Article pubs.acs.org/JPCC
Electrical and Optical Properties of Conductive and Transparent ITO@PMMA Nanocomposites Elen Poliani S. Arlindo,† Juliana A. Lucindo,† Carlos M. O. Bastos,‡ Paulo D. Emmel,‡ and Marcelo O. Orlandi*,†,§ †
Departament of Physics and Chemistry, São Paulo State University, CEP 15385-000, P.O. Box 31, Ilha Solteira, SP, Brazil Department of Physics, Federal University of São Carlos, CEP 13565-905, São Carlos, SP, Brazil § Department of Physical Chemistry, São Paulo State University, CEP 14801-970, P.O. Box 355, Araraquara, SP, Brazil ‡
ABSTRACT: In this work, nanocomposite films of indium tin oxide (ITO) nanowires in a PMMA matrix were obtained by tape casting. The electrical, optical and morphological properties of films were studied as a function of the amount of wires inserted in the composite, and it was used 1, 2, 5, and 10 wt %. Results confirmed that films transmittance decreases as the concentration of wires increases, attaining a minimum transmittance of 55% for 10 wt % of filler. On the other hand, the electrical resistance of composites was found to decrease by increasing the filler amount and the dc characterization indicate that percolation occurs for about 5 wt % of wires. The morphological studies carried out by TEM were considered to be in good agreement with the electrical results and confirm that for 5% of filler, the ITO nanostructures are in contact with one another inside the polymer. Moreover, we made computational simulation of 1D structures in a general matrix and it was found that percolation should occur for about 12 wt %. Although computational results indicate higher amount of wires necessary for percolation than we found experimentally, both results illustrate that using one-dimensional nanostructures as filler in composites enables obtaining percolation for a smaller amount of filler than when using, for instance, nanoparticles. Therefore, the simple processing technique employed here can be used to obtain transparent and conductive composites with several useful applications.
1. INTRODUCTION Nanomaterials are defined as materials that have at least one of their dimensions in the nano scale. These materials have been extensively studied in the past decade because of their interesting properties and the opportunities opened to test theories in the quantum range. In addition, the new or superior properties presented by nanomaterials permit them to be used technologically in almost all areas of knowledge.1−7 One interesting approach in the area of materials is the possibility of combining two or more phases which are immiscible at the microscopic level, but that result in a material with better properties than its constituents. This type of material is called a composite, and composites can have polymer, ceramic or metallic matrix. Polymer matrix nanocomposites have attracted special attention because they can be, for instance, lightweight and extremely strong depending on their constituents. Furthermore, this class of materials may possess antagonistic properties such as transparency in the visible range and conductivity, which renders it to be considered special. Usually, composites possess more than one improved property for a special application, thus referred to as multifunctional materials, and in order to have some control over the desired property the matrix and fillers should be carefully chosen.8−13 © 2012 American Chemical Society
PMMA is a widely used commercial polar polymer because of its excellent transparence in the visible range of electromagnetic radiation; it is lightweight and possesses good mechanical properties. Notwithstanding, the extremely low conductivity of PMMA can be a drawback depending on its application. In order to overcome this limitation conductive filler can be inserted into the PMMA matrix. However, a metallic or carbon based filler can induce opacity to the PMMA and/or jeopardize the esthetical characteristic of the material. Therefore, in the pursuit of transparent and conductive fillers some ceramics with these desired characteristics, such as ATO, ITO, and FTO, have been used, and composites with nanoparticles of these materials have been widely studied.14−17 Among these ceramics, special attention has been given to ITO because of its high conductivity and transparency values. In addition, the use of nanostructures enables attaining percolation threshold limits in composites for lower amounts of filler when compared to macrocomposites, and studies on high aspect ratio nanostructures such as carbon nanotubes (CNTs) confirm attainment of percolation for less than 1 wt % Received: April 2, 2012 Published: May 8, 2012 12946
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of filler. Hence, the use of high aspect ITO nanostructures to obtain nanocomposites should be examined.18 The main goal of this work is to develop a method of introducing different amounts of high aspect ratio ITO nanostructures (nanowires) inside a polymeric matrix (PMMA) followed by a study of its electrical, optical and morphological properties. Composites studied were characterized by UV−vis, dc (two and four probes) electrical measurements and by scanning and transmission electron microscopy (SEM and TEM, respectively). Moreover, a computational simulation of 1D structures in a general matrix was performed to know, theoretically, what is percolation threshold of nanowires. Finally, results were discussed in detail.
