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Zero-Dimensional Plastic Phase Transition Iron-Based Compounds with High T c and Switchable SHG Responses
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Anisotropy is crucial for birefringence (Δn) in optical materials, but optimizing it remains a formidable challenge (Δn >0.3). Supramolecular frameworks incorporating π‐conjugated components are promising for achieving enhanced birefringence because of their structural diversity and inherent anisotropy. Herein, we first synthesized (C6H6NO2)⁺Cl⁻ (NAC) and then constructed a halogen‐bonded supramolecular framework I⁺(C6H4NO2)⁻ (INA) by halogen aliovalent substitution of Cl⁻ with I⁺. The organic moieties are protonated and deprotonated nicotinic acid (NA), respectively. The antiparallel arrangement of birefringent‐active units in NAC and INA leads to significant differences in the bonding characteristics between the interlayer and intralayer domains. Moreover, the [O⋅⋅⋅I⁺⋅⋅⋅N] halogen bond in 1D [I⁺(C6H4NO2)⁻] chain exhibits stronger interactions and stricter directionality, resulting in a more pronounced in‐plane anisotropy between the intrachain and interchain directions. Consequently, INA exhibits exceptional birefringent performance, with a value of 0.778 at 550 nm, twice that of NAC (0.363 at 550 nm). This value significantly exceeds those of commercial birefringent crystals, such as CaCO3 (0.172 at 546 nm), and is the highest reported value among ultraviolet birefringent crystals. This work presents a novel design strategy that employs halogen bonds as connection sites and modes for birefringent‐active units, opening new avenues for developing high‐performance birefringent crystals.
REBa2Cu3O7‐x (REBCO) exhibits outstanding qualities such as a high irreversible field (7 T) and strong current‐carrying capacity (77 K, 10⁶ A cm⁻²), making it ideal for high magnetic field applications such as fusion, accelerators, and nuclear magnetic resonance. However, the degradation of in‐field critical current density (Jc) due to vortex motion is a significant hurdle. Research indicates that improving vortex pinning can notably enhance Jc in REBCO, showing promise for its future high‐field applications. This review explores vortex pinning mechanisms, artificial pinning centers (APCs), and how environmental conditions affect pinning characteristics, highlighting advancements in vortex pinning for REBCO in magnetic fields. Different methods for introducing APCs are discussed and compared for evaluating their efficacy in achieving optimal nanoparticle distributions and consequent superconducting performance improvements, underscoring the importance of nano‐defect features (type, size, and distribution) in optimizing the superconducting properties of the REBCO. Additionally, the review tackles current challenges, shares recent research breakthroughs, and outlines future research directions for designing pinning landscapes to further improve REBCO performance.
Bismuth telluride‐based devices are capable of converting low‐quality thermal energy into electrical power via the Seebeck effect. This transformative process not only extends the spectrum of energy utilization but also significantly amplifies energy efficiency. This review serves as a comprehensive guide, elucidating the intricate design considerations essential for optimizing bismuth telluride‐based devices in both electrical and structural design. By exploring various application scenarios, it identifies critical parameters crucial for device effectiveness. Furthermore, the current landscape of thermoelectric (TE) devices is meticulously analyzed, synthesizing their developmental trajectory and contrasting it with stringent design requirements. Through this comprehensive analysis, it pinpoints key challenges that impede the maximal performance of existing TE devices. Envisioning the trajectory of bismuth telluride‐based TE materials, this review makes projections regarding their future application trends. Traversing through contemporary mechanisms and technologies, it offers practical solutions and potential avenues aimed at enhancing the efficiency of TE devices. Ultimately, this discourse endeavors to provide invaluable insights, furnishing a roadmap for the advancement and refinement of TE devices in the years ahead. By proposing feasible solutions and charting plausible directions, it aspires to stimulate innovation and drive transformative progress in the domain of TE materials and science.
Interfacial layers (ILs) are prerequisites to form the selective charge transport for high‐performance organic photovoltaics (OPVs) but mostly result in considerable parasitic absorption loss. Trimming the ILs down to a mono‐molecular level via the self‐assembled monolayer is an effective strategy to mitigate parasitic absorption loss. However, such a strategy suffers from inferior electrical contact with low surface coverage on rough surfaces and poor producibility. To address these issues, here, the self‐assembled interlayer (SAI) strategy is developed, which involves a thin layer of 2–6 nm to form a full coverage on the substrate via both covalent and van der Waals bonds by using a self‐assembled molecule of 2‐(9H‐carbazol‐9‐yl) (2PACz). Via the facile spin coating without further rinsing and annealing process, it not only optimizes the electrical and optical properties of OPVs, which enables a world‐record efficiency of 20.17% (19.79% certified) but also simplifies the tedious processing procedure. Moreover, the SAI strategy is especially useful in improving the absorbing selectivity for semi‐transparent OPVs, which enables a record light utilization efficiency of 5.34%. This work provides an effective strategy of SAI to optimize the optical and electrical properties of OPVs for high‐performance and solar window applications.
Organic-inorganic metal halides (OIMHs) possessing switchable optical and electrical properties holds significant promise for applications in multifunctional sensors, switching devices, and information storage. However, the challenge lies in achieving effective...
