PFM (piezoresponse force microscopy)-aided design for molecular ferroelectrics

Metal-Free Perovskite Ferroelectrics with the Most Equivalent Polarization Axes

Ferroelectricity in metal-free perovskites (MFPs) has emerged as an academic hotspot for their lightweight, eco-friendly processability, flexibility, and degradability, with considerable progress including large spontaneous polarization, high Curie temperature, large piezoelectric response, and tailoring coercive field. However, their equivalent polarization axes as a key indicator are far from enough, although multiaxial ferroelectrics are highly preferred for performance output and application flexibility that profit from as many equivalent polarization directions as possible with easier reorientation. Here, by implementing the synergistic overlap of regulating anionic geometries (from spherical I- to octahedral [PF6]- and to tetrahedral [ClO4]- or [BF4]-) and cationic asymmetric modification, we successfully designed multiaxial MFP ferroelectrics CMDABCO-NH4-X3 (CMDABCO = N-chloromethyl-N'-diazabicyclo[2.2.2]octonium; X = [ClO4]- or [BF4]-) with the lowest P1 symmetry. More impressively, systemic characterizations indicate that they possess 24 equivalent polarization axes (Aizu notations of 432F1 and m3̅mF1, respectively)─the maximum number achievable for ferroelectrics. Benefiting from the multiaxial feature, CMDABCO-NH4-[ClO4]3 has been demonstrated to have excellent piezoelectric sensing performance in its polycrystalline sample and prepared composite device. Our study provides a feasible strategy for designing multiaxial MFP ferroelectrics and highlights their great promise for use in microelectromechanical, sensing, and body-compatible devices.

SHG Assisted Mixed-Phases Anatomizing in a Molecular Ferroelectric

Anatomizing mixed-phases, referring to analyzing the mixing profiles and quantifying the phases' proportions in a material, which is of great significance in the genuine applications. Here, by using second-harmonic generation (SHG) polarimetry and piezoresponse force microscopy (PFM) techniques, this work elucidates the contributions and distributions of two different symmetric phases mixed in an archetype monoaxial molecular ferroelectric, diisopropylammonium chloride (DIPACl). The two competing phases are preferred in thermodynamics or kinetic process respectively, and this work evidences the switching behavior between the two competing phases facilitated by an external electrical field as opposed to a heating process. This research contributes novel insights into phase engineering in the field of molecular ferroelectrics and is poised to serve as a potent analytical tool for subsequent applications.

Biodegradable Ferroelectric Molecular Plastic Crystal HOCH2(CF2)7CH2OH Structurally Inspired by Polyvinylidene Fluoride

Ferroelectric materials, traditionally comprising inorganic ceramics and polymers, are commonly used in medical implantable devices. However, their nondegradable nature often necessitates secondary surgeries for removal. In contrast, ferroelectric molecular crystals have the advantages of easy solution processing, lightweight, and good biocompatibility, which are promising candidates for transient (short-term) implantable devices. Despite these benefits, the discovered biodegradable ferroelectric materials remain limited due to the absence of efficient design strategies. Here, inspired by the polar structure of polyvinylidene fluoride (PVDF), a ferroelectric molecular crystal 1H,1H,9H,9H-perfluoro-1,9-nonanediol (PFND), which undergoes a cubic-to-monoclinic ferroelectric plastic phase transition at 339 K, is discovered. This transition is facilitated by a 2D hydrogen bond network formed through O-H···O interactions among the oriented PFND molecules, which is crucial for the manifestation of ferroelectric properties. In this sense, by reducing the number of -CF2- groups from ≈5 000 in PVDF to seven in PFND, it is demonstrated that this ferroelectric compound only needs simple solution processing while maintaining excellent biosafety, biocompatibility, and biodegradability. This work illuminates the path toward the development of new biodegradable ferroelectric molecular crystals, offering promising avenues for biomedical applications.

Experimental Observation of the Fully Ferroelectric-Fully Ferroelastic Effect in Multiferroic Hybrid Perovskites

Since the concept of "multiferroic" was first proposed in 1968, the coupling effect between different ferroic orders has attracted great interest in energy, information, and biomedical fields. However, the fully ferroelectric-fully ferroelastic effect has never been experimentally observed in hybrid perovskites, even though this effect was predicted to exist half a century ago. Realizing such cross-linking effects of polarization vectors and strain tensors has always been a huge challenge because of the complex difference in these two ferroic origins. Here, we report a multiferroic with full ferroelectricity and full ferroelasticity in two-dimensional (2D) hybrid perovskites based on ferroelectrochemistry. The dynamic molecular reorientations endow (cyclohexanemethylaminium)2PbCl4 with a desired symmetry change of 4̅2mFmm2 at a Curie temperature of 411.8 K. More strikingly, the switchable evolution of ferroelastic domains was directly observed under the control of either electric or mechanical fields, which is the first experimental observation of a fully ferroelectric-fully ferroelastic effect in hybrid perovskites. This work would provide new insights into understanding the intrinsic cross-linking mechanism between ferroelectricity and ferroelasticity toward the development of multichannel interactive microelectronic devices.

