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Synthesis and photoluminescent properties of CaMoO4 nanostructures at room temperature
Materials Letters 64 (2010) 602–604
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and photoluminescent properties of CaMoO4 nanostructures at room temperature Yongkui Yin 1, Yan Gao 1, Yuzeng Sun 1, Baibin Zhou ⁎, Lin Ma, Xiang Wu, Xu Zhang State Key Laboratory of Physical and Chemical Materials, Harbin Normal University, Harbin 150025, PR China
a r t i c l e
i n f o
Article history: Received 7 December 2008 Accepted 7 December 2009 Available online 16 December 2009 Keywords: Nanostructures Microemulsions Photoluminescence Molybdates
a b s t r a c t CaMoO4 nanostructures with different morphologies, such as ellipsoid-like, spindle-like, and sphere-like, were successfully synthesized in a simple cationic surfactant-CTAB-microemulsion system at room temperature. The molar ratio (w) of H2O to CTAB and the concentration of reactants played important roles in the morphological control of CaMoO4 nanostructures. A possible mechanism was proposed for the selective formation of the different morphologies. The CaMoO4 nanostructures exhibited excellent photoluminescence properties with the same new green emission peaks at 495 nm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Scheelite-type CaMoO4 has been of continuous research interest ever since the beginning of the 1970s because of its ability to produce green luminescence, which forms the basis of its wide use as phosphors, laser materials, and scintillation detectors [1–3]. Traditional ways to synthesize CaMoO4 usually need high temperature and harsh reaction conditions, such as the Czochralski technique [4], coprecipitation method [5], combustion synthesis [6], and solid-state reaction [7]. However, CaMoO4 powders prepared by these ways are relatively large with inhomogeneous morphology and composition, which make them hard to exhibit the ideal photoluminescence properties. To overcome this disadvantage, several new approaches have been used to synthesize CaMoO4 nanostructures, including complex polymerization [8], molten salt [9], microemulsion [10], the citrate complex [11], and pulsed laser ablation [12]. The obtained CaMoO4 nanostructures exhibit the good photoluminescence properties with emission peaks at 578 nm [8], 508 nm [9], 480 nm [10,11], and 430 nm [12], respectively. In order to extend their application range, the synthesis of CaMoO4 nanostructures with new emission peaks is much needed. In this paper, we report the synthesis of CaMoO4 nanostructures with various morphologies in a simple cationic surfactant-CTABmicroemulsion system. By carefully controlling the fundamental experimental parameters such as the molar ratio of H2O to CTAB (defined w) and the concentration of reactants, CaMoO4 nanostructures with morphologies of ellipsoid-like, spindle-like and sphere-like ⁎ Corresponding author. E-mail address: [email protected] (B. Zhou). 1 These authors contributed equally to this work. 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.12.014
can be efficiently achieved. The CaMoO4 nanostructures exhibited the same new green emission peaks at 495 nm with excellent photoluminescence properties. 2. Experimental procedure 2.1. Synthesis All chemical reagents were of analytical grade. Typically, CaMoO4 nanoparticles were prepared by the following procedure. First, two types of microemulsion solutions were prepared by solubilizing aqueous CaCl2 and Na2MoO4 solutions of identical concentration into cyclohexane/cetyltrimethylammonium bromide (CTAB)/1-pentanol microemulsion system (cyclohexane, 25 ml; CTAB, 1 g; 1-pentanol, 1 ml), respectively. After substantial stirring, the above two different microemulsion solutions with equivalent volume were mixed and stirred for another 15 min. The resulting mixture was then kept at room temperature for 12 h. After all reactions were completed, the precipitate was collected by centrifuging, washed several times with absolute ethanol and distilled water, and dried in air at room temperature. 2.2. Characterization X-ray powder diffraction pattern (XRD) of the product was collected on a Y-2000 automated X-ray diffractometer with Cu Kα radiation (λ = 0.1505 nm). The morphology and size of products were observed by HITACHI S4800 field emission scanning electron microscopy (FE-SEM). The fluorescence spectra of products were recorded on a Perkin-Elmer LS 55 photoluminescence spectrophotometer (PL) at room temperature.
