Ferroelectric Structure Research Articles

Conic sections in ferroelectric nematics: Experiments and mathematical modeling

The domain structure of a fluid ferroelectric nematic is dramatically different from the domain structure of solid ferroelectrics since it is not restricted by rectilinear crystallographic axes and planar surface facets. We demonstrate that thin films of a ferroelectric nematic seeded by colloidal inclusions produce domain walls (DWs) in the shape of conics such as a parabola. These conics reduce the bound charge within the domains and at the DWs. An adequate description of the domain structures requires one to analyze the electrostatic energy, which is a challenging task. Instead, we demonstrate that a good approximation to the experimentally observed polydomain textures is obtained when the divergence of spontaneous polarization—which causes the bound charge—is heavily penalized by assuming that the elastic constant of splay in the Oseen-Frank energy is much larger than those for twist and bend. The model takes advantage of the fact that the polarization vector is essentially parallel to the nematic director throughout the sample. Published by the American Physical Society 2024

Ferroelectric Perovskite/MoS2 Channel Heterojunctions for Wide-Window Nonvolatile Memory and Neuromorphic Computing.

Ferroelectric materials commonly serve as gate insulators in typical field-effect transistors, where their polarization reversal enables effective modulation of the conductivity state of the channel material, thereby realizing non-volatile memory. Currently, novel 2D ferroelectrics unlock new prospects in next-generation electronics and neuromorphic computation. However, the advancement of these materials is impeded by limited selectivity and narrow memory windows. Here, new concepts of 2D ferroelectric perovskite/MoS2 channel heterostructures field-effect transistors are presented, in which 2D ferroelectric perovskite features customizable band structure, few-layered ferroelectricity, and submillimeter-size monolayer wafers. Further studies reveal that these devices exhibit unique charge polarity modulation (from n- to p-type channel) and remarkable nonvolatile memory behavior, especially record-wide hysteresis windows up to 177V, which enables efficient imitation of biological synapses and achieves high recognition accuracy for electrocardiogram patterns. This result provides a device paradigm for future nonvolatile memory and artificial synaptic applications.

Structural, Magnetic, and Dielectric Properties of Laser-Ablated CoFe2O4/BaTiO3 Bilayers Deposited over Highly Doped Si(100)

Laser ablation was used to successfully fabricate multiferroic bilayer thin films, composed of BaTiO3 (BTO) and CoFe2O4 (CFO), on highly doped (100) Si substrates. This study investigates the influence of BaTiO3 layer thickness (50–220 nm) on the films’ structural, magnetic, and dielectric properties. The dense, polycrystalline films exhibited a tetragonal BaTiO3 phase and a cubic spinel CoFe2O4 layer. Structural analysis revealed compression of the CoFe2O4 unit cell along the growth direction, while the BaTiO3 layer showed a tetragonal distortion, more pronounced in thinner BTO layers. These strain effects, attributed to the mechanical interaction between both layers, induced strain-dependent wasp-waisted behavior in the films’ magnetic hysteresis cycles. The strain effects gradually relaxed with increasing BaTiO3 thickness. Raman spectroscopy and second harmonic generation studies confirmed BTO’s non-centrosymmetric ferroelectric structure at room temperature. The displayed dielectric permittivity dispersion was modeled using the Havriliak–Negami function combined with a conductivity term. This analysis yielded relaxation times, DC conductivities, and activation energies. The observed BTO relaxation time behavior, indicative of small-polaron transport, changed significantly at the BTO ferroelectric Curie temperature (Tc), presenting activation energies Eτ in the 0.1–0.3 eV range for T < Tc and Eτ > 0.3 eV for T > Tc. The BTO thickness-dependent Tc behavior exhibited critical exponents ν ~ 0.82 consistent with the 3D random Ising universality class, suggesting local disorder and inhomogeneities in the films. This was attributed to the composite structure of BTO grains, comprising an inner bulk-like structure, a gradient strained layer, and a disordered surface layer. DC conductivity analysis indicated that CoFe2O4 conduction primarily occurred through hopping in octahedral sites. These findings provide crucial insights into the dynamic dielectric behavior of multiferroic bilayer thin films at the nanoscale, enhancing their potential for application in emerging Si electronics-compatible magneto-electric technologies.

