A novel green emitting phosphor of Eu2+ doped Ca7(PO4)2(SiO4)2 was synthesized and its photoluminescence properties were investigated for application in UV LEDs. XRD was used to identify sample phase. Diffuse reflection spectra and photoluminescence spectra were used to investigate its photoluminescence properties. Ca7(PO4)2(SiO4)2:Eu2+ showed an absorption ranging from 240 to 440 nm in ultraviolet range and a broad green emission band peaked at 522 nm. The concentration quenching mechanism and the key parameters for the fabrication of WLEDs, such as the temperature dependent photoluminescence and CIE value had also been studied.
© 2014 Optical Society of America
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 . 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 . 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 . 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 . Nagelschmidtite [Ca7(PO4)2(SiO4)2 (C7P2S2)] was reported as the stable phase of α-Ca2SiO4 by T. Mikouchi, et al , 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.  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.  and potential application of C7P2S2:Eu2+ in LEDs are reported in this paper.
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 Ca7-xEux(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+.
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 .
Figures 3(a) and 3(b) present the normalized PLE (λem = 522 nm) and PL (λex = 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. , in which two distinct emission bands of Eu2+ appeared. Two small lines marked by pink color are assigned to the 5D0-7FJ (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  and this will be explained latter.
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 (Rc) 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.:22]). According to Dexter’s theory, there are three multipole–multipole interactions: dipole–dipole (d-d), dipole–quadrupole (d-q) and quadrupole–quadrupole (q-q), respectively. The emission intensity (I) of multipolar interaction can be determined by the change of the emission intensity from the emitting level with multipolar interaction, which follows the equation given below [23, 24]:Fig. 4 with a slope of −1.81. Therefore, the value of θ is calculated as approximately 6, which indicates that the d-d interaction is the concentration quenching mechanism of Eu2+ emission in the C7P2S2:Eu2+.
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.
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.
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 . 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.
To further verify the thermal quenching behavior and to reckon up the activation energy (ΔE) for thermal quenching, the Arrhenius equation  is fitted to the thermal quenching data of C7P2S2:0.03Eu2+.
Where, I0 is the initial emission intensity, IT 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/IT)-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) . The results demonstrate that C7P2S2:Eu2+ is a promising green-emitting phosphor.
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.
This work was supported by the National Science Foundation for Distinguished Young Scholars (No. 50925206) and the Research Fund for the Doctoral Program of Higher Education (No. 20120211130003).
References and links
1. C. Feldmann, T. Justel, C. Ronda, and P. Schmidt, “Inorganic luminescent materials: 100 years of research and application,” Adv. Funct. Mater. 13(7), 511–516 (2003). [CrossRef]
3. H. Zhao, G. Liu, J. Zhang, R. Arif, and N. Tansu, “Analysis of Internal quantum efficiency and current injection efficiency in III-nitride light-emitting diodes,” J. Disp. Technol. 9(4), 212–225 (2013). [CrossRef]
4. G. Liu, J. Zhang, C. Tan, and N. Tansu, “Efficiency-droop suppression by using large-bandgap AlGaInN thin barrier layers in InGaN quantum-well light-emitting diodes,” IEEE Photonics J. 5(2), 2201011 (2013). [CrossRef]
5. R. Arif, Y. Ee, and N. Tansu, “Polarization engineering via staggered InGaN quantum wells for radiative efficiency enhancement of light emitting diodes,” Appl. Phys. Lett. 91(9), 091110 (2007). [CrossRef]
6. S. Choi, M. Ji, J. Kim, H. Kim, M. Satter, P. Yoder, J. Ryou, R. Dupuis, A. Fischer, and F. Ponce, “Efficiency droop due to electron spill-over and limited hole injection in III-nitride visible light-emitting diodes employing lattice-matched InAlN electron blocking layers,” Appl. Phys. Lett. 101(16), 161110 (2012). [CrossRef]
