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

非技术总结该奖项支持计算和理论研究，以及挠性电学教育。如果对一块电绝缘材料施加应变梯度，例如通过弯曲它，就会在材料上产生电压。这种被称为挠性电效应的效应引起了人们的注意，因为这种效应有可能用于执行器、挠度传感器和能量收集器等技术应用。此外，挠曲电性也很重要，因为现代纳米级电子设备可能包含很大的无意应变梯度，因此挠曲电性可能在它们的性能中发挥关键作用。在发展对挠曲电的定量理解方面，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挠曲电响应。在这个项目中，PI将建立在他与合作者共同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研究更复杂的材料中的挠曲电响应。这项研究的目的是更好地了解什么材料的性质会导致特别大的挠曲电响应，这可以用于技术应用。这也将有助于识别具有抑制的挠曲电响应的材料，这在需要减轻无意应变梯度影响的情况下将是有用的。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的不同方面提供了一个理想的教育和指导平台。技术总结该奖项支持计算和理论研究以及挠曲电学教育。挠曲电效应，即由应变梯度引起的极化，在所有绝缘体中都是普遍存在的。随着器件缩小到微米和纳米尺度，可能会出现大的应变梯度，因此挠曲电效应可能在它们的性能中发挥重要作用。此外，柔性电还可以被用于材料机电操纵的新范例，例如开发由非压电成分构成的压电“超材料”，或者铁电极化的机械切换。在这项工作中，PI将利用最近发展的密度泛函微扰理论方法来探索和阐明复合材料中的挠曲电性物理，以准确和有效地计算挠曲电性系数。PI将研究两种材料体系中的挠曲电响应，目的是解决有关挠曲电通常如何在材料中表现的重要的公开问题。PI将专注于二维范德华结合材料，包括氮化硼和过渡金属二卤化物，以及对称性低于立方母体结构的扭曲的钙钛矿氧化物。PI将系统地探索对称性、力学和介电性质如何影响弯曲电响应，以及如何通过形成异质结构或超晶格或改变表面属性来测量或操纵这种响应。在这项工作中进行的材料研究，将能够识别具有大的柔电响应的特定材料和材料系统，这些材料和材料系统具有可能对应用有用的大的柔电响应，以及那些响应小的材料和材料系统，这些响应在非故意存在梯度并且必须缓解柔电的应用中是必要的。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.