Accumulation Of Negative Charge Research Articles

Experimental Verification of Multipactor Discharge Dynamics Between Ferrite Dielectric and Metal

The underlying mechanisms of multipactor between dielectric and metal and the discharge threshold of practical dielectric components have been investigated numerically and experimentally in this paper. The reduction in secondary electron yield (SEY) and the change in surface condition are achieved using the microstructure processing technology on both of ferrite dielectric and metal. Combined with the electromagnetic particle-in-cell method, the SEY model fit with the experimental data is utilized to simulate multipactor in three groups of ferrite circulators with different surface conditions. Simulation and experimental results demonstrate that the multipactor threshold of the circulator is increased by a factor of 2 through the SEY reduction on the ferrite and is increased by a factor of more than 8, from 380 W to more than 3400 W, through the SEY reduction on both of the ferrite and the metal. Different multipactor discharge mechanisms, including two-side multipactor with positive charge accumulation, singe-side multipactor with negative charge accumulation, and multipactor suppression effect, exist in circulators with the same physical structure and different surface conditions.

Experimental and Computational Studies Delineate the Role of Asparagine 177 in Hydride Transfer for E. coli Thymidylate Synthase.

Thymidylate synthase (TSase), an enzyme responsible for the de novo biosynthesis of 2'-deoxythymidine 5'-monophosphate (thymidylate, dTMP) necessary for DNA synthesis, has been a drug target for decades. TSase is a highly conserved enzyme across species ranging from very primitive organisms to mammals. Among the many conserved active site residues, an asparagine (N177, using Escherichia coli residues numbering) appears to make direct hydrogen bonds with both the C4=O4 carbonyl of the 2'-deoxyuridine 5'-monophosphate (uridylate, dUMP) substrate and its pyrimidine ring's N3. Recent studies have reassessed the TSase catalytic mechanism, focusing on the degree of negative charge accumulation at the O4 carbonyl of the substrate during two critical H-transfers - a proton abstraction and a hydride transfer. To obtain insights into the role of this conserved N177 on the hydride transfer, we examined its aspartic acid (D) and serine (S) mutants - each of which is expected to alter hydrogen bonding and charge stabilization around the C4=O4 carbonyl of the 2'-deoxyuridine 5'-monophosphate (uridylate, dUMP) substrate. Steady-state kinetics, substrate binding order studies and temperature-dependency analysis of intrinsic KIEs for the hydride transfer step of the TSase catalytic cycle suggest the active site of N177D is not precisely organized for that step. A smaller disruption was observed for N177S, which could be rationalized by partial compensation by water molecules and rearrangement of other residues toward preparation of the system for the hydride transfer under study. These experimental findings are qualitatively mirrored by QM/MM computational simulations, thereby shedding light on the sequence and synchronicity of steps in the TSase-catalyzed reaction. This information could potentially inform the design of mechanism-based drugs targeting this enzyme.

Relaxation processes in solid methane pre-irradiated with an electron beam

Abstract The relaxation processes in CH4 solids irradiated by an electron beam were studied with a focus on post-irradiation phenomena. Because of absence of thermally stimulated luminescence (TSL) from pure solid methane we applied current activation spectroscopy method – thermally stimulated exoelectron emission (TSEE). Two broad features were detected near 18 and 43 K. Their relative intensity appeared to depend on irradiation time. The experiments performed with a variable voltage applied to an electrode for current detection first revealed significant accumulation of uncompensated negative charge in pre-irradiated methane films. The TSEE current was detected at negative voltages up to −35 V. Concurrently with thermally stimulated yield of electrons the total yield of thermally stimulated post-desorption (TSpD) from pre-irradiated solid methane film was detected. Correlation of TSEE and TSpD yields is discussed and compared with the temperature behavior of solid methane subjected to a neutron flux.

Enhanced photocatalytic activity of PTCDI-C60 via π–π interaction

The π–π stacking of self-assembled PTCDI and π–π interactions between self-assembled PTCDI and C60 result in fast transfer of the photogenerated carriers and reduced carrier recombination. The π–π interaction lowers the position of valence band and narrows the band gap, thus leading to a stronger redox ability and a broad spectral response. The organic photocatalyst PTCDI-C60 can not only degrading phenol at a rate of 0.216 h−1, which is 8.24 times higher than that of pure PTCDI, but also producing oxygen at a rate of 22.5 μmol g−1 h−1. Moreover, the presence of C60 stabilizes the composite to decrease the accumulation of negative charge. The high catalytic activity can potentially be utilized in the fields of environmental and energy applications.

