Epoxy Systems

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Surfaces, Interfaces, and Applications

Strain Engineering in Highly Wrinkled CVD-Graphene/ Epoxy Systems George Anagnostopoulos, George Paterakis, Ioannis Polyzos, Panagiotis - Nektarios Pappas, Konstantinos Kouroupis-Agalou, Nicola Mirotta, Alessandra Scida, Vincenzo Palermo, John Parthenios, Konstantinos Papagelis, and Costas Galiotis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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ACS Applied Materials & Interfaces

Strain Engineering in Highly Wrinkled CVDGraphene/ Epoxy Systems George Anagnostopoulos1,*, George Paterakis1, Ioannis Polyzos1, Panagiotis-Nektarios Pappas1, Kostantinos Kouroupis-Agalou2, Nicola Mirotta2, Alessandra Scidà2, Vincenzo Palermo2, 3, John Parthenios1, Konstantinos Papagelis1,4 and Costas Galiotis1,5,* 1Institute

of Chemical Engineering Sciences, Foundation for Research and Technology – Hellas (FORTH/ ICE-HT), Patras 265 04, Greece

2

ISOF - Istituto per la Sintesi Organica e la Fotoreattivita—Consiglio Nazionale delle Ricerche, via Gobetti 101 - 40129 Bologna, Italy 3Department

of Industrial and Materials Science, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden

4Department

of Solid State Physics, School of Physics, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece 5Department

of Chemical Engineering, University of Patras, Patras 26504, Greece

Keywords: CVD graphene; epoxy resin; Raman spectroscopy; wrinkles; electrical resistance

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ABSTRACT Chemical vapor deposition (CVD) is regarded as a promising fabrication method for the automated, large-scale, production of graphene and other 2D materials.

However, its full

commercial exploitation is limited by the presence of structural imperfections such folds, wrinkles and even cracks that downgrade its physical and mechanical properties. For example, as shown here by means of Raman spectroscopy, the stress transfer from an epoxy matrix to CVD graphene is on average 30% of that of exfoliated monolayer graphene of over 10 μm in dimensions. However, in terms of electrical response, the situation is reversed; the resistance has been found here to decrease by the imposition of mechanical deformation possibly due to the opening up of the structure and the associated increase of electron mobility. This finding paves the way for employing CVD graphene/epoxy composites or coatings as conductive “networks” or bridges in cases for which the conductivity needs to be increased or at least retained when the system is under deformation. The tuning/control of such systems and their operative limitations are discussed here.


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Introduction Graphene is a carbon allotrope that has received worldwide attention due to its unique electrical, mechanical optical and thermal properties 1-3. Because of graphene’s inherent flexibility, it is an ideal material to be used in applications involving flexible substrates, such applications include photonics, optoelectronics, and organic electronics

4-6.

It is evident that in this context, the

fabrication of large-area graphene sheets of high quality as well the transfer of graphene films onto a foreign substrate is an essential process particularly for stretchable electronics based on polymers 7. There are several ways to produce single or a few layer graphenes 8; however chemical vapor deposition (CVD) growth - established as an important method in 2010, where a 30 in. graphene sheet was produced by a Korean group 5 is widely used because it can produce large-area9, high quality graphene at relatively low cost 10. The CVD method yields reliable production of largearea, high-quality graphene films on variety metal substrates11-13. More recently, Yamanda et al. 14

developed a fabrication method that combines plasma CVD at relatively low substrate

temperatures and a roll-to-roll process for graphene mass production, while SONYTM

6

has

reported a CVD synthesis method and a direct roll-to-roll transfer process using photocurable epoxy resin that allows the fabrication of a 100-m-long graphene transparent conductive film. As is now well established, CVD graphene sheets are prone to structural imperfections such as the formation of complex network of wrinkles, ripples and folds15-17. The origin of these defects is attributed to the thermal mismatch between graphene and the metal substrate

18

upon cooling

from high temperatures, due to differences in the thermal expansion coefficients and possibly to the mechanical strain generated by transferring the CVD graphene to suitable substrates 19-21.



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Recently, it has been shown by means of Raman spectroscopy that in a CVD graphene/ PET system used in prototype flexible displays 22, a microstructure is formed consisting of hexagonal array of islands of flat monolayer graphene separated by wrinkled or folded material 23. The size of these graphene “islands” governs the stress transfer process upon tensile loading23-24. Moreover, atomic force microscopy (AFM) has shown that the electrical current drops exactly along the wrinkles, since they seem to be acting as potential barriers for charge carriers leading to carrier scattering and increased contact resistance 25. In this work, a commercial aerospace-grade epoxy resin was selected as a substrate for studying the electromechanical response of simply-supported rectangular specimens of a CVD graphene/epoxy coating. It has been proved over the years that those thermoset materials, used extensively in structural composite applications, offer a unique combination of properties, such as high flexibility, thermal stability, moderate toughness, and good durability under service conditions 26. At the same time, many efforts are focusing on incorporating graphene in polymer matrices, by utilizing it as an alternative sensing material due to its combination of excellent mechanical properties 27, as well as its high electrical conductivity 28. For example, these materials could be used as “self-sensing” composite systems for the monitoring the performance of automotive or aeronautical structures29. In our case, by employing a one-shot method of fabrication, for which the transfer step is not required 23, specimens with a single layer of CVD graphene encaptured on the surface of the epoxy resin were produced. Up to now, the majority of works reported are examining the sensing capabilities of graphene mostly in flexible polymer systems poly(ethylene terephthalate) (PET)

