Formation Of Positive Charge Research Articles

Large-volume CdZnTe bar detectors characterized by laser-induced transient currents

In this work, we studied the free-carrier transport properties and space-charge formation/polarization in a CdZnTe bar-shaped radiation detector using the Laser-Induced Transient Current technique. We found out that a steady-state space charge ranging from 8 × 108 to 2.1 × 109 cm−3 is formed throughout the detector at about 0.5 s following biasing. The measured current waveforms were modeled by Monte Carlo simulations after taking into account the electric and weighting fields inside the detector and shielding box. Modeling of the unpolarized waveforms revealed an exceptionally high electron mobility-lifetime product μeτe≥0.095cm2V−1. The observed formation of positive space charge in the biased detector and linear scaling of the current waveform shapes on applied bias revealed that the detector polarization is attributed to carrier injection at the anode. Measurements in pulsed and DC bias in modified electrode geometry proved the surface-charge formation.

Computationally design aspects of superbasic amidine-arsinine 1-oxide binary frameworks

Novel Brønsted superbases (1–10) were designed on the basis of arsinine 1-oxide core by changing the species of proton absorption site, type and situation of electron-donating substituents in the gas phase and CH3CN medium. The conjugate acids of the designed structures are stabilized through positive charge distribution and/or formation of an intramolecular hydrogen bond. To explore the various structural aspects of the designed molecules for reaching the higher basicity values in gas phase and solution phase in CH3CN solvent, proton affinity (PA), gas phase basicity (GB), pKa, and Nucleus-IndependentChemicalShifts 1 Å above the arsinine 1-oxide ring center along with zz axis (NICS(1)zz) parameters as well as natural bond orbital (NBO) charges calculated and obtained for each structure. The basicity strength of proposed structures was evaluated using B3LYP/6–311 + G(d,p) level of theory. The protonation of all designed structures was studied from both possible sites in gas and CH3CN phase and PA, GB, pKa, and NICS(1)zz values were calculated for each case. It was found that most designed compounds demonstrate superbasicity characteristics and especially amidine-arsinine 1-oxide core structure including two phosphazene electron donating groups (10) shows highest superbasicity properties. The most important finding of the study was the considerable synergistic effect between amidine and arsinine 1-oxide moieties for creating remarkable basicity strength. Aromaticity alterations of the arsinine 1-oxide ring also was monitored during protonation process.

Temporally programmed polymer – solvent interactions using a chemical reaction network

Out of equilibrium operation of chemical reaction networks (CRNs) enables artificial materials to autonomously respond to their environment by activation and deactivation of intermolecular interactions. Generally, their activation can be driven by various chemical conversions, yet their deactivation to non-interacting building blocks remains largely limited to hydrolysis and internal pH change. To achieve control over deactivation, we present a new, modular CRN that enables reversible formation of positive charges on a tertiary amine substrate, which are removed using nucleophilic signals that control the deactivation kinetics. The modular nature of the CRN enables incorporation in diverse polymer materials, leading to a temporally programmed transition from collapsed and hydrophobic to solvated, hydrophilic polymer chains by controlling polymer-solvent interactions. Depending on the layout of the CRN, we can create stimuli-responsive or autonomously responding materials. This concept will not only offer new opportunities in molecular cargo delivery but also pave the way for next-generation interactive materials.

Performance and mechanism on hydrogen evolution of two-dimensional boron nitride under mechanical vibration

Hexagonal boron nitride (h-BN) is a graphite-like two-dimensional material with a piezoelectric nanostructure and thus is extensively applied. In this study, piezoelectric catalytic hydrogen production by h-BN was realized at the H2 yield of 824.281 μmol/g/h. After 5 repeated experiments, the H2 yield did not drop significantly, indicating 2D h-BN has high piezoelectric catalytic ability and stability. Since h-BN can directly convert mechanical energy into chemical energy, deformation after machinery vibration will induce the formation of surface positive charge q+ and negative charge q-. The negative charge concentrates on the conduction band and binds with hydrogen ion to form hydrogen gas that can escape. It displays the huge potential of two-dimensional piezoelectric materials in relieving energy pressure.

