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The crystallinity and the photoluminescent properties of spray pyrolized ZnO phosphor containing Eu2+ and Eu3+ ions
Journal of Physics and Chemistry of Solids 65 (2004) 1843–1847 www.elsevier.com/locate/jpcs
The crystallinity and the photoluminescent properties of spray pyrolized ZnO phosphor containing Eu2C and Eu3C ions Camellia Panatarani, I. Wuled Lenggoro, Kikuo Okuyama* Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi Hiroshima 739-8527, Japan Received 25 December 2003; revised 12 March 2004; accepted 9 June 2004
Abstract A europium doped ZnO (ZnO:Eu) particle was directly synthesized by the spray pyrolysis method. The crystal structure of samples was designated by the europium ion and the synthesis temperature. We identified the coexistence of Eu2C and Eu3C ions in the as prepared ZnO, which was strongly influenced by the doping concentration and the synthesis temperature. With addition of a 0.5 mol% concentration of europium ions, only the Eu2C ion existed in particles, while both Eu2C and Eu3C ions existed in sample using 1 mol% or higher concentration of europium ions. Changing the wavelength of the excitation source, we also found that both the blue and red luminescence can be obtained. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Optical materials; B. Chemical synthesis; C. X-ray diffraction; D. Crystallinity; D. Luminescence
1. Introduction The development of advanced display and lighting technology such as field-emission displays (FEDs) and plasma display panels (PDPs) requires phosphor which has a high efficiency and low degradation [1,2]. Since sulfidebased phosphor is known to easily degrade at high current densities, the research and development of oxide-based phosphors have become very important. Zinc oxide (ZnO) in film and powder forms is one of the most efficient oxide-based phosphors in both photoluminescence (PL) and cathode luminescence (CL) [3–5,12]. Versatile ZnO which has a wide band gap (3.3 eV) [3,8], has potential use in low-voltage applications [4,10]. In addition, ZnO has promising applications in the near future in mercury-free fluorescent lighting, white light emitting diodes (LEDs) and liquid crystal displays (LCDs) [8]. In order to enhance the luminescent properties of ZnO, the doped ZnO in binary or ternary systems, such as ZnO:Zn [7], ZnO:Eu [7–12], ZnO:Mn, ZnO:V, ZnO:W, ZnO:(Y,Eu), and ZnO:(W,Mg) [12] has been investigated. * Corresponding author. Tel.: C81-82-4247716; fax: C81-82-4247850. E-mail address: [email protected] (K. Okuyama). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.008
The ZnO:Eu system is very interesting because there is no energy transferred from the ZnO host to europium ions [10] and both the host (ZnO) and the doping (Eu) emit different colors. It implies a possibility for producing more than one base color of a ZnO:Eu luminescence system [11]. The number of base color, of course, depends on the material sources and synthesis conditions. Abdullah et al. succeeded in synthesizing three base colors of ZnO:Eu prepared by an in situ solution method using a very high concentration of LiOH. Indeed, that trivalent europium ion can be reduced into divalent europium ion [1,13]. Since both ions also emit different colors, the coexistence of Eu2C and Eu3C ions in a ZnO host is interesting and will be investigated. In the experiment reported here, the ZnO:Eu system was first synthesized by spray pyrolysis (SP) method without further treatment. The selection of SP for the preparation of materials was due to simplicity. In the SP method, zinc acetate solution and europium nitrate as the europium ion source were mixed as the starting solution. All chemicals were decomposed inside a micron-droplet at a high temperature while they were in the gas phase. Because the reaction of materials occurred at a homogeneous condition in micron sized droplets, there were no losses in material.
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2. Experimental The materials of zinc acetate dihydrate (99.9%) and europium (III) nitrate hexahydrate (99.9%) were purchased from Wako Chemicals. Both chemicals were used without further purification. Ultra pure water was used as a solvent. The concentration of europium nitrate was varied, while the total concentrations of the precursor solutions were fixed at 0.1 M. The apparatus used was the same as that reported in our previous paper [6]. The precursor solution was sprayed ultrasonically into a hot wall tubular reactor under 1 l/min N2 flow. The reactor was operated at a high temperature to activate europium dopants. The particles were collected by using an electrostatic precipitator. The photoluminescent (PL) characteristics of the prepared particles were characterized by means of spectrofluorophotometer (RF-5300 PC, Shimadzu) in which a xenon short arc lamp was used as an excitation source. All PL spectra were measured at room temperature under air. The crystallinity of the products was characterized by means of X-ray diffraction (XRD, Rint 2200V, Rigaku) and the morphology was observed using a field emission scanning electron microscope (FE-SEM, S-5000, Hitachi).