5, 30-32 7, 21.

such as poly(dimethyl siloxane) (PDMS)

16, 33-34,

Even for more rigid substrates such poly(methyl


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methacrylate) (PMMA)

35

or epoxy resin

36,

graphene has been utilized as a coating

31, 37

for

strain sensor applications. Only very recently the effect of cyclic loading has been investigated in graphene rigid polymers 38. Since the sensing response of graphene is directly connected to the interface between graphene and the host polymer matrix 23, we have employed AFM to assess the integrity of the interface upon cyclic loading. The results showed that the initial wrinkling morphology of the CVD graphene layer alters significantly after each loading cycle. In contrast, the electrical resistance showed an initial hysteresis during the 1st cycle, while in the 2nd cycle is drastically reduced. This indicates that the electron mobility is improved upon repeating tensile loading possibly as a result of straightening out of wrinkles and/or folds that contribute to electron scattering prior to loading. Such results may have important implications for the use of CVD graphene coatings as sensors and conductive networks 39 in structural composite applications.

Results and Discussion Evaluating the morphology and the interface integrity of the CVD graphene coating/epoxy resin system We have followed a one-shot fabrication method (see also Experimental and Supporting Information) in which a commercial epoxy resin system is spread on the top of a single layer of graphene on copper foil. After curing, etching and rinsing, the single layer of CVD graphene is encaptured on the surface of the epoxy resin (Figure S1) and therefore its morphology replicates the characteristics of the graphene sheet obtained after cooling to Room Temperature (RT).



Such configurations are depicted in Figure 1a, where by means of a Scanning Electron Micrograph (SEM) the morphology of the CVD graphene encaptured on the surface of the epoxy resin is shown. In Figure 1b the microphotograph corresponds to a region defined by the white rectangle shown in Figure 1a. Regions of well adhered, crumbled and wrinkled graphene are magnified to show clearly the morphology of the as-supported graphene specimen. (a)

(b)

Figure 1: (a) SEM microphotograph of an area of a CVD graphene sheet transferred onto the epoxy resin substrate where cracks (

), holes (○) and well adhered regions (x) are identified.

Features forming parallel lines correspond to terraces, steps and edges that resemble the surface of polycrystalline copper replicated during polymer curing (b) SEM microphotograph of a region defined by the white rectangle shown in (a). Regions of well adhered, crumbled and wrinkled graphene as obtained from cooling from high temperatures are shown. The effect of an applied stress field upon the observed morphological features has been examined by conducting AFM measurements prior and after mechanical loading. As shown in the AFM image of Figure 2a, the topography of the surface of the polycrystalline copper wafer (see also Figure S2b) that was used for the growth of graphene has been fully engraved into the graphene/ polymer system.


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Figure 2: (a) AFM topography of a 5.3 x 5.3 μm2 area on the graphene/ epoxy resin system where terraces and step edges, formed by the polycrystalline copper wafer (see also Figure S2b) that was used for the growth of graphene, are shown and (b) the corresponding peak force error image where the wrinkling network can be clearly distinguished from the texture of the epoxy resin surface. In both images the wrinkles are denoted with colored triangles. Scale bar: 1 μm Since the AFM tip only probes the surface of the graphene/polymer coating it is not easy to distinguish the morphological features of graphene vis-à-vis the morphology of the polymer itself. However, since the polymer was deposited directly on the grown CVD graphene it is assumed that both exhibit identical morphological features (transferred by the copper substrate). The existence of single layer graphene can only be detected by the established wrinkling network with a height of about 3 nm on average as shown in Figure 2a. The corresponding 3D image of the polymer surface texture is given in Figure S2a. Fainted wrinkles in Figure 2a with heights



lower than 3 nm are clearly distinguished in the respective peak force error image shown in Figure 2b. As stated in the literature, due to the thermal mismatch present during growth of CVD graphene on copper substrate, strain energy appears40, which it is greater than the adhesion energy, the graphene sheet can form wrinkles41. The produced wrinkles are created both during growth and transfer processes, and are very hard to eradicate, since graphene prefers to relax in planecompression at the expense of interfacial energy40. Such a behavior is attributed to the delamination and bending energy appeared in wrinkles41. Furthermore, wrinkling in CVD graphene has been investigated in detail, regarding their typical height and distribution due to the influence of thermal mismatch and substrate crystallographic plane.

