Effect of Yb3+ concentration on the green-yellow upconversion emission of SrGe4O9:Er3+ phosphors

This work presents the structural, morphological and luminescent, properties of SrGe4O9 (SGO):Er3+,Yb3+ phosphors. These phosphors were synthesized by simple combustion synthesis and subsequently annealed at 1100 degrees C. The XRD patterns revealed that all the SGO samples doped with Yb3+ concentrations from 2 to 10 at.% presented a trigonal pure phase (the Er3+ concentration was fixed to 1 at.%). The morphology of the SGO samples was analyzed by scanning electron microscopy and found that they are formed by microparticles with irregular shapes and average sizes in the range of 0.2 mu m-3 mu m. The luminescence measurements of the SGO:Er3+,Yb3+ samples showed the presence of two main emission bands at 551 nm (green) and at 662 nm (red) under excitation at 980 nm, which are associated to Er3+ transitions. For Yb concentration of 2 and 3 at.% the green band dominated, but the red band became more intense for Yb concentrations above 5 at.%. As result, the CIE coordinate changed from the green to the yellow region. The increase for the Yb content from 2 to 10 at.% also enhanced of the NIR emission of Er3+approximate to 5 times and the maximum upconversion emission was observed for 8% of Yb concentration. Further, the surface of the SGO samples was analyzed by the FTIR technique in order to find OH groups which are common luminescent quenching centers, but these groups were not detected on the samples. Since the SGO samples presented tunable emission, absence of OH groups on their surface and stable crystalline structure for high Yb dopant concentrations, they could be good candidates as phosphors for solid state lighting or displays applications.