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Synthesis and photoluminescence properties of (La, Pr) co-doped InVO4 phosphor
Microelectronic Engineering 148 (2015) 10–13
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
EmergingTechnologies2015
Synthesis and photoluminescence properties of (La, Pr) co-doped InVO4 phosphor Hung-Rung Shih a, Kuan-Ting Liu b, Lay-Gaik Teoh c, Li-Kai Wei d, Yee-Shin Chang d,⁎ a
Department of Mechanical and Computer-Aided Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan Department of Electronic Engineering, Cheng Shiu University, Kaohsiung 347, Taiwan Department of Mechanical Engineering, National Pingtung University of Science and Technology, Neipu, Pingtung 912, Taiwan d Department of Electronic Engineering, National Formosa University, Huwei, Yunlin, 632, Taiwan b c
a r t i c l e
i n f o
Article history: Received 9 June 2015 Received in revised form 22 July 2015 Accepted 25 July 2015 Available online 2 August 2015 Keywords: Indium orthovanadate Praseodymium Photoluminescence Phosphor Sol–gel method
a b s t r a c t (La3+, Pr3+) co-doped InVO4 phosphors were prepared using a sol–gel method. For the (In1 − xPrx)VO4 (x = 0–0.1) system, X-ray powder diffraction (XRD) patterns show that all of the peaks are attributed to the orthorhombic phase, and the scanning electron microscopy (SEM) images show that there is no obvious difference for the particle morphology between various Pr3+ ions doping. No emission peak characteristics of Pr3+ ions were observed under an excitation of 326 nm. In the (In0.97 − yLayPr0.03)VO4 (y = 0–0.97) system, XRD patterns indicated that the crystal structure changed from the orthorhombic InVO4 structure to the monoclinic LaVO4 structure when the In3+ ions were gradually substituted by the La3+ ions. The location of excitation peaks shifted to short wavelength area (blue shift), and the characteristic f–f transition of Pr3+ ion was observed. With increasing La3+ ion concentration, a series of emission peaks assigned to the 3P0 → 3H4,5,6, 1D2 → 3H4, and 3P0 → 3F2 characteristic transitions of Pr3+ ion were observed in the emission spectra under an excitation of 315 nm, and this indicated that the La3+ ion can act as a sensitizer for the (In0.97 − yLayPr0.03)VO4 phosphor. The Commission Internationale de I'Eclairage (CIE) color chromaticity coordinates of (In0.97 − yLayPr0.03)VO4 phosphors vary with the La3+ ion concentration from the yellowish light region, though the green-yellow and finally to the near-white light area as the La3+ ion concentration increased. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Oxide phosphors have recently gained much attention for their many applications, such as white light emitting diodes, derived from their higher chemical stability than sulfide phosphors [1–5]. Two companies, Nichia Chemical and Osram, already control many of the patents on phosphors, leading other manufacturers to invest in threewavelength mixed white lights and the development of novel phosphors. However, these phosphors have poor CIE coordinates due to the lack of a red emitting component [6]. Recently, much attention has been given to single-phased white light emitting phosphors [7], which have great potential for white light LED applications, and the emission efficiency could be improved by both a new host material and synthesis techniques. The lanthanide ion Pr3 + has an [Xe]4f2 configuration with energy less than 25,000 cm−1 for all 4f2 energy level and the Pr3+ ion emission in the visible region is caused due to the transition of the 3P0 level which contains two dominant transitions from the fluorescent 3P0 level to the
⁎ Corresponding author. E-mail address: [email protected] (Y.-S. Chang).
http://dx.doi.org/10.1016/j.mee.2015.07.007 0167-9317/© 2015 Elsevier B.V. All rights reserved.
