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Photoluminescence studies of red-emitting Y5Mo2O12: Eu3+, Sm3+ as a near-UV convertible phosphor
Materials Letters 114 (2014) 4–6
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Photoluminescence studies of red-emitting Y5Mo2O12: Eu3 þ , Sm3 þ as a near-UV convertible phosphor Jin Ye a,b,n, Zheng Jinju c a
Department of Applied Physics, School of Optoelectronic Information, Chongqing University of Technology, 69 Hongguang Street, Chongqing 400054, China College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China c Institute of Materials, Ningbo University of Technology, Ningbo 315016, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 June 2013 Accepted 12 September 2013 Available online 27 September 2013
Rare earth doped Y5Mo2O12 has been prepared by a solid-state reaction at 650 1C. For Eu3 þ single-doped Y5Mo2O12, the luminescence intensity changes with the concentration of Eu3 þ ions, and Y5Mo2O12: 0.5Eu3 þ exhibits the most intense red-emission under near UV excitation. For the Eu3 þ , Sm3 þ co-doped system under 405 nm excitation responsible for the 6H5/2-4K11/2 transition of Sm3 þ , the f–f transitions of both Eu3 þ and Sm3 þ are observed in photoluminescence spectra and the intensities of the main emission line (5D0-7F2 transition of Eu3 þ at 612 nm) are strengthened owing to the energy transition from Sm3 þ to Eu3 þ . The energy transfer from Sm3 þ ions to Eu3 þ ions is to be proved by comparing the luminescence intensity of Eu3 þ ions single doped sample with that of Eu3 þ and Sm3 þ co-doped sample. The luminescence is improved owing to Sm3 þ ions' introduced into Y5Mo2O12: Eu3 þ and luminescence intensities are about 7 times under 272 nm excitation and about 1.5 times under 403 nm excitation. & 2013 Elsevier B.V. All rights reserved.
Keywords: Luminescence Phosphors Y5Mo2O12: Eu3 þ Sm3 þ
1. Introduction As the most challenging application to replace traditional incandescent and fluorescent lamps, white light-emitting diodes (LEDs) have particularly received much attention, which offer several advantages over conventional fluorescent lamps such as stability, energy saving, compactness, efficient light output, longer lifetime, and being environment friendly [1,2]. However, white LEDs have low color-rendering index (CRI) because of their weak emission intensity in red spectral region. As a possible solution, a separate red-emitting phosphor can be used to compensate for the red deficiency in light output. The red phosphor for LED chips is commercially still limited to Eu2 þ -activated alkaline-earth sulfide, which has poor chemical stability and low luminescence efficiency [3]. Nitride red phosphors are considered to be promising for white LEDs due to their chemical stability and high efficiency [4]. However, these materials are of high cost and tough to synthesize for application, despite the fact that a new method has been developed [5]. Eu3 þ doped molybdate has been investigated as a potential red-emitting phosphor because it exhibits more stable physical and chemical properties. The red emission is originated from 5D0–7F2 transition of Eu3 þ and the NUV excitation performs at around 395 nm through 7F0–5L6 absorption of Eu3 þ . Some investigations on enhancing the luminescence intensity of CaMoO4: Eu3 þ were reported by introducing Li þ , Na þ , n Corresponding author at: Department of Applied Physics School of Optoelectronic Information, 69 Hongguang Street, Chongqing 400054, China. Tel./fax: þ 86 236 2563 051. E-mail address: [email protected] (J. Ye).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.09.048
K þ and Bi3 þ ions into the phosphor [6–8]. In our previous work [9], Sm3 þ was added to CaMoO4:Eu3 þ to generate additional NUV excitation line at 405 nm, originating from 6H5/2–4K11/2 absorption of Sm3 þ based on the performance of energy transfer from Sm3 þ to Eu3 þ . The significance of the transfer is to extend the excitation lines in the spectral range 390–410 nm so as to take advantage of all spectral components of the NUV LED excitation source. Energy transfer from Sm3 þ to Eu3 þ was also observed in other redemitting phosphors Na0.5Sm0.1Eu0.4WO4 [10], NaEu(MoO4)2 [11] and other molybdate [12]. In this work, the luminescence properties of Y5Mo2O12:Sm3 þ , 3þ Eu was demonstrated. For Eu3 þ single-doped Y5Mo2O12, the optimal concentration of Eu3 þ ions is 10 mol%. The energy transfer from Sm3 þ ions to Eu3 þ ions is to be proved by comparing the luminescence intensity of Eu3 þ ions singly doped sample with that of Eu3 þ and Sm3 þ co-doped sample. For the Eu3 þ , Sm3 þ co-doped system under 405 nm excitation responsible for the 6H5/2-4K11/2 transition of Sm3 þ , both Eu3 þ and Sm3 þ f–f transitions are observed in the luminescence spectra and the intensities of the main emission line (5D0-7F2 transition of Eu3 þ at 612 nm) are strengthened owing to the energy transition from Sm3 þ to Eu3 þ . The luminescence is improved owing to Sm3 þ ions' introduced in Y5Mo2O12: Eu3 þ and the luminescence intensities are about 7 times under 272 nm excitation and about 1.5 times under 403 nm excitation. 