Soft Ferroelectrics Research Articles

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.

Bulk photovoltaic effect in ferroelectric nematic liquid crystals.

The bulk photovoltaic (BPV) effect in ferroelectric liquid crystals is of increasing scientific interest owing to its great potential for light-energy conversion. The ferroelectric nematic phase exhibits a huge spontaneous polarization that can be aligned to a preferred direction. In this Letter, we investigate the tensorial properties of the BPV effect in the planarly aligned ferroelectric nematic phase of the liquid crystalline material RM734. A steady-state short-circuit photocurrent of ~160 pA and an open-circuit photovoltage of ~50 mV were observed in a cell with a thickness of 5.5 µm under the illumination of ultraviolet light without any bias voltage. Based on the photocurrent measurements in different electrode configurations, the non-zero elements of the BPV tensor were obtained. The BPV effect is attributed to the combination of the spontaneous polarization and the asymmetric distribution of photoinduced charge carriers. This study not only provides an understanding of the bulk PV mechanism in soft ferroelectrics but also promises a wide range of unprecedented, to the best of our knowledge, benefits for light harvesting to engineer marketable photovoltaic devices.

Modelling of Electro-Viscoelastic Materials through Rate Equations.

Models of dielectric solids subject to large deformations are established by following a thermodynamic approach. The models are quite general in that they account for viscoelastic properties and allow electric and thermal conduction. A preliminary analysis is devoted to the selection of fields for the polarization and the electric field; the appropriate fields are required to comply with the balance of angular momentum and to enjoy the Euclidean invariance. Next, the thermodynamic restrictions for the constitutive equations are investigated using a wide set of variables allowing for the joint properties of viscoelastic solids, electric and heat conductors, dielectrics with memory, and hysteretic ferroelectrics. Particular attention is devoted to models for soft ferroelectrics, such as BTS ceramics. The advantage of this approach is that a few constitutive parameters provide a good fit of material behaviour. A dependence on the gradient of the electric field is also considered. The generality and the accuracy of the models are improved by means of two features. The entropy production is regarded as a constitutive property per se, while the consequences of the thermodynamic inequalities are made explicit by means of representation formulae.

Probing Temperature‐Induced Phase Transitions at Individual Ferroelectric Domain Walls

AbstractFerroelectric domain walls have emerged as one of the most fascinating objects in condensed matter physics due to the broad variability of functional behaviors they exhibit. However, the vast majority of domain walls studies have been focused on bias‐induced dynamics and transport behaviors. Here, the scanning probe microscopy approach based on piezoresponse force microscopy (PFM) with a dynamically heated probe, combining local heating and local biasing of the material is introduced. This approach is used to explore the thermal polarization dynamics in soft Sn2P2S6 ferroelectrics, and allows for the exploration of phase transitions at individual domain walls. The strong and weak modulation regimes for the thermal PFM are introduced. The future potential applications of heated probe approach for functional SPM measurements including piezoelectric, elastic, microwave, and transport measurements are discussed.

Fluid Layered Ferroelectrics with Global C∞v Symmetry.

Ferroelectricity in fluid materials, which allows free rotation of molecules, is an unusual phenomenon raising cutting‐edge questions in science. Conventional ferroelectric liquid crystals have been found in phases with low symmetry that permit the presence of spontaneous polarization. Recently, the discovery of ferroelectricity with high symmetry in the nematic phase has attracted considerable attention. However, the physical mechanism and molecular origin of ferroelectricity are poorly understood and a large domain of macroscopically oriented spontaneous polarization is difficult to fabricate in the ferroelectric nematic phase. This study reports new fluid layered ferroelectrics with the C∞v symmetry in which nearly complete orientation of the spontaneous polarization remains stable under zero electric field without any orientation treatment. These ferroelectrics are obtained by simplifying the molecular structure of a compound with a known ferroelectric nematic phase, although the simplification reduced the dipole moment. The results provide useful insights into the mechanism of ferroelectricity due to dipole–dipole interactions in molecular assemblies. The new ferroelectric materials are promising for a wide range of applications as soft ferroelectrics.

