Synthesis of monodisperse NaYF4:Yb, Tm@SiO2 nanoparticles with intense ultraviolet upconversion luminescence

In this paper, the NaYF4:Yb, Tm upconversion (UC) nanoparticles (NPs) were synthesized using a solvothermal approach, and the core-shell structured NaYF4:Yb, Tm@SiO2 NPs, coated with a thin layer of SiO2 on the surface of the NaYF4:Yb, Tm NPs, were prepared by a typical Stöber method. X-ray diffraction (XRD), transmission electron microscopy (TEM), and luminescence spectroscopy were applied to characterize these samples. The obtained core-shell structured NaYF4:Yb, Tm@SiO2 NPs exhibited a perfect spherical morphology with narrow size distribution and smooth surfaces. Under 980 nm excitation, NaYF4:Yb, Tm and NaYF4:Yb, Tm@SiO2 samples showed intense ultraviolet UC luminescence, which originated from the 1D2 --> 3H6, 1I6 --> 3F4 transitions of Tm3+. These NPs have great potential for applications as fluorescent labels, imaging probes, optical storage, photodynamic therapy (PDT) in deep tissue, and solid-state lasers.