Controlled Formation of Surface Patterns in Metal Films Deposited on

Research Article www.acsami.org

Controlled Formation of Surface Patterns in Metal Films Deposited on Elasticity-Gradient PDMS Substrates Senjiang Yu,*,† Yadong Sun,† Yong Ni,*,‡ Xiaofei Zhang,† and Hong Zhou† †

Department of Physics, China Jiliang University, Hangzhou 310018, P.R. China Department of Modern Mechanics, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China

‡

S Supporting Information *

ABSTRACT: Controlled surface patterns are useful in a wide range of applications including flexible electronics, elastomeric optics, fluidic channels, surface engineering, measurement technique, biological templates, stamps, and sensors. In this work, we report on the controlled formation of surface patterns in metal films deposited on elasticity-gradient polydimethylsiloxane (PDMS) substrates. Because of the temperature gradient during the curing process, the PDMS substrate in each sample successively changes from a purely liquid state at one side to a purely elastic state at the opposite side. It is found that surface folds appear in the liquid or viscous PDMS region while wrinkles form in the elastic region. In the transition region from the liquid to elastic PDMS, a nested pattern (i.e., the coexisting of folds and wrinkles) can be observed. The folding wave is triggered by the intrinsic stress during the film deposition and its wavelength is independent of the film thickness. The wrinkling wave is induced by the thermal compression after deposition and its wavelength is proportional to the film thickness. The report in this work could promote better understanding of the effect of substrate elasticity on the surface patterns and fabrication of such patterns (folds and wrinkles) by tuning the substrate property. KEYWORDS: thin film, surface pattern, fold, wrinkle, elasticity gradient

1. INTRODUCTION Surface patterns induced by residual stress or mechanical compression can be widely observed in natural and artificial systems such as geologic structures, textile products, plant leaves, flowers, and fruits, animal skins, biological tissues, elastic sheets, films, coatings, graphenes, nanotubes, etc. For film− substrate systems, the surface patterns are strongly dependent on the substrate property and stress level. If the substrate is rigid, the film tends to delaminate from the substrate under the compression. Because of the mixed-mode related interfacial adhesion, the delaminating zone usually does not expand infinitely but it is localized. In the film surface, various interesting delaminating patterns, such as circular, straightsided, varicose, and telephone cord structures can be observed.1−3 If the substrate is elastic, the film and substrate can undergo a conformal deformation under the compression and the film remains well attached to the substrate. In the film surface, various homogeneous wrinkling patterns such as © XXXX American Chemical Society

labyrinths, herringbones, stripes, chessboards and ripples are frequently observed.4−6 The morphologies of the wrinkling patterns are closely related to the stress anisotropy and stress evolution in the film. The recent studies also showed that the wrinkles can change into the localized buckle-driven delaminations when the substrate rigidity increases or the interfacial adhesion decreases.7,8 If the substrate is a liquid or very soft gel, the elastic film tends to form homogeneous shallow wrinkles (with small aspect ratio) under a modest compression.9,10 As the compression increases greatly, the wrinkle amplitude (or aspect ratio) increases first, and then the homogeneous wrinkling mode is broken and finally the localized folding mode (the film penetrates into the substrate deeply) appears.10,11 Another Received: December 17, 2015 Accepted: February 9, 2016

A

DOI: 10.1021/acsami.5b12369 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces localized surface mode, namely, ridging (the film deforms upward), has also been reported although it is relatively infrequent in experiment.12,13 Furthermore, the localized delaminating mode can also be observed in the elastic films resting on liquid or soft gel substrates when the compression is much large.14,15 During the development of the localized modes (folding, ridging or delaminating), the surface wrinkles decay gradually and finally disappear completely.10−15 It is wellknown that the stress energy is evenly distributed on the wrinkled surface, but it is concentrated near the localized surface modes.16 Both the homogeneous wrinkles and localized stress modes have been extensively investigated experimentally and theoretically in the past decades. The morphological characteristics, mode selections, transition behaviors and mechanical mechanisms of these patterns are now well established. Especially, the controlling of the surface patterns via tuning the mechanical compression has been investigated deeply.10−15 However, a systematically experimental investigation on the controlling of the surface patterns via tuning the substrate property remains insufficient up to now. In this work, we report on the controlled formation of surface patterns in metal films deposited on elasticity-gradient PDMS substrates. The substrates were prepared by establishing a temperature gradient across the sample surface during the curing process. In each sample, the PDMS substrate successively changes from a purely liquid state to a purely elastic state in spatial. Then the metal films were sputtering deposited on the elasticity-gradient PDMS substrates. The surface morphologies of the films are strongly dependent on the position, that is, the substrate property. Two types of surface patterns, that is, folds and wrinkles, are observed in the films. The folding wave appears in the liquid or viscous PDMS region and results from the intrinsic stress during the film deposition. The wrinkling wave forms in the elastic PDMS region and is induced by the thermal stress after deposition. Our experimental technique can be developed to effectively maneuver the surface patterns via tuning the substrate property. Controlled surface patterns are beneficial for a wide range of technological applications.

Figure 1. (a) Experimental setup showing the preparation of elasticitygradient PDMS substrates. A liquid PDMS layer was first spin-coated onto a rectangular glass slide. Then the sample was placed onto a 100 °C hot plate for 10 min when one short edge of the glass slide was lifted up by an insulated plastic box. (b) Evolution of the temperature T at the PDMS surface with the distance from the top edge of the glass slide x with varied curing times.

