Cylinder Phase

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Interfacial Energy-Controlled Top Coats for Gyroid-Cylinder Phase Transitions of PS-b-PDMS Block Copolymer Thin Films In Hyu Ryu, Yongjoo Kim, Yeon Sik Jung, Jong Sung Lim, Caroline A Ross, and Jeong Gon Son ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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ACS Applied Materials & Interfaces

Interfacial Energy-Controlled Top Coats for Gyroid-Cylinder Phase Transitions of PS-bPDMS Block Copolymer Thin Films In Hyu Ryu†, ‡,+, YongJoo Kim§,+, Yeon Sik Jungǁ, Jong Sung Lim ‡, Caroline A. Ross¶, and Jeong Gon Son†,* †

Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST),

Seoul, 02792, South Korea ‡

Department of Chemical and Biomolecular Engineering, Sogang University, Seoul, 04107,

South Korea §

ǁ

KAIST Institute for NanoCentury, KAIST, Daejeon, 34141, South Korea

Department of Materials Science & Engineering, KAIST, Daejeon, 34141, South Korea

¶

Department of Materials Science & Engineering, Massachusetts Institute of Technology,

Cambridge, 02139, United States

KEYWORDS: Gyroid thin film, Block copolymer self-assembly, Sub-10 nm patterning, Top coat, Phase transition, SCFT simulation

ACS Paragon Plus Environment

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ABSTRACT

Block copolymers (BCPs) with a high Flory-Huggins interaction parameter (χ) can form welldefined sub-10 nm periodic structures and can be used as a template for fabrication of various functional nanostructures. However, the large difference of surface energy between the blocks commonly found in high-χ BCPs makes it is challenging to stabilize a useful gyroid morphology in thin film form. Here, we use an interfacial-energy-tailored top-coat on a blended film of a polystyrene-block-polydimethylsiloxane (PS-b-PDMS) BCP and a low-molecular-weight PDMS homopolymer with a hydrophilic end functional group. The top coat consisted of a random mixture of 40% hydrolyzed poly(vinyl acetate)-random-poly(vinly alcohol) (PVA-r-PVAc, PVA40) and PVAc homopolymer. At the optimized top-coat composition, gyroid nanostructures with sub-10-nm strut width can be achieved down to ~125 nm film thickness, which is only 3 times the lattice parameter of the gyroid structure. This is in marked contrast with a mixed morphology of gyroid and cylinders obtained for other compositions of the top coat. Selfconsistent field theoretic (SCFT) simulations were used to understand the effect of the interfacial energy between the top coat and BCP/homopolymer blends on the phase transition behavior of the BCP/homopolymer films.

Introduction The self-assembly of a diblock copolymer (BCP) is a spontaneous microphase separation phenomenon which leads to arrays of periodic microdomains with sizes of a few nm and above. The blocks assemble into microdomains with specific geometry such as body-centered cubic


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spheres, hexagonal cylinders, double gyroid, or lamellae. Directed self-assembly (DSA)1 of BCP thin films has been investigated for semiconductor device manufacturing, forming for example sub-15 nm line/dot patterns2,3 useful for making fin field-effect transistors (FinFETs),4 feature sizes which are below the diffraction limit of photolithography.5–7 Gyroid morphology BCPs have also been explored as long-range interconnected nanoscale templates for other applications including 3D photonic or phononic crystals,8 metamaterials,9 nanoporous membranes,10,11 and ordered bulk hetero-junction solar cells.12 The morphologies of bulk diblock copolymers are determined by the volume fraction of each block and the segregation strength between the two blocks, χN, where χ is the Flory-Huggins interaction parameter and N is the degree of polymerization. The period of the self-assembled pattern scales with N2/3 in the strong segregation regime. High-χ BCPs such as polystyreneblock-polydimethylsiloxane

(PS-b-PDMS)13–15

and

poly(cyclohexylethylene)-block-

poly(ethylene oxide)16 are attractive due to their ability to microphase-separate even at very low N (i.e. low molecular weights), leading to the formation of highly ordered sub-10 nm features with narrow interfaces between the microdomains. To control the morphology of a BCP, an order-to-order phase transition can be induced in bulk states, for example by control of temperature17 or solvent annealing with a selective solvent.18 In a thin films, the orientation of the microdomains is also important and may be controlled by the wetting conditions at the top surface and bottom interface between the BCP film and substrate. The different blocks in BCP films generally have different surface/interfacial energies on the top surface and bottom interface, resulting in a preferential wetting of one block and a parallel orientation of the microdomains with respect to the substrate.19,20 To obtain perpendicular orientation of BCP microdomains,21–26 a thermodynamically neutral surface or interface for both


