Superior Transverse Piezoelectricity in a Halide Perovskite Molecular Ferroelectric Thin Film

Biodegradable Ferroelectric Molecular Plastic Crystal HOCH2(CF2)7CH2OH Structurally Inspired by Polyvinylidene Fluoride

Ferroelectric materials, traditionally comprising inorganic ceramics and polymers, are commonly used in medical implantable devices. However, their nondegradable nature often necessitates secondary surgeries for removal. In contrast, ferroelectric molecular crystals have the advantages of easy solution processing, lightweight, and good biocompatibility, which are promising candidates for transient (short-term) implantable devices. Despite these benefits, the discovered biodegradable ferroelectric materials remain limited due to the absence of efficient design strategies. Here, inspired by the polar structure of polyvinylidene fluoride (PVDF), a ferroelectric molecular crystal 1H,1H,9H,9H-perfluoro-1,9-nonanediol (PFND), which undergoes a cubic-to-monoclinic ferroelectric plastic phase transition at 339 K, is discovered. This transition is facilitated by a 2D hydrogen bond network formed through O-H···O interactions among the oriented PFND molecules, which is crucial for the manifestation of ferroelectric properties. In this sense, by reducing the number of -CF2- groups from ≈5 000 in PVDF to seven in PFND, it is demonstrated that this ferroelectric compound only needs simple solution processing while maintaining excellent biosafety, biocompatibility, and biodegradability. This work illuminates the path toward the development of new biodegradable ferroelectric molecular crystals, offering promising avenues for biomedical applications.

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.

Piezoelectrically-activated antibacterial catheter for prevention of urinary tract infections in an on-demand manner

Catheter-associated urinary tract infection (CAUTI) is a common clinical problem, especially during long-term catheterization, causing additional pain to patients. The development of novel antimicrobial coatings is needed to prolong the service life of catheters and reduce the incidence of CAUTIs. Herein, we designed an antimicrobial catheter coated with a piezoelectric zinc oxide nanoparticles (ZnO NPs)-incorporated polyvinylidene difluoride-hexafluoropropylene (ZnO-PVDF-HFP) membrane. ZnO-PVDF-HFP could be stably coated onto silicone catheters simply by a one-step solution film-forming method, very convenient for industrial production. In vitro, it was demonstrated that ZnO-PVDF-HFP coating could significantly inhibit bacterial growth and the formation of bacterial biofilm under ultrasound-mediated mechanical stimulation even after 4 weeks. Importantly, the on and off of antimicrobial activity as well as the strenth of antibacterial property could be controlled in an adaptive manner via ultrasound. In a rabbit model, the ZnO-PVDF-HFP-coated catheter significantly reduced the incidence CAUTIs compared with clinically-commonly used catheters under assistance of ultrasonication, and no side effect was detected. Collectively, the study provided a novel antibacterial catheter to prevent the occurrence of CAUTIs, whose antibacterial activity could be controlled in on-demand manner, adaptive to infection situation and promising in clinical application.

Biodegradable ferroelectric molecular crystal with large piezoelectric response

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 HOCH2(CF2)3CH2OH [2,2,3,3,4,4-hexafluoropentane-1,5-diol (HFPD)] with a large piezoelectric coefficient d33 of ~138 picocoulombs per newton and piezoelectric voltage constant g33 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 d33 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.

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

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

Photophysical Behavior of Triethylmethylammonium Tetrabromoferrate(III) under High Pressure

The pressure-induced properties of hybrid organic-inorganic ferroelectrics (HOIFs) with tunable structures and selectable organic and inorganic components are important for device fabrication. However, given the structural complexity of polycrystalline HOIFs and the limited resolution of pressure data, resolving the structure-property puzzle has so far been the exception rather than the rule. With this in mind, we present a collection of in situ high-pressure data measured for triethylmethylammonium tetrabromoferrate(III), ([N(C2H5)3CH3][FeBr4]) (EMAFB) by unraveling its flexible physical and photophysical behavior up to 80 GPa. Pressure-driven X-ray diffraction and Raman spectroscopy disclose its soft and reversible structural distortion, creating room for delicate band gap modulation. During compression, orange turns dark red at ∼2 GPa, and further compression results in piezochromism, leading to opaque black, while decompressed EMAFB appears in an orange hue. Assuming that the mechanical softness of EMAFB is the basis for reversible piezochromic control, we present alternations in the electronic landscape leading to a 1.22 eV band narrowing at 20.3 GPa while maintaining the semiconducting character at 72 GPa. EMAFB exhibits an emission enhancement, manifested by an increase of photoluminescence up to 17.3 GPa, correlating with the onsets of structural distortion and amorphization. The stimuli-responsive behavior of EMAFB, exhibiting stress-activated modification of the electronic structure, can enrich the physical library of HOIFs suitable for pressure-sensing technologies.