2. EXPERIMENTAL SECTION 2.1. Substrate Preparation. The soft substrates used in our experiment were polydimethylsiloxane (PDMS, Dow Corning’s Sylgard 184). The prepolymer and cross-linker were mixed with 10:1 volume ratio. After stirring well, the PDMS was put into a low pressure chamber for several minutes to remove the air bubbles completely. Then the liquid PDMS was spin-coated onto clean glass slides with the size of about 25 × 12 mm2 (see Figures 1 and 2). The thickness of PDMS layer was in the range of 15−20 μm. The experiment shows that the liquid PDMS can cross-link gradually and turn into viscous or elastic material in the ambient temperature. But the natural curing process is very slow and it usually needs 2−3 days to cross-link the liquid PDMS completely. Temperature rising can promote the curing process greatly and the property of PDMS was strongly dependent on the curing temperature and curing time.17−19 In our experiment, the sample was placed onto a hot plate with 100 °C to cross-link the liquid PDMS, as shown in Figure 1a. During the curing process, one short edge of the glass slide was lifted up by an insulated plastic box. The temperature should increase from the top edge to bottom edge of the glass slide. We adopted an infrared thermometer (Smart Sensor AR852B+, with the temperature range of −50−700 °C) to measure the surface temperature point-by-point. The temperature distributions in the sample with varied curing times are shown in Figure 1b. Here we define the distance from the top edge of glass slide as x, as shown in Figure 2. It is found that the temperature is almost

Figure 2. (a) Overviews of nickel films with varied thicknesses deposited on the elasticity-gradient PDMS substrates. From left to right, the film thicknesses are 2.5, 11.3, 20, 37.5, 90, and 180 nm, respectively. The temperature during the curing process (i.e., the PDMS elasticity) increases from the top edge to bottom edge. (b) An enlarged view of the nickel film with the thickness h = 15 nm. independent of the curing time. That is to say, a stable temperature field across the PDMS surface can build up within a short time (about 1 min) due to the good thermal conductivity of glass slide. When the distance x increases, the temperature increases from about 60 °C to near 100 °C. As a result, the cross-linking degree of PDMS increases successively with increasing the distance x. Meanwhile, the property of PDMS is also strongly determined by the curing time.17−19 On the basis of many experimental investigations, we can obtain a perfect elasticity-gradient substrate when the curing time was tuned at 10 min. In this case, the PDMS near the bottom edge of glass slide has turned into completely elastic material, while it remains a liquid (or viscous) B


Research Article

ACS Applied Materials & Interfaces

Figure 3. (a−f) Typical surface morphologies at different positions taken by the optical microscopy (in reflection mode) for the nickel film with h = 5 nm (for details see Figure S1). The data appearing in the bottom-left corners represent the distance x. All images have the same size of 70 × 53 μm2.

Figure 4. (a−f) Typical surface morphologies at different positions taken by the optical microscopy for the nickel film with h = 30 nm (for details see Figure S2). The data appearing in the bottom-left corners represent the distance x. All images have the same size of 70 × 53 μm2. state near the top edge. After curing, the PDMS substrates cooled down to room temperature, and then they were put into the vacuum chamber for film deposition. 2.2. Film Fabrication. The films were prepared by a direct current (DC) magnetron sputtering technique at room temperature. The sputtering target was a piece of nickel disk (purity 99.9%) with the diameter of 60 mm and the thickness of 3 mm. Note that the film material is not restricted to nickel. Similar experimental phenomena can be found in a variety of metals, including nickel, iron, chromium, silver, cobalt, copper, etc. Before sputtering, the residual gas pressure was first pumped below 2 × 10−4 Pa. The argon gas (purity 99.99%) was then filled into the chamber and its pressure keeps to 0.5 Pa during the film deposition. The distance from the target to the substrate was about 80 mm. The deposition rate of the film was about 15 nm/min. The deposition time was controlled precisely by a computer and it ranged from 10 s to 20 min. 2.3. Characterization. The overviews of the samples were taken by a common camera (Nikon D5200). The surface morphologies of the films were investigated by an optical microscopy (Leica DMLM), equipped with a CCD camera. The 3D information and sectional profiles were scanned by an atomic force microscopy (AFM, Dimension 3100, Veeco) operated in tapping mode. The collected data were on square 256 × 256 arrays of pixels and the scanning areas ranged from 5 × 5 to 90 × 90 μm2. Water contact angles were measured using contact angle goniometer (OCA30, Dataphysics, Germany) with deionized water droplets size of 5 μL.

3. RESULTS AND DISCUSSION 3.1. Surface Morphologies. The overviews of metal films (here they are nickel) with varied thicknesses deposited on the elasticity-gradient PDMS substrates are shown in Figure 2a. Figure 2b shows an enlarged view of the metal film with the film thickness h = 15 nm. We find that the film surface near the top edge of glass slide is uniformly gray under the camera, while color stripes can be observed near the bottom edge. When the film thickness increases, the front of the color stripe propagates toward the top edge gradually. If the film thickness is large enough, the color strip becomes indistinct. The color strips usually result from the interference of light owing to periodic surface patterns such as wrinkles. It is suggested that the metal films deposited on the elasticity-gradient PDMS substrates should possess characteristic surface patterns. Figures 3−5 show the typical surface morphologies at different positions in the nickel films with h = 5, 30, and 120 nm, respectively. The detailed evolutional behaviors of the surface patterns from the top edge to bottom edge are shown in Figures S1−S3. Note that the image size shown in Figure 5 is 5 times larger than that shown in Figures 3 and 4. Our experiment shows that the metal films deposited on the elasticity-gradient PDMS substrates are usually frosted. The surface patterns are strongly dependent on the position (i.e., the PDMS elasticity) and the film thickness. In general, if the film thickness is small, various undulation patterns spread across the entire film surface, as shown in Figures 3 and 4. If the C


Research Article


Figure 5. (a−f) Typical surface morphologies at different positions taken by the optical microscopy for the nickel film with h = 120 nm (for details see Figure S3). The data appearing in the bottom-left corners represent the distance x. All images have the same size of 350 × 265 μm2.

