Change In Electron Concentration Research Articles

Time-Frequency Characteristics of Ionospheric Clutter in High Frequency Surface Wave Radar during Typhoon Muifa

The ionospheric clutter in High Frequency Surface Wave Radar (HFSWR) is the reflection of electromagnetic waves from the ionosphere back to the receiver, which should be suppressed as much as possible for the primary purpose of target detection in HFSWR. However, ionospheric clutter contains vast quantities of ionospheric state information. By studying ionospheric clutter, some of the relevant ionospheric parameters can be inferred, especially during the period of typhoons, when the ionospheric state changes drastically affected by typhoon-excited gravity waves, and utilizing the time-frequency characteristics of ionospheric clutter at typhoon time, information such as the trend of electron concentration changes in the ionosphere and the direction of the typhoon can be obtained. The results of the processing of the radar data showed the effectiveness of this method.

Highly tunable negative permittivity of carbon nanofiber/alumina metacomposites at different external temperatures

Effective and precise modulation of negative permittivity behavior still deserved further research. Herewith, alumina (Al2O3) metacomposites with various carbon nanofiber (CNF) contents were fabricated using hot-press sintering, and the effects of CNF content and external temperature on the electrical properties of metacomposites were investigated. As the CNF content increased, a three-dimensional interconnected CNF network was formed in the composites, resulting in the conversion of the impedance characteristic and the conductivity mechanism. The composites with CNF content of 2 wt% exhibited negative permittivity behavior, owing to the low-frequency plasma state of free electrons in the CNF conducting networks, which could be well simulated by the Drude model. Excellent electromagnetic shielding performance was observed in the composites with high CNF contents at Ku band, and shielding effectiveness reached more than 30 dB with a reflection-dominated mechanism. Furthermore, the temperature-dependences of the electrical properties of the composites were explored. Both the negative permittivity and conductivity of the composites increased with external temperature rising, which were mainly attributed to the changes in free electron concentration and mobility. It was worth noting that the values of negative permittivity varied very little, which provided an effective method for precise regulation of negative permittivity. This work not only enriched the adjustment method of negative permittivity but also helped to expand the application field of metacomposites.

Densification kinetics and microstructural evolution of binder jet printed and sintered porous Ni-Mn-Ga magnetic shape-memory alloys

Binder Jet Printing (BJP) of polycrystalline magnetic shape-memory alloys (MSMAs) allows to create complex geometries from pre-alloyed powders, while avoiding elemental evaporation and internal stresses. However, to increase functionality, polycrystalline MSMAs require high porosity and large grains to decrease internal constraints to twin boundary motion. This study focuses on the microstructural evolution of BJPed porous Ni-Mn-Ga during isothermal sintering to understand the densification mechanisms taking place at different temperatures (1070, 1080 and 1090 ℃) over time (0–8 h) in order to tailor the microstructure to regimes of large, interconnected porosity and large grains. Stereology was used to determine the sintering mechanisms as defined by Coble's model for intermediate stage sintering. Densification and grain size increased over time for all temperatures at different rates, while pore size decreased to a minimum near 1 h sintering. Sintering at 1070 ℃ resulted in an interplay of densifying and coarsening mechanisms, while at 1080 ℃ the dominating mechanism was volume diffusion and at 1090 ℃ grain boundary diffusion. No changes in electron concentration occurred during sintering, and thus, transformation temperatures and magnetization remained nearly constant. Transformation enthalpy and Gibbs free energy showed an inverse behavior to pore size, indicating larger pores reduce transformation energy.

