Electric Field Screening Effect Research Articles

Correlation between charge and current gain in LGADs treated with IR lasers

This study investigates the correlation between the gain, defined as the increase of measured charge generated from the absorption of incident radiation, and the gain measured as the increase of sensor leakage current in Low Gain Avalanche Detectors (LGAD) before and after neutron irradiations. LGADs exhibit high signal-to-noise ratios for minimum ionizing particles and will be used in high-energy physics experiments, mainly related to timing applications. Transient Current Technique (TCT) measurements were conducted with LGADs and PIN diodes. The electric field screening effect, caused by free and trapped carriers, is identified as the main reason for the differences measured in gain determined from the increase in leakage current and collected charge. These effects are more significant in irradiated LGAD sensors. The experimental results confirm expectations, demonstrating a growing spread between the two measured gains with fluence. The study enables the prediction of charge gain from the leakage current measurements, which are easier to conduct.

Reducing Screening Effect in Close-Packed Si Field Emitter Arrays

Electric field screening effect in close-packed semiconductor field emitter arrays (FEAs) changes the current distribution among emitter tips, such that those on the edges of the array emit much higher current than those in the middle, leading to poor reliability characteristics. Using analysis, numerical simulations, and experimental verification, three different techniques to suppress electric field screening and achieve uniform current distribution are proposed. These techniques are: 1) utilizing very sharp emitter tips; 2) reducing the anode–cathode distance (ACD) to be in the order of emitter pitch; and 3) using extremely low-doped but long field emitter tips. By adopting these techniques together, a more uniform emission from the array and a high emission current density can be achieved. FEAs with 160 K emitter tips with different ACDs and emitter sharpness were fabricated using electron beam lithography. In an array with sharp emitter tips, reliable emission with a maximum current of 0.7 mA (4.4 nA/tip) and a maximum current density of 1.75 A/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{2}}$</tex-math> </inline-formula> was achieved.

Electric fields in a counterflow nonpremixed flame: measurement and simulation

In electric field modified flames, the electric body force on fluid elements can play a role in modifying the flow field, affecting flame characteristics by this modified flow motion. Numerical studies have developed ion kinetic mechanisms and appropriate transport models for charged species, validating them with a voltage-current trend in 1D premixed flames. Recent experimental approaches have measured the electric field by adopting the Electric Field Induced Second Harmonic generation (EFISH) technique. However, the quantification has turned out very challenging due to the inherent distortion in the EFISH signal, as well as inhomogeneous temperature and concentration fields in the combustion field. Here, we propose measurement and calibration schemes to quantify the EFISH signal in a laminar counterflow nonpremixed flame and present comparison with numerical results using an in-house multi-physics CFD (Computational Fluid Dynamics) code. Overall, the quantified electric fields agreed well with those from numerical simulation, specifically capturing null electric fields near the flame in the sub-saturated regime due to the electric field screening effect. In the saturated regime, notable discrepancy was found in a fuel stream when electrons moved through it: experiment indicated a significant number of negative ions in the fuel stream, whereas numerical results predicted negligible negative ions, due to the implemented ion-mechanism. This suggested that the experimentally obtained electric fields may serve as validation data for modeling studies to improve transport models and ion-mechanism. In-situ measurement of charged species in the presence of external electric fields should be a future work.

Enhancement of electrohydrodynamic force with AC bias voltage in three-electrode dielectric barrier discharge plasma actuators

