Fabrication of polypyrrole micropatterns through microchannel-confined electropolymerization and their electrical conductivities

Electrochimica Acta 54 (2009) 4253–4257

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Fabrication of polypyrrole micropatterns through microchannel-confined electropolymerization and their electrical conductivities Yiqing Lu a , Guangxia Shen b , Chongjun Zhao a , Shouwu Guo b,∗ a

School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, PR China National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang District, Shanghai 200240, PR China b

a r t i c l e

i n f o

Article history: Received 22 January 2009 Received in revised form 24 February 2009 Accepted 24 February 2009 Available online 9 March 2009 Keywords: Electropolymerization Electric conductive polymer Polypyrrole micropattern Static potential polymerization Microchannel

a b s t r a c t Creating micropatterns of electrically conductive polymers on solid substrates is important for the low-cost construction of organic microelectronic devices. This work develops a novel strategy for the preparation of large-area polypyrrole (Ppy) micropatterns through area-selected in situ electropolymerization of pyrrole within microchannels. The effects on micropattern formation of electropolymerization procedures such as dynamic potential polymerization (DPP), static potential polymerization (SPP), and constant current polymerization (CCP), the solvent, and the polymerization time were studied systematically. The electrical conductivities of the Ppy micropatterns were measured and compared with a homogeneous Ppy thin film synthesized under the same conditions. Given the straightforward and versatile nature of this method, it is expected to contribute greatly to the convenient fabrication of low-cost organic microelectronic devices. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Patterned electrically conducting polymer structures have attracted great attention for possible applications in the development of microelectronic circuits [1], smart data storage cards [2], polymer dispersed liquid crystal displayers [3], and microsensors [4]. So far, several strategies, based upon photolithography [5], electron beam writing [6], ink-jet printing [7], dip-pen nanolithography [8], and microcontact printing [9,10] have been developed for preparing the electrically conducting polymer patterns. However, these methods usually involve high-cost setups and restrict the feature dimensions and morphologies of patterns, two properties that limit their practical applications in the fabrication of lowcost electric organic microelectronics. This area has been recently reviewed [11]. Using a pre-patterned substrate surface as a template, electrically conducting polymer patterns could be prepared through area-selected in situ polymerization [12–14]; however, prepatterning the substrate surface on a micro- or nanometer length scale usually requires a complicated procedure. Among the conventional conducting polymers, polypyrrole (Ppy), owing to its unique electrical conductivity performance and good thermal and environmental stability [15,16], is considered as one of the most promising organic electric conducting mate-

rials. However, Ppy is insoluble in most solvents and decomposes upon melting; thus, the preparation of well-defined Ppy micro- or nanopatterns using the aforementioned lithography and writing methods is challenging. On the other hand, Ppy can be synthesized conveniently via oxidative polymerization or electropolymerization of the pyrrole monomer [17–23]. Therefore, area-selected in situ polymerization on certain substrate surfaces may afford a new opportunity for patterning Ppy. Herein, we present a novel strategy for preparation of large-area Ppy micropatterns through area-selected in situ electropolymerization of pyrrole within microchannels. The microchannels were constructed by placing a poly(dimethylsiloxane) (PDMS) mold with microtrenches onto an ITO substrate. The as-synthesized Ppy micropatterns were characterized comprehensively by atomic force microscopy (AFM), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR). The Ppy micropatterns were transferred successfully from ITO to electrically insulating substrates by simple contact adhesion, and the electrical conductivities of the Ppy micropatterns were studied using a four-point probe system. 2. Experimental 2.1. Chemicals and materials

∗ Corresponding author. Tel.: +86 21 34206915; fax: +86 21 34206915. E-mail address: [email protected] (S. Guo). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.02.074

Pyrrole (99%, Acros Organics Company, Belgium), acetone (AR, Linfeng Chemical Reagent Company, Shanghai, China), ethanol and

