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

Structural, Magnetic, and Magnetostriction Properties of Flexible, Nanocrystalline CoFe2O4 Films Made by Chemical Processing

We report on the simple, single-step, and cost-effective fabrication, characterization, and performance evaluation of cobalt ferrite (CoFe₂O₄; CFO) nanocrystalline (NC) thin films on a flexible mica substrate. The chemical solution-based drop-casting method employed to fabricate crystalline CFO films and their characterization was performed by studying the phase formation, surface morphology, and magnetic parameters, while sensor applicability was evaluated using combined magnetic and magnetostrictive properties. X-ray diffraction (XRD) indicates the single-phase and nanocrystalline nature of CFO films, where the crystallite size is ∼60 nm. The optimum conditions employed resulted in CFO NC films with surface particles exhibiting a spherical shape morphology with a homogeneous size distribution, as revealed by scanning electron microscopy analyses. Raman spectroscopic characterization of the chemical bonding indicates all of the active bands that are characteristic of the ferrite phase confirm the spinel structure, which is in agreement with XRD studies. The saturation magnetization (M _S) and coercivity (H _C), which are extracted from the field-dependent magnetization data, of CFO NC films were found to be 15.8 emu/g and 1.6 kOe, respectively, while the first-order magnetocrystalline anisotropy constant K ₁ was ∼1.07 × 10⁶ erg/cm³. The magnetostriction strain curve indicates that the CFO NC films exhibit a strain value of ∼86 ppm at an applied magnetic field of 8 kOe, indicating their suitability for flexible sensor devices.

Coupled straintronic-optoelectronic effect in Mott oxide films

Controlling the electronic properties of complex oxides by an external stimulus is of significant importance for exploring exotic quantum states and developing modern electronic devices with low-level energy consumption. Here, we demonstrate the electro-photo double control of electronic transport for Mott insulating LaVO₃ thin films deposited onto ferroelectric 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ substrates. An electrically driven linear electroresistance effect is acquired at room temperature through the converse piezoelectric response, which can be optically regulated by 79%. Furthermore, the visible light-activated photoresistance response can be efficiently tuned by piezo strain. These results demonstrate that the strain-excited effect and photo-generated effect strongly correlated with each other, mediated by lattice-charge-orbital coupling. Our work points to an effective strategy for realizing the coupled straintronic-optoelectronic effect in hybrid correlated oxide/ferroelectric systems and delivering multi-field tunable low-dissipation versatile electronic and photonic devices.

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.

Flexible Lead-Free Ba0.5Sr0.5TiO3/0.4BiFeO3-0.6SrTiO3 Dielectric Film Capacitor with High Energy Storage Performance

Ferroelectric thin film capacitors have triggered great interest in pulsed power systems because of their high-power density and ultrafast charge–discharge speed, but less attention has been paid to the realization of flexible capacitors for wearable electronics and power systems. In this work, a flexible Ba_0.5Sr_0.5TiO₃/0.4BiFeO₃-0.6SrTiO₃ thin film capacitor is synthesized on mica substrate. It possesses an energy storage density of W_rec ~ 62 J cm⁻³, combined with an efficiency of η ~ 74% due to the moderate breakdown strength (3000 kV cm⁻¹) and the strong relaxor behavior. The energy storage performances for the film capacitor are also very stable over a broad temperature range (−50–200 °C) and frequency range (500 Hz–20 kHz). Moreover, the W_rec and η are stabilized after 10⁸ fatigue cycles. Additionally, the superior energy storage capability can be well maintained under a small bending radius (r = 2 mm), or after 10⁴ mechanical bending cycles. These results reveal that the Ba_0.5Sr_0.5TiO₃/0.4BiFeO₃-0.6SrTiO₃ film capacitors in this work have great potential for use in flexible microenergy storage systems.

Photoassisted Electron-Ion Synergic Doping Induced Phase Transition of n-VO2/p-GaN Thin-Film Heterojunction

As a typical correlated metal oxide, vanadium dioxide (VO₂) shows specific metal-insulator transition (MIT) properties and demonstrates great potential applications in ultrafast optoelectronic switch, resistive memory, and neuromorphic devices. Effective control of the MIT process is essential for improving the device performance. In the current study, we have first proposed a photoassisted ion-doping method to modulate the phase transition of the VO₂ layer based on the photovoltaic effect and electron-ion synergic doping in acid solution. Experimental results show that, for the prepared n-VO₂/p-GaN nanojunction, this photoassisted strategy can effectively dope the n-VO₂ layer by H⁺, Al³⁺, or Mg²⁺ ions under light radiation and trigger consecutive insulator-metal-insulator transitions. If combined with standard lithography or electron beam etching processes, selective doping with nanoscale size area can also be achieved. This photoassisted doping method not only shows a facile route for MIT modulation via a doping route under ambient conditions but also supplies some clues for photosensitive detection in the future.

Piezoelectricity in Excess of 800 pC/N over 400 °C in BiScO3-PbTiO3-CaTiO3 Ceramics

Ultrasonic sensors are widely applied in industries near room temperature; however, their application at high temperature is still a challenge mainly due to the lack of high-performance piezoelectric ceramics. Here, the 0.364BiScO₃-0.636PbTiO₃-0.005CaTiO₃ ceramic exhibits excellent piezoelectric performances at 20-440 °C. Its piezoelectric coefficient d₃₃ increases from 475 pC/N at 20 °C to 853 pC/N at 360 °C, and then it gradually decreases to 669 pC/N at 440 °C. Furthermore, the planar electromechanical coupling factor k_p gradually increases from 0.59 at 20 °C to 0.67 at 200 °C, and then it remains at a stable value of 0.65-0.67 at 150-350 °C. These achievements are because the ceramic morphotropic phase boundaries have a flat Gibbs free energy versus polarization curve and a wide temperature range. Since the piezoelectric ceramic shows satisfactory piezoelectric properties at 20-440 °C, the corresponding ultrasonic sensors can in situ monitor many high-temperature devices, such as engines, wheels, drills, boilers, etc.

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.

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.

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.