High-Performance Nanomaterials as Phosphors for Light-Emitting DiodesView this Special Issue
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Concentration Dependence of Luminescent Properties for : Red Phosphor and Its Charge Compensation
Sr2TiO4:Eu3+ phosphors using M+ (M = Li+, Na+, and K+) as charge compensators were prepared by the solid-state reaction. The powders were investigated by powder X-ray diffraction (XRD) and photoluminescence spectra (PL) to study the phase composition, structure, and luminescent properties. The results showed that Li+ ion was the best charge compensator. The phase was Sr2TiO4 when the doping concentration was small (%). When reached 15.0%, the phase turned into Sr3Ti3O7 because of the structure damage. The phosphor could be effectively excited by ultraviolet (365, 395 nm) and blue light (465 nm), and thenitemitted intense red light that peaked at around 620 nm (5D0→7F2). In addition, the emission of 700 nm (5D0→7F4) enhanced the red light color purity. The CIE chromaticity coordinates of samples with the higher red emission were between (0.650, 0.344) and (0.635, 0.352). Doped layered titanate Sr2TiO4:Eu3+ is a promising candidate red phosphor for white LEDs which can be suited for both near-UV LED chip and blue LED chip.
White light-emitting diodes (LEDs) are considered to be next-generation lighting devices. They have many advantages such as energy saving, environment friendliness, and small size [1–3]. Phosphor conversion method is a principal method in all the technologies to achieve white light owing to its easy achievement, high efficiency, and low cost.
The combination of blue chip and yellow phosphors has already been developed and is commercially available, but the lack of red emitting makes the color rendering index (CRI) lower . The other tricolor white LEDs consisting of red, green, and blue (RGB) phosphors excited with a UV chip emitting 400 nm also have challenges. The luminescence intensity of commercial red phosphors is much weaker than green and blue phosphors. For excellent color render index, both methods need efficient red phosphors that should have the excitation wavelength matching with the emission of the blue LEDs ( = 440–470 nm) or the UV LEDs ( = 350–410 nm). Therefore, the development of a red phosphor with high luminance and satisfactory chemical stability is a key technology for achieving warm white LEDs.
As a result, a kind of red phosphor with perovskite structure attracts much attention [5–7]. Perovskite structure MTiO3:Pr3+ (M = Ca, Sr, and Ba) phosphors emitted intense red light at 610 nm when excited by UV-light [8, 9]. Its intensity was greatly enhanced by adding charge compensation agents such as Al3+, Li+, and Na+ [10–12]. The optimized excitation wavelength of CaTiO3:Eu3+ was 400 nm, suitable for near-ultraviolet (N-UV) LED chip and emitted red light at 618 nm [13, 14]. However, the intensity is not so high that more-efficient red phosphors are needed to achieve an acceptable efficiency for white LEDs. Layered perovskite compounds have longer distance between layers so that they have bigger doping concentration. Therefore, layered perovskite compounds are good host materials for phosphors. Sr2TiO4 is a typical layered perovskite compound. According to these, trivalent Europium ion-activated Sr2TiO4 phosphor is prepared by solid-state reaction, and its luminescence properties are investigated to see whether it has the potential to be a red phosphor for N-UV or blue light LED chip.
2.1. Sample Preparation
All powder samples were synthesized through the solid-state reaction technique. High-purity SrCO3, TiO2 (analytical grade) and Eu2O3 (>99.99%) were mixed thoroughly in alcohol by ball milling and then dried. Appropriate amounts of Li2CO3, Na2CO3, or K2CO3 (analytical grade) were added as the charge compensators. The synthesis was performed at 1100°C for 2 h under air atmosphere in electric tube furnace. Series of (Sr1−2xEuxMx)2TiO4 (M = Li+, Na+, K+; x (mol%) = 5.0, 10.0, 15.0, 20.0, 25.0) powders were prepared. SrCO3 and Eu2O3 come from Sinopharm Chemical Reagent Co., Ltd., land Li2CO3, Na2CO3, K2CO3 and TiO2 come from Shanghai Lingfeng Chemical Reagent CO., LTD.
