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Friction and wear characteristics of multi-layer graphene films investigated by atomic force microscopy
Surface & Coatings Technology 205 (2011) 4864–4869
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Friction and wear characteristics of multi-layer graphene films investigated by atomic force microscopy Li-Yu Lin a, 1, Dae-Eun Kim a,⁎, Whan-Kyun Kim b, Seong-Chan Jun b a b
Center for Nano-Wear, Department of Mechanical Engineering, Yonsei University, Seoul 120-749, South Korea Nano Electromechanical Device Lab, Department of Mechanical Engineering, Yonsei University, Seoul 120-749, South Korea
a r t i c l e
i n f o
Article history: Received 10 August 2010 Accepted in revised form 26 April 2011 Available online 4 May 2011 Keywords: AFM Friction Graphene Tribology Wear
a b s t r a c t Friction and wear characteristics of multi-layer graphene films deposited on a Si substrate by mechanical exfoliation were investigated by atomic force microscopy (AFM). The graphene films consisted of a few layers of carbon basal plane. The number of graphene layers was determined by AFM and Raman spectroscopy. For the AFM friction measurement, loads in the range of − 5 to 30 nN were applied on the Si tip that slid against the graphene specimen. It was found that graphene films exhibited much lower friction (from 0.36 to 0.62 nN) than bare Si surface (from 1.1 to 4.3 nN) when the applied loads ranged from 3 to 30 nN. The wear characteristics were also assessed using the AFM. Detectable wear of graphene was generated when sliding was performed for 100 cycles under 5 μN applied load. The wear mechanism of graphene was proposed to be due to breakage of in-plane bonds between carbon atoms and shearing at the interface of graphene layers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the past several years, graphene has attracted much attention in the materials community because of its unique mechanical [1,2] and electrical [3,4] properties. Graphene is a monolayer of carbon atoms with two-dimensional (2D) honeycomb lattice, which is the basic building block for 0D buckyballs, 1D nanotubes, and 3D graphite. Besides the single layer graphene, bi-layer and few-layer graphene are also known to possess remarkable properties [3,5,6]. As far as preparation of graphene is concerned, a few methods have been used such as mechanical exfoliation, liquid-phase exfoliation, and epitaxial growth by chemical vapor deposition [7]. Currently, it still is a challenge to control the thickness and dimension of graphene on an arbitrary substrate. However, with continued efforts by numerous researchers in developing various graphene deposition processes this limitation is expected to be overcome in the future. The Young's modulus and strength of graphene have been investigated by nanoindentation using an AFM [1] as well as through simulation of its tensile and compressive properties [8]. These studies demonstrated that graphene has excellent mechanical properties with a Young's modulus of 1 TPa and a breaking strength of 130 GPa. Furthermore, electronic properties and quantum Hall effects of graphene have been investigated [9]. Single layer graphene exhibited remarkably high electron mobility (15,000 cm 2 V − 1 s − 1), which was quite independent of temperature. ⁎ Corresponding author. Tel.: + 82 2 2123 2822; fax: + 82 2 312 2159. E-mail address: [email protected] (D.-E. Kim). 1 Present address: Department of Mechanical and Aerospace Engineering, The George Washington University, Ashburn, VA 20147, USA. 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.04.092
Compared with the works on general properties of graphene, the tribological investigation of graphene has been quite limited. Since graphene is expected to play a wider role in nanotechnology, it is of interest to clearly understand its micro/nano-scale tribological characteristics as well. An AFM is an excellent tool for investigating nanotribological phenomena due to its extremely high lateral and vertical force resolutions [10–12]. The fundamental tribological characteristics of graphene have been investigated by using an AFM in an attempt to understand its mechanical behaviors under sliding contact situations. It has been reported in literatures that frictional properties of graphene are affected by the number of graphene layers [13–16]. These literatures showed that friction of graphene decreased with increasing number of graphene layers. The variation of friction with the number graphene layers was attributed to the interplay of surface attractive forces [13], the effect of electron-phonon coupling [14] as well as to puckering effect [16]. In these researches, normal loads less than 250 nN were used and the frictional properties were investigated under positive applied loads without causing bond breakage of graphene. Although the frictional properties and mechanisms of graphene have been proposed, wear resistant properties have not yet been studied. The wear characteristics of graphene may be affected by the number of graphene layers since it was previously shown that graphene with a few number of layers (1–4 layers) exhibited different friction properties than thicker films [13–16]. Also, graphene layers with more than 5 layers exhibited friction properties similar to those of bulk graphite [15,16]. In this work, friction and wear characteristics of multi-layer graphene films were investigated by an AFM under a range of applied normal loads. The multi-layer graphene specimens used for the friction and wear measurements consisted of 6–15 layers. Since the attractive force between the AFM tip and the specimen surface can be
L.-Y. Lin et al. / Surface & Coatings Technology 205 (2011) 4864–4869
a significant contribution to the effective applied load at the nanoscale, both negative and positive loads were applied to the AFM tip. As a comparison, the friction of bare Si surface was also measured under the same experimental conditions. In order to measure the wear resistant properties of graphene, normal loads up to 10 μN were applied on Si AFM probe sliding against graphene specimens up to 100 reciprocating cycles. Wear properties of graphene were characterized by analyzing the wear tracks on the specimen surface using an AFM.