Electrically switchable chiral nonlinear optics in an achiral ferroelectric 2D van der Waals halide perovskite.

Two-dimensional (2D) van der Waals (vdW) semiconductors play a key role in developing nanoscale nonlinear optical devices. 2D Ruddlesden-Popper lead halide perovskites (RPPs) expand the potential of using 2D vdW semiconductors in nonlinear optical applications because they exhibit electrically switchable and chiral second-order optical nonlinearity originating from the emergence of ferroelectricity and chirality. However, electrically switchable chiral nonlinear optics has not yet been realized because of the difficulty in electrically manipulating chiral structures. Here, we demonstrate that chiral second-harmonic generation (SHG) can be electrically induced and switched in an achiral biaxial ferroelectric 2D RPP. We observe reversible and continuous electrical switching of SHG circular dichroism and large nonlinear chiroptical activity. Polarization-resolved SHG imaging reveals that electrical poling induces the ferroelectric multidomain structure arising from the biaxial nature of the material, and the planar chirality appears. Our findings show a simple electrical control of the nonlinear chiroptical responses and establish chiral nonlinear optics based on ferroelectric 2D RPPs.

A ferroelectric capacitor with an asymmetric double-layered ferroelectric structure comprising a liquid-delivery MOCVD Pb(Zr, Ti)O3 layer and a sputter-deposited La-doped Pb(Zr, Ti)O3 for highly reliable FeRAM

We have developed a double-layered ferroelectric capacitor comprising a liquid-delivery metal–organic chemical vapor deposition-based Pb(Zr, Ti)O3 (PZT) lower layer and a sputter-deposited La-doped Pb(Zr, Ti)O3 (PLZT) upper layer. This structure is designed to achieve a large polarization in the stacked-type capacitor of FeRAM. Ferroelectric capacitors with noble metal electrodes, which are patterned by sputter-etching due to the difficulty of standard chemical reactive etching typically exhibit sloped sidewalls. Consequently, their polarization values, determined by the actual capacitor area, depend on their geometric shape. Our developed capacitor exhibits a 35% increase in polarization compared to an all-sputter-deposited PLZT capacitor. This improvement is due to the reduced total thickness of the developed capacitor achieved by simplifying the bottom electrode structure, which results in steeper capacitor sidewalls. Furthermore, our developed capacitor demonstrates excellent retention margin even after 3000 h of baking at 150 °C, and with a potential endurance of 1014 cycles as predicted by voltage acceleration measurements.

Peptide programming of supramolecular vinylidene fluoride ferroelectric phases.

Ferroelectric structures have spontaneous macroscopic polarization that can be inverted using external electric fields and have potential applications including information storage, energy transduction, ultralow-power nanoelectronics1,2 and biomedical devices3. These functions would benefit from nanoscale control of ferroelectric structure, the ability to switch polarization with lower applied fields (low coercive field) and biocompatibility. Soft ferroelectrics based on poly(vinylidene fluoride) (PVDF)4-6 have a thermodynamically unstable ferroelectric phase in the homopolymer, complex semi-crystalline structures, and high coercive fields. Here we report on ferroelectric materials formed by water-soluble molecules containing only six VDF repeating units covalently conjugated to a tetrapeptide, with the propensity to assemble into the β-sheet structures that are ubiquitous in proteins. This led to the discovery of ribbon-shaped ferroelectric supramolecular assemblies that are thermodynamically stable with their long axes parallel to both the preferred hydrogen-bonding direction of β-sheets and the bistable polar axes of VDF hexamers. Relative to a commonly used ferroelectric copolymer, the biomolecular assemblies exhibit a coercive field that is two orders of magnitude lower, as the result of supramolecular dynamics, and a similar level of remnant polarization, despite having a peptide content of 49 wt%. Furthermore, the Curie temperature of the assemblies is about 40 °C higher than that of a copolymer containing a similar amount of VDF. This supramolecular system was created using a biologically inspired strategy that is attractive in terms of sustainability and that could lead to new functions for soft ferroelectrics.

