Chiral multiferroicity in two-dimensional hybrid organic-inorganic perovskites.

Chiral multiferroics offer remarkable capabilities for controlling quantum devices at multiple levels. However, these materials are rare due to the competing requirements of long-range orders and strict symmetry constraints. In this study, we present experimental evidence that the coexistence of ferroelectric, magnetic orders, and crystallographic chirality is achievable in hybrid organic-inorganic perovskites [(R/S)-ß-methylphenethylamine]2CuCl4. By employing Landau symmetry mode analysis, we investigate the interplay between chirality and ferroic orders and propose a novel mechanism for chirality transfer in hybrid systems. This mechanism involves the coupling of non-chiral distortions, characterized by defining a pseudo-scalar quantity, ξ = p ⋅ r ( p represents the ferroelectric displacement vector and r denotes the ferro-rotational vector), which distinguishes between (R)- and (S)-chirality based on its sign. Moreover, the reversal of this descriptor's sign can be associated with coordinated transitions in ferroelectric distortions, Jahn-Teller antiferro-distortions, and Dzyaloshinskii-Moriya vectors, indicating the mediating role of crystallographic chirality in magnetoelectric correlations.

In-Plane Ferrielectric Order in van der Waals ß'-In2Se3.

van der Waals ferroic materials exhibit rich potential for implementing future generation functional devices. Among these, layered ß'-In2Se3 has fascinated researchers with its complex superlattice and domain structures. As opposed to ferroelectric α-In2Se3, the understanding of ß'-In2Se3 ferroic properties remains unclear because ferroelectric, antiferroelectric, and ferroelastic characteristics have been separately reported in this material. To develop useful applications, it is necessary to understand the microscopic structural properties and their correlation with macroscopic device characteristics. Herein, using scanning transmission electron microscopy (STEM), we observed that the arrangement of dipoles deviates from the ideal double antiparallel antiferroelectric character due to competition between antiferroelectric and ferroelectric structural ordering. By virtue of second-harmonic generation, four-dimensional STEM, and in-plane piezoresponse force microscopy, the long-range inversion-breaking symmetry, uncompensated local polarization, and net polarization domains are unambiguously verified, revealing ß'-In2Se3 as an in-plane ferrielectric layered material. Additionally, our device study reveals analogous resistive switching behaviors of different types owing to polarization switching, defect migration, and defect-induced charge trapping/detrapping processes.

Near-90° Switch in the Polar Axis of Dion-Jacobson Perovskites by Halide Substitution.

Ferroelectricity in two-dimensional hybrid (2D) organic-inorganic perovskites (HOIPs) can be engineered by tuning the chemical composition of the organic or inorganic components to lower the structural symmetry and order-disorder phase change. Less efforts are made toward understanding how the direction of the polar axis is affected by the chemical structure, which directly impacts the anisotropic charge order and nonlinear optical response. To date, the reported ferroelectric 2D Dion-Jacobson (DJ) [PbI4]2- perovskites exhibit exclusively out-of-plane polarization. Here, we discover that the polar axis in ferroelectric 2D Dion-Jacobson (DJ) perovskites can be tuned from the out-of-plane (OOP) to the in-plane (IP) direction by substituting the iodide with bromide in the lead halide layer. The spatial symmetry of the nonlinear optical response in bromide and iodide DJ perovskites was probed by polarized second harmonic generation (SHG). Density functional theory calculations revealed that the switching of the polar axis, synonymous with the change in the orientation of the sum of the dipole moments (DMs) of organic cations, is caused by the conformation change of organic cations induced by halide substitution.

Strain-Driven Solid-Solid Crystal Conversion in Chiral Hybrid Pseudo-Perovskites with Paramagnetic-to-Ferromagnetic Transition.

