Photoluminescent properties of Eu3+ and Dy3+ ions doped MgGa2O4 phosphors

Photoluminescent properties of Eu3+ and Dy3+ ions doped MgGa2O4 phosphors

Journal of Physics and Chemistry of Solids 74 (2013) 196–199 Contents lists available at SciVerse ScienceDirect Journal of Physics and Chemistry of ...

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Journal of Physics and Chemistry of Solids 74 (2013) 196–199

Contents lists available at SciVerse ScienceDirect

Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs

Photoluminescent properties of Eu3 þ and Dy3 þ ions doped MgGa2O4 phosphors Hai Liu, Lixin Yu n, Fuhai Li Nanchang University, Department of Materials Science and Engineering, 999 Xuefu Road, Nanchang 330031, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 April 2012 Received in revised form 13 September 2012 Accepted 16 September 2012 Available online 5 October 2012

Several concentrations Eu3 þ -doped and Dy3 þ -doped MgGa2O4 phosphors were prepared successfully by two-step firing synthesis. The sintered samples were characterized by means of X-ray diffraction (XRD) and fluorescence spectrophotometer. Little amount of rare earth doped will not change the host matrix structure and the maximum of the emission or excitation intensity of these phosphors will decrease as the concentration increasing for concentration quenching. The emissions of Eu3 þ caused by the transitions of 5 D0-7Fj (j¼ 0, 1, 2, 3, 4) were observed. And there are three groups of emission at 480 nm, 575 nm and 665 nm occurring at the spectrum of Dy3 þ ions, which shows that MgGa2O4 phosphors doped with nanostructures ions have the potentiality to be applied for white LEDs applications. & 2012 Elsevier Ltd. All rights reserved.

Keywords: D. Luminescence D. Optical properties

1. Introduction Just because white light-emitting-diodes (W-LEDs) have a longer lifetime, consume a lower energy, have a higher reliability and other advantages, they have been widely used into the class of lighting field and show a high possibility to replace conventional lamps, such as incandescent lamps and fluorescent lamps, perhaps becoming the fourth-generation lighting resource [1–3]. As for the W-LEDs composited by blue light and yellow light, there are several problems such as lower color-rendering index or lower luminous efficiency just because of lack of red light. Nowadays, thanks to the higher energy of near ultraviolet (UV) light to excite phosphor, we can obtain white light admixed with full-color-emission, which behaves more outstanding qualities than other phosphors [1,4]. Rare earth (RE) ions play a very important role in the improvement of phosphors’ optical properties. The emission of phosphor almost originates from impurities, while the absorption of energy takes place due to the host lattice or impurities [5,6]. And the emission of RE caused by f–f transitions will be different from that of d–f, whose emission spectra consists of sharp lines, while the d–f transitions show broad bands. As is often thought that the RE doped into the phosphors could improve the CRI and the energy efficiency [5]. In addition, because Eu3 þ ions (5D0) and Dy3 þ ions are hypersensitive to the hosts and local microstructures, Eu3 þ ions and doped hosts are universally applied to commercial phosphors and fluorescent probe.

n

Corresponding author. Tel.: þ86 791 83969329. E-mail address: [email protected] (L. Yu).

0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2012.09.006

Although as above described, the white light consisting of two kinds of colors has such those problems, thus the traditional sulfide phosphors are unstable under electron beam exposure [7,8]. So exploring the phosphors without sulfide is urgent for commercial application. In the present work, we chosen the MgGa2O4 doped with RE ions as the candidate to produce white light. It is hopeful that the RE ions could replace the Ga3 þ ions sites, which occupy the eight tetrahedral sites and sixteen octahedral sites because both the RE and Ga3 þ ions have same electronegativity and similar atomic size [7,9]. In our work, MgGa2O4 polycrystals doped with Eu3 þ and Dy3 þ were synthesized successfully via the solid state method. The research on MgGa2O4 doped other ions and relative results have been reported by many researchers [6–7,9–11] and MgGa2O4 doped with Eu3 þ and Dy3 þ , as the author known, has few reports on it. The result that the occurrence of two intense luminescent transitions from the 6H15/2 and 6H13/2 terminal levels in the blue region near 480 nm and in yellow region near 580 nm of Dy3 þ indicates that the MgGa2O4 phosphors doped with RE ions have the potentiality to be used into the field of W-LEDs applications [1,12–13].

