Bromine-Substitution-Induced High-Tc Two-Dimensional Bilayered Perovskite Photoferroelectric

Realization of Tunable Ferroelectricity in Perovskite Crystals for Multidirectional Self-Driven Photodetection

Halide perovskite ferroelectrics endowed with a distinctive spontaneous polarization effect have been regarded as prospective electroactive materials and are prevalently utilized in solar cells, photoelectric detection, and other domains. Among them, multipolar-axis ferroelectrics featuring multiple equivalent polarization directions are particularly desirable for diverse areas of applications. Nevertheless, the design and regulation of multipolar axis perovskite ferroelectrics remains a significant challenge. Here, guided by the strategy of layer regulation, we successfully designed and regulated a series of 2D homologous perovskites OA2Csn -1PbnBr3 n +1 (OA = n-octylammonium, n = 1-3). Notably, OA2Csn -1PbnBr3 n +1 exhibits layer-dependent ferroelectricity: OA2PbBr4 exhibits non-ferroelectricity, OA2CsPb2Br7 displays uniaxial ferroelectricity, while OA2Cs2Pb3Br10 has multiaxial ferroelectricity. Moreover, the devices fabricated based on OA2Cs2Pb3Br10 achieve high-performance self-driven photodetection in multiple directions. This precise layer-regulation strategy offers an efficient approach to obtaining and regulating multipolar-axis perovskite ferroelectrics, presenting the potential for next-generation optoelectronic devices.

Tuning Interlayer Couplings and Stabilizing 2D Perovskite Lattices through Intercalation Chemistry

Two-dimensional (2D) organic-inorganic hybrid lead halide perovskites are promising semiconductors for optoelectronics, spintronics, and ferroelectrics due to their versatile structural and physical properties enabled by a variety of organic spacer cations. While previous research has focused on new spacer cations for templating 2D perovskite structures and influencing their properties, the intercalation of functional molecules within the organic layers has been less explored. Here, we demonstrate the intercalation of iodine within the organic sublattice as an effective tool to tune interlayer electronic interactions and stabilize 2D perovskite structures that would otherwise not form. We synthesized and determined the single-crystal structures of seven new iodine-intercalated 2D perovskites with varying spacer cations and inorganic compositions. The intercalated iodine bridges neighboring inorganic layers via halogen bonding with the apical iodides, leading to interlayer vibrational and electronic couplings. The iodine intercalation enhances the lattice rigidity, which decreases phonon-phonon scattering and exciton-phonon coupling. Adjusting the inorganic composition further tunes the electronic band structures, because iodine's frontier orbitals contribute differently to the band edge states, leading to varied band alignments and photoluminescence quenching behaviors. Moreover, a decreased anisotropic emission polarization is observed after iodine intercalation due to the decreased in-plane confinement of the excitons. Our results demonstrate iodine intercalation as a powerful tool for tuning the structural and optoelectronic properties of 2D perovskites.

Novel lead-free bismuth-based perovskite-like (BrC5H13N)3Bi2Br9: synthesis, structural investigations and optoelectronic properties

Lead-free organic-inorganic hybrid perovskites have attracted increasing attention owing to their advantages of reduced toxicity, photo-detectability, switchable dielectric device application, ferroelectric properties and distinctive optical characteristics. Despite their promising features, the chemical engineering of hybrid perovskites remains a challenge, as identifying the appropriate strategies is essential to achieve the desired properties such as controlled bandgap energy and phase transition behaviours. Numerous approaches have been explored to optimize these characteristics. In this study, we employed halogenation of the organic component as a targeted strategy to enhance the stability and performance of hybrid perovskite materials. This approach enabled the successful synthesis of a non-centrosymmetric halobismuthate (BrC5H13N)3Bi2Br9 compound (BrC5H13N+: (2-bromoethyl)trimethylammonium), which exhibited excellent optic and electric properties and crystallized in the non-polar P212121 space group. The inorganic framework was precisely arranged with [Bi2Br9]3- polyhedra that were face-shared and separated by organic cations, resulting in an A3B2X9-type structure. Additionally, the compound (BrC5H13N)3Bi2Br9 possessed an indirect band gap of 2.58 eV, which suggests this material's semiconductor character. Photoluminescence (PL) studies revealed that the compound exhibited a broad band emission at about 730 nm. The electrical properties as a function of frequencies and temperatures showed the contribution of the grain and grain boundary to conduction, and AC conductivity confirmed the semiconductor behaviour. The activation energy suggested the combination of ionic and electronic conduction. These findings enrich the understanding on the behaviour of A3B2X9-type low-dimensional hybrids and holds promise in extending the application of lead-free hybrids to the field of ferroelectric, electric and optic materials.

