Directed Self-Assembly of Asymmetric Block Copolymers in Thin Films

Feb 1, 2018 - In summary, we demonstrate that highly ordered BCP nanostructures can be obtained on PTFE grating patterns, produced by a physical rubbi...
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Directed Self-Assembly of Asymmetric Block Copolymers in Thin Films Driven by Uniaxially Aligned Topographic Patterns Dong-Eun Lee, Jaegeon Ryu, Dongki Hong, Soojin Park, Dong Hyun Lee, and Thomas P. Russell ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08226 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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Directed Self-Assembly of Asymmetric Block Copolymers in Thin Films Driven by Uniaxially Aligned Topographic Patterns

Dong-Eun Lee1, Jaegeon Ryu2, Dongki Hong2, Soojin Park2, Dong Hyun Lee1*, and Thomas P. Russell3*

1

Department of Polymer Science and Engineering, Dankook University, 152 Jukjeon-ro, Suji-gu, Yongin-si, Gyeonggi-do, 16890, Republic of Korea.

2

Department of Energy Engineering, School of Energy & Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea. 3

Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003, USA.

Corresponding Author *To whom correspondence should be addressed. E-mail: [email protected]; Telephone: 413-577-1516; Fax: 413-577-1510. *To whom correspondence should be addressed. E-mail: [email protected]; Telephone: +8231-3009-3589; Fax: +82-31-8021-7218.

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Abstract : We present a simple, versatile approach to generate highly ordered nanostructures of block copolymers (BCPs) using rubbed surfaces. A block of poly(tetrafluoroethylene) (PTFE) was dragged across a flat substrate surface above the melting point of PTFE transferring a highly aligned PTFE topographic pattern to the substrate. Si wafer, glass, and polyimide films were used as substrates. Thin films of cylinder-forming asymmetric polystyrene-block-poly(2-vinyl pyridine) copolymers (S2VPs) were solvent-annealed on the surfaces having the transferred surface pattern to induce their directed self-assembly (DSA). Cylinders of P2VP oriented normal to the surface are markedly aligned along the rubbing direction and used as templates to generate extremely uniform arrays of various metallic nanoparticles of gold, silver, and platinum over a large area.

KEYWORDS : block copolymer, directed self-assembly, thin films, solvent-annealing, and topographic patterns.

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Block copolymers (BCPs) are composed of two or more chemically distinct polymers covalently linked together, so that they self-assemble into arrays of ordered microdomains, including spherical, cylindrical, and lamellae, depending on the relative volume fractions (f) of each component, the Flory-Huggins segmental interaction parameter (߯), the rigidity of the blocks, and the molecular chain length (N).1 These well-defined BCP nanostructures have been intensively investigated as a bottom-up approach for the generation of templates and scaffolds for the fabrication of storage media, catalysts, nanoreactors, and low dielectric constant materials.2-5 While BCPs exhibit a well-organized array of nanostructures with short-range order, long-range order over macroscopic areas has been difficult to achieve, restricting their use in addressable media. Directed self-assembly (DSA) strategies, including shear alignment, electric or magnetic field alignment, rapid photo-thermal annealing, zone annealing, patterned interfacial interaction, solvent vapor annealing, templated-assisted assembly, and zone casting have been developed to promote long-range ordering of the microdomains.6-17 However, these approaches have only been used over relatively small areas due either to the technical complexities in patterning a large area or the cost or time required to execute the patternings.18,19 Even though this method is the most effective way to generate ordering of BCP thin films with near perfect lateral order, it has met with difficulties in practical use. Russell and coworkers reported a strategy to generate macroscopic arrays of cylindrical microdomains with an orientation of the microdomains parallel to or normal to the films surface, using the reconstructed surface of a miscut sapphire or silicon single crystal substrate, characterized by a saw tooth topographic pattern on the surface where the pitch and orientation of the saw tooth pattern is dictated by the crystal lattice of the substrate, essentially translating the atomic orientation of the lattice to macroscopic length scales.20-22 This method affords an effective and inexpensive way to guide

