- Email: [email protected]
Structural characterization of β-SiC nanowires synthesized by direct heating method
Materials Science and Engineering C 26 (2006) 805 – 808 www.elsevier.com/locate/msec
Structural characterization of h-SiC nanowires synthesized by direct heating method Yunho Baek, YongHwan Ryu, Kijung Yong * Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea Available online 9 November 2005
Abstract Crystal structure of h-SiC nanowires was investigated using Raman spectroscopy, FT-IR, XRD, transmission electron microscopy and selected area electron diffraction. Cubic h-SiC nanowires were synthesized by heating NiO catalyzed Si substrates with WO3 and graphite mixed powders in the growth temperature of 1000 – 1100 -C. HRTEM image showed atomic arrangements of the grown SiC nanowires with a main growth direction of [111]. Raman spectra showed two characteristic peaks at 796 cm 1 and 968 cm 1, which are corresponding to transversal optic mode and longitudinal optic mode of h-SiC, respectively. Also, FT-IR absorption spectroscopy showed a SiC characteristic absorption band at ¨792 cm 1. D 2005 Elsevier B.V. All rights reserved. Keywords: SiC nanowires; Direct heating method; Solid – liquid – solid (SLS) mechanism
1. Introduction One-dimensional nanomaterials have become the focus of intensive research owing to their unique applications in mesoscopic physics and the fabrication of nanodevices. In particular, silicon carbide (SiC) nanowires have attracted considerable attention in recent years due to their novel physical properties and potential applications in nano-electronic and nano-optic devices [1,2]. Various synthesis methods have been used to grow SiC nanowires. Those methods include physical evaporation [3], chemical vapor deposition [4] and laser ablation method [5]. In most previous methods, Si/C sources are delivered on the substrate and nucleation of reactant products induces nanowire growth on the substrate. Vapor – liquid –solid (VLS) or vapor – solid (VS) mechanisms have been proposed for these growth methods [6,7]. Recently simple direct heating methods have been developed to synthesize SiC nanowires [8 –11]. In these methods, catalyst-dispersed Si substrates are heated at high temperature above 1000 -C with carbon sources. Yang et al. have obtained a high density of SiC nanowires by heating a-
* Corresponding author. Fax: +82 54 279 8298. E-mail address: [email protected] (K. Yong). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.09.083
C:Fe coated Si substrates [9]. On the other hand, Kim et al. heated Fe/Si substrates at 1100 -C in the ambient of methane/ hydrogen gas to produce SiC nanowires [10]. Also, Ryu et al. have synthesized crystalline SiC nanowires by heating NiO catalyzed Si in reductive environment, which was produced by the carbothermal reaction of WO3 by C [11]. The carbothermal reduction of WO3/C provides carbon source to grow SiC nanowires. Instead of VLS or VS mechanism, a solid– liquid – solid (SLS) mechanism has been proposed for direct heating methods. In this report, we focus on the structural characterization of SiC nanowires, which were synthesized by direct heating of NiO/Si substrates. The crystal structure of the SiC nanowires was investigated by Raman spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction, and Fourier transformed infrared spectroscopy (FTIR). 2. Experiment The growth of the h-SiC nanowires coated with the amorphous SiO2 was carried out in a tube furnace. The substrates used in our experiments were 3 V-cm n-type Si (100) wafers (1 1 cm2). The silicon substrates were dipped in the Ni(NO3)2/ethanol solution (0.01 M) after being cleaned in
806
Y. Baek et al. / Materials Science and Engineering C 26 (2006) 805 – 808
an ultrasonic acetone bath for 20 min. After drying in the air, the substrates were put in a long quartz tube and purged by 1000 sccm Ar (high-purity argon, 99.99%) flow for 1 h. The nanowires were synthesized in the temperature range of 1000 –1100 -C for 3 h under a constant 500 sccm Ar flow. To synthesize the core-shell nanowires, the WO3 and graphite mixed powders were used to provide reductive environment. A carbothermal reduction of WO3 by C produces CO/CO2 reducing agents [12] and also carbon sources. After cooling down to room temperature, the surface of the Si substrate was covered with a white colored deposit. To remove the SiO2 shell layer from the core-shell nanowires, a dilute HF acid solution was used. The as-prepared core-shell nanowires were immersed in a 5% HF aqueous solution for 40 s and rinsed with de-ionized water for 2 min. The HF-treated oxide-free SiC nanowires were immediately dried in an Ar stream. The characterization of the nanowires was done ex situ by SEM, HRTEM, XRD, FT-IR transmittance spectra(400 –4000 cm 1, 4 cm 1 resolution, MIDAC M1200) and Raman spectroscopy (Ar-ion laser excitation with a wavelength of 514.5 nm, 800 mm focal length monochromator, Jobin-Yvon LabRam high resolution).
