Spatial Separation Of Electrons Research Articles

Multi-channel charge transfer in self-supporting B-g-C3Nx/Bi2S3/CdS dual S-scheme heterojunction toward enhanced photothermal-photocatalytic performance

Improving the charge separation efficiency of catalysts is a significant challenge in photocatalysis. To solve this problem, we design a three-dimensional self-supported B-g-C3Nx/Bi2S3/CdS dual S-scheme heterojunction photocatalytic system mimicking natural photosynthesis to achieve multi-channel rapid charge transfer. The dual S-scheme heterojunction induces double interface electric fields on the surface of ultra-thin B-g-C3Nx nanosheets, which can drive charge carriers to migrate directionally along multiple channels and suppress carrier recombination through spatial separation of electrons and holes. Furthermore, the three-dimensional interconnected open framework of B-g-C3Nx/Bi2S3/CdS enhances light absorption, touching off intense photothermal effects and near-field temperature rise, thereby improving the chemical reaction kinetics. Benefiting from the efficient charge separation and near-field temperature rise, the B-g-C3Nx/Bi2S3/CdS exhibits a photocatalytic oxytetracycline degradation rate of 98.7% and H2 evolution rate of 4.78 mmol g−1 h−1. This work provides new insights into the construction of efficient S-scheme photocatalysts with multiple-channel charge transfer characteristics.

Electric Field-Induced Quenching of MAPbI3 Photoluminescence in PeLED Architecture.

Photoluminescence (PL) measurements are a widely used technique for the investigation of perovskite-based materials and devices. Although electric field-induced PL quenching provides additional useful information, this phenomenon is quite complex and not yet clearly understood. Here, we address the PL quenching of methylammonium lead iodide (MAPbI3) perovskite in a light-emitting diode (PeLED) architecture. We distinguish two quenching mechanisms: (a) indirect quenching by slow irreversible or partially reversible material changes that occur gradually under the applied light and electric field and (b) direct quenching by the influence of the electric field on the charge carrier densities, their spatial distributions, and radiative recombination rates. Direct quenching, observed under the abrupt application of negative voltage, causes a decrease of the PL intensity. However, the PL intensity then partially recovers within tens of milliseconds as mobile ions screen the internal electric field. The screening time increases to hundreds of seconds at low temperatures, indicating activation energies for ion motion of about 80 meV. On the other hand, ultrafast time-resolved PL measurements revealed two main phases of direct quenching: an instantaneous reduction in the radiative carrier recombination rate, which we attribute to the electron and hole displacement within individual perovskite grains, followed by a second phase lasting hundreds of picoseconds, which is due to the charge carrier extraction and spatial separation of electron and hole "clouds" within the entire perovskite layer thickness.

Electron Transport Properties in High Electron Mobility Transistor Structures Improved by V-Pit Formation on the AlGaN/GaN Interface

This work suggests new morphology for the AlGaN/GaN interface which enhances electron mobility in two-dimensional electron gas (2DEG) of high-electron mobility transistor (HEMT) structures. The widely used technology for the preparation of GaN channels in AlGaN/GaN HEMT transistors is growth at a high temperature of around 1000 °C in an H2 atmosphere. The main reason for these conditions is the aim to prepare an atomically flat epitaxial surface for the AlGaN/GaN interface and to achieve a layer with the lowest possible carbon concentration. In this work, we show that a smooth AlGaN/GaN interface is not necessary for high electron mobility in 2DEG. Surprisingly, when the high-temperature GaN channel layer is replaced by the layer grown at a temperature of 870 °C in an N2 atmosphere using TEGa as a precursor, the electron Hall mobility increases significantly. This unexpected behavior can be explained by a spatial separation of electrons by V-pits from the regions surrounding dislocation which contain increased concentration of point defects and impurities.

Spatial Separation of Electrons and Holes among ZnO Polar {0001} and {101̅0} Facets for Enhanced Photocatalytic Performance.

