Storage of an electric field for photocurrent generation in ferroelectric-functionalized organic devices

Organic radical ferroelectric crystals with martensitic phase transition

Organic martensitic compounds are an emerging type of smart material with intriguing physical properties including thermosalient effect, ferroelasticity, and shape memory effect. However, due to the high structural symmetry and limited design theories for these materials, the combination of ferroelectricity and martensitic transformation has rarely been found in organic systems. Here, based on the chemical design strategies for molecular ferroelectrics, we show a series of asymmetric 1,4,5,8-naphthalenediimide derivatives with the homochiral amine and 2,2,6,6-tetramethylpiperidine-N-oxyl components, which adopt the low-symmetric polar structure and so allow ferroelectricity. Upon H/F substitution, the fluorinated compounds exhibit reversible ferroelectric and martensitic transitions at 399 K accompanied by a large thermal hysteresis of 132 K. This large thermal hysteresis with two competing (meta)-stable phases is further confirmed by density functional theory calculations. The rare combination of martensitic phase transition and ferroelectricity realizes the bistability with two different ferroelectric phases at room temperature. Our finding provides insight into the exploration of martensitic ferroelectric compounds with potential applications in switchable memory devices, soft robotics, and smart actuators.

Enhanced Piezoelectric, Ferroelectric, and Electrostrictive Properties of Lead-Free (1-x)BCZT-(x)BCST Electroceramics with Energy Harvesting Capability

Next-generation electronics and energy technologies can now be developed as a result of the design, discovery, and development of novel, environmental friendly lead (Pb)-free ferroelectric materials with improved characteristics and performance. However, there have only been a few reports of such complex materials' design with multi-phase interfacial chemistry, which can facilitate enhanced properties and performance. In this context, herein, novel lead-free piezoelectric materials (1-x)Ba0.95 Ca0.05 Ti0.95 Zr0.05 O3 -(x)Ba0.95 Ca0.05 Ti0.95 Sn0.05 O3 , are reported, which are represented as (1-x)BCZT-(x)BCST, with demonstrated excellent properties and energy harvesting performance. The (1-x)BCZT-(x)BCST materials are synthesized by high-temperature solid-state ceramic reaction method by varying x in the full range (x = 0.00-1.00). In-depth exploration research is performed on the structural, dielectric, ferroelectric, and electro-mechanical properties of (1-x)BCZT-(x)BCST ceramics. The formation of perovskite structure for all ceramics without the presence of any impurity phases is confirmed by X-ray diffraction (XRD) analyses, which also reveals that the Ca2+ , Zr4+ , and Sn4+ are well dispersed within the BaTiO3 lattice. For all (1-x)BCZT-(x)BCST ceramics, thorough investigation of phase formation and phase-stability using XRD, Rietveld refinement, Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), and temperature-dependent dielectric measurements provide conclusive evidence for the coexistence of orthorhombic + tetragonal (Amm2 + P4mm) phases at room temperature. The steady transition of Amm2 crystal symmetry to P4mm crystal symmetry with increasing x content is also demonstrated by Rietveld refinement data and related analyses. The phase transition temperatures, rhombohedral-orthorhombic (TR-O ), orthorhombic- tetragonal (TO-T ), and tetragonal-cubic (TC ), gradually shift toward lower temperature with increasing x content. For (1-x)BCZT-(x)BCST ceramics, significantly improved dielectric and ferroelectric properties are observed, including relatively high dielectric constant εr ≈ 1900-3300 (near room temperature), εr ≈ 8800-12 900 (near Curie temperature), dielectric loss, tan δ ≈ 0.01-0.02, remanent polarization Pr ≈ 9.4-14 µC cm-2 , coercive electric field Ec ≈ 2.5-3.6 kV cm-1 . Further, high electric field-induced strain S ≈ 0.12-0.175%, piezoelectric charge coefficient d33 ≈ 296-360 pC N-1 , converse piezoelectric coefficient( d 33 ∗ ) ave ${( {d_{33}^*} )}_{{\rm{ave}}}$ ≈ 240-340 pm V-1 , planar electromechanical coupling coefficient kp ≈ 0.34-0.45, and electrostrictive coefficient (Q33 )avg ≈ 0.026-0.038 m4 C-2 are attained. Output performance with respect to mechanical energy demonstrates that the (0.6)BCZT-(0.4)BCST composition (x = 0.4) displays better efficiency for generating electrical energy and, thus, the synthesized lead-free piezoelectric (1-x)BCZT-(x)BCST samples are suitable for energy harvesting applications. The results and analyses point to the outcome that the (1-x)BCZT-(x)BCST ceramics as a potentially strong contender within the family of Pb-free piezoelectric materials for future electronics and energy harvesting device technologies.

