Stabilizing Fluoride Phosphors: Surface Modification by Atomic Layer Deposition

The use of an Al2O3- or TiO2-coated layer, deposited by using atomic layer deposition (ALD), was explored to improve the adhesion properties of fluoride phosphor particles. K2SiF6:Mn4+ was investigated as a benchmark fluoride phosphor. Mn4+-doped fluoride phosphors show a narrow red emission band centered at around 630 nm, making them suitable candidates for the red component in white light-emitting diodes. In lighting applications, red fluoride phosphors can lower the correlated color temperature without sacrificing the luminous efficacy of the radiation. In displays, their saturated red emission enlarges the color gamut. Although attractive optical properties are provided by fluoride phosphors, a remaining hurdle is their sensitivity to moisture, which leads to phosphor degradation. Protective hydrophobic shells can be used to enhance the moisture stability of phosphor materials. For fluoride phosphors, the surface fluorine inhibits a good adhesion between the core and shell, potentially leading to delamination of the shell with time. Ideally, the thin ALD-deposited seed layers provide an OH-saturated particle surface, offering an enhanced adhesion to outer moisture barrier layers or hydrophobic encapsulants. The results obtained indicate that the Al2O3 seed layers suffered from delamination and blistering after deposition, related to surface reactions during ALD. In contrast to Al2O3, the TiO2 layer could be grown with high uniformity and conformality. In a final step, the ALD-coated fluoride powders were successfully made hydrophobic using a nonchlorinated hydrophobic precursor.