Full-color emitting crystal engineered Sr3Al1−xSixO4+xF1−x: Eu2+/3+ oxyfluorides for developing bendable lighting composites

Abstract

A series of full-color emitting Eu2+/Eu3+-coexisted Sr2.9Al1−xSixO4+xF1−x: 0.1Eu2+/3+ (SASixOF: Eu2+/3+) oxyfluorides were synthesized by annealing the solid precursors in oxygen-deficient atmosphere. The structural changes in Sr3AlO4F (SAOF) owing to the Si4+ ions’ doping were visualized from the Rietveld refinement analysis. The substitution of Si4+ ions in Al sites contracted the AlO4 tetrahedra and could enlarge the Sr sites, and enabled the suitable occupation of Eu2+ ions in the Sr1 sites. Eventually, the X-ray photoelectron spectroscopy studies confirmed the valence conversion of europium ions from its trivalent to the divalent state owing to the Si4+ ions doping in SAOF: Eu2+/3+. Photoluminescence studies of SASixOF: Eu2+/3+ showed a bluish emission band at 482 nm for the 4f-5d transition of Eu2+ ions along with several sharp peaks above 550 nm owing to the intra f-f transition of Eu3+ ions. Increasing the Si4+ ions’ concentration subsequently, enhanced the Eu3+ to Eu2+ conversion rate decreased their emission intensity ratio, owing to which the emission color chromaticity was also tuned from orange-red (CIE: 0.48, 0.29) to nearly white (CIE: 0.30, 0.26) and eventually to the bluish region (CIE: 0.18, 0.23). The nearly white light-emitting composition SASi0.03OF: Eu2+/3+ and the intense bluish light-emitting optimum SASi0.06OF: Eu2+/3+ phosphors were further chosen for fabricating flexible composites based on phosphor and castor oil (CO). At 150 °C, the composite showed almost double emission than the phosphor powders owing to the thermal encapsulation of the powders provided by the CO matrix. The obtained composite started to degrade at a temperature as high as 300 °C. Therefore, the composite made with near white emitting SASi0.03OF: Eu2+/3+ phosphor was integrated with a 372 near UV-LED which showed intense cool white emission with the CIE of (0.29, 0.33), CCT of 7562 K, and CRI of 89. The above studies broadly suggested the adaptability of the obtained composites for flexible lighting applications.