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Controllable Fabrication of Large-Area Wrinkled Graphene on a Solution Surface Wenjun Chen, Xuchun Gui, Binghao Liang, Ming Liu, Zhiqiang Lin, Yuan Zhu, and Zikang Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00137 • Publication Date (Web): 13 Apr 2016 Downloaded from http://pubs.acs.org on April 19, 2016

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ACS Applied Materials & Interfaces

Controllable Fabrication of Large-Area Wrinkled Graphene on a Solution Surface Wenjun Chen1, Xuchun Gui1,2*, Binghao Liang1, Ming Liu1, Zhiqiang Lin1, Yuan Zhu1 and Zikang Tang1, 3* 1

State Key Lab of Optoelectronic Materials and Technologies, School of Electronics and

Information Technology, Sun Yat-sen University, Guangzhou, 510275, P. R. China 2

Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay,

Kowloon, Hong Kong, China 3

Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da

Universidade, Taipa, Macau, China

Corresponding authors: [email protected]; [email protected]

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Abstract Wrinkles, which are folds or creases in a material, are unavoidable to be formed in graphene, whatever the graphene is prepared by micromechanical exfoliation from graphite or chemical vapor deposition (CVD). However, the controllable formation and structures of graphene with nanoscale wrinkles remains a big challenge. Here, we report a liquid-phase shrink method to controllably fabricate large-area wrinkled graphene (WG). The CVD prepared graphene self-shrinks into a WG on an ethanol solution surface. By modifying the concentration of the ethanol solution, we can easily and efficiently obtain WG with a uniform distribution of wrinkles with different heights. The WG shows high stretchability and can withstand more than 100 % tensile strain and up to 720° twist. Furthermore, electromechanical response sensors based on double-layer stack of WG show ultrahigh sensitivity. This simple, effective and environmentally friendly liquid-phase shrink method will pave a way for the controllable formation of WG, which is an ideal candidate for application in highly-stretchable and highly-sensitive electronic devices. Keywords: graphene; wrinkles; interface interaction; morphological controls; stretchable sensors

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Introduction With its extraordinary mechanical, electronic and optical properties, graphene, as a promising two-dimension (2D) material, is expected to be widely applied to flexible electronic devices.1-3 However, strictly 2D flat graphene crystals are thermodynamically unstable. Wrinkles or folds in the third dimension of graphene form to minimize the total free energy, which enables the stable existence of graphene.4 Theoretical calculations have indicated that when the length or width of graphene exceeds ten nanometers, it could form a self-wrinkling structure.5 Wrinkles could also impart some special properties to graphene.6-8 For example, the local bending/curvature induced by wrinkles can dramatically change the local chemical reactivity and the charge transport behavior of graphene.9, 10 As a result of the wrinkles’ tip facilitating electronic emission,11 graphene with corrugations is seen as an ideal candidate for field emission applications.12 Additionally, wrinkled structures can also improve the stretchability of graphene-based flexible electronic devices.1, 13,14 Usually, wrinkles on mechanically exfoliated graphene are formed randomly with a height of about 1 nm.15 It is about several nanometers for the chemical vapor deposition (CVD) synthesized large-scale graphene.10 The wrinkles form in the CVD prepared graphene mainly for the following reasons: (i) never completely flat growth substrate, including surface steps,16 grain boundaries17 and wrinkling morphology18; (ii) different thermal expansion coefficient between graphene and substrate, which triggers the formation of wrinkles during the cooling process;17 (iii) during the etching of the metal substrate and transferring process of the grown graphene to target substrate, wrinkles and folds can be formed or released.18-20 Therefore, whether the graphene is prepared by CVD or micromechanical exfoliation,15 3



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wrinkles form and distribute randomly, unavoidably and uncontrollably. Although graphene transferred on a pre-strained PDMS or a substrate with man-made ordered-array pillars can form ordered micro wrinkles, the height of these wrinkles are usually on the order of micrometers.14, 21-23 For instance, graphene have to be stamped on a pre-stretched flexible polymer to form crumples by controlling the pre-strains and relaxation of the polymer substrate. But, the preparation of this crumpled graphene films relies on the processed substrate.22 Essential wrinkle-structured graphene research has not progressed past theoretical calculations. It remains a challenging question on how to controllably fabricate graphene with nanoscale wrinkles and folds. Here, we develop a simple, environmental-friendly method, using a liquid-phase process, to fabricate large-area graphene with controllable nanoscale wrinkles. The CVD prepared graphene can self-shrink to form wrinkled graphene (WG) on the ethanol solution surface. With modifying the concentration of the ethanol solution, we can easily and efficiently fabricate WG with a uniform distribution of wrinkles at different heights. The WG shows a high stretchability, which is approximately five times than that of flat graphene. The WG can undergo more than 100 % tensile strain and up to 720° twist. Additionally, the double-layer stack of WG possessed a combination of high stretchability electromechanical response.

