Ferroelectric switching and electrochemistry of pyrrole substituted trialkylbenzene‐1,3,5‐tricarboxamides

We explore a new approach to organic ferroelectric diodes using a benzene‐tricarboxamide (BTA) core connected with C10 alkyl chains to pyrrole groups, which can be polymerized to provide a semiconducting ferroelectric material. The compound possesses a columnar hexagonal liquid crystalline (LC) phase and exhibits ferroelectric switching. At low switching frequencies, an additional process occurs, which leads to a high hysteretic charge density of up to ～1000 mC/m². Based on its slow rate, the formation of gas bubbles, and the emergence of characteristic polypyrrole absorption bands in the UV–Vis–NIR, the additional process is identified as the oxidative polymerization of pyrrole groups, enabled by the presence of amide groups. Polymerization of the pyrrole groups, which is essential to obtain semiconductivity, is limited to thin layers at the electrodes, amounting to ～17 nm after cycling for 21 h. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 673–683     

Polarization‐Driven Self‐Powered Photodetection in a Single‐Phase Biaxial Hybrid Perovskite Ferroelectric

Self‐powered photodetection driven by ferroelectric polarization has shown great potential in next‐generation optoelectronic devices. Hybrid perovskite ferroelectrics that combine polarization and semiconducting properties have a promising position within this portfolio. Herein, we demonstrate the realization of self‐powered photodetection in a new developed biaxial ferroelectric, (EA)₂(MA)₂Pb₃Br₁₀ ( 1 , EA is ethylammonium and MA is methylammonium), which displays high Curie temperature (375 K), superior spontaneous polarization (3.7 μC cm^?2), and unique semiconducting nature. Strikingly, without an external energy supply, 1 exhibits an direction‐selectable photocurrent with fascinating attributes including high photocurrent density (≈4.1 μA cm^?2), high on/off switching ratio (over 10⁶), and ultrafast response time (96/123 μs); such merits are superior to those of the most active ferroelectric oxide BiFeO₃. Further studies reveal that strong inversion symmetry breaking in 1 provides a desirable driving force for carrier separation, accounting for such electrically tunable self‐powered photoactive behaviors. This work sheds light on exploring new multifunctional hybrid perovskites and advancing the design of intelligent photoelectric devices.     

A Molecular Ferroelectric Showing Room‐Temperature Record‐Fast Switching of Spontaneous Polarization

Fast switching of spontaneous polarization (P_s) is one of the most essential requirements for ferroelectrics used in the field of data storage. However, in contrast to inorganic counterparts, the low operating frequency (<500 Hz) for molecular ferroelectrics severely hinders their large‐scale applications. Herein, for the first time, we achieved the room‐temperature fastest switching of the P_s in a new molecular ferroelectric, N‐methylmorpholinium trinitrophenolate ( 1 ), which displays notable ferroelectricity (P_s=3.2 μc cm^?2). Strikingly, electric polarizations of 1 have been switched under a record‐high frequency of 263 kHz, and this performance remains stable without any obvious fatigue after ca. 2×10⁵ switching cycles. To our knowledge, 1 is the first organic ferroelectric to switch polarization at such a high operating frequency, exceeding the majority of organic ferroelectrics, which opens up new possibilities for its potential in the field of non‐volatile memory.     

Exploring a Lead‐free Semiconducting Hybrid Ferroelectric with a Zero‐Dimensional Perovskite‐like Structure

Perovskite lead halides (CH₃NH₃PbI₃) have recently taken a promising position in photovoltaics and optoelectronics because of remarkable semiconducting properties and possible ferroelectricity. However, the potential toxicity of lead arouses great environmental concern for widespread application. A new chemically tailored lead‐free semiconducting hybrid ferroelectric is reported, N‐methylpyrrolidinium)₃Sb₂Br₉ ( 1 ), which consists of a zero‐dimensional (0‐D) perovskite‐like anionic framework connected by corner‐ sharing SbBr₆ coordinated octahedra. It presents a large ferroelectric spontaneous polarization of approximately 7.6 μC cm^?2, as well as notable semiconducting properties, including positive temperature‐dependent conductivity and ultraviolet‐sensitive photoconductivity. Theoretical analysis of electronic structure and energy gap discloses a dominant contribution of the 0‐D perovskite‐like structure to the semiconducting properties of the material. This finding throws light on the rational design of new perovskite‐like hybrids, especially lead‐free semiconducting ferroelectrics.     

