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Dynamic Sliding Enhancement on Friction and Adhesion of Graphene, Graphene Oxide and Fluorinated Graphene Xingzhong Zeng, Yitian Peng, Mengci Yu, Haojie Lang, Xing'an Cao, and Kun Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19518 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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ACS Applied Materials & Interfaces

Dynamic Sliding Enhancement on Friction and Adhesion of

Graphene,

Graphene

Oxide

and

Fluorinated

Graphene Xingzhong Zeng1, Yitian Peng1,2*, Mengci Yu1, Haojie Lang1, Xing’an Cao1, Kun Zou1,2 1

College of Mechanical engineering, Donghua University, Shanghai 201620, China

2

Engineering Research Center of Advanced Textile Machinery, Donghua University, Ministry of

Education, Shanghai 201620, China

ABSTRACT: Graphene and functionalized graphene are promising candidates as ultrathin solid lubricants to deal with the adhesion and friction in micro and nanoelectromechanical systems (MEMS/NEMS). Here, the dynamic friction and adhesion characteristics of pristine graphene (PG), graphene oxide (GO) and fluorinated graphene (FG) were comparatively studied using atomic force microscopy (AFM). The friction as a function of load shows nonlinear characteristic on GO with strong adhesion and linear characteristic on PG and FG with relatively weak adhesions. An adhesion enhancement phenomenon that the slide-off force after dynamic friction sliding is larger than the pull-off force is observed. The degree of adhesion enhancement increases with the increasing surface energy, accompanied by a corresponding increase in transient friction strengthening effect. The dynamic adhesion and friction enhancements are attributed to the coupling of dynamic tip sliding and surface hydrophilic properties. The atomic-scale stick-slip behaviors confirm the interfacial interaction is enhanced during dynamic sliding, and the enhancing degree depends on the surface hydrophilic properties. These findings demonstrate the adhesive strength between the contact surfaces can be enhanced in the dynamic friction process, which needs careful attention in the interface design of MEMS/NEMS. KEYWORDS: adhesion; friction; atomic force microscopy (AFM); graphene; graphene oxide (GO); fluorinated graphene (FG)

1

*Corresponding author. Tel.: Fax: +86 21 67874297. E-mail address: [email protected] (Y.T. Peng).

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1. INTRODUCTION Due to the surface effect at the micro and nano scales, the surface forces including adhesion and friction play an important role in micro and nano electromechanical system (MEMS/NEMS), which could cause severe adhesion failure, friction problems and limit the further miniaturization of MEMS/NEMS1-3. Also, the consequences of adhesion at the nanoscale could change the frictional performance and cause undesirable effects, such as inducing stiction, reducing reliability and accelerating wear4, 5. Graphene has attracted tremendous attention to be an effective ultrathin solid lubricant because of its remarkable mechanical and tribological properties6-9. The functionalized graphenes including graphene oxide (GO) and fluorinated graphene (FG) may also be good candidates as solid lubricants to MEMS/NEMS due to their layer structure and similar lubrication properties derived from graphite10-13. Controlling the interfacial friction and adhesion is important for the application of functionalized graphenes in MEMS/NEMS, which inevitably requires a better understanding of their friction and adhesion characteristics. The friction of graphene with several layers was dependent on the thickness, and it increased monotonically with the decrease of the thickness14, 15. The thickness-dependent puckering effect14 and evolution of interfacial contact quality between the tip and graphene15 were the two main mechanisms for the thickness dependence of friction. The substrate underlying the graphene also had a great effect on the friction of graphene16-20. The intimate contact between the graphene and substrate decreased the friction and suppressed the thickness dependence of friction18, while the soft substrate increased the friction of graphene and enhanced the thickness dependence of friction19. Controlling the surface properties and structure of graphene could adjust the adhesion and friction 2


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characteristics21. Both the friction and adhesion of graphene increased with the increase of the surface hydrophilicity or surface energy21. The functionalized graphene demonstrated different friction and adhesion behaviors22-24. The friction of GO did not have a thickness dependence due to its strong surface hydrophilicity22. FG had higher nanoscale friction than graphene under UHV atomic force microscopy (AFM) measurements, which was attributed to the fluorination induced increase in out-of-plane stiffness23 or in interfacial potential corrugation24. Adhesion was considered as an important factor to affect the nanoscale friction behaviors. The friction generally increased as the interfacial adhesion increased25, 26. When the adhesion between the contact surfaces was relatively weak, the nanoscale friction increased linearly with the load26, 27. But the strong adhesion between the contact surfaces changed the dependence of friction on load from linear to nonlinear27, and even made the friction independent of load28. Meanwhile, the sufficiently strong adhesion could lead to some distinctive frictional phenomena, such as the negative friction coefficient29 and the significant friction hysteresis for loading versus unloading directions30. In contrary, the dynamic friction process between the contact surfaces could also affect their adhesion. For atomically thin two-dimensional materials, the tip sliding process could significantly improve the interfacial contact quality and contact area14, 15, which led to an increase in the interfacial interaction and then the adhesion31. Therefore, it is indispensable and meaningful to further investigate the complicate relation of adhesion and friction. The adhesion is generally a combination of capillary force, van der Waals force, electrostatic force and force due to the chemical bonds32. At ambient conditions with relatively high humidity,

