Power-dependent upconversion quantum yield of NaYF₄:Yb³⁺,Er³⁺ nano- and micrometer-sized particles - measurements and...

Photophysical studies of nonlinear lanthanide-doped photon upconverting nanoparticles (UCNPs) increasingly used in biophotonics and photovoltaics require absolute measurements of the excitation power density (P)-dependent upconversion luminescence (UCL) and luminescence quantum yields (Φ_UC) for quantifying the material performance, UCL deactivation pathways, and possible enhancement factors. We present here the P-dependence of the UCL spectra, Φ_UC, and slope factors of the different emission bands of representative 25 nm-sized oleate-capped β-NaYF₄:17% Yb³⁺, 3% Er³⁺ UCNPs dispersed in toluene and as powder as well as Φ_UC of 3 μm-sized upconversion particles (UCμP), all measured with a newly designed integrating sphere setup, enabling controlled variation of P over four orders of magnitude. This includes quantifying the influence of the beam shape on the measured Φ_UC and comparison of experimental Φ_UC with simulations utilizing the balancing power density model of the Andersson-Engels group and the simulated Φ_UC of UCμP from the Berry group, underpinned by closely matching decay kinetics of our UC material. We obtained a maximum Φ_UC of 10.5% for UCμP and a Φ_UC of 0.6% and 2.1% for solid and dispersed UCNPs, respectively. Our results suggest an overestimation of the contribution of the purple and an underestimation of that of the red emission of β-NaYF₄:Yb³⁺,Er³⁺: microparticles by the simulations of the Berry group. Moreover, our measurements can be used as a guideline to the absolute determination of UCL and Φ_UC.