Excitation power dependent population pathways and absolute quantum yields of upconversion nanoparticles in different...

The rational design of brighter upconversion nanoparticles (UCNPs) requires a better understanding of the radiationless deactivation pathways in these materials. Here, we demonstrate the potential of excitation power density (P)-dependent studies of upconversion (UC) luminescence intensities, slope factors, and absolute quantum yields (Φ_UC) of popular β-NaYF₄:20% Yb³⁺,2% Er³⁺ UCNPs of different surface chemistries in organic solvents, D₂O, and water as a tool to gain deeper insight into the UC mechanism including population and deactivation pathways particularly of the red emission. Our measurements, covering a P regime of three orders of magnitude, reveal a strong difference of the P-dependence of the ratio of the green and red luminescence bands (I_g/r) in water and organic solvents and P-dependent population pathways of the different emissive energy levels of Er³⁺. In summary, we provide experimental evidence for three photon processes in UCNPs, particularly for the red emission. Moreover, we demonstrate changes in the excited population dynamics via bi- and triphotonic processes dependent on the environment, surface chemistry, and P, and validate our findings theoretically.