The Evolution of Quantum Confinement in CsPbBr3 Perovskite Nanocrystals

Abstract

Colloidal nanocrystals (NCs) of lead halide perovskites are considered highly promising materials that combine the exceptional optoelectronic properties of lead halide perovskites with tunability from quantum confinement. But can we assume that these materials are in the strong confinement regime? Here, we report an ultrafast transient absorption study of cubic CsPbBr3 NCs as a function of size, compared with the bulk material. For NCs above ∼7 nm edge length, spectral signatures are similar to the bulk material–characterized by state-filling with uncorrelated charges–but discrete new kinetic components emerge at high fluence due to bimolecular recombination occurring in a discrete volume. Only for the smallest NCs (∼4 nm edge length) are strong quantum confinement effects manifest in TA spectral dynamics; focusing toward discrete energy states, enhanced bandgap renormalization energy, and departure from a Boltzmann statistical carrier cooling. At high fluence, we find that a hot-phonon bottleneck effect ...