Modification of Cellulose Acetate Membranes by Unipolar Corona Discharge for Separation of Oil-Water Emulsion

The separation of water-oil emulsion with cellulose membranes modified with a unipolar corona discharge at voltage of 5–25 kV and time of 1–5 min was investigated. The results of the X-ray diffraction analysis and of the electrostatic field parameters measurements showed a decrease in crystallinity from 0.29 to 0.27 and the formation of positive charges on the surface of the sample, while the formation of a double electric layer according to dielectric spectrometry data was not detected. During the separation of the model water-oil emulsions, an increase in efficiency was revealed as 80% to 98% and the separation productivity from 15 dm³/(m²·h) to 35 dm³/(m²·h) after treatment in the field of a unipolar corona discharge of cellulose acetate membranes, which is explained by a change in the supramolecular and chemical structure of the latter.

High-efficiency InGaN blue LEDs with reduced positive sheet polarization.

The formation of positive sheet polarization charges at the interface of the last quantum barrier (QB) and the conventional p-type electron-blocking layer (EBL) creates significant band bending, leading to severe electron leakage and poor hole injection in III-nitride light-emitting diodes. We report that the positive sheet polarization charges are mitigated by employing a lattice matched AlGaN last QB. Electron leakage is dramatically reduced due to the increased effective conduction band height at the last QB and EBL. Furthermore, it favors hole injection into the active region due to the reduced effective valance band height for EBL.

Space charge formation in the high purity semi-insulating bulk 4H–silicon carbide

We investigated the carrier transport and space charge formation in the high purity semi-insulating 4H-SiC bulk single crystal. Using the Laser–induced Transient Current Technique we observed anomalous short current waveform oscillations that are caused by very short carrier lifetime ≤ 1 ns. We conclude from a detailed inspection of current waveform shape completed by the analysis of collected charge that the electron mobility decreases in an increasing electric field. The saturation value of electron drift velocity 8.7 × 106 cm2s-1 was evaluated. The Hall effect measurement revealed the n-type electrical conductivity and dominant donor energy of ED = 0.85 eV. We also observed the blocking character of prepared Au contacts, which induce the electron depletion and positive space charge formation in biased sample. The continuous increase of the space charge in a wide time interval of 10-1 s – 104 s, attested to testified on a presence of a dispersed impurity band localized below the Fermi energy. Based on these findings, we proposed a theoretical model describing the observed phenomena very well.

DFT+U study of the electronic structure changes of WO3 monoclinic and hexagonal surfaces upon Cu, Ag, and Au adsorption. Applications for CO adsorption

A periodic DFT+U study about the adsorption of Cu, Ag, or Au on the monoclinic and hexagonal (001) surface of WO3 is presented. The scope is to elucidate the changes in the electronic structure of these materials upon adsorption of Cu, Ag and Au, and to provide some insights about the adsorption of CO. The adsorbed Cu, Ag, and Au atoms provokes the formation of positive charge on the surfaces by transferring electrons to the oxide. The charge is located at the bottom of the conduction band, which causes the Fermi level to move into the conduction band and the materials become semimetals. The CO molecule is weakly adsorbed on the clean surfaces. The adsorption provokes a compression of the surface's states at the conduction band due to the appearance of new states belonging to the CO molecule. This result is a slight stabilization of the bottom of the conduction band, which is translated into a reduction of the band gap upon CO adsorption on the clean surfaces. On the other hand, the CO molecule is also chemisorbed on the doped surfaces through a back-donation bond. The doped surfaces states at the conduction band are also compressed by the appearance of new states belonging to the CO molecule, and the states at the Fermi level seems to increase. The results provide good insights for the application of WO3 in gas sensing chemistry and photocatalysis.

