Electron-hole Pair Generation Research Articles

Extracellular polymeric substances altered the physicochemical properties of molybdenum disulfide nanomaterials to mitigate its toxicity to Chlorella vulgaris

Although the toxic effects of two-dimensional nanomaterials (2D-NMs) have been widely reported, the influence of extracellular polymeric substances (EPS) on the environmental fate and risk of 2D-NMs in aquatic environments is largely unknown, and the processes and mechanisms involved remain to be revealed. Herein, we investigated the impact of EPS secreted by microalgae (Chlorella vulgaris (C. vulgaris)) on the environmental transformation and risk of molybdenum disulfide (MoS2). We found that the attachment of EPS increased the thickness of MoS2 (from 2 nm to 5 nm), changed it from a monolayer sheet to a fuzzy multilayer structure, and promoted the formation of defects on MoS2. The blue-shift of the peak associated with the plasmon resonances in the 1 T phase and the generation of electron-hole pairs suggested that EPS altered the surface electronic structure of MoS2. EPS interacted mainly with the S atoms on the 1 T phase, and the attachment of EPS promoted the oxidation of MoS2. The reduction in hydrodynamic diameter (Dh) and the decrease in zeta potential indicated that EPS inhibited the agglomeration behavior of MoS2 and enhanced its dispersion and stability in aqueous media. Notably, EPS reduced the generation of free radicals (superoxide anion (•O2−), singlet oxygen (1O2), and hydroxyl radicals (•OH−)). Furthermore, EPS mitigated the toxicity of MoS2 to C. vulgaris, such as attenuated reduction in biomass and chlorophyll content. Compared to pristine MoS2, MoS2 + BG11 + EPS exhibited weaker oxidative stress, membrane damage and lipid peroxidation. The adsorption of EPS on MoS2 surface reduced the attachment sites of MoS2, making MoS2 less likely to be enriched on the cell surface. The findings have significant contribution for understanding the interactions between EPS and MoS2 in aquatic ecosystems, providing scientific guidance for risk assessment of 2D-NMs.

Luminescence of Cr-doped β-Ga2O3 thin films

The results of investigation of optical excitation, photoluminescence (PL) and cathodoluminescence (CL) spectra in Cr-doped β-Ga2O3 thin films are presented. The broad bands due to the generation of electron-hole pairs and transitions from the 4А2 ground level to the 4Т1 and 4Т2 excited states in Cr3+ ions are observed in the photoexcitation spectra. The R-lines and phonon repetitions of R-lines are observed on the luminescence spectra against the background of a broad structureless band with a maximum around 700 nm, caused by the 4T2 – 4A2 transitions in Cr3+ ions. The crystal field force Dq was determined and the Stokes and anti-Stokes repetitions of R-lines were interpreted.

Deciphering the Mechanisms of Photo-Enhanced Catalytic Activities in Plasmonic Pd-Au Heteromeric Nanozymes for Colorimetric Analysis.

The growing demand for highly active nanozymes in various fields has led to the development of several strategies to enhance their activity. Plasmonic enhancement, a strategy used in heterogenous catalysis, represents a promising strategy to boost the activity of nanozymes. Herein, Pd-Au heteromeric nanoparticles (Pd-Au dimers) with well-defined heterointerfaces have been explored as plasmonic nanozymes. As a model system, the Pd-Au dimers with integrated peroxidase (POD)-like activity and plasmonic activity are used to investigate the effect of plasmons on enhancing the activity of nanozymes under visible light irradiation. Mechanistic studies revealed that the generation of hot electron-hole pairs plays a dominant role in plasmonic effect, and it greatly enhances the decomposition of H2 O2 to the reactive oxygen species (ROS) intermediates (•OH, •O2 - and 1 O2 ), leading to elevated POD-like activity of the Pd-Au dimers. Finally, the Pd-Au dimers are applied in the plasmon-enhanced colorimetric method for the detection of alkaline phosphatase, exhibiting broad linear range and low detection limit. This study not only provides a straightforward approach for regulating nanozyme activity through plasmonic heterostructures but also sheds light on the mechanism of plasmon-enhanced catalysis of nanozymes.

