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Zinc sulfide/molybdenum disulfide (ZnS@MoS 2) nanorods composites were synthesized by straightforward hydrothermal method. Experimental investigations revealed that the kinetic coefficient (k) of ZnS@MoS 2 was 0.0667 min−1 within 30 min, with a degradation efficiency of 88.6 % toward methylene blue dye in presence of both stirring and light. These performances were notably higher compared to conditions with either light or stirring irradiation alone. Moreover, ZnS@MoS 2 composites not only had excellent catalytic activity but also good cyclic stability. The catalytic mechanism of ZnS@MoS 2 for degradation of MB organic pollutant was further discussed based on relevant characterization. Under both stirring and light conditions, the strong piezoelectric field effectively promoted charge carriers separation and reduced the electron-hole recombination rate, thus enhancing catalytic degradation efficiency for ZnS@MoS 2. This research holds significant importance for the development of high-performance catalysts in wastewater treatments field. [Display omitted] • A novel ZnS@MoS 2 nanorods composites catalyst was rationally designed. • ZnS@MoS 2 exhibited superior catalytic activity and stability toward degradation of methylene blue dye. • Excellent catalytic performance was mainly ascribed to the efficient charge carriers separation. • A possible catalytic mechanism of ZnS@MoS 2 was clarified. [ABSTRACT FROM AUTHOR]

A unique ZnIn 2 S 4 /BiFeO 3 piezoelectric enhanced step-scheme heterojunction was fabricated; the synergistic enhancement of the internal electric field generated in the heterojunction and the alternating piezoelectric field generated in BiFeO 3 induced an exceptional hydrogen evolution rate under the co-excitation of ultrasonic treatment and visible-light irradiation. [Display omitted] An emerging approach that employs both light and vibration energy on binary photo-/piezoelectric semiconductor materials for efficient hydrogen (H 2) evolution has garnered considerable attention. ZnIn 2 S 4 (ZIS) is recognized as a promising visible-light-activated photocatalyst. However, its effectiveness is constraint by the slow separation dynamics of photoexcited carriers. Density functional theory (DFT) predictions have shown that the integration of piezoelectric BiFeO 3 (BFO) is conducive to the reduction of the H 2 adsorption free energy (ΔG H*) for the photocatalytic H 2 evolution reaction, thereby enhancing the reaction kinetics. Informed by theoretical predictions, piezoelectric BFO polyhedron particles were successfully synthesized and incorporated with ZIS nanoflowers to create a ZIS/BFO heterojunction using an ultrasonic-assisted calcination method. When subjected to simultaneous ultrasonic treatment and visible-light irradiation, the optimal ZIS/BFO piezoelectric enhanced (piezo-enhanced) heterojunction exhibited a piezoelectric photocatalytic (piezo-photocatalytic) H 2 evolution rate approximately 6.6 times higher than that of pristine ZIS and about 3.0 times greater than the rate achieved under light-only conditions. Moreover, based on theoretical predictions and experimental results, a plausible mechanism and charge transfer route for the enhancement of piezo-photocatalytic performance were studied by the subsequent piezoelectric force microscopy (PFM) measurements and DFT calculations. The findings of this study strongly confirm that both the internal electric field of the step-scheme (S-Scheme) heterojunction and the alternating piezoelectric field generated by the vibration of BFO can enhance the transportation and separation of electron-hole pairs. This study presents a concept for the multipath utilization of light and vibrational energy to harness renewable energy from the environment. [ABSTRACT FROM AUTHOR]

Photocatalysis represents a powerful way for air purification. The key problem is how to design powerful and stable photocatalysts. This work developed a novel triple functional BaTiO 3 /g-C 3 N 4 /Polytetrafluoroethylene (PTFE) /Cu photocatalyst. The wind-induced piezoelectric field from BaTiO 3 , together with the BaTiO 3 /g-C 3 N 4 heterojunctions, facilitated the separation of photoelectron-hole pairs. Meanwhile, the wind-generated surface charges via triboelectric nanogenerator (TENG) from PTFE/Cu film promoted the formation of ·O 2 - and adsorption of formaldehyde (HCHO) via electrostatic attraction. As a result, the as-prepared BaTiO 3 /g-C 3 N 4 /PTFE/Cu thin film exhibited high activity in degradation of trace HCHO in air under wind blowing and sunlight irradiating, which was 1.5 times higher than pure g-C 3 N 4. Moreover, it also displayed strong stability against leaching owing to the PTFE, leading to a long lifetime during photocatalysis. By spraying films of piezoelectric photocatalysts on car and train surfaces, this study provides an efficient and durable strategy for self-cleaning and air-purification under sunlight-irradiation and wind-blowing. [Display omitted] • Sunlight-driven photocatalytic HCHO degradation on BaTiO 3 /g-C 3 N 4 /PTFE/Cu. • Wind-induced piezoelectric field to inhibit photoelectron-hole recombination. • Wind-induced triboelectric nanogenerator (TENG) to generate O 2 - radicals. • Co-promoting effects on the photocatalyst activity and durability. • Self-cleaning and air-purification on the car and train surface. [ABSTRACT FROM AUTHOR]

