Flexible Mica Research Articles

The Strain‐Modulated Single‐Mode Laser of Perovskite Microsheets with Grooves on Ultrathin Flexible Mica

AbstractInorganic perovskites have become a widely investigated candidate for fabrication of high‐performance micro‐devices due to their excellent optoelectronic properties and stability to the environment. Especially, their excellent lasing characteristics as a micro/nanolaser source, combined with flexible substrates, have great potential in the field of wearable or foldable photonic device applications. Here, the high‐quality CsPbBr3 perovskite microsheets with stress‐induced grooves are directly prepared on the ultrathin fluorine mica by the chemical vapor deposition method, and the single‐mode lasers are realized in these large‐size (>15 × 15 µm) microsheets. Thanks to the flexibility of mica, bending induced tensile strain on CsPbBr3 microsheet causes the laser mode blueshift continuously and reversibly with 15.3 meV %−1, this can be well explained by the Lorentz oscillator model under lasing conditions. Furthermore, by using the first‐principles calculation, it is clarified that the bandedge blueshift originates from the distortion of {PbBr6}4− octahedra and the consequential increase of the bond length of Pb‐Br3 and the Pb‐Br3‐Pb bond angle. This work opens up a new way to realize on‐chip single‐mode lasers with microstructures, and is helpful for the application of flexible photonic device in the fields of integrated on‐chip sensors.

Flexible La0.67Sr0.33MnO3:ZnO Nanocomposite Thin Films Integrated on Mica

The integration of functional oxide thin films on flexible substrates is critical for their application in flexible electronics. Here, to achieve flexible perovskite manganite oxide film with excellent low-field magnetoresistance (LFMR) effect, textured La0.67Sr0.33MnO3 (LSMO):ZnO nanocomposite film was deposited on a flexible mica substrate with ZnO buffer using pulsed laser deposition (PLD). Compared to the polycrystalline LSMO:ZnO nanocomposite film directly deposited on mica without buffer, the LSMO:ZnO/ZnO/mica sample exhibits larger saturation magnetization (164 emu/cm3) and higher Curie temperature (∼319 K), which results from the crystallinity and strain in the LSMO phase. In addition, the LSMO:ZnO/ZnO/mica film presents a high MR value of ∼39% at 10 K under 1 T. Furthermore, the good mechanical stretchability and property stability of the nanocomposite thin films have been demonstrated with mechanical bending.

Flexible lead-free film capacitor based on BiMg0.5Ti0.5O3-SrTiO3 for high-performance energy storage

Flexible dielectric film capacitors with high performance of energy storage has shown great promise as a solution to the flexibility and stability of modern electronics and electric power systems. Herein, a novel relaxor-ferroelectric BiMg0.5Ti0.5O3-xSrTiO3 (BMT-xSTO, x = 0.1, 0.2, 0.3 and 0.4) thin film capacitors are obtained via one-step fabrication on flexible mica substrates. A superior energy storage density of 109.7 J cm−3 and a pretty high efficiency of 80.6% are simultaneously achieved in the BMT-0.3STO film capacitor. At the same time, the energy storage performance can be stable in the temperature range of 25 to 200° C, the wide frequency range of 500 Hz to 10 kHz, and even after 108 electrical cycles. In addition, under the bending radii from 16 mm to 4 mm, there is no obvious variation in performance even after 104 mechanical fatigue cycles, indicating the super flexibility and stability of the film capacitors. The prominent properties of the BMT-based film capacitor are mainly attributed to the improvement of relaxation behavior by introducing appropriate STO, which are explained by the domain evolution via piezo-response force microscopy and phase field simulation. The present work suggests a new way to obtain lead-free and flexible dielectric film capacitors for flexible energy storage technology.

Preparation of a Vertical Graphene-Based Pressure Sensor Using PECVD at a Low Temperature.