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blocks has been developed, such as a random copolymer brush,23,27 neutral top coat28 and surfactants25, or external fields such as a kinetic solvent gradient approach29–32, mechanical flow33,34, electric field35 and surface chemical patterning36. For the case of a BCP which has a 3dimensional isotropic gyroid structure in bulk, surface/interface energy differences tend to suppress the gyroid in thin films, and gyroid-structured films are only reported for thicknesses of several hundred nanometers or more, such as polyisoprene-block-polystyrene-block-polyethylene oxide (PI-b-PS-b-PEO) gyroid films12,37 and polystyrene-block-poly(L-lactide) (PS-b-PLLA) gyroid films38,39. We found that bulk gyroid PS-b-PDMS exhibited perforated lamellae structures in very thin film geometry.40 We showed previously that perpendicular (i.e. out-of-plane) orientation of the lamellar or cylindrical microdomains of a high-χ PS-b-PDMS thin film could be obtained,31,41–43 despite the large difference in surface energies γ between the two blocks (γPS ~ 40.8 mN·m-1 and γPDMS ~ 20.4 mN·m-1) and high incompatibility between the blocks (χ=0.14 at 25 ºC).13,44 We produced perpendicular PS-b-PDMS cylinders (with a top layer of in-plane cylinders) by the control of solvent evaporation during the solvent annealing process,31,45 and fully perpendicular PS-bPDMS cylinders by introducing a poly(vinyl acetate)-random-poly(vinyl alcohol) (PVAc-rPVA) layer as a neutral top coat (∆γ < 0.5 mN·m-1)41. Here, we present the stabilization of the gyroid morphology of a PS-b-PDMS (9.5 kg·mol-1-b5.2 kg·mol-1, fPDMS ~ 0.40) in films with a thickness range of 100 nm to 300 nm under solvent annealing by systematically controlling interfacial energies between the BCP and PVAc-r-PVA top coat. A variation in interfacial energy was realized by changing the mixing ratio of PVAc homopolymer and 40% hydrolyzed PVAc-r-PVA (PVA40) in the top coat. To extend the window of stability of the gyroid phase in the BCP46 and to lower the interfacial hydrophobicity


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of the PDMS domain, we add 10 wt% of hydroxyl-terminated low-molecular weight PDMS homopolymer (PDMS-OH) to the PS-b-PDMS. As a result, we obtained gyroid morphology with a sub-10 nm strut-width in films with a thickness down to ~125 nm at an optimized blending ratio of PVAc/PVA40 in the top coat, and observed a phase transition from the gyroid to cylinders by changing the BCP film thickness and PVAc/PVA40 mixing ratio away from the optimized blending ratio. Self-consistent field theory (SCFT) supports the experimental observations and reveals the critical effects of the interfacial energy between the BCP and top coat and the thickness of the BCP film to the stabilization of the gyroid morphology.

Results and discussion Figure 1 shows the schematic of the strategy to produce a cylinder or gyroid morphology in PS-b-PDMS (9.5k-b-5.2k) blended with 10 wt% of hydroxyl-terminated PDMS homopolymer through the variation of top-coat composition. Blending in the homopolymer changes the volume fraction of PDMS in thin film and widens the window of the gyroid phase in the phase diagram by releasing the chain deformation energy at highly curved points of the gyroid morphology.46,47 A slightly reduced hydrophobicity of the PDMS microdomains is also expected from the PDMSOH addition. For an interfacial-energy-controllable top coat, a poly(vinyl acetate) (PVAc) homopolymer and 40% hydrolyzed poly(vinyl acetate)-random-poly(vinyl alcohol) (PVA40) blend system was chosen. By using blends of PVAc/PVA40, surface and interface energy can be controlled through changing the mixing ratio of PVAc/PVA40. The surface energy (γ = γd + γp, where γd is the dispersion component and γp is the polar component) of PVAc and pure PVA are 24.5+12 mN·m-1,44 and 29.1+22.2 mN·m-1,48 respectively, and by interpolation, the surface