First Observation of Negative Capacitance in Molecular Ferroelectric Thin Films

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

Chiral Two-Dimensional Hybrid Organic-Inorganic Perovskites for Piezoelectric Ultrasound Detection

Hybrid organic-inorganic perovskites (HOIPs) have exhibited striking application potential in piezoelectric energy harvesting and sensing due to their high piezoelectricity, light weight, and solution processability. However, to date, the application of piezoelectric HOIPs in ultrasound detection has not yet been explored. Here, we report the synthesis of a pair of chiral two-dimensional piezoelectric HOIPs, R-(4-bromo-2-butylammonium)2PbBr4 and S-(4-bromo-2-butylammonium)2PbBr4 [R-(BrBA)2PbBr4 and S-(BrBA)2PbBr4], which show low mechanical strength and significant piezoelectric strain coefficients that are advantageous for mechanoelectrical energy conversion. Benefiting from these virtues, the R-(BrBA)2PbBr4@PBAT and S-(BrBA)2PbBr4@PBAT [PBAT = poly(butyleneadipate-co-terephthalate)] composite films show prominent underwater ultrasound detection performance with a transmission effectivity of 12.0% using a 10.0 MHz probe, comparable with that of a polyvinylidene fluoride (PVDF) device fabricated in the same conditions. Density functional theory calculations reveal that R-(BrBA)2PbBr4 and S-(BrBA)2PbBr4 have a beneficial acoustic impedance (5.07-6.76 MRayl) compatible with that of water (1.5 MRayl), which is responsible for the facile ultrasound-induced electricity generation. These encouraging results open up new possibilities for applying piezoelectric HOIPs in underwater ultrasound detection and imaging technologies.

Persistent and Quasipersistent Spin Textures in Halide Perovskites Induced by Uniaxial Stress

Persistent spin textures are highly desirable for applications in spintronics, as they allow for long carrier spin lifetimes. However, they are also rare, as they require a delicate balance between spin-momentum coupling parameters. We used density functional theory simulations to predict the possibility of achieving these desirable spin textures through the application of uniaxial stress. Hybrid organic-inorganic perovskite MPSnBr3 (MP = CH3PH3) is a ferroelectric semiconductor which exhibits persistent spin textures near its conduction band minimum and mostly Rashba type spin textures in the vicinity of its valence band maximum. Application of uniaxial stress leads to the gradual evolution of the valence band spin textures from mostly Rashba type to quasipersistent ones under a tensile load and to pure Rashba or quasipersistent ones under a compressive load. The material exhibits flexibility, a rubber-like response, and both positive and negative piezoelectric constants. A combination of such properties may create opportunities for flexible and rubbery spintronic devices.

Analysis of the Structure, Thermal, and Molecular Dynamics of Organic-Inorganic Hybrid Crystal at Phases IV, III, II, and I: [NH2(CH3)2]2CdBr4

A comprehensive understanding of the physicochemical properties of organic-inorganic hybrids is essential for their solid-state lighting applications. Therefore, a single crystal of [NH2(CH3)2]2CdBr4 was grown; the crystal structure was monoclinic, and the phase transition temperatures for the four phases IV, III, II, and I were 383 K (TC1), 417 K (TC2), and 427 K (TC3). Furthermore, the chemical shifts caused by the local field around 1H, 13C, 14N, and 113Cd changed continuously with temperature, especially near TC1, indicating that the local environment changes with temperature. Owing to the large change in 113Cd chemical shifts, the coordination geometry of Br around Cd in the CdBr4 tetrahedra changes near TC1. Therefore, it is proposed that Br plays a significant role in the N-H···Br hydrogen bond. Finally, the spin-lattice relaxation time T1ρ, representing the energy transfer around the 1H and 13C atoms of the cation, changed significantly with temperature. The activation energies obtained from the T1ρ results were two times larger at high temperatures than at low temperatures. This study provides an understanding of the fundamental properties of organic-inorganic hybrid compounds to broaden their applications.

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.