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used for the friction force measurement was determined to be 53 N/ m. Though the spring constants used to quantify the normal and lateral forces were derived based on validated equations, it should be noted that manufacturing tolerance of the cantilever may lead to inaccuracy in the spring constants. The output voltage signal of friction force microscopy (FFM) obtained from the AFM during the friction measurement was transformed into friction force ( f ) by using the following equation [18,19]:
2. Specimens and experimental method The graphene films used in the experiments were deposited on a Si substrate with native oxide layer by the mechanical exfoliation method. Mechanical exfoliation method is a simple procedure in which thin flakes of graphene is detached from the bulk graphite by repeatedly using an adhesive tape [3]. Using this method, high-quality 2D crystalline graphene of a few micrometers in size can be acquired quite successfully. The graphene layers on the tape were transferred onto the Si substrate by contacting the two surfaces. The number of graphene layers was identified by AFM and Raman spectroscopy. The frictional properties of graphene and bare silicon were measured using an AFM under the applied loads from 30 to −5 nN. Initially, the AFM tip was made to contact the specimen surface at a high load and the friction force was measured over a distance of 20 μm for two cycles. Then, the cantilever was retracted slightly to lower the applied load and the friction force was measured again. This process was repeated until the tip got detached from the specimen surface due to excessive negative load. The schematic of the contact between AFM tip and specimen surface under positive and negative loads is shown in Fig. 1. The wear tests were performed by scanning the Si AFM tip against the graphene specimen in the contact mode and triggering the vector scan option. All of the measurements were carried out in ambient atmosphere with a temperature of 24 °C and a relative humidity of about 15%. For characterizing the surface morphology of specimens, all of the AFM images were obtained in the contact mode. Two types of AFM silicon cantilevers, PPP-CONTR and PPP-NCLR, used in this work were supplied by Nanosensor™. Based on the geometry of cantilevers supplied by the manufacturer, the normal spring constant was determined by using the following equation [17]: KN =
Ewt 3 4l3
ð1Þ
where E is the Young's modulus of Si, and w, t, L are the width, thickness, and length of the cantilever, respectively. The calculated values of the normal spring constants were similar to the nominal values of 0.12 and 42 N/m for PPP-CONTR and PPP-NCLR cantilever, respectively. In order to obtain the friction force between the tip and the specimen, the lateral spring constant, KL (N/m), of the AFM cantilever was calculated according to the following equation [17]: KL =
2 Gwt 3 1 h + t =2 3L
ð2Þ
where G is the shear modulus of Si and h is the height of the tip. The lateral spring constant of the PPP-CONTR AFM cantilever which was
Fig. 1. Schematic of AFM tip/specimen contact under negative and positive applied loads.
f =
0:4·h·KL ·V L·SDIF
ð3Þ
where 0.4 is the gain of torsion/deflection ratio that is determined by the equipment (Seiko Instruments Inc. SPA-400), SDIF is the normal deflection sensitivity (40 mV/nm), and V is the output voltage of FFM (mV). 3. Results and discussion 3.1. Identification of the number of graphene layers Before conducting the tribological tests on the graphene specimens it was necessary to characterize the thickness of the graphene patches on the Si substrate. As the building block of graphite, graphene has an extremely small thickness. The theoretical thickness of a single layer graphene is reported to be 0.35 ± 0.01 nm [20]. Optical microscopy is a quick and effective method to identify the location of graphene on the Si substrate since contrasting colors can be seen for different thickness of graphene layers. However, optical microscopy is not a quantitative method to determine the exact thickness of the graphene layer. On the other hand AFM has an atomic scale resolution for determining the surface topography and can be used to identify the number of graphene layers. However, the measured thickness of graphene may be larger than the theoretical value due to the instrumental offset that is caused by the different surface interaction forces between the substrate and graphene [20– 22]. Although the thickness of graphene specimen measured by AFM involves instrumental offset, the step height between a specific region and the adjacent region of the graphene specimen can be obtained without instrumental offset. Thus, AFM can be used to quantify the wear depth of graphene with adequate precision. Raman spectroscopy has been used as a nondestructive method for identification of the number of graphene layers [21]. For a graphene film with less than seven layers, the second order D peak (D′ peak) differs in shape, width, and position depending on the number of graphene layers [23–25]. The D′ peak of a monolayer graphene is a single peak, while the D′ peak of a few-layer graphene consists of subpeaks. The splitting of D′ peak as a function of the number of graphene layers can be calculated through a fitting process involving the Lorentzian peaks [25]. The D′ peaks of a bilayer graphene can be split into four subpeaks, while the splitting of a multilayer graphene becomes more complicated. The D′ peak of a multilayer graphene is decomposed to two subpeaks and the difference in Raman shift between the two subpeaks increases with the number of graphene layers. Fig. 2 shows the optical images and Raman spectra of the graphene patch. It is evident that different contrasts seen on the graphene surface in the optical image (Fig. 2a) are due to the varying thicknesses of the graphene film. For Raman spectra, which were acquired by using a laser excitation wavelength of 514 nm, the measurements were performed at six sites marked in Fig. 2a. As shown in Fig. 2b, two characteristic peaks are the G peak at ~1580 cm − 1 and the D′ peak at ~2700 cm − 1. The D′ peaks are plotted in Fig. 2c for the six different sites on the specimen shown in Fig. 2a. The subpeaks shown in dotted curves are determined by Lorentzian peak fitting. The number shown on the left side of each graph represents the difference of the two innermost subpeaks at the
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Fig. 2. (a) Optical image of graphene film on Si substrate. (b) Raman spectra of the graphene specimen acquired with a laser excitation wavelength of 514 nm. (c) Raman spectra showing D′ peaks for the six different sites on the graphene specimen. The dotted curves show the Lorentzian peaks obtained by using the data fitting method. The solid curves are the experimental data. The number of graphene layers is indicated on the right side of each graph.
respective site on the specimen. The difference between the two subpeaks is referred to as the splitting value of D′ peak. It can be seen that the splitting values are different for the respective sites. According to the splitting values obtained from the Raman spectroscopy [25], the results show that the number of graphene layers at site 1 is determined as 2 layers, and sites 3 and 5 consist of three layers. Also, it is deducted from the splitting value of D′ peak that sites 2 and 4
consist of 6–7 layers, and site 6 with the maximum splitting value of 33.1 cm − 1 consists of 8–9 layers. The number of graphene layers obtained from the Raman analysis was compared with the results obtained from the AFM measurements. Fig. 3 shows the AFM image of the specimen and the 2D profiles at six different regions. The thickness at the six sites was determined from the 2D profiles also shown in the figure. The dotted
Fig. 3. AFM image of the graphene specimen, 2D profiles at the six different regions and the thickness at the six sites indicated on the image.