lower 3H6 and 3H4 states [8]. The emission spectra of Pr are strongly modified in different hosts, which can be red, green, and even blue emission [9,10]. In effect, the luminescent properties of phosphors are strongly dependent on the crystal structure and grain size of the host materials. It has been reported that a fine grain size could enhance the emission efficiency and intensity of phosphors [11,12]. The sol–gel process is an attractive route that starts from molecular precursors and forms an oxide network via inorganic polymerization reactions, offering both product and processing advantages, such as high purity, ultrahomogeneity, and reduction of the sintering temperature. Materials with a wide bandgap, such as InMO4 (M = V, Nb, Ta) [13], have been reported to be active in the splitting of water into its components H2 and O2. Most of these photocatalysts, however, can only respond to ultraviolet (UV) light. One of these, Indium orthovanadate (InVO4) has an orthorhombic structure with the lattice parameters of a = 5.765 Å, b = 8.542 Å, and c = 6.592 Å. InVO4-based materials have received considerable attention in view of their potential applications in various fields, such as counter electrodes in electrochromic devices [14,15] and as photocatalysts [16]. In this investigation, InVO4 is used as a host material, and the (La3+, Pr3 +) co-doped InVO4 phosphors are synthesized using a sol–gel
H.-R. Shih et al. / Microelectronic Engineering 148 (2015) 10–13
method. The effect of (La3+, Pr3+) ions co-doped on the crystallinity and luminescent properties of InVO4 powders are studied. 2. Experimental procedure 2.1. Preparation of samples The (La3+, Pr3+) ion co-doped InVO4 phosphors were synthesized by the sol–gel method using ammonium metavanadate (NH4OH), Indium acetate [In(CH3CO2)3], praseodymium acetate [Pr(CH3CO2)3], and lanthanum acetate [La(CH3CO2)3]. Starting materials with the purity of 99.99% were from Aldrich Chemical Company, Inc. First, ammonium metavanadate was dissolved with ammonia in deionized water, and the yttrium acetate and europium acetate were separately dissolved in deionized water. When the precursor was completely dissolved in the solution, predetermined amounts of citric acid and ethylene glycol (equal molar ratio) were added. Citric acid and ethylene glycol (propionic acid) were then added to this as a chelating agent and stabilizing agent, respectively. The amounts of citric acid and ethylene glycol were determined by the ratio of citric acid to metal cations. The powders obtained were calcined at 950 °C in air for 6 h. 2.2. Characterization The effect of (La3+, Pr3 +) ion co-doped on the structure of InVO4 powders was studied using X-ray powder diffractometry (XRD, Rigaku Dmax-33 X-ray diffractometer), with Cu-Kα radiation, a source voltage of 30 kV and a current of 20 mA, to identify the possible phases formed after heat-treatment. The surface morphology was determined using a high-resolution scanning electron microscope (HR-SEM, S-4200, Hitachi). A Hitachi U-3010 UV visible spectrophotometer was used to measure the optical absorption behavior of the (La, Pr) co-doped InVO4 which were placed inside a closed quartz glass and measured from 200 to 700 nm at room temperature. Both the excitation and the luminescence spectra of the phosphors were recorded on a Hitachi F-4500 fluorescence spectrophotometer, using a 150 W xenon arc lamp as the excitation source, at room temperature. 3. Results and discussion 3.1. Phases in samples Fig. 1 shows the XRD patterns for (In1 − xPrx)VO4 (x = 0–0.1) powders calcined at 950 °C for 6 h in air. In a low doping concentration, it is seen that all of the diffraction peaks are attributed to an orthovanadate InVO4 crystal structure and there are no impurity peaks, which is
Fig. 1. X-ray diffraction pattern for (In1 − xPrxVO4, x = 0–0.1) powders calcined at 950 °C for 6 h.
