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Energy transfer and thermal regression mechanism of color-tunable phosphor Sr8La2(PO4)3.5(SiO4)2(BO4)0.5BO2: Ce3+, Tb3+
Journal of Alloys and Compounds 654 (2016) 8e14
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Energy transfer and thermal regression mechanism of color-tunable phosphor Sr8La2(PO4)3.5(SiO4)2(BO4)0.5BO2: Ce3þ, Tb3þ Zhipeng Ci a, b, **, Bingzheng Xu c, Peidian Li a, Qinjia Chen a, Xuemin Li a, Lili Han a, Jiachi Zhang a, b, *, Xiaoyi Hu a, Yuhua Wang a, b a b c
Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, Lanzhou 730000, China School of Electronic and Information Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2015 Received in revised form 9 September 2015 Accepted 11 September 2015 Available online 14 September 2015
A series of phosphosilicate phosphors Sr8La2(PO4)3.5(SiO4)2(BO4)0.5BO2: Ce3þ, Tb3þ are synthesized by solid-state reaction for the first time. The XRD Rietveld refinement presents that the compound crystallizes in a trigonal crystal system with space group P3 (No. 147). Under the excitation of 340 nm, commonly blue and yellowish green emissions from Ce3þ and Tb3þ are detected and can generate the color-tunable light with chromaticity coordinates from (0.188, 0.095) to (0.351, 0.517) by changing the ratio of Ce3þ/Tb3þ. By the photoluminescence spectra, decay times and energy transfer efficiency, the mechanism of energy transfer between Ce3þ and Tb3þ has been carefully investigated. With the increase of temperature, the Ce3þ or Tb3þ single doped sample shows an excellent thermal property, but when the Ce3þ and Tb3þ are co-doped into the host, the thermal property of sample seriously degenerates. This result indicates that the change of temperature can affect the energy transfer between Ce3þ and Tb3þ strongly and based on the configurational coordinate diagram, this phenomenon is explained reasonably. © 2015 Elsevier B.V. All rights reserved.
Keywords: Optical materials Luminescence Optical properties Thermal properties
1. Introduction In the last few decades, there has been a phenomenal growth in investigation of the field of luminescence, and significant progress has been made in solid state lighting [1]. The breakthrough in the development of high performance light-emitting diodes (LEDs) via improvement in both internal quantum efficiency [2e4] and extraction efficiency [5] has resulted in practical excitation sources applicable for phosphor-converted (pc) white LEDs. Currently, pc LEDs are obtained by two main types: (a) a blue LED chip and a Y3Al5O12:Ce3þ (YAG: Ce) phosphor, (b) an UV LED chip and two, three, even four phosphors [6,7]. However, the implementation of a blue chip with YAG: Ce suffers from poor color rendition and narrow visible range; The means of UV-LED with several phosphors also have many serious problems, such as the poor luminous efficiency resulted from the reabsorption and the lower intensity of the
* Corresponding author. Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China. ** Corresponding author. Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China. E-mail addresses: [email protected] (Z. Ci), [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.jallcom.2015.09.094 0925-8388/© 2015 Elsevier B.V. All rights reserved.
red phosphor. A useful solution is to develop a single composition color-tunable phosphor, especially white-emitting phosphor, because of small color aberration, high color rendering, and low cost. One of the strategies for generating white light from singlecomposition phosphors is by co-doping sensitizer and activator into a crystalline matrix, using the principle of energy transfer (ET) from sensitizer to activator, such as Eu2þ to Mn2þ [8], Ce3þ to Tb3þ [9], Ce3þ to Eu2þ or Ce3þ to Mn2þ [10,11]. Apatite structure is an important branch of phosphate system, which are represented by the general formula M10(ZO4)6X2 with M ¼ Ca2þ, Ba2þ, Mg2þ, Sr2þ, Pb2þ, Naþ, Kþ, La3þ, etc.; Z ¼ P5þ, As5þ, V5þ, Si4þ, etc.; and X ¼ F, Cl, Br, I, OH, O2, etc., and shows the wide range of tolerance of this structure type to chemical substitutions [12,13]. In addition, there are also reports on linear [BO2] groups taking the position of X [14]. Sr10(PO4)5.5(BO4)0.5(BO2) was first discovered by Chen etc. [15] in 2010. Sr10(PO4)5.5(BO4)0.5(BO2) is a derivative of the apatite crystal structure. Sr2þ occupies the Wyckoff positions 2d (Sr1, Sr2) and 6 g (Sr3). [PO4]3 tetrahedra (6 g) are partially replaced by [BO4]5 groups. The linear [BO2] units are located within the channels formed by Sr3 and running along the three-fold inversion axis. The space group symmetry of the title compound is reduced to P3 by displacement of the [(P þ B)O4] tetrahedra destroying the mirror plane characteristic for the parent
Z. Ci et al. / Journal of Alloys and Compounds 654 (2016) 8e14
apatite crystal structure (P63/m). In this structure, [SiO4]4 could replace [PO4]3 to form a solid solution, and further influence the crystal field and the Nephelauxetic Effect with the introduction of the rear earth ions, which will probably cause the diversity of luminescence properties. Therefore, in this paper, we have synthesized a series of phosphors (Sr0.8La0.2xy)10[(PO4)3.5(SiO4)2 (BO4)0.5](BO2) (SLPSB): xCe3þ, yTb3þ (0.001 x 0.04, 0.01 y 0.10) and carefully investigated the structure, photoluminescence and thermal properties as well as the ET phenomenon between the sensitizer and activator. 2. Experimental 2.1. Materials and synthesis All the powder samples were synthesized by the traditional solid-state reaction method. The starting materials were SrCO3 (A.R. 99.9%), La2O3 (A.R. 99.9%), H2SiO3 (A.R. 99.9%), (NH4)2HPO4 (A.R. 99.9%), H3BO3 (A.R. 99.9%), CeO2 (4 N 99.99%) and Tb4O7 (4 N 99.99%). The stoichiometric raw materials were ground thoroughly in an agate mortar and then heated to 873 K in air for 5 h. Subsequently the preheated mixture was ground again and fired to 1553 K for 8 h in an alumina crucible under N2eH2 (8%) atmosphere in horizontal tube furnaces. Finally the as-synthesized samples were slowly cooled to room temperature inside the tube furnace under H2eN2 flow. 2.2. Measurements and characterization The crystal structure of the synthesized samples was identified by using a Rigaku D/Max-2400 X-ray diffractometer (XRD) with Nifiltered Cu Ka radiation. The photoluminescence (PL) and PL excitation (PLE) spectra of the samples were measured by using an FLS920T fluorescence spectrophotometer equipped with a 450 W Xe light source and double excitation monochromators. The PL decay curves were measured by an FLS-920T fluorescence spectrophotometer with nF900 ns Flashlamp as the light source. All of the measurements were performed at room temperature. Thermal quenching was tested using a heating apparatus (TAP-02) in combination with PL equipment. 