Longitudinal and Lateral Integration of Conducting Polymer Nanowire

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Longitudinal and Lateral Integration of Conducting Polymer Nanowire Arrays via Block-Copolymer-Templated Electropolymerization Hideaki Komiyama, Motonori Komura, Yuka Akimoto, Kaori KAMATA , and Tomokazu Iyoda Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 05 Jun 2015 Downloaded from http://pubs.acs.org on June 8, 2015

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Chemistry of Materials

Longitudinal and Lateral Integration of Conducting Polymer Nanowire Arrays via Block-Copolymer-Templated Electropolymerization Hideaki Komiyama,*,†,‡ Motonori Komura,† Yuka Akimoto,‡ Kaori Kamata,†,‡ Tomokazu Iyoda*,†,‡ †Division of Integrated Molecular Engineering, Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. ‡Iyoda Supra-Integrated Material Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency (JST), 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. KEYWORDS Block Copolymer, Chemical template, Conducting polymer, Electropolymerization, Nanowire array, Integrated nanostructures

ABSTRACT: Pyrrole (Py) and thiophene derivatives were electropolymerized on a microphase-separated block copolymer [PEO-bPMA(Az)]-coated ITO electrode in order to form corresponding conducting polymer nanostructures of perpendicularly oriented and hexagonally arranged cylinders as a template without the removal of one of the domains. Polypyrrole (PPy) nanowires grew through PEO cylindrical domains oriented perpendicular to the electrode to afford an array with an aspect ratio of up to 140 and a density of up to 4.4 × 1012 wires/in.2. Long, straight π-stacked crystalline structures assigned to PPy main chains were observed in high-resolution TEM images of the individual wires. Additional elaborately crafted electropolymerization was demonstrated on a nanometer scale: (i) Stepwise electropolymerization of Py and 3,4-ethylenedioxythiophene (EDOT) afforded segmented PPy-poly(3,4-ethylenedioxythiophene) (PEDOT) and PEDOT-PPy wires with nanoheterojunctions and (ii) one-pot electropolymerization of a mixture of Py and 2,2′-bithiophene (BiTh), polymerized selectively and non-selectively in the PEO cylindrical domains, gave laterally composition-modulated structures, i.e., a PPy-poly(2,2′-bithiophene) (PBiTh) copolymer [P(Py-co-BiTh)] in the PEO cylindrical domains and PBiTh in the surrounding PMA(Az) domains. Both longitudinally segmented and laterally mosaic nanostructures of the conducting polymers can be fabricated just by utilizing the relative solubility of the monomers in the individual domains, herein called a chemical affinity template.

Introduction Nanostructured conducting polymers have drawn much attention in a wide range of research fields as functional molecular materials whose properties can be engineered directly from their molecular design. Such nanostructures allow one to control intrinsic molecular properties such as conductivity, which is normally hidden by the entangled molecular morphology in the bulk material. Therefore, conducting polymer materials with intrinsic properties that are made available by controlling the nanostructure are of particular interest for applications such as electrochromic devices,1, 2 energy storage,3, 4 sensors,5 field emission devices,6 actuators,7 and photovoltaics.8, 9 Nanostructured conducting polymers have been fabricated so far using template-free synthesis,10-12 soft-template synthesis using surfactants,13-15 micelles,16, 17 DNA,18, 19 and hard-template synthesis using track-etched membranes,20-22 anodized aluminium oxide (AAO) membranes,23-30 and block copolymer templates.31-33 In the block copolymer templating process, conducting polymer nanorods are fabricated by selective polymerization in one of the microphase-separated domains. The electropolymerization basically employs porous cylinders that are prepared by additional treatment for removal of one of the domains.34 Kim and co-workers have demonstrated electropolymerization of Py, EDOT, and 3-hexylthiophene in porous poly(styrene-b-methyl methacrylate) (PS-bPMMA).31 Kuila and Stamm reported the fabrication of a conduct-

ing polyaniline nanorod array by electropolymerization using a porous poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) thin film.32, 33 The nanorods were 10 nm in diameter with a 25 nm periodicity. 32, 33 32, 33 32, 33 32, 33 In the chemical polymerization procedure, an amphiphilic diblock copolymer thin film without etching of one of the domains is generally used as a template.35-37 Ishizu et al. fabricated PPy selectively in horizontally oriented poly(2-vinylpyridine) lamellar domains in poly(styrene-b-2-vinylpyridine).35 In previous studies, we developed amphiphilic liquid-crystalline block copolymers, PEOm-b-PMA(Az)n, whose thin films have anomalous orientation-defined microphase-separated nanostructures, i.e., hexagonally arranged PEO cylindrical domains surrounded by PMA(Az) domains perpendicular to the surfaces of various kinds of substrates such as silicon wafers, glass, polymers, metals, and so on just after thermal annealing (chemical structure is shown in Figure S1, Supporting Information).38-41 Note that no surface pretreatment, such as surface neutralization to adjust the surface energy between the substrate surface and the polymer,42 is required. Another characteristic is that the cylindrical domains are not physical pores; instead they are filled with under-cooled PEO at room temperature. PEOcontaining block copolymers directly work as soft template.43 We utilized the PEOm-b-PMA(Az)n template to fabricate nanoscale functional materials by using the PEO cylindrical domains as ethereal permeable channels.44-47

