A Sequential Process of Graphene Exfoliation and Site-Selective

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Functional Nanostructured Materials (including low-D carbon)

Sequential Exfoliation of Graphene and Site-Selective Copper/graphene Metallization Enabled by Multifunctional 1-Pyrenebutyric Acid Tetrabutylammonium Salt Jie Zhao, Chenyu Wen, Rui Sun, Shi-Li Zhang, Biao Wu, and Zhi-Bin Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21162 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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ACS Applied Materials & Interfaces

Sequential Exfoliation of Graphene and Site-Selective Copper/graphene Metallization Enabled by Multifunctional 1Pyrenebutyric Acid Tetrabutylammonium Salt Jie Zhao1,2, Chenyu Wen1, Rui Sun3, Shi-Li Zhang1, Biao Wu2, Zhi-Bin Zhang1* 1 Division of Solid-State Electronics, Department of engineering sciences, Uppsala University, SE-751 21 Uppsala, Sweden 2 Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, People’s Republic of China 3 Division of Nanotechnology and Functional Materials, Department of engineering sciences, Uppsala University, SE-751 21 Uppsala, Sweden

*Corresponding author E-mail: [email protected] Abstract This paper reports a procedure leading to a shear exfoliation of pristine few-layer graphene flakes in water and a subsequent site-selective formation of Cu/graphene films on polymer substrates, both of which are enabled by employing the water soluble 1pyrenebutyric acid tetrabutylammonium salt (PyB-TBA). The exfoliation with PyB-TBA as enhancer leads to as-deposited graphene films dried at 90 oC that are characterized by electrical conductivity of 110 S/m. Due to the good affinity of the tetrabutylammonium cations to the catalyst PdCl42-, electroless copper deposition selectively in the graphene films is initiated, resulting in a self-aligned formation of highly conductive Cu/graphene films at room temperature. The excellent solution-phase and low-temperature processability, self-

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aligned copper growth and high electrical conductivity of the Cu/graphene films have permitted fabrication of several electronic circuits on plastic foils, thereby indicating their great potential in compliant, flexible and printed electronics. Key words Graphene, electroless copper deposition, solution-phase processing, self-aligned metallization, flexible electronics


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Introduction Over the past decade, great advancement in the development of flexible and stretchable electronics has been made.1-5 The mechanically deformable electronics can be applied in the development of wearable electronic devices for real-time healthcare monitoring and sensing, wearable displays, electronic skins, implantable devices smart textiles, deformable antenna, and energy conversion and storage devices.6-12 For flexible electronic devices, conductors of high electrical conductivity and high fatigue strength that are capable of withstanding repeated mechanical deformation are crucial. To date, metals particularly silver and copper are still the predominant conductive materials in printed flexible electronics due to their high electrical conductivity.13-18 Their deposition is normally carried out by means of (1) depositing and sintering nanoparticles (NPs) or (2) metal-organic decomposition (MOD).19-22 However, silver and copper have low fatigue strength which can limit the long-term reliability of a flexible device. Temperature normally higher than 150 oC for sintering NPs is required. The needed thermal budget is unwanted for temperature sensitive substrates. Silver is not abundant in earth and is thus expensive. Although copper is much cheaper, it is easily oxidized at nanometer dimension. Therefore, it is vital to find solutions to remedy the above challenges or to realize metal replacement. Carbon nanomaterials, e.g., graphene, have emerged as promising alternatives as a single-crystal graphene exhibits extraordinary electrical, electronic, mechanical and thermal properties.23-35 Graphene have very high fatigue strength and fatigue resistance, which are highly desirable properties in flexible electronics.3640

Challenges for the application of graphene remain in different aspects. First, how to achieve

efficient exfoliation of pristine one- and few-layer graphene flakes from graphite still remains a bottleneck. Most of the claimed graphene products in global markets should be considered as graphite particles.41 In addition, several issues in forming thin films with graphene flakes via printing persist. The presence of polymer residues requires relative high temperature (



