Bright Colloidal Deep-Blue-Emissive Europium Halide Nanocrystals

Halide perovskite nanocrystals (PNCs) are gaining significant attention as light-emitting materials due to their high color purity and facile color tuning, coupled with the ease of solution-based processing. However, a significant limitation for industrialization lies in the toxic nature of Pb-based perovskites. Additionally, the photoluminescence quantum yield (PLQY) and stability of blue-emitting PNCs have lagged behind compared to the red and green counterparts.<br/>To address these challenges, europium (Eu)-based nanocrystals, which emit deep-blue light, have been proposed as a key eco-friendly alternative. However, the inherent properties of Eu precursors: (i) low solubility and (ii) strong organic coordination, present significant obstacles for colloidal synthesis of Eu-based nanocrystals. So far, reported colloidal Eu-based PNCs have not achieved a high PLQY exceeding 6%.<br/>In this study, we present a systematic synthetic strategy of colloidal Eu-based nanocrystals with high PLQYs. We controlled the chemical interaction between the precursors and ligands, along with modulation of reaction temperature and duration. These controlled reactions led to successful synthesis of deep-blue-emitting Eu²⁺-doped CsBr, Cs₄EuBr₆/CsBr and CsEuBr₃ nanocrystals. Notably, the synthesized colloidal CsEuBr₃ nanocrystals exhibited a high PLQY of ~40% and a narrow spectral linewidth. Furthermore, we investigated the chemical reactions between the precursors and ligands in-depth, suggesting the synthetic mechanisms of the colloidal Eu-based nanocrystals. This study introduces great potential of Eu-based nanocrystals for solution-processable non-toxic blue light emitters exhibiting both high PLQY and high color purity.