Ferroelectric Research Articles

Photocatalytic performance of Au/Bi4Ti3O12 composite improved by synergistic piezoelectric effect and exciton-plasmon

Bi4Ti3O12 (BiTO) is a typical bismuth-layered Aurivillius ferroelectric material for its potential application in photocatalysis. In this study, Au/BiTO hybrid was fabricated by precipitating Au nanoparticles onto BiTO nanoflake using a sodium citrate reduction route. This integration has led to a remarkable enhancement in the photocatalytic efficiency, which is attributed to the synergistic coupling of piezoelectric effect of BiTO with the localized surface plasmon resonance (LSPR) of Au nanoparticles. The incorporation of Au nanoparticles onto the BiTO matrix has significantly boosted the light absorption and charge carrier separation, thereby augmenting the photocatalytic activity. Notably, Au/BiTO-2 composite exhibits exceptional photocatalytic performance, demonstrating the complete degradation of RhB within 40 min under full-spectrum light irradiation. Moreover, Au/BiTO-2 displays a robust photocatalytic action against other organic pollutants, including methyl orange (MO), ofloxacin (OFLX) and tetracycline hydrochloride (TC). Further enhancement of photocatalytic process is achieved when combing light irradiation with ultrasonic excitation. The ultrasonic waves break the static spontaneous polarization through alternating piezoelectric potential, which accelerates the catalytic reaction. The photocatalytic action rate of Au/BiTO-2 toward RhB reaches a value as high as 0.0984 min−1, which is nearly 2.3 folds increasement compared to that of pristine BiTO. The possible synergistic mechanism caused by piezoelectric effect and exciton-plasmon effect is discussed in detail. The work not only enriches our understanding of the underlying principles but also supply a new strategy to design high-performance catalysts in the field of environmental remediation and energy conversion.

Circuit Level Implementation of Negative Capacitance Source Pocket Double Gate Tunnel FET for Low Power Applications

This manuscript presents a pioneering study on enhancing analog and radio frequency performance through the implementation of negative capacitance source pocket double gate tunnel field-effect transistor. By integrating a ferroelectric material into the gate stack and introducing a fully depleted n-type pocket near the source/channel junction, we achieved significant enhancements in key metrics such as ON current (ION), switching ratio, subthreshold swing (SS), and various analog/RF parameters like transconductance (gm), cutoff frequency (fT) when compared to existing literature. Additionally, we extend our analysis to circuit-level applications such as inverter and 5-stage ring oscillator. Our findings reveal an impressive inverter delay of 1.09 ps with a gain of 104, as well as a ring oscillator operating at a frequency of 500 GHz. These results position the proposed device as an ideal candidate for high-speed, low-power applications.

Resistive switching properties in polycrystalline LiNbO3 thin films

LiNbO3 (LNO) is currently intensively studied as an important ferroelectric material. In this work, polycrystalline LNO films were prepared through a sputtering technique, and their ferroelectricity-related resistive switching property was investigated using a device structure of PtSi/SiO2/LNO/Pt. The device exhibits a volatile resistance switching property at lower positive sweeping voltages and a stable bipolar nonvolatile switching property at higher sweeping voltages. The resistive switching mechanism of the device is discussed based on the domain wall conductivity characteristics of the polycrystalline LNO thin films. The PtSi/SiO2/LNO/Pt memristor device has potential applications in memory and artificial neural synapses.

Formation and energetics of head-to-head and tail-to-tail domain walls in hafnium zirconium oxide

180∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document} domains walls (DWs) of head-to-head/tail-to-tail (H–H/T–T) type in ferroelectric (FE) materials are of immense interest for a comprehensive understanding of the FE attributes as well as harnessing them for new applications. Our first principles calculation suggests that such DW formation in hafnium zirconium oxide (HZO) based FEs depends on the unique attributes of the HZO unit cell, such as polar-spacer segmentation. Cross pattern of the polar and spacer segments in two neighboring domains along the polarization direction (where polar segment of one domain aligns with the spacer segment of another) boosts the stability of such DWs. We further show that low density of oxygen vacancies at the metal-HZO interface and high work function of metal electrodes are conducive for T–T DW formation. On the other hand, high density of oxygen vacancy and low work function of metal electrode favor H–H DW formation. Polarization bound charges at the DW get screened when band bending from depolarization field accumulates holes (electrons) in T–T (H–H) DW. For a comprehensive understanding, we also investigate their FE nature and domain growth mechanism. Our analysis suggests that a minimum thickness criterion of domains has to be satisfied for the stability of H–H/T–T DW and switching of the domains through such DW formation.

Three-dimensional domain identification in a single hexagonal manganite nanocrystal

The three-dimensional domain structure of ferroelectric materials significantly influences their properties. The ferroelectric domain structure of improper multiferroics, such as YMnO3, is driven by a non-ferroelectric order parameter, leading to unique hexagonal vortex patterns and topologically protected domain walls. Characterizing the three-dimensional structure of these domains and domain walls has been elusive, however, due to a lack of suitable imaging techniques. Here, we present a multi-peak Bragg coherent x-ray diffraction imaging determination of the domain structure in single YMnO3 nanocrystals. We resolve two ferroelectric domains separated by a domain wall and confirm that the primary atomic displacements occur along the crystallographic c-axis. Correlation with atomistic simulations confirms the Mexican hat symmetry model of domain formation, identifying two domains with opposite ferroelectric polarization and adjacent trimerization, manifesting in a clockwise arrangement around the hat’s brim.

Monolithic three-dimensional hafnia-based artificial nerve system

Artificial nerve systems that mimic the tactile perception ability of human skin could be adopted in the realization of devices for a hyperconnected society, including prosthetic devices, interfaces for virtual reality, and smart sensors. The system consists of a pressure sensor, memory, and signal processing circuit, which possess the capability of generating sensory neuron-like signals. Although technological advances have been made in semiconductor-based memory and transistors over the decades, sensor technology is still relying on micro-electromechanical systems, which are hard to realize miniaturization/high-density and high-performance technology. Here, we demonstrate a monolithic three-dimensional integrated artificial nerve on the basis of ferroelectric HfxZr1-xO2 for use in the core element of neuromorphic memory and a self-powered piezoelectric pressure sensor. The morphotropic phase boundary (MPB) state of hafnia film was formed between orthorhombic and tetragonal phases for high piezoelectric performance. The MPB-based sensor was integrated with self-rectifying ferroelectric tunnel junction memory and silicon-based ring oscillator. We realized a frequency-modulated pulse-like synaptic signal with pressure detection ability over a wide range of 1–50 kPa and a good sensitivity of 0.35 mV/kPa. Also, we demonstrate the real-time frequency-modulated operation of the artificial nerve system by adjusting the functional properties of the hafnia films.

Design, modelling and experiments on a multisource energy harvester based on a single ferroelectric ceramic

Abstract Energy harvesting is a promising technique that can provide renewable and clean energy for the wireless sensor nodes. However, the solar, mechanical and thermal energies in our living environment are not always available due to the day/night, the weather and working conditions. Therefore, energy harvesting for a single energy source cannot provide a stable and continuous energy supply. Here, a multisource energy harvester based on a single material/structure (PLZT-Sb) is presented for scavenging solar, thermal, and mechanical energies simultaneously or individually. And then the output energy mathematical model is established and proved experimentally. The enhanced energy generations with the peak voltage of 1.9kV and peak current of 200nA are achieved by the unique integration of multi-effects, which can drive 139 LEDs. This work demonstrates an innovative approach for developing multisource energy harvester in a single ferroelectric material on the basis of the coupled multi-physics fields.