Organic martensitic compounds are an emerging type of smart material with intriguing physical properties including thermosalient effect, ferroelasticity, and shape memory effect. However, due to the high structural symmetry and limited design theories for these materials, the combination of ferroelectricity and martensitic transformation has rarely been found in organic systems. Here, based on the chemical design strategies for molecular ferroelectrics, we show a series of asymmetric 1,4,5,8-naphthalenediimide derivatives with the homochiral amine and 2,2,6,6-tetramethylpiperidine-N-oxyl components, which adopt the low-symmetric polar structure and so allow ferroelectricity. Upon H/F substitution, the fluorinated compounds exhibit reversible ferroelectric and martensitic transitions at 399 K accompanied by a large thermal hysteresis of 132 K. This large thermal hysteresis with two competing (meta)-stable phases is further confirmed by density functional theory calculations. The rare combination of martensitic phase transition and ferroelectricity realizes the bistability with two different ferroelectric phases at room temperature. Our finding provides insight into the exploration of martensitic ferroelectric compounds with potential applications in switchable memory devices, soft robotics, and smart actuators.
In this study, we identify a novel class of second-order nonlinear optical (NLO) crystals, non-π-conjugated piperazine (H10C4N2, PIP) metal halides, represented by two centimeter-sized, noncentrosymmetric organic–inorganic metal halides (OIMHs), namely H12C4N2CdI4 (P212121) and H11C4N2CdI3 (Cc). H12C4N2CdI4 is the first to be prepared, and its structure contains a CdI4 tetrahedron, which led to a poor NLO performance, including a weak and non-phase-matchable second harmonic generation (SHG) response of 0.5 × KH2PO4 (KDP), a small birefringence of 0.047 @1064 nm and a narrow bandgap of 3.86 eV. Moreover, H12C4N2CdI4 is regarded as the model compound, and we further obtain H11C4N2CdI3via the replacement of CdI4 with a highly polarizable CdNI3 tetrahedron, which results in a sharp enhancement of SHG response and birefringence. H11C4N2CdI3 exhibits a promising NLO performance including 6 × KDP, 4.10 eV, Δn = 0.074 @1064 nm and phase matchability, indicating that it is the first OIMH to simultaneously exhibit strong SHG response (>5 × KDP) and a wide bandgap (>4.0 eV). Our work presents a novel direction for designing high-performance NLO crystals based on organic–inorganic halides and provides important insights into the role of the hybridized tetrahedron in enhancing the SHG response and birefringence.
The bulk anomalous photovoltaic (BAPV) effect of acentric materials refers to a distinct concept from traditional semiconductor‐based devices, of which the above‐bandgap photovoltage hints at a promise for solar‐energy conversion. However, it is still a challenge to exploit new BAPV‐active systems due to the lacking of knowledge on the structural origin of this concept. BAPV effects in single crystals of a 2D lead‐free double perovskite, (BBA)2CsAgBiBr7 (1, BBA = 4‐bromobenzylammonium), tailored by mixing aromatic and alkali cations in the confined architecture to form electric polarization are acquired here. Strikingly, BAPV effects manifested by above‐bandgap photovoltage (VOC) show unique attributes of directional anisotropy and positive dependence on electrode spacing. The driving source stems from orientations of the polar aromatic spacer and Cs⁺ ion drift, being different from the known built‐in asymmetry photovoltaic heterojunctions. As the first demonstration of the BAPV effect in the double perovskites, the results will enrich the family of environmentally green BAPV‐active candidates and further facilitate their new optoelectronic application.
Molecular antiferroelectrics (AFEs) have taken a booming position in the miniaturization of energy storage devices due to their low critical electric fields. However, regarding intrinsic competitions between dipolar interaction and steric hindrance, it is a challenge to exploit room-temperature molecular AFEs with high energy storage efficiency. Here, we present a new 2D hybrid perovskite-type AFE, (i-BA)2(FA)Pb2Br7 (1), which shows ultrahigh energy storage efficiencies at room temperature. Most strikingly, the typical double P-E hysteresis loops afford an ultrahigh storage efficiency up to ∼91% at low critical electric fields (E cr = 41 kV cm-1); this E cr value is much lower than those of state-of-the-art AFE oxides, revealing the potential of 1 for miniaturized energy-storage devices. In terms of the energy storage mechanism, the dynamic ordering and antiparallel reorientation of organic cations trigger its AFE-type phase transition at 303 K, thus giving a large spontaneous electric polarization of ∼3.7 μC cm-2, while the increasement of steric hindrance of the organic branched-chain i-BA+ spacer cations stabilizes its antipolar sublattices. To the best of our knowledge, this exploration of achieving ultrahigh energy storage efficiency at such a low critical electric field is unprecedented in the AFE family, which paves a pathway for miniaturized energy storage applications.