grids by inserting the grid into the suspension of ITO and PMMA in THF which was dried at room temperature. To carry out electrical measurements of the nanowires, a quartz substrate with interdigitated gold electrodes made by lithograph was used. A micromanipulator (Wentworth, model MP1008) was then used to connect the substrate to a stabilized voltage source (Keithley, model 237). For the two and four probe electrical measurements in the composites, a resistivity measurement system (Signatone, model Pro-4) and the Keithley stabilized source were used simultaneously. The optical characterization of the composite films (about 20 μm of thickness) was performed by UV−vis spectroscopy (Varian, model Cary 50) in the range of 200 to 1000 nm. It should be highlighted that the optical transmittance in the visible range is an important parameter for the application of these films. 2.4. Computational Simulation to Achieve Percolation. To study, theoretically, the percolation threshold of fibers in a general matrix, we developed a Fortran program which calculates the intersection of random distributed fibers in a matrix. In this program it is possible to change the box size, which in this case is the matrix; and the number, size and rotation angle of fibers. The program associates a number to each fiber and has a subroutine to verify percolation,20,21 and when different wires intersect they assume the same number, which was chosen as the lower one. So, when two parallel sides of the box have wires with the same number, percolation occurred. In this study the box size was kept constant because all films done by tape casting have the same size, and we studied the percolation as a function of size and number of fibers (the rotation angle of fibers was left between −180 to +180 o for all simulations).
2. EXPERIMENTAL SECTION 2.1. Growth of ITO Nanostructures. The ITO nanowires were synthesized by a carbothermal reduction method using the coevaporation of oxides.18 In this work, the starting materials used were SnO2 (Sigma-Aldrich, 99.9% of purity) and In2O3 (Sigma-Aldrich, 99.99% of purity) mixed in a molar proportion of 1:1 with the reducing agent, which in the present study was the carbon black (Union Carbide, > 99% of purity). In a typical procedure both mixtures were placed in separate alumina boats which were then inserted side by side in the hot zone of a tube furnace (EDG, model FT-HI 40). The synthesis was performed at 1150 °C for 1 h with the extremities of the tube sealed with aluminum cover and PTHF o-rings. A N2 gas flux of 80 sccm was kept throughout the synthesis to carry on vapor to colder region of the tube. Details of the ITO nanostructures synthesis are given in refs 18 and 19. 2.2. Preparation of Nanocomposites. To prepare the nanocomposites, the material collected after the synthesis was dispersed using an ultrasonic cleaner in an alcohol medium. Then, the PMMA (Sigma-Aldrich, average molecular weight of 12000 g/mol) was dissolved in Tetrahydrofuran (THF) and a known amount of ITO nanowires was inserted into the solution at 1, 2, 5, and 10 wt % in relation to the amount of PMMA. The suspension was kept under stirring for 15 min and then deposited over a glass substrate using the tape casting process at room temperature. After the removal of films from the glass, no further processing was necessary to characterize the nanocomposites. 2.3. Characterization of Nanowires and Nanocomposite. The wool-like material collected after the carbothermal reduction synthesis was analyzed by X-ray diffraction (Shimadzu, model XRD 6000) using the Cu Kα radiation in the range of 20 to 60°. The morphological characterization of the materials was then performed using a field emission gun scanning electron microscope (FEG-SEM; Jeol, models JSM 6330F and JSM 7500 F). The morphology, structure and chemical ratio of the cations in the ITO nanowires were also analyzed by transmission electron microscopy (TEM; Jeol JSM 3010 operated at 300 kV) equipped with energy dispersive X-ray spectroscopy (EDS). The same technique (TEM, using a Philips CM 200 microscope operated at 200 kV) was used to study the percolation of wires inside the polymeric matrix. In order to prepare the samples for SEM, some drops of a suspension of ITO nanowires in an alcohol medium were deposited on conductive and plane silicon substrate. For TEM characterization of wires, one drop of the same suspension was deposited on a carbon coated Cu grid, while for the composite characterization, a composite film was produced on the Cu
3. RESULTS AND DISCUSSION 3.1. ITO Nanostructures. After the synthesis of the ITO structures, a wool-like material was collected inside the tube in a region between 450 and 500 °C. Figure 1 presents the XRD pattern of the collected material. All the peaks in the diffractogram can be indexed by the Al, In0,2Sn0,8 (JCPDS card #48-1547), and In2O3 (JCPDS card #6-416) phases. It is
Figure 1. XRD pattern of the collected material after the synthesis. 12947
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Figure 2. FEG-SEM images of typical ITO nanowires with square cross section and the width distribution of wires.
