Upconversion nanoparticles: influence of the host lattices on crystallographic and luminescent properties

Ytterbium/erbium co-doped NaYF4, NaGdF4, CaF2, and LaF3 upconversion nanoparticles (UCNPs) were synthesized by hydrothermal process. X-ray diffraction pattern (XRD), Thermogravimetric analysis (TGA), Fourier transforms infrared (FTIR), UV/visible, and photoluminescence spectroscopic techniques were employed to determine the crystal structure, grain size, phase pureness, thermal stability, surface functional groups, optical properties, and UC luminescence properties. CaF2 host lattice presented a cubic face-centered structure, whereas NaYF4, NaGdF4, and LaF3 host matrices exhibited a hexagonal phase of the materials. The lattice parameters were calculated from the reflection peak positions. NaYF4 host material exhibited higher thermal stability, and LaF3 was the least among the presented host matrices. FTIR spectra display the presence of carboxylic moieties which assist in the formation of colloidal solution. Absorption spectra illustrated excellent colloidal dispersibility and optical properties of the UCNPs in an aqueous solvent. Energy bandgap values were estimated from the UV/visible spectra graph plotted between (& alpha;h & nu;)(2) vs photon energy (h & nu;) are to be 5.64, 5.50, 5.82, and 5.71 eV for the NaYF4:Yb/Er, NaGdF4:Yb/Er, CaF2:Yb/Er, and LaF3:Yb/Er UCNPs, respectively. UC luminescence spectra were recorded to investigate the impact of the host lattices on the luminescence intensities. In the presented results, NaYF4 host matrix produces the highest emission intensity, and the LaF3 host lattice was the least among the observed results. The findings of these non-functionalized UCNPs will be highly useful in luminescent-based biomedical applications.

Automated summary

Ytterbium/erbium co-doped NaYF4, NaGdF4, CaF2, and LaF3 upconversion nanoparticles (UCNPs) were synthesized by a hydrothermal process. XRD, TGA, FTIR, UV/vis, and photoluminescence spectroscopy were used to study their crystal structure, thermal stability, surface functional groups, optical properties, and UC luminescence properties. NaYF4 host material exhibited the highest thermal stability and the NaYF4 host matrix had the highest emission intensity among the observed results. These non-functionalized UCNPs have great potential in luminescent-based biomedical applications.