Revealing the Role of Organic Ligands in Deep-Blue-Emitting Colloidal Europium Bromide Perovskite Nanocrystals.

Europium halide perovskites are promising candidates for environmentally benign blue-light emitters with their narrow emission line width. However, the development of high-photoluminescence quantum yield (PLQY) colloidal europium halide perovskite nanocrystals (PNCs) is hindered by limited synthetic methods and elusive reaction mechanisms. Here, we provide an effective synthetic route for achieving high-PLQY deep-blue-emitting colloidal CsEuBr PNCs. Using two Br-organic ligand precursors, oleylammonium bromide (OLAMHBr) and trioctylphosphine dibromide (TOPBr), we identified distinct phase evolution routes involving Eu:CsBr, CsEuBr, and CsEuBr. The OLAMHBr precursor initially promotes the formation of the Eu:CsBr phase, which reorganizes into the CsEuBr perovskite phase via proton transfer. In contrast, the TOPBr precursor induces the formation of core/shell CsEuBr/CsBr PNCs, which subsequently transform into CsEuBr through nucleophilic addition. The TOPBr route exhibited enhanced CsEuBr phase homogeneity, resulting in a significantly higher PLQY (40.5%; full width at half-maximum (fwhm) = 24 at 430 nm), compared to the OLAMHBr route (16.5% at 418 nm). Notably, the phase-pure CsEuBr PNCs demonstrated a world-record PLQY among the reported blue-emitting lead-free PNCs that exhibit a narrow emission line width (fwhm <25 nm). This work highlights the significant role of organic ligands in the colloidal synthesis of CsEuBr PNCs and their potential as nontoxic, solution-processable blue-light emitters.