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Polar molecular crystals hold a promise for controlling bulk physical properties originated in their unique switchable polarity via structural transformation. However, the mechanisms for switching polarization are mainly limited to displacive and disorder-order phase transitions, which rarely involve the reconstruction of chemical bonds. Here, we have switched and tuned electric polarization in a bimetallic halide, (Neopentylammonium)4AgBiBr8 (1), as verified by light-excited pyroelectric effect. Most notably, its Ag-Br coordination bonds show a zipper-like dynamic switching behavior from the 'locked' to 'unlocked' state, namely, reconstruction of chemical bonds. Coupling with the dynamic ordering of organic cations, this bond-switching transition makes a contribution to switchable polarization of 1. As expected, its polarity creates pyroelectric effect for self-driven X-ray detection with high sensitivity (3.8×103 μC Gy-1 cm-2) and low limit of detection (4.8 nGy s-1). This work on the bond-switching mechanism provides an avenue to design polar molecular candidate for smart optoelectronic devices., (© 2024 Wiley‐VCH GmbH.)

The concept of ferroelectric polarons is proposed to partially explain the exceptional optoelectronic properties observed in lead halide perovskites (LHPs). It is intriguing but unclear how this proposal, which involves local or transient polarizations, applies in general to 2D LHPs with long-range ferroelectricity. Here, this work presents a pioneering time-domain experimental investigation of polarons in ferroelectric (IA) 2 (MA) 2 Pb 3 Br 10 (IMPB; IA is isoamylammonium and MA is methylammonium) using transient absorption spectroscopy. Compared to non-ferroelectric LHPs, IMPB exhibits several distinct polaronic properties closely associated with macroscopic polarizations of ferroelectricity, including a prolonged polaron formation time (≈1.1 ps), a Stark splitting of the bleaching (≈63 meV), and a giant polaron Mott density (≈7.6 × 10 18 cm -3 ). These findings broaden the realm of 2D polaron systems and reveal the decisive role of static/unidirectional polarizations on polaron physics in 2D LHPs., (© 2024 The Author(s). Advanced Science published by Wiley‐VCH GmbH.)

Birefringent crystal, which has the capacity to manipulate polarization light, holds an indispensable position in optics and optoelectronics, while it remains challenging to fulfill the modulation of birefringence. Here, we present wide spectral photo-pyroelectric effect in a silver-based hybrid pyroelectric, (N-CHM)Ag 2 I 3 (N-CHM=N-cyclohexylmethylamine), serving as a feasible strategy to regulate birefringence through light stimuli. As the first silver-based hybrid pyroelectric, (N-CHM)Ag 2 I 3 exhibits strong room-temperature photo-pyroelectricity with a large polarization of ~3.23 μC/cm 2 and high voltage responsivity of ~0.96 m 2 /C across the UV-NIR spectral region. Strikingly, the photomodulation of its in-plane birefringence is established through pyroelectric effect, giving a saturation value of ~1.68×10 -2 that is among the highest level achieved to date. This study on the birefringence photomodulation of lead-free hybrid pyroelectric is anticipated to boost future development of new smart optical and optoelectronic devices., (© 2024 Wiley-VCH GmbH.)

Antiferroelectric (AFE) has emerged as a promising branch of electroactive materials, due to intriguing physical attributes stemming from the electric field-induced antipolar-to-polar phase transformation. However, the requirement of extremely high electric field strength to switch adjacent sublattice polarization poses great challenges for exploiting new molecular AFE system. Although photoirradiation is striking as a noncontact and nondestructive manipulation tool to optimize physical properties, optical control of antiferroelectricity still remains unexplored. Here, by adopting light-sensitive I 3 - anion into 2D perovskite family, we design a new I 3 - -intercalated molecular AFE of (t-ACH) 2 EA 2 Pb 3 I 10 (I 3 ) 0.5  ⋅ ((H 3 O)(H 2 O)) 0.5 (1, t-ACH=trans-4-aminomethyl-1-cyclohexanecarboxylate, EA=ethylammonium). The I 3 - -intercalating gives an ultra-narrow band gap of 1.65 eV with strong absorption. In term of AFE structure, the anti-parallel alignment of electric dipoles results in a large spontaneous polarization of 4.3 μC/cm 2 . Strikingly, 1 merely shows AFE behaviour in the dark even under ultrahigh voltage, while the field-induced ferroelectric state can be facilely obtained upon visible illumination. Such unprecedented visible-photo-assisted phase switching ascribes to the incorporation of photoactive I 3 - anions that reduces AFE-to-ferroelectric switching barrier. This pioneering work on the photo-assisting transformation of ferroic orders paves a way to develop future photoactive materials with potential applications., (© 2024 Wiley-VCH GmbH.)

2D organic-inorganic hybrid perovskites (OIHPs) have become one of the hottest research topics due to their excellent environmental stability and unique optoelectronic properties. Recently, the ferroelectricity and thermochromism of 2D OIHPs have attracted increasing interests. Integrating ferroelectricity and thermochromism into perovskites can significantly promote the development of multichannel intelligent devices. Here, a novel 2D Dion-Jacobson OIHP of the formula (3AMP)PbI 4 (where 3AMP is 3-(aminomethyl)pyridinium) is reported, which has a remarkable spontaneous polarization value (Ps) of 15.6 µC cm -2 and interesting thermochromism. As far it is known, such a large Ps value is the highest for 2D OIHPs recorded so far. These findings will inspire further exploration and application of multifunctional perovskites., (© 2023 Wiley‐VCH GmbH.)

Metal-free molecular antiferroelectric (AFE) holds a promise for energy storage on account of its unique physical attributes. However, it is challenging to explore high-curie temperature (T c ) molecular AFEs, due to the lack of design strategies regarding the rise of phase transition energy barriers. By renewing the halogen substitution strategy, we have obtained a series of high-T c molecular AFEs of the halogen-substituted phenethylammonium bromides (x-PEAB, x=H/F/Cl/Br), resembling the binary stator-rotator system. Strikingly, the p-site halogen substitution of PEA + cationic rotators raises their phase transition energy barrier and greatly enhances T c up to ~473 K for Br-PEAB, on par with the record-high T c values for molecular AFEs. As a typical case, the member 4-fluorophenethylammonium bromide (F-PEAB) shows notable AFE properties, including high T c (~374 K) and large electric polarization (~3.2 μC/cm 2 ). Further, F-PEAB also exhibits a high energy storage efficiency (η) of 83.6 % even around T c , catching up with other AFE oxides. This renewing halogen substitution strategy in the molecular AFE system provides an effective way to design high-T c AFEs for energy storage devices., (© 2024 Wiley‐VCH GmbH.)