Localized and Intense Energy Conversion in the Diffusion Region of Asymmetric Magnetic Reconnection

AbstractWe analyze a high‐resolution simulation of magnetopause reconnection observed by the Magnetospheric Multiscale mission and explain the occurrence of strongly localized dissipation with an amplitude more than an order of magnitude larger than expected. Unlike symmetric reconnection, wherein reconnection of the ambient reversed magnetic field drives the dissipation, we find that the annihilation of the self‐generated, out‐of‐plane (Hall) magnetic field plays the dominant role. Electrons flow along the magnetosheath separatrices, converge in the diffusion region, and jet past the X‐point into the magnetosphere. The resulting accumulation of negative charge generates intense parallel electric fields that eject electrons along the magnetospheric separatrices and produce field‐aligned beams. Many of these features match Magnetospheric Multiscale observations.

Application of [Pt(II)(Tetra-Tert-Butylsalophen)] Complex within Organic Devices: Deep Red Emission, Bistable Light-Emitting Diodes and Operational Stability

This paper focuses on the Negative Differential Resistance (NDR) we observed on organic light-emitting diodes (OLEDs) using [Pt(II)(tetra-tert-butylSalophen)] as host, since this Pt(II) complex displays a deep-red emission (λmax = 660 nm). Electrical characterizations of monolayer devices have shown that doping Tris-(8-hydroxyquinoline)aluminum (Alq3) as matrix emissive layer with this complex, leads to the modulation of the charge transport properties highlighted by Negative Differential Resistance (NDR). Upon electrical driving stresses, the conductivity of active layer can be switched between two electrical states (ON and OFF) with a figure of merit higher than 103. By adding an electron-blocking layer, we demonstrated that the NDR trend is closely related to negative charge accumulation within Alq3 leading to the modification of electronic properties in the vicinity of anode/active layer interface. The NDR phenomenon is interpreted in terms of space charge polarization (SCP) linked to charge trapping/untrapping mechanism as a consequence of the polarization/depolarization of the Pt(II) complex. Under electrical driving stresses, the performance of the devices which include the Pt(II) complex, are stabilized. A schematic model is proposed to depict the SCP responsible for NDR and decrease-resetting behaviors observed in these devices.

Copper–Palladium Tetrapods with Sharp Tips as a Superior Catalyst for the Oxygen Reduction Reaction

AbstractThe development of superior non‐Pt catalysts for the oxygen reduction reaction (ORR) is highly desirable but still challenging. Herein, we prepared 50‐nm CuPd tetrapods with catalytic performance towards the alkaline ORR that was higher than that of Pd nanoparticles, CuPd nanoparticles, CuPd tetrahedrons, and 7‐nm CuPd tetrapods. The mass activity of 50‐nm CuPd tetrapods was 0.29 A mgPd−1, which was superior to that of commercial Pt/C and most Pd‐based catalysts reported to date. Mechanistic studies revealed that the enhanced performance could be attributed to the synergy of the alloy effect and sharp‐tip effect, both of which enable the accumulation of negative charges on the Pd atoms at the vertices of the tetrapods. The high negative charge density of the Pd atoms at the vertices lowers the Pd d‐band center and, thus, its oxygen adsorption energy. This work provides a pathway to improve the activity of ORR catalysts by modulating their electronic structures, which opens up new sight to design promising non‐Pt catalysts for the ORR.

External electric field driven modification of the anomalous and spin Hall conductivities in Fe thin films on MgO(001)

The effects of applying external electric fields to the anomalous and spin Hall conductivities in Fe thin-film models with different layer thicknesses on MgO(001) are investigated by using first-principles calculations. We observe that, for the considered systems, the application of positive electric field associated with the accumulation of negative charges on the Fe side generally decreases (increases) the anomalous (spin) Hall conductivities. The mapping of the Hall conductivities within the two-dimensional Brillouin zone shows that the electric-field-induced modifications are related to the modification of the band structures of the atoms at the interface with the MgO substrate. In particular, the external electric field affects the Hall conductivities via the modifications of the ${d}_{xz},{d}_{yz}$ orbitals, in which the application of positive electric field pushes the minority-spin states of the ${d}_{xz},{d}_{yz}$ bands closer to the Fermi level. Better agreement with the anomalous Hall conductivity for bulk Fe and a more realistic scenario for the electric field modification of Hall conductivities are obtained by using the thicker layers of Fe on MgO (${\mathrm{Fe}}_{3}$/MgO and ${\mathrm{Fe}}_{5}$/MgO).

Substituent Effects on the Reactivity of Cyclic Tertiary Sulfamidates.