SOI FinFET Design Optimization for Radiation Hardening and Performance Enhancement

This work proposes Total Ionizing Dose (TID) hardening techniques compatible with conventional 14-nm-node silicon-on-insulator (SOI) FinFETsf process flows through performing 3-dimensional (3-D) simulations based on technology computer-aided design (TCAD) tools. The simulation results reveal a significantly critical TID impact induced by trapped charges in the buried oxide (BOX) and the spacer with calibration against 14 nm SOI FinFETfs experimental data (error < 6%). Inspired by the physical interpretation, an optimization technique featuring an optimized gate structure, spacer length, and substrate bias is designed. The optimized gate structure is utilized to enhance the local gate-to-channel coupling at the bottom and reduce the generation and capture of electron-hole pairs. By reducing the spacer length, a lower sensitive volume in the spacer can effectively suppress the TID response. The setting of the negative substrate bias greatly improves the subthreshold characteristics, weakening the TID effect in the BOX. By adopting the combined optimization including these techniques, the threshold voltage shift (VTH) induced by a 5 Mrad(SiO2) irradiation can be reduced to 21 mV, whereas VTH is 45 mV with only gate structure optimized and 97 mV without hardening. Meanwhile, the ION/IOFF after radiation increases to 1 × 107, which is at least four orders of magnitude better than the original device. Meanwhile, subthreshold swing (SS) is reduced from 81 mV/dec to 71 mV/dec, and Drain Induced Barrier Lowering (DIBL) is reduced from 120 mV/V to 99 mV/V, respectively. The combined optimization design is demonstrated as an effective method to improve the tolerance against TID irradiation without compromising performance, promoting 14 nm SOI FinFET’s application in future harsh space environments.

(Invited) Effect of Gaseous Tritium Environments on Betavoltaic Device Longevity

Direct betavoltaic energy conversion is a specialized energy harvesting technology, converting beta radiation from a radiation source directly into electricity using a semiconductor. Of the most common beta emitting isotopes, tritium betavoltaics hold promise owing to the high specific activity of solid tritium compounds, low shielding requirements and relatively high availability. Betavoltaic devices offer great promise to produce continuous quantities of nanowatt to microwatt power over the course of several years, particularly for low-power sensor, medical, and space applications where sunlight is too sparse for solar cell use or where battery replacement is challenging.Canadian Nuclear Laboratories (CNL) is currently developing betavoltaic devices based on tritium. CNL has unique facilities to produce, fabricate and test betavoltaic devices. Computational techniques have been used to address challenges of longevity and electron-hole pair generation in semiconductor materials, in particular (In,Ga)P. Long-term studies on test wafers in different tritium-containing environments and the effects on power output and longevity will also be discussed.

Spray coated of newly La-doped SnS2 thin films for photocatalytic degradation application

In the present study, we present a comprehensive investigation on the synthesis and thorough characterization of La (2 and 4 at. %)-doped 2D-SnS2 thin films, which were prepared by a spray-coating method with a clear molecular ink. These thin films were specifically targeted for their potential application in photodegradation. The impact of La doping on the structural, morphological, mechanical, and electrical properties of the thin films was analyzed using a range of appropriate techniques. Additionally, the mechanical properties were assessed through Vickers micro-indentation. The results indicate that the pristine SnS2 films predominantly exhibit a preferred (101) crystal planes, while the introduction of La doping results in the appearance of a secondary Sn2S3 phase. The bandgap of the films was measured to range from 2 eV to 2.22 eV. Moreover, the thermoelectric power analysis revealed that the films exhibit n-type characteristics, and the mechanical properties showed an increase in hardness with La doping. Notably, the photocatalytic degradation rate was found to be 58.2%, 60.27%, and 62.92% for pristine SnS2, La (2 at.%)-SnS2, and La (4 at.%)-SnS2, respectively, when tested against Malachite green dyes under visible light. This enhancement in degradation efficiency can be attributed to the presence of La3+ ions, which promote electron-hole pair generation.

Boosting Photocatalytic Performance of ZnO Nanowires via Building Heterojunction with g-C3N4.

