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Sm3+ (Ln=Y, Gd)
Solid State Communications 131 (2004) 65–69 www.elsevier.com/locate/ssc
Novel red phosphors for solid state lighting; the system BixLn12xVO4; Eu3þ/Sm3þ (Ln ¼ Y, Gd) S. Neeraja,b, N. Kijimab,c, A.K. Cheethama,b,* a Materials Research Laboratory, University of California, Santa Barbara, CA 93106-5121, USA Mitsubishi Chemicals Center for Advanced Materials, University of California, Santa Barbara, CA 93106, USA c Inorganic Functional Material Laboratory, MCC-Group Science & Technology Research Center, Mitsubishi Chemical Corporation, 1000, Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan b
Received 10 March 2004; accepted 22 March 2004 by C.N.R. Roa
Abstract Solid solutions of vanadates of formula BixLn12xVO4 (Ln ¼ Y, Gd) doped with Eu3þ or Sm3þ ions have been synthesized by solid-state reactions. Intense red/orange-red luminescence is obtained in these samples on excitation in the broad chargetransfer band in the near UV. The excitation in the Eu3þ levels leads to much less intense red emission. These materials could find applications as red phosphors for solid-state white lighting devices utilizing GaN-based excitation in the near UV. q 2004 Elsevier Ltd. All rights reserved. PACS: 78.55. 2 m; 78.55.Hx; 83.80.Pc Keywords: E. Luminescence; D. Optical properties
1. Introduction Recently, solid-state lighting based on GaN semiconductors has made remarkable breakthroughs in efficiency. GaN-based diodes emit bright violet-blue light, which can be used to pump longer wavelength phosphors. Nakamura pioneered this approach at Nichia Chemical Co. and the first white light emitting diodes (LEDs) became commercially available in 1997 [1,2]. These white LEDs can be obtained by combining a InGaN blue LED emitting at 465 nm with a broad-band yellow phosphor, e.g. (Y12xGdx)3(Al12yGay)5O12 (YAG:Ce) [1,2]. The variation of x and y can be used to produce broad emission from 510 and 580 nm, leading to a high color rendering index and a luminous efficiency of about 32 lm/W at the present time [3]. In addition to the blue LED/yellow phosphor approach, white light can be produced by a variety of other approaches, including color mixing of three LED emissions (e.g. combining discrete blue, green, and red LEDs [4]) or * Corresponding author. Fax: þ 1-805-893-8502. E-mail address: [email protected] (A.K. Cheetham). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.03.050
the pumping of red, green and blue phosphors with a deep blue/UV LED or laser diode (LD) [4]. We believe that nitride-based vertical cavity surface emitting lasers (VCSELs), coupled with phosphors optimized for violet or near-UV absorption, have great potential for high-efficiency solid-state lighting [5]. The red, green and blue phosphors that are currently used in conventional fluorescent lighting have been optimized for excitation by other means, such as by using the UV emission from a mercury discharge, for which the characteristic wavelengths are 185 and 254 nm [6]. Hence, one of the challenges for the new generation of lighting based upon GaN lies in the development of novel families of phosphors that are optimized for excitations at longer wavelengths in the near UV (350 – 400 nm). The current phosphor materials of choice for solid-state lighting based upon near UV GaN-LEDs are Y2O2S:Eu3þ for red, ZnS:(Cuþ, Al3þ) for green, and BaMgAl10O17:Eu2þ (BAM) for blue [7]. Unfortunately, the efficiency of the Y2O2S:Eu3þ red phosphor is about 8 times less than that of the green and blue phosphors, so it is necessary to use a phosphor mixture containing 80% red, 10% green and 10% blue in order to obtain good color rendering. In addition, the
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lifetime of the Y2O2S:Eu3þ material is inadequate under extended UV irradiation. For both of these reasons, therefore, there is an urgent need to make superior red phosphors that absorb in the near UV and emit in the red. Our strategy to solve this problem has focused initially on materials that have broad and intense charge-transfer (C-T) absorption bands in the near UV and are therefore capable of efficiently capturing the emission from a GaNbased LED or LD over a range of wavelengths. Such materials include vanadates, molybdates, tungstates, etc. where the oxygen to metal C-T bands are very intense. Following the excitation in the UV, the energy is transferred to the activator ion by a non-radiative mechanism (in the case of red emission, we shall use Eu3þ, as described below). Interesting candidates for red phosphors include materials containing Eu3þ (f6), which fluoresces via a 5D0 to 7F2 transition at ,612 nm when the ion is present in a non-centrosymmetric site. An example of such a system would be Eu-doped YVO4 [7], which is used as a cathodoluminescent material in color television screens. This material may be appropriate for the present application, since the C-T band of the vanadate ion is well placed in the UV. Other candidates include Eu-doped molybdates and tungstates, such as Y2(WO4)3, Y2(MoO4)3 and NaLn(WO4)2 [8]. We have explored Bi3þ-containing rare-earth vanadates in which 6s2 ! 6s6p excitations in the bismuth ion can be used to harvest the near-UV light. In the case of the BixLn12xVO4 (Ln ¼ Gd, Eu) system, for example, structural and spectral studies have shown the existence of two ranges of solid solution [9,10]. One has a tetragonal structure of the zircon type ð0 , x , 0:64Þ and the other has a monoclinic structure of the scheelite-related fergusonite type ðx . 0:93Þ: Coordination of the cations in these oxides is such that V is coordinated to four oxygen atoms forming a tetrahedron and Ln (or Bi) to eight oxygen atoms from different tetrahedra. In this communication, we report the luminescence behaviors of the solid solutions of BixLn12xVO4:Eu (Ln ¼ Y, Gd), BixLn12xVO4:Sm and correlate them with the available structural information. In addition, we have also investigated the effect of co-doping Tb(III)/Pr(III) on the Eu(III) emission in these solid solutions and compared the emission intensities of these samples with the standard red phosphor (Y2O2S:Eu3þ).
