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Structure and luminescence of Dy3+ doped CaO–B2O3–SiO2 glasses
Author’s Accepted Manuscript Structure and luminescence of Dy3+ doped CaOB2O3-SiO2 glasses Yan Hao, Ju Cao
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S0921-4526(16)30137-5 http://dx.doi.org/10.1016/j.physb.2016.04.017 PHYSB309439
To appear in: Physica B: Physics of Condensed Matter Received date: 13 January 2016 Revised date: 16 March 2016 Accepted date: 15 April 2016 Cite this article as: Yan Hao and Ju Cao, Structure and luminescence of Dy 3+ doped CaO-B2O3-SiO2 glasses, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2016.04.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structure and luminescence of Dy3+ doped CaO-B2O3-SiO2 glasses Yan Hao, Ju Cao State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang, 621010, China
Abstract The present work reports structure and luminescence of Dy3+ doped CaOB2O3-SiO2 glasses prepared by melt quenching technique. The presence of various stretching and bending vibrations of different borate and silicate groups were identified from FTIR spectral measurements. The optical absorption and luminescence spectra were also measured, and their emission spectra exhibit two intense emission bands at around 485 nm (blue) and 577 nm (yellow) corresponds to 4
4
F9/2→6H15/2 and
F9/2→6H13/2 transitions, respectively. The emission spectra were characterized through
CIE 1931 color chromaticity diagram to explore its suitability for W-LED applications. Furthermore, the proper Y2O3 could change local structure of glass, which makes the UV absorption edge shift to longer wavelength, and it’s easier to transfer energy from host to Dy3+ and then enhance the emission of Dy3+.
Key words: Dy3+, structure, glass, luminescence
1. Introduction Light Emitting Diodes (LEDs) are found to be better alternative to the argon mercury discharge fluorescent lamps for general lighting purpose [1]. Especially, White-Light Emitting Diodes (W-LEDs) make remarkable breakthrough in the field of
solid state lighting technology over the usual incandescent and fluorescent lamps for general lighting purpose due to their unique properties like low power consumption, high brightness, longer lifetime, compactness and excellent low-temperature performance [2]. Recent years, Trivalent Rare Earth (RE3+) ion doped glasses are found to be a bright future applying for W-LEDs due to their excellent homogeneity, variety of form and size, easy to fabricate and high thermal stability [3]. Among the rare earth ions, the Dy3+ ion is prominent for W-LEDs applications due to its two intense emission bands 470-500 nm (4F9/2→6H15/2) and 570-600 nm (4F9/2→6H13/2), which are approximately white in total. Therefore, Dy3+ doped glasses have received many attentions recently [4-7]. CaO-B2O3-SiO2 glass has the advantages of uniform, transparent, good stability, easy preparation, low temperature and preparation process. Zhang et al [8-10] reported Ce3+/Tb3+/Sm3+, Ce3+/Tb3+/Eu3+ doped CaO-B2O3-SiO2 glasses and achieved white light by adjusting the concentration of rare earth ions, and they think CaO-B2O3-SiO2 glass is an ideal matrix for rare earth ion doped luminescent materials. Liu [11] also reported CaO-B2O3-SiO2 glass ceramics doped with Dy3+ was prepared through sol-gel process, and its chromaticity coordinates was right in white-light regions. In this paper, rare earth oxide Y2O3 was induced to CaO-B2O3-SiO2 glasses doped with Dy3+, and the effect of Y’s and Dy’s concentration on the glass structure and luminescent properties were studied.
2. Experimental All glass samples were prepared by high-temperature melting method, and the chemical compositions of glasses list in Table 1. High purity CaO, H3BO3, SiO2, Y2O3
and Dy2O3 were used as starting materials. In each batch about 10 g raw materials were mixed homogeneously and then melted at 1200 ℃ for 2h in an electric furnace. The melt was then slowly cooled to room temperature and the transparent glasses were obtained. Then glass samples were ground to powders for easy to measuring their properties. The obtained glasses were identified by X-ray diffraction (XRD) measurements using Ni-filtered Cu Kα radiation. The infrared spectrum was measured by Spectrum One Fourier transform infrared absorption spectrometer. The optical absorption spectra were measured by Shimadzu UV-3150 UV-VIS-NIR spectrophotometer. The excitation and emission spectra were measured by PerkinElmer LS 55 fluorescence spectrophotometer. All measurements were carried out at room temperature.
