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

Two-Dimensional Spin-Crossover Molecular Solid Solutions with Tunable Transition Temperatures across 90 K

Spin-crossover (SCO) materials exhibit remarkable potential as bistable switches in molecular devices. However, the spin transition temperatures (Tc) of known compounds are unable to cover the entire ambient temperature spectrum, largely limiting their practical utility. This study reports an exemplary two-dimensional SCO solid solution system, [FeIII(H0.5LCl)2-2x(H0.5LF)2x]·H2O (H0.5LX = 5-X-2-hydroxybenzylidene-hydrazinecarbothioamide, X = F or Cl, x = 0 to 1), in which the adjacent layers are adhered via hydrogen bonding. Notably, the Tc of this system can be fine-tuned across 90 K (227-316 K) in a linear manner by modulating the fraction x of the LF ligand. Elevating x results in strengthened hydrogen bonding between adjacent layers, which leads to enhanced intermolecular interactions between adjacent SCO molecules. Single-crystal diffraction analysis and periodic density functional theory calculations revealed that such a special kind of alteration in interlayer interactions strengthens the FeIIIN2O2S2 ligand field and corresponding SCO energy barrier, consequently resulting in increased Tc. This work provides a new pathway for tuning the Tc of SCO materials through delicate manipulation of molecular interactions, which could expand the application of bistable molecular solids to a much wider temperature regime.

Dual Circularly Polarized Luminescence (CPL) and Piezoelectric Responses in Self-Assembled Chiral Nanostructures Derived from a Dipeptide Based Piezorganogel

Next-generation medical and consumer electrical devices require soft, flexible materials. Piezoelectric materials, capable of converting mechanical stress into electrical energy, are of interest across various fields. Chiral nanostructures, with inherent chirality, have emerged as potential piezoelectric materials. Peptide-based materials, known for self-assembly and stimuli responsiveness, hold promise for the utilization of chiral nanostructures. When combined with luminescent chromophores, peptides can generate aggregation-induced chiroptical effects like Circularly Polarized Luminescence (CPL) and Circular Dichroism (CD). In this study, a chiral organogel, L,L-1 is synthesized, and its self-assembly, mechanical properties, and chiroptical features are examined. The organogel exhibits thermo-reversible and thixotropic behavior, forming fibrillar networks and 2D-sheets upon cooling. CD spectroscopy reveals aggregation-induced chirality on pyrene chromophore, resulting in CPL with glum values of 3.0 (± 0.2) × 10-3 and 3.1 (± 0.2) × 10-3 for L,L-1 and D,D-1, respectively. Notably, the 2D-sheets exhibit an enhanced piezoelectric response (d33 ≈76.0 pm V-1 ) compared to the fibrillar network (d33 ≈64.1 pm V-1 ). Introducing an electron-deficient molecule into the solution forms a Charge-transfer (CT) complex, modulating the piezoelectric response to d33 ≈52.44 pm V-1 . This study offers a promising approach to optoelectronics design, presenting a chiral system with both CPL and piezoelectric responses, opening new possibilities for innovative applications.

In situ recrystallization of zero-dimensional hybrid metal halide glass-ceramics toward improved scintillation performance

Zero-dimensional (0D) hybrid metal halide (HMH) glasses are emerging luminescent materials and have gained attention due to their transparent character and ease of processing. However, the weakening of photoluminescence quantum efficiency from crystal to glass phases poses limitations for photonics applications. Here we develop high-performance glass-ceramic (G-C) scintillators via in situ recrystallization from 0D HMH glass counterparts composed of distinct organic cations and inorganic anions. The G-C scintillators maintain excellent transparency and exhibit nearly 10-fold higher light yields and lower detection limits than those of glassy phases. The general in situ recrystallization within the glass component by a facile heat treatment is analyzed via combined experimental elaboration and structural/spectral characterization. Our results on the development of G-Cs can initiate more exploration on the phase transformation engineering in 0D HMHs, and therefore make them highly promising for large-area scintillation screen applications.

