Scalable Graphene Electro-Patterning, Functionalization, and Printing

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Scalable Graphene Electro-Patterning, Functionalization, and Printing Junha Park, Hong-Ki Park, and Jaewu Choi* Quantum Information Display Laboratory, Department of Information Display, Kyung Hee University, 1 Heogi-Dong Dongdaemoon-Gu, Seoul 130-701, South Korea ABSTRACT: Scalable direct graphene patterning and functionalization approaches, which are essential to harness the excellent physical properties of graphene for various biological, chemical, electronic, and optical device applications, are demonstrated in this study. These are achieved by employing simple scalable affordable electrochemical methods. First, graphene was patterned and functionalized in multiple concentric ring shapes and this is realized by controlling the liquid droplet size and the polarity of the applied bias voltage, respectively. Second, the pattern transfer from a mask to graphene is also conducted with assistance of a photoresist (PR) pattern on graphene. The unprotected graphene by the PR was etched by the electrochemical process. Finally, the direct pattern writing on graphene is demonstrated by dragging a liquid droplet at a bias voltage. This maskless direct patterning process of graphene can be extended to a large scale graphene patterning by employing a computerized xyz translation stage like an inkjet printer.

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INTRODUCTION

and chemical properties of graphene and for in situ analyzing the graphene devices. Wetting, electrowetting and electrochemical behaviors of graphene have been largely investigated by several research groups.17−24 Freestanding graphene is hydrophobic but the wetting properties of graphene on substrates depend on the supporting substrates.25 Furthermore, the fracturing of graphene on specific substrates is also casually observed after the electrowetting experiment.26−32 Additionally, spontaneous intercalation of water or electrolyte under graphene has been observed.33−35 On the basis of these previous reports, which provide the clue of that graphene can be intentionally patterned by electrochemical methods, this study investigates, proposes and demonstrates novel scalable direct patterning methods of graphene simply using water and electricity without requiring of vacuum and any capital instruments as shown in Figure 1. For demonstration, chemical vapor deposition (CVD) method synthesized graphene is transferred on silicon substrates with 300 nm thick thermal oxide. The liquid droplet is not only just spread on the graphene surface but also permeated between the graphene and substrate at the triple point of solid, liquid and air. With increasing the bias voltage with time, the permeated tiny liquid droplets (graphene blisters) spread and are merged underneath the graphene. This develops a significant stress at the graphene blisters and causes to initiate the fracture of the graphene at the weakest bonding sites such as pentagonhexagon bonding sites or impurity sites.27,31,36−38 This study shows that the intercalation of water under graphene at the

The two-dimensional honeycomb structure of carbon atoms known as graphene has potential in diverse device applications because of its unique and excellent physical properties at low energies such as two-dimensional crystalline and electronic structure, massless charge carriers, highest charge carrier mobility, unique optical properties, etc.1−4 With capabilities of large scale graphene synthesis and transfer methods,5−7 the large scale application of graphene for biological, chemical, and electronic optical sensors and devices typically requires scalable patterning methods of graphene at affordable cost.3,4,8,9 Thus, various graphene patterning methods have been developed.10−16 These include electron beam lithography, helium ion beam lithography, reactive ion etching, oxygen plasma etching, nanoimprint lithography, photolithography, reduction of graphene oxide, scanning probe lithography, laser lithography, and electrochemical patterning using colloidal templates.10−16 These approaches show the strength in the fine graphene patterning. However, these methods require several processing steps for patterning with capital instruments as well as it is inconvenient to large scale patterning of graphene. Thus, with conjunction with the above-mentioned fine patterning technologies, facile, scalable and affordable graphene patterning methods with less process requirements are highly desirable for future large scale graphene device applications. Graphene device fabrication processes frequently include wet graphene transfer processes, which may be unavoidable for large scale graphene applications. As the extension, this study shows that large scale graphene patterning can be achieved by employing a wet electrochemical process. The wet electrochemical methods are applicable not only for graphene patterning, but also for functionalization to control the physical © XXXX American Chemical Society

Received: May 15, 2017 Revised: June 14, 2017 Published: June 26, 2017 A

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Figure 1. Schematics for (a) patterning and functionalization process in a ring shape by a droplet with controllable size, (b) PR-assisted patterning process by a controlled dipping method, and (c) scalable maskless direct patterning process by motorized dragging a droplet on graphene such as ink jet printers.

