Bioinspired Fabrication of Free-Standing ... - ACS Publications

Bioinspired Fabrication of Free-Standing Conducting Films with Hierarchical Surface Wrinkling Patterns Xiu Yang,†,§ Yan Zhao,‡,§ Jixun Xie,†,§ Xue Han,† Juanjuan Wang,† Chuanyong Zong,† Haipeng Ji,† Jingxin Zhao,† Shichun Jiang,† Yanping Cao,*,‡ and Conghua Lu*,† †

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China

‡

S Supporting Information *

ABSTRACT: Mechanical instability has been shown to play an important role in the formation of wrinkle structures in biofilms, which not only can adopt instability modes as templates to regulate their 3D architectures but also can tune internal stresses to achieve stable patterns. Inspired by nature, we report a mechanical−chemical coupling method to fabricate freestanding conducting films with instability-driven hierarchical micro/nanostructured patterns. When polypyrrole (PPy) film is grown on an elastic substrate via chemical oxidation polymerization, differential growth along with in situ self-reinforcing effect induces stable wrinkle patterns with different scales of wavelengths. The self-reinforcing effect modifies the internal stresses, hence PPy films with intact wrinkles can be removed from substrates and further transferred onto target substrates for functional device fabrication. To understand the buckling mechanics, we construct a model which reveals the formation of hierarchical wrinkle patterns. KEYWORDS: surface patterns, free-standing, polypyrrole, mechanical properties, surface wrinkling

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purely elastic instability largely depend on the substrate constraint, and thus they may disappear when the substrate is removed.12,14,18−20 In this sense, it is a tremendous challenge, in principle, to fabricate free-standing films with stable surface patterns by the mechanical-instability-based strategy. In nature, diverse surface patterns exist. Understanding the correlation between the growth and form has long been a central issue of developmental biology. Many previous studies have demonstrated that mechanical instability can play a role in pattern formation in some natural systems. This includes the hierarchical patterns observed in biofilms,21,22 the gyrification patterns of the cerebral cortex of mammalian brains,23 fingerprints,24 and the topological conformation of some fruits and plants.25 Unlike engineering materials or structures, these living systems may adopt instability modes as templates to regulate their surface patterns, and they are able to alter the microstructures and tune the internal stresses during the growth to achieve stable patterns.21−25 Therefore, these natural systems mainly employ mechanical/biochemical coupling to

urface patterning across different length scales is of great importance from a scientific and technological view.1−3 As a nonlithographic surface patterning method, surface wrinkling has attracted considerable interest in the past two decades due to its simplicity, low cost, and applicability to arbitrary surface geometries.4−7 This technique relies on the mechanical instability of the materials or structures.8,9 For instance, in a composite system with a stiff film resting on a compliant substrate, surface wrinkling may occur to lower the total potential energy when the system is subjected to in-plane compression with the compressive stress beyond a certain critical value.8,9 The surface-wrinkling-based patterning technique has found wide applications ranging from optical− electronic conversion devices,10,11 optical microlenses,12 ultrahigh surface-enhanced Raman scattering,13 and reversible channels14 to smart surfaces with tunable properties.15−17 A key feature in this technique is that the surface patterns depend on the internal stresses in the system, which can be a merit because the wrinkling patterns can be widely tuned by manipulating the stress states.4−12,18 However, this feature could be an obvious disadvantage in the cases where stable surface patterns should be maintained.12,19,20 For example, surface wrinkles in a film/substrate system induced by the © XXXX American Chemical Society

Received: January 22, 2016 Accepted: March 4, 2016

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DOI: 10.1021/acsnano.6b00509 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the process to prepare a free-standing PPy film with hierarchical wrinkles: growth of PPy film on PDMS substrate (I); casting the PVA layer (II); peeling the PVA/PPy film from the substrate (III); water dissolving PVA (IV,V). The free-standing film was further transferred to target substrates (VI).