Solid Solutions of Plastic/Ferroelectric Crystals: Toward Tailor-Made Functional Materials

Plastic crystals that show ferroelectricity are highly promising materials for a wide range of applications. Their inherent remarkable malleability and highly symmetric cubic structures in the plastic crystal phase ensure that their ferroelectricity and related properties are retained in their bulk polycrystals. To develop functional materials based on such plastic/ferroelectric crystals, methods to tune their properties for specific applications are required. Here, we report the preparation of solid solutions of plastic/ferroelectric ionic crystals by mixing crystals with a common anion but different cations, or crystals with a common cation but different anions, which allows a continuous modification of the Curie temperature of the ferroelectric system over a range of 100 K. This adjustment of the Curie temperature allows the flexible tuning of the pyroelectric properties of the solid solutions, including a significant enhancement of room-temperature performance. The solid solutions also exhibit morphotropic phase boundaries in the composition-temperature phase diagrams, which shows an abrupt change in crystal structures with a variation of composition. This study showcases a simple and versatile property-tuning method that can be expected to pave the way for major progress in the development of materials based on plastic/ferroelectric crystals, which will eventually advance to the stage of pursuing tailor-made functional materials with desired properties.

Ferroelectric Texture of Individual Barium Titanate Nanocrystals

Ferroelectric materials display exotic polarization textures at the nanoscale that could be used to improve the energetic efficiency of electronic components. The vast majority of studies were conducted in two dimensions on thin films that can be further nanostructured, but very few studies address the situation of individual isolated nanocrystals (NCs) synthesized in solution, while such structures could have other fields of applications. In this work, we experimentally and theoretically studied the polarization texture of ferroelectric barium titanate (BaTiO3, BTO) NCs attached to a conductive substrate and surrounded by air. We synthesized NCs of well-defined quasicubic shape and 160 nm average size that conserve the tetragonal structure of BTO at room temperature. We then investigated the inverse piezoelectric properties of such pristine individual NCs by vector piezoresponse force microscopy (PFM), taking particular care to suppress electrostatic artifacts. In all of the NCs studied, we could not detect any vertical PFM signal, and the maps of the lateral response all displayed larger displacement amplitude on the edges with deformations converging toward the center. Using field phase simulations dedicated to ferroelectric nanostructures, we were able to predict the equilibrium polarization texture. These simulations revealed that the NC core is composed of 180° up and down domains defining the polar axis that rotate by 90° in the two facets orthogonal to this axis, eventually lying within these planes forming a layer of about 10 nm thickness mainly composed of 180° domains along an edge. From this polarization distribution, we predicted the lateral PFM response, which was revealed to be in very good qualitative agreement with the experimental observations. This work positions PFM as a relevant tool to evaluate the potential of complex ferroelectric nanostructures to be used as sensors.

Molecular orbital breaking in photo-mediated organosilicon Schiff base ferroelectric crystals

Ferroelectric materials, whose electrical polarization can be switched under external stimuli, have been widely used in sensors, data storage, and energy conversion. Molecular orbital breaking can result in switchable structural and physical bistability in ferroelectric materials as traditional spatial symmetry breaking does. Differently, molecular orbital breaking interprets the phase transition mechanism from the perspective of electronics and sheds new light on manipulating the physical properties of ferroelectrics. Here, we synthesize a pair of organosilicon Schiff base ferroelectric crystals, (R)- and (S)-N-(3,5-di-tert-butylbenzylidene)-1-((triphenylsilyl)oxy)ethanamine, which show optically controlled phase transition accompanying the molecular orbital breaking. The molecular orbital breaking is manifested as the breaking and reformation of covalent bonds during the phase transition process, that is, the conversion between C = N and C-O in the enol form and C-N and C = O in the keto form. This process brings about photo-mediated bistability with multiple physical channels such as dielectric, second-harmonic generation, and ferroelectric polarization. This work further explores this newly developed mechanism of ferroelectric phase transition and highlights the significance of photo-mediated ferroelectric materials for photo-controlled smart devices and bio-sensors.

Organic-Inorganic Hybrid Ferroelectric and Antiferroelectric with Afterglow Emission

Luminescent ferroelectrics are holding exciting prospect for integrated photoelectronic devices due to potential light-polarization interactions at electron scale. Integrating ferroelectricity and long-lived afterglow emission in a single material would offer new possibilities for fundamental research and applications, however, related reports have been a blank to date. For the first time, we here achieved the combination of notable ferroelectricity and afterglow emission in an organic-inorganic hybrid material. Remarkably, the presented (4-methylpiperidium)CdCl3 also shows noticeable antiferroelectric behavior. The implementation of cationic customization and halogen engineering not only enables a dramatic enhancement of Curie temperature of 114.4 K but also brings a record longest emission lifetime up to 117.11 ms under ambient conditions, realizing a leapfrog improvement of at least two orders of magnitude compared to reported hybrid ferroelectrics so far. This finding would herald the emergence of novel application potential, such as multi-level density data storage or multifunctional sensors, towards the future integrated optoelectronic devices with multitasking capabilities.