A ferroelectric nematic liquid crystal vitrified at room temperature

ABSTRACT Most of the current highly polar rod-shaped molecules that form ferroelectric nematic (NF) phase do so only at elevated temperatures and multicomponent mixtures are generally needed to obtain a broad and room temperature range NF phase. In this work, we describe the synthesis, phase characterisation and measurement of various physical properties of a new ferroelectric nematic compound 4-[(4-nitrophenoxy)carbonyl]phenyl 2-isopropoxy-4-methoxybenzoate (RT11165). The molecular structure of RT11165 with a 2-isopropoxy group differs only by a substitution of the 2-methoxy group found in the prototype ferroelectric nematic material 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). This small structure change produces a rather dramatic change in phase behaviour leading to an NF phase from 63°C down to room temperature. Below about 45°C the rotational viscosity of RT11165 increases critically and the temperature dependence indicates a glass transition at ~19°C. The transparent and polar glassy state of RT11165, which should be also piezoelectric, is a good candidate for energy storage, piezoecatalysis, data storage and other applications.

Tuning structural, dielectric, and ferroelectric properties of BTO-based ceramics through dual-site substitution

Different strategies for tuning the properties of ferroelectric materials have been employed for advancing electromechanical applications. Among several material synthesis techniques explored, site substitution is an effective approach. The present work focuses on Ba1-xCaxSnyTi1-yO3 (BCST) ceramics prepared by solid-state sintering route with different substitution levels (x = 0, 0.05 and y = 0, 0.09). The impact of site substitution on the properties of Barium Titanate (BTO), phase purity, structural analysis, chemical composition, dielectric, and ferroelectric properties have been studied. An ionic radii difference at A-site and B-site substitution is a key factor to achieve significant changes in physical and structural properties. X-ray diffraction patterns ensure the formation of pure phase perovskite structure in all the compositions. Raman spectroscopy indicates Ca substitution not only occurs at the A-site but also within the oxygen octahedra, adversely affecting the properties. Ca and Sn act as strong grain growth inhibitors, leading to a fine-grained and dense microstructure in BCST. While Ca substitution has a limited impact on the Curie temperature (TC), Sn substitution lowers TC in BST (54 °C) and BCST (50 °C) compared to BTO (126 °C). The Sn substitution results in the coexistence of multiple phases (rhombohedral, tetragonal and orthorhombic) in BST, enhancing its properties (ε' = 6118 at RT, Pr = 5.50 μC/cm2, Q11 = 0.060 m4/C2, d*33 = 526 pm/V) in comparison to BTO. These results acknowledge the role of substituents in lattice disruption, paving the way for chemistry-based materials design in the field of dielectric, actuator and energy storage applications.

Nonreciprocal and Nonvolatile Electric-Field Switching of Magnetism in van der Waals Heterostructure Multiferroics.

Multiferroic materials provide robust and efficient routes for the control of magnetism by electric fields, which have been diligently sought after for a long time. Construction of two-dimensional (2D) vdW multiferroics is a more exciting endeavor. To date, the nonvolatile manipulation of magnetism through ferroelectric polarization still remains challenging in a 2D vdW heterostructure multiferroic. Here, we report a van der Waals (vdW) heterostructure multiferroic comprising the atomically thin layered antiferromagnet (AFM) CrI3 and ferroelectric (FE) α-In2Se3. We demonstrate anomalously nonreciprocal and nonvolatile electric-field control of magnetization by ferroelectric polarization. The nonreciprocal electric control originates from an intriguing antisymmetric enhancement of interlayer ferromagnetic coupling in the opposite ferroelectric polarization configurations of α-In2Se3. Our work provides numerous possibilities for creating diverse heterostructure multiferroics at the limit of a few atomic layers for multistage magnetic memories and brain-inspired in-memory computing.

