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A facile route to FeS nanowires via an infiltration process
Solid State Communications 140 (2006) 325–328 www.elsevier.com/locate/ssc
A facile route to FeS nanowires via an infiltration process Wei Wang a,b , Sheng-Yue Wang a,∗ , Ke-Yu Wang b , You-Lu Gao a , Mei Liu a a Department of Physics, Southeast University, Nanjing 210096, PR China b College of Physics and Technology, Nanjing Normal University, Nanjing 210097, PR China
Received 17 May 2006; received in revised form 1 September 2006; accepted 5 September 2006 by A. Pinczuk Available online 22 September 2006
Abstract Iron sulfide (FeS) nanowires were synthesized by the reaction between FeCl2 ·4H2 O and dimethyl sulfoxide (DMSO) in absolute alcohol at 200 ◦ C via an infiltration process in anodic aluminum oxide template (AAO). In this process, DMSO served as both strong infiltrator and sulfur source. The samples were characterized by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectra. Magnetic hysteresis analysis showed that the as prepared FeS nanowires were ferromagnetic, and the ferromagnetic nanowires turned superparamagnetic when their diameters decreased to about 20 nm. This work demonstrates a very simple method for obtaining FeS nanowires and may open a route to the synthesis of other 1D metallic chalcogenides. c 2006 Elsevier Ltd. All rights reserved.
PACS: 85.40.U; 75.50; 81.20.-n; 77.84.Bw Keywords: A. Chalcogenides; A. Nanowires; B. Infiltration; D. Magnetic properties
1. Introduction In the past decade, the discovery of one-dimensional (1D) nanostructured materials, including nanotubes [1], nanowires [2], nanocoaxial cable [3] and nanobelts [4], has stimulated great interest, because of both their interesting physical properties and their wide range potential applications in nanodevices. Owing to the remarkable simplicity of the process and potential applications in nanosystems, the template method, including using hard or soft templates, is still an important and promising route [5,6]. Up to now, the universal and effective assembly techniques using templates mainly focused on electrochemical techniques [7–9], which were proven to be low cost and high yield for preparing metal and alloy nanowires [10,11]. However, the electrochemical technique was not a feasible method for preparing compounds such as metal oxide or sulfide nanowires. For example, Fe3 O4 nanowires were recently synthesized by an electroprecipitation technique in a template [12], but the ratio of ∗ Corresponding author. Tel.: +86 25 83791835; fax: +86 25 83792868.
E-mail address: [email protected] (S.-Y. Wang). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.09.009
length to diameter was only about 3. Therefore, it is a challenge for chemists to develop simpler and more effective methods for preparing 1D nanostructured materials such as metal chalcogenides, for which it is more feasible to combine microsystem or nanosystem assembly [13]. Dimethyl sulfoxide (DMSO) has been used as an intermediate for preparing nanotube arrays electrochemically [14,15], and as a sulfur source for synthesizing metal chalcogenide [16– 18]. Our recent research showed that anions and cations can be conveyed and then go through the tunnel of AAO carried by DMSO at a speed of 1–2 µm/min. In addition, the degasification procedure which was usual required as reported in the literature [13,19] is not necessary in our method. Recently, a series of reports showed that the solvent DMSO has perfect infiltration properties for complete filling of the porous surface [20]. However, how to control the decomposition of DMSO during infiltration is still a sticking point. In this paper, a facile infiltration method is reported, for preparing FeS nanowires in AAO template. As a feature, DMSO served as not only the sulfur source but also a strong infiltrator for preparing FeS nanowires. Owing to the use of ammonia gas, the main chemical reaction takes place when the infiltration process has finished. Moreover, the microstructure,
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W. Wang et al. / Solid State Communications 140 (2006) 325–328
channel modification, and electric conductivity of the templates are not required in this method. 2. Experiment All the chemicals were analytical grade and purchased from Shanghai Chemical Reagents. The AAO templates with ‘anodisc’ 0.2 µm diameter were from Whatman International Ltd Company. In a typical procedure, first AAO templates were placed in an enclosed NH3 ambient for an appropriate time, and FeCl2 ·4H2 O was dissolved into the mixed solution of absolute alcohol and DMSO (volume ratio of 4:6). Then the mixture containing Fe2+ with 0.1 mol/L concentration was slowly dropped onto one side surface of the AAO template. The entire infiltration of DMSO was carried out in NH3 atmosphere. Simultaneously argon gas was used to prevent the oxidation of Fe2+ in the system. When the infiltration process finished, the templates were put into a quartz tube oven and heated up to 150 ◦ C or 200 ◦ C for various times, and then cooled to room temperature naturally. X-ray diffractometry (XRD) was performed on a Rigaku D/Max-RA mode X-ray diffractometer. The morphology of the nanowires was recorded with a JEM-200CX transmission electron microscope (TEM) and a JSM-5610LV-VANTAGE scanning electron microscope (SEM). The Raman spectrum was measured with a JY-HR800 laser Raman spectrometer at room temperature. Studies of the magnetic properties were carried out using a Lakeshore 7307-9309 vibrating sample magnetometer (VSM).
