1. Introduction

Phosphor-converted white light emitting diodes (pc-WLEDs) are greatly considered as the fourth-generation light source due to their excellent properties such as brightness, long lifetime, energy efficiency [1, 2]. WLEDs are usually based on the combination of a blue InGaN chip [3–6] and a yellow phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce). However, they have low color rendering index (CRI) and high correlated color temperatures (CCTs) due to the lack of the red component. For CCTs less than 6000 K, the CRI of typical high efficacy pc-WLEDs is less than 80, in comparison to that of 100 for incandescent and 82-85 of fluorescent lamps [7]. For solving this problem, the red phosphors which can be efficiently excited by blue light are required and such phosphors are usually based on nitride [8]. However, almost all the nitride-type phosphors require critical preparation conditions, such as high temperature, high pressure and expensive raw materials. Besides, with increasing the amount of electrical current, the hue of WLEDs changes from white to blue because the yellow phosphors thermal quench then the blue light from InGaN chip emission dominates. The development of ultraviolet (UV) LED chip based on AlGaN [9–11] mixed with red, green and blue phosphors (RGB) can avoid above drawbacks since the UV light is invisible. Nowadays, UV LED chips combined with RGB phosphors have been extensively researched. Accordingly, the eventual performance of UV LEDs strongly depends on the luminescence properties of the phosphors used. Thus, the exploring new RGB phosphors excited by UV light has developed rapidly and has become one of the hot research topics in the phosphor field.

The potential application of rare earth doped silicate materials in UV LEDs have been widely studied [12–14] because of high chemical and physical stability and low cost synthesis. Eu²⁺ doped alkaline earth silicate materials have been especially of great interest. In consideration of the similar ionic radius and the same charge, Eu²⁺ diffuses more easily into an alkaline earth silicate host and emits light, such as β-Ca₂SiO₄:Eu²⁺ (β-C₂S:Eu²⁺), in which Eu²⁺ prefers to occupying the Ca²⁺ sites and emits bright green light [12]. Whereas the isomer of β-Ca₂SiO₄ i.e. α′-Ca₂SiO₄ and α-Ca₂SiO₄ are not stable unless adding the raw material Ca₃(PO₄)₂ [15, 16]. Recently, the Eu²⁺ doped Ca₁₅(PO₄)₂(SiO₄)₆ (C₁₅P₂S₆), which is the stable phase of α′-Ca₂SiO₄ was reported as a good candidate of green emitting phosphor for UV LEDs [16]. Nagelschmidtite [Ca₇(PO₄)₂(SiO₄)₂ (C₇P₂S₂)] was reported as the stable phase of α-Ca₂SiO₄ by T. Mikouchi, et al [17], which is explained by the substitution of one of SiO₄ tetrahedra and Ca sites of α-Ca₂SiO₄ by PO₄ tetrahedron and Ca vacancy (V), respectively according to the reaction: 2 SiO₄ + Ca → 2 PO₄ + V. Considering above information, C₇P₂S₂ may act as promising host lattice for Eu²⁺. Ref. [18] reports yellowish-green emitting phosphor of C₇P₂S₂:Eu²⁺ synthesized by sol-gel method. Since the wet chemical method was used in the synthesis process, the original synthesis route of C₇P₂S₂ may be too critical to realize wide application in the production process. Herein C₇P₂S₂:Eu²⁺ was synthesized by solid-state reaction. The different photoluminescence properties from that of Ref. [18] and potential application of C₇P₂S₂:Eu²⁺ in LEDs are reported in this paper.

2. Experimental

Using solid-state reaction method Ca₇(PO₄)₂(SiO₄)₂:xEu²⁺ were prepared. The original materials were CaCO₃ (99.5%), Eu₂O₃ (99.99%), NH₄H₂PO₄ (AR), SiO₂ (AR). The raw materials with Ca_7-xEu_x(PO₄)₂(SiO₄)₂ (0.5% ≤ x ≤ 7%) were mixed thoroughly using ethanol as a solvent. Then, the mixtures were fired at 1500 °C under H₂/N₂ condition for 6 h. Finally, the samples were cooled to room temperature in the furnace and ground with an agate mortar.

Rigaku D/Max-2400 X-ray diffractometer (XRD) was employed to check the phase of the samples. The diffuse reflection spectra were obtained by an UV-vis spectrophotometer (PE lambda 950) using BaSO₄ as a standard reference. Both excitation (PLE) and emission (PL) spectra were measured by Horiba Jobin Yvon Fluorlog-3 spectrofluorometer system. Temperature dependent PL was carried out using PL spectra and a standard TAP-02 high temperature fluorescence controller.