Piezoferroic: Multi-stacked hard/soft Pb(Ti,Zr)O3 films deposited through wet chemical method

Improving the piezoelectric performance of thin film piezoelectric devices is vital for their future application. In this research, the multi-stacked hard/soft piezoelectric lead zirconate titanate (PZT) films with improved ferroelectric properties are introduced. The films are achieved by sol-gel dip coating deposition of the layers of PZT doped with niobium (PNZT) and iron (PFZT) as soft and hard ferroelectrics, respectively. Different sequences of the layer deposition are applied to form various multi-stacked hard/soft composites. The films demonstrate near stoichiometry composition and high quality crack-free surfaces. The polarization-electric field loops illustrate that the composite can not only enhance the polarization significantly compared to the soft PNZT, but also maintain high coercive field and provide square shaped polarization-electric field loops. The results show that the remanent polarization of 94.1 μC cm−2 along with the coercive field of 5.9 kV/cm are achieved in the 10-layer double PFZT/double PNZT multi-stacked film, which are much improved compared to PZT, PFZT, and PNZT films. Moreover, dielectric constant of 2535 at 1 kHz is obtained in this sample. These intriguing properties of hard/soft composite can arise from polarization interaction of composite layers and diffusion ions through the layers.

Carrier-mediated control over the soft mode and ferroelectricity in BaTiO3

We calculate the effects of conduction band electrons (CBEs), introduced by doping or photoexcitation, on the ferroelectricity and phonon dynamics of ${\mathrm{BaTiO}}_{3}$ (BTO). We show that CBEs destabilize ferroelectricity, which would lower the Curie temperature and coercive field, and might help to improve the speed or efficiency with which polarization domains can be switched in ferroelectric devices. We show that CBEs lower the frequencies of the ${A}_{1}$ soft and ferroelectric modes in BTO's ferroelectric phases, and raise the soft-mode frequency in its paraelectric phase. We also show that femtosecond laser pulses could be used to selectively excite a coherent ${A}_{1}$ ferroelectric-mode phonon. This would allow this much-studied excitation to be monitored by pump-probe spectroscopy as it decays into other modes. We show that many of the properties of doped and undoped BTO have simple and intuitive explanations, within an ionic picture of BTO's bonding, if it is assumed that ferroelectricity is not driven by long-range interactions, but by the attraction between Ti and O neighbors. Most of the effects of CBEs are consequences of them reducing ions' charges and increasing their polarizabilities, thereby weakening the Ti-O attraction. As the CBE density increases, so does the density of delocalized interstitial electrons, which would increase conductivity. We argue that a polar metallic phase exists if the threshold for metallic conductivity can be reached before the CBEs make polar distortions energetically unfavorable by critically weakening the Ti-O attraction.

Dielectric and electro-mechanic nonlinearities in perovskite oxide ferroelectrics, relaxors, and relaxor ferroelectrics

The polarization and strain response of ferroelectric materials at electric fields below the macroscopic coercive field is of paramount importance for the operation of many electronic devices. The response of real ferroelectric and related materials is, in general, complex and difficult to interpret. The reason for this is that many processes in a ferroelectric material contribute to its properties, often concurrently. Examples include the motion of ferroelectric and ferroelastic domains, the presence of domains within domains, the dynamics of different types of polar nano-entities, the interaction of polar nano-entities (e.g., polar nanoregions in relaxors) with the strain and polarization within domains, motion of defects, and rearrangement of defect clusters and their interaction with polarization and strain. One signature of these processes is nonlinearity of the strain and polarization. Most ferroelectrics exhibit nonlinear response at all practical field levels, meaning that the apparent material coefficients depend on the amplitude of the driving excitation. In this paper, we show that an investigation of nonlinear behavior is a sensitive way to study various mechanisms operating in dielectric and piezoelectric materials. We review the basic formalism of the nonlinear description of polarization and strain, give a physical interpretation of different terms, and illustrate this approach on numerous examples of relaxors, relaxor ferroelectrics, hard and soft ferroelectrics, and morphotropic phase boundary compositions. An experimental approach based on a lock-in technique that is well suited for such studies is also discussed.

