Metal-free three-dimensional perovskite ferroelectrics

Record-high thermal stability achieved in a novel single-component all-organic ferroelectric crystal exhibiting polymorphism

Traditionally, lead and heavy metal containing inorganic oxides dominate the area of ferroelectricity. Although, recently, lightweight non-toxic organic ferroelectrics have emerged as excellent alternatives, achieving higher temperature up to which the ferroelectric phase can persist has remained a challenge. Moreover, only a few of those are single-component molecular ferroelectrics and were discovered upon revisiting their crystal structures. Here we report a novel phenanthroimidazole derivative, which not only displays notable spontaneous and highly stable remnant polarizations with a low coercive field but also retains its ferroelectric phase up to a record-high temperature of ∼521 K. Subsequently, the crystal undergoes phase transition to form non-polar and centrosymmetric polymorphs, the first study of its kind in a single-component ferroelectric crystal. Moreover, the compound exhibits a significantly high thermal stability. Given the excellent figures-of-merit for ferroelectricity, this material is likely to find potential applications in microelectronic devices pertaining to non-volatile memory.

Mechanically interlocked architecture aids an ultra-stiff and ultra-hard elastically bendable cocrystal

Molecular crystals are not known to be as stiff as metals, composites and ceramics. Here we report an exceptional mechanical stiffness and high hardness in a known elastically bendable organic cocrystal [caffeine (CAF), 4-chloro-3-nitrobenzoic acid (CNB) and methanol (1:1:1)] which is comparable to certain low-density metals. Spatially resolved atomic level studies reveal that the mechanically interlocked weak hydrogen bond networks which are separated by dispersive interactions give rise to these mechanical properties. Upon bending, the crystals significantly conserve the overall energy by efficient redistribution of stress while perturbations in hydrogen bonds are compensated by strengthened π-stacking. Furthermore we report a remarkable stiffening and hardening in the elastically bent crystal. Hence, mechanically interlocked architectures provide an unexplored route to reach new mechanical limits and adaptability in organic crystals. This proof of concept inspires the design of light-weight, stiff crystalline organics with potential to rival certain inorganics, which currently seem inconceivable.

Nanoscale Effects in One-Dimensional Columnar Supramolecular Ferroelectrics

Organic ferroelectrics have been actively developed with the goal of fabricating environmentally friendly and low-cost memory devices. The remanent polarization of hydrogen-bonded organic ferroelectrics approaches that of the inorganic ones. Nanoscale fabrication of organic ferroelectrics is an essential aspect of high-density memory devices. A pyrene derivative with four tetradecylamide (-CONHC₁₄ H₂₉ ) chains (1) formed an amide-type N-H⋅⋅⋅O hydrogen-bonded one-dimensional (1D) column, which demonstrated ferroelectricity in the discotic hexagonal columnar (Col_h ) liquid crystalline phase through the inversion of the orientation of the hydrogen-bonded chains. On the contrary, similar chiral pyrene derivatives bearing 3,7-dimethyl-1-octhylamide chains (S-2 and R-2) did not indicate the Col_h phase and ferroelectricity. Homogeneous mixed liquid crystals (1)_1-x (S-2)_x (i.e., between the ferroelectric 1 and the non-ferroelectric S-2) enable the control of the nanoscale aggregation state of the organic ferroelectrics, resulting in a nanoscale effect of the 1D supramolecular ferroelectrics. Ferroelectric mixed liquid crystals (1)_1-x (S-2)_x were observed at x≦0.03, where one S-2 molecule was inserted after every thirty-three 1 molecule in the mixed liquid crystal (1)₃₃ (S-2). An average (1)₃₄ length of approximately 12 nm was required to maintain the 1D ferroelectricity, which was similar to the nanoscale limit of inorganic ferroelectrics, such as hafnium oxide thin film (≈15 nm).

