Polyol-mediated synthesis and photoluminescent properties of Ce ³⁺ and/or Tb ³⁺-doped LaPO ₄ nanoparticles

Rare-earth ion (Ce ³⁺, Tb ³⁺) doped LaPO ₄ nanoparticles were prepared by the polyol method and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), UV-vis absorption spectroscopy, photoluminescence (PL) spectroscopy, and lifetimes. The results of XRD indicate that the as-prepared nanoparticles are well-crystallized at 160°C and assigned to the monoclinic monazite structure of the LaPO ₄ phase. The obtained LaPO ₄:Ce ³⁺, Tb ³⁺ nanoparticles are spherical with narrow size distribution and average size of 20 nm. The doped rare-earth ions show their characteristic emission in LaPO ₄ nanoparticles, i.e., Ce ³⁺ 5d-4f and Tb ^{3+ 5}D ₄- ⁷F _J (J = 6-3) transitions, respectively. The optimum doping concentration for Tb ³⁺ in La _0.8-xCe _0.2Tb _xPO ₄ nanoparticles is determined to be 15 mol% (x = 0.15). The luminescence decay curves of Ce ³⁺ in LaPO ₄:Ce ³⁺ and LaPO ₄:Ce ³⁺, Tb ³⁺ nanoparticles present a single-exponential behavior, and the lifetimes (τ) of Ce ³⁺ decrease with increasing Tb ³⁺ concentrations (at the constant Ce ³⁺ concentration) in LaPO ₄:Ce ³⁺, Tb ³⁺ nanoparticles due to the energy transfer from Ce ³⁺ to Tb ³⁺. The energy-transfer efficiency from Ce ³⁺ to Tb ³⁺ was calculated, which depends on the doping concentrations of Tb ³⁺ if the concentration of Ce ³⁺ is fixed.