Langmuir 2002, 18, 7253-7257
7253
Micrometer to Nanometer Patterns of Polypyrrole Thin Films via Microphase Separation and Molecular Mask Imsun Seo,† Myoungho Pyo,‡ and Gyoujin Cho*,† Department of Chemical Engineering and Nanotechnology Center, Sunchon National University, 315 Maegok Sunchon, Chonnam 540-742, Korea, and Department of Chemistry and Nanotechnology Center, Sunchon National University, 315 Maegok Sunchon, Chonnam 540-742, Korea Received February 28, 2002. In Final Form: July 4, 2002 We report a new method to control both the nucleation and growth of polypyrrole (Ppy) for the formation of the patterns from micrometer to nanometer sizes using adsorbed sodium dodecyl sulfate bilayers as a molecular mask. Polystyrene (PS) and poly(4-vinylpyridine) blend and block poly(styrene-b-4-vinylpyridine), which have microphase-separated domains in micrometer and nanometer scales, respectively, were used as templates. With the molecular mask, the resulting Ppy can be selectively grown up to 50 nm only along the PS domains.
Introduction Conducting polymers are being used to replace both conventional metals and inorganic materials in a variety applications, especially in microelectronic devices.1 To be effectively applied in microelectronic devices, the conducting polymers must be easily fabricated into micro- to nanoscale patterns. However, the construction of conducting polymer-based microelectronic devices has been impeded by the technical challenges, mainly due to the intractability of conducting polymers.2 Although it has been claimed that processibility of conducting polymers can be improved by using novel monomers, judiciously chosen dopants, and colloid systems,3,4 the selective deposition of conducting polymers in micro- to nanometer scales is still far from being satisfactory. Techniques for the patterning of conducting polymers have been recently reviewed.5 For the formation of micropatterns of conducting polymers, photolithography,6 e-beam writing,7 laser writing,8 surface-templated deposition,9 modified microcontact printing,10 screen printing,11 and inkjet printing12 have been successfully employed. However, photolithographic, e-beam writing, and laser † Department of Chemical Engineering and Nanotechnology Center. ‡ Department of Chemistry and Nanotechnology Center.
(1) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. H. Handbook of conducting polymers, 2nd ed.; Dekker: New York, 1998; Chapter 32. (2) Reynolds, J. R.; Pomerantz, M. In Eletroresponsive Molecular and Polymeric Systems; Skotheim, T., Ed.; Marcel Dekker: New York, 1991; Vol. 2, p 187. (3) Nguyen, M. T.; Leclerc, M.; Diaz, A. F. Trends Polym. Sci. (Cambridge, U.K.) 1995, 3, 186. Joo, J.; Lee, J. K.; Hong, J. K.; Baeck, J. S.; Epstein, A. J.; Jang, K. S.; Suh, J. S.; Oh, E. S. Macromolecules 1998, 31, 1. 479. McCarthy, G. P.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 1997, 13, 3686. (4) Cho, G.; Fung, B. M.; Glatzhofer, D. T.; Lee, J. S.; Shul, Y. G. Langmuir 2001, 17, 456. (5) Holdcroft, S. Adv. Mater. 2001, 13, 1753. (6) Bargon, J.; Behnck, W.; Weidenbrueck, T.; Ueno, T. Synth. Met. 1991, 41, 1111. (7) Magnus Persson, S. H.; Kyreklev, P.; Inganas, O. Adv. Mater. 1996, 8. 405. (8) Abdou, M. S. A.; Xie, Z. W.; Leung, A. M. Synth. Met. 1992, 52, 159. (9) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Chem. Mater. 1995, 7, 526. Rzsnyai, L. F.; Wrighton, M. S. Langmuir 1995, 11, 3913. (10) Huang, Z.; Wang, P. C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. M. Langmuir 1997, 13, 6480. (11) Garnier, F.; Hadjlaoui, R.; Yasser, A.; Srivastava, P. Science 1994, 265, 1684. (12) Hebner, T. R.; Sturm, J. C. Appl. Phys. Lett. 1998, 73, 1775.