3. Results and discussion 3.1. Crystal structure analysis The results of Rietveld refinement of SLPSB host implemented with the crystallographic information files identified by previous reports [15] are plotted in Fig. 1(a) by using the GSAS program. The black crosses and red solid line depict the observed and calculated patterns, respectively; the as-obtained goodness of fit parameter c2 ¼ 2.38 and Rwp ¼ 8.21% and Rp ¼ 7.19% can ensure the phase purity. The compound crystallizes in a trigonal crystal system with space group P3 (No. 147), and its cell parameters are a ¼ b ¼ 9.680(1) Å, c ¼ 7.252(7) Å. The detailed crystallographic data of SLPSB is listed in Tables 1e3. In the structure, there are 12 crystallographically independent atoms in the unit cell, including 3 SL (SL ¼ 4/5Sr þ 1/5La), 1 P, 1 Si, 2 B and 5 O atoms as shown in the inset of Fig. 1(a). SL1 and SL2 are nine-fold coordinated (d(SL1eO) ¼ 2.5197e3.0790 Å, d(SL2eO) ¼ 2.5115e2.7283 Å) with an average distance of 2.7419 and 2.5848 Å, respectively. SL3 is surrounded by seven oxygen atoms forming a distorted pentagonal bipyramid (d(SL3eO) ¼ 2.2405e2.8717 Å) with an average distance of 2.5666 Å. P, Si and B (B1) adopt the tetrahedral groups which are isolated from each other. The remaining B (B2) ions occupy the Wyckoff positions 1b and form the linear BO 2 units, which are
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located within the channels formed by SL3. In phosphate compounds, many studies indicated that the Ln3þ occupies exclusively the (6 h) sites [16,17]. However, when silicate groups are incorporated into the apatite structure, the case might be different. In Sr10xLax(PO4)6x(SiO4)xO (x ¼ 0, 2, 4) materials, K. Boughzala et al. reported the occupancy ratios of La3þ in the different sites and pointed out that Ln3þ will be distributed into the three sites with a marked preference for the 6 h site [18,19]. As a derivative of the apatite crystal structure SLPSB, with the introduction of silicate groups, the La3þ would similarly occupy the different Wyckoff positions 2d (SL1, SL2) and 6 g (SL3). Meanwhile, because the Sr2þ and La3þ fully and complementarily occupy all the different sites, the Sr2þ will also occupy the different Wyckoff positions 2d (SL1, SL2) and 6 g (SL3). Thereby, when the Ce3þ and Tb3þ are introduced into the host, they should be distributed into the three sites with a remarkable preference for one site. The SEM image of SLPSB: 0.01Ce3þ, 0.03Tb3þ clearly presents that the grains have the irregular shape of blocky particles with a size of about 5e10 mm. Fig. 1(b) shows the XRD patterns of the samples SLPSB, SLPSB: 0.01Ce3þ, SLPSB: 0.03Tb3þ and SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.1). All the observed diffraction peaks are well indexed to those of SLPSB and no second phase is observed, indicating that the doping ions do not cause significant changes in the host structure. Since the radius of Tb3þ is smaller than those of Sr2þ and La3þ, with the doping of Tb3þ, the diffraction peaks of the SLPSB: 0.01Ce3þ, xTb3þ samples show a slight shift to larger 2q angles compared to those of the pure SLPSB. The relative difference in ionic radii between matrix cations and dopant ions Ce3þ and Tb3þ is listed in Table 4. 3.2. Photoluminescence property analysis Fig. 2(a) shows the PLE spectrum of SLPSB: 0.01Ce3þ monitored at 415 nm and PL spectra of SLPSB: xCe3þ (0.001 x 0.04) excited by 351 nm. It is obviously observed that SLPSB: xCe3þ can be efficiently excited by UV light and emit the blue light from 5de4f transition of Ce3þ. With the increase of Ce3þ, the PL intensity gradually increases. When the concentration of Ce3þ is 1%, the PL intensity reaches the maximum. The optimal concentration (1%) is referred to as “critical concentration”, which is due to ET from one activator to another until energy sink in the lattice is reached. To understand the ET mechanism of SLPSB: 0.01Ce3þ, it is useful to know the critical distance Rc of ET between Ce3þ. Blasse suggested that the critical distance Rc(CeeCe) could be calculated by the critical concentration of the activator ion [20]:
Rc z2
3V 4pxc N
1=3 (1)
where V is the volume of the unit cell, xc is the critical concentration of the activator ion, and N is the number of cations of per unit cell. For the SLPSB host, N ¼ 10, xc ¼ 0.01, and V ¼ 588.56 Å3, and therefore the Rc(CeeCe) value is calculated to 22.40 Å. This result indicates that the ET between Ce3þ and Ce3þ is not via exchange interaction mechanism but electric multipolar interaction, and thus leads to a stronger blue emission from Ce3þ [21,22]. Fig. 2(b) shows the PL and PLE spectra of SLPSB: 0.01Ce3þ. With the change of excited and monitored wavelength, the highest PL peak shifts from 398 to 432 nm and the highest PLE peak shifts from 330 to 380 nm, which also indicate that Ce3þ should occupy the different sites. In Fig. 2(c), under the excitation of 255, 352 and 377 nm, SLPSB: 0.03Tb3þ presents the yellowish green emission from Tb3þ 5D4e7FJ (J ¼ 3, 4, 5, 6) characteristic transitions. According to Dexter's theory [23], an efficient ET requires a partial overlap between the PLE spectrum of the activator and the PL spectrum of the sensitizer
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Fig. 1. (a) Experimental (crosses), calculated (solid line) and difference (bottom) results of XRD refinement of SLPSB host; The inset shows the structure diagram of SLPSB according to the refinement results and SEM image of SLPSB: 0.01Ce3þ, 0.03Tb3þ; (b) The XRD patterns of the samples SLPSB, SLPSB: 0.01Ce3þ, SLPSB: 0.03 Tb3þ and SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.1).