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the surface adhesion is mainly dominated by the capillary force due to the capillary condensation and adsorption of thin water films on surfaces with relatively high surface energy32,

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. The

hydrogen bonds formed between the contact surfaces could also improve the adhesion34. While for two-dimensional materials, the surface adhesion can also be affected by interfacial contact quality and contact area18. It was found that the adhesion of graphene decreased with the increase of the bonding strength between graphene and underlying substrate18. The increased bonding strength reduced the dynamic flexibility of graphene, which decreased the interfacial contact quality and contact area between the tip and graphene, and thus the adhesion decreased. Generally, the adhesion between the AFM tip and material surface was measured by the conventional contact and separation measurement32, 35, in which the pull-off force was determined by recording a force-distance curve. However, the adhesion between the contact surfaces in the dynamic friction process cannot be characterized by the conventional pull-off force measurement with static characteristic32. In some cases, the adhesion between the AFM tip and surface in the dynamic sliding process was characterized from the friction versus load, which the load corresponding to the separation point of the tip from surface was regarded as the dynamic adhesion36. But the difference of adhesions obtained from the conventional pull-off force measurement and the dynamic tip sliding process has not been explored. Here, the nanoscale friction and adhesion characteristics of pristine graphene (PG), graphene oxide (GO) and fluorinated graphene (FG) were comparatively studied using AFM. The relationship between friction and load was found to be nonlinear on GO and linear on PG and FG. An adhesion enhancement phenomenon that the slide-off force was larger than the pull-off force was observed. 4


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Plasma treatment was introduced to tune the surface hydrophilic properties of PG. The degree of adhesion enhancement increased with the increase of surface energy, accompanied by a strong friction strengthening effect. The adhesion and friction enhancements were attributed to the coupling of dynamic tip sliding and surface hydrophilic properties from the atomic-scale stick-slip behaviors.

2. EXPERIMENTAL METHODS PG was detached from the highly oriented pyrolytic graphite (HOPG) and then transferred to a n-doped Si substrate covered with 300 nm-thick SiO2 layer generated by dry oxidation via the mechanical exfoliation method37. Before transfer, the SiO2/Si substrates were sonicated in acetone, ethanol and deionized water successively for 10 minutes, and then dried with nitrogen. GO was synthesized according to our previous report9. The obtained GO was exfoliated by ultrasonic treating in deionized (DI) water for 20 min to form a stable GO suspension, and then dripped on the SiO2/Si substrate and dried with nitrogen. FG powder (CF2, Nanjing XFNANO Materials TECH Co., Ltd, China) was exfoliated in ethanol solution for 20 min to form a stable FG suspension, and then dripped on the SiO2/Si substrate and dried with nitrogen. The identification and structural features of PG, GO and FG were characterized by Raman spectroscopy (inVia Reflex) with a 533 nm laser wavelength. The chemical structure and elemental composition information of these samples were determined by X-ray photoelectron spectroscopy (XPS, PHI-5000 Versa-Probe) at a pass energy of 29.4eV, using monochromatic Al-Ka irradiation. 5



Their surface hydrophilicity and hydrophobicity were measured by determining the water contact angle (WCA) with a deionized water droplet of volume 3 µl via contact angle meter (OCA15EC). The values were taken the average of three repeat measurements, and the error was below 3°. Direct voltage glow-discharge plasma treatment (PDC-32G-2, Harrick Plasma, USA) was applied to tune the surface hydrophilicity of PG. The power for plasma treatment was set to 6.5 W and the treatment time was set to 1 s and 2 s, considering these parameters only changed the surface hydrophilicity but not the structure21. The location and thickness of PG, GO and FG were determined by optical microscopy (Motic/PSM1000) and AFM (MFP-3D, Asylum Research). The corresponding topographies were obtained by AFM in tapping mode using silicon probes with a nominal normal spring constant of 0.2 N m-1 and tip radius of about 8 nm (PPP-LFMR, Nanosensors). Note that in AFM imaging, relatively high free amplitude and low setpoint were chosen to make the measurements in the repulsive regime, thus to reduce the uncertainty of thickness determination. The surface roughness (Ra) measurement was performed on the AFM topographies with the area of 400 nm × 400 nm. The quantitative force was obtained by calibrating the normal and lateral forces of AFM tips via the noncontact method38. PG, GO and FG with the same layers (three layers) were selected to perform the friction and adhesion measurements. The friction versus load was measured in the area of 400 nm × 400 nm with the scanning velocity of 2 µm s-1. The friction under each load was obtained by the friction loop, which corresponds to a complete trace and retrace scan over the same line, and the friction equals to the half difference between the lateral forces obtained through trace and retrace scanning. The quantitative friction was based on the average of the values measured 6


