Molecular Engineering to Minimize the Sheet Resistance Increase of

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J. Phys. Chem. C 2009, 113, 16915–16920

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Molecular Engineering to Minimize the Sheet Resistance Increase of Single-Walled Carbon Nanotube/Binder Hybrid Conductive Thin Films Joong Tark Han, Sun Young Kim, Hee Jin Jeong, Seung Yol Jeong, and Geon-Woong Lee* Nano Carbon Materials Research Group, Korea Electrotechnology Research Institute, Changwon, 641-120 Korea ReceiVed: May 6, 2009; ReVised Manuscript ReceiVed: July 30, 2009

We propose a strategy to enhance the optoelectrical properties of the single walled carbon nanotube (SWNT)/ binder hybrid thin films through control of the intermolecular interaction between the nanotube and the binder material. Aiming at this goal, silane sol having various functional groups (hydroxyl, methyl, vinyl, phenyl) were used as model binder materials. As the degree of the interfacial interaction increases, we can expect that the sheet resistances of the films will be increased by the enhanced junction resistance. However, the SWNT/ phenyl-functionalized silane (PTMS) hybrid thin films show a significant enhancement in the optoelectrical properties compared with that of other films containing methyl or vinyl groups, even though the PTMS forms the strongest π-π interaction of them with the nanotubes and fully covered the top surface of the film. We suggest that this phenomenon can be explained by bridging of the nanotube bundles with PTMS by the strong π-π interaction, which was demonstrated by the large D/G ratio, broad fwhm, and enhanced Breit-Wigner-Fano line of G band in the Raman spectra. 1. Introduction Carbon nanotube (CNT)-based transparent conductive coating technology has potential applications in electrostatic dissipation (ESD), electromagnetic interference (EMI) shielding, and transparent film heating as well as in the development of alternative electrode materials for touch panels and e-papers in display technologies, solar cells, flexible electronic devices, automobiles, and optical devices.1-10 Research into methods for the fabrication of single-walled carbon nanotube (SWNT)-based transparent conductive films has mainly focused on the effects of the material properties of the nanotubes, including purity, diameter, chirality, defects, metallicity, and doping on the resulting electronic structures of the SWNTs.10-13 Moreover, the organic materials such as conjugated polymer, block copolymer, polyelectrolyte, pyrene, DNA, and so forth were used considering the dispersion and stabilization of the CNTs in different solvent media and polymer matrix.14 However, if the mechanical and interfacial properties, for example, barrier properties, of transparent and highly conductive thin films based on SWNTs are to be improved, binder materials such as crosslinkable polymers, silane compounds, or titanium compounds, and so forth should be added to the SWNT solution.15 Apart from the electronic properties of the nanotubes, the intermolecular interactions between the nanotubes and the binder material is therefore another important factor in such applications because they can affect the degree of dispersion of the SWNTs and tunneling through the insulating layer around the nanotubes. However, there have been few studies of the effects of the intermolecular interactions between SWNT surfaces and binder materials on the optoelectrical properties of SWNT/binder hybrid thin films. Herein, we focus on controlling the optoelectrical properties of the SWNT/binder hybrid thin films by manipulating the intermolecular interactions at the interface between the nanotubes and the functional groups of the binder materials with * To whom correspondence should be addressed. E-mail: gwleephd@ keri.re.kr. Fax: +82-55-280-1590. Tel: +82-55-280-1677.