By means of Raman spectroscopy it has been found that a copper surface with

crystallographic orientation (111) can be employed to obtain a monolayer graphene with low defect density42. Based on the above, the recorded inhomogeneity in graphene conformation onto the polymer surface (epoxy matrix) as resulted from the wrinkling network and revealed in both SEM and AFM images (Figures 1 and 2), is correlated directly with Raman spectroscopic data acquired with a spatial resolution of 1 μm, by analyzing the profiles of G and 2D Raman graphene bands. Relevant Raman spectra data of specimen A taken from a sampling area of 1500 x 500 μm2 derived from 900 data points are given in Figures S4, S5 and Table S1, whereas a detailed statistical analysis is presented in Figures S6. The position of 2D Raman band, Pos(2D), has a mean value of 2610.5  5.2 cm-1 indicating the presence of both compression and/or doping43 (Figure S6a). If we reasonably consider that the


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Pos(2D) shift (~15 cm-1) is due to the imposition of biaxial strain during the production process 40,

the corresponding compressive biaxial strain, using sensitivity values of +148 cm-1/% for

strain 40, is of the order of~-0.10% (Table S2). For this amount of strain, the calculated shift of position of G Raman band, Pos(G), if attributed entirely to biaxial compression should be 6 cm-1 (Table S2), using the corresponding relation for the doubly degenerate in-plane Raman-active G phonon 44. These results indicate that ~40% of the total shift of Pos(G) is mechanically related, while the rest is attributed to unintentional doping. Interestingly, the corresponding statistical analysis introduces two distinct groups of Pos(G) observed in Figure S6c, with wavenumber ranged between 1584-1593 cm-1 (Group A) and 1592-1601 cm-1 (Group B). The obtained bimodal distribution results from the apparent inhomogeneity of the examined system. As for the Raman linewidths, useful interpretations can be extracted regarding doping, strain, disorder and number of layers in graphene45. For CVD synthesized graphene, the corresponding value of full width at high maximum for the 2D Raman band, FWHM(2D), (32.7  6.2 cm-1) at rest is larger compared to monolayer exfoliated flakes (24 cm-1) of doping from the metal substrate

46

46

resulting from the presence

and growth-induced strain, which induces topographic

folding and rippling of the membrane 47. Moreover, the interaction of the graphene membrane with the liquid epoxy resin during curing induces additional strain which gives rise to a FWHM(2D) of 60 cm-1 (Figure S6b)

48.

The

majority of FWHM(2D) values are distributed between 27-39 cm-1 (~94% of the mapping points), while locations with values larger than 45-50 cm-1 (~ 5% of the mapping points) are attributed to thicker graphene (bi-layer or even multilayer islands) 49-52.



Similarly, the full width at high maximum for the G Raman band, FWHM(G), exhibits a mean value of 18.4  7.4 cm-1 (Figure S6d) also higher than that of the exfoliated graphene 43, which implies different levels of doping and strain along the examined area 53-54. Additionally, the ratio of heights for both G and 2D Raman bands, I(2D)/I(G), which can be used as a qualitative indication for doping 54, appears to have relative low value (1.2  0.9) (Figure S6e) compared to low doping value of ~ 3.5 for single layer graphene supported on Si/SiO2 54. Similar statistical analysis has been done for all the examined specimens, while the mechanical integrity and the stress transfer characteristics of specimens B and C are assessed by combination of tensile loading and in situ Raman spectroscopy.

More specifically, the dependence of

Pos(2D) with applied strain is investigated by recording Raman data over an area of 15 x15 μm2 (16 data points) for both specimens (B and C). The 2D peak data correspond to points with an initial FWHM(2D) of the order of ~ 30 cm-1 (Figure S7). The residual compressive strain appeared on the CVD graphene /epoxy resin system for both specimens (B and C) is of the order of 0.10 % (Table S2). As shown in Figure 3a, for Specimen B, Pos(2D) shifts to lower values at a rate of ~-18 cm-1/% up to 1.4% of strain, while Pos(G) redshifts at a rate of -5.5 cm-1/% (Figure 3b). As for specimen C similar trends are also observed (Figure S8a, c).


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(a)



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(b) Figure 3: (a) Pos(2D) and (b) Pos(G) as a function for the applied strain for specimen B. In both studied specimens, the observed shift rates are much lower than those observed in an exfoliated graphene/PMMA system, indicating less effective stress transfer whereas interfacial shear sliding

55

appears at higher strain levels (1.0-1.4%).

procedure for which no transfer step is required

23

This is due to the fabrication

and also to the fact that the single layer of

CVD graphene is fully encaptured on the surface of the epoxy resin thus, leading to a better quality of graphene/ epoxy interface 36. However, due to the inherent steps and wrinkles, presented in Figures 1, 2, the CVD graphene is not actually deformed as it would be if it were a perfect flat exfoliated specimen; thus, by


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comparing these results with those obtained from a perfect monolayer53 it is estimated that for applied strain of 1.0%, the graphene sheet appears to be subjected to an average strain of order of ~ 0.30 %. Furthermore, for specimen B, at 1.40 % external strain the encapsulated CVD graphene is strained by 0.40%, which is the maximum strain value reached, beyond which a strain relaxation is observed (Figure 3a). Similarly, it was found recently23, that during deformation of CVD graphene on polyester substrate, the rate of shift of the Raman 2D peak wavenumber per unit strain was less than 25% of that of flat flakes of mechanically exfoliated graphene. Thus, the conclusion here is that the wrinkled morphology of CVD graphene affects the stress transfer efficiency

23

and therefore

their effective Young’s moduli are only a fraction of the 1 TPa value of the pristine exfoliated graphene. In our case, for which a CVD graphene sheet with folds, wrinkles and cracks (Figure 1) is simply-supported on epoxy resin substrate, strain variations seems to be more significant and spread over a much larger area, leading to a greater value range of FWHM(2D) as compared to the value of unstrained exfoliated monolayer graphene. By implementing external loading, those inhomogeneities are evidently amplified as observed by the increase of the FWHM(2D) shift rates up to 30 cm-1/% (Figure S8c), while in case of exfoliated graphenes the corresponding FWHM shift rates do not exceed the value of 15 cm-1/% 56. As follows, since only a percentage of the applied (external) strain field is imparted to graphene sheet, it was considered essential to evaluate the reliability of the measured strain on the examined system by employing strain gauges and the Stoney’s approximation57-58 assuming that the CVD graphene is consider as a uniform one atom thick film. Since the thickness of graphene



is much smaller than the epoxy matrix substrate, the estimated strain can be extracted as a function of the curvature of the epoxy resin layer. We used the Stoney’s approximation

59

and electrical resistance strain gauges to assess the

validity of the strain transmitted by the 4-point-bend device and the strain of the composite system itself. The results showed that the slopes between the two set of data ranged from 0.89  0.06 (Stoney’s approximation) and 1.14  0.03 (strain gauges) which indicate a reasonable agreement between the various methods employed (Figure S10).

Effect of applied strain upon the stress transfer efficiency of the CVD graphene/ epoxy system

The dependence of Pos(2D) and Pos(G) as a function of the applied strain, for two loadingunloading deformation cycles, is investigated by Raman mapping an area of 15 x15 μm2 (16 data points in total) on the surface of specimen D (Figure 4).


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-1

Pos(2D) (cm )

2625

Specimen D

2620

rst

1 cycle

2615 2610 2605

-1

slope: -18.7 cm /%

2600 2625 -1

Pos(2D) (cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nd

2 cycle

2620 2615 2610 2605 2600

-1

slope: -11.1 cm /%

0.0

0.2

0.4

0.6

0.8

Applied strain (%) (a)


1.0

1.2


Specimen D rst

-1

Pos(G) (cm )

1600

1 cycle

1596 1592 -1

slope: -6.4 cm /%

1588 1600 nd

2 cycle

-1

Pos(G) (cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1596 1592 -1

slope: -5.3 cm /%

1588

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Applied strain (%) (b) Figure 4: (a) Pos(2D) and (b) Pos(G) as a function of the applied strain for two deformation cycles implemented on the CVD graphene/epoxy resin system (Specimen D) for a 785 nm laser excitation. Each data point is the average of the 16 mapping points. The black circles and the red squares correspond to loading and unloading procedure, respectively. Also, the red dot lines are least-squares line fits, while the pink solid lines act as guides to the eye. During loading (Figure 4a) (1rst cycle), Pos(2D) shifts to lower values at a rate of -18.7 cm-1/% up to 1.0% of applied strain, while the FWHM(2D) increases almost linearly with strain at a rate of 16.5 cm-1/% (Figure S9a). Similarly, the Pos(G) red-shifts at a rate of -6.4 cm-1/% (Figure 4b) and its FWHM(G) increases almost linearly at a rate of 7.0 cm-1/% (Figure S9b). During


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unloading, a non-linear blue-shift of Pos(2D) is observed (Figure 4a). In a similar loading situation on a flexible display graphene based prototype, where CVD graphene has been embedded into two thin polymer films, the Pos(2D) red-shifts during loading and reverses back to its initial values during unloading22. By the 2nd loading cycle the same trend is observed, but the corresponding shift rates for Pos(2D) are relatively lower than the 1st loading cycle, attributed to changes of the epoxy resin substrate. As it is depicted in Figure S13, by implementing a 4-point bending testing with a low displacement rate (see also Supporting Information), after a complete loading-unloading cycle, the epoxy resin exhibits plastic deformation of order of ~0.1%. In similar works, where graphene is subjected to cyclic tensile loading, as the applied strain to the substrate increases, the relative compressive strain that graphene experiences on unloading introduces a high density of buckling ridges. As it will be discussed below, an increased amount of wrinkling seems to be present within the AFM topographic images obtained after each deformation cycle.

Evaluating interface changes by nano-topography mapping The topographic AFM image shown in Figure 5a was recorded on specimen D from an area of 14 x 14 μm2 prior to loading and after the 1st and 2nd loading cycles. Importantly, the AFM scanned area (Figure 5a) was carefully chosen as being part of the same Raman mapping area presented in Figure 4. The 3D topographic images taken from different scanning windows within the examined region (Figure 5a) prior to loading (Figure 5b), after the 1st (Figure 5c) and 2nd loading cycles (Figure 5d), respectively are also presented for comparison.



