Elucidating the Physics of Flexoelectricity Through First-Principles Calculations of Complex Materials

该奖项支持柔性电的计算和理论研究以及教育。如果将应变梯度施加到一块电绝缘材料上，例如通过弯曲它，将在材料上产生电压。这种效应被称为柔性电，由于可能用于执行器、偏转传感器和能量收集器等技术应用而引起了人们的关注。此外，柔性电是重要的，因为现代纳米级电子器件可能包含很大的无意应变梯度，因此柔性电可能在其特性中起关键作用。在开发柔性电的定量理解中，一个关键的缺失因素是缺乏有效和准确的计算方法来预测材料的柔性电响应。在这个项目中，PI将建立在他与合作者开发的预测方法的基础上，并将其应用于探索和表征比以前理论上研究的更复杂的材料中的柔性电响应。这项研究的目标是更好地了解哪些材料特性会导致特别大的柔性电响应，这可以用于技术应用。这也将有助于识别具有抑制挠曲电响应的材料，这将在需要减轻无意应变梯度影响的情况下非常有用。该项目的研究活动为研究生和本科生在凝聚态物理、材料科学和计算科学等各个方面的教育和指导提供了理想的平台。该奖项支持柔性电的计算和理论研究以及教育。挠曲电效应，即由应变梯度引起的电极化，在所有绝缘体中都是普遍存在的。当器件缩小到微纳米尺度时，可能会出现大的应变梯度，因此挠曲电效应可能在其性能中起重要作用。此外，柔性电可以用于材料机电操纵的新范例，例如由非压电成分构建的压电“超材料”的开发，或铁电极化的机械开关。在这项工作中，PI将探索和阐明复杂材料中的挠性电的物理学，利用最近开发的密度泛函微扰理论方法准确有效地计算挠性电系数。PI将研究两种材料系统中的挠性电响应，目的是解决与挠性电如何在材料中普遍表现有关的重大开放问题。PI将重点研究二维范德华键合材料，包括氮化硼和过渡金属二硫族化合物，以及对称性低于立方母结构的“扭曲”钙钛矿氧化物。PI将系统地探索对称性、力学和介电性质如何影响柔性电响应，以及如何通过形成异质结构或超晶格或修改表面性质来测量或操纵这种响应。在这项工作中进行的材料研究将能够识别具有大挠曲电响应的特定材料和材料系统，这些材料和材料系统可能对应用有用，以及那些具有小响应的材料和材料系统，这些材料和材料系统在无意中存在梯度并且必须减轻挠曲电的应用中是必要的。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。

NONTECHNICAL SUMMARYThis award supports computational and theoretical research, and education on flexoelectricity.If a strain gradient is applied to a piece of an electrically insulating material, for example by bending it, a voltage will be created across the material. This effect, known as flexoelectricity, has attracted attention because of the possibility for technological applications such as actuators, deflection sensors, and energy harvesters. Also, flexoelectricity is important since modern nanoscale electronic devices may contain large unintentional strain gradients, and thus flexoelectricity may play a crucial role in their properties. A pivotal missing ingredient in developing a quantitative understanding of flexoelectricity has been the lack of an efficient and accurate computational methodology to predict the flexoelectric response of a material. In this project, the PI will build on predictive methodology he has developed with collaborators and apply it to explore and characterize the flexoelectric response in materials more complex than could be theoretically investigated before. The goal of this research is to better understand what materials properties lead to an especially large flexoelectric response, which can be utilized for technological applications. This will also help identify materials with a suppressed flexoelectric response, which will be useful in cases where the effects of unintentional strain gradients need to be mitigated. The research activities in this project serve as an ideal platform for the education and mentoring of graduate and undergraduate students in diverse aspects of condensed matter physics, materials science, and computational science. TECHNICAL SUMMARYThis award supports computational and theoretical research, and education on flexoelectricity.The flexoelectric effect, where electrical polarization is induced by a strain gradient, is universal in all insulators. As devices shrink to the micro and nanoscale, large strain gradients can occur, and therefore the flexoelectric effect may play a significant role in their properties. Also, flexoelectricity can be exploited for novel paradigms of electromechanical manipulation of materials, such as the development of piezoelectric "metamaterials" constructed from nonpiezoelectric constituents, or mechanical switching of ferroelectric polarization. In this work, the PI will explore and elucidate the physics of flexoelectricity in complex materials, utilizing recently developed density functional perturbation theory methodology for accurately and efficiently calculating flexoelectric coefficients. The PI will investigate the flexoelectric response in two materials systems with the goal of addressing significant open questions relating to how flexoelectricity is generally manifested in materials. The PI will focus on two-dimensional, van der Waals bonded materials including, boron nitride and the transition-metal dichalcogenides, and "distorted" perovskite oxides with lower symmetry than the cubic parent structure. The PI will systematically explore how symmetry, mechanical, and dielectric properties influence the flexoelectric response, and how this response can be measured or manipulated by forming heterostructures or superlattices, or modifying surface properties. The materials study performed in this work, will enable the identification of specific materials and material systems that have large flexoelectric responses which may be useful for applications, as well as those with small responses, which are necessary in applications where gradients are present unintentionally and flexoelectricity must be mitigated.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.