Simultaneous 980 and 800 nm Orthogonal Upconversion and Ligand-Sensitized Downshifting of Triply Excited and Multicolor-Emitted Core–Multishell Nanocrystals for High-Level Anticounterfeiting Applications

In this study, we synthesized four types of core-pentad shell nanocrystals (NCs) through a layer-by-layer thermal decomposition method. Each NC was doped with distinct lanthanide ions in specific layers to achieve tricolor emissions by orthogonal dual upconversion under 980 and 800 nm excitation, as well as downshifting through ligand sensitization. To prevent energy transfer between the two upconversion emissive layers, an inert interlayer was introduced. Consequently, the NCs exhibited two distinct emission colors for the two different NIR excitations. Nd ions were incorporated for 800 nm excitation and separated by a Yb layer with the activator layer to eliminate cross relaxations between activator ions (Er or Tm) and Nd ions. The NCs were further capped with the 2,6-PDA ligand to sensitize Eu or Tb ions by UV excitation. Diverse patterns and matrices were screen-printed by utilizing these triply excited emissive NCs. These unclonable printed patterns hold excellent potential for advanced anticounterfeiting applications.