1. Introduction

White LED (Light Emitting Diode) is a promising strategy for energy conservation in solid-state lighting due to the advantages such as high efficiency, long lifetimes, low power consumptions, and mercury-free design. In general, white LEDs are generated by not only combining the light of several colored LEDs, but also comprising a blue or ultraviolet LED with phosphors (called phosphor-converted white LEDs).

The first demonstration of phosphor-converted white LEDs was achieved by combination of a blue LED chip and a yellow phosphor YAG:Ce (Y₃Al₅O₁₂:Ce³⁺) [1]. However, the application of YAG:Ce-converted white LEDs faces some disadvantages such as low color rendering index (CRI), saturation of luminous efficacy at high drive levels [2], and color drift by different drive conditions [3]. These problems may be solved by comprising one ultraviolet LED and two phosphors, because of their high luminous flux and color stability under high operating current [2, 3]. For example, Narukawa et al. have reported a white LED, which is combined near-ultraviolet LED, blue phosphors, and yellow phosphors, exhibits better performances of color temperature, color rendering, and luminous efficacy, in comparison with that comprised by a blue LED and a yellow phosphor [2]. Phosphor-converted white LEDs has also been achieved by using an ultraviolet LED with tricolor (red, green, and blue) phosphors [4]. Based on this idea, high CRI (>95) has been obtained by mixing multichromatic phosphors with near ultraviolet LEDs [5]. Under this condition, high efficient blue emitting phosphors are essential for white LEDs with high CRI.

BaMgAl₁₀O₁₇:Eu²⁺ is a traditional efficient blue phosphor for fluorescent lamps and also can act as a blue component in white LEDs. The internal quantum efficiency is over 90%, and the external quantum efficiency is close to 70% [6]. However, BaMgAl₁₀O₁₇:Eu²⁺ degrades more rapidly due to the oxidation of Eu²⁺ from heat treatment in air during lamp processing [7], and the concentration of Eu²⁺ is much high (10 mol %), which raises the cost.

Recently, there are some reports of new efficient blue phosphors for white LEDs such as Ca₂PO₄Cl:Eu²⁺ [8], NaSrBO₃:Ce³⁺ [6], and Sr₅(PO₄)₂SiO₄:Ce³⁺ [9]. Lately, Ca₄P₂O₉:Eu²⁺ has been reported as a red-emitting phosphor for solid-state lighting [10]. Generally, Ce³⁺ has a relatively smaller crystal field splitting in d orbital level than Eu²⁺ in the same host matrix, and Ce³⁺ activated phosphors usually exhibit high efficiency such as the commercial yellow phosphor YAG:Ce. Thus, efficient green or blue emitting may be got from Ce³⁺ doped Ca₄P₂O₉.

In this study, luminescence properties of Ca₄P₂O₉:Ce³⁺ and (Ca,Sr)₄P₂O₉:Ce³⁺ were investigated. Ca₄P₂O₉:Ce³⁺ was found to exhibit good thermal stability and high efficiency, suggesting that it can be as a candidate blue phosphor for white LEDs.

2. Experimental

All specimens were prepared by a solid-state reaction method using CaCO₃ (AR), SrCO₃ (AR), NH₄H₂PO₄ (AR), and CeO₂ (4N) as starting materials. Stoichiometric amounts of the raw materials were weighed and mixed in an agate mortar. The mixed powders were first sintered at 900 °C for 3 h in air to decompose the carbonate. After a second homogenization in the mortar the samples were heated up to 1400 °C and kept at this temperature for 6 h in reductive atmosphere (5 vol% H₂ and 95 vol% N₂).

The crystalline phase of products were identified using X-ray diffraction (XRD; Ultima III, Rigaku) with Cu Kα radiation (λ = 0.154 nm). The photoluminescence spectra were recorded with VARIAN Cary Eclipse. Photoluminescence quantum yield was measured using the integrating sphere approach on a Horiba Jobin Yvon FM-4P-TCSPC instrument. During the measurement of quantum yield, a Xe lamp was used and the excitation wavelength was centered at 340 nm with a half band width of 5 nm. The emission spectrum was from 350 to 600 nm. The integral excitation intensity and the integral emission intensity were measured by the integrating sphere. The quantum yield of the phosphor was determined by the ratio of the number of photons in the emission spectrum to that in the excitation spectrum.