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the reference electrode, respectively. The static potential polymerization (SPP) method was utilized. 2.4. Characterization of the Ppy micropatterns AFM images of as-fabricated Ppy patterns were acquired on a Multimode Nanoscope V scanning probe microscopy system using the tapping mode. The SEM images of the patterns were obtained using a FEI Sirion 200 field-emission scanning electron microscope (FESEM). FT-IR spectroscopy measurements were performed on a Nicolet 570 spectrometer. The electrical conductivities of Ppy micropatterns were measured using a digital four-point probe system (SZ-82) from a telecommunications instrument plant (Suzhou, China). 3. Results and discussion 3.1. Fabrication and characterization of Ppy micropatterns

Fig. 1. Schematic representation of the procedure for Ppy micropattern fabrication in microchannels on the surface of ITO through electropolymerization.

methanol (AR, Zhenxing Chemical Company, Shanghai, China), LiClO4 (AR, Sinopharm Chemical Reagent Co., Ltd., China), ITOcoated glass slides (Nanbo Display Apparatus Technology Co., Ltd., Shenzhen, China), and a PDMS [poly(dimethylsiloxane)] kit including dimethylsiloxane monomer and curing agent (Sylgard 184, Dow Corning,) were used as received. 2.2. Microchannel construction The microchannels were constructed simply by firmly placing an appropriate PDMS mold with microtrenches onto an ITO surface (see Fig. 1). The PDMS molds were prepared following a procedure from the literature [24]. Briefly, dimethylsiloxane monomer and curing agent were mixed at volume ratio of 10:1 under vigorous stirring and then poured onto a mold with microstructure patterns maintained at the bottom of a glass beaker. After releasing the air bubbles trapped in the mixture of dimethylsiloxane monomer and curing agent by tapping the beaker gently, the beaker was transferred into an oven with temperature of 80 ◦ C, and kept for 6 h. After the high temperature curing, PDMS was peeled off the mold, rinsed with methanol thoroughly, and then dried using nitrogen gas. Indium tin oxide (ITO) was cut into small slides (∼0.6 × 2 cm) and cleaned by sonicating in acetone, ethanol, and distilled water, respectively, and finally dried by nitrogen gas. 2.3. Electropolymerization To prepare Ppy micropatterns through microchannel-confined electropolymerization, a typical three-electrode cell system was used. The microchannel forming PDMS/ITO was the working electrode, platinum wire was the counter electrode, and Ag/AgCl was

The procedure for preparation of the Ppy micropatterns is shown schematically in Fig. 1. The ratio of width to depth of the microtrenches on the PDMS mold is important for constructing the microchannels through which the polymerization solution can flow smoothly. In this work, a PDMS mold with averaged microtrench dimensions of 1.2 ␮m and 200 nm in width and depth, respectively, was used. Additionally, the ITO surface is moderately hydrophilic to water and polar organic solvents; hence the polarization solution could flow through the microchannels easily. To achieve appropriate conditions for the electropolymerization of pyrrole on ITO, three different synthetic methods, dynamic potential polymerization (DPP, in which the electrical potential was increased gradually from 0.2 V to 0.8 V during the polymerization), static potential polymerization (SPP, in which the electrical potential was increased rapidly from 0.2 V to 0.8 V at the beginning of the polymerization), and constant current polymerization (CCP, in which the electrical current was kept constant at 1 × 10−4 A during the polymerization) were first investigated. The corresponding cyclic voltammetry (CV) curves of Ppy thin films fabricated through the aforementioned three different electropolymerization procedures, but using the same polymerization solution (0.2 M of pyrrole and 0.5 M of LiClO4 ), were recorded during the polymerization. As depicted in Fig. 2, although a pair of redox peaks appears in each CV curve showing the typical electrochemical activity of Ppy thin film, the CV curves of the thin films obtained from DPP and CCP have relatively less current than that from SPP, implying that the Ppy synthesized through the SPP procedure should have better electrical conductivity. Thus, the SPP procedure was employed for the preparation of Ppy micropatterns in this work. The electrolyte and solvent usually affect the conductivity of the polymer synthesized through electropolymerization [25,26]. In this work, LiClO4 was utilized as an electrolyte, and the mixture of methanol and water was used as a solvent for polymerization. The reason for choosing the mixture of methanol and water as the electropolymerization solvent is that methanol is a good solvent for pyrrole monomer and is widely used for Ppy synthesis, and it has also been demonstrated that Ppy synthesized in methanol has a relatively high electrical conductivity [27]; however, the solubility of LiClO4 in pure methanol is too low to form an electrolyte solution; therefore, a water and methanol mixture was employed for the Ppy electropolymerization. The SPP procedure was chosen for the preparation of Ppy micropatterns in the PDMS/ITO microchannel based on the aforementioned electrochemical properties of Ppy thin film. In a typical process, a reaction solution was used containing 0.05 M of pyrrole monomer and 0.5 M of LiClO4 in the mixture of MeOH/H2 O (8:2,

