TOF SIMS analysis and enhanced UVB photoluminescence by energy transfer from Pr~(3+) to Gd~(3+) in Ca_3(PO_4)_2:Gd~(3+),Pr~(3+) phosphor prepared by urea assisted combustion

摘要

Ca_3(PO_4)_2:Gd~(3+),Pr~(3+) phosphors with different concentrations of Gd~(3+) and Pr~(3+) were prepared by combustion process using metal nitrates as precursors and urea as fuel. The phosphors were characterized by X-ray diffraction (XRD), Scanning electron microscopy, Time of Flight-Secondary Ion Mass Spectrometry, ultraviolet-visible and photoluminescence (PL) spectroscopies. The XRD patterns were consistent with the standard rhombohedral phase of Ca_3(PO_4)_2 referenced in JCPDS Card No. 70-2065. The average crystallite size calculated using the Scherrer equation at different urea masses was in the range of ~60-120nm. The PL excitation spectra of Ca_3(PO_4)_2:Gd~(3+) and Ca_3(PO_4)_2:Pr~(3+) exhibited peaks at 220-280 and 300-490 nm associated with the f -> f transitions of Gd3* and Pr3* respectively. The ultraviolet B (UVB) emission resulting from the ~6P_(7/2)-> ~8S_(7/2) transition of Gd~(3+) was observed at 313 nm when the Ca_3(PO_4)_2:Gd~(3+) phosphor was excited at a wavelength of 274 nm using a monochromatized xenon lamp. Upon Pr~(3+) co-doping, the excitation peaks due to Gd~(3+) and Pr~(3+) f -> f transitions were suppressed and an intense broad excitation peak ascribed to the 4f ->4f5d transitions of Pr~(3+) was observed at 227 nm. The peak intensity of the UVB emission at 313 nm was shown to improve considerably when the Gd~(3+) and Pr~(3+) co-doped systems were excited at the wavelength of 227 nm suggesting that the Pr~(3+) is a good sensitizer of the 313 nm narrow line UVB emission from Gd~(3+).