This article provides a comprehensive overview of piezo‐ and ferro‐electric materials based on organic molecules and organic–inorganic hybrids for mechanical energy harvesting. Molecular (organic and organic–inorganic hybrid) piezo‐ and ferroelectric materials exhibit significant advantages over traditional materials due to their simple solution‐phase synthesis, light‐weight nature, thermal stability, mechanical flexibility, high Curie temperature, and attractive piezo‐ and ferroelectric properties. However, the design and understanding of piezo‐ and ferroelectricity in organic and organic–inorganic hybrid materials for piezoelectric energy harvesting applications is less well developed. This review describes the fundamental characterization of piezo‐ and ferroelectricity for a range of recently reported organic and organic–inorganic hybrid materials. The limits of traditional piezoelectric harvesting materials are outlined, followed by an overview of the piezo‐ and ferroelectric properties of organic and organic–inorganic hybrid materials, and their composites, for mechanical energy harvesting. An extensive description of peptide‐based and other biomolecular piezo‐ and ferroelectric materials as a biofriendly alternative to current materials is also provided. Finally, current limitations and future perspectives in this emerging area of research are highlighted. This perspective aims to guide chemists, materials scientists, and engineers in the design of practically useful organic and organic–inorganic hybrid piezo‐ and ferroelectric materials and composites for mechanical energy harvesting.

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Recent Advances in Organic and Organic–Inorganic Hybrid Materials for Piezoelectric Mechanical Energy Harvesting

Two-dimensional (2D) materials exhibit remarkable mechanical properties, enabling their applications as flexible and stretchable ultrathin devices. As the origin of several extraordinary mechanical behaviors, ferroelasticity has also been predicted theoretically in 2D materials, but so far lacks experimental validation and investigation. Here, we present the experimental demonstration of 2D ferroelasticity in both exfoliated and chemical-vapor-deposited β'-In2Se3 down to few-layer thickness. We identify quantitatively 2D spontaneous strain originating from in-plane antiferroelectric distortion, using both atomic-resolution electron microscopy and in situ X-ray diffraction. The symmetry-equivalent strain orientations give rise to three domain variants separated by 60° and 120° domain walls (DWs). Mechanical switching between these ferroelastic domains is achieved under ≤0.5% external strain, demonstrating the feasibility to tailor the antiferroelectric polar structure as well as DW patterns through mechanical stimuli. The detailed domain switching mechanism through both DW propagation and domain nucleation is unraveled, and the effects of 3D stacking on such 2D ferroelasticity are also discussed. The observed 2D ferroelasticity here should be widely available in 2D materials with anisotropic lattice distortion, including the 1T' transition metal dichalcogenides with Peierls distortion and 2D ferroelectrics such as the SnTe family, rendering tantalizing potential to tune 2D functionalities through strain or DW engineering.

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Two-dimensional ferroelasticity in van der Waals β'-In2Se3.

The mechanical properties of crystalline materials are crucial knowledge for their screening, design, and exploitation. Density functional theory (DFT), remains one of the most effective computational tools for quantitatively predicting and rationalising the mechanical response of these materials. DFT predictions have been shown to quantitatively correlate to a number of experimental techniques, such as nanoindentation, high-pressure X-ray crystallography, impedance spectroscopy, and spectroscopic ellipsometry. Not only can bulk mechanical properties be derived from DFT calculations, this computational methodology allows for a full understanding of the elastic anisotropy in complex crystalline systems. Here we introduce the concepts behind DFT, and highlight a number of case studies and methodologies for predicting the elastic constants of materials that span ice, biomolecular crystals, polymer crystals, and metal–organic frameworks (MOFs). Key parameters that should be considered for theorists are discussed, including exchange–correlation functionals and dispersion corrections. The broad range of software packages and post-analysis tools are also brought to the attention of current and future DFT users. It is envisioned that the accuracy of DFT predictions of elastic constants will continue to improve with advances in high-performance computing power, as well as the incorporation of many-body interactions with quasi-harmonic approximations to overcome the negative effects of calculations carried out at absolute zero.

