Nanotexturing of Conjugated Polymers via One-Step Maskless

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Nanotexturing of Conjugated Polymers via One-step Maskless Oxygen Plasma Etching for Enhanced Tunable Wettability Youhua Jiang, Jian Xu, Junghoon Lee, Ke Du, Eui-Hyeok Yang, Myoung-Woon Moon, and Chang-Hwan Choi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01593 • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 17, 2017

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Nanotexturing of Conjugated Polymers via One-step Maskless Oxygen Plasma Etching for Enhanced Tunable Wettability

Youhua Jiang1, Jian Xu1, Junghoon Lee1, Ke Du1, Eui-Hyeok Yang1, Myoung-Woon Moon2, and Chang-Hwan Choi1*

1

Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, New Jersey

07030, USA 2

Materials and Life Science Division, Korea Institute of Science and Technology (KIST), Seoul,

02792, Korea Rep.

*

To whom correspondence should be addressed. Tel.: +1-201-216-5579; Fax: +1-201-216-8315; e-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

A one-step maskless oxygen plasma etching process is investigated to nanopattern conjugated polymer, dodecylbenzenesulfonate doped polypyrrole (PPy(DBS)), and to examine the effects of nanostructures on the inherent tunable wettability of the surface and the droplet mobility. Etching characteristics such as the geometry and dimension of the nanostructures are systematically examined for the etching power and duration. The mechanism of self-formation of verticallyaligned dense-array pillared nanostructures in the one-step maskless oxygen plasma etching process is also investigated. Results show that the lateral dimensions such as periodicity and diameter of the pillared nanostructures are insensitive to the etching power and duration, whereas the length and aspect ratio of the nanostructures increase with them. X-ray photoelectron spectroscopy analysis and thermal treatment of the polymer reveal that the co-deposition of impurities on the surface resulted from the holding substrate is the primary reason for the selfformation of nanostructures during the oxygen plasma etching, while the local crystallinity subject to thermal treatment has minor effect on the lateral dimensions. Retaining the tunable wettability (oleophobicity) for organic droplets during the electrochemical redox (i.e., reduction and oxidization) process, the nanotextured PPy(DBS) surface shows significant enhancement of the droplet mobility, compared to the flat PPy(DBS) surface with no nanotextures, by making the surface superoleophobic (i.e., in a Cassie-Baxter wetting state). Such enhancement of the tunable oleophobicity and droplet mobility of the conjugated polymer will be of great significance in many applications such as microfluidics, lab-on-a-chip devices, and water/oil treatment.

KEYWORDS: Plasma etching, nanostructures, conjugated polymer, redox, wettability, droplet


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INTRODUCTION

Conjugated polymers have been widely applied in industrial and clinical practices, such as polymer-based photovoltaics,1 electroluminescence,2 logic circuits,3 chemical sensors,4 and proteolytic remodeling.5 Recently, tunable wettability and droplet mobility have also been reported for conjugated polymer surfaces doped with functional surfactants, such as dodecyl benzenesulfonate-doped polypyrrole (PPy(DBS)) surfaces.6,7 By simply applying low voltages (~1

V),

PPy(DBS)

surfaces

surrounded

by

electrolyte

undergo

electrochemical

oxidation/reduction (redox) processes, causing the reorientation and release of the surfactant DBS- dopants, which modifies the surface oleophobicity and droplet-electrolyte interfacial tension in a tunable way. Using this principle, in situ controllable wettability and mobility of organic droplets on PPy(DBS) surfaces were achieved during redox,6,7 potentially benefiting applications such as microfluidics, lab-on-chip devices, and water treatment.8-10 Meanwhile, it should be noted that polymers featured with vertically-aligned dense-array and high-aspect-ratio nanostructures often show enhanced functionalities, which is mostly due to the dramatic increase of the surface area, in many applications including cell separation,11 proton exchange membrane,12 supercapacitor,13 organic light-emitting diode (OLED),14 ferroelectric,15 and solar cells.16 However, the effects of nanostructures on the functionalities of conjugated polymers, particularly, those of PPy(DBS) surfaces on the tunable oleophobicity and droplet mobility have not yet been systematically investigated, which is mostly due to the inefficiency or complexity associated with the nanopatterning processes of the conjugated polymer materials, especially to cover a relatively large area. One main approach that has been explored for the nanopatterning of polymeric substrates is based on non-lithographic techniques such as the pyrolysis of a pre-patterned material,11


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polymerization,13 and infiltration15 and coating16 of polymers on pre-structured surfaces. However, those techniques require high temperature,11 complicated process,13 and one-off prestructured templates.15,16 Other approaches are based on lithographic techniques, such as electron beam lithography,17 nanoimprint lithography,18 nanosphere lithography,19 and laser interference lithography.20 Electron beam lithography and nanoimprint lithography can pattern polymeric substrates to have well-ordered and high-aspect-ratio nanostructures with controllable dimensions. However, electron beam lithography is a slow serial process, which limits the scalability. Nanoimprint lithography requires careful characterizations of patterning conditions such as pressure and temperature for given polymeric materials. It also requires additional fabrication steps to remove residual polymer layer after imprinting. Nanosphere lithography is an inexpensive and scalable technique; however, uniform deposition of nanosphere masks over a relatively large surface area is challenging. Laser interference lithography is maskless lithographic process that can pattern well-ordered polymer nanostructures over a large area; however, it is challenging to fabricate nanostructures with a feature size less than 50 nm. In contrast, it has recently been shown that a simple one-step maskless O2 plasma etching process can create vertically-aligned high-aspect-ratio nanostructures on various types of polymer surfaces including commercialized photoresist materials such as SU-8,14,21-23 and NR7,21 polypyrrole (PPy),14 polyethylene terephthalate (PET),24-27 poly(methyl methacrylate) (PMMA),14,21,26 polystyrenes (PS),14,26 polypropylene (PP),26 polypropylene terephthalate (PPT),26 and polybutylene terephthalate (PBT)26. For example, polymer nanostructures of sub-50 nm in size and the aspect ratio up to 20 have successfully been demonstrated using the one-step maskless O2 plasma etching process with NR-7, SU-8, and PMMA.21 Local surface curvature,14 self-masking effects (co-deposition of impurities during the plasma etching process),21-24 and