Figure 2. Optical (a,f), SE(M) (b,c), SE(L) (d,g), and AFM (e) images of the in situ wrinkling PPy film on planar (a−e) and patterned (f,g) PDMS(n:1) substrates. n:1 = 10:1 (a−e) and 30:1 (f,g). Frame h shows the dependence of the primary/secondary wavelengths and root mean square of the wrinkling film on the modulus of the PDMS(n:1) substrate. The error bars are the standard deviation of the measurements.

film on the wrinkled configuration causes hierarchical wrinkle patterns with two-scale wavelengths. To understand the wrinkling mechanics, a computational model has been built to track the occurrence of surface wrinkling and the formation of hierarchical patterns. Unlike a purely elastic instability, surface wrinkling in our system is accompanied by an in situ self-reinforcing effect which modifies the internal stress and stabilizes the wrinkling patterns. We have shown that, after substrate removal, a free-standing PPy film with stable wrinkling patterns is achieved, which can be further transferred onto desirable target surfaces. The current strategy greatly extends the application fields of the wrinkling-based technique and offers competing opportunities for the design of advanced film materials and corresponding devices.

pattern and stabilize the rich 3D architectures. Typically, biofilms are antibiotic-resistant bacterial aggregates that are able to trigger hospital-acquired infections.22 Gene expression regulates cell division and differentiation. The cell death/ growth and fluid migration can modulate the microstructure and elastic parameters of the biofilm and generate internal stresses. When the induced internal compressive stress/strain exceeds the critical value, surface wrinkling occurs.21,22,26 Further growth and death of cells in the buckled structure lead to stress and generate hierarchical wrinkling patterns. Inspired by the above mechanical−chemical processes involved in the pattern formation of biofilms, we present here a simple yet robust technique to fabricate free-standing films with instability-driven hierarchical micro/nanostructured patterns. Growth of polypyrrole (PPy) films on polydimethylsiloxane (PDMS) substrates via a chemical oxidation polymerization leads to wrinkling of the film. Further differential growth of the B


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Figure 3. AFM images of the in situ wrinkled PPy film from a reaction time of t = 3 (a), 4 (b), 10 (c), 40 (d), and 120 min (e). Frame f shows the evolution of the absorbance and film thickness h with t. Frame g shows the dependence of the primary/secondary wavelengths on h. The error bars are the standard deviation of the measurements.

RESULTS AND DISCUSSION Figure 1 illustrates the process to prepare a free-standing PPy film with stabilized hierarchical wrinkling patterns. First, growth of the PPy film on a PDMS substrate is performed via a chemical oxidation polymerization of pyrrole. In situ surface wrinkling combined with the self-reinforcing effect leads to stable wrinkles with two-scale wavelengths (Figure 1(I)). Subsequently, we cast a precursor poly(vinyl alcohol) (PVA) layer conformally on the wrinkled PPy/PDMS system (Figure 1(II)) and then peel the PVA/PPy film from the PDMS substrate (Figure 1(III)). Finally, we put the detached PVA/ PPy film into water to dissolve the PVA layer to obtain the freestanding PPy film (Figure 1(IV,V)). The resulting free-standing film with the intact two-scale patterns is further transferred to target surfaces (Figure 1(VI)). Surface adsorption polymerization has been well-utilized for the preparation of PPy films on various substrates.27,28 Here, growth of the PPy film on the PDMS substrate is achieved by direct immersion of the substrate into a strong acidic solution of pyrrole. After polymerization at 2−5 °C for the reaction time t of 60 min, we obtain a black PPy film on the substrate surface (inset of Figure 2a). The optical microscopy image shows that the air-dried PPy film has a wormlike morphology with an average periodicity of ∼3.48 μm (Figure 2a). In order to reveal the detailed morphological information, the film is examined by scanning electron microscopy (SEM) equipped with the mixed detector/imaging model (SE(M)) (Figure 2b,c) and the lower detector/imaging model (SE(L)) (Figure 2d). SE(M) and SE(L) are utilized because the former is more sensitive to surface compositional information along with a higher imaging resolution, while the latter is more sensitive to surface topographical information. The recorded SE(M) images show that the PPy film is decorated with intertwined stripes with the average periodicity (λ1) of 0.65 μm (Figure 2b). Meanwhile, each stripe is composed of densely packed 60−70 nm sized nanoparticles (Figure 2c). By contrast, the SE(L) image demonstrates that the intertwined stripes with λ1 of ∼0.65 μm are further organized into a larger wavelength (i.e., λ2 = ∼3.48 μm) of periodical labyrinth patterns (Figure 2d). The