Experimental Methods Growth of Graphene: Graphene was prepared on 25-µm copper foil (99.8 metal basis, Alfa Aesar) by the method of ambient-pressure CVD. In our growth system, a piece of Cu foil was put into a quartz reaction tube with 25-mm inner. Then the growth chamber was heated from 4


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ambient temperature to 1000℃ under the mixture of Ar (200 sccm) and H2 (35 sccm) in 40 minutes. After that the copper foil was annealed at 1000℃ for 15 minutes, CH4 (30 sccm) was introduced for 5-min growth of graphene. Subsequently, the Cu was quickly pulled out of the high-temperature zone and cooled down to ambient temperature under Ar (200 sccm) and H2 (35 sccm) protection. Structural Characterization: SEM images were captured using a Hitachi S-4800 operated at 3kV. TEM images were carried out using a JEOL JEM-2010HR at a 200kV accelerating voltage. Optical microscopy images were taken by a Zeiss Axio CSM 700. AFM images were characterized using a Veeco Edge. Raman spectra and mappings were collected by a HORIBA JY HR800 with 633-nm laser. Diameter of the laser spot is ~2µm and increment of Raman mappings is 200 nm. Transmittance measurements were performed using a UV-visible spectrometer (Maya2000 Pro, Ocean Optics) with unpolarized light provided by a fiber-coupled light source with spot size of approximately 3 mm in diameter. Sheet resistances of samples were measured by Hall effect system (Accent HL-5500). Graphene samples were transferred on a quartz for hall-resistivity measurement. The size of pristine graphene is approximate 15 × 15 mm2 and the length and width of all the devices based on as-prepared WG with different RMS are 4 to 10 mm. Four same probes (with diameter of micron scale) as electrodes were directly posed and contact with the sample at the place with the distance of ~1 mm to the edge, as illustrated in Figure S1. Manufacture and Test of Electromechanical Sensor: Polydimethylsiloxane (PDMS) flexible substrate (1-mm thick) was first treated by air-plasma to increase its hydrophilicity. Then, WG samples were directly transferred on PDMS substrate and dried at 60℃ in an oven for 5



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formation of sensor. For double-layer electromechanical sensor, one more WG sample was stacked on the former one, which had been transferred on PDMS previously, then dried in the oven. The electricity test was carried out by two-electrodes using a Keithley 2400 with operated voltage of 1 V. And silver wires were utilized as electrodes. Resistance changes of samples were calculated from corresponding I-V characteristics. For the experiments of tension strain, one end of the WG was fixed, and another side is connected with an electrical control translation system with the step resolution is 1.25 µm. For the test of the resistance changes of single-layer WG under tension and twist, silver wires were connected to both ends of the WG. And for the test of the resistance changes of devices based on double-layer stack WG under twist, silver wires were connected at the upper and lower layers WG. For the twist experiments, one end of the WG was fixed, and another side is connected with an electrical control rotation system with the rotation resolution is 0.0025°.

Results and discussion The schematics for controllable fabrication of WG in solution are illustrated in Figure 1a. First, the as-grown graphene was directly put onto a ferric chloride (FeCl3) solution for etching of copper substrate. After etching, the graphene was floated on the surface of the solution. It was then transferred onto deionized water (DI water) using a glass slide to clear remaining ferric chloride solution and other impurities. After clearing several times, the graphene still maintained its initial shape and floated on the DI water surface (Figure S2, Supporting Information). The clean graphene (CG) can be directly transferred onto the target substrate, such as a square slice or silicon wafer. If the clean graphene is transferred onto an 6


