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Healing Graphene Defects using Selective Electrochemical Deposition: Toward Flexible and Stretchable Devices Taeshik Yoon, Jae-Han Kim, Jun Hyung Choi, Dae Yool Jung, Ick-Joon Park, Sung-Yool Choi, Nam Sung Cho, Jeong-Ik Lee, Young-Duck Kwon, Seungmin Cho, and Taek-Soo Kim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b07098 • Publication Date (Web): 29 Dec 2015 Downloaded from http://pubs.acs.org on December 31, 2015

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Healing Graphene Defects using Selective Electrochemical Deposition: Toward Flexible and Stretchable Devices Taeshik Yoon1, Jae-Han Kim1, Jun Hyung Choi1, Dae Yool Jung2, Ick-Joon Park2, SungYool Choi2,5, Nam Sung Cho3, Jeong-Ik Lee3, Young-Duck Kwon4, Seungmin Cho4, and Taek-Soo Kim1,5* 1

Department of Mechanical Engineering, KAIST, Daejeon 34141, Korea 2

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Department of Electrical Engineering, KAIST, Daejeon 34141, Korea

Electronics and Telecommunications Research Institute, Daejeon 34129, Korea 4 5

Hanwha Techwin, Seongnam 13201, Korea

Graphene Research Center (GRC), KAIST, Daejeon 34141, Korea

To whom correspondence should be addressed. *E-mail: [email protected]

KEYWORDS: Graphene, Defect, Healing, Electrochemical deposition, Flexible and stretchable electronics

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ABSTRACT: Graphene produced by chemical-vapor-deposition inevitably has defects such as grain boundaries, pinholes, wrinkles, and cracks, which are the most significant obstacles for the realization of superior properties of pristine graphene. Despite efforts to reduce these defects during synthesis, significant damages are further induced during integration and operation of flexible and stretchable applications. Therefore, defect healing is required in order to recover the ideal properties of graphene. Here, the electrical and mechanical properties of graphene are healed based on the selective electrochemical deposition on graphene defects. By exploiting the high current density on the defects during the electrodeposition, metal ions such as silver and gold can be selectively reduced. The process is universally applicable to conductive and insulating substrates, because graphene can serve as a conducting channel of electrons. The physically filled metal on the defects improves the electrical conductivity and mechanical stretchability by means of reducing contact resistance and crack density. The healing of graphene defects is enabled by the solution-based room temperature electrodeposition process, which broadens the use of graphene as an engineering material.


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Graphene appears to hold great promise as a next generation material for a wide range of engineering applications;1,

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however, the unveiling of its defects is exposing the mismatch

between ideals and reality. It is revealed that the graphene defects impede the ballistic electronic transport,3,

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lower the mechanical strength,5 and disturb the impermeable characteristic.6

Intrinsic defects such as grain boundaries and pinholes are created as the grain grows from randomly nucleated carbon seeds, which generally occurs during the chemical vapor deposition (CVD) process.7 Wrinkles, cracks, and residues are formed during the integration of graphene, mostly due to the transfer and lamination processes.8, 9 The formation of these defects should be suppressed during the fabrication process, but defects are also generated by mechanical or surface damage during the operation of graphene devices. Therefore, control of defects is crucial in determining the quality of graphene. There have been many endeavors to control and minimize graphene defects;10-12 however, it is considered that defects are inevitable in the integration of graphene and therefore methods of healing these defects should be explored. The healing of damaged graphene preserves the intrinsic electrical and mechanical properties of graphene without compromising the quality, and it ensures long-term durability in flexible and stretchable electronic devices. Recent research demonstrated defect healing processes, which improved the electrical conductivity through selective metal deposition on the defects. The selective chemisorption of metal was enabled by atomic layer deposition (ALD).13, 14 However, the ALD process is limited to specific coating materials and substrates. Moreover, the ALD requires high-cost equipment, and has low deposition rate, which hinder the commercialization of the process. The other approach used the redox reaction between copper substrate and gold.15 Despite the high selectivity on defects, the reaction cannot be applied to non-metal substrate such as polymer and ceramic, therefore the