Emerging Halide Perovskite Ferroelectrics

Halide perovskites have gained tremendous attention in the past decade owing to their excellent properties in optoelectronics. Recently, a fascinating property, ferroelectricity, has been discovered in halide perovskites and quickly attracted widespread interest. Compared with traditional perovskite oxide ferroelectrics, halide perovskites display natural advantages such as structural softness, low weight, and easy processing, which are highly desirable in applications pursuing miniaturization and flexibility. This review focuses on the current research progress in halide perovskite ferroelectrics, encompassing the emerging materials systems and their potential applications in ferroelectric photovoltaics, self-powered photodetection, and X-ray detection. The main challenges and possible solutions in the future development of halide perovskite ferroelectric materials are also attempted to be pointed out.

Anion-Dependent Polarization and Piezoelectric Power Generation in Hybrid Halide MAPbX3 (X = I, Br, and Cl) Thin Films with Out-of-Plane Structural Adjustments

Anion-dependent differences in the electromechanical energy harvesting capability of perovskite halides have not been experimentally demonstrated thus far. Herein, anion-dependent piezoelectricity and bending-driven power generation in high-quality methylammonium lead halide MAPbX₃ (X = I, Br, and Cl) thin films are explored; additionally, anisotropic in situ strain is imposed to improve energy harvesting under tensile bending. After applying the maximum in situ strain of -0.73% for all the halide thin films, the MAPbI₃ thin-film harvester exhibited a peak voltage/current of ≈23.1 V/≈1703 nA as the best values, whereas MAPbBr₃ and MAPbCl₃ demonstrated ≈5.6 V/≈176 nA and ≈3.3 V/≈141 nA, respectively, under identical bending conditions. Apart from apparent ferroelectricity of tetragonal MAPbI₃ , origin of the piezoelectricity in both cubic MAPbBr₃ and MAPbCl₃ is explored as being related to organic-inorganic hydrogen bonding, lattice distortion, and ionic migration, with experimental supports of effective piezoelectric coefficient and grain boundary potential. Conclusively, piezoelectricity of the cubic halides is assumed to be due to their soft polarity modes and relatively low elastic modulus with vacancies contributing to space-charge polarization. In the case of ferroelectric MAPbI₃ , the distortion of PbI₆ octahedra and atomic displacement within each octahedron are quantitatively estimated.

Crystal structures, phase transitions, and nuclear magnetic resonance of organic-inorganic hybrid [NH2(CH3)2]2ZnBr4 crystals

Organic-inorganic hybrid [NH₂(CH₃)₂]₂ZnBr₄ crystals were grown via slow evaporation, and their monoclinic structure was determined using single-crystal X-ray diffraction (XRD). The two phase transition temperatures at 401 K (T _C1) and 436 K (T _C2) were defined using differential scanning calorimetry and powder XRD. In the nuclear magnetic resonance spectra, a small change was observed in the ¹H chemical shifts for NH₂, ¹³C chemical shifts for CH₃, and ¹⁴N resonance frequency for NH₂ near T _C1. ¹H spin-lattice relaxation times T _1ρ and ¹³C T _1ρ for NH₂ and CH₃, respectively, rapidly decreased near T _C1, suggesting that energy was easily transferred. NH₂ in the [NH₂(CH₃)₂]⁺ cation was significantly influenced by the surrounding environments of ¹H and ¹⁴N, indicating a change in the N-H⋯Br hydrogen bond with the coordination geometry of the ZnBr₄ anion. These fundamental properties open efficient avenues for the development of organic-inorganic hybrids, thus qualifying them for practical applications.

Ferroelectric hybrid organic-inorganic perovskites and their structural and functional diversity

Molecular ferroelectrics have gradually aroused great interest in both fundamental scientific research and technological applications because of their easy processing, light weight and mechanical flexibility. Hybrid organic-inorganic perovskite ferroelectrics (HOIPFs), as a class of molecule-based ferroelectrics, have diverse functionalities owing to their unique structure and have become a hot spot in molecular ferroelectrics research. Therefore, they are extremely attractive in the field of ferroelectrics. However, there seems to be a lack of systematic review of their design, performance and potential applications. Herein, we review the recent development of HOIPFs from lead-based, lead-free and metal-free perovskites, and outline the versatility of these ferroelectrics, including piezoelectricity for mechanical energy-harvesting and optoelectronic properties for photovoltaics and light detection. Furthermore, a perspective view of the challenges and future directions of HOIPFs is also highlighted.

