Polarization-induced Electric Fields Research Articles

Effect of Electric Field on Carbon Encapsulation and Catalytic Activity of Pd for Efficient Formic Acid Decomposition

Utilizing carbon coated ferroelectric materials to introduce polarization‐induced electric field (PIEF) stimulates research of electric field‐assisted catalysts for various reactions. However, effect of PIEF on carbon coating mechanism has not been studied. Herein, tourmaline nanoparticles (TNPs) with spontaneous dipole moments were applied as the PIEF supplier and sucrose was utilized as the carbon precursor to synthesize carbon coated TNPs (TNP@SC) to uncover the influence of PIEF on the carbon coating and catalytic activity of Pd toward formic acid decomposition (FAD). PIEF enhanced the adsorption capacity of TNPs for the caramelized intermediate species and increased H+ concentration by facilitating water ionization. Polymerization of adsorbed caramelized intermediate species on TNPs was accelerated. Uniformly coated carbon layer with more defects, larger specific surface area, and higher porosity was coated on TNP. Such surface properties of the carbon layer were beneficial for strongly anchoring ultrafine Pd nanoparticles. Owing to the specific properties of carbon layer and existence of PIEF, Pd/TNP@SC exhibited higher FAD activity than the catalysts absent of PIEF. Stronger PIEF leaded to higher initial turnover frequency. This study provides guidance to supply electric field for catalysis.

Synergistic Interactions of Bulk Polarization and Built-In Electric Field Inducing 2D/2D S-Scheme Homojunction Toward Enhanced Photocatalytic Performance.

The rational design of S-scheme photocatalysts, achieved by serially integrating two different semiconductors, represents a promising strategy for efficient charge separation and amplified photocatalytic performance, yet it remains a challenge. Herein, ZnIn2S4 (ZIS) and oxygen-doped ZnIn2S4 (O-ZIS) nanosheets are chosen to construct a homojunction catalyst architecture. Theoretical simulations alongside comprehensive in situ and ex situ characterizations confirm that ZIS and O-ZIS with noncentrosymmetric layered structures can generate a polarization-induced bulk-internal electric field (IEF) within the crystal. A robust interface-IEF is also created by the strong interfacial interaction between O-ZIS and ZIS with different work functions. Owing to these features, the O-ZIS/ZIS homojunction adopts an S-scheme directional charge transfer route, wherein photoexcited electrons in ZIS and holes in O-ZIS concurrently migrate to their interface and subsequently recombine. This enables spatial charge separation and provides a high driving force for both reduction and oxidation reactions simultaneously. Consequently, such photocatalyst exhibits an H2 evolution rate up to 142.9µmol h-1 without any cocatalysts, which is 4.6- and 3.4-fold higher than that of pristine ZIS and O-ZIS, respectively. Benzaldehyde is also produced as a value-added oxidation product with a rate of 146.9µmol h-1. This work offers a new perspective on the design of S-scheme systems.

Strain modulation effect of superlattice interlayer on InGaN/GaN multiple quantum well

The strong piezoelectric field in InGaN/GaN heterostructure quantum wells severely reduces the light emission efficiency of multiple quantum well (MQW) structures. To address this issue, a strain modulation interlayer is commonly used to mitigate the piezoelectric polarization field and improve the luminescence performance of the devices. To investigate the influence and mechanism of strain modulation in the InGaN/GaN superlattice (SL), epitaxial wafers with an n-type InGaN/GaN SL interlayer sample, and their corresponding control samples are prepared. The measured temperature-dependent photoluminescence (PL) spectra of the epitaxial wafers, show that the introduction of an SL interlayer leads to a shorter-wavelength emission and enhancement of internal quantum efficiency. As the temperature increases, a blue shift of the PL peak is observed. However, for the sample with an SL interlayer, the blue shift of the PL peak with temperature increasing is relatively small. Electroluminescence (EL) experiments indicate that the introduction of an SL interlayer significantly increases the integrated intensity of the EL peak and reduces its full width at half maximum. These phenomena collectively indicate that the incorporation of a superlattice interlayer can partly suppress the quantum-confined Stark effect (QCSE) that affects the light emission efficiency. Theoretical calculations show that the introduction of a superlattice strain layer before growing an active multiple quantum well can weaken the polarization-induced built-in electric field in the active quantum well, reduce the tilt of the energy band in the multiple quantum well active region, increase the overlap of electron and hole wave functions, enhance the emission probability, shorten the radiative recombination lifetime, and promote competition between radiative recombination and non-radiative recombination, thereby achieving higher recombination efficiency and improving light emission intensity. This study provides experimental and theoretical evidence that the strain modulation SL interlayer can effectively improve the device performance and offer guidance for optimizing the structural design of devices.