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process is not viable after the transfer process. These approaches were focused on the healing of intrinsic defects formed in synthesis, however the healing of extrinsic damages was not considered. Furthermore, the healing effects on the mechanical property and stability have not reported yet. Here, we demonstrate the healing of graphene defects realized through the selective electrochemical reduction of metal ions. The metals were selectively deposited on graphene defects, which is universally applicable on conductive metal and insulating polymer or ceramic substrates. The electrical conductivity and mechanical stretchability of damaged graphene were improved after the filling of physical gaps by the electrodeposited metal. External forces such as mechanical strain and surface scratch were induced on graphene, followed by the healing process; it was found that the electrical conductivity was restored with visual markers for the induced cracks. RESULTS AND DISCUSSION Chemical-vapor-deposited (CVD) graphene7,

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was used in this study, because synthesized

graphene is exposed to numerous defect nucleation processes. The graphene samples were classified according to the contact surfaces: one is as-grown graphene on copper and the other is transferred graphene on insulating substrates such as glass or polymer. The transferred graphene was made from as-grown graphene via the transfer process, which is composed of bonding with the thermal release film, etching the metal, and releasing to the target substrate.9 Electrochemical reduction of silver on graphene defects. Here, we demonstrate the selective healing of graphene defects based on electrodeposition,16-18 as depicted in Fig. 1a. Silver nitrate was selected as metal salt to utilize the high electrical


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conductivity of silver, and an aqueous solution was made with a concentration from 10-3 to 10-5 M. As depicted in Fig. 1a, the cathode was connected to the graphene surface and the anode was connected to the graphite rod with a DC power supply. Graphite was selected for the anode material because it has sufficient conductivity and is insensitive to oxidation. Silver ions were reduced on the cathode, while oxygen molecules evolved on the anode. The electrochemical reaction requires a potential difference, which can be determined using the standard electrode potential and the Nernst equation,19 as plotted in Fig. S1. In our experiments, the concentration was varied from 10-3 to 10-5 M, and the resulting cell potential was –0.193 and –0.312 V, respectively. As a result, the required electrical potential for the non-spontaneous reaction is less than one volt, but for a rapid response, the potential was varied from –1 to –5 V. The intrinsic defects such as grain boundaries and pinholes can be selectively healed by the electrodeposition on both conductive and insulating substrates. The as-grown graphene on copper (Fig. 1b and 1c) and the transferred graphene on glass (Fig. 1d and 1e) were observed using a scanning electron microscope. It was found that the silver was partially deposited under certain conditions: when the concentration was less than 10-3 M and the applied potential was less than –5 V, the effects of the concentration and potential are discussed in Figure S7 and S8. After the healing process, the silver ions were selectively reduced on the graphene defects with specific mesh structures, and the elemental analysis is presented in Figure S2. The micrometersized mesh is the line defects on the synthesized graphene, which are primarily grain boundaries or cracks. The configuration of deposited silver is point-like on copper, while it is line-like on glass substrate. The deposition mechanisms on conductive and insulating substrates are different, and it will be further discussed in the following figure. In previous research, the graphene defects were visualized via oxidation or corrosion of the underlying metal.20-22 These methods cannot be


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applied to the transferred graphene, because the target substrate is typically polymer or ceramic, which is more inert compared with metal. Furthermore, these methods can induce oxidation, exfoliation, and cracking on the graphene and metal. However, the electrodeposition method relies on the reduction process, which does not degrade the quality of graphene and can be adopted on any substrate. Selective electrodeposition mechanisms. The electrodeposition of metal on graphene enables the optical classification of intrinsic defects in graphene. As shown in Figure 2, the two types of graphene show its own defects configuration which is classified into pinhole and line defects. The deposition mechanism is slightly different between conductive and insulating substrates. In the case of conductive substrate, electron flows in low resistance path which is copper rather than graphene. On the graphene covered region, only small portion of electron can flow outward to graphene surface because of the contact resistance between copper and graphene. When electrons encounter the defect sites, the electrons freely move outward to copper-solution interface, and which causes the reduction of silver ions on copper. Therefore, the entire area of line defects and pinhole were covered with silver. This phenomenon can be also explained by the spontaneous galvanic reaction between silver and copper.23 The copper tends to be oxidized by giving electrons, while the silver is reduced by accepting the electrons. However, in this research, we applied potential to accelerate and maximize the deposition of metal. It was also confirmed with Raman spectra that pinhole and line defect were fully covered with silver, as shown in Figure 2b. The deposited silver has strong Raman peak24, 25 similar to the D and G peaks of graphene.26 The general Raman spectrum of graphene including 2D peak is obtained on the pristine graphene grain, while strong silver peak


