Multicolor output and shape controlled synthesis of lanthanide-ion doped fluorides upconversion nanoparticles.

A general and facile approach for tailoring the multicolor output and shapes of lanthanide-ion doped fluoride upconversion nanoparticles (UCNPs) within a given composition is presented. By adjusting the temperature and time in the thermolysis procedure, the color output and shapes of NaYF(4):20%Yb, 2%Er UCNPs can be readily manipulated. The nanoparticles were characterized through the use of transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and upconversion luminescence spectroscopy. It is shown that the relative intensities of green emissions gradually increased with the rise of temperature and prolongation of growth time under excitation of 980 nm, which resulted in multicolor output of NaYF(4):20%Yb, 2%Er UCNPs. Simultaneously, the shapes for UCNPs can also be controlled. TEM images, estimated micro-stress by Williamson-Hall methodology and a series of control experiments and analyses reveal that crystallinity is mainly responsible for the multicolor output of UCNPs. Based on the above method, the tailoring of color output is also successfully realized in Ho(3+) and Tm(3+) ions. It is expected that this method may be used to tune the physical properties of other nanoparticles, and these multicolored UCNPs are promising for applications in multiplexed bioimaging, biodetection, display, other optical technologies, etc.