CAREER: Realizing strain-induced phase boundaries in lead-free ferroelectric oxide heterostructures and membranes

NON-TECHNICAL SUMMARY Enhanced electrical and electromechanical responses in ferroelectric materials often arise at chemically induced phase boundaries where multiple structures coexist—for example, the well-known morphotropic phase boundary in lead zirconate titanate. At these phase boundaries, external stimuli, such as electric fields, can trigger interconversion between different structures, leading to colossal physical responses that can be utilized for various functional applications. However, these chemically induced phase boundaries usually involve complex chemical compositions that introduce disorder and heterogeneity. Additionally, these materials often contain toxic lead, raising significant environmental and health concerns. To address these challenges, the research team aims to develop strain-engineering pathways to create phase boundaries in lead-free ferroelectric oxide heterostructures and membranes beyond the traditional chemical method. These new strain modalities open opportunities to explore, manipulate, and harness novel functionalities in oxide materials for next-generation applications. Aligned with the research, this program supports the launch of a teacher-training workshop for the special education teachers in K-12 public schools and offers research internships for high school students. These efforts aim to break barriers to STEM careers for students to promote a stronger workforce. Additionally, this program develops hands-on and online course modules to foster greater interest and access in functional thin-film materials research for undergraduates and graduate students. TECHNICAL SUMMARY This program aims to advance strain engineering beyond traditional heteroepitaxy to create strain-induced phase boundaries in lead-free ferroelectric thin-film heterostructures and membranes. By leveraging anisotropic epitaxy and dynamic strain tuning, the program seeks to generate coexisting and bridging phases with enhanced dielectric, piezoelectric, and ferroelectric properties in lead-free sodium niobate heterostructures and membranes. This program employs atomic-scale epitaxy and chemical lift-off techniques to synthesize epitaxial heterostructures and membranes while applying mechanical tuning to control the competition between nearly degenerate polymorphs near phase boundaries. A broad set of structural and property characterization tools are utilized to establish strain-structure-property relationships, enabling the rational design and precise control of strain to unlock new phases with superior electrical and electromechanical performance. In parallel, the program integrates educational initiatives to foster a stronger STEM workforce, with a particular focus on engaging students. These initiatives include teacher-training workshops for K-12 special education teachers, research internships for high school students and undergraduate students, as well as hands-on and online course modules focused on functional thin-film materials. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.