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

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.