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corresponding standard deviations of 9.5, 12.8 and 10.5 nm, respectively, indicating that the particles were monodisperse in size. 3.2. Effect of the reaction conditions on the morphology and size of CaMoO4 nanostructures
Fig. 1. XRD pattern of a spherical CaMoO4 sample.
3. Results and discussion 3.1. Characterization of CaMoO4 nanostructures Because all samples with different shapes have same composition, only the XRD pattern of the spherical product is shown as an example. As shown in Fig. 1, all the peaks can be indexed to be pure tetragonal scheelite structure of CaMoO4 with cell parameters of a = 5.226 Å and c = 11.43 Å (JCPDS Card no. 29-351). No other peaks of impurities are detected in the synthesized products. Fig. 2a–c shows typical FE-SEM images of CaMoO4 nanostructures with morphologies of ellipsoid-like, spindle-like and sphere-like obtained at different reaction conditions, respectively, indicating that large quantity and good uniformity were achieved. The average major axis size of nanoellipsoids is 132 nm while the average minor axis size is 85 nm (Fig. 2a). From Fig. 2b, we can see that the average major and the minor axis sizes of CaMoO4 nanospindles are 246 and 90 nm, respectively. The CaMoO4 nanospheres have an average diameter of 576 nm (Fig. 2c). Statistics on the major axis size of nanoellipsoids and nanospindles and the diameter of nanospheres were taken by measuring SEM images from the same samples as in Fig. 2a, b, and c. As shown in Fig. 2d–f (Gaussian distribution), the average major axis size of nanoellipsoids and nanospindles and the average diameter of nanospheres are 132, 246, and 576 nm, respectively, with
The morphology and size of the products were found to depend on the w value and the concentration of reactants, in which the concentration of CTAB was keep at a constant of 0.1 M. When the w value was increased from 10 to 40, nanoellipsoids (Fig. 2a), with an average major axis of 132 nm and minor axis of ∼ 85 nm, evolved into the nanospheres with an average diameter of 576 nm (Fig. 2c). After the concentration of reactants was changed from 0.1 to 1 M, the average major axis size of the nanoellipsoids increased from 132 nm to 246 nm, whereas the minor axis size keeps almost unchangeable (∼ 90 nm), which make nanoellipsoids evolve into nanospindles (Fig. 2b). The above results show that with the increase of the w value or the concentration of reactants, the morphologies of products changed gradually from ellipsoid-like to sphere-like or spindle-like, respectively, and the sizes of products were increased. 3.3. Formation mechanism Based on the above experimental results, it can be concluded that the w value and the reactants concentration have a significant effect on the morphology and size of final products. Generally, after two microemulsion solutions containing Ca2+ and MoO2− were mixed together, 4 CaMoO4 nucleation and irreversible micellar fusion may be coincident and result in the formation of CaMoO4 nuclei in center. Thus the ends of the nuclei have already turned into a water-rich region while the central region became a water-poor region. When w = 10 was used, which was considered to be moderate water content [13], the fuse rate between two spherical droplets would be relatively slow and dynamic exchange induced the fuse significantly faster at the ends of the CaMoO4 nuclei than the central region covered by the surfactant. This process may result in the formation of ellipsoid-like nanostructures [13] and small changes along the minor axis direction when w was fixed. In addition, a relatively high concentration led to the formation of nanospindles. This was probably because when a higher concentration of reactants was used, much more CaMoO4 nuclei would be produced in the microemulsion droplets, therefore CaMoO4 nanoellipsoids that formed within the
Fig. 2. Typical FE-SEM images of the products obtained at (a) w = 10, [CaCl2] = 0.1 M, (b) w = 10, [CaCl2] = 1.0 M, and (c) w = 40, [CaCl2] = 0.1 M and the corresponding size statistics for the major axis of (d) nanoellipsoids and (e) nanospindles and the diameter of (f) nanospheres in Fig. 2a–c, respectively.