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In the current study, we attained ultrafine fabrication of conducting polymer nanostructures through PEO-filled cylindrical domains of a microphase-separated PEOm-b-PMA(Az)n template by electropolymerization. We investigated (1) the domain selectivity of electropolymerization in the template, (2) the growth behavior of PPy nanowires via current–time (i–t) curves and field emission scanning electron microscopic (FE-SEM) images, (3) the controllability of the size of the PPy nanowire array, and (4) the microscopic and spectroscopic characteristics and electrical conductivity of the PPy nanowire array. Lastly, additional hierarchically integrated nanostructures of the conducting polymer nanowire array, i.e., longitudinally segmented PPy-PEDOT and PEDOT-PPy nanowire arrays and laterally mosaic nanostructures of P(Py-co-BiTh) and PBiTh composites, were fabricated by stepwise and one-pot electropolymerization methods, which has never been demonstrated so far, by using a block copolymer template. Results and Discussion Electropolymerization via block-copolymer-coated electrode First, let us discuss the electropolymerization by using ITO electrodes coated with the homopolymers PEO and PMA(Az) as segments of the block copolymer, PEOm-b-PMA(Az)n. Py, BiTh, and EDOT were electropolymerized in propylene carbonate (PC) under potentiostatic conditions at 1.10 V, 1.35 V, and 1.40 V vs. Ag/AgCl, respectively, on the PEO114- and PMA(Az)139-coated ITO electrodes (430 nm and 130 nm thick, respectively). Figure 1a shows the i–t curve for the electropolymerization of Py. A large anodic current flew in the PEO114-coated electrode, whereas very little current flew in the PMA(Az)139-coated electrode. The PEO114-coated electrode turned black, but little color change was observed for the PMA(Az)139-coated electrode. Py penetrated into the PEO114 film and was electropolymerized on the electrode surface to give a PPy/PEO composite, whereas no electropolymerization occurred in the PMA(Az)139 film due to the low solubility of Py. However, electropolymerization of BiTh proceeded in both the PEO114- and PMA(Az)139-coated electrodes. A similar magnitude of anodic current was observed in both electrodes, probably due to the similar solubility of BiTh in both the PEO and PMA(Az) films (Figure 1b). Also, both electrodes turned black, suggesting the formation of PBiTh/PEO and PBiTh/PMA(Az) composites, respectively. The i–t curve of electropolymerization of EDOT behaved as an intermediate between the Py and BiTh cases. A larger current was observed in the PEO114-coated electrode, whereas less than half that current was observed in the PMA(Az)139-coated electrode (Figure. 1c). The

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resulting PEO114-coated electrode looked darker than the PMA(Az)139-coated electrode. PEDOT grew more in the PEO114 film than in the PMA(Az)139 film. The above observations demonstrate electropolymerization of Py selectively in the PEO domains of the PEOm-b-PMA(Az)n template and electropolymerization of BiTh non-selectively in both the PEO and PMA(Az) domains (Figure 1d). A EDOT was found to show higher chemical affinity to PEO than PMA(Az) homopolymers from the resulting current intensities, meaning that PEDOT will give higher density distribution in the PEO domains of PEOm-b-PMA(Az)n template. Next, PEO114-b-PMA(Az)67 was spin coated onto an ITO electrode for the electropolymerization of Py, EDOT, and BiTh (characterization of the polymer is described in Figure S1, Supporting Information). Thermal annealing at 140 °C for 6 h gave the characteristic microphase-separated nanostructure, in which the hexagonally arranged PEO cylindrical domains were perpendicularly aligned and surrounded by the PMA(Az) domains in a smectic liquid crystalline phase,48 which was confirmed by atomic force microscope (AFM) observation and grazing incidence small angle X-ray scattering (GISAXS) measurement (Figure S2, Supporting Information). The linear sweep voltanmograms of these monomers were shown in Figure S3, Supporting Information. The electropolymerizations were carried out under potentiostatic conditions. Figure 2 shows crosssectional FE-SEM images of the PEO114-b-PMA(Az)67-coated ITO electrodes after electropolymerization of Py, EDOT, and BiTh. In case of electropolymerization of Py and EDOT (Figure 2a,b), bright stripes are found across the template films, whereas no stripes are observed in case of BiTh (Figure 2c). The top surfaces of the templates are covered with fluffy sediments in the Py and EDOT cases. After electropolymerization of Py to fill within PEO cylindrical domains (detail is described in the next section), the template was well rinsed with chloroform and toluene so as to dissolve and characterize the template by 1H-NMR and ultraviolet-visible (UV-vis) spectroscopy. A 98% of PEO114-b-PMA(Az)67 was recovered from the wash solutions (Figures S4 and S5, Supporting Information). After removal of the template by dissolving it with toluene, a fibrillar structure remained on the electrode (Figure 3a). The diameter was quite similar to that of PEO cylinder. The remaining fibril was identified as PPy by Raman spectroscopy (Figure 3b and Table S1, Supporting Information). Raman spectrum of the fibril is reasonably consistent with that of PPy bulk film synthesized by electropolymerization without the template, including the characteristic vibrations of C=C stretching (1581 cm-1) and antisymmetrical C-N stretching (1384

Figure 1. i–t curves for electropolymerizations of (a) Py, (b) BiTh, and (c) EDOT on PEO114- (black) and PMA(Az)139- (red) coated ITO electrodes. (d) Schematic illustration of selectivity of monomers of Py, EDOT, and BiTh to PEO and PMA(Az) homopolymers.


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Figure 2. Cross-sectional FE-SEM images of the PEO114-bPMA(Az)67 templates after electropolymerization of (a) Py, (b) EDOT, and (c) BiTh under potentiostatic conditions of 1.10 V, 1.40 V, and 1.35 V (vs. Ag/AgCl), respectively. These samples were observed without a metal coating. Scale bars indicate 200 nm.

