The effects of dopant concentration and excitation intensity on the upconversion and downconversion emission processes of β-NaYF4:Yb3+,Er3+ nanoparticles

The dopant concentration of lanthanide ions in photon-upconversion nanoparticles (UCNPs) remains one of the key points to boost these nanomaterials’ brightness and, therefore, their application development. Here, we analyzed the effect of Er3+ and Yb3+ dopant concentration of β-NaYF4:Yb3+,Er3+ nanoparticles on the visible upconversion and near-infrared downconversion luminescence intensity. To maintain the size and the morphological properties of the nanoparticles we used a total dopant concentration of 22% while varying the ratio of Yb3+ to Er3+ ions from 0 to 10. A huge increase in luminescence takes place as the Yb/Er ratio increases following a power-law behavior, being this luminescence enhancement greater at low excitation intensities. Above an Yb/Er ratio around two, saturation occurs with a slight peak when this ratio is around four. Simulations using a rate equation model showed that upconversion luminescence (UCL) is mainly produced by energy transfer between neighboring Er3+ ions at low Yb/Er ratios, while at high ratios, energy transfer from Yb3+ to Er3+ ions dominates. However, downconversion luminescence (DCL) is produced at all analyzed ratios, except 0, by this last mechanism.