Effects of surface modification on optical properties and thermal stability of K₂SiF₆:Mn⁴⁺ red phosphors by deposition of an ultrathin Al₂O₃ layer using gas-phase deposition in a fluidized bed reactor

The feasibility of coating K₂SiF₆:Mn⁴⁺ phosphor particles with an Al₂O₃ layer, in order to enhance the optical properties and improve the chemical and thermal stability, has been studied. Two types of K₂SiF₆:Mn⁴⁺ phosphor particles have been coated with a thin (3-25 nm) Al₂O₃ layer using atomic layer deposition in a fluidized bed reactor. The Al₂O₃ coating layer does not have any significant effect on the spectral excitation and emission features, but the emission intensity of conventional K₂SiF₆:Mn⁴⁺ (KSF-1) decreases, which is ascribed to the formation of undesirable MnO₂. The thermal quenching of the KSF-1 phosphor in an inert atmosphere is reduced by the Al₂O₃ coating layer. Degradation during the deposition of Al₂O₃ is prevented by using K₂SiF₆:Mn⁴⁺ particles with an undoped K₂SiF₆ shell (KSF-2). The Al₂O₃ coating layer has a positive effect on the stability of both the KSF-1 and KSF-2 phosphors in a water environment, as the Al₂O₃ layer acts as a barrier against the hydrolysis of K₂SiF₆. In air, however, water present in the Al₂O₃ coating layer enhances the degradation of the phosphor at elevated temperatures.