One pot synthesis and systematic study of the photophysical and magnetic properties and thermal sensing of α and β-phase NaLnF4 and β-phase core@shell nanoparticles

In this work we report a modified one-pot thermal decomposition method for the synthesis of NaLnF(4) nanoparticles (Ln = Gd, Yb). This modified method allows fine-tuning of the alpha and beta crystalline phases for both light (Gd) and heavy (Yb) lanthanides and also an easy route to the hierarchical nanostructuration of nanoparticles. The core@shell nanostructuration obtained via a step-by-step addition of the shell precursor was demonstrated by HRTEM and STEM/EDS analysis and it greatly improves the upconversion luminescence intensity of the nanoparticles. Under NIR irradiation, the visible Er3+ emission shows high thermal sensitivity in the temperature range of 83 to 323 K, reaching 9.52% K-1 at 83 K for the beta-NaGd0.94Pr0.02Er0.02Yb0.02F4@3NaY(0.8)Yb(0.2)F(4) sample. This is one of the highest values reported so far for upconversion thermal sensing using NaLnF(4) nanoparticles. We report for the first time, a comparison between the magnetic properties of alpha and beta-NaGdF4:Pr:Er:Yb nanoparticles. This comparison shows that the alpha-NaLnF(4) samples systematically presented higher magnetization values than the beta-NaLnF(4). Our work provides a controlled method of synthesis that resulted in NaLnF(4) nanoparticles with a high sensitivity for nanothermometry applications at low temperatures. The observed dependence of the magnetic properties on the crystalline phase demonstrated here clearly shows that further studies on the magnetic properties of these fluorides should be encouraged.