One-Dimensional Ferroelectric Nanostructures: Synthesis, Properties, and Applications

Above-Room-Temperature Ferroelectricity and Giant Second Harmonic Generation in 1D vdW NbOI3

The realization of spontaneous ferroelectricity down to the one-dimensional (1D) limit is both fundamentally intriguing and practically appealing for high-density ferroelectric and nonlinear photonics. However, the 1D vdW ferroelectric materials are not discovered experimentally yet. Here, the first 1D vdW ferroelectric compound NbOI3 with a high Curie temperature TC > 450 K and giant second harmonic generation (SHG) is reported. The 1D crystalline chain structure of the NbOI3 is revealed by cryo-electron microscopy, whereas the 1D ferroelectric order originated from the Nb displacement along the Nb-O chain (b-axis) is confirmed via obvious electrical and ferroelectric hysteresis loops. Impressively, NbOI3 exhibits a giant SHG susceptibility up to 1572 pm V-1 at a fundamental wavelength of 810 nm, and a further enhanced SHG susceptibility of 5582 pm V-1 under the applied hydrostatic pressure of 2.06 GPa. Combing in situ pressure-dependent X-ray diffraction, Raman spectra measurements, and first-principles calculations, it is demonstrated that the O atoms shift along the Nb─O atomic chain under compression, which can lead to the increased Baur distortion of [NbO2I4] octahedra, and hence induces the enhancement of SHG. This work provides a 1D vdW ferroelectric system for developing novel ferroelectronic and photonic devices.

Orthorhombic Polar Phase in Sodium Niobate Nanoribbons

Ferroelectric materials exhibit switchable spontaneous polarization below Curie's temperature, driven by octahedral distortions and rotations, as well as ionic displacements. The ability to manipulate polarization coupled with persistent remanence, drives diverse applications, including piezoelectric devices. In the last two decades, nanoscale exploration has unveiled unique material properties influenced by morphology, including the capability to manipulate polarization, patterns, and domains. This paper focuses on the characterization of nanometric sodium niobate (SN) synthesized from metallic niobium through alkali hydrothermal treatment, utilizing electron microscopy techniques, including high-resolution differential phase contrast (DPC) in scanning transmission electron microscopy (STEM). The material exhibits a nanoribbon structure forming a tree root-like network. The study identifies crystallographic phase, atomic columns displacement directions, and surface features, such as exposed planes and the absence of particular atomic columns. The high sensitivity of integrated DPC images proves crucial in overcoming observational challenges in other STEM modes. These observations are essential for potential applications in electronic, photocatalytic, and chemical reaction contexts.

Piezoelectric Biomaterials Inspired by Nature for Applications in Biomedicine and Nanotechnology

Bioelectricity provides electrostimulation to regulate cell/tissue behaviors and functions. In the human body, bioelectricity can be generated in electromechanically responsive tissues and organs, as well as biomolecular building blocks that exhibit piezoelectricity, with a phenomenon known as the piezoelectric effect. Inspired by natural bio-piezoelectric phenomenon, efforts have been devoted to exploiting high-performance synthetic piezoelectric biomaterials, including molecular materials, polymeric materials, ceramic materials, and composite materials. Notably, piezoelectric biomaterials polarize under mechanical strain and generate electrical potentials, which can be used to fabricate electronic devices. Herein, a review article is proposed to summarize the design and research progress of piezoelectric biomaterials and devices toward bionanotechnology. First, the functions of bioelectricity in regulating human electrophysiological activity from cellular to tissue level are introduced. Next, recent advances as well as structure-property relationship of various natural and synthetic piezoelectric biomaterials are provided in detail. In the following part, the applications of piezoelectric biomaterials in tissue engineering, drug delivery, biosensing, energy harvesting, and catalysis are systematically classified and discussed. Finally, the challenges and future prospects of piezoelectric biomaterials are presented. It is believed that this review will provide inspiration for the design and development of innovative piezoelectric biomaterials in the fields of biomedicine and nanotechnology.

Piezo-enhanced near infrared photocatalytic nanoheterojunction integrated injectable biopolymer hydrogel for anti-osteosarcoma and osteogenesis combination therapy

Preventing local tumor recurrence while promoting bone tissue regeneration is an urgent need for osteosarcoma treatment. However, the therapeutic efficacy of traditional photosensitizers is limited, and they lack the ability to regenerate bone. Here, a piezo-photo nanoheterostructure is developed based on ultrasmall bismuth/strontium titanate nanocubes (denoted as Bi/SrTiO3), which achieve piezoelectric field-driven fast charge separation coupling with surface plasmon resonance to efficiently generate reactive oxygen species. These hybrid nanotherapeutics are integrated into injectable biopolymer hydrogels, which exhibit outstanding anticancer effects under the combined irradiation of NIR and ultrasound. In vivo studies using patient-derived xenograft models and tibial osteosarcoma models demonstrate that the hydrogels achieve tumor suppression with efficacy rates of 98.6 % and 67.6 % in the respective models. Furthermore, the hydrogel had good filling and retention capabilities in the bone defect region, which exerted bone repair therapeutic efficacy by polarizing and conveying electrical stimuli to the cells under mild ultrasound radiation. This study provides a comprehensive and clinically feasible strategy for the overall treatment and tissue regeneration of osteosarcoma.