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semicrystalline domains25-27 have been suggested as the origins of the formation of the highaspect-ratio nanostructures on the polymer surfaces. However, the exact mechanism of the selfformation of nanostructures in such processes still remains unclear. Moreover, the polymer materials examined for such O2 plasma etching processes have mostly been limited to simple (i.e., non-conjugated) polymers. The fabrication of nanostructures of conjugated polymers, such as PPy(DBS) which is a promising polymer material doped with functional surfactants in many fluidic applications, especially using the scalable one-step maskless O2 plasma etching technique, has not yet been reported. Moreover, the functionalities of the PPy(DBS) surfaces examined so far were on flat surfaces with no decoration of nanostructures.6,7 Thus, the effects of nanostructures on the tunable wettability and mobility of organic droplets on the PPy(DBS) surfaces has not yet been explored and understood. In this work, we demonstrate the nanopatterning of the conjugated polymer, PPy(DBS), with the one-step maskless O2 plasma etching process. The effects of etching power and duration on the formation of nanostructures and their geometries and dimensions are systematically investigated. Particularly, we also investigate the mechanism of the self-formation of the nanostructures by examining the co-deposited impurities on the etched surfaces and the effects of semicrystalline domains of the polymer surfaces subject to pre-heat treatment on the resultant nanostructural morphology. As an application, we also investigate how such nanostructured PPy(DBS) surfaces enhance its tunable wettability and droplet mobility compared to flat (nonstructured) surfaces, by examining the changes of the apparent contact angles and the sliding angles of organic droplets during the redox processes.

MATERIALS AND METHODS


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Fabrication of PPy(DBS) Nanostructures Schematics of the fabrication processes of PPy(DBS) nanostructures using a maskless one-step oxygen plasma etching technique are shown in Figure 1. Frist, 10-nm chromium (Cr) and 30-nm gold (Au) films (Figure 1b) were sequentially deposited on a polished silicon substrate (1 cm × 1 cm) by electron beam evaporation (Explorer 14, Denton Vacuum, Moorestown, NJ, USA). Secondly, a PPy(DBS) layer was deposited on the Cr/Au-coated silicon substrate by electropolymerization. The Cr/Au-coated silicon substrate, as a working electrode, was submerged in a solution containing 0.1 M pyrrole (reagent grade, 98%, Sigma-Aldrich, St. Louis, MO, USA) and 0.1 M sodium dodecylbenzenesulfonate (technical grade, Sigma-Aldrich, St. Louis, MO, USA). Another Cr/Au-coated silicon substrate (5 cm × 5 cm) and a saturated calomel electrode (SCE, Fisher Scientific Inc., Pittsburgh, PA, USA) were also used as the counter and reference electrodes, respectively. The electropolymerization of PPy(DBS) on the Cr/Au-coated silicon was done by applying a voltage of 0.8 V (vs. SCE) using potentiostat (263A, Princeton Applied Research, Oak Ridge, TN, USA) until the accumulated charge density reached 1000 mC.cm-2, corresponding to the PPy(DBS) film thickness of 4.7 µm (Figure 1c). The PPy(DBS) surface was then cleaned by rinsing with deionized water and dried in air for 24 hours before further characterization or fabrication of nanostructures. Then, the prepared PPy(DBS) sample was attached on a silicon wafer (4 inches in diameter), which served as a holding substrate (Figure 1d), and loaded into the etching chamber of an inductively coupled plasma (ICP, HiEtch, BMR Technology Corp., Anaheim, CA, USA). Under the fixed O2 flow rate of 30 sccm (standard cubic centimeter per minute) and chamber pressure of 20 Pa, different etching powers (50, 100, 150, and 200 W) and times (120, 240, 360, 480, and 600 s) were applied for the nanopatterning and characterizations of the PPy(DBS) surface. A helium gas flow


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of 10 sccm was also introduced at the back of the holding substrate to maintain the constant temperature of 20 °C during the etching process.

Characterization of PPy(DBS) Nanostructures The etched PPy(DBS) surfaces were coated with a thin layer (~1.8 nm) of Au/Pd alloy to observe the structural morphology using a scanning electron microscope (SEM, Auriga small dual-beam FIB-SEM, Carl Zeiss, Jena, Germany). Captured SEM images were analyzed to determine the averaged geometrical dimensions (e.g., center-to-center pitch, diameter, length, and aspect ratio of the nanostructures). The co-deposition of impurities on the PPy(DBS) surfaces during the etching, which causes the self-formation of nanostructures, was analyzed by X-ray photoelectron spectroscopy (XPS, Sigma Probe, Thermo Scientific, Waltham, MA, USA). The changes of surface chemical states were measured on the etched surfaces with different powers (i.e., 50, 100, 150, and 200 W) at a given etching time (480 s). The high-resolution (0.1 eV) core-level spectra were obtained using Al K-α (1486.7 eV) with 400 μm of beam diameter. To investigate the effect of polymer thermal history on the formation of nanostructures, the substrates coated with PPy(DBS) surfaces (Figure 1c) were also pre-heated at different temperatures (100, 200, and 300 ℃) in an oven (Ultra-Clean 100, Lab-Line) for 1 h, and then cooled down at room temperature (~22 ℃) for 1 day before the ICP etching.