11
because there are no charge compensation issues when the Pr3+ ions substitute the In3+ ions in the InVO4 lattice, and they can easily form solid solutions. After increasing the Pr3 + ion concentrations further, an additional phase was observed. It was due to the difference of ion radii for Pr3+ ion (1.32 Å) and In3+ ion (0.9 Å), and the optimum doping concentration for x = 0.03. Fig. 2 shows the XRD patterns for In1 − yLayPr0.03VO4 (y = 0–0.97) powders calcined at 950 °C for 6 h in air. As can be seen, no second phase exists, and the structure was changed gradually from the orthorhombic InVO4 structure to the monoclinic LaVO4 structure as the La3+ ion concentration was increased. 3.2. Microstructures Fig. 3 shows the SEM micrographs for In0.97Pr0.03VO4 doped with various concentrations of La3+ ion. There is no obvious difference for the particle morphology between various Pr3 + ions doping. For In0.97Pr0.03VO4 phosphor, the particle shape and sizes are not regular and uniform, but have a smooth surface morphology. As the La3+ ion concentration increases, the particle shape remains irregular, but the size appears to decrease. When all the In3+ ions have been substituted by the La3+ ions, the particle size is smallest and the distribution is more homogeneous. In general, smoother particle surface, more uniform distribution, and more homogeneous particle size will enhance the luminescence efficiency of phosphors. 3.3. Optical properties Fig. 4 shows (a) the excitation and emission spectrum of InVO4, and (b) the emission spectrum of InVO4 doped with different Pr3+ ion concentrations. The excitation spectrum indicates that the InVO4 host has a wide band between 240 and 360 nm, centered at about 326 nm, which is attributed to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO3− 4 anionic group [17–19]. In addition, under an excitation of 326 nm, there is an emission peak observed at charge transfer (VCT) transi526 nm which originates from the VO3− 4 tion [20], and which indicates that InVO4 is a self-activating phosphor. For the InVO4:Pr3+ system shown in Fig. 4(b), under an excitation of 326 nm, no Pr3+ ion characteristic emission peaks were observed. Different concentrations of Pr3+ ions did not change the wave shape, but did change the intensities for the emission spectra. The optimum Pr3+ ion concentration is 3 mol%. Fig. 5 shows the photoluminescence excitation spectra for (In0.97Pr0.03)VO4 doped with various La3+ ion concentrations calcined at 950 °C for 6 h (λem = 489 nm, 3P0 → 3H4 transition of Pr3 + ion). For La3 + ion free-(In0.97Pr0.03)VO4 phosphor, a broad band from the excitation spectra can be seen in the near UV range 300–350 nm with
Fig. 2. X-ray diffraction pattern for (In1 − yLayPr0.03VO4, y = 0–0.97) powders calcined at 950 °C for 6 h.
12
H.-R. Shih et al. / Microelectronic Engineering 148 (2015) 10–13
Fig. 3. SEM micrographs for (a) In0.97Pr0.03VO4, (b) (In0.87La0.1Pr0.03)VO4, (c) (In0.47La0.5Pr0.03)VO4, (d) La0.97Pr0.03VO4 phosphors calcined at 950 °C for 6 h.
its center at 326 nm, and no f–f transition of the activator Pr3+ ion was observed. The broad band centered at 326 nm is assigned to the charge transfer state originating from the oxygen ligand to the central vanadi3+ ion was substitutum atom inside VO3− 4 group [17–19]. When the In 3+ ed by the La ion, the location of excitation peaks shifted to the short wavelength area (blue shift), and the 3P0 → 3H4 for Pr3+ ion f–f transition was observed. This was because the host structure had changed from the orthorhombic InVO4 structure to the monoclinic LaVO4 structure. In addition, both of the excitation peak intensities were obtained when the In3+ ions were substituted fully by the La3+ ions. Fig. 6 shows the luminescence spectra for different concentrations of La3 + ion-doped (In0.97Pr0.03)VO4 phosphors, calcined at 950 °C for 6 h, under a deep ultraviolet excitation (315 nm). For La3 + free (In0.97Pr0.03)VO4, y = 0, no emission peak characteristics of Pr3+ ions
Fig. 4. (a) Excitation and emission spectrum of InVO4, (b) emission spectrum of InVO4 doped with different Pr3+ ion concentrations.
Fig. 5. Photoluminescence excitation spectra for (In0.97 − yLay)Pr0.03VO4 phosphor calcined at 950 °C for 6 h.
