The past 10 years of molecular ferroelectrics: structures, design, and properties.

Ferroelectricity, which has diverse important applications such as memory elements, capacitors, and sensors, was first discovered in a molecular compound, Rochelle salt, in 1920 by Valasek. Owing to their superiorities of lightweight, biocompatibility, structural tunability, mechanical flexibility, etc., the past decade has witnessed the renaissance of molecular ferroelectrics as promising complementary materials to commercial inorganic ferroelectrics. Thus, on the 100th anniversary of ferroelectricity, it is an opportune time to look into the future, specifically into how to push the boundaries of material design in molecular ferroelectric systems and finally overcome the hurdles to their commercialization. Herein, we present a comprehensive and accessible review of the appealing development of molecular ferroelectrics over the past 10 years, with an emphasis on their structural diversity, chemical design, exceptional properties, and potential applications. We believe that it will inspire intense, combined research efforts to enrich the family of high-performance molecular ferroelectrics and attract widespread interest from physicists and chemists to better understand the structure-function relationships governing improved applied functional device engineering.

Rational Design of 2D Metal Halide Perovskites with Low Congruent Melting Temperature and Large Melt-Processable Window.

Metal halide perovskites (MHPs) have garnered significant attention due to their distinctive optical and electronic properties, coupled with excellent processability. However, the thermal characteristics of these materials are often overlooked, which can be harnessed to cater to diverse application scenarios. We showcase the efficacy of lowering the congruent melting temperature (Tm) of layered 2D MHPs by employing a strategy that involves the modification of flexible alkylammonium through N-methylation and I-substitution. Structural-property analysis reveals that the N-methylation and I-substitution play pivotal roles in reducing hydrogen bond interactions between the organic components and inorganic parts, lowering the rotational symmetry number of the cation and restricting the residual motion of the cations. Additional I···I interactions enhance intermolecular interactions and lead to improved molten stability, as evidenced by a higher viscosity. The 2D MHPs discussed in this study exhibit low Tm and wide melt-processable windows, e.g., (DMIPA)2PbI4 showcasing a low Tm of 98 °C and large melt-processable window of 145 °C. The efficacy of the strategy was further validated when applied to bromine-substituted 2D MHPs. Lowering the Tm and enhancing the molten stability of the MHPs hold great promise for various applications, including glass formation, preparation of high-quality films for photodetection, and fabrication of flexible devices.

First Observation of Negative Capacitance in Molecular Ferroelectric Thin Films.

On the path of persisting Moore's Law, one of the biggest obstacles is the "Boltzmann tyranny," which defines the lower limit of power consumption of individual transistors. Negative capacitance (NC) in ferroelectrics could provide a solution and has garnered significant attention in the fields of nanoelectronics, materials science, and solid-state physics. Molecular ferroelectrics, as an integral part of ferroelectrics, have developed rapidly in terms of both performance and functionality, with their inherent advantages such as easy fabrication, mechanical flexibility, low processing temperature, and structural tunability. However, studies on the NC in molecular ferroelectrics are limited. In this study, the focus is centered on the fabricated high-quality thin films of trimethylchloromethyl ammonium trichlorocadmium(II), and a pioneering investigation on their NC responses is conducted. The findings demonstrate that the NC exhibited by molecular ferroelectrics is comparable to that of conventional HfO2 -based ferroelectrics. This underscores the potential of molecular material systems for next-generation electronic devices.

Volume-Confined Fabrication of Large-Scale Single-Crystalline Molecular Ferroelectric Thin Films and Their Applications in 2D Materials.

With outstanding advantages of chemical synthesis, structural diversity, and mechanical flexibility, molecular ferroelectrics have attracted increasing attention, demonstrating themselves as promising candidates for next-generation wearable electronics and flexible devices in the film form. However, it remains a challenge to grow high-quality thin films of molecular ferroelectrics. To address the above issue, a volume-confined method is utilized to achieve ultrasmooth single-crystal molecular ferroelectric thin films at the sub-centimeter scale, with the thickness controlled in the range of 100-1000 nm. More importantly, the preparation method is applicable to most molecular ferroelectrics and has no dependency on substrates, showing excellent reproducibility and universality. To demonstrate the application potential, two-dimensional (2D) transitional metal dichalcogenide semiconductor/molecular ferroelectric heterostructures are prepared and investigated by optical spectroscopic method, proving the possibility of integrating molecular ferroelectrics with 2D layered materials. These results may unlock the potential for preparing and developing high-performance devices based on molecular ferroelectric thin films.

