Room-Temperature Anisotropic Actuation Driven by a Synergistic Order-Disorder and Displacive Phase Transition in a Ferroelectric Crystal.

Actuating materials convert different forms of energy into mechanical responses. To satisfy various application scenarios, they are desired to have rich categories, novel functionalities, clear structure-property relationships, fast responses, and, in particular, giant and reversible shape changes. Herein, we report a phase transition-driven ferroelectric crystal, (rac-3-HOPD)PbI3 (3-HOPD = 3-hydroxypiperidine cation), showing intriguingly large and anisotropic room-temperature actuating behaviors. The crystal consists of rigid one-dimensional [PbI3] anionic chains running along the a-axis and discrete disk-like cations loosely wrapping around the chains, leaving room for anisotropic shape changes in both the b- and c-axes. The shape change is switched by a ferroelectric phase transition occurring at around room temperature (294 K), driven by the exceptionally synergistic order-disorder and displacive phase transition. The rotation of the cations exerts internal pressure on the stacking structure to trigger an exceptionally large displacement of the inorganic chains, corresponding to a crystal lattice transformation with length changes of +24.6% and -17.5% along the b- and c-axis, respectively. Single crystal-based prototype devices of circuit switches and elevators have been fabricated by exploiting the unconventional negative temperature-dependent actuating behaviors. This work provides a new model for the development of multifunctional mechanically responsive materials.

Shape Shifting and Locking in Mechanically Responsive Organic-Inorganic Hybrid Materials for Thermoelastic Actuators.

The construction of mechanically responsive materials with reversible shape-shifting, shape-locking, and stretchability holds promise for a wide range of applications in fields such as soft robotics and flexible electronics. Here, we report novel thermoelastic one-dimensional organic-inorganic hybrids (R/S-Hmpy)PbI3 (Hmpy=2-hydroxymethyl-pyrrolidinium) to show mechanical responses. The single crystals undergo two phase transitions at 310 K and 380 K. When heated to 380 K, they show shape-shifting and expansion along the b-axis by about 13.4 %, corresponding to a larger deformation than that of thermally activated shape memory alloys (8.5 %), and exhibit a strong actuation force. During the cooling process, the stretched crystal shape maintains and a shape-locking phenomenon occurs, which is lifted when the temperature decreases to 305 K. Meanwhile, due to the introduction of chiral ions, the thermal switching shows a 10-fold second-order nonlinear switching contrast (common values typically below 3-fold). This study presents a thermoelastic actuator based on shape-shifting and -locking of organic-inorganic hybrids for the first time. The dielectric and nonlinear optical switching properties of organic-inorganic hybrids broaden the range of applications of mechanically responsive crystals.

Unveiling Chiral Perovskite CD Signal Scaling: Discerning Authentic and Counterfeit Signals through Sample-State Analysis.

Circular dichroism (CD) spectroscopy is a well-known and powerful technique widely used for distinguishing chiral enantiomers based on their differential absorbance of the right and left circularly polarized light. With the increasing demand for solid-state chiral optics, CD spectroscopy has been extended to elucidate the chirality of solid-state samples beyond the traditional solution state. However, due to the sample preparation differential, the CD spectra of the same compound measured by different researchers may not be mutually consistent. In this study, we employ solution, powder, thin-film, and single-crystal samples to explore the challenges associated with CD measurements and distinguish between genuine and fake signals. Rational fabrication of the solid-state samples can effectively minimize the macroscopic anisotropic nature of the samples and thereby mitigate the influence of linear dichroism (LD) and linear birefringence (LB) effects, which arise from anisotropy-induced differences in the absorbances and refractive indices. The local anisotropic and overall isotropic features of the high-quality thin-film sample achieve an optically isotropic state, which exhibits superior CD signal repeatability at the front and back sides at different angles by rotating the sample along the light path. In addition, sample thickness-induced CD signal overload and absorption saturation pose more severe challenges than the LBLD-induced amplified CD signal but are rarely focused on. The CD signal overload in the deep UV region leads to the presence of fake signals, while absorption saturation results in a complete loss of the CD signal. These findings help obtain accurate CD signals by a well-fabricated optically isotropic sample to avoid LDLB and optimize the sample thickness to avoid fake signals and no signals.

