Developing Spin and Polarization Order in 2D Hybrid Perovskites

Metal-halide perovskite (MHP) semiconductor materials have demonstrated a booming rise in optoelectronic performance, spanning IR to the ionizing radiation regimes. MHP crystals are largely enabled by avenues of multifunctionality, such as strong light absorption/emission, charge carrier conduction, and multiferroic properties, which are cooperatively engineered to break theoretical conversion efficiency limits. Without doubt, curiosity accumulates around the fundamental material physics of their broadly tunable optical, electronic, and magnetic properties. This work aims to impact the fundamental understanding of polarization and spin order in MHPs, while developing material solutions towards sensing and imaging from the IR to high energy (X-ray, gamma-ray) radiation.

Bulk single crystals and single-crystalline like films of 2D-phase MHP are prepared with crystal chemistries that host multiferroic properties (ferroelectric-ferroelastic-ferromagnetic), exhibit strong light absorption/emission, and support carrier conduction. The broken inversion symmetry and strong orbital polarization of 2D-phase MHPs promote spatial ordering of excitonic states and spin-dependent Rashba band splitting, leading to excited state polarization and spin order. Several MHPs are developed as platforms to investigate the fundamental mechanisms involved in photo-ferroelectric and photo-ferromagnetic coupling. Ultra-fast spectroscopic measurements, such as transient absorption and time-resolved photoluminescence with temperature and light polarization dependence, reveals tunable coupling of photoexcited excitons. Further in-situ optical characterization uncovers extraordinary excited state dynamics in ferroelectric 2D-phase MHPs. Active control of the spin degree of freedom in ferroelectrics is shown using circularly polarized photoexcitation into the Rashba bands, where the resulting spin-polarized states are found to be spin-operability through varying interfacial spin. This optical-magnetic coupling behavior is further investigated through the magnetic and chemical depth sensitive polarized neutron reflectometry technique. Collectively, this work explores the microscopic structure-property behaviors of MHP to develop optical control and coupling with multifunctional properties toward breaking technical boundaries in optoelectronic devices.