Flexible pressure sensors have received much attention due to their widespread potential applications in electronic skins, health monitoring, and human–machine interfaces. Graphene and its derivatives hold great promise for two-dimensional sensing materials, owing to their superior properties, such as atomically thin, transparent, and flexible structure. The high performance of most graphene-based pressure piezoresistive sensors relies excessively on the preparation of complex, post-growth transfer processes. However, the majority of dielectric substrates cannot hold in high temperatures, which can induce contamination and structural defects. Herein, a credibility strategy is reported for directly growing high-quality vertical graphene (VG) on a flexible and stretchable mica paper dielectric substrate with individual interdigital electrodes in plasma-enhanced chemical vapor deposition (PECVD), which assists in inducing electric field, resulting in a flexible, touchable pressure sensor with low power consumption and portability. Benefitting from its vertically directed graphene microstructure, the graphene-based sensor shows superior properties of high sensitivity (4.84 KPa−1) and a maximum pressure range of 120 KPa, as well as strong stability (5000 cycles), which makes it possible to detect small pulse pressure and provide options for preparation of pressure sensors in the future.

Flexible fast responding solar-blind photodetectors based on (TmGa)2O3 films grown on mica

As an ultra-wide bandgap semiconductor, gallium oxide (Ga2O3) holds great application potential in deep-ultraviolet (DUV) photodetectors. While the performance of photodetectors based on crystalline Ga2O3 thin films grown on hard substrates has been continuously improved, photodetectors based on amorphous Ga2O3 grown on less-stringent substrates in a more convenient and accessible way emerged as alternative technology and received increasing attention. Herein, we choose thulium (Tm) for doping and grow amorphous Tm-Ga2O3 films on non-lattice-matched flexible mica substrates. Thanks to the larger bandgap of Tm2O3 (∼6.5 eV) and stronger Tm–O bond, the (TmxGa1−x)2O3 films possess broadened bandgap and lessened oxygen vacancies compared to pure Ga2O3. Consequently, the photodetectors that were produced based on these amorphous (TmxGa1−x)2O3 films exhibit high performances with both low dark current and fast response speed (36.47 pA and 0.07 s at x = 0.05) and well maintain the performance after multiple cycles of bending at radius as small as 5 mm. This work sheds light on the development of flexible devices based on amorphous (TmxGa1−x)2O3 for solar-blind DUV detection.

Significantly enhanced energy storage density and efficiency in flexible Bi3.15Nd0.85Ti3O12 thin film via periodic dielectric layers

Flexible energy storage based on ferroelectric capacitors enjoys high power density and rapid respond time, but the polarization fatigue problems limit its long-term reliability. Bi3.15Nd0.85Ti3O12 (BNT) is a lead-free ferroelectric material with fatigue-free properties, which is promising for applications in long-term ferroelectric devices. Nevertheless, the applications of BNT thin films for energy storage are restricted by their poor energy efficiency and low energy density. In this work, flexible BNT thin films with different numbers of SrTiO3 (STO) insert layers are fabricated on flexible mica substrates by an exquisitely designed dual-target pulsed laser deposition process, and the energy storage performances are effectively optimized via interface engineering. By inserting periodic STO dielectric layers with repetition periods of 100, the energy storage density (Wrec) and efficiency (η) are improved to ∼24.26 J cm−3 and ∼71.93%, which are, respectively, increased to 287% and 132% compared with the pure phase BNT thin film. Importantly, the improved Wrec and η can be well maintained under large bending deformation (bending radius as small as r = 4 mm) and within a wide temperature range (25–175 °C), suggesting its good stability and reliability. These results show that the involvement of periodic dielectric layers in BNT thin films can significantly enhance energy storage density and efficiency and effectively promote its applications in future flexible energy storage devices.

