- Email: [email protected]
Fabrication and tribological properties of nanogrids on CVD-grown graphene
Accepted Manuscript Title: Fabrication and tribological properties of nanogrids on CVD-grown graphene Authors: Yan Jiang, Yu Sun, Juan Song PII: DOI: Reference:
S0968-4328(16)30385-7 http://dx.doi.org/doi:10.1016/j.micron.2017.03.005 JMIC 2405
To appear in:
Micron
Received date: Revised date: Accepted date:
28-12-2016 9-3-2017 10-3-2017
Please cite this article as: Jiang, Yan, Sun, Yu, Song, Juan, Fabrication and tribological properties of nanogrids on CVD-grown graphene.Micron http://dx.doi.org/10.1016/j.micron.2017.03.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication and tribological properties of nanogrids on CVD-grown graphene Yan Jiang *, Yu Sun, Juan Song School of Material Science and Engineering, Jiangsu university, Zhenjiang, China, 212013 *E-mail: [email protected]
Research Highlights
Regular nanogrids are fabricated on CVD-grown graphene by atomic force microscopy tip-induced local oxidation.
Formation of nanogrids is related to the duration of applied voltage and structure of graphene.
Friction measurement with AFM can clearly distinguish the borders of nanogrid on graphene.
Adhesion force of graphene with nanogrids decreases due to the increase of surface roughness.
Abstract We have used atomic force microscopy (AFM) combined with an external
sourcemeter to make patterns on CVD-grown graphene, and regular nanogrids have been fabricated on graphene by AFM tip-induced local oxidation. The friction and adhesion properties of graphene with the nanogrids have been characterized by the lateral mode of AFM and the results show that the friction force on the borders of nanogrid is higher than that on the normal area of graphene, yet the adhesion force on the nanogrids decrease slightly due to the change of surface roughness. Keywords graphene; atomic force microscopy; nanogrid; friction; adhesion
1. Introduction
As an atomically thin two-dimension material of carbon atoms packed into a honeycomb lattice, graphene possesses many unique physical properties, such as superior mechanical strength (130 GPa) ( Lee et al., 2008), a fascinating carrier mobility (~106 cm2V-1s-1) (Wang et al., 2009) and high optical transparency (Nair et al., 2008), that make it a suitable candidate for micro-nano electric and photonic device applications. Many of these applications require graphene to be patterned into ribbons, gaps, or other complex structures. Several methods have been reported for patterning graphene, such as scratching (Lu et al., 2010), dip-pen nanolithography (Hirtz et at., 2013), electron beam irradiation (Withers et al., 2011; Cagliani et al., 2015), self-assembly of block copolymers (Li et al., 2015; Kim et al., 2013), nonlithographic fabrication through template (Tai et al., 2011) and local anodic oxidation lithography (Neubeck et al., 2010; Wei et al., 2010; Weng et al., 2008; Giesbers et al., 2008). Among these methods, local oxidation lithography has been successfully used for fabricating nanodevices based on semiconductors and is also considered as a promising technique for patterning graphene, because no other impurities except for oxygen could be introduced and the edges of graphene can be controlled well by this method. To date, simple nanolines can be fabricated easily by common local oxidation lithography of AFM, which has been used to form insulating trenches in graphene flakes to fabricate nanodevices with subnanometer precision (Weng et al., 2008). However, fabrication of complex patterns such as arrays of nanodots by local oxidation needs to control the AFM scanner to move precisely with customed computer program, which is usually rather time-consuming. So improving local oxidation lithography of AFM is necessary in order to make the fabrication of complex pattern to be easier. Compared to the electrical properties of graphene, the tribological properties of graphene have received less attention. Since tribological issues like adhesion, friction and wear may badly influence the reliability and performance of nanodevices and graphene is expected to play a wider role in nanodevices, it is important to clearly understand its tribological characteristics as well. It has been reported that frictional properties of graphene are affected by the number of graphene layers and the substrate (Li et al., 2010). Friction of graphene monotonically increased as the number of layers decreased due to the puckering effect (Lee et al., 2010) and the interplay of surface attractive forces (Lee et al., 2009). The thickness dependence can be suppressed if the graphene is strongly bound to the substrate. Moreover, friction measurements have been also used to characterize variable reduction of graphene oxide (GO) achieved by an AFM tip-based thermochemical nanolithography method (Marsden et al., 2013). The surface friction is reduced as the GO is replaced with graphene by thermal reduction and the border between the graphene and graphene oxide can be clearly distinguished by friction measurements. So distinguishing between graphene and graphene oxide by friction measurements will be helpful to control the edges of nanopatterned graphene based on the local oxidation lithography and improve the local oxidation process of graphene.