A novel dielectric barrier discharge (DBD) plasma-actuator module with an exposed electrode and two covered electrodes was developed to enhance electrohydrodynamic force generation based on the concept that it separates the ionization and acceleration processes. The conventional three-electrode configuration of the DBD plasma actuator suffers from unexpected spark discharge between the exposed electrodes, thereby failing to strengthen the electric field intensity for accelerating charged particles or generating a stable ionic wind. In this study, a third electrode was embedded in the dielectric layer to prevent spark discharge. Furthermore, an alternating current (AC) waveform was employed as the bias voltage, which was applied to the third electrode, instead of the direct current (DC) voltage used in a conventional DBD plasma actuator. Induced flow visualization using particle image velocimetry technique revealed that the DC bias voltage forms a weak ionic wind in the proposed DBD plasma actuator owing to the electric field screening effect, and the ionic wind periodically appears when the polarity of the voltage is reversed by applying an AC-bias voltage. The velocity of the ionic wind increases with increasing frequency and the AC bias voltage amplitude. Also, decreasing the distance between the second and third electrodes results in ionic wind enhancement. The results obtained in this study provide insights into the drastic improvement in the performance of DBD plasma actuators with the enhancement of the electric field intensity for charged particle acceleration.

Nanoscale electric field imaging with an ambient scanning quantum sensor microscope

Nitrogen-vacancy (NV) center in diamond is a promising quantum sensor with remarkably versatile sensing capabilities. While scanning NV magnetometry is well-established, NV electrometry has been so far limited to bulk diamonds. Here we demonstrate imaging external alternating (AC) and direct (DC) electric fields with a single NV at the apex of a diamond scanning tip under ambient conditions. A strong electric field screening effect is observed at low frequencies. We quantitatively measure its frequency dependence and overcome this screening by mechanically oscillating the tip for imaging DC fields. Our scanning NV electrometry achieved an AC E-field sensitivity of 26 mV μm−1 Hz−1/2, a DC E-field gradient sensitivity of 2 V μm−2 Hz−1/2, and sub-100 nm resolution limited by the NV-sample distance. Our work represents an important step toward building a scanning-probe-based multimodal quantum sensing platform.

Improving experimental procedures for assessing electrical properties of advanced liquid crystal materials

ABSTRACT Electrical measurements of liquid crystals are a standard part of their material characterisation. Typically, such measurements are carried out using a sandwich-like cell of a single thickness. In this paper, we show that interactions between ions and substrates of a liquid crystal cell result in the dependence of the direct current electrical conductivity of liquid crystal materials on the cell thickness. The obtained experimental results combined with modelling point to the existence of ions of several types and to the competition between ion-releasing and ion-trapping processes in liquid crystal cells. We also propose to use a multi-electrode twin-cell that allows the visualisation of the electric field screening effect in liquid crystals and its mitigation by means of ferroelectric nanoparticles.

High-performance optoelectronic devices based on van der Waals vertical MoS2/MoSe2 heterostructures

Monolayer MoS2 is a direct band gap semiconductor with large exciton binding energy, which is a promising candidate for the application of ultrathin optoelectronic devices. However, the optoelectronic performance of monolayer MoS2 is seriously limited to its growth quality and carrier mobility. In this work, we report the direct vapor growth and the optoelectronic device of vertically-stacked MoS2/MoSe2 heterostructure, and further discuss the mechanism of improved device performance. The optical and high-resolution atomic characterizations demonstrate that the heterostructure interface is of high-quality without atomic alloying. Electrical transport measurements indicate that the heterostructure transistor exhibits a high mobility of 28.5 cm2/(V·s) and a high on/off ratio of 107. The optoelectronic characterizations prove that the heterostructure device presents an enhanced photoresponsivity of 36 A/W and a remarkable detectivity of 4.8 × 1011 Jones, which benefited from the interface induced built-in electric field and carrier dependent Coulomb screening effect. This work demonstrates that the construction of two-dimensional (2D) semiconductor heterostructures plays a significant role in modifying the optoelectronic device properties of 2D materials.

Effect of low-frequency electric field screening on motility of human sperm.

