Probing the Crystal Structure and Formation Mechanism of Lanthanide-Doped Upconverting Nanocrystals

Lanthanide (Ln)-doped upconverting nano crystals (UCNCs), such as NaLnF(4) (with Ln = lanthanide), constitute an important class of nanoscale materials due to their capacity to convert near-infrared photons into near ultraviolet or visible light. Although under intense investigation for more than a decade, UCNCs have been relatively underexplored especially regarding their crystal structure and mechanisms of formation in organic media. The former is needed to explain the relationship between atomic scale structure and upconversion (UC) properties of UCNCs local symmetry for 4f-4f transition probability, Ln(3+) distances for energy migration), while the latter is essential to finely tune the size, morphology, chemical composition, and architecture of well-defined upconverting nanostructures, which constitute the experimental levers to modify the optical properties. In this contribution, we use synchrotron-based diffraction experiments coupled to Rietveld and pair distribution function (PDF) analyses to understand the formation of NaGdF4:Yb:Er UCNCs in organic media and to investigate their crystal structure. Our results reveal a complex mechanism of the formation of NaGdF4:Yb:Er UCNCs based on chemical reactions involving molecular clusters and in situ-generated, crystalline sodium fluoride at high temperature. Additionally, a detailed crystallographic investigation of NaGdF4:Yb:Er UCNCs is presented. Our Rietveld and PDF analyses show that the space group P (6) over bar is the one that best describes the crystal structure of NaGdF4:Yb:Er UCNCs contrary to what has been recently proposed. Further, our Rietveld and PDF data reveal the formation of bulk-like crystal structure down to 10 nm with limited distortions. The results presented in this paper constitute an important step toward the comprehensive understanding of the underlying picture that governs UC properties of lanthanide-doped nanostructures.