Flexible ferroelectric element based on van der Waals heteroepitaxy

Growth of Single Crystalline 2D Materials beyond Graphene on Non-metallic Substrates

The advent of 2D materials has ushered in the exploration of their synthesis, characterization and application. While plenty of 2D materials have been synthesized on various metallic substrates, interfacial interaction significantly affects their intrinsic electronic properties. Additionally, the complex transfer process presents further challenges. In this context, experimental efforts are devoted to the direct growth on technologically important semiconductor/insulator substrates. This review aims to uncover the effects of substrate on the growth of 2D materials. The focus is on non-metallic substrate used for epitaxial growth and how this highlights the necessity for phase engineering and advanced characterization at atomic scale. Special attention is paid to monoelemental 2D structures with topological properties. The conclusion is drawn through a discussion of the requirements for integrating 2D materials with current semiconductor-based technology and the unique properties of heterostructures based on 2D materials. Overall, this review describes how 2D materials can be fabricated directly on non-metallic substrates and the exploration of growth mechanism at atomic scale.

Giant tunnelling electroresistance in atomic-scale ferroelectric tunnel junctions

Ferroelectric tunnel junctions are promising towards high-reliability and low-power non-volatile memories and computing devices. Yet it is challenging to maintain a high tunnelling electroresistance when the ferroelectric layer is thinned down towards atomic scale because of the ferroelectric structural instability and large depolarization field. Here we report ferroelectric tunnel junctions based on samarium-substituted layered bismuth oxide, which can maintain tunnelling electroresistance of 7 × 105 with the samarium-substituted bismuth oxide film down to one nanometer, three orders of magnitude higher than previous reports with such thickness, owing to efficient barrier modulation by the large ferroelectric polarization. These ferroelectric tunnel junctions demonstrate up to 32 resistance states without any write-verify technique, high endurance (over 5 × 109), high linearity of conductance modulation, and long retention time (10 years). Furthermore, tunnelling electroresistance over 109 is achieved in ferroelectric tunnel junctions with 4.6-nanometer samarium-substituted bismuth oxide layer, which is higher than commercial flash memories. The results show high potential towards multi-level and reliable non-volatile memories.

Multifunctional Flexible Ferroelectric Thin Films with Large Electrocaloric Effect and High Energy Storage Performance

Flexible ferroelectric films with high polarization hold great promise for energy storage and electrocaloric (EC) refrigeration. Herein, we fabricate a lead-free Mn-modified 0.75 Bi(Mg0.5Ti0.5)O3-0.25 BaTiO3 (BMT-BTO) thin film based on a flexible mica substrate. Excellent EC performance with maximum adiabatic temperature change (ΔT ∼23.5 K) and isothermal entropy change (ΔS ∼33.1 J K-1 kg-1) is achieved in the flexible BMT-BTO film, which is attributed to the local structural transition and lattice disorder near 90 °C. Meanwhile, a good energy storage density of ∼70.6 J cm-3 and a quite high efficiency of ∼82% are realized in the same ferroelectric film, accompanied by excellent stability of frequency and electric fatigue (500-10 kHz and 108 cycles). Furthermore, there is no apparent variation in performance under different bending strains. These prominent properties indicate that the multifunctional BMT-BTO ferroelectric film is a promising candidate for applications of flexible energy storage and EC refrigeration.

Radiation-Hardened and Flexible Pb(Zr0.53Ti0.47)O3 Piezoelectric Sensor for Structural Health Monitoring

Piezoelectric sensors are excellent damage detectors that can be applied to structural health monitoring (SHM). SHM for complex structures of aerospace vehicles working in harsh conditions is frequently required, posing challenging requirements for a sensor's flexibility, radiation hardness, and high-temperature tolerance. Here, we fabricate a flexible and lightweight Pb(Zr0.53Ti0.47)O3 piezoelectric film on flexible KMg3(AlSi3O10)F2 substrate via van der Waals (vdW) heteroepitaxy, endowing it with robust ferroelectric and piezoelectric properties under low energy-high flux protons (LE-HFPs) radiation (1015 p/cm2). More importantly, the Pb(Zr0.53Ti0.47)O3 film sensor maintains highly stable damage monitoring sensitivity on an aluminum plate under harsh conditions of LE-HFPs radiation (1015 p/cm2, flat structure), high temperature (175 °C, flat structure), and mechanical fatigue (bending 105 cycles under a radius of 5 mm, curved structure). All these superior qualities are suggested to result from the outstanding film crystal quality due to vdW epitaxy. The flexible and lightweight Pb(Zr0.53Ti0.47)O3 film sensor demonstrated in this work provides an ideal candidate for real-time SHM of aerospace vehicles with flat and complex curve-like structures working in harsh aerospace environments.

Engineering Heat Transport Across Epitaxial Lattice-Mismatched van der Waals Heterointerfaces

Artificially engineered 2D materials offer unique physical properties for thermal management, surpassing naturally occurring materials. Here, using van der Waals epitaxy, we demonstrate the ability to engineer extremely insulating thermal metamaterials based on atomically thin lattice-mismatched Bi2Se3/MoSe2 superlattices and graphene/PdSe2 heterostructures with exceptional thermal resistances (70-202 m2 K/GW) and ultralow cross-plane thermal conductivities (0.012-0.07 W/mK) at room temperature, comparable to those of amorphous materials. Experimental data obtained using frequency-domain thermoreflectance and low-frequency Raman spectroscopy, supported by tight-binding phonon calculations, reveal the impact of lattice mismatch, phonon-interface scattering, size effects, temperature, and interface thermal resistance on cross-plane heat dissipation, uncovering different thermal transport regimes and the dominant role of long-wavelength phonons. Our findings provide essential insights into emerging synthesis and thermal characterization methods and valuable guidance for the development of large-area heteroepitaxial van der Waals films of dissimilar materials with tailored thermal transport characteristics.

