A Flexible Composite Mechanical Energy Harvester from a Ferroelectric Organoamino Phosphonium Salt

A Highly Electrostrictive Salt Cocrystal and the Piezoelectric Nanogenerator Application of Its 3D-Printed Polymer Composite

Ionic cocrystals with hydrogen bonding can form exciting materials with enhanced optical and electronic properties. We present a highly moisture-stable ammonium salt cocrystal [CH3C6H4CH(CH3)NH2][CH3C6H4CH(CH3)NH3][PF6] ((p-TEA)(p-TEAH)·PF6) crystallizing in the polar monoclinic C2 space group. The asymmetry in (p-TEA)(p-TEAH)·PF6 was induced by its chiral substituents, while the polar order and structural stability were achieved by using the octahedral PF6- anion and the consequent formation of salt cocrystal. The ferroelectric properties of (p-TEA)(p-TEAH)·PF6 were confirmed through P-E loop measurements. Piezoresponse force microscopy (PFM) enabled the visualization of its domain structure with characteristic "butterfly" and hysteresis loops associated with ferro- and piezoelectric properties. Notably, (p-TEA)(p-TEAH)·PF6 exhibits a large electrostrictive coefficient (Q33) value of 2.02 m4 C-2, higher than those found for ceramic-based materials and comparable to that of polyvinylidene difluoride. Furthermore, the composite films of (p-TEA)(p-TEAH)·PF6 with polycaprolactone (PCL) polymer and its gyroid-shaped 3D-printed composite scaled-up device, 3DP-Gy, were prepared and evaluated for piezoelectric energy-harvesting functionality. A high output voltage of 22.8 V and a power density of 118.5 μW cm-3 have been recorded for the 3DP-Gy device. Remarkably, no loss in voltage outputs was observed for the (p-TEA)(p-TEAH)·PF6 devices even after exposure to 99% relative humidity, showcasing their utility under extremely humid conditions.

Biodegradable ferroelectric molecular crystal with large piezoelectric response

Transient implantable piezoelectric materials are desirable for biosensing, drug delivery, tissue regeneration, and antimicrobial and tumor therapy. For use in the human body, they must show flexibility, biocompatibility, and biodegradability. These requirements are challenging for conventional inorganic piezoelectric oxides and piezoelectric polymers. We discovered high piezoelectricity in a molecular crystal HOCH2(CF2)3CH2OH [2,2,3,3,4,4-hexafluoropentane-1,5-diol (HFPD)] with a large piezoelectric coefficient d33 of ~138 picocoulombs per newton and piezoelectric voltage constant g33 of ~2450 × 10-3 volt-meters per newton under no poling conditions, which also exhibits good biocompatibility toward biological cells and desirable biodegradation and biosafety in physiological environments. HFPD can be composite with polyvinyl alcohol to form flexible piezoelectric films with a d33 of 34.3 picocoulombs per newton. Our material demonstrates the ability for molecular crystals to have attractive piezoelectric properties and should be of interest for applications in transient implantable electromechanical devices.

Perspectives on recent advancements in energy harvesting, sensing and bio-medical applications of piezoelectric gels

The development of next-generation bioelectronics, as well as the powering of consumer and medical devices, require power sources that are soft, flexible, extensible, and even biocompatible. Traditional energy storage devices (typically, batteries and supercapacitors) are rigid, unrecyclable, offer short-lifetime, contain hazardous chemicals and possess poor biocompatibility, hindering their utilization in wearable electronics. Therefore, there is a genuine unmet need for a new generation of innovative energy-harvesting materials that are soft, flexible, bio-compatible, and bio-degradable. Piezoelectric gels or PiezoGels are a smart crystalline form of gels with polar ordered structures that belongs to the broader family of piezoelectric material, which generate electricity in response to mechanical stress or deformation. Given that PiezoGels are structurally similar to hydrogels, they offer several advantages including intrinsic chirality, crystallinity, degree of ordered structures, mechanical flexibility, biocompatibility, and biodegradability, emphasizing their potential applications ranging from power generation to bio-medical applications. Herein, we describe recent examples of new functional PiezoGel materials employed for energy harvesting, sensing, and wound dressing applications. First, this review focuses on the principles of piezoelectric generators (PEGs) and the advantages of using hydrogels as PiezoGels in energy and biomedical applications. Next, we provide a detailed discussion on the preparation, functionalization, and fabrication of PiezoGel-PEGs (P-PEGs) for the applications of energy harvesting, sensing and wound healing/dressing. Finally, this review concludes with a discussion of the current challenges and future directions of P-PEGs.