2. Experimental
from the furnace, cooled, finely ground and reheated at 800 8C for another 24 – 48 h to complete the reaction. The rare-earth vanadate compounds are white powders (yellow or orange when there is unreacted vanadium oxide), which become yellowish with increasing Bi-content. The Eu3þ dopant concentration (added as Eu2O3) was about 5 mol% (of the Ln3þ þ Bi3þ) in each of the syntheses. For the synthesis of BixLn12xVO4:Sm, a similar procedure was followed with the dopant atom being Sm3þ (added as Sm2O3) and the doping concentration was about 5 mol% (of the Ln3þ þ Bi3þ). The final products were characterized by powder X-ray diffraction, solid-state MAS NMR, photoluminescence and photoexcitation spectroscopy. 2.2. Spectroscopy Emission and excitation spectra were recorded utilizing a Spex Fluorolog 2 spectro-fluorimeter equipped with a Hamamatsu R928A water-cooled photomultiplier tube configured for photon counting and interfaced with a computer running Spex DM3000f software. Emission spectra were corrected for PMT response. Both emission and excitation spectra were corrected for lamp intensity variation by the ratio method with a Rhodamine-B dye in 1,2-Propanediol (8 g/l). A 380 nm cut-off filter was placed in front of the emission monochromator to block scattered excitation light. The emissions were usually monitored in the front face configuration.
3. Results 3.1. X-ray diffraction studies on BixLn12xVO4 samples doped with Eu3þ/Sm3þ Structural studies of the system BixLn12xVO4 have shown the existence of two ranges of solid solution, as described above [9]. The study of the mixed LnVO4 – BiVO4 (Ln ¼ Y, Gd) samples doped with Eu3þ or Sm3þ shows the expected change in the XRD pattern with increasing bismuth concentration (Fig. 1). Samples with x # 0:65 show a powder pattern corresponding to the tetragonal zircon phase (Fig. 1(a) – (d)), whereas for x . 0:65; the powder pattern indicates that these samples contain a mixture of both monoclinic fergusonite and tetragonal zircon phases (Fig. 1(e) and (f)). The pure BiVO4 doped with Eu3þ shows the powder pattern corresponding to the monoclinic form (Fig. 1(g)).
2.1. Materials For the syntheses of BixLn12xVO4:Eu (Ln ¼ Y, Gd) stoichiometric mixtures of Ln2O3, Bi2O3, NH4VO3, and Eu2O3 were ground together and heated in an alumina crucible at 600 8C for 24 h. The products were then removed
3.1.1. Spectral properties of BixLn12xVO4 (Ln ¼ Y, Gd) samples doped with Eu3þ In general, the photoluminescence spectra of YVO4 – BiVO4 doped with Eu3þ ions show strong 5D0 ! 7F2 (614, 618 nm) and 5D0 ! 7F1 (593 nm) emission lines on
S. Neeraj et al. / Solid State Communications 131 (2004) 65–69
Fig. 1. Powder X-ray diffraction patterns of BixY12xVO4:Eu samples.
excitation at both 254 and 365 nm (Fig. 2). The excitation spectrum for monitoring the 5D0 ! 7F2 emission of Eu3þ shows a broad charge-transfer band along with a sharp 7 F0 ! 5L6, Eu3þ line at 394 nm, the latter being a relatively weak spectral feature when compared to the broad C-T band (Fig. 3). The excitation spectrum in pure YVO4 doped with Eu3þ shows a broad absorption maximum centered around , 310 nm, and a band edge at 340 nm; however the absorption in the short-wavelength UV (240 –275 nm) is very low. In the case of BixY12xVO4:Eu3þ the band edge shifts to longer wavelengths with increasing bismuth concentration. For example, the, band edge in the excitation spectrum moves from ,340 nm (for pure YVO4, x ¼ 0) to , 420 nm for the x ¼ 0:45 bismuth rich sample (Fig. 3). This increase occurs due to extra absorption involving the
Fig. 2. Photoluminescence spectra of BixY12xVO4:Eu samples (lexc ¼ 365 nm).
