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.     

A Semiconducting Organic–Inorganic Hybrid Perovskite‐type Non‐ferroelectric Piezoelectric with Excellent Piezoelectricity

Piezoelectric materials are a class of important functional materials applied in high‐voltage sources, sensors, vibration reducers, actuators, motors, and so on. Herein, [(CH₃)₃S]₃[Bi₂Br₉]( 1 ) is a brilliant semiconducting organic–inorganic hybrid perovskite‐type non‐ferroelectric piezoelectric with excellent piezoelectricity. Strikingly, the value of the piezoelectric coefficient d₃₃ is estimated as ≈18 pC N^?1. Such a large piezoelectric coefficient in non‐ferroelectric piezoelectric has been scarcely reported and is comparable with those of typically one‐composition non‐ferroelectric piezoelectrics such as ZnO (3pC N^?1) and much greater than those of most known typical materials. In addition, 1 exhibits semiconducting behavior with an optical band gap of ≈2.58 eV that is lower than the reported value of 3.37 eV for ZnO. This discovery opens a new avenue to exploit molecular non‐ferroelectric piezoelectric and should stimulate further exploration of non‐ferroelectric piezoelectric due to their high stability and low loss characteristics.     

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.     

High‐Tc Enantiomeric Ferroelectrics Based on Homochiral Dabco‐derivatives (Dabco=1,4‐Diazabicyclo[2.2.2]octane)

1,4‐Diazabicyclo[2.2.2]octane (dabco) and its derivatives have been extensively utilized as building units of excellent molecular ferroelectrics for decades. However, the homochiral dabco‐based ferroelectric remains a blank. Herein, by adding a methyl (Me) group accompanied by the introduction of homochirality to the [H₂dabco]²⁺ in the non‐ferroelectric [H₂dabco][TFSA]₂ (TFSA=bis(trifluoromethylsulfonyl)ammonium), we successfully designed enantiomeric ferroelectrics [R and S‐2‐Me‐H₂dabco][TFSA]₂. The two enantiomers show two sequential phase transitions with transition temperature (T_c) as high as 405.8 K and 415.8 K, which is outstanding in both dabco‐based ferroelectrics and homochiral ferroelectrics. To our knowledge, [R and S‐2‐Me‐H₂dabco][TFSA]₂ are the first examples of dabco‐based homochiral ferroelectrics. This finding opens an avenue to construct dabco‐based homochiral ferroelectrics and will inspire the exploration of more eminent enantiomeric molecular ferroelectrics.     

A Molecular Thermochromic Ferroelectric

Molecular ferroelectrics have attracted considerable interests because of their easy and environmentally friendly processing, low acoustical impedance and mechanical flexibility. Herein, a molecular thermochromic ferroelectric, N,N′‐dimethyl‐1,4‐diazoniabicyclo[2.2.2]octonium tetrachlorocuprate(II) ([DMe‐DABCO]CuCl₄) is reported, which shows both excellent ferroelectricity and intriguing thermochromism. [DMe‐DABCO]CuCl₄ undergoes a ferroelectric phase transition from Pca2₁ to Pbcm at a significantly high Curie temperature of 413 K, accompanied by a color change from yellow to red that is due to the remarkable deformation of [CuCl₄]^2? tetrahedron, where the ferroelectric and paraelectric phases correspond to yellow and red, respectively. Combined with multiple bistable physical properties, [DMe‐DABCO]CuCl₄ would be a promising candidate for next‐generation smart devices, and should inspire further exploration of multifunctional molecular ferroelectrics.     

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.     

Efficient Bottom‐Up Preparation of Graphene Nanoribbons by Mild Suzuki–Miyaura Polymerization of Simple Triaryl Monomers

Herein an efficient bottom‐up solution‐phase synthesis of N=9 armchair graphene nanoribbons (GNRs) is described. Catalyzed by Pd(PtBu₃)₂, Suzuki–Miyaura polymerization of a simple and readily available triaryl monomer provides a novel GNR precursor with a high molecular weight and excellent solubility. Upon cyclodehydrogenation, the resulting GNR exhibits semiconducting properties with an approximately 1.1 eV band gap (LUMO: ?3.52 eV; HOMO: ?4.66 eV) as characterized by UV/Vis‐NIR spectroscopy and cyclic voltammetry.     

