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

In the semiconducting perovskite materials family, the cesium-lead-chloride compound (CsPbCl) supports robust excitons characterized by a blue-shifted transition and the largest binding energy, thus presenting a high potential to achieve demanding solid-state room-temperature photonic or quantum devices. Here we study the fundamental emission properties of cubic-shaped colloidal CsPbCl nanocrystals (NCs), examining in particular individual NC responses using micro-photoluminescence in order to unveil the exciton fine structure (EFS) features. Within this work, NCs with average dimensions ⟨⟩ ≈ 8 nm (α = , , ) are studied with a level of dispersity in their dimensions that allows disentangling the effects of size and shape anisotropy in the analysis. We find that most of the NCs exhibit an optical response under the form of a doublet with crossed polarized peaks and an average inter-bright-state splitting, Δ ≈ 1.53 meV, but triplets are also observed though being a minority. The origin of the EFS patterns is discussed in the frame of the electron-hole exchange model by taking into account the dielectric mismatch at the NC interface. The different features (large dispersity in the Δ values and occasional occurrence of triplets) are reconciled by incorporating a moderate degree of shape anisotropy, observed in the structural characterization, by preserving the relatively high degree of the NC lattice symmetry. The energy distance between the optically inactive state and the bright manifold, Δ, is also extracted from time-resolved photoluminescence measurements (Δ ≈ 10.7 meV), in good agreement with our theoretical predictions.