Grain boundaries (GBs) in 2D materials are arrays of dislocations that strongly influence electronic, optical, mechanical, and transport properties. In monolayer MoS2, Mo 5|7 and S 5|7 dislocation cores are structurally established, yet how their distinct mobilities govern GB migration and grain growth has remained unresolved. Here, we directly image and quantify GB mobility in polycrystalline monolayer MoS2 at the level of individual dislocation cores. An atomic-scale in-situ heating study reveals that mobile GBs consist solely of Mo 5|7 dislocations, whereas immobile GBs contain S 5|7 kink sites that suppress GB motion. Dark-field TEM grain-growth measurements show that mobile GBs drive rapid coarsening, while S 5|7-pinned immobile GBs arrest microstructural evolution. Molecular dynamics simulations correlate this mobility contrast with distinct local mechanical and energetic environments around Mo 5|7 and S 5|7 cores, providing an atomistic origin for their different migration kinetics. These findings establish a direct structure-kinetics-growth relationship for GB evolution in monolayer MoS2 and demonstrate a broadly applicable in-situ electron-microscopy platform for probing defect-driven microstructural dynamics in 2D materials.

Active symmetry control - a central challenge in materials science, particularly in ferroelectrics - is achieved via mechanically assisted poling (MAP) guided by thermodynamics and phase - field modeling. This approach yields extraordinary piezoelectric coefficients (about 5,000 pC/N at 24 degC; 11,700 pC/N at 58 degC) together with about 65% optical transmittance in a classic relaxor ferroelectric, Pb(Mg1/3Nb2/3)O3-PbTiO3. Mechanical suppression of undesirable phases stabilizes a reconstructed symmetry with highly ordered domains, verified by multiple characterization techniques. The strategy is validated across several distinct ferroelectric systems. To demonstrate its practical utility, we fabricate a transparent dual-modal wearable sensor integrating continuous blood pressure monitoring via piezoelectricity with photoplethysmographic SpO2 detection, enabling high-fidelity physiological tracking. This work establishes mechanically assisted symmetry reconstruction as a pathway to multifunctional optoelectronic materials and compact wearable health technologies.

The existence and stability of phases in ferroelectric solid solutions under hydrostatic pressure are studied by using a phenomenological theory. Phase diagrams, with the consideration of phase coexistence, are calculated based on the criterion of the Hessian matrix at various temperatures. Results have shown that rhombohedral (R) phase and tetragonal (T) phase at morphotropic phase boundary (MPB) exhibit the highest stability in resisting the depolarization caused by hydrostatic pressure. Enhanced piezoelectricity exists near the phase boundaries, where ferroelectric–paraelectric phase transition occurs. Besides, average piezoelectric coefficient d33 of domains with the coexistence of R phase and T phase is relative high at MPB due to the sharp increase of piezoelectricity close to the phase transition.

Piezoelectric performance of relaxor ferroelectric crystals is closely related to the domain size and microstructure. This work uses autocorrelation analysis to study the domain size of direct current poling (DCP) and alternating current poling (ACP) PMN-0.30PT crystals with [001]-oriented rhombohedral (R) strip domain structures. The average horizontal domain size (HDS) shows good agreement with the short-range correlation lengths for the polarizing light microscopy (PLM) images. Domain size was controlled by ACP frequency, with the optimal conditions determined from the scaling relationship extracted from the dynamic hysteresis loops. The piezoelectric coefficient d(33) of the [001]-poled PMN-0.30PT crystals under an optimal ACP (5 kV(rms)/cm, 10 Hz, 10 cycles) was 2740 pC/N with < xi> of 760 ( +/- 20) nm, which was 65 % higher than DCP (5 kV/cm, 5 min) with < xi> of 1170 ( +/- 50) nm, respectively. The new analysis method offers a quantitative assessment of domain sizes and its effect on the macro-performance.

The ferroelectric tunneling junction,with a metal-ferroelectric ultra-thin film-metal structure,has different tunneling resistance states through polarization manipulation,leading to potential applications in next-generation information storage devices with low-power consumption,fast reading/writing speed,high storage density,and non-volatility.However,the ferroelectric thin films still experience high-temperature rises with re-duced stability due to high driving fields,and reducing the driving electric field is crucial for designing ferroe-lectric tunneling devices.The ferroelectric thin films with coexisting domains have lowered barriers and de-creased driving electric fields for domain switching,which are achieved through substrate manipulation.Herein the substrate effects on the driving field,the tunneling resistance switching ratio and the tunneling properties,were studied based on the WKB approximation combined with the Landau phenomenological theory.The results show that,the ferroelectric tunnel junction with coexisting domains exhibits 3 resistive states corresponding to out-of-plane and in-plane polarizations.The effective driving electric field can be reduced to 25 MV/m,which is 76%lower than that with 2 resistive single domains.The proposed theoretical framework provides a fundamen-tal understanding of the formation of multi-state and reduction of the driving field for low-energy,multi-resist-ance ferroelectric storage devices.

