Preparation And Luminescent Properties Of Phosphors Excited By NUV Light For White LEDs

At present, using a NUV LED chip coated red, green and blue phosphors has become the emphasis direction to generate white LED. Therefore, to explore tricolor phosphors that can be excited effectively by near UV LEDs (NUV) has attracted substantial attention. However, these types of tricolor phosphors are not much, being the drawbacks of low luminous efficiency and bad thermal stability. To solve the problems, two types of phosphors obtained by RE ions doped in orthosilicate and meta-aluminate hosts with stable thermal and chemical properties were prepared, and size, morphology and dispersion of the phosphors particles are improved by using chemical synthesis method in the paper. The effects of crystal structure, electronic band structure, and synthesis method on luminescent properties were systematically investigated. The main achievements are as the follows:1. A system of Rare earth doped orthsilicate phosphors:(1)M2Si04:Eu3+(M=Ca, Sr, Ba) red-emitting phosphors were synthesized by solid state reaction and these luminescent properties were investigated. M2Si04have two different cation sites:M(â… )(asymmetric) and M(â…¡)(symmetric). In the Ba2Si04:Eu3+, Eu3+mainly occupied at M(I) cation, causing the613nm emission of5D0â†'7F2strongest. In the Ca2SiO4:Eu3+and Sr2SiO4:Eu3+, it was equal probability that Eu3+occupied at M(☉) and M(â…¡) site, which made the transition5D0to7F1(591nm) was as strong as the transition5D0to7F2(613nm). Ba2SiO4:Eu3+phosphor had stronger excitation and emission intensity with better red color purity when compared with Sr2Si04:Eu3+and Ca2SiO4:Eu3+phosphors.(2)The influence of annealing temperature, Eu3+dopant content and charge compensation (Li+, Na+, K+) on intensities of Sr2Si04:Eu3+and Ba2SiO4:Eu3+were studied. When annealing temperature was1300℃, the emission intensity of the samples reached maximum.The quenching content of Eu3+was6%and8%in the Sr2SiO1:Eu3+and Ba2SiO4:Eu3+,respectively. The charge compensator R+(R+=Li+,Na+and K_)injecting into the host efficiently enhanced the luminescence intensity,and the emission intensity of the phosphor doping L+was higher than that of Na+or K+(3)Sr2SiO4:Eu+and SiO2@Sr2SiO4:Eu3+core-shell phosphor were prepared by sol-gel method. Compared with the sample synthesized by solid state reaction, Sr2SiO4:Eu3+had possessed low crystal tempreture, strong emission intensity. SiO2@Sr2SiO4:Eu3+was composed of uniform coating and smooth shell, the emission intensity of SiO2@Sr2SiO4:Eu3+core-shell phosphor was higher than Sr2SiO4:Eu3+ (4) The luminescent properties of Sr2SiO4:Ce3+and Sr2SiO4:Eu2+were investigated. Two samples showed two emission peaks under NUV excitation. The two emission peaks of Sr2SiO4:Ce3+were406nm and436nm, respectively. The two emission peaks of Sr2SiO4:Eu2+were463nm and535nm. Ce3+/Eu2+ions on Sr(I) Sites gave short wavelength emissions, while Ce3+/Eu2+ions on Sr(II) Sites showed an long emission band. With the increasing of Eu content, the blue emission intensity of Sr2SiO4:Eu2+decreased, whereas the green emission intensity increasing. Red shift of emission band from535nm to569nm has been aehieved with the increase of Eu2+content. The electronic structure and host absportion were performed by first principles.2. Eu2+doped meta-aluminate phosphors:(1)The SrAl2O4:Eu2+green-emitting phosphors were synthesized by sol-gel method, and their luminescent properties were investigated with changing the reducing temperature, the concentration of the activator. The highest intensity of SrAl2O4:Eu2+phosphor was achieved by annealing at1200℃and the Eu2+content of8mol%. Over1300℃, however, the Sr4Al14O25phase appeared, inducing a small blue-shift in the emission peak (519nm-509nm).(2) The effects of the host component variety through substituting cation in the host lattice on Eu2+emission spectra were researched. The substitution of B3+for Al3+ions in SrAl2O4host resulted in the transformation of phase from SrAl2O4to Sr4Al14O25as well as to SrB2Al2O7, which led to the blue-shift (519nm-467nm).When Ca2+and Ba2+ions replaced Sr2+ions in the host, SrAl2O4 CaAl2O4and BaAl2O4solid solution can be formed throughout the whole composition range. So, the blue, blue-green and green emitting of Eu2+can be tuned in the hosts.3. Other red borate phosphors:The luminescent properties of LiSrBO3:xEu3+phosphors were studied. This phosphor can be effectively excited by ultraviolet (395nm) and exhibit a satisfactory red performance peaked at around612nm. The introduction of charge compensator Li+, Na+and K+can further enhance emission intensity.The introduction of Al3+improved the color purity of LiSrBO3:Eu3+. When Al3+content was2%, the chromatieity coordinates was (0.652,0.348).