Mn⁴⁺-Doped Fluoride Nanocrystals Enable High-Resolution Red-Emitting X-ray Imaging Screens

Mn4+-doped fluorides are a growing class of narrow-band red-emitting phosphors for lighting and display technologies. However, we lack a method for synthesizing these materials as colloidal nanocrystals (NCs), making their solution processing difficult and limiting their potential applications. Herein, we propose a colloidal approach to the preparation of Mn4+-doped inorganic hexafluoride (AxB′F6:Mn4+, in which A = K, Cs and B′ = Al, Si, Sn) NCs with narrow size distributions. Our method can be extended to different matrices and dopant compositions. Furthermore, it permits the fine-tuning of the Mn4+ concentration in the NCs. The AxB′F6:Mn4+ NCs exhibit narrow-band bright-red emission with a photoluminescence quantum yield of up to ∼73% and long-term stability, and they can form colloidal solutions in various polar and nonpolar solvents. The solution processability of the NCs and their small size enabled the fabrication of transparent and flexible NC-polymer composite films, which we were fashioned into X-ray imaging scintillators. These scintillators exhibit excellent performance with a high X-ray imaging resolution of 20 lp/mm, and a relatively low detection limit of 720 nGy/s. Our synthesis strategy makes way for the exploration of a new class of red-emitting colloidal NCs for various downconversion applications.