(a)
1 m
(b) [111] 0.25nm
3. Results and discussion A large quantity of SiO2-coated SiC nanowires was synthesized by heating Si substrate with WO3/C mixed powders. As-grown nanowires were treated by HF solution to remove thin silicon oxide sheath layers. Although the oxide layer can act as protective layer for core SiC nanowires, the removal of the oxide layer is required for many applications in the nanoelectronic device fabrication. Fig. 1 (a) shows SEM image of the HF treated nanowires. SEM image clearly reveals a uniform diameter distribution in the range 20¨50 nm. The lengths of nanowires were over several tens of micrometers. The surface of nanowires was clean without any particles. No metal catalyst was found at the tip of the grown nanowires. The atomic structure of nanowires was investigated using high-resolution transmission electron microscopy (HRTEM). Fig. 1 (b) is a typical image of the synthesized nanowires. The atomic arrangements of the SiC core-nanowire were clearly seen in the HRTEM image. It shows the (111) fringes perpendicular to the wire axis are on average separated by 0.25 nm, indicating the single crystalline SiC nanowires growth along the [111] direction. The inset of Fig. 1 (b) is the selected-area electron diffraction (SAED) pattern, which can be indexed to the cubic h-SiC structure. Energy dispersion spectroscopy (EDS) indicated that the nanowires were mainly composed of Si and C (not shown). No evidence of Ni, W or other impurities was found in the EDS spectrum. Some of the samples showed growth direction such as [100] and also several grain boundaries in a single nanowire. Fig. 1 (c) shows a high density of planar defects and stacking faults within a SiC nanowire. The SAED indicates a streaking pattern, which is characteristic feature of a stacking fault
2nm
(c)
2nm Fig. 1. (a) SEM image of the HF treated h-SiC nanowires directly grown from NiO/Si by heating with the WO3/C powder. (b) HRTEM image of the SiC nanowire showing that the distance between two fringes is about 0.25 nm close to the (111) spacing of h-SiC indicating a preferential growth in [111] direction. Inset is the SAED pattern of SiC nanowire. (c) HRTEM image of the SiC nanowire, revealing the presence of stacking faults. Inset shows a streaking diffraction pattern characteristic of stacking faults.
Y. Baek et al. / Materials Science and Engineering C 26 (2006) 805 – 808
807
4000
Intensity (arb.units)
3500
TO
3000
2500 LO 2000
1500
1000 600
700
800
900
1000
Raman Shift ( cm
-1
1100
1200
1300
)
Fig. 2. Room temperature Raman spectrum from SiC nanowires. TO and LO vibration modes were observed at 796 and 968 cm
structure. The stacking faults were found in some of very thin nanowires. Fig. 2 shows the Raman spectrum of the SiC nanowires, which was measured at room temperature after exciting with the 514 nm line of an argon ion laser. It is well known that Raman scattering provides a particular insights to the microstructure and conformation of materials. Fig. 2 shows two characteristic peaks at 796 cm 1 and 968 cm 1, which are corresponding to transversal optic mode (TO) and longitudinal optic mode (LO) of h-SiC, respectively. The narrow TO feature indicates that the grown SiC nanowires have a high crystalline quality. A shoulder at around 765 cm 1 was found in the spectrum, which may originate from the stacking faults of the SiC nanowires.
1
, respectively.
The XRD spectrum of the HF treated nanowires is shown in Fig. 3. It shows a typical XRD pattern of the h-SiC crystal (JCPDS card No.: 29-1129), which was obtained from SiC nanowires grown on the Si substrate. The grown SiC nanowires showed a main growth direction of [111]. Peaks corresponding to the Si substrate and the stacking fault (sf) of SiC were also found. FTIR spectrum of the SiC nanowires is shown in Fig. 4. An absorption peak at 792 cm 1 indicates the transversal optic (TO) mode (Si – C stretching vibration) and a shoulder at 954 cm 1 is corresponding to the longitudinal optic (LO) vibration mode. A significant asymmetry of the peak is thought to be partly due to the polycrystalline nature of the SiC nanowires. Inset is a magnified spectrum in the wave number range of
Intensity [a.u.]