[24,25]. From Fig. 2(c), we can find that there is a remarkable overlap between emission band of Ce3þ (donor) and absorption band of Tb3þ (acceptor). In Fig. 2(d), monitored at 542 nm, as the highest PL peak for Tb3þ but not for Ce3þ, the PLE spectrum of SLPSB: 0.01Ce3þ, 0.03Tb3þ is very similar with that of SLPSB: 0.01Ce3þ monitored at 415 nm, not that of SLPSB: 0.03Tb3þ, which implies that Ce3þ makes some contributions to the Tb3þ characteristic peak at 542 nm, i.e., an effective resonance-type ET takes
place between Ce3þ and Tb3þ. Fig. 3(a) reveals the PL spectra of SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.1). Upon 340 nm excitation, the emission spectra of Ce3þ, Tb3þ co-doped SLPSB consists of not only the broad violetblue emission band centered at 405 nm from Ce3þ, but also a series of strong yellowish green line emissions from Tb3þ. With increasing Tb3þ contents, the emission intensity of Ce3þ gradually declines, but that of Tb3þ gradually increases, which further
Z. Ci et al. / Journal of Alloys and Compounds 654 (2016) 8e14 Table 1 Crystal structural data and lattice parameters of SLPSB. Formula
Sr8La2(PO4)3.5(SiO4)2(BO4)0.5BO2
Crysta system Space group Lattice a/Å b/Å c/Å a/ o b/o g/o Cell volume (V/Å3) R values
Hexagonal P3(147)
c
9.680(1) 9.680(1) 7.252(7) 90 90 120 588.56 Rwp ¼ 8.21% Rp ¼ 7.19% 2.38
2
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indicates that the high light output of Tb3þ actually comes from the ET from Ce3þ to Tb3þ. In order to investigate the ET mechanism from Ce3þ to Tb3þ, the decay times of Ce3þ in SLPSB: Ce3þ, xTb3þ are tested and depicted in Fig. 3(b). We can see that the decay curves of Ce3þ deviate gradually from a single exponential rule with the increase of Tb3þ. The effective lifetimes of the decay curves of Ce3þ can be evaluated using the Eq. (2) [26]:
Z
∞
tIðtÞdt t ¼ Z0 ∞
(2) IðtÞdt
0
The calculated decay times are determined to be 35.49, 31.04, 27.73, 21.83, 17.75, and 12.98 ns in Fig. 3 (b). According to Dexter's formulation [23], the ET rate is given by:
Table 2 Crystallographic data of SLPSB determined by the XRD Rietveld refinement at the room temperature. Atom
Wyck.
x/a
y/b
z/c
SL1 SL2 SL3 P Si B1 O1 O2 O3 O4 O5 B2
2d 2d 6g 6g 6g 6g 6g 6g 6g 6g 2c 1b
1/3 1/3 0.23221 0.38884 0.38884 0.38884 0.33005 0.59728 0.33044 0.37933 0 0
2/3 2/3 0.0154 0.37376 0.37376 0.37376 0.47413 0.482 0.28007 0.26213 0 0
0.01111 0.49879 0.25185 0.23577 0.23577 0.23577 0.22513 0.26581 0.08613 0.48367 0.32247 1/2
Q PðRÞf b A R tD
Z
fD ðEÞFA ðEÞ dE Ec
(3)
where tD is the decay time of the donor emission, QA is the total absorption cross section of the acceptor ion, R is the distance between the donor and the acceptor, and b and c are parameters dependent on the type of ET. The probability functions fD(E) and FA(E) represent the observed shapes of the donor emission band and the acceptor absorption band, respectively. Thus, according to Eq. (3), the ET rate P is in inverse proportion to the decay time tD. The result that decay lifetime of the Ce3þ decreases monotonically with the increase of Tb3þ also further proves the ET from Ce3þ to Tb3þ. The energy transfer efficiency hCeeTb can be expressed by:
Table 3 Selected interatomic distances in the crystal structure of SLPSB. Atom contacts
Å
Atom contacts
O1 SL1 SL1 O1 SL1 O1 SL1 O2 SL1 O2 SL1 O2 SL1 O3 SL1 O3 SL1 O3 SL2 O1 SL2 O1 SL2 O1 SL2 O2 SL ¼ 4/5 Sr þ 1/5 La, Z* ¼ 7/12 P þ 4/12 Si þ 1/12 B1
2.5197 2.5201 2.5205 2.6268 2.6269 2.6274 3.0780 3.0786 3.0790 2.7116 2.7119 2.7122 2.5302
SL2 SL2 SL2 SL2 SL2 SL3 SL3 SL3 SL3 SL3 SL3 SL3
Table 4 Relative difference in ionic radii (Dr (%) ¼ 100 [Rm (CN)-Rd (CN)]/Rm (CN)) between matrix cations and dopant ions Ce3þ and Tb3þ. Ions
Radius/Å
CN
Dr(%)
Ce3þ Sr2þ La3þ Ce3þ Sr2þ La3þ Tb3þ Sr2þ La3þ Tb3þ Sr2þ La3þ
1.196 1.31 1.216 1.07 1.21 1.10 1.095 1.31 1.216 0.98 1.21 1.10
9 9 9 7 7 7 9 9 9 7 7 7
0 8.70 1.64 0 11.57 2.73 0 16.41 9.95 0 19.01 10.91
h¼1
ts ts0
Å O2 O2 O4 O4 O4 O1 O2 O3 O3 O4 O4 O5
2.5304 2.5309 2.5115 2.5122 2.5125 2.7283 2.4009 2.5484 2.7946 2.8717 2.2405 2.3817
(4)
where tS0 is the lifetime of Ce3þ in the absence of Tb3þ and tS is the lifetime of Ce3þ in the presence of Tb3þ. The decay lifetime values are used for calculation, and the results are presented in Fig. 3(c). For the samples SLPSB: 0.01Ce3þ, xTb3þ (0 x 0.1), the critical distance Rc(CeeTb) for ET from the Ce3þ to Tb3þ can be also estimated by the Eq. (1). In this case, the critical concentration (xc), at which the PL intensity of Ce3þ is one half of that in the sample in the absence of Tb3þ, is 0.05 in Fig. 3(a). Therefore, the critical distance (Rc(CeeTb)) is calculated to be 13.10 Å, which also indicates that the ET between Ce3þ and Tb3þ is via electric multipolar interaction [21,22]. Fig. 3(d) illustrates the Commission Internationale de
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Fig. 2. (a) The PLE spectrum of SLPSB: 0.01Ce3þ and PL spectra of SLPSB: xCe3þ (0.001 x 0.04); The inset shows the PL intensities of SLPSB: xCe3þ as a function of x; (b) The PL and PLE spectra at different excited and monitored wavelength of SLPSB: 0.01Ce3þ; (c) The PL spectra of SLPSB: 0.03Tb3þ under the excitation of different wavelength and the overlap between the PLE spectrum of SLPSB: 0.03Tb3þ and the PL spectrum of SLPSB: 0.01Ce3þ; (d) The PLE spectra of SLPSB: 0.01Ce3þ, SLPSB: 0.03Tb3þ and SLPSB: 0.01Ce3þ, 0.03Tb3þ monitored at different PL peaks.
Fig. 3. (a) The PL spectra of SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.1) excited at 340 nm; (b) The decay curves of SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.1) excited at 340 nm and monitored at 405 nm; (c) The variation of hT and t with the increase of Tb3þ; (d) The CIE chromaticity coordinates of SLPSB: 0.01Ce3þ, SLPSB: 0.03Tb3þ and SLPSB: Ce3þ, xTb3þ (0.01 x 0.07).