A folded π-system with supramolecularly oriented dipoles: single-component piezoelectric relaxor with NLO activity

Organic molecules with an active dipole moment have a natural propensity to align in an antiparallel fashion in the solid state, resulting in zero macroscopic polarization. This primary limitation makes the material unresponsive to switching with electric fields, mechanical forces, and to intense laser light. A single-component organic material that bestows macroscopic dipole-driven electro-mechanical and optical functions, e.g., piezoelectric, ferroelectric and nonlinear optical (NLO) activity, is unprecedented due to the design challenges imparted by crystal symmetry and dipole orientations. Herein we report a crystalline organic material that self-assembles with a polar order (P ₁), and is endowed with a high piezoelectric coefficient (d ₃₃-47 pm V^-1), as well as ferroelectric and Debye-type relaxor properties. In addition, it shows second harmonic generation (SHG) activity, which is more than five times that of the benchmark potassium dihydrogen phosphate. Piezoelectric force microscopy (PFM) images validated electro-mechanical deformations. Piezoresponse force spectroscopy (PFS) studies confirmed a signature butterfly-like amplitude and a phase loop. To the best of our knowledge, this is the first report of a folded supramolecular π-system that manifests unidirectionally oriented dipoles and exhibits piezoelectricity, ferroelectricity, and has excellent ability to generate second harmonic light. These findings can herald new design possibilities based on folded architectures to explore opto-, electro- and mechano-responsive multifaceted functions.

Exploring Helical Peptides and Foldamers for the Design of Metal Helix Frameworks: Current Trends and Future Perspectives

Flexible and biocompatible metal peptide frameworks (MPFs) derived from short and ultra-short peptides have been explored for the storage of greenhouse gases, molecular recognition, and chiral transformations. In addition to short flexible peptides, peptides with specifically folded conformations have recently been utilized to fabricate a variety of metal helix frameworks (MHFs). The secondary structures of the peptides govern the structure-assembly relationship and thereby control the formation of three-dimensional (3D)-MHFs. Particularly, the hierarchical structural organization of peptide-based MHFs has not yet been discussed in detail. Here, we describe the recent progress of metal-driven folded peptide assembly to construct 3D porous structures for use in future energy storage, chiral recognition, and biomedical applications, which could be envisioned as an alternative to the conventional metal-organic frameworks (MOFs).

A Ferroelectric Aminophosphonium Cyanoferrate with a Large Electrostrictive Coefficient as a Piezoelectric Nanogenerator

Hybrid materials possessing piezo- and ferroelectric properties emerge as excellent alternatives to conventional piezoceramics due to their merits of facile synthesis, lightweight nature, ease of fabrication and mechanical flexibility. Inspired by the structural stability of aminophosphonium compounds, here we report the first A₃ BX₆ type cyanometallate [Ph₂ (ⁱ PrNH)₂ P]₃ [Fe(CN)₆ ] (1), which shows a ferroelectric saturation polarization (P_s ) of 3.71 μC cm^-2 . Compound 1 exhibits a high electrostrictive coefficient (Q₃₃ ) of 0.73 m⁴  C^-2 , far exceeding those of piezoceramics (0.034-0.096 m⁴  C^-2 ). Piezoresponse force microscopy (PFM) analysis demonstrates the polarization switching and domain structure of 1 further confirming its ferroelectric nature. Furthermore, thermoplastic polyurethane (TPU) polymer composite films of 1 were prepared and employed as piezoelectric nanogenerators. Notably, the 15 wt % 1-TPU device gave a maximum output voltage of 13.57 V and a power density of 6.03 μW cm^-2 .

