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Perpendicular Orientation of Diblock Copolymers Induced by Confinement between Graphene Oxide Sheets Ki-In Choi, Tae-Ho Kim, Yeonhee Lee, Hyeri Kim, Hoyeon Lee, Guangcui Yuan, Sushil K. Satija, Jae Hak Choi, Hyungju Ahn, and Jaseung Koo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03991 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Perpendicular Orientation of Diblock Copolymers Induced by Confinement between Graphene Oxide Sheets

Ki-In Choi,1,4,∥ Tae-Ho Kim,1,∥ Yeonhee Lee,2 Hyeri Kim,1 Hoyeon Lee,1 Guangcui Yuan,3 Sushil K. Satija,3 Jae-Hak Choi,4 Hyungju Ahn,5 Jaseung Koo1,*

1

Neutron Science Center, Korea Atomic Energy Research Institute (KAERI), Daejeon, 34057, Korea 2

3

Advanced Analysis Center, Korea Institute of Science & Technology, Seoul 02792, Korea

Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899 USA 4

Department of Organic Materials Engineering, Chungnam Nation University, Daejeon, 34134, Korea

5

Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology, Pohang 37673, Korea

*Corresponding author. Email: [email protected]; Tel.: +82 42 868 8436; Fax: +82 42 868 4629 ∥

K.-I. Choi and T.-H. Kim contributed equally to this work.

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Abstract We have studied an orientation structure of self-assembled block copolymers (dPS-bPMMA) of deuterated polystyrene (dPS) and poly(methyl methacrylate) (PMMA) confined between graphene oxide (GO) surfaces. The results of combination techniques, such as neutron reflectivity, time-of-flight secondary-ion mass spectrometry, grazing-incidence small-angle X-ray scattering and scanning electron microscopy, show that self-assembled domains of the block copolymers in thin films near the GO sheets are oriented perpendicular to the surface of the GO monolayers, in contrast to the horizontal lamellar structure of the copolymer thin film in the absence of the GO monolayers. This is due to the amphiphilic nature of the GO, which leads to a non-preferential interaction of both dPS and PMMA blocks. Double-sided confinement with GO monolayers further extends the ordering behavior of the dPS-b-PMMA thin films. Continuous vertical orientation of the block copolymer thin films is also obtained in the presence of alternating GO layers within thick copolymer films.

“For Table of Contents use only”

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Introduction Graphene oxide (GO) has been widely used as a precursor of graphene for various applications of graphene-based composite materials.1-3 One of the key advantages of GO for these applications is its amphiphilic nature, which can be altered by manipulating the functionalization of the hydrophobic carbon basal planes with the hydrophilic oxygen groups.4,5 Depending on the oxidization or reduction procedures, the surface energy of the GO can be tailored to the properties required for each practical application.6-8 Hence, GO can function as a two-dimensional (2D) surfactant molecule that improves the compatibility of polymer blends and stabilizes the colloidal structure for emulsion polymerization systems because of the non-preferential attractive interactions of GO with hydrophilic and hydrophobic molecules.9,10 Novel amphiphilic GO sheets also serve as a neutral surface for block copolymers (BCPs), inducing a vertical orientation of their self-assembled domain by tuning the surface energy of the GO. Kim et al. suggest that the amphiphilicity of GO improves the surface energy and leads to better wettability for both polystyrene (PS) and poly(methyl methacrylate) (PMMA) blocks of the copolymers on the GO surface.11 They also used wrinkled GO morphologies to obtain customized three-dimensional (3D) self-assembled nanopatterning of BCPs.12,13 As a result of the unique flexibility and transparency of GO, oriented nanopatterned BCP films can be transferred onto arbitrary surfaces with a variety of 3D structures. This orientation control of the BCP thin films confined near the solid filler surface is important in terms of practical applications. Alexandre et al. and Ha et al. reported that the ordered microstructure of the BCP matrix in composites containing sheet-like nanoscale fillers significantly influences the physical properties, for example, the highly anisotropic 3