Figure 3. (a) AFM phase image of the PPy nanowires after removal of the template with toluene. (b) Raman spectra of the corresponding PPy nanowires (red) and PPy bulk film synthesized by electropolymerization without the template (black).

cm-1),49, 50 apart from a minor difference in intensity ratio probably due to a difference in transition moment between the fibril and the bulk film. We therefore conclude here that the bright stripes seen in the FE-SEM image should be assigned to the PPy nanowires. In Figure 3a, PPy nanowires were agglomerated, due to capillary forces working in adjacent nanowires during solvent evaporation, as is known to occur in the AAO template processes.51 We also confirmed the formation of the PEDOT nanowires and PBiTh thin film in the template (Figure S6 and Table S1, Supporting Information). PPy and PEDOT grew selectively through the PEO cylindrical domains of the template and then over-grew to cover the whole template surface. On the other hand, PBiTh grew indiscriminately in both the PEO and PMA(Az) domains. Such nonselective domain electropolymerization proceeded in the cases of N-methylpyrrole, N-ethylpyrrole, 3-heptylpyrrole, 3-octylpyrrole, 3-hexylthiophene, and 3,4′-dihexyl-2,2′-bithiophene in this study. It is thus apparent that Py and EDOT are PEO domain-selective monomers, whereas BiTh and the others are not. Unfortunately, it is impossible to fabricate PBiTh

nanowire, expectable for optoelectronic applications,31 by the current templating system. Also we have not yet found any PMA(Az) domain-selective monomers as the opposite case of Py and EDOT. However, it can be noted here that the PEO-b-PMA(Az) film on the electrode worked as a “chemical affinity” template, which is based on the relative solubility of the monomer in the PEO and PMA(Az) domains. Especially, the characteristics of our method is selectivity to the PEO domain, leading to fabrication of nanocylindrical features. Since the PEO is known as aprotic polar solvent in organic synthesis, a rule of the monomers exhibiting the PEO selectivity was closely associated to their solubility to polar solvents. The present “chemical affinity” results from the PEO and PMA(Az) domains connecting the electrode surface with the electrolytic solution containing the concerned solutes such as the monomers used here, which are quite different from conventional porous masks such as track-etched membranes,20-22 AAO membranes,23-30 and porous block copolymer templates.31-33 There are two advantages of this “chemical affinity” template: First, pore fabrication is not needed. Second, various affinity-dependent electropolymerization processes can be designed, as will be demonstrated in the last section. Growth behavior of PPy nanowires in the template Figure 4a shows the i–t curve for the electropolymerization of Py using the PEO114-b-PMA(Az)67-coated ITO electrode. The oscillatory i–t curve includes four characteristic stages, marked as I–IV, that are quite different from that of a bare ITO electrode (Figure S7, Supporting Information). The anodic current increased rapidly within 5 s (stage I), then immediately decreased to approximately 150 μA/cm2 over 5–20 s of electrolysis time (stage II). It increased again over 20–50 s (stage III), and finally decayed gradually to reach a steady state of approximately 300 μA/cm2 after 50 s of electrolysis time (stage IV). The PPy nanowires grew across the PEO114-bPMA(Az)67 template from the ITO electrode surface, shown as bright stripes in Figures 4c–4e. Figure 4c shows a cross-sectional image in which the PPy nanowires reached ~70% of the length of the PEO cylindrical domains at 3.3 mC/cm2 of passed charge (Q) in stage II. The length of the PPy nanowires was proportional to the passed charge during stage II. At 4.3 mC/cm2 of Q in stage III, the tips of the PPy nanowires appeared from the top surface of the template film and then grew hemispherically to form a mushroomshaped structure on the top of the cylindrical domains (Figure 4d). At 10.0 mC/cm2 of Q in stage III, these PPy mushroom-shaped structures merged to form fluffy sediments, finally covering the whole surface as a continuous film (Figure 4e). In addition, the growth of the sediments on the template was monitored via AFM measurement (Figure S8, Supporting Information). Figure 4b shows the in-plane profiles of GISAXS measurements of the template after electropolymerization at 3.3 mC/cm2 of Q in stage II. The corresponding GISAXS 2D images were shown in Figure S9, Supporting Information. The series of intense peaks are assigned to the hexagonal cylindrical arrangement with scattering vectors of 1, √3, √4, √7, and √9 and a periodicity of 27.8 nm, values that are consistent with those found in the template with a periodicity of 26.9 nm. The peak intensity was enhanced after electropolymerization owing to the increased difference in electron density between the PPy nanowires and the PMA(Az) matrix. In stage I, a uniform PPy thin layer was formed at the interface between the ITO electrode and template



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owing to the temporal penetration of the electrolytic solution between them (see Figure S10, Supporting Information). Because of the limited continuous supply of Py, the PPy nanowires started to grow through the PEO cylinders (stage II). The swelling of the template allows to grow the PPy nanowires in PEO-filled cylinders, since the expansion of the PEO cylindrical domains and the thickening of template were observed in TEM and AFM measurements, respectively (Figure S11, Supporting Information). The PPy growth behavior at each stage is illustrated in Figure 4f and Figure S12, Supporting Information. Stage I is the formation of a uniform PPy thin layer between the template and ITO electrode. Stage II is PPy nanowire growth in the PEO cylindrical domains up to the top surface of the template. The rapidly decreased current resulted from an insufficient supply of Py in the PEO cylindrical domains from the electrolytic solution. In stage III, PPy grows hemispherically from the tips of the PPy nanowires. The observed current increases because the effective surface area of the PPy increases. In stage IV, hemispherically grown PPy becomes a continuous film, forming an overlayer similar to what occurs in bulk electropolymerization. A similar oscillatory i–t curve was obtained in the electropolymerization of EDOT, whereas the nonselective domain electropolymerization of BiTh did not show such behavior (Figure S13, Supporting Information). Feature size control of PPy nanowire array

