Exciton Fine Structure of CsPbCl₃ Nanocrystals: An Interplay of Electron-Hole Exchange Interaction, Crystal Structure, Shape Anisotropy, and Dielectric Mismatch

In the semiconducting perovskite materials family, thecesium-lead-chloridecompound (CsPbCl3) supports robust excitons characterizedby a blue-shifted transition and the largest binding energy, thuspresenting a high potential to achieve demanding solid-state room-temperaturephotonic or quantum devices. Here we study the fundamental emissionproperties of cubic-shaped colloidal CsPbCl3 nanocrystals(NCs), examining in particular individual NC responses using micro-photoluminescencein order to unveil the exciton fine structure (EFS) features. Withinthis work, NCs with average dimensions & LeftAngleBracket;L (& alpha;)& RightAngleBracket; & AP; 8 nm (& alpha; = x, y, z) are studied with a levelof dispersity in their dimensions that allows disentangling the effectsof size and shape anisotropy in the analysis. We find that most ofthe NCs exhibit an optical response under the form of a doublet withcrossed polarized peaks and an average inter-bright-state splitting,& UDelta;( BB ) & AP; 1.53 meV, but tripletsare also observed though being a minority. The origin of the EFS patternsis discussed in the frame of the electron-hole exchange modelby taking into account the dielectric mismatch at the NC interface.The different features (large dispersity in the & UDelta;( BB ) values and occasional occurrence of triplets) arereconciled by incorporating a moderate degree of shape anisotropy,observed in the structural characterization, by preserving the relativelyhigh degree of the NC lattice symmetry. The energy distance betweenthe optically inactive state and the bright manifold, & UDelta;( BD ), is also extracted from time-resolved photoluminescencemeasurements (& UDelta;( BD ) & AP; 10.7meV), in good agreement with our theoretical predictions.