Spatial separation of electrons and holes is critical for improving their photocatalytic performance, which is ascribed to the suppressed photoinduced carriers’ recombination among facets. In this work, the ZnO–Au–MnOx heterogeneous nanostructure photocatalyst was prepared by photodepositing Au and MnOx on the ZnO polar {0001} and {101̅0} crystal facets, respectively. The photocatalytic performance of ZnO–Au–MnOx was higher than ZnO and ZnO–Au for the degradation of rhodamine B dye under UV light irradiation. Due to the potential difference between different crystal planes of zinc oxide, electrons and holes will migrate to different crystal planes of zinc oxide. This will lead to the deposition of Au and MnOx on different crystal facets of zinc oxide. The efficient photoinduced carrier separation of ZnO–Au–MnOx resulted in the high photocatalytic activity, which is well supported by photoelectrochemical and photoluminescence analyses. The intermediated species formed during the reaction were investigated by high performance liquid chromatography. The reaction mechanism was investigated by radical trapping experiments and electron spin resonance analysis. The special structure of selective deposition of redox cocatalysts on the different facets should be promising and intriguing for designing highly efficient photocatalysts.

Regimented Charge Transport Phenomena in Semiconductive Self-Assembled Rhenium Nanotubes

Photoconductivity, a crucial property, determines the potential of semiconductor materials for use in optoelectronic and photocatalytic device applications. The one-dimensional metal-organic nanotube semiconducting material [{Re(CO)3}6(bho)(phpy)6]n (MBT 1, where bho is benzene-1,2,3,4,5,6-hexaoate and phpy is 4-phenylpyridine) reported herein exhibits record photocurrent responses at a broad spectral range. MBT 1 is comprised of a unique nanotube structure that is composed of six rhenium sites, six 4-phenylpyridine ligands, and a benzene-1,2,3,4,5,6-hexaoate unit. The highly organized self-assembled molecular bamboo tube MBT 1 displays semiconducting characteristics with a low activation energy of 1.63 meV. The alternating current (AC) and direct current (DC) conductivities of pellet devices are approximately 10-4 S/cm. For a single-crystal device, DC conductivity was found to be 1.5 S/cm, an unprecedented 10 000 times higher. The bandgap of MBT 1 was determined to be 1.03 eV, consistent with the theoretically estimated value of 1.2 eV. Theoretical calculations suggest that the unique structural architecture of MBT 1 allows for effective charge transport, which is facilitated by the spatial separation of electrons and holes that MBT 1 contains. This also eliminates fast charge recombination. The findings are not only chemically and fundamentally important but also have great potential for applications in innovative nano-optoelectronics.

Surface discharge of bulk materials investigated from change of charge trap characteristics of polyimide single molecular chain

Surface discharge is one of the reasons for insulation failure. Polyimide (PI) is used in gas-solid insulation of high-frequency electric power equipment. Therefore, based on density functional theory, the effects of single molecular chain structure, energy level, density of states, electrostatic potential, excited state and other micro parameters under external electric field on trap formation and surface discharge of both PI and polar- group- OH<sup>– </sup>affected PI are discussed from the atomic and molecular level. The results show that the structure of PI is crimped and the dipole moment increases under external electric field, which is easy to accumulate charges to form space charge center, especially after the introduction of polar group OH<sup>–</sup>. In the PI molecules, hole traps are formed in the benzene ring region, and electron traps are formed in the imide ring region. The number of electron trap energy levels is large, in which the space charge trap depth gradually deepens with the increase of external electric field. After the introduction of polar group OH<sup>–</sup>, the excitation energy of PI molecules decreases, which makes the electrons inside the molecules excited easily. The spatial separation of electrons and holes decreases with the increase of electric field, which is conducive to the recombination of holes and electrons to emit photons.

Construction of Low-Cost Z-Scheme Heterostructure Cu2 O/PCN for Highly Selective CO2 Photoreduction to Methanol with Water Oxidation.

Solar-driven CO2 reaction with water oxidation into alcohols represents a promising approach to achieve real artificial photosynthesis. However, rapid recombination of photogenerated carriers seriously restricts the development of artificial photosynthesis. Herein, a facile method is explored to construct low-cost Z-Scheme heterostructure Cu2 O/polymeric carbon nitride (PCN) by in situ growth of Cu2 O hollow nanocrystal on PCN. The protective PCN layer and Z-schematic charge flow can make robust Cu2 O/PCN photocatalysts, and the spatial separation of electrons and holes with high redox potentials of ECB (-1.15eV) and EVB (1.65eV) versus NHE can efficiently drive CO2 photoreduction to methanol in pure water, which is further confirmed by DFT calculation. The Z-scheme heterostructure Cu2 O/PCN exhibits a high methanol yield of 276µmol g-1 in 8 h with ca. 100% selectivity, much superior to that of isolated Cu2 O and PCN, and all the reported Cu2 O-based heterostructures. This work provides a unique strategy to efficiently and selectively promote the conversion of CO2 and H2 O into high-value chemicals by constructing a low-cost Z-scheme heterostructure.

