Synthesis and optical properties of red blue emitting sr2mgsi2o7 eu 3 + eu 2+ phosphors for white led

Journal of Science: Advanced Materials and Devices (2016) 204e208 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original article Synthesis and optical properties of red/blue-emitting Sr2MgSi2O7:Eu3ỵ/Eu2ỵ phosphors for white LED Tong Thi Hao Tam a, b, Nguyen Duy Hung a, Nguyen Thi Kim Lien a, Nguyen Duc Trung Kien a, Pham Thanh Huy a, * a b Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hanoi 10000, Viet Nam National Economics University (NEU), No 207 Giai Phong Street, Hanoi 10000, Viet Nam a r t i c l e i n f o a b s t r a c t Article history: Received June 2016 Received in revised form 12 June 2016 Accepted 12 June 2016 Available online 18 June 2016 Phosphor-converted white light emitting diodes (white LEDs) have received great attention in recent years since they have several excellent features such as high lumen output, low power consumption, long lifetime and environmentally friendly In this work, we report the co-precipitation synthesis of red/blue Sr2MgSi2O7:Eu3ỵ/Eu2ỵ phosphors with various Eu doping concentration The results show that the obtained Sr2MgSi2O7:Eu3ỵ/Eu2ỵ phosphors have good crystallinity and emit strong red (Sr2MgSi2O7:Eu3ỵ) and blue (Sr2MgSi2O7:Eu2ỵ) emissions under near UV light excitation The sharp emission peaks at 577, 590, 612, 653, and 701 nm corresponded to the typical 5D0 / 7Fj (j ¼ 0,1,2,3,4) transitions of Eu3ỵ, and the blue emission peaking at 460 nm is attributed to the typical 4f65d1-4f7 transition of Eu2ỵ in the same Sr2MgSi2O7 host lattice Both phosphors can be well excited in the wavelength range of 260e400 nm where the near UV-LED is well matched The above results suggest that the Sr2MgSi2O7:Eu3ỵ/Eu2ỵ phosphors are promising red/blue-emitting phosphors for the application in near UV pumped phosphorconverted white LEDs © 2016 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Sr2MgSi2O7:Eu3ỵ/Eu2ỵ Photoluminescence Red and blue emitting phosphor Introduction Phosphors are widely used in solid-state lighting, especially for the phosphor-converted light emitting diode (white LED) in which yellow light-emitting phosphor (such as YAG:Ce3ỵ) are pumped by GaN chips to generate white light [1] On the one side, the current white LEDs show several advantages over incandescent and fluorescent lamps including low operating voltage, low energy consumption, long lifetime … However, on the other side, this kind of white LED shows relatively low color-rendering index (CRI), and high color temperature due to lack of a red-light emitting component [2,3] So far, one solution to these problems has been to fabricate a white LED with high color rendering by combining red, green and blue emitting tricolor phosphors pumped by a near UVLED [4,5] Therefore, extensive efforts have been made to develop new blue and red phosphors with light luminous efficiency, good color, and high CRI [6] * Corresponding author Tel.: ỵ84 36230435; fax: ỵ84 36230293 E-mail address: huy.phamthanh@hust.edu.vn (P.T Huy) Peer review under responsibility of Vietnam National University, Hanoi Recently, the alkaline earth silicates based-phosphors (alkermanite phosphors) have been reported as one of the most essential luminescent materials due to their excellent thermal and chemical stability and high brightness Particularly, Sr2MgSi2O7 is a good candidate for UV-LED application since it has a rigid tetragonal structure and strong absorption band in UV region [7,8] It is well known that Europium (Eu) is the most common rare earth to be used as an activator in phosphors Eu3ỵ ion is a preferable