One-Dimensional High-Entropy Compounds

One-dimensional (1D) high-entropy compounds (HECs) with subnano diameters are highly attractive because long-range electron delocalization may occur along the high-entropy atomic chain, which results in extraordinary properties. Nevertheless, synthesizing such 1D HECs presents a substantial challenge, and the physicochemical attributes of these novel structures remain ambiguous. Herein, we developed a comelting-filling-freezing-modification (co-MFFM) method for synthesizing 1D high-entropy metal phosphide (HEP) by simultaneously encapsulating various metal cations within single-walled carbon nanotubes (SWCNTs) followed with a phosphorization process. The resulting 1D HEP nanowires confined within SWCNTs exhibit crucial features, including an ultrafine, high-entropy, and amorphous structure, along with a core-shell arrangement. The SWCNT as a shell could donate π electrons to 1D HEP for enhanced electron delocalization and protect 1D HEP as an atomically single-layered protective covering, thus boosting high electrocatalytic activity and stability. Moreover, the co-MFFM method demonstrates scalability for mass production and displays universal applicability to the synthesis of various 1D HECs.

Ferroelectricity of ice nanotube forests grown in three-dimensional graphene: the electric field effect

Generating diverse ferroelectric ice nanotubes (NTs) efficiently has always been challenging, but matters in nanomaterial synthesis and processing technology. In the present work, we propose a method of growing ice NT forests in a single cooling process. A three-dimensional (3D) graphene structure was selected to behave as a representative container in which a batch of (5, 0) ice NTs was formed simultaneously under the cooling process from molecular dynamics simulation. Other similar 3D graphene structures but with different hole configurations, like uniform triangle or both triangle and pentagon, were also tested, revealing that ice NTs with different tube indices, i.e. both (3, 0) and (5, 0), could also be formed at the same time. Intriguingly, the orientations of the dipole moments of the water molecules of an ice NT formed were independent of each other, making the net ferroelectricity of the whole system weakened or even cancelled. An electric field could help change the orientation of the water molecules of the already obtained ice NTs and even twist the tube to be a spiral (5, 1) one if it was applied during the cooling process, such that the net ferroelectricity was greatly improved. The underlying physical mechanism of all phase transition phenomena, including the improvement of the ferroelectricity under an electric field, were explored in depth from the phase transition curves and structural point of view. The obtained results are of significant application value for improving the preparation efficiency of nano-ferroelectric materials, which are prosperous in nano-devices.

Effect of lithium doping on the structural, conduction mechanism and dielectric property of MnNbO4

The development of multifunctional materials is an exceptional research area, which is aimed at enhancing the versatility of materials according to their wide fields of application. Special interest was devoted here to lithium (Li)-doped orthoniobate ANbO₄ (A = Mn), in particular, the new material Li_0.08Mn_0.92NbO₄. This compound was successfully synthesized by a solid-state method and characterized using various techniques, including X-ray diffraction (XRD), which confirmed the successful formation of an ABO₄ oxide with an orthorhombic structure and the Pmmm space group. The morphology and elemental composition were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The vibrational study (Raman) at room temperature confirmed the existence of the NbO₄ functional group. The effects of frequency and temperature on the electrical and dielectric properties were studied using impedance spectroscopy. In addition, the diminishing of the radius of semicircular arcs in the Nyquist plots (-Z'' vs. Z') showed the semiconductor behavior of the material. The electrical conductivity followed Jonscher's power law and the conduction mechanisms were identified. The electrical investigations showed the dominant transport mechanisms in the different frequency and temperature ranges, proposing the correlated barrier hopping (CBH) model in the ferroelectric phase and the paraelectric phase. The temperature dependence in the dielectric study revealed the relaxor ferroelectric nature of Li_0.08Mn_0.92NbO₄, which correlated the frequency-dispersive dielectric spectra with the conduction mechanisms and their relaxation processes. The results demonstrate that Li-doped Li_0.08Mn_0.92NbO₄ could be used both in dielectric and electrical applications.

Ferroelectric Transistors for Memory and Neuromorphic Device Applications

Ferroelectric materials have been intensively investigated for high-performance nonvolatile memory devices in the past decades, owing to their nonvolatile polarization characteristics. Ferroelectric memory devices are expected to exhibit lower power consumption and higher speed than conventional memory devices. However, non-complementary metal-oxide-semiconductor (CMOS) compatibility and degradation due to fatigue of traditional perovskite-based ferroelectric materials have hindered the development of high-density and high-performance ferroelectric memories in the past. The recently developed hafnia-based ferroelectric materials have attracted immense attention in the development of advanced semiconductor devices. Because hafnia is typically used in CMOS processes, it can be directly incorporated into current semiconductor technologies. Additionally, hafnia-based ferroelectrics show high scalability and large coercive fields that are advantageous for high-density memory devices. This review summarizes the recent developments in ferroelectric devices, especially ferroelectric transistors, for next-generation memory and neuromorphic applications. First, the types of ferroelectric memories and their operation mechanisms are reviewed. Then, issues limiting the realization of high-performance ferroelectric transistors and possible solutions are discussed. The experimental demonstration of ferroelectric transistor arrays, including 3D ferroelectric NAND and its operation characteristics, are also reviewed. Finally, challenges and strategies toward the development of next-generation memory and neuromorphic applications based on ferroelectric transistors are outlined.

