3d tailored crumpling of block‐copolymer lithography on

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3D Tailored Crumpling of Block-Copolymer Lithography on Chemically Modifi ed Graphene

Ju Young Kim , Joonwon Lim , Hyeong Min Jin , Bong Hoon Kim , Seong-Jun Jeong , Dong Sung Choi , Dong Jun Li , and Sang Ouk Kim*

Dr. J. Y. Kim, J. Lim, H. M. Jin, Dr. B. H. Kim, Dr. S.-J. Jeong, D. S. Choi, D. J. Li, Prof. S. O. Kim National Creative Research Initiative Center for Multi-Dimensional Directed Nanoscale Assembly Department of Material Science & Engineering KAIST , Daejeon 34141 , Republic of Korea E-mail: [email protected]

DOI: 10.1002/adma.201504590

(CMG) has been introduced as an excellent fl exible substrate for artifi cial mechanical modulation, along with numerous advantages, including atomic scale fl atness, surface energy tunability, thermal and chemical stability, which are crucial for the effective overlay of nanoscale patterning. [ 11,13 ] Direc-tional control of the mechanical stress on CMG layer effectively modifi es the 3D delaminated structure from random to aligned crumpling, [ 7c ] and BCP nanopattern overlaid on CMG also experiences identical controlled transformation. Notably, this customized crumpling of self-assembled nanopattern dramati-cally increases the areal pattern density. The maximum six-fold multiplication of pattern density is demonstrated. The syner-gistic combination of BCP lithography and tailored crumpling of CMG layer attains remarkably high catalytic behavior for hydrogen evolution reaction (HER). Areal density enhancement of metal catalyst particles and facile removal of hydrogen bub-bles on the crumpled surface accomplish unprecedented level of high catalytic activity as well as long term reliability.

Figure 1 A illustrates the procedure for customized crum-pling of self-assembled nanopatterns. First, CMG layer was prepared by the reduction of solution cast graphene oxide (GO) thin fi lm. The surface energy of CMG could be fi nely con-trolled by the reduction process for the vertical orientation of self-assembled nanodomains of polystyrene- block -poly(methyl methacrylate) (PS- b -PMMA) BCPs. [ 13 ] Lamellar or cylindrical PS- b -PMMA BCPs were overlaid on the CMG by spin-casting and thermally annealed for self-assembly. For the application of controlled mechanical stress to the bilayer fi lm, shrinkage fi lm was employed as the bottom substrate material. The shrinkage fi lm is entropy-driven contractible polymer substrate. [ 14 ] Owing to the mechanical robustness and compliance of CMG, the self-assembled nanopatterns/CMG bilayer could be success-fully transferred onto a shrinkage fi lm (Figure S1 and S2, Sup-porting Information). [ 11 ] The following substrate shrinkage was performed in a highly controllable manner: isotropic or aniso-tropic. For isotropic shrinkage, the sample was simply annealed at ≈180 °C for 2–3 min without any mechanical stress. The fi lm underwent spontaneous lateral thermal shrinkage down to ≈40% of original dimension. As shown in Figure 1 B, mechan-ical instability induced by the substrate shrinkage caused the delamination of BCP/CMG bilayer from bottom shrinkage fi lm to form a fl ower-like 3D crumpled morphology.

Anisotropic uniaxial shrinkage was performed by anchoring one side of the shrinkage fi lm. Figure 1 C showed the resultant uniaxially crumpled structure. Subsequent shrinkage in the orthogonal direction was also possible. Such sequential crumplings in different orientations may create hierarchical crumpled structures with different characteristic length scales