Hybrid organic-inorganic perovskites (HOIPs) are promising stimuli-responsive materials (SPMs) owing to their molecular softness and tailorable structural dimensionality. The design of mechanically responsive HOIPs requires an in-depth understanding of how lattice strain induces intermolecular rearrangement that impacts physical properties. While chirality transfer from an organic cation to an inorganic lattice is known to influence chiral-optical properties, its effect on strain-induced phase conversion has not been explored. As opposed to achiral or racemic organic cations, chiral organic cations can potentially afford a new dimension in strain-responsive structural change. Herein, we demonstrate that mechanical strain induces a solid phase crystal conversion in chiral halide pseudo-perovskite single crystals (R/S)-(FE)2CuCl4 (FE = (4-Fluorophenyl)ethylamine) from a 0D isolated CuCl4 tetrahedral to 1D corner-sharing CuFCl5 octahedral framework via the incorporation of Cu···F interaction and N-H···F hydrogen bonding. This strain-induced crystal-to-crystal conversion involves the connection of neighboring 0D CuCl4 tetrahedra via Cu2+-Cl--Cu2+ linkages as well as the incorporation of a F-terminated organic cation as one of the X atoms in BX6 octahedra, leading to a reduced band gap and paramagnetic-to-ferromagnetic conversion. Control experiments using nonchiral or racemic perovskite analogs show the absence of such solid phase conversion. To demonstrate pressure-sensitive properties, the 0D phase is dispersed in water-soluble poly(vinyl alcohol) (PVA) polymer, which can be applied to a large-scale pressure-induced array display on fibrous Spandex substrates via a screen-printing method.

Growth of bilayer MoTe2 single crystals with strong non-linear Hall effect.

The reduced symmetry in strong spin-orbit coupling materials such as transition metal ditellurides (TMDTs) gives rise to non-trivial topology, unique spin texture, and large charge-to-spin conversion efficiencies. Bilayer TMDTs are non-centrosymmetric and have unique topological properties compared to monolayer or trilayer, but a controllable way to prepare bilayer MoTe2 crystal has not been achieved to date. Herein, we achieve the layer-by-layer growth of large-area bilayer and trilayer 1T' MoTe2 single crystals and centimetre-scale films by a two-stage chemical vapor deposition process. The as-grown bilayer MoTe2 shows out-of-plane ferroelectric polarization, whereas the monolayer and trilayer crystals are non-polar. In addition, we observed large in-plane nonlinear Hall (NLH) effect for the bilayer and trilayer Td phase MoTe2 under time reversal-symmetric conditions, while these vanish for thicker layers. For a fixed input current, bilayer Td MoTe2 produces the largest second harmonic output voltage among the thicker crystals tested. Our work therefore highlights the importance of thickness-dependent Berry curvature effects in TMDTs that are underscored by the ability to grow thickness-precise layers.

Intercalation-driven ferroelectric-to-ferroelastic conversion in a layered hybrid perovskite crystal.

Two-dimensional (2D) organic-inorganic hybrid perovskites have attracted intense interests due to their quantum well structure and tunable excitonic properties. As an alternative to the well-studied divalent metal hybrid perovskite based on Pb2+, Sn2+ and Cu2+, the trivalent metal-based (eg. Sb3+ with ns2 outer-shell electronic configuration) hybrid perovskite with the A3M2X9 formula (A = monovalent cations, M = trivalent metal, X = halide) offer intriguing possibilities for engineering ferroic properties. Here, we synthesized 2D ferroelectric hybrid perovskite (TMA)3Sb2Cl9 with measurable in-plane and out-of-plane polarization. Interestingly, (TMA)3Sb2Cl9 can be intercalated with FeCl4 ions to form a ferroelastic and piezoelectric single crystal, (TMA)4-Fe(iii)Cl4-Sb2Cl9. Density functional theory calculations were carried out to investigate the unusual mechanism of ferroelectric-ferroelastic crossover in these crystals.

Giant room temperature electrocaloric effect in a layered hybrid perovskite ferroelectric: [(CH3)2CHCH2NH3]2PbCl4.