Characterization of Wettability The tunable wettability and mobility of organic droplets on the PPy(DBS) surfaces with or without nanostructures during redox was examined by measuring the changes of the apparent contact angles (θi) and the sliding angles (θs) of a micro-droplet (~1.5 µL) of dichloromethane


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(DCM, ≥99.8%, Sigma-Aldrich, St. Louis, MO, USA). The nanostructured PPy(DBS) surfaces were tested in 1 day after the O2 plasma etching to minimize the potential change of the hydrophobicity induced by O2 plasma treatment. For the in-situ control of electrochemical redox process, the experiments were performed under the immersion within electrolyte. A 25 mL 0.1 M sodium nitrate (NaNO3, ≥99.0%, Sigma-Aldrich, St. Louis, MO, USA) solution was used as the electrolyte, where the DCM droplet-electrolyte interfacial tension is 27.8 mN/m. A platinum (Pt) mesh of 13 mm × 35 mm in size and a saturated calomel electrode (SCE) were used as the counter and reference electrode, respectively. The PPy(DBS) surface was initially oxidized for 15 s by applying the potential of +0.6 V (vs. SCE). Then, a DCM droplet (~1.5 µL) was placed on the oxidized PPy(DBS) surface and the contact angle was recorded and analyzed using a goniometer system (Model 250, Ramé-hart, Netcong, NJ, USA). The sliding angle is defined as the angle of the tilted surface, where the droplet slides off. As for the PPy(DBS) surface under reduction, a DCM droplet (~1.5 µL) was placed on the oxidized surface, followed by reducing the surface at the potential of -0.9 V (vs. SCE) and the change of contact angles were recorded and analyzed simultaneously. Sliding angles of droplets on reduced surfaces were measured by tilting the oxidized surface at different angles first and then reducing it. Advancing (θa) and receding (θr) angles were measured to be the angles at the front and rear ends of the sliding droplet, respectively. Contact angle hysteresis (CAH) is defined as the deviation between the advancing and receding angles. The measurements were repeated consecutively for five redox cycles on each sample to examine the repeatability.

RESULTS AND DISCUSSION

Morphology of Fabricated Nanostructures


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Figure 2 shows the SEM images of the PPy(DBS) surfaces etched by O2 plasma with different powers (50, 100, 150, and 200 W) for different durations (120, 240, 360, 480, and 600 s). The O2 plasma with the relatively low power of 50 W (Figure 2a) did not show significant formation of nanostructures on the PPy(DBS) surface, compared to the higher powers (i.e., 100, 150, and 200 W), even with an increase in the etching time. Especially for low etching times, no evident difference of the surface morphology was found, compared to the PPy(DBS) surfaces before O2 plasma etching (see Figure S1 in Supporting Information). Only when the etching time is relatively long enough (e.g., 600 s), the formation of nanoscale bumps was noticeable (Figure 2a-v). When the etching power was increased to 100 W (Figure 2b), the effect of O2 plasma etching on surface morphologies became more evident. While only small bumps (50-80 nm in length) were found on the surfaces for the etching time of 120 and 240 s (Figures 2b-i and 2b-ii), longer nanostructures (>200 nm) were formed by the O2 plasma etching for more than 360 s (Figures 2b-iii to 2b-v). In case of the etching power of 150 W (Figure 2c), the PPy(DBS) surface with etching time of 120 s (Figure 2c-i) showed significant difference from those by using lower etching powers (50 and 100 W, Figures 2a-i and 2b-i). In the cases of 50 and 100 W for 120 s, the nanoscale bumps were formed on the mountain-like microscale patterns of the PPy(DBS) surfaces which resulted from the electropolymerization process.6 However, the mountain-like microscale patterns were not found after etching for 120 s with 150 W, while nanoscale bumps were fabricated similarly with the cases of lower powers. The microscale roughness of the initial PPy(DBS) surface is regarded to be removed by the O2 plasma etching with the relatively high power (150 W). Obviously longer nanostructures (> 300 nm) were fabricated on the PPy(DBS) surfaces with longer etching times more than 240 s (Figures 2c-ii to


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2c-v). A higher etching power of 200 W (Figure 2d) resulted in the formation of nanostructures even only for 120 s (Figure 2d-i), showing the significantly increased etch rate. In general, the nanostructure length dramatically increased with the increase of etching time. However, a longer etching time (e.g., more than 480 s) started to result in the noticeable undercut, causing some nanostructures to collapse (Figures 2d-iv and 2d-v). In addition, the longer nanostructures (Figures 2c-v and 2d-v) are less straight than the shorter ones (Figures 2b-v, 2c-iii, and 2d-iii), which is attributed to the undercut which would make the high-aspectratio slender nanostructures mechanically unstable. The geometric parameters (i.e., center-to-center pitches, diameters, lengths, and aspect ratios) of the nanostructures, estimated by the SEM images, are shown in Figure 3. The pitch and diameter were measured from the roots of nanostructures. All the values were obtained from averages of at least 5 measurements at different locations of different samples. The variation of the center-to-center pitches (Figure 3a) was not significant (within the range of 48-55 nm) despite the different etching powers and times. Similarly, the variation of the diameter of the nanostructures (Figure 3b) was also insignificant (within the range of 26-33 nm), regardless of the etching time and power. The results indicate the independency of the lateral dimensions of the nanostructures on the etching power and time. Although the length of nanostructures fabricated under 50 W (Figure 3c) is almost indifferent to the etching time due to the insufficient etching power, the higher etching powers (i.e., 100, 150, and 200 W) resulted in the almost linear increase of the nanostructure length with the etching time, i.e., from ~9 nm/min at 50 W to ~42 nm/min at 100 W to ~80 nm/min at 150 W to ~100 nm/min at 200 W. The aspect ratios (length/diameter) of the nanostructures with respect to the etching time (Figure 3d) showed the similar trend with lengths, since the nanostructure


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diameter was almost constant. In this study, the nanostructures of PPy(DBS) up to ~1034 nm in length and ~32 in aspect ratio could be formed at the highest etching power (200 W) and etching time (600 s) employed. Although the limit of the nanostructure length or aspect ratio has not been examined in this study, the result suggests that they can be increased furthermore with the increase of the etching power and time as long as the undercut issue is minimized. It is regarded that adding fluorine-based gas in the etching gases, which can coat and passivate the sidewalls of the polymer nanostructures during plasma etching,14,22 can reduce the undercut of nanostructures and improve the anisotropic etching characteristics.

