Twisting Photons with Chiral Metal-Halide Semiconductors

Chiral metal-halide semiconductors (MHS) have recently developed as promising candidates for spin- and polarization-resolved optoelectronic devices. Although several chiral MHS with rich chemical and structural diversity have been reported lately, the fundamental mechanisms governing their chiroptical activity, namely, circularly polarized absorption and emission, remain elusive. In this talk, I will discuss our recent progress in understanding and tuning the chiroptical activity in chiral MHS. I will first discuss how the chirality is transferred from organic to inorganic component through asymmetric covalent bonding interactions. Their endowed molecular chirality was then studied by circular dichroism (CD). However, we found that the previously reported “apparent” CD in chiral MHS thin films is not an intrinsic chiroptical property, but rather, arising from an interference between the film’s linear birefringence and linear dichroism. We verify the presence of LB and LD effects in both one-dimensional and zero-dimensional chiral MHS thin films. We then establish spectroscopic methods 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 is then uncovered, which is well described by the chirality-induced spin–orbit coupling in the chiral structures. Meanwhile, we found that high CD signals do not necessarily lead to high circularly polarized luminescence as most of the current chiral MHS display very low photoluminescence quantum yields (PLQY). We will then discuss the reasons of low PLQY in these materials. Finally, we will show our strategies to turn on the circularly polarized luminescence by introducing extrinsic self-trapped excitons in a 0D chiral MHS, and by tuning the inorganic layer thickness in perovskite nanoplatelets.