Synthetic Variation and Structural Trends in Layered Two-Dimensional Alkylammonium Lead Halide Perovskites

We report the cooling-induced crystallization of layered two-dimensional (2D) lead halide perovskites with controllable inorganic quantum-well thicknesses (n = 1, 2, 3, and 4), organic-spacer chain lengths (butyl-, pentyl-, and hexylammonium), A-site cations (methylammonium and formamidinium), and halide anions (iodide and bromide). Using single-crystal X-ray diffraction, we refined crystal structures for the iodide family as a function of these compositional parameters and across their temperature-dependent phase transitions. In general, lower-symmetry crystal structures, increasing extents of organic-spacer interdigitation, and increasing organic-spacer corrugation tilts are observed at low temperature. Moreover, greater structural distortions are observed in lead halide octahedra closest to the organic-spacer layer, and higher-n structures exhibit periodic variation in Pb-I bond lengths. These structural trends are used to explain corresponding temperature-dependent changes in the photoluminescence spectra. We also provide detailed guidance regarding the combination of synthetic parameters needed to achieve phase-pure crystals of each composition and discuss difficulties encountered when trying to synthesize particular members of the 2D perovskite family containing formamidinium or cesium as the A-site cation. These results provide a foundation for understanding structural trends in 2D lead halide perovskites and the effects these trends have on their thermal, electronic, and optical properties.