Transformation of free-standing ZnO nanorods upon Er doping

Erbium doped zinc oxide (ZnO:Er) powder was synthesized by hydrothermal growth. The morphology changed from undoped nanorods to microrods. The effect of Er doping on luminescence properties of ZnO:Er was observed. Furthermore, annealing in air at elevated temperatures up to 700 degrees C results in additional modification of defect states and related luminescence properties. In particular, exciton-related UV emission band and the defect-related red emission band appear in all annealed samples. Er3+ infrared (IR) photoemission, the typical I-4(13/2) -> I-4(15/2) transition, not observed in as grown samples with low Er concentration, occurs at room temperature in all ZnO:Er samples after annealing in air at elevated temperatures. Oppositely, the electron paramagnetic resonance (EPR) signal of Er3+ is getting weaker upon the annealing at elevated temperatures, disappearing completely at about 500 degrees C. The intensity of the g = 1.96 EPR spectrum, typical for ZnO structures, is getting decreased and almost disappears after the annealing at 500 degrees C but increasing again after the annealing at 600 degrees C and 700 degrees C. X-ray photoelectron spectroscopy and IR Raman spectroscopy suggested that annealing in air causes diffusion of Zn from the surface to the bulk and oxidation of Er to Er3+ in Er2O3 and Er(OH)(x) forms.