H.-R. Shih et al. / Microelectronic Engineering 148 (2015) 10–13
13
Table 1 Commission Internationale de I'Eclairage (CIE) color coordinates for (In0.97 − yLay) Pr0.03VO4 (y = 0–0.97) self-activated phosphors excited at 315 nm. (In0.97 − yLay)Pr0.03VO4 y=
Color coordinates
0 0.1 0.3 0.5 0.7 0.97
x = 0.348, y = 0.475 x = 0.341, y = 0.477 x = 0.348, y = 0.469 x = 0.337, y = 0.452 x = 0.340, y = 0.408 x = 0.388, y = 0.367
and this indicated that the La3 + ion can act as a sensitizer for the (In0.97 − yLayPr0.03)VO4 phosphor. The color tone changes from greenyellow to the yellowish region, and then to the near-white light region as the In3+ ions were substituted gradually by the La3+ ions. Fig. 6. Photoluminescence emission spectra for (In0.97 − yLay)Pr0.03VO4 phosphor calcined at 950 °C for 6 h.
Acknowledgments were observed, but a wide emission band can be observed between 450 and 650 nm, centered at ~526 nm. As discussed earlier, this is caused by 3+ ion concenthe VO3− 4 charge transfer (VCT) transition [20]. As the La tration increases, the broad emission band is eliminated and a series of emission peaks assigning to the 3P0 → 3H4,5,6, 1D2 → 3H4, and 3 P0 → 3 F2 characteristic transitions of Pr3+ ion can be observed [21]. These results indicate that the La3 + ion can act as a sensitizer in the (In0.97 − yLayPr0.03)VO4 phosphor. The Commission Internationale de I'Eclairage (CIE) color coordinates for (In0.97 − yLay)Pr0.03VO4 (y = 0–0.97) self-activated phosphors excited at 315 nm are shown in Table 1. As the La3 + ion concentration increases, the color tone changes gradually from yellowish light region (0 mol% La3+ ion doping, x = 0.348, y = 0.475), though green-yellow (50 mol% La3+ ion doping, x = 0.337, y = 0.452) and finally to the near-white light region (97mol% La3+ ion doping, x = 0.388, y = 0.367). 4. Summary A color-tunable (La3+, Pr3+) co-doped InVO4 self-activated phosphor is synthesized using a sol–gel method. No Pr3+ ion characteristic emission peaks of the (In1 − xPrx)VO4 (x = 0–0.1) phosphor were observed under an excitation of 326 nm. For La3+-doped (In0.97Pr0.03)VO4 system, XRD patterns indicated that the crystal structure changed from the orthorhombic InVO4 structure to the monoclinic LaVO4 structure as the La3+ ion concentration increased. With increasing the La3+ ion concentration, a series of emission peaks assigned to the 3P0 → 3H4,5,6, 1 D2 → 3H4, and 3P0 → 3 F2 characteristic transitions of Pr3 + ion were observed in the emission spectra under an excitation of 315 nm,
The authors would like to thank the National Science Council of the Republic of China for financially supporting this research, under contract No. (101-2221-E-150-034-MY3). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
W.T. Hsu, W.H. Wu, C.H. Lu, Mater. Sci. Eng. B 104 (2003) 40. B.S. Tsai, Y.H. Chang, Y.C. Chen, J. Mater. Res. 19 (5) (2004) 1504. Y.S. Chang, H.J. Lin, Y.C. Li, Y.L. Chai, Y.Y. Tsai, J. Solid State Chem. 180 (2007) 3076. R.H. Mauch, Appl. Surf. Sci. 92 (1996) 589. A. Kudo, K. Omori, H. Kato, J. Am. Chem. Soc. 121 (49) (1999) 11459. K. Toda, Y. Kawakmi, S.I. Kousaka, Y. Ito, A. Komeno, K. Uematsu, M. Sato, IEICE Trans. Electron. E89–C (10) (2006) 1406. X.M. Liu, C.K. Lin, J. Lin, Appl. Phys. Lett. 90 (2007) (081904-1-081904-3). C. Pedrini, D. Bouttet, C. Dujardin, B. Moine, I. Dafinei, P. Lecoq, M. Koselja, K. Blazek, Opt. Mater. 3 (1994) 81. W. Piper, J. DeLuca, F. Ham, J. Lumin. 8 (1974) 344. J. Sommerdijk, A. Bril, A. De Jager, J. Lumin. 8 (1974) 341. Y.S. Chang, J. Electron. Mater. 37 (7) (2008) 1024. G. Wakefield, H.A. Keron, P.J. Dobson, J.L. Hutchison, J. Colloid Interface Sci. 215 (1999) 179. D.K. Williams, B. Bihari, B.M. Tissue, J.M. McHale, J. Phys. Chem. B 102 (1998) 916. Z. Zou, J. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 625. B. Orel, A. Surca Vuk, U.O. Krasovec, G. Drazic, Electrochim. Acta 46 (2001) 2059. A.S. Vuk, U.O. Krasovec, B. Orel, P. Colomban, J. Electrochem. Soc. 148 (6) (2001) 231. J. Ye, Z. Zou, M. Oshikiri, A. Matsushita, M. Shimoda, M. Imai, T. Shishido, Chem. Phys. Lett. 356 (2002) 221. C. Hsu, R.C. Powell, J. Lumin. 10 (5) (1975) 273. H.W. Zhang, X.Y. Fu, S.Y. Niu, G.Q. Sun, Q. Xin, Solid State Commun. 132 (2004) 527. Y. Luo, Z.G. Xia, B.F. Lei, Y.G. Liu, RSC Adv. 3 (2013) 22206. N. Kristianpoller, A. Shmilevich, D. Weiss, R. Chen, N. Khaidukov, Radiat. Meas. 33 (2001) 637.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