A Two-Dimensional Hybrid Lead Bromide Ferroelectric Semiconductor with an Out-of-Plane Polarization.

A two-dimensional (2D) organic-inorganic hybrid perovskite (OIHP) material with out-of-plane ferroelectricity is the key to the miniaturization of vertical-sandwich-type ferroelectric optoelectronic devices. However, 2D OIHP ferroelectrics with out-of-plane polarization are still scarce, and effective design strategies are lacking. Herein, we report a novel 2D Dion-Jacobson perovskite ferroelectric semiconductor synthesized by a rigid-to-flexible cationic tailoring strategy, achieving an out-of-plane polarization of 1.7 µC/cm2 and high photoresponse. Integrating out-of-plane ferroelectricity with excellent photoelectric properties affords a promising platform to investigate ferroelectricity-related effects in vertical optoelectronic devices.

Structural Descriptors to Correlate Pb Ion Displacement and Broadband Emission in 2D Halide Perovskites.

2D hybrid lead halide perovskites exhibit versatile photoluminescent behaviors for narrowband to broadband emissions (BBEs) and have become attractive candidates for potential applications such as solid-state lighting. Establishing the relationship between the perovskite structural distortion and BBE is key but challenging in designing and optimizing the perovskite luminophores. Conventional attention is given to analyzing the intra-octahedron distortion of the [PbX6]4- (X = halide) unit that has not yet provided a clear structure-luminescence relationship. Herein, we introduce a descriptor, Pb displacement, to describe the inter-octahedron distortion to clarify the structure-emission relationship. The displacement of adjacent Pb centers represents the lattice distortion, which determines the broadband/narrowband emission instead of the octahedron distortion itself. We find a kite-type quadrilateral rule in (001) type 2D perovskites, that is, the degree to which the four octahedral central ions deviate from a square relates to the BBE. The kite-type arrangement of the Pb ions usually corresponds to the BBEs due to the large structure distortions. In contrast, the square-type arrangement of the Pb ions corresponds to the narrowband emissions because of the small distortions. The distortion descriptor magnifies the distortion scale, making it larger than the conventional one for the intra-octahedron distortion, which matches the general concept of excitons based on the scale of the crystal lattice. Therefore, the set of structural descriptors is better to correlate the perovskite structures and emission properties.

[(4AMTP)PbBr2]2PbBr4: a Nontypical Cation-Coordinated Perovskite Showing Deep-Blue Emissions and Blue-Light Photoelectric Response.

Metal-halide perovskites have been extensively investigated due to their fascinating structural diversity and outstanding optoelectronic properties. We report the photoluminescence and photoelectric response behavior of a nontypical Dion-Jacobson perovskite [(4AMTP)PbBr2]2PbBr4 (4AMTP = 4-aminomethyltetrahydropyran cation), featuring neutral PbBr2 layers incorporated into the cationic layers between the anionic [PbBr4]2- layers. A 4AMTP-coordinated PbBr2 unit constitutes the unique nontypical dication layer. The seven-coordinated PbBr6O unit forms a largely distorted decahedron with a short distance from the inorganic [PbBr4]2- layer. The photoluminescence behavior of [(4AMTP)PbBr2]2PbBr4 shows crystal size dependence. Larger crystals show purplish-blue emission with two peaks at 414 and 431 nm, whereas smaller grain size (<50 µm) crystals demonstrate a single emission peak at 425 nm. Meanwhile, it shows a blue-light photoelectric response under the illumination of 405 nm with an on/off ratio of 175 for single-crystal devices and 97 for thin-film devices. The photoresponse performance is proportional to the molar extinction coefficient of the absorption band edge.

Structural Phase Transitions and Dielectric Switching in a Series of Organic-Inorganic Hybrid Perovskites ABX3 (X=ClO4- or BF4- ).