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through repeating the friction loop three times. The adhesions were determined from two different ways, one is through the conventional contact and separation measurement without tip sliding (called conventional “pull-off force” measurement) and the other is through the friction versus load in the tip sliding process (proposed as the tip sliding “slide-off force” measurement here). In the tip sliding slide-off force measurement, the load gradually decreased from the maximum load until the tip separated from the surface completely and the load corresponding to the separation point was considered as the slide-off force between the tip and surface. Five different adjacent areas were selected for the adhesion measurement, and ten repeated measurements were performed on each area. The errors of the adhesions were calculated as the standard deviation of ten conventional pull-off force measurements or ten tip sliding slide-off force measurements. The load used for the conventional pull-off force measurement was kept the same with the maximum load used for the tip sliding slide-off force measurement, whose value was set to 20 nN. The velocity of conventional pull-off force measurement was set to 2 µm s-1, which was the same as the velocity in tip sliding slide-off force measurement. All experiments were performed in ambient conditions (20-25ºC and 40-50% R.H).

3. RESULTS AND DISCUSION 3.1 Characterizations of PG, GO and FG Figures 1(a), (b) and (c) show the typical AFM topographic images measured on PG, GO and FG, and the cross-sectional height profiles corresponding to the red solid lines are shown in Figures 7



1(d), (e) and (f), respectively. From the height profile shown in Figure 1(d), the thicknesses of the two steps on PG are about 1.1 nm and 1.5 nm, respectively. The theoretical thickness of a single-layer PG was reported to be about 0.35±0.01 nm39. The measured thickness of single-layer PG on substrate by AFM is usually somewhat larger than the theoretical value due to the instrumental offset that is caused by the uncertainty of AFM and the inhomogeneous surface interactions between the graphene and substrate39, 40. In the repulsive measurement regime, the thickness of a single-layer PG on SiO2/Si substrate appears to be about 0.4-0.5 nm40. Therefore, here the number of layers corresponding to thicknesses of 1.1 nm and 1.5 nm are about 3 layers and 4 layers, respectively. Figure 1(g) demonstrates the Raman spectrum measured on PG, whose characteristic peaks, G peak (1580 cm-1) and 2D peak (2670 cm-1), are obviously detected. Meanwhile, the absence of D peak at 1350 cm-1 denotes the structure of PG is relatively perfect. Five adjacent areas labeled as ‘A1’ to ‘A5’ in the topographic image are used for the following friction and adhesion measurements. According to the height profile in Figure 1(e), the thickness of GO is determined to be about 2.0 nm. It is known that single-layer GO is significantly thicker than single-layer PG, due to the presence of carbon and oxygen covalent bonds. Considering the thickness of single-layer GO is about 0.7 nm, the thickness of 2.0 nm corresponds to 3 layers of GO41, 42. Figure 1(h) shows the Raman spectrum measured on GO, the 2D peak demonstrates a significant change compared with PG, which denotes that its graphitic structure is destroyed by the oxidation process. Also, a prominent defect-activated D peak is detected due to the extensive oxygen-containing functional groups. Similarly, the thickness of FG is about 2.6 nm based on the height profile in Figure 1(f). 8


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This also corresponds to 3 layers of FG when considering the thickness of single-layer FG is about 0.8-1.0 nm43, 44. The measured Raman spectrum on FG is similar to that measured on GO, as shown in Figure 1(i). In addition, according to the intensity ration of D peak and G peak that reveals the degree of defect45, the GO and FG have similar degrees of defect.

Figure 1. AFM topographical images measured on (a) PG, (b) GO and (c) FG. (d-f) Height profiles along the red solid lines in the corresponding topographical images. (g-i) Raman spectra measured on the areas with green asterisks in the corresponding topographical images. ‘A1’-‘A5’ are the areas used for adhesion and friction measurements.

Figures 2(a), (b) and (c) demonstrate the XPS survey spectra of PG, GO and FG, respectively. Two characteristic peaks corresponding to C 1s and O 1s at binding energy of about ∼284 eV and ∼532 eV are observed for all the three surfaces. The O 1s peaks of PG and FG are relatively weak, and they are attributed to the oxygen and water vapor adsorbed during the exposure of the surfaces 9



to air. The GO demonstrates a strong O 1s peak, which means the surface of GO is terminated with a large number of oxygen-containing functional groups and exhibits hydrophilicity. While the FG demonstrates a strong F 1s peak, which indicates the surface of FG is terminated with a great deal of fluorine-containing functional groups and exhibits hydrophobicity. The C 1s spectra of the three surfaces are expanded in Figure 2(d-f). According to the Gaussian fitting results of C 1s, there are plenty of oxygen-containing and fluorine-containing functional groups on the surface of GO and FG, respectively.

Figure 2. XPS survey spectra of (a) PG, (b) GO and (c) FG. Expanded C 1s XPS spectra of (d) PG, (e) GO and (f) FG.