triple point is effectively controlled by applying a bias voltage. And at the same time, physical or chemical modification (oxidation) can be occurred according to the bias polarity. On the basis of this, this study demonstrates three graphene patterning methods as shown in Figure 1: (1) patterning and functionalization of graphene in ring shape by controlling the droplet size and bias polarity (Figure 1a), (2) pattern transfer on graphene using prepatterned PR pattern (Figure 1b), and (3) direct patterning (writing) of graphene by dragging a droplet on graphene using an electrically addressed tip (Figure 1c). These maskless direct printing methods can be applied to the development of printed biosensors, chemical sensors, electronics, optics and the other sensor and device technologies, etc.

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EXPERIMENTAL METHODS Graphene on a 35 μm thick copper foil was synthesized by a typical chemical vapor deposition (CVD) method.5 After the copper foil was annealed at 1000 °C for 30 min in the gas mixture flow of 100 sccm Ar and 50 sccm H2, the copper foil was annealed at 1000 °C for 8 min in the 5 sccm CH4 flow for 8 min. PMMA (poly(methyl 2-methylpropenoate)) (40 g L−1) dispersed in 1,2-dichlorobenzene solution was spin-coated at 2000 rpm on the synthesized graphene. After that, the copper foils employed for the growth was etched using a 0.7 M FeCl3 solution. The PMMA−graphene stacks were cleaned up using a modified Radio Corporation of America (RCA) clean method.6 Then the stacks were transferred on 1.5 cm × 1.5 cm silicon (Si(100)) substrates with a 300 nm thick silicon oxide layer. After annealing them at 150 °C for 15 min, the PMMA layer was removed with acetone at 60 °C for 1 h. After that, the samples were annealed at 350 °C in the mixed gas environment of Ar and H2 for 2 h to reduce PMMA residues. Figure 2 shows the electrowetting configuration employed in this study. Ag paste as an electrode was put on one edge of a graphene sheet. The employed aqueous electrolyte consists of 1 mM or 1 M KCl (1 M KCl for contact angle measurement and anodic configuration, and 1 mM KCl for cathodic configuration to reduce the formation of AuCl as a byproduct). The electrolyte droplets (5 μL per drop) were directly dropped on graphene with a micropipettor (Jencons Accurate 480−203) and pipet tips of inner diameter ∼0.7 mm and the outer diameter ∼1 mm. A gold wire (30 μm in diameter) as another

Figure 2. Schematic configuration for graphene electrowetting experiments. The bias voltage between the graphene/SiO2/Si and the gold wire electrode was scanned with alternating bias polarity while the graphene is grounded.

electrode was immersed in the electrolyte as shown in Figure 2. A bias voltage is applied between the graphene and the droplet while the graphene is grounded. The droplet was monitored by a CCD camera equipped with a microscopic lens system. The bias voltage was scanned from 0 to 3 V with 0.2 V step, from 0 to −4 V with −0.2 V step. Current−voltage characteristics were investigated with a Keithley 6430 source meter with a two electrodes configuration. For the ring patterning shown in Figure 1a, the droplet sizes are controlled by varying the number of drops. For the PR assisted patterning study shown in Figure 1b, Micro Chemicals AZ1512 PR was spin-coated on graphene/silicon samples at 3000 rpm. The PR was patterned using photomasks, which were printed by a laser toner printer on transparent PET films. To etch the graphene uncovered by the PR pattern, a dipping method is employed. The silver paste for contact and the silicon substrate were grasped by a stainless tweezer together (the other side of the silicon was insulated by SiO2 and paraffin film). And the tweezer was connected to 1 mM KCl solution through the source meter. To prevent water from touching the tweezer, the samples slowly dipped to the solution by a vertical translation stage at 3 V applied bias voltage. After the samples completely dipped, they were cleaned by blowing water using B