Electrocaloric effect driven by electric fields displays great potential in realizing highly efficient solid-state refrigeration. Nevertheless, most known electrocaloric materials exhibit relatively poor cooling performance near room temperature, which hinders their further applications. The emerging family of hybrid perovskite ferroelectrics, which exhibits superior structural diversity, large heat exchange and broad property tenability, offers an ideal platform. Herein, we report an exceptionally large electrocaloric effect near room temperature in a designed hybrid perovskite ferroelectric [(CH3)2CHCH2NH3]2PbCl4, which exhibits a sharp first-order phase transition at 302 K, superior spontaneous polarization (>4.8 µC/cm2) and relatively small coercive field (<15 kV/cm). Strikingly, a large isothermal entropy change ΔS of 25.64 J/kg/K and adiabatic temperature change ΔT of 11.06 K under a small electric field ΔE of 29.7 kV/cm at room temperature are achieved, with giant electrocaloric strengths of isothermal ΔS/ΔE of 0.86 J·cm/kg/K/kV and adiabatic ΔT/ΔE of 370 mK·cm/kV, which is larger than those of traditional ferroelectrics. This work presents a general approach to the design of hybrid perovskite ferroelectrics, as well as provides a family of candidate materials with potentially prominent electrocaloric performance for room temperature solid-state refrigeration.

Bromine-Substitution-Induced High-Tc Two-Dimensional Bilayered Perovskite Photoferroelectric.

High-Curie-temperature (Tc) ferroelectrics have exhibited broad applications in optoelectronic devices. Recently, two-dimensional multilayered perovskite ferroelectrics with excellent photoelectric attributes are attracting increasing interest as new systems of photoferroelectrics. However, the effective tuning of the Tc value of a multilayered perovskite photoferroelectric system still remains a huge challenge. Here, by a halogen substitution strategy to introduce bromine atoms on n-propylamine cations, the hybrid perovskite photoferroelectric (3-bromopropylaminium)2(formamidinium)Pb2Br7 (BFPB) with a high Tc value (348.5 K) was obtained. It is notable that BFPB adopts a two-dimensional bilayered inorganic framework, with tight linking to the organic cation by C-Br···Br-Pb halogen···halogen interactions and N-H···Br hydrogen bonds. Intriguingly, in comparison with the prototypical compound (n-propylaminium)2(formamidinium)Pb2Br7, a remarkable augmentation of 85.2 K in the resulting Tc value of BFPB is clearly observed, which further broadens the temperature range of its application. In combination with the remarkable ferroelectric and semiconducting attributes, the reversible bulk photovoltaic effect was realized in single crystals of BFPB. This finding can not only enhance the hybrid perovskite ferroelectric family but also further promote the photoelectric application of ferroelectrics.

(C3H9NI)4AgBiI8: a direct-bandgap layered double perovskite based on a short-chain spacer cation for light absorption.

A new iodide layered double perovskite (C3H9NI)4AgBiI8 (IPAB) has been developed based on a short-chain spacer cation, which is the first homologous compound in iodide double perovskites that adopt the Ruddlesden-Popper structure type. Importantly, IPAB is a promising environmentally friendly alternative to the recently rapidly progressing lead halide semiconductors owing to its narrow direct-bandgap of 1.87 eV and excellent stability.

Large-Area Exfoliated Lead-Free Perovskite-Derivative Single-Crystalline Membrane for Flexible Low-Defect Photodetectors.