Al2O3 buffer-facilitated epitaxial growth of high-quality ZnO/ZnS core/shell nanorod arrays.

II-VI semiconductor heterojunctions show huge potential for application in nanodevice fabrication due to their type-II alignments owing to the better spatial separation of electrons and holes. However, the hetero-epitaxial growth of high-quality heterostructures is still a challenge, especially for materials with large lattice mismatch. In this work, well-aligned single-crystalline ZnO/ZnS core/shell nanorod arrays were obtained by introducing an Al2O3 buffer layer. It is interesting that the nature of the ZnS layer varies with the thickness of the Al2O3 layer. When Al2O3 is less than 2 nm, the interaction between the substrate and epilayer is strong enough to penetrate through the buffer layer, enabling the growth of ZnS on Al2O3-coated ZnO nanorod arrays. On the basis of detailed characterization, a rational growth mechanism of the core/shell heterostructure is proposed, in which the Al2O3 interlayer can eliminate voids due to the Kirkendall effect around the interface and accommodate a misfit dislocation between the inner ZnO and outer ZnS, resulting in more sufficient strain relaxation in the epitaxy. In addition, cathodoluminescence measurements demonstrate that the optical properties of the ZnO/ZnS heterostructure could be effectively improved by taking advantage of the thin Al2O3. The I-V curves characterized by PeakForce tunneling atomic force microscopy reveal that the heterostructure shows a typical rectifying behavior and good photoresponse to ultraviolet light. These findings may provide a reasonable and effective strategy for the growth of highly lattice-mismatched heterostructure arrays buffered by the Al2O3 layer, broadening the options for fabricating heterojunctions and promoting their applications in optoelectronic devices.

Trajectory Analysis in a Collector with Multistage Energy Recovery for a DEMO Prototype Gyrotron. Part I. Idealized Magnetic Field Distribution

This paper presents the results of simulation of the collector of a prototype gyrotron designed for the DEMO project. Trajectory analysis in a collector with four-stage recovery of the residual beam energy based on the method of spatial separation of electrons in crossed azimuthal magnetic and axial electric fields was carried out. In this part of the research, the azimuthal magnetic field was formed using a conductor located on the axis of the device. The study was carried out for a spent electron beam with a particle velocity and coordinate distribution close to those obtained in experiments with high-power gyrotrons. As a result of optimizing the geometry and potentials of the collector sections, an overall efficiency of the gyrotron higher than 80% was achieved, which is close to the maximum efficiency with ideal separation of electron beam fractions with different energies. The data obtained will be used to design a toroidal solenoid for creating an azimuthal magnetic field.

Development of a Collector with Multistage Recuperation for Gyrotron

A four-stage collector system is developed for the experimental gyrotron at the Peter the Great St. Petersburg Polytechnic University based on spatial separation of electrons with different energies in crossed electric and magnetic fields. The conditions of use of this system for the efficient recuperation of electron residual energy in a spent beam are determined. The numerical simulation of the recuperator and the analysis of the electric and magnetic field distribution in the gyrotron collector region are accomplished. Theoretical estimations and trajectory analysis of the helical electron beam show that the system developed provides the electron residual energy recuperation necessary to achieve total gyrotron efficiency over 70%.

