Positive Charge Buildup Research Articles

Impact of Rh–CeOx interaction on CO oxidation mechanisms

We have investigated the impact of electronic metal–support interaction on catalytic properties of Rh/CeOx system, i.e. its capacity for the reverse and direct oxygen spillover, in the context of CO oxidation. The studies were carried out on two types of samples consisting of Rh particles supported on polycrystalline nearly stoichiometric and partially reduced cerium oxide films using temperature programmed reaction, temperature programmed desorption, and conventional X-ray and resonant photoelectron spectroscopies. We have found that the electronic Rh–CeOx interaction leads to a buildup of a net positive charge on Rh particles supported on stoichiometric cerium oxide, and a net negative charge on Rh particles on reduced cerium oxide. The effect of the Rh–CeOx interaction is manifested in suppression of the reverse and direct oxygen spillover on Rh particles supported on partially reduced ceria.

Lophine derivatives as activators in peroxyoxalate chemiluminescence.

Lophine and four of its derivatives were used as activators (ACTs) of the chemiluminescent peroxyoxalate (PO) reaction of bis(2,4,6-trichlorophenyl)oxalate with H2O2, catalysed by imidazole. Kinetic emission assays have shown that with the tested compounds the reaction mechanism, regarding the formation of the high energy intermediate (HEI) of the PO reaction, occurs as previously seen for commonly used ACTs. A bimolecular interaction of the compounds with the HEI leads to chemiexcitation through the chemically initiated electron exchange luminescence (CIEEL) mechanism, as confirmed by a linear free-energy correlation between the relative catalytic rate constants and the oxidation potentials of the compounds. The yields of excited state formation and light emission, in the range of 10(-2)-10(-3) E mol(-1), are comparable to the ones seen with commonly used ACTs. A Hammett plot with ρ = -0.90 indicates the buildup of a partial positive charge on the transition step of the catalytic process, consistent with the formation of a radical cation of the ACT, being an additional validation of the CIEEL mechanism in this system.

Graphics processing unit accelerated three-dimensional model for the simulation of pulsed low-temperature plasmas

A 3-dimensional particle-in-cell/Monte Carlo collision simulation that is fully implemented on a graphics processing unit (GPU) is described and used to determine low-temperature plasma characteristics at high reduced electric field, E/n, in nitrogen gas. Details of implementation on the GPU using the NVIDIA Compute Unified Device Architecture framework are discussed with respect to efficient code execution. The software is capable of tracking around 10 × 106 particles with dynamic weighting and a total mesh size larger than 108 cells. Verification of the simulation is performed by comparing the electron energy distribution function and plasma transport parameters to known Boltzmann Equation (BE) solvers. Under the assumption of a uniform electric field and neglecting the build-up of positive ion space charge, the simulation agrees well with the BE solvers. The model is utilized to calculate plasma characteristics of a pulsed, parallel plate discharge. A photoionization model provides the simulation with additional electrons after the initial seeded electron density has drifted towards the anode. Comparison of the performance benefits between the GPU-implementation versus a CPU-implementation is considered, and a speed-up factor of 13 for a 3D relaxation Poisson solver is obtained. Furthermore, a factor 60 speed-up is realized for parallelization of the electron processes.

A leaky membrane and a sodium transporter at life's great divergence.

A leaky membrane and a sodium transporter at life's great divergence.

Mercaptobenzoic acid-palladium(0) complexes as active catalysts for S-benzylation with benzylic alcohols via (η3-benzyl)palladium(ii) cations in water

Mercaptobenzoic acid-palladium(0) complexes show high catalytic activity for S-benzylation with benzylic alcohols via the (η(3)-benzyl)palladium(II) cation in water. Notably, these palladium(0) complexes could play an important role in formation of active (η(3)-benzyl)palladium(II) cation complexes followed by S-benzylation. Hammett studies on the rate constants of S-benzylation by various substituted alcohols show good correlation between log(kX/kH) and the σ(+) value of the respective substituents. From the slope, negative ρ values are obtained, suggesting that there is a build-up of positive charge in the transition state. Water plays an important role in the catalytic system for sp(3) C-O bond activation and stabilization of the activated Pd(II) cation species. The catalytic system can be performed using only 2.5 mol% Pd2(dba)3 without the phosphine ligand or other additives.