2D Multilayered Perovskite Ferroelectric with Halogen Bond Induced Interlayer Locking Structure toward Efficient Self-Powered X-Ray Detection

2D Ruddlesden-Popper (RP) hybrid perovskite ferroelectrics have emerged as a promising class of direct X-ray detection materials. However, their intrinsic van der Waals gaps result in weak interlayer interactions that destabilize the layered motifs impacting the stability of the X-ray detector. Thus, it is crucial but remains toughly challenge to enhance interlayer interactions exploring stable RP perovskite ferroelectric X-ray detectors. Here, halogen bond is proposed to enhance the interlayer interactions of RP perovskite ferroelectrics obtaining a 2D trilayered ferroelectric, (BrPA)2(EA)2Pb3Br10 (BEPB, BrPA = 3-bromopropylaminium; EA = ethylammonium). Strikingly, the strong Br···Br halogen bonds lock cations to the inorganic skeletons, and C─H···Br hydrogen bonds bridge adjacent spacing sheets, which effectively improves structural stability and suppresses ion migration. The typical P-E hysteresis loops reveal its concrete ferroelectric behaviors, giving a large polarization of ≈7.3 µC cm-2. Consequently, the BEPB-based X-ray detector results in a high sensitivity of 562.6 µC Gy-1 cm-2 at 0 V bias, and most importantly, it exhibits low baseline drift and exceptional environmental stability. As far as is known, halogen bond strengthening 2D multilayered ferroelectric to achieve stable and efficient X-ray detection is unprecedented, which sheds light on the future design of stable optoelectronic devices toward practical applications.

Pressure-Driven Circularly Polarized Luminescence Enhancement and Chirality Amplification

Achieving ultrahigh-color-purity circularly polarized luminescence (CPL) in low-dimensional chiral perovskites is challenging due to strong electron-phonon coupling caused by lead halide octahedral distortion. Herein, the circularly polarized piezoluminescence behaviors of six novel chiral perovskites, (S/R-3-XPEA)2PbBr4 (PEA = phenethylamine; X = F, Cl, Br), were systematically investigated. Upon compression, (S/R-3-ClPEA)2PbBr4 exhibits significant piezofluorochromic behaviors, transforming from yellow CPL to ultrahigh-color-purity deep-blue CPL. At 2.5 GPa, the deep-blue CPL intensity increases by an order of magnitude and its luminescence asymmetry factors (glum) are amplified from the initial ±0.03 to ±0.1. (S/R-3-BrPEA)2PbBr4 presents a similar piezochromic response, realizing deep-blue CPL at 1.7 GPa, while (S/R-3-FPEA)2PbBr4 retains a yellow CPL under high pressure. High-pressure structural characterization and theoretical calculations confirm that pressure-enhanced halogen bonds reduce the penetration depth of S/R-3-BrPEA+ and S/R-3-ClPEA+ into the [PbBr6]4- frameworks, significantly suppressing electron-phonon coupling and increasing magnetic transition dipole moment in (S/R-3-BrPEA)2PbBr4 and (S/R-3-ClPEA)2PbBr4, which are responsible for the ultrahigh-purity deep-blue CPL and chirality amplification, respectively.

Effect of Halide Tuning on the Structural, Dielectric, and Optical Properties of Two-Dimensional 2-Chloroethylammonium Lead Halides

Layered hybrid organic-inorganic lead halides have gained a lot of attention for optoelectronic applications. A notable subset within this category is perovskites comprising halogenated amines since they may exhibit reduced band gap or polar order. We synthesized three compounds comprising 2-chloroethylammonium (CEA+) cations, with the chemical formula CEA2PbX4 (X = Cl, Br, I). X-ray diffraction studies show that at room temperature (RT), CEA2PbBr4 and CEA2PbI4 crystallize in Pbnm symmetry, with ordered CEA+ cations. CEA2PbBr4 and CEA2PbI4 undergo one structural phase transition (PT) into a disordered Pmnm phase near 315 and 360 K, respectively. CEA2PbCl4 shows a different packing of CEA+ with the organic chains oriented perpendicularly to the perovskite layers. It undergoes two PTs at 332 and 203 K from the high-temperature (HT) disordered I4/mmm phase to the partially ordered intermediate Pbnm phase and completely ordered low-temperature (LT) phase of the unknown space group. All compounds emit photoluminescence (PL): orange, yellow-green, and yellow for CEA2PbI4, CEA2PbCl4, and CEA2PbBr4, respectively, and bromide exhibits a very high quantum efficiency of 48%. Overall, our findings show that halide engineering strongly modulates hydrogen and halogen bonding strength, affecting the structural arrangement of building units, molecular dynamics, and thus optoelectronic properties.