Insight into Interfacial Polarization for Enhancing Piezoelectricity in Ferroelectric Nanocomposites

The interfacial effect is widely used to optimize the properties of ferroelectric nanocomposites, however, there is still a lack of direct evidence to understand its underlying mechanisms limited by the nano size and complex structures. Here, taking piezoelectricity, for example, the mechanism of interfacial polarization in barium titanate/poly(vinylidene fluoride-ran-trifluoroethylene) (BTO/P(VDF-TrFE)) nanocomposite is revealed at multiple scales by combining Kelvin probe force microscope (KPFM) with theoretical stimulation. The results prove that the mismatch of permittivity between matrix and filler leads to the accumulation of charges, which in turn induces local polarization in the interfacial region, and thus can promote piezoelectricity independently. Furthermore, the strategy of interfacial polarization to enhance piezoelectricity is extended and validated in other two similar nanocomposites. This work uncovers the mechanism of interfacial polarization and paves newfangled insights to boost performances in ferroelectric nanocomposites.

Large-Area Flexible Memory Arrays of Oriented Molecular Ferroelectric Single Crystals with Nearly Saturated Polarization

Molecular ferroelectrics (MFs) have been proven to demonstrate excellent properties even comparable to those of inorganic counterparts usually with heavy metals. However, the validation of their device applications is still at the infant stage. The polycrystalline feature of conventionally obtained MF films, the patterning challenges for microelectronics and the brittleness of crystalline films significantly hinder their development for organic integrated circuits, as well as emerging flexible electronics. Here, a large-area flexible memory array is demonstrated of oriented molecular ferroelectric single crystals (MFSCs) with nearly saturated polarization. Highly-uniform MFSC arrays are  prepared on large-scale substrates including Si wafers and flexible substrates using an asymmetric-wetting and microgroove-assisted coating (AWMAC) strategy. Resultant flexible memory arrays exhibit excellent nonvolatile memory properties with a low-operating voltage of <5 V, i.e., nearly saturated ferroelectric polarization (6.5 µC cm^-2 ), and long bending endurance (>10³ ) under various bending radii. These results may open an avenue for scalable flexible MF electronics with high performance.

Temperature Stable Piezoelectric Imprint of Epitaxial Grown PZT for Zero-Bias Driving MEMS Actuator Operation

In piezoelectric transducer applications, it is common to use a unipolar operation signal to avoid switching of the polarisation and the resulting nonlinearities of micro-electromechanical systems. However, semi-bipolar or bipolar operation signals have the advantages of less leakage current, lower power consumption and no additional need of a DC-DC converter for low AC driving voltages. This study investigates the potential of using piezoelectric layers with an imprint for stable bipolar operation on the basis of epitaxially grown lead zirconate titanate cantilevers with electrodes made of a metal and metal oxide stack. Due to the manufacturing process, the samples exhibit high crystallinity, rectangular shaped hysteresis and a high piezoelectric response. Furthermore, the piezoelectric layers have an imprint, indicating a strong built-in field, which shifts the polarisation versus electric field hysteresis. To obtain the stability of the imprint, laser doppler vibrometry and switching current measurements were performed at different temperatures, yielding a stable imprinted electric field of -1.83 MV/m up to at least 100 °C. The deflection of the cantilevers was measured with a constant AC driving voltage while varying the DC bias voltage to examine the influence of the imprint under operation, revealing that the same high deflection and low nonlinearities, quantified by the total harmonic distortion, can be maintained down to low bias voltages compared to unipolar operation. These findings demonstrate that a piezoelectric layer with a strong imprint makes it possible to operate with low DC or even zero DC bias, while still providing strong piezoelectric response and linear behaviour.