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Fig. 3. Excitation spectra of BixY12xVO4:Eu samples for the 614 nm Eu3þ emission.
Bi– O component in addition to the V– O C-T bands. This increase seems to be monotonic only when less than half ðx ¼ 0:4 – 0:45Þ of the yttrium ions are substituted by Bi3þ ions. For substitutions exceeding x , 0:45; the longwavelength C-T band diminishes in intensity and disappears completely for compositions with x . 0:65: However, the absorption in the short-wavelength (240– 275 nm) seems to increase gradually (Fig. 3). The photoluminescence spectra of GdVO4 – BiVO4 doped with Eu3þ again show the characteristic spectral lines for Eu3þ emission. The excitation spectrum for monitoring the 5D0 ! 7F2 emission of Eu3þ shows a small absorption in the short-wavelength region (240 –275 nm), a broad charge-transfer band at lower energies, and a sharp but weak 7F0 ! 5L6 Eu3þ line at 394 nm (Fig. 4). The
Fig. 4. Excitation spectra of BixGd12xVO4:Eu samples for the 613 nm Eu3þ emission.
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excitation spectrum in pure GdVO4 doped with Eu3þ shows a band edge at ,350 nm, but in the BixGd12xVO4:Eu3þ system the band edge shifts to longer wavelengths with increasing bismuth concentration, moving to ,425 nm for the x ¼ 0:4 sample (Fig. 4). Again, this increase seems to be monotonic only when less than half of the gadolinium ions are substituted by Bi3þ ions. For substitutions exceeding x ¼ 0:4; the long-wavelength C-T band again decreases in intensity, as in the yttrium system, though the shortwavelength (240 –275 nm) absorption increases gradually. 3.1.2. Spectral properties of BixY12xVO4 doped with Sm3þ Sm3þ as an activator in YVO4 is known to have high emission efficiency. There is also considerable similarity in the emission colors of both Sm3þ (orange red) and Eu3þ (red) in the vanadate lattice. The photoluminescence spectra YVO4 – BiVO4 doped with Sm3þ ions show the main emission lines for Sm3þ as 564 (4G5/2 2 6H5/2), 601 (4G5/2 2 6H7/2) and 645, 654 nm (4G5/2 2 6H9/2) on excitation at 363.8 nm (Fig. 5). The excitation spectrum for monitoring the 4G5/2 ! 6H9/2 emission of Sm3þ shows a weak absorption band at short wavelengths (240– 275 nm), a broad C-T band centered around , 320 nm, plus the sharp Sm3þ lines (Fig. 6). The excitation spectrum in pure YVO4 doped with Sm3þ shows a broad maximum centered around , 320 nm with a band edge at 350 nm. In the case of the Y12xBixVO4:Sm3þ samples, the C-T band and the band edge shift to longer wavelengths with increasing bismuth concentration. As the bismuth concentration increases, the band edge in the excitation spectrum moves from 340 nm (for pure YVO4 x ¼ 0) to ,415 nm for ðx ¼ 0:4Þ bismuth rich samples. As with the other systems described above, this increase seems to be monotonic only when less than half ðx , 0:4Þ of the yttrium ions are substituted by Bi3þ ions. For substitutions exceeding x ¼ 0:45; the long-wavelength
Fig. 6. Excitation spectra of BixY12xVO4:Sm samples for the 645 nm Sm3þ emission.
C-T band decreases drastically in intensity, whereas the absorption at short wavelengths (240 – 275 nm) increases gradually (Fig. 6). In Table 1, we list the comparison of emission intensities of the BixY12xVO4:Eu3þ ions to that of the standard Y2O2S:Eu3þ red phosphor on excitation at 400 nm. The samples containing low concentrations of Bi3þ show a decreased Eu3þ emission when compared to that of the standard red phosphor when excited under these conditions, whereas the samples containing bismuth x $ 0:2 show a much enhanced emission, the highest being in sample containing bismuth x ¼ 0:3: The increase in the emission intensity of Eu3þ with Bi3þ content is due the energy transfer from Bi3þ center to Eu3þ. Increased bismuth content decreases the efficiency of this energy transport from the matrix to Eu3þ, perhaps by distorting the former. Such distortions may arise from the chemical and electronic nature of Y3þ and Bi3þ ions. Table 1 Comparison of emission intensities of the Y12xBixVO4:Eu3þ ions to that of the standard Y2O2S:Eu3þ, Sm3þ red phosphor
Fig. 5. Photoluminescence spectra of BixY12xVO4:Sm samples (lexc ¼ 363:8 nm).