Ferroelectric properties and Raman spectroscopy of the [(C4H9)4N]3Bi2Cl9 compound

The exploration of ferroelectric hybrid materials is highly appealing due to their great technological significance. They have the potential to conserve power and amazing applications in information technology. In line with this, we herein report the development of a [(C₄H₉)₄N]₃Bi₂Cl₉ tetra-alkyl hybrid compound that exhibits ferroelectric properties. The phase purity was confirmed by Rietveld refinement of the X-ray powder diffraction pattern. It crystallizes, at room temperature, in the monoclinic system with the P2₁/n space group. The outcome of temperature dependence of the dielectric constant proved that this compound is ferroelectric below approximately 238 K. The dielectric constants have been fitted using the modified Curie–Weiss law and the estimated γ values are close to 1. This confirms classical ferroelectric behavior. Raman spectroscopy is efficiently utilized to manifest the origin of the ferroelectricity, which is ascribed to the dynamic motion of cations as well as distortion of the anions. Moreover, the analysis of the wavenumbers and the half-width for δ_s(Cl–Bi–Cl) and ω(CH₂) modes, based on the order–disorder model, allowed us to obtain the thermal coefficient and activation energy near the para-ferroelectric phase transition.

The exploration of ferroelectric hybrid materials is highly appealing due to their great technological significance.

Toward the Targeted Design of Molecular Ferroelectrics: Modifying Molecular Symmetries and Homochirality

Although the first ferroelectric discovered in 1920 is Rochelle salt, a typical molecular ferroelectric, the front-runners that have been extensively studied and widely used in diverse applications, such as memory elements, capacitors, sensors, and actuators, are inorganic ferroelectrics with excellent electrical, mechanical, and optical properties. With the increased concerns about the environment, energy, and cost, molecular ferroelectrics are becoming promising supplements for inorganic ferroelectrics. The unique advantages of high structural tunability and homochirality, which are unavailable in their inorganic counterparts, make molecular systems a good platform for manipulating ferroelectricity. Remarkably, based on the Neumann's principle and the Curie symmetry principle defining the group-to-subgroup relationship, we have found some outstanding high-temperature molecular ferroelectrics, like diisopropylammonium bromide (DIPAB) with a large spontaneous polarization up to 23 μC/cm² ( Fu, D. W.; et al. Science 2013 , 339 , 425 ). However, their application potential is severely limited by the uniaxial nature, leading to major issues in finding proper substrates for thin-film growth and achieving high thin-film performance. Inspired by the commercialized inorganic ferroelectrics like Pb(Zr, Ti)O₃ (PZT), where the multiaxial nature contributes greatly to the optimized ferroelectric and piezoelectric performance, developing high-temperature multiaxial molecular ferroelectrics is an imminent task. In this Account, we review our recent research progress on the targeted design of multiaxial molecular ferroelectrics. We first propose the "quasi-spherical theory", a phenomenological theory based on the Curie symmetry principle, to modify the spherical cations to a low-symmetric quasi-spherical geometry for acquiring the highly symmetric paraelectric phase and the polar ferroelectric phase of multiaxial ferroelectrics simultaneously. Besides the sizes and weights of the cation and anion, the intermolecular interactions are particularly crucial for decelerating the molecular rotation at low temperature to reasonably induce ferroelectricity. It means that the momentums of the cation and anion should be matched, so we describe the "momentum matching theory". In particular, introducing homochirality, a superiority of molecular materials over the inorganic ones, was demonstrated as an effective approach to increase the incidence of ferroelectric crystal structures. Thanks to the striking chemical variability and structure-property flexibility of molecular materials, our research efforts outlined in this Account have led to and will further motivate the richness and the application exploration of high-temperature, high-performance multiaxial molecular ferroelectrics, along with the implementation and perfection of the targeted design strategies.

