- Email: [email protected]
Facile synthesis and photoluminescence properties of dumbbell-like Ln-doped BaWO4 (Ln = Nd, Er, Yb) microstructures
Materials Letters 64 (2010) 1503–1505
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
Facile synthesis and photoluminescence properties of dumbbell-like Ln-doped BaWO4 (Ln = Nd, Er, Yb) microstructures Suying Hou a, Yan Xing a,⁎, Hong Ding b, Xianchun Liu a, Bo Liu a, Xiujuan Sun a a b
College of Chemistry, Northeast Normal University, Changchun 130024, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 1 December 2009 Accepted 2 April 2010 Available online 9 April 2010
a b s t r a c t Dumbbell-like microstructure of Ln-doped BaWO4 (Ln = Nd, Er, Yb) has been successfully synthesized via a very simple precipitation technique at room temperature without any templates or catalyst. The synthesized products were systematically studied by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Such a simple one-pot solution-based process may be extended to fabricate complex hierarchical micro- and nanostructures of other functional materials. Additionally, the near-infrared luminescence of Ln-doped BaWO4 (Ln = Nd, Er, Yb) dumbbells was discussed in detail. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, tungstate materials have aroused much interest due to their luminescence behavior, structural properties, and potential applications in various fields [1,2]. Barium tungstate (BaWO4) with a tetragonal scheelite-type structure is a very promising material in the electro-optical industry because of its emission of blue luminescence [3]. In addition, because of its interesting stimulated Raman scattering (SRS) properties, BaWO4 is also a potential material for designing allsolid-state lasers emitting radiation in a specific spectral region and will have applications for medical treatment, up-conversion lasers and spectroscopy [4,5]. Due to these important properties, many efforts have been made for the synthesis of BaWO4 with a desired structure and morphology, such as 1D uniform nanorods [6], 2D nanosheets [7] and microparticles with a complex form [8–11]. However, most of these methods need either templates (such as surfactant, polymer, etc.) or complex equipment. Recently, Wang et al. reported the synthesis of well-crystallized BaWO4 powders with different morphologies via a simple template-free method [12]. The alkaline-earth metal tungstates AWO4 (A = Ca2+, Sr2+, Ba2+) have been extensively investigated as a self-activating phosphor emitting blue or green light under ultraviolet or X-ray excitation [13,14]. And lanthanide elements are known for their unique luminescence properties. Recently, AWO4 were reported to be efficient luminescent hosts for rare-earth [15–17]. When the lanthanide luminescence occurs in the near-infrared regions, these elements may be used as fluorescers in the design of signal
⁎ Corresponding author. Tel.: + 86 431 86105529; fax: + 86 431 85099108. E-mail address: [email protected] (Y. Xing). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.04.004
amplification for optical communication [18], to compensate light lost during transmission. Up to date, a number of rare-earth (Nd3+, Tm3+, Er3+, Ho3+ and Yb3+)-doped AWO4 (A = Ca2+, Sr2+, Ba2+) crystals have been successfully synthesized [19–24]. However, to the best of knowledge, few studies have been carried out to fabricate hierarchical micro/nanocomposite structures of lanthanide-doped alkaline-earth metal tungstates. Herein, a simple precipitation technique was proposed to synthesize dumbbell-like Ln3+ (Ln = Nd, Er, Yb)-doped BaWO4 microstructures without using any template at room temperature. Near-infrared luminescent properties of lanthanide ion doped BaWO4 microstructures were also investigated. 2. Experimental All chemical reagents were of analytical grade and used as received without further purification. Typically, 1.52 mmol of Ba(NO3)2 and 0.08 mmol of Ln(NO3)3·6H2O (Ln = Er, Nd, Yb) were first added to 8 mL distilled water at room temperature, NaOH solution (0.5 M) was used to adjust the pH of the mixture. Then 8 mL of 0.2 M Na2WO4 (1.6 mmol) was added dropwise to the above solution under stirring. The suspension containing white precipitates was stirred for a further 15 min. Finally, the resulting precipitates were filtered, washed with distilled water and absolute ethanol, and dried at 50 °C in air. X-ray powder diffraction (XRD) analysis was measured on a Siemens D5005 Diffractometer with CuKα radiation (λ=0.15418 nm). Fieldemission scanning electron microscopy (FESEM) images were obtained with an XL30 ESEM FEG microscope. Transmission electron microscopic (TEM) images were obtained on a JEM-2100F microscope with an accelerating voltage of 200 kV. Room-temperature photoluminescence (PL) spectra were recorded on a Fluorolog-3 with a 400 W xenon lamp as the excitation source at room temperature.
