Spatial Separation Of Carriers Research Articles

Synergistic Effect of the Sulfur Vacancy and Schottky Heterojunction on Photocatalytic Uranium Immobilization: The Thermodynamics and Kinetics.

Not only a critical matter in the nuclear fuel cycle but uranium is also a global contaminant with both radioactive and chemical toxicity. Reducing soluble hexavalent uranium [U(VI)] to relatively nonimmigrated tetravalent uranium [U(IV)] by photocatalytic technologies is recognized as a highly promising strategy for avoiding environmental pollution and re-extracting uranium resources from nuclear wastewater. Herein, we have designed a heterojunction photocatalyst constructed from the carbon aerogels (CA) and the CdS nanoflowers with an S-vacancy (CA@CdS-SV). With the S-vacancy and heterojunction being synergized, the U(VI) removal rate exceeded 97% in 40 min without the addition of any sacrificial agents. As impacted by the synergistic effects of the S-vacancy and heterojunction, thermodynamics and kinetics revealed that photogenerated electrons were first captured via shallow traps generated by vacancies on CdS-SV and then transferred to the CA surfaces through the heterojunction to realize the spatial separation of carriers, thereby achieving a satisfactory performance. This work is considered to underpin the improvement of U(VI) immobilization by exploiting the synergistic effect of vacancy engineering and the Schottky heterojunction from the perspective of thermodynamics and kinetics.

Dependence of InGaN Quantum Well Thickness on the Nature of Optical Transitions in LEDs.

The design of the active region is one of the most crucial problems to address in light emitting devices (LEDs) based on III-nitride, due to the spatial separation of carriers by the built-in polarization. Here, we studied radiative transitions in InGaN-based LEDs with various quantum well (QW) thicknesses—2.6, 6.5, 7.8, 12, and 15 nm. In the case of the thinnest QW, we observed a typical effect of screening of the built-in field manifested with a blue shift of the electroluminescence spectrum at high current densities, whereas the LEDs with 6.5 and 7.8 nm QWs exhibited extremely high blue shift at low current densities accompanied by complex spectrum with multiple optical transitions. On the other hand, LEDs with the thickest QWs showed a stable, single-peak emission throughout the whole current density range. In order to obtain insight into the physical mechanisms behind this complex behavior, we performed self-consistent Schrodinger–Poisson simulations. We show that variation in the emission spectra between the samples is related to changes in the carrier density and differences in the magnitude of screening of the built-in field inside QWs. Moreover, we show that the excited states play a major role in carrier recombination for all QWs, apart from the thinnest one.

Tailoring the band alignment and magnetic and optical properties of g-C3N4/WSe2 van der Waals heterostructures by vacancies and atomic doping

Tunable band structure and effective spatial separation of carriers are important for designing good optoelectronic devices. In this work, effects of interfacial defect including vacancies and [Formula: see text]- or [Formula: see text]-type doping on the band alignment and magnetic and optical properties of g-C3N4/WSe2 van der Waals heterostructures are systematically investigated under the premise of spin-orbit coupling density-functional theory. The results illustrate that the pristine g-C3N4/WSe2 heterostructure exhibits intrinsic type-I band alignment with a direct bandgap of 1.101 eV, and the absorption of UV-visible light can be effectively improved compared with that of isolated WSe2 and g-C3N4 monolayers. The N vacancies as well as [Formula: see text], [Formula: see text] and [Formula: see text] doping produce asymmetrical spin-up and spin-down states, bringing about the magnetization and even magnetic half-metallicity characteristics ([Formula: see text] and [Formula: see text] doping). The band edge positions of g-C3N4/WSe2 heterostructure can be also well adjusted by introducing vacancies and [Formula: see text]- or [Formula: see text]-type doping in the g-C3N4 layer, accompanied by the transition of band alignment from types I to II, which will remarkably promote the separation of photogenerated electron-hole pairs. The defective g-C3N4/WSe2 heterostructures can be considered as a promising candidate for spintronics and optoelectronic devices.

