Luminescence Spectroscopy and Visible Upconversion Properties of Er3+ in ZnO Nanocrystals

Er 3+ -doped ZnO nanocrystals with hexagonal wurtzite structure were prepared by addition of LiOH to Zn−O−Er precursors in ethanol. The well-known green hot band from the 2 H 11/2 → 4 I 15/2 transition in the 520 − 540 nm region and Stokes emissions were investigated. And the blue, green, and red upconversion emissions were observed from the annealed samples under near-infrared light (808 nm) excitation. The upconversion emission intensity ratio of the hypersensitive 2 H 11/ 2 → 4 I 15/2 to 4 S 3/2 → 4 I 15/2 transition ( I 530 nm / I 550 nm ) in the sample annealed at 400 °C is different from that in the sample annealed at 700 °C, which is attributed to the variation of the local structure around Er 3+ ions. The upconversion emission spectra and the pump power dependence of the luminescence intensities are provided to explain the upconversion mechanisms. These results confirm that visible upconversion emissions of Er 3+ in the ZnO nanocrystals are mainly produced via two-photon excited-state absorption (ESA) and energy transfer upconversion (ETU) processes.