200

oC)

to make printed graphene films conductive.42-46

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Poor control over the

microstructure in the films and the inferior physical contact among the individual graphene flakes are responsible for the degraded electrical and thermal conductivities.47,48 One viable solution to high performance printable conductors is to exploit synergetic effect by combining graphene with metals, ideally copper in a good manner. Recently, it has been demonstrated that a multilayer structures with alternating copper film and monolayer graphene sheet exhibits ultra-high strength at 1.5 GPa as compared to that of bulk copper (4080 MPa). The underlying mechanism is ascribed to blocking the dislocation propagation across the metal–graphene interface.49 Cu/graphene composites with graphene flakes uniformly distributed in the copper matrix have been studied for potential applications in 3D integration and electrofriction materials.50-52 The incorporation of graphene can increase the fatigue strength of copper by 400%.53 However, it is a critical issue on how a Cu/graphene structure in a controllable and scalable manner is produced. To disperse graphene in copper matrix, molecular level mixing (MLM) and electrochemical (EC) deposition have been used. The former method starts from forming CuO on graphene oxides (GO) followed by reducing both copper oxide and graphene oxide.52. The latter method is conducted in electrolyte of copper mixed with GO,51 which requires a conductive surface prior to the electrochemical deposition. An additional drawback in MLM and EC methods is associated with the use of GO. The high-density defect in GO is hard to be removed by ordinary reduction methods. In this work, we demonstrate a facile method to yield site-selective and self-aligned Cu/graphene films using pristine few-layer graphene flakes. By employing water-soluble 1pyrenebutyric acid tetrabutylammonium salt (PyB-TBA), an efficient shear exfoliation of few-layer graphene flakes from graphite in water with PyB-TBA as exfoliation enhancer, stable aqueous pristine graphene dispersion with PyB-TBA as stabilizer, and self-aligned formation of Cu/graphene films with PyB-TBA as reaction site provider have been readily


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achieved. The excellence in processability and performance of the Cu/graphene films has permitted fabrication of several electronic circuits on plastic foils, thereby indicating their great potential in compliant, flexible and printed electronics. Experimental section Materials Graphite flakes, 1-pyrenebutyric acid, tetrabutylammonium hydroxide solution, sodium hydroxide, ammonium tetrachloropalladate (II), potassium sodium tartrate tetrahydrate, copper (II) sulfate pentahydrate, formaldehyde solution were purchased from Sigma-Aldrich and used as-received. Exfoliation of graphene and patterned graphene film deposition 500 mg 1-pyrenebutyric acid (1.73 mmol) was added to an aqueous solution (20 mL) of tetrabutylammonium hydroxide solution (3.47 mL, 3.47 mmol). After stirring for 2 hrs, the solution became clear. Then 2.5 g graphite was added to the solution and followed by further stirring for 2 days. After that, the solution was moved to a clean beaker and more deionized (DI) water ( 300 mL) was added for shear mixing. The speed of shear mixing was 5000 rpm and the processing time was 30 min. The dispersion was kept for 2 hrs after shear mixing to allow the non-exfoliated graphite powder to sediment. The upper part of solution was collected and then was centrifuged at 3000 rpm for 20 min. The upper solution was decanted and moved into a bottle. A vacuum filtration was operated subsequently to collect the exfoliated graphene flakes. After washing with DI water for several times, the “cake” of exfoliated graphene flakes was re-suspended in DI water with short-period sonication resulting in aqueous graphene dispersions for thin film deposition. For patterning in graphene thin film deposition, a shadow mask made of cut plastic foil was pasted to poly(ethylene



terephthalate) (PET), graphene dispersion was then drop casted and dried at 65 °C for 15 min for material characterizations. The adhesive mask was removed afterwards to obtain patterned graphene films. To measure sheet resistance of the Cu/graphene, graphene films with bridge sheet resistance structure pattern were prepared by drop-casting on PET substrate, which pattern was defined by the tape mask. The graphene films for sheet resistance measurement were baked in air at 90 oC for 1 hr to remove moisture residue. Electroless deposition of copper The patterned substrates were immersed in a 5 mM (NH4)2PdCl4 aqueous solution for 20 min, followed by rinsing with DI water for several times to remove the non-immobilized PdCl42- species. The electroless deposition of copper was performed in a plating bath including a 1:1 mixture of freshly Fehling's reagent solution (0.012 g/mL NaOH, 0.029 g/mL potassium sodium tartrate tetrahydrate and 0.013 g/mL CuSO4·5H2O) and formaldehyde solution (9.5 uL/mL). The net reaction between formaldehyde and the copper (II) ions in Fehling's solution is: 2HCHO + Cu2+ + 4OH− → 2HCOO− + Cu0 + H2 + 2H2O. After electroless deposition, the samples were washed with DI for several times, dried by blowing with nitrogen gun and keeping on hot plate at 40 °C. Characterizations Raman spectroscopy was conducted on a Renishaw Raman spectroscope with laser wavelength of 532 nm and 20× lens. Powder X-ray diffraction (XRD) measurements were carried out in a Bruker D8 advance XRD Twin-Twin instrument (Bruker, Bremen, Germany) with Cu-Kα radiation (λ=1.5418 Å). The measurement was carried out in the 2θ range from 10° to 80° with a step size of 0.03° and a measuring time of 1 sec per step. The X-ray photoelectron spectroscopy (XPS) experiments were conducted on a PHI Quantera II scanning XPS microprobe. The spectra were calibrated against the C 1s peak at 284.8 eV for


