White light-emitting calcium tungstate microspheres synthesized via co-precipitation at room temperature and application to UV-LED chip

Crystalline CaWO ₄ was synthesized by a co-precipitation method at room temperature. XRD peak at a position like that in the reference pattern, and the signal was detected from the (112) plane. Raman signals suggested the presence of symmetrical vibrations of the WO ₄ bond. The absorption was observed in the UV region, and the bandgap calculated from the Tauc plot was 4.47 eV. When the sample was excited at 230 nm, blue emission with a peak at 430 nm was observed. Crystalline CaWO ₄ was doped with Dy ³⁺ ions to synthesize a white light-emitting phosphor. The addition of Dy ³⁺ ions altered the crystal structure and shifted the Raman signals slightly. The sample was excited at 230 nm, both 430 nm and 575 nm emissions were observed, and the intensity of the 575 nm emission increased with increasing Dy ³⁺ concentration. The CaWO ₄: _0.1Dy ³⁺ sample showed the highest luminescence intensity, and excessive doping decreased the luminescence intensity owing to concentration quenching. The photoluminescence decay lifetime initially increased with increasing Dy ³⁺ concentration but decreased at concentrations greater than 0.1 mol% because of concentration quenching. The synthesized white light emitting CaWO ₄:Dy ³⁺ powder was mixed with PDMS to form a flexible composite. The composite could be easily bent with a finger and emitted white light when applied on a UV-LED chip.