In this paper, local oxidation lithography based on an atomic force microscopy (AFM) combining with an external sourcemeter is used to pattern graphene grown by chemical vapor deposition, obtaining regular nanogrids on the graphene surface. Moreover, we have measured the friction and adhesion properties of graphene with nanogrids by AFM. 2. Experimental
Graphene was fabricated by chemical vapor deposition (CVD) onto copper foil and transferred to SiO2/Si wafers. In the transferring process, a PMMA solution was spin-coated onto the top side of the as-prepared graphene sample. The Cu under graphene film could be dissolved after being dipped into an iron chloride solution and then the graphene-coated PMMA was washed with DI water in order to get rid of the remaining metal flakes. The resulting PMMA/graphene layer was placed onto the SiO2/Si substrate and finally, the PMMA was removed by acetone. Characterization of CVD-grown graphene was performed by Raman spectroscopy and AFM. The Raman spectra were recorded with a DXR micro-Raman spectrometer (Thermo Fisher Scientific) with an incident laser beam wavelength of 532 nm. The AFM measurements were carried out with a MFP-3D AFM (Asylum Research) in contact mode using Pt-coated silicon cantilever (AC240 TM, Olympus) under ambient conditions. Nanopatterning of graphene was performed in air by AFM combining with a Keithley sourcemeter and the relative humidity was about 70%. Two edges of graphene films were respectively connected through Au contacts to two output ports of sourcemeter. When a voltage was outputted through the sourcemeter, the current limited to 125 μA flowed from one edge of graphene films to the other one; at the same time the AFM probe scanned on the graphene with contact mode and regular nanogrids could be produced immediately. Topography, friction and adhesive properties of nanogrids were characterized by AFM. The torsional force constant of AFM cantilever was not calibrated and the friction forces were represented by voltage signals since the friction forces in newton are proportional to the torsional force constant of cantilever and the voltage signal. 3. Results and discussion
3.1 Characterization of CVD-grown graphene Figure1 shows the Raman spectra of graphene on the SiO2/Si substrate. One can see that there are not any peaks observed on the Raman spectra of the SiO2 substrate in the range of 1000 ~3000 cm-1 , while for graphene a sharp and strong peak can be easily observed around 1581 cm-1, commonly called G band, corresponding to in-plane carbon-atom stretching vibrations.
The 2D band occurs at around 2690 cm-1, with a lower intensity and wider peak compared with the G band, which indicates that the sample consists of more than one atomic layer of graphene. Moreover, the D band, which is related to the defects and disorders in graphene, appears around 1350 cm-1, indicating the existence of some defects in the graphene sample (Graf et al., 2007).
The AFM images of SiO2/Si substrate and graphene surface are shown in figure 2. The surface roughness of SiO2/Si substrate is measured to be 0.8 nm while the roughness of surface coating with graphene is about 0.2 nm. The folds and wrinkles are clearly visible in graphene and the gap in the topography image refers to the substrate. Figure 2(c) shows crosssection profiles along the dotted line across the graphene and substrate in figure 2(b). The thickness of graphene, i.e. the height of step between the graphene and the substrate, is measured to be 0.4~1.1 nm through the cross-section profiles. Taking into account the Raman spectrum of graphene and the thickness of single layer, the number of graphene can be determined to be 2~4. 3.2 Fabrication of nanogrids on the graphene Figure 3(a) and (b) respectively show the AFM topography of graphene before and after a voltage of 0.15 V is applied to the graphene through the sourcemeter. It is observed that regular anogrids are produced when the voltage is applied to the two edges of graphene during the AFM scanning on the graphene surface with the contact mode. 3D topography image of nanogrids, as shown in figure3(c), indicates that the borders of nanogrids are protrusive. According to the cross-section profile of figure 3(d) along the red line in figure 3(b), it is obtained that the width and height of nanogrid borders are about 90 nm and 1 nm, respectively. Since the experiment was performed in ambient air, it can be inferred that the protrusive borders result from local oxidation of graphene induced by AFM tip under the electric field. When the AFM tip approaches the graphene surface, water vapor in air condenses to form a liquid bridge between the tip and the graphene surface. Furthermore, if a voltage is applied to the graphene and the AFM tip is grounded through the system, a high electric field is induced between the AFM tip and graphene surface, and this electric field can induce local electrochemical oxidation