Facile construction of tourmaline@MOFs composites via micron-scale electrical polarization to enhance photodegradation of gaseous formaldehyde

The functional composition approach for promoting metal–organic framework (MOF)-based photo-catalysis has usually been hampered by grain incompatibility, resulting in an insufficient contact interface. Herein, spontaneously polarized tourmaline (TM) was proposed to be combined with MOFs to generate a polarization-induced electric field on a micron-scale, which could improve HCHO photocatalytic degradation performance. Based on the micron-scale electrical polarization strategy, facile micron-interface construction in TM-ZrO2@NU66 (NH2-UiO-66, UiO = University of Oslo) and TM-TiO2@NM125 (NH2-MIL-125, MIL = Materials Institute Lavoisier) dramatically improved the remnant polarization (2.8 and 2.4 times), charge transfer rate (3.8 and 5.9 times), and •OH/•O2-/h+ formation. Consequently, two TM@MOFs exhibited 6–90 times higher formaldehyde removal rates than previously reported state-of-the-art photocatalysts. The fabricated composites exhibited a high mineralization rate, deep formaldehyde removal (>80 % of total organic carbon (TOC) removal within 150 min), and outstanding recycling stability. This study narrates an innovative and straightforward method for creating composites based on MOFs, significantly reducing the challenges associated with ensuring MOF compatibility with other functional materials.

Hydroxyl-groups engineering of triazole-based polymers for efficient photocatalytic hydrogen peroxide production

Photocatalytic hydrogen peroxide (H2O2) production via indirect two-electrons oxygen reduction reaction (indirect 2e−ORR) is severely limited by the unsatisfactory O2/H+ mass transfer and slow interfacial charge transfer efficiency. Herein, side-engineered triazole-based polymers (TZPs) are reported for efficient photocatalytic H2O2 production without sacrificial agent. The photoelectric properties, mass transfer and photocatalytic activities can be tuned by the distribution of hydroxyl groups on the benzenetricarboxaldehyde ring of TZPs. Theoretical calculations reveal that the asymmetric 2-hydroxyl-1,3,5-benzenetricarboxaldehyde monomer renders TZP-OE the largest dipole moment (2.5814 Debya) and the lowest first excited state index (3.654 eV). Moreover, the hydroxyl group optimizes the H2O wettability of TZPs. Benefiting from the enlarged intermolecular polarization-induced built-in electric field and excellent hydrophilicity, TZP-OE exhibits excellent photocatalytic H2O2 evolution rate of 1618 μmol/L under visible light irradiation, which is 3.5 times that of TZP-NE. Therefore, side-group engineering provides a new approach for the regulation of catalyst structure and photocatalytic activity.

Solution-Grown MAPbBr3 Single Crystals for Self-Powered Detection of X-rays with High Energies above One Megaelectron Volt.

Perovskite single crystals are actively studied as X-ray detection materials with enhanced sensitivity. Moreover, the feasibility of using perovskites for self-powered devices such as photodetectors, UV detectors, and X-ray detectors can significantly expand their application range. In this work, the charge carrier transport and photocurrent properties of MAPbBr3 single crystals (MSCs) are improved by the mechanochemical surface treatment using glycerin combined with an additional electrode design that forms an ohmic contact. The sensitivity of MSC-based detectors and pulse shape generated by X-rays are enhanced at various bias voltages. The synthesized MSC detectors generate direction-dependent photocurrents, which indicate the presence of a polarization-induced internal electric field. In addition, photocurrent signals are produced by X-rays with energies greater than 1 MeV under a zero-bias voltage. This work demonstrates a high application potential of perovskites as self-powered detectors for X-rays with energies exceeding 1 MeV.

