Revisiting the measured luminescence lifetimes in Yb3+,Er3+-co-doped upconversion nanocrystals

Constructing the core-shell structure is the most popular method for improving the luminescence quantum efficiency of rare earth upconversion nanocrystals. The inert shell serves to prevent the energy quenching from the sensitizer or activator ions to the surface defects or ligands. This structure allows of a higher optimized sensitizer doping ratio to enhance the absorption capacity without concentration quenching. This was demonstrated by the increased luminescence emission intensity and extended lifetime. Here we investigated the mechanism behind the variation of luminescence lifetimes in several typical Yb3+-co-doped upconversion nanocrystals by combining theoretical analyses based on rate equations with experimental results. Only in Yb3+,Er3+-co-doped nanocrystals with strong energy back transfer processes, the rise of Yb3+ doping ratio leads to an acceleration of the energy back transfer rate from Er3+ to Yb3+, and resulted in a longer Yb3+ excited state (2F5/2(Yb3+)) lifetime. The Er3+ second excited state (4S3/2(Er3+)) lifetime is always about one-half of that of the Yb3+ excited state. These results revealed that the extended luminescence lifetime is not the direct result of the suppressed quenching, and provided an alternative protocol for the fine tuning of the lifetime of upconversion luminescence.