Ferroelectric Lithography in Single-Component Organic Enantiomorphic Ferroelectrics

Organic ferroelectrics are flexible, lightweight, and bio-friendly, promising for bio-harmonized electronic devices, while their ferroelectric lithography remains relatively unexplored. Here, by introducing homochirality and ZE photoisomerization, we obtained a pair of organic enantiomorphic ferroelectrics, di(benzylamino)-substituted derivatives of muconic acids, the first ferroelectrics in the muconic family. Their ferroelectric and chiral features were confirmed by the polarization-electric field hysteresis loops and circular dichroism spectra, respectively. Piezoresponse force microscopy measurements demonstrate that the desired domain structure can be precisely achieved by applying a local electric field on a predefined pattern in their thin films. Moreover, thermogravimetric analyses reveal that their ferroelectricity can persist up to above 550 K. The precise pattern lithography and excellent thermal stability make them competitive candidates for ferroelectric lithography.

Enhanced photocatalytic performance of heterogeneous hydrotalcite by spontaneously polarized ferroelectric

Two-dimensional photocatalytic materials have attracted great attention due to their large specific surface area and abundant active sites. Suppressing the recombination of photo-excited carriers is an effective approach to improve the performances of photocatalytic materials. Herein, we introduced ferroelectric PbTiO₃ into the two-dimensional layered double hydroxides (LDHs) to improve the carrier separation efficiency and photocatalytic performances. A built-in electric field was generated in the polarized PbTiO₃, resulting in the improvement of the carrier separation efficiency and the promotion of the lifetime of photo-excited carriers in the LDHs-PbTiO₃ composites. As a result, the LDHs-PbTiO₃ composites showed the decent photocatalytic performances towards water splitting under visible light irradiation. The oxygen production rate of the proposed LDHs-PbTiO₃ composites was almost twice than that of pristine LDHs. These results have addressed the significance of photo-excited carriers in photocatalytic materials. This approach could undoubtedly provide the valuable information in design and construction of high efficiency photocatalysts.

Optical Control of Polarization Switching in a Single-Component Organic Ferroelectric Crystal

The optical control of polarization switching is attracting tremendous interest because photoirradiation stands out as a nondestructive, noncontact, and remote-control means beyond an electric or strain field. The current research mainly uses various photoexcited electronic effects to achieve the photocontrol polarization, such as a light-driven flexoelectric effect and a photovoltaic effect. However, since photochromism was discovered in 1867, the structural phase transition caused by photoisomerization has never been associated with ferroelectricity. Here, we successfully synthesized an organic photochromic ferroelectric with polar space group Pna2₁, 3,4,5-trifluoro-N-(3,5-di-tert-butylsalicylidene)aniline, whose color can change between yellow and orange via laser illumination. Its dielectric permittivity and spontaneous polarization can be switched reversibly with a photoinduced phase transition triggered by structural photoisomerization between the enol form and the trans-keto form. To our knowledge, this is the first photoswitchable ferroelectric crystal to achieve polarization switching through a structural phase transition triggered by photoisomerization. This finding paves the way toward photocontrol of smart materials and biomechanical applications in the future.

Exploring a Fatigue-Free Layered Hybrid Perovskite Ferroelectric for Photovoltaic Non-Volatile Memories

Through a functional unit-transmutation strategy, a fatigue-free layered hybrid perovskite ferroelectric (C₆ H₅ CH₂ NH₃ )₂ CsPb₂ Br₇ (BCPB) has been developed, which demonstrates stable spontaneous polarization (P_s ) of 6.5 μC cm^-2 and high Curie temperature up to 425 K. Meanwhile, BCPB shows splendid bulk photovoltaic effect (BPVE) properties with noticeable zero-bias photocurrent density (5 μA cm^-2 ), and high on/off switching ratio of current (over 3×10⁵ ); these merits even overmatch the most known ferroelectric semiconductor BiFeO₃ . The unique structure with self-regulated net electrical charged layers gives rise to the fatigue-free feature of P_s and BPVE (no significant fatigue after 10⁸ polarity switching cycles), promoting the potential applications of BCPB in photovoltaic non-volatile memories. This work offers an efficient approach for exploring fatigue-free semiconducting ferroelectrics as well as excavates their further applications in next-generation electronic devices.

