The cyan-emitting BaSi₂O₂N₂:Eu²⁺ phosphor is a promising narrow-band and high-efficiency luminescent material used in wide-color-gamut white light-emitting diodes (wLEDs). However, its serious degradation under thermal attacks hinders its practical applications and needs to be improved. Herein, we proposed to deposit a nano-sized Al₂O₃ film around each BaSi₂O₂N₂:Eu²⁺ particle through atomic layer deposition (ALD) in a fluidized bed reactor to improve its thermal stability. Thermal gravimetric analysis results showed that the Al₂O₃ layer with a thickness of only 11 nm had an obvious anti-oxidization effect, by which the oxidation temperature in air of the Al₂O₃ coated phosphor was largely increased from ∼550 to ∼750 °C. Moreover, the Al₂O₃ coated phosphor remained 93% of its luminescence intensity in comparison to 73% of the uncoated one when degraded under water-steam at 200 °C for 24 h. The oxidization of both the BaSi₂O₂N₂ host matrix and the doped Eu²⁺ ions was reduced by the Al₂O₃ layer. Meanwhile, the wLEDs fabricated with the Al₂O₃ coated phosphor showed a luminous flux of 3 times higher than that of the uncoated one when aged under 100 mA for 300 h. The greatly improved thermal degradation property of BaSi₂O₂N₂:Eu²⁺ phosphor and the reliability of the wLEDs indicate that the ALD approach could be a feasible route to produce uniform and nano layers on phosphors and enhance their stability.

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.

The red-emitting Sr ₂ Si ₅ N ₈ :Eu ²⁺ phosphor with a superior quantum efficiency and suitable emission spectrum has been widely used as a promising down-conversion material in white light-emitting diodes. However, its thermal degradation under high temperature handicaps its large scale application, and therefore must be reduced. Here, we proposed to increase the thermal stability of Sr ₂ Si ₅ N ₈ :Eu ²⁺ by coating a nanometer-order Al ₂ O ₃ film on each phosphor particle using an atomic layer deposition approach in a fluidized bed reactor. The deposited Al ₂ O ₃ layer was quite uniform and conformal when using O ₃ as the oxidizer, and its thickness could be controlled by the dosage type, deposition temperature and cycle numbers, which largely affects the photoluminescence properties and thermal degradation of the title phosphor. Thermal gravimetric analysis results showed that the oxidation temperature of the coated phosphor increased from 700 to 850 °C, suggesting that the coating layer has the function of anti-oxidation. Meanwhile, the coated phosphor particle surface became hydrophobic. Consequently, the thermal degradation of phosphor powders in air at 200 °C was greatly reduced and the stability of the fabricated LEDs with coated powders was also improved. Prospectively, the proposed approach provides a new strategy to improve the thermal stability of other phosphors.