Figure 6. (a−h) Typical surface morphologies at different positions taken by the atomic force microscopy (AFM) for the nickel film with h = 5 nm (for details see Figure S4). The green arrows in (a) represent the folding patterns of the film. The data appearing in the bottom-left corners represent the distance x. All images have the same size of 20 × 20 μm2. (i) Comparison of the sectional profiles at different positions. Note that the profile lines are shifted artificially along the vertical coordinate for clarity.

film thickness is larger than a critical value (about 90 nm for nickel film), cracks start to form near the bottom edge (i.e., the completely elastic PDMS region). Meanwhile, the undulation pattern decays gradually and finally the film surface is flattened, as shown in Figures 5 and S3. To detect more structural details of the surface undulations, the film morphologies at different positions and corresponding sectional profiles (in this case h = 5 nm) were taken by an atomic force microscopy (AFM), as shown in Figure 6. Sequential AFM images are also shown in Figure S4. It is clear that two types of surface waves can be distinguished. Near the top edge, the surface wave is much disordered, including the wave orientation and wave period. This surface wave is defined as folding wave since film folds can be seen clearly (see the arrows in Figure 6a). As the distance increases, the size of the folding wave decreases gradually. At x ≈ 19 mm, a nested pattern appears because another large surface wave starts to form.20,21 Then this large wave becomes more pronounced while the small wave (folding wave) decays gradually and finally disappears completely. The large wave is disordered in orientation but its period is well-defined. This pattern is surface wrinkling and it is defined as wrinkling wave. The transition from the folding wave to wrinkling wave and corresponding sectional profiles are shown in Figure S5. Because the wrinkling wave is well periodic, the interference fringes of light can be seen clearly in the wrinkling zone. But

the folding wave does not have a good periodicity, and thus uniformly gray color is observed in the folding zone, as shown in Figure 2. As the film thickness increases, the front of interference fringes propagates toward the top edge gradually, meaning that the wrinkling zone is enlarged accordingly. If the film thickness is much large, the cracks start to form in the film and the wrinkles decay gradually, which makes the interference fringes become indistinct. To further understand the evolution behaviors of these surface patterns, we measured the dependences of the average period (or wavelength) λ and root-mean-square (RMS) surface roughness Rq on the distance x, as shown in Figure 7. Note that although the folding wave does not have a good periodicity, its average wavelength can be effectively determined by measuring a large number of values. It is found that the fold wavelength and wrinkle wavelength both decrease steadily with increasing the distance. The folding wave has a biggest wavelength at the top edge (x = 0). Then it decays gradually with increasing the distance and finally disappears at about x = 20 mm. The wrinkling wave starts to form at x ≈ 18 mm. As the distance increases, the wrinkle wavelength first decays quickly and then the decay rate slows down gradually. The coexisting zone for folding and wrinkling patterns (shaded area in Figure 7) is comparatively narrow (about 2 mm for h = 5 nm). On the other hand, as the distance x increases, the RMS surface roughness Rq decreases steadily in the folding zone. Then it D


Research Article


varied film thicknesses. As we have mentioned above, the cracks start to form when the film thickness is beyond a critical value (about h = 90 nm for nickel film). The occurrence of cracking pattern can change the surface morphologies of the film greatly. Therefore, we only measured the fold wavelength and wrinkle wavelength below the critical film thickness (h < 90 nm). It is found that the wrinkle wavelength is much larger than the fold wavelength at the same position, especially in the case of large film thickness. The fold wavelengths for varied film thicknesses are almost coincident with one another. The enlarged view shown in Figure 8b indicates that the folding wave is independent of the film thickness. Generally, the fold wavelength decays from about 3.5 to 0.5 μm with increasing the distance. On the other hand, the wrinkle wavelength is strongly dependent on the film thickness. The enlarged view shown in Figure 8c indicates that the wrinkle wavelength usually has a larger value for larger film thickness at the same position. Figure 8d shows the dependence of the wrinkle wavelength in the vicinity of the bottom edge (x = 24.5 mm) on the film thickness h. It is clear that the wrinkle wavelength increases approximately linearly with the film thickness. In the experiment, we also measured the position where the wrinkles start to form (namely, x1), the position where the folds finally disappear (namely, x2) and their difference Δx = x2−x1 for various film thicknesses, as shown in Figure S6. We find that as the film thickness increases, the positions x1 and x2 both decrease steadily. The difference Δx is almost unchanged when h < 40 nm. But it increases obviously when h > 40 nm, indicating that the coexisting zone for folding and wrinkling patterns has a larger width for larger film thickness. 3.2. Formation Mechanisms. Surface wrinkling and folding can be widely observed in natural and artificial systems such as tectonic plates, ground surfaces, plant fruits, animal skins, biological tissues, sheets, films, coatings, graphenes etc.