Deformation mismatch in layered structures commonly leads to mechanical instability, such as wrinkling or crumpling. [ 1 ] Lung structure, [ 2 ] wrinkled cell membrane, [ 3 ] and fi ngerprint [ 4 ] found in the realm of nature are generated by such mechanical instability and demonstrate noticeable advantages, such as large surface area and stress tolerance for osmotic swelling or cell growth. [ 1d ] Similar benefi ts have also been exploited for artifi cial devices, for instance, ultrasensitive sensor with high surface area, [ 5 ] stretchable or fl exible electronics made from stress-dis-tributing buckled connector, [ 6 ] and so on. [ 7 ] Another remarkable aspect of the mechanical instability is the effective transforma-tion of in-plane 2D structure into 3D morphology. This feature can be judiciously exploited to attain the complex structures, hardly obtainable by conventional photolithography. [ 8 ] Unfor-tunately, fabrication of the 3D complex structures based on mechanical instability remains in the micrometer-scale range thus far. In general, nanoscale fabrication stringently requires chemically and thermally stable ultrafl at substrate geometry and thereby are generally incompatible to mechanically soft foundation, which is the key requirement for the effective mechanical modulation. [ 1d ]

Block-copolymer (BCP) lithography is a well-established, self-assembly based route to dense periodic nanopatterns with molecular level precision. [ 9 ] It has successfully demon-strated laterally ordered sub 10 nm scale line or dot patterns exploiting the genuine principles of directed self-assembly. [ 10 ] Nonetheless, BCP lithography has been principally exploited for 2D nanopatterning thus far. Conventional processing steps including the spin casting of BCP fi lm and subsequent thermal/solvent annealing for BCP chain diffusion strictly require hard and fl at substrate geometry to maintain the uniform thickness of BCP template upon processing steps. [ 11 ] How to address such an inherent restriction and establish effective process for 3D structure is an urgent requirement for the further advance of nanoscale patterning towards the next generation 3D and fl exible device architectures. [ 12 ]

In this work, we present customized 3D self-assembled nanopatterning enabled by the controlled crumpling of gra-phene based substrate layer. Chemically modifi ed graphene

Adv. Mater. 2016, 28, 1591–1596

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(Figure 1 D). Upon the shrinkage and crumpling process, BCP fi lms maintain the tight contact to the crumpled CMG without signifi cant damage in the nanopattern structure.

In general, mechanical instability in bilayer system strongly depends on the degree of compression. [ 15 ] For weak compres-sion regime, wrinkles are formed without delamination. Above certain critical compression, upper thin layer is locally delami-nated from bottom substrate and laterally fi led up or folded. In our system with the shrinkage ratio of ≈0.4, upper BCP/CMG bilayer was dominantly delaminated and crumpled (Figure S3, Supporting Information). The average width and height of delaminated and crumpled CMG were measured to be ≈1.1 and

≈1.8 µm, respectively (Figure S4, Supporting Information). Based on a typical delamination model for bilayer compression, [ 15 ] the width and height of delaminated part could be calculated to be 1.3–1.7 and 2.5–3.3 µm, respectively (Supporting Information). The minor deviation between the experimental and calculated results could be deduced from the variation of thickness and mechanical properties of CMG layer, inhomogeneous interfacial energy and so on. We note that the width and height of delami-nation can be readily controlled by several parameters such as the thickness of each layer and shrinkage ratio. [ 1b , 15,16 ]

The functional nanomaterials prepared by the pattern transfer of BCP morphology can be also crumpled in the

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Figure 1. A) Schematic illustration of tailor crumpling of self-assembled nanopattern/CMG. B–D) SEM images of isotropically (B), uniaxially (C), and hierarchically (D) (two times in orthogonal directions) crumpled lamellae BCP nanotemplate/CMG.

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desired fashion. In this work, typical pattern transfer for BCP lithography involved with PMMA selective etching and metal deposition and lift-off process has been used. Pd nanowire arrays and Au nanoparticle arrays were prepared. The resultant functional nanopattern/CMG bilayer was transferred onto shrinkage fi lm. Tailored crumpling was performed in a similar way with BCP fi lm/CMG bilayer ( Figure 2 A–F and Figure S3, Supporting Information). It is noteworthy that diverse other pattern transfer methodologies, such as ion immersion, [ 17 ] spin-casting of precursor solution, [ 18 ] atomic layer deposition [ 10c , 19 ] are also available for the desired nanomaterial formation.

