34 © Adama Science & Technology University https://ejssd.astu.edu.et Ethiopian Journal of Science and Sustainable Development (EJSSD) p-ISSN 1998-0531 Volume 5 (1), 2018 Synthesis and Luminescent Characteristics of (Ca4-x-y- zBaySrz)(PO4)2O:xEu 2+ Phosphors as a Potential Application for White-Light Emitting Diode G. Deressa School of Applied Natural Science, Department of Applied Chemistry, Adama Science and Technology University, Adama 1888, Ethiopia Email: Abstract A novel tunable red- to yellow- emitting phosphor,(Ca4-x-y-zBaySrz)(PO4)2O:xEu 2+ is reported that displays a broad emission from 500 to 800nm, and its emission can be adjusted from red to yellow by changing Ba 2+ and Sr 2+ doping concentration. X-ray powder diffraction analysis confirmed the phase formation. Excitation and emission spectra, and concentration dependence of emission intensity of the phosphor were investigated. The results showed that with increasing Ba 2+ /Sr 2+ concentration, the emission peak wavelength blue-shift from 594 to 567 nm, and the color can be tuned from red to yellow. When single- phase (Ca2.95Ba0.5Sr0.5)(PO4)2O:0.05Eu 2+ phosphor is pumped by a blue InGaN light-emitting diode we obtain white light with color rending index between 80.0 and 88.0 and color temperatures between 3500 and 5450 K, suggesting that this material is competitive as a color conversion material for solid state lighting. Keywords: White light-emitting diode; Phosphors; Tunable emitting 1. Introduction The commercialized white light emitting diodes (WLEDs) can be generated by combining a blue-emitting InGaN-LED chip and a yellow-emitting garnet phosphor Y 3 A l 5 O 1 2 :C e 3 + ( YA G:C e 3 + ) (Bachmann et al., 2009; Schubert et al., 2005 and Deressa et al., 2015). However, such white light does not have sufficient color rendering properties. The color-rendering index (CRI) remains to be improved due to the lack of a full color component from the yellow phosphor. Recently, several WLED devices fabricated using ultraviolet (UV) chip coupled with a blend of tunable (blue-to-yellow) emitting and red-emitting phosphor, which exhibited favorable properties, Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 35 © Adama Science & Technology University https://ejssd.astu.edu.et including tunable correlated color temperature (CCT), tunable Commission International del’ Eclairage (CIE) chromaticity coordinates, and excellent CRI values (Deressa et al., 2015; Setlur et al., 2010; and Zhou et al., 2014).Unfortunately, the emission tunable range of the phosphor with a single activated ion has been reported is limited to blue to yellow. Changing the cation ratio of orthophosphate phosphate can distort the lattices structure due to change in crystal field of the host that causes the broadening spectra of the luminescence.This is also a promising host for lighting and display due to its good thermal stability and high luminescent efficiencies. The Eu2+-activated OrthophosphatePhosphates are good candidates as host structures and offer a number of merits, such as high chemical and low synthesis temperature and physical stability, and they exhibit interesting luminescence (Yang et al., 2004; Zhou et al., 2013). The motivation of this work was to invesigate a tunable red to yellow-emitting and efficient inorganic phosphors (Ca4-x-y- zBaySrz)(PO4)2O:xEu 2+) that could be applied in UV or blue LEDbased solid state white light sources. Orthophosphate-phosphor doped with rare earth ions is widely explored as red, green and blue (RGB) phosphors owing to advantages of relatively low sintering temperature, good chemical stability, and satisfactory absorption in the ultraviolent (UV) to blue region (Zhou et al., 2013; Huang et al., 2008). Ca4(PO4)2O:Eu 