Low External Electric Field Research Articles

In-depth theoretical analysis of the influence of an external electric field on charge transport parameters.

It is important to develop materials with environmental stability and long device shelf life for use in organic field-effect transistors (OFETs). The microscopic, molecular-level nature of the organic layer in OFETs is not yet well understood. The stability of geometric and electronic structures and the regulation of the external electric field (EEF) on the charge transport properties of four typical homogeneous organic semiconductors (OSCs) were investigated by density functional theory (DFT). The results showed that under the EEF, the structural changes in single-bond linked oligomers were more sensitive and complex than those of condensed molecules, and there were non-monotonic changes in their reorganization energy (λ) during charge transport under an EEF consisting of decreases and then increases (Series D). The change in λ under an EEF can be preliminarily and qualitatively determined by the change in the frontier molecular orbitals (FMOs) - the number of C-atoms with nonbonding characteristics. For single-bonded molecules, the transfer integral is basically unchanged under a low EEF, but it will greatly change at a high EEF. Because the structure and properties of the molecule will greatly change under different EEFs, the effect of an EEF should be fully considered when determining the intrinsic mobility of OSCs, which could cause a deviation 0.3-20 times in mobility. According to detailed calculations, one heterogeneous oligomer, TH-BTz, was designed. Its λ can be greatly reduced under an EEF, and the change in the energy level of FMOs can be adjusted to different degrees. This study provides a reasonable idea for verification of the experimental mobility value and also provides guidance for the directional design of stable high-mobility OSCs.

Stereo‐Active Lone Pairs Induced Giant Polarization in a 2D Ge‐Based Halide Perovskite Antiferroelectric

AbstractAntiferroelectrics, characterized by electrically controlled antipolar‐polar phase transformation, have attracted tremendous attention as a class of promising electroactive materials for assembling electronic devices. The emerging two‐dimensional (2D) halide perovskites with superior compositional diversity offer an ideal platform for exploring electroactive materials, whereas lead‐free antiferroelectric counterparts are still scarcely reported. Herein, for the first time, a new lead‐free 2D germanium iodide perovskite antiferroelectric (i‐BA)2CsGe2I7 (1, i‐BA is iso‐butylammonium) has been presented, which exhibits a high Curie temperature (Tc) up to 403 K. Remarkably, benefiting from the lone pair stereochemical activity in Ge2+ induced large structural distortion and Cs+ ion off‐center displacement, 1 shows well‐defined double P–E hysteresis loops in a wide temperature range with a giant maximum polarization up to 18.8 µC cm−2, which achieves a new high record among molecular antiferroelectrics. Moreover, under a low external electric field of 22.5 kV cm−1, the antipolar‐polar phase transformation in 1 affords a recoverable energy storage density Wrec of 0.27 J cm−3 and high storage efficiency up to 79.76%. Such lead‐free halide perovskite antiferroelectric with intriguing antiferroelectric behaviors, including high Tc, large polarization and remarkable energy storage properties, is exciting, which provides an alternative candidate for high‐performance antiferroelectrics for environmentally friendly electronic devices.

Stability of moving solitons in trans -polyacetylene in an electric field

In this work we study the dynamics and stability of charged solitons in trans-polyacetylene (tPA), and revisit the issue of the stability of these non-linear excitations under the effect of an external electric field applied parallel to the polymer. Using the formalism of the Su-Schrieffer-Heeger (SSH) model, we solve the coupled dynamical equations for electrons and classical nuclei at the mean-field level and in the regime of low external electric field $E$, where the dynamics of the moving soliton is adiabatic. Analyzing observable quantities in real space and frequency space, we identify the microscopic mechanisms triggering the dynamical instabilities of the soliton. In addition, we put forward the definition of a proper quantitative measure of its stability, an issue which to the best of our knowledge has remained an open question. Besides its intrinsic interest from the fundamental point of view, our work might be relevant for the design of novel organic electronic devices based on soliton-mediated transport.

