Laser-Directed Self-Assembly of Highly Aligned Lamellar and

Feb 8, 2018 - We carried out the laser writing process with lamella-forming (Mn: 25–26 kg mol–1) and cylinder-forming (Mn: 36–10.5 kg mol–1) B...
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Laser-Directed Self-Assembly of Highly Aligned Lamellar and Cylindrical Block Copolymer Nanostructures: Experiment and Simulation Daeseong Yong,† Hyeong Min Jin,‡ Sang Ouk Kim,*,‡ and Jaeup U. Kim*,† †

Department of Physics, School of Natural Science, UNIST, Ulsan 44919, Republic of Korea National Creative Research Initiative Center for Multi-Dimensional Directed Nanoscale Assembly, Department of Materials Science and Engineering, KAIST, Daejeon 34141, Republic of Korea

‡

S Supporting Information *

ABSTRACT: Laser photothermal annealing is emerging as a promising strategy for directed self-assembly of block copolymers along with its unique advantages, such as area selectivity, solventfree ultrafast process, and highly oriented nanopattern formation without substrate prepatterning. We investigate laser-induced highly aligned lamellar and cylindrical self-assembled nanostructure formation by means of simulation as well as experiment. Selfassembled surface-perpendicular lamellar or surface-parallel cylindrical nanodomains in PS-b-PMMA thin films could be aligned by lateral steady scan of focused laser irradiation to attain excellent long-range order over 10 μm length scale. For the systematic understanding of the experimental observation, quasi-static simulation employing successive self-consistent field theory calculation has been developed. Miniaturized simulations of experimental systems could confirm a strong tendency for lamellar domains to grow in the direction of laser scanning. Cylindrical self-assembled domains exhibit similar behaviors provided that the surface prefers one block and the block copolymer film thickness is moderate.

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INTRODUCTION Block copolymer (BCP) self-assembly can generate various types of nanostructures with sub-50 nm size,1−3 and it has received much attention as a complementary strategy for conventional photolithography4−6 due to ultrafine resolution, high scalability, and low price. However, because of entropy and incomplete annealing, polygrain structures with many defects are spontaneously formed during the BCP selfassembly, and controlling long-range order has been a longstanding challenge for its practical applications. To date, various approaches for directed self-assembly including permanent fields (chemoepitaxy,7,8 graphoepitaxy5,9−14) or dynamic external fields (thermal field,15−17 shear field,18,19 magnetic field,20,21 electric field22−25) have been proposed. Among these strategies, zone annealing methods using temporal and spatial thermal field allow continuous process of directed self-assembly without a preguidance pattern.17,26 In particular, localized photothermal laser heating allows the formation of an extremely high thermal field, and thus several research groups reported the directed self-assembly of BCPs using laser zone annealing process.27−31 This extreme high thermal field from localized laser heating creates highly ordered BCP structures, and our experimental team demonstrated lateral ordering of surface-perpendicular lamellar structure using laser writing with process temperature (Tpeak) higher than the order−disorder transition temperature (TODT) of the BCP system.15,16 © XXXX American Chemical Society

In this report, we experimentally demonstrate laser writing directed self-assembly of surface-perpendicular lamellae as well as surface-parallel cylinders, and their domain alignment mechanism is studied by quasi-static simulation adopting selfconsistent field theory (SCFT) method. A few theoretical methods have been previously suggested for the modeling of the zone annealing process. In one simulation, Yang and coworkers32 solved the time-dependent Ginzburg−Landau type equation for BCP evolution, and they found that lamellae parallel to the scanning direction were eventually chosen at small enough scanning velocity though lamellae perpendicular to the scanning direction were initially selected at faster scanning velocity. A similar transition was reported in a research using SCFT under time- and space-dependent mobility field.26 Dynamic self-consistent field theory (DSCFT) can also be an attractive theoretical tool for the study of the BCP domain evolution. Zhang, Lin, and co-workers recently reported a few zone annealing simulations using dynamic SCFT.33−35 In their earlier work using two-dimensional DSCFT, the formation of lamellae parallel to the scanning direction is observed for symmetric BCPs at small enough scanning velocity,33 and they also reported lamellae perpendicular to the scanning direction Received: December 13, 2017 Revised: January 9, 2018

