Tunable white-light-emitting Sr2−xCaxNb2O7:Pr3+ phosphor by adjusting the concentration of Ca2+ ion

Optical Materials 36 (2014) 1093–1096

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Optical Materials journal homepage: www.elsevier.com/locate/optmat

Tunable white-light-emitting Sr2xCaxNb2O7:Pr3+ phosphor by adjusting the concentration of Ca2+ ion Juan Geng a, Yonghu Chen b, Guangrui Gu a, Lianhua Tian a,⇑ a b

Department of Physics, Yanbian University, Yanji 133002, China Department of Physics, University of Science and Technology of China, Hefei 230026, China

a r t i c l e

i n f o

Article history: Received 12 October 2013 Received in revised form 6 January 2014 Accepted 10 January 2014 Available online 4 March 2014 Keywords: Sr2xCaxNb2O7:Pr3+ White light Nonradiative relaxation

a b s t r a c t The photoluminescence (PL) properties of Sr2xCaxNb2O7:Pr3+ were investigated in this study. The system exhibited three different states when Sr2+ ion was replaced by Ca2+ ion gradually. The excitation spectra 7 showed the absorptions of the charge transfer (CT) transition of NbO6 and intervalence charge transfer (IVCT) transition of Pr3+ ? Nb5+. The transitions from 3H4 to 3PJ (J = 0, 1, and 2) of Pr3+ were located at 452, 474, and 490 nm. The main excitation peak changed with the phase transformations. The PL spectra of Sr2xCaxNb2O7:Pr3+ excited with 320 nm showed two intense regions in the wavelength of 480–517 nm (blue-green) and 585–640 nm (orange-red). With increasing content of Ca2+ ion, the orange-red band enhanced and the Commission International de l’Echairage (CIE) moved from blue-green to white and then to purple. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Recently, there has been increasing attention in the development of generating white light and many efforts have been made to achieve white-light-emitting phosphors. Generally, white light originates from a combination of multiband light emission through multi- or single-component phosphor-conversion upon irradiation with ultraviolet or blue light. However, these white phosphors still have some drawbacks such as decrease in device efficiency, poisonous components, or difficult adjustment of optical properties [1]. Alternatively, it is possible to generate white light through a strategy of combining different emissions of Pr3+ ion in a single host lattice, as the emission color of Pr3+ depends strongly on the host lattice [2]. Niobates have been intensively studied for their potential applications in ferroelectric, dielectric, piezoelectric, and photocatalytic fields [3–6]. Rare earths are doped in many niobates host materials as luminous centers to design a large number of luminescent materials [7–10]. Sr2Nb2O7 has been selected as the host lattice in this study. Sr2Nb2O7 belongs to the perovskite-like layered structure family. The general formula is AnBnO3n+2 (n = 4 for A2B2O7). Their structure is characterized by corner-shared BO6 octahedra and 12-coordinated A cations within the perovskite-like layers, which are linked by A cations at their boundaries [11,12]. A series of different concentrations of Ca2+ ion doped Sr2Nb2O7 powders were ⇑ Corresponding author. Tel.: +86 433 2733938. E-mail address: [email protected] (L. Tian). http://dx.doi.org/10.1016/j.optmat.2014.01.009 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.

synthesized to prepare Sr2xCaxNb2O7 matrix. Photoluminescence properties of Sr2xCaxNb2O7:Pr3+ phosphors were investigated in this study. By adjusting the concentration of Ca2+ ion, the samples exhibited two dominant emissions of Pr3+, and then gave out white light. 2. Experimental The Sr2xCaxNb2O7:Pr3+(0.5%) phosphors were prepared by the conventional solid-state reaction method. Stoichiometric amounts of CaCO3 (99.99%), SrCO3 (99.99%), Nb2O5 (99.99%), and Pr2O3 (99.99%) were homogeneously mixed. The mixture was placed in an alumina crucible, transferred into the furnace, and annealed at 1350 °C in air for 2 h to obtain the compounds. The crystal structure of the final products were investigated by XRD with Cu Ka (k = 0.154 056 nm) radiation on a TD-2500 X-ray diffractometer. The excitation and photoluminescence spectra were measured by a Hitachi F-7000 fluorescence spectrofluorometer. The FT-Raman scattering data were collected on a Thermo Nicolet FT-Raman 960 spectrometer. The CIE chromaticity coordinates were calculated from the PL spectra of the phosphor samples. All the measurements were performed at room temperature. 3. Results and discussion 3.1. X-ray diffraction Fig. 1 shows the XRD patterns of Sr2xCaxNb2O7:Pr3+(0.5%) (x = 0, 0.5, 1.0, 1.5, and 2.0) obtained by the solid-state reaction.