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mechanical behavior of a nanocomposite.14,15 These nanofillers also accelerate the kinetics of formation of phase-separated microstructures.16,17 Confined BCPs formed a parallel orientation of lamellar domains relative to an embedded sheet from the theoretical study.18,19 In addition, these microdomain structures are important for graphene-based nanoelectronics, such as photovoltaic devices and organic light emission devices.3,20-22 One of key factors in improving the power conversion efficiency in BCP solar cell systems is controlling the orientation structure of the copolymer in the active layer confined between graphene-based electrode surfaces.23,24 In this study, we prepared BCP thin films sandwiched between GO monolayer as a model BCP composite system to measure the accurate assembled structure of confined BCPs, including their perpendicular orientation between GO layers. Langmuir–Blodgett (LB) and Langmuir–Schaefer (LS) techniques were employed to deposit the top and bottom GO layers, respectively, on the spin-coated BCP thin films. Neutron reflectivity (NR) and time-of-flight secondary-ion mass spectrometry (ToF-SIMS) were used to probe the ordering structure of BCPs confined in GO sheets along the film depth in overall film surface area. The domain orientation was also confirmed by grazing-incidence small-angle X-ray scattering (GISAXS) and scanning electron microscopy (SEM). We compared the microdomain orientation of BCPs for one-sided confinement and double-sided confinement, namely, the film on the GO monolayer and that between two GO layers, respectively. Furthermore, the GO layers were embedded into thick BCP films to check whether the intercalated GO layers continuously affected the orientation of BCP thin films.

Experimental Section 4


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GO particles dispersed in deionized (DI) water were obtained from graphite flakes (Sigma–Aldrich, St. Louis, MO, USA) treated by Hummers and Offeman’s method.25 Deuterated polystyrene-block-poly(methyl methacrylate) (dPS-b-PMMA, Mw = 37 KDa/48 KDa, polydispersity index =1.07) and all solvents were purchased from Polymer Source Inc. (Dorcal, Quebec, Canada) and Sigma–Aldrich (St. Louis, MO, USA), respectively. For utilization as substrates, polished silicon (Si (100)) wafers from Shin-Etsu (Tokyo, Japan) were partitioned to 1.5-cm2 slices. In order to make their surface hydrophobic, these slices were cleaned by sonication in DI water treated with UV/ozone to remove unspecified contaminants, and then immersed in dilute hydrofluoric acid (HF) solution (H2O: HF, 10:1) to etch the oxide layer. The etched wafers were then thoroughly washed with DI water and dried in a nitrogen stream. To prepare dPS-b-PMMA thin films confined between GO monolayers on the substrate, the LS and LB techniques were applied for the bottom and top layer of the GO,26 as in Figure 2b below. Moreover, to induce the orientation of selfassembled structures, the prepared films were annealed at 221 °С (calibrated values) for 1 hr under vacuum conditions (10-4 Torr). This is already equilibrium condition. The dPS-bPMMA thin films on the GO monolayer exhibit a similar morphology after annealed for 5 hrs at the same temperature (Figure S1). For the multi-layered dPS-b-PMMA films confined by GO monolayers, a dPS-b-PMMA solution in toluene was spin-coated on a UV/ozone-treated Si wafer. The spin-coated films were then carefully floated on the surface of a DI water bath and promptly transferred onto the prepared thin films confined between the GO monolayers. After dry of the films under vacuum for 3 hrs, LS techniques to deposit GO monolayers on top of the samples were performed successively. For tertiary formation of dPS-b-PMMA films confined between GO layers, the above procedure was repeated. To obtain reduced graphene oxide (rGO), GO films on a Si wafer were annealed at 80 °С for 24 h in hydrazine 5


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vapor. After hydrazine treatment, these films were heated to 400 °С under an argon atmosphere for 3 h and cooled to room temperature. For the morphological investigations, GO monolayers and annealed dPS-b-PMMA thin films with and without GO monolayers on the bottom were characterized by using AFM (Nanoscope IIIa; Veeco Instruments Inc., NY, USA) by using the noncontact tapping mode and a silicon nitride tip (size = 5 nm). To compare the surface hydrophobicity of samples, water contact angles were measured with a contact angle meter (Phoenix 150, Surface Electro Optics, Korea). NR was used to investigate the interfacial structures of annealed dPS-b-PMMA thin films and their multilayers confined between GO surfaces. The specular NR measurements were performed with an NG7 reflectometer and PBR beamline at the Cold Neutron Facility of the National Institute of Standards and Technology (Gaithersburg, MD, USA. The NR data were measured as a function of the wavevector transfer, qz (~4π/λsinθ), for which θ was the grazing angle of the incidence neutron beam via specular NR experiments. The obtained NR data were corrected for footprints and background, and analysis of them was conducted through the computational reflectivity profiles by using a Parratt formalism.27 The computational profiles were calculated by using a Levenberg–Marquardt nonlinear leastsquares method by adjusting the thickness, scattering-length densities, and interfacial width of the unknown layers with the least-squares statistic (χ2). Unpolarized Raman spectra were recorded for the GO monolayer. An InVia Raman microscope (Renishaw Ltd., Gloucestershire, UK) was used with an excitation source (incident power of 2 mW) of the 633-nm line from a He–Ne laser. Negative-ion ToF-SIMS analyses were performed by using an ION-TOF GmbH spectrometer (Munster, Germany) equipped with Bi+ and Cs+ primary ion beam sources for 6