Figure 7. Dependences of the average wavelength λ (a) and rootmean-square (RMS) surface roughness Rq (b) on the distance x. The hollow squares (left) and solid stars (right) in (a) represent the fold wavelength and wrinkle wavelength, respectively. The shaded area represents the coexistence of folding and wrinkling patterns. The nickel film thickness is h = 5 nm.

increases somewhat during the transition from the folding wave to wrinkling wave. In the wrinkling zone, the surface roughness decreases slightly with increasing the distance. Figure 8a shows the dependences of the average fold wavelength and wrinkle wavelength on the distance x with

Figure 8. (a) Dependences of the average fold wavelength (left) and wrinkle wavelength (right) on the distance x with varied film thicknesses. (b) Enlarged view showing the evolution of the fold wavelength with the distance. The solid line is a guide to the eye. (c) Enlarged view showing the evolution of the wrinkle wavelength with the distance. The solid line is a guide to the eye for the film with h = 60 nm. (d) Dependence of the wrinkle wavelength λ in the vicinity of the bottom edge (x = 24.5 mm) on the film thickness h. The solid line is a linear fit to the experimental data. E


Research Article


temperature of the PDMS substrate would increase obviously during deposition. Therefore, the PDMS substrate thermally expands and places the metal film under a tensile stress. As the deposition time (or film thickness) increases, the tensile stress increases accordingly. When the tensile stress is beyond a critical value, cracks start to form in the film. The initiation and morphology of the cracks are strongly dependent on the interfacial adhesion between the film and the substrate.19,23−25 For completely elastic PDMS substrate, the interfacial adhesion is strong. The tensile stress is released by the formation of new intersectional cracks.19 Therefore, the cracks are dense but narrow in the elastic PDMS region (near the bottom edge). For liquid PDMS substrate, the interfacial adhesion is weak. The tensile stress is released by the sliding of the broken film pieces.23 Therefore, the cracks are sparse but broad in the liquid PDMS region (near the top edge). Furthermore, the liquid or viscous PDMS can effectively accommodate the tensile stress because of the flowing deformation of the substrate, and thus the cracks always start from the elastic PDMS region. On the basis of the above analysis, we can conclude that the folding wave forms at the early stage of film deposition. Its morphology is almost unchanged during the subsequent deposition process of the film. If the film thickness is large enough, the cracks start to form near the bottom edge due to the tensile stress stored up during deposition. The cracks propagate from the bottom edge to top edge gradually with further increasing the film thickness. After deposition, the wrinkling wave appears in the elastic PDMS region owing to the thermal compression. The morphologies of the folding and wrinkling waves near the cracks are shown in Figure S7. Since the formation of folding wave is earlier than that of cracking pattern, the folding morphology remains unchanged after the crack forms, as shown in Figure S7a. However, the formation of wrinkling wave is later than that of cracking pattern. The occurrence of the crack can change the local stress field greatly. Near the crack edge, the isotropic or equi-biaxial stress can change into a quasi-uniaxial stress which is parallel to the crack edge.4,19 Therefore, the wrinkles are always aligned perpendicular to the crack edge, as shown in Figures 5e and S7b. To further verify the formation mechanisms of the folding and wrinkling waves, we have performed a simple test. In this test, the film sample was placed on a hot plate and they were placed under the optical microscopy. Then the temperature of the hot plate was increasing gradually and the morphological evolution of the film was recorded in situ by the optical microscopy. It is found that the wrinkling wave decays gradually with increasing the temperature. When the temperature is above ∼150 °C, the wrinkling wave disappears completely. However, the folding wave is almost unchanged even when the temperature is above 200 °C. This simple test further proves that the folding wave results from the surface instability during the film deposition while the wrinkling wave is driven by the thermal compression after deposition. 3.3. Theoretical Analysis. An interesting report about the observed folding wave in our experiment is the independency of the film thickness as shown in Figure 8b. We believe that the periodicity of the folding wave is inherited from the periodicity of the fastest growing wrinkling of a compressed elastic film on the viscoelastic substrate.26,27 There exists a critical wavelength expressed as

Usually, the wrinkling and folding patterns form under a compression originating from the residual stress or constrained growth. The surface morphologies of thin films are closely related to the substrate elasticity and compression level. For elastic substrates, the films are susceptible to forming periodic wrinkles under the compression.4−6 For liquid or very soft gel substrates, the films tend to form shallow wrinkles under a modest compression while they can evolve into localized folds9,10 (sometimes ridges11,12 or delaminations13,14) when the compression increases greatly. The weak interfacial adhesion and fluid inelastic deformation play important roles in determining the folding morphology. In our experiment, we suggest that the folding wave is triggered by the intrinsic stress during the film deposition while the wrinkling wave is induced by the thermal compression after deposition. During deposition, the high energy metal atoms and other particles (with the energy of several electron volts) bombard onto the PDMS surface. For completely elastic PDMS, its surface can keep flat under the particle bombardment due to the strong cross-linking between the PDMS molecules. For liquid PDMS, however, its surface is susceptible to generating instability during deposition because the cross-linking between the PDMS molecules is very weak. Therefore, the folding wave forms in the liquid or viscous PDMS region. This pattern is strongly dependent on the cross-linking degree of the PDMS, and thus the fold wavelength and surface roughness both decrease greatly with increasing the distance x, as shown in Figures 7 and 8. According to many previous studies, the temperature of the sample (especially the substrate surface) can increase obviously during the film deposition owing to the heat radiation and particle bombardment.4,22 After deposition, the sample cools down to the room temperature and the PDMS substrate thermally contracts, placing the metal film under a compressive stress. For completely elastic PDMS, the interfacial adhesion between the metal film and the PDMS substrate is very strong. The thermal compression is relieved by the formation of homogeneous wrinkles with well-defined wavelengths. Because the thermal stress is isotropic or equi-biaxial, the wrinkles are disordered in orientation (they are also called as labyrinths).4−6 For liquid PDMS, the interfacial adhesion is very weak. The thermal compression can be effectively released by the sliding of the film and the flowing deformation of the substrate.23 Therefore, the wrinkling wave is not visible in the liquid PDMS region although it is pronounced in the elastic PDMS region. In the transition region from liquid PDMS to elastic PDMS, the folding wave first forms during the film deposition. Then the thermal compression induces the formation of the wrinkling wave after deposition and thus the coexisting of folding and wrinkling patterns occurs. Because the film deposition can further cross-link the PDMS due to the heat and particle bombardment, the elastic PDMS region (corresponding to the wrinkling zone) expands gradually with increasing the film thickness, as shown in Figure 2. Our experiment also shows that if the nickel film thickness is beyond 90 nm, the cracks start to form near the bottom edge for the elasticity-gradient sample. As the film thickness further increases, the cracks can spread from the bottom edge to top edge gradually, as shown in Figure 2a. Generally, the cracks are dense but narrow near the bottom edge, while they are sparse but broad near the top edge. We suggest that the cracks appeared in our experiment are driven by the thermal stress during the film deposition. As we have mentioned above, the

