pubs.acs.org/Langmuir © 2010 American Chemical Society
Electrochemical Patterning of Transparent Single-Walled Carbon Nanotube Films on Plastic Substrates Kwi Nam Han,† Cheng Ai Li,† Byunghee Han,† Minh-Phuong Ngoc Bui,† Xuan-Hung Pham,† Jaebum Choo,† Mark Bachman,‡ G. P. Li,‡ and Gi Hun Seong*,† † Department of Bionano Engineering, Hanyang University, Ansan 426-791, South Korea, and Integrated Nanosystems Research Facility, Department of Electrical Engineering and Computer Science, and Department of Biomedical Engineering, University of California, Irvine, California 92697
‡
Received December 9, 2009. Revised Manuscript Received February 24, 2010 We report a new patterning method for single-walled carbon nanotubes (SWCNTs) films on flexible, transparent poly(ethylene terephthalate) using electrochemical etching in an aqueous electrolyte solution. Electrochemical etching of the SWCNT films patterned with photoresist polymer was accomplished in a three-electrode system, and the electrochemically patterned SWCNT films were then characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The voltammetry curve showed that SWCNTs underwent drastic oxidation above an applied potential of 1.315 V with the generation of gas bubbles, and the oxidation current became constant above 2.6 V due to the mass transfer limit. SEM images showed that the networks of SWCNTs in the area protected with the photoresist polymer had no damage and vivid connections were obvious, while the connections and shapes of SWCNTs in the area exposed to electrochemical etching were indistinct and slightly damaged. In the Raman spectra of the area protected with the photoresist polymer and the exposed SWCNT area, the intensity ratio of the D-line to the G-line increased from 0.077 to 1.136, which indicated that the ordered carbons of the SWCNT film gradually became amorphous carbons due to electrochemical etching. For optimal patterning, the electrochemical etchings of SWCNT films were performed under various conditions (the applied potential, pH of the electrolyte solution, and electrolyte concentration). An applied potential of 3.0 V in 0.1 M NaCl electrolyte solution (pH 7.0) was optimal for homogeneous electrochemical patterning of SWCNT films. In an electrochemiluminescence reaction, the SWCNT films patterned by this technique could be used successfully as flexible and transparent electrodes.
Introduction Single-walled carbon nanotubes (SWCNTs) have attracted significant research interest in many fields due to their remarkable electrical, mechanical, thermal, and optical properties.1 In particular, the excellent properties of SWCNT make them more suitable for use in transparent and flexible electrodes and able to better satisfy the requirements of high conductivity and transparency than can indium tin oxide.2,3 Conductive SWCNT *Corresponding author. Tel: þ82-31-400-5202; Fax: þ82-31-436-8148; E-mail address:
[email protected] (Gi Hun Seong).
(1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787– 792. (2) 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. (3) 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. (4) 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. (5) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622–625. (6) Patolsky, F.; Weizmann, Y.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 2113–2117. (7) Kauffman, D. R.; Star, A. Angew. Chem., Int. Ed. 2008, 47, 6550–6570. (8) Ito, T.; Sun, L.; Crooks, R. M. Anal. Chem. 2003, 75, 2399–2406. (9) Henriquez, R. R.; Ito, T.; Sun, L.; Crooks, R. M. Analyst 2004, 129, 478–482. (10) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Nature 2003, 424, 654–657. (11) Durkop, T.; Getty, S. A.; Cobas, E.; Fuhrer, M. S. Nano Lett. 2004, 4, 35–39. (12) Pasquier, A. D.; Unalan, H. E.; Kanwal, A.; Miller, S.; Chhowalla, M. Appl. Phys. Lett. 2005, 87, 203511(1-3) (13) Guldi, D. M.; Rahman, G. M. A.; Prato, M.; Jux, N.; Qin, S.; Ford, W. Angew. Chem., Int. Ed. 2005, 44, 2015–2018. (14) Jung, Y. J.; Kar, S.; Talapatra, S.; Soldano, C.; Viswanathan, G.; Li, X.; Yao, Z.; Ou, F. S.; Avadhanula, A.; Vajtai, R.; Curran, S.; Nalamasu, O.; Ajayan, P. M. Nano Lett. 2006, 6, 413–418.