3. Results and discussion The FTIR spectra of all samples are shown in Fig.1, different Y2O3 content (Fig.1 (a)) and different Dy2O3 content (Fig.1 (b)). The spectra exhibit five main absorption bands, B-O stretching of [BO3] units in 1600-1200 cm-1, vibration of boron-oxygen rings in 1300-1200 cm-1, Si-O stretching of [SiO4] units and B-O stretching of [BO4] units in 1200-780 cm-1, B-O-B bending of [BO3] units in 780-640 cm-1 and Si-O-Si bending in 540-400 cm-1 [12], and the position of all absorption peaks and their attributions list in Table 2. It’s clear that there is no new absorption peaks appeared when Y2O3 is introduced to this system and with the increase of Y2O3 the vibration of boron–oxygen rings at about 1235 cm-1 becomes stronger and the absorption peak at about 1047 cm-1 shifts to shorter wavenumbers (988 cm-1). Based on glass related theory, Y2O3 acts as the network modifier and can be attracted by borate network, and
provides free oxygen to [BO3] clusters, and then form [BO4] clusters, which are negatively charged and can’t connect each other in direct. Therefore, [BO4] clusters can connect [BO3] clusters and form boron–oxygen rings, which are accordance with enhancement of the vibration of boron-oxygen rings in 1300-1200 cm-1. It was reported that [SiO4] tetrahedron (Qn) with different bridging oxygen number n had different wavenumbers in the range of 800-1200 cm-1, Q4, Q3, Q2, Q1 and Q0 vibration wavenumbers were in 1200, 1100, 1000, 900 and 850cm-1, respectively, and cationic type, temperature and pressure was not at the same condition would cause a little differences in the wavenumbers [13]. Furthermore, the vibration wavenumbers at about 1089 and 975 cm-1 are due to the absorptions of Q3 and Q2 cluster, and it is evident that with the increase of Y2O3 the absorptions of Q2 cluster enhance greatly and make the absorption band shift to shorter wavenumbers. Accordingly, it can be concluded that the increase of Y2O3 would destroy bridge oxygen and cause the number of non-bridging oxygen (NBO) increased. While with the increase of Dy2O3, the position of each absorption peak has no evident change and the vibrations of boron-oxygen rings in 1300-1200 cm-1 become weaker. The optical absorption spectra of the Dy3+ doped glasses are shown in Fig.2, different Y2O3 content (Fig.2 (a)) and different Dy2O3 content (Fig.2 (b)). The spectra exhibit 12 absorption bands in the UV–VIS–NIR region located at 223, 325, 348, 366, 390, 447, 477, 748, 799, 891, 1078, 1257 and 1672 nm originates due to the host and the electric dipole (ED) transition from the 6H15/2 ground state to the various excited states of Dy3+ such as 4M17/2, 6P7/2, 6P5/2, 4I13/2, 4I15/2, 4F9/2, 6F3/2, 6F7/2, 6F5/2, 6F9/2 (6H7/2), 6
F11/2 (6H9/2) and 6H11/2, respectively. The absorption band of Dy3+ at around 1257 nm
corresponds to the hypersensitive transition (6H15/2→6F11/2, 6H7/2) which shows higher
intensity compared to the other transitions of Dy3+ and obey the selection rules │∆S│= 0,│∆L│≤ 2 and │∆J│≤ 2 [14]. It’s clear that the absorptions of the host and Dy3+ for sample Y10 were enhanced evidently. According to the results of FTIR spectra, Y2O3 could destroy bridge oxygen and cause the number of non-bridging oxygen increased. J. Krogh-Mogh [15] reported the ultraviolet (UV) absorption edge of pure oxide glass was corresponding to the valence electron transition of the oxygen ions in the glass structure, and it was determined by the chemical bond of network. When Y2O3 was introduced into the glass network, the chemical bond of network decreased because of bridge oxygen rupturing and forming NBO and its electronic excitation energy decreased. Generally weaken this chemical bonds makes the UV absorption edge shift to longer wavelength. According to Fig.2, the main position of host absorption had no evident change, but its right edge from 280 nm to 350 nm became higher, which is due to more NBO in glass structure. It means that proper Y2O3 could change local structure of glass, which makes the