Photorefractive effect of ferroelectrics refers to the light-induced change of refractive index, which is an optical controlling avenue in holographic storage and image processing. For most ferroelectrics, however, the small photorefractive effect (10 -5 -10 -4 ) hinders their practical application and it is urgent to exploit new photorefractive system. Here, for the first time, strong photorefractive effects are achieved in a 2D metal-halide ferroelectric, [CH 3 (CH 2 ) 3 NH 3 ] 2 (CH 3 NH 3 )Pb 2 Cl 7 (1), showing large spontaneous polarization (≈4.1 µC cm -2 ) and wide optical bandgap (≈3.20 eV). Notably, under light irradiation, 1 enables a large variation of refractive indices up to ≈ 1× 10 -3 , being one order higher than the existing materials and comparable to the state-of-the-art inorganic ferroelectrics. This intriguing photorefractive behavior involves with the sharp variation of polarization caused by photo-pyroelectricity. As the first report of 2D metal-halide photorefractive ferroelectric, this work sheds light on optical controlling of physical properties in electric-ordered materials., (© 2023 Wiley-VCH GmbH.)

Ferroelectric photovoltaics driven by spontaneous polarization (P s ) holds a promise for creating the next-generation optoelectronics, spintronics and non-volatile memories. However, photoactive ferroelectrics are quite scarce in single homogeneous phase, owing to the severe P s fatigue caused by leakage current of photoexcited carriers. Here, through combining inorganic and organic components as building blocks, we constructed a series of ferroelectric semiconductors of 2D hybrid perovskites, (HA) 2 (MA) n-1 Pb n Br 3n+1 (n=1-5; HA=hexylamine and MA=methylamine). It is intriguing that their Curie temperatures are greatly enhanced by reducing the thickness of inorganic frameworks from MAPbBr 3 (n=∞, T c =239 K) to n=2 (T c =310 K, ΔT=71 K). Especially, on account of the coupling of room-temperature ferroelectricity (P s ≈1.5 μC/cm 2 ) and photoconductivity, n=3 crystal wafer was integrated as channel field effect transistor that shows excellent a large short-circuit photocurrent ≈19.74 μA/cm 2 . Such giant photocurrents can be modulated through manipulating gate voltage in a wide range (±60 V), exhibiting gate-tunable memory behaviors of three current states ("-1/0/1" states). We believe that this work sheds light on further exploration of ferroelectric materials toward new non-volatile memory devices., (© 2023 Wiley-VCH GmbH.)

Photoferroelectrics, especially emerging halide perovskite ferroelectrics, have motivated tremendous interests owing to their fascinating bulk photovoltaic effect (BPVE) and cross-coupled functionalities. However, solid solutions of halide perovskite photoferroelectrics with controllable structure and enhanced performance are scarcely explored. Herein, through mixing cage cation, a homogeneous halide perovskite photoferroelectric PA 2 FA x MA 1-x Pb 2 Br 7 solid solution (PA, FA and MA are CH 3 CH 2 CH 2 NH 3 + , NH 2 CHNH 2 + and CH 3 NH 3 + , 0≤x≤1) has been developed, which demonstrates tunable Curie temperature in a wide range of 263-323 K and excellent optoelectrical features. As the component adjusted to x=0.7, the bulk crystal demonstrates ultrahigh pyroelectric coefficient up to 1.48 μC cm -2  K -1 around room temperature. Strikingly, benefiting from the light-induced pyroelectricity and remarkable BPVE, a self-powered and sensitive photodetector based solid solution crystals with boosted responsivity and detectivity over than 1300 % has been achieved. This pioneering work sheds light on the exploration of photoferroelectric solid solutions towards high-performance photoelectronic devices., (© 2023 Wiley-VCH GmbH.)

2D Dion-Jacobson (DJ) phase hybrid perovskites have shown great promise in the photoelectronic field owing to their outstanding optoelectronic performance and superior structural rigidity. However, DJ phase lead-free double perovskites are still a virgin land with direct X-ray detection. Herein, we have designed and synthesized a new DJ phase lead-free layered double perovskite of (HIS) 2 AgSbBr 8 (1, HIS 2+  = histammonium). Centimeter-sized (18 × 10 × 5 mm 3 ) single crystals of 1 are successfully grown via the temperature cooling technique, exhibiting remarkable semiconductive characteristics such as a high resistivity (2.2 × 10 11  Ω cm), a low trap state density (3.56 × 10 10 cm -3 ), and a large mobility-lifetime product (1.72 × 10 -3 cm 2 V -1 ). Strikingly, its single-crystal-based X-ray detector shows a high sensitivity of 223 µC Gy -1 air cm -2 under 33.3 V mm -1 , a low detection limit (84.2 nGy air s -1 ) and superior anti-fatigue. As far as we know, we firstly demonstrates the potential of 2D DJ phase lead-free hybrid double perovskite in X-ray detection, showing excellent photoelectric response and operational stability. This work will pave a promising pathway to the innovative application of hybrid perovskites for eco-friendly and efficient X-ray detection., (© 2023 Wiley-VCH GmbH.)