Emerging Halide Perovskite Ferroelectrics

Halide perovskites have gained tremendous attention in the past decade owing to their excellent properties in optoelectronics. Recently, a fascinating property, ferroelectricity, has been discovered in halide perovskites and quickly attracted widespread interest. Compared with traditional perovskite oxide ferroelectrics, halide perovskites display natural advantages such as structural softness, low weight, and easy processing, which are highly desirable in applications pursuing miniaturization and flexibility. This review focuses on the current research progress in halide perovskite ferroelectrics, encompassing the emerging materials systems and their potential applications in ferroelectric photovoltaics, self-powered photodetection, and X-ray detection. The main challenges and possible solutions in the future development of halide perovskite ferroelectric materials are also attempted to be pointed out.

Piezoelectric materials and techniques for environmental pollution remediation

With the rapid development of industrialization and agriculture, a series of critical imminent environmental problems and water pollution have caught wide attention from the public and society. Piezoelectric catalysis technology with piezoelectric materials is a green and environmental method that can efficiently improve the separation of electron-hole pairs, then generating the active substances such as OH, H₂O₂ and O₂^-, which can degrade water pollutants. Therefore, we firstly surveyed the piezoelectric catalysis in piezoelectric materials and systematically concluded and emphasized the relationship between piezoelectric materials and the piezoelectric catalytic mechanism, the goal to elucidate the effect of polarization on piezoelectric catalytic performance and enhance piezoelectric catalytic performance. Subsequently, the applications of piezoelectric materials in water treatment and environmental pollutant remediation were discussed including degradation of organic pollutants, removal of heavy mental ions, radionuclides, bacteria disinfection and water splitting for H₂ generation. Finally, the development prospects and future outlooks of piezoelectric catalysis were presented in detail.

Utilizing Strain-Engineered Stable Halide Perovskite for Interfacial Interaction with Molecular Dipoles To Enhance Ferroelectric Switching and Piezoresponse in Polymer Composite Nanofibers

Mechanical and solar to electrical energy conversion using piezo- and ferroelectric and photovoltaic effects may be a practical answer to the rising energy demand. In this quest, piezoelectric polymer poly(vinylidene fluoride-hexafluoroproylene) (P(VDF-HFP)) has gained interest due to its superior piezo- and ferroelectricity. In photovoltaic applications, inorganic halide perovskite (IHP) of CsPbI₃ is considered a prime model compound. However, its application is limited because of the photoactive perovskite phase instability at ambient conditions. Here, we report the in situ synthesis of the stable perovskite γ-CsPbI₃ through an electrospinning process at room temperature, encapsulated within a ferroelectric copolymer poly(vinylidene fluoride-hexafluoroproylene) (P(VDF-HFP)) as a composite nanofiber. Computational calculation using density functional theory (DFT) reveals that tensile strain plays a critical role in the dynamical stabilization of γ-CsPbI₃. This tensile strain is triggered by the electrospinning process, which aids in the formation and growth of γ-CsPbI₃. In the CsPbI₃-P(VDF-HFP) composite nanofiber, γ-CsPbI₃ nucleates the polar β-crystalline phase in P(VDF-HFP), which results in the intrinsic piezo- and ferroelectric characteristics. The γ-CsPbI₃ aids in preferable molecular dipole orientation, resulting in improved nanoscale piezo- and ferroelectric properties. The composite nanofiber features a higher piezoelectric d₃₃ magnitude (∼30 pm/V) and lower decay constant for polarized domains (τ_composite ≈ 17). The composite was utilized as a piezoelectric nanogenerator to demonstrate human physiological motion monitoring in self-power mode. The relevant pressure sensitivities of 81 and 40 mV/kPa for the low-pressure (<0.6 kPa) and high-pressure (>0.6 to 12 kPa) ranges, respectively, promise its suitability in the health care sector.

One-Dimensional Ferroelectric Nanoarrays with Wireless Switchable Static and Dynamic Electrical Stimulation for Selective Regulating Osteogenesis and Antiosteosarcoma

Preventing local tumor recurrence and simultaneously improving bone-tissue regeneration are in great demand for osteosarcoma therapy. However, the current therapeutic implants fail to selectively suppress tumor growth and enhance osteogenesis, and antitumor therapy may compromise osseointegration of the bone implant. Here, based on the different responses of bone tumor cells and osteoblasts to different electric stimulations, we constructed ferroelectric BaTiO₃ nanorod arrays (NBTO) on the surface of titanium implants with switchable dynamic and static electrical stimulation for selective bone-tumor therapy and bone tissue regeneration. Polarized NBTO (PNBTO) generated a sustained dynamic electrical stimulus in response to wireless ultrasonic irradiation ("switch-on"), which disrupted the orientation of the spindle filaments of the tumor cell, blocked the G2/M phase of mitosis, and ultimately led to tumor cell death, whereas it had almost no cytotoxic effect on normal bone cells. Under the switch-off state, PNBTO with a high surface potential provided static electrical stimulation, accelerating osteogenic differentiation of mesenchymal stem cells and enhancing the quality of bone regeneration both in vitro and in vivo. This study broadens the biomedical potential of electrical stimulation therapy and provides a comprehensive and clinically feasible strategy for the overall treatment and tissue regeneration in osteosarcoma.