Recovery and Delay of Impurity-Induced Poisoning of Oxygen Exchange Kinetics in Mixed Conducting Oxides Via Controlled Surface Acidity

The oxygen exchange kinetics in mixed conducting oxide surfaces are critical to a variety of energy-related technologies, including solid oxide fuel/electrolyser cells (SOFCs/SOECs), permeation membranes, batteries, and sensors. Such devices, however, commonly suffer from performance degradation from impurity-poisoning during long-term operation, for example, chromia poisoning in SOFCs from metal interconnects or silica from furnace refractories. The acidity/basicity of binary additives has recently been found to be a sensitive descriptor for determining the oxygen exchange coefficient (kchem ) of mixed ionic/electronic conducting oxides (MIEC), systematically varing by over 6 orders of magnitude via controlled surface infiltration with binary oxides. These huge variations in kchem are attributed to changes in surface electron concentration induced by the additives.Inspired by these ideas, in this work we first demonstrate the ability to recover the oxygen exchange kinetics of MIEC oxides (e.g. Pr0.1Ce0.9O2-δ (PCO) and Sr(Ti,Fe)O3 (STF)) by counteracting acidic infiltrant’s impact on degradation, e.g. by chromia or silica, by compensating with a basic infiltrant like lithia or calcia. Further, we further demonstrate the ability to enhance impurity tolerance of MIEC oxides by pre-coating basic additives onto the electrodes. Such pre-treatments not only markedly improve the initial kchem of the pristine MIEC oxides, but markedly lower the rate of degradation induced by subsequent coatings with acidic poisons. Electrical conductivity relaxation measurements on porous MIEC specimens were used to examine the impact of infiltrants on kchem . In parallel, the area-specific resistance of screen-printed symmetric cells (cathode|electrolyte|cathode) were examined by electrochemical impedance spectroscopy.

Electroacoustic Effect and Electrical Superconductivity of Metals

The paper discusses the conditions under which superconductivity of metals would be detected at atmospheric pressure and at room temperature. One of the possible directions of research to achieve these conditions is to change the phonon spectrum of metals in order to increase the Debye temperature, and therefore to increase the temperature of the superconducting transition Tc. According to the idea of the work and to the estimates made in it, the maximum frequency of phonons nmax in metals can be increased by the short-term action of an external constant electric field (electric pulse). The duration of the pulse should be (10–5–10–7) s. The voltage of the constant electric field source is U » 1V. A decrease in the crystal lattice parameter should be accompanied by an increase in the maximum phonon frequency nmax and, accordingly, by an increase in the Debye temperature. The characteristic size of the sample, in which the change in electron concentration can be realized, should not exceed the length of electron shielding in metals, i. e. ≈ 10–8 m. The sequential action of a certain number of electric field pulses can maintain a superconducting state in a metal sample for some finite time.

(Invited) Non-Enzymatic Glucose Sensor Fabricated By Ni-Nanowires Decorated Graphene Gated FETs

In this study, an innovative non-invasive glucose sensor made of nickel nanowire-decorated graphene-gated field-effect transistors is demonstrated. Due to the redox reaction between nickel nanowires and glucose molecules, electron exchange occurs when Ni(III) reacts with glucose to form Ni(II) and gluconolactone. Changes in electron concentration are amplified by transistors at the bottom, improving detection sensitivity. In this study, the electrical properties of glucose sensor fabricated from nickel nanowire/graphene/gold surfaces was studied. Since the CVD-grown graphene film has excellent electrical conductivity, the electrons generated by the redox reaction can be quickly and evenly dispersed. The results show that the glucose sensor with graphene film has good current stability, low detection limit and good linear relationship between current and concentration. The demonstrated nickel wire-decorated graphene-gated FET biosensor can be used for the quantification of glucose with a linear range of 10-50 mM and a detection limit of 51nM.

