Judd-Ofelt parameters and energy transfer rates of α-NaYF4:Eu3+– a theoretical and experimental study

Abstract

We report the analysis of Europium (Eu3+) ion-doped NaYF4 thin films from an experimental and theoretical complementary point of view. The pristine and Eu3+ doped-NaYF4 thin films were obtained by electrochemical deposition in the cubic phase (α-NaYF4), which were fully characterized by X-ray diffraction, scanning electron microscopy, and photoluminescence techniques. The emission spectrum of α-NaYF4:Eu3+ exhibits two intense bands at 610 and 628 nm for 5D0 → 7F2 transition with an excitation energy source at 245 nm. The experimental emission spectrum of α-NaYF4:Eu3+ thin films suggests that the D4h could be the local point symmetry around Eu3+ because of the number of sublevels in 5D0 → 7F1,2 transitions. The LUMPAC software was used to calculate the excited states, Judd-Ofelt (J-O) parameters, and energy transfer rates (ETR) from ligand to Eu3+ semi-empirically, using the local point group symmetry proposed in this work. The obtained J-O parameters are Ω2 = 15.39 × 10−20 cm2, Ω4 = 2.14 × 10−20 cm2, and Ω6 = 0.1275 × 10−20 cm2, where Ω6 was calculated by the QDC method despite of 5D0→7F6 transition is not observed in the experimental emission spectrum. The calculated energy transfer rates give the energy transitions from ligand S1→T (singlet→triplet) to T→Eu3+. Finally, the analysis of molecular orbitals derived from time dependent-DFT calculations confirms that the electronic transfer occurs from F− → Eu3+ (from ligand to Eu3+).