Positive direction of polarization-induced electric field improves formic acid electrooxidation on Pd

Positive direction of polarization-induced electric field improves formic acid electrooxidation on Pd

Lattice-matched AlInN/GaN bottom DBR impact on GaN-based vertical-cavity-surface-emitting laser diodes: systematical investigations.

In this paper, by using advanced numerical models, we investigate the impact of the AlN/GaN distributed Bragg reflector (DBR) and AlInN/GaN DBR on stimulated radiative recombination for GaN-based vertical-cavity-surface-emitting lasers (VCSELs). According to our results, when compared with the VCSEL with AlN/GaN DBR, we find that the VCSEL with AlInN/GaN DBR decreases the polarization-induced electric field in the active region, and this helps to increase the electron-hole radiative recombination. However, we also find that the AlInN/GaN DBR has a reduced reflectivity when compared with the AlN/GaN DBR with the same number of pairs. Furthermore, this paper suggests that more pairs of AlInN/GaN DBR will be set, which helps to even further increase the laser power. Hence, the 3dB frequency can be increased for the proposed device. In spite of the increased laser power, the smaller thermal conductivity for AlInN than AlN results in the earlier thermal droop in the laser power for the proposed VCSEL.

Self-powered dual-wavelength polarization-sensitive photodetectors based on ZnO/BiFeO3 heterojunction

The ferroelectric-photovoltaic (FE-PV) devices have received growing attention for the FE polarization-induced internal electric field to facilitate the separation and transport of the photogenerated electron-hole (e-h) pairs in the FE semiconductors. However, the low photocurrent output of the FE-PV devices is still a key challenge for the application and development of these devices. Here, ZnO/BiFeO3 (BFO) heterojunction photodetectors (PDs) are designed to utilize the built-in electric field at the heterojunction interface of ZnO nanoparticles (NPs)/BFO and the spontaneous FE polarization field of BFO to promote the separation and transport of the photogenerated carriers. ZnO nanorod arrays (NRs) grown on ZnO NPs provide a directional transport channel for the photogenerated electrons. ZnO/BFO heterojunction devices demonstrate a self-powered photoresponse in the ultraviolet (UV) and visible (vis) dual-wavelength. The devices show a responsivity of 1.32 mA/W, response time (τrise/τdecay) of 12.6/44.9 ms at 420 nm light illumination. In addition, the polarization imaging function of ZnO/BFO PDs is explored by extending the polarization photocurrent mapping values of a single PD unit to a complex array of optical information image processing utilizing the polarization-sensitive properties of BFO without external bias.

Boosting Photocatalytic Water Oxidation on Photocatalysts with Ferroelectric Single Domains.

Ferroelectric materials are considered as promising photocatalysts due to their efficient charge separation via apolarization-induced built-in electric field. However, the polydomain structures hinder spatial charge separation and transfer due to the cancellation of polarization vectors in the domains. Inthiswork, taking BiFeO3 (BFO) as a prototype, single-domain BFO nanosheets with visible-light absorption are prepared, as evident by piezoresponse force microscopy (PFM), spatially resolved surface photovoltage spectroscopy (SRSPS), and photodeposition experiments. The single-domain BFO nanosheets show nine times activity in photocatalytic water oxidation reaction under visible-light irradiation, compared with that of the polydomain BFO particles. With the asymmetric driving force for charge separation in a single domain, selective deposition of cocatalysts further enhances the photocatalytic activity of single-domain ferroelectric BFO nanosheets. These results demonstrate the role of thesingle-domain structure in constructing the driving force of charge separation in ferroelectric photocatalysts. The fabrication of single-domain structures in ferroelectric photocatalysts to achieve enhanced photocatalytic activity offers a path to efficiently utilize the photogenerated charges in solar energy conversion.

Systematic Errors of Electric Field Measurements in Ferroelectrics by Unit Cell Averaged Momentum Transfers in STEM.

