| As the novel fourth-generation light source,white light emitting diodes(LEDs)exhibit the advantaged of energy conservation and environmental protection,which has replaced filament lamp,fluorescent lamp and high pressure gas discharge lamp.For white LEDs,phosphors play a key role in achieving optimal white light with excellent color rendering index and high correlated color temperature.Therefore,it will be significant for a high quality LEDs to develop a novel phosphor with excellent luminescent properties and thermal stability.Among them,rare earth doped oxynitride phosphors,such asSrSi2O2N2:Eu2+phosphor have been received more and more attention due to the high luminous efficiency,excellent chemical stability and thermal stability,and broad excitation ranges matched well with blue and near UV LED.However,it is still a great challenge to prepare SrSi2O2N2:Eu2+phosphor with single phase by traditional solid-state reaction or other synthetic methods.The impurity phases in SrSi2O2N2:Eu2+ phosphor will decrease the luminescence intensity and thermal stability.Therefore,it is more significant to synthesize pure phase of SrSi2O2N2:Eu2+ with expected thermal stability.Furthermore,most of the reported literatures are focused on the preparation method of SrSi2O2N2:Eu2+ and thermal stability(or thermal stability mechanism)of the fluorescent material is seldom concerned,which is important for the lifetime of LED light sources.In this paper,the green-emitting SrSi2O2N2:Eu2+ phosphors with single phase have been successfully synthesized by a simple two-step method.During the first step process,Sr2SiO4:Eu3+ precursor was formed and emitted red light under UV light excitation.The excessive amount of Si3N4 in reactants promoted the formation of pure SrSi2O2N2 phase when molar ratio ofSi3N4/Sr2SiO4 was located in the range of 1.1-1.5.The luminescence intensities of SrSi2O2N2:Eu2+ phosphors were enhanced by Si3N4/Sr2SiO4 molar ratio of1.1.Phase analysis,luminescence properties and thermal stability of the samples were intensively investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS)and TGA/DSC.The emission intensity of the phosphors increases firstly with increasing Eu2+ concentration(x)and reach the strongest intensity for emission peak at 527 nm when x = 0.02,then the emission intensity decreases due to the concentration quenching.The emission bands of the green phosphors ranged from 526 nm to 537 nm with the changes of x(0.01-0.14).The excitation spectra monitored at 526 nm show a broad band from 300 nm to 470 nm.The intense green emission bands-526 nm are observed in the emission spectra under the excitation of 370 n,,which is attributed to 4f65d-4f7 transition of Eu2+ ions.The thermal stability of SrSi2O2N2:Eu2+ has been investigated by a calcined method in air for 4 h at temperature of 400 ?,which will be in favor of the understanding of thermal degradation or thermal oxidation of phosphor.It can be seen from TG curves that an appreciable weight gain begins at 800 ? and a massive weight gain appears after 900 ?owing to the oxidation of phosphor.The thermal oxidation temperature is confirmed as 915 ?by extension lines.Based on the results of TG-DSC,Sr0.9sSi2O2N2:0.05Eu2+ phosphor was sintered at 1200? in air and crystalline SrSiO3 phase was found after the thermal oxidation process.As evidenced by XPS spectra,trivalent europium ions were identified on the surface of the as-synthesized samples with approximate theoretical atomic ratio of Eu/Sr and play a key role to avoid oxidation of Eu2+ ions in the bulk during a heat-treatment process.Therefore,the thermal oxidation of SrSi2O2N2:Eu2+ phosphors was attributed to the formation of the SrSiO3 layer on the surface of SrSi2O2N2 particles. |
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