To demonstrate further controllability of 3D nanopattern, lat-erally ordered self-assembled nanopatterns were also crumpled. As a typical example, disposable photoresist patterns formed by photolithography guided the lateral orientation of BCP nanodomains based on graphoepitaxy principle. [ 20 ] Ordered 3D nanopattern was realized by the subsequent pattern transfer and shrinkage process (Figure 2 G). Other nanodomain

alignment principles exploiting chemical nanopatterns, [ 9c ] shear force, [ 21 ] electric fi eld [ 22 ] can be also employed to prepare later-ally ordered nanopatterns.

Our crumpled nanopattern can be utilized for many different applications, such as stretchable conductor, [ 6a , c , 7c ] transmittance-tunable optodevice, [ 7e ] and strain sensor. [ 7b ] In this work electro-chemical catalytic application is tested based on the potential advantages of crumpled nanopatterns, such as ultradense and evenly distributed loading of metal nanoparticles, electrical con-ductivity of CMG layer, and open porous geometry of crumpled structure. By virtue of these advantages, HER was investigated as a prototype electrochemical reaction. [ 23 ]

Hexagonal Pt nanoparticle array/CMG bilayer was pre-pared by BCP lithography and subsequent pattern transfer (Figure 2 D). For control experiment, conventional Pt/C reference was also tested. [ 23b ] Au thin fi lm with ≈25 nm thick-ness was used as a current collector to ensure the electrical conductivity over entire electrode area (Figure S5, Supporting

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Figure 2. A–F) SEM images of Pd nanowire (A–C) and Au nanodot arrays (D–F), fabricated from BCP nanotemplates, on isotropically (A,D), uniaxially (B,E), and hierarchically (C,F) crumpled CMG layers. G) Schematic of 1D crumpling of well-aligned BCP/CMG (left), and corresponding SEM image with different magnifi cations (middle, right). The right image is a magnifi ed view for the area defi ned by the white box in the middle image.


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Information). We note that with the additional thin Au layer, the 3D crumpling behavior of CMG structure was well-reproduced. As presented in Figure 3 A,B, our crumpled Pt nanocatalyst array/CMG exhibited a higher current density (≈120 mA cm −2 @ 0.1 V) and lower Tafel slope (24 mV dec −1 ) than fl at Pt nano-catalyst array/CMG. Also, the crumpled nanocatalyst platform showed superior performance over conventional Pt/C (≈70 mA cm −2 @ 0.1 V, 34 mV dec −1 ) in any aspect of per-formance measure. Direct scanning electron microscopy (SEM) analysis reveals that our crumpled nanocatalyst has a signifi cantly higher catalyst loading level than Pt/C reference (Figure S6, Supporting Information). Nonetheless, simple high loading of catalyst nanoparticles does not fully account for the superior HER performance of crumpled nanocatalyst. The char-acteristic performance of fl at Pt nanocatalyst array/CMG simply multiplied by 6 (areal enhancement factor by shrinkage) does not reach that of crumpled nanocatalysts, particularly in high current density range (Figure S7, Supporting Information).

To account for the extra enhancement, we focused on the low fl uctuation of current density for crumpled nanocatalysts than others (Figure 3 C,D). In HER, hydrogen gas bubbles are generated during reaction. If the generated hydrogen bubbles

remain at the catalyst surface, they will block the interface between hydronium in water and catalyst surface and thus diminish the effective active area of catalysts. [ 23a , 24 ] Interest-ingly, the reduction and recovery of catalytic area by bubble formation and migration induce the fl uctuation of polarization curve. For fl at Pt nanocatalysts, the fl uctuation is clear at high potential region, indicating the ineffi cient hydrogen bubble migration (Figure 3 D). Similar behavior is also observed for Pt/C (Figure 3 D). By contrast, crumpled nanocatalyst does not show signifi cant fl uctuation of current density, despite the enormous generation of hydrogen bubbles. This behavior can be directly confi rmed by naked eye. Figure 4 A,B and Movie S1 and S2, Supporting Information, show the typical catalyst sur-face during HER. At 0 s (Figure 4 A,B), HER reaction started. For fl at nanocatalyst, the generated hydrogen bubbles tend to remain at catalyst surface. The bubble size continuously increases and it is detached as its size reaches a critical dimen-sion, which is governed by the competition between buoyancy of bubble and interfacial energy of catalyst surface with bubble (Figure 4 A). By contrast, at crumpled nanocatalyst surface small bubbles are detached as soon as they are formed. While the variation of effective catalytic area by bubble formation and

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Figure 3. A) Hydrogen evolution reaction of Pt nanocatalyst/crumpled CMG (squares), Pt nanocatalyst/fl at CMG (circles), and Pt/C (triangles), and B) corresponding Tafel slope. C,D) Magnifi ed views for the marked regions in (A).