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SiO2(300 nm)/Si as a function of applied bias voltages to the counter gold electrod with respect to the grounded graphene. Typically the contact angle measurements were conducted with the samples prepared in 12 h ago. Right after dropping of the 1 M KCl liquid droplets on graphene/SiO2(300 nm)/Si, the initial contact angles of the aqueous droplets were ranged from ∼70 to ∼80 deg at zero applied bias voltage as shown in Figure 3a. Further, in the laboratory environmental conditions such as °C temperature of 24 °C and relative humidity of 44%, the contact angle is reduced due to the evaporation of the liquid droplet as shown in Figure 3b. The typical reduction rate was 0.18 deg/s. Unexpectedly the contact angle reduction of the droplet on graphene was happened without changing of the inplane radius as shown in Figure 3b. This indicates that pinning is occurred at the edge of the droplet. This is attributed to that the graphene is bent out of the graphene plane at the triple point of graphene, liquid, and air due to the relatively high surface tension at the triple point.37 As the applied bias voltage increases, the contact angles of the electrolyte drops on graphene decrease as well as the radius of the liquid electrolyte increases as shown in Figure 3a. When the magnitude of the bias voltage is less than 2 V, the contact angle variation is relatively small and symmetric. However, the magnitude of the bias voltage is higher than 2 V, the contact angle is dramatically changed and the detailed behavior depends on the polarity of the applied bias voltage. When the graphene acts as a cathode (graphene cathodic configuration), the variation rate of the contact angle per voltage is relatively high compared to that when the graphene does as an anode (graphene anodic configuration). Further, the contact angle variation was not reversible. Even though these kinds of behaviors have been observed on HOPG,19 this clearly suggests that the contact angle variation at high voltages does not follow the Young−Lippmann’s relationship.18,19 2. Ring Patterning. 2.1. Single Ring Patterning and Functionalization. After electrowetting experiment with water droplets, the water droplets on graphene are washed out by D.I. water and followed by N2 gas blow as shown in Figure 1a. The

N2 gas. Then remained PR patterns were removed by dipping in acetone for 1 h (Figure 1b). For graphene patterning by printing method as shown in Figure 1c, a 200 μL pipet tip filled with 1 mM KCl solution was hold by a tong on a micrometer XYZ stage. And the end of the pipet tip (1 mm of the outer diameter, 0.7 mm of the inner diameter) was placed on the surface of graphene with 0.2 mm gap. Then let water in the tip keep in contact with the surface, then the tip was horizontally moved at 5 V bias voltage and drew a “⊂” shape. The samples were processed using the same method mentioned above (Figure 1c). The Raman spectra of graphene samples were taken with a T64000 JobinYvon system with a laser of 532 nm in wavelength and 0.6 mW in optical power.

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RESULTS AND DISCUSSION 1. Electrowetting and Pinning. Figure 3a shows the measured contact angles of the aqueous droplets on graphene/

Figure 3. (a) Bias voltage dependent contact angles of 1 M KCl aqueous droplets (5 μL) on graphene by varying the applied bias voltages of the gold wire elctrode from 0 V to a positive voltage (A and B) and a negative voltage (C and D) with respect to the grounded graphene with a few representing optical photographs on the righthand sides (dashed arrows indicating the corresponding contact angles). (b) Optical photographs taken at 0 and 900 s later after the deposition of the water droplets on graphene.

Figure 4. Photograph of graphene samples after electrowetting experiments (a) at -3 V (anodic configuration) and (b) at +3 V (cathodic configuration) for 60 s taken before and while moisturizing them by breath blowing. The light blue area corresponds to SiO2 layer while the dark blue area does to graphene layer. The detailed views of patterned graphene in the aqueous (c) 1 M KCl and (d) 1 mM KCl electrolyte droplets on graphene/SiO2/Si at an anodic (- 3 V) and a cathodic (+3 V) configurations, respectively. (e) Raman spectra taken from the graphene region covered with droplets before (black) and after electro-wetting experiments with 1 mM KCl aqueous drops at graphene cathodic configuation (+3 V, blue) and anodic configuation (−3 V, red). C