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energy of PVA40 is calculated to be 26.3+16.1 mN·m-1. The surface energy of blended films was obtained from a linear combination of those of PVAc and PVA40. For the substrate, we chose UV/ozone-treated Si wafers which can provide a near-neutral surface41 and thus can promote the perpendicular orientation of cylindrical PS-b-PDMS under solvent annealing condition with acetone.31 The BCP/homopolymer solution in cyclohexane was spin-coated on the Si wafer (see methods section for details). The films were fully dried in a vacuum oven at room temperature for 1 hr, then a PVAc/PVA40 solution in acetic acid was spin-coated onto the BCP films. A brief Ar plasma treatment was carried out on the bilayer films in order to crosslink the top surface of the top coat49 to avoid dewetting of the top coat during the solvent annealing, but this treatment did not affect the top coat/BCP interface. The bilayer films were solvent annealed in a vapor of acetone (P where P/Ps ≈ 0.95 and Ps is the saturated vapor pressure, ~ 184.6 mmHg at 20 °C)50 and slowly dried in the chamber until the liquid solvent was fully evaporate, in order to reduce solvent gradient effects during the drying process. The top coat is more swellable and permeable than the BCP so is believed to have minor effects on the solvent uptake in the BCP41. The dried films were rinsed in acetic acid to remove the top coat. To observe the self-assembled nanostructures, O2 reactive ion etching (RIE) was performed to selectively etch the PS and oxidize the PDMS microdomains to form a silica-rich structure. Clrearly for the isotropic gyroid phase in thin films, some level of neutrality is needed since both blocks are present at the interface. Therefore, we firstly estimated the neutrality of the PVAc/PVA40 top coats on the PS-b-PDMS films using simple surface/interfacial energy calculation. We use Wu’s method for calculating interfacial energy changes between semi-dilute polymer solutions with the harmonic-mean.44 From this, effective interfacial energies between


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various top coats and BCPs in the swollen state51 were calculated in Table 1 for blend ratios of PVAc and PVA40 from 10:0 to 0:10. For the calculation, the surface energies of PS (33.9+6.9 mN·m-1), PDMS (19.55+0.85 mN·m-1), and acetone (18+4.3 mN·m-1)44,52 with measured swelling ratios of films during the solvent annealing as presented in Table 2 were also considered. The surface energy of PS is much higher than that of PDMS due to the extremely low polar component of PDMS (~0.85 mN·m-1), while the interfacial energy of PS/top coat is lower than that of PDMS/top coat because of higher hydrophilicity of PVAc/PVA40. As the ratio of PVAc increased, the estimated interfacial energy difference is gradually decreased and the top coat would approach a neutral condition for both blocks. It also should be considered that, in the swollen state, estimated effective interfacial energies between the blocks and the top coats are significantly decreased compared with those in the dry state because of the screening effect resulting from the solvent molecules at the interface between the top coat and BCP film. However, in the range from pure PVAc to pure PVA40, the effective interfacial energy of PDMS with the top coats is always higher than that of PS, which means that a perfectly neutral condition cannot be realized for the neat PS-b-PDMS. We thus decided to add hydroxylterminated PDMS homopolymer for increasing the hydrophilicity of the PDMS domain and reducing the PDMS/top coat interfacial energy. The surface energy of 0.8 kg·mol-1 PDMS-OH can be estimated as 21.5 mN·m-1, slightly higher than bulk PDMS, based on the estimation of the surface energy of end-functionalized polymers.53 The amphiphilic property of the PDMS-OH also can lead to its segregation mainly to the interfaces of the BCP films which effectively increase the hydrophilicity of the PDMS domain. If the interfacial energy of PDMS/top coat decreases to some extent with the addition of PDMS-OH, the interface affinity between BCPs and top coats can be controlled from neutral to preferable to either block of BCPs.


