3D Chiral Perovskitoid-to-Perovskite Reconstructive Phase Transition Designed Molecular Ferroelectric Thin-Film with...

The realization of pure out-of-plane polarization in ferroelectric thin films is crucial for next-generation high-performance microelectronics and miniaturized devices. Molecular ferroelectrics, which allow facile and large-area thin-film fabrication via solution processing, have gained tremendous attention in recent years. However, to date, molecular ferroelectric thin films with pure out-of-plane polarization have never been reported due to the inconsistency of their random orientation with the polarization direction. Herein, 3D chiral alkali metal halide ferroelectrics R/S-AQCsI₃ (AQ = 3-aminoquinolinium) are designed, of which thin films exhibit a highly preferred (00l) orientation. Notably, R/S-AQCsI₃ can undergo an unprecedented reconstructive phase transition from hybrid 3D perovskitoid to perovskite structure while maintaining the thin films with the (00l) preferential orientation. More importantly, such a topological rearrangement of the 3D inorganic [CsI₃]_n ^2- framework triggers chiral AQ²⁺ cation reorientation in the lattice, which changes the polarization direction from b-axis in the polar perovskitoid phase to c-axis in the perovskite phase, enabling the variation from pure in-plane to pure out-of-plane polarization in the thin-film. To the knowledge, this is the first report of molecular ferroelectric thin films with pure out-of-plane polarization. This work opens up an avenue for achieving pure out-of-plane polarization in molecular ferroelectric thin films.