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at the cathodic configuration but discontinuously and narrowly at the anodic configuration. Second, the graphene in the interior of the ring is oxidized at the anodic configuration but no reaction at the cathodic configuration. 2.2. Multiple Ring Patterning and Functionalization. As indicated by the voltage dependent contact angle variation shown in Figure 3a and the distinct image contrasts shown in Figure 4, parts a and b, the response of the graphene to the applied bias voltage depends on the applied bias voltage magnitude, its polarity and scan direction as shown in Figures 3 and 4. The cyclic voltammogram shown in Figure 5a shows that the current is relatively low when the magnitude of the applied bias

optical images of the electrowetted samples are shown in parts a and b of Figure 4 when the graphene acts as an anode and a cathode, respectively. The left and right optical images of parts a and b of Figure 4 are taken before and during moisturizing the surface with breath blow, respectively. The enhanced contrast with breath blow is attributed to the difference in the hydrophilic properties of graphene and silicon oxide. When the graphene is used as an anode, the mark of the droplet edge is barely visible as a relatively bright light blue. The relatively bright light blue region corresponding to silicon oxide is shown around the ring pattern. However, when the graphene is employed as a cathode, the clear ring pattern with a width of ∼1 mm is shown in Figure 4b. Further, the interior of the ring shows the same color with that of the unreacted outer region. Figure 4c shows the detailed views of the patterned graphene with a 1 M KCl aqueous droplet at the applied bias voltage of −3 V (anodic configuration) while Figure 4d does the patterned graphene with a 1 mM KCl liquid droplet at +3 V (cathodic configuration). The graphene patterns obtained by the two configurations show distinctive characteristics. First of all, the patterned area at the anodic configuration is narrow and discontinuous as shown in Figure 4c (Figure 4a). However, the patterned area at the cathodic configuration is wide and continuous as shown in Figure 4d even though the ionic electrolyte concentration is 1000 times less compared to that employed at the anodic configuration. As shown in Figure 3a, the contact angle modulation at the cathodic configuration is large compared to that at the anodic configuration. This suggests that the graphene patterning is directly correlated to the contact angle modulation. The patterning at the anodic configuration does not show any directional property. However, the pattern formed at the cathodic configuration implies that the patterning was progressed along the radial direction. The graphene fragments are connected to the inner ring edge and becomes smaller in radial direction. However, the outer edge is clearly trimmed. This suggests that the fracturing and tearing was initiated from the inner edge and it propagates outward. Further, this clearly indicates that the patterning process was not occurred in the graphene region covered with the liquid droplet. Figure 4e shows the Raman spectra taken from the dried graphene samples after a bias voltage applied in the electrowetting configuration (see the Experimental Methods). When the graphene is used as a cathode (at +3 V, cathodic configuration), there is no significant variation in the Raman spectra (blue) from those of the pristine graphene (black) as shown in Figure 4e. This indicates that there is no chemical modification of graphene under the positive bias voltage at which the tearing of graphene was observed. However, when the graphene is employed as an anode (at -3 V, anodic configuration), the graphene G band peak (around 1580 cm−1) intensity was reduced but the D band peak (1350 cm−1) intensity was significantly developed as shown in Figure 4e (red). This indicates that the defects of graphene dramatically increase with the applied negative bias voltage. The resistance of the anodized sample largely increases to several megaohms from a few kiloohms. When the bias voltage is reversely biased, the resistance reduces to several tens of kiloohms. This suggests that the oxidation and reduction of graphene is quasireversible. There are two distinctions of the behaviors in the anodic and cathodic configurations. First, the degree of the ring edge patterning is occurred uniformly and widely over the ring edge

Figure 5. (a) Cyclic voltammogram. Voltage is scanned from 0 to 4 V (①), to 0 V (②), to −4 V (③) and to 0 V (④) with scan rate of 0.5 V s−1 and step voltage size of 0.067 V step−1. The multiple ring patterns ((b) 8 rings and (c) 4 rings) are formed by controlling the droplet size with multiple drops of 1 M KCl aqueous solution at −4 V (b) and 1 mM KCl aqueous solution at +3 V (c), respectively. At each drop, the bias voltage is approximately applied for 20 s.

voltage is less than 2 V. This relatively wide low current region corresponds to the nonfaradic zone.19,23,24 But the current significantly increases when the bias voltage is higher than 3 V in magnitude. As shown in Figure 3a, in this nonfaradic region, the contact angle variation is minor compared to that in the high voltage region, in which the current dramatically increases with the bias voltage. It is believed that the faradic reaction is related to the evolution of hydrogen and oxygen due to the electrolysis.19,23,24 On the basis of the visual observation using a microscope, bubbling was observed at the employed thin Au wire electrode. However, the bubbling in the graphene region was not visible and this can be attributed to the relatively small amount hydrogen generation over the wide graphene region. The reaction at the electrodes plays the role in the contact angle variation. Further with a relatively high concentrated electrolyte solution (∼1 M), insoluble yellow AuCl (gold(I) chloride) particles were formed at a high positive bias voltage (>2 V). Thus, to reduce the AuCl formation, 1 mM KCl solution is mostly utilized for patterning in this study. Parts b and c of Figure 5 show the multiple ring patterns, which were formed by increasing the droplet size using multiple drops under bias voltages of −4 and +3 V, respectively. When D