Self-Powered Filterless Narrowband UV Photodetection Triggered by Asymmetric Charge Carrier Generation in a Wide-Bandgap Halide Perovskite Ferroelectric

Narrowband photodetection with selective light detection in ultraviolet (UV) range is particularly pronounced in specialized applications such as targeted wavelength imaging and UV-phototherapy. In contrast to conventional strategies, ferroelectric materials with pronounced bulk photovoltaic effect (BPVE) provide a novel asymmetric carrier generation concept for achieving filterless spectrally selective photodetection. Herein, for the first time, the realization of self-powered filterless narrowband UV photodetection is demonstrated in bulk single crystals of a newly developed halide perovskite ferroelectric, 2FEA2EA2Pb2Cl10 (2FEEPC), which exhibits a wide bandgap of 3.36 eV and a remarkable spontaneous polarization (≈3.5 µC cm-2). Benefiting from the strongly distorted inorganic framework with asymmetric delocalized electronic states, 2FEEPC demonstrates a highly wavelength-dependent BPVE along the polar direction, showing a high narrowband detecting performance with a response peak at UV wavelength of 355 nm and a full-width-at-half-maximum (FWHM) of ≈15 nm. More interestingly, the ferro-pyro-phototronic effect of 2FEEPC endows a colossal gain of up to 600% for photopyroelectric current. This work presents an unprecedented approach to achieve self-powered filterless narrowband photodetection, thereby shedding light on the development of photosensitive ferroelectrics for spectrally photoelectric applications.

Halogen Substitution Strategy for Exploiting High-Performance Molecular Ferroelectrics

Molecular ferroelectrics are emerging as a robust family of electric-ordered materials due to their distinct structural flexibility, molecular tunability, and versatility. In recent years, diverse chemical design approaches have significantly contributed to discovering and optimizing ferroelectric performances of molecule-based ferroelectric systems. Notably, halogen substitution is one of the most effective strategies for inducing symmetry breaking and optimizing the dipole moments and potential energy barriers. In this minireview, we have summarized recent significant advances of halogen substitution strategy in molecule-based ferroelectrics, including organic-inorganic hybrids and metal-free molecular systems. Subsequently, we discuss the underlying mechanism of halogen substitution to improve ferroelectric performances, including the generation of spontaneous polarization, enhancement of Curie temperature, and bandgap engineering. Finally, the future directions in designing and modulating molecular ferroelectrics by halogen substitution strategy are also highlighted.

Exploring Aqueous Solution-Processed Pseudohalide Rare-Earth Double Perovskite Ferroelectrics toward X-Ray Detection with High Sensitivity

Three-dimensional (3D) pseudohalide rare-earth double perovskites (PREDPs) have garnered significant attention for their versatile physical properties, including ferroelectricity, ferroelasticity, large piezoelectric responses, and circularly polarized luminescence. However, their potential for X-ray detection remains unexplored, and the low Curie temperature (TC) limits the performance window for PREDP ferroelectrics. Here, by applying the chemical regulation strategies involving halogen substitution on the organic cation and Rb/Cs substitution to the PREDP [(R)-M3HQ]2RbEu(NO3)6 [(R)-M3HQ=(R)-N-methyl-3-hydroxylquinuclidinium] with a low TC of 285 K, a novel 3D PREDP ferroelectric [(R)-CM3HQ]2CsEu(NO3)6 [(R)-CM3HQ=(R)-N-chloromethyl-3-hydroxylquinuclidinium] are successfully synthesized, for which the TC reaches 344 K. More importantly, such a strategy endowed [(R)-CM3HQ]2CsEu(NO3)6 with notable X-ray detection capabilities. Centimeter-sized [(R)-CM3HQ]2CsEu(NO3)6 single crystals fabricated from aqueous solutions demonstrated a sensitivity of 1307 μC Gyair -1 cm-2 and a low detectable dose rate of 152 nGyair s-1, the highest sensitivity reported for hybrid double perovskite ferroelectric detectors. This work positions PREDPs as promising candidates for the next generation of eco-friendly optoelectronic materials and also offers substantial insights into the interaction between structure, composition, and functionality in ferroelectric materials.

Unusual Triple-State Switching of Thermally Induced Birefringence in a Two-Dimensional Perovskite Ferroelectric

Birefringent crystals hold a significant position in optical and optoelectronic fields due to their capability to control polarized light. Despite various chemical strategies devoted to designing birefringent crystals, it remains a challenge to switch and manipulate birefringence under physical stimuli. Here we present an unusual triple-state switching of birefringence in a 2D perovskite ferroelectric, (N-methylcyclohexylammonium)2PbCl4 (1), which exhibits two reversible phase transitions at 361 and 373 K. The in-plane birefringence of 1 (Δnbc) shows three distinctive states inside the bc plane, namely, the low-, high-, and zero-Δnbc states. Strikingly, a huge augmentation of Δnbc is solidly confirmed up to ∼400% between its low and high states, far beyond other birefringent materials. The origin of this triple-state switching of birefringence involves the variation of the ferroelastic strain and domain in the vicinity of the phase transition. As an entirely new mode of switching birefringence, this work facilitates the further development of new intelligent nonlinear optics.

