Ferroelectric Research Articles

Enhancing photoelectrochemical properties of α-Fe2O3 using Zr-doped HfO2 ferroelectric nanoparticles

Hematite (α-Fe2O3) as a semiconductor photoanode for photoelectrochemical (PEC) water splitting is a promising candidate material. However, its PEC performance is limited by the slow charge transfer kinetics at the α-Fe2O3/electrolyte interface. Ferroelectric materials can be used as a tool for carrier separation in photoelectrochemical water splitting due to their spontaneous polarization electric field characteristics. Here, we deposited Hf0.5Zr0.5O2 (HZO) ferroelectric nanoparticles on the surface of α-Fe2O3 and use the spontaneous polarization field of ferroelectric HZO to promote the separation of photogenerated carriers, promote the charge transfer from α-Fe2O3 to the electrolyte, and enhance the PEC of hematite. After depositing the HZO nanoparticles, the photogenerated holes are efficiently extracted from α-Fe2O3 and then directed into the electrolyte to participate in water oxidation reaction at α-Fe2O3/HZO interfaces, which enhanced the charge separation efficiency by 58 % compared to that of pure α-Fe2O3. Consequently, the α-Fe2O3/HZO photoanode exhibits a 65 % boost in photocurrent density, reaching up to 0.28 mA cm−2 at 1.23 V versus the reversible hydrogen electrode (VRHE) compared to that of pure α-Fe2O3. This study introduces a method to engineer the photoanode/ferroelectric interface, enhancing carrier separation and thereby optimizing PEC water splitting performance.

Investigation of structural, morphological, dielectric, optical, and ferroelectric-energy storage properties of [(1-x) BCT-(x) BZT] electroceramics

Extensive research and advancements have been progressed by the vast potential of eco-friendly dielectric capacitors in high-level electronic elements and high-performance pulsed power systems thus far. Ferroelectric ceramic materials exhibiting relaxor properties and significant dielectric constants are prime candidates for attaining exceptional energy storage capabilities. Herein, a lead-free (1-x) Ba0.9Ca0.1TiO3–xBaZr0.15Ti0.85O3 (abbreviated as (1-x) BCT-xBZT) solid solution system has been synthesized via the conventional solid-state reaction technique, with varying levels of BZT doping. The X-ray diffraction study confirmed pure perovskite structure formation in the prepared ceramics, and the coexistence of orthorhombic (O) (space group- Amm2) and tetragonal (T) (space group-P4mm) phases has been detected through the Rietveld refinement technique at room temperature. Rietveld refinement data and associated analysis highlight the gradual shift of crystal symmetry from Amm2 to P4mm as the x content increases. Replacing BZT in Ca-modified BaTiO3 has led to significant enhancements in microstructural properties and an increase in the width of the optical band gap. This, in turn, enhances energy storage capabilities compared to pure BCT. Incorporating BZT into BCT has led to the evolution of relaxor ceramics, a crucial development for achieving elevated levels of recoverable energy storage density. In this study, the solid solution comprising 0.6BCT-0.4BZT exhibits distinct traits, including a diffuse phase transition, high breakdown strength, shallow dielectric loss (tanδ <0.007), and significantly high dielectric constant. Due to enhanced electrical characteristics, the 0.6BCT-0.4BZT possesses very high saturation polarization of ∼48 μCcm−2, a total energy density of 1.71 J/cm3, and a recoverable energy density of 1.1 J/cm3. In addition, the optimized ceramics demonstrate concurrent achievement of high-frequency stability (10–250 Hz), fatigue resistance (up to 104 cycles), and favorable temperature stability (20–160°C). This study showcased a potential material candidate suitable for energy storage devices functioning within low to medium-electric fields.