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adventitious carbon. Scanning electron microscope (SEM) images and energy dispersive spectroscopy (EDS) were recorded on Zeiss 1530. Transmission electron microscopy (TEM) images were recorded with a FEI Titan Themis operated at 200 kV. To prepare graphene samples for the TEM analysis, small portion of the dispersion of PyB-1COO− modified graphene was diluted with ethanol and the resultant mixture was drop-casted on a TEM Cu grid. The sheet resistance was measured by means of four-probe technique and capacitance were measured by Keysight B1500A (Keysight Technologies, USA). The resistance change of Cu/graphene upon bending tests was recorded by Oscilloscope (MSOX2002A, Keysight Technologies, USA). The plastic masks for patterned graphene film deposition were designed by AutoCAD and prepared by Plotter Cutter.

Figure 1. Schematic illustration of shear exfoliation and stabilization of few-layer graphene flakes in water using PyB-TBA as exfoliation enhancer and dispersion stabilizer, respectively, as well as the photograph of a vial containing aqueous graphene solution.

Results and discussion In Figure 1, the shear exfoliation of graphene from graphite in water in the presence of PyB-TBA is sketched. Here, PyB-TBA act as molecular wedges to assist in cleaving the graphene sheets from the graphite by shear force.54 Firstly, edge expansion of the graphene



sheets can take place with PyB-TBA wedge during the magnetic stirring of the mixture of graphite and PyB-TBA in water (Step 1). The expansion facilitates cleaving of the graphene layer by shear force and allows PyB-TBA to intercalate between individual layers in the graphite during shear exfoliation (Step 2). The PyB-TBA form π–π interaction on the fresh surface of graphene.55 When the exfoliation completes, the resultant graphene flakes are well dispersed in water with PyB-TBA as stabilizer (see the photo in Figure 1). The two-fold functions of PyB-TBA, i.e., wedge and stabilizer, are due to the π–π interaction between the PyB-TBA and the basal plane of graphene. Computational simulations according to the reported literature values demonstrated that pyrene derivatives have stronger interaction and lower coefficient of friction than that of bilayer graphene.56,57 In addition, the TBA+ cations add cation–π interaction with graphene. It was found that the exfoliated graphene dispersion is stable for a few days under ambient condition. Raman spectroscopy is a powerful tool to identify the number of layers and structural defects of graphene.58-60 The Raman spectrum from graphene thin films shows three typical peaks of graphene (Figure 2a): D peak (1347 cm-1), G peak (1579 cm-1), and 2D peak (2688 cm-1). The ID/IG ratio of 0.52 is primarily ascribed to the edge effect of the graphene flakes and the presence of PyB-TBA on graphene. As the shape and position of 2D peak are sensitive to the number of graphene layers,61,62 de-convolution of the 2D peak was performed as shown in the insert of Figure 2a which reveals that the majority of the graphene flakes have 3-4 layers. XPS measurement shows that the carbon 1s spectrum locates at around 285 eV (Figure 2b), which is a characteristic peak of graphitic carbon. The spectrum is resolved as: C (sp2) at 284.8 eV, C (sp3) at 285.5 eV, C=O at 287.6 eV, O-C=O at 288.8 eV, and π–π* at 290.5 eV.63,64 One can see that the C (sp2) from graphene dominates the spectrum while the existence of C (sp3), C=O, and O-C=O groups is primarily attributed to PyB-TBA in the film. The presence of PyB-TBA is further confirmed by the FT-IR result (Figure S3). Several peaks


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in the range of 2800-3000 cm−1 originate from CH2 and CH3 in the PyB-TBA and the peak at 1693 cm−1 ascribes to C=O. The peaks at 1568, 1410, 840 and 708 cm−1 correspond to the characteristic signals of benzene while the peak at 1022 cm−1 can be attributed to C–O vibrations.