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was observed on the line defect and pinhole. The pinhole spectrum showed strong silver peaks without any 2D peak of graphene. Furthermore, the electrodeposition is also applicable on insulating substrates such as polymer or ceramic. The current path is different from that in the conductive substrate because graphene is the only channel to transport electrons. The defect sites are electrically disconnected, and therefore the charge distribution on graphene is affected by the defect geometries. Based on the Coulomb’s law,27 the charge distribution in various geometries can be numerically analyzed with the moment method.28-30 It is obvious that charges are concentrated on geometric boundary such as edges or vertices, which yields the selective electrodeposition of metal. As shown in the pinhole of Figure 2c, the silver was partially deposited along edges, while the entire area of pinhole was covered on graphene-copper. It was also confirmed with Raman spectroscopy that the pinhole spectrum does not show any peak. The selective electrodeposition mechanisms on conductive and insulating substrates were investigated, and which ensures the healing of graphene defects on any substrate. Healing of mechanical and electrical properties in graphene Through exploiting this selective electrodeposition, the graphene defects can be healed. In order to maximize the performances of graphene such as the electron transport and load support, the defects should be minimized. Here, the defects in graphene were used as functional spaces, and the electrodeposition enabled the healing of electrical conductivity and mechanical stretchability. Using the selective growth characteristic, silver can fill the physical gap between the separated graphene domains. The healed graphene domains have a higher electrical conductivity through reducing the contact resistance, and they support high mechanical deformation without fractures.


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The healing of electrical conductivity and mechanical stretchability were demonstrated, as shown in Figure 3. The mechanical stretchability represents the resistance to fracture, and cracks are the most critical defects in graphene; thus, the density of the cracks should be reduced in order to obtain high stretchability. As depicted in Figure 3a, cracks can be physically filled and reconnected via electrodeposited metal. The selective healing reduces the crack density and contact resistance, which enables uniform load and charge transfer. The multilayer graphene was synthesized on a Ni-deposited silicon wafer, and then the graphene-Ni thin film was mechanically peeled in water through weakening the adhesion of the Ni-SiO2 interface.31 After the etching of the Ni film, the multilayer graphene could glide freely on the liquid surface, and it was tensile tested by gripping both ends,32 as depicted in Figure 3c and 3d. The tensile strain was applied to the graphene film until a fracture occurred. The results in Figure 3b show that the healed graphene had a higher elongation at break, because the overall crack density was decreased and each graphene grain was strongly tied by the metal particles. The electrical resistance was also measured by transferring the multilayer graphene onto a PET substrate. The integration of graphene requires the lamination process, and which induces numerous cracks on graphene. However, after the healing process, the sheet resistance was significantly reduced because the electrodeposited metals filled the cracks. It was further demonstrated that the electrical conductivity of defective graphene is tunable by controlling the healing conditions. The monolayer graphene was transferred onto PET and was healed using the electrodeposition process. In Figure 3e and 3f, the changes of the sheet resistance are plotted as a function of the concentration, voltage, and time. The molar concentration of silver nitrate was varied from 10–3 to 10–5 M, the reduction time was varied from 1 to 60 minutes, and the applied voltage was varied from –2.5 to –5 V. The plots show that


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the resistance was decreased as the time, and voltage increased at the high concentration, 10–3 M and 10–4 M. In the most extreme case, the resistance was decreased by two orders of magnitude. The deposited silver reconnected the graphene grains and reduced the contact resistance, which improved the overall conductivity in films. However, the low concentration results of 10–5 M showed the increased resistance. The electrochemical reaction is accompanied with the evolution of hydrogen gas at cathode; therefore the graphene-substrate interface would be partially delaminated by the bubbles.33 The other reason is the chemical doping effect on graphene. The adsorption of hydrogen ions on graphene induces n-doping,34 which in turn reduces the conductivity of the p-doped transferred graphene. Further research is required regarding the effect of hydrogen bubbles and ions. Healing damaged graphene. The healing process also can be applied to strain damaged graphene. Nowadays, flexible and stretchable characteristics are required in advanced electronic devices. These devices should sustain high strain without failure; otherwise, the induced defect such as cracks should be controlled to ensure long-term reliability. However, cracks are inevitable because graphene exhibits brittle behavior, and therefore the damage should be treated by the healing process. Figure 4b to 4e shows the tensile strained and healed graphene on a PET substrate. The electrical resistance was monitored with respect to the applied strain, as plotted in Figure S3. Figure 4b to 4d present the metal-connected cracks under each strain: the induced cracks were perpendicular to the strain direction and the density was increased as the strain increased. The sheet resistance is plotted in Fig. 4e. After the strain damage, the resistance was significantly increased. However, the cracks on the graphene were effectively healed and the resistance was recovered.


