Luminescence Enhancement of Mn⁴⁺-Activated Fluorides via a Heterovalent Co-Doping Strategy for Monochromatic Multiplexing

Mn⁴⁺ non-equivalent doped fluorides with high color purity red emission and relatively short decay time are crucial for wide color gamut displays and emerging applications, whereas the low quantum efficiency (QE) restrains their further applications. Herein, the luminescence of Mn⁴⁺ non-equivalent doped fluoride K₂NaAlF₆:Mn⁴⁺ (KNAF:Mn⁴⁺) is significantly enhanced via a heterovalent co-doping strategy, where the luminescence intensity is obviously increased by ∼85%, but the decay time is almost unchanged. The experimental characterization and density functional theory (DFT) calculations provide an understanding of the luminescence enhancement mechanism of heterovalent co-doping, which is enabled by simultaneously improving the stability of Mn⁴⁺ and reducing the number of quenching centers (defects and impurities). Combining the short-decay-time (τ = 4.03 ms) emission KNAF:Mn⁴⁺, Mg2+ and long-decay-time (τ = 9.23 ms) emission K₂SiF₆:Mn⁴⁺, a novel monochromatic multiplexing mode in the millisecond order is presented, which can be decoded not only in high-efficiency by a digital camera but also with a high security. This work provides a new optical multiplexing for the information security applications and also inspires the design of high-efficiency Mn⁴⁺-activated luminescent materials.