A white light emitting phosphor Sr1.5Ca0.5SiO4:Eu3+, Tb3+, Eu2+ for LED-based near-UV chip: Preparation, characterization and luminescent mechanism

A white light emitting phosphor Sr1.5Ca0.5SiO4:Eu3+, Tb3+, Eu2+ for LED-based near-UV chip: Preparation, characterization and luminescent mechanism

Journal of Luminescence 131 (2011) 2697–2702 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/lo...

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Journal of Luminescence 131 (2011) 2697–2702

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

A white light emitting phosphor Sr1.5Ca0.5SiO4:Eu3 þ , Tb3 þ , Eu2 þ for LED-based near-UV chip: Preparation, characterization and luminescent mechanism Xi Chen, Junfeng Zhao, Liping Yu, Chunying Rong, Chengzhi Li, Shixun Lian n Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education), Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha, Hunan 410081, China

a r t i c l e i n f o

abstract

Article history: Received 15 October 2010 Received in revised form 24 June 2011 Accepted 27 June 2011 Available online 7 July 2011

In this work, we report preparation, characterization and luminescent mechanism of a phosphor Sr1.5Ca0.5SiO4:Eu3 þ ,Tb3 þ ,Eu2 þ (SCS:ETE) for white-light emitting diode (W-LED)-based near-UV chip. Co-doped rare earth cations Eu3 þ , Tb3 þ and Eu2 þ as aggregated luminescent centers within the orthosilicate host in a controlled manner resulted in the white-light phosphors with tunable emission properties. Under the excitation of near-UV light (394 nm), the emission spectra of these phosphors exhibited three emission bands: one broad band in the blue area, a second band with sharp lines peaked in green (about 548 nm) and the third band in the orange–red region (588–720 nm). These bands originated from Eu2 þ 5d-4f, Tb3 þ 5D4-7FJ and Eu3 þ 5D0-7FJ transitions, respectively, with comparable intensities, which in return resulted in white light emission. With anincrease of Tb3 þ content, both broad Eu2 þ emission and sharp Eu3 þ emission increase. The former may be understood by the reduction mechanism due to the charge transfer process from Eu3 þ to Tb3 þ , whereas the latter is attributed to the energy transfer process from Eu2 þ to Tb3 þ . Tunable white-light emission resulted from the system of SCS:ETE as a result of the competition between these two processes when the Tb3 þ concentration varies. It was found that the nominal composition Sr1.5Ca0.5SiO4:1.0%Eu3 þ , 0.07%Tb3 þ is the optimal composition for single-phased white-light phosphor. The CIE chromaticity calculation demonstrated its potential as white LED-based near-UV chip. & 2011 Elsevier B.V. All rights reserved.

Keywords: Sr1.5Ca0.5SiO4:Eu3 þ Tb3 þ Eu2 þ White-light emitting phosphor Near-UV chip excitation

1. Introduction Recently, much attention has been paid to the development of white-light emitting diodes (W-LED) [1–3] because of their wide application in consumer electronics as well as in solid lighting. A stable white light has been obtained through a blue LED (GaN chip) precoated with a yellow phosphor (Y1  aGda)3(Al1  bGab)5 O12:Ce3 þ (YAG:Ce) [4] and has been widely used in various applications such as full-color displays, liquid crystal display back lighting and traffic signals [5]. However, this kind of white light has a low color rendering index (Rao80) because the yellow light emission from the phosphor YAG:Ce lacks sufficient red emission. So, white light generation has been proposed to combine LED chip with three-phased [6] (red [7–9], green [10] and blue [11]) phosphors, but in this three-color-converter system, the blue emission efficiency is poor because of the strong re-absorption problem of the blue light by the red and green emitting phosphors.

n

Corresponding author. Tel./fax: þ 86 731 8865345. E-mail address: [email protected] (S. Lian).