Collective control of a vortex array in a ferroelectric ultrathin film

Recently, the observation of ferroelectric vortex arrays has triggered the investigation of topological domain structures and their characteristics. Vortices are typical topological domain structures with chirality in nanoscale ferroelectric materials. The chirality of a single vortex in a nanodot can be easily manipulated, but the collective control of a vortex array is exceptionally difficult and has not yet been realized. This Letter proposes an effective scheme for the collective control of a vortex array and investigates it via phase-field simulations. The results indicate that the collective control of a vortex array with bidirectional switching can be realized by introducing a bending film with periodic large curvatures under alternative electric fields. Furthermore, a general rule for determining the electrically controllable chirality of ferroelectric vortices is proposed. This Letter demonstrates the feasibility of the collective control of vortex arrays and provides insights for developing ferroelectric nanoelectronic devices based on vortex arrays.

Deterministic Orientation Control of Ferroelectric HfO2 Thin Film Growth by a Topotactic Phase Transition of an Oxide Electrode.

The scale-free ferroelectricity with superior Si compatibility of HfO2 has reawakened the feasibility of scaled-down nonvolatile devices and beyond the complementary metal-oxide-semiconductor (CMOS) architecture based on ferroelectric materials. However, despite the rapid development, fundamental understanding, and control of the metastable ferroelectric phase in terms of oxygen ion movement of HfO2 remain ambiguous. In this study, we have deterministically controlled the orientation of a single-crystalline ferroelectric phase HfO2 thin film via oxygen ion movement. We induced a topotactic phase transition of the metal electrode accompanied by the stabilization of the differently oriented ferroelectric phase HfO2 through the migration of oxygen ions between the oxygen-reactive metal electrode and the HfO2 layer. By stabilizing different polarization directions of HfO2 through oxygen ion migration, we can gain a profound understanding of the oxygen ion-relevant unclear phenomena of ferroelectric HfO2.

Synthesis, atomic structure and electronic properties of ferroelectric AgBiP2Se6 ultrathin flakes

Layered transition metal thiophosphate ABP2X6(A = Ag or Cu, B = V, Cr, In or Bi, X = S or Se) as a promising 2D ferroelectric material, has attracted great attention nowadays. In this work, ultrathin AgBiP2Se6 sheets down to 6 nm were synthesized for the first time. Atomic force microscopy, double spherical aberration scanning TEM and Raman spectra were performed to investigate the morphology, atomic structure and vibration modes of the samples. The first-principles calculations utilizing density-functional theory(DFT) were also used to analyze the electronic properties, band structures and projected density of states (PDOS) for both ferroelectric (FE) and paraelectric(PE) states of AgBiP2Se6 monolayer.

Halide Perovskite Inducing Anomalous Nonvolatile Polarization in Poly(vinylidene fluoride)-based Flexible Nanocomposites

Ferroelectric materials have important applications in transduction, data storage, and nonlinear optics. Inorganic ferroelectrics such as lead zirconate titanate possess large polarization, though they are rigid and brittle. Ferroelectric polymers are light weight and flexible, yet their polarization is low, bottlenecked at 10 μC cm−2. Here we show poly(vinylidene fluoride) nanocomposite with only 0.94% of self-nucleated CH3NH3PbBr3 nanocrystals exhibits anomalously large polarization (~19.6 μC cm−2) while retaining superior stretchability and photoluminance, resulting in unprecedented electromechanical figures of merit among ferroelectrics. Comprehensive analysis suggests the enhancement is accomplished via delicate defect engineering, with field-induced Frenkel pairs in halide perovskite stabilized by the poled ferroelectric polymer through interfacial coupling. The strategy is general, working in poly(vinylidene fluoride-co-hexafluoropropylene) as well, and the nanocomposite is stable. The study thus presents a solution for overcoming the electromechanical dilemma of ferroelectrics while enabling additional optic-activity, ideal for multifunctional flexible electronics applications.

Optimal parameter space for stabilizing the ferroelectric phase of Hf0.5Zr0.5O2 thin films under strain and electric fields

Abstract Hafnia-based ferroelectric materials, like Hf0.5Zr0.5O2 (HZO), have received tremendous attention owing to their potentials for building ultra-thin ferroelectric devices. The orthorhombic(O)-phase of HZO is ferroelectric but metastable in its bulk form under ambient conditions, which poses a considerable challenge to maintaining the operation performance of HZO-based ferroelectric devices. Here, we theoretically addressed this issue that provides parameter spaces for stabilizing the O-phase of HZO thin-films under various conditions. Three mechanisms were found to be capable of lowering the relative energy of the O-phase, namely, more significant surface-bulk portion of (111) surfaces, compressive c-axis strain, and positive electric fields. Considering these mechanisms, we plotted two ternary phase diagrams for HZO thin-films where the strain was applied along the in-plane uniaxial and biaxial, respectively. These diagrams indicate the O-phase could be stabilized by solely shrinking the film-thickness below 12.26 nm, ascribed to its lower surface energies. All these results shed considerable light on designing more robust and higher-performance ferroelectric devices.

