Excess Electron Concentration Research Articles

Cross-domain growth and angle-dependent interlayer coupling of twisted bilayer MoS2

Twisted 2D transition metal dichalcogenides (TMDCs) play a significant role in the development of twistronics. However, it is still challenging to prepare high-quality twisted TMDCs by current stacking or folding techniques. Herein, we propose a cross-domain chemical vapor deposition method to synthesize twisted bilayer MoS2 through precisely controlling the supply of molybdenum precursor. It is found that the top layer of a bilayer MoS2 grain maintains its original orientation even when it crosses over to neighboring monolayer MoS2 grain. This suggests that the van der Waals epitaxy can be prevented with the assistance of covalent bonds. Furthermore, the interlayer coupling strength reaches a maximum value at the twisted angle (θ) of 0° or 60° and a minimum at θ = 30°. Moreover, the evolution of in-plane shear mode and out-of-plane breathing mode obtained from low-frequency Raman spectroscopy reveals atomic reconstructions of the moiré pattern. Meanwhile, the shift of the indirect bandgap exhibits an angle dependence consistent with the interlayer coupling strength, which likely comes from the mixing of pz orbitals. The change in A−/A intensity ratio is not mainly originated from the trion binding energy, but the excess electron concentration. Our results offer a feasible approach to prepare high-quality twisted TMDCs and provide a good platform for studying twistronics and related phenomena.

Theoretical Analysis of InGaN Solar Energy Converters Based on Photon-Enhanced Thermionic Emission

Photon-enhanced thermionic emission (PETE) is an efficient solar energy conversion mechanism that combines photovoltaic effects and thermionic emissions. In this study, a diffusion–emission model of electrons for the InGaN cathode was deduced based on one-dimensional continuity equations. The temperature dependence of the excess electron concentration, current density, and conversion efficiency at different cathode electron affinities was simulated, and the performance of the PETE converter under isothermal and nonisothermal state was compared. The results show that the improvement in conversion efficiency under isothermal condition was limited by the increase in anode temperature and reached the maximum of ~22% at an electron affinity of 0.56–0.59 eV and the operating temperature of 710–740 K. When the anode temperature was 500 K, the conversion efficiency increased with the increase in the electron affinity and exceeded the maximum value of the isothermal state at 0.6 eV. We explored the behavior of the converter at bias voltages as well as the determination of the maximum conversion efficiency point. The open-circuit voltage in the isothermal state was lower than that in the nonisothermal state, and the output voltage at the maximum conversion efficiency was eventually greater than the flat-band voltage.

Ag Nanoparticles/AgX (X=Cl, Br and I) Composites with Enhanced Photocatalytic Activity and Low Toxicological Effects

AbstractPeriodic structures induced by electron irradiation are a unique phenomenon when electron beams irradiate on the surface of some materials. These periodic structures have potential for technological applications. However, the fuzzy nature of the electron‐induced structuring hinders its further exploration in such applications. In this paper, novel Ag nanoparticle/AgX (X=Cl, Br and I) composites, with enhanced photocatalytic activity and low toxicological effects, were prepared, for the first time, using electron beam irradiation. The remarkable advantage of this approach is that the Ag nanoparticles/AgX composites can be easily prepared in one‐step without the need for high‐pressure conditions, surfactants, ionic liquids, or reducing agents. Furthermore, our method does not involve any toxic substances, which makes the as‐synthesized samples highly applicable for technological applications. The structure, morphology and physicochemical properties of the Ag nanoparticles/AgX composites were studied using various characterization techniques. Using first‐principles calculations based on density functional theory and the quantum theory of atoms in molecules, we reveal how the concentration of excess electrons in the AgX materials induces the formation of the Ag nanoparticles under electron beam irradiation. These results extend the fundamental understanding of the atomic process underlying the mechanism of Ag−X bond rupture observed during the transformation induced via electron irradiation of the AgX crystals by increasing the total number of electrons in the bulk structure. Thus, our findings provide viable guidance for the realization of new materials for the degradation of contaminated wastewater with low toxicity.