Enhanced Charge Collection by Single Ion Strike in AlGaN/GaN HEMTs

Transient currents in AlGaN/GaN High Electron Mobility Transistors (HEMTs) induced by focused ion beams are measured. Enhanced charge collection occurs in the on- and pinch-off states of AlGaN/GaN HEMTs. The first-order mechanism of enhancement is associated with bipolar and back-channel effects. In addition, we suggest that the temporal positive charge buildup contributes to the enhanced charge collection. The effect of displacement damage on the enhanced charge collection is also discussed.

Evaluation of test structures for the novel n+-in-p pixel and strip sensors for very high radiation environments

Radiation-tolerant n+-in-p silicon sensors were developed for use in very high radiation environments. Novel n+-in-p silicon strip and pixel sensors and test structures were fabricated, tested and evaluated, in order to understand the designs implemented. The resistance between the n+ implants (interstrip resistance), the electric potential of the p-stop, and the punch-through-protection (PTP) onset voltage were measured before and as a function of fluence after irradiation. The technology computer-aided design (TCAD) simulations were used to understand the radiation damage and fluence dependence of the structures. The decrease in the interstrip resistance is a consequence of increased leakage current. The decrease in the electric potential of the p-stop results from a build-up of positive charge in the silicon–silicon oxide interface. The decrease and subsequent increase in the PTP onset voltages results from the interface charge build-up and an increase in acceptor states.

Total fluence influence on the detected magnitude of neutron burst using proportional detectors

The measurement of very short neutron bursts, when individual neutrons cannot be counted in the usual manner, is possible with proportional detectors (such as 3 He) taking the integration of the total electric charge due to many overlapped interactions, as the measure of the amount of the neutron signal. This method requires a correction related to the total amount of neutrons that interacted with the detector. This correction originates in the well-known build-up of positive electric charge too slow to be dislodged from the detection volume during the neutron burst. This causes self-shielding of the applied electric field with the ensuing reduction of the charge multiplication process in the gas, described in the literature. Short neutron bursts from a plasma focus device and a conventional isotopic neutron source were employed in the experimental phase and the known theory was applied in the analysis, which justifies assigning the observed effects to the space-charge shielding of the externally applied electric field. This work introduces a correction to the neutron yield derived from the registered electric charge, through a model of collected charge reduction as a function of total neutrons measured.

Cation–π induced aggregation of water-soluble [PtII(diimine)(Ln-S,O)]+complexes studied by1H DOSY NMR and TEM: from ‘dimer aggregates’ in acetonitrile to nano-aggregates (‘metallogels’) in water

(1)H NMR chemical shift concentration dependence as well as the diffusion coefficients from DOSY NMR of mixed ligand [Pt(II)(1,10-phenanthroline)(N-pyrrolidyl-N-(2,2-dimethylpropanoyl)thiourea)]Cl ([Pt(II)(phen)(L(1)-S,O)]Cl) dissolved in mixtures of acetonitrile-water in the range 0-30% (v/v) D(2)O-CD(3)CN shows that the complex cation (M(+) = [Pt(II)(phen)(L(1)-S,O)](+)) aggregates to form dimers, 2M(+) ⇌ {M(+)}(2), with association constants ranging from K(D)(CD(3)CN) = 17 ± 2 M(-1) to K(D)(30% (v/v) D(2)O-CD(3)CN) = 71 ± 8 M(-1) at 299.3 K, presumably via non-covalent cation-π interactions. Experimental data are consistent with an 'offset' face-to-face cation-π stacking arrangement of the planar cation. However in water-rich solvent mixtures from >30% (v/v) D(2)O-CD(3)CN to pure D(2)O, the extent of aggregation significantly increases until a critical aggregation concentration (CAC) is reached, estimated to be 9.6 and 10.3 mM from (1)H NMR chemical shift concentration dependence and DOSY NMR measurements respectively. Above the CAC the formation of nano-structures formulated as {[Pt(II)(phen)(L(1)-S,O)](+)}(n)Cl(-)(y) (n, y > 2) is indicated. DOSY studies show a significant decrease of the average diffusion coefficient D(obs) as a function of increasing concentration of [Pt(II)(phen)(L(1)-S,O)]Cl in D(2)O. The aggregation number (N) estimated from hydrodynamic volumes of the mononuclear [Pt(II)(phen)(L(1)-S,O)](+) cation (V(H)(0)), and those V(H) estimated from D(obs) (N = V(H)/V(H)(0)) as a function of total complex concentration, ranges from ~2 to ~735 in pure D(2)O. Above the CAC well defined nano-structures which may be loosely termed "metallogels" could be characterized by means of transmission electron microscopy. As expected the addition of NaCl appears to increase the extent of aggregate formation, by presumably stabilizing the formation of nano-sized {[Pt(II)(phen)(L(1)-S,O)](+)}(n)Cl(-)(y) aggregates preventing excessive positive electrostatic charge build-up.

