Facile and cost-effective technique to control europium oxidation states in glassy fluorophosphate matrices with tunable photoluminescence

Abstract

Inorganic fluorophosphate glasses doped with Eu$$^{2+}$$ 2 + /Eu$$^{3+}$$ 3 + are potential candidates for phosphors for commercial white LEDs. This report presents a fast, inexpensive and effective method of controlling the relative concentrations of Eu$$^{2+}$$ 2 + /Eu$$^{3+}$$ 3 + photoluminescent centers in these glasses. The technique consists of a fast quenching of the melt of initial reagents under appropriate conditions. Eu$$^{2+}$$ 2 + /Eu$$^{3+}$$ 3 + ratio was controlled by carrying out the melting under a reducing atmosphere at a temperature between 1000 and 1200 $$^\circ$$ ∘ C for periods of 5 to 15 minutes. The reducing atmosphere was provided by a ’double crucible’ technique and did not require special gas lines during the synthesis. The samples were studied by several complementary experimental methods (X-ray diffractometry—XRD, X-ray photoelectron spectroscopy—XPS, photoluminescence—PL—and photoluminescence excitation—PLE—spectroscopies as well as optical transmission spectroscopy). It was shown that the syntheses resulted in amorphous materials with different relative Eu$$^{2+}$$ 2 + /Eu$$^{3+}$$ 3 + concentration ratios, strongly dependent on the preparation conditions: the temperature and the time of melting in a reducing atmosphere. Moreover, changes in these ratios strongly affected the materials’ PL and PLE spectra. Demonstration of reproducible smooth transition from amaranth to blue luminescence color, with white in between, was the most spectacular result of this work.