3. Results and discussion

Figure 1 shows the XRD patterns of C₇P₂S₂:xEu²⁺ (0.5% ≤ x ≤ 7%). All the diffraction peaks can be well indexed to the single C7P2S2 phase according to the standard data of C₇P₂S₂ (JCPDS#110676).The result indicates that Eu²⁺ has been successfully incorporated into the host lattice. Considering the ionic radii and valence state, Eu²⁺ should occupy the sites of the Ca²⁺.

 

Fig. 1 XRD patterns of C₇P₂S₂:xEu²⁺ (0.5% ≤ x ≤ 7%).

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Figure 2 shows the diffuse reflection spectra of C₇P₂S₂:xEu²⁺ (0.5% ≤ x ≤ 7%). There are two obvious absorption bands in the wavelength range of 213-316 nm and 316-440 nm, respectively. Even at x = 1%, the body color already turns light-green, owing to the 4f-5d absorption transition at the Eu²⁺ center. The absorption intensities increase with increase Eu²⁺ concentration confirming the above attribution. Besides, the absorption edge gradual red shift with increase of Eu²⁺ concentration. The red shift in the absorption edge can be ascribed to the changes in the optical band gap of C₇P₂S₂ with Eu²⁺ doping [19].

 

Fig. 2 Diffuse reflection spectra of C₇P₂S₂:xEu²⁺ (0.5% ≤ x ≤ 7%).

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Figures 3(a) and 3(b) present the normalized PLE (λ_em = 522 nm) and PL (λ_ex = 365 nm) spectra of C₇P₂S₂:xEu²⁺ (0.5% ≤ x ≤ 7%). The PLE spectra have two obvious broad bands in the wavelength range of 240-316 nm and 316-440 nm, respectively, which correspond to the 4f-5d transition of Eu²⁺ occupying Ca²⁺ sites in C₇P₂S₂. The strong and broad PLE spectra in ultraviolet range indicate C₇P2S₂:Eu²⁺ could well match the UV LED chips. Small line in the pink color is assigned to the ⁷F₀-⁵L₆ transition of Eu³⁺. Besides, analogous red shift with diffuse reflection spectra is found in PLE spectra, as shown in Fig. 2(a). The emission band of Eu²⁺ shows an asymmetric shape centered at 522 nm in the long wavelength region attributed to the ⁴f₆⁵d-⁴f₇ transition of Eu²⁺, which indicates the existence of different luminescent centers. The PL spectra are very different from Ref. [18], in which two distinct emission bands of Eu²⁺ appeared. Two small lines marked by pink color are assigned to the ⁵D₀-⁷F_J (J = 2, 4) transitions of Eu³⁺, corresponding to the PLE spectra. This phenomenon demonstrates that the Eu³⁺ ions are hardly been reduced totally to Eu²⁺ in C₇P₂S₂ host. Room temperature Stokes shift is calculated by the lowest 5d absorption and emission maxima, and the value is 9829 cm⁻¹. Larger Stokes shift may cause larger thermal quenching [7] and this will be explained latter.

 

Fig. 3 PLE (λ_em = 522 nm) (a) and PL (λ_ex = 365 nm) (b) spectra of C₇P₂S₂:x3Eu²⁺.

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With increasing Eu²⁺ concentration, the shape of all emission spectra has no significant changes except for the emission intensity. The brightness increases with the increase of dopant concentration until 3%, and then decreases due to concentration quenching. If we consider energy transfer between two identical centers, the critical transfer distance (R_c) is defined as the distance for which the probability of energy transfer equals the probability of radioactive emission of Eu²⁺, as pointed out by Blasse [20, 21], shown as following Eq.:

 

Fig. 4 The dependence of lgI/x on lgx in C₇P₂S₂:Eu²⁺.

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Table 1 summarizes the characteristics of C₇P₂S₂:Eu²⁺ and some other typical Eu²⁺ doped Ca-Si/P-O compounds. Based on the comparison, we can see that all Eu²⁺ doped Ca-Si-P-O compounds show green emitting under UV light excitation no matter what detailed structure is. Thus, the common characteristics of these [PO₄] and [SiO₄] contained hosts should be responsible for similar photoluminescence properties. The full width at half maximum (FWHM) of C₇P₂S₂:Eu²⁺ is greatly wider than others, proving Eu²⁺ occupies more sites in C₇P₂S₂ host than other host.

Table 1. Characteristics of C7P2S2:Eu²⁺ and Eu²⁺ doped Ca-Si-P-O compounds.

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In order to further investigate the possible practical application of the C₇P₂S₂:Eu²⁺, some key parameters, such as CIE value and the temperature dependent PL have also been measured and studied. The CIE chromaticity coordinates for C₇P₂S₂:0.03Eu²⁺ sample excited at 365 nm are calculated to be (0.315, 0.429), shown in Fig. 5. The photo image of C₇P₂S₂:Eu²⁺ under UV lamp is also shown in Fig. 5.