Ferroics in Hybrid Organic-Inorganic Perovskites: Fundamentals, Design Strategies, and Implementation

Hybrid organic-inorganic perovskites (HOIPs) afford highly versatile structure design and lattice dimensionalities; thus, they are actively researched as material platforms for the tailoring of ferroic behaviors. Unlike single-phase organic or inorganic materials, the interlayer coupling between organic and inorganic components in HOIPs allows the modification of strain and symmetry by chirality transfer or lattice distortion, thereby enabling the coexistence of ferroic orders. This review focuses on the principles for engineering one or multiple ferroic orders in HOIPs, and the conditions for achieving multiferroicity and magnetoelectric properties. The prospects of multilevel ferroic modulation, chiral spin textures, and spin orbitronics in HOIPs are also presented.

A Metal-Free Molecular Ferroelectric [4-Me-cyclohexylamine]ClO4 Introduced by Boat and Chair Conformations of Cyclohexylamine

Organic ferroelectrics have received a great deal of interest due to their exclusive properties. However, organic ferroelectrics have not been fully explored, which hinders their practical application. Here, we presented a novel metal-free organic molecular ferroelectric [4-MCHA][ClO4 ] (1) (4-MCHA=trans-4-methylcyclohexylamine), which exhibits an above-room-temperature of 328 K. Strikingly, the single crystal structure analysis of 1 shows that the driving force of phase transition is related to the interesting chair-boat conformation change of 4-MCHA cation, in addition to the order-disorder transition of ClO4 - anion. Using piezoelectric response force microscopy (PFM), the presence of domains and the implemented polarization switching were clearly observed, which explicitly determined the presence of room-temperature ferroelectricity of 1. As far as we know, the ferroelectric phase transition mechanism attributed to the conformational change in a trans isomeric cation is very rare. This research enriched the path of designing ferroelectric materials and smart materials.

Design and Piezoelectric Energy Harvesting Properties of a Ferroelectric Cyclophosphazene Salt

Cyclophosphazenes offer a robust and easily modifiable platform for a diverse range of functional systems that have found applications in a wide variety of areas. Herein, for the first time, it reports an organophosphazene-based supramolecular ferroelectric [(PhCH2 NH)6 P3 N3 Me]I, [PMe]I. The compound crystallizes in the polar space group Pc and its thin-film sample exhibits remnant polarization of 5 µC cm-2 . Vector piezoresponse force microscopy (PFM) measurements indicated the presence of multiaxial polarization. Subsequently, flexible composites of [PMe]I are fabricated for piezoelectric energy harvesting applications using thermoplastic polyurethane (TPU) as the matrix. The highest open-circuit voltages of 13.7 V and the maximum power density of 34.60 µW cm-2 are recorded for the poled 20 wt.% [PMe]I/TPU device. To understand the molecular origins of the high performance of [PMe]I-based mechanical energy harvesting devices, piezoelectric charge tensor values are obtained from DFT calculations of the single crystal structure. These indicate that the mechanical stress-induced distortions in the [PMe]I crystals are facilitated by the high flexibility of the layered supramolecular assembly.

Organic radical ferroelectric crystals with martensitic phase transition

Organic martensitic compounds are an emerging type of smart material with intriguing physical properties including thermosalient effect, ferroelasticity, and shape memory effect. However, due to the high structural symmetry and limited design theories for these materials, the combination of ferroelectricity and martensitic transformation has rarely been found in organic systems. Here, based on the chemical design strategies for molecular ferroelectrics, we show a series of asymmetric 1,4,5,8-naphthalenediimide derivatives with the homochiral amine and 2,2,6,6-tetramethylpiperidine-N-oxyl components, which adopt the low-symmetric polar structure and so allow ferroelectricity. Upon H/F substitution, the fluorinated compounds exhibit reversible ferroelectric and martensitic transitions at 399 K accompanied by a large thermal hysteresis of 132 K. This large thermal hysteresis with two competing (meta)-stable phases is further confirmed by density functional theory calculations. The rare combination of martensitic phase transition and ferroelectricity realizes the bistability with two different ferroelectric phases at room temperature. Our finding provides insight into the exploration of martensitic ferroelectric compounds with potential applications in switchable memory devices, soft robotics, and smart actuators.

Revealing the Polarizations of Molecular Ferroelectrics via SHG Polarimetry at the Nanoscale

Multifarious molecular ferroelectrics with multipolar axial characteristics have emerged in recent years, enriching the scenarios for energy harvesting, sensing, and information processing. The increased polar axes have enhanced the urgency of distinguishing different polarization states in material design, mechanism exploration, etc. However, conventional methods hardly meet the requirements of in situ, fast, microscale, contactless, and nondestructive features due to their inherent limitations. Herein, SHG polarimetry is introduced to probe the multioriented polarizations on a nanosized multiaxial molecular ferroelectric, i.e., TMCM-CdCl3 nanoplates, as an example. Combined with the analysis of the second-order susceptibility tensor, SHG polarimetry could serve as an effective method to detect the polarization orders and domain distributions of molecular ferroelectrics. Profiting from the full-optical feature, SHG polarimetry can even be performed on samples covered by transparent mediums, 2D materials, or thin metal electrodes. Our research might spark further fundamental studies and expand the application boundaries of next-generation ferroelectric materials.