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growth is suppressed (Figure 4 B), measured current density does not show signifi cant fl uctuation.

For the quantitative comparison of surface wettability between fl at and crumpled catalysts, contact angle measure-ment was performed. We note that the gas bubble growth at catalytic surface in aqueous media is relevant to the receding mode of contact angle measurement (Figure S8, Supporting Information). Both phenomena are involved with the triple line of gas, liquid and solid. For both cases, volume of gas phase increases as much as that of liquid phase decreases. Thus, it can be inferred that high receding contact angle makes large contact area with gas phase which should be overcome for bubble to be detached and low receding contact angle mini-mizes the contact region for gas phase with same volume, which enables easy migration of bubble as buoyancy of small bubble can facilely overcome the adhesion from small contact area (Figure S8, Supporting Information).

Figure 4 C,D and Movie S3 and S4, Supporting Informa-tion, present the receding contact angle measurement for fl at

and crumpled catalysts. While fl at nanocatalyst showed the receding contact angle of ≈21°, crumpled nano catalyst exhib-ited a good wettability with the receding contact angle of ≈6°. The low receding contact angles are due to the residual hydro-philic functional group at CMG surface, such as hydroxyl, and carboxyl groups. [ 25 ] The enhanced hydrophilic property of crumpled structure can be explained by Wenzel model, [ 26 ] which signifi es that high roughness makes hydrophilic one more hydrophilic. Eventually, the more hydrophilic nature of crumpled surface tries to minimize the contact area between hydrogen bubbles and catalyst surface. Therefore, tiny hydrogen bubbles readily leave from the catalyst surface, as soon as they are formed. By contrast, substantial bubble growth is required for the detachment at fl at surface due to the large interfacial adhesion between catalyst surface and hydrogen gas medium. Taken together, this result clarifi es that for the highly effi cient electrocatalytic behavior spatial arrange-ment of catalyst particles is signifi cant as well as the catalytic property of catalyst itself.

Figure 4. A,B) Photograph of surface of fl at Pt nanocatalyst/CMG (A) and crumpled Pt nanocatalyst/CMG (B). C,D) Receding water contact angles at the surfaces of fl at Pt nanocatalyst/CMG (C) and crumpled Pt nanocatalyst/CMG (D).


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Adv. Mater. 2016, 28, 1591–1596


In summary, we have demonstrated 3D self-assembled nano-patterning by customized crumpling of CMG layer. Tailored CMG crumpling driven by the controlled lateral shrinkage of bottom substrates enabled the effective conversion of 2D self-assembled BCP nanopatterns into complex 3D morphologies. The resultant ultra-dense nanopatterning platform could be suc-cessfully exploited for the high performance gas-releasing elec-trochemical catalysis, assisted by controlled wetting property of tailored crumpled surface. This 3D pattern transformation principle based on substrate crumpling is generally compatible to many different 2D nanopatterning methods, including con-ventional photolithography and nanoimprint, and thereby can be broadly applicable to novel applications, such as photonics, sensors and energy harvesting. Besides, taking advantage of the surface reactivity and patternability of CMG crumpling layer, our approach could also be integrated with other self-assembly systems, such as DNA and colloids for a broad range of pattern length scales and different functionalities.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the National Creative Research Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale Assembly (2015R1A3A2033061) and the Hybrid Interface Materials Research Group (Global Frontier project, 2014M3A6B1075032) of the National Research Foundation of Korea (MSIP).

Received: September 17, 2015 Revised: October 17, 2015

Published online: December 14, 2015

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3d tailored crumpling of block‐copolymer lithography on

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