Luminescence properties of Eu3+ doped CaBi2Ta2O9 bismuth layered-structure ferroelectrics

Materials Research Bulletin 48 (2013) 4301–4306

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Luminescence properties of Eu3+ doped CaBi2Ta2O9 bismuth layered-structure ferroelectrics Rui-Rui Cui a, Chao-Yong Deng a,*, Xin-Yong Gong a, Xu-Cheng Li a, Jian-Ping Zhou b a b

Department of Electronic Science, College of Science, Guizhou University, Guiyang, Guizhou 550025, China College of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 13 March 2013 Accepted 27 June 2013 Available online 4 July 2013

Eu3+ doped CaBi2Ta2O9 bismuth layered-structure ferroelectrics were synthesized by a solid state reaction method. Photoluminescence spectra exhibited a novel red-emitting centered at 615 nm under the excitation of 465 nm. The luminescence intensity of Ca1xEuxBi2Ta2O9 increased with the increase of Eu3+ concentration until x = 0.15. Then the luminescence intensity decreased with the continuous increase of Eu3+ concentration. It is calculated that the thermal activation energy was 0.3 eV in the research on the thermal stability of the sample Ca0.85Eu0.15Bi2Ta2O9. In addition, an enhancement of luminescence intensity was observed in the sample doped with Sr (Ba, Nb). ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Optical materials A. Layered compounds B. Luminescence B. Optical properties

1. Introduction In recent years, bismuth layered-structure ferroelectrics (BLSFs) have drawn much attention because of their desirable ferroelectric, dielectric, piezoelectric, pyroelectric, electrooptic and photocatalytic properties [1,2]. ABi2Ta2O9 (A = Ca, Sr, Ba) is one of the BLSFs, and belong to the family of Aurivillius compounds with a general formula of (Bi2O2)2+ (Am1BmO3m+1)2 (m = 2), where A is a 12coordinate cation and B is a 6-coordinate cation and m is the number of pseudo-perovskite units in the pseudo-perovskite layers [3]. The structure of ABi2Ta2O9 consists of two perovskite units sandwiched between bismuth oxide layers [4–6]. Rare earth ions doped BLSFs become a new class of luminescent materials with promising photoluminescence (PL) properties. These materials not only have excellent ferroelectric property, but also present marvelous luminescent performance [7,8]. This makes it possible to use BLSFs as a potential advanced material in the design of next generation novel multifunctional optoelectronic devices. It is known that the PL spectra of luminescence material are determined by the electronic structure of the doped rare earth, while the width and relative intensity of the spectra depend on the crystal symmetry of the host matrix. The study of PL is important from both application and fundamental point of view. Doping in perovskite units of BLSFs is usually used to enhance the dielectric

* Corresponding author. Tel.: +86 851 8290870; fax: +86 851 8290870. E-mail address: [email protected] (C.-Y. Deng). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.06.053

and ferroelectric properties [9–11]. The PL properties of Pr, Er, Tm, Tb, Ho and Yb doped CaBi4Ti4O15 [12], Bi4Ti3O12 [13,14] and (MgCa)2Bi4Ti5O20 [15,16] have been investigated. According to these reports, rare earth ions occupy the A site of BLSFs hosts and show a strong PL due to f–f transitions. The Eu3+ ions are known as excellent optical probes for their sensitivity on the surrounding chemical environment, which also plays an important role in modern lighting field based on their 4f–4f or 5d–4f transitions. As far as we know, few reports have been found on the PL properties of CaBi2Ta2O9. Especially, the research on Eu3+ doped CaBi2Ta2O9 (CBTO: Eu3+) have not been reported yet. Therefore, the structure of novel luminescence material is designed as Ca1xEuxBi2Ta2O9 (x = 0.05, 0.1, 0.15, 0.2 and 0.25). The microstructure and PL properties were studied as a function of Eu3+ ion concentration and sintering temperature, respectively. It was found that the luminescence intensity could be improved by partial substituting in perovskite units. The thermal stability of Ca0.85Eu0.15Bi2Ta2O9 was also investigated in details. 2. Experimental The Ca1xEuxBi2Ta2O9 (x = 0.05, 0.1, 0.15, 0.2 and 0.25) were synthesized by a solid state reaction method. The starting materials were CaCO3 (99.99%), Bi2O3 (99.999%), Ta2O5 (99.99%) and Eu2O3 (99.9%). The stoichiometric amounts of starting materials were ground in an agate mortar thoroughly. Then the mixtures were placed in alumina crucibles and sintered at 1200 8C for 4 h in air. The sintered mixtures were ground again for measurement.