SE(L) result is in good accord with that of atomic force microscopy (AFM), from which we can roughly identify that these short stripes of λ1 are tangled to form hierarchical textures of λ2 (Figure 2e). When we replace the planar PDMS substrate with a physically patterned stamp (Supporting Information Figure S1a−e), the labyrinth patterns on the film evolve into ordered ones, with the orientation perpendicular to the introduced stamp boundary (Figure 2f). For the surface patterns far from the boundary, they retain the initial disordered orientation (Supporting Information Figure S1f,g). On the basis of the optical and 3D laser images of the oriented patterns (Supporting Information Figure S1f,h), we validate the existence of the secondary patterns with λ2. Interestingly, the characteristic two-scale wavelengths, that is, the primary λ1 and the secondary λ2 (denoted by the number in Figure 2g), can be simultaneously identified on the SE(L) image (Figure 2g). Evidently, both the secondary patterns with λ2 and the primary patterns with λ1 are oriented and perpendicular to the introduced boundary (Figure 2g). This enhanced wrinkle orientation stemming from the effect of boundary conditions4,12,14,19,20,29 clearly indicates that the as-formed hierarchical patterns belong to the stress-relief surface wrinkles. Additionally, no obvious difference exists in the wrinkling morphologies of the wet film, regardless if it is subjected to water washing or not (Supporting Information Figure S2). This implies that surface wrinkling takes place during polymerization growth of PPy film on the substrate rather than in the water washing and air drying. Namely, the in situ surface wrinkling leads to the two-scale wavelengths of wrinkle patterns. In this study, the growth of the film is constrained by the substrate, and the film senses the compression through interfacial shear, which leads to surface wrinkling. If tension is imposed on the film through interfacial shear, for instance, by stretching the substrate, film cracking may occur as shown in a previous study.30 In order to reveal the involved wrinkling mechanism, a series of experiments are performed. First, the effect of the substrate is explored. When a glass slide is used as the substrate, the deposited film has distinct morphologies (Supporting InformaC


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with λ1 and λ2. It is found that complicated relations exist between them (Figure 3g). For instance, when h < 160 nm corresponds to t < 10 min, λ1 increases with h. When 160 nm < h < 500 nm (i.e., 10 min < t < 60 min), λ1 arrives at the saturated value of 0.67 μm. Meanwhile the newly formed λ2 increases with h. When h > 500 nm (i.e., t > 1 h), λ2 arrives at the saturated value of ∼4 μm, although h still increases slowly until t = 7 h. In other words, the dependence of λ1 and λ2 on h is diverged from the expected relation of eq 1. This divergence implies the occurrence of the unusual wrinkling mechanism, which will be discussed in detail subsequently. To further investigate the physics underlying the formation of the multiscale surface wrinkles, finite element (FE) simulations are performed using the commercial software ABAQUS. The film and substrate are both modeled as an incompressible neo-Hookean material. More than 120 000 quadratic plane strain hybrid elements (CPE8RH) are adopted to simulate the wrinkling pattern evolution induced by the differential growth. In experiments, the substrate does not grow and the film undergoes constrained growth with the film becoming thicker and thicker. The deformation gradient tensor can be expressed as39−43

tion Figure S3a,b). These morphologies should be attributed to the delamination blisters,31 supported by the SEM image (Supporting Information Figure S3a,b). These delaminationdriven blisters are in good agreement with previous reports in the case where rigid substrates are applied.31,32 On the other hand, when we vary the modulus of the PDMS substrate (Es(n:1)), the corresponding surface morphologies also change. Here, Es(n:1) is simply adjusted by the weight ratio (n:1) of the base/curing agent for PDMS.33 The hierarchical wrinkles become increasingly obvious with the decrease of Es(n:1) (Figure 2 and Supporting Information Figure S4). Furthermore, both λ1 and λ2 increase with the decrease of Es(n:1) (Figure 2h), as predicted by the following theoretical solution34,35 λ = 2πh[Ef̅ /(3Es̅ )]1/3

(1)