3. Results and discussion 3.1. The crystallinity of ZnO:Eu samples The XRD pattern of white powders obtained from various concentrations of europium ions had the preferential
Fig. 1. XRD patterns of samples prepared at the synthesis temperature of 1100 8C (*europium nitrate concentration).
Fig. 2. XRD patterns of samples prepared by adding 9 mol% europium nitrate to the precursor solution at different synthesis temperatures.
growth along the [101] direction (Fig. 1). Pure ZnO showed the zincite structure in conformity with the JCPDS 36-1451 as reported in Refs. [5,6]. A very high degree of crystallinity resulted from the use of the high synthesis temperature (1100 8C) to promote the crystal growth. The addition of a small amount of europium ions (up to 1 mol%) into the precursor solution, did not change the crystal structure of the samples. There was no evidence of new peaks presented in the XRD pattern of these samples. However by increasing the concentration of europium ions, new peaks appeared at 2qz28, 33 and 428. By comparing the XRD patterns of samples with the JCPDS 34-0072, it suggested that Eu3C ions existed in the samples prepared by the additional 3 mol% or more of europium nitrate. Moreover, these phenomena indicated that the samples prepared from the high concentration of europium ions formed a composite. The influence of the synthesis temperature on the XRD patterns of sample prepared from the additional high concentration of europium ions (9 mol%) is shown in Fig. 2. Increasing the synthesis temperatures up to 1100 8C, strongly enhanced the formation of the Eu2O3 compound. In the X-ray patterns of samples prepared at low synthesis temperatures (750 and 950 8C), all peak positions were corresponded in accordance with ZnO reference (JCPDS 36-1451). On the other hand, the peaks associated with ZnO phase (JCPDS 36-1451) were very broad from the sample prepared at 1100 8C. Moreover, peaks with strong intensity appeared at 2qz28 and 338. Although the peaks associated with Eu2O3 compound (JCPDS 34-0072) in the region between 28 and 338 did not appear for the sample prepared at 1100 8C, however the peaks of Eu2O3 phase were exist for the sample prepared at higher temperature (1300 8C). We assumed that the strong peaks at 2qz28 and 338 from
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Fig. 3. SEM images of samples prepared by adding europium nitrates at different concentrations and synthesis temperatures. (a) Pure ZnO, 1100 8C, (b) ZnO:Eu (9 mol%), 1100 8C, (c) ZnO:Eu (9 mol%), 630 8C and (d) ZnO:Eu (1 mol%), 1300 8C.
the sample prepared at 1100 8C associated with Eu2O3 compound. The degree of crystalinity of the sample prepared at 750 8C was lower than that of 950 8C. This indicated that by increasing 200 8C of synthesis temperature (from 750 to 950 8C), the degree of crystallinity can be improved. FE-SEM pictures of particles in high magnifications are shown in Fig. 3. The europium doped ZnO prepared at high synthesis temperatures (1100 8C or above, Fig. 3a, b and d) shows the highly agglomerated form. The presence of the europium ion in the ZnO did not greatly
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Fig. 5. PL spectra of samples prepared at different synthesis temperatures. The spectra were measured under the excitations of (a) 321 and (b) 466 nm, respectively.
influence the size and the morphology of particles (Fig. 3a and b). The agglomerate form of particles was obtained because the synthesis temperature was very high. Fig. 3c shows the europium doped ZnO prepared from the additional 9 mol% europium ion at a 630 8C synthesis temperature. Nearly spherical, 600–1000 nm sized particles were obtained. 3.2. Photoluminescence properties of ZnO:Eu samples The role of europium ion concentration and the synthesis temperature on the PL properties of samples are shown by Figs. 4–6. Figs. 4a, 5a and 6a show the PL spectra of samples excited using a wavelength of 321 nm. Figs. 4b, 5b and 6b show the PL spectra of
Fig. 4. PL spectra of samples prepared by adding different europium nitrate concentrations to the precursor solution. The spectra were measured under the excitation of (a) 321 and (b) 466 nm, respectively.
