Tunable Upconversion Emissions from Lanthanide-doped Monodisperse-NaYF4 Nanoparticles

We report upconversion multicolor tuning based on uniform -NaYF4:Yb/Tm/Er nanoparticles. The as-synthesized nanoparticles with an average diameter of 25nm are well dispersed in a wide range of nonpolar solvents including hexane, cyclohexane, dichloromethane, and toluene. These nanoparticles show intense upconversion emissions and the color output can be precisely modulated by adjusting activator ratios of Tm3+ to Er3+. Dopant-concentration dependent emission properties of the triply doped particle system are also investigated. In addition, we demonstrate that these nanoparticles can be readily transferred to polar solvents such as ethanol and water by growing a thin silica layer (10nm) around the particles, providing potential applications in biological labeling and imaging.