The charge states’ conversion of the activator ions in CaF₂:Eu nanophosphors

ABSTRACT

According to stationary X-ray-excited luminescence spectra and thermally stimulated luminescence spectra of CaF₂:Eu nanophosphors, it was found that Eu³⁺ → Eu²⁺ conversion can occur during thermal annealing of fine-grained (d = 25 nm) nanoparticles in the 200–800°C range, which is accompanied by an increase in their size within 40–189 nm. An important role of the exciton mechanism of Eu²⁺ luminescence excitation was revealed according to the temperature dependence of X-ray-excited luminescence spectra of CaF₂:Eu nanoparticles of 114 nm size. The maximum of the X-ray-excited luminescence light output of CaF₂:Eu nanophosphors in the Eu²⁺ ions’ emission band was traced out at 400–500°C annealing temperature and at the size of nanoparticles of 114–180 nm. The subsequent growth of the annealing temperatures, particularly in the 800–1000°C range, causes the reduction of X-ray-excited luminescence light output because of the increment of lattice defects’ concentration due to a sharp increase in the size of nanoparticles and their agglomeration.

KEYWORDS:

• Eu³⁺ → Eu²⁺ conversion
• CaF₂:Eu nanoparticles
• thermal annealing
• X-ray-excited luminescence spectra
• thermally stimulated luminescence

Disclosure statement

No potential conflict of interest was reported by the authors.