Enhanced highly thermal-humidity reliability of (Sr,Ca)AlSiN₃:Eu²⁺ phosphors via rapid plasma-assisted silica surface coating

(Sr, Ca)AlSiN₃:Eu²⁺ (SCASN) phosphors are widely applied in white LEDs to enhance the red spectral component, thereby improving spectral coverage and potentially contributing to a higher color rendering index. However, the rapid degradation of SCASN phosphors under highly thermal and thermal-humidity conditions restricts broader application. In this study, silica-coated SCASN phosphors were prepared via rapid plasma-assisted film deposition. The 54 nm silica coating significantly altered the wettability, optical properties, and stability of the phosphors. After the silica surface coating, the water contact angle increased from 20° to 125.5°, indicating enhanced hydrophobicity. Further photoluminescence (PL) test results demonstrated an improvement in intensity for the coated phosphor compared to the uncoated phosphor. Moreover, after aging at 100°C and 100% relative humidity for 336 hours, the PL intensity of the uncoated phosphor decreased to 65% of its original value, while the coated phosphor maintained 88.5%. This excellent stability is attributed to the protective silica coating, which acts as a barrier against moisture penetration and prevents the hydrolysis of the phosphor surface. First-principles density functional theory calculations further demonstrated that unsaturated O-Si groups formed during HDMSO decomposition create a silica coating, reducing the surface H₂O adsorption energy from -2.544 eV to -0.062 eV. The plasma-assisted deposition method effectively enhances phosphor stability, promoting the development of durable and high-performance LED phosphors.