3. Results and discussion

Ca₄P₂O₉ is crystallized in a monoclinic unit cell (space group P21) with lattice parameters of a = 7.018 Å, b = 11.98 Å, c = 9.469 Å, and β = 90.88°. All the Ce-doped Ca₄P₂O₉ samples have similar XRD pattern to pure phase Ca₄P₂O₉ (JCPDS No. 25-1137), and no impurity phase is observed, as shown in Fig. 1(a). Ca²⁺ ions in Ca₄P₂O₉ can be divided into two types, one eight-coordinated Ca²⁺ and seven seven-coordinated Ca²⁺ [10, 11]. The phase of (Ca,Sr)₄P₂O₉:Ce³⁺ binary system were investigated, and their XRD patterns of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ (y = 0-1) are displayed in Fig. 1(b). The samples remain the monoclinic Ca₄P₂O₉ phase for y = 0-0.1 samples with XRD peaks shifted to lower angle, since the ionic radius of Sr²⁺ ion is larger than that of Ca²⁺ ion. The XRD peaks around 28° and 32° for the samples (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ (y = 0.2-0.3) shift change significantly, accompanying with the disappearance of some diffraction peaks ascribed to Ca₄P₂O₉ phase. When y equals 0.4, diffraction peaks of Sr₅(PO₄)₃OH emerge obviously. In the case of the samples with y = 0.5, 0.75 and 1, the diffraction peaks are mainly ascribed to Sr₅(PO₄)₃OH, and two peaks (marked with asterisk) of impurity SrO exist in the sample with y = 1. For the samples with y = 0.4 and 0.5, some peaks attributed to solid solution phase remain, especially for the sample with y = 0.4. It can be concluded that pure Sr₄P₂O₉ is hardly obtained under the existence of water vapor which is from the environment and the reaction products [12].

 

Fig. 1 (a) XRD patterns of Ca₄P₂O₉:xCe³⁺ (x = 0-0.01) compared with the indexed JCPDS No. 25-1137 for Ca₄P₂O₉; (b) XRD patterns of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ (y = 0-1) compared with the indexed JCPDS No. 25-1137 (Ca₄P₂O₉), No. 20-1208 (Sr₄P₂O₉) and No. 33-1348 (Sr₅(PO₄)₃OH).

Download Full Size | PDF

Figure 2(a) displays the photoluminescence excitation and emission spectra of the Ca₄P₂O₉: Ce³⁺ phosphor at room temperature. A strong excitation band and a relatively weak one from Ce³⁺ excitation appeared at about 340 and 270 nm, respectively. The 5d orbital is strongly affected by the crystal field of the host matrix and splits into several sub-bands. Thus, these two excitation bands at about 340 and 270 nm are caused by the transitions from the ground state to different 5d splitting state of Ce³⁺. The strong excitation band of ca. 340 nm indicates that this phosphor can be pumped effectively by UV-LED. The broad emission spectrum from 360 to 520 nm shows non-Gaussian symmetry. As mentioned above, there exist two types of Ca²⁺ ions, eight-coordinated Ca²⁺ and seven-coordinated Ca²⁺, and Ce³⁺ ions are expected to randomly occupy Ca²⁺ ion sites [10]. Different substitution sites of Ce³⁺ may lead to different emission band (shape or center position), furthermore, the emission peaks are due to the electronic transitions from the lowest 5d level to two different 4f ground state levels. Thus, coupling the possible effects of different substitution sites and different transition levels, a non-Gaussian symmetric emission band is observed.