SiC(111)
Si substrate SiC(220)
sf
0
20
SiC(311)
SiC(200)
40
60
80
100
2 Theta Fig. 3. XRD pattern of the SiCNWs extracted from the core-shell nanowires after HF treatment. The crystal phases (111), (200), (220), and (311) of SiC are denoted, and sf indicates a diffraction peak due to the stacking faults in the SiC crystal. A XRD peak from the bare Si substrate is also shown.
Y. Baek et al. / Materials Science and Engineering C 26 (2006) 805 – 808
Transmittance (arb.units)
Transmittance(arb.units)
808
Si-O (1106)
1180
1160
1140
1120
1100
1080
1060
1040
wavelength(cm-1)
LO
TO Si-C(792)
1300
1200
1100
1000
900
800
700
600
wavenumber (cm-1) Fig. 4. FTIR spectrum of SiC nanowires. Peaks corresponding to TO and LO vibration were denoted. Inset shows a magnified spectrum in the range of 1040¨1180 cm 1.
1040¨1180 cm 1. It shows a weak peak at 1106 cm 1, which is attributed to the stretching vibration of Si –O. These results indicate that the HF-treated SiC nanowires still have very small amounts of silicon oxide. Ex situ IR characterization was not performed right after HF treatment, which may cause the formation of a very thin native oxide on SiC nanowires. Our HRTEM, Raman, XRD and FTIR results indicated that the directly grown SiC nanowires on Si had a high crystalline cubic SiC nanowire structure synthesized by a simple heating method of NiO catalyzed Si substrates. 4. Conclusions Highly crystalline SiC nanowires were synthesized by direct heating of NiO catalyzed Si substrate. The grown h-SiC nanowires were characterized by SEM, HRTEM, XRD, Raman spectroscopy and FTIR. Atomic arrangements of the grown nanowires were clearly seen in HRTEM images. Main growth direction was [111] and polycrystalline feature was also found. Some of the very thin nanowires exhibited stacking faults, which were detected by HRTEM, XRD, and Raman spectra. Raman and FTIR spectra showed TO/LO vibration modes characteristic of h-SiC, respectively.
Acknowledgments This work was supported by grant No. RTI04-01-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE). References [1] Y. Yao, S.T. Lee, F.H. Li, Chem. Phys. Lett. 381 (2003) 9. [2] H.J. Li, Z.J. Li, A.L. Meng, K.Z. Li, X.N. Zhang, Y.P. Xu, J. Alloys Compd. 352 (2003) 279. [3] Z.S. Wu, S.Z. Deng, J. Chen, J. Zhou, Appl. Phys. Lett. 80 (2002) 3829. [4] H.F. Zhang, C.M. Wang, S.L. Wang, Nano. Lett. 2 (2002) 941. [5] W. Shi, Y. Zheng, H. Peng, N. Wang, C.S. Lee, S.T. Lee, J. Am. Chem. Soc. 83 (2000) 3228. [6] S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, H.W. Shim, E.K. Suh, C.J. Lee, Chem. Phys. Lett. 363 (2002) 134. [7] H.F. Yan, Y.J. Xing, Q.L. Hang, D.P. Yu, Y.P. Wang, J. Xu, Z.H. Xi, S.Q. Feng, Chem. Phys. Lett. 323 (2000) 224. [8] S.C. Lyu, O.H. Cha, E.K. Suh, H. Ruh, H.J. Lee, C.J. Lee, Chem. Phys. Lett. 367 (2003) 136. [9] T.H. Yang, C.H. Chen, A. Chatterjee, H.Y. Li, J.T. Lo, C.T. Wu, K.H. Chen, L.C. Chen, Chem. Phys. Lett. 379 (2003) 155. [10] H.Y. Kim, J. Park, H. Yang, Chem. Commun. (2003) 256. [11] Y.H. Ryu, Y.J. Tak, Kijung Yong, Nanotechnology 16 (2005) 370. [12] G.A. Swift, R. Koc, J. Mater. Sci. 36 (2001) 803.
