Colloidal synthesis and remarkable enhancement of the upconversion luminescence of BaGdF5:Yb3+/Er3+ nanoparticles by active-shell modification

BaGdF5:Yb3+/Er3+ colloidal nanoparticles (NPs) with tunable average particle diameters from 3 to 10 nm in different solvents were first prepared by thermal decomposition method. X-Ray diffraction (XRD), transmission electron microscopy (TEM) and upconversion (UC) luminescence spectra were used to characterize the resulting samples. After coating the active shell (containing Yb3+) around the BaGdF5:Yb3+/Er3+ NPs, not only their morphology and monodisperse properties were well maintained but also the relative UC luminescence intensity was enhanced remarkably. In contrast to the pure BaGdF5:Yb3+/Er3+ core, the luminescence intensity of the active-shell coated NPs can be enhanced several hundred times. Unexpectedly, the luminescence intensity of the active-core/active-shell NPs was about four times as high as that of β-NaYF4:Yb3+/Er3+ NPs under the same doping concentration and measurement conditions. Moreover, the luminescence colors of the active-core/active-shell NPs can be tuned from green to yellow by multilayer active-shell coating under 980 nm laser excitation.