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Self-catalytic VLS growth and optical properties of single-crystalline GeO2 nanowire arrays
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Materials Letters 62 (2008) 1010 – 1013 www.elsevier.com/locate/matlet
Self-catalytic VLS growth and optical properties of single-crystalline GeO2 nanowire arrays Yong Su a , Xuemei Liang a,⁎, Sen Li a , Yiqing Chen a , Qingtao Zhou a , Song Yin a , Xia Meng a , Mingguang Kong b b
a School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui, 230009, PR China Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, PR China
Received 14 June 2007; accepted 19 July 2007 Available online 25 July 2007
Abstract Large-scale GeO2 nanowire arrays were successfully synthesized by a simple thermal evaporation method with the mechanism of Self-catalytic Vapor–Liquid–Solid growth. Its morphology and microstructure were determined by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and photoluminescence spectroscopy (PL). The onedimensional GeO2 nanostructures are extremely sensitive to electron irradiation, and the nanowires were transformed from crystal into amorphous when performing high-resolution transmission electron microscopy (HRTEM). The nanowires have diameters of 100 nm, length of 800 nm and the spherical particles terminating at the tip of the nanowires have diameters of about 500 nm. Two blue emission peaks at 448 nm and 471 nm and a violet emission peak at 411 nm were observed in the room-temperature photoluminescence measurements. © 2007 Elsevier B.V. All rights reserved. Keywords: Germanium oxide; Nanomaterials; Optical material and properties; Luminescence
1. Introduction Quasi one-dimensional nano-sized materials have peculiar structural characteristics and size effects. They often show some excellent physical properties compared with bulk ones. Germanium oxide (GeO2) is an important material for optical fibers [1]. GeO2-based glass can be widely used in optical devices because of its novel structural, optical and vibratory properties [2–4]. In recent years various methods have been used to prepare one-dimensional GeO2 nanostructures, such as GeO2 whiskers synthesized by laser ablation [5], GeO2 nanowires synthesized by physical evaporation [6] and by carbothermal reduction reaction [7], GeO2 nanorods prepared by carbon nanotubes template [8], GeO2 nanotubes synthesized by vapor phase reaction [9], GeO2 nanoneedles grown by
⁎ Corresponding author. Tel.: +86 551 2901365; fax: +86 551 2901362. E-mail address: [email protected] (X. Liang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.07.034
thermal deposition [10] and so on. But there are few reports about the synthesis of GeO2 nanowire arrays. Today one of the most important challenges in physical and material science is the preparation of ordered nanostructure arrays with controlled properties and dimensions [11]. The GeO2 nanowire arrays may have more potential applications in nanodevice with diverse functions. In this letter, GeO2 nanowire arrays have been grown by a simple thermal evaporation method. 2. Experimental method The synthesis of GeO2 nanowire arrays was carried out in a horizontal tube furnace with two ends. One end is a copper cold finger and the other end is an inlet of carrier gas. The source material was the high purity Ge powders (99.999%) which was put into an alumina boat. Then the boat was placed in a quartz tube that was inserted into the horizontal tube furnace subsequently, the source material must be kept in the middle of the furnace. Prior to heating, the reaction chamber was pumped to a vacuum of 10− 3 Torr. The carrier gas (Ar + 5%H2)
Y. Su et al. / Materials Letters 62 (2008) 1010–1013
was introduced through the tube at a flow rate of 60 standard cubic centimeters per minute (sccm). The source material was rapidly heated to 1050 °C from room temperature within 10 min and was kept at 1050 °C for 40 min. A piece of Si (111) wafer was used as substrate, which was located at downstream position of source material. The distance between the source material and substrate was 1 cm. So the deposition temperature is about 950 °C. We switched the carrier gas (Ar + 5%H2) for Ar + 10%O2 when the temperature reached 1050 °C. Subsequently, the furnace was cooled down to room temperature. The chamber pressure was kept at about 300 Torr in the entire process. After cooling down, a white fluffy product could be observed on the substrate. The morphology and microstructures of the products were characterized by FE-SEM (JEOL, JSM-6700F) and HRTEM (JEOL-2010). The chemical composition of the products was analyzed by EDS. PL at room temperature was measured by He–Cd laser with a wavelength of 260 nm, a power of 5– 15 mW as the excitation source. 3. Results and discussion FE-SEM image shown in Fig. 1(a) reveals that the products consist of a large quantity of nanowires well-aligned. Most of the nanowires
Fig. 1. FE-SEM images of GeO2 nanowire arrays. (a) An image of products at a low-magnification. (b) A high-magnification image of products.