J. Geng et al. / Optical Materials 36 (2014) 1093–1096

Intensity [a.u.]

1094

(g)

Ca2Nb2O7 (JCPDS No. 74-0390)

(f)

x = 2.0

(e)

x = 1.5

increasing concentration of Ca2+ ion [13]. It was confirmed that the solid solution of Sr2xCaxNb2O7 was formed with the introduction of Ca2+ ions into Sr2Nb2O7 until x = 0.5. However, over x = 0.5, the system exhibited both the Sr2Nb2O7 and Ca2Nb2O7 phases and formed a two-phase mixture. While the value of x exceeded 1.5, the system came into being a solid solution of Sr2xCaxNb2O7 again with the monoclinic Ca2Nb2O7 dominating.

(d)

x = 1.0

3.2. Photoluminescence properties of Sr2xCaxNb2O7:Pr3+

(c)

x = 0.5

(b)

x=0

(a)

Sr2Nb2O7 (JCPDS No. 70-0114)

20

30

40

50

60

2θ [Degree]

Intensity [a.u.]

Fig. 1. XRD patterns of (a) Sr2Nb2O7 (JCPDS No. 70-0114), (b–f) Sr2xCaxNb2O7:Pr3+(0.5%) (x = 0, 0.5, 1.0, 1.5, and 2.0), and (g) Ca2Nb2O7 (JCPDS No. 74-0390).

(d)

x = 0.5

(c)

x = 0.3

(b)

x = 0.1

(a)

x=0

27

30

33

The excitation spectra of Sr2xCaxNb2O7:Pr3+(0.5%) (x = 0, 0.5, 1.0, 1.5, and 2.0) monitored at 608 nm are shown in Fig. 3. The excitation spectra of Sr2xCaxNb2O7:Pr3+(0.5%) consisted of two broad bands and a series of small peaks. The first band located at 7 higher energy corresponded to the CT transition of NbO6 complex, whereas the second, located at lower energy, was ascribed to the IVCT transition of Pr3+ ? Nb5+ [14,15]. The IVCT transitions of Pr3+ ? Nb5+ for Sr2Nb2O7:Pr3+(0.5%) and Ca2Nb2O7:Pr3+(0.5%) were calculated by the empirical equation of Philippe Boutinaud located at 307 and 327 nm, respectively [15]. The experimental values just fell in the theoretical values, as shown in Fig. 3. The weak excitation peaks at 452, 474, and 490 nm could be attributed to the 3 H4 ? 3P2, 3H4 ? 3P1, and 3H4 ? 3P0 transitions of Pr3+, respectively [16]. On the other hand, the excitation band positions of Sr2xCax Nb2O7:Pr3+(0.5%) (x = 0, 0.5, 1.0, 1.5, and 2.0) were similar even for the two extreme samples: Sr2Nb2O7:Pr3+(0.5%) and Ca2Nb2O7: Pr3+(0.5%), as shown in Fig. 3. However, the intensity of excitation band was enhanced with the introduction Ca2+ ion into Sr2Nb2O7: Pr3+. Furthermore, the excitation band edge moved from 350 to 390 nm with the introduction of Ca2+ ion. The photoluminescence spectra of Sr2Nb2O7:Pr3+(0.5%) and Ca2Nb2O7:Pr3+(0.5%) are shown in Fig. 4. Both the phosphors exhibited dominant luminescence at two wavelength regions, i.e. 480–517 nm (blue-green) and 585–640 nm (orange-red) [10]. The first emission could be assigned to the 3P0 ? 3H4 transition of Pr3+ [16], while the later one could be attributed to an overlap of the 1D2 ? 3H4 and 3P0 ? 3H6 transitions of Pr3+ [17]. In addition, a broad emission band centered at 440 nm was observed in Ca2Nb2O7:Pr3+(0.5%) in Fig. 4(b). This might be assigned to the emission of CaNb2O6 [18]. The weak emission peaks at 535 and 548 nm could be attributed to the 3P1 ? 3H5 and 3P0 ? 3H5 transitions of Pr3+ ion, respectively [16]. The intensity ratios between blue-green and orange-red emission of Sr2xCaxNb2O7:Pr3+(0.5%), IG/IR, versus the value of x, are

2θ [Degree] 1400

Fig. 2. XRD patterns of (a–d) Sr2xCaxNb2O7:Pr3+(0.5%) (x = 0, 0.1, 0.3, and 0.5) from 25° to 34°.