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films have been used in field emission displays,4 sensors,5-9 thin film transistors,10,11 and transparent electrodes for optoelectronic devices.12-14 The successful implementation of SWCNT films for various applications requires high-quality and high-resolution patterning at defined positions, with large-scale control of location and orientation. Several methods including chemical vapor deposition growth patterning,15,16 inkjet printing,17,18 electrophoresis deposition,19 chemically anchored deposition,20 laser irradiation,21 and contact transfer22,23 have been developed to pattern carbon nanotube films.24-27 In the previous report, we demonstrated a patterning method of SWCNT film using (15) 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. (16) 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. (17) Song, J. W.; Kim, J.; Yoon, Y. H.; Choi, B. S.; Kim, J. H.; Han, C. S. Nanotechnology 2008, 19, 095702(1-6) (18) 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. (19) Gao, B.; Yue, G. Z.; Qiu, Q.; Cheng, Y.; Shimoda, H.; Fleming, L.; Zhou, O. Adv. Mater. 2001, 13, 1770–1773. (20) Jung, M. S.; Jung, S. O.; Jung, D. H.; Ko, Y. K.; Jin, Y. W.; Kim, J. M.; Jung, H. T. J. Phys. Chem. B 2005, 109, 10584–10589. (21) 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. (22) Meitl, M. A.; Zhou, Y.; Gaur, A.; Jeon, S.; Usrey, M. L.; Strano, M. S.; Rogers, J. A. Nano Lett. 2004, 4, 1643–1647. (23) Kumar, A.; Pushparaj, V. L.; Kar, S.; Nalamasu, O.; Ajayan, P. M.; Baskaran, R. Appl. Phys. Lett. 2006, 89, 163120(1-3) (24) Behnam, A.; Noriega, L.; Choi, Y.; Wu, Z.; Rinzler, A. G.; Ural, A. Appl. Phys. Lett. 2006, 89, 093107(1-3) (25) Li, X.; Zhang, L.; Wang, X.; Shimoyama, I.; Sun, X.; Seo, W. S.; Dai, H. J. Am. Chem. Soc. 2007, 129, 4890–4891. (26) Park, J. U.; Meitl, M. A.; Hur, S. H.; Usrey, M. L.; Strano, M. S.; Kenis, P. J. A.; Rogers, J. A. Angew. Chem., Int. Ed. 2006, 45, 581–585. (27) Ko, H.; Jiang, C.; Shulha, H.; Tsukruk, V. V. Chem. Mater. 2005, 17, 2490– 2493.
Published on Web 03/17/2010
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O2-plasma dry etching.28 This technique including conventional photolithography and subsequent O2-plasma treatment allows excellent reproducibility and is compatible with polymeric substrates because of low processing temperatures. However, it requires expensive plasma equipments and high maintenance costs. Herein, we describe a new patterning method for the production of SWCNT films on flexible, transparent poly(ethylene terephthalate) (PET) using electrochemical etching in an aqueous electrolyte solution. Electrochemical etching of the SWCNT films patterned with photoresist polymer was accomplished in a three-electrode system, and the electrochemically patterned SWCNT films were then characterized by scanning electron microscopy (SEM) and Raman spectroscopy. For optimal patterning, the electrochemical etchings of SWCNT films were performed under various conditions (different applied voltages, different pH electrolyte solutions and electrolyte concentrations). To demonstrate the potential use of the patterned SWCNT films as transparent electrodes, an electrochemiluminescence (ECL) reaction was carried out using the patterned SWCNT films.
