The average electron detachment times for the long-lived molecular anions of 3-benzelidenephthalide and phenolphthalein were measured to be 150 µs at 120 °C and 560 µs at 190 °C, respectively. The long-lived molecular anions of phenolphthalein are ascribed to an isomer formed by ring opening. The present results suggest that, opposite to phenolphthalein, polymeric materials based on 3-benzylidenephthalide cannot be switched to a high conductive state due to different mechanisms of stabilization of their long-lived molecular anions.

Nonlinear Electron Transport and Effects of Resistive Switching in Iron Nanoparticle Metal Oxide/Metal Composites

Voltammetric methods are used to study longitudinal electron transport and the effects of resistive switching in films of nanoparticle metal oxide (Fe/FeOx)/metal composites grown by magnetron sputtering (in an argon atmosphere) and the ensuing iron oxidation. Formation of iron nanograins and intergrain oxide is controlled using the methods of electron microscopy, Raman spectroscopy, and X-ray diffraction. Analysis of voltammetric characteristics of the composite shows the linear ohmic mode of electron transport at low voltages and the nonlinear electron transport mode at high voltages. The resistive switching of the composite from the low conductivity state (LRS) into the high conductivity state (HRS) accompanied by formation of magnetite Fe3O4 regions in the intergrain FeOx oxide characteristic for memristor materials is registered at an increase in voltage. At a further increase in voltage, the reverse process is registered: the switching of the studied film from the high conductivity state in the low conductivity state due to oxidation of intergrain magnetite to maghemite. As the breakdown voltage is registered on the surface of such a composite, a system of oriented elongated ghematite nanoparticles with the average diameter of 45 nm and length of up to 2 μm is formed.

Dielectric relaxation and AC conductivity behaviour of Se80Te15Bi5/PVA nanocomposite film

Abstract An analytical approach has been applied to investigate the dielectric properties and ac conductivity behaviour of a new polymer nanocomposite (PNC) of polyvinyl alcohol (PVA) doped with nanocolloids of Se80Te15Bi5 glass over a frequency range of 100 Hz–5 MHz for temperatures ranging from 298 K to 448 K. An increase in dielectric constant and relaxation time for PNC has been seen as compared to pure PVA. The Cole-Cole model of relaxation shows good agreement with experimental data. The ac conductivity (σac) behaviour has been found to follow the Jonscher's power law Aωs. Prevailing mechanism of conduction has been confirmed to be correlated barrier hopping (CBH) type. The synthesized PNC has been used as an active layer in the fabrication of a memory device. Electrical characterization of the device has been done by current-voltage (I–V) and capacitance-voltage (C–V) measurements. The switching between high conductive (ON) and low conductive (OFF) states of the device may be attributed to charge trapping and de-trapping in Se80Te15Bi5 nanocolloids. The retention characteristic of the device, determined by capacitance-time (C-t) measurements indicates its good reliability and stability.

Towards In-Situ Electron Microscopy Studies of Flash Sintering

Flash sintering, a special case of electric field-assisted sintering, results in accelerated densification at lower temperatures than conventional sintering methods. However, the mechanisms remain elusive despite the wide application potential. In-situ electron microscopy studies reveal shrinkage of ZnO green bodies due to both heating and heating/biasing but show no obvious effect of the current on the behavior. In contrast, thin epitaxial ZnO films deposited on an Al2O3 substrate undergo a clear flash event during in-situ voltage application in the TEM, providing the first observation of flash sintering of a thin film. The specimen was captured in the high conductivity state where grain boundary motion was observed. The microscopic origins of the high conductivity state could not be detected, but may have the same underlying physical origin as the high conductivity memristive state.

Potential-Induced High-Conductance Transport Pathways through Single-Molecule Junctions.

Employing single molecules as electronic circuit building blocks is one promising approach to electronic device miniaturization. We report single-molecule junction formation where the orientation of molecules can be controlled externally by the working electrode potential. The scanning tunneling microscopy break junction (STM-BJ) method is used to bridge tetrafluoroterephthalic acid (TFTPA) and terephthalic acid (TPA) molecules between the Au(111) electrode and the STM tip to measure the single-molecule conductance through the junction. When the Au(111) electrode is at negative potentials (with respect to the zero-charge potential), a highly ordered and flat-oriented superstructure forms, allowing for direct contact between the π system of the benzene ring of the molecules and the Au(111) electrode, leading to junction formation with no anchoring group involvement. Our first-principles nonequilibrium Green's function (NEGF) computation shows a flat configuration yields a conductance that is 3 orders of magnitude larger than for a molecule vertically connected to the electrodes via anchoring groups. Conductances of 0.24 ± 0.04 and 0.22 ± 0.02 G0 are experimentally measured with the flat configurations of TFTPA and TPA, respectively. These values are at least 2 orders of magnitude higher than the experimental values previously reported for the conductance of TPA bridged through carboxylic acid anchoring groups (3.8 × 10-4-3.2 × 10-3 G0). In contrast, a positively charged surface triggers an order-disorder transition eliminating the high-conductance states, most likely because the formation of the flat-oriented junction is prevented. The dependence of TFTPA conductance on the electrode potential (electrode Fermi level) suggests a LUMO mediated transport mechanism. Calculation confirms the lack of an effect of the addition of an electron-withdrawing group are investigated.