Major Electron Research Articles

On closed-shell interactions between heavy main-group elements.

A series of di- and polymetal complexes involving closed-shell, heavy main-group atoms and ions shows a selection of special physical properties. These involve short metal-metal contacts, low entropies of formation and, most interestingly, strong Raman bands at low wavenumbers. These results together with the constitution of the coordination compounds, where the majority of electrons are assembled on the highly polarizable metal atoms and ions, experimental results have been interpreted in terms of direct, partial covalent metal-metal bonding. Previous theoretical studies have challenged this view and instead attributed the obvious attractive forces involved to secondary-type of interactions, such as dispersion. This study utilizes a multitude of theoretical tools, such as natural bond order (NBO) and natural energy decomposition analysis (NEDA), non-covalent interaction (NCI) analysis, electron localization functions (ELFs), and atoms-in-molecules (AIM) to characterize the interactions in models comprising closed-shell dimers, as well as experimentally studied ring and cage systems constituting the main reason for the hypotheses on metal-metal interactions. The results show that all experimental results can be attributed to the covalent interactions between the electron-rich, metal centers and the bridging anions in ring and cage coordination compounds of high symmetry, where the experimentally observed effects can be traced to the combination of covalent interactions between the metal centers and the anions along the edges of the ring and cage systems in combination with the cooperative effects generated by the high symmetry of these ring and cage systems.

Investigation on the Behavior of Electrons in the Bullet as Well as Its Dependence on Applied Voltage and H2O Concentration in the Atmospheric-Pressure Ar/H2O Plasma Jets

This article presents a systematic investigation on the behavior of electrons in the bullet in the Ar/H2O plasma jet in increasing the applied voltage and water vapor concentration based on a needle-plate discharge configuration and a 1-D particle-in-cell Monte Carlo method. The scattering angle of the electron in the bullet is described using the angle between the electron scattering direction and the bullet propagation direction. The following results are obtained. The evolution characteristic of the angle spectrum of electrons in the bullet in increasing the applied voltage is the same as that in increasing water vapor concentration, namely, the electrons scattered toward the target get increasingly less. When increasing the applied voltage, the OH density, the electron density, and the average electron energy $E_{\mathrm {ave}}$ increase. However, the increase in water vapor concentration leads to an evidently increasing OH density and a decrease in both the electron density and $E_{\mathrm {ave}}$ . In the range of electron scattering angles below 90° in the bullet, the percentage of electrons with energy above 3.27 eV (the threshold of inducing dissociative electron attachment to water molecule) increases with increasing applied voltage but presents a slight decrease in increasing water vapor concentration. The majority of electrons to most probably interact with the target are of energies higher than 3.27 eV. Finally, a notable effect of H2O admixture is that it gives rise to the decrease in not only $E_{\mathrm {ave}}$ but also the electron density in the bullet, especially the latter decreases evidently.

Impact ionization and intervalley electron scattering in InSb and InAs induced by a single terahertz pulse

Electronic properties of InSb and InAs are sensitive to electric field due to their narrow forbidden energy gaps and big difference in effective masses of electrons in different conduction band valleys. Here we report impact ionization processes and redistribution of electrons between the Γ, L and X valleys induced by a single ultrashort terahertz (THz) pulse at 80 K temperature. Monte Carlo simulation revealed that electron motion in this case has near ballistic character. The threshold electric field of impact ionization increases as the THz pulse gets shorter, and the process of impact ionization essentially raises cooling rate of hot electrons. The L valley gets mainly occupied by electrons in InSb while the X valley holds the majority of electrons in InAs at strong electric fields, respectively above 20 kV/cm and 90 kV/cm. The calculated results are in good agreement with the available experimental data.

Influence of operating parameters on high‐pressure microhollow cathode discharge with a cylindrical hole

AbstractAs a promising candidate for generating large‐scale plasma at high pressure, microhollow cathode discharge (MHCD) with a cylindrical hole has been numerically simulated at an argon pressure higher than 50 Torr by a two‐dimensional particle‐in‐cell Monte Carlo collision method. Results indicate that MHCD is operated in a confined phase at the beginning, which transits to an expanded phase and reaches a steady‐state at about 160 ns. In a steady‐state MHCD, the majority of electrons have a low energy of several eV, and some electrons have a high energy up to 480 eV. High‐energy electrons mainly appear adjacent to the cathode surface. With increasing applied voltage, there is a transition from the confined phase to the expanded phase for a steady‐state MHCD. Moreover, maximal density of charged particles increases for the expanded phase with increasing applied voltage and argon pressure or decreasing hole diameter. Besides, discharge morphology has been investigated in detail with varying operating parameters. All these results have been compared with those simulated by the fluid model.

