Laser-Directed Self-Assembly of Highly Aligned Lamellar andCylindrical Block Copolymer Nanostructures: Experiment andSimulationDaeseong 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 Scienceand Engineering, KAIST, Daejeon 34141, Republic of Korea

*S Supporting Information

ABSTRACT: Laser photothermal annealing is emerging as apromising strategy for directed self-assembly of block copolymersalong with its unique advantages, such as area selectivity, solvent-free ultrafast process, and highly oriented nanopattern formationwithout substrate prepatterning. We investigate laser-inducedhighly aligned lamellar and cylindrical self-assembled nanostruc-ture formation by means of simulation as well as experiment. Self-assembled surface-perpendicular lamellar or surface-parallelcylindrical nanodomains in PS-b-PMMA thin films could be aligned by lateral steady scan of focused laser irradiation toattain 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 simulationsof 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 copolymerfilm thickness is moderate.

■ INTRODUCTION

Block copolymer (BCP) self-assembly can generate varioustypes of nanostructures with sub-50 nm size,1−3 and it hasreceived much attention as a complementary strategy forconventional photolithography4−6 due to ultrafine resolution,high scalability, and low price. However, because of entropyand incomplete annealing, polygrain structures with manydefects are spontaneously formed during the BCP self-assembly, and controlling long-range order has been a long-standing challenge for its practical applications.To date, various approaches for directed self-assembly

including permanent fields (chemoepitaxy,7,8 graphoepi-taxy5,9−14) or dynamic external fields (thermal field,15−17

shear field,18,19 magnetic field,20,21 electric field22−25) havebeen proposed. Among these strategies, zone annealingmethods using temporal and spatial thermal field allowcontinuous process of directed self-assembly without apreguidance pattern.17,26 In particular, localized photothermallaser heating allows the formation of an extremely high thermalfield, and thus several research groups reported the directedself-assembly of BCPs using laser zone annealing process.27−31

This extreme high thermal field from localized laser heatingcreates highly ordered BCP structures, and our experimentalteam demonstrated lateral ordering of surface-perpendicularlamellar structure using laser writing with process temperature(Tpeak) higher than the order−disorder transition temperature(TODT) of the BCP system.15,16

In this report, we experimentally demonstrate laser writingdirected self-assembly of surface-perpendicular lamellae as wellas surface-parallel cylinders, and their domain alignmentmechanism is studied by quasi-static simulation adopting self-consistent field theory (SCFT) method. A few theoreticalmethods have been previously suggested for the modeling ofthe zone annealing process. In one simulation, Yang and co-workers32 solved the time-dependent Ginzburg−Landau typeequation for BCP evolution, and they found that lamellaeparallel to the scanning direction were eventually chosen atsmall enough scanning velocity though lamellae perpendicularto the scanning direction were initially selected at fasterscanning velocity. A similar transition was reported in aresearch using SCFT under time- and space-dependentmobility field.26

Dynamic self-consistent field theory (DSCFT) can also be anattractive theoretical tool for the study of the BCP domainevolution. Zhang, Lin, and co-workers recently reported a fewzone annealing simulations using dynamic SCFT.33−35 In theirearlier work using two-dimensional DSCFT, the formation oflamellae parallel to the scanning direction is observed forsymmetric BCPs at small enough scanning velocity,33 and theyalso reported lamellae perpendicular to the scanning direction

Received: December 13, 2017Revised: January 9, 2018

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for slightly asymmetric BCPs. In a later work, they tested theefficiency of the zone annealing when the Flory−Hugginsinteraction parameter χN at the annealed zone is still in theordered regime.34 They found that the combination ofchemoepitaxy and zone annealing can greatly enhance thelamellar alignment, and this idea of the dual-field system isfurther pursued using three-dimensional DSCFT simulation.35

Most of the above researches performed two-dimensionalsimulation due to the high computational cost of the BCPevolution calculation, and thus dependence on the filmthickness was not accountable. The naive expectation is thatthe two-dimensional morphology is extended to a three-dimensional structure by the growth of the surface-perpendicular lamellae. Even though the aforementionedthree-dimensional DSCFT simulation35 confirmed that this isoften a valid prediction, it also revealed a few nontrivialthickness-dependent behaviors; in general, when all otherparameters are fixed, the lamellar alignment degrades as thethickness increases. Moreover, in a two-dimensional simulation,cylindrical domains only appear as hexagonally arranged dots,and their growth parallel to the scanning direction cannot bestudied even though it is a possible consequence of the laserscanning. In order to address these issues, we also performquasi-static simulations using three-dimensional SCFT calcu-lation and investigate the thickness dependence of the zoneannealing.