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DOI: 10.1021/acs.macromol.7b02645 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules for slightly asymmetric BCPs. In a later work, they tested the efficiency of the zone annealing when the Flory−Huggins interaction parameter χN at the annealed zone is still in the ordered regime.34 They found that the combination of chemoepitaxy and zone annealing can greatly enhance the lamellar alignment, and this idea of the dual-field system is further pursued using three-dimensional DSCFT simulation.35 Most of the above researches performed two-dimensional simulation due to the high computational cost of the BCP evolution calculation, and thus dependence on the film ̈ expectation is that thickness was not accountable. The naive the two-dimensional morphology is extended to a threedimensional structure by the growth of the surfaceperpendicular lamellae. Even though the aforementioned three-dimensional DSCFT simulation35 confirmed that this is often a valid prediction, it also revealed a few nontrivial thickness-dependent behaviors; in general, when all other parameters are fixed, the lamellar alignment degrades as the thickness increases. Moreover, in a two-dimensional simulation, cylindrical domains only appear as hexagonally arranged dots, and their growth parallel to the scanning direction cannot be studied even though it is a possible consequence of the laser scanning. In order to address these issues, we also perform quasi-static simulations using three-dimensional SCFT calculation and investigate the thickness dependence of the zone annealing.

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Figure 1. (a) Schematic illustration of laser zone annealing BCP selfassembly. (b) Two-dimensional temperature profile of localized laser heating. (c) Temperature as a function of distance along the x-axis. pulse duration 200 ns) was used for the laser writing assembly of BCPs. The laser beam was focused into an elliptical shape (diameter dx: 100 μm; dy: 600 μm; and typical power density: 2.33 × 10−5 W/ μm2), and it was irradiated directly onto the sample. From this typical localized laser photothermal effect, a temperature gradient up to 1.35 K/μm was generated on the CMG layer (Figure 1b,c).31 The laser scanned the sample in the lateral direction, and the scan velocity was carefully controlled from 100 to 5000 nm/s by a linear motorized stage [PRO165LM-0400 (Aerotech)] (Figure S6).

MATERIALS AND EXPERIMENTAL METHODS

Materials. All BCPs were purchased from Polymer Source Inc. and used without purification. Short random copolymers were synthesized by nitroxide-mediated living radical polymerization.36 Graphite powder, toluene, and sulfuric acid (H2SO4) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl), hydrogen peroxide (H2O2, 30% aqueous solution), potassium permanganate (KMnO4), and isopropyl alcohol (C3H8O, 99.5%) were purchased from Junsei Chemical Co., Ltd. Ruthenium tetroxide (RuO4) staining agent was purchased from Electron Microscopy Sciences. Preparation of Chemically Modified Graphene Substrate and BCP Thin Films. Few-layered graphene oxide (GO) film was spin-coated with the gentle blowing of N2 gas at target mother substrate, such as bare glass or quartz. The thickness of GO film could be controlled by the concentration of GO solution and spin RPM. Then GO films were chemically modified by (i) thermal treatment (700 °C, 1 h) or (ii) chemical reduction under hydrazine monohydrate vapor for 1 h.37 This chemically modified graphene (CMG)38 layer served as a surface energy modifier controlling the orientation of the BCP structure as well as photothermal conversion layer.39 The degree of CMG reduction, which can be easily controlled by reduction temperature, enabled adjustment of the surface energy.40 In the case of the lamellar structure, CMG was adjusted to the neutral surface condition for surface-perpendicular orientation, while in the case of the cylindrical structure, CMG was adjusted to deviate from the neutral surface condition for surface-parallel orientation of cylinders. On the CMG (transparency: 89.2% (Figure S5); thickness: ∼2 nm)/glass substrate, BCP films were spin-coated with toluene solution. In particular, symmetric lamella-forming poly(styrene)-b-poly(methyl methacrylate) (PS-b-PMMA) copolymers (Mn: 25−26 kg mol−1) were blended with short PS-r-PMMA neutral random copolymers (Mn: 17 kg mol−1) in a weight ratio of 7:3. Asymmetric cylinderforming PS-b-PMMA copolymers (Mn: 36−10.5 kg mol−1) were used for surface-parallel cylinder study. All the BCP thin films (thickness: 100−500 nm) were formed by spin-casting with toluene solution with a BCP concentration of 2−8 wt %. Laser Writing BCP Assembly Process on CMG/Glass Substrate. Figure 1a presents the laser writing assembly of PS-bPMMA BCP domains. Focused near-IR laser beam (ytterbium pulsed fiber laser with wavelength 1064 nm, pulse frequency 300 kHz, and