Wide applications of personal consumer electronics have tended to cause a huge demand for smart and portable electronics, featuring mechanical flexibility, lightweight, and environmental friendliness. However, most of the recently reported flexible photodetectors based on microcrystalline and amorphous components commonly suffer from severe drawbacks, including plenty of grains, boundaries, and surface defects. Here, we present a new lead-free chiral perovskite-derivative light absorber of (aminoguanidinium)3Bi2I9 (AG3Bi2I9), which displays a narrow direct band gap of about 1.89 eV. High-quality bulk single crystals were successfully grown with dimensions up to 24 × 12 × 5 mm3. Emphatically, as-grown bulk single crystals are easy to be exfoliated for large-area ultrathin wafers with an exfoliated area up to 0.6 cm2, showing promise for low-defect flexible optoelectronic applications. The remarkable surface smoothness and crystalline quality of single-crystalline thin layers were further confirmed by TEM, HRTEM, AFM, single-crystalline X-ray diffraction, and space-charge limited current (SCLC) measurements. As expected, the planar photodetectors based on flexible exfoliated wafers are first fabricated and exhibit notable photoelectric performance. This work represents an important step forward as it offers an effective strategy for the fabrication of high-quality large-area flexible exfoliated wafer devices.

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

Polarized light detection is attracting increasing attention for its wide applications ranging from optical switches to high-resolution photodetectors. Two-dimensional (2D) hybrid perovskite-type ferroelectrics combining inherent light polarization dependence of bulk photovoltaic effect (BPVE) with excellent semiconducting performance present significant possibilities. Now, the BPVE-driven highly sensitive polarized light detection in a 2D trilayered hybrid perovskite ferroelectric, (allyammonium)2 (ethylammonium)2 Pb3 Br10 (1), is presented. It shows a superior BPVE with near-band gap photovoltage of ca. 2.5 V and high on/off switching ratio of current (ca. 104 ). Driven by the superior BPVE, 1 exhibits highly sensitive polarized light detection with a polarization ratio as high as ca. 15, which is far more beyond than those of structural anisotropy-based monocomponent devices. This is the first realization of BPVE-driven polarized light detection in hybrid perovskite ferroelectrics.

Exploring a lead-free organic-inorganic semiconducting hybrid with above-room-temperature dielectric phase transition.

Recently, organic-inorganic hybrid lead halide perovskites have attracted great attention for optoelectronic applications, such as light-emitting diodes, photovoltaics and optoelectronics. Meanwhile, the flexible organic components of these compounds give rise to a large variety of important functions, such as dielectric phase transitions. However, those containing Pb are harmful to the environment in vast quantities. Herein, a lead-free organic-inorganic hybrid, (C6H14N)2BiCl5 (CHA; C6H14N+ is cyclohexylaminium), has been successfully developed. As expected, CHA exhibits an above-room-temperature solid phase transition at 325 K (T c), which was confirmed by the differential scanning calorimetry measurement and variable temperature single crystal X-ray diffraction analyses. Further analyses indicate the phase transition is mainly governed by the order-disorder transformation of organic cyclohexylaminium cations. Interestingly, during the process of phase transition, the dielectric constant (ε') of CHA shows an obvious step-like anomaly, which displays a low dielectric constant state below T c and a high dielectric constant state above T c. Furthermore, variable temperature conductivity combined with theoretical calculations demonstrate the notable semiconducting feature of CHA. It is believed that our work will provide useful strategies for exploring lead-free organic-inorganic semiconducting hybrid materials with above room temperature dielectric phase transitions.

Tunable optical absorption in lead-free perovskite-like hybrids by iodide management.

We present an instructional design strategy to tune the optical absorption ability of organic-inorganic hybrids by managing the internal iodide state. Three bismuth-based hybrids with different internal iodide states were synthesized and display tunable bandgaps from 1.91 eV to 1.87 eV and finally to 1.59 eV.

Tailored Synthesis of an Unprecedented Pb-Mn Heterometallic Halide Hybrid with Enhanced Emission.

