The Internal Structure of Lanthanide-Doped Nanoparticles and the Effect of High-Temperature Annealing on Their Luminescent Properties

Abstract

Up-converting nanoparticles are widely studied for a wide range of applications based on their unique optical properties, with NaYF4 nanoparticles doped with Yb3+ and Er3+ receiving particular attention. While developing this material in nanoparticle form extends their potential applications, the resulting nanoparticles have proven less efficient up-converters than their bulk counterpart. Reported up-conversion quantum yields are significantly lower, even when very thick shells were grown to eliminate quenching by surface defects and surface-bound molecules. This raises the question whether the internal structure of these particles contributes to the lower quantum yield. In our work, we investigated the internal structure of NaYF4:Yb3+ and Er3+ NPs using high-resolution scanning energy dispersive X-ray spectroscopy, generating two-dimensional elemental maps. We deduced that the ions are not distributed homogeneously in the nanoparticles as made via a colloidal synthesis route. Heating the nanoparticles to temperatures used to anneal bulk crystals resulted in a homogeneous distribution, but an increase in emission intensity under similar measurement conditions was not observed. Vibrational spectroscopy showed the presence of OH– in dried nanoparticles, which might act as an internal quencher.