- Email: [email protected]
SiO2 nanocables and nanosprings synthesized by catalyst-free method
ARTICLE IN PRESS
Physica E 31 (2006) 9–12 www.elsevier.com/locate/physe
SiC/SiO2 nanocables and nanosprings synthesized by catalyst-free method W.M. Zhou, Z.X. Yang, F. Zhu, Y.F. Zhang National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Institute of Micro and Nano Science and Technology, Shanghai Jiaotong University, Shanghai 20030, PR China Received 4 July 2005; accepted 30 August 2005 Available online 8 November 2005
Abstract High-yield and high-purity SiC/SiO2 nanocables and nanosprings were synthesized without the presence of catalyst by high frequencyinduction heating of SiO and activated carbon fibers in the temperature of 1450 1C for 10 min. The as-prepared nanocables consist of a 10–25 nm diameter single-crystalline core wrapped with an amorphous SiO2 shell of a diameter of 10–20 nm. The nanosprings diameter and pitch of as-grown SiC/SiO2 are 25 and 10 nm, respectively. The formation mechanism was analyzed by thermodynamics calculation combined with SEM, TEM and HRTEM. r 2005 Elsevier B.V. All rights reserved. PACS: 81.05.Hd; 81.07.Bc; 81.07.Vb; 81.10.Bk Keywords: SiC; Nanocable; Nanospring; Induction heating
1. Introduction Recently, one-dimensional structures such as wires, rods, belts, and tubes have become the focus of intensive research because of their unique applications in functional materials and the fabrication of the nanoscale devices [1,2]. Among these materials, SiC is especial potential material for large band-gap semiconductor (Eg ¼ 2.4 V) and reinforcement of composite with its superior electronic, physical and chemical properties [3–5], making it an excellent candidate for high-temperature microelectronic devices. Various synthesis methods have been addressed for SiC nanostructure, including reaction between magnesium silicate and carbon tetrachloride [6], carbonthermal reduction [7], conversion of CNT to SiC by reacting with SiO [8], and chemical vapor deposition [9–11], and so on. However, among these methods, most of as-prepared products has a lesser yield or metal catalyst contamination, or it is even
Corresponding author. Tel.: +86 21 6293 3294; fax: +86 21 6282 3631.
E-mail address: [email protected] (W.M. Zhou). 1386-9477/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2005.08.018
time-consuming, which limits a further study and applications in many fields. In this paper, we present a simple way for the fabrication of SiC/SiO2 nanocables and nanosprings without the metal catalyst being grown by heating SiO power with homemade induction heated system composed of high-purity graphite coated with an activated carbon fiber thermoinsulating layer. The novel structures of nanosprings without catalyst introduction have never been reported before. The synthesis reaction was performed in a vertical quartz tube furnace at 1450 1C in Ar gas. After cooling, large quantities of light blue products were deposited on the activated carbon fiber which acted as carbon sources. The nanocables consist of a 10–25 nm diameter core wrapped with an amorphous SiO2 layer. The nanosprings diameter and pitch of as-grown SiC are 25 and 10 nm, respectively. 2. Experimental The experiment was conducted in a vertical highfrequency induction furnace including quartz tube (of outer diameter 80 mm and length 120 cm) and an inductive
ARTICLE IN PRESS W.M. Zhou et al. / Physica E 31 (2006) 9–12
10
Fig. 2. XRD pattern of the as-prepared products. Fig. 1. Schematic diagram of the high frequency induction furnace.
heat cylinder which the schematic diagram of it is shown in Fig. 1. A 2 g raw SiO powder (purity 99.9%) was placed in high-purity graphite crucible wrapped by an activated carbon fiber thermo-insulating layer. After evacuation of the system at 3 103 Torr, an entraining Ar gas flow at a flow rate of 100 sccm is introduced and a total pressure in the tube in the range 50–100 Torr. The temperature of crucible is rapidly increased to 1450 1C and maintained for 10 min. After the system was cooled to room temperature with Ar gas, large quantities of light blue products were coated on an activated carbon fiber thermo-insulating layer. After completing the reaction, the collected materials were characterized using X-ray diffraction (XRD, CuKa radiation), field emission scanning electron microscopy (FESEM) equipped with energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM, at 300 kV) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM2010F).
Fig. 3. (a) SEM image of as-prepared samples grown at 1450 1C for 20 min by SiO powder and carbon fiber. (b) EDS spectrum recorded from individual nanostructure reveals only Si, C and O.