2. Experimental All samples were synthesized by solid-state reactions. Raw materials were Na2MoO4 2H2O (analytical reagent, AR), Y2O3 (AR),
J. Ye, Z. Jinju / Materials Letters 114 (2014) 4–6
Sm2O3 and Eu2O3 (99.99%). Proper amounts of the raw materials were weighed and mixed in an agate mortar. The mixed powders were heated at 600 1C for 4 h. Phase purity was examined by XRD using a Rigaku Dmax 2000 X-ray diffractometer. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded at room temperature using a Horiba Jobin Yvon Fluromax-4P spectrophotometer.
3. Results and discussion In order to characterize the phase purity and crystallinity of the samples, the XRD patterns of the products were measured. As a representative for all these samples, the XRD pattern of Y5Mo2O12:0.02Sm3 þ phosphor is shown in Fig. 1. All the samples doped with low Eu3 þ concentration display the same diffraction patterns, which appeared in JCPDS card no. 78-1078, corresponding to the intrinsic diffraction patterns of Y5Mo2O12. No peaks of impure phases are observed. This fact suggests that the resultant powders are pure and of single phase. The excitation spectrum of Y5Mo2O12:Eu3 þ phosphor while monitoring at 612 nm corresponding to the 5D0-7F2 transition of Eu3 þ is shown in Fig. 2. It can be seen that the excitation spectra consist of 2 parts: one is an intense, broad band from 200 to 350 nm, and another is sharp lines from 350 to 500 nm. The broad bands in the UV region may contain the charge transfer excitation of Eu3 þ ions and the energy-transfer transition from molybdate groups to Eu3 þ ions, as shown in Fig. 2. It is assigned to the charge transfer (CT) band of Eu3 þ -O2 around 250 nm and the CT band of Mo6 þ -O2 around 310 nm [13]. In most of the literature, the contribution of the two components cannot be distinguished due to spectral overlap [14]. The sharp lines correspond to the characteristic f-f transitions of Eu3 þ ions within its 4f6 configuration.
5
They are ascribed to 7F0-5D4, 7F0-5GJ, 5L7, 7F0-5L6, 7F0-5D3, and 7 F0-5D2 transitions of Eu3 þ ion. Fig. 3 displays the emission spectrum of Y5Mo2O12 phosphors doped with different Eu3 þ ion concentrations excited under 394 nm near UV light. As can be seen, the emission spectrum essentially consists of intense and sharp lines ranging from 570 to 750 nm, which are attributed to the 5D0-7FJ (J¼ 0–4) transitions. The electric dipole allowed transition would be dominant when the Eu3 þ ion occupied the lattice site of non-centrosymmetric environment in the scheelite phases according to electronic transition selection rules [15]. So, the intensity of 5D0-7F2 (electric dipole transition) was found to be much stronger than that of 5D0-7F1 (magnetic dipole transition). The major emission of Y5Mo2O12:Eu3 þ phosphors is located at 612 nm, which is red. The doping concentration of luminescent centers is an important factor influencing the phosphor performance [16]. Therefore, it is necessary to confirm the optimum doping concentration. The emission intensities of 5D0-7FJ (J¼0–4) transitions of Y5Mo2O12 phosphors are different with various Eu3 þ ion concentrations. The intensity enhances with the increase of doping concentration and reaches a maximum at 5 mol% Eu3 þ ion doping concentration. The concentration quenching occurs at a higher concentration. Blasse et al. proposed that the quenching mechanism was associated with the exchange interaction, which results in the energy transfer and ultimately quenches the emission from the 5D0 level of the Eu3 þ ion [17]. Fig. 4 shows the emission spectra of Y5Mo2O12: 0.1Eu3 þ , 0.01Sm3 þ and Y5Mo2O12: 0.01Sm3 þ excited by 405 nm near UV light. As shown in the lower part, the characteristic emission of
Fig. 3. Emission spectra of Y5Mo2O12: xEu3 þ (x¼ 0.01, 0.03, 0.05, 0.08, 0.1) phosphors (λex ¼ 394 nm).
