Synthesis and luminescence properties of strontium silicate phosphors by using a microwave-assisted sintering

Abstract

The thesis reports the preparation of Eu2+ ions doped strontium silicate (Sr2SiO4, Sr3SiO5) phosphors by using microwave-assisted sintering method. In the case of greenish-yellow emitting Sr2SiO4:Eu2+ phosphors, the effect of microwave-assisted sintering time on the structural and luminescent properties was investigated. The XRD patterns confirmed their monoclinic structure of β-Sr2SiO4 with the space group of P21/n. The full width at a half maximum (FWHM) of the diffraction peaks was narrower with increasing the microwave-assisted sintering time from 10 to 50 min, indicating the improved crystallinity of Sr2SiO4:Eu2+. The scanning electron microscope (SEM) images exhibited the irregular morphology of Sr2SiO4:Eu2+ particles with the sizes in the range of 1~3 ㎛. The photoluminescence excitation (PLE) spectra of Sr2SiO4:Eu2+ exhibited an intense broad band between 260 to 485 nm region with a band maximum at 361 nm, this broad band is attributed to the 4f8→4f75d1 transition of Eu2+ ions. Upon excitation at 361 nm, PL spectra showed intense greenish-yellow emission centered at 541 nm. As microwave-assisted sintering time increased from 10 to 50 min, the PL intensity also increased. However, the PL intensity decreased when the microwave-assisted sintering time further increased above 50 min, resulting the optimal microwave-assisted sintering time was 50 min. Upon excitation at 361 nm, the quantum efficiency (QE) of the optimized Sr2SiO4:Eu2+ is 63.27% and CIE chromaticity value was (0.306, 0.477). In the case of yellowish-orange emitting Sr3-xSiO5:xEu2+ (x=0.01~0.05) phosphors, the effect of Eu2+ ion concentrations in the Sr3SiO5 was investigated. The XRD patterns confirmed their tetragonal structure with a space group of P4/ncc. The SEM images exhibited the irregular morphology of Sr3SiO5:Eu2+ particles with the sizes between 1~3 ㎛. PLE spectra of Sr3SiO5:Eu2+ exhibited intense and broad band with two dominant peaks of 354 and 396 nm in the ultraviolet region (UV) due to the 4f8→4f75d1 transition of Eu2+ ions. Under the excitation at 354 and 396 nm, the PL spectra showed a strong yellowish-orange emission peaking at 575 nm. It was observed that the PL intensity increased as the concentration of Eu2+ ion increased. The optimal concentration of Eu2+ ion was found to be 0.04 mol. The CIE chromaticity values of optimized Sr3SiO5:Eu2+ were (0.428, 0.457) under the excitation wavelength of 354 nm and (0.466, 0.486) under the excitation wavelength of 396 nm, respectively. The results suggest that microwave-assisted sintering process is a promising approach for synthesis of phosphors due to the drastic reduction on rapid heating without extra reductive agents.