EmergingTechnologies2015
Synthesis and photoluminescence properties of (La, Pr) co-doped InVO4 phosphor Hung-Rung Shih a, Kuan-Ting Liu b, Lay-Gaik Teoh c, Li-Kai Wei d, Yee-Shin Chang d,⁎ a
Department of Mechanical and Computer-Aided Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan Department of Electronic Engineering, Cheng Shiu University, Kaohsiung 347, Taiwan Department of Mechanical Engineering, National Pingtung University of Science and Technology, Neipu, Pingtung 912, Taiwan d Department of Electronic Engineering, National Formosa University, Huwei, Yunlin, 632, Taiwan b c
a r t i c l e
i n f o
Article history: Received 9 June 2015 Received in revised form 22 July 2015 Accepted 25 July 2015 Available online 2 August 2015 Keywords: Indium orthovanadate Praseodymium Photoluminescence Phosphor Sol–gel method
a b s t r a c t (La3+, Pr3+) co-doped InVO4 phosphors were prepared using a sol–gel method. For the (In1 − xPrx)VO4 (x = 0–0.1) system, X-ray powder diffraction (XRD) patterns show that all of the peaks are attributed to the orthorhombic phase, and the scanning electron microscopy (SEM) images show that there is no obvious difference for the particle morphology between various Pr3+ ions doping. No emission peak characteristics of Pr3+ ions were observed under an excitation of 326 nm. In the (In0.97 − yLayPr0.03)VO4 (y = 0–0.97) system, XRD patterns indicated that the crystal structure changed from the orthorhombic InVO4 structure to the monoclinic LaVO4 structure when the In3+ ions were gradually substituted by the La3+ ions. The location of excitation peaks shifted to short wavelength area (blue shift), and the characteristic f–f transition of Pr3+ ion was observed. With increasing La3+ ion concentration, a series of emission peaks assigned to the 3P0 → 3H4,5,6, 1D2 → 3H4, and 3P0 → 3F2 characteristic transitions of Pr3+ ion were observed in the emission spectra under an excitation of 315 nm, and this indicated that the La3+ ion can act as a sensitizer for the (In0.97 − yLayPr0.03)VO4 phosphor. The Commission Internationale de I'Eclairage (CIE) color chromaticity coordinates of (In0.97 − yLayPr0.03)VO4 phosphors vary with the La3+ ion concentration from the yellowish light region, though the green-yellow and finally to the near-white light area as the La3+ ion concentration increased. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Oxide phosphors have recently gained much attention for their many applications, such as white light emitting diodes, derived from their higher chemical stability than sulfide phosphors [1–5]. Two companies, Nichia Chemical and Osram, already control many of the patents on phosphors, leading other manufacturers to invest in threewavelength mixed white lights and the development of novel phosphors. However, these phosphors have poor CIE coordinates due to the lack of a red emitting component [6]. Recently, much attention has been given to single-phased white light emitting phosphors [7], which have great potential for white light LED applications, and the emission efficiency could be improved by both a new host material and synthesis techniques. The lanthanide ion Pr3 + has an [Xe]4f2 configuration with energy less than 25,000 cm−1 for all 4f2 energy level and the Pr3+ ion emission in the visible region is caused due to the transition of the 3P0 level which contains two dominant transitions from the fluorescent 3P0 level to the
⁎ Corresponding author. E-mail address: [email protected] (Y.-S. Chang).