at the graphene surface/water interfaces, which causes a transition of the local graphene to graphene oxide (Masubuchi et al., 2011). When graphene is oxidized to graphene oxide, sp2 carbon bonds of graphene convert into sp3 carbon bonds that can ripple the carbon skeleton, thereby increasing the local sheet thickness (Wei et al., 2010) and resulting in the protrusive nanogrid borders on the graphene surface. Figure 3(b) and (d) also show nanogrids with a remarkable periodicity. The periodicity of nanogrids is related to the applied voltage, which can be confirmed by force-distance curve measurements. Figure 4 (a) and (b) show force-distance curves with and without the applied voltage, respectively. The horizontal part of curve keeps stable and no distance dependent forces act on the AFM probe when no voltage is applied. While in the case of applied voltage, the horizontal part of curve shows periodic fluctuations, indicating a periodic electrostatic force introduced by the applied electric field affects the cantilever. The acting distance of electrostatic force is measured to be about 26.4 nm and the scanning speed of force-distance curve is 1984 nm/s, so the acting duration of electrostatic force is determined to be 13.3 ms. When nanogrids were formed, the scanning speed was 6565 nm/s. In this case, the acting
distance of voltage in single duration is 87 nm, which is consistent with the width of nanogrid border. The results indicate that regular nanogrids are caused by the periodic voltage applied to the graphene and also suggest that different patterns may be fabricated by changing the duration of applied voltage. In experiments, we also notice that the oxidation of graphene occurs on the initial part of scanning area. Nanogrids just take over about 1/3 of scanning area and no changes on the upper part of scanning area are observed, as shown in figure 3(b). This is likely associated with the local structure of graphene. According to the Raman spectra of graphene, there are some defects existing in the graphene. The defects have important influence on the water molecules adsorption on the graphene surface and electron transport in graphene (Kou et al., 2011; Jiang et al., 2008), which correlates highly with the process of AFM-based anodic oxidation. The defects are rather random in the CVD-grown graphene, which may result in the uneven adsorption of water on the surface of graphene. Moreover, the amount of water decreases as the oxidation progress, also resulting in the end of the oxidation process. So the influence of applied voltage on the different area of graphene is inconsistent even under the same experiment conditions and the quality of graphene is very important to the fabrication of nanostructure on the graphene surface using the local anodic oxidation technique. 3.3 Friction and adhesion properties of nanogrid surface The friction properties can be measured by recording the lateral forces in contact mode AFM. Figure 5 shows the topography and simultaneously acquired friction images. In comparison to the topography, the friction image in figure 5(b) shows a much sharper contrast and clearer borderlines of nanogrids. The friction force on the borders is significantly higher than that on other normal area of graphene. This is because the borders are composed of the graphene oxide, the friction coefficient of which is much higher than graphene (Wei et al., 2010). It can be seen from figure 5(c) that the friction force on the nanogrid borders and other normal area is respectively about 9 mV and 1.3 mV, indicating the friction force increases nearly 6 times after the graphene transits into graphene oxide.
Figure 6(a) and (b) show the topography of graphene with nanogrids and corresponding adhesive force map, respectively. It can be seen that the area with nanogrids is a little darker than that without nanogrids. To compare the adhesive forces of two areas, the adhesive forces of 96 points, respectively obtained at the areas with and without nanogrids, are used to perform the Gaussian statistical analysis and the results are shown in figure 6(c) and (d). It is found that the adhesive forces on the normal area without nanogrids obey Gaussian distribution well. For the area with nanogrids, the adhesive forces show a wider distribution compared to the normal area because there are not only graphene but also graphene oxide existing in this area. A Kolmogorov–Smirnov test (K–S test) has also been performed to compare the two groups of adhesive force and the results show that there is significant difference between them. Mean adhesive force is 11.92±0.44 nN on the normal area and it is slightly reduced to be 11.55±0.58 nN on the area with nanogrids. It is usually believed that