writing techniques involve irradiation and solvent dissolution processes that may deteriorate physical properties of conjugated polymers. One of the proper ways to circumvent this problem is based on a bottom-up method13 in which molecular-scale or nanometer-scale components are self-assembled or chemically synthesized into nanosized structures with desired patterns. Since the bottomup method does not include any etching and dissolution processes, the conducting polymers can retain the desirable electronic properties. In this approach, the nucleation and growth of conducting polymers are regioselectively controlled on the designated surface. The surface-template deposition and modified microcontact printing techniques are typical examples of the bottom-up method, although the patterned features are usually in micrometer ranges. To attain nanopatterns of conducting polymers, two techniques using atomic force microscopy (AFM)14,15 and microphase separated block copolymer as templates have been reported.16 In the AFM method, nanopatterns of conducting polymers can be formed because the deposition of conducting polymers is localized on the patterns traced by the AFM tip. In the method of using microphaseseparated block copolymers as templates, the conducting polymer is regioselectively nucleated and grown along one domain of the diblock copolymers because of the surface energy difference between two domains of the microphaseseparated block copolymer. However, those methods still have some problems to be addressed: it is difficult to form complex nanopatterns using the AFM method, and the process is relatively expensive. The use of block copolymers as templates was successful for two-dimensional nanopatterning, but we found that the influence of the structured surface morphology of the block copolymer becomes weaker as the structures build up, so that the boundaries between patterns become coalesced gradually as the thickness increased. In view of these problems, techniques that can control feature sizes in the desired micrometer to nanometer ranges through simple and inexpensive processes are highly demanded to keep pace with emerging nanotechnology. (13) Cobden D. H. Nature 2001, 409, 32. (14) Cai, Z. W.; Gao, J. S.; Xie, Z. X.; Tian, Z. Q.; Mar, B. W. Langmuir 1998, 14, 2508. (15) Maynor, B. W.; Filocamo, S. F.; Grinstaff, M. W.; Liu, J. J. Am. Chem. Soc. 2002, 124, 522. (16) Ishizu, K.; Honda, K.; Kanbara, T.; Yamamoto, T. Polymer 1994, 35, 4901. Goren, M.; Lennox, R. B. Nano Lett. 2001, 1, 735.
10.1021/la025685q CCC: $22.00 © 2002 American Chemical Society Published on Web 09/06/2002
7254
Langmuir, Vol. 18, No. 20, 2002
Letters
Scheme 1. Schematic Diagram for the Polypyrrole Patterning via Molecular Masks
Theoretically, patterns of conducting polymers can be obtained if the nucleation and growth are allowed in specific regions but prevented in other regions. If one can construct masks to prevent the nucleation and growth selectively in designated regions, the patterns will form in the unmasked regions. To apply this concept to the formation of conducting polymer patterns, we employ microphase-separated polymer blends or block copolymers as templates, which allow the formation of the masks only on designated regions. While polymer blends can be used to generate the phase-separated morphology of a micrometer scale, block copolymers enable the construction of masks in a nanometer scale. For templates of micrometer and nanometer scales, blends of polystyrene (PS)/ poly(4-vinylpyridine) (4PV) and block copolymers of poly(styrene-b-4vinylpyridine) (PS-b-4PV) were used, respectively. A common feature for both polymer blends and block copolymers is that the two kinds of phaseseparated domains impart different properties on the surface so that the masks can be formed regioselectively via favored interactions with a specific domain. Using simple coating methods, it is easy to attain thin films with heterogeneous surfaces from polymer blends. However, to obtain microphase-separated nanodomains with heterogeneous surfaces, the nanodomains should be oriented normal to the surface of block copolymer films. To obtain microphase-separated nanodomains normal to the surface of block copolymer films, it is necessary to generate neutral surface using random copolymers anchored to the substrate,17 casting the films with a thickness less than Lo (equilibrium period),17 or by using substrates with low surface energy such as self-assembled hydrocarbon monolayer.18 In this work, carbon-coated mica disks were used as substrate to attain the low surface energy. We chose polypyrrole (Ppy) as a standard conducting polymer for this study since it is stable19 under ambient conditions and has a wide range of applications.20 It is known that, depending on the ionic properties, surfactant bilayers adsorbed on substrates can act as a two(17) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. J. Science 1997, 275, 1458. Huang, E.; Rockford, L.; Russell, T. P.; Hawker, C. J.; Mays, J. Nature 1998, 395, 757. (18) Heier, J.; Kramer, E. J.; Walheim, S.; Krausch, G. Macromolecules 1997, 30, 6610. (19) See, for example, the Proceedings of the 1992 International Conference on Synthetic Metals (ICSM ’92) Synth. Met. 1993, 55-57. (20) Miksa, B.; Slomkowski, S. Colloid Polym. Sci. 1995, 273, 47. Aquino-Binag, C. N.; Kumar, N.; Lamb, R. N. Chem. Mater. 1996, 8, 2579.