H/F-Substitution-Induced Homochirality for Designing High-Tc Molecular Perovskite Ferroelectrics

A ferroelectric with a high phase-transition temperature (T_c ) is an indispensable condition for practical applications. Over the past decades, both strain engineering and the isotope effect have been found to effectively improve the T_c within ferroelectric material systems. However, the former strategy seems to prefer working in inorganic ferroelectric thin films, while the latter is also limited to some certain systems, such as hydrogen-bonded ferroelectrics. It is noted that a mono-fluorinated molecule is geometrically very similar to its parent molecule and the substitution of H by an F atom can introduce a chiral center on the molecule to template or stabilize polar structures. Significantly, the barrier of rotation of the fluorinated organic molecules is raised, resulting in a remarkable increase in T_c . Herein, by applying the molecular design strategy of H/F substitution to the organic-inorganic perovskite ferroelectric (pyrrolidinium)CdCl₃ with a low T_c of 240 K, two high-T_c chiral perovskite ferroelectrics, (R)- and (S)-3-F-(pyrrolidinium)CdCl₃ are successfully synthesized, for which the T_c reaches 303 K. The significant enhancement of 63 K in T_c extends the ferroelectric working temperature range to room temperature. This finding provides a new effective way to regulate the T_c in ferroelectrics and to design high-T_c molecular ferroelectrics.

H/F substituted perovskite compounds with above-room-temperature ferroelasticity: [(CH3)4P][Cd(SCN)3] and [(CH3)3PCH2F][Cd(SCN)3]

An organic-inorganic perovskite compound [(CH₃)₄P][Cd(SCN)₃] (1) and its fluorine-substituted product [(CH₃)₃PCH₂F][Cd(SCN)₃] (2) exhibit ferroelastic phase transitions above room temperature. The very close van der Waals radii of H and F atoms ensure isomorphism of the crystal structures. However, the higher phase transition temperature, stronger ferroelastic spontaneous strain value and dielectric properties of 2 can possibly be explained by differences in the electronegativity between F and H atoms.

Lead-free double perovskites Cs2InCuCl6 and (CH3NH3)2InCuCl6: electronic, optical, and electrical properties

Searching for alternatives to lead-containing metal halide perovskites, we explored the properties of indium-based inorganic double perovskites Cs₂InMX₆ with M = Cu, Ag, Au and X = Cl, Br, I, and of its organic-inorganic hybrid derivative MA₂InCuCl₆ (MA = CH₃NH₃⁺) using computation within Kohn-Sham density functional theory. Among these compounds, Cs₂InCuCl₆ and MA₂InCuCl₆ were found to be potentially promising candidates for solar cells. Calculations with different functionals provided the direct band gap of Cs₂InCuCl₆ between 1.05 and 1.73 eV. In contrast, MA₂InCuCl₆ exhibits an indirect band gap between 1.31 and 2.09 eV depending on the choice of exchange-correlation functional. Cs₂InCuCl₆ exhibits a much higher absorption coefficient than that calculated for c-Si and CdTe, common semiconductors for solar cells. Even MA₂InCuCl₆ is predicted to have a higher absorption coefficient than c-Si and CdTe across the visible spectrum despite the fact that it is an indirect band gap material. The intrinsic charge carrier mobilities for Cs₂InCuCl₆ along the L-Γ path are predicted to be comparable to those for MAPbI₃. Finally, we carried out calculations of the band edge positions for MA₂InCuCl₆ and Cs₂InCuCl₆ to offer guidance for solar cell heterojunction design and optimization. We conclude that Cs₂InCuCl₆ and MA₂InCuCl₆ are promising semiconductors for photovoltaic and optoelectronic applications.

A Nickel(II) Nitrite Based Molecular Perovskite Ferroelectric

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

Chiral CdSe nanoplatelets as an ultrasensitive probe for lead ion sensing

As opposed to traditional photoluminescence and ultra-violet based optical sensing, we present here a sensing system based on resolved optically active polarization with promising applications. It is based on the ultrathin CdSe nanoplatelets (NPLs) when modified with either l or d-cysteine molecules (l/d-cys) as bio-to-nano ligands. The chiral ligand transfers its chiroptical activity to the achiral nanoplatelets with an anisotropy factor of ∼10-4, which unlocks the chiral excitonic transitions and allows lead ion detection with a limit of detection (LOD) as low as 4.9 nM. Simulations and modelling based on time-dependent density functional theory (TD-DFT) reveal the chiral mechanism of l/d-cys capped CdSe NPLs. The presented CD-based sensing system illustrates an alternative possibility of using chiral CdSe NPLs as competitive chiral sensors for heavy metal ion detection.