Exploiting the Bulk Photovoltaic Effect in a 2D Trilayered Hybrid Ferroelectric for Highly Sensitive Polarized Light Detection

Polarized light detection is attracting increasing attention for its wide applications ranging from optical switches to high‐resolution photodetectors. Two‐dimensional (2D) hybrid perovskite‐type ferroelectrics combining inherent light polarization dependence of bulk photovoltaic effect (BPVE) with excellent semiconducting performance present significant possibilities. Now, the BPVE‐driven highly sensitive polarized light detection in a 2D trilayered hybrid perovskite ferroelectric, (allyammonium)₂(ethylammonium)₂Pb₃Br₁₀ ( 1 ), is presented. It shows a superior BPVE with near‐band gap photovoltage of ca. 2.5 V and high on/off switching ratio of current (ca. 10⁴). Driven by the superior BPVE, 1 exhibits highly sensitive polarized light detection with a polarization ratio as high as ca. 15, which is far more beyond than those of structural anisotropy‐based monocomponent devices. This is the first realization of BPVE‐driven polarized light detection in hybrid perovskite ferroelectrics.     

Bandgap engineering of indenofluorene‐based conjugated copolymers with pendant donor‐π‐acceptor chromophores for photovoltaic applications

Three narrow‐band‐gap conjugated copolymers based on indenofluorene and triphenylamine with pendant donor‐π‐acceptor chromophores were successfully synthesized by post‐functionalization approach. All the polymers have good solubility in common solvents and excellent thermal stability. The photophysical properties, energy levels and band gaps of the polymers were well manipulated by introducing different acceptor groups onto the end of their conjugated side chains. By using different acceptor groups, the band gaps of the polymers were narrowed from 1.86 to 1.53 eV by lowering their lowest unoccupied molecular orbital levels, whereas their relatively deep highest occupied molecular orbital levels of approximately ?5.35 eV were maintained. Bulk‐heterojunction solar cells with these polymers as electron donors and (6,6)‐phenyl‐C₇₁‐butyric acid methyl ester as acceptor showed power conversion efficiencies as high as 3.1% and high open circuit voltages more than 0.88 eV. The relationships between the performance and film morphology, energy levels, charge mobilities were discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Designing and Constructing a High‐Temperature Molecular Ferroelectric by Anion and Cation Replacement in a Simple Crown Ether Clathrate

Molecular ferroelectrics have displayed a promising future since they are light‐weight, flexible, environmentally friendly and easily synthesized, compared to traditional inorganic ferroelectrics. However, how to precisely design a molecular ferroelectric from a non‐ferroelectric phase transition molecular system is still a great challenge. Here we designed and constructed a molecular ferroelectric by double regulation of the anion and cation in a simple crown ether clathrate, 4 , [K(18‐crown‐6)]⁺[PF₆]^?. By replacing K⁺ and PF₆^? with H₃O⁺ and [FeCl₄]^? respectively, we obtained a new molecular ferroelectric [H₃O(18‐crown‐6)]⁺[FeCl₄]^?, 1 . Compound 1 undergoes a para‐ferroelectric phase transition near 350 K with symmetry change from P21/n to the Pmc21 space group. X‐ray single‐crystal diffraction analysis suggests that the phase transition was mainly triggered by the displacement motion of H₃O⁺ and [FeCl₄]^? ions and twist motion of 18‐crown‐6 molecule. Strikingly, compound 1 shows high a Curie temperature (350 K), ultra‐strong second harmonic generation signals (nearly 8 times of KDP), remarkable dielectric switching effect and large spontaneous polarization. We believe that this research will pave the way to design and build high‐quality molecular ferroelectrics as well as their application in smart materials.     