Fig. 1. (a) Schematic diagram of the infiltration assembly of FeS nanowires and (b) SEM image of a cross section showing FeS nanowires in AAO template.
3. Results and discussion It is well known that acid ambient speeds up DMSO decomposition, while alkaline ambient restrains DMSO decomposition. Thus, in order to harmonize the decomposition and infiltration of DMSO, ammonia gas was employed to restrain the DMSO from premature decomposition in our experiments. The scheme in Fig. 1 illustrates the procedure for fabricating FeS nanowires. The reaction process can be expressed as follows: 150 ◦ C
(1) (CH3 )2 SO −−−→ CH3 SH + CH2 O 150 ◦ C
(2) CH3 SH + CH3 CH2 OH −−−→ CH3 OC2 H5 + H2 S 200 ◦ C
(3) FeCl2 + H2 S −−−→ FeS + HCl. The samples were identified as being polycrystalline FeS by XRD (iron sulfide, orthorhombic, JCPDS file 76-0963, a = ˚ b = 6.930 A, ˚ and c = 5.825 A; ˚ calculated values: 6.935 A, ˚ b = 7.059 A, ˚ and c = 5.587 A), ˚ as shown in a = 6.858 A, Fig. 2. It is found that there is high preferential crystal growth along the h211i direction when the reaction is carried out at 150 ◦ C for 0.5 h first and then increased to 200 ◦ C for 10 h (Fig. 2a). To further confirm the composition of the product, the Raman spectra of FeS samples were also recorded at room temperature. The spectrum consists of two sets of peaks, one at 213, 282 cm−1 for FeS, which is consistent with the reported values [21], and the other at 136, 338, 677, 710 cm−1 , also
Fig. 2. XRD patterns of samples prepared at (a) 150 ◦ C for 0.5 h and then heated up to 200 ◦ C for 10 h; (b) 200 ◦ C for 10 h. Ammonia gas adsorption times for (a) and (b) are both 10 min before annealing. Mean diameters: D a ≈ D b ≈ 200 nm.
for the FeS, which has not been observed in many reported Raman spectra of single-crystalline or polycrystalline FeS samples [22]. The Raman shifts of the latter may derive from the surface phonon modes of different microstructures, between nanocrystalline and single-crystalline or bulk polycrystalline FeS [23]. Fig. 3(a) shows the representative TEM images of FeS nanowires prepared by the typical procedures. It is found that the nanowire diameter is tunable by controlling the adsorption time of the ammonia gas. For example, with the increase of
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W. Wang et al. / Solid State Communications 140 (2006) 325–328
Fig. 4. Magnetic hysteresis curves of FeS nanowires with different diameters measured at room temperature. D a ≈ 200 nm, D b ≈ 60 nm, D c ≈ 20 nm. Table 1 Magnetic property data for the samples from Fig. 4 Magnetic property
Curve a
Curve b
Curve c
Ms (emu/g) Coercive force (kOe)
117.83 0.314
95.54 0.209
50.96 0
of 60 and 200 nm show ferrimagnetism. No obvious shape anisotropy of the nanowires is observed. When the diameters decrease to about 20 nm, however, the nanowires exhibit superparamagnetism. This can be explained as follows: when the diameters of nanowires are smaller than the critical size of FeS single domains, the ferrimagnetic nanowires will become single-domain structures [24]. Influenced by thermal disturbance under the measured magnetic field, the magnetic properties of the nanowires are thus converted to superparamagnetism. Further work should be done to clarify the relation between magnetic properties and microstructures. Fig. 3. TEM images of nanowires prepared by using different adsorption times for ammonia gas before annealing: (a) 10 min, (b) 1 h and (c) more than 20 h.
