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Single-Walled Carbon Nanotubes/Polymer Composite Electrodes Patterned Directly from Solution Jingbo Chang, Choolakadavil Khalid Najeeb, Jae-Hyeok Lee, and Jae-Ho Kim* Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Republic of Korea ABSTRACT: This work describes a simple technique for direct patterning of single-walled carbon nanotube (SWNT)/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOTPSS) composite electrodes in a large area on a substrate based on the solution transfer process by microcontact printing using poly(dimethylsiloxane) (PDMS) stamps. Various shapes of SWNT/PEDOTPSS composite patterns, such as line, circle, and square, can be easily fabricated with high pattern fidelity and structural integrity. The single parallel line pattern device exhibits high electrical conductivity (0.75 105 S/m) and electronic stability because of alignment of nanotubes and big-size SWNT bundles (∼5 nm). The electromechanical study reveals that the composite patterns show ∼1% resistance change along SWNT alignment direction and ∼5% resistance change along vertical alignment direction after 200 bend cycles. Our approach provides a facile, low-cost method to pattern transparent conductive SWNT/polymer composite electrodes and demonstrates a novel platform for future integration of conducting SWNT/polymer composite patterns for optoelectronic applications.
1. INTRODUCTION The development of solution-processable single-walled carbon nanotube (SWNT)/polymer composites for electronic devices is essential for applying to organic electronics technology such as flexible flat-panel display devices,1 batteries,2 energy harvesting and new optoelectronic devices,3 and thin film transistors.4 As these and other applications of SWNT/polymer films become more sophisticated, it is important to develop efficient strategies for controlling the architecture, geometry, and positioning of arrays at the micrometer and nanometer scale to tailor the film properties and functionality. One important challenge for nanodevice applications is to pattern carbon nanotube/organic nanocomposites in a large scale. Patterning of carbon nanotube/thiol suspensions have been reported using both “top-down” and “bottom-up” techniques.5,6 Recent progress in patterning of SWNT thin films includes SWNT growth on prepatterned catalysts,7,8 adsorption onto a chemically modified surface,9 microcontact printing,10 photolithography,11 and manipulation with external fields and flows.12 Most recently, patterning of SWNT films over large areas in a simple, reliable, and convenient manner has been achieved by dry transfer of horizontally aligned structures grown by chemical vapor deposition.13 According to previously described techniques for patterning SWNT films, some of them are costly and also limited to a few different kinds of nanostructures, some generally yield low coverage14,15 from aqueous solution or repeated depositions to increase SWNT film density, and some rely on chemical modification of the SWNTs1620 which may degrade the electrical performance of the tubes. However, rational design of SWNT patterns with both high density and ordered orientation is highly r 2011 American Chemical Society
desirable to exploit the excellent electronic and mechanical properties of SWNTs. In our previous report, we have successfully fabricated SWNT patterns by contacting and peeling off poly(dimethylsiloxane) (PDMS) stamps from the substrates via microcontact printing.21 This method specifically required that prepatterned SWNTs should be well unidirectionally aligned as a monolayer film on the substrate. In another recent report, we demonstrated direct patterning of SWNT/polymer composites by inkjet printing and evaluated effect of surface modification of SWNTs on the electrical properties of line patterns; however, the feature size of the pattern by this method is limited to a width of ∼100 μm.22 In order to further scale down the feature size to nanometer scale and to construct various shapes, herein, we demonstrate a solution transfer process to fabricate SWNT/polymer composite patterns by microcontact printing.2325 SWNT-conducting polymer composites have attracted special interest because they not only demonstrate enhanced properties of individual components but also generate additional functionalities, thereby facilitating various electronic and photonic applications. Among various electrical conductive organic polymers, poly(3,4-ethylenedioxythiophene) (PEDOT), as a member of poly(thiophene) family, has drawn wide interest in supercapacitor applications because of its fast charge/discharge ability,26 high room temperature conductivity, and high thermal and chemical stability. Especially, nanocomposite films of carbon Received: March 14, 2011 Revised: May 2, 2011 Published: May 10, 2011 7330
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Figure 1. Schematic illustration of fabrication of SWNT/polymer patterns by ink transfer: (a) PDMS stamp, (b) PDMS stamp with filled ink, (c) contact between the filled stamp and the substrate and an optical image of ink filled a channel (inset), (d) removal of the stamp.
nanotubes with PEDOTpoly(styrenesulfonate) (PSS) has greatly attracted the attention of researchers as transparent conductive films or hole-extraction conducting layers for organic photovoltaic devices.2729 To obtain good electrical properties, the resulting SWNT/PEDOTPSS pattern films must be composed of clean, aligned, and undamaged tubes with controlled surface coverage. Therefore, the development of SWNT/PEDOTPSS composite patterns without degradation of SWNT properties and construction technique that relies on solutionbased processing plays an important role in its practical applications. In particular, the controlled design of such patterns with good ordered nanostructures for desired applications is a key challenge in micro/nanoelectromechanical systems. Our technique can easily generate transparent nanometer-scale electrodes, which allows complex nanostructures to be directly fabricated over a large area using a SWNT/PEDOTPSS ink.
