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Postsynthetic vapor-phase anion exchange in the ferromagnetic two-dimensional (2D) hybrid metal-halide perovskite, PEA 2 CrCl 4 (PEA + = phenethylammonium), is reported. Anion exchange using vaporous trimethylsilyl bromide (TMS-Br) is shown to drive complete conversion of solution-processed PEA 2 CrCl 4 polycrystalline thin films to PEA 2 CrBr 4 . Low-temperature magnetic circular dichroism spectroscopy indicates ferromagnetic ordering in these PEA 2 CrCl 4 and PEA 2 CrBr 4 films. Via partial anion exchange of exfoliated flakes of PEA 2 CrCl 4 single crystals, we demonstrate that it is possible to generate abrupt lateral PEA 2 CrCl 4 /PEA 2 CrBr 4 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 MA 2 CrCl 4 (MA + = methylammonium), which instead show 3D exchange. Comparison of anion-exchange reactions in PEA 2 CrCl 4 , PEA 2 MnCl 4 , and PEA 2 PbCl 4 shows that 2D bromide diffusion is slowest in PEA 2 CrCl 4 , 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.

Moiré superlattices of layered transition metal dichalcogenides are proven to host periodic electron crystals due to strong correlation effects. These electron crystals can also be intertwined with intricate magnetic phenomena. In this Letter, we present our findings on the moiré exchange effect, resulting from the modulation of local magnetic moments by electron crystals within well-aligned WSe_{2}/WS_{2} heterobilayers. Employing polarization-resolved magneto-optical spectroscopy, we unveil a high-energy excitonic resonance near one hole per moiré unit cell (v=-1), which possesses a giant g factor several times greater than the already very large g factor of the WSe_{2} A exciton in this heterostructure. Supported by continuum model calculations, these high-energy states are found to be dark excitons brightened through Umklapp scattering from the moiré mini-Brillouin zone. When the carriers form a Mott insulating state near v=-1, the Coulomb exchange between doped carriers and excitons forms an effective magnetic field with moiré periodicity. This moiré exchange effect gives rise to the observed giant g factor for the excitonic Umklapp state.

Cesium lead halide nanostructures have highly tunable optical and optoelectronic properties. Establishing precise control in forming perovskite single-crystal nanostructures is key to unlocking the full potential of these materials. However, studying the growth kinetics of colloidal cesium lead halides is challenging due to their sensitivity to light, electron beam, and environmental factors like humidity. In this study, in situ observations of CsPbBr 3 -particle dynamics were made possible through extremely low dose liquid cell transmission electron microscopy, showing that oriented attachment is the dominant pathway for the growth of single-crystal CsPbBr 3 architectures from primary nanocubes. In addition, oriented assembly and fusion of ligand-stabilized cubic CsPbBr 3 nanocrystals are promoted by electron beam irradiation or introduction of polar additives that both induce partial desorption of the original ligands and polarize the nanocube surfaces. These findings advance our understanding of cesium lead halide growth mechanisms, aiding the controlled synthesis of other perovskite nanostructures.