Non-conventional mechanism of ferroelectric fatigue via cation migration

Developing fatigue-resistant ferroelectrics using interlayer sliding switching

Ferroelectric materials have switchable electrical polarization that is appealing for high-density nonvolatile memories. However, inevitable fatigue hinders practical applications of these materials. Fatigue-free ferroelectric switching could dramatically improve the endurance of such devices. We report a fatigue-free ferroelectric system based on the sliding ferroelectricity of bilayer 3R molybdenum disulfide (3R-MoS2). The memory performance of this ferroelectric device does not show the wake-up effect at low cycles or a substantial fatigue effect after 106 switching cycles under different pulse widths. The total stress time of the device under an electric field is up to 105 s, which is long relative to other devices. Our theoretical calculations reveal that the fatigue-free feature of sliding ferroelectricity is due to the immobile charge defects in sliding ferroelectricity.

Three-Dimensional Visualization of Oxygen-Vacancy Migration and Redistribution in Ca-Substituted BiFeO3

Ionic movement has received renewed attention in recent years, particularly in the field of ferroelectric oxides, since it is intrinsically linked to chemical reaction kinetics and ferroelectric phase stability. The associated surface electrochemical processes coupled local ionic transport with an applied electric bias, exhibiting very high ionic mobility at room temperature based on a simple electrostatics scenario. However, few studies have focused on the applied-polarity dependence of ionic migration with directly visualized maps. Here, we use incorporated experiments of conductive scanning probe microscopy and time-of-flight secondary ion mass spectrometry to investigate oxygen ionic migration and cation redistribution in ionic oxides. The local concentrations of oxygen vacancies and other cation species are visualized by three-dimensional mappings, indicating that oxygen vacancies tend to be ejected toward the surface. An accumulation of oxygen vacancies and ionic redistribution strongly depend on tip polarity, thus corroborating their role in the electrochemical process. This work illustrates the interplay between ionic kinetics and electric switching.

Influence of water quality factors on cake layer 3D structures and water channels during ultrafiltration process

The three-dimensional (3D) structure of the cake layer, which could be influenced by water quality factors, plays a significant role in the ultrafiltration (UF) efficiency of water purification. However, it remains challenging to precisely reveal the variation of cake layer 3D structures and water channel characteristics. Herein, we systematically report the variation in the cake layer 3D structure at the nanoscale induced by key water quality factors and reveal its influence on water transport, in particular the abundance of water channels within the cake layer. In comparison with pH and Na+, Ca2+ played more significant role in determining cake layer structures. The sandwich-like cake layer, which was induced by the asynchronous deposition of humic acids and sodium alginate (SA), shifted to an isotropic structure when Ca2+ was present due to the Ca2+ bridging. In comparison with the sandwich-like structure, the isotropic cake layer has higher fractions of free volume (voids) and more water channels, leading to a 147% improvement in the water transport coefficient, 60% reduction in the cake layer resistance, and 21% increase in the final membrane specific flux. Our work elucidates a structure-property relationship where improving the isotropy of the cake layer 3D structure is conducive to the optimization of water channels and water transport within cake layers. This could inspire tailored regulation strategies for cake layers to enhance the UF efficiency of water purification.

Cake layer 3D structure regulation to optimize water channels during Al-based coagulation-ultrafiltration process