Spatial carrier separation in cobalt phosphate deposited ZnIn2S4 nanosheets for efficient photocatalytic hydrogen evolution

Developing photocatalyst with effective charge separation and fast surface reaction kinetics is crucial to high-efficiency photocatalytic H2 evolution. Herein we spatially separate the reductive and oxidative reaction sites of ZnIn2S4 nanosheet and accelerate the hole transfer process by simply depositing cobalt phosphate (CoHxPOy, noted as Co-Pi), a non-precious co-catalyst, on its basal plane. Theoretical calculations combined with comprehensive characterizations reveal that Zn vacancies induced deep local energy levels serve as hole trap states to pin the photogenerated holes on (001) surface of ZnIn2S4. Upon Co-Pi deposition, Co atoms preferentially bond with S atoms nearby the Zn vacancies of ZnIn2S4. Taking advantage of the formation of Co-S bonds, Co-Pi acts as a hole receptor to extract the trapped holes effectively. As the photocatalytic activity of ZnIn2S4 is caused by active S atoms on its edge side, deposition of Co-Pi on planar side of nanosheets helps electron-hole migration to different facets. As a result of inert facet activation and spatial carrier separation, Co-Pi/ZnIn2S4 exhibits a superior photocatalytic H2 evolution rate of 10.19 mmol g-1h−1 under visible light, which is superior to most of reported ZnIn2S4-based photocatalysts. This work will provide novel insights into the activation of inert basal planes of two-dimensional photocatalysts.

α-Fe2O3/ZnO heterostructure for enhanced photocatalytic and antibacterial activity

This paper reports the accomplishment of a noticeable enhancement in the photodegradation and antibacterial activity of α-Fe2O3 by a heterostructure formation between α-Fe2O3 and ZnO flakes. The heterostructure yields 97% photodegradation of methylene blue pollutant in water within 180 min, which is a fourfold increase compared to that observed with a pure α-Fe2O3 nanostructure. Further characterisations indicate that the improved results are attributable to the reduction in optical band gap, the increased electrical conductivity, the improved spatial separation of carriers and the enhanced amount of available hydroxyl radicals for the reaction. The stability and reusability of the photocatalysts are confirmed for four consecutive cycles of operations. The phase identification and structural studies are done using x-ray diffraction, transmission electron microscopy and x-ray photoelectron spectroscopy in conjunction with surface morphology from field emission scanning electron microscopy. The enhancement of room temperature electrical conductivity by five times and band gap reduction from 1.98 eV to 1.75 eV for the heterostructures, when compared with α-Fe2O3 nanostructures, are obtained from voltage–current measurements and optical reflectance spectra.

Strain‐Dependent Band Structures and Electronic Properties in Sb/Bi Lateral Heterostructures Calculated by First Principles

2D material structures have drawn much attention because of the unique characteristics of carriers confined in 2D planes. Various structures have been fabricated for high‐performance optoelectronic devices. Herein, via first principles, lateral heterostructures (LHSs) based on antimony (Sb) and bismuth (Bi) are predicted and band structures affected under strain are calculated. For Sb/Bi LHSs, zigzag and armchair atomic configurations are considered. By altering the number of atoms on two sides of the heterostructures, the Sb/Bi LHSs exhibit narrow bandgaps. Moreover, external compressive and tensile strains induce transitions from indirect to direct band structures and further compress the bandgap energy into the midinfrared range. Partial density‐of‐states analysis indicates that, under the applied strains, the valence band mainly comprises Bi 6p states and Sb 5p states. In addition, charge density distributions indicate that electrons are localized at Bi atoms, whereas holes are localized at Sb atoms, thus exhibiting spatial separation of carriers. A narrow‐bandgap material in which the band structure and electronic structure characteristics have great potential for infrared optoelectronic applications is proposed herein.

In situ construction of a direct Z-scheme AgBr/α-Ag2WO4 heterojunction with promoted spatial charge migration and photocatalytic performance

A hierarchical AgBr/α-Ag2WO4 Z-scheme heterojunction was facially constructed for greatly improved photocatalytic activity towards pollutant elimination due to promoted spatial separation of carriers with high redox capacity.