Switchable polarization makes ferroelectric materials widely usable in nonvolatile memories and electronic nanodevices. The coercive electric field, determining the polarization switching, is crucially important for the design and usage of ferroelectric devices. To prevent leakage and dielectric breakdown of the devices, it is necessary to reduce the coercive electric field. In this work, we propose a theoretical model to study the domain switching properties of epitaxial PbTiO 3 films within the framework of Landau's phenomenological theory, especially the multistep polarization switching in ferroelectric thin films near a phase boundary. Results indicate that substrate strains can significantly affect the domain structures and the domain switching process. Under specific substrate strain conditions (tensile misfit strain of 0%-0.8%), polydomains exist and 180 degrees polarization switching can be achieved via a two-step 90 degrees switching process. The coercive field required for a multistep switching process can be decreased by about 50% in comparison to that for single domains. The approach of reducing the electric field presents significant opportunities for the design and enhancement of devices based on ferroelectric materials across various applications.

Qualification of polarization can be realized either on a macroscopic scale as an average property by P-E hysteresis measurements or on a nano/micro scale by piezoelectric force microscopy, transmission electron microscopy, scanning electron microscopy, and so on. However, visualization and qualification of polarization distribution in the micron to millimeter scale is still a challenge. Polarizing light microscopy (PLM) is often used in the study of ferroelectric domain structures mainly for domain patterns. A phenomenon called "chromatic polarization" has been observed in transparent ferroelectric crystals by using a crossed-PLM system viewed with white light, which contains rich information about local polarization distribution. In this study, an automatic full-angle light intensity detection (AFALID) algorithm combined with colorimetry is developed to analyze the distribution of nonuniform local spontaneous polarization distribution in transparent ferroelectric single crystals. Temperature-dependent spontaneous polarizations from the color analysis for PMN-0.36PT single crystals with single tetragonal domain state are in good coincidence with those extracted from temperature-dependent hysteresis loops and pyroelectric current measurements. We further apply this method to quantify the nonuniform domain distributions with nano-indentations. This non-contact and non-destructive characterization can provide fast and automatic detection of polarization distributions in ferroelectric materials.

Industrial application of lead-free piezoelectric ceramics is prevented by intrinsic thermal instability. Herein, we propose a method to achieve outstanding thermal stability of converse piezoelectric constant () in lead-free potassium sodium niobate (KNN)-based ceramics by inducing a synergistic interaction between the grain size and polar configuration. Based on computational methods using phase-field simulations and first-principles calculations, the relationship between the grain size and polar configuration is demonstrated, and the possibility of achieving improved thermal stability in fine grains is suggested. A set of KNN systems is presented with meticulous dopant control near the chemical composition at which the grain size changes abnormally. Comparing the two representative samples with coarse and fine grains, significant enhancement in the thermal stability of is exhibited up to 300 °C in the fine grains. The origin of the thermal superiority in fine-grained ceramics is identified through an extensive study from a microstructural perspective. The thermal stability is realized in a device by successfully demonstrating the temperature dependence of piezoelectricity. It is notable that this is the first time that lead-free piezoelectric ceramics are able to achieve exceptionally stable piezoelectricity up to 300 °C, which actualizes their applicability as piezoelectric devices with high thermal stability.

Optical microscopy is an essential characterization technique that is used as an initial step in the study of domain structures and phase transitions of ferroelectric materials. Image analysis aided by advanced computer algorithms is urgently required to enable an improved accuracy and higher efficiency of data processing. In this study, an automatic full-angle light intensity detection algorithm is developed to study polarizing light microscopy images of domain structures and the phase evolution process is decoded using the scale invariant feature transform method. This data processing algorithm is then used to characterize the phase transition process in relaxor ferroelectric PMN-0.29PT single crystals with complex domain structures. An emerging in-plane tetragonal phase is tracked during the rhombohedral (R) to tetragonal (T) phase transition, which is associated with peaks present in the temperature-dependent dielectric curve. Compared to the traditional polarizing light microscopy characterization method, this computer-aided algorithm enables a quicker and more informative analysis of domain evolutions in ferroelectric single crystals.