Figure 5: (a) AFM image of an area of 14 x 14 μm2 on specimen D, which was carefully chosen to lie within the Raman mapping region selected in Figure 4. This picture corresponds to the initial status of the examined system (prior loading). 3D AFM images of the examined area in the (b) green, (c) red and (d) violet rectangular are corresponding to the unstrained, 1st loading and 2nd loading status of the sample, respectively. As shown (blue triangle), a wrinkle normal to the direction of applied load is formed after the 1st loading cycle. This becomes more intense at the end of the 2nd cycle and is accompanied by the formation of an adjacent lateral wrinkle (red triangle) at a distance of ~150 nm. Scale bar: 1 μm As has been shown previously (Figure 2), the graphene sheet follows closely the texture of the polymer surface and therefore it is difficult to be discerned by AFM imaging (Figure 5b). However, in Figure 5c (after 1st loading), a wrinkle can be clearly seen (denoted with the blue triangle) having a mean height and width of about 3 and 21 nm, respectively. The reason for the


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appearance of premature compressive failure60 upon unloading stems from the gradual strain relaxation of graphene upon loading. During uploading the matrix (epoxy resin substrate) is forced to return to zero strain, which in effect puts the inclusion (graphene sheet) in compression. The wrinkling direction is perpendicular to the strain axis as shown in Figures 5a and c61. After the 2nd loading – unloading cycle, a new wrinkle is observed being almost parallel to the previous one at a distance of about 1.2 μm, with height and width of about 3 nm and 23 nm on average, respectively.

Such graphene wrinkles can be considered as buckle-induced

delaminations, resulting from prior strain relaxation upon loading.62 This description is supported by the corresponding adhesion map of the substrate topography shown in Figure 2a. The adhesion forces between the AFM tip and the surface (see also explanation in Figure S2) are much higher than that on the wrinkles indicating that the graphene layer is tightly bound on the texture of the underline substrate. The smaller the adhesion forces on the wrinkles, the more graphene is detached from the substrate and screens the tip/surface interactions63 (Figure S2).

Effect of applied strain upon the electrical resistance of the CVD graphene/ epoxy system As mentioned earlier both the CVD production process, as its transfer to the epoxy substrate, are responsible for the formation of its wrinkling morphology. Moreover, it seems that additional wrinkling results from the cyclic loading (Figure 5). In general the presence of wrinkles and folds in CVD graphene degrades its electrical performance due to the inherent anisotropy in the wrinkle/ fold charge transport and therefore its resistivity17. Ni et al.64 showed that the current growth and transfer methods of CVD graphene leads to quasi-periodic nano-ripple arrays in graphene, similar to those studied elsewhere65 (Figure 5).



Such high-density of ripples and wrinkles not only causes anisotropy in charge transport but also sets limits on both the sheet resistance and the charge mobility in graphene even in the absence of grain boundaries.

To assess the above conjecture, we have conducted two different

mechanical tests while recording the relative change of the electrical resistance, ΔR/Ro, during the mechanical loading/unloading of the CVD graphene system. In the first test, that involved two (2) loading-unloading cycles, the experiment was conducted under quasi-static conditions to allow the simultaneous acquisition of Raman spectra and resistance measurements. It is also worth noting that the strain values were recorded both by electrical resistance strain gauges and also calculated through the Stoney formula5, 57 (see also Supporting Information). The results of ΔR/Ro over strain for both cycles are presented in Figure 6. As seen, the electrical resistance decreases by the application of tensile strain by 0.4%45, reaching a plateau which is retained for strains up to ~1%. As mentioned above this effect is attributed to the opening up of the wrinkled structure that gives rise to more efficient electron mobility. Upon unloading, the resistance starts increasing up to 0.8% of strain, due to the formation of wrinkles at new locations as found by the AFM observations (Figure 5). By further decreasing the strain, the resistance of the sample remains constant at a value higher than its initial Ro, which is expected due to formation of new wrinkles/ folds upon unloading. This picture is consistent with the Raman data presented in Figure 4, in which a shift of both Pos(G) and Pos(2D) to higher values is obtained, indicative of the presence of higher compression in the graphene sheet at the end of the loading cycle. The net result of this deformation cycle is the broad hysteresis loop (width of ~ 0.6) in the values of ΔR/Ro (Figure 6a).


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0.8 0.6

Loading Loading (Stoney's formula) Unloading Unloading (Stoney's formula)

(a)

ΔR/R0

0.4

Cycle 1

0.2 0.0 -0.2 -0.4 -0.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4

(b)

Loading Loading (Stoney's formula) Unloading Unloading (Stoney's formula)

ΔR/R0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cycle 2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Applied strain (%) Figure 6: The relative variation of electrical resistance, ΔR/Ro, for specimen D as a function of the applied strain (solid hatched shapes) and the strain obtained according to Stoney’s approximation (see also Supporting Information) for a film-on foil structure (non-hatched shapes) for (a) loading-unloading cycle 1 and (b) loading-unloading cycle 2 (quasi-static mechanical testing). The sample resistance in undeformed state was Ro = 7.16  0.08 kΩ. The black and red solid only serve as a visual guidance. However, after the completion of a second loading cycle, the hysteresis width appears to be significantly narrower (decreased by ~65%)

(Figure 6b).