Enhancement and Function of the Piezoelectric Effect in Polymer Nanofibers

The realization of intelligent, self-powered components and devices exploiting the piezoelectric effect at large scale might greatly contribute to improve our efficiency in using resources, albeit a profound redesign of the materials and architectures used in current electronic systems would be necessary. Piezoelectricity is a property of certain materials to generate an electrical bias in response to a mechanical deformation. This effect enables energy to be harvested from strain and vibration modes, and to sustain the power of actuators, transducers, and sensors in integrated networks, such as those necessary for the Internet of Thing. Polymers, combining structural flexibility with lightweight construction and ease of processing, have been largely used in this framework. In particular, the poly(vinylidene fluoride) [PVDF, (CH₂CF₂) _n ] and its copolymers exhibit strong piezoelectric response, are biocompatibile, can endure large strains and can be easily shaped in the form of nanomaterials. Confined geometries, improving crystal orientation and enhancing piezoelectricity enable the fabrication of piezoelectric nanogenerators, which satisfy many important technological requirements, such as conformability, cheap fabrication, self-powering, and operation with low-frequency mechanical inputs (Hz scale). This account reports on piezoelectric polymer nanofibers made by electrospinning. This technique enables the formation of high-aspect-ratio filaments, such as nanowires and nanofibers, through the application of high electric fields (i.e., on the order of hundreds of kV/m) and stretching forces to a polymeric solution. The solution might be charged with functional, organic or inorganic, fillers or dopants. The solution is then fed at a controlled flow rate through a metallic spinneret or forms a bath volume, from which nanofibers are delivered. Fibers are then collected onto metallic surfaces, and upon a change of the collecting geometry, they can form nonwovens, controlled arrays, or isolated features. Nanofibers show unique features, which include their versatility in terms of achievable chemical composition and chemico-physical properties. In addition, electrospinning can be up-scaled for industrial production. Insight into the energy generation mechanism and how the interaction among fibers can be used to enhance the piezoelectric performance are given in this paper, followed by an overview of fiber networks as the active layer in different device geometries for sensing, monitoring, and signal recognition. The use of biodegradable polymers, both natural and synthetic, as critically important building blocks of the roadmap for next-generation piezoelectric devices, is also discussed, with some representative examples. In particular, biodegradable materials have been utilized for applications related to life science, such as the realization of active scaffolds and of electronic devices to be placed in intimate contact with living tissues and organs. Overall, these materials show many relevant properties that can be of very high importance for building next-generation, sustainable energy harvesting, self-rechargeable devices and electronic components, for use in several different fields.

Achieving circularly polarized luminescence and large piezoelectric response in hybrid rare-earth double perovskite by a chirality induction strategy

Chirality, an intrinsic property of nature, has received increased attention in chemistry, biology, and materials science because it can induce optical rotation, ferroelectricity, nonlinear optical response, and other unique properties. Here, by introducing chirality into hybrid rare-earth double perovskites (HREDPs), we successfully designed and synthesized a pair of enantiomeric three-dimensional (3D) HREDPs, [(R)-N-methyl-3-hydroxylquinuclidinium]₂RbEu(NO₃)₆ (R1) and [(S)-N-methyl-3-hydroxylquinuclidinium]₂RbEu(NO₃)₆ (S1), which possess ferroelasticity, multiaxial ferroelectricity, high quantum yields (84.71% and 83.55%, respectively), and long fluorescence lifetimes (5.404 and 5.256 ms, respectively). Notably, the introduction of chirality induces the coupling of multiaxial ferroelectricity and ferroelasticity, which brings about a satisfactory large piezoelectric response (103 and 101 pC N^-1 for R1 and S1, respectively). Moreover, in combination with the chirality and outstanding photoluminescence properties, circularly polarized luminescence (CPL) was first realized in HREDPs. This work sheds light on the design strategy of molecule-based materials with a large piezoelectric response and excellent CPL activity, and will inspire researchers to further explore the role of chirality in the construction of novel multifunctional materials.

Bioelectricity-coupling patches for repairing impaired myocardium

Cardiac abnormalities, which account for extensive burdens on public health and economy, drive necessary attempts to revolutionize the traditional therapeutic system. Advances in cardiac tissue engineering have expanded a highly efficacious platform to address cardiovascular events, especially cardiac infarction. Current efforts to overcome biocompatible limitations highlight the constructs of a conductive cardiac patch to accelerate the industrial and clinical landscape that is amenable for patient-accurate therapy, regenerative medicine, disease modeling, and drug delivery. With the notion that cardiac tissue synchronically contracts triggered by electrical pulses, the cardiac patches based on conductive materials are developed and treated on the dysfunctional heart. In this review, we systematically summarize distinct conductive materials serving as the most promising alternatives (conductive nanomaterials, conductive polymers, piezoelectric polymers, and ionic electrolytes) to achieve electric signal transmission and engineered cardiac tissues. Existing applications are discussed considering how these patches containing conductive candidates are fabricated into diverse forms with major strategies. Ultimately, we try to define a new concept as a bioelectricity-coupling patch that provides a favorable cardiac micro-environment for cardiac functional activities. Underlying challenges and prospects are presented regarding industrial processing and cardiovascular treatment of conductive patch progress. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Cardiovascular Disease.

