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.