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A yellow emitting phosphor Dy:Bi4Si3O12 crystal for LED application
Materials Letters 135 (2014) 176–179
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A yellow emitting phosphor Dy:Bi4Si3O12 crystal for LED application Bobo Yang, Jiayue Xu n, Yan Zhang, Yaoqing Chu, Meiling Wang, Yuxian Wen Institute of Crystal Growth, School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 April 2014 Accepted 25 July 2014 Available online 4 August 2014
Dy-doped bismuth silicate (Bi4Si3O12, BSO) crystals were grown by the modified vertical Bridgman method. The luminescence properties for light emitting diode (LED) were investigated. Efficient energy transition from Bi ions to Dy ions has been established by photoluminescence investigation upon UV excitation and results in blue, yellow and red emissions at about 487 nm, 574 nm and 662 nm respectively. Photoluminescence intensity varies with excitation wavelength and the doping concentration of Dy3 þ , and Chromaticity coordinates does not changes distinctly. The samples exhibit stable yellow light by varying the relative concentration of Dy3 þ and even when the excitation wavelengths changes from UV to n-UV. These results indicate that Dy:Bi4Si3O12 is a highly yellow emitting crystal with potential application in yellow LEDs. & 2014 Elsevier B.V. All rights reserved.
Keywords: Crystal growth Luminescence Optical materials and properties Dy:Bi4Si3O12 LED
1. Introduction As a potential candidate for replacing conventional incandescent and fluorescent lamps, light emitting diodes (LEDs) has attracted intensive attention in recent years due to the advantages of long lifetime, saving energy, high efficiency, reliability, and its environmental-friendly characteristics [1,2]. During the past few years, more and more cities have begun replacing traditional streetlights (typically high pressure sodium bulbs) with newer light-emitting diode bulbs (LEDs). Unfortunately, exposure to the light of blue-rich or ‘white’ LED bulbs can disrupt natural circadian rhythms in humans and wildlife [3]. Therefore, it is necessary to use yellow rich LED streetlights. In addition, Yellow LED is widely used in architectural lighting, stage lighting and decorative lighting. As white-LED is assembled by combining a blue chip with yellow-emitting phosphor [4,5], yellow LED is also can be assembled by fixing the yellow emitting phosphor onto the UVLED chip. Significant recent works had been reported in advancing the deep UV and mid UV emitter technology by using high Alcontent AlGaN [6–10] and AlInN [11,12] alloys. A brief discussion on this emitter technology is important to indicate the rapid progress in deep and mid UV emitter technology [6–12], which enables the practical implementation of UV-LEDs. The powder phosphors for LED applications are usually embedded in epoxy resins. However, there are many drawbacks in this type of LEDs. For instance, there is a large difference in refractive indices of the phosphors. The heat resistance of the epoxy resins is usually poor.
n
Corresponding author. E-mail address: [email protected] (J. Xu).
http://dx.doi.org/10.1016/j.matlet.2014.07.167 0167-577X/& 2014 Elsevier B.V. All rights reserved.
In addition, the refractive index mismatch between the phosphors and the epoxy results in light scattering [13]. Compared with phosphors, single crystals exhibit good anti-light irradiation, as well as good thermal, mechanical, and chemical stability. The rigid cyclic symmetric structure of single crystals results in the high luminous efficiency of active ions. The excellent thermal, mechanical, chemical, and optical properties of single crystals are beneficial for LED applications to obtain high stability, long lifetime, high luminous efficiency, and good color. However, only a few reports have focused on rare earth-doped single crystals for LED. The visible luminescence of Dy3 þ (4f9) ion mainly consists of two intense bands in the blue (470–500 nm) and yellow (570– 600 nm) regions, which are associated with the 4F9/2–6H15/2 and 4 F9/2–6H13/2 transitions, respectively. The Dy3 þ ions as the luminescence center, have attracted much attention because of its light emission [14–16]. Bismuth silicate (Bi4Si3O12, BSO) has emerged as an excellent scintillation material to be employed in Nuclear and high-energy physics experiments [17–19]. Superior properties, such as high hardness, large specific heat, small thermal expansion, high optical damage threshold, and high optical transmittance, render BSO crystal the potential to function as a phosphor material for LED. Bi3 þ in BSO shows a broad band-like absorption property. And Bi3 þ ions are very good sensitizers of luminescence, which can efficiently absorb the UV light and transfer the energy to the luminescence center [20,21]. In addition, the cost of growing BSO crystals is much lower than growing YAG crystals as the lower cost of feed materials and lower melting point. Our group has studied the growth and properties of pure BSO and rare earth doped BSO crystals for several years, large and high quality crystals could be grown by the modified vertical Bridgman method [22–26]. In this letter, we report the growth of Dy3 þ -doped
B. Yang et al. / Materials Letters 135 (2014) 176–179
Bi4Si3O12 single crystal by Bridgman method and demonstrate the capability of generating yellow light under excitation at UV light.