L'Eclairage (CIE) chromaticity coordinates of SLPSB: 0.01Ce3þ, SLPSB: 0.03Tb3þ, SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.07), which were calculated based on the corresponding PL spectra. ET makes it possible to obtain both the blue emission of Ce3þ and the yellowish
green emission of Tb3þ in a single host. Thus, we can simply change the doping content of Tb3þ to adjust the color tone so as to meet the requirement of different illumination applications. The CIE chromaticity coordinates of SLPSB: 0.01Ce3þ, xTb3þ (x ¼ 0.01 and 0.03)
Z. Ci et al. / Journal of Alloys and Compounds 654 (2016) 8e14
are found to be located in the white region, with CIE chromaticity coordinates (0.234, 0.246) and (0.281, 0.345), respectively. Although the chromaticity coordinates are not very close to the standard white (0.33, 0.33), which would not be recommended to serve as room lighting sources, the current white emission could serve as outdoor lighting or back light of a mobile telephone. 3.3. Temperature-dependent photoluminescence behavior A comprehensive understanding of the thermal quenching of phosphors in the process of the phosphors application is indispensable because many devices suffer from thermal problems. Numerous investigations have discussed the thermal quenching behaviors [27e29]. Two competing factors are in prevail, one is the activation energy of non-radiative relaxation, the other is the rate of temperature-induced direct tunneling, which prevents emissive transition between the different activator ions excited state and the ground state in host [29]. The temperature dependent PL spectra from 20 to 250 C of SLPSB: 0.03Tb3þ, SLPSB: 0.01Ce3þ and SLPSB: 0.01Ce3þ, 0.03Tb3þ excited by 340 nm are shown in Fig. 4(a), (b) and (c), respectively. With the increasing temperature, the PL intensities of all samples gradually decline. For the Ce3þ or Tb3þ single doped SLPSB, the PL intensities at 250 C are 87.3% and 71.8% of their initial intensities at 20 C. However, for the Ce3þ and Tb3þ co-doped sample, the PL intensity of Ce3þ and Tb3þ in SLPSB: 0.01Ce3þ, 0.03Tb3þ declines rapidly and at 250 C drop to 29.8% and 32.1% of those at 20 C. This result indicates that the change of temperature can affect the ET between Ce3þ and Tb3þ strongly. Based on the configurational coordinate diagram in Fig. 5, the abnormal phenomenon can be explained reasonably. The curves g(1,2) and 7FJ are the ground states of Ce3þ and Tb3þ. The curves e and 5DJ are the excited states of Ce3þ and Tb3þ. The points A and B are the lowest positions of the excited states 7FJ and e. To simplify the discussion, we assume that the
13
crossing points among the excited states and ground states of Ce3þ and Tb3þ can be expressed by only one capital letter. The points M are the crossing point of g(1,2) and e. P is the crossing point of e and 5 DJ. DE1, DE2 and DE3 are the energy differences of B to P, B to M and A to P, respectively. Under the excitation of the UV light, the electrons are excited to the excited states from g(1,2), 7FJ to e, 5DJ. For SLPSB: 0.03Tb3þ and SPLSB: 0.01Ce3þ, most of the electrons return to the ground states by radiative transitions to obtain the characteristic emitting of Tb3þ and Ce3þ at the room temperature, but for SLPSB: 0.01Ce3þ, 0.03Tb3þ, the electrons of e would likely overcome the energy barrier DE1 as well under the electron-phonon coupling, and then accomplish energy transfer from Ce3þ to Tb3þ along the pink way ➀ at the room temperature. With the increase of temperature, for the Ce3þ or Tb3þ single doped SLPSB, the electrons of e and 5DJ could be difficult to overcome the energy barriers between their own excited states and ground states, thus result in an excellent thermal properties as shown in Fig. 4(a) and (b). However, for the Ce3þ, Tb3þ co-doped SLPSB, more electrons of e could overcome the energy barrier DE1 and DE2 to transfer the energy to 5DJ or return to g(1,2) along the pink way ➀ and green way ② from the crossing points P and M due to the stronger electron-phonon coupling. This process could lead to the continuous decrease of electrons of e, thus the PL intensity of Ce3þ more rapidly declines than that in SLPSB: 0.01Ce3þ. But in the discussion above, we have known that the PL intensity of Tb3þ depends strongly on Ce3þ. Although the enhanced electron-phonon coupling is helpful to electron transfer from e to 5 DJ, the decrease of electron number in e due to electron directtunneling from e to g(1,2) could still lead to the decline of Ce3þeTb3þ ET efficiency, then result in the rapid decline of Tb3þ intensity, which can be also confirmed by the similar downtrend of Ce3þ and Tb3þ PL intensities in SLPSB: 0.01Ce3þ, 0.03Tb3þ in Fig. 4(d). In addition, in Fig. 4(c), we also observed that the decline of the Ce3þ PL intensity from 200 to 250 C becomes slower than that below 200 C. It might be because the possibility of the back-
Fig. 4. The PL spectra of (a) SLPSB: 0.03Tb3þ, (b) SLPSB: 0.01Ce3þ, (c) SLPSB: 0.01Ce3þ, 0.03Tb3þ under various temperatures; (d) The dependence of normalized PL intensities on temperature for the typical phosphors.
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Fig. 5. The configurational coordinate diagram of the excited and ground states of Ce3þ and Tb3þ.
tunneling of electrons from 5DJ to e along the blue way ③ gradually increases under the stronger phonon vibration with the further increasing temperature, depressing the degradation of the thermal properties of Ce3þ in SLPSB: 0.01Ce3þ, 0.03Tb3þ.
expenses of the central university and Open Project of Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University. References
4. Conclusions In summary, a simple solid-state route was adopted to fabricate a series of phosphosilicate phosphors SLPSB: Ce3þ, Tb3þ. Their crystal structure, PL and PLE spectra, decay times, CIE chromaticity coordinates and thermal quenching are measured to evaluate its PL properties. The luminescence analysis demonstrates that the phosphors SLPSB: Ce3þ, Tb3þ can be efficiently excited by the UV light from 240 to 400 nm, and simultaneously emit the blue light from Ce3þ and the yellowish green light from Tb3þ. By adjusting the ratio of Ce3þ/Tb3þ, the color-tunable light with chromaticity coordinates from (0.188, 0.095) to (0.351, 0.517) can be obtained. The spectral characteristic and decay times indicate that the efficient ET occurs between Ce3þ and Tb3þ. With the increasing temperature, the PL intensities of all samples gradually decline. The PL intensities of SLPSB: 0.03 Tb3þ and SLPSB: 0.03Ce3þ at 250 C is 87.3% and 71.8% of their initial intensities at 20 C. However, when Ce3þ and Tb3þ are co-doped into SLPSB host, the PL intensities of Ce3þ, Tb3þ in SLPSB: 0.01Ce3þ, 0.03Tb3þ degenerate rapidly and at 250 C drop to 29.8% and 32.1% of those at 20 C. In addition, we also observed that the decline of the Ce3þ PL intensity from 200 to 250 C becomes slower than that below 200 C. These results indicate that the change of temperature can affect the energy transfer between Ce3þ and Tb3þ strongly. Based on the configurational coordinate diagram, an underlying mechanism is proposed to explain the abnormal changing trend, which could be used in the discussion of the thermal properties of multiple activator co-doped phosphors as reference. Acknowledgments This work was supported by National Natural Science Funds of China (No. 51302121), the basic scientific research business
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Energy transfer and thermal regression mechanism of color-tunable phosphor Sr8La2(PO4)3.5(SiO4)2(BO4)0.5BO2: Ce3þ, Tb3þ Zhipeng Ci a, b, **, Bingzheng Xu c, Peidian Li a, Qinjia Chen a, Xuemin Li a, Lili Han a, Jiachi Zhang a, b, *, Xiaoyi Hu a, Yuhua Wang a, b a b c
Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, Lanzhou 730000, China School of Electronic and Information Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2015 Received in revised form 9 September 2015 Accepted 11 September 2015 Available online 14 September 2015
A series of phosphosilicate phosphors Sr8La2(PO4)3.5(SiO4)2(BO4)0.5BO2: Ce3þ, Tb3þ are synthesized by solid-state reaction for the first time. The XRD Rietveld refinement presents that the compound crystallizes in a trigonal crystal system with space group P3 (No. 147). Under the excitation of 340 nm, commonly blue and yellowish green emissions from Ce3þ and Tb3þ are detected and can generate the color-tunable light with chromaticity coordinates from (0.188, 0.095) to (0.351, 0.517) by changing the ratio of Ce3þ/Tb3þ. By the photoluminescence spectra, decay times and energy transfer efficiency, the mechanism of energy transfer between Ce3þ and Tb3þ has been carefully investigated. With the increase of temperature, the Ce3þ or Tb3þ single doped sample shows an excellent thermal property, but when the Ce3þ and Tb3þ are co-doped into the host, the thermal property of sample seriously degenerates. This result indicates that the change of temperature can affect the energy transfer between Ce3þ and Tb3þ strongly and based on the configurational coordinate diagram, this phenomenon is explained reasonably. © 2015 Elsevier B.V. All rights reserved.