Two-dimensional (2D) lead halide perovskites are a family of materials at the heart of solar cell, light-emitting diode, and photodetector technologies. This perspective leads to a number of synthetic efforts toward materials of this class, including those with prescribed polar architectures. The methylhydrazinium (MHy+) cation was recently presumed to have an unusual capacity to generate non-centrosymmetric perovskite phases, despite its intrinsically nonchiral structure. Here, we witness this effect once again in the case of the Ruddlesden–Popper perovskite phase of formula MHy2PbCl4. MHy2PbCl4 features three temperature-dependent crystal phases, with two first-order phase transitions at T1 = 338.2 K (331.8 K) and T2 = 224.0 K (205.2 K) observed in the heating (cooling) modes, respectively. Observed transitions involve a transformation from high-temperature orthorhombic phase I, with the centrosymmetric space group Pmmn, through the room-temperature modulated phase II, with the average structure being isostructural to I, to the low-temperature monoclinic phase III, with non-centrosymmetric space group P21. The intermediate phase II is a rare example of a modulated structure in 2D perovskites, with Pmmn(00γ)s00 superspace symmetry and modulation vector q ≅ 0.25c*. MHy2PbCl4 beats the previous record of MHy2PbBr4 in terms of the shortest inorganic interlayer distance in 2D perovskites (8.79 Å at 350 K vs 8.66 Å at 295 K, respectively). The characteristics of phase transitions are explored with differential scanning calorimetry, dielectric, and Raman spectroscopies. The non-centrosymmetry of phase III is confirmed with second harmonic generation (SHG) measurements, and polarity is demonstrated by the pyroelectric effect. MHy2PbCl4 also exhibits thermochromism, with the photoluminescence (PL) color changing from purplish-blue at 80 K to bluish-green at 230 K. The demonstration of polar characteristics for one more member of the methylhydrazinium perovskites settles a debate about whether this approach can present value for the crystal engineering of acentric solids similar to that which was recently adopted by a so-called fluorine substitution effect.
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]2CrCl4 ([DFCBA]2CrCl4), 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.
Ferroelectric (FE) and antiferroelectric (AFE) materials are used for several memory-related and energy-related applications. Perovskite materials (e.g., bulk ceramics) remain the most common materials for many applications. However, due to large deposition thickness, these materials are not appropriate for future miniaturized devices. In 2011, FE and AFE properties were reported in Si-doped HfO2 thin films. HfO2-based FE and AFE materials have several advantages over conventional materials, such as ultrathin deposition thickness (in range of nanometers), compatibility with existing Si semiconductor technology, and suitability for the integration within 3-D nanostructures. Therefore, fluorite-structured materials can be appropriate for miniaturized devices. These fluorite-structured materials are extensively studied for memory and energy-related applications. The first review on this topic was published after four years of discovering the FE and AFE properties in these materials. From the past decade, a lot of research has been reported about the detailed mechanism and application of these materials. This review insightfully discusses the progress in the research of fluorite-structured materials and critically discusses some potential applications. The authors will also discuss some challenges, extract new knowledge, and suggest promising future research directions of these materials.
Molecular ferroelectrics have been widely adopted as alternatives to traditional ceramic and polymer ferroelectrics due to their outstanding piezoelectric and ferroelectric properties, as well as their unique advantages of flexibility and low acoustic impedance, among others. However, rationally designing and optimizing molecular ferroelectrics with high performance remain a significant challenge. This review gives a brief introduction on the very recent approaches for the targeted design of molecular ferroelectrics through H/F substitution. H/F substitution comes into being the guidelines for designing and optimizing molecular ferroelectrics as well as the blueprint for the discovery of new functionalities for practical applications. Finally, a perspective view of future research on the H/F-substituted molecular ferroelectrics is provided.
Chiral hybrid organic–inorganic perovskites (HOIPs) have been well developed for circularly polarized light (CPL) detection, while new members that target at solar–blind ultraviolet (UV) region remain completely unexplored. Here, an effective design strategy to demonstrate circular polarization‐sensitive solar–blind UV photodetection by growing wide‐bandgap chiral HOIP [(R)‐MPA]2PbCl4 ((R)‐MPA = methylphenethylammonium) single crystals onto silicon wafers, with well‐defined heterostructures, is reported. The solid mechanical and electrical connection between the chiral HOIP and silicon wafer results in strong built‐in electric field at heterojunction, providing a desirable driving force for separating/transporting carriers generated under CPL excitation at 266 nm. Unexpectedly, during such a transport process, not only the chirality of HOIP crystal is transferred to the heterostructure, but also the circular polarization sensitivity is significantly amplified. Consequently, anisotropy factor of the resultant detectors can reach up to 0.4 at zero bias, which is much higher than that of the pristine single‐phase chiral HOIP (≈0.1), reaching the highest among the reported CPL–UV photodetectors. As far as we know, the integration of chiral HOIP crystals with silicon technology is unprecedent, which paves a way for designing boosted‐performance CPL detectors in solar–blind UV region as well as for other advanced optoelectronic devices.
Atomically thin two‐dimensional (2D) bismuth oxychalcogenides (Bi2O2X, X = S, Se, Te) have recently attracted extensive attention in the material research community due to their unique structure, outstanding long‐term ambient stability, and high carrier mobility, which enable them as promising candidates for high‐performance electronic and optoelectronic applications. Herein, we present a comprehensive review on the recent advances of 2D bismuth oxychalcogenides research. We start with an introduction of their fundamental properties including crystal structure and electronic band structure. Next, we summarize the common techniques for synthesizing these 2D structures with high crystallinity and large lateral size. Furthermore, we elaborate on their device applications including transistors, artificial synapses, optical switch and photodetectors. The last but not the least, we summarize the existing challenges and prospects for this emerging 2D bismuth oxychalcogenides field.