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grow vertically on Si (111) wafer, and a few of them keep a certain angle with substrate. The high-magnification SEM image, Fig. 1(b), shows that there are spherical particles terminating at the ends of the nanowires .The nanowires are with uniform diameters of about 100 nm. But the diameters of spherical particles are not uniform. Observing carefully, we can find that the surfaces of the nanowires and particles are all smooth. To obtain more details about the structure and composition of the as-synthesized nanowire arrays, TEM, SAED, and EDS measurements were performed. The TEM image is shown in Fig. 2(a). The diameter and length of the nanowire are about 100 nm and 800 nm respectively. The diameter of the spherical particles is about 500 nm. When performing the HRTEM, we found that electron diffraction spots disappeared completely, which indicate the transformation from crystal into amorphous. Zhang et al. [8] and Bai et al. [12] have ever reported respectively that GeO2 nanowires and nanorods are very sensitive to electron irradiation and can transform quickly from single crystal into amorphous in seconds under electron beam irradiation. In our experiment, this transformation occurred when performing HRTEM. Here electron energy of electron diffraction may be not strong enough to excite this transformation compared with that of HRTEM. The electron diffraction pattern in Fig. 2(a) shows cube structure of the asgrown GeO2 nanowires. But because of the lack of the HRTEM image, we can't get detailed information about the growth direction of the nanowires. The EDS spectra of the nanowire in Fig. 2(b) indicate that the component of the nanowire is GeO2, while the particle atop the nanowire is Ge with trace of O (about 3% in content) as shown in Fig. 2 (c). The O may come from the oxidized layer of the particles or adsorbed oxygen on the surface of the particle. Generally speaking, two growth mechanisms exist, i.e. VLS (vapor– liquid–solid) and VS (vapor–solid), to explain the growth of onedimensional nanostructures in such a thermal evaporation process. In the VLS process, a transition metal particle (such as gold, iron, etc.) capped at the tip of the nanowires serves as the catalytic active site, which is almost characteristic of VLS growth [13–16]. Spherical particles can be clearly observed at the tip of the nanowires in our product, so the growth of GeO2 nanowire arrays is dominated by the VLS process. Commonly, in the VLS process, Au, Fe etc. were added specially as metal catalyst. But in our experiment, the catalyst Ge was the source material, what's more, the as-grown product is GeO2 nanowires. This growth mechanism is called Self-catalytic VLS growth [17,18]. The growth process of the GeO2 nanowire arrays could be divided into four steps: (1) With the increase of the temperature, Ge vapor is formed at high-temperature zone. (2) The Ge vapor is driven by flowing gas and deposited on the surface of the Si wafer to form Ge liquid droplets at the low-temperature zone which will serve as liquid nucleus for VLS growth of GeO2 nanowire arrays. (3) The Ge vapor is oxidized to form GeO2 in the oxygen environment when reaching 1050 °C. Ge liquid droplets will grow larger for adsorbing the formed GeO2, meanwhile, they likely react with the oxygen to form GeO2 directly. The GeO2 will separate out to form GeO2 crystalline nuclei due to supersaturating. (4) The GeO2 crystalline nuclei grow up gradually to form GeO2 nanowires. The possible growth scenario is shown in Fig. 3. Fig. 4 is photoluminescence (PL) spectra of the products at room temperature. It is clearly seen that the products emit violet light with peak at 411 nm and bright blue light with two peaks at 448 nm and 471 nm. Zhang et al. [9] observed an intensive peak at 448 nm for the GeO2 nanorods and nanotubes. They presented that the blue light emission can be attributed to oxygen vacancies and oxygen– germanium vacancies centers. Wu et al. [7] reported that peak at 485 nm of as-synthesis GeO2 nanowires is also caused by the oxygen vacancies and oxygen–germanium vacancies centers. The two
1012
Y. Su et al. / Materials Letters 62 (2008) 1010–1013
Fig. 2. (a) TEM images of a nanowire, the insets show its corresponding SAED pattern. (b) EDS spectra of the nanowire. (c) EDS spectra of the spherical particle of the nanowire.
intensive peaks mentioned above are similar to the two peaks of blue emission detected in our experiment. We use the Si wafer as substrate, it is likely formed SiOx films containing Ge and GeO2 crystalloid on the substrate, so the 411 nm peak can be explained by the mechanism used in the fused silica with peaks at 400 nm (3.1 eV) [19].
emission peaks at 448 nm and 471 nm and a violet emission peak at 411 nm which was never reported and we speculate it may be attributed to the Si substrate.