the adsorption time (or dosage) of ammonia gas from 10 min to more than 20 h before annealing, the diameters of the as prepared nanowires decrease dramatically from the initial 200 nm to 20 nm, using 200 nm pore size AAO templates (Fig. 3(b)–(c)). The result indicates that ammonia gas adsorption is an effective factor for obtaining thinner nanowires with a fixed pore size template. When the diameter decreases to as little as about 20 nm, however, there is no further diameter reduction on extending the adsorption time, i.e. to more than 24 h. So this simple method might be a potential route to preparing extremely thin nanowires using templates or nanodevices. We must emphasize that the AAO templates from Whatman have uniform pore size, with about 20 nm allowable error. Fig. 4 shows magnetic hysteresis curves of FeS nanowires measured at room temperature. Detailed magnetic property data are listed in Table 1. The nanowires with diameters
4. Conclusions A simple infiltration method is introduced for preparing FeS nanowires in AAO templates. The diameter of the nanowires can be reduced in a fixed pore size AAO template. The process consists of ion infiltration, DMSO pyrolysis and crystal growth of FeS in nanochannels. Magnetic studies indicate that the FeS nanowires show ferrimagnetism when the diameters are more than 20 nm, and will turn superparamagnetic when the diameters are reduced below the critical size of single domains. This is expected to be a revelatory technique for the synthesis of other 1D metal chalcogenides, and even for nanonetwork or nanoelectrocircuit assembly. Acknowledgments This work was supported by Project 50572016 of the National Natural Science Foundation of China, and BK2004076 of the Natural Science Foundation of Jiangsu Province.
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W. Wang et al. / Solid State Communications 140 (2006) 325–328
References [1] [2] [3] [4] [5]
[6] [7] [8] [9]
[10] [11] [12]
S. Iijima, Nature 354 (1991) 56. G. Fasol, Science 280 (1998) 545. Y. Zhang, K. Suenaga, C. Colliex, Science 281 (1998) 973. W.P. Zhang, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. (a) Y.-J. Han, L.M. Wysocki, M.S. Thanawala, T. Siegrist, J. Aizenberg, Angew. Chem. Int. Ed. 44 (2005) 2; (b) G. Fasol, K. Runge, Appl. Phys. Lett. 70 (1997) 2467; (c) E. Braun, Y. Eichen, U. Sivan, G. Ben-Yoseph, Nature 391 (1998) 775. B. Gates, Y.Y. Wu, Y.D. Yin, P.D. Yang, Y.N. Xia, J. Am. Chem. Soc. 123 (2001) 11500. C. Ruan, M. Paulose, O.K. Varghese, G.K. Mor, C.A. Grimes, J. Phys. Chem. B 109 (2005) 15754. C.L. Lu, C. Guan, Y.F. Liu, Y.R. Cheng, B. Yang, Chem. Mater. 17 (2005) 2448. (a) R. Chen, D. Xu, G. Guo, Y. Tang, Chem. Phys. Lett. 377 (2003) 205; (b) R.Z. Chen, D.S. Xu, G.L. Guo, L.L. Gui, Electrochem. Commun. 5 (2003) 579. G. Meng, A. Cao, J.Y. Cheng, A. Vijayaraghavan, Y.J. Jung, M. Shima, P.M. Ajayan, J. Appl. Phys. 97 (2005) 064303. C.R. Martin, Science 266 (1994) 1961. C. Terrier, M. Abid, C. Arm, S. Serrano-Guisan, L. Gravier, J.-Ph. Ansermet, J. Appl. Phys. 98 (2005) 086102.