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Y. Yin et al. / Materials Letters 64 (2010) 602–604
may greatly depend on the particle distribution. Hence, the enhancement of photoluminescence intensity of ellipsoid-like nanostructure can be attributed to the narrow particle size distribution and homogeneous particle morphology. More interestingly, it is clearly seen that there are two weak shoulder peaks in the CaMoO4 emission spectra, which can be explained by the existence of Frenkel defects structure (oxygen ion shifted to the inter-site position with simultaneous creation of vacancy) in the surface layers of nanostructures [16,17]. However, the true factor influencing the PL property is not still clear, which need further investigation. 4. Conclusions
Fig. 3. PL spectra of CaMoO4 nanostructures with morphologies of (a) ellipsoid-like, (b) spindle-like, and (c) sphere-like obtained at the same conditions as in Fig. 2a–c, respectively.
ellipsoidal microemulsion droplets naturally had relatively long major axes. When the w value was high, the fuse rate between two spherical droplets would be very fast and the obtained microemulsion droplets may be nearly spherical, resulting in the formation of CaMoO4 nanospheres with large size [13,14].
In summary, CaMoO4 nanostructures with different morphologies, such as ellipsoid-like, spindle-like and sphere-like, have been synthesized in a simple cationic microemulsion system at room temperature. The morphologies of CaMoO4 nanostructures can be controlled by adjusting the molar ratio of H2O to CTAB and the concentration of reactants. The obtained ellipsoid-like CaMoO4 nanostructures exhibited excellent photoluminescence property, which might hold potential application as a phosphor. Acknowledgements This work was supported by the National Natural Science Foundation of China (nos. 20371014, 20671026 and 20971032). References
3.4. Photoluminescence property The room temperature PL spectra of the products are shown in Fig. 3. It is generally assumed that the measured emission spectrum of CaMoO4 is mainly attributed to the charge-transfer transitions within the MoO42− complex [1,11]. With an excitation at 240 nm, all CaMoO4 nanostructures with different morphologies exhibited the same new green emission peaks at 495 nm. The emission peak positions are different from those previously reported in the case of other CaMoO4 nanostructures [8–12], such as 578 nm [8], 508 nm [9], 480 nm [10,11], and 430 nm [12]. Otherwise, the intensity of PL emission of nanoellipsoids (narrow size distribution) is stronger than that of nanospheres and nanospindles (relatively wide size distribution). Generally, the photoluminescence intensity of phosphors depends strongly on the particle shape and distribution [15]. However, in our work, all the products we obtained are uniform with different morphologies, the photoluminescence intensity
[1] Groenink JA, Hakfoort C, Blasse G. Phys Status Solidi A 1979;54:329–36. [2] Chandrasekhar BK, White WB. Mater Res Bull 1990;25:1513–8. [3] Cho WS, Yashima M, Kakihana M, Kudo A, Sakata T, Yoshimura M. J Am Ceram Soc 1997;80:765–9. [4] Grasser R, Pitt E, Scharmann A, Zimmerer G. Phys Status Solidi B 1975;69:359–68. [5] Paski EF, Blades MW. Anal Chem 1988;60:1224–30. [6] Yang P, Yao GQ, Lin JH. Inorg Chem Commun 2004;7:389–91. [7] Sleight AW. Acta Crystallogr B: Struct Crystallogr Cryst Chem 1972;28:2899–902. [8] Marques APA, Longo VM, Melo DMA, Pizani PS, Leite ER, Varela JA, et al. J Solid State Chem 2008;181:1249–57. [9] Wang YG, Ma JF, Tao JT, Zhu XY, Zhou J, Zhao ZQ, et al. Ceram Int 2007;33:693–5. [10] Gong Q, Qian XF, Ma XD, Zhu ZK. Cryst Growth Des 2006;6:1821–5. [11] Ryu JH, Yoon JW, Lim CS, Oh WC, Shim KB. J Alloy Compd 2005;390:245–9. [12] Ryu JH, Choi BG, Yoon JW, Shim KB, Machi K, Hamada KJ. J Lumin 2007;124:67–70. [13] Cao MH, Wang YH, Guo CX, Qi YJ, Hu CW. Langmuir 2004;20:4784–6. [14] Sun LN, Cao MH, Wang YH, Sun GB, Hu CW. J Cryst Growth 2006;289:231–5. [15] Hong GY, Jeon BS, Yoo YK, Yoo JS. J Electro Soc 2001;148:161–6. [16] Ryu JH, Koo SM, Yoon JW, Lim CS, Shim KB. Mater Lett 2006;60:1072–5. [17] Annenkov A, Auffray E, Korzhik M, Lecoq P, Peigneux JP. Phys Status Solidi A 1998;170:47–62.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and photoluminescent properties of CaMoO4 nanostructures at room temperature Yongkui Yin 1, Yan Gao 1, Yuzeng Sun 1, Baibin Zhou ⁎, Lin Ma, Xiang Wu, Xu Zhang State Key Laboratory of Physical and Chemical Materials, Harbin Normal University, Harbin 150025, PR China
