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 Qm 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.

Discovery of Ferroelectricity in the Fullerene Adduct C60S8.

Fullerenes offer versatile functionalities and are promising materials for a widespread range of applications from biomedicine and energy to electronics. Great efforts have been made to manipulate the symmetries of fullerene and its derivatives for studying material properties and novel effects, such as ferroelectricity with polar symmetry; however, no documentary report has been obtained to realize their ferroelectricity. Here, for the first time, we demonstrated clear ferroelectricity in a fullerene adduct formed by C60 and S8. More is different: the combination of the most symmetric molecule C60 with the highest Ih symmetry and molecule S8 with high D4d symmetry resulted in the polar C60S8 adduct with a low crystallographic symmetry of the C2v (mm2) point group at room temperature. The presented C60S8 undergoes polar-to-polar ferroelectric phase transition with the mm2Fm notation, whose ferroelectricity was confirmed by a ferroelectric hysteresis loop and ferroelectric domain switching. This finding opens up a new functionality for fullerenes and sheds light on the exploration of more ferroelectric fullerenes.

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.

A Homochiral Fulgide Organic Ferroelectric Crystal with Photoinduced Molecular Orbital Breaking.

Thermally triggered spatial symmetry breaking in traditional ferroelectrics has been extensively studied for manipulation of the ferroelectricity. However, photoinduced molecular orbital breaking, which is promising for optical control of ferroelectric polarization, has been rarely explored. Herein, for the first time, we synthesized a homochiral fulgide organic ferroelectric crystal (E)-(R)-3-methyl-3-cyclohexylidene-4-(diphenylmethylene)dihydro-2,5-furandione (1), which exhibits both ferroelectricity and photoisomerization. Significantly, 1 shows a photoinduced reversible change in its molecular orbitals from the 3 π molecular orbitals in the open-ring isomer to 2 π and 1 σ molecular orbitals in the closed-ring isomer, which enables reversible ferroelectric domain switching by optical manipulation. To our knowledge, this is the first report revealing the manipulation of ferroelectric polarization in homochiral ferroelectric crystal by photoinduced breaking of molecular orbitals. This finding sheds light on the exploration of molecular orbital breaking in ferroelectrics for optical manipulation of ferroelectricity.

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.

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.

Homochiral Multiferroic Cyanido-Bridged Dimetallic Complexes Assembled by C-F⋠⋠⋠K Interactions.

Cyanido-bridged dimetallic complexes are attracting attention due to their varied structures and properties. However, homochiral cyanido-bridged dimetallic complexes are rare, and making them ferroelectric is a great challenge. Introducing C-F⋅⋅⋅K interactions between the guest chiral cations and the host [KFe(CN)6 ]2- framework, gives three-dimensional cyanido-bridged dimetallic multiferroics, [R- and S-3-fluoropyrrolidinium]2 [KFe(CN)6 ] (R- and S-3-FPC). The mirror-symmetric vibrational circular dichroism (VCD) signal shows their enantiomeric nature. R- and S-3-FPC crystallize in the same chiral-polar space group P21 at 298 K. Piezoresponse force microscopy (PFM), polarizing optical microscopy, and temperature-dependent second-harmonic generation (SHG) measurements show their multiferroic properties (the coexistence of ferroelectricity and ferroelasticity), in line with the Aizu notation of 222F2. R-3-FPC shows excellent ferroelectricity with saturated polarization up to 9.4 µC cm-2 .

Unprecedented Ferroelectricity and Ferromagnetism in a Cr2+ -Based Two-Dimensional Hybrid Perovskite.

Organic-inorganic hybrid perovskites (OIHPs) have gained tremendous interest for their rich functional properties. However, the coexistence of more than one of ferroelectricity, ferromagnetism and ferroelasticity has been rarely found in OIHPs. Herein, we report a two-dimensional Cr2+ -based OIHP, [3,3-difluorocyclobutylammonium]2 CrCl4 ([DFCBA]2 CrCl4 ), which shows both ferroelectricity and ferromagnetism. It undergoes a 4/mmmFm type ferroelectric phase transition at a temperature as high as 387 K and shows multiaxial ferroelectricity with a saturate polarization of 2.1 µC cm-2 . It acts as a soft ferromagnet with a Curie temperature of 32.6 K. This work throws light on the exploration of OIHPs with the coexistence of ferroelectricity and ferromagnetism for applications in future multifunctional smart devices.