Conductance quantization control and neuromorphic properties in Pt-nanoparticle incorporated HfAlOx alloy memristor

In this work, platinum nanoparticles have been utilized to achieve better control of conductance quantization for high-density memory storage nonvolatile memory applications. Here, atomic layer deposited Pt-nanoparticles are sandwiched between HfAlOx switching layers. An Au/Ti/HfAlOx/Pt-NP/HfAlOx/ITO resistive random-access memory (RRAM) device exhibits bipolar resistive switching SET/RESET properties at a very low external electric field with memory window >10 and an endurance of >103 cycles. With a very slow voltage sweep rate (0.002 V/step) during current–voltage characteristics under both SET and RESET conditions, a controlled stepwise increase/decrease in distinct conductance quantization behavior with integer and half-integer multiples was observed. This phenomenon predicts atomic contact formation and rupture of oxygen vacancies between conductive filaments and Pt-NPs. Control of post-synaptic conductance properties with modulation of pre-spike width, number, and frequency showed the robustness of the RRAM device studied here. Gradual, controlled change in conductance obtained under dc and pulse conditions in the experiments is very promising for next-generation multi-level high-density storage RRAM devices to develop artificial electric synapses for neuromorphic applications.

Electric-field induced fluctuations in laser generated plasma plume

The effect of an external electric field on laser-generated plasma has been studied. It is observed that the laser-generated plasma can be used for the ignition of a spark in the presence of a low voltage external electric field. An eight-fold emission intensity enhancement in Cu I spectral lines are measured as compared to the signal intensity in the absence of an external electric field. The plasma parameters remain the same initially, up to a few microseconds after the generation of plasma, and this feature makes it more interesting for the quantitative analysis of any sample using laser induced breakdown spectroscopy (LIBS). In the presence of an external electric field, fluctuations (contraction and expansion) in the laser-generated plasma are observed which increase the plasma decay time and consequently result in enhanced signal intensity.

Controlling reaction rate of phase transfer hydrogenation of acetophenone by application of low external electric field

AbstractThe controllable mass transfer and reaction rate for phase transfer hydrogenation of acetophenone across a well‐defined boundary were investigated. The effect of solvent was found important and 1‐butanol exhibited the best performance among the five investigated homologous alcohol solvents, consistent with its higher solubility in water and greater dielectric constant. Initial reaction rates increased with increasing electric potential, consistent with enhanced mass transfer across the aqueous/organic boundary. At longer reaction times, deactivation was apparent. It correlated with increasing voltage and is ascribed to lower equilibrium concentration of reactive species at the interface. External control over reaction rate was demonstrated by switching the applied electric potential over the course of the reaction. Effects of external electric field on enantioselectivity were also explored with reversal field direction. The changes correlate with catalyst decomposition.

Electrostatically enhanced catalytic phase transfer hydrogenation of acetophenone under low external electric field

The oriented external electric field (OEEF) promoted phase transfer catalysis (PTC) reaction is reported for the first time. It was demonstrated experimentally that an OEEF can act as promoter or inhibitor for the phase transfer hydrogenation of acetophenone depending on its direction. Organic solvents showed differing influences on the reaction, attributed to different electrical conductivities. A non-linear dependence of product yield on OEEF potential was also discovered. This work presents a methodology for promoting PTC reactions in the absence of a chemical phase transfer agent and shows promise for significantly improving the performance of organic reactions in biphasic liquid systems.

Reduced graphene oxide biosensor platform for the detection of NT-proBNP biomarker in its clinical range

Reduced graphene oxide (rGO) thin films can be exploited as highly sensitive transducer layers and integrated in interdigital micro-electrode systems for biosensing processes. The distinctive bipolar characterisitics of rGO thin films can be modulated by a very low external electric field due to the electrostatic charges of biomolecules. These charges lead to a fast response in the readout signals of rGO based ion sensitive field-effect transistors (ISFETs). The characterisitc changes of rGO ISFETs enable a fast, accurate and reproducible detection of biomolecules. The biosensing mechanism offers a fast and label-free approach for analyte detection in contrast to the classical ELISA method.In this contribution, we introduce a reproducible fabrication process of rGO based field-effect transistors on wafer level. The sensors are functionalized as biosensors to measure N-terminal pro-brain natriuretic peptide (NT-proBNP) in human serum within its clinical range. Our optimized rGO sensor shows very promising electrical properties and can be considered as a proof of concept study for the detection of various analytes. The easy and cost-effective fabrication as well as the versatile usability make this new technological platform an auspicious tool for different sensing applications in future.