Figure 4. (a) i–t curve for electropolymerization of Py by using PEO114-b-PMA(Az)67 template. (b) GISAXS in-plane profiles of PEO114-b-PMA(Az)67 template with (red) and without (black) PPy nanowires (stage II). Cross-sectional FE-SEM images of the template after electropolymerization with a passed charge of (c) 3.3 mC/cm2 in stage II, (d) 4.3 mC/cm2 in stage III, and (e) 10.0 mC/cm2 in stage III. The images are tilted at 15°. These samples were observed without a metal coating. Scale bars are 150 nm. (f) Schematic illustration of growth behavior of PPy in PEO-b-PMA(Az) template. (before) Template coated on ITOcoated glass electrode. (stage I) Formation of PPy thin layer at the interface between ITO electrode and the template. (stage II) Selective growth of PPy nanowires thorough PEO cylinders. (stage III) Hemispherical growth from the individual tips of PPy nanowires. (stage IV) Bulk growth of PPy overlayer on the template.

The PEOm-b-PMA(Az)n template has remarkable potential to control the PEO cylinder diameter and its center-to-center distance (periodicity) by modifying the molecular weight and/or volume fraction of PEO.52 Three kinds of block copolymers with different molecular weights of each segment, PEO40-b-PMA(Az)17, PEO114-bPMA(Az)45, and PEO272-b-PMA(Az)94, were adopted to achieve the size control of the PPy nanowire array (characterization of these polymers is described in Figures S1b– S1d, Supporting Information). The feature size of the PEO cylinders was systematically tuned with (m, n) in the number of repeated units of each block in the template, specifically for (diameter, periodicity): (40, 17), (6 nm, 13 nm); (114, 45), (9 nm, 24 nm); (272, 94), (15 nm, 34 nm) (Figure S14, Supporting Information). The electropolymerization of Py was employed in stage II, where PPy nanowires grew halfway in the PEO cylinders. The upper parts of the PPy nanowires inside the template came out by repeated partial dissolution of the template from the top surface with a mixed solvent of toluene and acetone [3/7 (v/v)], called solvent etching here.53 Figure 5 shows AFM height images of the PPy nanowires in the PEO40-b-PMA(Az)17, PEO114-bPMA(Az)45, and PEO272-b-PMA(Az)94 templates after solvent etching. The dimple array of the PEO cylindrical domains was changed to a pit array in a hexagonal arrangement. The diameters of the PPy nanowires in the PEO40-b-PMA(Az)17, PEO114-b-PMA(Az)45, and PEO272-b-PMA(Az) 94 templates were 7, 10, and 17 nm, respectively, as estimated from the AFM images. A linear relationship of periodicity of the PPy nanowires in the template and the PEO cylinders in the template was obtained with a transcription efficiency of 1.03 ± 0.01, implying satisfactory transcription of the tens of nanometersscale periodic patterning (Figure 5d). Thus, the PEOm-b-PMA(Az)n thin film functioned as a template for electropolymerization, and the hexagonally arranged PEO cylindrical domains were transferred to the corresponding PPy nanowire array nanostructures. The PEO40b-PMA(Az)17 template gave an ultrafine PPy nanowire array, with a


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Figure 5. Tapping mode AFM height images of PPy nanowires in (a) PEO40-b-PMA(Az)17, (b) PEO114-b-PMA(Az)45, and (c) PEO272-b-PMA(Az)94 templates. Insets are the corresponding Fourier fast transform (FFT) images collected from the whole observed area. (d) Relationship of center-to-center distance between the PPy nanowires and PEO cylinders in the templates.

nanowire diameter of 7 nm, a periodicity of 13 nm, and a density of 4.4 × 1012 wires/in.2, which has been never achieved before. Another advantage of the PEOm-b-PMA(Az)n template is perpendicularly aligned PEO cylindrical domains with high aspect ratio, reaching several hundred.40 PEO114-b-PMA(Az)67 templates with thickness of 50, 85, 100, 120, 165, 385, 700, and 1400 nm were prepared by controlling the spin-coating conditions. The electropolymerization was carried out up to stage IV of the i–t curve. All the PEO cylinders were filled with PPy nanowires and the top surface of the template was covered with a PPy overlayer (Figure 6). The specific stages in i–t curves were observed in the similar manner to Figure 4a (Figure S15, Supporting Information). The amount of passed charge (Qfill) needed to form the PPy nanowires (stage II) was estimated from the individual i–t curves. A linear relationship between the value of Qfill and the thickness of the template used, corresponding to the length of the PPy nanowires, was obtained over a wide range of thicknesses (Figure 6a). Figures 6b and 6c show typical