0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.06.056

To overcome problems like this for W-LED, single-phased whitelight (SPWL) phosphors [1,2,6,12–21] have been an active research area in the study of luminescent materials. Although singlecomposition white-light phosphors for near-UV or UV excitation have been reported in the literature, satisfactory solution are much harder and novel phosphor is still desirable. Alkaline earth orthosilicate M2SiO4 (M ¼Ca, Sr, Ba) phosphors for W-LED have been intensively studied due to their high chemical stability [1,16,22–30] since M2SiO4 doped with divalent europium (Eu2 þ ) was first investigated by Kim et al. [16]. Choi et al. [1] recently reported that (Ba,Ca)2SiO4:Eu2 þ , Mn2 þ phosphor emits warm white light under ultraviolet light excitation. In addition, an enhanced and tunable excitation and emission intensity of Sr2SiO4:Eu2 þ phosphor can be obtained by co-doped Ba2 þ /Mg2 þ to change the crystal field and covalence of the host [24]. Therefore, emission color and efficiency of orthosilicate phosphors can be fine-tuned either by the substitution of different cations in the host or by co-doped activators. Rare earth ions have been playing an important role in phosphor duo to the abundant emission colors based on their 4f-4f or 5d-4f transitions, such as the trivalent europium ion

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(Eu3 þ ) is a red-emitting activator due to its 5D0-7F1,2 transitions [31] and the terbium ion (Tb3 þ ) is a green-emitting activator due to its 5D4-7F0 transitions [32]. In addition to the above sharp emission lines originating from 4f-4f transition, these transitions from higher 5d levels, such as bivalent europium ion (Eu2 þ ) 5d-4f transitions, are often observed depending on the host lattice [33] and the doping concentration of Eu2 þ , leading to the broad blue emission. After an appropriate selection of the host lattice, doping concentration of Eu3 þ , Tb3 þ and preparation conditions, it should be possible to achieve simultaneously emission of the red, green and blue colors from the 4f-4f transitions of Eu3 þ , Tb3 þ and the 5d-4f transitions of Eu2 þ , respectively, with comparable intensities. Hence, the tricolor emission bands of the phosphor co-doped with Eu3 þ , Tb3 þ and Eu2 þ combine to give out white light. Nevertheless, no report of such work has been found in the previous literature. In the present work, we report the first realization of such an idea. We demonstrate that it is possible to obtain SPWL phosphor via chemical approaches with appropriate tuning of activator contents of Eu3 þ , Tb3 þ and Eu2 þ in Sr1.5Ca0.5SiO4 host lattices. The luminescent mechanism for the co-doped Eu3 þ , Tb3 þ and Eu2 þ cations in Sr1.5Ca0.5SiO4 has also been discussed.

The mixture was placed in the tube furnace and calcined at 1200 1C in air for 4 h to obtain the host of phosphor and make rare earth ions Eu3 þ and Tb3 þ into the host lattice to get SCS:aEu3 þ , bTb3 þ . Then, the obtained powder (A1) was divided into five parts, the four parts were fired at the same temperature in reducing atmosphere (5% H2 and 95% N2) for 15, 30, 45 and 60 min (samples A2–A5 in Table 1), respectively, in order for the Eu3 þ ions in the host lattice to be partially reduced to Eu2 þ to obtain a final product of SCS:(a–c) Eu3 þ , bTb3 þ , cEu2 þ . The formation of alkaline earth orthosilicate phosphors by the solid state reaction can be represented by the following equation with the assumption that complete conversion has occurred 1.5SrCO3 þ 0.5CaCO3 þH2SiO3 þ aEuCl3 þbTbCl3-Sr1.5Ca0.5SiO4: (a–c) Eu3 þ , bTb3 þ , cEu2 þ þ gaseous by-product The crystalline phase of the prepared samples was examined by the X-ray diffractometry with Cu Ka radiation at 40 kV and 300 mA (Rigaku D/MAX-2500 X-ray diffractometry, Tokyo, Japan). Particle sizes and shapes were measured by scanning electron microscopy (SEM) (JSM-5600LV). Excitation and emission spectra of the samples were measured using Hitachi F-4500 luminescence spectrofluorometer, equipped with 175 W xenon lamp as an excitation source, 400V photomultiplier tube voltage and UV390 nm filter.

