Erbium Environment in ZnO:Er Polycrystalline Fibers Produced by Electrospinning

Abstract

Erbium-doped polycrystalline ZnO fibers were prepared by electrospinning. Organo-metallic Zn and Er (0.5, 1.0 and 2.0 % Er/Zn ratio) precursors dissolved in methanol, DI water and acetic acid were forced through a syringe needle. An electric field of 20 kV over 16 cm was used to promote the growth of fibers from the needle to a rotating drum. The resulting material consisted of entangled fibers with 0.5 to 1 μm typical diameter. The as-prepared samples present faint Er³⁺ luminescence at 1.5 μm when excited by the 488 nm line of an Ar⁺ laser. This luminescence intensity increases up to 25 times when the samples are annealed for 30 min at 500 °C in air. The effect of annealing over the Er environment was determined by EXAFS measurements at the Er L₃ edge. In all as-prepared samples Er is coordinated to 8 ± 0.5 oxygen atoms at up to 2.36 ± 0.02 Å interatomic separation. After annealing the coordination is reduced to 5.9 ± 0.5 and the atomic separation becomes 2.30 ± 0.02 Å. This is comparable to Er₂O₃ where the coordination is 6 and the first neighbor distance is 2.26 ± 0.02 Å. The overcoordination in the as-prepared samples is consistent with the Er L₃ edge at 8361.7 ± 0.5 eV compared to 8360.2 ± 0.5 eV in the annealed samples and 8359.9 ± 0.5 eV in Er₂O₃. In conclusion, the annealing of electrospun ZnO:Er fibers makes the Er environment more similar to that of Er₂O₃ increasing the luminescence efficiency.