[Display omitted] • Designed a series of Bi 2 WO 6 /R-COFs (R=H, F, CH 3 O) heterojunctions. • Synergy of ligand engineering and piezoelectric field improves carrier utilization. • Bi 2 WO 6 /CH 3 O-COFs achieved 93% degradation for sulfamethoxazole within 48 min. • Performance of Bi 2 WO 6 /CH 3 O-COFs is in the middle to upper range of other catalysts. Covalent organic frameworks (COFs) have attracted significant attention in photocatalytic water purification due to their structural designability and sufficiently negative conduction band edges. However, the limited oxidation capacity of single COFs and the low utilization rate of photogenerated carriers result in suboptimal catalytic activity. To address these challenges, we propose a two-level regulation strategy that involves ligand engineering and the introduction of a piezoelectric field to enhance the photocatalytic performance of COF-based photocatalysts. In this study, we combined the piezoelectric material Bi 2 WO 6 with COFs modified with different ligands to design a series of Bi 2 WO 6 /R-COFs (R=H, F, CH 3 O) heterojunctions. Modifying COFs with –OCH 3 functional group tuned the local electron distribution, thereby remarkably improving photogenerated electrons transfer rate constant. Besides, the incorporation of Bi 2 WO 6 generated a piezoelectric field under ultrasound, which provides a strong driving force for photocatalytic. Meanwhile, the construction of the heterojunctions offered multi-channel for the generation of reactive oxygen species. As a result, the photocatalytic degradation rate of Bi 2 WO 6 /CH 3 O-COFs reached 93.5 % toward sulfamethoxazole within 48 min under ultrasound-coupled light irradiation, which is nearly three times higher than Bi 2 WO 6 /H-COFs. Furthermore, Bi 2 WO 6 /CH 3 O-COFs demonstrated superior degradation performance for other pollutants, including p-chlorophenol, bisphenol A, tetracycline, and phenol, with degradation efficiencies of 99.0 %, 98.4 %, 93.5 % and 88.1 %, respectively. This work proposes a two-level modulation strategy that significantly enhances the antibiotic degradation performance of COF-based catalysts, offering a new perspective for the design of photocatalysts for environmental remediation. [ABSTRACT FROM AUTHOR]

The internal electric field induced by a piezoelectric effect is a promising method to suppress rapid electron–hole recombination of photocatalysis for environmental remediation. Herein, we fully utilized both piezoelectric and photocatalytic properties of Bi2WO6 nanoplates for the removal of organic pollutants. The degradation rate constant of piezo-photocatalysis is dramatically boosted to 0.077 min–1 by the Bi2WO6 nanoplates, which is 4.2-fold of the photocatalysis alone. The superior catalytic activity of Bi2WO6 nanoplates was attributed to the accelerated electron–hole pair separation efficiency associated with the enhancement of the piezoelectric effect-induced internal electric field. Based on the theoretical calculation, the bandgap of Bi2WO6 nanoplates would decrease from 1.99 to 1.67 eV under mechanical strains, indicating that broader light harvesting and faster charge transfer can be achieved with the assistance of the piezoelectric field. Accordingly, multiple reactive oxygen species (·O2– and ·OH) are dominant for the degradation activity based on the quenching experiments and EPR analysis, outperforming piezocatalysis and photocatalysis. We have provided a new strategy to improve the catalytic decontamination performance. [ABSTRACT FROM AUTHOR]

In this study, the effects of the piezoelectric polarization field have been investigated on the spectral and power characteristics of In0.2Ga0.8N/GaN superluminescent light-emitting diodes. The Schrödinger and Poisson equations, the rate equations in the multiple quantum well active region and separate confinement heterostructure layers, and the optical propagating equations have been solved in the presence of the piezoelectric field. The results have been compared with results of the case of without piezoelectric field. According to the results, in the presence of piezoelectric field, the red-shift occurs in the spectra, and the width of spectrum increases. Also, the piezoelectric field decreases the peak intensity of spectrum and modal gain of the device. [ABSTRACT FROM AUTHOR]

Abstract Three InGaN/GaN MQWs samples with varying GaN cap layer thickness were grown by metalorganic chemical vapor deposition (MOCVD) to investigate the optical properties. We found that a thicker cap layer is more effective in preventing the evaporation of the In composition in the InGaN quantum well layer. Furthermore, the quantum-confined Stark effect (QCSE) is enhanced with increasing the thickness of GaN cap layer. In addition, compared with the electroluminescence measurement results, we focus on the difference of localization states and defects in three samples induced by various cap thickness to explain the anomalies in room temperature photoluminescence measurements. We found that too thin GaN cap layer will exacerbates the inhomogeneity of localization states in InGaN QW layer, and too thick GaN cap layer will generate more defects in GaN cap layer.

Three similar‐structure InGaN/AlGaN multiple‐quantum‐well near‐ultraviolet (NUV) light‐emitting diodes (LEDs) are utilized to investigate the microscopic effect of defects on strain relaxation. Consistent correlations among the crystal quality, the piezoelectric field (FPZ), the internal quantum efficiency (IQE), and the bandgap shrinkage of NUV LEDs are obtained by investigating the macroscopic characterizations. The difference in crystal quality (or the defect density) of NUV LEDs is found by the ideality factor, the emission microscope image, the Shockley–Read–Hall coefficient, and the IQE. Electroreflectance spectra are used to calculate FPZ of NUV LEDs. FPZ, the IQE, and the peak‐wavelength shift at driving currents are increased with the samples' crystal quality compared to the reference sample. Also, FPZ, the IQE, and the peak‐wavelength shift are decreased with the increase in samples' defect densities. A similar result is found for the bandgap shrinkage. This effect significantly indicates that the strain relaxation is induced by defects. Herein, a model that systematically explains the observed changes in macroscopic properties of NUV LEDs is proposed. [ABSTRACT FROM AUTHOR]

