Tuning the upconversion luminescence of cubic KMnF₃:Yb³⁺/Er³⁺ nanocrystals through inert lanthanide ion doping

Multicolor lanthanide-doped upconversion nanoparticles have attracted extensive interest due to their application in biosensing, bioimaging, and therapeutics. Herein, a facile strategy of inert gadolinium or yttrium ion doping is proposed to implement tunable multicolor upconversion luminescence in a conventional single red emitting KMnF₃ matrix through engineering the local crystal structure and controlling the energy exchanging transfer process. Variation of the Gd³⁺ or Y³⁺ doping content in the KMnF₃:Yb³⁺/Er³⁺ lattice induces a gradual morphological transformation from cube to polyhedron, which arises from the increased lattice binding. The enlarged crystal cell size and the engineered local crystal structure enable the confinement of energy transfer as well as back transfer between Er³⁺ emitters and Mn²⁺ ions. Moreover, the partial replacement of Mn²⁺ ions by Gd³⁺ or Y³⁺ ions also weakens the intense interaction between Er³⁺ and Mn²⁺ ions. Based on the above functions caused by the precise control of the inert ion doping amount, a palette of multicolor UC luminescence shifting from red to green can be produced and immediately recognized by the naked eye. The results here provide the possibility of realizing simultaneous control of the physical dimensions as well as the luminescence properties of lanthanide-doped upconversion systems.