Ferroelectric coordination metal complexes based on structural and electron dynamics

Ferroelectrics that display electrically invertible polarisation are attractive materials because of their potential for wide-ranging applications. To date, considerable effort has thus been devoted towards developing ferroelectric materials, particularly those comprising organic/inorganic compounds. In these systems, structural dynamics such as atomic displacement and reorientation of polar ions/molecules play a key role in the generation of reversible spontaneous polarisation. Although there are many reports concerned with organic/inorganic ferroelectrics, ferroelectrics based on coordination metal complexes have been largely unexplored despite their often unique electronic and spin state properties. In this feature article, we discuss recent progress involving coordination metal complex-based ferroelectrics where the reversible polarisation originates not only from structural dynamics (represented by proton transfer, molecular motion, and liquid crystalline behaviour) but also from electron dynamics (represented by electron transfer and spin crossover phenomena) occurring at the metal centre. Furthermore, unique synergy effects (i.e. magnetoelectric coupling) resulting from the structural and electron dynamics are described. We believe that this review pertaining to ferroelectric coordination metal complexes provides new insights for fabricating further advanced functional materials such as multiferroics and spintronics.

A small-molecule organic ferroelectric with piezoelectric voltage coefficient larger than that of lead zirconate titanate and polyvinylidene difluoride

Piezoelectric materials that generate electricity when deforming are ideal for many implantable medical sensing devices. In modern piezoelectric materials, inorganic ceramics and polymers are two important branches, represented by lead zirconate titanate (PZT) and polyvinylidene difluoride (PVDF). However, PVDF is a nondegradable plastic with poor crystallinity and a large coercive field, and PZT suffers from high sintering temperature and toxic heavy element. Here, we successfully design a metal-free small-molecule ferroelectric, 3,3-difluorocyclobutanammonium hydrochloride ((3,3-DFCBA)Cl), which has high piezoelectric voltage coefficients g₃₃ (437.2 × 10⁻³ V m N⁻¹) and g₃₁ (586.2 × 10⁻³ V m N⁻¹), a large electrostriction coefficient Q₃₃ (about 4.29 m⁴ C⁻²) and low acoustic impedance z₀ (2.25 × 10⁶ kg s⁻¹ m⁻²), significantly outperforming PZT (g₃₃ = 34 × 10⁻³ V m N⁻¹ and z₀ = 2.54 × 10⁷ kg s⁻¹ m⁻²) and PVDF (g₃₃ = 286.7 × 10⁻³ V m N⁻¹, g₃₁ = 185.9 × 10⁻³ V m N⁻¹, Q₃₃ = 1.3 m⁴ C⁻², and z₀ = 3.69 × 10⁶ kg s⁻¹ m⁻²). Such a low acoustic impedance matches that of the body (1.38–1.99 × 10⁶ kg s⁻¹ m⁻²) reasonably well, making it attractive as next-generation biocompatible piezoelectric devices for health monitoring and “disposable” invasive medical ultrasound imaging.

A small-molecule organic ferroelectric (3,3-DFCBA)Cl has high piezoelectric voltage coefficients g₃₃ (437.2 × 10⁻³ V m N⁻¹), a large electrostriction coefficient Q₃₃, and low acoustic impedance z₀, far beyond that of PZT and PVDF.