The photopyroelectric effect in ferroelectrics has shown great potential for application in infrared detection and imaging. One particular subclass is broadband with dielectric bistability, which allows for large pyroelectric figures-of-merit (FOMs). Herein, an improper high-T c perovskite ferroelectric, (IA) 2 (EA) 2 Pb 3 Cl 10 (1, where IA is isoamylammonium and EA is ethylammonium) is presented, in which spontaneous polarization (P s ) stems from the dynamic ordering of organic cations and the tilting of distorted PbCl 6 octahedra. Notably, 1 displays unusual dielectric bistability with small variations in the temperature-dependent dielectric constants near T c  = 392 K; this bistable attribute endows large pyroelectric FOMs with peak voltage efficiency (F V  = 1.7×10 -2  cm 2 µC -1 ) and sensitivity (F D  = 3.9×10 -4 Pa -1/2 ). These F V and F D parameters, beyond those of their proper counterparts, make 1 a promising candidate for infrared photodetection. As expected, the broadband photopyroelectric effects observed in 1 covered the ultraviolet to infrared-II spectral region (266-1950 nm). Such P s -directed photoactivities overcome the optical bandgap limitation and allow for wide-wave photodetection. As an innovative study on improper ferroelectricity, light is shaded here on the targeted engineering of new electrically ordered candidate materials for smart optoelectronic devices., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Two-dimensional (2D) hybrid perovskite materials have been widely used for polarization-sensitive photodetection due to their fascinating optical and physical attributes. However, studies on those materials that enable strong polarized-light activities under a weak-light condition remain quite scarce. Here, by tailoring aromatic cation into 3D prototype, we have successfully obtained a new 2D hybrid perovskite, (FPEA) 2 (MA)Pb 2 Br 7 (1, where FPEA is 4-fluorophenethylammonium and MA is methylammonium). The alternative alignment of inorganic and organic structural components results in significant anisotropy, including optical absorption and electric conductivity. The coupling effect of these anisotropic properties in 1 gives rise to strong dichroic activities toward detecting polarized light. Especially, under weak light intensity (∼330 nW/cm 2 ), it can still generate a large polarization ratio up to 1.35, which is even higher than those of some typical 2D materials (i. e., GeSe ∼1.09). Besides, single crystal-based photodetector of 1 displays fascinating detecting performances, including large photocurrent on/off ratio (∼10 4 ), fast response time (∼154/182 μs) and excellent antifatigued stability. These findings disclose the potentials of 1 as a robust candidate for detecting weak polarized light, which has practical applications in the field of polarized optoelectronics., (© 2023 Wiley-VCH GmbH.)

2D Fe-chalcogenides have drawn significant attention due to their unique structural phases and distinct properties in exploring magnetism and superconductivity. However, it remains a significant challenge to synthesize 2D Fe-chalcogenides with specific phases in a controllable manner since Fe-chalcogenides have multiple phases. Herein, a molecular sieve-assisted strategy is reported for synthesizing ultrathin 2D iron sulfide on substrates via the chemical vapor deposition method. Using a molecular sieve and tuning growth temperatures to control the partial pressures of precursor concentrations, hexagonal FeS, tetragonal FeS, and non-stoichiometric Fe 7 S 8 nanoflakes can be precisely synthesized. The 2D h-FeS, t-FeS, and Fe 7 S 8 have high conductivities of 5.4 × 10 5 S m -1 , 5.8 × 10 5 S m -1 , and 1.9 × 10 6 S m -1 . 2D tetragonal FeS shows a superconducting transition at 4 K. The spin reorientation at ≈30 K on the non-stoichiometric Fe 7 S 8 nanoflakes with ferrimagnetism up to room temperature has also been observed. The controllable synthesis of various phases of 2D iron sulfide may provide a route for synthesizing other 2D compounds with various phases., (© 2023 Wiley-VCH GmbH.)

As a momentum-independent spin configuration, persistent spin texture (PST) could avoid spin relaxation and play an advantageous role in spin lifetime. Nevertheless, manipulation of PST is a challenge due to the limited materials and ambiguous structure-property relationships. Herein, we present electrically switchable PST in a new 2D perovskite ferroelectric, (PA) 2 CsPb 2 Br 7 (where PA is n-pentylammonium), which has a high Curie temperature of 349 K, evident spontaneous polarization (3.2 μC cm -2 ) and a low coercive electric field of 5.3 kV cm -1 . The combination of symmetry-breaking in ferroelectrics and effective spin-orbit field facilitates intrinsic PST in the bulk and monolayer structure models. Strikingly, the directions of spin texture are reversible by switching the spontaneous electric polarization. This electric switching behavior relates to the tilting of PbBr 6 octahedra and the reorientation of organic PA + cations. Our studies on ferroelectric PST of 2D hybrid perovskites afford a platform for electrical spin texture manipulation., (© 2023 Wiley-VCH GmbH.)

The bulk anomalous photovoltaic (BAPV) effect of acentric materials refers to a distinct concept from traditional semiconductor-based devices, of which the above-bandgap photovoltage hints at a promise for solar-energy conversion. However, it is still a challenge to exploit new BAPV-active systems due to the lacking of knowledge on the structural origin of this concept. BAPV effects in single crystals of a 2D lead-free double perovskite, (BBA) 2 CsAgBiBr 7 (1, BBA = 4-bromobenzylammonium), tailored by mixing aromatic and alkali cations in the confined architecture to form electric polarization are acquired here. Strikingly, BAPV effects manifested by above-bandgap photovoltage (V OC ) show unique attributes of directional anisotropy and positive dependence on electrode spacing. The driving source stems from orientations of the polar aromatic spacer and Cs + ion drift, being different from the known built-in asymmetry photovoltaic heterojunctions. As the first demonstration of the BAPV effect in the double perovskites, the results will enrich the family of environmentally green BAPV-active candidates and further facilitate their new optoelectronic application., (© 2023 Wiley-VCH GmbH.)

Broadband photodetection has shown a great promise for diverse applications, while the realization of plateau photoresponse from ultraviolet (UV) to near-infrared (NIR) spectral region is very challenging. Herein, we exploit photoexcited pyroelectric effect in a chiral hybrid perovskite, (N, N-dimethylcyclohexylammonium)PbBr 3 (1), serving as a new pathway to drive broadband photoactivities. It is a room-temperature pyroelectric with large polarization of ≈6.4 μC cm -2 and high pyroelectric figure-of-merits (F V =1.0×10 -2  cm 2   μC -1 and F D =7.1×10 -5  Pa -1/2 ). Strikingly, light-induced pyroelectric effect arising from spontaneous polarization is observed in 1, which cover UV (266 nm) to NIR-II (1950 nm) full spectral region. The broadband photoresponses actualized by pyroelectricity break the limit of optical band gap. As the first demonstration of photo-pyroelectricity covering UV-to-NIR spectral region in hybrid perovskites, this work paves a pathway to assemble high-performance smart devices., (© 2022 Wiley-VCH GmbH.)

