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

Abstract

Study on photon emission of Ce3+-doped phosphors is essential for advanced optoelectronic application. However, it remains quite difficult to design near-ultraviolet excitable Ce3+-activated silicate blue/cyan-emitting phosphors because of the combined influence of iconicity of Ce3+, crystal field strength, and nephelauxetic effect. Herein, Ce3+ incorporated into a proper host LiYSiO4 (LYS) phosphor is mechanistically investigated and shown efficient blue/cyan photoluminescence (PL) owing to Ce3+ 5d1 electronic state experiencing appropriate crystal field splitting. The comprehensive analysis based on structural refinement, dynamic/static spectra, and density functional theory calculations indicates that Ce3+ 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 Ce3+ → Tb3+ → Sm3+, systematic PL tuning in a large color gamut is realized by further codoping with the green- and red-emissions of Tb3+ and Sm3+ in the phosphor LYS: Ce3+/Tb3+/Sm3+. The direct ET from Ce3+ → Sm3+ is difficult to occur due to the metal-metal charge transfer effect. Of note, the PL thermal-quenching of Ce3+ is mainly associated with multiphonon relaxation, while the quenching of Tb3+/ Sm3+ 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 Tb3+/Sm3+, 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: Ce3+/Tb3+/Sm3+ demonstrates that the phosphor is promising for high-quality of light.