Experimental Evidence for a Triboluminescent Antiperovskite Ferroelectric: Tris(trimethylammonium) catena‐Tri‐μ‐chloro‐manganate(II) Tetrachloromanganate(II)

The perovskite structure is rich in ferroelectricity. In contrast, ferroelectric antiperovskites have been scarcely confirmed experimentally since the discovery of M₃AB‐type antiperovskites in the 1930s. Ferroelectricity is now revealed in an organic–inorganic hybrid X₃AB antiperovskite structure, which exhibits a clear ferroelectric phase transition 6/mmmF6mm with a high Curie point of 480 K. The physical properties across the phase transition are obviously changed along with the symmetry requirements, providing solid experimental evidence for the ferroelectric phase transition. More interestingly, the discovered antiperovskite shows intense photoluminescence and triboluminescence properties. The confirmation of the triboluminescent ferroelectric antiperovskite will open new avenues to explore excellent optoelectronic properties in the antiperovskite family.     

Structure–Property Relationship of Supramolecular Ferroelectric [H‐66dmbp][Hca] Accompanied by High Polarization,Competing Structural Phases,and Polymorphs

Three polymorphic forms of 6,6′‐dimethyl‐2,2′‐bipyridinium chloranilate crystals were characterized to understand the origin of polarization properties and the thermal stability of ferroelectricity. According to the temperature‐dependent permittivity, differential scanning calorimetry, and X‐ray diffraction, structural phase transitions were found in all polymorphs. Notably, the ferroelectric α‐form crystal, which has the longest hydrogen bond (2.95 Å) among the organic acid/base‐type supramolecular ferroelectrics, transformed from a polar structure (space group, P2₁) into an anti‐polar structure (space group, P2₁/c) at 378 K. The non‐ferroelectric β‐ and γ‐form crystals also exhibited structural rearrangements around hydrogen bonds. The hydrogen‐bonded geometry and ferroelectric properties were compared with other supramolecular ferroelectrics. A positive relationship between the phase‐transition temperature (T_C) and hydrogen‐bond length (<d>) was observed, and was attributed to the potential barrier height for proton off‐centering or order/disorder phenomena. The optimized spontaneous polarization (P_s) agreed well with the results of the first‐principles calculations, and could be amplified by separating the two equilibrium positions of protons with increasing <d>. These data consistently demonstrated that stretching <d> is a promising way to enhance the polarization performance and thermal stability of hydrogen‐bonded organic ferroelectrics.     

Low band‐gap modulation of isoindigo‐based copolymers toward high open‐circuit voltage of polymer solar cells

Designing low band‐gap‐conjugated polymers coupled with low HOMO levels attracts great attention in the field of polymer solar cells (PSCs). By using donor–acceptor (D‐A) copolymerization strategy, we designed and synthesized a series of low band‐gap copolymers with deep HOMO levels via introducing an isoindigo (IID) acceptor unit in the copolymers with the donor unit of fluorene (F) (PIID‐F), carbazole (Cz) (PIID‐Cz), thiophene (Th) (PIID‐Th), dithiophene (DTh) (PIID‐DTh), or dithienosilole (DTS) (PIID‐DTS). The HOMO level of the copolymers, measured by electrochemical cyclic voltammetry, varies from ?5.3 eV to ?5.8 eV, depending on different donor units in the copolymers. However, the LUMO levels of all the copolymers are fixed at about ?3.6 eV, which is mainly determined by IID acceptor unit due to its strong electron‐withdrawing ability. The new results will provide an effect help in designing IID based molecular structures. Among the copolymers, PIID‐DTS has a low band gap of 1.58 eV and possesses a low‐lying HOMO energy level of ?5.33 eV. The PSCs based on PIID‐DTS as donor and PC₇₀BM as acceptor exhibited a high open‐circuit voltage (V_oc) of 0.93 V and a primary power conversion efficiency of 2.45%. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3477–3485     

Sonophotocatalytic treatment of diazinon using visible light‐driven Ce:Cu‐1,4‐BDOAH2 photocatalyst in a batch‐mode process: Response surface methodology and optimization