Photoelectrochemical Kinetics: Hydrogen Evolution on p-Type Semiconductors

The theory of intensity-modulated photocurrent spectroscopy (IMPS) is extended to consider the two-electron two-proton hydrogen evolution reaction at p-type semiconductor photocathodes. A generalized reaction scheme is described, and two limiting cases are analyzed. The first involves photoelectrochemical desorption of hydrogen by electron/proton transfer and the second chemical desorption by reaction of adsorbed hydrogen atoms. Expressions are derived for the IMPS responses in the two cases as well as for the modulated surface electron concentration, which is related to the quasi Fermi level at the interface. The surface electron concentration expressions can be used to interpret electrical, optical, or microwave experiments that detect the excess electron concentration. The IMPS responses calculated for the two limiting cases are fitted to a simplified IMPS model that gives rate constants kt nd kr for charge transfer and recombination respectively. kt and kr are then related to the rate constants describing the two limiting schemes for the light-driven hydrogen evolution reaction.

Degenerately n-Doped Colloidal PbSe Quantum Dots: Band Assignments and Electrostatic Effects

We present a spectroscopic study of colloidal PbSe quantum dots (QDs) that have been photodoped to introduce excess delocalized conduction-band (CB) electrons. High-quality absorption spectra are obtained for these degenerately doped QDs with excess electron concentrations up to ∼1020 cm-3. At the highest doping levels, electrons have completely filled the 1Se orbitals of the CB and partially populated the higher-energy 1Pe orbitals. Spectroscopic changes observed as a function of carrier concentration permit an unambiguous assignment of the second excitonic absorption maximum to 1Ph-1Pe transitions. At intermediate doping levels, a clear absorption feature appears between the first two excitonic maxima that is attributable to parity-forbidden 1Sh,e-1Pe,h excitations, which become observable because of electrostatic symmetry breaking. Redshifts of the main excitonic absorption features with increased carrier concentration are also analyzed. The Coulomb stabilization energies of both the 1Sh-1Se and 1Ph-1Pe excitons in n-doped PbSe QDs are remarkably similar to those observed for multiexcitons with the same electron count. The origins of these redshifts are discussed.

Defect chemistry and transport properties of SrCo1 − xTaxO2.5 + δ as a promising oxygen electrocatalyst for reversible solid oxide fuel cells

In this study, we report on thermodynamic and transport properties of SrCo1−xTaxO2.5+δ (SCT) derived from a defect chemistry model encompassing oxygen interstitials as the ionic point defect and holes/excess electrons as the electronic point defects. The results show that SCT can be reasonably modelled as a large-polaron itinerant hole-conductor with a constant mobility at high oxygen stoichiometry (2.5+δ). At low oxygen stoichiometry, electronic carriers tend to be localized small-polarons with a trapping center at TaCo··. With the established defect model, a complete picture of electron hole concentration p=p(T, Po2), excess electron concentration n=n(T, Po2) and oxygen nonstoichiometry δ=δ(T, Po2) is mapped out, from which p=p(T) under a constant δ is derived. The latter further reveals the true activation energy for hole-conduction under a constant δ and yields thermodynamic data for the oxygen incorporation reaction creating oxygen interstitials. These fundamental data are also compared with SrCo1−xNbxO2.5+δ (SCN), suggesting that SCT is a better hole-conductor with higher hole concentration for oxygen electrocatalysis.

Electron Excess Doping and Effective Schottky Barrier Reduction on the MoS2/h-BN Heterostructure.