Structural characterization of h-SiC nanowires synthesized by direct heating method Yunho Baek, YongHwan Ryu, Kijung Yong * Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea Available online 9 November 2005
Abstract Crystal structure of h-SiC nanowires was investigated using Raman spectroscopy, FT-IR, XRD, transmission electron microscopy and selected area electron diffraction. Cubic h-SiC nanowires were synthesized by heating NiO catalyzed Si substrates with WO3 and graphite mixed powders in the growth temperature of 1000 – 1100 -C. HRTEM image showed atomic arrangements of the grown SiC nanowires with a main growth direction of [111]. Raman spectra showed two characteristic peaks at 796 cm 1 and 968 cm 1, which are corresponding to transversal optic mode and longitudinal optic mode of h-SiC, respectively. Also, FT-IR absorption spectroscopy showed a SiC characteristic absorption band at ¨792 cm 1. D 2005 Elsevier B.V. All rights reserved. Keywords: SiC nanowires; Direct heating method; Solid – liquid – solid (SLS) mechanism
1. Introduction One-dimensional nanomaterials have become the focus of intensive research owing to their unique applications in mesoscopic physics and the fabrication of nanodevices. In particular, silicon carbide (SiC) nanowires have attracted considerable attention in recent years due to their novel physical properties and potential applications in nano-electronic and nano-optic devices [1,2]. Various synthesis methods have been used to grow SiC nanowires. Those methods include physical evaporation [3], chemical vapor deposition [4] and laser ablation method [5]. In most previous methods, Si/C sources are delivered on the substrate and nucleation of reactant products induces nanowire growth on the substrate. Vapor – liquid –solid (VLS) or vapor – solid (VS) mechanisms have been proposed for these growth methods [6,7]. Recently simple direct heating methods have been developed to synthesize SiC nanowires [8 –11]. In these methods, catalyst-dispersed Si substrates are heated at high temperature above 1000 -C with carbon sources. Yang et al. have obtained a high density of SiC nanowires by heating a-
* Corresponding author. Fax: +82 54 279 8298. E-mail address: [email protected] (K. Yong). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.09.083
C:Fe coated Si substrates [9]. On the other hand, Kim et al. heated Fe/Si substrates at 1100 -C in the ambient of methane/ hydrogen gas to produce SiC nanowires [10]. Also, Ryu et al. have synthesized crystalline SiC nanowires by heating NiO catalyzed Si in reductive environment, which was produced by the carbothermal reaction of WO3 by C [11]. The carbothermal reduction of WO3/C provides carbon source to grow SiC nanowires. Instead of VLS or VS mechanism, a solid– liquid – solid (SLS) mechanism has been proposed for direct heating methods. In this report, we focus on the structural characterization of SiC nanowires, which were synthesized by direct heating of NiO/Si substrates. The crystal structure of the SiC nanowires was investigated by Raman spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction, and Fourier transformed infrared spectroscopy (FTIR). 2. Experiment The growth of the h-SiC nanowires coated with the amorphous SiO2 was carried out in a tube furnace. The substrates used in our experiments were 3 V-cm n-type Si (100) wafers (1 1 cm2). The silicon substrates were dipped in the Ni(NO3)2/ethanol solution (0.01 M) after being cleaned in
806
Y. Baek et al. / Materials Science and Engineering C 26 (2006) 805 – 808
an ultrasonic acetone bath for 20 min. After drying in the air, the substrates were put in a long quartz tube and purged by 1000 sccm Ar (high-purity argon, 99.99%) flow for 1 h. The nanowires were synthesized in the temperature range of 1000 –1100 -C for 3 h under a constant 500 sccm Ar flow. To synthesize the core-shell nanowires, the WO3 and graphite mixed powders were used to provide reductive environment. A carbothermal reduction of WO3 by C produces CO/CO2 reducing agents [12] and also carbon sources. After cooling down to room temperature, the surface of the Si substrate was covered with a white colored deposit. To remove the SiO2 shell layer from the core-shell nanowires, a dilute HF acid solution was used. The as-prepared core-shell nanowires were immersed in a 5% HF aqueous solution for 40 s and rinsed with de-ionized water for 2 min. The HF-treated oxide-free SiC nanowires were immediately dried in an Ar stream. The characterization of the nanowires was done ex situ by SEM, HRTEM, XRD, FT-IR transmittance spectra(400 –4000 cm 1, 4 cm 1 resolution, MIDAC M1200) and Raman spectroscopy (Ar-ion laser excitation with a wavelength of 514.5 nm, 800 mm focal length monochromator, Jobin-Yvon LabRam high resolution).
