Enhancing thermal degradation stability of BaSi<sub>2</sub>O<sub>2</sub>N<sub>2</sub>:Eu<sup>2</sup><sup>+</sup> for white light-emitting diodes by ultra-thin Al<sub>2</sub>O<sub>3</sub> layer via atomic layer deposition

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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.