Spectral Behavior of Bias-Dependent Photocurrent and Photoluminescence in Sputtered ZnO Layers

The bias-dependent behavior of the photocurrent (PC) and photoluminescence (PL) of sputtered ZnO layers has been investigated. Based on PC spectroscopy results, the PC intensity of the observed free exciton increased strongly up to electric field of 60 V/cm, after which its rate of increase slightly reduced due to disturbance of field-assisted dissociation of radical ion pairs, which leads to photocarrier generation. Thus, the energy of excitonic PC peaks showed a tendency to red-shift with increasing electric field, being attributed to the induced Stark effect. Therefore, it is concluded that the strong interaction between free excitons and photogenerated PC carriers leads to displacement or widening of the spectrum. In the PL measurements, near-band-edge (NBE) and violet emissions were observed. With increasing electric field, two PL emissions were progressively quenched. The combined PL/PC results reveal that the PL ions associated with the NBE and violet emissions readily interact with the PC carriers of photogenerated electrons and holes. This behavior reduces the recombination ratio and the lifetime of PL ions. So, the PL intensity quenching originates from a decrease in the number of carriers participating in recombination. Consequently, we find that the quenching mechanism of the NBE and violet emissions is strongly related to low external electric field.

Ab initio investigation on electronic properties of the adenine molecule contacted with gold electrodes: effects of an external electric field

A theoretical investigation using density functional theory is performed on the electronic properties of a model molecular wire under an external electric field (EF), such as electronic structure and geometry. The molecular wire is modeled as 4Au/S-Adenine-S/4Au where Adenine is a base of deoxyribonucleic acid molecule acting as a molecular bridge. According to our results the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases with exerting low EF. The application of EF may increase the molecular conjugation and the induced dipole moment. Moreover, the spatial distribution of frontier molecular orbitals moves from a fully delocalized form over whole molecular wire to a partially localized in terminal side which is dependent to EF strength. Accumulated natural bond orbital (NBO) charge on Au, S and NH2 group show that there is a close relationship among these factors and HOMO–LUMO gap. We would also expect an ohmic behavior of the current through the model wire.

Field-Emission Mechanism of Island-Shaped Graphene–BN Nanocomposite

Recent experiments have shown that gap opening and work function modulating in graphenes have been obtained by substituting C atoms with B or N atoms. Especially, some efforts have been set on the fabrication of semiconducting graphene nanocomposites. Here we propose an island-shaped graphene–BN nanocomposite for potential applications as field-emission electron sources. The field emission mechanism of the graphene–BN has been investigated by using density functional theory calculations. Our results show that graphene has a high level of emission currents with a low external electric field. The combination of BN nanocage and graphene makes use of the curvature effect and the nitrogenation that induce local charge enhancement. The presence of an BN nanocage allows for an important enhancement of the field emission properties. The graphene–BN nanofabrication perhaps leads to a unique hybrid platform consisting of graphene and nonmetal, paving the way in future field-emission electron sources.

Modelling the influence of hydrogen bond network on chemical shielding tensors description. GIAO-DFT study of WALP23 transmembrane α-helix as a test case

Density Functional Theory (B3LYP/6-31G(d,p)) calculations of (15)N amide and (13)C carbonyl NMR chemical shielding tensors have been performed on WALP23trans-membrane alpha-helix peptide and compared to solid state NMR experiment performed on [(13)C(1)-Ala(13), (15)N-Leu(14)] specifically labelled peptide powder sample. Using either theoretical results obtained on the whole peptide or experimental data as reference, several simplest chemical models have been explored in order to reduce the computational cost while maintaining good theoretical accuracy. From this study, it appears that the hydrogen bond (N-H...O=C) network that exists in the alpha-helix has a major influence on the chemical shielding tensor and more specifically on the carbonyl (13)C sigma(22) eigenvalue. We show that a small truncated WALP_7 model is not adequate for (13)C(1) NMR description. The application of an external electric field in order to model the hydrogen bond network allows calculating chemical shielding tensors with accurate eigenvalues while the associated eigenvectors are largely modified. Finally, a 23 residues polyglycine peptide that includes the Alanine and Leucine residues for which NMR parameters must be calculated is proposed as the chemical model. This model is sufficient to mostly reproduce the calculation performed on WALP23 with major gain in computational time. Moreover, the application of a low intensity external electric field allows reaching the experimental accuracy for the determination of the eigenvalues.