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The most extreme case of strain damage is folding. Folding induces localized high strain on films and substrates, which is accompanied with the plastic deformation of the substrate. The graphene on PET film was folded and healed as shown in Figs. 4f and 4g. The folded graphene-PET was permanently deformed and electrically disconnected. However, after the electrodeposition process, the folded graphene was healed, and this was confirmed through turning on an LED. It was demonstrated that the strain damaged graphene can be healed through the electrodeposition process, which is a significant opportunity for the realization of graphene in flexible, stretchable, and foldable electronic devices. Surface scratch is a crucial issue, particularly in diffusion barrier or tactile devices, where graphene is exposed to the outer surface. Graphene is prone to be scratched or torn due to its weak adhesion, and this induces delamination failure. It was demonstrated that the scratched graphene on PET can be healed using the modified electrodeposition process, as depicted in Figure S4. The scratch on graphene-PET surface was made with the cutting plotter (Jaguar IV, GCC Company, USA). The letters “HEALING” were programmed in the software, and the steel cutting blade scribed the surface with 150 gram force. The modified electrodeposition process does not need to be immersed in a solution; therefore, the healing agent is selectively applied on the scratches. As shown in Figure 4h, the silver nitrate solution was dropped on the scratch, and an alternating current was applied to both sides of the graphene surface. Due to the alternating current, electrons were concentrated on both scratched edges and it reduced the silver ions; the entire video clip can be viewed in movie S1. As shown in Figure 4i, the 40 µm gap scratches were successfully re-connected and visualized via the selective reduction of the silver ions. The modified electrodeposition process has several practical advantages including that the specific anode electrode is not required, the healing can be applied on a local region by positioning the


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droplets, and therefore the healing solution can be saved. It is anticipated that the spraying of droplets with a current source enables the local healing of surface damage on graphene. Electrochemical deposition of gold. The benefit of the electrodeposition method is that any electroplating materials can be applied to graphene. Here, gold was also demonstrated for the healing of intrinsic defects in graphene, as seen in the dark-field optical image Figure 5. A gold sulfite aqueous solution was used for the electrodeposition, and the gold was selectively deposited on the graphene defects, which is similar to the silver in Figure 2. The left side image is the original graphene on copper, and the white spots are partially oxidized copper. As shown in the right side, the graphene grain boundaries were easily detectable via the electrodeposited gold. The graphene grain boundaries are independent of the copper grain boundaries, and the graphene grain size is five to ten microns. The defects of the transferred graphene on glass were also healed, as seen in Figure S6. These findings imply that the selective electrodeposition method can be used universally for graphene healing. For example, the metal filled graphene defects will enhance the performance of the gas diffusion barrier, by reducing voids. It is expected that the electrodeposition can restore the ideal characteristics of graphene and provide novel functionality in graphene, which is a new opportunity in various applications. CONCLUSIONS In conclusion, we have shown the healing of graphene defects using the electrochemical reduction of metals. The selective deposition of metal was attributed to the current concentration on the defects, and it was successfully applied on copper, glass, and PET substrates over a large area. The mechanical stretchability of the free-standing multilayer graphene was directly


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measured, and the healed graphene exhibited increased stretchable behavior. Moreover, it was demonstrated that strained and scratched graphene could recover its electrical conductivity. The facile solution-based room temperature electrodeposition process is applicable to the production, integration, and operation of graphene devices and the findings provide a new approach to tune graphene defects.