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lines on the AFM image indicate the scan lines for the 2D profiles. The region with the minimum thickness of 1.5 nm is located at site 1 and the region with the maximum thickness of 4.0 nm is located at site 6. It can be seen that the thickness obtained from AFM is higher than what is predicted from the number of layers obtained from the Raman analysis multiplied by the theoretical thickness of a single graphene layer or 0.35 nm. For instance, at site 1 the number of layers obtained from Raman analysis is two, and hence the predicted thickness is 0.70 nm. The discrepancy in the theoretical and measured thickness is due to the instrumental offset of ~ 0.8 nm. Also, the step height between site 1 and site 3 is 0.5 nm, corresponding to the measured thickness of a single graphene layer. 3.2. Friction and wear of graphene films Friction tests were performed using the graphene specimen with a thickness of 6 nm prepared by the method described previously. Fig. 4 shows the AFM image and 2D profile of the specimen used in the friction measurements. Fig. 5a shows the raw data of friction measurement under 5.4 nN applied load. Fig. 5b shows the friction force obtained from the raw AFM friction data for graphene and bare Si specimens under various positive and negative applied loads. The applied load was pre-set to 5.4 nN and the load was reduced by incrementally retracting the cantilever. The friction forces shown in Fig. 5b were obtained on the same track for all positive and negative loads until the AFM tip got detached from the sample surface. The maximum negative applied load corresponded to the force at which the tip got detached from the specimen surface. This force for graphene (−5.3 nN) was slightly higher than that of Si (−5.1 nN). As expected, it can be seen that the friction force increased as the applied load was increased. Also, the friction of graphene was much lower than that of bare Si in almost the entire range of applied load except for the extreme end of the negative load region. The nonzero value of the friction force below zero applied load was due to the presence of interfacial adhesion which served to generate an overall positive effective load. The mechanism of interfacial adhesion is quite complicated and it is typically caused by surface forces such as van der Waals, capillary, electrostatic, and chemical bonding forces. Considering the experimental conditions and the nature of the specimens used in this work the effects of electrostatic and chemical bonding forces are expected to be low. Since the experiments were carried out in ambient environment, it is most likely that capillary and van der Waals forces dominated the adhesion force. Although these forces are
Fig. 5. (a) The raw friction data measured with an AFM in contact mode. (b) and (c) friction force of graphene and Si substrate under various negative and positive applied loads.
Fig. 4. AFM image and 2D profile of the graphene specimen used the friction measurements.
applicable in the entire range of applied loads, their contribution to the resulting friction force becomes most profound at negative applied loads. Thus, even under a negative applied load, a positive friction force can be generated as shown in Fig. 5b and c shows the friction force of graphene and Si substrate for applied loads in the range of 6 to 30 nN. It can be seen that the friction force of graphene only increased slightly from 0.42 to 0.62 nN when the applied load was increased from 6 to 3 nN. In the past, Bragg [26] described that the low frictional property of graphite was attributed to weak bonding between the basal planes, which led to the lubricious property of graphite. In addition, the friction and wear characteristics of graphite are strongly influenced by the environment. Compared with water vapor and oxygen environments that resulted in low frictional properties of most carbon materials, dry and vacuum environments led to relatively high friction and wear [27,28]. It was proposed that graphite basal plane had an intrinsically
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Fig. 6. (a) Optical image, (b) AFM image and (c) Raman spectra of the graphene specimen used for the wear tests.
low surface energy and water vapor was required for passivation of graphite surface to maintain low friction [29]. As the building block of graphite, graphene also exhibited low friction both in atmospheric environment [13] and an ultra high vacuum condition [14]. The graphene specimens used for the friction measurements in this work consisted of only a few layers. There was no evidence of basal-plane slip during the friction measurement since the surface morphology and thickness of graphene remained unchanged after the friction test. Thus, the frictional energy dissipation was postulated to be mostly concentrated at the sliding interface between the tip and graphene surface. The low friction of graphene was therefore, attributed to the intrinsic properties of graphene. In order to assess the wear characteristics of graphene, the AFM Si tip was slid against the graphene specimen under higher applied loads than the load range used for the friction experiments. Fig. 6 shows the optical image, AFM image and Raman spectra of the specimen used for the wear tests. AFM analysis showed that the thickness of graphene specimen was 2.8 nm. From the Raman spectra, it could be seen that the splitting value of D′ peak was about 28.9 and the number of the graphene layers calculated from this value was 6–7 layers which corresponded to a thickness of about 2.3 nm. The difference in the thickness values was within the instrumental offset of 0.8 nm mentioned previously.
Fig. 7 shows the surface morphology of the graphene specimen before and after the wear tests. The solid-black lines on the AFM image of Fig. 7a indicate the wear test tracks on the graphene specimen. The eight lines with a length of 5 μm run parallel to each other at an interval of 1 μm. The AFM tip was slid against the graphene specimen along each line for 100 sliding cycles under various applied loads ranging from 10 to 3 μN. The track number 1 and 8 correspond to 10 and 3 μN applied load, respectively. The AFM image in Fig. 7b shows the surface morphology of graphene specimen after 100 sliding cycles on each track. The image shows that some region of graphene was fractured and folded along the scanning direction. Fig. 8 shows the AFM image of the wear tracks with higher magnification and the cross-sections of the wear tracks. Six distinct wear tracks marked from 1 to 6 in Fig. 8a can be found on the graphene specimen. The tracks in Fig. 8b, which are marked from 1 to 8, correspond to applied loads from 10 to 3 μN, respectively. It can be seen that the width of wear tracks was about 200 nm. The relatively wide wear track width was attributed to the creep effect of the piezoelectric AFM scanner during the 100 cycles of reciprocating tip motion as well as wear of the tip. The depth of wear tracks as a function of applied load is shown in Fig. 9. It shows that the wear depth was in the range of 1.14 to 0.47 nm. Theoretically, the wear depth of graphene was expected to be in multiples of the thickness of a single graphene layer of 0.35 nm. However, the measured depth values did not exactly correspond to multiples of the single layer thickness. This is considered to be due to measurement artifacts. Notwithstanding the existing artifacts, wearout of a single layer graphene was detectable since relative step height on graphene could be obtained at the atomic-scale. Thus, the extent of graphene wear could be evaluated based on the estimated number of layers removed during the wear test under a given applied load. Considering the measured wear depth between 0.47 and 1.14 nm, the worn layers of the graphene possibly consisted of 1–3 layers. Applied load of 5 μN resulted in the wear depth of about 0.47 nm which corresponded roughly to one layer of graphene. Similarly, under 7–9 μN of applied load, two layers, and under 10 μN of applied load, three layers of graphene were worn out. The results of wear tests suggested that 5 μN could be considered as the critical load at which a single layer of graphene was removed after 100 cycles of sliding. From the wear characteristics of graphene described above, the wear mechanism may be proposed. It is obvious that a single layer of graphene which consists of a plane of carbon atoms cannot be worn off gradually with increasing number of sliding cycles. Thus, it was postulated that the layer got removed at once due to a certain event that led to excessive contact stress that was sufficient to overcome the strength of graphene. This event may be introduction of a nanoparticle between the tip and the graphene specimen that may lead to extremely high contact stress due to three body abrasion or the
Fig. 7. AFM images of graphene specimen (a) before and (b) after wear test.