2. EXPERIMENTAL SECTION Materials. The single-walled carbon nanotubes (SWNTs) synthesized by arc discharge process were purchased from Iljin Nanotech Co, Korea. Sodium dodecylbenzenesulfonate (SDBS) and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) polymer (PEDOTPSS) aqueous solution (1.3 wt %) were purchased from Sigma-Aldrich. Sylgard 184 silicone elastomer (DC184A) and curing agent (DC-184B) were purchased from Dow Corning. SWNT/PEDOTPSS Ink Preparation. A probe-type high-power ultrasound sonicator (Sonics, VCS 750, Vibra cell) was used for the preparation of SWNT dispersions. A SDBS solution (1 mg/mL, 20 mL) was prepared in deionized water and stirred for 20 min. After immersing SWNTs (5 mg) into SDBS solution, it was sonicated for 10 min with a power sonic tip (750 W, 38%). Icewater cooling was used throughout to prevent heating of the sample. After sonication, dispersed nanotubes were obtained. For preparation of composite ink, 10 mL of 1.3 wt % PEDOTPSS polymer aqueous solution was mixed with 20 mL of SWNT solution and then sonicated for 5 min. A uniform SWNT/ PEDOT composite solution was formed. Fabrication of PDMS Stamps. PDMS stamp fabrication for this study was carried out by uniformly mixing 30 g of the Sylgard 184
silicone elastomer (DC-184A) and 3 g of a curing agent (DC-184B) and then stirring for about 10 min. The mixture solution was casted into the structured master. The gas bubbles caused by mixing were degassed in a vacuum chamber at 10 Torr for 30 min. The degassed PDMS was cured in an oven at 80 °C for 3 h. The cured PDMS was easy to peel away from the master, and then a PDMS stamp was formed. Characterization. SWNT/polymer composite patterns were characterized by scanning electron microscopy (SEM) using a Philips XL30FEG-SEM. Raman spectra of SWNT dispersion, polymer, and the composite samples were recorded on a Dongwoo 320i instrument (Andor Technology) at an excitation wavelength of 785 nm with an acquisition time of 1 s. The surface treatment of samples was contacted by a UVozone cleaner (UVO cleaner model 42, Jelight Co.). The optical property was measured by ultravioletvisible (UVvis) transmittance spectroscopy (JASCO-V530 UVvisNIR spectrometer (Japan)). Atomic force microscope (AFM) images were obtained using a silicon AFM tip with XE-100 instrument (Park system, Korea) in noncontact mode at atmospheric condition. The currentvoltage characteristics of composite patterns were measured by using a Keithley 2612 System SourceMeter.
3. RESULTS AND DISCUSSION A direct printing technique based on a SWNT/conducting polymer complex ink transfer process has been developed for realizing low-cost and large-area electronic devices. In this printing method, nanometer-scale patterns were constructed by using PDMS stamps to transfer ink onto the silicon substrates. The PDMS stamps were fabricated by casting a degassed mixture of the DC-184A and the DC-184B on the structured master. After curing it, the PDMS stamp was peeled away from the master and features were formed as shown in Figure 1a. The composite ink can be loaded into concave troughs of PDMS stamps as indicated in Figure 1b. After filling PDMS stamps with a suitable ink by sinking stamps into ink solution, only recessed areas in the concave troughs were filled with the ink, and the ink solution on the outer surface of the stamp was removed by using a glass stick. The stamp with the filled ink was then brought into 7331
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Figure 2. (a) SEM image of SWNT/PEDOTPSS composites. (b) Raman spectra of SWNT dispersion, PEDOTPSS aqueous solution, and SWNT/PEDOTPSS composite ink.