worth to mention that peaks of In1.875Sn0.125O3 phase (JCPDS card #89-4597) superpose the In2O3 ones and the only way to distinguish these phases is using data refinement or chemical analysis or electrical measurements. In this work we used the last two characterizations. The Al peaks are related to the aluminum sample holder used in the XRD measurements and have no correlation with the ITO sample. Moreover, the relative intensities between the In2O3 and the In0,2Sn0,8 peaks confirm that the former is present in greater amount. Regarding the In2O3 phase, it is easy to recognize that the peak at 2θ = 35.5° is much higher than it should be for a nonoriented powder, meaning that the sample has a preferential orientation for the (400) planes with an interplanar distance of 2.5 Å. It is important to notice that no peak was observed for tin oxide phases. In fact, following the phase diagram of In2O3 and SnO2,22 up to about 15 mol % of tin atoms can be in the solid solution in the In2O3 matrix without any secondary phase. According to the literature23−25 the tin rich metallic phase should be related to particles which control the growth mechanism of structures. Figure 2 illustrates the FEG-SEM
characterization of the material obtained after the synthesis. It is possible to observe that this material is composed of onedimensional structures with well-defined edges; its size is homogeneous along their lengths and there are no apparent superficial defects. Figure 2c shows a SEM image from which it is possible to observe that the synthesized structures have square cross section. This square cross section is expected for the In2O3 phase, known to be cubic, if the growth occurs in (h00) planes, and it is important to notice that the XRD result showed preferential orientation for planes of this family. Hereinafter, these square cross-section nanostructures will be called nanowires. The SEM images also illustrated that most nanowires have a metallic sphere at one extremity, thus suggesting that the growth mechanism of wires follows a selfcatalytic vapor−liquid−solid (VLS) process. Figure 2d presents the width distribution of wires. This figure clearly depicts a single modal distribution and indicates that most of wires have width between 51 to 100 nm. These findings are in good agreement with previous reports about ITO nanostructures.5 12948
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obtained in the XRD metallic phase. The reason for obtaining an Indium rich structure through a tin rich liquid sphere is explained elsewhere.19 It is important to mention that these tin rich metallic spheres must be in liquid phase at the wire formation temperature (450 °C), suggesting that the growth of nanowires follows a selfcatalytic VLS process. Figure 5 presents the two probe electrical response for the ITO nanowires. The electrical current is observed to vary
Figure 3 presents TEM images of nanowires, confirming they have a homogeneous width along their length. The high
Figure 3. (a) TEM and (b) HRTEM images of a typical ITO nanowire. Inset in panel b is the Fourier-transform of the HRTEM image.
resolution micrograph in Figure 3b and its Fourier-transformed image show that each wire is single-crystalline and grows in the In2O3 structure. Moreover, both the interplanar distance in the HRTEM image and the electron diffraction indexation confirm that wires grow in the (200) planes of the indium oxide phase. This result agrees with the XRD analysis which showed preferential orientation for this family of planes and also explains the origin of square cross section obtained. In order to ascertain that tin-doped In2O3 nanowires were synthesized, the energy dispersive X-ray spectroscopy (EDS) technique was used to examine the chemical composition of wires and spheres. EDS spectra of both the wire and the sphere are given in Figure 4. In this case, because the X-ray L lines of In and Sn atoms are superposed, the X-ray K lines in a TEM microscope were used. The EDS analysis showed that the wires are constituted of indium, tin and oxygen atoms with a In:Sn proportion of 89:11 at%, thus indicating this method was successful in obtaining ITO nanostructures. Meanwhile, the sphere chemical analysis showed the sphere is a Sn rich Sn−In alloy with a In:Sn proportion of 22:78 at%. The EDS results for the metallic spheres support those of the XRD, because the proportion between cations obtained by EDS is close to that
Figure 5. Two probe electrical response for ITO nanowires; the inset shows a FEG-SEM image of the approach used to perform the electrical measurements.