L.-Y. Lin et al. / Surface & Coatings Technology 205 (2011) 4864–4869
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Fig. 9. The wear depth of multi-layer graphene as a function of applied loads.
resistance that makes it an attractive material for various applications in nanotechnology. Acknowledgments This CRI work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010-0018289). Fig. 8. (a) AFM image of wear tracks and (b) the cross-sections of wear tracks.
References presence of a defect in the graphene layer along the path of the AFM tip motion. The wear sequence may be described by breakage of the in-plane bonds between carbon atoms along the edge of the wear track and shearing between the graphene layers underneath the wear track. It has been reported that the intrinsic strength of graphene obtained by indentation method was 130 GPa [1]. Also, based on the simulation results of tensile strength of graphene, the breaking strength of zigzag and armchair graphene was calculated to be 107 GPa and 90 GPa, respectively [30]. Thus, for wear to occur, at some point in the sliding process stress exceeding these values must be generated within the graphene layers. Such stress levels may be reached if the contact stress is sufficiently high. Under higher applied loads, the bonds of the inplane carbon atoms in the sub-layers of graphene may be broken to result in bi- or multi-layer graphene wear. Quantification of stress levels necessary to achieve such a condition remains as a topic for further research. 4. Summary Friction and wear characteristics of multi-layer graphene films were investigated by using an AFM. The multi-layer graphene films used for friction and wear measurements consisted of 6–15 layers. The friction properties of graphene were measured under negative and positive applied loads ranging from − 5 to 30 nN. The results showed that graphene exhibited much lower friction than the Si substrate. The wear test results indicated that a significant wear of graphene was generated at the critical applied load of 5 μN after 100 cycles of sliding. It was proposed that the wear of graphene was due to breakage of the in-plane bonds between carbon atoms and shearing at the interface of the graphene layers caused by a certain event that led to excessive contact stress during the sliding process. Despite its low thickness, graphene has superb frictional properties and relatively high wear
[1] C.G. Lee, X.D. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385. [2] C. Gómez-Navarro, M. Burghard, K. Kern, Nano Lett. 8 (2008) 2045. [3] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [4] C. Stampfer, E. Schurtenberger, F. Molitor, J. Guttinger, T. Ihn, K. Ensslin, Nano Lett. 8 (2008) 2378. [5] K.S. Novoselov, E. McCann, S.V. Morozov, V.I. Fal'ko, M.I. Katsnelson, U. Zeitler, D. Jiang, F. Schedin, A.K. Geim, Nat. Phys. 2 (2006) 177. [6] Y.B. Zhang, J.P. Small, W.V. Pontius, P. Kim, Appl. Phys. Lett. 86 (2005) 073104. [7] M. Taghioskoui, Mater. Today 12 (2009) 34. [8] Y.W. Gao, P. Hao, Physica E 41 (2009) 1561. [9] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [10] I.H. Sung, D.E. Kim, Ultramicroscopy 107 (2007) 1. [11] L.Y. Lin, J.M. Seo, M.C. Jeong, K.J. Koo, D.E. Kim, J.M. Myoung, Mater. Sci. Eng. A 460–461 (2007) 370. [12] H.J. Kim, D.E. Kim, Int. J. Precis. Eng. Manuf. 10 (2009) 141. [13] H.S. Lee, N.S. Lee, Y.H. Seo, J.H. Eom, S.W. Lee, Nanotechnology 20 (2009) 325701. [14] T. Filleter, J.L. McChesney, A. Bostwick, E. Rotenberg, K.V. Emtsev, Th. Seyller, K. Horn, R. Bennewitz, Phys. Rev. Lett. 102 (2009) 086102. [15] C.G. Lee, X.D. Wei, Q.Y. Li, R. Carpick, J.W. Kysar, J. Hone, Phy. Status Solidi B 246 (2009) 2562. [16] C.G. Lee, Q.Y. Li, W. Kalb, X.Z. Liu, H. Berger, R.W. Carpick, J. Hone, Science 328 (2010) 76. [17] E. Tocha, H. Schonherr, G.J. Vancso, Langmuir 22 (2006) 2340. [18] W.S. Choi, J.H. Heo, I.S. Chung, B.Y. Hong, Thin Solid Films 475 (2005) 287. [19] M.W. Bai, K. Kato, N. Umehara, Y. Miyake, J.G. Xu, H. Tokisue, Surf. Coat. Technol. 126 (2000) 181. [20] A. Gupta, G. Chen, P. Joshi, S. Tadigadapa, P.C. Eklund, Nano Lett. 6 (2006) 2667. [21] Z.H. Ni, H.M. Wang, J. Kasim, H.M. Fan, T. Yu, T.H. Wu, Y.P. Feng, Z.X. Shen, Nano Lett. 7 (2007) 2758. [22] P. Nemes-Incze, Z. Osvath, K. Kamaras, L.P. Biro, Carbon 46 (2008) 1435. [23] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Phys. Rep. 473 (2009) 51. [24] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (2006) 187401. [25] D. Graf, F. Molitor, K. Ensslin, C. Stampfer, A. Jungen, C. Hierold, L. Wirtz, Nano Lett. 7 (2007) 238. [26] W.H. Bragg, An Introduction to Crystal Analysis, Bell London, , 1928. [27] H. Zaidi, F. Robert, D. Paulmier, Thin Solid Films 264 (1995) 46. [28] B.K. Yen, Wear 192 (1996) 208. [29] B.K. Yen, B.E. Schwickert, M.F. Toney, Appl. Phys. Lett. 84 (2004) 4702. [30] H. Zhao, K. Min, N.R. Aluru, Nano Lett. 8 (2009) 3012.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Friction and wear characteristics of multi-layer graphene films investigated by atomic force microscopy Li-Yu Lin a, 1, Dae-Eun Kim a,⁎, Whan-Kyun Kim b, Seong-Chan Jun b a b
Center for Nano-Wear, Department of Mechanical Engineering, Yonsei University, Seoul 120-749, South Korea Nano Electromechanical Device Lab, Department of Mechanical Engineering, Yonsei University, Seoul 120-749, South Korea
a r t i c l e
i n f o
Article history: Received 10 August 2010 Accepted in revised form 26 April 2011 Available online 4 May 2011 Keywords: AFM Friction Graphene Tribology Wear