2+ is novel red-emitting phosphate phosphors (Kottaisamy et al., 1994). To the best of our knowledge, its tunable emitting properties have not been reported in the literature. In this paper, we investigated a tunable red- to yellow- emitting (Ca4-x-y-zBaySrz) Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 36 © Adama Science & Technology University https://ejssd.astu.edu.et (PO4)2O:xEu 2+ phosphor. This phosphor was excited by light in UV to blue region and showed tunable red-to-yellow emission. The optical properties of Orthophosphate phosphate phosphors were systematically investigated by means of photoluminescence excitation (PLE) and emission (PL) spectra, thermal stability, Raman, and applications for white light emitting diodes (white-LEDs). 2. Materials and Methods 2.1. Preparation (Ca4-x-y- zBaySrz) (PO4)2O:xEu2+ phosphate phosphors Powder samples of Orthophosphate phosphors were prepared by a solid-state reaction. MCO3 (99.99%), [M = Ca, Sr and Ba], (NH4)2HPO4 (99.0%), and Eu2O3 (99.99%) are weighed, thoroughly mixing and ground in an agate mortar. The mixed powders were then transferred into an alumina crucible, and pre- sintered in a furnace at 500 ℃ for 2 hours in air to eliminate the water and decompose the carbonate. The pre-sintered samples were subsequently cooled down to room temperature and fully ground to form a homogeneous mixture. Then the mixture was re-sintered at a high temperature of 1000 to 1300 ℃ for 4 hours under a reducing atmosphere (5%H2/95%N2) for 4 hours. After sintering, the samples were cooled down naturally to room temperature in the furnace. The acquired sintered products were pulverized for further measurements. The nominal molar compositions of the phosphors are: Ca4-x-yBaySrz(PO4)2O:XEu 2+ (0.01 ≤ X ≤ 0.10, 0 ≤Y ≤ 2.5, and 0 ≤ Z ≤ 2.5)) (Setlur et al., 2010) 2.2. Materials Characterization The as-prepared samples were characterized by using the following instruments; Scanning electron microscope (SEM: LEO SUPRA 55, Carl Zeiss) attached Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 37 © Adama Science & Technology University https://ejssd.astu.edu.et with an energy dispersive X-ray spectrometer (EDX), X-ray diffractometer (M18XHF - SRA, Mac Science), X-ray diffraction (XRD) analysis using a Rigaku D/MAX 2500 with Cu K radiation. The measurements of photoluminescence (PL) and photoluminescence excitation (PLE) spectra are carried out using a DARSA PRO-5200 fluorescence spectrophotometer equipped with a xenon lamp as the excitation light source. The thermal quenching characteristics are measured in the temperature range of 25-200 ℃. Raman spectra of (Ca, Sr, Ba)4(PO4)2O: Eu 2+ were carried out at room temperature in the range of 200 to 2000 cm-1 using Agiltron, Raman Spectrometer, which is having excitation Nd:YAG laser) a excitation laser source (1064 nm). 2.4. LED Lamp Fabrication. The WLEDs were fabricated using a blue-LED chip with a 450 nm excitation wavelength. First of all, the chip was assembled on a LED frame and care was taken to prevent the formation of air bubbles between the chip and frame, because it can damage the LED when a forward bias current is applied. The semi-transparent silicone epoxy was prepared by mixing two optical encapsulates, i.e., OE-6630 A (base) and OE- 6630 B (catalyst) in 1:2 ratio. Afterwards, the required quantity of (Ca2.95Ba0.50Sr0.50Eu0.05)(PO4)2O phosphor powders were mixed with the silicone epoxy, kept in a mixer and later kept in a desiccator to remove the air bubbles. Finally, it was poured in a dispenser tube. The phosphor mixed epoxy was later dispensed onto the LED frame and kept in an oven (@ 120 °C, for 90 min) to harden, and eventually the WLEDs were characterized by using an OL770 multi-channel spectroradiometer attached with an integrating sphere (Deressa et al., 2015). Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 38 © Adama Science & Technology University https://ejssd.astu.edu.et 3. Results and Discussion 3.1 Phosphor Synthesis and Characterization. Fig.1 shows the results of the data collection and structure refinements for (Ca2.95 Ba0.50 Sr0.50 Eu0.05) (PO4)2O. The single crystal structure data of Ca4(PO4)2O (ICSD no. 2631) was performed as starting reference to approach the actual crystal structure. The final converged weighted-profile of Rexp=6.99, Rwp= 9.16, Rp=7.09 and GOF=1.31 is shown in Table 1. The (Ca2.95 Ba0.50 Sr0.50 Eu0.05) (PO4)2O crystallizes in a monoclinic unit cell with space group P21(4) space group with Z = 4. In the crystal structure of Ca4(PO4)2O, the Ca 2+ ions have eight different coordination environments, only Ca(6) ion is eight-coordinated, and the other Ca2+ ions are seven-coordinated (See Fig. .1 (b) Table 1: Rietveld refinement and crystal data of Rietveld enhancement and crystal data of (Ca2.95Ba0.50Sr0.50Eu0.05)(PO4)2O phosphors Formula Ca3.95(PO4)2O:0.05Eu 2+ Cryst. syst. P21(4) – monoclinic Crystal Density (g/cm3) 7.229 Units, Z 4 a (Å) 7.012 (11) b (Å) 11.979 (13) c (Å) 9.481 (9) V (Å3) 797.32576(15) β (deg) 90.6(2) Rexp (%) 6.89 Rwp (%) 9.14 Rp (%) 7.06 GOF 1.29 Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 39 © Adama Science & Technology University https://ejssd.astu.edu.et Figure1: (a) Observed (black), calculated (red), and difference (green) synchrotron XRD profiles for the Rietveld refinement of (Ca2.85Ba0.50 Sr0.50) (PO4)2O:0.05Eu 2+. Bragg reflections are indicated with tick marks. (b) Crystal structure of Ca4(PO4)2O unit cell viewed in a- direction (Denget al., 2013). The XRD patterns of (Ca4-y- zBaySrz)(PO4)2O:0.05Eu 2+ together with the Joint Committee on Powder Diffraction Standards (JCPDS) card No. 73-1379 are shown in Fig. 2. When the Sr2+ doping content (z), the diffraction peaks of the obtained sample can be indexed to the standard data except for little shift, indicating that these samples almost are single-phase. When the Ba2+ doping content (y), the diffraction peaks of the obtained sample can be indexed to the standard data except for little shift, indicating that these samples almost are single-phase. Eu2+/Ba2+/Sr2+ are incorporated in the host lattice (Yang et al., 2004). When the Ba2+ doping content (y) is higher than 0.10, the intensity of some crystal planes diffraction peaks increased gradually, such as (200), (210) and (212) which indicated that with the increase of Ba2+ ions substiturion, resulted in corresponding crystal face preferred growth. Additionally, the diffraction peaks exhibited a shift toward smaller angles with rising doping content of the Ba2+ ion, Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 40 © Adama Science & Technology University https://ejssd.astu.edu.et which may be related to the substitution of smaller Ca2+ by the lager Ba2+. The micromorphology of the crystalline (Ca2.95 Ba0.50 Sr0.50 Eu0.05) (PO4)2O phosphor sample observed by SEM is shown in Fig. 3. It is observed that the particles have smooth morphology and the diameters are ranging from 15 to 20 μm. The elemental composition of (Ca2.95 Ba0.50 Sr0.50 Eu0.05) (PO4)2O sample verified by EDS shows (wt %) Ca 35.28%, Ba 3.23%, Sr 3.38, Eu 2.25 %, P 16.34 % and O 38.91%. The composition suggested by EDS is consistent with stoichiometric weight ratio with reasonable relative error. Figure 2: XRD patterns of (Ca4-y-zBaySrz)(PO4)2O:0.05Eu 2+ phosphors Figure 3: (a) SEM image and (b) EDS profile of (Ca2.95Ba0.50 Sr0.50Eu0.05) (PO4)2O sample Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 41 © Adama