Experimental Section Homogeneous SWCNT films were fabricated using a vacuum filtration method. Briefly, the SWCNT mixture (Topnanosys Co., 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 electrochemical etching in a threeelectrode system. 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 SWCNT film prepatterned with photoresist polymer was placed into a cell with a large SWCNT film (a counter electrode) and Ag/AgCl (saturated in 3 M NaCl) (a reference electrode) following the application of a positive voltage (þ3.0 V versus Ag/ AgCl). After electrochemical etching, the SWCNT films were then immersed in the tetrahydrofuran solution to completely remove the photoresist polymer from the SWCNT films. This was followed by washing with ethanol and deionized water and drying under nitrogen gas. Raman measurements were performed at room temperature with a 17 mW He-Ne laser at an excitation frequency of 633 nm. Raman spectra were acquired from different spots in the patterned sample, and an integration time of 10 s was used for all Raman measurements. The electrical and optical properties of the patterned SWCNT films were characterized by a four probe setup and UV-vis spectroscopy, respectively. The morphological changes were investigated by field emission-scanning electron microscopy (FE-SEM) at an accelerating voltage of 15 kV. To demonstrate the ability of the patterned SWCNT films to function as transparent electrodes, a three-electrode ECL reaction was carried out using a CHI600C electrochemical analyzer. The patterned SWCNT film, Pt wire, and Ag/AgCl electrode were used as the working electrode, counter electrode, and reference electrode, respectively. A mixture consisting of 1 mM Ru(bpy)32þ and 5 mM TPA dissolved in PBS buffer (pH 7.0) was used to fill the reaction chamber. Chronoamperometry measurements were (28) Han, K. N.; Li, C. A.; Bui, M. N.; Seong, G. H. Langmuir 2010, 26, 598–602.
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Figure 1. (A) Schematic of the electrochemical patterning of SWCNT films. (B) A typical voltammogram obtained from SWCNT films in an aqueous 0.1 M NaCl electrolyte solution. Inset: The current vs time curve obtained for anodization of SWCNT films at the potential of 3.0 V in 0.1 M NaCl solution. (C) Optical microscope image of the patterned SWCNT film on the PET flexible substrate. The scale bar represents 300 μm. taken to ensure maintenance of a steady 1.0 V potential bias between the working and counter electrodes. The ECL signal beneath the SWNCTs film was recorded by fluorescent microscopy.
Results and Discussion To fabricate large-area homogeneous SWCNT films, we used the vacuum filtration method.29 The thickness of SWCNT films was controlled to be ∼100 nm, and suitable resistivities and transparencies were achieved by adjusting the concentration and volume of the SWCNT suspension. The average resistivity and transparency of the fabricated flexible SWCNT films was 400 ohm/sq and 80%, respectively (the transparency of PET (29) de Heer, W. A.; Bacsa, W. S.; Ch^atelain, A.; Gerfin, T.; Humphrey-Baker, R.; Forro, L.; Ugarte, D. Science 1995, 268, 845–847.
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Figure 2. SEM images of SWCNTs (A) protected with a photoresist polymer, (B) exposed to electrochemical etching, and (C) in the interface between (A) and (B). Inset: SEM image of the interface between (A) and (C). (D) Raman spectra of SWCNT films recorded from SWCNTs (a) protected with a photoresist polymer, (b) exposed to electrochemical etching, and (c) in the interface. The scale bars represent 1 μm.
film itself was 89%). The SWCNT films showed high flexibility with negligible change in resistivity upon hard bending. For electrochemical patterning of the SWCNT films, we used photolithography followed by electrochemical etching (Supporting Information, Figure 1). The excellent compatibility of SWCNT films with photolithography allows precise and high-resolution features to be defined.30 Electrochemical etching of the SWCNT films patterned with photoresist polymer was accomplished in a three-electrode system, in which the photoresist polymer-patterned SWCNT film (a working electrode) was placed into a cell with a large SWCNT film (a counter electrode) and Ag/AgCl (saturated in 3 M NaCl) (a reference electrode), followed by the application of a positive voltage (þ3.0 V vs Ag/AgCl), as shown in Figure 1A. During electrochemical etching, carbon nanotubes beneath the patterned photoresist polymer were protected from etching by anodization of SWCNTs while the exposed carbon nanotubes were destroyed. Figure 1B shows a typical voltammogram obtained from SWCNT films in an aqueous 0.1 M NaCl electrolyte solution. Electrochemical oxidation of carbon-based materials involving coal, carbon black, and diamond-like carbon leads to the conversion of solid carbon to volatile CO2 at the anode.31-35 The respective half-cell reactions of SWCNT can be (30) Lu, S.; Panchakesan, B. Appl. Phys. Lett. 2006, 88, 253107(1-3) (31) Coughlin, R. W.; Farooque, M. Nature 1979, 279, 301–303. (32) Coughlin, R. W.; Farooque, M. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 211–219. (33) M€uhl, T.; Myhra, S. Nanotechnology 2007, 18, 155304(1-6) (34) Barisci, J. N.; Wallace, G. G.; Baughman, R. H. J. Electroanal. Chem. 2000, 488, 92–98. (35) Ito, T.; Sun, L.; Crooks, R. M. Electrochem. Solid-State Lett. 2003, 6, C4–7.