Hexagonal Boron Nitride Seed Layer-Assisted van der Waals Growth of BaSnO3 Perovskite Films

We have succeeded in obtaining BaSnO3 perovskite thin films with remarkable near-infrared luminescence by van der Waals growth. The films were grown on quartz glass substrates by pulsed laser deposition using hexagonal boron nitride as the seed layer, and their crystallinity was confirmed by X-ray diffraction and cross-sectional transmission electron microscopy. The near-infrared emission of the grown film exhibited a broad emission peak centered at 920 nm. The transparency of the BaSnO3 film (thickness = 1000 nm)/ hexagonal boron nitride /double-sided optically polished quartz glass substrate was approximately 90% at approximately 500 nm with or without the BaSnO3 film. Films showing remarkable near-infrared emission and high transparency obtained by van der Waals-type growth could be used in practical wavelength conversion devices that improve the efficiency of Si single-crystal solar cells. The hexagonal boron nitride seed layer supporting the van der Waals growth is an effective method for high-quality crystal growth of films. It can be used for perovskite-type oxides with many functionalities.

Flexible but Refractory Single-Crystalline Hyperbolic Metamaterials

The fabrication of flexible single-crystalline plasmonic or photonic components in a scalable way is fundamentally important to flexible electronic and photonic devices with high speed, high energy efficiency, and high reliability. However, it remains a challenge. Here, we have successfully synthesized flexible single-crystalline optical hyperbolic metamaterials by directly depositing refractory nitride superlattices on flexible fluorophlogopite-mica substrates with magnetron sputtering. Interestingly, these flexible hyperbolic metamaterials show dual-band hyperbolic dispersion of dielectric constants with small dielectric losses and high figures of merit in the visible to near-infrared ranges. More importantly, the optical properties of these nitride-based flexible hyperbolic metamaterials show remarkable stability during 1000 °C heating or after being bent 1000 times. Therefore, the strategy developed in this work offers an easy and scalable route for fabricating flexible, high-performance, and refractory plasmonic or photonic components, which can significantly expand the applications of current electronic and photonic devices.

Recent Advancements in Physiological, Biochemical, and Multimodal Sensors Based on Flexible Substrates: Strategies, Technologies, and Integrations

Flexible wearable devices have been widely used in biomedical applications, the Internet of Things, and other fields, attracting the attention of many researchers. The physiological and biochemical information on the human body reflects various health states, providing essential data for human health examination and personalized medical treatment. Meanwhile, physiological and biochemical information reveals the moving state and position of the human body, and it is the data basis for realizing human-computer interactions. Flexible wearable physiological and biochemical sensors provide real-time, human-friendly monitoring because of their light weight, wearability, and high flexibility. This paper reviews the latest advancements, strategies, and technologies of flexibly wearable physiological and biochemical sensors (pressure, strain, humidity, saliva, sweat, and tears). Next, we systematically summarize the integration principles of flexible physiological and biochemical sensors with the current research progress. Finally, important directions and challenges of physiological, biochemical, and multimodal sensors are proposed to realize their potential applications for human movement, health monitoring, and personalized medicine.

Applications of remote epitaxy and van der Waals epitaxy

Epitaxy technology produces high-quality material building blocks that underpin various fields of applications. However, fundamental limitations exist for conventional epitaxy, such as the lattice matching constraints that have greatly narrowed down the choices of available epitaxial material combinations. Recent emerging epitaxy techniques such as remote and van der Waals epitaxy have shown exciting perspectives to overcome these limitations and provide freestanding nanomembranes for massive novel applications. Here, we review the mechanism and fundamentals for van der Waals and remote epitaxy to produce freestanding nanomembranes. Key benefits that are exclusive to these two growth strategies are comprehensively summarized. A number of original applications have also been discussed, highlighting the advantages of these freestanding films-based designs. Finally, we discuss the current limitations with possible solutions and potential future directions towards nanomembranes-based advanced heterogeneous integration.

Growth, structure, and morphology of van der Waals epitaxy Cr1+δTe2 films

The preparation of two-dimensional magnetic materials is a key process to their applications and the study of their structure and morphology plays an important role in the growth of high-quality thin films. Here, the growth, structure, and morphology of Cr_1+δTe₂ films grown by molecular beam epitaxy on mica with variations of Te/Cr flux ratio, growth temperature, and film thickness have been systematically investigated by scanning tunneling microscopy, reflection high-energy electron diffraction, scanning electron microscope, and X-ray photoelectron spectroscopy. We find that a structural change from multiple phases to a single phase occurs with the increase in growth temperature, irrespective of the Cr/Te flux ratios, which is attributed to the desorption difference of Te atoms at different temperatures, and that the surface morphology of the films grown at relatively high growth temperatures (≥ 300 °C) exhibits a quasi-hexagonal mesh-like structure, which consists of nano-islands with bending surface induced by the screw dislocations, as well as that the films would undergo a growth-mode change from 2D at the initial stage in a small film thickness (2 nm) to 3D at the later stage in thick thicknesses (12 nm and 24 nm). This work provides a general model for the study of pseudo-layered materials grown on flexible layered substrates.

Interlayer Coupling Enhanced Energy Storage Performance in a Flexible BMT-BTO/BMT Multilayer Ferroelectric Film Capacitor

Flexible ferroelectric capacitors with high energy density and storage efficiency are highly desirable in the next generation of flexible electronic devices. To develop high-performance ferroelectric capacitors, a conventional approach is chemical modification. Here, a novel approach of interlayer coupling is proposed to achieve high energy storage performance in BiMg_0.5Ti_0.5O₃-BaTiO₃/BiMg_0.5Ti_0.5O₃ (BMT-BTO/BMT)_N multilayer ferroelectric films fabricated on flexible mica substrates via a sol-gel coating method. The interlayer electrostatic coupling between the ferroelectric BMT and relaxor ferroelectric BMT-BTO layers leads to small remnant polarization and large breakdown field strength, resulting in an outstanding energy storage density of ∼106.8 J cm^-3 and a good efficiency of ∼75.6% in the multilayer thin films. Further, the energy storage performance remains stable in a wide range of temperatures (25-200 °C) and frequencies (500 Hz to 10 kHz) after 10⁸ electrical loading cycles. The energy storage performance also has no obvious deterioration when the multilayer film experiences 10⁴ mechanical bending cycles with a bending radius of 4 mm. The approach proposed in the present work should be generally implementable in other multilayer flexible ferroelectric capacitors and offers a novel avenue to enhance energy storage performance by tuning the interlayer coupling.

