Patterning of Single-Walled Carbon Nanotube Films on Flexible

Sep 8, 2009 - Department of Applied Chemistry, Hanyang University, Ansan 425-791, South Korea. Received June 13, 2009. Revised Manuscript Received ...
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Patterning of Single-Walled Carbon Nanotube Films on Flexible, Transparent Plastic Substrates Kwi Nam Han, Cheng Ai Li, Minh-Phuong Ngoc Bui, and Gi Hun Seong* Department of Applied Chemistry, Hanyang University, Ansan 425-791, South Korea Received June 13, 2009. Revised Manuscript Received August 5, 2009 We report a simple patterning method for single-walled carbon nanotubes (SWCNTs) films on flexible, transparent poly(ethylene terephthalate) using an O2-plasma technique in a capacitively coupled plasma (CCP) system. The homogeneous SWCNT films in a large area were fabricated by the vacuum filtration method. The plasma patterning process of SWCNT films includes conventional photolithography and subsequent O2-plasma treatment. During the plasma treatment, SWCNTs underneath the patterned photoresist polymer are protected from etching and damage by O2-plasma while the exposed SWCNTs are destroyed. The morphological changes and the effect of plasma treatment on the chemical properties of SWCNT films were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The physical properties of SWCNT films such as transparency and conductivity were systematically characterized under various plasma conditions. In an electrochemiluminescence reaction, the SWCNT films patterned by the CCP system-based O2-plasma treatment could be used as flexible and transparent electrodes.

1. Introduction Carbon nanotubes (CNTs) have become increasingly relevant to a variety of engineering applications due to their excellent electrical, mechanical, thermal, and optical properties.1-4 The outstanding properties of CNTs have established CNT films as a viable new class of optically transparent and electrically conductive materials5,6 suitable for applications such as field emission displays,7-9 sensors,10-12 thin film transistors,13,14 and transparent electrodes for optoelectronic devices.15,16 The successful implementation of CNT films for various applications requires high-quality and high-resolution patterning at defined positions, with large-scale control of location and orientation. Many attempts have been made to pattern CNT films. These efforts *Corresponding author: Tel þ82-31-400-5499; Fax þ82-31-407-3863; e-mail [email protected].

(1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787–792. (2) Geng, H. Z.; Kim, K. K.; So, K. P.; Lee, Y. S.; Chang, Y.; Lee, Y. H. J. Am. Chem. Soc. 2007, 129, 7758–7759. (3) Hu, L.; Hecht, D. S.; Gruner, G. Nano Lett. 2004, 4, 2513–2517. (4) Fujiwara, A.; Matsuoka, Y.; Suematsu, H.; Ogawa, N.; Miyano, K.; Kataura, H.; Maniwa, Y.; Suzuki, S.; Achiba, Y. Carbon 2004, 42, 919–922. (5) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, 1273–1276. (6) Zhang, M.; Fang, S.; Zakhidov, A. A.; Lee, S. B.; Aliev, A. E.; Williams, C. D.; Atkinson, K. R.; Baughman, R. H. Science 2005, 309, 1215–1219. (7) Rinzler, A. G.; Hafner, J. H.; Nikolaev, P.; Nordlander, P.; Colbert, D. T.; Smalley, R. E.; Lou, L.; Kim, S. G.; Tomanek, D. Science 1995, 269, 1550–1553. (8) Yoon, B. J.; Hong, E. H.; Jee, S. E.; Yoon, D. M.; Shim, D. S.; Son, G. Y.; Lee, Y. J.; Lee, K. H.; Kim, H. S.; Park, C. G. J. Am. Chem. Soc. 2005, 127, 8234– 8235. (9) Sveningsson, M.; Morjan, R. E.; Nerushev, O.; Campbell, E. E. B. Carbon 2004, 42, 1165–1168. (10) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622–625. (11) Patolsky, F.; Weizmann, Y.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 2113–2117. (12) Snow, E. S.; Perkins, F. K.; Houser, E. J.; Badescu, S. C.; Reinecke, T. L. Science 2005, 307, 1942–1945. (13) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Nature 2003, 424, 654–657. (14) Durkop, T.; Getty, S. A.; Cobas, E.; Fuhrer, M. S. Nano Lett. 2004, 4, 35– 39. (15) Pasquier, A. D.; Unalan, H. E.; Kanwal, A.; Miller, S.; Chhowalla, M. Appl. Phys. Lett. 2005, 87, 203511. (16) Guldi, D. M.; Rahman, G. M. A.; Prato, M.; Jux, N.; Qin, S.; Ford, W. Angew. Chem., Int. Ed. 2005, 44, 2015–2018.