3+

λem = 608 nm Sr2-xCaxNb2O7:Pr (0.5%)

x=0 x = 0.5 x = 1.0 x = 1.5 x = 2.0

1200

Intensity [a.u.]

At room temperature, Sr2Nb2O7 belongs to the orthorhombic Cmc21 group space with cell parameters a = 3.933 Å, b = 26.726 Å, c = 5.683 Å and V = 597.4 Å3. Ca2Nb2O7 has a monoclinic P21 space group with cell parameters a = 7.697 Å, b = 13.385 Å, c = 5.502 Å, c = 98.34° and V = 560.8 Å3 [11,12]. It can be seen that the system Sr2xCaxNb2O7:Pr3+(0.5%) has several phases depending on the compositions. The diffraction peaks of Sr2Nb2O7:Pr3+(0.5%) (x = 0) and Ca2Nb2O7:Pr3+(0.5%) (x = 2) are fundamental, consistent with the Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 70-0114 (Sr2Nb2O7) and 74-0390 (Ca2Nb2O7), respectively. The enlarged XRD patterns of Sr2xCaxNb2O7:Pr3+(0.5%) (x = 0, 0.1, 0.3, and 0.5) from 25° to 34° are shown in Fig. 2. The system is found to form a nearly single-phase Sr1.5Ca0.5Nb2O7:Pr3+(0.5%) compound until x = 0.5. The diffraction peaks shifted to the larger-angle side with the increasing concentration of Ca2+ ion. It is speculated that the cell volume of Sr2xCaxNb2O7 was decreased with the

(PLE)

1000 800 600 400 200 0 200

300

400

500

Wavelength [nm] Fig. 3. PLE spectra of Sr2xCaxNb2O7:Pr3+(0.5%) (x = 0, 0.5, 1.0, 1.5, and 2.0) monitored at 608 nm.

1095

J. Geng et al. / Optical Materials 36 (2014) 1093–1096

3+

(a)

(a) Sr2Nb2O7:Pr (0.5%)

3+

Sr2Nb2O7:Pr (0.5%) -1

Intensity [a.u.]

Normalized Intensity

846.14 cm

3+

(b) Ca2Nb2O7:Pr (0.5%)

(b)

3+

Ca2Nb2O7:Pr (0.5%)

-1

1033.76 cm 350 400 450 500 550 600 650 700

Wavelength [nm] 200

Fig. 4. PL spectra of (a) Sr2Nb2O7:Pr3+(0.5%) and (b) Ca2Nb2O7:Pr3+(0.5%) excited at 320 nm.

of Sr2Nb2O7:Pr3+(0.5%) and Ca2Nb2O7:Pr3+(0.5%). According to the FT-Raman spectra of (a) Sr2Nb2O7:Pr3+(0.5%) and (b) Ca2Nb2O7: Pr3+(0.5%) in Fig. 5, the maximum phonon energy of Ca2Nb2O7: Pr3+(0.5%) was much larger than that of Sr2Nb2O7:Pr3+(0.5%). Therefore, we considered that the rate WNR of the nonradiative relaxation process in Ca2Nb2O7:Pr3+(0.5%) was higher than that in Sr2Nb2O7:Pr3+(0.5%). This made the 3P0 ? 1D2 nonradiative relaxation more effective with partial quenching of the blue-green emission from 3 P0 and giving rise to orange-red emission from 1D2 with the increase in Ca2+ ion. As a result, the relative intensities of the two clusters could be tuned by different doping levels of Ca2+ ion and blended to give out white light. The changes in CIE chromaticity coordinates with different contents of Ca2+ ion were calculated, and are plotted in Fig. 6. As can be seen, the coordinates of Sr2xCax Nb2O7:Pr3+(0.5%) systematically shifted from blue-green to white and then to purple with increasing content of Ca2+ ion. The CIE coordinate of SrCaNb2O7:Pr3+(0.5%) (i.e., x = 1.0) was (0.331, 0.326),

0.9 0.7

where bel = 10 s and a = 4.5(±1)  10 cm [22]. The energy gap DE between 3P0 and 1D2 were assumed to be 3500 cm1 in both

