Optimizing the composition of LaF 3 :Yb,Tm upconverting nanoparticles synthesised by the co-precipitation method to improve the emission intensity.

LaF ₃ :1-6 mol%Yb ³⁺ ,0.1 mol%Tm ³⁺ nanoparticles with upconversion properties were synthesised by the co-precipitation method. Particles were characterized by transmission electron microscopy coupled with energy-dispersive spectrometry, inductively coupled plasma optical emission spectrometry, X-ray diffraction, and simultaneous thermogravimetry and differential thermal analysis. Upconversion properties were investigated by fluorescence spectroscopy, using 980 nm laser light excitation. The results show that the particles had a hexagonal LaF ₃ crystal structure. Crystallite and particle sizes decreased with increasing Yb ³⁺ content and the average crystallite size changed between 18 and 36 nm, while the average particle diameter was 25-52 nm. The highest upconversion emission intensity at the 480 nm emission peak could be reached with an optimal Yb ³⁺ content of 3 mol% and an Yb/Tm ratio of ∼75. These upconverting nanoparticles, prepared using a cheap and environmentally friendly co-precipitation method and containing relatively low dopant ion concentrations, will be helpful in a variety of promising fields, such as sensing, solar cells and security applications.