A New Hybrid Lead-Free Metal Halide Piezoelectric for Energy Harvesting and Human Motion Sensing

Hybrid organic-inorganic piezoelectrics have attracted attention due to their simple synthesis, mechanical flexibility, and designability, which have promising application potential in flexible sensing and self-powered energy harvesting devices. Although some hybrid piezoelectrics are discovered, most of their structures are limited by the perovskite-type and often contain lead. Herein, the synthesis, structure, and piezoelectric properties of a new hybrid lead-free metal halide, (BTMA)₂ CoBr₄ (BTMA = benzyltrimethylammonium) are reported. The experimental and theoretical results demonstrate that this material simply composed of [CoBr₄ ]^2- tetrahedra and BTMA⁺ cations exhibits significant piezoelectricity (d₂₂ = 5.14, d₂₅ = 12.40 pC N^-1 ), low Young's and shear moduli (4.11-17.56 GPa; 1.86-7.91 GPa). Moreover, the (BTMA)₂ CoBr₄ /PDMS (PDMS = polydimethylsiloxane) composite thin films are fabricated and optimized. The 10% (BTMA)₂ CoBr₄ /PDMS-based flexible devices show attractive performance in energy harvesting with an open-circuit voltage of 19.70 V, short-circuit current of 4.24 µA, and powder density of 11.72 µW cm^-2 , catching up with those of piezoelectric ceramic composites. Meanwhile, these film devices show excellent capability in accurately sensing human body motions, such as finger bending and tapping. This work demonstrates that (BTMA)₂ CoBr₄ and related piezoelectric lead-free halides can be promising molecular materials in modern energy and sensing applications.

Tunable hybrid perovskites with Narrow bandgap and Multistage phase transition properties: 2,2-difluoroethylamine·Antimony Hexabromide

Zero-dimensional perovskites have received extensive attention in recent years due to their unique structures, such as large exciton binding energy, strong quantum confinement effect, and good stability. Precisely construct target...

Engineering Elastic Properties of Isostructural Molecular Perovskite Ferroelectrics via B-Site Substitution

Managing elastic properties of ABX₃ type molecular perovskite ferroelectrics is critical to their future applications since these parameters determine their service durability and reliability in devices. The abundant structural and chemical viability of these compounds offer a convenient way to manipulate their elastic properties through a facile chemical approach. Here, the elastic properties and high-pressure behaviors of two isostructural perovskite ferroelectrics, MDABCO-NH₄ I₃ and MDABCO-KI₃ (MDABCO = N-methyl-N'-diazabicyclo[2.2.2]octonium) is systematically investigated, via the first principles calculations and high-pressure synchrotron X-ray diffraction experiments. It is show that the simple replacement of NH₄ ⁺ by K⁺ on the B-site respectively results in up to 48.1%, 52.4%, and 56.3% higher Young's moduli, shear moduli and bulk moduli, which is attributed to the much stronger KI coordination bonding than NH₄ …I hydrogen bonding. These findings demonstrate that it is possible to tune elastic properties of molecular perovskite ferroelectrics via simply varying the framework assembling interactions.

Acoustic Properties of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have attracted significant attention in the past two decades due to their diverse physical properties and associated functionalities. Although numerous advances have been made, the acoustic properties of MOFs have attracted very little attention. Here, we systematically investigate the acoustic velocities and impedances of 19 prototypical MOFs via first-principle calculations. Our results demonstrate that these MOFs exhibit a wider range of acoustic velocities, higher anisotropy, and lower acoustic impedances than their inorganic counterparts, which are ascribed to their structural diversity and anisotropy, as well as low densities. In addition, the piezoelectric properties, which are intimately related to the acoustic properties, were calculated for 3 MOFs via density functional perturbation theory, which reveals that MOFs can exhibit significant piezoelectricity due to the ionic contribution. Our work provides a comprehensive study of the fundamental acoustic properties of MOFs, which could stimulate further interest in this new exciting field.