activator for red phosphors with sharp emission peaks in the red region (from 570 to 700 nm) caused by the 5D0 / 7Fj (J ¼ 0, 1, 2, 3, 4) transitions of the trivalent state, while Eu2ỵ is the most frequently used activator in the blue phosphors and its emission usually consist of a broad band due to transitions from the 4f65d to the 4f7 ground state Additionally, Eu2ỵ ion can emit light from the UV to the infrared with broad band emitting luminescence on different host matrices since the involved 5d orbital of Eu2ỵ ion is external and strongly influenced by the crystal field [9,10] Until now, the phosphors based on Sr2MgSi2O7 host lattice were prepared by different methods such as solid-state reaction, hydrothermal, solegel methods or combustion processing with ultrasonic dispersion technique [11e14] Among the various synthesis methods, the co-precipitation method is known to http://dx.doi.org/10.1016/j.jsamd.2016.06.009 2468-2179/© 2016 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/) T.T Hao Tam et al / Journal of Science: Advanced Materials and Devices (2016) 204e208 205 produce phosphor powders with uniform, narrow size distribution, and homogeneous distribution of the activator ions [15] It is important to note that in most of the previous research, to synthesis the Sr2MgSi2O7:Euỵ2 phosphor, the precursor powders were normal sintered in reduced gas environment in a one-step synthesis process, therefore only the blue emitting phosphors can be obtained In this work, we present the results of our study on red/blue phosphors based on Eu-doped Sr2MgSi2O7 prepared by coprecipitation method Initially, Eu3ỵ-doped phosphor was synthesized as a red emitting phosphor, and its structure and luminescent properties were investigated as a function of the sintering temperature Lately, Eu2ỵ doped phosphor was obtained by reducing the corresponding Eu3ỵ phosphor in forming gas environment Moreover, the inuence of Eu3ỵ doping concentration on the luminescent properties of the phosphors was also investigated Experimental The Sr2MgSi2O7:Eu3ỵ phosphor was synthesized by a coprecipitation reaction In this reaction, nitrate salts Sr(NO3)2, Mg(NO3)2$6H2O, tetraethylorthosilicate (C2H5O)4Si (TEOS), and europium oxide Eu2O3 were used as precursors All these chemicals were of analytic grade The raw materials were weighed according to the nominal composition of Sr2-xMgSi2O7:xEu3ỵ (x ẳ 0.02, 0.03, and 0.04) Sr(NO3)2.4H2O and Mg(NO3)2$6H2O were mixed in distilled water A stoichiometric amount of TEOS and Eu2O3 was also dissolved in ethanol and HNO3, respectively The solutions were stirred until the solution became transparent, after which they were mixed and continuously stirred for h Subsequently, an appropriate amount of NH4OH was added to the solution to enable precipitation The precipitate and solution were continuously stirred to obtain a white viscous gel Then, centrifugal force was applied to allow the resulting precursor to be separated The separated precursor was washed with DI water for several times After drying at 200  C for 24 h, the dry powder was calcined in air at various temperatures for h to receive the Sr2-xMgSi2O7:xEu3ỵ phosphors To produce the Sr2-xMgSi2O7:xEu2ỵ phosphors, the corresponding Sr2-xMgSi2O7:xEu3ỵ phosphor was subjected to ion reduction in the mixture of H2/N2 (10%/90%) gas at different temperatures for h The phase purity of the phosphors was identified by X-ray diffraction (XRD) pattern Measurements were carried out on a D8/ Advance-Bruker diffractometer with CuKa radiation (l ¼ 1.5403 Å) The scan rate was kept at s/step at a scattering angle range of 20e70 The Raman spectra