Ultrastrong up-conversion luminescence in inert-active-inert sandwich structure with critical inert-core size

Lanthanide-based up-conversion materials exhibit abundant emission colors under infrared excitation; however, enhancing their up-conversion luminescence efficiency remains a hot topic and a challenge. Here, we show ultrastrong up-conversion luminescence in an inert-active-inert sandwich structure with critical core sizes. It is observed that the emission intensity of the sandwich structure (NaYF4@NaYbF4:0.5 %Tm@ NaYF4) is equal to similar to 129 times of that in the NaYF4@NaYbF4:0.5 %Tm core/shell nanoparticles and 64 times of that in the NaYbF4:0.5 %Tm@Y core/shell nanoparticles. Moreover, it is more than a tenfold enhancement compared to the well-known classic core-shell structures (NaYF4:20 %Yb, 0.5 %Tm@NaYF4). By restricting the activator to 2D planes, their interactions with vacancies that are randomly distributed in 3D space are minimized. As a result, the energy transfer rate of Yb3+ defects is suppressed in this sandwich structure, while further effectively maintaining or enhancing the energy transfer from Yb3+ to the activator. Worth mentioning is that the strongest upconversion luminescence is observed in the inert-active-inert sandwich structure with a critical inert core size of approximately 85 nm, which is in good agreement with the inflexion point of the calculated the effective excitation volume per unit cross-section.