Structural and Functional Diversity in Lead-Free Halide Perovskite Materials

Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community holds different views toward the toxic lead in these high-performance perovskites, it is certainly preferred to replace lead with nontoxic, or at least less-toxic, elements while maintaining the superior properties. Here, the design rules for lead-free perovskite materials with structural dimensions from 3D to 0D are presented. Recent progress in lead-free halide perovskites is reviewed, and the relationships between the structures and fundamental properties are summarized, including optical, electric, and magnetic-related properties. 3D perovskites, especially A₂ B⁺ B³⁺ X₆ -type double perovskites, demonstrate very promising optoelectronic prospects, while low-dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic, and multifunctional applications. Furthermore, based on these structure-property relationships, strategies for multifunctional perovskite design are proposed. The challenges and future directions of lead-free perovskite applications are also highlighted, with emphasis on materials development and device fabrication. The research on lead-free halide perovskites at Linköping University has benefited from inspirational discussions with Prof. Olle Inganäs.

Growth and Property Investigations of Two Organic–Inorganic Hybrid Molecular Crystals with High Thermal Stability: 4-Iodoanilinium perchlorate 18-crown-6 and 4-Iodoanilinium Borofluorate 18-crown-6

Two new organic–inorganic hybrid molecular single crystals, 4-Iodoanilinium perchlorate 18-crown-6 (1) and 4-Iodoanilinium borofluorate 18-crown-6 (2), with large sizes and high thermal stability were successfully synthesized by solution method. Their structures, phase purities, thermal stability, dielectric, absorption and fluorescence spectra were systematically investigated for potential applications. Compounds 1 and 2 crystallize in orthorhombic crystal system, in same space group, namely Pnma. The thermal measurements shown 1 and 2 maintain high thermal stability up to 150 °C. The temperature dependency of dielectric constant was studied, and no distinct anomaly was observed. The band gap were calculated to be 3.38 eV and 3.57 eV for 1 and 2, respectively, slightly smaller than those of layer perovskite (benzylammonium)2PbCl4 semiconducting materials, which have potential applications in optoelectronic detection field. The investigations throw light on the semiconductor properties of organic–inorganic hybrid crown type material and provide two types of crown compounds with high thermal stability.

Electro-Optic Modulation in Hybrid Metal Halide Perovskites

Rapid and efficient conversion of electrical signals to optical signals is needed in telecommunications and data network interconnection. The linear electro-optic (EO) effect in noncentrosymmetric materials offers a pathway to such conversion. Conventional inorganic EO materials make on-chip integration challenging, while organic nonlinear molecules suffer from thermodynamic molecular disordering that decreases the EO coefficient of the material. It has been posited that hybrid metal halide perovskites could potentially combine the advantages of inorganic materials (stable crystal orientation) with those of organic materials (solution processing). Here, layered metal halide perovskites are reported and investigated for in-plane birefringence and linear electro-optic response. Phenylmethylammonium lead chloride (PMA₂ PbCl₄ ) crystals are grown that exhibit a noncentrosymmetric space group. Birefringence measurements and Raman spectroscopy confirm optical and structural anisotropy in the material. By applying an electric field on the crystal surface, the linear EO effect in PMA₂ PbCl₄ is reported and its EO coefficient is determined to be 1.40 pm V^-1 . This is the first demonstration of this effect in hybrid metal halide perovskites, materials that feature both highly ordered crystalline structures and solution processability. The in-plane birefringence and electro-optic response reveal that layered perovskite crystals could be further explored for potential applications in polarizing optics and EO modulation.