598 DOI: 10.1021/la9021273

include chemical vapor deposition growth patterning,17,18 inkjet printing,19,20 electrophoresis deposition,21,22 chemically anchored deposition,23 and laser irradiation.24 However, these methods require complicated fabrication processes, such as the use of electrical field gradients, substrate modification, and tedious thickness control processes. In addition, there may be limitations in the resolution and reproducibility of the resulting CNT films. The plasma etching technique is one of the CNT patterning techniques, which has been used widely for the surface activation of various materials25 as well as for removal of carbon-based organic material from substrate surfaces.26 This method has several advantages over other patterning techniques: (1) it can be scaled up to produce large quantities for commercial use, (2) it results in high feature resolutions and sharp pattern edges, (3) the processing time is short, and (4) it offers good reproducibility and reliability. Recently, several research groups have reported the use of plasma etching techniques incorporating inductively coupled plasma (ICP) systems or reactive ion etching (RIE) systems to etch CNTs.27-29 ICP systems produce high ion densities that can (17) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Science 1996, 274, 1701–1703. (18) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105–1107. (19) Song, J. W.; Kim, J.; Yoon, Y. H.; Choi, B. S.; Kim, J. H.; Han, C. S. Nanotechnology 2008, 19, 095702. (20) Kordas, K.; Mustonen, T.; Toth, G.; Jantunen, H.; Lajunen, M.; Soldano, C.; Talapatra, S.; Kar, S.; Vajtai, R.; Ajayan, P. M. Small 2006, 2, 1021–1025. (21) Boccaccini, A. R.; Cho, J.; Roether, J. A.; Thomas, B. J. C.; Jane Minay, E.; Shaffer, M. S. P. Carbon 2006, 44, 3149–3160. (22) Gao, B.; Yue, G. Z.; Qiu, Q.; Cheng, Y.; Shimoda, H.; Fleming, L.; Zhou, O. Adv. Mater. 2001, 13, 1770–1773. (23) Jung, M. S.; Jung, S. O.; Jung, D. H.; Ko, Y. K.; Jin, Y. W.; Kim, J.; Jung, H. T. J. Phys. Chem. B 2005, 109, 10584–10589. (24) Terrones, M.; Grobert, N.; Olivares, J.; Zhang, J. P.; Terrones, H.; Kordatos, K.; Hsu, W. K.; Hare, J. P.; Townsend, P. D.; Prassides, K.; Cheetham, A. K.; Kroto, H. W.; Walton, D. R. M. Nature 1997, 388, 52–55. (25) Holl€ander, A.; Thome, J.; Keusgen, M.; Degener, I.; Klein, W. Appl. Surf. Sci. 2004, 235, 145–150. (26) Egitto, F. D. Pure Appl. Chem. 1990, 62, 1699–1708. (27) Lu, S.; Panchapakesan, B. Appl. Phys. Lett. 2006, 88, 253107. (28) Lustig, S. R.; Boyes, E. D.; French, R. H.; Gierke, T. D.; Harmer, M. A.; Hietpas, P. B.; Jagota, A.; McLean, R. S.; Mitchell, G. P.; Onoa, G. B.; Sams, K. D. Nano Lett. 2003, 3, 1007–1012. (29) Behnam, A.; Choi, Y.; Noriega, L.; Wu, Z.; Kravchenko, I.; Rinzler, A. G.; Ural, A. J. Vac. Sci. Technol. B 2007, 25, 348–354.

