Enhanced up-conversion emissions in ZnO-LiYbO2:RE3+ (RE = Er or Ho) hybrid phosphors through surface modification

• The obtained hybrid phosphors exhibited strong red or green dominated UC emission.
• Significantly enhanced UC emissions can be achieved after surface modification.
• The results indicated that surface states played a vital role on the UC emissions.
• Surface modification was proved to be an effective way to passivate surface states.

The ZnO-LiYbO2:RE3+ (RE = Er or Ho) hybrid phosphors were synthesized by a sol–gel method and then surface modified with ZnO via a seed deposition method. The structure, morphology and element distribution of the hybrid phosphors were studied by X-ray powder diffraction (XRD), field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectroscopy (EDS), respectively. Under 980 nm excitation, the ZnO-LiYbO2:RE3+ (RE = Er or Ho) hybrid phosphors exhibited strong red or green dominated up-conversion (UC) emissions and the emission intensities were further dramatically enhanced after being surface modified with ZnO. The transform infrared (FT-IR) measurement indicated that surface states, including surface dangling bonds and surface defects, can greatly affect the UC emission properties. Further pump power dependence of UC emission intensity revealed that the two-photon processes were involved in the red and green emissions in both the ZnO-LiYbO2:Er3+ and the ZnO-LiYbO2:Ho3+ hybrid phosphors, and then the possible UC mechanisms were discussed.

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