Explaining the influence of dopant concentration and excitation power density on the luminescence and brightness of β-NaYF4:Yb3+,Er3+ nanoparticles: Measurements and simulations

We assessed the influence of Yb3+ and Er3+ dopant concentration on the relative spectral distribution, quantum yield (phi(UC)), and decay kinetics of the upconversion luminescence (UCL) and particle brightness (B-UC) for similarly sized (33 nm) oleate-capped beta-NaYF4:Yb3+,Er3+ upconversion (UC) nanoparticles (UCNPs) in toluene at broadly varied excitation power densities (P). This included an Yb3+ series where the Yb3+ concentration was varied between 11%-21% for a constant Er3+ concentration of 3%, and an Er3+ series, where the Er3+ concentration was varied between 1%-4% for a constant Yb3+ concentration of 14%. The results were fitted with a coupled rate equation model utilizing the UCL data and decay kinetics of the green and red Er3+ emission and the Yb3+ luminescence at 980 nm. An increasing Yb3+ concentration favors a pronounced triphotonic population of F-4(9/2) at high P by an enhanced back energy transfer (BET) from the (4)G(11/2) level. Simultaneously, the Yb3+-controlled UCNPs absorption cross section overcompensates for the reduction in phi(UC) with increasing Yb3+ concentration at high P, resulting in an increase in B-UC. Additionally, our results show that an increase in Yb3+ and a decrease in Er3+ concentration enhance the color tuning range by P. These findings will pave the road to a deeper understanding of the energy transfer processes and their contribution to efficient UCL, as well as still debated trends in green-to-red intensity ratios of UCNPs at different P.