Wireless Deep Brain Stimulation by Ultrasound-Responsive Molecular Piezoelectric Nanogenerators.

Implantable neural stimulation devices are becoming prevalent in bioelectronic medicine for the precise treatment of various clinical diseases. Nevertheless, the limited lifespan and buckling size of the implanted devices remain significant obstacles for chronic clinical application. In this study, we developed an ultrasound-driven battery-free neurostimulator based on a high-performance mini-sized nanogenerator and demonstrated its successful application for the deep-brain-stimulation (DBS) therapy of Parkinson's disease in a rat model. This soft piezoelectric-triboelectric hybrid nanogenerators (PTNG) are made of porous thin-films of molecular piezoelectric materials, which have great advantages of facile, scalable, low-temperature, and flexible processing. Without any bucky accessory control circuits, the subcutaneously implanted soft PTNG can function as a wirelessly powered neurostimulator, allowing for the adjustment of stimulation parameters through external programmable ultrasound pulses. This DBS electroceutical application of energy-harvesting thin-film devices based on molecular piezoelectric materials provides valuable insight into the development of a soft high-performance bioelectronic device.

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.

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 (Ps), low coercive field (Ec), 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 Ps of 24.14 µC/cm2 (on a par with BaTiO3), a low Ec 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 Ps 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 Ec. Our work brings the comprehensive ferroelectric performances of OIHPs to a comparable level with commercial inorganic ferroelectric perovskites.

The First Demonstration of Strain-Controlled Periodic Ferroelectric Domains with Superior Piezoelectric Response in Molecular Materials.

Achieving a periodic domain structure in ferroelectric materials to tailor the macroscopic properties or realize new functions has always been a hot topic. However, methods to construct periodic domain structures, such as epitaxial growth, direct writing by scanning tips, and the patterned electrode method, are difficult or inefficient to implement in emerging molecular ferroelectrics, which have the advantages of lightweight, flexibility, biocompatibility, etc. An efficient method for constructing and controlling periodic domain structures is urgently needed to facilitate the development of molecular ferroelectrics in nanoelectronic devices. In this work, it is demonstrated that large-area, periodic and controllable needle-like domain structures can be achieved in thin films of the molecular ferroelectric trimethylchloromethyl ammonium trichlorocadmium (TMCM-CdCl3 ) upon the application of tensile strain. The domain evolution under various tensile strains can be clearly observed, and such processes are accordingly identified. Furthermore, the domain wall exhibits a superior piezoelectric response, with up to fivefold enhancement compared to that of the pristine samples. Such large-area tunable periodic domain structure and abnormally strong piezoresponse are not only of great interests in fundamental studies, but also highly important in the future applications in functional molecular materials.

Giant electrocaloric effect in a molecular ceramic.

The electrocaloric effect (ECE) is an efficient and environmentally friendly method for solid-state refrigeration driven by an electric field. However, disregarding the ECE performance, the mass of materials also limits the amount of energy transferred in the cooling process. While molecular ECE materials have been attracting intensive attention with their excellent ECE properties, most reported molecular compounds can only be utilized in the form of thin films or single crystals. Unlike inorganic ceramics, molecular thin films and single crystals are very difficult to prepare in a large amount, which greatly restrains the future application of those materials. In this work, we report an excellent molecular ECE material in the form of polycrystalline molecular ceramics. Such molecular ceramics are composed of plastic molecular ferroelectrics, and can fulfil the requirement of large mass, easy processing, excellent performance and low energy consumption. Our molecular ceramic of HQReO4 (HQ: protonated quinuclidine) demonstrates an isothermal entropy change of 5.8 J K-1 kg-1 and an adiabatic temperature change of 3.1 K. Notably, by a simple low-temperature pressing process without added adhesives (about 373 K), an HQReO4 molecular ceramic block can be obtained, and its ECE performance is observed to be comparable to that of single crystals, for the first time. This work proposes a new application form for molecular electrocaloric materials, which opens up new ideas for solid-state refrigeration.

