Hydrothermal assisted fabrication of ZnWO4:Yb3+/Er3+ nanophosphors with color tunable upconversion luminescence of enhanced thermal stability

ZnWO 4 :Yb 3+ /Er 3+ upconversion (UC) nanophosphors were successfully synthesized by hydrothermal reaction at 180 ℃ and subsequent calcination at 700 ℃ for crystallinity improvement and better incorporation of dopant ions. The phosphors were analyzed to emit green (∼533 nm; 2 H 11/2 → 4 I 15/2 transition of Er 3+ ), green (∼546 nm; 4 S 3/2 → 4 I 15/2 ) and red (∼660 nm; 4 F 9/2 → 4 I 15/2 ) light under 980 nm laser excitation, all through a two-photon process. The incorporation of Na + and K + , especially K + , was found to remarkably modify the UC performance of Er 3+ , in terms of red/green intensity ratio (luminescence color), fluorescence lifetime and thermal stability of luminescence, which was rationalized by considering 4 F 7/2 + 4 I 11/2 → 4 F 9/2 cross relaxation and lattice defects/local structure. The optimal ZnWO 4 :0.08Yb 3+ ,0.02Er 3+ phosphor, which contains ∼ 0.97 at.% Na + and 0.16 at.% K + , was demonstrated to have the potential for optical temperature sensing with the thermally coupled 2 H 11/2 / 4 S 3/2 energy levels of Er 3+ .