Inorganic light‐emitting diodes (LEDs) possess exceptional advantages for achieving wide wavelength emission ranges. However, certain material limitations should be overcome to alleviate the green gap between InGaN and AlInGaP‐based LEDs. In this study, monolithic red, green, and bluish‐green emissions are achieved by exploiting the blueshift phenomenon, caused by the band‐filling and piezoelectric screening effects. Different‐sized LEDs are designed to control the injection current density on the same wafer. The smaller the size of the LED, the shorter the emission wavelength, resulting in multi‐color emission (from red to bluish‐green). As a result, red, green, and white emissions are individually and simultaneously obtained using dichromatic LED systems, comprising three red and one green LED cell. In addition, a wide range of visible light colors are achieved by adjusting the size of the LEDs and the applies injection current. [ABSTRACT FROM AUTHOR]

The piezoelectric properties of two-dimensional semiconductor nanobubbles present remarkable potential for application in flexible optoelectronic devices, and the piezoelectric field has emerged as an efficacious pathway for both the separation and migration of photogenerated electron-hole pairs, along with inhibition of recombination. However, the comprehension and control of photogenerated carrier dynamics within nanobubbles still remain inadequate. Hence, this study is dedicated to underscore the importance of in situ detection and detailed characterization of photogenerated electron-hole pairs in nanobubbles to enrich understanding and strategic manipulation in two-dimensional semiconductor materials. Utilizing frequency modulation kelvin probe force microscopy (FM-KPFM) and strain gradient distribution techniques, the existence of a piezoelectric field in monolayer WS 2 nanobubbles was confirmed. Combining w/o and with illumination FM-KPFM, second-order capacitance gradient technique and in situ nanoscale tip-enhanced photoluminescence characterization techniques, the interrelationships among the piezoelectric effect, interlayer carrier transfer, and the funneling effect for photocarrier dynamics process across various nanobubble sizes were revealed. Notably, for a WS 2 /graphene bubble height of 15.45 nm, a 0 mV surface potential difference was recorded in the bubble region w/o and with illumination, indicating a mutual offset of piezoelectric effect, interlayer carrier transfer, and the funneling effect. This phenomenon is prevalent in transition metal dichalcogenides materials exhibiting inversion symmetry breaking. The implication of our study is profound for advancing the understanding of the dynamics of photogenerated electron-hole pair in nonuniform strain piezoelectric systems, and offers a reliable framework for the separation and modulation of photogenerated electron-hole pair in flexible optoelectronic devices and photocatalytic applications.

Three InGaN/GaN MQWs samples with varying GaN cap layer thickness were grown by metalorganic chemical vapor deposition (MOCVD) to investigate the optical properties. We found that a thicker cap layer is more effective in preventing the evaporation of the In composition in the InGaN quantum well layer. Furthermore, the quantum-confined Stark effect (QCSE) is enhanced with increasing the thickness of GaN cap layer. In addition, compared with the electroluminescence measurement results, we focus on the difference of localization states and defects in three samples induced by various cap thickness to explain the anomalies in room temperature photoluminescence measurements. We found that too thin GaN cap layer will exacerbates the inhomogeneity of localization states in InGaN QW layer, and too thick GaN cap layer will generate more defects in GaN cap layer. [ABSTRACT FROM AUTHOR]

The piezoelectric field in InGaN/AlGaN-based multiple-quantum-well (MQW) near-ultraviolet light-emitting diodes (LEDs) was measured by electroreflectance and photocurrent spectroscopies. At forward and reverse biases, both electroreflectance and photocurrent spectra were studied by utilizing similar-structure samples with indium contents of ~4.5%, ~5.5%, ~6.5%, and ~7.5%. The influence of MQW and superlattice structures on the electroreflectance spectra was interactively identified. The relation between the defects and the diffusion of Mg acceptors through the defects into the MQWs was also observed and systematically confirmed through the capacitance-voltage characteristics. The effects of diffused Mg acceptors on electroreflectance spectra, depletion width, and the piezoelectric field were found. The strain relaxation caused by the defects was also systematically investigated. The calculated piezoelectric fields of these samples were in good agreement with the theoretically calculated values. [ABSTRACT FROM AUTHOR]

In this paper, anatase TiO2 nanoparticles (14 nm) were coated on the surface of piezoelectric PbTiO3 single crystal fibers by a secondary hydrothermal method, thus a p‐n junction was formed at the interface between TiO2 and PbTiO3. The PbTiO3 fibers were exposed to an external force (by ultrasound) to generate a dynamic piezoelectric electric field in the crystal, which incessantly separates the carriers and improves the photocatalytic efficiency of TiO2. When the ultrasonic frequency is 40 kHz, the TiO2/PbTiO3 fibers have the highest photocatalytic degradation efficiency (96%) for methylene orange (MO), which is up to 60% improvement compared to TiO2 nanoparticles. [ABSTRACT FROM AUTHOR]

Highlights • The optical properties of InAlAs/InP structures have been investigated. • We report the effect of V/III ratio and substrate polarity on the optical properties of InAlAs/InP structures. • The type II recombination has shown a strong dependence on V/III ratio and substrate polarity. • PR signals corresponding to Franz-Keldysh Oscillation (FKO) have been observed, in our samples. Abstract InAlAs alloy were grown on a InP (3 1 1) substrate with different polarity, by metalorganic chemical vapor deposition (MOCVD) growth. Photoluminescence (PL) and Photoreflectance (PR) measurements have been carried out, in our samples, in order to study the V/III flux ratio effect in type-II heterostructures. It is found that, with increasing excitation power density, the PL line was shifted at higher energy side and no saturation of its associated PL intensity was observed. It is a fingerprint of type II transition emission. Yet, the type II interface recombination has shown a strong dependence on AsH3 overpressure and substrate polarity. Therefore, the arsenic overpressures (V/III) affect the piezoelectric (PZ) field and the As/P exchange in our investigated structures. Indeed, we have observed an opposite behavior of type II energy transition shift from A (Indium In face) to B (Phosphorus P face) polarity substrate in respect to V/III flow ratio variation. This fact has been explained as an atomic terminated surface (In or P) in InP substrate. In the other side, PR signals corresponding to Franz-Keldysh Oscillation (FKO) have been observed, in InAlAs/InP (3 1 1). The analysis of their period has allowed one to determine the value of the PZ field in the samples. This result enables us to extract the V/III ratio and the substrate polarity effect on the type II transition shift, in such heterostructures. [ABSTRACT FROM AUTHOR]

