This MWN research focuses on new insights into the fundamental nature of ferroelectric domain walls developed by this joint US/Ukraine team over the past two years. In particular, the team leverages the comprehensive theory, simulation and advanced experimental framework they have developed for quantitative piezoelectric force microscopy (PFM). Using this framework, they have discovered unexpected broadening of ferroelectric domain walls over tens of nanometers. Analytical theory and phase field modeling, predicts that even a few nanometers of broadening can dramatically change the macroscale properties such as threshold fields for wall motion, by many orders of magnitude. The team predicts unusual magnetic-like domain walls in ferroelectrics. Such walls can be engineered to be extremely broad, (100's nm), and their dynamical properties under electric fields, and hence their impact on macroscale properties are presently unexplored. Using Scanning Spectroscopy Piezoelectric Force Microscopy (SSPFM), and optical second harmonic generation-near field scanning optical microscopy (SHG-NSOM), the team is exploring this mysterious new world of domain walls. Broadly speaking, the US team (Penn State and Oak Ridge National Labs) focuses on the experimental and phase-field simulations of such ferroelectric walls, while the Ukranian team (National Academy of Sciences, Ukraine) is developing the theoretical framework.
The development of quantitative PFM and SHG-NSOM imaging techniques that combine theory, numerical simulations and cutting-edge experimental techniques are expected to have a much broader impact that extends beyond ferroelectrics, to other fields of materials science, chemistry and life sciences. This US/Ukranian team is an excellent example of a genuine international collaboration that started rather spontaneously a few years ago between the PIs, and has been very productive. This proposal will provide funds to energize and sustain this spontaneous effort by supporting undergraduate and graduate students to work and collaborate in a global context, support extended visits across the Atlantic by PIs and students alike, further interactions between a university (Penn State), a national lab (Oak-Ridge National Lab) and a national academy (NAS-Ukraine), support organization of an annual international workshop on Piezoelectric Force Microscopy and summer workshops in nonlinear optical microscopy, and provide research opportunities for women and underrepresented groups.
This MWN award is co-funded by DMR-EPM, DMR-OSP, and OISE Eurasia Region.
Project Goals: Ferroelectrics are materials with a built in electric polarization that can be switched by an electric field. They are widely used in ultrasound imaging, radar, precision actuation in biosurgical devices and atomic force microscopes, and in optical modulators that power the internet. Understanding and controlling ferroelectric domains, regions of uniform polarization, is critical to all these functions. This project aimed to understand the internal structure of the walls that separate different ferroelectric domains, and how they respond to external stimuli. This topic has been the subject of study for over 60 years. Yet new insights are emerging as a result of advances in our theoretical understanding, as well as in experimental techniques, both of which this project furthered in the past 4 years, as described in more detail below. A second key goal of this project was to develop a strong transatlantic collaboration with Drs. Anna Morozovska, and Eugene Eliseev, from the National Academy of Sciences of Ukraine. The collaboration, the opportunities it afforded for student training and mentorship was an invaluable part of the project goal. Students and Publications: Overall, 9 graduate students (two international exchange students from China) and 2 postdocs were trained by this project. 6 PhDs were granted. Two of the graduating students (Vasudeva rao Aravind, and Amit Kumar) have gone on to become faculty members in Clarion University, PA ( primarily undergraduate teaching institution) and Queen's University, Belfast, respectively. The project resulted in 42 peer reviewed publications, of which 22 were joint publications with Ukrain. The work resulted in over a hundred talks and presentations at professional conferences, about half of which were invited. Research Highlights: We discovered that ferroelectric walls are not purely Ising type walls, as previously throught, but rather have Bloch and Neel like mixed character as well. We discovered that this character arises from a phenomenon called the flexoelectric effect inside the walls, where strong gradients in polarization ans strain exists. As a consequence a large charge density exists in such walls, making them conductive. Fundamental mechanisms leading to conductivity at walls were discovered. In materials composed of polyhedral units such as oxygen octahedra in ferroelectric perovskites, a new phenomenon, namely flexo-roto effect was discovered that leads to polarization in seemingly nonpolar materials. We discovered a new class of phase transitions in ferroelectrics called thermotropic phase boundaries (TPB), which exist even in classic ferroelectrics known for many years. Across a thermal phase transition, stresses and fields arising from domain walls can stabilize a new low symmetry phase that is not normally predicted by thermodynamics. This new phase is a slight distortion of the parent phase, but it has optical and piezoelectric properties that are enhanced 400% over the parent phases! Techniques: We have significantly advanced the state-of-the-art in four techniques: Optical Second Harmonic Generation Microscopy (SHG μscopy), Piezoelectric Force Microscopy (PFM), Scanning Xray Diffraction Nanoprobe (SXDN), and Phase-field Modeling. SHG μscopy was developed in the PI lab that led to the discovery of TPBs described above. PFM is a widely popular technique to image polar materials today, but the origin of the signal and its modeling was still lacking. We have developed excellent theoretical models and numerical techniques to simulate these signals and understand the quantitiative information PFM in reveals. SXDN is a cutting-edge technique at Advanced Photon Source, Argonne, IL that uses a 30nm rastering Xray beam to do nanoscale diffraction. We have performed some of the first quantitative studies with this system, which has helped image the new low symmetry phase mentioned above, and helped resolve disputes surrounding this phase for many years. We have developed state-of-the art phase-field modeling codes that can model complex microstructures in all sorts of materials, ranging from metallic alloys, to ceramics, semiconductors, and ferroelectrics. Recently we have expanded the code to include flexoelectric effect for the first time. International and National Collaborations: The work described above would not have been possible without our international and National collaborations. The theoretical ideas surrounding many of the phenomena revolving around flexoelectric effects, domain wall conductivity, new low symmetry phases, and non-Ising walls were initiated by this international exchange with Ukraine. It has been an extremely dynamic and productive collaboraiton, involving everyday emails, and skype sessions, that has not needed a structured approach to a collaboration - it has been rather spontaneous and highly productive. PFM collaboraitons with Dr. Sergei Kalinin at Oak Ridge National Labs, and SXDN collaboration with Dr. Martin Holt at Argonne has been invaluable in advancing the science and experimental techniques.