4. Conclusion
This work was financially supported by the National Natural Science Foundation of China (NSFC, Grant No. 20671027) and by the Nature Science Foundation of Anhui province of China (Grant No. 050440904).
GeO2 nanowire arrays with spherical particles terminating at the ends are reported. The growth mechanism of the nanowire arrays is proposed for a Self-catalytic VLS growth, for Ge acts both as catalyst and source material. The diameters of the nanowires are about 100 nm uniformly and that of the particle are about hundreds of nanometers. GeO2 nanostructure is very sensitive to electron diffraction. The PL spectra shows two blue
Fig. 3. Growth scenario for the GeO2 nanowire: (I) vaporization of Ge, (II) condensation of Ge vapor to form Ge droplets, (III) absorption of GeO2 vapor and O, and crystal nucleation, (IV) crystal growth.
Acknowledgments
Fig. 4. PL spectra of the GeO2 nanowire arrays at room temperature.
Y. Su et al. / Materials Letters 62 (2008) 1010–1013
References [1] [2] [3] [4] [5]
H.S. Jang, D.H. Kim, H.R. Lee, S.Y. Lee, Mater. Lett. 59 (2005) 1526. P. Fisher, F. Waldner, Solid State Commun. 44 (1982) 657. S.L. Hou, D.S. Oliver, Appl. Phys. Lett. 18 (1971) 325. M. Couzi, J.R. Vignalou, G. Boulon, Solid State Commun. 20 (1976) 461. Y.H. Tang, Y.F. Zhang, N. Wang, I. Bello, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 74 (1999) 3824. [6] Z.G. Bai, D.P. Yu, H.Z. Zhang, Y. Ding, Y.P. Wang, X.Z. Gai, Q.L. Hang, G.C. Xiong, S.Q. Feng, Chem. Phys. Lett. 303 (1999) 311. [7] X.C. Wu, W.H. Song, B. Zhao, Y.P. Sun, J.J. Du, Chem. Phys. Lett. 349 (2001) 210. [8] Y.J. Zhang, J. Zhu, Q. Zhang, Y.J. Yan, N.L. Wang, X.Z. Zhang, Chem. Phys. Lett. 317 (2000) 504.
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[9] Z. Jiang, T. Xie, G.Z. Wang, X.Y. Yuan, C.H. Ye, W.P. Cai, G.W. Meng, G.H. Li, L.D. Zhang, Mater. Lett. 59 (2005) 416. [10] P. Hidalgo, B. Méndez, J. Piqueras, Nanotechnology 16 (2005) 2521. [11] A.K. Menon, B.K. Gupta, Nanostruct. Mater. 11 (1999) 965. [12] Z.G. Bai, D.P. Yu, H.Z. Zhang, Y. Ding, Y.P. Wang, X.Z. Gai, Q.L. Hang, G.C. Xiong, S.Q. Feng, Chem. Phys. Lett. 303 (1999) 311. [13] J. Zhang, L. Zhang, Solid. State. Commun. 122 (2002) 493. [14] Z.W. Pan, Z.R. Dai, Z.L. Wang, Appl. Phys. Lett. 80 (2002) 309. [15] C.H. Liang, G.W. Meng, Y. Lei, F. Phillipp, L.D. Zhang, Adv. Mater. 13 (2001) 1330. [16] X.C. Jiang, T. Herricks, Y. Xia, Nano. Lett. 2 (2002) 1333. [17] Y.Q. Chen, et al., Chem. Phys. Lett. 369 (2003) 16. [18] Y.M. Liu, Y.Q. Li, S.S. Fan, Chem. Phys. Lett. 375 (2003) 632. [19] L.N. Skuija, A.N. Trukhin, Phys. Rev. B 39 (1989) 3909.
