Controls and evidence for assigning europium emission in Eu-treated CsPbCl₃ nanocrystals

Lanthanide doping of semiconductor nanocrystals is widely pursued to enhance luminescence via photosensitization, and lead halide perovskite nanocrystals have been frequently reported as effective hosts due to their large absorption cross sections. Here we synthesized cubic-CsPbCl₃ nanocrystals in the presence of Eu³⁺ precursors, which have been reported to exhibit strong photosensitized Eu³⁺ emission, and reproduce the emission motifs reported for Eu-treated CsPbCl₃ nanocrystals. Using centrifugation fractionation together with control experiments and spectral comparisons, we find that the ensemble Eu³⁺ line emission is dominated by Eu-ligand complex byproducts that persist through standard processing. Transmission electron microscopy confirms the presence of cubic CsPbCl₃ nanocrystals, and spot energy-dispersive X-ray spectroscopy detects Eu on individual cubic particles. Single-particle spectroscopy further reveals dilute emissive particles exhibiting anticorrelated host emission and an anomalous broad band, which we assign to a lanthanide-centered charge-transfer state (CTS), indicating a competing pathway that disfavors Eu³⁺ f-f emission from the perovskite host. Overall, these results decouple evidence for Eu-containing CsPbCl₃ nanocrystals from the origin of ensemble Eu³⁺ line emission, motivate purification and identification protocols, and demonstrate the utility of single-particle spectroscopy for assessing lanthanide-doped semiconductor nanocrystals.