We investigate the differences in optoelectronic performances of InGaN/AlGaN multiple-quantum-well (MQW) near-ultraviolet light-emitting diodes by using samples with different indium compositions. Various macroscopic characterizations have been performed to show that the strain-induced piezoelectric field (FPZ), the crystal quality, and the internal quantum efficiency increase with the sample's indium composition. This improved performance is owing to the carrier recombination at relatively defect-free indium-rich localized sites, caused by the local in-plane potential-energy fluctuation in MQWs. The potential-energy fluctuation in MQWs are considered to be originating from the combined effects of the inhomogeneous distribution of point defects, FPZ, and indium compositions. [ABSTRACT FROM AUTHOR]

Piezo-photocatalysis is a frontier technology for converting mechanical and solar energies into crucial chemical substances and has emerged as a promising and sustainable strategy for N 2 fixation. Here, for the first time, defects and piezoelectric field are synergized to achieve unprecedented piezo-photocatalytic nitrogen reduction reaction (NRR) activity and their collaborative catalytic mechanism is unraveled over BaTiO 3 with tunable oxygen vacancies (OVs). The introduced OVs change the local dipole state to strengthen the piezoelectric polarization of BaTiO 3 , resulting in a more efficient separation of photogenerated carrier. Ti 3+ sites adjacent to OVs promote N 2 chemisorption and activation through d-π back-donation with the help of the unpaired d-orbital electron. Furthermore, a piezoelectric polarization field could modulate the electronic structure of Ti 3+ to facilitate the activation and dissociation of N 2 , thereby substantially reducing the reaction barrier of the rate-limiting step. Benefitting from the synergistic reinforcement mechanism and optimized surface dynamics processes, an exceptional piezo-photocatalytic NH 3 evolution rate of 106.7 µmol g -1  h -1 is delivered by BaTiO 3 with moderate OVs, far surpassing that of previously reported piezocatalysts/piezo-photocatalysts. New perspectives are provided here for the rational design of an efficient piezo-photocatalytic system for the NRR., (© 2023 Wiley-VCH GmbH.)

In this study, we compared the device performance of GaInN-based green LEDs grown on c-plane sapphire substrates with a conventional low temperature GaN buffer layer to those with a sputtered-AlN buffer layer. The light output power and leakage current characteristics were significantly improved by just replacing the buffer layer with a sputtered-AlN layer. To understand the origin of the improvement in performance, the electrical and optical properties were compared by means of electro-reflectance spectroscopy, I–V curves, electroluminescence spectra, L–I curves, and internal quantum efficiencies. From the analysis of the results, we concluded that the improvement is mainly due to the mitigation of strain and reduction of the piezoelectric field in the multiple quantum wells active region. [ABSTRACT FROM AUTHOR]

Abstract In this paper we have studied comprehensively annealed and non annealed single quantum well (QW) light emitting diode (LED) structures with different doping concentrations in the barriers. Computations have been carried out through the self-consistent solutions of Schrödinger and Poison equations. Wide changes in the transition energies (TE) and the transition probabilities (TP) are observed due to the doping variation. On annealing, with the increase of diffusion length, the results show that depending on the doping concentration, the TE increases monotonically as well as it goes through red-blue shifts. It is also observed that, for lower doping, TP increases and/or decreases with the diffusion length. The variation of TP also depends on the operating current. After a critical doping in the barriers, on annealing, the optical performances of QW LEDs deteriorate. Through change in doping and annealing, the optical performance of the InGaN/GaN QW LEDs may be improved. This information should be of importance to the optoelectronic designer. Highlights • We have studied annealed and non annealed single QW LED structures with different doping concentrations in the barriers. • Depending on doping,on annealing,the transition energy increases monotonically as well as it goes through red-blue shifts. • At the lower doping, transition probability increases and/or decreases with the diffusion length depending on the current. • After a critical doping in the barriers, on annealing, the optical performances of QW LEDs deteriorate. • Below the critical doping, on annealing, the transition probability significantly increases at higher current. [ABSTRACT FROM AUTHOR]

Although that the continuous tunability of InGaN/GaN QW LEDs, carries the promise of a significant impact in optoelectronics, the reduction of the square of the overlap of electron and hole wave functions ( M e h 2 ) in InGaN/GaN QW LEDs, under certain conditions, is a sizable problem, difficult to overcome. Theoretical investigations have been carried out on the incorporation of Indium (In) in the GaN barrier layers, with an aim of increasing the overlap of electron and hole wave functions. Rigorous studies through the self consistent solution of Schrödinger and Poison equations expose some new and striking results. With suitable doping, the inclusion of In in the barriers can increase M e h 2 to more than two times that of a conventional InGaN/GaN QW LED. In in the barrier along with doping may be suitably utilized to tailor the transition energy and M e h 2 with current density, as desired. The transition energy and the M e h 2 may be made to have a positive or a negative slope with current density or they may be made fairly constant. This paper will outline the theoretical details, computational methodologies, the parameters used, and the striking new results with suitable depictions and discussions. These new information ought to be interesting for current optoelectronics. [ABSTRACT FROM AUTHOR]

