Thin‐Film Ferroelectrics

Abstract

non-centrosymmetric structures which can possess a spontaneous electric polarization which can be controlled using applied electric fields (Figure 1). Ferroelectrics themselves are inherently hierarchical materials-wherein picometer ionic displacements give rise to polarization which can collectively extend over millimeters or self-organize into complex mesoscopic structures or collectively reorient under applied stimuli (e.g., electric fields, temperature, or stress). Understanding these complex behaviors necessitates a multilevel approach, wherein atomistic, microscopic, mesoscopic, and macroscopic properties are studied in concert. Parallel advances in synthesis, characterization, and simulation have enabled such multimodal studies and provided a methodology through which a multitude of ferroelectric functionalities can now be achieved and studied.The promise of utilizing these functionalities in a number of applications kick-started the modern era of ferroelectric research in the mid-20th century. [1] Looking further back, in the 1920s, the ability to switch the polarization of sodium potassium tartrate tetrahydrate (commonly known as Rochelle salt) was first observed, along with dielectric and piezoelectric anomalies near the ferroelectric transition-now called the Curie point. In the 1940s, as part of the accelerated research push associated with World War II, ferroelectric BaTiO 3 was discovered by accident. When modifying TiO 2 with BaO to enhance its dielectric properties, a record-high dielectric permittivity was discovered and subsequent studies demonstrated a hysteretic switchable polarization. This discovery ushered in a new understanding of ferroelectricity as more than a rare phenomenon associated with salts that contained hydrogen bonding, but rather as a phenomenon that could exist in simple oxides like perovskites. Over the subsequent decades, the number of ferroelectric compositions exploded, particularly within the perovskite oxides, introducing new chemistries including LiNbO 3 and the PbZr x Ti 1−x O 3 system. Meanwhile, theoretical descriptions of ferroelectricity were advanced through lattice-dynamical models invoking a soft-mode optical phonon. Studies of the piezoelectric, thermodynamic, and optical properties in ferroelectric ceramics allowed for their deployment in a number of applications including piezoelectric sensors and pyroelectric infrared detectors. By the 1960s, increased interest in using ferroelectric polarization for nonvolatile memory was driving research into Over the last 30 years, the study of ferroelectric oxides has been revolutionized by the implementation of epitaxial-thin-film-based studies, which have driven many advances in the understanding of ferroelectric physics and the realization of novel polar structures and functionalities. New questions have motivated the development of advanced synthesis, characterization, and simulations of epitaxial thin films and, in turn, have provided new insights and applications across the micro-, meso-, and macroscopic length sc...

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SnS/MoS2 van der Waals heterojunction for in‐plane ferroelectric field‐effect transistors with multibit memory and logic characteristics

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