Fig. 1. XRD pattern of Y5Mo2O12: 0.02 Sm3 þ .
Fig. 2. Excitation spectra of Y5Mo2O12: xEu3 þ (x ¼0.01, 0.03, 0.05, 0.08, 0.1) phosphors (λem ¼ 612 nm).
Fig. 4. Emission spectra of Y5Mo2O12: 0.1Eu, 0.01Sm3 þ and Y5Mo2O12: 0.01Sm3 þ (λex ¼ 405 nm).
6
J. Ye, Z. Jinju / Materials Letters 114 (2014) 4–6
Fig. 5. Emission spectrum of Y5Mo2O12: 0.1Eu, 0.01 Sm3 þ and Y5Mo2O12: 0.1Eu (a: λex ¼ 272 nm; b: 403 nm).
Sm3 þ ions is expressed as sharp lines. For the Sm3 þ and Eu3 þ codoped samples, the most intense luminescence peak at 612 nm corresponds to the 5D0-7F2 transition of Eu3 þ ions while the Sm3 þ ions are excited by 405 nm. We infer that there is energy transfer from Sm3 þ ions to Eu3 þ ions, as reported before [10]. As shown in Fig. 5, the energy transfer from Sm3 þ to Eu3 þ ions is clear. When the co-doped sample is excited at 272 nm, the characteristic emissions of both Eu3 þ and Sm3 þ can be observed, as shown in Fig. 5a, indicating that the charge transfer state of Eu3 þ –O2 and MoO42 group can transfer energy to both of Eu3 þ and Sm3 þ . While the co-doped sample excited at 403 nm is responsible for the 6H5/2-4K11/2 transition of Sm3 þ , besides the fluorescence of Sm3 þ , the fluorescence from 5D0-7F2 transition of Eu3 þ is also observed, indicating the occurrence of energy transfer from Sm3 þ to Eu3 þ . For Eu3 þ ions single doped sample, the integrated luminescence intensity of 5D0-7F2 transition is lower than that of Eu3 þ ions in the co-doped sample. The luminescence is improved owing to Sm3 þ ions' introduced in Y5Mo2O12: Eu3 þ and the luminescence intensities are about 7 times under 272 nm excitation and about 1.5 times under 403 nm excitation. 4. Conclusions In conclusion, Eu3 þ , Sm3 þ singly doped and co-doped Y5Mo2O12 have been prepared by a solid-state reaction at 650 1C and the luminescence intensity is investigated. The photoluminescence spectra show that Y5Mo2O12:0.1Eu3 þ , 0.01Sm3 þ exhibits the most intense red-emission under near UV excitation. In the Eu3 þ , Sm3 þ co-doped system, both Eu3 þ and Sm3 þ f–f transitions are observed in the luminescence spectra, and the intensities of the main emission line (5D0-7F2 transition of Eu3 þ at 612 nm) are strengthened compared with the Eu3 þ ions single doped sample because of the energy transition from Sm3 þ to Eu3 þ . The luminescence is improved owing to Sm3 þ ions introduced in Y5Mo2O12: Eu3 þ and the luminescence intensities are about 7 times under 272 nm excitation and about 1.5 times under 403 nm excitation. With the
broadened absorption around 400 nm and strengthened emission intensity at 612 nm, the phosphor Y5Mo2O12:0.1Eu3 þ , 0.01Sm3 þ is a promising application for UV-LED.