a r t i c l e
i n f o
Article history: Received 7 December 2008 Accepted 7 December 2009 Available online 16 December 2009 Keywords: Nanostructures Microemulsions Photoluminescence Molybdates
a b s t r a c t CaMoO4 nanostructures with different morphologies, such as ellipsoid-like, spindle-like, and sphere-like, were successfully synthesized in a simple cationic surfactant-CTAB-microemulsion system at room temperature. The molar ratio (w) of H2O to CTAB and the concentration of reactants played important roles in the morphological control of CaMoO4 nanostructures. A possible mechanism was proposed for the selective formation of the different morphologies. The CaMoO4 nanostructures exhibited excellent photoluminescence properties with the same new green emission peaks at 495 nm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Scheelite-type CaMoO4 has been of continuous research interest ever since the beginning of the 1970s because of its ability to produce green luminescence, which forms the basis of its wide use as phosphors, laser materials, and scintillation detectors [1–3]. Traditional ways to synthesize CaMoO4 usually need high temperature and harsh reaction conditions, such as the Czochralski technique [4], coprecipitation method [5], combustion synthesis [6], and solid-state reaction [7]. However, CaMoO4 powders prepared by these ways are relatively large with inhomogeneous morphology and composition, which make them hard to exhibit the ideal photoluminescence properties. To overcome this disadvantage, several new approaches have been used to synthesize CaMoO4 nanostructures, including complex polymerization [8], molten salt [9], microemulsion [10], the citrate complex [11], and pulsed laser ablation [12]. The obtained CaMoO4 nanostructures exhibit the good photoluminescence properties with emission peaks at 578 nm [8], 508 nm [9], 480 nm [10,11], and 430 nm [12], respectively. In order to extend their application range, the synthesis of CaMoO4 nanostructures with new emission peaks is much needed. In this paper, we report the synthesis of CaMoO4 nanostructures with various morphologies in a simple cationic surfactant-CTABmicroemulsion system. By carefully controlling the fundamental experimental parameters such as the molar ratio of H2O to CTAB (defined w) and the concentration of reactants, CaMoO4 nanostructures with morphologies of ellipsoid-like, spindle-like and sphere-like ⁎ Corresponding author. E-mail address: [email protected] (B. Zhou). 1 These authors contributed equally to this work. 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.12.014
can be efficiently achieved. The CaMoO4 nanostructures exhibited the same new green emission peaks at 495 nm with excellent photoluminescence properties. 2. Experimental procedure 2.1. Synthesis All chemical reagents were of analytical grade. Typically, CaMoO4 nanoparticles were prepared by the following procedure. First, two types of microemulsion solutions were prepared by solubilizing aqueous CaCl2 and Na2MoO4 solutions of identical concentration into cyclohexane/cetyltrimethylammonium bromide (CTAB)/1-pentanol microemulsion system (cyclohexane, 25 ml; CTAB, 1 g; 1-pentanol, 1 ml), respectively. After substantial stirring, the above two different microemulsion solutions with equivalent volume were mixed and stirred for another 15 min. The resulting mixture was then kept at room temperature for 12 h. After all reactions were completed, the precipitate was collected by centrifuging, washed several times with absolute ethanol and distilled water, and dried in air at room temperature. 2.2. Characterization X-ray powder diffraction pattern (XRD) of the product was collected on a Y-2000 automated X-ray diffractometer with Cu Kα radiation (λ = 0.1505 nm). The morphology and size of products were observed by HITACHI S4800 field emission scanning electron microscopy (FE-SEM). The fluorescence spectra of products were recorded on a Perkin-Elmer LS 55 photoluminescence spectrophotometer (PL) at room temperature.
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corresponding standard deviations of 9.5, 12.8 and 10.5 nm, respectively, indicating that the particles were monodisperse in size. 3.2. Effect of the reaction conditions on the morphology and size of CaMoO4 nanostructures
Fig. 1. XRD pattern of a spherical CaMoO4 sample.