Cu–1,4‐benzenedioxyacetic acid (Cu‐1,4‐BDOAH₂) with a narrow band gap (2.52 eV) was synthesized and doped with Ce to afford Ce:Cu‐1,4‐BDOAH₂ as an efficient photocatalyst with narrower band gap (2.39 eV). The prepared Cu‐1,4‐BDOAH₂ and Ce:Cu‐1,4‐BDOAH₂ were characterized using Fourier transform infrared, energy‐dispersive X‐ray, diffuse reflectance spectroscopies, scanning electron microscopy and X‐ray diffraction. The sonophotocatalytic degradation of diazinon was carried out in a batch‐mode reactor using visible light‐driven Ce:Cu‐1,4‐BDOAH₂ photocatalyst as well as ultrasonic irradiation. The narrow band gap of the photocatalyst means that it can be activated under visible light illumination. The effects of operational parameters such as initial diazinon concentration (5–25 mg l^?1), pH (2–10), photocatalyst dosage (10–30 mg) and irradiation time (10–30 min) on the sonophotocatalytic degradation efficiency were investigated using central composite design under response surface methodology. The optimization process was studied using desirability function and the results indicated 99.8% degradation, which was obtained at optimum values of 25 mg l^?1, 6, 20 mg and 20 min for the initial concentration of diazinon, pH, photocatalyst dosage and irradiation time, respectively. Reusability experiments of Ce:Cu‐1,4‐BDOAH₂ photocatalyst showed that it is quite stable with excellent catalytic activity even after five cycles.     

Two‐Dimensional Haeckelite NbS2: A Diamagnetic High‐Mobility Semiconductor with Nb4+ Ions

In all known Group 5 transition‐metal dichalcogenide monolayers (MLs), the metal centers carry a spin, and their ground‐state phases are either metallic or semiconducting with indirect band gaps. Here, on grounds of first‐principles calculations, we report that the Haeckelite polytypes 1 S ‐NbX₂ (X=S, Se, Te) are diamagnetic direct‐band‐gap semiconductors even though the Nb atoms are in the 4+ oxidation state. In contrast, 1 S ‐VX₂ MLs are antiferromagnetically coupled indirect‐band‐gap semiconductors. The 1 S phases are thermodynamically and dynamically stable but of slightly higher energy than their 1 H and 1 T ML counterparts. 1 S ‐NbX₂ MLs are excellent candidates for optoelectronic applications owing to their small band gaps (between 0.5 and 1 eV). Moreover, 1 S ‐NbS₂ shows a particularly high hole mobility of 2.68×10³ cm² V⁻¹ s⁻¹, which is significantly higher than that of MoS₂ and comparable to that of WSe₂.     

Experiment‐Driven Modeling of Crystalline Phosphorus Nitride P3N5: Wide‐Ranging Implications from a Unique Structure

Nitridophosphates have emerged as advanced materials due to their structural variability and broad technical applicability. Their binary parent compound P₃N₅, a polymeric network of corner‐ and edge‐sharing PN₄ tetrahedra with N 2 and N 3 sites, is a particularly interesting example. We present a study of the band gap and electronic structure of α‐P₃N₅ by using soft X‐ray spectroscopy measurements and DFT calculations. The band gap, which is crucial for all applications, is measured to be 5.87±0.20 eV. This agrees well with the calculated, indirect band gap of 5.21 eV. The density of states are found to show dramatic variation between the nonequivalent N sites and a high degree of covalency. Coupled to these results is what is, to our knowledge, the largest core hole shift reported to date for a soft X‐ray absorption spectrum. We propose an intuitive bonding scheme for α‐P₃N₅ that explains the observed band gap and unique density of states, while providing a framework for predicting these properties in known and yet to be discovered PN compounds. We briefly consider the implications of these results for new low‐dimensional P and PN materials, which alongside graphene, could become important materials for nanoelectronics.     