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organic solution again, such as ethanol/water solution, it will quickly shrink at the air-liquid interface within a few seconds. The air-liquid interface provides a constrained environment for the self-shrinking of graphene and has been widely exploited for the preparation of nanoparticle thin films24-27. Subsequently, the WG can float freely on the surface of organic solution. The target substrate, which should be processed in air-plasma to improve its hydrophilicity before, is wedged into the solution by intercalation to “fish out” the WG on it and dried in an oven at 60℃. Ultimately, we obtain a WG with a network of graphene islands separated by wrinkles with a height of tens to hundreds of nanometers, as illustrated by the three-dimensional (3D) schematic of Figure 1b. Photographs (Figure S2) give concrete details of the above preparation process and show the CVD grown graphene shrank in ethanol solution. However, if the as-prepared WG is transferred from ethanol solution to DI Water again, it cannot spread out, which means the wrinkled structures are irreversible. During the procedures of CVD growth, etching and transferring, wrinkles and folds were unavoidably formed and released (Figure S3). Scanning electron microscopy (SEM) image (Figure 1c) shows the existence of random folds in our CG attached on a silicon substrate, similar to that previously observed with most CVD prepared graphene.28-30 These random wrinkles with a height of about 10 nm are known to be formed mostly owing to uneven growth substrate. In comparison, the WG shrunk in organic solution (volume ratio of ethanol to DI water is 1:1) has controllably higher and denser folded wrinkles (Figure 1d). High-magnification SEM and transmission electron microscopy (TEM) images (Inset of Figure 1d and Figure S4) show that the folded structures are similar with stacking of graphene layers.31 According to the images of atomic force microscopy (AFM), it is further 7



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proved that higher and uniform wrinkles can be obtained using the liquid-phase shrink method, shown in Figure 1e and 1f, which correspond to the SEM images provided in Figure 1c and d, respectively. Root mean square (RMS) is introduced to characterize the roughness of graphene.32 The RMS for the CG on a silicon substrate is only 8 nm, while it is almost double (15 nm) for the WG prepared in a 1:1 vol.% ethanol solution. The 3D view (Figure 1g) of the WG clearly and directly exhibits uniform and higher wrinkles with sharp tips, which results in an increasing RMS, compared with CG (Figure S3). Graphene also shrunk in other organic solutions, such as acetone solution, acetic acid solution (Figure S5 and Figure S6), etc, and a similar phenomenon is observed. In the following discussion, we will systematically discuss the shrinking action of graphene in an ethanol solution. The height, RMS and distribution of the wrinkles in graphene can be controlled by simply changing the concentration of the ethanol solution. Figure 2a-c show the SEM images of the WG, which was fabricated in ethanol solution with a concentration of 1.2:1, 1.6:1 and 2:1 vol. % respectively. More SEM and AFM images of the WG shrunk in other concentrations of the ethanol solution are supplied in Figure S7 and Figure S8. Increasing the concentration of the ethanol solution results in smaller graphene islands and both the height and RMS of the wrinkles increase (Figure 2d-g). When the concentration of ethanol is 2:1 vol.%, parts of the wrinkles resemble the multilayer stacking of graphene (Figure 2f), and the height and RMS arrive 125 and 50 nm, respectively. But the wrinkles still have a uniform distribution (Figure 2g). The height of WG is in the range of several nanometers to hundred nanometers, which is much higher than that induced by graphene preparation. Therefore, wrinkles formation during the process of etching of the metal substrate and transfer in our 8


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experiment can be negligible. But, the height of the wrinkles is lower than the folded graphene formed by uniaxially and biaxially pre-strained PDMS, the height of which is up to several microns.22 When the concentration exceeds 2:1 vol.%, the graphene could be gathered to form a lump of graphene (Figure S9). The formation of wrinkles also results in a decrease of the graphene’s area. The remaining area after shrinkage is about 60 % in a 1:1 vol. % ethanol solution (Figure 2h). The remaining area decreased approximately linearly with the increasing concentration of the ethanol solution. The decrease of the remaining area results from the formation of more wrinkles per unit area at a higher concentration. Optical transmission and sheet resistance measurements were taken on CG and WG. It shows that the CG (RMS of 8 nm) maintains a transmittance of above 96 % over a wavelength range of 400 to 800 nm (Figure 3a), similar with most reports of graphene.33 The transmittance at 550 nm of WG with RMS of 15 to 50 nm is 84 % to 60 %, respectively (Figure 3b). These characteristics are similar with those observed for the overlapping of the flat monolayer graphene, which suggests the thickness of the WG becomes double, triple or even higher multiple, which results in the decrease of optical transmittance. Sheet resistance values were measured by a Hall effect system. The relationship between RMS and sheet resistance is plotted in Figure 3b. The resistance was 305 Ω/sq for RMS of 34 nm (transmittance of 70%), and increased to 985 Ω/sq for RMS of 15 nm (transmittance of above 84%). More importantly, we were able to control the highly transparent WG to maintain its initial rectangular shape (Figure 3c), as compared with the agglomerate reported in a previous article,34 which is favorable for its application in flexible and transparent devices. Raman spectroscopy was employed to determine the structural information of the 9