2. Experimental section Eu2O3 and Tb4O7 were 99.99% in purity. CaCO3 (A.R.), SrCO3 (A.R.), H2SiO3 (A.R.) and Li2CO3 (A.R.) were used as raw materials. Stoichiometric Eu2O3 and Tb4O7 were dissolved in dilute HCl under vigorous stirring condition with the pH value of the solution kept between 2 and 3. A LED phosphor consists of alkaline earth orthosilicate co-activated by Eu3 þ and Tb3 þ , whose design formula is Sr1.5Ca0.5SiO4:aEu3 þ , bTb3 þ , denoted as SCS:aEu3 þ , bTb3 þ . The numbers, a and b, in the nominal composition of SCS:aEu3 þ , bTb3 þ indicate the concentration of different metal ions on a molar basis added before firing. The polycrystalline phosphor was synthesized by a two-stage solid-state reaction. Three series of samples with varying rare earth metal ions were prepared. Raw materials were added according to the ratio listed in Table 1 (suitable amount Li2CO3 was added as charge compensation). In the preparation, the complete burning process was achieved by a two-stage solid-state reaction: Firstly, the starting materials were mixed together in agate mortar with required molar ratio.

3. Results and discussion 3.1. Crystal structure Fig. 1 shows the representative X-ray diffraction pattern for the prepared samples. All diffraction peaks can be indexed to the reported data of Sr1.5Ca0.5SiO4 that possesses an orthorhombic crystal structure with space group of Pmnb with its lattice para˚ b¼7.037 A˚ and c ¼9.644 A˚ (JCPDS card No. 77meters a ¼5.647 A, 1618) [34]. Only a single phase belonging to Sr1.5Ca0.5SiO4 was observed, whether the dram rare earth ions had been added or not (Fig. 1), indicating that this single phase has been fully developed through our preparation procedures and the small amount of Eu3 þ , Tb3 þ and Eu2 þ rare earth cations successfully doped into the host did not change the lattice structure. The inset in Fig. 1 shows the scanning electron microscopy micrograph of

Table 1 The doping concentration of Eu3 þ (a) and Tb3 þ (b) ions and optimized preparation conditions for designed composition Sr1.5Ca0.5SiO4: (a–c)%Eu3 þ , b%Tb3 þ , c%Eu2 þ phosphor. Samples

A1 A2 A3 A4 A5 B1 B2 B3 B4 B5 B6 B7 C1 C2 C3 C4 C5 C6

Doped concentration

Atmosphere and firing conditions

CIE x

y

0.559 0.3698 0.425 0.225 0.225 0.247 0.221 0.214 0.229 0.241 0.257 0.251 0.293 0.288 0.301 0.321 0.359 0.357

0.331 0.309 0.449 0.301 0.419 0.246 0.259 0.266 0.335 0.380 0.423 0.410 0.241 0.265 0.299 0.322 0.347 0.367

Eu3 þ (a)

Tb3 þ (b)

Step 1

1.0 1.0 1.0 1.0 1.0 0 0 0 0 0 0 0 1.0 1.0 1.0 1.0 1.0 1.0

0 0 0 0 0 0.01 0.05 0.10 0.20 0.30 0.35 0.40 0.02 0.03 0.05 0.07 0.10 0.15

Air, Air, Air, Air, Air, Air, Air, Air, Air, Air, Air, Air, Air, Air, Air, Air, Air, Air,

1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C,

Step 2 4h 4h 4h 4h 4h 4h 4h 4h 4h 4h 4h 4h 4h 4h 4h 4h 4h 4h

/ 95%N2 þ5%H2, 95%N2 þ5%H2, 95%N2 þ5%H2, 95%N2 þ5%H2, / / / / / / / 95%N2 þ5%H2, 95%N2 þ5%H2, 95%N2 þ5%H2, 95%N2 þ5%H2, 95%N2 þ5%H2, 95%N2 þ5%H2,