λc = F

2πh −12εm(1 + v)

(1) DOI: 10.1021/acsami.5b12369 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces where h is the film thickness, ν is the Poisson’s ratio, and εm the residual compressive strain formed during the deposition. The predicted wavelength seems to be linearly proportional to the film thickness, in contrast to the result shown in Figure 8b. However, we note that the residual compression is not constant in the film with varying the thickness during the deposition.28 Sputtering is a frequently used technique to prepare thin films in science and engineering fields. Recently, Roth et al. fabricated thickness gradient metal films on polymer substrates by using sputtering technique.29,30 Schwartzkopf et al. observed the metal films sputter deposited on polymer substrates can be divided into four growth processes: nucleation, difusionmediated growth, adsorption-driven growth and grain growth.31,32 Because of the nonequilibrium growth, the stress origin and evolution for sputter deposited films are comparatively complicated. According to the recent theoretical study,33,34 the compressive strain in sputter deposited films is mainly contributed to the fast adatom insertion into the grain boundary during the film deposition and its equilibrium value is a function of the film thickness εm ≈ −

⎛d ⎞ 4h tanh⎜ ⎟ ⎝ 4h ⎠ d

increasing the curing temperature. Therefore, the wrinkle wavelength λ decreases steadily with increasing the distance x, as shown in Figure 8c. The evolution of wrinkle wavelength with the substrate elasticity in our experiment is similar to the previous studies.37 At the same position (e.g., x = 24.5 mm), the elastic modulus of the PDMS is a constant, and thus the wrinkle wavelength λ is directly proportional to the film thickness h, as shown in Figure 8d. To verify the evolution behavior of the wrinkling wave with the film thickness, the metal films were also deposited on completely cross-linked PDMS substrates (no elasticity gradient). In this case, the liquid PDMS was first spin-coated onto square glass slides with the size of about 10 × 10 mm2. Then the samples were cured at 100 °C (no temperature gradient) for more than 1 h to cross-link the PDMS completely.17−19 The typical surface morphologies of the nickel films with varied thicknesses deposited on the completely elastic PDMS substrates taken by the AFM are shown in Figure S8. The dependences of the wrinkle wavelength λ and RMS surface roughness Rq on the film thickness are also shown in Figure S9. It is clear that the wrinkle wavelength λ increases linearly with the film thickness h, similar to the result shown in Figure 8d. Furthermore, the wrinkling wavelength shown in Figure 8d is almost equal to that shown in Figure S9a for the same film thickness, indicating that the PDMS near the bottom edge for the elasticity-gradient sample has turned into the completely elastic material. According to eq 4, the wrinkle amplitude A is proportional to the wavelength and the square root of compressive strain. The wrinkle wavelength λ and compressive strain ε both increase with the film thickness, and thus the surface roughness (or wrinkle amplitude) increases approximately linearly with increasing the film thickness, as shown in Figure S9b. 3.4. Surface Hydrophobicity. Controlled surface patterns have many potential applications including flexible electronics,38,39 elastomeric optics, 40 micro- or nanofluidic channels,41 marine antifouling,42 hydrophobicity,43,44 smart adhesion,45 photonics,46 measurement technique,47,48 and self-assembly microstructures.49 As a sample of applications, we have investigated the hydrophobicity of the metal films deposited on the elasticity-gradient PDMS substrates.50,51 The snapshots for the water droplets on the nickel film surface with h = 5 nm at different positions are shown in Figure 9a. The dependence of the water contact angle θ on the distance x for varied film thicknesses is shown in Figure 9b. We find that as the distance x increases, the contact angle θ decreases steadily, mainly due to the decreasing of the surface roughness (see Figure 7b). For the film with h = 5 nm, the contact angle decreases from about 125° near the top edge to below 80° near the bottom edge. Near the top edge, the contact angles for different film thicknesses are almost coincident because the folding wave is independent of the film thickness. In the elastic PDMS region, the contact angles for different film thicknesses are separated greatly. Usually, the contact angle increases with increasing the film thickness at the same position, which is mainly due to the increasing of the surface roughness with the film thickness (see Figure S9b). It is clear that one can control the hydrophobicity of the film surface by simply tuning the substrate property and the film thickness. It should be noted here that the contact angle remains very small even near the top edge for superhydrophobic surface. To generate a superhydrophobic surface, further experimental attempts are needed, including decreasing the substrate elasticity, improving the

(2)

with d the average grain size. Substituting eq 2 into eq 1, the result shows that the wavelength can be insensitive to the film thickness in some values of d. Alternative insight into explain the independence of the folding wave on the film thickness is the study of the wrinkle-to-fold transitions in the swelled hydrogel layers with material properties varying in thickness direction.35,36 The predicted critical wavelength shows the independency of the metal film thickness. To confirm this mechanism, future work need to check the gradient substrate modulus in thickness direction in the current material system. On the other hand, it is well-known that the wrinkling phenomenon can be understood by using a simple stress model. The film−substrate system is usually treated as a thin, stiff film resting on an infinite thickness, elastic substrate. On the basis of the principle of energy minimization (including stretching and bending energies of the film and elastic energy of the substrate), the equilibrium wrinkle wavelength λ and amplitude A can be expressed as ⎡ E (1 − ν 2) ⎤1/3 s ⎥ λ = 2πh⎢ f ⎣ 3Es(1 − νf 2) ⎦