The development of a stable and highly active photocatalyst has garnered significant attention in the field of wastewater treatment. In this study, a novel technique involving a facile stirring method was devised to fabricate an array of g-C3N4/ZnO nanowire (ZnO NW) composites. Through the introduction of g-C3N4 to augment the generation of electron-hole pairs upon exposure to light, the catalytic efficacy of these composites was found to surpass that of the pristine ZnO NWs when subjected to simulated sunlight. The photocatalytic performance of a 20 mg·L-1 methylene blue solution was found to be highest when the doping rate was 25 wt%, resulting in a degradation rate of 99.1% after 60 min. The remarkable enhancement in catalytic efficiency can be ascribed to the emergence of a captivating hetero-junction at the interface of g-C3N4 and ZnO NWs, characterized by a harmoniously aligned band structure. This alluring arrangement effectively curtailed charge carrier recombination, amplified light absorption, and augmented the distinct surface area, culminating in a notable boost to the photocatalytic prowess. These findings suggest that the strategic engineering of g-C3N4/ZnO NW heterostructures holds tremendous promise as a pioneering avenue for enhancing the efficacy of wastewater treatment methodologies.

Spin-exchange carrier multiplication in manganese-doped colloidal quantum dots.

Carrier multiplication is a process whereby a kinetic energy of a carrier relaxes via generation of additional electron-hole pairs (excitons). This effect has been extensively studied in the context of advanced photoconversion as it could boost the yield of generated excitons. Carrier multiplication is driven by carrier-carrier interactions that lead to excitation of a valence-band electron to the conduction band. Normally, the rate of phonon-assisted relaxation exceeds that of Coulombic collisions, which limits the carrier multiplication yield. Here we show that this limitation can be overcome by exploiting not 'direct' but 'spin-exchange' Coulomb interactions in manganese-doped core/shell PbSe/CdSe quantum dots. In these structures, carrier multiplication occurs via two spin-exchange steps. First, an exciton generated in the CdSe shell is rapidly transferred to a Mn dopant. Then, the excited Mn ion undergoes spin-flip relaxation via a spin-conserving pathway, which creates two excitons in the PbSe core. Due to the extremely fast, subpicosecond timescales of spin-exchange interactions, the Mn-doped quantum dots exhibit an up-to-threefold enhancement of the multiexciton yield versus the undoped samples, which points towards the considerable potential of spin-exchange carrier multiplication in advanced photoconversion.

Enhancing photodetection performance of UV photodetectors with stacked Pt/NiO dual capping layers on IGZO thin-film transistors

This paper proposes a solution to mitigate the trade-off between dark and photocurrents in the indium gallium zinc oxide (IGZO) thin-film transistor (TFT) applications, such as ultraviolet photodetectors, by using a stacked Pt/NiO dual capping layer (CL). The Pt CL forms a Schottky contact with the IGZO channel, which maximizes the depletion width on the channel layer and allows the use of a thicker channel to suppress both dark current and channel resistance. On the other hand, the NiO CL forms a pn heterojunction with the IGZO channel, which provides additional space for generating electron–hole pairs and is forward biased by the photovoltaic voltage under UV irradiation, resulting in a further negative shift in the threshold voltage and a significant increase in photocurrent. Experimental results show that the proposed CL scheme exhibits excellent photoresponsivity, photosensitivity, and specific detectivity in a 40 nm-thick IGZO TFT, with the values of 1888 A/W, 3.37 × 108 A/A, and 3.99 × 1016 Jones, respectively, which are about 55%, 83%, and 68% higher than a traditional 30 nm-thick IGZO TFT using only NiO CL under 275 nm UV irradiation.

A DFT study of CuCl-XSe (X=Ga, In) vdW heterostructure as an efficient photocatalysts for hydrogen production

The photocatalytic efficiency of monolayer materials can be greatly improved by constructing two-dimensional van der Waals heterostructures. In this work, the electronic properties and photocatalytic mechanism of the CuCl/XSe (X = In, Ga) vdW heterostructure with a stable structure are explored via the density functional theory. Our results indicate that the band gap of CuCl/InSe (1.241 eV) vdW heterostructure and CuCl/GaSe (1.334 eV) vdW heterostructure and large optical absorbance promote sunlight absorption and electron-hole pair generation. The built-in electric field formed at the interface accelerates the reorganization of photogenerated holes in XSe and photogenerated electrons in CuCl, which results in the photogenerated electrons and holes in the heterostructure accumulate in CB of XSe and VB of CuCl, respectively. The spatial separation of photoexcited electrons and holes prolongs the lifespan of carriers and enhances the photocatalytic efficiency of the CuCl/XSe vdW heterostructure. Meanwhile, the biaxial strain can effectively modify the band gaps. Finally, outstanding visible and near-infrared light absorption abilities mainly from ∼400 to around 1400 nm may guide the CuCl/XSe vdW heterojunction to a promises applications in photovoltaics.