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QUASI-STATIC SCFT SIMULATIONS The laser scanning process explained so far is modeled by a quasi-static simulation using SCFT (see Supporting Information for the details). The PS-b-PMMA film is assumed to be an incompressible melt of AB diblock copolymers, and its A (PS) fraction f is set to 0.5 and 0.7 for the lamella- and cylinderforming BCPs, respectively. The BCP is regarded as a Gaussian chain with N segments each having statistical segment length a. For the initialization of the thin film morphology, we follow the standard SCFT recipe for BCP melt41−47 except that the effect of the temperature change is accounted as the change of χ. Because temperature is now a function of the position relative to the laser center, we use a position-dependent function χ(r) for the mean potential field calculation. The experimental domain ordering occurs via a nearmacroscopic scale of rearrangement, but due to computational limitation, we can only perform simulations on a much smaller area covering only 20−30 BCP periods. Even after such a miniaturization, a fully dynamic simulation is an unrealistic target; thus, we adopt a quasi-static simulation schematically described in Figure 2. In this method, a position-dependent χ(r) parameter is introduced to represent the laser irradiated area. It is essentially the disordered part of the film as shown with green colors in the figure. Initially, a starting morphology of the BCP thin film is obtained by an SCFT calculation with a small random initial field (Figure 2a). Once the system converges to a certain morphology, we turn on the laser which will immediately create the disordered region (Figure 2b). The next step is to move the back ODT line slightly to the right. Using the previously obtained morphology as a new input, SCFT calculation is performed to find a slightly evolved morphology (Figure 2c). By repeating this process, the B

DOI: 10.1021/acs.macromol.7b02645 Macromolecules XXXX, XXX, XXX−XXX

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Initially, the 25−26 kg mol−1 PS-b-PMMA BCPs create randomly oriented surface-perpendicular lamellar phase. As elliptically shaped focused laser beam was irradiated directly onto the sample, the beam scanned the sample in the lateral direction with 100 nm/s speed. When the lateral scanning of the laser beam proceeded at a speed below 250 nm/s, the BCP film underwent a gradual temperature change which resulted in a morphological transition, as illustrated in Figure 1a. This lateral scanning of the laser beam spontaneously aligned the surface-perpendicular lamellar nanodomains along the scan direction, which can be confirmed by the SEM snapshot of the thin film surface exhibited in Figure 3a for a sample with film thickness 300 nm. For the case of cylindrical nanodomains shown in Figure 3b, the film thickness was 80 nm, and the surface-parallel cylindrical nanodomains were also aligned along the scan direction. At the initial stage, surface-perpendicular lamellar and surface-parallel cylindrical nanodomains were randomly oriented. As the scan progressed and the laser beam center reached to the given area of the film, the temperature increased enough to exceed TODT of the BCP system, and the local area was in a disordered state. This effect created an order/disorder boundary (ODT line) following the approximate shape of the laser. As the beam was further scanned, the beam center moved away and the temperature of the area fell back below TODT. This process initiated a directed self-assembly of the BCP domains along the back boundary of the disordered region which became the front boundary of the ordered region. The quasi-static simulation method can be meaningful only when the experimental sample is evolving through an almost quasi-static process. The most important experimental evidence is that the degree of domain alignment was critically dependent upon the laser scan velocity v. When v > 1000 nm/s, domain orientations were weakly correlated, but preferred alignment along laser scan direction became stronger at v < 1000 nm/s. Highly oriented morphology with low defect density was observed below 250 nm/s,31 and we obtained the best domain alignment at the slowest laser scan velocity, 100 nm/s. It suggests that at a given instance the BCP domains have enough time to reorient themselves to find the locally metastable morphology. A similar trend was observed in our quasi-static