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Fig. 2. Cyclic voltammetry (CV) curves of Ppy thin films synthesized by (a) dynamic potential (DPP), (b) static potential (SPP), and (c) constant current (CCP) polymerizations. The polymerization solution contains 0.2 M of pyrrole monomer and 0.5 M of LiClO4 in a mixture of MeOH/H2 O (8:2, by volume). The CV’s sweep rate is 0.02 V/s.

in volume). The electropolymerization time was set to 7 min. The supplied electric potential was set to 0.8 V (increased rapidly from 0.2 V to 0.8 V at the beginning of the polymerization) [28]. After the polymerization, the PDMS/ITO electrode was picked out from the polymerization solution, and the PDMS stamp was carefully peeled off the ITO substrate. The Ppy micropatterns formed on the ITO surface were rinsed with distilled water several times and dried at room temperature. The topographic morphologies were characterized using AFM and SEM, as shown in Figs. 3 and 4, respectively. The micropatterns consist of parallel lines. The average width and height of the Ppy were about 1.2 ␮m and 126 nm, respectively, measured from the AFM images. This is consistent with the dimensions

of the microtrenches of the PDMS molds (1.2 ␮m and 200 nm in width and depth, respectively) that we used. To investigate the solvent effect, the micropatterns were fabricated in the methanol/water mixtures with a volume ratio of 6:4, while the polymerization time and the concentrations of pyrrole monomer and electrolyte (LiClO4 ) were maintained at 7 min, 0.05 M and 0.5 M, respectively. Fig. 4a shows a typical SEM image of the as-synthesized Ppy microstructure patterns on the ITO surface. It clearly shows that the dimensions of the Ppy lines are almost the same as those of the Ppy lines synthesized in an 8:2 methanol/water mixture (see Fig. 3). We also prepared the Ppy patterns in the 2:8 methanol/water mixture, and an analogous

Fig. 3. Tapping mode AFM image (height) of Ppy micropatterns on an ITO substrate prepared via static potential polymerization.

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Fig. 4. SEM images of Ppy patterns prepared via SPP on an ITO substrate with polymerization times of 7 (a), and 10 (b) min, respectively.

phenomenon was observed. This suggested that the Ppy could be obtained in methanol/water mixtures with a relatively wide range of methanol to water ratios. Fig. 4b shows the SEM images of Ppy micropatterns synthesized under the same polymerization solution, but with a longer polymerization time of 10 min. From the SEM image, it was found that the lateral dimension of the Ppy lines is the same as that of the lines obtained with shorter polymerization time; however, the packing density of Ppy nanoparticles in each line is relatively higher. This indicates that the dimension of the Ppy lines can be confined by the microchannels. The enlarged SEM images (inset at upper-right in Fig. 4a and b) show that each Ppy line seems to be composed of a large number of Ppy nanoparticles. However, the SEM images show only the topographic morphology of the Ppy, and the “nanoparticles” appearing in the SEM image might be the top ends of Ppy nanorods. Considering that pyrrole polymerization was initiated on the ITO surface within microchannels, we speculate that the Ppy lines consist of either the aggregation of a large number of Ppy nanoparticles [29] or, more probably, closely packed Ppy nanorods that are aligned with their longer axes perpendicularly to the ITO surface [30]. The chemical composition of the as-synthesized Ppy micropatterns was characterized via FT-IR spectroscopy. As shown in Fig. 5, the peak at 780 cm−1 is from the C H out plane bending vibration [31,32], the 1080–1300 cm−1 peaks correspond to the C H in plane vibration, the 1450 cm−1 peak represents the conjugated stretching vibration of C N [33], the 1550 cm−1 peak is attributed to the stretching of C C in the pyrrole rings, and the 3400 cm−1 peak corresponds to the N H stretching vibration. The results

