Anion Exchange and Lateral Heterostructure Formation in Ferromagnetic PEA₂Cr(Cl,Br)₄ Two-Dimensional Perovskites

Postsynthetic vapor-phase anion exchange in the ferromagnetic two-dimensional (2D) hybrid metal-halide perovskite, PEA₂CrCl₄ (PEA⁺ = phenethylammonium), is reported. Anion exchange using vaporous trimethylsilyl bromide (TMS-Br) is shown to drive complete conversion of solution-processed PEA₂CrCl₄ polycrystalline thin films to PEA₂CrBr₄. Low-temperature magnetic circular dichroism spectroscopy indicates ferromagnetic ordering in these PEA₂CrCl₄ and PEA₂CrBr₄ films. Via partial anion exchange of exfoliated flakes of PEA₂CrCl₄ single crystals, we demonstrate that it is possible to generate abrupt lateral PEA₂CrCl₄/PEA₂CrBr₄ magneto-heterointerfaces. Kinetic studies reveal that lateral heterostructure formation is dictated by rapid edge-site halide exchange followed by slower intralayer bromide diffusion, and there is negligible interlayer (3D) bromide or TMS-Br diffusion. The importance of the bulky PEA⁺ interlayer cation in suppressing 3D diffusion is highlighted by parallel anion-exchange experiments on MA₂CrCl₄ (MA⁺ = methylammonium), which instead show 3D exchange. Comparison of anion-exchange reactions in PEA₂CrCl₄, PEA₂MnCl₄, and PEA₂PbCl₄ shows that 2D bromide diffusion is slowest in PEA₂CrCl₄, attributed to the antiferrodistortive ordering found in this composition. In addition to demonstrating both postsynthetic composition control and heterostructure formation in ferromagnetic Cr-based 2D perovskites for the first time, these results also advance our fundamental understanding of ion-exchange processes in this relatively unexplored family of 2D perovskites, broadening opportunities for investigation and control of novel spin effects in low-dimensional metal-halide perovskites.