a b s t r a c t Friction and wear characteristics of multi-layer graphene films deposited on a Si substrate by mechanical exfoliation were investigated by atomic force microscopy (AFM). The graphene films consisted of a few layers of carbon basal plane. The number of graphene layers was determined by AFM and Raman spectroscopy. For the AFM friction measurement, loads in the range of − 5 to 30 nN were applied on the Si tip that slid against the graphene specimen. It was found that graphene films exhibited much lower friction (from 0.36 to 0.62 nN) than bare Si surface (from 1.1 to 4.3 nN) when the applied loads ranged from 3 to 30 nN. The wear characteristics were also assessed using the AFM. Detectable wear of graphene was generated when sliding was performed for 100 cycles under 5 μN applied load. The wear mechanism of graphene was proposed to be due to breakage of in-plane bonds between carbon atoms and shearing at the interface of graphene layers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the past several years, graphene has attracted much attention in the materials community because of its unique mechanical [1,2] and electrical [3,4] properties. Graphene is a monolayer of carbon atoms with two-dimensional (2D) honeycomb lattice, which is the basic building block for 0D buckyballs, 1D nanotubes, and 3D graphite. Besides the single layer graphene, bi-layer and few-layer graphene are also known to possess remarkable properties [3,5,6]. As far as preparation of graphene is concerned, a few methods have been used such as mechanical exfoliation, liquid-phase exfoliation, and epitaxial growth by chemical vapor deposition [7]. Currently, it still is a challenge to control the thickness and dimension of graphene on an arbitrary substrate. However, with continued efforts by numerous researchers in developing various graphene deposition processes this limitation is expected to be overcome in the future. The Young's modulus and strength of graphene have been investigated by nanoindentation using an AFM [1] as well as through simulation of its tensile and compressive properties [8]. These studies demonstrated that graphene has excellent mechanical properties with a Young's modulus of 1 TPa and a breaking strength of 130 GPa. Furthermore, electronic properties and quantum Hall effects of graphene have been investigated [9]. Single layer graphene exhibited remarkably high electron mobility (15,000 cm 2 V − 1 s − 1), which was quite independent of temperature. ⁎ Corresponding author. Tel.: + 82 2 2123 2822; fax: + 82 2 312 2159. E-mail address: [email protected] (D.-E. Kim). 1 Present address: Department of Mechanical and Aerospace Engineering, The George Washington University, Ashburn, VA 20147, USA. 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.04.092
Compared with the works on general properties of graphene, the tribological investigation of graphene has been quite limited. Since graphene is expected to play a wider role in nanotechnology, it is of interest to clearly understand its micro/nano-scale tribological characteristics as well. An AFM is an excellent tool for investigating nanotribological phenomena due to its extremely high lateral and vertical force resolutions [10–12]. The fundamental tribological characteristics of graphene have been investigated by using an AFM in an attempt to understand its mechanical behaviors under sliding contact situations. It has been reported in literatures that frictional properties of graphene are affected by the number of graphene layers [13–16]. These literatures showed that friction of graphene decreased with increasing number of graphene layers. The variation of friction with the number graphene layers was attributed to the interplay of surface attractive forces [13], the effect of electron-phonon coupling [14] as well as to puckering effect [16]. In these researches, normal loads less than 250 nN were used and the frictional properties were investigated under positive applied loads without causing bond breakage of graphene. Although the frictional properties and mechanisms of graphene have been proposed, wear resistant properties have not yet been studied. The wear characteristics of graphene may be affected by the number of graphene layers since it was previously shown that graphene with a few number of layers (1–4 layers) exhibited different friction properties than thicker films [13–16]. Also, graphene layers with more than 5 layers exhibited friction properties similar to those of bulk graphite [15,16]. In this work, friction and wear characteristics of multi-layer graphene films were investigated by an AFM under a range of applied normal loads. The multi-layer graphene specimens used for the friction and wear measurements consisted of 6–15 layers. Since the attractive force between the AFM tip and the specimen surface can be
L.-Y. Lin et al. / Surface & Coatings Technology 205 (2011) 4864–4869
a significant contribution to the effective applied load at the nanoscale, both negative and positive loads were applied to the AFM tip. As a comparison, the friction of bare Si surface was also measured under the same experimental conditions. In order to measure the wear resistant properties of graphene, normal loads up to 10 μN were applied on Si AFM probe sliding against graphene specimens up to 100 reciprocating cycles. Wear properties of graphene were characterized by analyzing the wear tracks on the specimen surface using an AFM.