host UV absorptions edge shift to longer wavelength, and it is closer to Dy’s absorption. For the samples with different Dy3+ concentration, there is no evident change of the absorption position and sample Dy2.5 has the biggest absorption coefficience. The excitation and emission spectra of all samples are shown in Fig. 3, different Y2O3 content (Fig.3 (a)) and different Dy2O3 content (Fig.3 (b)). The excitation wavelength (λex) for the emission spectra and the monitoring wavelength (λem) for the excitation spectra are also provided. There are nine peaks observed in the excitation spectra. The excitation peaks at about 290 nm and 300 nm are attributed to the host according to the absorption spectra (Fig.2). The other excitation peaks represent the transitions of Dy3+ from 6H15/2 to 4M17/2 (317 nm), 6P7/2 (348 nm), 6P5/2
(363 nm), 4I13/2 (386 nm), 4G11/2 (426nm), 4I15/2 (450 nm), and 4F9/2 (475 nm), respectively. It is clear that the excitation peak at 300 nm of sample Y10 becomes evident. This is because that the chemical bond of network decreased when the proper Y2O3 was introduced into the glass network, which makes the UV absorption edge shift to longer wavelength. According to Dexter theory [16], it can be concluded that there exists an effective energy transfer process from host to Dy3+ from the overlap among the excitation band from 280 to 330 nm observed in Fig.3. The three emission bands centered at 485, 576 and 632 nm in the emission spectra originate from the transitions of 4F9/2→6H15/2, 4F9/2→6H13/2 and 4F9/2→6H11/2, respectively. It is evident that sample Y10 shows the highest luminescence intensity, which is accordance with the result of excitation spectra. The optimum Dy2O3 concentration is 2.5%, and after that the luminescence intensity decreases sharply, and this is due to concentration quenching effect, which is caused by a non-radioactive energy transfer process between the neighboring Dy3+ ions when the Dy3+ concentration reaches at a critical value. [17] And Fig.4 shows the energy level diagram along with absorption, excitation and emission transitions in this glass. Based on the theory of colorimetry, we can easily deduce the chromaticity coordinates for all samples from their emission spectra, as shown in Fig. 3 (c) and (d). It can be seen that the color coordinates of all samples lie in the white light region and are nearer to the equilibrium energy points (x=0.33 and y=0.33) located at the center of the white light region in the CIE 1931 color chromaticity diagram. This illustrates the fact that, the studied glasses are most appropriate for white LED applications.
4. Conclusions In summary, we have investigated the structure and luminescence of Dy3+ doped CaO-B2O3-SiO2 glasses, and the influence of Y2O3 to this glass was also studied. It was identified that the increase of Y2O3 would destroy bridge oxygen and cause the number of non-bridging oxygen (NBO) increased, and proper Y2O3 makes the host UV absorption edge shift to longer wavelength and it is closer to Dy3+ absorption, which is easier to transfer energy from host to Dy3+, and then enhances its emission. When Y2O3 is 10% and Dy2O3 is 2.5%, the glass has the optimum luminescence intensity.
Acknowledgements This work was supported by Project for Science and Technology for Sichuan Province (14ZA0102) and Project for Science and technology innovation team of Southwest University of Science and Technology (14tdfk01).
References [1] D. Balaji, K. Kavirasu, A. Durairajan, S. Moorthy Babu, Photoluminescence properties of novel Sm3+ and Dy3+ co-activated CsGd(WO4)2 phosphors, J. Alloys Compd. 637 (2015) 350–360. [2] R. Vijayakumar, G.Venkataiah, K.Marimuthu, White light simulation and luminescence studies on Dy3+ doped Zinc borophosphate glasses, Physica B 457 (2015) 287–295. [3] Utegulov, Zhandos Nurpeisovich, 2003 Oklahoma State University Structural characterization of rare-earth doped soda magnesia alumina silica glasses for holographic storage: Brillouin, Raman and NMR spectroscopy studies.