Direction Dependent Ferroelectricity and Conductivity in a Single Crystal 2D Halide Double Perovskite

Halide ferroelectric materials have garnered a lot of interest because of their distinctive electrical and structural characteristics. In this study, the design and development of a new non-centrosymmetric 2D layered halide double perovskite material, Cl1.14Br2.86PA4AgInBr8 (CPAIn) is reported. This material shows ferroelectric properties above room temperature, with a Curie temperature of 190 °C. This behavior is achieved through the substitution of the halogenated A-site organic linker, 3-chloropropylammonium. CPAIn exhibits anisotropic ferroelectric behavior with higher spontaneous polarization of 6.25 µC cm-2 along the perpendicular direction to the octahedral layers, whereas the value decreases to 0.174 µC cm-2 between sheets. While using bottom contact to study the nature of polarity within a sheet, the P-E loop displays capacitive loop. The nature and value of polarization is highly direction dependent, and to further understand the mechanism of conduction, a combination of temperature-dependent impedance studies and poling dependent conductivity techniques are employed. These directional dependent properties hold immense potential in memory devices, sensors and photovoltaics, piezoelectric devices and energy storage.

Exploiting the Cationic Size Effect to Improve the Curie Temperature of Hybrid Perovskites Photoferroelectric Semiconductors

Ferroelectric materials with Curie temperature (Tc) below room temperature severely limit their practical applications. Although research on hybrid perovskite photoferroelectrics is ongoing, effective regulation of Tc still poses significant challenges. Herein, we utilized the cationic size effect to successfully regulate the Tc of hybrid perovskite photoferroelectric semiconductors. As the perovskitizer was replaced by a smaller-sized MA+ (methylammonium) with a larger-sized EA+ (ethylammonium), not only was the ferroelectricity of the hybrid perovskite well maintained but the Tc of (PA)2(MA)2Pb3Br10 (315 K) to (PA)2(EA)2Pb3Br10 (385 K) (PA is n-propylaminium) increased by 70 K, which was mainly due to the significant increase in the energy barriers that the system needed to overcome during the phase transition. Subsequently, we achieved efficient self-powered X-ray detection through the ferroelectric-induced bulk photovoltaic effect (BPVE) in (PA)2(EA)2Pb3Br10. The devices based on (PA)2(EA)2Pb3Br10 single crystals exhibit an outstanding sensitivity of 95 μC Gy-1 cm-2 and a low detection limit of 239 nGy s-1 at 0 V bias under X-ray radiation. This study provides an effective approach for designing and constructing high-temperature multilayer photoferroelectric semiconductors in the future.

Directed Assembly of Ordered Mixed-Spacer Quasi-2D Halide Perovskites through Homomeric Chains of Intermolecular Bonds

Two-dimensional (2D) halide perovskites (HPs) are of significant interest to researchers because of their natural structural frameworks and intriguing optoelectronic properties. However, the direct fabrication of ordered mixed-spacer quasi-2D HPs remains challenging. Herein, a synthetic strategy inspired by the principle of supramolecular synthons is employed for the self-assembly of a series of ordered mixed-spacer bilayered HPs. The key innovation involves the introduction of intermolecular hydrogen bonds using a bifunctional 3-aminopropionitrile cation. Three homogeneous n = 2 structures are obtained, with a subtly ordered perovskite connected by two distinct types of organic cation layers, resulting in a recurrent ABAB' stacking sequence. These three compounds exhibit attractive semiconducting properties. Moderate bandgaps in the range of 2.70 to 2.76 eV with an absorption wavelength range of 448-459 nm exhibit excellent photoelectric response. Moreover, the ordered structures facilitate excellent polarization-sensitive photodetection, with an impressive on/off ratio of 103. The response speed ranged from 298 to 381 µs, and the out-of-plane polarization-related dichroism ratio is determined to be 1.19. Such ordered mixed-spacer bilayered perovskites have not been reported. These results enrich the HPs system and play a significant role in the direct assembly of novel perovskites with ordered structures.