Composition
Emission intensity (lexc ¼ 400 nm)
Y2O2S:Eu,Sm (Eu0.04, Sm0.3) Y0.95Eu0.05VO4 Y0.90Bi0.05Eu0.05VO4 Y0.85Bi0.10Eu0.05VO4 Y0.75Bi0.20Eu0.05VO4 Y0.70Bi0.25Eu0.05VO4 Y0.65Bi0.30Eu0.05VO4 Y0.60Bi0.35Eu0.05VO4 Y0.55Bi0.40Eu0.05VO4 Y0.50Bi0.45Eu0.05VO4
3.3 0.13 0.15 0.12 0.72 0.77 1.2 0.81 0.97 0.71
S. Neeraj et al. / Solid State Communications 131 (2004) 65–69
The BixY12xVO4 samples codoped with Eu3þ, Tb3þ or Eu , Pr3þ ions also show a similar comparison to the standard red phosphor when their emission intensities were measured at 400 nm. No energy transfer from the Tb3þ or Pr3þ was observed as expected in the beginning, though the reasons to this effect are still unclear. Further study in this regard is in progress. 3þ
4. Discussions Datta et al. showed that for the YVO4:Eu, Bi system, increasing amount of Bi3þ ions in the lattice, the excitation bands shift to longer wavelengths [11]. The 6s2 ! 6s6p transition of Bi3þ ions often contributes to the luminescence of bismuth-doped phosphors [12]. We believe that the extra absorption and the band shift towards longer wavelengths in the excitation spectrum of LnVO4 – BiVO4 samples doped with Eu3þ or Sm3þ involves the absorption due to Bi3þ ions. Such extra absorption has also been witnessed in other cases when ns2 ions have been introduced in other host lattices absorbing in the UV region, for example, CaWO4 – PbWO4, Y2WO6 – Bi [12]. The more puzzling observation however, is that the excitation spectra of the Bi12xLnxVO4 samples doped with Eu3þ or Sm3þ show a decrease in the long wavelength absorption above a critical bismuth concentration. Literature reports on bismuth, antimony niobates and tantalates have concluded that if ns2 ions (Sb3þ, Bi3þ) are in a more favorable, asymmetrical coordination, the outer electrons (ns2) tend to be more localized and stabilized, so their influence on the luminescence of the surrounding centers is smaller [13]. The excitation spectra of the samples with low bismuth content ðx # 0:4Þ can be explained by the fact that Bi3þ ions are present in a more symmetrical (zircon-type) environment, and hence the 6s2 electrons are on top of the 2p level of O22 as a result of which the excitation band shifts to longer wavelengths. Beyond a certain limit increasing bismuth content ð0:4 , x , 0:64Þ; we observe a drastic reduction in the longwavelength absorption in the excitation spectra, even though the samples are still with in the composition regime of the zircon phase ð0 , x , 0:64Þ: The luminescence behavior of the Bi12xLnxVO4:Eu/Sm samples with 0:4 , x , 0:64 must stem from distortion of the bismuth environments, as more Bi is introduced into the zircon structure.
5. Conclusions The present findings have three consequences with respect to red phosphors for solid state lighting: 1. By codoping bismuth and activator ions (Eu/Sm) in
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rare-earth vanadates, we are able to shift the C-T band edge from ,340 to , 420 nm, making the Bi12xLnxVO4:Eu/Sm system suitable for devices in which the excitation is in this wavelength range. 2. These phosphors can be excited by a UV or near UV GaN-LED operating anywhere with in , 300– 400 nm range to emit red light which can be combined with green and blue phosphors to produce white light. 3. Our vanadates have excellent thermal stability since they are synthesized at 800 8C. Unlike the yttrium oxysulfide they also have excellent hydrolytic stability. 4. Though not mentioned in this communication our experiments with Bi12xYxVO4 samples doped with Er3þ and Dy3þ have led to green phosphors, which show extra long wavelength UV absorption when compared to the pure YVO4 system. We are extending this work to other host systems in order to find the optimum material for the red phosphor application.
