Nanoscale Observation of Time‐Dependent Domain Wall Pinning as the Origin of Polarization Fatigue

The microscopic mechanism of polarization fatigue (i.e., a loss of switchable polarization under electrical cycling) remains one of the most important long‐standing problems in ferroelectric communities. Although there are numerous proposed fatigue models, a consensus between the models and experimental results is not reached yet. By using modified‐piezoresponse force microscopy, nanoscale domain switching dynamics are visualized for different fatigue stages in epitaxial PbZr_0.4Ti_0.6O₃ capacitors. Systematic time‐dependent studies of the domain nucleation and evolution reveal that domain wall pinning, rather than nucleation inhibition, is the primary origin of fatigue. In particular, the evolution of domain wall pinning process during electrical cycling, from the suppression of sideways domain growth in early fatigued stages to the blockage of forward domain growth in later stages, is directly observed. The pinning of forward growth results in a nucleation‐limited polarization switching and a significant slowdown of the switching time in the severely fatigued samples. The direct nanoscale observation of domain nucleation and growth dynamics elucidates the importance of evolution of the domain wall pinning process in the fatigue of ferroelectric materials.     

Origin of Hysteresis in CH3NH3PbI3 Perovskite Thin Films

Organic–inorganic hybrid perovskite solar cells are attracting the attention of researchers owing to the high level of performance they exhibit in photovoltaic device applications. However, the attainment of an even higher level of performance is hindered by their anomalous current–voltage (I–V) hysteresis behavior. Even though experimental and theoretical studies have suggested that the perovskite materials may have a ferroelectric nature, it is still far from being fully understood. In this study, the origin of the hysteresis behavior in CH₃NH₃PbI₃ perovskite thin films is investigated. The behavior of ferroelectricity using piezoresponse force microscopy is first examined. Then, by comparing the scan‐rate‐dependent nano/macroscopic I–V curves, it is found that ion migration assisted by the grain boundaries is a dominant origin of I–V hysteresis from a macroscopic viewpoint. Consequently, the observations suggest that, even though ferroelectricity exists in the CH₃NH₃PbI₃ perovskite materials, ion migration primarily contributes to the macroscopic I–V hysteresis. The presented results can provide fundamental guidelines to the resolution of hysteresis issues in organic–inorganic hybrid perovskite materials.     

Influence of a Single Grain Boundary on Domain Wall Motion in Ferroelectrics

Epitaxial tetragonal 425 and 611 nm thick Pb(Zr_0.45Ti_0.55)O₃ (PZT) films are deposited by pulsed laser deposition on SrRuO₃‐coated (100) SrTiO₃ 24° tilt angle bicrystal substrates to create a single PZT grain boundary with a well‐defined orientation. On either side of the bicrystal boundary, the films show square hysteresis loops and have dielectric permittivities of 456 and 576, with loss tangents of 0.010 and 0.015, respectively. Using piezoresponse force microscopy (PFM), a decrease in the nonlinear piezoelectric response is observed in the vicinity (720–820 nm) of the grain boundary. This region represents the width over which the extrinsic contributions to the piezoelectric response (e.g., those associated with the domain density/configuration and/or the domain wall mobility) are influenced by the presence of the grain boundary. Transmission electron microscope (TEM) images collected near and far from the grain boundary indicate a strong preference for (101)/(01) type domain walls at the grain boundary, whereas (011)/(01) and (101)/(01) are observed away from this region. It is proposed that the elastic strain field at the grain boundary interacts with the ferro‐electric/elastic domain structure, stabilizing (101)/(01) rather than (011)/(01) type domain walls, which inhibits domain wall motion under applied field and decreases non‐linearity.