were recorded on a Horiba Jobin Yvon LabRAM HR-800 spectrometer using HeeNe laser (632.8 nm) with a power density of 215 W/cm2 A high-resolution mode of 1.2 cmÀ1 was used Morphology was taken with a JSM-7600F (Jeol Co., Japan) field emission scanning electron microscope (FESEM) PL and PLE were measured on a NANO LOG spectrofluorometer PL and PLE spectra were obtained by using a 450 W xenon light source with a spectral resolution of about nm Results and discussion Fig shows XRD patterns of the product sintered at 900, 1100, 1200, and 1300  C for h It can be seen that until temperature of 1200  C, the main crystalline phase in the powder is Si2SiO4, beside the Sr2MgSi2O7 and Sr3MgSi2O8 phases with smaller portion Contents of the desired Sr2MgSi2O7 phase and the secondary Sr3MgSi2O8 phase increase with increasing the sintering temperature At the temperature of 1300  C, intensity of the diffraction peaks related to the Sr2SiO4 phase decreased abruptly, in contrast to the strong increase Fig XRD patterns of Sr2MgSi2O7:Eu3ỵ phosphors sintered at 900, 1100, 1200, and 1300  C for h Closed square, closed circle (grey color) and closed circle (red color) denote XRD peak positions of Sr2MgSi2O7, Sr2SiO4, Sr3MgSi2O8, respectively of the Sr2MgSi2O7 phase These results indicate that the presence and dominance of the Sr2MgSi2O7 phase can only be obtained in the sample sintered at 1300  C Our results are similar to those reported by Kwon et al., in which the Sr2SiO4 phase in the Sr2MgSi2O7 phosphor (synthesized by a conventional solid-state reaction method) disappeared only after sintering at 1300  C or higher [16] The XRD pattern Fig also confirms the tetragonal structure of the Sr2MgSi2O7 host lattice The main phase Sr2MgSi2O7 has tetragonal crystal structure Sr2ỵ ion in this crystal structure occupies a unique position (position symmetry Cs) with eight neighboring O2À ions and the SreO distance is 2.662 Å in average When Eu is doped into Sr2MgSi2O7, Eu ions are expected to replace the position of Sr2ỵ in the crystal network because of the excellent compatibility ionic radius of Eu3ỵ and Sr2ỵ, 1.25 and 1.26 Å, respectively [17e19] To evaluate the possibility of replacing the Eu3ỵ ions on the position of Sr ions in the host lattice, Raman spectra measurement of the doped 4% Eu3ỵ and undoped Sr2MgSi2O7 host were carried out The Raman spectra taken at room temperature are shown in Fig For the host lattice (curve a), Raman peaks are observed at 901, 652, 315, 220, 201 and 153 cmÀ1 The peaks correspond to the stretching vibrations of the SieO and SieSi bonds of the Si2O7 group [20] For the doped Sr2MgSi2O7:4%Eu3ỵ sample, the Raman spectrum (curve b) is similar to that of the undoped Sr2MgSi2O7 sample, no other peaks were found This result implies that the Eu3ỵ dopant ion was not substituted on the Si4ỵ site, and did not change the unite cell volume and SiOSi angle, instead they were incorporated in to host lattice by replacing the Sr2ỵ sites The morphology of the phosphors was characterized by FESEM Fig show FESEM images of the as-received phosphor (dry powder) (a) and the Sr2MgSi2O7:Eu3ỵ phosphor sintered at 1300  C for h (b) The dry powder show clusters of particles with variety shapes and sizes, whereas the Sr2MgSi2O7:Eu3ỵphosphor exhibits needle-like shape particles with an average length of about micron The chemical composition of the Sr2MgSi2O7:3%Eu3ỵ phosphor has been measured using energy dispersive X-ray spectroscopy (EDS) The result of the EDS analysis is shown in Fig 3(c) which is representing the composition of the phosphor powder studies Fig illustrates the photoluminescence (PL) spectra of