When using the unit cell average of first moment data from four-dimensional scanning transmission electron microscopy (4D-STEM) to characterize ferroelectric materials, a variety of sources of systematic errors needs to be taken into account. In particular, these are the magnitude of the acceleration voltage, STEM probe semi-convergence angle, sample thickness, and sample tilt out of zone axis. Simulations show that a systematic error of calculated electric fields using the unit cell averaged momentum transfer originates from violation of point symmetry within the unit cells. Thus, values can easily exceed those of potential polarization-induced electric fields in ferroelectrics. Importantly, this systematic error produces deflection gradients between different domains seemingly representing measured fields. However, it could be shown that for PbZr0.2Ti0.8O3, many adjacent domains exhibit a relative crystallographic mistilt and in-plane rotation. The experimental results show that the method gives qualitative domain contrast. Comparison of the calculated electric field with the systematic error showed that the domain contrast of the unit cell averaged electric fields is mainly caused by dynamical scattering effects and the electric field plays only a minor role, if present at all.

A polarization mismatched p-GaN/p-Al0.25Ga0.75N/p-GaN structure to improve the hole injection for GaN based micro-LED with secondary etched mesa

A low hole injection efficiency for InGaN/GaN micro-light-emitting diodes (μLEDs) has become one of the main bottlenecks affecting the improvement of the external quantum efficiency (EQE) and the optical power. In this work, we propose and fabricate a polarization mismatched p-GaN/p-Al0.25Ga0.75N/p-GaN structure for 445 nm GaN-based μLEDs with the size of 40 × 40 μm2, which serves as the hole injection layer. The polarization-induced electric field in the p-GaN/p-Al0.25Ga0.75N/p-GaN structure provides holes with more energy and can facilitate the non-equilibrium holes to transport into the active region for radiative recombination. Meanwhile, a secondary etched mesa for μLEDs is also designed, which can effectively keep the holes apart from the defected region of the mesa sidewalls, and the surface nonradiative recombination can be suppressed. Therefore, the proposed μLED with the secondary etched mesa and the p-GaN/p-Al0.25Ga0.75N/p-GaN structure has the enhanced EQE and the improved optical power density when compared with the μLED without such designs.

Ultrasensitive and high-speed AlGaN/AlN solar-blind ultraviolet photodetector: a full-channel-self-depleted phototransistor by a virtual photogate

High sensitivity, high solar rejection ratio, and fast response are essential characteristics for most practical applications of solar-blind ultraviolet (UV) detectors. These features, however, usually require a complex device structure, complicated process, and high operating voltage. Herein, a simply structured n-AlGaN/AlN phototransistor with a self-depleted full channel is reported. The self-depletion of the highly conductive n-AlGaN channel is achieved by exploiting the strong polarization-induced electric field therein to act as a virtual photogate. The resulting two-terminal detectors with interdigital Ohmic electrodes exhibit an ultrahigh gain of 1.3 × 10 5 , an ultrafast response speed with rise/decay times of 537.5 ps/3.1 μs, and an ultrahigh Johnson and shot noise (flicker noise) limited specific detectivity of 1.5 × 10 18 ( 4.7 × 10 16 ) Jones at 20-V bias. Also, a very low dark current of the order of ∼ pA and a photo-to-dark current ratio of above 10 8 are obtained, due to the complete depletion of the n - Al 0.5 Ga 0.5 N channel layer and the high optical gain. The proposed planar phototransistor combines fabrication simplicity and performance advantages, and thus is promising in a variety of UV detection applications.

Alleviated built-in electric field in the active region of AlGaN deep-ultraviolet light-emitting diodes with locally embedded p-i-n junctions.

The strong polarization-induced electric field in the multi-quantum well region reduces the radiative recombination rates by separating the electron and hole wave functions, which is one of the most detrimental factors that is to blame for the low luminous efficiency of AlGaN deep-ultraviolet light-emitting diodes (DUV LEDs). In this work, we redesigned the active region by incorporating Si and Mg doping at the vicinity of the quantum wells, forming a series of embedded p-i-n junctions in the multi-quantum well region. The additional electric field induced by the fixed charges from the embedded doping-induced junctions can effectively compensate for the intrinsic polarization-induced electric fields in the quantum well region and give rise to the improved overlap of hole and electron wave function, hence enhancing the radiative recombination rates and the external quantum efficiency and optical power of DUV LEDs. The mechanism behind the alleviated polarization electric field is comprehensively discussed and analyzed. The embedded p-i-n junctions can also alter the band diagram structure of the active region, decrease the effective barrier heights for holes, and diminish the electron leakage into the p-type region. In addition, different thicknesses and doping concentrations of the embedded p- and n- layers were designed, and their influence on the performance of DUV LEDs was numerically analyzed. The proposed structure with embedded p-i-n junctions provides an alternative way to achieve efficient DUV LEDs.

