Energy Migration Upconversion in Manganese(II)-Doped Nanoparticles

We report the synthesis and characterization of cubic NaGdF₄:Yb/Tm@NaGdF₄:Mn core-shell structures. By taking advantage of energy transfer through Yb→Tm→Gd→Mn in these core-shell nanoparticles, we have realized upconversion emission of Mn²⁺ at room temperature in lanthanide tetrafluoride based host lattices. The upconverted Mn²⁺emission, enabled by trapping the excitation energy through a Gd³⁺ lattice, was validated by the observation of a decreased lifetime from 941 to 532 μs in the emission of Gd³⁺ at 310 nm (⁶P_7/2→⁸S_7/2). This multiphoton upconversion process can be further enhanced under pulsed laser excitation at high power densities. Both experimental and theoretical studies provide evidence for Mn²⁺ doping in the lanthanide-based host lattice arising from the formation of F^- vacancies around Mn²⁺ ions to maintain charge neutrality in the shell layer. On the up: The upconversion emission of Mn²⁺ ions can be realized in NaGdF₄:Yb/Tm@NaGdF₄:Mn core-shell nanoparticles by utilizing energy migration through the gadolinium sublattice (see figure). The multiphoton upconversion process can be further enhanced under pulsed laser excitation at high power densities. The ability of the Mn²⁺-doped nanoparticles to easily undergo oxidization makes them attractive for use in hydrogen peroxide detection.