[13] (a) P.D. Yang, T. Deng, D. Zhao, P. Feng, D. Pine, B.F. Chmelka, G.M. Whitesides, G.D. Stucky, Science 282 (1998) 2244. (b) B. Messer, J.H. Song, P.D. Yang, J. Am. Chem. Soc. 122 (2000) 10232. (c) Y. Huang, X.F. Duan, C.M. Lieber, Science 291 (2001) 630. [14] Q. Huang, L. Gao, Nanotechnology 15 (2004) 1855. [15] J.D. Klein, R.D. Herrick, D. Palmer, M.J. Sailor, C.J. Brumlik, C.R. Martin, Chem. Mater. 5 (1993) 902. [16] S.K. Chakarvarti, J. Micromech. Microeng. 3 (1993) 57. [17] M. Holzinger, J. Abraham, P. Whelan, R. Graupner, L. Ley, F. Hennrich, M. Kappes, A. Hirsch, J. Am. Chem. Soc. 125 (2003) 856. [18] (a) M. Nath, A. Choudhury, A. Kundu, C.N.R. Rao, Adv. Mater. 15 (2003) 2098. (b) P.P. Chin, J. Ding, J.B. Yi, B.H. Liu, J. Alloys Comp. 390 (2005) 255. [19] Y. Li, Q.M. Zhang, F.Z. Huang, J.H. Wan, Z.N. Gu, Chinese J. Inorg. Chem. 18 (2002) 79. [20] R. Red´on, A. V´azquez-Olmos, M.E. Mata-Zamora, A. Ord´on˜ ez-Medrano, F. Rivera-Torres, J.M. Saniger, J. Colloid Interface Sci. 287 (2005) 664. [21] (a) J. Lara, T. Blunt, P. Kotvis, A. Riga, W.T. Tysoe, J. Phys. Chem. B 102 (1998) 1703. (b) A. Boughriet, B. Ouddane, C. Cordier, J. Laureyns, J. Chem. Soc. 94 (1998) 3677. [22] H.L. Park, W.B. White, Phys. Status Solidi K 69 (1987) 144. [23] D.S. Chun, C.M. Dai, W.P. Hsieh, C.T. Tsai, J. Appl. Phys. 69 (1991) 8402. [24] C.P. Bean, J. Appl. Phys. 30 (1959) 120.
A facile route to FeS nanowires via an infiltration process Wei Wang a,b , Sheng-Yue Wang a,∗ , Ke-Yu Wang b , You-Lu Gao a , Mei Liu a a Department of Physics, Southeast University, Nanjing 210096, PR China b College of Physics and Technology, Nanjing Normal University, Nanjing 210097, PR China
Received 17 May 2006; received in revised form 1 September 2006; accepted 5 September 2006 by A. Pinczuk Available online 22 September 2006
Abstract Iron sulfide (FeS) nanowires were synthesized by the reaction between FeCl2 ·4H2 O and dimethyl sulfoxide (DMSO) in absolute alcohol at 200 ◦ C via an infiltration process in anodic aluminum oxide template (AAO). In this process, DMSO served as both strong infiltrator and sulfur source. The samples were characterized by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectra. Magnetic hysteresis analysis showed that the as prepared FeS nanowires were ferromagnetic, and the ferromagnetic nanowires turned superparamagnetic when their diameters decreased to about 20 nm. This work demonstrates a very simple method for obtaining FeS nanowires and may open a route to the synthesis of other 1D metallic chalcogenides. c 2006 Elsevier Ltd. All rights reserved.
PACS: 85.40.U; 75.50; 81.20.-n; 77.84.Bw Keywords: A. Chalcogenides; A. Nanowires; B. Infiltration; D. Magnetic properties
1. Introduction In the past decade, the discovery of one-dimensional (1D) nanostructured materials, including nanotubes [1], nanowires [2], nanocoaxial cable [3] and nanobelts [4], has stimulated great interest, because of both their interesting physical properties and their wide range potential applications in nanodevices. Owing to the remarkable simplicity of the process and potential applications in nanosystems, the template method, including using hard or soft templates, is still an important and promising route [5,6]. Up to now, the universal and effective assembly techniques using templates mainly focused on electrochemical techniques [7–9], which were proven to be low cost and high yield for preparing metal and alloy nanowires [10,11]. However, the electrochemical technique was not a feasible method for preparing compounds such as metal oxide or sulfide nanowires. For example, Fe3 O4 nanowires were recently synthesized by an electroprecipitation technique in a template [12], but the ratio of ∗ Corresponding author. Tel.: +86 25 83791835; fax: +86 25 83792868.
