Anisotropic Friction of Wrinkled Graphene Grown by Chemical Vapor

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Anisotropic friction of wrinkled graphene grown by chemical vapor deposition Fei Long, Poya Yasaei, Wentao Yao, Amin Salehi-Khojin, and Reza Shahbazian-Yassar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017

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ACS Applied Materials & Interfaces

Anisotropic friction of wrinkled graphene grown by chemical vapor deposition Fei

Long1,

Poya

Yasaei2,

Wentao

Yao1,

Amin

Salehi-Khojin2*,

Reza

Shahbazian-Yassar 1, 2* 1 Department of Mechanical Engineering and Engineering Mechanics, Michigan Technological University, Houghton, Michigan 49931, USA 2 Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, USA. KEYWORDS: graphene, friction, anisotropy, chemical vapor deposition, atomic force microscopy

Abstract Wrinkle structures are commonly seen on graphene grown by chemical vapor deposition (CVD) method due to the different thermal expansion coefficient between graphene and its substrate. Despite the intensive investigations focusing on the electrical properties, the nano-tribological properties of wrinkles and the influence of wrinkle structures on the wrinkle free graphene remain less understood. Here, we report the observation of anisotropic nanoscale frictional characteristics depending on the orientation of wrinkles in CVD-grown graphene. Using friction force microscopy, we found that the coefficient of friction perpendicular to the wrinkle direction was ~194% compare to that of parallel direction. Our systematic investigation shows that ripples and ‘puckering’ mechanism, which dominate the friction of exfoliated graphene, plays 1

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even a more significant role in the friction of wrinkled graphene grown by CVD. The anisotropic friction of wrinkled graphene suggests a new way to tune graphene friction property by nano/micro-structure engineering, such as introducing wrinkles.

1. Introduction The frictional property of two dimensional (2D) materials at nanoscale has significant importance not only for the fundamental understanding of nano-tribology1, but also for the design and application of nano-electrical and mechanical systems (NEMs)

2-4

.

Recent friction force microscopy (FFM) studies have revealed that the frictional properties of exfoliated graphene and other 2D materials on Si/SiO2 substrate strongly depend on the number of layers. In general, it is observed that monolayer sheets exhibit higher friction than multilayers which is attributed to a higher elastic compliance and moderate layer-substrate adhesion 5. Theoretical modeling of the single asperity contact between a sliding atomic force microscope probe and graphene has shown that monolayer graphene locally buckles at the probe contact area which effectively increases the friction5-6. This phenomenon is referred to as ‘puckering’ effect. As the 2D materials get thicker, the interlayer adhesion and bending stiffness increases which depresses the puckering and results in lower friction. For sharp probes, Brownian dynamics (BD) simulations suggested the contributing mechanism of this layer dependency can also be the decreased sample deformation energy due to increased local contact stiffness7. In addition to the thickness dependency, residual stress in exfoliated monolayer graphene sheets introduces ripple structures when transferring to 2


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Si/SiO2 substrate, which lead to anisotropic frictional behavior8-10. The friction is reported to be lower along the ripple lines because the out-of-plane puckering is prohibited by the higher bending stiffness, with 215% anisotropy

10

. Despite the

intensive studies on exfoliated graphene, the nano-tribological properties of CVD-grown graphene remain less understood

1, 11-12

. CVD is currently the most

promising method for the growth of large area graphene 13 . Due to the negative thermal expansion coefficient in graphene and the positive values in typical growth substrates (e.g., copper), ‘wrinkles’ are commonly formed within the grains of monolayer graphene during the cooling process14-16. These wrinkles are believed to alter the electrical and mechanical properties of graphene 17-19. In our current work, CVD-grown graphene with highly anisotropic wrinkles were synthesized and transferred onto the Si/SiO2 substrate. The effects of wrinkle structures on the friction property of graphene were systematically investigated using FFM under ambient conditions. Experimental friction measurements revealed strong dependency on the orientation of wrinkles. In detail, lower friction was observed along the wrinkle direction, while higher friction was measured perpendicular to the wrinkle direction. The anisotropy is on average ~194%, which is similar to exfoliated graphene, suggesting that ripples and ‘puckering’ mechanism also dominates the friction of CVD-grown graphene.