http://dx.doi.org/10.1016/j.mee.2015.07.007 0167-9317/© 2015 Elsevier B.V. All rights reserved.
lower 3H6 and 3H4 states [8]. The emission spectra of Pr are strongly modified in different hosts, which can be red, green, and even blue emission [9,10]. In effect, the luminescent properties of phosphors are strongly dependent on the crystal structure and grain size of the host materials. It has been reported that a fine grain size could enhance the emission efficiency and intensity of phosphors [11,12]. The sol–gel process is an attractive route that starts from molecular precursors and forms an oxide network via inorganic polymerization reactions, offering both product and processing advantages, such as high purity, ultrahomogeneity, and reduction of the sintering temperature. Materials with a wide bandgap, such as InMO4 (M = V, Nb, Ta) [13], have been reported to be active in the splitting of water into its components H2 and O2. Most of these photocatalysts, however, can only respond to ultraviolet (UV) light. One of these, Indium orthovanadate (InVO4) has an orthorhombic structure with the lattice parameters of a = 5.765 Å, b = 8.542 Å, and c = 6.592 Å. InVO4-based materials have received considerable attention in view of their potential applications in various fields, such as counter electrodes in electrochromic devices [14,15] and as photocatalysts [16]. In this investigation, InVO4 is used as a host material, and the (La3+, Pr3 +) co-doped InVO4 phosphors are synthesized using a sol–gel
H.-R. Shih et al. / Microelectronic Engineering 148 (2015) 10–13
method. The effect of (La3+, Pr3+) ions co-doped on the crystallinity and luminescent properties of InVO4 powders are studied. 2. Experimental procedure 2.1. Preparation of samples The (La3+, Pr3+) ion co-doped InVO4 phosphors were synthesized by the sol–gel method using ammonium metavanadate (NH4OH), Indium acetate [In(CH3CO2)3], praseodymium acetate [Pr(CH3CO2)3], and lanthanum acetate [La(CH3CO2)3]. Starting materials with the purity of 99.99% were from Aldrich Chemical Company, Inc. First, ammonium metavanadate was dissolved with ammonia in deionized water, and the yttrium acetate and europium acetate were separately dissolved in deionized water. When the precursor was completely dissolved in the solution, predetermined amounts of citric acid and ethylene glycol (equal molar ratio) were added. Citric acid and ethylene glycol (propionic acid) were then added to this as a chelating agent and stabilizing agent, respectively. The amounts of citric acid and ethylene glycol were determined by the ratio of citric acid to metal cations. The powders obtained were calcined at 950 °C in air for 6 h. 2.2. Characterization The effect of (La3+, Pr3 +) ion co-doped on the structure of InVO4 powders was studied using X-ray powder diffractometry (XRD, Rigaku Dmax-33 X-ray diffractometer), with Cu-Kα radiation, a source voltage of 30 kV and a current of 20 mA, to identify the possible phases formed after heat-treatment. The surface morphology was determined using a high-resolution scanning electron microscope (HR-SEM, S-4200, Hitachi). A Hitachi U-3010 UV visible spectrophotometer was used to measure the optical absorption behavior of the (La, Pr) co-doped InVO4 which were placed inside a closed quartz glass and measured from 200 to 700 nm at room temperature. Both the excitation and the luminescence spectra of the phosphors were recorded on a Hitachi F-4500 fluorescence spectrophotometer, using a 150 W xenon arc lamp as the excitation source, at room temperature. 3. Results and discussion 3.1. Phases in samples Fig. 1 shows the XRD patterns for (In1 − xPrx)VO4 (x = 0–0.1) powders calcined at 950 °C for 6 h in air. In a low doping concentration, it is seen that all of the diffraction peaks are attributed to an orthovanadate InVO4 crystal structure and there are no impurity peaks, which is
Fig. 1. X-ray diffraction pattern for (In1 − xPrxVO4, x = 0–0.1) powders calcined at 950 °C for 6 h.