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the BCP microdomains since all lithographic processes are eliminated by essentially combining two self-assembly processes, i.e. the surface reconstruction and the self-assembly of the BCP. Here, we show that a macroscopic array of highly ordered arrays of poly(2-vinylpyridine) cylindrical microdomains oriented normal to the surface can be generated by using uniaxially aligned topographic patterns of poly(tetrafluoroethylene) (PTFE) on various substrates. By dragging a PTFE block across the substrate, a soft grating pattern of PTFE, oriented in the dragging direction, is produced by the transfer of PTFE to the substrate surface.23-26 These grating patterns can effectively guide the BCP microdomains, so that the long-range ordering of BCP thin films can be achieved during solvent-annealing process. By introducing various metal precursors to the BCP thin films, highly organized arrays of metal nanoparticles with uniform size can be produced. This strategy represents a simple, effective route to obtain macroscopic order on virtually any surface.

RESULT AND DISCUSSION Figures 1(a) and 1(b) show AFM images of the PTFE grating patterns on flat Si substrates fabricated at different temperatures. These results clearly show highly aligned PTFE grating patterns along the rubbing direction and their dimensions could be varied by changing the temperatures used in the rubbing process. With increasing the temperature from 310 ℃ to 350 ℃, the pitch of the grating patterns increased from 68 nm to 248 nm while their heights ranged from 10 nm to 30 nm, respectively. The fabrication of grating patterns of soft polymeric materials occurs by a friction transfer as the two surfaces slide past each other. It was confirmed that PTFE was readily oriented along the rubbing direction. The transfer can be regulated by

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changing the temperature, dragging rate, and loading pressure. PTFE has a low coefficient of friction but a high rate of wear, since its chain is rigid, assuming a helical conformation due to steric effect of the fluorine atoms along main chain.23-26 When a block of PTFE is dragged over a flat surface under a defined pressure at temperatures close to the melting point, the PTFE is transferred to the substrate and the PTFE chains are oriented in the sliding direction. As shown by others, the transfer does not result in a uniform coating of PTFE but, rather, a highly aligned grating pattern with a pitch and amplitude regularly governed by the rubbing process.23-26 Figure S1 shows the XRD reflection patterns of both the deposited PTFE grating patterns and the bulk PTFE, respectively. As can be seen, the peaks of the bulk PTFE were observed at 18.1 °, 31.57 °, 37.14 ° and 41.34 °, corresponding to the (100) and (110) lattice planes of the hexagonal unit cell.27,28 A strong reflection at 18.1 ° was observed for the PTFE grating patterns used in this study. Other reflections were absent due to the highly oriented nature of the polymer in the grating, the resultant orientation of the crystal lattice, and the orientation of the diffraction vector normal to the film surface, and the paucity of material on the surface. Consequently, by drawing a PTFE block across the surface at a controlled temperature and rubbing rate with constant loading pressure, a topographical grating pattern of PTFE could be produced, as shown in Figure 1. Even though the alignment is very high, imperfections are evident with some PTFE (white arrows) grating patterns being 3 ~ 4 times larger than the others. Minimization of such defects is important, since they give rise to low surface energy areas where the BCP solution can dewet during coating. From our AFM results, a temperature of 340 ℃, ~ 10 ℃ higher than the melting point of PTFE (Figure S2), was found to be optimal for obtaining uniform arrays of PTFE grating patterns. Details of the PTFE grating patterns used in this study are summarized in Figure 1(c). This result show that the larger crystalline strands of PTFE in bulk phase can be

