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

LaF:1-6 mol%Yb,0.1 mol%Tm 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 crystal structure. Crystallite and particle sizes decreased with increasing Yb 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 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.