Direct observation of elemental fluctuation and oxygen octahedral distortion-dependent charge distribution in high entropy oxides

The enhanced compositional flexibility to incorporate multiple-principal cations in high entropy oxides (HEOs) offers the opportunity to expand boundaries for accessible compositions and unconventional properties in oxides. Attractive functionalities have been reported in some bulk HEOs, which are attributed to the long-range compositional homogeneity, lattice distortion, and local chemical bonding characteristics in materials. However, the intricate details of local composition fluctuation, metal-oxygen bond distortion and covalency are difficult to visualize experimentally, especially on the atomic scale. Here, we study the atomic structure-chemical bonding-property correlations in a series of perovskite-HEOs utilizing the recently developed four-dimensional scanning transmission electron microscopy techniques which enables to determine the structure, chemical bonding, electric field, and charge density on the atomic scale. The existence of compositional fluctuations along with significant composition-dependent distortion of metal-oxygen bonds is observed. Consequently, distinct variations of metal-oxygen bonding covalency are shown by the real-space charge-density distribution maps with sub-ångström resolution. The observed atomic features not only provide a realistic picture of the local physico-chemistry of chemically complex HEOs but can also be directly correlated to their distinctive magneto-electronic properties

Enhanced polarization and abnormal flexural deformation in bent freestanding perovskite oxides

Recent realizations of ultrathin freestanding perovskite oxides offer a unique platform to probe novel properties in two-dimensional oxides. Here, we observe a giant flexoelectric response in freestanding BiFeO3 and SrTiO3 in their bent state arising from strain gradients up to 3.5 × 107 m−1, suggesting a promising approach for realizing ultra-large polarizations. Additionally, a substantial change in membrane thickness is discovered in bent freestanding BiFeO3, which implies an unusual bending-expansion/shrinkage effect in the ferroelectric membrane that has never been seen before in crystalline materials. Our theoretical model reveals that this unprecedented flexural deformation within the membrane is attributable to a flexoelectricity–piezoelectricity interplay. The finding unveils intriguing nanoscale electromechanical properties and provides guidance for their practical applications in flexible nanoelectromechanical systems

Nanoscale electronic properties of ferroelectric heterostructures studied via four-dimensional scanning transmission electron microscopy

Due to their intrinsic polarization, ferroelectric materials have a variety of applications in electronic, optical, and electromechanical devices. In addition, modern thin film growth techniques such as molecular beam epitaxy and pulsed laser deposition have made it possible to grow heterostructures with atomically sharp interfaces leading to the discovery of many new phenomena and functional properties stemming from the coupling between the ferroelectric and the substrate material. In the study of these thin films, transmission electron microscopy has been a powerful technique for high resolution structural and chemical characterization of these heterostructures. With the development of fast pixelated detectors, four-dimensional scanning transmission electron microscopy (4D STEM) has led to the emergence of new techniques for directly interrogating the electronic properties of ferroelectrics with atomic resolution. This opens new opportunities for studying both the mechanisms for how macroscopic properties can emerge from atomic scale electronic phenomena and the fundamental physics of ferroelectricity. In this work, new analysis techniques based on 4D STEM electric field and charge density imaging are developed and applied to reveal the electronic properties of ferroelectric heterostructures.First, we systematically study the effect of sample thickness and electron probe defocus on the electric field measured in 4D STEM and demonstrate that the technique can be extended to samples up to 20 nm thickness. Next, new quantitative analysis techniques for 4D STEM charge density images are developed. These techniques are applied to a BiFeO3-SrTiO3 (ferroelectric-insulator) heterostructure, which reveals the local electronic properties of the interface. Through a combination of methods, we showed that there is charge accumulation at the interface due to an asynchronous change in the structural polarization and the charge distribution across the interface. Applying 4D STEM electric field imaging to PbTiO3-SrTiO3 superlattice, we found that asymmetric features in atomic scale images originate from the charge distribution in perovskite ferroelectrics and while nanometer-resolution images reveal the total electric field of the material system. Finally, an in-depth study of how Bader charge analysis, adapted for use with 4D STEM charge density images, can be leveraged to understand the bonding properties of PbTiO3. These results demonstrate the potential of 4D STEM for investigating the electronic properties of materials and provide new insights into the behavior of ferroelectric heterostructures