A flexible artificial intrinsic-synaptic tactile sensory organ

Imbuing bio-inspired sensory devices with intelligent functions of human sensory organs has been limited by challenges in emulating the preprocessing abilities of sensory organs such as reception, filtering, adaptation, and sensory memory at the device level itself. Merkel cells, which is a part of tactile sensory organs, form synapse-like connections with afferent neuron terminals referred to as Merkel cell-neurite complexes. Here, inspired by structure and intelligent functions of Merkel cell-neurite complexes, we report a flexible, artificial, intrinsic-synaptic tactile sensory organ that mimics synapse-like connections using an organic synaptic transistor with ferroelectric nanocomposite gate dielectric of barium titanate nanoparticles and poly(vinylidene fluoride-trifluoroethylene). Modulation of the post-synaptic current of the device induced by ferroelectric dipole switching due to triboelectric-capacitive coupling under finger touch allowed reception and slow adaptation. Modulation of synaptic weight by varying the nanocomposite composition of gate dielectric layer enabled tuning of filtering and sensory memory functions.

The recent progress of triboelectric nanogenerator-assisted photodetectors

Since 2012, triboelectric nanogenerator (TENG) has attracted significant interest from researchers in the field of energy conversion due to its unique output characteristics of high voltage, pulse and low current. In addition, recent advancements have demonstrated that photodetection platforms based on TENG exhibit great advantages such as being simple, low-cost, portable, with high sensitivity, high response, etc, and are environment friendly. Here, this article provides a comprehensive review on the state-of-the-art photodetectors based on TENG in recent years, and a detailed introduction to the structural design and potential mechanisms. It mainly focuses on self-powered photodetectors (including photodetectors as a load resistance of a TENG and photosensitive materials such as tribo-layer of TENG) and the modulation of photodetectors based on TENG (including utilizing the voltage of TENG as well as triboelectric microplasma). Finally, we put forward some perspectives and outlook, including structure engineering and mechanism guidance, for the future development of simple, high-performance and portable photodetectors based on TENG.

Large Piezoelectric Response in Hybrid Rare-Earth Double Perovskite Relaxor Ferroelectrics

Piezoelectric materials are technologically important, and the most used are perovskite ferroelectrics. In recent years, more and more emerging areas have put forward new requirements for piezoelectric materials, such as light weight, low acoustic impedance, good flexibility, and biocompatibility. In this context, hybrid organic-inorganic perovskite ferroelectrics have emerged as promising supplements, because they combine attractive features of inorganic and organic materials. Among them, hybrid double-metal perovskites have recently been found to exhibit excellent ferroelectricity. However, their potential as piezoelectric materials has not been exploited. Here, we describe large piezoelectric response in hybrid rare-earth double perovskite relaxor ferroelectrics (RM3HQ)₂RbLa(NO₃)₆ and (RM3HQ)₂NH₄La(NO₃)₆ (RM3HQ = R-N-methyl-3-hydroxylquinuclidinium). They are simultaneously ferroelectric and ferroelastic crystals, with the R3 ferroelectric phase and P2₁3 paraelectric phase. We found that ferroelectric polar microdomains and paraelectric nonpolar regions coexist in a wide temperature range through variable-temperature piezoresponse force microscopy images. The two-phase coexistence reveals low energy barriers of transitions between the two phases and between the polar microdomains with different polarization directions. These lead to the easy polarization rotation of the polar microdomains upon applying a stress and, accordingly, the large piezoelectric response up to 106 pC N^-1 for (RM3HQ)₂RbLa(NO₃)₆. This finding represents a significant step toward novel applications of piezoelectric materials based on lead-free hybrid perovskites.