 

Fig. 2 (a) Photoluminescence excitation (monitored at 420 nm) and emission (excited at 340 nm) spectra of Ca₄P₂O₉:0.005Ce³⁺; inset shows the emission intensity (I) as a function of the Ce³⁺ concentration (x mol %); (b) the dependence of lg (I/x) on lg (x) according to Eq. (2).

Download Full Size | PDF

The photoluminescence emission intensity of Ca₄P₂O₉:xCe³⁺ depends on the Ce³⁺ concentration, as shown in the inset of Fig. 2(a). The emission intensity (excited at 340 nm) increases with the Ce³⁺ doping concentration until a maximum reaches at x = 0.005. As the Ce³⁺ concentration further increases, the emission intensity decreases due to concentration quenching process. In general, the concentration quenching originates from the energy migration among the activator ions. The excitation energy will be lost at a killer center in the energy migration process, resulting in luminescence quenching. The critical distance (R_c), pointed out by Blasse [13], is defined as the distance for which the probability of energy transfer equals the probability of radiative emission of the activator ions. According to Eq. (1),

Excitation and normalized emission spectra of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ (y = 0-0.4) are shown in Fig. 3. The excitation spectra in Fig. 3(a) are composed of two excitation bands which are similar to the excitation spectrum in Fig. 2(a), accompanying with the decrease of the excitation intensity with Sr content (y). The only difference is that the weak excitation band position at short wavelength has changed when y exceeds 0.2, which is consistent with the structure change in Fig. 1(b). The emission spectra shape and intensity (inset in Fig. 3(b)) are almost the same for the samples with y = 0-0.1. As y further increases to 0.2, the emission peak is broadened, accompanying with dramatic decrease of the intensity (inset in Fig. 3(b)), but almost the same shape has been observed for the samples with y = 0.2-0.4. It is noted that the change of the emission shape and intensity of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ (y = 0-0.4) depends on the change of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ structure shown in Fig. 1(b). The compound Sr₄P₂O₉ was reported by Bauer and Balz, who found it to be isostructural with the analogous calcium compound Ca₄P₂O₉ [12]. The samples with low concentrations of Sr remain the monoclinic structure analogous to Ca₄P₂O₉, which leads to almost the same emission spectra for y = 0-0.1 (Fig. 3). As the Sr concentration increases to some value (y>0.2 in this case), obvious change in the structure of (Ca_1-ySr_y)₄P₂O₉ has been observed, especially for y>0.4 (Sr₅(PO₄)₃OH dominates). Thus, the emission spectra of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ are broadened. However, when y exceeds 0.4, Sr₅(PO₄)₃OH has formed, and the emission can hardly be observed.

 

Fig. 3 (a) Excitation spectra of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ (y = 0-0.4); (b) The normalized emission spectra of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ (y = 0-0.4) under the excitation of 340 nm, the inset shows the emission intensity versus y.

Download Full Size | PDF

It is to our interest that why the emission intensity of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ for y = 0.2-0.4 decreases dramatically compared with those for y = 0-0.1. Two main possible mechanisms may be ascribed to this decrease. One is the existence of OH group in the lattice, quenching the excitation energy; the other is the difference of the activation energy for the thermal quenching of Ce³⁺ which is due to the structure difference. Hydroxyl-quenching influences obviously the luminescence of rare earth ions [19]. Rare earth ions coupled to OH groups can be as quenchers [20]. The more quenchers are, the more possible excitation energy quenches through energy transfer from the activators to quenchers. On the other side, the change of the matrix structure has great effect on the 5d orbitals of Ce³⁺. And difference of the activation energy for the thermal quenching of Ce³⁺ may be caused by structure change. Thus, the decrease of luminescence with the increase in the Sr concentration may due to large thermal quenching at room temperature. To investigate this, temperature quenching experiment of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ (y = 0 and 0.25) was performed from 77 to 400 K (Fig. 4). The temperature dependence of the luminescence can be described by the Arrhenius equation as follows:

 

Fig. 4 Temperature dependent emission intensity of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ for y = 0 and 0.25 respectively.