Science & Technology University https://ejssd.astu.edu.et The Raman vibrational frequencies of v2, v3, and v4 are follows the same phenomenal. The shift in the observed Raman frequencies is believed to be due a tightening or enlargement of the molecular PO4 3- ion as oxygen atoms of neighboring molecules are either drawn near or pushed apart on substitution of cations of different size. The bond length of Ba-O/ Sr-O are longer than the C- O in the crystal structure of Ba4 PO)2O,Sr4PO4)2O and Ca4PO4)2O, respectively. The crystal structure of (Ca2.95Ba0.50Sr0.50) (PO4)2O:0.05Eu 2+ formed from mixed crystal structure of Ba4PO4)2O,Sr4PO4)2O and Ca4PO4)2O as observed from XRD and Raman properties of the structures, Both XRD and Raman spectra of the host (Ca2.95Ba0.50Sr0.50) (PO4)2O found between these three hosts of Ba5(PO4)3Cl and Sr5(PO4)3Cl lattices. This indicate that the lattices of (Ca2.95 Ba0.50 Sr0.50) (PO4)2O host combined from an expanded Ba-O/Sr-O and contracted Ca-O bonds which create different environments around the sites of Eu2+ ions doped in the lattice of (Ca2.95 Ba0.50Sr0.50)(PO4)2O phosphor and cause for the blue shifts and spectral broadening of Eu2+ activated (Ca2.95 Ba0.50 Sr0.50) (PO4)2O phosphors (Setlur et al., 2010; Im et al., 2010). The excitation and emission spectrum of (Ca2.95 Ba0.50 Sr0.50) (PO4)2O:0.05Eu 2+ are shown in Fig. 5. Under 450 nm excitation, there are the broad asymmetric emission spectrum peaking at 567 nm with four broad emission bands centered at 567, 608, 632 and 667 nm, respectively. The broad emission shows that Eu2+ has more than one emission center in Ca4(PO4)2O which belongs to the typical emission of Eu2+ ions ascribed to 5d→4f transitions. By Gaussian deconvolution, the emission spectrum of (Ca2.95 Ba0.50 Sr0.50)(PO4)2O:0.05Eu 2+ can be well-decomposed into eight Gaussian proflies peaking at 534 Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 42 © Adama Science & Technology University https://ejssd.astu.edu.et nm, 556 nm, 567 nm, 592, 608 nm, 632 nm, 667 nm, 673 nm and 674nm, which can be ascribed to eight different Ca2+ sites occupied by Eu2+ ions. The refinement shows that the structure of Ca4(PO4)2O contains eight crystallographically distinct Ca2+ sites that can be occupied by Eu2+. The Eu2+ ions substituting Ca2+ ions in the site which has shorter Ca-O band distance are expected to experience stronger crystal field strength corresponding to a longer wavelength emission with larger Stoke shift, when the crystal environments are analogous. Fig. 5illustrates eight kinds of cations ions coordinated with oxygen atoms, corresponding to the eight Gaussian profiles in the emission spectrum (Schubert et al., 2005; Deressa et al., 2015 and Wu et al., 2011). It is also observed that the excitation spectrum monitoring by 594 nm exhibits a different spectral profile, which means that the Ba/Sr substitute Ca at diferent site in the lattics and emission bands should be ascribed to different sites occupied by Eu2+ ions, which is also in accordance with the above analysis on the crystal structure. Figure 4. Raman Spectra of (Ca4-y-zBaySrz) (PO 4)2O:0.05Eu2+ phosphor Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 43 © Adama Science & Technology University https://ejssd.astu.edu.et The excitation spectrum shows a broad absorption from 300 to 500 nm with different maximum in the range, which corresponds to the 4f7→4f65d1 transition of the Eu2+ ions. (Ca3.95-y-zBaySrz)(PO4)2O:0.05Eu 2+ can be efficiently excited by blue light (350~480 nm), which is very advantageous for application in the W-LEDs combined with highly efficient blue-InGaN chips (Fig. 6b) (Zhouet al., 2014; Dorenboset al., 2013 and Zhou et al., 2013). Figure 5: Emission spectra