Upon loading, the graphene’s


resistance remains almost constant up to 1.2%.

On unloading, small variations of the

corresponding measured resistance values are observed. It is, thus, expected that after a number of loading-unloading cycles the hysteresis will gradually vanish. The second experiment was quasi-dynamic in an attempt to increase the loading-unloading frequency. In particular, the strain was applied stepwise, and at each step, the strain was kept constant for 15 s. The loading stopped at a displacement of 1.8 mm corresponding to a strain of 1.4%. The same protocol was followed for the unloading cycle as the specimen returned back to zero strain. In total four (4) loading-unloading cycles were conducted on a different CVD graphene/epoxy resin system, while the resistance of each sample was continuously monitored (see also Experimental Section and Supporting Information) (Figure S11). Indicative results from the quasi-dynamic test are shown in Figure 7, where the ΔR/Ro as a function of strain for the CVD graphene/epoxy resin system for the first and third loading-unloading cycles is presented, respectively. The ΔR/Ro of the examined system as a function of strain exhibits a nonlinear behavior that resembles the behavior of the quasi-static experiment for which a broad hysteresis was found (Figure 6). Moreover for moderate strains of 0.4 - 0.6% the sample resistance decreases by about 43% of its initial value at zero (Figure 7). It seems that at low applied strain levels, the morphology of the graphene is opening up resulting to a better charge transport. Significantly, by increasing the number of loading-unloading cycles, the hysteresis decreases drastically and at the third cycle, it is completely vanished (cycle 3).


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(a)

(b)

Figure 7: The relative variation of electrical resistance, ΔR/Ro, of the epoxy resin/CVD graphene system as a function of the applied strain for the (a) first and (b) third loading-unloading cycles, respectively for the quasi-dynamic mechanical testing. The black and red solid lines only serve as a visual guidance. At this point, it is important to assess the strain sensing capability of the graphene sheet within the strain range of 0.0-0.6 %. In Figure 8 the strain sensitivity (G) or gauge factor (GF) of the graphene sheet, defined as ΔR/R0 = Gε66, is presented as a function of the loading/ unloading cycles for both independent tests (quasi-static and dynamic).



Figure 8: The gauge factor (GF) as a function of the number of the loading-unloading cycles estimated for dynamic (red symbols) and quasi-static (black symbols) tests, respectively. The dotted red line acts as a guide to the eye. As it is stated elsewhere

67,

the value of GF depends on material type, reaching for metallic

materials around 2-4 while 80-170 or -95 to -110 for semiconductor strain gauges. In the case of graphene related materials (GRM), depending on application (e.g. strain and tactile sensors, large area sensing) 31, type 7, 16, 34, 36 (CVD graphene, HOPG, GO, graphene- polymer nanocomposites) and level of deformation, differences of two orders of magnitude in the GF values are obtained. In this work, the GF values obtained are relatively high at least for the quasi static test. However, they seem to decrease as the external load is continuously applied (ε > 0.6%), and this attributed to interface degradation due to increase of the wrinkle density.


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To study a practical application of the system as an electromechanical sensor, we examined its response in continuous operation for a significant amount of loading-unloading cycles (see also Experimental Section), for which the maximum applied displacement, at the middle of the specimen, was 0.5 mm, corresponding to 0.5% of strain on the outer surface (Figure S12). Within that particular range, electrical resistance can take its minimum values, while the epoxy resin substrate is within its elastic region (Figure S12 and Table S3). The experiment consisted of two identical routines (1 and 2) with varying applied load frequencies, while the electrical the resistance was continuously monitored. As shown in Figure 9, the graphene response to the applied loading is quite remarkable. The relative variation of graphene’s electrical resistance, ΔR/Ro follows in parallel the deformation changes of the resin substrate induced by the externally applied load. Especially, for the six (6) loading-unloading cycles, the same number of peaks and valleys are clearly distinguished for the graphene’s resistance. Any deviations observed can be attributed to the non-perfect quality of the CVD graphene sheet deposited onto the epoxy resin substrate (Figure 1). As the loading-unloading frequency is increased, a similar trend is observed for the resistance. Therefore, it seems that CVD graphene follows up the applied load with no particular lag, since during the time interval between the applied cycles (30 sec), the resistance recorded returns back to its initial value (Ro = 13.4 ± 0.3 kΩ) denoting the operational efficiency of the examined graphene sensing system; thus, no additional wrinkling is formed. Even after the completion of a second routine (Figure 8), which corresponds to almost 400 consequent cycles of loadingunloading, the CVD graphene still returns to its initial resistance value.

This is the first

demonstration of graphene as an electromechanical sensor (See Supporting Video 1, Supporting Information).