2. Results and discussion Monolayer graphene samples were synthesized on copper foils by the ambient pressure CVD technique20-21, and then transferred on Si/SiO2 substrate using the 3



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poly(methyl methacrylate) (PMMA)-assisted technique as previously described22. Scanning electron microscopy (SEM) was employed to check the sample morphology. As illustrated in Figure 1a, the low magnification SEM image shows partially covered individual hexagonal graphene grains. Figure 1b gives the Raman spectra on the graphene sample. The I2D/IG ratio is close to 2, confirming that the graphene is monolayer. Figure 1c shows a typical AFM topography of the graphene samples on the Si/SiO2 substrate. Most wrinkles are formed along the same direction, while only a few of them sitting approximately orthogonal. The preferred wrinkle orientation is referred to as high-density wrinkle direction in the following context as indicated with red arrow in Fig. 1c. Figure 1d shows higher magnification AFM topography of the wrinkles along high-density direction. The size of the wrinkles varies from 1 to 5 nm in height, 7 to 18 nm in width, and up to microns in length. More importantly, most of the wrinkles are almost parallel with each other at the sub-micron scale which result in a unique structural anisotropy at the nano/microscopic scale. FFM was employed to investigate the frictional property of the wrinkled graphene. In FFM, the cantilever scan direction is perpendicular to its axial direction. The friction force between the AFM tip and the sample produces a torsional moment that laterally twist the cantilever (Figure S1). This lateral twisting signal is usually coupled with the sample topography and introduces artifacts. These artifacts can be minimized by subtracting the forward scan signal by the backward scan signal 10, 23-24. Experimentally, the half width of the lateral force loop formed by the forward and backward scan is commonly used as a measure of the frictional force (Figure S2). 4


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Two FFM measurements were carried out by aligning the high-density wrinkle direction perpendicular and parallel to the scanning direction of the probe, respectively. The topography images are shown in Figure 2a and c. The scanning direction of the probe is horizontal. The corresponding friction force images were captured simultaneously with the topography and are shown in Figure 2b and d, respectively. Wrinkles perpendicular and parallel to the probe scanning direction are indicated with green and yellow arrows, respectively. When the wrinkles were aligned perpendicular to the probe scanning direction, they exhibited higher friction (brighter contrast in Figure 2b) than the wrinkle free graphene. However, when the wrinkles were aligned parallel to probe scanning direction, they exhibited lower friction (darker contrast in Figure 2d) than the wrinkle free graphene. The friction of wrinkles can be explained by the mechanism proposed as follows. There is interlayer adhesion within the wrinkle structure due to van der Waals interaction as shown in Figure S3. This interlayer adhesion keeps the stable wrinkle morphology from collapsing. For the perpendicular probe scan direction with respect to the wrinkle, the interlayer adhesion does not prohibit the bending deformation. So the contact area between the probe and wrinkle is much larger than the single asperity contact between the probe and wrinkle free graphene, as illustrated in Figure 2e. Therefore, higher friction was observed. When the probe scans parallel to the wrinkle and especially when the probe scans right on top of the wrinkle as shown in Figure 2f, the probe needs to overcome the interlayer adhesion in order to create deformation. Because the wrinkle structure is more rigid vertically due to the interlayer adhesion, less deformation is allowed. As a 5



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result, lower friction was observed. In addition to the direction dependency of wrinkle friction, we observed that the friction of the wrinkle free graphene depends strongly on the high-density wrinkle direction. Friction force profiles obtained from the red and blue dashed lines in Figure 2b and d are shown in Figure 3a with red and blue solid lines, respectively. The peaks in the profiles correspond to the wrinkles perpendicular to the probe scan direction. The values between the peaks correspond to the friction of the wrinkle free graphene areas. Under the same normal load of 150 nN, the friction of the wrinkle free graphene was greater when measured perpendicular to the high-density wrinkle direction (red solid line), compared to that measured in parallel direction (blue solid line). More importantly, the friction force profile of the wrinkle free graphene exhibits minor peaks perpendicular to the high-density wrinkle direction, while the profile is much smoother along the parallel direction. The increased waviness in the friction signal suggests the existence of ripple texture on the wrinkle free graphene due to the inhomogeneous adhesion between the graphene and its substrate. Moreover, the observation also implies that the direction of the ripple texture is aligned along the wrinkles. This anisotropic surface texture results in an anisotropic out-of-plane bending stiffness, which is more rigid along the parallel direction than the perpendicular direction. As shown in Figure 3a, when the probe scans perpendicular to the ripple lines, the monolayer graphene can be easily deformed and puckered around the probe due to the lower bending stiffness. The increased contact area leads to higher friction, as can be seen in Figure 3a that the area enclosed by red lines is 6