Enhanced fluoride removal from drinking water in wide pH range using La/Fe/Al oxides loaded rice straw biochar

Abstract A novel and highly efficient adsorbent was prepared by loading La/Fe/Al oxides onto rice straw biochar (RSBC), which was tested for the ability to remove fluoride from drinking water. Characterized by SEM, XRD, Zeta potential and FTIR, it was found that the ternary metal oxides were successfully loaded on the surface of biochar in amorphous form, resulting in the formation of hydroxyl active adsorption sites and positive charges, which played a synergistic role in fluoride removal. Through batch adsorption tests, key factors including contact time, initial fluoride concentration, initial pH and co-existing anions effects were investigated. Results showed that the tri-metallic modified biochar (La/Fe/Al-RSBC) had excellent fluoride removal performance with an adsorption capacity of 111.11 mg/g. Solution pH had little impact on the removal of fluoride, the adsorbent retained excellent fluoride removal capacity in a wide pH range of 3.0–11.0. The co-existing anions had almost no effect on the fluoride removal by La/Fe/Al-RSBC. In addition, La/Fe/Al-RSBC could be regenerated and reused. Electrostatic adsorption and ion exchange were responsible for this adsorption behavior. These findings suggested the broad application prospect of a prepared biochar adsorbent based on rare earth and aluminum impregnation for the fluoride removal from drinking water.

Improved Performance of Electron Blocking Layer Free AlGaN Deep Ultraviolet Light-Emitting Diodes Using Graded Staircase Barriers.

To prevent electron leakage in deep ultraviolet (UV) AlGaN light-emitting diodes (LEDs), Al-rich p-type AlxGa(1−x)N electron blocking layer (EBL) has been utilized. However, the conventional EBL can mitigate the electron overflow only up to some extent and adversely, holes are depleted in the EBL due to the formation of positive sheet polarization charges at the heterointerface of the last quantum barrier (QB)/EBL. Subsequently, the hole injection efficiency of the LED is severely limited. In this regard, we propose an EBL-free AlGaN deep UV LED structure using graded staircase quantum barriers (GSQBs) instead of conventional QBs without affecting the hole injection efficiency. The reported structure exhibits significantly reduced thermal velocity and mean free path of electrons in the active region, thus greatly confines the electrons over there and tremendously decreases the electron leakage into the p-region. Moreover, such specially designed QBs reduce the quantum-confined Stark effect in the active region, thereby improves the electron and hole wavefunctions overlap. As a result, both the internal quantum efficiency and output power of the GSQB structure are ~2.13 times higher than the conventional structure at 60 mA. Importantly, our proposed structure exhibits only ~20.68% efficiency droop during 0–60 mA injection current, which is significantly lower compared to the regular structure.

Double network hydrophobic starch based amphoteric hydrogel as an effective adsorbent for both cationic and anionic dyes

A double-network hydrophobic-hydrogel was successfully synthesized. Here epichlorohydrin (ECH) and N, N-methylene bis-acrylamide act as the crosslinkers for starch and poly(acrylic acid), respectively. The synthesized hydrogel having both positive and negative charge exhibited amphoteric properties and employed for the exclusion of anionic and cationic dyes from waste aqueous solution. The formation of positive charge occurs via reaction of triethylamine with the open up epoxy ring of ECH while carboxylate ion of poly(acrylic acid) gives the negative charge in the hydrogel. The formation of double cross-linking and the presence of quaternized alkyl chains lead towards hydrophobicity with much lower swelling ability. As an adsorbent, the hydrogel was able to remove both cationic dye (methylene blue) and anionic dye (congo red) with adsorption maxima of 133.65 mg/g and 64.73 mg/g, respectively. This study showed that the synthesized hydrogel offers enormous potential as the toxic dye adsorbent for effluent treatment of industrial wastewater.