The reactivity of cyclic tertiary sulfamidates derived from α-methylisoserine strongly depends on the substitution at the C and N termini. These substrates are one of the very few examples able to undergo nucleophilic ring opening at a quaternary carbon with complete inversion of the configuration, as demonstrated both experimentally and computationally. When the sulfonamide is unprotected, the characteristic ring-opening reaction is completely silenced, which explains that the majority of the ring-opening reactions reported in the literature invoke N-alkyl or N-carbonyl-protected sulfamidates. Accumulation of negative charge at the NSO3 moiety in the transition state, especially when the sulfonamide NH is deprotonated, drastically raises the activation barrier for the nucleophilic attack. On the other hand, ester groups at the carboxylic position favor ring opening, whereas amides allow competition between the substitution and elimination pathways. Using pyridine as a nucleophilic probe, we have demonstrated both experimentally and computationally that a proper selection of the substitution scheme can enhance the synthetic scope of α-methylisoserine-derived sulfamidates, switching off and on the nucleophilic ring-opening in a controlled manner. This is particularly convenient for hybrid α/β-peptide synthesis, as demonstrated recently by our group.

Laser-induced charge separation in organic nanofibers: A joint experimental and theoretical investigation

Abstract Charge generation, recombination, and transport in organic semiconductors are fundamental processes that dictate the performance for a range of organic devices. Here we used electrostatic force microscopy combined with laser scanning microscopy to study the carrier distribution in individual organic nanofibers that result from localized illumination: NaT2 (5,5′-bis(naphth-2-yl)-2,2′-bithiophene) crystalline nanofibers were illuminated with a continuous wave, focused laser beam with above-band gap photon energy. The localized illumination causes a local negative charge accumulation at the illumination position, while the remaining part of the nanofiber exhibits a positive charge. We propose that this inhomogeneous distribution of photogenerated holes and electrons along the nanofiber axis is due to a surface-trap induced exciton dissociation and the difference in charge carrier mobilities for hole and electrons. We use a one-dimensional, drift-diffusion based model to rationalize the observations and give an improved understanding of the factors governing the charge build-up.

Improving Electrical Stability and Ideality in Organic Field‐Effect Transistors by the Addition of Fullerenes: Understanding the Working Mechanism

Electrical instability and nonideality due to undesirable electron injection are often‐encountered problems for high‐mobility organic field‐effect transistors (OFETs) with low‐bandgap polymer semiconductors. Due to electron trapping and the resulting accumulation of negative charges on the silicon dioxide dielectric, transfer curves deviate from ideality characteristics and double‐slopes are observed as the devices are operated for extended periods of time. One way to circumvent those is to use an electron‐acceptor additive, such as fullerene and its derivatives. This work interprets the mechanisms of how fullerene derivatives suppress electron transport and electrical instability while maintaining high hole mobility in p‐type OFETs. This study shows that hole transport of the active layer is uninterrupted upon the addition of the electron acceptors. Most importantly, the added fullerene derivatives out‐compete SiO2 to acquire electrons that are injected into the polymers. Electrical instability and double‐slope induced from electron trapping at SiO2 surface are thereby suppressed.

Exploring New Mechanisms for Effective Antimicrobial Materials: Electric Contact-Killing Based on Multiple Schottky Barriers.

The increasing threat of multidrug-resistance organisms is a cause for worldwide concern. Progressively microorganisms become resistant to commonly used antibiotics, which are a healthcare challenge. Thus, the discovery of new antimicrobial agents or new mechanisms different from those used is necessary. Here, we report an effective and selective antimicrobial activity of microstructured ZnO (Ms-ZnO) agent through the design of a novel star-shaped morphology, resulting in modulation of surface charge orientation. Specifically, we find that Ms-ZnO particles are composed of platelet stacked structure, which generates multiple Schottky barriers due to the misalignment of crystallographic orientations. We also demonstrated that this effect allows negative charge accumulation in localized regions of the structure to act as "charged domain walls", thereby improving the antimicrobial effectiveness by electric discharging effect. We use a combination of field emission scanning electron microscopy (FE-SEM), SEM-cathodoluminescence imaging, and Kelvin probe force microscopy (KPFM) to determine that the antimicrobial activity is a result of microbial membrane physical damage caused by direct contact with the Ms-ZnO agent. It is important to point out that Ms-ZnO does not use the photocatalysis or the Zn2+ released as the main antimicrobial mechanism, so consequently this material would show low toxicity and robust stability. This approach opens new possibilities to understand both the physical interactions role as main antimicrobial mechanisms and insight into the coupled role of hierarchical morphologies and surface functionality on the antimicrobial activity.