0.6

560 Green

-0.1 -0.1 0.0

purp le

ue bl

red d re

0.0

le

3.30 2.91 1.95 1.46 1.01 0.66

rp

608 608 610 611 613 614

0.1

blue

ng e

ish

490 490 491 491 493 495

nish gree

or a

pu

0 0.1 0.5 1.0 1.5 2.0

0.2

2 345 6 7 8

l rp

IG/IR

0.3

1

d

Main peaks of orange-red emission (nm)

en

blue green

pu

Main peaks of blue-green emission (nm)

bluis h gr e

re

x

y

0.5 0.4

1: x = 0 2: x = 0.1 3: x = 0.3 4: x = 0.5 5: x = 1.0 6: x = 1.5 7: x = 1.7 8: x = 2.0

530

0.8

3

Table 1 Main peaks and relative intensity ratios (IG/IR) of the blue-green and orange-red emission of Sr2xCaxNb2O7:Pr3+(0.5%) (x = 0, 0.1, 0.5, 1.0, 1.5, and 2.0) pumped at 320 nm.

1200

Fig. 5. FT-Raman spectra of (a) Sr2Nb2O7:Pr3+(0.5%) and (b) Ca2Nb2O7:Pr3+(0.5%).

ð1Þ

1

1000

lo w

7

800

ye l

W NR ¼ bel exp½aðDE  2hxmax Þ

600

Raman shift [cm-1]

h yellowis green

shown in Table 1. When Sr2+ ion was replaced by Ca2+ ion gradually, both the peaks shifted to lower energy. With the increase in x in Sr2xCaxNb2O7:Pr3+(0.5%), the average bond length of Sr/Ca–O became shorter, which resulted in stronger crystal field and led to the red shift of the emissions [19]. IG/IR decreased with the increasing x, indicating that the orange-red emission enhanced faster than the blue-green emission. The performance of the compound depends on the relative positions of the IVCT states and of the two emitting levels. As the IVCT state is at lower energy in Ca2Nb2O7:Pr3+(0.5%) than that of in Sr2Nb2O7:Pr3+(0.5%), the probability of nonradiative relaxation from IVCT state to 1D2 level in Ca2Nb2O7:Pr3+(0.5%) is larger than that in Sr2Nb2O7:Pr3+(0.5%). Therefore, the IG/IR of Ca2Nb2O7:Pr3+(0.5%) is much less than the IG/IR of Sr2Nb2O7:Pr3+(0.5%). However, the quenching of 3P0 emission cannot be fully explained by the relative positions of the IVCT states. Multiphonon relaxation also contributes to the quenching of 3 P0 level. The rate of 3P0 ? 1D2 multiphonon relaxation depended on the maximum phonon energy (⁄xmax) of the host lattice and on the energy gap (DE) between 3P0 and the highest Stark level of the 1D2 multiplet [20,21]. The rate WNR of the nonradiative relaxation process could be roughly estimated using the modified exponential energy gap law of van Dijk and Schuurmans [22]:

400

470

0.1

0.2

0.3

x

0.4

0.5

0.6

0.7

0.8

Fig. 6. CIE chromaticity diagram for Sr2xCaxNb2O7:Pr3+(0.5%) (x = 0, 0.1, 0.3, 0.5, 1.0, 1.5, 1.7, and 2.0) under 320 nm excitation.

1096

J. Geng et al. / Optical Materials 36 (2014) 1093–1096

which was the closest to the pure white point (0.33, 0.33). Qi et al. [10] considered that the Eu3+ phosphors used over 10 transitions to blend a white light will limit the output intensity of the mixed light, and that the Pr3+ phosphors with only two dominant transitions are expected to be more efficient. Therefore, Sr2xCaxNb2O7:Pr3+(0.5%) is a promising white-light-emitting phosphor. 4. Conclusion In conclusion, with the introduction of Ca2+ ion into Sr2xCax Nb2O7:Pr3+, the major excitation band and emission peaks changed with the system phase transformations. The PL spectra of Sr2xCax Nb2O7:Pr3+ showed two main wavelength regions. Both of the relative positions of the IVCT states and the multiphonon relaxation have contributed to the partial quenching of the blue-green emission from 3P0 level and give rise to the orange-red emission from 1D2 level. With varying content of Ca2+ ion, the blue-green and orange-red emission of Sr2xCaxNb2O7:Pr3+ could be mixed to give out white light. Thus, Sr2xCaxNb2O7:Pr3+(0.5%) is a promising white-light-emitting phosphor. Acknowledgments This study was supported by the National Natural Science Foundations of China (51362028, 11164031 and 51272224) and the Nature Science Fund of Science and Technology Department of Jilin Province (20130101035JC).

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