NSF-Europe: Atomic Defects and Their Dramatic Influence on Nanoscale Electrical, Elastic, and Optical Properties in Ferroelectrics

本研究的核心目标是探索畴壁与原子缺陷结构之间的静态和动态相互作用，以及它们对铁电体纳米级电学、弹性和光学性质的影响。铁电体在其晶体结构中具有内置的电极化，可以通过外场进行切换。在经典铁电体铌酸锂（LiNbO3）和钽酸锂（LiTaO3）中，pi最近发现晶体中存在少量的非化学计量，在宏观尺度上（如矫顽力场、内部场、畴结构、晶格应变和光学性质）发生了巨大的数量级变化。单个铁电畴壁的纳米级局部结构也表现出广泛的应变区域、电场和光学双折射（超过微米），这与原子尖锐的反平行壁（纳米）的理论期望相反。实验证据表明，点缺陷复合体是造成这种差异的主要原因。这些发现揭示了从纳米到宏观尺度上原子缺陷和铁电晶格之间相互作用的一系列基本问题。采用集中的多方面方法，结合实验和理论工具。该方法包括利用“设计”探针离子的光学和磁波谱对缺陷进行实验探测，结合使用x射线同步加速器成像和近场光学和扫描探针显微镜探测畴壁的局部结构，如应变、局部电场、极化梯度和纳米级光学性质。这些实验研究将与使用电子结构和原子水平方法的点缺陷复合物、畴壁及其相互作用的原子建模紧密结合。铁电材料有许多应用，例如用于眼科手术的微钻，用于高速互联网的高速光调制器，潜艇中的水下压力传感器，以及杂货店柜台上的条形码阅读器。在这些应用中，通过添加少量可以显著改变宏观性质的掺杂剂来操纵材料。这种经验主义的知识体系在今天的工业中得到了充分利用，但在这些点缺陷如何在纳米尺度上发挥作用以及这些相互作用如何扩大到影响宏观尺度特性方面，缺乏一个基本的基础。这项工作旨在达到一定程度的理解，使基于科学的策略能够“设计”具有所需性能的材料。本项目由材料研究部（陶瓷）和国际办公室（西欧）共同资助，是美国国家科学基金会与欧洲材料研究合作项目（NSF 02-135）。该项目正在与应用物理和光通信小组（教授）合作进行。德国帕德博恩大学的Sohler， Zrenner, and Noe)和德国波恩大学的应用光学小组（Prof. Buse）。

The central goal of this proposal is to probe the static and dynamic interactions between domain walls and atomic defects structure, and their influence on the nanoscale electrical, elastic and optical properties of ferroelectrics. Ferroelectrics have built-in electrical polarization in their crystal structure that can be switched by an external field. In classic ferroelectrics lithium niobate, LiNbO3 and lithium tantalate, LiTaO3, the PIs have recently discovered dramatic order-of-magnitude changes in the macroscale properties (such as coercive fields, internal fields, domain structure, lattice strain and optical properties) with small amounts of non-stoichiometry in the crystals. The nanoscale local structure of a single ferroelectric domain wall also exhibits wide regions of strain, electric fields and optical birefringence (over micrometers) that are contrary to the theoretical expectations from an atomically sharp antiparallel wall (nanometer). Experimental evidence suggests point defect complexes as the main reason for this discrepancy. These discoveries open up a host of fundamental questions about the interaction between atomic defects and ferroelectric lattice from nano-to-macro scales. A focused multifaceted approach is used combining experimental and theoretical tools. This approach consists of experimentally probing defects using optical and magnetic spectroscopy of "designer" probe ions, combined with probing the local structure of domain walls such as strains, local electric fields, polarization gradient, and nanoscale optical properties using X-ray synchrotron imaging, and near-field optical and scanning probe microscopies. These experimental studies will be closely coupled with atomistic modeling of point defect complexes, domain walls, and their interactions using electronic-structure and atomic-level approaches. There are many applications of ferroelectric materials, such as micro-drills used in eye surgery, high speed optical modulators for a fast internet, underwater pressure sensors in submarines, and bar-code readers at grocery checkout counters. In these applications, the material is manipulated by adding small amounts of dopants that can dramatically alter the macroscopic properties. This largely empirical body of knowledge is exploited in industry today but lacks a fundamental grounding in precisely how these point defects function on a nanoscale and how these interactions scale up to influence macroscale properties. This work aims at a level of understanding that would enable science-based strategies to "design" materials with desired properties. This NSF project is co-funded by the Division of Materials Research (Ceramics) and the International Office (Western Europe) as a Cooperative Activity in Materials Research between the NSF and Europe (NSF 02-135). This project is being carried out in collaboration with the Applied Physics and Optical Communications groups (Profs. Sohler, Zrenner, and Noe) at the University of Paderborn, Germany and the Applied Optics Group (Prof. Buse) at the University of Bonn, Germany.