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Multiscale, Hierarchical Patterning of Graphene by Conformal Wrinkling Won-Kyu Lee,† Junmo Kang,† Kan-Sheng Chen,† Clifford J. Engel,‡ Woo-Bin Jung,‡ Dongjoon Rhee,† Mark C. Hersam,*,†,‡,§ and Teri W. Odom*,†,‡ †

Department of Materials Science and Engineering, ‡Department of Chemistry, and §Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: This paper describes how delamination-free, hierarchical patterning of graphene can be achieved on prestrained thermoplastic sheets by surface wrinkling. Conformal contact between graphene and the substrate during strain relief was maintained by the presence of a soft skin layer, resulting in the uniform patterning of three-dimensional wrinkles over large areas (>cm2). The graphene wrinkle wavelength was tuned from the microscale to the nanoscale by controlling the thickness of the skin layer with 1 nm accuracy to realize a degree of control not possible by crumpling, which relies on delamination. Hierarchical patterning of the skin layers with varying thicknesses enabled multiscale graphene wrinkles with predetermined orientations to be formed. Significantly, hierarchical graphene wrinkles exhibited tunable mechanical stiffness at the nanoscale without compromising the macroscale electrical conductivity. KEYWORDS: Graphene, hierarchical patterning, texturing, polystyrene, wrinkles, conductive atomic force microscopy phene patterning is achieved by sandwiching a soft fluoropolymer skin layer between as-synthesized graphene and prestrained polystyrene substrates. Because the thickness of the skin is controllable with one-nanometer accuracy, the resulting graphene wrinkle wavelengths can be tuned from tens of nanometers to several micrometers. Moreover, because graphene wrinkles only occur on the skin layer regions, patterning the substrate with different skin thicknesses results in multiscale graphene wrinkles with different wavelengths and orientations side-by-side. In this manner, the mechanical stiffness of graphene can be locally tuned as a function of wrinkle wavelength while maintaining consistent electrical conductivity. Figure 1a depicts the process for conformal wrinkling of graphene on thermoplastic substrates. First, prestrained, thermoplastic polystyrene (PS) was treated with CHF3 plasma gas in a reactive ion etching (RIE) system. The CHF3 plasmamediated polymerization formed a soft skin CFx layer on PS with tunable thickness (h).15 Second, using an ethyl cellulose (EC) carrier film, cm2-areas of graphene were transferred onto the soft skin layer (Figure S1). Because the surface energy of the EC film was comparable to that of the CFx skin layer on PS, the adhesion between the carrier film and substrate was enough for graphene transfer (Figure S2). The EC film was then

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atterning three-dimensional (3D) structure into twodimensional (2D) graphene is important for applications in flexible electrodes,1 stretchable electronics,2 sensors and actuators,3,4 and energy storage devices.5−8 In particular, the crumpling of graphene via strain-relief of polymeric substrates has been pursued due to its ability to achieve large-area (>cm2) texturing. Graphene crumples can be generated spontaneously after strain relief of prestrained elastomer or viscoelastic substrates.4,9 The resulting 3D textured graphene has exhibited increased surface area and chemical reactivity,10 tunable hydrophobicity,4,11 and tailored optical transmittance.4,12 Moreover, the new functionalities induced by crumpling have been demonstrated without compromising the electrical conductivity of graphene.4,9,13 The 3D texturing of graphene results from delamination and irregular buckling of graphene from a strained substrate.4,12,14 Because of the weak interfacial energy between the substrate and graphene, nonuniform crumpling results after strain relief.14 Delaminated buckling is limited as a general approach for the 3D patterning of one-atom-thick graphene, however, because (1) the resulting feature sizes have been limited to less than 100 nm; (2) feature ordering cannot be controlled; and (3) the design of hierarchical graphene patterns is not possible. Hierarchical structuring is critical for modulating the mechanical, chemical, and optical properties of graphene on a single surface. Here we demonstrate a conformal wrinkling process that can generate hierarchical graphene architectures. Multiscale gra© XXXX American Chemical Society

Received: August 13, 2016 Revised: September 30, 2016

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DOI: 10.1021/acs.nanolett.6b03415 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Fabrication of graphene nanowrinkles. (a) Scheme of the process to fabricate 1D and 2D graphene nanowrinkles. (b) Height and (c) current mapping of 2D wrinkles at the PS/graphene boundary by conductive AFM.