Theoretical Prediction of Chiral 3D Hybrid Organic-Inorganic Perovskites

Hybrid organic-inorganic perovskites (HOIPs), in particular 3D HOIPs, have demonstrated remarkable properties, including ultralong charge-carrier diffusion lengths, high dielectric constants, low trap densities, tunable absorption and emission wavelengths, strong spin-orbit coupling, and large Rashba splitting. These superior properties have generated intensive research interest in HOIPs for high-performance optoelectronics and spintronics. Here, 3D hybrid organic-inorganic perovskites that implant chirality through introducing the chiral methylammonium cation are demonstrated. Based on structural optimization, phonon spectra, formation energy, and ab initio molecular dynamics simulations, it is found that the chirality of the chiral cations can be successfully transferred to the framework of 3D HOIPs, and the resulting 3D chiral HOIPs are both kinetically and thermodynamically stable. Combining chirality with the impressive optical, electrical, and spintronic properties of 3D perovskites, 3D chiral perovskites is of great interest in the fields of piezoelectricity, pyroelectricity, ferroelectricity, topological quantum engineering, circularly polarized optoelectronics, and spintronics.

[C6N2H18][SbI5]: A Lead-free Hybrid Halide Semiconductor with Exceptional Dielectric Relaxation

[C₆N₂H₁₈][SbI₅] (1), a novel metal halide semiconductor with dielectric relaxation behavior, has been successfully synthesized, in which the cavities between the one-dimensional [SbI₅] _n^2- polyanions are occupied by 2-methyl-1,5-pentanediammonium (2-MPDA) cations. 1 undergoes a reversible solid-state phase transition at T_C = 192.7 K and shows a step-like dielectric anomaly. Interestingly, above T_C, distinct dielectric dispersion in a wide temperature range is also witnessed. Remarkably, the solid state UV-vis diffuse reflectance spectrum of 1 exhibits a slightly gentler absorption edge at about 650 nm; that is, 1 adopts an indirect band gap with 1.92 electron volts. The combined narrow band gap, strong photoconductivity effect, and excellent dielectric relaxation might shed light on the exploitation of lead-free hybrid metal halide molecular materials with promising application prospects in thermoresponsive relaxation-type dielectric materials and photovoltaic conversion devices.

The First 2D Homochiral Lead Iodide Perovskite Ferroelectrics: [R- and S-1-(4-Chlorophenyl)ethylammonium]2 PbI4

2D organic-inorganic lead iodide perovskites have recently received tremendous attention as promising light absorbers for solar cells, due to their excellent optoelectronic properties, structural tunability, and environmental stability. However, although great efforts have been made, no 2D lead iodide perovskites have been discovered as ferroelectrics, in which the ferroelectricity may improve the photovoltaic performance. Here, by incorporating homochiral cations, 2D lead iodide perovskite ferroelectrics [R-1-(4-chlorophenyl)ethylammonium]₂ PbI₄ and [S-1-(4-chlorophenyl)ethylammonium]₂ PbI₄ are successfully obtained. The vibrational circular dichroism spectra and crystal structural analysis reveal their homochirality. They both crystalize in a polar space group P1 at room temperature, and undergo a 422F1 type ferroelectric phase transition with transition temperature as high as 483 and 473.2 K, respectively, showing a multiaxial ferroelectric nature. They also possess semiconductor characteristics with a direct bandgap of 2.34 eV. Nevertheless, their racemic analogue adopts a centrosymmetric space group P2₁ /c at room temperature, exhibiting no high-temperature phase transition. The homochirality in 2D lead iodide perovskites facilitates crystallization in polar space groups. This finding indicates an effective way to design high-performance 2D lead iodide perovskite ferroelectrics with great application prospects.

Bioferroelectric Properties of Glycine Crystals

Biological ferroelectric materials have great potential in biosensing and disease diagnosis and treatment. Glycine crystals form the simplest bioferroelectric materials, and here we investigate the polarizations of its β- and γ-phases. Using density functional theory, we predict that glycine crystals can develop polarizations  even larger than those of conventional inorganic ferroelectrics. Further, using systematic molecular dynamics simulations utilizing polarized crystal charges, we predict the Curie temperature of γ-glycine to be 630 K, with a required coercive field to switch its polarization states of 1 V·nm^-1, consistent with experimental evidence. This work sheds light on the microscopic mechanism of electric dipole ordering in biomaterials, helping in the material design of novel bioferroelectrics.