Precise multi-axis operation is essentially required in orthodontics for tooth movement. Despite the development of flexible multi-dimensional force sensors that have effectively perceived multi-dimensional forces, they still face the challenge of simultaneously 3D force and moment in a single flexible force sensor. Six-dimensional force perception can successfully operate objects that critically rely on directional tracking and accurate monitoring of complex multi-axis stimuli. To realize the integration of sensing units with the perception of six-dimensional force under even a soft touch, we design a flexible six-dimensional force sensor with tenon-and-mortise interlocking structures inspired by traditional Chinese ancient architecture. This unique structure enables conjunction of deformation, which can be studied and decoded by Deep Neural Networks (DNN) with six-dimensional force, including forces and rotating moments in the x, y, and z directions. This soft sensor with minimal size (7 x 7 x 7 mm(3)) and high detection accuracy (the DNN error is below 10(-4)) can be used in orthodontic treatment for precise correction with a full collection of orthodontic force. This unique flexible six-dimensional force sensor provides a new strategy for the design of multi-dimensional force sensors, paving the way for the development of intelligent robotics, interactive human-machine interfacing, and advanced prosthetics.

The sophistication, adaptability, and complexity of biological systems have provided enormous inspiration and have been a continuous source of numerous innovations. Soft living organisms like drosera capensis have amazing predatory behavior that can capture prey of ideal size, enabling them to interact with environmental stimuli efficiently. Mimicking such natural intelligence in artificial systems with systematical functions of multiple information perception, neuronal transmission, and adaptive motility remains a grand challenge. Here, a biomimetic drosera capensis is reported that is capable of multifunctional self-sensing, automatic regulation, and adaptive actuation in response to diverse stimuli with intelligent predation capability in an entirely closed-loop fashion. The functional system heterogeneously integrates the thermal-responsive soft actuator as the muscle-like motor and flexible tactile, strain, and piezoelectric multimodal sensors as somatosensory receptors. With the synergistic effect of multifunctional sensing and fast actuating schemes, the artificial drosera capensis deconvolutes multiple characteristics of the catching process (e.g., strain rate, magnitude, and direction) and thus holds impressive predatory behavior for ideal-sized prey. This electronically innervated artificial drosera capensis with multimodal sensing and self-regulated actuating capability through the closed-loop control of sensing and actuating system paves the way for the development of adaptive soft robots.

Phase and domain structures in ferroelectric materials play a vital role in determining their dielectric and piezoelectric properties. Ferroelectric thin films with coexisting multiple domains or phases often have fascinating high sensitivity and ultrahigh physical properties. However, the control of the coexisting multiple domains is still challenging, thus necessitating the theoretical prediction. Here, we studied the phase coexistence and the domain morphology of PbTiO3 epitaxial films by using a Landau–Devonshire phenomenological model and canonic statistical method. Results show that PbTiO3 films can exist in multiple domain structures that can be diversified by the substrates with different misfit strains. Experimental results for PbTiO3 epitaxial films on different substrates are in good accordance with the theoretical prediction, which shows an alternative way for further manipulation of the ferroelectric domain structures.

Relaxor-based ferroelectric single crystal with lower PbTiO3 content has complex phase transitions and mixed ferroelectric and relaxor characteristics, making it very challenging for the construction of Landau free energy expansion crucially used for theoretical investigation of this system. In this work, relaxor behavior in 0.78Pb(Mg1/3Nb2/3)O3-0.22PbTiO3 (PMN-0.22PT) single crystals was well suppressed through careful experimental design, leaving preserved single domain state for the Landau parameter extraction in this rhombohedral-phase composition. Using these extracted Landau free energy expansion parameters, energy profile and phase coexistence were theoretically studied and compared with those of PMN-0.30PT and PMN-0.36PT single crystals. Results show that rhombohedral phase dominates in PMN-0.22PT, and tetragonal phase dominates in PMN-0.36PT around room temperature. While, rhombohedral phase and tetragonal phase coexist in MPB composition PMN-0.30PT with small discrepancy of both free energies and polarization vectors.

Searching for lead-free piezoelectric materials with a large piezoelectricity and excellent thermal stability has been a major concern in both scientific research and practical applications. To understand the mechanism of high piezoelectricity and its temperature-dependent behavior in lead-free materials, we focus here on the tetragonal (K,Na,Li)(Nb,Ta)O3 single crystal and investigate the intrinsic d33* along arbitrary directions as well as its evolution with temperature. The synergistic influence of several factors (narrow tetragonal temperature interval, sharp tetragonal-cubic phase transition, and large PS below TC) leads to a strong anisotropy in the d33* profile, while a high d33* is obtained around θ = 45° over a wide temperature range. This work comprehensively reveals the physical mechanism of piezoelectric anisotropy in lead-free materials, which provides vital information to design high-performance lead-free piezoelectric materials through orientation engineering and lattice manipulation, which is expected to benefit a wide range of piezoelectric materials.