Epitaxial (110)-oriented La0.7Sr0.3MnO3 film directly on flexible mica substrate

Manufacture and characterizations of perovskite-mica van der Waals epitaxy heterostructures are a critical step to realize the application of flexible devices. However, the fabrication and investigation of the van der Waals epitaxy architectures grown on mica substrates are mainly limited to (111)-oriented perovskite functional oxide thin films up to now and buffer layers are highly needed. In this work, we directly grew La0.7Sr0.3MnO3 (LSMO) thin films on mica substrates without using any buffer layer. By the characterizations of x-ray diffractometer and scanning transmission electron microscopy, we demonstrate the epitaxial growth of the (110)-oriented LSMO thin film on the mica substrate. The LSMO thin film grown on the mica substrate via van der Waals epitaxy adopts domain matching epitaxy instead of conventional lattice matching epitaxy. Two kinds of domain matching relationships between the LSMO thin film and mica substrate are sketched by Visualization for Electronic and STructural Analysis software and discussed. A decent ferromagnetism retains in the (110)-oriented LSMO thin film. Our work demonstrates a new pathway to fabricate (110)-oriented functional oxide thin films on flexible mica substrates directly.

Manganese-doped transparent conductive magnetic indium oxide films integrated on flexible mica substrates with high mechanical durability

Recent advances in flexible electronic devices have stimulated demands for the fabrication of multifunctional materials on highly flexible substrates. However, flexible diluted magnetic semiconductor films have been inadequately reported thus far. In this study, I grew polycrystalline manganese (Mn)-doped indium oxide (In2O3) films on flexible mica substrates at three deposition temperatures (Td) using the pulsed laser deposition method. All the investigated Mn-doped In2O3 films exhibited high optical transparency, high electrical conductivity, room-temperature ferromagnetism, and excellent mechanical durability. The present experimental results demonstrate that Td considerably influences the optical, electrical, and magnetic properties as well as the mechanical durability of the flexible Mn-doped In2O3 films. Based on the robustness of inorganic mica at high deposition temperatures, this detailed investigation provides a strategy for fabricating flexible diluted magnetic semiconductor films, the properties of which could be carefully optimized by widely varying the Td.

Mechanically induced enhancement and modulation of upconversion photoluminescence by bending lanthanide-doped perovskite oxides.

We report experimental studies of the bending strain impact on the upconversion processes in Yb3+, Er3+, and Mn2+ co-doped BaTiO3 (BTO) thin films with mica as the flexible substrate. Bending strain induces strong enhancement and modulation of the upconversion emission in doped BTO thin films. Because the unshielded 3d5 configuration of Mn2+ is more susceptible to crystal field changes, the introduction of an Mn2+ ion further promotes the strain-induced modulation effect. The upconversion intensity is amplified by six times at bending strain ε = 1.83% in BTO:Yb3+/Er3+/Mn2+ thin films. These results demonstrate the opportunity of rendering an upconversion emission through integrating lanthanide-doped ferroelectric films with flexible mica, especially by incorporating an Mn2+ ion.

Mechanically enhanced magnetism in flexible semitransparent CuFe2O4/mica epitaxial heterostructures

Mechanical strain tuning of magnetic properties is crucial for developing flexible electronic and spintronic devices. Here, (111)-oriented CuFe2O4 (CuFO) thin films are epitaxially grown on flexible mica substrates, and the influence of the mechanical strain on magnetic properties is investigated by altering the curvature of the CuFO/mica structures. Both the in-plane and out-of-plane saturation magnetic moments increase monotonously with the decrease of bending radius, which can be ascribed to the mechanical strain-induced Jahn-Teller distortion and much easier neighboring ferromagnetic domain switching in the CuFO films. Particularly, the residual magnetization can be dramatically enhanced by mechanical strain with a large gauge factor of 321.25, demonstrating effective strain tuning of magnetism. Moreover, the CuFO/mica structures exhibit strain tunable parallel magnetic anisotropy and excellent mechanical antifatigue properties. This work implies a great potential for promissing applications in flexible spintronics and electronics.

Flexible Ferroelectric Hafnia-Based Synaptic Transistor by Focused-Microwave Annealing.