Mechanisms of Self-Formation of Nanostructures No significant dependency of the lateral dimensions of the nanostructures (pitch and diameter) on the etching power and duration suggests that the mechanism of the self-formation of nanostructures during the oxygen plasma etching also does not depend on the etching power and duration. However, the true mechanism of the self-formation of nanostructures during the oxygen plasma etching has not been known clearly yet. Estimated from the literatures available,14,21,22,24-27 Table 1 compares the lateral dimensions (structural pitch and diameter) of nanostructures of various types of polymer materials fabricated primarily by O2 plasma etching processes, especially with respect to the different mechanisms of the self-formation of nanostructures proposed. Table 1 shows that the resultant nanostructural pitches and diameters even for the same polymer materials vary significantly, reported by different groups. For example, the nanostructural pitch and diameter of PPy were reported to be ~500 and ~300 nm, respectively, in [14], whereas those of the PPy(DBS) examined in this study are ~50 and ~30 nm. Those of SU-8 films were reported to be ~600 and ~300 nm in [14], but ~200 and ~150 nm in


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[22], and ~100 and ~40 nm in [21]. Those of PMMA films were reported to be ~300 and ~150 nm in [14], but ~200 and ~100 nm in [21], and ~80 and ~40 nm in [26]. Those of PS films were reported to be ~400 and ~200 nm in [14], but ~50 and ~20 nm in [26]. Also, those of PET films were reported to be ~50 and ~30 nm in [25-27], but ~250 and 40-110 nm in [24]. Local surface roughness or curvature has been proposed as a possible reason for the selfformation of nanostructures.14 However, as shown in Figures 2b-ii and 2b-iii, nanostructures were formed uniformly on the PPy(DBS) surface irrespective of the microscale local surface protrusions. Moreover, the structural diameters of the PPy(DBS) nanostructures found in this study (~30 nm) is much smaller than that of nanostructures reported to be caused by local surface curvature (150-300 nm). Thus, it is regarded that the local surface curvature would not be associated with the results obtained in this study. Instead, the other mechanisms are considered as potential primary reasons for the self-formation of nanostructures during the O2 plasma etching, including the co-depositions of impurities and the surface crystallinity of the polymer materials. First, we have verified the co-depositions of impurities that can serve as hard etching masks during the etching process. The deposited inorganic impurities, such as sodium, potassium, aluminum, and antimony, were previously reported either from the residuals in the etching chamber or from the etched polymers.21-23,28 The main source of the impurities protecting the target surface from the plasma etching was also found to be from the holding substrate (the substrate beneath the samples to fit into the etching chamber).24 And the variation of nanostructure diameters (~110 nm vs. ~40 nm) of PET was reported to be induced by the different sputter yields of impurities (Ag vs. Si).24 To investigate the efficacy of the codepositions of impurities on the self-formation of nanostructures on the PPy(DBS) surface, we


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have performed XPS analysis on the nanopatterned PPy(DBS) surfaces etched with different powers (50, 100, 150, and 200 W) at a fixed time (480 s). The PPy(DBS) surfaces without O2 plasma treatment was also analyzed for comparison. The holding substrate used in this work during the oxygen plasma etching was a silicon wafer. Figures 4a and 4b show the high resolution of XPS core level of Si2p and O1s, respectively. No silicon was found on PPy(DBS) surfaces before etching (noted as “initial” in Figure 4), whereas a peak (at 533.5 eV) for C-O or -OH bonds, which are not easy to deconvolution, is found in O1s spectra. After O2 plasma etching, signals for silicon in Si2p spectra were detected and the signal intensity increased with the increase of etching power. This result indicates that more silicon impurities were deposited onto the PPy(DBS) surface with the increase of etching power. The Si2p spectra have been deconvoluted into three peaks of Si2+ (at 101.4 eV), Si3+ (at 102.5 eV), and Si4+ (at 103.5 eV) corresponding to SiO, Si2O3, and SiO2, respectively. Meanwhile, O1s spectra have been deconvoluted into two peaks for C-O or -OH bonds (at 533.5 eV) and Si-O bond (at 532.5 eV). Figures 4c and 4d show the area ratio (%) of each peak for Si2p and O1s spectra, respectively. The areas of Si3+ and Si4+ peaks increase with the increase of etching power, while the peak of Si2+ decreases. Accordingly, the area of the peak for Si-O bonding in O1s spectra is enhanced with the increase of etching power. Higher etching power of O2 plasma would promote the sputtering of silicon impurities as well as the reaction between silicon and oxygen, thus the atomic compositions of Si and O on the surface increase with the etching power, as shown in Figure 4e. In addition, since the higher etching power of O2 plasma also enhances the oxidation of silicon, the higher oxidation state of silicon oxides (e.g., Si2O3 and SiO2) are deposited on the PPy(DBS) surfaces with greater etching power, resulting in the decrease of Si2+ peak with the increase of power.


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These results verify that the silicon oxides (SiO, Si2O3, and SiO2) are deposited onto the PPy(DBS) surface to serve as the etching mask. The silicon impurities could be from the etching chamber walls21,22,28 or the holding substrate24 generated by the sputtering by the plasma ions. In this study, the XPS scan only detected silicon as the metallic element, indicating that the codeposited impurities were primarily from the holding substrate of silicon (see Figure S2 in Supporting Information). It is regarded that the sputtering and co-deposition of impurities would be more significant from the holding substrate than the etching chamber walls, since the holding substrate is closer to the sample and more perpendicular to the plasma than the chamber walls. In addition, as shown in Figure 2, the coexistence of the different lengths of nanostructures (from nanobumps to short and long nanostructures) suggests that the formation of nanostructures was continuous during the O2 plasma etching process resulting from the continuous co-deposition of impurities. It should also be noted in Table 1 that although the lateral dimensions of the nanostructures reported by the different groups varied significantly, those fabricated by the same research group and under the same etching conditions (e.g., using the same etching chambers and holding substrates), which would provide the co-deposition of the similar sizes of impurities, showed similar sizes. For example, regardless of polymer types, the nanostructure diameter reported by Morber et al. lies within the range of 150-300 nm,14 while that by del Campo’s group does within the range of 20-40 nm,25-27 and that by Du et al. does within the range of 40-100 nm.21 Those results further support the co-deposition of impurities for inducing the selfformation of nanostructures during the oxygen plasma etching. In the work by Morber et al.,14 the formation of nanostructures on PS films by plasma etching after stamping a grid (copper) is