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analysis and sputtering, respectively. For the mass data acquisitions, a pulsed 25-keV Bi+ primary ion beam was used to desorb and ionize species from surface of the dPS-b-PMMA thin films with and without GO confinement. In order to induce the charge compensation of the films, pulsed and low-energy electrons were applied. A Cs+ ion source at 3 keV bombarded the surface of the samples with an incident angle of 45°. The target current for the Cs+ ion source with a raster size of 100 × 100 µm2 was continuously maintained at 13.5 nA. For GISAXS and SEM measurements, the PMMA component was entirely removed by the UV irradiation (at the 250 nm for 1 hr) and rinsing with acetic acid to enhance the phase contrast between the two blocks in dPS-b-PMMA thin films. The GISAXS experiment was performed at the 9A beamline of Pohang Accelerator Laboratory (PAL), Korea. The operation conditions were chosen as a wavelength (λ) of 1.12 Å and the sample-to-detector distance of 4.428 m. The incident angle (αi) was set at 0.12 °. Field emission scanning electron microscopy (Magellan400, FEI, USA) was operated with an accelerating voltage of 15kV. To improve the phase contrast by increasing electron density, the PS component was stained by osmium tetroxide (OsO4) before measuring SEM.

Results and Discussion The dPS-b-PMMA thin films confined between the GO monolayers were prepared. We applied the LS and LB techniques for the bottom and top layers of the GO monolayers, respectively.26 A GO suspension in water was used as a subphase, and an amine-terminated alkyl surfactant monolayer, i.e., octadecylamine (ODA), was spread from a chloroform solution (1 mg/mL) onto the surface of the GO suspension as illustrated in Figure 1a. The GO sheets in the water were then able to float to the liquid–gas interface because of the 7


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interaction between the positively charged amine groups on ODA and the oxygenated groups on the GO sheet. Full coverage of the GO monolayer was obtained upon compression to 20 mN/m during surface pressure-area isotherm measurement (Figure S2). The monolayer at the interface was then transferred to large silicon substrates by LS deposition. We measured the surface morphology by using AFM. Figure 1b shows that the GO sheets form a closely packed monolayer by compression with the barriers. The thickness of the layers was measured as 1.35 nm, which indicates a single layer of GO.26,28,29 The presence of GO in the films was confirmed with Raman spectroscopy after deposition of the GO monolayers on the Si substrates, as shown in Figure 1c. Typical peaks of GO sheets, such as the G, D, G+D, and 2D bands exhibited from the spectrum,30 indicating that the GO monolayers were stably

Figure 1. (a) Schematics of the GO monolayer formation by using the LangmuirSchaefer(LS) technique; (b) AFM image of the GO monolayer transferred onto the Si substrate by LS deposition; (c) Raman spectrum of the GO monolayer.

