Strong up-conversion emissions in ZnO:Er³⁺, ZnO:Er³⁺–Yb³⁺ nanoparticles and their surface modified counterparts
2011
Meng, Xiuqing | Liu, Chaoren | Wu, Fengmin | Li, Pu
ZnO:Er³⁺ and ZnO:Er³⁺–Yb³⁺ nanoparticles (NPs) are fabricated by a sol–gel method, afterwards parts of which are separated and surface modified in Mo(NO₃)₃ solution. Analyses on phase and structure based on X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques indicate that Er³⁺ and Yb³⁺ are incorporated into the ZnO lattice successfully and after Mo treatment, a thin layer of MoO₃ forms on the NPs surface, forming core/shell structures. Raman scattering spectra reveal the existence of ZnMoO₄ in the shell part. Visible up-conversion (UC) is observed in all the samples, with Mo treated and untreated ZnO:Er³⁺ emitting dominant but relatively weak red light, corresponding to ⁴F₉/₂–⁴I₁₅/₂ transition of Er³⁺. In Yb³⁺-codoping systems, the integral UC intensity is enhanced obviously though red emission still dominates the UC spectra before surface modification. In the Mo treated system, ZnO:Er³⁺–Yb³⁺/MoO₃, green emission is increased while the red is suppressed in comparison to ZnO:Er³⁺–Yb³⁺, with the intensity of green to red ratio (GRR) changing from 0.25 to 8. A novel phenomenon is discovered that the green emissions in our samples involve three-photon processes.
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