Atomistic simulation of doping effects on growth and charge transport in Si/Ag interfaces in high-performance solar cells

We present the results of a first-principles atomistic simulation study of the effects of phosphorus doping on the silver/silicon interface as found in high-performance solar cells. Calculating the interfacial stabilities of the (110)/(110) and (111)/(111) interfaces we demonstrate how the presence of phosphorus increases the nucleation rate of silver crystallites and how the relative stabilities of the interfaces depend on the doping. We then calculate the electronic structure of the interfaces, demonstrating how the presence of phosphorus leads to a buildup of positive charge in the silicon and an opposite negative charge in the silver. Finally we show how this charge buildup significantly affects the $n$-type Schottky barriers at the interfaces, in both cases lowering the Schottky barrier by more than 100 meV.

Normal-incidence Electron Gun alignment method for negative ion analysis on insulators by magnetic sector SIMS

On Cameca magnetic sector SIMS (Secondary Ion Mass Spectrometer) instruments, detection of negative secondary ions during analysis of insulating materials represents one of the more challenging uses of the instrument. This is because the highest sensitivity is achieved using a positive Cesium primary ion beam to sputter a sample held at a negative potential. The combination of positive charge from the primary beam and the acceleration of negative ions from the sample surface results in the build-up of positive charge in the sputtered area. The Normal-incidence Electron Gun (NEG) is used to deliver electrons to neutralize the positive charge build-up. At typical conditions, the electron beam diameter is rather small (∼150μm), so it is critical to align the NEG to overlay the analysis area as closely as possible. A new method has been developed to utilize cathodoluminescence of gallium nitride (GaN) to visually align the electron gun. This approach is shown to result in reproducible analyses of insulating phases, including depth profiling of oxide on semiconductor materials, bulk analyses for hydrogen, and oxygen isotope ratio microanalyses.

Increase of interface and bulk density of states in amorphous-indium-gallium-zinc-oxide thin-film transistors with negative-bias-under-illumination-stress time

The evolution with time of interface trap density and bulk density of states in amorphous-indium-gallium-zinc-oxide thin-film transistors (TFTs), for negative-bias-under-illumination-stress (NBIS), is traced. Based on the combined analysis of TFT current-voltage and capacitance-voltage characteristics, position of Fermi energy, flat band voltage, interface trap density, and gap state density per unit energy are investigated as function of NBIS time and applied gate voltage. These key parameters help to identify the degradation phenomena responsible for the negative threshold voltage shift caused by NBIS. In particular, the interface trap density becomes more positive; from 0.03 × 1011/cm2 to 0.65 × 1011/cm2, while the gap trap density per unit energy also increases after NBIS, supporting defect creation in the bulk and build-up of positive charge at the gate insulator/active-layer interface as the mechanism responsible for NBIS instability.

Defect level distributions and atomic relaxations induced by charge trapping in amorphous silica

We compute the distribution of electronic levels of native defects in amorphous silica from total energy differences of charge-state density functional theory calculations over an ensemble of atomic structures. The predicted distributions reproduce results from trap spectroscopy by charge injection experiments, validating the calculations. Furthermore, our study characterizes the experimentally inaccessible contributions of individual defect types to the overall distribution. Computed electron and hole trapping levels provide insight into the positive charge buildup in bulk silica observed in negative-bias-temperature-instability, an important degradation mechanism of metal-oxide-semiconductor devices.