Ferroelectric Behavior of a Hexamethylenetetramine‐Based Molecular Perovskite Structure

We report the development of a molecular ferroelectric material inspired by the hexamethylenetetramine (hmta) non‐centrosymmetric molecular rotator. The bromide salt of diprotonated hmta (hmtaH₂) crystalized as (hmtaH₂)(NH₄)Br₃ in a metal‐free ABX₃ perovskite‐type structure, in which the A and B sites are occupied by hmtaH₂²⁺ and ammonium cations, respectively. The compound crystallized in the Pma2 polar space group. A distorted polar perovskite structure formed owing to the distortion of {(NH₄)Br₆} octahedrons that are stabilized through the formation of NH???Br hydrogen bonds and the orientational ordering of positive charges on the non‐centrosymmetric hmtaH₂ molecules. This spontaneous polarization exhibited ferroelectric behavior with a nominally high Curie temperature (>400 K), in which the electrical switching of polarization originates from the rotation of the hmtaH₂ unit.     

Ferroelectric behavior and polarization mechanism in odd‐odd polyamide 11,11

Ferroelectric and piezoelectric behavior in odd‐odd polyamide 11,11 are successfully detected for the first time. The maximum coercive field (E_c) of 90 MV/m, and a remnant polarization (P_r) of 40 mC/m² are obtained at room temperature. A piezoelectric strain coefficient (d₃₃) value as high as ?3.9 pC/N has been found in stretched polyamide 11,11 film. The structural change of samples before and after poling is investigated by wide‐angle X‐ray diffraction patterns, differential scanning calorimetry, and Fourier transform infrared spectroscopy. The results indicate that the nature of the ferroelectricity originates from amide group dipoles in the γ‐form crystal regions. Hysteresis behavior appears to result from the crystallites reversal mechanism. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1094–1099     

Switchable Charge‐Transfer in the Photoelectrochemical Energy‐Conversion Process of Ferroelectric BiFeO3 Photoelectrodes

Instead of conventional semiconductor photoelectrodes, herein, we focus on BiFeO₃ ferroelectric photoelectrodes to break the limits imposed by common semiconductors. As a result of their prominent ferroelectric properties, the photoelectrodes are able to tune the transfer of photo‐excited charges generated either in BiFeO₃ or the surface modifiers by manipulating the poling conditions of the ferroelectric domains. At 0 V vs Ag/AgCl, the photocurrent could be switched from 0 μA cm^?2 to 10 μA cm^?2 and the open‐circuit potential changes from 33 mV to 440 mV, when the poling bias of pretreatment is manipulated from ?8 V to +8 V. Additionally, the pronounced photocurrent from charge injection of the excited surface modifiers could be quenched by switching the poling bias from +8 V to ?8 V.     

A High‐Temperature Molecular Ferroelectric Zn/Dy Complex Exhibiting Single‐Ion‐Magnet Behavior and Lanthanide Luminescence

Multifunctional molecular ferroelectrics are exciting materials synthesized using molecular chemistry concepts, which may combine a spontaneous electrical polarization, switched upon applying an electric field, with another physical property. A high‐temperature ferroelectric material is presented that is based on a chiral Zn²⁺/Dy³⁺ complex exhibiting Dy³⁺ luminescence, optical activity, and magnetism. We investigate the correlations between the electric polarization and the crystal structure as well as between the low‐temperature magnetic slow relaxation and the optical properties.     

Room‐Temperature Ferroelectric Material Composed of a Two‐Dimensional Metal Halide Double Perovskite for X‐ray Detection

Although two‐dimensional (2D) metal–halide double perovskites display versatile physical properties due to their huge structural compatibility, room‐temperature ferroelectric behavior has not yet been reported for this fascinating family. Here, we designed a room‐temperature ferroelectric material composed of 2D halide double perovskites, (chloropropylammonium)₄AgBiBr₈, using an organic asymmetric dipolar ligand. It exhibits concrete ferroelectricity, including a Curie temperature of 305 K and a notable spontaneous polarization of ≈3.2 μC cm^?2, triggered by dynamic ordering of the organic cation and the tilting motion of heterometallic AgBr₆/BiBr₆ octahedra. Besides, the alternating array of inorganic perovskite sheets and organic cations endows large mobility‐lifetime product (μτ=1.0×10^?3 cm² V^?1) for detecting X‐ray photons, which is almost tenfold higher than that of CH₃NH₃PbI₃ wafers. As far as we know, this is the first study on an X‐ray‐sensitive ferroelectric material composed of 2D halide double perovskites. Our findings afford a promising platform for exploring new ferroelectric materials toward further device applications.     