Potential Visible-Light Driven PtO₂/GaN vdW Hetero-Bilayer Photocatalysts for Water Splitting Using First-Principles

Novel two-dimensional (2D) PtO2/GaN van der Waals (vdW) hetero-bilayers (HBL) are studied here for photocatalytic water splitting (PWS) application under first-principles density functional theory (DFT). We proposed six HBLs due to the atomic orientational variations and two of them are found dynamically stable confirmed by phonon dispersion curves. The two stable HBLs, HBL1, and HBL6 also show negative binding energy depicted by the interlayer distance-dependent binding energy curves. Among them, HBL1 has the lowest binding energy, suggesting the exothermic practicability of the material. Electronically both materials show a visible ranged indirect bandgap of ~2.65 (2.69) eV for HBL 1 (HBL6), lowered by ~2 times compared to their intrinsic constituents (2D PtO2, 2D GaN). The bandgaps also have type-II band orientation, which is highly required for efficient spatial carrier separation in photocatalytic water splitting (PWS) applications. The optical properties of the HBLs were also calculated, and it’s found that the HBLs have $\sim 2\times 10 ^{5}{\mathrm {cm}}^{-1} $ of perovskite material-like absorption coefficient in the visible spectrum, a key requirement for efficient photocatalysis. Reflectivity is as low as ~7% in the visible spectrum, suggesting the low-loss nature of the materials. Photocatalytic band-edges with type-II band alignments show sufficient kinetic overpotential for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in both HBLs, suggesting effective water-splitting capacity. Moreover, we have explored the biaxial strain-induced tunability of the electronic bandgap, absorption coefficients, and photocatalytic band edges. They all found responsive due to homogeneous biaxial strain and show bandgap-lowering, absorption coefficient visible shifting, and band-edges tuning from compressive to tensile strains in the −6 % to +6% range. These studies suggest that the novel PtO2/GaN vdW layered material can be a probable efficient material for visible-light-driven photocatalytic water-splitting technology.

Covalently Bonded Bi2O3 Nanosheet/Bi2WO6 Network Heterostructures for Efficient Photocatalytic CO2 Reduction

The highly efficient spatial carrier separation and abundant active sites are significant for CO2 photocatalytic reduction efficiency. Herein, an ultrathin Bi2O3 nanosheet/Bi2WO6 network heterostructure is successfully synthesized via a covalently bonded epitaxial growth strategy. Due to cosharing of the Bi–O tetrahedron between Bi2O3 and Bi2WO6, this heterostructure exhibits a compact interface, which can provide a highway for the charge transfer and then boost their separation. Moreover, the pores generated from the hierarchical structure afford abundant exposed photocatalytic active sites. Thus, the optimal Bi2O3/Bi2WO6 heterostructure (BOBWO-12) shows superior photocatalytic performance for CO2 reduction with an ultrahigh selectivity as well as the removal of RhB. Notably, the photocatalytic CO generation rate over BOBWO-12 reaches up to 17.39 μmol·g–1·h–1, which is 18.0 and 4.2 times higher than that of bulk-Bi2WO6 and Bi2WO6 nanosheets, respectively, and the selectivity is of about 95.4%. Moreover, the Bi2O3/Bi2WO6 heterostructure also exhibits improved long-term stability, resulting from the firm heterointerface. Our studies present a conventient avenue to construct a high-efficiency heterostructure photocatalyst with a firm interface by the covalently bonded epitaxial growth method for CO2 reduction.

Hollow MoSe2@Bi2S3/CdS Core-Shell Nanostructure as Dual Z-Scheme Heterojunctions with Enhanced Full Spectrum Photocatalytic-Photothermal Performance