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used for the friction force measurement was determined to be 53 N/ m. Though the spring constants used to quantify the normal and lateral forces were derived based on validated equations, it should be noted that manufacturing tolerance of the cantilever may lead to inaccuracy in the spring constants. The output voltage signal of friction force microscopy (FFM) obtained from the AFM during the friction measurement was transformed into friction force ( f ) by using the following equation [18,19]:
2. Specimens and experimental method The graphene films used in the experiments were deposited on a Si substrate with native oxide layer by the mechanical exfoliation method. Mechanical exfoliation method is a simple procedure in which thin flakes of graphene is detached from the bulk graphite by repeatedly using an adhesive tape [3]. Using this method, high-quality 2D crystalline graphene of a few micrometers in size can be acquired quite successfully. The graphene layers on the tape were transferred onto the Si substrate by contacting the two surfaces. The number of graphene layers was identified by AFM and Raman spectroscopy. The frictional properties of graphene and bare silicon were measured using an AFM under the applied loads from 30 to −5 nN. Initially, the AFM tip was made to contact the specimen surface at a high load and the friction force was measured over a distance of 20 μm for two cycles. Then, the cantilever was retracted slightly to lower the applied load and the friction force was measured again. This process was repeated until the tip got detached from the specimen surface due to excessive negative load. The schematic of the contact between AFM tip and specimen surface under positive and negative loads is shown in Fig. 1. The wear tests were performed by scanning the Si AFM tip against the graphene specimen in the contact mode and triggering the vector scan option. All of the measurements were carried out in ambient atmosphere with a temperature of 24 °C and a relative humidity of about 15%. For characterizing the surface morphology of specimens, all of the AFM images were obtained in the contact mode. Two types of AFM silicon cantilevers, PPP-CONTR and PPP-NCLR, used in this work were supplied by Nanosensor™. Based on the geometry of cantilevers supplied by the manufacturer, the normal spring constant was determined by using the following equation [17]: KN =
Ewt 3 4l3
ð1Þ
where E is the Young's modulus of Si, and w, t, L are the width, thickness, and length of the cantilever, respectively. The calculated values of the normal spring constants were similar to the nominal values of 0.12 and 42 N/m for PPP-CONTR and PPP-NCLR cantilever, respectively. In order to obtain the friction force between the tip and the specimen, the lateral spring constant, KL (N/m), of the AFM cantilever was calculated according to the following equation [17]: KL =
2 Gwt 3 1 h + t =2 3L
ð2Þ
where G is the shear modulus of Si and h is the height of the tip. The lateral spring constant of the PPP-CONTR AFM cantilever which was
Fig. 1. Schematic of AFM tip/specimen contact under negative and positive applied loads.
f =
0:4·h·KL ·V L·SDIF
ð3Þ
where 0.4 is the gain of torsion/deflection ratio that is determined by the equipment (Seiko Instruments Inc. SPA-400), SDIF is the normal deflection sensitivity (40 mV/nm), and V is the output voltage of FFM (mV). 3. Results and discussion 3.1. Identification of the number of graphene layers Before conducting the tribological tests on the graphene specimens it was necessary to characterize the thickness of the graphene patches on the Si substrate. As the building block of graphite, graphene has an extremely small thickness. The theoretical thickness of a single layer graphene is reported to be 0.35 ± 0.01 nm [20]. Optical microscopy is a quick and effective method to identify the location of graphene on the Si substrate since contrasting colors can be seen for different thickness of graphene layers. However, optical microscopy is not a quantitative method to determine the exact thickness of the graphene layer. On the other hand AFM has an atomic scale resolution for determining the surface topography and can be used to identify the number of graphene layers. However, the measured thickness of graphene may be larger than the theoretical value due to the instrumental offset that is caused by the different surface interaction forces between the substrate and graphene [20– 22]. Although the thickness of graphene specimen measured by AFM involves instrumental offset, the step height between a specific region and the adjacent region of the graphene specimen can be obtained without instrumental offset. Thus, AFM can be used to quantify the wear depth of graphene with adequate precision. Raman spectroscopy has been used as a nondestructive method for identification of the number of graphene layers [21]. For a graphene film with less than seven layers, the second order D peak (D′ peak) differs in shape, width, and position depending on the number of graphene layers [23–25]. The D′ peak of a monolayer graphene is a single peak, while the D′ peak of a few-layer graphene consists of subpeaks. The splitting of D′ peak as a function of the number of graphene layers can be calculated through a fitting process involving the Lorentzian peaks [25]. The D′ peaks of a bilayer graphene can be split into four subpeaks, while the splitting of a multilayer graphene becomes more complicated. The D′ peak of a multilayer graphene is decomposed to two subpeaks and the difference in Raman shift between the two subpeaks increases with the number of graphene layers. Fig. 2 shows the optical images and Raman spectra of the graphene patch. It is evident that different contrasts seen on the graphene surface in the optical image (Fig. 2a) are due to the varying thicknesses of the graphene film. For Raman spectra, which were acquired by using a laser excitation wavelength of 514 nm, the measurements were performed at six sites marked in Fig. 2a. As shown in Fig. 2b, two characteristic peaks are the G peak at ~1580 cm − 1 and the D′ peak at ~2700 cm − 1. The D′ peaks are plotted in Fig. 2c for the six different sites on the specimen shown in Fig. 2a. The subpeaks shown in dotted curves are determined by Lorentzian peak fitting. The number shown on the left side of each graph represents the difference of the two innermost subpeaks at the
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Fig. 2. (a) Optical image of graphene film on Si substrate. (b) Raman spectra of the graphene specimen acquired with a laser excitation wavelength of 514 nm. (c) Raman spectra showing D′ peaks for the six different sites on the graphene specimen. The dotted curves show the Lorentzian peaks obtained by using the data fitting method. The solid curves are the experimental data. The number of graphene layers is indicated on the right side of each graph.
respective site on the specimen. The difference between the two subpeaks is referred to as the splitting value of D′ peak. It can be seen that the splitting values are different for the respective sites. According to the splitting values obtained from the Raman spectroscopy [25], the results show that the number of graphene layers at site 1 is determined as 2 layers, and sites 3 and 5 consist of three layers. Also, it is deducted from the splitting value of D′ peak that sites 2 and 4
consist of 6–7 layers, and site 6 with the maximum splitting value of 33.1 cm − 1 consists of 8–9 layers. The number of graphene layers obtained from the Raman analysis was compared with the results obtained from the AFM measurements. Fig. 3 shows the AFM image of the specimen and the 2D profiles at six different regions. The thickness at the six sites was determined from the 2D profiles also shown in the figure. The dotted
Fig. 3. AFM image of the graphene specimen, 2D profiles at the six different regions and the thickness at the six sites indicated on the image.