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, Rs -tert-butanesulfinamide (Rs -tBuSA) and Ss -tert-butanesulfinamide (Ss -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.

3D-printed polymer composite devices based on a ferroelectric chiral ammonium salt for high-performance piezoelectric energy harvesting

Three-dimensional printing (3DP) is an emerging technology to fabricate complex architectures, necessary to realize state-of-the-art flexible and wearable electronic devices. In this regard, top-performing devices containing organic ferro- and piezoelectric compounds are desired to circumvent significant shortcomings of conventional piezoceramics, e.g. toxicity and high-temperature device processibility. Herein, we report on a 3D-printed composite of a chiral ferroelectric organic salt {[Me3CCH(Me)NH3][BF4]} (1) with a biodegradable polycaprolactone (PCL) polymer that serves as a highly efficient piezoelectric nanogenerator (PENG). The ferroelectric property of 1 originates from its polar tetragonal space group P42, verified by P-E loop measurements. The ferroelectric domain characteristics of 1 were further probed by piezoresponse force microscopy (PFM), which gave characteristic 'butterfly' and hysteresis loops. The PFM amplitude vs. drive voltage measurements gave a relatively high magnitude of the converse piezoelectric coefficient for 1. PCL polymer composites with various weight percentages (wt%) of 1 were prepared and subjected to piezoelectric energy harvesting tests, which gave a maximum open-circuit voltage of 36.2 V and a power density of 48.1 μW cm-2 for the 10 wt% 1-PCL champion device. Furthermore, a gyroid-shaped 3D-printed 10 wt% 1-PCL composite was fabricated to test its practical utility, which gave an excellent output voltage of 41 V and a power density of 56.8 μW cm-2. These studies promise the potential of simple organic compounds for building PENG devices using advanced manufacturing technologies.

Precise Design of Molecular Ferroelectrics with High TC and Tunable Band Gap by Molecular Modification

Molecular ferroelectric materials are widely applied in piezoelectric converters, non-volatile memorizers, and photovoltaic devices due to their advantages of adjustable structure, lightweight, easy processing, and environmental friendliness. However, designing multifunctional molecular ferroelectrics with excellent properties has always been a great challenge. Herein, a multiaxial molecular ferroelectric is successfully designed by modifying the quasi-spherical cation dabco with CuBr₂ to obtain halogenated [Bretdabco]CuBr₄ (Bretdabco = N-bromoethyl-N'-diazabicyclo [2.2.2]octane), which crystallizes in polar point groups (C₆). Typical ferroelectric behaviors featured by the P-E hysteresis loop and switched ferroelectric domain are exhibited. Notably, the molecular ferroelectric shows a high T_C of 460 K, which is rare in the field and could greatly expand the application range of this material. In addition, the band gap is adjustable through the regulation of halogen. Both the UV absorption spectra and theoretical calculations indicate that the molecular ferroelectrics belong to a direct band gap (2.14 eV) semiconductor. This tunable and narrow band gap semiconductor molecular ferroelectric material with high T_C can be utilized more effectively in the study of optoelectronics and sensors, including piezoelectric energy harvesters. This research may provide a promising approach for the development of multiaxial molecular ferroelectrics with a tiny band gap and high T_C.

Molecular Design of a Metal-Nitrosyl Ferroelectric with Reversible Photoisomerization

The development of photo-responsive ferroelectrics whose polarization may be remotely controlled by optical means is of fundamental importance for basic research and technological applications. Herein, we report the design and synthesis of a new metal-nitrosyl ferroelectric crystal (DMA)(PIP)[Fe(CN)₅(NO)] (1) (DMA = dimethylammonium, PIP = piperidinium) with potential phototunable polarization via a dual-organic-cation molecular design strategy. Compared to the parent non-ferroelectric (MA)₂[Fe(CN)₅(NO)] (MA = methylammonium) material with a phase transition at 207 K, the introduction of larger dual organic cations both lowers the crystal symmetry affording robust ferroelectricity and increases the energy barrier of molecular motions, endowing 1 with a large polarization of up to 7.6 μC cm^-2 and a high Curie temperature (T_c) of 316 K. Infrared spectroscopy shows that the reversible photoisomerization of the nitrosyl ligand is accomplished by light irradiation. Specifically, the ground state with the N-bound nitrosyl ligand conformation can be reversibly switched to both the metastable state I (MSI) with isonitrosyl conformation and the metastable state II (MSII) with side-on nitrosyl conformation. Quantum chemistry calculations suggest that the photoisomerization significantly changes the dipole moment of the [Fe(CN)₅(NO)]^2- anion, thus leading to three ferroelectric states with different values of macroscopic polarization. Such optical accessibility and controllability of different ferroelectric states via photoinduced nitrosyl linkage isomerization open up a new and attractive route to optically controllable macroscopic polarization.