■ MATERIALS AND EXPERIMENTAL METHODSMaterials. All BCPs were purchased from Polymer Source Inc. and

used without purification. Short random copolymers were synthesizedby nitroxide-mediated living radical polymerization.36 Graphitepowder, toluene, and sulfuric acid (H2SO4) were purchased fromSigma-Aldrich. Hydrochloric acid (HCl), hydrogen peroxide (H2O2,30% aqueous solution), potassium permanganate (KMnO4), andisopropyl alcohol (C3H8O, 99.5%) were purchased from JunseiChemical Co., Ltd. Ruthenium tetroxide (RuO4) staining agent waspurchased from Electron Microscopy Sciences.Preparation of Chemically Modified Graphene Substrate

and BCP Thin Films. Few-layered graphene oxide (GO) film wasspin-coated with the gentle blowing of N2 gas at target mothersubstrate, such as bare glass or quartz. The thickness of GO film couldbe 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 monohydratevapor for 1 h.37 This chemically modified graphene (CMG)38 layerserved as a surface energy modifier controlling the orientation of theBCP structure as well as photothermal conversion layer.39 The degreeof CMG reduction, which can be easily controlled by reductiontemperature, enabled adjustment of the surface energy.40 In the case ofthe lamellar structure, CMG was adjusted to the neutral surfacecondition for surface-perpendicular orientation, while in the case of thecylindrical structure, CMG was adjusted to deviate from the neutralsurface condition for surface-parallel orientation of cylinders. On theCMG (transparency: 89.2% (Figure S5); thickness: ∼2 nm)/glasssubstrate, BCP films were spin-coated with toluene solution. Inparticular, symmetric lamella-forming poly(styrene)-b-poly(methylmethacrylate) (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 cylinder-forming PS-b-PMMA copolymers (Mn: 36−10.5 kg mol−1) were usedfor surface-parallel cylinder study. All the BCP thin films (thickness:100−500 nm) were formed by spin-casting with toluene solution witha BCP concentration of 2−8 wt %.Laser Writing BCP Assembly Process on CMG/Glass

Substrate. Figure 1a presents the laser writing assembly of PS-b-PMMA BCP domains. Focused near-IR laser beam (ytterbium pulsedfiber laser with wavelength 1064 nm, pulse frequency 300 kHz, and

pulse duration 200 ns) was used for the laser writing assembly ofBCPs. The laser beam was focused into an elliptical shape (diameterdx: 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 typicallocalized laser photothermal effect, a temperature gradient up to 1.35K/μm was generated on the CMG layer (Figure 1b,c).31 The laserscanned the sample in the lateral direction, and the scan velocity wascarefully controlled from 100 to 5000 nm/s by a linear motorized stage[PRO165LM-0400 (Aerotech)] (Figure S6).

■ QUASI-STATIC SCFT SIMULATIONSThe laser scanning process explained so far is modeled by aquasi-static simulation using SCFT (see Supporting Informa-tion for the details). The PS-b-PMMA film is assumed to be anincompressible melt of AB diblock copolymers, and its A (PS)fraction f is set to 0.5 and 0.7 for the lamella- and cylinder-forming BCPs, respectively. The BCP is regarded as a Gaussianchain with N segments each having statistical segment length a.For the initialization of the thin film morphology, we follow thestandard SCFT recipe for BCP melt41−47 except that the effectof the temperature change is accounted as the change of χ.Because temperature is now a function of the position relativeto the laser center, we use a position-dependent function χ(r)for the mean potential field calculation.The experimental domain ordering occurs via a near-