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larger than that enclosed by blue lines. Moreover, as the probe moves forward, the work done by the probe is converted to the strain energy stored in the puckered graphene. When the strain energy overcomes the friction force, it suddenly dissipates, resulting in the minor peaks observed in Figure 3a red lines. On the other hand, the bending stiffness along the ripple lines is higher. Therefore, puckering is prohibited as illustrated in Figure 3c. So the friction along the parallel direction is not only less but also more uniform as observed in Figure 3a (blue lines). We also noticed that the lateral force peaks produced by wrinkles can be well reproduced in both forward and backward scans, but the minor peaks on wrinkle free graphene area exhibited variations. It has been reported that the sliding of an AFM tip on graphene surface would induce changes of graphene’s interfacial geometry, which locally delaminates the graphene layer on the substrate25. Although this sliding history dependency was developed to explain the adhesion variation between an AFM tip and graphene, we believe similar mechanism also plays an important role in our FFM experiments. As illustrated in Figure S4, when the AFM tip scans in the forward direction, it applies uniaxial stress to the graphene along the scanning direction. The friction redistributes the graphene residual strain along the same direction, which creates rippling texture in the scan direction26. This in effect increases the bending rigidity, therefore, the displacement of graphene is reduced when the tip scans in the backward direction, resulting in lower friction detection. However, this redistribution of the residual strain does not change the overall anisotropic friction of the wrinkle free graphene, because the wrinkle structures dominate over the sliding history of the AFM tip in our 7



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experiments. The results support our discussion that the frictions of wrinkles and wrinkle free graphene have different origins. The friction of wrinkles is originated from their morphology, while the friction of wrinkle free graphene is due to anisotropic residual strain. Further computational simulations based on FEM and MD will be performed for the detailed understanding of the underlying mechanism. To further characterize the anisotropic friction of CVD-grown graphene, we investigated the dependency of friction on the applied normal load as shown in Figure 4a. Six parallel samples were measured, and each data point gives the average value of friction forces measured from 10 wrinkle-free areas. The friction curves measured perpendicular to the high-density wrinkle direction are shown in red, while frictions measured along the parallel direction are shown in blue. Consistent with our previous observation, the friction is on average higher along the perpendicular direction (red curves) than that along the parallel direction (blue curves). More importantly, the slopes of the curves correspond to the coefficient of friction (COF). The average COF value measured along the perpendicular direction was 0.035, which was ~194% than the COF value of 0.018 measured along the parallel direction. The magnitude of the friction along with the COFs shows strong anisotropic dependency on the wrinkle direction. The probe wear is always a concern in FFM. To measure the probe wear quantitatively, a polycrystalline Titanium roughness sample is used as AFM tip characterizer27. The basic principle of this characterization process is shown in Figure S5 (a-b). The AFM image obtained on the sharp features can be used to evaluate the 8


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tip morphology. The images of a new probe and 3 probes after the friction measurements are shown in Figure S5 (c-f). The corresponding tip profiles and the estimated tip radii are shown in Figure S5 (g). The radius of the new tip is = 11.52 nm, which agrees with the nominal value of 8-12 nm provided by the manufacturer. The tip radius after the friction measurement is 13.8 ± 0.4 nm, which is ~20% larger than the new tip. This result indicates moderate wear of the AFM probes. However, comparing to the ~194% anisotropic friction of graphene, the tip wear only has a minor influence on the experiments and it will not change our conclusions. In order to confirm that the observed anisotropic friction was induced by wrinkle structures instead of the graphene intrinsic atomic lattice, the COF values of the same area along various wrinkle orientations were measured. If the COF show 60° periodicity, then the anisotropy could be originated from the hexagonal carbon lattice. On the other hand, if the COF show 180° periodicity, then the anisotropy must be induced by the wrinkle structures. Figure 4b shows the measured COFs, as well as the fitting curve of ∗ sin|| , where θ = ±90° indicates that the probe scans perpendicular to high-density wrinkle direction and θ = 0° corresponds to parallel direction. The data fits well with a periodicity of 180° which shows strong evidence that the anisotropic friction observed on CVD grown graphene is indeed induced by wrinkle structures. In addition, we measured the friction at the graphene/SiO2 boundary as shown in Figure 5. Figure 5a,c,d,f are the topography images, and b,d,f,h are the corresponding 9



