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

Abstract

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.

Popular Abstract

When we imagine light, we usually think of brightness or colour. Light can also be seen as energy packets called photons, which have their own ”twist” or ”polarization”. These twists can be associated with the spin of electrons in certain materials, which can be controlled with light. As these spins can be controlled using light, it opens the door to novel research and technologies where information is stored and processed not only through charge, but also through spin. This interesting field is known as opto-spintronics.

This thesis work delves into the ultrafast realm to study time-resolved excitons and spin dynamics in chiral two-dimensional lead halide perovskites. Chiral means the mirror images of the structure or material do not match perfectly, and a classic example is our hand (left and right). This handedness characteristic means the material interacts with light differently when compared with left- or right-circularly polarized light. It is like how a right glove does not fit our left hand. It is not just a curiosity, but this is a unique behaviour of chirality that influences the behaviour of electron spins. This phenomenon is called chirality-induced spin selectivity or CISS.

We fabricated thin films of chiral and non-chiral (racemic) perovskite to explore selective chirality and compared their features. Films are usually made using a solution-based process, and basic characterizations are done to confirm the growth of crystals and other properties. X-ray Diffraction (XRD) was used to check their structure, and chiroptical activity or circular dichroism (CD) by shining circularly polarized light and measuring the differential absorbance. The chiral thin film samples absorbed one twist of light more than the other, whereas the racemic sample showed no such difference. Then, using modified ultrafast circularly (left/co-circular, or right/counter-circular) polarized pump-probe spectroscopy, we observed how spins and excitons behave over a picosecond scale (trillionths of a second).

• author: Bhagwat, Pratik Bharat
• supervisor: Tönu Pullerits; Sankaran Ramesh
• organization: Department of Physics
• course: PHYM03 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy; Chemistry; Technology and Engineering
• keywords: Ultrafast spectroscopy, circularly polarized pump-probe spectroscopy, transient absorption (TA), chirality, perovskite, X-ray diffraction (XRD), circular dichroism (CD), exciton, spin dynamics.
• language: English
• id: 9211781
• date added to LUP: 2025-09-16 08:29:17
• date last changed: 2025-09-16 08:29:17