the adhesive force between an AFM tip and a sample surface includes the capillary force, van der Waals force, electrostatic force and chemical bonding force. In ambient air, the capillary force is a main contributor to the adhesion and closely related to surface wettability (Ou et al., 2010). It has been confirmed that the roughness of sample surface has strong influence on its wettability (Kubiak et al., 2011; Nosonovsky et al., 2008; Uelzen et al., 2003). For hydrophilic surface, the wettability increases with the roughness of sample surface. While hydrophobic surface will be more hydrophobic with the increase of surface roughness (Uelzen et al., 2003). According to Kim’s report, the water contact angel of CVD-grown graphene transferred to SiO2 is 93.8°for Cu-grown graphene (Kim et al., 2011), indicating the surface of graphene is hydrophobic. When nanogrids are formed on the graphene surface, the surface roughness changes from 0.24 nm to 0.32 nm, resulting in the decrease of surface wettability and the capillary force, and finally the adhesive force between the AFM tip and sample surface decreases. 4. Conclusions Regular nanogirds have been fabricated on the few-layer CVD-grown graphene by AFM based on local anidoc oxidation. Friction images of graphene with nanogrids show clear a border between original graphene and oxided graphene that involves higher friction. Measurements of adhesive force indicate that the adhesive force for the normal area of graphene is a little larger than that for the area with nanogrids due to the increase of surface roughness.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 11102075 and 61205128) and Open Fund of Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education (No. INMD-2014M02). References Cagliani, A., Lindvall, N., Larsen, M. B. B. S., Mackenzie, D. M., Jessen, B. S., Booth, T. J., Bøggild, P., 2015. Defect/oxygen assisted direct write technique for nanopatterning graphene. Nanoscale, 7, 6271-6277. Giesbers, A. J. M., Zeitler, U., Neubeck, S., Freitag, F., Novoselov, K. S., Maan, J. C., 2008. Nanolithography and manipulation of graphene using an atomic force microscope. Solid State Commun., 147, 366-369. Graf, D., Molitor, F., Ensslin, K., Stampfer, C., Jungen, A., Hierold, C., Wirtz, L., 2007. Spatially resolved Raman spectroscopy of single-and few-layer graphene. Nano lett., 7, 238-242. Hirtz, M., Oikonomou, A., Georgiou, T., Fuchs, H., Vijayaraghavan, A., 2013. Multiplexed biomimetic lipid membranes on graphene by dip-pen nanolithography. Nat. commun., 4,2591. Jiang, Y., Guo, W., 2008. Convex and concave nanodots and lines induced on HOPG surfaces by AFM voltages in ambient air. Nanotech., 19, 345302.
Kim, J. Y., Kim, B. H., Hwang, J. O., Jeong, S. J., Shin, D. O., Mun, J. H.,Choi, Y. J., Jin H. M., Kim, S. O., 2013. Flexible and Transferrable Self‐Assembled Nanopatterning on Chemically Modified Graphene. Adv. Mater., 25, 1331-1335. Kim, K. S., Lee, H. J., Lee, C., Lee, S. K., Jang, H., Ahn, J. H., Kim, J.H., Lee, H. J., 2011. Chemical vapor deposition-grown graphene: the thinnest solid lubricant. ACS nano, 5, 5107-5114. Kou, L., Tang, C., Guo, W., Chen, C., 2011. Tunable magnetism in strained graphene with topological line defect. ACS nano, 5, 1012-1017. Kubiak, K. J., Wilson, M. C. T., Mathia, T. G., Carval, P., 2011. Wettability versus roughness of engineering surfaces. Wear, 271, 523-528. Lee, C., Li, Q., Kalb, W., Liu, X. Z., Berger, H., Carpick, R. W., Hone, J., 2010. Frictional characteristics of atomically thin sheets. Science, 328, 76-80. Lee, C., Wei, X., Kysar, J. W., Hone, J., 2008. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science, 321, 385-388. Lee, H., Lee, N., Seo, Y., Eom, J., Lee, S., 2009. Comparison of frictional forces on graphene and graphite. Nanotechnology, 20, 325701. Li, Q., Lee, C., Carpick, R. W., Hone, J., 2010. Substrate effect on thickness‐dependent friction on graphene. Phys. Status Solidi B, 247, 2909-2914. Li, T., Wang, Z., Schulte, L., Ndoni, S., 2016. Substrate tolerant direct block copolymer nanolithography. Nanoscale, 8, 136-140. Lu, G., Zhou, X., Li, H., Yin, Z., Li, B., Huang, L., Boey, F., Zhang, H., 2010. Nanolithography of single-layer graphene oxide films by atomic force microscopy. Langmuir, 26, 6164-6166. Marsden, A. J., Phillips, M., Wilson, N. R., 2013. Friction force microscopy: a simple technique for identifying graphene on rough substrates and mapping the orientation of graphene grains on copper. Nanotechnology., 24, 255704. Masubuchi, S., Arai, M., Machida, T., 2011. Atomic force microscopy based tunable local anodic oxidation of graphene. Nano lett., 11, 4542-4546. Nair, R. R., Blake, P., Grigorenko, A. N., Novoselov, K. S., Booth, T. J., Stauber, T., Peres, N.M.R., Geim, A. K., 2008. Fine structure constant defines visual transparency of graphene. Science, 320, 1308-1308. Neubeck, S., Freitag, F., Yang, R., Novoselov, K. S., 2010. Scanning probe lithography on graphene. Phys. Status Solidi B, 247, 2904-2908. Nosonovsky, M., Bhushan, B., 2008. Roughness-induced superhydrophobicity: a way to design nonadhesive surfaces. J. Phys-Condens.Mat., 20, 225009.