Figure 2. Raman spectrum, a), and carbon 1s XPS, b), of the shear exfoliated graphene films. The C1s spectrum is fitted using the graphitic carbon and PyB-TBA signals.

TEM analysis was performed to characterize the shear exfoliated few-layer graphene flakes. As shown in Figure 3, the lateral size of the flakes is in the range from 200 to 400 nm (Figure 3a). The TEM images in Figure 3c and 3d show 4- and 2-layer graphene flake, respectively. Bi-layer to four-layer graphene flakes were commonly observed in the TEM imaging. The TEM result is well in agreement with the above Raman analysis. Most the flakes are covered by PyB-TBA which gives rise to the observed disorder feature. Some areas in graphene flakes are free of PyB-TBA and exhibit ordered atomic arrangement.



Figure 3. Selected TEM images of graphene flakes prepared by PyB-TBA assisted shear exfoliation of graphite a) to d) with cross-section images as inset for a four-layer (c) and bi-layer (d) graphene flakes, respectively.

It was found that the use of PyB-TBA as dispersion stabilizer results in conductive graphene thin films deposited from the dispersion and dried at low temperature. One asdeposited film with thickness of ~1.8 m dried at 90 oC exhibits sheet resistance ~5 kΩ/□. As a result, a DC conductivity of 110 S/m is obtained. Graphene films deposited from dispersions with polymer stabilizers baked at such low temperature are typically non-conducting, as shown in our previous report.65 The relatively good conductivity obtained at such low temperature implies that the remaining PyB-TBA between neighbouring individual graphene flakes is very thin so that carriers can easily transport through different individual layers. It deserves to mention that the processing temperature, e.g., at most 90 oC, is fully compatible with most the temperature sensitive substrate materials like plastics, rubbers and papers. On the patterned graphene films with PET foils as substrates, self-aligned electroless copper deposition was readily achieved by following the procedure as illustrated in Figure 4.


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The procedure started by attaching a plastic shadow mask onto a PET foil (Figure 4a). A drop of graphene dispersion was casted and dried at 65 oC (Figure 4b). After removal of the mask, the patterned graphene film on the PET was formed (Figure 4c). Subsequently, the sample was immersed in the (NH4)2PdCl4 aqueous solution (photo between Figure 4c and Figure 4d). This was followed by rinsing with DI water to remove the un-immobilized catalytic PdCl42species. The samples were finally dried by nitrogen blowing (Figure 4d). Since the quaternary ammonium groups (QA+) show good affinity to PdCl42- species according to the reported literatures,66-68 the TBA+ cations should be able to immobilize the PdCl42- species, which can initiate the electroless copper growth. To prove this, the samples were immersed in a copper electroless plating bath (photo between Figure 4d and Figure 4e). Afterwards, the samples were rinsed with DI water and drying with nitrogen gun. As observed in the photo of Figure 4e, copper deposition that was self-aligned to the patterned graphene films was achieved. Self-alignment has been a standard metallization step used in conventional silicon technology as it dramatically simplifies the device fabrication. Silicide formation, i.e., silicidation, occurs for the metals (e.g., Ti, Ni, Co) in direct contact with the underlying Si, but not in contact with SiO2. The unreacted metal on SiO2 can be selectively removed. In this work, self-alignment was enabled by the presence of PyB-TBA in the graphene films. As the un-immobilized PdCl42- on the substrate was easily removed by water rinsing, the self-alignment in the copper growth was thus readily realized. An insightful investigation of the copper growth in the graphene films was provided by the SEM imaging (Figure 5). The morphology of graphene films exhibited flake feature of around 200 to 400 nm in size prior to the copper deposition (Figure 5a). After the copper deposition, the film surface was fully coated with copper and was clearly flattened (Figure 5b, Figure S4). The cross-section SEM images in Figure 5c-d and EDS in Figure S5-S6 shows that the copper growth mainly occurred on the surface of graphene films, forming a distinct