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3. Results and discussion

Intensity (a.u.)

x=0.25

3.1. Effect of Eu3+ ion concentration

20

30

40

50

x=0.05

60

70

(3113) (335)

(2210)

(1113) (135) (315)

(208) (220) (2010)

(119)

(006) (111) (113) (008) (115) (200) (202) (0010)

x=0.15

80

2 (degree) Fig. 1. X-ray diffraction patterns of Ca1xEuxBi2Ta2O9 (x = 0.05, 0.15 and 0.25).

The crystal structure of the CBTO: Eu3+ was identified by X-ray diffractometer (XRD) with CuKa radiation (model D/Max-RA, Rigaku). Particles size and shape were observed by a field emission scanning electron microscope (FE-SEM) (model JEM-2000EX). The excitation and emission spectra were recorded by a fluorescence spectrophotometer (Horiba FluoroMax-4). The temperature dependent properties were examined by a variable temperature device measured from 25 8C to 300 8C and each target temperature stayed for 10 min.

100

(a)

em

=615 nm 7

5

F0

L6

Intensity (a.u.)

80 60 7

40 7

F0

20

F0 5L7 5 D4

x=0.15 x=0.2 x=0.25 x=0.1 x=0.05 5

7

F0

0 350

7

5

F0

D2

In order to investigate the effect of Eu3+ doping concentration on the PL intensity, the samples Ca1xEuxBi2Ta2O9 (x = 0.05, 0.1, 0.15, 0.2 and 0.25) were synthesized, respectively. Fig. 1 shows XRD patterns of Ca1xEuxBi2Ta2O9 sintered at 1200 8C. As shown in Fig. 1, all of the diffraction peaks match well with the standard diffraction card (JCPDS 72-2365) for CaBi2Ta2O9, no impurities were detected which indicated that a pure orthorhombic phase of CaBi2Ta2O9 was obtained. Fig. 2 shows the excitation and emission spectra of Ca1xEuxBi2Ta2O9 (x = 0.05, 0.1, 0.15, 0.2 and 0.25). All excitation spectra were obtained monitored at 615 nm, which corresponds to the transition of Eu3+ (5D0 ! 7F2). The excitation peaks in the range of 350–500 nm are attributed to the intra-4f6 transitions of Eu3+. The two sharp peaks at 397 and 465 nm are ascribed to the transition of 7F0 ! 5L6 and 7F0 ! 5D2, which matches well with the near UV and blue LED chips. The three weak peaks at 362, 385 and 417 nm are attributed to the transition of 7F0 ! 5D4, 7F0 ! 5L7 and 7 F0 ! 5D3, respectively [17]. The emission spectra were measured with the excitation at 465 nm (Fig. 2(b)). The emission peaks in the range of 550– 750 nm are corresponding to radiative transitions from the 5D0 to 7Fj ( j = 0–4) levels of Eu3+ [18]. The red emission centers at 615 nm and yellow emission centers at 592 nm could be corresponding to 5D0 ! 7F2 electric dipolar transition and 5 D0 ! 7F1 magnetic dipolar transition of the 4f-electronic transitions of Eu3+, respectively [19]. The four emission peaks at 683, 696, 702 and 707 nm are typical emission of Eu3+, owing to the splitting of 5D0 ! 7F4. The weak emission peaks at 580 and 653 nm arise from 5D0 ! 7F0 transition and 5D0 ! 7F3 transition, respectively. It was observed that the PL intensity of Ca1xEuxBi2Ta2O9 enhanced with the increasing of Eu3+ ion concentration (x  0.15). When the Eu3+ ion concentration continuously increased (x > 0.15), the PL intensity decreased

D3

400

450

500

Wavelength (nm) 100

(b)

ex

=465 nm 5 7 D0

F2

x=0.15 x=0.2 x=0.25 x=0.1 x=0.05 5

80

Intensity (a.u.)

5

D0

7

F1

60 40 20

5

D0

0 550

7

F0

5

D0

600

D0

7

F4

7

F3

650

700

750

Wavelength (nm) Fig. 2. (a) Excitation and (b) emission spectra of Ca1xEuxBi2Ta2O9(x = 0.05, 0.1, 0.15, 0.2 and 0.25). (lex = 465 nm, lem = 615 nm).

Fig. 3. The CIE chromaticity diagram of Ca1xEuxBi2Ta2O9 (x = 0.05, 0.1, 0.15, 0.2 and 0.25).

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Ca0.85Eu0.15Bi2Ta2O9

Intensity (a.u.)

1400

1200

1000 20

30

40

50

60

70

80

2 (degree) Fig. 4. XRD patterns of Ca0.85Eu0.15Bi2Ta2O9 sintered at 1000, 1200 and 1400 8C, respectively.