Acknowledgment This work is financially supported by the National Nature Science Foundation of China (11104366 and 11104365), and the Natural Science Foundation Project of Chong Qing (Grant Nos. CSTC2011jjA50015 and CSTC2011BB4112). References [1] Huang CH, Chan TS, Liu WR, Wang DY, Chiu YC, Yeh YT, et al. Journal of Materials Chemistry 2012;22:20210. [2] Suehiro T, Xie RJ, Hirosaki N. Industrial and Engineering Chemistry Research 2013;52(22):7453–6. [3] Hu YS, Zhuang WD, Ye HQ, Wang DH, Zhang SS, Huang XW. Journal of Alloys and Compounds 2005;390:226. [4] Nang W., British UK Patent, GB 2373368; 2002. [5] Xie RJ, Hirosaki N. Science and Technology of Advanced Materials 2007;8:588. [6] Wang JG, Jing XP, Yan CH, Lin JH. Journal of the Electrochemical Society 2005;152:G186. [7] Liu J, Lian HZ, Shi CS. Optical Materials 2007;29:1591. [8] Yan SX, Zhang JH, Zhang X, Lü SZ, Ren XG, Nie ZG, et al. Journal of Physical Chemistry C 2007;111:13256. [9] Jin Y, Zhang JH, Lü SZ, Zhao HF, Zhang X, Wang XJ. Journal of Physical Chemistry C 2008;112:5860. [10] Holloway Jr. WW, Kestigian M. Journal of the Optical Society of America 1966;56:1171. [11] Wang Z, Liang H, Gong M, Su Q. Electrochemical and Solid-State Letters 2005;8:H33. [12] Zhang L, Zhou L, Han P, Wang L, Zhang Q. Journal of Materials Chemistry C 2013;1:54–7. [13] Tian Y, Qi X, Wu X, Hua R, Chen B. Journal of Physical Chemistry C 2009;113:10767–72. [14] Wen F, Zhao X, Huo H, Chen JS, S.L. E, Zhang JH. Materials Letters 2002;55 (3):152–7. [15] Blasse G. Journal of Chemical Physics 1969;45(7):2356–60. [16] Boyer D, Bertrand G, Mahiou R. Journal of Luminescence 2003;104(4):229–37. [17] Berdowski P, VanKeulen J, Blasse G. Journal of Solid State Chemistry 1986;63 (1):86–8.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Photoluminescence studies of red-emitting Y5Mo2O12: Eu3 þ , Sm3 þ as a near-UV convertible phosphor Jin Ye a,b,n, Zheng Jinju c a
Department of Applied Physics, School of Optoelectronic Information, Chongqing University of Technology, 69 Hongguang Street, Chongqing 400054, China College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China c Institute of Materials, Ningbo University of Technology, Ningbo 315016, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 June 2013 Accepted 12 September 2013 Available online 27 September 2013
Rare earth doped Y5Mo2O12 has been prepared by a solid-state reaction at 650 1C. For Eu3 þ single-doped Y5Mo2O12, the luminescence intensity changes with the concentration of Eu3 þ ions, and Y5Mo2O12: 0.5Eu3 þ exhibits the most intense red-emission under near UV excitation. For the Eu3 þ , Sm3 þ co-doped system under 405 nm excitation responsible for the 6H5/2-4K11/2 transition of Sm3 þ , the f–f transitions of both Eu3 þ and Sm3 þ are observed in photoluminescence spectra and the intensities of the main emission line (5D0-7F2 transition of Eu3 þ at 612 nm) are strengthened owing to the energy transition from Sm3 þ to Eu3 þ . The energy transfer from Sm3 þ ions to Eu3 þ ions is to be proved by comparing the luminescence intensity of Eu3 þ ions single doped sample with that of Eu3 þ and Sm3 þ co-doped sample. The luminescence is improved owing to Sm3 þ ions' introduced into Y5Mo2O12: Eu3 þ and luminescence intensities are about 7 times under 272 nm excitation and about 1.5 times under 403 nm excitation. & 2013 Elsevier B.V. All rights reserved.
