Manipulating Chirality of 2D Lead Halide Perovskites via Mixed Chiral and Achiral Organic Ligands

Hybrid organic-inorganic halide perovskites have emerged as an important family of materials for optoelectronics with broader applications in solar cells, photodetectors, light-emitting diodes (LEDs), and lasers. As ionic semiconductors, the flexible crystal structures and tunable compositions make it possible to rationally design hybrid halide perovskites with desired properties. Chiral halide perovskites demonstrate such possibility and the breakthrough with respect to the development of a new class of chiral semiconductors, which opens up new applications of hybrid perovskites to chiroptoelectronics, ferroelectrics, and spintronics. One type of chiral perovskites is based on two-dimensional (2D) hybrid organic-inorganic halide perovskites by incorporating chiral organic ligands between achiral inorganic single layers composed of corner-sharing metal-halide octahedra. As a new class of chiral semiconductors, the chirality of hybrid 2D perovskites is attributed to the symmetry-breaking in the inorganic framework induced by the enantiopure chiral organic cations via asymmetric hydrogen bonding interactions, which transfers the structural chirality across the organic-inorganic interface. In this work, we demonstrate that the chirality of 2D halide perovskites can be widely tuned via incorporation of mixed chiral and achiral organic ligands in the organic layer. A broad range of alkyl and aryl chiral and achiral cations are used in forming 2D chiral halide perovskites. Additionally, we also synthesized semiconducting organic cations and integrated into the 2D chiral halide perovskites. To reveal the origin of the chirality, we conducted density functional theory (DFT) calculations to gain the insights of hydrogen bonding and octahedral structure as well as their relationship to the electronic band structure and spin spilt. We also conducted time dependent DFT to calculate circular dichroism (CD) spectra and to understand the correlation of rotatory strength at the orbital symmetry level. We performed temperature dependent synchrotron powder X-ray diffraction (PXRD) and pair distribution function (PDF) measurements and analysis to reveal the structural properties. To understand the chiroptoelectronics embedded in these new chiral perovskites, we conducted temperature dependent circularly polarized photoluminescent (CPPL) and conventional and circularly polarized transient absorption spectroscopy (TAS) measurements. While many phenomena are unknown and needed further investigations, this work provides a new way to manipulate chirality of 2D perovskites, which could lead to broader applications.