Hafnia-based ferroelectric memory devices with excellent ferroelectricity, low power consumption, and fast operation speed have attracted considerable interest with the ever-growing desire for nonvolatile memory in flexible electronics. However, hafnia films are required to perform a high temperature (>500 °C) annealing process for crystallization into the ferroelectric orthorhombic phase. It can hinder the integration of hafnia ferroelectric films on flexible substrates including plastic and polymer, which are not endurable at high temperatures above 300 °C. Here, we propose the extremely low-temperature (∼250 °C) process for crystallization of Hf0.5Zr0.5O2 (HZO) thin films by applying a focused-microwave induced annealing method. HZO thin films on a flexible mica substrate exhibits robust remnant polarization (2Pr ∼ 50 μC/cm2), which is negligibly changed under bending tests. In addition, the electrical characteristics of a HZO capacitor on the mica substrate were evaluated, and ferroelectric thin film transistors (Fe-TFTs), using a HZO gate insulator, were fabricated on mica substrates for flexible synapse applications. Symmetric potentiation and depression characteristics are successfully demonstrated in the Fe-TFT memory devices, and the synaptic devices result in high recognition accuracy of 91.44%. The low-temperature annealing method used in this work are promising for forming hafnia-based Fe-TFT memory devices as a building block on a flexible platform.

Flexoelectric-induced photovoltaic effects and tunable photocurrents in flexible LaFeO3 epitaxial heterostructures

By engineering strain gradients in dielectrics, the flexoelectric effect can be created, which yields interesting physical properties via electromechanical coupling. Here, we report flexoelectric-induced photovoltaic effects in centrosymmetric LaFeO3 thin-film heterostructures grown on flexible mica substrates, in which partial relaxation of lattice-mismatch strain against LaAlO3 stretching layers results in giant strain gradients and pronounced electrical polarizations. The flexoelectric polarization modulates band alignment and leads to significant photovoltaic effects with a short-circuit current density of ∼0.4 mA/cm2 and an open circuit voltage of ∼ -0.45 V in Pt/LaFeO3/LaNiO3 devices. In addition, by concavely/convexly bending the mica substrate, mechanical strain gradients give rise to bi-directionally tunable photocurrents, in which continuously change of short-circuit current density with a magnitude of ∼100% and good reproducibility in repetitive bending operations are observed in the Pt/LaFeO3/LaNiO3 devices. The present work demonstrates an approach to design self-powered photoelectric devices with an electromechanical degree of freedom based on the flexoelectric effect in flexible thin-film heterostructures.

Growth of Metastable α-Ga<sub>2</sub>O<sub>3</sub> Epitaxial Thin Film on Flexible Synthetic Mica by Insertion α-Fe<sub>2</sub>O<sub>3</sub> Buffer Layer

Metastable α-Ga2O3 thin films with a corundum structure were grown epitaxially on flexible synthetic mica. The κ-Ga2O3 thin films were grown in a temperature range of 450‒600 °C, without a buffer layer. In contrast, the α-Ga2O3 thin films were grown in a wider temperature range of 350‒600 °C, by inserting corundum-structured α-Fe2O3 buffer layers. X-ray diffraction (XRD) rocking curve measurements revealed that the α-Ga2O3 thin film grown at 425 °C had the highest degree of crystallinity. Cross-sectional transmission electron microscopy (TEM) observations and an XRD φ scan revealed that the epitaxial relationship between the thin film and the substrate via the buffer layer was (0001) α-Ga2O3 [11‒20] || (001) synthetic mica [100]. Furthermore, when TEM observation was performed close to the surface of the α-Ga2O3 thin film at a high magnification, a lattice image derived from the out-of-plane (0001)-plane orientation was observed. However, it was also revealed that the α-Ga2O3 thin films did not have a single crystal structure but rather an in-plane domain structure. By conducting selected area electron diffraction (SAED) at the interface area, it was determined that the α-Fe2O3 buffer layer was polycrystalline. This implies that α-Ga2O3 thin films were epitaxially grown while forming the in-plane domains on the polycrystalline buffer layers. The results of this study indicate the flexible applications of α-Ga2O3 thin films, which have significant potential for use as power sources and deep-ultraviolet devices.