(3)

⎛ ε ⎞1/2 A = h⎜ ⎟ ∝ λ ε ⎝ εc ⎠

(4)

Here E is the elastic modulus, ε is the compressive strain, 2/3 2 1 ⎡ 3E (1 − νf ) ⎤ εc = 4 ⎢⎣ s is the critical strain for wrinkling, the 2 ⎥ Ef (1 − νs ) ⎦ subscripts f and s refer to the film and substrate, respectively. In our experiment, the thickness of the PDMS layer is in the range of 15−20 μm, while the film thickness is only tens nanometers. It is clear that the assumption of infinite thickness substrate is validated and the above stress model can explain the wrinkling wave appeared in our experiment. Given that the mechanical parameters of the films are constant, the wrinkle wavelength λ is proportional to the film thickness h, but inversely proportional to the substrate elastic modulus as λ ∝ Es−1/3. As we have mentioned above, the cross-linking degree (or elastic modulus) of the PDMS increases obviously with G



■

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b12369. Details of evolution behaviors of the surface patterns from the top edge to bottom edge in the metal films deposited on elasticity-gradient PDMS substrates; the details of transition behaviors from the folding wave to wrinkling wave; morphological comparison of the folding and wrinkling patterns near the cracks; and the details of evolution behaviors of the wrinkling patterns with increasing the film thickness for completely elastic PDMS substrates (PDF)

■

Figure 9. (a) Snapshots for the water droplets on the nickel film surface with h = 5 nm at different positions. The distance x increases from left to right. (b) Dependence of the water contact angle θ on the distance x for varied film thicknesses.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

particle bombardment during film deposition, designing the hierarchical surface, tuning the mechanical compression, etc.

ACKNOWLEDGMENTS We thank Xiaofei Xiao, Qixiang Chen, and Miaogen Chen for useful discussions and technical assistance. This work was supported by the National Natural Science Foundation of China (Grant Nos. 11204283, 11104054, 11132009, 11222219) and the Fundamental Research Funds for the Central Universities and the Collaborative Innovation Center of Suzhou Nano Science and Technology.

4. CONCLUSIONS In summary, the elasticity-gradient PDMS substrates have been prepared via establishing a temperature gradient across the sample surface during the curing process. The PDMS substrate successively changes from a purely liquid state to a purely elastic state in spatial for each sample. Various surface patterns of the metal films deposited on the elasticity-gradient PDMS substrates are described and discussed in detail. It is found that the surface patterns are strongly dependent on the sample position, i.e., the substrate property. In general, the folding wave appears in the liquid or viscous PDMS region while the wrinkling wave forms in the elastic region. The coexisting of the folding and wrinkling waves can be observed in the transition region from the liquid to elastic PDMS. The folding wave forms at the early stage of the film deposition due to the surface instability from the particle bombardment. Its sizes (including wavelength and amplitude) decrease greatly with increasing the substrate elasticity, but are independent of the film thickness. The wrinkling wave is induced by the thermal compression after deposition. Its sizes decrease with increasing the substrate elasticity but increase linearly with the film thickness. If the film thickness is large enough, the crack patterns can form during deposition due to the thermal tensile stress. The crack can change the local thermal stress and thus the wrinkle morphologies change greatly. The hydrophobicity of the film surface is found to decrease with increasing the substrate elasticity, but increase with the film thickness. We anticipate that the metal films deposited on the elasticity-gradient PDMS substrates will provide a platform for the fundamental researches on various surface patterns including folding, wrinkling, delaminating, cracking and their coexistence (or transition). The controlled formation of surface patterns via simply tuning the substrate property should be beneficial for a wide range of technological applications in flexible electronics, micro- or nanofluid channels, optical devices, biological templates, microelectromechanical systems etc.

■

REFERENCES

(1) Gioia, G.; Ortiz, M. Delamination of Compressed Thin Films. Adv. Appl. Mech. 1997, 33, 119−192. (2) Velankar, S. S.; Lai, V.; Vaia, R. A. Swelling-Induced Delamination Causes Folding of Surface-Tethered Polymer Gels. ACS Appl. Mater. Interfaces 2012, 4, 24−29. (3) Yu, S. J.; Xiao, X. F.; Chen, M. G.; Zhou, H.; Chen, J.; Si, P. Z.; Jiao, Z. W. Morphological Selections and Dynamical Evolutions of Buckling Patterns in SiAlNx Films: From Straight-Sided to Telephone Cord or Bubble Structures. Acta Mater. 2014, 64, 41−53. (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) Cai, S.; Breid, D.; Crosby, A. J.; Suo, Z.; Hutchinson, J. W. Periodic Patterns and Energy States of Buckled Films on Compliant Substrates. J. Mech. Phys. Solids 2011, 59, 1094−1114. (6) Yu, S. J.; Ni, Y.; He, L. H.; Ye, Q. L. Tunable Formation of Ordered Wrinkles in Metal Films with Controlled Thickness Gradients Deposited on Soft Elastic Substrates. ACS Appl. Mater. Interfaces 2015, 7, 5160−5167. (7) Mei, H.; Huang, R.; Chung, J. Y.; Stafford, C. M.; Yu, H. H. Buckling Modes of Elastic Thin Films on Elastic Substrates. Appl. Phys. Lett. 2007, 90, 151902. (8) Pan, K.; Ni, Y.; He, L. H.; Huang, R. Nonlinear Analysis of Compressed Elastic Thin Films on Elastic Substrates: From Wrinkling to Buckle-Delamination. Int. J. Solids Struct. 2014, 51, 3715−3726. (9) Huang, J.; Davidovitch, B.; Santangelo, C. D.; Russell, T. P.; Menon, N. Smooth Cascade of Wrinkles at the Edge of A Floating Elastic Film. Phys. Rev. Lett. 2010, 105, 038302. (10) Pocivavsek, L.; Dellsy, R.; Kern, A.; Johnson, S.; Lin, B.; Lee, K. Y. C.; Cerda, E. Stress and Fold localization in Thin Elastic Membranes. Science 2008, 320, 912−916. H


