Concentration and temperature dependent upconversion luminescence of CaWO4: Er3+, Yb3+ 3D microstructure materials

We have successfully synthesized upconversion CaWO4:Er3+,Yb3+ 3D microstructure materials by employing a hydrothermal method, while using different concentrations of the precursors (CaCl2, Na2WO4). By adjusting the concentration of the precursors the shape of the CaWO4:Er3+,Yb3+ particles changed from dumbbell-like to spherical, the size first decreased and then increased with the precursor concentration change. The morphology of the obtained samples was uniform except for the 10 mmol CaWO4:Er3+,Yb3+ sample, where the particles were aggregated. XRD measurements were carried out to confirm that the structure of the CaWO4 products prepared at different precursor concentrations remained unchanged. The products synthesized at different concentrations had different exothermic peaks, therefore different heat-treatment temperatures were used. Regarding the upconversion luminescence properties, the intensities of the different samples without heat treatment were almost identical, only the relative intensity of the green emission and the red emission changed. In addition, by adjusting the heat-treatment temperature from 300 degrees C to 1000 degrees C, for the 5 mmol CaWO4:Er3+,Yb3+ sample the relative intensity of the green and the red emission slowly increased and then decreased. As a result of this altered ratio of the two emission peaks the corresponding CIE coordinates also changed. For comparison, the 10 mmol CaWO4:Er3+,Yb3+ sample heat treated from 400 degrees C to 1100 degrees C exhibited similar luminescence behaviour, except that the difference in intensity between the S-4(3/2) -> I-4(15/2) and F-4(9/2) I-4(15/2) peaks was larger with temperature change than observed for the 5 mmol CaWO4:Er3+,Yb3+ sample. The H-2(11/2) -> I-4(15/2), S-4(3/2) -> I-4(15/2) and F-4(9/2) -> I-4(15/2) transitions all can be explained by a two-photon absorption process.