Download Full Size | PDF

The obtained E_a by fitting the experiment data for the sample with y = 0 and 0.25 is 0.033 and 0.027 eV, respectively. The activation energy of Ca₄P₂O₉:0.005Ce³⁺ is very close to that of (Ca_0.75Sr_0.25)₄P₂O₉:0.005Ce³⁺. Hence, the decrease of the luminescence of (Ca_1-ySr_y)₄P₂O₉:0.005Ce³⁺ with higher Sr concentration (y>0.2) is not ascribed to the difference of the activation energy for the thermal quenching. Therefore, the existence of OH groups account for the decrease in the luminescence (inset in Fig. 3(b)).

Small thermal quenching is a critical factor for phosphors in the character of chromaticity and brightness in lighting at high temperature. Thermal quenching of the luminescent property of Ca₄P₂O₉:0.005Ce³⁺ from room temperature to 450 K has been investigated as shown in Fig. 5. No obvious shift is observed in the emission band, suggesting stable chromaticity coordinates by varying the temperature. The relative integrated emission intensity of Ca₄P₂O₉:0.005Ce³⁺ decreases marginally with the increase of the temperature, and remains 90% of the original intensity (at 300 K) when at 400 K. As the temperature increases further, the relative integrated emission intensity begins to drop obviously. Stable chromaticity property and small decrease of the intensity with temperature indicate good thermal stability of Ca₄P₂O₉:0.005Ce³⁺ phosphor.

 

Fig. 5 Temperature dependence of the integrated emission intensity normalized with respect to the value at 300 K.

Download Full Size | PDF

Bright blue emission of Ca₄P₂O₉:0.005Ce³⁺ is shown to our naked eyes with CIE chromaticity coordinates (0.157, 0.094). And the chromaticity can be tuned by forming the solid solution of Ca₄P₂O₉ and Sr₄P₂O₉. Cyanine color is observed from the emission of (Ca_0.75Sr_0.25)₄P₂O₉:0.005Ce³⁺ with CIE chromaticity coordinates (0.164, 0.168). The colors of these two phosphors are displayed in Fig. 6. The optimal phosphor has a high external quantum yield up to 64.8% (excited at 340 nm), which is very close to that of commercial product BaMgAl₁₀O₁₇:Eu²⁺ (69.6%) (excited at 365 nm) [6]. However, the activation center concentration in Ca₄P₂O₉:Ce³⁺ (0.5 mol %) in this study is much lower than that of BaMgAl₁₀O₁₇:Eu²⁺ (11.6 mol %) [21]. Ca₄P₂O₉:Ce³⁺ can act as an efficient promising blue phosphor for white LEDs.

 

Fig. 6 CIE chromaticity coordinates of Ca₄P₂O₉:0.005Ce³⁺ and (Ca_0.75Sr_0.25)₄P₂O₉:0.005Ce³⁺ phosphors.

Download Full Size | PDF

4. Conclusion

Ca₄P₂O₉:Ce³⁺, as a blue-emitting phosphor, was synthesized by a solid-state reaction. A non-Gaussian symmetric broad emission band centered at 420 nm with CIE chromaticity coordinates (0.157, 0.094) was observed. Concentration quenching effect of Ca₄P₂O₉:xCe³⁺ showed that the optimal concentration is at x = 0.005. The quenching mechanism is due to the energy transfer among the nearest neighbor ions. The chromaticity can be tuned by substituting part of Sr for Ca, changing the structure of this system. And Cyanine color was obtained with CIE chromaticity coordinates (0.164, 0.168) from the phosphor (Ca_0.75Sr_0.25)₄P₂O₉:0.005Ce³⁺. Ca₄P₂O₉:Ce³⁺ phosphor displayed a good thermal stability, and 90% of the initial intensity at room temperature (300 K) remained as the temperature increased to 400 K. A high external quantum yield, 64.8%, was also performed in this phosphor, which indicated that Ca₄P₂O₉:Ce³⁺ can be as a promising candidate blue-emitting phosphor for white LEDs.