Under-oil superhydrophilic desert sand layer was used for efficient gravity-directed water-in-oil emulsions separation with high flux.

Polar topological texture has become an emerging research field for exotic phenomena and potential applications in reconfigurable electronic devices. We report toroidal topological texture self-organized in a ferroelectric polymer, poly(vinylidene fluoride-ran-trifluoroethylene) [P(VDF-TrFE)], that exhibits concentric topology with anticoupled chiral domains. The interplay among the elastic, electric, and gradient energies results in continuous rotation and toroidal assembly of the polarization perpendicular to polymer chains, whereas relaxor behavior is induced along polymer chains. Such toroidal polar topology gives rise to periodic absorption of polarized far-infrared (FIR) waves, enabling the manipulation of the terahertz wave on a mesoscopic scale. Our observations should inform design principles for flexible ferroic materials toward complex topologies and provide opportunities for multistimuli conversions in flexible electronics.

Recent realizations of ultrathin freestanding perovskite oxides offer a unique platform to probe novel properties in two-dimensional oxides. Here, we observe a giant flexoelectric response in freestanding BiFeO3 and SrTiO3 in their bent state arising from strain gradients up to 3.5 × 107 m−1, suggesting a promising approach for realizing ultra-large polarizations. Additionally, a substantial change in membrane thickness is discovered in bent freestanding BiFeO3, which implies an unusual bending-expansion/shrinkage effect in the ferroelectric membrane that has never been seen before in crystalline materials. Our theoretical model reveals that this unprecedented flexural deformation within the membrane is attributable to a flexoelectricity–piezoelectricity interplay. The finding unveils intriguing nanoscale electromechanical properties and provides guidance for their practical applications in flexible nanoelectromechanical systems.

Ferroelectric domain wall memories have been proposed as a promising candidate for nonvolatile memories, given their intriguing advantages including low energy consumption and high-density integration. Perovskite oxides possess superior ferroelectric prosperities but perovskite-based domain wall memory integrated on silicon has rarely been reported due to the technical challenges in the sample preparation. Here, we demonstrate a domain wall memory prototype utilizing freestanding BaTiO3 membranes transferred onto silicon. While as-grown BaTiO3 films on (001) SrTiO3 substrate are purely c-axis polarized, we find they exhibit distinct in-plane multidomain structures after released from the substrate and integrated onto silicon due to the collective effects from depolarizing field and strain relaxation. Based on the strong in-plane ferroelectricity, conductive domain walls with reading currents up to nanoampere are observed and can be both created and erased artificially, highlighting the great potential of the integration of perovskite oxides with silicon for ferroelectric domain wall memories.

Perovskite ferroelectric oxides are usually considered to be brittle materials, however, recent work [Dong et al., Science 366, 475 (2019)] demonstrated the super-elasticity in the freestanding BaTiO3 thin films. This property may originate from the ferroelectric domain evolution during the bending, which is difficult to observe in experiments. Therefore, understanding the relation among the bending deformation, thickness of the films, and the domain dynamics is critical for their potential applications in flexible ferroelectric devices. Here, we reported the dynamics of ferroelectric polarization in the freestanding BaTiO3 ultrathin films in the presence of large bending deformation up to 40˚ using phase-field simulation. The ferroelectric domain evolution reveals the transition from the flux-closure to a/c domains with "vortex-like" structures, which caused by the increase of out-of-plane ferroelectric polarization. Additionally, by varying the film thickness in the identical bending situation, we found the a/c phase with "vortex-like" structure emerges only as the film thickness reached 12 nm or higher. Results from our investigations provide instructive information for the microstructure evolution of bending ferroelectric perovskite oxide films, which could serve as guide for the future application of ferroelectric films on flexible electronic devices.

The complex micro-/nanoscale wrinkle morphology primarily fabricated by elastic polymers is usually designed to realize unique functionalities in physiological, biochemical, bioelectric, and optoelectronic systems. In this work, we fabricated inorganic freestanding BaTiO 3 ferroelectric thin films with zigzag wrinkle morphology and successfully modulated the ferroelectric domains to form an in-plane (IP) superstructure with periodic surface charge distribution. Our piezoresponse force microscopy (PFM) measurements and phase-field simulation demonstrate that the self-organized strain/stress field in the zigzag-wrinkled BaTiO 3 film generates a corresponding pristine domain structure. These domains can be switched by tip-induced strain gradient (flexoelectricity) and naturally form a robust and unique "braided" inplane domain pattern, which enables us to offer an effective and convenient way to create a microscopic ferroelectric superstructure. The corresponding periodic surface potential distribution provides an extra degree of freedom in addition to the morphology that could regulate cells or polar molecules in physiological and bioelectric applications.

Oxide nano-springs have attracted many research interests because of their anti-corrosion, hightemperature tolerance, oxidation resistance, and enhanced-mechanic-response from unique helix structures, enabling various nano-manipulators, nano-motors, nano-switches, sensors, and energy harvesters. However, preparing oxide nano-springs is a challenge for their intrinsic nature of lacking elasticity. Here, we developed an approach for preparing self-assembled, epitaxial, ferroelectric nanosprings with built-in strain due to the lattice mismatch in freestanding La 0.7 Sr 0.3 MnO 3 /BaTiO 3 (LSMO/BTO) bilayer heterostructures. We nd that these LSMO/BTO nano-springs can be extensively pulled or pushed up to their geometry limits back and forth without breaking, exhibiting super-scalability with full recovery capability. The phase-eld simulations reveal that the excellent scalability originates from the continuous ferroelastic domain structures, resulting from twisting under co-existing axial and shear strains. In addition, the oxide hetero-structural springs exhibit strong resilience due to the limited plastic deformation nature and the built-in strain between the bilayers. This discovery provides an alternative way for preparing and operating functional oxide nano-springs that can be applied to various technologies.

Solomon rings, upholding the symbol of wisdom with profound historical roots, had been widely used as decoration in ancient architecture and clothes. Yet, it was only recently discovered that such topological structures can be formed by self-organization in biological/chemical molecules, liquid crystals, etc. Here, we report the observation of polar Solomon rings in a ferroelectric nanocrystal, which are composed of two interwoven vortices and mathematically equivalent to a 412 link in topology. By combining piezoresponse force microscopy observations and phase-field simulations, we demonstrate the reversible switching between polar Solomon rings and vertex textures by an electric field. The two types of topological polar textures exhibit distinct absorption of terahertz infrared waves, which can be utilized in infrared displays with a nanoscale resolution. Our study establishes, both experimentally and computationally, the existence and electric manipulation of polar Solomon rings, a new form of topological polar structures, which may provide a simple way for fast, robust, and high-resolution optoelectronic devices.