Flexible Strategy of Epitaxial Oxide Thin Films

Applying functional oxide thin films to flexible devices is of great interests within the rapid development of information technology. The challenges involve the contradiction between the high-temperature growth of high-quality oxide films and low melting point of the flexible supports. This review summarizes the developed methods to fabricate high-quality flexible oxide thin films with novel functionalities and applications. We start from the fabrication methods, e.g. direct growth on flexible buffered metal foils and layered mica, etching and transfer approach, as well as remote epitaxy technique. Then, various functionalities in flexible oxide films will be introduced, specifically, owing to the mechanical flexibility, some unique properties can be induced in flexible oxide films. Taking the advantages of the excellent physical properties, the flexible oxide films have been employed in various devices. Finally, future perspectives in this research field will be proposed to further develop this field from fabrication, functionality to device.

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Different methods to fabricate flexible oxide thin films have been introduced

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Physical functionalities of flexible oxide thin films have been demonstrated

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Various applications of flexible oxide thin films have been discussed

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Future perspectives of flexible oxide thin films have been proposed

High Energy Storage Performance of All-Inorganic Flexible Antiferroelectric-Insulator Multilayered Thin Films

With the increasingly high requirements for wearable and flexible devices, traditional inorganic capacitors cannot meet the flexible demand of next-generation electronic devices. In this work, the energy storage property of all-inorganic flexible films has been systematically studied. PbZrO₃ (PZO) and Al₂O₃ (AO) are selected as the antiferroelectric layer and insulating layer, respectively. The heterostructured films are prepared on the fluorphlogopite (F-Mica) substrate by chemical solution deposition. The microstructure, polarization behavior, and energy storage performances are investigated. The results demonstrate that the AO/PZO/AO/PZO/AO (APAPA) multilayered thin film possesses a greatly improved energy storage density (W_rec) of 28.1 J/cm³ with an excellent energy storage efficiency (η) of 80.1%, which is ascribed to the enhanced breakdown strength and large difference in polarization. Furthermore, the capacitive films exhibit good stability under a wide working temperature range of 25-140 °C and an electric fatigue endurance of 10⁷ cycles. Besides, the energy storage performances are almost unchanged after 10⁴ bending cycles, demonstrating an excellent mechanical bending endurance. This work sheds light on the preparation technology and improvement of the dielectric energy storage performance for all-inorganic flexible multilayered thin films.

3D Conformal Fabrication of Piezoceramic Films

Piezoceramic films are an essential class of energy-conversion materials that have been widely used in the electronics industry. Although current methods create a great freedom for fabricating high-quality piezoceramic films, it requires well-controlled synthesis conditions, including special high-cost equipment and planar substrates particularly. The limited substrate selections hinder the applications of piezoceramic films in 3D conformal structures where most objects possess complex curvilinear surfaces. To overcome such limitations, a fast, energy-efficient, and cost-effective approach, named flame treated spray (FTS) coating, is developed for preparing piezoceramic films on free-form surfaces. The flame treatment significantly enhances the hydrophilicity of a substrate, assisting in forming a uniform and continuous thin film. The followed spray coating deposits hundreds of nanometers to several micrometers thick films on 3D free-form surfaces. Given the size controllability and arbitrary surface compatibility of the FTS method, a highly conformal piezoelectric tactile sensor array (4 × 4) is assembled on a spherical surface for mimicking robot fingers and an on-site thin-film sensor on the wing of an aircraft model to monitor the vibration in real-time during flight. The FTS film deposition offers a highly promising methodology for the application of functional thin-film from micro- to marcoscale devices, regardless of conformal problems.

Van der Waals Epitaxial Growth of ZnO Films on Mica Substrates in Low-Temperature Aqueous Solution

In this article, we demonstrate the van der Waals (vdW) epitaxial growth of ZnO layers on mica substrates through a low-temperature hydrothermal process. The thermal pretreatment of mica substrates prior to the hydrothermal growth of ZnO is essential for growing ZnO crystals in epitaxy with the mica substrates. The addition of sodium citrate into the growth solution significantly promotes the growth of ZnO crystallites in a lateral direction to achieve fully coalesced, continuous ZnO epitaxial layers. As confirmed through transmission electron microscopy, the epitaxial paradigm of the ZnO layer on the mica substrate was regarded as an incommensurate van der Waals epitaxy. Furthermore, through the association of the Mist-CVD process, the high-density and uniform distribution of ZnO seeds preferentially occurred on mica substrates, leading to greatly improving the epitaxial qualities of the hydrothermally grown ZnO layers and obtaining flat surface morphologies. The electrical and optoelectrical properties of the vdW epitaxial ZnO layer grown on mica substrates were comparable with those grown on sapphire substrates through conventional solution-based epitaxy techniques.

Ultrahigh Energy Storage Performance of Flexible BMT-Based Thin Film Capacitors

Ferroelectric thin film capacitors have attracted increasing attention because of their high energy storage density and fast charge-discharge speed, but less attention has been paid to the realization of flexible capacitors for wearable electronics and power systems. In this work, flexible xMn-BiMg_0.5 Ti_0.7 O₃ (xMn-BMT_0.7 ) thin film capacitors with ultrahigh energy storage density and good stability are deposited on mica substrate. The introduction of excess TiO₂ with an amorphous structure contributes to the forming of the polar nano regions, resulting in the reduced ferroelectric hysteresis. In order to further improve the energy storage performance, Mn doping increases the polarization by regulating chemical pressure in the lattices and inhibits the valence change of Ti⁴⁺ . Especially in the 1.5% Mn-BMT_0.7 film capacitor, an ultrahigh energy storage density of 124 J cm^-3 and an outstanding efficiency of 77% are obtained, which is one of the best energy storage performances recorded for ferroelectric capacitors. In addition, the flexible ferroelectric film capacitor also exhibits good thermal stability (25-200 °C), high frequency reliability (500 Hz-10 kHz), excellent electrical (10⁸ cycles), and mechanical (10⁴ cycles) fatigue properties. This work is expected to pave the way for the application of BMT-based thin film capacitors in flexible energy storage systems.