Organic-inorganic lead halide hybrids have attracted extensive interest in solid-state lighting, due to their superior color tunability and low-cost solution processing. However, the relatively low photoluminescence quantum efficiency (PLQE) is a common issue for most bulk lead halide hybrids. Inspired by the intriguing luminescence properties of heterometallic complexes, we rationally developed an unprecedented two-dimensional (2D) Pb-Mn heterometallic halide hybrid, (C5H14N2)2Pb4MnCl14 (2), through a precisely tailored synthetic approach based on (C5H14N2)2Pb5Cl14 (1). Intriguingly, 2 features a unique 2D heterometallic halide layer configuration, in which the strong quantum confinement facilitates efficient energy transfer from bound excitons to d-states of Mn2+, resulting in highly sensitized Mn2+emission. The PLQE of 2 is up to 32%, considerably higher than that of pristine 1 (less than 1%). Moreover, 2 presents significant environmental and thermal stability, benefiting from its cluster feature. To our best knowledge, this is the first example of construction of a Pb-Mn heterometallic halide hybrid with bulk highly efficient red emission. This work provides a way to enhance the PLQE of lead halide hybrids via sensitization in rationally designed heterometallic halide hybrids.

An Unprecedented Biaxial Trilayered Hybrid Perovskite Ferroelectric with Directionally Tunable Photovoltaic Effects.

Multiaxial molecular ferroelectrics, in which multiple-directional switching of spontaneous polarization creates diverse properties, have shown many intriguing advantages, making them indispensable complements to conventional inorganic oxides. Despite recent blooming advances, multiaxial molecular ferroelectric with bulk photovoltaic effects still remains a huge blank. Herein, we report a biaxial lead halide ferroelectric, EA4Pb3Br10 (1, EA = ethylammonium), which adopts the unique trilayered perovskite motif with a high Curie temperature of ∼384 K. Particularly, for 1, the distinct symmetry breaking with 4/ mmmF mm2 species leads to the emergence of four equivalent polarization directions in the ferroelectric phase. Based on its biaxial nature, the bulk photovoltaic effect of 1 can be facilely tuned between such multiple directions through electric poling. As far as we know, this is the first report on biaxial hybrid perovskite ferroelectric showing directionally tunable photovoltaic activity. This work provides an avenue to control the bulk physical properties of multiaxial molecular ferroelectrics, and highlights their potential for further applications in the field of smart devices.

Exploration of Chiral Organic-Inorganic Hybrid Semiconducting Lead Halides.

Organic-inorganic lead halides have recently emerged as promising alternatives to conventional optoelectronic materials, considering their intriguing physical properties. However, organic-inorganic lead halides featuring chirality are seldom explored. Here, a pair of enantiomorphic organic-inorganic hybrid semiconducting lead halides, (R-C5 H14 N2 )PbBr4 (1R) and (S-C5 H14 N2 )PbBr4 (2S), were successfully obtained with the templating of chiral amines. These compounds adopt distinct one-dimensional infinite quantum helices formed by edge-shared transformative lead bromide octahedra. Notably, 1R and 2S present mirror circular dichroism (CD) signals due to the chirality transfer of the enantiopure amines. Furthermore, 1R and 2S exhibit phase-matchable quadratic nonlinear response and typical semiconducting behaviours. This work highlights the potential of lead halides as a new kind of chiral semiconducting materials in spintronic and chiral optical applications.

Discovery of an Above-Room-Temperature Antiferroelectric in Two-Dimensional Hybrid Perovskite.

Antiferroelectric materials have been regarded as a promising candidate for electronic energy storage devices, due to their natural double polarization versus electric field ( P- E) hysteresis loops. Currently, two-dimensional organic-inorganic hybrid perovskites with structural diversity and tunability, have received blooming interests, whereas above-room-temperature antiferroelectrics are still unreported in this perovskite system. Herein, for the first time, we successfully acquire a two-dimensional Ruddlesden-Popper hybrid perovskite antiferroelectric, ((CH3)2CHCH2NH3)2CsPb2Br7 (1), which shows an above-room-temperature Curie temperature at 353 K, trigging by the synergistic dynamic motion of inorganic Cs atoms and organic isobutylammonium cations. Intriguingly, the antiferroelectricity of 1 existing over a wide temperature range of 298-353 K are revealed by the distinct double P- E hysteresis loops. Besides, 1 possesses remarkable energy storage efficiency up to 69%, comparable to those of some reported inorganic antiferroelectric ceramics, promoting 1 potential application in energy storage devices. This work provides an avenue to construct novel antiferroelectric materials for high-performance electronic device applications.