3. Results and discussion 3.1. Characterization of CaMoO4 nanostructures Because all samples with different shapes have same composition, only the XRD pattern of the spherical product is shown as an example. As shown in Fig. 1, all the peaks can be indexed to be pure tetragonal scheelite structure of CaMoO4 with cell parameters of a = 5.226 Å and c = 11.43 Å (JCPDS Card no. 29-351). No other peaks of impurities are detected in the synthesized products. Fig. 2a–c shows typical FE-SEM images of CaMoO4 nanostructures with morphologies of ellipsoid-like, spindle-like and sphere-like obtained at different reaction conditions, respectively, indicating that large quantity and good uniformity were achieved. The average major axis size of nanoellipsoids is 132 nm while the average minor axis size is 85 nm (Fig. 2a). From Fig. 2b, we can see that the average major and the minor axis sizes of CaMoO4 nanospindles are 246 and 90 nm, respectively. The CaMoO4 nanospheres have an average diameter of 576 nm (Fig. 2c). Statistics on the major axis size of nanoellipsoids and nanospindles and the diameter of nanospheres were taken by measuring SEM images from the same samples as in Fig. 2a, b, and c. As shown in Fig. 2d–f (Gaussian distribution), the average major axis size of nanoellipsoids and nanospindles and the average diameter of nanospheres are 132, 246, and 576 nm, respectively, with
The morphology and size of the products were found to depend on the w value and the concentration of reactants, in which the concentration of CTAB was keep at a constant of 0.1 M. When the w value was increased from 10 to 40, nanoellipsoids (Fig. 2a), with an average major axis of 132 nm and minor axis of ∼ 85 nm, evolved into the nanospheres with an average diameter of 576 nm (Fig. 2c). After the concentration of reactants was changed from 0.1 to 1 M, the average major axis size of the nanoellipsoids increased from 132 nm to 246 nm, whereas the minor axis size keeps almost unchangeable (∼ 90 nm), which make nanoellipsoids evolve into nanospindles (Fig. 2b). The above results show that with the increase of the w value or the concentration of reactants, the morphologies of products changed gradually from ellipsoid-like to sphere-like or spindle-like, respectively, and the sizes of products were increased. 3.3. Formation mechanism Based on the above experimental results, it can be concluded that the w value and the reactants concentration have a significant effect on the morphology and size of final products. Generally, after two microemulsion solutions containing Ca2+ and MoO2− were mixed together, 4 CaMoO4 nucleation and irreversible micellar fusion may be coincident and result in the formation of CaMoO4 nuclei in center. Thus the ends of the nuclei have already turned into a water-rich region while the central region became a water-poor region. When w = 10 was used, which was considered to be moderate water content [13], the fuse rate between two spherical droplets would be relatively slow and dynamic exchange induced the fuse significantly faster at the ends of the CaMoO4 nuclei than the central region covered by the surfactant. This process may result in the formation of ellipsoid-like nanostructures [13] and small changes along the minor axis direction when w was fixed. In addition, a relatively high concentration led to the formation of nanospindles. This was probably because when a higher concentration of reactants was used, much more CaMoO4 nuclei would be produced in the microemulsion droplets, therefore CaMoO4 nanoellipsoids that formed within the
Fig. 2. Typical FE-SEM images of the products obtained at (a) w = 10, [CaCl2] = 0.1 M, (b) w = 10, [CaCl2] = 1.0 M, and (c) w = 40, [CaCl2] = 0.1 M and the corresponding size statistics for the major axis of (d) nanoellipsoids and (e) nanospindles and the diameter of (f) nanospheres in Fig. 2a–c, respectively.
