Thermally Robust Narrow‐Band Deep‐Blue Emission From Eu 2+ ‐Activated Zero‐Dimensional Cs 2 ZnBr 4 Nanocrystals

High‐color‐purity deep‐blue emitters are essential for advanced display technologies targeting the Rec. 2020 standard, where narrow‐band emission is critical for achieving wide color gamut and accurate color reproduction. However, simultaneously achieving narrow emission, high efficiency, and robust stability in metal halide nanocrystals remains highly challenging. Here, a rigid 0D host‐lattice engineering strategy is demonstrated to enable thermally robust narrow‐band deep‐blue emission in Eu 2+ ‐activated Cs 2 ZnBr 4 nanocrystals. The structurally isolated [ZnBr 4 ] 2– tetrahedra create localized tetrahedral Eu 2+ luminescent centers that effectively suppress electron‐phonon coupling and spectral broadening while supporting efficient 5 d ‐4 f emission. As a result, the nanocrystals exhibit intense deep‐blue photoluminescence centered at 450 nm with a narrow full width at half maximum (FWHM) of 21 nm. Notably, the emitters retain 86.5% of their emission intensity at 150°C relative to 30°C and also show excellent photostability and environmental stability. When integrated into white light‐emitting devices, the resulting devices achieve a wide color gamut covering 127.1% of NTSC and 94.9% of Rec. 2020. This work establishes a viable strategy for developing thermally robust and high‐color‐purity emitters for next‐generation display backlight applications.