Sr2-xTbxSi5N8 phosphors were prepared via the solid-state reaction process. Analytical-grade Sr3N2, Si3N4, and Tb4O7 were mixed according to the chemical formula of Sr2-xTbxSi5N8 (x = 0.02-0.10). The mixtures were ball-milled using zirconia balls for 2 h in an argon-filled glove box. Then the mixed powders were placed in molybdenum crucibles and calcined at 1300oC-1600oC for 8 h in a reducing atmosphere (10 vol% H2 and 90 vol% N2).
The structural analysis of the obtained samples was carried out using X-ray diffractometer (Rigaku, Ultima IV) with a standard CuK? X-ray source. The PDXL program was used to refine the crystal structure. The morphology, particle sizes, and elemental distribution were investigated using a field effect scanning electron microscope (FESEM, JEOL JSM- 7610F). TEM Rigaku, D/max 2550, JEOL-2010 transmission electron microscopy was used to investigate the microstructures of the synthesized phosphors. For TEM measurement, the specimens were prepared via the double-beam focused ion beam (FIB). The specimen preparation process by using FIB is shown in Figure 1. Figure 1 (a) displays the selected area of the powders and the picked out laminar specimens for TEM is shown in Figure 1 (b). The perpendicular view to the ion beam direction of the picked out thin foils are shown in Figure 1 (c) and (d), respectively. The indexing of the TEM diffraction spot and corresponding characterizations were carried out via CrysTBox software 27. The photoluminescence characteristics of the prepared phosphors were investigated using a fluorescence spectrophotometer (Hitachi, F-4500) with a Xe lamp as the excitation source. X-ray photoelectron spectroscopy (XPS) measurement was carried out with a theta probe angle-resolved XPS System (Thermo Scientific).
For fabricating WLEDs, the as-prepared phosphors were mixed with YAG: Ce3+ phosphors and dispersed in transparent silicon resin to prepare phosphor mixtures. The mixtures were then coated on 460 nm InGaN-based LED chips to fabricate LED devices. The electroluminescence characteristics of fabricated LEDs were measured using a CCE spectrophotometer (BRC112E). The Commission Internationale de I’Eclairage (CIE) coordinates were converted from the photoluminescence spectra using the color calculator software.