Enhanced upconversion luminescence and optical thermometry in Er3+/Yb3+ heavily doped ZrO2 by stabilizing in the monoclinic phase

Abstract

Er³⁺ and Yb³⁺ heavily doped monoclinic ZrO₂ (m-ZrO₂) stabilized by Nb⁵⁺ is prepared by solid-state reactions. The upconversion luminescence (UCL) of concentration optimized m-ZrO₂:2%Er³⁺,9%Yb³⁺ shows a yellow emission which is found to be 4 times stronger than the red UCL of the optimized cubic ZrO₂:2%Er³⁺,14%Yb³⁺. It is revealed that monoclinic ZrO₂ has faster radiative transition rates of Er³⁺ and Yb³⁺ than cubic ZrO₂ because of the lower crystal symmetry in the monoclinic phase that enables higher 4f–5d mixing of rare earth ions. The enhanced radiative transition allows 9%Yb³⁺ in the monoclinic phase to have a similar absorption of 980 nm excitation light to that of 14%Yb³⁺ in the cubic phase, thus retaining the lower energy back transfer from Er³⁺ in the green emitting state to Yb³⁺ and lower concentration quenching of Yb³⁺ in the monoclinic phase. The green and red dual-color optical thermometry behavior of m-ZrO₂ has been explored. A large detectable temperature range, at which the absolute sensitivity is high, is realized through adopting this complementary optical thermometer. Repeated thermal experiments prove the stability of the m-ZrO₂ as a temperature probe. Our results indicate that heavily doped m-ZrO₂ is an excellent host for efficient luminescence of rare earth ions.