Figure 2. Schematic illustration of the disordered region (green color) as laser writing progresses.

simulation continues until the laser moves across the simulation area as shown in Figure 2d. In short, our simulation moves from one metastable state to another as time goes on, and this is the reason why we call it a quasi-static simulation.

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RESULTS AND DISCUSSION We carried out the laser writing process with lamella-forming (Mn: 25−26 kg mol−1) and cylinder-forming (Mn: 36−10.5 kg mol−1) BCPs. For the former case of lamella-forming BCPs (Figure 3a), short neutral copolymers were blended for the reduction of effective χN of the system,48 and a subsidiary role of defect melting is also expected.48 With them, TODT is lowered so that low-energy laser photothermal treatment can create a disordered region around the irradiation center. We did not mix random copolymers with the cylinder-forming BCPs (Figure 3b) because they become disordered at relatively higher χN or relatively lower temperature.

Figure 3. (a) Surface-perpendicular lamellar and (b) surface-parallel cylindrical morphologies at disorder melting front during the laser zone annealing process. C

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Figure 4. Laser writing simulation of a 50R0 × 50R0 two-dimensional lamella-forming (χN = 20, f = 0.5) BCP film. (a) to (d) correspond to the four steps exhibited in Figure 2. In the green region, χN = 4.

other width of laser zone produces a similar result provided that the zone width is much larger than L0. Also, repeated simulations with random initial conditions always produce similar morphologies, and change of f in the range between 0.4 and 0.5 produces lamellar domains well-aligned in the scan direction. As mentioned earlier, the long-range order starts to be degraded as scan velocity increases, but no abrupt alignment direction transition is observed. From the observation that our simulation exhibits better alignment at quasi-static evolution with smaller steps, it is natural to predict that slow scan velocity is important for the well-aligned nanostructures even though the step size in our simulation is not directly translated to the experimental velocity. It is interesting that previous twodimensional DSCFT simulation suggested that a scan velocity of 2.6 μm/s is enough to create well-aligned nanostructures33 while our experiments show that much slower scan velocity is recommended for the best alignment. This difference may be due to the dimensionality of the system considering that slower scan speed is required for the alignment of thick BCP films.35 In the earlier simulations, periodic boundaries are adopted and column-shaped laser sweeps the entire sample, but our simulation does not use this method. Before the laser scan begins, wormlike structures are initially created, and our test shows that it is important to let the wormlike structures to surround the laser scanned area (Figure S9). Free energy comparison in the Supporting Information suggests that the grain boundaries between the aligned and wormlike region may play a key role in the determination of the alignment direction (Figure S10). There exists one additional driving force for the lamellar growth in the scan direction. In the current simulation, the

simulation in which we controlled the movement of the laser irradiated area between quasi-static steps. Only when the step size approached to the minimum possible value, we obtained simulation result consistent with the experiment at 100 nm/s. It strongly suggests that both the experiment and theory are in the quasi-static regime for the films in this paper. In order to model the lamella-forming BCP experiment using two-dimensional SCFT calculation, we use symmetric BCPs with χN = 20 when the laser is turned off. This value is close to the estimation for the experimental 25−26 kg mol−1 PS-bPMMA.49 We perform simulations in a two-dimensional rectangular region of lateral length 50R0 and width 50R0 with periodic boundary conditions, where R0 = aN1/2 is √6 times the radius of gyration Rg. The natural BCP period at χN = 20 is L0 = 1.65R0, and thus the system size corresponds to approximately 30L0. Inside this box, we miniaturize the entire experimental situation including the laser scanned area as shown in Figure 4. Initially, the area is filled with wormlike surface-perpendicular lamellar domains (Figure 4a). The laser is then turned on (Figure 4b), and the scanned area moves to the right. As the laser writing continues, the domains begin to align along the scan direction, and Figures 4c,d show the well-aligned lamellar domains found at a later stage (Video S1). One earlier two-dimensional simulation modeling zone annealing reported strong f-dependent morphology transition,33 and other works claimed the existence of scan-velocitydependent alignment direction transition.26,32 However, our simulation shows that the observed long-range order is insensitive to small changes of simulation parameters and/or initial conditions. For example, the y-directional width of the laser scan is 30R0 (≈18L0) for the current simulation, but any D