Figure 2. Characteristics of graphene crumples and wrinkles. (a) SEM image of graphene crumples formed by 2D strain. The inset scheme and the yellow dots show the points of delamination between the crumple valleys and the underlying PS substrate. SEM images show tunable wavelengths of (b) disordered 2D wrinkles and (c) ordered 1D wrinkles. (d) Quantification of λ for graphene and PS wrinkles as a function of skin thickness (h). (e) Raman spectra of graphene wrinkles and crumples with different orientations.

Young’s moduli of the skin layer and the substrate.15−17 Wrinkle orientation can be further controlled by selecting the direction of global strain relief; uniaxial strain (ε1D) formed highly ordered 1D wrinkles, and biaxial strain (ε2D) generated

removed after rinsing with ethanol. Finally, the prestrain in the system was relieved by heating the substrate above the PS glass transition temperature (Tg) to generate the graphene wrinkles. Wrinkles were formed because of the difference between the B

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Figure 3. Local patterning of graphene wrinkles with controlled quasi-ordering. (a) Schematic illustration of skin patterning followed by graphene transfer and selective wrinkle formation. (b) SEM images of the boundary between patterned wrinkles with and without graphene and (c) the resulting architecture consisting of graphene wrinkles and crumples side-by-side. (d−g) Controlled quasi-ordering of graphene wrinkles and (h) calculated FFT order parameters (SFFT) as a function of λ/w.

are important (Figure S6). Graphene wrinkles showed a defined quasi-periodicity of features (i.e., the wrinkle wavelength (λ)),18 both for ε2D and ε1D (Figure 2b,c). Typically, λ is linearly proportional to the skin layer thickness (h).18 For the case of a soft skin layer on PS, h can be precisely controlled by changing the CHF3 RIE treatment time to enable continuous tunability of λ.15 We also found that the λ of single-crystalline graphene wrinkles was very similar to wrinkles in polycrystalline CVD graphene (Figure S7). Because of the uniformity of the graphene wrinkles, λ can be quantified for both 1D (λ1D) and 2D (λ2D) features by performing a fast Fourier transform (FFT) of scanning electron microscopy (SEM) images15 (Figure 2d). Because h can be controlled with ±1 nm accuracy,15 graphene wrinkles with arbitrary wavelengths can be achieved in 1D and 2D. By increasing h from 2 to 50 nm, λ2D linearly increased from 90 to 560 nm, and when h increased up to 100 nm, microscale graphene wrinkles were obtained. A similar trend was observed for 1D graphene wrinkles in that λ1D linearly increased from 70 to 900 nm as h increased from 2 to 50 nm. λ for 1D and 2D wrinkles are different under the same h because the strain in 2D (ε2D) is defined differently from that in 1D (ε1D) (Methods). Graphene can also act as an effective skin layer for wrinkling, and as expected, the λ of graphene wrinkles was larger than PS wrinkles for the same h (CFx) (Figure S8). By subtracting λ of PS wrinkles from that of graphene wrinkles formed at the same

random 2D wrinkles. The conformal wrinkling of graphene was reversible by stretching the as-fabricated wrinkles above Tg of the PS (Figure S3). The boundary between graphene wrinkles and PS wrinkles was characterized by conductive atomic force microscopy (CAFM). C-AFM simultaneously mapped topography (Figure 1b) and local current flow (Figure 1c) of the graphene wrinkles. For ε2D ∼ 0.4, the height image shows uniform patterning of nanoscale wrinkles across the entire surface with the boundary between graphene wrinkles and PS wrinkles being undetectable in the height image. However, the current image clearly shows the boundary since graphene is significantly more conductive than PS. The height and current images together confirm that graphene achieved conformal contact with the underlying PS wrinkles during macroscopic strain relief. We further verified conformal contact by investigating the statistical distribution of feature heights for PS and graphene wrinkles (Figure S4). In contrast to conformal graphene wrinkles on the skin layer, delaminated graphene crumples formed on untreated PS (Figure 2a). Under ε2D = 0.4, the points of delamination between the valleys of graphene crumples and the PS were randomly distributed. Delaminated 1D crumples were also induced by uniaxial strain (ε1D) of 0.5 (Figure S5). Under the same strain, the graphene wrinkles showed larger surface areas than that of the crumples indicating that conformal wrinkling has the potential for applications where surface area increases C