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wrinkles. The Raman spectra of the CG and WG are shown in Figure 4a. The lack of a visible D band suggests that the pristine graphene (RMS 8 nm) is free of defects.35, 36 However, for the WG, the intensity of the D peak increases and the intensity ratio of I2D/IG decreases with an increase of RMS (Figure 4b). The intensity ratio of ID/IG and I2D/IG are 0.96 and 0.69, respectively, for the WG with an RMS of 50 nm. The decrease of I2D/IG results from the overlapping of the graphene layers in WG, which is similar with multilayer stack of graphene.1, 37 At the same time, the misorientation or the absence of AB- or ABC-stacking among the overlapping layers may introduce defects to the WG,38 resulting in the increase of ID/IG. Wrinkles result in the appearance of the D′ peak (~1620 cm-1), which is strongly related to defects and structural mismatches,39 as seen in Figure 3a. In this case, the peak intensity ratio of the D′ peak to G peak (ID′/IG) represents the degree of scrolling of the wrinkles. Moreover, ID′/IG is positively correlated to the RMS of WG (Figure 3b). Thus, the emergence of the D′ peak offers another effective method, namely the distribution of the peak intensity of D′ (ID′), to characterize the WG. Under optical microscopy, we can see that both the CG (Figure 4c) and WG (Figure 4d and e) are uniform over a large area (larger than 180 µm × 180 µm) without cracks. Raman mappings of these samples, corresponding to the selected area of 20 µm × 20 µm, show a uniform intensity distribution of ID′, indicating homogeneous wrinkle formation over a large area (Figure 4f-h). All these results further confirm that uniform wrinkles can be obtained controllably using the liquid-phase shrink method. The formation of wrinkles in graphene is a fast and spontaneous process that occurs on the surface of the solution, owing to the reduction of the surface and interface energy of the 10


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graphene/solution system. This is different from the cases where wrinkle formation relies on the substrate’s morphology18, 21, 40 and in-plane shear loading.22, 41, 42 We thought that the wrinkle formation includes at least two dynamic factors. Firstly, when the graphene is transferred from DI water to the organic solvent (ethanol in our work), the difference of surface tensions between organic solvent and DI water is a main motivation (the mechanism schematics are shown in Figure S10), which is expressed by an inequality simplified from Young’s equation:34 γsl + γgs ≥ γgl

(1)

where γsl, γgs and γgl represent the surface tension between solid (graphene film) and liquid, gas and solid, and gas and liquid, respectively. The smaller surface tension of ethanol (22.3 mN/m), in comparison to that of DI water (72.7 mN/m), meets the requirement of the above inequality and triggers the formation of wrinkles in graphene. The surface tension of ethanol solution with different concentration is measured and figured, as shown in Figure S10c. The same shrinking phenomena also can be found in acetone or acetic acid solutions due to their low surface tensions (23.7 mN/m and 27.6 mN/m respectively). However, this does not explain why more wrinkles in graphene were formed in acetone and acetic acid solutions (Figure S5 and Figure S6), compared with that formed in an ethanol solution with the same concentration, even though ethanol has a lower surface tension than acetone and acetic acid solutions (Figure 2a). The second dynamic force for the formation of wrinkles is the solution evaporation. Acetone, acetic acid and higher concentration ethanol solutions evaporate faster, resulting in a higher distribution density of wrinkles in graphene. Here, we introduce Rayleigh-Bénard convection (the mechanism schematics are shown in Figure S10d), which is 11



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inspired by the high evaporation rate of the solute (ethanol in our work) and results in a temperature difference ∆Th between the surface and bottom.43 In our experiment, a thermocouple is used to measure the ∆Th. The Rayleigh number Ra is expressed by the following equation:

