Luminescence spectroscopy and visible upconversion properties of Er3+ in ZnO nanocrystals

Er3+-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 H-2(11/2) --> I-4(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 H-2(11/2) --> I-4(15/2) to S-4(3/2) --> I-4(15/2) transition (I-530nm/I-550nm) in the sample annealed at 400degreesC is different from that in the sample annealed at 700degreesC, which is attributed to the variation of the local structure around Er3+ 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 Er3+ in the ZnO nanocrystals are mainly produced via two-photon excited-state absorption (ESA) and energy transfer upconversion (ETU) processes.