1504
S. Hou et al. / Materials Letters 64 (2010) 1503–1505
Fig. 1. XRD patterns of the samples (a) Yb3+- (b) Er3+- and (c) Nd3+-doped BaWO4 obtained at room temperature.
3. Results and discussion The phase compositions and phase structures of Ln3+-doped BaWO4 (Ln = Nd, Er, Yb) microcrystals were examined by X-ray powder diffraction (XRD). As shown in Fig. 1, all of the detectable peaks can be indexed to almost the same positions as those from the tetragonal structured BaWO4 with unit cell dimensions a = 0.561 nm, c = 1.271 nm (JCPDS card No. 43-0646), which indicates the introduction of 5% Ln3+-doped BaWO4 did not destroy the tetragonalphase structure. The strong and sharp diffraction peaks indicate that the as-obtained products are well-crystalline. The morphologies of the as-synthesized products at room temperature were examined by field-emission scanning electron microscopy (FESEM). A large number of well-defined dumbbell-like shapes with a length of about 5–8 µm were formed as shown in Fig. 2a.
No other morphologies can be detected, indicating a high yield of these Ln3+-doped BaWO4 dumbbells. A high-magnification SEM image (Fig. 2b) shows that each dumbbell is actually composed of nanorods branched out radically from the revolutional axis of the dumbbell with an average diameter of about 800 nm. Also the surface of the dumbbells is smooth. Fig. 2c shows a typical TEM image of one single dumbbell, revealing that the dumbbell is constructed by nanorods, which is in accordance with the SEM images. The fast Fourier transform (FFT) (inset of Fig. 2c) pattern exhibits that the dumbbell-like structure is crystalline in nature. A representative HRTEM image obtained from the tip of a nanorod is shown in Fig. 2d. The lattice fringes are clearly visible with a spacing of 0.542 nm and 0.203 nm, which agree well with the lattice spacing of (101) and (220) of tetragonal Ln3+-doped BaWO4, respectively. The optical properties of Ln3+ (Ln = Nd, Er, Yb)-doped BaWO4 samples have been systematically investigated in detail. Note that this doping process alters neither the crystal structures nor the shapes of the host materials. The emission spectrum of the BaWO4:Er dumbbells was obtained by exciting the sample at 275 nm. The broad emission band extending from 1440 to 1650 nm and centered at 1532 nm is shown in Fig. 3 and is attributed to the transition from the first excited state (4I13/2) to the ground state (4I15/2) of the partially filled 4f shell of Er3+. Erbium-doped materials have been the subject of much interest for many years, because the transition around 1540 nm is in the right position of the third telecommunication window [25]. The NIR emission spectra of BaWO4:Nd and BaWO4:Yb are depicted in Fig. 3, which were collected by excitation at 277 and 275 nm, respectively. The NIR spectrum of BaWO4:Nd consists of three bands centered at λ=889, 1061, and 1333 nm, which are attributed to the f–f transitions of 4 F3/2 (emitting level)→ 4I9/2, 4F3/2 → 4I11/2, and 4F3/2 → 4I13/2, respectively. The solid materials containing the Nd3+ ion have been regarded as the most popular luminescent materials in laser system applications, because the strongest transition at 1060 nm has potential application for laser emission [26]. In the NIR spectrum of BaWO4:Yb, the prominent 977 nm emission band can be observed, which is assigned to the 2F5/2 → 2F7/2 transition of the Yb3+ ion. It should also be noted that the Yb3+ ion
Fig. 2. FESEM (a–b), TEM (c), FFT (inset of c) and HRTEM (d) images of the as-prepared samples.