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represented as SWCNTðsÞ þ 2H2 OðlÞ f 4Hþ ðaqÞ þ 4e þ CO2 ðgÞ ðanode reaction; oxidationÞ 4Hþ ðaqÞ þ 4e - f 2H2 ðgÞ ðcathode reaction; reductionÞ The overall reaction between SWCNT and water can be expressed as SWCNTðsÞ þ 2H2 OðlÞ f CO2 ðgÞ þ 2H2 ðgÞ This redox reaction occurs catalytically in the presence of Fe3þ ions that are included during the synthesis of SWCNT. The voltammetry curve showed that the SWCNT films underwent drastic oxidation above the applied potential of 1.315 V, and that gas bubbles were generated. When the potential was high enough (higher than 2.6 V), the oxidation current became constant because water was used as the reactant molecules.36 The current vs time curve obtained for anodization of SWCNTs is shown in the inset of Figure 1B. The sharp current falloff indicates that electrochemical oxidation, which results in the production of CO2, led to corrosion of the SWCNTs. The current then decreased rapidly the moment that percolation within the SWCNT network was broken by the electrochemical etching. The current falloff depended highly on the applied potential, electrolyte, the (36) Dhooge, P. M.; Stilwell, D. E.; Park, S. M. J. Electrochem. Soc. 1982, 129, 1719–24.
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Figure 3. Effect of the applied potential on the electrochemical etching of SWCNT films. (A) Plot of current vs time in response to different applied potentials in 0.1 M NaCl electrolyte solution: (a) 1.5 V, (b) 2.0 V, (c) 2.5 V, (d) 3.0 V, and (e) 4.0 V. Optical microscope images of the SWCNT films patterned electrochemically at potentials of (B) 3.0 V, (C) 2.0 V, and (D) 4.0 V. The scale bars represent 300 μm.
size of pattern, and its shape. Figure 1C shows an optical microscope image of the patterned SWCNT films on the PET flexible substrate. The SWCNT films were treated by electrochemical etching under typical conditions of a cell potential of 3.0 V in 0.1 M NaCl electrolyte solution. The clear pattern demonstrated the effectiveness of SWCNT patterning by this method. Interestingly, the interface between the area protected with the photoresist polymer and the exposed SWCNT area was more transparent than the area protected by the photoresist polymer and the exposed SWCNT area. The exposed SWCNT films underwent etching due to their electrochemical reactions with H2O, and the electrons generated during this process passed through the SWCNT network (Supporting Information, Figure 2). However, the resistivity of the exposed SWCNTs increased gradually during the electrochemical etching process, leading to a decline in the flow of electrons through the SWCNT network. Finally, the exposed SWCNT area reached the electric percolation critical point, and oxidation of the SWCNTs occurred mainly at the interface, because the interface had a more favorable electron path than did the exposed SWCNTs. Therefore, the SWCNTs at the interface where electrons can be easily transported underwent electrochemical etching continuously, and the boundary line was therefore more transparent than were the areas protected with the photoresist polymer and that of the exposed SWCNT. For more detailed characterization of the electrochemically patterned SWCNT films, they were investigated by SEM and Raman spectroscopy (Figure 2). In the SWCNT area protected with the photoresist polymer, the networks of SWCNTs appeared to have no damage, and vivid connections were obvious, while the Langmuir 2010, 26(11), 9136–9141
connections and shapes of SWCNTs in the area exposed to electrochemical etching were indistinct and slightly damaged. The transparency and resistivity of the exposed SWCNT area were 82-83% and 2000-2500 ohm/sq, respectively. In our previous patterning method of SWCNT film using O2 plasma, SWCNTs in the area exposed to plasma could be almost removed by controlling plasma treatment power and time.28 For example, when SWCNT films were treated at 400 W for 5 min, the transparency of the SWCNT film was improved to 86% and it was impossible to measure resistivity as it was above the measurement range of the instrument (∼100 MΩ). The morphology of the SWCNTs in the interface between the area protected with the photoresist polymer and the exposed SWCNT area resembled that of amorphous carbon. The destruction of whole SWCNTs in the interface resulted from oxidation, which converted SWCNTs into volatile CO2. In the Raman spectra of the area protected with the photoresist polymer and the exposed SWCNT area, the strong peak at 1595 cm-1 (G-line), which indicated graphite/ordered carbons in SWCNTs, decreased substantially, while the amorphous/disordered carbon peak at 1322 cm-1 (D-line) increased by more than 30%. The intensity ratio of the D-line to the G-line increased from 0.077 to 1.136, which indicated that the ordered carbons of the SWCNT film gradually became amorphous carbons due to electrochemical etching. In addition, in the unprotected area, the amorphous peak (1322 cm-1) at the interface disappeared almost completely and the graphite peak (1595 cm-1) decreased by approximately 95% compared to the area protected with the photoresist polymer. These results indicate that most of the SWCNTs and amorphous carbons on the PET substrate were converted to volatile CO2. DOI: 10.1021/la904642k