linearly with the applied voltage for both positive and negative values of bias and the curve passes through the origin, which is expected for an Ohmic material. The slope of the plot represents the resistance of the wire which in this case is about 102 Ω. After carrying out geometric parameters correction, a conductivity of about 1.1 × 106 S/m was obtained (resistivity of about 90 μΩ cm). This value is found to be far higher than for pure In2O3 nanowires, which is about 102 S/m,6 and similar to ones obtained in other works6,26 for ITO nanowires. This means that the doping of Sn atoms in the In2O3 matrix increases the conductivity of nanowires, and this behavior confers to ITO characteristics for special applications such as
Figure 4. EDS spectra: (a) ITO nanowire and (b) metallic sphere presents at the ITO wire extremity. 12949
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observation should be related to the spheres at nanowires extremities since ITO does not have any absorption at this wavelength,28,29 and this effect is more noticeable for composites with higher amounts of filler. Table 1 presents the two-probe electrical results for nanowires and composites. The values clearly show that the
transparent and conductive devices, which we will explored in detail in the next section. 3.2. Nanocomposites. Nanocomposites filled with ITO nanowires were produced using the quantities of filler already described in the Experimental Section. Figure 6 shows typical
Table 1. Two Probe Electrical Results for Nanowires and Composites films PMMA PMMA/ITO 1% PMMA/ITO 2% PMMA/ITO 5% PMMA/ITO 10% ITO nanowire
Figure 6. PMMA and ITO@PMMA photos of films with different filler amounts.
R (2 probe) 3.5 4.0 3.6 2.5 5.3 2.2
× × × × × ×
1010 Ω 1010 Ω 1010 Ω 104 Ω 103 Ω 102 Ω
photograph of composite films. It is easy to observe that films present a good transparency although, as expected, the pure PMMA film is found to be the most transparent one. As the amount of filler increases, the transparency is found to decrease, which is a characteristic behavior of composites. The transmittance of films was measured quantitatively using a UV−vis spectrometer and the resulting plot is given in Figure 7. The figure confirms that pure PMMA film transmits about
insertion of 1 and 2 wt % of filler does not alter the electrical resistance of pure PMMA. However, an abrupt fall in the resistance is observed for 5 wt % of filler thus indicating the start of percolation. On increasing the filler amount to 10 wt % the electrical resistance of the composite was found to be only 1 order of magnitude greater than the nanowire one, and at this point good percolation was reached for the composite. Figure 8
Figure 7. UV−vis spectra for pure PMMA and for nanocomposites; the inset shows a zoom in the visible spectrum.
Figure 8. Plots of the transmittance at 550 nm and the electrical resistance of composites as function of the amount of nanowires.
95% visible light and has a characteristic absorption at 275 nm, attributed to carbonyl groups in the polymer macromolecule.27 For a wavelength below 257 nm, the transmittance is observed to drop abruptly. This absorption is known as the optical window of the PMMA, where the light energy is high enough to excite electrons. Three changes are noticed in the UV−vis results when ITO nanowires are inserted in the composite. First, the absorption at 275 was found to increase as the amount of nanowires was increased. This was attributed to the interaction effect of the polymer with the wires; second, as already observed qualitatively in Figure 6, the transmittance of visible light was found to decrease as the amount of nanowires in the composite increases, with the minimum transmittance reaching about 55%, which is considered a good value for several practical applications. Finally, composite samples were found to present a small absorption band at 780 nm. This
summarizes these findings and also shows the UV−vis average transmittance at 550 nm. While the UV−vis data presents an almost linear decreasing of transmittance as the amount of nanowires increased, the electrical response has an abrupt drop at 5 wt % of filler. So, Figure 8 shows that the effect of nanowires insertion is more pronounced in the electrical response than in the optical one, and this is a good result in order to obtain transparent and conductive materials. For those samples that presented percolation, it was possible to measure the electrical resistance using the four probe approach which represents a more accurate method. Values obtained from this method are given in Table 2. From the four probe method results, it was observed that the composite with 10 wt % of filler has about the same electrical resistance of nanowires, confirming the occurrence of good percolation. The electrical resistance corrected by the geometric parameters 12950
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Table 2. Four Probe Electrical Results and Conductivity for Composites films
R (4 probe)
σ
PMMA/ITO 5% PMMA/ITO 10%
9.4 × 103 Ω 5.3 × 102 Ω
10.7 S/m 23.8 S/m