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friction force images of 0°, 60°, 120° and 180°, respectively. Figure 5i shows the friction force profiles of the same graphene/SiO2 boundary area with various scan angles. The graphene show no friction anisotropy near the boundary. We attribute this to the weak adhesion between graphene and SiO2 substrate at the boundary area. As can be seen from the topography images, the graphene near the boundary is slightly scratched after multiple scans. In other words, at the boundary area the morphology of graphene changes significantly, and wrinkle structure no longer dominates the friction. At areas far from the boundary, no obvious scratch can be seen on graphene, therefore, in order to study the wrinkle induced anisotropic friction, all the data in the manuscript was measured away from the boundary areas. The coefficient of friction of the SiO2 substrate is shown in Figure 5j. The SiO2 substrate does not exhibit anisotropic friction and the average COF is 0.251 which is an order of magnitude higher than that of graphene. More importantly, the orientations of the graphene grains are random during CVD, although the wrinkles are aligned in cooling process. Therefore, measuring several grains on the same substrate would eliminate the possibility that the anisotropic friction is induced by the various alignments with respect to the substrates. Also, we transferred the graphene onto several SiO2 substrates with random orientations, and observed the same anisotropic friction of graphene. This indicates that the frictional property of the substrate does not play a dominate role in our experiments. Furthermore, the environmental conditions may also alter the frictional property of graphene, such as humidity, temperature, and vacuum or ambient. While we believe that these factors are not likely to induce 10


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anisotropic nature to the friction on graphene, it is important to deconvolute these effects in future studies of friction on graphene and other two-dimensional materials.

3. Conclusions In summary, we systematically investigated the friction of monolayer CVD-grown graphene with FFM. Experimental results show that the frictional behavior of CVD-grown graphene depends on the orientation of wrinkle structures. Higher friction was observed perpendicular to the wrinkle direction while lower friction was observed parallel to the wrinkle direction. The friction anisotropy can be as high as ~194%, indicating that anisotropic nano/micro-structures such as wrinkles have significant impact on the frictional property of monolayer graphene. The observed anisotropic friction is originated from the formation of ripples and can be explained by the ‘puckering’ mechanism. Our results suggest the potential of tuning graphene friction by nano/micro-structure engineering, which could benefit the applications such as nano/micro-electromechanical systems where tunable friction is desirable.

4. Methods 4.1 Graphene synthesis- The graphene growth was carried out using an ambient pressure chemical vapor deposition (AP-CVD) method, similar to our previous reports22, 28. In short, the copper substrates (from Alfa Aesar, product no. 46365) were cleaned in 10% hydrochloric acid (in deionized water) for 10-20 minutes and 11



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eventually rinsed in acetone and isopropanol, dried under N2 flow, and loaded into the CVD chamber. Next, the chamber was evacuated (down to 1 mtorr) and purged with 5% hydrogen in argon forming gas to reach the ambient pressure. The samples were heated up to 1,050 ℃ under the same flow for 60 min. Next, 20 p.p.m. of methane (CH4) was added for 40-90 minutes, and the chamber was then rapidly cooled down to room temperature. The samples was then coated with poly(methyl methacrylate) (PMMA) and floated on copper etchant (CE-100) for 24 hours, then floated on several DI water bathes to remove the solvent residue. Finally, the floating samples were scooped out by the target (Si/SiO2) substrates and annealed in vacuum at 350

for 2 hours under

flow of 5% forming gas to remove the polymer residue and enhance the adhesion to the surface. 4.2 AFM- The experimental setup is shown in Figure S1. All AFM experiments were carried out with Dimension ICON system (Bruker, CA.) in ambient conditions. RFESP (Bruker, CA.) cantilevers were selected with nominal spring constant of 3 N/m. The normal and lateral spring constants were calibrated using Sader’s method29. The calibrated normal and torsional spring constants of 3 cantilevers are 4.5±0.3 N/m and 185±10 N/m, respectively. AFM images were analyzed with Nanoscope Analysis software (Bruker, CA) and the line profiles were replotted with Origin 9.0. When measuring friction at various probe scanning angles, the graphene samples were physically rotated while the probe scanning direction remained unchanged. 4.3 SEM- All SEM images are obtained with an integrated Carl Zeiss microscope in a Raith e-LiNE plus electron beam lithography system. We used the acceleration 12