Synthesis and characterization of novel photoconducting carbazole derivatives in main-chain polymers for photorefractive applications

We have prepared hole-transporting polymers that have carbazole and 3,3′-dicarbazole in the main chain by polycondensation based on Fridel–Craft reaction via cations generated by proton of p-toluene sulfonic acid catalyst. In the device, they display high diffraction efficiency at low external electric field. The DiCz composite had a high diffraction efficiency of 82.4% at 52 V/μm and a photorefractive response time constant of 1.18 s at an electric field of 50 V/μm and a writing intensity of 60 mW/cm 2. The samples have excellent properties with respect to optical quality and shelf-lifetime.

Rapid demulsification of dense oil-in-water emulsion by low external electric field: I. Experimental evidence

Rapid demulsification of dense oil-in-water (O/W) emulsions has been found to be induced by applying a low electric field of the order of 1–10 V/cm. The demulsification simultaneously takes place over the entire space between two electrodes. Dilute O/W emulsions are demulsified only near an electrode after being condensed by electrophoresis of the droplets, which indicates that the mutual contact of oil droplets through thin water layers is necessary for the rapid demulsification. Increase of the concentration of electrolyte accelerates the demulsification, whereas increase of the concentration of an ionic surfactant retards the demulsification. O/W emulsions stabilized with a nonionic polymer surfactant are not demulsified. Water-in-oil (W/O) emulsions are not demulsified even the chemical components are the same as those of the O/W emulsion. Application of an alternating electric field with the frequency higher than 10 kHz does not cause the demulsification. All this evidence strongly suggests that the applied low electric field causes the rearrangement of surface charges on oil droplets, which results in the reduction of the height of an energy barrier for the coalescence of the droplets.

Carbazole-based Photorefractive Organic Glass Showing High Diffraction Efficiency at Low External Electric Field

Photorefractive organic glass, 9-(2-ethyl-hexyl)-3-[2-(4-nitro-phenyl)-vinyl]-9H-carbazole (EHCN), showing large coupling gain and high diffraction efficiency at low external electric field was synthesized and characterized. EHCN showed the glass transition at 25°C and the good thermal stability up to 400°C. Photorefractivity of EHCN was investigated by using 100 µm thick films (S1, S2, S3) doped with 0%, 0.5%, and 1% 2,4,7-trinitrofluorenone (TNF), respectively. Two-beam coupling net gain of 50.8 cm-1 was obtained for the 0.5% TNF-doped EHCN sample (S2) at 40 V/µm field. The highest internal diffraction efficiency, obtained from the degenerate four-wave mixing at 30 V/µm field, reached 90% for the 1% TNF-doped EHCN sample (S3).

Two-color infrared detection using intersubband transitions in multiple step quantum wells with superlattice barriers

A dual-band multiple quantum well infrared photodetector has been fabricated for the near and mid-infrared detection. The structure consists of InGaAs/GaAs step quantum wells separated by AlGaAs/GaAs superlattice barriers. The step quantum wells are designed to have two bound states and a quasi-continuum above the superlattice barriers. The bound-to-bound and bound-to-continuum transitions are employed for the dual-band detection. The superlattice barriers are used to extract the photocurrent from the bound-to-bound transition with a relatively low external electric field, which keeps the dark current low. This work demonstrates the use of a new quantum well structure for the fabrication of multicolor quantum well infrared photodetectors.

Solution of the linearized Boltzmann equation for the two-dimensional electron gas in heterojunctions.

The Boltzmann equation at low external electric field with dominant polar-optical-phonon scattering is accurately solved for temperatures above 77 K. Electron transport properties that include intersubband and intrasubband transitions for the two-dimensional electron gas in ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As-GaAs heterojunctions are obtained numerically.

An Air Filter Made from an Electret and a Conductor

Electrically charged filter fibers, and, in particular, electret fibers, can be used in air filters. Planar electrets have a high internal and a low external electric field; but only external fields contribute to filtration, and so if electrets are used in filters, the fundamental problem is to increase the external field. In a conventional electret filter this problem is solved simply by reducing the width of the electret sheet by making it into fibers, but an alternative solution is to place conductors close to the surface of the electret. A filter with a low resistance to airflow can be produced by this means, but the level of charge that can be sustained on the electret is limited by dielectric breakdown in the air between dielectric and conductor. The total charge on such a filter has been measured microscopically by X-irradiation, and macroscopically with a probe; the two methods give similar results. The electric field inside the filter has been calculated by a self-consistent method. Specimen results for filtration efficiency are given.