macroscopic scale of rearrangement, but due to computationallimitation, we can only perform simulations on a much smallerarea covering only 20−30 BCP periods. Even after such aminiaturization, a fully dynamic simulation is an unrealistictarget; thus, we adopt a quasi-static simulation schematicallydescribed in Figure 2. In this method, a position-dependentχ(r) parameter is introduced to represent the laser irradiatedarea. It is essentially the disordered part of the film as shownwith green colors in the figure. Initially, a starting morphologyof the BCP thin film is obtained by an SCFT calculation with asmall random initial field (Figure 2a). Once the systemconverges to a certain morphology, we turn on the laser whichwill immediately create the disordered region (Figure 2b). Thenext 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 evolvedmorphology (Figure 2c). By repeating this process, the

Figure 1. (a) Schematic illustration of laser zone annealing BCP self-assembly. (b) Two-dimensional temperature profile of localized laserheating. (c) Temperature as a function of distance along the x-axis.

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

■ RESULTS AND DISCUSSIONWe carried out the laser writing process with lamella-forming(Mn: 25−26 kg mol−1) and cylinder-forming (Mn: 36−10.5 kgmol−1) BCPs. For the former case of lamella-forming BCPs(Figure 3a), short neutral copolymers were blended for thereduction of effective χN of the system,48 and a subsidiary roleof defect melting is also expected.48 With them, TODT islowered so that low-energy laser photothermal treatment cancreate a disordered region around the irradiation center. We didnot mix random copolymers with the cylinder-forming BCPs(Figure 3b) because they become disordered at relatively higherχN or relatively lower temperature.

Initially, the 25−26 kg mol−1 PS-b-PMMA BCPs createrandomly oriented surface-perpendicular lamellar phase. Aselliptically shaped focused laser beam was irradiated directlyonto the sample, the beam scanned the sample in the lateraldirection with 100 nm/s speed. When the lateral scanning ofthe laser beam proceeded at a speed below 250 nm/s, the BCPfilm underwent a gradual temperature change which resulted ina morphological transition, as illustrated in Figure 1a. Thislateral scanning of the laser beam spontaneously aligned thesurface-perpendicular lamellar nanodomains along the scandirection, which can be confirmed by the SEM snapshot of thethin film surface exhibited in Figure 3a for a sample with filmthickness 300 nm. For the case of cylindrical nanodomainsshown in Figure 3b, the film thickness was 80 nm, and thesurface-parallel cylindrical nanodomains were also aligned alongthe scan direction.At the initial stage, surface-perpendicular lamellar and

surface-parallel cylindrical nanodomains were randomly ori-ented. As the scan progressed and the laser beam centerreached to the given area of the film, the temperature increasedenough to exceed TODT of the BCP system, and the local areawas in a disordered state. This effect created an order/disorderboundary (ODT line) following the approximate shape of thelaser. As the beam was further scanned, the beam center movedaway and the temperature of the area fell back below TODT.This process initiated a directed self-assembly of the BCPdomains along the back boundary of the disordered regionwhich 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 almostquasi-static process. The most important experimental evidenceis that the degree of domain alignment was critically dependentupon the laser scan velocity v. When v > 1000 nm/s, domainorientations were weakly correlated, but preferred alignmentalong laser scan direction became stronger at v < 1000 nm/s.Highly oriented morphology with low defect density wasobserved below 250 nm/s,31 and we obtained the best domainalignment at the slowest laser scan velocity, 100 nm/s. Itsuggests that at a given instance the BCP domains have enoughtime to reorient themselves to find the locally metastablemorphology. A similar trend was observed in our quasi-static