Antiferroelectric Anisotropy of Epitaxial PbHfO3 Films for Flexible Energy Storage

AbstractDielectric capacitors are widely studied for power supply systems because they can quickly store and release electrical energy. Among various kinds of dielectric materials, antiferroelectrics show promising features of high energy‐storage density and efficiency. In this study, epitaxial antiferroelectric PbHfO3 films with different orientations are fabricated, in which remarkable anisotropies of polarization and energy storage properties are discovered. With the optimization of film orientation, much‐improved energy density and excellent high‐temperature efficiency are achieved in the PbHfO3 films. Moreover, the PbHfO3 films are fabricated onto flexible mica substrates, which exhibit excellent property robustness against mechanical bending. This study provides a fundamental understanding of the anisotropic antiferroelectric behaviors of epitaxial PbHfO3 films and provides a generalizable pathway for flexible energy‐storage dielectric capacitors.

ALD Deposited ZnO:Al Films on Mica for Flexible PDLC Devices.

In this work, highly conductive Al-doped ZnO (AZO) films are deposited on transparent and flexible muscovite mica substrates by using the atomic layer deposition (ALD) technique. AZO-mica structures possess high optical transmittance at visible and near-infrared spectral range and retain low electric resistivity, even after continuous bending of up to 800 cycles. Structure performances after bending tests have been supported by atomic force microscopy (AFM) analysis. Based on performed optical and electrical characterizations AZO films on mica are implemented as transparent conductive electrodes in flexible polymer dispersed liquid crystal (PDLC) devices. The measured electro-optical characteristics and response time of the proposed devices reveal the higher potential of AZO-mica for future ITO-free flexible optoelectronic applications.

Heteroepitaxy of flexible piezoelectric Pb(Zr0.53Ti0.47)O3 sensor on inorganic mica substrate for lamb wave-based structural health monitoring

Active Lamb wave detection is an effective and simple monitoring method for structural health monitoring (SHM). Piezoelectric lead zirconate titanate (PZT) ceramics are most commonly used as transducers to excite and receive Lamb wave. However, due to their hardness, thickness and brittleness, PZT ceramics are severely limited in detecting damage on curved surface structures. Herein, we report that flexible PZT film sensor is an excellent candidate for receiving Lamb wave used in Lamb wave-based SHM of aircraft. An improved piezoelectric coefficient d33 (~130 pm V−1) is obtained in epitaxial PZT films grown on flexible inorganic mica substrates via van der Waals heteroepitaxy. Superior stable ferroelectricity and piezoelectricity retain in flexible PZT film after bending 104 cycles under a radius of 8 mm. As proven by experiment and simulation, flexible PZT film is demonstrated as an ultrasonic sensor with high sensitivity to be bonded on the curved aluminum plate for Lamb wave-based damage monitoring. Our work demonstrates that flexible PZT films have enormous potential for real-time light-weight and high-sensitivity receiver sensors for the SHM of aging aircraft.

Formation and physical properties of the self-assembled BFO–CFO vertically aligned nanocomposite on a CFO-buffered two-dimensional flexible mica substrate

Engineering the interfaces between materials of different structures and bonding nature in a well-controlled fashion has been playing a key role in developing new devices with unprecedented functionalities. In particular, direct growth of nanostructures on van der Waals substrates not only is essential for fully exploiting the potential of a wide variety of self-assembled nano-sized heterostructures but also can expand the horizons for electronic and photonic applications that involve nanostructures of specific composition and geometry. In the present work, we demonstrate the epitaxial growth of a self-assembled vertically aligned nanocomposite of magnetoelectric oxides on a flexible substrate via van der Waals epitaxy, which evidently adds an additional dimension of flexibility to similar thin-film heteroepitaxy architectures that have been mostly realized on rigid lattice-matched substrates. It is noted that the utilization of buffer layers is essential for obtaining high-quality flexible thin films with vertically aligned nanocomposite architecture. We believe that this route can provide alternative options for developing flexible thin-film devices with heteroepitaxy architectures of other functional materials.