Materials Letters 62 (2008) 1010 – 1013 www.elsevier.com/locate/matlet
Self-catalytic VLS growth and optical properties of single-crystalline GeO2 nanowire arrays Yong Su a , Xuemei Liang a,⁎, Sen Li a , Yiqing Chen a , Qingtao Zhou a , Song Yin a , Xia Meng a , Mingguang Kong b b
a School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui, 230009, PR China Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, PR China
Received 14 June 2007; accepted 19 July 2007 Available online 25 July 2007
Abstract Large-scale GeO2 nanowire arrays were successfully synthesized by a simple thermal evaporation method with the mechanism of Self-catalytic Vapor–Liquid–Solid growth. Its morphology and microstructure were determined by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and photoluminescence spectroscopy (PL). The onedimensional GeO2 nanostructures are extremely sensitive to electron irradiation, and the nanowires were transformed from crystal into amorphous when performing high-resolution transmission electron microscopy (HRTEM). The nanowires have diameters of 100 nm, length of 800 nm and the spherical particles terminating at the tip of the nanowires have diameters of about 500 nm. Two blue emission peaks at 448 nm and 471 nm and a violet emission peak at 411 nm were observed in the room-temperature photoluminescence measurements. © 2007 Elsevier B.V. All rights reserved. Keywords: Germanium oxide; Nanomaterials; Optical material and properties; Luminescence
1. Introduction Quasi one-dimensional nano-sized materials have peculiar structural characteristics and size effects. They often show some excellent physical properties compared with bulk ones. Germanium oxide (GeO2) is an important material for optical fibers [1]. GeO2-based glass can be widely used in optical devices because of its novel structural, optical and vibratory properties [2–4]. In recent years various methods have been used to prepare one-dimensional GeO2 nanostructures, such as GeO2 whiskers synthesized by laser ablation [5], GeO2 nanowires synthesized by physical evaporation [6] and by carbothermal reduction reaction [7], GeO2 nanorods prepared by carbon nanotubes template [8], GeO2 nanotubes synthesized by vapor phase reaction [9], GeO2 nanoneedles grown by
⁎ Corresponding author. Tel.: +86 551 2901365; fax: +86 551 2901362. E-mail address: [email protected] (X. Liang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.07.034
thermal deposition [10] and so on. But there are few reports about the synthesis of GeO2 nanowire arrays. Today one of the most important challenges in physical and material science is the preparation of ordered nanostructure arrays with controlled properties and dimensions [11]. The GeO2 nanowire arrays may have more potential applications in nanodevice with diverse functions. In this letter, GeO2 nanowire arrays have been grown by a simple thermal evaporation method. 2. Experimental method The synthesis of GeO2 nanowire arrays was carried out in a horizontal tube furnace with two ends. One end is a copper cold finger and the other end is an inlet of carrier gas. The source material was the high purity Ge powders (99.999%) which was put into an alumina boat. Then the boat was placed in a quartz tube that was inserted into the horizontal tube furnace subsequently, the source material must be kept in the middle of the furnace. Prior to heating, the reaction chamber was pumped to a vacuum of 10− 3 Torr. The carrier gas (Ar + 5%H2)
Y. Su et al. / Materials Letters 62 (2008) 1010–1013
was introduced through the tube at a flow rate of 60 standard cubic centimeters per minute (sccm). The source material was rapidly heated to 1050 °C from room temperature within 10 min and was kept at 1050 °C for 40 min. A piece of Si (111) wafer was used as substrate, which was located at downstream position of source material. The distance between the source material and substrate was 1 cm. So the deposition temperature is about 950 °C. We switched the carrier gas (Ar + 5%H2) for Ar + 10%O2 when the temperature reached 1050 °C. Subsequently, the furnace was cooled down to room temperature. The chamber pressure was kept at about 300 Torr in the entire process. After cooling down, a white fluffy product could be observed on the substrate. The morphology and microstructures of the products were characterized by FE-SEM (JEOL, JSM-6700F) and HRTEM (JEOL-2010). The chemical composition of the products was analyzed by EDS. PL at room temperature was measured by He–Cd laser with a wavelength of 260 nm, a power of 5– 15 mW as the excitation source. 3. Results and discussion FE-SEM image shown in Fig. 1(a) reveals that the products consist of a large quantity of nanowires well-aligned. Most of the nanowires
Fig. 1. FE-SEM images of GeO2 nanowire arrays. (a) An image of products at a low-magnification. (b) A high-magnification image of products.