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Figure 5. SEM image of surface-perpendicular lamellar domains aligned by laser zone annealing. (a) Macroscopic image of laser writing BCP selfassembly. (b) Orientation mapping image from center to 50 μm out of center shows that highly aligned surface-perpendicular lamellar domains are created along the laser scan direction over 20 μm distance from the center. (c) Orientation order parameter as a function of distance from the center. (d−f) Magnified SEM images of surface-perpendicular lamellae at each spot in (b).

randomly oriented wormlike lamellar domains surround the laser scanned area, nullifying the effect of the periodic boundary at the top and bottom directions. One may consider lamellar domains parallel to the ODT line which are likely to be at least metastable. These domains must inevitably be created one after another, which implies that the morphological evolution must overcome a free energy hill for each domain creation. On the other hand, the aligned lamellar domains in Figure 4 can continuously grow without any obstacles, and this is a preferable strategy for the system to expand the long-ranged ordered area. For the verification of the degree of long-range order, we display the orientation mapping image of an area covering 50 μm distance from the laser scan center (Figures 5a,b). The color mapping of the orientation order shows that the longrange order is excellent over 10 μm length, and it is slowly deteriorating afterward. It is also verified by the orientation order parameter (Figure 5c) and SEM images. In Figure 5d, surface-perpendicular lamellar domains are well aligned along the scan direction. At 20 μm away from the center, the domains are aligned in diagonal directions and the alignment gradually begins to collapse (Figure 5e). The domain orientations are almost random above 40 μm of distance (Figure 5f). In our miniaturized simulation, the detailed large-scale influence of the laser curvature is difficult to verify directly because the experimental laser curvature corresponds to a tiny bending of the laser shape. Our simulation in Figure S12 uses laser curvature much larger than the experimental value, but the lamellar domain ordering along the scan direction is maintained to a reasonable level. Thickness-dependent BCP morphologies are commonly observed, and it is important to check if the surfaceperpendicular lamellar morphology is preferred regardless of the commensurability of the film. The top-view SEM images and orientation mappings for samples with thicknesses from 100 to 500 nm are displayed in Figures 6a−d and Figure S7, demonstrating that the lamellar alignment is excellent regardless of the thickness of the film. However, the actual three-dimensional structure of the film is not a simple extension of the two-dimensional picture. Cross-sectional SEM images displayed in Figures 6e,f reveal that tilting and bending of the

Figure 6. SEM image of surface-perpendicular lamellar domains for the case with film thicknesses (a) 100, (b) 200, (c) 400, and (d) 500 nm. Orientation mapping for each case is shown as an inset. (e) and (f) are cross-sectional SEM images (tilted angle) for samples with thicknesses 200 and 400 nm, respectively.

surface-perpendicular lamellae are commonly observed, and they become more significant as the film thickness increases. In addition, we need to consider the surface-parallel lamellar morphology as a candidate phase, and thus simulation of the E

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Figure 7. Three-dimensional quasi-static simulation of laser writing on a 30R0 × 30R0 area with thicknesses (a) 0.8L0 (Video S2), (b) 3.0L0, (c) 1.5L0, and (e) 2.3L0. (d) and (f) show (c) and (e) at different angles, respectively, after leaving only B-rich (B fraction >0.55) regions.

