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

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 (Phi(UC)) for quantifying the material performance, UCL deactivation pathways, and possible enhancement factors. We present here the P-dependence of the UCL spectra, Phi(UC), and slope factors of the different emission bands of representative 25 nm-sized oleate-capped beta-NaYF4: 17% Yb3+, 3% Er3+ UCNPs dispersed in toluene and as powder as well as Phi(UC) of 3 mu m-sized upconversion particles (UC mu P), 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 Phi(UC) and comparison of experimental Phi(UC) with simulations utilizing the balancing power density model of the Andersson-Engels group and the simulated Phi(UC) of UC mu P from the Berry group, underpinned by closely matching decay kinetics of our UC material. We obtained a maximum Phi(UC) of 10.5% for UC mu P and a Phi(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 beta-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 Phi(UC).