Tailoring Exciton Spin Dynamics via Quantum Confinement in CsPbBr3 Nanoplatelets.

Understanding how quantum confinement governs spin-resolved dynamics is essential for the development of perovskite-based spin photonics. Here, we systematically investigate exciton spin relaxation in thickness-tunable CsPbBr3 nanoplatelets via circularly polarized transient absorption spectroscopy. At early delay times (∼0.4 ps), state-filling and Pauli exclusion lead to photoinduced absorption signals on opposite energetic sides of the exciton bleach under different circular polarizations. The same polarization spectra exhibit a thickness-dependent sign reversal on the low-energy side of the exciton bleach, arising from the shifting balance between band gap renormalization and rapid radiative recombination. Furthermore, the spin lifetime increases from 0.27 ps for 2-monolayer nanoplatelets to 1.22 ps for 6-monolayer nanoplatelets due to reduced spatial wave function overlap and suppressed exchange interactions. This work provides a crucial framework for designing perovskite gain media and spin-optoelectronic devices.