three-dimensional morphology is important. For this, we choose a rectangular region of lateral length 30R0 and width 30R0, and we use various film thicknesses Lz as explained below. For fast calculation, we adopt the Neumann boundary conditions in all directions. Even though the modeling of surfaces at z = 0 and Lz may create subtle entropically driven chain segregation and morphology transition,50−55 such effects are likely to be small in the current simulation because the interfacial energy at the grain boundary is expected to be dominant. Considering the advantage of surface interaction assignment at the interfacial layer, use of Neumann boundary condition can be an appropriate policy for this problem. Apart from the size, the biggest difference from the twodimensional case is that the surface-parallel lamellar morphology competes with the surface-perpendicular one, and its preference strongly depends on the film thickness. Figure 7a shows the BCP morphology in a film of thickness 0.8L0. At this thickness, the BCP domains are always perpendicular to the substrate, and they follow the scan direction in the laser swept area. The situation is more interesting when the film thickness is exactly integer multiples of 0.5L0, satisfying the commensurability condition of the surface-parallel morphology. At 1.5L0 thickness (Figure 7c), the surface-parallel lamellar morphology now competes with the surface-perpendicular morphologies, and more complicated patterns are observed outside the laser scanned area. Note that the same simulations with different initial conditions often produce a perfectly aligned morphology, and we present this figure just to explain all the morphologies we observed. Figure 7d exhibits its inner domain structure, showing that somewhat tilted surface-perpendicular domains coexist with the occasional surface-parallel ones. Even though

the lamellar alignment along the scan direction is slightly interfered, the overall long-range order remains the same, and this trend continues for simulations of other thicknesses, 2.3L0 (Figure 7e) and 3.0L0 (Figure 7b). The inner domain structure of the 2.3L0 case is shown in Figure 7f, and one can confirm that the lamellar domains maintain strong tendency to follow the scan direction, while the tilting and bending of the lamellae become more pronounced. This result is consistent with the experimental cross-sectional views shown in Figure 6. Earlier three-dimensional DSCFT research also reported similar alignment behavior.35 In their dual-field approach, the bottom surface preference is the driving force of the domain alignment, and thus the alignment is better at the bottom surface. In our simulation, the surrounding wormlike domains guide the alignment in the swept area, and we do not expect such a height dependent behavior. In our experiment, the maximum temperature gradient was 1.35 K/μm, and it is regarded as a relatively high gradient. Converting it to the χN gradient,49 it corresponds to 3.0 × 10−2/μm. Another important gradually changing parameter is the height variation, which is measured to be 0.152 nm/μm (Figure S8b). Both parameter changes are small at the scale of the simulation box size which is estimated to be 530 nm for the three-dimensional simulation, but significant changes can occur over the real experimental scale which is hundreds of micrometers. A few previous simulations suggest that the former, χN gradient, is expected to degrade the lamellar domain alignment,26,33 but the latter is known to enhance the lamellae growth along the direction of the thickness variation.56,57 Such parameter spaces are not explored in the current work because our focuses are on other parameters and their effects on the F

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Figure 8. Three-dimensional quasi-static simulation of laser writing on a 30R0 × 30R0 area of cylinder-forming (χN = 20, f = 0.7) BCP film. (a) Surface of a film with thickness 3.0L0 (Video S3) and (b) its inner domain structure viewed at a different angle. Surface of a film with thickness (c) 2.3L0 and (d) 2.0L0. (e) Case with both the top and bottom surfaces preferring PMMA (thickness 1.3L0) and (f) its inner domain structure viewed at a different angle.

film thickness was preferable for the lamellar nanodomains, but now it is desirable to make the film thickness fitting for the given number of cylindrical layers. If the morphology contains perfectly aligned hexagonal cylinders, the preferable thickness would be integer multiples of L0/2 or L0√3/2,58,59 but other film thicknesses may be accommodated by slightly adjusting the cylinder-to-cylinder distance.60 In the current simulation, the surface preference to the PS domain often creates a PS monolayer at the bottom, adding another reason why the preferable thickness cannot be simply estimated. In our test with various film thicknesses, the commensurability strongly affects the resulting morphology when the film is too thin, Lz < 2.0L0. Above this thickness, the morphology seems to depend less on the thickness, as shown by Figures 8c,d. In our simulation with extremely thick films, the effect of the bottom surface preference is eventually lost and cannot be delivered to the top surface, but the 80 nm film used for our cylinderforming BCP experiment fits in the regime where good alignment along the laser scan direction is expected. Unlike the lamella-forming BCP system, the experiment of the cylindrical system has an additional control parameter, the surface interaction. To see if fine-tuning of the surface interaction is necessary for the cylinder alignment, we perform a few simulations with various surface interaction combinations, and Figures 8e,f show the case that both surfaces prefer PMMA. The surface preference attracts PMMA domains toward the upper surface to create surface-parallel cylinders. As laser scan proceeds, those domains eventually align along the scan direction. Our tests using various strengths and combinations of surface interaction reveal that the most important role of the

