Mechanical displacement in commonly used piezoelectric materials is typically restricted to linear or biaxial in nature and to a few percent of the material dimensions. Here, we show that free-standing BaTiO3 membranes exhibit non-conventional electromechanical coupling. Under an external electric field, these superelastic membranes undergo controllable and reversible "sushi-rolling-like" 180° folding-unfolding cycles. This crease-free folding is mediated by charged ferroelectric domains, leading to a giant > 3.8 and 4.6 µm displacements for a 30-nm thick membrane at room temperature and 60 °C, respectively. Further increasing the electric field above the coercive value changes the fold curvature, hence augmenting the effective piezoresponse. Finally, it is found that the membranes fold with increasing temperature followed by complete immobility of the membrane above the Curie temperature, allowing us to model the ferroelectric-domain origin of the effect.The electromechanical power conversion of piezoelectrics is the basis for a broad range of sensing, actuating, and communication technologies, including ultrasound imaging and cellular phones. 1-3 Recent interest in electromechanical energy harvesting 4,5 as well as in flexible electronics for wearable devices, 6,7 nano motors, 8 and medical applications 9-11 raises a need for flexible piezoelectric materials and devices. Modern applications of piezoelectrics hinge on thin films, 12-14 however, the substrate in such geometries is typically rigid, preventing the development of flexible devices. Flexible piezoelectric devices are therefore typically based on either nanowires 4 or on thin-film systems, but with substrates that have been designed especially for such applications. 15,16 Most piezoelectric applications rely on lead-based materials, which exhibit strong piezoelectric coefficients. Nevertheless, the toxicity of these materials is undesirable for environmental considerations, while it also disqualifies them for medical or wearable applications. Likewise, traditional thin-film geometries limit the electromechanical excitation modes. That is, usually, uniaxial electric field results in either parallel or perpendicular uniaxial or biaxial mechanical deformation (or vice versa).Nevertheless, the interest in flexible-electronic technologies raises a need for advanced electromechanical excitation modes, e.g., for motorized devices, including microscale aerial vehicles. 17 Substrate removal for piezoelectric films or membranes augments their functional properties, 18-21 mainly thanks to mechanically-induced ferroic-domain reorganization. 22 However, the preparation of completely stand-alone substrate-free films has remained a challenge. Lu et al. 23 demonstrated lately a general method to prepare oxide materials in the form of membranes, i.e., continuous free-standing thin films with no substrate. More recently, Dong et al. 24 used this method to process BaTiO3 membranes, which is a well-known lead-free piezoelectric and ferroelectric material. This work show...

In ferroelectricity, atomic-scale dipole moments interact collectively to produce strong electromechanical coupling and switchable macroscopic polarization. Hence, the functionality of ferroelectrics emerges at a solid-solid phase transformation that is accompanied by a sudden disappearance of an inversion symmetry. Much effort has been put to understand the ferroelectric transition at the polarization length scale. Nevertheless, the dipole-moment origin of ferroelectricity has remained elusive. Here, we used variable-temperature high-resolution transmission electron microscopy to reveal the dipole-moment dynamics during the ferroelectric-to-paraelectric transition. We show that the transition occurs when paraelectric nuclei of the size of a couple of unit cells emerge near the surface. Upon heating, the cubic phase sidewalk grows towards the bulk. We quantified the nucleation barrier and show dominancy of mechanical interactions, helping us demonstrate similarities to predictions of domain nucleation during electric field switching. Our work motivates dynamic atomic-scale characterizations of solid-solid transitions in other materials.

No abstract

Hybrid-halide perovskite (HHP) films exhibit exceptional photo-electric properties. These materials are utilized for highly efficient solar cells and photoconductive technologies. Both ion migration and polarization have been proposed as the source of enhanced photoelectric activity, but the exact origin of these advantageous device properties has remained elusive. Here, we combined microscale and device-scale characterization to demonstrate that polarization-assisted conductivity governs photoconductivity in thin HHP films. Conductive atomic force microscopy under light and variable temperature conditions showed that the photocurrent is directional and is suppressed at the tetragonal-to-cubic transformation. It was revealed that polarization-based conductivity is enhanced by light, whereas dark conductivity is dominated by non-directional ion migration, as was confirmed by large-scale device measurements. Following the non-volatile memory nature of polarization domains, photoconductive memristive behavior was demonstrated. Understanding the origin of photoelectric activity in HHP allows designing devices with enhanced functionality and lays the grounds for photoelectric memristive devices.3 Hybrid-halide perovskites (HHP), specifically methylammonium lead iodide (MAPbI3), garner much interest in recent years thanks to their unique electro-optic and photo-electric properties.These materials are therefore utilized for competitive technologies, such as high efficiency solar cells, as well as self-powered photodetectors and devices. [1][2][3][4][5] The typical light-assisted electronhole pairing and separation in semiconductors 6 might not suffice to support the observed photoconductivity in HHPs. Hence, additional mechanisms have been proposed as governing the photoconductivity in these materials. Traditionally, the advantageous properties of HHP devices are attributed to ion migration, which is a result of the high ion mobility in these semiconducting materials. 7,8 Recent accumulated literature discusses the existence of polarization and polarization domains in these materials, especially in highly strained geometries, such as thin films. 9-16 It has been proposed that the high photocurrents stem from polarization-based conductivity, which is a competing mechanism for ion migration. 17 Hence, it is currently uncertain what mechanism governs the advantageous material and device properties.Upon electric-field poling, both ion migration and polarization effects are observable. 3,18,19 Both mechanisms can also support previously reported anomalous photovoltaic behavior, such as high photo-voltage and a hysteretic current-voltage curve is also supported by. 19,20 This causes significant challenges in decoupling the two effects, hence hindering the realization of the fundamental mechanism that governs the functionality in these materials, as well as encumbering their technological development. While polarization effects provide more selfpowered and stable capacity for devices, 11,19 ion migration is transient in nature...

In this work, slanted, kinked, and straight silicon nanowires (SiNWs) are fabricated on Si(111) and (100) substrates using a facile two-step metal-assisted chemical etching nanofabrication technique. We systematically investigated the effect of crystallography, morphology of Ag catalyst, and composition of etchant on the etch profile of Ag catalyst on Si(111) and (100) substrates. We found that the movement of AgNPs inside the Si is determined by physiochemical events such as Ag/Ag interaction, Ag/Si contact, and diffusion kinetics. Further, from detailed TEM and micro-Raman spectroscopy analyses, we demonstrate that the metal catalyst moves in the crystallographically preferred etching direction (viz., <100>) only when the interface effect is not predominant. Further, the metal-assisted chemical etching (MACE) system is highly stable at low-concentration plating and etching solutions, but at high concentrations, the system loses its stability and becomes highly random, leading to the movement of Ag catalyst in directions other than ⟨100⟩. In addition, our studies reveal that Ag nanostructures growth on Si(111) and (100) substrates through galvanic displacement is controlled by substrate symmetry and surface bond density. Finally, we demonstrate that by using an optimized balance between the Ag morphology and concentration of the etchant, the angle in slanted SiNWs, kink position in kinked SiNWs, and aspect ratio of straight SiNWs can be controlled judiciously, leading to enhanced optical absorption in the broadband solar spectrum.