2. Experiment The starting chemicals employed in this experiment are magnesium nitrate hexahydrate (AR), gallium oxide (3 N) and europium oxide (4 N) or dysprosium oxide (4 N). All chemicals are not further purified. The molar ratios of RE (Eu3 þ or Dy3 þ ) to MgGa2O4 were 1%, 2% and 4%. Corresponding samples are labeled with Ga0.01Eu, Ga0.02Eu, Ga0.04Eu, Ga0.01Dy, Ga0.02Dy and Ga0.04Dy, respectively. First, all required chemicals was well

H. Liu et al. / Journal of Physics and Chemistry of Solids 74 (2013) 196–199

mixed with a small amount of ethanol. Then the samples were polished for 1 h at room temperature. To evaporate the ethanol, the chemical obtained were dried at a certain temperature of 50 1C for several hours. Second, the dry powder was sent into the muffle furnace then heated to 900 1C for 1 h. Finally, the powder was heated to 1100 1C for 2 h. Then we obtained MgGa2O4:Eu3 þ and MgGa2O4:Dy3 þ phosphors. The crystallization process and phase identification of the calcined powders were studied by powder X-ray diffraction (XRD), operating with a Cu-target tube (l ¼0.15418 nm, with 40 kV and 250 mA) as radiative source and a graphite monochrometer, and the photoluminescence properties of the samples were recorded by F4600 fluorescence spectrophotometer at room temperature. A 150 W xenon lamp with light passed through a monochromator was used for excitation source. The resolution of F-4600 is 72 nm. As comparison, MgGa2O4 being absent RE ions was also prepared and characterized in the same procedure.

3. Results and discussion 3.1. XRD results The X-ray powder diffraction patterns of all samples are shown in Fig. 1. It can be seen that MgGa2O4:Eu3 þ and MgGa2O4:Dy3 þ could form stable solid state polycrystals at 1100 1C. It can be seen that the structures of RE doped gallate can be assigned to the spinel structure. No additional phases are observed. Almost diffraction

5000 311

Ga.

3000

440

511

Ga.0.02Dy Ga.0.04Dy

422

400

220

Ga.0.01Dy

222

Intensity (cps)

4000

2000 1000 0 20

40

60 2θ (degrees)

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Ga. Ga.0.01Eu

peaks can be attributed to the cubic MgGa2O4 phase (JCPDS No.780551). Due to the difference in ions radius between Ga3 þ and Eu3 þ or Dy3 þ ions, RE doped gallate hosts should replace the Ga3 þ in the lattice sites to make the crystallinity decreased as the Eu3 þ or Dy3 þ content increased [8,14]. But from the XRD results, the exist of RE ions has little influence on the crystalline structure of the phosphor. These results indicate that the RE ions enter the MgGa2O4 lattice sites [15].

3.2. Luminescent properties of MgGa2O4:Eu3 þ Fig. 2 shows the excitation spectra of the MgGa2O4:Eu3 þ phosphors monitored the emissions at 505 nm and 620 nm. It can be seen as monitoring the mission at 505 nm, there exists a broad band absorption peak at 240 nm in both MgGa2O4 and MgGa2O4:Eu3 þ , which indicates that the broad band absorption is due to the MgGa2O4 hosts. As monitoring the emission at 620 nm, there is a broad band ranging from 250 nm to 350 nm peaking at 285 nm, which is assigned to the charge transfer band (CTB) absorption from 2p orbital of O2 ions to the 4f vacant orbital of Eu3 þ ions [4]. It can be seen clearly that the peak of CTB for different Eu3 þ concentrations evidently changes, 280 nm in Ga.0.01 Eu, 258 nm in Ga.0.02 Eu and 291 nm in Ga.0.04 Eu, respectively. As known, the CTB depends on the local environments around Eu3 þ ions. The above results indicate that the local structures surround Eu3 þ ions evidently change because of the different Eu3 þ contents. The sharp lines ranging from 350 nm to 450 nm are with the direct excitation of f–f shell transitions of Eu3 þ . For different Eu3 þ concentrations, the strongest absorption locates at about 395 nm, which contributes to the 7F0-5L6 transitions in the near-UV region. The f–f transitions in the Eu3 þ , 4f6 configuration in longer spectral region with 7F0-5D4 (360 nm), 7F0-5L7 (380 nm), and 7F0-5D3 (410 nm) is also marked in Fig. 2. The activator concentration will affect the luminescent properties of the phosphor seriously, which could be seen clearly from Fig. 2, especially MgGa2O4 with the molar ratio 2% of Eu3 þ ions. The origination of the blue shift of CTB is perhaps due to that electrons are very difficult to be transferred from O2 orbital to the Ga3 þ or Gan þ ion at the strong degree of covalency of metal–O ligand bond [16]. Fig. 3 shows the emission spectra of MgGa2O4:Eu3 þ power phosphor at room temperature at 280 nm excitation. The 5D0–7FJ transitions (J¼0, 1, 2, 3, 4) ranging from 550 nm to 750 nm are observed. Among them, the 5D0–7F2 transitions (about 620 nm) are the strongest. This replies that the red emission is much stronger than the orange emission of Eu3 þ ions in MgGa2O4 phosphors. Also, we can see both MgGa2O4 and MgGa2O4:Eu3 þ having a broad band at the wavelength of about 505 nm, which is due to the emission of MgGa2O4 [17–19].