the aim of minimizing the contact resistance at the nanotube junction. We show in this paper that there is a clear correlation between the optoelectrical properties of the SWNT/binder thin films and the intermolecular interaction between the nanotube and the binder material. To achieve this goal, silane precursors with various functional groups (hydroxyl, methyl, vinyl, and phenyl) were tested as model binder materials. The use of sol-gel chemistry to modify the properties of the gel with functionalized silane precursors has significant advantages.16 The effect of the intermolecular interaction at the interface between the nanotubes and model binder materials on optoelectrical properties of the films was systematically investigated by analyzing the microscope images and Raman spectra. 2. Experimental Methods The SWNTs produced by arc-discharge method (P3) was purchased from Carbon Solution Inc. and used as received, which are functionalized with carboxylic groups by nitric acid treatment. P3 was dispersed in ethanol for 1 h in an ultrasonic bath at a definite concentration, following the treatment with a high pressure homogenizer and bath sonicator. The solubility of P3 in ethanol was determined by UV-vis spectroscopy (CARY 5000). The silane sol solution was prepared as follows. Typically, a TEOS sol was prepared by mixing 5 g of tetraethoxysilane (from Aldrich), 2 g of water, 50 mL ethanol, and 100 µL of 12.1 M HCl, and a continuous sol-gel reaction of it at 60 °C. Then, in order to remove water from the prepared sol, the solvent was exchanged with dimethylformamide in a rotary evaporator under vacuum. Other silane sols were prepared with the same method. The silane sol solution was added to the CNT solution at a definite concentration. The fabrication of CNT/silane hybrid film was achieved by automatic spray coater (Fujimori Co., NVD200) with a nozzle of 1.2 mm diameter at room temperature. The prepared CNT/silane hybrid thin films were then heated under vacuum for 3 h (at 100 °C) in order to remove the remaining chemicals and cure the silane binder. The pristine SWNT films were also treated at the same condition to keep the same condition. Energy dispersive X-ray

10.1021/jp9042073 CCC: $40.75  2009 American Chemical Society Published on Web 09/08/2009

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Figure 1. A schematic diagram of the intermolecular interactions between SWNTs and silane.

analysis (EDAX) was performed to confirm the existence of the silane molecules in the film. The corresponding images of the resulting films were obtained by scanning electron microscopy (SEM, HITACHI S4800) and atomic force microscopy (Digital Instruments Multimodes). The sheet resistance was measured by four probe tester (Loresta, MCP-T610). The Raman spectra were measured to characterize the structure and the dispersion state of nanotubes in the film at room temperature using a high resolution Raman spectrometer (LabRAM HR800 UV) under excitation at wavelengths λ of 633 and 514 nm. The Raman spectra results were averaged by measuring from five different points of each sample to minimize the data error. 3. Results and Discussion We can easily expect that the tested silane materials have various intermolecular interactions with the nanotube surfaces with strengths in the order of tetraethoxysilane (TEOS), methyltrimethoxysilane (MTMS), vinyltrimethoxysilane (VTMS), and phenyltrimethoxysilane (PTMS), as shown in Figure 1. The unpaired electrons of the hydroxyl groups of the TEOS sol can just polarize the negative charge on the nanotube surface and do not form the favorable interaction.17 Hydrophobic interactions can arise between the methyl groups in the MTMS sol and the nanotube surface.18 The vinyl groups in VTMS and the phenyl groups in PTMS can interact with SWNT surfaces via π-π interactions.19,20 It seems likely that PTMS would provide the most favorable surface for the CNTs because of the strongest π-π interaction. This interaction is expected to affect the intertube or interbundle distance in the SWNT/binder thin films, which can be directly correlated with the electrical properties of the SWNT/binder film because the sheet resistance of the film results from the intrinsic resistance of the SWNTs and the contact resistance at the junctions between the nanotubes. We expect that when the binder material is intercalated between the SWNTs or the SWNT bundles, the junction resistance will increase. From this point of view, it is expected that the sheet resistance of SWNT/PTMS will be the highest because the phenyl rings in PTMS can interact strongly with the nanotubes via π-π interactions, so the nanotubes will be well dispersed in the film. Moreover, the transmittance and sheet resistance of the spray-coated CNT/binder films are expected to depend on the amount of deposited CNTs and binder material, and on the ratio of CNT and binder. From a plot of sheet resistance as a function of binder content, it is evident that a critical binder content (Xc) exists above which the sheet resistance increases dramatically.15 To determine Xc, the effects of varying the binder content in the SWNT/silane hybrid films on their optoelectrical properties were investigated. The strength of the interaction between MTMS and the nanotubes is moderate, so it was used as the binder material in this initial experiment. To prepare the CNT/