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because of the concentration quenching effect [20,21]. As a result, the optimal concentration of Eu3+ ions for Ca1xEuxBi2Ta2O9 is x = 0.15, and the shape and position of emission peaks are not influenced by Eu3+ ion concentration. It is known that the crystal field environment can also influence luminescence efficiency. The transition of 5D0 ! 7F2 of Eu3+ is very sensitive to the variation of environment [22,23]. The electronic dipole transition 5D0 ! 7F2 is forbidden in a centrosymmetric crystal, whereas the magnetic dipole transition 5D0 ! 7F1 is independent on crystal field symmetry [24,25]. In order to study the symmetry of the local crystal field of the Eu3+ sites in the CaBi2Ta2O9 host matrix, the parameter IAS defined as IAS = I (5D0 ! 7F2)/I (5D0 ! 7F1) was calculated. It is generally admitted that the ratio indicates the degree of asymmetry in the vicinity of Eu3+ ions. The IAS values for the samples x = 0.05, 0.1, 0.15, 0.2 and 0.25 were 1.52, 1.65, 1.52, 1.64 and 1.65, respectively, which indicated that Eu3+ was at a non-centrosymmetric site [26]. Fig. 3 shows the Commission Internationale de I’Eclairage (CIE) chromaticity coordinates of Ca1xEuxBi2Ta2O9 (x = 0.05, 0.1, 0.15, 0.2

Fig. 5. SEM images of Ca0.85Eu0.15Bi2Ta2O9 sintered at (a) 1000 8C, (b) 1200 8C and (c) 1400 8C, respectively.

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ex

1200 ºC

=465 nm

Intensity (a.u.)

100

Intensity (a.u.)

80 60

1200 1300 1100 1400 1000

40 20 0 550

600

1000 1100 1200 1300 1400 Temperature (ºC)

650

700

750

Wavelength (nm) Fig. 6. Emission spectra of Ca0.85Eu0.15Bi2Ta2O9 sintered at different temperatures. The inset is the plot of relative character emission (5D0 ! 7F2) peak intensity.

and 0.25). As the concentration of Eu3+ ions increases, the CIE coordinates (X, Y) varies systematically from x = 0.05 (0.58, 0.408), x = 0.1 (0.609, 0.386), x = 0.15 (0.612, 0.383), x = 0.2 (0.621, 0.376) to x = 0.25 of (0.624, 0.374). These colors belong to the orange and red regions. With increasing activator concentration, the emission color shifts to a longer wavelength region slightly. This phenomenon could be explained by the change of crystal field surrounding the Eu3+ ions [27].

the standard diffraction pattern data of CaBi2Ta2O9 (JCPDS 722365), which indicated that a pure CaBi2Ta2O9 phase can be formed in this temperature range. Fig. 5 illustrates the SEM images of the as-sintered samples. One can easily see that, with the increasing sintering temperature, the particles gradually grow up (Fig. 5(a)–(c)). The average particles size of the sample sintered at 1000 8C is about 3 mm, and the average size is about 5 mm when the sintering temperature is 1200 8C. When the sintering temperature is 1400 8C, the vitreous material is formed. Fig. 6 shows the emission spectra of Ca0.85Eu0.15Bi2Ta2O9 sintered at different temperatures. It can be seen that the emission intensity of CBTO: Eu3+ is strongly affected by the sintering temperature. As the sintering temperature increases, the particles size grows larger (Fig. 5), which reduces the boundary surface between particles and minimizes light scattering. Therefore, the emission intensity enhanced gradually till T = 1200 8C. If the sintering temperature goes up continuously, the generating of vitreous material will reduce the emission efficiency. Therefore, it is concluded that the optimal sintering temperature is 1200 8C. Fig. 7 is the CIE chromaticity coordinates of Ca0.85Eu0.15Bi2Ta2O9 sintered at different temperatures. As the sintering temperature

3.2. Effect of sintering temperature The effect of sintering temperature on microstructure and PL properties of Ca0.85Eu0.15Bi2Ta2O9 were also studied. A series of samples sintered at 1000, 1100, 1200, 1300 and 14008C were synthesized, respectively. Fig. 4 shows the XRD patterns of the assintered Ca0.85Eu0.15Bi2Ta2O9. The patterns data matches well with

Fig. 7. The CIE chromaticity diagram of Ca0.85Eu0.15Bi2Ta2O9 sintered at different temperatures.

Fig. 8. (a) Emission spectra of Ca0.85Eu0.15Bi2Ta2O9 measured at different temperatures (from 25 8C to 300 8C). The inset plots the emission (5D0 ! 7F2) peak intensity. (b) ln[I0/I(T)  1] versus 1/(kBT) plot.