(a)
1 m
(b) [111] 0.25nm
3. Results and discussion A large quantity of SiO2-coated SiC nanowires was synthesized by heating Si substrate with WO3/C mixed powders. As-grown nanowires were treated by HF solution to remove thin silicon oxide sheath layers. Although the oxide layer can act as protective layer for core SiC nanowires, the removal of the oxide layer is required for many applications in the nanoelectronic device fabrication. Fig. 1 (a) shows SEM image of the HF treated nanowires. SEM image clearly reveals a uniform diameter distribution in the range 20¨50 nm. The lengths of nanowires were over several tens of micrometers. The surface of nanowires was clean without any particles. No metal catalyst was found at the tip of the grown nanowires. The atomic structure of nanowires was investigated using high-resolution transmission electron microscopy (HRTEM). Fig. 1 (b) is a typical image of the synthesized nanowires. The atomic arrangements of the SiC core-nanowire were clearly seen in the HRTEM image. It shows the (111) fringes perpendicular to the wire axis are on average separated by 0.25 nm, indicating the single crystalline SiC nanowires growth along the [111] direction. The inset of Fig. 1 (b) is the selected-area electron diffraction (SAED) pattern, which can be indexed to the cubic h-SiC structure. Energy dispersion spectroscopy (EDS) indicated that the nanowires were mainly composed of Si and C (not shown). No evidence of Ni, W or other impurities was found in the EDS spectrum. Some of the samples showed growth direction such as [100] and also several grain boundaries in a single nanowire. Fig. 1 (c) shows a high density of planar defects and stacking faults within a SiC nanowire. The SAED indicates a streaking pattern, which is characteristic feature of a stacking fault
2nm
(c)
2nm Fig. 1. (a) SEM image of the HF treated h-SiC nanowires directly grown from NiO/Si by heating with the WO3/C powder. (b) HRTEM image of the SiC nanowire showing that the distance between two fringes is about 0.25 nm close to the (111) spacing of h-SiC indicating a preferential growth in [111] direction. Inset is the SAED pattern of SiC nanowire. (c) HRTEM image of the SiC nanowire, revealing the presence of stacking faults. Inset shows a streaking diffraction pattern characteristic of stacking faults.
Y. Baek et al. / Materials Science and Engineering C 26 (2006) 805 – 808
807
4000
Intensity (arb.units)
3500
TO
3000
2500 LO 2000
1500
1000 600
700
800
900
1000
Raman Shift ( cm
-1
1100
1200
1300
)
Fig. 2. Room temperature Raman spectrum from SiC nanowires. TO and LO vibration modes were observed at 796 and 968 cm
structure. The stacking faults were found in some of very thin nanowires. Fig. 2 shows the Raman spectrum of the SiC nanowires, which was measured at room temperature after exciting with the 514 nm line of an argon ion laser. It is well known that Raman scattering provides a particular insights to the microstructure and conformation of materials. Fig. 2 shows two characteristic peaks at 796 cm 1 and 968 cm 1, which are corresponding to transversal optic mode (TO) and longitudinal optic mode (LO) of h-SiC, respectively. The narrow TO feature indicates that the grown SiC nanowires have a high crystalline quality. A shoulder at around 765 cm 1 was found in the spectrum, which may originate from the stacking faults of the SiC nanowires.
1
, respectively.
The XRD spectrum of the HF treated nanowires is shown in Fig. 3. It shows a typical XRD pattern of the h-SiC crystal (JCPDS card No.: 29-1129), which was obtained from SiC nanowires grown on the Si substrate. The grown SiC nanowires showed a main growth direction of [111]. Peaks corresponding to the Si substrate and the stacking fault (sf) of SiC were also found. FTIR spectrum of the SiC nanowires is shown in Fig. 4. An absorption peak at 792 cm 1 indicates the transversal optic (TO) mode (Si – C stretching vibration) and a shoulder at 954 cm 1 is corresponding to the longitudinal optic (LO) vibration mode. A significant asymmetry of the peak is thought to be partly due to the polycrystalline nature of the SiC nanowires. Inset is a magnified spectrum in the wave number range of
Intensity [a.u.]