A series of organic-inorganic hybrid perovskites ABX3 (A=diprotonated 1,4-diazabicyclo[2.2.2]octane or piperazine; B=Na+ or K+ ; X=ClO4- or BF4- ) has been synthesized. They feature a cubic cage-like host-guest structures of which A is the cationic guest residing in the anionic cage B8 X12 , B is the vertex of the cage with variable coordination numbers between six and twelve, and X is the bridging ligand with mono- and/or bidentate coordination modes. The extended Goldschmidt tolerance factor t is used to describe the phase stability of the compounds. Differential scanning calorimetry, variable-temperature structural analyses, and dielectric measurements reveal that order-disorder transitions of the A guest and/or X bridging ligand are supposed to be responsible for structural phase transitions and dielectric switching in the compounds.

Coupled Kite-to-Square Distortion Transition and Physical Properties in 2D Lead Halide Perovskite.

2D lead halide perovskites showcase diverse electrical and optoelectrical properties due to their adaptable structural distortion, which dictates the symmetry characteristics of the material. To accommodate the geometric shape of the cation, the inorganic layer of the 2D perovskite often undergoes specific distortions such as lead-halide bond length elongation/compression and lead atom displacement. The resultant distortion manifests as a quadrilateral shape formed by Pb atoms from four adjacent four octahedrons. The degree of distortion increases as the quadrilateral deviates further from a square shape and vice versa. This quadrilateral shape not only visually represents the magnitude of distortion but also confirms its direction. During the transition from kite to square distortion under external stimuli, the positions of the Pb atoms vividly illustrate the symmetry-breaking process, corresponding to a shift from high to low symmetry states. The electrical and optoelectronic properties, including ferroelectricity, pyroelectricity, piezoelectricity, nonlinear optical properties, and characteristics related to bulky photovoltaic effects, some of them exhibit direction dependence nature. This perspective employed a visible structural distortion approach to elucidate symmetry breaking and coupling distortion transitions with eight optoelectronic physical properties in 2D layered perovskite. We review recent research advancements and outline current challenges that help us to understand the structure-property relationship of 2D perovskite.

Coupled Ferroelectricity and Optical Activity in Optically Active Ferroelectrics.

Ferroelectricity was initially discovered in 1921 in Rochelle salt (potassium sodium tartrate tetrahydrate), a chiral compound containing a chiral unit. However, the inherent relationship between coupled ferroelectricity and optical activity in chiral ferroelectrics derived from achiral units, as well as in polar ferroelectrics, remains insufficiently explored. In this regard, we propose a fresh concept of optically active ferroelectrics, specifically those crystallizing in seven optically active point groups (1, 2, 4, 3, 6, m, mm2). Subsequently, we elucidate the mechanism of coupled ferroelectricity and optical activity, emphasizing the cooperative interplay of chirality and polarity flipping under the influence of an electric field. Finally, we expound on the applications of this principle for the in situ generation of chiral enantiomers and polar isomers, thereby providing valuable insights into the chiral/polar research community and advancing our comprehension of ferroelectricity.

Tuning ferroelectric phase transition temperature by enantiomer fraction.

Tuning phase transition temperature is one of the central issues in phase transition materials. Herein, we report a case study of using enantiomer fraction engineering as a promising strategy to tune the Curie temperature (TC) and related properties of ferroelectrics. A series of metal-halide perovskite ferroelectrics (S-3AMP)x(R-3AMP)1-xPbBr4 was synthesized where 3AMP is the 3-(aminomethyl)piperidine divalent cation and enantiomer fraction x varies between 0 and 1 (0 and 1 = enantiomers; 0.5 = racemate). With the change of the enantiomer fraction, the TC, second-harmonic generation intensity, degree of circular polarization of photoluminescence, and photoluminescence intensity of the materials have been tuned. Particularly, when x = 0.70 - 1, a continuously linear tuning of the TC is achieved, showing a tunable temperature range of about 73 K. This strategy provides an effective means and insights for regulating the phase transition temperature and chiroptical properties of functional materials.

Circularly Polarized Luminescence in Zero-Dimensional Antimony Halides: Structural Distortion Controlled Luminescence Thermometer.