2. Experimental High purity Dy2O3 (44N), SiO2 (44N) and Bi2O3 (45N) were used as starting materials for Dy-doped BSO single crystals growth. These starting materials were weighed according to stoichiometric equation ratio of formula (Bi1 xDyx)4Si3O12 (x¼0.005, 0.01, 0.015, 0.02) and mixed in a ball mill coated with polyethylene for 8 h. The mixture was then held at 750 1C for 8 h. After sintering, these chemicals were ground to powder and mixed again in the ball mill for 3 h. The samples were then put into the aluminum oxide crucible and held at 850 1C for 12 h to prepare Dy3 þ :Bi4Si3O12 polycrystalline materials. The feed materials were then loaded into the Pt crucible to grow single crystal in Bridgman furnace. In this work, The furnace temperature was kept at 1075 1C (50 1C higher than the melting point of BSO) to ensure complete melting. The crystal growth rate was 0.2–0.5 mm/h. The orientation of the seed crystal was chosen to be (0 0 1) in all our growth runs. Dy3 þ :Bi4Si3O12 crystals ϕ25 mm 100 mm size were successfully grown. The X-ray powder diffraction of Dy3 þ :Bi4Si3O12 crystal was collected with crushed clear crystals, using a D/max-2200 PC type diffractometer (Rigaku Co. Ltd., Tokyo,
177
Japan) at room temperature. After the confirmation of the obtained phase, grown single crystals were cut along the growth axis and polished into wafers of 2 mm thickness for the measurements of optical properties. The room temperature excitation and emission spectra of the samples were carried out with Edinburgh Instruments FLS920 spectrophotometer, using Xenon lamp as a light source.
3. Results and discussion The XRD pattern of the Dy3 þ -doped BSO samples is shown in Fig. 1(a). Based on the PDF#35–1007, the XRD spectrum shows that the diffraction peaks and relative intensity of the crystal samples are very similar to those of BSO. This result indicates that doping with Dy3 þ has a negligible effect on diffraction data. Thus, the crystals have a pure orthorhombic phase, and the current doping level does not cause any obvious peak shift or second phase. The rare earth ion dopant Dy3 þ substitutionally enters the Bi3 þ sites. Since the ionic radius of Dy3 þ ( 0.0912 nm) is slightly smaller than that of Bi3 þ ( 0.103 nm), lattice constants of Dy3 þ : Bi4Si3O12 samples became smaller with the increase of Dy concentration as shown in Fig. 1(b). Fig. 2 shows photoluminescence emission and excitation spectra of Dy-doped BSO crystal samples at room temperature. From
Fig. 1. (a) Powder X-ray diffraction patterns of the Dy doped BSO crystals; (b) Lattice constants of the grown Dy:BSO single crystals plotted against Dy content.
Fig. 2. (a) Emission spectra and (b) excitation spectra of the Dy-doped BSO crystals.