A chiral switchable photovoltaic ferroelectric 1D perovskite

Spin and valley degrees of freedom in materials without inversion symmetry promise previously unknown device functionalities, such as spin-valleytronics. Control of material symmetry with electric fields (ferroelectricity), while breaking additional symmetries, including mirror symmetry, could yield phenomena where chirality, spin, valley, and crystal potential are strongly coupled. Here we report the synthesis of a halide perovskite semiconductor that is simultaneously photoferroelectricity switchable and chiral. Spectroscopic and structural analysis, and first-principles calculations, determine the material to be a previously unknown low-dimensional hybrid perovskite (R)-(-)-1-cyclohexylethylammonium/(S)-(+)-1 cyclohexylethylammonium) PbI₃. Optical and electrical measurements characterize its semiconducting, ferroelectric, switchable pyroelectricity and switchable photoferroelectric properties. Temperature dependent structural, dielectric and transport measurements reveal a ferroelectric-paraelectric phase transition. Circular dichroism spectroscopy confirms its chirality. The development of a material with such a combination of these properties will facilitate the exploration of phenomena such as electric field and chiral enantiomer-dependent Rashba-Dresselhaus splitting and circular photogalvanic effects.

Two-Dimensional Organic-Inorganic Hybrid Rare-Earth Double Perovskite Ferroelectrics

As a major branch of hybrid perovskites, two-dimensional (2D) hybrid double perovskites are expected to be ideal systems for exploring novel ferroelectric properties, because they can accommodate a variety of organic cations and allow diverse combinations of different metal elements. However, no 2D hybrid double perovskite ferroelectric has been reported since the discovery of halide double perovskites in the 1930s. Based on trivalent rare-earth ions and chiral organic cations, we have designed a new family of 2D rare-earth double perovskite ferroelectrics, A₄M^IM^III(NO₃)₈, where A is the organic cation, M^I is the alkaline metal or ammonium ion, and M^III is the rare-earth ion. This is the first time that ferroelectricity is realized in 2D hybrid double perovskite systems. These ferroelectrics have achieved high-temperature ferroelectricity and photoluminescent properties. By varying the rare-earth ion, variable photoluminescent properties can be achieved. The results reveal that the 2D rare-earth double perovskite systems provide a promising platform for achieving multifunctional ferroelectricity.

Ferroelectricity and Rashba Effect in a Two-Dimensional Dion-Jacobson Hybrid Organic-Inorganic Perovskite

Hybrid organic-inorganic perovskites (HOIPs) are a new generation of high-performance materials for solar cells and light emitting diodes. Beyond these applications, ferroelectricity and spin-related properties of HOIPs are increasingly attracting interests. The presence of strong spin-orbit coupling, allied with symmetry breaking ensured by remanent polarization, should give rise to Rashba-type splitting of electronic bands in HOIP. However, the report of both ferroelectricity and Rashba effect in HOIP is rare. Here we report the observation of robust ferroelectricity and Rashba effect in two-dimensional Dion-Jacobson perovskites.

Gate-Tunable In-Plane Ferroelectricity in Few-Layer SnS

Ultrathin ferroelectrics hold great promise for modern miniaturized sensors, memories, and optoelectronic devices. However, in most ferroelectric materials, polarization is destabilized in ultrathin films by the intrinsic depolarization field. Here we report robust in-plane ferroelectricity in few-layer tin sulfide (SnS) 2D crystals that is coupled anisotropically to lattice strain. Specifically, the intrinsic polarization of SnS manifests as nanoripples along the armchair direction due to a converse piezoelectric effect. Most interestingly, such nanoripples show an odd-and-even effect in terms of its layer dependence, indicating that it is highly sensitive to changes in inversion symmetry. Ferroelectric switching is demonstrated in field-effect transistor devices fabricated on ultrathin SnS films, in which a stronger ferroelectric response is achieved at negative gate voltages. Our work shows the promise of 2D SnS in ultrathin ferroelectric field-effect transistors as well as nanoscale electromechanical systems.

