Ferroelectric polymer nanocomposite alignment layer in twisted nematic liquid crystal devices for reducing switching voltage

Twisted nematic liquid crystal device (TNLCD) was fabricated using a ferroelectric zinc oxide (ZnO)-doped polyimide alignment layer. The ferroelectric nanoparticle can produce a local electric field, which can trigger the orientation of liquid crystal molecule and reduces the switching voltage. The uniform dispersion of ferroelectric ZnO nanoparticles in the alignment layer was studied using field emission scanning electron microscopy and atomic force microscopy. The ferroelectric property of ZnO-doped polyimide was investigated using dynamic contact electrostatic force microscopy. An increased local electric field due to the presence of nano ZnO was confirmed with the help of scanning tunnelling microscopy. An augmentation of capacitance was observed with an increase in concentration, which substantiates the reduction of switching voltage of TNLCD with the modification of ferroelectric nanoparticle-doped alignment layer.     

Dielectric and Ferroelectric Properties of Sol-Gel Derived YMnO3 Films

There has been considerable interest in ferroelectric (FE) films especially for non-volatile memories and ultra high density DRAM applications. Such FE films typically consist of lead zirconate titanate (PZT) with novel oxide contacts, or layered perovskite such as Sr2Bi2TaO9. Recently, there have been reports of sputtered YMnO3 films which exhibit a single polarization axis and do not contain any volatile species of Pb or Bi. Single crystal YMnO3 exhibits satisfactory polarization (6 C/cm2) and low coercive field (<20 kV/cm). Additionally, the dielectric constant of YMnO3 is quite low (<30) which should facilitate ferroelectric switching. In this study, sol-gel derived YMnO3 films were prepared on platinized Si wafers and their dielectric and ferroelectric properties were characterized. Their electrical properties will be discussed with respect to Y/Mn stoichiometry ratio, hexagonal phase development and processing conditions. The potential of YMnO3 as a material in non-volatile memories is evaluated.     

The First Enantiomeric Stereogenic Sulfur-Chiral Organic Ferroelectric Crystals

Chiral ferroelectric crystals with intriguing features have attracted great interest and many with point or axial chirality based on the stereocarbon have been successively developed in recent years. However, ferroelectric crystals with stereogenic heteroatomic chirality have never been documented so far. Here, we discover and report a pair of enantiomeric stereogenic sulfur-chiral single-component organic ferroelectric crystals, R_s-tert-butanesulfinamide (R_s-tBuSA) and S_s-tert-butanesulfinamide (S_s-tBuSA) through the deep understanding of the chemical design of molecular ferroelectric crystals. Both enantiomers adopt chiral-polar point group 2 (C₂) and exhibit mirror-image relationships. They undergo high-temperature 432F2-type plastic ferroelectric phase transition around 348 K. The ferroelectricity has been well confirmed by ferroelectric hysteresis loops and domains. Polarized light microscopy records the evolution of the ferroelastic domains, according with the fact that the 432F2-type phase transition is both ferroelectric and ferroelastic. The very soft characteristics with low elastic modulus and hardness reveals their excellent mechanical flexibility. This finding indicates the first stereosulfur chiral molecular ferroelectric crystals, opening up new fertile ground for exploring molecular ferroelectric crystals with great application prospects.     

钛酸铅与偏氟乙烯-三氟乙烯共聚物形成的铁电复合物相变过程的热释电研究

研究了偏氟乙烯-三氟乙烯共聚物与钛酸铅形成的铁电复合物PT/P[VDF(70)-TrFE(30)]的热释电流行为。P[VDF(70)-TrFE(30)]室温下存在两个铁电相,即较无序的铁电相和较有序的铁电相,升温经过各自的相转变点后转变成较有序的顺电相和较无序的顺电相。热释电流谱上出现在95℃和108℃的两个电流峰,分别由两个铁电相的结构陷阱以及部分取向的偶极所贡献。采用Tc以上温度极化并冷却到不同温度的方法可明确鉴别出该试样两个顺电相降温转变过程,转变点分别为62℃和50℃。实验证实陷阱仅存在于铁电相而不存在于顺电相中。钛酸铅的引入虽不导致新电流峰出现,但可使共聚物的Tc降低、电流加大。热释电流方法可非常灵敏地跟踪铁电共聚物的相变过程。     

Ferroelectric LC-elastomers

Ferroelectric liquid crystalline elastomers combine the rubber elasticity of polymer networks with liquid crystalline phases and ferroelectric ordering. Ferroelectric switching leads therefore to a deformation of the polymer network and an elastic stress. The coupling between both effects can be varied by changing the topology of the netpoints.     