It is of great importance to accurately determine the full matrix parameters of the piezoelectric materials using as few samples as possible. Here we performed a simple and widely applicable method to accurately determine the full matrix parameters of [001]C-poled K(Ta,Nb)O3 single crystal. Only one sample was used throughout the experiment procedure, which eliminated inconsistency from multi-sample testing. We also improved the accuracy of results by inversing the impedance spectra of two inequivalent crystallographic directions, [001]C and [010]C. Based on this technique, variation of full matrix parameters with temperature were obtained using one sample. This work is of great significance for the characterization of novel piezoelectric materials and the evaluation of the performance of piezoelectric devices at various temperature.

Phase transitions of Pb(Mg1/3Nb2/3)O-3-0.34PbTiO(3) (PMN-0.34 PT) have been studied by using dielectric constant variations, polarization-electric field hysteresis loops, and polarizing light microscopy (PLM). Even without the assistance of electric field, orthorhombic (O) phase exists in the [001](C)-oriented PMN-0.34 PT single crystals during the heating process, which is confirmed by the polarizer/analyzer extinction angle from full-angle in situ PLM observation combined with the grayscale image analysis. The phase transition sequence is monoclinic (M-C) -> O -> M-C -> tetragonal (T) -> cubic (C), which is different from the commonly accepted M-C -> T -> C sequence. The existence of the O phase accompanies with much enhanced dielectric and piezoelectric performance, which indicates a new mechanism to produce large piezoelectricity in PMN-PT single crystals. (C) 2020 Elsevier B.V. All rights reserved.

Temperature-dependent dielectricity and polarization of [001]C-oriented 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 relaxor-based ferroelectric single crystals were studied by using a combined method of the X-ray diffraction, dielectric spectrum, polarization-electric (P-E) field hysteresis loops, and the Landau-phenomenological theory. Results show that the room-temperature rhombohedral phase experiences a rhombohedral-monoclinic-tetragonal coexisting state, then transforms to tetragonal phase, and finally to cubic phase during the zero-field-heating process. The six-order Landau-type thermal expansion parameters for the tetragonal phase were determined by using the typical characteristics at the cubic-tetragonal phase transition in the temperature range from 91 °C to 113 °C. The calculated dielectric curve, polarizations, and P-E loops fit well with the experimental results. The phase stability and piezoelectricity are further studied and compared with those of the PMN-0.36PT single crystal. The provided methods and obtained Landau parameters can be used for further studies on the relaxor-based ferroelectric single crystals.

Ferroelastic switching in ferroelectric/multiferroic oxides plays a crucial role in determining their dielectric, piezoelectric, and magnetoelectric properties. In thin films of these materials, however, substrate clamping is generally thought to limit the electric-field- or mechanical-force-driven responses to the local scale. Here, we report mechanical-force-induced large-area, non-local, collective ferroelastic domain switching in PbTiO3 epitaxial thin films by tuning the misfit-strain to be near a phase boundary wherein c/a and a1/a2 nanodomains coexist. Phenomenological models suggest that the collective, c-a-c-a ferroelastic switching arises from the small potential barrier between the degenerate domain structures, and the large anisotropy of a and c domains, which collectively generates much larger response and large-area domain propagation. Large-area, non-local response under small stimuli, unlike traditional local response to external field, provides an opportunity of unique response to local stimuli, which has potential for use in high-sensitivity pressure sensors and switches.

Domain wall motions mainly affect all kinds of properties of ferroelectric materials, such as piezoelectricity, dielectric response, and mechanical loss, and the extrinsic contributions associated with domain wall motions have always been an important issue. In this study, the reversible and irreversible extrinsic contributions to the dielectric properties of [011](C)-oriented 0.27Pb(In1/2Nb1/2)O-3-0.46Pb(Mg1/3Nb2/3)O-3-0.27PbTiO(3):Mn single crystals have been extracted by the Rayleigh analysis. We found that in the unpoled samples, the extrinsic contributions of reversible and irreversible domain wall motions to dielectric properties significantly reduced, whereas after poling, only the irreversible extrinsic contribution decreased. The pinning effect in the 2R domain structure is much weaker than that in the 4R domain structure, leading to the low enhancement of Q(m) and a slight decrease in piezoelectricity caused by acceptor doping in 2R domain structure. This study explores the domain wall dynamics of acceptor-doped single crystals and mainly guides on further performance optimization in PbTiO3-based relaxor single crystals.