of (Ca2.95Ba0.50Sr0.50)(PO4)2O:0.05Eu2+ (λex = 450nm) measured emission (black line), fitted curve (red line) and deconvoluted Gaussian curve Fig. 6(a) shows room temperature normalized PL spectra of (Ca3.95- yBay)(PO4)2O: 0.05Eu 2+ phosphors for an excitation wavelength of 450 nm. The emission intensity at 594nm shifted to shorter wavelength as Ba2+/Sr2+ substituted in the lattices of Ca2+ ion. In general, the electronic configuration of Eu2+ is 4f7 and 4f65d1 at the ground and excited state, respectively, and the luminescence of Eu2+ is ascribed to the 4f65d1into 4f75d0 transition which results in the broad band emission. The emission wavelength of Eu2+ depends strongly on the structure of the host crystal through the crystal field splitting of the 5d band and the ground state for the Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 44 © Adama Science & Technology University https://ejssd.astu.edu.et 4f7electronic configuration (Zhou et al., 2013; Kottaisamy et al., 1994 and Huang et al., 2008). Figure 6: (a) Emission spectra (λex= 450nm) and, (b) Excitation (λem= 594) of (Ca3.95-y-zBaySrz) (PO4)2O: 0.05Eu 2+). As stated above, the positions of the 4f6d1 levels are much more influenced by the outer crystal field interaction than the 4f7 levels and highly depend on the crystalline environment around Eu2+ ion, so significant optical changes are expected if local structure around the Eu2+ center is different (Schubert et al., 2005; Deressa et al., 2015). In case of as-synthesized Ca3.95-y-zBaySrz)(PO4)2O:0.05Eu 2+ phosphors, the environment around Eu2+ ions have uniform crystal structures. The unit cell volume of Ca4(PO4)2O is less than Ba4(PO4)2O and Sr4(PO4)2O hosts due to ionic size difference of Sr2+/Ba2+ and Ca2+ ions and the ionic size of Eu2+ less than that of Sr2+and Ba2+ ions, upon Eu2+ doping in to the above hosts. The is Ba4(PO4)2O and Sr4(PO4)2O less than slightly distortion than Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 45 © Adama Science & Technology University https://ejssd.astu.edu.et Ca4(PO4)2O hosts because of the size difference between Eu2+ and Ba2+ /Sr2+ bigger than the size of Ca2+ ions. Therefore, PL spectrum of the dopant Eu2+ ion experienced strong crystal fields (blue shift) and low symmetry (broaden) in C a 4 - y - z B a y S r z ) ( P O 4 ) 2 O than Ca4(PO4)2O:0.05Eu 2+ phosphors, due to local structure (symmetry) difference around Eu2+ center in the respective hosts. When content of Ba2+/Sr2+ ions increased from (y = 0.1 to 1.0, and z = 0.1 to 1.0) the emission peak positions gradually move toward longer wavelengths from 549 nm to 594 nm, and in addition, the FWHM of Eu2+ ions emission broaden from 140 nm to 210 nm (see Fig. 6(a)). For (Ca3.95- yBay)(PO4)2O:0.05Eu 2+ phosphors, about 70 nm shift and 51 nm broadening allow highly color- tunable phosphors by changing the content of Ba2+/Sr2+ ions (Ronda et al., 2008; Sommerdijk et al., 1974 and Wegh et al., 1999). This indicate that the lattices of (Ca2.95Ba0.50Sr0.50)(PO4)2O host combined from an expanded Ba-O/Sr-O and contracted Ca-O bonds which create different environments around the sites of Eu2+ ions doped in the lattice of (Ca2.95Ba0.50Sr0.50)(PO4)2O phosphor and cause for the blue shifts and spectral broadening of Eu2+ activated (Ca2.95 Ba0.50 Sr0.50) (PO4)2O phosphors. Fig. 7 shows the emission spectra of Ca4-x(PO4)2O:xEu 2+ with different Eu2+ concentration. It can be seen from the Fig., the optimal Eu2+ content was about x= 0.10. When the content of Eu2+ ions was over x = 0.05, concentration quenching occurred and emission intensity decreased with increasing Eu2+ ions concentration. Moreover, emission wavelength as well as emission intensity of the Ca4- x(PO4)2O:xEu 2+ phosphors were