In order to characterize the surface wettability of PG, GO and FG, the measurements of water contact angle (WCA) were performed (see Supporting information). The measured WCA values on GO, PG and FG are about 45±2.8°, 85.1±1.4° and 115±1.2°, respectively. Oxygen-containing functional groups make the GO surface more hydrophilic, reducing the WCA of PG. In contrast, 10


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fluorination of PG increases the surface hydrophobicity with a corresponding increase of the WCA. Therefore, it can be concluded that the surfaces of GO and FG have strong hydrophilic and hydrophobic properties, respectively. According to the characterizations, PG is nearly defect-free, but GO and FG have relatively high degrees of defect. GO has strong hydrophilic property due to the presence of oxygen-containing functional groups, and FG has strong hydrophobic property due to the presence of fluorine-containing functional groups. Three layers of PG, GO and FG will be selected for the friction and adhesion measurements.

3.2 Nanoscale friction and adhesion characteristics of PG, GO and FG Figure 3 demonstrates the friction versus load measured on the PG, GO and FG. The frictions of PG, GO and FG all increase with the increase of the load. And the GO and PG show the largest and the smallest frictions, respectively. In order to clearly compare the friction difference between PG, GO and FG, the friction data after the tip separating from the surface have been omitted here. The omitted locations have been marked by the corresponding dotted lines in Figure 3(a), which indicates the load corresponding to the separation of tip and surface. In a sense, this load can be considered as the adhesion in the dynamic friction process based on the definition of adhesion. Because the determination method of adhesion through friction versus load is different from the conventional pull-off force measurement determined by recording a force-distance curve, it is proposed as the ‘tip sliding slide-off force measurement’ here. And the adhesion determined by this 11



method is called as the ‘slide-off force’. Form Figure 3(a), the slide-off forces of PG, GO and FG are about 12.5 nN, 19.5 nN and 7.5 nN, respectively. Figure 3(b) shows the friction versus load of PG and FG extracted from Figure 3(a). The friction versus load can be linearly fitted, as the green solid lines shown. Meanwhile, the friction of PG is somewhat smaller than that of FG. The linear relationships between friction and load of PG and FG are consistent with other studies23, 46. The difference in friction between PG and FG was also obtained previously23, 24, although the PG in their studies was the chemical vapor deposition (CVD)-grown graphene and their experiments were conducted in ultrahigh vacuum. The FG has strong surface hydrophobicity, but the fluorination induced increase in out-of-plane stiffness23 or increase in interfacial potential corrugation24 could lead to the friction of FG larger than PG. As for the friction versus load on GO shown in Figure 3(c), it demonstrates different frictional behaviors when compared with PG and FG. The friction of GO shows a power function relation with the load, as the power fitting curve shown. The fitted power function is, f=0.996(L+18.817)0.56, where f is the friction and L is the load. The nonlinear relationship between friction and load could be attributed to the adhesion effect, when considering the adhesion is an important factor affecting the relationship between friction and load27. It was indicated that the relationship between friction and load turned into nonlinear when the adhesion was relatively strong. Meanwhile, the power exponent of 0.56 is very close to the power exponent of 2/3, as reported in the literature27. The different friction behaviors on PG, GO and FG indicate the interfacial interaction between the tip and the corresponding surface is also different.

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Figure 3. Friction versus load. (a) Comparison of PG, FG and GO. (b) Comparison of PG and FG. (c) GO. The green lines are the linear fitting lines and the olive curve is the power fitting curve. The friction data after the tip separating from the surface have been omitted.

From the characterization results, the surface properties of PG, GO and FG are different. The adhesion can be significantly affected by the surface properties. The friction behaviors on PG, GO and FG indicate the friction and adhesion are closely related in the dynamic sliding process. Therefore, the relation between friction and adhesion are further studied by measuring the adhesions on PG, GO and FG with different surface properties through two aspects. One is the conventional pull-off force measurement by recording a force-distance curve, and the other is the tip sliding slide-off force measurement proposed above by measuring the friction versus load in the dynamic tip sliding process. Figures 4(a-d), (e-h) and (i-l) show the measured adhesions on PG, GO and FG, respectively. Five different adjacent areas are selected for the adhesion measurement, and ten repeated measurements are performed on each area. Figures 4(a, e, i) and (c, g, k) show the pull-off forces measured by the conventional pull-off force measurement before and after the tip sliding slide-off force measurements, respectively. Figure 4(b, f, j) shows the plot of friction versus load in the dynamic tip sliding process, which is used to determine the slide-off force (as the pink dotted line 13