cross-sectional FE-SEM images of the 385-nm- and 1400-nm-thick templates, respectively, after electropolymerization. The PPy overlayer appeared to have been peeled from the top surface of the template as a result of cleavage during the preparation of the cross section. An obtuse zigzag structure with an angle of approximately 30° was found along the 1400-nm-long PPy nanowire array (Figure 6c), whereas the other PPy nanowires with a template thickness of less than 385 nm appeared to be straight (Figure 6b). A similar zigzag structure was observed in the PEO cylinders of the template with a thickness of less than 1 μm, which can be explained at present by a kind of structural conflict between the long persistent length of the PEO cylindrical domains and the tilting structure of the azobenzene mesogens in a smectic C liquid crystalline state at room temperature.40 Actually, such long PPy nanowires appeared to grow, faithfully tracing the zigzag structure of the long PEO cylindrical domains. It came as a surprise that all the 1400-nm-long PEO cylindrical domains worked as both efficient diffusion channels of Py and nanoconfined reactors for its electropolymerization, implying that the PEO cylindrical domains were connected at both ends with the electrode and electrolytic solution. The PPy nanowires were 10 nm in diameter and their aspect ratio reached 140. To the best of our knowledge, this is the highest aspect ratio among nanowire-structured materials fabricated through the block copolymer templating process. In 2008, Kim and co-workers first fabricated PPy nanorods with an aspect ratio of 4 (25 nm in diameter and 100 nm long) using a nanoporous PS-b-PMMA template.31 Kuila and Stamm fabricated a polyaniline nanorod array with an aspect ratio of 3 (10 nm in diameter and 30 nm long) through the use of a nanoporous PS-b-P4VP template.32, 33 By using a commercially available AAO template, Joo et al. reported an aspect ratio of 197 in PEDOT nanowires (200 nm in diameter and 39.3 μm long).27 Separately, we succeeded in fabricating the thinnest PPy nanowire array 7 nm in diameter, with a periodicity of 13 nm, and 100 nm long by using a PEO40-b-PMA(Az)17 template, as described above. The surface density of the PPy nanowires reached 4.4 × 1012 wires/in.2 Characterization of PPy nanowires The PEO in the cylinders was in an under-cooled state.47 A question arises here whether the observed PPy nanowires were pure PPy or a composite with PEO segments, i.e., whether the PEO segments were included or excluded from the PPy nanowires. As discussed in the first section, 98% of PEO114-b-PMA(Az)67 was recovered from the electropolymerized template. Consequently, the growing PPy nanowires pushed aside the PEO segments to the surrounding PMA(Az) domains. We have never observed any buckling,

Figure 6. (a) The relationship between amount of the passed charge (Qfill) to form PPy nanowires (stage II) and thickness of PEO114-bPMA(Az)67. Cross-sectional FE-SEM images of PPy nanowires in the template with thicknesses of (b) 385 nm and (c) 1.4 μm. These samples were observed without a metal coating. Scale bars are 400 nm.



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Figure 7. HR-TEM images of a single PPy nanowire at (a) low and (b) high magnifications. Inset is FFT image obtained from marked area. delamination, and lift-up of the template during and even after electropolymerization. The thickening of the template probably removes the pressure to the PMA(Az) domains (Figure S11, Supporting Information). High-resolution transmission electron microscope (HR-TEM) images of singly dispersed PPy nanowires were taken. The nanowires tended to aggregate, sometimes with partial overlaps. Figure 7 shows an HR-TEM image of a single nanowire found at the edge of the aggregate. Figure 7b shows a magnified image of the white square shown in Figure 7a. Longitudinal fringe patterns with spacing of 3.3 Å are clearly observed within the nanowire. The spacing of 3.3 Å corresponds to that of π–π stacking reported for crystalline PPy.54 The fringe patterns are observed over 160 nm, and they are neither bent nor broken (Figure S16, Supporting Information). They may be assigned to PPy main chains fully extended and packed over approximately 400 pyrrole units. By electropolymerization through an AAO template, Yan et al. fabricated 220-nm-diameter PPy nanowires but characterized them as an amorphous phase.55 It is also interesting to note a fringe structure in the nanowire tip (Figure 7a), in which the dark lines look closed around the tip, which will require further study to understand it. The longitudinal lattice structures in the crystalline PPy nanowires are thought to be caused by a so-called confined effect upon electropolymerization of pyrrole, known as condensation polymerization,31, 56 in PEO cylindrical domains with high aspect ratios. The deposition of PPy chains is restricted in the PEO cylinders, although the cylinder diameter (10 nm) is much larger than the stacking distance (0.33 nm). The formation of PPy chains exposed to electrolytic solution can work as electrode for the further growth of PPy, which contributes to the long-range growth, even though the PEO forms long channel from the surface of ITO electrode. We speculate that it is due to interfacial tension between PPy chains and PEO chains of cylinder wall. Further improvements in handling single PPy nanowires will allow us to reveal their intrinsic electric and mechanical properties. Figure 8 shows the ultraviolet-visible-near infrared (UV-visNIR) spectra of the PPy nanowires partially filled in the PEO cylinders (stage II) before and after removal of the PEO114-b-PMA(Az)67 template. The spectrum of the PPy nanowires before removal of the template was obtained by subtracting the ITO electrode with the template. An intense absorption band at approximately 1.0 eV is assigned to the transition from the valence band (VB) to the bonding bipolaron in doped PPy. Two other absorption bands at approximately 2.7 eV and 3.6 eV are assigned to the electronic transitions from the VB to the antibonding bipolaron and the π–π* transition,

Figure 8. UV-vis-NIR spectra of PPy nanowires in the PEO114-bPMA(Az)67 template (blue), PPy nanowires after removal of the template (red), and PPy bulk film (black). The spectrum of the PPy nanowires before removal of the template was obtained by subtracting the ITO electrode with the template. The spectra of the PPy nanowires after removal of the template and that of the PPy bulk film were obtained by subtracting the ITO electrode.