1200 1C, 1200 1C, 1200 1C, 1200 1C,

15 min 30 min 45 min 60 min

1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C, 1200 1C,

30 min 30 min 30 min 30 min 30 min 30 min

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Fig. 1. XRD patterns of sample C4 in Table 1.

the SCS:1.0%Eu3 þ , 0.10%Tb3 þ sample (C4 in Table 1), which consists of aggregated particles with size smaller than 20 mm. 3.2. The luminescent properties of Eu3 þ /Eu2 þ -doped Sr1.5Ca0.5SiO4 The luminescent properties from Sr1.5Ca0.5SiO4:Eu was very sensitive to the preparation conditions, such as heat-treatment time in reduction atmosphere. Photoluminescence (PL) properties of singly doped Eu3 þ , Eu2 þ and co-doped Eu3 þ and Eu2 þ in Sr1.5Ca0.5SiO4 were systematically investigated. Fig. 2(a)–(c) shows the excitation and emission spectra of phosphor A1–A5 (in Table 1), and the dependents of the emission intensities of Eu3 þ (5D0-7F2) and Eu2 þ (4f-5d) and Eu2 þ -emission peaks of phosphors A1–A5 on the reduction time. For phosphor A1, sharp peaks were observed, which can be explained in terms of internal Eu3 þ 4f-4f transitions, with the peaks at 591, 615 and 700 nm assigned to 5D0-7FJ (J¼1, 2, 4) of Eu3 þ ions, respectively [35]. For phosphor A4 and A5, the emission spectrum shows a broad band at the range of 400–700 nm centered about 480–484 nm excited by 394 nm, which is believed to be associated with the electronic transition of 4f65d1-4f7 of the Eu2 þ ions [33,36]. Therefore, the excitation and emission spectra of SCS:Eu3 þ and SCS:Eu2 þ can be used to evaluate the reduction extent of Eu3 þ ion in Sr1.5Ca0.5SiO4 and its luminescence behaviors. However, for phosphor A2 and A3, both sharp lines and a broad band are clearly observed. This result is due to the fact that part of the doped Eu3 þ ions in the host crystal were reduced to Eu2 þ ions, leading to Eu3 þ and Eu2 þ ions co-excitation in the system of Sr1.5Ca0.5SiO4. Besides the sharp lines from the transition of 5D0-7FJ (J¼1, 2, 4) by Eu3 þ ions, an additional broad emission band peaked at 525 nm is also observed if excited at 452 nm. With the increase of reduction time, the emission intensity of Eu2 þ increase and wavelength position shifts insignificantly to the shorter-wavelength region from 525 to 482 nm. In comparison with emission intensity of Eu3 þ , when the reduction time exceeds 0.5 h, the emission intensity of Eu3 þ ion is very low. In other words, the optimum time (30 min) is more efficient to obtain emission for Eu2 þ ion, which is almost equal to Eu3 þ ion