Ou, J., Wang, J., Liu, S., Mu, B., Ren, J., Wang, H., Yang, S., 2010. Tribology study of reduced graphene oxide sheets on silicon substrate synthesized via covalent assembly. Langmuir, 26, 15830-15836. Tai, G. A., Wang, K., Sun, Z., Yin, J., Ng, S. M., Zhou, J.,Yan F., Leung, C.W., Wong, K. H., Guo, W., Lau, S. P., 2011. Nonlithographic fabrication of crystalline silicon nanodots on graphene. J Phys. Chem. C, 116, 532-537. Uelzen, T., Müller, J., 2003. Wettability enhancement by rough surfaces generated by thin film technology. Thin Solid Films, 434, 311-315. Wang, S., Ang, P. K., Wang, Z., Tang, A. L. L., Thong, J. T., Loh, K. P., 2009. High mobility, printable, and solution-processed graphene electronics. Nano lett., 10, 92-98. Wei, Z., Wang, D., Kim, S., Kim, S. Y., Hu, Y., Yakes, M. K., Laracuente, A. R., Dai, Z., Marder, S. R., Berger, C., King, W. P., Heer, W. A.,Sheehan, P.E.,Riedo, E., 2010. Nanoscale tunable reduction of graphene oxide for graphene electronics. Science, 328, 1373-1376. Weng, L., Zhang, L., Chen, Y. P., Rokhinson, L. P., 2008. Atomic force microscope local oxidation nanolithography of graphene. Appl. Phys. Lett., 93, 093107. Withers, F., Bointon, T. H., Dubois, M., Russo, S., Craciun, M. F., 2011. Nanopatterning of fluorinated graphene by electron beam irradiation. Nano lett., 11, 3912-3916.
Figure 1 Raman spectra of CVD-grown graphene and SiO2/Si substrate.
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Figure 2 AFM topography image of SiO 2/Si substrate (a) and graphene CVD-grown graphene (b). (c) Corresponding cross-section profiles along the dotted line in (b).
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Figure 3 AFM topography images of graphene before (a) and after (b) nanogrids are formed. (c) 3D topography image of graphene with nanogrids. (d) cross-section profile along the red line in (b).
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Figure 4 Force versus distance curves obtained by contact model AFM without voltage applied (a) and with a voltage of 0.15 V applied to the graphene (b).
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Figure 5 AFM topography image (a) and simultaneously obtained friction image (b) of nanogrids on the graphene. (c) cross-section profile along the red dotted line in (b)
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Figure 6 AFM topography image(a) and corresponding adhesion force map (b) of graphene with nanogrids.(c)and (d) statistical analysis histograms of adhesion force on the normal area and
nanogrids area of graphene, respectively.
S0968-4328(16)30385-7 http://dx.doi.org/doi:10.1016/j.micron.2017.03.005 JMIC 2405
To appear in:
Micron
Received date: Revised date: Accepted date:
28-12-2016 9-3-2017 10-3-2017
Please cite this article as: Jiang, Yan, Sun, Yu, Song, Juan, Fabrication and tribological properties of nanogrids on CVD-grown graphene.Micron http://dx.doi.org/10.1016/j.micron.2017.03.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication and tribological properties of nanogrids on CVD-grown graphene Yan Jiang *, Yu Sun, Juan Song School of Material Science and Engineering, Jiangsu university, Zhenjiang, China, 212013 *E-mail: [email protected]
Research Highlights
Regular nanogrids are fabricated on CVD-grown graphene by atomic force microscopy tip-induced local oxidation.
Formation of nanogrids is related to the duration of applied voltage and structure of graphene.
Friction measurement with AFM can clearly distinguish the borders of nanogrid on graphene.
Adhesion force of graphene with nanogrids decreases due to the increase of surface roughness.