The crystalline phases of synthesized powders were determined by X-ray diffraction (XRD, D/Max2500, Rigaku, Japan) using Cu Kα radiation ( Å) in the range of – with a step size of 0.02°. The photoluminescence spectra of the phosphors were measured using a fluorescent spectrophotometer (FL3-221, HOROBA, Jobin Yvon, France) at room temperature.
3. Results and Discussion
3.1. The Choice of Charge Compensators
Eu3+ ions are expected to replace Sr2+ ions. It would be difficult to keep charge balance in the lattice. Therefore, Eu3+ ions may not be fully introduced into Sr2+ sites in order to keep charge balance. Eu3+ may exist in Eu2O3 state, and it would lead to the decrease of emission intensity. This problem can be solved by adding charge compensators. Li+, Na+, and K+ ions are always chosen as charge compensators for phosphors [15, 16]. Because the radii of Li+, Na+, and K+ are small which are easy to enter into lattice, and they are all of +1 valence which is convenient for charge compensation. Therefore, Li+, Na+, and K+ were added in the same molar weight as Eu3+ to act as the charge compensators to improve the luminescence intensity of Sr2TiO4:Eu3+.
Figure 1 shows the excitation and emission spectra of (Sr0.8Eu0.1M0.1)2TiO4 phosphor with different charge compensators. The shape and positions were similar in the PL and PLE spectra for all the samples. Excitation and emission intensities were enhanced obviously after adding charge compensators. Before adding charge compensators, Eu3+ could not substitute Sr2+ sites totally in order to maintain chemically neutral Sr2TiO4:Eu3+ phosphor. Some Eu3+ could not act as activated ions, and the impurity phase could interdict the energy transfer between matrix and activated ions and restrain the Sr2TiO4 grains growth during the sintering process . After adding charge compensators, Eu3+ ions were able to be fully introduced into Sr2+ sites, and charge compensators and vacancy could substitute Sr2+ sites so that SrO and Eu2O3 could react with TiO2 to a great extent. In addition, Li2CO3, Na2CO3, and K2CO3 can act as flux agents to promote the formation of luminescence materials polycrystal. Compared with Na+ and K+, the intensity was enhanced greatly when adding Li+ as charge compensator. These phenomena are assigned to the fact that the ionic radius of Li+ (0.92 Å) to the one of Sr2+ (1.31 Å) is the smallest, the one of Na+ (1.24 Å) being second, and the one of K+ (1.55 Å) is the biggest so that it develops the distortion grade different, which impacts on the luminous properties and crystal structures . Thus, Li+ is selected as charge compensator in the following experiments.
3.2. XRD of (Sr1−2xEuxLix)2TiO4 (x (mol%) = 0.0, 5.0, 10.0, 15.0, 20.0) Powders
Figure 2 shows the XRD patterns of Eu3+ ions doped Sr2TiO4 phosphors with different Li+ concentration as charge compensator. At first, when the doping concentration was small (), all the peaks were indexed by Sr2TiO4 without any impurity. After reached 10.0%, there appeared some Sr3Ti2O7 peaks, but Sr2TiO4 was the major phase. When , Sr3Ti2O7 turned to be the major phase, because the structure of Sr2TiO4 was damaged with the increase of Eu3+ and Li+. Until came to 20.0%, the phase composition consisted of Sr3Ti2O7 and some impurities, because the content of Sr2+ was too little and the radius of Li+ was too small to support the whole structure. After doping Eu3+, peaks shifted to higher 2θ values relatively, which indicated that Eu3+ had occupied the sites of Sr2+, because the radius of Eu3+ is 1.12 Å (CN = 9), smaller than that of Sr2+ (1.31 Å, CN = 9). When Eu3+ occupied the site of Sr2+, the lattice would shrink so that the diffraction peaks would shift to high diffraction angles compared to pure Sr2TiO4 and Sr3Ti2O7 .
