Upconversion improvement by the reduction of Na + -vacancies in Mn 2+ doped hexagonal NaYbF 4 :Er 3+ nanoparticles

Hexagonal-phase NaYbF 4 :Er 3+ upconversion nanoparticles (UCNPs) have been synthesized via a co-precipitation method in high-boiling-point solvents, and remarkably enhanced upconversion luminescence, particularly in red emission bands (650–670 nm) in NaYbF 4 :Er 3+ UCNPs, has been achieved by Mn 2+ doping. The underlying reason for luminescence enhancement by Mn 2+ doping is explored by a series of controlled experiments, and a mechanism of enhancement based on the decrease of Na + -vacancies and organic adsorption is proposed. The Mn 2+ substitution disturbs the equilibrium of the charge and crystal lattice in the hexagonal-phase NaYbF 4 :Er 3+ UCNPs, which makes the Na + -vacancies that quenched luminescence become filled with Na + or Mn 2+ to offset the imbalance of the charge and electron cloud distortion. In addition, the Mn 2+ doping at the surface of UCNPs could reduce the organic adsorption on the surface of the UCNPs by an extra F − ion on the grain surface resulting in luminescence enhancement. Therefore, the Mn 2+ -doping approach provides a facile strategy for improvement of luminescence, which will impact on the field of bioimaging based on UCNP nanoprobes.