Excitation power-dependent kinetics of up-conversion luminescence in single Er3⁺/Yb3⁺-doped nanocrystals

The up-conversion luminescence observed at the level of single nanoemitters reveals a wealth of processes that are not apparent in bulk materials. One such example is the experimental result discussed in this work, demonstrating that the shape of the decay profile of red up-conversion luminescence in individual NaYF4:Er³⁺/Yb³⁺ nanocrystals, and consequently estimated decay rates, strongly depend on the laser excitation power. To explain this, we solved a set of kinetic rate equations for the Er³⁺/Yb³⁺ system. Referring to the geometry of a nanocrystal, we assumed the presence of two populations of excited Er³⁺ ions to distinguish between emitters located in the nanocrystal’s center and those close to its surface. Additionally, we postulated that ions situated near the surface are subject to quenching by surfactants, with the efficiency of this quenching depending on the ambient temperature, which can be affected by the excitation laser itself. The proposed kinetic model accurately describes the experimental luminescence decay profiles and further allows the estimation of the emitter’s effective temperature across a wide range of excitation powers and various nanocrystal geometries, such as core-shell.