Two-DimensionalDion–Jacobson FerroelectricSemiconductors with an Out-of-Plane Polarization and Ultraviolet Photoluminescence

Two-dimensional hybrid lead halides are promising materials for a wide range of applications. Among their most desirable subclasses are luminescent ferroelectrics, especially those exhibiting out-of-plane polarization, a property that enables the miniaturization of ferroelectric and integrated optoelectronic devices. In this work, we expand the still very small family of hybrid perovskites with out-of-plane polarization by introducing two members: the single-layered (MPDA)PbCl4 and the bilayered (MPDA)(MA)Pb2Cl7 (where MPDA = N-methyl-1,3-propanediamine). We show that both compounds exhibit efficient second-harmonic generation (SHG) at room temperature and a wide transparency range down to the ultraviolet. (MPDA)PbCl4 undergoes one structural phase transition from high-temperature phase I (space group C2/c) to room-temperature phase II (space group Pn). Two crystal phases are also present for (MPDA)(MA)Pb2Cl7: the high-temperature phase I (space group C222) and the room-temperature phase II (space group Pn). X-ray diffraction data revealed the ferroelectric nature of the room-temperature phases II and the presence of out-of-plane polarization. The ferroelectricity of (MPDA)PbCl4 was further confirmed by the observation of ferroelectric hysteresis loops. Optical studies revealed that (MPDA)PbCl4 and (MPDA)(MA)Pb2Cl7 exhibit narrow and broad emission bands with maxima near 354 and 410 nm, respectively. Overall, our findings show that the use of the MPDA2+ cation and the mixed-cation approach has led to the discovery of hybrid lead chloride perovskites exhibiting the coexistence of out-of-plane spontaneous polarization, SHG activity, and tunable photoluminescence.