Two-Dimensional Lead(II) Halide-Based Hybrid Perovskites Templated by Acene Alkylamines: Crystal Structures, Optical Properties, and Piezoelectricity
A series of two-dimensional (2D) hybrid organic–inorganic perovskite (HOIP) crystals, based on acene alkylamine cations (i.e., phenylmethylammonium (PMA), 2-phenylethylammonium (PEA), 1-(2-naphthyl)methanammonium (NMA), and 2-(2-naphthyl)ethanammonium (NEA)) and lead(II) halide (i.e., PbX 4 2–, X = Cl, Br, and I) frameworks, and their corresponding thin films were fabricated and examined for structure–property relationship. Several new or redetermined crystal structures are reported, including those for (NEA) 2 PbI 4, (NEA) 2 PbBr 4, (NMA) 2 PbBr 4, (PMA) 2 PbBr 4, and (PEA) 2 PbI 4 . Non-centrosymmetric structures from among these 2D HOIPs were confirmed by piezoresponse force microscopy—especially noteworthy is the structure of (PMA) 2 PbBr 4, which was previously reported as centrosymmetric. Examination of the impact of organic cation and inorganic layer choice on the exciton absorption/emission properties, among the set of compounds considered, reveals that perovskite layer distortion (i.e., Pb–I–Pb bond angle between adjacent PbI 6 octahedra) has a more global effect on the exciton properties than octahedral distortion (i.e., variation of I–Pb–I bond angles and discrepancy among Pb–I bond lengths within each PbI 6 octahedron). In addition to the characteristic sharp exciton emission for each perovskite, (PMA) 2 PbCl 4, (PEA) 2 PbCl 4, (NMA) 2 PbCl 4, and (PMA) 2 PbBr 4 exhibit separate, broad “white” emission in the long wavelength range. Piezoelectric compounds identified from these 2D HOIPs may be considered for future piezoresponse-type energy or electronic applications.