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 °C to 1000 °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 °C to 1100 °C exhibited similar luminescence behaviour, except that the difference in intensity between the ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 peaks was larger with temperature change than observed for the 5 mmol CaWO₄:Er³⁺,Yb³⁺ sample. The ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions all can be explained by a two-photon absorption process. - Graphical abstract: The influence of precursor concentration and heat-treatment temperature on the morphology, and upconversion luminescence properties of CaWO₄: Er³⁺, Yb³⁺3D microstructure materials, has been investigated.