The Stokes Shift and Exciton Fine Structure in Strongly Confined CsPbBr 3 Perovskite Nanoplatelets.

The Stokes shift is negligible in bulk perovskites but large in two-dimensional (2D) CsPbBr 3 perovskites. The issue has attracted a lot of discussion, but it remains controversial. Here, we report the temperature-dependent absorption and photoluminescence (PL) spectra of CsPbBr 3 perovskite nanoplatelets (NPLs). We observe a temperature-dependent Stokes shift changing from 26 to 41 meV. This phenomenon was attributed to the exciton fine structure according to the great difference in peak width. The triple bright exciton levels all participate in the absorption process and result in a wide absorption peak, while only the lowest exciton level contributes to photon emission and exhibits a relatively narrow PL peak. The PL decay curves also present the characterization of bright and dark exciton couplings at low temperatures. The splitting of triple bright excitons is induced by the morphology anisotropy of the 2D structure, so the large Stokes shift is proposed to be an intrinsic property of 2D perovskites.