Alloying n‐Butylamine into CsPbBr3 To Give a Two‐Dimensional Bilayered Perovskite Ferroelectric Material

Cesium‐lead halide perovskites (e.g. CsPbBr₃) have gained attention because of their rich physical properties, but their bulk ferroelectricity remains unexplored. Herein, by alloying flexible organic cations into the cubic CsPbBr₃, we design the first cesium‐based two‐dimensional (2D) perovskite ferroelectric material with both inorganic alkali metal and organic cations, (C₄H₉NH₃)₂CsPb₂Br₇ ( 1 ). Strikingly, 1 shows a high Curie temperature (T_c=412 K) above that of BaTiO₃ (ca. 393 K) and notable spontaneous polarization (ca. 4.2 μC cm^?2), triggered by not only the ordering of organic cations but also atomic displacement of inorganic Cs⁺ ions. To our knowledge, such a 2D bilayered Cs⁺‐based metal–halide perovskite ferroelectric material with inorganic and organic cations is unprecedented. 1 also shows photoelectric semiconducting behavior with large “on/off” ratios of photoconductivity (>10³).     

Nano‐Ferroelectric for High Efficiency Overall Water Splitting under Ultrasonic Vibration

Piezocatalysis, converting mechanical vibration into chemical energy, has emerged as a promising candidate for water‐splitting technology. However, the efficiency of the hydrogen production is quite limited. We herein report well‐defined 10 nm BaTiO₃ nanoparticles (NPs) characterized by a large electro‐mechanical coefficient which induces a high piezoelectric effect. Atomic‐resolution high angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) and scanning probe microscopy (SPM) suggests that piezoelectric BaTiO₃ NPs display a coexistence of multiple phases with low energy barriers and polarization anisotropy which results in a high electro‐mechanical coefficient. Landau free energy modeling also confirms that the greatly reduced polarization anisotropy facilitates polarization rotation. Employing the high piezoelectric properties of BaTiO₃ NPs, we demonstrate an overall water‐splitting process with the highest hydrogen production efficiency hitherto reported, with a H₂ production rate of 655 μmol g^?1 h^?1, which could rival excellent photocatalysis system. This study highlights the potential of piezoelectric catalysis for overall water splitting.     

An Above‐Room‐Temperature Ferroelectric Organo–Metal Halide Perovskite: (3‐Pyrrolinium)(CdCl3)

Hybrid organo–metal halide perovskite materials, such as CH₃NH₃PbI₃, have been shown to be some of the most competitive candidates for absorber materials in photovoltaic (PV) applications. However, their potential has not been completely developed, because a photovoltaic effect with an anomalously large voltage can be achieved only in a ferroelectric phase, while these materials are probably ferroelectric only at temperatures below 180 K. A new hexagonal stacking perovskite‐type complex (3‐pyrrolinium)(CdCl₃) exhibits above‐room‐temperature ferroelectricity with a Curie temperature T_c=316 K and a spontaneous polarization P_s=5.1 μC cm^?2. The material also exhibits antiparallel 180° domains which are related to the anomalous photovoltaic effect. The open‐circuit photovoltage for a 1 mm‐thick bulky crystal reaches 32 V. This finding could provide a new approach to develop solar cells based on organo–metal halide perovskites in photovoltaic research.     

A Displacive‐Type Metal Crown Ether Ferroelectric Compound: Ca(NO3)2(15‐crown‐5)

Following the Curie symmetry principle and Aizu rule, we discovered there is a centrosymmetric‐to‐noncentrosymmetric phase transition in Ca(NO₃)₂(15‐crown‐5) at T_c=205 K. The transition was confirmed by differential scanning calorimetry and second harmonic generation measurements. The transition gives rise to excellent ferroelectricity, such as a giant dielectric anomaly, with faster polarization switching (5×10^?5 s) of up to 10⁷ times without showing fatigue. The ferroelectric mechanism is attributable to the coordination environmental distortion of the central Ca atom. This finding can throw light on the further research in metal–organic ferroelectrics.