Remarkable Enhancement of Piezoelectric Performance by Heavy Halogen Substitution in Hybrid Perovskite Ferroelectrics.

Piezoelectric materials that enable electromechanical conversion have great application value in actuators, transducers, sensors, and energy harvesters. Large piezoelectric (d33) and piezoelectric voltage (g33) coefficients are highly desired and critical to their practical applications. However, obtaining a material with simultaneously large d33 and g33 has long been a huge challenge. Here, we reported a hybrid perovskite ferroelectric [Me3NCH2Cl]CdBrCl2 to mitigate and roughly address this issue by heavy halogen substitution. The introduction of a large-size halide element softens the metal-halide bonds and reduces the polarization switching barrier, resulting in excellent piezoelectric response with a large d33 (∼440 pC/N), which realizes a significant optimization compared with that of previously reported [Me3NCH2Cl]CdCl3 (You et al. Science2017, 357, 306-309). More strikingly, [Me3NCH2Cl]CdBrCl2 simultaneously shows a giant g33 of 6215 × 10-3 V m/N, far exceeding those of polymers and conventional piezoelectric ceramics. Combined with simple solution preparation, easy processing of thin films, and a high Curie temperature of 373 K, these attributes make [Me3NCH2Cl]CdBrCl2 promising for high-performance piezoelectric sensors in flexible, wearable, and biomechanical devices.

Inch-Size Molecular Ferroelectric Crystal with a Large Electromechanical Coupling Factor on Par with Barium Titanate.

Molecular ferroelectrics with large piezoelectric responses have long been sought for their advantages of light weight, mechanical flexibility, and easy preparation, in contrast to the widely used inorganic counterparts. Representatively, a molecular ferroelectric crystal [Me3NCH2Cl]CdCl3 (TMCM-CdCl3) has been found to show a large piezoelectric coefficient d33 of 220 pC/N exceeding that of BaTiO3 (You et al. Science2017, 357, 306-309). However, although the d33 of molecular ferroelectrics has achieved great progress, their electromechanical coupling factor k33, which is essential for various piezoelectric applications, including ultrasonic transducers and actuators, was rarely explored and is far below the level of inorganic ferroelectrics. The major reason for this situation is the great challenge of growing large-size crystals which is a key limiting factor for measuring k33. Here, we grew inch-size crystals of organic-inorganic perovskite ferroelectric TMCM-CdCl3 with a high d33 (383 pC/N) for investigating its piezoelectric responses including the k33 (0.483) by the resonance method. Such high k33 (0.483) is much larger than those of other molecular ferroelectrics and competitive with that of BaTiO3 (0.5). In addition, TMCM-CdCl3 has a low elastic modulus of 13.03 GPa, an order of magnitude lower than that of BaTiO3. This finding sheds light on the exploration of large electromechanical coupling factors in molecular ferroelectrics for potential applications in flexible and portable piezoelectric devices.

Axial-Chiral BINOL Multiferroic Crystals with Coexistence of Ferroelectricity and Ferroelasticity.

Chirality exists everywhere from natural amino acids to particle physics. The introduction of point chirality has recently been shown to be an efficient strategy for the construction of molecular ferroelectrics. In contrast to point chirality, however, axial chirality is rarely used to design ferroelectrics so far. Here, based on optically active 1,1'-bi-2-naphthol (BINOL), which has been applied extensively as a versatile chiral reagent in asymmetric catalysis, chiral recognition, and optics, we successfully design a pair of axial-chiral BINOL multiferroics, (R)-BINOL-DIPASi and (S)-BINOL-DIPASi. They experience a 2F1-type full ferroelectric/ferroelastic phase transition at a high temperature of 362 and 363 K, respectively. Piezoelectric force microscopy and polarization-voltage hysteresis loops demonstrate their ferroelectric domains and domain switching, and polarized light microscopy visualizes the evolution of stripe-shaped ferroelastic domains. The axial-chiral BINOL building block promotes the generation of the polar structure and ferroelectricity, and the organosilicon component increases the rotational energy barrier and thus the phase transition temperature. This work presents the first axial-chiral high-temperature multiferroic crystals, offering an efficient path for designing molecular multiferroics through the introduction of axial chirality.