Ra =

ρ gα∆Th 3 h , ηκ

(2)

where ρ is the density of solution, g is the gravitational acceleration, α is the thermal expansion coefficient, η is the kinematic viscosity, k is the thermal diffusivity, h is the height from the surface to the under surface. For example, for the ethanol solution with concentration of 1:1 vol.%, it is measured h = 20 mm, ∆Th= 2℃. Taking α= 750×10-6 ℃-1, η= 1.074×10-3 Pa·s and k= 7×10-8 m2·s-1 of the values of ethanol, we can calculate the Rayleigh number of the solution to be about Ra= 1251.4, which exceeds the critical Rayleigh number Rc= 1100.8.44, 45 Therefore, a vertically convective flow should occur in the solution, inducing graphene shrinkage and wrinkle formation. As the concentration of ethanol solution increases, ethanol evaporates faster, which leads to a higher ∆Th and Ra. Hence, more wrinkles can be obtained, which agrees with the observations from our experiments. In addition to the good optical and electrical properties of WG, wrinkles also improve the stretchability of graphene, which inspires its applications in transparent and highly-stretchable devices under tension and twist. Schematics of these two working patterns and the device structure are shown in Figure 5a and Figure S11a, respectively. Figure 5b shows the resistance changes versus tensile strain for WG with different RMS. The resistance change increased about 22 times when the CG was stretched to 20 %. This means that the 12


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sample has produced large cracks at this strain and the stretchability of the CG is lower than 20%. A similar resistance change has been observed in several other flat graphene under stretching.46 However, the WG are much more stretchable owing to the wrinkles or folds in the graphene providing the possibility for expansion. For the WG with RMS of 26 nm, the resistance changed slightly under a tensile stain of 40%. And it even still conductive under more than 100% strain, which demonstrates wrinkled structures increase the working range of devices. The resistance change is also reversible under cyclic loading-unloading (Figure S12). Gauge factor of graphene with different RMS were calculated and figured (Figure S13). It further indicated that the WG exhibits higher stretchability comparing with CG and other reported structures47, 48. We can draw a similar conclusion from the torsion of the WG (Figure 5c). Schematics of WG working under torsion are shown in Figure S14. WG with a RMS of 26 nm will still be conductive with up to two-circle twist (720 °), compared with only a 100° twist for CG. Additionally, the resistance slightly increases (less than 5%) even under a 120° twist, which means a high anti-deformation ability of the WG. For further understanding, torsion level is utilized to study the performance of the samples under twist49 (Figure S15a). This WG may be the ideal candidate to be applied in electronics with the requisite of having to operate under large deformation. Furthermore, after tension and twist, there are cracks on the surface of CG, resulted in the increases of the resistance. However, for the WG samples, it has not any physical degradation, and its resistance still maintains its initial value. The WG also can be used as sensitive electromechanical sensors. We have constructed a double-layer WG, in which the wrinkles or tips from each layer could be connected, forming a conductive path (see schematic in Figure 5a and optical microscopy images in Figure S16). 13



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The device structure is shown in Figure S11b. The resistance change is more sensitive for double-layer WG under the same twist angle, as compared with that observed for single-layer WG (Figure 5d). Under 180° twist, the resistance changes 200% of the sensor based on double-layer WG with RMS of 26 nm, which is almost ten times than that of single-layer WG (11%). After 300° twist, the resistance change of double-layer WG increases sharply, which is even more than 35 times than single-layer WG. The twist-induced misconnection of previously contacting wrinkles between two layers of WG is likely to be the reason for the decreasing conductivity and increase of resistance. More importantly, the resistance changes nearly linearly over a large range of twist angles (0~300°), which is favorable for the use in sensors. Cyclic loading and unloading of this flexible device under torsion (twist angle of 5°), the double-layer WG shows reversible resistance change relative to the initial value (inset of Figure 5d). This proves its stability and much higher sensitivity in detection of small angle twist. Similar results can be obtained from other WG samples with different RMS (Figure S17). In general, the combination of higher sensitivity and comparable stretchability guarantees numerous applications of layers-stacked of WG such as flexible, stretchable and sensitive electronics. Conclusions In this report, we introduce a new liquid-phase shrink method for the controllable formation of large-scale WG with uniform nanoscale wrinkles. Through changing the concentration of the ethanol solution, we are able to easily and efficiently obtain WG with a certain RMS. Most importantly, the as-prepared sample still maintains its initial regular shape, which facilitates its applications. Theoretically, the surface tension and Rayleigh-Bénard 14


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convection are introduced to clarify the formation mechanism in order to further precisely control the fabrication of wrinkles. Electromechanical characteristics of highly-transparent (transmittance at 550 nm is more than 70%) WG samples prepared by this method clearly exhibit the high stretchability (work under ~100% tensile strain or ~720° twist). Additionally, sensors based on a double-layer stack of WG show the combination of high stretchability and sensitivity under torsion. We believe that using this method, more organic solvents with different surface tensions and evaporation rates can be used to control fabrication of WG, which ensures its application to a much wider range of fields. The wrinkled structure in graphene also could bring particular mechanical, and thermal properties for graphene. It is worthy and important studying for the wrinkled graphene in the future.