[4] Selvi S., Venkataiah G., Arunkumar S., Muralidharan G., Marimuthu K., Structural and luminescence studies on Dy3+ doped lead boro-telluro-phosphate glasses, Physica B 454 (2014) 72–81. [5] Zhu CF , Wang J, Zhang MM, Ren XR, Shen JX, Yue YZ, Eu-, Tb-, and DyDoped Oxyfluoride Silicate Glasses for LED Applications, J. Am. Ceram. Soc. 97 (2014) 854-861. [6] R. Vijayakumar, G. Venkataiah, K. Marimuthu, Structural and luminescence studies on Dy3+ doped boro-phosphate glasses for white LED's and laser applications, J. Alloys Compd. 652 (2015) 234–243. [7] Anjaiah G, Rasool SKN, Kistaiah P, Spectroscopic and visible luminescence properties of rare earth ions in lead fluoroborate glasses, J. Lumin. 159 (2015) 110-118. [8] Yanbin Zhang, Zhenfeng Zhu, Yinpo Qiao. Luminescence properties of Ce3+/ Tb3+/Eu3+ triply-doped CaO-B2O3-SiO2 glasses for white light emitting diodes, Mater. Lett. 93 (2013) 9-11. [9] Zhenfeng Zhu, Yanbin Zhang, Yinpo Qiao, Hui Liu, Dianguang Liu. Luminescence properties of Ce3+/Tb3+/Sm3+ co-doped CaO-SiO2-B2O3 glasses for white light emitting diodes, J. Lumin. 134 (2013) 724-728. [10] Zhenfeng Zhu, Yanbin Zhang, Yinpo Qiao, Dianguang Liu, Bingqing Wang, Zhichun Zhang. Full color and tunable white emitting in ternary Ce/Tb/Sm co-doped CaO-B2O3-SiO2 glasses, J. Non. Cryst. Solids 358 12-13 (2012) 1550-1553. [11] Shixiang Liu, Xingyun Li, Xianglong Yu, Zhidong Chang, Ping Che, Ji Zhou, Wenjun Li, A route for white LED package using luminescent low-temperature co-fired ceramics J. Alloys Compd. 655 (2016) 203–207. [12] Yin Cheng, Hanning Xiao, Chen Shuguang, Bingzhong Tang, Structure and
crystallization of B2O3–Al2O3–SiO2 glasses, Physica B 404 (2009) 1230. [13] Wang Mitang，Cheng Jinshu，Li Mei，He Feng, Effect of Y2O3 Dopant on Structure and Viscosity of Silicate Glass and Melt, J. Chin. Ceramic Soc. 41 1 (2013) 115-121. [14] Sk.Mahamuda, K.Swapna, P.Packiyaraj, A.SrinivasaRao, G.Vijaya Prakash, Lasing potentialities and white light generation capabilities of Dy3+ doped oxy-fluoroborate glasses, J. Lumin., 153 (2014) 382–392. [15] J. Krogh-Mogh, The structure of vitreous and liquid boron oxide, J. Non-Cryst. Solids, 1 (1969) 269-284. [16] D.L. Dexter, Theory of Concentration Quenching in Inorganic Phosphors, J. Chem. Phys. 22 (1954) 1063. [17] Z.P.Ci, Q.S.Sun, S.C.Qin, M.X.Sun, X.J.Jiang, X.D.Zhang, Y.H.Wang, Warm white light generation from a single phase Dy3+ doped Mg2Al4Si5O18 phosphor for white UV-LEDs, Phys. Chem. Chem. Phys. 16 (2014) 11597–11602.
Figure Caption Fig.1 FTIR spectra of Dy3+ doped glasses Fig.2 Optical absorption spectra of Dy3+ doped glasses Fig.3 Excitation, Emission spectra and CIE color chromaticity diagram of Dy3+ doped glasses Fig.4 Energy level diagrams of Dy3+ (b)
Q
3
Q
Relative Intensity (a.u.)
Relative Intensity (a.u.)