Abstract We have used atomic force microscopy (AFM) combined with an external
sourcemeter to make patterns on CVD-grown graphene, and regular nanogrids have been fabricated on graphene by AFM tip-induced local oxidation. The friction and adhesion properties of graphene with the nanogrids have been characterized by the lateral mode of AFM and the results show that the friction force on the borders of nanogrid is higher than that on the normal area of graphene, yet the adhesion force on the nanogrids decrease slightly due to the change of surface roughness. Keywords graphene; atomic force microscopy; nanogrid; friction; adhesion
1. Introduction
As an atomically thin two-dimension material of carbon atoms packed into a honeycomb lattice, graphene possesses many unique physical properties, such as superior mechanical strength (130 GPa) ( Lee et al., 2008), a fascinating carrier mobility (~106 cm2V-1s-1) (Wang et al., 2009) and high optical transparency (Nair et al., 2008), that make it a suitable candidate for micro-nano electric and photonic device applications. Many of these applications require graphene to be patterned into ribbons, gaps, or other complex structures. Several methods have been reported for patterning graphene, such as scratching (Lu et al., 2010), dip-pen nanolithography (Hirtz et at., 2013), electron beam irradiation (Withers et al., 2011; Cagliani et al., 2015), self-assembly of block copolymers (Li et al., 2015; Kim et al., 2013), nonlithographic fabrication through template (Tai et al., 2011) and local anodic oxidation lithography (Neubeck et al., 2010; Wei et al., 2010; Weng et al., 2008; Giesbers et al., 2008). Among these methods, local oxidation lithography has been successfully used for fabricating nanodevices based on semiconductors and is also considered as a promising technique for patterning graphene, because no other impurities except for oxygen could be introduced and the edges of graphene can be controlled well by this method. To date, simple nanolines can be fabricated easily by common local oxidation lithography of AFM, which has been used to form insulating trenches in graphene flakes to fabricate nanodevices with subnanometer precision (Weng et al., 2008). However, fabrication of complex patterns such as arrays of nanodots by local oxidation needs to control the AFM scanner to move precisely with customed computer program, which is usually rather time-consuming. So improving local oxidation lithography of AFM is necessary in order to make the fabrication of complex pattern to be easier. Compared to the electrical properties of graphene, the tribological properties of graphene have received less attention. Since tribological issues like adhesion, friction and wear may badly influence the reliability and performance of nanodevices and graphene is expected to play a wider role in nanodevices, it is important to clearly understand its tribological characteristics as well. It has been reported that frictional properties of graphene are affected by the number of graphene layers and the substrate (Li et al., 2010). Friction of graphene monotonically increased as the number of layers decreased due to the puckering effect (Lee et al., 2010) and the interplay of surface attractive forces (Lee et al., 2009). The thickness dependence can be suppressed if the graphene is strongly bound to the substrate. Moreover, friction measurements have been also used to characterize variable reduction of graphene oxide (GO) achieved by an AFM tip-based thermochemical nanolithography method (Marsden et al., 2013). The surface friction is reduced as the GO is replaced with graphene by thermal reduction and the border between the graphene and graphene oxide can be clearly distinguished by friction measurements. So distinguishing between graphene and graphene oxide by friction measurements will be helpful to control the edges of nanopatterned graphene based on the local oxidation lithography and improve the local oxidation process of graphene.
In this paper, local oxidation lithography based on an atomic force microscopy (AFM) combining with an external sourcemeter is used to pattern graphene grown by chemical vapor deposition, obtaining regular nanogrids on the graphene surface. Moreover, we have measured the friction and adhesion properties of graphene with nanogrids by AFM. 2. Experimental
Graphene was fabricated by chemical vapor deposition (CVD) onto copper foil and transferred to SiO2/Si wafers. In the transferring process, a PMMA solution was spin-coated onto the top side of the as-prepared graphene sample. The Cu under graphene film could be dissolved after being dipped into an iron chloride solution and then the graphene-coated PMMA was washed with DI water in order to get rid of the remaining metal flakes. The resulting PMMA/graphene layer was placed onto the SiO2/Si substrate and finally, the PMMA was removed by acetone. Characterization of CVD-grown graphene was performed by Raman spectroscopy and AFM. The Raman spectra were recorded with a DXR micro-Raman spectrometer (Thermo Fisher Scientific) with an incident laser beam wavelength of 532 nm. The AFM measurements were carried out with a MFP-3D AFM (Asylum Research) in contact mode using Pt-coated silicon cantilever (AC240 TM, Olympus) under ambient conditions. Nanopatterning of graphene was performed in air by AFM combining with a Keithley sourcemeter and the relative humidity was about 70%. Two edges of graphene films were respectively connected through Au contacts to two output ports of sourcemeter. When a voltage was outputted through the sourcemeter, the current limited to 125 μA flowed from one edge of graphene films to the other one; at the same time the AFM probe scanned on the graphene with contact mode and regular nanogrids could be produced immediately. Topography, friction and adhesive properties of nanogrids were characterized by AFM. The torsional force constant of AFM cantilever was not calibrated and the friction forces were represented by voltage signals since the friction forces in newton are proportional to the torsional force constant of cantilever and the voltage signal. 3. Results and discussion
3.1 Characterization of CVD-grown graphene Figure1 shows the Raman spectra of graphene on the SiO2/Si substrate. One can see that there are not any peaks observed on the Raman spectra of the SiO2 substrate in the range of 1000 ~3000 cm-1 , while for graphene a sharp and strong peak can be easily observed around 1581 cm-1, commonly called G band, corresponding to in-plane carbon-atom stretching vibrations.