3.3. Luminescent Properties of (Sr1−xEuxLix)2TiO4 (x (mol%) = 5.0, 10.0, 15.0, 20.0) Phosphors
Figure 3(a) presents the excitation spectra of (Sr1−xEuxLix)2TiO4 under different Eu3+ concentrations ( = 615 nm). The excitation spectra consisted of a wide excitation band from 325 to 425 nm and some sharp line peaks of characteristic transitions of Eu3+. The broad band was a charge transfer band (CTB) which was caused by several charge transition. Figure 3(b) shows the results of Gaussian peak fitting of the PLE spectra of (Sr0.7Eu0.15Li0.15)2TiO4. Peak 1 (350 nm) was ascribed to the O→Ti of phase Sr3Ti2O7, peak 2 (380 nm) was ascribed to the O→Ti of phase Sr2TiO4, and peak 3 (362 nm) was ascribed to the O→Eu. The sum of deconvoluted curves (red dash line) was almost fitted with observed line (black solid line). The other intense 395, 465 and weak 415 nm excitation peaks related the intra-4f transitions from ground sate 7F0 to 5L6, 5D2, and 5D3, respectively. The shape and position were similar except that the intensity of absorption varied with the increase of Eu3+ concentration. What interests us is that the intensity of CTB is stronger than that of intra-4f transitions at lower Eu3+ concentration (), and it mainly performs wide band absorption of matrix. Then the intensity of CTB decreases with the increase of Eu3+ doping concentration. However, characteristic sharp line spectra increase continuously until , and it performs 395 and 465 nm intense linear excitation peaks. With the increase of Eu3+ and Li+, the phase composition changes from Sr2TiO4 to Sr3Ti2O7 to some impurities, and impurity is bad for luminescence intensity. However, both intensity of CTB and intra-4f transitions are high when Eu3+ doped in layered titanate, and there is reason to believe that Sr2TiO4:Eu3+ and even Sr3Ti2O7:Eu3+ are promising red phosphors for white LEDs.
Figures 4(a), 4(b), and 4(c) are the emission spectra of (Sr1−xEuxLix)2TiO4 excited by 365, 395, and 465 nm, respectively. The shape and position of the emission spectra were similar, and they all emitted dominated intense 620 nm (5D0→7F2) red light and weak 578 nm (5D0→7F1), 700 nm (5D0→7F4) light. 5D0→7F2 and 5D0→7F4 are electric dipole transitions of Eu3+, which are very sensitive to the local environment around the Eu3+, and depend on the symmetry of the crystal field. As we know, the transitions are forbidden when Eu3+ occupies centrosymmetric sites. The structure of Sr2TiO4 is tetragonal K2NiF4 type, and perovskite layers are interleaved with SrO layers. In Sr2TiO4, all Sr ions occupy positions between the perovskite layers (9-oxygen-ion-coordinated sites with symmetry) . Therefore, it indicates that Eu3+ has occupied the noncentersymmetrical sites of Sr2+. Compared with usual luminescence of Eu3+ (594, 615 nm) , the luminescence of Eu3+ in layered perovskite has some red shift which performs better red light color purity and higher luminescent intensity. In addition, the emission of 700 nm in some degree enhances red light color purity, too. Those are more beneficial to make it become a red light compensation material for white LEDs.