A mixed-cation 3D lead bromide hybrid perovskite (NMDAP) 2 FAPb 4 Br 13 (1, NMDAP 2+ = N-methyl-1,3-diaminopropanium, FA + = formamidinium) is tailored by incorporating the large NMDAP 2+ cation with the small FA + ion into 3D FAPbBr 3 . Structurally, the small FA + ions occupy the prototypical cavities formed by distorted corner-sharing PbBr 6 octahedra as in FAPbBr 3 , while the bulky NMDAP 2+ ions are confined to the expanded lattice which is defined by a new structural motif composed of eight corner-sharing PbBr 6 octahedra and four edge-sharing octahedra. This localized lattice expansion from FAPbBr 3 creates a new branch of the intriguing 3D hybrid perovskite family, breaking the limits of the conventional Goldschmidt tolerance factor rule. Moreover, a high two-photon absorption (2AP) coefficient of 60.8 cm MW -1 was demonstrated to near-infrared (NIR) 800 nm illumination, enabling sensitive NIR photoresponses with large on/off ratio (≈10 4 ) as well as favorable stability., (© 2022 Wiley-VCH GmbH.)

Polarized-light detection in the deep ultraviolet solar-blind region is indispensable for optoelectronic applications. 2D hybrid perovskite ferroelectrics with inherent anisotropy and bulk photovoltaic effect (BPVE) have become a robust candidate in this portfolio. We here report a wide band-gap 2D perovskite ferroelectric, (isobutylammonium) 2 (methylamium)Pb 2 Cl 7 (1), in which the BPVE behaves as self-driving source for solar-blind ultraviolet polarized-light detection. It exhibits a large dichroism ratio (≈15.7) and superior BPVE-directed photocurrent (≈3.6 μA cm -2 , under 266 nm). These attributes facilitate excellent self-powered polarized-light detection of high on/off contrast (≈10 3 ) and polarization ratio (≈2.5), beyond those of inorganic oxides (e.g., ZnO, 1.47; GaN, 1.38). This study highlights potential of 2D perovskite ferroelectrics for new optoelectronic application., (© 2022 Wiley-VCH GmbH.)

Ferro-pyro-phototronic (FPP) effect is a triple coupling of ferroelectricity, light-induced pyroelectricity, and photo-excitation, which holds a bright promise for next-generation modern optoelectronic devices. However, except for few oxides (e.g., BaTiO 3 ), new FPP-active candidates remain extremely scarce due to the knowledge lacking on the underlying role of three coupling components. By tailoring the interlayered spacers, the authors present a series of 2D cesium-based perovskite ferroelectrics, (A') 2 CsPb 2 Br 7 (where A'-site cation is organic spacer), showing remarkable FPP-active properties. As expected, the dynamic ordering and reorientation of spacers along with atomic displacement of Cs + in the perovskite cavity lead to their ferroelectric polarizations. Particularly, exceptional FPP properties are created through this cooperation; the most FPP-active candidate (n-hexylammonium) 2 CsPb 2 Br 7 endows a giant contrast up to 1500% for photopyroelectric current to photovoltaic signal. This figure-of-merit is far beyond most inorganic oxide counterparts, such as ≈110% for BaTiO 3 . Further, the electric switching and controlling of FPP directions confirm a crucial role of ferroelectric polarization to this coupling effect. To the authors' best knowledge, this is the first study on an FPP-active candidate of 2D hybrid perovskites, which affords a new avenue to design ferroelectrics with targeted physical properties and forward their potentials to smart optoelectronic device application., (© 2022 Wiley-VCH GmbH.)

Two-dimensional (2D) hybrid perovskites are recently emerging as a potential family of semiconductors for versatile optoelectronic applications. Currently, the "perovskitizer" moieties are rigidly limited to small-size cations, while few 2D metal-halides containing guanidinium cations inside perovskite cages have been studied for photodetection. Herein, we present a new 2D hybrid perovskite, (i-BA) 2 (G)Pb 2 I 7 (where G is guanidinium and i-BA is isobutylammonium), which adopts a bilayered framework of {GPb 2 I 7 }. Single-crystal structure analyses disclose that G cations act as the perovskitizer, confined in the flexible perovskite cages formed by the distorted PbI 6 octahedra. Such inorganic sheets are crucial to the superior semiconducting properties and optical bandgap, as verified by the density functional theory calculation. Furthermore, its planar crystal-array photodetector shows fascinating photoelectric performance, including a quite low dark current (∼4.6×10 -11  A), a large current switching ratio (∼1.0×10 3 ), and a notable photo-responsivity of ∼0.72 A W -1 , suggesting great potential of (i-BA) 2 (G)Pb 2 I 7 for photodetection., (© 2021 Wiley-VCH GmbH.)

Two-dimensional (2D) organic-inorganic hybrid perovskites, benefiting from their natural anisotropy of quantum-well motifs and optical properties, have shown remarkable polarization-dependent responses superior to the 3D counterparts. Here, for the first time, multiwavelength polarization-sensitive detectors were fabricated by using single crystals of a guanidine-based 2D hybrid perovskite, (BA) 2 (GA)Pb 2 I 7 (where BA + is n-butylammonium and GA + is guanidium). Its unique 2D quantum-well structure results in strong crystallographic-dependence of optical absorption. Strikingly, our crystal-based photodetector exhibits a prominent photocurrent dichroic ratio (I max /I min ) of ∼2.2 at 520 nm, higher than the typical 2D inorganic materials (GeSe, ∼1.09, PdSe 2 , ∼1.8). In addition, notable dichroic ratios of 1.29 and 1.23 at 405 nm and 637 nm are also created for the multiwavelength polarized-light detection. The prominent detecting performances, including low dark current (1.6×10 -11  A), considerable on/off ratio (∼2×10 3 ), high photodetectivity (∼3.3×10 11 Jones) and responsivity (∼12.01 mA W -1 ), make (BA) 2 (GA)Pb 2 I 7 a promising candidate for polarized-light detection. This work sheds light on the rational engineering of new 2D hybrid perovskites for the high-performance optoelectronic device applications., (© 2021 Wiley-VCH GmbH.)

Two-dimensional (2D) organic-inorganic hybrid perovskite (OIHP) ferroelectrics have attracted widespread interest in the field of optoelectronics due to the combination of excellent semiconducting and ferroelectric properties. The Curie temperature (T C ), below which ferroelectricity exists, is a crucial parameter for ferroelectrics. However, the lack of research on T C tuning of 2D OIHP ferroelectrics hinders their further progress. Here, through incorporating ethylammonium (EA) as cage-confined rotators, we obtained two 2D OIHP ferroelectrics, (IBA) 2 (EA)Pb 2 Br 7 (2L; IBA=isobutylammonium), and (IBA) 2 (EA) 2 Pb 3 Br 10 (3L). Intriguingly, T C is successfully tuned from 326 K (2L) to 370 K (3L) with increasing layer thickness. Structural and computational analyses suggest that the improvement of T C is due to the higher phase-transition energy barrier triggered by the cage-confined EA rotators with increased layer thickness. This work suggests that EA is an effective "cage-confined rotator" to rationally design high-T C 2D OIHP ferroelectrics., (© 2020 Wiley-VCH GmbH.)

