NaGdF:Eu nanocrystalline: an in-depth study of energy transfer processes and Judd-Ofelt analysis using the luminescence excitation spectrum.

NaGdF nanocrystalline doped with different concentrations of Eu ions were synthesized using the precipitation method. The structure and morphology of the material were investigated through the measurements of the XRD patterns and SEM images, respectively. The optical properties of the NaGdF:Eu nanocrystalline were studied in the framework of the Judd-Ofelt theory in which the parameters were calculated by two methods: the traditional method using the luminescence spectra and the self-referenced method using the luminescence excitation spectra. In NaGdF:Eu nanocrystalline, the Gd ions in the lattice act as sensitizer centers for the luminescence of Eu ions under excitation at 272 and 310 nm. The energy transfer process from Gd to Eu causes the emission enhancement of Eu ions. Upon excitation by the characteristic wavelengths of Gd, the luminescence efficiency of the Eu ions in NaGdF:Eu is affected by two mechanisms: the emission of Gd ions and the trapping of excited energy by the Eu ions. The energy transfer between Eu ions was also discussed in detail. This process leads to the enhancement of the luminescence bands originating from the D level. The dominant interaction between the Eu ions in the energy transfer process is the dipole-dipole mechanism, which is determined by fitting the decay curve of the D level to the Inokuti-Hirayama model.