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Figure 6. PR assisted graphene patterns with the letters of “KHUKHUKHU”, “QID” and “KHU”. (a and b) Photographic images before and during moisturizing with breath blow, respectively. (c and d) Microscope images of the graphene edges at bias voltages of 5 and 10 V when the aqueous droplet is located near to the graphene edge, respectively. (c) There are a lot of tiny blisters under graphene (graphene blisters). The larger graphene blisters are distributed as the nearer the edge. As the intercalation of water progressed, the graphene edge is detached from the substrate, floated on the water surface and torn off. The red arrows indicate the dented point of the torn graphene blister rim and are related to the strong attachments of the graphene to the substrate through the graphene impurities and the silicon suboxides.

Figure 7. (a) Schematic for maskless graphene patterning. (b) Patterned graphene with a Korean character “⊂” with/without a dot by dragging the droplet on graphene using a manual xy stage. (c) Contrast enhanced photographic images formed by moisturizing with breath blow. The shapes were formed with dragging speed of ∼0.5 mm s−1. Water spilled out of the tip of the microppipette onto the substrate as the tip moves along on the surface. Graphene residues inside of the patterns appears and this depends on the dragging speed relative to the droplet edge fragment speed.

shown in Figure 5 and 6. Thus, in the employed dipping method, the patterning is happened only at the liquid surface. Thus, the dipping speed was controlled (100 to 10 μm s−1) and it depends on the applied bias voltages. The outcomes are shown in Figure 6, parts a and b. In Figure 6b, the dark blue area corresponds to the graphene removed area by the electrical removable process as shown in Figure 6a. Parts c and d of Figure 6 show the intercalation of the liquid between the graphene and the substrate from the liquid droplet located at the graphene edge. When the liquid droplet located at the edge of the graphene is applied with an electrical potential, the liquid is intercalated underneath the graphene in the form of tiny droplets rather than continuous liquid film as shown in Figure 6c. As a result, graphene blisters are formed. As increasing the number of the diffused tiny droplets and merging themselves due to the high surface energy of water, the graphene blisters grow. One can find out the dark spots (marked by arrows in Figure 6d) around the rim of the large droplets. This indicates that the binding between graphene and substrate is not uniform and the van der Waals interaction

the graphene is used as an anode, the relatively thin rings are formed and the color of the ring interior is different from that of the unreacted zone. However, when the graphene is employed as a cathode, the wide multiple rings are formed while the color of the ring interior is the same as that of the unreacted region as expected. This is consistent with the Raman spectra shown in Figure 4e. 3. Advanced Patterning Methods. 3.1. PR Assisted Dipping Patterning. Typical photolithography patterning of graphene with a desired pattern is customarily using PR patterns.10−12 To demonstrate the pattern transfer of a predefined pattern on mask to graphene, the PR assisted graphene patterning method is developed as shown in Figure 1b and Figure 6. First the pattern is lithographically predefined on the PR coated on graphene, and then the pattern is transferred to the graphene by electrochemically removing the unprotected graphene area by dipping the graphene with the PR pattern in the aqueous solution under a bias voltage as shown in Figure 6. This dipping is required because the graphene is patterned only at the triple points as explained and E

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Figure 8. Schematic diagrams for the proposed patterning mechanism. (a) Graphene on hydrophilic silicon oxide substrate with a very thin water interfacial layer is shown. (b) The surface tension at the triple points is high enough to induce the bending of the graphene at the droplet edge. γlv is the surface energy of water. When graphene is biased with a negative voltage (cathodic configuration) with respect to the conter gold electrode, the electrostatic force increases and (c) causes the hydrated cations to move to the graphene, (d) causes the hydrated cations to permeate through graphene defects or pores and the graphene blisters to become larger with molecular hydrogen evolution (H2), and (e) causes the tearing of the graphene blisters, where the torn graphene is floated on the aqueous droplet while the inner edge is attached to the substrates. When graphene is biased with a positive voltage (anodic configuration), (f) the electrostatic force increases and causes the anions to move to the graphene, and the anions are reacted with graphene (GO), (g) the hydrated anions are permeated through graphene defects or pores and this increases the graphene blisters, and (h) this causes the tearing of the graphene blisters, and the torn graphene is floated on the aqueous droplet while the inner edge remained attached to the substrates.