Here, E̅ is the plane strain modulus given by E̅ = E(1 − ν ), E is the Young’s modulus, and ν is the Poisson ratio. Subscripts f and s refer to the film and substrate, respectively, and h is the film thickness. The root mean square (rms) roughness of the in situ wrinkling film estimated from the AFM image also increases with the decrease of Es(n:1) (Figure 2h). Furthermore, the substrate’s modulus plays a more important role in the resulting morphologies than the substrate’s surface energy, which is revealed by the comparison result shown in Supporting Information Figure S3. Undoubtedly, these results demonstrate that the in situ two-scale wrinkles are strongly dependent on the elastic properties of the substrate, which in turn convincingly supports the conclusion of the stress-relief nature of the above in situ hierarchical patterns. Furthermore, in our case, the oxidant applied (e.g., ammonium persulfate) has no obvious effect on the formation of hierarchical wrinkling patterns (Supporting Information Figure S5), although previous publications have reported that the formation of hierarchical microstructures in the PPy films depends on the oxidant being used during polymerization.36 The influence of reaction time t on the resulting morphologies has been investigated systematically. The AFM and SEM images show that the PPy film from t = 3 min is composed of 60−70 nm sized nanoparticles, and no obvious wrinkling is identified (Figure 3a and Supporting Information Figure S6a). When t = 4 min, surface wrinkling with one characteristic wavelength (i.e., λ1) occurs (Figure 3b and Supporting Information Figure S6b). When t = 10 min, the primary wrinkles with λ1 start to interwind each other, and the secondary bifurcation with λ2 is observed (Figure 3c and Supporting Information Figure S6c). With further elongation of t, the primary wrinkles remain almost unchanged, but the secondary wrinkles grow noticeably (Figure 3d,e and Supporting Information Figures S6d−f and S7). Here, we track the corresponding evolution of the UV−vis absorption spectrum (Supporting Information Figure S8). It is noted that the main absorption peak occurs at ∼455 nm, which is assigned to the π−π* transition of the PPy chain.37 The plot of the absorbance at 455 nm as a function of t indicates that the polymerization growth of PPy film on the substrate can be roughly divided into three stages: approximately linear growth (t < ∼1 h), slow growth (1 h < t < 7 h), and saturated growth (t > 7 h) (Figure 3f). This growth behavior generally conforms to results of the previous report.38 The variation in the film thickness h with t shows a similar trend (Figure 3f). Here, h is measured indirectly with the details shown in Supporting Information SI-1. However, this indirect approach is a rough approximation of film thickness. We examine the relation of h 2

F = Fe·Fg

(2)

where Fe is the elastic deformation gradient and Fg is the additional deformation gradient due to growth/swelling. In our simulation, the film is assumed to undergo isotropic growth and Fg is given by ⎡g ⎤ ⎢ g ⎥ Fg = ⎢ ⎥ ⎢⎣ g ⎥⎦

(3)

where g is the growth/swelling factor that is related to the growth strain εg by g = 1 + εg. The analogy between the volumetric growth model and the thermal stress model has been illustrated;40 therefore, thermal expansion is used in this study to simulate the differential growth. To simulate the sequential wrinkling phenomenon in experiments, the scheme proposed by Cao and Hutchinson44 is adopted. The function of “model change” in ABAQUS is used to add a stress-free film on the wrinkled surface to simulate the film growth. The film materials are appended on the substrate surface layer by layer (Figure 4). In experiments, no additional mechanical straining and no heating/cooling are imposed on the film/substrate system. Therefore, the compressive stress required for the in situ wrinkling should come from the different swelling between the growing PPy film and the PDMS substrate in the mixed growth solution. It is known that the PDMS substrate is relatively inert to the solution. Under the same conditions, the PPy film can be swollen by the reaction medium, due to electrostatic repulsions between charged sites of the oxidized PPy chain and the penetration of counterions into the PPy matrix to maintain the system electroneutrality45,46 (Figure 1). Here, the saturated swelling strain εg is set at 10% for each layer. The results of FE simulations for sequential wrinkling are shown in Figure 4 and Supporting Information movie 1. When the compressive stress σ in the film due to the constraint is greater than the critical value σc, surface wrinkling with a critical wavelength of about 0.5 μm occurs (Figure 4b). With the film becoming thicker and thicker, the secondary instability arises (Figure 4f), with the wavelength (∼3 μm) being significantly greater than that of the primary bifurcation. The wrinkle D