Predicting Dielectric Properties of Ferroelectric Materials with Point Defects by a Phase-Field Model

Predicting Dielectric Properties of Ferroelectric Materials with Point Defects by a Phase-Field Model

Influence of sintering temperature on electrocaloric and energy storage properties of sol-gel derived lead-free BaHf0.20Ti0.80O3 ferroelectric ceramics for sustainable energy solutions

Ferroelectric materials have emerged as intriguing contenders for a variety of applications in the ongoing effort to look for sustainable energy solutions, including energy storage and solid-state cooling systems. In this work, a systematic investigation has been conducted on the microstructural, dielectric, electrocaloric, and energy storage characteristics of sol-gel elaborated BaHf0.20Ti0.80O3 (BHT) ferroelectric ceramics. The structural investigation by Raman and X-ray diffraction indicated that when the sintering temperature increases, the tetragonality of BHT ceramics reduces, and the evolution of the cubic phase increases. The evolution of the microstructure in BHT ceramics is found to be significantly reliant on the sintering temperature. With increasing temperature of sintering, the bulk density of BHT ceramics increases. The improved dielectric and ferroelectric properties were attained in BHT ceramics at higher sintering temperatures. The BHT ceramic sintered at 1350 °C exhibited a maximum electrocaloric temperature change of ΔT ∼0.6 °C with a corresponding efficiency ΔT/ΔE of 0.3 K mm/kV near ambient temperature under a low electric field of 20 kV/cm. The BHT ceramic sintered at 1450 °C exhibited excellent room temperature recoverable energy density of JRec ∼67.84 10−3J/cm3 and high energy storage efficiency of η ∼75.13 % under a low electric field of 20 kV/cm. The highest JRec ∼73.22 10−3J/cm3 with corresponding η of 64.60 % were obtained in BHT ceramic sintered at temperature 1350 °C. These findings broaden the possibility of exploring BHT-based systems as efficient lead-free ferroelectrics for both solid-state refrigeration and energy storage applications.

Surface ferromagnetism of lead-free ferroelectric bismuth sodium titanate materials

The role of complex surface defect on the magnetic at the (110) surface of bismuth sodium titanate (Bi0.5Na0.5TiO3) was discussed based on the first-principles calculation. The first-principle calculations for various types of surface defects exhibited the existence of magnetic moments for selected chemical and position defects. Specifically, Na and Bi vacancies induced large magnetic moments of 0.52 µB/f.u and 0.50 µB/f.u, respectively, which were larger than that of Ti vacancies of 0.01 µB/f.u. Interestingly, oxygen vacancies did not induce local magnetic moments. Furthermore, significant magnetic moments of 0.50 µB/f.u and 0.49 µB/f.u were obtained for Na and Bi interstitial defects, while the local magnetic moments were slightly achieved around 0.03 µB/f.u and 0.04 µB/f.u for Ti and O interstitial defects, respectively. Anti-site defects between Bi and Na at A-site in perovskite ABO3 structure exhibited magnetic moments of 0.55 µB/f.u for Na anti-site at Bi-site and 0.39 µB/f.u for Bi anti-site at Na-site. Interestingly, anti-site defects between the A-site and B-site in perovskite ABO3 structure resulted in larger magnetic moments, with values of 0.57 µB/f.u and 0.53 µB/f.u obtained for Ti anti-site defects at the Bi-site and Na-site, respectively. Additionally, magnetic moments of 0.50 µB/f.u and 0.54 µB/f.u were achieved for Bi and Na anti-site defects at the Ti-site, respectively. We expected that our work further contributed to the understanding of the role of surface defects in the magnetism of Bi0.5Na0.5TiO3 materials in integrating ferromagnetic properties into lead-free ferroelectric materials for smart electronic device applications.

Stochasticity in ferroelectric memory devices with different bottom electrode crystallinity

This study comprehensively analyzes of the influence of bottom electrode crystallinity on the stochastic noise in ferroelectric tunnel junctions (FTJs). We perform a comparative analysis of stochastic read noise in FTJ devices with crystalline silicon (FTJc-Si) and poly-silicon (FTJpoly-Si) bottom electrodes by utilizing low-frequency noise spectroscopy and multi-frequency conductance measurements. The results indicate that FTJpoly-Si exhibits a 50 % higher read current fluctuation in the high-resistance state (HRS) at high read currents compared to FTJc-Si due to differences in interface trap density and the subsequent barrier height fluctuations. Although the crystallinity of the bottom electrode affects the noise in the HRS, the noise in the low-resistance state (LRS) remains consistent with the consequence of bulk-limited Poole-Frenkel emission. These findings clarify the relationship between bottom electrode crystallinity and stochastic noise in FTJs, providing crucial insights for the design and operation of future ferroelectric-based memory.