Ferroelectric Films

The preparation and properties of ferroelectric films are reviewed. Specific attention is directed to ferroelectric films prepared by wet chemical methods. Emphasis is placed on the microstructure and properties of SBT films for memory applications, and their dependence on chemistry and processing, as well as on the effects of chemistry and processing on the properties of piezoelectric and pyroelectric films. Emphasis is placed on recent results obtained in our laboratories, including modeling of the ferroelectric behavior of films and experimental results in each of these areas. Also considered is the use of ferroelectric films in memory, piezoelectric and pyroelectric devices of various types, with specific note made of the recently-announced use of pyroelectric sensors in automobiles.     

聚氨酯与铁电陶瓷粉复合物的电致感应性能研究

利用双辊制备了一系列热塑性弹性体聚氨酯（PU）和铁电钛酸锆酸铅盐（PZT）的电感应PZT／PU复合物。X－衍射和SE镜结果显示,复合物中,在铁电相中PZT陶瓷粉呈现结晶态并且均匀分布在PU基体中,复合物的弹性模量和相对电容率随着复合物中PZT体积分数增加而升高,在高压电场下,低PZT含量的复合物呈现负的电致伸缩应力,当PZT体积分数增加到6％以上时,在某个临界电场下,复合物呈现出电致伸缩应力反转的特性,即电致伸缩应力从负转变为正,并且发生应力反转的临界电场随着复合物中PZT体积分数的增加而降低,PU的电致伸缩特性与PZT在高电场下极化反转效应结合,导致了PZT／PU复合物这种有趣的性能,因此PZT／PU复合物将在高电压场方面作为传感器开关材料得到广泛应用。     

Visualization of Ferroelectric Domains in Thin Films of Molecular Materials Using Confocal Micro-Raman Spectroscopy

Ferroelectrics are an important class of functional materials. Among all their unique properties, the study of their ferroelectric domains and domain walls is of great interest due to their importance in ferroelectric applications. There are many methods to characterize ferroelectric domains, namely, scanning probe microscopy, optical microscopy, electron microscopy, etc. Currently, newly emerged molecular ferroelectrics are attracting much attention from chemists, physicists and researchers in material sciences due to their structural flexibility, light mass, simple fabrication, etc. However, for the characterization of molecular ferroelectric domains, most conventional methods require either a complicated preparation process or direct contact between physical probes and material surfaces, limiting the development of molecular ferroelectric materials. In this report, we have demonstrated that confocal micro-Raman spectroscopy, as a nondestructive and noncontact in-situ method, is very suitable for studying the ferroelectric polarization and structures of domains in molecular ferroelectrics. Taking recently reported molecular ferroelectric trimethylchloromethyl ammonium trichlorocadmium(II) (TMCM-CdCl₃) as an example, the non-180° domains have been characterized and visualized at different temperatures. Such a simple and extendable method requires minimum sample preparation, which would further benefit the research of molecular ferroelectric domain engineering and promote the miniaturization and integration of molecular ferroelectric films.     

Influence of Poly (methyl metacrylate) Addition on Resistive Switching Performance of P3HT/P(VDF-TrFE) Blend Films

Organic semiconducting/ferroelectric blend films attracted much attention due to their elec-trical bistability and rectification properties and thereof the potential in resistive memory devices. Blend films were usually deposited from solution, during which phase separation oc-curred, resulting in discrete semiconducting phase whose electrical property was modulated by surrounding ferroelectric phase. However, phase separation resulted in rough surface and thus large leakage current. To further improve electrical properties of such blend films, poly(methyl metacrylate) (PMMA) was introduced as additive into P3HT/P(VDF-TrFE) semiconducting/ferroelectric blend films in this work. It indicated that small amount of PMMA addition could effectively enhance the electrical stability to both large electrical stress and electrical fatigue and further improve retention performance. Overmuch PMMA addition tended to result in the loss of resistive switching property. A model on the con-figuration of three components was also put forward to well understand our experimental observations.     

Ferroelectric liquid crystal composites based on the porous stretched polyethylene films

Electro-optically active polymer–liquid crystal composites based on ferroelectric liquid crystals and stretched porous polyethylene films were developed. The alignment of ferroelectric liquid crystals incorporated into the porous polyethylene films with average porous diameter of around 200 nm was observed and studied. It was shown experimentally that these samples containing ferroelectric liquid crystals are flexible electro-optical films exhibiting a saturation electric field near 2·10⁷ Vm^?1 and a response time of about 30 μs under the action of the saturation field. A simple theoretical model of ferroelectric liquid crystal molecules' complete reorientation in electric fields inside pores of the films has been proposed and confirmed experimentally.     