Design and Performance Enhancement of a GaAs-Based Homojunction Solar Cell Using Ga0.5In0.5P as a Back Surface Field (BSF): A Simulation Approach

The GaAs semiconductor is a solar energy promising material for photovoltaic applications due to its good optical and electronic properties. In this work, a homojunction GaAs solar cell with AlxGa1-xAs and GayIn1-yP solar energy materials as window and back surface field (BSF) layers, respectively, was simulated and investigated using SCAPS-1D software. The performance of the GaAs-based solar cell is evaluated for different proportions ofxandy, which allowed us to obtain the values of 0.8 and 0.5 forxandy, respectively, as the best values for high performance. We then continued the optimization by taking into account some parameters of the solar cell, such as thickness, doping, and bulk defect density of the p-GaAs base, n-GaAs emitter, and Ga0.5In0.5P BSF layer. Solar cell efficiency increases with emitter thickness, but the recombination phenomenon is more pronounced than that of electron-hole pair generation in the case of a thicker base. The effect of variation in the work function of the back contact has also been studied, and the best performance is for a platinum (Pt) electrode. The optimized GaAs-based solar cell achieves a power conversion efficiency of 35.44% (JSC=31.52 mA/cm2,VOC=1.26 V,FF=89.14%) and a temperature coefficient of -0.036%/°C. These simulation results provide insight into the various ways to improve the efficiency of GaAs-based solar cells.

Rational Synthesis of Au-CdS Composite Photocatalysts for Broad-Spectrum Photocatalytic Hydrogen Evolution.

Incorporation of plasmonic metal nanomaterials can significantly enhance the visible light response of semiconductor photocatalysts via localized surface plasmon resonance (LSPR) mechanisms. However, the surfaces of plasmonic metal nanomaterials are often covered with surfactant molecules, which is undesired when the nanomaterials are used for photocatalytic hydrogen evolution, since surfactant molecules could significantly compromise the nanomaterials' cocatalyst functionalities by blocking the active sites and/or by inhibiting the surface charge transfer process. Herein, we demonstrate a method that assembles Au nanoparticles (NPs) into Au colloidosomes (AuCSs) without modifying their surfaces with surfactants. The resulting AuCSs were then coupled with CdS for the formation of Au-CdS composite photocatalysts through an in situ deposition method. The assembly of Au NPs induced a broader and stronger LSPR response for AuCSs, while the absence of surfactants allowed them to act efficiently as cocatalysts. This essentially enhanced the electron-hole pair generation rate and further their utilization efficiency, leading to an extremely high hydrogen evolution rate of 235.8 mmol·g-1·h-1 under simulated sunlight excitation.

DFT and experimental investigations on the photocatalytic mechanism of C3N4-MQDs/BWO heterojunction for Tetracycline degradation

BackgroundThe development of efficient antibiotic degradation catalysts has attracted much attention from researchers, but little is known about the intrinsic relationship between the geometry, electronic structure, and photocatalytic activity of the catalysts. MethodsHerein, a magnetic recoverable C3N4-MQDs/BWO heterojunction was successful synthesized by combining C3N4 magnetic quantum dots with Bi2WO6 nanosheets through a hydrothermal method. Significant FindingsThe prepared CMBW-2 (C3N4-MQDs/BWO with 2% C3N4-MQDs) was applied to degraded TC (tetracycline) solution of 20mg·L−1 under visible light irradiation, presenting 100% conversion within 120 min, and the reaction rate is 13.8 times and 2.3 times that of Bi2WO6 and g-C3N4/Bi2WO6, respectively. Its stability tests show an unchanged activity after five cycles for a long 600 min visible light irradiation. Various characterization results confirm that the good catalytic performance originates from the formation of a heterojunction to facilitate the separation of generated electron–hole pairs, while its larger specific surface area provides more active adsorption sites. DFT results show that the Bi2WO6(001)/C3N4(002) heterostructure is a staggered band arrangement structure whose VBM (valance band maximum) is mainly inhabited by atomic orbitals of Bi2WO6, while its CBM (conduction band minimum) is mainly inhabited by atomic orbitals of C3N4(002). Based on DFT and experimental results, an indirect Z-scheme is proposed, in which Fe3O4 nanoparticles are regarded as an electronic conductive bridge to realize rapid electron transfer. Moreover, the C3N4-MQDs/BWO heterojunction has high potential for practical applications in the removal of emerging contaminants in sewerage because of its excellent stability and sufficient magnetism.