1.6

ΔR/Ro

Routine 2

Routine 1

1.2

(a)

0.8 0.4 0.0 -0.4

Displacement (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

100

200

300

400

500

600

700

800

900 1000 1100 1200 1300

0.8 (b) 0.7 0.6 6 cycles 12 cycles 60 cycles 120 cycles 6 cycles 12 cycles 60 cycles 120 cycles 0.5 0.4 0.3 0.2 0.1 0.0 0

100

200

300

400

500

600

700

800

900 1000 1100 1200 1300

Time (sec) Figure 9: (a) Τhe recorded relative variation of electrical resistance, ΔR/Ro, of the epoxy resin/CVD graphene system as a function of time as it follows (b) the applied loading-unloading cycles (6, 12, 60, 120 loading-unloading cycles per 2 mins time each) for two sequences (routine 1 and 2), with 90 sec difference from each other. The maximum applied displacement was 0.5 mm, corresponding to 0.5% of strain (Figure S12), where graphene’s resistance appears its optimum performance (Figure 6, 7)


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Conclusions The electromechanical stability of a CVD graphene sheet as a structural coating on a commercial epoxy resin substrate has been investigated by means of AFM nano-mechanical mapping and Raman microscopy. CVD graphene appears to have wrinkles with variable densities and morphologies at the nanoscale. By mechanical cyclic loading, these wrinkles accommodate less compressive strain with higher wavelengths, while the strain transfer efficiency between polymer and graphene seems not to be affected dramatically and follows the changes in the mechanical properties of resin upon cycling loading. Moreover, the random wrinkling in such hybrid materials renders the graphene sheet resistance variable when their density and morphological characteristics alter. Training the material, however, by cyclic mechanical loading at low deformations can restore its electrical resistance to almost constant values. This study provides a simple and novel method for monitoring structural polymers such as epoxy resins, applied to aeronautical applications, by using graphene’s rapid electromechanical response presented here. Such hybrid materials could be potentially exploited as real time sensors for applications where the fatigue life of a structure made by composite materials is an important issue.



Experimental Method Fabrication of the CVD monolayer graphene on the epoxy resin system Initially, liquefied Araldite® LY 5052/Aradur® 5052 epoxy system (mixing ratio 100/38 parts by weight) is spread on the top of a single layer of CVD Graphene on copper foil. A PDMS mask is also attached to give specific dimensions (70(L) x 15(W) x 2(T) mm3) to the final product (Figure S1). The curing of the resin is performed in open air environment (room temperature) for 24 hours. After curing, the PDMS mask is removed. The excess of copper outside the boundaries of the epoxy resin is cut. After, the sample is transferred to a FeCl3 solution bath for 5 hours in order to etch the copper (Cu) foil. The final product is inserted into HCl acid bath for 30 min, to remove any Cu residuals remaining on the surface. After the etching process, the samples are rinsed with water and ethanol and the single layer of graphene is encaptured on the surface of the epoxy resin. More details are also given in Supporting Information S1. Raman measurements In order to check the influence of the morphology of the substrate on graphene properties, Raman mapping took place.

Spectra were taken with at 785 nm (1.58 eV) laser using a

MicroRaman (InVia Reflex, Rensihaw, UK) set-up. The laser power was kept below 1.5 mW on the sample to avoid laser-induced local heating, while an Olympus MPLN100x objective (NA = 0.90) was used to focus the beam on the samples. The obtained spectra of the pure epoxy resin and the graphene/resin sample are presented in Figure S5. The vibrational assignments of all the peaks observed within the range of 1000 – 3000 cm-1 for both the epoxy resin and graphene and presented in Table S1 68-69.


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Mechanical tensile tests: A. Flexural measurements The flexural properties of the neat epoxy resin were measured according to ASTM D7264/D7264M-7. Rectangular neat resin specimens with specific dimensions (70(L) x 15(W) x 2(T) mm3) were carefully mounted on an in-house 3 point-bending grip apparatus. By applying the load to the specimens at a crosshead rate of 1 mm/mm, simultaneous load-deflection data were recorded. An extracted stress-strain curve as well the flexural strength and strain and the corresponding modulus are presented in the Supporting Information (Figure S12 and Table S3). B. Cyclic deformation I. Quasi-static deformation on CVD graphene/epoxy resin system The top surface of the monolayer CVD graphene/epoxy resin system was subjected to tension using a four-point bending jig. By flexing the epoxy beam upwards by means of an adjustable screw connected to a set of gears, the desired deflection δ was achieved. Slow travelling of the lever results in accurate incremental steps of applied strains to the specimen. In fact, it has been calculated that a full turn corresponds to 0.20 % of applied strain according to the following equation (ASTM D 672).

 ( )  4.36

t L2

(1)

where δ is the deflection (manually applied) on the epoxy resin bar, L the length of supporting span and t the thickness of the epoxy resin bar . The validity of equation 1 has been confirmed by independent strain measurements using minute strain gauges within the middle area of pure epoxy resin beams. Finally, since the relationship



between the 2D peak and strain has already been established earlier44, the external strain can also be estimated from the shift of the 2D peak at each increment of strain with reference to its position at 0% strain. In order to conduct Raman mapping during loading, the four-point bending apparatus was placed on a three-axis piezoelectric translation stage that was operated on three orthogonal axes by a Thorlabs Inc piezoelectric controller.