contact with the underlying Si wafer as illustrated in Figure 1c. The inset of Figure 1c shows an optical image of ink filled inside a PDMS channel, which clearly reveals that the shape of ink surface is crescent. When the ink inside the channel brought into contact with the substrate, two protruding sides of the crescent ink were first touched receiving substrates, and then the ink was released along walls of troughs. This kind of convective transport consecutively brings rigid nanotubes from liquid phase to the substrate. During the transportation process, solvent is gradually evaporating, and therefore self-assembly and self-aligned nanotube patterns are formed.30 Because of capillary action, the two surfaces (PDMS stamp and substrate) can contact tightly. As the liquid evaporates, the attractive capillary force gradually increases, pulling the two surfaces to match well and adjusting to form uniform pattern morphology. The majority of the liquid layer initially evaporates through the open sides between the stamp and the substrate, while the remainder permeates through the stamps. After removing PDMS stamps, the SWNT/polymer composite parallel line patterns were formed as shown in Figure 1d. A SWNT/polymer composite ink was prepared by mixing PEDOTPSS aqueous solution with SWNTSDBS dispersion, and this mixture was then ultrasonicated for 5 min to obtain a homogeneous solution. The characterization of a SWNT/PEDOTPSS composite film and uniform aqueous composite solution were conducted by SEM and Raman spectroscopy, respectively. Figure 2a shows a SEM image of SWNT/PEDOTPSS composite thin film drop dried on a Si substrate, which demonstrates a diameter of SWNTs is about 5 nm and most of nanotubes are well aligned in the polymer matrix.31 Analysis of the Raman spectra of these nanotubes before and after mixing with PEDOTPSS polymers was performed with an excitation wavelength of 785 nm. The SWNTs exhibit a weak disorder band (D-band) at ca. 1290 cm1 and a strong tangential mode band (G-band) at ca. 1572 cm1, as shown in Figure 2b. The weak D-band indicates that the sample contains a small amount of disordered sp2 carbons and that the nanotube walls have a low number of defects. A representative Raman absorption band of PEDOTPSS sample was observed at ca. 1241 and 1407 cm1. The composites demonstrate a small shift of the G-band from 1572 to 1570 cm1 due to the interaction with the PEDOTPSS, and the intensity is reduced monotonically due to the orientation of SWNTs in the polymer matrix. The role of the
Figure 3. SEM images of SWNT/PEDOTPSS composite patterns: (a) a large area of SWNT composite parallel line patterns, (b) highresolution patterns, (c) two pattern lines, (d) a 600 nm wide single aligned SWNT composite pattern, and (e) a single 1.0 μm and (f) 1.3 μm wide line pattern.
PEDOTPSS in the composite solution is for assistance to form consecutive patterns, in which the conductive polymer most likely acts as an “inter-tube junction bridge” for holding and connecting nanotubes each other on substrates when solvent is evaporating.32 The SWNT/PEDOTPSS composite parallel line patterns are clearly seen in Figure 3a, and a center-to-center spacing between parallel line patterns is about 80 μm as shown in Figure 3b. From Figure 3c,d a width of a single pattern is ∼600 nm, and the observation of smooth sidewalls confirms that the dimension of the pattern fundamentally limits the resolution of patterns. Furthermore, the SWNT/polymer composite patterns clearly showed alignment of nanotubes in the patterns. The surface morphology reveals that the ink exhibits only modest lateral spreading on the receiving substrate. The 7332
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Figure 4. SEM images of complex patterns: (a) fan-shaped patterns, (b) parallel line patterns, (c) circular patterns, and (d) square patterns.
Figure 5. (a) Series of CA graphs on the PDMS patterns with different UVO treatment time: 0, 10, and 30 min. (b) CA graph on the underlying Si substrate. SEM images of pattern quality on the Si substrate with treatment time of (c) 10 min and (d) 30 min.