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voltage of 10 kV and working distance of 10 mm for imaging. 4.4 Raman- The Raman spectra are obtained with a Horiba LabRAM HR Evolution confocal Raman microscope with a 532 nm laser wavelength, 1200 g/mm grating, and a 100× objective lens. The laser power was in the range of 1-4 mW.

ASSOCIATED CONTENT Supporting Information. Additional experimental data and analysis graphs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Author Contributions F.L. and R.S.Y. conceived the idea. F.L. and W.T.Y. performed FFM measurements and R.S.Y supervised them. P.Y. synthesized graphene samples, performed SEM and Raman characterizations and A.S.K supervised him. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENT R.S.Y., F.L. and W.T.Y. acknowledge financial support from the National Science Foundation (Award No. CMMI-1200383). A.S.K. and P.Y. acknowledge the financial support from the National Science Foundation (EFMA-1542864). References (1) Kim, K. S.; Lee, H. J.; Lee, C.; Lee, S. K.; Jang, H.; Ahn, J. H.; Kim, J. H.; Lee, H. J. Chemical Vapor Deposition-Grown Graphene: The Thinnest Solid Lubricant. ACS Nano 2011, 5, 5107-5114. (2) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451-10453. (3) Meric, I.; Han, M. Y.; Young, A. F.; Ozyilmaz, B.; Kim, P.; Shepard, K. L. Current Saturation in Zero-Bandgap, Top-Gated Graphene Field-Effect Transistors. Nat. Nanotechnol. 2008, 3, 654-659. (4) Chen, C.; Rosenblatt, S.; Bolotin, K. I.; Kalb, W.; Kim, P.; Kymissis, I.; Stormer, H. L.; Heinz, T. F.; Hone, J. Performance of Monolayer Graphene Nanomechanical Resonators with Electrical Readout. Nat. Nanotechnol. 2009, 4, 861-867. (5) Lee, C.; Li, Q.; Kalb, W.; Liu, X.-Z.; Berger, H.; Carpick, R. W.; Hone, J. Frictional Characteristics of Atomically Thin Sheets. Science 2010, 328, 76-80. (6) Ye, Z.; Tang, C.; Dong, Y.; Martini, A. Role of Wrinkle Height in Friction Variation with Number of Graphene Layers. J. Appl. Phys. 2012, 112, 116102. (7) Smolyanitsky, A.; Killgore, J. P.; Tewary, V. K. Effect of Elastic Deformation on Frictional Properties of Few-Layer Graphene. Phys. Rev. B. 2012, 85, 035412. (8) Choi, J. S.; Chang, Y. J.; Woo, S.; Son, Y. W.; Park, Y.; Lee, M. J.; Byun, I. S.; Kim, J. S.; Choi, C. G.; Bostwick, A.; Rotenberg, E.; Park, B. H. Correlation between Micrometer-Scale Ripple Alignment and Atomic-Scale Crystallographic Orientation of Monolayer Graphene. Sci. Rep. 2014, 4, 7263. (9) Rastei, M. V.; Heinrich, B.; Gallani, J. L. Puckering Stick-Slip Friction Induced by a Sliding Nanoscale Contact. Phys. Rev. Lett. 2013, 111, 084301. (10) Choi, J. S.; Kim, J. S.; Byun, I. S.; Lee, D. H.; Lee, M. J.; Park, B. H.; Lee, C.; Yoon, D.; Cheong, H.; Lee, K. H.; Son, Y. W.; Park, J. Y.; Salmeron, M. Friction Anisotropy–Driven Domain Imaging on Exfoliated Monolayer Graphene. Science 2011, 333, 607-610. (11) Egberts, P.; Han, G. H.; Liu, X. Z.; Johnson, A. T. C.; Carpick, R. W. Frictional Behavior of Atomically Thin Sheets: Hexagonal-Shaped Graphene Islands Grown on Copper by Chemical Vapor Deposition. ACS Nano 2014, 8, 5010-5021. (12) Demirbaş, T.; Baykara, M. Z. Nanoscale Tribology of Graphene Grown by Chemical Vapor Deposition and Transferred onto Silicon Oxide Substrates. J. Mater. Res. 2016, 31, 1914-1923. (13) Yazyev, O. V.; Chen, Y. P. Polycrystalline Graphene and Other Two-Dimensional Materials. Nat. Nanotechnol. 2014, 9, 755-767. (14) Hattab, H.; N’Diaye, A. T.; Wall, D.; Klein, C.; Jnawali, G.; Coraux, J.; Busse, C.; van Gastel, R.; Poelsema, B.; Michely, T.; Meyer zu Heringdorf, F. J.; Horn-von Hoegen, M. Interplay of Wrinkles, Strain, and Lattice Parameter in Graphene on Iridium. Nano Lett. 2012, 12, 678-682. (15) Paronyan, T. M.; Pigos, E. M.; Chen, G.; Harutyunyan, A. R. Formation of Ripples in Graphene as a Result of Interfacial Instabilities. ACS Nano 2011, 5, 9619-9627. (16) Zhang, Y.; Gao, T.; Gao, Y.; Xie, S.; Ji, Q.; Yan, K.; Peng, H.; Liu, Z. Defect-Like Structures of Graphene on Copper Foils for Strain Relief Investigated by High-Resolution Scanning Tunneling 14


