Efficient multicolor tunability of ultrasmall ternary-doped LaF3 nanoparticles: energy conversion and magnetic behavior

Abstract

Luminescence-tunable multicolored LaF₃:xCe³⁺,xGd³⁺,yEu³⁺ (x = 5; y = 1, 5, 10, and 15 mol%) nanoparticles have been synthesized via a low cost polyol method. Powder X-ray diffraction and high-resolution transmission electron microscopy studies confirm the hexagonal phase of the LaF₃:xCe³⁺,xGd³⁺,yEu³⁺ nanophosphors with average sizes (oval shape) ranging from 5 to 7 nm. Energy-dispersive X-ray spectroscopy analyses show the uniform distribution of Ce³⁺, Gd³⁺, and Eu³⁺ dopants in the LaF₃ host matrix. The photoluminescence spectra and electron paramagnetic resonance measurements guarantee the presence of Eu²⁺, corroborated through DC susceptibility measurements of the samples displaying paramagnetic behavior at 300 K, whereas weak ferromagnetic ordering is shown at 2 K. The non-radiative energy transfer processes from the 4f(²F_5/2) → 5d state (Ce³⁺) to the intraconfigurational 4f excited levels of rare earth ions and simultaneous emissions in the visible region from the 4f⁶5d¹ (Eu²⁺) and ⁵D₀ (Eu³⁺) emitting levels, leading to overlapped broad and narrow emission bands, have been proclaimed. The energy transfer mechanism proposes involvement of the Gd³⁺ ion sub-lattice as the bridge and finally trapping by Eu^2+/3+, upon excitation of the Ce³⁺ ion. The calculation of experimental intensity parameters (Ω_2,4) has been discussed and the highest emission quantum efficiency (η = 85%) of the Eu³⁺ ion for the y = 10 mol% sample is reported. The advantageous existence of the Eu²⁺/Eu³⁺ ratio along with variously doped nanomaterials described in this work, results in tunable emission color in the blue-white-red regions, highlighting the potential application of the samples in solid-state lighting devices, scintillation devices, and multiplex detection.