E-mail address: [email protected] (S.-Y. Wang). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.09.009
length to diameter was only about 3. Therefore, it is a challenge for chemists to develop simpler and more effective methods for preparing 1D nanostructured materials such as metal chalcogenides, for which it is more feasible to combine microsystem or nanosystem assembly [13]. Dimethyl sulfoxide (DMSO) has been used as an intermediate for preparing nanotube arrays electrochemically [14,15], and as a sulfur source for synthesizing metal chalcogenide [16– 18]. Our recent research showed that anions and cations can be conveyed and then go through the tunnel of AAO carried by DMSO at a speed of 1–2 µm/min. In addition, the degasification procedure which was usual required as reported in the literature [13,19] is not necessary in our method. Recently, a series of reports showed that the solvent DMSO has perfect infiltration properties for complete filling of the porous surface [20]. However, how to control the decomposition of DMSO during infiltration is still a sticking point. In this paper, a facile infiltration method is reported, for preparing FeS nanowires in AAO template. As a feature, DMSO served as not only the sulfur source but also a strong infiltrator for preparing FeS nanowires. Owing to the use of ammonia gas, the main chemical reaction takes place when the infiltration process has finished. Moreover, the microstructure,
326
W. Wang et al. / Solid State Communications 140 (2006) 325–328
channel modification, and electric conductivity of the templates are not required in this method. 2. Experiment All the chemicals were analytical grade and purchased from Shanghai Chemical Reagents. The AAO templates with ‘anodisc’ 0.2 µm diameter were from Whatman International Ltd Company. In a typical procedure, first AAO templates were placed in an enclosed NH3 ambient for an appropriate time, and FeCl2 ·4H2 O was dissolved into the mixed solution of absolute alcohol and DMSO (volume ratio of 4:6). Then the mixture containing Fe2+ with 0.1 mol/L concentration was slowly dropped onto one side surface of the AAO template. The entire infiltration of DMSO was carried out in NH3 atmosphere. Simultaneously argon gas was used to prevent the oxidation of Fe2+ in the system. When the infiltration process finished, the templates were put into a quartz tube oven and heated up to 150 ◦ C or 200 ◦ C for various times, and then cooled to room temperature naturally. X-ray diffractometry (XRD) was performed on a Rigaku D/Max-RA mode X-ray diffractometer. The morphology of the nanowires was recorded with a JEM-200CX transmission electron microscope (TEM) and a JSM-5610LV-VANTAGE scanning electron microscope (SEM). The Raman spectrum was measured with a JY-HR800 laser Raman spectrometer at room temperature. Studies of the magnetic properties were carried out using a Lakeshore 7307-9309 vibrating sample magnetometer (VSM).
Fig. 1. (a) Schematic diagram of the infiltration assembly of FeS nanowires and (b) SEM image of a cross section showing FeS nanowires in AAO template.
3. Results and discussion It is well known that acid ambient speeds up DMSO decomposition, while alkaline ambient restrains DMSO decomposition. Thus, in order to harmonize the decomposition and infiltration of DMSO, ammonia gas was employed to restrain the DMSO from premature decomposition in our experiments. The scheme in Fig. 1 illustrates the procedure for fabricating FeS nanowires. The reaction process can be expressed as follows: 150 ◦ C
(1) (CH3 )2 SO −−−→ CH3 SH + CH2 O 150 ◦ C
(2) CH3 SH + CH3 CH2 OH −−−→ CH3 OC2 H5 + H2 S 200 ◦ C
(3) FeCl2 + H2 S −−−→ FeS + HCl. The samples were identified as being polycrystalline FeS by XRD (iron sulfide, orthorhombic, JCPDS file 76-0963, a = ˚ b = 6.930 A, ˚ and c = 5.825 A; ˚ calculated values: 6.935 A, ˚ b = 7.059 A, ˚ and c = 5.587 A), ˚ as shown in a = 6.858 A, Fig. 2. It is found that there is high preferential crystal growth along the h211i direction when the reaction is carried out at 150 ◦ C for 0.5 h first and then increased to 200 ◦ C for 10 h (Fig. 2a). To further confirm the composition of the product, the Raman spectra of FeS samples were also recorded at room temperature. The spectrum consists of two sets of peaks, one at 213, 282 cm−1 for FeS, which is consistent with the reported values [21], and the other at 136, 338, 677, 710 cm−1 , also
Fig. 2. XRD patterns of samples prepared at (a) 150 ◦ C for 0.5 h and then heated up to 200 ◦ C for 10 h; (b) 200 ◦ C for 10 h. Ammonia gas adsorption times for (a) and (b) are both 10 min before annealing. Mean diameters: D a ≈ D b ≈ 200 nm.