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

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

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simulation in which we controlled the movement of the laserirradiated area between quasi-static steps. Only when the stepsize approached to the minimum possible value, we obtainedsimulation result consistent with the experiment at 100 nm/s. Itstrongly suggests that both the experiment and theory are inthe 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 BCPswith χN = 20 when the laser is turned off. This value is close tothe estimation for the experimental 25−26 kg mol−1 PS-b-PMMA.49 We perform simulations in a two-dimensionalrectangular region of lateral length 50R0 and width 50R0 withperiodic boundary conditions, where R0 = aN1/2 is √6 timesthe radius of gyration Rg. The natural BCP period at χN = 20 isL0 = 1.65R0, and thus the system size corresponds toapproximately 30L0. Inside this box, we miniaturize the entireexperimental situation including the laser scanned area asshown in Figure 4. Initially, the area is filled with wormlikesurface-perpendicular lamellar domains (Figure 4a). The laser isthen turned on (Figure 4b), and the scanned area moves to theright. As the laser writing continues, the domains begin to alignalong the scan direction, and Figures 4c,d show the well-alignedlamellar domains found at a later stage (Video S1).One earlier two-dimensional simulation modeling zone

annealing reported strong f-dependent morphology transi-tion,33 and other works claimed the existence of scan-velocity-dependent alignment direction transition.26,32 However, oursimulation shows that the observed long-range order isinsensitive to small changes of simulation parameters and/orinitial conditions. For example, the y-directional width of thelaser scan is 30R0 (≈18L0) for the current simulation, but any

other width of laser zone produces a similar result provided thatthe zone width is much larger than L0. Also, repeatedsimulations with random initial conditions always producesimilar morphologies, and change of f in the range between 0.4and 0.5 produces lamellar domains well-aligned in the scandirection. As mentioned earlier, the long-range order starts tobe degraded as scan velocity increases, but no abrupt alignmentdirection transition is observed. From the observation that oursimulation exhibits better alignment at quasi-static evolutionwith smaller steps, it is natural to predict that slow scan velocityis important for the well-aligned nanostructures even thoughthe step size in our simulation is not directly translated to theexperimental velocity. It is interesting that previous two-dimensional DSCFT simulation suggested that a scan velocityof 2.6 μm/s is enough to create well-aligned nanostructures33

while our experiments show that much slower scan velocity isrecommended for the best alignment. This difference may bedue to the dimensionality of the system considering that slowerscan speed is required for the alignment of thick BCP films.35

In the earlier simulations, periodic boundaries are adoptedand column-shaped laser sweeps the entire sample, but oursimulation does not use this method. Before the laser scanbegins, wormlike structures are initially created, and our testshows that it is important to let the wormlike structures tosurround the laser scanned area (Figure S9). Free energycomparison in the Supporting Information suggests that thegrain boundaries between the aligned and wormlike region mayplay 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

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 foursteps exhibited in Figure 2. In the green region, χN = 4.

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randomly oriented wormlike lamellar domains surround thelaser scanned area, nullifying the effect of the periodic boundaryat the top and bottom directions. One may consider lamellardomains parallel to the ODT line which are likely to be at leastmetastable. These domains must inevitably be created one afteranother, which implies that the morphological evolution mustovercome a free energy hill for each domain creation. On theother hand, the aligned lamellar domains in Figure 4 cancontinuously grow without any obstacles, and this is apreferable strategy for the system to expand the long-rangedordered area.For the verification of the degree of long-range order, we

display the orientation mapping image of an area covering 50μm distance from the laser scan center (Figures 5a,b). Thecolor mapping of the orientation order shows that the long-range order is excellent over 10 μm length, and it is slowlydeteriorating afterward. It is also verified by the orientationorder parameter (Figure 5c) and SEM images. In Figure 5d,surface-perpendicular lamellar domains are well aligned alongthe scan direction. At 20 μm away from the center, the domainsare aligned in diagonal directions and the alignment graduallybegins to collapse (Figure 5e). The domain orientations arealmost random above 40 μm of distance (Figure 5f). In ourminiaturized simulation, the detailed large-scale influence of thelaser curvature is difficult to verify directly because theexperimental laser curvature corresponds to a tiny bending ofthe laser shape. Our simulation in Figure S12 uses lasercurvature much larger than the experimental value, but thelamellar domain ordering along the scan direction is maintainedto a reasonable level.Thickness-dependent BCP morphologies are commonly

observed, and it is important to check if the surface-perpendicular lamellar morphology is preferred regardless ofthe commensurability of the film. The top-view SEM imagesand orientation mappings for samples with thicknesses from100 to 500 nm are displayed in Figures 6a−d and Figure S7,demonstrating that the lamellar alignment is excellentregardless of the thickness of the film. However, the actualthree-dimensional structure of the film is not a simple extensionof the two-dimensional picture. Cross-sectional SEM imagesdisplayed in Figures 6e,f reveal that tilting and bending of the

surface-perpendicular lamellae are commonly observed, andthey 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