shown). The measured average pull-off force and slide-off force are compared in Figure 4(d, h, l). For the case of PG, the average pull-off force is about 10.5 nN before the tip sliding slide-off force measurement (Figure 4(a)). However, the tip does not separate from the PG surface at the negative load of -10.5 nN in the sliding friction process. The average slide-off force is about 12.5 nN, which is larger than the pull-off force (Figure 4(b)). Here the slide-off force has increased by 19.0% relative to the pull-off force. Meanwhile, the measured average pull-off force after the tip sliding measurement is about 10.6 nN (Figure 4(c)), which is nearly the same with that before the tip sliding measurement. The consistent pull-off forces measured before and after the tip sliding slide-off force measurements indicate that the tip sliding does not affect the pull-off force but the slide-off force. Here the relation that the slide-off force is larger than the pull-off force indicates an adhesion enhancement phenomenon. And it also indicates that the friction process can in turn affect the interfacial adhesion. The adhesion enhancement phenomenon has been observed on PG with different thicknesses and under different experimental parameters. It can be concluded that the adhesion between the tip and PG surface in the friction process is always larger than that in the conventional pull-off force measurement. This means the adhesive strength between the contact surfaces can be enhanced by the relative sliding in the friction process. For the case of GO, the average pull-off force and slide-off force are about 16.4 nN (Figures 4(e)) and 20.1 nN (Figures 4(f)), respectively, which are much larger than those measured on PG. And the measured average pull-off force after the tip sliding measurement is nearly unchanged, whose value is about 16.6 nN (Figure 4(g)). The slide-off force on GO has increased by 23%

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relative to the pull-off force, which is also larger than that on PG. The much larger slide-off force relative to pull-off force further indicates the adhesion enhancement phenomenon, which the relative sliding in the friction process enhances the adhesion between the contact surfaces. The larger pull-off force on GO relative to that on PG can be attributed to the enhanced surface hydrophilicity and surface energy of GO21. The increased difference between slide-off force and pull-off force in GO indicates the surface properties play an important role in the adhesion enhancement. The average pull-off force and slide-off force measured on FG are about 8.2 nN (Figures 4(i)) and 8.1 nN (Figures 4(j)), respectively, which are both smaller than those measured on PG and GO. Meanwhile, the pull-off forces are also nearly unchanged before and after the slide-off force measurements (Figures 4(k)). However, unlike the PG and GO, the pull-off force and slide-off force measured on FG are almost the same (Figures 4(l)). The significantly reduced adhesion and adhesion enhancement result from the strong surface hydrophobicity of FG, which will be discussed later. It is suggested that making graphene super hydrophobic can decrease the effect of the friction process on the adhesion between the contact surfaces. In addition, although the friction of FG is larger than PG, the measured adhesions including pull-off force and slide-off force are smaller than PG. This is because the adhesion is mainly dominated by the surface properties, but the friction is also related to many other factors, such as the out-of-plane stiffness and the structure characteristic23, 24.

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Figure 4. Adhesions measured on (a-d) PG, (e-h) GO and (i-l) FG. (a, e, i) Pull-off force measured before the tip sliding slide-off force measurement, (b, f, j) slide-off force measured in the tip sliding process, (c, g, k) pull-off force measured after the tip sliding slide-off force measurement and (d, h, l) comparison of pull-off force and slide-off force. The pink dotted lines in (b, f, j) point out the average slide-off forces.

Above researches indicate an adhesion enhancement phenomenon that the adhesion between the contact surfaces is enhanced in the friction process. The adhesion enhancement is closely related to the surface properties, which it is the largest on GO with strong surface hydrophilicity and the smallest on FG with strong hydrophobicity. Thus it can be concluded that the adhesion enhancement increases with increase of the surface energy. In order to further confirm the dependence of adhesion enhancement on the surface energy, plasma treatment is introduced to tune the surface hydrophilicity of PG. The plasma treatment time here is set as 1 s and 2 s to only change the surface hydrophilicity but not the structure of PG21. Figure 5 shows the measured pull-off force and slide-off force on the PG before and after the

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plasma treatment. It can be seen from Figures 5(a) and (b) that the pull-off force and friction both increase gradually as the treatment time increases from 0 s to 2 s, which is consistent with our previous study21. It showed that the adhesion and friction increased with the increase of the plasma treatment time21. The linear relationship between friction and load changes into nonlinear as the treatment time increases. Meanwhile, the slide-off force also increases with the increase of the treatment time, as the dotted lines shown. Figure 5(c) concludes the quantitative slide-off force versus treatment time. The slide-off force is somewhat larger than the pull-off force and it also increases with the increase of the treatment time. The difference between the slide-off force and pull-off force, namely the adhesion enhancement, is concluded in Figure 5(d). The degree of adhesion enhancement increases from 19.0% to 20.4% with the treatment time increases from 0 s to 2 s. Based on our previous study21, the surface energy of PG increases with the increase of the plasma treatment time, which leads to the adhesion and friction between the tip and surface have the similar increasing trend. Here, not only the pull-off force and slide-off force increase with the increase of the treatment time, but also the degree of adhesion enhancement. Therefore, it can be concluded that the adhesion enhancement in the friction process is closely related to the surface properties, whose degree increases with the increase of the surface energy.

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Figure 5. Adhesions measured on the plasma treated PG. (a) Pull-off force versus plasma treatment time, (b) friction versus load under different time of plasma treatment, (c) slide-off force versus treatment time and (d) degree of adhesion enhancement versus treatment time.

