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

Abstract

ZnWO₄:Yb³⁺/Er³⁺ 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; ²H_11/2 → ⁴I_15/2 transition of Er³⁺), green (∼546 nm; ⁴S_3/2 → ⁴I_15/2) and red (∼660 nm; ⁴F_9/2 → ⁴I_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 Er³⁺, in terms of red/green intensity ratio (luminescence color), fluorescence lifetime and thermal stability of luminescence, which was rationalized by considering ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 cross relaxation and lattice defects/local structure. The optimal ZnWO₄:0.08Yb³⁺,0.02Er³⁺ 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 ²H_11/2/⁴S_3/2 energy levels of Er³⁺.