The NanoMax three-axis flexure stage can provide

nanometric positioning on the three orthogonal axes.

At each strain level, the stage was

translated with a step of 5 μm with the simultaneous collection of Raman spectra within an area of 15 x 15 μm2 (16 total points). II.

Quasi-static deformation on the epoxy resin substrate

The influence of the external applied strain on the examined system was studied by designing a mechanical test, which simulated the quasi static tensile loading conditions that took place in the four-point bending apparatus in terms of strain and time. The overall duration of the test, as well as the final strain level, were determined by the corresponding testing conditions during the Raman spectra acquisition. The applied quasi-static consisted of a full strain-controlled loading−unloading cycle at a maximum strain of 1.5%. The displacement rate was calculated to be 0.03 mm/min, using the specimen specifications of ASTM D7264/D7264M-7 standard. III.

Quasi-dynamic deformation

The quasi-dynamic straining of the examined polymer/graphene system was carried out by using the specially designed in-house four point bending apparatus (according to ASTM D7264), mounted on a MTS mini-Bionix 848 servo-hydraulic testing frame. The testing apparatus was


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capable of applying a fully strain-controlled external load via the embedded LVDT system, while the MTS TestStar 40 controller provided a high rate real-time recording of the experiment parameters (time, displacement and force). The simultaneous measurement of the corresponding resistance of the graphene film was accomplished using a KEITHLEY instrument, as described in a following section. Each experiment consisted of a set of four loading-unloading cycles, reaching a maximum strain on the outer surface of the specimen of 1.4%. Loading and unloading procedure followed a twelve step routine in each direction, with adequate dwell time (15s) in order to ensure proper resistance measurements between steps. All experiments were conducted under a constant room temperature of 23 oC. IV.

Continuous cyclic deformation

The experiment of the cyclic deformation of the examined hybrid graphene/polymer specimens was carried out using the abovementioned MTS servo-hydraulic setup with a three point bending mounting fixture. The mounting apparatus was fabricated in-house following the specifications of the ASTM D7264 standard. In the specific three point bending setup the loading point lies in the center of the upper surface of the specimen, consequently the sample was placed with the deposited graphene film on the bottom surface in order to avoid any contact during the test. The experiment parameters were set in order to ensure that the outer surface of the specimen (where the graphene film was deposited) operates in the strain range of 0% to 0.5%. Simultaneous electrical resistivity measurements were conducted as described in the previous section. Each experiment consisted of two identical dynamic loading routines. Each routine included the execution of consecutive cyclic loading between 0% and 0.5% maximum strain at



Page 32 of 43

four different frequencies: 0.05, 0.1, 0.5 and 1.0 Hz. Before altering the loading rate, the routine applied a ‘relaxation’ step of 30s to ensure that the specimen returns to its initial undeformed state. All experiments were conducted under a constant room temperature of 23 oC. Resistance measurements In the vicinity of mapping area, two copper electrodes (Kemptron Ltd copper tape with conductive adhesive for EMI shielding), with a length of 1 mm and thickness 0.035 mm, were attached on the graphene sheet at fixed distance of 10 mm, covering all the width of the sample. At each applied strain level, the corresponding resistance was recorded using a KEITHLEY 2420 3A sourcemeter. Atomic Force Microscopy measurements AFM images were taken using a Dimension Icon (Bruker Nano) AFM with a Nanoscope V controller using the PeakForceTM Tapping Mode. PeakForceTM images up to a force set point of 100 nN were taken using ScanAsyst Air probes, or ATESP probes (both from Bruker). By introducing PeakForceTM quantitative nano-mechanical mapping (QNM) AFM mode

70,

it was

allowed to obtain spatial mapping of the nano-mechanical properties of the single layer CVD graphene on the epoxy resin surface.


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ASSOCIATED CONTENT Supporting Information Further experimental data and explanations are given upon: S1. Growing epoxy resin on single layer of graphene, S2. PeakForceTM (QNM) mapping, S3. Spectroscopic data, S4. Statistical analysis of graphene quality prior to mechanical characterization, S5. Defining the influence of unintentional doping on the shift variation of G peak, S6. Spectra of 2D and 2G for data points with an initial FWHM(2D) of order ~ 30 cm-1, S7. Dependence of Pos(2D), Pos(G), FWHM(2D) and FWHM(G) with applied strain for all examined specimens, S8. Stoney’s formula assumptions, S9. Experimental set-up of the dynamic test, S10. Flexural properties of the epoxy resin. This material is available free of charge via the Internet at http:// pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Whom all correspondence should be sent to: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.



ACKNOWLEDGEMENT The research leading to these results has received funding from research project “Graphene Core 1, GA: 696656 - Graphene-based disruptive technologies”, as well by “Graphene Core 2, GA: 785219”, which are implemented under the EU-Horizon 2020 Research & Innovation Actions (RIA) and is financially supported by EC-financed parts of the Graphene Flagship. The Patras group also acknowledges the support of the ERC Advanced Grant “Tailor Graphene” (no: 321124). In addition, it is acknowledged PhD candidate Nick Koutroumanis for the flexural measurements of the epoxy substrate.


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