Fig. 6. PL spectra of samples prepared by adding 1 mol% europium nitrate to the precursor solution with different synthesis temperatures under excitation of (a) 321 and (b) 466 nm, respectively.
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samples recorded at a 466 nm excitation. The aim of using 466 nm wavelength of excitation was to confirm the transition originated from Eu3C [10]. The intensity of the PL peaks was affected by the addition of the europium ions into the precursor solution. In Fig. 4a, the position of the PL peak of ZnO prepared with the addition 0.5–9 mol% of europium ions centered at three positions, i.e. 364, 399 and 468 nm, respectively. These peaks resulted from the 5d/4f transition. The emission curve is very broad due to the splitting of d-state. We suggested such shape was caused by the presence of Eu2C ions. The peak intensity centered at 364 and 399 nm was strongly influenced by increasing the concentration of europium ion. The highest peaks at the positions of 364 and 399 nm were obtained at the sample prepared from 0.1 mol% europium ion. On the other hand, at the higher concentration (3 mol% or more) of the europium ion, the intensity of PL peaks at 364 and 399 nm were lowered by increasing the concentration of europium ion. These indicate that the addition of the 3 mol% europium ion exceeded the quenching point of the doping concentration. The europium ions easily take the trivalent state in oxide, such as in Y2O3 and CaO [7], however these ions have tendency take the divalent state [1,13]. Cocivera et al. reported that the reduction of Eu3C/Eu2C occurred in the alkaline earth carbonate thin films by annealing alkaline earth containing Eu3C ions under air. The effectiveness of the reduction process was affected by the cations of the host lattice [13]. The reduction of trivalent to divalent ions possibly occurred in the material due to a charge compensation [13]. In addition, the reduction of trivalent to divalent state of europium ions easily occurred because the 4f/5d transitional energy is higher than the charge-transfer state (CTS) [2]. In Fig. 4b, there are no peaks related to the Eu3C ion which appeared in the samples prepared with the addition of 0.5 mol% europium ions. The small evidence of a red PL peak at 613 nm appeared from samples prepared with the addition of 1 mol% europium ion (Fig. 4b), and on the other hand peak at 364 nm also strongly existed from the same sample (Fig. 4a). These results indicate that Eu2C and Eu3C ions coexistence in the sample prepared with the additional 1 mol% europium nitrate. The addition of higher (more than 1 mol%) europium ions concentration into precursor solution increases the intensity of PL at 613 nm. These results indicated that the Eu3C ions exist at the samples prepared with the addition 1 mol% or higher concentration of europium ion. The peak position at 613 nm related to the 5D0/7F2 transition. Because of the differences in the electrical charge and the ionic radii, the Eu3C ions can not replace the Zn2C ions. The large amount of Eu3C creates a new component, i.e. Eu2O3. Regarding the XRD pattern as shown in the Fig. 1, it is clear that the luminescent peaks located at the red region in Fig. 3b originated from the Eu2O3 component in the composite form.
The effect of synthesis temperature on the PL properties was studied on the samples prepared with the additional higher concentration (9 mol%) of the europium ion. These effects are shown in Fig. 5. In the sample prepared at 630 8C, there are tendencies in the co-existence of Eu2C and Eu3C in the matrix. The very low peak intensities correspond to Eu2C and Eu3C are present in the blue region (Fig. 5a) and red region (Fig. 5b) respectively. Both spectra were influenced by the synthesis temperature. By using the synthesis temperature of 750 8C or above, the intensity of PL peak at 399 and 468 nm increased. In addition, a new peak at 364 nm appeared and the peak at 399 nm decreased in the samples prepared at 1100 8C. By using the excitation of 466 nm, the intensity of the red peak (w613 nm) also increased by raising the synthesis temperature. The peak position at 612 nm shifted to 613 and 611 nm for the samples prepared at 950 and 1100 8C, respectively. The peak at 611 nm became sharper by increasing the synthesis temperature and a new peak at 628 nm presented in the sample prepared at 1100 8C. Yamashita et al. observed the coexistence of both Eu2C and Eu3C ions from the trivalent europium doped CaO sample. When CaO:Eu is prepared in N2, most Eu ions are introduced as Eu3C into CaO. On the other hand, the Eu2C center increases when the CaO:Eu is prepared in H2 [14]. The influence of the synthesis temperature on the samples prepared with a low concentration (1 mol%) of europium ion was also investigated. The PL characteristics of the samples are shown by Fig. 6. Under the excitation of 321 nm, the entire PL peaks, especially at the blue (400–500 nm) region, strongly increased by raising the synthesis temperature from 1100 to 1300 8C. The PL peaks at the blue region were intensified approximately seven times; however, this peak is very broad. We suggested that the broading effect was caused by two reasons; first, the peak originated from the overlap of those centered at 463, 468 and 473 nm and secondly, the peak related to d-state emission which is increased by raising the synthesis temperature. The increases in the blue luminescent peaks indicate that the synthesis temperature is a very important factor in order to activate the divalent europium ion.