The human body is constantly exposed to an extremely low electromagnetic field (ELF-EMF), in particular at 50Hz, emitted by power lines, domestic distribution lines, electrical appliances, etc. It is assumed that the increase in electromagnetic exposure may cause adverse effects upon human health, as well as raising concerns regarding the impact on human fertility. The aim of this in vitro study was to investigate the influence of ELF-EMF with a frequency of 50 Hz on the motility of human sperm. At the same time, the effectiveness of the dielectric screen constructed by ADR Technology ® in absorbing the emitted radiation was examined. Semen samples of 20 patients were exposed to the influence of an extremely low electromagnetic field. After 5, 15 and 30 min., spermatozoa motility was analysed using a computer-assisted spermatozoa motility analysis system. The following sperm motility parameters were examined: 1) velocity straight linear motility; 2) cross-beat frequency; 3) lateral head displacement; 4) homogeneity of progressive motility velocity. It was found that the ELF-EMF presented a negative effect on the motility of human spermatozoa. A significant decrease in spermatozoa motility speed and a significant increase in lateral head deviation values were observed under the influence of the electromagnetic field. ELF-EMF did not show an effect on either lateral head displacement or homogeneity of progressive motility velocity. A positive effect of the dielectric screen ADR Technology® was found. This effect compensated spermatozoa motility changes induced with ELF-EMF.

Clarifying photoluminescence decay dynamics of self-assembled quantum dots

We studied the temperature-dependent photoluminescence (PL) and time-resolved PL spectra of multilayer CdTe/ZnTe quantum dots (QDs) to understand their carrier dynamics. We demonstrated a method of enhancing the confinement of carriers in CdTe QDs by modulating the number of stacked layers, leading to enhanced acoustic phonons up to 67 μeV and reducing the optical phonon coupling to 20 meV with an average phonon energy of 20 meV. The temperature-dependent decay time could be explained using a simple model of the thermal redistribution of carrier states. Thermal escape from hole states during multiphonon scattering occurred only at high temperatures, whereas blue shifts and enhanced PL intensity were expected to enhance the electron–phonon coupling and confinement-induced mixing among discrete state and continuum states with separation energies of 3.5–7.4 meV. Time-resolved PL measurements probed the electric field screening effect as a function of the strain distribution in QDs and was established to be 2.5 ± 0.2 MV/cm.

Field emission characteristics of a small number of carbon fiber emitters

This paper reports an experiment that studies the emission characteristics of small number of field emitters. The experiment consists of nine carbon fibers in a square configuration. Experimental results show that the emission characteristics depend strongly on the separation between each emitter, providing evidence of the electric field screening effects. Our results indicate that as the separation between the emitters decreases, the emission current for a given voltage also decreases. The authors compare the experimental results to four carbon fiber emitters in a linear and square configurations as well as to two carbon fiber emitters in a paired array. Voltage-current traces show that the turn-on voltage is always larger for the nine carbon fiber emitters as compared to the two and four emitters in linear configurations, and approximately identical to the four emitters in a square configuration. The observations and analysis reported here, based on Fowler-Nordheim field emission theory, suggest the electric field screening effect depends critically on the number of emitters, the separation between them, and their overall geometric configuration.

Understanding selectivity in radio frequency and microwave sorting of porphyry copper ores

Continuous high-throughput microwave treatment followed by infrared thermal imaging (MW-IR) has previously been shown to provide attractive separations for a number of porphyry copper ores, leading to rejection of a large proportion of barren fragments from ore-grade material or concentration of copper values from waste-grade material. However, the efficacy of the sorting process is reduced by the presence of hydrated clays and pyrite. Literature measurements have shown differences in the conductivity of pyrite and copper sulphides such as chalcopyrite at radio frequencies. In this work the potential of using radio frequency (RF) heating to exploit these differences and achieve improved selectivity between copper and iron sulphides, is investigated. For the first time a novel bulk materials handling and presentation method that facilitates even heating of angular ore fragments in parallel plate RF systems is discussed. The fragment-by-fragment thermal response of five ore samples under equivalent pilot MW-IR and RF-IR processing conditions is evaluated, showing that there is an increase in selectivity in the heating of hydrated clay minerals in RF compared to microwave. It is suggested, again for the first time, that selectivity in the microwave processing of ores containing semi-conducting minerals is due predominantly to magnetic absorption (induction heating) caused by eddy currents associated with the magnetic field component of electromagnetic energy. In radio frequency processing, where electric field is the dominant component, heating of semi-conducting minerals is limited by the electric field screening effect. This effect is demonstrated using synthetic fragments. Thermal response profiles of synthetic fragments show that approximately 2.5 times the mass of sulphide minerals to hydrated clay minerals would result in an equal temperature increase for microwave heated fragments in which the microwave-heating minerals are evenly disseminated throughout the matrix. This understanding provides the foundations for development of models incorporating different thermal responses for individual heated phases, alongside other textural and treatment variables, that can be used to predict how close to intrinsic sortability ores will perform in MW-IR and RF-IR without the need for extensive processing trials.