4. Mechanism of Patterning. On the basis of the abovementioned experimental observations, electrowetting behaviors, edge pinning effect at triple points, ring edge patterning, electrochemical response, intercalation at graphene edge, and hydrophilicity of substrate, the patterning mechanism can be proposed as below (Figure 8). First of all, the employed graphene/SiO2(300 nm)/Si samples in this study were exposure to ambient atmosphere for 12 h after annealing them at 350 °C in argon and hydrogen atmospheres. As a result, the environmental water can be intercalated into the gap between the hydrophilic silicon dioxide substrate and the graphene, and the very thin water layer between graphene and silicon oxide can be formed30,31,33,34 as shown in Figure 8a. The water intercalation can be happened through the graphene edge and the nanopores of the CVD graphene. The intercalated water molecules could show a distinct behavior from the bulk water in in the formation of water clusters due to the unique synergetic effect of the hydrophilicity of the substrate and graphene and the confined geometry.40,41 In particular, the CVD grown graphene is defective and porous.5−7 This makes the water intercalation easier and faster in the CVD grown graphene on hydrophilic substrates. This is evidenced by the defect induced pinning process shown in Figure 3b, in which the graphene is bent outward the surface at the triple point as shown in Figure 8b. The dry adhesion energy between graphene and silicon oxide substrates mediated by van der Waals interaction is known as high as 450 mJ m−2 even though it decreases with the number of graphene layer.36 This strong adhesion is attributed to the conformal property of graphene to the relatively rough surface of the substrate as well as to the possible discrete bonding of graphene to the defective silicon oxide. However, when the surface of the SiO2/Si substrate is wet with molecularly thin water layer, the adhesion energy between graphene and the substrate becomes significantly reduced as low as 60 mJ m−2.39

between them is not whole story. One can expect a chemical bonding between defective graphene and the nonstochiometric silicon oxide.36,38 As the droplets underneath the graphene grow with merging of the tiny droplets, the graphene is floated on the liquid surface while the rim is tightly bonded to the substrate (black spots). As a result, a strong stress is developed at the graphene and is proportional to the droplet size. As the stress becomes higher than the elastic limit, the fragment of graphene is started from the weakest bonding sites (defective bonding sites) in the carbon network.27 It is known that the CVD graphene is defective as indicated by the Raman spectra shown in Figure 4e. Once the fragment is developing at the weak bonding sites, the fragment is propagated until the intercalation is stopped due to the balance of the external force with the elastic force. 3.2. Scalable Direct Printing. Ultimately, the patterning of graphene should be largely directly achieved with any desired pattern shape and size and minimal post process without using any masks. Figure 1c shows the scalable direct patterning process by a computerized movable tip or a movable stage. The patterns shown in Figure 7, parts b and c, were obtained by manually moving the tip on a XYZ microstage (Figure 7a). This study indicates that the large scale direct printing without any masks can be done using the proposed conceptual scalable graphene printer as shown in Figure 1c. In this maskless scalable graphene printing, the minimum feature size can be controlled by the droplet size, which largely depends on the opening aperture size of the employed pipet tip19,30 (the employed pipet tip inner diameter ∼0.7 mm and the outer diameter ∼1 mm in this study), dwelling time or sweeping speed of the pipet, applied bias voltages, electrolyte type, substrate type, and substrate temperature, etc. With a suitable choice of the liquid and ions, the graphene also can be functionalized with desired physical properties, pattern shape, and size. This can be extended to the roll to roll graphene pattering in future. F

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gas is possible even though the hydrogen evolution was not visible. The hydrogen gas formed underneath the graphene (bubble) largely increases the stress compared to that the liquid only drop (blister).31 As a result, the pattern becomes relatively wider and continuous. The depletion of water between graphene and substrate causes the stronger adhesion and this makes the patterning process almost stopped due to the balance among the forces.

