Twisting Photons with Chiral Metal-Halide Perovskites

Over the past few decades, metal-halide perovskites have garnered widespread research interest due to their extraordinary semiconductive properties and facile synthetic routes. In recent years, the incorporation of chiral organic cations into the perovskite lattice structure induces symmetry breaking. Thus, these perovskites crystallize in chiral space group, with helical packing pattern in the structure. It offers opportunities to precisely control the behavior (i.e. angular momentum) of photons, phonons and electrons within the perovskite. Thus, chiral perovskites exhibit attractive chiroptical and spintronic properties, which involves the polarization of light and electrons. Since chiral perovskites being reported, the unusual chiroptical and spintronic properties have been extensively investigated. However, several key questions remain unsolved. For instance, whether the genuine circular dichroism in chiral perovskite thin film is interfered by external factors during the measurement; the structure-CD relationship in chiral perovskite is still underexplored; whether magnetic doping strategy in bulk chiral perovskite will generate emerging properties; the relationship between chiroptical and spintronic properties within chiral 2D perovskites is unknown. This thesis summarizes my three PhD projects, which aim to solve the questions being aroused above. In the first project, through spectroscopic measurement and Muller matrix analysis, we discovered that the previously reported “apparent” anisotropy factor measured from circular dichroism (CD) in chiral MHS thin films is not an intrinsic chiroptical property, dimensional and zero-dimensional chiral MHS thin films. Spectroscopic methods was established to decouple the genuine CD from other spurious contributions, which allows a quantitative comparison of the intrinsic chiroptical activity across different chiral MHS. The relationship between the structure and the genuine chiroptical activity was then uncovered, which is well described by the chirality-induced spin–orbit coupling in the chiral structures. Our study unveils the macroscopic origin of chiroptical activity of chiral MHS and provides design principles for obtaining high anisotropic factors for future chiral optoelectronic applications. In the second project, we synthesized a Mn2+-doped chiral two-dimensional (2D) perovskite, We found that efficient energy transfer from the chiral host to the Mn2+ dopants was observed. This energy transfer process gives rise to circularly polarized luminescence from the excited state of Mn2+ (4T1 → 6A1), exhibiting a photoluminescence quantum yield up to 24% and a dissymmetry factor of 11%. The exciton fine structures of undoped and Mn2+-doped (R-MPA)2PbBr4 were further studied through magnetic circular dichroism (MCD) spectroscopy. Our analysis shows that chiral organic cations lead to an exciton fine structure splitting energy as large as 5.0 μeV, and the splitting is further increased upon Mn2+ doping. Our results reveal the strong impacts of molecular chirality and magnetic dopants on the exciton structures of halide perovskites. In the third project, we used magnetic circular dichroism to probe the exciton spin splitting in a series of chiral 2D perovskites. Our results show that the anisotropy factor of circular dichroism is indeed proportional to the exciton spin splitting energy, with larger splitting energy yielding larger anisotropy factors. Further structural analysis showed that the splitting energy is closely correlated with both the in-plane and out-of-plane distortion structural parameters of the inorganic lattice. Our work provides an important mechanistic understanding of chiroptical activity and establishes the structure–property relationship for 2D chiral perovskites