Structural determination, energy transfer and color-tunable photoluminescence of stable LiYSiO4: Ce/Tb/Sm Phosphors toward nUV pumped wLEDs application

Study on photon emission of Ce ³⁺-doped phosphors is essential for advanced optoelectronic application. However, it remains quite difficult to design near-ultraviolet excitable Ce ³⁺-activated silicate blue/cyan-emitting phosphors because of the combined influence of iconicity of Ce ³⁺, crystal field strength, and nephelauxetic effect. Herein, Ce ³⁺ incorporated into a proper host LiYSiO ₄ (LYS) phosphor is mechanistically investigated and shown efficient blue/cyan photoluminescence (PL) owing to Ce ³⁺ 5d ₁ electronic state experiencing appropriate crystal field splitting. The comprehensive analysis based on structural refinement, dynamic/static spectra, and density functional theory calculations indicates that Ce ³⁺ ions are energetically favored in octahedral Y site as compared with Li site in the host, resulting in attractive PL properties. According to the principle of color superposition and via cascading energy transfer (ET) from Ce ³⁺ → Tb ³⁺ → Sm ³⁺, systematic PL tuning in a large color gamut is realized by further codoping with the green- and red-emissions of Tb ³⁺ and Sm ³⁺ in the phosphor LYS: Ce ³⁺/Tb ³⁺/Sm ³⁺. The direct ET from Ce ³⁺ → Sm ³⁺ is difficult to occur due to the metal-metal charge transfer effect. Of note, the PL thermal-quenching of Ce ³⁺ is mainly associated with multiphonon relaxation, while the quenching of Tb ³⁺/ Sm ³⁺ is primarily due to the cross relaxation (CR) process via dipole-dipole (d-d) interaction. The appropriate distance between the two nearest adjacent Y sites in the host together with the ET are conducive to weaken the CR process of Tb ³⁺/Sm ³⁺, leading to high PL efficiency. Finally, a prototype pc- wLED assembled via a remote ‘capping’ packaging strategy and by using only the stably composition-optimized white-emitting LYS: Ce ³⁺/Tb ³⁺/Sm ³⁺ demonstrates that the phosphor is promising for high-quality of light.