We experimentally and theoretically investigate the relationship between the electroreflectance (ER) and photocurrent (PC) spectra, and how they can be utilized to estimate the flat-band voltage and the bandgap energy of the InGaN/GaN-based quantum-well (QW) structure in blue light-emitting diodes. With theoretical modeling of ER and PC spectra, we calculate the changes in both the refractive index ( \Delta \textn ) and optical absorption ( $\Delta \alpha )$ spectra from experimental ER and PC data by using the Kramers–Kronig relation. Then, we compare \Delta \textn (or $\Delta \alpha )$ spectra obtained differently from the ER and PC data and try to comprehend their physical meanings interactively. From these combined studies, we propose an exact method of determining the flat-band voltage, the piezoelectric field, the emission energy, the effective bandgap energy, and the Stokes shift of a QW structure under the quantum-confined Stark effect (QCSE). [ABSTRACT FROM PUBLISHER]

Nitride-based heterostructures with InGaN/GaN multiple quantum wells in active region were investigated by electroreflectance spectroscopy. Two spectral lines with the stable difference between their phase parameters and slightly different energies were observed in the electroreflectance spectra under various bias voltages. Using models taking into account interference effects it was shown that electroreflectance signal at low amplitude of modulation voltage originates from the first and the last quantum wells in active region. The difference between built-in electric field strengths in these two quantum wells were estimated as 270–320 kV/cm. [ABSTRACT FROM AUTHOR]

The InGaN/GaN multiple quantum well structures on sapphire substrates were observed by scanning electron microscope (SEM) using obliquely polished samples. From the contrast change across the p-n junction, piezoelectric fields were deduced. The direction of the piezoelectric field was consistent with the theoretical prediction, but the strength was smaller. Strain-stress calculation showed that strain relief along the surface is not significant; therefore, it is not responsible for the smaller value of the obtained field strength. As one cause, carrier distribution in the wells under the steady-state condition was pointed out. Potential profile along the surface was different for the samples polished from the surface and from the substrate. This was confirmed by the SEM contrast profile. [ABSTRACT FROM AUTHOR]

A hurdle encountered in the InGaN/GaN LED, is, that the overlap of electrons and holes decreases significantly under certain conditions which deteriorates the performance of the LED. Several methods are being studied for increasing the overlap of electrons and holes of which introduction of a staggered InGaN/GaN QW, in place of the rectangular QW seems promising. In this paper we have studied comprehensively, different forms of the staggered QW; single sided and symmetric, with different Indium compositions in the well. Computations have been carried out through the self-consistent solutions of Schrödinger and Poison equations. We have studied the band structures, fields, the carrier distributions, the transition energies and the overlap of the electron and hole wave functions with current densities. It has been found that the overlap of electrons and holes increases significantly for the staggered QWs at the same current. The best results are obtained for the symmetrically staggered QWs, where the overlap increases even up to three times that of the rectangular QW and using this structure the operating current may be decreased by more than two orders of magnitude to obtain the same transition energy. The theoretical and computational results will be presented with suitable discussions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Build 1T@2H-MoS 2 /Bi 2 S 3 heterojunction to degrade organic pollutants in the dark. • Mechanical force can induce 1T@2H-MoS 2 /Bi 2 S 3 to establish a piezoelectric field. • The piezoelectric field further promotes free carrier separation in heterojunctions. • The piezoelectric field bends the Fermi level to induce free carrier rearrangement. • Free carriers gather at high redox potential sites to participate in the reaction. In recent years, odd-layer 2H-MoS 2 nanosheets have proved to be promising piezoelectric catalysts to solve environmental pollution problems. To further improve the 2H-MoS 2 piezoelectric performance, 1T@2H-MoS 2 /Bi 2 S 3 heterostructures were prepared using Bi 2 WO 6 as a precursor. Compared with 1T@2H-MoS 2 , 1T@2H-MoS 2 /Bi 2 S 3 piezoelectric degradation of methylene blue is significantly improved under dark and ultrasonic conditions. It is attributed to the free carrier separation being facilitated through the type-II heterojunction, and the electron transport paths are optimized by high conductivity 1T-MoS 2. In the radical scavenging experiment and ESR measurement, the hydroxyl radicals produced by 1T@2H-MoS 2 /Bi 2 S 3 are more than those produced by 1T@2H-MoS 2 in the piezoelectric catalysis process. It indicates that the piezoelectric field can realize the free carrier rearrangement by adjusting the band structure. Thus, free carriers in the 1T@2H-MoS 2 /Bi 2 S 3 heterojunction have the opportunity to shift to the position with high redox potential to unlock the higher redox capability compared with the lower redox potential of the original 1T@2H-MoS 2 and Bi 2 S 3. In conclusion, this study shows that the piezoelectric field can regulate the free carrier rearrangement in the type-II heterojunction, allowing the type-II heterojunction can not only efficiently separate free carriers but also maintain a high redox potential. [ABSTRACT FROM AUTHOR]

In this study, we compared the device performance of GaInN-based green LEDs grown on c-plane sapphire substrates with a conventional low temperature GaN buffer layer to those with a sputtered-AlN buffer layer. The light output power and leakage current characteristics were significantly improved by just replacing the buffer layer with a sputtered-AlN layer. To understand the origin of the improvement in performance, the electrical and optical properties were compared by means of electro-reflectance spectroscopy, I⁻V curves, electroluminescence spectra, L⁻I curves, and internal quantum efficiencies. From the analysis of the results, we concluded that the improvement is mainly due to the mitigation of strain and reduction of the piezoelectric field in the multiple quantum wells active region.