Figure 5. SEM image of surface-perpendicular lamellar domains aligned by laser zone annealing. (a) Macroscopic image of laser writing BCP self-assembly. (b) Orientation mapping image from center to 50 μm out of center shows that highly aligned surface-perpendicular lamellar domains arecreated 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).

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

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three-dimensional morphology is important. For this, wechoose a rectangular region of lateral length 30R0 and width30R0, and we use various film thicknesses Lz as explained below.For fast calculation, we adopt the Neumann boundaryconditions in all directions. Even though the modeling ofsurfaces at z = 0 and Lz may create subtle entropically drivenchain segregation and morphology transition,50−55 such effectsare likely to be small in the current simulation because theinterfacial energy at the grain boundary is expected to bedominant. Considering the advantage of surface interactionassignment at the interfacial layer, use of Neumann boundarycondition can be an appropriate policy for this problem.Apart from the size, the biggest difference from the two-

dimensional case is that the surface-parallel lamellar morphol-ogy competes with the surface-perpendicular one, and itspreference strongly depends on the film thickness. Figure 7ashows the BCP morphology in a film of thickness 0.8L0. At thisthickness, the BCP domains are always perpendicular to thesubstrate, and they follow the scan direction in the laser sweptarea. The situation is more interesting when the film thicknessis exactly integer multiples of 0.5L0, satisfying the commensur-ability condition of the surface-parallel morphology. At 1.5L0

thickness (Figure 7c), the surface-parallel lamellar morphologynow competes with the surface-perpendicular morphologies,and more complicated patterns are observed outside the laserscanned area. Note that the same simulations with differentinitial conditions often produce a perfectly aligned morphology,and we present this figure just to explain all the morphologieswe observed. Figure 7d exhibits its inner domain structure,showing that somewhat tilted surface-perpendicular domainscoexist with the occasional surface-parallel ones. Even though

the lamellar alignment along the scan direction is slightlyinterfered, the overall long-range order remains the same, andthis trend continues for simulations of other thicknesses, 2.3L0(Figure 7e) and 3.0L0 (Figure 7b). The inner domain structureof the 2.3L0 case is shown in Figure 7f, and one can confirmthat the lamellar domains maintain strong tendency to followthe scan direction, while the tilting and bending of the lamellaebecome more pronounced. This result is consistent with theexperimental cross-sectional views shown in Figure 6. Earlierthree-dimensional DSCFT research also reported similaralignment behavior.35 In their dual-field approach, the bottomsurface preference is the driving force of the domain alignment,and thus the alignment is better at the bottom surface. In oursimulation, the surrounding wormlike domains guide thealignment in the swept area, and we do not expect such aheight 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 isthe height variation, which is measured to be 0.152 nm/μm(Figure S8b). Both parameter changes are small at the scale ofthe simulation box size which is estimated to be 530 nm for thethree-dimensional simulation, but significant changes can occurover the real experimental scale which is hundreds ofmicrometers. A few previous simulations suggest that theformer, χN gradient, is expected to degrade the lamellar domainalignment,26,33 but the latter is known to enhance the lamellaegrowth along the direction of the thickness variation.56,57 Suchparameter spaces are not explored in the current work becauseour focuses are on other parameters and their effects on the

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.