respectively.57, 58 The maxima of the 1.0 eV and 3.6 eV absorptions are in good agreement with those of bulk PPy, but the 2.7 eV absorption band is shifted to the higher energy region as compared with that of bulk PPy appeared at 2.5 eV. These absorbances could be underestimated owing to the highly oriented PPy main chains parallel to the incident light. After removal of the template by dissolving it with toluene, the PPy nanowires were agglomerated as shown in Figure 3a. In the absorption spectrum of the collapsed PPy nanowire array (Figure 8, red line), the absorbances of these three peaks increased as compared with the sample before etching of the template (Figure 8, blue line), since the laid transition dipole moments became responsive to the electric field of the incident light. Conductive AFM measurement of the PPy nanowire array with the template was carried out to estimate the longitudinal conductivity (σ) of the individual PPy nanowires. Figure 9a shows typical I–V curves with high (red) and low (blue) conductivities, assuming a diameter of 10 nm and a length of 80 nm for each PPy nanowire, in a range obeying Ohm’s law. Under the same conditions, the I–V curve (black) of the PPy bulk film gave (4.4 ± 1.8) × 10−1 S/cm of conductivity. It should be noted here that both higher and lower conductivities than that of the PPy bulk are imaged on the PPy nanowires grown in the PEO cylindrical domain and the PMA(Az) matrix domain, respectively. Figure 9b shows a histogram of the conductivity distribution at 80 point measurements. A bimodal distribution was obtained. The lower distribution with a tail to the higher conductivity region may be explained by possible partial contact of the Ircoated cantilever to the PPy nanowire. The conductivities derived from the red and blue I–V curves in Figure 9a are located at the maxima of both distribution curves as representative measurements. The average conductivity of the single PPy nanowire is 4.0 ± 1.5 S/cm, which is ten times higher than that of the PPy bulk film. The higher conductivity may be explained by the well-aligned crystalline PPy main chains shown in Figure 7. We had expected still higher conductivity than that of the stretched PPy bulk film,59 considering its crystalline nanostructure and fully doped state as characterized by X-ray


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Longitudinal and Lateral Integration of Conducting Polymer Nanowire Array Both long reaction channels of the PEO cylindrical domains and affinity-dependent electropolymerization allow one to fabricate additional hierarchically integrated nanostructures of the conducting polymer nanowire array.

Figure 9. (a) Typical I–V curves of PPy nanowires in the template (red, PPy nanowire; blue, PMA(Az) matrix) and the PPy bulk film (black) under conducting AFM operation. Insets give enlarged curves for low-current region and an illustration of the conducting AFM setup. (b) Conductivity (σ) histogram of PPy nanowires in the template. Conductivity was calculated from resistance of I–V curves (slope of I–V curve between bias voltage of −0.05 V and 0.05 V).

photon spectroscopy (XPS) spectra (Figure S17, Supporting Information) and the fact that at present we suspect poor electrical contact between the AFM cantilever and the top of the PPy nanowire. More importantly, the large ratio (3.8 × 105) of maximal and minimal conductivities (9.0 S/cm and 2.4 × 10−5 S/cm, respectively) in the histogram allows us to conclude that the ultra-anisotropic nanostructure of the PPy nanowire array was completely transferred from the PEOm-b-PMA(Az)n template film in terms of the nanostructure and conducting property.

Using a 180-nm-thick PEO114-b-PMA(Az)67 template, stepwise electropolymerization of Py and EDOT was attempted in order to fabricate longitudinally segmented nanowire arrays of PPy-PEDOT and PEDOT-PPy, as Figure 10 illustrates. The Py was electropolymerized to form PPy nanowires halfway up the PEO cylinders in a PC solution containing 10 mM Py and 100 mM LiClO4 at 1.10 V vs. Ag/AgCl. After the electrolytic solution was replaced by another PC solution containing 10 mM EDOT and 100 mM LiClO4, electropolymerization was employed at 1.40 V vs. Ag/AgCl. Figure 10b shows high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the cross section of the resulting template. The nanowire array is clearly shown as bright lines in HAADF mode. The energy dispersive X-ray spectroscopies (EDS, Figure 10c) revealed that a sulfur with signal at 2.3 keV was detected near the top surface (solid-line rectangle in Figure 10b), but not near the substrate (broken-line rectangle in Figure 10b). In the second electropolymerization, the PEDOT nanowires grew from the tips of the PPy nanowires inside the PEO cylinders, defined herein as PPyPEDOT longitudinally segmented nanowires. The fluffy overlayer is assigned to PEDOT. Inversely, PEDOT-PPy longitudinally segmented nanowires, shown in Figure 10d, could be also fabricated by stepwise electropolymerization first of EDOT and then of Py (Figure 10e). The EDS signal of sulfur was observed near the substrate, but not near the top surface (Figure 10f) in the opposite manner to

Figure 10. (a, d) Schematic illustrations of stepwise electropolymerization for fabrication of longitudinally integrated conducting polymer nanowires. HAADF-STEM images of (b) PPy-PEDOT- and (e) PEDOT-PPy-segmented nanowires in the PEO114-b-PMA(Az)67 template. (c, f) EDS spectra on the upper and lower areas in the corresponding HAADF-STEM images.