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CrystalExplorer is a native cross-platform program supported on Windows, MacOS and Linux with the primary function of visualization and investigation of molecular crystal structures, especially through the decorated Hirshfeld surface and its corresponding two-dimensional fingerprint, and through the visualization of void spaces in the crystal via isosurfaces of the promolecule electron density. Over the past decade, significant changes and enhancements have been incorporated into the program, such as the capacity to accurately and quickly calculate and visualize quantitative intermolecular interactions and, perhaps most importantly, the ability to interface with the Gaussian and NWChem programs to calculate quantum-mechanical properties of molecules. The current version, CrystalExplorer21, incorporates these and other changes, and the software can be downloaded and used free of charge for academic research.
Hybrid bismuth-containing halides are emerging as alternative candidates to lead-containing perovskites for light-harvesting applications, as Bi3+ is isoelectronic with Pb2+ and the presence of an active lone pair of electrons is expected to result in outstanding charge-carrier transport properties. Here, we report a family of one binary and three ternary iodobismuthates containing 1,4-diazabicyclo[2.2.2]octane (DABCO). These materials have been prepared solvothermally and their crystal structures, thermal stability, and optical properties determined. Reactions carried out in the presence of bismuth iodide and DABCO produced (C6H12N2)BiI3 (1), which consists of hybrid ribbons in which pairs of edge-sharing bismuth octahedra are linked by DABCO ligands. Short I···I contacts give rise to a three-dimensional network. Similar reactions in the presence of copper iodide produced (C8H17N2)2Bi2Cu2I10(2) and [(C6H13N2)2BiCu2I7](C2H5OH) (3) in which either ethylated DABCO cations (EtDABCO)+ or monoprotonated DABCO cations (DABCOH)+ are coordinated to copper in discrete tetranuclear and trinuclear clusters, respectively. In the presence of potassium iodide, a unique three-dimensional framework, (C6H14N2)[(C6H12N2)KBiI6] (4), was formed, which contains one-dimensional hexagonal channels approximately 6 Å in diameter. The optical band gaps of these materials, which are semiconductors, range between 1.82 and 2.27 eV, with the lowest values found for the copper-containing discrete clusters. Preliminary results on the preparation of thin films are presented.
Plastic crystals are a class of compounds composed of small molecules with globular structures. Plastic crystals exhibit various unique features, most of which arise from the rotator motion and orientational disorder of the constituent molecules. We recently discovered that some plastic crystals exhibit ferroelectricity in lower-temperature phases, including the room-temperature phase. The crystals, i.e., plastic/ferroelectric molecular crystals, have demonstrated several unique features that differ from conventional molecular ferroelectrics. In particular, the materials have achieved ferroelectric performance for the first time as bulk polycrystals of small-molecule crystals. These features are attributed to the multiaxial ferroelectricity and facile preparation of monolithic bulk polycrystals, which are impossible to achieve for conventional molecular crystals. Numerous related molecular ferroelectrics have been developed, rendering plastic/ferroelectric crystals an emerging class of molecular ferroelectrics. In this perspective, we have outlined the development and unique features of plastic/ferroelectric molecular crystals, focusing particularly on their ferroelectricity and related properties in bulk polycrystalline forms.
P–n photovoltaic junctions are essential building blocks for optoelectronic devices for energy conversion. However, this photovoltaic efficiency has almost reached its theoretical limit. Here, a brand‐new excitonic photovoltaic effect in 2D CsPbBr3/CdS heterostructures is revealed. These heterostructures, synthesized by epitaxial growth, display a clean interface and a strong interlayer coupling. The excitonic photovoltaic effect is a function of both the built‐in equilibrium electrical potential energy and the chemical potential energy, which is generated by the significant concentration gradient of electrons and holes at the heterojunction interface. Excitingly, this novel photovoltaic effect results in a large open‐circuit voltage of 0.76 V and a high power conversion efficiency of 17.5%. In addition, high photodetection performance, including a high photoswitch ratio (Ilight/Idark) of 105 and a fast response rate of 23 µs are obtained. These findings provide a new platform for photovoltaic applications. 2D CsPbBr3/CdS flakes are achieved via epitaxial growth. The as‐synthesized heterostructures display high‐quality crystallization and strong interlayer coupling at the interface. Meanwhile, the device exhibits excellent excitonic photovoltaic effects, including large open‐circuit voltage (≈0.76 V), high power conversion efficiency (≈17.5%). In addition, a fast response time of 23 µs achieved under zero bias for photodetection.