Keywords: Optical materials Luminescence Optical properties Thermal properties
1. Introduction In the last few decades, there has been a phenomenal growth in investigation of the field of luminescence, and significant progress has been made in solid state lighting [1]. The breakthrough in the development of high performance light-emitting diodes (LEDs) via improvement in both internal quantum efficiency [2e4] and extraction efficiency [5] has resulted in practical excitation sources applicable for phosphor-converted (pc) white LEDs. Currently, pc LEDs are obtained by two main types: (a) a blue LED chip and a Y3Al5O12:Ce3þ (YAG: Ce) phosphor, (b) an UV LED chip and two, three, even four phosphors [6,7]. However, the implementation of a blue chip with YAG: Ce suffers from poor color rendition and narrow visible range; The means of UV-LED with several phosphors also have many serious problems, such as the poor luminous efficiency resulted from the reabsorption and the lower intensity of the
* Corresponding author. Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China. ** Corresponding author. Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China. E-mail addresses: [email protected] (Z. Ci), [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.jallcom.2015.09.094 0925-8388/© 2015 Elsevier B.V. All rights reserved.
red phosphor. A useful solution is to develop a single composition color-tunable phosphor, especially white-emitting phosphor, because of small color aberration, high color rendering, and low cost. One of the strategies for generating white light from singlecomposition phosphors is by co-doping sensitizer and activator into a crystalline matrix, using the principle of energy transfer (ET) from sensitizer to activator, such as Eu2þ to Mn2þ [8], Ce3þ to Tb3þ [9], Ce3þ to Eu2þ or Ce3þ to Mn2þ [10,11]. Apatite structure is an important branch of phosphate system, which are represented by the general formula M10(ZO4)6X2 with M ¼ Ca2þ, Ba2þ, Mg2þ, Sr2þ, Pb2þ, Naþ, Kþ, La3þ, etc.; Z ¼ P5þ, As5þ, V5þ, Si4þ, etc.; and X ¼ F, Cl, Br, I, OH, O2, etc., and shows the wide range of tolerance of this structure type to chemical substitutions [12,13]. In addition, there are also reports on linear [BO2] groups taking the position of X [14]. Sr10(PO4)5.5(BO4)0.5(BO2) was first discovered by Chen etc. [15] in 2010. Sr10(PO4)5.5(BO4)0.5(BO2) is a derivative of the apatite crystal structure. Sr2þ occupies the Wyckoff positions 2d (Sr1, Sr2) and 6 g (Sr3). [PO4]3 tetrahedra (6 g) are partially replaced by [BO4]5 groups. The linear [BO2] units are located within the channels formed by Sr3 and running along the three-fold inversion axis. The space group symmetry of the title compound is reduced to P3 by displacement of the [(P þ B)O4] tetrahedra destroying the mirror plane characteristic for the parent
Z. Ci et al. / Journal of Alloys and Compounds 654 (2016) 8e14
apatite crystal structure (P63/m). In this structure, [SiO4]4 could replace [PO4]3 to form a solid solution, and further influence the crystal field and the Nephelauxetic Effect with the introduction of the rear earth ions, which will probably cause the diversity of luminescence properties. Therefore, in this paper, we have synthesized a series of phosphors (Sr0.8La0.2xy)10[(PO4)3.5(SiO4)2 (BO4)0.5](BO2) (SLPSB): xCe3þ, yTb3þ (0.001 x 0.04, 0.01 y 0.10) and carefully investigated the structure, photoluminescence and thermal properties as well as the ET phenomenon between the sensitizer and activator. 2. Experimental 2.1. Materials and synthesis All the powder samples were synthesized by the traditional solid-state reaction method. The starting materials were SrCO3 (A.R. 99.9%), La2O3 (A.R. 99.9%), H2SiO3 (A.R. 99.9%), (NH4)2HPO4 (A.R. 99.9%), H3BO3 (A.R. 99.9%), CeO2 (4 N 99.99%) and Tb4O7 (4 N 99.99%). The stoichiometric raw materials were ground thoroughly in an agate mortar and then heated to 873 K in air for 5 h. Subsequently the preheated mixture was ground again and fired to 1553 K for 8 h in an alumina crucible under N2eH2 (8%) atmosphere in horizontal tube furnaces. Finally the as-synthesized samples were slowly cooled to room temperature inside the tube furnace under H2eN2 flow. 2.2. Measurements and characterization The crystal structure of the synthesized samples was identified by using a Rigaku D/Max-2400 X-ray diffractometer (XRD) with Nifiltered Cu Ka radiation. The photoluminescence (PL) and PL excitation (PLE) spectra of the samples were measured by using an FLS920T fluorescence spectrophotometer equipped with a 450 W Xe light source and double excitation monochromators. The PL decay curves were measured by an FLS-920T fluorescence spectrophotometer with nF900 ns Flashlamp as the light source. All of the measurements were performed at room temperature. Thermal quenching was tested using a heating apparatus (TAP-02) in combination with PL equipment. 3. Results and discussion 3.1. Crystal structure analysis The results of Rietveld refinement of SLPSB host implemented with the crystallographic information files identified by previous reports [15] are plotted in Fig. 1(a) by using the GSAS program. The black crosses and red solid line depict the observed and calculated patterns, respectively; the as-obtained goodness of fit parameter c2 ¼ 2.38 and Rwp ¼ 8.21% and Rp ¼ 7.19% can ensure the phase purity. The compound crystallizes in a trigonal crystal system with space group P3 (No. 147), and its cell parameters are a ¼ b ¼ 9.680(1) Å, c ¼ 7.252(7) Å. The detailed crystallographic data of SLPSB is listed in Tables 1e3. In the structure, there are 12 crystallographically independent atoms in the unit cell, including 3 SL (SL ¼ 4/5Sr þ 1/5La), 1 P, 1 Si, 2 B and 5 O atoms as shown in the inset of Fig. 1(a). SL1 and SL2 are nine-fold coordinated (d(SL1eO) ¼ 2.5197e3.0790 Å, d(SL2eO) ¼ 2.5115e2.7283 Å) with an average distance of 2.7419 and 2.5848 Å, respectively. SL3 is surrounded by seven oxygen atoms forming a distorted pentagonal bipyramid (d(SL3eO) ¼ 2.2405e2.8717 Å) with an average distance of 2.5666 Å. P, Si and B (B1) adopt the tetrahedral groups which are isolated from each other. The remaining B (B2) ions occupy the Wyckoff positions 1b and form the linear BO 2 units, which are
9