3.3 The mechanism for the adhesion enhancement An adhesion enhancement phenomenon is observed on PG, GO and FG, and the degree of adhesion enhancement increases with the increasing surface energy. The phenomenon that tip sliding enhancing adhesion was also observed in the previous research of Carpick et al.31. It was demonstrated that the tip sliding was able to induce the local delamination of top graphene layer, and change the interfacial geometry of graphene31. They thought exposing the graphene to air during the process of the sample preparation and the steps required to identify graphene can result in aging of the top graphene layer, leading to an enhanced tip-graphene interaction that exceeds the graphene-graphene interlayer interaction or graphene-substrate interfacial interaction. But the 19



delamination was only a possible conjecture. The adhesion measurement in the tip sliding process in our case is different from that in Carpick et al. In their case, the tip was continuously pre-slid on the graphene surface first, and then the conventional pull-off force measurements were performed for the first moment after the tip stopped sliding on the surface. The way of adhesion measurements that conventional contact and separation did not change before and after the tip sliding. While in our case, the measuring principle of slide-off force is significantly different. The load is gradually reduced until the tip separates from the surface during the tip sliding friction process, and the negative load corresponding to the separating point is considered as the slide-off force. Meanwhile, according to the research of Cannara et al., the delamination of the topmost atomic layers could result in an effective negative friction coefficient that the friction increased with the decrease of load29. However, here the measured friction always decreases with the decrease of load as shown in Figure 3, the negative coefficient of friction never appears. More importantly, the pull-off forces measured by conventional contact and separation before and after the tip sliding are almost identical on the three surfaces. In addition, considering the stronger GO interlayer interaction and the weaker tip-FG interaction, the delamination of the topmost layer in GO and FG cannot be so significantly like that in Carpick et al. and Cannara et al. Therefore, it can be concluded that the local delamination of top atomic layer is not the main reason for the adhesion enhancement here. The surface roughness (Ra) measured on the three surfaces are in the same range of about 0.1-0.15 nm, which indicates the effect of surface roughness can be ignored. The number of layers

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of PG, GO and FG are all three layers. The applied maximum loads and velocities for the pull-off force and slide-off force measurements are the same, which eliminates the influences of load and velocity. The most striking divergence of PG, GO and FG lies in their surface properties or surface energies, according to the XPS and WCA characterizations. Meanwhile, the adhesion enhancement on plasma treated PG further indicates the surface property is the dominant factor. The WCA can not only be used to characterize the surface properties but also to reflect the surface energy18, 47. Generally, the surface energy increases with the decrease of WCA. Based on the Young equation48, γSV =γLV cos θ +γSL , it can evaluate the surface energy. Where γSV , γLV , γSL represent the solid surface free energy, liquid surface free energy and solid-liquid interfacial energy, respectively. θ is the WCA between the solid surface and liquid. However, it is difficult to quantitatively assess the surface energies of PG, GO and FG, because the solid-liquid interfacial energy (γSL ) varies with the solid surface. Therefore, the work of adhesion (Wa)49, Wa =γSV +γLV − γSL = γLV (1 + cosθ) is calculated here, which eliminates the effect of γSL .and can be used to characterize the interaction strength between the tip and surface. Supposing the value of γLV =72.7 mJ m-21 and combing the measured WCA, the calculated work of adhesion on PG, GO and FG are about 79.0 mJ m-2, 124.1 mJ m-2 and 42.0 mJ m-2, respectively. The relative magnitude of work of adhesion on PG, GO and FG is consistent with above measured degree of adhesion enhancement. Figures 6(a) and (b) conclude the measured degree of adhesion enhancement on PG, GO and FG and the calculated work of adhesion. Note that the degree of adhesion enhancement on FG is

21



nearly zero. The degree of adhesion enhancement demonstrates a similar changing trend with the work of adhesion from PG to FG. It indicates that the interfacial interaction can be increased more significantly when the tip sliding on the hydrophilic GO compared with the hydrophobic FG. Also, the work of adhesion between the tip and the plasma treated PG is also calculated by referring the related WCA in our previous study21, which is concluded in Figure 6(c). The work of adhesion and degree of adhesion enhancement (Figure 5(d)) both increase with the increase of the treatment time, namely they increase with the increasing surface energy.

Figure 6. (a) Degree of adhesion enhancement and (b) work of adhesion on PG, GO and FG. (c) Work of adhesion on plasma treated PG.

Based on the above analysis and characterization, the mechanism for the adhesion enhancement is proposed. The difference of slide-off force and pull-off force on PG, GO and FG could attribute to the coupling of tip sliding and surface properties. The tip sliding on graphene can lead to the adhesive puckering deformation of graphene14 and improve the interfacial contact quality of tip and graphene15 due to the flexibility of graphene. Thus the contact area and interfacial interaction increase significantly, which leads to the tip separate from the graphene difficultly. While in the sudden contact and separation pull-off force measurement, only a few tip atoms come into interacting with the graphene in a relatively short time, which results in a small contact area 22