Materials emitting circularly polarized luminescence (CPL) have been intensively studied for their promising applications in various fields. However, developing tunable and responsive CPL materials in a wide wavelength range remains a great challenge. Here, a pair of chiral (R,R/S,S-DCDA)3Sb2Cl12 (DCDA = dimethyl-1,2-cyclohexanediamine divalent cation) shows efficient broadband yellow emission with a photoluminescence (PL) quantum yield of 27.6% with a CPL asymmetry factor of 3 × 10-3. The associated chiroptical activity is attributed to the efficient chiral transfer as well as the self-trapped exciton emission originating from the large distortion of the inorganic blocks. Notably, (R,R/S,S-DCDA)3Sb2Cl12 exhibits a large red-shift emission exceeding 100 nm upon lowering temperature. An excellent linear correlation of the PL wavelength on temperature indicates that the compounds can be used as PL thermometers, which originates from a temperature-dependent linear structural distortion of the [SbCl6] emitter. This work inspires the potential utilization of CPL-emitting materials as responsive light sources.

cis/trans-Isomeric Cation Tuning Photoluminescence and Photodetection in 2D Perovskites.

Cationic components in the organic-inorganic hybrid perovskites (OIHPs) play an important role in the arrangement and tilting of the inorganic part that is responsible for semiconducting, luminescent, and photoelectronic properties. Herein, we report two 2D OIHP compounds, (cis-4ACHO)2(H3OBr)PbBr4 (1) and (trans-4ACHO)2(H3OCl)PbBr4 (2) (4ACHO = 4-aminocyclohexanol), showing both photoluminescence (PL) and photodetection (PD) that are tuned by the cis- and trans configurational isomerism of 4ACHO. Crystals of 1 and 2 exhibit similar packing structures but with different crystallographic symmetries. Compound 2 displays a broadband white-light emission with a higher PL efficiency (6.6%) than 1 (2.1%) that emits narrowband blue light while the PD property of 1 is better than 2 with a higher on/off ratio under the same conditions. The PL and PD of the two compounds show a seesaw relationship, which provides a new perspective for understanding the PL and PD properties in OIHPs.

Near-room-temperature martensitic actuation profited from one-dimensional hybrid perovskite structure.

Martensitic transformation, usually accompanied by ferroelastic and thermoelastic behaviors, is an interesting and useful mechanical-related property upon external stimuli. For molecular crystals, however, martensitic systems to show reversible stimuli-actuation behaviors are still limited because of a lack of designability and frequent crystal collapse due to large stress releases during the transformations. Here, a one-dimensional hybrid perovskite semiconductor (NMEA)PbI3 (NMEA = N-methylethylammonium) was prepared by following a dimensionality reduction design principle. The crystal undergoes reversible ferroelastic and thermoelastic martensitic transformations, which are attributed to weak intermolecular interactions among the chains that easily trigger the interchain shearing movement. The actuation behavior occurring during the phase transition is very close to room temperature and demonstrated to behave as a mechanical actuator for switching. This work provides an effective approach to designing molecular actuators with promising applications in next-generation intelligence devices.

Chiral Rashba Ferroelectrics for Circularly Polarized Light Detection.

Direct detection of circularly polarized light (CPL) is a challenging task due to limited materials and ambiguous structure-property relationships that lead to low distinguishability of the light helicities. Perovskite ferroelectric semiconductors incorporating chirality provide new opportunities in dealing with this issue. Herein, a pair of 2D chiral perovskite ferroelectrics is reported, which have enhanced CPL detection performance due to interplays among lattice, photon, charge, spin, and orbit. The chirality-transfer-induced chiral&polar ferroelectric phase enhances the asymmetric nature of the photoactive sublattice and achieves a switchable self-powered detection via the bulk photovoltaic effect. The single-crystal-based device exhibits a CPL-sensitive detection performance under 430 nm with an asymmetric factor of 0.20 for left- and right-CPL differentiation, about two times that of the pure chiral counterparts. The enhanced CPL detection performance is ascribed to the Rashba-Dresselhaus effect that originates from the bulk inversion asymmetry and strong spin-orbit coupling, shown with a large Rashba coefficient, which is demonstrated by density functional theory calculation and circularly polarized light excited photoluminescence measurement. These results provide new perspectives on chiral Rashba ferroelectric semiconductors for direct CPL detection and ferroelectrics-based chiroptics and spintronics.