Sr2MgSi2O7:4%Eu3ỵ phosphor samples sintered at different temperatures in the range of 900e1300  C Under the near UV excitation of 360 nm, a broad blue emission band centered around 430e470 nm and several sharp lines in the orangeered region 206 T.T Hao Tam et al / Journal of Science: Advanced Materials and Devices (2016) 204e208 Fig Raman spectra of the Sr2MgSi2O7 (a) and Eu3ỵ-doped Sr2MgSi2O7 (b) phosphors sintered at 1300  C in air ambient for h peaking at about 577, 590, 612, 653, and 701 nm The sharp red emission lines should be ascribed to the transitions within the 4f6 configuration of Eu3ỵ These lines corresponds to the 5D0 / 7F0, D0 / 7F1, 5D0 / 7F2, 5D0 / 7F3 and 5D0 / 7F4 transitions of Eu3ỵ, respectively [21] Here, the emission line at 612 nm is attributed to the electric dipole transition (5D0 / 7F2), while the emission around 590 nm is assigned to the magnetic dipole transition (5D0 / 7F1), which is sensitive to site symmetry According to the parity selection rule, when the Eu3ỵ ions are located at the site with an inversion symmetric center, the 5D0 / 7F1 magnetic dipole transition is permitted, which results in orangeered emission around 590 nm In the other case, if the Eu3ỵ ions located at the site without an inversion symmetric center, because the opposite parity 5d configuration is mixed into 4f configuration, the parity selection rule is able to lifted, and fef forbidden transition is partially allowed, the hypersensitive 5D0 / 7F2 electric dipole transition will be permitted, which results in red emission around 612 nm [13,21,22] Thus, the observation of the strongest emission peak at 612 nm in our phosphor may indicate that Eu3ỵ ions mainly occupy non-inversion symmetric center in the host lattice For the broad blue emission band, it is known that Eu2ỵ presents a broad emission band peaking at around 460 nm due to the 4f65d1 to 4f7 transition of Eu2ỵ (8S7/2-7Fj, j ẳ 0, 1, 2, 3, and 4) Since no reduction process has been carried out with the phosphor, it is quite possible that during the high temperature sintering, a small amount of Eu3ỵ ions were reduced to Eu2ỵ ions that leads to the blue emission [23e26] Longer sintering time and higher temperature can enhance this ion transformation It can also be seen from Fig that while the PL intensity of the red emissions increased with increasing sintering temperature, the peak position and the shape of the blue band change arbitrary with increasing temperature Since the blue emission bands related to Eu2ỵ ions is sensitive to the host lattice environment, the change of the blue emission band with sintering temperature may indicate the change of the crystalline phases in the sample as observed from XRD results In our opinion, the increase of the PL intensity of the red emission is related to the higher content of the Sr2MgSi2O7 phase upon increasing temperature from 900 to 1300  C Fig FESEM images of the as-received powder (dry powder) (a) and the Sr2MgSi2O7:3%Eu3ỵ phosphor powder sintered at 1300  C in air ambient for h (b) and EDS spectrum (c) of the Sr2MgSi2O7:3%Eu3ỵ phosphor after sintering Fig shows the excitation spectrum (PLE monitored at 612 nm) of the Sr2MgSi2O7:Eu3ỵ phosphor The PLE spectrum covers a wide region between 350 and 600 nm revealing that the phosphor can be excited by near UV at 360, 381, 393 and 463 nm Such excitation wavelengths are well matched with near UV-LED excitation wavelength, indicating a great potential for white LED application Further, the strong excitation peak at 463 nm points out that the Sr2MgSi2O7:Eu3ỵ phosphor can also be used in the blue LED pumped white LED To investigate the effect of Eu3ỵ doping concentration on PL intensity of Sr2MgSi2O7 phosphors synthesized by co-precipitation method, the emission spectra of the phosphors at various Eu3ỵ concentrations (x ¼ 0.02, 0.03, and 0.04) are presented in Fig The emission intensity increases until x ¼ 0.03 and then decreases as a result of enhanced dipoleedipole interaction This optimal Eu3ỵ concentration is lower than that reported in the literature [17] T.T Hao Tam et al / Journal of Science: Advanced Materials and Devices (2016) 204e208 Fig Photoluminescence spectra of Sr2MgSi2O7:4%Eu3ỵ powders sintered at different temperatures from 900 to 1300  C in air ambient for h under 360 nm near UV excitation Fig Excitation spectrum (PLE) of Sr2MgSi2O7:Eu3ỵ phosphor (monitor for 612 nm emission line) 2ỵ The PL and PLE spectra of Sr2MgSi2O7:Eu phosphors sintered at 1300  C for h and reduced at 1300 for h are shown in Fig The PL spectra show a broad emission band in the blue peaking at 462 nm under the excitation wavelength of 360 nm This emission is due to the 4f65d1e4f7 transition of Eu2ỵ ions in the host lattice Also, the PLE spectrum monitored at 460 nm is shown in Fig It is shown that the blue emission band can be efficiently excited by both UV and near UV excitation source from 260 to 415 nm Thus, the Sr2MgSi2O7:Eu2ỵ phosphor obtained in this work matches well the excitation wavelength of the near UV LED chip Conclusions Sr2MgSi2O7:Eu3ỵ/Eu2ỵ phosphors were prepared by the coprecipitation method followed by sintering at 1300  C for h in air ambient (Sr2MgSi2O7:Eu3ỵ) and reduced at 1300  C for h in forming gas environment (Sr2MgSi2O7:Eu2ỵ) The Sr2MgSi2O7:Eu3ỵ phosphor shows strong red emission peaking at 612 nm that can be excited by both near UV (360, 381, 393 nm) and blue (463 nm) LED 207 Fig PL spectra of Sr2MgSi2O7:xEu3ỵ phosphors with different Eu3ỵ doping concentration (x ẳ 0.02, 0.03 and 0.04) under optimum excitation wavelength of 393 nm Fig PL (lex ẳ 360 nm) and PLE spectra of Sr2MgSi2O7:Eu2ỵ phosphor sintered at 1300  C in air ambient for h and reduced at 1300  C for h in forming gas of 10%H2/ 90%N2 environment The Sr2MgSi2O7:Eu2ỵ phosphor emits strong blue light peaking at 462 nm and can be excited by both UV and near UV-LED These results suggested that the Sr2MgSi2O7:Eu3ỵ/Eu2ỵ phosphors have high potential for phosphor-converted white LED application Acknowledgments This work was supported by the National Program on Technology Innovation, project number DM.06.DN/13 This paper is dedicated to PETER BROMMER e a former physicist of the University of Amsterdam and good friend of the Vietnamese physicists References [1] S Lee, S Seo, Optimization of yttrium aluminum garnet: Ce3ỵ phosphors for white light-emitting diodes by combinatorial chemistry method, J Electrochem Soc 149 (2002) J85eJ88, http://dx.doi.org/10.1149/1.1511755 [2] Y.X Pan, M.M Wu, Q.J Su, Tailored photoluminescence of YAG: Ce3ỵ phosphor through various methods, J Phys Chem Solids 65 (2004) 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(Sr2MgSi2O7: Eu2 ỵ) The Sr2MgSi2O7: Eu3 ỵ phosphor shows strong red emission peaking at 612 nm that can be excited by both near UV (36 0, 38 1, 39 3 nm) and blue (4 63 nm) LED 207 Fig PL spectra of Sr2MgSi2O7: xEu3ỵ... Sr2MgSi2O7: Eu3 ỵ /Eu2 ỵ phosphors were prepared by the coprecipitation method followed by sintering at 130 0  C for h in air ambient (Sr2MgSi2O7: Eu3 ỵ) and reduced at 130 0  C for h in forming gas... Sr2MgSi2O7: xEu3ỵ phosphors with different Eu3 ỵ doping concentration (x ẳ 0.02, 0. 03 and 0.04) under optimum excitation wavelength of 39 3 nm Fig PL (lex ¼ 36 0 nm) and PLE spectra of Sr2MgSi2O7: Eu2 ỵ phosphor