dimensional medium either to promote the growth of Ppy4,21 or to prevent its nucleation and mask its growth.22 When sodium dodecyl sulfate (SDS) is adsorbed and formed bilayers on the substrate, Ppy cannot grow on the substrate;22 when cetylpyridinium chloride (CPC) is used instead of SDS under the same conditions, Ppy can form ultrathin films.4 On thin films of polymer blend (PS/4PV) or block copolymer (PS-b-4PV) coated on a substrate, the pyridine rings in 4PV can be easily protonated to become positively charged so that SDS will be strongly adsorbed on the protonated 4PV units through the ionic interaction between the negatively charged headgroup of SDS and positively charged 4PV units and form the bilayers while the monolayers will be formed on PS units through the weak van der Waals interaction between hydrophobic PS surfaces and the tail of SDS. The adsorbed SDS bilayers act as a molecular mask, and the hydrophobic domain (PS) can attract pyrrole from the medium to form Ppy patterns (Scheme 1). Experimental Section For polymer blend templates, a mixture of PS (Mw ) 350 000 g/mol, Aldrich) and 4PV (Mw ) 60 000 g/mol, Aldrich) with unit molar ratio of 1:1 was dissolved in chloroform. The solution was spun-cast on a freshly peeled mica disk to form an approximately 100 nm polymer film. Phase separation was achieved by annealing the film at 150 °C under nitrogen gas for 5 h. For copolymer templates, poly(styrene-b-4-vinylpyridine) (PS-b-4PV), with Mw of 58 000 mol/g and PD of 1.1, was prepared using anionic polymerization.23 The ratio of PS/4PV was 0.95 calculated from integrating the 1H NMR peaks of PS and 4PV. PS-b-4PV was dissolved into chloroform (1 mg/mL), and the films were cast on a carbon-coated mica disk. The disk was dipped into solution, solvent was evaporated under ambient condition, and the dried disk was annealed under nitrogen at 150 °C for 3 h. The overall phase domain morphologies of microphase separated PS/4PV blend and PS-b-4PV on the disk were determined by applying contact AFM (Park Scientific Autoprobe CP) using a silicon nitride tip with a spring constant of 0.05 N/m. The thin films were treated with 0.1 M of hexanoic acid solution for 30 s to protonate the 4PV sites. The acid-treated PS/4PV blend and PS-b-4PV films were rinsed thoroughly with deionized (21) Cho, G.; Glatzhofer, D. T.; Fung, B. M.; Yuan, W. L.; O’Rear, E. A. Langmuir 2000, 16, 44424. Yuan, W. L.; O’Rear, E. A.; Cho, G..; Funkhouser, G.. P.; Glatzhofer, D. T. Thin Solid Films 2001, 385, 96. (22) Funkhouser, G. P.; Arevalo, M. P.; Glatzhofer, D. T.; O’Rear, E. A. Langmuir 1995, 11, 1443. (23) Thurmond, K. B.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1997, 119, 6656.
Letters
Langmuir, Vol. 18, No. 20, 2002 7255
Figure 2. AFM images for the regioselectively grown polypyrrole thin films on PS domain: (A) for topological surface image; (B) for 3-D image of (A). masked on PS-b-4PV, the presence of Ppy was confirmed by measuring the conductivity increases. A conventional four-probe method was utilized.