The 2D band occurs at around 2690 cm-1, with a lower intensity and wider peak compared with the G band, which indicates that the sample consists of more than one atomic layer of graphene. Moreover, the D band, which is related to the defects and disorders in graphene, appears around 1350 cm-1, indicating the existence of some defects in the graphene sample (Graf et al., 2007).
The AFM images of SiO2/Si substrate and graphene surface are shown in figure 2. The surface roughness of SiO2/Si substrate is measured to be 0.8 nm while the roughness of surface coating with graphene is about 0.2 nm. The folds and wrinkles are clearly visible in graphene and the gap in the topography image refers to the substrate. Figure 2(c) shows crosssection profiles along the dotted line across the graphene and substrate in figure 2(b). The thickness of graphene, i.e. the height of step between the graphene and the substrate, is measured to be 0.4~1.1 nm through the cross-section profiles. Taking into account the Raman spectrum of graphene and the thickness of single layer, the number of graphene can be determined to be 2~4. 3.2 Fabrication of nanogrids on the graphene Figure 3(a) and (b) respectively show the AFM topography of graphene before and after a voltage of 0.15 V is applied to the graphene through the sourcemeter. It is observed that regular anogrids are produced when the voltage is applied to the two edges of graphene during the AFM scanning on the graphene surface with the contact mode. 3D topography image of nanogrids, as shown in figure3(c), indicates that the borders of nanogrids are protrusive. According to the cross-section profile of figure 3(d) along the red line in figure 3(b), it is obtained that the width and height of nanogrid borders are about 90 nm and 1 nm, respectively. Since the experiment was performed in ambient air, it can be inferred that the protrusive borders result from local oxidation of graphene induced by AFM tip under the electric field. When the AFM tip approaches the graphene surface, water vapor in air condenses to form a liquid bridge between the tip and the graphene surface. Furthermore, if a voltage is applied to the graphene and the AFM tip is grounded through the system, a high electric field is induced between the AFM tip and graphene surface, and this electric field can induce local electrochemical oxidation at the graphene surface/water interfaces, which causes a transition of the local graphene to graphene oxide (Masubuchi et al., 2011). When graphene is oxidized to graphene oxide, sp2 carbon bonds of graphene convert into sp3 carbon bonds that can ripple the carbon skeleton, thereby increasing the local sheet thickness (Wei et al., 2010) and resulting in the protrusive nanogrid borders on the graphene surface. Figure 3(b) and (d) also show nanogrids with a remarkable periodicity. The periodicity of nanogrids is related to the applied voltage, which can be confirmed by force-distance curve measurements. Figure 4 (a) and (b) show force-distance curves with and without the applied voltage, respectively. The horizontal part of curve keeps stable and no distance dependent forces act on the AFM probe when no voltage is applied. While in the case of applied voltage, the horizontal part of curve shows periodic fluctuations, indicating a periodic electrostatic force introduced by the applied electric field affects the cantilever. The acting distance of electrostatic force is measured to be about 26.4 nm and the scanning speed of force-distance curve is 1984 nm/s, so the acting duration of electrostatic force is determined to be 13.3 ms. When nanogrids were formed, the scanning speed was 6565 nm/s. In this case, the acting
distance of voltage in single duration is 87 nm, which is consistent with the width of nanogrid border. The results indicate that regular nanogrids are caused by the periodic voltage applied to the graphene and also suggest that different patterns may be fabricated by changing the duration of applied voltage. In experiments, we also notice that the oxidation of graphene occurs on the initial part of scanning area. Nanogrids just take over about 1/3 of scanning area and no changes on the upper part of scanning area are observed, as shown in figure 3(b). This is likely associated with the local structure of graphene. According to the Raman spectra of graphene, there are some defects existing in the graphene. The defects have important influence on the water molecules adsorption on the graphene surface and electron transport in graphene (Kou et al., 2011; Jiang et al., 2008), which correlates highly with the process of AFM-based anodic oxidation. The defects are rather random in the CVD-grown graphene, which may result in the uneven adsorption of water on the surface of graphene. Moreover, the amount of water decreases as the oxidation progress, also resulting in the end of the oxidation process. So the influence of applied voltage on the different area of graphene is inconsistent even under the same experiment conditions and the quality of graphene is very important to the fabrication of nanostructure on the graphene surface using the local anodic oxidation technique. 3.3 Friction and adhesion properties of nanogrid surface The friction properties can be measured by recording the lateral forces in contact mode AFM. Figure 5 shows the topography and simultaneously acquired friction images. In comparison to the topography, the friction image in figure 5(b) shows a much sharper contrast and clearer borderlines of nanogrids. The friction force on the borders is significantly higher than that on other normal area of graphene. This is because the borders are composed of the graphene oxide, the friction coefficient of which is much higher than graphene (Wei et al., 2010). It can be seen from figure 5(c) that the friction force on the nanogrid borders and other normal area is respectively about 9 mV and 1.3 mV, indicating the friction force increases nearly 6 times after the graphene transits into graphene oxide.