Phase Diagram of a Strained Ferroelectric Nanowire

Ferroelectric materials manifest unique dielectric, ferroelastic, and piezoelectric properties. A targeted design of ferroelectrics at the nanoscale is not only of fundamental appeal but holds the highest potential for applications. Compared to two-dimensional nanostructures such as thin films and superlattices, one-dimensional ferroelectric nanowires are investigated to a much lesser extent. Here, we reveal a variety of the topological polarization states, particularly the vortex and helical chiral phases, in loaded ferroelectric nanowires, which enable us to complete the strain–temperature phase diagram of the one-dimensional ferroelectrics. These phases are of prime importance for optoelectronics and quantum communication technologies.

Small-diameter p-type SnS nanowire photodetectors and phototransistors with low-noise and high-performance

P-type nanostructured photodetectors and phototransistors have been widely used in the field of photodetection due to their excellent electrical and optoelectronic characteristics. However, the large dark current of p-type photodetectors will limit the detectivity. Herein, we synthesized small-diameter single-crystalline p-type SnS nanowires (NWs) and then fabricated single SnS NW photodetectors and phototransistors. The device exhibits low noise and low dark current, and its noise current power is as low as 2.4 × 10^-28A². Under 830 nm illumination and low power density of 0.12 mW cm^-2, the photoconductive gain, responsivity and detectivity of the photodetector are as high as 3.9 × 10², 2.6 × 10²A W^-1and 1.8 × 10¹³Jones, respectively, at zero gate voltage. The rise and fall time of response are about 9.6 and 14 ms. The experimental results show that the small-diameter p-type SnS NWs have broad application prospects in high-performance and low-power photodetectors with high sensitivity, fast response speed and wide spectrum detection in the future.

Ferroelectrics in photocatalysis

The rapid development of industrialization and population brings water and air pollution and energy crisis. Solar driven catalysis is expected to relieve above issues. However, low efficiency of solar conversion limited by poor light harvesting and serious charge recombination of semiconductors and high surface reaction barriers is far from the satisfactory of industrial request. Ferroelectrics have been considered as promising photocatalysts to overcome these shortcomings. Herein, perovskite ferroelectrics such as BaTiO 3 , PbTiO 3 and LiNbO 3 , layered bismuth-based ferroelectrics like BiFeO 3 , Bi 2 WO 6 , Bi 2 MoO 6 , etc. and other ferroelectrics have been introduced, and their crystal structure, polarity source and synthetic method have been highlighted. Then, the research progress of ferroelectrics for photocatalysis has been summarized, including pollution degradation, water splitting and CO 2 reduction. Finally, the current challenges and future prospects of ferroelectric photocatalysts have been provided. The purpose of this review is not only to provide a timely summary for the application of ferroelectrics in photocatalysis, but also to present a deep insight and guideline for the future research works of ferroelectrics .

Piezoelectric A15B16C17 Compounds and Their Nanocomposites for Energy Harvesting and Sensors: A Review

Interest in pyroelectrics and piezoelectrics has increased worldwide on account of their unique properties. Applications based on these phenomena include piezo- and pyroelectric nanogenerators, piezoelectric sensors, and piezocatalysis. One of the most interesting materials used in this growing field are A¹⁵B¹⁶C¹⁷ nanowires, an example of which is SbSI. The latter has an electromechanical coupling coefficient of 0.8, a piezoelectric module of 2000 pC/N, and a pyroelectric coefficient of 12 × 10⁻³ C/m²K. In this review, we examine the production and properties of these nanowires and their composites, such as PAN/SbSI and PVDF/SbSI. The generated electrical response from 11 different structures under various excitations, such as an impact or a pressure shock, are presented. It is shown, for example, that the PVDF/SbSI and PAN/SbSI composites have well-arranged nanowires, the orientation of which greatly affects the value of its output power. The power density for all the nanogenerators based upon A¹⁵B¹⁶C¹⁷ nanowires (and their composites) are recalculated by use of the same key equation. This enables an accurate comparison of the efficiency of all the configurations. The piezo- and photocatalytic properties of SbSI nanowires are also presented; their excellent ability is shown by the high reaction kinetic rate constant (7.6 min⁻¹).

Fundamentals and Design-Led Synthesis of Emulsion-Templated Porous Materials for Environmental Applications

Emulsion templating is at the forefront of producing a wide array of porous materials that offers interconnected porous structure, easy permeability, homogeneous flow-through, high diffusion rates, convective mass transfer, and direct accessibility to interact with atoms/ions/molecules throughout the exterior and interior of the bulk. These interesting features together with easily available ingredients, facile preparation methods, flexible pore-size tuning protocols, controlled surface modification strategies, good physicochemical and dimensional stability, lightweight, convenient processing and subsequent recovery, superior pollutants remediation/monitoring performance, and decent recyclability underscore the benchmark potential of the emulsion-templated porous materials in large-scale practical environmental applications. To this end, many research breakthroughs in emulsion templating technique witnessed by the recent achievements have been widely unfolded and currently being extensively explored to address many of the environmental challenges. Taking into account the burgeoning progress of the emulsion-templated porous materials in the environmental field, this review article provides a conceptual overview of emulsions and emulsion templating technique, sums up the general procedures to design and fabricate many state-of-the-art emulsion-templated porous materials, and presents a critical overview of their marked momentum in adsorption, separation, disinfection, catalysis/degradation, capture, and sensing of the inorganic, organic and biological contaminants in water and air.