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also attributed to this mechanism because the impurities might transmit onto the PS film from the grid and serve as etching inhibitors. Previously, semicrystalline domains, which are affected by the thermal and mechanical treatments of the polymer films, were also proposed as a main mechanism for the formation of nanostructures in O2 plasma etching.25-27 The previous works showed that amorphous domain had a higher etching rate than the crystalline domain by O2 plasma, which resulted in the islandlike structures and formed the high-aspect-ratio nanostructures with the increase of etching time. However, it should be noted that the variation of crystal sizes reported was less than 15 nm.26 Thus, the crystallinity cannot explain the significant variation of nanostructure sizes of the same polymer materials shown in Table 1. Moreover, for the given amorphous polymer which does not exhibit crystalline structures, high-aspect-ratio nanostructures with varying sizes (e.g., from ~40 to ~150 nm for PMMA, and from ~20 to ~200 nm for PS) were also shown, which disputes the primary role of surface crystallinity on the self-formation of nanostructures during the O2 plasma etching. In this study, to modify the crystallinity of PPy(DBS) films and investigate the effects of the modified crystallinity on the morphology of self-formed nanostructures, heat treatments with different temperatures were employed before the application of O2 plasma etching. Based on the glass transition temperature of polypyrrole (PPy, 160-170 °C),29 the electropolymerized PPy(DBS) surfaces (Figure 1c) were heat-treated at 100, 200, and 300 °C for 1 h by ovens and cooled down to room temperature. No melting was observed during the heat treatment, which is in accordance with the literature.29 Then, those heat-treated surfaces and a surface without the heat treatment (as a control) were etched by O2 plasma within the chamber on the same holding substrate of silicon simultaneously and resultant surfaces morphologies were compared, as


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shown in Figure 5. Regardless of the etching times (240 s vs. 600 s at 200 W) and the heat treatment temperatures, the same morphological types (i.e., discrete pillar-like nanostructures) were formed on the PPy(DBS) surfaces. Moreover, the pitch and diameter of the nanostructures did not show significant variations despite the different heat treatment temperatures, when they were measured at the base (root) region of the nanostructures. This result suggests that the surface thermal history does not (at least not the main reason) affect the lateral dimensions (i.e., pitch and diameter) of the self-formed nanostructures. Figure 5 also shows that nanostructures with different lengths still coexist on the PPy(DBS) surfaces even with the different pre-heat treatment histories. It further supports the more dominant role of the co-deposition of impurities on the self-formation of high-aspect-ratio nanostructures during the O2 plasma etching than the crystallinity of polymer films. Meanwhile, the thermal history apparently influenced the shape of the nanostructures. For the surface without pre-heat treatment, dense and straight pillar structures were formed by the O2 plasma etching at relatively short etching time (240 s at 200 W), as shown in Figure 5a. On contrary, with the pre-heat treatment, the nanostructures were inclined to bend and form bundled pillars even at the short etching time (240 s at 200 W), making them look less dense (Figures 5b-d). For the surfaces with elongated etching time (600 s at 200 W), longer nanostructures were formed on the surfaces regardless of the pre-heating temperatures, as shown in Figures 5e-h, which is in good agreement with the previous report.26 However, the bending and bundling of the high-aspect-ratio nanostructures were increasingly significant with the preheat treatment temperatures, which made them look less dense as well. Nonetheless, the result still indicates that the lateral dimensions of the polymer nanostructures such as the pattern periodicity (pitch) should be determined by the density or


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species of the co-deposited impurities sputtered from the holding substrate materials. Thus, it suggests that it should be possible to modulate the pitch of the polymer nanostructures by controlling the density or species of the co-deposited impurities on the polymer surface, which can be done by putting dummy materials near the holding substrate22 or depositing a thin layer of various metal materials on the holding substrate24 or even directly on the polymer surface30 before the plasma etching.

Effects of Nanostructures on Tunable Wettability To investigate the effects of nanostructures on the tunable wettability and mobility of organic droplets on the PPy(DBS) surfaces under the immersion within electrolyte, the apparent contact angles and the sliding angles of DCM droplets (interfacial tension between DCM droplet and electrolyte: 27.8 mN/m) on the PPy(DBS) surface featured with the nanostructures etched at 200 W for 600 s were examined during the redox processes and compared with the PPy(DBS) surface with no nanostructures, as shown in Figure 6 and summarized in Table 2. On the surface without nanostructures, the DCM droplet under oxidization showed a contact angle of 89 ± 5° (Figure 6a) and got pinned on the surface with no roll-off even the surface was tilted vertically (Figure 6b). During the reduction, the contact angle of the DCM droplet increased to 153 ± 3° along with the flattening of the droplet. The droplet rolled off from the reduced surface at the sliding angle of 0.4° and the contact angle hysteresis (difference between advancing and receding contact angles) was measured to be 9 ± 4°, indicating the dramatic change in the droplet mobility via the redox process. The tunable wettability (i.e., the increase of a contact angle) and mobility (i.e., transition from the pinning mode to sliding) of the DCM droplet on the PPy(DBS) surface is due to the


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reorientation and the release of DBS- surfactant dopants to the droplet-electrolyte interface during the redox process.6 When the surface is reduced, the DBS- molecules get reoriented to expose the sulfonic acid group instead of dodecyl chain to the solid-electrolyte interface, making the surface relatively more oleophobic. At the same time, a minute amount of DBS- molecules get released from the PPy chains and accumulate on droplet-electrolyte interface, causing a dramatic decrease of the droplet-electrolyte interfacial tension. The decreased interfacial tension further makes the droplet flattened due to the increased Bond number (i.e., the increased effect of the gravitational body force relative to the interfacial tension force).6,31 As for the DCM droplet on the PPy(DBS) surfaces with nanostructures, the initial contact angle under oxidation (148 ± 5°) was much greater than that on the surface without nanostructures (89 ± 5°). The dramatic increase of the contact angle even under the same oxidation state is due to the high-aspect-ratio pillared nanostructures, which support the droplet being in the Cassie-Baxter wetting state (i.e., the surrounding electrolyte liquid gets entrapped between the nanostructures so that the DCM droplet only partially contacts to the tip surface of the PPy(DBS)).32 In addition, the discrete pillared nanostructures reduce the effective contact line density and the pinning so that the contact angle hysteresis and the retention force between the droplet and the surface are greatly decreased.33 As a result, the DCM droplet on the oxidized surface featured with nanostructures also rolled off at the moderate sliding angle of 30 ± 3°, whereas that on the oxidized surface with no nanostructures was completely pinned even at the vertically titled angle of 90° (Figure 6b). The contact angle hysteresis on the oxidized surface with nanostructures was 38 ± 10°, which is also significantly lower than that on the oxidized PPy(DBS) surfaces with no nanostructures where the contact angle hysteresis could not even be measured due to the complete pinning. Such a low sliding angle and a contact angle hysteresis