Cooperative Nature of Ferroelectricity in Two-Dimensional Hybrid Organic-Inorganic Perovskites

Two-dimensional (2D) ferroelectric (FE) hybrid organic-inorganic perovskites (HOIPs) are promising for potential applications as miniaturized flexible ferroelectric/piezoelectric devices. Recently, several 2D HOIPs [e.g., Ruddlensden-Popper type HOIP BA₂PbCl₄ (BA = C₆H₅CH₂NH₃⁺)] were reported to possess room-temperature ferroelectricity. However, the underlying microscopic mechanisms for ferroelectricity in 2D HOIPs remain elusive. Here, by performing first-principles calculations and symmetry mode analysis, we demonstrate that there exists a cooperative coupling between A-site organic molecules and B-site inorganic Pb²⁺ ions that is essential to the ferroelectricity in 2D BA₂PbCl₄. The nonpolar ground state of the closely related compounds BA₂PbBr₄ and BA₂PbI₄ can also be explained in terms of the weakened cooperative coupling. We further predict that 2D BA₂PbF₄ displays in-plane ferroelectricity with a higher Curie temperature and larger electric polarization. Our work not only reveals the unusual FE mechanism in 2D HOIPs but also provides a solid theoretical basis for the rational design of 2D multifunctional materials.

Chiral Switchable Low-Dimensional Perovskite Ferroelectrics

Low-dimensional hybrid organic-inorganic perovskites (HOIPs) possess more localized electronic states and narrower conduction and valence bands to promote self-trapping of excitons and stronger exciton emission; therefore, they are widely used as building blocks for various applications in the fields of optoelectronics, photovoltaics, light-emitting diodes, luminescence, fluorescence, and so forth. Despite the past decades of intensive study, the discovered low-dimensional chiral HOIPs are rare as of the 1D chiral HOIP single crystals reported in 2003, as well as the low-dimensional chiral HOIP ferroelectrics are particularly scarce since the first chiral two-dimensional (2D) and/or one-dimensional (1D) HOIP ferroelectrics reported. Herein, two new low-dimensional HOIPs with the same conformational formula [R-MPA]₂CdCl₄ (R-MPA⁺ = (R)-(-)-1-methyl-3-phenylpropylamine) were successfully synthetized by means of regulating the stoichiometric proportion of R-MPA and CdCl₂ in two ways of 1:1 (1) and 2:1 (2). By combining single-crystal X-ray diffraction, circular dichroism (CD) spectroscopy, differential scanning calorimetry, temperature-dependent dielectric constant, temperature-dependent second-harmonic generation (SHG) effect, polarization-dependent SHG response, and P-E hysteresis loop, we reveal that 1 is a 1D nonchiral molecular ferroelectric and 2 is the first zero-dimensional (0D) chiral ferroelectric with distinct CD signals; meanwhile, 2 exhibits increased properties of high-T_c, large dielectric constant, SHG isotropy, and ferroelectricity than that of 1. These results not only shed light on the high tunability of the low-dimensional HOIP ferroelectrics but also open up an avenue to explore multifunctional chiral ferroelectrics.

Multifunctional rare earth molecular ferroelectrics with a piezoelectric response: ((nBu)4N)3[Ce(NO3)4(SCN)2]((CH3CH2CH2CH2)4N = tetrabutylammonium)

A new type of multipolar rare earth molecular ferroelectric: ((nBu)4N)3[Ce(NO3)4(SCN)2] (BuCH3CH2CH2CH2)4N), undergoes a high-temperature ferroelectric phase transition, possesses flexible switchable SHG effect, moderate spontaneous polarization and a narrow band gap.

A high-temperature halide perovskite molecular ferroelastic with evident dielectric switching

Under the quasi-spherical strategy, a hybrid halide perovskite (TMTB)CdCl₃ is designed and synthesized and shows evident high-temperature ferroelastic phase transition and dielectric switching.

A polar oxyhalogen-vanadate compound (C5NH13Cl)2VOCl4 with optical and two-staged dielectric switch behavior

Polar organic-inorganic hybrid materials have been applied in ferroelectricity, as well as in devices based on nonlinear optical and piezoelectricity properties. Here, we used a rectangular pyramid structure of [VOCl4]2- to construct a polar compound (C5NH13Cl)2VOCl4 (1). Compound 1 crystallized in the monoclinic P21 space group. Coexistence of nonlinear optical switching behavior (space-group change from P21 to P21/n) and two-staged thermosensitive dielectric switching properties could be achieved under the stimulus of temperature. Our findings provide an effective approach for construction of polar materials.