178
B. Yang et al. / Materials Letters 135 (2014) 176–179
Fig. 3. (a) Emission spectrum (inset is the partial energy level diagram) and (b) CIE chromatic coordinate diagram of 1.0 mol% Dy-doped BSO crystals at different excitation wavelengths (288, 323, 348, 363, 390 nm) (inset is the polished sample under UV light).
the fluorescence spectra under the excitation of 288 nm, we can find that there are three emission peaks located at 487 nm, 574 nm and 662 nm bands for the Dy-doped BSO crystals, corresponding to the transitions of 4F9/2-6H15/2, 4F9/2-6H13/2 and 4F9/2-6H11/2, respectively. While there is only one broad emission band peaking at about 480 nm for the undoped BSO sample. The intrinsic emission peak of BSO located at 480 nm is weakened and the characteristic emission peaks of Dy3 þ ions located at 487 nm, 574 nm and 662 nm are enhanced as the concentration of Dy3 þ is increased. It is observed that the excitation spectrum monitored at 574 nm of Dy-doped BSO crystal samples at room temperature consists of a broad band in a wavelength range between about 220 nm and 310 nm corresponding to oxygen to bismuth charge transfer band. Above about 290 nm, which corresponds to the band gap of BSO, all the excitation peaks can be assigned to the 4f 4f transitions of Dy3 þ . According to the phenomenon shown in the PL and PLE spectra, the doping concentration quenching occurs when addition of Dy3 þ reaches more than 1 mol% due to Dy cross relaxation processes. The emission spectra of the 1 mol% Dy3 þ -doped BSO crystal under excitation at 288, 323, 348, 365 and 390 nm were measured. Fig. 3(a) shows that the intensity of the emission spectra of the crystal excited under 288 nm is much stronger than excited under other light. It is indicated that the energy absorbed by Bi atoms then is transferred to Dy. The emission mechanism of BSO: Dy3 þ is depicted in the partial energy level diagram shown in the inset of Fig. 3(a). The CIE chromaticity coordinates for the different concentration of the Dy3þ -doped BSO crystal under 288-nm excitation were calculated using the corresponding emission spectrum in Fig. 2 and are presented in Table 1. Upon excitation at 288 nm, all the samples exhibit yellow light. Therefore, emission color remains barely unchanged by varying the relative concentration of Dy3 þ based on the excitation and emission spectra, as well as CIE chromaticity coordinates. In this work, the CIE chromaticity coordinates of the 1 mol% Dy3þ -doped BSO crystal under excitation at 288, 323, 348, 365 and 390 nm were illustrated in Fig. 3(b). The coordinates are calculated using the corresponding emission spectrum in Fig. 3(a). As can be seen from Fig. 3(b), the emission color does not changes distinctly, it also exhibit stable yellow light when the excitation wavelengths changes from 288 nm to 390 nm. The results presented show that doping of BSO with Dy3þ results in a highly yellow emitting crystal.
Table 1 Chromaticity coordinates of Dy:BSO crystals at different concentrations (λex ¼288 nm). Concentration of Dy3 þ (%)
x-coordinate
y-coordinate
0.5 1 1.5 2
0.4218 0.4324 0.4287 0.4334
0.4414 0.4485 0.445 0.4481
4. Conclusions In summary, Dy3 þ -doped Bi4Si3O12 single crystals were grown by Bridgman method. The intrinsic emission peak of BSO located at 480 nm is weakened and the characteristic emission peaks of Dy3 þ ions located at 487 nm, 574 nm and 662 nm are enhanced as the concentration of Dy3 þ is increased. The doping concentration quenching occurs when addition of Dy3 þ reaches more than 1 mol% due to Dy cross relaxation processes. The Dy3 þ -doped Bi4Si3O12 crystals exhibit stable yellow light when the excitation wavelengths changes from UV to n-UV. The results presented reveal the potential application of Dy3 þ -doped Bi4Si3O12 crystal as a yellow emitting phosphor in yellow light generation. Acknowledgments This work was supported by the National Natural Science Foundation of China (51342007), the National Key Basic Research Program (2011CB612310), the Shanghai Municipal Science and Technology Commission (11JC1412400). References [1] Schubert EF, Kim JK. Solid-state light sources getting smart. Science 2005;308:1274. [2] Kimura N, Sakuma K, Hirafune S, Asano K, Hiroski N, Xie RJ. Extrahigh color rendering white light-emitting diode lamps using oxynitride and nitride phosphors excited by blue light-emitting diode. Appl Phys Lett 2007;90: 051109. [3] Fabio F, Pierantonio C, Christopher DE, David MK, Abraham H. Limiting the impact of light pollution on human health, environment and stellar visibility. J Environ Manag 2011;92(10):2714.