Keywords: Luminescence Phosphors Y5Mo2O12: Eu3 þ Sm3 þ
1. Introduction As the most challenging application to replace traditional incandescent and fluorescent lamps, white light-emitting diodes (LEDs) have particularly received much attention, which offer several advantages over conventional fluorescent lamps such as stability, energy saving, compactness, efficient light output, longer lifetime, and being environment friendly [1,2]. However, white LEDs have low color-rendering index (CRI) because of their weak emission intensity in red spectral region. As a possible solution, a separate red-emitting phosphor can be used to compensate for the red deficiency in light output. The red phosphor for LED chips is commercially still limited to Eu2 þ -activated alkaline-earth sulfide, which has poor chemical stability and low luminescence efficiency [3]. Nitride red phosphors are considered to be promising for white LEDs due to their chemical stability and high efficiency [4]. However, these materials are of high cost and tough to synthesize for application, despite the fact that a new method has been developed [5]. Eu3 þ doped molybdate has been investigated as a potential red-emitting phosphor because it exhibits more stable physical and chemical properties. The red emission is originated from 5D0–7F2 transition of Eu3 þ and the NUV excitation performs at around 395 nm through 7F0–5L6 absorption of Eu3 þ . Some investigations on enhancing the luminescence intensity of CaMoO4: Eu3 þ were reported by introducing Li þ , Na þ , n Corresponding author at: Department of Applied Physics School of Optoelectronic Information, 69 Hongguang Street, Chongqing 400054, China. Tel./fax: þ 86 236 2563 051. E-mail address: [email protected] (J. Ye).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.09.048
K þ and Bi3 þ ions into the phosphor [6–8]. In our previous work [9], Sm3 þ was added to CaMoO4:Eu3 þ to generate additional NUV excitation line at 405 nm, originating from 6H5/2–4K11/2 absorption of Sm3 þ based on the performance of energy transfer from Sm3 þ to Eu3 þ . The significance of the transfer is to extend the excitation lines in the spectral range 390–410 nm so as to take advantage of all spectral components of the NUV LED excitation source. Energy transfer from Sm3 þ to Eu3 þ was also observed in other redemitting phosphors Na0.5Sm0.1Eu0.4WO4 [10], NaEu(MoO4)2 [11] and other molybdate [12]. In this work, the luminescence properties of Y5Mo2O12:Sm3 þ , 3þ Eu was demonstrated. For Eu3 þ single-doped Y5Mo2O12, the optimal concentration of Eu3 þ ions is 10 mol%. The energy transfer from Sm3 þ ions to Eu3 þ ions is to be proved by comparing the luminescence intensity of Eu3 þ ions singly doped sample with that of Eu3 þ and Sm3 þ co-doped sample. For the Eu3 þ , Sm3 þ co-doped system under 405 nm excitation responsible for the 6H5/2-4K11/2 transition of Sm3 þ , both Eu3 þ and Sm3 þ f–f transitions are observed in the luminescence spectra and the intensities of the main emission line (5D0-7F2 transition of Eu3 þ at 612 nm) are strengthened owing to the energy transition from Sm3 þ to Eu3 þ . The luminescence is improved owing to Sm3 þ ions' introduced in Y5Mo2O12: Eu3 þ and the luminescence intensities are about 7 times under 272 nm excitation and about 1.5 times under 403 nm excitation. 2. Experimental All samples were synthesized by solid-state reactions. Raw materials were Na2MoO4 2H2O (analytical reagent, AR), Y2O3 (AR),
J. Ye, Z. Jinju / Materials Letters 114 (2014) 4–6
Sm2O3 and Eu2O3 (99.99%). Proper amounts of the raw materials were weighed and mixed in an agate mortar. The mixed powders were heated at 600 1C for 4 h. Phase purity was examined by XRD using a Rigaku Dmax 2000 X-ray diffractometer. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded at room temperature using a Horiba Jobin Yvon Fluromax-4P spectrophotometer.