Figure 2. Transmittance vs Rs for (a) SWNT/MTMS hybrid films containing various amounts of MTMS binder, and (b) for SWNT/silane hybrid films with 50 wt % of binder.

binder dispersion solution, SWNTs functionalized with carboxyl groups (P3 nanotubes from Carbon Solution Inc.) were dispersed in ethanol by using sonication and a high pressure homogenizer. The SWNT solution has a maximum concentration of 350 mg/ L, as indicated by UV-vis absorption spectroscopy (Supporting Information Figure S1); this solution was diluted to a concentration of 200 mg/L to ensure its long-term stability after mixing with the silane sol. The silane sol was prepared in ethanol, water, and HCl at 60 °C. A stable P3/MTMS sol solution was then prepared by mixing the CNT solution directly with the silane sol and sonicating the mixture for 1 h. We used a simple spray coating in order to fabricate the SWNT/binder thin films with a transmittance at 550 nm of 50-97%. P3/MTMS solutions with various silane contents (0, 25, 50, 75 wt %) were deposited on piranha-treated glass substrates. The existence of the silane

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Figure 3. SEM images of P3/MTMS hybrid films containing various amounts of CNTs: (a) 100, (b) 75, (c) 50, and (d) 25 wt %.

molecules in the film prepared on PET substrate was confirmed by EDAX analysis as shown in Supporting Information Figure S2. Figure 2A shows a plot of the transmittance versus sheet resistance of the P3/MTMS films with various binder contents. Both parameters were found to decrease with increases in the amount of spray-coated CNT/binder solution. Note that the sheet resistance increases dramatically at a certain binder content. In this system, the critical binder content, Xc, is approximately 50 wt %. Figure 3 shows typical scanning electron microscopy images of P3/MTMS films fabricated using a spray-coater on piranha-treated substrates. It was found that above the critical binder content the CNTs were fully covered with MTMS sol, which increases the contact resistance between the CNT network and probe and decreases the tunneling between the CNTs through the insulating binder layer between CNT bundles. Thereafter, the CNT/silane solutions were prepared with a silane content of 50 wt % and were deposited with the spraycoater on the substrates. The coating solutions were prepared with the same method as used for P3/MTMS. Figure 2B shows the sheet resistance-transmittance curve for the P3/silane sol hybrid films containing the various silane binders for a wide range of film thicknesses. In the case of P3/TEOS, it was expected that the sheet resistance of the films would not be much higher than that of pristine P3 films because the strength of the van der Waals interaction at an intermolecular nanotube junction is larger than that of the interaction between a nanotube and the oxygen or hydroxyl groups of a TEOS sol. As expected, it was found that the sheet resistances of the films were gradually increased in the order of P3/TEOS, P3/MTMS, and P3/VTMS films. However, it is noticeable that the sheet resistance of P3/ PTMS films is lower than that of P3/MTMS films. The lowest sheet resistance of P3/TEOS films comparing with other SWNT/ binder thin films can be explained by the exposed CNT surface and the closely connected nanotube bundles as shown Figures 4A and 5A, while the SWNTs were fully covered with VTMS in SWNT/VTMS films having the highest sheet resistance (Figures 4C and 5C). However, note that the sheet resistance of P3/PTMS films is much lower than that of P3/VTMS films, even though the CNTs appear to be well distributed and covered with the binder material (Figures 4D and 5D). It has previously been reported that aromatic molecules such as the phenylterminated silane used here interact and bind selectively to metallic SWNTs because their polarizability is larger than that of the semiconducting nanotubes.20 Therefore, we suggest that

