Synthesis of green and yellow phosphors for white LED by spray pyrolysis

Abstract

An ultrasonic spray pyrolysis process was applied to the preparation of green and yellow candidates for white light emitting diode (LED). Flux materials and co-dopants were introduced to improve their luminescent properties in spray pyrolysis. Green emitting Eu-doped calcium magnesium chlorosilicate $[Ca_8Mg(SiO_4)_4Cl_2:Eu^{2+}]$ phosphor was prepared by spray pyrolysis and its luminescent properties were optimized by changing the ca precursor type, the Eu concentration and the post-treatment temperature. It was found that the ratio of calcium nitrate and calcium chloride, which were used as the source of Ca and Cl, respectively, significantly affects the PL intensity and the particle morphology. PL intensity was monotonically increased by increasing calcium chloride content with respect to calcium nitrate content was increased in the spray solution due to increases in the crystallinity and the phase purity of $Ca_8Mg(SiO_4)_4Cl_2:Eu^{2+}$ phosphor. $Ca_8Mg(SiO_4)_4Cl_2:Eu^{2+}$ was gradually turned from a spherical shape to a polyhedral shape of micron size as the calcium chloride content was increased in the spray solution. The concentration quenching of $Eu^{2+}$ was observed at 0.375 mol% of Ca ion. The optimized $Ca_8Mg(SiO_4)_4Cl_2:Eu^{2+}$ phosphor, which showed a broad emission spectrum from 460 to 550 nm with a peak position at 502 nm (x=0.1102, y=0.5325), had a high brightness under the excitation by short- and long-wavelength ultraviolet (UV) lights. To improve its PL intensity, yttrium and gadolinium were introduced as co-dopant materials. The excitation property in the range of long wavelength (280 nm ≥) was enhanced by co-doping. $Ca_8Mg(SiO_4)_4Cl_2:Eu^{2+}$ phosphor with co-dopant had 31 ~ 36 % higher PL intensity than that without co-dopant. The highest PL intensity was achieved when the co-doping concentration of yttrium was about 20 mol% of the doping concentration of europium. Another green emitting barium strontium silicate [$Ba...