1011
grow vertically on Si (111) wafer, and a few of them keep a certain angle with substrate. The high-magnification SEM image, Fig. 1(b), shows that there are spherical particles terminating at the ends of the nanowires .The nanowires are with uniform diameters of about 100 nm. But the diameters of spherical particles are not uniform. Observing carefully, we can find that the surfaces of the nanowires and particles are all smooth. To obtain more details about the structure and composition of the as-synthesized nanowire arrays, TEM, SAED, and EDS measurements were performed. The TEM image is shown in Fig. 2(a). The diameter and length of the nanowire are about 100 nm and 800 nm respectively. The diameter of the spherical particles is about 500 nm. When performing the HRTEM, we found that electron diffraction spots disappeared completely, which indicate the transformation from crystal into amorphous. Zhang et al. [8] and Bai et al. [12] have ever reported respectively that GeO2 nanowires and nanorods are very sensitive to electron irradiation and can transform quickly from single crystal into amorphous in seconds under electron beam irradiation. In our experiment, this transformation occurred when performing HRTEM. Here electron energy of electron diffraction may be not strong enough to excite this transformation compared with that of HRTEM. The electron diffraction pattern in Fig. 2(a) shows cube structure of the asgrown GeO2 nanowires. But because of the lack of the HRTEM image, we can't get detailed information about the growth direction of the nanowires. The EDS spectra of the nanowire in Fig. 2(b) indicate that the component of the nanowire is GeO2, while the particle atop the nanowire is Ge with trace of O (about 3% in content) as shown in Fig. 2 (c). The O may come from the oxidized layer of the particles or adsorbed oxygen on the surface of the particle. Generally speaking, two growth mechanisms exist, i.e. VLS (vapor– liquid–solid) and VS (vapor–solid), to explain the growth of onedimensional nanostructures in such a thermal evaporation process. In the VLS process, a transition metal particle (such as gold, iron, etc.) capped at the tip of the nanowires serves as the catalytic active site, which is almost characteristic of VLS growth [13–16]. Spherical particles can be clearly observed at the tip of the nanowires in our product, so the growth of GeO2 nanowire arrays is dominated by the VLS process. Commonly, in the VLS process, Au, Fe etc. were added specially as metal catalyst. But in our experiment, the catalyst Ge was the source material, what's more, the as-grown product is GeO2 nanowires. This growth mechanism is called Self-catalytic VLS growth [17,18]. The growth process of the GeO2 nanowire arrays could be divided into four steps: (1) With the increase of the temperature, Ge vapor is formed at high-temperature zone. (2) The Ge vapor is driven by flowing gas and deposited on the surface of the Si wafer to form Ge liquid droplets at the low-temperature zone which will serve as liquid nucleus for VLS growth of GeO2 nanowire arrays. (3) The Ge vapor is oxidized to form GeO2 in the oxygen environment when reaching 1050 °C. Ge liquid droplets will grow larger for adsorbing the formed GeO2, meanwhile, they likely react with the oxygen to form GeO2 directly. The GeO2 will separate out to form GeO2 crystalline nuclei due to supersaturating. (4) The GeO2 crystalline nuclei grow up gradually to form GeO2 nanowires. The possible growth scenario is shown in Fig. 3. Fig. 4 is photoluminescence (PL) spectra of the products at room temperature. It is clearly seen that the products emit violet light with peak at 411 nm and bright blue light with two peaks at 448 nm and 471 nm. Zhang et al. [9] observed an intensive peak at 448 nm for the GeO2 nanorods and nanotubes. They presented that the blue light emission can be attributed to oxygen vacancies and oxygen– germanium vacancies centers. Wu et al. [7] reported that peak at 485 nm of as-synthesis GeO2 nanowires is also caused by the oxygen vacancies and oxygen–germanium vacancies centers. The two
1012
Y. Su et al. / Materials Letters 62 (2008) 1010–1013
Fig. 2. (a) TEM images of a nanowire, the insets show its corresponding SAED pattern. (b) EDS spectra of the nanowire. (c) EDS spectra of the spherical particle of the nanowire.
intensive peaks mentioned above are similar to the two peaks of blue emission detected in our experiment. We use the Si wafer as substrate, it is likely formed SiOx films containing Ge and GeO2 crystalloid on the substrate, so the 411 nm peak can be explained by the mechanism used in the fused silica with peaks at 400 nm (3.1 eV) [19].
emission peaks at 448 nm and 471 nm and a violet emission peak at 411 nm which was never reported and we speculate it may be attributed to the Si substrate.