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lines on the AFM image indicate the scan lines for the 2D profiles. The region with the minimum thickness of 1.5 nm is located at site 1 and the region with the maximum thickness of 4.0 nm is located at site 6. It can be seen that the thickness obtained from AFM is higher than what is predicted from the number of layers obtained from the Raman analysis multiplied by the theoretical thickness of a single graphene layer or 0.35 nm. For instance, at site 1 the number of layers obtained from Raman analysis is two, and hence the predicted thickness is 0.70 nm. The discrepancy in the theoretical and measured thickness is due to the instrumental offset of ~ 0.8 nm. Also, the step height between site 1 and site 3 is 0.5 nm, corresponding to the measured thickness of a single graphene layer. 3.2. Friction and wear of graphene films Friction tests were performed using the graphene specimen with a thickness of 6 nm prepared by the method described previously. Fig. 4 shows the AFM image and 2D profile of the specimen used in the friction measurements. Fig. 5a shows the raw data of friction measurement under 5.4 nN applied load. Fig. 5b shows the friction force obtained from the raw AFM friction data for graphene and bare Si specimens under various positive and negative applied loads. The applied load was pre-set to 5.4 nN and the load was reduced by incrementally retracting the cantilever. The friction forces shown in Fig. 5b were obtained on the same track for all positive and negative loads until the AFM tip got detached from the sample surface. The maximum negative applied load corresponded to the force at which the tip got detached from the specimen surface. This force for graphene (−5.3 nN) was slightly higher than that of Si (−5.1 nN). As expected, it can be seen that the friction force increased as the applied load was increased. Also, the friction of graphene was much lower than that of bare Si in almost the entire range of applied load except for the extreme end of the negative load region. The nonzero value of the friction force below zero applied load was due to the presence of interfacial adhesion which served to generate an overall positive effective load. The mechanism of interfacial adhesion is quite complicated and it is typically caused by surface forces such as van der Waals, capillary, electrostatic, and chemical bonding forces. Considering the experimental conditions and the nature of the specimens used in this work the effects of electrostatic and chemical bonding forces are expected to be low. Since the experiments were carried out in ambient environment, it is most likely that capillary and van der Waals forces dominated the adhesion force. Although these forces are
Fig. 5. (a) The raw friction data measured with an AFM in contact mode. (b) and (c) friction force of graphene and Si substrate under various negative and positive applied loads.
Fig. 4. AFM image and 2D profile of the graphene specimen used the friction measurements.
applicable in the entire range of applied loads, their contribution to the resulting friction force becomes most profound at negative applied loads. Thus, even under a negative applied load, a positive friction force can be generated as shown in Fig. 5b and c shows the friction force of graphene and Si substrate for applied loads in the range of 6 to 30 nN. It can be seen that the friction force of graphene only increased slightly from 0.42 to 0.62 nN when the applied load was increased from 6 to 3 nN. In the past, Bragg [26] described that the low frictional property of graphite was attributed to weak bonding between the basal planes, which led to the lubricious property of graphite. In addition, the friction and wear characteristics of graphite are strongly influenced by the environment. Compared with water vapor and oxygen environments that resulted in low frictional properties of most carbon materials, dry and vacuum environments led to relatively high friction and wear [27,28]. It was proposed that graphite basal plane had an intrinsically
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Fig. 6. (a) Optical image, (b) AFM image and (c) Raman spectra of the graphene specimen used for the wear tests.
low surface energy and water vapor was required for passivation of graphite surface to maintain low friction [29]. As the building block of graphite, graphene also exhibited low friction both in atmospheric environment [13] and an ultra high vacuum condition [14]. The graphene specimens used for the friction measurements in this work consisted of only a few layers. There was no evidence of basal-plane slip during the friction measurement since the surface morphology and thickness of graphene remained unchanged after the friction test. Thus, the frictional energy dissipation was postulated to be mostly concentrated at the sliding interface between the tip and graphene surface. The low friction of graphene was therefore, attributed to the intrinsic properties of graphene. In order to assess the wear characteristics of graphene, the AFM Si tip was slid against the graphene specimen under higher applied loads than the load range used for the friction experiments. Fig. 6 shows the optical image, AFM image and Raman spectra of the specimen used for the wear tests. AFM analysis showed that the thickness of graphene specimen was 2.8 nm. From the Raman spectra, it could be seen that the splitting value of D′ peak was about 28.9 and the number of the graphene layers calculated from this value was 6–7 layers which corresponded to a thickness of about 2.3 nm. The difference in the thickness values was within the instrumental offset of 0.8 nm mentioned previously.
Fig. 7 shows the surface morphology of the graphene specimen before and after the wear tests. The solid-black lines on the AFM image of Fig. 7a indicate the wear test tracks on the graphene specimen. The eight lines with a length of 5 μm run parallel to each other at an interval of 1 μm. The AFM tip was slid against the graphene specimen along each line for 100 sliding cycles under various applied loads ranging from 10 to 3 μN. The track number 1 and 8 correspond to 10 and 3 μN applied load, respectively. The AFM image in Fig. 7b shows the surface morphology of graphene specimen after 100 sliding cycles on each track. The image shows that some region of graphene was fractured and folded along the scanning direction. Fig. 8 shows the AFM image of the wear tracks with higher magnification and the cross-sections of the wear tracks. Six distinct wear tracks marked from 1 to 6 in Fig. 8a can be found on the graphene specimen. The tracks in Fig. 8b, which are marked from 1 to 8, correspond to applied loads from 10 to 3 μN, respectively. It can be seen that the width of wear tracks was about 200 nm. The relatively wide wear track width was attributed to the creep effect of the piezoelectric AFM scanner during the 100 cycles of reciprocating tip motion as well as wear of the tip. The depth of wear tracks as a function of applied load is shown in Fig. 9. It shows that the wear depth was in the range of 1.14 to 0.47 nm. Theoretically, the wear depth of graphene was expected to be in multiples of the thickness of a single graphene layer of 0.35 nm. However, the measured depth values did not exactly correspond to multiples of the single layer thickness. This is considered to be due to measurement artifacts. Notwithstanding the existing artifacts, wearout of a single layer graphene was detectable since relative step height on graphene could be obtained at the atomic-scale. Thus, the extent of graphene wear could be evaluated based on the estimated number of layers removed during the wear test under a given applied load. Considering the measured wear depth between 0.47 and 1.14 nm, the worn layers of the graphene possibly consisted of 1–3 layers. Applied load of 5 μN resulted in the wear depth of about 0.47 nm which corresponded roughly to one layer of graphene. Similarly, under 7–9 μN of applied load, two layers, and under 10 μN of applied load, three layers of graphene were worn out. The results of wear tests suggested that 5 μN could be considered as the critical load at which a single layer of graphene was removed after 100 cycles of sliding. From the wear characteristics of graphene described above, the wear mechanism may be proposed. It is obvious that a single layer of graphene which consists of a plane of carbon atoms cannot be worn off gradually with increasing number of sliding cycles. Thus, it was postulated that the layer got removed at once due to a certain event that led to excessive contact stress that was sufficient to overcome the strength of graphene. This event may be introduction of a nanoparticle between the tip and the graphene specimen that may lead to extremely high contact stress due to three body abrasion or the
Fig. 7. AFM images of graphene specimen (a) before and (b) after wear test.