Two Non-π-Conjugated Deep-UV Nonlinear Optical Sulfates.

The non-π-conjugated sulfate system has long been overlooked as potential deep-UV nonlinear optical (NLO) materials. Here we report two asymmetric anhydrous sulfates, namely, NH4NaLi2(SO4)2 (Ι) and (NH4)2Na3Li9(SO4)7 (Π), which consist of non-π-conjugated [SO4]2- anions. Their single crystals can be readily grown by a facile evaporation method from water solution. Both sulfates are transparent down to the deep-UV region. Interestingly, there is a large NLO gap between I and Π, with phase-matching NLO responses of 1.1 and 0.5 times that of the benchmark KH2PO4, respectively. The first-principles studies reveal that the non-π-conjugated [SO4]2- anions are the dominate NLO-active groups, and the large NLO gap between I and Π can be ascribed to the nonbonding O 2 p orbitals of different orientations in the crystallographically independent S1O4 groups. This work provides an innovative non-π-conjugated source that is distinct from the traditional π-conjugated ones for deep-UV NLO materials.

Abrupt Structural Transformation in Asymmetric ABPO4F (A = K, Rb, Cs).

An asymmetric structure is the necessary requirement for second-order nonlinear-optical (NLO) materials, which have important applications in modern science and technology. Here we report two isostructural asymmetric compounds, RbBPO4F and CsBPO4F. Both compounds crystallize in cubic space group P213 (No. 198) with three-dimensional (3D) gismondine-like structures. Remarkably, in spite of the same basic structural units BO3F and PO4, both structures are distinct from the previously reported derivative KBPO4F, which crystallizes in a monoclinic space group Cc (No. 9) with a two-dimensional (2D)-layered structure. Careful structural analysis reveals that this structural transformation (from a monoclinic 2D structure to cubic 3D structures) should be aroused by the different alkaline ionic radii. To the best of our knowledge, such an abrupt structural transformation by alkaline elements is reported in all-inorganic asymmetric compounds for the first time. The structural transformation from 2D to 3D structures is favorable to eliminate the layered growth habit. This study will shed deep insight in the structural modulation of asymmetric compounds.

Two-Dimensional Hybrid Perovskite-Type Ferroelectric for Highly Polarization-Sensitive Shortwave Photodetection.

Two-dimensional (2D) materials have been well developed for polarization-sensitive photodetection, while new 2D members used in shortwave region (>2.5 eV) still remain scarce. The family of 2D hybrid perovskite ferroelectrics, in which the coupling of spontaneous polarization ( Ps) and light benefits dissociation of photoinduced carriers, has shown great potential in this portfolio. Here, we report a new 2D hybrid perovskite ferroelectric, [CH3(CH2)3NH3]2(CH3NH3)Pb2Br7 (1), which exhibits a superior Ps of 3.6 µC/cm2 and a relatively wide bandgap (∼2.55 eV). The unique 2D perovskite motif results in an intrinsic anisotropy of optical absorption (the ratio αc/αa ≈ 1.9 at 405 nm), involving its polarization-sensitive activity. As expected, the strongest photoresponses were observed along the c-axis (i.e., parallel to Ps), along with a large dichroism ratio ( Iphc/ Ipha ≈ 2.0) and highly sensitive detectivity up to ∼109 Jones. Further, crystal-device of 1 shows a fast responding rate (∼20 µs) and excellent antifatigued merits. As pioneering work, 1 is the first polarization-sensitive ferroelectric in the new branch of 2D hybrid perovskites. Such intriguing behaviors make 1 a potential candidate for the shortwave polarized-light detection, which also sheds light on new functionalities for future optoelectronic application of hybrid perovskites.