Divalent Europium-containing colloidal metal halide nanocrystals for light-emitting applications.

Lanthanide-based inorganic nanomaterials have been widely utilized as luminescent materials for broad-ranging applications in lighting, display, and optoelectronic devices. Among lanthanide elements, divalent europium (Eu²⁺) has recently gained significant attention owing to its excellent photoluminescence (PL) properties, such as a short radiative decay lifetime, narrow PL bandwidth, and wide emission range from ultraviolet to near-infrared. Particularly, colloidal metal halide nanocrystals (MHNCs) offer unique advantages as inorganic hosts for Eu²⁺ owing to their excellent phase purity, chemical and optical stability, and colloidal stability for facile integration via solution processes. In addition, the PL properties of Eu²⁺, originating from the parity-allowed 4f-5d transitions, can be precisely controlled by tuning the phase and compositions of MHNCs. Therefore, an in-depth understanding of the Eu²⁺ PL mechanism and synthesis of phase-pure MHNCs is essential for the advancement of Eu²⁺-based MHNCs as novel emitters. This review summarizes recent developments in Eu²⁺-based colloidal MHNCs and their PL properties. First, the local factors affecting the luminescence properties of Eu²⁺ in inorganic hosts are discussed. Subsequently, recent advances in the synthesis of Eu²⁺-based MHNCs using different host-dopant frameworks, their optical proprieties, and applications are outlined. This comprehensive review provides valuable insights for designing high-performance emitters, particularly for achieving deep-blue emission in light-emitting diodes and high-energy scintillators.