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We firstly observed the top surface morphology of 170-nm-thick PS-b-PDMS films with various blending ratios of the top coat, as shown in Figure 2. In the case of pure PVAc (10:0) top coat/BCP bilayer film (see Figure 2(a)), in-plane oriented cylinders were dominant. BCPs with a top coat with mixing ratio of 8:2 and 6:4 exhibited a mixed morphology of gyroid and cylindrical phases as shown in Figure 2(b) and Figure 2(c). When the mixing ratio of PVAc/PVA40 reached 4:6, a gyroid phase was observed throughout the BCP film in Figure 2(d). The strut-width of the PDMS gyroid was approximately 9 nm, which is similar to the width of cylindrical patterns (~ 9 nm) from our previous study of 16 kg·mol-1 PS-b-PDMS annealed in acetone vapor.31,41–43 It is worth noting that the interfacial tension difference between the blocks and the top coat with 4:6 mixing ratio is not exactly zero, and the interface is slightly PS-favored. Because the volume fraction of the BCP is not 50%, an isotropic gyroid phase is realized when the top coat is slightly preferential to the majority block (PS). This is consistent with previous studies of different neutral conditions of perpendicular lamellae/cylinder phase of PS-b-PMMA BCPs on random copolymer brushes with varying composition.27 Further addition of PVA40 to the top coat induced a mixed morphology of cylinder and gyroid at a mixing ratio of 2:8 (Figure 2(e)) and a pure PVA40 top coat produced an in-plane cylinder morphology as shown in Figure 2(f). For monitoring the morphology changes of the BCP films over a wide area, grazing incidence small angle X-ray scattering (GISAXS) and cross-sectional SEM experiments with the 170 nm thick BCP films were performed, Figure 3. The GISAXS patterns of a BCP film annealed with a top coat with 4:6 mixing ratio in Figure 3(a) exhibits clear diffraction spots which are located along the qz axis at qy = 0.194, 0.263, 0.304, 0.334, 0.348 nm-1, matching with (121) and at qy = 0.203, 0.331, 0.389, 0.406 nm-1, matching with (220) diffraction peaks of a gyroid structure with {121} orientation.17,54,55 Two sets of diffraction patterns at the same qy appear from the reflected


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and transmitted X-ray beam, respectively. The lattice parameter of the gyroid structures (αG) was ) peak (,() ~ 0.406 nm-1) and obtained as 43.7 nm from the horizontally diffracted ( αG=( ∙ √)/,() . The large number of diffraction spots in the GISAXS patterns indicate that highly ordered and preferentially oriented gyroid structures were the dominant morphology. Figure 3(c) is the GISAXS patterns from the BCP films with pure PVAc top coat, and shows strong (11) diffraction peaks at qy = 0.292 nm-1 from in-plane hexagonally packed cylinder structures and very faint peaks at qy = 0.352 nm-1 from perpendicularly oriented hexagonal cylinders. The domain spacing of the cylindrical morphology is ~18.6 nm. The pure PVAc top coat promotes an in-plane cylinder morphology but perpendicular orientation is also formed, attributed to the solvent gradient during the drying process31. In the case of the gyroid/cylinder mixed morphology, in which the top coat has 8:2 mixing ratio, Figure 3(b), the GISAXS patterns show a superposition of the gyroid diffraction peaks at qy = 0.194, 0.263, 0.304, 0.334, 0.348 nm1

and in-plane cylinder diffraction peaks at qy = 0.292 nm-1 marked with a red arrow in Figure

3(b). For clearer viewing of the reported peak positons, the line profiles at the Yoneda maximum from the gyroid, mixed and cylinder GISAXS data were also presented in Figure S1. Based on the GISAXS and SEM results, it is clear that the PS-b-PDMS thin films can exhibit gyroid or cylinder morphologies as a function of the top coat composition. The morphology is present throughout the BCP film thickness. The complete behavior of morphology vs. top coat PVAc/PVA40 mixing ratio and thickness of BCP film is shown in Figure 4. It is noteworthy that the bulk gyroid morphology was present for all film thicknesses over 300 nm regardless of the top coat. However, as film thickness decreased, the interfacial interaction with the top coat starts to play a critical role in determining the morphology. For top coats with preferential wetting of either PS or PDMS, in-plane cylinders are favored, but for a top coat mixing ratio of around 6:4