4. Conclusion The reduction of the Eu3C/Eu2C ions occurred in the particles prepared by the addition of a low concentration of europium ion. This reduction changed the base color of luminescence from red to blue. Blue luminescence can be enhanced by increasing the synthesis temperature. At a high concentration of europium ion, the Eu3C created the Eu2O3 component in the ZnO–Eu2O3 composite.
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Acknowledgements This study was supported in part by the NEDO’s Nanotechnology Particle Project from the Ministry of Economy, Trade and Industry of Japan, Grant-in Aids from the Ministry of Education, Culture, Sports, Science and Technology of Japan (K.O. and I.W.L.) and financial support from Hashiya Scholarship Foundation (C.P.). The authors thank Mr Mikrajudin Abdullah for his helpful discussion and Mr Yasumori Kuromizu for his assistance during the experiments and characterization.
References [1] G. Blasse, Luminescent Materials, Springer, New York, 1994. [2] W.M. Yen, S. Shionoya (Eds.), Phosphor Handbook, CRC Press, Boca Raton, FL, 1998. [3] K.H. Yoon, J.Y. Cho, Photoluminescence characteristics of zinc oxide thin films prepared by spray pyrolysis technique, Mater. Res. Bull. 35 (2000) 39–46. [4] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J. Caruso, M.J. Hampden-Smith, T.T. Kodas, Green photoluminescence efficiency and free carrier density in ZnO phosphor powders prepared by spray pyrolysis, J. Lumin. 75 (1997) 11–16.
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[5] S.A. Studenikin, N. Golego, M. Cocivera, Fabrication of green and orange photoluminescent, undoped ZnO films using spray pyrolysis, J. Appl. Phys. 84 (1998) 2287–2294. [6] C. Panatarani, I.W. Lenggoro, K. Okuyama, Synthesis of single crystalline ZnO nanoparticles by salt assisted spray pyrolysis, J. Nanoparticles Res. 5 (2003) 47–53. [7] Y. Hayashi, H. Narahara, T. Uchida, T. Noguchi, S. Ibuki, Photoluminescence of Eu-doped ZnO phosphor, Jpn. J. Appl. Phys. 34 (1995) 1878–1882. [8] R.N. Bhargava, V. Chhabra, T. Som, A. Ekimov, N. Taskar, Quantum confined atoms of doped ZnO nanocrystals, Phys. Status Solidi (b) 229 (2002) 897–901. [9] Y.-K. Park, J.-I. Han, M.G. Kwak, H. Yang, S.-H. Ju, W.-S. Cho, Effect of coupling structure of Eu on the photoluminescent characteristics for ZnO:EuCl3 phosphor, Appl. Phys. Lett. 72 (1998) 668–670. [10] S.A.M. Lima, F.A. Sigoli, M.R. Davolos, M. Jafelicci Jr., Europium (III)-containing zinc oxide from Pechini method, J. Alloys Compd 344 (2002) 280–284. [11] M. Abdullah, T. Morimoto, K. Okuyama, Generating blue and red luminescence from ZnO/poly(ethylene glycol) nanocomposites prepared by an in-situ method, Adv. Funct. Mater. 13 (2003) 1–5. [12] V.Z. Mardkovich, H. Hayashi, M. Haemori, T. Fukumura, M. Kawasaki, Discovery and optimization of new ZnO-based phosphors using a combinatorial method, Adv. Funct. Mater. 13 (2003) 519–524. [13] J. Hao, J. Gao, M. Cocivera, Tuning of the blue emission from europium-doped alkaline earth chloroborate thin films activated in air, Appl. Phys. Lett. 82 (2003) 2778–2780. [14] N. Yamashita, Coexistence of the Eu2C and Eu3C centers in the CaO:Eu powder phosphor, J. Electrochem. Soc. 140 (1993) 840–843.