Effects of Measurement Temperature on Radioluminescence Processes in Cerium-Activated Silica Glasses for Dosimetry Applications.

Cerium-doped-silica glasses are widely used as ionizing radiation sensing materials. However, their response needs to be characterized as a function of measurement temperature for application in various environments, such as in vivo dosimetry, space and particle accelerators. In this paper, the temperature effect on the radioluminescence (RL) response of Cerium-doped glassy rods was investigated in the 193-353 K range under different X-ray dose rates. The doped silica rods were prepared using the sol-gel technique and spliced into an optical fiber to guide the RL signal to a detector. Then, the experimental RL levels and kinetics measurements during and after irradiation were compared with their simulation counterparts. This simulation is based on a standard system of coupled non-linear differential equations to describe the processes of electron-hole pairs generation, trapping-detrapping and recombination in order to shed light on the temperature effect on the RL signal dynamics and intensity.

Nanoporous GaN Photoanode Decorated with Plasmonic Au Nanoparticles for Enhanced Solar Water Splitting

Photoelectrochemical (PEC) water splitting is a promising approach to generating eco‐friendly hydrogen energy from water. Despite long‐lasting efforts, the existing methods still suffer limitations in the design of photoelectrodes with optimal structures for efficient operation. Herein, a novel porous gallium nitride (PGaN) photoanode decorated with plasmonic Au nanoparticles (AuNPs) is fabricated. After microstructural optimization, this composite photoanode improves the water‐splitting efficiency from 0.036% to 0.30% and exhibits a 2.7 times improvement of the photocurrent density in comparison with the planar GaN photoanode. The significant improvement of the water‐splitting efficiency is attributed to the synergistic combination of the porous structure with the plasmonic AuNPs that greatly benefits electron–hole pairs generation, separation, and charge‐carrier transport. The photoanode is robust and easy to fabricate, and this work provides an idea for future exploration of durable and efficient GaN‐based photoelectrodes.

Impact of photovoltaic effect on performance enhancement of triboelectric nanogenerator for energy harvesting applications

The photo-induced triboelectric nanogenerator (PTENG) has been explored as new generation energy technology, which is enabled by coupling triboelectric and photo-induced charge carriers. PTENG shows a dual work mechanism that is attributed to the synergetic outcome of the photovoltaic and triboelectric effects. This study aims to provide a new strategy for triboelectric nanogenerator devices to achieve high output performance for various smart electronic device applications. Aluminum and n-type silicon were chosen as two different materials for triboelectric measurements. Different techniques were adopted to obtain triboelectric output parameters through the in-house developed triboelectric setup. AFM and Kelvin probe force microscopy imaging techniques were employed to obtain surface roughness and to measure the change in surface potential under the illumination of red (630 nm) laser light. A significant improvement in surface potential of around 40 mV was observed under laser illumination. Triboelectric characterization has been performed to obtain open circuit voltage (VOC) and short circuit current (ISC) with and without light illumination at different speeds of the motor. VOC and Isc were found to be 0.8 V and 2.2 nA, respectively, at 50 rpm speed without any illumination, which was further enhanced to 1.8 V and 5.5 nA, respectively, under the influence of laser light illumination. An increase in the motor speed to 100 rpm results in higher VOC (2.4 V) and higher ISC (3.4 nA) as compared to 50 rpm at no illuminating condition. However, it enhances to 3.4 V and 6.0 nA in illuminating conditions. This is attributed to the generation of charge carrier due to triboelectrification, which is further enhanced because of the photovoltaic effect wherein the generation of electron-hole pair occurs due to the shining of light on the side of semiconductors. These synergetic effects have tremendous potential in sustainable energy harvesting.