Organic-inorganic metal halide perovskites (most notably CH 3 NH 3 PbI 3 ) have demonstrated remarkable physical attributes for photovoltaic and diverse optoelectronic applications, whereas concerns about toxicity owing to the use of lead in the chemical composition still motivate further exploration of new, nontoxic candidates. Lead-free halide double perovskites (HDPs), designed by the rational chemical substitution of Pb 2+ with other nontoxic candidate elements, have recently attracted interest as a fascinating alternative to their Pb-based counterparts. Herein, recent advances in crystal structures, physical properties, and versatile optoelectronic applications of lead-free HDPs, such as solar cells, photodetectors, X-ray detectors, and light-emitting diodes, are reviewed. Perspectives to improve the physical and photoelectric properties of existing HDP materials are also discussed and will favor future development of new, lead-free HDP candidates., (© 2020 Wiley-VCH GmbH.)

Polarized-light detection in solar-blind ultraviolet region is indispensable for optoelectronic applications, whereas new 2D candidates targeted at solar-blind UV range remain extremely scarce. 2D hybrid perovskite ferroelectrics that combine polarization and semiconducting properties are of increasing interest. Here, using the 3D-to-2D dimensional reduction of CH 3 NH 3 PbCl 3 , we designed a multilayered hybrid perovskite ferroelectric, (CH 3 CH 2 NH 3 ) 2 (CH 3 NH 3 ) 2 Pb 3 Cl 10 , which shows spontaneous polarization and a high Curie temperature (390 K) comparable with that of BaTiO 3 (393 K). The wide band gap (ca. 3.35 eV) and anisotropic absorbance stemming from its intrinsic 2D motif, greatly favor its polarization-sensitive activity in UV region. The device displays excellent polarization-sensitive behavior under 266 nm, along with a large dichroic ratio (ca. 1.38) and high on/off current ratio (ca. 2.3×10 3 )., (© 2020 Wiley-VCH GmbH.)

Perovskite hybrids of lead organometal halides, (e. g., CH 3 NH 3 PbX 3 , X=Cl, Br, I) have gained wide attention since their spectacular progress on optoelectronic technologies in recent years. However, it still remains the fundamental problem, namely, the existence of the toxic, bioaccumulative, polluting element Pb and the instability of humidity. In particular, the use of toxic Pb severely limits the broad applications and commercialization of lead-containing optoelectronic devices, and finding an alternative that preserves the unique optoelectronic properties of the lead halide perovskite is highly urgent. Bi 3+ exhibits isoelectronic behavior similar to that of Pb 2+ in terms of ion radius and electronegativity, which is expected as an environmentally friendly alternative for lead. However, 1D Bi-based hybrid organic-inorganic light-absorbing materials with pyridinium are not numerous. Although great progress has been made, the research on photoresponse behavior are still rarely reported. Here, we present a lead-free organic-inorganic hybrid absorber with photoconductive response, (C 5 H 7 N 2 )BiI 4 (1, C 5 H 7 N 2 + is 2-aminopyridinium), which displays a narrow optical bandgap of ∼1.8 eV. Equally importantly, 1 not only features highly applicable properties, but is also able to absorb across most of the UV/Vis region of the solar spectrum. Furthermore, high-quality single crystal with sizes up to 7×6×4 mm 3 was successfully grown. In addition, 1 exhibited extremely low dark current (2×10 -11  A) at the bias of 10 V, which indicates that the indicator of 1 is an order of magnitude lower than other reported perovskite photodetectors. Moreover, photocurrent on/off switches is faster (∼670 ms), indicating that 1 can be a promising photosensitive switch. These intriguing properties, namely the lead-free composition, a small band gap, and the fast photoconductive response, suggest promising applications in the field of optoelectronics., (© 2020 Wiley-VCH GmbH.)

Multiphoton absorption (MPA) has been utilized for important technological applications. High-order multiphoton harvesting (e.g., five-photon absorption, 5PA) exhibits unique properties that could benefit biophotonics. Within this field, perovskite oxide ferroelectrics (e.g., BaTiO 3 ) enable low-order optical nonlinearities of 2PA/3PA processes. However, it is challenging to obtain efficient, high-order 5PA effects. Herein, for the first time, giant and broadband MPA properties are presented in the 2D hybrid perovskite ferroelectric (IA) 2 (MA) 2 Pb 3 Br 10 (1; IA = isoamylammonium and MA = methylammonium), where multiphoton-excited optical nonlinearities related to different MPA mechanisms over a broadband range of 550-2400 nm are observed. Strikingly, its 5PA absorption cross-section (σ 5 ) reaches up to 1.2 × 10 -132 cm 10 s 4 photon -4 (at 2400 nm), almost 10 orders larger than some state-of-the-art organic molecules and a record-high value among all known ferroelectrics. This unprecedented 5PA effect results from the quantum-confined motif of inorganic trilayer sheets (wells) and organic cations (barriers) in 1. Moreover, its large ferroelectric polarization of 5 µC cm -2 could promote modulation of MPA effects under external electric fields. As far as it is known, this is the first report on giant, broadband high-order MPA properties in ferroelectrics, which provides potential, novel electric-ordered materials for next-generation biophotonic applications., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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) 4 AgBiBr 8 , 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 AgBr 6 /BiBr 6 octahedra. Besides, the alternating array of inorganic perovskite sheets and organic cations endows large mobility-lifetime product (μτ=1.0×10 -3  cm 2  V -1 ) for detecting X-ray photons, which is almost tenfold higher than that of CH 3 NH 3 PbI 3 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., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