METHODS Electrochemical deposition. The silver nitrate (AgNO3) was used for the electrochemical deposition, which was purchased from OCI Company Ltd. in Korea. A deionized water of 100 ml in a beaker was prepared, and the salt was dissolved by stirring the solution in room temperature. Especially, the beaker was thoroughly cleaned with sonication because some contaminants may affect the electrodeposition. A graphene sample was prepared in 15 × 15 mm size, and additional silver paste was coated on the both edges of the specimen to make stable electrical contact. A graphite rod having 2-mm diameter was prepared for an anode material. A DC power supply was prepared for the electrodeposition, cathode was connected to the graphene sample and anode was connected to the graphite rod. The applied voltage was controlled during the electrodeposition, and the both electrodes were separated by a 43-mm distance. Raman and SEM characterization. For the characterization of graphene, we utilized Raman spectroscopy. The ARAMIS (Horiba Jobin Yvon, France) equipment was used, and an Ar ion laser beam (514.5 nm) was irradiated on


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graphene surfaces. The obtained spectrum ranged from 1200 cm-1 to 3000 cm-1, and each spectrum was acquired twenty times. A field-emission scanning electron microscope (FE-SEM) was used for the observation of electrodeposited graphene surfaces. The 10 kV electron beam with 2500 × magnification was used to observe the electrodeposited metals on graphene. Strain damaging on graphene-PET composite. A graphene was transferred on PET substrate to measure the electrical resistance change during the tensile of specimen. The sample was fabricated in 3 × 35 mm size, and using the customized grip, electrical resistance was measured during the loading and unloading a specimen. A highprecision micromechanical test system (Delaminator Adhesion Test System; DTS Company, USA) was used to applying strain on graphene-PET composite, while the electrical resistance was measured using a probe machine (2000 Multimeter, KEITHLEY Instruments, USA). The applied strain rate was 5 µms-1. The measured electrical resistance was converted to a sheet resistance by dividing the aspect ratio of the specimen.


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Figure 1. Defects healing of the as-grown and transferred graphene using selective electrochemical deposition of silver. (a) A schematic of the deposition process in aqueous silver nitrate solution. The graphene on the substrate was connected to the cathode, and a direct current was applied. (b), (c) The SEM images of before and after the healing on graphene-copper, respectively. (d), (e) The SEM images of before and after the healing on graphene-glass, respectively.


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Figure 2. Selective electrodeposition on conductive metal and insulating glass. (a), (b) An optical image with schematic and Raman spectra of selectively deposited silver on graphenecopper. Pinhole was fully covered with silver. (c), (d) An optical image with schematic and Raman spectra of selectively deposited silver on graphene-glass. The edge of pinhole was covered with silver as line defects.


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Figure 3. Electrical and mechanical healing of defects in graphene. (a) A schematic of the healing effect on graphene. The deposited metal prevents stress concentration and generates current paths. (b) The measured elongation at break and sheet resistance of the multilayer graphene. The healed multilayer graphene exhibited higher mechanical stretchability and electrical conductivity through reducing the cracks and contact resistance. (c), (d) Photographs of the floating-based tensile testing of the multilayer graphene, before and after the healing, respectively. (e), (f) The sheet resistance change of the transferred monolayer graphene-PET is plotted as functions of the healing time, and solution concentration. The applied voltage was –2.5 V and –5 V, respectively.


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Figure 4. Damaged graphene and its recovery of electrical conductivity via selective healing. (a) A schematic of the damaged graphene on a PET substrate. The strain damage was induced by stretching and folding, and the surface damage was induced by scratching. (b)-(d) Optical images of the healed graphene after being stretched for 2.5%, 5%, and 10%, respectively. The induced cracks were visualized via the deposition of silver, and the density was increased as the strain increased. (e) The sheet resistance change of the damaged and healed graphene in each strain condition. (f), (g) Photographs of the folded and healed graphene, respectively. The folded graphene-PET was damaged with severe plastic deformation. The healed graphene recovered its electrical conductivity and was demonstrated by LED. (h) A photograph of the scratched graphene on the PET: the scratch was not visible before the healing process. (i) A photograph presents the visualized letters on the graphene after the healing process, and the inset shows the re-connected scratches by the deposited silver.


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Figure 5. Healed graphene defects on copper via the selective electrodeposition of gold. The dark-field optical microscope image indicates the gold deposited graphene grain boundaries on the right side, while the left side image only shows the original copper grain morphologies.


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Acknowledgement. This work was supported by the Graphene Materials and Components Development Program of MOTIE/KEIT (10044412, Development of basic and applied technologies for OLEDs with graphene), by the Basic Science Research Program (2015R1A1A1A05001115), Nano-Material Technology Development Program (2012M3A7B4049807), and Global Ph.D. Fellowship Program (2013H1A2A1032731) funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea, and also by the High-Risk High-Return research project, KAIST. The multilayer graphene was also supplied by JUSUNG Engineering, Korea.

Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI.

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