11
because there are no charge compensation issues when the Pr3+ ions substitute the In3+ ions in the InVO4 lattice, and they can easily form solid solutions. After increasing the Pr3 + ion concentrations further, an additional phase was observed. It was due to the difference of ion radii for Pr3+ ion (1.32 Å) and In3+ ion (0.9 Å), and the optimum doping concentration for x = 0.03. Fig. 2 shows the XRD patterns for In1 − yLayPr0.03VO4 (y = 0–0.97) powders calcined at 950 °C for 6 h in air. As can be seen, no second phase exists, and the structure was changed gradually from the orthorhombic InVO4 structure to the monoclinic LaVO4 structure as the La3+ ion concentration was increased. 3.2. Microstructures Fig. 3 shows the SEM micrographs for In0.97Pr0.03VO4 doped with various concentrations of La3+ ion. There is no obvious difference for the particle morphology between various Pr3 + ions doping. For In0.97Pr0.03VO4 phosphor, the particle shape and sizes are not regular and uniform, but have a smooth surface morphology. As the La3+ ion concentration increases, the particle shape remains irregular, but the size appears to decrease. When all the In3+ ions have been substituted by the La3+ ions, the particle size is smallest and the distribution is more homogeneous. In general, smoother particle surface, more uniform distribution, and more homogeneous particle size will enhance the luminescence efficiency of phosphors. 3.3. Optical properties Fig. 4 shows (a) the excitation and emission spectrum of InVO4, and (b) the emission spectrum of InVO4 doped with different Pr3+ ion concentrations. The excitation spectrum indicates that the InVO4 host has a wide band between 240 and 360 nm, centered at about 326 nm, which is attributed to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO3− 4 anionic group [17–19]. In addition, under an excitation of 326 nm, there is an emission peak observed at charge transfer (VCT) transi526 nm which originates from the VO3− 4 tion [20], and which indicates that InVO4 is a self-activating phosphor. For the InVO4:Pr3+ system shown in Fig. 4(b), under an excitation of 326 nm, no Pr3+ ion characteristic emission peaks were observed. Different concentrations of Pr3+ ions did not change the wave shape, but did change the intensities for the emission spectra. The optimum Pr3+ ion concentration is 3 mol%. Fig. 5 shows the photoluminescence excitation spectra for (In0.97Pr0.03)VO4 doped with various La3+ ion concentrations calcined at 950 °C for 6 h (λem = 489 nm, 3P0 → 3H4 transition of Pr3 + ion). For La3 + ion free-(In0.97Pr0.03)VO4 phosphor, a broad band from the excitation spectra can be seen in the near UV range 300–350 nm with
Fig. 2. X-ray diffraction pattern for (In1 − yLayPr0.03VO4, y = 0–0.97) powders calcined at 950 °C for 6 h.
