The Y2O2S:Er3+@Y2O2S:Yb3+,Ho3+ core-shell up-conversion (UC) nanoparticles were successfully synthesized by the homogeneous co-precipitation method. The Y2O2S:Er3+@Y2O2S:Yb3+,Ho3+ core-shell nanoparticles exhibit bright green emissions under 980 nm excitation, while the triple-ion doped Y2O2S:Er3+,Yb3+,Ho3+ sample presents mainly red emissions. The intensity ratio of green-to-red emission of the core-shell and conventional triple-ion doped samples are 2.8 and 0.3, respectively. Investigations on the UC mechanisms show that emissions from Er3+ and Ho3+ ions are achieved simultaneously in the core-shell nanoparticles. This is due to the efficient energy transfers of Yb3+→Ho3+ within the shell layer and Yb3+→Er3+ between the shell and the core. While the triple-ion doped Y2O2S: Er3+,Yb3+,Ho3+ sample exhibits mainly the emissions of Er3+ along with weak luminescence of Ho3+ ion. Since the cross relaxation between Er3+ and Ho3+ ions in the Y2O2S:Er3+,Yb3+,Ho3+ nanoparticles can effectively suppress the emissions of Ho3+ ions. Yet, in the core-shell structure, this cross relaxation can be successfully restrained in the core-shell structure where Er3+ is in the core and Ho3+ is in the shell. Therefore, the construction of core-shell structure can improve the luminescence efficiency and provide a route for adjustment of emission color.
Upconversion luminescence and temperature sensing properties of NaGd(WO4)2:Yb3+/Er3+@SiO2 core–shell nanoparticles