Ferroelectric Metal-Organic Framework as a Host Material for Sulfur to Alleviate the Shuttle Effect of Lithium-Sulfur Battery

Ferroelectricity has an excellent reversible polarization conversion behavior under an external electric field. Herein, we propose an interesting strategy to alleviate the shuttle effect of lithium-sulfur battery by utilizing ferroelectric metal-organic framework (FMOF) as a host material for the first time. Compared to other MOF with same structure but without ferroelectricity and commercial carbon black, the cathode based on FMOF exhibits a low capacity decay and high cycling stability. These results demonstrate that the polarization switching behaviors of FMOF under the discharge voltage of lithium-sulfur battery can effectively trap polysulfides by polar-polar interactions, decrease polysulfides shuttle and improve the electrochemical performance of lithium-sulfur battery.

High-resolution structural mapping and single-domain switching kinetics in 2D-confined ferroelectric nanodots for low-power FeRAM

Ferroelectric nanostructures have received much attention because they can be used for the next generation of ferroelectric random-access memory (FeRAM) in flexible electronic devices. Manipulation of domain reversal in ferroelectric nanostructures is extremely important, but rarely studied. Herein, we present generic and reusable fabrication of 2D-confined P(VDF-TrFE) nanodots with an integration density of up to 4 Gbit per inch2, and then investigate the structural maps and the corresponding domain switching kinetics of P(VDF-TrFE) nanodots by atomic force microscope-based (AFM-based) technology. Meanwhile, their storage features, such as precise programmability and data stability, are well characterized by piezoresponse force microscopy (PFM). Remarkably, the ferroelectric crystals in single-confined P(VDF-TrFE) nanodots simultaneously aligned in a plane over the whole patterned region. 2D-confined P(VDF-TrFE) 50 : 50 nanodots has high-temperature ferroelectric (HT FE) phase with all-trans conformations, which endows them with excellent memory characteristics, such as a low operating voltage of 3 V, a short domain nucleation of 100 ms (by V = 10 V), a fast domain growth, an excellent writing-erasing repeatability, and a long retention time. Compared with normal ferroelectric materials, like P(VDF-TrFE) 70 : 30, approximately 150% ratio of energy loss and a 5-fold duration for domain nucleation can be saved. Especially, written domains were well confined in the P(VDF-TrFE) 50 : 50 nanodots, which attains precise programmability on a single nanodot. Our systematic study provides an alternative route for the fabrication of ferroelectric nanostructures that are worth considering for the next generation of flexible FeRAM in all-organic nanoelectronic devices.

SbSI Composites Based on Epoxy Resin and Cellulose for Energy Harvesting and Sensors-The Influence of SBSI Nanowires Conglomeration on Piezoelectric Properties

In this paper, ferroelectric antimony sulfoiodide (SbSI) nanowires have been used to produce composites for device fabrication, which can be used for energy harvesting and sensors. SbSI is a very useful material for nanogenerators and nanosensors in which the high values of the piezoelectric coefficient (d₃₃ = 650 pC/N) and the electromechanical coefficient (k₃₃ = 0.9) are essential. Alternatively, cellulose and epoxy resin were matrix materials in these composites, whereas SbSI nanowires fill the matrix. Piezoelectric response induced by vibrations has been presented. Then, a composite with an epoxy resin has been used as an element to construct a fiber-reinforced polymer piezoelectric sensor. For the first time, comparison of piezoelectric properties of cellulose/SbSI and epoxy resin/SbSI nanocomposite has been presented. The influence of concentration of SbSI nanowires for properties of epoxy resin/SbSI nanocomposite and in a fiber-reinforced polymer based on them has also been shown. Results of aligning the SbSI nanowires in the epoxy matrix during a curing process have been presented as well.

Peculiar piezoelectricity of atomically thin planar structures

The emergence of piezoelectricity in two-dimensional (2D) materials has represented a milestone towards employing low-dimensional structures for future technologies. 2D piezoelectric materials possess unique and unprecedented characteristics that cannot be found in other morphologies; therefore, the applications of piezoelectricity can be substantially extended. By reducing the thickness into the 2D realm, piezoelectricity might be induced in otherwise non-piezoelectric materials. The origin of the enhanced piezoelectricity in such thin planes is attributed to the loss of centrosymmetry, altered carrier concentration, and change in local polarization and can be efficiently tailored via surface modifications. Access to such materials is important from a fundamental research point of view, to observe the extraordinary interactions between free charge carriers, phonons and photons, and also with respect to device development, for which planar structures provide the required compatibility with the large-scale fabrication technologies of integrated circuits. The existence of piezoelectricity in 2D materials presents great opportunities for applications in various fields of electronics, optoelectronics, energy harvesting, sensors, actuators and biotechnology. Additionally, 2D flexible nanostructures with superior piezoelectric properties are distinctive candidates for integration into nano-scale electromechanical systems. Here we fundamentally review the state of the art of 2D piezoelectric materials from both experimental and theoretical aspects and report the recent achievements in the synthesis, characterization and applications of these materials.