Layered hexagonal boron nitride (h-BN) thin film is a dielectric that surpasses carrier mobility by reducing charge scattering with silicon oxide in diverse electronics formed with graphene and transition metal dichalcogenides. However, the h-BN effect on electron doping concentration and Schottky barrier is little known. Here, we report that use of h-BN thin film as a substrate for monolayer MoS2 can induce ∼6.5 × 1011 cm-2 electron doping at room temperature which was determined using theoretical flat band model and interface trap density. The saturated excess electron concentration of MoS2 on h-BN was found to be ∼5 × 1013 cm-2 at high temperature and was significantly reduced at low temperature. Further, the inserted h-BN enables us to reduce the Coulombic charge scattering in MoS2/h-BN and lower the effective Schottky barrier height by a factor of 3, which gives rise to four times enhanced the field-effect carrier mobility and an emergence of metal-insulator transition at a much lower charge density of ∼1.0 × 1012 cm-2 (T = 25 K). The reduced effective Schottky barrier height in MoS2/h-BN is attributed to the decreased effective work function of MoS2 arisen from h-BN induced n-doping and the reduced effective metal work function due to dipole moments originated from fixed charges in SiO2.

Creating Excess Electrons at the Anatase TiO2(101) Surface

Excess electrons facilitate redox reactions at the technologically relevant anatase TiO2(101) surface. The availability of these electrons is related to the defect concentration at the surface. We present two-photon (2PPE, 3.10–3.54 eV) and ultraviolet (UPS, 21.2 & 40.8 eV) photoemission spectroscopy measurements evidencing an increased concentration of excess electrons following electron bombardment at room temperature. Irradiation-induced surface oxygen vacancies are known to migrate into the sub-surface at this temperature, quickly equilibrating the surface defect concentration. Hence, we propose that the irradiated surface is hydroxylated. Peaks in UPS difference spectra are observed centred 8.45, 6.50 and 0.73 eV below the Fermi level, which are associated with the 3σ and 1π hydroxyl molecular orbitals and Ti 3d band gap states, respectively. The higher concentration of excess electrons at the hydroxylated anatase (101) surface may increase the potential for redox reactions.

Radiative recombination in GaN/InGaN heterojunction bipolar transistors

We report an electroluminescence (EL) study on npn GaN/InGaN heterojunction bipolar transistors (HBTs). Three radiative recombination paths are resolved in the HBTs, corresponding to the band-to-band transition (3.3 eV), conduction-band-to-acceptor-level transition (3.15 eV), and yellow luminescence (YL) with the emission peak at 2.2 eV. We further study possible light emission paths by operating the HBTs under different biasing conditions. The band-to-band and the conduction-band-to-acceptor-level transitions mostly arise from the intrinsic base region, while a defect-related YL band could likely originate from the quasi-neutral base region of a GaN/InGaN HBT. The IB-dependent EL intensities for these three recombination paths are discussed. The results also show the radiative emission under the forward-active transistor mode operation is more effective than that using a diode-based emitter due to the enhanced excess electron concentration in the base region as increasing the collector current increases.

Structural and optical characterization of nonpolar (10–10) m-InN/m-GaN epilayers grown by PAMBE

Plasma-assisted molecular beam epitaxy growth of (10–10) m-InN/(10–10) m-GaN was carried out on bare (10–10) m-sapphire substrate. The high resolution X-ray diffraction studies confirmed the orientation of the as-grown films. Nonpolar InN layer was grown at different growth temperatures ranging from 390°C to 440°C and the FWHM of rocking curve revealed good quality film at low temperatures. An in-plane relationship was established for the hetrostructures using phi-scan and a perfect alignment was found for the epilayers. Change of morphology of the films grown at different temperatures was observed using an atomic force microscopy technique showing the smoothest film grown at 400°C. InN optical band gap was found to be vary from 0.79–0.83eV from absorption spectra. The blue-shift of absorption edge was found to be induced by excess background electron concentration.

Stepped Current-Voltage Relation and THz Oscillations in GaN MOSFET Due to Optical Phonon Emission: Monte Carlo simulation

Electron transport and drain current noise in wurtzite GaN MOSFET have been studied by Monte Carlo particle simulation that simultaneously solves the Boltzmann transport and pseudo-2D Poisson equations. It is shown that at positive gate voltages giving excess electron concentration in the n-region of the channel, drain current self-oscillations in the THz frequency range up to 6 THz are possible. These self-oscillations are driven by an electron plasma instability. Additionally a step-like drain current dependence on drain bias is demonstrated.