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> σc, the in situ primary surface wrinkling with λ1 occurs (e.g., t = 4 min, Figure 3b). This swelling-induced surface wrinkling with a swelling degree on the film higher than that on the substrate has been identified in several previous reports.14,18,47 The subsequent film growth proceeds on the surface and the interior of the wrinkled film, yielding hybrid structures with an increase of the film thickness. In other words, in the hybrid film, a partial film has not taken part in the former self-wrinkling. However, the whole hybrid film can be swollen in the reaction medium, leading to the spontaneous modulation of the stress field in the wrinkling film and thus the wrinkling pattern. As a result, it can be seen that λ1 scales linearly with h at the beginning stage (i.e., h < 200 nm, Figure 3g). In the above hybrid film, the subsequent film growth takes place not only on the wrinkling surface but also inside the asswollen wrinkling bulk film. It is supported by the control experiment shown in Supporting Information Figure S9. In the case of conformal deposition on the wrinkling topography, the subsequently grown film performs as a surface mask to stabilize the underlying wrinkling patterns. In the case of internal growth, the subsequently grown film interpenetrates into the wrinkling bulk film, resulting in an “interpenetrated polymer network” and thus stabilizing the wrinkling structures. The latter stabilization mechanism is similar to the previously reported photopolymerization of the swelling agent to lock the swelling-induced wrinkling patterns to some extent.12,19,20 However, the current stabilization is performed during the in situ wrinkling without external swelling agent and without additional photopolymerization employed. Most importantly, the self-reinforcing effect modifies the internal stress and induces the formation of the stable two-scale wrinkling patterns. The ratio of the internal growth related to the surface conformal growth will decrease along with the reaction because of the decrease in the monomer concentration and the increase in the osmotic pressure from the enhanced stability of the selflocked film. In this case, λ1 remains unchanged although h increases further (i.e., h > 200 nm, Figure 3g). Namely, the fine modulation of λ1 with the variation of h has been terminated. Simultaneously, the whole composite film is still swollen by the

Figure 4. Finite element simulation of hierarchical wrinkles generated by the differential growth of the film on the soft substrate. (a) Substrate; (b−g) number of PPy layers varies from 1 to 6. Modulus ratio of the film to the substrate is taken as μf/μs = 12. Here, μ is the initial shear modulus. In the simulations, the PPy layer was “deposited” onto the (composite) substrate layer by layer. The thickness of each layer is 50 nm. Scale bar: 2 μm.

amplitude of the larger scale increases as the film grows (Figure 4g). The hierarchical wrinkling patterns are similar to those observed in experiments; however, a quantitative comparison of the wrinkling wavelengths given by simulations with those from experiments is difficult because an accurate determination of the modular ratio and the film thickness is challenging. On the basis of the above experimental and simulation results, we propose the following in situ self-reinforcing surface wrinkling mechanism. During polymerization growth of PPy film on the free PDMS substrate in the mixed solution, the different swelling between them induces the required σ. Once σ

Figure 5. Digital (a−c), optical (d), and SEM (e) images of the in situ wrinkling PPy film that was peeled from the PDMS substrate with the help of a PVA layer (a), followed by water dissolution of the PVA layer to yield a free-standing one (b) and final transfer onto target substrates such as a plastic sheet (c) and PDMS stamp (d,e). E


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limitations in the selection of underlying substrates in previous methods, in addition to the conundrum of the wrinkle stability, which is directly related to the device performance/life. Additionally, the transfer printing strategy can be simply repeated many times to generate novel devices with multilayers of transferred films. Finally, these stable wrinkles can be combined with other patterns for the fabrication of complex multiscale architectures, as shown in Supporting Information Figures S12 and S13. The in situ self-reinforcing wrinkling strategy is inspired by the wrinkled morphology observed in biofilms; in this sense, our material may serve as a model system to understand the rich and complex multiscale architectures of bacterial biofilms on various substrates.