(Fig. 2c). These results demonstrate that the phosphor emission wavelength and luminescent intensity can be fine tuned by adjusting the mole ratio of Eu3 þ to Eu2 þ through controlling the reducing atmosphere in the preparation procedure. Experimental results show that a partial reduction from Eu3 þ to Eu2 þ in Sr1.5Ca0.5SiO4 is possible, which provides the freedom for us to adjust emissions of the SCS phosphors to accommodate different purposes. 3.3. The lumininescent properties of Tb3 þ -doping Sr1.5Ca0.5SiO4 There have already been many studies in the literature on the photoluminescence of Tb3 þ in M2SiO4 (where M¼Ca2 þ , Sr2 þ , Ba2 þ ) phosphors [37]. In this paper, the Sr1.5Ca0.5SiO4 phosphor doped with Tb3 þ were synthesized as listed in Table 1 (labeled by B1–B7). Fig. 3 shows the excitation and emission spectra of Sr1.5Ca0.5SiO4:bTb3 þ (b¼0.01–0.40 mol). As shown in Fig. 3, for Tb3 þ -doped Sr1.5Ca0.5SiO4, upon excitation by the characteristic wavelength 284 nm, sharp emission lines peaked from 380 to 650 nm were observed. For the line emission bands of Tb3 þ , it is generally accepted that weak sharp lines between 380 and 475 nm originate from 5D3-7FJ (J¼4, 5, 6) and lines between 475 and 650 nm originate from 5D4-7FJ (J¼3, 4, 5, 6) of the 4f-4f transition of Tb3 þ ions [37]. In particular, the highest sharp line peaked at 544 nm is characteristic of 5D4-7F5 of Tb3 þ 4f-4f transitions [38]. The inset of Fig. 3 displays the impact of the concentration of Tb3 þ on the luminescent intensity of SCS:Tb3 þ . It can be seen that the emission intensity first increases with the increasing Tb3 þ concentration (b), reaching the maximum at b¼ 0.35%, then it does not change with further increase in Tb3 þ concentration. Experimental results show that SCS:0.35%Tb3 þ phosphor is green emitting phosphor suitable for the near-UV chip. 3.4. The luminescent properties of Sr1.5Ca0.5SiO4:Eu3 þ , Tb3 þ , Eu2 þ (SCS:ETE) The above experimental results, exhibit that the emission of the phosphor SCS:Eu3 þ , Eu2 þ is peaked at red and blue area but

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Fig. 3. Excitation and emission spectra of B7 sample in Table 1. The inset of Fig. 3(b) shows the Tb3 þ concentration (b) dependence of the PL intensities of Tb3 þ (5D4-7F5) in Sr1.5 Ca0.5SiO4: bTb3 þ under excitation at 284 nm.

Fig. 2. The excitation (a) and emission (b) spectra of sample A1–A5 in Table 1, and the dependents of the emission intensities of Eu3 þ (5D0-7F2) and Eu2 þ (4f-5d) and Eu2 þ -emitting peaks of phosphors A1–A6 on the reduction time (c).

that of the phosphor SCS:Tb3 þ is peaked at green area, suggest that tricolor (red, green and blue) emitting bands may be obtained when Eu3 þ , Eu2 þ and Tb3 þ are co-excited in the single-phase Sr1.5Ca0.5SiO4, leading to the white light emitting. To this end, the phosphors were prepared by two steps as listed in Table 1 (labeled by C1–C7). As shown in Fig. 4, The high-energy absorptions of Tb3 þ at wavelength shorter than 300 nm become very weak in the absorption spectra, and the dominant absorption peaks shift insignificantly to the longer-wavelength region from 315 to 400 nm for SCS:ETE. The excitation band locates at wavelengths between 300 and 400 nm, significantly overlapping with the excitation of Eu2 þ ion, almost the Eu3 þ excitation is independent. Therefore, the excitation at 397 nm is not the best for Tb3 þ and Eu2 þ , but it is the most effective to enhance the red emission of Eu3 þ . Fig. 5a shows one broad excitation band in the ultraviolet region and emission peaks at red, blue and green areas are found, suggesting that the phosphor is suitable for near-UV excitation [39] and undergoes multicolor emission.

Fig. 4. Excitation spectra of phosphor sample C6 in Table 1.