Results and Discussion
Figure 1. AFM images for the phase-separated film of PS/ 4PV blend: (A) for the topological surface image with crosssectional analysis; (B) for the 3-D image of (A); (C) for the image after selective dissolution of 4PV. water and dried at room temperature. The acid-treated disks were placed vertically in 4 dram vials and covered with 10 mL of the solution containing 0.04 mM of SDS. The vials were placed in a bath at 25 °C for 1 h. Then, 6 µL of pyrrole was added into the vial. After another hour, an equimolar amount of ammonium peroxydisulfate (based on the amount of pyrrole) was added as a concentrated solution. After a designated reaction time (4 h for PS/4PV blend and 5 min to 1 h for PS-b-4PV after adding ammonium peroxodisulfate), the disks were removed, rinsed with distilled water to wash off the residues of adsorbed SDS, and air-dried overnight. The surface topology changes after Ppy deposition were compared using contact AFM. The electroactivity of Ppy on SDS-masked PS/4PV blend was also investigated to confirm that the topographic changes (obtained from AFM) are originated from Ppy deposition. To study the electroactivity, Au was sputter-coated on Ppy and Au-coated Ppy layers were detached from SDS-masked PS/4PV after immersing them in toluene at 80 °C for 1 h. Cyclic votammograms were obtained using a BAS CV-50W potentiostat. For Ppy deposited on SDS-
Parts A and B of Figure 1 show the topographical and three-dimensional AFM images of the large-scaled phaseseparated thin films of the PS/4PV blend with its crosssectional analysis. The spinodal structures24 are homogeneously dispersed on the surface with typical dimensions of 50 nm in height and 1-2 µm in diameter. The spinodal structures were observed only when the unit molar ratio of PS to 4PV was 1:1. To find out whether the spinodal structures are 4PV or PS, the PS/4PV-coated disk was immersed into a mixed solution of pyridine/tetrahydrofuran (volume ratio 2:1) for 30 min, and the surface morphology of the resulting disk was re-examined using AFM. As shown in Figure 1C, the spinodal structures were almost all gone, with only some traces left. Because 4PV is selectively dissolved in the mixed solvent, we conclude that the protruded spinodal structures are 4PV domains, and the flat surface is the PS domains. As another test, a PS/4PV-coated disk was dipped in a 1 N hexanoic acid solution for 30 min. Because 4PV can react with the carboxylic acid groups of hexanoic acid to form pyridinium hexanoate moieties, the contact angle changed dramatically from 150° to 80°. In contrast, PS films alone showed a contact angle of 160° before and after the acid treatment. These results confirm that the protruded spinodal structures on the surface are indeed 4PV domains, and the chemical nature of surface is heterogeneous. Treating the PS/4PV blend samples with hexanoic acid did not cause morphological changes (the AFM images are not shown). After the acid-treated disks were immersed into the surfactant solution for 1 h, the hexanoate anions are replaced by the dodecyl sulfate anions, which can form (24) Karim, A.; Douglas, J. F.; Nisato, G.: Liu, K. W.; Amis, E. J. Macromolecules 1999, 32, 5917.
7256
Langmuir, Vol. 18, No. 20, 2002
Letters
Figure 3. Cyclic voltammograms of Ppy deposited on SDSmasked PS/4PV in a 0.1 M NaCl aqueous solution at (A) 25, (B) 50, (C) 100, (D) 200, and (E) 400 mV s-1.
Figure 5. AFM images for Ppy nanopatterns: (A) 5 × 5 µm2, grown for 5 min; (B) 10 × 10 µm2, grown for 10 min; (C) 2 × 2 µm2, grown for 10 min; (D) cross-sectional analysis of image C.
Figure 4. AFM images of PS-b-4PV film (A) before and (B) after acid treatment.
bilayers (Scheme 1). After the addition of pyrrole into the solution followed by ammonium peroxydisulfate, the oxidative polymerization of pyrrole was initiated. The solution slowly turned black, and precipitated Ppy particles were observed. After 1 h, the disks were gently rinsed with distilled water and dried under an ambient condition overnight. The characterization of the morphology of the Ppy film formed on the PS/4PV-coated disks was undertaken using AFM, and the topological surface and threedimensional AFM images are presented in Figure 2. In all regions, the protruded spinodal structures were all buried by the Ppy deposition onto the film. These structures are reproducible and are not due to contamination by impurities. Furthermore, from diffuse reflectance IR (Shimazu 7100) on the Ppy patterned sample, the characteristic Ppy stretching bands were detected at 3300 cm-1 (NH stretching) and 1500 cm-1 (CdC stretching). The presence of Ppy was further confirmed from cyclic voltammograms. Figure 3 demonstrates the current responses of surface-deposited materials, which causes the topographical changes, during potential cycling at various scan rates. The voltammograms show typical
behaviors of Ppy, including scan rate dependence, indicative of the selective deposition of Ppy.25 The observed morphology can be explained by the regioselective growth of Ppy only onto the PS domains, but not onto the 4PV domains, because of the following consideration. Initially, SDS is adsorbed on the PS domains by the interaction of the hydrophobic tail to the PS surface to form monolayers, but the interaction is weak, and the adsorbed SDS can be replaced by pyrrole from the aqueous phase. Because pyrrole is attracted to the PS domains due to low surface energy, Ppy can preferentially nucleate and grow on the PS domains after the polymerization is initiated. On the contrary, the bilayers of SDS formed on the positively charged surface of the 4PV domains are quite sturdy and would not be replaced by pyrrole molecules; consequently they can act as masks to prevent the nucleation and growth of Ppy22 on the spinodal structures. The same procedure can be used to form nanopatterns of Ppy, except that microphase-separated PS-b-4PV instead of PS/4PV blend is used as templates. As determined from AFM studies (Figure 4A), the nanodomains of 4PV in PS-b-4PV appear as lamellar domains with 55 (25) Pyo, M.; Reynolds, J. R. J. Phys. Chem. 1995, 99, 8249. Diaz, A. F.; Castillo, J. I.; Logan, J. A.; Lee, W.-Y. J. Electroanal. Chem. 1981, 129, 115.