Acknowledgements This work made use of facilities supported by the MRSEC program of the National Science Foundation under award No. DMR00-80034
References [1] S. Nakamura, MRS Bulletin, 29, 1997. [2] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997. [3] R. Mueller-Mach, G.O. Mueller, IEEE. J. Sel. Top. Quantum 8 (2002) 339. [4] S. Muthu, F.J.P. Schuurmans, M.D. Pashley, IEEE. J. Sel. Top. Quantum Electron. 8 (2002) 333. [5] D.A. Steigerwald, J.C. Bhat, D. Collins, R.M. Fletcher, M.O. Holcomb, M.J. Ludowise, P.S. Martin, S.L. Rudaz, IEEE. J. Sel. Top. Quantum 8 (2002) 310. [6] G. Blasse, B.C. Grabmeier, Luminescent Materials, Springer, Berlin, 1994. [7] M. Shinoya, W.M. Yen, Phosphor HandBook, CRC Press, Boca Raton, FL, 1999. [8] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2– 6. [9] J. Ghamri, H. Baussart, M. Lebras, J.M. Leroy, Ann. Chim. Sci. Mater. 15 (1990) 111. [10] J. Ghamri, H. Baussart, M. Lebras, J.M. Leroy, J. Phys. Chem. Solids 50 (1989) 1237. [11] R.K. Datta, J. Electrochem. Soc. 114 (1967) 1057. [12] G. Blasse, A. Bril, J. Chem. Phys. 48 (1968) 217. [13] M. Wiegel, W. Middel, G. Blasse, J. Mater. Chem. 5 (1995) 981.
Novel red phosphors for solid state lighting; the system BixLn12xVO4; Eu3þ/Sm3þ (Ln ¼ Y, Gd) S. Neeraja,b, N. Kijimab,c, A.K. Cheethama,b,* a Materials Research Laboratory, University of California, Santa Barbara, CA 93106-5121, USA Mitsubishi Chemicals Center for Advanced Materials, University of California, Santa Barbara, CA 93106, USA c Inorganic Functional Material Laboratory, MCC-Group Science & Technology Research Center, Mitsubishi Chemical Corporation, 1000, Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan b
Received 10 March 2004; accepted 22 March 2004 by C.N.R. Roa
Abstract Solid solutions of vanadates of formula BixLn12xVO4 (Ln ¼ Y, Gd) doped with Eu3þ or Sm3þ ions have been synthesized by solid-state reactions. Intense red/orange-red luminescence is obtained in these samples on excitation in the broad chargetransfer band in the near UV. The excitation in the Eu3þ levels leads to much less intense red emission. These materials could find applications as red phosphors for solid-state white lighting devices utilizing GaN-based excitation in the near UV. q 2004 Elsevier Ltd. All rights reserved. PACS: 78.55. 2 m; 78.55.Hx; 83.80.Pc Keywords: E. Luminescence; D. Optical properties
1. Introduction Recently, solid-state lighting based on GaN semiconductors has made remarkable breakthroughs in efficiency. GaN-based diodes emit bright violet-blue light, which can be used to pump longer wavelength phosphors. Nakamura pioneered this approach at Nichia Chemical Co. and the first white light emitting diodes (LEDs) became commercially available in 1997 [1,2]. These white LEDs can be obtained by combining a InGaN blue LED emitting at 465 nm with a broad-band yellow phosphor, e.g. (Y12xGdx)3(Al12yGay)5O12 (YAG:Ce) [1,2]. The variation of x and y can be used to produce broad emission from 510 and 580 nm, leading to a high color rendering index and a luminous efficiency of about 32 lm/W at the present time [3]. In addition to the blue LED/yellow phosphor approach, white light can be produced by a variety of other approaches, including color mixing of three LED emissions (e.g. combining discrete blue, green, and red LEDs [4]) or * Corresponding author. Fax: þ 1-805-893-8502. E-mail address: [email protected] (A.K. Cheetham). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.03.050
the pumping of red, green and blue phosphors with a deep blue/UV LED or laser diode (LD) [4]. We believe that nitride-based vertical cavity surface emitting lasers (VCSELs), coupled with phosphors optimized for violet or near-UV absorption, have great potential for high-efficiency solid-state lighting [5]. The red, green and blue phosphors that are currently used in conventional fluorescent lighting have been optimized for excitation by other means, such as by using the UV emission from a mercury discharge, for which the characteristic wavelengths are 185 and 254 nm [6]. Hence, one of the challenges for the new generation of lighting based upon GaN lies in the development of novel families of phosphors that are optimized for excitations at longer wavelengths in the near UV (350 – 400 nm). The current phosphor materials of choice for solid-state lighting based upon near UV GaN-LEDs are Y2O2S:Eu3þ for red, ZnS:(Cuþ, Al3þ) for green, and BaMgAl10O17:Eu2þ (BAM) for blue [7]. Unfortunately, the efficiency of the Y2O2S:Eu3þ red phosphor is about 8 times less than that of the green and blue phosphors, so it is necessary to use a phosphor mixture containing 80% red, 10% green and 10% blue in order to obtain good color rendering. In addition, the