Investigation of the anomalous hump phenomenon in amorphous InGaZnO thin-film transistors

In this work, we investigated an anomalous hump in the bottom-gate amorphous-InGaZnO thin-film transistors. The hump in the subthreshold region appeared at elevated temperature and disappeared after N2 atmosphere annealing. The anomalous hump phenomenon is attributed to the existence of both main current path composed by electron and parasitic current path induced by positive charges formed by adsorbed H2O molecules. With the increase of the testing temperature, the hump effect becomes significant since the formation of positive charges is strengthened by heat. This hump can be weakened by negative bias temperature instability stress at high temperature due to the charge-trapping mechanism.

A 120-kV electron welding gun with plasma cathode: vacuum and non-vacuum application

The article studies the generation of small-diameter focused electron beams using electron-optical system of a welding gun with plasma cathode. The results show that the beams always form in the electron-optical system with plasma cathode when the gas in the region of beam emission, acceleration and transportation is present. The presence of the gas causes positive volumetric charge formation during the passage of electrons. This charge electrically neutralizes the beam, and in some cases can lead to overcompensation of its electron charge. The compensation of the volumetric charge in the region of beam formation and acceleration is the principal discrepancy of an electron-optical system with plasma cathode from a thermal cathode system. It is demonstrated that the brightness of the electron beam from a plasma cathode gun meets that of the beams from a thermal cathode gun. The paper describes the electron gun featuring voltage up to 120 kV and presents images of polished samples of weld penetration in stainless steel and titanium in vacuum. The gun served as the basis of a device for ejecting electron beam into the atmosphere. It is shown that the pressure rise up to the atmospheric one is provided through two gas-dynamic stages. The main characteristics of the atmospheric gun are given.

(Invited) Unified Understanding of the Shockley Stacking Fault Formation in 4H-SiC Crystals under Thermal Equilibrium and Non-Thermal Equilibrium Conditions Based on the Quantum Well Action Concept

It is well-known that SiC crystal deficiencies are delaying the market penetration of outstandingly superior SiC power devices. Efforts to date have centred on eradicating extended defects from 4H-SiC bulk crystals and epitaxial thin films. Among the extended defects existing in these materials, basal plane stacking faults (SFs) are one of the most serious concerns in SiC power devices because they are formed during the forward-bias operation of 4H-SiC PiN diodes and detrimental to SiC power devices, such as SiC MOSFETs as well as SiC BJTs and PiN diodes. In this phenomenon, i.e., the so-called “bipolar degradation”, single Shockley-type SFs (SSSFs) expand via the recombination of minority carriers (holes) at the partial dislocations in the Si core, which encompass the SSSFs. Recombination-enhanced dislocation glide (REDG) is widely accepted as the most probable kinematic mechanism of this phenomenon, and several other kinematic and energetic effects have been discussed in conjunction with the REDG mechanism.Another negative effect of SF formation in SiC crystals is the spontaneous formation of double Shockley SFs (DSSFs) in heavily nitrogen-doped 4H-SiC crystals, which largely increases the resistivity of the crystals. When the nitrogen concentration exceeds the critical value (2–3 × 1019 cm−3), DSSFs are spontaneously formed in the crystals during high-temperature annealing (> 1100°C). DSSFs have been shown to heal (shrink) upon annealing at even higher temperatures (1800°C), and the co-doping of aluminum acceptors tends to reduce the density of DSSFs in heavily nitrogen-doped 4H-SiC crystals.The above-mentioned two phenomena can be explained based on the concept of quantum well action (QWA). In the QWA, electrons in the conduction band of 4H-SiC enter the SF-induced quantum well, consequently decreasing the electronic energy of the system. However, from the viewpoint of thermodynamics, the two phenomena differ from each other. Whereas DSSF formation is a thermodynamically equilibrium process, SSSF expansion in SiC PiN diodes occurs under non-equilibrium conditions, such as the forward biasing of the PiN diode or ultraviolet light illumination of the materials. In this respect, it is worthy to note that SSSF expansion only occurs under non-equilibrium conditions and has never been observed in thermal equilibrium even when the nitrogen concentration in 4H-SiC crystals exceeds 1 × 1020 cm−3.In this paper, I will try to clarify the physical origin of the driving force of the SSSF expansion during the forward-biasing of 4H-SiC PiN diodes via theoretical investigation of the energetics of the SF formation in 4H-SiC crystals under non-equilibrium conditions. Unified understanding of the SF expansion under thermal equilibrium and non-thermal equilibrium conditions will also be attempted based on the QWA concept. The similarity and dissimilarity in underlying physics between the SF formation in forward-biased 4H-SiC PiN diodes and that in heavily nitrogen-doped 4H-SiC crystals during high temperature annealing are discussed, taking into account the electron trapping into the SF and the resulting positive charge formation in the materials. The difference in the dimensionality of the electron systems in SFs (two-dimensions) and bulk 4H-SiC (three-dimensions) provides different energy dependencies of the density of states for electrons in the two systems and plays an important role in the temperature dependence of the SF expansion in 4H-SiC crystals.