Research Article

ACS Applied Materials & Interfaces (11) Kim, P.; Abkarian, M.; Stone, H. A. Hierarchical Folding of Elastic Membranes under Biaxial Compressive Stress. Nat. Mater. 2011, 10, 952−957. (12) Cao, C.; Chan, H. F.; Zang, J.; Leong, K. W.; Zhao, X. Harnessing Localized Ridges for High-Aspect-Ratio Hierarchical Patterns with Dynamic Tunability and Multifunctionality. Adv. Mater. 2014, 26, 1763−1770. (13) Jin, L.; Takei, A.; Hutchinson, J. W. Mechanics of Wrinkle/ Ridge Transitions in Thin Film/Substrate Systems. J. Mech. Phys. Solids 2015, 81, 22−40. (14) Wagner, T. J. W.; Vella, D. Floating Carpets and the Delamination of Elastic Sheets. Phys. Rev. Lett. 2011, 107, 044301. (15) Chen, Y. C.; Crosby, A. J. High Aspect Ratio Wrinkles via Substrate Prestretch. Adv. Mater. 2014, 26, 5626−5631. (16) Witten, T. A. Stress Focusing in Elastic Sheets. Rev. Mod. Phys. 2007, 79, 643−675. (17) Xu, F.; Zhu, Y. Highly Conductive and Stretchable Silver Nanowire Conductors. Adv. Mater. 2012, 24, 5117−5122. (18) Song, F.; Ren, D. Stiffness of Cross-Linked Poly (Dimethylsiloxane) Affects Bacterial Adhesion and Antibiotic Susceptibility of Attached Cells. Langmuir 2014, 30, 10354−10362. (19) Yu, S. J.; Zhang, X. F.; Xiao, X. F.; Zhou, H.; Chen, M. G. Wrinkled Stripes Localized by Cracks in Metal Films Deposited on Soft Substrates. Soft Matter 2015, 11, 2203−2212. (20) Efimenko, K.; Rackaitis, M.; Manias, E.; Vaziri, A.; Mahadevan, L.; Genzer, J. Nested Self-Similar Wrinkling Patterns in Skins. Nat. Mater. 2005, 4, 293−297. (21) Yoo, P. J.; Lee, H. H. Evolution of A Stress-Driven Pattern in Thin Bilayer Films: Spinodal Wrinkling. Phys. Rev. Lett. 2003, 91, 154502. (22) Yu, S. J.; Zhang, Y. J.; Zhou, H.; Chen, M. G.; Zhang, X. F.; Jiao, Z. W.; Si, P. Z. Spontaneous Formation of Hierarchical Wrinkles in Cr Films Deposited on Silicone Oil Drops with Constrained Edges. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2013, 88, 042401. (23) Cai, P. G.; Yu, S. J.; Xu, X. J.; Chen, M. G.; Sui, C. H.; Ye, G. X. Growth Mechanism and Stress Relief Patterns of Ni Films Deposited on Silicone Oil Surfaces. Appl. Surf. Sci. 2009, 255, 8352−8358. (24) Marthelot, J.; Roman, B.; Bico, J.; Teisseire, J.; Dalmas, D.; Melo, F. Self-Replicating Cracks: A Collaborative Fracture Mode in Thin Films. Phys. Rev. Lett. 2014, 113, 085502. (25) Seghir, R.; Arscott, S. Controlled Mud-Crack Patterning and Self-Organized Cracking of Polydimethylsiloxane Elastomer Surfaces. Sci. Rep. 2015, 5, 14787. (26) Huang, R.; Suo, Z. Wrinkling of A Compressed Elastic Film on A Viscous Layer. J. Appl. Phys. 2002, 91, 1135−1142. (27) Huang, R.; Im, S. H. Dynamics of Wrinkle Growth and Coarsening in Stressed Thin Films. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2006, 74, 026214. (28) Freund, L. B.; Suresh, S. Thin Film Materials: Stress, Defect Formation and Surface Evolution; Cambridge University Press: Cambridge, 2004. (29) Roth, S. V.; Burghammer, M.; Riekel, C.; Müller-Buschbaum, P.; Diethert, A.; Panagiotou, P.; Walter, H. Self-Assembled Gradient Nanoparticle-Polymer Multilayers Investigated by An Advanced Characterization Method: Microbeam Grazing Incidence X-Ray Scattering. Appl. Phys. Lett. 2003, 82, 1935−1937. (30) Roth, S. V.; Walter, H.; Burghammer, M.; Riekel, C.; Lengeler, B.; Schroer, C.; Kuhlmann, M.; Walther, T.; Sehrbrock, A.; Domnick, R.; Müller-Buschbaum, P. Combinatorial Investigation of the Isolated Nanoparticle to Coalescent Layer Transition in A Gradient Sputtered Gold Nanoparticle Layer on Top of Polystyrene. Appl. Phys. Lett. 2006, 88, 021910. (31) Schwartzkopf, M.; Buffet, A.; Körstgens, V.; Metwalli, E.; Schlage, K.; Benecke, G.; Perlich, J.; Rawolle, M.; Rothkirch, A.; Heidmann, B.; Herzog, G.; Müller-Buschbaum, P.; Röhlsberger, R.; Gehrke, R.; Stribeck, N.; Roth, S. V. From Atoms to Layers: in Situ Gold Cluster Growth Kinetics during Sputter Deposition. Nanoscale 2013, 5, 5053−5062.

