Upconversion improvement by the reduction of Na+-vacancies in Mn2+ doped hexagonal NaYbF4:Er3+ nanoparticles

Abstract

Hexagonal-phase NaYbF₄:Er³⁺ 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 NaYbF₄:Er³⁺ UCNPs, has been achieved by Mn²⁺ doping. The underlying reason for luminescence enhancement by Mn²⁺ 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 Mn²⁺ substitution disturbs the equilibrium of the charge and crystal lattice in the hexagonal-phase NaYbF₄:Er³⁺ UCNPs, which makes the Na⁺-vacancies that quenched luminescence become filled with Na⁺ or Mn²⁺ to offset the imbalance of the charge and electron cloud distortion. In addition, the Mn²⁺ 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 Mn²⁺-doping approach provides a facile strategy for improvement of luminescence, which will impact on the field of bioimaging based on UCNP nanoprobes.