Letters
nm interspacing,26 which is slightly larger than the theoretically calculated27 nanodomain size of 36 nm due to the artifact of the AFM tip. The thickness of PS-b-4PV films on the surface of carbon-coated mica disks was determined to be 20 nm using R-step and contact AFM. After the acid treatment, the nanodomains were swollen and showed 80 nm interspacings (Figure 4B). From the cross-sectional analysis of AFM images of panels A and B of Figure 4, the mean height of protruded units (most likely 4PV domains) was 2.22 and 2.64 nm, respectively, before and after acid treatment. The protruded domains were raised by 0.42 nm because of the reaction of hexanoic acid with the pyridine units. The acid-treated PS-b-4PV films were immersed into aqueous phases containing SDS and pyrrole. The immersion times after the initiation of polymerization ranged from 10 to 60 min, and the temperature was kept at 25 °C. After the sample was taken out from the aqueous solution, it was rinsed with distilled water. For a relatively short reaction time (5 min), both bare nanodomains of PS-b-4PV films (Ra in Figure 5A) and Ppy-grown regions (Rb in Figure 5A) can be seen in the AFM image. For a longer reaction time (10 min), distinct nanopatterns are observed (panels B and C of Figure 5). By comparison of the images in Figure 5A and Figure 5B, it is reasonable to conclude that Ppy starts to nucleate along the nanodomains of PS and completely nucleate onto all of the PS nanodomains after about 10 min. The reason for the regioselective nucleation of Ppy along the PS block is that the SDS bilayers adsorbed on the 4PV domains act as molecular masks and prevent the nucleation and growth (26) Heier, J.; Kramer, I. J.; Walheim, S.; Krausch, G.. Macromolecules 1997, 30, 6610. Li, Z.; Zhao, W.; Liu, Y.; Rafailovich, M. H.; Sokolov, J. J. Am. Chem. Soc. 1996, 118, 10892. (27) Hashimoto, T.; Shibayama, M.; Kawai, H. Macromolecules 1980, 13, 1237.
Langmuir, Vol. 18, No. 20, 2002 7257
of Ppy. However, at longer reaction times, Ppy can overgrow and cover all the nanometer-scale domains, because there is no favored growth direction to the decreased interfacial energy so that Ppy can grow along any direction. As a result, the features of patterning become blurred after 20 min of growth time and are completely lost after 30 min. On the basis of the crosssectional analysis of the AFM image (Figure 5D), the height of Ppy patterning is estimated to be about 8 nm with a width of 60 nm for samples prepared with 10 min of growth. The conductivity for Ppy nanopatterns grown for 10 min was also investigated by a conventional fourprobe method. The conductivity of 33 S cm-1 was obtained. Although direct comparison of the conductivity with previously reported values is impossible, this suggests that Ppy was deposited along the PS domains to form a continuous phase. It should be mentioned that the conductivity of Ppy-free PS-b-4PV could not be measured with our experimental setup due to high resistivity. In summary, we have demonstrated a new method to fabricate patterns of Ppy films with micrometer to nanometer scales using regioselectively adsorbed SDS bilayers as molecular masks to prevent the nucleation and growth of Ppy. The microphase-separated films of polymer blend and block copolymer were adopted as micrometer and nanometer scale templates, respectively, to control the adsorption of SDS from aqueous phase. These results open up possibilities of simple and inexpensive patterning processes not only for organic polymers but also for inorganic materials. Acknowledgment. This work was supported by Korea Research Foundation (KRF-2000-041-D00213) for which we are grateful. LA025685Q