Intrinsic Control of Interlayer Exciton Generation in Van der Waals Materials via Janus Layers

We demonstrate the possibility of engineering the optical properties of transition metal dichalcogenide heterobilayers when one of the constitutive layers has a Janus structure. We investigate different MoS2@Janus layer combinations using first-principles methods including excitons and exciton-phonon coupling. The direction of the intrinsic electric field from the Janus layer modifies the electronic band alignments and, consequently, the energy separation between dark interlayer exciton states and bright in-plane excitons. We find that in-plane lattice vibrations strongly couple the two states, so that exciton-phonon scattering may be a viable generation mechanism for interlayer excitons upon light absorption. In particular, in the case of MoS2@WSSe, the energy separation of the low-lying interlayer exciton from the in-plane exciton is resonant with the transverse optical phonon modes (40 meV). We thus identify this heterobilayer as a prime candidate for efficient generation of charge-separated electron-hole pairs.

Oxygen vacancy-rich C/Ti3C2/(001)TiO2 hollow microspheres and the photocatalytic degradation of organic pollutants

C/Ti3C2/(001)TiO2 hollow microspheres were prepared by combining the template method, self-assembly and hydrothermal method. Anatase TiO2 nanosheets with rich oxygen vacancies formed on the microspheres and exposed active (001) facets due to the spherical surface. TC was removed 92.3% by C/Ti3C2/(001)TiO2-HMs within 160 min, 28.4% by TiO2 and 49.5% by (001)TiO2/Ti3C2 under the same condition. The high removal efficiency of C/Ti3C2/(001)TiO2-HMs could be because the oxygen vacancies exposed on the surface of (001)TiO2 afforded more active sites, extended the light absorption range and promoted the generation of electron-hole pairs. The composite C/Ti3C2/(001)TiO2 structure could suppress the recombination of electron-hole pairs and prolong their lifetime. The degradation pathway of TC was discussed. The TC removal efficiency of C/Ti3C2/(001)TiO2-HMs was confirmed by treating the wastewater from the Slender West Lake. The production principle of oxygen vacancy could be used for preparing MXene composites with excellent catalytic properties and the C/Ti3C2/(001)TiO2-HMs could be a potential choice for treating wastewater.

Exciton Manipulation for Enhancing Photoelectrochemical Hydrogen Evolution Reaction in Wrinkled 2D Heterostructures.

2D materials' junctions have demonstrated capabilities as metal-free alternatives for the hydrogen evolution reaction (HER). To date, the HER has been limited to heterojunctions of different compositions or band structures. Here, the potential of local strain modulation based on wrinkled 2D heterostructures is demonstrated, whichhelpsto realize photoelectrocatalytically active junctions. By forming regions of high and low tensile strain in wrinkled WS2 monolayers, local modification of their band structure and internal electric field due to piezoelectricity is realized in the lateral direction. This structure produces efficient electron-hole pair generation due to light trapping and exciton funneling toward the crest of the WS2 wrinkles and enhances exciton separation. Additionally, the formation of wrinkles induces an air gap in-between the 2D layer and substrate, which reduces the interfacial scattering effect and consequently improves the charge-carrier mobility. A detailed study of the strain-dependence of the photocatalytic HER process demonstratesa 2-fold decrease in the Tafel slope and a 30-fold enhancement in exchange current density. Finally, optimization of the light absorption through functionalization with quantum dots produces unprecedented photoelectrocatalytic performance and provides a route toward the scalable formation of strain-modulated WS2 nanojunctions for future green energy generation.

Electro-optic fs pulsed laser deflection in KTN crystals using UV illumination.

UV-illuminated, paraelectric-phased potassium tantalate niobate (KTN) single crystals mitigate the beam deformation effects of femtosecond pulsed lasers in KTN deflectors. UV light illumination can control the amount of trapped charge present and minimize domain inversion in KTN deflectors, owing to its generated electron-hole pairs. This enables high beam quality deflection of fs pulsed lasers, with access to larger deflection angles, deflection speeds, and modulation switching ratios. These results enable the use of KTN deflectors in many fs pulsed laser applications and hasten the advancement of fs applications that require these deflection qualities.