Physiochemical characterization of highly biocompatible, and colloidal LaF3:Yb/Er upconversion nanoparticles

Photoluminescence Investigations of Dy3+-Doped Silicate Xerogels and SiO2-LaF3 Nano-Glass-Ceramic Materials

In this work, the series of Dy³⁺-doped silicate xerogels were synthesized by sol-gel technique and further processed at 350 °C into SiO₂-LaF₃:Dy³⁺ nano-glass-ceramic materials. The X-ray diffraction (XRD) measurements, along with the thermal analysis, indicated that heat-treatment triggered the decomposition of La(TFA)₃ inside amorphous sol-gel hosts, resulting in the formation of hexagonal LaF₃ phase with average crystal size at about ~10 nm. Based on the photoluminescence results, it was proven that the intensities of blue (⁴F_9/2 → ⁶H_15/2), yellow (⁴F_9/2 → ⁶H_13/2), and red (⁴F_9/2 → ⁶H_11/2) emissions, as well as the calculated yellow-to-blue (Y/B) ratios, are dependent on the nature of fabricated materials, and from fixed La³⁺:Dy³⁺ molar ratios. For xerogels, the emission was gradually increased, and the τ(⁴F_9/2) lifetimes were elongated to 42.7 ± 0.3 μs (La³⁺:Dy³⁺ = 0.82:0.18), however, for the sample with the lowest La³⁺:Dy³⁺ molar ratio (0.70:0.30), the concentration quenching was observed. For SiO₂-LaF₃:Dy³⁺ nano-glass-ceramics, the concentration quenching effect was more visible than for xerogels and started from the sample with the highest La³⁺:Dy³⁺ molar ratio (0.988:0.012), thus the τ(⁴F_9/2) lifetimes became shorter from 1731.5 ± 5.7 up to 119.8 ± 0.4 μs. The optical results suggest, along with an interpretation of XRD data, that Dy³⁺ ions were partially entered inside LaF₃ phase, resulting in the shortening of Dy³⁺-Dy³⁺ inter-ionic distances.