2D materials with inherent attributes of structural anisotropy have been well applied in the field of polarization-sensitive photodetection. However, to explore new 2D members with strong polarized-light responses still remains a challenge. Herein, by alloying diamine molecule into the 3D prototype of CsPbBr 3 , a new Dion-Jacobson (DJ) type 2D perovskite of (HDA)CsPb 2 Br 7 (1, where HDA 2+ is 1,6-hexamethylenediammonium), containing both inorganic Cs metal and organic cations is designed. The natural anisotropy characteristics of 1 are solidly elucidated by analyzing crystal structure, electric conductivity, and optical properties. Strikingly, distinct polarization-sensitive responses are observed in 1, owing to its strong anisotropy of optical absorption (the ratio of α c /α b ≈ 2.2). Consequently, crystal-based detectors of 1 exhibit fascinating photo-activities to polarized-light, including high detectivity (1.5 × 10 9 Jones), large dichroism ratio (I ph c /I ph b ≈ 1.6) and fast responding rate (200 µs). All these polarization-sensitive performances along with intriguing phase stability make 1 a potential candidate for polarized-light detection. This work paves a pathway toward new functionalities of DJ-type 2D hybrid perovskites for their future optoelectronic device applications., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

3D perovskite CsPbBr 3 has recently taken a blooming position for optoelectronic applications. However, due to the lack of natural anisotropy of optical attributes, it is a great challenge to fulfil polarization-sensitive photodetection. Here, for the first time, we exploited dimensionality reduction of CsPbBr 3 to tailor a 2D-multilayered hybrid perovskite, (TRA) 2 CsPb 2 Br 7 (1, in which TRA is (carboxy)cyclohexylmethylammonium), serving as a potential polarized-light detecting candidate. Its unique quantum-confined 2D structure results in intrinsic anisotropy of electrical conductivity, optical absorbance, and polarization-dependent responses. Particularly, it exhibits remarkable dichroism with the photocurrent ratio (I pc /I pa ) of ≈2.1, being much higher than that of the isotropic CsPbBr 3 crystal and reported CH 3 NH 3 PbI 3 nanowire (≈1.3), which reveals its great potentials for polarization-sensitive photodetection. Further, crystal-based detectors of 1 show fascinating responses to the polarized light, including high detectivity (>10 10 Jones), fast responding time (≈300 μs), and sizeable on/off current ratios (>10 4 ). To our best knowledge, this is the first study on 2D Cs-based hybrid perovskite exhibiting strong polarization-sensitivity. The work highlights an effective pathway to explore new polarization sensitive candidates for hybrid perovskites and promotes their future electronic applications., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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) 2 Pb 3 Br 10 (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. 10 4 ). 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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

By dimensional reduction of the 3D motif of Cs 2 AgBiBr 6 , a lead-free 2D hybrid double perovskite, (i-PA) 2 CsAgBiBr 7 (1, i-PA=isopentylammonium), was successfully designed. It adopts a quantum-confined bilayered structure with alternating organic and inorganic sheets. Strikingly, the unique 2D architecture endows it highly anisotropic nature of physical properties, including electric conductivity and optical absorption (the ratio α b /α c =1.9 at 405 nm). Such anisotropy attributes result in the strong polarization-sensitive responses with large dichroic ratios up to 1.35, being comparable to some 2D inorganic materials. This is the first study on the hybrid double perovskites with strong polarization sensitivity. A crystal device of 1 also exhibits rapid response speed (ca. 200 μs) and excellent stabilities. The family of 2D hybrid double perovskites are promising optoelectronic candidates, and this work paves a new pathway for exploring new green polarization-sensitive materials., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Halide double perovskites have recently bloomed as the green candidates for optoelectronic applications, such as X-ray detection. Despite great efforts, the exploration of promising organic-inorganic hybrid double perovskites toward X-ray detection remains unsuccessful. Now, single crystals of the lead-free hybrid double perovskite, (BA) 2 CsAgBiBr 7 (BA + is n-butylammonium), featuring the unique 2D multilayered quantum-confined motif, enable quite large μτ (mobility-lifetime) product up to 1.21×10 -3  cm 2  V -1 . This figure-of-merit realized in 2D hybrid double perovskites is unprecedented and comparable with that of CH 3 NH 3 PbI 3 wafers. (BA) 2 CsAgBiBr 7 crystals also exhibit other intriguing attributes for X-ray detection, including high bulk resistivity, low density of defects and traps, and large X-ray attenuation coefficient. Consequently, a vertical-structure crystal device under X-ray source yields a superior sensitivity of 4.2 μC Gy air -1  cm -2 ., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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) 2 Pb 3 Br 10 (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 10 6 ), and ultrafast response time (96/123 μs); such merits are superior to those of the most active ferroelectric oxide BiFeO 3 . 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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