Ga.0.02Eu Ga.0.04Eu

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422

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Ga.0.04Eu Ga. Ga.0.01Eu Ga.0.02Eu

2.0 1.6 1.2 0.8

0 20

40

60 2θ (degrees)

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100

Fig. 1. XRD pattern of MgGa2O4 doped with Eu3 þ and Dy3 þ in different content.

250

300 Wavelength (nm)

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Fig. 2. Excitation spectrum of MgGa2O4:Eu3 þ power phosphor (lem ¼ 505 nm and 620 nm).

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H. Liu et al. / Journal of Physics and Chemistry of Solids 74 (2013) 196–199

12

30 Ga.0.01Eu Ga.0.02Eu Ga.0.04Eu Ga

8

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Ga0.04Dy Ga0.01Dy

25

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3 5D -7F 0 1 5D -7F 0 3

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Fig. 5. Excitation spectrum of MgGa2O4:Dy3 þ power phosphor (lem ¼ 580 nm).

Ga.0.01Eu

2

4 4 15/2- I13/2, F7/2

10

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5 D0-7F2

4 6 15/2- M15/2, P7/2

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Fig. 3. Emission spectrum of MgGa2O4:Eu3 þ power phosphor (lex ¼280 nm).

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6H

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0 550

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500 550 600 Wavelength (nm)

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Fig. 6. Emission spectrum of MgGa2O4:Dy3 þ power phosphor (lex ¼240 nm).

Fig. 4. Emission spectrum of MgGa2O4:Eu3 þ power phosphor (lex ¼ 395 nm).

Fig. 4 is the emission spectra under 395 nm excitation. The sharp and narrow peaks are expected because of the shielding effect of 4f electrons by 5s and 5p electrons in the outer shells of Eu3 þ ions [20]. A magnetic dipole transitions is unexpected at symmetry site along with a forced electric dipole transition, and the forced electric dipole transitions 5D0-7F2 is induced by the lack of inversion symmetry at the Eu3 þ site. That is to say most Eu3 þ ions do not occupy the inversion symmetry centers [5,8]. The strongest peaks are observed at about 615 nm, which due to the 5D0-7F2 transitions. The corresponding transitions of other peaks are also marked in Fig. 4. The result of emission configuration in Fig. 4 is as same as in Fig. 3. By comparing the three curve lines in the figure, we can know that the strongest relative intensity of MgGa2O4:Eu3 þ is the host doping with 0.01 M ratio of Eu3 þ . In other words, when the concentration of Eu3 þ increases, the intensity decreases instead. This result is due to the concentration quenching via exchange interaction [4,8,20]. 3.3. Optical properties of MgGa2O4:Dy3 þ Because Dy3 þ ions have blue and yellow emissions. The ratio of yellow emission to blue emissions (Y/B) is proper, white light maybe obtained. Excitation spectra of MgGa2O4:Dy3 þ power phosphor monitoring the emission at 580 nm is shown in Fig. 5. The band from 220 nm to 280 nm is perhaps caused by the host. The location of host absorption for different Dy3 þ contents evidently changes [21,22].