A Ferroelectric-Photovoltaic Effect in SbSI Nanowires

A ferroelectric-photovoltaic effect in nanowires of antimony sulfoiodide (SbSI) is presented for the first time. Sonochemically prepared SbSI nanowires have been characterized using high-resolution transmission electron microscopy (HRTEM) and optical diffuse reflection spectroscopy (DRS). The temperature dependences of electrical properties of the fabricated SbSI nanowires have been investigated too. The indirect forbidden energy gap E_gIf = 1.862 (1) eV and Curie temperature T_C = 291 (2) K of SbSI nanowires have been determined. Aligned SbSI nanowires have been deposited in an electric field between Pt electrodes on alumina substrate. The photoelectrical response of such a prepared ferroelectric-photovoltaic (FE-PV) device can be switched using a poling electric field and depends on light intensity. The photovoltage, generated under λ = 488 nm illumination of P_opt = 127 mW/cm² optical power density, has reached U_OC = 0.119 (2) V. The presented SbSI FE-PV device is promising for solar energy harvesting as well as for application in non-volatile memories based on the photovoltaic effect.

Nonvolatile Memory Based on Molecular Ferroelectric/Graphene Field Effect Transistor

Ferroelectric thin films are extensively attractive as next-generation nonvolatile memories. Recently, molecular ferroelectrics (MFe), as an emerging new class, have been a new research focus because of their desirable characteristics such as good solution processability, tunable chemical properties, and bio-friendly compositions. However, traditional uniaxial MFe only possess one polar axis which greatly limits their application, as it requires restricted orientational control in single crystal. To achieve macroscopic ferroelectricity and thus fully realize technological advantages of MFe, development of multiaxes is imperative to maximize effective polarization in specific crystallographic orientations. Herein, we present an early exploration on polycrystalline multiaxial MFe thin films of [Hdabco][ReO₄] with a two-dimensional graphene hybrid nonvolatile memory device. The polarization switching of MFe is experimentally realized by the nonvolatile modulation of two current states in graphene. Such a hybrid device can exhibit large memory window ∼35 V implying its great potential in memory applications. Hence, by taking the advantages of multiple polarization axes of MFe, the low cost and large area MFe/graphene hybrid memory manifests new possibilities for the integration of these materials as flexible next generation memory devices.

A Ferroelectric/Electrochemical Modulated Organic Synapse for Ultraflexible, Artificial Visual-Perception System

Human eyes undertake the majority of information assimilation for learning and memory. Transduction of the color and intensity of the incident light into neural signals is the main process for visual perception. Besides light-sensitive elements that function as rods and cones, artificial retinal systems require neuromorphic devices to transform light stimuli into post-synaptic signals. In terms of plasticity timescale, synapses with short-term plasticity (STP) and long-term potentiation (LTP) represent the neural foundation for experience acquisition and memory formation. Currently, electrochemical transistors are being researched as STP-LTP devices. However, their LTP timescale is confined to a second-to-minute level to give unreliable non-volatile memory. This issue limits multiple-plasticity synapses with tunable temporal characteristics and efficient sensory-memory systems. Herein, a ferroelectric/electrochemical modulated organic synapse is proposed, attaining three prototypes of plasticity: STP/LTP by electrochemical doping/de-doping and ferroelectric-LTP from dipole switching. The device supplements conventional electrochemical transistors with 10000-second-persistent non-volatile plasticity and unique threshold switching properties. As a proof-of-concept for an artificial visual-perception system, an ultraflexible, light-triggered organic neuromorphic device (LOND) is constructed by this synapse. The LOND transduces incident light signals with different frequency, intensity, and wavelength into synaptic signals, both volatile and non-volatile.

