Revisiting the Luminescence Decay Kinetics of Energy Transfer Upconversion

Energy transfer upconversion (ETU) can efficiently upconvert near-infrared photons into higher-energy photons. Although a comprehensive understanding of ETU is fundamental to the design of ETU materials, the basic excited-state decay kinetics of ETU remains a complicated problem. Here we unravel the mechanism underlying ETU decay in benchmark β-NaYF₄:Er³⁺ and β-NaYF₄:Ln³⁺/Yb³⁺ (Ln = Er, Ho, Tm) ETU microcrystals by combining rate equation analyses with ETU decay measurements. The results show that all of the excited states of one ETU system decay concordantly, with the ETU decay of the emitting state determined by only its intrinsic decay and the product of the ETU decays of the two intermediate states directly responsible for the emitting-state photon upconversion. This general mechanism may serve as a basic rule for excited-state kinetics in upconversion microparticles and nanoparticles, which could provide detailed insight into ETU processes and guide the design of efficient ETU materials.