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increases, the CIE chromaticity coordinates (X, Y) varies from (0.54, 0.4408) of T = 1000 8C to (0.6211, 0.376) of T = 1400 8C. With the increasing of sintering temperature, a slight shift in emission color to a longer wavelength region occurs. 3.3. Thermal stability of Ca0.85Eu0.15Bi2Ta2O9 The thermal behavior of Ca0.85Eu0.15Bi2Ta2O9 was studied in Fig. 8. As measurement temperature increases, the emission intensity decreases sharply. The emission intensity ratio (I(T)/ I0  100%) is 82% at 100 8C. And the intensity ratio drops to 45% when the temperature increases to 200 8C. As explained in Ref. [27], this is a thermal quenching effect. Between the ground state and excited state, the excited luminescence centers are thermally activated through phonon interactions and then released in the form of thermal through the crossing point. The nonradiative transition probability by thermal activation is strongly affected by temperature, therefore, leads to the decreasing of emission intensity [28]. The thermal quenching behavior of temperature effect can be explained by the following equation based on the model of Struck and Fonger [29–31]:

IðTÞ ¼

I0 ; ½1 þ cexpðE=kB TÞ

where I(T) is the intensity at a given temperature, I0 is the initial intensity, c is a constant, kB is Boltzmann’s constant and E is the thermal activation energy for thermal quenching. Based on the data of emission spectra (Fig. 8(a)), ln[I0/I(T)  1] versus 1/(kBT) was ploted in Fig. 8(b). According to the linear fitting of its slope, the thermal activation energy E is 0.3 eV. 3.4. Effect of doping in perovskite units In order to investigate the influence of the partial substitution in perovskite units on the luminescence properties of CBTO: Eu3+, four samples were obtained under the same process. Fig. 9 shows the emission spectra of Ca0.85Eu0.15Bi2Ta2O9, (Ca0.9Sr0.1)0.85Eu0.15Bi2Ta2O9, (Ca0.9Ba0.1)0.85Eu0.15Bi2Ta2O9 and Ca0.85Eu0.15Bi2(Ta0.9Nb0.1)2O9, respectively. It was found that ascribe to the change of the lattice symmetry, the partial substitution improved the emission intensity. As it was reported, the bond angle of Ta-OTa increases with increasing A2+ cation size [32,33]. When the Ca2+ (1.34 A˚) ions partially substituted by bigger ions of Sr2+ (1.44 A˚) or Ba2+ (1.61 A˚), the structural distortion becomes obvious. This

=465 nm

ex

100

Nb Intensity (a.u.)

80

Ba Sr

No doped

60 40 20 0 575

600

625

650

Wavelength (nm) Fig. 9. Emission spectra of Ca0.85Eu0.15Bi2Ta2O9, (Ca0.9Sr0.1)0.85Eu0.15Bi2Ta2O9, (Ca0.9Ba0.1)0.85Eu0.15Bi2Ta2O9 and Ca0.85Eu0.15Bi2(Ta0.9Nb0.1)2O9, respectively).

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structural distortion allows stronger transitions of Eu3+ to occur and makes an enhancement of PL intensity. The PL intensity of Ca0.85Eu0.15Bi2(Ta0.9Nb0.1)2O9 is enhanced based on similar reason. So the partial substitution in perovskite units is an effective method to improve the luminescence properties. The CIE chromaticity coordinates (X, Y) of (Ca0.9Sr0.1)0.85Eu0.15Bi2Ta2O9, (Ca0.9Ba0.1)0.85Eu0.15Bi2Ta2O9 and Ca0.85Eu0.15Bi2(Ta0.9Nb0.1)2O9 are (0.613, 0.3826), (0.611, 0.3843) and (0.6097, 0.3853). Compared with the value (0.611, 0.3842) of Ca0.85Eu0.15Bi2Ta2O9, the luminescence region has no obvious change. 4. Conclusions In the present work, a novel red-emitting material of Ca1xEuxBi2Ta2O9 (x = 0.05, 0.1, 0.15, 0.2 and 0.25) were synthesized by a solid state reaction method. The XRD patterns showed that a pure CaBi2Ta2O9 phase was obtained by this method. Investigation on the effect of sintering temperature indicated that the optimal sintering temperature was 1200 8C. The radiative transitions from the 5D0 to 7Fj (j = 0–4) levels of Eu3+ were observed from the emission spectra. When the concentration of Eu3+ ions x was 0.15, Ca1xEuxBi2Ta2O9 had the strongest emission intensity. The thermal activation energy was calculated to be 0.3 eV in the research of thermal stability. In addition, the partial substitution in perovskite units showed an enhancement of emission intensity. Eu3+ doped CaBi2Ta2O9 bismuth layeredstructure ferroelectrics would be potential in multifunctional optoelectronic devices. Acknowledgements This work was supported by the Science Research Plan Funds of Guizhou Province of China (Nos. 2012-3005, 2010-4005,2009-15, 2010-2134, 2011-2016) and Fundamental Research Funds for the Guiyang City (No. 2012101-2-4).

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