Cationic Water-Soluble Conjugated Polyelectrolytes/Graphene Oxide Nanocomposites as Efficient Green Hole Injection Layers in Organic Light Emitting Diodes

The current research presents using a nanocomposite comprising of a cationic conjugated polyelectrolyte (CPE), poly[(2,5-bis(2-(N,N-diethylammonium bromide)ethoxy)-1,4-phenylene)-alt-1,4-phenylene] or (PPPNEt2·HBr), with graphene oxide (GO) as a new hole injection layer (HIL) for organic light emitting diodes. It is demonstrated that using the designed ionically functionalized water-soluble conjugated polymers instead of polyethylene dioxythiophene:polystyrenesulfonate (PEDOT:PSS) is a promising approach to overcome the strong acidic nature of PEDOT:PSS besides excluding its nonconductive PSS part. As the other aspiration of this work, we introduce a good partner for dissolving and spin-casting of GO as a simple and economic technique to use the hole conductive and electron blocking nature of GO in the hole injection portion of assembled devices. Using this new binary blend showed enhanced charge carrier mobility, good electroluminescence, and J–V characteristics in comparison with conventional devices. Such improvement is interpreted with induced ion space charge of HIL at the interface and resulting electric field screening effect due to ion migration.

Study of the electric field screening effect for field emitter arrays

Understanding the influence of the interaction of emitters on the field emission performance of a field emitter array is of great importance for the design of a multi-emitter device. In this work, a screening factor S is defined to quantify the effect of electric field screening due to the interaction of emitters in an array. A three-dimensional finite integration technique is used to study the electric field screening effect for Spindt-type field emitter arrays which have potential applications in flatscreen displays and high-power vacuum electronics. The dependences of the electric field screening on the tip-to-tip spacing, half-angle of the emitter, height of the emitter and radius of the tip’s curvature are analyzed. The effects of the variations in the size of the emitter’s geometry on the most efficient arrangement to achieve maximum emission efficiency in an array are discussed.

Experimental Study of Electric Field Screening of Four Carbon Fiber Cathodes in a Linear Array Configuration

Electric field screening caused by the proximity of individual field emitters in a field emitter array influences the current that can be obtained. This paper describes the image with four carbon fiber field emitters at a 500 μm separation in a linear configuration to isolate and quantify the effect of electric field screening.

The electric field screening and crossing point shift effects in coated carbon nanotubes

We report here the theoretical explanation of the experimentally observed electric field screening and crossing point shift effects in glycine-isopropanol-coated single-walled carbon nanotubes. The external electric field screening effect was illustrated in the framework of the analytical solution of the Laplace equation and Langevin–Debye theory. In this approach, we utilized the polarizability tensor of the glycine-isopropanol molecular complex calculated ab initio by the Density Functional Theory. The shift of the crossing point of the original p-type semiconductor curve for nanotube with the final metallic line was explained from ab initio results of the electron charge transfer from nanotube to the coating layer. The results of calculations agreed with the experimental data very well and an encouraging relationship was found.