enabled calculating the conductivity of composites. While the wires were observed to show a metallic-like behavior and high conductivity, the composites present only reasonable conductivity. This observed difference is due to the method used to calculate the conductivity; for nanowires the two probe measurement was used, while for the composites, the fourprobe measurement was employed. To estimate the amount of filler theoretically necessary to achieve percolation, a Fortran routine was developed. For calculations, it was assumed a fixed size square box, with 1000 pixels in length, and we vary the size and the amount of filler. Figure 9 shows a plot of average amount of filler necessary to achieve percolation as function of wire length (wire width was set as 1 pixel). In the same way which other reports,30 this plot clearly shows that it is easier to achieve percolation using high aspect ratio materials (like 1D nanowires), and the average of percolation goes asymptotically to a limit value (20 wt %) as the wire size increases. This value is higher than the one we found experimentally to achieve percolation, and some errors source are the average value obtained and the fixed wire length used in the program: it is distributed in the obtained sample. So, using the wire length of 400 pixels (average percolation at 23 wt %) we simulate the insertion of different amount of wires in the matrix. Figure 9b shows a typical simulation with 20 wires (12.4 wt %) where is possible to see that percolation was achieved in the horizontal direction. This value is much closer to the experimental one to get percolation of filler in the polymer matrix. Figure 10 presents low magnification TEM images of the composites for different amount of filler. In all images the typical structure of PMMA can be clearly seen,31 while for composites with 1 and 2 wt %, some isolated nanowires can also be observed. This result is in good agreement with the electrical ones since isolated wires could not decrease the
Figure 10. TEM images for the nanocomposite films: (a) 1 wt %, (b) 2 wt %, (c) 5 wt % ,and (d) 10 wt % of ITO nanowires.
electrical resistance of the composites. Furthermore, it is important mentioning that although for 1 and 2 wt % filler systems only isolated wires were observed, these wires were more commonly found to occur in 2 wt % filler systems than in 1 wt % filler systems, as expected. For the composite with 5 wt % of ITO, the TEM image clearly illustrates that percolation was reached in some parts of the composite. This result also supports the electrical results, which indicate the percolation initiated for this amount of filler. For the sample with greater amount of ITO percolation was also observed as shown in Figure 10d. This kind of image is easily obtained during the TEM analysis thus explaining the reason for the smaller resistance obtained for this composite. On the basis of the exposed analyses and observations, it can be concluded that it is possible to produce transparent and conductive composites by inserting conductive ITO nanowires in a polymeric matrix. This result is important because transparent and conductive films have a wide range of potential
Figure 9. (a) Plot of average number of wires to achieve percolation as function of the wire length and (b) typical simulation of wires with horizontal percolation. 12951
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(15) Hoshi, Y; Kato, H; Funatsu, K. Thin Solid Films 2003, 445 (2), 245−250. (16) Manifacier, J. C. Thin Solid Films 1982, 90 (3), 297−308. (17) Ginley, D. S.; Bright, C. MRS Bull. 2000, 25 (8), 15−18. (18) Orlandi, M. O.; Aguiar, R; Lanfredi, A. J. C.; Longo, E; Varela, J. A.; Leite, E. R. Appl. Phys. A: Mater. Sci. Process. 2005, 80 (1), 23−25. (19) Orlandi, M. O., Lanfredi, A. J. C., Longo, E. In Microscopy: Science, Technology, Applications and Education, Microscopy Book Series Vol. 4; Méndes-Vilas, A, Dias, J., Ed.; Formatex: Spain, 2011; pp 1667−1673. (20) Balberg, I; Binenbaum, N. Phys. Rev. B 1983, 28 (7), 3799− 3812. (21) Balberg, I; Binenbaum, N.; Anderson, C. H. Phys. Rev. Lett. 1983, 51 (18), 1605−1608. (22) Enoki, H; Echigoya, J; Suto, H. J. Mater. Sci. 1991, 26 (15), 4110−4115. (23) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4 (5), 89. (24) Xia, Y; Yang, P; Sun, Y; Wu, Y; Mayers, B; Gates, B; Yin, Y; Kim, F; Yan, H. Adv. Mater. 2003, 15 (5), 353−389. (25) Mohammad, S. N. Nano Lett. 2008, 8 (5), 1532−1538. (26) Chiu, S. P.; Chung, H. F.; Lin, Y. H.; Kai, J. J.; Chen, F. R.; Lin, J. J. Nanotechnology 2009, 20, 105203. (27) Zidan, H. M.; Elnader, M. A. Phys. B 2005, 355 (1−4), 308− 317. (28) Alam, M. J.; Cameron, D. C. Surf. Coat. Technol. 2001, 142 (144), 776−780. (29) Lee, J. Y.; Yang, J. W.; Chae, J. H.; Park, J. H.; Choi, J. I.; Park, H. J.; Kim, D. Opt. Commun. 2009, 282 (12), 2362−2366. (30) Taya, M. Electronic Composites: Modeling, Characterization, Processing and MEMS Applications; Cambridge University Press: New York, 2008. (31) Yeh, J. M.; Chang, K. C.; Peng, C. W.; Lai, M. C.; Hung, C. B.; Hsu, S. C.; Hwang, S. S.; Lin, H. R. Mater. Chem. Phys. 2009, 115 (2− 3), 744−750.