(a)
(b) [111]
3. Results and discussion The XRD pattern of as-prepared samples is shown in Fig. 2. All the peaks were indexed as b-SiC. From Fig. 3, which shows the SEM images and EDS of the sample, we see that the average diameters of nanowires range from 10 to 50 nm and have lengths up to 10 mm. From an energy dispersion spectroscopy (EDS), only three elements, Si, C and O, could be found. No other impurities were detected in the nanostructures. TEM and HRTEM were used to further investigate the morphology. In low-magnification TEM images of the sample (Fig. 4), the nanostructures were found and classified into two basic kinds: nanocables and nanosprings. Similar heterostructures, such as ZnS/SiC nanocables, BN-coated SiC, and carbon-coated SiC nanowires,
0.25nm 25nm (c)
5nm (d)
50nm
5nm
Fig. 4. (a) and (c) are typical TEM images for SiC/SiO2 nanocables and nanosprings. And (b) and (d) are HRTEM images for (a) and (c) nanostructures, respectively.
ARTICLE IN PRESS W.M. Zhou et al. / Physica E 31 (2006) 9–12
SiOðgÞ þ 2CðsÞ ! SiCðsÞ þ COðgÞ;
(1)
2SiOðgÞ þ 3CðsÞ ! 2SiCðsÞ þ CO2ðgÞ;
(2)
SiOðgÞ þ 3COðgÞ ! SiCðsÞ þ 2CO2 ðgÞ:
(3)
According to thermodynamics, the Gibbs-free energy DG(T) and the formation enthalpy DH(T) of a reaction at temperature T calculated using thermodynamic data from Ref. [17] is given in Fig. 5. It can be seen from Fig. 5 that the formation enthalpy DH(T) of reactions (1), (2) and (3) are very large, which indicates that lots of heat has been released during reaction. Moreover, the Gibbs free energy DG(T) of the three reactions are all negative in the results, which indicates the reactions can take place. At 1450 1C, the molar Gibbs-free energy of reaction (3) is positive. So in this result, reactions (1) and (2) are mainly formation mechanism. These data suggest that SiC should be main products in our experiments. Our experimental results are
−450 −400
∆H KJ/mol
−350 −300 −250
Equation (1)
−200
Equation (2)
−150
Equation (3)
−100 −50 200
400
600
800
1000
1200
1400
1600
T/K −400
∆G KJ/mol
and biaxial and coaxial SiC/SiO2 nanowires were synthesized by high temperature heating [12–15]. Fig. 4(a) is a typical smooth and straight nanocable composed of a single-crystalline core of a diameter of 10–25 nm coated with an amorphous silicon oxide layer. The atomic arrangements of SiC/SiO2 nanocable were exploited by HRTEM as shown in Fig. 4(b). An electron beam is perpendicular to the surface of the sheet. The lattice fringes show the image characteristics of b-SiC crystal, in which a d-spacing of 0.25 nm (indicated by parallel lines) corresponds to the (1 1 1) plane spacing, indicating nanowires growth along [1 1 1] direction. The growth direction is indexed as [1 1 1] shown in Fig. 4(b). A similar result is reported by other groups [14,15]. Additionally, some nanosprings composed of a core and an amorphous silicon oxide layer are shown in Figs. 4(c) and (d). The diameter of the nanospring is 25 nm and that of the nanospring pitch of 10 nm, this amounts to diameter/ pitch ratio of 2.5. Moreover, the total nanostructures consist of transition region from SiC/SiO2 nanocable to SiC/SiO2 nanospring then to SiC/SiO2 nanocable and its total length in Fig. 4(c) is 310 nm with a nanospring of 150 nm. TEM dark-images indicate the nanospring is composed of a crystalline core and an amorphous coating. In previous paper, the important role of the catalyst in the formation of SiC/SiO2 nanosprings was elucidated by Zhang et al. [7], which is different from our experiment without catalyst. Helical spring structure is the most fundamental structural configuration for DNA and many biological proteins, which are due to van der Waals force and hydrogen bonding [16]. These shapes of nanostructures will be found to be a great potential in application in electronic circuits and light emitting devices as important in building nanospring-based transducers and actuators, and tunable functional components for MEMS and NEMS [17]. The following possible chemical reaction mechanism has been proposed for the synthesis of SiC nanostructures:
11
−350
Equation (1)
−300
Equation (2) Equation (3)
−250 −200 −150 −100 −50 0 200
400
600
800
1000
1200
1400
1600
T/K Fig. 5. (a) The change of enthalpy DH(T) as a function of temperature for Eqs. (1), (2) and (3). (b) The change of Gibbs free energy DG(T) as a function of temperature for Eqs. (1), (2) and (3).