Enhanced energy storage performance in Bi4Ti3O12 thin films with oxygen depletion layers

The imprint effect in ferroelectric materials can significantly enhance the performance of energy storage devices. Bi4Ti3O12 (BTO) and oxygen-deficient Bi4Ti3O11.2 (DBTO) thin films were deposited on single-crystal Nb-doped SrTiO3 substrates using pulsed laser deposition. In stark contrast, multilayer DBTO/BTO thin films incorporating an oxygen-deficient layer were also fabricated, with the DBTO layers having thicknesses of 5 nm and 10 nm, respectively. Both BTO and DBTO thin films exhibited epitaxial growth and c-axis-oriented crystallinity. Regarding ferroelectric properties, DBTO/BTO capacitors with an oxygen-deficient layer demonstrated a pronounced imprint effect in their ferroelectric hysteresis loops, compared to the BTO and DBTO capacitors. This effect was particularly noticeable in the DBTO/BTO capacitor with a 10 nm oxygen-deficient layer, as opposed to the thin film with a 5 nm oxygen-deficient layer. We suggest that the imprint effect can be attributed to localized electric field variations and imbalances in charge transport at the interface between the oxygen-depleted layer and the remaining thin film. Consequently, the imprint effect observed in the DBTO/BTO capacitors contributed to an enhancement in energy storage density and efficiency, compared to BTO and DBTO capacitors. This study demonstrates the potential of multilayer DBTO/BTO thin films in advancing capacitive energy storage systems.

Large electrocaloric effect near room temperature induced by domain switching in ferroelectric nanocomposites

The solid-state refrigeration technique based on the electrocaloric effect (ECE) of ferroelectric materials has been regarded as a promising alternative to vapor compression systems due to its advantages of high efficiency and easy miniaturization. However, the small adiabatic temperature change (ATC) and narrow operating temperature range of ferroelectric materials are key obstacles for their practical applications of ECE refrigeration. To improve the ECE performance of ferroelectric polymer poly(vinylidene fluoride) [P(VDF-TrFE)], PbZr1−xTixO3 (PZT) nanoparticles with larger polarization is herein introduced to form ferroelectric nanocomposites. The phase-field simulation is employed to investigate the dynamic hysteresis loops and corresponding domain evolution of the ferroelectric nanocomposites. The temperature-dependent ATC values are calculated using the indirect method based on the Maxwell relation. The appearance of the double hysteresis loop is observed in P(VDF-TrFE) nanocomposite filled with PbZr0.1Ti0.9O3 nanoparticles [P(VDF-TrFE)–PZT0.9], which is mainly caused by a microscopic domain transition from single domain to polar vortex. Compared to the P(VDF-TrFE), enhanced ATC values associated with the domain transition are unveiled in P(VDF-TrFE)–PZT0.9, and the temperature range of excellent ECE is also effectively broadened. In addition, as the component x of filled PZT nanoparticles increases to cross the morphotropic phase boundary (MPB), the maximum ATC value shows a significant increase. The results presented in this work not only explain the mechanism of domain transition induced excellent ECE in the P(VDF-TrFE)–PZT nanocomposite, but also stimulate future studies on enhancing ECE of P(VDF-TrFE) by introducing ferroelectric nanofillers.