Discrete interactions between a few interlayer excitons trapped at a MoSe2\u2013WSe2 heterointerface

Inter-layer excitons (IXs) in hetero-bilayers of transition metal dichalcogenides (TMDs) represent an exciting emergent class of long-lived dipolar composite bosons in an atomically thin, near-ideal two-dimensional (2D) system. The long-range interactions that arise from the spatial separation of electrons and holes can give rise to novel quantum, as well as classical multi-particle correlation effects. Indeed, first indications of exciton condensation have been reported recently. In order to acquire a detailed understanding of the possible many-body effects, the fundamental interactions between individual IXs have to be studied. Here, we trap a tunable number of dipolar IXs (NIX ~ 1–5) within a nanoscale confinement potential induced by placing a MoSe2–WSe2 hetero-bilayer (HBL) onto an array of SiO2 nanopillars. We control the mean occupation of the IX trap via the optical excitation level and observe discrete sharp-line emission from different configurations of interacting IXs. The intensities of these features exhibit characteristic near linear, quadratic, cubic, quartic and quintic power dependencies, which allows us to identify them as different multiparticle configurations with NIX ~ 1–5. We directly measure the hierarchy of dipolar and exchange interactions as NIX increases. The interlayer biexciton (NIX = 2) is found to be an emission doublet that is blue-shifted from the single exciton by ΔE = (8.4 ± 0.6) meV and split by 2J = (1.2 ± 0.5) meV. The blueshift is even more pronounced for triexcitons ((12.4 ± 0.4) meV), quadexcitons ((15.5 ± 0.6) meV) and quintexcitons ((18.2 ± 0.8) meV). These values are shown to be mutually consistent with numerical modelling of dipolar excitons confined to a harmonic trapping potential having a confinement lengthscale in the range ell approx 3 nm. Our results contribute to the understanding of interactions between IXs in TMD hetero-bilayers at the discrete limit of only a few excitations and represent a key step towards exploring quantum correlations between IXs in TMD hetero-bilayers.

Biexciton Condensation in Electron-Hole-Doped Hubbard Bilayers: A Sign-Problem-Free Quantum MonteCarlo Study.

The bilayer Hubbard model with electron-hole doping is an ideal platform to study excitonic orders due to suppressed recombination via spatial separation of electrons and holes. However, suffering from the sign problem, previous quantum MonteCarlo studies could not arrive at an unequivocal conclusion regarding the presence of phases with clear signatures of excitonic condensation in bilayer Hubbard models. Here, we develop a determinant quantum MonteCarlo algorithm for the bilayer Hubbard model that is sign-problem-free for equal and opposite doping in the two layers and study excitonic order and charge and spin density modulations as a function of chemical potential difference between the two layers, on-site Coulomb repulsion, and interlayer interaction. In the intermediate coupling regime and in proximity to the SU(4)-symmetric point, we find a biexcitonic condensate phase at finite electron-hole doping, as well as a competing (π,π) charge density wave state. We extract the Berezinskii-Kosterlitz-Thouless transition temperature from superfluid density and a finite-size scaling analysis of the correlation functions and explain our results in terms of an effective biexcitonic hard-core boson model.

Optical characterization of type-II ZnO/ZnS multiple quantum wells grown by atomic layer deposition

ZnO/ZnS multiple-quantum wells (MQWs) were grown on Si substrates by atomic layer deposition, and their optical properties were investigated. Light emission well below the band-to-band transition energies of constituent materials was observed, which was attributed to the type-II band alignment of ZnO/ZnS MQWs. The type-II transition showed a significant blue shift with increasing excitation power, which originated from modified band bending caused by spatial separation of electrons and holes as electrons are confined in the ZnO conduction band quantum well and holes are confined in the ZnS valence band quantum well. The conduction band offset of the ZnO/ZnS heterojunction was determined to be 1.58 eV simulated from the type-II emission of the MQWs with controlled layer thicknesses. The type-II transition provides an important opportunity to utilize visible light emission and absorption for various optical and energy harvesting devices using the ZnO/ZnS material system.