Highly Stable MOF-Type Lead Halide Luminescent Ferroelectrics

Lead halide molecular ferroelectrics represent an important class of luminescent ferroelectrics, distinguished by their high chemical and structural tunability, excellent processability and distinctive luminescent characteristics. However, their inherent instability, prone to decomposition upon exposure to moisture and light, hinders their broader ferroelectric applications. Herein, for the first time, we present a series of isoreticular metal-organic framework (MOF)-type lead halide luminescent ferroelectrics, demonstrating exceptional robustness under ambient conditions for at least 15 months and even when subjected to aqueous boiling conditions. Unlike conventional metal-oxo secondary building units (SBUs) in MOFs adopting highly centrosymmetric structure with limited structural distortion, our lead halide-based MOFs occupy structurally deformable [Pb2X]+ (X=Cl-/Br-/I-) SBUs that facilitate a c-axis-biased displacement of Pb2+ centers and substantially contribute to thermoinducible structural transformation. Importantly, this class of MOF-type lead halide ferroelectrics undergo ferroelectric-to-paraelectric phase transitions with remarkably high Curie temperature of up to 505 K, superior to most of molecular ferroelectrics. Moreover, the covalent bonding between phosphorescent organic component and the light-harvesting inorganic component achieves efficient spin-orbit coupling and intersystem crossing, resulting in long-lived afterglow emission. The compelling combination of high stability, ferroelectricity and afterglow emission exhibited by lead halide MOFs opens up many potential opportunities in energy-conversion applications.

A Lead-Free Ferroelectric 2D Dion-Jacobson Tin Iodide Perovskite

2D hybrid organic-inorganic halide perovskites emerge as a new class of 2D semiconductors with the potential to combine excellent optoelectronic properties with symmetry-enabled properties such as ferroelectricity. Although many lead-based ferroelectric 2D halide perovskites are reported, there is yet to be a conclusive report of ferroelectricity in tin-based 2D perovskites. Here, the structures and properties of a new series of 2D Dion-Jacobson (DJ) Sn perovskites: (4AMP)SnI4, (4AMP)(MA)Sn2I7, and (4AMP)(FA)Sn2I7 (4AMP = 4-(aminomethyl)piperidinium, MA = methylammonium, and FA = formamidinium), are reported. Structural characterization reveals that (4AMP)SnI4 is polar with in-plane spontaneous polarization whereas (4AMP)(MA)Sn2I7 and (4AMP)(FA)Sn2I7 are centrosymmetric. Further, (4AMP)SnI4 displays second harmonic generation (SHG) and polarization-electric field hysteresis measurements confirm it is ferroelectric with a spontaneous polarization of 10.0 µC cm-2 at room temperature. (4AMP)SnI4 transitions into a centrosymmetric structure above 367 K. As the first direct experimental observation of the spontaneous ferroelectric polarization of a Sn-based 2D hybrid perovskite, this work opens up environmentally friendly 2D tin halide perovskites for ferroelectricity and other physical property studies.

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.

Exciton engineering of 2D Ruddlesden-Popper perovskites by synergistically tuning the intra and interlayer structures

Designing two-dimensional halide perovskites for high-performance optoelectronic applications requires deep understanding of the structure-property relationship that governs their excitonic behaviors. However, a design framework that considers both intra and interlayer structures modified by the A-site and spacer cations, respectively, has not been developed. Here, we use pressure to synergistically tune the intra and interlayer structures and uncover the structural modulations that result in improved optoelectronic performance. Under applied pressure, (BA)2(GA)Pb2I7 exhibits a 72-fold boost of photoluminescence and 10-fold increase of photoconductivity. Based on the observed structural change, we introduce a structural descriptor χ that describes both the intra and interlayer characteristics and establish a general quantitative relationship between χ and photoluminescence quantum yield: smaller χ correlates with minimized trapped excitons and more efficient emission from free excitons. Building on this principle, we design a perovskite (CMA)2(FA)Pb2I7 that exhibits a small χ and an impressive photoluminescence quantum yield of 59.3%.

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

Room-Temperature Anisotropic Actuation Driven by a Synergistic Order-Disorder and Displacive Phase Transition in a Ferroelectric Crystal

Actuating materials convert different forms of energy into mechanical responses. To satisfy various application scenarios, they are desired to have rich categories, novel functionalities, clear structure-property relationships, fast responses, and, in particular, giant and reversible shape changes. Herein, we report a phase transition-driven ferroelectric crystal, (rac-3-HOPD)PbI3 (3-HOPD = 3-hydroxypiperidine cation), showing intriguingly large and anisotropic room-temperature actuating behaviors. The crystal consists of rigid one-dimensional [PbI3] anionic chains running along the a-axis and discrete disk-like cations loosely wrapping around the chains, leaving room for anisotropic shape changes in both the b- and c-axes. The shape change is switched by a ferroelectric phase transition occurring at around room temperature (294 K), driven by the exceptionally synergistic order-disorder and displacive phase transition. The rotation of the cations exerts internal pressure on the stacking structure to trigger an exceptionally large displacement of the inorganic chains, corresponding to a crystal lattice transformation with length changes of +24.6% and -17.5% along the b- and c-axis, respectively. Single crystal-based prototype devices of circuit switches and elevators have been fabricated by exploiting the unconventional negative temperature-dependent actuating behaviors. This work provides a new model for the development of multifunctional mechanically responsive materials.