Dielectric Properties of P(VDF-TrFE-CTFE) Composites Filled with Surface-Coated TiO2 Nanowires by SnO2 Nanoparticles

Nanocomposites containing inorganic fillers embedded in polymer matrices have exhibited great potential applications in capacitors. Therefore, an effective method to improve the dielectric properties of polymer is to design novel fillers with a special microstructure. In this work, a combination of hydrothermal method and precipitation method was used to synthesize in situ SnO₂ nanoparticles on the surface of one-dimensional TiO₂ nanowires (TiO₂ NWs), and the TiO₂NWs@SnO₂ fillers well-dispersed into the poly (vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)] polymer. Hybrid structure TiO₂NWs @SnO₂ introduce extra interfaces, which enhance the interfacial polarization and the dielectric constant. Typically, at 10 vol.% low filling volume fraction, the composite with TiO₂NWs @SnO₂ shows a dielectric constant of 133.4 at 100 Hz, which is almost four times that of polymer. Besides, the TiO₂ NWs prevents the direct contact of SnO₂ with each other in the polymer matrix, so the composites still maintain good insulation performance. All the improved performance indicates these composites can be widely useful in electronic devices.

Advancing Versatile Ferroelectric Materials Toward Biomedical Applications

Ferroelectric materials (FEMs), possessing piezoelectric, pyroelectric, inverse piezoelectric, nonlinear optic, ferroelectric-photovoltaic, and many other properties, are attracting increasing attention in the field of biomedicine in recent years. Because of their versatile ability of interacting with force, heat, electricity, and light to generate electrical, mechanical, and optical signals, FEMs are demonstrating their unique advantages for biosensing, acoustics tweezer, bioimaging, therapeutics, tissue engineering, as well as stimulating biological functions. This review summarizes the current-available FEMs and their state-of-the-art fabrication techniques, as well as provides an overview of FEMs-based applications in the field of biomedicine. Challenges and prospects for future development of FEMs for biomedical applications are also outlined.

Core/Shell Piezoelectric Nanofibers with Spatial Self-Orientated β-Phase Nanocrystals for Real-Time Micropressure Monitoring of Cardiovascular Walls

Implantable pressure biosensors show great potential for assessment and diagnostics of pressure-related diseases. Here, we present a structural design strategy to fabricate core/shell polyvinylidene difluoride (PVDF)/hydroxylamine hydrochloride (HHE) organic piezoelectric nanofibers (OPNs) with well-controlled and self-orientated nanocrystals in the spatial uniaxial orientation (SUO) of β-phase-rich fibers, which significantly enhance piezoelectric performance, fatigue resistance, stability, and biocompatibility. Then PVDF/HHE OPNs soft sensors are developed and used to monitor subtle pressure changes in vivo. Upon implanting into pig, PVDF/HHE OPNs sensors demonstrate their ultrahigh detecting sensitivity and accuracy to capture micropressure changes at the outside of cardiovascular walls, and output piezoelectric signals can real-time and synchronously reflect and distinguish changes of cardiovascular elasticity and occurrence of atrioventricular heart-block and formation of thrombus. Such biological information can provide a diagnostic basis for early assessment and diagnosis of thrombosis and atherosclerosis, especially for postoperative recrudescence of thrombus deep within the human body.

Room-Temperature Ferroelectricity in Group-IV Metal Chalcogenide Nanowires

The realization of low-dimensional ferroelectrics is both fundamentally intriguing and practically appealing, to be used in nanoscale devices. Here, GeS and SnS nanowires are predicted to be one-dimensional (1D) ferroelectrics with inversion symmetry spontaneously broken by soft optical modes. Despite the low dimensionality, the estimated Curie point for GeS nanowires is above room temperature, benefiting experimental detection and suggesting realistic applications. To this end, further aspects of these 1D ferroelectrics are also examined, revealing the domain wall localization, switchable carrier mobility, and practically effective shieling by confining the nanowires inside the carbon nanotubes, all together potentially useful for nanoscale ferroelectric devices of broad interest.

Structure and High Performance of Lead-Free (K0.5Na0.5)NbO3 Piezoelectric Nanofibers with Surface-Induced Crystallization at Lowered Temperature

Lead-free potassium and sodium niobate (KNN) nanofiber webs with random and aligned configurations were prepared by the electrospinning process from polymer-modified chemical solution. The crystallization process, structure, composition, dielectric, ferroelectric, and piezoelectric properties of the nanofibers and nanofiber webs were investigated. Theoretical analysis and experimental results showed that the surface-induced heterogeneous nucleation resulted in the remarkable lower crystallization temperature for the KNN nanofibers with the {100} orientation of the perovskite phase in contrast to the bulk KNN gel and thus well-controlled chemical stoichiometry. Low dielectric loss, large electric polarization, and high piezoelectric performance were obtained in the nanofiber webs. In particular, the aligned nanofiber web exhibited further improved piezoelectric strain and voltage coefficients and higher FOM than their thin film counterparts and is promising for high-performance electromechanical sensor and transducer applications.