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the gyroid is stabilized even for thin films. Therefore there is an extended regime of gyroid morphology with the 6:4 top coat surrounded by a regime of mixed morphology of gyroid and cylinder phases, then a cylindrical morphology without gyroids as shown in Figure 4. The gyroid formed under the 6:4 top coat for thickness down to ~125 nm, which is only about 3 times the gyroid lattice parameter αG. Self-consistent field theory was applied to BCP/homopolymer blends with a tunable top surface in order to elucidate the self-assembly behavior of the system. In our simulation, the BCP is modeled with χN=22.5 and volume fraction of PDMS fPDMS=0.325, and the PDMS homopolymer constitutes a volume fraction of 0.1 to match the experimental condition (10% weight percent of PDMS homopolymer). In the bulk model system, a stable gyroid phase is obtained for a unit cell length of 8.0Rg which corresponds to 43.7 nm from GISAXS data, where Rg is the radius of gyration of the BCP. A cylinder morphology is obtained when the unit cell length deviates from 43.7 nm as shown in Figure S2. To find the wetting condition that produces an isotropic gyroid phase in a thin film, we simulated the polymer blend confined between two impenetrable slabs with different wetting conditions near the surface. The thickness of the top and bottom slabs was 20 nm to ensure that the top and bottom of polymer film do not interact due to the periodic boundary conditions. As shown in Figure S3, at wPS=6.25 and wPS=7.5 (slightly favorable wetting conditions to the PS block near surface and bottom), a stable gyroid is obtained, but cylinders appeared when wPS is lower than 6.25 and higher than 7.5. Here wPS is the field difference between PS and PDMS in the field theoretic description of the system (see methods section for details). For a systematic study of phase behavior of BCP and homopolymer blends under the various top coat conditions, we simulated polymer blends under various thickness and values of wPS near


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the top of the film. Figure 5 shows an example of the phase behavior of the BCP/homopolymer blend as a function of wPS at 87.4 nm film thickness which is the commensurate condition for the gyroid morphology in the bulk blend. Compared to the case of the 43.7 nm thick film, the BCP/homopolymer blend self-assembles into gyroid structures for a wider range of wPS (from 5.0 to 7.5) because the 87.4 nm thick film is less confined in the out-of-plane direction and can more easily follow the bulk morphology. Interestingly, at wPS=8.75 from Figure 5(c), a mixed morphology of gyroid and cylinder phases (one or two layers of cylinders on top and gyroid at the bottom) appeared. Since the wetting condition at the top of the film is more attractive to PS, PS is present at the top interface of film which results in cylinder structures near the top, but the gyroid forms nearer the bottom surface of the film. For wPS lower than 5.0 (Figure 5(a)) and higher than 8.75 (Figure 5(d)), the free energy of forming cylinders throughout the film is lower than that of the mixed morphology due to the stronger interface affinity to either PS or PDMS. From this study, we expect that the window of top coat conditions within which gyroid and gyroid/cylinder mixed morphology form will be widened when the film thickness is increased and the film behaves more like the bulk. We also expect that at incommensurate thicknesses, the top coat window for gyroid and mixed morphology will be narrowed. This feature is well represented in Figure 6 which is the complete morphology phase diagram of BCP/homopolymer blends as functions of film thickness and wPS. As expected, the regime of gyroid and mixed morphology is increased as film thickness is increased especially for the thicknesses where commensurability condition is satisfied (film thickness of 43.7, 87.4, 131.1, 174.8, and 218.5 nm). For the points where the commensurability condition is not satisfied, the thickness range for gyroid and mixed morphology is narrowed; however, the range is generally widened as film thickness is increased. These simulation results are well matched to the experimental results from


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Figure 4, and this systematic study of BCP/homopolymer blends under a tunable top coat will provide a pathway to promote continuous BCP gyroid structures even for thin films.