A Homochiral Fulgide Organic Ferroelectric Crystal with Photoinduced Molecular Orbital Breaking

Thermally triggered spatial symmetry breaking in traditional ferroelectrics has been extensively studied for manipulation of the ferroelectricity. However, photoinduced molecular orbital breaking, which is promising for optical control of ferroelectric polarization, has been rarely explored. Herein, for the first time, we synthesized a homochiral fulgide organic ferroelectric crystal (E)-(R)-3-methyl-3-cyclohexylidene-4-(diphenylmethylene)dihydro-2,5-furandione ( 1 ), which exhibits both ferroelectricity and photoisomerization. Significantly, 1 shows a photoinduced reversible change in its molecular orbitals from the 3 π molecular orbitals in the open-ring isomer to 2 π and 1 σ molecular orbitals in the closed-ring isomer, which enables reversible ferroelectric domain switching by optical manipulation. To our knowledge, this is the first report revealing the manipulation of ferroelectric polarization in homochiral ferroelectric crystal by photoinduced breaking of molecular orbitals. This finding sheds light on the exploration of molecular orbital breaking in ferroelectrics for optical manipulation of ferroelectricity.     

Halide Double Perovskite Ferroelectrics

Halide double perovskites have recently emerged as a promising environmentally friendly optoelectronic and photovoltaic material for their inherent thermodynamic stability, high defect tolerance, and appropriate band gaps. However, to date, no ferroelectric material based on halide double perovskites has been discovered. Herein, by hetero‐substitution of lead and cation intercalation of n‐propylamine, the first halide double perovskite ferroelectric, (n‐propylammonium)₂CsAgBiBr₇ ( 1 ), is reported and it exhibits distinct ferroelectricity with a notable saturation polarization of about 1.5 μC cm^?2. More importantly, single‐crystal photodetectors of 1 exhibit extraordinary performance with containing high on/off ratios of about 10⁴, fast response rates of 141 μs, and detectivity as high as 5.3×10¹¹ Jones. This finding opens a new way to design high‐performance perovskite ferroelectrics, and provides a viable approach in the search for stable and lead‐free optoelectronic materials as an alternative to the lead‐containing system.     

Lead‐Free Halide Double Perovskite Cs2AgBiBr6 with Decreased Band Gap

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs₂AgBiBr₆, shows attractive optical and electronic features, making it promising for high‐efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal‐engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs₂AgBiBr₆ under ambient conditions. The band‐gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first‐principles calculations indicate that enhanced Ag–Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band‐gap narrowing effect. This work provides new insights for achieving lead‐free double perovskites with suitable band gaps for optoelectronic applications.     

Dithieno[3,2‐b:2′,3′‐d]pyrrole–alkylthiophene–benzo[c][1,2,5]thiadiazole‐based highly stable and low band gap polymers for polymer light‐emitting diodes

A series of new low band gap π‐conjugated polymers containing N‐alkyldithieno[3,2‐b:2′,3′‐d]pyrrole, benzo[c][1,2,5]thiadiazole, and alkylthiophenes are reported. The polymerization condition was standardized and the use of CuO to obtain high‐molecular‐weight polymer was also realized. The molecular weight of the polymers was found to be in the range of 45,000–53,000. All the polymers were found to be soluble in most of the common organic solvents, such as chloroform, dichloromethane, THF, and chlorobenzene with excellent film forming properties. The λ_max of the polymers was found to be in the range of 687–663 nm with band gap in the range of 1.35–1.43 eV. The oxidation potential of the polymers from cyclic voltammetry was determined to be 0.5–0.75 V. The HOMO levels of the above synthesized polymers were found to be between 5.24 and 5.54 eV. All the polymers exhibited a PL emission in between 755 and 773 nm. The polymers were found to be thermally stable above 277 °C with only a 5% weight loss. From the thermal stability values, it is expected that the current set of polymers are stable enough for the application in electronic devices. To realize the potential use of the polymers, EL devices were fabricated and found to show red emission with comparatively low threshold voltage. A brightness of 54 cd m⁻² for the device with polymer PC could be reached. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6514–6525, 2009