Near-90° Switch in the Polar Axis of Dion-Jacobson Perovskites by Halide Substitution

Ferroelectricity in two-dimensional hybrid (2D) organic-inorganic perovskites (HOIPs) can be engineered by tuning the chemical composition of the organic or inorganic components to lower the structural symmetry and order-disorder phase change. Less efforts are made toward understanding how the direction of the polar axis is affected by the chemical structure, which directly impacts the anisotropic charge order and nonlinear optical response. To date, the reported ferroelectric 2D Dion-Jacobson (DJ) [PbI₄]^2- perovskites exhibit exclusively out-of-plane polarization. Here, we discover that the polar axis in ferroelectric 2D Dion-Jacobson (DJ) perovskites can be tuned from the out-of-plane (OOP) to the in-plane (IP) direction by substituting the iodide with bromide in the lead halide layer. The spatial symmetry of the nonlinear optical response in bromide and iodide DJ perovskites was probed by polarized second harmonic generation (SHG). Density functional theory calculations revealed that the switching of the polar axis, synonymous with the change in the orientation of the sum of the dipole moments (DMs) of organic cations, is caused by the conformation change of organic cations induced by halide substitution.

Superior ferroelectricity and nonlinear optical response in a hybrid germanium iodide hexagonal perovskite

Abundant chemical diversity and structural tunability make organic-inorganic hybrid perovskites (OIHPs) a rich ore for ferroelectrics. However, compared with their inorganic counterparts such as BaTiO₃, their ferroelectric key properties, including large spontaneous polarization (P_s), low coercive field (E_c), and strong second harmonic generation (SHG) response, have long been great challenges, which hinder their commercial applications. Here, a quasi-one-dimensional OIHP DMAGeI₃ (DMA = Dimethylamine) is reported, with notable ferroelectric attributes at room temperature: a large P_s of 24.14 μC/cm² (on a par with BaTiO₃), a low E_c below 2.2 kV/cm, and the strongest SHG intensity in OIHP family (about 12 times of KH₂PO₄ (KDP)). Revealed by the first-principles calculations, its large P_s originates from the synergistic effects of the stereochemically active 4s² lone pair of Ge²⁺ and the ordering of organic cations, and its low kinetic energy barrier of small DMA cations results in a low E_c. Our work brings the comprehensive ferroelectric performances of OIHPs to a comparable level with commercial inorganic ferroelectric perovskites.

Tuning the luminescent properties of a three-dimensional perovskite ferroelectric (Me-Hdabco)CsI3via Sn(II) doping

As promising functional materials, organic-inorganic hybrid metal halide perovskites have attracted significant interest because of their excellent photovoltaic performance. However, although considerable efforts have been made, three-dimensional (3D) metal halide perovskites beyond lead halides have been rarely reported. Herein, a new 3D organic-inorganic hybrid ferroelectric material (Me-Hdabco)CsI₃ (1, Me-Hdabco = N-methyl-1,4-diazoniabicyclo[2.2.2]octane) was synthesized and characterized. 1 underwent a ferroelectric to paraelectric phase transition at T_c = 441 K, which was investigated by differential scanning calorimetry (DSC), dielectric measurements, and variable temperature structural analyses. Moreover, 1 shows a clear ferroelectric domain switching recorded by piezoelectric force microscopy. More interestingly, the pristine colorless crystal of 1 has no photoluminescence properties, while 10% Sn(II):(Me-Hdabco)CsI₃ shows intense photoluminescence with a quantum yield of 8.90% under UV excitation. This finding will open up a new avenue to probe organic-inorganic hybrid multifunctional materials integrated ferroelectric and photoluminescence.

A Poling-Free Supramolecular Crown Ether Compound with Large Piezoelectricity

Supramolecular host-guest ferroelectrics based on solution processing are highly desirable because they are generally created with intrinsic piezoelectricity/ferroelectricity and do not need further poling. Poly(vinylidene fluoride) (PVDF) in the electric-active beta phase after stretching/annealing still shows no piezoelectric response unless poled. Although many supramolecular host-guest ferroelectrics have been discovered, their piezoelectricity is relatively small. Based on H/F substitution, we reported a supramolecular host-guest compound [(CF₃-C₆H₄-NH₃)(18-crown-6)][TFSA] (CF₃-C₆H₄-NH₃ = 4-trifluoromethylanilinium, TFSA = bis(trifluoromethanesulfonyl)ammonium) with a remarkable piezoelectric response of 42 pC/N under no poling condition. The introduction of F atoms increases phase transition temperature, polar axes, second harmonic generation (SHG) intensity, and piezoelectric coefficient d₃₃. To our knowledge, such a large piezoelectric performance of [(CF₃-C₆H₄-NH₃)(18-crown-6)][TFSA] makes its d₃₃, piezoelectric voltage coefficient g₃₃, and mechanical quality factor Q_m the highest among the reported supramolecular host-guest ferroelectric compounds and even larger than the values of PVDF. This work provides inspiration for optimizing piezoelectricity on molecular materials.

Ferroelectric Ionic Molecular Crystals with Significant Plasticity and a Low Melting Point: High Performance in Hot-Pressed Polycrystalline Plates and Melt-Grown Crystalline Sheets

Among ferroelectric crystals based on small molecules, plastic/ferroelectric crystals are currently receiving particular attention because they can be used as bulk polycrystals. Herein, we show that an ionic molecular ferroelectric crystal, guanidinium tetrafluoroborate, exhibits significant malleability and multiaxial ferroelectricity despite the absence of a plastic crystal phase. Powder samples of this crystal can be processed into transparent bulk crystalline plates either by press-forming or by melt-growing. The plates show high ferroelectric performance and related properties, demonstrating the largest hitherto reported spontaneous polarization for bulk polycrystals of small-molecule-based ferroelectrics. Owing to the ready availability of large-scale materials and processability into various bulk crystalline forms, this ferroelectric crystal represents a highly promising functional material that will boost research on diverse applications as bulk crystals.