We investigate the differences in optoelectronic performances of InGaN/AlGaN multiple-quantum-well (MQW) near-ultraviolet light-emitting diodes by using samples with different indium compositions. Various macroscopic characterizations have been performed to show that the strain-induced piezoelectric field (FPZ), the crystal quality, and the internal quantum efficiency increase with the sample’s indium composition. This improved performance is owing to the carrier recombination at relatively defect-free indium-rich localized sites, caused by the local in-plane potential-energy fluctuation in MQWs. The potential-energy fluctuation in MQWs are considered to be originating from the combined effects of the inhomogeneous distribution of point defects, FPZ, and indium compositions.

Internal polarizations field which take place in quantum structures of group-III nitrides have an important consequence on their optical properties. Optical properties of wurtzite AlGaN/GaN quantum well (QW) structures grown by MBE and MOCVD on c-plane sapphire substrates have been investigated by means of photoluminescence (PL) and time resolved photoluminescence (TRPL) at low-temperature. PL spectra exhibit a blue-shifted emission of AlGaN/GaN quantum well (QW) nanostructures by decreasing the barrier width contrary to the arsenide system. The trend of the barrier-width dependence of the internal polarization field is reproduced by using simple electrostatic arguments. In addition the effect of well width variation on the optical transition and decay time of GaN MQWs have been investigated and it has been shown that the screening of the piezoelectric field and the electron-hole separation are strongly dependent on the well thickness and have a profound effect on the optical properties of the GaN/AlGaN MQWs.

InGaN based light emitting diodes (LEDs) suffer from the adverse influence of band tilt due to the piezoelectric field induced quantum-confined Stark effect (QCSE). We demonstrate incremental recovering of QCSE induced band tilt along with its impact on the performance of III-nitride-based green LEDs (at 532.5 nm) by applying external stress. The external tensile stress determined by peak shift in Raman spectroscopy ranges from 0 to 0.95 GPa as the wafer curvature increases from 0 to 2.32 m −1 . Compared to the pristine LEDs wafer, the PL emission peak undergoes an increase in intensity of 61%, blue shift of 11.5 nm and peak narrowing of 5.5 nm through bending the wafer to the maximum curvature of 2.94 m −1 . These results all verify the adverse effects due to the presence of the QCSE effect in green LEDs, which can be effectively suppressed by the external tensile stress, thus improving both the radiative recombination rate and electron-hole wave function overlaps. Under optimal conditions, the light output power is enhanced by 12.4% in chips on epi-wafer (COW) form under the wafer to the maximum curvature of 2.94 m −1 without affecting the voltage required to reach the same current density. This work sheds light on implementation of the facile design in real LEDs devices to achieve significant increase in light output without involving any complicated transferring process onto flexible substrates. [ABSTRACT FROM AUTHOR]

Stress states in GaN epilayers grown on Si (1 1 1) and c -plane sapphire, and their effects on built-in piezoelectric field induced by compressive stress in InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) were investigated using the electroreflectance (ER) spectroscopic technique. Relatively large tensile stress is observed in GaN epilayers grown on Si (1 1 1), while a small compressive stress appears in the film grown on c -plane sapphire. The InGaN/GaN MQWs of LED on c-plane sapphire substrate has a higher piezoelectric field than the MQWs of LEDs on Si (1 1 1) substrate by about 1.04 MV/cm. The large tensile stress due to lattice mismatch with Si (1 1 1) substrate is regarded as external stress. The external tensile stress from the Si substrate effectively compensates for the compressive stress developed in the active region of the InGaN/GaN MQWs, thus reducing the quantum-confined Stark effect (QCSE) by attenuating the piezoelectric polarization from the InGaN layer. [ABSTRACT FROM AUTHOR]

In this paper, we present a procedure and model for determining the electric field F w q that governs the quantum-confined Stark effect (QCSE) in each quantum well of an unbiased [111]-oriented InGaAs/InGaAsP multi-quantum well semiconductor optical amplifier (MQW-SOA) with a p-i-n diode structure. For a given input power value, we estimated the electric field F w q by substituting into simple theoretical expressions the experimentally determined output power, some structural parameters, and the voltage provided by a digital multimeter operating in diode test mode connected between the electrodes of the amplifier. The model results were used to estimate the Stark shift of the MQW-SOA excitonic resonances, which were blue-shifted by 5.21 meV when the input power was varied from –15.9 to 5.17 dBm. The excellent agreement between the experimental and calculated results allowed the validation of the model. To the authors' knowledge, this is the first time that it has been experimentally demonstrated the presence of excitonic resonances in an MQW-SOA and the blue shift that they exhibit when the input power alters the QCSE. Finally, we used our model to investigate the impact of simultaneously modifying the strain in the quantum wells and the p-i-n junction built-in electric field (F p i n) on the variability of F w q with respect to the input power. In fact, for zero input power, by modifying F p i n , we compensate for the fluctuation that F w q undergoes when the strain is altered. Every time the strain is changed, and F w q is compensated, a new operating regime is obtained. We found that the larger the operating regime strain, the less input power is required to vary | F w q | by a given amount. For the MQW-SOA used in this work, about half the input power would be required to induce a 1 MV/m change in | F w q | if an operating regime with a strain of –1.52 replaced one with a strain of –1.33. • A study of the QCSE in an unbiased [111]-oriented MQW-SOA is presented. • The electric field that governs the QCSE is practically determined for any input power. • An approximation for the spatial separation of electron–hole pairs in QWs is proposed. • Stark blue shifts of the 1s hh exciton are experimentally demonstrated in an MQW-SOA. • The impact of modifying the strain and the PIN field on the QCSE is investigated. [ABSTRACT FROM AUTHOR]