The First Chiro-Inositol Organosilicon Ferroelectric Crystal.

Organosilicons have been used extensively in aerospace, electronics, food, medicine and other fields, due to their low viscosity, hydrophobicity, corrosion resistance, non-toxic, and physiologically inert features. Despite extensive interest, however, organosilicon ferroelectric crystals have never been found. Here, by using the chemical design strategy, we successfully obtained a molecular ferroelectric D-chiro-inositol-SiMe3 with polar P43 symmetry, whose spontaneous polarization can be electrically switchable on thin film. The introduction of organosilicon groups endows the thin films with excellent softness, ductility and flexibility (extremely low hardness of 72.8 MPa and small elastic modulus of 5.04 GPa) that are desirable for biomedical and human-compatible applications. As the first case of organosilicon ferroelectric crystal to date, this work offers a new structural paradigm for molecular ferroelectrics, and highlights their potential for flexible bio-electronic applications.

Large Electrostrictive Coefficient in a Two-Dimensional Hybrid Perovskite Ferroelectric.

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are attracting tremendous interest for their great scientific and technological potential in photovoltaics and optoelectronics. Although the ferroelectricity in 2D HOIPs has been greatly developed, however, to date no phosphonium-based 2D HOIP ferroelectrics have yet been found. Meanwhile, electrostriction plays an important role in the electromechanical behavior of ferroelectrics, while it has never been reported for 2D HOIP ferroelectrics. Here, we present the first phosphonium-based 2D HOIP ferroelectric (EATMP)PbBr4 (EATMP = (2-aminoethyl)trimethylphosphanium) with a direct bandgap of 2.84 eV. Notably, (EATMP)PbBr4 possesses a high Curie temperature of 534 K, which is the highest among all reported 2D HOIP ferroelectrics. Moreover, it exhibits a large electrostrictive coefficient of about 3.96 m4 C-2 as well, far exceeding those of PVDF (1.3 m4 C-2) and inorganic ones (∼0.034-0.096 m4 C-2). With excellent ferroelectric and piezoelectric properties and the merit of easy fabrication, (EATMP)PbBr4 shows great potential in applications for future smart devices of actuators, transducers, and sensors.

Record Enhancement of Curie Temperature in Host-Guest Inclusion Ferroelectrics.

Solid-state molecular rotor-type materials such as host-guest inclusion compounds are very desirable for the construction of molecular ferroelectrics. However, they usually have a low Curie temperature (Tc) and uniaxial nature, severely hindering their practical applications. Herein, by regulating the anion to control "momentum matching" in the crystal structure, we successfully designed a high-temperature multiaxial host-guest inclusion ferroelectric [(MeO-C6H4-NH3)(18-crown-6)][TFSA] (MeO-C6H4-NH3 = 4-methoxyanilinium, TFSA = bis(trifluoromethanesulfonyl)ammonium) with the Aizu notation of mmmFm. Compared to the parent uniaxial ferroelectric [(MeO-C6H4-NH3)(18-crown-6)][BF4] with a Tc of 127 K, the introduction of larger TFSA anions brings a lower crystal symmetry at room temperature and a higher energy barrier of molecular motions in phase transition, giving [(MeO-C6H4-NH3)(18-crown-6)][TFSA] multiaxial ferroelectricity and a high Tc up to 415 K (above that of BaTiO3). To our knowledge, such a record temperature enhancement of 288 K makes its Tc the highest among the reported crown-ether-based ferroelectrics, giving a wide working temperature range for applications in data storage, temperature sensing, actuation, and so on. This work will provide guidance and inspiration for designing high-Tc host-guest inclusion ferroelectrics.

Two-Dimensional Hybrid Perovskite Ferroelectric Induced by Perfluorinated Substitution.

