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

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Up-converting nanoparticles are widely studied for a wide range of applications based on their unique optical properties, with NaYF 4 nanoparticles doped with Yb 3+ and Er 3+ 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 NaYF 4:Yb 3+ and Er 3+ 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.