Intense red emitting monoclinic LaPO4:Eu3+ nanoparticles: host–dopant energy transfer dynamics and photoluminescence properties

Abstract

LaPO₄ nanoparticles were synthesized using complex polymerization method and characterized systematically using X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. Varied concentration of europium ion is doped in the LaPO₄ lattice and optical properties and Judd–Ofelt analysis were investigated. It is observed that LaPO₄ nanoparticles give violet–blue emission when irradiated with UV light. On doping europium ion the band gap of LaPO₄ decreases. Based on DFT calculations it is proposed that energy range over which d and f states of Eu are distributed is coincident with the valence band of LaPO₄ so causing an efficient energy transfer from LaPO₄ to europium ion. The actual site symmetry for europium ion in lanthanum orthophosphate was also evaluated as D_2d based on the Stark splitting pattern although it is C₁ for La³⁺ in LaPO₄. The critical energy-transfer distance for the Eu³⁺ ions was evaluated, based on which the quenching mechanism was verified to be an electric multipolar interaction. It is also observed that the red emission intensity of LaPO₄:Eu³⁺ (2.0 mol%) is almost 85% of a commercial red phosphor, which clearly demonstrates that the as-prepared samples are promising red phosphors under near-UV for use in white-light emitting diodes.