The influence of strain and barrier/well thickness ratio on recombination processes in multi-quantum well (MQW) Al 0.1 Ga 0.9 N/GaN structures was investigated using X-ray diffraction and Raman and photoluminescence spectroscopies. The deformation state of the wells and barriers was determined. In addition, the value of the polarization fields, the density of polarization charges, and the positions of energy levels for optical transitions within the quantum wells were calculated. It was established that compressive strain in the buffer layer as well as in the layers of the MQWs with respect to the buffer layer lead to the piezoelectric fields having equal sign in the well and the barrier. As a result, the recombination of donor–acceptor pairs dominates over transitions between electron and hole states in the quantum well. [ABSTRACT FROM AUTHOR]

The emission spectrum of the conventional InGaN/GaN quantum well (QW) diode changes remarkably with the operating current. The degree of change depends on the constituent parameters of the QW. A detailed investigation has been carried out through the self-consistent solutions of the Schrödinger and Poisson equations to elucidate the change of the emission energy and the transition probability with the operating current and to explore how the changes depend on the well width of the QW and the In mole fraction. It is found that choosing suitable values of the mentioned parameters, the emission energy may be kept almost constant with current, or it may be tuned over a wide range through current. The two possibilities have immense importance for optoelectronic device applications. [ABSTRACT FROM AUTHOR]

The optical band-gap has been investigated in detail in (InAs)GaAs/AlGaAs HEMTs structures using a photoluminescence spectroscopy. A power dependent PL study allows highlighting the different causes of the shift band-gap. We can explain the shift by more effects: quantum confined Stark effect (QCSE), band-gap renormalisation (BGR) and Burstein–Moss (BM) effects. We are interested in these effects based on the carriers’ behaviour inside GaAs and InAs/GaAs channels. These effects were affected by carrier concentration depending on Si-δ-doping density and AlGaAs spacer layer thickness as well as the strain relaxation effect. The photoluminescence characterisations have been studied in the 1–10 W cm −2 power range. [ABSTRACT FROM AUTHOR]

Based on the assumption that the physical fields close to the V-notch vertex are expressed by the series asymptotic expansions, the evaluation of singularity exponents for piezoelectric V-notches is turned into solving the characteristic values of ordinary differential equations under given boundary conditions, which are a set of equations with variable coefficients and solved by the interpolating matrix method developed by part of the authors before. The singularity analysis for V-notches under in plane and out of plane conditions is taken into account. Numerical results show that all the singularity exponents and the characteristic angular functions can be evaluated synchronously without the need of solving the transcendental equations iteratively. The present method is not only suitable for the singularity analysis of V-notches, but also for cracks and interface ends, without encountering the problem of ill-posed equations when the complex potential function method for the singularity analysis of V-notches is degenerated to analyze the singularity of cracks. [ABSTRACT FROM AUTHOR]

The built-in piezoelectric field induced by compressive stress in InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) was investigated using the electric field dependent electroreflectance (ER) spectroscopic method. InGaN/GaN MQW structures were prepared on sapphire substrates of different thicknesses. Thinning the sapphire substrate enables control of the compressive stress by changing the curvature of the wafer bowing. The wafer bowing-induced mechanical stress alters the piezoelectric field in the InGaN/GaN MQW. The flat band voltage, estimated by measuring the applied reverse bias voltage that induces a 180° phase shift in the ER spectra, was decreased from −11.21 V to −10.51 V by thinning the sapphire substrate thickness from 200 to 60 μm. To calculate the piezoelectric field (F pz ) from the compensation voltage, the depletion width was obtained from the capacitance–voltage measurement. The F pz estimated from the energy shift in ER peak in a bias range from 0 to −12 V was changed by 110 kV/cm. [ABSTRACT FROM AUTHOR]

By die-attaching GaN-based light-emitting diodes (LEDs) on Si substrates with eutectic AuSn solder, the external quantum efficiency of the LEDs was enhanced by 3.5% at a current input of 120 A/cm2. The enhancement of the external quantum efficiency of the die-attached LED chips is attributed to a reduction in compressive stress and piezoelectric fields in quantum wells after eutectic AuSn die-attachment. Raman and photoluminescence analyses were used to estimate the reduction in compressive stress and piezoelectric field in quantum wells, which is 277.8 MPa and 0.056 MV/cm, respectively, after AuSn die-attachment. [ABSTRACT FROM PUBLISHER]

Although the high performance device designs based on piezotronic effect have obtained immense progress, the unique electronic properties of third-generation semiconductor are still required to be deeply illustrated when it is accessible to reveal novel physical states or go beyond performance limitation of traditional semiconductor devices. Strain-produced piezoelectric polarization in the third-generation semiconductors provides a strong power to manipulate the interfacial electronic behaviors particular for two-dimensional electron gas (2DEG) which plays an important role in various exotic quantum states as well as realistic applications. In this study, we base on AlGaN/GaN heterostructure to comparatively explore the impact of piezotronic effect and other effects on the 2DEG. The electron concentration, electrical field, and ground states of the 2DEG are investigated by performing the self-consistent calculation. The results demonstrate that similar to the modulation of gate voltage, stress is also an effective tool to control electron concentration of 2DEG by interfacial piezoelectric field. In contrast with the long-range electric field provided by gate voltage, stress-induced piezoelectric field is very strong short-range field with the level of MV/cm, indicating a more precise mean to control 2DEG. Moreover, piezotronic effect can sufficiently raise the density of state of quantum ground state by decreasing energy level, and thus can be used to enhance Rashba spin-orbit coupling in wide-gap piezoelectric semiconductors. This study provides a deep insight of using piezotronic effect to control the 2DEG, and opens an avenue for manipulating spin degree of freedom. We base on AlGaN/GaN heterostructure to comparatively explore the impact of piezotronic effect and other effects on the 2DEG. The results demonstrate that stress-induced piezoelectric field is very strong short-range field with the level of MV/cm, indicating that stress is a more precise operating way to control 2DEG in comparison with gate voltage. Moreover, piezotronic effect can sufficiently raise the density of state of quantum ground state by decreasing energy level, and thus can be used to enhance Rashba spin-orbit coupling in wide-gap piezoelectric semiconductors. [Display omitted] • Two-dimensional electron gas can be effectively controlled by piezoelectric properties. • Piezoelectric polarization offers a strong short-range electric field to precisely modulate electronic properties. • Piezotronic effect has potential to enhance spin-orbit coupling in third-generation semiconductors. [ABSTRACT FROM AUTHOR]

