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

The perfect energy level overlap of H, S, and F in Er ions with those of F, F/S, and F in adjacently codoped Ho ions allows efficient interenergy transfer. Therefore, in addition to routine activators, Er or Ho can further act as sensitizers to transfer the upconverted energy to nearby Ho or Er, resulting in enhanced upconversion luminescence due to the emission overlap. Proper codoping of Er/Ho or Ho/Er 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 Er or Ho-doped host materials. Indeed, the so-far strongest red upconversion emission under 1532 nm excitation was obtained in LiYF:Er/Ho@LiYF nanoparticles and Ho-sensitized Er upconversion emissions excited by 1150 nm laser was simultaneously discovered. With great enhancement compared with that of singly Ho doped counterparts, this work demonstrates the generality and rationality of our design strategy.