Enhancing piezoelectric coefficient and thermal stability in lead-free piezoceramics: insights at the atomic-scale

Given the highly temperature-sensitive nature of the polymorphic phase boundaries, attaining excellent piezoelectric coefficient with superior temperature stability in lead-free piezoceramics via direct compositional design remains a formidable challenge. We demonstrate the synergistic improvement of piezoelectric coefficient and thermal stability in lead-free piezoceramics via atomic-scale local ferroelectric structure design. Via modulation of the local Landau energy barrier at doping sites, we effectively mitigate fluctuations in piezoelectric d33. Our approach achieves an impressive d33 of ~430 pC/N with a minimal temperature fluctuation range (△d33 ~ 7%) across the room temperature to 100 °C in potassium sodium niobate ceramics. Further optimization through annealing extends this temperature up to 150 °C (△d33 ~ 8%) while maintaining a high d33 of ~380 pC/N, rivaling the performance of classic temperature stable lead zirconate titanate. This work establishes a framework for addressing the dilemma between high piezoelectric coefficient and inadequate temperature stability in lead-free piezoceramics.

Electronic and Transport Engineering of A-Type Antiferromagnets with Ferroelectric Sandwich Structure: Toward Multistate Nonvolatile Memory Applications.

Achieving higher-order multistates with mutual interstate switching at the nanoscale is essential for high-density storage devices; yet, it remains a significant challenge. Here, we demonstrate that integrating A-type antiferromagnetic semiconductors sandwiched between ferroelectric layers is an effective strategy to achieve high-performance multistate data storage. Taking the Sc2CO2/VSi2P4 bilayer (bi-VSi2P4)/Sc2CO2 van der Waals multiferroic heterostructure as an example, our first-principles calculations show that by switching the polarization direction of the upper and bottom ferroelectric Sc2CO2 layers, antiferromagnetic bi-VSi2P4 can exhibit four distinct states with different band structures. The intriguing band structure engineering stems from the polarization-field-induced band shift and interface charge transfer. Accordingly, the proposed Sc2CO2/bi-VSi2P4/Sc2CO2-based multiferroic device can achieve four different resistance states, accompanied by fully spin-polarized currents and giant tunneling electroresistance ratios. Our results propose a viable strategy for realizing nonvolatile electrical control of antiferromagnets at the nanoscale and provide insights into the development of advanced memories.

Origin of zero thermal expansion in an average cubic structure in Pb-free relaxor ferroelectrics

This study presents “K0.5Na0.5NbO3-based” Pb-free smart material 0.80(K0.5Na0.5NbO3)–0.20(Ba0.9Sr0.1TiO3) (KBST20) as exhibiting zero thermal expansion (ZTE) at low temperatures (T≤ 100 K) with long-range cubic symmetry stable over a wide temperature range (9 K ≤T≤ 500 K). The linear coefficient of the thermal expansion (αl) obtained from temperature-dependent neutron diffraction data is in the range of 0.255–5.75 × 10−6 K–1 (9–500 K), which is rarely observed for Pb-free materials possessing long-range cubic symmetry. The temperature-dependent dielectric data of KBST20 exhibits a strong relaxational behavior with high frequency dispersion (ΔT≈ 27 K), suggesting the presence of polar phased regions known as polar nano regions. The ZTE has been attributed to enhanced correlations among PNRs exhibiting ferroelectrostriction. Furthermore, temperature-dependent Raman scattering data reveal polar monoclinic distortion at short ranges rather than cubic symmetry at long ranges. In addition, the intensity of Raman modes increases with the decrease in temperature, suggesting enhancement of the polar phase at low temperatures, which consequently leads to zero thermal expansion in KBST20.

