Power- and energy-dependent photoluminescence of Eu3+ incorporated and segregated ZnO polycrystalline nanobelts synthesized by a facile combustion method followed by heat treatment

Abstract

A simple and efficient combustion method has been developed for pure and doped ZnO polycrystalline nanobelts. Eu³⁺ can form a supersaturated solid solution in the nanobelts annealed at 600 °C. Photogenerated electrons cannot transfer effectively from the ZnO conduction band (CB) to the excited states of incorporated Eu³⁺ ions and exchange energy. However, incorporated Eu³⁺ can form a trap level occurring at about 90 meV below the CB minimum, thus photogenerated electrons can relax rapidly from the CB to the shallow level trap and then recombine with the free-holes of the valence band edge, becoming a dominant emission with increasing excitation power density. The heavy doping not only results in excitonic localization, but induces Auger quenching and Fano effects at higher excitation densities. Due to the solid solubility limit, however, excess Eu³⁺ can segregate from the ZnO matrix as Eu₂O₃ precipitates and form a ZnO/Eu₂O₃ hybrid structure upon annealing at 1000 °C. Moreover, the overlap between ZnO emission bands and Eu₂O₃ absorption lines results in an efficient energy transfer from ZnO host to Eu₂O₃ clusters and an appearance of a dominant ⁵D₀–⁷F₂ emission line in PL spectra.