Nonvolatile optical phase shift in ferroelectric hafnium zirconium oxide

A nonvolatile optical phase shifter is a critical component for enabling the fabrication of programmable photonic integrated circuits on a Si photonics platform, facilitating communication, computing, and sensing. Although ferroelectric materials such as BaTiO3 offer nonvolatile optical phase shift capabilities, their compatibility with complementary metal-oxide-semiconductor fabs is limited. Hf0.5Zr0.5O2 is an emerging ferroelectric material, which exhibits complementary metal-oxide-semiconductor compatibility. Although extensively studied for ferroelectric transistors and memories, its application to photonics remains relatively unexplored. Here, we show the optical phase shift induced by ferroelectric Hf0.5Zr0.5O2. We observed a negative change in refractive index at a 1.55 μm wavelength in a pristine device regardless of the direction of the applied electric field. The nonvolatile phase shift was only observed once in a pristine device. This non-reversible phase shift can be attributed to the spontaneous polarization within the Hf0.5Zr0.5O2 film along the external electric field.

Ferroelectric polymorphic phenomena in the layered antiferromagnet Cu(OH)2

Ferroic orders and their associated structural phase transitions are paramount in the understanding of a multitude of unconventional condensed matter phenomena. On this note, our investigation focuses on the polymorphic ferroelectric (FE) phase transitions of Copper(II) hydroxide, Cu(OH)2, considering an antiferromagnetic ground state. By employing the first-principles studies and group theory analysis, we have provided a systematic theoretical investigation of vibrational properties in the hypothetical Cmcm high-symmetry phase to unveil the symmetry-allowed ferroic phases. We identified a non-polar to polar ( Cmc21 ) phase transition, in which the displacive transformation is primarily responsible for the phase change induced by two B1u ( i.e. Γ2− ) phonon modes within the centrosymmetric phase. We also observed the existence of two polar structures with the same space group and different degrees of polarization ( i.e. Ps = 3.06 µC·cm−2 and Ps = 42.41 µC·cm−2), emerging from the high symmetry non-polar structure. According to the structural analysis the FE order, of a geometric nature, is driven by the Γ2− mode in which the O- and H-sites displacements lead the polar distortion with a minor contribution from the Cu-sites. Interestingly, the 3d 9:Cu2+ Jahn–Teller distortion coupled with the orientational shifts of O–H atoms enhances the polarization.

Interface engineering of composite systems: Focusing on the compatibility of reduced graphene oxide and Bi0.8La0.1Ba0.1Fe0.9Ti0.1O3 hybrid systems

The composite of reduced graphene oxide and perovskite-based ferroelectric materials offers exciting opportunities to enhance the optical performance of optoelectronic devices. The present work reporting rGO modified x(Bi0.8La0.1Ba0.1Fe0.9Ti0.1O3)-1-x(rGO) (La-BBFT/rGO) composite to investigate their structural, morphological, electrical, magnetic and optical properties by using robust solid-state and spun calcination method. X-Ray diffraction patterns confirm the rhombohedral crystal structure with space group (R3c) of both samples with the presence of rGO as second phase. The functional groups present in reduced graphene oxide (rGO) can act as nucleation sites for crystallization, leading to the formation of more grains and Grain boundaries. The ferroelectric phase transition was observed in La-BBFT/rGO composites at comparatively low temperature near 250 °C. The impedance spectroscopy confirms the non-Debye behaviour of relaxation process. The electric and magnetic remanence shows the compatibility of the switching state. Additionally, the optical band gap energy of both samples was evaluated and found to be in the range of 2.0 to 1.9eV. Incorporation of rGO in La-BBFT/rGO, were marginally improved, the dialectic, impedance, and optical properties, which could be beneficial for electronic and optical components in the opto-electronic industries.

