Preparation of Eu3+-Y3+ co-doping red-emitting phosphors for white-light emitting diodes (W-LEDs) application and investigation of their optical characteristics
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Eu3+-Y3+ co-doping Ca0.54Sr0.34−1.5x Eu0.08Y x (MoO4) y (WO4)1−y (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20. y = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0), have been prepared and their luminescent properties are investigated to seek a new red-emitting phosphor for ultraviolet-light emitting diodes chips. In absorption efficiency, luminescent intensity and chromaticity coordinates, Ca0.54Sr0.22(MoO4)0.2(WO4)0.8:0.08Eu3+, 0.08Y3+ phosphor is better than commercial Y2O2S:Eu3+ phosphor excited by 390–405 nm light emitting diodes chip.
KeywordsLuminescent Property White Lead Color Render Index Yellow Phosphor Luminescent System
Phosphors research are immensely important due to their applications in some fields such as lamp, plasma display panels (PDPs), field emission displays (FEDs) and solid-state lighting (SSL). White light based on light-emitting diodes (LEDs) already begin to replace traditional incandescent lamps and expected to replace fluorescent lamps in the near future. Now it is considered being a new generation of lighting owing to their high efficiency, reliability, pollution-free and low energy consumption [1, 2]. In 1998, the first white LED has been fabricated using blue LED with yellow phosphor YAG:Ce3+ .
In order to improve the efficiency of phosphors, crystal chemical substitutions have been carried out in the hosts lattice, such as YAG:Ce3+ ((Y1−a Gd a )3(Al1−b Ga b )5O12:Ce3+). However, the spectral composition of the light created by the conventional two- band white LED (blue LED combined with yellow phosphor) differs from that of natural white light due to halo effect of blue/yellow color separation and poor color rendering index (CRI) caused by the lack of red emission .
The other approach to obtain white light is to combine UV-LED/laser diode with blue, green and red (BGR) phosphors . High performance white LEDs have been fabricated by using NUV-LED with two or three kinds of blue, green, yellow, orange and red phosphors (BGYOR), which are based on SrS- and ZnS-based long wavelength phosphors . Conventional red phosphors that are used for LED application are sulfide-based phosphors(CaS:Eu2+, SrY2S4:Eu2+, Y2O2S:Eu3+ and ZnCdS:Cu,Al), of which thermal/chemical stability is poor and decomposed compositions release bad smell under extended UV-irradiation. However, their emission efficiency, such as Y2O2S:Eu3+ red phosphor, is less than that of the green and blue phosphors. It is reported Eu3+-substituted phosphors containing WO4 and MoO4 tetrahedra with scheelite-related structures [7, 8].
The efficiency of charge transfer (CT) band (due to MoO4/WO4 tetrahedra) is not good enough to give high emission intensity for near-UV excitation. So we have explored the possibility of Eu3+-Sm3+, Eu3+-Gd3+and Eu3+-La3+ co-doped double hosts MoO4–WO4 tetrahedra structure as potential red-emitting-phosphor candidates for white LEDs [9, 10, 11, 12, 13]. It is found that the emission efficiency of phosphors are improved obviously. In order to improve the emission property of phosphors, in this paper, we shall synthesize Eu3+-Y3+ co-activated double hosts MoO4–WO4 tetrahedra structure Ca0.54Sr0.34−1.5x Eu0.08Y x (MoO4) y (WO4)1−y phosphor by solid-state reaction in air and investigate its luminescent properties.
The stoichiometric reactants were first ground and pre-fired at 500 °C for 2 h, and then heated at 600–1,100 °C in air for 3 h and the powders were obtained. Then the luminescence properties of the product were studied, NH4Cl employed as flux.
2.2 X-ray powder diffraction and photoluminescence characterization
The structure of the product was recorded by X-ray powder diffraction (XRD) employing CuKα radiation at 40 kv and 250 mA. A step size of 0.02°(2θ) was used with a scan speed of 4°/min. Excitation and emission spectra, excitation and emission slits set at 2.5 nm, were measured by using a Hitachi F-4600 spectrometer equipped with a 150 W-xenon lamp under a working voltage of 500 V. All the measurements were performed at room temperature.
3 Results and discussion
3.1 X-ray powder diffraction analysis
3.2 SEM patterns analysis
3.3 Photoluminescence analysis
3.4 Red-emitting phosphors employment
The intensive emission of LED chip at ~400 nm can be observed in Fig. 6, which is benefitable to obtain a white-light LED by combining this phosphor with appropriate blue and green phosphors. From the application angle, one good mono-color LED phosphor with luminescent property should own three characteristics. Above all, the phosphor must have efficient absorption at ~400 nm. Secondly, the phosphor exhibits high luminous intensity under ~400 nm excitation. Last but not least, the chromaticity coordinates of the phosphor is close to the NSTC standard values.
In a word, it is investigated that the luminescent property of Ca0.54Sr0.22(MoO4)0.2(WO4)0.8:0.08Eu3+, 0.08Y3+ phosphor is better than that of commercial available Y2O2S:Eu3+ phosphor in the above three facets.
Through pre-heat, changing Y3+ concentration and Mo6+(W6+) content and adding appropriate NH4Cl content(employed as flux), the luminescent properties of phosphors can be improved. Ca0.54Sr0.22(MoO4)0.2(WO4)0.8:0.08Eu3+, 0.08Y3+ phosphor is better than commercial available Y2O2S:Eu3+ phosphor with 390–405 nm LED chip in absorption efficiency, luminescent intensity and the chromaticity coordinates. it is investigated that the red phosphors, Eu3+-Y3+ co-doped Ca0.54Sr0.22Eu0.08Y0.08(MoO4)0.2(WO4)0.8 is an excellent red-emitting phosphors for LED.
This work was funded by research grants from the Science and Technology Projects of Liaoning Province (Grant No.2006223004) of People’s Republic of China, and the Educational Department Funds of Liaoning Province (Grant No.05L329) of People’s Republic of China and Anhui Provincial International Cooperation of Science and Technology Agency (Grant No.09080703019) of People’s Republic of China.
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