Photoinduced charge redistribution and its influence on excitonic states in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures

The photoinduced charge redistribution in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures under different conditions of optical excitation has been investigated using scanning probe microscopy and optical spectroscopy in the temperature range from 5 to 300 K. Excitation of the samples by radiation with a photon energy greater than the band gap of Zn(Cd)Se leads to the accumulation of electrons in quantum wells, which is detected using scanning spreading resistance microscopy. For moderate excitation densities (up to 25 W/cm2) and at temperatures ranging from 80 to 100 K, the density of a quasi-two-dimensional electron gas formed in quantum wells is several orders of magnitude higher than the density of electron-hole pairs generated by the excitation radiation. The excess electron concentration in the quantum well leads to a broadening of the exciton resonances and to an increase in the relative intensity of the donor-bound exciton emission line and also determines the increase in the luminescence quantum yield with increasing excitation intensity. An additional illumination with a photon energy less than the band gap of Zn(Cd)Se decreases the concentration of excess electrons in quantum wells. The influence of the additional illumination is observed at a temperature of approximately 100 K and almost completely suppressed at 5 K. The obtained results are explained in terms of the formation of a potential barrier for electrons at the ZnMgSSe/GaAs interface and by the specific features of recombination processes in the electron-hole system containing impurity centers with different charge states.

Effects of photoinduced charge redistribution on excitonic states in Zn(Cd)Se/ZnMgSSe quantum wells

Photoinduced charge redistribution processes in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well structures are studied using steady-state photoluminescence, photoreflectance, and scanning spreading resistance microscopy with an additional illumination. It is shown that an above-barrier optical pumping leads to the accumulation of electrons in the quantum wells. The resulting concentration of excess electrons in the quantum wells is several orders of magnitude higher than the concentration of photoexcited electron-hole pairs. These excess electrons induce broadening of excitonic resonances and, furthermore, cause an enhancement in the photoluminescence quantum yield and an increase in the relative intensity of the bound-exciton emission line. The additional below-barrier illumination at temperatures about 100 K leads to a decrease in the excess electron concentration in the quantum wells. The observed phenomena are explained in terms of a simple model considering the formation of a barrier in the conduction band near the ZnMgSSe/GaAs heterointerface.

Monte Carlo Simulation of Noise and THz Generation in InP FET at Excess of Electrons in Channel

Electron transport and drain current noise in field effect transistor with nnn InP channel have been studied by Monte Carlo particle simulation which simultaneously solves the Boltzmann transport and pseudo-2D Poisson equations. It is shown that at gate voltages giving excess electron concentration in n-region of channel the drain current self-oscillations in THz frequency range are possible. The self-oscillations are driven by electron plasma instability.

Mg‐doped InN and InGaN – Photoluminescence, capacitance–voltage and thermopower measurements

AbstractThe bandgap range of InGaN extends from the near‐IR (InN, 0.65 eV) to the ultraviolet. To exploit this wide tuning range in light generation and conversion applications, pn junctions are required. The large electron affinity of InN (5.8 eV) leads to preferential formation of native donor defects, resulting in excess electron concentration in the bulk and at surfaces and interfaces. This creates difficulties for p‐type doping and/or measuring of the bulk p‐type activity. Capacitance–voltage measurements, which deplete the n‐type surface inversion layer, have been used to show that Mg is an active acceptor in InN and Inx Ga1–x N for 0.2 < x < 1.0, i.e. over the entire composition range. Mg acceptors can be compensated by irradiation‐induced native donors. Thermopower measurements were used to provide definitive evidence that Mg‐doped InN has mobile holes between 200 K and 300 K. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ce4(P1-xSix)3-z: A First Example for the Stabilization of the Anti-Th3P4 Type Structure by Substitution in the Non-Metal Substructure

