CAREER: Structural dynamics and optoelectronics of anharmonic soft semiconductors

Nontechnical Description Metal halide perovskites are a promising class of materials with device applications such as solar cells and light-emitting diodes. Their exceptional electronic properties are coupled with inexpensive, low-temperature solution processing. This combination is surprising, given that defects can be introduced during film formation or through impurities. This CAREER project is motivated by the need to better understand these materials and to advance the science and technology of soft semiconductors. The key to understanding the exceptional properties of halide perovskites lies with their structural dynamics, that is, how atoms composing a material interact with one another through vibrations. Unlike conventional semiconductors, structural dynamics in halide perovskites are anharmonic, arising from long-ranged and damped nonlinear interactions in a soft lattice. This research project aims to establish fundamental concepts as to how soft semiconductors function with extreme lattice anharmonicity. The PI will investigate these materials with a combination of advanced optical and vibrational spectroscopy, coupled with computational modeling. Insights developed in this research will help to comprehend collective motions in these emerging materials and enable their customization. The educational focus of this project embraces and leverages the rise of computation for the teaching and learning of physical chemistry. Computation as a medium and approach will be incorporated into instruction, curricular and extracurricular projects, and student research. The PI will also explore the visualization and sonification of scientific data to bridge science and art, bringing this to students and the general public. Technical Description This research proposal focuses on metal halide perovskites, a relatively new class of soft semiconductors with strongly anharmonic structural dynamics in sharp contrast to conventional quasi-harmonic semiconductors. In this yet uncharted territory, we need to rethink how electronic and structural degrees of freedom couple with each other. Gaps in our understanding exist that call for basic knowledge and new concepts to fully appreciate the origin, extent, and consequence of a soft anharmonic lattice in optoelectronic materials. To bridge these gaps, the PI seeks to quantify universal lattice anharmonicity and its impact on charge carrier relaxation in halide perovskite soft semiconductors; characterize ferroelastic domain wall optoelectronics in halide perovskite soft semiconductors; and identify new two-dimensional and molecular soft semiconductors. The project features correlated spectroscopic investigations across multiple time and length scales. Terahertz frequency lattice dynamics will be probed and correlated to optical range electronic transitions as well as elastic properties in the static limit. Emphasis will be placed on distilling universal design principles from metal halide perovskites and generalizing these insights to other emerging materials. This project is jointly funded by the Electronic and Photonic Materials program and the Established Program toStimulate Competitive Research (EPSCoR). 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.