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and poor interfacial contact quality. Therefore, the slide-off force is larger than the pull-off force, causing the adhesion enhancement. The tip sliding induced puckering deformation and improved interfacial contact quality are also dependent on the surface properties. The graphene surface with high surface energy could further increase the puckering deformation and interfacial contact quality in the tip sliding friction process, which results in a more significant adhesion enhancement. Also, chemical sorption as a result of the increasing surface energy brings in a water molecule layer32, which further increases the adhesion enhancement. While the surface with very low surface energy does not have such effects, thus the adhesion enhancement reduces. Figure 7 demonstrates the schematic diagrams of pull-off force and slide-off force measurements on GO and FG, as well as the contact geometries and interfacial interactions. According to our previous study32, for GO with strong surface hydrophilicity in ambient conditions, the pull-off force is mainly composed of capillary force and van der Waals force, as shown in Figure 7(b). The water molecular layer adsorbed on GO will facilitate the growth of water bridge between the tip and GO surface50, which aggravates the effect of capillary force. Thus it can infer that the capillary force is larger than the van der Waals force here. While in the tip sliding slide-off force measurement, the tip sliding can increase the puckering deformation, which increases the interfacial contact area. Then the capillary force and van der Waals force will be enlarged due to the increased interfacial contact area. In addition, the accumulation of the oxygen-containing functional groups on GO could reinforce the hydrogen bonds interactions between Si tip and GO in the long time tip sliding process47, due to the presence of hydroxyl groups on the surface of Si tip covered thin layer of native oxide. Therefore, the interfacial interactions between tip and GO in the slide-off force 23



measurement include not only the enhanced capillary force and van der Waals force, but also the hydrogen bonds interaction, as shown in Figure 7(c). Thus the tip is more difficult to separate from the GO surface in the tip sliding slide-off force measurement than the sudden pull-off force measurement, and then the slide-off force is much larger than the pull-off force, resulting in a significant adhesion enhancement.

Figure 7. Schematic diagrams of pull-off force and slide-off force measurements on (a-c) GO and (d-f) FG. The contact geometries and interfacial interactions correspond to (b, e) conventional pull-force measurement and (c, f) tip sliding slide-off force measurement.

The capillary force and hydrogen bond interaction can be greatly eliminated in the interface of 24


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tip and FG due to the strong hydrophobicity of FG. The van der Waals interaction becomes the dominant factor in both the conventional pull-off force measurement and tip sliding slide-off force measurement, as shown in Figure 7(e) and 7(f). Meanwhile, the significantly decreased interfacial interaction impedes the puckering deformation of FG, and thus the interfacial contact area in the slide-off force measurement is similar to that in the pull-off force measurement, which makes the interfacial interactions nearly the same in the two cases. Therefore, the adhesion enhancement is nearly disappeared. The interfacial interaction between the tip and the corresponding surface can be reflected in the atomic-scale stick-slip behaviors14, 15, 18, 51. For a few layers of graphene, there is a transient friction strengthening effect that the friction increases with the increasing scan distance in the initial a few atomic periods, and the degree of friction strengthening is proportional to the interfacial interaction14, 15, 51. Figures 8(a) and (b) show the lateral force versus scan distance measured on PG with the thicknesses of 1.1 nm and 1.5 nm, respectively. The transient friction strengthening effect is clearly observed in the two cases, as the dotted lines marked. In order to clearly compare the degree of friction strengthening effect, the lateral force peak value in each stick-slip period is extracted out based on the method in our previous studies51, as shown in Figure 8(e). By linearly fitting the extracted peak values, the corresponding slope can be obtained, which represents the degree of friction strengthening. The slopes obtained on PG with the thicknesses of 1.1 nm and 1.5 nm are about 0.394 and 0.292, respectively, which means the degree of friction strengthening decreases slightly with the increase of the thickness, consistent with other studies14, 15. The transient friction strengthening originates from the puckering deformation of PG14 and evolution of the 25



interfacial contact quality15. When the tip begins to slide on the PG surface, the puckering deformation and interfacial contact quality gradually increase, thus increasing the interfacial interaction. The atomic-scale friction strengthening effect denotes the tip sliding can increase the interfacial interaction between the tip and PG, as reported in other studies31. However, the sudden contact and separation in the conventional pull-off force measurement cannot cause the interfacial interaction to increase, because there is no tip sliding. The enhanced interaction can hold the tip on PG strongly, which larger force is needed to separate the tip from the surface than that in the conventional pull-off force measurement. Figure 8(c) shows the atomic-scale stick-slip behaviors of the tip on GO. It displays a significant static friction, as the green solid lines indicated, which is not observed on PG. The great surface disorder of GO makes the stick-slip movement less obvious52, but the friction strengthening effect on GO is still obvious, as the purple dotted lines marked. The extracted lateral force peak values and the corresponding linear fitting line are concluded in Figure 8(f). The slope of the linear fitting line (1.428) is much larger than that obtained on PG, which means the degree of friction strengthening on GO is much greater than that on PG. The relatively significant static friction and the seriously increased degree of friction strengthening indicate that the interfacial interaction between the tip and GO is much larger than that between the tip and PG and it can be significantly increased in the dynamic tip sliding process. While for FG, neither the transient friction strengthening effect nor the static friction occur on FG, as shown in Figure 8(d). It means the tip sliding barely enhances the interfacial interaction between the tip and FG. Thus the pull-off force and slide-off force will be nearly the same, in which the adhesion enhancement disappears. 26


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Figure 8. Lateral force versus scan distance showing the atomic-scale stick-slip behaviors measured on (a, b) PG, (c) FG and (d) GO. (e, f) Lateral force peak values extracted from the lateral force trace curves. The thicknesses of PG are: (a) 1.1 nm and (b) 1.5 nm, respectively. The applied load is about 5 nN.