Conclusion In conclusion, transitions between gyroid and cylinder morphology were found in 14 kg·mol-1 PS-b-PDMS thin films over a range of film thickness from 125 nm to 275 nm by introducing an interfacial-tension-tailored top coat and blending with hydroxyl-terminated short-chain PDMS homopolymers. The interfacial energy of the top coat was manipulated via the blending ratio of PVAc homopolymer and 40% hydrolyzed PVAc-r-PVA (PVA40). At the 4:6 blending ratio, isotropic gyroid thin films with sub-10 nm strut-width could be achieved in films as thin as ~125 nm thickness, which is only 3 times the lattice parameter of the gyroid and the thinnest gyroid film reported to date. The self-consistent field theory simulations reproduces the effect of an interfacial-tension-tailored top coat on the phase behavior of the BCP films. This approach suggests a new route to stabilization of the gyroid morphology and a more generalized method for realizing bulk-like isotropic gyroid structures of thin BCP films, simply by controlling the interfacial energy between top coat and BCP. This method can enlarge the opportunities for using gyroid structures as a periodic, interconnected network templates for various applications including metamaterials, photonic/phononic crystals, and high-performance membranes.


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Methods Section Preparation

of

top

coat/block

copolymer

bilayer

films:

Polystylene-block-

polydimethylsiloxane (PS-b-PDMS, Mn ~ 9.5k-b-5.2k, PDI=1.08, fPDMS = 0.40) and hydroxylterminated polydimethylsiloxane homopolymer (Mn ~ 800 g·mol-1) was purchased from Polymersource, Inc. (in Quebec, Canada). Polyvinyl acetate (PVAc, Mn ~ 100 kg·mol-1) and hydrolyzed 40% (PVA40, Mn ~ 72 kg·mol-1) was purchased from Sigma Aldrich, Inc. and Polyscience, Inc., respectively. 2.5~4.5 wt% of 9:1 PS-b-PDMS and PDMS homopolymer mixed solution in cyclohexane was spin-coated onto a UV-ozone treated Si wafer (30 min, UVO cleaner, Ahtech LTS Co., Ltd.). By varying the concentration and spin-rate, the thickness of the films was varied from 100 nm to 300 nm. The BCP films were fully dried in a vacuum oven at room temperature in 1 hr. For the top coats, 4 wt% of PVAc/PVA40 mixed solution in acetic acid was spin-coated onto the previously prepared BCP films. The thickness of top coat film was approximately ~350 nm. After spin-coating, the top coat film was treated with Ar plasma (20 sccm, 20 mTorr, 10 s) for chemical modification at the top surface of top coat to avoid dewetting phenomena of top coat during the solvent annealing process. Thicknesses of the films were measured by spectral reflectomtry (F20, Filmetrics Inc.). Self-assembly of block copolymer films: The films were placed in a 9.3 ml volume chamber with a small leak and exposed to acetone vapor by placing 1.25 ml of acetone in the chamber. The relative vapor pressure of acetone (P/Ps) was approximately 0.95. After the full evaporation of acetone in the chamber (~1 hour), the films were taken out from the chamber and the top coat was selectively removed using acetic acid rinsing. The residual solvent was removed under vacuum. The BCP film was treated with O2 RIE (30 s, 90 W, 6 sccm) to selectively remove the PS domain and oxidize the PDMS to leave a topographical pattern.


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Structural characterization: The morphology of the etched BCP film was observed by a field emission-scanning electron microscope (FE-SEM, JSM-7500F, JEOL) operated at 10 kV. The film was coated with Pt film (10 mA, 20 s). The GISAXS measurements were carried out at the 3C SAXS Beamline of the Pohang Light Source to obtain information on the BCP morphology in the direction of film thickness. The 2D detector with 2048 pixels x 2048 pixels was used to collect data. The incident angle α was approximately 0.08º, which is slightly above the critical angle of the block copolymer film (α =0.06º). The wavelength of the X-rays was 1.2096 Å and the sample-to-detector distance was ~2 m. Self-consistent field theoretic (SCFT) simulation: Gyroid forming PS-b-PDMS block copolymers and PDMS homopolymer blends are simulated with appropriate top coat using the hybrid particle-field simulation method56 which is further modified for addition of homopolymers.57 Top coat and substrate are modeled as impenetrable slabs with thickness of 20 nm to ensure that top and bottom of polymer film do not affect each other. We varied the affinity of the top coat by modeling a polymer brush which attracted PS under the top coat, i.e. the PS field is increased by 0.1wPS and PDMS field is lowered by 0.1wPS, to ensure selectivity near top coat while wPS is fixed to 7.5 at the bottom of the surface. The simulations were performed using the Lattice Boltzmann diffusion equation solver optimized for graphical processing units.58

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.xxx


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The line profiles at the Yoneda maximum from 2D GISAXS, Detailed phase behaviors of BCPs on different unit cell size and wetting condition from the SCFT simulation. Figures S1, S2, S3 (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. +These authors contributed equally.

