Phase Switching as the Origin of Large Piezoelectric Response in Organic-Inorganic Perovskites: A First-Principles Study

The past 10 years of molecular ferroelectrics: structures, design, and properties

Ferroelectricity, which has diverse important applications such as memory elements, capacitors, and sensors, was first discovered in a molecular compound, Rochelle salt, in 1920 by Valasek. Owing to their superiorities of lightweight, biocompatibility, structural tunability, mechanical flexibility, etc., the past decade has witnessed the renaissance of molecular ferroelectrics as promising complementary materials to commercial inorganic ferroelectrics. Thus, on the 100th anniversary of ferroelectricity, it is an opportune time to look into the future, specifically into how to push the boundaries of material design in molecular ferroelectric systems and finally overcome the hurdles to their commercialization. Herein, we present a comprehensive and accessible review of the appealing development of molecular ferroelectrics over the past 10 years, with an emphasis on their structural diversity, chemical design, exceptional properties, and potential applications. We believe that it will inspire intense, combined research efforts to enrich the family of high-performance molecular ferroelectrics and attract widespread interest from physicists and chemists to better understand the structure-function relationships governing improved applied functional device engineering.

Persistent and Quasipersistent Spin Textures in Halide Perovskites Induced by Uniaxial Stress

Persistent spin textures are highly desirable for applications in spintronics, as they allow for long carrier spin lifetimes. However, they are also rare, as they require a delicate balance between spin-momentum coupling parameters. We used density functional theory simulations to predict the possibility of achieving these desirable spin textures through the application of uniaxial stress. Hybrid organic-inorganic perovskite MPSnBr3 (MP = CH3PH3) is a ferroelectric semiconductor which exhibits persistent spin textures near its conduction band minimum and mostly Rashba type spin textures in the vicinity of its valence band maximum. Application of uniaxial stress leads to the gradual evolution of the valence band spin textures from mostly Rashba type to quasipersistent ones under a tensile load and to pure Rashba or quasipersistent ones under a compressive load. The material exhibits flexibility, a rubber-like response, and both positive and negative piezoelectric constants. A combination of such properties may create opportunities for flexible and rubbery spintronic devices.

The First Demonstration of Strain-Controlled Periodic Ferroelectric Domains with Superior Piezoelectric Response in Molecular Materials

Achieving a periodic domain structure in ferroelectric materials to tailor the macroscopic properties or realize new functions has always been a hot topic. However, methods to construct periodic domain structures, such as epitaxial growth, direct writing by scanning tips, and the patterned electrode method, are difficult or inefficient to implement in emerging molecular ferroelectrics, which have the advantages of lightweight, flexibility, biocompatibility, etc. An efficient method for constructing and controlling periodic domain structures is urgently needed to facilitate the development of molecular ferroelectrics in nanoelectronic devices. In this work, it is demonstrated that large-area, periodic and controllable needle-like domain structures can be achieved in thin films of the molecular ferroelectric trimethylchloromethyl ammonium trichlorocadmium (TMCM-CdCl₃ ) upon the application of tensile strain. The domain evolution under various tensile strains can be clearly observed, and such processes are accordingly identified. Furthermore, the domain wall exhibits a superior piezoelectric response, with up to fivefold enhancement compared to that of the pristine samples. Such large-area tunable periodic domain structure and abnormally strong piezoresponse are not only of great interests in fundamental studies, but also highly important in the future applications in functional molecular materials.

Remarkable Enhancement of Piezoelectric Performance by Heavy Halogen Substitution in Hybrid Perovskite Ferroelectrics

Piezoelectric materials that enable electromechanical conversion have great application value in actuators, transducers, sensors, and energy harvesters. Large piezoelectric (d₃₃) and piezoelectric voltage (g₃₃) coefficients are highly desired and critical to their practical applications. However, obtaining a material with simultaneously large d₃₃ and g₃₃ has long been a huge challenge. Here, we reported a hybrid perovskite ferroelectric [Me₃NCH₂Cl]CdBrCl₂ to mitigate and roughly address this issue by heavy halogen substitution. The introduction of a large-size halide element softens the metal-halide bonds and reduces the polarization switching barrier, resulting in excellent piezoelectric response with a large d₃₃ (∼440 pC/N), which realizes a significant optimization compared with that of previously reported [Me₃NCH₂Cl]CdCl₃ (You et al. Science2017, 357, 306-309). More strikingly, [Me₃NCH₂Cl]CdBrCl₂ simultaneously shows a giant g₃₃ of 6215 × 10^-3 V m/N, far exceeding those of polymers and conventional piezoelectric ceramics. Combined with simple solution preparation, easy processing of thin films, and a high Curie temperature of 373 K, these attributes make [Me₃NCH₂Cl]CdBrCl₂ promising for high-performance piezoelectric sensors in flexible, wearable, and biomechanical devices.