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Y. Yin et al. / Materials Letters 64 (2010) 602–604
may greatly depend on the particle distribution. Hence, the enhancement of photoluminescence intensity of ellipsoid-like nanostructure can be attributed to the narrow particle size distribution and homogeneous particle morphology. More interestingly, it is clearly seen that there are two weak shoulder peaks in the CaMoO4 emission spectra, which can be explained by the existence of Frenkel defects structure (oxygen ion shifted to the inter-site position with simultaneous creation of vacancy) in the surface layers of nanostructures [16,17]. However, the true factor influencing the PL property is not still clear, which need further investigation. 4. Conclusions
Fig. 3. PL spectra of CaMoO4 nanostructures with morphologies of (a) ellipsoid-like, (b) spindle-like, and (c) sphere-like obtained at the same conditions as in Fig. 2a–c, respectively.
ellipsoidal microemulsion droplets naturally had relatively long major axes. When the w value was high, the fuse rate between two spherical droplets would be very fast and the obtained microemulsion droplets may be nearly spherical, resulting in the formation of CaMoO4 nanospheres with large size [13,14].
In summary, CaMoO4 nanostructures with different morphologies, such as ellipsoid-like, spindle-like and sphere-like, have been synthesized in a simple cationic microemulsion system at room temperature. The morphologies of CaMoO4 nanostructures can be controlled by adjusting the molar ratio of H2O to CTAB and the concentration of reactants. The obtained ellipsoid-like CaMoO4 nanostructures exhibited excellent photoluminescence property, which might hold potential application as a phosphor. Acknowledgements This work was supported by the National Natural Science Foundation of China (nos. 20371014, 20671026 and 20971032). References
3.4. Photoluminescence property The room temperature PL spectra of the products are shown in Fig. 3. It is generally assumed that the measured emission spectrum of CaMoO4 is mainly attributed to the charge-transfer transitions within the MoO42− complex [1,11]. With an excitation at 240 nm, all CaMoO4 nanostructures with different morphologies exhibited the same new green emission peaks at 495 nm. The emission peak positions are different from those previously reported in the case of other CaMoO4 nanostructures [8–12], such as 578 nm [8], 508 nm [9], 480 nm [10,11], and 430 nm [12]. Otherwise, the intensity of PL emission of nanoellipsoids (narrow size distribution) is stronger than that of nanospheres and nanospindles (relatively wide size distribution). Generally, the photoluminescence intensity of phosphors depends strongly on the particle shape and distribution [15]. However, in our work, all the products we obtained are uniform with different morphologies, the photoluminescence intensity
[1] Groenink JA, Hakfoort C, Blasse G. Phys Status Solidi A 1979;54:329–36. [2] Chandrasekhar BK, White WB. Mater Res Bull 1990;25:1513–8. [3] Cho WS, Yashima M, Kakihana M, Kudo A, Sakata T, Yoshimura M. J Am Ceram Soc 1997;80:765–9. [4] Grasser R, Pitt E, Scharmann A, Zimmerer G. Phys Status Solidi B 1975;69:359–68. [5] Paski EF, Blades MW. Anal Chem 1988;60:1224–30. [6] Yang P, Yao GQ, Lin JH. Inorg Chem Commun 2004;7:389–91. [7] Sleight AW. Acta Crystallogr B: Struct Crystallogr Cryst Chem 1972;28:2899–902. [8] Marques APA, Longo VM, Melo DMA, Pizani PS, Leite ER, Varela JA, et al. J Solid State Chem 2008;181:1249–57. [9] Wang YG, Ma JF, Tao JT, Zhu XY, Zhou J, Zhao ZQ, et al. Ceram Int 2007;33:693–5. [10] Gong Q, Qian XF, Ma XD, Zhu ZK. Cryst Growth Des 2006;6:1821–5. [11] Ryu JH, Yoon JW, Lim CS, Oh WC, Shim KB. J Alloy Compd 2005;390:245–9. [12] Ryu JH, Choi BG, Yoon JW, Shim KB, Machi K, Hamada KJ. J Lumin 2007;124:67–70. [13] Cao MH, Wang YH, Guo CX, Qi YJ, Hu CW. Langmuir 2004;20:4784–6. [14] Sun LN, Cao MH, Wang YH, Sun GB, Hu CW. J Cryst Growth 2006;289:231–5. [15] Hong GY, Jeon BS, Yoo YK, Yoo JS. J Electro Soc 2001;148:161–6. [16] Ryu JH, Koo SM, Yoon JW, Lim CS, Shim KB. Mater Lett 2006;60:1072–5. [17] Annenkov A, Auffray E, Korzhik M, Lecoq P, Peigneux JP. Phys Status Solidi A 1998;170:47–62.