Lead-based organic-inorganic hybrid perovskite materials are widely applied in solar cells due to their excellent optoelectronic properties. These materials usually appear ferroelectricity with the built-in electric field, which can be used to improve the power conversion efficiency (PCE) of solar cells. However, the toxicity of lead has severely hampered their applications. Hence, it is of utmost significance to explore novel organic-inorganic hybrid perovskite materials with non-toxicity, ferroelectricity, and narrow bandgap at room temperature to enhance the PCE for broad practical applications. Herein, we reported a lead-free hybrid molecular ferroelectric material, i.e. (C5N2H9)3Bi2I9, with zero-dimensional perovskite-like structure. (C5N2H9)3Bi2I9 undergoes first-order ferroelectric phase transition with a Curie temperature of 327 K. Moreover, the dielectric constants of (C5N2H9)3Bi2I9 exhibit a step-like anomaly varied between the high-temperature paraelectric phase and the low-temperature ferroelectric phase. Furthermore, (C5N2H9)3Bi2I9 demonstrates a narrow bandgap of 2.1 eV. Hence, the as-reported novel member of the organic-inorganic hybrid family renders great promise for optoelectronic devices.
In the quest for organic-inorganic metal halides (OIMHs) that harmoniously combine large second harmonic generation (SHG) efficiency with broad bandgaps, our study introduces a series of noncentrosymmetric and polar piperazine...
Transient implantable piezoelectric materials are desirable for biosensing, drug delivery, tissue regeneration, and antimicrobial and tumor therapy. For use in the human body, they must show flexibility, biocompatibility, and biodegradability. These requirements are challenging for conventional inorganic piezoelectric oxides and piezoelectric polymers. We discovered high piezoelectricity in a molecular crystal HOCH 2 (CF 2 ) 3 CH 2 OH [2,2,3,3,4,4-hexafluoropentane-1,5-diol (HFPD)] with a large piezoelectric coefficient d 33 of ~138 picocoulombs per newton and piezoelectric voltage constant g 33 of ~2450 × 10 ⁻³ volt-meters per newton under no poling conditions, which also exhibits good biocompatibility toward biological cells and desirable biodegradation and biosafety in physiological environments. HFPD can be composite with polyvinyl alcohol to form flexible piezoelectric films with a d 33 of 34.3 picocoulombs per newton. Our material demonstrates the ability for molecular crystals to have attractive piezoelectric properties and should be of interest for applications in transient implantable electromechanical devices.
Since the switchable spontaneous polarization of ferroelectric materials endows it with many useful properties such as a large pyroelectric coefficient, switchable spontaneous polarization, and semiconductor, it has a wide range of application prospects, and the research of high-performance molecular ferroelectric materials has become a hot spot. We obtained a 0D organic-inorganic hybrid ferroelectric [(CH3)3NCH2CH2CH3]2FeCl4 (1) with well-defined ferroelectric domains and excellent domain inversion and exhibited a relatively large spontaneous polarization (Ps = 9 μC/m-2) and a Curie temperature (Tc) of 394 K. Furthermore, compound 1 belongs to the non-centrosymmetrical space group Cmc21 and has a strong second-harmonic generation signal. Interestingly, we also performed magnetic tests on 1, which confirmed that it is a magnetic material. This work provides clues for exploring the application of high-performance molecular ferroelectric materials in future multifunctional smart devices.
Compared with uniaxial molecular ferroelectrics, multiaxial ferroelectrics have better application prospects because they are no longer subject to the single-crystal form and have been pursued in recent years. Halogen engineering refers to the adjustment of halogens in materials at the atomic level, which can not only explore multiaxial ferroelectrics but also help to improve piezoelectrics, recently. In this work, we successfully synthesized and characterized three multiaxial plastic ferroelectrics through the precise molecular design from I to Cl, confirming the increase of the number of polar axes of ferroelectrics from 3 to 6, the increase of second-harmonic generation density from 2.1 times to nearly 6 times of monopotassium phosphate, and the increase of piezoelectric coefficient by 140%. This systematic work has proved that halogen engineering can not only enrich the family of multiaxial plastic ferroelectrics but also promote the further development of nonlinear optical and piezoelectric materials.
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.
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.
Plastic crystals, as a molecular material with multiple functions, have become a research hotspot in the exploration of new ferroelectric crystal compounds, especially due to their unique solid-solid phase transition properties. Based on this, we synthesized a new 0D organic-inorganic hybrid Fe-based plastic ferroelectric [(CH3)3NCH2CHCH2]FeCl4 (1), which has a high-temperature phase transition point of 393 K, obvious ferroelectric domains, and spontaneous polarization and has been tested by dielectric and piezoelectric power microscopy (PFM) and ultraviolet absorption (UV-vis). At room temperature, it crystallizes in the space group Cmc21 and has an obvious SHG switch. In addition, compound 1 also has an optical narrow band gap of 2.45 eV, indicating that compound 1 is a high-quality semiconductor material. This work advances the development of plastic ferroelectrics and provides an avenue for exploring the frequency-doubling response and optoelectronic properties of high-temperature plastic ferroelectrics.
