CAREER: Hybrid Bronzes: Mixed-Valence Hybrid Metal Oxides as a Tunable Material Platform

PART 1: Non-Technical Summary Renewable energy technologies such as solar cells, batteries, and fuel cells are critical for addressing urgent global energy demand in a sustainable manner. These systems rely on materials that are highly stable and, as a function of their ordered structures, display important properties for energy-related use such as efficient light absorption and the ability to easily conduct electrical current. However, it is challenging and costly to synthesize and modify important crystalline solids such as metal oxides. The chemical tuning of molecules, on the other hand, is more precise and less energy intensive. With this CAREER award, supported by the Solid State and Materials Chemistry program in NSF’s Division of Materials Research, the principal investigator and his research group investigate how to combine the best qualities of molecules and materials by developing design principles for an emerging new class of organic-inorganic materials called hybrid bronzes. These easily synthesized, low-cost, and air-/water-stable compounds achieve atomic-level integration of metal oxide layers with molecules having adjustable functions, thereby providing a tunable material platform that can cater to numerous desired applications. This work elucidates structure-property relationships governing the electronic behavior of hybrid bronzes to inform their ultimate implementation in energy-related technologies. Furthermore, this interdisciplinary research program trains undergraduate and graduate students as the future STEM workforce and develops a multi-faceted instructional video platform called "Lab Hacks" that seeks to lower resource and knowledge barriers in STEM education and research. PART 2: Technical Summary Hybrid bronzes are bulk crystalline materials that combine alternating layers of (1) mixed-valence metal oxide sheets featuring tunable charge-carrier densities and band gaps and (2) molecular arrays with the potential for chemical-, redox-, and photo-activity. Here, the term "bronze" refers to the metallic luster that quasi-free electrons impart to reduced metal oxides and it is these mobile carriers that ultimately enable such electronic versatility. To advance the hybrid bronze platform toward energy-related use, it is necessary to understand and subsequently control their redox activity, light absorption, and charge transport. Hybrid bronzes also represent versatile model systems that can probe questions regarding two-dimensional solid-state phenomena. With this CAREER award, supported by the Solid State and Materials Chemistry program in NSF’s Division of Materials Research, the principal investigator and his research group leverage mild aqueous self-assembly reactions to produce bulk crystalline hybrid bronzes with a fine degree of synthetic control. A suite of diffraction-based, spectroscopic, and electronic characterization techniques including high-pressure methods are then employed to elucidate structure-property relationships. Specifically, evaluation of systematically varied molecular structure-directing effects illuminates how charge transport is dictated within inorganic layers. Principles governing stimulus-driven charge transfer phenomena between molecules and layers are explored through optoelectronic, electrochemical, and charge transport analysis, followed by iterative molecular tuning. Further, pressure/strain-induced structure changes are employed as a unique approach to dictating electronic property transitions within hybrid bronzes. Overall, this work reveals connections between the structures and electronic behaviors of hybrid bronzes, including emergent phenomena, to demonstrate design rules enabling their customization. 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.