Double hysteresis (P-E) loops in the transparent composite containing BaTiO3 at room temperature

A transparent composite sample containing BaTiO3 was synthesized by using the melting quenching method. The presence of a glassy phase in this composite sample was detected through XRD analysis, and this was further confirmed by DSC study. TEM and SAED analyses provided evidence that the BaTiO3 nanoparticles/clusters are embedded within the borate glass matrix, establishing the composite nature of this sample. FTIR spectrum of the present sample revealed that the glass matrix is composed of two structural groups (BO3 with NBO’s, and BO4), along with the distinct groups for BaTiO3. XPS spectra of the present composite sample indicated the presence of more than one type of boron, barium, titanium and oxygen. DSC and dielectric studies of the present composite sample revealed the presence of the phase transition temperature (Tc). Dielectric constant (ɛr) and dielectric loss (tan δ) curves of the present composite sample displayed an anomaly peak in the vicinity of TC. The optical transmission spectrum of the present composite sample exhibit two transmission bands of Ti3+ (3d1) ions in tetragonal distorted sites. At room temperature, the present transparent composite sample exhibited double hysteresis loops for BaTiO3 at low electric fields. The results obtained can be used for the development of lead-free ferroelectric material.

Significant Enhancement of Electrocaloric Effect in Ferroelectric Polycrystalline Ceramics Through Grain Boundary Barrier Engineering

AbstractA key challenge currently for the new ferroelectric refrigeration with high efficiency and environmental friendliness lies in the urgent demand for ferroelectric materials with huge electrocaloric effects (ECE). Ferroelectric polycrystalline ceramics with high ECE stand out as one of the most promising candidates for electrocaloric cooling applications. However, the grain boundary network, as a barrier for the cross‐transmission of charged carriers, widely exists in electrocaloric polycrystalline ceramics and is often neglected in favor of focusing more on composition regulation and structural design. Herein, a grain boundary barrier engineering is proposed that regulates the Schottky barrier at the grain boundary network in the Ba0.8Zr0.2TiO3 ceramics by a maneuverable annealing process and clarifies its critical role in enhancing the ECE of polycrystalline ceramics. As a result, a substantial enhancement of the EC performance (from 0.68 to 1.63 K at 50 °C and 80 kV cm−1, ≈2.4 times) has been achieved in the annealed Ba0.8Zr0.2TiO3 ceramics with a lower Schottky barrier. The microstructural and electrical characterization reveals that the lower Schottky barrier in the grain boundary network facilitates the domain switching and electronic transition, hence resulting in enhanced polarization response and EC performance.

Applying the Wake-Up-like Effect to Enhance the Capabilities of Two-Dimensional Ferroelectric Field-Effect Transistors.

For traditional ferroelectric field-effect transistors (FeFETs), enhancing the polarization domain of bulk ferroelectric materials is essential to improve device performance. However, there has been limited investigation into the enhancement of polarization field in two-dimensional (2D) ferroelectric material such as CuInP2S6 (CIPS). In this study, similar to bulk ferroelectric materials, CIPS exhibited enhanced polarization field upon application of external cyclic voltage. Moreover, unlike traditional ferroelectric materials, the polarization enhancement of CIPS is not due to redistribution of the defect but rather originates from a mechanism: the long-distance migration of Cu ions. We termed this mechanism the "wake-up-like effect". After incorporating the wake-up-like effect into the graphene/CIPS/WSe2 FeFET device, we successfully increased the hysteresis window and enhanced the current on/off ratio by 4 orders of magnitude. Moreover, the FeFET yielded remarkable achievements, such as multilevel nonvolatile memory with 21 distinct conductance levels, a high on/off ratio exceeding 106, a long retention time exceeding 103 s, and neuromorphic computing with 93% accuracy at recognizing handwritten digits. Introducing the wake-up-like effect to 2D CIPS may pave the way for innovative approaches to achieve advanced multilevel nonvolatile memory and neuromorphic computing capabilities for next-generation micro-nanoelectronic devices.