Modeling of Changes in the Resistivity of Semi Insulating Gallium Phosphide under the Influence of Lighting

The article presents the results of a simulation of changes in gallium phosphide (GaP) resistivity under the influence of lighting. The adopted model of the defect structure is presented along with the defect parameters. Initial conditions created on the basis of a tested material sample, labeled GaP-1, made of monocrystals of semi-insulating gallium phosphide (SI GaP), are presented. The simulation methodology and the created model of the kinetics equations are described. As a result of the simulation, the values of the photocurrent and the electron-hole pair generation coefficient G were assigned to data obtained experimentally depending on the carrier lifetime coefficient τ. Changes in resistivity and concentration of electrons and holes in the bands for gallium phosphide with a structure consisting of five defects are presented. The proposed simulation method can be used to calculate switch-on and -off times and photocurrent values for the semiconductor materials used to construct PCSS (photoconductive semiconductor switches) and other electronic devices.

Effect of locally-gradient Ni@NiTe2 inclusions on the Seebeck coefficient of Bi2Te3 + xNi composites

The Bi2Te3 + xNi composites with x = 0.00, 0.50, 0.85, 1.00, 1.25 and 1.50 wt% were prepared by spark plasma sintering method. Owing to high-temperature diffuse redistribution of matrix and filler atoms, filler inclusions in Bi2Te3 matrix are forming as locally-gradient Ni@NiTe2 inclusions. Since electrical conductivity of Ni is much higher as compared to that for Bi2Te3, the composites are electrically-inhomogeneous semiconductors. With increasing x, fraction of NiTe2 shell increases, and fraction of Ni core decreases, although all the composites with different x were sintered at the same temperature. This feature is originated from local overheating of the filler inclusions due to flowing pulse electrical current through the electrically-inhomogeneous medium. Diffusion coefficient of Ni inside Bi2Te3, estimated from analysis of Ni concentration profiles near the Ni@NiTe2 inclusions, enhances with increasing x. By using energy activation of the diffusion, local overheating temperatures of the filler inclusions were extracted. The x-dependent changes in electron concentration of the composites are related to competition of increasing in number of the filler inclusions and decreasing in fraction of Ni cores inside the inclusions. Effect of the filler content on the Seebeck coefficient of the composites was analysed by the Pisarenko relationship, plotted for simple parabolic band model. Magnetic electron scattering by ferromagnetic Ni cores in the Ni@NiTe2 inclusions can contribute to the Seebeck coefficient via relevant changes in scattering factor.

Highly Sensitive Sub-ppm CH3COOH Detection by Improved Assembly of Sn3O4-RGO Nanocomposite.

Detection of sub-ppm acetic acid (CH3COOH) is in demand for environmental gas monitoring. In this article, we propose a CH3COOH gas sensor based on Sn3O4 and reduced graphene oxide (RGO), where the assembly of Sn3O4-RGO nanocomposites is dependent on the synthesis method. Three nanocomposites prepared by three different synthesis methods are investigated. The optimum assembly is by hydrothermal reactions of Sn4+ salts and pre-reduced RGO (designated as RS nanocomposite). Raman spectra verified the fingerprint of RGO in the synthesized RS nanocomposite. The Sn3O4 planes of (111), (210), (130), (13¯2) are observed from the X-ray diffractogram, and its average crystallite size is 3.94 nm. X-ray photoelectron spectroscopy on Sn3d and O1s spectra confirm the stoichiometry of Sn3O4 with Sn:O ratio = 0.76. Sn3O4-RGO-RS exhibits the highest response of 74% and 4% at 2 and 0.3 ppm, respectively. The sensitivity within sub-ppm CH3COOH is 64%/ppm. Its superior sensing performance is owing to the embedded and uniformly wrapped Sn3O4 nanoparticles on RGO sheets. This allows a massive relative change in electron concentration at the Sn3O4-RGO heterojunction during the on/off exposure of CH3COOH. Additionally, the operation is performed at room temperature, possesses good repeatability, and consumes only ~4 µW, and is a step closer to the development of a commercial CH3COOH sensor.