2D Ruddlesden-Popper (RP) halide perovskites with natural multiple quantum well structures are an ideal platform to integrate into vertical heterostructures, which may introduce plentiful intriguing optoelectronic properties that are not accessible in a single bulk crystal. Here, we report liquid-phase van der Waals epitaxy of a 2D RP hybrid perovskite (4,4-DFPD)2PbI4 (4,4-DFPD is 4,4-difluoropiperidinium) on muscovite mica and fabricate a series of perovskite-perovskite vertical heterostructures by integrating it with a second 2D RP perovskite R-NPB [NPB = 1-(1-naphthyl)ethylammonium lead bromide] sheets. The grown (4,4-DFPD)2PbI4 nanobelt array can be multiple layers to unit-cell thin and are crystallographically aligned on the mica substrate. An interlayer photo emission in this R-NPB/(4,4-DFPD)2PbI4 heterostructure with a lifetime of about 25 ns at 120 K has been revealed. Our demonstration of epitaxial (4,4-DFPD)2PbI4 array grown on mica via liquid-phase van der Waals epitaxy provides a paradigm to prepare orderly distributed 2D RP hybrid perovskites for further integration into multiple heterostructures. The discovery of a new interlayer emission in the R-NPB/(4,4-DFPD)2PbI4 heterostructure enriches the basic understanding of interlayer charge transition in halide perovskite systems.
The electron-phonon (e-ph) interaction in lead halide perovskites (LHPs) plays a role in a variety of physical phenomena. Unveiling how the local lattice distortion responds to charge carriers is a critical step toward understanding the e-ph interaction in LHPs. Herein, we advance a fundamental understanding of the e-ph interaction in LHPs from the perspective of stereochemical activity of 6s2 lone-pair electrons on the Pb2+ cation. We demonstrate a model system based on three LHPs with distinctive lone-pair activities for studying the structure-property relationships. By tuning the A-cation chemistry, we synthesized single-crystal CsPbBr3, (MA0.13EA0.87)PbBr3 (MA+ = methylammonium; EA+ = ethylammonium), and (MHy)PbBr3 (MHy+ = methylhydrazinium), which exhibit stereo-inactive, dynamic stereo-active, and static stereo-active lone pairs, respectively. This gives rise to distinctive local lattice distortions and low-frequency vibrational modes. We find that the e-ph interaction leads to a blue shift of the band gap as temperature increases in the structure with the dynamic stereo-active lone pair but to a red shift in the structure with the static stereo-active lone pair. Furthermore, analyses of the temperature-dependent low-energy photoluminescence tails reveal that the strength of the e-ph interaction increases with increasing lone-pair activity, leading to a transition from a large polaron to a small polaron, which has significant influence on the emission spectra and charge carrier dynamics. Our results highlight the role of the lone-pair activity in controlling the band gap, phonon, and polaronic effect in LHPs and provide guidelines for optimizing the optoelectronic properties, especially for tin-based and germanium-based halide perovskites, where stereo-active lone pairs are more prominent than their lead counterparts.
Multifunctional materials have always been an attractive research area, but how to combine multiple excellent properties in one compound remains a considerable challenge. Organic-inorganic hybrid compounds are widely used in the design of such materials due to their rich properties and flexible assembly. Herein, two new manganese(II)-based organic-inorganic hybrid compounds, (C6NH16)2MnBr4 (1) and (C7NH18)2MnBr4 (2), are prepared by the solution method. Compounds 1 and 2 both emit extremely strong green light under UV excitation, with high quantum yields of 45.93 and 50.98%, respectively. In addition, reversible solid-state phase transitions and obvious switchable dielectric properties are shown at 378/366 and 361/352 K, respectively. The coexistence of the dual stimulus-response characteristics of temperature and light in compounds 1 and 2 opens a new path for exploring more multifunctional phase transition materials.
Host-guest inclusion compounds with multistage reversible phase transitions are ideal materials for constructing molecular ferroelectrics. However, how to accurately design a molecular ferroelectric from a non-ferroelectric phase transition to a...
Two-dimensional (2D) Ruddlesden-Popper hybrid perovskites (RPHPs) have shown enormous potentials for photoelectric applications, owing to their advantages of structural diversity and unique physical properties. Despite extensive studies, the cage-incorporation of...
Birefringent materials are vital materials to modulate the polarization of light, and play a key role in plorization devices such as linear optical devices, optical communication devices, and fiber optic sensors. It is still a challenge to design excellent birefringent materials. Herein, we report an organic-inorganic hybrid oxalate birefringent material, (CN4H7)SbC2O4F2(H2O)0.5, by introducing organic delocalized π-conjugated [CN4H7]+ and [C2O4]2- groups, and stereochemical active inorganic SbO4F2 polyhedra. (CN4H7)SbC2O4F2(H2O)0.5 exhibits a large birefringence (Δn = 0.126@546 nm) that is almost equal to that of the well-known birefringent material α-BaB2O4. Theoretical calculations reveal that the distinguished birefringence should stem from the synergistic arrangement of π-conjugated [CN4H7]+ and [C2O4]2- planar groups, and highly distorted SbO4F2 polyhedra with a stereochemically active lone pair. The synergistic effect of π-conjugated systems and the lone pair electrons greatly boosts the birefringence, which is helpful for the development of high-performance birefringent materials.
