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 (650â670 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 Fâ 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.