Machine learning surrogate for 3D phase-field modeling of ferroelectric tip-induced electrical switching

Phase-field modeling offers a powerful tool for investigating the electrical control of the domain structure in ferroelectrics. However, its broad application is constrained by demanding computational requirements, limiting its utility in inverse design scenarios. Here, we introduce a machine-learning surrogate to accelerate 3D phase-field modeling of tip-induced electrical switching. By dynamically handling the boundary conditions, the surrogate achieves accurate reproduction of switching trajectories under various tip locations and applied voltages. With stable predictions throughout entire morphological evolution pathways and a relative error inferior to 10% compared to direct solvers, the model efficiently emulates intricate switching sequences. By successfully replicating the boundary conditions, the presented framework strides towards a holistic surrogate for the ferroelectric phase field. With up to 2500-fold speed-ups over classical methods, our approach opens the path for the tractable design of the domain structure and the resolution of realistic inverse problems.

Evolution of Ferroelectricity in Sr0.6Ba0.4Nb2O6-BaTiO3 Solid Solution with a Strong Electrocaloric Effect.

In conventional knowledge, ferroelectric solid solutions were formed between members belonging to the same crystal structure family. Since both tungsten bronze and perovskite structures are constructed by connecting the corner-sharing oxygen octahedra, it offers a possibility for formatting an unusual solid solution between these two families. Herein, (1 - x)Sr0.6Ba0.4Nb2O6-xBaTiO3, (1 - x)SBN-xBT, solid solutions were synthesized and the solution mechanism was resolved from a structure viewpoint. With increasing BT content, the solid solution persists of tetragonal tungsten bronze structure, but the lattice parameter a (= b) decreases whereas c increases, resulting in the significant reduction of grains anisotropy. The ferroelectric-relaxor phase transition temperature shows a monotonic increase as x increases. However, the ferroelectricity evolution is not monotonous as a function of BT content because of the competitive effects of Ba and Ti on the property. As a result, the x = 0.10 ceramic shows the strongest ferroelectricity and a remarkable electrocaloric effect of 1.4 K near room temperature. This work challenges the traditional view of solid solution formation and provides an alternative way to modulate the structure and properties of ferroelectrics.

Giant Piezoelectric Effects of Topological Structures in Stretched Ferroelectric Membranes.

Freestanding ferroelectric oxide membranes emerge as a promising platform for exploring the interplay between topological polar ordering and dipolar interactions that are continuously tunable by strain. Our investigations combining density functional theory (DFT) and deep-learning-assisted molecular dynamics simulations demonstrate that DFT-predicted strain-driven morphotropic phase boundary involving monoclinic phases manifest as diverse domain structures at room temperatures, featuring continuous distributions of dipole orientations and mobile domain walls. Detailed analysis of dynamic structures reveals that the enhanced piezoelectric response observed in stretched PbTiO_{3} membranes results from small-angle rotations of dipoles at domain walls, distinct from conventional polarization rotation mechanism and adaptive phase theory inferred from static structures. We identify a ferroelectric topological structure, termed "dipole spiral," which exhibits a giant intrinsic piezoelectric response (>320 pC/N). This helical structure, possessing a rotational zero-energy mode, unlocks new possibilities for exploring chiral phonon dynamics and dipolar Dzyaloshinskii-Moriya-like interactions.

Fast selective edge-enhanced imaging with topological chiral lamellar superstructures.

Edge detection is a fundamental operation for feature extraction in image processing. The all-optical method has aroused growing interest owing to its ultra-fast speed, low energy consumption and parallel computation. However, current optical edge detection methods are generally limited to static devices and fixed functionality. Herein, we propose a fast-switchable scheme based on a ferroelectric liquid crystal topological structure. The self-assembled chiral lamellar superstructure, directed by the azimuthally variant photo-alignment agent, can be dynamically controlled by the polarity of the external electric field and respectively generates the vector beams with nearly orthogonal polarization distribution. Even after thousands of cycles, the horizontal and vertical edges of the object are selectively enhanced with an ultra-fast switching time of ∼57 μs. Broadband edge-enhanced imaging is efficiently demonstrated. This work extends the ingenious building of topological heliconical superstructures and offers an important glimpse into their potential in the emerging frontiers of optical computing for artificial intelligence.

