@article{Kaiser2019, 
author = {Martin Kaiser and Christian Würth and Marco Kraft and Tero Soukka and Ute Resch-Genger},
title = {Explaining the influence of dopant concentration and excitation power density on the luminescence and brightness of β-NaYF4: Yb3+, Er3+ nanoparticles: Measurements and simulations},
year = {2019},
journal = {Nano Research},
volume = {12},
number = {8},
pages = {1871-1879},
keywords = {nanoparticles, upconversion, quantum yield, color tuning, dopant concentration, rate equation model},
url = {https://www.sciopen.com/article/10.1007/s12274-019-2450-4},
doi = {10.1007/s12274-019-2450-4},
abstract = {We assessed the influence of Yb3+ and Er3+ dopant concentration on the relative spectral distribution, quantum yield (ƵUC), and decay kinetics of the upconversion luminescence (UCL) and particle brightness (BUC) for similarly sized (33 nm) oleate-capped β-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%Ƀ21% for a constant Er3+ concentration of 3%, and an Er3+ series, where the Er3+ concentration was varied between 1%Ƀ4% 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 4F9/2 at high P by an enhanced back energy transfer (BET) from the 4G11/2 level. Simultaneously, the Yb3+-controlled UCNPs absorption cross section overcompensates for the reduction in ƵUC with increasing Yb3+ concentration at high P,  resulting in an increase in BUC. 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.}
}