B. Yang et al. / Materials Letters 135 (2014) 176–179
[4] Sheu JK, Chang SJ, Kuo CH, Su YK, Wu LW, Lin YC, et al. White-light emission from near UV InGaN-GaN LED chip precoated with blue/green/red phosphors. IEEE Photon Technol Lett 2003;15:18. [5] Yukio N, Junya N, Takahiko S, Kouichiro D, Takao Y, Takashi M. Ultra-high efficiency white light emitting diodes. Jpn J Appl Phys 2006;45:L1084–6. [6] Zhang J, Zhao HP, Tansu N. Effect of crystal-field split-off hole and heavy-hole bands crossover on gain characteristics of high Al-content AlGaN quantum well lasers. Appl Phys Lett 2010;97:111105. [7] Zhang J, Zhao HP, Tansu N. Large optical gain AlGaN-delta-GaN quantum wells laser active regions in mid- and deep-ultraviolet spectral regimes. Appl Phys Lett 2011;98:171111. [8] Taniyasu Y, Kasu M. Polarization property of deep-ultraviolet light emission from C-plane AlN/GaN short-period superlattices. Appl Phys Lett 2011;99: 251112. [9] Pecora EF, Zhang W, Nikiforov AY, Zhou L, Smith DJ, Yin J. Sub-250 nm roomtemperature optical gain from AlGaN/AlN multiple quantum wells with strong band-structure potential fluctuations. Appl Phys Lett 2012;100:061111. [10] Zhang J, Tansu N. Engineering of AlGaN-delta-GaN quantum-well gain media for mid- and deep-ultraviolet lasers. IEEE Photon J 2013;5:2600209. [11] Liu G, Zhang J, Li XH, Huang GS, Paskova T, Evans KR, et al. Metalorganic vapor phase epitaxy and characterizations of nearly-lattice-matched AlInN alloys on GaN/sapphire templates and free-standing GaN substrates. J Crys Growth 2012;340:66–73. [12] Chung RB, Wu F, Shivaraman R, Keller S, DenBaars SP, Speck JS, et al. Growth study and impurity characterization of AlxIn1 xN grown by metal organic chemical vapor deposition. J Crys Growth 2011;324:163–7. [13] Cui ZG, Jia GH, Deng DG, Hua YJ, Zhao SL, Huang LH. Synthesis and luminescence properties of glass ceramics containing MSiO3:Eu2 þ (M¼Ca, Sr, Ba) phosphors for white LED. J Lumin 2012;132:153–60. [14] Klimczak M, Malinowski M, Sarnecki J, Piramidowicz R. Luminescence properties in the visible of Dy:YAG/YAG planar waveguides. J Lumin 2009;129: 1869–73.
179
[15] Sun XY, Huang SM, Gong XS, Gao QC, Ye ZP, Cao CY. Spectroscopic properties and simulation of white-light in Dy3 þ -doped silicate glass. J Non-Cryst Solids 2010;356:98–101. [16] Xu XH, He QL, Yan LT. White-light long persistent and photo-stimulated luminescence in CaSnSiO5:Dy3 þ . J Alloys Compd 2013;574:22–6. [17] Shimizu H, Miyahara F, Hariu H, Hayakawa T, Ishikawa T, Itaya M, et al. First beam test on a BSO electromagnetic calorimeter. Nucl Instrum Methods Phys Res A 2005;550:258–66. [18] Ishikawa T, Fujimura H, Hashimoto R, Kaida S, Kasagi J, Kitazawa R, et al. A detailed test of a BSO calorimeter with 100–800 MeV positrons. Nucl Instrum Methods Phys Res A 2012;694:348–60. [19] Akchurin N, Alwarawrah M, Cardini A, Ciapetti G, Ferrari R, Franchino S, et al. Dual-readout calorimetry with crystal calorimeters. Nucl Instrum Methods Phys Res A 2009;598(3):710–21. [20] Liu XM, Lin J. Enhanced luminescence of gadolinium niobates by Bi3 þ doping for field emission displays. J Lumin 2007;122–123:700. [21] Xiao XZ, Yan B. Hybrid precursors synthesis and optical properties of LnNbO4: Bi3 þ blue phosphors and Bi3 þ sensitizing of on Dy3 þ 's luminescence in YNbO4 matrix. J Alloys Compd 2006;421:252–7. [22] Zhang Y, Xu JY, Lu BL. Spectroscopic properties of Dy3 þ : Bi4Si3O12 single crystal. J Alloys Compd 2014;582:635–9. [23] Xu JY, Wang H, He QB, Shen H, Shimizu HJ, Xiang WD. Bridgman growth of Bi4Si3O12 scintillation crystals. J Chin Ceram Soc 2009;37:295–8. [24] Zhang Y, Xu JY, He QB, Lu BL. Bridgman growth and characterization of Bi4(GexSi1 x)3O12 mixed crystals. J Cryst Growth 2013;362:121–4. [25] Zhang Y, Xu JY, Shao PF. Growth and spectroscopic properties of Yb:BSO single crystal. J Cryst Growth 2011;318:920–3. [26] Shen H, Xu JY, Ping WJ, He QB, Zhang Y, Jin M, et al. Mechanical and thermal properties of Bi4Si3O12 single crystals. Chin Phys Lett 2012;29:076501.