First-principles predictions of HfO2-based ferroelectric superlattices

The metastable nature of the ferroelectric phase of HfO2 is a significant impediment to its industrial application as a functional ferroelectric material. In fact, no polar phases exist in the bulk phase diagram of HfO2, which shows a dominant non-polar monoclinic ground state. As a consequence, ferroelectric orthorhombic HfO2 is stabilized either kinetically or via epitaxial strain. Here, we propose an alternative approach, demonstrating the feasibility of thermodynamically stabilizing polar HfO2 in superlattices with other simple oxides. Using the composition and stacking direction of the superlattice as design parameters, we obtain heterostructures that can be fully polar, fully antipolar or mixed, with improved thermodynamic stability compared to the orthorhombic polar HfO2 in bulk form. Our results suggest that combining HfO2 with an oxide that does not have a monoclinic ground state generally drives the superlattice away from this non-polar phase, favoring the stability of the ferroelectric structures that minimize the elastic and electrostatic penalties. As such, these diverse and tunable superlattices hold promise for various applications in thin-film ferroelectric devices

Nonlinear Effects of Hydrophobic Confinement on the Electronic Structure and Dielectric Response of Water.

Fundamental studies of the dielectrics of confined water are critical to understand the ion transport across biological and synthetic nanochannels. The relevance of these fundamental studies, however, surmounts the difficulty of probing water's dielectric constant as a function of a fine variation in confinement. In this work, we explore the computational efficiency of machine learning potentials to derive the confinement effects on the dielectric constant, polarization, and dipole moment of water. Our simulations predict an enhancement of the axial dielectric constant of water under extreme confinement, arising from either the formation of ferroelectric structures of ordered water or larger dipole fluctuations facilitated by the disruption of water's H-bond network. Our study highlights the impact of hydrophobic nanoconfinement on the dielectric constant and on the ionic and electronic structure of water molecules, pointing to the importance of geometric flexibility and electronic polarizability to properly model confinement effects on water.

High performance near ultraviolet ray detector by cluster-wrapped surface structure in ferroelectrics

High-sensitivity detectors for near-ultraviolet rays are of great importance in civil and military fields. Recently, great efforts have been made to enhance the sensitivity, but mainly at the cost of increasing energy consumption or bulk volume. High-performance near-ultraviolet ray detector was fabricated using ZnO clusters compounded in ferroelectric Bi4.5Na0.5Ti4O15 composites, which exhibit ultrahigh sensitivity detectivity (D*), linear dynamic response (LDR) and photoconductance σ(λ) of up to 3.9 × 1011 Jones, 52.04 dB, and 3.9 × 10-10 S/m, respectively. The three-parameter criterions are over a wide wavelength range larger than that of a semiconductor. High external quantum efficiency 4.6 % is to ensure the self-powered device. Such a brilliant virtue is attributed to the artificial transport layer by the superior ZnO cluster surface-wrapped structure, which effectively transports the electrons or holes induced by photons. These pioneering findings indicate that the ZnO cluster surface-wrapped structure may be useful for the assembly of ferroelectric high-performance UV photoelectronic devices in the near future.

Single or Vortex Ferroelectric and Ferromagnetic Domain Nanodot Array of Magnetoelectric BiFe0.9Co0.1O3.