S. Hou et al. / Materials Letters 64 (2010) 1503–1505
1505
Acknowledgments This work was supported by Jilin Province Foundation (20090518), Training Fund of NENU'S Scientific Innovation Project (No. NENUSTC07004) and the Fundamental Research Funds for the Central Universities (09QNJJ 012). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2010.04.004. References
Fig. 3. Emission spectra for Ln-doped BaWO4 (Ln = Nd, Er, Yb) dumbbell-like microstructures: (a) BaWO 4 :Er (λ ex = 275 nm, blue line); (b) BaWO 4 :Nd (λex = 277 nm, red line); (c) BaWO4:Yb (λex = 275 nm, green line).
emission band is not a single sharp band, but an envelope of bands arising at the lower energy side than the primary 977 nm band. This may be the result of splitting of the energy levels of the Yb3+ ion as a consequence of crystal field effects. The Yb3+ ion has some advantages for laser emission due to its very simple energy level scheme. 4. Conclusions In summary, hierarchical dumbbell-like Ln3+ (Ln = Nd, Er, Yb)doped BaWO4 microstructures have been successfully synthesized via a simple solution process without using any template at room temperature. The good near-infrared luminescent properties of Ln3+doped BaWO4 (Ln = Nd, Er, Yb), together with their interesting morphology, open the field for potential applications in telecommunication and laser emission.
[1] Yu SH, Liu B, Mo MS, Huang JH, Liu XM, Qian YT. Adv Funct Mater 2003;13:639–47. [2] Shi YH, Feng SH, Cao CS. Mater Lett 2000;44:215–8. [3] Xie B, Wu Y, Yang J, Li FQ, Wu J, Yuan SW, Yu WC, Qian YT. J Cryst Growth 2002;235:283–6. [4] Nikl M, Bohacek P, Mihokova E, Kobayashi M, Ishii M, Usuki Y, Babin V, Stolovich A, Zazubovich S, Bacci M. J Lumin Sci 2000;87–89:1136–9. [5] Černy P, Zverev PG, Jelinkova H, Basiev TT. Opt Commun 2000;177:397–404. [6] Kwan S, Kim F, Akana J, Yang PD. Chem Commun 2001:447–8. [7] Luo ZJ, Li HM, Xia JX, Zhu WS, Guo JX, Zhang BB. Mater Lett 2007;61:1845–8. [8] Zhao XF, Li TK, Xi YY, Dickon HLN, Yu JG. Crys Growth Des 2006;6:2210–3. [9] Wang R, Liu C, Zeng J, Li KW, Wang H. J Solid State Chem 2009;182:677–84. [10] Shi HT, Qi LM, Ma JM, Cheng HM. J Am Chem Soc 2003;125:3450–1. [11] Zhang X, Xie Y, Xu F, Tian XB. J Colloid Inter Sci 2004;274:118–21. [12] Wang XM, Xu HY, Wang H, Yan H. J Cryst Growth 2005;284:254–61. [13] Thongtem T, Phuruangrat A, Thongtem S. Appl Surf Sci 2008;254:7565–9. [14] Cavalcante LS, Sczancoski JC, Espinosa JWM, Varela JA, Pizani PS, Longo E. J Alloys Compd 2009;474:195–200. [15] Liao JS, Qiu B, Wen HR, Chen JL, You WX. Mater Res Bull 2009;44:1863. [16] Su YG, Li LP, Li GS. Chem Mater Des 2008;20:6060–7. [17] Lou XM, Chen DH. Mater Lett 2008;62:1681–4. [18] Kawa M, Frechet JMJ. Chem Mater 1998;10:286–96. [19] Lin XS, Chen JL, Zhuang NF, Zhao B, Chen JZ. J Cryst Growth 2005;277:223–7. [20] Ivleva LI, Basiev TT, Boronina IS, Zverev PG, Osiko VV, Polozkov NM. Opt Mater Des 2003;23:439–42. [21] Jia GH, Tu CY, You ZY, Li JF, Zhu ZJ, Wang Y, Wu BC. J Cryst Growth 2004;273:220–5. [22] Mirov IS, Fedorov VV, Moskalev IS, Martyshkin DV, Beloglovski SY, Burachas SF, Saveliev YA, Tseitline AM. Opt Lett 2008;31:94–101. [23] Šulc J, Jelínková H, Basiev TT, Doroschenko ME, Ivleva LI, Osiko VV, Zverev PG. Opt Mater Des 2007;30:195–7. [24] Liao JS, Qiu B, Wen HR, Chen JL, You WX, Liu LB. J Alloys Compd 2009;487:758–62. [25] Sun LN, Zhang HJ, Yu JB, Yu SY, Peng CY, Dang S, Guo XM, Feng J. Langmuir 2008;24:5500–7. [26] Song SY, Zhang Y, Feng J, Xing Y, Lei YQ, Fan WQ, Zhang HJ. Cryst Growth Des 2009;9:848–52.
