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

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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.