Inadequate separation of carriers is still major obstacles to the practical application of photocatalysis. The reasonable design of semiconductor photocatalysts is expected to break through this bottleneck. Herein, MoSe2@Bi2S3/CdS is designed by solvothermal-hydrothermal strategy. Thanks to the dual Z-Scheme heterojunction conforms to the characteristics of multi-channel charge transfer which favor the spatial separation of carriers. MoSe2@Bi2S3/CdS hollow structure increases the utilization rate of light source, the core-shell structure increases the interface area and the active sites. Moreover, full spectrum absorption endues MoSe2@Bi2S3/CdS with unpredictable photothermal effect, which can touch off near-field temperature rise to facilitate photocatalytic process. Profiting from the synergistic facilitation effect, MoSe2@Bi2S3/CdS exhibits excellent photocatalytic activity in the hydrogen evolution and pollutants removal of Cr (VI) and 2,4,6-trichlorophenol (TCP), which are up to 11.84 mmol/g/h, 98.7% and 99.2%, respectively. This dual functional MoSe2@Bi2S3/CdS photocatalyst may paves new opportunities for solving the energy crisis and environmental pollution.

Mechanisms of impurity-enhanced spatial carrier separation and photocatalytic activity of F–N codoped m-ZrO2 nanocrystals

Metal oxide photocatalysts have high chemical stability in aqueous solutions, but their photocatalytic activity is usually restricted by poor electron–hole separation. Although impurity doping has been demonstrated to be an effective way to tune photocatalytic properties, the mechanisms of enhanced photocatalytic activity remain ambiguous. Here we show that appropriately incorporated impurities can markedly improve the spatial carrier separation and photocatalytic activity. This concept is demonstrated by experimental and density functional theory studies on the effects of fluorine (F) and nitrogen (N) dopants on photocatalytic H2 evolution performance of m-ZrO2 nanocrystals. We found that F readily enters the lattice of m-ZrO2 to form F substitutional defect (FO) with low defect formation energy (DFE), but N has difficulty entering the lattice to form N substitutional defect (NO) unless N is codoped with F. The F doping doubled the photocatalytic activity of m-ZrO2 without reducing its bandgap, which is attributed to the enhanced spatial carrier separation caused by defect FO. The F–N codoping quadrupled the photocatalytic activity of m-ZrO2, with the enhanced photocatalytic activities attributed to the synergetic effect of the impurity-enhanced spatial carrier separation and the increased light absorption. We observed that F–N codoping enhances photocatalytic performance by affecting both separation and recombination of the carriers: (1) F–N codoping enhances spatiotemporal separation of electrons and holes, and (2) F–N codoping reduces spatial overlap between the electron and hole distributions and inhibits recombination of the carriers. Our findings may provide novel insights into designing high-performance photocatalysts.

Enhanced carriers separation efficiency in g-C3N4 modified with sulfonic groups for efficient photocatalytic Cr(VI) reduction

Rapid carrier transport and high carrier separation efficiency are key factors in improving photocatalytic performance. Herein, a novel sulfonic groups modified g-C3N4 photocatalyst was constructed by solution impregnation and evaporation method. The strong electronegativity of the sulfonic acid group provides a guide for the migration of electrons to the surface, thereby promoting the transport and separation of photogenerated carriers. In addition to that, the sulfonic acid group can also achieve spatial separation of carriers on the surface and serve as an active site. Based on the above merits, the sulfonic-acid-modified g-C3N4 exhibited a significantly enhanced visible light reduction Cr(VI) activity which is 2.9 times higher than that of pure g-C3N4.The resent work provides a new strategy for the surface modification of photocatalysts, which contributes to construct high performance photocatalysts.

Fine Structure and Spin Dynamics of Linearly Polarized Indirect Excitons in Two-Dimensional CdSe/CdTe Colloidal Heterostructures

Heterostructured two-dimensional colloidal nanoplatelets are a class of material that has attracted great interest for optoelectronic applications due to their high photoluminescence yield, atomically tunable thickness, and ultralow lasing thresholds. Of particular interest are laterally heterostructured core-crown nanoplatelets with a type-II band alignment, where the in-plane spatial separation of carriers leads to indirect (or charge transfer) excitons with long lifetimes and bright, highly Stokes shifted emission. Despite this, little is known about the nature of the lowest energy exciton states responsible for emission in these materials. Here, using polarization-controlled, steady-state, and time-resolved photoluminescence measurements, at temperatures down to 1.6 K and magnetic fields up to 30 T, we study the exciton fine structure and spin dynamics of archetypal type-II CdSe/CdTe core-crown nanoplatelets. Complemented by theoretical modeling and zero-field quantum beat measurements, we find the bright-exciton fine structure consists of two linearly polarized states with a fine structure splitting ∼50 μeV and an indirect exciton Landé g-factor of 0.7. In addition, we show the exciton spin lifetime to be in the microsecond range with an unusual B-3 magnetic field dependence. The discovery of linearly polarized exciton states and emission highlights the potential for use of such materials in display and imaging applications without polarization filters. Furthermore, the small exciton fine structure splitting and a long spin lifetime are fundamental advantages when envisaging CdSe/CdTe nanoplatelets as elementary bricks for the next generation of quantum devices, particularly given their ease of fabrication.