ACKNOWLEDGMENT We gratefully acknowledge financial support from the Global Frontier Research Program (2011-0032156) funded by the Korean Government (MEST), the R&D Convergence Program of NST (National Research Council of Science & Technology) of the Republic of Korea, and the Korea Institute of Science and Technology (KIST) internal project. The experimental support by the staffs at the 3C beamline of the Pohang Light Source is also gratefully acknowledged.


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Table 1. Surface and calculated interfacial energies of PS and PDMS with air and various ratios of PVAc/PVA top coats corresponding to the melt state, and effective surface and interfacial energies corresponding to the swollen state.

γ

PS-Top

PS-air

PS-PVAc

8:2

6:4

4:6

2:8

PS-PVA40

Melt

40.7

2.93

3.20

3.52

3.88

4.26

4.67

Swollen

32.1

1.044

1.018

1.016

1.049

1.13

1.28

PDMSPVAc

8:2

6:4

4:6

2:8

PDMSPVA40

(mN··m-1)

γ

PDMS-Top PDMS-air -1

(mN··m ) Melt

20.4

10.2

11.1

12.0

12.9

13.8

14.7

Swollen

20.9

2.41

2.72

3.10

3.55

4.08

4.73

Table 2. Swelling ratios (the thickness ratio between the swelled film (ts) and the pristine film (t0)) of the PS-b-PDMS, PS and PDMS films and PVAc/PVA top coat films under P/Ps = 0.95 of acetone vapor at 22 ºC.

Swelling ratio (ts/t0)

PS-b-PDMS

PS

PDMS

Acetone (P/Ps ~ 0.95)

1.32

1.37

1.18

PVAc

PVA40

2.01

1.61


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Figure 1. Schematic images of morphology changes of a thin film BCP with interfacial energycontrolled top coats under a solvent annealing process.


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Figure 2. SEM image of top coat/BCP films with ~170 nm thickness after solvent annealing and removing the top coat. The morphologies evolve from cylinder to gyroid to cylinder, according to the blending ratio of the PVAc/PVA40 top coat: (a) 10:0 , (b) 8:2, (c) 6:4, (d) 4:6, (e) 2:8, and (f) 0:10. The thickness of the top coat was ~350 nm.


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Figure 3. (a-c) GISAXS scattering patterns and (d-f) cross-section SEM images of 170 nm thick BCP films annealed under various 350 nm thick PVAc/PVA40 top coats. (a and d) Gyroid structure formed under 4:6 top coat. Circles and squares denote the diffraction peaks from the gyroid (121) and (220), and red and black denote the peaks from reflected and transmitted X-ray beams, respectively. (b and e) Gyroid/cylinder mixed structures from 8:2 top coat. The red arrow indicates the scattering peak from the in-plane oriented cylinders. (c and f) BCP film with 10:0 top coat. The red hexagon indicates the in-plane cylinders and the faint black hexagon peak originates from perpendicularly oriented cylinders.


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Figure 4. Experimental phase diagram of a solvent-annealed blend of 90 wt% PS-b-PDMS and 10 wt% of PDMS-OH homopolymer, showing the gyroid, cylinder and gyroid/cylinder mixed morphologies of thin films as a function of blending ratio of PVAc/PVA40 top coat and film thickness of BCP.


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Figure 5. SCFT simulation of the phase behavior of 87.4 nm thick films of BCP/homopolymer blends as a function of wetting condition of top coat, wPS for (a) wPS = 2.5, (b) wPS = 6.25, (c) wPS = 8.75, and (d) wPS = 12.5. Green plates represent boundaries of impenetrable slabs defining the film thickness and the blue curve represents the isosurface of 50 % corresponding to the combined volume fraction of the PDMS homopolymer and the PDMS block. At the bottom of the figure, the morphology is shown as a function of wPS from 0 to 15.


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Figure 6. Phase diagram of the BCP/homopolymer morphology as a function of film thickness and wPS from SCFT simulations. Green, red, and blue squares represent cylinders, gyroid and gyroid/cylinder mixed phases respectively.


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