(a)
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Dy1.0 Dy1.5 Dy2.0 Dy2.5 Dy3.0
Y0 Y10 Y20 Y30
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Wavenumbers (cm )
Fig.1
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host
-1
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4
M17/2
0.04
6
6
P7/2
P5/2
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I13/2
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6 4
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I15/2
F3/2
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375
F5/2 F7/2 6F ,6H 9/2 7/2
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Absorption Coefficience (cm )
Absorption Coefficience (cm )
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Dy1.0 Dy1.5 Dy2.0 Dy2.5 Dy3.0
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F5/2 6F
7/2
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6
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ex= 348 nm
Relative Intensity (a.u.)
em= 485 nm
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G11/2 I15/2 F9/2
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em= 485nm
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ex=348 nm
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Fig.3
Fig.4
Table 1. Composition of CaO-B2O3- SiO2- Dy2O3 glasses (wt %) Table 2. Vibration types of the main absorption bands in samples
Table 1. sample
CaO
B2O3
SiO2
Y2O3
Dy2O3
Y0 Y10 Y20 Y30 Dy1.0 Dy1.5 Dy2.0 Dy2.5 Dy3.0
20 20 20 20 20 20 20 20 20
48 42 36 30 42 42 42 42 42
32 28 24 20 28 28 28 28 28
0 10 20 30 10 10 10 10 10
1.5 1.5 1.5 1.5 1.0 1.5 2.0 2.5 3.0
Table 2. Absorption bands(cm-1) 460-470 600-850 900-1080 1080 1200-1300 1200-1600
Vibration types Bending vibration Si-O-Si Bending vibration of B–O–B in [BO3] triangle, Shrink vibration of [AlO4] Stretching vibration of B–O–B in [BO4] tetrahedron Antisymmetric stretching vibration of Si-O-Si Vibration of boron–oxygen rings Stretching vibration of B–O–B in [BO3] triangles
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PII: DOI: Reference:
S0921-4526(16)30137-5 http://dx.doi.org/10.1016/j.physb.2016.04.017 PHYSB309439
To appear in: Physica B: Physics of Condensed Matter Received date: 13 January 2016 Revised date: 16 March 2016 Accepted date: 15 April 2016 Cite this article as: Yan Hao and Ju Cao, Structure and luminescence of Dy 3+ doped CaO-B2O3-SiO2 glasses, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2016.04.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structure and luminescence of Dy3+ doped CaO-B2O3-SiO2 glasses Yan Hao, Ju Cao State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang, 621010, China
Abstract The present work reports structure and luminescence of Dy3+ doped CaOB2O3-SiO2 glasses prepared by melt quenching technique. The presence of various stretching and bending vibrations of different borate and silicate groups were identified from FTIR spectral measurements. The optical absorption and luminescence spectra were also measured, and their emission spectra exhibit two intense emission bands at around 485 nm (blue) and 577 nm (yellow) corresponds to 4
4
F9/2→6H15/2 and
F9/2→6H13/2 transitions, respectively. The emission spectra were characterized through
CIE 1931 color chromaticity diagram to explore its suitability for W-LED applications. Furthermore, the proper Y2O3 could change local structure of glass, which makes the UV absorption edge shift to longer wavelength, and it’s easier to transfer energy from host to Dy3+ and then enhance the emission of Dy3+.
Key words: Dy3+, structure, glass, luminescence
1. Introduction Light Emitting Diodes (LEDs) are found to be better alternative to the argon mercury discharge fluorescent lamps for general lighting purpose [1]. Especially, White-Light Emitting Diodes (W-LEDs) make remarkable breakthrough in the field of
solid state lighting technology over the usual incandescent and fluorescent lamps for general lighting purpose due to their unique properties like low power consumption, high brightness, longer lifetime, compactness and excellent low-temperature performance [2]. Recent years, Trivalent Rare Earth (RE3+) ion doped glasses are found to be a bright future applying for W-LEDs due to their excellent homogeneity, variety of form and size, easy to fabricate and high thermal stability [3]. Among the rare earth ions, the Dy3+ ion is prominent for W-LEDs applications due to its two intense emission bands 470-500 nm (4F9/2→6H15/2) and 570-600 nm (4F9/2→6H13/2), which are approximately white in total. Therefore, Dy3+ doped glasses have received many attentions recently [4-7]. CaO-B2O3-SiO2 glass has the advantages of uniform, transparent, good stability, easy preparation, low temperature and preparation process. Zhang et al [8-10] reported Ce3+/Tb3+/Sm3+, Ce3+/Tb3+/Eu3+ doped CaO-B2O3-SiO2 glasses and achieved white light by adjusting the concentration of rare earth ions, and they think CaO-B2O3-SiO2 glass is an ideal matrix for rare earth ion doped luminescent materials. Liu [11] also reported CaO-B2O3-SiO2 glass ceramics doped with Dy3+ was prepared through sol-gel process, and its chromaticity coordinates was right in white-light regions. In this paper, rare earth oxide Y2O3 was induced to CaO-B2O3-SiO2 glasses doped with Dy3+, and the effect of Y’s and Dy’s concentration on the glass structure and luminescent properties were studied.