Parasitic transport paths in two-well scattering-assisted terahertz quantum cascade lasers

Using nonequilibrium Green’s functions, possible parasitic paths are identified in two-well scattering-assisted terahertz quantum cascade lasers operating at 3.5 THz. The majority of electrons in the upper laser state can escape through these paths, causing a 66% loss of population inversion at 50 K. Three types of paths are clarified: one is responsible for non-selective injection via LO-phonon scattering, the other two lead to leakages via high-lying states to downstream periods by sequential tunneling. Finally, several ways of suppressing these paths are suggested by having small oscillator strength (<0.3), or employing asymmetric structure.

Anomalous Trapping of Low Pitch Angle Electrons by Coherent Whistler Mode Waves

AbstractChorus emissions composed of coherent whistler mode waves are responsible for pitch angle scattering of energetic electrons. This scattering is closely related to energetic electron precipitation into the atmosphere, contributing to pulsating auroras. Conventionally, energetic electrons are considered to satisfy the cyclotron resonance condition over the energy range of a few to tens of kiloelectron volts and are scattered toward the loss cone by waves. However, previous simulation studies indicate that low pitch angle electrons tend to be scattered away from the loss cone by coherent whistler mode waves. We examine the mechanism of anomalous trapping at low pitch angles, deriving a particle equation with low pitch angle assumptions. An additional term that is conventionally neglected represents the Lorentz force caused by the wave magnetic field and the parallel particle velocity. Therefore, due to the large v‖×Bw Lorentz force, low pitch angle electrons satisfying the cyclotron resonant condition are scattered away from the loss cone and effectively trapped by waves. We perform test particle simulations in a one‐dimensional dipole magnetic field with a whistler mode wave model and reproduce the anomalous trapping of electrons. The simulation results show that the majority of electrons at high and moderate pitch angles are scattered toward low pitch angle regions while low pitch angle electrons are strongly scattered toward high pitch angle regions. Consequently, a coherent chorus element produces a bump in the electron pitch angle distribution.

First-principles comparative study of UN and Zr corrosion

We studied surface corrosion effects on Zr and UN using first-principles density functional theory-based calculations. We focused on the energetics of Zr (1000), UN (100) and UN (110) surfaces, exposed to water and oxygen. Average distance between the terminating UN (100) surface and bulk increases due to the presence of additional oxygen content, as well as for the (110) surface. The average distance between the surface layer and bulk is greater in the (110) surface than the (100) surface after water adsorption. Oxygen concentration determines whether H2 or oxynitrde is formed on the (110) surface. Local density of states and partial charge density show the bonding between the UN surfaces and adsorbates. From an electronic energy of −2 eV to the Fermi level, the majority of electrons are found to be localized around U atoms. Electron localization function calculations further reveal the corrosion mechanism details.

Exciton dynamics in cation-exchanged CdSe/PbSe nanorods: The role of defects

Cation exchange occurs via defect initiated solid-state diffusion, a process that can lead to defect formations. The effect of such inherent defect formation on carrier dynamics of cation-exchanged heterostructures remains poorly understood. Herein, we report exciton dynamics in type II CdSe/PbSe heterostructure nanorods formed via cation exchange. The majority of electrons in CdSe domains decays in 5ps due to ultrafast carrier trapping. The defect generated by cation exchange can be healed by annealing the as-synthesized CdSe/PbSe heterostructure nanorods. This study suggests a strategy for improving properties of heteronanostructures prepared by cation exchange for applications in photovoltaics and photocatalysis.

Review: can diet influence the selective advantage of mitochondrial DNA haplotypes?

This review explores the potential for changes in dietary macronutrients to differentially influence mitochondrial bioenergetics and thereby the frequency of mtDNA haplotypes in natural populations. Such dietary modification may be seasonal or result from biogeographic or demographic shifts. Mechanistically, mtDNA haplotypes may influence the activity of the electron transport system (ETS), retrograde signalling to the nuclear genome and affect epigenetic modifications. Thus, differential provisioning by macronutrients may lead to selection through changes in the levels of ATP production, modulation of metabolites (including AMP, reactive oxygen species (ROS) and the NAD+/NADH ratio) and potentially complex epigenetic effects. The exquisite complexity of dietary influence on haplotype frequency is further illustrated by the fact that macronutrients may differentially influence the selective advantage of specific mutations in different life-history stages. In Drosophila, complex I mutations may affect larval growth because dietary nutrients are fed through this complex in immaturity. In contrast, the majority of electrons are provided to complex III in adult flies. We conclude the review with a case study that considers specific interactions between diet and complex I of the ETS. Complex I is the first enzyme of the mitochondrial ETS and co-ordinates in the oxidation of NADH and transfer of electrons to ubiquinone. Although the supposition that mtDNA variants may be selected upon by dietary macronutrients could be intuitively consistent to some and counter intuitive to others, it must face a multitude of scientific hurdles before it can be recognized.