Second, when electrolyte liquid droplet is placed on the graphene/SiO2(300 nm)/Si samples, the initial contact angles were measured with some distribution as shown in Figure 3a. This is related to the quality of the CVD graphene and the uneven water intercalation. Third, the electrostatic force driven contact angle variation is small and symmetric in the nonfaradic region as shown in Figure 3a. This suggests that the pinning process is still prominent at the low bias voltages. However, the applied bias voltage becomes high beyond the nonfaradic region,23 the contact angle variation becomes significantly large and depends on the polarity of the bias voltage. This suggests that the faradic reaction, which depends on the bias polarity,22 plays the key role in the wetting, intercalation as well as graphene patterning. When the electrostatic potential is applied as shown in Figure 8c and 8f, the graphene is more bent out while the water molecules underneath the graphene are collected at the bent region due to the space created from the bent. At negative bias, as shown in Figure 8c, the depletion of water between graphene and substrate increases the adhesion energy between them. However, at positive bias, as shown in Figure 8f, the graphene is oxidized. Fourth, with further increasing of the bias voltage, the water permeation underneath the CVD graphene at the triple point is very plausible33,34 because the CVD graphene is defective and porous unlike the mechanically cleaved highly crystalline graphene as shown in parts d and g of Figure 8. Thus, the water molecules can intercalate through the defect or the defect edge. The water drop underneath the graphene can be merged because the relatively high surface energy of water21 is 72 mJ m−2 while the wet adhesion energy of graphene39 is known as 60 mJ m−2 and the surface energy of silicon oxide36 is 115−200 mJ m−2. With competition of the surface energy, the water droplet becomes larger. So the tension at the triple point makes a spatial gap between graphene and the SiO2 layer and then water can be intercalated. This is why the intercalation was observed only at the edge (triple point) of the droplet and the detachment occurred in a radial form. The water droplet formation in the nanospace between the graphene and the hydrophilic substrate can be considered as a synergetic effect of the nanoconfinement of water and the hydrophilicity of the substrate and the graphene.40,41 Fifth, the stress developed at the graphene due to the larger graphene blisters initiates the cracks at the weak bonding sites such as detect sites, pore edges, pentahepta ring structures.26,27 With increasing of the applied bias voltage, the fracture is developed. However, the cracking becomes slower and saturated at the given bias voltage because the adhesion between graphene and the substrate becomes stronger owing to the dry contact formation of graphene to the substrate while the bias voltage is applied. Sixth, the patterning depends on the bias polarity. It is attributed to the faradic reaction at the graphene.22,23 In the anodic configuration, the graphene becomes oxidized with the surrounding water molecules. There are water on the graphene as well as water under the graphene. Particularly, when the water under the graphene is participating at the reaction, the bonding between graphene and the substrate becomes stronger with a possible chemical bonding formation of C−O−Si.38 This strongly suggests that the patterning at the anodic configuration is small and discontinuous compared to that at the cathodic configuration as shown in Figure 8h. However, at the cathodic configuration in the faradic regime, the evolution of hydrogen

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CONCLUSION In this study, scalable direct graphene patterning is demonstrated by employing a simple electrochemical method. This allows maskless direct patterning and functionalization of CVD graphene on SiO2 (300 nm)/Si. The triple point of garphene, liquid and air, and the hydrophilic substrates play the significant role in the intercalation of water underneath the graphene as well as the patterning process. The liquid intercalation and graphene functionalization depend on the polarity of the applied bias voltage. This approach can be extended to the large scale direct graphene patterning and functionalization. A large scale direct writing or patterning method is proposed by employing a computer controlled dragging method of the droplet under a bias voltage. This allows to pattern large scale graphene, which is loaded on a computer controlled xyz stage, with combination of a movable tip in the xyz direction. The patterned feature size can be controlled with the droplet size, pipet tip size, applied bias voltage, biasing and dwelling time, or scan speed such as with an inkjet printer. This can be extended to a roll to roll direct writing/patterning system.

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AUTHOR INFORMATION

Corresponding Author

*(J.C.) E-mail: [email protected]. ORCID

Jaewu Choi: 0000-0002-4648-9054 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Kyung Hee University in 2012 (KHU-20121737). REFERENCES

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DOI: 10.1021/acs.jpcc.7b04639 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b04639 J. Phys. Chem. C XXXX, XXX, XXX−XXX