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A yellow emitting phosphor Dy:Bi4Si3O12 crystal for LED application Bobo Yang, Jiayue Xu n, Yan Zhang, Yaoqing Chu, Meiling Wang, Yuxian Wen Institute of Crystal Growth, School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 April 2014 Accepted 25 July 2014 Available online 4 August 2014
Dy-doped bismuth silicate (Bi4Si3O12, BSO) crystals were grown by the modified vertical Bridgman method. The luminescence properties for light emitting diode (LED) were investigated. Efficient energy transition from Bi ions to Dy ions has been established by photoluminescence investigation upon UV excitation and results in blue, yellow and red emissions at about 487 nm, 574 nm and 662 nm respectively. Photoluminescence intensity varies with excitation wavelength and the doping concentration of Dy3 þ , and Chromaticity coordinates does not changes distinctly. The samples exhibit stable yellow light by varying the relative concentration of Dy3 þ and even when the excitation wavelengths changes from UV to n-UV. These results indicate that Dy:Bi4Si3O12 is a highly yellow emitting crystal with potential application in yellow LEDs. & 2014 Elsevier B.V. All rights reserved.
Keywords: Crystal growth Luminescence Optical materials and properties Dy:Bi4Si3O12 LED
1. Introduction As a potential candidate for replacing conventional incandescent and fluorescent lamps, light emitting diodes (LEDs) has attracted intensive attention in recent years due to the advantages of long lifetime, saving energy, high efficiency, reliability, and its environmental-friendly characteristics [1,2]. During the past few years, more and more cities have begun replacing traditional streetlights (typically high pressure sodium bulbs) with newer light-emitting diode bulbs (LEDs). Unfortunately, exposure to the light of blue-rich or ‘white’ LED bulbs can disrupt natural circadian rhythms in humans and wildlife [3]. Therefore, it is necessary to use yellow rich LED streetlights. In addition, Yellow LED is widely used in architectural lighting, stage lighting and decorative lighting. As white-LED is assembled by combining a blue chip with yellow-emitting phosphor [4,5], yellow LED is also can be assembled by fixing the yellow emitting phosphor onto the UVLED chip. Significant recent works had been reported in advancing the deep UV and mid UV emitter technology by using high Alcontent AlGaN [6–10] and AlInN [11,12] alloys. A brief discussion on this emitter technology is important to indicate the rapid progress in deep and mid UV emitter technology [6–12], which enables the practical implementation of UV-LEDs. The powder phosphors for LED applications are usually embedded in epoxy resins. However, there are many drawbacks in this type of LEDs. For instance, there is a large difference in refractive indices of the phosphors. The heat resistance of the epoxy resins is usually poor.
n
Corresponding author. E-mail address: [email protected] (J. Xu).
http://dx.doi.org/10.1016/j.matlet.2014.07.167 0167-577X/& 2014 Elsevier B.V. All rights reserved.
