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Two-dimensional metal halide perovskite nanocrystals are attractive for their blue-shifted absorption relative to their 3D counterparts, narrow emission line widths, high absorption cross sections, and high radiative recombination. Herein, we report a ligand mediated anion exchange reaction that transforms CsPbCl3nanocubes into anisotropic CsPbX3nanoplates (NPs) through adding a mixture of dodecanethiol (DDT) and AlX3(X = Cl, Br, I) to a solution of CsPbCl3nanocrystals. The anion-exchanged nanocrystals consist of mostly plate-shaped CsPbX3nanocrystals along {010} facets. Although DDT or AlX3treatment alone results in increased photoluminescence quantum yield, ΦPL, the combination of both DDT and AlX3leads to the formation of plate-shaped nanocrystals, higher ΦPL, and improved stability as compared to when only one of these is used. We suspect nanocrystal exfoliation driven by ligand penetration and self-assembly both play important roles in transforming the parent nanocubes into NPs. The NPs are highly emissive and stable due to the introduction of both an anion source and surface ligand. Nanoplates synthesized through DDT-AlX3ligand mediated anion exchange show photoluminescence quantum yields of 47 and 65% for violet and blue emitters (CsPbCl3and CsPbCl3–xBrx), respectively, near 100% for green emitters (CsPbBr3), and 81% for red emitters (CsPbI3), while maintaining good long-term stability.

Two-dimensional halide perovskites (2D-HPs) are of significant interest for their applications in optoelectronic devices. Part of this increased interest in 2D-HPs stems from their increased stability relative to their three-dimensional (3D) counterparts. Here, the origin of higher stability in 2D-HPs is mainly attributed to the bulky ammonium cation layers, which can act as a blocking layer against moisture and oxygen ingression and ion diffusion. While 2D-HPs have demonstrated increased stability, it is not clear how the structure of the ammonium ions impacts material stability. Herein, we investigate how the structure of ammonium cations, including three n-alkyl ammoniums, phenethylammonium (PEA) and five PEA derivatives, anilinium (An), benzylammonium (BzA), and cyclohexylmethylammonium (CHMA), affects the crystal structure and air, water, and oxygen stability of 2D tin halide perovskites (2D-SnHPs). We find that stability is influenced by several factors, including the molecular packing and intermolecular interactions in the organic layer, steric effects around the ammonium group, the orientation distribution of the 2D sheets, and the hydrophobicity of the perovskite film surface. With superior hydrophobicity, strong interactions between organic molecules, and a high extent of parallel oriented inorganic sheets, the 2-(4-trifluoromethyl-phenyl)-ethylammonium (4-TFMPEA) ion forms the most stable 2D-SnHP among the 12 ammonium cations investigated.