Tuning Long-Lived Mn(II) Upconversion Luminescence through Alkaline-Earth Metal Doping and Energy-Level Tailoring

Crystal-site engineering of hexagonal-phase NaGdF₄:Mn through alkaline-earth metal (A²⁺) doping is used to tune the upconversion luminescence of the Mn²⁺ dopants. Experimental and calculated results show the ability of A²⁺ doping to alter the occupancy site of Mn²⁺ ion from Na⁺ to Gd³⁺, thus enabling an emission change from green (520 nm) to yellow (583 nm) upon excitation at 980 nm. The yellow emission of Mn²⁺ shows a long emission lifetime of 65 ms, more than three times longer than the green emission of Mn²⁺ (20 ms). The combination of tunable long-lived Mn²⁺ emission with lanthanide emission at a single-particle level provides a convenient route to triple transient upconversion color codes upon dynamic excitation, which offers an attractive optical feature particularly suitable for efficient document encoding.