reaction medium. When the swelling-induced critical wrinkling conditions are satisfied, the second surface wrinkling with λ2 will happen on the first wrinkled film with λ1, resulting in the two-scale patterns (Figures 2−4). This in situ self-reinforcing wrinkling is highly suitable for a large-area surface patterning (e.g., 7 × 7 cm2, inset in Figure 2a), irrespective of surface geometries of the applied substrate (Figure 2f,g). Due to the greatly enhanced stability of the in situ wrinkling, the wrinkled PPy film can be peeled from the PDMS substrate and becomes free-standing. As illustrated in Figures 1(I) and 2a−g, the large-scale wrinkling occurs on PPy film when it is grown on the substrate. Then a concentrated PVA solution is cast on the wrinkled PPy film, and a solidified precursor PVA film (∼30 μm thick) is conformally attached on the wrinkled profile (Figure 1(II)). When we peel the PVA film, the underlying wrinkled PPy film is also detached from the PDMS substrate (Figure 1(III) and Figure 5a). Consequently, a freestanding PPy film is obtained after dissolution of the PVA layer in water (Figure 1(IV,V) and Figure 5b). As expected, the primary/secondary wrinkle morphologies are maintained on the free-standing film (Figure 5d,e), although the substrate constraint has been removed and the PVA layer has been fully dissolved (Supporting Information Figure S10). For comparison, the same procedure is applied to the following two typical cases. One is the heating-induced wrinkling on a polystyrene (PS)/PDMS system (Supporting Information Figure S11a). The other is the three-scale wavelengths of wrinkles on the PPy/PDMS system, which comes from the well-organized in situ two-scale wrinkles and the external solvent swellinginduced ones (Supporting Information Figures S12 and S13 and the related discussion in Supporting Information SI-2). Once the substrate constraint was removed, no wrinkles remain on the free-standing PS film (Supporting Information Figure S11b). In the latter case, the in situ two-scale wrinkles are maintained, while the external solvent swelling-induced third wrinkling disappears (Supporting Information Figure S14). The above comparative results strongly demonstrate the decisive role of the self-reinforcement in the enhanced stability of the in situ two-scale wrinkles. Since the free-standing wrinkling PPy film is spread out in the water, it can be easily transferred to any target substrates, such as ITO, a plastic sheet (Figure 5c), and a PDMS stamp (Figure 5d,e). Meanwhile, the initial two-scale wrinkle morphologies are steadily maintained on the transferred PPy film (Figure 5d,e). This combined strategy composed of the in situ self-reinforcing wrinkling and the subsequent transfer printing enables us to fabricate functional surfaces/devices. For example, combined with the enhanced stability and the good conductivity, the transferred wrinkling PPy film on target substrates has great potential for applications such as in advanced gas/solvent sensors, photovoltaic cells, and a new generation of biomaterials with outstanding comprehensive performances.48−50

METHODS Preparation of the PDMS(n:1) Substrate. A polydimethylsiloxane substrate was prepared by mixing the base/curing agent (Sylgard 184, Dow Corning) at a designed weight ratio of n:1 (e.g., n:1 = 5:1, 10:1, 20:1, 30:1, and 40:1). After being degassed for 1 h, the mixed base/ curing agent was poured into a culture dish and baked at 60 °C for 6 h to obtain the elastic PDMS(n:1) substrate. Unless specified otherwise, the applied PDMS(n:1) substrate is from the weight ratio of base/curing agent of 10:1, that is, PDMS(10:1). The patterned PDMS substrates were prepared by pouring the mixed base/curing agent onto structured silicon masters. When the curing was over, the PDMS stamps with negative patterns were peeled carefully from the silicon masters. Growth of PPy Film on the PDMS(n:1) Substrate with In Situ Self-Reinforcing Surface Wrinkling. Typically, under continuous stirring at 2−5 °C, a pyrrole monomer was added dropwise into HCl solution to obtain a mixture solution composed of 0.1 M pyrrole and 1 M HCl (simply referred to A). Similarly, the mixture solution composed of 0.2 M FeCl3 and 1 M HCl was also prepared (simply referred to B). Subsequently, the PDMS(n:1) substrate was immersed into the mixed solution of A and B with a volume ratio of 1:1. After a designed reaction time (t) at 2−5 °C, the PPy-film-grown PDMS substrate was taken out, followed by water washing and air drying. Fabrication of a Free-Standing PPy Film with Stabilized Wrinkling Patterns and Subsequent Transfer onto a Target Substrate. A 10% aqueous solution of poly(vinyl alcohol) (J&K Scientific Ltd, Mw = 20000−30000), was cast on the wrinkling PPy film that was grown on the PDMS substrate. After being dried in air, the PVA film coupled with the wrinkled PPy film was carefully peeled from the PDMS substrate. Subsequently, the PVA/PPy film (∼30 μm thick) was floated in water to dissolve the PVA layer for the formation of free-standing PPy film. Finally, target substrates such as an ITO slide and a PDMS stamp were partially immersed in the water. Once the free PPy film was attached, the substrate was pulled out of the water slowly with simultaneous transfer of the film on its surface. Characterization. Surface patterns of the samples were characterized by an inverted Observer A1 microscope (Zeiss, Germany), atomic force microscope (Agilent 5500 AFM/SPM) in tapping mode, and a laser confocal microscope (LEXT OLS4100, Olympus). The samples without metal sputtering were observed by a scanning electron microscope (Hitachi S-4800) equipped with different detectors and imaging models. A T6 New Century spectrophotometer was used to record the UV−vis absorption spectrum of the grown PPy film from different reaction time t.