in good consistency with this prediction. In typical VLS process, the metal catalyst plays an importance role in onedimensional nanostructure. In the results, no catalyst could be seen in TEM images and no liquor occurred; thus the VS process was proposed. Firstly, carbon fiber acting as carbon source was circumvented by gas SiO, which was carried out by Ar gas and nucleated on a cooler carbon fiber with its large surface aiding in the nucleation process, and the reactions between SiO and C lead to SiC nucleation. As the concentration of SiC becomes sufficiently high, they aggregate into small clusters [2]. With an increasing supply of SiO being carried to the nucleation regions, the nanowires along the directions of the least stable plane grow when they get superstaturated. Secondly, an amorphous silicon oxide shell might have two reasons: at elevated temperature, SiC nanostructures oxidate by leakage of chamber; on the other hand, the unreacted SiO and O2 take place to form amorphous silicon oxide, which deposits on the nearest SiC nanowires [7]. Finally, there might be a stress which results from hampering of forming gaseous CO and CO2, and dislocation during the growth of
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W.M. Zhou et al. / Physica E 31 (2006) 9–12
nanostructures, thus forming SiC/SiO2 nanosprings. The structure defects (Figs. 4(b) and (d)) were responsible for growth morphologies, thereby minimizing the surface-free energy from the energy point of view. To have a comprehensive understanding of the growth process, the formation mechanism of nanosprings is evolved. As for the occurrence of the two morphologies in the same experiment, it could be caused by temperature gradient in the activated carbon fiber thermo-insulating layer with rapid heating and cooling. 4. Conclusion Large-scale SiC/SiO2 nanocables and nanosprings are easily obtained with SiO powder and activated carbon fibers by high frequency induction heating at low cost. The catalyst-free of as-prepared nanocables and nanosprings are different from results reported elsewhere. By means of XRD, SEM, EDS and TEM (HRTEM), nanocables and nanosprings have been characterized and discussed in detail. The chemical reactions of SiC formation are proposed. Acknowledgment This work is supported by the National Natural Science Foundation of China no. 50272039, and by the Developing
Foundation of Shanghai Science and Technology Grant no. 0452nm056.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Z.L. Wang, Adv. Mater. 12 (2003) 1295. Y.N. Xia, P.D. Yang, Adv. Mater. 15 (2003) 353. H.K. Seong, Appl. Phys. Lett. 85 (2004) 1256. W. Yang, H. Araki, A. Kohyama, S. Thaveethavorn, H. Suzuki, T. Noda, J. Am. Ceram. Soc. 87 (2004) 1720. K.T. Yeon, H.S. Soo, L.H. Mo, Mater. Trans. 45 (2004) 1442. Y.C. Ying, Y.L. Gu, J. Solid State Chem. 177 (2004) 4163. G.W. Meng, L.D. Zhang, C.M. Mo, J. Mater. Res . 13 (1998) 2533. H. Dai, E.W. Wong, Y. Lu, S. Fan, C.M. Lieber, Nature 375 (1995) 769. H.F. Zhang, C.M. Wang, S.L. Wang, Nano Lett. 2 (2002) 941. H.J. Choi, H.K. Seong, J.C. Lee, Y.M. Sung, J. Cryst. Growth 269 (2004) 472. K.W. Wong, X.T. Zhou, F.C.K. Au, H.L. Lai, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 75 (1999) 2918. Y.H. Ryu, B.T. Park, Y.H. Song, K.J. Yong, J. Cryst. Growth 271 (2004) 99. J.Q. Hu, Y. Bando, J.H. Zhan, D. Golberg, Appl. Phys. Lett. 85 (2004) 2932. C.C. Tang, Y.S. Bando, Appl. Phys. Lett. 83 (2003) 659. Z.L. Wang, Z.R. Dai, R.P. Gao, Z.G. Bai, Appl. Phys. Lett. 77 (2000) 3349. X.Y. Kong, Z.L. Wang, Nano. Lett. 3 (2003) 1625. Y. Liang, Y. Che, Handbook of Hermodynamics of Inorganic Substance, Dongbei University Press, 1994.