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easily transferred at higher temperature during the rubbing process due to the higher mobility of PTFE chains. An asymmetric polystyrene-block-poly(2-vinylpyridiene) copolymer (S2VP245k; Mn = 245 kg/mol) having P2VP cylindrical microdomains (fP2VP = 0.29) in a PS matrix was used to investigate the ability of the PTFE grating patterns to direct the lateral orientation of the array of self-assembled microdomains. Since PTFE has a very low surface energy (γ = 18 ~ 20 mJ/m2), it is difficult to prepare uniform BCP thin films on a PTFE coated the surface. The surface of the PTFE grating patterns was treated by an oxygen plasma prior to spin-coating to enhance wettability of the BCP thin films.29,30 As seen in Figure S3, with increasing exposure times to O2 plasma from 0 to 60 sec, the water contact angle decreased from 102 ° to 25 °. Contact angles < 90 ° are wettability with water (hydrophilic), while large contact angles (> 90 °) are not (hydrophobic).31 Therefore, spin-coating of BCP solutions was done surfaces that were O2 plasma-treated for more than 15 seconds. It should be noted that the height reduction of the PTFE grating patterns was about 6 nm after O2 plasma treatment. A thin film of S2VP245k was spin-coated onto the PTFE grating patterns from a toluene solution. The thickness of the resulting BCP thin films was ~ 48 nm. As shown in Figure 2(a), the initial spin-coated S2VP245k film showed ill-defined spherical domains, ~ 34 nm in diameter, arising from the micellar structures of the BCP in toluene, since toluene is a selective solvent for PS. Grooves are seen from the underlying PTFE grating patterns. The fast Fourier transform (FFT) image in the inset of Figure 2(a) shows a broad halo, characteristic of the irregular array of the BCP microdomains. The morphologies of S2VP245k thin films were developed by solvent vapor annealing in tetrahydrofuran (THF) vapor. A hexagonally packed array of P2VP cylindrical microdomains oriented normal to the surface was found after 1 hour of solvent-annealing time,

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as shown in Figure 2(b). Interestingly, despite some defects, the AFM results show that the underlying PTFE patterns can effectively guide the self-assembly of the microdomains. The inset of Figure 2(b) also shows six distinct spots, characteristic of a hexagonal array of P2VP microdomains with short-range order. With a longer annealing time of 6 hours, the hexagonal array of P2VP microdomains is well-developed and highly aligned with the underlying PTFE grating over a very large area (5 µm x 5 µm), as shown in Figure 2(c). Here, the average pitch of the PTFE grating patterns was ~147 nm with a height of 21.8 nm, while the domain spacing and center-to-center distance of BCP cylinders were 79.6 nm and 92 nm, respectively. The FFT in the inset of Figure 2(c) shows the six-fold symmetry, characteristic of the hexagonal lattice packing of the P2VP cylindrical microdomains where the orientation of the lattice is preserved over large distances. In Figure S4(a) and S4(b), two SEM images of aligned BCP microdomains were taken from two different sites, separated by ~ 20 µm. To confirm the long-range lateral order of the BCP microdomains, seven SEM images of a surface-reconstructed BCP thin film were spliced together as shown in Figure S4(c) and S4(d). Russell and coworkers reported that the faceted surfaces of sapphire could be used to align BCP microdomains with long-range lateral ordering and theoretically argued that the orientation arose from packing constraints of the BCPs within the facets, i.e. entropic in origin.20,21 A similar argument can be made here. A Voronoi diagram can be used to assess the long-range ordering of the cylindrical microdomains.32,33 The Voronoi diagrams in Fiugre 3(b) and 3(d), corresponding to AFM images of the solvent annealed BCP thin films on a flat Si substrate (Figure 3(a)) and a substrate with the grating patterns (Figure 3(c)) indicate that the number of five and seven nearest neighbor defects was markedly reduced and that degree of lateral order increased substantially.

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The lateral ordering of P2VP cylinders was also affected by the thickness of BCP thin films. Figure 4(a) and 4(b) show AFM images of solvent-annealed S2VP245k thin films, 25 nm and 112 nm in thickness, respectively, on a grating pattern having a pitch of ~ 128 nm and an amplitude of ~17 nm. For the 25 nm thick film, vertically oriented P2VP cylindrical microdomains aligned along the grating are seen, even though the thin film did not cover fully the surface. For the 112 nm thick film, only a short-range order is observed. Therefore, there is a film thickness where lateral order can be optimized.20,21 While the use of topographic patterning to promote the lateral ordering of BCP microdomains is not new, the simplicity in the preparation of the grating pattern makes this strategy a very simple, yet elegant route to enhance the lateral ordering. In addition, the process should not be substrate specific. Figures S5 show AFM images of PTFE grating patterns and solvent-annealed S2VP245k thin films on a glass substrate (Figures S5(a) and S5(b)) and a polyimide film (Figure S5(c) and S5(d)), respectively. Well-defined PTFE grating patterns were produced on both substrates and the lateral ordering of P2VP cylindrical microdomains was markedly improved. If the PTFE block is pressed onto a rotating disk, grating patterns with a varying curvature can be produced. Shown in Figure 5 is the case where the grating pattern with a 7 cm radius of curvature was produced on a Si substrate. AFM images of solvent vapor annealed S2VP245k film from three different regions (Zone (1), Zone (2), and Zone (3) are shown in Figure 5(b), 5(c), and 5(d), respectively.). As can be seen, laterally ordered hexagonally packed P2VP cylindrical microdomains were obtained. It should be noted that, by changing the annealing condition, cylindrical microdomains oriented parallel to the substrate surface can be obtained which, in turn, can be guided by the topographic patterning.