Sensitive and selective CO2 gas sensor based on CuO/ZnO bilayer thin-film architecture

A CuO/ZnO (C/Z) bilayer thin film was fabricated with a porous top CuO layer to facilitate a sensitive and selective response towards CO2 gas. Such a sensor architecture allowed optimum oxygen and CO2 gas adsorption in the interfacial region. The C/Z thin-film sensor exhibited a good response (47%) for 2500 ppm CO2 at 375 °C as opposed to CuO (15%) at 300 °C and ZnO (16%) at 350 °C. The sensor was selective to CO2 in respect of CO and CH4 gases at 375 °C with selectivity factor κCO2 ∼ 5 and ∼ 8 for CO and CH4 respectively. By analyzing the conductance-time transients for the gas, the adsorption behavior of CO2 on the heterogenous C/Z bilayer thin-film sensor was established. CO2 obeyed an extended Freundlich model of adsorption. Theoretical analysis of the said adsorption model was performed through which the activation energy (EA) and heat of adsorption (Q) of CO2 gas were estimated. A complementary relationship between EA and Q was established. It was shown that EA decreases with increasing concentration from 123.95 to 108.36 kJ/mol for 1000–2500 ppm CO2 for energetically heterogeneous surfaces. Alternatively, Q values increase with increasing concentration from 59.73 to 71.65 kJ/mol for 500–2500 ppm CO2. The CO2 sensing mechanism was elucidated based on surface defects for CuO and ZnO. CO2 sensing in the C/Z bilayer thin-film sensor was controlled by the adsorption of oxygen forming a space charge layer at the surface and interface of the p-n heterojunction and by band-bending as a result of the change of electron concentration across the junction.

Ga2O3(Sn) Oxides for High-Temperature Gas Sensors.

Gallium(III) oxide is a promising functional wide-gap semiconductor for high temperature gas sensors of the resistive type. Doping of Ga2O3 with tin improves material conductivity and leads to the complicated influence on phase content, microstructure, adsorption sites, donor centers and, as a result, gas sensor properties. In this work, Ga2O3 and Ga2O3(Sn) samples with tin content of 0–13 at.% prepared by aqueous co-precipitation method were investigated by X-ray diffraction, nitrogen adsorption isotherms, X-ray photoelectron spectroscopy, infrared spectroscopy and probe molecule techniques. The introduction of tin leads to a decrease in the average crystallite size, increase in the temperature of β-Ga2O3 formation. The sensor responses of all Ga2O3(Sn) samples to CO and NH3 have non-monotonous character depending on Sn content due to the following factors: the formation of donor centers and the change of free electron concentration, increase in reactive chemisorbed oxygen ions concentration, formation of metastable Ga2O3 phases and segregation of SnO2 on the surface of Ga2O3(Sn) grains.

Formation of epitaxial InSb films on semi-insulating GaAs(100) by explosive thermal evaporation: their structure and electrical properties

In the present work, the influence of the deposition temperature of InSb films on semi-insulating GaAs(100) on their phase composition, crystal perfection and electrical properties was investigated. The InSb films of various extent of crystal perfection are formed by means of explosive thermal deposition of InSb on semi-insulating GaAs(100) substrates in the temperature range of 375–460 °C. X-ray diffraction analysis established that the films are heteroepitaxial. It is shown that an increase in the deposition temperature of InSb films from 375 to 460 °C leads to a change in the film surface roughness (Ra) from 3.4 to 19.1 nm. The Hall voltage sensitivity to the magnetic field of InSb films varies in the range of 500–1500 mV/T. The electron concentration (n) and mobility (μ) changes in the range of 2 ⋅ 1016 – 6 ⋅ 1016 cm–3, 10 ⋅ 103 – 21 ⋅ 103 cm2/(V ⋅ s). The formed InSb films on semi-insulating GaAs(100) substrate are of practical interest for the manufacture of highly sensitive miniature Hall devices.

Enhanced Carrier-Exciton Interactions in Monolayer MoS2 under Applied Voltages.