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Density functional theory predictions of the mechanical properties of crystalline materials

The coexistence of multiferroic orders has attracted increasing attention for its potential applications in multiple-state memory, switches, and computing, but it is still challenging to design single-phase crystalline materials hosting multiferroic orders at above room temperature. By utilizing versatile ABX3-type perovskites as a structural model, we judiciously introduced a polar organic cation with easily changeable conformations into a tetrafluoroborate-based perovskite system, and successfully obtained an unprecedented molecular perovskite, (homopiperazine-1,4-diium)[K(BF4)3], hosting both ferroelectricity and ferroelasticity at above room temperature. By using the combined techniques of variable-temperature single-crystal X-ray structural analyses, differential scanning calorimetry, and dielectric, second harmonic generation, and piezoresponse force microscopy measurements, we demonstrated the domain structures for ferroelectric and ferroelastic orders, and furthermore disclosed how the delicate interplay between stepwise changed dynamics of organic cations and cooperative deformation of the inorganic framework induces ferroelectric and ferroelastic phase transitions at 311 K and 455 K, respectively. This instance, together with the underlying mechanism of ferroic transitions, provides important clues for designing advanced multiferroic materials based on organic–inorganic hybrid crystals.

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Room-temperature ferroelectric and ferroelastic orders coexisting in a new tetrafluoroborate-based perovskite

Organic Enantiomeric Ferroelectrics with High Piezoelectric Performance: Imidazolium l- and d-Camphorsulfonate

Materials that can produce large controllable strains are widely used in shape memory devices, actuators and sensors Great efforts have been made to improve the strain outputs of various material systems Among them, ferroelastic transitions underpin giant reversible strains in electrically-driven ferro/piezoelectrics and thermally- or magneticallydriven shape memory alloys However, large-strain ferroelastic switching in conventional ferroelectrics is very challenging while magnetic and thermal controls are not desirable for applications Here, we demonstrate an unprecedentedly large shear strain up to 215 % in a hybrid ferroelectric, C6H5N(CH3)3CdCl3 The strain response is about two orders of magnitude higher than those of top-performing conventional ferroelectric polymers and oxides It is achieved via inorganic bond switching and facilitated by the structural confinement of the large organic moieties, which prevents the undesired 180-degree polarization switching Furthermore, Br substitution can effectively soften the bonds and result in giant shear piezoelectric coefficient (d35 ~ 4800 pm/V) in Br-rich end of the solid solution, C6H5N(CH3)3CdBr3xCl3(1-x) The superior electromechanical properties of the compounds promise their potential in lightweight and high energy density devices, and the strategy described here should inspire the development of next-generation piezoelectrics and electroactive materials based on hybrid ferroelectrics

Ferroelastic-switching-driven colossal shear strain and piezoelectricity in a hybrid ferroelectric

Materials that can produce large controllable strains are widely used in shape memory devices, actuators and sensors1,2, and great efforts have been made to improve the strain output3–6. Among them, ferroelastic transitions underpin giant reversible strains in electrically driven ferroelectrics or piezoelectrics and thermally or magnetically driven shape memory alloys7,8. However, large-strain ferroelastic switching in conventional ferroelectrics is very challenging, while magnetic and thermal controls are not desirable for practical applications. Here we demonstrate a large shear strain of up to 21.5% in a hybrid ferroelectric, C6H5N(CH3)3CdCl3, which is two orders of magnitude greater than that in conventional ferroelectric polymers and oxides. It is achieved by inorganic bond switching and facilitated by structural confinement of the large organic moieties, which prevents undesired 180° polarization switching. Furthermore, Br substitution can soften the bonds, allowing a sizable shear piezoelectric coefficient (d35 ≈ 4,830 pm V−1) at the Br-rich end of the solid solution, C6H5N(CH3)3CdBr3xCl3(1−x). The electromechanical properties of these compounds suggest their potential in lightweight and high-energy-density devices, and the strategy described here could inspire the development of next-generation piezoelectrics and electroactive materials based on hybrid ferroelectrics. Reversible strains are widely used in high-technology systems, with piezoelectrics showing fast response but low strain. Here, ferroelectric C6H5N(CH3)3CdCl3 is shown to produce a strain of 21.5%, two orders of magnitude larger than other piezoelectrics, due to organic molecules preventing 180° polarization switching.