Towards wake-up free ferroelectrics and scaling: Al-doped HZO and its crystallographic texture

Ferroelectric (FE) hafnium zirconium oxide (HZO) is an excellent candidate for data storage applications. However, it has some reliability limitations such as imprint and retention. Herein, we explore Al doping of HZO to overcome these limitations. FE behavior is tuned by the aluminum (Al) concentrations in the films and by annealing temperature. A correlation is done between electrical behavior, crystallographic texture, and FE phases determined by grazing-incidence X-ray diffraction (GIXRD) measurements. Reduced coercive field (2Ec) values and wake-up free HZO-based ferroelectrics are explored. We show the tunability of remanent polarization (2Pr) and 2Ec with respect to Al-doping concentration and anneal temperature, hence crystallographic texture.

The atomic configuration and metallic state of extrinsic defects in Nb-doped BiFeO3 thin films

Defects in ferroelectric materials can cause microscopic strain and electron doping, which opens up new possibilities for unique physical properties in nanoelectronics. However, comprehending the relationship between these deep-buried defect configurations and the resulting physical properties is a long-term challenge. Here, utilizing the integrated differential phase imaging technique equipped in scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy, we show the full element-resolved atomic configuration of characteristic extrinsic defects in Nb-doped epitaxial BiFeO3 thin films. Atomic-resolution characterization reveals that the Nb doping substitutes Fe at the center of an oxygen octahedron and creates Fe vacancies near the defect core, leading to the formation of one perovskite cell-wide BiNbO3 chain that is expanded by 19 % in perovskite cell volume and rotated by 45° relative to the matrix lattice. Further quantitative analysis of the defect demonstrates that the ferroelectric polarization distribution around the dopant-induced defects displays a “head-to-head” configuration, indicating the accumulation of negative charges. Supported by electron energy loss spectroscopy and density functional theory calculations, the linear defect leads to one-dimensional metallic channels embedded within the ferroelectric BiFeO3 matrix. These atomic-scale findings establish the structure-functionality relationship of extrinsic defect in Nb-doped BiFeO3 thin films, providing new insight into their fundamental understanding and potential use in low-dimensional nanoelectronics devices.

Effect of poling on photocatalytic activity with SrBi4Ti4O15 catalyst

AbstractThis research article is related to a bismuth oxide‐layered pseudo‐perovskite ferroelectric material and its capabilities in the area of photocatalysis. SrBi4Ti4O15 (SBT) catalyst was prepared via a solid‐state reaction followed by calcination at 950°C sustained over 4 h. X‐ray diffraction, X‐ray photoelectron spectroscopy, bandgap analysis, Raman spectroscopy, and scanning electron microscopy confirmed the successful synthesis of the SBT catalyst. The photocatalysis capability was checked on a methylene blue (MB) model dye. The poling of the SBT catalyst was performed under 2 kV/mm of electric field. The poling of the catalyst material played a crucial role in enhancing the photocatalytic activity. Moreover, the effect of volume (k(v)), initial concentration (k(ic)) of the MB dye, amount of catalyst (k(c)), and intensity (k(i)) of the light were examined and analyzed by the mathematical model. This study suggested that higher intensity and amount of catalyst enhance photocatalytic activity, whereas higher concentration and volume of dye suppress the photocatalytic activity. The amount of catalyst showed the highest effect on the photocatalytic activity. The rate constant is influenced by these factors and the order of influence is k(c) &gt; k(i) &gt; k(v) ≃ k(ic).

Confinement and magnetic-field effect on chiral ferroelectric nematic liquid crystals in Grandjean-Cano wedge cells.