The interplay of blocking properties with charge and potential redistribution in thin carbon-doped GaN on n-doped GaN layers

In carbon-doped GaN (GaN:C) buffers used in a GaN-on-Si technology, the buffer is embedded in between transition and channel layers. This makes the analysis of buffer properties difficult due to e.g., carrier injection from or potential drop at these adjacent layers. Here, we analyze capacitance- and current-voltage characteristics of 200–300 nm thick GaN:C ([C] = 1019 cm−3) layers which are embedded between a top metal electrode and bottom n-doped GaN (n-GaN). Such structures allow a better potential control in GaN:C and thus determination of the band diagram quantitatively. The accumulation of negative charge (concentration up to 6 × 1017 cm−3) with bias is observed in GaN:C at both polarities. For biases Vappl < +1.7 V at the top electrode, negative charges accumulate in GaN:C near to its interface with n-GaN so that GaN:C exhibits no potential drop and blocks leakage current. For Vappl > +1.7 V, accumulated negative charges in GaN:C raise an energy barrier of ∼1.1 eV for electron injection from n-GaN to GaN:C. This causes a potential drop in GaN:C leading to a significant leakage current increase. The Fermi level pinning in GaN:C at a commonly referred acceptor at EV + 0.7(±0.2) eV is extracted only from electrostatic considerations. The occupancy change of carbon acceptors is attributed to trapping processes where the dislocation-related conductive paths are supposed to be involved in carrier transport from the top metal electrode to the carbon defect.

XPS valence band study of Na‐silicate glasses: energetics and reactivity

High‐resolution X‐ray photoelectron spectroscopy (XPS) of valence bands of vitreous silica (SiO2(vit)), and Na‐silicate glasses are divisible into 3 sub‐bands. These are the O 2s band, the lower valence band (LVB), and the upper valence band (UVB). Many peaks of the LVB and UVB have line widths (full width at half maximum) of 1.2–1.7 eV, which are commensurate with widths of core level O 1s lines, indicating weak dispersion of these valence band peaks. Two types of oxygen exist in the glasses. There is oxygen bridging 2 Si atoms (bridging oxygen [BO]), and O bonded to Si and Na (nonbridging oxygen [NBO]). The addition of Na to siliceous glasses diminishes electronic density of states of the LVB and enhances density of states of the UVB, a consequence of quenching Si‐BO σ‐bonds of the LVB, creating Si‐NBO σ‐bonds in the UVB, and creating atomic‐like O 2px,y nonbonding orbitals at the top of the UVB or highest occupied molecular orbitals. The process destabilizes the valence bands of the glasses by 2 to 6 eV per Si‐NBO bond formed. All orbitals are destabilized with increased Na2O content. There is effectively complete transfer of Na 3s electrons to NBO but this charge is redistributed among all atoms of the tetrahedron via the 4 Si‐O σ bonds (sp3‐O 2px character). The BE of the Si 2p line decreases more rapidly with Na2O content than any other line monitored, and this preferential accumulation of negative charge on Si causes Si‐O bonds to become weaker through decrease in the force constant of the bonds. The durability of Na‐silicate glasses decreases in response. The destabilization of the highest occupied molecular orbitals makes the Na‐rich glasses more susceptible to attack by reagents such as H+, OH−, and H2O. Copyright © 2017 John Wiley & Sons, Ltd.

Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO3

In this study, we investigated the electronic surface structure of donor-doped strontium titanate. Homoepitaxial 0.5 wt. % donor-doped SrTiO3 thin films were analyzed by in situ near ambient pressure X-ray photoelectron spectroscopy at a temperature of 770 K and oxygen pressures up to 5 mbar. Upon exposure to an oxygen atmosphere at elevated temperatures, we observed a rigid binding energy shift of up to 0.6 eV towards lower binding energies with respect to vacuum conditions for all SrTiO3 core level peaks and the valence band maximum with increasing oxygen pressure. The rigid shift is attributed to a relative shift of the Fermi energy towards the valence band concomitant with a negative charge accumulation at the surface, resulting in a compensating electron depletion layer in the near surface region. Charge trapping effects solely based on carbon contaminants are unlikely due to their irreversible desorption under the given experimental conditions. In addition, simple reoxygenation of oxygen vacancies c...