Characteristics of Hot Hole Injection, Trapping, and Detrapping in Gate Oxide of Polycrystalline Silicon Thin-Film Transistors

The characterization and modeling of the hysteresis phenomenon observed in the off-state regime of n-channel polycrystalline thin-film transistors are presented. The shift in Id–Vg characteristics between upward and downward scan were measured under various bias conditions and ambient temperatures. The localized positive charge build-up induced by band-to-band hot hole injection is responsible for the mechanism for reducing the off-leakage current, and it is recovered through the thermal emission of trapped holes, which is enhanced by self-heating of the device during the on-current conduction. On the basis of this model, the internal device characteristics related to the thermal and oxide trap properties are analyzed.

Behavior of Ta2O5-Si Capacitors with Different Gate Electrode under Constant Current Stress

The electrical characteristics of MOS capacitors with Ta2O5 as oxide and with different gate electrodes(Al, Au, W, TiN) are investigated using high frequency capacitance-voltage (C-V) and current–voltage (I-V)measurements. The influence of deposition techniques of gate electrode (reactive sputtering and evaporation)and the type of electrode material (different work functions) are observed. Charged trapping properties werestudied by measuring the gate voltage shift due to trapped charge generation in order to investigate theresponse to constant current stress (CCS) under various current/time conditions at room temperature. Theresults showed that the presence of pre-existing electron traps leads the changes of time dependent voltageduring gate injected CCS in the initial stage, followed by the slow positive charge build-up, same for all thestructures and characteristic for this oxide. Gate-induced defects due to ther rate of reaction of gate electrodewith the oxide are responsible for different behavior of the structures with different gate electrodes, observedin the initial stage of CCS. The Au-gated devices appear to be the most susceptible to the constant currentstress degradation. They also have highest values of capacitance and lowest leakage currents. Key Words: Gate electrode; Dielectric properties; Oxides; Constant current stress

Edge Effects in Self-Heating-Related Instabilities in p-Channel Polycrystalline-Silicon Thin-Film Transistors

Self-heating-related instabilities have been investigated in p-channel polycrystalline-silicon thin-fllm transistor, showing an anomalous transconductance (gm) increase. The gm increase is a fingerprint of edge effects, resulting from a buildup of positive trapped charge in the gate oxide at the channel edges. This was confirmed by the annihilation of such positive charges obtained by sequential hot-carrier bias-stress experiments. From the analysis of the edge effects in devices with different channel lengths, we were able, using 2-D numerical simulations, to deter mine the size of the defected edge regions to be 400 nm.

Self-neutralized ion beam

A vacuum arc ion source provides high current beams of metal ions that have been used both for accelerator injection and for ion implantation, and in both of these applications the degree of space charge neutralization of the beam is important. In accelerator injection application, the beam from the ion source may be accelerated further (post-acceleration), redirected by a bending magnet(s), or focused with magnetic or electrostatic lenses, and knowledge of the beam space charge is needed for optimal design of the optical elements. In ion implantation application, any build-up of positive charge in the insulating targets must be compensated by a simultaneous flux of cold electrons so as to provide overall charge neutrality of the target. We show that in line-of-sight ion implantation using a vacuum arc ion source, the high current ion beam carries along its own background sea of cold electrons, and this copious source of electrons provides a “self-neutralizing” feature to the beam. Here we describe experiments carried out in order to demonstrate this effect, and we provide an analysis showing that the beam is space-charge-neutralized to a very high degree

ChemInform Abstract: Gold(I)‐Catalyzed Enantioselective Intermolecular Hydroarylation of Allenes with Indoles and Reaction Mechanism by Density Functional Theory Calculations.