Carrier-exciton interactions in two-dimensional transition metal dichalcogenides (TMDs) is one of the crucial elements for limiting the performance of their optoelectronic devices. Here, we have experimentally studied the carrier-exciton interactions in a monolayer MoS2-based two-terminal device. Such two-terminal device without a gate electrode is generally considered as invalid to modulate the carrier concentration in active materials, while the photoluminescence peak exhibits a red shift and decay with increasing applied voltages. Time-resolved photoluminescence spectroscopy and photoluminescence multipeak fittings verify that such changes of photoluminescence peaks result from enhanced carrier-exciton interactions with increasing electron concentration induce the charged exciton increasing. To characterize the level of the carrier-exciton interactions, a quantitative relationship between the Raman shift of out-of-plane mode and changes in electron concentration has been established using the mass action model. This work provides an appropriate supplement for understanding the carrier-exciton interactions in TMD-based two-terminal optoelectronic devices.

Sensitive and Selective Detection of Pb2+ Ions Using 2,5-Dimercapto-1,3,4-Thiadiazole Functionalized AlGaN/GaN High Electron Mobility Transistor

We report sensitive and selective AlGaN/GaN High Electron Mobility Transistor (HEMT)-based sensor for Lead ion (Pb2+) detection. The gate region of the HEMT was functionalized by 2,5-dimercapto-1,3,4-thiadiazole (DMTD). The response of the sensor is observed by monitoring drain to source current ( $\text{I}_{\textsf {DS}}$ ) for different concentrations of Pb2+ ions at a fixed drain to source voltage ( $\text{V}_{\textsf {DS}}$ ). Our sensor reaches the lower detection limit of 0.018 ppb, which is much lower than the standard detection limit recommended by the World Health Organization (WHO) for drinking water. Furthermore, the sensor exhibited a rapid response time of ~4 seconds and high sensitivity of $0.607~\mu \text{A}$ /ppb. Moreover, the selectivity analysis was performed and found that the sensor was highly selective towards Pb2+ ions. The change in electron concentration at 2-dimensional electron gas (2DEG) upon the capture of Pb2+ ions at gate region by DMTD, causes a change in the $\text{I}_{\textsf {DS}}$ , which showed excellent sensing response towards Pb2+ ions. The highly sensitive, selective, and rapid detection of Pb2+ ions paves the way for stable sensing performance based on DMTD functionalized AlGaN/GaN HEMT sensor.

High-density electron doping of SmNiO3 from first principles

Recent experimental work has realized a new insulating state of samarium nickelate (SmNiO$_3$), accessible in a reversible manner via high-density electron doping. To elucidate this behavior, we use the first-principles density functional theory (DFT) + U method to study the effect of added electrons on the crystal and electronic structure of SmNiO$_3$. First, we track the changes in the crystal and electronic structure with added electrons compensated by a uniform positive background charge at concentrations of $\frac{1}{4}$, $\frac{1}{2}$, $\frac{3}{4}$, and 1 electrons per Ni. The change in electron concentration does not rigidly shift the Fermi energy; rather, the added electrons localize on NiO$_6$ octahedra causing an on-site Mott transition and a change in the density of states resulting in a large gap between the occupied and unoccupied Ni $e_g$ orbitals at full doping. This evolution of the density of states is essentially unchanged when the added electrons are introduced by doping with interstitial H or Li ions.

Enhancement of gas sensing by implantation of Sb-ions in SnO2 nanowires

We describe gas sensing improvements of SnO2 nanowire (NW) gas sensors by Sb-ion implantation. SnO2 NWs were grown through a vapor-liquid-solid growth technique and Sb-ion implantation was performed by an ion implanter at different doses of 2 × 1013, 2 × 1014, and 2 × 1015 ion/cm2. The morphology and chemical compositions were confirmed by different characterization techniques. Gas sensing results demonstrated the gas sensing improvement by Sb-implantation, where the lowest dose gas sensor showed the best performance towards tested gases. However, high doses led to a reduced sensing response. We suggested the related sensing mechanism based not only on change of electron concentration by substituting Sn4+ ions to Sb+5 ions, but also on generation of surface defects. This study can open new possibilities to use ion-implantation as promising way to enhance the sensing capability of different gas sensors.

