Unusual Thermal Quenching of Photoluminescence from an Organic-Inorganic Hybrid [MnBr4 ]2- -based Halide Mediated by Crystalline-Crystalline Phase Transition.

The ability to generate and manipulate photoluminescence (PL) behavior has been of primary importance for applications in information security. Excavating novel optical effects to create more possibilities for information encoding has become a continuous challenge. Herein, we present an unprecedented PL temporary quenching that highly couples with thermodynamic phase transition in a hybrid crystal (DMML)2 MnBr4 (DMML=N,N-dimethylmorpholinium). Such unusual PL behavior originates from the anomalous variation of [MnBr4 ]2- tetrahedrons that leads to non-radiation recombination near the phase transition temperature of 340 K. Remarkably, the suitable detectable temperature, narrow response window, high sensitivity, and good cyclability of this PL temporary quenching will endow encryption applications with high concealment, operational flexibility, durability, and commercial popularization. Profited from these attributes, a fire-new optical encryption model is devised to demonstrate high confidential information security. This unprecedented optical effect would provide new insights and paradigms for the development of luminescent materials to enlighten future information encryption.

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.

Introducing Ferroelasticity into 1D Hybrid Lead Halide Semiconductor by Halogen Substitution Strategy.

Organic-inorganic hybrid lead halide perovskites (OLHPs), represented by (CH3 NH3 )PbI3 , are one of the research focus due to their exceptional performance in optoelectronic applications, and ferroelastic domain walls are benign to their charge carrier transport that is confirmed recently. Among them, the 1D OLHPs feature better stability against desorption and moisture, but related 1D ones possessing ferroelasticity are rarely investigated and reported so far. In this work, the 1D ferroelastic semiconductor (N-iodomethyl-N-methyl-morpholinium)PbI3 ((IDMML)PbI3 ) is prepared successfully by introducing successively halogenate atoms from Cl, Br to I into the organic cation of the prototype (N,N-dimethylmorpholinium)PbI3 ((DMML)PbI3 ). Notably, (IDMML)PbI3 shows the narrow bandgap energy (≈2.34 eV) according to the ultraviolet-visible absorption spectrum and the theoretical calculation, and possesses the evident photoconductive characteristic with the on/off ratio of current of ≈50 under the 405 nm light irradiation. This work provides a new case for the ferroelastic OLHPs and will inspire intriguing research in the field of optoelectronic.

Homochiral Chemistry Strategy to Trigger Second-Harmonic Generation and Dual Dielectric Switches.

Switchable materials have attracted enormous interest due to their promising applications in important fields such as sensing, electronic components, and information storage. Nevertheless, obtaining multifunctional switching materials is still a problem worth investigating. Herein, by incorporating (Rac-, L-, D-2-amino-1-propanol) as the templating cation, we have obtained (Rac-, L-, D-HTMPA)CdCl3 (HTMPA = 1-hydroxy-N, N, N-trimethyl-2-propanaminium). We have adopted a chiral chemistry strategy that causes (Rac-HTMPA)CdCl3 in the central symmetric space to crystallize in the chiral space group. Based on the modulation of the homochiral strategy, (L-, D-HTMPA)CdCl3 shows a dual phasic transition at 269 and 326 K and a switchable second-harmonic generation response. In addition, (L-, D-HTMPA)CdCl3 is chiral switchable material to exhibit stable dual dielectric and second-harmonic generation (SHG) switches. This work provides an approach to exploring multifunctional chiral switchable materials.

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 .

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.

In Situ Observation of Ferroelastic Domain and Phase Transition in a Three-Dimensional Molecular Crystal.

Massive efforts have been devoted to designing molecular ferroic materials by molecular modification. For molecular ferroelastic, previous work is focused on the temperature-dependent ferroelastic domains, however, few are related to controlling the ferroelastic domain by the stress. Inspired by the "quasi-spherical theory" and fluorination effect, we designed a more flexible (MedabcoF)2+ (MedabcoF=1-fluoro-4-methyl-1,4-diazoniabicyclo[2.2.2]octane) cation by introducing a methyl group and a fluorine atom at the two symmetrical ends of the Dabco (1,4-diazoniabicyclo[2.2.2]octane) and synthesized a hybrid 3D perovskite (MedabcoF)Rb(BF4 )3 (1) which displays three reversible phase transitions accompanying dual ferroelastic behavior. Besides, it not only exhibits ferroelastic domains switching by the thermal stimulation, and the sensitive reaction of in situ domains under the stress of it is also realized. This work not only achieves a force-controlled ferroelastic domain but develops a more profound comprehension of the relationship between the thermal motion behavior of guest cations and the intriguing properties of materials.

Organometallic-Based Hybrid Perovskite Piezoelectrics with a Narrow Band Gap.