One remarkable feature of the above Sr1.5Ca0.5SiO4 phosphor is that its emission wavelength and luminescent intensity can be fine tuned by adjusting the mole ratio among Eu3 þ , Eu2 þ and Tb3 þ through controlling the atmosphere and the doping concentration of Tb3 þ in the preparation process, which provides the freedom in adjusting emissions of the Sr1.5Ca0.5SiO4 phosphor to

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white emission (black dot d: x¼ 0.321, y¼0.322), which is very close to the standard white (x¼ 0.33, y¼0.33) leading to a novel single-phased white-light emitting phosphor for LED-based nearUV chip. 3.5. The luminescent mechanism of SCS:Eu3 þ , Tb3 þ , Eu2 þ

Fig. 5. (a) Photoluminescence spectra of Sr1.5Ca0.5SiO4:1%Eu3 þ , b% Tb3 þ phosphors at different Tb3 þ concentration (dot a-f is agreement with b¼ 0.02, 0.03, 0.05, 0.07, 0.10, 0.15) excited by lex ¼ 394 nm and dependence of the energy transfer efficiency ZT on Tb3 þ content. (b) The corresponding CIE chromaticity diagram.

accommodate different purposes in applications. This prominent feature is useful in practice because one can adjust the phosphor to obtain different color emissions. Fig. 5a displays the dependence of the emission wavelength and relative intensity of one series of phosphors (C1–C7) on the nominal composition of two doping cations, Eu3 þ and Tb3 þ . These spectra show that Tb3 þ does not noticeably change the peak wavelength but significantly affect the emission intensity of Eu2 þ after optimizing the preparation process, resulting in the emission color varying from cold white to warm white. The white emission can be further confirmed by the CIE (Commission International de I’Eclairage 1931 chromaticity) coordinates for the emission spectra of Sr1.5Ca0.5SiO4:1%Eu3 þ , bTb3 þ . As shown in Fig. 5b, upon excitation by 394 nm, the ca. CIE chromaticity coordinates (in Table 1) of phosphors C1–C7 vary from cold white (x¼ 0.293, y ¼0.241) to warm white (x ¼0.357, y¼0.367) (black dots a-f ) with changing of Tb3 þ concentration (b) from 0.02–0.15 mol; when b¼0.07 mol, phosphor C4 (nominal composition Sr1.5Ca0.5SiO4:1.0% Eu3 þ , 0.07%Tb3 þ ) gives the pure

The luminescent mechanism of Sr1.5Ca0.5SiO4 co-doped with Eu2 þ , Tb3 þ and Eu3 þ can be illustrated by the energy levels (Fig. 6). As shown in Fig. 6, The excited electrons from the 7 F0-5L6 transition of Eu3 þ at 394 nm yields the emission spectrum (Fig. 5a), which consists of all the emission sharp lines from the 5D0 excited state to the 7FJ ground state of Eu3 þ , i.e., 5 D0-7F0 (578 nm), 5D0-7F1 (587 nm), 5D0-7F2 (612 nm), 5 D0-7F3 (650 nm), 5D0-7F4 (700 nm) from the 5D4 excited state to the 7FJ ground state of Tb3 þ , i.e., 5D4-7F5 (544 nm), and a broad band range of 400–540 nm centered at 462 nm from the 5d excited state to the 8S7/2 ground state (4f-5d) of Eu2 þ . The above emissions have convincingly shown that white light emission of SCS:ETE originated from the combination of Eu3 þ , Tb3 þ , Eu2 þ multicolor emitting. On the other hand, the energy transfer occur from 5d level of excited Eu2 þ ions to 5D4 level of Tb3 þ ion, enhancing 5D4-7F5 emitting. From Fig. 5a, it can be seen that the luminescence of Ca0.5Sr1.5SiO4:1%Eu3 þ , b%Tb3 þ (b¼0.02–0.15) phosphors can be tuned in the white region by changing the doping concentration of Tb3 þ , under excitation by a 394 nm source. Fig. 5a suggests that the luminescence intensities of the blue band (l ¼480 nm), green band (l ¼545 nm) and red band (l ¼ 612 nm) vary in accordance with the Tb3 þ concentration. All spectra consist of a relatively broad blue emission band, a weak green band and a series of red bands. It is clearly seen that in Eu3 þ and Tb3 þ co-doped materials, blue emission band from Eu2 þ 5d-4f transition appeared as Eu3 þ ions were partly reduced. As also shown in Figs. 3a and 2b, based on the observed significant overlap between the excitation spectrum of Tb3 þ and emission spectrum of Eu2 þ , the effective resonance type energy transfer is expected to take place from Eu2 þ to Tb3 þ . As the Tb3 þ concentration increases, the intensity of the blue emission band significantly decreases, but the change of red emission band is not significant (Fig. 5a). This indicates that the quantity of energy can transfer from Eu2 þ to Tb3 þ with increasing Tb3 þ concentration. Hence, we can be sure about the efficient energy transfer from Eu2 þ to Tb3 þ in SCS:ETE phosphor, which is the same as Tb3 þ and Eu2 þ co-doping with (Sr, Ba)2SiO4 phosphor [40]. The energy transfer efficiency (ZT) from Eu2 þ to Tb3 þ has been discussed by