A High Working Temperature Multiferroic Induced by Inverse Temperature Symmetry Breaking

Molecular-based multiferroic materials that possess ferroelectric and ferroelastic orders simultaneously have attracted tremendous attention for their potential applications in multiple-state memory devices, molecular switches, and information storage systems. However, it is still a great challenge to effectively construct novel molecular-based multiferroic materials with multifunctionalities. Generally, the structure of these materials possess high symmetry at high temperatures, while processing an obvious order-disorder or displacement-type ferroelastic or ferroelectric phase transition triggered by symmetry breaking during the cooling processes. Therefore, these materials can only function below the Curie temperature (Tc), the low of which is a severe impediment to their practical application. Despite great efforts to elevate Tc, designing single-phase crystalline materials that exhibit multiferroic orders above room temperature remains a challenge. Here, an inverse temperature symmetry-breaking phenomenon was achieved in [FPM][Fe3(μ3-O)(μ-O2CH)8] (FPM stands for 3-(3-formylamino-propyl)-3,4,5,6-tetrahydropyrimidin-1-ium, which acts as the counterions and the rotor component in the network), enabling a ferroelastoelectric phase at a temperature higher than Tc (365 K). Upon heating from room temperature, two-step distinct symmetry breaking with the mm2Fm species leads to the coexistence of ferroelasticity and ferroelectricity in the temperature interval of 365-426 K. In the first step, the FPM cations undergo a conformational flip-induced inverse temperature symmetry breaking; in the second step, a typical ordered-disordered motion-induced symmetry breaking phase transition can be observed, and the abnormal inverse temperature symmetry breaking is unprecedented. Except for the multistep ferroelectric and ferroelastic switching, this complex also exhibits fascinating nonlinear optical switching properties. These discoveries not only signify an important step in designing novel molecular-based multiferroic materials with high working temperatures, but also inspire their multifunctional applications such as multistep switches.

Building Block-Inspired Hybrid Perovskite Derivatives for Ferroelectric Channel Layers with Gate-Tunable Memory Behavior

Ferroelectric photovoltaics driven by spontaneous polarization (Ps ) holds a promise for creating the next-generation optoelectronics, spintronics and non-volatile memories. However, photoactive ferroelectrics are quite scarce in single homogeneous phase, owing to the severe Ps fatigue caused by leakage current of photoexcited carriers. Here, through combining inorganic and organic components as building blocks, we constructed a series of ferroelectric semiconductors of 2D hybrid perovskites, (HA)2 (MA)n-1 Pbn Br3n+1 (n=1-5; HA=hexylamine and MA=methylamine). It is intriguing that their Curie temperatures are greatly enhanced by reducing the thickness of inorganic frameworks from MAPbBr3 (n=∞, Tc =239 K) to n=2 (Tc =310 K, ΔT=71 K). Especially, on account of the coupling of room-temperature ferroelectricity (Ps ≈1.5 μC/cm2 ) and photoconductivity, n=3 crystal wafer was integrated as channel field effect transistor that shows excellent a large short-circuit photocurrent ≈19.74 μA/cm2 . Such giant photocurrents can be modulated through manipulating gate voltage in a wide range (±60 V), exhibiting gate-tunable memory behaviors of three current states ("-1/0/1" states). We believe that this work sheds light on further exploration of ferroelectric materials toward new non-volatile memory devices.

Centimeter-Size Single Crystals of Halide Perovskite Photoferroelectric Solid Solution with Ultrahigh Pyroelectricity Boosted Photodetection

Photoferroelectrics, especially emerging halide perovskite ferroelectrics, have motivated tremendous interests owing to their fascinating bulk photovoltaic effect (BPVE) and cross-coupled functionalities. However, solid solutions of halide perovskite photoferroelectrics with controllable structure and enhanced performance are scarcely explored. Herein, through mixing cage cation, a homogeneous halide perovskite photoferroelectric PA2 FAx MA1-x Pb2 Br7 solid solution (PA, FA and MA are CH3 CH2 CH2 NH3 + , NH2 CHNH2 + and CH3 NH3 + , 0≤x≤1) has been developed, which demonstrates tunable Curie temperature in a wide range of 263-323 K and excellent optoelectrical features. As the component adjusted to x=0.7, the bulk crystal demonstrates ultrahigh pyroelectric coefficient up to 1.48 μC cm-2  K-1 around room temperature. Strikingly, benefiting from the light-induced pyroelectricity and remarkable BPVE, a self-powered and sensitive photodetector based solid solution crystals with boosted responsivity and detectivity over than 1300 % has been achieved. This pioneering work sheds light on the exploration of photoferroelectric solid solutions towards high-performance photoelectronic devices.

