Luminescent behaviour of SrF2 and CaF2 crystals doped with Eu ions under different annealing temperatures

Highlights: • Nanocrystalline alkaline earth fluoride materials influence the Eu ion emission stability. • Annealing treatment enhanced the stability of Eu emission. • Existence of impurities in a fluoride material alters the Eu emission characteristics. -- Abstract: The photoluminescent (PL) properties of europium (Eu) doped SrF₂ and CaF₂ nanoparticles have been investigated after annealed in an Ar/H₂ atmosphere at different temperatures for possible down-conversion applications in solar cells. PL investigations revealed that the Eu doped samples exhibited emission from both Eu²⁺ and Eu³⁺ ions. The emission spectrum's shape of the Eu²⁺ doped SrF₂ sample stayed unchanged, unlike the Eu²⁺ doped CaF₂ sample. Temperature treatments significantly affected the Eu³⁺ emission. A complete reduction of Eu³⁺ to Eu²⁺ occurred during heating under H₂ atmosphere. Degradation of the Eu²⁺ emission intensity occurred during prolonged laser irradiation of the annealed SrF₂ as-prepared samples, but it was stable at 800 °C annealing temperature. These results demonstrated that temperature treatment destroyed the surface trap centers and enhanced the photoemission stability of the Eu²⁺.