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
Facile synthesis and photoluminescence properties of dumbbell-like Ln-doped BaWO4 (Ln = Nd, Er, Yb) microstructures Suying Hou a, Yan Xing a,⁎, Hong Ding b, Xianchun Liu a, Bo Liu a, Xiujuan Sun a a b
College of Chemistry, Northeast Normal University, Changchun 130024, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 1 December 2009 Accepted 2 April 2010 Available online 9 April 2010
a b s t r a c t Dumbbell-like microstructure of Ln-doped BaWO4 (Ln = Nd, Er, Yb) has been successfully synthesized via a very simple precipitation technique at room temperature without any templates or catalyst. The synthesized products were systematically studied by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Such a simple one-pot solution-based process may be extended to fabricate complex hierarchical micro- and nanostructures of other functional materials. Additionally, the near-infrared luminescence of Ln-doped BaWO4 (Ln = Nd, Er, Yb) dumbbells was discussed in detail. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, tungstate materials have aroused much interest due to their luminescence behavior, structural properties, and potential applications in various fields [1,2]. Barium tungstate (BaWO4) with a tetragonal scheelite-type structure is a very promising material in the electro-optical industry because of its emission of blue luminescence [3]. In addition, because of its interesting stimulated Raman scattering (SRS) properties, BaWO4 is also a potential material for designing allsolid-state lasers emitting radiation in a specific spectral region and will have applications for medical treatment, up-conversion lasers and spectroscopy [4,5]. Due to these important properties, many efforts have been made for the synthesis of BaWO4 with a desired structure and morphology, such as 1D uniform nanorods [6], 2D nanosheets [7] and microparticles with a complex form [8–11]. However, most of these methods need either templates (such as surfactant, polymer, etc.) or complex equipment. Recently, Wang et al. reported the synthesis of well-crystallized BaWO4 powders with different morphologies via a simple template-free method [12]. The alkaline-earth metal tungstates AWO4 (A = Ca2+, Sr2+, Ba2+) have been extensively investigated as a self-activating phosphor emitting blue or green light under ultraviolet or X-ray excitation [13,14]. And lanthanide elements are known for their unique luminescence properties. Recently, AWO4 were reported to be efficient luminescent hosts for rare-earth [15–17]. When the lanthanide luminescence occurs in the near-infrared regions, these elements may be used as fluorescers in the design of signal
⁎ Corresponding author. Tel.: + 86 431 86105529; fax: + 86 431 85099108. E-mail address: [email protected] (Y. Xing). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.04.004
amplification for optical communication [18], to compensate light lost during transmission. Up to date, a number of rare-earth (Nd3+, Tm3+, Er3+, Ho3+ and Yb3+)-doped AWO4 (A = Ca2+, Sr2+, Ba2+) crystals have been successfully synthesized [19–24]. However, to the best of knowledge, few studies have been carried out to fabricate hierarchical micro/nanocomposite structures of lanthanide-doped alkaline-earth metal tungstates. Herein, a simple precipitation technique was proposed to synthesize dumbbell-like Ln3+ (Ln = Nd, Er, Yb)-doped BaWO4 microstructures without using any template at room temperature. Near-infrared luminescent properties of lanthanide ion doped BaWO4 microstructures were also investigated. 2. Experimental All chemical reagents were of analytical grade and used as received without further purification. Typically, 1.52 mmol of Ba(NO3)2 and 0.08 mmol of Ln(NO3)3·6H2O (Ln = Er, Nd, Yb) were first added to 8 mL distilled water at room temperature, NaOH solution (0.5 M) was used to adjust the pH of the mixture. Then 8 mL of 0.2 M Na2WO4 (1.6 mmol) was added dropwise to the above solution under stirring. The suspension containing white precipitates was stirred for a further 15 min. Finally, the resulting precipitates were filtered, washed with distilled water and absolute ethanol, and dried at 50 °C in air. X-ray powder diffraction (XRD) analysis was measured on a Siemens D5005 Diffractometer with CuKα radiation (λ=0.15418 nm). Fieldemission scanning electron microscopy (FESEM) images were obtained with an XL30 ESEM FEG microscope. Transmission electron microscopic (TEM) images were obtained on a JEM-2100F microscope with an accelerating voltage of 200 kV. Room-temperature photoluminescence (PL) spectra were recorded on a Fluorolog-3 with a 400 W xenon lamp as the excitation source at room temperature.
