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 f 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 f 4 I 15/2 to 4 S 3/2 f 4 I 15/2 transition (I 530nm /I 550nm ) 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.
IntroductionIn recent years, rare-earth (RE) doped nanocrystals have attracted a great deal of attention due to particle-size dependent optical properties, 1-3 which lead to their potential application in phosphors, 1,4 display monitors, 5 optical communication, 6,7 and two-photon fluorescence imaging. 8 Furthermore, RE ions, especially trivalence erbium, are much suitable for the conversion of infrared to visible light (upconversion) due to a favorable electronic energy level structure. The excited states ( 4 I 9/2 and 4 I 11/2 ) of Er 3+ ion with long lifetimes can easily be populated with near-infrared (NIR) radiation. The availability of low-cost NIR laser diode brings much convenience for the realization of upconversion emissions. Moreover, upconversion emissions of Er 3+ -doped nanocrystals have been achieved in different crystal structure hosts, such as cubic ZrO 2, Y 2 O 3 , Lu 2 O 3 , BaTiO 3 ; 7-15 tetragonal La 2 (MoO 4 ) 3 ; 16 and anatase and rutile TiO 2 . 8,17 Fluorescence labeling is very useful practice in biological applications, such as immunoassay, DNA sequencing, and clinical diagnostics. 18-24 Ultrasensitive detection of nucleic acid marked with upconversion phosphors (UCP) has been reported by Rijke et al. 21 As fluorescence labeling material compared with conventional fluorophores (e.g., Cy5, TMR), 18,24 UCP with no photobleaching shows higher sensitivity and lower background, where high-luminescence efficiency and homogeneous nanometer size distribution are critically important for detection of biomolecules (protein, DNA, RNA) and immunoassay. This work is motivated by not only the purpose of the Er 3+ -doped nanocrystals as UCP for fluorescence label but also the