Optical Spectroscopy of Eu³⁺ Doped ZnO Nanocrystals

Abstract

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