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adsorbed on Si substrates. The dPS-b-PMMA thin films were prepared on the GO monolayer surface by spin coating at 2500 rpm. Even after spin coating of the copolymer layers, the GO monolayers were maintained on the substrates without delamination or detachment, which was checked with AFM after removal of the polymer layers by rinsing with toluene. The top GO monolayers were also deposited from the liquid–gas interface on the spin-coated dPS-bPMMA thin-film surface by the LB technique. In this manner, the copolymer thin films make contact with the bottom and top GO surfaces rather than the ODA-coated side, so there is no effect of the ODA molecules on the self-assembled structure of the BCP. The dPS-b-PMMA thin films confined between GO sheets, as illustrated in Figure 2b, were annealed at 221 °C for 1 hr. The AFM image in Figure S3 shows top GO monolayer on the BCP thin film maintains even after annealing. Underneath the GO monolayer, the vertically-oriented finger print pattern can be seen in the gap between the GO platelets. We employed NR and ToF-SIMS techniques to investigate the self-assembled microdomain structure of the BCP sandwiched between GO monolayers. If one block in the BCP is deuterated (for example, the deuterated PS block in the dPS-b-PMMA used for this study), one can determine the self-assembled microdomain structure of the thin films by characterizing the deuterium distribution along the film depth. Figure 2 shows the specular neutron reflectivity of the dPS-b-PMMA thin film sandwiched between GO monolayers and a control sample without GO. Because of the high contrast between the dPS and hydrogenated PMMA blocks in terms of scattering length density (SLD), we can probe the lamellar orientation structure relative to the film surface in a non-destructive way. The NR data were collected as a function of the wavevector transfer, qz = (4π/λsinθi), by changing the incident (θi) and exit (θf) angles but maintaining θi = θf. The reflectivity profiles for dPS-b-PMMA in 9


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the absence and presence of the GO sheets are shown in Figures 2e and f, respectively. The main difference between the two profiles is the Bragg reflections, which are only seen for the control sample, dPS-b-PMMA on a silicon substrate without GO. This indicates that a multilayered structure within the film comprised alternating dPS and PMMA layers parallel

Figure 2. Schematic representations of (a) a dPS-b-PMMA thin film on the Si substrate and (b) a dPS-b-PMMA thin film confined between GO sheets. SLD profiles of (c) a dPS-b-PMMA thin film on the Si substrate and (d) a dPS-b-PMMA thin film confined between GO sheets. Neutron reflectivity data of (e) a dPS-b-PMMA thin film on the Si substrate and (f) a dPS-b-PMMA thin film confined between GO sheets.

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to the substrate surface. The high-frequency oscillation evident in the data arises from the total thickness (t) of the specimen, calculated to be 928 Å. For horizontal lamella orientation, t=(n+1/2)L0, where n is an integer and L0 is a lamellar spacing. n and L0 were obtained to be 2 and 371 Å, respectively. The total thickness of the specimen was verified independently by an ellipsometry measurement. The slight difference in the thicknesses of the dPS and PMMA domains is because of the differences in the molecular weights and mass densities of the blocks.31 The solid lines in Figures 2e and f are the fit by using the SLD profiles in Figures 2c and d, respectively, where PS is preferentially located at the air/copolymer interface because of the lower surface energy of dPS, and PMMA is preferentially located at the Si surface, most likely because of interactions between PMMA and the substrate. On the other hand, dPS-b-PMMA (302 Å thick) sandwiched between GO sheets exhibits the oscillation with regular frequency without any Bragg reflection. From the SLD file, the overall dPS-b-PMMA film was found to have almost constant SLD along the depth. This indicates that the dPS and PMMA domains orient vertically relative to the Si surface. We found similar results from the ToF-SIMS measurements. The negative-ion ToFSIMS mass spectra of dPS-b-PMMA thin films on Si substrates and the corresponding thin films (302 Å thick) sandwiched between GO sheets were collected after annealing at 221 °C for 1 hr which (Figure 3), AFM experiments as a function of annealing time showed, is sufficient time for equilibrium to be reached. We obtained a 2D reconstructed ToF-SIMS image of specific fragment ion intensities, such as deuterium (2H-; m/z 2.014), from the PMMA blocks and hydrogen (H-) from the dPS blocks, through the course of depth profiling. By using the Cs+ primary-ion bombardment (3 keV), the secondary-ion counts from the copolymer thin films were obtained as a function of the sputtering time (corresponding to a depth from the film surface). From the ToF-SIMS images, we found that, for the copolymer 11


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thin films on the substrates, deuterium (2H-)- and hydrogen (H-)-dominant layers were alternately stacked parallel to the surface of the substrate (Figure 3a).32,33 The depth profile also showed alternating maximum and minimum positions as a result of the distribution of deuterium and hydrogen along the depth as a function of the film depth (Figure 3b). This indicates the parallel orientation of microdomains relative to the surface of the substrates. However, for the copolymer thin films sandwiched between GO monolayers, an almost homogeneous distribution of the deuterium and hydrogen is revealed in the film (Figure 3c). The depth profile also has an almost constant number of secondary ions of both deuterium and hydrogen because of the vertical orientation of the microdomains in the presence of the GO (Figure 3d). The H- depth profile of the GO sandwiched-BCP thin film showed slightly higher intensity of hydrogen ion near the surface (Figures 3d and 9d). This is probably due to matrix effect.34 The top GO layers on BCP thin films enhance the ejection of hydrogen ions

