This grant provides funding for a combined theoretical and experimental study to understand the emergence of size-dependent ?giant? piezoelectricity in ferroelectric nanostructures. Modeling approaches ranging from quantum mechanical atomistic calculations to continuum methods will be employed to understand the basic science underpinning this phenomenon. Flexure, artificial cracks and indentation based experiments on inhomogeneously strained nanostructures will be conducted to verify the predictions.
The expected benefits of the program will be both educational and societal. Piezoelectricity, the development of electrical polarization upon mechanical stimulus and thus the conversion of mechanical to electrical energy, has numerous applications ranging from simple consumer products such as cell phones to complex technological advancements: atomic force microscopy, sonars and artificial muscles among many others. Accordingly, anticipated research outcomes are in next generation sensors and actuators, energy harvesting, artificial muscles that exhibit simultaneously large motion and force and an enhanced understanding of how to design multi-functionality. The educational impact is that both undergraduate and graduate students will be trained in emerging and interdisciplinary research that falls at the intersections of materials science, mechanics, and physics. The grant puts specific emphasis on research experience for high school students and undergraduate students and in particular female scholars. Research findings will be incorporated into courses, modules specifically targeted towards grade school students and broadly disseminated through conference presentations, scholarly publications, and the PI?s websites.
In this project, the overarching goal was to theoretically and experimentally investigate the emergence of size-dependent "giant" piezoelectricity at the nanoscale. Preliminary theoretical work (verified by quantum mechanical based atomistic calculations) of one of the PIs had indicated a peculiar (and tantalizing) scaling of the effective piezoelectric constants of certain types of nanostructures resulting in, for example, an increase in nearly 400 % for 400 nm tetragonal-phase piezoelectric BaTiO3 beams under bending conditions and the development of appreciable piezoelectricity in 5 nm nano-beams of the otherwise non-piezoelectric cubic-phase BaTiO3. During the course of this study, we discovered a novel indentation size effect in ferroelectrics due to flexoelectricity. We have found that let alone electromechanical coupling, even the elastic modulus of a ferroelectric (under indentation) gets renormalized (up to twice the bulk value at small contact sizes) due to flexoelectricity. The discovered size-effect was verified by experiments. Using a combination of a theoretical framework and atomistic calculations, we have highlighted the concept of surface piezoelectricity that can be used to interpret the piezoelectricity of nanostructures. Focusing on three specific material systems (ZnO, SrTiO3 and BaTiO3), we have elucidated the renormalization of apparent piezoelectric behavior at small scales. In a rather interesting interplay of symmetry and surface effects, we have shown that nanostructures of certain non-piezoelectric materials may also exhibit piezoelectric behavior. Finally, for the case of ZnO, using a comparison with first principles calculations, we also comment on the fidelity of the widely-used core-shell interatomic potentials to capture non-bulk electro-mechanical response. (3) Through experiments on SrTiO3, a non-ferroelectric, but highly flexoelectric material, we were able to conclusively show that the indentation size effect observed in our experiments earlier is NOT due to dislocation activity but primarily due to flexoelectricity. Seven peer-reviewed journal papers have emerged from this effort. As part of the broader impact, we recruited female graduate students, developed a short course on nanotechnology, incorporated the state-of-the-art findings in advanced graduate courses and widely disseminated the results through conference presentations, invited talks and peer-review journal papers.
