Strong up-conversion emissions in ZnO:Er3+, ZnO:Er3+–Yb3+ nanoparticles and their surface modified counterparts

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.

Graphical abstractRoom temperature up-conversion PL properties from sample B to C1, which indicates that the green emission of sample C enhanced greatly after MoO3 surface modification.Figure optionsDownload full-size imageDownload high-quality image (106 K)Download as PowerPoint slideHighlights► ZnO:Er3+/MoO3 and ZnO:Er3+Yb3+/MoO3 core/shell structures. ► Core/shell structures obtained by surface modified ZnO:Er3+ and ZnO:Er3+Yb3+ NPs. ► Core/shell structures improve green up conversion (UC) emissions greatly in the NPs. ► Green UC emissions in ZnO: Er3+Yb3+/MoO3 involve three-photon processes.