Figure 3. 3D ToF-SIMS images and depth profiles of (a, b) a dPS-b-PMMA thin film on the Si substrate and (c, d) a dPS-b-PMMA thin film confined between GO sheets. 12


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during SIMS measurement. Hence, it is not because of the segregation of the PMMA. In addition, all profiles have transient regions at the early stage of sputtering because of unstable sputtering yield. This effect can be eliminated by the sacrificial layer. We additionally deposited the PS layer on the GO sandwiched-BCP thin films and measured the ToF-SIMS (Figure S4). Although NR and ToF-SIMS have the advantage of measuring the overall structural information along the entire film depth, these techniques provide indirect evidences for vertical orientation of BCP microdomains. For the NR technique, specular reflectivity from the sample surface is generally measured as a function of the wave vector component qz using a point detector. As a result, the SLD profiles along the film depth perpendicular to the surface are obtained and no lateral information is accessible. Also, in case of the homogenous distribution of the deuterium along the film depth from NR and ToF-SIMS results, other domain structures can be possible rather than vertical orientation. However, grazing incidence small angle neutron scattering (GISANS) technique uses a two-dimensional position sensitive detector which allows to measure diffusely scattered information as a function of qxy, yielding in-plane

lateral structure of the films.35-37 In case of one-sided

confinement, there are several other techniques besides the GISANS for characterizing the surface morphology of perpendicularly-oriented BCP patterns. The AFM has been widely used to measure the top surface structure of finger printed patterns.38 In the case of PS-bPMMA, the surface moduli are different for each microdomain; the friction mode of the AFM can provide clearer image of the self-assembled pattern than topographic mode. After selectively etching one block (PMMA block) from the film by UV, the internal structure of the microdomain pattern can be measured using a scanning electron microscope (SEM) and grazing incidence small angle X-ray scattering (GISAXS). They require destructive 13


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processing of the film, but are suitable methods to simultaneously probe surfaces and inner structures. We also used GISAXS technique to characterize the structure of dPS-b-PMMA thin films on GO monolayers (one-sided confinement). This technique provides structural information of nanostructures on BCP films. To obtain full-depth information of structures, the incident angle (αi) was set at 0.12 °. Figure 4a and b show the GISAXS patterns for dPSb-PMMA thin films on the GO monolayer and the in-plane intensity profiles as a function of qxy, respectively, with different thickness of 302 and 404 Å. The in-plane intensity profiles were collected at qr,z = 0 (qr,z is radial diffraction profile with the intensity integrated along zaxis), and the integrating width in ∆qr,z was 7.0 × 10-4 Å. Before GISAXS measurement, the PMMA component of dPS-b-PMMA films was entirely removed by the UV irradiation and

Figure 4. (a) 2D GISAXS patterns of dPS-b-PMMA thin films with thickness of 302 (top), and 404 Å (bottom) on the GO monolayer; (b) in-plane GISAXS intensity profiles of a dPS-b-PMMA thin film on the GO monolayer. 14


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rinsing with acetic acid. A perpendicular structure was observed along the in-plane (i.e., horizontal direction) scattering (qxy = 0.0146 Å-1) with the weak higher order peaks, whereas no apparent peak appeared along the out-of-plane (i.e., perpendicular direction) scattering on both film thickness. The plots of GISAXS images of the line scans along the horizon of reflecting point were shown in Figure 4b. The multiple-order Bragg-rod peaks at scattering vector ratios of higher order peaks and the primary peaks (qxy/qxy*) = 1:2:3 is showed on the both of plots, indicating that high ordered lamellar structures are developed inside of dPS-bPMMA thin films. We also used the AFM and SEM to evidence the perpendicular orientation by measuring the surface and inside structure of dPS-b-PMMA thin films (302 Å thick) on GO monolayers (one-sided confinement). For the control sample, lamellar microdomains were oriented parallel to the substrate, as evidenced by the typical hole and island pattern in the AFM measurement (Figure 5a).39 However, in the case of the sample with the GO surface, the AFM results reveal fingerprint patterns, which indicate a perpendicular orientation of the microdomains (Figure 5b). From SEM results, we also found that this perpendicular orientation occurs both at the film surface and inside film. Figures 6a and b show the SEM images of the self-assembled dPS-b-PMMA films of different thickness on the GO monolayer. These findings are consistent with the results of NR and ToF-SIMS which exhibit the homogeneous distribution of deuterium, indicating the vertical orientation of the BCP (Figures S5 and S6). The combination of our techniques (i.e., NR, ToF-SIMS, GISAXS, AFM, and SEM) is appropriate approach to investigate the both parallel and vertical orientation of confined thin BCP films at the surface as well as inside of the films.