Strain tuning on Van der Waals negative capacitance transistors

Negative differential capacitance induced by the energy barrier formed by the nonlinear ferroelectric insulating layer during the phase transition in negative capacitance field effect transistors (NCFETs) can break through the fundamental thermionic limit of the metal-oxide-semiconductor field effect transistors (MOSFETs), which is set at a subthreshold swing (SS) of 60 mV/dec at room temperature for low-power switching. Strain engineering has been shown effective for modulating both intrinsic and coupled properties of ferroelectric materials. However, while the modulation of negative capacitance (NC) by strain primarily remains within theoretical predictions, there is a lack of experimental studies to explore its effect on NCFETs. This study introduces strain to NCFETs by utilizing the magnetostriction of Terfenol-D upon the applied magnetic field. Results reveal a decrease in SSmin from 43.5 mV/dec to 15.2 mV/dec with a compressive strain increase from 0 to −0.005%. Additionally, theoretical results by using time-evolving Ginzburg-Landau equations and Kirchhoff's laws demonstrate that compressive strain enhances both the amplitude and duration of transient NC in CuInP2S6, while vice versa under tensile strain. This work offers valuable insights for the design of NCFET devices.

High energy density in artificial heterostructures through relaxation time modulation.

Electrostatic capacitors are foundational components of advanced electronics and high-power electrical systems owing to their ultrafast charging-discharging capability. Ferroelectric materials offer high maximum polarization, but high remnant polarization has hindered their effective deployment in energy storage applications. Previous methodologies have encountered problems because of the deteriorated crystallinity of the ferroelectric materials. We introduce an approach to control the relaxation time using two-dimensional (2D) materials while minimizing energy loss by using 2D/3D/2D heterostructures and preserving the crystallinity of ferroelectric 3D materials. Using this approach, we were able to achieve an energy density of 191.7 joules per cubic centimeter with an efficiency greater than 90%. This precise control over relaxation time holds promise for a wide array of applications and has the potential to accelerate the development of highly efficient energy storage systems.

Diffraction contrast of ferroelectric domains in DPC STEM images.

Differential phase contrast scanning transmission electron microscopy (DPC STEM) is a powerful technique for directly visualizing electromagnetic fields inside materials at high spatial resolution. Electric field observation within ferroelectric materials is potentially possible by DPC STEM, but concomitant diffraction contrast hinders the quantitative electric field evaluation. Diffraction contrast is basically caused by the diffraction-condition variation inside a field-of-view, but in the case of ferroelectric materials, the diffraction conditions can also change with respect to the polarization orientations. To quantitatively observe electric field distribution inside ferroelectric domains, the formation mechanism of diffraction contrast should be clarified in detail. In this study, we systematically simulated diffraction contrast of ferroelectric domains in DPC STEM images based on the dynamical diffraction theory, and clarify the issues for quantitatively observing electric fields inside ferroelectric domains. Furthermore, we conducted experimental DPC STEM observations for a ferroelectric material to confirm the influence of diffraction contrast predicted by the simulations.

High-concentration doping effects of aliovalent Al and Ga on ferroelectric properties of BaTiO3 Films

BaTiO3 is a promising ferroelectric material with controllable properties through elemental doping. Doping aliovalent elements such as Al3+ and Ga3+ into the Ti4+ sites of BaTiO3 has been also studied, demonstrating the potential enhancement of its ferroelectric properties. However, the doping concentration has been limited to small values. In this study, we significantly expand the doping levels of aliovalent Al3+ and Ga3+ ions on Ti4+ sites in BaTiO3 films up to 20 % using substrate-induced strain effect, and investigate their effects on the structural, ferroelectric, and dielectric properties. Ga 20 %-doped films exhibit room-temperature ferroelectricity with a spontaneous polarization of 20 μC/cm2, closely resembling BaTiO3 (24 μC/cm2). In contrast, ferroelectricity disappears in Al 20 %-doped BaTiO3 films even at 10 K, indicating the importance of size of dopants for maintaining the ferroelectricity. Regarding the structure, Al and Ga doping promote the formation of a locked phase; the observed critical thickness, 20–100 nm, is much thicker than that of non-doped BaTiO3 film, 10 nm. The c/a – 1 ratio in the locked phase reaches 7 %. Because of the coexistence of the relaxed and locked phases, the doped films exhibit flexoelectricity, generating a spontaneous electric field along the out-of-plane direction.