The crystallinity and the photoluminescent properties of spray pyrolized ZnO phosphor containing Eu2C and Eu3C ions Camellia Panatarani, I. Wuled Lenggoro, Kikuo Okuyama* Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi Hiroshima 739-8527, Japan Received 25 December 2003; revised 12 March 2004; accepted 9 June 2004
Abstract A europium doped ZnO (ZnO:Eu) particle was directly synthesized by the spray pyrolysis method. The crystal structure of samples was designated by the europium ion and the synthesis temperature. We identified the coexistence of Eu2C and Eu3C ions in the as prepared ZnO, which was strongly influenced by the doping concentration and the synthesis temperature. With addition of a 0.5 mol% concentration of europium ions, only the Eu2C ion existed in particles, while both Eu2C and Eu3C ions existed in sample using 1 mol% or higher concentration of europium ions. Changing the wavelength of the excitation source, we also found that both the blue and red luminescence can be obtained. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Optical materials; B. Chemical synthesis; C. X-ray diffraction; D. Crystallinity; D. Luminescence
1. Introduction The development of advanced display and lighting technology such as field-emission displays (FEDs) and plasma display panels (PDPs) requires phosphor which has a high efficiency and low degradation [1,2]. Since sulfidebased phosphor is known to easily degrade at high current densities, the research and development of oxide-based phosphors have become very important. Zinc oxide (ZnO) in film and powder forms is one of the most efficient oxide-based phosphors in both photoluminescence (PL) and cathode luminescence (CL) [3–5,12]. Versatile ZnO which has a wide band gap (3.3 eV) [3,8], has potential use in low-voltage applications [4,10]. In addition, ZnO has promising applications in the near future in mercury-free fluorescent lighting, white light emitting diodes (LEDs) and liquid crystal displays (LCDs) [8]. In order to enhance the luminescent properties of ZnO, the doped ZnO in binary or ternary systems, such as ZnO:Zn [7], ZnO:Eu [7–12], ZnO:Mn, ZnO:V, ZnO:W, ZnO:(Y,Eu), and ZnO:(W,Mg) [12] has been investigated. * Corresponding author. Tel.: C81-82-4247716; fax: C81-82-4247850. E-mail address: [email protected] (K. Okuyama). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.008
The ZnO:Eu system is very interesting because there is no energy transferred from the ZnO host to europium ions [10] and both the host (ZnO) and the doping (Eu) emit different colors. It implies a possibility for producing more than one base color of a ZnO:Eu luminescence system [11]. The number of base color, of course, depends on the material sources and synthesis conditions. Abdullah et al. succeeded in synthesizing three base colors of ZnO:Eu prepared by an in situ solution method using a very high concentration of LiOH. Indeed, that trivalent europium ion can be reduced into divalent europium ion [1,13]. Since both ions also emit different colors, the coexistence of Eu2C and Eu3C ions in a ZnO host is interesting and will be investigated. In the experiment reported here, the ZnO:Eu system was first synthesized by spray pyrolysis (SP) method without further treatment. The selection of SP for the preparation of materials was due to simplicity. In the SP method, zinc acetate solution and europium nitrate as the europium ion source were mixed as the starting solution. All chemicals were decomposed inside a micron-droplet at a high temperature while they were in the gas phase. Because the reaction of materials occurred at a homogeneous condition in micron sized droplets, there were no losses in material.
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2. Experimental The materials of zinc acetate dihydrate (99.9%) and europium (III) nitrate hexahydrate (99.9%) were purchased from Wako Chemicals. Both chemicals were used without further purification. Ultra pure water was used as a solvent. The concentration of europium nitrate was varied, while the total concentrations of the precursor solutions were fixed at 0.1 M. The apparatus used was the same as that reported in our previous paper [6]. The precursor solution was sprayed ultrasonically into a hot wall tubular reactor under 1 l/min N2 flow. The reactor was operated at a high temperature to activate europium dopants. The particles were collected by using an electrostatic precipitator. The photoluminescent (PL) characteristics of the prepared particles were characterized by means of spectrofluorophotometer (RF-5300 PC, Shimadzu) in which a xenon short arc lamp was used as an excitation source. All PL spectra were measured at room temperature under air. The crystallinity of the products was characterized by means of X-ray diffraction (XRD, Rint 2200V, Rigaku) and the morphology was observed using a field emission scanning electron microscope (FE-SEM, S-5000, Hitachi).