The variation in cake layer three-dimensional (3D) structures and related water channel characteristics induced by coagulation pretreatment remains unclear; however, gaining such knowledge will aid in improving ultrafiltration (UF) efficiency for water purification. Herein, the regulation of cake layer 3D structures (3D distribution of organic foulants within cake layers) by Al-based coagulation pretreatment was analyzed at the micro/nanoscale. The sandwich-like cake layer of humic acids and sodium alginate induced without coagulation was ruptured, and foulants were gradually uniformly distributed within the floc layer (toward an isotropic structure) with increasing coagulant dosage (a critical dosage was observed). Furthermore, the structure of the foulant-floc layer was more isotropic when coagulants with high Al₁₃ concentrations were used (either AlCl₃ at pH 6 or polyaluminum chloride, in comparison with AlCl₃ at pH 8 where small-molecular-weight humic acids were enriched near the membrane). These high Al₁₃ concentrations lead to a 48.4% higher specific membrane flux than that seen for UF without coagulation. Molecular dynamics simulations revealed that with increasing Al₁₃ concentration (Al₁₃: 6.2% to 22.6%), the water channels within the cake layer were enlarged and more connected, and the water transport coefficient was improved by up to 54.1%, indicating faster water transport. These findings demonstrate that facilitating an isotropic foulant-floc layer with highly connected water channels by coagulation pretreatment with high-Al₁₃-concentration coagulants (having a strong ability to complex organic foulants) is the key issue in optimizing the UF efficiency for water purification. The results should provide further understanding of the underlying mechanisms of coagulation-enhancing UF behavior and inspire precise design of coagulation pretreatment to achieve efficient UF.

Exploring a Fatigue-Free Layered Hybrid Perovskite Ferroelectric for Photovoltaic Non-Volatile Memories

Through a functional unit-transmutation strategy, a fatigue-free layered hybrid perovskite ferroelectric (C₆ H₅ CH₂ NH₃ )₂ CsPb₂ Br₇ (BCPB) has been developed, which demonstrates stable spontaneous polarization (P_s ) of 6.5 μC cm^-2 and high Curie temperature up to 425 K. Meanwhile, BCPB shows splendid bulk photovoltaic effect (BPVE) properties with noticeable zero-bias photocurrent density (5 μA cm^-2 ), and high on/off switching ratio of current (over 3×10⁵ ); these merits even overmatch the most known ferroelectric semiconductor BiFeO₃ . The unique structure with self-regulated net electrical charged layers gives rise to the fatigue-free feature of P_s and BPVE (no significant fatigue after 10⁸ polarity switching cycles), promoting the potential applications of BCPB in photovoltaic non-volatile memories. This work offers an efficient approach for exploring fatigue-free semiconducting ferroelectrics as well as excavates their further applications in next-generation electronic devices.

Direct observation of nanoscale dynamics of ferroelectric degradation

Failure of polarization reversal, i.e., ferroelectric degradation, induced by cyclic electric loadings in ferroelectric materials, has been a long-standing challenge that negatively impacts the application of ferroelectrics in devices where reliability is critical. It is generally believed that space charges or injected charges dominate the ferroelectric degradation. However, the physics behind the phenomenon remains unclear. Here, using in-situ biasing transmission electron microscopy, we discover change of charge distribution in thin ferroelectrics during cyclic electric loadings. Charge accumulation at domain walls is the main reason of the formation of c domains, which are less responsive to the applied electric field. The rapid growth of the frozen c domains leads to the ferroelectric degradation. This finding gives insights into the nature of ferroelectric degradation in nanodevices, and reveals the role of the injected charges in polarization reversal.

Local Probe Comparison of Ferroelectric Switching Event Statistics in the Creep and Depinning Regimes in Pb(Zr_{0.2}Ti_{0.8})O_{3} Thin Films

Ferroelectric materials provide a useful model system to explore the jerky, highly nonlinear dynamics of elastic interfaces in disordered media. The distribution of nanoscale switching event sizes is studied in two Pb(Zr_{0.2}Ti_{0.8})O_{3} thin films with different disorder landscapes using piezoresponse force microscopy. While the switching event statistics show the expected power-law scaling, significant variations in the value of the scaling exponent τ are seen, possibly as a consequence of the different intrinsic disorder landscapes in the samples and of further alterations under high tip bias applied during domain writing. Importantly, higher exponent values (1.98-2.87) are observed when crackling statistics are acquired only for events occurring in the creep regime. The exponents are systematically lowered when all events across both creep and depinning regimes are considered-the first time such a distinction is made in studies of ferroelectric materials. These results show that distinguishing the two regimes is of crucial importance, significantly affecting the exponent value and potentially leading to incorrect assignment of universality class.