We explore the structure and magnetic-field response of edge dislocations in Grandjean-Cano wedge cells filled with chiral mixtures of the ferroelectric nematic mesogen DIO. Upon cooling, the ordering changes from paraelectric in the cholesteric phase N^{*} to antiferroelectric in the smectic SmZ_{A}^{*} and to ferroelectric in the cholesteric N_{F}^{*}. Dislocations of the Burgers vector b equal to the helicoidal pitch P are stable in all three phases, while dislocations with b=P/2 exist only in the N^{*} and SmZ_{A}^{*}. The b=P/2 dislocations split into pairs of τ^{-1/2}λ^{+1/2} disclinations, while the thick dislocations b=P are pairs of nonsingular λ^{-1/2}λ^{+1/2} disclinations. The polar order makes the τ^{-1/2} disclinations unstable in the N_{F}^{*} phase, as they should be connected to singular walls in the polarization field. We propose a model of transformation of the composite τ^{-1/2} line-wall defect into a nonsingular λ^{-1/2} disclination, which is paired up with a λ^{+1/2} line to form a b=P dislocation. The SmZ_{A}^{*} behavior in the in-plane magnetic field is different from that of the N_{F}^{*} and N^{*}: the dislocations show no zigzag instability, and the pitch remains unchanged in the magnetic fields up to 1 T. The behavior is associated with the finite compressibility of smectic layers.

Enhancement of ferroelectricity in chemical-solution-deposited ZrO2 thin films through fine phase transition control

Recent studies have highlighted the ferroelectric potential of ZrO2 thin films, which offer advantages over HfO2-based ferroelectric materials, such as lower thermal budget and greater natural abundance. In this research, pure ZrO2 ferroelectric thin films were fabricated on Si substrates using chemical solution deposition with all-inorganic precursors. We explored the impacts of film thickness and HfO2 seed layers on phase transition and ferroelectric properties. Notably, an 11 nm thick ZrO2 film exhibited ferroelectric behavior with a remanent polarization of 12.6 μC/cm2. Due to the combined effects of tensile stress and surface energy, the increase in thickness to 30 nm led to a phase transition from the orthorhombic phase to the monoclinic phase and a resultant decrease in remanent polarization. Additionally, the introduction of a HfO2 seed layer facilitated the crystallization of the ferroelectric o-phase, enhancing the remanent polarization to 16.5 μC/cm2 in our ∼8 nm thick ZrO2 thin film with a ∼2 nm thick HfO2 seed layer. These findings provide valuable insights into optimizing ferroelectric properties in ZrO2 thin films through phase transition control.

Strain-induced ferromagnetism and magneto-electric coupling in two-dimensional ferroelectric ZnIn2S4

Two-dimensional (2D) multiferroic materials are currently in high demand due to their significant potential for applications in the field of high-density data storage devices. However, due to the different requirements for generating ferroelectricity and magnetism, 2D multiferroic materials are rare. In this study, we propose that applying strain can induce magnetism in 2D ferroelectric materials with special electronic structures, thereby creating 2D multiferroics. Taking 2D ZnIn2S4 as an example, it shows robust ferroelectricity with an appropriate switching barrier (79.3 meV), and the out-of-plane ferroelectric polarization is 0.0322 C/m2. Applying biaxial tensile strain can change the energy of the flatband near the Fermi level, ultimately resulting in self-doping phenomena and leading to Stoner-type itinerant ferromagnetism. The reversal of ferroelectric polarization in ZnIn2S4 bilayer and ZnIn2S4–In2Se3 heterostructure can manipulate the magnetic moment of the system, exhibiting significant magnetoelectric coupling phenomena. Our findings provide a pathway for designing 2D ferromagnetic and multiferroic materials.

Background suppression structured illumination microscopy based on polarization modulation

We report a structured illumination microscopy (SIM) reconstruction method based on polarization modulation (PM), which can suppress background noise by utilizing the polarization-sensitivity properties of fluorescent dipoles. The proposed method employs a ferroelectric liquid crystal (FLC) spatial light modulator (SLM) to generate multidirectional interference patterns, and uses a combination of two liquid crystal variable phase retarders (LCVRs) and a quarter-wave plate (QWP) for flexible and rapid polarization modulation. The results of simulations and experiments demonstrate that the combination of SIM and PM effectively suppresses background noise in SIM with a slight improvement in resolution. This technique also solves the problem of structural distortion that occasionally exists in SIM and improves the accuracy of the reconstruction.