Abstract
Tetragonal structured Sr3AlO4F is highly strained as reported from its global instability index estimation. Moreover, our results of X-ray photoelectron spectroscopy (XPS) also ascertained that the structure of Sr3AlO4F is highly strained with oxygen vacancies. Herein, aliovalent substitutions of divalent Sr ions with trivalent Ln (Ln = Gd/Y) ions were carried out to improve the stability of Sr3AlO4F lattice, which subsequently enhanced the photoluminescence in a series of Sr2.9-3x/2LnxAlO4F: 0.1Eu3+ phosphors. All the phosphors showed intense red-orange emission (5D0 → 7F1,2) at excitation with UV and near-UV light. The critical concentrations of Gd3+ and Y3+ up to which the Eu3+ emission intensities increased linearly were observed to be x = 0.09 and x = 0.07, respectively. Nevertheless, further enhancement in the Eu3+ luminescence of the optimized phosphors was realized by subsequently annealing in low oxygen atmospheres. The enhancement in oxygen deficiency during the post-annealing in Ar or vacuum led the energy transfer (O2--Eu3+) to a greater extent which afterward increased the Eu3+ luminescence. The optimized Sr2.765Gd0.09AlO4F: 0.1Eu3+ and Sr2.795Y0.07AlO4F: 0.1Eu3+ phosphors showed high red color purity (~99%), as well as CIE coordinates of (0.62, 0.38), indicated that these phosphors could be appropriate red-emitting components for making flexible optical films for many lighting devices. Therefore, flexible polydimethylsiloxane based films were also fabricated using optimized Sr2.765Gd0.09AlO4F: 0.1Eu3+ phosphor. The electroluminescence of a flexible PDMS-phosphor composite film showed an intense and pure red color with good thermal stability suggesting its suitability in flexible lighting and display devices.
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