Wafer-Scale Epitaxy of Flexible Nitride Films with Superior Plasmonic and Superconducting Performance

Transition-metal nitrides (e.g., TiN, ZrN, TaN) are incredible materials with excellent complementary metal-oxide semiconductor compatibility and remarkable performance in refractory plasmonics and superconducting quantum electronics. Epitaxial growth of flexible transition-metal nitride films, especially at the wafer scale, is fundamentally important for developing high-performance flexible photonics and superconducting electronics, but the study is rare thus far. This work reports the high-quality epitaxy of 2-in. titanium nitride (TiN) films on flexible fluorophlogopite-mica (F-mica) substrates via reactive magnetron sputtering. Combined measurements of spectroscopic ellipsometry and electrical transport reveal the superior plasmonic and superconducting performance of TiN/F-mica films owing to the high single crystallinity. More interestingly, the superconductivity of these flexible TiN films can be manipulated by the bending states, and enhanced superconducting critical temperature T_C is observed in convex TiN films with in-plane tensile strain. Density functional theory calculations reveal that the strain can tune the electron-phonon interaction strength and the resultant superconductivity of TiN films. This study provides a promising route toward integrating scalable single-crystalline transition-metal nitride films with flexible electronics for high-performance plasmonics and superconducting electronics.

High-Temperature and Flexible Piezoelectric Sensors for Lamb-Wave-Based Structural Health Monitoring

Piezoelectric sensors can be utilized in Lamb-wave-based structural health monitoring (SHM), which is an effective method for aircraft structural damage detection. However, due to the inherent stiffness, brittleness, weight, and thickness of piezoelectric ceramics, their applications in aircraft structures with complex curved surfaces are seriously restricted. Herein, we report a flexible, light-weight, and high-performance BaTiO₃:Sm₂O₃/SrRuO₃/SrTiO₃/mica film sensor that can be used in high-temperature SHM of aircraft. Enhanced ferroelectric Curie temperature (487 °C) and piezoelectric coefficient d₃₃ (120-130 pm/V) are achieved in BaTiO₃, which can be attributed to the tensile strain developed by stiff Sm₂O₃ nanopillars. Stable ferroelectricity and piezoelectricity are retained up to 150 °C. The flexible BaTiO₃:Sm₂O₃/SrRuO₃/SrTiO₃/mica film is validated as an ultrasonic sensor with high sensitivity and stability for damage monitoring on aircraft structures with the curved surface ranging from 25 to 150 °C. Our work demonstrates that flexible and light-weight BaTiO₃:Sm₂O₃/SrRuO₃/SrTiO₃/mica film sensors can be employed as high-temperature piezoelectric sensors for real-time SHM of aircraft structures with complex curved surfaces.

Strain Control of Phase Transition and Exchange Bias in Flexible Heusler Alloy Thin Films

The practical applications for the distinctive functions of metamagnetic Heusler alloys, such as magnetic shape memory effect, various caloric effects, etc., strongly depend on the phase transition temperatures. Here, flexible Heusler alloy Ni-Mn-Sn films have been deposited on mica substrates by pulsed laser deposition with a Ti buffer layer. Clear ferromagnetic (FM) transition followed by the martensitic transformation at around room temperature and exchange bias (EB) with a blocking temperature of 70 K are observed. Under the application of both tensile and compressive strains by bending the mica substrates, all the characteristic temperatures of Ni-Mn-Sn films, including the FM transition temperature, martensitic transformation temperature, and blocking temperature of EB, are significantly increased by about 10 K. Furthermore, EB field and coercivity are both strongly strengthened, which is mainly caused by the simultaneous enhancement of FM and anti-FM Mn-Mn coupling because of their shortened separations by strain and verified by the Monte Carlo simulation results. The strain controlling for structural and magnetic properties provides efficient manipulation for Heusler alloy-based magnetic devices.

Transfer-Free PZT Thin Films for Flexible Nanogenerators Derived from a Single-Step Modified Sol-Gel Process on 2D Mica

Piezoelectric materials enable emerging self-powered wearable and implantable devices. The sol-gel technology with the lowest cost for large-scale production has shown its potential for producing high-quality PZT thin films. However, fabricating PZT films with a sufficient thickness for different application scenarios requires a long and repeated coating and heat-treatment process. The traditional solution-based method usually requires at least 20 coating cycles to fabricate 2 μm-thick PZT thin films. To save cost and improve fabrication efficiency, we develop a simplified thin-film fabrication method assisted by PZT powder. The new method can fabricate 2 μm-thick PZT films in a single step, one spin coating and annealing. Experiments indicate that the powder-based PZT thin films have porous structures and outstanding piezoelectric performances. The measured d₃₃ of the powder-based PZT thin film is 47 pm/V. Both solution-based and powder-based PZT thin films show high flexibility and good fatigue resistance. Furthermore, we explore 2D mica as the substrate and achieve the transfer-free fabrication of flexible PZT thin-film nanogenerators that effectively simplify the complicated physical or chemical thin-film lift-off processes. The nanogenerator prototypes demonstrate the capability of accurately monitoring dynamic responses of flexible and soft human tissues.

Periodic Wrinkle-Patterned Single-Crystalline Ferroelectric Oxide Membranes with Enhanced Piezoelectricity

Self-assembled membranes with periodic wrinkled patterns are the critical building blocks of various flexible electronics, where the wrinkles are usually designed and fabricated to provide distinct functionalities. These membranes are typically metallic and organic materials with good ductility that are tolerant of complex deformation. However, the preparation of oxide membranes, especially those with intricate wrinkle patterns, is challenging due to their inherently strong covalent or ionic bonding, which usually leads to material crazing and brittle fracture. Here, wrinkle-patterned BaTiO₃ (BTO)/poly(dimethylsiloxane) membranes with finely controlled parallel, zigzag, and mosaic patterns are prepared. The BTO layers show excellent flexibility and can form well-ordered and periodic wrinkles under compressive in-plane stress. Enhanced piezoelectricity is observed at the sites of peaks and valleys of the wrinkles where the largest strain gradient is generated. Atomistic simulations further reveal that the excellent elasticity and the correlated coupling between polarization and strain/strain gradient are strongly associated with ferroelectric domain switching and continuous dipole rotation. The out-of-plane polarization is primarily generated at compressive regions, while the in-plane polarization dominates at the tensile regions. The wrinkled ferroelectric oxides with differently strained regions and correlated polarization distributions would pave a way toward novel flexible electronics.