Giant polarization in super-tetragonal thin films through interphase strain

Strain engineering has emerged as a powerful tool to enhance the performance of known functional materials. Here we demonstrate a general and practical method to obtain super-tetragonality and giant polarization using interphase strain. We use this method to create an out-of-plane-to-in-plane lattice parameter ratio of 1.238 in epitaxial composite thin films of tetragonal lead titanate (PbTiO₃), compared to 1.065 in bulk. These thin films with super-tetragonal structure possess a giant remanent polarization, 236.3 microcoulombs per square centimeter, which is almost twice the value of known ferroelectrics. The super-tetragonal phase is stable up to 725°C, compared to the bulk transition temperature of 490°C. The interphase-strain approach could enhance the physical properties of other functional materials.

Photovoltage Reversal in Organic Optoelectronic Devices with Insulator-Semiconductor Interfaces

Photoinduced space-charges in organic optoelectronic devices, which are usually caused by poor mobility and charge injection imbalance, always limit the device performance. Here we demonstrate that photoinduced space-charge layers, accumulated at organic semiconductor-insulator interfaces, can also play a role for photocurrent generation. Photocurrent transients from organic devices, with insulator-semiconductor interfaces, were systematically studied by using the double-layer model with an equivalent circuit. Results indicated that the electric fields in photoinduced space-charge layers can be utilized for charge generation and can even induce a photovoltage reversal. Such an operational process of light harvesting would be promising for photoelectric conversion in organic devices.

Metal-free three-dimensional perovskite ferroelectrics

Inorganic perovskite ferroelectrics are widely used in nonvolatile memory elements, capacitors, and sensors because of their excellent ferroelectric and other properties. Organic ferroelectrics are desirable for their mechanical flexibility, low weight, environmentally friendly processing, and low processing temperatures. Although almost a century has passed since the first ferroelectric, Rochelle salt, was discovered, examples of highly desirable organic perovskite ferroelectrics are lacking. We found a family of metal-free organic perovskite ferroelectrics with the characteristic three-dimensional structure, among which MDABCO (N-methyl-N'-diazabicyclo[2.2.2]octonium)-ammonium triiodide has a spontaneous polarization of 22 microcoulombs per square centimeter [close to that of barium titanate (BTO)], a high phase transition temperature of 448 kelvins (above that of BTO), and eight possible polarization directions. These attributes make it attractive for use in flexible devices, soft robotics, biomedical devices, and other applications.

Spatially Probed Plasmonic Photothermic Nanoheater Enhanced Hybrid Polymeric-Metallic PVDF-Ag Nanogenerator

Surface plasmon-based photonics offers exciting opportunities to enable fine control of the site, span, and extent of mechanical harvesting. However, the interaction between plasmonic photothermic and piezoresponse still remains underexplored. Here, spatially localized and controllable piezoresponse of a hybrid self-polarized polymeric-metallic system that correlates to plasmonic light-to-heat modulation of the local strain is demonstrated. The piezoresponse is associated to the localized plasmons that serve as efficient nanoheaters leading to self-regulated strain via thermal expansion of the electroactive polymer. Moreover, the finite-difference time-domain simulation and linear thermal model also deduce the local strain to the surface plasmon heat absorption. The distinct plasmonic photothermic-piezoelectric phenomenon mediates not only localized external stimulus light response but also enhances dynamic piezoelectric energy harvesting. The present work highlights a promising surface plasmon coordinated piezoelectric response which underpins energy localization and transfer for diversified design of unique photothermic-piezotronic technology.

Visualization of Room-Temperature Ferroelectricity and Polarization Rotation in the Thin Film of Quinuclidinium Perrhenate

Recently, a plastic crystal of quinuclidinium perrhenate (HQReO_{4}) was reported to have the feasibility of controlling the crystallographic orientation in the grown crystal, but the corresponding temperature window is only about 22 K (345-367 K). Such a narrow window and uncertain ferroelectricity at room temperature would extremely limit its application potential. In this report, we prepared a large area high-quality polycrystalline thin film of HQReO_{4} and for the first time observed ferroelectricity in the temperature range from 298 to 367 K. Density functional theory calculations revealed the origin of room-temperature ferroelectricity is ascribed to the collaborative flipping of HQ (protonated quinuclidine) and ReO_{4}^{-}, which is dynamically preferred in the presence of a N─H⋯O hydrogen bond. A local piezoresponse force microscopy measurement was also employed to study the mechanisms of multiaxial polarization rotation and domain dynamics. By extending the ferroelectric temperature window to room temperature and the extraordinary thin-film processability, HQReO_{4} would certainly become a suitable candidate for next generation ferroelectric materials.