A first rare-earth phosphide silicide Ce4(P(1-x)Si(x))(3-z) and its analogues with La, Pr, and Nd were synthesized and characterized. The compounds crystallize in the anti-Th3P4 structure type. The cerium compound shows a mixed occupation of the 12a site with Si and P and possesses a wide homogeneity range with respect to x and z variation. The electronic configuration of Ce, deduced from magnetic susceptibility and X-ray absorption spectroscopy data, remains 4f(1) (Ce3+) independently from x and z. The cerium valence and the phase stability region are discussed employing electronic band-structure calculation and chemical bonding analysis with electron localization function. Atomic interactions are shown to remain nearly unchanged, while the change of the excess electron concentration with P/Si substitution is considered to play the main role for the stabilization of the structural motif.

A unified analytical model for charge transport in Heterojunction Bipolar Transistors

A unified analytical charge transport model of HBTs, which is applicable for a wide variety of emitter-base (e–b) structures of HBTs, viz. abrupt or graded heterojunctions and p–n junctions displaced into wider or narrower band gap material is proposed. This is a thermionic field diffusion model, which considers thermionic emission and tunneling at the e–b heterojunction and diffusion in the quasi-neutral base regions. The Fermi-level splitting is considered while calculating the space charge region (SCR) recombination currents, which in turn is taken into account to calculate excess electron concentration at the edge of the depletion region in the base side of the e–b p–n junction. Closed form analytical expressions for the terminal currents are obtained, which has been implemented in a circuit simulator. The accuracy of the model is established by comparison with numerical simulation results and experimental data.

FROM NEXT NEAREST NEIGHBOR SITE PERCOLATION TO CONTINUUM PERCOLATION: APPLICATION TO HIGH-Tc SUPERCONDUCTORS

In high-Tc superconductors dopant atoms supply holes or excess electrons, and electric conduction is established in the neighborhood of the dopant. We propose that percolation of these conducting areas leads to global electric conduction. To investigate these processes numerical procedures to simulate continuum percolation are developed. The relation between the concentration of connecting discs and the fraction of the area of the plane covered by all discs is computed in the whole range between next nearest neighbor site percolation and continuum percolation. This method is applied to investigate underdoped copper oxides where small hole or excess electron concentrations are insufficient to establish electric conduction. The results of this study are applied to the model of mobile charge carriers which consist of or are accompanied by diffusing d-electrons. Our investigations show that with increasing doping the Neel temperature of the antiferromagnetism vanishes and spin glass states appear in accordance with experiments. Furthermore, the peaks in the specific heat become very broad. This broadening due to randomness and loss of translation invariance has been observed in nuclear magnetic resonance peaks of doped superconducting cuprates.

Exciton and Charged Exciton Absorption in Asymmetric Double Quantum Well Structures

We present absorption studies of neutral and negatively charged excitons in nominally undoped CdTe-based asymmetric double quantum well structure. The origin of excess electron concentration is shown to be related to hole trapping by deep centers. An additional illumination with sapphire laser is used to change excess electron concentration in the well. We apply a phenomenological model to extract transition intensities and energies. The dependence of the dissociation energy on electron concentration is roughly linear. The extrapolation of dissociation energy to zero concentration yields a value of 3.8 meV.

Dynamic of Spin Triplet and Singlet Trions in a GaAs Quantum Well

In this paper we discuss the dynamics of the spin-triplet and spin-singlet states of the negatively charged exciton in a 300 A quantum well (QW) under a perpendicular magnetic field. The singlet spin state shows similar behaviour to that already observed for the trion at 0 T. However, the triplet state (which is supposed to be optically inactive) shows a very long decay time at low electron concentrations which shortens upon adding carriers to the well. For this reason we have modeled the perturbation to the trion triplet wavefunction caused by the excess electrons, and found an enhancement of its oscillator strength with electron density, which follows a cubic power law. The same power law is also found for the experimental variation of the triplet decay rate (proportional to the trion oscillator strength). This justifies the fact that the trion triplet state, in presence of an excess electron concentration, can have a finite oscillator strength.