Density Functional Theory Study of ZnIn2S4 and CdIn2S4 Polymorphs Using Full‐Potential Linearized Augmented Plane Wave Method and Modified Becke–Johnson Potential

ZnIn2S4 and CdIn2S4 are ternary sulfide semiconductors with attracting photoelectric and catalytic properties, which are closely related with their band structure. The full‐potential linearized augmented plane wave (FP‐LAPW) method is used to calculate the band structures of cubic and hexagonal polymorphs of ZnIn2S4 and CdIn2S4. Among the four systems studied, the hexagonal CdIn2S4 structure with a space group P63mc (No. 186) is newly predicted, and the phonon calculation shows that its structure is dynamically stable. Owing to the well‐known bandgap underestimation problem of local density approximation (LDA) and generalized gradient approximation (GGA), the effects of Hubbard model and modified Becke–Johnson (mBJ) potential to band structure are investigated. The calculated outcomes of cubic ZnIn2S4 and cubic CdIn2S4 with mBJ‐GGA + U are in good agreement with the experimental data. For hexagonal ZnIn2S4, it is shown that electronic states in high valence bands and low conduction bands have different components. The origin of discrepancy between the theoretical bandgap of hexagonal ZnIn2S4 and experimental value obtained from optical absorption measurement is discussed based on the spatial separation of electron and hole states and optical transition probability.

Optimal construction of WO3·H2O/Pd/CdS ternary Z-scheme photocatalyst with remarkably enhanced performance for oxidative coupling of benzylamines

In this work, an all-solid-state Z-scheme photocatalyst of WO3·H2O/Pd/CdS ternary heterostructures was constructed by introducing a small amount of Pd into the optimized WO3·H2O/CdS binary system, where Pd nanoparticles (NPs) were sandwiched between WO3·H2O nanosheets and CdS NPs. The WO3·H2O/Pd/CdS ternary heterostructures exhibited remarkably enhanced photocatalysis towards the oxidative coupling of benzylamines to imines, compared with the other photocatalysts of WO3·H2O, CdS, WO3·H2O/CdS, WO3·H2O/Pd, and WO3·H2O/CdS/Pd. The wavelength-dependent photocatalysis of WO3·H2O/Pd/CdS is in good agreement with its light-harvesting intensity at the corresponding wavelengths. The photoelectrochemical results reflected the enhanced charge carrier separation in WO3·H2O/Pd/CdS, and the photocatalytic H2 evolution confirmed the highly efficient Z-scheme charge transfer in this ternary photocatalyst. Therefore, the WO3·H2O/Pd/CdS ternary heterostructures possess distinct advantages of extended light harvesting region, spatial separation of electron and holes, and elevated redox power, which synergistically contribute to the superior photocatalysis. The Pd NPs sandwiched between WO3·H2O and CdS acted as not only an electron mediator to facilitate the interfacial charge separation but also a cocatalyst to accelerate the selective oxidation of benzylamine. This work demonstrates a rational design of highly efficient Z-scheme photocatalysts and presents some revelations about the photocatalytic mechanism for selective organic transformations.

Tunable Ferroelectricity in Ruddlesden-Popper Halide Perovskites.

Ruddlesden-Popper (RP) halide perovskites are the new kids on the block for high-performance perovskite photovoltaics with excellent ambient stability. The layered nature of these perovskites offers an exciting possibility of harnessing their ferroelectric property for photovoltaics. Adjacent polar domains in a ferroelectric material allow the spatial separation of electrons and holes. Presently, the structure-function properties governing the ferroelectric behavior of RP perovskites are an open question. Herein, we realize tunable ferroelectricity in 2-phenylethylammonium (PEA) and methylammonium (MA) RP perovskite (PEA)2(MA) n̅-1Pb n̅I3 n̅+1. Second harmonic generation (SHG) confirms the noncentrosymmetric nature of these polycrystalline thin films, whereas piezoresponse force microscopy and polarization-electric field measurements validate the microscopic and macroscopic ferroelectric properties. Temperature-dependent SHG and dielectric constant measurements uncover a phase transition temperature at around 170 °C in these films. Extensive molecular dynamics simulations support the experimental results and identified the correlated reorientation of MA molecules and ion translations as the source of ferroelectricity. Current-voltage characteristics in the dark reveal the persistence of hysteresis in these devices, which has profound implications for light-harvesting and light-emitting applications. Importantly, our findings disclose a viable approach for engineering the ferroelectric properties of RP perovskites that may unlock new functionalities for perovskite optoelectronics.

Development of multistage energy recovery system for gyrotrons

A four-stage depressed collector based on spatial separation of electrons with different energies in the crossed electric and magnetic fields was developed for the experimental SPbPU gyrotron. Modeling of the system of electron energy recovery and analysis of the distributions of electric and magnetic fields in the gyrotron collector region were performed. As a result of the theoretical estimations and the trajectory analysis of the helical electron beam, it is shown that the developed system provides recovery of the residual electron energy necessary to achieve the total efficiency of the gyrotron exceeding 70 %.