Anisotropic Ion Migration and Electronic Conduction in van der Waals Ferroelectric CuInP2S6

Van der Waals (vdW) thio- and seleno-phosphates have recently gained considerable attention for the use as "active" dielectrics in two-dimensional/quasi-two-dimensional electronic devices. Bulk ionic conductivity in these materials has been identified as a key factor for the control of their electronic properties. However, direct evidence of specific ion species' migration at the nanoscale, particularly under electric fields, and its impact on material properties has been elusive. Here, we report on direct evidence of a phase-selective anisotropic Cu-ion-hopping mechanism in copper indium thiophosphate (CuInP₂S₆) through detailed scanning probe microscopy measurements. A two-step Cu-hopping path including a first intralayer hopping (in-plane) and second interlayer hopping (out-of-plane) crossing the vdW gap is unveiled. Evidence of electrically controlled Cu ion migration is further verified by nanoscale energy-dispersive X-ray spectroscopy (EDS) mapping. These findings offer new insight into anisotropic ionic manipulation in layered vdW ferroelectric/dielectric materials for emergent vdW electronic device design.

Enhanced ferroelectricity in NaNbO3–LaCoO3:Mn epitaxial thin film

A (001)-oriented NaNbO3–LaCoO3:Mn ferroelectric thin film with enhanced tetragonal distortion and large remanent polarization (31 μC cm−2) achieved by chemical modification was successfully deposited on a SrRuO3-buffered LaAlO3 substrate.

Recent Progress in the Nanoscale Evaluation of Piezoelectric and Ferroelectric Properties via Scanning Probe Microscopy

Piezoelectric and ferroelectric materials have garnered significant interest owing to their excellent physical properties and multiple potential applications. Accordingly, the need for evaluating piezoelectric and ferroelectric properties has also increased. The piezoelectric and ferroelectric properties are evaluated macroscopically using laser interferometers and polarization-electric field loop measurements. However, as the research focus is shifted from bulk to nanosized materials, scanning probe microscopy (SPM) techniques have been suggested as an alternative approach for evaluating piezoelectric and ferroelectric properties. In this Progress Report, the recent progress on the nanoscale evaluation of piezoelectric and ferroelectric properties of diverse materials using SPM-based methods is summarized. Among the SPM techniques, the focus is on recent studies that are related to piezoresponse force microscopy and conductive atomic force microscopy; further, the utilization of these two modes to understand piezoelectric and ferroelectric properties at the nanoscale level is discussed. This work can provide guidelines for evaluating the piezoelectric and ferroelectric properties of materials based on SPM techniques.

Interfacial dielectric layer as an origin of polarization fatigue in ferroelectric capacitors

Origins of polarization fatigue in ferroelectric capacitors under electric field cycling still remain unclear. Here, we experimentally identify origins of polarization fatigue in ferroelectric PbZr_0.52Ti_0.48O₃ (PZT) thin-film capacitors by investigating their fatigue behaviours and interface structures. The PZT layers are epitaxially grown on SrRuO₃-buffered SrTiO₃ substrates by a pulsed laser deposition (PLD), and the capacitor top-electrodes are various, including SrRuO₃ (SRO) made by in-situ PLD, Pt by in-situ PLD (Pt-inPLD) and ex-situ sputtering (Pt-sputtered). We found that fatigue behaviour of the capacitor is directly related to the top-electrode/PZT interface structure. The Pt-sputtered/PZT/SRO capacitor has a thin defective layer at the top interface and shows early fatigue while the Pt-inPLD/PZT/SRO and SRO/PZT/SRO capacitor have clean top-interfaces and show much more fatigue resistance. The defective dielectric layer at the Pt-sputtered/PZT interface mainly contains carbon contaminants, which form during the capacitor ex-situ fabrication. Removal of this dielectric layer significantly delays the fatigue onset. Our results clearly indicate that dielectric layer at ferroelectric capacitor interfaces is the main origin of polarization fatigue, as previously proposed in the charge injection model.