K2SiF6:Mn4+@K2SiF6 phosphor with remarkable negative thermal quenching and high water resistance for warm white LEDs

Poor water resistance and luminescent thermal stability remain great challenges for commercial narrow red emitting Mn4+-doped fluoride phosphors for white light emitting diodes (WLEDs). In this work, a simple coating method is developed to synthesize K2SiF6:0.06Mn4+@0.10K2SiF6 red emitting composite phosphor with excellent water resistance, luminescent thermal stability and high efficiency. After immersing into water for 300 min, the optimal sample maintains 88% of the initial integrated PL intensity, whereas that of uncoated control sample steeply drops down to 1%. A remarkable negative-thermal-quenching effect is observed that the integrated PL intensities at 120, 150, 180 and 210 °C are 176, 198, 214 and 213% of that initial one at 30 °C. The mechanism of above behaviors are discussed and proposed: The K2SiF6 shell layer not only prevents the hydrolysis of Mn4+ by water and but also cuts off the transfer of energy to surface defects, leads to the increase of radiation transition probability faster than that of non-radiation transition with the increase of temperature. These findings not only illustrate the multiple effects of coating K2SiF6 on the luminescent performances of K2SiF6:Mn4+ red emitting phosphor, but also provide a facile strategy to improve the water resistance as well as luminescent thermal stability of Mn4+-doped fluoride phosphors for warm WLEDs in the future.