Physica E 31 (2006) 9–12 www.elsevier.com/locate/physe
SiC/SiO2 nanocables and nanosprings synthesized by catalyst-free method W.M. Zhou, Z.X. Yang, F. Zhu, Y.F. Zhang National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Institute of Micro and Nano Science and Technology, Shanghai Jiaotong University, Shanghai 20030, PR China Received 4 July 2005; accepted 30 August 2005 Available online 8 November 2005
Abstract High-yield and high-purity SiC/SiO2 nanocables and nanosprings were synthesized without the presence of catalyst by high frequencyinduction heating of SiO and activated carbon fibers in the temperature of 1450 1C for 10 min. The as-prepared nanocables consist of a 10–25 nm diameter single-crystalline core wrapped with an amorphous SiO2 shell of a diameter of 10–20 nm. The nanosprings diameter and pitch of as-grown SiC/SiO2 are 25 and 10 nm, respectively. The formation mechanism was analyzed by thermodynamics calculation combined with SEM, TEM and HRTEM. r 2005 Elsevier B.V. All rights reserved. PACS: 81.05.Hd; 81.07.Bc; 81.07.Vb; 81.10.Bk Keywords: SiC; Nanocable; Nanospring; Induction heating
1. Introduction Recently, one-dimensional structures such as wires, rods, belts, and tubes have become the focus of intensive research because of their unique applications in functional materials and the fabrication of the nanoscale devices [1,2]. Among these materials, SiC is especial potential material for large band-gap semiconductor (Eg ¼ 2.4 V) and reinforcement of composite with its superior electronic, physical and chemical properties [3–5], making it an excellent candidate for high-temperature microelectronic devices. Various synthesis methods have been addressed for SiC nanostructure, including reaction between magnesium silicate and carbon tetrachloride [6], carbonthermal reduction [7], conversion of CNT to SiC by reacting with SiO [8], and chemical vapor deposition [9–11], and so on. However, among these methods, most of as-prepared products has a lesser yield or metal catalyst contamination, or it is even
Corresponding author. Tel.: +86 21 6293 3294; fax: +86 21 6282 3631.
E-mail address: [email protected] (W.M. Zhou). 1386-9477/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2005.08.018
time-consuming, which limits a further study and applications in many fields. In this paper, we present a simple way for the fabrication of SiC/SiO2 nanocables and nanosprings without the metal catalyst being grown by heating SiO power with homemade induction heated system composed of high-purity graphite coated with an activated carbon fiber thermoinsulating layer. The novel structures of nanosprings without catalyst introduction have never been reported before. The synthesis reaction was performed in a vertical quartz tube furnace at 1450 1C in Ar gas. After cooling, large quantities of light blue products were deposited on the activated carbon fiber which acted as carbon sources. The nanocables consist of a 10–25 nm diameter core wrapped with an amorphous SiO2 layer. The nanosprings diameter and pitch of as-grown SiC are 25 and 10 nm, respectively. 2. Experimental The experiment was conducted in a vertical highfrequency induction furnace including quartz tube (of outer diameter 80 mm and length 120 cm) and an inductive
ARTICLE IN PRESS W.M. Zhou et al. / Physica E 31 (2006) 9–12
10
Fig. 2. XRD pattern of the as-prepared products. Fig. 1. Schematic diagram of the high frequency induction furnace.
heat cylinder which the schematic diagram of it is shown in Fig. 1. A 2 g raw SiO powder (purity 99.9%) was placed in high-purity graphite crucible wrapped by an activated carbon fiber thermo-insulating layer. After evacuation of the system at 3 103 Torr, an entraining Ar gas flow at a flow rate of 100 sccm is introduced and a total pressure in the tube in the range 50–100 Torr. The temperature of crucible is rapidly increased to 1450 1C and maintained for 10 min. After the system was cooled to room temperature with Ar gas, large quantities of light blue products were coated on an activated carbon fiber thermo-insulating layer. After completing the reaction, the collected materials were characterized using X-ray diffraction (XRD, CuKa radiation), field emission scanning electron microscopy (FESEM) equipped with energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM, at 300 kV) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM2010F).
Fig. 3. (a) SEM image of as-prepared samples grown at 1450 1C for 20 min by SiO powder and carbon fiber. (b) EDS spectrum recorded from individual nanostructure reveals only Si, C and O.
