Constructing a photoferroelectric semiconductor by regulating non-covalent interactions through halogen substitution

The advantages of molecular ferroelectrics lie in the "designability" and "multifunctionality", and the molecular-level regulation ability has opened up a brand-new dimension for ferroelectric materials. Non-covalent interactions play a crucial role in the construction of molecular ferroelectrics. However, there remain significant challenges in balancing the strength and reversibility of non-covalent interactions, as well as achieving long-range ordered arrangements. Therefore, a systematic study of non-covalent interactions in the structure is the key to construct high-performance molecular ferroelectrics. Here, we introduced halogenated amines with large dipole moments into adjacent inorganic layers to regulate the non-covalent interactions in the structure, thereby inducing the generation of ferroelectricity. Through a halogen substitution strategy to introduce chlorine (Cl) atoms on PA⁺ (n-propylaminium) cations, hybrid perovskite photoferroelectric semiconductor (Cl-PA)₂PbBr₄ (Cl-PA⁺ is 3-chloropropylaminium) with large piezoelectric response (d ₃₃ = 36 pC/N) and high Curie temperature (T _c = 365 K) was obtained. Compared with non-ferroelectric (PA)₂PbBr₄ (μ _PA = 1.2 D), the larger dipole moment (μ _Cl-PA = 3.3 D) and the directional ordered arrangement of Cl-PA⁺ in (Cl-PA)₂PbBr₄ synergistically induce its ferroelectricity. More importantly, when Cl replaces H, it affects the hydrogen bond network between the organic cation and the inorganic layer, enhancing the dynamic freedom of the Cl-PA⁺ cations, making the structure of (Cl-PA)₂PbBr₄ more prone to phase transitions when the temperature changes. The hydrogen bonding and halogen-halogen interactions in (Cl-PA)₂PbBr₄ lead to the directional and ordered arrangement of Cl-PA⁺ cations, breaking the centrosymmetric structure and synergistically promoting the generation of ferroelectricity. This work has confirmed the significance of non-covalent interactions in the construction of ferroelectrics.