Chiral binuclear gold(I) phosphine complexes catalyze enantioselective intermolecular hydroarylation of allenes with indoles in high product yields (up to 90%) and with moderate enantioselectivities (up to 63% ee). Among the gold(I) complexes examined, better ee values were obtained with binuclear gold(I) complexes, which displayed intramolecular Au(I)-Au(I) interactions. The binuclear gold(I) complex 4c [(AuCl)(2)(L3)] with chiral biaryl phosphine ligand (S)-(-)-MeO-biphep (L3) is the most efficient catalyst and gives the best ee value of up to 63%. Substituents on the allene reactants have a slight effect on the enantioselectivity of the reaction. Electron-withdrawing groups on the indole substrates decrease the enantioselectivity of the reaction. The relative reaction rates of the hydroarylation of 4-X-substituted 1,3-diarylallenes with N-methylindole in the presence of catalyst 4c [(AuCl)(2)(L3)]/AgOTf [L3 = (S)-(-)-MeO-biphep], determined through competition experiments, correlate (r(2) = 0.996) with the substituent constants σ. The slope value is -2.30, revealing both the build-up of positive charge at the allene and electrophilic nature of the reactive Au(I) species. Two plausible reaction pathways were investigated by density functional theory calculations, one pathway involving intermolecular nucleophilic addition of free indole to aurated allene intermediate and another pathway involving intramolecular nucleophilic addition of aurated indole to allene via diaurated intermediate E2. Calculated results revealed that the reaction likely proceeds via the first pathway with a lower activation energy. The role of Au(I)-Au(I) interactions in affecting the enantioselectivity is discussed.

Gold(I)‐Catalyzed Enantioselective Intermolecular Hydroarylation of Allenes with Indoles and Reaction Mechanism by Density Functional Theory Calculations

Chiral binuclear gold(I) phosphine complexes catalyze enantioselective intermolecular hydroarylation of allenes with indoles in high product yields (up to 90%) and with moderate enantioselectivities (up to 63% ee). Among the gold(I) complexes examined, better ee values were obtained with binuclear gold(I) complexes, which displayed intramolecular Au(I)-Au(I) interactions. The binuclear gold(I) complex 4c [(AuCl)(2)(L3)] with chiral biaryl phosphine ligand (S)-(-)-MeO-biphep (L3) is the most efficient catalyst and gives the best ee value of up to 63%. Substituents on the allene reactants have a slight effect on the enantioselectivity of the reaction. Electron-withdrawing groups on the indole substrates decrease the enantioselectivity of the reaction. The relative reaction rates of the hydroarylation of 4-X-substituted 1,3-diarylallenes with N-methylindole in the presence of catalyst 4c [(AuCl)(2)(L3)]/AgOTf [L3 = (S)-(-)-MeO-biphep], determined through competition experiments, correlate (r(2) = 0.996) with the substituent constants σ. The slope value is -2.30, revealing both the build-up of positive charge at the allene and electrophilic nature of the reactive Au(I) species. Two plausible reaction pathways were investigated by density functional theory calculations, one pathway involving intermolecular nucleophilic addition of free indole to aurated allene intermediate and another pathway involving intramolecular nucleophilic addition of aurated indole to allene via diaurated intermediate E2. Calculated results revealed that the reaction likely proceeds via the first pathway with a lower activation energy. The role of Au(I)-Au(I) interactions in affecting the enantioselectivity is discussed.

ChemInform Abstract: Imination of Sulfides and Sulfoxides with Sulfonylimino‐λ3‐bromane under Mild, Metal‐Free Conditions.

Exposure of sulfides and sulfoxides to trifluoromethanesulfonylimino(aryl)-lambda(3)-bromane in dichloromethane at 0 degrees C results in a facile transfer of the sulfonylimino group to sulfur atoms and affords N-triflylsulfilimines and -sulfoximines in high yields under transition-metal-free conditions. Imination of (R)-methyl p-tolyl sulfoxide proceeded with predominant retention of configuration at the stereogenic sulfur center. The Hammett plot afforded rho values of -0.58 for para-substituted thioanisoles and -0.49 for their equivalent sulfoxides, which suggests a buildup of positive charge on the sulfur atoms of sulfides and sulfoxides in the transition state. Calculations suggest a bimolecular nucleophilic-substitution mechanism on the negatively charged nitrogen atom of the sulfonylimino-lambda(3)-bromane, which involves the attack of a sulfide from the opposite side to bromine(III).