two-layer structure. Our EDS analysis reveals that Cu ions permeated inside the underlying graphene films a certain degree which likely occurred during the initial Cu growth in the aqueous solution. Once the continuous Cu film was formed, the permeation of Cu ions was hindered. By measuring the total sheet resistance and thickness of an as-deposited Cu/graphene film, an effective DC conductivity at 7.9 × 105 S/m was obtained. This value is two orders of magnitude lower than that of bulk Cu (5.8 × 107 S/m).69 With the measured thickness and DC conductivity of underlying graphene film, the grown Cu layer with 0.25 m in thickness exhibits DC conductivity at 7.8×106 S/m.

Figure 4. Schematic illustration of the procedure used for site-selective and self-aligned electroless deposition of copper on the graphene films on PET substrates where the real pattern is represented by three lines in the sketch. a-c) The formation of patterned graphene film on PET substrate, and d-e) electroless deposition of copper.

In order to prove the significant role of TBA+ cations in the site-selective, thus selfaligned, electroless deposition of copper, a control experiment was carried out. Aqueous fewlayer graphene dispersions were prepared with addition of PyB-TBA, 1-pyrenebutyric acid sodium salt (PyB-Na), and (Hydroxypropyl) methyl cellulose (HMC), respectively, under the same condition. PyB-Na has been used as graphene exfoliation enhancer in a ball-milling process70 and HMC is water soluble cellulose normally used as graphene dispersion stabilizer. The as-deposited films from the three dispersions undergo the electroless deposition of copper.


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As shown in Figure S1, well defined Cu/graphene films were formed only on the TBA+ based graphene film. As comparison, very few copper nanoparticles were deposited on the Na+ cations based graphene film and no copper were found on the HMC based graphene film. The comparison result demonstrates that TBA+ cations play a crucial role in the site-selectively electroless copper deposition by providing binding sites for the catalytic species.

Figure 5. Selected SEM images of, in top view a), a film of PyB-TBA assisted exfoliation of few-layer graphene flakes; b) the electroless deposited copper layer on a graphene film; c-d) cross-section SEM images of an asdeposited Cu/graphene film where the copper layer is indicated by the dash line and “G” represents the graphene-flake layer.



Figure 6. a) XRD spectrum of Cu/graphene film. b) Relative resistances of Cu/graphene film versus bending cycles in bending test. The insert images are the circuit diagram of the bending tests and photographs of the leased and compressed sample. c) Photographs and schematic illustration of sandwich structural capacitor; d) capacitance versus pressing/releasing cycles.

The facile solution-phase and low temperature processing, and self-alignment of Cu/graphene film with the high effective DC conductivity indicate the promising application in flexible electronics. An additional merit lies in the absence of copper oxide and cuprous oxide in the electroless deposited copper, as shown in the XRD result (Figure 6a). The peaks at 43.4°, 50.5°, 74.2° degrees correspond to the (111), (200), and (220) crystal planes of Cu, respectively.67 The resultant Cu/graphene film is electrically robust against mechanical bending. This is supported by the observation that the original resistance of a Cu/graphene film on a PET foil can be completely restored even the sample underwent repeated sever


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mechanical bending as shown in Figure 6b. For this test, a simple circuit illustrated in the insert of Figure 6b where the Cu/graphene belt (5 mm × 50 mm) is connected in series with a resistor and a DC voltage source was employed. The voltage on the resistor was monitored by oscilloscope so that the resistance change of the Cu/graphene film can be extracted (shown as the photos in Figure 6b). The Cu/graphene belt is rather mechanically robust as its resistance was changed by only 5% after the sample was repeatedly bended for 1000 cycles (Figure S7). To demonstrate a real application of the self-aligned Cu/graphene, a flexible capacitor array was prepared through sandwiching a thin layer of a polydimethylsiloxane (PDMS) layer by two PET foils containing parallel Cu/graphene lines, as shown in Figure 6c. By pressing the fabricated capacitor manually, the compressed PDMS layer leads to a significant increase in capacitance by 50 %, i.e., from ~12 pF to ~18 pF (Figure 6d). The capacitance immediately goes back to the original level after releasing. In Figure 6d, it further displays that the change of capacitance responds differently to the manual pressing with different pressure and frequency, which indicates the potential applications in the emerging areas such as wearable electronics, Internet of Things, and robots. In order to further demonstrate the versatile application of Cu/graphene as flexible electrical interconnects and wirings, a simple light-emitted diode (LED) circuit (Figure 7a) and a more complicated 8-LED sequentially illuminated circuit (Figure 7b) were designed and fabricated on PET foil. A button battery and a switch were interconnected with the LEDs via the Cu/graphene interconnects, as shown in Figure 7a. The LED was illuminated when the switch was turned on. The brightness of the LED remained unchanged when the circuit was bended (Figure 7a). In the LED sequentially illuminated circuit (Figure 7b), 555 Timer is used to generate a square wave with frequency varied by tuning a rheostat. The square wave is fed to D-triggers (74HC74) to generate 3-bit counting output. Then it is finally delivered to 3-8 decoder (74HC138), which 8-bit output drives LEDs (Figure 7b). The continuously counting