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Figure 4. Formation of graphene wrinkles on hierarchically patterned skin layers. (a) Iteration of the skin-patterning processes to fabricate hierarchical graphene wrinkles (double masking process as a representative case). SEM images showing hierarchically textured graphene on the patterned skin layers using (b) double and (c) triple masking process.

h, the contribution of graphene in increasing λ of the PS wrinkles can be quantified. When h was ∼2 nm, the presence of graphene increased λ2D of PS wrinkles by ∼200% and λ1D by ∼130%. Graphene did not act as an effective skin layer when h exceeded 100 nm. Also, we found that λ increased as the number of graphene layers increased for fixed h and strain (Figure S9), where λ2D of four-layered graphene was ∼110% larger than that of single-layer graphene. Raman spectroscopy on graphene wrinkles and graphene crumples revealed compressive strain in the graphene after wrinkling or crumpling (Figure 2e). Because the graphene Raman G peak overlaps with the background peaks of PS (Figure S10), we used the 2D band to characterize graphene under macroscopic strain relief. 19 In general, intrinsic compressive strain in graphene results in a blue shift of the 2D band, and where the resulting 2D peak shift is a measure of the amount of strain.9,20,21 Compared to flat graphene, a blue shift of the 2D band occurred in graphene wrinkles and crumples for both 1D and 2D features. Notably, the blue shift of graphene wrinkles was larger than that of graphene crumples for all surface-relief features. Hence, the compressive strain in graphene during macroscopic strain relief was larger in conformal wrinkling compared to crumpling (Figure S11). Because the total amount of global strain for wrinkling and crumpling on PS was the same, conformal wrinkling provides a larger window to engineer strain in graphene. Graphene wrinkles at different strain were investigated to understand the structural evolution of graphene by conformal wrinkling (Figure S12). When the CFx skin layer was relatively thin (h ∼ 6 nm), λ linearly decreased from 160 to 75 nm as ε1D increased from 0.3 to 0.75. Interestingly, in the high strain regime (ε1D > 0.7), a second generation of wrinkles with larger periodicity formed as a route to nonlinear strain relief17 and resulted in hierarchical self-similar wrinkles. When h was larger than 20 nm, λ also decreased as ε1D increased up to 0.7. Notably, under moderate global strain (ε1D ∼ 0.5) self-contacts between wrinkles started to appear (Figure S13). At high strain (ε1D > 0.7), instead of forming self-similar structures, a wrinkleto-fold transition was followed by significant surface flattening (Figure S12). In this case, the strain was concentrated into a