 

Fig. 5 CIE coordinate and the digital photo of C₇P₂S₂:0.03Eu²⁺ under UV lamp.

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Thermal stability of phosphors used in LEDs is of importance as it has a considerable effect on the light output and CRI. In general, phosphors suffer a decline in the process of increase temperature. Figure 6(a) shows temperature dependent PL of C₇P₂S₂:0.03Eu²⁺ under 365 nm excitation, inset displays the brightness as a function of temperature. The brightness of C₇P₂S₂:0.03Eu²⁺ decreases to 34.1% of its initial value at 140°C. The possible causes for thermally activated nonradioactive transitions of Eu²⁺ are photoionization into the host lattice conduction band or level crossing between the lowest energy 5d level and the 4f ground states [7]. As calculated above, the room temperature Stokes shift is 9829 cm⁻¹. Considering larger Stokes shift, level crossing will play a critical role in thermal quenching. Larger Stokes shift reduces the energy barrier for nonradioactive crossovers from 5d level to the 4f ground states, resulting in the stronger quenching. Enhanced quenching in C₇P₂S₂:0.03Eu²⁺ should be explained by two parts: One reason is increasing the concentration of Eu³⁺ with temperature increase, shown the pink part in Fig. 6(a). The other reason is the vacancy in C₇P₂S₂. Eu³⁺ and vacancy provide alternate quenching paths in the host lattice. Additionally, the emission wavelength shows an obvious blue shift with increasing temperature (emission peak from 522 nm to 493 nm as temperature from 20 °C to 250 °C). Both blue shift and increase of Eu³⁺ concentration result in emission color changed from green to cyan, shown in Fig. 6(b) the blue shift found in C₇P₂S₂:Eu²⁺ should be explained by thermally active phonon-assisted excitation from lower energy sublevel to high-energy sublevel in the excited states of Eu²⁺ [27, 28]. It is further proved that there should be one more different Eu²⁺ ions sites in the C₇P₂S₂ host.

 

Fig. 6 (a) Temperature dependent PL spectra of C₇P₂S₂:0.03Eu²⁺, inset shows the brightness versus temperature (λex = 365 nm), (b) CIE coordinate versus temperature.

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To further verify the thermal quenching behavior and to reckon up the activation energy (ΔE) for thermal quenching, the Arrhenius equation [29] is fitted to the thermal quenching data of C₇P₂S₂:0.03Eu²⁺.

Where, I₀ is the initial emission intensity, I_T is emission intensity at different temperatures. c is a constant for a certain host, k is the Boltzmann constant (8.625 × 10⁻⁵ eV). Figure 7 plots the relationship of Ln[(I₀/I_T)-1] versus 1000/T for C₇P₂S₂:0.03Eu²⁺, which is linear with a slope of −3.359. According to above equation, ΔE is calculated to be 0.29 eV in C₇P₂S₂:0.03Eu²⁺. It is generally known that activation energy signifies the energy gap between the lowest excited state level of Eu²⁺ and the bottom of the conduction band, and is closely related to the thermally activated energy transfer process. The activation energy for thermal quenching in the C₇P₂S₂:Eu²⁺ is higher than other silicate phosphates like C₁₅P₂S₆:Eu²⁺ (ΔE = 0.213 ev) [30]. The results demonstrate that C₇P₂S₂:Eu²⁺ is a promising green-emitting phosphor.

 

Fig. 7 The Arrhenius fitting of the emission intensity of C₇P₂S₂:0.03Eu²⁺ and the calculated ΔE for thermal quenching.

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4. Conclusion

C₇P₂S₂:xEu²⁺ were synthesized by solid state reaction and their photoluminescence properties were characterized. C₇P₂S₂:xEu²⁺ showed a strong and broad absorption in the UV regions. Under 365 nm excitation, all samples gave bright green emission peaked at 522 nm. The best Eu²⁺ concentration in C₇P₂S₂ is found to be 3%. The concentration quenching mechanism is investigated to be d-d interaction. Temperature dependent PL and CIE values of C₇P₂S₂:0.03Eu²⁺ had been investigated in detail. When temperature is at 140 °C, the brightness is 34.1% of its initial value with an obviously blue shift and an increased Eu³⁺ intensity. The activation energy for thermal quenching of C₇P₂S₂: Eu²⁺ was calculated to be 0.29 eV, which was higher than other silicate phosphates. All the characteristics suggest that C₇P₂S₂: Eu²⁺ was a promising green-emitting phosphor candidate in UV LEDs.