Does Ce3+ ion doping in CaF2:Tb3+ exhibit more luminescence than CeF3:Tb3+?

Abstract

This article reveals the experimental evidence explaining why Tb³⁺ doping in CaF₂:Ce³⁺ has better luminescence intensity than Tb³⁺ doping in CeF₃. It also discusses the changes in the crystal structure and luminescence when Ce³⁺ ions are over-doped in CaF₂:Tb³⁺ (Ce³⁺ = 15–23 at.%; Tb³⁺ = 5 at.%) nanocrystals. Although both CeF₃ and CaF₂ as hosts have low phonon vibrations, CaF₂ has a larger bandgap than CeF₃. Due to this, the luminescence of Ln³⁺ doped in CaF₂ is expected to be better. This has been confirmed by time-resolved luminescence studies, where the decay lifetime of Tb³⁺ is longer in CaF₂:Ce³⁺–Tb³⁺ than in CeF₃:Tb³⁺, indicating that CaF₂ is a better luminescent host than CeF₃. At 15 at% Ce³⁺ and above, the luminescence intensity of Tb³⁺ falls as the concentration of Ce³⁺ ions increases. The energy transfer from Ce³⁺ to Tb³⁺ occurs in both the hosts. XRD (X-ray diffraction) and TEM (transmission electron microscopy) analyses confirm the formation of CeF₃ (hexagonal) in the case of over-doping of Ce³⁺ in the CaF₂ (cubic) host. The ratio of CaF₂ and CeF₃ phase concentration decreases with the increase of co-doping of Ce³⁺. The decay dynamics of Tb³⁺ ions in CaF₂:Ce³⁺,Tb³⁺ (Ce³⁺ = 15–23 at.%; Tb³⁺ = 5 at.%) nanocrystals are determined, and after careful analysis, the roles of the 2 different host crystals (CaF₂ and CeF₃) in the decay parameters are observed. The observed luminescence quantum yield (QY) value for Ce³⁺–Tb³⁺ doped in the CaF₂ host (∼44%) is more than that for Tb³⁺ doped in the CeF₃ host (∼28%).