SiC(111)
Si substrate SiC(220)
sf
0
20
SiC(311)
SiC(200)
40
60
80
100
2 Theta Fig. 3. XRD pattern of the SiCNWs extracted from the core-shell nanowires after HF treatment. The crystal phases (111), (200), (220), and (311) of SiC are denoted, and sf indicates a diffraction peak due to the stacking faults in the SiC crystal. A XRD peak from the bare Si substrate is also shown.
Y. Baek et al. / Materials Science and Engineering C 26 (2006) 805 – 808
Transmittance (arb.units)
Transmittance(arb.units)
808
Si-O (1106)
1180
1160
1140
1120
1100
1080
1060
1040
wavelength(cm-1)
LO
TO Si-C(792)
1300
1200
1100
1000
900
800
700
600
wavenumber (cm-1) Fig. 4. FTIR spectrum of SiC nanowires. Peaks corresponding to TO and LO vibration were denoted. Inset shows a magnified spectrum in the range of 1040¨1180 cm 1.
1040¨1180 cm 1. It shows a weak peak at 1106 cm 1, which is attributed to the stretching vibration of Si –O. These results indicate that the HF-treated SiC nanowires still have very small amounts of silicon oxide. Ex situ IR characterization was not performed right after HF treatment, which may cause the formation of a very thin native oxide on SiC nanowires. Our HRTEM, Raman, XRD and FTIR results indicated that the directly grown SiC nanowires on Si had a high crystalline cubic SiC nanowire structure synthesized by a simple heating method of NiO catalyzed Si substrates. 4. Conclusions Highly crystalline SiC nanowires were synthesized by direct heating of NiO catalyzed Si substrate. The grown h-SiC nanowires were characterized by SEM, HRTEM, XRD, Raman spectroscopy and FTIR. Atomic arrangements of the grown nanowires were clearly seen in HRTEM images. Main growth direction was [111] and polycrystalline feature was also found. Some of the very thin nanowires exhibited stacking faults, which were detected by HRTEM, XRD, and Raman spectra. Raman and FTIR spectra showed TO/LO vibration modes characteristic of h-SiC, respectively.
Acknowledgments This work was supported by grant No. RTI04-01-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE). References [1] Y. Yao, S.T. Lee, F.H. Li, Chem. Phys. Lett. 381 (2003) 9. [2] H.J. Li, Z.J. Li, A.L. Meng, K.Z. Li, X.N. Zhang, Y.P. Xu, J. Alloys Compd. 352 (2003) 279. [3] Z.S. Wu, S.Z. Deng, J. Chen, J. Zhou, Appl. Phys. Lett. 80 (2002) 3829. [4] H.F. Zhang, C.M. Wang, S.L. Wang, Nano. Lett. 2 (2002) 941. [5] W. Shi, Y. Zheng, H. Peng, N. Wang, C.S. Lee, S.T. Lee, J. Am. Chem. Soc. 83 (2000) 3228. [6] S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, H.W. Shim, E.K. Suh, C.J. Lee, Chem. Phys. Lett. 363 (2002) 134. [7] H.F. Yan, Y.J. Xing, Q.L. Hang, D.P. Yu, Y.P. Wang, J. Xu, Z.H. Xi, S.Q. Feng, Chem. Phys. Lett. 323 (2000) 224. [8] S.C. Lyu, O.H. Cha, E.K. Suh, H. Ruh, H.J. Lee, C.J. Lee, Chem. Phys. Lett. 367 (2003) 136. [9] T.H. Yang, C.H. Chen, A. Chatterjee, H.Y. Li, J.T. Lo, C.T. Wu, K.H. Chen, L.C. Chen, Chem. Phys. Lett. 379 (2003) 155. [10] H.Y. Kim, J. Park, H. Yang, Chem. Commun. (2003) 256. [11] Y.H. Ryu, Y.J. Tak, Kijung Yong, Nanotechnology 16 (2005) 370. [12] G.A. Swift, R. Koc, J. Mater. Sci. 36 (2001) 803.