J. Phys. Chem. C, Vol. 113, No. 39, 2009 16917 Rs of P3/PTMS is lower than that of P3/VTMS because of the interconnection between the nanotubes or nanotube bundles with phenyl functionalized silane sol by strong π-π interactions, which can cause the decrease of the junction contact resistance. To provide the evidence of the bridging of the nanotubes with PTMS sol, Raman spectroscopy was used at two laser excitation wavelengths (1.96 and 2.41 eV). In a strongly aggregated state, for example, CNT network films without binder materials, van der Waals interactions would be essentially between bundles, whereas in the CNT/binder thin film, interactions between bundles and functional groups of the binder materials can influence the Raman features. Introducing dopants into nanotubes is one approach to enhance the conductivity of the CNT network films by tailoring the electronic properties of SWNT films. The interaction of the SWNTs with electron donors or electron acceptors can affect the in-plane Raman-active vibrations (G band) and the radial breathing mode (RBM) bands.21 In our system, it was observed that the position of RBM peak at ∼199 cm-1 is not affected by the binder materials because the bundle size was not changed after addition of the binder materials. Moreover, the G+ band is slightly downshifted by the addition of silane binder materials (Figures 6A and Supporting Information S3), which indicate that the functional groups act as weak electron-donating groups (CH3, vinyl, phenyl) and the sheet resistances of the SWNT/silane films are not mainly affected by the charge transfer effect.22 Therefore, we suggest that the dispersion state or distance between the nanotube bundles in the thin films can affect dominantly on the conductivity of the CNT network films. The line-width and D/G ratio for the G+ band can be an indicative of the degree of aggregation or bundling of the nanotubes. The enhanced resonance processes in the Raman scattering G band might be due to the exfoliation of the nanotubes, resulting in decrease of D/G ratio of G band.23 The D/G ratio of the pristine P3 film at 1.96 eV was found to be 0.224, which is altered by the addition of the silane binders (Figure 6A). The D/G ratio is 0.136 in the case of VTMS, in which the SWNTs are well dispersed. However, note that the D/G ratio of the P3/PTMS film (0.167) is larger than those of P3/MTMS (0.148) and P3/VTMS (0.136). Moreover, the relative intensities of RBM bands are significantly changed after hybridization with the silane binders. RBM peaks for laser excitation at 1.96 eV were depressed depending on the tube diameter, which means that the functionalized silane binders interact with the nanotubes favorably (Figure 7). Figure 6D reports the relationship between the ratios ID/IRBM and ID/IG for laser excitation at 2.41 eV, in which a rather good linear relationship was obtained. This result indicate that the aggregation state or the interbundle distance of bundles in the thin film network affected the in-plane Raman-active vibrations (G-band) and radial breathing mode (RBM) in nanotubes, assuming the disorder defects are constants after hybridization. From analyzing the full-width half-maximum (fwhm) of the G+ band, the dispersion state of the CNTs or CNT bundles can also be estimated. As shown in Figure 6A, the fwhm of the G+ band of the films exhibits a similar trend with D/G ratio. The trend in the sheet resistance for the various silane binders is the inverse of that in the D/G ratio and the fwhm. These results therefore provide strong evidence that notably, the interbundle distance in the SWNT/PTMS film is not much different from that in the pristine and SWNT/TEOS films. The SWNT bundles are presumably bridged by the strong interactions between the CNTs and the phenyl groups of PTMS, which contribute to the

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Figure 4. AFM images of P3/silane hybrid films: (a) P3/TEOS, (b) P3/MTMS, (c) P3/VTMS, and (d) P3/PTMS films.

Figure 5. SEM images of P3/silane hybrid films: (a) P3/TEOS, (b) P3/MTMS, (c) P3/VTMS, and (d) P3/PTMS films.

enhanced conductivity of the SWNT networks even though the CNTs are fully covered with insulating materials from the top view image.