located within the channels formed by SL3. In phosphate compounds, many studies indicated that the Ln3þ occupies exclusively the (6 h) sites [16,17]. However, when silicate groups are incorporated into the apatite structure, the case might be different. In Sr10xLax(PO4)6x(SiO4)xO (x ¼ 0, 2, 4) materials, K. Boughzala et al. reported the occupancy ratios of La3þ in the different sites and pointed out that Ln3þ will be distributed into the three sites with a marked preference for the 6 h site [18,19]. As a derivative of the apatite crystal structure SLPSB, with the introduction of silicate groups, the La3þ would similarly occupy the different Wyckoff positions 2d (SL1, SL2) and 6 g (SL3). Meanwhile, because the Sr2þ and La3þ fully and complementarily occupy all the different sites, the Sr2þ will also occupy the different Wyckoff positions 2d (SL1, SL2) and 6 g (SL3). Thereby, when the Ce3þ and Tb3þ are introduced into the host, they should be distributed into the three sites with a remarkable preference for one site. The SEM image of SLPSB: 0.01Ce3þ, 0.03Tb3þ clearly presents that the grains have the irregular shape of blocky particles with a size of about 5e10 mm. Fig. 1(b) shows the XRD patterns of the samples SLPSB, SLPSB: 0.01Ce3þ, SLPSB: 0.03Tb3þ and SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.1). All the observed diffraction peaks are well indexed to those of SLPSB and no second phase is observed, indicating that the doping ions do not cause significant changes in the host structure. Since the radius of Tb3þ is smaller than those of Sr2þ and La3þ, with the doping of Tb3þ, the diffraction peaks of the SLPSB: 0.01Ce3þ, xTb3þ samples show a slight shift to larger 2q angles compared to those of the pure SLPSB. The relative difference in ionic radii between matrix cations and dopant ions Ce3þ and Tb3þ is listed in Table 4. 3.2. Photoluminescence property analysis Fig. 2(a) shows the PLE spectrum of SLPSB: 0.01Ce3þ monitored at 415 nm and PL spectra of SLPSB: xCe3þ (0.001 x 0.04) excited by 351 nm. It is obviously observed that SLPSB: xCe3þ can be efficiently excited by UV light and emit the blue light from 5de4f transition of Ce3þ. With the increase of Ce3þ, the PL intensity gradually increases. When the concentration of Ce3þ is 1%, the PL intensity reaches the maximum. The optimal concentration (1%) is referred to as “critical concentration”, which is due to ET from one activator to another until energy sink in the lattice is reached. To understand the ET mechanism of SLPSB: 0.01Ce3þ, it is useful to know the critical distance Rc of ET between Ce3þ. Blasse suggested that the critical distance Rc(CeeCe) could be calculated by the critical concentration of the activator ion [20]:
Rc z2
3V 4pxc N
1=3 (1)
where V is the volume of the unit cell, xc is the critical concentration of the activator ion, and N is the number of cations of per unit cell. For the SLPSB host, N ¼ 10, xc ¼ 0.01, and V ¼ 588.56 Å3, and therefore the Rc(CeeCe) value is calculated to 22.40 Å. This result indicates that the ET between Ce3þ and Ce3þ is not via exchange interaction mechanism but electric multipolar interaction, and thus leads to a stronger blue emission from Ce3þ [21,22]. Fig. 2(b) shows the PL and PLE spectra of SLPSB: 0.01Ce3þ. With the change of excited and monitored wavelength, the highest PL peak shifts from 398 to 432 nm and the highest PLE peak shifts from 330 to 380 nm, which also indicate that Ce3þ should occupy the different sites. In Fig. 2(c), under the excitation of 255, 352 and 377 nm, SLPSB: 0.03Tb3þ presents the yellowish green emission from Tb3þ 5D4e7FJ (J ¼ 3, 4, 5, 6) characteristic transitions. According to Dexter's theory [23], an efficient ET requires a partial overlap between the PLE spectrum of the activator and the PL spectrum of the sensitizer
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Z. Ci et al. / Journal of Alloys and Compounds 654 (2016) 8e14
Fig. 1. (a) Experimental (crosses), calculated (solid line) and difference (bottom) results of XRD refinement of SLPSB host; The inset shows the structure diagram of SLPSB according to the refinement results and SEM image of SLPSB: 0.01Ce3þ, 0.03Tb3þ; (b) The XRD patterns of the samples SLPSB, SLPSB: 0.01Ce3þ, SLPSB: 0.03 Tb3þ and SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.1).
[24,25]. From Fig. 2(c), we can find that there is a remarkable overlap between emission band of Ce3þ (donor) and absorption band of Tb3þ (acceptor). In Fig. 2(d), monitored at 542 nm, as the highest PL peak for Tb3þ but not for Ce3þ, the PLE spectrum of SLPSB: 0.01Ce3þ, 0.03Tb3þ is very similar with that of SLPSB: 0.01Ce3þ monitored at 415 nm, not that of SLPSB: 0.03Tb3þ, which implies that Ce3þ makes some contributions to the Tb3þ characteristic peak at 542 nm, i.e., an effective resonance-type ET takes
place between Ce3þ and Tb3þ. Fig. 3(a) reveals the PL spectra of SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.1). Upon 340 nm excitation, the emission spectra of Ce3þ, Tb3þ co-doped SLPSB consists of not only the broad violetblue emission band centered at 405 nm from Ce3þ, but also a series of strong yellowish green line emissions from Tb3þ. With increasing Tb3þ contents, the emission intensity of Ce3þ gradually declines, but that of Tb3þ gradually increases, which further
Z. Ci et al. / Journal of Alloys and Compounds 654 (2016) 8e14 Table 1 Crystal structural data and lattice parameters of SLPSB. Formula
Sr8La2(PO4)3.5(SiO4)2(BO4)0.5BO2
Crysta system Space group Lattice a/Å b/Å c/Å a/ o b/o g/o Cell volume (V/Å3) R values
Hexagonal P3(147)
c
9.680(1) 9.680(1) 7.252(7) 90 90 120 588.56 Rwp ¼ 8.21% Rp ¼ 7.19% 2.38
2
11
indicates that the high light output of Tb3þ actually comes from the ET from Ce3þ to Tb3þ. In order to investigate the ET mechanism from Ce3þ to Tb3þ, the decay times of Ce3þ in SLPSB: Ce3þ, xTb3þ are tested and depicted in Fig. 3(b). We can see that the decay curves of Ce3þ deviate gradually from a single exponential rule with the increase of Tb3þ. The effective lifetimes of the decay curves of Ce3þ can be evaluated using the Eq. (2) [26]:
Z
∞
tIðtÞdt t ¼ Z0 ∞
(2) IðtÞdt
0
The calculated decay times are determined to be 35.49, 31.04, 27.73, 21.83, 17.75, and 12.98 ns in Fig. 3 (b). According to Dexter's formulation [23], the ET rate is given by:
Table 2 Crystallographic data of SLPSB determined by the XRD Rietveld refinement at the room temperature. Atom
Wyck.