Origin of Ferroelectricity in Two Prototypical Hybrid Organic-Inorganic Perovskites

Hybrid organic-inorganic perovskite (HOIP) ferroelectrics are attracting considerable interest because of their high performance, ease of synthesis, and lightweight. However, the intrinsic thermodynamic origins of their ferroelectric transitions remain insufficiently understood. Here, we identify the nature of the ferroelectric phase transitions in displacive [(CH₃)₂NH₂][Mn(N₃)₃] and order-disorder type [(CH₃)₂NH₂][Mn(HCOO)₃] via spatially resolved structural analysis and ab initio lattice dynamics calculations. Our results demonstrate that the vibrational entropy change of the extended perovskite lattice drives the ferroelectric transition in the former and also contributes importantly to that of the latter along with the rotational entropy change of the A-site. This finding not only reveals the delicate atomic dynamics in ferroelectric HOIPs but also highlights that both the local and extended fluctuation of the hybrid perovskite lattice can be manipulated for creating ferroelectricity by taking advantages of their abundant atomic, electronic, and phononic degrees of freedom.

A New Hybrid Lead-Free Metal Halide Piezoelectric for Energy Harvesting and Human Motion Sensing

Hybrid organic-inorganic piezoelectrics have attracted attention due to their simple synthesis, mechanical flexibility, and designability, which have promising application potential in flexible sensing and self-powered energy harvesting devices. Although some hybrid piezoelectrics are discovered, most of their structures are limited by the perovskite-type and often contain lead. Herein, the synthesis, structure, and piezoelectric properties of a new hybrid lead-free metal halide, (BTMA)₂ CoBr₄ (BTMA = benzyltrimethylammonium) are reported. The experimental and theoretical results demonstrate that this material simply composed of [CoBr₄ ]^2- tetrahedra and BTMA⁺ cations exhibits significant piezoelectricity (d₂₂ = 5.14, d₂₅ = 12.40 pC N^-1 ), low Young's and shear moduli (4.11-17.56 GPa; 1.86-7.91 GPa). Moreover, the (BTMA)₂ CoBr₄ /PDMS (PDMS = polydimethylsiloxane) composite thin films are fabricated and optimized. The 10% (BTMA)₂ CoBr₄ /PDMS-based flexible devices show attractive performance in energy harvesting with an open-circuit voltage of 19.70 V, short-circuit current of 4.24 µA, and powder density of 11.72 µW cm^-2 , catching up with those of piezoelectric ceramic composites. Meanwhile, these film devices show excellent capability in accurately sensing human body motions, such as finger bending and tapping. This work demonstrates that (BTMA)₂ CoBr₄ and related piezoelectric lead-free halides can be promising molecular materials in modern energy and sensing applications.

Flexible Piezoelectricity of Two-Dimensional Materials Governed by Effective Berry Curvature

Two-dimensional piezoelectric materials have been regarded as ideal candidates for flexible and versatile nanoelectromechanical systems, yet their fundamental piezoelectric mechanisms remain to be fully understood. Employing joint theoretical-statistical analyses, we develop universal models for quantifying the piezoelectricity of three-coordinated honeycomb-like monolayers at the atomistic level. The theoretical model within the framework of modern polarization theory suggests that the distribution of effective Berry curvature is essential for interpreting the relation between microscopic/electronic structures and piezoelectric properties. The statistical model based on DFT high-throughput calculation reveals that 2D piezoelectricity crucially depends on the effective mass, bandgap, and atomic distance along the rotation axis. Implementing stress and doping is demonstrated to be effective for optimizing piezoelectricity. Such findings provide valuable guidelines for designing 2D piezoelectric materials.