Revealing the Role\nof Organic Ligands in Deep-Blue-Emitting\nColloidal Europium Bromide Perovskite Nanocrystals

Europium halide perovskites are promising candidates\nfor environmentally\nbenign blue-light emitters with their narrow emission line width.\nHowever, the development of high-photoluminescence quantum yield (PLQY)\ncolloidal europium halide perovskite nanocrystals (PNCs) is hindered\nby limited synthetic methods and elusive reaction mechanisms. Here,\nwe provide an effective synthetic route for achieving high-PLQY deep-blue-emitting\ncolloidal CsEuBr<sub>3</sub> PNCs. Using two Br-organic ligand precursors,\noleylammonium bromide (OLAMHBr) and trioctylphosphine dibromide (TOPBr<sub>2</sub>), we identified distinct phase evolution routes involving\nEu<sup>2+</sup>:CsBr, Cs<sub>4</sub>EuBr<sub>6</sub>, and CsEuBr<sub>3</sub>. The OLAMHBr precursor initially promotes the formation of\nthe Eu<sup>2+</sup>:CsBr phase, which reorganizes into the CsEuBr<sub>3</sub> perovskite phase via proton transfer. In contrast, the TOPBr<sub>2</sub> precursor induces the formation of core/shell Cs<sub>4</sub>EuBr<sub>6</sub>/CsBr PNCs, which subsequently transform into CsEuBr<sub>3</sub> through nucleophilic addition. The TOPBr<sub>2</sub> route\nexhibited enhanced CsEuBr<sub>3</sub> phase homogeneity, resulting\nin a significantly higher PLQY (40.5%; full width at half-maximum\n(fwhm) = 24 at 430 nm), compared to the OLAMHBr route (16.5% at 418\nnm). Notably, the phase-pure CsEuBr<sub>3</sub> PNCs demonstrated\na world-record PLQY among the reported blue-emitting lead-free PNCs\nthat exhibit a narrow emission line width (fwhm <25 nm). This work\nhighlights the significant role of organic ligands in the colloidal\nsynthesis of CsEuBr<sub>3</sub> PNCs and their potential as nontoxic,\nsolution-processable blue-light emitters.