The emission intensity was found to increase with the increase of Eu3+ concentration up to 10.0 mol% ( nm). While excited at 395 and 465 nm, the best doping concentration was 15.0 mol%. Because, when , the phase is Sr2TiO4 and group has strong absorption at UV light region , it has the strongest luminescence intensity when excited at 365 nm. However, when , the phase is Sr3Ti2O7. The crystal system of Sr3Ti2O7 is tetragonal, and double perovskite layers are interleaved with SrO layers. In Sr3Ti2O7, some Sr ions occupy positions between the perovskite layer (9-oxygen-ion-coordinated sites with symmetry), and the other Sr ions occupy positions in the center of the perovskite layer (12-oxygen-ion-coordinated sites with symmetry) . In spite of the 12-oxygen-ion-coordinated sites in Sr3Ti2O7, 9-oxygen-ion-coordinated sites are vast majority; therefore, it emits characteristic intense red light of Eu3+ which is sensitive to the surrounding symmetry of the crystal field. Usually, Eu3+ doping concentration is less than 5.0% in normal phosphor. However, the doping concentration of Eu3+ is up to or even over 10.0% in Sr2TiO4 and Sr3Ti2O7 host. The main reason is that layered perovskite has SrO layers interleaved in perovskite layers, and it makes bigger space between layers so that they have bigger doping concentration which leads to the higher luminescence intensity. In conclusion, layered titanate is a good matrix for phosphors, and Eu3+ doped layered titanate is a good red phosphor for white LEDs which can be suited for both near-UV LED chip and blue LED chip.
The samples that are emitting most intense red light respectively excited at 365, 395, and 465 nm are chosen to calculate their CIE chromaticity coordinates. The corresponding CIE chromaticity coordinates using symbol “” to indicate their positions are shown in Figure 5, and they change between (0.650, 0.344) and (0.635, 0.352), which is due to the variability of the relative intensities of 620 nm and 700 nm mainly. They are all close to coordinates of the “ideal red” which is (0.67, 0.33) . In addition, their CIE chromaticity coordinates are rather close the edge of CIE diagram, indicating that this kind of phosphors shows better color purity in solid-state lighting.
3.4. SEM of (Sr1−xEuxLix)2TiO4 (x (mol%) = 5.0, 10.0, 15.0, 20.0) Phosphor Powders
It is well known that crystallinity and surface morphology of phosphors has a strong effect on the luminescence properties. Figure 6 shows the SEM morphology of (Sr1−xEuxLix)2TiO4 with different Eu3+ concentration. The particles enlarged with the increase of Eu3+ and Li+ concentration, and all presented irregular polygon. The particle size was about 1 μm when and kept around 5 μm when reached 10.0%, 15.0%, and 20.0% which corresponded with the requirements of particle size to phosphor. It has been established that Li+ was conducive to the formation of crystalline phase, and it is mainly because of the function of Li2CO3 flux . However, the special doping concentration of active Eu3+ also has some influence on the particle size .
In this paper, Sr2TiO4:Eu3+, M+ (M = Li+, Na+, and K+) phosphors were synthesized through the solid-state reaction. M+ as charge compensators led to the increase of emission intensity, and Li+ was confirmed to be the best charge compensator. The excitation spectra consisted of a wide excitation band and some sharp line peaks of characteristic transitions of Eu3+ when Eu3+ doping concentration is low (), the phase is Sr2TiO4, and the intensity excited by CTB (365 nm) is stronger than that of intra-4f transitions. After reached 15.0%, the structure damaged and the phase became Sr3Ti2O7. The intensity excited by CTB became weaker and characteristic emission peaks of Eu3+ stronger when excited at 395 and 465 nm. Because Sr2TiO4 has strong absorption at UV light region, while Sr3Ti2O7 has two layers and there are more noncentersymmetrical Sr2+ sites for substituting. Those phosphors all exhibited intense 620 nm (5D0→7F2) red light and weak 578 nm (5D0→7F1), 700 nm (5D0→7F4) light. The CIE chromaticity coordinates of three samples that are emitting most intense red light change between (0.650, 0.344) and (0.635, 0.352), close to coordinates of the “ideal red” which is (0.67, 0.33). The particle size meets the demands of phosphor. Doped layered titanate Sr2TiO4:Eu3+ is a promising candidate red phosphor for white LEDs which can be suited for both near-UV LED chip and blue LED chip.
The authors would like to thank the generous financial support from National Defense Fundamental Research of China (6134502), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Research and Innovation Program for College Graduates of Jiangsu Province (CXZZ12_0410), and National Natural Science Foundation of China (51202111).
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