Investigation of the ferroelectric domain dynamics and its effects on macroscopic behaviors using a synchrotron X-ray photon correlation spectroscopy

铁电（FE）材料具有自发极化特性，可以在电场作用下进行切换。这些材料由畴组成，即极化方向相同的区域。FE材料已用于各种电子器件，如红外传感器、执行器和超声波换能器。虽然众所周知，改变畴结构可以提高FE材料的机电和介电性能，但对于优化宏观材料性能的理想畴尺寸或畴壁密度（即相邻畴之间的边界区域）存在知识空白。本研究旨在建立一种基于x射线光子相关光谱（XPCS）的原位表征方法。XPCS可以在中尺度长度（1到几十微米）内从几毫秒到几百秒的时间范围内研究材料的原子结构的动力学。这个范围对于解释原子现象对宏观行为的影响至关重要，而且目前现有的技术，如x射线衍射和压电响应力显微镜，无法有效地获得。本文提出的方法比较了外加电场作用下x射线图像的位移。位移程度将进行统计分析，以揭示材料中的机电响应机制，包括通过连续域变形或离散域壁运动。这项研究的结果将为进一步研究畴壁密度和畴尺寸对FE材料性能的影响提供上述方法可行性的概念证明。此外，该提案还包括旨在促进代表性不足的少数群体（URM）职业发展的教育计划。这些计划包括K-12外展计划，为大学生整合STEM课程，并为URM团体的高中生和大学生提供研究指导。本研究旨在通过概念验证研究来研究铁电（FE）畴工程背后的基本机制，区分与机电（即压电）响应相关的内在（即域扩展和偶极子旋转）和外在（即离散域壁运动）相关的x射线散射行为。提出的x射线光子相关光谱（XPCS）方法为研究介观（1 ~几十微米）范围内的畴和畴壁效应提供了一种全新的方法，介观是解释原子现象对宏观材料行为影响的临界长度尺度。相比之下，现有的基于x射线衍射的技术可以通过相应衍射峰的相对强度来估计宏观尺度上的畴切换。此外，理论估计仅限于计算中使用的衍射峰可以完美描述的域结构。同样，虽然压电响应力显微镜可以可视化FE畴模式，但由于扫描速度的限制，很难在中尺度上表征原子结构的动态变化。相比之下，XPCS在一个帧中获取散射信号，在几毫秒到几百秒的时间内跟踪所有位置的模式移动。本项目将使用Pb(Mg1/3Nb2/3)O3-PbTiO3单晶，测试XPCS解相关函数的统计分布是否能够区分内源和外源响应的影响。目标是开发一种“双场”XPCS方法和一种统计分析工具，用于评估内在和外在机制对压电和介电性能的贡献。该项目还旨在通过涉及材料科学、x射线表征和数据科学的多学科研究，培养下一代工程师和科学家。研究成果也将用于教育K-12，本科，以及研究生水平的学生来自代表性不足的少数群体通过各种举措，如外展活动和创新课程的努力。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARYFerroelectric (FE) materials show spontaneous polarization that can be switched by an electric field. These materials consist of domains, the regions where the polarization is oriented in the same directions. FE materials have been used in various electronic devices such as infrared sensors, actuators, and ultrasound transducers. While it is well recognized that the modification of the domain configurations can enhance the electromechanical and dielectric properties of the FE materials, there is a knowledge gap regarding what is the ideal domain size or domain wall density (i.e., boundary regions between adjacent domains) to optimize the macroscopic material properties. The proposed research aims to develop an in-situ characterization method based on the X-ray photon correlation spectroscopy (XPCS). XPCS can investigate the dynamics of the atomic structures of materials over a range of time from milliseconds to hundreds of seconds in the mesoscale length (1 to tens of micrometers). This range is a critical to explaining the effect of atomistic phenomena on the macroscopic behavior and not efficiently accessible by the current existing techniques, such as X-ray diffraction and piezoresponse force microscopy. The newly proposed method compares the shift of X-ray images under applied electric field. The degree of shift will be statistically analyzed to reveal the mechanisms for electromechanical response in the material, including through continuous domain deformation or discrete domain wall motion. The outcome of this research will be a proof-of-concept of the viability of the above approach for further investigations of the effects of domain wall density and domain size on the FE material properties. Additionally, this proposal includes educational plans aimed at promoting STEM careers among underrepresented minority (URM) groups. These plans involve K-12 outreach initiatives, integrating STEM curriculum for university students, and providing research mentorship to high school and college students from URM groups.TECHNICAL SUMMARYThe proposed research aims to investigate the fundamental mechanisms behind the ferroelectric (FE) domain engineering through proof-of-concept research distinguishing X-ray scattering behaviors associated with the intrinsic (i.e., domain extension and dipole rotation) and extrinsic (i.e., discrete domain wall motion) contributions to the electromechanical (i.e., piezoelectric) response. The proposed approach by X-ray photon correlation spectroscopy (XPCS) provides a fundamentally new method to investigate the domain and domain wall effects in the mesoscopic (one to tens of microns) range, which is a critical length scale explaining the effect of atomistic phenomena on the macroscopic material behavior. For comparison, the existing techniques based on X-ray diffraction can estimate domain switching on a macroscopic scale, by using the relative intensity of the corresponding diffraction peak. Furthermore, the theoretical estimation is limited to the domain configuration that can be perfectly described by the diffraction peaks used in the calculation. Similarly, while piezoresponse force microscopy can visualize FE domain patterns, it is difficult to characterize the dynamic atomic structure changes at the mesoscale, due to the scanning speed limitations. In contrast, XPCS acquires the scattering signals in a frame to track the pattern shift in all locations over milliseconds to hundreds of seconds. This project will use Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals to test if the statistical distributions of the XPCS decorrelation functions can distinguish between the effects of intrinsic and extrinsic responses. The goal is to develop a 'two-field' XPCS method and a statistical analysis tool for assessing the contributions of intrinsic and extrinsic mechanisms to the piezoelectric and dielectric properties. This project also aims to educate the next generation of engineers and scientists through multidisciplinary research involving materials science, X-ray characterization, and data science. The research outcome will be also used to educate K-12, undergraduate, as well as graduate-level students from underrepresented minority groups through various initiatives, such as outreach activities and innovative curricular efforts.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.