Stability and formation of Inversion Boundaries in ZnO: DFT and experimental screening for new IB-forming dopants

The proposed project deals with the stability and formation of chemically induced domain walls, which recently received a lot of attention in the design of functional materials. Because they cause a sudden 2D discontinuity in bulk properties, known as quantum wells, they represent an ideal tool for tailoring the physical properties of materials. As a case study of chemically induced domain walls in this project, we choose inversion boundaries (IBs) in ZnO, which is a low cost non-toxic, n–type semiconductor with a polar wurtzite structure, that exhibits an excellent combination of electronic and optical properties, including an appropriate band-gap structure, chemical resistance, and thermodynamic stability, making it adaptable for a range of applications in electronics. IBs are chemically induced domain walls across which the crystal polarity is reversed. They have been linked to improved electron transport, increased phonon scattering, and increased height of the Schottky barrier which is reflected on optoelectronic, piezotronic, thermoelectric and varistor properties of ZnO–based materials. Further, they have been related to yet unexplained p–type conduction of ZnO, and have clearly been proven to cause anisotropic and exaggerated grain growth, which was successfully employed to control microstructure development of ZnO–based ceramics. ZnO is a highly adaptive model system for the incorporation of a wide range of dopants that change the electronic structure of ZnO, and some of these are known to produce complex types of IBs on various lattice planes or layered structures with multiple IBs. The key objective of this project will be to resolve fundamental questions related to the stability and formation of IBs in ZnO that will help to disentangle their role in collective physical properties. The first research challenge will be to identify the mechanisms of dopant incorporation and the driving force influencing IB formation. IB formation will be studied by an originally devised methodology that involves theoretical approaches based on DFT, structural design, and targeted experiments to study the formation of IBs with selected dopants including in–situ and high–resolution electron microscopy with the associated spectroscopic methods to study the structure and chemistry of IBs at the atomic scale. The proposed research methodology has never been attempted before, and it represents a cutting-edge approach to studying chemically induced domain boundaries and their formation mechanisms. The second research challenge will be to identify new IB–forming dopants through screening of other elements of the periodic system for potential candidates in a high throughput study. The final challenge will be the prediction of physical properties. The principal investigator of this postdoctoral project is Dr. Vesna Ribić. With her work on the stability of IB in Sb2O3–doped ZnO, she made a significant breakthrough in the studies of domain walls in materials by combining theoretical and experimental approaches. Her pioneering work that involves quantum chemical calculations in predicting the most stable IB structure inspired a group of theoreticians at Materials Center Leoben (MCL) to follow–up on the research challenge and developed an innovative theoretical approach enabling comparison of IB formation energies across different chemistries. This led to mutual collaboration between researchers to perform a few critical tests and with confidence plot the outline for this postdoctoral project. The combined expertise of both groups has the potential to produce high-impact scientific discoveries and develop innovative research methodologies and solutions in the course of the project.