12
H.-R. Shih et al. / Microelectronic Engineering 148 (2015) 10–13
Fig. 3. SEM micrographs for (a) In0.97Pr0.03VO4, (b) (In0.87La0.1Pr0.03)VO4, (c) (In0.47La0.5Pr0.03)VO4, (d) La0.97Pr0.03VO4 phosphors calcined at 950 °C for 6 h.
its center at 326 nm, and no f–f transition of the activator Pr3+ ion was observed. The broad band centered at 326 nm is assigned to the charge transfer state originating from the oxygen ligand to the central vanadi3+ ion was substitutum atom inside VO3− 4 group [17–19]. When the In 3+ ed by the La ion, the location of excitation peaks shifted to the short wavelength area (blue shift), and the 3P0 → 3H4 for Pr3+ ion f–f transition was observed. This was because the host structure had changed from the orthorhombic InVO4 structure to the monoclinic LaVO4 structure. In addition, both of the excitation peak intensities were obtained when the In3+ ions were substituted fully by the La3+ ions. Fig. 6 shows the luminescence spectra for different concentrations of La3 + ion-doped (In0.97Pr0.03)VO4 phosphors, calcined at 950 °C for 6 h, under a deep ultraviolet excitation (315 nm). For La3 + free (In0.97Pr0.03)VO4, y = 0, no emission peak characteristics of Pr3+ ions
Fig. 4. (a) Excitation and emission spectrum of InVO4, (b) emission spectrum of InVO4 doped with different Pr3+ ion concentrations.
Fig. 5. Photoluminescence excitation spectra for (In0.97 − yLay)Pr0.03VO4 phosphor calcined at 950 °C for 6 h.
H.-R. Shih et al. / Microelectronic Engineering 148 (2015) 10–13
13
Table 1 Commission Internationale de I'Eclairage (CIE) color coordinates for (In0.97 − yLay) Pr0.03VO4 (y = 0–0.97) self-activated phosphors excited at 315 nm. (In0.97 − yLay)Pr0.03VO4 y=
Color coordinates
0 0.1 0.3 0.5 0.7 0.97
x = 0.348, y = 0.475 x = 0.341, y = 0.477 x = 0.348, y = 0.469 x = 0.337, y = 0.452 x = 0.340, y = 0.408 x = 0.388, y = 0.367
and this indicated that the La3 + ion can act as a sensitizer for the (In0.97 − yLayPr0.03)VO4 phosphor. The color tone changes from greenyellow to the yellowish region, and then to the near-white light region as the In3+ ions were substituted gradually by the La3+ ions. Fig. 6. Photoluminescence emission spectra for (In0.97 − yLay)Pr0.03VO4 phosphor calcined at 950 °C for 6 h.
Acknowledgments were observed, but a wide emission band can be observed between 450 and 650 nm, centered at ~526 nm. As discussed earlier, this is caused by 3+ ion concenthe VO3− 4 charge transfer (VCT) transition [20]. As the La tration increases, the broad emission band is eliminated and a series of emission peaks assigning to the 3P0 → 3H4,5,6, 1D2 → 3H4, and 3 P0 → 3 F2 characteristic transitions of Pr3+ ion can be observed [21]. These results indicate that the La3 + ion can act as a sensitizer in the (In0.97 − yLayPr0.03)VO4 phosphor. The Commission Internationale de I'Eclairage (CIE) color coordinates for (In0.97 − yLay)Pr0.03VO4 (y = 0–0.97) self-activated phosphors excited at 315 nm are shown in Table 1. As the La3 + ion concentration increases, the color tone changes gradually from yellowish light region (0 mol% La3+ ion doping, x = 0.348, y = 0.475), though green-yellow (50 mol% La3+ ion doping, x = 0.337, y = 0.452) and finally to the near-white light region (97mol% La3+ ion doping, x = 0.388, y = 0.367). 4. Summary A color-tunable (La3+, Pr3+) co-doped InVO4 self-activated phosphor is synthesized using a sol–gel method. No Pr3+ ion characteristic emission peaks of the (In1 − xPrx)VO4 (x = 0–0.1) phosphor were observed under an excitation of 326 nm. For La3+-doped (In0.97Pr0.03)VO4 system, XRD patterns indicated that the crystal structure changed from the orthorhombic InVO4 structure to the monoclinic LaVO4 structure as the La3+ ion concentration increased. With increasing the La3+ ion concentration, a series of emission peaks assigned to the 3P0 → 3H4,5,6, 1 D2 → 3H4, and 3P0 → 3 F2 characteristic transitions of Pr3 + ion were observed in the emission spectra under an excitation of 315 nm,
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