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further indicate that the DCM droplet sits on the nanostructured PPy(DBS) surface in the CassieBaxter wetting state, instead of the Wenzel state where the retention between the droplet and surface should increase dramatically due to the increase of the solid-droplet contact line density (i.e., with the nanostructures wetted by the DCM droplet instead of the electrolyte liquid).34 The nanostructured PPy(DBS) surface also allowed the change of its wettability and droplet mobility during reduction. The droplet also got flattened during the reduction due to the predominance of the gravitational force over the interfacial tension force as a result of the release of the DBS- molecules from the surface followed by the accumulation onto the dropletelectrolyte interface. However, in term of the change of the contact angle, the difference was less prominent compared to the case without nanostructures. While the contact angle of the reduced PPy(DBS) surface with nanostructures (158 ± 3°) was higher than that on the surface without nanostructures (153 ± 3°), the reduction of the oxidized surface with nanostructures increased the droplet contact angle no more than by ~10° (from 148° to 158°), which was much lower than that on the PPy(DBS) surface with no nanostructures (~64°, from 89° to 153°) (Table 2). The oxidized surface with nanostructures already sustained the Cassie-Baxter wetting state for the DCM droplet with a high contact angle, thus the effect of the surface chemistry change (the enhancement of oleophobicity) and the release of DBS- molecules during the reduction did not result in a significant modification of the droplet contact angle. It should also be noted that while the contact angle hysteresis (4 ± 2°) and sliding angle (0.2 ± 0.1°) of the DCM droplet on the reduced PPy(DBS) surface with nanostructures are lower than those on the reduced surface with no nanostructures (9 ± 4° and 0.4 ± 0.1°, respectively), the differences are not as dramatic as in the case of the oxidized surface. It indicates that the impacts of the nanostructures of the PPy(DBS) surface on the oleophobicity and the droplet


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mobility are more significant in the oxidized state than the reduced one. It is because the release of DBS- molecules during the reduction, which decreases the droplet-electrolyte interfacial tension, is the more dominant effect on the contact angle increase and the droplet mobility than the change of the wetting state and the contact line density by the nanostructures. Nonetheless, the results clearly show that the high-aspect-ratio pillared nanostructures result in the significant decrease of the retention of the DCM droplet on the PPy(DBS) surface in both oxidation and reduction states. Moreover, the tunable wettability and mobility of the DCM droplet on the nanostructured PPy(DBS) surfaces during redox were repeatable for full the five cycles tested in this study, indicating the functionality (the switchable wettability) of the PPy(DBS) surfaces was not evidently compromised by the O2 plasma etching. Such advantages resulted from the nanostructures should be of great significance in droplet manipulation such as in situ control of the droplet mobility,7 where the droplet motion can be facilitated by the decreased solid-droplet contact line density and pinning33 as a result of the suspended droplet (i.e., Cassie-Baxter state) on the nanostructure tips.

CONCLUSIONS

We have successfully demonstrated a simple, large-scale, and efficient way to fabricate high-aspect-ratio nanostructures on a conjugated polymer, PPy(DBS) surfaces, by employing an one-step maskless O2 plasma etching method. The effects of etching power and duration on the geometric parameters of the nanostructures (i.e., pitch, diameter, length, and aspect ratio) were systematically investigated. Nanostructure lengths and aspect ratios increased with the increase of etching power and duration, whereas the pitches and diameters remained almost constant. In this work, the PPy(DBS) nanostructures of a length up to ~1 μm and an aspect ratio up to ~32


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were successfully fabricated by the one-step O2 plasma etching. The primary mechanism of the self-formation of nanostructures was corroborated to the co-deposition of impurities on the surfaces sputtered from the holding substrate by the analysis of XPS core level spectra, which explains the reason why the pitches and diameters of the nanostructures were not dependent on the O2 plasma etching parameters or thermal history of the polymer. Other evidences also support that the co-deposition of impurities should be the main reason for the self-formation of nanostructures, including the coexistence of the nanostructures of different lengths and the similar lateral dimensions of the nanofeatures etched from the same etching conditions despite different polymer types. The results illuminate that efforts should be primarily made on the modification of deposited impurities in the maskless O2 plasma etching processes of polymers to control the morphology of resultant nanostructures for tailored applications. In addition, we have demonstrated the potential application of PPy(DBS) surfaces decorated with the high-aspectratio nanostructures for tunable wettability and droplet mobility. The droplet manipulation under redox processes was facilitated by the high-aspect-ratio nanostructures on the PPy(DBS) surfaces, compared to the flat surface with no nanostructures, because the solid-droplet contact line density was significantly reduced by the nanostructures supporting the Cassie-Baxter wetting state. This study paves the way to better understand the formation of nanostructures on polymer surfaces by the one-step maskless O2 plasma etching process and offers potential applications of this technique to realize functional polymer materials such as the surfactantdoped conjugated polymer PPy(DBS) to be equipped with tunable superoleophobicity, which can yield a broad range of impact on droplet-based applications such as microfluidics, lab-on-chips, surface cleaning, and oil recovery.


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ASSOCIATED CONTENT Supporting Information. SEM images of the morphology of PPy(DBS) surfaces before O2 plasma etching subjected to thermal treatment at temperatures of 100, 200, and 300 °C for 1 h and that without thermal treatment are available at Figure S1. XPS scan of PPy(DBS) surfaces etched by O2 plasma is available at Figure S2.