3. Results and discussion 3.1. The crystallinity of ZnO:Eu samples The XRD pattern of white powders obtained from various concentrations of europium ions had the preferential
Fig. 1. XRD patterns of samples prepared at the synthesis temperature of 1100 8C (*europium nitrate concentration).
Fig. 2. XRD patterns of samples prepared by adding 9 mol% europium nitrate to the precursor solution at different synthesis temperatures.
growth along the [101] direction (Fig. 1). Pure ZnO showed the zincite structure in conformity with the JCPDS 36-1451 as reported in Refs. [5,6]. A very high degree of crystallinity resulted from the use of the high synthesis temperature (1100 8C) to promote the crystal growth. The addition of a small amount of europium ions (up to 1 mol%) into the precursor solution, did not change the crystal structure of the samples. There was no evidence of new peaks presented in the XRD pattern of these samples. However by increasing the concentration of europium ions, new peaks appeared at 2qz28, 33 and 428. By comparing the XRD patterns of samples with the JCPDS 34-0072, it suggested that Eu3C ions existed in the samples prepared by the additional 3 mol% or more of europium nitrate. Moreover, these phenomena indicated that the samples prepared from the high concentration of europium ions formed a composite. The influence of the synthesis temperature on the XRD patterns of sample prepared from the additional high concentration of europium ions (9 mol%) is shown in Fig. 2. Increasing the synthesis temperatures up to 1100 8C, strongly enhanced the formation of the Eu2O3 compound. In the X-ray patterns of samples prepared at low synthesis temperatures (750 and 950 8C), all peak positions were corresponded in accordance with ZnO reference (JCPDS 36-1451). On the other hand, the peaks associated with ZnO phase (JCPDS 36-1451) were very broad from the sample prepared at 1100 8C. Moreover, peaks with strong intensity appeared at 2qz28 and 338. Although the peaks associated with Eu2O3 compound (JCPDS 34-0072) in the region between 28 and 338 did not appear for the sample prepared at 1100 8C, however the peaks of Eu2O3 phase were exist for the sample prepared at higher temperature (1300 8C). We assumed that the strong peaks at 2qz28 and 338 from
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Fig. 3. SEM images of samples prepared by adding europium nitrates at different concentrations and synthesis temperatures. (a) Pure ZnO, 1100 8C, (b) ZnO:Eu (9 mol%), 1100 8C, (c) ZnO:Eu (9 mol%), 630 8C and (d) ZnO:Eu (1 mol%), 1300 8C.
the sample prepared at 1100 8C associated with Eu2O3 compound. The degree of crystalinity of the sample prepared at 750 8C was lower than that of 950 8C. This indicated that by increasing 200 8C of synthesis temperature (from 750 to 950 8C), the degree of crystallinity can be improved. FE-SEM pictures of particles in high magnifications are shown in Fig. 3. The europium doped ZnO prepared at high synthesis temperatures (1100 8C or above, Fig. 3a, b and d) shows the highly agglomerated form. The presence of the europium ion in the ZnO did not greatly
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Fig. 5. PL spectra of samples prepared at different synthesis temperatures. The spectra were measured under the excitations of (a) 321 and (b) 466 nm, respectively.
influence the size and the morphology of particles (Fig. 3a and b). The agglomerate form of particles was obtained because the synthesis temperature was very high. Fig. 3c shows the europium doped ZnO prepared from the additional 9 mol% europium ion at a 630 8C synthesis temperature. Nearly spherical, 600–1000 nm sized particles were obtained. 3.2. Photoluminescence properties of ZnO:Eu samples The role of europium ion concentration and the synthesis temperature on the PL properties of samples are shown by Figs. 4–6. Figs. 4a, 5a and 6a show the PL spectra of samples excited using a wavelength of 321 nm. Figs. 4b, 5b and 6b show the PL spectra of
Fig. 4. PL spectra of samples prepared by adding different europium nitrate concentrations to the precursor solution. The spectra were measured under the excitation of (a) 321 and (b) 466 nm, respectively.
Fig. 6. PL spectra of samples prepared by adding 1 mol% europium nitrate to the precursor solution with different synthesis temperatures under excitation of (a) 321 and (b) 466 nm, respectively.