Ferroelectric Behavior of an Octahedral Metal-Ligand Cage and Its 2D-Connected Cage Framework

Supramolecular systems hold great potential as ferroelectric materials because they are easy to prepare and do not require toxic and environmentally damaging elements. However, directing the self-assembly process of a supramolecular array to yield polarizable solids is still challenging. Here, we describe induced ferroelectricity in a supramolecular framework of metal-organic cages that are supported by a flexible tripodal ligand (NHCH₂ -(3-Py))₃ PO (TPPA). Ferroelectric responses on the discrete cage [Cu₆ (H₂ O)₁₂ (TPPA)₈ ](NO₃ )₁₂  ⋅ 45H₂ O (1) and its 2D-connected framework [{Cu₆ Cl₄ (H₂ O)₆ (TPPA)₈ }(NO₃ )₈  ⋅ 60H₂ O]_n (2) yielded well-resolved rectangular hysteresis loops at room temperature with remnant polarization values of 27.27 and 29.09 μC/cm² , respectively. Thermal hysteresis measurements (THM) and capacitance-voltage (C-V) plots further corroborate the ferroelectric behavior in these compounds. The polarization in them is due to the displacements of solvated molecules and nitrate ions in the pockets of these frameworks.

Harnessing Plasticity in an Amine-Borane as a Piezoelectric and Pyroelectric Flexible Film

We demonstrate that trimethylamine borane can exhibit desirable piezoelectric and pyroelectric properties. The material was shown to be able operate as a flexible film for both thermal sensing, thermal energy conversion and mechanical sensing with high open circuit voltages (>10 V). A piezoelectric coefficient of d₃₃ ≈10-16 pC N^-1 , and pyroelectric coefficient of p≈25.8 μC m^-2  K^-1 were achieved after poling, with high pyroelectric figure of merits for sensing and harvesting, along with a relative permittivity of ϵ 33 σ ≈ 6.3.

Record-high thermal stability achieved in a novel single-component all-organic ferroelectric crystal exhibiting polymorphism

Traditionally, lead and heavy metal containing inorganic oxides dominate the area of ferroelectricity. Although, recently, lightweight non-toxic organic ferroelectrics have emerged as excellent alternatives, achieving higher temperature up to which the ferroelectric phase can persist has remained a challenge. Moreover, only a few of those are single-component molecular ferroelectrics and were discovered upon revisiting their crystal structures. Here we report a novel phenanthroimidazole derivative, which not only displays notable spontaneous and highly stable remnant polarizations with a low coercive field but also retains its ferroelectric phase up to a record-high temperature of ∼521 K. Subsequently, the crystal undergoes phase transition to form non-polar and centrosymmetric polymorphs, the first study of its kind in a single-component ferroelectric crystal. Moreover, the compound exhibits a significantly high thermal stability. Given the excellent figures-of-merit for ferroelectricity, this material is likely to find potential applications in microelectronic devices pertaining to non-volatile memory.

Hydrogen-bonded organo-amino phosphonium halides: dielectric, piezoelectric and possible ferroelectric properties

Molecular ferroelectric materials are an exciting class of materials for potential applications in energy and electronics. Herein, we report examples of hydrogen-bonded binary salts of diphenyl diisopropylamino phosphonium halides [Ph2(iPrNH)2P]·X [DPDP·X, X = Cl, Br, I] which show dielectric, piezoelectric and NLO properties and some potentially ferroelectric attributes at room temperature. The phosphonium bromide salt was prepared by bromination of the phosphine precursor Ph2PCl and its subsequent treatment with isopropyl amine. The chloride and iodide salts were synthesized by the halogen exchange reaction of the bromide salt. The variable temperature single crystal X-ray analysis indicates the retention of the polar non-centrosymmetric phase of these materials for a wide range of temperatures from 100 to 400 K and above. All these assemblies were shown to exhibit 1D H-bonded chain structures along the crystallographic b-axis. The P-E loop measurements of these salts gave curves similar to those of non-linear leaky dielectric materials. However, the vertical piezoresponse force microscopy (V-PFM) analyses showed the existence of polarizable domain inversions indicating the possibility of ferroelectric behaviour in these materials. The temperature dependent dielectric measurements on these salts support the absence of phase transition temperatures in these assemblies. Also, bias-dependent PFM studies reveal their piezoelectric nature as the obtained converse piezoelectric coefficients are consistent with the d33 values obtained by the direct quasi-static methods.