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Fig. 9. The wear depth of multi-layer graphene as a function of applied loads.
resistance that makes it an attractive material for various applications in nanotechnology. Acknowledgments This CRI work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010-0018289). Fig. 8. (a) AFM image of wear tracks and (b) the cross-sections of wear tracks.
References presence of a defect in the graphene layer along the path of the AFM tip motion. The wear sequence may be described by breakage of the in-plane bonds between carbon atoms along the edge of the wear track and shearing between the graphene layers underneath the wear track. It has been reported that the intrinsic strength of graphene obtained by indentation method was 130 GPa [1]. Also, based on the simulation results of tensile strength of graphene, the breaking strength of zigzag and armchair graphene was calculated to be 107 GPa and 90 GPa, respectively [30]. Thus, for wear to occur, at some point in the sliding process stress exceeding these values must be generated within the graphene layers. Such stress levels may be reached if the contact stress is sufficiently high. Under higher applied loads, the bonds of the inplane carbon atoms in the sub-layers of graphene may be broken to result in bi- or multi-layer graphene wear. Quantification of stress levels necessary to achieve such a condition remains as a topic for further research. 4. Summary Friction and wear characteristics of multi-layer graphene films were investigated by using an AFM. The multi-layer graphene films used for friction and wear measurements consisted of 6–15 layers. The friction properties of graphene were measured under negative and positive applied loads ranging from − 5 to 30 nN. The results showed that graphene exhibited much lower friction than the Si substrate. The wear test results indicated that a significant wear of graphene was generated at the critical applied load of 5 μN after 100 cycles of sliding. It was proposed that the wear of graphene was due to breakage of the in-plane bonds between carbon atoms and shearing at the interface of the graphene layers caused by a certain event that led to excessive contact stress during the sliding process. Despite its low thickness, graphene has superb frictional properties and relatively high wear
[1] C.G. Lee, X.D. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385. [2] C. Gómez-Navarro, M. Burghard, K. Kern, Nano Lett. 8 (2008) 2045. [3] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [4] C. Stampfer, E. Schurtenberger, F. Molitor, J. Guttinger, T. Ihn, K. Ensslin, Nano Lett. 8 (2008) 2378. [5] K.S. Novoselov, E. McCann, S.V. Morozov, V.I. Fal'ko, M.I. Katsnelson, U. Zeitler, D. Jiang, F. Schedin, A.K. Geim, Nat. Phys. 2 (2006) 177. [6] Y.B. Zhang, J.P. Small, W.V. Pontius, P. Kim, Appl. Phys. Lett. 86 (2005) 073104. [7] M. Taghioskoui, Mater. Today 12 (2009) 34. [8] Y.W. Gao, P. Hao, Physica E 41 (2009) 1561. [9] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [10] I.H. Sung, D.E. Kim, Ultramicroscopy 107 (2007) 1. [11] L.Y. Lin, J.M. Seo, M.C. Jeong, K.J. Koo, D.E. Kim, J.M. Myoung, Mater. Sci. Eng. A 460–461 (2007) 370. [12] H.J. Kim, D.E. Kim, Int. J. Precis. Eng. Manuf. 10 (2009) 141. [13] H.S. Lee, N.S. Lee, Y.H. Seo, J.H. Eom, S.W. Lee, Nanotechnology 20 (2009) 325701. [14] T. Filleter, J.L. McChesney, A. Bostwick, E. Rotenberg, K.V. Emtsev, Th. Seyller, K. Horn, R. Bennewitz, Phys. Rev. Lett. 102 (2009) 086102. [15] C.G. Lee, X.D. Wei, Q.Y. Li, R. Carpick, J.W. Kysar, J. Hone, Phy. Status Solidi B 246 (2009) 2562. [16] C.G. Lee, Q.Y. Li, W. Kalb, X.Z. Liu, H. Berger, R.W. Carpick, J. Hone, Science 328 (2010) 76. [17] E. Tocha, H. Schonherr, G.J. Vancso, Langmuir 22 (2006) 2340. [18] W.S. Choi, J.H. Heo, I.S. Chung, B.Y. Hong, Thin Solid Films 475 (2005) 287. [19] M.W. Bai, K. Kato, N. Umehara, Y. Miyake, J.G. Xu, H. Tokisue, Surf. Coat. Technol. 126 (2000) 181. [20] A. Gupta, G. Chen, P. Joshi, S. Tadigadapa, P.C. Eklund, Nano Lett. 6 (2006) 2667. [21] Z.H. Ni, H.M. Wang, J. Kasim, H.M. Fan, T. Yu, T.H. Wu, Y.P. Feng, Z.X. Shen, Nano Lett. 7 (2007) 2758. [22] P. Nemes-Incze, Z. Osvath, K. Kamaras, L.P. Biro, Carbon 46 (2008) 1435. [23] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Phys. Rep. 473 (2009) 51. [24] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (2006) 187401. [25] D. Graf, F. Molitor, K. Ensslin, C. Stampfer, A. Jungen, C. Hierold, L. Wirtz, Nano Lett. 7 (2007) 238. [26] W.H. Bragg, An Introduction to Crystal Analysis, Bell London, , 1928. [27] H. Zaidi, F. Robert, D. Paulmier, Thin Solid Films 264 (1995) 46. [28] B.K. Yen, Wear 192 (1996) 208. [29] B.K. Yen, B.E. Schwickert, M.F. Toney, Appl. Phys. Lett. 84 (2004) 4702. [30] H. Zhao, K. Min, N.R. Aluru, Nano Lett. 8 (2009) 3012.