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Microscopy. ACS Nano 2011, 5, 4014-4022. (17) Bao, W.; Miao, F.; Chen, Z.; Zhang, H.; Jang, W.; Dames, C.; Lau, C. N. Controlled Ripple Texturing of Suspended Graphene and Ultrathin Graphite Membranes. Nat. Nanotechnol. 2009, 4, 562-566. (18) Zhang, K.; Arroyo, M. Understanding and Strain-Engineering Wrinkle Networks in Supported Graphene through Simulations. J. Mech. Phys. Solids 2014, 72, 61-74. (19) Vázquez de Parga, A. L.; Calleja, F.; Borca, B.; Passeggi, M. C. G.; Hinarejos, J. J.; Guinea, F.; Miranda, R. Periodically Rippled Graphene: Growth and Spatially Resolved Electronic Structure. Phys. Rev. Lett. 2008, 100, 056807. (20) Vlassiouk, I.; Regmi, M.; Fulvio, P.; Dai, S.; Datskos, P.; Eres, G.; Smirnov, S. Role of Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene. ACS Nano 2011, 5, 6069-6076. (21) Yu, Q.; Jauregui, L. A.; Wu, W.; Colby, R.; Tian, J.; Su, Z.; Cao, H.; Liu, Z.; Pandey, D.; Wei, D.; Chung, T. F.; Peng, P.; Guisinger, N. P.; Stach, E. A.; Bao, J.; Pei, S.-S.; Chen, Y. P. Control and Characterization of Individual Grains and Grain Boundaries in Graphene Grown by Chemical Vapour Deposition. Nat. Mater. 2011, 10, 443-449. (22) Yasaei, P.; Kumar, B.; Hantehzadeh, R.; Kayyalha, M.; Baskin, A.; Repnin, N.; Wang, C.; Klie, R. F.; Chen, Y. P.; Král, P.; Salehi-Khojin, A. Chemical Sensing with Switchable Transport Channels in Graphene Grain Boundaries. Nat. Commun. 2014, 5, 4911. (23) Deng, Z.; Smolyanitsky, A.; Li, Q.; Feng, X.-Q.; Cannara, R. J. Adhesion-Dependent Negative Friction Coefficient on Chemically Modified Graphite at the Nanoscale. Nat. Mater. 2012, 11, 1032-1037. (24) Marsden, A. J.; Phillips, M.; Wilson, N. R. Friction Force Microscopy: A Simple Technique for Identifying Graphene on Rough Substrates and Mapping the Orientation of Graphene Grains on Copper. Nanotechnology 2013, 24, 255704. (25) Liu, X.-Z.; Li, Q.; Egberts, P.; Carpick, R. W. Nanoscale Adhesive Properties of Graphene: The Effect of Sliding History. Adv. Mater. Interfaces 2014, 1, 1300053. (26) Bao, W.; Miao, F.; Chen, Z.; Zhang, H.; Jang, W.; Dames, C.; Lau, C. N. Controlled Ripple Texturing of Suspended Graphene and Ultrathin Graphite Membranes. Nat. Nanotechnol. 2009, 4, 562-566. (27) Vorselen, D.; Kooreman, E. S.; Wuite, G. J. L.; Roos, W. H. Controlled Tip Wear on High Roughness Surfaces Yields Gradual Broadening and Rounding of Cantilever Tips. Sci. Rep. 2016, 6, 36972. (28) Yasaei, P.; Fathizadeh, A.; Hantehzadeh, R.; Majee, A. K.; El-Ghandour, A.; Estrada, D.; Foster, C.; Aksamija, Z.; Khalili-Araghi, F.; Salehi-Khojin, A. Bimodal Phonon Scattering in Graphene Grain Boundaries. Nano Lett. 2015, 15, 4532-4540. (29) Green, C. P.; Lioe, H.; Cleveland, J. P.; Proksch, R.; Mulvaney, P.; Sader, J. E. Normal and Torsional Spring Constants of Atomic Force Microscope Cantilevers. Rev. Sci. Instrum. 2004, 75, 1988-1996.