Flexible transparent solar blind ultraviolet photodetector based on amorphous Ga<sub>2</sub>O<sub>3</sub> grown on mica substrate

Solar-blind deep-ultraviolet (UV) photodetectors (PDs) based on the super-wide bandgap semiconductor material Ga<sub>2</sub>O<sub>3</sub> is one of the hot topics of current research, but how to prepare high-performance Ga<sub>2</sub>O<sub>3</sub>-based solar-blind PDs in the field of flexible and transparent optoelectronics still faces challenges. In this work, an amorphous Ga<sub>2</sub>O<sub>3</sub> film with high transmittance is grown on a flexible mica substrate by using the radio frequency magnetron sputtering technology. On this basis, using AZO as an electrode material, a transparent metal-semiconductor-metal (MSM) structured solar-blind deep ultraviolet photodetector based amorphous Ga<sub>2</sub>O<sub>3</sub> film is fabricated, and the performance of PD in the planar state and after multiple bending are systematically compared and analyzed. The results show that the amorphous Ga<sub>2</sub>O<sub>3</sub> based transparent PD has ultra-high visible light transparency and shows good solar-blind ultraviolet photoelectric characteristics. The responsivity of the PD under 254 nm light is 2.69 A/W, and the response time and the recovery time are 0.14 s and 0.31 s, respectively. After bending 300 times, the PD has a photoresponse behavior similar to its planar state, and the performance of the PD has no obvious attenuation phenomenon, showing good flexibility and stability. This work proves that AZO can be used as the electrode material of the next generation of flexible and visible light transparent Ga<sub>2</sub>O<sub>3</sub> based photodetectors, and provides a reference for developing the high-performance flexible and transparent solar-blind deep ultraviolet photodetectors.

Large-area ReS2 monolayer films on flexible substrate for SERS based molecular sensing with strong fluorescence quenching

Two-dimensional (2D) transition metal dichalcogenides materials have recently been explored as possible substrates for surface-enhanced Raman spectroscopy (SERS). However, in comparison to the conventional noble-metal-based counterpart, most of the 2D materials have very small area for SERS measurement, which is a big limitation toward practical applications. Herein, we presented our study of SERS using substrate-scale continuous ReS2 monolayer films on flexible mica substrate. We demonstrated that the charge transfer between the target molecules and the atomically thin ReS2 can be modulated by the underlying substrate of ReS2, resulting in greatly different suppressing level of FL background. Kelvin probe force microscopy revealed a large difference of surface potential between ReS2/SiO2 and ReS2/mica, suggesting that the strong FL quenching of molecular on ReS2/mica might be attributed to the inert surface of layered mica. The large-area ReS2 monolayer films on mica show a detection limit of R6G molecule around 10−7 M and a robust SERS performance after an exposure in air for one month and repeated bending for 1000 times. Our work explores the practical application of 2D ReS2 for molecular detection via SERS technique.

The microstructure and ferroelectric properties of PbZr0.52Ti0.48O3 films on mica substrates

PbZr0.52Ti0.48O3 (PZT) tetragonal ferroelectric films have been fabricated on flexible mica and rigid silicon substrates respectively with a bottom electrode of LaNiO3 (LNO) films via a low-cost sol-gel process. The microstructure and ferroelectric properties of both films were systematically investigated. The quantitative analysis results of XRD and TEM indicate that the PZT film on Si substrate suffer from residual in-plane tensile stress, while a virtually strain-free film can be obtained on mica substrate. The PZT film on mica substrate has a lower local coercive voltage studied by a piezoelectric response force microscope. It is the weak van der Waals interaction between the film and the two-dimensional mica substrate that reduce the substrate clamping effect and contribute to a more free response to the electric field for domain walls. A large measured residual polarization of ~52 μC/cm2 was obtained for the film on mica substrate, and the P−E loops can remain stable after bending cycles of 1000 times. The flexible PZT thin film is expected to have a strong potential application in the next generation of wearable devices.