Abstract: Crystal orientation effects on intersubband transition properties of (11n)-oriented ZnCdTe/ZnTe quantum dots (QDs) were investigated by using the continuum elasticity and effective mass theories. In a range of the crystal angle below 30°, the subband energy difference is nearly independent of the crystal angle. On the other hand, it gradually increases with crystal angle when the angle exceeds 30° and becomes a maximum near the angle ( ) corresponding to a (111) crystal orientation due to the piezoelectric field effect. The transition matrix element becomes a maximum near the angle and a minimum near the crystal angle . [Copyright &y& Elsevier]

Abstract: InGaN-based photovoltaic structures with different indium contents in InGaN active layer were investigated. The peak wavelengths of the photoluminescence (PL) spectra were measured at 464.3nm and 525.4nm for the blue-photovoltaic device (B-PV) and the green-photovoltaic device (G-PV) structures, respectively. The flat-band voltage was observed at −8V through the lower temperature (10K) bias-dependent PL measurement in B-PV device, but the flat-band voltage cannot be observed in G-PV device when the reverse bias was larger than −15V. The piezoelectric field (PZ) of the G-PV device was larger than the B-PV device through the bias-dependent PL measurement that was caused by the large lattice mismatch between the GaN and InGaN layers with high indium content. The small open-circuit voltage and the larger short-circuit current density (Jsc) were measured in the G-PV device compared with the B-PV device. The large Jsc value of the G-PV device was caused by the large photocurrent escaped from the PZ-field induced tilted-band structure in InGaN active layer. The InGaN-based PV devices with a large indium content and a lager PZ field had potential for the InGaN photovoltaic device applications. [Copyright &y& Elsevier]

Abstract: Differential phase contrast microscopy has become known as a high resolution imaging technique for magnetic micro-structures in the past. The method senses the local induction by measuring the deflection of the probe beam after it passes through a specimen area carrying a magnetic field. Little attention has been paid, however, to the fact that this technique is also capable of measuring electric fields. An application of the technique to measure piezoelectric polarization fields inside multi-layered structures such as quantum wells is demonstrated. For this purpose, piezoelectric fields within non-centrosymmetric crystal structures, based on GaN/InGaN/GaN quantum wells, are investigated. It can be shown that the technique is sensitive to these fields and yields detailed information about the field distribution. The specific information and experimental limitations as well as artefacts of the technique will be discussed in detail and first measurements are shown. The main advantages turn out to be high sensitivity for electric fields, combined with a very high resolution, which is limited only by the STEM probe size. Another advantage is the large achievable field of view. [Copyright &y& Elsevier]

The internal electric field due to the piezoelectric effect in an InGaN/GaN multiple quantum-well structure is measured via photoluminescence (PL), electroreflectance (ER), and photocurrent (PC) spectroscopies, and the measurement results are compared with each other and with theoretically calculated values. The flat-band voltage is estimated by measuring the applied bias voltage that induces the maximum in PL peak energy by the quantum-confined Stark effect, 180^\circ phase shift in the ER spectrum, and the smallest band tail state in the PC spectrum. The internal electric fields estimated by the PL, PC, and ER spectra are -1.81, -2.12\pm 0.14, and -2.04∼MV/cm, respectively. The measured piezoelectric fields are in good agreement with theoretically calculated values. Possible factors affecting piezoelectric field measurements are discussed from various perspectives. [ABSTRACT FROM AUTHOR]

Electron holography was used to determine the piezoelectric fields across an InGaN/GaN quantum-well structure. Holograms were taken with the sample intentionally tilted such that adjacent layers overlapped. The phase changes in the overlapping regions were analyzed to determine the piezoelectric fields in each well. It was shown that the piezoelectric field was strongest at the central part of the quantum-well structure. The field strength averaged over eight InGaN wells was ~2.2 MV/cm. [ABSTRACT FROM AUTHOR]

Abstract: Recent experimental investigations revealed that the biaxial stress in thin InGaN layers grown on thick GaN layer induces a large piezoelectric field along [0001] orientation that causes red-shift in optical transitions and reduction in oscillator strengths because of spatial separation of the electron and hole wave functions. In this Letter based on theoretical modeling we determined the well width z-dependent effect on red-shifted quantum-confined Stark effect (QCSE) in GaN/In x Ga1− x N () strained quantum well structures. Analyses are based on the solution of Schrödinger equation in a finite well including the internal piezoelectric electric field (F) due to the strained polarization as the perturbation potential. Our theoretical results show: (1) the red-shift in optical transition has a quadratic well-width form as it is for infinite wells (Davies, 1998) , (2) assuming the model based on a carrier effective mass dependence on the width of quantum wells, , fits the experimental data (Takeuchi et al., 1997) much more accurate compare to the model with constant effective mass, . [Copyright &y& Elsevier]