3. Results and discussion In order to characterize the phase purity and crystallinity of the samples, the XRD patterns of the products were measured. As a representative for all these samples, the XRD pattern of Y5Mo2O12:0.02Sm3 þ phosphor is shown in Fig. 1. All the samples doped with low Eu3 þ concentration display the same diffraction patterns, which appeared in JCPDS card no. 78-1078, corresponding to the intrinsic diffraction patterns of Y5Mo2O12. No peaks of impure phases are observed. This fact suggests that the resultant powders are pure and of single phase. The excitation spectrum of Y5Mo2O12:Eu3 þ phosphor while monitoring at 612 nm corresponding to the 5D0-7F2 transition of Eu3 þ is shown in Fig. 2. It can be seen that the excitation spectra consist of 2 parts: one is an intense, broad band from 200 to 350 nm, and another is sharp lines from 350 to 500 nm. The broad bands in the UV region may contain the charge transfer excitation of Eu3 þ ions and the energy-transfer transition from molybdate groups to Eu3 þ ions, as shown in Fig. 2. It is assigned to the charge transfer (CT) band of Eu3 þ -O2 around 250 nm and the CT band of Mo6 þ -O2 around 310 nm [13]. In most of the literature, the contribution of the two components cannot be distinguished due to spectral overlap [14]. The sharp lines correspond to the characteristic f-f transitions of Eu3 þ ions within its 4f6 configuration.
5
They are ascribed to 7F0-5D4, 7F0-5GJ, 5L7, 7F0-5L6, 7F0-5D3, and 7 F0-5D2 transitions of Eu3 þ ion. Fig. 3 displays the emission spectrum of Y5Mo2O12 phosphors doped with different Eu3 þ ion concentrations excited under 394 nm near UV light. As can be seen, the emission spectrum essentially consists of intense and sharp lines ranging from 570 to 750 nm, which are attributed to the 5D0-7FJ (J¼ 0–4) transitions. The electric dipole allowed transition would be dominant when the Eu3 þ ion occupied the lattice site of non-centrosymmetric environment in the scheelite phases according to electronic transition selection rules [15]. So, the intensity of 5D0-7F2 (electric dipole transition) was found to be much stronger than that of 5D0-7F1 (magnetic dipole transition). The major emission of Y5Mo2O12:Eu3 þ phosphors is located at 612 nm, which is red. The doping concentration of luminescent centers is an important factor influencing the phosphor performance [16]. Therefore, it is necessary to confirm the optimum doping concentration. The emission intensities of 5D0-7FJ (J¼0–4) transitions of Y5Mo2O12 phosphors are different with various Eu3 þ ion concentrations. The intensity enhances with the increase of doping concentration and reaches a maximum at 5 mol% Eu3 þ ion doping concentration. The concentration quenching occurs at a higher concentration. Blasse et al. proposed that the quenching mechanism was associated with the exchange interaction, which results in the energy transfer and ultimately quenches the emission from the 5D0 level of the Eu3 þ ion [17]. Fig. 4 shows the emission spectra of Y5Mo2O12: 0.1Eu3 þ , 0.01Sm3 þ and Y5Mo2O12: 0.01Sm3 þ excited by 405 nm near UV light. As shown in the lower part, the characteristic emission of
Fig. 3. Emission spectra of Y5Mo2O12: xEu3 þ (x¼ 0.01, 0.03, 0.05, 0.08, 0.1) phosphors (λex ¼ 394 nm).
Fig. 1. XRD pattern of Y5Mo2O12: 0.02 Sm3 þ .
Fig. 2. Excitation spectra of Y5Mo2O12: xEu3 þ (x ¼0.01, 0.03, 0.05, 0.08, 0.1) phosphors (λem ¼ 612 nm).
Fig. 4. Emission spectra of Y5Mo2O12: 0.1Eu, 0.01Sm3 þ and Y5Mo2O12: 0.01Sm3 þ (λex ¼ 405 nm).
6
J. Ye, Z. Jinju / Materials Letters 114 (2014) 4–6
Fig. 5. Emission spectrum of Y5Mo2O12: 0.1Eu, 0.01 Sm3 þ and Y5Mo2O12: 0.1Eu (a: λex ¼ 272 nm; b: 403 nm).