(a)
(b) [111]
3. Results and discussion The XRD pattern of as-prepared samples is shown in Fig. 2. All the peaks were indexed as b-SiC. From Fig. 3, which shows the SEM images and EDS of the sample, we see that the average diameters of nanowires range from 10 to 50 nm and have lengths up to 10 mm. From an energy dispersion spectroscopy (EDS), only three elements, Si, C and O, could be found. No other impurities were detected in the nanostructures. TEM and HRTEM were used to further investigate the morphology. In low-magnification TEM images of the sample (Fig. 4), the nanostructures were found and classified into two basic kinds: nanocables and nanosprings. Similar heterostructures, such as ZnS/SiC nanocables, BN-coated SiC, and carbon-coated SiC nanowires,
0.25nm 25nm (c)
5nm (d)
50nm
5nm
Fig. 4. (a) and (c) are typical TEM images for SiC/SiO2 nanocables and nanosprings. And (b) and (d) are HRTEM images for (a) and (c) nanostructures, respectively.
ARTICLE IN PRESS W.M. Zhou et al. / Physica E 31 (2006) 9–12
SiOðgÞ þ 2CðsÞ ! SiCðsÞ þ COðgÞ;
(1)
2SiOðgÞ þ 3CðsÞ ! 2SiCðsÞ þ CO2ðgÞ;
(2)
SiOðgÞ þ 3COðgÞ ! SiCðsÞ þ 2CO2 ðgÞ:
(3)
According to thermodynamics, the Gibbs-free energy DG(T) and the formation enthalpy DH(T) of a reaction at temperature T calculated using thermodynamic data from Ref. [17] is given in Fig. 5. It can be seen from Fig. 5 that the formation enthalpy DH(T) of reactions (1), (2) and (3) are very large, which indicates that lots of heat has been released during reaction. Moreover, the Gibbs free energy DG(T) of the three reactions are all negative in the results, which indicates the reactions can take place. At 1450 1C, the molar Gibbs-free energy of reaction (3) is positive. So in this result, reactions (1) and (2) are mainly formation mechanism. These data suggest that SiC should be main products in our experiments. Our experimental results are
−450 −400
∆H KJ/mol
−350 −300 −250
Equation (1)
−200
Equation (2)
−150
Equation (3)
−100 −50 200
400
600
800
1000
1200
1400
1600
T/K −400
∆G KJ/mol
and biaxial and coaxial SiC/SiO2 nanowires were synthesized by high temperature heating [12–15]. Fig. 4(a) is a typical smooth and straight nanocable composed of a single-crystalline core of a diameter of 10–25 nm coated with an amorphous silicon oxide layer. The atomic arrangements of SiC/SiO2 nanocable were exploited by HRTEM as shown in Fig. 4(b). An electron beam is perpendicular to the surface of the sheet. The lattice fringes show the image characteristics of b-SiC crystal, in which a d-spacing of 0.25 nm (indicated by parallel lines) corresponds to the (1 1 1) plane spacing, indicating nanowires growth along [1 1 1] direction. The growth direction is indexed as [1 1 1] shown in Fig. 4(b). A similar result is reported by other groups [14,15]. Additionally, some nanosprings composed of a core and an amorphous silicon oxide layer are shown in Figs. 4(c) and (d). The diameter of the nanospring is 25 nm and that of the nanospring pitch of 10 nm, this amounts to diameter/ pitch ratio of 2.5. Moreover, the total nanostructures consist of transition region from SiC/SiO2 nanocable to SiC/SiO2 nanospring then to SiC/SiO2 nanocable and its total length in Fig. 4(c) is 310 nm with a nanospring of 150 nm. TEM dark-images indicate the nanospring is composed of a crystalline core and an amorphous coating. In previous paper, the important role of the catalyst in the formation of SiC/SiO2 nanosprings was elucidated by Zhang et al. [7], which is different from our experiment without catalyst. Helical spring structure is the most fundamental structural configuration for DNA and many biological proteins, which are due to van der Waals force and hydrogen bonding [16]. These shapes of nanostructures will be found to be a great potential in application in electronic circuits and light emitting devices as important in building nanospring-based transducers and actuators, and tunable functional components for MEMS and NEMS [17]. The following possible chemical reaction mechanism has been proposed for the synthesis of SiC nanostructures:
11
−350
Equation (1)
−300
Equation (2) Equation (3)
−250 −200 −150 −100 −50 0 200
400
600
800
1000
1200
1400
1600
T/K Fig. 5. (a) The change of enthalpy DH(T) as a function of temperature for Eqs. (1), (2) and (3). (b) The change of Gibbs free energy DG(T) as a function of temperature for Eqs. (1), (2) and (3).