An Effective Strategy of Introducing Chirality to Achieve Multifunctionality in Rare-Earth Double Perovskite Ferroelectrics

The hybrid rare-earth double perovskite (HREDP) system provides great convenience for the construction of multifunctional materials. However, suffering from the high symmetry of their intrinsic structure, HREDPs face the challenges in the realization and optimization of ferroelectric and piezoelectric properties. For the first time, after a systematic investigation of the chirality transformation principle, it is found that the introduction of chirality is an efficient strategy for the targeted construction of multifunctionality, which simultaneously increases the possibility of obtaining multiaxial ferroelectricity and ferroelasticity, and effectively realizes a large piezoelectric response. Moreover, chirality induced ferroelasticity will also achieve excellent magnetic or optical response driven by pressure-sensitive. To verify the feasibility of the above ideas, by using rare-earth ions (Ce³⁺ ) and suitable chiral organic cations, a new HREDP, (R-N-methyl-3-hydroxylquinuclidinium)₂ RbCe(NO₃ )₆ (R1) is successfully designed, in which ferroelasticity, multiaxial ferroelectricity, satisfactory piezoelectric response, and the pressure-driven single-ion magnetics switch are simultaneously achieved for the first time. This work shows that the induction of chirality and the HREDP system provide an effective strategy and ideal platform for the expansion and optimization of the functions in perovskite ferroelectrics.

Achieving circularly polarized luminescence and large piezoelectric response in hybrid rare-earth double perovskite by a chirality induction strategy

Chirality, an intrinsic property of nature, has received increased attention in chemistry, biology, and materials science because it can induce optical rotation, ferroelectricity, nonlinear optical response, and other unique properties. Here, by introducing chirality into hybrid rare-earth double perovskites (HREDPs), we successfully designed and synthesized a pair of enantiomeric three-dimensional (3D) HREDPs, [(R)-N-methyl-3-hydroxylquinuclidinium]₂RbEu(NO₃)₆ (R1) and [(S)-N-methyl-3-hydroxylquinuclidinium]₂RbEu(NO₃)₆ (S1), which possess ferroelasticity, multiaxial ferroelectricity, high quantum yields (84.71% and 83.55%, respectively), and long fluorescence lifetimes (5.404 and 5.256 ms, respectively). Notably, the introduction of chirality induces the coupling of multiaxial ferroelectricity and ferroelasticity, which brings about a satisfactory large piezoelectric response (103 and 101 pC N^-1 for R1 and S1, respectively). Moreover, in combination with the chirality and outstanding photoluminescence properties, circularly polarized luminescence (CPL) was first realized in HREDPs. This work sheds light on the design strategy of molecule-based materials with a large piezoelectric response and excellent CPL activity, and will inspire researchers to further explore the role of chirality in the construction of novel multifunctional materials.

Chemical design of homochiral heterocyclic organic ferroelectric crystals

Single component organic ferroelectrics of spirooxazacamphorsultam derivatives, 1-SSR and 1-RRS, exhibit well-defined ferroelectricity (P_s = 2.2 μC cm^-2) and piezoelectricity (d₃₃ = 10 pC N^-1) below their melting point. More importantly, they possess a low acoustic impedance value of 2.7 × 10⁶ kg s^-1 m^-2, which is well-matched with body tissues.

Two-Step Dielectric Responsive Organic-Inorganic Hybrid Material with Mid-Band Light Emission

Hybrid organic-inorganic perovskite (HOIP) have received tremendous scientific attention because of the phase transition and photovoltaic properties. However, achieving the special perovskite structure with both two-step dielectric response and luminescence characteristics is rarely reported. Herein, we report an organic-inorganic hybrid perovskite, [(BA)₂  ⋅ PbI₄ ] (Compound 1, BA=n-butylamine) by introducing flexible organic cations (HBA⁺ ), with direct mid-band gap as 2.28 eV. Interestingly, this material exhibits two-step reversible dielectric response at 350 K and 460 K (in heating process), respectively. Besides, the photoluminescence was found: it emits charming green light under 365 nm lamp (Photoluminescence quantum yield is 9.52 %). The outstanding two-step dielectric response and luminescence characteristics of this compound might pave the way for the application of dielectric and ferroelectric functional materials in temperature sensors and mechanical switches.