Beyond Substrates: Strain Engineering of Ferroelectric Membranes

Strain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available high-quality substrates. Here, using the ferroelectric BaTiO₃ , production of precisely strain-engineered, substrate-released nanoscale membranes is demonstrated via an epitaxial lift-off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide-metal/ferroelectric/oxide-metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75 to 425 °C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10 kV cm^-1 and improving switching times to <5 ns for a 20 µm diameter capacitor in a 100-nm-thick film). In devices integrated on flexible polymers, enhanced room-temperature dielectric permittivity with large mechanical tunability (a 90% change upon ±0.1% strain application) is demonstrated. This approach paves the way toward the fabrication of ultrafast CMOS-compatible ferroelectric memories and ultrasensitive flexible nanosensor devices, and it may also be leveraged for the stabilization of novel phases and functionalities not achievable via direct epitaxial growth.

Flexible Quasi-van der Waals Ferroelectric Hafnium-Based Oxide for Integrated High-Performance Nonvolatile Memory

Ferroelectric memories with ultralow-power-consumption are attracting a great deal of interest with the ever-increasing demand for information storage in wearable electronics. However, sufficient scalability, semiconducting compatibility, and robust flexibility of the ferroelectric memories remain great challenges, e.g., owing to Pb-containing materials, oxide electrode, and limited thermal stability. Here, high-performance flexible nonvolatile memories based on ferroelectric Hf0.5Zr0.5O2 (HZO) via quasi-van der Waals heteroepitaxy are reported. The flexible ferroelectric HZO exhibits not only high remanent polarization up to 32.6 µC cm-2 without a wake-up effect during cycling, but also remarkably robust mechanical properties, degradation-free retention, and endurance performance under a series of bent deformations and cycling tests. Intriguingly, using HZO as a gate, flexible ferroelectric thin-film transistors with a low operating voltage of ±3 V, high on/off ratio of 6.5  ×  105, and a small subthreshold slope of about 100 mV dec-1, which outperform reported flexible ferroelectric transistors, are demonstrated. The results make ferroelectric HZO a promising candidate for the next-generation of wearable, low-power, and nonvolatile memories with manufacturability and scalability.

van der Waals oxide heteroepitaxy for soft transparent electronics

The quest for multifunctional, low-power and environment friendly electronics has brought research on materials to the forefront. For instance, as the emerging field of transparent flexible electronics is set to greatly impact our daily lives, more stringent requirements are being imposed on functional materials. Inherently flexible polymers and metal foil templates have yielded limited success due to their incompatible high-temperature growth and non-transparency, respectively. Although the epitaxial-transfer strategy has shown promising results, it suffers from tedious and complicated lift-off-transfer processes. The advent of graphene, in particular, and 2D layered materials, in general, with ultrathin scalability has revolutionized this field. Herein, we review the direct growth of epitaxial functional oxides on flexible transparent mica substrates via van der Waals heteroepitaxy, which mitigates misfit strain and substrate clamping for soft transparent electronics applications. Recent advances in practical applications of flexible and transparent electronic elements are discussed. Finally, several important directions, challenges and perspectives for commercialization are also outlined. We anticipate that this promising strategy to build transparent flexible optoelectronic devices and improve their performance will open up new avenues for researchers to explore.

Exchange Bias in a La0.67Sr0.33MnO3/NiO Heterointerface Integrated on a Flexible Mica Substrate

Flexible electronics integrating spintronics are of great potential in the areas of lightweight and flexible personal electronics. The integration of ferromagnetic and other functional oxides on flexible mica substrates is crucial for the proposed computer technology. In this work, we demonstrate the successful integration of a ferromagnetic-antiferromagnetic nanocomposite of La_0.67Sr_0.33MnO₃ (LSMO)/NiO with unique perpendicular exchange bias properties on a flexible mica substrate. Utilization of multiple sets of buffer layers has been attempted to overcome the large mismatch between the film and the substrate and to achieve high-quality nanocomposite growth on mica. Exchange bias of ∼200 and ∼140 Oe for the applied magnetic field perpendicular and parallel to the film surface, respectively, has been achieved and attributed to the strongly coupled vertical ferromagnetic/antiferromagnetic interfaces. Such nanocomposite thin films exhibit excellent structural robustness and reliability under a cyclic bending test. This work demonstrates the enormous potential of integrating complex two-phase multifunctional oxides on mica for future flexible wearable personal devices.

Towards a Large-Area Freestanding Single-Crystal Ferroelectric BaTiO3 Membrane

The fabrication and transfer of freestanding single-crystal ferroelectric membranes deserve intensive investigations as to their potential applications in flexible wearable devices, such as flexible data storage devices and varied sensors in E-skin configurations. In this report, we have shown a comprehensive study approach to the acquisition of a large-area freestanding single-crystal ferroelectric BaTiO3 by the Sr3Al2O6 scarification layer method. By controlling the thickness of the BaTiO3 and Sr3Al2O6, the exposed area of the Sr3Al2O6 interlayer, and the utilization of an additional electrode La2/3Sr1/3MnO3 layer, the crack density on the freestanding BaTiO3 can be dramatically decreased from 24.53% to almost none; then, a more than 700 × 530 μm2 area high-quality freestanding BaTiO3 membrane can be achieved. Our results offer a clear and repeatable technology routine for the acquisition of a flexible large-area ferroelectric membrane, which should be instructive to other transition metal oxides as well. Our study can confidently boost flexible device fabrication based on single-crystal transition metal oxides.

Ferroelectric Tunnel Junctions: Modulations on the Potential Barrier

Recently, ferroelectric tunnel junctions (FTJs) have attracted considerable attention for potential applications in next-generation memories, owing to attractive advantages such as high-density of data storage, nondestructive readout, fast write/read access, and low energy consumption. Herein, recent progress regarding FTJ devices is reviewed with an emphasis on the modulation of the potential barrier. Electronic and ionic approaches that modulate the ferroelectric barriers themselves and/or induce extra barriers in electrodes or at ferroelectric/electrode interfaces are discussed with the enhancement of memory performance. Emerging physics, such as nanoscale ferroelectricity, resonant tunneling, and interfacial metallization, and the applications of FTJs in nonvolatile data storage, neuromorphic synapse emulation, and electromagnetic multistate memory are summarized. Finally, challenges and perspectives of FTJ devices are underlined.

