Synthesis, Judd-Ofelt analysis and energy transfer mechanism in β-NaYGdF4: Eu3+ microphosphors

Abstract

A series of β-Na(Y0.95-x,Gd0.05)F4: xEu3+ (x = 0 − 0.15) microphosphors with minimal concentration of Gd3+is developed using modified hydrothermal techniques. The phase purity, structure, morphology, elemental distributions and surface chemical compositions are investigated by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis and X-ray photoelectron spectroscopy analysis. Characteristics photoluminescence emissions of Gd3+ (6P7/2 →8S7/2) and Eu3+ (5D0 → 7Fj) are observed in the doped and codoped phosphors by indirect and direct excitations at 272 nm (of Gd3+) and 394 nm (of Eu3+) respectively. Strong PL emissions at 310 nm of Gd-doped β- NaYF4 at an excitation of 272 nm suggests its possibility in phototherapy applications. The observed Ω2 > Ω4 from the Judd-Ofelt model suggests a local asymmetricity around Eu3+ ions in the studied microphosphors. The emission peak intensity at 615 nm varies with Eu3+concentrations and quenching occurs at higher doping level. Moreover, the emission spectra and luminescence lifetime based on Inokuti-Hirayama model reveals an efficient energy transfer from Gd3+ to Eu3+ is mainly mediated through dipole-dipole interaction. A detail schematic representation of the energy transfer process between activator ions is also documented. Comparatively a better emission color tunability and color purity (90.08%) of red emission is achieved in Gd3+-Eu3+ doped phosphors for indirect excitation of Eu3+ ions at 272 nm. The effective tunability of the photophysical properties of these UV excitable phosphors suggest its applicability in fabricating wLED using UVchips, spectral converters for solar cell and bio-labeling etc.