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lifetime of the Y2O2S:Eu3þ material is inadequate under extended UV irradiation. For both of these reasons, therefore, there is an urgent need to make superior red phosphors that absorb in the near UV and emit in the red. Our strategy to solve this problem has focused initially on materials that have broad and intense charge-transfer (C-T) absorption bands in the near UV and are therefore capable of efficiently capturing the emission from a GaNbased LED or LD over a range of wavelengths. Such materials include vanadates, molybdates, tungstates, etc. where the oxygen to metal C-T bands are very intense. Following the excitation in the UV, the energy is transferred to the activator ion by a non-radiative mechanism (in the case of red emission, we shall use Eu3þ, as described below). Interesting candidates for red phosphors include materials containing Eu3þ (f6), which fluoresces via a 5D0 to 7F2 transition at ,612 nm when the ion is present in a non-centrosymmetric site. An example of such a system would be Eu-doped YVO4 [7], which is used as a cathodoluminescent material in color television screens. This material may be appropriate for the present application, since the C-T band of the vanadate ion is well placed in the UV. Other candidates include Eu-doped molybdates and tungstates, such as Y2(WO4)3, Y2(MoO4)3 and NaLn(WO4)2 [8]. We have explored Bi3þ-containing rare-earth vanadates in which 6s2 ! 6s6p excitations in the bismuth ion can be used to harvest the near-UV light. In the case of the BixLn12xVO4 (Ln ¼ Gd, Eu) system, for example, structural and spectral studies have shown the existence of two ranges of solid solution [9,10]. One has a tetragonal structure of the zircon type ð0 , x , 0:64Þ and the other has a monoclinic structure of the scheelite-related fergusonite type ðx . 0:93Þ: Coordination of the cations in these oxides is such that V is coordinated to four oxygen atoms forming a tetrahedron and Ln (or Bi) to eight oxygen atoms from different tetrahedra. In this communication, we report the luminescence behaviors of the solid solutions of BixLn12xVO4:Eu (Ln ¼ Y, Gd), BixLn12xVO4:Sm and correlate them with the available structural information. In addition, we have also investigated the effect of co-doping Tb(III)/Pr(III) on the Eu(III) emission in these solid solutions and compared the emission intensities of these samples with the standard red phosphor (Y2O2S:Eu3þ).
2. Experimental
from the furnace, cooled, finely ground and reheated at 800 8C for another 24 – 48 h to complete the reaction. The rare-earth vanadate compounds are white powders (yellow or orange when there is unreacted vanadium oxide), which become yellowish with increasing Bi-content. The Eu3þ dopant concentration (added as Eu2O3) was about 5 mol% (of the Ln3þ þ Bi3þ) in each of the syntheses. For the synthesis of BixLn12xVO4:Sm, a similar procedure was followed with the dopant atom being Sm3þ (added as Sm2O3) and the doping concentration was about 5 mol% (of the Ln3þ þ Bi3þ). The final products were characterized by powder X-ray diffraction, solid-state MAS NMR, photoluminescence and photoexcitation spectroscopy. 2.2. Spectroscopy Emission and excitation spectra were recorded utilizing a Spex Fluorolog 2 spectro-fluorimeter equipped with a Hamamatsu R928A water-cooled photomultiplier tube configured for photon counting and interfaced with a computer running Spex DM3000f software. Emission spectra were corrected for PMT response. Both emission and excitation spectra were corrected for lamp intensity variation by the ratio method with a Rhodamine-B dye in 1,2-Propanediol (8 g/l). A 380 nm cut-off filter was placed in front of the emission monochromator to block scattered excitation light. The emissions were usually monitored in the front face configuration.
3. Results 3.1. X-ray diffraction studies on BixLn12xVO4 samples doped with Eu3þ/Sm3þ Structural studies of the system BixLn12xVO4 have shown the existence of two ranges of solid solution, as described above [9]. The study of the mixed LnVO4 – BiVO4 (Ln ¼ Y, Gd) samples doped with Eu3þ or Sm3þ shows the expected change in the XRD pattern with increasing bismuth concentration (Fig. 1). Samples with x # 0:65 show a powder pattern corresponding to the tetragonal zircon phase (Fig. 1(a) – (d)), whereas for x . 0:65; the powder pattern indicates that these samples contain a mixture of both monoclinic fergusonite and tetragonal zircon phases (Fig. 1(e) and (f)). The pure BiVO4 doped with Eu3þ shows the powder pattern corresponding to the monoclinic form (Fig. 1(g)).
2.1. Materials For the syntheses of BixLn12xVO4:Eu (Ln ¼ Y, Gd) stoichiometric mixtures of Ln2O3, Bi2O3, NH4VO3, and Eu2O3 were ground together and heated in an alumina crucible at 600 8C for 24 h. The products were then removed
3.1.1. Spectral properties of BixLn12xVO4 (Ln ¼ Y, Gd) samples doped with Eu3þ In general, the photoluminescence spectra of YVO4 – BiVO4 doped with Eu3þ ions show strong 5D0 ! 7F2 (614, 618 nm) and 5D0 ! 7F1 (593 nm) emission lines on
S. Neeraj et al. / Solid State Communications 131 (2004) 65–69
Fig. 1. Powder X-ray diffraction patterns of BixY12xVO4:Eu samples.