In addition, the refractive index mismatch between the phosphors and the epoxy results in light scattering [13]. Compared with phosphors, single crystals exhibit good anti-light irradiation, as well as good thermal, mechanical, and chemical stability. The rigid cyclic symmetric structure of single crystals results in the high luminous efficiency of active ions. The excellent thermal, mechanical, chemical, and optical properties of single crystals are beneficial for LED applications to obtain high stability, long lifetime, high luminous efficiency, and good color. However, only a few reports have focused on rare earth-doped single crystals for LED. The visible luminescence of Dy3 þ (4f9) ion mainly consists of two intense bands in the blue (470–500 nm) and yellow (570– 600 nm) regions, which are associated with the 4F9/2–6H15/2 and 4 F9/2–6H13/2 transitions, respectively. The Dy3 þ ions as the luminescence center, have attracted much attention because of its light emission [14–16]. Bismuth silicate (Bi4Si3O12, BSO) has emerged as an excellent scintillation material to be employed in Nuclear and high-energy physics experiments [17–19]. Superior properties, such as high hardness, large specific heat, small thermal expansion, high optical damage threshold, and high optical transmittance, render BSO crystal the potential to function as a phosphor material for LED. Bi3 þ in BSO shows a broad band-like absorption property. And Bi3 þ ions are very good sensitizers of luminescence, which can efficiently absorb the UV light and transfer the energy to the luminescence center [20,21]. In addition, the cost of growing BSO crystals is much lower than growing YAG crystals as the lower cost of feed materials and lower melting point. Our group has studied the growth and properties of pure BSO and rare earth doped BSO crystals for several years, large and high quality crystals could be grown by the modified vertical Bridgman method [22–26]. In this letter, we report the growth of Dy3 þ -doped
B. Yang et al. / Materials Letters 135 (2014) 176–179
Bi4Si3O12 single crystal by Bridgman method and demonstrate the capability of generating yellow light under excitation at UV light.
2. Experimental High purity Dy2O3 (44N), SiO2 (44N) and Bi2O3 (45N) were used as starting materials for Dy-doped BSO single crystals growth. These starting materials were weighed according to stoichiometric equation ratio of formula (Bi1 xDyx)4Si3O12 (x¼0.005, 0.01, 0.015, 0.02) and mixed in a ball mill coated with polyethylene for 8 h. The mixture was then held at 750 1C for 8 h. After sintering, these chemicals were ground to powder and mixed again in the ball mill for 3 h. The samples were then put into the aluminum oxide crucible and held at 850 1C for 12 h to prepare Dy3 þ :Bi4Si3O12 polycrystalline materials. The feed materials were then loaded into the Pt crucible to grow single crystal in Bridgman furnace. In this work, The furnace temperature was kept at 1075 1C (50 1C higher than the melting point of BSO) to ensure complete melting. The crystal growth rate was 0.2–0.5 mm/h. The orientation of the seed crystal was chosen to be (0 0 1) in all our growth runs. Dy3 þ :Bi4Si3O12 crystals ϕ25 mm 100 mm size were successfully grown. The X-ray powder diffraction of Dy3 þ :Bi4Si3O12 crystal was collected with crushed clear crystals, using a D/max-2200 PC type diffractometer (Rigaku Co. Ltd., Tokyo,
177
Japan) at room temperature. After the confirmation of the obtained phase, grown single crystals were cut along the growth axis and polished into wafers of 2 mm thickness for the measurements of optical properties. The room temperature excitation and emission spectra of the samples were carried out with Edinburgh Instruments FLS920 spectrophotometer, using Xenon lamp as a light source.