(32) Schwartzkopf, M.; Santoro, G.; Brett, C. J.; Rothkirch, A.; Polonskyi, O.; Hinz, A.; Metwalli, E.; Yao, Y.; Strunskus, T.; Faupel, F.; Müller-Buschbaum, P.; Roth, S. V. Real-Time Monitoring of Morphology and Optical Properties during Sputter Deposition for Tailoring Metal−Polymer Interfaces. ACS Appl. Mater. Interfaces 2015, 7, 13547−13556. (33) Shin, J. W.; Chason, E. Compressive Stress Generation in Sn Thin Films and the Role of Grain Boundary Diffusion. Phys. Rev. Lett. 2009, 103, 056102. (34) Bhandakkar, T. K.; Chason, E.; Gao, H. Analytical Model of Transient Compressive Stress Evolution during Growth of High Diffusivity Thin Films on Substrates. Philos. Mag. 2010, 90, 3037− 3048. (35) Wu, Z.; Bouklas, N.; Huang, R. Swell-Induced Surface Instability of Hydrogel Layers with Material Properties Varying in Thickness Direction. Int. J. Solids Struct. 2013, 50, 578−587. (36) Im, S. H.; Huang, R. Evolution of Wrinkles in ElasticViscoelastic Bilayer Thin Films. J. Appl. Mech. 2005, 72, 955−961. (37) Wilder, E. A.; Guo, S.; Lin-Gibson, S.; Fasolka, M. J.; Stafford, C. M. Measuring the Modulus of Soft Polymer Networks via A Buckling-Based Metrology. Macromolecules 2006, 39, 4138−4143. (38) Rogers, J. A.; Someya, T.; Huang, Y. Materials and Mechanics for Stretchable Electronics. Science 2010, 327, 1603−1607. (39) Sun, Y.; Choi, W. M.; Jiang, H.; Huang, Y. Y.; Rogers, J. A. Controlled Buckling of Semiconductor Nanoribbons for Stretchable Electronics. Nat. Nanotechnol. 2006, 1, 201−207. (40) Yu, C.; O’Brien, K.; Zhang, Y. H.; Yu, H.; Jiang, H. Tunable Optical Gratings Based on Buckled Nanoscale Thin Films on Transparent Elastomeric Substrates. Appl. Phys. Lett. 2010, 96, 041111. (41) Ohzono, T.; Monobe, H.; Shiokawa, K.; Fujiwara, M.; Shimizu, Y. Shaping Liquid on A Micrometre Scale Using Microwrinkles as Deformable Open Channel Capillaries. Soft Matter 2009, 5, 4658− 4664. (42) Efimenko, K.; Finlay, J.; Callow, M. E.; Genzer, J.; et al. Development and Testing of Hierarchically Wrinkled Coatings for Marine Antifouling. ACS Appl. Mater. Interfaces 2009, 1, 1031−1040. (43) Kim, Y. H.; Lee, Y. M.; Lee, J. Y.; Ko, M. J.; Yoo, P. J. Hierarchical Nanoflake Surface Driven by Spontaneous Wrinkling of Polyelectrolyte/Metal Complexed Films. ACS Nano 2012, 6, 1082− 1093. (44) Li, Y.; Dai, S.; John, J.; Carter, K. R. Superhydrophobic Surfaces from Hierarchically Structured Wrinkled Polymers. ACS Appl. Mater. Interfaces 2013, 5, 11066−11073. (45) Chan, E. P.; Smith, E. J.; Hayward, R. C.; Crosby, A. J. Surface Wrinkles for Smart Adhesion. Adv. Mater. 2009, 20, 711−716. (46) Kim, J. B.; Kim, P.; Pégard, N. C.; Oh, S. J.; Kagan, C. R.; et al. Wrinkles and Deep Folds as Photonic Structures in Photovoltaics. Nat. Photonics 2012, 6, 327−332. (47) Stafford, C. M.; Harrison, C.; Beers, K. L.; Karim, A.; Amis, E. J.; VanLandingham, M. R.; Kim, H. C.; Volksen, W.; Miller, R. D.; Simonyi, E. E. A Buckling-Based Metrology for Measuring the Elastic Moduli of Polymeric Thin Films. Nat. Mater. 2004, 3, 545−550. (48) Chan, E. P.; Kundu, S.; Lin, Q.; Stafford, C. M. Quantifying the Stress Relaxation Modulus of Polymer Thin Films via Thermal Wrinkling. ACS Appl. Mater. Interfaces 2011, 3, 331−338. (49) Yoo, P. J. Invited Paper: Fabrication of Complexly Patterned Wavy Structures using Self-Organized Anisotropic Wrinkling. Electron. Mater. Lett. 2011, 7, 17−23. (50) Lamberti, A.; Virga, A.; Rivolo, P.; Angelini, A.; Giorgis, F. Easy Tuning of Surface and Optical Properties of PDMS Decorated by Ag Nanoparticles. J. Phys. Chem. B 2015, 119, 8194−8200. (51) Feng, J. T.; Zhao, Y. P. Influence of Different Amount of Au on the Wetting Behavior of PDMS Membrane. Biomed. Microdevices 2008, 10, 65−72.

I


Controlled Formation of Surface Patterns in Metal Films Deposited on

Recommend Documents