excitation at both 254 and 365 nm (Fig. 2). The excitation spectrum for monitoring the 5D0 ! 7F2 emission of Eu3þ shows a broad charge-transfer band along with a sharp 7 F0 ! 5L6, Eu3þ line at 394 nm, the latter being a relatively weak spectral feature when compared to the broad C-T band (Fig. 3). The excitation spectrum in pure YVO4 doped with Eu3þ shows a broad absorption maximum centered around , 310 nm, and a band edge at 340 nm; however the absorption in the short-wavelength UV (240 –275 nm) is very low. In the case of BixY12xVO4:Eu3þ the band edge shifts to longer wavelengths with increasing bismuth concentration. For example, the, band edge in the excitation spectrum moves from ,340 nm (for pure YVO4, x ¼ 0) to , 420 nm for the x ¼ 0:45 bismuth rich sample (Fig. 3). This increase occurs due to extra absorption involving the
Fig. 2. Photoluminescence spectra of BixY12xVO4:Eu samples (lexc ¼ 365 nm).
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Fig. 3. Excitation spectra of BixY12xVO4:Eu samples for the 614 nm Eu3þ emission.
Bi– O component in addition to the V– O C-T bands. This increase seems to be monotonic only when less than half ðx ¼ 0:4 – 0:45Þ of the yttrium ions are substituted by Bi3þ ions. For substitutions exceeding x , 0:45; the longwavelength C-T band diminishes in intensity and disappears completely for compositions with x . 0:65: However, the absorption in the short-wavelength (240– 275 nm) seems to increase gradually (Fig. 3). The photoluminescence spectra of GdVO4 – BiVO4 doped with Eu3þ again show the characteristic spectral lines for Eu3þ emission. The excitation spectrum for monitoring the 5D0 ! 7F2 emission of Eu3þ shows a small absorption in the short-wavelength region (240 –275 nm), a broad charge-transfer band at lower energies, and a sharp but weak 7F0 ! 5L6 Eu3þ line at 394 nm (Fig. 4). The
Fig. 4. Excitation spectra of BixGd12xVO4:Eu samples for the 613 nm Eu3þ emission.
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excitation spectrum in pure GdVO4 doped with Eu3þ shows a band edge at ,350 nm, but in the BixGd12xVO4:Eu3þ system the band edge shifts to longer wavelengths with increasing bismuth concentration, moving to ,425 nm for the x ¼ 0:4 sample (Fig. 4). Again, this increase seems to be monotonic only when less than half of the gadolinium ions are substituted by Bi3þ ions. For substitutions exceeding x ¼ 0:4; the long-wavelength C-T band again decreases in intensity, as in the yttrium system, though the shortwavelength (240 –275 nm) absorption increases gradually. 3.1.2. Spectral properties of BixY12xVO4 doped with Sm3þ Sm3þ as an activator in YVO4 is known to have high emission efficiency. There is also considerable similarity in the emission colors of both Sm3þ (orange red) and Eu3þ (red) in the vanadate lattice. The photoluminescence spectra YVO4 – BiVO4 doped with Sm3þ ions show the main emission lines for Sm3þ as 564 (4G5/2 2 6H5/2), 601 (4G5/2 2 6H7/2) and 645, 654 nm (4G5/2 2 6H9/2) on excitation at 363.8 nm (Fig. 5). The excitation spectrum for monitoring the 4G5/2 ! 6H9/2 emission of Sm3þ shows a weak absorption band at short wavelengths (240– 275 nm), a broad C-T band centered around , 320 nm, plus the sharp Sm3þ lines (Fig. 6). The excitation spectrum in pure YVO4 doped with Sm3þ shows a broad maximum centered around , 320 nm with a band edge at 350 nm. In the case of the Y12xBixVO4:Sm3þ samples, the C-T band and the band edge shift to longer wavelengths with increasing bismuth concentration. As the bismuth concentration increases, the band edge in the excitation spectrum moves from 340 nm (for pure YVO4 x ¼ 0) to ,415 nm for ðx ¼ 0:4Þ bismuth rich samples. As with the other systems described above, this increase seems to be monotonic only when less than half ðx , 0:4Þ of the yttrium ions are substituted by Bi3þ ions. For substitutions exceeding x ¼ 0:45; the long-wavelength
Fig. 6. Excitation spectra of BixY12xVO4:Sm samples for the 645 nm Sm3þ emission.