2. Experimental All glass samples were prepared by high-temperature melting method, and the chemical compositions of glasses list in Table 1. High purity CaO, H3BO3, SiO2, Y2O3
and Dy2O3 were used as starting materials. In each batch about 10 g raw materials were mixed homogeneously and then melted at 1200 ℃ for 2h in an electric furnace. The melt was then slowly cooled to room temperature and the transparent glasses were obtained. Then glass samples were ground to powders for easy to measuring their properties. The obtained glasses were identified by X-ray diffraction (XRD) measurements using Ni-filtered Cu Kα radiation. The infrared spectrum was measured by Spectrum One Fourier transform infrared absorption spectrometer. The optical absorption spectra were measured by Shimadzu UV-3150 UV-VIS-NIR spectrophotometer. The excitation and emission spectra were measured by PerkinElmer LS 55 fluorescence spectrophotometer. All measurements were carried out at room temperature.
3. Results and discussion The FTIR spectra of all samples are shown in Fig.1, different Y2O3 content (Fig.1 (a)) and different Dy2O3 content (Fig.1 (b)). The spectra exhibit five main absorption bands, B-O stretching of [BO3] units in 1600-1200 cm-1, vibration of boron-oxygen rings in 1300-1200 cm-1, Si-O stretching of [SiO4] units and B-O stretching of [BO4] units in 1200-780 cm-1, B-O-B bending of [BO3] units in 780-640 cm-1 and Si-O-Si bending in 540-400 cm-1 [12], and the position of all absorption peaks and their attributions list in Table 2. It’s clear that there is no new absorption peaks appeared when Y2O3 is introduced to this system and with the increase of Y2O3 the vibration of boron–oxygen rings at about 1235 cm-1 becomes stronger and the absorption peak at about 1047 cm-1 shifts to shorter wavenumbers (988 cm-1). Based on glass related theory, Y2O3 acts as the network modifier and can be attracted by borate network, and
provides free oxygen to [BO3] clusters, and then form [BO4] clusters, which are negatively charged and can’t connect each other in direct. Therefore, [BO4] clusters can connect [BO3] clusters and form boron–oxygen rings, which are accordance with enhancement of the vibration of boron-oxygen rings in 1300-1200 cm-1. It was reported that [SiO4] tetrahedron (Qn) with different bridging oxygen number n had different wavenumbers in the range of 800-1200 cm-1, Q4, Q3, Q2, Q1 and Q0 vibration wavenumbers were in 1200, 1100, 1000, 900 and 850cm-1, respectively, and cationic type, temperature and pressure was not at the same condition would cause a little differences in the wavenumbers [13]. Furthermore, the vibration wavenumbers at about 1089 and 975 cm-1 are due to the absorptions of Q3 and Q2 cluster, and it is evident that with the increase of Y2O3 the absorptions of Q2 cluster enhance greatly and make the absorption band shift to shorter wavenumbers. Accordingly, it can be concluded that the increase of Y2O3 would destroy bridge oxygen and cause the number of non-bridging oxygen (NBO) increased. While with the increase of Dy2O3, the position of each absorption peak has no evident change and the vibrations of boron-oxygen rings in 1300-1200 cm-1 become weaker. The optical absorption spectra of the Dy3+ doped glasses are shown in Fig.2, different Y2O3 content (Fig.2 (a)) and different Dy2O3 content (Fig.2 (b)). The spectra exhibit 12 absorption bands in the UV–VIS–NIR region located at 223, 325, 348, 366, 390, 447, 477, 748, 799, 891, 1078, 1257 and 1672 nm originates due to the host and the electric dipole (ED) transition from the 6H15/2 ground state to the various excited states of Dy3+ such as 4M17/2, 6P7/2, 6P5/2, 4I13/2, 4I15/2, 4F9/2, 6F3/2, 6F7/2, 6F5/2, 6F9/2 (6H7/2), 6
F11/2 (6H9/2) and 6H11/2, respectively. The absorption band of Dy3+ at around 1257 nm
corresponds to the hypersensitive transition (6H15/2→6F11/2, 6H7/2) which shows higher