for the FeS, which has not been observed in many reported Raman spectra of single-crystalline or polycrystalline FeS samples [22]. The Raman shifts of the latter may derive from the surface phonon modes of different microstructures, between nanocrystalline and single-crystalline or bulk polycrystalline FeS [23]. Fig. 3(a) shows the representative TEM images of FeS nanowires prepared by the typical procedures. It is found that the nanowire diameter is tunable by controlling the adsorption time of the ammonia gas. For example, with the increase of
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W. Wang et al. / Solid State Communications 140 (2006) 325–328
Fig. 4. Magnetic hysteresis curves of FeS nanowires with different diameters measured at room temperature. D a ≈ 200 nm, D b ≈ 60 nm, D c ≈ 20 nm. Table 1 Magnetic property data for the samples from Fig. 4 Magnetic property
Curve a
Curve b
Curve c
Ms (emu/g) Coercive force (kOe)
117.83 0.314
95.54 0.209
50.96 0
of 60 and 200 nm show ferrimagnetism. No obvious shape anisotropy of the nanowires is observed. When the diameters decrease to about 20 nm, however, the nanowires exhibit superparamagnetism. This can be explained as follows: when the diameters of nanowires are smaller than the critical size of FeS single domains, the ferrimagnetic nanowires will become single-domain structures [24]. Influenced by thermal disturbance under the measured magnetic field, the magnetic properties of the nanowires are thus converted to superparamagnetism. Further work should be done to clarify the relation between magnetic properties and microstructures. Fig. 3. TEM images of nanowires prepared by using different adsorption times for ammonia gas before annealing: (a) 10 min, (b) 1 h and (c) more than 20 h.
the adsorption time (or dosage) of ammonia gas from 10 min to more than 20 h before annealing, the diameters of the as prepared nanowires decrease dramatically from the initial 200 nm to 20 nm, using 200 nm pore size AAO templates (Fig. 3(b)–(c)). The result indicates that ammonia gas adsorption is an effective factor for obtaining thinner nanowires with a fixed pore size template. When the diameter decreases to as little as about 20 nm, however, there is no further diameter reduction on extending the adsorption time, i.e. to more than 24 h. So this simple method might be a potential route to preparing extremely thin nanowires using templates or nanodevices. We must emphasize that the AAO templates from Whatman have uniform pore size, with about 20 nm allowable error. Fig. 4 shows magnetic hysteresis curves of FeS nanowires measured at room temperature. Detailed magnetic property data are listed in Table 1. The nanowires with diameters
4. Conclusions A simple infiltration method is introduced for preparing FeS nanowires in AAO templates. The diameter of the nanowires can be reduced in a fixed pore size AAO template. The process consists of ion infiltration, DMSO pyrolysis and crystal growth of FeS in nanochannels. Magnetic studies indicate that the FeS nanowires show ferrimagnetism when the diameters are more than 20 nm, and will turn superparamagnetic when the diameters are reduced below the critical size of single domains. This is expected to be a revelatory technique for the synthesis of other 1D metal chalcogenides, and even for nanonetwork or nanoelectrocircuit assembly. Acknowledgments This work was supported by Project 50572016 of the National Natural Science Foundation of China, and BK2004076 of the Natural Science Foundation of Jiangsu Province.