1504
S. Hou et al. / Materials Letters 64 (2010) 1503–1505
Fig. 1. XRD patterns of the samples (a) Yb3+- (b) Er3+- and (c) Nd3+-doped BaWO4 obtained at room temperature.
3. Results and discussion The phase compositions and phase structures of Ln3+-doped BaWO4 (Ln = Nd, Er, Yb) microcrystals were examined by X-ray powder diffraction (XRD). As shown in Fig. 1, all of the detectable peaks can be indexed to almost the same positions as those from the tetragonal structured BaWO4 with unit cell dimensions a = 0.561 nm, c = 1.271 nm (JCPDS card No. 43-0646), which indicates the introduction of 5% Ln3+-doped BaWO4 did not destroy the tetragonalphase structure. The strong and sharp diffraction peaks indicate that the as-obtained products are well-crystalline. The morphologies of the as-synthesized products at room temperature were examined by field-emission scanning electron microscopy (FESEM). A large number of well-defined dumbbell-like shapes with a length of about 5–8 µm were formed as shown in Fig. 2a.
No other morphologies can be detected, indicating a high yield of these Ln3+-doped BaWO4 dumbbells. A high-magnification SEM image (Fig. 2b) shows that each dumbbell is actually composed of nanorods branched out radically from the revolutional axis of the dumbbell with an average diameter of about 800 nm. Also the surface of the dumbbells is smooth. Fig. 2c shows a typical TEM image of one single dumbbell, revealing that the dumbbell is constructed by nanorods, which is in accordance with the SEM images. The fast Fourier transform (FFT) (inset of Fig. 2c) pattern exhibits that the dumbbell-like structure is crystalline in nature. A representative HRTEM image obtained from the tip of a nanorod is shown in Fig. 2d. The lattice fringes are clearly visible with a spacing of 0.542 nm and 0.203 nm, which agree well with the lattice spacing of (101) and (220) of tetragonal Ln3+-doped BaWO4, respectively. The optical properties of Ln3+ (Ln = Nd, Er, Yb)-doped BaWO4 samples have been systematically investigated in detail. Note that this doping process alters neither the crystal structures nor the shapes of the host materials. The emission spectrum of the BaWO4:Er dumbbells was obtained by exciting the sample at 275 nm. The broad emission band extending from 1440 to 1650 nm and centered at 1532 nm is shown in Fig. 3 and is attributed to the transition from the first excited state (4I13/2) to the ground state (4I15/2) of the partially filled 4f shell of Er3+. Erbium-doped materials have been the subject of much interest for many years, because the transition around 1540 nm is in the right position of the third telecommunication window [25]. The NIR emission spectra of BaWO4:Nd and BaWO4:Yb are depicted in Fig. 3, which were collected by excitation at 277 and 275 nm, respectively. The NIR spectrum of BaWO4:Nd consists of three bands centered at λ=889, 1061, and 1333 nm, which are attributed to the f–f transitions of 4 F3/2 (emitting level)→ 4I9/2, 4F3/2 → 4I11/2, and 4F3/2 → 4I13/2, respectively. The solid materials containing the Nd3+ ion have been regarded as the most popular luminescent materials in laser system applications, because the strongest transition at 1060 nm has potential application for laser emission [26]. In the NIR spectrum of BaWO4:Yb, the prominent 977 nm emission band can be observed, which is assigned to the 2F5/2 → 2F7/2 transition of the Yb3+ ion. It should also be noted that the Yb3+ ion
Fig. 2. FESEM (a–b), TEM (c), FFT (inset of c) and HRTEM (d) images of the as-prepared samples.