uniform alignment of nanotubes in the matrix possesses higher electrical conductivities and mechanical properties of SWNT filled composites.33 This can be useful in electronics and many other applications, where room-temperature transfer of aligned SWNTs can be integrated into devices that utilize the excellent anisotropic electrical and thermal properties of SWNTs. Moreover, this transfer process is not only limited to SWNT/polymer composites, as similar contact transfer could be carried out using any material with similar inks. However, the ability to manipulate aligned SWNT structures to form patterns has a substantial impact on the future of carbon nanotube-based applications. The widths of patterns were found to be dependent on the concentration of composite inks. For high concentration of the SWNT/ polymer ink (2-fold), the width increases to ∼1.0 μm from 600 nm in Figure 3e, and 3-fold increase of ink concentration shows about 1.3 μm sized patterns in Figure 3f. Through additional printing steps, many more complex nanostructures can be constructed. Diverse nanostructures of SWNT/PEDOTPSS composite based on ink transfer process were fabricated as shown in Figure 4. These SEM images clearly
show that the transferred patterns retain the features of the stamps. Figure 4a shows SEM images of SWNT/PEDOTPSS patterns fabricated using fan-shaped PDMS stamps. The very narrow SWNT/PEDOTPSS parallel line patterns (width: 200 nm) were prepared by using fine PDMS stamps (narrow features, center-to-center spacing: 30 μm) in Figure 4b. As shown in Figure 4c,d, nanoscale complex patterns, such as square patterns and circular patterns, were also successfully fabricated, demonstrating that various shapes can be fabricated with high pattern fidelity and structural integrity. In the patterning process, to further quantify printed feature morphology, we investigated the relationship of surface wettability between the PDMS stamp and the underlying (receiving) substrate, which determine the capability of loading and releasing SWNT/PEDOTPSS composite ink, respectively. The solution wettability in the PDMS stamps was controlled by varying the UVO treatment time as shown in Figure 5a. To transfer ink well, the substrate should have high solution wettability for receiving ink as shown in Figure 5b. We can produce PDMS stamps with different contact angles (CAs). The pattern quality is strongly 7333
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Figure 6. (a) Transmittance of the parallel line patterns with varying widths (600, 800, 1200 nm) on the glass. (b) Electrical property of patterns with different widths. Inset: optical photograph of SWNT/PEDOT patterns connected by two probes. (c, d) Current versus gate voltage (ISDVG) characteristics of a SWNT FET device, obtained with VSD = 1 V before O3 treatment and after O3 treatment for 3 h. Inset of (c): AFM image of a single pattern.
related to the CA of the solution on the PDMS stamps. Discontinuous patterns as shown in Figure 5c are observed on the PDMS surface with CA above 45°, when the PDMS stamp was treated with UVO for less than 30 min. As a comparison, prolonged UVO treatment decreases CA and increases quality of patterns dramatically. Consequently, low CA contributes to the high quality of patterns, and CA < 45° is required to ensure continuous patterns as shown in Figure 5d. For the SWNT/PEDOTPSS printed on the substrate, the optical transmittance of parallel line pattern electrodes with different widths such as 600, 1000, and 1300 nm was measured by UVvis spectroscopy as illustrated in Figure 6a. The pattern with 80 μm center-to-center spacing and feature size of 1.3 μm showed transmittance as high as 88% in the visible to nearinfrared region and increases to 96% as the width decreases to 600 nm. The linear currentvoltage characteristic of patterns (length: 310 μm) measured for the SWNT parallel line patterns with different widths along the alignment direction by directly connecting the two Au probes of a Keithley 2612 source meter indicated that the nanotubes in the parallel line pattern are in ohmic contact. An optical microscopic image of parallel line SWNT/PEDOTPSS composite patterns directly connected by two probes is shown in the inset of Figure 6b. A high current flow was observed when the voltage was applied through the electrodes along the nanotube alignment direction. Figure 6b shows electrical properties of patterns with different widths. It is clear that the conductivity of patterns increases when the width of patterns increases. The transmittance and conductivity of the pattern are dependent on the pattern width, which in turn determined by the concentration of the SWNT solution when same feature sized PDMS stamps are used. Obviously, higher concentration SWNT solutions will produce wider patterns, which possess higher conductivity but less transparency.
Chen et al. reported a strategy to improve the conductivity and transmittance of PEDOTPSS/multiwalled carbon nanotube composite films through the surface modification of carbon nanotubes by chemical oxidation with HNO3 followed by treatment with sorbitol and sonic agitation up to 6 h to achieve uniform dispersion of nanotubes in polymer matrix.34 However, in order to obtain the composite pattern film with high electrical conductivity for certain applications, the nanotubes should be long and with fewer defects. Therefore, to preserve the SWNT surface intact and to avoid the issues of shortening the nanotubes by acidic treatment, in our approach we used a low-power sonication for dispersing SWNTs with the assistance of the surfactant. For adjusting the trade-off between conductivity and transmittance of composite pattern films, the SWNTs properties (length, defect), dispersity, and concentration of nanotubes should be controlled. Owing to the coexistence of metallic and semiconducting carbon nanotubes, to investigate the property of integration of nanotube electronics, we also carried out measurement of SWNT field-effect transistors (SWNT FETs) with a width of 600 nm and a length of 310 μm in the channel on a Si/SiO2 substrate. An ISDVG curve in Figure 6c shows that the conductivity almost remains unchanged by gate bias flow, indicating that the SWNT/PEDOTPSS composites possess strong metallic property. The inset of Figure 6c clearly shows that the height of patterns is about 102 nm from the height profile and width is about 600 nm, which was measured by atomic force microscopy (AFM) from bare area to the pattern on the substrate. We calculated electrical conductivity (σ) using the equation σ ¼ IL=US where I is current (15 μA), L is length (310 μm), U is applied voltage (1 V), and S represents area (600 nm 102 nm) of cross section of a single pattern. Using this equation, the conductivity was 7334
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Figure 7. Electromechanical measurements made on the paralell pattern film with the pattern width of 600 nm. (a) Pattern resistance versus radius of curvature for one bending in tension on the PET substrate and bending direction indicated in inset. (b) Mean pattern resistance as a function of bending cycles and optical image of SWNT patterns on the PET substrate (inset).