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Figure 11. (a) Schematic illustration of one-pot electropolymerization using the PEO-b-PMA(Az) template to fabricate laterally integrated conducting polymer nanowires. (b) BF-TEM image of ultra-thin section of PEO114-b-PMA(Az)67 template after one-pot electropolymerization of Py and BiTh. (c) HAADF-STEM image of the ultra-thin section. Inset is EDS point analysis of sulfur (S Kα, solid line) and nitrogen (N Kα, dashed line) along yellow profiles (~5 nm step). Figure 10c. The above characterization implies that we succeeded in fabricating heterojunctions of two different conducting polymer nanowires 10 nm in diameter and 180 nm long. It should be noted that such heterojunction nanowire arrays (1 × 1012 wires/in.2 of surface density) are connected perpendicularly to a macroscopic ITO electrode, which can be initially achieved by using the perpendicularly aligned PEO long cylindrical domains characteristic of the PEO-b-PMA(Az) template. Further characterization of the junction between PPy and PEDOT is undergoing. Another type of hierarchically integrated conducting nanowire array was fabricated by utilizing affinity-dependent electropolymerization. As discussed in the first section with Figure 1, Py and EDOT were polymerized selectively in the PEO cylinders, whereas BiTh was polymerized equally in both the PEO cylindrical and PMA(Az) matrix domains, resulting in one-pot electropolymerization of a mixture of Py and BiTh. Random copolymerization occurred in the PEO cylinders to give P(Py-co-BiTh) nanowires, whereas PBiTh grew in the PMA(Az) matrix. As a result, a laterally mosaic composition-modulated conducting polymer composite with the template, i.e., a P(Py-co-BiTh) nanowire array surrounded by PBiTh, was achieved. The one-pot electropolymerization was performed in a PC solution containing 10 mM Py, 10 mM BiTh, and 100 mM LiClO4 under potentiostatic condition at 1.35 V (vs. Ag/AgCl), where both Py and BiTh were electropolymerized (Figure 11a). Figure 11b shows a bright-field transmission electron microscope (BF-TEM) image of the cross section of the resulting template. The nanowires with dark contrast penetrated into the PEO cylindrical domains across the template without causing damage. This indicates that the one-pot electropolymerization efficiently progressed through the microphase-separated nanostructures in the template. EDS point analyses on sulfur and nitrogen were carried out across the stripe, shown as the row of yellow circles in Figure 11c. The electron beam diameter was 1 nm and the point analyses were taken at 5 nm intervals. Both the sulfur (solid line) and nitrogen (broken line) EDS profiles are shown in the inset. The EDS signals on sulfur and nitrogen showed oscillatory and complementary profiles along the analyzing points across the stripe. The atomic concentration of nitrogen in the

PPy nanowires was higher than that in the PMA(Az) domains containing the azobenzene moieties, and so the bright lines in the HAADF mode can be assigned to the PEO cylindrical domains. On the bright lines, the sulfur and nitrogen signals are low and high, respectively, and inversely high and low, respectively, on the dark lines. It is suggested that both Py and BiTh were electropolymerized in the PEO cylindrical domains (bright lines) to give P(Py-co-BiTh) random copolymer nanowires and that PBiTh grew in the PMA(Az) matrix domains (dark lines). At 1.35 V vs. Ag/AgCl, which is more anodic than usual in the electropolymerization of Py (1.10 V), a larger fraction of PPy in the P(Py-co-BiTh) nanowires would be expected. On the other hand, BiTh was electropolymerized dominantly in the PMA(Az) domains. The resulting conducting polymer in the template may be described as a compositionally modulated hexagonal nanostructure, i.e., P(Py-co-BiTh) nanowires surrounded by a PBiTh matrix, both of which co-exist in the “flexible” template. This mosaic nanostructured conducting polymer composite cannot be achieved by electropolymerization utilizing conventional nanoporous templates such as one-domain-removed block copolymers and AAO reported previously. The present template can be regarded as a chemical affinity template. Conclusion We succeeded in fabricating conducting polymer nanowire arrays by using liquid crystalline block-copolymer-templated electropolymerization without removal of one of the domains. Depending on the relative solubility of the monomers in the PEO and PMA(Az) domains, PPy and PEDOT nanowires grew selectively in the PEO cylindrical domains, whereas PBiTh grew nonselectively in both the PEO and PMA(Az) domains. The structural features of the PPy nanowire array in this study were diameters of 7–17 nm, center-to-center distances of 13–35 nm, and lengths of 50–1400 nm, attaining 4.4 × 1012 wires/in.2 of maximal surface density (7 nm diameter and 13 nm periodicity) and a maximal aspect ratio of 140 (1.4 μm long and 10 nm in diameter). These characteristics were controlled by controlling the PEO-b-PMA(Az), the film thickness, and the amount of charge passed. Ten-times-higher conductivity (4.0 ± 1.5 S/cm) of a single PPy nanowire as compared to that of a PPy bulk film was obtained as a result of well-aligned PPy main chains in the longitudinal