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

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

Flexible Robust and High-Density FeRAM from Array of Organic Ferroelectric Nano-Lamellae by Self-Assembly

Ferroelectric memories are endowed with high data storage density by nanostructure designing, while the robustness is also impaired. For organic ferroelectrics favored by flexible memories, low Curie transition temperature limits their thermal stability. Herein, a ferroelectric random access memory (FeRAM) is demonstrated based on an array of P(VDF-TrFE) lamellae by self-assembly. Written data shows enhanced thermal endurance up to 90 °C and undergoes 12 thermal cycles between 30 and 80 °C with little volatilization. The promoted thermal stability is attributed to pinning effect at interfaces between grain boundaries and lamellae, where charged domain walls and charged defects are coupled. These results provide a strategy for improving robustness of organic flexible FeRAMs, and reveal an attracting coupling effect between different phases of ferroelectric polymer.

A molecular perovskite solid solution with piezoelectricity stronger than lead zirconate titanate

Piezoelectric materials produce electricity when strained, making them ideal for different types of sensing applications. The most effective piezoelectric materials are ceramic solid solutions in which the piezoelectric effect is optimized at what are termed morphotropic phase boundaries (MPBs). Ceramics are not ideal for a variety of applications owing to some of their mechanical properties. We synthesized piezoelectric materials from a molecular perovskite (TMFM)x(TMCM)1–xCdCl3 solid solution (TMFM, trimethylfluoromethyl ammonium; TMCM, trimethylchloromethyl ammonium, 0 ≤ x ≤ 1), in which the MPB exists between monoclinic and hexagonal phases. We found a composition for which the piezoelectric coefficient d33 is ~1540 picocoulombs per newton, comparable to high-performance piezoelectric ceramics. The material has potential applications for wearable piezoelectric devices.

Organic enantiomeric high-T c ferroelectrics

For nearly 100 y, homochiral ferroelectrics were basically multicomponent simple organic amine salts and metal coordination compounds. Single-component homochiral organic ferroelectric crystals with high-Curie temperature (T _c) phase transition were very rarely reported, although the first ferroelectric Rochelle salt discovered in 1920 is a homochiral metal coordination compound. Here, we report a pair of single-component organic enantiomorphic ferroelectrics, (R)-3-quinuclidinol and (S)-3-quinuclidinol, as well as the racemic mixture (Rac)-3-quinuclidinol. The homochiral (R)- and (S)-3-quinuclidinol crystallize in the enantiomorphic-polar point group 6 (C ₆) at room temperature, showing mirror-image relationships in vibrational circular dichroism spectra and crystal structure. Both enantiomers exhibit 622F6-type ferroelectric phase transition with as high as 400 K [above that of BaTiO₃ (T _c = 381 K)], showing very similar ferroelectricity and related properties, including sharp step-like dielectric anomaly from 5 to 17, high saturation polarization (7 μC/cm²), low coercive field (15 kV/cm), and identical ferroelectric domains. Their racemic mixture (Rac)-3-quinuclidinol, however, adopts a centrosymmetric point group 2/m (C _2h), undergoing a nonferroelectric high-temperature phase transition. This finding reveals the enormous benefits of homochirality in designing high-T _c ferroelectrics, and sheds light on exploring homochiral ferroelectrics with great application.

Dielectric Relaxation and Beyond Limiting Behavior of Alternating-Current Conductivity in a Supermolecular Ferroelectric

A cyclen-based hybrid supermolecule crystal, [(FeCl₂ )(cyclen)]Cl (1), where cyclen=1,4,7,10-tetraazacyclododecane, was prepared using a liquid-liquid diffusion approach. The variable crystal structures exhibit that compound 1 belongs to an orthorhombic crystal system, Pna2₁ space group (point group C_2V ) in the temperature range of 150-400 K. This hybrid supermolecule shows a dielectric relaxation behavior around room temperature, and the ferroelectric nature of 1 has been directly verified by hysteresis measurements. In addition, the AC (alternating current) conductivity study reveals that the 1 displays a beyond limiting behavior. These interesting findings are for the first time reported in the field of supermolecular ferroelectrics. This study may open a new way to construct supermolecular ferroelectrics and give insights into their conductor behavior.