alignments of two- and three-dimensional lamellar and cylindrical domains. Note that even though the thickness variation is not significant over hundreds of L0, it will inevitably affect the commensurability of the film. Even when the original film thickness is integer multiples of 0.5L0 and surface-parallel lamellar domains are occasionally chosen, they will eventually be unfavorable. It provides one reason why the surface-parallel lamellar domains occasionally observed in the simulation are not observable in our experiments. Now let us turn our attention to the simulation of cylinderforming BCP thin films. One is obligated to go into the threedimensional simulation because two-dimensional simulation can only produce surface-perpendicular cylinders. Let us consider the case with χN = 20 and f = 0.7, for which the natural cylinder-to-cylinder distance in bulk is L0 = 1.69R0. In order to apply the substrate preference to the majority phase (PS), surface interaction is imposed at the bottom surface (see Supporting Information for details). As a result, PS predominantly occupies the bottom layer, and the surfaceperpendicular cylindrical phase is strongly suppressed. Surfaceparallel cylinders can still be freely oriented, but the film with thickness 3.0L0 (Figure 8a) shows a clear alignment in the direction of the laser movement, and the inner layers also exhibit good alignments (Figure 8b). Note that even though the top view of the thin film is similar to that of the lamella-forming BCPs, there exists a clear difference for the alignment condition. For the lamellae, neutralized surface interaction enhances surface-perpendicular morphologies, but now surface preferential interaction promotes surface-parallel cylinders. Also, noncommensurable G

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surface interaction is to break the symmetry and initiate the surface-parallel cylinder growth. The exact strength of the surface interaction has a relatively minor influence on the final morphology.

CONCLUSIONS In this study, laser writing directed BCP self-assemblies of surface-perpendicular lamellae and surface-parallel cylinders on CMG films have been demonstrated by experiment and quasistatic simulation using SCFT. Laser writing with extreme thermal field driven by localized photothermal heating enables anomalous long-range alignment of polymeric self-assembly patterns. Using simulations, systematic analysis with respect to various factors affecting the assembly behavior has been carried out to provide a fundamental understanding of the alignment mechanism. With enhancement of process efficiency by introducing laser interference fringe or parallel line beam array, this fab-friendly laser writing process, which has enormous advantages including single-step orientation controllability and roll-to-roll process compatibility, is expected to open up new industrial potentials of BCP self-assembly. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02645. Additional experimental methods and data, additional theoretical methods and simulation data, Figures S1−S12 (PDF) Video S1 (AVI) Video S2 (AVI) Video S3 (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.O.K.). *E-mail: [email protected] (J.U.K.). ORCID

Hyeong Min Jin: 0000-0001-5326-1413 Sang Ouk Kim: 0000-0003-1513-6042 Jaeup U. Kim: 0000-0002-2853-2784 Author Contributions

D.Y. and H.M.J. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (2014R1A2A1A11054430 and 2017R1A2B4012377). H.M.J. and S.O.K. were supported by the Global Frontier Hybrid Interface Materials (GFHIM) (2013M3A6B1078874), and the Nano-Material Technology Development Program (2016M3A7B4905613) through the NRF funded by the MSIP. This research used high performance computing resources of the UNIST Supercomputing Center. H

DOI: 10.1021/acs.macromol.7b02645 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b02645 Macromolecules XXXX, XXX, XXX−XXX