Ferroelectric-enhanced BiVO4-BiFeO3 photoelectrocatalysis for efficient, stable and large-current-density oxygen evolution

The efficient BiVO4 photoanodes converting light to charge carriers and further oxidizing water to oxygen in PEC system is the gordian technical barrier for water spliltting, because that BiVO4 remain suffering from sluggish water oxidation kinetic, severe surface recombination and inefficient carrier separation. Firstly, the suitable cocatalysts loading can solve the sluggish kinetics of surface reaction and serious recombination of photogenerated charge carriers. Secondly, constructing heterostructures composed of multi-semiconductors can improve the separation efficiency of charge carriers and light absorption range. On the other hand, the existence of the built-in electric field can also provide a driving force for the transport of photoinduced charge carriers, thus enhancing separation efficiency. Herein, a ferroelectric enhanced photoelectrocatalysis system, by taking BiVO4 and Co3O4 as photocatalyst and cocatalyst, and coupling the ferroelectric material BiFeO3 to form BiVO4-BiFeO3 heterojunction was developed. The Co3O4 cocatalyst provided active sites for OER and BiVO4-BiFeO3 heterojunction promoted carrier separation. Besides of the regulation of heterojunction structure, BiFeO3 could also form the local internal electric field through ferroelectric polarization at a low voltage, which further promoted carrier separation and increased photocurrent. The as-prepared Co3O4/BiVO4-BiFeO3 sample exhibited remarkable electrocatalytic and photoelectrochemical activity for OER in 1 M KOH, an impressive photocurrent density at 1.23 V vs. RHE was achieved under AM 1.5 G light (2.24 mA/cm2), which was approximately double higher than those of Co3O4/BiVO4 and Co3O4/BiFeO3, respectively. Importantly, after the ferroelectric polarization of BiFeO3 was performed, photocurrent densities of 4.51 mA/cm2 at 1.23 V and 1280 mA/cm2 at 1.86 V vs. RHE were obtained under AM 1.5 G light, attesting the polarization-induced electric field contributed to enhance photoelectrochemical catalytic performance. The large-current-density OER obtained was also an important preponderance for the practicability.

Ferro-Pyro-Phototronic Effect in Monocrystalline 2D Ferroelectric Perovskite for High-Sensitive, Self-Powered, and Stable Ultraviolet Photodetector.

2D hybrid perovskite ferroelectrics have drawn great attention in the field of photodetection, because the spontaneous polarization-induced built-in electric field can separate electron-hole pairs, and makes self-powered photodetection possible. However, most of the 2D hybrid perovskite-based photodetectors focused on the detection of visible light, and only a few reports realized the self-powered and sensitive ultraviolet (UV) detection using wide bandgap hybrid perovskites. Here, 2D ferroelectric PMA2PbCl4 monocrystalline microbelt (MMB)-based PDs are demonstrated. By using the ferro-pyro-phototronic effect, the self-powered Ag/Bi/2D PMA2PbCl4 MMB/Bi/Ag PDs show a high photoresponsivity up to 9 A/W under 320 nm laser illumination, which is much higher than those of previously reported self-powered UV PDs. Compared with responsivity induced by the photovoltaic effect, the responsivity induced by the ferro-pyro-phototronic effect is 128 times larger. The self-powered PD also shows fast response and recovery speed, with the rise time and fall time of 162 and 226 μs, respectively. More importantly, the 2D PMA2PbCl4 MMB-based PDs with Bi/Ag electrode exhibit significant stability when subjected to high humidity, continuous laser illumination, and thermal conditions. Our findings would shed light on the ferro-pyro-phototronic-effect-based devices, and provide a good method for high-performance UV detection.