Gate-Tunable Tunneling Transistor Based on a Thin Black Phosphorus-SnSe2 Heterostructure.

Tunneling field-effect transistors (TFETs) are of considerable interest owing to their capability of low-power operation. Here, we demonstrate a novel type of TFET which is composed of a thin black phosphorus–tin diselenide (BP–SnSe2) heterostructure. This combination of 2D semiconductor thin sheets enables device operation either as an Esaki diode featuring negative differential resistance (NDR) in the negative gate voltage regime or as a backward diode in the positive gate bias regime. Such tuning possibility is imparted by the fact that only the carrier concentration in the BP component can be effectively modulated by electrostatic gating, while the relatively high carrier concentration in the SnSe2 sheet renders it insensitive against gating. Scanning photocurrent microscopy maps indicate the presence of a staggered (type II) band alignment at the heterojunction. The temperature-dependent NDR behavior of the devices is explainable by an additional series resistance contribution from the individual BP and SnSe2 sheets connected in series. Moreover, the backward rectification behavior can be consistently described by the thermionic emission theory, pointing toward the gating-induced formation of a potential barrier at the heterojunction. It furthermore turned out that for effective Esaki diode operation, care has to be taken to avoid the formation of positive charges trapped in the alumina passivation layer.

Transient dynamics of packet-like space charge in low-density polyethylene at high temperatures

Among features observed in polyethylene-based materials under high fields, packet-like space charge are frequently reported but without complete theory explaining their formation and propagation. The present contribution reports transient dynamics of packet-like space charge in low-density polyethylene (LDPE) under multi-field coupling conditions based on pulsed electro-acoustic (PEA) method. The measurements were carried out under −125 kV/mm at 20 °C, 40 °C, and 60 °C. Space charge results reveal systematic occurrence of positive packet-like space charge in all samples generated at the anode and then moved toward the cathode. It is observed that higher temperatures contribute to the migration of packet-like space charge and field distortion. Based on the detailed injection model of packet-like space charge, it is shown that electrode injection plays a major role in the formation of positive packet-like space charge, the total amount and depth not only depend on the temperature, but also the negative charges dynamics. Packet-like space charge phenomena at different temperatures still feature NDM relationship with the nonlinear increase of migrate velocity as the rise of temperature.

Study of the effect of passivation layers on capacitance of AlGaN/GaN heterostructures

MOCVD grown AlGaN/GaN heterostructures on sapphire and silicon substrates have been studied. The capacity–voltage characteristic have been measured in the 200Hz–1MHz frequency range with a planar position of mercury and second probe on the specimen surface. The shape of the typical C–V curves for the heterostructures with upper undoped i-AlGaN and i-GaN layers 1,5–2,5nm in thickness have been analyzed. The appearance of a typical peak on the C–V curves upon a change from the depletion region to the accumulation region has been registered in some structures with an i-GaN layer thickness of 5nm and more at low frequencies (f<50–200kHz). The height of this peak increased with a reduction of frequency. It has been found experimentally that the frequency at which the peak is registered can depend on the dislocation density in heterostructures. Possible explanation of the peak formation and band diagram transformation in these structures under an applied electric field have been presented. We show that the application of a Si3N4 passivation layer results in the formation of additional positive charge.

