The European Ceramic Society


Oct 1, 2021

YCN Research in Spot: Characterization of Domain Walls in ferroelectric materials down to atomic level by Oana Condurache

Domain walls (DWs) are a special type of interphase that separate regions of homogeneous polarization vector orientation in a ferroelectric material.

Over the last decade, DWs have come into the spotlight and a new paradigm for future devices has opened up: DWs electronics. What, one might ask, makes DWs so attractive for the future electronics? Firstly, DWs are 2D features with a width that ranges from nanoscale to atomic scale. In addition, they can have properties that are distinct from the domain matrix (e.g., increased electrical conductivity in an otherwise insulating domain matrix). But what is really exciting in terms of functionality is that DWs are not static, but can potentially be moved, erased, and recreated using an external electric field.  
To make DWs-based electronics real, a comprehensive study of their properties is required. In my PhD study, I characterize DWs in ferroelectric materials down to the atomic level using a state-of-the-art microscopic technique called Scanning Transmission Electron Microscopy (STEM). My work is carried out under the supervision of Prof. Dr. Andreja Benčan and Prof. Dr. Goran Dražić. The materials of choice are in the vast majority lead-free ferroelectrics. We study the morphology, structure and chemistry of DWs at the atomic level and try to understand if and how these properties are interconnected.  
In addition to static studies of DWs, we are working to understand their dynamics by employing in-situ voltage bias STEM. The motivation behind such studies is that the dynamics of DWs is activated only in the presence of the external fields (mainly electric field). In-situ electron microscopy is a relatively new field for which there is not yet a comprehensive reference literature to rely on. Thus, in our work we pay special attention to the optimization of the experimental setup. This includes simulations of the electric field distribution in the sample geometry used and special care for sample preparation by Focused Ion Beam.  

a) Schematic representation of domains and DW in a ferroelectric material. The black arrow represents polarization vector. 
b) Illustration of a STEM image (Low Angle Annular Dark Field imaging mode) where ferroelectric domain contrast emerges. In this example, needle-like domains can be seen in potassium sodium niobate single crystal. 
c) Illustration of an atomic resolution image of a DW (High Angle Annular Dark Field imaging mode) in a perovskite ferroelectric (with the chemical formula ABO3). The image is acquired with the electron beam along the [100] pseudo-cubic crystallographic direction. In this representation, atomic columns corresponding to A and B sites are seen. The displacement of B site atomic column in respect to A-site sublattice is a quantity proportional to the polarization vector and, as it can be seen, it points on a different direction on each side of the DW (yellow arrow). 


PhD  Student: Oana-Andreea Condurache  
Electronic Ceramics Department, Jozef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia 

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