Nanodots composed of multiferroic cobalt-substituted BiFeO3, a ferroelectric ferromagnet at room temperature, are fabricated by pulsed laser deposition using anodized porous alumina as masks. The obtained nanodots are approximately 60 nm in diameter, more than 10 nm in thickness, and approximately 70 Gbit/in.2 in density. Piezoresponse and magnetic force microscopies show both ferroelectricity and ferromagnetism with a single-domain nature. It is also found that the dot with 190 nm diameter had multidomain vortex ferroelectric and magnetic structures indicating the strong magnetoelectric coupling. The single-domain cobalt-substituted BiFeO3 nanodots are suitable for verifying magnetization reversal by the electric field, which is the first step in the development of low-power-consumption nonvolatile magnetic memory devices.

Poling induced changes in polarization domain structure of polymer ferroelectrics for application in organic thin film transistors

While electrical poling of organic ferroelectrics has been shown to improve device properties, there are challenges in visualizing accompanying structural changes. We observe poling induced changes in ferroelectric domains by applying differential phase contrast (DPC) imaging in the scanning transmission electron microscope, a method that has been used to observe spatial distributions of electromagnetic fields at the atomic scale. In this work, we obtain DPC images from unpoled and electrically poled polyvinylidene fluoride trifluorethylene films and compare their performance in polymer thin film transistors. The vertically poled films show uniform domains throughout the bulk compared to the unpoled film with a significantly higher magnitude of the overall polarization. Thin film transistors comprising a donor–acceptor copolymer as the active semiconductor layer show improved performance with the vertically poled ferroelectric dielectric film compared with the unpoled ferroelectric dielectric film. A poling field of 80–100 MV/m for the dielectric layer yields the best performing transistors; higher than 100 MV/m is seen to degrade the transistor performance. The results are consistent with a reduction in deleterious charge carrier scattering from ferroelectric domain boundaries or interfacial dipoles arising from electrical poling.

BCl3/Cl2 plasma etching process to fabricate a ferroelectric gate structure for device integration

Despite the significant potential of ferroelectric devices in overcoming the challenges faced by conventional high-k-based CMOS devices owing to the scaling of CMOS processes, most ferroelectric devices are not implemented in practical circuits yet. For practical application, integrating them into a circuit is essential, and the development of a reliable etching process is crucial for the integration of individual devices into circuits. Therefore, this study proposes a process for etching hafnium zirconium oxide (HZO)-based gate stacks to fabricate a gate structure and integrate HZO-based devices into circuits. First, poly-Si/TiN/HZO/TiN/SiO2 was deposited on a Si substrate and etched via Cl2 and BCl3/Cl2 plasma etchings. Cl2 plasma etching was found to be less effective, whereas BCl3/Cl2 plasma etching exhibited a higher etching rate. The optimal etching time for the BCl3/Cl2 plasma at which the entire stack was successfully removed was 50 s. Furthermore, the optimal ratio of Ar:Cl2:BCl3 that resulted in minimal damage to the Si surface was determined to be 1:1:3. These results led to the successful formation of an HZO-based gate structure and provided the potential to integrate ferroelectric devices into the circuit, thereby enabling their practical utilization.

Curvature conservation and conduction modulation for symmetric charged ferroelectric domain walls

Curvature conservation is a common feature for the physical properties with topological constraints, from the macroscopic cosmic string loops to the microscopic quantized vortex rings in the superfluid. Interestingly, ferroelectric domain-wall curvature is important to determine its conduction, which is the key parameter for designing domain-wall nanoelectronic devices. Here, we demonstrate the curvature conservation of the charged domain walls confined in ferroelectric topological structures with specific domain symmetry by combining piezoelectric force microscopy, conducting atomic force microscopy, and phase-field simulations. Significantly, the intrinsic conductivity of the charged domain walls exhibits an inverse functional relationship with the local domain-wall curvature in the ferroelectric BiFeO3 nano-islands, which can also be precisely and continuously modulated by applying an electric field. This work provides us with an insightful understanding of the conduction mechanism of charged domain walls confined in ferroelectric topological structures with a high degree of spatial symmetry, which paves the way for the application of charged domain walls as flexible conducting channels with continuously changing resistance states to be used for ferroelectric domain-wall memristors.