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Figure 4. (A) Plot of current vs time in response to different electrolyte solutions. SWCNT films were etched electrochemically at a constant potential of 3.0 V. Optical microscope images of the SWCNT films patterned electrochemically in (B) 0.1 M NaOH electrolyte solution and (C) 0.1 M H2SO4 electrolyte solution. (D) Luminescence image of an SWCNT film after electrochemical etching in 0.1 M H2SO4 electrolyte solution. This image was obtained after ECL reaction in the mixture of Ru(bpy)32þ and TPA. The scale bars represent 300 μm.
Figure 3A demonstrates the dependence of electrochemical etching on the applied voltage, which varied between 1.5 and 4.0 V in 0.1 M NaCl solution (pH 7.0). At a potential less than 1.5 V, no change of oxidation current was observed for a long time. This indicates that, at this potential, electrochemical etching scarcely occurred or was too slow for the patterning of SWCNT films. The current falloff time became gradually faster in the range of potential from 1.5 to 2.5 V as the applied voltage increased and was then constant at potentials greater than 2.5 V. These results can be explained by the mass transfer limit. The electrochemical reaction is an interfacial process in which reduction or oxidation occurs at the electrode-solution interface. Molecules or ions in solution must be transported to the electrode surface for the electrochemical event to occur. Therefore, the transport of H2O and Hþ from the solution phase to the surface of electrode is important for the oxidation of SWCNTs. The pattern of the SWCNT films was well-defined at a potential of 3.0 V in 0.1 M NaCl solution (Figure 3B), but incorrect patterns were seen at potentials of 2.0 or 4.0 V. The SWCNT films etched at a potential of 2.0 V were patterned irregularly (Figure 3C). It appears that low oxidation potentials cause insufficient oxidation of SWCNT on PET substrates. When the SWCNT film was etched electrochemically at a potential of 4.0 V, the width of the patterns became thinner than that of the photoresist polymer patterned on the SWCNTs (Figure 3D). This observation suggests that excessively high potentials cause explosive oxidation of SWCNTs, and that at these high potentials, even SWCNTs protected with photoresist polymer can be etched, although the etching rate is equal to that at a potential of 3.0 V. On the basis of these results, a potential of 9140 DOI: 10.1021/la904642k
3.0 V appears to be optimal for effective and homogeneous electrochemical etching of SWCNT films. To investigate the dependence of SWCNT etching on the pH of the electrolyte solution, the electrochemical etching of SWCNTs was carried out at constant potential of 3.0 V in acidic, basic, and neutral electrolyte solutions (Figure 4). Electrochemical etching was more effective in a neutral electrolyte solution than in a basic or acidic electrolyte solution. The SWCNT films etched in a basic electrolyte solution (NaOH) demonstrated unclean patterns because basic solutions dissolve the photoresist polymer during electrochemical etching. When SWCNT films were etched in acidic (H2SO4) or neutral (NaCl) electrolyte solutions, the resulting films had clear patterns, as shown in the optical images. However, a luminescence image of the SWCNT film etched in an acidic electrolyte solution revealed rough-edged patterns compared to the film etched in a neutral electrolyte solution (Figure 4D and Figure 5C). The current vs time curves obtained for anodization of SWCNTs in neutral and basic electrolyte solutions were similar, while the curve for the acidic electrolyte solution showed a fast current falloff, indicating a fast electrochemical etching rate. These results can be explained by considering the ionic conductivity of the solutions and the use of Fe3þ ions as catalysts. The divalent electrolyte H2SO4 has a relatively higher ionic conductivity than those of the monovalent electrolytes NaCl or NaOH. This high ionic conductivity offers a good electron path for electrochemical reactions. Interestingly, when H2SO4 was used as an electrolyte, the etching rate was two times faster than that observed for other divalent electrolytes such as Na2SO4 (data not shown). We believe that this observation is due to the fact that Langmuir 2010, 26(11), 9136–9141