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs), which possess the merits of good material stability, structural diversity, and ease of fabrication, are highly desirable for widespread applications of ferroelectrics, solar cells, and electroluminescent devices. Although some molecular design strategies toward ferroelectrics have been proposed, however, it is still a great challenge to precisely induce and optimize the ferroelectricity in 2D HOIPs. Here, for the first time through perfluorinated substitution strategy, we successfully design a high-performance 2D HOIP ferroelectric, (perfluorobenzylammonium)2PbBr4, exhibiting more obvious second harmonic generation intensity, larger piezoelectric response, more polar axes, larger spontaneous polarization of 4.2 µC cm-2, and higher Curie temperature of 440 K than those of parent (benzylammonium)2PbBr4. Compared to the selective effect of monofluorinated substitution on different positions of the benzene ring, where (3-fluorobenzylammonium)2PbBr4 and (4-fluorobenzylammonium)2PbBr4 are not ferroelectrics, the pioneering perfluorinated substitution is more universal and effective for targeted design of aromatic ferroelectrics. This work offers an efficient strategy for precisely designing high-performance 2D HOIP ferroelectrics.

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.

A Three-Dimensional Lead Halide Perovskite-Related Ferroelectric.

Three-dimensional (3D) organic-inorganic lead halides represented by [CH3NH3]PbI3 perovskite have attracted great interest for their diverse functional properties and promising optoelectronic applications. However, 3D lead halides are still very rare and their ferroelectricity remains controversial. Here, we report an unprecedented 3D lead halide perovskite-related ferroelectric [2-trimethylammonioethylammonium]Pb2Cl6 ([TMAEA]Pb2Cl6), which contains a 3D lead chloride framework of corner- and edge-sharing PbCl6 octahedral, with the [TMAEA]+ cations occupying the voids of the framework. [TMAEA]Pb2Cl6 shows a ferroelectric-to-paraelectric phase transition with the Curie temperature as high as 412 K, a typical ferroelectric hysteresis loop at 293 K with a spontaneous polarization of 1 µC/cm2, and a clear ferroelectric domain switching. To the best of our knowledge, [TMAEA]Pb2Cl6 is the first 3D lead halide showing such an excellent ferroelectricity. Additionally, it also exhibits a semiconducting property with a direct band gap of 3.43 eV. This finding enriches the family of 3D hybrid lead halides and inspires the exploration of 3D lead halide ferroelectrics.

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.

Two-Dimensional Organic-Inorganic Perovskite Ferroelectric Semiconductors with Fluorinated Aromatic Spacers.

Two-dimensional (2D) organic-inorganic perovskites (OIPs), with improved material stability over their 3D counterparts, are highly desirable for device applications. It is their considerable structural diversity that offers an unprecedented opportunity to engineer materials with fine-tuning functionalities. The isosteric substitution of hydrogen by an electronegative fluorine atom has been proposed as a useful route to improve the photovoltaic performance of 2D OIPs, whereas its valuable role in developing ferroelectricity is still waiting for further exploration. Herein, for the first time we applied fluorinated aromatic cations in extending the family of 2D OIP ferroelectrics, and successfully obtained [2-fluorobenzylammonium]2PbCl4 as a high-performance ferroelectric semiconductor. The failures in the nonferroelectric [4-fluorobenzylammonium]2PbCl4 and [3-fluorobenzylammonium]2PbCl4 demonstrate that the selective introduction of fluorine in correct structural positions is particularly essential. This work represents an unprecedented proof-of-concept in the use of fluorinated aromatic cations for the targeted design of excellent 2D OIP ferroelectrics, and is believed to inspire the future development of low-cost, high-efficiency, and stable device applications.

A Nickel(II) Nitrite Based Molecular Perovskite Ferroelectric.

The X-site ion in organic-inorganic hybrid ABX3 perovskites (OHPs) varies from halide ion to bridging linkers like HCOO- , N3 - , NO2 - , and CN- . However, no nitrite-based OHP ferroelectrics have been reported so far. Now, based on non-ferroelectric [(CH3 )4 N][Ni(NO2 )3 ], through the combined methodologies of quasi-spherical shape, hydrogen bonding functionality, and H/F substitution, we have successfully synthesized an OHP ferroelectric, [FMeTP][Ni(NO2 )3 ] (FMeTP=N-fluoromethyl tropine). As an unprecedented nitrite-based OHP ferroelectric, the well-designed [FMeTP][Ni(NO2 )3 ] undergoes the ferroelectric phase transition at 400 K with an Aizu notation of 6/mmmFm, showing multiaxial ferroelectric characteristics. This work is a great step towards not only enriching the molecular ferroelectric families but also accelerating the potential practical applications.