Ferroelectric block copolymers

A block copolymer consisting of polystyrene and a side chain ferroelectric liquid crystalline polymer was synthesized using polymer analogous chemistry on a monodisperse poly(styrene-b-isoprene). Composition was adjusted to give lamellar microstructure after addition of the mesogenic side groups. If placed in an LC cell without orientation of domains, no ferroelectric response was observed. After shearing the thin film, presumably due to alignment of lamellae, a bistable ferroelectric switching could be detected.     

Ageing and fatigue of lead titanate and lead zirconate titanate thin ferroelectric films

The phenomena of ageing and fatigue have been experimentally investigated in lead titanate and lead zirconate titanate thin ferroelectric films for samples on different substrates and with different materials of the measuring electrodes. A certain broadening of the dielectric permittivity peak is observed for the films on the silicon substrate after a year keeping without external actions. The lead titanate films on corundum substrates did not demonstrate visible changes in structural and dielectric parameters in the course of this time. In the course of repeated cycling the reduction of switching polarization in the lead titanate and lead zirconate titanate films on silicon substrates takes place at considerably greater number of cycles as compared to the same films on corundum substrates under the identical conditions. The above changes of dielectric and switching characteristics can be relevant to the changes in the domain structure of the materials under investigation in the process of their ageing and repeated switching. The reason for the acceleration of the ageing processes in the films on corundum substrates could be either an increase in the absolute magnitude of the switching field or an increase of the internal bias field, that facilitate the migration of oxygen vacancies in the films with the perovskite structure to the electrode–ferroelectric surface with the consequent fixation of domain walls.     

Ferroelectric and dielectric behaviour of Bi0.92La0.08FeO3 multiferroic thin films prepared by soft chemistry route

Bi_0.92La_0.08FeO₃ (BLFO) thin films were grown on platine substrates by the soft chemical route. Ferroelectric and dielectric behaviors of BLFO films deposited by spin-coating technique and annealed at 773 K for 2 h in air atmosphere were explained. BLFO thin films obtained presents a rhombohedral structure. The BLFO films present dielectric and ferroelectric behaviors with dielectric permittivity and dielectric loss of approximately 81 and 0.0144 at 1 kHz. The Au/BLFO/Pt capacitor shows a hysteresis loop with remnant polarization of 20.6 μC/cm² and coercive field of 53.88 kV/cm. The polarization switching and the fatigue behavior of the BLFO films were significantly enhanced.     

The Effect of Substrate on the Properties of Non-volatile Ferroelectric P(VDF-TrFE)/P3HT Memory Devices

Ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))/semiconducting poly(3-hexyl thiophene) (P3HT) blend systems have drawn great attention with their potential use for electronic applications, particularly non-volatile memory devices. It is essential to grasp a full understanding of the crystallization habits of the two polymers on different substrates for purposeful control of the structures of the blend and therefore the properties of the devices. Here, the effects of structure and morphology of the blend films generated at different substrate surfaces on the ferroelectric and switching properties of related devices are reported. It is identified that P(VDF-TrFE)/P3HT blend films prepared on graphene substrate show not only an obvious optimization in the ferroelectric behavior of P(VDF-TrFE), but also an enhancement of the charge transport within P3HT domains. By employing sandwich structure constructed by silver electrode and P3HT/P(VDF-TrFE) blend film on graphene substrate, high-performance ferroelectric memory devices have been obtained, which exhibit a great electrical switching behavior with high ON/OFF ratio of about 1000 and low coercive voltage of approximately 5 V. These findings provide useful guidance for fabricating high-performance ferroelectric memory devices.

     

The study of local dual‐peak vibratory properties in ferroelectric P(VDF‐TrFE) films

In this article, we reported the studies on the local dual‐peak vibrations in ferroelectric P(VDF‐TrFE) films, induced by an alternating voltage with its amplitude high enough to cause polarization reversal. A further study showed that this dual‐peak vibration was caused by ferroelectric switching process. The buildup of these dual‐peak vibrations had been detected, and the asymmetrical evolution of local butterfly loops was observed. It was believed that the observed time delay during the developments of dual‐peak vibrations and butterfly loops should be attributed to the electrode effect. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3282–3287, 2006     

Salicylideneaniline is a Photoswitchable Ferroelectric Crystal

Since the discovery of the first ferroelectric Rochelle salt, most ferroelectrics have been investigated showing thermally triggered symmetry-breaking phase transition. Although photochromism arising from geometrical isomerization was reported as early as 1867, such photoswitchable ferroelectric crystals have scarcely been developed to date. Herein, we report that salicylideneaniline is a photochromic ferroelectric crystal. Upon photoirradiation, the dielectric constant shows obvious switching between high and low dielectric states, and more importantly, the ferroelectric polarization demonstrates quick and reversible switching. This work opens the gate to developing photoswitchable ferroelectrics, which holds great potential for applications in optically controlled smart devices.     