Published on Web 09/08/2009

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Figure 1. Schematic of the patterning of SWCNT films by a CCP system.

yield high ion energies for fast etch rates and good etch selectivity. However, when polymer is used as a substrate, ICP systems are unsuitable because the high temperatures resulting from plasma generation in ICP systems can cause polymer substrates such as poly(ethylene terephthalate) (PET) and poly(methyl methacrylate) (PMMA) to melt or deform easily. In addition, ICP systems are not appropriate for etching CNTs on polymer substrates because the plasma with high ion energies can etch polymer substrates.29 RIE systems also present some disadvantages for etching CNTs on polymer substrates. The reactive ions accelerated by radiofrequency bias in RIE systems are highly effective at physically etching PET substrate, and PET substrates exposed to RIE plasma demonstrated significant adverse changes in topology and increased surface roughness.30,31 In contrast to the ICP and RIE systems, capacitively coupled plasma (CCP) systems are particularly suitable for use with polymer substrates because their low ion energies cannot etch polymer surfaces. In addition, the homogeneity of the patterns produced over large areas using CCP is greater than those produced by ICP systems. This homogeneity is due to the uniformity of plasma density resulting from CCP system processes. Herein, we report a simple patterning method for the production of SWCNT films on flexible, transparent plastic substrates such as PET using an O2-plasma technique in a CCP system. The physical and chemical properties of SWCNT films (transparency, conductivity, and chemical changes) were systematically characterized under various plasma conditions for applications involving large-area flexible displays, flexible solar panels, and touch screens. An electrochemiluminescence (ECL) reaction was carried out on the patterned SWCNT films to confirm that it could function for use in electrodes.

2. Experimental Section The homogeneous SWCNT films were fabricated by a vacuum filtration method. Briefly, the SWCNT mixture (Topnanosys co., (30) Powell, H. M.; Lannutti, J. J. Langmuir 2003, 19, 9071–9078. (31) Beake, B. D.; Ling, J. S. G.; Leggett, G. J. J. Mater. Chem. 1998, 8, 1735– 1742.

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South Korea) was sonicated for 1 h and then centrifuged at 14 000 rpm for 10 min. The presuspended solution was further diluted by a factor of 50 with deionized water and filtered through an anodic aluminum oxide membrane of 0.2 μm pore size. The alumina membrane under the SWCNT thin layer was easily removed in 3 M NaOH solution, and the SWCNT thin layer was then transferred to a flexible PET film directly after adjusting the solution to neutral pH using deionized water. SWCNT films were patterned using a standard photolithography method and subsequent O2-plasma treatment in a CCP system (Figure 1). A positive photoresist polymer (AZ4620) was spin-coated onto the produced SWCNT films at 1500 rpm for 1 min, followed by exposure to UV light (∼365 nm) through a designed mask and development with AZ400K solution. The SWCNTs film prepatterned with photoresist polymer was placed between 14  14 in. radio-frequency plate electrodes. O2-plasma treatments in the CCP system were performed at 70 mTorr chamber pressure, power in the range of 100-500 W, and a substrate reflective frequency of 13.56 MHz for O2-plasma treatment times between 1 and 5 min. After treatments, the remaining photoresist polymer on the SWCNT films was removed with an ethanol solution. The morphological changes and the effect of plasma treatment on the chemical properties of SWCNT films were investigated by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The electrical and optical properties of the patterned SWCNT films were characterized by a four-probe setup and UV-vis spectroscopy, respectively. To demonstrate the ability of the patterned SWCNT films to function as electrodes, a three-electrode ECL reaction was carried out using a CHI600C electrochemical analyzer (CH Instruments, Inc.). The patterned SWCNT film, Pt wire, and Ag/AgCl electrode were used as a working electrode, a counter electrode, and a reference electrode, respectively. A mixture consisting of 1 mM Ru(bpy)32þ and 5 mM tripropylamine (TPA) dissolved in phosphate buffered saline (pH 7.0) was used to fill the reaction chamber. A chronoamperometry measurement was performed to maintain a steady 1.0 V potential bias between working and counter electrodes. The ECL signal was collected beneath the SWNCT films by fluorescent microscopy. DOI: 10.1021/la9021273

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Figure 3. SEM image of SWCNT film treated by O2-plasma at a power of 400 W for (a) 1, (b) 3, and (c) 5 min. (d) SEM image of pristine SWCNT films. (e) SEM image of SWCNT film that carbon nanotubes are almost removed after O2-plasma treatment at a power of 500 W for 5 min. The scale bar represents 100 nm.

Figure 2. Optical images of the patterned SWCNT films. SWCNTs deposited on flexible PET substrate were patterned with O2plasma under 500 W power and 70 mTorr chamber pressure for 5 min. The scale bar represents 300 μm.