4. Conclusion
This work was financially supported by the National Natural Science Foundation of China (NSFC, Grant No. 20671027) and by the Nature Science Foundation of Anhui province of China (Grant No. 050440904).
GeO2 nanowire arrays with spherical particles terminating at the ends are reported. The growth mechanism of the nanowire arrays is proposed for a Self-catalytic VLS growth, for Ge acts both as catalyst and source material. The diameters of the nanowires are about 100 nm uniformly and that of the particle are about hundreds of nanometers. GeO2 nanostructure is very sensitive to electron diffraction. The PL spectra shows two blue
Fig. 3. Growth scenario for the GeO2 nanowire: (I) vaporization of Ge, (II) condensation of Ge vapor to form Ge droplets, (III) absorption of GeO2 vapor and O, and crystal nucleation, (IV) crystal growth.
Acknowledgments
Fig. 4. PL spectra of the GeO2 nanowire arrays at room temperature.
Y. Su et al. / Materials Letters 62 (2008) 1010–1013
References [1] [2] [3] [4] [5]
H.S. Jang, D.H. Kim, H.R. Lee, S.Y. Lee, Mater. Lett. 59 (2005) 1526. P. Fisher, F. Waldner, Solid State Commun. 44 (1982) 657. S.L. Hou, D.S. Oliver, Appl. Phys. Lett. 18 (1971) 325. M. Couzi, J.R. Vignalou, G. Boulon, Solid State Commun. 20 (1976) 461. Y.H. Tang, Y.F. Zhang, N. Wang, I. Bello, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 74 (1999) 3824. [6] Z.G. Bai, D.P. Yu, H.Z. Zhang, Y. Ding, Y.P. Wang, X.Z. Gai, Q.L. Hang, G.C. Xiong, S.Q. Feng, Chem. Phys. Lett. 303 (1999) 311. [7] X.C. Wu, W.H. Song, B. Zhao, Y.P. Sun, J.J. Du, Chem. Phys. Lett. 349 (2001) 210. [8] Y.J. Zhang, J. Zhu, Q. Zhang, Y.J. Yan, N.L. Wang, X.Z. Zhang, Chem. Phys. Lett. 317 (2000) 504.
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[9] Z. Jiang, T. Xie, G.Z. Wang, X.Y. Yuan, C.H. Ye, W.P. Cai, G.W. Meng, G.H. Li, L.D. Zhang, Mater. Lett. 59 (2005) 416. [10] P. Hidalgo, B. Méndez, J. Piqueras, Nanotechnology 16 (2005) 2521. [11] A.K. Menon, B.K. Gupta, Nanostruct. Mater. 11 (1999) 965. [12] Z.G. Bai, D.P. Yu, H.Z. Zhang, Y. Ding, Y.P. Wang, X.Z. Gai, Q.L. Hang, G.C. Xiong, S.Q. Feng, Chem. Phys. Lett. 303 (1999) 311. [13] J. Zhang, L. Zhang, Solid. State. Commun. 122 (2002) 493. [14] Z.W. Pan, Z.R. Dai, Z.L. Wang, Appl. Phys. Lett. 80 (2002) 309. [15] C.H. Liang, G.W. Meng, Y. Lei, F. Phillipp, L.D. Zhang, Adv. Mater. 13 (2001) 1330. [16] X.C. Jiang, T. Herricks, Y. Xia, Nano. Lett. 2 (2002) 1333. [17] Y.Q. Chen, et al., Chem. Phys. Lett. 369 (2003) 16. [18] Y.M. Liu, Y.Q. Li, S.S. Fan, Chem. Phys. Lett. 375 (2003) 632. [19] L.N. Skuija, A.N. Trukhin, Phys. Rev. B 39 (1989) 3909.