single fold, and the surrounding wrinkles decreased in amplitude.16,22 In addition to global control of the graphene wrinkle wavelength and orientation in 1D and 2D, wrinkles and crumples can be achieved side-by-side by patterning the skin layer. Figure 3a depicts the skin-patterning process. First, the PS surface was masked with poly(vinylpyrrolidone) (PVP) by inverse solvent-assisted nanoscale embossing (inSANE) using PDMS molds.23 After CHF3 RIE treatment and removal of the PVP masks, the CFx skin only formed on the exposed PS regions. Then, graphene was transferred onto the patterned skin using the same process in Figure 1a. As a result of the soft nature of the skin, there were no defects or tearing of the graphene at the skin-PS step edges and conformal contact was achieved (Figure S14). After strain relief of the PS substrate, graphene wrinkles were generated selectively on the patterned skin regions (Figure 3b). Similar to the trends observed in unpatterned skin layers (Figure 2b,c), λ within the patterned regions can be tuned by changing h. As expected, graphene crumples were formed on regions without the patterned skin to produce surfaces with both crumples and wrinkles (Figure 3c). Step edges at the boundaries of the patterned skin regions acted as strain-relief structures and affected the local orientation of graphene wrinkles (Figure 3d−h). Wrinkle patterns tend to order only near the strain relief features and become disordered as the distance from the relief structure increases.18 By changing λ and feature spacing (w), the orientation of graphene wrinkles on the patterned skin could be systematically controlled, thus enabling the design of hierarchical structures. We quantified the wrinkle orientation ordering using a figure of merit called the fast Fourier transform order parameter (SFFT).16 With a uniform skin layer, SFFT of 1D and 2D wrinkles were ∼0.7 and ∼0, respectively. Because the local strain effect on the wrinkle orientation scales with λ,16,25 SFFT is correlated with the ratio λ/w, regardless of the absolute values of λ and w. In this case, w was held constant (w = 5 μm) while λ was changed from 0.04w to 0.3w (ε2D = 0.4). When λ/w increased from 0.04 to 0.16, SFFT linearly increased from 0 to 0.55. Interestingly, as λ/w became larger than 0.16, SFFT saturated at ∼0.7, suggesting that graphene wrinkles were highly ordered. Above λ/w > 0.3, D

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Figure 5. Local electrical properties of hierarchical graphene wrinkles. (a) Height and current mapping of textured graphene formed by the triple patterning process in Figure 4c. (b) Local I−V curves for different domains having different feature size and orientation. Each domain number in (a) corresponds with the I−V curve in (b).

Figure 6. Characterization of the mechanical properties on hierarchically patterned graphene wrinkles. Force−distance curves between the AFM tip and the peaks of wrinkles in (a) area 1 (λ1 ∼ 160 nm), (b) area 2 (λ2 ∼ 300 nm), and (c) area 3 (λ3 ∼ 450 nm).

the graphene wrinkles collapsed into folds with higher SFFT near 0.8. The side-by-side patterns of graphene wrinkles and graphene crumples provide an opportunity to investigate local compressive strain. By Raman mapping the 2D bands, we visualized the local strain distribution in patterned graphene (Figure S15). Similar to the trend in unpatterned (singlewavelength) graphene wrinkles and graphene crumples in a single skin layer (Figure 2e), the blue shift of the 2D band from wrinkles was larger than crumples. Interestingly, the local strain from conformal graphene wrinkles was always larger than graphene crumples (Figure S16). Because soft skin layers can be deposited conformally on surfaces with arbitrary topologies,24,26 hierarchical formation of patterned skin regions with different h was possible through multiple masking and RIE treatment processes (Figure 4a). After global strain relief (ε2D ∼ 0.4), hierarchical architectures consisting of multiscale graphene wrinkle wavelengths and

different orientations were produced. Figure 4b shows hierarchically textured graphene wrinkles by a double skinpatterning process. In areas that were masked twice by PVP, graphene crumples were generated because there was no skin layer. In contrast, in the patterned skin areas with h1 ∼ 15 nm (from a single PVP masking) and h1 + h2 ∼ 30 nm (no PVP mask), graphene wrinkles with λ1 ∼ 160 nm and λ2 ∼ 300 nm were formed, selectively. The SFFT of graphene wrinkles with λ2 was ∼0.13 while that with λ1 was ∼0.55, which suggested that patterned dimensions (λ/w ratio) affected wrinkle orientation. Compared to the disordered 2D (SFFT ∼ 0) and ordered 1D wrinkles (SFFT ∼ 0.7) on a uniform skin layer, the orientations of graphene wrinkles on hierarchically patterned skins were quasi-ordered. By adding one more masking cycle, we formed four different areas of features (Figure 4c). In addition to the regions with crumples, and λ1 ∼ 160 nm and λ2 ∼ 300 nm, the largest λ3 ∼ 450 nm was formed in areas with the thickest skin layer (h1 + h2 + h2 ∼ 45 nm). The geometry of the mask E