x/a
y/b
z/c
SL1 SL2 SL3 P Si B1 O1 O2 O3 O4 O5 B2
2d 2d 6g 6g 6g 6g 6g 6g 6g 6g 2c 1b
1/3 1/3 0.23221 0.38884 0.38884 0.38884 0.33005 0.59728 0.33044 0.37933 0 0
2/3 2/3 0.0154 0.37376 0.37376 0.37376 0.47413 0.482 0.28007 0.26213 0 0
0.01111 0.49879 0.25185 0.23577 0.23577 0.23577 0.22513 0.26581 0.08613 0.48367 0.32247 1/2
Q PðRÞf b A R tD
Z
fD ðEÞFA ðEÞ dE Ec
(3)
where tD is the decay time of the donor emission, QA is the total absorption cross section of the acceptor ion, R is the distance between the donor and the acceptor, and b and c are parameters dependent on the type of ET. The probability functions fD(E) and FA(E) represent the observed shapes of the donor emission band and the acceptor absorption band, respectively. Thus, according to Eq. (3), the ET rate P is in inverse proportion to the decay time tD. The result that decay lifetime of the Ce3þ decreases monotonically with the increase of Tb3þ also further proves the ET from Ce3þ to Tb3þ. The energy transfer efficiency hCeeTb can be expressed by:
Table 3 Selected interatomic distances in the crystal structure of SLPSB. Atom contacts
Å
Atom contacts
O1 SL1 SL1 O1 SL1 O1 SL1 O2 SL1 O2 SL1 O2 SL1 O3 SL1 O3 SL1 O3 SL2 O1 SL2 O1 SL2 O1 SL2 O2 SL ¼ 4/5 Sr þ 1/5 La, Z* ¼ 7/12 P þ 4/12 Si þ 1/12 B1
2.5197 2.5201 2.5205 2.6268 2.6269 2.6274 3.0780 3.0786 3.0790 2.7116 2.7119 2.7122 2.5302
SL2 SL2 SL2 SL2 SL2 SL3 SL3 SL3 SL3 SL3 SL3 SL3
Table 4 Relative difference in ionic radii (Dr (%) ¼ 100 [Rm (CN)-Rd (CN)]/Rm (CN)) between matrix cations and dopant ions Ce3þ and Tb3þ. Ions
Radius/Å
CN
Dr(%)
Ce3þ Sr2þ La3þ Ce3þ Sr2þ La3þ Tb3þ Sr2þ La3þ Tb3þ Sr2þ La3þ
1.196 1.31 1.216 1.07 1.21 1.10 1.095 1.31 1.216 0.98 1.21 1.10
9 9 9 7 7 7 9 9 9 7 7 7
0 8.70 1.64 0 11.57 2.73 0 16.41 9.95 0 19.01 10.91
h¼1
ts ts0
Å O2 O2 O4 O4 O4 O1 O2 O3 O3 O4 O4 O5
2.5304 2.5309 2.5115 2.5122 2.5125 2.7283 2.4009 2.5484 2.7946 2.8717 2.2405 2.3817
(4)
where tS0 is the lifetime of Ce3þ in the absence of Tb3þ and tS is the lifetime of Ce3þ in the presence of Tb3þ. The decay lifetime values are used for calculation, and the results are presented in Fig. 3(c). For the samples SLPSB: 0.01Ce3þ, xTb3þ (0 x 0.1), the critical distance Rc(CeeTb) for ET from the Ce3þ to Tb3þ can be also estimated by the Eq. (1). In this case, the critical concentration (xc), at which the PL intensity of Ce3þ is one half of that in the sample in the absence of Tb3þ, is 0.05 in Fig. 3(a). Therefore, the critical distance (Rc(CeeTb)) is calculated to be 13.10 Å, which also indicates that the ET between Ce3þ and Tb3þ is via electric multipolar interaction [21,22]. Fig. 3(d) illustrates the Commission Internationale de
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Fig. 2. (a) The PLE spectrum of SLPSB: 0.01Ce3þ and PL spectra of SLPSB: xCe3þ (0.001 x 0.04); The inset shows the PL intensities of SLPSB: xCe3þ as a function of x; (b) The PL and PLE spectra at different excited and monitored wavelength of SLPSB: 0.01Ce3þ; (c) The PL spectra of SLPSB: 0.03Tb3þ under the excitation of different wavelength and the overlap between the PLE spectrum of SLPSB: 0.03Tb3þ and the PL spectrum of SLPSB: 0.01Ce3þ; (d) The PLE spectra of SLPSB: 0.01Ce3þ, SLPSB: 0.03Tb3þ and SLPSB: 0.01Ce3þ, 0.03Tb3þ monitored at different PL peaks.
Fig. 3. (a) The PL spectra of SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.1) excited at 340 nm; (b) The decay curves of SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.1) excited at 340 nm and monitored at 405 nm; (c) The variation of hT and t with the increase of Tb3þ; (d) The CIE chromaticity coordinates of SLPSB: 0.01Ce3þ, SLPSB: 0.03Tb3þ and SLPSB: Ce3þ, xTb3þ (0.01 x 0.07).