Table 1 The resistances (unit, ) of as-synthesized micropatterns and the continuous Ppy thin films. Samples Resistances of patterns Resistances of continuous thin films

1 280 573

2 281 679

3 362 413

4 400 413

5 438 1170

unambiguously show that the micropatterns are composed of Ppy. 3.2. Conductivities of the Ppy patterns The as-prepared micropatterns could be peeled off when they were pressed against with Scotch tape, elastomer, and even glass slides. In this work, to avoid any influence of the ITO on the electric conductivity measurement of the Ppy micropatterns, the Ppy micropatterns were transferred from ITO onto the surface of an insulated glass slide via simple contact adhesion [34]. For electric conductivity measurement, a four-point probe was carefully placed on the Ppy micropattern surface, and three different sites of each sample were chosen to determine the electrical resistances. For comparison, the electrical resistance of the surrounding continuous thin films was measured at the same time. The detailed electrical resistance data of five samples prepared under the same conditions are shown in Table 1. All the listed resistance data are the averaged data obtained from three different sites. From the data, we found that electrical resistances varied with the samples, which might be due to the four-point probe setup alignment, but in all cases, the resistances measured on the Ppy micropatterns are smaller than those of the surrounding continuous Ppy films. A similar phenomenon was observed by another research group [35]. This is not fully understood at the moment, but we think that the order and packing density of the Ppy nanorods in the micropatterns might play an important role in their electrical conductivities. 4. Conclusion

Fig. 5. FT-IR spectrum of a Ppy micropattern.

In summary, micropatterns composed of parallel Ppy lines have been successfully prepared through area-selected in situ electropolymerization confined within microchannels. The topographic morphologies, chemical compositions, and electrical conductivities of the as-synthesized micropatterns were studied systematically. We found that the Ppy micropatterns had smaller electrical resistances than the continuous thin film of Ppy prepared under the same conditions. The well-arranged pattern structure and relatively high electrical conductivity make the micropattern possible for low-cost organic electronic device applications. This synthetic

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strategy should be general and applicable to the preparation of micropatterns of other conductive polymers. Acknowledgements This work was supported by the National High Technology Research and Development Program (863 program) of China (No. 2006AA04Z309), the National Basic Research Program (973 program) of China (No. 2007CB936000) and the National Natural Science Foundation of China (Nos. 20774029 and 20873084) References [1] P. Wang, R.E. Lakis, A.G. MacDiarmid, Thin Solid Films 516 (2008) 2341. [2] A. Dodabalapur, J. Laquindanum, H.F. Katz, Z. Bao, Appl. Phys. Lett. 69 (1996) 4227. [3] Z. Huang, P.C. Wang, J. Feng, A.G. MacDiarmid, Synth. Met. 85 (1997) 1375. [4] M. Xue, Y. Zhang, Y. Yang, T. Cao, Adv. Mater. 20 (2008) 1. [5] J. Bargon, W. Behnck, T. Weidenbruck, T. Ueno, Synth. Met. 41 (1999) 1111. [6] S.H. Magnus, P. Kyreklev, O. Inganas, Adv. Mater. 8 (1996) 405. [7] A. Morrin, O. Ngamnac, E. O’Malley, N. Kent, S.E. Moulton, G.G. Wallace, M.R. Smyth, A.J. Killard, Electrochim. Acta 53 (2008) 5092. [8] J. Lim, C.A. Mirkin, Adv. Mater. 14 (2002) 1474. [9] W. Choi, O. Park, Curr. Appl. Phys. 6 (2006) 695. [10] T. Granlund, T. Nyberg, L.S. Roman, M. Svenson, O. Inganas, Adv. Mater. 12 (2000) 269. [11] S. Holdcroft, Adv. Mater. 13 (2001) 1753. [12] Z. Huang, P. Wang, A.G. Macdiarmid, Y. Xia, G.M. Whitesides, Langmuir 13 (1997) 6480.

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