Figure 6(a) and (b) show the topography of graphene with nanogrids and corresponding adhesive force map, respectively. It can be seen that the area with nanogrids is a little darker than that without nanogrids. To compare the adhesive forces of two areas, the adhesive forces of 96 points, respectively obtained at the areas with and without nanogrids, are used to perform the Gaussian statistical analysis and the results are shown in figure 6(c) and (d). It is found that the adhesive forces on the normal area without nanogrids obey Gaussian distribution well. For the area with nanogrids, the adhesive forces show a wider distribution compared to the normal area because there are not only graphene but also graphene oxide existing in this area. A Kolmogorov–Smirnov test (K–S test) has also been performed to compare the two groups of adhesive force and the results show that there is significant difference between them. Mean adhesive force is 11.92±0.44 nN on the normal area and it is slightly reduced to be 11.55±0.58 nN on the area with nanogrids. It is usually believed that
the adhesive force between an AFM tip and a sample surface includes the capillary force, van der Waals force, electrostatic force and chemical bonding force. In ambient air, the capillary force is a main contributor to the adhesion and closely related to surface wettability (Ou et al., 2010). It has been confirmed that the roughness of sample surface has strong influence on its wettability (Kubiak et al., 2011; Nosonovsky et al., 2008; Uelzen et al., 2003). For hydrophilic surface, the wettability increases with the roughness of sample surface. While hydrophobic surface will be more hydrophobic with the increase of surface roughness (Uelzen et al., 2003). According to Kim’s report, the water contact angel of CVD-grown graphene transferred to SiO2 is 93.8°for Cu-grown graphene (Kim et al., 2011), indicating the surface of graphene is hydrophobic. When nanogrids are formed on the graphene surface, the surface roughness changes from 0.24 nm to 0.32 nm, resulting in the decrease of surface wettability and the capillary force, and finally the adhesive force between the AFM tip and sample surface decreases. 4. Conclusions Regular nanogirds have been fabricated on the few-layer CVD-grown graphene by AFM based on local anidoc oxidation. Friction images of graphene with nanogrids show clear a border between original graphene and oxided graphene that involves higher friction. Measurements of adhesive force indicate that the adhesive force for the normal area of graphene is a little larger than that for the area with nanogrids due to the increase of surface roughness.
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Figure 1 Raman spectra of CVD-grown graphene and SiO2/Si substrate.
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Figure 2 AFM topography image of SiO 2/Si substrate (a) and graphene CVD-grown graphene (b). (c) Corresponding cross-section profiles along the dotted line in (b).
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Figure 3 AFM topography images of graphene before (a) and after (b) nanogrids are formed. (c) 3D topography image of graphene with nanogrids. (d) cross-section profile along the red line in (b).
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Figure 4 Force versus distance curves obtained by contact model AFM without voltage applied (a) and with a voltage of 0.15 V applied to the graphene (b).
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Figure 5 AFM topography image (a) and simultaneously obtained friction image (b) of nanogrids on the graphene. (c) cross-section profile along the red dotted line in (b)
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Figure 6 AFM topography image(a) and corresponding adhesion force map (b) of graphene with nanogrids.(c)and (d) statistical analysis histograms of adhesion force on the normal area and
nanogrids area of graphene, respectively.