3. Results and Discussion To fabricate the homogeneous SWCNT films over a large area, the vacuum filtration method was adopted.5 The thickness of SWCNT films was controlled as ∼100 nm for being most suitable resistivity and transparency. The averaged resistivity and transparency of the fabricated flexible SWCNT films were 400 ohm/sq and 80%, respectively. SWCNT films showed high flexibility with negligible change in resistivity at hard bending. Other methods of CNT film production including drop-drying from solvent, airbrushing, and Langmuir-Blodgett deposition present severe limitations in terms of the film quality or production. However, the vacuum filtration method guarantees the homogeneity of the CNT films by the process itself.5 In addition, the film thickness can be controlled with nanoscale precision by adjusting the CNTs concentration and volume of the suspension. The plasma patterning process of SWCNT film includes photolithography and subsequent O2-plasma treatment in a CCP system. The excellent compatibility of SWCNTs with photolithography allows precise and high-resolution features to be defined.27 During the plasma treatment, SWCNTs underneath the patterned photoresist polymer are protected from etching and damage by O2-plasma while the exposed SWCNTs are destroyed. The optical microscope images of the patterned SWCNT films on the PET flexible substrate are shown in Figure 2. The area patterned by O2-plasma treatment is more transparent than the area protected with the 600 DOI: 10.1021/la9021273

photoresist polymer. The clear patterns demonstrated the effectiveness of SWCNT patterning by this method. For a more detailed characterization, the morphological changes of the SWCNTs were investigated by SEM (Figure 3). In a pristine SWCNT image, the ordered SWCNTs are touching each other. However, as the O2-plasma treatment time is increased, the morphology of the SWCNTs changes substantially into amorphous carbon. SWCNTs are known to be chemically resistant and highly stabile. However, structural defects in the SWCNTs can form during purification and/or ultrasonic dispersion steps. These defects can be attacked by the O2-plasma, resulting in the destruction of whole SWCNTs. The destruction starts gradually from these defects and results in the formation of volatile CO2, CO, and H2O.27 To determine the effect of plasma treatment on the chemical properties of SWCNT film, we recorded XPS spectra which are presented in Figure 4. Carbon atoms disappeared after O2-plasma treatment compared with pristine SWCNT film. These spectra reveal that O2 radicals generated from the plasma cause damage to the SWCNT structure, resulting in the release of volatile species such as CO2, CO, and H2O from the damaged SWCNTs. To define the subtle chemical change that occurs in SWCNTs, the C 1s spectrum of O2-plasma-treated SWCNT film was compared with that of pristine SWCNT film (Figure S1 in the Supporting Information). The content of sp2 carbon decreased from 62.6% to 40.1% after plasma treatment, while the content of sp3 carbon increased from 19.1% to 26.4%. This observation suggests that O2 radicals broke π-bonding among the SWCNTs, resulting in the conversion of SWCNTs to amorphous carbon, as shown in the SEM images. Changes in the physical properties of SWCNT films according to various plasma treatment powers and time were characterized because the electrical conductivity and transmittance of patterned SWCNT films are important physical properties that need to be considered when SWCNT films are used in optoelectronic devices such as flexible displays and touch screens. There were no changes in sheet resistivity and transparency in the SWCNT area protected by photoresist polymer before and after O2-plasma treatments. Figure 5a shows the effect of O2-plasma treatment power on the resistivity of SWCNT film exposed to O2-plasma. SWCNT films were treated with O2-plasma under different powers from 200 to 500 W for between 1 and 5 min. The resistivity of SWCNT film increased as the power increased, but when SWCNTs films were treated at 400 W for 5 min and at 500 W over 3 min, it was impossible to measure resistivity as it was above the measurement range of the instrument (∼100 MΩ). In comparison Langmuir 2010, 26(1), 598–602

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Figure 4. XPS survey spectra of SWCNT film (a) before and (b) after oxygen plasma treatment. SWCNT films were fabricated on silicon substrates (Si3N4) to avoid the interference by carbon atoms on PET substrates and treated under the following conditions: 70 mTorr chamber pressure, 500 W power, and a treatment time of 3 min.