Acknowledgements This work was partially supported by Guangdong Natural Science Funds for Distinguished Young Scholar (Grant No. 2014A030306022), Pearl River S&T Nova Program of Guangzhou (No. 2014J2200066).

Supporting Information Available: Process of fabrication of WG, additional SEM, TEM and AFM images of the clean graphene and WG, SEM images of WG fabricated in acetone/water solution and acetic acid/water solution, Optical characterization of double-layer-stacking WG, 15-nm-RMS WG-based electromechanical sensor. The Supporting Information is available free of charge on the ACS Publications website.

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Strain

Sensing

Application.

Adv.

10.1002/adfm.201504717. 19


Funct.

Mater.

2016,

DOI:


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49. Yang, T.; Wang, Y.; Li, X.; Zhang, Y.; Li, X.; Wang, K.; Wu, D.; Jin, H.; Li, Z.; Zhu, H. Torsion Sensors of High Sensitivity and Wide Dynamic Range Based on a Graphene Woven Structure. Nanoscale 2014, 6, 13053.

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Figure 1. Description of wrinkled graphene (WG). (a) Schematic illustration of the preparation of WG in ethanol solution. (b) 3D schematic of as-prepared WG. (c, d) SEM images of CG and WG (prepared in 1:1 vol.% ethanol solution) on a silicon substrate, respectively. Inset shows high-magnification SEM image of the WG. (e, f) AFM images and surface analysis of the CG and WG, respectively. (g) 3D views of WG.

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(g)

150

R M S : 50 nm

[nm]

100 50 0 -5 0 150 R M S : 34 nm

[nm]

100 50 0 -5 0 150 R M S : 23 nm

[nm]

100 50 0 -5 0

150 RMS: 8 nm 100

[nm]

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50 0 -50

-100

0

5

10

15

20

[µm]

Figure 2. Structural characterization of WG. (a-c) SEM images of WG with RMS of 23 nm, 34 nm and 50 nm, respectively. Inset shows the high-magnification SEM image of corresponding sample. (d-f) 3D view of AFM characterization of WG with RMS 23, 34 and 50 nm, respectively. (g) Surface analysis of WG with different RMS. (h) Remaining area after shrinkage (black line) and RMS (blue line) of WG versus the concentration of the ethanol solution.

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Figure 3. Optical and electrical properties of WG. (a) Optical transmittance spectra of WG with different RMS. (b) Transmittance at wavelength 550 nm (black line) and sheet resistance (red line) versus RMS of WG. (c) Macroscopic photographs of WG with RMS of 8, 15, 23 and 34 nm.

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) b (

) a ( D

0.5

2D

G D'

1.8

50 nm 0.4

ID'/IG

34 nm 26 nm 23 nm

1.5

0.3 1.2 0.2

15 nm

I2D/IG

45 nm

Intensity(a.u.)

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0.9

0.1

8 nm 0.0

1200

1600

2000

2400

2800

0.6 0

10

20

30

40

50

RMS (nm)

-1

Raman shift(cm )

Figure 4. Raman characterization of WG. (a) Raman spectra of WG with different RMS. D peak (~1320 cm-1), G peak (~1590 cm-1), D′ peak (~1620 cm-1) and 2D peak (~2660 cm-1) are marked. (b) I D′/IG (black line) and I2D/IG (red line) versus RMS of WG. (c-e) Optical microscopy images of WG, showing a uniform distribution of wrinkles over a larger area. (f-h) Raman mapping of the selected sections in c-e respectively. Integration wave number is from 1610 to 1630 cm-1. Excitation laser wavelength is 633 nm.

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) d (

) c (

15 70

RMS: 8 nm 15 nm 26 nm

Double-layer 26 nm Single-layer 26 nm

60

12

50 ∆R/R0 (%)

4

m n 6 2 : S M R

5

o 5 t s i w T

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∆R/R0

∆R/R0

3 2

30

9

20 10

6

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s r e yr ae l y a el l be ul og n Di S

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500

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o

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Angle of twist( )

Figure 5. Electromechanical response of WG. (a) Schematic of WG under tension or stacking. (b) Resistance changes of WG with RMS of 8, 15 and 26 nm under tension. (c) Resistance changes of WG under different twist angle. (d) Comparison of resistance changes between double-layer-stacking and single-layer WG with RMS of 26 nm under twist. Inset is cyclic loading and unloading with a twist angle of 5°.

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