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

非技术描述：压电性和挠性电性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扮演着重要的角色，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。本项目的目的是通过实验验证理论预测，这两种效应都可能在非常薄的1到2纳米量级的薄膜中表现出数量级的增强。对一种二维片状氧化物材料进行了研究。在这类材料中，应变会对其性能产生巨大影响。这个项目的目的是研究这种2D材料系统中的电子运动如何受到应变的控制。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则。该项目为招募和培训来自少数族裔群体的研究生和本科生提供机会，使他们拥有纳米科学前沿领域合成和表征2D氧化物纳米材料的知识和经验。研究成果被用于对高中教师和学生的外展。该项目还创建了开放访问的在线代码，用于计算压电学、半导体和压电学的国际社区。技术描述：原子计算预测了纳米厚度的独立二维(2D)材料中的压电性和挠性电效应将有数量级的增强。这种强烈的应变激发极化可能会通过压电效应极大地影响它们的半导体性质。然而，由于缺乏合适的物质对象，纳米级压电体和挠性电效应的实验研究远远落后于理论研究。PI的团队最近创造了独立的纳米厚度的单晶氧化锌纳米片，为研究这种材料的压电性、挠性和压电性提供了一个独特的平台。本研究旨在研究二维氧化锌纳米片中的这些现象，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激发极化是如何调节半导体性质的。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，应用于单独应变的氧化锌纳米片上，可以定量估计不同晶体取向的压电系数和挠曲电系数。此外，通过设计和表征氧化锌纳米片基晶体管和二极管，探索了二维受限压电和半导体沟道中与应变相关的界面电子能量和电子输运特性。

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.