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can be electrochemically recycled at room temperature and is soluble in a variety of solvents.37,38 In the ECL reaction, Ru(bpy)32þ is electrochemically oxidized to Ru(bpy)33þ in the presence of tripropylamine (TPA) (Figure 5A).39 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. 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 fluorescent microscopy located below the device was used to capture light emitted from the patterned SWCNT electrodes simultaneously. All of the reactions occurred only on the working electrode surface (anode). Figure 5B,C presents an optical image and a luminescence image of the patterned SWCNT film after the ECL reaction, respectively. Red light was emitted from the patterned SWCNT electrode surface without leakage, as shown in the line-scan of the chemiluminescent signal. This signal had homogeneous signal intensity, and the width of the line-scan at each SWCNT electrode was the same as that of the patterned SWCNT electrode. This simple experiment indicates that our electrochemical etching method can be successfully applied to pattern SWCNT transparent electrodes.
Conclusions
Figure 5. (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. SWCNT films were etched electrochemically at the potential of 3.0 V in 0.1 M NaCl solution. The inset is an ECL line scan at the location indicated by the dashed line in (C).
Fe3þ ions were used as catalysts during SWCNT synthesis.36 Fe3þ ions dissolve more easily in a strong acidic solution than in a neutral or basic solution. Therefore, the presence of large amounts of Fe3þ ions in the H2SO4 solution would reduce the activation energy of SWCNT oxidation, resulting in overactive-etching of SWCNTs covered with photoresist polymer due to the increase in the etching rate. Another notable observation was that the etching rate in NaOH or H2SO4 electrolyte solution increased as the concentration of electrolyte in these solutions increased due to the higher ionic conductivity of these solutions. However, the etching rate in a high concentration NaCl electrolyte solution was reduced because of a competitive reaction between SWCNT oxidation and chlorine oxidation (Supporting Information, Figure 3). An ECL reaction was carried out on the patterned SWCNT film to confirm that it could function as a transparent electrode. Ru(bpy)32þ has been the most extensively studied and used compound in ECL systems because it provides strong luminescence,
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We introduced a novel patterning method for SWCNT films on plastic substrates based on electrochemical etching in an aqueous electrolyte solution. The voltammetry curve showed that SWCNTs undergo drastic oxidation above an applied potential of 1.315 V with the generation of gas bubbles; the oxidation current became constant above 2.6 V due to the mass transfer limit. The applied potential, pH of the electrolyte solution, and electrolyte concentration all affected the etching rate of SWCNT films. We found that an applied potential of 3.0 V in 0.1 M NaCl electrolyte solution was optimal for homogeneous electrochemical patterning of SWCNT films. The SWCNT films patterned by this technique were applied successfully as flexible transparent electrodes, as shown by the ECL reaction. We expect that this simple and versatile patterning technique will facilitate fabrication of SWCNT film-based electronic and optoelectronic devices such as large-area flexible displays, flexible solar panels, and touch screens. Acknowledgment. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (No. 2009-0058925 and No. R11-2008044-01003-0). We also acknowledge the financial support of the Ministry of Knowledge Economy (MKE) and the Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Strategic Technology. Supporting Information Available: Schematic of the electrochemical patterning of SWCNT films and effect of electrolyte concentration on etching of SWCNT films. This material is available free of charge via the Internet at http://pubs.acs.org. (37) Fan, F. F.; Bard, A. J. Nano Lett. 2008, 8, 1746–1749. (38) Zhan, W.; Bard, A. J. Anal. Chem. 2007, 79, 459–463. (39) Marquette, C. A.; Blum, L. J. Anal. Bioanal. Chem. 2008, 390, 155–168.
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