Optical Spectroscopy of Eu³⁺ Doped ZnO Nanocrystals

Abstract

The energy levels and local structures of Eu<sup>3+</sup> incorporated in the lattice and surface sites of ZnO nanocrystals\nwere investigated based on the high-resolution fluorescence spectra at 10 K. Radiative emissions from <sup>5</sup>D<sub>1</sub>\nwere first observed for Eu<sup>3+</sup> at the lattice site of ZnO. It is shown that the site symmetry of Eu<sup>3+</sup> at the lattice\nsite descends from <i>C</i><sub>3</sub><i><sub>v</sub></i> to <i>C</i><i><sub>s</sub></i> or <i>C</i><sub>1</sub>, whereas Eu<sup>3+</sup> ions at the surface occupy more disordered sites of the\nlowest symmetry <i>C</i><sub>1</sub>. The luminescence decay of <sup>5</sup>D<sub>0</sub> at the lattice site, showing a rise time and longer lifetime,\nbehaves distinctly from that of the surface sites. Because of a small filling factor (52%) of nanoparticles, the\n<sup>5</sup>D<sub>0</sub> lifetime of Eu<sup>3+</sup> is significantly affected by the surrounding medium, which can be well interpreted with\nthe virtual-cavity model. The Judd−Ofelt intensity parameters of Eu<sup>3+</sup> in ZnO nanocrystals were determined,\nwith Ω<sub>2,4,6</sub> values of (9.59, 8.11, <0.25) and (21.51, 2.30, <0.25) in units of 10<sup>-20</sup> cm<sup>2</sup> for Eu<sup>3+</sup> at the\nsurface and lattice sites, respectively. A defect-mediated energy transfer from the ZnO band gap to Eu<sup>3+</sup> was\nobserved. The growth mechanism for the incorporation of Eu<sup>3+</sup> into the ZnO lattice was also revealed.