Soft Ferroelectrics Enabling High-Performance Intelligent Photo Electronics.

Soft ferroelectrics based on organic and organic-inorganic hybrid materials have gained much interest among researchers owing to their electrically programmable and remnant polarization. This allows for the development of numerous flexible, foldable, and stretchable nonvolatile memories, when combined with various crystal engineering approaches to optimize their performance. Soft ferroelectrics have been recently considered to have an important role in the emerging human-connected electronics that involve diverse photoelectronic elements, particularly those requiring precise programmable electric fields, such as tactile sensors, synaptic devices, displays, photodetectors, and solar cells for facile human-machine interaction, human safety, and sustainability. This paper provides a comprehensive review of the recent developments in soft ferroelectric materials with an emphasis on their ferroelectric switching principles and their potential application in human-connected intelligent electronics. Based on the origins of ferroelectric atomic and/or molecular switching, the soft ferroelectrics are categorized into seven subgroups. In this review, the efficiency of soft ferroelectrics with their distinct ferroelectric characteristics utilized in various human-connected electronic devices with programmable electric field is demonstrated. This review inspires further research to utilize the remarkable functionality of soft electronics.

Local hard and soft pinning of 180° domain walls in BaTiO3 probed by in situ transmission electron microscopy

We report on the electric field response of 180 degree nanodomain walls in BaTiO$_3$ using in situ electrical biasing in transmission electron microscopy (TEM). The sample is biased on a micro-device designed for reliable testing whose key attributes are confirmed by finite element calculations. The presence of weakly charged zig-zag domain walls at room temperature is attributed to the geometric confinement of the device. The motion of the domain walls under the applied electric field allows to extract local P-E loops where distinct domain wall pinning in deep and random energy potential profiles, characteristic for hard and soft ferroelectrics, respectively, are observed. Hard domain wall pinning results in asymmetrical loops typical for "hard" ferroelectrics while the soft domain wall pinning follows Rayleigh-like behaviour. All effects are measured locally and directly from the imaged domain structure.

The Soft Molecular Polycrystalline Ferroelectric Realized by the Fluorination Effect.

For a century ferroelectricity has attracted widespread interest from science and industry. Inorganic ferroelectric ceramics have dominated multibillion dollar industries of electronic ceramics, ranging from nonvolatile memories to piezoelectric sonar or ultrasonic transducers, whose polarization can be reoriented in multiple directions so that they can be used in the ceramic and thin-film forms. However, the realization of macroscopic ferroelectricity in the polycrystalline form is challenging for molecular ferroelectrics. In pursuit of low-cost, biocompatible, and mechanically flexible alternatives, the development of multiaxial molecular ferroelectrics is imminent. Here, from quinuclidinium perrhenate, we applied fluorine substitution to successfully design a multiaxial molecular ferroelectric, 3-fluoroquinuclidinium perrhenate ([3-F-Q]ReO4), whose macroscopic ferroelectricity can be realized in both powder compaction and thin-film forms. The fluorination effect not only increases the intrinsic polarization but also reduces the coercive field strength. More importantly, it is also, as far as we know, the softest of all known molecular ferroelectrics, whose low Vickers hardness of 10.5 HV is comparable with that in poly(vinylidene difluoride) (PVDF) but almost 2 orders of magnitude lower than that in BaTiO3. These attributes make it an ideal candidate for flexible and wearable devices and biomechanical applications.