Infrared-pump electronic-probe of methylammonium lead iodide reveals electronically decoupled organic and inorganic sublattices

Organic-inorganic hybrid perovskites such as methylammonium lead iodide (CH₃NH₃PbI₃) are game-changing semiconductors for solar cells and light-emitting devices owing to their defect tolerance and exceptionally long carrier lifetimes and diffusion lengths. Determining whether the dynamically disordered organic cations with large dipole moment benefit the optoelectronic properties of CH₃NH₃PbI₃ has been an outstanding challenge. Herein, via transient absorption measurements employing an infrared pump pulse tuned to a methylammonium vibration, we observe slow, nanosecond-long thermal dissipation from the selectively excited organic mode to the inorganic sublattice. The resulting transient electronic signatures, during the period of thermal-nonequilibrium when the induced thermal motions are mostly concentrated on the organic sublattice, reveal that the induced atomic motions of the organic cations do not alter the absorption or the photoluminescence response of CH₃NH₃PbI₃, beyond thermal effects. Our results suggest that the attractive optoelectronic properties of CH₃NH₃PbI₃ mainly derive from the inorganic lead-halide framework.

Structural phase transition and dielectric anisotropy properties of a lead-free organic–inorganic hybrid

We presented a new inorganic–organic hybrid compound, which exhibits phase transition and dielectric anisotropy properties.

Designing a new family of oxonium-cation based structurally diverse organic–inorganic hybrid iodoantimonate crystals

We report proton-bound oxonium cation based iodoantimonate hybrid organic–inorganic crystals with diverse structure–property relationships.

Multilevel States of Nano-Electromechanical Switch for a PUF-Based Security Device

A nano-electromechanical (NEM) switch using multilevel states based on the high security physical unclonable function (PUF) is proposed and experimentally demonstrated. Using the asymmetric random stiction of a silicon nanowire (SiNW), the conventional binary state is simply expanded to a quaternary-state encryption key without increasing chip area. The multiple states are determined by the asymmetrically bent direction and stiction of the SiNW. The experimental results show that the fabricated NEM-PUF with multistates retains unique, random, and robust characteristics, while the key capacity is doubled, even with the same array size footprint.

Mechanical properties of the ferroelectric metal-free perovskite [MDABCO](NH4)I3

We here probe the mechanical properties of the metal-free perovskite [MDABCO](NH₄)I₃, a material that recently has been discovered as promising ferroelectric.

Near-room-temperature tunable dielectric response induced by dual phase transitions in a lead-free hybrid: (C3H8N)2SbBr5

We report a new lead-free organic–inorganic hybrid that undergoes the dual phase transitions near room temperature.

Effect of carbon-skeleton isomerism on the dielectric properties and proton conduction of organic cocrystal compounds assembled from 1,2,4,5-benzenetetracarboxylic acid and piperazine derivatives

Two isostructural 2D supramolecular cocrystal compounds show different dielectric responses and proton conductivities due to the alteration of the carbon-skeleton of piperazine derivatives.

Metal-free perovskites for non linear optical materials

We identify the existence of nonlinear optical (NLO) activity in a number of novel ABX₃-type metal-free perovskites, where A is a highly tuneable organic cation, B is a NH₄ cation and X is a halide anion.

(H2dabco)[Na(BF4)3]: an ABX3-type inorganic–organic hybrid perovskite compound exhibiting dielectric switching above room-temperature

The (H₂dabco)[Na(BF₄)₃] undergoes a static-to-dynamic phase transition at 403/386 K. Crystal structure analysis reveals that H₂dabco²⁺ and/or BF₄⁻ undergo disordering.

Above room-temperature dielectric and nonlinear optical switching materials based on [(CH3)3S]2[MBr4] (M = Cd, Mn and Zn)

Three organic–inorganic hybrid compounds based on [Me₃S]⁺ cations exhibit sensitive dielectric and nonlinear optical switching characteristics above room temperature.