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This vertical orientation can be attributed to the amphiphilic surface properties of GO. Hence, GO acts as a neutral surface that possesses surface energy and is capable of nonpreferential interactions with the PS and PMMA domains. Therefore, the PMMA blocks interact with the oxidized domain of GO through hydrogen bonding, whereas the PS blocks interact with the unoxidized basal plane through van der Waals forces and π–π stacking.3 We

Figure 5. (a) AFM images of hole (top) and island (bottom) of a dPS-b-PMMA thin film on the Si substrate; (b) AFM height (top) and phase (bottom) images of a dPS-b-PMMA thin film on the GO monolayer. 16


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changed the surface energy of the GO by reduction with hydrazine and thermal annealing at 400 °C in an Ar atmosphere. Raman spectra were measured before and after reduction treatment of GO sheets. After the treatments, the intensity ratios of between G and D bands and between 2D and G + D bands, IG/ID and I2D/IG+D were increased respectively (Figure S7) After reduction, structural imperfections due to adhesion of hydroxyl and epoxide groups on the carbon base plane were recovered to graphite structure. Figure 7a shows that the contact angle was increased from 53.8 ° to 71.5 ° after reduction of GO. The decreased surface energy of reduced GO (rGO) ruins the vertical orientation of the microdomains, as observed from the AFM measurements (Figure 7b). Mansky et al. have reported that the random copolymer (RCP) brush on a substrate can be used as a neutral surface with the water contact

Figure 6. SEM images of assembled dPS-b-PMMA films with different thicknesses of (a) 302 and (b) 404 Å on GO monolayers. 17


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angle of around 65° for vertical orientation of BCP thin films.40 Note that the water contact angle of the GO was obtained to be 53.8° for vertical orientation. This discrepancy is probably due to additional π-π interaction of the GO with the PS. The PS contact angle of the GO monolayer (5.7°), which we previously reported in ref 9, is similar to that on the neutral RCP surface to facilitate vertical orientation.40 Interestingly, this perpendicular orientation of the dPS-b-PMMA thin films can be further extended through double-sided confinement by the GO sheet, relative to single-sided confinement. For example, if the dPS-b-PMMA film thickness, d, is greater than 404 Å, the domains in the film are not fully oriented to the GO monolayer with one-sided confinement, as seen from the AFM result in Figure 8. However, dPS-b-PMMA thin films confined between GO sheets, i.e., with both-sided confinement, can be oriented vertically for the

Figure 7. (a) Water contact angle data for GO (53.8°) and rGO (71.5°); (b) AFM images of dPS-b-PMMA thin films (302 Å thick) on the GO and rGO monolayers. 18


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thicker BCP film. We determined the ToF-SIMS spectra of the dPS-b-PMMA thin film (d = 865 Å) on the GO monolayer and the thin film confined between GO sheets (Figure 9). From the results, the dPS-b-PMMA thin films confined between two GO monolayers (both-sided confinement) exhibit the same distribution of hydrogen and deuterium along the depth, which indicates vertical orientation of microdomains in the film (Figures 9c and d) whereas the BCP thin films on the GO monolayer (one-sided confinement) have non-homogeneous distribution of hydrogen and deuterium (Figures 9a and b), thereby exhibiting imperfect vertical orientation of microdomain from the AFM result (Figure 8). Previously, several groups have reported computational simulation studies on BCP assembled structure in confined polymer systems between solid walls.41-44 They investigated