Modeling and Design of High-Efficiency Single-Photon Sources

Solid-state sources capable of emitting single photons on demand are of great interest in quantum information applications. Ideally, such a source should emit exactly one photon into the collection optics per trigger, the emitted photons should be indistinguishable, and the source should be electrically driven. Several design strategies addressing these requirements have been proposed. In the cavity-based source, light emission is controlled using resonant cavity quantum electrodynamics effects, whereas in the waveguide-based source, broadband electric field screening effects are employed to direct the light emission into the optical mode of interest. For all the strategies, accurate modeling and careful optical engineering is required to achieve high performance. In this study, we discuss the models and numerical techniques used to analyze such structures. The physical effects governing the light emission profile and the possibilities of tailoring it as well as the mechanisms governing the coherence are elucidated. The major design strategies pursued to optimize the single-photon source performance and the remaining challenges are reviewed.

Molecular-size-distribution-dependent aggregation of humic substances by Na(I), Ag(I), Ca(II), and Eu(III)

Molecular-size-distribution (MSD)-dependent aggregation of Fluka and International Humic Substance Society humic acids (HAs) in the presence of Na(I), Ca(II), Ag(I), and Eu(III) were studied using high-performance size-exclusion chromatography. Larger molecular size fractions (>8000Da) rich in aliphatic carbons were observed to aggregate in the presence of Na(I) and Ca(II), whereas both large and small molecular size fractions concurrently aggregated in the presence of Ag(I) and Eu(III). In view of the non-specific binding of Na(I) and Ca(II) with HAs, the electric-field screening effect is probably the main cause of HA aggregation by Na(I) and Ca(II), preserving the intrinsic hydrophobicity of the HAs. Reduced surface charges and the hydrophobic heterogeneities of different MSD fractions of HAs by direct binding of Ag(I) and Eu(III) are postulated to induce insignificant dependence of aggregation on the different MSD fractions.

Separation of top and bottom surface conduction in Bi2Te3 thin films

Quantum spin Hall (QSH) systems are insulating in the bulk with gapless edges or surfaces that are topologically protected and immune to nonmagnetic impurities or geometric perturbations. Although the QSH effect has been realized in the HgTe/CdTe system, it has not been accomplished in normal 3D topological insulators. In this work, we demonstrate a separation of two surface conductions (top/bottom) in epitaxially grown Bi2Te3 thin films through gate dependent Shubnikov–de Haas (SdH) oscillations. By sweeping the gate voltage, only the Fermi level of the top surface is tuned while that of the bottom surface remains unchanged due to strong electric field screening effects arising from the high dielectric constant of Bi2Te3. In addition, the bulk conduction can be modulated from n- to p-type with a varying gate bias. Our results on the surface control hence pave a way for the realization of QSH effect in topological insulators which requires a selective control of spin transports on the top/bottom surfaces.

Experimental study of electric field screening by the proximity of two carbon fiber cathodes

This paper describes the first experiments that use only two carbon fiber field emitters with different separations to quantify and isolate the effect of electric field screening. Experiments show that when the separation between the two carbon fiber cathodes decreases, both the effective field enhancement factor, βeff, and the current emission decreases. For a two-emitter geometry, our experiment suggests a height of approximately 1.5 times the separation between the two cathodes as the optimum ratio to optimize the emitted current. The paper shows the analysis of the turn on voltage of the field emitters for different separations. The authors compare experimental data with Fowler–Nordheim field emission theory and particle-in-cell simulation, showing good agreement between experiment, theory, and modeling.

Enhanced field emission from clustered TiO2 nanotube arrays

Field emission properties of clustered titanium dioxide (TiO2) nanotube arrays have been studied and compared with those of dense, highly aligned TiO2 nanotube arrays. It was found that clustered nanotube arrays showed significant increase of field emission current density (sevenfold) and field enhancement factor (fivefold) compared to regular shape, highly aligned forest-like TiO2 nanotube arrays, which can be explained by the reduction of electric field screening effects. Clustered TiO2 nanotubes arrays were achieved by electrochemical oxidation of titanium sheet in electrolyte consisting of diethylene or ethylene glycol solvents and ammonium fluorine, while the well aligned nanotube arrays were grown using glycerol.