2D hybrid perovskites have shown great promise in the photodetection field, due to their intriguing attributes stemming from unique structural architectures. However, the great majority of detectors based on this 2D system possess a relatively low response speed (≈ms), making it extremely urgent to develop new candidates for superfast photodetection. Here, a new organic-inorganic hybrid perovskite, (PA) 2 (FA)Pb 2 I 7 (EFA, where PA is n-pentylaminium and FA is formamidine), which features the 2D Ruddlesden-Popper type perovskite framework that is composed of the corner-sharing PbI 6 octahedra is reported. Significantly, photodetectors fabricated on highly oriented thin films, which exhibit a perfect orientation parallel to 2D inorganic perovskite layers, exhibit a superfast response time up to ≈2.54 ns. To the best of the knowledge, this figure-of-merit catches up with that of the top-ranking commercial materials, and sets a new record for 2D hybrid perovskite photodetectors. Moreover, extremely high photodetectivity (≈1.73 × 10 14 Jones, under an incident power intensity of ≈46 µW cm -2 ), considerable switching ratios (>10 3 ), and low dark current (≈10 pA) are also achieved in the detector, indicating its great potential for high-efficiency photodetection. These results shed light on the possibilities to explore new 2D candidates for assembling future high-performance optoelectronic devices., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Inorganic-organic hybrid perovskites, especially two-dimensional (2D) layered halide perovskites, have attracted significant attention due to their unique structures and attractive optoelectronic properties, which open up a great opportunity for next-generation photosensitive devices. Herein, we report a new 2D bilayered inorganic-organic hybrid perovskite, (C 6 H 13 NH 3 ) 2 (NH 2 CHNH 2 )Pb 2 I 7 (HFA, where C 6 H 13 NH 3 + is hexylaminium and NH 2 CHNH 2 + is formamidinium), which exhibits a remarkable photoresponse under broadband light illumination. Structural characterizations demonstrate that the 2D perovskite structure of HFA is constructed by alternant stacking of inorganic lead iodide bilayered sheets and organic hexylaminium layers. Optical absorbance measurements combined with density functional theory (DFT) calculations suggest that HFA is a direct band gap semiconductor with a narrow band gap (E g ) of ≈2.02 eV. Based on these findings, photodetectors based on HFA crystal wafer are fabricated, which exhibit fascinating optoelectronic properties including large on/off current ratios (over 10 3 ), fast response speeds (τ rise =310 μs and τ decay =520 μs) and high responsivity (≈0.95 mA W -1 ). This work will contribute to the design and development of new two-dimensional bilayer inorganic-organic hybrid perovskites for high-performance photosensitive devices., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Nonlinear optical (NLO) switches driven by a solid-state structural phase transition have attracted extensive attention; however, above-room-temperature solid-state NLO switch materials are still sparse. Herein, we report an above-room-temperature tin halide organic-inorganic hybrid quadratic NLO switchable material, N-methylpyrrolidinium trichloride stannite ([C 5 H 12 N]SnCl 3 , MPSC). The MPSC crystal exhibits a phase-matchable NLO property that is 1.1 times that of KH 2 PO 4 (KDP) and NLO switching behavior, changing from a high second harmonic generation (SHG) response to a low SHG response at 383 K, thereby demonstrating its prospective applications in the field of nonlinear optics. Variable-temperature crystal structural analysis combined with theoretical calculations revealed that the large NLO response stems from the inorganic SnCl 3 moiety, whereas the high-performance NLO switching properties mainly originate from the order/disorder transformation of the N-methylpyrrolidinium. This work provides a new approach to designing and exploring new high-performance quadratic NLO switches involving tin halide organic-inorganic hybrids., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Photosensitive organic-inorganic hybrid ferroelectrics are emerging as a new class of promising materials with a broad spectral response (e.g., visible light, UV light, and IR light) for optoelectronic applications. However, spectrally customized photoresponses in ferroelectrics have not been achieved. Herein, we report a photosensitive organic-inorganic hybrid ferroelectric, (diisobutylammonium) 3 ClCoCl 4 (1), with a notable spontaneous polarization of approximately 4.6 μC cm -2 and high Curie temperature (T c =372.5 K). More interestingly, it features spectrally selective absorption at specific wave bands ranging from 550 to 700 nm with a large absorption coefficient, which was assigned to the d-d transition of Co 2+ from the 4 A 2 to the 4 T 1 (P) state. It is emphasized that 1 shows spectrally customized photosensitivity with distinct photocurrent generation under light illumination at 550-700 nm. This is the first hybrid photosensitive ferroelectric with a spectrally customized photoresponse. This finding should lead to innovative applications of hybrid photosensitive ferroelectrics, especially towards spectrally selective optoelectronic devices., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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. 2×10 5 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., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Cesium-lead halide perovskites (e.g. CsPbBr 3 ) have gained attention because of their rich physical properties, but their bulk ferroelectricity remains unexplored. Herein, by alloying flexible organic cations into the cubic CsPbBr 3 , we design the first cesium-based two-dimensional (2D) perovskite ferroelectric material with both inorganic alkali metal and organic cations, (C 4 H 9 NH 3 ) 2 CsPb 2 Br 7 (1). Strikingly, 1 shows a high Curie temperature (T c =412 K) above that of BaTiO 3 (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 (>10 3 )., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Organic-inorganic hybrid perovskites, with the formula ABX 3 (A=organic cation, B=metal cation, and X=halide; for example, CH 3 NH 3 PbI 3 ), have diverse and intriguing physical properties, such as semiconduction, phase transitions, and optical properties. Herein, a new ABX 3 -type semiconducting perovskite-like hybrid, (hexamethyleneimine)PbBr 3 (1), consisting of one-dimensional inorganic frameworks and cyclic organic cations, is reported. Notably, the inorganic moiety of 1 adopts a perovskite-like architecture and forms infinite columns composed of face-sharing PbBr 6 octahedra. Strikingly, the organic cation exhibits a highly flexible molecular configuration, which triggers an above-room-temperature phase transition, at T c =338.8 K; this is confirmed by differential scanning calorimetry (DSC), specific heat capacity (C p ), and dielectric measurements. Further structural analysis reveals that the phase transition originates from the molecular configurational distortion of the organic cations coupled with small-angle reorientation of the PbBr 6 octahedra inside the inorganic components. Moreover, temperature-dependent conductivity and UV/Vis absorption measurements reveal that 1 also displays semiconducting behavior below T c . It is believed that this work will pave a potential way to design multifeatured perovskite hybrids by utilizing cyclic organic amines., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Perovskite hybrids of lead organometal halides, most notably CH 3 NH 3 PbI 3 , have shown extremely promising applications in the field of optoelectronics, because of their remarkable semiconducting and light-absorbing properties. However, two key issues-the toxicity of lead and the poor ambient instabilities-have restrained their further commercialization. Herein, we have designed a new stable polar lead-free hybrid material by utilizing the strategy of cation substitution, [(CH 3 ) 3 N] 3 Bi 2 I 9 (1), which adopts the 0D inorganic perovskite-like architecture by face-sharing BiI 6 octahedra. It is interesting that 1 displays excellent absorbing properties with a narrow optical band gap of ≈2.0 eV, and positive temperature-dependent conductivity confirms its semiconducting behaviors. In addition, 1 has good phase stability against decomposition under ambient conditions, much superior to that of CH 3 NH 3 PbI 3 . This work suggests the potential of 1 as a lead-free semiconducting absorber with high phase stability for photoelectric applications., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Two-dimensional (2D) layered hybrid perovskites have shown great potential in optoelectronics, owing to their unique physical attributes. However, 2D hybrid perovskite ferroelectrics remain rare. The first hybrid ferroelectric with unusual 2D multilayered perovskite framework, (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) 2 Pb 3 Br 10 (1), has been constructed by tailored alloying of the mixed organic cations into 3D prototype of CH 3 NH 3 PbBr 3 . Ferroelectricity is created through molecular reorientation and synergic ordering of organic moieties, which are unprecedented for the known 2D multilayered hybrid perovskites. Single-crystal photodetectors of 1 exhibit fascinating performances, including extremely low dark currents (ca. 10 -12  A), large on/off current ratios (ca. 2.5×10 3 ), and very fast response rate (ca. 150 μs). These merits are superior to integrated detectors of other 2D perovskites, and compete with the most active CH 3 NH 3 PbI 3 ., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A new high-temperature, hybrid, phase-transition material, 1-methylpiperidinium tetrabromozincate (1), that shows a reversible transition at 345 K was synthesized. Differential scanning calorimetry and specific heat capacity measurements confirmed this reversible transformation with a large heat hysteresis of 25 K, which describes a typical first-order phase transition in 1. The dielectric constant exhibited a steplike anomaly and showed high and low dielectric states in the high- and room-temperature phases, respectively, and therefore, this hybrid might be considered as a potential switchable dielectric material. The variable-temperature powder X-ray diffraction patterns displayed remarkable shifts between the experimental patterns at the two different phases. Single-crystal X-ray diffraction analyses at various temperatures revealed that the origin of this transformation could be attributed to disordering of the bromine atoms in the anion and the nitrogen atom of the cation. The cation also assumed a conformational change, which was likely induced by the disordered nitrogen atom. The conformational onset of the transformation of the cation from a planar conformer into a relaxed chair also occurred upon decreasing the temperature below transition point; thus, the combined order-disorder and conformational change induced the structural transformation and the change in symmetry., (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Perovskite lead halides (CH3 NH3 PbI3 ) 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)3 Sb2 Br9 (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., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Perovskite-type ferroelectrics composed of organometallic halides are emerging as a promising alternative to conventional photovoltaic devices because of their unique photovoltaic effects (PVEs). A new layered perovskite-type photoferroelectric, bis(cyclohexylaminium) tetrabromo lead (1), is presented. The material exhibits an exceptional anisotropy of bulk PVEs. Upon photoexcitation, superior photovoltaic behaviors are created along its inorganic layers, which are composed of corner-sharing PbBr6 octahedra. Semiconducting activity with remarkable photoconductivity is achieved in the vertical direction, showing sizeable on/off current ratios (>10(4) ), which compete with the most active photovoltaic material CH3 NH3 PbI3 . In 1 the temperature-dependence of photovoltage coincides fairly well with that of polarization, confirming the dominant role of ferroelectricity in such highly anisotropic PVEs. This finding sheds light on bulk PVEs in ferroelectric materials, and promotes their application in optoelectronic devices., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Ultrahigh pyroelectric figures of merit are achieved in a new phase-transition material, di-n-butylaminium trifluoroacetate, of which the peak values are an order of magnitude larger than those of their inorganic counterparts. Such an attractive behavior of pyroelectric detectivity is strongly related to its distinct bistable dielectric behavior, which recalls excellent thermoelectric response in organic molecular phase-transition systems., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Herein, a facile strategy for the controllable synthesis of BiVO4@C core-shell nanoparticles on reduced graphene oxide (RGO) is reported. The BiVO4 particle size can be controlled in the process by adjusting the volume ratio of glycerol in the sol-gel solution. The glycerol layers adsorbed on BiVO4 (BiVO4@glycerol) made it possible to form hydrogen bonds between BiVO4@glycerol and graphene oxide with the assistance of ultrasound. After thermal treatment, glycerol adsorbed on the BiVO4 particles formed amorphous carbon shells to link the particles and RGO. As a result, the obtained RGO-BiVO4@C nanocomposite showed a five times higher rate in O2 evolution from water under visible-light irradiation. Also, it demonstrated a six times higher photocatalytic performance enhancement than that of pure BiVO4 in the degradation of Rhodamine B. The enhanced performance is attributed to the carbon shells that restrict the growth of BiVO4 , the reduced graphene oxide that improves the electronic conductivity of the composite, and importantly, the bonds formed between the carbon shells and RGO that reduce the recombination loss of photogenerated charges effectively. The strategy is simple, effective, and can be extended to other ternary oxides with controlled size and high performance., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Dipropylammonium picrate (compound 1) undergoes a first-order phase transition at 185.0 K. Differential scanning calorimetry (DSC) measurements show reversible anomaly peaks with a thermal hysteresis of 10.2 K. Dielectric responses with distinct anisotropy and striking anomaly further confirm the phase transition. Detailed structural analyses demonstrate that the phase transition in 1 is dominated by order-disorder transformations of the dipropylammonium (DPA) cations and the picrate anions. Moreover, the unique distortions of the terminal propyl moieties in the flexible chain-like DPA cation also play an important role in this structural phase transition. All these results indicate that 1 is a potential phase-transition material., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A new molecular phase transition material, [PhCH(2) NH(CH(3))(2)](2) C(2)O(4)⋅H(2)C(2)O(4), which undergoes a reversible phase transition at 151.6 K, has been successfully synthesized. Differential scanning calorimetry (DSC), specific heat capacity, and dielectric measurements confirm its reversible phase transition with a large thermal hysteresis of 15.1 K, demonstrating that the phase transition is typical first order. Variable-temperature single-crystal X-ray diffraction analyses reveal that the order-disorder transformations of carboxy oxygen atoms induce the structural phase transition. A slight reorientation of the oxalic acid unit is discovered to accompany the ordering of carboxy oxygen atoms at low temperature. The DSC measurement result of the deuterated analog is different to that of 1, indicating that proton dynamic motions in hydrogen bonds also contribute to the phase transition., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A new glycine-based reversible phase transition compound diglycine methanesulfonate (1) has been successfully synthesized. Differential scanning calorimetry (DSC) measurements of 1 showed a pair of broad peaks around 134 K (T(c)=phase transition temperature) with a slight thermal hysteresis during the heating/cooling cycle, thereby indicating that this compound undergoes a reversible second-order phase transition. Dielectric measurements further confirmed the phase transition and revealed a switchable response to the ambient temperature change for the dielectric constants of 1, namely, the dielectric constants have a distinctive step-like anomaly switching between a high dielectric state in the room temperature phase (RTP) and a low state in the low temperature phase (LTP). Variable-temperature single-crystal X-ray diffraction analyses show that 1 undergoes an atypical transition from the space group P2₁/m in the RTP to P2₁/c in the LTP. The origin of the switchable dielectric phase transition is ascribed to the movement of the moieties in 1 from the equilibrium position, and this stems from the twisting of the molecules in the compound. We believe that these findings will be useful in exploring switchable dielectric phase transition materials., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)