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

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 C-1 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.