Optical Spectroscopy of Eu³⁺ Doped ZnO Nanocrystals

Abstract

The energy levels and local structures of Eu3+ 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 5D1 were first observed for Eu3+ at the lattice site of ZnO. It is shown that the site symmetry of Eu3+ at the lattice site descends from C3v to Cs or C1, whereas Eu3+ ions at the surface occupy more disordered sites of the lowest symmetry C1. The luminescence decay of 5D0 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 5D0 lifetime of Eu3+ is significantly affected by the surrounding medium, which can be well interpreted with the virtual-cavity model. The Judd−Ofelt intensity parameters of Eu3+ in ZnO nanocrystals were determined, with Ω2,4,6 values of (9.59, 8.11, <0.25) and (21.51, 2.30, <0.25) in units of 10-20 cm2 for Eu3+ at the surface and lattice sites, respectively. A defect-mediated energy transfer from the ZnO band gap to Eu3+ was observed. The growth mechanism for the incorporation of Eu3+ into the ZnO lattice was also revealed.