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W. Wang et al. / Solid State Communications 140 (2006) 325–328
References [1] [2] [3] [4] [5]
[6] [7] [8] [9]
[10] [11] [12]
S. Iijima, Nature 354 (1991) 56. G. Fasol, Science 280 (1998) 545. Y. Zhang, K. Suenaga, C. Colliex, Science 281 (1998) 973. W.P. Zhang, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. (a) Y.-J. Han, L.M. Wysocki, M.S. Thanawala, T. Siegrist, J. Aizenberg, Angew. Chem. Int. Ed. 44 (2005) 2; (b) G. Fasol, K. Runge, Appl. Phys. Lett. 70 (1997) 2467; (c) E. Braun, Y. Eichen, U. Sivan, G. Ben-Yoseph, Nature 391 (1998) 775. B. Gates, Y.Y. Wu, Y.D. Yin, P.D. Yang, Y.N. Xia, J. Am. Chem. Soc. 123 (2001) 11500. C. Ruan, M. Paulose, O.K. Varghese, G.K. Mor, C.A. Grimes, J. Phys. Chem. B 109 (2005) 15754. C.L. Lu, C. Guan, Y.F. Liu, Y.R. Cheng, B. Yang, Chem. Mater. 17 (2005) 2448. (a) R. Chen, D. Xu, G. Guo, Y. Tang, Chem. Phys. Lett. 377 (2003) 205; (b) R.Z. Chen, D.S. Xu, G.L. Guo, L.L. Gui, Electrochem. Commun. 5 (2003) 579. G. Meng, A. Cao, J.Y. Cheng, A. Vijayaraghavan, Y.J. Jung, M. Shima, P.M. Ajayan, J. Appl. Phys. 97 (2005) 064303. C.R. Martin, Science 266 (1994) 1961. C. Terrier, M. Abid, C. Arm, S. Serrano-Guisan, L. Gravier, J.-Ph. Ansermet, J. Appl. Phys. 98 (2005) 086102.
[13] (a) P.D. Yang, T. Deng, D. Zhao, P. Feng, D. Pine, B.F. Chmelka, G.M. Whitesides, G.D. Stucky, Science 282 (1998) 2244. (b) B. Messer, J.H. Song, P.D. Yang, J. Am. Chem. Soc. 122 (2000) 10232. (c) Y. Huang, X.F. Duan, C.M. Lieber, Science 291 (2001) 630. [14] Q. Huang, L. Gao, Nanotechnology 15 (2004) 1855. [15] J.D. Klein, R.D. Herrick, D. Palmer, M.J. Sailor, C.J. Brumlik, C.R. Martin, Chem. Mater. 5 (1993) 902. [16] S.K. Chakarvarti, J. Micromech. Microeng. 3 (1993) 57. [17] M. Holzinger, J. Abraham, P. Whelan, R. Graupner, L. Ley, F. Hennrich, M. Kappes, A. Hirsch, J. Am. Chem. Soc. 125 (2003) 856. [18] (a) M. Nath, A. Choudhury, A. Kundu, C.N.R. Rao, Adv. Mater. 15 (2003) 2098. (b) P.P. Chin, J. Ding, J.B. Yi, B.H. Liu, J. Alloys Comp. 390 (2005) 255. [19] Y. Li, Q.M. Zhang, F.Z. Huang, J.H. Wan, Z.N. Gu, Chinese J. Inorg. Chem. 18 (2002) 79. [20] R. Red´on, A. V´azquez-Olmos, M.E. Mata-Zamora, A. Ord´on˜ ez-Medrano, F. Rivera-Torres, J.M. Saniger, J. Colloid Interface Sci. 287 (2005) 664. [21] (a) J. Lara, T. Blunt, P. Kotvis, A. Riga, W.T. Tysoe, J. Phys. Chem. B 102 (1998) 1703. (b) A. Boughriet, B. Ouddane, C. Cordier, J. Laureyns, J. Chem. Soc. 94 (1998) 3677. [22] H.L. Park, W.B. White, Phys. Status Solidi K 69 (1987) 144. [23] D.S. Chun, C.M. Dai, W.P. Hsieh, C.T. Tsai, J. Appl. Phys. 69 (1991) 8402. [24] C.P. Bean, J. Appl. Phys. 30 (1959) 120.