S. Hou et al. / Materials Letters 64 (2010) 1503–1505
1505
Acknowledgments This work was supported by Jilin Province Foundation (20090518), Training Fund of NENU'S Scientific Innovation Project (No. NENUSTC07004) and the Fundamental Research Funds for the Central Universities (09QNJJ 012). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2010.04.004. References
Fig. 3. Emission spectra for Ln-doped BaWO4 (Ln = Nd, Er, Yb) dumbbell-like microstructures: (a) BaWO 4 :Er (λ ex = 275 nm, blue line); (b) BaWO 4 :Nd (λex = 277 nm, red line); (c) BaWO4:Yb (λex = 275 nm, green line).
emission band is not a single sharp band, but an envelope of bands arising at the lower energy side than the primary 977 nm band. This may be the result of splitting of the energy levels of the Yb3+ ion as a consequence of crystal field effects. The Yb3+ ion has some advantages for laser emission due to its very simple energy level scheme. 4. Conclusions In summary, hierarchical dumbbell-like Ln3+ (Ln = Nd, Er, Yb)doped BaWO4 microstructures have been successfully synthesized via a simple solution process without using any template at room temperature. The good near-infrared luminescent properties of Ln3+doped BaWO4 (Ln = Nd, Er, Yb), together with their interesting morphology, open the field for potential applications in telecommunication and laser emission.
[1] Yu SH, Liu B, Mo MS, Huang JH, Liu XM, Qian YT. Adv Funct Mater 2003;13:639–47. [2] Shi YH, Feng SH, Cao CS. Mater Lett 2000;44:215–8. [3] Xie B, Wu Y, Yang J, Li FQ, Wu J, Yuan SW, Yu WC, Qian YT. J Cryst Growth 2002;235:283–6. [4] Nikl M, Bohacek P, Mihokova E, Kobayashi M, Ishii M, Usuki Y, Babin V, Stolovich A, Zazubovich S, Bacci M. J Lumin Sci 2000;87–89:1136–9. [5] Černy P, Zverev PG, Jelinkova H, Basiev TT. Opt Commun 2000;177:397–404. [6] Kwan S, Kim F, Akana J, Yang PD. Chem Commun 2001:447–8. [7] Luo ZJ, Li HM, Xia JX, Zhu WS, Guo JX, Zhang BB. Mater Lett 2007;61:1845–8. [8] Zhao XF, Li TK, Xi YY, Dickon HLN, Yu JG. Crys Growth Des 2006;6:2210–3. [9] Wang R, Liu C, Zeng J, Li KW, Wang H. J Solid State Chem 2009;182:677–84. [10] Shi HT, Qi LM, Ma JM, Cheng HM. J Am Chem Soc 2003;125:3450–1. [11] Zhang X, Xie Y, Xu F, Tian XB. J Colloid Inter Sci 2004;274:118–21. [12] Wang XM, Xu HY, Wang H, Yan H. J Cryst Growth 2005;284:254–61. [13] Thongtem T, Phuruangrat A, Thongtem S. Appl Surf Sci 2008;254:7565–9. [14] Cavalcante LS, Sczancoski JC, Espinosa JWM, Varela JA, Pizani PS, Longo E. J Alloys Compd 2009;474:195–200. [15] Liao JS, Qiu B, Wen HR, Chen JL, You WX. Mater Res Bull 2009;44:1863. [16] Su YG, Li LP, Li GS. Chem Mater Des 2008;20:6060–7. [17] Lou XM, Chen DH. Mater Lett 2008;62:1681–4. [18] Kawa M, Frechet JMJ. Chem Mater 1998;10:286–96. [19] Lin XS, Chen JL, Zhuang NF, Zhao B, Chen JZ. J Cryst Growth 2005;277:223–7. [20] Ivleva LI, Basiev TT, Boronina IS, Zverev PG, Osiko VV, Polozkov NM. Opt Mater Des 2003;23:439–42. [21] Jia GH, Tu CY, You ZY, Li JF, Zhu ZJ, Wang Y, Wu BC. J Cryst Growth 2004;273:220–5. [22] Mirov IS, Fedorov VV, Moskalev IS, Martyshkin DV, Beloglovski SY, Burachas SF, Saveliev YA, Tseitline AM. Opt Lett 2008;31:94–101. [23] Šulc J, Jelínková H, Basiev TT, Doroschenko ME, Ivleva LI, Osiko VV, Zverev PG. Opt Mater Des 2007;30:195–7. [24] Liao JS, Qiu B, Wen HR, Chen JL, You WX, Liu LB. J Alloys Compd 2009;487:758–62. [25] Sun LN, Zhang HJ, Yu JB, Yu SY, Peng CY, Dang S, Guo XM, Feng J. Langmuir 2008;24:5500–7. [26] Song SY, Zhang Y, Feng J, Xing Y, Lei YQ, Fan WQ, Zhang HJ. Cryst Growth Des 2009;9:848–52.