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Figure 1. (a) Typical SEM image of CVD grown graphene after transfer on Si/SiO2 substrate. (b) Raman spectra obtained from the single crystalline graphene with I2D/IG ratio of ~2, which is characteristic for monolayer graphene. (c) AFM topography of monolayer graphene on Si/SiO2 substrate. Most wrinkles are aligned along the same direction, which is referred to high-density wrinkle direction as indicated with red arrow. (d) Zoom-in AFM topography of wrinkles along high-density wrinkle direction.

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Figure 2. (a) Topography and (b) friction force mapping when the probe scans perpendicular to the high-density wrinkle direction. (c) Topography and (d) friction force mapping when the probe scans parallel to the high-density wrinkle direction. Wrinkles perpendicular and parallel to the probe scanning direction are indicated with green and yellow arrows, respectively. The friction force profiles along the red and blue dashed lines are shown in Figure 3. Figures in (e) and (f) are schematics of the interaction between the probe and wrinkle along perpendicular and parallel direction, respectively.

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Figure 3. (a) Friction force profiles from the dashed lines illustrated in Figure 2b and 2d. Both forward and backward scan profiles are shown. The sharp peaks correspond to the high friction of wrinkles. Perpendicular to the probe scan direction, the wrinkle free graphene exhibits minor peaks originated from ripples and puckers, as indicated on the red profile. Meanwhile on the blue profile, no obvious ripples are observed. The schematics in (b) and (c) show graphene ‘puckering’ at the probe contact area. More graphene deformation is produced perpendicular to the ripple lines due to the lower bending stiffness.

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Figure 4. (a) Friction force as a function of normal load. The friction of graphene perpendicular to the high-density wrinkle direction is on average higher than that along the parallel direction. The coefficient of friction (COF) is measured as the slope of the curves. The average COF value perpendicular to the high-density wrinkle direction is 0.035, and the COF value of parallel direction is 0.018. The anisotropy is 194%. (b) COFs as a function of angle between probe scanning direction and high-density wrinkle direction. = ±90° corresponds to perpendicular and = 0° corresponds to parallel direction. The data is well fitted by a sinusoidal regression which shows 180° periodicity.

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Figure 5. (a) Graphene friction at the graphene/SiO2 boundary area with various scan angles. (a)(c)(e)(g) the topography images, (b)(d)(f)(h) the friction force images of 0°, 60°, 120° and 180° respectively. (i) friction force profiles at the graphene/SiO2 boundary area with various scan angles. (j) Coefficient of friction (COF) on Si/SiO2 substrate as a function of AFM probe scan angle. The average COF is 0.251 which is an order of magnitude higher than that of graphene. More importantly, the COF does not show dependency of scanning angle. Scale bar 1 µm.

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Anisotropic Friction of Wrinkled Graphene Grown by Chemical Vapor

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