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samples recorded at a 466 nm excitation. The aim of using 466 nm wavelength of excitation was to confirm the transition originated from Eu3C [10]. The intensity of the PL peaks was affected by the addition of the europium ions into the precursor solution. In Fig. 4a, the position of the PL peak of ZnO prepared with the addition 0.5–9 mol% of europium ions centered at three positions, i.e. 364, 399 and 468 nm, respectively. These peaks resulted from the 5d/4f transition. The emission curve is very broad due to the splitting of d-state. We suggested such shape was caused by the presence of Eu2C ions. The peak intensity centered at 364 and 399 nm was strongly influenced by increasing the concentration of europium ion. The highest peaks at the positions of 364 and 399 nm were obtained at the sample prepared from 0.1 mol% europium ion. On the other hand, at the higher concentration (3 mol% or more) of the europium ion, the intensity of PL peaks at 364 and 399 nm were lowered by increasing the concentration of europium ion. These indicate that the addition of the 3 mol% europium ion exceeded the quenching point of the doping concentration. The europium ions easily take the trivalent state in oxide, such as in Y2O3 and CaO [7], however these ions have tendency take the divalent state [1,13]. Cocivera et al. reported that the reduction of Eu3C/Eu2C occurred in the alkaline earth carbonate thin films by annealing alkaline earth containing Eu3C ions under air. The effectiveness of the reduction process was affected by the cations of the host lattice [13]. The reduction of trivalent to divalent ions possibly occurred in the material due to a charge compensation [13]. In addition, the reduction of trivalent to divalent state of europium ions easily occurred because the 4f/5d transitional energy is higher than the charge-transfer state (CTS) [2]. In Fig. 4b, there are no peaks related to the Eu3C ion which appeared in the samples prepared with the addition of 0.5 mol% europium ions. The small evidence of a red PL peak at 613 nm appeared from samples prepared with the addition of 1 mol% europium ion (Fig. 4b), and on the other hand peak at 364 nm also strongly existed from the same sample (Fig. 4a). These results indicate that Eu2C and Eu3C ions coexistence in the sample prepared with the additional 1 mol% europium nitrate. The addition of higher (more than 1 mol%) europium ions concentration into precursor solution increases the intensity of PL at 613 nm. These results indicated that the Eu3C ions exist at the samples prepared with the addition 1 mol% or higher concentration of europium ion. The peak position at 613 nm related to the 5D0/7F2 transition. Because of the differences in the electrical charge and the ionic radii, the Eu3C ions can not replace the Zn2C ions. The large amount of Eu3C creates a new component, i.e. Eu2O3. Regarding the XRD pattern as shown in the Fig. 1, it is clear that the luminescent peaks located at the red region in Fig. 3b originated from the Eu2O3 component in the composite form.
The effect of synthesis temperature on the PL properties was studied on the samples prepared with the additional higher concentration (9 mol%) of the europium ion. These effects are shown in Fig. 5. In the sample prepared at 630 8C, there are tendencies in the co-existence of Eu2C and Eu3C in the matrix. The very low peak intensities correspond to Eu2C and Eu3C are present in the blue region (Fig. 5a) and red region (Fig. 5b) respectively. Both spectra were influenced by the synthesis temperature. By using the synthesis temperature of 750 8C or above, the intensity of PL peak at 399 and 468 nm increased. In addition, a new peak at 364 nm appeared and the peak at 399 nm decreased in the samples prepared at 1100 8C. By using the excitation of 466 nm, the intensity of the red peak (w613 nm) also increased by raising the synthesis temperature. The peak position at 612 nm shifted to 613 and 611 nm for the samples prepared at 950 and 1100 8C, respectively. The peak at 611 nm became sharper by increasing the synthesis temperature and a new peak at 628 nm presented in the sample prepared at 1100 8C. Yamashita et al. observed the coexistence of both Eu2C and Eu3C ions from the trivalent europium doped CaO sample. When CaO:Eu is prepared in N2, most Eu ions are introduced as Eu3C into CaO. On the other hand, the Eu2C center increases when the CaO:Eu is prepared in H2 [14]. The influence of the synthesis temperature on the samples prepared with a low concentration (1 mol%) of europium ion was also investigated. The PL characteristics of the samples are shown by Fig. 6. Under the excitation of 321 nm, the entire PL peaks, especially at the blue (400–500 nm) region, strongly increased by raising the synthesis temperature from 1100 to 1300 8C. The PL peaks at the blue region were intensified approximately seven times; however, this peak is very broad. We suggested that the broading effect was caused by two reasons; first, the peak originated from the overlap of those centered at 463, 468 and 473 nm and secondly, the peak related to d-state emission which is increased by raising the synthesis temperature. The increases in the blue luminescent peaks indicate that the synthesis temperature is a very important factor in order to activate the divalent europium ion.