Sm3 þ ions is expressed as sharp lines. For the Sm3 þ and Eu3 þ codoped samples, the most intense luminescence peak at 612 nm corresponds to the 5D0-7F2 transition of Eu3 þ ions while the Sm3 þ ions are excited by 405 nm. We infer that there is energy transfer from Sm3 þ ions to Eu3 þ ions, as reported before [10]. As shown in Fig. 5, the energy transfer from Sm3 þ to Eu3 þ ions is clear. When the co-doped sample is excited at 272 nm, the characteristic emissions of both Eu3 þ and Sm3 þ can be observed, as shown in Fig. 5a, indicating that the charge transfer state of Eu3 þ –O2 and MoO42 group can transfer energy to both of Eu3 þ and Sm3 þ . While the co-doped sample excited at 403 nm is responsible for the 6H5/2-4K11/2 transition of Sm3 þ , besides the fluorescence of Sm3 þ , the fluorescence from 5D0-7F2 transition of Eu3 þ is also observed, indicating the occurrence of energy transfer from Sm3 þ to Eu3 þ . For Eu3 þ ions single doped sample, the integrated luminescence intensity of 5D0-7F2 transition is lower than that of Eu3 þ ions in the co-doped sample. The luminescence is improved owing to Sm3 þ ions' introduced in Y5Mo2O12: Eu3 þ and the luminescence intensities are about 7 times under 272 nm excitation and about 1.5 times under 403 nm excitation. 4. Conclusions In conclusion, Eu3 þ , Sm3 þ singly doped and co-doped Y5Mo2O12 have been prepared by a solid-state reaction at 650 1C and the luminescence intensity is investigated. The photoluminescence spectra show that Y5Mo2O12:0.1Eu3 þ , 0.01Sm3 þ exhibits the most intense red-emission under near UV excitation. In the Eu3 þ , Sm3 þ co-doped system, both Eu3 þ and Sm3 þ f–f transitions are observed in the luminescence spectra, and the intensities of the main emission line (5D0-7F2 transition of Eu3 þ at 612 nm) are strengthened compared with the Eu3 þ ions single doped sample because of the energy transition from Sm3 þ to Eu3 þ . The luminescence is improved owing to Sm3 þ ions introduced in Y5Mo2O12: Eu3 þ and the luminescence intensities are about 7 times under 272 nm excitation and about 1.5 times under 403 nm excitation. With the
broadened absorption around 400 nm and strengthened emission intensity at 612 nm, the phosphor Y5Mo2O12:0.1Eu3 þ , 0.01Sm3 þ is a promising application for UV-LED.
Acknowledgment This work is financially supported by the National Nature Science Foundation of China (11104366 and 11104365), and the Natural Science Foundation Project of Chong Qing (Grant Nos. CSTC2011jjA50015 and CSTC2011BB4112). References [1] Huang CH, Chan TS, Liu WR, Wang DY, Chiu YC, Yeh YT, et al. Journal of Materials Chemistry 2012;22:20210. [2] Suehiro T, Xie RJ, Hirosaki N. Industrial and Engineering Chemistry Research 2013;52(22):7453–6. [3] Hu YS, Zhuang WD, Ye HQ, Wang DH, Zhang SS, Huang XW. Journal of Alloys and Compounds 2005;390:226. [4] Nang W., British UK Patent, GB 2373368; 2002. [5] Xie RJ, Hirosaki N. Science and Technology of Advanced Materials 2007;8:588. [6] Wang JG, Jing XP, Yan CH, Lin JH. Journal of the Electrochemical Society 2005;152:G186. [7] Liu J, Lian HZ, Shi CS. Optical Materials 2007;29:1591. [8] Yan SX, Zhang JH, Zhang X, Lü SZ, Ren XG, Nie ZG, et al. Journal of Physical Chemistry C 2007;111:13256. [9] Jin Y, Zhang JH, Lü SZ, Zhao HF, Zhang X, Wang XJ. Journal of Physical Chemistry C 2008;112:5860. [10] Holloway Jr. WW, Kestigian M. Journal of the Optical Society of America 1966;56:1171. [11] Wang Z, Liang H, Gong M, Su Q. Electrochemical and Solid-State Letters 2005;8:H33. [12] Zhang L, Zhou L, Han P, Wang L, Zhang Q. Journal of Materials Chemistry C 2013;1:54–7. [13] Tian Y, Qi X, Wu X, Hua R, Chen B. Journal of Physical Chemistry C 2009;113:10767–72. [14] Wen F, Zhao X, Huo H, Chen JS, S.L. E, Zhang JH. Materials Letters 2002;55 (3):152–7. [15] Blasse G. Journal of Chemical Physics 1969;45(7):2356–60. [16] Boyer D, Bertrand G, Mahiou R. Journal of Luminescence 2003;104(4):229–37. [17] Berdowski P, VanKeulen J, Blasse G. Journal of Solid State Chemistry 1986;63 (1):86–8.