Tailoring local coordination structure of the Er ions for tuning the up-conversion multicolor luminescence.

The regulation of the local structure around Er ions is an important channel for adjusting the characteristic of up-conversion luminescence. In this paper, the cubic-phased Er:CaF crystals with different Er doping concentrations were fabricated with temperature gradient technique (TGT) method and the effect of the local coordination structure of the Er ions on its luminescence performance was investigated. The local coordination structure of Er ions was simulated by density functional theory. The computational results show that clusters evolve from low order to high order with the increase of Er ion doping concentration. In this evolution process, the local structure transforms from cubic structure to the co-existence of cubic and lower symmetric square anti-prism structures. Meanwhile, the distance between Er ions in the cluster decreased first and then increased slightly, and in dimers and trimers this distance reached the minimum. Under 980 nm excitation, with the increase of Er ion concentration, the intensity ratios of the red and green emissions of Er:CaF first increased from 0.61 to 42.03 and then decreased to 12.11. The corresponding up-conversion luminescence gamut was adjusted from monochrome green to red to red-yellow. This work provides a new thread for realizing upconversion multicolor luminescence by regulating the clusters of rare earth ions.