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

The perfect energy level overlap of 2H11/2, 4S3/2, and 4F9/2 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.