Dielectric properties of composite materials based on P(VDF-TrFE) copolymer and deuterated triglicyne sulfate crystal

The film structures on the base of soft and crystalline ferroelectrics (poly(vinylidene fluoride - trifluoroethylene) copolymer and deuterated triglycine sulfate) were produced. The volume content of the DTGS microcrystal particles embedded in the polymer matrix of the composite films was up to 10%. It was found crystalline phase changes in the polymer matrix at the DTGS inclusions presence. The DTGS crystal particles increased dielectric permittivity and electrical conductivity of the composite films. The temperature dependences of the dielectric permittivity indicated increasing the phase transition temperature of polymer matrix at the DTGS inclusions growth.

Controlling the soft self-assembly of 1,3,4-oxadiazoles by carbosilane end-groups

Herein we report the soft self-assembly of 1,3,4-oxadiazoles with carbosilane end groups in a series of polar and mirror-symmetry broken liquid crystalline (LC) phases. Most compounds are based on the new 4-[4-(5-hydroxyphenyl)-1,3,4-oxadiazol-2-yl)]benzoate building block and involve carbosilane end-groups. Compounds combining one silylated and one alkylated end form a sequence of lamellar, modulated lamellar and columnar phases with developing polar order and even leading to soft ferroelectrics. Silyl groups at both ends gives rise to a modulated lamellar phase with temperature induced transition to a hexagonal columnar phase, with the first observation of a polar switching in a columnar phase based on the oxadiazole heterocycle. Spontaneous mirror symmetry breaking (“dark conglomerate”) is observed after removal of the silyl group(s). The tilt correlation mode of the lamellar phases is controlled by combining a cyano-substituted with a silylated end. Overall, this work contributes to the understanding of the soft self-assembly of oxadiazole based and silylated bent-core mesogens, presenting new properties and potentiality for the oxadiazole materials. Also, it provides general guidelines for the targeted molecular design of functional soft matter with tailored morphologies and properties.

Influence of polarization effect by electric field on energy storage

Based on the Devonshire theory, the E-induced effects of polarization, dielectrics, hysteresis loop, and energy storage are investigated. Numerical simulations confirm that the ferroelectrics with soft ferroelectricity with properties of small and E-sensitive polarization, abrupt decrease of dielectric peak with increasing E, sloped and slender hysteresis loop with central narrow when approaching Tc, and symmetry energy storage peak, much less than Tc, towards high temperature with increasing E, can be the candidate of high energy storage materials.

Epitaxially strained SnTiO3 at finite temperatures**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574246, 51390472, U1537210, and 11564010), the National Basic Research Program of China (Grant No. 2015CB654903), the Natural Science Foundation of Guangxi Zhuang Autonomous Region (Grant Nos. GA139008 and AA138162), and the “111” Project of China (Grant No. B14040).

By combining the effective Hamiltonian approach and direct ab initio computation, we obtain the phase diagram of SnTiO3 with respect to epitaxial strain and temperature. This demonstrates the complex features of the phase diagram and provides an insight into this system, which is a presumably simple perovskite. Two triple points, as shown in the phase diagram, may be exploited to achieve high-performance piezoelectric effects. Despite the inclusion of the degree of freedom related to oxygen octahedron tilting, the ferroelectric displacements dominate the structural phases over the whole misfit strain range. Finally, we show that SnTiO3 can change from hard to soft ferroelectrics with the epitaxial strain.

Ferroelectricity by Bose-Einstein condensation in a quantum magnet.

The Bose–Einstein condensation is a fascinating phenomenon, which results from quantum statistics for identical particles with an integer spin. Surprising properties, such as superfluidity, vortex quantization or Josephson effect, appear owing to the macroscopic quantum coherence, which spontaneously develops in Bose–Einstein condensates. Realization of Bose–Einstein condensation is not restricted in fluids like liquid helium, a superconducting phase of paired electrons in a metal and laser-cooled dilute alkali atoms. Bosonic quasi-particles like exciton-polariton and magnon in solids-state systems can also undergo Bose–Einstein condensation in certain conditions. Here, we report that the quantum coherence in Bose–Einstein condensate of the magnon quasi particles yields spontaneous electric polarization in the quantum magnet TlCuCl3, leading to remarkable magnetoelectric effect. Very soft ferroelectricity is realized as a consequence of the O(2) symmetry breaking by magnon Bose–Einstein condensation. The finding of this ferroelectricity will open a new window to explore multi-functionality of quantum magnets.