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.

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.

Molecular Design of Three-Dimensional Metal-Free A(NH4)X3 Perovskites for Photovoltaic Applications.

The intense research activities on the hybrid organic-inorganic perovskites (HOIPs) have led to the greatly improved light absorbers for solar cells with high power conversion efficiency (PCE). However, it is still challenging to find an alternative lead-free perovskite to replace the organohalide lead perovskites to achieve high PCE. This is because both previous experimental and theoretical investigations have shown that the Pb2+ cations play a dominating role in contributing the desirable frontier electronic bands of the HOIPs for light absorbing. Recent advances in the chemical synthesis of three-dimensional (3D) metal-free perovskites, by replacing Pb2+ with NH4 +, have markedly enriched the family of multifunctionalized perovskites (Ye et al., Science2018, 361, 151-155). These metal-free perovskites possess the chemical formula of A(NH4)X3, where A is divalent organic cations and X denotes halogen atoms. Without involving transition-metal cations, the metal-free A(NH4)X3 perovskites can entail notably different frontier electronic band features from those of the organohalide lead perovskites. Indeed, the valence and conduction bands of A(NH4)X3 perovskites are mainly attributed by the halogen atoms and the divalent A2+ organic cations, respectively. Importantly, a linear relationship between the bandgaps of A(NH4)X3 perovskites and the lowest unoccupied molecular orbital energies of the A2+ cations is identified, suggesting that bandgaps can be tailored via molecular design, especially through a chemical modification of the A2+ cations. Our comprehensive computational study and molecular design predict a metal-free perovskite, namely, 6-ammonio-1-methyl-5-nitropyrimidin-1-ium-(NH4)I3, with a desirable bandgap of ∼1.74 eV and good optical absorption property, both being important requirements for photovoltaic applications. Moreover, the application of strain can further fine-tune the bandgap of this metal-free perovskite. Our proposed design principle not only offers chemical insights into the structure-property relationship of the multifunctional metal-free perovskites but also can facilitate the discovery of highly efficient alternative, lead-free perovskites for potential photovoltaic or optoelectronic applications.

Confinement-Driven Ferroelectricity in a Two-Dimensional Hybrid Lead Iodide Perovskite.

Organic-inorganic hybrid perovskites (OIHPs) hold a great potential for scientific and technological endeavors in the field of ferroelectrics, solar cells, and electroluminescent devices, because of their structural diversity, low cost of manufacture, and ease of fabrication. However, lead iodide perovskite ferroelectrics with narrow band gap have rarely been reported. Here, we present a new two-dimensional (2D) layered lead iodide perovskite ferroelectric, (4,4-DFHHA)2PbI4 (4,4-DFHHA = 4,4-difluorohexahydroazepine), with a spontaneous polarization (Ps) of 1.1 µC/cm2 at room temperature, a direct bandgap of 2.32 eV, and a high Curie temperature Tc of 454 K (beyond that of BaTiO3, 393 K). On the basis of the nonferroelectrics (HHA)I, (4-FHHA)I, and (4,4-DFHHA)I (HHA = hexahydroazepine, 4-FHHA = 4-fluorohexahydroazepine), we assembled them with PbI2 to form lead iodide perovskites. Because the space between adjacent one-dimensional (1D) chains is relatively large and the confinement effect is not obvious, the cations are still in a disordered state, and 1D OIHPs (HHA)PbI3 and (4-FHHA)PbI3 are also nonferroelectrics at room temperature. In the confined environment of the 2D PbI42- framework for (4,4-DFHHA)2PbI4, the 4,4-DFHHA cations become ordered, and their asymmetric distribution leads to the spontaneous polarization. This work offers an efficient strategy for enriching the family of lead iodide perovskite ferroelectrics through the confinement effect and should inspire further exploration of the interplay between ferroelectricity and photovoltaics.

Bistable State of Protons for Low-Voltage Memories.