Fig. 6. Schematic energy levels of the Eu3 þ –Tb3 þ –Eu2 þ system showing possible energy transfer mechanisms (pathways).

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Paulose et al. [41] and can be expressed in the following equation: I ZT ¼ 1 s Iso where Is and Iso are the luminescence intensity of the sensitizer (Eu2 þ ) with and without activator (Tb3 þ ). In the present work, the energy transfer efficiency increases with increasing Tb3 þ concentration as shown in the inset of Fig. 5a. In addition, the charge transfer among the Tb3 þ , Eu2 þ and 3þ Eu ions has been discussed in many studies [42–45] and can be explained in the following equation: Eu3 þ þTb3 þ ¼Eu2 þ þTb4 þ Therefore, another energy transfer process in the SCS:ETE system is attributed to the charge transfer from Tb3 þ to Eu3 þ , holding the equilibrium of the concentration of Tb3 þ , Eu3 þ and Eu2 þ ions in the Ca0.5Sr1.5SiO4 host lattice and leading to stable white-light emission. Therefore, we can infer there are energy transfer processes from Eu2 þ to Tb3 þ to Eu3 þ in the system of Sr1.5Ca0.5SiO4:Eu3 þ , Tb3 þ , Eu2 þ . 4. Conclusion A novel single-phased white-light phosphor Sr1.5Ca0.5SiO4:Eu3 þ , Tb , Eu2 þ (SCS:ETE) has been prepared by the two-step hightemperature solid state process. An appropriate amount of Eu3 þ ions in nominal composition Sr1.5Ca0.5SiO4:1.0%Eu3 þ , 0.07%Tb3 þ was reduced to Eu2 þ ions under reduced atmosphere in the second step. So the luminescent colors of phosphor SCS:ETE are tunable by adjusting the concentration of co-doped Eu3 þ , Tb3 þ ions as raw materials and by controlling the ratio of Eu3 þ , Tb3 þ and Eu2 þ concentrations within the solid solution (Sr1.5Ca0.5SiO4) through the preparation condition in the firing process. Under the excitation of near-UV light (394 nm), its emission spectra exhibit a broad band in blue area, sharp lines peaked in green (about 544 nm) and orange–red (578–720 nm) regions, which correspond to Eu2 þ 5d-4f transitions, Tb3 þ 5D4-7FJ and Eu3 þ 5D0-7FJ transitions, respectively. The blue emission of Eu2 þ and the green-emission of Tb3 þ decreased and the yellow and red emission of Eu3 þ increased as the concentration of Tb3 þ increased in the Tb3 þ –Eu3 þ co-doped phosphors, which suggested that there are energy transfers from Eu2 þ to Tb3 þ to Eu3 þ . The result of CIE calculations shows that the SCS:1.0%Eu3 þ , 0.07%Tb3 þ phosphor, upon excitation by 394 nm, provides white emission (x¼0.321, y¼0.322) that is very close to the standard white (x¼0.33, y¼0.33), which shows it is a potential phosphor for applications in white LED and field emission display devices. 3þ

Acknowledgments This work is partially supported by the National Natural Science Foundation of China (Grant nos. 20971042, 50772035), by the Science and Technology Office of Education Department of Hunan Province (no. 10A070).

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