The gamma irradiation responses of yttrium oxide capacitors and first assessment usage in radiation sensors

Co-60 gamma irradiation responses of the Y2O3 MOS capacitors were investigated, and initial assessment of the Y2O3 dielectrics used in gamma radiation sensors was discussed. We analyzed the effects of applied radiation from flat-band and mid-gap voltage shifts, and also capacitance–voltage measurements were obtained before and after irradiation. It has been observed that the measured capacitance is almost constant with irradiation and the basic modification in flat band shifts toward more positive voltages due to negative charge accumulation, thanks to trap centers in the MOS capacitors. The reason of negative charge trapping in the devices structure may be attributed to ionized Yttrium atoms and cluster of the oxygen vacancies occurred by irradiation. Also, a linear dose flat band relation has been observed, and irradiation sensitivity was found to be 10.8±0.43mV/Gy for Y2O3 calculated for five different capacitors, which is more sensitive than the conventional SiO2 dielectric layers. The higher sensitivity is probably due to the high trapped efficiency in the Y2O3 dielectrics. On the other hand, the generated oxide traps densities increase with irradiation while interface state density trend varies by irradiation. This behavior for interface states was attributed to the passivation of the dielectric layer from the semiconductor. The charge accumulation in the MOS capacitors is in the order of 1010–1011 cm−2 for the given dose range. This did not cause any significant device degradation through its operation. Consequently, the irradiation does not significantly affect the device operation. Especially, for radiation measurements system with linear dose performance and sensitivity, Y2O3 may be a promising future gate dielectric material candidate for radiation sensors in given radiation dose range.

Highly Efficient Organic Photocatalyst with Full Visible Light Spectrum through π-π Stacking of TCNQ-PTCDI.

Self-assembled TCNQ-PTCDI composite photocatalysts can not only degrade phenol at a rate of 0.154 h-1, which is 10.4 times higher than that of pure PTCDI, but also produce oxygen at a rate of ∼14 μmol·g-1·h-1 without cocatalysts. The π-π interactions between TCNQ and PTCDI result in fast transfer of carriers and reduced carrier recombination. The interaction lowers the valence band and narrows the band gap, thus leading to a stronger oxidizability and a broad spectral response (∼730 nm). Moreover, the presence of TCNQ stabilizes the composite to decrease the accumulation of negative charge, which results in an excellent stability of the composite. The high catalytic activity can potentially be utilized in the fields of environmental and energy applications.

Space charge characterization in biaxially oriented polypropylene films

Biaxially oriented polypropylene (BOPP) is one kind of the most significant dielectric films for energy storage capacitors. The operating electric field up to 650 kV/mm for the BOPP film can lead to numerous space charge injection, which would be very dangerous to the capacitor. In this investigation, space charge behavior in the BOPP films under various operating temperatures and DC electric fields was studied by the thermal pulse method. At room temperature, the asymmetric bi-polar space charge injection was observed in the BOPP films under DC electric field, and the accumulation of negative charge become dominant gradually with the increase of the experimental temperature. At temperature 70 °C, only negative space charge can be observed in the whole bulk, which could mainly be attributed to the mutual shifting of the opposite polarity injected charge with different amounts. As the consequence, the local electric field generated by accumulated space charge can be up to 80% of the applied field.

Solitary waves in dusty plasmas with weak relativistic effects in electrons and ions

Two distinct classes of dust ion acoustic (DIA) solitary waves based on relativistic ions and electrons, dust charge Zd and ion-to-dust mass ratio Q’ = mi/md are established in this model of multicomponent plasmas. At the increase of mass ratio Q’ due to increase of relativistic ion mass and accumulation of more negative dust charges into the plasma causing decrease of dust mass, relativistic DIA solitons of negative potentials are abundantly observed. Of course, relativistic compressive DIA solitons are also found to exist simultaneously. Further, the decrease of temperature inherent in the speed of light c causes the nonlinear term to be more active that increases the amplitude of the rarefactive solitons and dampens the growth of compressive solitons for relatively low and high mass ratio Q’, respectively. The impact of higher initial streaming of the massive ions is observed to identify the point of maximum dust density Nd to yield rarefactive relativistic solitons of maximum amplitude.

Elucidating the evolution of the current-voltage characteristics of planar organometal halide perovskite solar cells to an S-shape at low temperature

The temperature dependence of initial transient current-voltage (I-V) characteristic of planar perovskite solar cell by one-step solution process is investigated. An S-shaped I-V characteristic emerges in response to low temperature and the photovoltaic parameters drop dramatically. This is mainly attributed to the increasing amount of negative charges accumulating in the TiO2/CH3NH3PbI3 interface region and the reduced built-in field separating the photo-generated carriers in the absorber layer. The influence of negative charge accumulation can be represented by two extra diodes that are in series with a conventional solar cell circuit model at low temperature whereas it acts as a resistor with low resistivity above room temperature. These findings help to understand the charge transport mechanism in perovskite solar cells.