Engineering of atomic-scale flexoelectricity at grain boundaries

Mei Wu,Xiaomei Li,Xuedong Bai,Xin-Zheng Li,Xiaoyue Gao,Jingmin Zhang,Bo Han,Ke Qu,Yuanwei Sun,Kaihui Liu,Ruixue Zhu,Xiaowei Zhang,Peng Gao

doi:10.1038/s41467-021-27906-0

Abstract

Flexoelectricity is a type of ubiquitous and prominent electromechanical coupling, pertaining to the electrical polarization response to mechanical strain gradients that is not restricted by the symmetry of materials. However, large elastic deformation is usually difficult to achieve in most solids, and the strain gradient at minuscule is challenging to control. Here, we exploit the exotic structural inhomogeneity of grain boundary to achieve a huge strain gradient (~1.2 nm−1) within 3–4-unit cells, and thus obtain atomic-scale flexoelectric polarization of up to ~38 μC cm−2 at a 24° LaAlO3 grain boundary. Accompanied by the generation of the nanoscale flexoelectricity, the electronic structures of grain boundaries also become different. Hence, the flexoelectric effect at grain boundaries is essential to understand the electrical activities of oxide ceramics. We further demonstrate that for different materials, altering the misorientation angles of grain boundaries enables tunable strain gradients at the atomic scale. The engineering of grain boundaries thus provides a general and feasible pathway to achieve tunable flexoelectricity.

Highlights

Flexoelectricity is a type of ubiquitous and prominent electromechanical coupling, pertaining to the electrical polarization response to mechanical strain gradients that is not restricted by the symmetry of materials
For a wellbonded symmetric Grain boundaries (GBs), the atomic configuration of the GB core is a natural trapezoidal shape with a significantly inhomogeneous strain distribution in space. Such disrupted atomic bonding is usually confined to a few unit cells around the GB core, which is expected to introduce huge strain gradients[23]
When extrapolated to SrTiO3 (STO) GBs, huge strain gradients exist in the 22.6° GB core, and even larger strain gradients are observed in the 36.8° GB, exhibiting that tunable flexoelectricity can be achieved by altering the misorientation angle of GBs in different materials

Summary

Introduction

Flexoelectricity is a type of ubiquitous and prominent electromechanical coupling, pertaining to the electrical polarization response to mechanical strain gradients that is not restricted by the symmetry of materials. In practice, validating this speculation is challenging, as GBs are of atomic size and have complex structures, while conventional macroscopic characterizations and measurements only provide collective information about the bulk material and may be influenced by other irrelevant factors, such as surface effects[24,25] We verify this speculation and develop a general strategy to generate atomic-scale flexoelectric polarization via GB engineering. For a 24° tilt LaAlO3 (LAO) GB, we directly visualize the atomic arrangements by advanced atomically resolved scanning transmission electron microscopy (STEM) and spectroscopy techniques and find a huge strain gradient (~1.2 nm−1) and a remarkable flexoelectric effect that induces atomic displacement up to 81.4 ± 10.5 pm within 3–4-unit cells around the GB In such a confined GB region, a large local polarization (~38 μC cm−2, estimated by first-principles density functional theory (DFT) calculations) exists, forming a “head-to-head” polarization configuration. The tunability via GB engineering may open vistas in nanoelectronics and nanoelectromechanical systems

Methods

Results

Conclusion