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alignments of two- and three-dimensional lamellar andcylindrical domains. Note that even though the thicknessvariation is not significant over hundreds of L0, it will inevitablyaffect the commensurability of the film. Even when the originalfilm thickness is integer multiples of 0.5L0 and surface-parallellamellar domains are occasionally chosen, they will eventuallybe unfavorable. It provides one reason why the surface-parallellamellar domains occasionally observed in the simulation arenot observable in our experiments.Now let us turn our attention to the simulation of cylinder-

forming BCP thin films. One is obligated to go into the three-dimensional simulation because two-dimensional simulationcan only produce surface-perpendicular cylinders. Let usconsider the case with χN = 20 and f = 0.7, for which thenatural cylinder-to-cylinder distance in bulk is L0 = 1.69R0. Inorder to apply the substrate preference to the majority phase(PS), surface interaction is imposed at the bottom surface (seeSupporting Information for details). As a result, PSpredominantly occupies the bottom layer, and the surface-perpendicular cylindrical phase is strongly suppressed. Surface-parallel cylinders can still be freely oriented, but the film withthickness 3.0L0 (Figure 8a) shows a clear alignment in thedirection of the laser movement, and the inner layers alsoexhibit 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 cleardifference for the alignment condition. For the lamellae,neutralized surface interaction enhances surface-perpendicularmorphologies, but now surface preferential interactionpromotes surface-parallel cylinders. Also, noncommensurable

film thickness was preferable for the lamellar nanodomains, butnow it is desirable to make the film thickness fitting for thegiven number of cylindrical layers. If the morphology containsperfectly aligned hexagonal cylinders, the preferable thicknesswould be integer multiples of L0/2 or L0√3/2,58,59 but otherfilm thicknesses may be accommodated by slightly adjusting thecylinder-to-cylinder distance.60 In the current simulation, thesurface preference to the PS domain often creates a PSmonolayer at the bottom, adding another reason why thepreferable thickness cannot be simply estimated. In our testwith various film thicknesses, the commensurability stronglyaffects the resulting morphology when the film is too thin, Lz <2.0L0. Above this thickness, the morphology seems to dependless on the thickness, as shown by Figures 8c,d. In oursimulation with extremely thick films, the effect of the bottomsurface preference is eventually lost and cannot be delivered tothe top surface, but the 80 nm film used for our cylinder-forming BCP experiment fits in the regime where goodalignment along the laser scan direction is expected.Unlike the lamella-forming BCP system, the experiment of

the cylindrical system has an additional control parameter, thesurface interaction. To see if fine-tuning of the surfaceinteraction is necessary for the cylinder alignment, we performa 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 theupper surface to create surface-parallel cylinders. As laser scanproceeds, those domains eventually align along the scandirection. Our tests using various strengths and combinationsof surface interaction reveal that the most important role of the

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 viewedat a different angle.

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

■ CONCLUSIONSIn this study, laser writing directed BCP self-assemblies ofsurface-perpendicular lamellae and surface-parallel cylinders onCMG films have been demonstrated by experiment and quasi-static simulation using SCFT. Laser writing with extremethermal field driven by localized photothermal heating enablesanomalous long-range alignment of polymeric self-assemblypatterns. Using simulations, systematic analysis with respect tovarious factors affecting the assembly behavior has been carriedout to provide a fundamental understanding of the alignmentmechanism. With enhancement of process efficiency byintroducing laser interference fringe or parallel line beamarray, this fab-friendly laser writing process, which hasenormous advantages including single-step orientation con-trollability and roll-to-roll process compatibility, is expected toopen up new industrial potentials of BCP self-assembly.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.macro-mol.7b02645.

Additional experimental methods and data, additionaltheoretical methods and simulation data, Figures S1−S12(PDF)Video S1 (AVI)Video S2 (AVI)Video S3 (AVI)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: sangouk.kim@kaist.ac.kr (S.O.K.).*E-mail: jukim@unist.ac.kr (J.U.K.).

ORCIDHyeong Min Jin: 0000-0001-5326-1413Sang Ouk Kim: 0000-0003-1513-6042Jaeup U. Kim: 0000-0002-2853-2784Author ContributionsD.Y. and H.M.J. contributed equally.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was supported by Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT & FutureP l ann ing (MSIP) (2014R1A2A1A11054430 and2017R1A2B4012377). H.M.J. and S.O.K. were supported bythe Global Frontier Hybrid Interface Materials (GFHIM)(2013M3A6B1078874), and the Nano-Material TechnologyDevelopment Program (2016M3A7B4905613) through theNRF funded by the MSIP. This research used high perform-ance computing resources of the UNIST SupercomputingCenter.

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