Surface science of bulk-terminated cubic perovskite oxides

Perovskite oxides possess great potential for many applications, such as (photo)catalysis, fuel cells, electronics or spintronics. The bottleneck for their practical use lies in detailed understanding of the materials surfaces at the atomic scale: There is an inherent problem with the stability of perovskites at operating conditions, and a detailed understanding of their interaction with the ambient environment is currently lacking. We have recently pioneered how to prepare well-defined surfaces of two cubic perovskites, KTaO3 and SrTiO3, suitable for surface-science studies [Science 359, 572 (2018)]. These surfaces represent a perfect starting point for applying the analytical techniques of surface science and studying the detailed interaction of these materials with molecules from the external atmosphere. Understanding these interactions is essential for optimizing catalytic reactions in applications like fuel cells or photocatalytic light harvesting. Further, the bulk-terminated surfaces used in this work provide access to bulk properties of the materials, such as special electronic states or ferroelectricity. Detailed understanding of these properties aims towards opening new routes in oxide electronics and enabling new generation of electronic devices operating beyond binary logic.

The project was focused on the surface science of cubic perovskites, motivated by the prospective use of these materials in catalysis, electronics and production of renewable fuels. Perovskites have a ternary chemical composition, i.e., they consist of at least three chemical elements. There are wide options of tuning their chemical and physical properties by choosing and mixing the composition of the material, while keeping its cubic crystal structure. The ternary composition makes perovskites attractive for various applications, but represents a challenge for surface science: Perovskites can adopt a myriad of surface terminations. To overcome this problem, we have decided to work on surfaces prepared by cleaving and developed new methods for preparing well-defined surfaces that follow the bulk crystal structure. With this approach, we used a combination of atomically-resolved microscopies and first-principle calculations to answer key questions that arise in the field, using prototypical perovskites SrTiO3 and KTaO3. The first major result is a detialed analysis of the complex electronic structure of these materials. At the surface, we have observed a fine competition between delocalized electrons forming 2D electron gases, and localized electrons (polarons) of various sizes. These results have been published in Nature communications, one of the most prestigeous interndisciplinary journals. Antoher important result is understanding how molecules bind to the surface: Perovskites hold a unique quality that allows tuning the bonding strength of adsorbed molecules and possibly regulate the catalytic properties of the material. This result has been recently published in Science Advances.