In-Plane Ferroelectric Tin Monosulfide and Its Application in a Ferroelectric Analog Synaptic Device

Two-dimensional ferroelectrics is attractive for synaptic device applications because of its low power consumption and amenability to high-density device integration. Here, we demonstrate that tin monosulfide (SnS) films less than 6 nm thick show optimum performance as a semiconductor channel in an in-plane ferroelectric analogue synaptic device, whereas thicker films have a much poorer ferroelectric response due to screening effects by a higher concentration of charge carriers. The SnS ferroelectric device exhibits synaptic behaviors with highly stable room-temperature operation, high linearity in potentiation/depression, long retention, and low cycle-to-cycle/device-to-device variations. The simulated device based on ferroelectric SnS achieves ∼92.1% pattern recognition accuracy in an artificial neural network simulation. By switching the ferroelectric domains partially, multilevel conductance states and the conductance ratio can be obtained, achieving high pattern recognition accuracy.

Bending Strain-Tailored Magnetic and Electronic Transport Properties of Reactively Sputtered γ'-Fe4N/Muscovite Epitaxial Heterostructures toward Flexible Spintronics

The strain modulation on the magnetic and electronic transport properties of the ferromagnetic films is one of the hot topics due to the practical applications in flexible and wearable spintronic devices. However, the large strain-induced saturation magnetization and resistance change is not easy to achieve because most of the ferromagnetic films deposited on flexible substrates are polycrystalline or amorphous. Here, the flexible epitaxial γ'-Fe₄N/mica films are fabricated by facing-target reactive sputtering. At a tensile strain with a radius of curvature (ROC) of 3 mm, the saturation magnetization (M_s) of the γ'-Fe₄N/mica film is tailored significantly with a maximal variation of 210%. Meanwhile, the magnetic anisotropy was broadly tunable at different strains, where the out-of-plane M_r/M_s at a tensile strain of ROC = 2 mm is six times larger than that at the unbent state. Besides, the strain-tailored longitudinal resistance R_xx and anomalous Hall resistivity ρ_xy appear where the drop of R_xx (ρ_xy) reaches 5% (22%) at a tensile strain of ROC = 3 mm. The shift of the nitrogen position in the γ'-Fe₄N unit cell at different bending strains plays a key role in the strain-tailored magnetic and electronic transport properties. The flexible epitaxial γ'-Fe₄N films have the potential applications in magneto- and electromechanical wearable spintronic devices.

Continuously controllable photoconductance in freestanding BiFeO3 by the macroscopic flexoelectric effect

Flexoelectricity induced by the strain gradient is attracting much attention due to its potential applications in electronic devices. Here, by combining a tunable flexoelectric effect and the ferroelectric photovoltaic effect, we demonstrate the continuous tunability of photoconductance in BiFeO3 films. The BiFeO3 film epitaxially grown on SrTiO3 is transferred to a flexible substrate by dissolving a sacrificing layer. The tunable flexoelectricity is achieved by bending the flexible substrate which induces a nonuniform lattice distortion in BiFeO3 and thus influences the inversion asymmetry of the film. Multilevel conductance is thus realized through the coupling between flexoelectric and ferroelectric photovoltaic effect in freestanding BiFeO3. The strain gradient induced multilevel photoconductance shows very good reproducibility by bending the flexible BiFeO3 device. This control strategy offers an alternative degree of freedom to tailor the physical properties of flexible devices and thus provides a compelling toolbox for flexible materials in a wide range of applications.

Ultralow Power Wearable Heterosynapse with Photoelectric Synergistic Modulation

Although the energy consumption of reported neuromorphic computing devices inspired by biological systems has become lower than traditional memory, it still remains greater than bio-synapses (≈10 fJ per spike). Herein, a flexible MoS2-based heterosynapse is designed with two modulation modes, an electronic mode and a photoexcited mode. A one-step mechanical exfoliation method on flexible substrate and low-temperature atomic layer deposition process compatible with flexible electronics are developed for fabricating wearable heterosynapses. With a pre-spike of 100 ns, the synaptic device exhibits ultralow energy consumption of 18.3 aJ per spike in long-term potentiation and 28.9 aJ per spike in long-term depression. The ultrafast speed and ultralow power consumption provide a path for a neuromorphic computing system owning more excellent processing ability than the human brain. By adding optical modulation, a modulatory synapse is constructed to dynamically control correlations between pre- and post-synapses and realize complex global neuromodulations. The novel wearable heterosynapse expands the accessible range of synaptic weights (ratio of facilitation ≈228%), providing an insight into the application of wearable 2D highly efficient neuromorphic computing architectures.

Mechanically tunable exchange coupling of Co/CoO bilayers on flexible muscovite substrates

The employment of flexible muscovite substrates has given us the feasibility of applying strain to heterostructures dynamically by mechanical bending. In this study, this novel approach is utilized to investigate strain effects on the exchange coupling in ferromagnetic Co and anti-ferromagnetic CoO (Co/CoO) bilayers. Two different Co/CoO bilayer heterostructures were grown on muscovite substrates by oxide molecular beam epitaxy, with the CoO layer being purely (111)- and (100)-oriented. The strain-dependent exchange coupling effect can only be observed on Co/CoO(100)/mica but not on Co/CoO(111)/mica. The origin of this phenomenon is attributed to the anisotropic spin re-orientation induced by mechanical bending. The strain-dependent magnetic anisotropy of the bilayers determined by anisotropic magnetoresistance measurements confirms this conjecture. This study elucidates the fundamental understanding of how magnetic exchange coupling can be tuned by externally applied strain via mechanical bending and, hence, provides a novel approach for implementing flexible spintronic devices.