A Systematic Exploration of InGaN/GaN Quantum Well-Based Light Emitting Diodes on Semipolar Orientations -=SUP=-*-=/SUP=-

Light-emitting diodes (LEDs) based on group III-nitride semiconductors (GaN, AlN, and InN) are crucial elements for solid-state lighting and visible light communication applications. The most widely used growth plane for group III-nitride LEDs is the polar plane (c-plane), which is characterized by the presence of a polarization-induced internal electric field in heterostructures. It is possible to address long-standing problems in group III-nitride LEDs, by using semipolar and nonpolar orientations of GaN. In addition to the reduction in the polarization-induced internal electric field, semipolar orientations potentially offer the possibility of higher indium incorporation, which is necessary for the emission of light in the visible range. This is the preferred growth orientation for green/yellow LEDs and lasers. The important properties such as high output power, narrow emission linewidth, robust temperature dependence, large optical polarization ratio, and low-efficiency droop are demonstrated with semipolar LEDs. To harness the advantages of semipolar orientations, comprehensive studies are required. This review presents the recent progress on the development of semipolar InGaN/GaN quantum well LEDs. Semipolar InGaN LED structures on bulk GaN substrates, sapphire substrates, free-standing GaN templates, and on Silicon substrates are discussed including the bright prospects of group III-nitrides. Keywords: Group III-nitride semiconductor, semipolar, light-emitting diodes, InGaN/GaN quantum well.

Piezoelectric polarization-induced internal electric field manipulation of the photoelectrochemical performance in Nd, Co codoped BiFeO3

Investigation of the synergistic mechanism of element doping and piezoelectric polarization to improve the catalytic activity of BiFeO3.

Polarization-induced internal electric field to manipulate piezo-photocatalytic and ferro-photoelectrochemical performance in bismuth ferrite nanofibers

Developing lead-free ferroelectrics BiFeO3 with polarized electric field for tuning charge-transport properties in piezo-photocatalytic and ferro-photoelectrochemical (PEC) is highly desired but also challenging, especially defects such as impurity phases and oxygen vacancies lead to the weak polarization and large leakage current of BiFeO3. Here, we used a facile electrospinning strategy to modify BiFeO3 nanofibers by A-site Pr ion and B-site Mn ion co-doping. In this way, the concentrations of oxygen vacancies and valence of Fe3+ to Fe2+ were significantly inhibited, and the morphotropic phase boundary (MPB) of the rhombohedral (R) to tetragonal (T) phase was obtained, resulting in better ferroelectric performances and lower leakage current. Thus, BiPrFeMnO3 nanofibers was able to generate a large piezoelectric potential through magnetic stirring (piezoelectric effect) and light irradiation (photocatalytic effect), resulting in superior piezo-photocatalytic performance with a degradation rate of 0.1352 min−1 for rhodamine B, which was 8.29, 4.3 and 4.2 times higher than that of BiFeO3, BiPrFeO3 and BiFeMnO3, respectively. In addition, optimized PEC performance by controlling the polarization state was observed in BiPrFeMnO3. The photocurrent could be effectively tuned by more than 16 times (8.2 −131.2 μA·cm−2 at 0 V vs Ag/AgCl) under irradiation of simulated sunlight by tuning the poling voltage between + 4 and − 4 V. Meanwhile, the onset potential switched from − 0.16 to − 0.18 V, which was favorable for the PEC reactions. Our present work gives a clear understanding of the role of ferroelectric polarization and solar energy conversion and provides a way to develop highly efficient piezo-/ferroelectric nanomaterials.

Near-infrared electroluminescence of AlGaN capped InGaN quantum dots formed by controlled growth on photoelectrochemical etched quantum dot templates

Near-infrared electroluminescence of InGaN quantum dots (QDs) formed by controlled growth on photoelectrochemical (PEC) etched QD templates is demonstrated. The QD template consists of PEC InGaN QDs with high density and controlled sizes, an AlGaN capping layer to protect the QDs, and a GaN barrier layer to planarize the surface. Scanning transmission electron microscopy (STEM) of Stranski–Krastanov (SK) growth on the QD template shows high-In-content InGaN QDs that align vertically to the PEC QDs due to localized strain. A high-Al-content Al 0.9 Ga 0.1 N capping layer prevents the collapse of the SK QDs due to intermixing or decomposition during higher temperature GaN growth as verified by STEM. Growth of low-temperature (830°C) p-type layers is used to complete the p-n junction and further ensure QD integrity. Finally, electroluminescence shows a significant wavelength shift (800 nm to 500 nm), caused by the SK QDs’ tall height, high In content, and strong polarization-induced electric fields.