4. Conclusion The reduction of the Eu3C/Eu2C ions occurred in the particles prepared by the addition of a low concentration of europium ion. This reduction changed the base color of luminescence from red to blue. Blue luminescence can be enhanced by increasing the synthesis temperature. At a high concentration of europium ion, the Eu3C created the Eu2O3 component in the ZnO–Eu2O3 composite.
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Acknowledgements This study was supported in part by the NEDO’s Nanotechnology Particle Project from the Ministry of Economy, Trade and Industry of Japan, Grant-in Aids from the Ministry of Education, Culture, Sports, Science and Technology of Japan (K.O. and I.W.L.) and financial support from Hashiya Scholarship Foundation (C.P.). The authors thank Mr Mikrajudin Abdullah for his helpful discussion and Mr Yasumori Kuromizu for his assistance during the experiments and characterization.
References [1] G. Blasse, Luminescent Materials, Springer, New York, 1994. [2] W.M. Yen, S. Shionoya (Eds.), Phosphor Handbook, CRC Press, Boca Raton, FL, 1998. [3] K.H. Yoon, J.Y. Cho, Photoluminescence characteristics of zinc oxide thin films prepared by spray pyrolysis technique, Mater. Res. Bull. 35 (2000) 39–46. [4] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J. Caruso, M.J. Hampden-Smith, T.T. Kodas, Green photoluminescence efficiency and free carrier density in ZnO phosphor powders prepared by spray pyrolysis, J. Lumin. 75 (1997) 11–16.
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[5] S.A. Studenikin, N. Golego, M. Cocivera, Fabrication of green and orange photoluminescent, undoped ZnO films using spray pyrolysis, J. Appl. Phys. 84 (1998) 2287–2294. [6] C. Panatarani, I.W. Lenggoro, K. Okuyama, Synthesis of single crystalline ZnO nanoparticles by salt assisted spray pyrolysis, J. Nanoparticles Res. 5 (2003) 47–53. [7] Y. Hayashi, H. Narahara, T. Uchida, T. Noguchi, S. Ibuki, Photoluminescence of Eu-doped ZnO phosphor, Jpn. J. Appl. Phys. 34 (1995) 1878–1882. [8] R.N. Bhargava, V. Chhabra, T. Som, A. Ekimov, N. Taskar, Quantum confined atoms of doped ZnO nanocrystals, Phys. Status Solidi (b) 229 (2002) 897–901. [9] Y.-K. Park, J.-I. Han, M.G. Kwak, H. Yang, S.-H. Ju, W.-S. Cho, Effect of coupling structure of Eu on the photoluminescent characteristics for ZnO:EuCl3 phosphor, Appl. Phys. Lett. 72 (1998) 668–670. [10] S.A.M. Lima, F.A. Sigoli, M.R. Davolos, M. Jafelicci Jr., Europium (III)-containing zinc oxide from Pechini method, J. Alloys Compd 344 (2002) 280–284. [11] M. Abdullah, T. Morimoto, K. Okuyama, Generating blue and red luminescence from ZnO/poly(ethylene glycol) nanocomposites prepared by an in-situ method, Adv. Funct. Mater. 13 (2003) 1–5. [12] V.Z. Mardkovich, H. Hayashi, M. Haemori, T. Fukumura, M. Kawasaki, Discovery and optimization of new ZnO-based phosphors using a combinatorial method, Adv. Funct. Mater. 13 (2003) 519–524. [13] J. Hao, J. Gao, M. Cocivera, Tuning of the blue emission from europium-doped alkaline earth chloroborate thin films activated in air, Appl. Phys. Lett. 82 (2003) 2778–2780. [14] N. Yamashita, Coexistence of the Eu2C and Eu3C centers in the CaO:Eu powder phosphor, J. Electrochem. Soc. 140 (1993) 840–843.