Colloidal CsPbBr ₃ Nanoplatelets at the Single-Particle Level: An Optical and Theoretical Study

We investigate the exciton fine structure of strongly confined colloidal CsPbBr₃ nanoplatelets at the single-particle level using polarization-resolved micro-photoluminescence and energy- and time-resolved spectroscopy. Bright-bright and bright-dark exciton splittings increase from 1 to 3 meV and 12 to 21 meV, respectively, as NPL thickness varies from 2 to 3 monolayers. Phonon-assisted relaxation pathways via dark excitons are also identified. Effective mass modeling, including finite barrier potential, dielectric confinement, crystal field symmetry, and electron-hole exchange interaction, accurately reproduces our measurements. Our theoretical predictions align remarkably well with prior spectroscopic studies, emphasizing the decisive influence of tetragonal and orthorhombic crystal fields together with in-plane anisotropy on excitonic splittings. This combined experimental-theoretical framework provides design principles for tailoring the exciton symmetry and energy levels and controlling the polarization-selective emission, paving the way for their optimized integration into next-generation photonic and optoelectronic devices.