Toward Multifunctional Electronics: Flexible NBT-Based Film with a Large Electrocaloric Effect and High Energy Storage Property

Advances in smart and wearable devices are driving innovations in multifunctional flexible materials at a tremendous pace. Here, drawing support from the unique flexible fluorophlogopite mica platform, we present a promising all-inorganic bendable Mn-modified 0.65(0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃)-0.35SrTiO₃ (NBBST) film with dual use in electrocaloric (EC) refrigeration and energy storage via a cost-effective transfer-free process. An appreciable room-temperature EC effect with adiabatic temperature change of 12 K and isothermal entropy of 18 J K^-1 kg^-1 was realized in the NBBST film, which benefits from the large change in dipolar ordering near depolarization temperature. Also, the film exhibits a broad operating temperature span over 25 °C because of its relaxor feature. Most importantly, the film can maintain a high EC performance either under bending deformation at 5 mm radius or after undergoing 10⁴ bending-unbending cycles. Meanwhile, the flexible NBBST film possesses good energy storage property with a recoverable energy density of 56 J cm^-3 and an efficiency of 66%. This is the first example of a lead-free all-inorganic multifunctional film capacitor toward the flexible EC refrigeration and energy storage devices. This work shows bright prospects in the emerging flexible e-market.

Multifunctional All-Inorganic Flexible Capacitor for Energy Storage and Electrocaloric Refrigeration over a Broad Temperature Range Based on PLZT 9/65/35 Thick Films

Multifunctional capacitors can efficiently integrate multiple functionalities into a single material to further down-scale state-of-the-art integrated circuits, which are urgently needed in new electronic devices. Here, an all-inorganic flexible capacitor based on Pb_0.91La_0.09 (Zr_0.65Ti_0.35)_0.9775O₃ (PLZT 9/65/35) relaxor ferroelectric thick film (1 μm) was successfully fabricated on LaNiO₃/F-Mica substrate for application in electrostatic energy storage and electrocaloric refrigeration simultaneously. The flexible PLZT 9/65/35 thick film presents a desirable breakdown field of 1998 kV/cm, accompanied by a superior recoverable energy density (W_rec) of 40.2 J/cm³. Meanwhile, the thick film exhibits excellent stability of energy-storage performance, including a broad operating temperature (30-180 °C), reduplicative charge-discharge cycles (1 × 10⁷ cycles), and mechanical bending cycles (2000 times). Moreover, a large reversible adiabatic temperature change (ΔT) of 18.0 °C, accompanied by an excellent electrocaloric strength (ΔT/ΔE) of 22.4 K cm/V and refrigerant capacity (RC) of 11.2 J/cm³, is obtained at 80 °C in the flexible PLZT 9/65/35 thick film under the moderate applied electric field of 850 kV/cm. All of these results shed light on a flexible PLZT 9/65/35 thick film capacitor that opens up a route to practical applications in microenergy-storage systems and on-chip thermal refrigeration of advanced electronics.

All-Inorganic Flexible Ba0.67Sr0.33TiO3 Thin Films with Excellent Dielectric Properties over a Wide Range of Frequencies

With rapid advances in flexible electronics and communication devices, flexible dielectric capacitors exhibiting high permittivity, low loss, and large electric-field tunability over a wide frequency range have attracted increasing attention. Here, a large-scale Ba_0.67Sr_0.33TiO₃ (BST) dielectric thin film sandwiched between SrRuO₃ (SRO) bottom electrode and Pt top electrode is fabricated on a flexible mica substrate. The mica/SRO/BST/Pt capacitor exhibits a dielectric constant (ε_r') of more than 1200, a loss tangent [tan(δ)] as low as 0.16, and a tunability of 67% at low frequencies around 10 kHz. Simultaneously, the capacitor can retain an ε_r' of 540 and a tan(δ) of 0.07 at microwave frequencies, e.g., 18.6 GHz. Moreover, even when the capacitor is bent to a small radius of 5 mm or undergoes 12 000 bending cycles (at 5 mm radius), almost no deterioration in ε_r', tan(δ), and tunability is observed. The excellent dielectricity and mechanical flexibility and durability endow the mica/SRO/BST/Pt capacitor with huge potential for flexible electronic and microwave applications.

Oxide Heteroepitaxy-Based Flexible Ferroelectric Transistor

With the rise of Internet of Things, the presence of flexible devices has attracted significant attention owing to design flexibility. A ferroelectric field-effect transistor (FeFET), showing the advantages of high speed, nondestructive readout, and low-power consumption, plays a key role in next-generation technology. However, the performance of these devices is restricted since conventional flexible substrates show poor thermal stability to integrate traditional ferroelectric materials, limiting the compatibility of wearable devices. In this study, we adopt flexible muscovite mica as a substrate due to its good thermal properties and epitaxial integration ability. A flexible FeFET composed of oxide heteroepitaxy on muscovite is realized by combining an aluminum-doped zinc oxide film as the semiconductor channel layer and a Pb(Zr_0.7Ti_0.3)O₃ film as the ferroelectric gate dielectric. The excellent characteristics of the transistor together with superior thermal stability and mechanical flexibility are demonstrated through various mechanical bending and temperature measurements. The on/off current ratio of the FeFET is higher than 10³, which based on the field effect in the transfer curve. The smallest bending radius that can be achieved is 5 mm with a cyclability of 300 times and a retention of 100 h. This study opens an avenue to use oxide heteroepitaxy to construct a FeFET for next-generation flexible electronic systems.

Integration of bulk materials with two-dimensional materials for physical coupling and applications

Hybrid heterostructures are essential for functional device systems. The advent of 2D materials has broadened the material set beyond conventional 3D material-based heterostructures. It has triggered the fundamental investigation and use in applications of new coupling phenomena between 3D bulk materials and 2D atomic layers that have unique van der Waals features. Here we review the state-of-the-art fabrication of 2D and 3D heterostructures, present a critical survey of unique phenomena arising from forming 3D/2D interfaces, and introduce their applications. We also discuss potential directions for research based on these new coupled architectures.