Poling-Written Ferroelectricity in Bulk Multiferroic Double-Perovskite BiFe0.5Mn0.5O3.

We present a comprehensive study of the electrical properties of bulk polycrystalline BiFe0.5Mn0.5O3, a double perovskite synthesized in high-pressure and high-temperature conditions. BiFe0.5Mn0.5O3 shows an antiferromagnetic character with TN = 288 K overlapped with an intrinsic antiferroelectricity due to the Bi(3+) stereochemical effect. Beyond this, the observation of a semiconductor-insulator transition at TP ≈ 140 K allows one to define three distinct temperature ranges with completely different electrical properties. For T > TN, electric transport follows an ordinary thermally activated Arrhenius behavior; the system behaves as a paramagnetic semiconductor. At intermediate temperatures (TP < T < TN), electric transport is best described by Mott's variable range hopping model with lowered dimensionality D = 1, stabilized by the magnetic ordering process and driven by the inhomogeneity of the sample on the B site of the perovskite. Finally, for T < TP, the material becomes a dielectric insulator, showing very unusual poling-induced soft ferroelectricity with high saturation polarization, similar to the parent compound BiFeO3. Under external electric poling, the system irreversibly evolves from antiferroelectric to polar arrangement.

Tuning Photoluminescence Response by Electric Field in Electrically Soft Ferroelectrics.

We show that an electrically soft ferroelectric host can be used to tune the photoluminescence (PL) response of rare-earth emitter ions by external electric field. The proof of this concept is demonstrated by changing the PL response of the Eu^{3+} ion by electric field on a model system Eu-doped 0.94(Na_{1/2}Bi_{1/2}TiO_{3})-0.06(BaTiO_{3}). We also show that new channels of radiative transitions, forbidden otherwise, open up due to positional disorder in the system, which can as well be tuned by electric field.

The structural and multiferroic properties of (Bi1−xLax)(Fe0.95Co0.05)O3 ceramics

La and Co co-doped BiFeO 3 ((Bi 1− x La x )(Fe 0.95 Co 0.05 )O 3 ( x =0, 0.10, 0.20, 0.30)) ceramics were prepared by tartaric acid modified sol–gel method. The X-ray diffraction patterns indicate a transition from rhombohedral structure to tetragonal structure at x =0.20, which has been confirmed by the Raman measurements. The band gap increases with increasing x to 0.20, and then decreases with further increasing x to 0.30. The structural transition has significant effects on the multiferroic properties. The remnant magnetization and saturate ferromagnetic magnetization decrease abruptly with increasing x to 0.10, and then gradually increase with further increasing x up to 0.30. The coercivity is significantly reduced with increasing La doping concentration. The ferroelectricity has been improved by La doping, and the polarization increases with increasing x to 0.10, then decreases with further increasing x up to 0.30. The simultaneous coexistence of soft ferromagnetism and ferroelectricity at room temperature in tetragonal Bi 0.70 La 0.30 Fe 0.95 Co 0.05 O 3 indicates the potential multiferroic applications.

Structural and electrical characterization of Bi9−xTi3Mn5+xO27

Solid solutions of bismuth layer structured Bi9−xTi3Mn5+xO27 (x = 0–3) system were prepared by the solid-state reaction technique using component oxides. X-ray powder diffraction, scanning electron microscopy, dielectric constant, variation of magnetization with magnetic field measurement techniques are used to characterize the compounds. The substitution of Mn3+ at Fe3+ site in Bi9−xTi3Mn5+xO27 was found to shift the Curie temperature towards the low-temperature region. The relationship between atomic displacement of B site ions (Ti4+, Fe3+, Mn3+) and the Curie temperature is discussed. Diffused type of phase transition was observed with a very high loss leading to the formation of soft ferroelectrics.