Targeted regulation and optimization of multifunctional phase transition materials by novel void occupancy engineering

As an innovative form of stimulus-response materials, organic-inorganic hybrid phase transition materials have become a wonderful contender in the field of functional electronic equipment due to their versatile structure, intensive functions and straightforward preparation. However, the targeted regulation and optimization of the electrical/optical response, along with the establishment of regular structure-performance relationships, pose significant challenges in meeting the diverse demands of practical applications over an extended period. Herein, we conducted a systematic investigation into the role of lattice void occupancy in regulating phase transition temperature (Tp) and related optical/electrical bistability. By taking hybrid material [TMEA][Cd(SCN)3] featuring a flexible ammonium cation [TMEA]+ (TMEA = ethyltrimethylammonium) as the prototype, we successfully synthesized three phase transition materials, namely [DEDMA][Cd(SCN)3], [TEMA][Cd(SCN)3] and [TEA][Cd(SCN)3] (DEDMA = diethyldimethylammonium, TEMA = triethylmethylammonium, and TEA = tetraethylammonium), and the excellent regulation of the physical properties of these compounds was achieved through subtle engineering of void occupancy. More strikingly, [TEA][Cd(SCN)3] exhibits remarkable bistable properties in terms of dielectric and nonlinear optical responses (with second-harmonic generation intensity reaching 2.5 times that of KDP). This work provides a feasible avenue to reasonably customise organic-inorganic hybrid phase transition materials and finely adjust their intriguing functionalities.

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.

Reversible structural transformations and color-tunable emissions in organic manganese halides

Herein, we for the first time report a reversible conversion between green-emissive [DMPZ]MnCl4 and red-emissive [DMPZ]4(MnCl6)(MnCl4)2·(H2O)2 (DMPZ = 1,4-dimethylpiperazine) using kinetic and thermodynamic controlling strategies. Significantly, the synchronous structural and emission transformations in single-component organic manganese halides with adjustable emission colors are highlighted.

Paper-based sensor based on lead-free manganese halide for the determination of water content in organic solvents

A highly stable and luminescent lead-free manganese(II) halide hybrid MnBr₄(TMN)₂ (C₃₄H₄₂Br₄MnN₄) was designed and synthesized by introducing a large cationic organic spacer. The MnBr₄(TMN)₂ displays high luminescence with quantum yields up to 77% and possesses turn-off fluorescence behavior (E_x/E_m=365/546 nm) for water. These properties make the MnBr₄(TMN)₂ a promising candidate as an alternative indicator for the detection of water with potential applications for the fabrication of LEDs. Herein, a paper-based sensor based on MnBr₄(TMN)₂ is described for the determination of water content in organic solvents. The mechanism of water sensing can be tentatively explained by fluorescence quenching originating from the destruction of water due to the Mn-Br bonds of MnBr₄(TMN)₂. The MnBr₄(TMN)₂-based paper sensor exhibits an excellent discrimination ability of water content in the range 0-25.0% with a detection limit of 0.27%. Satisfactory recoveries (94.91±4.09% to 103.23±2.38%) are obtained in spiked ethanol solvent samples, which demonstrate that the MnBr₄(TMN)₂-based paper sensor is capable of detecting water content in real ethanol solvent samples.

Zero-Dimensional Hybrid Cuprous Halide of [BAPMA]Cu2Br5 as a Highly Efficient Light Emitter and an X-Ray Scintillator

Lead halide perovskites have been explored as a new kind of promising X-ray with wide applications in radiation-associated fields, but low light yield and serious toxicity extremely restrict further applications. To address these issues, we herein demonstrated one new zero-dimensional (0D) organic-inorganic hybrid cuprous halide of [BAPMA]Cu₂Br₅ (BAPMA = N,N-Bis(3-aminopropyl) methylamine) containing discrete [Cu₄Br₁₀]^6- tetramers as excellent lead-free scintillators. Upon UV light excitation, [BAPMA]Cu₂Br₅ displays highly efficient broadband yellowish-green light emission with one dominant peak at 526 nm, a large Stokes shift of 244 nm, and a high photoluminescent quantum yield of 53.40%. Significantly, this broadband light emission can also be excited by higher-energy X-ray as radioluminescence with a high scintillation light yield of 43,744 photons/MeV. The detection limit of 0.074 μGy_air/s is also far less than the required value for regular medical diagnostics of 5.5 μGy_air/s. The solution-assembled hybrid structure facilely enables the [BAPMA]Cu₂Br₅-based scintillation screen to display high-performance X-ray imaging with a spatial resolution of 15.79 lp/mm showcasing potential application in X-ray radiography. In brief, combined merits of low toxicity and cost, negligible self-absorption, a low detection limit, considerable light yield, and spatial resolution highlight the excellent scintillation performance of 0D hybrid cuprous halide.