3. Results and discussion The XRD pattern of the Dy3 þ -doped BSO samples is shown in Fig. 1(a). Based on the PDF#35–1007, the XRD spectrum shows that the diffraction peaks and relative intensity of the crystal samples are very similar to those of BSO. This result indicates that doping with Dy3 þ has a negligible effect on diffraction data. Thus, the crystals have a pure orthorhombic phase, and the current doping level does not cause any obvious peak shift or second phase. The rare earth ion dopant Dy3 þ substitutionally enters the Bi3 þ sites. Since the ionic radius of Dy3 þ ( 0.0912 nm) is slightly smaller than that of Bi3 þ ( 0.103 nm), lattice constants of Dy3 þ : Bi4Si3O12 samples became smaller with the increase of Dy concentration as shown in Fig. 1(b). Fig. 2 shows photoluminescence emission and excitation spectra of Dy-doped BSO crystal samples at room temperature. From
Fig. 1. (a) Powder X-ray diffraction patterns of the Dy doped BSO crystals; (b) Lattice constants of the grown Dy:BSO single crystals plotted against Dy content.
Fig. 2. (a) Emission spectra and (b) excitation spectra of the Dy-doped BSO crystals.
178
B. Yang et al. / Materials Letters 135 (2014) 176–179
Fig. 3. (a) Emission spectrum (inset is the partial energy level diagram) and (b) CIE chromatic coordinate diagram of 1.0 mol% Dy-doped BSO crystals at different excitation wavelengths (288, 323, 348, 363, 390 nm) (inset is the polished sample under UV light).
the fluorescence spectra under the excitation of 288 nm, we can find that there are three emission peaks located at 487 nm, 574 nm and 662 nm bands for the Dy-doped BSO crystals, corresponding to the transitions of 4F9/2-6H15/2, 4F9/2-6H13/2 and 4F9/2-6H11/2, respectively. While there is only one broad emission band peaking at about 480 nm for the undoped BSO sample. The intrinsic emission peak of BSO located at 480 nm is weakened and the characteristic emission peaks of Dy3 þ ions located at 487 nm, 574 nm and 662 nm are enhanced as the concentration of Dy3 þ is increased. It is observed that the excitation spectrum monitored at 574 nm of Dy-doped BSO crystal samples at room temperature consists of a broad band in a wavelength range between about 220 nm and 310 nm corresponding to oxygen to bismuth charge transfer band. Above about 290 nm, which corresponds to the band gap of BSO, all the excitation peaks can be assigned to the 4f 4f transitions of Dy3 þ . According to the phenomenon shown in the PL and PLE spectra, the doping concentration quenching occurs when addition of Dy3 þ reaches more than 1 mol% due to Dy cross relaxation processes. The emission spectra of the 1 mol% Dy3 þ -doped BSO crystal under excitation at 288, 323, 348, 365 and 390 nm were measured. Fig. 3(a) shows that the intensity of the emission spectra of the crystal excited under 288 nm is much stronger than excited under other light. It is indicated that the energy absorbed by Bi atoms then is transferred to Dy. The emission mechanism of BSO: Dy3 þ is depicted in the partial energy level diagram shown in the inset of Fig. 3(a). The CIE chromaticity coordinates for the different concentration of the Dy3þ -doped BSO crystal under 288-nm excitation were calculated using the corresponding emission spectrum in Fig. 2 and are presented in Table 1. Upon excitation at 288 nm, all the samples exhibit yellow light. Therefore, emission color remains barely unchanged by varying the relative concentration of Dy3 þ based on the excitation and emission spectra, as well as CIE chromaticity coordinates. In this work, the CIE chromaticity coordinates of the 1 mol% Dy3þ -doped BSO crystal under excitation at 288, 323, 348, 365 and 390 nm were illustrated in Fig. 3(b). The coordinates are calculated using the corresponding emission spectrum in Fig. 3(a). As can be seen from Fig. 3(b), the emission color does not changes distinctly, it also exhibit stable yellow light when the excitation wavelengths changes from 288 nm to 390 nm. The results presented show that doping of BSO with Dy3þ results in a highly yellow emitting crystal.