Octahedral-shaped perovskite CaCu3Ti4O12 with dual defects and coexposed {(001), (111)} facets for visible-light photocatalysis

The finite light absorption and rapid charge carrier recombination are two major bottlenecks in semiconductor photolysis. Introducing defects and exposed facets at photocatalysts can significantly improve photoefficiency due to the synergetic effects of extending light response and effective usage of photoinduced charge carriers. However, the prime influential factors in surface defects and predominantly exposed facets that determine photocatalytic performance are still unclear. Herein, to combine the advantages of defects and exposed facets, we, for the first time, report a surfactant-free synthesis and boosted photolysis of a single-crystal CaCu3Ti4O12 octahedron with dual defects (oxygen and metal deficiencies of Cu+ and Ti3+) and coexposed ({001}, {111}) facets by a facile molten salt approach for visible light antibiotic decomposition. The utilized molten salt KCl acts as a structure directing agent, decreasing surface energy to form the octahedron shapes and coexposed facets. As-obtained sample displays a superior efficiency enhancement for antibiotic degradation, approximately 69 times faster than that of its counterpart, defect free CaCu3Ti4O12. Density functional theory (DFT) calculations and photodecomposition results show that existing of coexposed facets contributes to spatial separation of carriers, while dual defects improve light absorption, affect charge density and thus promote carrier transfer. The vacancy formation energies of oxygen and titanium defects on exposed facets were calculated. Moreover, dual defects, especially the Ti3+ species are greatly affected by defect dipoles of CaCu3Ti4O12 with distorted units of CuO4 and TiO6, in which TiO6 clusters make greater contributions on dipole-induced internal field changes. Finally, two concepts of "surface heterojunction" and "defect dipole" are revealed to unveil the synergetic roles of surface chemistry and defect engineering in tuning a photoactivity of crystal based inorganic semiconductors. With dual defects, predominantly coexposed facets and unique crystal shapes, this work reports a strategy and direction for rational synthesis of highly efficient visible light driven catalysts and provides new insights for unearthing the surface chemistry and defect engineering of inorganic photocatalysts.

Optimization of Ohmic Contacts to n -Type GaAs Nanowires

III-V nanowires are comprehensively studied because of their suitability for optoelectronic quantum technology applications. However, their small dimensions and the spatial separation of carriers from the wire surface render electrical contacting difficult. Systematically studying ohmic contact formation by diffusion to $n$-doped GaAs nanowires, we provide a set of optimal annealing parameters for Pd/Ge/Au ohmic contacts. We reproducibly achieve low specific contact resistances of $\sim2\times10^{-7}\,\Omega\text{cm}^2$ at room temperature becoming an order of magnitude higher at $T\simeq4.2\,$K. We provide a phenomenological model to describe contact resistances as a function of diffusion parameters. Implementing a transfer-matrix method, we numerically study the influence of the Schottky barrier on the contact resistance. Our results indicate that contact resistances can be predicted using various barrier shapes but further insights into structural properties would require a full microscopic understanding of the complex diffusion processes.

Enhanced performance of doped BiOCl nanoplates for photocatalysis: understanding from doping insight into improved spatial carrier separation

Doped BiOCl nanoplates enclosed with (001) and (110) facets were fabricated to demonstrate the role of doping in promoting spatial carrier separation.