applications, and practical tests can be performed with this system in order to examine its technological viability.
4. CONCLUSION ITO nanowires were successfully synthesized by a carbothermal reduction process with the coevaporation of oxides, and these wires were used as filler in PMMA polymer. The electrical response showed that ITO wires have a high conductivity (106 S/m), but incorporation of filler up to 2% does not change the composite electrical resistance, and percolation began at 5 wt % of filler. The optical response of the composite showed that the transmittance decreases as the amount of filler increases, but this effect is almost linear. It means that the filler insertion has a greater influence on the electrical than on the optical response of the composite. The TEM analysis supported the electrical results showing that the beginning of percolation occurs for 5 wt % of filler and increasing the filler amount to 10 wt % does not significantly alter the morphology of the composite. The results presented here show that the insertion of a low concentration of ITO nanowires in PMMA polymer enables obtaining transparent and conductive composites.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank the financial support of Brazilian funding agencies FAPESP and CNPq. TEM facilities were by LME-LNLS and IQ-UNESP. SEM facilities were provided by LMA-IQ-UNESP.
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REFERENCES
(1) Burda, C; Chen, X; Radha, N; El-Sayed, A. Chem. Rev. 2005, 105 (4), 1025−1102. (2) Johnson, M. C.; Aloni, S; Mccready, D. E.; Bourret-Courchesne, E. D. Cryst. Growth. Des. 2006, 6 (8), 1936−1946. (3) Yan, Y; Zhou, L; Zhang, J; Zeng, H; Zhang, Y; Zhang, L. J. Phys. Chem. C 2008, 112 (28), 10412−10417. (4) Peng, X. S.; Wang, Y. W.; Zhang, J; Wang, X. F.; Zhao, L. X.; Meng, G. W.; Zhang, L. D. Appl. Phys. A: Mater. Sci. Process. 2002, 74 (3), 437−439. (5) Chen, Y. Q.; Jiang, J; Wang, B; Hou, J. G. J. Phys. D Appl. Phys. 2004, 37 (23), 3319−3322. (6) Wan, Q; Dattoli, E. N.; Fung, W. Y.; Guo, W; Chen, Y; Pan, X; Lu, W. Nano Lett. 2006, 6 (12), 2909−2915. (7) Yan, Y; Zhou, L. Appl. Phys. A: Mater. Sci. Process. 2008, 92 (2), 401−405. (8) Carotenuto, G; Valente, M; Sciume, G; Valente, T; Pepe, G; Ruotolo, A; Nicolais, L. J. Mater. Sci. 2006, 41 (17), 5587−5592. (9) Winey, K. I.; Vaia, R. A. MRS Bull. 2007, 32 (4), 314−321. (10) Tans, S. J.; Devoret, M. H.; Dai, H; Thess, A; Smalley, R. E.; Geerligs, L. J.; Dekker, C. Nature 1997, 386 (6624), 474−477. (11) Mintmire, J. W.; Dunlap, B. I.; White, C. T. Phys. Rev. Lett. 1992, 68 (5), 631−634. (12) Nozaki, T; Karatsu, T; Ohnishi, K; Okazaki, K. Carbon N. Y. 2010, 48 (1), 232−238. (13) Kim, D. O.; Lee, M. H.; Lee, J. H.; Lee, T. W.; Kim, K. J.; Lee, Y. K.; Kim, T; Choi, H. R.; Koo, J. C.; Nam, J. D. Org. Electron. 2008, 9 (1), 1−13. (14) Shanthi, E; Dutta, V; Banerjee, A; Chopra, K. L. J. Appl. Phys. 1980, 51 (12), 6243−6251. 12952
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