Table 1 Chromaticity coordinates of Dy:BSO crystals at different concentrations (λex ¼288 nm). Concentration of Dy3 þ (%)
x-coordinate
y-coordinate
0.5 1 1.5 2
0.4218 0.4324 0.4287 0.4334
0.4414 0.4485 0.445 0.4481
4. Conclusions In summary, Dy3 þ -doped Bi4Si3O12 single crystals were grown by Bridgman method. The intrinsic emission peak of BSO located at 480 nm is weakened and the characteristic emission peaks of Dy3 þ ions located at 487 nm, 574 nm and 662 nm are enhanced as the concentration of Dy3 þ is increased. The doping concentration quenching occurs when addition of Dy3 þ reaches more than 1 mol% due to Dy cross relaxation processes. The Dy3 þ -doped Bi4Si3O12 crystals exhibit stable yellow light when the excitation wavelengths changes from UV to n-UV. The results presented reveal the potential application of Dy3 þ -doped Bi4Si3O12 crystal as a yellow emitting phosphor in yellow light generation. Acknowledgments This work was supported by the National Natural Science Foundation of China (51342007), the National Key Basic Research Program (2011CB612310), the Shanghai Municipal Science and Technology Commission (11JC1412400). References [1] Schubert EF, Kim JK. Solid-state light sources getting smart. Science 2005;308:1274. [2] Kimura N, Sakuma K, Hirafune S, Asano K, Hiroski N, Xie RJ. Extrahigh color rendering white light-emitting diode lamps using oxynitride and nitride phosphors excited by blue light-emitting diode. Appl Phys Lett 2007;90: 051109. [3] Fabio F, Pierantonio C, Christopher DE, David MK, Abraham H. Limiting the impact of light pollution on human health, environment and stellar visibility. J Environ Manag 2011;92(10):2714.
B. Yang et al. / Materials Letters 135 (2014) 176–179
[4] Sheu JK, Chang SJ, Kuo CH, Su YK, Wu LW, Lin YC, et al. White-light emission from near UV InGaN-GaN LED chip precoated with blue/green/red phosphors. IEEE Photon Technol Lett 2003;15:18. [5] Yukio N, Junya N, Takahiko S, Kouichiro D, Takao Y, Takashi M. Ultra-high efficiency white light emitting diodes. Jpn J Appl Phys 2006;45:L1084–6. [6] Zhang J, Zhao HP, Tansu N. Effect of crystal-field split-off hole and heavy-hole bands crossover on gain characteristics of high Al-content AlGaN quantum well lasers. Appl Phys Lett 2010;97:111105. [7] Zhang J, Zhao HP, Tansu N. Large optical gain AlGaN-delta-GaN quantum wells laser active regions in mid- and deep-ultraviolet spectral regimes. Appl Phys Lett 2011;98:171111. [8] Taniyasu Y, Kasu M. Polarization property of deep-ultraviolet light emission from C-plane AlN/GaN short-period superlattices. Appl Phys Lett 2011;99: 251112. [9] Pecora EF, Zhang W, Nikiforov AY, Zhou L, Smith DJ, Yin J. Sub-250 nm roomtemperature optical gain from AlGaN/AlN multiple quantum wells with strong band-structure potential fluctuations. Appl Phys Lett 2012;100:061111. [10] Zhang J, Tansu N. Engineering of AlGaN-delta-GaN quantum-well gain media for mid- and deep-ultraviolet lasers. IEEE Photon J 2013;5:2600209. [11] Liu G, Zhang J, Li XH, Huang GS, Paskova T, Evans KR, et al. Metalorganic vapor phase epitaxy and characterizations of nearly-lattice-matched AlInN alloys on GaN/sapphire templates and free-standing GaN substrates. J Crys Growth 2012;340:66–73. [12] Chung RB, Wu F, Shivaraman R, Keller S, DenBaars SP, Speck JS, et al. Growth study and impurity characterization of AlxIn1 xN grown by metal organic chemical vapor deposition. J Crys Growth 2011;324:163–7. [13] Cui ZG, Jia GH, Deng DG, Hua YJ, Zhao SL, Huang LH. Synthesis and luminescence properties of glass ceramics containing MSiO3:Eu2 þ (M¼Ca, Sr, Ba) phosphors for white LED. J Lumin 2012;132:153–60. [14] Klimczak M, Malinowski M, Sarnecki J, Piramidowicz R. Luminescence properties in the visible of Dy:YAG/YAG planar waveguides. J Lumin 2009;129: 1869–73.
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