found to be ∼0.75 105 S/m. For investigation of electronic stability, we treated our devices using UVO for 3 h. It is well-known that O3 is strong gas phase oxidant which induces rehybridization defects to converse sp2 to sp3 sites of SWNTs and lead to metal to semiconductor conversion.3540 Figure 6d shows the metallic SWNT FET exhibited weak gate bias dependence after O3 treatment, thus indicating that the conductivity does not have significant difference under O3 treatment. A little increase in the channel current on/off ratio was observed from ∼1 before treatment to 6.7 after O3 treatment. This means big-diameter SWNTs (5 nm) possess almost metallic property and electronic stability.41 Finally, we note that these composite patterns can be easily transferred to any substrate of interest and be useful as flexible electrode materials. This kind of high conductive composite patterns, which are easy to fabricate, is considerable to be used as electrodes in applications such as integrated circuits. In this case, we monitored the resistance of parallel line patterns during a bend cycle from an initial radius of curvature of 9 mm to a final radius of 3 mm before being relaxed on the flexible poly(ethylene terephthalate) (PET) substrates. Figure 7a shows the line pattern conductivity versus radius of curvature during bending along y-axis (parallel line pattern direction) and x-axis (perpendicular line pattern direction), which are schematically illustrated in the inset. It is clear that the line pattern conductivity varies ∼2% during bending in tension along y-axis, while ∼8% along x-axis. However, these composite line patterns show stability along y axis during one bend cycle, and it is necessary to explore their stability over many bend cycles. The pattern was bent to be semicircular (0 f π radian) over 200 times. It is clear from Figure 7b that the resistance varied ∼1% along y-axis, while 5% along x-axis. The optical image of bent SWNT patterns on the PET substrate is shown in inset of Figure 7b. In comparison, the line pattern is much more stable in tension along y-axis, which indicates nanotube alignment contributes for electromechanical stability.
4. CONCLUSIONS In summary, we have demonstrated the solution-based direct patterning of SWNT/PEDOTPSS electrodes by microcontact
printing. The crescent shapes of ink filled in the PDMS channels determine the connection between ink and receiving substrates should be protrudent parts of the ink. It means that the ink will release along trough walls of PDMS stamps, so that this kind of process transfers rigid SWNT composites from ink solution onto the underlying substrates to form aligned nanotube patterns. The size of SWNT composite patterns depends on concentration of the composite ink and is not restricted by the feature size of PDMS stamps. Therefore, this method could be convenient to make very narrow SWNT composite patterns. The effect of pattern quality was investigated by different surface wettability, where CA < 45° is needed to ensure continuous patterns. Those kinds of uniform patterns possess high transparency, conductivity, and electromechanical stability. It is also possible to fabricate all kinds of high-performance n- and p-type transistors using small sized (∼1 nm) SWNTs or SWNT/conjugated polymer composites. Our approach may open up new avenues for fabricating printed electronic and optoelectronic devices.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was supported by a grant (06K1401-00411) from the Center for Nanoscale Mechatronics & Manufacturing, one of the 21st Century Frontier Research Programs, which are supported by the Ministry of Education, Science and Technology, Korea. In addition, we thank the BK21 program of molecular science and technology in Ajou University. ’ REFERENCES (1) Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T. Nature Mater. 2009, 8, 494–499. (2) Chen, J.; Liu, Y.; Minett, A. I.; Lynam, C.; Wang, J.; Wallace, G. G. Chem. Mater. 2007, 19 (15), 3595–3597. 7335
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