with pristine SWCNTs films, this increase indicates that the SWCNT films lost their conducting characteristics and became nonconducting. Sheet resistivity also increased dramatically from 1 MΩ to over 100 MΩ when SWCNTs films were treated at 400 W for 5 min and at 500 W over 3 min. The electric percolation threshold might explain the reason why the sheet resistivity increased dramatically. The connection of SWCNTs gradually deceased in the range between 100 Ω and 1 MΩ. However, over 1 MΩ, the network of SWCNTs is likely to have decreased below the electric percolation critical point, resulting in no electrons moving on to the next SWCNT. Figure 5b shows the changes in transparency and sheet resistivity according to O2-plasma treatment time at power of 400 W. The transparency of the SWCNT film was about 79% before plasma treatment but improved to 86% with increasing treatment times. The transparency of PET film itself is 89%. Even though SWCNTs were not etched all the way down to the substrate by CCP systems, as shown in the SEM images, O2-plasma treatment increased the transparency of the SWCNT film to almost the original transparency of PET film. An ECL reaction was carried out on the patterned SWCNT films to confirm that it could function as a transparent electrode. Among all ECL systems, Ru(bpy)32þ has been the most extensively studied and used compound because it provides strong luminescence, can be electrochemically recycled at room temperature, and is dissolved in a variety of solvents. Because of the reversible nature of the Ru2þ/3þ anodic redox couple, TPA was used as an “oxidative-reductive” coreactant to generate ECL. Langmuir 2010, 26(1), 598–602

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Figure 5. (a) Effects of O2-plasma treatment power on the resistivity of SWCNT films. SWCNT films were treated with O2-plasma under a power ranging from 200 to 500 W between 1 and 5 min. (b) Changes in sheet resistivity and transparency according to O2plasma treatment time at a power of 400 W.

In the Ru(bpy)32þ/TPA system, a strong reductant is generated during TPA oxidation that is able to react with the oxidized form of Ru(bpy)33þ to produce Ru(bpy)32þ*.32 Electrochemical oxidation of TPA followed by spontaneous deprotonation of an oxidation product generates a powerful reducing agent. Ru(bpy)32þ emits light at 620 nm when it relaxes to the ground state. Figure 6a demonstrates the order of ECL reactions occurring on the patterned SWCNT films. The patterned SWCNT electrode (a working electrode) was placed into a cell in a dark box with clean platinum wire (a counter electrode) and Ag/AgCl (saturated in 3 M NaCl) (a reference electrode). This device emits ECL from the patterned SWCNT electrodes following the application of a positive voltage (þ1 V vs Ag/AgCl). A CCD camera located below the device captures light emitted from the patterned SWCNT electrodes simultaneously. All of the reactions occur only on the working electrode (anode). Parts b and c of Figure 6 are an optical image of the patterned SWCNT film and a luminescence image of the patterned SWCNT film after the ECL reaction, respectively. Red light is emitted on the SWCNT electrode surface without leakage, as shown in the line scan of the chemiluminescent signal in Figure 6d. This signal has homogeneous signal intensity, and the width of the line scan at each SWCNT electrode is the same as that of the patterned SWCNT electrode. This simple experiment indicates that the CCP systembased O2-plasma treatment can be successfully applied to pattern a SWCNT transparent electrode. (32) Marquette, C.; Blum, L. Anal. Bioanal. Chem. 2008, 390, 155–168.

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Figure 6. (a) Schematic of ECL reactions on the patterned SWCNT films. (b) Optical image of the patterned SWCNT electrodes. (c) Luminescence image of the patterned SWCNT electrodes after the ECL reaction. (d) ECL line scan at the location indicated by the dashed part in (c).

4. Conclusions In conclusion, we have demonstrated how a CCP-based plasma technique can be used to pattern SWCNTs on flexible and transparent PET substrates. This technique offers high resolution and excellent reproducibility compared to other methods. When SWCNTs films were treated at 400 W for 5 min and at 500 W for 3 min, the resistivity of the SWCNT film increased dramatically until the electric percolation critical point. The transparency of SWCNT film improved up to 86% by increasing the treatment time at a power of 400 W. The SWCNT films patterned by this technique can be used successfully as flexible transparent electrodes as shown by the ECL reaction. We expect that this simple and

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versatile patterning technique will facilitate fabrication of SWCNT film-based electronic and optoelectronic devices. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grants funded by the Korea government (MEST) (No. R01-2008-000-10877-0 and No. R11-2008-044-01003-0). Supporting Information Available: XPS C1s spectrum of CNT before (a) and after (b) oxygen plasma treatment. This material is available free of charge via the Internet at http://pubs.acs.org.

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