Large Phase-Transition Temperature Enhancement Achieved in a Layered Lead Iodide Hybrid Crystal by H/F Substitution

Organic-inorganic hybrid metal halides with structural flexibility and solution processability have been widely investigated for different application scenarios. However, the effective construction of phase-transition materials with a high phase-transition temperature (Ttr) for potential practical applications remains a great challenge, and reports on the regulation of Ttr with significant enhancement have been rare. In this manuscript, we have realized a large Ttr increase of 148 K in a layered hybrid lead iodide crystal (4-FTMBA)4Pb3I10 (4-FTMBA = 4-fluoro-N,N,N-trimethylbenzenaminium) by the H/F substitution strategy. Compared to the parent (TMBA)4Pb3I10 (TMBA = N,N,N-trimethylbenzenaminium), H/F substitution preserves the structural framework and crystal symmetry in (4-FTMBA)4Pb3I10. The introduction of heavier fluorine will significantly increase the motion barrier for the order-disorder transition, resulting in the remarkably improved Ttr. Temperature-dependent crystal structures, Raman spectra, and dielectric analyses well support the phase-transition behavior. In addition, evident thermochromism with a tunable direct band gap in (4-FTMBA)4Pb3I10 has been observed using UV-vis spectra. To the best of our knowledge, the achieved Ttr enhancement of 148 K by H/F substitution is the highest among the organic-inorganic hybrid lead halide phase-transition materials. This finding would greatly inspire the rational design of functional materials with high performance.

Elucidating Structure-Property Correlation in Perovskitoid and Antiperovskite Piperidinium Manganese Chloride

In the world of semiconductors, organic-inorganic hybrid (OIH) halide perovskite is a new paradigm. Recently, a zealous effort has been made to design new lead-free perovskite-like OIH halides, such as perovskitoids and antiperovskites, for optoelectronic applications. In this context, we have synthesized a perovskitoid compound (Piperidinium)MnCl₃ (compound 1) crystallizing in an orthorhombic structure with infinite one-dimensional (1D) chains of MnCl₆ octahedra. Interestingly, this compound shows switchable dielectric property governed by an order-disorder structural transition. By controlling the stoichiometry of piperidine, we have synthesized an antiperovskite (Piperidinium)₃Cl[MnCl₄] (compound 2), the inverse analogue of a perovskite, consisting of zero-dimensional (0D) MnCl₄ tetrahedra. This type of organic-inorganic hybrid antiperovskite halide is unique and scarce. Such a dissimilarity in lattice dimensionality and Mn²⁺ ion coordination ensues fascinating photophysical and magnetic properties. Compound 1 exhibits red emission with a photoluminescence quantum yield (PLQY) of ∼28%. On the other hand, the 0D antiperovskite compound 2 displays green emission with a higher PLQY of 54.5%, thanks to the confinement effect. In addition, the dimensionality of the compounds plays a vital role in the exchange interaction. As a result, compound 1 shows an antiferromagnetic ground state, whereas compound 2 is paramagnetic down to 1.8 K. This emerging structure-property relationship in OIH manganese halides will set the platform for designing new perovskites and antiperovskites.

Halogen engineering of organic-inorganic hybrid perovskites displaying nonlinear optical, fluorescence properties and phase transition

In order to meet the needs of social development, increasing research attention has been paid to multifunctional molecular-based phase-transition materials. The traditional phase-transition materials with a single functional property can be transformed into magnificent ones by adding additional functional properties-for instance photoluminescence and magnetic order- because having two or more functional properties simultaneously greatly broadens the fields of their applications. At present, there are very few multifunctional phase-transition materials showing excellent performance, and the crystal structure design and performance optimization of materials still need to be studied in depth. Herein, we report the development of two organic-inorganic hybrid materials: (MBA)₂ZnI₄ (1, MBA = 4-methoxybenzylammonium) with switchable dielectricity and a high phase-transition temperature (T_c = 359.55 K), and (MBA)₂ZnBr₄ (2) with green luminescence (λ_exc = 314 nm) and nonlinear optical properties (0.75× KDP). A two-dimensional (2D) fingerprint analysis of the Hirshfeld surface plots revealed a significant difference between the hydrogen-bonding interaction before the phase transition and that afterwards. The two compounds were further verified, from energy band structure calculations, to be direct-band-gap semiconductors. In conclusion, this work has provided a viable strategy, involving the application of chemical modifications, for designing various functional materials.

