Strong up-conversion emissions in ZnO:Er3+, ZnO:Er(3+)-Yb3+ nanoparticles and their surface modified counterparts.

ZnO:Er(3+) and ZnO:Er(3+)-Yb(3+) nanoparticles (NPs) are fabricated by a sol-gel method, afterwards parts of which are separated and surface modified in Mo(NO(3))(3) solution. Analyses on phase and structure based on X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques indicate that Er(3+) and Yb(3+) are incorporated into the ZnO lattice successfully and after Mo treatment, a thin layer of MoO(3) forms on the NPs surface, forming core/shell structures. Raman scattering spectra reveal the existence of ZnMoO(4) in the shell part. Visible up-conversion (UC) is observed in all the samples, with Mo treated and untreated ZnO:Er(3+) emitting dominant but relatively weak red light, corresponding to (4)F(9/2)-(4)I(15/2) transition of Er(3+). In Yb(3+)-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:Er(3+)-Yb(3+)/MoO(3), green emission is increased while the red is suppressed in comparison to ZnO:Er(3+)-Yb(3+), 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.