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

Postsynthetic vapor-phase anion exchange in the ferromagnetic two-dimensional (2D) hybrid metal-halide perovskite, PEACrCl (PEA = phenethylammonium), is reported. Anion exchange using vaporous trimethylsilyl bromide (TMS-Br) is shown to drive complete conversion of solution-processed PEACrCl polycrystalline thin films to PEACrBr. Low-temperature magnetic circular dichroism spectroscopy indicates ferromagnetic ordering in these PEACrCl and PEACrBr films. Via partial anion exchange of exfoliated flakes of PEACrCl single crystals, we demonstrate that it is possible to generate abrupt lateral PEACrCl/PEACrBr 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 MACrCl (MA = methylammonium), which instead show 3D exchange. Comparison of anion-exchange reactions in PEACrCl, PEAMnCl, and PEAPbCl shows that 2D bromide diffusion is slowest in PEACrCl, 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.