Integrated Reversible Thermochromism, High Tc , Dielectric Switch and Narrow Band Gap in One Multifunctional Ferroic

Organic-inorganic Hybrid (OIH) materials for multifunctional switchable applications have attracted enormous attention in recent years due to their excellent optoelectronic properties and good structural tunability. However, it still remains challenging to fabricate one simple OIH compound with multi-functionals properties, such as dielectric switching, thermochromic properties, semiconductor characteristics and ferroelasticity. Under this context, we successfully synthesized [2-(2-fluorophenyl)ethan-1- ammonium]₂ SnBr₆ (compound 1), which has a higher phase transition temperature of 427.7 K. Additionally, it exhibits a semiconducting property with an indirect band gap of 2.36 eV. Combining ferroelastic, narrow band gap, thermochromic, and dielectric properties, compound 1 can be considered as a rarely reported multi-functional ferroelastic material, which is expected to give inspiration for broadening the applications in the smart devices field.

Robust Spin-Dependent Anisotropy of Circularly Polarized Light Detection from Achiral Layered Hybrid Perovskite Ferroelectric Crystals

Circularly polarized light (CPL) detection has sparked overwhelming research interest for its widespread chiroptoelectronic and spintronic applications. Ferroelectric materials, especially emerging layered hybrid perovskite ferroelectrics, exhibiting striking bulk photovoltaic effect (BPVE) present significant possibilities for CPL detection by a distinctive working concept. Herein, for the first time, we demonstrate the realization of robust angular anisotropy of CPL detection in a new layered hybrid perovskite ferroelectric crystal (CPA)₂FAPb₂Br₇ (1, CPA is chloropropylammonium, FA is formamidinium), which crystallized in an optically active achiral polar point group. Benefiting from the notable spontaneous polarization (5.1 μC/cm²) and excellent semiconducting characteristics, single crystals of 1 exhibit remarkable BPVE under light illumination, with a high current on/off switching ratio (ca. 10³). More intriguingly, driven by the angular carrier drift originating from spin-dependent BPVE in optically active ferroelectrics, 1 displays highly sensitive self-powered CPL detection performance, showing a robust angular anisotropy factor up to 0.98, which is far more than those achieved by material intrinsic chirality. This work provides an unprecedented approach for realizing highly sensitive CPL detection, which sheds light on the further design of optically active ferroelectrics for chiral photonic applications.

Unprecedented Self-Powered Visible-Infrared Dual-Modal Photodetection Induced by a Bulk Photovoltaic Effect in a Polar Perovskite

Visible-infrared dual-modal light harvesting is crucial for various optoelectronic devices, particularly for solar cells and photodetectors. Hybrid metal-halide perovskites are recently emerging for visible-infrared dual-modal photodetection owing to their prominent multiphoton absorption and carrier transport performances. However, they work relying on an applied external power source or complicated heterostructures. It is still a difficult task to realize visible-infrared dual-modal self-powered photoresponse induced by a bulk photovoltaic effect (BPVE) in a single material. In this work, we constructed a polar multilayered perovskite, (Br-BA)₂(EA)₂Pb₃Br₁₀ (BEP; EA⁺ = ethylammonium, and Br-BA⁺ = 4-brombutylammonium). Notably, the polar feature endows BEP with a BPVE. In addition, BEP presents a distinctive two-photon activity arising from the layered quantum-well structure. Benefitting from these striking characteristics, self-powered visible-infrared dual-modal photodetection is realized, and a direct self-powered detection of 800 nm light with a photocurrent of 2.1 nA cm^-2 is achieved. This work will inspire the design of desired photoelectric materials with a BPVE for high-performance self-powered visible-infrared dual-modal photodetection.

2D lead-free organic–inorganic hybrid exhibiting dielectric and structural phase transition at higher temperatures

A novel switchable molecular dielectric material [3-3-difluorocyclobutylammonium]2CdCl4 was synthesized. It shows a reversible phase transition at 353.95 K and rapid switching and reversibility between high and low dielectric states for several cycles.

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).

Tuning Dielectric Transitions in Two-Dimensional Organic-Inorganic Hybrid Lead Halide Perovskites

Organic-inorganic hybrid metal halide perovskites possessing unique two-dimensional (2D)-layered structures have been demonstrated with excellent molecular tunability and stability, especially the promising semiconductor properties for solar cell applications. In this work, three 2D lead halide organic-inorganic hybrid perovskites (IAA)₂PbX₄ (IAA = isoamylammonium cation and X = Cl, Br, and I) were synthesized by employing a solution processing method and demonstrate distinct tuning solid-state phase transitions coupled with dielectric responses, as well as light absorption properties. Among the title perovskites, the phase transition temperature decreases gradually, and their band gap also indicates a narrowing trend. The results are mainly derived from slight changes in the crystal structure by halogen regulation. These findings might provide an effective crystal engineering strategy for exploring high-performance functional perovskite 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.