intensity compared to the other transitions of Dy3+ and obey the selection rules │∆S│= 0,│∆L│≤ 2 and │∆J│≤ 2 [14]. It’s clear that the absorptions of the host and Dy3+ for sample Y10 were enhanced evidently. According to the results of FTIR spectra, Y2O3 could destroy bridge oxygen and cause the number of non-bridging oxygen increased. J. Krogh-Mogh [15] reported the ultraviolet (UV) absorption edge of pure oxide glass was corresponding to the valence electron transition of the oxygen ions in the glass structure, and it was determined by the chemical bond of network. When Y2O3 was introduced into the glass network, the chemical bond of network decreased because of bridge oxygen rupturing and forming NBO and its electronic excitation energy decreased. Generally weaken this chemical bonds makes the UV absorption edge shift to longer wavelength. According to Fig.2, the main position of host absorption had no evident change, but its right edge from 280 nm to 350 nm became higher, which is due to more NBO in glass structure. It means that proper Y2O3 could change local structure of glass, which makes the host UV absorptions edge shift to longer wavelength, and it is closer to Dy’s absorption. For the samples with different Dy3+ concentration, there is no evident change of the absorption position and sample Dy2.5 has the biggest absorption coefficience. The excitation and emission spectra of all samples are shown in Fig. 3, different Y2O3 content (Fig.3 (a)) and different Dy2O3 content (Fig.3 (b)). The excitation wavelength (λex) for the emission spectra and the monitoring wavelength (λem) for the excitation spectra are also provided. There are nine peaks observed in the excitation spectra. The excitation peaks at about 290 nm and 300 nm are attributed to the host according to the absorption spectra (Fig.2). The other excitation peaks represent the transitions of Dy3+ from 6H15/2 to 4M17/2 (317 nm), 6P7/2 (348 nm), 6P5/2
(363 nm), 4I13/2 (386 nm), 4G11/2 (426nm), 4I15/2 (450 nm), and 4F9/2 (475 nm), respectively. It is clear that the excitation peak at 300 nm of sample Y10 becomes evident. This is because that the chemical bond of network decreased when the proper Y2O3 was introduced into the glass network, which makes the UV absorption edge shift to longer wavelength. According to Dexter theory [16], it can be concluded that there exists an effective energy transfer process from host to Dy3+ from the overlap among the excitation band from 280 to 330 nm observed in Fig.3. The three emission bands centered at 485, 576 and 632 nm in the emission spectra originate from the transitions of 4F9/2→6H15/2, 4F9/2→6H13/2 and 4F9/2→6H11/2, respectively. It is evident that sample Y10 shows the highest luminescence intensity, which is accordance with the result of excitation spectra. The optimum Dy2O3 concentration is 2.5%, and after that the luminescence intensity decreases sharply, and this is due to concentration quenching effect, which is caused by a non-radioactive energy transfer process between the neighboring Dy3+ ions when the Dy3+ concentration reaches at a critical value. [17] And Fig.4 shows the energy level diagram along with absorption, excitation and emission transitions in this glass. Based on the theory of colorimetry, we can easily deduce the chromaticity coordinates for all samples from their emission spectra, as shown in Fig. 3 (c) and (d). It can be seen that the color coordinates of all samples lie in the white light region and are nearer to the equilibrium energy points (x=0.33 and y=0.33) located at the center of the white light region in the CIE 1931 color chromaticity diagram. This illustrates the fact that, the studied glasses are most appropriate for white LED applications.
4. Conclusions In summary, we have investigated the structure and luminescence of Dy3+ doped CaO-B2O3-SiO2 glasses, and the influence of Y2O3 to this glass was also studied. It was identified that the increase of Y2O3 would destroy bridge oxygen and cause the number of non-bridging oxygen (NBO) increased, and proper Y2O3 makes the host UV absorption edge shift to longer wavelength and it is closer to Dy3+ absorption, which is easier to transfer energy from host to Dy3+, and then enhances its emission. When Y2O3 is 10% and Dy2O3 is 2.5%, the glass has the optimum luminescence intensity.
Acknowledgements This work was supported by Project for Science and Technology for Sichuan Province (14ZA0102) and Project for Science and technology innovation team of Southwest University of Science and Technology (14tdfk01).