ASSOCIATED CONTENT

CONCLUSION In conclusion, we report a simple and versatile wrinkling-based method to fabricate free-standing films with stable hierarchical micro/nanostructured patterns. The surface patterns can be well-tuned by controlling the deposition time, monomer concentration, and the substrate modulus. We show that the free-standing films with stable wrinkles are available after substrate removal and can be further transferred onto arbitrary target substrates. This transfer printing strategy addresses the

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b00509. Figures S1−S14 and detailed discussions of the film thickness measurement (SI-1) and results of Figures S12 and S13 (SI-2) (PDF) Movie 1: FE simulation of formation of hierarchical wrinkles (MOV) F


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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

X.Y., Y.Z., and J.X.X. contributed equally to this work. C.H.L. conceived the project. Y.P.C. conceived and designed the theory. X.Y. carried out the experiments. Y.Z. built the computational model and performed the simulations. J.X.X. performed the SEM characterization. X.H. and J.J.W. carried out the AFM measurements. C.H.L. and Y.P.C. prepared the manuscript. All authors contributed to the data interpretation, discussions, and writing.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS C.H.L. acknowledges the financial support from the Natural Science Foundation of China (Nos. 21374076, 21574099) and Tianjin Research Program of Application Foundation and Advanced Technology (No. 13JCYBJC17100). Y.P.C. acknowledges the financial support from NSFC (Nos. 11172155, 11572179). REFERENCES (1) Assender, H.; Bliznyuk, V.; Porfyrakis, K. How Surface Topography Relates to Materials’ Properties. Science 2002, 297, 973−976. (2) Shyu, T. C.; Damasceno, P. F.; Dodd, P. M.; Lamoureux, A.; Xu, L.; Shlian, M.; Shtein, M.; Glotzer, S. C.; Kotov, N. A. A Kirigami Approach to Engineering Elasticity in Nanocomposites Through Patterned Defects. Nat. Mater. 2015, 14, 785−789. (3) Xu, S.; Yan, Z.; Jang, K.-I.; Huang, W.; Fu, H.; Kim, J.; Wei, Z.; Flavin, M.; McCracken, J.; Wang, R.; et al. Assembly of Micro/ Nanomaterials into Complex, Three-Dimensional Architectures by Compressive Buckling. Science 2015, 347, 154−159. (4) Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides, G. M. Spontaneous Formation of Ordered Structures in Thin Films of Metals Supported on an Elastomeric Polymer. Nature 1998, 393, 146−149. (5) Genzer, J.; Groenewold, J. Soft Matter with Hard Skin: From Skin Wrinkles to Templating and Material Characterization. Soft Matter 2006, 2, 310−323. (6) Schweikart, A.; Fery, A. Controlled Wrinkling as a Novel Method for the Fabrication of Patterned Surfaces. Microchim. Acta 2009, 165, 249−263. (7) Rodríguez-Hernández, J. Wrinkled Interfaces: Taking Advantage of Surface Instabilities to Pattern Polymer Surfaces. Prog. Polym. Sci. 2015, 42, 1−41. (8) Cerda, E.; Mahadevan, L. Geometry and Physics of Wrinkling. Phys. Rev. Lett. 2003, 90, 074302. (9) Li, B.; Cao, Y.-P.; Feng, X.-Q.; Gao, H. Mechanics of Morphological Instabilities and Surface Wrinkling in Soft Materials: A Review. Soft Matter 2012, 8, 5728−5745. (10) Rogers, J. A.; Someya, T.; Huang, Y. Materials and Mechanics for Stretchable Electronics. Science 2010, 327, 1603−1607. (11) Liu, Z.; Fang, S.; Moura, F.; Ding, J.; Jiang, N.; Di, J.; Zhang, M.; Lepró, X.; Galvão, D.; Haines, C.; et al. Hierarchically Buckled SheathCore Fibers for Superelastic Electronics, Sensors, and Muscles. Science 2015, 349, 400−404. (12) Chan, E. P.; Crosby, A. J. Fabricating Microlens Arrays by Surface Wrinkling. Adv. Mater. 2006, 18, 3238−3242. (13) Bae, H. J.; Bae, S.; Park, C.; Han, S.; Kim, J.; Kim, L. N.; Kim, K.; Song, S. H.; Park, W.; Kwon, S. Biomimetic Microfingerprints for Anti-Counterfeiting Strategies. Adv. Mater. 2015, 27, 2083−2089. G


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