Влияние отжига на свойства слоев Ge : Sb/Si(001) с концентрацией сурьмы выше уровня ее равновесной растворимости в германии

The impact of rapid thermal treatment on electrical and luminescent properties of Ge:Sb/Si(001) layers which have Sb concentration much higher than the equilibrium solubility limit was investigated. Local modifications of such properties throughout the structure’s depth were revealed using precise wet chemical etching. It was obtained that at relatively low annealing temperatures ( 500°С) the changes of electron concentration and photoluminescence response may occur without any remarkable diffusion-related redistribution of dopant atoms. For the relatively high ( 700°С) annealing temperatures the changes of electrical and optical properties after anneal are caused by the significant Sb bulk diffusion and desorption from the sample surface. In particular Sb bulk diffusion leads to the formation of doped regions in the initially undoped areas which further contribute to the resulting structure conductivity and its photoluminescence signal.

Comprehensive Study of the Effect of Plasma Stream on Heat-Resistant Materials

An experimental set-up with a unified system of synchronized control signals was constructed to study the interaction of high-enthalpy plasma jet with the surface of heat-resistant materials. The analysis of air–plasma stream interaction with technical (isotropic) graphite is performed. The air–plasma jet has a temperature of 7000–8000 K in the interaction zone, this interaction is accompanied by destruction of the graphite material and lasts for 100–200 s. Using methods like two-positioning visualization, micropyrometry and spectroscopy the following data are obtained: temperature fields on the graphite sample’s surface and their temporal and spatial changes, dynamics of the sample’s mass loss rate, changes in the parameters (temperature and concentration of electrons, heavy particles temperature) of the incoming plasma stream.

Magnetization plateau as a result of the uniform and gradual electron doping in a coupled spin-electron double-tetrahedral chain.

The double-tetrahedral chain in a longitudinal magnetic field, whose nodal lattice sites occupied by the localized Ising spins regularly alternate with triangular plaquettes with the dynamics described by the Hubbard model, is rigorously investigated. It is demonstrated that the uniform change of electron concentration controlled by the chemical potential in a combination with the competition between model parameters and the external magnetic field leads to the formation of one chiral and seven nonchiral phases at the absolute zero temperature. Rational plateaux at one-third and one-half of the saturation magnetization can also be identified in the low-temperature magnetization curves. On the other hand, the gradual electron doping results in 11 different ground-state regions that distinguish from each other by the evolution of the electron distribution during this process. Several doping-dependent magnetization plateaux are observed in the magnetization process as a result of the continuous change of electron content in the model.

Linear and nonlinear characteristics of time-resolved photoluminescence modulation by terahertz pulse**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574105, 61475054, 61405063, and 61177095), the Hubei Science and Technology Agency Project, China (Grant No. 2015BCE052), and the Fundamental Research Funds for the Central Universities, China (Grant No. 2017KFYXJJ029).

The linear and nonlinear characteristics of time-resolved photoluminescence (PL) of n-type bulk semiconductor GaAs modulated with terahertz (THz) pulse are studied by using an ensemble Monte Carlo (EMC) method. In this paper the center energy valley ( valley) electron concentration changes with the pulse delay time, sampling time and the outfield are mainly discussed. The results show that the sampling time and the THz field should exceed certain thresholds to effectively excite photoluminescence quenching (PLQ). Adopting a direct current (DC) field makes the sampling time threshold shortened and the linear range of THz field-modulation PL expanded. Moreover, controlling the sampling time and the outfield intensity can improve the linear quality: with forward time, the larger outfield is used; with backward time, the smaller outfield is used. This study can provide a theoretical basis of THz field linear modulation in a larger range for new light emitting devices.