Chemically tunable photoresponse of ultrathin polypyrrole

Building a complex structure system of conductive polymers without a complicated fabricating process is a long-awaited goal to improving the functional photoresponse properties of conductive polymers. In this study, we demonstrate that the photoresponse of polypyrrole (PPy)-based photodetector devices with an ultrathin polymer layer can be chemically modulated by simply immersing the devices into an alkaline solution. After alkaline treatment, the pyrrole unit transforms into a quinoid structure. Characteristics of current-voltage reveal an increased photosensitivity with several orders of magnitude when decreasing the applied bias voltage. Furthermore, ultrathin PPy belts with a width of 100 nm exhibit ultra-high photosensitivites of roughly 1000 (unit) and photoresponsivities of 54.3 A W^-1 due to the high surface area ratio of the nanobelts.

Bandgap Engineering of Lead-Halide Perovskite-Type Ferroelectrics

Semiconducting ferroelectricity is realized in hybrid perovskite-type compounds (cyclohexylammonium)2 PbBr4-4 x I4 x (x = 0-1). By adjusting the composition x, the bandgap is successfully tuned from previously reported 3.65 eV to as low as 2.74 eV, and the excellent ferroelectricity was kept intact. This finding may contribute to improving the photoelectronic and/or photovoltaic performance of hybrid perovskite-type compounds.

Factors affecting the polarity and magnitude of photoresponse of transient photodetectors

Understanding the factors that govern the polarity of response of transient photodetectors provides a strategy for optimization of their photoresponsivity.

High-performance organic broadband photomemory transistors exhibiting remarkable UV-NIR response

High-performance organic broadband photomemory transistors by photogenerated minority carrier trapping and accumulation kinetics.

Polarization-sensitive broadband photodetector using a black phosphorus vertical p-n junction

The ability to detect light over a broad spectral range is central to practical optoelectronic applications and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here, we demonstrate a broadband photodetector using a layered black phosphorus transistor that is polarization-sensitive over a bandwidth from ∼400 nm to 3,750 nm. The polarization sensitivity is due to the strong intrinsic linear dichroism, which arises from the in-plane optical anisotropy of this material. In this transistor geometry, a perpendicular built-in electric field induced by gating can spatially separate the photogenerated electrons and holes in the channel, effectively reducing their recombination rate and thus enhancing the performance for linear dichroism photodetection. The use of anisotropic layered black phosphorus in polarization-sensitive photodetection might provide new functionalities in novel optical and optoelectronic device applications.

A lead-halide perovskite molecular ferroelectric semiconductor

Inorganic semiconductor ferroelectrics such as BiFeO₃ have shown great potential in photovoltaic and other applications. Currently, semiconducting properties and the corresponding application in optoelectronic devices of hybrid organo-plumbate or stannate are a hot topic of academic research; more and more of such hybrids have been synthesized. Structurally, these hybrids are suitable for exploration of ferroelectricity. Therefore, the design of molecular ferroelectric semiconductors based on these hybrids provides a possibility to obtain new or high-performance semiconductor ferroelectrics. Here we investigated Pb-layered perovskites, and found the layer perovskite (benzylammonium)₂PbCl₄ is ferroelectric with semiconducting behaviours. It has a larger ferroelectric spontaneous polarization P_s=13 μC cm⁻² and a higher Curie temperature T_c=438 K with a band gap of 3.65 eV. This finding throws light on the new properties of the hybrid organo-plumbate or stannate compounds and provides a new way to develop new semiconductor ferroelectrics.

Lead-halide perovskite compounds have seen a considerable interest for their optoelectronic properties. Here, the authors discover a ferroelectric halide perovskite compound as an alternative pathway towards designing semiconductor ferroelectrics.