Polarization inscription in ferroelectric (111) PZT and (100) SBT films

Ferroelectric thin films form an equilibrium domain structure compatible with their respective crystallographic symmetry. In tetragonal (111) PZT, 90° domains prevail; in (pseudo-tetragonal) (100) SBT both 90° and 180° domains are present. The size of 90° domains has been measured for e.g., PZT as slabs of 15 nm width. Domain size is a result of stress minimization in the film during the paraelectric (PE) → ferroelectric (FE) transition. A precise and regular domain pattern for (111) PZT and (100) SBT films has been investigated in detail by TMSFM. Single domains can be addressed mechanically with the tip of an AFM. Such single domain switching corresponds to a data storage density of 200 Gbit/inch². Applications of ferroelectric and high- paraelectric materials for e.g., non-volatile data storage replacing DRAM devices or as sensors in infrared cameras are increasingly becoming popular.     

Chaotic transients in surface-stabilized smectic C* cells induced by magnetic field

Recent theoretical developments regarding the understanding of weakly chaotic transients in ferroelectric liquid crystals (FLCs), induced by electric field, are studied in terms of the interaction with magnetic field. Our research is related with the nonlinear dynamical system represented by a thin film of surface-stabilized FLC in smectic C* phase, and subjected by the swinging magnetic field. The computation of the Lyapunov exponents from the dynamic equation for the director field reveals that the director dynamics exhibits limit cycle, hyperchaotic attractor and strange attractor behavior in the dissipative nonlinear media. The transients between director’s phase space trajectories can be handled by the magnetic field parameters. The fundamental understanding of the director dynamics may have a valuable contribution to the applications of thin liquid crystal films.     

Ferroelectric Polarization Modulated Facet-selective Charge Separation in Bi4NbO8Cl Single Crystal for Boosting Visible-light Driven Bifunctional Water Splitting

Developing bifunctional water-splitting photocatalysts is meaningful, but challenged by the harsh requirements of specific-facet single crystals with spatially separated reactive sites and anisotropic charge transfer paths contributed by well-built charge driving force. Herein, tunable ferroelectric polarization is introduced in Bi₄NbO₈Cl single crystal nanosheets to strengthen the orthogonal charge transfer channels. By manipulating the in-plane polarization from octahedral off-centering of Nb⁵⁺ and out-of-plane polarization from lone pair electron effect of anisotropic Bi³⁺, both the fast charge recombination in bulk catalyst and the process of charge trapping into surface states can be effectively modulated. Collaborating with modest polarization electric field and facet junction induced built-in electric field, cooperative charge tractive force is constructed, which reinforces the spatial separation and migration of photogenerated electrons and holes to {110} reductive site facet and {001} oxidation site facet, respectively. While excessive polarization charges impair the facet-selective charge separation characteristics and conversely promote charge recombination on the surface. As a result, polarity-optimized Bi₄NbO₈Cl shows an excellent H₂ and O₂ evolution rate of 54.21 and 36.08 μmol ⋅ h⁻¹ in the presence of sacrificial reagents under visible light irradiation. This work unveils the function of ferroelectric polarization in tuning the intrinsic facet-selective charge transfer process of photocatalysts.     

Ferroelectric lyo-mesophase of banana-shaped molecules

Preliminary results are reported on the first ferroelectric 'lyo-mesophases' of banana-shaped molecules. We studied the effect of the non-polar solvents p-xylene and m-xylene on the macroscopic properties of banana-shaped molecules having a B₇ phase. It was observed that by adding only 15% of solvent the isotropic-B₇ transition temperature was lowered from 130°C to 70°C and the ferroelectric electro-optical switching range was extended from 1-2°C to more than 50°C, thus maintaining the switching even at room temperature. The xylene molecules act as spacers, maintaining larger separations between the banana-molecules, thus reducing the polar packing order. These observations provide unique opportunities for tuning the intermolecular interactions of banana-shaped molecules and modifying their macroscopic properties.