Role of the optical pulse repetition rate in the efficiency of terahertz emitters

Excitation of n-GaAs and p-InAs terahertz emitters by the series of optical pulses is studied by ensemble Monte Carlo simulations. It is found that the spatial separation of photoexcited electrons and holes dramatically reduces the recombination intensity in n-GaAs emitter, the operation of which is based on the surface field effect. The spatial separation of carriers does not affect the recombination intensity in p-InAs emitter, the operation of which is based on the photo-Dember effect. Therefore, the recovery time of equilibrium state after optical pulse in n-GaAs emitter significantly exceeds the corresponding recovery time in p-InAs emitter. This fact leads to a substantial reduction of photocurrent amplitude in n-GaAs emitter excited by the optical pulse series at high repetition rate.

Effect of Growth Temperature on the Luminescence Properties of InP/GaP Short-Period Superlattice Structures

The optical properties of InP/GaP short-period superlattice (SPS) structures grown at various temperatures from <TEX>$400^{\circ}C$</TEX> to <TEX>$490^{\circ}C$</TEX> have been investigated by using temperature-dependent photoluminescence (PL) and emission wavelength-dependent time-resolved PL measurements. The PL peak energy for SPS samples decreases as the growth temperature increases. The decreased PL energy of ~10 meV for the sample grown at <TEX>$425^{\circ}C$</TEX> compared to that for <TEX>$400^{\circ}C$</TEX>-grown sample is due to the CuPt-B type ordering, while the SPS samples grown at <TEX>$460^{\circ}C$</TEX> and <TEX>$490^{\circ}C$</TEX> exhibit the significant reduction of the PL peak energies due to the combined effects of the formation of lateral composition modulation (LCM) and CuPt-B type ordering. The SPS samples with LCM structure show the enhanced carrier lifetime due to the spatial separation of carriers. This study represents that the bandgap energy of InP/GaP SPS structures can be controlled by varying growth temperature, leading to LCM formation and CuPt-B type ordering.

Hydrothermal synthesis of high-quality type-II CdTe/CdSe core/shell quantum dots with dark red emission.

A hydrothermal method was used to synthesize type-II CdTe/CdSe core/shell quantum dots (QDs) using the thilglycolic acid (TGA) capped CdTe QDs as cores, which show a number of advantages. Because of the spatial separation of carriers the low excited states of CdTe/CdSe QDs, they exhibit many novel properties that are fundamentally different from the type-I QDs. On the other hand, our experiment results show that the wave function of the hole of the exciton in the CdTe core extends well into the CdSe shell. The results also reveal that a thick shell can confine the electrons inside the particles and thereby improve the PL efficiency and prolong the lifetime of the core/shell QDs. We use the UV-vis absorption and fluorescence spectrum measurements on growing particles in detail. We found that the fluorescence of the CdTe/CdSe QDs was strongly dependent on the thick of the shell and size of the core as well as the unique type-II heterostructure, which make the type-II core/shell QDs more suitable in photovoltaic or photoconduction applications.

Transport and Photoelectric Effects in Structures with Ge and SiGe Nanoclusters Grown on Oxidized Si (001)

Crystalline germanium nanoclusters (NCs) are grown by a molecular-beam epitaxy technique on chemically oxidized Si (100) surface at 700oC. Deposition of silicon on the surface with Ge nanoclusters leads to surface reconstruction and formation of polycrystalline diamond-like Si coverage, while nanoclusters core becomes tetragonal SiGe alloy. Possible mechanisms for nanoclusters growth are discussed. Selective photoexcitation of Ge or SiGe nanoclusters or space-charge layer of underlying Si allows to observe two non-equilibrium steady-states with higher and lower conductivity values as compared to the equilibrium one. The persistent photoconductivity (PPC) behaviour was observed after excitation of electron-hole pairs in Si (001) substrate. This effect may be attributed to spatial carrier separation by macroscopic fields in the depletion layer of the near-surface Si. Decreasing of surface conductivity, driven by optical recharging of NCs and Si/SiO2 interface states, is observed in the spectral range from 0.6 to 1.0 eV. Conductivity drop is discussed in the terms of hole accumulation by Ge-NC states enhancing the local-potential variations and, therefore, decreasing the surface conductivity of p-Si.