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patterns, the number of masking and RIE cycles, and RIE treatment time (or h) can be arbitrarily selected, thus allowing nearly unlimited tunability over the resulting hierarchical structure. To investigate how hierarchical structuring affected the electrical properties of graphene, local current flow was measured by C-AFM (Figure 5). The height and current images of multiscale graphene wrinkles in Figure 4c show that the electrical properties of graphene are preserved even under structural hierarchy (Figure 5a). The current images confirmed that the current level was similar across the boundaries between domains with different λ and crumples and verified that graphene did not tear after patterning and strain-relief (Figure S17). For more precise measurements of local conductivity of graphene wrinkles with λ1 (area 1), λ2 (area 2), λ3 (area 3), and crumples (area 4), the gold C-AFM tip was positioned over specific areas for current−voltage (I−V) characterization (Figure 5b). I−V curves from specific spots were linear because of the high-quality Ohmic contact between the tip and graphene. The calculated resistances from the I−V curves at the four different areas were comparable and confirmed that the local conductivity of the graphene was maintained both in the wrinkled and crumpled forms. Furthermore, hierarchically patterned graphene surfaces showed tunable mechanical properties (Figure 6). In particular, force−distance curves were measured on the patterned graphene surface to investigate the mechanical stiffness of the multiscale graphene wrinkles. The slope of the tip−retraction curve during indentation can be considered as a measure of the stiffness of nanostructures.27 On the peaks of the wrinkles, the slope increased by ∼19% as λ increased from λ1 ∼ 160 nm (area 1) to λ3 ∼ 450 nm (area 3), which suggests that the stiffness of graphene on the skin was enhanced by ∼560% (Methods). The stiffness of the graphene wrinkles in area 3 was similar to that of the flat skin layer because the surface rigidity saturated as λ increased above 400 nm (Figure S18). The hysteresis of the force−distance curves was also compared for the different graphene wrinkles. Notably, the hysteresis decreased as λ increased because less energy dissipation occurred while the tip was pushing the stiffer wrinkles. In the valleys of all patterned graphene wrinkles, however, there was no hysteresis (Figure S19), indicating that these graphene regions were rigid for all λ. Furthermore, the adhesion forces between the surface and tip was nearly invariant (∼10 nN on valleys and ∼18 nN on peaks) for all graphene wrinkles (areas 1, 2, and 3) (Figure S20). In contrast, a much smaller adhesive force (∼3 nN) was measured on the graphene crumples as expected since these features are delaminated from the surface (Figure S21). In summary, we have demonstrated an approach for hierarchically patterning graphene in 3D. The feature size, orientation, and structural hierarchy of graphene were controlled uniformly and by design using conformal wrinkling with a soft skin layer. By patterning the multiscale wrinkles sideby-side with delaminated graphene crumples, the nanoscopic mechanical stiffness and surface adhesion of graphene were locally tuned while maintaining electrical conductivity. With rational design of the hierarchical structuring, conformal wrinkling allows exquisite tailoring of graphene for advanced applications, such as sensors and nanobio interfaces, where tunable mechanical properties and surface interactions are necessary. This approach is also likely to be generalizable to the emerging family of postgraphene 2D materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b03415. Methods, calculations, and additional figures and references (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected]. *Email: [email protected]. Present Address

(W.-B.J.) Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Yuseong-gu, Daejeon 305-338, Republic of Korea. Author Contributions

W.-K.L. and J.K. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF CMMI-1462633), the Office of Naval Research (ONR N00014-13-1-0172), and the Northwestern University Materials Research Science and Engineering Center (MRSEC, NSF DMR-1121262). This work made use of the Northwestern University Micro/Nano Fabrication Facility (NUFAB), which is supported by the State of Illinois and Northwestern University, and the Northwestern University Atomic and Nanoscale Characterization Experimental Center (NUANCE), which is supported by the NSF-MRSEC (NSF DMR-1121262). W.-K.L. gratefully acknowledges support from the Ryan Fellowship and the Northwestern University International Institute for Nanotechnology.

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