Fig. 6 shows the emission spectrum of MgGa2O4:Dy3 þ at 240 nm excitation. From Fig. 6, there is a broad band occurring at 430 nm, which shows weakest intensity at the concentration of 0.02. The weak band is probably due to the emission from MgGa2O4. When the samples are excited at 350 nm in Fig. 7, there are three groups of emission at 480 nm, 575 nm and 665 nm, which can be assigned to the 4F9/2-6H15/2, 4F9/2-6H13/2 and 4F9/2-6H11/2 transitions of Dy3 þ ions, respectively, corresponding to blue, yellow and red light [22–27], as shown in Fig. 7. Among the three emission peaks, the 4F9/2-6H13/2 emission belongs to hypersensitive transition which is strongly influenced by outside environments of Dy3 þ [3,22,26,28–30]. This could be explained clearly from the picture. At the wavelength of 480 nm and 665 nm, the maximum increases as the dopant of Dy3 þ decreasing, while at 575 nm, the dopant of 2% shows the strongest intensity. This phenomenon is maybe caused by concentration quenching via the cross-relaxation [22,30–32]. So, we can adjust the yellow to blue integral intensity values to produce pure white light, which shows the potentiality to be used into the field of WLEDs applications.

4. Conclusion Various concentrations Eu3 þ -doped and Dy3 þ -doped MgGa2O4 phosphors were prepared and characterized. Eu3 þ and Dy3 þ ions all could form a stable solid state with MgGa2O4. And RE ions enter the lattice site, which makes the XRD having similar patterns. The

H. Liu et al. / Journal of Physics and Chemistry of Solids 74 (2013) 196–199

7 4

F9/2->6H13/2

6

Ga0.02Dy

Intensity (a.u.)

5

Ga0.01Dy 4F

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Ga0.04Dy

6 9/2-> H15/2

4F

3

9/2->

6H

11/2

2 1 0 450

525

600 675 Wavelength (nm)

750

Fig. 7. Emission spectrum of MgGa2O4:Dy3 þ power phosphor (lex ¼ 350 nm).

phosphor doped with Eu3 þ shows a bright red emission at 625 nm,which belongs to the electric dipole transition (5D0-7F2) of Eu3 þ ions. There are three groups of emission at 480 nm, 575 nm and 665 nm occurring in the emission spectra of MgGa2O4:Dy power phosphor (lex ¼350 nm), which probably can be applied for white LEDs. Both Eu3 þ -doped and Dy3 þ -doped MgGa2O4 will show the phenomenon of concentration quenching via the exchange interaction. In other words, the intensities of phosphors decrease as the concentrations arising at a certain wavelength of excitation or emission. But it also needs more studies about the MgGa2O4 doped with RE for the white LEDs applications. References [1] C.F. Guo., D.X. Huang., Q. Su., Methods to improve the fluorescence intensity of CaS:Eu2 þ red-emitting phosphor for white LED, Mater. Sci. Eng., B 130 (2006) 189–193. [2] Q. Luo., Qiao. Xs, X.P. Fan., et al., Luminescence behavior of Ce3 þ and Dy3 þ codoped oxyfluoride glasses and glass ceramics containing LaF3 nanocrystals, J. Appl. Phys. 105 (2009) 043506. [3] M. Jayasimhadri., K.W. Jang., H.S. Lee., et al., White light generation from Dy3 þ -doped ZnO–B2O3–P2O5 glasses, J. Appl. Phys. 106 (2009) 013105. [4] C.H. Liang., Y.C. Chang., Photoluminescence properties of Eu3 þ -doped BaY2ZnO5 phosphors under near-ultraviolet irradiation, J. Mater. Res. 25 (5) (2010) 850–856. [5] C.R. Ronda., T. Justel., H. Nikol., Rare earth phosphors: fundamentals and applications, J. Alloys Compd. 275–277 (1998) 669–676. [6] D. Jia., W.M. Yen., Enhanced VK3 þ center afterglow in MgAl2O4 by doping with Ce3 þ , J. Lumin. 101 (2003) 115–121. [7] B. Yasoda., R.P. Sreekanth Chakradhar., et al., Electron paramagnetic resonance and luminescent properties of Mn2 þ : MgGa2O4 phosphor, J. Appl. Phys. 98 (2005) 053910. [8] B.S. Tsai., B.H. Chang., Y.C. Chen., Nanostructured red-emitting MgGa2O4:Eu3 þ phosphors, J. Mater. Res. 19 (5) (2004) 1504–1508.

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