Photoluminescence and time-resolved-luminescence of CaWO4:Dy3+ phosphors

Abstract

The 4I15/2/4F9/2 states of Dy3+ ion are attractive as they have been confirmed to be thermally linked and thus can be used for luminescent ratiometric thermometry. Traditionally, this thermally coupled property is confirmed by using the Boltzmann distribution formula, which is, however, not so reliable as the fitted energy gap between thermally coupled states is sometimes different from the real value. Here we present a more hard evidence to support that the 4I15/2/4F9/2 states of Dy3+ ion are indeed in thermal equilibrium. The time resolved luminescence originating from the 4I15/2/4F9/2 states of Dy3+ ion in calcium tungstate host has been studied as a function of temperature from 333 to 773 K. It is found that the 4I15/2/4F9/2 states always own the same lifetime over the whole experimental temperature range, which is increased from 211 to 238 μs, suggesting undoubtedly that the 4I15/2/4F9/2 states are thermally coupled all the time. As an added benefit, it is confirmed that lifetime thermometry can be exploited by using the 4I15/2/4F9/2 states. In the end, its sensing performance, i.e. relative sensitivity, is compared with the temperature sensing method that depends on the luminescence intensity ratio between the 4I15/2-6H15/2 and 4F9/2-6H15/2 transitions of Dy3+ ion. The results reveal that the lifetime thermometry has a lower thermal sensitivity.