7. A. Setlur, W. Heward, M. Hannah, and U. Happek, “Incorporation of Si4+-N3- into Ce3+-doped garnets for warm white LED phosphors,” Chem. Mater. 20(19), 6277–6283 (2008). [CrossRef]
8. R. Xie, N. Hirosaki, T. Suehiro, F. Xu, and M. Mitomo, “A Simple, Efficient Synthetic Route to Sr2Si5N8:Eu2+-Based Red Phosphors for White Light-emitting Diodes,” Chem. Mater. 18(23), 5578–5583 (2006). [CrossRef] [PubMed]
9. J. Zhang, H. Zhao, and N. Tansu, “Effect of crystal-field split-off hole and heavy-hole bands crossover on gain characteristics of high Al-content AlGaN quantum well lasers,” Appl. Phys. Lett. 97(11), 111105 (2010). [CrossRef]
10. J. Zhang and N. Tansu, “Engineering of AlGaN-Delta-GaN Quantum-Well Gain Media for Mid- and Deep-Ultraviolet Lasers,” IEEE Photonics J. 5(2), 2600209 (2013). [CrossRef]
11. Y. Taniyasu and M. Kasu, “Polarization property of deep-ultraviolet light emission from C-plane AlN/GaN short-period superlattices,” Appl. Phys. Lett. 99(25), 251112 (2011). [CrossRef]
12. Y. Luo, D. Jo, K. Senthil, S. Tezuka, M. Kakihana, K. Toda, T. Masaki, and D. Yoon, “Synthesis of high efficient Ca2SiO4:Eu2+ green emitting phosphor by a liquid phase precursor method,” J. Solid State Chem. 189, 68–74 (2012). [CrossRef]
13. M. Saradhi and U. Varadaraju, “Photoluminescence Studies on Eu2+-Activated Li2SrSiO4: a Potential Orange-Yellow Phosphor for Solid-State Lighting,” Chem. Mater. 18(22), 5267–5272 (2006). [CrossRef]
14. M. Lim, J. Park, C. Kim, H. Park, and M. Han, “Luminescence characteristics of green light emitting Ba2SiO4:Eu2+ phosphor,” J. Mater. Sci. Lett. 22(19), 1351–1353 (2003). [CrossRef]
15. W. Fix, H. Heymann, and R. Heinke, “Subsolidus Relations in the System 2CaO·SiO2-3CaO·P2O5,” J. Am. Ceram. Soc. 52(6), 346–347 (1969). [CrossRef]
16. S. Hur, H. Song, H. Roh, D. Kim, and K. Hong, “A novel green-emitting Ca15(PO4)2(SiO4)6:Eu2+ phosphor for applications in n-UV based w-LEDS,” Mater. Chem. Phys. 139(2-3), 350–354 (2013). [CrossRef]
17. W. Eysel and T. Hahn, “Polymorphism and solid solution of Ca2GeO4 and Ca2SiO4,” Zeitschrift fur Kristallographic, Bd. 131(1-6), 322–341 (1970). [CrossRef]
18. D. Wei, Y. Huang, and H. Seo, “Novel yellowish-green emitting luminescence in Ca7Si2P2O16:Eu2+ phosphor,” Mater. Res. Bull. 48(9), 3614–3619 (2013). [CrossRef]
19. R. Xie, N. Hirosaki, M. Mitomo, Y. Yamamoto, T. Suehiro, and K. Sakuma, “Optical Properties of Eu2+ in γ-SiAlON,” J. Phys. Chem. B 108(32), 12027–12031 (2004). [CrossRef]
20. G. Blasse, “Energy Transfer in Oxidic Phosphors,” Phys. Lett. 28(6), 444–445 (1968). [CrossRef]
21. Z. Xia, R. Liu, K. Huang, and V. Drozd, “Ca2Al3O6F:Eu2+: a green-emitting oxyfluoride phosphor for white light-emitting diodes,” J. Mater. Chem. 22(30), 15183–15189 (2012). [CrossRef]
22. D. Dexter, “A theory of sensitized luminescence in solids,” J. Chem. Phys. 21(5), 836 (1953). [CrossRef]
23. L. Uitert, “Characterization of Energy transfer interactions between rare earth ions,” J. Electrochem. Soc. 114(10), 1048–1053 (1967). [CrossRef]
24. P. Dorenbos, “Thermal quenching of Eu2+ 5d–4f luminescence in inorganic compounds,” J. Phys. Condens. Matter 17(50), 8103–8111 (2005). [CrossRef]
25. H. Yu, D. Deng, Y. Li, S. Xu, Y. Li, C. Yu, Y. Ding, H. Li, H. Yin, and Q. Nie, “Electronic strcuture and luminescent properties of Ca5(PO4)2(SiO4):Eu2+ green emitting phosphor for white light emitting diodes,” Opt. Commun. 289, 103–108 (2013). [CrossRef]
26. H. Roh, S. Hur, H. Song, I. Park, D. Yim, D. Kim, and K. Hong, “Luminescence properties of Ca5(PO4)2(SiO4): Eu2+ green phosphor for near UV-based white LED,” Mater. Lett. 70, 37–39 (2012). [CrossRef]
27. J. Kim, Y. Park, S. Kim, J. Choi, and H. Park, “Temperature-dependent emission spectra of M2SiO4:Eu2+ (M= Ca, Sr, Ba) phosphors for green and greenish white LEDs,” Solid State Commun. 133(7), 445–448 (2005). [CrossRef]
28. Z. Xia, X. Wang, Y. Wang, L. Liao, and X. Jing, “Synthesis, structure, and thermally stable luminescence of Eu2+ doped Ba2Ln(BO3)2Cl (Ln = Y, Gd and Lu) host compounds,” Inorg. Chem. 50(20), 10134–10142 (2011). [CrossRef] [PubMed]
29. S. Zhang, Y. Nakai, T. Tsuboi, Y. Huang, and H. J. Seo, “The thermal stabilities of luminescence and microstructures of Eu2+-doped KBaPO4 and NaSrPO4 with β-K2SO4 type structure,” Inorg. Chem. 50(7), 2897–2904 (2011). [CrossRef] [PubMed]
30. S. Hur, H. Song, H. Roh, D. Kim, and K. Hong, “Enhancement of fluorescence by Ce3+ doping in green-emitting Ca15(PO4)2(SiO4)6:Eu2+ phosphor for UV-based w-LEDs,” Ceram. Int. 39(8), 9791–9795 (2013). [CrossRef]