AUTHOR INFORMATION Corresponding Author * Chang-Hwan Choi, Tel.: +1-201-216-5579; Fax: +1-201-216-8315; e-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work has been supported in part by National Science Foundation Awards (ECCS1202269 and CMMI-1462499) and an American Chemical Society Petroleum Research Fund (PRF# 56455-ND5). The authors thank Dr. Tseng-Ming Chou for this assistance on sputter coating for SEM imaging within the Laboratory for Multiscale Imaging at Stevens Institute of Technology.


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Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Brédas, J. L.; Lögdlund, M.; Salaneck, W. R. Electroluminescence in Conjugated Polymers. Nature 1999, 397, 121-128.

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Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; LangeveldVoss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; De Leeuw, D. M. Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685-688.

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Thomas Iii, S. W.; Joly, G. D.; Swager, T. M. Chemical Sensors Based on Amplifying Fluorescent Conjugated Polymers. Chem. Rev. 2007, 107, 1339-1386.

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Xu, W.; Xu, J.; Choi, C. H.; Yang, E. H. In Situ Control of Underwater-Pinning of Organic Droplets on a Surfactant-Doped Conjugated Polymer Surface. ACS Appl. Mater. Interfaces 2015, 7, 25608-25617.

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Xu, W.; Xu, J.; Li, X.; Tian, Y.; Choi, C. H.; Yang, E. H. Lateral Actuation of an Organic Droplet on Conjugated Polymer Electrodes: Via Imbalanced Interfacial Tensions. Soft Matter 2016, 12, 6902-6909.

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Yuan, J.; Liu, X.; Akbulut, O.; Hu, J.; Suib, S. L.; Kong, J.; Stellacci, F. Superwetting Nanowire Membranes for Selective Absorption. Nat. Nanotechnol. 2008, 3, 332-336.


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Liu, X.; Ye, Q.; Yu, B.; Liang, Y.; Liu, W.; Zhou, F. Switching Water Droplet Adhesion Using Responsive Polymer Brushes. Langmuir 2010, 26, 12377-12382.

(10) Yao, X.; Gao, J.; Song, Y.; Jiang, L. Superoleophobic Surfaces with Controllable Oil Adhesion and Their Application in Oil Transportation. Adv. Funct. Mater. 2011, 21, 42704276. (11) Jaramillo, M. D. C.; Torrents, E.; Martínez-Duarte, R.; Madou, M. J.; Juárez, A. On-Line Separation of Bacterial Cells by Carbon-Electrode Dielectrophoresis. Electrophoresis 2010, 31, 2921-2928. (12) Ko, T. J.; Her, E. K.; Shin, B.; Kim, H. Y.; Lee, K. R.; Hong, B. K.; Kim, S. H.; Oh, K. H.; Moon, M. W. Water Condensation Behavior on the Surface of a Network of Superhydrophobic Carbon Fibers with High-Aspect-Ratio Nanostructures. Carbon 2012, 50, 5085-5092. (13) Xu, J.; Wang, K.; Zu, S. Z.; Han, B. H.; Wei, Z. Hierarchical Nanocomposites of Polyaniline Nanowire Arrays on Graphene Oxide Sheets with Synergistic Effect for Energy Storage. ACS Nano 2010, 4, 5019-5026. (14) Morber, J. R.; Wang, X.; Liu, J.; Snyder, R. L.; Wang, Z. L. Wafer-Level Patterned and Aligned Polymer Nanowire/ Micro- and Nanotube Arrays on Any Substrate. Adv. Mater. 2009, 21, 2072-2076. (15) Oh, S.; Kim, Y.; Choi, Y. Y.; Kim, D.; Choi, H.; No, K. Fabrication of Vertically WellAligned P(VDF-TrFE) Nanorod Arrays. Adv. Mater. 2012, 24, 5708-5712. (16) Mariani, G.; Wang, Y.; Wong, P. S.; Lech, A.; Hung, C. H.; Shapiro, J.; Prikhodko, S.; ElKady, M.; Kaner, R. B.; Huffaker, D. L. Three-Dimensional Core-Shell Hybrid Solar Cells Via Controlled in Situ Materials Engineering. Nano Lett. 2012, 12, 3581-3586.


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(17) Beckwith, K. S.; Cooil, S. P.; Wells, J. W.; Sikorski, P. Tunable High Aspect Ratio Polymer Nanostructures for Cell Interfaces. Nanoscale 2015, 7, 8438-8450. (18) Mårtensson, T.; Carlberg, P.; Borgström, M.; Montelius, L.; Seifert, W.; Samuelson, L. Nanowire Arrays Defined by Nanoimprint Lithography. Nano Lett. 2004, 4, 699-702. (19) Cheung, C. L.; Nikolić, R. J.; Reinhardt, C. E.; Wang, T. F. Fabrication of Nanopillars by Nanosphere Lithography. Nanotechnology 2006, 17, 1339-1343. (20) Wathuthanthri, I.; Liu, Y.; Du, K.; Xu, W.; Choi, C. H. Simple Holographic Patterning for High-Aspect-Ratio Three-Dimensional Nanostructures with Large Coverage Area. Adv. Funct. Mater. 2013, 23, 608-618. (21) Du, K.; Wathuthanthri, I.; Liu, Y.; Kang, Y. T.; Choi, C. H. Fabrication of Polymer Nanowires Via Maskless O2 Plasma Etching. Nanotechnology 2014, 25, 165301. (22) Chen, M. H.; Chuang, Y. J.; Tseng, F. G. Self-Masked High-Aspect-Ratio Polymer Nanopillars. Nanotechnology 2008, 19, 505301. (23) De Volder, M. F. L.; Vansweevelt, R.; Wagner, P.; Reynaerts, D.; Van Hoof, C.; Hart, A. J. Hierarchical Carbon Nanowire Microarchitectures Made by Plasma-Assisted Pyrolysis of Photoresist. ACS Nano 2011, 5, 6593-6600. (24) Ko, T. J.; Oh, K. H.; Moon, M. W. Plasma-Induced Hetero-Nanostructures on a Polymer with Selective Metal Co-Deposition. Adv. Mater. Interfaces 2015, 2, 1400431. (25) Wohlfart, E.; Fernández-Blázquez, J. P.; Knoche, E.; Bello, A.; Pérez, E.; Arzt, E.; Del Campo, A. Nanofibrillar Patterns by Plasma Etching: The Influence of Polymer Crystallinity and Orientation in Surface Morphology. Macromolecules 2010, 43, 99089917.


