Ultralow Strain-Induced Emergent Polarization Structures in a Flexible Freestanding BaTiO3 Membrane

The engineering of ferroic orders, which involves the evolution of atomic structure and local ferroic configuration in the development of next-generation electronic devices. Until now, diverse polarization structures and topological domains are obtained in ferroelectric thin films or heterostructures, and the polarization switching and subsequent domain nucleation are found to be more conducive to building energy-efficient and multifunctional polarization structures. In this work, a continuous and periodic strain in a flexible freestanding BaTiO3 membrane to achieve a zigzag morphology is introduced. The polar head/tail boundaries and vortex/anti-vortex domains are constructed by a compressive strain as low as ≈0.5%, which is extremely lower than that used in epitaxial rigid ferroelectrics. Overall, this study c efficient polarization structures, which is of both theoretical value and practical significance for the development of next-generation flexible multifunctional devices.

Freestanding Oxide Membranes for Epitaxial Ferroelectric Heterojunctions

Since facile routes to fabricate freestanding oxide membranes were previously established, tremendous efforts have been made to further improve their crystallinity, and fascinating physical properties have been also reported in heterointegrated freestanding membranes. Here, we demonstrate our synthetic recipe to manufacture highly crystalline perovskite SrRuO₃ freestanding membranes using new infinite-layer perovskite SrCuO₂ sacrificial layers. To accomplish this, SrRuO₃/SrCuO₂ bilayer thin films are epitaxially grown on SrTiO₃ (001) substrates, and the topmost SrRuO₃ layer is chemically exfoliated by etching the SrCuO₂ template layer. The as-exfoliated SrRuO₃ membranes are mechanically transferred to various nonoxide substrates for the subsequent BaTiO₃ film growth. Finally, freestanding heteroepitaxial junctions of ferroelectric BaTiO₃ and metallic SrRuO₃ are realized, exhibiting robust ferroelectricity. Intriguingly, the enhancement of piezoelectric responses is identified in freestanding BaTiO₃/SrRuO₃ heterojunctions with mixed ferroelectric domain states. Our approaches will offer more opportunities to develop heteroepitaxial freestanding oxide membranes with high crystallinity and enhanced functionality.

A Two-Dimensional Superconducting Electron Gas in Freestanding LaAlO3/SrTiO3 Micromembranes

Freestanding oxide membranes constitute an intriguing material platform for new functionalities and allow integration of oxide electronics with technologically important platforms such as silicon. Sambri et al. recently reported a method to fabricate freestanding LaAlO₃/SrTiO₃ (LAO/STO) membranes by spalling of strained heterostructures. Here, we first develop a scheme for the high-yield fabrication of membrane devices on silicon. Second, we show that the membranes exhibit metallic conductivity and a superconducting phase below ∼200 mK. Using anisotropic magnetotransport we extract the superconducting phase coherence length ξ ≈ 36-80 nm and establish an upper bound on the thickness of the superconducting electron gas d ≈ 17-33 nm, thus confirming its two-dimensional character. Finally, we show that the critical current can be modulated using a silicon-based backgate. The ability to form superconducting nanostructures of LAO/STO membranes, with electronic properties similar to those of the bulk counterpart, opens opportunities for integrating oxide nanoelectronics with silicon-based architectures.

Enhanced Metal-Insulator Transition in Freestanding VO2 Down to 5 nm Thickness

Ultrathin freestanding membranes with a pronounced metal-insulator transition (MIT) have huge potential for future flexible electronic applications as well as provide a unique aspect for the study of lattice-electron interplay. However, the reduction of the thickness to an ultrathin region (a few nm) is typically detrimental to the MIT in epitaxial films, and even catastrophic for their freestanding form. Here, we report an enhanced MIT in VO₂-based freestanding membranes, with a lateral size up to millimeters and the VO₂ thickness down to 5 nm. The VO₂ membranes were detached by dissolving a Sr₃Al₂O₆ sacrificial layer between the VO₂ thin film and the c-Al₂O₃(0001) substrate, allowing the transfer onto arbitrary surfaces. Furthermore, the MIT in the VO₂ membrane was greatly enhanced by inserting an intermediate Al₂O₃ buffer layer. In comparison with the best available ultrathin VO₂ membranes, the enhancement of MIT is over 400% at a 5 nm VO₂ thickness and more than 1 order of magnitude for VO₂ above 10 nm. Our study widens the spectrum of functionality in ultrathin and large-scale membranes and enables the potential integration of MIT into flexible electronics and photonics.

Freestanding Double-Layer MoO3/CNT@S Membrane: A Promising Flexible Cathode for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have been regarded as a promising candidate of secondary batteries to satisfy the enormous demand for electric vehicles and energy storage applications. However, Li-S batteries still suffer from severe capacity fading due to the shuttle effect of lithium polysulfides. Here, we develop a freestanding double-layer MoO₃/carbon nanotube@S (FMC@S) membrane by hydrothermal and suction filtration strategy, without polymer binder and current collector substrate. FMC@S contains a polysulfide blocking layer and an active material layer. Except for S content, the two layers have the same components and are integrated together, so there is no distinct interface between the two layers, which can facilitate ion and electron transport. As a result, the FMC@S cathode delivers promising capacity retention and rate capability. The hierarchical integrated design provides a new strategy to develop high-performance flexible cathodes for Li-S batteries.

A MEMS device capable of measuring near-field thermal radiation between membranes

For sensors constructed by freestanding membranes, when the gap between a freestanding membrane and the substrate or between membranes is at micron scale, the effects of near-field radiative heat transfer on the sensors' thermal performance should be considered during sensor design. The radiative heat flux is transferred from a membrane to a plane or from a membrane to a membrane. In the current study of the near-field thermal radiation, the scanning probe technology has difficulty in making a membrane separated at micron scale parallel to a plane or another membrane. A novel MEMS (micro electromechanical system) device was developed by sacrificial layer technique in this work to realize a double parallel freestanding membrane structure. Each freestanding membrane has a platinum thin-film resistor and the distance between the two membranes is 1 m. After evaluating the electrical and thermal characteristics of the lower freestanding membrane, experimental measurements of near-field radiative heat transfer between the lower membrane and the upper membrane were carried out by setting the lower membrane as a heat emitter and the upper membrane as a heat receiver. The near-field radiative heat transfer between the two membranes was validated by finding a larger-than-blackbody radiative heat transfer based on the experimental data.