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Ferroelastic-switching-driven large shear strain and piezoelectricity in a hybrid ferroelectric

Self-assembly of metal nanocrystals is able to create a gap of sub-nanometer distance for concentrating incoming light by the strong coupling of surface plasmon resonance, known as a 'hot spot'. Although the plasmonic property of silver is better than other metals in the visible range, the superior Raman enhancement of silver compared to gold is still under debate. To provide direct evidence, in this work, we studied the silver adsorption on assembled gold nanorods (AuNRs) using in situ surface-enhanced Raman scattering (SERS) measurements. The self-assembled AuNR multimers were used as the SERS substrate, where the 4-mercaptophenol (MPh) molecules in our experiment played dual roles as both probe molecules for the Raman scattering and linking molecules for the AuNR assembly in a basic environment. Silver atoms were adsorbed on the surface of gold nanorod assemblies by reduction of Ag+ anions. The stability of the adsorbed silver was guaranteed by the basic environment. We monitored the SERS signal during the silver adsorption with a home-built in situ Raman spectroscope, which was synchronized by recording the UV-vis absorption spectra of the reaction solution to instantly quantify the plasmonic effect of the silver adsorption. Although a minor change was found in the plasmonic resonance wavelength or intensity, the measured SERS signal at specific modes faced a sudden increase by 2.1 folds during the silver adsorption. The finite element method (FEM) simulation confirmed that the silver adsorption corresponding to the plasmonic resonance variation gave little change to the electric field enhancement. We attributed the mode-specific enhancement mechanism of the adsorption of silver to the chemical enhancement from charge transfer (CT) for targeting molecules with a specific orientation. Our findings provided new insights to construct SERS substrates with higher enhancement factor (EF), which hopefully would encourage new applications in the field of surface-enhanced optical spectroscopies.

https://iopscience.iop.org/article/10.1088/1361-6528/ab8400/pdf

In situ monitoring of silver adsorption on assembled gold nanorods by surface-enhanced Raman scattering.

Finite-Temperature Dynamics in Cesium Lead Iodide Halide Perovskite

In the present work, Bi0.86Sm0.14FeO3 ceramics with near Pna21 single-phase structure were obtained by a repeated longtime sintering process, and the dielectric, ferroelectric, and magnetic characteristics were determined together with the magnetoelectric coupling coefficient. The evolution of Pna21 symmetry was revealed by transmission electron microscope analysis, and the ferroelectric transition from Pbnm to Pna21 was determined by differential scanning calorimetry analysis together with the dielectric characterization, and the Curie temperature was determined as 195°C. The saturated polarization–electric field (P–E) hysteresis loop was obtained with Pr = 12.2 μC cm−2, and the unlocking of ferromagnetism was confirmed by the apparent magnetization–magnetic field (M–H) hysteresis loop with Mr = 77.4 emu mol−1. The giant electric field–controlled magnetism was due to the Pna21/R3c field–induced transition, which was confirmed by both structure analysis and theoretical calculation, and the change of remanent magnetization under an electric field was achieved up to 44.8 emu mol−1 (57.9%), which was much bigger than that of rare-earth (RE)-substituted BiFeO3 ceramics without optimizing Pna21 phase fraction. The present results might significantly deepen the physical understanding of Pna21 phase and subsequently provide new hints on further enhancing electric field–controlled magnetism in RE-substituted BiFeO3 multiferroic ceramics. 

Yehui Zhang

Papers

Ferroelastic-switching-driven colossal shear strain and piezoelectricity in a hybrid ferroelectric

Ferroelastic-switching-driven large shear strain and piezoelectricity in a hybrid ferroelectric

In situ monitoring of silver adsorption on assembled gold nanorods by surface-enhanced Raman scattering.

Finite-Temperature Dynamics in Cesium Lead Iodide Halide Perovskite

Giant electric field‐controlled magnetism in Bi 0.86 Sm 0.14 FeO 3 multiferroic ceramics with Pna 2 1 symmetry