Epitaxial patterned Bi2FeCrO6 nanoisland arrays with room temperature multiferroic properties

Epitaxial multiferroic Bi2FeCrO6 nanoisland arrays with a lateral size of ∼100 nm have been successfully fabricated by patterned SiO2 template-assisted pulsed laser deposition. The as-grown island structure exhibits promising multiferroic properties (i.e. ferroelectric and magnetic) even at nanometer dimensions at room temperature. This work demonstrates an effective strategy to fabricate high-density nonvolatile ferroelectric/multiferroic memory devices.

A Ferroelectric-Photovoltaic Effect in SbSI Nanowires

A ferroelectric-photovoltaic effect in nanowires of antimony sulfoiodide (SbSI) is presented for the first time. Sonochemically prepared SbSI nanowires have been characterized using high-resolution transmission electron microscopy (HRTEM) and optical diffuse reflection spectroscopy (DRS). The temperature dependences of electrical properties of the fabricated SbSI nanowires have been investigated too. The indirect forbidden energy gap E_gIf = 1.862 (1) eV and Curie temperature T_C = 291 (2) K of SbSI nanowires have been determined. Aligned SbSI nanowires have been deposited in an electric field between Pt electrodes on alumina substrate. The photoelectrical response of such a prepared ferroelectric-photovoltaic (FE-PV) device can be switched using a poling electric field and depends on light intensity. The photovoltage, generated under λ = 488 nm illumination of P_opt = 127 mW/cm² optical power density, has reached U_OC = 0.119 (2) V. The presented SbSI FE-PV device is promising for solar energy harvesting as well as for application in non-volatile memories based on the photovoltaic effect.

An Ultrasonic Fabrication Method for Epoxy Resin/SbSI Nanowire Composites, and their Application in Nanosensors and Nanogenerators

In this manuscript, a new fabrication technology for epoxy resin/antimony sulpho-iodide (SbSI) nanowire composites is presented. SbSI nanowires, with lateral dimensions of 10 nm to 100 nm and lengths up to several micrometres, have been synthesised using ultrasound irradiation. The prepared SbSI nanowires have been bound with epoxy resin in a mass ratio of 1:4, and then ultrasound irradiation has been used again for homogenization of the mixture. The fabricated epoxy resin/SbSI nanowire composites, due to the piezoelectric properties of SbSI (electromechanical coefficient k₃₃ = 0.9, and piezoelectric coefficient d_V = 0.9 × 10⁻⁹ C/N) may be used as an active layer in nanosensors and nanogenerators. The preliminary investigations of epoxy resin/SbSI nanowire composites for sound excitation (frequency f = 175 Hz; L = 90 dB), vibrations (f = 24 Hz; A = 1 mm; F = 0.73 N), and shock wave (p = 6 bar), allowed for the determination of the composite’s open circuit voltage: 0.0153 V_RMS, 0.166 V_RMS, and 4.51 V_p-p, respectively. Maximum power output densities of 0.45 nW/cm³ and 860 nW/cm³ have been achieved for excitation by sound and vibration, respectively, for a 0.6 mm thick layer of composite.

Composition and morphology tuning during hydrothermal synthesis of SrxBa1−xNb2O6 tetragonal tungsten bronzes studied by in situ X-ray diffraction

Advanced in situ techniques provide knowledge about crystal growth mechanisms of Sr_xBa_1−xNb₂O₆ facilitating the design of the microstructure and the determination of stoichiometry.

Approaching Piezoelectric Response of Pb-Piezoelectrics in Hydrothermally Synthesized Bi0.5(Na1- xK x)0.5TiO3 Nanotubes

A large piezoelectric coefficient of 76 pm/V along the diameter direction, approaching that of lead-based piezoelectrics, is observed in hydrothermally synthesized Pb-free Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ nanotubes. The 30-50 nm diameter nanotubes are formed through a scrolling and wrapping mechanism without the need of a surfactant or template. A molar ratio of KOH/NaOH = 0.5 for the mineralizers yields the Na/K ratio of ∼0.8:0.2, corresponding to an orthorhombic-tetragonal (O-T) phase boundary composition. X-ray diffraction patterns along with transmission electron microscopy analysis ascertain the coexistence of orthorhombic and tetragonal phases with (110) and (001) orientations along the nanotube length direction, respectively. ²³Na NMR spectroscopy confirms the higher degree of disorder in Bi_0.5(Na_{1- x}K _x)_0.5TiO₃ nanotubes with O-T phase coexistence. These findings present a significant advance toward the application of Pb-free piezoelectric materials.