Two-dimensional (2D) hybrid perovskites of Ruddlesden-Popper (RP) lattices are recently booming as a vigorous class of ferroelectrics, whereas their intrinsic van der Waals gaps exert weak interactions that destabilize the layered motifs. Thus, it is an urgent challenge to reduce interlayered energy gaps to allow an exploration of stable RP ferroelectrics. Here, we propose hydrogen bonds to reduce van der Waals gaps of 2D RP-type perovskites while the ferroelectricity is retained. For the first time, a homoconformational trans isomer has been alloyed as the spacing cation of the 2D ferroelectric (t-ACH)2(EA)2Pb3Br10 (1, where t-ACH is 4-aminomethyl-1-cyclohexanecarboxylate and EA is ethylammonium). Strikingly, the strong O-H···O hydrogen bonds link adjacent spacing sheets to build a quasi-RP motif with a lower energy gap. In terms of ferroelectricity, the mixed-cation alloying has a crucial role in spontaneous polarization (Ps), as verified by structure analyses, quadratic optical nonlinearity, and electric hysteresis loops. The ordering of t-ACH+ and EA+ cations induces mmmFm symmetry breaking, along with an unusual in-plane Ps value of ∼2.9 μC/cm2 in the ac plane. In combination with the anisotropic nature of its 2D motif, this ferroelectricity creates strong linearly polarized light sensitivity with a large dichroism ratio of ∼3.2, far beyond those of most of the inorganic 2D systems. As far as we know, a 2D RP-type ferroelectric with a trans isomer cationic spacer is unprecedented, and the concept of reducing energy gaps via H-bonding interactions will strengthen the layered perovskite structure and shed light on the rational design of stable ferroelectrics toward photoelectric applications.
[1,4-butanediammonium]BiI 5 is a lead-free organic–inorganic ferroelectric semiconductor showing a high Curie temperature and a small band gap.
Three-dimensional (3D) organic-inorganic lead halide hybrids have become a hot academic topic because of their various functional properties. However, 3D lead halide hybrid ferroelectrics are still very rare until now. Here, we report a new 3D lead halide perovskite-related ferroelectric, (EATMP)Pb2Br6 [EATMP = (2-aminoethyl)trimethylphosphanium]. Based on nonferroelectric CH3NH3PbBr3, by replacing PbBr6 octahedra with a Pb2Br10 dimer of edge-sharing octahedra as the basic building unit, the expanded 3D lead bromide perovskite analog was formed with the large [EATMP]2+ cations occupying the voids of framework. Notably, (EATMP)Pb2Br6 displays a direct bandgap of 2.81 eV, four polarization directions, and a high Curie temperature (Tc) of 518 K (much beyond that of BaTiO3, 393 K), which is the highest among all reported 3D organic-inorganic hybrid ferroelectrics. Such a high Tc may result from the high rotational energy barrier of cations induced by a larger molecular volume and relatively low crystal symmetry. Our work provides an efficient avenue to construct new 3D organic-inorganic lead halide hybrids and would inspire the further exploration of 3D lead halide ferroelectrics.
Ferroelectric materials have a variety of technological applications, as transducers, capacitors, sensors, etc. Great interest in molecular ferroelectrics has emerged because of their structural flexibility, tunability, and homochirality. However, the discoveries of molecular ferroelectrics are not abundant. The lack of chemical design is the main challenge in realizing new molecular ferroelectrics. Consequently, chemical design approaches, including the ideas of introducing quasi-spherical theory, homochirality, and H/F substitution, have been developed recently. Through these advanced methodologies, a wide range of ferroelectrics were successfully synthesized, changing the blind search into a targeted chemical design. In this Perspective, we aim to provide insight into the fundamental chemistry and physics of molecular ferroelectrics and propose the concept of "ferroelectrochemistry", concerned with the targeted design and performance optimization of molecular ferroelectrics from the chemical point of view. We start with the basic theories used in the modification of chemical structures for new molecular ferroelectrics, such as the quasi-spherical theory. After that, we focus on the fundamentals of homochirality from the perspective of chemistry and advantages of introducing a homochiral molecule within the scope of ferroelectrics. Further, we explore another design strategy, H/F substitution, as an analogue of the H/D isotope effect. The introduction of a F atom usually does not change the polar point group but may induce a minor structural disruption that enhances physical properties such as Curie temperature and spontaneous polarization. We hope our comprehensive studies on the targeted design and performance optimization strategies for molecular ferroelectrics may build up and enrich the content of ferroelectrochemistry.
Isotope effect usually brings improved performance or new features such as KD2PO4 and SrTi18O3 induced a higher phase transition temperature (Tc) and ferroelectricity respectively. However, the isotope effect seems to work in some limited systems and usually extremely toxic and unstable. Due to the close van der Waal’s radii and the similar steric parameters, the H/F substitution as isotope-like effect has been used on spherical amines for designing ferroelectrics. In this work, we reported a study of H/F substitution based on the chain amine in [NH3CH2CH3]3BiCl6. As expected, the substituted compound [NH3CH2CH2F]3BiCl6 successfully achieves the space group transformation from centrosymmetric to non-centrosymmetric system, triggering the second-order nonlinear optical properties. Equally importantly, the phase transition temperature increases significantly from 229 K to 361.5 K. The crystal structure analysis and 2D fingerprint plots prove that the excellent properties were stemmed from the increased force between organic cations and inorganic parts with temperature rising. Consequently, this work will open up the door for designing chain amine in hybrid materials to improve original performance or generate new performance.