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The highly ordered BCP thin films were also used as scaffolds for the fabrication of arrays of inorganic nanoparticles. Figure 6(a) shows an AFM image of surface-reconstructed BCP thin films having a pore diameter of ~32 nm after immersion in ethanol.34 Arrays of silica nanoparticle (80 nm in diameter) were obtained by coating PDMS on the surfaces of the reconstructed thin films followed by oxygen plasma treatment as shown Figure 6(b).35-37 The AFM and SEM images of Figure 6(c) and 6(d) show highly ordered arrays of silica nanoparticles over a 12 x 12 µm2 area. By immersing the BCP thin films in aqueous solutions containing metal precursors, like HAuCl4 and Na2PtCl4, the precursors are selectively located in P2VP cylindrical microdomains. After loading the metal precursors into the P2VP cylindrical microdomains, the precursors were converted into nanoparticles with diameters of ~34 nm by oxygen plasma treatment, as shown in Figure 6(e) and 6(f), respectively.38-42 The generation of the SiOx, Au, and Pt nanoparticles was confirmed by EDAX analysis, where characteristic peaks of those metals are evident. The peak positions of Si, Au, and Pt synthesized from the BCPs are 1.77, 2.06, 2.02 keV, respectively, in agreement with published results of others.43-45 By varying the molecular weight of the copolymer, the size of the microdomains and the pitch can be varied. The AFM image (height mode) of solvent-annealed S2VP77k (Mn,PS = 56,000 g/mol, Mn,P2VP = 21,000 g/mol, PDI = 1.06) thin films on PTFE grating patterns (Figure S7) showed that the orientation and lateral ordering of the copolymer can be maintained, while the size of the inorganic nanoparticles can be tailored.

CONCLUSIONS

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In summary, we demonstrate that highly ordered BCP nanostructures can be obtained on PTFE grating patterns, produced by a physical rubbing of PTFE over the surface of a substrate. The guiding ability of the grating, coupled with the enhanced ordering of BCP microdomains using solvent vapor annealing, were used to obtain highly ordered, oriented arrays of block copolymer microdomains. The nature of the substrate was not found to be limiting, with gratings being placed on silicon, glass and polymeric substrates. The copolymer microdomains were then used to generate arrays of inorganic materials, SiOx, Au, and Pt. The simplicity of the approach and the generality of the substrate make the rubbing process described and alternative, very inexpensive route to guiding the self-assembly of BCPs.

EXPERIMENTAL SECTION Materials. Si wafers (p-type, Si) were purchased from LG Siltron Inc. A bar of poly(tetrafluoro ethylene) (PTFE) was provided from IL WON ELPLA and used as it is. Two different molecular weights of polystyrene-block-poly(2-vinyl pyridine) copolymers (S2VP245k (Mn,PS = 175,000 g/mol, Mn,P2VP = 70,000 g/mol, PDI = 1.08) & S2VP77k (Mn,PS = 56,000 g/mol, Mn,P2VP = 21,000 g/mol, PDI = 1.06)) used in this experiment were purchased from Polymer Source. Toluene (SAMCHUN), ethanol (SAMCHUN), chloroform (Sigma-Aldrich), tetrahydrofuran (DUKSAN), isopropyl alcohol (IPA) (DUKSAN), and HPLC-grade water (J.T.BAKER) were used without any further purification. For synthesis of nanoparticle, linear poly(dimethyl siloxane) (PDMS,