Synthesis and characterization of rare-earth doped borates phosphors for application in solid state lighting
Lephoto, Mantwa Annah
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Inorganic borates have long been a focus of research for their variety of structure types, transparency to a wide range of wavelengths, high laser damage tolerance, and high optical quality. In the current study, borates such as BaB8O13 and LiBaBO3 were synthesized by using solution combustion method and solid state method respectively. These hosts were doped and co-doped with rare-earth ions such as europium (Eu3+), samarium (Sm3+), dysprosium (Dy3+), cerium (Ce3+) and non-rare-earth ion bismuth (Bi3+). The structure, particle morphology, stretching vibrations, photoluminescence and chemical composition of the materials were studied using different analytical techniques. The structure of the materials was studied using X-ray diffraction (XRD). Particle morphology was examined by scanning electron microscope (SEM) and transmission electron microscopy (TEM). The chemical composition analysis was carried out using energy dispersive spectrometer (EDS). The stretching frequency modes were examined using Fourier transform infrared spectroscopy (FTIR). The thermal analysis was carried out by thermogravimetric analysis (TGA). The optical properties of the materials were characterized using photoluminescence (PL) spectroscopy and ultraviolet-visible (UV-Vis) spectroscopy at room temperature. Thermoluminescence analysis was also carried out in this study. The XRD patterns of BaB8O13 doped with different rare-earths ions confirmed the formation of orthorhombic structure with cell parameters a = 8.550 Å, b = 17.350 Å and c = 13.211 Å. The patterns showed some extra peaks which were attributed to unreacted precursors. SEM images showed agglomeration of particles with irregular shapes. The infrared stretching frequencies detected in the spectral wavenumber range of 650 – 1600 cm-1 also confirmed the formation of the BaB8O13 host matrix. The chemical compositions from the EDS analysis confirmed the formation of the desired powder phosphors. From BaB8O13: Bi3+ powder phosphors, the broad PL emission due to 1S0 – 3P1 transitions of Bi3+ ions was observed at 548 nm in the green region of the visible spectrum under 271 nm excitation. The Commission International de I’Eclairage (CIE) coordinates of x = 0.3267 and y = 0.6004 suggest that the phosphor can be used as a source of green light in light emitting devices of different types. The decay spectra were also recorded and their characteristics showed that the phosphors consist of a single exponential decay process. The BaB8O13: Ce3+ powder phosphors showed PL emission at around 515 nm ascribed to 5d1 – 4f1 transition of Ce3+ after excitation at 270 nm. A standard CIE diagram derived from relative emissions from the powder phosphors suggested a unique emission concentrated in the green region, thus the phosphor serve as a potential source of green light in light emitting devices. BaB8O13: Eu3+ emits red light, and the strongest peak was located at 614 nm, which was attributed to the 5D0→7F2 transition of Eu3+. BaB8O13: Sm3+ produced red-orange light, and the major emission peak was located at 596 nm which was assigned to the 4G5/2→6H7/2 transition of Sm3+. When excited at 402 nm, the PL emission intensity from BaB8O13: 0.05Eu3+; 0.005Sm3+ at 614 nm was enhanced considerably compared to that of the sample without Sm3+, suggesting that energy was transferred from Sm3+ to Eu3+. The CIE coordinates of BaB8O13: 0.05Eu3+; 0.005Sm3+ powder phosphor (0.637, 0.362) were located in the red region indicating that the phosphor can be used as a source of red light in LEDs. The luminescence spectra of BaB8O13: Dy3+ excited by 350 nm showed two intense peaks at 480 nm and 574, corresponding to the 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions of Dy3+, respectively. According to the CIE coordinates, this phosphor has great potential as a single-component white-light-emitting phosphor for near-UV LEDs. The XRD patterns of the LiBaBO3 phosphors showed that they crystallized in a pure monoclinic phase. The scanning electron microscopy images showed that the powders were made up of fluffy needle-like particles that were randomly aligned. The band-gap of the LiBaBO3 host was estimated to be 3.33 eV from the UV-Vis absorption data. Blue emission was observed from the LiBaBO3 host which was ascribed to the self-activation of the host matrix. In addition, greenish-blue (493 nm) and red (613 nm) emissions were observed from europium-doped samples and were attributed to the emissions of Eu2+ and Eu3+, respectively. Furthermore, after co-doping with Bi3+, the emission intensity of Eu3+ located at 613 nm was significantly enhanced. From the CIE coordinates, the tunable color properties of LiBaBO3: Eu3+ indicated that the phosphors provide a potential to be a single component white light phosphor. LiBaBO3: Dy3+ showed three emission peaks at 482 nm (blue), 575 nm (yellow) and 664 nm (red) which were attributed to 4F9/2 – 6H15/2 , 4F9/2 – 6H13/2 and 4F9/2 – 6H11/2 transitions of Dy3+ respectively. The CIE chromatic coordinates and color-correlated temperature were also determined for this phosphor.