The effect of adhesion on friction is mainly to change the dependence of friction on load. As the interfacial adhesion increases, the friction behavior transforms from linear to nonlinear and finally the friction coefficient can even become negative. The dynamic friction process between the 27



contact surfaces can increase the interfacial interaction and then the adhesion. But the effect of friction process on adhesion is also dependent on the surface properties. The adhesion enhancement of graphene increases with the increase of surface energy. The friction strengthening effect shows an synchronous increase with the adhesion enhancement. It is essential to consider the adhesion between the contact surfaces could be enhanced in the relative sliding process, which may cause sever dynamic adhesion failure and friction problems.

4. CONCLUSIONS The friction and adhesion characteristics of PG, GO and FG were comparatively studied using AFM. The friction as a function of load shows nonlinear characteristics on GO with strong adhesion and linear characteristics on PG and FG with relatively weak adhesions. The adhesion enhancement phenomenon that the slide-off force is larger than the pull-off force is observed. Also, the degree of adhesion enhancement increases with the increasing surface energy, which is the largest on hydrophilic GO and is the smallest on hydrophobic FG. The large adhesion enhancement is accompanied by a strong transient friction strengthening effect during the initial atomic periods, originating from the coupling of dynamic tip sliding and surface hydrophilic properties. The atomic-scale stick-slip friction behaviors confirm the interfacial interaction is enhanced by the relative sliding. These findings demonstrate the adhesive strength between the contact surfaces can be enhanced during the dynamic friction process, which needs careful attention in the interface design of MEMS/NEMS. 28


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ASSOCIATED CONTENT Table S1 in the supporting information shows the measured WCA on PG, GO and FG.

AUTHOR INFORMATION The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (grant nos. 51675097, U1632128, 51775105), the Natural Science Foundation of Shanghai (grant no. 17ZR1400700) and the Fundamental Research Funds for the Central Universities (grant nos. 002232017G1-03, CUSF-DH-D-2018086).

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Figure 1. AFM topographical images measured on (a) PG, (b) GO and (c) FG. (d-f) Height profiles along the red solid lines in the corresponding topographical images. (g-i) Raman spectra measured on the areas with green asterisks in the corresponding topographical images. A1-A5 are the areas used for adhesion and friction measurements. 106x70mm (600 x 600 DPI)


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Figure 2. XPS survey spectra of: (a) PG, (b) GO and (c) FG. Expanded C 1s XPS spectra of: (d) PG, (e) GO and (f) FG. 81x41mm (600 x 600 DPI)



Figure 3. Friction versus load. (a) Comparison of PG, FG and GO. (b) Comparison of PG and FG. (c) GO. The green lines are the linear fitting lines and the olive curve is the power fitting curve. The friction data after the tip separating from the surface have been omitted. 44x12mm (600 x 600 DPI)


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90x51mm (600 x 600 DPI)



90x51mm (600 x 600 DPI)


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Figure 4. Adhesions measured on (a-d) PG, (e-h) GO and (i-l) FG. (a, e, i) Pull-off force measured before the tip sliding slide-off force measurement, (b, f, j) slide-off force measured in the tip sliding process, (c, g, k) pull-off force measured after the tip sliding slide-off force measurement and (d, h, l) comparison of pull-off force and slide-off force. The pink dotted lines in (b, f, j) point out the average slide-off forces. 90x51mm (600 x 600 DPI)



Figure 5. Adhesions measured on the plasma treated PG. (a) Pull-off force versus plasma treatment time, (b) friction versus load under different time of plasma treatment, (c) slide-off force versus treatment time and (d) degree of adhesion enhancement versus treatment time. 91x52mm (600 x 600 DPI)


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Figure 6. (a) Degree of adhesion enhancement and (b) work of adhesion on PG, GO and FG. (c) Work of adhesion on plasma treated PG. 39x9mm (600 x 600 DPI)



Figure 7. Schematic diagrams of pull-off force and slide-off force measurements on (a-c) GO and (d-f) FG. The contact geometries and interfacial interactions correspond to (b, e) conventional pull-force measurement and (c, f) tip sliding slide-off force measurement. 129x105mm (300 x 300 DPI)


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Figure 8. Lateral force versus scan distance showing the atomic-scale stick-slip behaviors measured on (a, b) PG, (c) FG and (d) GO. (e, f) Lateral force peak values extracted from the lateral force trace curves. The thicknesses of PG are: (a) 1.1 nm and (b) 1.5 nm, respectively. The applied load is about 5 nN. 171x184mm (300 x 300 DPI)



Graphical abstract 32x6mm (600 x 600 DPI)


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Dynamic Sliding Enhancement on the Friction and ... - ACS Publications

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