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(26) Fernández-Blázquez, J. P.; Del Campo, A. Templateless Nanostructuration of Polymer Surfaces. Soft Matter 2012, 8, 2503-2508. (27) Fernández-Blázquez, J. P.; Serrano, C.; Fuentes, C.; Del Campo, A. Distinct Nanopatterns on Dry Etched Semicrystalline Polymer Films Controlled by Mechanical Orientation. ACS Macro Lett. 2012, 1, 627-631. (28) Lee, C. G. N.; Kanarik, K. J.; Gottscho, R. A. The Grand Challenges of Plasma Etching: A Manufacturing Perspective. Journal of Physics D: Applied Physics 2014, 47. (29) Biswas, M.; Roy, A. Thermal, Stability, Morphological, and Conductivity Characteristics of Polypyrrole Prepared in Aqueous Medium. J. Appl. Polym. Sci. 1994, 51, 1575-1580. (30) Fang, H.; Wu, W.; Song, J.; Wang, Z. L. Controlled Growth of Aligned Polymer Nanowires. Journal of Physical Chemistry C 2009, 113, 16571-16574. (31) Xu, J.; Palumbo, A.; Xu, W.; Yang, E. H. Effects of Electropolymerization Parameters of PPy(DBS) Surfaces on the Droplet Flattening Behaviors During Redox. J. Phys. Chem. B 2016, 120, 10381-10386. (32) Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546-551. (33) Xu, W.; Choi, C. H. From Sticky to Slippery Droplets: Dynamics of Contact Line Depinning on Superhydrophobic Surfaces. Phys. Rev. Lett. 2012, 109, 24504. (34) Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988-994.


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Table 1. Comparison of lateral dimensions of polymer nanostructures fabricated by O2 plasma etching processes reported in the literatures.

Proposed mechanism

Polymer material

Etching gas

Etching power (W)

PMMA14 Local surface curvature

14

PPy

Ar, O2, & CF4

14

PS

SU-8

400

14 21

PMMA Co-deposition of impurities

50

Structural diameter (nm)

~300

~150

~500

~300

~400

~200

~600

~300

~200

~100

~100

~40

SU-8

21

SU-8

22

CF4 & O2

150

~200

~150

24

O2

N/A

~250

~110 & ~40

~50

~30

~80

~40

~50

~20

PET PET Semicrystalline domains

O2

Structural pitch (nm)

25-27 26

PMMA

O2

100

26

PS


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Table 2. Apparent contact angle, sliding angle, advancing/receding angle, and contact angle hysteresis (CAH) of a DCM droplet on PPy(DBS) surfaces without and with nanostructures at different redox states.

Oxidized Nanostructures

Reduced

θi (°)

θs (°)

θa/θr (°)

CAH (°)

θi (°)

θs (°)

θa/θr (°)

CAH (°)

Without

89 ± 5

Pinned

Pinned

Pinned

153 ± 3

0.4 ± 0.1

160/151 ± 2

9±4

With

148 ± 5

30 ± 3

161/123 ± 5

38 ± 10

158 ± 3

0.2 ± 0.1

161/157 ± 2

4±2


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Figure 1. Schematics of the fabrication processes of PPy(DBS) nanostructures. (a) Polished bare silicon substrate. (b) Thin films of Cr (10 nm) and Au (30 nm) are deposited successively on the bare silicon substrate. (c) PPy(DBS) film is coated on the Cr/Au layer by electropolymerization. (d) PPy(DBS) nanostructures are fabricated by the one-step maskless O2 plasma etching process.


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Figure 2. SEM images of the PPy(DBS) surfaces etched by O2 plasma with (a) 50, (b) 100, (c) 150, and (d) 200 W. The main images are 45o-tilted views and the insets are cross-sectional views. The five rows represent the surfaces etched with different etching times, namely 120 (i), 240 (ii), 360 (iii), 480 (iv), and 600 s (v). The scale bars in the main images and the insets are 1 μm and 500 nm, respectively.


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Figure 3. Characterizations of the geometric dimensions of the nanostructures of PPy(DBS) etched by the one-step maskless oxygen plasma process with various etching times and powers: (a) center-to-center pitch, (b) diameter, (c) length, and (d) aspect ratio (length to diameter).


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Figure 4. X-ray photoelectron spectroscopy (XPS) analysis results of PPy(DBS) surfaces etched by O2 plasma with different powers at a fixed duration (480 s). XPS core-level spectra for Si2p and O1s with respect to the etching power are shown in (a) and (b), respectively. The grey dots represent the raw data and the colored solid lines are the fitting lines. Detailed results for the area percentage changes of silicon and oxygen are shown in (c) and (d), respectively. Si2+, Si3+, and Si4+ are marked by blue, red, and green colors, respectively, in (a) and (c). Si-O and C-O or -OH are represented by blue and red colors, respectively, in (b) and (d). Atomic percentage changes of silicon and oxygen are shown in (e). Silicon and oxygen are represented by red-cross and light blue-cross bars, respectively.


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Figure 5. SEM images of the etched PPy(DBS) surfaces with pre-heat treament at different temperatures. (a) and (e): without heat treatment. (b) and (f): 100 °C. (c) and (g): 200 °C. (d) and (h): 300 °C. Surfaces with short etching time (200 W for 240 s) and long etching time (200 W for 600 s) are shown in (a)-(d) and (e)-(h), respectively. The scale bars in the main images and the insets are 1 µm and 500 nm, respectively.


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Figure 6. Tunable wettability and mobility of the DCM droplet on the PPy(DBS) surfaces featured with nanostructures under the immersion within electrolyte, compared to those with no nanostructures. (a) The changes of apparent contact angles with respect to redox cycle. (b) Droplet mobility (sliding behaviors) on oxidized and reduced surfaces.


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TOC Graphic

90°

30°

Not etched O2 Plasma

Substrate

Table of Content


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Nanotexturing of Conjugated Polymers via One-Step Maskless

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