Hybrid organic-inorganic perovskites (HOIPs) with the general formula ABX3 hold phenomenal research interest for their great scientific and technological potential in photovoltaic, piezoelectric, and electroluminescent devices. It is their considerable structural diversity that offers a good opportunity to build a variety of HOIP structures with various functionalities. However, no organometallic-based HOIP piezoelectrics have yet been found, despite the structural diversity and functional richness of organometallic compounds such as the ferrocene-based family. Here, for the first time, we report an organometallic-based HOIP piezoelectric, [(ferrocenylmethyl)trimethylammonium]PbI3. Benefitting from the stability of ferrocene-based cations, excellent piezoelectric performance, comparable to that of LiNbO3, can be obtained and optimized by tuning the anionic framework. The involvement of organometallic cations enables a narrow band gap of 2.37 eV, much lower than those of most HOIPs and some inorganic semiconductors. This work provides a new future direction for the study of perovskites and will inspire intriguing research on organometallic-based HOIP piezoelectrics.

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.

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.

Methylation Design Strategy to Trigger a Dual Dielectric Switch and Improve the Phase Transition Temperature.

Phase transitions of hybrid materials have aroused widespread concern and call for an in-depth study on its structure design, because the structure and characteristics are closely related, which promote potential applications in the field of temperature sensors, dielectric switches, and actuators. However, designing materials with multiple phase transitions and a high phase transition temperature (Tr) remains a huge challenge. In order to deal with this key hurdle, we tried to regulate the structural components and successfully synthesized [MASD]2[CdCl4] (1, MASD = 8-methyl-5-azoniaspiro[4,5]decane), which displays multiple phase transitions occurring at 273.8 K and 395.9 K separately. The Tr has significantly increased compared with the parent compounds reported previously. As the temperature sensitivity of compound 1 is constant at different frequencies, it can be applied for detectors or sensors under frequency-independent or wide frequency conditions. Moreover, methylation design strategy evidently triggered the dual dielectric switch and improved the Tr, which opens a new path for finding and adjusting ideal materials of multiple phase transition.

Unique Design Strategy for Dual Phase Transition That Successfully Validates Dual Switch Implementation in the Dielectric Material.

Dual phase transition/switch materials are a critical cornerstone of information storage and sensing. However, they are difficult to design successfully, and compared with materials showing single-switchable phase transitions, the dual ones retain many challenges by far. Therefore, the significance of a general strategy is far greater than an accidental success. Here, an efficient strategy combining branchlike Et3R and trunklike benzylamine analogues successfully validates dual-switch implementation in the dielectric materials. This inevitable success is based on our treelike analogue mentioned above in which amines with multiple branches can achieve a temperature-induced phase change. Exactly, (BCDA)2ZnBr4 [BCDA = benzyl-(2-chloroethyl)dimethylammonium] proves the regularity and undergoes two reversible phase transitions at 295.4 and 340.8 K, respectively. Variable-temperature single-crystal X-ray diffraction revealed that the generation of double phase transitions is caused by progressive changes of treelike BCDA+ as the temperature rises. Because the permittivity ε' of (BCDA)2ZnBr4 abruptly changed near the phase-transition temperatures, such physical properties make it have latent applicability. In short, the success of our strategy will inspire researches to discover more interesting dual phase transition/switch materials.

A Chiral Thermochromic Ferroelastic with Seven Physical Channel Switches.

Switchable materials play an invaluable role in signal processing and encryption of smart devices. The development of multifunctional materials that exhibit switching characteristics in multiple physical channels has attracted widespread attention. Now, two chiral thermochromic ferroelastic crystals (S-CTA)2 CuCl4 and (R-CTA)2 CuCl4 (CTA=3-chloro-2-hydroxypropyltrimethylammonium) have been prepared with switchable properties in dielectricity, conductivity, second harmonic generation (SHG), piezoelectricity, ferroelasticity, chiral, and thermochromic properties. Compared with traditional phase-transition materials with switching features, thermochromism brings additional spectral encryption possibilities for future information processing. To the best of our knowledge, this is the first chiral thermochromic ferroelastic that exhibits switching properties in seven physical channels. This work is expected to promote further exploration of multifunctional molecular switchable materials.

Higher-Temperature Dielectric Molecular Motor Induced by Unusual Chair-to-Rotator Motion.

With regard to the artificial molecular motor that was recognized with the 2016 Nobel Prize, this success proves the great scientific significance of rotary motor-type motion at the molecular level, which has been expected to play an invaluable role in the development of electronic information molecular materials. However, designing electronic information-critical high-temperature molecular motors has always been a huge challenge. Since we discovered [(CH3)3NCH2Cl]MnCl3, this cation rotation pattern with a motor-type motion structure has continued to attract our attention. Considering a strategy that combines molecular machines with dielectric theory, ( N, N-dimethylpiperidinium)CdCl3, the new dielectric molecular motor material that exhibits superior physical properties, could be considered to be an excellent dielectric switch based on its electric field and temperature. Crystal structure analyses reveal that the reversible phase transition is mainly induced by the unusual chair-to-rotator motion of cations. Because of the unprecedented leaping structural transition from P63/ mmc to P21/ c and the rotating motor-type motion structure, the material exhibits remarkable anisotropy and outstanding dielectric switching characteristics. These findings open a new avenue for the design and assembly of novel molecular motor materials in the field of electronic information.