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 bistablity with a double-well potential in DABCO-based ferroelectrics. A large deuterium isotope effect (∆T = ~53 K) not only proves that they are hydrogen-bonded ferroelectrics, but also extends the ferroelectric working temperature range to room temperature. The super-fast polarization switching of 100 kHz and ultra-low 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.
Polar crystal structures have attracted more and more attention, due to their unique characteristics, such as ferroelectricity, piezoelectricity, and nonlinear optical property, etc. However, the construction of polar materials is always accidental, and finding an effective synthesis strategy to construct polar materials remains a challenge. Herein, inorganic-organic hybrid compounds of [C7H14N][FeCl4] (1) ([C7H14N] = quinuclidinium cation) and [C7H14N][GeCl3] (2) were prepared, respectively, to verify the beneficial effect of polar anions on the construction of polar crystals. Compound 1 crystallized in the Pbca space group, while 2 belongs to the P43 space group at room temperature. Investigation into the structure of 2 reveals that the polarity of 2 derives from the triangular pyramid structure of [GeX3]- with lone pair electrons. Meanwhile, 2 undergoes a phase transition from the P43 space group to the center Pm3m at 385 K, leading to the optic-electric switching property. Thus, the present work exhibits the advantage of [GeX3]- as the inorganic constituent component in the hybrid polar materials and provides an effective approach for the construction of a polar molecule-based crystal with a switchable property.
Organic-inorganic ABX3 (A, B = cations, X = anion) hybrids with perovskite structure have recently attracted tremendous interest due to their structural tunability and rich functional properties, such as ferroelectricity. However, ABX3 hybrid ferroelectrics with other structures have rarely been reported. Here, we successfully designed an ABX3 hybrid ferroelectric [(CH3)3NCH2F]ZnCl3 with a spontaneous polarization of 4.8 μC/cm2 by the molecular modification of [(CH3)4N]ZnCl3 through hydrogen/halogen substitution. It is the first zinc halide ABX3 ferroelectric, which contains one-dimensional [ZnCl3]-
n
chains of corner-sharing ZnCl4 tetrahedra, distinct from the anionic framework of corner-sharing or face-sharing BX6 octahedra in the ABX3 perovskites. From zero dimension to one dimension, the high symmetry of ZnCl4 tetrahedra is broken, and all of them align along one direction to form a polar [ZnCl3]-
n
chain, beneficial to the generation of ferroelectricity. This finding provides an efficient polar anionic framework for enriching the family of hybrid ferroelectrics by assembling with various cations and should inspire further exploration of new classes of organic-inorganic ABX3 ferroelectrics.
Smart stimulus-responsive materials, which could automatically respond to external stimuli (thermal/optical/electric) and rapidly switch between high/low states (switch ON/OFF), have been an important carrier in high-tech fields such as smart electronic devices, the Internet of things, robots and so on. However, the effective design and development of these materials with simple-fabricated methods, multiple response modes and logical control features are still facing enormous challenges. Herein, the star family, organic-inorganic hybrid perovskite, embrace a new member, [C5H14N]2·[CdCl4] (1), which simultaneously displays rapid response in the channels of dielectric and fluorescence under the stimulus of thermal. Specifically, 1 exhibits phase transition below room temperature (Tc = 239 K, ΔT < 8 K) including thermal anomaly, dielectric transition, and the symmetry breaking. And the Mn-doped crystal could exhibit temperature-dependent red luminescence under UV excitation with high signal-to-noise ratio of 3. Besides, the dielectric/fluorescent dual response triggered by thermal all exhibit rapid response and high fatigue resistance. This finding will be a further step to the practical application of stimulus-responsive materials based on two-dimensional perovskites on smart electronic devices.
Piezoelectric sensors that can work in various conditions with superior performance are highly desirable with the arrival of the Internet of Things. For practical applications, large piezoelectric voltage coefficient g and 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 Vm N−1 [much beyond that of Pb(Zr,Ti)O3 (PZT) ceramics (20 to 40 × 10−3 Vm N−1), more than two times higher than that of poly(vinylidene fluoride) (PVDF, about 286.7 × 10−3 Vm N−1)]. Combined with those advantages of molecular ferroelectrics, such as light weight, easy and environment-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.
Organic-inorganic hybrid ferroelectric materials have attracted attention from researchers as promising candidates for functional materials, because they combine high performance and low cost. Herein, a new organic-inorganic hybrid ferroelectric material (C4H10N)6[InBr6][InBr4]3·H2O (1) was synthesized and characterized. The compound undergoes a paraelectric-ferroelectric phase transition at TC = 232 K, which is investigated by differential scanning calorimetry (DSC), dielectric measurements and variable-temperature structural analyses. Crystal structure analyses indicated that the disorder-order transition of Br atoms in [InBr4]- anions depending on temperature was the main factor driving the phase transition from Pbcn to Pna21. This finding will open up a new direction to probe the organic-inorganic hybrid molecular ferroelectric phase transition materials.