Rational Design of Molecular Ferroelectrics with Negatively Charged Domain Walls

Ferroelectric domains and domain walls are unique characteristics of ferroelectric materials. Among them, charged domain walls (CDWs) are a special kind of peculiar microstructure that highly improve conductivity, piezoelectricity, and photovoltaic efficiency. Thus, CDWs are believed to be the key to ferroelectrics' future application in fields of energy, sensing, information storage, and so forth. Studies on CDWs are one of the most attractive directions in conventional inorganic ferroelectric ceramics. However, in newly emerged molecular ferroelectrics, which have advantages such as lightweight, easy preparation, simple film fabrication, mechanical flexibility, and biocompatibility, CDWs are rarely observed due to the lack of free charges. In inorganic ferroelectrics, doping is a traditional method to induce free charges, but for molecular ferroelectrics fabricated by solution processes, doping usually causes phase separation or phase transition, which destabilizes or removes ferroelectricity. To realize stable CDWs in molecular systems, we designed and synthesized an n-type molecular ferroelectric, 1-adamantanammonium hydroiodate. In this compound, negative charges are induced by defects in the I^- vacancy, and CDWs can be achieved. Nanometer-scale CDWs that are stable at temperatures as high as 373 K can be "written" precisely by an electrically biased metal tip. More importantly, this is the first time that the charge diffusion of CDWs at variable temperatures has been investigated in molecular ferroelectrics. This work provides a new design strategy for n-type molecular ferroelectrics and may shed light on their future applications in flexible electronics, microsensors, and so forth.

The first salicylaldehyde Schiff base organic-inorganic hybrid lead iodide perovskite ferroelectric

A salicylaldehyde Schiff base hybrid lead iodide perovskite [SAPD]PbI₃ (SAPD = 1-((2-hydroxybenzylidene)amino)pyridin-1-ium) was found to show a robust nonlinear optical response and large spontaneous polarization. We expect this work to inspire researchers to investigate the optical control of ferroelectricity in hybrid perovskites.

An organic plastic ferroelectric with high Curie point

Plastic ferroelectrics, featuring large entropy changes in phase transitions, hold great potential application for solid-state refrigeration due to the electrocaloric effect. Although conventional ceramic ferroelectrics (e.g., BaTiO3 and KNbO3) have been widely investigated in the fields of electrocaloric material and catalysis, organic plastic ferroelectrics with a high Curie point (T c) are rarely reported but are of great importance for the sake of environmental protection. Here, we reported an organic plastic ferroelectric, (-)-camphanic acid, which crystallizes in the P21 space group, chiral polar 2 (C2) point group, at room temperature. It undergoes plastic paraelectric-to-ferroelectric phase transition with the Aizu notation of 23F2 and high T c of 414 K, showing large entropy gain (ΔS t = 48.2 J K-1 mol-1). More importantly, the rectangular polarization-electric field (P-E) hysteresis loop was recorded on the thin film samples with a large saturated polarization (P s) of 5.2 μC cm-2. The plastic phase transition is responsible for its multiaxial ferroelectric feature. This work highlights the discovery of organic multiaxial ferroelectrics driven by the motive of combining chirality and plastic phase transition, which will extensively promote the practical application of such unique functional materials.

Unusual high-temperature host–guest inclusion compound-based ferroelectrics with nonlinear optical switching and large spontaneous polarization behaviours

Host–guest inclusion compound-based ferroelectrics [Hcta-(18-crown-6)]+[BF4]− (1) and [Hcta-(18-crown-6)]+[ClO4]− (2) with a high Curie temperature (Tc = 403/394 K) and large spontaneous polarization (Ps = 5.7/4.7 μC cm−2) are reported.

High Curie temperature bismuth-based piezo-/ferroelectric single crystals of complex perovskite structure: recent progress and perspectives

The recent progress in high TC bismuth-based piezo-/ferroelectric single crystals is reviewed in terms of materials design, crystal growth, physical properties, crystal chemistry, and complex domain structures, and the future perspectives are discussed.

Dual-channel control of ferroelastic domains in a host–guest inclusion compound

By replacing [PF6]− with the larger [TFSA]−, the phase transition temperature is increased from 305 K to 342 K in a host–guest inclusion ferroelastic crystal, [(3,4-DFA)(18-crown-6)][TFSA], which can realize dual-channel (thermal and stress) control of ferroelastic domains.

An unprecedented azobenzene-based organic single-component ferroelectric

The first azobenzene-based organic single-component ferroelectric 2-amino-2′,4,4′,6,6′-pentafluoroazobenzene was designed, which shows an exceptionally high Curie temperature (Tc) of 443 K.

Highest-Tc single-component homochiral organic ferroelectrics

Organic single-component ferroelectrics with low molecular mass have drawn great attention for application in organic electronics. However, the discovery of high-T_c single-component organic ferroelectrics has been very scarce. Herein, we report a pair of homochiral single-component organic ferroelectrics (R)-10-camphorsulfonylimine and (S)-10-camphorsulfonylimine under the guidance of ferroelectric chiral chemistry. They crystallize in the chiral–polar space group P2₁, and their mirror image relations have been identified using vibrational circular dichroism spectra. They both exhibit 422F2 multiaxial ferroelectricity with T_c as high as 429 K. Besides, they possess superior acoustic impedance characteristics with a value of 2.45 × 10⁶ kg s⁻¹ m⁻², lower than that of PVDF. To our knowledge, enantiomeric (R and S)-10-camphorsulfonylimine show the highest T_c among the known organic single-component ferroelectrics and low acoustic impedance well matching with that of bodily tissues. This work promotes the development of high-performance organic single-component ferroelectrics and is of great inspiration to explore their application in next-generation flexible smart devices.

