Time-resolved Excitons and Spin Dynamics in Chiral 2D Lead Halide

Sammanfattning

The ability to manipulate spin states with light forms the basis of spin-based photonics. The development of semiconductors with tunable spin properties will enable devices that use spin for information storage, circular polarized light emission, etc. With the recent reports of a chirality-induced spin selectivity (CISS) effect in electrons, the question arises whether structural chirality provides a handle to tune spin dynamics in chiral semiconductors and nanomaterials. This thesis is focused on the investigation of ultrafast exciton and spin dynamics of chiral two-dimensional (2D) lead halide perovskite thin films using circularly polarized pump-probe or transient absorption (TA) spectroscopy. The chiral (R-MBA)2PbI4 and racemic (rac-MBA)2PbI4 2D thin films were prepared using simple solution-based methods. X-ray diffraction confirmed the formation of highly ordered 2D octahedral structures of lead iodide with chiral interlayers of methylbenzylammonium. A strong circular dichroism (CD) observed close to the excitonic resonance confirmed the optical chirality of the chiral samples, which were distinguished by the lack of a CD signal from the racemic samples. To understand the influence and role of chirality on spin relaxation mechanisms, ultrafast TA measurements were performed with different pump and probe pulse polarizations (co- and counter-circular, linear). Spectral analysis identifies three features: 2 positive photoinduced absorptions and 1 negative ground state photobleaching. With multi-exponential fitting of kinetics, distinct time decay components are identified, assigned to thermalization, exciton-exciton annihilation, exciton recombination, and trapping or de-trapping process. Comparative spin dynamics between chiral and racemic samples suggest differences in the spin relaxation process, even if their kinetics are similar. Both chiral and racemic exhibit longer-lived spin polarization around 400ps. These findings are critical to understanding spin-dependent processes in chiral metal halide perovskite materials. The experimental framework presented in the study further establishes a base for probing CISS phenomena and the optimization of spin relaxation processes in emerging opto-spintronic materials.