Nanometer-Scale Piezoelectric, Flexoelectric and Piezotronic Effects from 2D Piezoelectric Nanomaterials

非技术描述：压电和挠性电分别描述了电对均匀和非均匀机械应变的响应。这两种效应在现代机电系统中都起着重要作用，包括能量采集器、电力变压器、传感器、微天平、换能器和执行器。该项目旨在通过实验验证理论预测，即在1到2纳米尺度的极薄薄膜中，这两种效应可能会表现出数量级的增强。对二维（2D）片状氧化物材料进行了研究。在这种材料中，应变会对其性能产生巨大的影响。该项目旨在研究如何通过应变控制这种二维材料系统中的电子运动。从这项研究中获得的知识有可能揭示下一代传感器、执行器和能量收集设备的新材料和设计原理。该项目为招收和培养具有纳米科学前沿二维氧化物纳米材料合成和表征知识和经验的少数民族研究生和本科生提供了机会。并将研究结果应用于高中师生的外展活动中。该项目还创建了用于计算压电能带图的开放访问在线代码，为压电、半导体和压电电子学的国际社区提供服务。技术描述：原子计算预测了纳米厚独立二维（2D）材料的压电和挠曲电效应的数量级增强。这种强应变极化可能会通过压电效应极大地影响其半导体性能。然而，由于缺乏合适的材料对象，纳米尺度的压电和挠曲电效应的实验研究远远落后于理论研究。PI团队最近创造的独立纳米厚单晶ZnO纳米片为研究这种材料的压电、挠性和压电行为提供了一个独特的平台。本研究的目的是在二维ZnO纳米片中研究这些现象，以验证这两种效应在纳米尺度上的巨大增强的理论预测，并了解二维纳米材料系统中应变诱导极化如何调节半导体性能。基于原子力显微镜的技术，包括开尔文探针显微镜、静电力显微镜和压电力显微镜，应用于单独应变的ZnO纳米片上，可以定量估计沿不同晶体取向的压电和挠曲电系数。此外，通过设计和表征ZnO纳米片晶体管和二极管，探索了二维受限压电和半导体通道中与应变相关的界面电子能量学和电子输运性质。

Nontechnical Description: Piezoelectricity and flexoelectricity describe a response of electricity to uniform and non-uniform mechanical strains, respectively. Both effects play an important role in modern electromechanical systems, including energy harvesters, power transformers, sensors, microbalances, transducers, and actuators. This project aims to experimentally test the theoretical prediction that both effects may exhibit orders of magnitudes enhancement in very thin, on the order of 1 to 2 nanometer scale, films. Studies are conducted on a two-dimensional (2D) sheet-like oxide materials. In such materials, strain can impose tremendous impacts on their properties. This project aims to study how electron movement in such 2D material systems can be controlled by the strain. The knowledge gained from this research has the potential to disclose new materials and design principles for next-generation sensors, actuators, and energy harvesting devices. This project provides opportunities for recruiting and training graduate and undergraduate students from underrepresented minority groups with knowledge and experiences of synthesizing and characterizing 2D oxide nanomaterials on the frontier of nanoscience. The research results are utilized in outreach to high school teachers and students. This project also creates open-access online codes for calculating the piezotronic band diagrams, serving the international communities of piezoelectrics, semiconductors and piezotronics.Technical Description: Atomistic calculations have predicted an orders-of-magnitude enhancement of the piezoelectric and flexoelectric effects in nanometer-thick free-standing two-dimensional (2D) materials. This strong strain-induced polarization may drastically influence their semiconductor properties via the piezotronic effect. However, due to the lack of appropriate material objects, experimental study of the nanometer-scale piezoelectric and flexoelectric effects far lags behind the theoretical study. Free-standing nanometer-thick single-crystalline ZnO nanosheets recently created by the PI's team offer a unique platform for studying the piezoelectric, flexoelectric and piezotronic behavior of this material. The research aims at studying these phenomena in 2D ZnO nanosheets in order to verify the theoretical prediction of the gigantic enhancement of both effects in the nanometer scale and to understand how the semiconductor properties are tuned by the strain-induced polarization in 2D nanomaterial systems. Atomic force microscopy-based techniques, including Kelvin probe microscopy, electrostatic force microscopy, and piezoelectric force microscopy applied on individually strained ZnO nanosheets allow quantitative estimation of piezoelectric and flexoelectric coefficients along different crystal orientations. In addition, the strain-related interfacial electron energetics and electronic transport properties in 2D confined piezoelectric and semiconducting channels are explored by designing and characterizing ZnO nanosheet-based transistors and diodes.