A facile and general approach for the multicolor tuning of lanthanide-ion doped NaYF4 upconversion nanoparticles within a fixed composition

In this paper, we report a general and facile approach for subtly tuning the multicolor output of lanthanide-ion doped NaYF4 upconversion nanoparticles (UCNPs) within a given composition under single wavelength excitation of 980 nm. By adjusting the molar ratio of octadecylamine (OM) and oleamide in reaction solution under a thermolysis procedure, the color output of NaYF4:20%Yb,2%Er UCNPs can be fine tuned. X-Ray diffraction (XRD), transmission electron microscopy (TEM), upconversion photoluminescence spectroscopy, FT-IR and H-1-NMR were employed to characterize the obtained samples. H-1-NMR analysis and a series of control experiments have revealed that the tunability of multicolor output mainly resulted from different ratios of two coordinating ligands bonded onto the particle surface. We also synthesized NaYF4:20%Yb, 2%Er UCNPs with multicolor output in three pairs of mixtures, namely OM and oleic acid (OA), OM and N-octadecyloleamide (OOA), as well as OA and OOA, based on the above method. Moreover, this approach was further extended to the preparation of multicolored NaYF4:20%Yb, 2% Ho and NaYF4:20%Yb, 0.5%Tm UCNPs. It is expected these multicolored UCNPs have great potential for applications in biology, displays and other optical technologies.