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Figure Caption Fig.1 FTIR spectra of Dy3+ doped glasses Fig.2 Optical absorption spectra of Dy3+ doped glasses Fig.3 Excitation, Emission spectra and CIE color chromaticity diagram of Dy3+ doped glasses Fig.4 Energy level diagrams of Dy3+ (b)
Q
3
Q
Relative Intensity (a.u.)
Relative Intensity (a.u.)
(a)
2
Dy1.0 Dy1.5 Dy2.0 Dy2.5 Dy3.0
Y0 Y10 Y20 Y30
2000
1800
1600
1400
1200
1000
800
600
400
2000
1800
1600
-1
1400
1200
1000
800
-1
600
Wavenumbers (cm )
Wavenumbers (cm )
Fig.1
0.25
host
-1
0.15
0.10
0.06
4
M17/2
0.04
6
6
P7/2
P5/2
4
I13/2
0.02
0.00
275
300
325
350
(nm) 6
0.05
6 4
6
I15/2
F3/2
6
375
F5/2 F7/2 6F ,6H 9/2 7/2
400
425
(b)
0.10
-1
Absorption Coefficience (cm )
Absorption Coefficience (cm )
0.20
-1
Y0 Y10 Y20 Y30
0.08
Absorption Coefficience (cm )
0.25
(a)
-1
Absorption Coefficience (cm )
host
0.20
0.15
0.06
4
P7/2
P5/2
4
I13/2
0.00
0.10
250
275
300
325
350
375
400
425
450
(nm)
4
I15/24F9/2
0.05
H11/2
6 6
0.02
6 6
M17/2
0.04
6
F11/2, H9/2
Dy1.0 Dy1.5 Dy2.0 Dy2.5 Dy3.0
0.08
6
F3/2
F5/2 6F
7/2
6
6
F11/2, H9/2 6 F9/2, H7/2
6
4
F9/2
6
H11/2
0.00
0.00 200
400
600
800
1000
1200
(nm)
1400
1600
1800
2000
200
Fig.2
400
600
800
(nm) 1200 1000
1400
1600
1800
2000
4
(a)
H15/2
host 4
M17/2 6
P7/2 4
6
I13/2
P5/2
4
4
Y0 Y10 Y20 Y30
6
ex= 348 nm
Relative Intensity (a.u.)
em= 485 nm
Relative Intensity (a.u.)
F9/2
Y0 Y10 Y20 Y30
6
H13/2
6
H11/2
4
G11/2 I15/2 F9/2
300
350
400
450
(nm)
450
4
Dy1.0 Dy1.5 Dy2.0 Dy2.5 Dy3.0
em= 485nm
host 4
M17/2
6
P7/2
4
6
P5/2
300
350
I13/2
4
4 G11/2I15/2 F9/2
4
400
(nm)
450
500
F9/2 6
H15/2
550
(nm)
500 450
600
650
Dy1.0 Dy1.5 Dy2.0 Dy2.5 Dy3.0
ex=348 nm
Relative Intensity (a.u.)
(b)
Relative Intensity (a.u.)
500
6
H13/2
6
500
550
(nm)
600
H11/2
650
Fig.3
Fig.4
Table 1. Composition of CaO-B2O3- SiO2- Dy2O3 glasses (wt %) Table 2. Vibration types of the main absorption bands in samples
Table 1. sample
CaO
B2O3
SiO2
Y2O3
Dy2O3
Y0 Y10 Y20 Y30 Dy1.0 Dy1.5 Dy2.0 Dy2.5 Dy3.0
20 20 20 20 20 20 20 20 20
48 42 36 30 42 42 42 42 42
32 28 24 20 28 28 28 28 28
0 10 20 30 10 10 10 10 10
1.5 1.5 1.5 1.5 1.0 1.5 2.0 2.5 3.0
Table 2. Absorption bands(cm-1) 460-470 600-850 900-1080 1080 1200-1300 1200-1600
Vibration types Bending vibration Si-O-Si Bending vibration of B–O–B in [BO3] triangle, Shrink vibration of [AlO4] Stretching vibration of B–O–B in [BO4] tetrahedron Antisymmetric stretching vibration of Si-O-Si Vibration of boron–oxygen rings Stretching vibration of B–O–B in [BO3] triangles