Influence of solvothermal synthesis parameters on NaY0.65Gd0.15F4:Yb0.18Er0.02 UCNPs structural, morphological and optical characteristics

Monosized Up-Converting NanoParticles (UCNPs) with biocompatible surface and unique optical properties attract a great interest as new cell markers or drug delivery systems. The uppermost UC efficiency of β-NaYF4:Yb/Er phase is due to its hexagonal P63/m space group arrangement which could accommodate higher concentration of dopants at shorter distance. Stabilization of this phase in nanoparticles is usually achieved through thermal decomposition of organic precursors in the presence of solvents with a high boiling point. Here, for the same purpose, we used gadolinium co-doping during chitosan assisted solvothermal processing of inorganic precursor salts. Precursor concentration, solvent type, and synthesis time were varied in order to determine their influence on the β-NaY0.65Gd0.15F4:Yb0.18Er0.02 phase crystallization. The XRPD analysis showed that lower surplus of fluoride ions during synthesis leads to formation of Y0.65Gd0.15F4:Yb0.18Er0.02 orthorhombic phase, while the increase of fluoride content or prolongation of the processing time enhances formation α- NaY0.65Gd0.15F4:Yb0.18Er0.18 phase. Along with it, the changes of UCNPs morphology from spindle to spherical shape is detected. All samples emit intense green emission due to the (2H11/2, 4S3/2) → 4I15/2 electronic transitions, after been excited with infrared light (λ=978 nm)