Figure 8. AFM images of assembled dPS-b-PMMA films of with various thicknesses on GO monolayers: film thicknesses of (a) 302, (b) 350, (c) 404, (d) 426, (e) 560, and (f) 865 Å. 19


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the effects of the interactions between the polymer-solid wall and the layer thickness of the polymers between the walls on the orientation of BCP domains. In the case of strongly preferential interactions between the one block of BCP and wall, simulation results reveal that the parallel orientation can be obtained, whereas in the case of weakly preferential surface, the orientation morphologies can vary depending on whether film thicknesses are commensurate with L0. If the surfaces are neutral, however, perpendicular orientation is expected regardless of film thickness.42 The theoretical prediction for neutral surfaces agrees with our findings of BCP films confined between GO sheets, in which BCP thin films with different thicknesses exhibit the vertical orientation when confined between GO sheets. This also indicates that the top and bottom GO monolayers function as neutral surfaces for the dPS-b-PMMA.

Figure 9. 3D ToF-SIMS images and depth profiles of (a, b) a dPS-b-PMMA thin film on the GO monolayer and (c, d) a dPS-b-PMMA thin film (thickness = 865 Å) confined between GO sheets.

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Additionally, we investigated whether the vertical orientation of a thick BCP film could occur at the entire film depth in the presence of alternating GO layers parallel to the substrate surface in the film. We prepared multilayers consisting of three repeating layers of dPS-b-PMMA thin films (d = 379 Å) confined between GO sheets (total thickness is 1238 Å) and investigated whether the microdomains were oriented vertically in each polymer layer (Figure 10a). From the NR results, we found that the SLD values for all layers of dPS-bPMMA between the GO sheets are nearly the same (2.3 × 10-6 Å-2; Figure 10b). This also

Figure 10. (a) Neutron reflectivity data of a dPS-b-PMMA multilayer thin film confined between GO sheets; (b) the corresponding SLD profile of a dPS-b-PMMA multilayer thin film confined between GO sheets; (c) AFM images of a dPS-b-PMMA multilayer thin film confined between GO sheets. 21


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shows the perpendicular orientation of the copolymers induced by GO over the entire film thickness. This was also confirmed by the AFM measurement. We annealed the sample each time we deposited of BCP layers for the multilayer and measured the AFM for each BCP layer as shown in Figure 10c, where fingerprint patterns were observed in all cases. This suggests that the multiple GO layers intercalated in the BCP matrices can induce the vertical orientation of the self-assembled BCP domains perpendicular to the GO surfaces, even when the total thickness is ~125 nm.

Conclusion We measured the orientation structure of the dPS-b-PMMA thin films confined between two GO monolayers (both-sided confinement) using NR and ToF-SIMS techniques and compared the results with structure of the BCP thin films only on the bottom GO monolayer (one-sided confinement) measured by using AFM and SEM. In both cases, the BCPs are perpendicularly oriented relative to the surface of GO monolayer. This is due to the amphiphilic nature of the GO, which leads to a non-preferential interaction of both dPS and PMMA blocks. However, the both-sided confinement of the GO extended the vertical orientation even for the 865 Å-thick BCP thin film. In the case of one-sided confinement, vertical orientation was obtained to be incomplete even with the thickness of BCP thin films higher than 404 Å. This is due to the fact that the top and bottom GO monolayers function as a neutral surface for the dPS-b-PMMA. As a result, insertion of multiple GO layers into a thick BCP film facilitates continuous vertical orientation.

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ACKNOWLEDGEMENTS This work was supported primarily by a grant from by the National Research Foundation of Korea under Contract 2017M2A2A6A01019911. The identification of commercial products does not imply endorsement by the National Institute of Standards and Technology nor does it imply that these are the best for the purpose.

Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: AFM images of dPS-b-PMMA thin film on the GO monolayer annealed at 221 oC for 5 hrs; Surface pressure-area (π-A) isotherm of ODA on the surface of aqueous GO suspensions; an AFM image of GO monolayer on the dPS-b-PMMA after annealing at 221 oC for 1 hr; 3D Tof-SIMS images and depth profiles of a PS thin film on dPS-b-PMMA/GO; Neutron reflectivity profiles of the dPS-b-PMMA/GO, and their corresponding SLD profiles; 3D TofSIMS images and depth profiles of the dPS-b-PMMA/GO; and Raman spectra of GO and rGO sheets before and after the reduction.

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