A pair of enantiomeric organic ferroelectrics (R and S)-10-camphorsulfonylimine show the highest T_c among the known single-component organic ferroelectrics.

Post-synthetic modification within MOFs: a valuable strategy for modulating their ferroelectric performance

It is a great route designing new MOF ferroelectrics to enrich the scope of ferroelectrics or improving the ferroelectric performance to enhance the opportunity of applications through the strategy of post-synthetic modification (PSM).

Crown Ether Host-Guest Molecular Ferroelectrics

In recent years, molecular ferroelectrics have received great attention due to their low weight, mechanical flexibility, easy preparation and excellent ferroelectric properties. Among them, crown-ether-based molecular ferroelectrics, which are typically composed of the host crown ethers, the guest cations anchored in the crown ethers, and the counterions, are of great interest because of the host-guest structure. Such a structure allows the components to occur order-disorder transition easily, which is beneficial for inducing ferroelectric phase transition. Herein, we summarized the research progress of crown ether host-guest molecular ferroelectrics, focusing on their crystal structure, phase transition, ferroelectric-related properties. In view of the small spontaneous polarization and uniaxial nature, we outlook the chemical design strategies for obtaining high-performance crown-ether-based molecular ferroelectrics. This minireview will be of guiding significance for the future exploration of crown ether host-guest molecular ferroelectrics.

"May the Force Be with You!" Force-Volume Mapping with Atomic Force Microscopy

Information of the chemical, mechanical, and electrical properties of materials can be obtained using force volume mapping (FVM), a measurement mode of scanning probe microscopy (SPM). Protocols have been developed with FVM for a broad range of materials, including polymers, organic films, inorganic materials, and biological samples. Multiple force measurements are acquired with the FVM mode within a defined 3D volume of the sample to map interactions (i.e., chemical, electrical, or physical) between the probe and the sample. Forces of adhesion, elasticity, stiffness, deformation, chemical binding interactions, viscoelasticity, and electrical properties have all been mapped at the nanoscale with FVM. Subsequently, force maps can be correlated with features of topographic images for identifying certain chemical groups presented at a sample interface. The SPM tip can be coated to investigate-specific reactions; for example, biological interactions can be probed when the tip is coated with biomolecules such as for recognition of ligand-receptor pairs or antigen-antibody interactions. This review highlights the versatility and diverse measurement protocols that have emerged for studies applying FVM for the analysis of material properties at the nanoscale.

Optical Control of Polarization Switching in a Single-Component Organic Ferroelectric Crystal

The optical control of polarization switching is attracting tremendous interest because photoirradiation stands out as a nondestructive, noncontact, and remote-control means beyond an electric or strain field. The current research mainly uses various photoexcited electronic effects to achieve the photocontrol polarization, such as a light-driven flexoelectric effect and a photovoltaic effect. However, since photochromism was discovered in 1867, the structural phase transition caused by photoisomerization has never been associated with ferroelectricity. Here, we successfully synthesized an organic photochromic ferroelectric with polar space group Pna2₁, 3,4,5-trifluoro-N-(3,5-di-tert-butylsalicylidene)aniline, whose color can change between yellow and orange via laser illumination. Its dielectric permittivity and spontaneous polarization can be switched reversibly with a photoinduced phase transition triggered by structural photoisomerization between the enol form and the trans-keto form. To our knowledge, this is the first photoswitchable ferroelectric crystal to achieve polarization switching through a structural phase transition triggered by photoisomerization. This finding paves the way toward photocontrol of smart materials and biomechanical applications in the future.

A hybrid hydrochromic molecular crystal applicable to invisible ink with high reversibility

Highly reversible hydrochromic behavior is realized in a novel hybrid molecular crystal by controlling the gain and loss of coordinated water.

Unique cation-template three-dimensional hybrid material demonstrates dielectric switchable response

The strict tolerance space of three-dimensional (3D) crystalline structures is still a significant challenge in the area of switching dielectrics in comparison with lower-dimensional structures. Generally, the function of crystalline materials can be given or adjusted by controlling the environment in which synthesis takes place or the packing rearrangement. Using this method, special functional enhancements or changes in the dielectrics can be realized by improving the synthetic strategies. Here, a 3D switchable dielectric compound [MeHdabco]K(BF4)3 was achieved by employing the temperature selective effect. In particular, its structure is completely different from the usual 3D perovskite structure, which is constructed using two different cation-template frameworks. Moreover, the 3D [MeHdabco]K(BF4)3 shows a structural phase transition at 358 K. The thermal analysis (differential scanning calorimetry (DSC)) and X-ray diffractometry results provided evidence of these phase changes. This work provides a feasible strategy that can be used to achieve the different structures of an 'isomer', and enrich the method used for designing diverse functional materials.

Halogen regulation triggers NLO and dielectric dual switches in hybrid compounds with green fluorescence

An effective strategy of using halogens to modify organic–inorganic hybrid materials to obtain NLO switching characteristics, which is expected to be used for the directional adjustment of NLO switch activity.