Efficient upconversion fluorescence in NaYF4:Yb3+, Er3+/mNaYF4 multilayer core-shell nanoparticles

Here, we show a core-shell nanostructure which effectively improves NIR-to-visible upconversion quantum efficiency and luminescence intensity. The NaYF4:Yb3+, Er3+/mNaYF4 (m = 0, 1, 2, 3, 4, 5) core-shell nanoparticles are synthesized using a thermal decomposition method. The structure, morphology and fluorescence spectrum of the nanoparticles are confirmed by XRD, TEM and fluorometer. The average size of the as-prepared NaYF4:Yb3+, Er3+/mNaYF4 cores is about 8 nm while the thickness of the shells varies from 0.5 nm to 8 nm for different m values. With 980 nm excitation, the nanoparticles exhibit distinct emission peaks at 522 nm, 544 nm and 650 nm. It is noticed that an increasing shell thickness enhances their fluorescence intensity. The fluorescence lifetime and quantum yield of obtained multilayer NaYF4:Yb3+, Er3+/mNaYF4 nanoparticles are also discussed.