Design for Brighter Photon Upconversion Emissions via Energy Level Overlap of Lanthanide Ions

The perfect energy level overlap of ²H_11/2, ⁴S_3/2, and ⁴F_9/2 in Er³⁺ ions with those of ⁵F₃, ⁵F₄/⁵S₂, and ⁵F₅ in adjacently codoped Ho³⁺ ions allows efficient interenergy transfer. Therefore, in addition to routine activators, Er³⁺ or Ho³⁺ can further act as sensitizers to transfer the upconverted energy to nearby Ho³⁺ or Er³⁺, resulting in enhanced upconversion luminescence due to the emission overlap. Proper codoping of Er³⁺/Ho³⁺ or Ho³⁺/Er³⁺ obviously elevates the maximum doping concentration (thus producing additional upconverted photons) to a level higher than that causing luminescence quenching and significantly enhances upconversion emissions compared with those of singly Er³⁺ or Ho³⁺-doped host materials. Indeed, the so-far strongest red upconversion emission under 1532 nm excitation was obtained in LiYF₄:Er/Ho@LiYF₄ nanoparticles and Ho³⁺-sensitized Er³⁺ upconversion emissions excited by 1150 nm laser was simultaneously discovered. With great enhancement compared with that of singly Ho³⁺ doped counterparts, this work demonstrates the generality and rationality of our design strategy.