Computational Analysis and Characterization of Cockpit Environmental Control System of a Fighter Aircraft in Humid Environment

he Environmental Control System (ECS) is an important part of any fighter aircraft and has far reaching repercussions in terms of its capability to operate as an effective weapon system under adverse weatherconditions. The aircrew and majority of electronics depend on ECS to keep temperature, pressure and humidity levels within acceptable limits for efficient operation. These limits are explicitly defined in relevant MIL standards MIL-E-18927E(AS)[1]. In order to thoroughly analyze the ECS of under study aircraft[2, 3], Computational Fluid Dynamics (CFD) analysis techniques have been used. CFD techniques offer great flexibility since various conditions can be simulated and analyzed results can be used for required improvements in the system.

Below-Band-Gap Ionization of Liquid-to-Supercritical Ammonia: Geminate Recombination via Proton-Coupled Back Electron Transfer

Femtosecond multiphoton ionization experiments have been conducted on ammonia over a wide range of temperature (225 K ≤ T ≤ 490 K) and density (0.18 g/cm(3) ≤ ρ ≤ 0.7 g/cm(3)), thereby covering the liquid and supercritical phases. The experiments were carried out with excitation pulses having a wavelength of 400 nm, and the ionization was found to involve two photons. Therefore, the total ionization energy in this study corresponds to 6.2 eV, which is roughly 2 eV below the valence-to-conduction band gap of the fluid. The ionization generates solvated electrons, which have been detected through their characteristic near-infrared resonance, and must be facilitated through a coupling to nuclear degrees of freedom of the liquid. The recombination of the solvated electron with the geminate fragments was found to obey predominantly single-exponential kinetics with time constants between 500 fs and 1 ps. Only a very minor fraction of the photogenerated electrons is able to escape from the geminate recombination. The results indicate that the majority of electrons are injected into suitable trapping sites located between the first and second solvation shells of the initially ionized ammonia molecules. Such configurations can be considered as instantly reactive and facilitate an ultrafast barrierless electron annihilation. This process is found to exhibit a pronounced kinetic isotope effect, which indicates that the electronic decay is accompanied by the transfer of a proton. The sequence of ionization and recombination events can therefore be described appropriately as a proton-coupled electron transfer (PCET) followed by a proton-coupled back electron transfer (PCBET).

Deposited low temperature silicon GHz modulator

We demonstrate gigahertz electro-optic modulator fabricated on low temperature polysilicon using excimer laser annealing technique compatible with CMOS backend integration. Carrier injection modulation at 3 Gbps is achieved. These results open up an array of possibilities for silicon photonics including photonics on DRAM and on flexible substrates.

Bimolecular Auger Recombination of Electron–Hole Pairs in Two-Dimensional CdSe and CdSe/CdZnS Core/Shell Nanoplatelets

We have determined the Auger recombination kinetics of electrons and holes in colloidal CdSe-only and CdSe/CdS/ZnS core/shell nanoplatelets by time-resolved photoluminescence measurements. Excitation densities as high as an average of 18 electron–hole pairs per nanoplatelet were reached. Auger recombination can be described by second-order kinetics. From this we infer that the majority of electrons and holes are bound in the form of neutral excitons, while the fraction of free charges is much smaller. The biexciton Auger recombination rate in nanoplatelets is more than 1 order of magnitude smaller than for quantum dots and nanorods of equal volume. The latter is of advantage for application in lasers, light-emitting diodes, and photovoltaics.

Spin dependent 2D electron scattering by nanomagnets

The 2D scattering problem of an electron by a magnetized nanoparticle is solved in the Born approximation with account of the dipole–dipole interaction of the magnetic moments of electron and nanomagnet. The scattering amplitudes in this problem are the two-component spinors. They are obtained as functions of the electron spin orientation, the electron energy and show anisotropy in scattering angle. The initially polarized beam of electrons scattered by the nanomagnet consists of electrons with no spin flipped and spin flipped. The majority of electrons with no spin flipped are scattered by small angles. The majority electrons with spin flipped are scattered in the vicinity of the scattering angles π / 2 and 3 π / 2 . This can be used as one more method of controlling the spin currents.

A Comparison of Different Methods To Determine the Electron Diffusion Length in Dye-Sensitized Solar Cells

A new steady-state method for determination of the electron diffusion length in dye-sensitized solar cells (DSCs) is described and illustrated with data obtained using cells containing three different types of electrolyte. The method is based on using near-IR absorbance methods to establish pairs of illumination intensity for which the total number of trapped electrons is the same at open circuit (where all electrons are lost by interfacial electron transfer) as at short circuit (where the majority of electrons are collected at the contact). Electron diffusion length values obtained by this method are compared with values derived by intensity-modulated methods and by impedance measurements under illumination. The results indicate that the values of electron diffusion length derived from the steady-state measurements are consistently lower than the values obtained by the non-steady-state methods. For all three electrolytes used in the study, the electron diffusion length was sufficiently high to guarantee electron collection efficiencies greater than 90%. Measurement of the trap distributions by near-IR absorption confirmed earlier observations of much higher electron trap densities for electrolytes containing Li+ ions. It is suggested that the electron trap distributions may not be intrinsic properties of the TiO2 nanoparticles but may be associated with electron−ion interactions.