Characterizing monoclonal antibody structure by carboxyl group footprinting

Structural characterization of proteins and their antigen complexes is essential to the development of new biologic-based medicines. Amino acid-specific covalent labeling (CL) is well suited to probe such structures, especially for cases that are difficult to examine by alternative means due to size, complexity, or instability. We present here a detailed account of carboxyl group labeling (with glycine ethyl ester (GEE) tagging) applied to a glycosylated monoclonal antibody therapeutic (mAb). The experiments were optimized to preserve the structural integrity of the mAb, and experimental conditions were varied and replicated to establish the reproducibility of the technique. Homology-based models were generated and used to compare the solvent accessibility of the labeled residues, which include aspartic acid (D), glutamic acid (E), and the C-terminus (i.e., the target probes), with the experimental data in order to understand the accuracy of the approach. Data from the mAb were compared to reactivity measures of several model peptides to explain observed variations in reactivity. Attenuation of reactivity in otherwise solvent accessible probes is documented as arising from the effects of positive charge or bond formation between adjacent amine and carboxyl groups, the latter accompanied by observed water loss. A comparison of results with previously published data by Deperalta et al using hydroxyl radical footprinting showed that 55% (32/58) of target residues were GEE labeled in this study whereas the previous study reported 21% of the targets were labeled. Although the number of target residues in GEE labeling is fewer, the two approaches provide complementary information. The results highlight advantages of this approach, such as the ease of use at the bench top, the linearity of the dose response plots at high levels of labeling, reproducibility of replicate experiments (<2% variation in modification extent), the similar reactivity of the three target probes, and significant correlation of reactivity and solvent accessible surface area.

Investigation of the Passivation Layers Influence on Capacitance Characteristics of AlGaN/GaN Heterostructures

AlGaN/GaN heterostructures grown by MOCVD method on sapphire and silicon substrates were test subjects. The capacity− voltage characteristic measurements have been run in 200Hz — 1MHz frequency range at planar disposition of mercury and second probe on the sample surface. The shape of typical C−V curves for the heterostructures with the upper undoped i−AlGaN and i−GaN layers at thickness 15—25 A have been analyzed. The appearance of a typical peak on the C−V curves at changing from depletion region to accumulation region has been registered for some structures with thickness of i−GaN layer 50A at low frequencies (f < 50—200 kHz). The height of this peak increased with reduction of frequency. It has been found experimentally that frequency at which the peak is registered can depend on the dislocation density in heterostructures. Possible explanation of the peak formation and band diagram modifications in these structures under an applied electric field have been presented. We show that using a Si3N4 passivation layer results in the formation of additional positive charge.

Light‐Addressable Potentiometric Sensor with Nitrogen‐Incorporated Ceramic Sm2O3 Membrane for Chloride Ions Detection

A light‐addressable potentiometric sensor (LAPS) with ceramic samarium oxide (Sm2O3)‐sensing membrane treated by nitrogen plasma immersion ion implantation (PIII) has been proposed for chloride ion detection. For the pure Sm2O3‐LAPS, a potassium ion sensitivity of 39.21 mV/pK is obtained. With the nitrogen PIII treatment on Sm2O3‐sensing membrane, the N–O peak is observed by X‐ray photoelectron spectroscopy, implying the formation of positive charges, (N‐O)+ and (N‐O‐N)+, within the Sm2O3 film. The positive charges can attract the chloride ions to react with the surface sites of OH2+, achieving a superior chloride ion sensitivity of 36.17 mV/pCl for the Sm2O3‐LAPS with nitrogen PIII treatment. The LAPS structure with ceramic Sm2O3‐sensing membrane can be used for future biosensing applications, especially for the potassium and chloride ions detection in serum.