The Stokes Shift and Exciton Fine Structure in Strongly Confined CsPbBr3 Perovskite Nanoplatelets

The Stokes shift is negligible in bulk perovskites butlarge intwo-dimensional (2D) CsPbBr3 perovskites. The issue hasattracted a lot of discussion, but it remains controversial. Here,we report the temperature-dependent absorption and photoluminescence(PL) spectra of CsPbBr3 perovskite nanoplatelets (NPLs).We observe a temperature-dependent Stokes shift changing from 26 to41 meV. This phenomenon was attributed to the exciton fine structureaccording to the great difference in peak width. The triple brightexciton levels all participate in the absorption process and resultin a wide absorption peak, while only the lowest exciton level contributesto photon emission and exhibits a relatively narrow PL peak. The PLdecay curves also present the characterization of bright and darkexciton couplings at low temperatures. The splitting of triple brightexcitons is induced by the morphology anisotropy of the 2D structure,so the large Stokes shift is proposed to be an intrinsic propertyof 2D perovskites.