L'Eclairage (CIE) chromaticity coordinates of SLPSB: 0.01Ce3þ, SLPSB: 0.03Tb3þ, SLPSB: 0.01Ce3þ, xTb3þ (0.01 x 0.07), which were calculated based on the corresponding PL spectra. ET makes it possible to obtain both the blue emission of Ce3þ and the yellowish
green emission of Tb3þ in a single host. Thus, we can simply change the doping content of Tb3þ to adjust the color tone so as to meet the requirement of different illumination applications. The CIE chromaticity coordinates of SLPSB: 0.01Ce3þ, xTb3þ (x ¼ 0.01 and 0.03)
Z. Ci et al. / Journal of Alloys and Compounds 654 (2016) 8e14
are found to be located in the white region, with CIE chromaticity coordinates (0.234, 0.246) and (0.281, 0.345), respectively. Although the chromaticity coordinates are not very close to the standard white (0.33, 0.33), which would not be recommended to serve as room lighting sources, the current white emission could serve as outdoor lighting or back light of a mobile telephone. 3.3. Temperature-dependent photoluminescence behavior A comprehensive understanding of the thermal quenching of phosphors in the process of the phosphors application is indispensable because many devices suffer from thermal problems. Numerous investigations have discussed the thermal quenching behaviors [27e29]. Two competing factors are in prevail, one is the activation energy of non-radiative relaxation, the other is the rate of temperature-induced direct tunneling, which prevents emissive transition between the different activator ions excited state and the ground state in host [29]. The temperature dependent PL spectra from 20 to 250 C of SLPSB: 0.03Tb3þ, SLPSB: 0.01Ce3þ and SLPSB: 0.01Ce3þ, 0.03Tb3þ excited by 340 nm are shown in Fig. 4(a), (b) and (c), respectively. With the increasing temperature, the PL intensities of all samples gradually decline. For the Ce3þ or Tb3þ single doped SLPSB, the PL intensities at 250 C are 87.3% and 71.8% of their initial intensities at 20 C. However, for the Ce3þ and Tb3þ co-doped sample, the PL intensity of Ce3þ and Tb3þ in SLPSB: 0.01Ce3þ, 0.03Tb3þ declines rapidly and at 250 C drop to 29.8% and 32.1% of those at 20 C. This result indicates that the change of temperature can affect the ET between Ce3þ and Tb3þ strongly. Based on the configurational coordinate diagram in Fig. 5, the abnormal phenomenon can be explained reasonably. The curves g(1,2) and 7FJ are the ground states of Ce3þ and Tb3þ. The curves e and 5DJ are the excited states of Ce3þ and Tb3þ. The points A and B are the lowest positions of the excited states 7FJ and e. To simplify the discussion, we assume that the
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crossing points among the excited states and ground states of Ce3þ and Tb3þ can be expressed by only one capital letter. The points M are the crossing point of g(1,2) and e. P is the crossing point of e and 5 DJ. DE1, DE2 and DE3 are the energy differences of B to P, B to M and A to P, respectively. Under the excitation of the UV light, the electrons are excited to the excited states from g(1,2), 7FJ to e, 5DJ. For SLPSB: 0.03Tb3þ and SPLSB: 0.01Ce3þ, most of the electrons return to the ground states by radiative transitions to obtain the characteristic emitting of Tb3þ and Ce3þ at the room temperature, but for SLPSB: 0.01Ce3þ, 0.03Tb3þ, the electrons of e would likely overcome the energy barrier DE1 as well under the electron-phonon coupling, and then accomplish energy transfer from Ce3þ to Tb3þ along the pink way ➀ at the room temperature. With the increase of temperature, for the Ce3þ or Tb3þ single doped SLPSB, the electrons of e and 5DJ could be difficult to overcome the energy barriers between their own excited states and ground states, thus result in an excellent thermal properties as shown in Fig. 4(a) and (b). However, for the Ce3þ, Tb3þ co-doped SLPSB, more electrons of e could overcome the energy barrier DE1 and DE2 to transfer the energy to 5DJ or return to g(1,2) along the pink way ➀ and green way ② from the crossing points P and M due to the stronger electron-phonon coupling. This process could lead to the continuous decrease of electrons of e, thus the PL intensity of Ce3þ more rapidly declines than that in SLPSB: 0.01Ce3þ. But in the discussion above, we have known that the PL intensity of Tb3þ depends strongly on Ce3þ. Although the enhanced electron-phonon coupling is helpful to electron transfer from e to 5 DJ, the decrease of electron number in e due to electron directtunneling from e to g(1,2) could still lead to the decline of Ce3þeTb3þ ET efficiency, then result in the rapid decline of Tb3þ intensity, which can be also confirmed by the similar downtrend of Ce3þ and Tb3þ PL intensities in SLPSB: 0.01Ce3þ, 0.03Tb3þ in Fig. 4(d). In addition, in Fig. 4(c), we also observed that the decline of the Ce3þ PL intensity from 200 to 250 C becomes slower than that below 200 C. It might be because the possibility of the back-
Fig. 4. The PL spectra of (a) SLPSB: 0.03Tb3þ, (b) SLPSB: 0.01Ce3þ, (c) SLPSB: 0.01Ce3þ, 0.03Tb3þ under various temperatures; (d) The dependence of normalized PL intensities on temperature for the typical phosphors.
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Z. Ci et al. / Journal of Alloys and Compounds 654 (2016) 8e14
Fig. 5. The configurational coordinate diagram of the excited and ground states of Ce3þ and Tb3þ.
tunneling of electrons from 5DJ to e along the blue way ③ gradually increases under the stronger phonon vibration with the further increasing temperature, depressing the degradation of the thermal properties of Ce3þ in SLPSB: 0.01Ce3þ, 0.03Tb3þ.
expenses of the central university and Open Project of Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University. References
4. Conclusions In summary, a simple solid-state route was adopted to fabricate a series of phosphosilicate phosphors SLPSB: Ce3þ, Tb3þ. Their crystal structure, PL and PLE spectra, decay times, CIE chromaticity coordinates and thermal quenching are measured to evaluate its PL properties. The luminescence analysis demonstrates that the phosphors SLPSB: Ce3þ, Tb3þ can be efficiently excited by the UV light from 240 to 400 nm, and simultaneously emit the blue light from Ce3þ and the yellowish green light from Tb3þ. By adjusting the ratio of Ce3þ/Tb3þ, the color-tunable light with chromaticity coordinates from (0.188, 0.095) to (0.351, 0.517) can be obtained. The spectral characteristic and decay times indicate that the efficient ET occurs between Ce3þ and Tb3þ. With the increasing temperature, the PL intensities of all samples gradually decline. The PL intensities of SLPSB: 0.03 Tb3þ and SLPSB: 0.03Ce3þ at 250 C is 87.3% and 71.8% of their initial intensities at 20 C. However, when Ce3þ and Tb3þ are co-doped into SLPSB host, the PL intensities of Ce3þ, Tb3þ in SLPSB: 0.01Ce3þ, 0.03Tb3þ degenerate rapidly and at 250 C drop to 29.8% and 32.1% of those at 20 C. In addition, we also observed that the decline of the Ce3þ PL intensity from 200 to 250 C becomes slower than that below 200 C. These results indicate that the change of temperature can affect the energy transfer between Ce3þ and Tb3þ strongly. Based on the configurational coordinate diagram, an underlying mechanism is proposed to explain the abnormal changing trend, which could be used in the discussion of the thermal properties of multiple activator co-doped phosphors as reference. Acknowledgments This work was supported by National Natural Science Funds of China (No. 51302121), the basic scientific research business
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