Tunable Ferroelectricity in Ruddlesden-Popper Halide Perovskites

Ruddlesden-Popper (RP) halide perovskites are the new kids on the block for high-performance perovskite photovoltaics with excellent ambient stability. The layered nature of these perovskites offers an exciting possibility of harnessing their ferroelectric property for photovoltaics. Adjacent polar domains in a ferroelectric material allow the spatial separation of electrons and holes. Presently, the structure-function properties governing the ferroelectric behavior of RP perovskites are an open question. Herein, we realize tunable ferroelectricity in 2-phenylethylammonium (PEA) and methylammonium (MA) RP perovskite (PEA)₂(MA) _n̅-1Pb _n̅I_{3 n̅+1}. Second harmonic generation (SHG) confirms the noncentrosymmetric nature of these polycrystalline thin films, whereas piezoresponse force microscopy and polarization-electric field measurements validate the microscopic and macroscopic ferroelectric properties. Temperature-dependent SHG and dielectric constant measurements uncover a phase transition temperature at around 170 °C in these films. Extensive molecular dynamics simulations support the experimental results and identified the correlated reorientation of MA molecules and ion translations as the source of ferroelectricity. Current-voltage characteristics in the dark reveal the persistence of hysteresis in these devices, which has profound implications for light-harvesting and light-emitting applications. Importantly, our findings disclose a viable approach for engineering the ferroelectric properties of RP perovskites that may unlock new functionalities for perovskite optoelectronics.

Flexible, Temperature-Resistant, and Fatigue-Free Ferroelectric Memory Based on Bi(Fe0.93Mn0.05Ti0.02)O3 Thin Film

A recent hot-spot topic for flexible and wearable devices involves high-performance nonvolatile ferroelectric memories operating under compressive or tensile mechanical deformations. This work presents the direct fabrication of a flexible (Mn,Ti)-codoped multiferroic BiFeO₃ film capacitor with Pt bottom and Au top electrodes on mica substrate. The fabricated polycrystalline Bi(Fe_0.93Mn_0.05Ti_0.02)O₃ film on mica exhibits superior ferroelectric switching behavior with robust saturated polarization ( P_s ∼ 93 μC/cm²) and remanent polarization ( P_r ∼ 66 μC/cm²) and excellent frequency stability (1-50 kHz) and temperature resistance (25-200 °C), as well as reliable long-lifetime operation. More saliently, it can be safely bent to a small radius of curvature, as low as 2 mm, or go through repeated compressive/tensile mechanical flexing for 10³ bending times at 4 mm radius without any obvious deterioration in polarization, retention time at 10⁵ s, or fatigue resistance after 10⁹ switching cycles. These findings demonstrate a novel route to designing flexible BiFeO₃-based ferroelectric memories for information storage and data processing, with promising applications in next-generation smart electronics.

Flexible Metal-Insulator Transitions Based on van der Waals Oxide Heterostructures

Recently, flexible and wearable electronics are highly desirable because of their great potential in the next-generation information devices. In this work, we demonstrate the realization of the metal-insulator transition (MIT) effect in flexible rare-earth nickelate heterostructures. The NdNiO₃ thin films are grown on lattice-mismatched mica substrates along the pseudocubic (111) direction via the van der Waals heteroepitaxy, in which the MIT behaviors are induced and modulated by carefully controlling the lattice strain and the ionic valence state with SrTiO₃ and LaAlO₃ buffering layers. Enhanced MIT properties with sharp transition and significant resistivity change between the metallic and the insulating states are achieved in the NdNiO₃/LaAlO₃/SrTiO₃/mica heterostructures with appropriate in-plane tensile strain and suppressed concentration of Ni²⁺ ions. In addition, the proposed NdNiO₃-based heterostructures exhibit excellent flexibility with reliable MIT characteristics not only in statically concave/convex bending but also in dynamically bending cycling up to 1000 times. The present work provides a platform to design and fabricate new flexible devices integrated with the MIT effect.

Highly Stable In-Plane Microwave Magnetism in Flexible Li0.35Zn0.3Fe2.35O4(111) Epitaxial Thin Films for Wearable Devices

With the advances in artificial intelligence and communication technologies, flexible microwave magnetic materials have become essential for flexible microwave detectors, wearable microwave sensors, and flexible spintronics. Here, a highly stable in-plane (IP) ferromagnetic resonance (FMR) and a tunable out-of-plane (OOP) FMR character are demonstrated in the flexible microwave magnetic Li_0.35Zn_0.3Fe_2.35O₄ (LZFO) (111) epitaxial thin films under external mechanical bending. Both the IP FMR line width and resonance field ( H_r) are basically unchanged when the sample was bent under an external tensile or compressive bending strain and deformation. However, the OOP FMR spectra (including H_r and absorption peak) could be tuned by mechanical bending, i.e., the LZFO sample possesses two OOP FMR absorption peaks at an external bending curvature. Meanwhile, excellent mechanical antifatigue and mechanical retention characteristics have also been obtained in the LZFO sample. The highly stable IP and the tunable OOP FMR spectra in the same LZFO sample with excellent mechanical antifatigue character have a promising prospect in microwave magnetic devices and flexible spintronics.

Transparent Antiradiative Ferroelectric Heterostructure Based on Flexible Oxide Heteroepitaxy

In the era of Internet of Things, the demand for flexible and transparent electronic devices has shifted to the forefront of materials science research. However, the radiation damage to key performance of transparent devices under radiative environment remains as a critical issue. Here, we present a promising technology for nonvolatile transparent electronic devices based on flexible oxide heteroepitaxy. A direct fabrication of epitaxial lead lanthanum zirconate titanate on transparent flexible mica substrate with indium tin oxide electrodes is presented. The transparent flexible ferroelectric heterostructures not only retain their superior performance, thermal stability, reliability, and mechanical durability, but also exhibit remarkably robust properties against to a strong radiation exposure. Our study demonstrates an extraordinary concept to realize transparent flexible nonvolatile electronic devices for the design and development of next-generation smart devices with potential application in electronics, automotive, aerospace, and nuclear systems.

A Fabrication and Measurement Method for a Flexible Ferroelectric Element Based on Van Der Waals Heteroepitaxy

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on high-density data storage and low-power consumption capabilities. However, the high-quality oxide based nonvolatile memory on flexible substrates is often constrained by the material characteristics and the inevitable high-temperature fabrication process. In this paper, a protocol is proposed to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica. The versatile deposition technique and measurement method enable the fabrication of flexible yet single-crystalline non-volatile memory elements necessary for the next generation of smart devices.