Molecular ferroelectrics are attracting tremendous interest because of their easy and environmental-friendly processing, low acoustic impedance, and mechanical flexibility. Their ferroelectric mechanism is mainly ascribed to the order-disorder transition of molecules such as spherical 1,4-diazabicyclo[2.2.2] octane (DABCO) and quinuclidine. Here, we present two molecular ferroelectrics, [HDABCO][TFSA] and its deuterated one [DDABCO][TFSA] (TFSA = bis(trifluoromethylsulfonyl)ammonium), whose ferroelectricity is triggered by the proton ordering. This is the first time that the protons show a thermally fluctuated bistability with a double-well potential in DABCO-based ferroelectrics. A large deuterium isotope effect (ΔT = ∼53 K) not only proves that they are hydrogen-bonded ferroelectrics but also extends the ferroelectric working temperature range to room temperature. The superfast polarization switching of 100 kHz and ultralow coercive voltage of 1 V (far less than 5 V required for commercially available ferroelectric devices), benefiting from the low energy for proton transfer, allow [DDABCO][TFSA] a great potential for memory devices with low-voltage, high-speed operation. This work should inspire further exploration of hydrogen-bonded molecular ferroelectrics for flexible and wearable devices with the low-power information storage.

Observation of Vortex Domains in a Two-Dimensional Lead Iodide Perovskite Ferroelectric.

Topological defects, such as vortices and skyrmions, provide a wealth of splendid possibilities for new nanoscale devices because of their marvelous electronic, magnetic, and mechanical behaviors. Recently, great advances have been made in the study of the ferroelectric vortex in conventional perovskite oxides, such as BaTiO3 and BiFeO3. Despite extensive interest, however, no intriguing ferroelectric vortex structures have yet been found in organic-inorganic hybrid perovskites (OIHPs), which are desirable for their mechanical flexibility, ease of fabrication, and low acoustical impedance. We observed the robust vortex-antivortex topological configurations in a two-dimensional (2D) layered OIHP ferroelectric (4,4-DFPD)2PbI4 (4,4-DFPD is 4,4-difluoropiperidinium). This provides future directions for the study of perovskites and makes it a promising alternative for nanoscale ferroelectric devices in medical, micromechanical, and biomechanical applications.

Two-Dimensional Layered Perovskite Ferroelectric with Giant Piezoelectric Voltage Coefficient.

Piezoelectric sensors that can work under various conditions with superior performance are highly desirable with the arrival of the Internet of Things. For practical applications, a large piezoelectric voltage coefficient g and a high Curie temperature Tc are critical to the performance of piezoelectric sensors. Here, we report a two-dimensional perovskite ferroelectric (4-aminotetrahydropyran)2PbBr4 [(ATHP)2PbBr4] with a saturated polarization of 5.6 µC cm-2, high Tc of 503 K [above that of BaTiO3 (BTO, 393 K)], and extremely large g33 of 660.3 × 10-3 V m N-1 [much beyond that of Pb(Zr,Ti)O3 (PZT) ceramics (20 to 40 × 10-3 V m N-1), more than 2 times higher than that of poly(vinylidene fluoride) (PVDF, about 286.7 × 10-3 V m N-1)]. Combined with the advantages of molecular ferroelectrics, such as light weight, easy and environmentally friendly processing, and mechanical flexibility, (ATHP)2PbBr4 would be a competitive candidate for next-generation smart piezoelectric sensors in flexible devices, soft robotics, and biomedical devices.

A Molecular Thermochromic Ferroelectric.

Molecular ferroelectrics have attracted considerable interests because of their easy and environmentally friendly processing, low acoustical impedance and mechanical flexibility. Herein, a molecular thermochromic ferroelectric, N,N'-dimethyl-1,4-diazoniabicyclo[2.2.2]octonium tetrachlorocuprate(II) ([DMe-DABCO]CuCl4 ) is reported, which shows both excellent ferroelectricity and intriguing thermochromism. [DMe-DABCO]CuCl4 undergoes a ferroelectric phase transition from Pca21 to Pbcm at a significantly high Curie temperature of 413 K, accompanied by a color change from yellow to red that is due to the remarkable deformation of [CuCl4 ]2- tetrahedron, where the ferroelectric and paraelectric phases correspond to yellow and red, respectively. Combined with multiple bistable physical properties, [DMe-DABCO]CuCl4 would be a promising candidate for next-generation smart devices, and should inspire further exploration of multifunctional molecular ferroelectrics.