in good consistency with this prediction. In typical VLS process, the metal catalyst plays an importance role in onedimensional nanostructure. In the results, no catalyst could be seen in TEM images and no liquor occurred; thus the VS process was proposed. Firstly, carbon fiber acting as carbon source was circumvented by gas SiO, which was carried out by Ar gas and nucleated on a cooler carbon fiber with its large surface aiding in the nucleation process, and the reactions between SiO and C lead to SiC nucleation. As the concentration of SiC becomes sufficiently high, they aggregate into small clusters [2]. With an increasing supply of SiO being carried to the nucleation regions, the nanowires along the directions of the least stable plane grow when they get superstaturated. Secondly, an amorphous silicon oxide shell might have two reasons: at elevated temperature, SiC nanostructures oxidate by leakage of chamber; on the other hand, the unreacted SiO and O2 take place to form amorphous silicon oxide, which deposits on the nearest SiC nanowires [7]. Finally, there might be a stress which results from hampering of forming gaseous CO and CO2, and dislocation during the growth of
ARTICLE IN PRESS 12
W.M. Zhou et al. / Physica E 31 (2006) 9–12
nanostructures, thus forming SiC/SiO2 nanosprings. The structure defects (Figs. 4(b) and (d)) were responsible for growth morphologies, thereby minimizing the surface-free energy from the energy point of view. To have a comprehensive understanding of the growth process, the formation mechanism of nanosprings is evolved. As for the occurrence of the two morphologies in the same experiment, it could be caused by temperature gradient in the activated carbon fiber thermo-insulating layer with rapid heating and cooling. 4. Conclusion Large-scale SiC/SiO2 nanocables and nanosprings are easily obtained with SiO powder and activated carbon fibers by high frequency induction heating at low cost. The catalyst-free of as-prepared nanocables and nanosprings are different from results reported elsewhere. By means of XRD, SEM, EDS and TEM (HRTEM), nanocables and nanosprings have been characterized and discussed in detail. The chemical reactions of SiC formation are proposed. Acknowledgment This work is supported by the National Natural Science Foundation of China no. 50272039, and by the Developing
Foundation of Shanghai Science and Technology Grant no. 0452nm056.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Z.L. Wang, Adv. Mater. 12 (2003) 1295. Y.N. Xia, P.D. Yang, Adv. Mater. 15 (2003) 353. H.K. Seong, Appl. Phys. Lett. 85 (2004) 1256. W. Yang, H. Araki, A. Kohyama, S. Thaveethavorn, H. Suzuki, T. Noda, J. Am. Ceram. Soc. 87 (2004) 1720. K.T. Yeon, H.S. Soo, L.H. Mo, Mater. Trans. 45 (2004) 1442. Y.C. Ying, Y.L. Gu, J. Solid State Chem. 177 (2004) 4163. G.W. Meng, L.D. Zhang, C.M. Mo, J. Mater. Res . 13 (1998) 2533. H. Dai, E.W. Wong, Y. Lu, S. Fan, C.M. Lieber, Nature 375 (1995) 769. H.F. Zhang, C.M. Wang, S.L. Wang, Nano Lett. 2 (2002) 941. H.J. Choi, H.K. Seong, J.C. Lee, Y.M. Sung, J. Cryst. Growth 269 (2004) 472. K.W. Wong, X.T. Zhou, F.C.K. Au, H.L. Lai, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 75 (1999) 2918. Y.H. Ryu, B.T. Park, Y.H. Song, K.J. Yong, J. Cryst. Growth 271 (2004) 99. J.Q. Hu, Y. Bando, J.H. Zhan, D. Golberg, Appl. Phys. Lett. 85 (2004) 2932. C.C. Tang, Y.S. Bando, Appl. Phys. Lett. 83 (2003) 659. Z.L. Wang, Z.R. Dai, R.P. Gao, Z.G. Bai, Appl. Phys. Lett. 77 (2000) 3349. X.Y. Kong, Z.L. Wang, Nano. Lett. 3 (2003) 1625. Y. Liang, Y. Che, Handbook of Hermodynamics of Inorganic Substance, Dongbei University Press, 1994.