from 0 to 7 turns on the 8 LEDs circularly (video in supporting information). Details about the circuit design and working principle are provided in Figure S8 in the supporting information.

Figure 7. Photographs of a) the electronic circuit integrated with a button battery, a switch and a LED, as well as the robustness of the circuit when stretched and compressed; b) The 8-LED sequentially illuminated circuit integrated with eight LEDs, 555 Timer, D-triggers, 3-8 decoder (give time in each photo). c) The performance of the low frequency filter; the insert images are the photographs of the filter and the circuit diagram.

To demonstrate a fully printed circuit, a prototype of one-order RC low-pass filter on a PET substrate was fabricated. In the circuit, the electrical interconnects were made of Cu/graphene film and the resistor (10.45 kΩ) consists of few-layer graphene film achieved by drop-casting the graphene dispersion (the insert in Figure 7c). The capacitor was formed by drop-casting aqueous graphene oxide dispersion obtained from Graphenea between two interdigital electrodes made of Cu/graphene. As shown in Figure 7c, the cut-off frequency of the circuit is 690 kHz, from which the capacitance can be calculated to be 22.1 pF. This prototype of all carbon-based circuit provides one a solution-phase approach for preparing low-cost electronic components on flexible substrates at room temperature. It deserves to


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stress that the materials and the processing techniques developed in this work are compatible with the advanced printing techniques and roll-to-toll manufacturing. Conclusion As a summary, by using water soluble pyrene derivative, PyB-TBA, efficient shear exfoliation of pristine few-layer graphene flakes from graphite in water, stable graphene dispersions and site-selective formation of highly conductive Cu/graphene films have been subsequently achieved. PyB-TBA provides triple functions in the fabrication process: enhancer of graphene exfoliation, stabilizer in the resultant aqueous graphene dispersions and linkage sites for immobilizing PdCl42- species that catalyzes the electroless deposition of copper. Compared to the state of the art in graphene exfoliation and Cu/graphene formation, our method shows several distinct advantages. First, our exfoliation scheme leads to conductive graphene thin films (with conductivity at 110 S/m) dried at 90 oC at the most. Second, the graphene flakes in the Cu/graphene films are pristine, as opposed to use of rGO in the state of the art. Third, the formation of Cu/graphene is site-selective and self-aligned. Last but not the least, the Cu/graphene films can be realized on any surface, i.e., without requirement of conductive surface compared to the electrochemical deposition. In addition, it is highlighted that the procedure is carried out with water as media and under ambient conditions. The aforementioned advantages of our method facilitate the fabrication of electronic circuits using copper/graphene films as flexible interconnects on plastic foils. Supporting Information Supporting Information is available free of charge on the ACS Publications website at DOI: The comparison of electroless of copper on the surface of different stabilizer-based graphene, SEM image of the deposited copper nanoparticles, EDS, FT-IR spectrum, robust



test, the circuit diagram of the electronic circuit and different patterns of Cu/graphene composite (PDF). Acknowledgement The authors would like to acknowledge the financial support from the Swedish Foundation for Strategic Research (SSF Grant No: Dnr SE13-0061), the Swedish Research Council (No. 621-2014-5596) and Chinese Scholarship Council (CSC). The authors are grateful of Dr. Lars Riekehr for the assistance of TEM measurement.

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A Sequential Process of Graphene Exfoliation and Site-Selective

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