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direction, which were imaged using HR-TEM. By utilizing long PEO cylindrical domains and affinity-dependent electropolymerization, two additional types of hierarchically integrated nanostructures of the conducting polymer nanowire array were fabricated. One was longitudinally segmented PPy-PEDOT and PEDOT-Py nanowire arrays with heterojunctions by stepwise electropolymerization, and the other was a lateral mosaic nanostructure of P(Py-co-BiTh) and PBiTh, faithfully transferred from the microphase-separated nanostructure of the PEO-b-PMA template by one-pot electropolymerization of a mixture of Py and BiTh. Both longitudinal and lateral heterojunctions on a nanometer scale achieved by block-copolymertemplated electropolymerization are promising as ultrahigh-density nanostructured electrodes. Methods Electropolymerization using homopolymer- and block-copolymer-coated ITO electrode An ITO electrode was cut into 1 cm × 2 cm pieces for use as electrodes. The pieces were then washed in water and ethanol under ultrasonication. The PEO114- and PMA(Az)139-coated ITO electrodes were prepared by spin coating with 3 wt% chloroform and toluene solutions, respectively, at 2000 rpm for 30 s. The thicknesses of the PEO114 and PMA(Az)139 thin films on the ITOs were 430 nm and 130 nm, respectively. The PEO114 thin film was cured by electron beam at 60 kV and 30 μA for 200 s using an Min-Electron Beam (Ushio, Japan) prior to the electropolymerization so as to be insolubilized in PC. Typically, a 3 wt% toluene solution of PEOm-b-PMA(Az)n is spin coated onto the ITO electrode at 2000 rpm for 30 s. Note that no pretreatment of the substrate surface such as pre-coating of the random copolymer with a composition identical to that of the block copolymer film is required. Perpendicularly aligned and hexagonally arranged PEO cylindrical domains surrounded by liquid crystalline PMA(Az) domains were fabricated by thermal annealing at 140 °C for 6 h and used as the template in this study. Electropolymerization was carried out with a potentiostat/galvanostat (Ivium CompactStat, Ivium Technologies, Netherlands). The template- or homopolymer-coated ITO, Pt plate, and Ag/AgCl electrodes were used as the working, counter, and reference electrodes, respectively. Electropolymerization of Py, EDOT, and BiTh (10 mM) was carried out in a PC solution containing 0.1 M LiClO4 as the supporting electrolyte under potentiostatic conditions at 1.10 V, 1.40 V, and 1.35 V vs. Ag/AgCl, respectively. The whole area of the working electrode was 1 cm × 1 cm. After the electropolymerization, the thin film was rinsed with acetonitrile and dried at room temperature.

spring constant 2 N/m, resonant frequency 70 kHz, purchased from Asylum Research) was used for current–voltage (I–V) curve measurements. The conductivity (σ [S/cm]) was calculated from the following equation:

σ = (1/R)×(L/A) , where R, L, and A are the resistance [Ω], length [cm], and contact area of the AFM tip with the sample [cm2], respectively. R was obtained from the slope of the I–V curve in the region following Ohm’s law (−0.05 V to 0.05 V). The L values of the PPy nanowires and PPy bulk film were 80 nm and 70 nm, respectively, which were measured from the AFM images across a line scratched out in the samples. The A value was calculated using the Hertz theory. 60 The A value was smaller than the distance between the PPy nanowires (6.4 nm2). Raman spectra were recorded using a HoloLab 5000R series (Kaiser Optical Systems, USA). The excitation laser wavelength was 532 nm with 0.25 mW power on the sample. UV-vis and UV-vis-NIR spectra were taken using a SolidSpec-3700DUV system (Shimadzu, Japan). BF-TEM images were taken using an H-7100 system (Hitachi High-Technologies, Japan) at an accelerating voltage of 100 kV. HR-TEM and HAADF-STEM observations with EDS were performed using a JEM-2100F (JEOL, Japan) at an accelerating voltage of 200 kV. An ultra-thin section of the integrated conducting polymer nanowires in the template was obtained by the microtome method. After electropolymerization, the ITO layer was dissolved with the ITO etching solution to peel the template from the ITO electrode. The template was floated on the surface of water, and then picked up on a PET substrate (10 μm in thickness). The template on the PET substrate was embedded with room-temperature-cured epoxy resin. The ultra-thin section was obtained by using a UC6 ultra-microtome (Leica, Germany) with a diamond knife (DiATOME, tilted 45°). The cut section was picked up on the TEM grid with a carbon supporting membrane.

AUTHOR INFORMATION Corresponding Author (H.K.) [email protected] (T.I.) [email protected]

Present Addresses (M.K.) Department of Electrical and Electronics Engineering, National Institute of Technology, Numazu College, 3600 Ooka, Numazu, Shizuoka 410-8501, Japan.

Notes

Observations and measurements

The authors declare no competing financial interest.

The nanostructures of PPy, PEDOT, and PBiTh after electropolymerization were taken by FE-SEM (S-5200, Hitachi High-Technologies, Japan) at accelerating voltages of 1–5 kV. GISAXS measurements were performed using a Nano-Viewer (Rigaku, Japan) with a Cu Kα X-ray with a wavelength of λ = 1.54 Å. AFM observation was carried out using a Nanoscope IIIa MultiMode (Bruker, USA) in tapping mode. The cantilever had a 5 N/m spring constant and 119–154 kHz resonant frequencies. The conductive AFM measurements were performed using a Cypher system (Asylum Research, USA) in contact mode. An iridium-coated cantilever (ASYELEC-01,

ACKNOWLEDGMENT We thank Material Analysis Suzukake-dai Center, Technical Department, Tokyo Institute of Technology for the use of the JEM-2100F and the Cypher. We are grateful to Prof. Takashi Tatsumi and Dr. Toshiyuki Yokoi from Chemical Resources Laboratory of Tokyo Institute of Technology for the use of the S-5200. H.K. thanks the Japan Society for the Promotion of Science for a research fellowship for young scientists. K.K. gratefully acknowledges financial support by JST-PRESTO and Grant-



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in-Aid for Science Research in a Priority Area “Super-Hierarchical Structures” (No. 19022008) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

ASSOCIATED CONTENT Supporting Information Available. Experimental details and additional data. This material is available free of charge via the Internet at http://pubs.acs.org.

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Longitudinal and Lateral Integration of Conducting Polymer Nanowire

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