Piezo-generated charge mapping revealed through direct piezoelectric force microscopy

While piezoelectric and ferroelectric materials play a key role in many everyday applications, there are still a number of open questions related to their physics. To enhance our understanding of piezoelectrics and ferroelectrics, nanoscale characterization is essential. Here, we develop an atomic force microscopy based mode that obtains a direct quantitative analysis of the piezoelectric coefficient d33. We report nanoscale images of piezogenerated charge in a thick single crystal of periodically poled lithium niobate (PPLN), a bismuth ferrite (BiFO3) thin film, and lead zirconate titanate (PZT) by applying a force and recording the current produced by these materials. The quantification of d33 coefficients for PPLN (14 ± 3 pC per N) and BFO (43 ± 6 pC per N) is in agreement with the values reported in the literature. Even stronger evidence of the reliability of the method is provided by an equally accurate measurement of the significantly larger d33 of PZT.

Electric and Mechanical Switching of Ferroelectric and Resistive States in Semiconducting BaTiO3-δ Films on Silicon

Materials that can couple electrical and mechanical properties constitute a key element of smart actuators, energy harvesters, or many sensing devices. Within this class, functional oxides display specific mesoscale responses which often result in great sensitivity to small external stimuli. Here, a novel combination of molecular beam epitaxy and a water-based chemical-solution method is used for the design of mechanically controlled multilevel device integrated on silicon. In particular, the possibility of adding extra functionalities to a ferroelectric oxide heterostructure by n-doping and nanostructuring a BaTiO₃ thin film on Si(001) is explored. It is found that the ferroelectric polarization can be reversed, and resistive switching can be measured, upon a mechanical load in epitaxial BaTiO_3-δ /La_0.7 Sr_0.3 MnO₃ /SrTiO₃ /Si columnar nanostructures. A flexoelectric effect is found, stemming from substantial strain gradients that can be created with moderate loads. Simultaneously, mechanical effects on the local conductivity can be used to modulate a nonvolatile resistive state of the BaTiO_3-δ heterostructure. As a result, three different configurations of the system become accessible on top of the usual voltage reversal of polarization and resistive states.

"Self-Peel-Off" Transfer Produces Ultrathin Polyvinylidene-Fluoride-Based Flexible Nanodevices

Here, a new strategy, self-peel-off transfer, for the preparation of ultrathin flexible nanodevices made from polyvinylidene-fluoride (PVDF) is reported. In this process, a functional pattern of nanoparticles is transferred via peeling from a temporary substrate to the final PVDF film. This peeling process takes advantage of the differences in the work of adhesion between the various layers (the PVDF layer, the nanoparticle-pattern layer and the substrate layer) and of the high stresses generated by the differential thermal expansion of the layers. The work of adhesion is mainly guided by the basic physical/chemical properties of these layers and is highly sensitive to variations in temperature and moisture in the environment. The peeling technique is tested on a variety of PVDF-based functional films using gold/palladium nanoparticles, carbon nanotubes, graphene oxide, and lithium iron phosphate. Several PVDF-based flexible nanodevices are prepared, including a single-sided wireless flexible humidity sensor in which PVDF is used as the substrate and a double-sided flexible capacitor in which PVDF is used as the ferroelectric layer and the carrier layer. Results show that the nanodevices perform with high repeatability and stability. Self-peel-off transfer is a viable preparation strategy for the design and fabrication of flexible, ultrathin, and light-weight nanodevices.

Ferroelectric-Enhanced Polysulfide Trapping for Lithium-Sulfur Battery Improvement

A brand new polysulfide entrapping strategy based on the ferroelectric effect has been demonstrated for the first time. By simply adding the nano-ferroelectrics (BaTiO₃ nanoparticles) into the cathode, the heteropolar polysulfides can be anchored within the cathode due to the internal electric field originated from the spontaneous polarization BaTiO₃ nanoparticles, and thus significantly improving the cycle stability of Li-S batteries.

Morphology control and large piezoresponse of hydrothermally synthesized lead-free piezoelectric (Bi0.5Na0.5)TiO3 nanofibres

(Bi_0.5Na_0.5)TiO₃ nanofibers were synthesized hydrothermally, nanostructures were investigated and piezoelectric properties of single nanofibers were measured.

Hydrothermal synthesis of strontium titanate: thermodynamic considerations, morphology control and crystallisation mechanisms

The hydrothermal/solvothermal method is one of the most versatile synthetic routes for producing a large number of compounds. The thermodynamic aspects, the control of morphology and the crystallisation mechanisms are reviewed and discussed in this highlight, with special emphasis on the synthesis of SrTiO₃, as a model system.

Direct Writing of Patterned, Lead-Free Nanowire Aligned Flexible Piezoelectric Device

A high-performance flexible piezoelectric nanogenerator (PNG) is fabricated by a direct writing method, which acquires both patterned piezoelectric structure and aligned piezoelectric nanowires simultaneously. The voltage output of the as-prepared PNG is nearly 400% compared with that of the traditional spin-coated device due to the effective utilization of stress. This facile printing approach provides an efficient strategy for significant improvement of the piezoresponse.

Antimony sulphoiodide (SbSI), a narrow band-gap non-oxide ternary semiconductor with efficient photocatalytic activity

A highly crystalline 3D urchin-shaped SbSI with an ns² cationic electronic configuration displays very high and efficient photocatalytic degradation of organic pollutants.

Enhanced photocatalytic activity on polarized ferroelectric KNbO3

In this paper, we demonstrate the enhanced photodegradation of rhodamine B on polarized ferroelectric KNbO₃ (KNO) particles.