Structures and Electronic Properties of Domain Walls in BiFeO3 Thin Films

Domain walls (DWs) in ferroelectrics are atomically sharp and can be created, erased, and reconfigured within the same physical volume of ferroelectric matrix by external electric fields. They possess a myriad of novel properties and functionalities that are absent in the bulk of the domains, and thus could become an essential element in next-generation nanodevices based on ferroelectrics. The knowledge about the structure and properties of ferroelectric DWs not only advances the fundamental understanding of ferroelectrics, but also provides guidance for the design of ferroelectric-based devices. In this article, we provide a review of structures and properties of DWs in one of the most widely studied ferroelectric systems, BiFeO3 thin films. We correlate their conductivity and photovoltaic properties to the atomic-scale structure and dynamic behaviors of DWs

Probing the dynamics of nanoparticle formation from a precursor at atomic resolution

Control of reduction kinetics and nucleation processes is key in materials synthesis. However, understanding of the reduction dynamics in the initial stages is limited by the difficulty of imaging chemical reactions at the atomic scale; the chemical precursors are prone to reduction by the electron beams needed to achieve atomic resolution. Here, we study the reduction of a solid-state Pt precursor compound in an aberration-corrected transmission electron microscope by combining low-dose and in situ imaging. The beam-sensitive Pt precursor, K2PtCl4, is imaged at atomic resolution, enabling determination of individual (K, Pt, Cl) atoms. The transformation to Pt nanoclusters is captured in real time, showing a three-stage reaction including the breaking of the ionic bond, formation of PtCl2, and the reduction of the dual-valent Pt to Pt metal. Deciphering the atomic-scale transformation of chemicals in real time using combined low-dose and in situ imaging brings new possibility to study reaction kinetics in general

Probing the dynamics of nanoparticle formation from a precursor at atomic resolution.

Control of reduction kinetics and nucleation processes is key in materials synthesis. However, understanding of the reduction dynamics in the initial stages is limited by the difficulty of imaging chemical reactions at the atomic scale; the chemical precursors are prone to reduction by the electron beams needed to achieve atomic resolution. Here, we study the reduction of a solid-state Pt precursor compound in an aberration-corrected transmission electron microscope by combining low-dose and in situ imaging. The beam-sensitive Pt precursor, K2PtCl4, is imaged at atomic resolution, enabling determination of individual (K, Pt, Cl) atoms. The transformation to Pt nanoclusters is captured in real time, showing a three-stage reaction including the breaking of the ionic bond, formation of PtCl2, and the reduction of the dual-valent Pt to Pt metal. Deciphering the atomic-scale transformation of chemicals in real time using combined low-dose and in situ imaging brings new possibility to study reaction kinetics in general

In situ observation of domain wall lateral creeping in a ferroelectric capacitor

AbstractAs a promising candidate for next‐generation nonvolatile memory devices, ferroelectric oxide films exhibit many emergent phenomena with functional applications, making understanding polarization switching and domain evolution behaviors of fundamental importance. However, tracking domain wall motion in ferroelectric oxide films with high spatial resolution remains challenging. Here, an in situ biasing approach for direct atomic‐scale observations of domain nucleation and sideways motion is presented. By accurately controlling the applied electric field, the lateral translational speed of the domain wall can decrease to less than 2.2 Å s−1, which is observable with atomic resolution STEM imaging. In situ observations on a capacitor structured PbZr0.1Ti0.9O3/La0.7Sr0.3MnO3 heterojunction demonstrate the unique creeping behavior of a domain wall under a critical electric field, with the atomic structure of the creeping domain wall revealed. Moreover, the evolution of the metastable domain wall forms an elongated morphology, which contains a large proportion of charged segments. Phase‐field simulations unveil the competition between gradient, elastic, and electrostatic energies that decide this unique domain wall creeping and morphology variation. This work paves the way toward a complete fundamental understanding of domain wall physics and potential modulations of domain wall properties in real devices