Breit-Wigner-Fano (BWF) line on the lower energy side of the G-band can also give an evidence of the intertube connection because as reported, the asymmetric BWF line shape

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Figure 7. Radial breathing mode (RBM) bands of P3 and P3/silane hybrid films excited at 1.96 eV.

SWNTs can be fitted with one Lorentzian at ∼1580 cm-1 and a BWF feature in the region ∼1530-1560 cm-1. As shown in Supporting Information Figure S4, the G-band spectra for the pristine P3 and P3/silane films were satisfactorily fitted with six components with a BWF line shape with 1/q and Γ values (Supporting Information Table S1). It was also found that the proportion of metallic components that appears at an excitation of 1.96 eV increases to 58% in P3/VTMS (Figure 6B). The sheet resistance is strongly correlated with the metallic portion of the G-band.26 Similar trends were observed for the other P3/ silane films. 4. Conclusions

Figure 6. (a) The correlation of the Raman spectra at 1.96 eV (D/G ratio, fwhm of G+ band) with the Rs (with an optical transmittance of 85%) of pristine SWNT and SWNT/silane films. (b) Metallic components extracted from the G-band and G band shift at 1.96 eV. (c) shows the possible interactions between SWNTs and PTMS. (d) Correlation between the ratios ID/IRBM vs ID/IG at 2.41 eV.

and intensity are sensitive to interactions among nanotubes in bundles, or between nanotubes and surrounding molecules.24 At an excitation energy of 2.41 eV, the semiconducting SWNTs (E33S) were excited exclusively, and the metallic SWNTs (E11M) were excited in the pristine SWNTs at an excitation energy of 1.96 eV, as demonstrated by the presence of a broad BWF line on the lower energy side of the G-band in Supporting Information Figure S4.25 Note that the intensity of the BWF feature of P3/TEOS is similar to that of pristine P3, whereas those of the films of P3 with the other silane binders are lower. This result seems to support the assertion that the BWF intensity is strongly dependent on the degree of SWNT aggregation. The Raman spectrum at the excitation energy of 1.96 eV is a convolution of several peaks due to both semiconducting and metallic SWNTs. For semiconducting tubes, both G+ and Gare of Lorentzian shape. Metallic tubes exhibit broad BWF line shape for G-, whereas G+ exhibit Lorentzian shapes. It has been shown that s-SWNTs exhibit four peaks in this region, at ∼1607, 1592, 1569, and 1553 cm-1, whereas the spectrum for m-

We conclude that the intermolecular interaction at the interface between the nanotube and the binder has a substantial effect on the opto-electrical properties of the CNT-based conductive thin films, and that this effect can be used to optimize the formulation of the SWNT and binder coating solution. In order to clarify that effect, we controlled the intermolecular interactions at the interface between the nanotube and the insulating binder materials by using various functionalized silane sol, and compared the results obtained for these systems with those for the SWNT film without the binder. We found that by increasing the degree of the interfacial interaction from TEOS to VTMS, as expected, the sheet resistances of the thin films were increased by the enhanced junction resistance. However, the opto-electrical properties of the SWNT/PTMS thin films were better than that of the SWNT/MTMS having a smaller interaction force between the nanotube and binder. This phenomenon can be explained by bridging of the nanotubes with PTMS sol by the strong π-π interaction, which was demonstrated by the large D/G ratio, broad fwhm, and enhanced BWF line of G+ band in the Raman spectrum. Such a precise control of the optoelectrical properties of SWNT/binder films can be useful to fabricate the high performance conductive thin films, with ramifications for understanding the fundamental intermolecular interaction in carbon materials science. Acknowledgment. This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy and KERI, Republic of Korea. We also thank Dr. H. S. Lee at POSTECH for the experimental assistance. Supporting Information Available: UV-vis absorption spectra of the SWNT/silane solutions and EDAX and Raman

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