Modulus-Modulated All-Organic Core-Shell Nanofiber with Remarkable Piezoelectricity for Energy Harvesting and Condition Monitoring

The low piezoelectricity of piezoelectric polymers significantly restricts their applications. Introducing inorganic fillers can slightly improve the piezoelectricity of polymers, whereas it is usually at the cost of flexibility and durability. In this work, using a modulus-modulated core-shell structure strategy, all-organic nanofibers with remarkable piezoelectricity were designed and prepared by a coaxial electrospinning method. It was surprisingly found that the introduction of a nonpiezoelectric polymeric core (e.g., polycarbonate, PC) can result in 110% piezoelectric coefficient (d₃₃) enhancement in a poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) nanofiber. Accordingly, the all-organic PVDF-TrFE@PC core-shell nanofiber exhibits record-high energy-harvesting performance (i.e., 126 V output voltage, 710 mW m^-2 power density) among the reported organic piezoelectric materials. In addition, the excellent sensing capability of the core-shell nanofiber enabled us to develop a wireless vibration monitoring and analyzing system, which realizes the real-time vibration detection of a power transformer.

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 .

Lead-Free MDABCO-NH4I3 Perovskite Crystals Embedded in Electrospun Nanofibers

In this work, we introduce lead-free organic ferroelectric perovskite N-methyl-N'-diazabicyclo[2.2.2]octonium)-ammonium triiodide (MDABCO-NH₄I₃) nanocrystals embedded in three different polymer fibers fabricated by the electrospinning technique, as mechanical energy harvesters. Molecular ferroelectrics offer the advantage of structural diversity and tunability, easy fabrication, and mechanical flexibility. Organic-inorganic hybrid materials are new low-symmetry emerging materials that may be used as energy harvesters because of their piezoelectric or ferroelectric properties. Among these, ferroelectric metal-free perovskites are a class of recently discovered multifunctional materials. The doped nanofibers, which are very flexible and have a high Young modulus, behave as active piezoelectric energy harvesting sources that produce a piezoelectric voltage coefficient up to g_eff = 3.6 VmN^-1 and show a blue intense luminescence band at 325 nm. In this work, the pyroelectric coefficient is reported for the MDABCO-NH₄I₃ perovskite inserted in electrospun fibers. At the ferroelectric-paraelectric phase transition, the embedded nanocrystals display a pyroelectric coefficient as high as 194 × 10^-6 Cm^-2k^-1, within the same order of magnitude as that reported for the state-of-the-art bulk ferroelectric triglycine sulfate (TGS). The perovskite nanocrystals embedded into the polymer fibers remain stable in their piezoelectric output response, and no degradation is caused by oxidation, making the piezoelectric perovskite nanofibers suitable to be used as flexible energy harvesters.

Multifunctional Chiral 2D Lead Halide Perovskites with Circularly Polarized Photoluminescence and Piezoelectric Energy Harvesting Properties

Introducing the chiral spacers to two-dimensional (2D) lead halide perovskites (LHPs) enables them to exhibit circularly polarized photoluminescence (CPPL), which could have applications in chiral-optics and spintronics. Despite that a great deal of effort has been made in this field, the reported polarization degree of CPPL at ambient conditions is still very limited, and the integration of multiple functionalities also remains to be explored. Here we report the structures, CPPL, and piezoelectric energy harvesting properties of chiral 2D LHPs, [R-1-(4-bromophenyl)ethylaminium]₂PbI₄ (R-[BPEA]₂PbI₄) and [S-1-(4-bromophenyl)ethylaminium]₂PbI₄ (S-[BPEA]₂PbI₄). Our results show that these chiral perovskites are direct bandgap semiconductors and exhibit CPPL centered at ∼513 nm with a maximum degree of polarization of up to 11.0% at room temperature. In addition, the unique configurational arrangement of the chiral spacers is found to be able to reduce the interlayer π-π interactions and consequently result in strong electron-phonon coupling. Furthermore, the intrinsic chirality of both R-[BPEA]₂PbI₄ and S-[BPEA]₂PbI₄ enables them to be piezoelectric active, and their composite films can be applied to generate voltages and currents up to ∼0.6 V and ∼1.5 μA under periodic impacting with a strength of 2 N, respectively. This work not only reports a high degree of CPPL but also demonstrates piezoelectric energy harvesting behavior for realizing multifunctionalities in chiral 2D LHPs.