First-principles study on field emission properties of carbon nanocone

The first-principles calculation was performed to study the electron field emission properties of carbon nanocones （CNC） with different cone angles and geometrical structures. The results show that with the increase of the applied electric field （Eadd）, the electronic structure of CNC changes significantly, the density of states （DOS） shifts towards to low energy position. Especially, DOS at Ef increases and the pseudogap decreases respectively, and a majority of electrons congregate at the close end. The investigation of DOS, HOMO/LUMO and Mulliken population analysis indicates that CNC3 and CNC4 are very suited for electron field emission.

Electron energy distribution function and dense-gas ionization in the presence of a strong field

The mechanism for dense-gas ionization is analyzed in the case when the deceleration of electrons by gas can be neglected in the equation of motion of a single electron. An expression for the electron energy distribution function in the presence of a strong field is derived. The characteristic width of the distribution corresponds to the energy acquired by the electron at a length determined by the inverse Townsend coefficient. The electron energy distributions are calculated for various distances form the cathode. It is demonstrated that the distribution becomes independent of the coordinate at a distance from the cathode that is significantly greater than the inverse Townsend coefficient. In this case, the distribution coincides with the distribution obtained with analytical calculations. The absence of the coordinate dependence is realized even in the presence of an extremely strong field when, in accordance with the commonly accepted point of view, the majority of electrons are runaway electrons.

Particle Acceleration and Magnetic Field Generation in Electron‐Positron Relativistic Shocks

Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel, and other two-stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a three-dimensional relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic electron-positron jet front propagating into an ambient electron-positron plasma with and without initial magnetic fields. We find small differences in the results for no ambient and modest ambient magnetic fields. New simulations show that the Weibel instability created in the collisionless shock front accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. Furthermore, the nonlinear fluctuation amplitudes of densities, currents, and electric and magnetic fields in the electron-positron shock are larger than those found in the electron-ion shock studied in a previous paper at a comparable simulation time. This comes from the fact that both electrons and positrons contribute to generation of the Weibel instability. In addition, we have performed simulations with different electron skin depths. We find that growth times scale inversely with the plasma frequency, and the sizes of structures created by tine Weibel instability scale proportionally to the electron skin depth. This is the expected result and indicates that the simulations have sufficient grid resolution. While some Fermi acceleration may occur at the jet front, the majority of electron and positron acceleration takes place behind the jet front and cannot be characterized as Fermi acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying nonuniform, small-scale magnetic fields, which contribute to the electron s (positron s) transverse deflection behind the jet head. This small- scale magnetic field structure is appropriate to the generation of radiation from deflected electrons (positrons) as opposed to synchrotron radiation. The jitter radiation has different properties than synchrotron radiation calculated assuming a uniform magnetic field. The jitter radiation resulting from small-scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks.

Band Structure Effects on the Transient Electron Velocity Overshoot in GaN

Time-resolved electroabsorption measurements on an AlGaN/GaN heterojunction p-i-n photodiode have been used to study the transient electron velocity overshoot for transport in the c-direction in wurtzite GaN. The velocity overshoot increases with electric field up to ∼320 kV/cm, at which field a peak velocity of 7.25 x 10 7 cm/s is attained within the first 200 fs after photoexcitation. However, theoretical Monte Carlo calculations incorporating a GaN full-zone band structure show that the majority of electrons do not attain sufficient energy to effect intervalley transfer until they are subjected to higher fields (> 325 kV/cm). Insight into this behavior can be gleaned from the band nonparabolicity deduced from the constant energy surfaces in the Γ valley, which shows that the effective mass in the c-direction can be viewed as becoming larger at high k values.

Threshold voltage instability of a static RAM under 77-K operation

Threshold voltage instability of MOSFETs in an n-MOS static RAM under accelerated operating test conditions at 77 K has been studied. It is found that under high bias operations (supply voltage ≥ 7 V) at 77 K, a threshold voltage shift occurs, due to the presence of negative and positive charges in the gate oxide. The negative charge is caused by trapping the injected electrons in the pre-existing electron traps. The majority of electrons are thermally re-emitted, as the samples are warmed to room temperature. On the other hand, the presence of positive charges is a dominant factor in the threshold voltage shift under these test conditions. A simple quantitative model to explain the mechanism of positive charge generation responsible for the negative threshold-voltage shift is proposed. Some samples show that, from the analysis, the amount of saturated negative shift in threshold voltage can be estimated to be 100 mV at most.