Band alignment and carrier recombination of InAs/GaAs<sub>1−</sub><sub><italic>x</italic></sub>Sb<sub><italic>x</italic></sub> quantum dots affected by Sb content

The influences of Sb content on the band structure and carrier recombination dynamics of InAs/GaAs1− x Sb x quantum dots (QDs) have been investigated by both calculation and experimental demonstration. The calculation results using the eight-band model indicated that the band alignment of InAs/GaAs1− x Sb x QDs was type I at low Sb content and changed from type I to type II when Sb content increased from 0.08 to 0.14. During this phase, electrons were still localized in InAs QDs while holes were transferred from InAs QDs to the GaAsSb capping layer gradually, which resulted in the reduction of the overlap of electron and hole wave functions and carrier transition rate. To experimentally prove the calculation results, InAs/GaAs1− x Sb x QDs with various Sb contents were grown by molecular beam epitaxy. The photoluminescence (PL) measurement results confirmed the expected influence of Sb content on the band alignment of InAs/GaAs1− x Sb x QDs. For InAs/GaAs1− x Sb x QDs samples with Sb content of 0.15 and 0.2, the PL peak energy showed a blue shift with increasing excitation power, which is a signature of a type-II band alignment structure. On the other hand, the PL peak energy was independent of excitation power in the samples with Sb content of 0.05 and 0.1. Besides, the redshift with increasing Sb content became stronger and the PL intensity became weaker when Sb content was larger than 0.14. To investigate the carrier transition rate, time-resolved PL measurement was performed. The minority carrier lifetime increased from 0.41 to 14.3 ns when the band alignment of InAs/GaAs1− x Sb x QDs changed from type I to type II, which indicated that carrier recombination was inhibited. InAs/GaAs1− x Sb x QDs with type-II band alignment are preferred for intermediate band solar cells due to longer carrier lifetime resulting from the spatial separation of electrons and holes, which is beneficial to maintain the separation between conduction band, intermediate band and valence band quasi-Fermi levels.

Room-temperature electrical control of exciton flux in a van der Waals heterostructure.

Devices that rely on the manipulation of excitons-bound pairs of electrons and holes-hold great promise for realizing efficient interconnects between optical data transmission and electrical processing systems. Although exciton-based transistor actions have been demonstrated successfully in bulk semiconductor-based coupled quantum wells1-3, the low temperature required for their operation limits their practical application. The recent emergence of two-dimensional semiconductors with large exciton binding energies4,5 may lead to excitonic devices and circuits that operate at room temperature. Whereas individual two-dimensional materials have short exciton diffusion lengths, the spatial separation of electrons and holes in different layers in heterostructures could help to overcome this limitation and enable room-temperature operation of mesoscale devices6-8. Here we report excitonic devices made of MoS2-WSe2 van der Waals heterostructures encapsulated in hexagonal boron nitride that demonstrate electrically controlled transistor actions at room temperature. The long-lived nature of the interlayer excitons in our device results in them diffusing over a distance of five micrometres. Within our device, we further demonstrate the ability to manipulate exciton dynamics by creating electrically reconfigurable confining and repulsive potentials for the exciton flux. Our results make a strong case for integrating two-dimensional materials in future excitonic devices to enable operation at room temperature.

Acceleration of Solar Wind Particles Passing through the 3D Heliospheric Current Sheet

AbstractAdditional acceleration of protons and electrons passing through a 3D reconnecting current sheet (RCS) of the solar corona and heliosphere is investigated with PIC approach. The simulation confirms spatial separation of electrons and protons and generation of a polarisation electric field induced by separated particles. In the heliospheric current sheet with a weak magnetic field there are two populations of particles: transit and bounced. The transit particles (both protons and electrons) are accelerated to high energies while the bounced electrons fail to reach the midplane. Instead they form an electron cloud of horseshoe or medallion types at some distance D from its midplane, which is larger for bigger guiding field magnitudes. These energetic electrons and protons appearing near the HCS boundaries can be a great danger for the satellites crossing the sector boundaries.