C-T band decreases drastically in intensity, whereas the absorption at short wavelengths (240 – 275 nm) increases gradually (Fig. 6). In Table 1, we list the comparison of emission intensities of the BixY12xVO4:Eu3þ ions to that of the standard Y2O2S:Eu3þ red phosphor on excitation at 400 nm. The samples containing low concentrations of Bi3þ show a decreased Eu3þ emission when compared to that of the standard red phosphor when excited under these conditions, whereas the samples containing bismuth x $ 0:2 show a much enhanced emission, the highest being in sample containing bismuth x ¼ 0:3: The increase in the emission intensity of Eu3þ with Bi3þ content is due the energy transfer from Bi3þ center to Eu3þ. Increased bismuth content decreases the efficiency of this energy transport from the matrix to Eu3þ, perhaps by distorting the former. Such distortions may arise from the chemical and electronic nature of Y3þ and Bi3þ ions. Table 1 Comparison of emission intensities of the Y12xBixVO4:Eu3þ ions to that of the standard Y2O2S:Eu3þ, Sm3þ red phosphor
Fig. 5. Photoluminescence spectra of BixY12xVO4:Sm samples (lexc ¼ 363:8 nm).
Composition
Emission intensity (lexc ¼ 400 nm)
Y2O2S:Eu,Sm (Eu0.04, Sm0.3) Y0.95Eu0.05VO4 Y0.90Bi0.05Eu0.05VO4 Y0.85Bi0.10Eu0.05VO4 Y0.75Bi0.20Eu0.05VO4 Y0.70Bi0.25Eu0.05VO4 Y0.65Bi0.30Eu0.05VO4 Y0.60Bi0.35Eu0.05VO4 Y0.55Bi0.40Eu0.05VO4 Y0.50Bi0.45Eu0.05VO4
3.3 0.13 0.15 0.12 0.72 0.77 1.2 0.81 0.97 0.71
S. Neeraj et al. / Solid State Communications 131 (2004) 65–69
The BixY12xVO4 samples codoped with Eu3þ, Tb3þ or Eu , Pr3þ ions also show a similar comparison to the standard red phosphor when their emission intensities were measured at 400 nm. No energy transfer from the Tb3þ or Pr3þ was observed as expected in the beginning, though the reasons to this effect are still unclear. Further study in this regard is in progress. 3þ
4. Discussions Datta et al. showed that for the YVO4:Eu, Bi system, increasing amount of Bi3þ ions in the lattice, the excitation bands shift to longer wavelengths [11]. The 6s2 ! 6s6p transition of Bi3þ ions often contributes to the luminescence of bismuth-doped phosphors [12]. We believe that the extra absorption and the band shift towards longer wavelengths in the excitation spectrum of LnVO4 – BiVO4 samples doped with Eu3þ or Sm3þ involves the absorption due to Bi3þ ions. Such extra absorption has also been witnessed in other cases when ns2 ions have been introduced in other host lattices absorbing in the UV region, for example, CaWO4 – PbWO4, Y2WO6 – Bi [12]. The more puzzling observation however, is that the excitation spectra of the Bi12xLnxVO4 samples doped with Eu3þ or Sm3þ show a decrease in the long wavelength absorption above a critical bismuth concentration. Literature reports on bismuth, antimony niobates and tantalates have concluded that if ns2 ions (Sb3þ, Bi3þ) are in a more favorable, asymmetrical coordination, the outer electrons (ns2) tend to be more localized and stabilized, so their influence on the luminescence of the surrounding centers is smaller [13]. The excitation spectra of the samples with low bismuth content ðx # 0:4Þ can be explained by the fact that Bi3þ ions are present in a more symmetrical (zircon-type) environment, and hence the 6s2 electrons are on top of the 2p level of O22 as a result of which the excitation band shifts to longer wavelengths. Beyond a certain limit increasing bismuth content ð0:4 , x , 0:64Þ; we observe a drastic reduction in the longwavelength absorption in the excitation spectra, even though the samples are still with in the composition regime of the zircon phase ð0 , x , 0:64Þ: The luminescence behavior of the Bi12xLnxVO4:Eu/Sm samples with 0:4 , x , 0:64 must stem from distortion of the bismuth environments, as more Bi is introduced into the zircon structure.
5. Conclusions The present findings have three consequences with respect to red phosphors for solid state lighting: 1. By codoping bismuth and activator ions (Eu/Sm) in
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rare-earth vanadates, we are able to shift the C-T band edge from ,340 to , 420 nm, making the Bi12xLnxVO4:Eu/Sm system suitable for devices in which the excitation is in this wavelength range. 2. These phosphors can be excited by a UV or near UV GaN-LED operating anywhere with in , 300– 400 nm range to emit red light which can be combined with green and blue phosphors to produce white light. 3. Our vanadates have excellent thermal stability since they are synthesized at 800 8C. Unlike the yttrium oxysulfide they also have excellent hydrolytic stability. 4. Though not mentioned in this communication our experiments with Bi12xYxVO4 samples doped with Er3þ and Dy3þ have led to green phosphors, which show extra long wavelength UV absorption when compared to the pure YVO4 system. We are extending this work to other host systems in order to find the optimum material for the red phosphor application.
Acknowledgements This work made use of facilities supported by the MRSEC program of the National Science Foundation under award No. DMR00-80034
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