Lanthanide luminescence at the nanoscale has garnered considerable attention over the last few decades, with research on ternary fluoride nanoparticles focusing heavily on upconversion and to a lesser extent on radioluminescence. However, these nanomaterials have not yet been implemented commercially. This is likely, in part, due to the overwhelming emphasis on demonstrating potential applications, rather than understanding the fundamental mechanisms that drive these luminescence phenomena. As a result, the forbidden nature of the lanthanide 4f-4f transitions has hindered the widespread implementation of such nanoparticles.
To shed light on the complex nature of lanthanide-doped upconverting nanoparticles, studies on co-doped Yb3+ and Tm3+ nanoparticles were completed with varying host composition (including LiYF4, NaGdF4, and BaYF5) and activator dopant concentration (ranging from 0.1 to 2.0 mol%). The interionic spacing and site symmetry of the lanthanide ions was deemed to play an integral role in the relative intensity of each Tm3+ emission, indicating that different combinations are optimal for different applications. With the addition of an active shell doped with Tb3+, the energy transfer across the core/shell interface was evaluated next, establishing that a radiative energy transfer mechanism from the 1D2 excited state of Tm3+ was most prominent.
Influences known to affect the upconversion efficiency of nanomaterials were then evaluated on LiLuF4:Eu3+ radioluminescent nanoparticles. The results herein indicated that a greater material density and effective atomic number improved the efficiency of the radioluminescence process, while varying the dopant concentration was not as influential, when compared to direct ultraviolet excitation. Furthermore, the addition of Gd3+ as a sensitizer or employing core/shell structures did not prove advantageous to the radioluminescence intensity.
While luminescence lifetimes are measured to evaluate nonradiative energy transfer efficiencies between spectroscopically active species, proof-of-concepts herein demonstrate that they can also be employed to add a temporal component to various applications. These include upconversion nanothermometry, particle velocimetry, and covert information storage, all taking advantage of the long-lived excited state decay times of various lanthanide ions.