changed by varying the concentration of Eu2+ ions. As the concentration of Eu2+ ions in the Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 46 © Adama Science & Technology University https://ejssd.astu.edu.et host lattice was increased, the emission wavelength shifted slightly to a shorter wavelength. With an increase of the Eu2+ concentration x , the integrated emission intensity slightly increases with a breaking point x ≈ 0.10, which establish its higher emission efficiency. Upon excitation at a wavelength of 450 nm, the external quantum efficiency of (Ca2.95 Ba0.50 Sr0.50) (PO4)2 O:0.05Eu2+ and (Sr,Ba)2SiO4:Eu 2+ well known yellow phosphor are determined to be 38.7% and 70.9%, respectively. The lower quantum efficiencies of (Ca2.95 Ba0.50Sr0.50)(PO4)2O:0.05Eu 2+coul d be further enhanced by process optimization. 3.2. Application to White LEDs. Figure 8 shows photometric and colorimetric quantities of the white LEDs under the following applied currents: 20, 50, 100, 150, 200, 250, 300 and 350mA. When the applied current was 350mA, the white LED have CIE color coordinate of (0.3354, 0.3448) at a white light (Tc = 5500 K) and an excellent Ra of 86.The CIE color coordinates shifted towards the white light region along the Planckian locus, the value of Ra decreased from 91.6 to 86.0 and the luminous efficiency decreased from 21.5 to 16.5 lm/W as the applied current was increased. In comparison with the blue InGaN chip pumped with YAG:Ce3+ phosphor (Ra = 75, CCT=7756K), and LaSr2AlO5 phosphor (Ra = 85- 85, CCT = 4200-5500 K) (Deng et al., 2013; Huang et al., 2008), the white LEDs in this study shows higher Ra value and lower CCT value. Therefore, the (Ca, Ba, Sr)4(PO4)2O:Eu 2+ phosphors are promising for application in excellent Ra white LEDs. Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 47 © Adama Science & Technology University https://ejssd.astu.edu.et Figure 7: The emission spectra of Ca4-x(PO4)2O:xEu 2+ with different Eu2+ concentration Figure 8: CIE chromatic coordinates, Ra and luminous efficiency (the inset) of the white LED using (Ca2.95Ba0.50Sr0.50)(PO4)2O:0.05Eu 2+ under various applied currents. The Planckian locus line and the points corresponding to color temperatures are indicate 4. Conclusions In summary, a tunable red-to- yellow- emitting Ca3.95-y-zBaySrz) (PO4)2O:0.05Eu 2+ phosphors have been reported. The excitation spectrum shows broad band in the near-ultraviolet, ultraviolet and blue region, which matches well with blue chips (Ca, Ba, Sr)4 Deressa G. Ethiop. J. Sci. Sustain. Dev., 5 (1), 2018 48 © Adama Science & Technology University https://ejssd.astu.edu.et (PO4)2O:Eu 2+which is excitable over a broad range from 500 to 800nm when its emission can be adjusted from reg to yellow by changing Ba2+/Sr2+ doping concentration. By applying (Ca2.95Ba0.50Sr0.50) (PO4)2 O:0.05 Eu2+ phosphor on blue chip, we obtained a white LED device with high Ra of 86 and CCT value of 5450 K. Therefore, with the interesting tunable emission property, (Ca, Ba) 4(PO4)2O: Eu 2+ phosphor has great application potential as a good color conversion material for solid state lighting. Acknowledgment This research was supported by School of Applied Natural Science, Adama Science and Technology University under Eight Cycle ASTU Research Grant. The laboratory activities were done in the Department of Display Science and Engineering, Pukyong National University, Busan, 608-737, Republic of Korea. References Bachmann, V., Ronda, C., and Meijerink, A., (2009). Temperature Quenching of Yellow Ce3+ Luminescence in YAG:Ce, Chem. Mater., 21, 2077. Deressa, G., Park, K.W., Jeong, H.S., Lim, S.G., Kim, H.J., Jeong, Y.S., and Kim, J.S., (2015). 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